The proteasome as a druggable target with multiple therapeutic potentialities: Cutting and non-cutting edges.

Introduction

Under physiological conditions human cells express approximately 10,000 proteins that must be properly folded to carry out their biological functions ( Klaips, Jayaraj, & Hartl, 2018 ; Kulak, Geyer, & Mann, 2017 ). To comply with their role, some proteins retain a certain degree of structural flexibility which may render them more prone to misfolding and aggregation ( Chiti & Dobson, 2006 ; Ciechanover & Kwon, 2017 ). When proteins unfold, as a consequence of aging and/or environmental stress, or else are no functionally required, they undergo degradation to limit the threat raised by their maintenance ( Klaips et al., 2018 ). Thus, proteome fidelity (proteostasis) is achieved through a complex and multi-subcellular compartments network, which coordinates synthesis, folding, conformational upkeep and degradation ( Labbadia & Morimoto, 2015 ; Powers, Morimoto, Dillin, Kelly, & Balch, 2009 ).

Finding a universal definition of the proteostasis network (PN), which would encompass the structural composition, hierarchical organization and dynamics of recruitment of main actors is particularly challenging, mostly because the expression and the activity of many PN factors are tailored depending on the different physiological stimuli the cell may experience in the context of its tissue microenvironment. PN properties can be altered by physio-pathological and multi-factorial phenomena ( e.g., aging and/or environmental stress), or by mutations in PN components, which may lead to the onset/progression of different pathologies, including cancer, neurodegenerative disorders or other genetic diseases sustained by altered proteostasis ( Balch, Morimoto, Dillin, & Kelly, 2008 ; Labbadia & Morimoto, 2015 ; Powers et al., 2009 ).

A general and widely accepted view of the PN encompasses three major branches, namely: 1) protein synthesis, which adjusts the level of bulk proteins to cell demands; 2) protein folding, which is mediated by a vast repertoire of chaperones (now referred to as “chaperome”); 3) protein degradation, which allows the proteolytic removal of undesired proteins through two main intracellular proteolytic systems, namely Ubiquitin-Proteasome-System (UPS) and autophagy ( Ciechanover & Kwon, 2017 ; Klaips et al., 2018 ; Sala, Bott, & Morimoto, 2017 ). Furthermore, a myriad of regulatory proteins (such as transcription and metabolic factors, chromatin remodelling factors, and regulators of posttranslational modifications) act as PN auxiliary and coordinate the cross-talk between the PN compartments accounting for the afore mentioned plasticity of the PN ( Klaips et al., 2018 ; Labbadia & Morimoto, 2015 ).

Therefore, unlike early scientists, who considered proteins essentially stable and prone to only a minor “wear and tear” ( Schoenheimer, 1946 ; Schoenheimer, Ratner, & Rittenberg, 1939 ; Thibaudeau & Smith, 2019 ), it is now known that proteome is highly dynamic, and proteins constantly undergo turn over at different rates, according to their biological role ( Lecker, Goldberg, & Mitch, 2006 ; Thibaudeau & Smith, 2019 ).

In the 1950s, the discovery of autophagy-lysosome system as “intracellular exergonic digestive system” by de Duve and colleagues was the first step in understanding intracellular and extracellular protein breakdown ( De Duve, Gianetto, Appelmans, & Wattiaux, 1953 ; de Duve, Pressman, Gianetto, Wattiaux, & Appelmans, 1955 ; De Duve & Wattiaux, 1966 ; Sabatini & Adesnik, 2013). Over the same years, Simpson showed for the first time that intracellular proteolysis in mammalian cells requires energy, suggesting the existence of an additional mechanism of protein degradation ( Simpson, 1953 ). However, this observation was considered with scepticism, since hydrolysis of the peptide bond is exergonic, and there is no apparent thermodynamic advantage in energy use ( Wilkinson, 2005 ). However, the seminal Simpson's discovery found support in the 1970s, when Goldberg and colleagues identified a novel, cytosolic ATP-dependent proteolytic system ( Bigelow, Hough, & Rechsteiner, 1981 ; Etlinger & Goldberg, 1977 ; Goldberg, 1972 ; Goldberg & Dice, 1974; Goldberg & St John, 1976 ; Thibaudeau & Smith, 2019 ; Wilkinson, 2005 ). Some years later, Wilk and Orlowski purified a 700-kDa “multicatalytic proteinase complex”, which was able to cleave peptides after hydrophobic, acidic and basic residues, suggesting the existence of multiple active sites in its structure ( Wilk & Orlowski, 1980 ; Wilk & Orlowski, 1983 ). This “stacked donut ring” complex (which later was shown to be the 20S) was tnamed “proteasome”, and its orthologues were identified in all life domains ( Arrigo, Tanaka, Goldberg, & Welch, 1988 ; Tanaka et al., 1988; Tanaka, Waxman, & Goldberg, 1983 ; Thibaudeau & Smith, 2019 ). A milestone in protein degradation field was the discovery by Ciechanover and colleagues of a 8-kDa heat-stable protein, APF-1 (later renamed “ubiquitin”), whose ATP-dependent covalent conjugation with proteins targeted them for degradation by a downstream protease, that was then identified as the 26S proteasome ( Ciechanover, 2005 ; Ciechanover, 2013 ; Ciechanover, Finley, & Varshavsky, 1984 ; Ciechanover, Heller, Elias, Haas, & Hershko, 1980 ; Ciechanover, Hod, & Hershko, 2012 ; Hershko, Ciechanover, Heller, Haas, & Rose, 1980 ; Hershko, Eytan, Ciechanover, & Haas, 1982 ; Hough, Pratt, & Rechsteiner, 1986 ; Hough, Pratt, & Rechsteiner, 1987 ; Leestemaker & Ovaa, 2017 ; Varshavsky, 2006 ).

Over the last decade, the critical role played by UPS in the maintenance of protein homeostasis and its involvement in the pathogenesis of human diseases have been largely investigated. With respect to this, proteasome is now considered a crucial target for therapeutic intervention in many diseases, such as neurodegenerative, immune-related disorders and cancer.

In this review, we will first discuss the structure and function of proteasome under physiological conditions; then we will focus our attention on the alterations of the proteasome functionality involved in the onset and progression of neurodegeneration and cancer. Finally, we will summarize: 1) the FDA- and EMA-approved proteasome inhibitors that are used for cancer treatment as well as novel promising inhibitors currently investigated in preclinical studies and clinical trials; 2) proteasome activators as novel tools to treat neurodegenerative disorders.

Proteasome structure and function

General organization of UPS

The UPS is the major actor in the turn-over of most cellular soluble proteins, playing fundamental roles in several facets of cell life, such as cell cycle, apoptosis, DNA repair, antigen presentation, inflammation, cellular response to environmental stress, and morphogenesis of neuronal networks ( Glickman & Ciechanover, 2002 ; Kunjappu & Hochstrasser, 2014 ).

UPS displays a hierarchical organization which encompasses two intertwined and consecutive steps: 1) the covalent attachment of ubiquitin polymers to substrates; 2) degradation by the 26S proteasome of ubiquitin-tagged substrates, followed by the release of free and recyclable ubiquitin moieties along with oligopeptides of cleared protein ( Scheffner, Nuber, & Huibregtse, 1995 ; Glickman & Ciechanover, 2002 ; Grasso et al., 2017 ). Ubiquitin conjugation proceeds through a three-step mechanism. First, the ubiquitin-activating enzyme E1 activates ubiquitin in an ATP-dependent manner, generating a high-energy thiol ester intermediate. In the second step, activated ubiquitin is then transferred from E1 to one of several E2 enzymes (ubiquitin-conjugating enzymes), leading to the formation of another high-energy thiol ester intermediate. Finally, ubiquitin is conjugated to substrates by a ubiquitin (E3) ligase, which is responsible for substrate specificity ( Ciechanover, 2013 ; Glickman & Ciechanover, 2002 ; Hough et al., 1986 ; Leestemaker & Ovaa, 2017 ; Pao et al., 2018 ; Pickart, 2001 ; Windheim, Peggie, & Cohen, 2008 ). The end-point of UPS is the 26S complex (hereafter referred to as 26S), a multifunctional 2500 kDa proteolytic molecular machine, composed by the 20S proteasome core particle (CP, hereafter referred to as 20S), which houses the proteolytic activity. The 20S is capped by one or two 19S regulatory particle(s) (RP) (hereafter referred to as 19S), which carry out the ATP-dependent recognition, unfolding and translocation into the 20S of the poly-ubiquitinated substrate ( Ciechanover, 2005 ; Glickman & Ciechanover, 2002 ; Kunjappu & Hochstrasser, 2014 ; Pao et al., 2018 , see also 2.2 , 2.3 ). Over the last decades, several alternative regulators of 20S have been described, namely PA28 protein family and Blm10/PA200, whose structure, substrate specificities, and biological roles go beyond the scope of this review and are extensively reviewed elsewhere ( Rechsteiner & Hill, 2005 ; Tanaka, 2009 ; Huang & Chen, 2009 ; Kish-Trier & Hill, 2013 ; Cascio, 2014 ; Poot et al., 2014; Schmidt & Finley, 2014 ; Jiang, Zhao, & Qiu, 2018 ; Limanaqi, Biagioni, Gaglione, Busceti, & Fornai, 2019 ).

Although the initial dogma on proteasome recognition mechanism states that the 26S hydrolyses only proteins tagged with at least four ubiquitin molecules, emerging evidences show that poly-ubiquitin chains are not the unique signal. In fact, in some cases, multiple or single mono-ubiquitination appears to be sufficient to label a substrate for proteasomal degradation ( Kravtsova-Ivantsiv, Cohen, & Ciechanover, 2009 ; Shabek et al., 2012 ). Moreover, ornithine decarboxylase has been the first of a long series of protein substrates ( i.e., Rpn4, thymidylate synthase, myelin) reported to be degraded by the 26S regardless of ubiquitination ( Bercovich, Rosenberg-Hasson, Ciechanover, & Kahana, 1989 ; Chen, Barton, Chi, Clurman, & Roberts, 2007 ; Forsthoefel, Peña, Xing, Rafique, & Berger, 2004 ; Ju & Xie, 2004 ; Kudriaeva, Kuzina, Zubenko, Smirnov, & Belogurov, 2019 ; Li, Yuan, Pan, Liu, & Huang, 2016 ; Murakami et al., 1992 ; Rosenberg-Hasson, Bercovich, Ciechanover, & Kahana, 1989 ; Sheaff et al., 2000 ). This implies the existence of alternative molecular signals (also named “degrons”), such as specific amino acidic sequence or structural elements, that mediate proteasome recognition and degradation of substrates independently on their ubiquitination levels ( Baugh, Viktorova, & Pilipenko, 2009 ; Kudriaeva & Belogurov, 2019 ).

The biological significance of ubiquitin-independent degradation of substrates by the 26S is a topic deserving great attention in order to decipher its physiological meaning in tissue homeostasis. Two proposed explanations envisage that it could be “only” a remnant of evolution, or else it could be rather a mechanism that provides, under selected circumstances, an alternative strategy to overcome the de-regulation of the canonical ubiquitin-dependent pathway ( Erales & Coffino, 2014 ; Finley, 2009 ). In support of this second hypothesis, the turnover of Rpn4, a substrate and a transcriptional regulator of proteasome genes, is carried out through both ubiquitin-dependent and ubiquitin-independent pathways, providing the cell with an alternative mechanism to modulate the level of Rpn4 and of proteasome in the case of inappropriate ubiquitin conjugation ( Erales & Coffino, 2014 ; Hanna, Meides, Zhang, & Finley, 2007 ; Ju & Xie, 2004 ).

In this regard, an intriguing example of how ubiquitin-dependent and ubiquitin-independent pathways cooperate to survey cellular homeostasis comes from the regulation of the proteome of lipid droplets (LDs), that are ubiquitous, endoplasmic reticulum-derived storage organelles from which neutral lipids are rapidly mobilized in response to cellular demands. In fact, some proteins of LDs are degraded by proteasome through the canonical ubiquitination pathway, whereas some others are processed only when the “degron” signals become unmasked upon protein insertion into the lipid monolayer ( Bersuker & Olzmann, 2017 ). Interestingly, it has been reported that proteasome mediates ubiquitin-dependent degradation of patatin-like phospholipase domain-containing protein 3 (PNPLA3), whose sequence variant 148 M is resistant to ubiquitination and to proteasome degradation, and accumulates into LDs, contributing to non-alcoholic fatty liver disease pathogenesis ( Basu Ray, 2019 ; Kozlitina et al., 2014 ; Speliotes et al., 2011 ).

An additional issue in deciphering the mechanisms of proteasome degradation is the ubiquitin-independent degradation in vitro of macromolecular substrates by the uncapped 20S. In fact, several studies demonstrate that the 20S is able to degrade natively unfolded as well as oxidized and misfolded proteins ( Davies, 1993 ; Davies, 2001 ; Grune, Reinheckel, & Davies, 1996 ; Raynes, Pomatto, & Davies, 2016 ; Reinheckel et al., 1998 ; Shringarpure, Grune, Mehlhase, & Davies, 2003 ). Indeed, oxidative stress induces chemical alterations, bringing about conformational changes and exposure of hydrophobic residues on damaged protein surfaces ( Carrard, Bulteau, Petropoulos, & Friguet, 2002 ; Raynes et al., 2016 ). These surface hydrophobic patches stimulate, in an allosteric fashion, the translocation of the substrate into the 20S proteolytic chamber (see Section 2.2 for details) ( Coux, Tanaka, & Goldberg, 1996 ; Davies, 2001 ; Giulivi, Pacifici, & Davies, 1994 ; Kisselev, Kaganovich, & Goldberg, 2002 ), since under oxidative stress conditions this form is more stable than the 26S, which is quickly and reversibly inactivated likely through dissociation into free 20S and 19S particles ( Reinheckel et al., 1998 ; Reinheckel, Ullrich, Sitte, & Grune, 2000 ; Shringarpure et al., 2003 ; Wang et al., 2010 ; Wang et al., 2017 ). Moreover, also the E1-E2-E3 cascade is transiently inactivated during oxidative stress, supporting an ubiquitin-independent degradation of oxidized proteins ( Grune et al., 2011 ). Thus, the current view is that 20S activity on oxidized and damaged proteins might compensate for the loss of the ubiquitin-dependent activity of the 26S under redox imbalance.

Interestingly, the activity of 20S on these subsets of substrates might be assisted by PA28 which seems to increase its selectivity and activity ( Fabre et al., 2014 ; Grune et al., 2011 ; Pickering & Davies, 2012 ). This occurrence is further discussed below in regard to the neurodegenerative disorders, since up-regulation of PA28 of immune-proteasome ( i.e., an inducible proteasome subset expressed in hematopoietic cells, which plays a significant role in immunity, see Box 1) is a common feature of this class of human pathologies. Along with this, ubiquitin -independent degradation by 20S has been also demonstrated for substrates that, like oxidized proteins, have regions characterized by high hydrophobicity, such as tau protein and α-synuclein. These findings reinforce the role of 20S in the regulation of protein homeostasis independently from its association with RPs ( Asher, Tsvetkov, Kahana, & Shaul, 2005 ; Baugh et al., 2009 ; David et al., 2002 ; Dyson & Wright, 2005 ; Raynes et al., 2016 ; Tofaris, Layfield, & Spillantini, 2001 ).

The herein described picture underlines the complexity of proteasome heterogeneity, since proteasome composition, specificity and activity are flexible and finely regulated at multiple steps, including post-translational modification and regulatory factors ( i.e., proteasome interacting proteins) ( Morozov & Karpov, 2019 ; Tanaka, 2009; Tundo et al., 2017 ). Since the proteasome pathway is extremely dynamic and reflects cellular metabolic demands ( Dahlmann, 2016 ; Hirano, Kimura, & Kimura, 2016 ), it is not surprising that different proteasome forms may co-exist and fulfil different but interconnected functions that are not yet completely understood ( Morozov & Karpov, 2019 ).

As a matter of fact, it should be emphasized that, in vertebrates, proteasome has gained considerable tissue-specificity, as supported by the existence of alternative forms of proteasome, namely (see Box 1): immuno-proteasome, thymo-proteasome, and spermato-proteasome, in which constitutive catalytic subunits of 20S are replaced by inducible/tissue-specific homologs. This is a clear-cut example of evolutionary-based mechanisms for the refinement of intracellular proteolysis ( Kniepert & Groettrup, 2014 ; Morozov & Karpov, 2019 ; Murata et al., 2007 ; Qian et al., 2013 ; Tanaka, 2009 ; Uechi, Hamazaki, & Murata, 2014 ).

20S Core particle

20S structural arrangement

The 20S core particle, which belongs to the N-terminal nucleophilic (Ntn) hydrolase family, is a cylinder-like packed particle which contains four axial stacking heptameric rings, arranged into two outer α-rings and two inner β-rings ( i.e α1–7β1–7α1–7β1–7) ( Baumeister et al., 1988 ; Bochtler, Ditzel, Groll, Hartmann, & Huber, 1999 ; Groll et al., 1997 ; Groll et al., 2000 ; Kunjappu & Hochstrasser, 2014 ; Tanaka, 2009 ). Electron micrographic studies measured its molecular dimensions that are 160 Å in length and 120 Å in diameter ( Borissenko & Groll, 2007 ; Harris, 1968 ) ( Fig. 1 A). Eukaryotic 20S has a central channel, which houses proteolytic chambers distributed among six active β-subunits, three for each β-ring, namely: the chymotryptic-like (β5 subunit), the trypsin-like (β2) and caspase-like (β1) sites, which confer the property to preferentially cleave proteins after hydrophobic, basic and acidic residues, respectively ( Groll & Huber, 2003 ; Tanaka, 2009; Unno et al., 2002 ). Historically, the 20S was the first enzyme classified as threonine protease, in which the hydroxyl group of the N -terminal Thr1 acts as nucleophile in all active subunits ( Chen & Hochstrasser, 1995 ; Kisselev, Songyang, & Goldberg, 2000 ; Löwe et al., 1995 ). Thr1Oγ nucleophilic attack to the carbonyl carbon atom of the scissile peptide bond generates a first cleavage product, forming a covalent acyl-enzyme tetrahedral intermediate, followed by hydrolysis through the addition of a nucleophilic water molecule, which regenerates the functional active site and releases the second cleavage product ( Löwe et al., 1995 ; Marques, Palanimurugan, Matias, Ramos, & Dohmen, 2009 ; Groll et al., 1999 ; Huber et al., 2009). Conserved residues in the proximity of Thr1, involved in the proteolysis mechanism, are Glu/Asp17 and Lys33. Lys33, which forms a salt bridge with Asp17, is positively charged at neutral pH, contributing to lower the p K a of Thr1 amino group, so that this group can work as the proton acceptor required for the activation of Thr1 hydroxyl group ( Borissenko & Groll, 2007 ; Groll & Huber, 2003 ; Löwe et al., 1995 ) ( Fig. 1 B). Moreover, Ser129, Asp166, and Ser169 residues, which are close to Thr1, are required for structural stability of the proteolytic centre ( Borissenko & Groll, 2007 ; Chen & Hochstrasser, 1995 ; Heinemeyer, Fischer, Krimmer, Stachon, & Wolf, 1997 ; Löwe et al., 1995 ; Seemuller, Lupas, & Baumeister, 1996 ). Recently, a revised interpretation of the proteasome active site architecture has been proposed, according to which proteasome can be viewed as having two triads, both essentials for an efficient proteolysis, consisting of ( i ) Thr1, Lys33 and Asp/Glu17 residues and ( ii ) Thr1, Ser129 and Asp166 residues, respectively ( Huber et al., 2016 ). In this novel vision, Lys33 -NH 2 group is expected to act as the proton shuttle, while Asp17O γ orients Lys33 -NH 2 group, making it more prone to protonation, by raising its p K a ( Huber et al., 2016 ). The positive charge on Thr1-NH 3 + group, which is essential for the binding and stabilization of the amide nitrogen of incoming peptide substrates, is favoured by the close proximity of Ser129 and Asp166 residues, which increase its p K a value, this being a crucial step for the first cleavage and for proteolytic reaction progress. In conclusion, Lys33 and Asp17 seem to be required to deprotonate the Thr1 hydroxyl side chain, whereas Ser129 and Asp166 are needed to protonate the N-terminal amine group of Thr1 ( Huber et al., 2016 ; Vielberg, Bauer, & Groll, 2018 ) ( Fig. 1 B). It is important to emphasize that the names used to describe β-subunits catalytic activities do not reflect accurately the specificity of each active site, which is much broader, meaning that 20S function cannot be simply interpreted as the integration of the three different activities into a unique machine ( Bochtler et al., 1999 ; Groll & Huber, 2003 ). Accordingly, it has been reported that the substrate specificity is modulated not only by P 1 residue of the substrate, but also by the physical constraints of the substrate around proteasome active site ( Bogyo, Shin, McMaster, & Ploegh, 1998 ; Cardozo, Vinitsky, Michaud, & Orlowski, 1994 ; Dick et al., 1998 ; Groll et al., 1997 ; Groll & Huber, 2003 ; Groll, Nazif, Huber, & Bogyo, 2002 ). Moreover, biochemical analysis suggested a network of intricate interconnections among the three active sites ( i.e., the so called “bite and chew” mechanism), in which the chymotryptic-like site performs the first cleavage ( i.e., the “bite”), followed by a series of cleavage steps at the trypsin-like and caspase-like sites ( i.e., the “chewing”) ( Kisselev, Akopian, Castillo, & Goldberg, 1999 ; Śledź et al., 2013 ). Fig. 1 Structure of 20S. A. Structure of the 20S proteasome particle as viewed from the top (top panel) or the side (bottom panel). The protein backbone of the subunits is presented as ribbon. B. Active site of the threonine peptidase subunit (β5) of the proteasome. The protein backbone of the β5 subunit is represented as turquoise ribbon, catalytic residue (Thr1) and other residues that help to maintain the structural stability of the catalytic site (Lys33, Asp17, Ser129, Asp166 and Ser169) are represented as sticks. Polar interactions are indicated as black dashed lines together with the corresponding distances. C. Vertical cross-section of the 20S particle, the α-subunit rings are represented as red ribbons, the β-subunit rings as blue ribbons, the outline of the internal cavity and the internal “chambers” are highlighted with a black dashed line. Fig. 1

All catalytically active subunits ( i.e., β1, β2 and β5) are synthesized as inactive precursors, which gain their hydrolytic properties only after the proper assembly of two half-proteasome assemblies, thus generating the 20S active form (see Section 2.3 for details) ( Zwickl, Kleinz, & Baumeister, 1994 ; Brannigan et al., 1995 ; Seemüller et al., 1995 : Ditzel et al., 1998 ; Huber et al., 2016 ). During the final step of the proteasome maturation process, segments of the immature active sites are removed by autolysis between residues Thr1 and Gly1, a process closely related to the proteolysis mechanism ( Budenholzer, Cheng, Li, & Hochstrasser, 2017 ; Chen & Hochstrasser, 1995 ; Chen & Hochstrasser, 1996 ; Huber et al., 2016 ). The pro-peptides of different β subunits act then as “co-chaperones” during 20S assembly (see Section 2.3 for details) ( Budenholzer et al., 2017 ; Kunjappu & Hochstrasser, 2014 ). Noteworthy, a critical function of the pro-peptide sequence is to prevent the Nα- acetylation of catalytic Thr in β-subunits, which would block the active site function before the formation of the half-proteasome; thus, pro-peptides are removed only when the half-proteasomes are correctly assembled, an occurrence which impairs the access of Nα-acetyltransferase to active sites ( Arendt & Hochstrasser, 1999 ; Budenholzer et al., 2017 ; Groll et al., 1999 ; Schmidtke et al., 1996 ; Seemuller et al., 1996 ). Concerning the remaining four β-subunits, it should be remarked that subunits β3, β4, and β6 subunits, which lack of the nucleophilic threonine in position 1, are catalytically inactive, and attempts to render them proteolytically active through site-directed mutations have failed ( Chen et al., 1995 ; Groll et al., 1999 ; Heinemeyer et al., 1997 ). Conversely, the β7 subunit, which keeps conserved Thr1 and Gly-1 residues (like active subunits) remains inactive since during the maturation process the β7 pro-peptide is not cleaved at Thr 1, but at position Thr8, due to substitutions of Lys33 and Ser129 by Arg33 and Phe129, respectively. It has been also proposed that the β7 subunit shows a Ntn-hydrolase proteolytic activity at Thr8, even though the surroundings of the proposed active site differ significantly from those of other subunits, but its role is still unknown ( Borissenko & Groll, 2007 ; Unno et al., 2002 ).

In proteasome architecture, while β-rings contain the proteolytic active sites (as discussed above), the outer α-rings form a nearly flat surface that binds to RPs ( i.e., 19S, PA28, see Section 2.3 for details). In the free 20S (that is not engaged with the RP), N -terminal tails of the α-subunits point all inwards to the centre of the ring and neighbouring tails are tightly anchored by an intricate lattice of intra-subunits interactions, constituting “the gate”, which regulates the substrate access through a 13 Å entry pore into the antechamber ( i.e , at α7-β7 interface). This passageway keeps the substrate in an unfolded state, directing it toward the catalytic chamber ( i.e., at β7-β7 interface) ( Bajorek & Glickman, 2004 ; Gaczynska & Osmulski, 2014 ; Groll et al., 2000 ; Marques et al., 2009 ; Ruschak, Religa, Breuer, Witt, & Kay, 2010 ; Unno et al., 2002 ) ( Fig. 1 C). The insertion of the substrate through this “ N -terminal gate” is the rate-limiting step of proteasome activity and prevents unwanted protein degradation ( Akopian, Kisselev, & Goldberg, 1997 ; Groll et al., 2000 ; Köhler et al., 2001 ). In fact, RP binding induces the N-terminal tails displacement and opens the gate, facilitating the substrate translocation (see Section 2.3 ) ( Choi et al., 2016 ; Finley, Chen, & Walters, 2016 ; Marshall & Vierstra, 2019 ; Matyskiela, Lander, & Martin, 2013 ; Śledź et al., 2013 ). However, it is worth recalling that RP binding to 20S is not an absolute requirement for proteasome activation, since 20S can switch from an inactive “closed” conformation to an active “open” conformation spontaneously or after chemical treatment ( e.g., SDS) ( Bajorek & Glickman, 2004 ; Förster, Whitby, & Hill, 2003 ; Groll et al., 2000 ). Noteworthy, since the α3 tail points toward the centre of the channel and maintains a close interaction with all other N -termini of α subunits ( Köhler et al., 2001 ; Köhler, Bajorek, et al., 2001 ), the deletion of first nine residues in α3 subunit N -tail in yeast 20S (α3Δn) induces a general disorder in the neighbouring tails, stimulating the opening of the entry pore ( Köhler, Bajorek, et al., 2001 ; Köhler, Cascio, et al., 2001 ). Thus, the α3Δn mutant is in a constitutively activated “open” state and its basal proteolytic activity toward small peptides is consistently enhanced, as compared to that of wild-type (wt) 20S ( Bajorek & Glickman, 2004 ; Köhler, Bajorek, et al., 2001 ; Köhler, Cascio, et al., 2001 ). Conversely, the double mutant α3-α7Δn more efficiently degrades macromolecular substrates with respect to either single mutant, suggesting that the interaction between these opposite tails is crucial in the regulation of gate opening ( Bajorek, Finley, & Glickman, 2003 ; Bajorek & Glickman, 2004 ). Interestingly, the α3Δn mutation does not alter the assembly of 26S, as demonstrated by the evidence that the abundance and activity of mutant 26S are similar to those of wt- 26S ( Groll et al., 2000 ; Groll & Huber, 2003 ). Accordingly, human cell lines stably expressing α3ΔN subunits show enhanced activity of both free 20S and holoenzyme complexes. This turns out in an increase of the degradation rate of poly-ubiquitinated proteins, reinforcing the role of α3-mediated gate opening mechanism, and suggesting that the α3Δn holoenzyme could help cell to fight the proteotoxic stress ( Choi et al., 2016 ). These data envisage that RP binding to 20S stimulates a structural rearrangement similar to that induced by the deletion of the α3 tail.

20S biogenesis

Proteasome maturation refers to the process that drives the proper incorporation of individual subunits to assemble into a proteolytically active 26S. Whilst 19S assembly is largely uncovered yet (see Section 2.3.2 ), the 20S assembly has been uncoiled at a great molecular detail also by virtue of the extensive similarity between yeast and mammalian pathways. This similarity has widened the repertoire of methodological approaches suitable to uncover the molecular insights. In eukaryotes, the stepwise recruitment of individual α- and β-subunits to constitute a fully mature 20S requires the presence of five molecular chaperones, called Proteasome Assembly Chaperones (PAC1–4 in human, Pba1–4 in yeast) and Proteasome Maturation Protein (POMP in human, hUmp-1 in yeast) ( Hirano et al., 2006 ; Le Tallec et al., 2007 ; Ramos & Dohmen, 2008 ). These chaperones drive the sequential insertion of the subunits preventing the formation of off-target assemblies presumably through non-catalytic activities ( Burri et al., 2000 ; Fricke, Heink, Steffen, Kloetzel, & Krüger, 2007 ). First, PAC1–PAC2 and PAC3–PAC4 work as heterodimers in recruiting the α-subunits during the earliest steps of biogenesis, that is the α-ring formation ( Hirano et al., 2005 ; Hirano et al., 2006 ; Le Tallec et al., 2007 ; Matias, Ramos, & Dohmen, 2010 ; Wu et al., 2018 ). Very recent advances in the field propose that in human cells α4, α5, α6 and α7 subunits first assemble to form a core tetrameric α-ring intermediate (α4-α7), being driven by PAC3-PAC4 heterodimers, which localize at the inner side of the nascent α-ring ( Satoh et al., 2019 ; Wu et al., 2018 ). Recently, crystallographic data have allowed to identify a hydrophobic surface, surrounded by charged residues in PAC4, which is complementary to that of PAC3, thus providing a clue for the interaction between the two partners ( Kurimoto et al., 2017 ). Notably, PACs surface was also found to display a charge complementarity with α4 and α5 subunits, envisaging the first structural basis for the binding of the heterodimer PAC3–PAC4 to the nascent 20S ( Kurimoto et al., 2017 ).

Thereafter, the PAC1–PAC2 heterodimer binds the outer side of this assembly, favouring the recruitment of α1, α2 and α3 subunits, thus leading to the formation of a mature heptameric α-ring ( Wu et al., 2018 ). Besides correctly introducing the α-subunit, the presence of the chaperones prevents the formation of aberrant off-pathway α-ring dimers, an occurrence potentially favoured by the sticky properties of α-subunits, in particular α7, which is prone to form high MW homo-oligomers in vitro ( Kozai et al., 2017 ).

The α-ring is then the scaffold for subsequent insertion of the seven β-subunits through the contribution of POMP at the outer surface of the endoplasmic reticulum, which is, to date, the main intracellular localization where these events take place in human cells ( Fricke et al., 2007 ; Hoefer, Boneberg, Grotegut, Kusch, & Illges, 2006 ; Krüger, Kloetzel, & Enenkel, 2001 ; Witt et al., 2000 ). The β-ring assembly starts with pro-β2, followed by β3, β4, forming the 13S complex; once these subunits are inserted, the PAC3-PAC4 heterodimer is released and pro-β5, pro-β6 and pro-β1 subunits assemble ( Hirano et al., 2005 ; Hirano et al., 2006 ; Satoh et al., 2019 ). Remarkably, structural insights suggest that the pro-peptide is not merely involved in preventing the early activation of the catalytic Thr in the catalytically active subunit (see Section 2.1 ), but is necessary for further stepwise incorporation of subunits, likely through an allosteric mechanism. The pro-peptides of β2 and β5 are essential for recruitment and incorporation of β3 and β6, respectively, whereas the β5 pro-peptide is necessary for the specific interaction with POMP ( Hoefer et al., 2006 ). The ultimate step of β-ring formation is the pro-β7 insertion and the formation of a half 20S ( i.e., the 15S complex) which, upon dimerization, forms the mature 20S. Although it is proven that full activation of 20S requires a) shedding of the β-subunits pro-peptides, b) PAC1-PAC2 detachment and/or clearance and c) POMP clearance, it is not fully clear whether the degradation of the chaperones is carried out by the 20S itself or if PAC chaperones are actually cleaved or released intact to be recycled for further maturation processes. It is further widely envisaged that additional unidentified factors may take part in the maturation process with activities overlapping with those of PACs in dependence of metabolic needs of.

The deepening of the molecular insights of proteasome maturation, both in terms of transcriptional regulation and of dynamics of proteins interaction, is expected to offer a new perspective for the development of therapeutic strategies based on the modulation of proteasome availability in selected tissues ( Goldberg, Zhao, & Collins, 2015 ). Clinical and molecular studies envisage that increased POMP translation and bioavailability upon down-regulation of miR-101 (which targets POMP mRNA) is an oncogenic stimulus for breast cancer cells ( Zhang, Bi, Fan, Wang, & Bao, 2015 ). Thus, the consequent increased proteasome intracellular content would confer protection from the proteotoxic insult to which highly proliferating cells are likely exposed, favouring cell survival ( Zhang et al., 2015 ). Furthermore, POMP up-regulation enhanced the bulk proteasome activity under proteo-toxic conditions, providing a metabolic advantage under redox insult ( Chondrogianni & Gonos, 2007 ). As a matter of fact, recent genetic studies on POMP promoter have identified mutations at the 3’UTR region and splicing variants in different skin inflammatory disorders, such as CANDLE syndrome (Chronic Atypical Neutrophilic Dermatosis with Lipodystrophy and Elevated temperature) or proteasome-associated autoinflammatory syndrome (PRAAS). Furthermore, increased POMP levels were observed in psoriatic skin lesions ( Brehm et al., 2015 ; Dahlqvist et al., 2010 ; Ebstein, Poli Harlowe, Studencka-Turski, & Krüger, 2019 ; Morice-Picard et al., 2017 ; Poli et al., 2018 ; Zieba et al., 2017 ). In these diseases, a decrease of proteasome levels, consequent to an altered maturation, is envisaged to activate the Unfolded Protein Response (UPR) through the endoplasmic reticulum stress. Such a stressful condition is known to trigger an inflammatory stimulus, which, once chronic, would account for the pathogenesis of the disease ( Dahlqvist, Törmä, Badhai, & Dahl, 2012 ; Ebstein et al., 2019 ).

Conversely, PACs involvement in pathological processes is still controversial and, probably, largely unexplored for pharmacological purposes although a functional evaluation of thielocin B analogues as protein-protein interaction inhibitors of PAC3 homodimer has been recently evaluated ( Ohsawa et al., 2018 ). However, it must be considered that although the contribution of PACs to proteasome biogenesis (discussed above) would suggest that they are crucial for life, transgenic murine KO models for either PAC1 or PAC2 are viable with major anatomical and functional alterations limited to different brain regions ( Sasaki et al., 2010 ). This finding underlies that the biological activity of PACs is redundant, envisaging that either additional still unknown factors can vicariate PACs activity or the self-assembling properties of free α-subunits (documented to some extent in vitro ) is enough to promote the constitution of fully active 20S particles. More recently, our group reported a marked down-regulation of PAC1-PAC2 expression, along with that of α7 subunit, in primary cultures of skin fibroblasts isolated from subjects affected by Rett Syndrome (RTT), a sever neuro-developmental disorder ( Sbardella et al., 2020a ). These cells were characterized by two different non-sense early truncating mutations of MeCP2 ( i.e., a transcriptional repressor that is mutated in the vast majority of patients affected by the syndrome) and by a concomitant severe lack of mature proteasome particles ( Amir et al., 1999 ; Sbardella et al., 2020 ). Furthermore, silencing of MeCP2 expression in neuron-like cells resulted in a similar proteasome dysfunction, indicating an unprecedented role of this transcriptional regulator in proteasome biogenesis. Different approaches, including a revolutionary in vivo imaging system, suggest that only a small fraction of the intracellular proteasome particles is proteolytically active under physiological conditions ( Asano et al., 2015 ). Thus, proteasome content appears to exceed the amount necessary to sustain life. In this view, the brain abnormalities in PAC1 KO mouse might be interpreted as the consequence of a reduced proteasome biogenesis in the tissue that is known to be more vulnerable to dysregulation of intracellular proteostasis which is primarily handled by the UPS ( Sasaki et al., 2010 ). This suggestion would be even more fascinating if PAC1-PAC2 loss will be confirmed to occur also in the CNS of RTT, which is the tissue prevalently affected in syndrome onset and progression.

19S regulatory particles

19S structural arrangement

Gate opening is crucial in 20S function, and cells have evolved different regulators (see Box 1) which control this proteasome process ( Finley et al., 2016 ). The predominant and best characterized 20S activator is the 19S which interacts, in the presence of ATP, with one or both ends of the 20S to form proteasome holo-complexes, 26S ( i.e., single-capped) and 30S, respectively ( i.e., doubly-capped) ( Armon, Ganoth, & Hershko, 1990 ; Bard et al., 2018 ; Eytan, Ganoth, Armon, & Hershko, 1989 ; Liu et al., 2006 ; Marshall & Vierstra, 2019 ; Schmidt & Finley, 2014 ; Smith et al., 2005 ). These different proteasome assemblies ( i.e., 26S and 30S) coexist, together with free 20S, in cell cytosol, and are known to cleave ubiquitinated substrates, although their substrate specificity and different biological role remain a somewhat enigmatic issue in proteasome biology. The abundance of the three main proteasome populations ( i.e., 20S, 26S and 30S) seems to be finely modulated by the specific microenvironment in which the cell lives. This structural arrangement is carried out by different proteasome interacting proteins (PIPs) which can be classified either ( a ) extrinsic de-ubiquitinases (DUBs) (see Box 2) and/or ( b ) auxiliary proteasome regulators, Ecm29, HSP70 ( Tanaka, 2009 ). In this regard, Insulin-Degrading-Enzyme (IDE), a Zn 2+ protease, which behaves as a Heat-Shock Protein ( Tundo et al., 2013 ), has been reported not only to directly bind the 20S, but also to modulate its activity through allosteric mechanisms, envisaging that it may be a novel auxiliary proteasome regulator ( Sbardella et al., 2015 ; Tundo et al., 2017 ). Moreover, IDE was found to compete with 19S binding, modifying the distribution of different proteasome population in vitro ( Sbardella et al., 2018 ). However, despite the biological relevance, the exact molecular mechanism which drives proteasome population interconversion is still far from being satisfactorily elucidated.

Once bound over the axial 20S pores, the 19S RP carries out different functions, namely (see Section 2.1 ): (i) recognition and unfolding of ubiquitinated substrates; (ii) opening of the 20S pore; (iii) substrates entry into the 20S catalytic chamber; (iv) release of ubiquitin moieties during substrate degradation ( Collins & Goldberg, 2017 ; Finley & Prado, 2019 ; Marshall & Vierstra, 2019 ) ( Fig. 2 ). From the structural point of view, the 19S is made up by two different sub-components, the lid and the base, which form a conformationally dynamic complex ( Bajorek & Glickman, 2004 ; Budenholzer et al., 2017 ). Fig. 2 Overall organization of the proteasome 26S particle. Left: the “Core Particle” (20S proteasome) is represented as protein ribbons, in yellow and magenta the two α-subunit rings, in blue and green the two β-subunit rings. The two regulatory particles (19S proteasome), attached on both ends of the 20S particle, are represented as protein ribbons. The group of regulatory AAA-ATPases (Rpt1–6) are coloured in dark and pale green, the non-ATPase regulators (Rpn) are coloured in violet and orange. Right : close-up of the 26S regulatory particle with the various non-ATPase subunits highlighted and labelled. The particle is shown from various point of view: on the top panel it is shown from the top; on the lower panel two opposite side views are shown. Fig. 2

The base binds directly to the 20S, and it is formed by a ring of six paralogous AAA-ATPases, named Rpt1-Rpt6, and three non-ATPase subunits ( i.e., Rpn1, Rpn2, Rpn13), which provide multiple binding site for ubiquitin and ubiquitin-like proteins ( Bard et al., 2018 ; He et al., 2012 ; Husnjak et al., 2008 ; Marshall & Vierstra, 2019 ; Saeki & Tanaka, 2012 ; Shi et al., 2016 ). Rpt subunits associate into three pairs of heterodimers ( i.e., Rpt1-Rpt2, Rpt3-Rpt6, Rpt4-Rpt5), which then form the hetero-hexameric motor of proteasome. The C-terminal helical domains of Rpt1-Rpt2 subunits interact with Rpn1, while those of the heterodimer Rpt3-Rpt6 bind Rpn2. Rpn13 also interacts with the C-terminal residues of Rpn2 through its N-terminal pleckstrin-like receptor of ubiquitin (PRU) domain, whereas the C-terminal region of Rp4-Rpt5 extends out from the base body without interaction with other proteasome subunits, at least in the resting state ( Fig. 2 ) ( Beck et al., 2012 ; Budenholzer et al., 2017 ; Djuranovic et al., 2009 ; Hemmis, Heard, & Hill, 2019 ; Husnjak et al., 2008 ; Tomko, Funakoshi, Schneider, Wang, & Hochstrasser, 2010 ; VanderLinden, Hemmis, Yao, Robinson, & Hill, 2017 ; Zhang et al., 2009 ). The first identified ubiquitin receptor was Rpn10, that is not considered part of the base, but functions as a bridge between the lid and the base, stabilizing their interaction ( Aubin-Tam, Olivares, Sauer, Baker, & Lang, 2011 ; Beckwith, Estrin, Worden, & Martin, 2013 ; Erales, Hoyt, Troll, & Coffino, 2012 ; Maillard et al., 2011 ; Martin, Baker, & Sauer, 2008 ), as further suggested by the lid and base disassembly when Rpn10 is mutated ( Deveraux, Ustrell, Pickart, & Rechsteiner, 1994 ; Isasa et al., 2010 ; Keren-kaplan et al., 2016 ). Importantly, mono-ubiquitination of Rpn10, which is modulated by stressful conditions, regulates its association with proteasome, and thus proteasome activity and stability ( Budenholzer et al., 2017 ; Isasa et al., 2010 ; Keren-kaplan et al., 2016 ). An additional intrinsic ubiquitin receptor is the T1 toroidal region of the Rpn1 ( Elsasser, Chandler-Militello, Müller, Hanna, & Finley, 2004 ; Shi et al., 2016 ), which, like Rpn10 and Rpn13, also recognizes ubiquitin -like domains (UBLs) of extrinsic ubiquitin receptors ( i.e., HR23/Rad23, PLIC2/DsK2 and Ddi1), stimulating the proteasome-mediated degradation of ubiquitinated substrates ( Leggett et al., 2002 ; Raasi, Varadan, Fushman, & Pickart, 2005 ; Saeki, Saitoh, Toh-e, & Yokosawa, 2002 ; Shi et al., 2016 ; Spyracopoulos, 2016 ). It remains unclear why proteasome contains such an array of ubiquitin-binding receptors, and what differential roles they might play in substrate recognition and degradation ( Bard et al., 2018 ; Cundiff et al., 2019 ; Hamazaki, Hirayama, & Murata, 2015 ).

Upon recognition by intrinsic and extrinsic ubiquitin receptors, substrates are engaged with the AAA + motor of the highly dynamic Rpt1–6 hexameric ring that couples ATP hydrolysis to substrate unfolding and translocation, converting chemical energy into mechanical work ( de la Peña, Goodall, Gates, Lander, & Martin, 2018 ; Dong et al., 2019 ; Eisele et al., 2018 ). The C-terminal tails of Rpt2, Rpt3 and Rpt5 contain the conserved HbYX motif (see also Section 2.3.2 ) that fits into the groove between adjacent α-subunits of 20S inducing a conformational change into their N-termini which drives 20S gate opening ( Rabl et al., 2008 ; Smith et al., 2007 ). Mutational studies have indicated that functions of Rpt subunits are not redundant, but they cover different roles according to the vertical position they adopt in the hexamer, such that subunits located at the top ( i.e., Rpt3 and Rpt4) contribute to substrate engagement and translocation more than subunits located further down, like Rpt1 and Rpt2 ( Beckwith et al., 2013 ; Erales et al., 2012 ; Lander et al., 2012 ; Rubin, Glickman, Larsen, Dhruvakumar, & Finley, 1998 ; Wehmer et al., 2017 ).

The peripheral lid subcomplex, which reveals significant structural and sequence similarities with COP9 signalosome and eIF3, braces one side of the base and it is composed of nine ATPase subunits, namely Rpn3, Rpn5–9, Rpn11–12 and Sem1 (also named Rpn15). Among these subunits, six are PCI (proteasome-CSN-initiation factor 3) domain containing subunits ( i.e., Rpn3, Rpn5–7, Rpn9, and Rpn12), and two are MPN (Mpr1-Pad1 N-terminal) domain containing subunits ( i.e., Rpn8 and Rpn11) ( Beckwith et al., 2013 ; Erales et al., 2012 ; Schmidt & Finley, 2014 ). The most important functions of the lid subcomplex are the strengthening of 20S—19S interaction ( e.g., Rpn6 is a molecular clamp holding the core and regulatory subcomplexes together) and de-ubiquitination of substrates before their processing by the AAA-ATPase ( Pathare et al., 2012 ; Pathare et al., 2014 ). Cleavage of polyubiquitin chains enables Ub recycling into the cellular pool ( Budenholzer et al., 2017 ; Pathare et al., 2014 ) and it is carried out mostly by Rpn11, a Zn 2+ de-ubiquitinase essential for proteasome functions and cell viability located above the translocation channel, which removes the entire ubiquitin chains of the substrates before their entry into the ATPase translocation ring ( de Poot, Tian, & Finley, 2017 ). As a matter of fact, the close proximity of the N-terminal domain ring of the AAA + ATPases sterically prevents the cleavage inside ubiquitin moieties by Rpn11, which, therefore, removes polyubiquitin chains by hydrolysing the isopeptide bond (located at the base of the chain) between the lysine residue of the substrate and the C-terminus of the first ubiquitin monomer ( Yao & Cohen, 2002 ). The MPN domain of Rpn11 forms a heterodimer with the non-catalytic MPN domain of Rpn8, forming the minimal DUB-competent complex ( Pathare et al., 2014 ; Worden, Padovani, & Martin, 2014 ). Even though the Rpn11/Rpn8 dimer is active when isolated, it is significantly inhibited in the free lid sub-complex through its interaction with the neighbouring lid subunit Rpn5. During lid incorporation into the 26S proteasome, conformational rearrangements occur, activating the action of the de-ubiquitinase ( Dambacher, Worden, Herzik, Martin, & Lander, 2016 ; Verma et al., 2002 ; Worden et al., 2014 ; Yao & Cohen, 2002 ), which is crucial for the efficient substrate degradation. However, in order to prevent a premature ubiquitin chain removal, de-ubiquitination must be restricted to committed substrates that are engaged with the ATPase motor; therefore, the rate-limiting step in de-ubiquitination process is represented by an ubiquitin-linked conformational switch of Rpn11 Insert-1 loop from an inactive closed state to an active β-hairpin. This conformational change is activated by mechanical substrate translocation of AAA + motor ATPase, allowing a direct coupling of substrate de-ubiquitination and degradation ( Worden, Dong, & Martin, 2017 ). Beside Rpn11, the two extrinsic DUBs, Usp14 (see Box 2) and UchL5, which are also strongly associated with both Rpn1 and Rpn13, are involved in cleaving or editing of the ubiquitin chain from substrates ( Bard et al., 2018 ; Guterman & Glickman, 2004 ; Hamazaki et al., 2006 ; Lam, Xu, DeMartino, & Cohen, 1997 ; Qiu et al., 2006 ; Yao et al., 2006 ).

19S assembly

Unlike 20S, the heterogeneous and dynamic structural features of 19S demand a different and independent assembly process of the base and lid subcomplexes ( Budenholzer et al., 2017 ; Lander et al., 2012 ; Marshall & Vierstra, 2019 ; Tomko et al., 2015 ; Tomko & Hochstrasser, 2014 ).

Base assembly has been poorly studied and two separated, but not mutually exclusive, models have been proposed up to now. According to the first model, base assembly takes place regardless of lid subunits, finding its justification mainly through studies in E. coli . On the other hand, according to the second model, 20S would act as a scaffold for 19S biogenesis, as suggested by mutagenic and immunoprecipitation studies in yeast ( Beckwith et al., 2013 ; Funakoshi, Tomko, Kobayashi, & Hochstrasser, 2009 ; Li et al., 2017 ; Marshall & Vierstra, 2019 ; Park et al., 2013 ; Tomko et al., 2010 ). However, both models support the notion that an ordered recruitment of free base subunits is orchestrated by a set of dedicated chaperones, namely p27 (Nas2 in yeast), p28 (Nas6 in yeast) and S5b (Hsm3 in yeast), which join together couples of Rpts subunits, driving their correct insertion in human cells. Specifically, the interaction between the C-terminal domain of Rpt subunits with respective chaperones leads to three precursor modules formation ( i.e., p27-Rpt4-Rpt5, p28-Rpt3-Rpt6, and S5bRpt1-Rpt2) ( Le Tallec et al., 2007 , Funakoshi et al., 2009 ; Roelofs et al., 2009 ; Saeki, Toh-e, Kudo, Kawamura, & Tanaka, 2009 ; Tomko et al., 2010 ). Remarkably, none of these chaperons is crucial for cell viability, but multiple genetic deletions become lethal under proteotoxic conditions ( Budenholzer et al., 2017 ). Furthermore, the N-terminal domain of chaperone Adc17 binds Rpt6, mediating Rpt3-Rpt6 dimerization ( Hanssum et al., 2014 ; Rousseau & Bertolotti, 2016 ); thereafter, Adc17 early dissociates by heterodimers during the assembly process, whereas Hsm3, which contacts also Rpn1, dissociates from the lid only upon completion of 19S maturation ( Barrault et al., 2012 ; Funakoshi et al., 2009 ; Hanssum et al., 2014 ). Importantly, the p28-Rpt3-Rpt6 module also binds Rpn14, whereas the Nas6-Rpt3-Rpt6-Rpn14 module seems to form an intermediate with the p27-Rpt4-Rpt5 one. This intermediate module also interacts with Rpn2 and Rpn13, but p27 likely dissociates before the incorporation of S5b-Rpt1-Rpt2 module, since a complex displaying the two chaperones has never been observed ( Funakoshi et al., 2009 ; Saeki et al., 2009 ; Tomko et al., 2010 ). However, it must be remarked that the ordered series of events that lead to base assembly are speculative, and differences may exist between human and yeast ( Budenholzer et al., 2017 ), most data indicating that lid and the Rpn10 associate only when the base is completely formed ( Budenholzer et al., 2017 ; Funakoshi et al., 2009 ; Roelofs et al., 2009 ; Saeki et al., 2009 ; Tomko et al., 2010 ).

The lid assembly proceeds through a coordinated process, characterized by an ordered series of subcomplexes interaction (Tomko et al., 2011; Fukunaga, Kudo, Toh-e, Tanaka, & Saeki, 2010 ; Tomko et al., 2015 ; Estrin, Lopez-Blanco, Chacón, & Martin, 2013 ). Lid assembly begins with heterodimerization of Rpn8-Rpn11, which is then followed by Rpn6, Rpn5 and Rpn9 recruitment, leading to the release of the first lid module ( Estrin et al., 2013 ; Sharon, Taverner, Ambroggio, Deshaies, & Robinson, 2006 ). Unlike 20S and 19S base, no chaperones dedicated to assist the process have ever been identified, even though the intrinsically disordered Sem1 subunit of the lid seems to play a crucial role in linking Rpn3 and Rpn7 to form the heterotrimeric intermediate LP3, which is an early phase of lid biogenesis ( Bohn et al., 2013 ; Dambacher et al., 2016 ; Fukunaga et al., 2010 ; Sone, Saeki, Toh-e, & Yokosawa, 2004 ). Thereafter, the first lid module and LP3 associate to form LP2, creating a complete lid that only misses the Rpn12 subunit ( Estrin et al., 2013 ; Tomko & Hochstrasser, 2011 ; Yu et al., 2015 ). Hence, the last step is the incorporation of Rpn12, which fits its C-terminal helix into a helical bundle formed by clusters of C-termini of other Rpn subunits ( Marshall & Vierstra, 2019 ; Tomko et al., 2015 ). Rpn12 binding induces a conformational change to the rest of lid, which favours the association between lid and base ( Budenholzer et al., 2017 ; Tomko et al., 2015 ). During lid maturation, and possibly during lid and base connection, Rnp8/Rpn11 undergoes a conformational change, which leads to a rigid body rotation of the heterodimer, so that Rpn11 is located where it can deubiquitinate polyubiquitinated substrates before their entry in the ATPase channel ( Dambacher et al., 2016 ; Tomko et al., 2015 ). As mentioned in Section 2.3.1 , Rpn11 activity is inhibited by Rpn5, and further by Rpn8-Rpn9 interaction. When lid assembly is completed, the module Rpn8-Rpn11 rotates away from Rpn5, allowing Rpn11 activation ( Dambacher et al., 2016 ; Ehlinger et al., 2013 ).

The last step of the holo-enzyme formation is represented by the association between 19S and 20S. The key event is the 19S-mediated gate opening, which is driven by the insertion of C-terminal HbYX motifs of Rpt2, Rpt3, and Rpt5 into the 20S α-subunit pockets ( Park et al., 2013 ; Rabl et al., 2008 ; Smith et al., 2007 ; Tian et al., 2011 ). However, recently it has been shown that stable docking of HbYX motifs into the 20S is not sufficient to promote the gate opening. Accordingly, efficient gate opening has been proposed to occur only when Rpt1 and Rpt6 C-termini are engaged into the α-ring ( Eisele et al., 2018 ; Park et al., 2013 ; Sokolova, Li, Polovin, & Park, 2015 ). Moreover, Rpn6 binding to α2 subunit facilitates 20S—19S interaction ( Lander et al., 2012 ; Pathare et al., 2012 ).

Notably, 20S—19S association occurs spontaneously in vitro in the presence of ATP, whereas in cell models it seems influenced by a series of interacting proteins, such as HSP90, IDE, and Ecm29 ( Imai, Maruya, Yashiroda, Yahara, & Tanaka, 2003 ; Sbardella et al., 2018 ; Tundo et al., 2017 ; Yamano et al., 2008 ). The latter protein seems to play a particularly important role under stressful conditions, since it binds structural aberrant proteasome, repressing 20S—19S interaction ( De La Mota-Peynado et al., 2013 ; Lee et al., 2011 ; Lehmann, Niewienda, Jechow, Janek, & Enenkel, 2010 ; Panasenko & Collart, 2011 ; Park, Kim, Tian, Gygi, & Finley, 2011 ; Wang et al., 2017 ).

Structural conformation of the proteasome holoenzyme

A main breakthrough for understanding the structural basis of 26S came from a series of Cryo-EM studies on proteasome holoenzyme from different species, such as yeast, rat and humans ( Lander et al., 2012 ; Matyskiela et al., 2013 ; Unverdorben et al., 2014 ; Wehmer et al., 2017 ; Chen, Wu, & Shen, 2016 ; Huang et al., 2016; Wehmer & Sakata, 2016 ; Bard et al., 2018 ). These studies have revealed the existence of at least four distinct human 26S conformational states ( i.e., SA, SB, SC and SD, mirrored in yeast 26S by s1, s2, s3 and s4), that appear conserved among species. The numeric order of these main states is suggested by a structural comparison that reveals progressive and sequential movements from SA (s1) state through SB (s2) and SC (s3), to SD (s4), which is similar to SA ( Bard et al., 2018 ; Wehmer & Sakata, 2016 ). In all identified conformations, the architecture and structure of 20S remains essentially unaltered, whereas the two subcomplexes of 19S, the lid and the base (see Section 2.3.1 ) are highly dynamic, changing the relative orientation with respect to each other and to core particles; these movements are coupled to the functional cycle of 26S ( Chen et al., 2016 ; Unverdorben et al., 2014 ; Wehmer et al., 2017 ). However, despite the advance in knowledge on 26S structure, we have to recall that an intriguing aspect, which has never been deeply investigated, concerns the conformational transition, occurring after the binding of the first 19S, on the opposite end of 20S, where one free α-ring surface is available for the binding of a second 19S particle, which yields a double capped 30S proteasome, whose real structure, as well as the function, remains poorly understood ( Tundo, Sbardella, & Coletta, 2018 ).

The yeast s1 is a low energy ATP-bound ground state, that is assumed to be the primary substrate-binding conformation ( Matyskiela et al., 2013 ; Sledz et al., 2013; Unverboden et al., 2014; Wehmer et al., 2017 ; Ding et al., 2017; Chen et al., 2016 ; Zhu et al., 2017 ). In the s1 state, the 20S gate is closed, since the substrate translocation channel of ATPase ring is not aligned with the 20S gate, and the active site of Rpn11 is 25Ǻ away from substrate entry pore ( Eisele et al., 2018 ; Finley & Prado, 2019 ; Wehmer et al., 2017 ). The transition toward the s2 state is driven mainly by the lid rotation, which drives Rpn11 to a position above the central processing pore of the base. On the other hand, the progression from s2 to s3 is mediated by a rearrangement of Rpt1-Rpt6, wherefore N-ring of Rpts and AAA+ domains shift toward Rpn1, thus generating a wider channel aligned with core particle axial pore ( Matyskiela et al., 2013 ; Unverboden et al., 2014; Wehmer et al., 2017 ; Chen et al., 2016 ). Therefore, the s3 state is characterized by the axial alignment of the essential DUB Rpn11 (see also Section 2.3.1 ), 19S translocation channel, and 20S gate. However, in spite of these rearrangements and of the evidence that s2 and s3 states are primed for substrate degradation, the 20S gate is still mostly occluded, preventing substrate entry ( Matyskiela et al., 2013 ; Sledz et al., 2013; Bard et al., 2018 ; Finley and Prado 2019). The gate becomes fully opened only during the transition from s3 to s4, inducing the entry of the substrate into the catalytic core; thus, gate opening is a consequence of the insertion of “HbYX” motif of C-termini of Rpt2-Rpt3-Rpt5 subunits into 20S pocket (see Section 2.3.1 ). Stable docking of HbYX motifs into the 20S is insufficient to promote gate opening, which is completed only in the s4 state upon engagement of the C-termini of Rpt6 and Rpt1 into the α-ring ( Eisele et al., 2018 ; Finley & Prado, 2019 ). Besides these four states, further structural and biochemical studies have revealed recently the presence of two additional open gate states in yeast proteasome ( i.e., s5 and s6) ( Eisele et al., 2018 ). In the case of human 26S proteasome Cryo-EM studies showed that the substrate Sic1 PY 26 complex ( i.e., the Cdk inhibitor Sic1 from Saccharomyces cerevisiae with a Pro-Pro-Pro-Ser motif inserted into N-terminal) exists in seven conformational states, EA1, EA2, EB, EC1, EC2, ED1, ED2 ( Dong et al., 2019 ). EA1 and EA2 states represent two initial ubiquitin recognition states; EB2 is the “de-ubiquitination” state, in which the isopeptide bond between Rpn11 and substrate is close to the zinc-active site of Rpn11; EC1 and EC2 are conformations at the onset of substrate translocation; ED1 and ED2 carry on and complete substrate translocation ( Dong et al., 2019 ). Functional models of 26 activity, derived both from cryo-EM and biochemical analysis, couple ATP hydrolytic cycle to substrate translocation ( Matyskiela et al., 2013 ; de la Pena et al., 2018; Dong et al., 2019 ). Therefore, sequential ATP hydrolysis and phosphate release, which are coordinated within the ATPase motor, seem to supply “the power” to induce conformational changes that drive the substrate through the central pore (de la Pena et al., 2018; Eisele et al., 2018 ; Tundo et al., 2018 ; Dong et al., 2019 ). In agreement with a “rotatory” mechanism, a hydrolytic event in a single Rpt subunit is followed by another one in the nearby subunit, thus proceeding throughout the entire ring (de la Pena et al., 2018; Eisele et al., 2018 ; Tundo et al., 2018 ; Dong et al., 2019 ). In fact, it has been proposed that a specific Rpt subunit binds ATP and engages substrate at the uppermost position; then, this subunit hydrolyses ATP (when at the penultimate position of the staircase), releasing the phosphate moiety and disengaging from substrate, which proceeds to the next hydrolytic step ( Eisele et al., 2018 ; de la Pena et al., 2018). A disengaged subunit moves outward from the ring, where it can contact another segment of the translocating substrate. At the same time, the other substrate-engaged subunits carry out a coordinated and synergistic motion so that the substrate translocates by about two amino acids (~ 6 Ǻ) toward the 20S (de la Pena et al., 2018; Eisele et al., 2018 ; Dong et al., 2019 ; Finley & Prado, 2019 ; Majumder et al., 2019 ).

Importantly, cryo-electron tomography approaches have also visualized proteasome particles in their native conformation in living cells, allowing to have an insight on the percentage of different populations that harbour the cells ( Asano et al., 2015 ; Finley & Prado, 2019 ; Guo, He, Li, & Le, 2017 ). In intact hippocampal neurons, a molecular census of proteasome conformational states showed that, in the absence of proteotoxic stress, only 20% of the 26S was engaged in substrate processing, whereas the remaining portion was in the substrate-accepting ground state. It suggests that the capacity of the proteasome system is not fully exploited by the cell under physiological conditions ( Asano et al., 2015 ). Interestingly, poly-Gly-Ala (poly-GA) aggregates, which result from aberrant expansion of GGGGCC repeat in C9orf72 gene ( i.e., the most common genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia), recruits 26S molecules which are in the s4 state ( Guo et al., 2018 ), unlike the general pool of proteasome. However, since poly-GA are not favourable proteasome substrates, 26S sequestration and consequent inhibition has been proposed to cover a crucial role in neurodegeneration ( Finley and Prado, 2019 , Guo et al., 2017 ).

Proteasome in cancer progression

Proteostasis network in cancer

Over the last decades, hallmarks of cancer cells have been described to provide a sort of universal definition which would account for the multi-step development of human tumours ( Hanahan & Weinberg, 2000 ; Hanahan & Weinberg, 2011 ; Pack et al., 2014 ). These hallmarks, which are complementary features that enable tumour growth and metastatic dissemination, include proliferative signalling, growth suppressors inactivation, cell death resistance, replicative immortality, angiogenesis, invasiveness and dissemination, cell metabolism reprogramming and immune-surveillance evasion ( Hanahan & Weinberg, 2000 ; Hanahan & Weinberg, 2011 ; Shereen, Khan, Kazmi, Bashir, & Siddique, 2020 ; Tundo, Sbardella, Lacal, Graziani, & Marini, 2019 ). Recently, resistance to proteostasis unbalance has been proposed as a new malignant hallmark of cancer, envisaging the possibility that this acquired property cooperates with the other altered circuits to allow cancer cell survival, proliferation and dissemination ( Carvalho, Rodríguez, & Matthiesen, 2016 ; Dong & Cui, 2018 ; Klaips et al., 2018 ). Cancer cells, due to the rapid proliferation rate, are constantly under cellular stress with a consequent decrease of protein quality control ( Carvalho et al., 2016 ; Vahid, Thaper, & Zoubeidi, 2017 ). However, the unbalance of protein synthesis, folding, trafficking and degradation, which usually leads normal cells to death, does not induce the same fate in cancer cells that acquire and develop, during tumour progression, novel properties to promote their survival ( Calderwood, Khaleque, Sawyer, & Ciocca, 2006 ; Vahid et al., 2017 ).

In recent years, three main reasons have gained considerable insight as to why PNs are altered in human tumours, namely 1) genomic instability; 2) persistence of stressful conditions in tumour micro-environment, and 3) age-related proteome imbalance ( Dong & Cui, 2018 ).

First, cancer cell genome is highly unstable and builds up several point mutations in protein coding sequence and/or genome mutations ( e.g., large duplications, deletions, inversions, and translocations as well as altered copy numbers of entire chromosomes, such as aneuploidy). This may turn out in an inappropriate repression or activation of tumour suppressors and oncogenes, respectively, excessive protein synthesis, and/or translation of mutated proteins with altered folding, function and turn-over ( Benbrook & Long, 2012 ; Kim & Zaret, 2015 ; Vogelstein et al., 2013 ; Weaver & Cleveland, 2006 ). It has been estimated that over 90% of human solid tumours harbour aneuploidies that lead to an excess in protein synthesis ( Dai, Dai, & Cao, 2012 ; Weaver & Cleveland, 2006 ; Williams & Amon, 2009 ); indeed, this is a relevant issue mainly for proteins that become functional upon assembly in stoichiometric complexes such as in the case of ribosomes ( Deshaies, 2014 ). Therefore, genomic alterations support a proteostasis unbalance (also referred as proteotoxic crisis) that renders cancer cells more dependent than normal cells on PNs clearance mechanisms, including UPS ( Deshaies, 2014 ). Accordingly, yeast cells with one-third of single chromosomal aneuploidies are hypersensitive to proteasome inhibitors, and some cells “adapted” to aneuploidy harbour mutations that depress UPS activity ( Torres et al., 2007 ; Torres et al., 2010 ; Torres, Williams, & Amon, 2008 ).

Secondly, during tumour development, tumour cells are continuously exposed to a variety of extrinsic perturbations, such as nutrient deprivation, hypoxia, and acidosis. Despite this pressure, tumour cells successfully proliferate and efficiently withstand this challenge by adapting to the fluctuations of the microenvironment, reprogramming their proteome and fully exploiting the cell defence mechanisms against proteotoxic stress. Thus, ultimately, stressful conditions lead to a disruption of the proteostasis balance, which is associated to the promotion of malignant properties (such as invasiveness, immune surveillance escape, and metabolism reprogramming), achieving a plethora of PN alterations ( Oromendia & Amon, 2014 ; Dufey, Urra, & Hetz, 2015 ; Nam and Joe, 2019).

Last, pathological and physiological senility is considered a major risk factor for protein conformational diseases, including immunological and metabolic disorders, neurodegeneration and cancer ( Carrell & Lomas, 1997 ; Kikis, Gidalevitz, & Morimoto, 2010 ; van der Willik, Schagen, & Ikram, 2018 ). In fact, progressive exposure of stressors during aging induces accumulation of damaged and unfolded proteins which culminates in PNs alteration ( Dong & Cui, 2018 ; Sklirou, Papanagnou, Fokialakis, & Trougakos, 2018 ). Thus, in a vicious circle, unbalanced PNs lead to the proteotoxic crisis, which favours tumourigenesis ( Arnsburg & Kirstein-Miles, 2014 ; Miller, Drake, Naylor, Price, & Hamilton, 2014 ). As a matter of fact, in accordance with the proteotoxic crisis hypothesis, reprogramming the proteome might represent a novel therapeutic approach, since agents that target components of different PN pathways are expected to be more toxic for cancer cells than for normal cells ( Deshaies, 2014 ; Yuan et al., 2018 ). In the next paragraphs, we will review the biological rationale for targeting proteasome in the context of UPS as a strategy to treat cancer.

Degradation of cancer-related proteins by proteasome

A number of preclinical studies have reported alterations of proteasome expression and activity in different type of cancers, including haematological malignancies, lung, breast, pancreatic, head and neck, and thyroid cancers ( Adams, 2003 ; Arlt et al., 2009 ; Chen et al., 2005 ; Kumatori et al., 1990 ; Roeten, Cloos, & Jansen, 2018 ). The reason of this high proteasome activity is not well understood, even though it is likely linked to stressful conditions ( e.g., hypoxia, reperfusion, alteration of growth factors and cytokines levels), which evolve in the context of a heterogeneous tumour microenvironment. Deregulation of the proteasome activity can destabilize and/or disrupt the balance between tumour suppressors and oncoproteins, promoting cancer progression ( Chang & Ding, 2018 ; Kaplan, Torcun, Grune, Ozer, & Karademir, 2017 ; Ogiso, Tomida, Kim, & Tsuruo, 1999 ). An element of complexity in understanding the role of proteasome in carcinogenesis is also represented by the fact that most investigations are carried out in unsorted cancer cells, which do not include cancer stem cells ( Voutsadakis, 2017 ). Thus, cancer stem cell theory states that all tumour cells derive by a small percentage of cancer stem cells capable of repopulating tumours after therapy ( Hanahan & Weinberg, 2011 ; Simons & Clevers, 2011 ). Noteworthy, the proteasome function is decreased in these cells with respect to the bulk of tumour population, revealing that a better understanding of proteasome regulation in different cell sub-sets might unveil further opportunities in cancer therapy ( Banno et al., 2016 ; Voutsadakis, 2017 ). Despite the criticism, there are many key proteins, degraded by proteasome, that are involved in carcinogenesis; below are listed examples of proteins, which are considered crucial in cancer progression and are reported to mediate cell death after exposure to proteasome inhibitors ( Ciechanover et al., 2001 ; Evan & Vousden, 2001 ; Jang, 2018 ; Johnson, 2015 ; Soave, Guerin, Liu, & Dou, 2017 ) ( Fig. 3 ). Fig. 3 Regulation of NF-kB/E2F/Rb and p53/p21 pathways by proteasome. NF-kB/E2F/Rb pathway : under unstimulated conditions, NF-kB is kept inactive in the cytosol by IkB inhibitor. Different stimuli ( e.g., cytokines, stressors, Pathogen Associated Molecular Patterns), generically indicated as green spheres on membrane receptors, phosphorylate and activate IKKB γ subunit which thereafter phosphorylate IkB through the kinase activity held by the α and β subunits. Phosphorylated IkB is ubiquitinated and degraded by the 26S proteasome. Free NF-kB dimers translocate into the nucleus, where transcription of target genes occurs. Cyclin D1 (CCDN1) expression allows the cyclin D1-Cdk4/6 complex to form. This complex phosphorylates Rb protein, inducing its detachment from EF2 transcription factor. Free EF2 enters the nucleus and transcribes cyclin E ( not shown ), cyclin A and genes involved in DNA synthesis: this triggers the progression toward the S-phase. p53/p21 pathway : under physiological conditions, p53 degradation is predominantly orchestrated by the E3-ligase MDM2, which promotes its poly-ubiquitination, and, thus, its degradation by the 26S proteasome. A number of stimuli activate the p53 pathway, inducing its tetramerization and translocation into the nucleus. Herein, p53 triggers the transcription of pro-apoptotic factors ( i.e., Noxa and Bax, not shown ) and CdkI p21. When expressed, p21 binds to: (i) Cdk2/cyclin E (CCNE1) complex, blocking the entry of the cell into the S phase; (ii) cyclin B (CCNB1)/Cdk1 complex, leading to a growth arrest in the G2 phase; (iii) PCNA, inhibiting DNA replication. p21 levels are also modulated through the ubiquitin-dependent degradation by the 26S, and further by a ubiquitin -independent pathway by the 20S. Figure legend is restricted to NF-kB and p53, whose mechanisms of transcription induction is not sketched. PIs stands for Proteasome Inhibitors and the red arrows indicate the steps of NF-kB, p53 and p21 turn-over which are blocked by this class of drugs. Fig. 3

NF-kB

NF-kB is a crucial transcription factor that induces the expression of a wide range of genes involved in cell proliferation, apoptosis, inflammation and angiogenesis ( Karin, Cao, Greten, & Li, 2002 ; Qureshi et al., 2018 ; Wu & Shi, 2013 ). Alteration of NF-kB pathway has been documented in a series of human tumours, including breast, lung, prostate, pancreatic cancer and melanoma, as well as in haematological malignancies, such as Hodgkin's/Non-Hodgkin's lymphoma and multiple myeloma ( Aggarwal, 2004 ; Braun et al., 2006 ; Johnson, 2015 ; Karin & Greten, 2005 ; Kim, Hawke, & Baldwin, 2006 ; Perkins, 2012 ; Van Waes, 2007 ). It is generally accepted that NF-kB promotes cancer progression by inhibiting apoptosis, and that also chemo- and radiotherapy treatments activate NF-kB signalling, inducing acquired resistance to conventional cancer therapy ( Baldwin, 2001 ; Nakanishi & Toi, 2005 ; Wu & Shi, 2013 ). Under unstimulated conditions, NF-kB homo- or hetero-dimers are sequestered in an inactive form in the cytoplasm by its inhibitor IkB. Different stimuli, including stress and chemotherapy, activate IkB kinase ( i.e., IKKB) that phosphorylates IkB, leading to its ubiquitination and degradation by the proteasome. Free NF-kB dimers then translocate into the nucleus wherein they induce the transcription of target genes ( Baldwin, 1996 ; Schwartz, Hernandez, & Mark Evers, 1999 ; Traenckner et al., 1995 ) ( Fig. 3 ). Mammals express five NF-κB proteins, namely RelA (p65), RelB, c-Rel, p50 and p52. Proteasome is involved in the maturation process of p50 and p52, which are synthesized as large precursors of p105 and p100 respectively ( Beinke & Ley, 2004 ; Fan & Maniatis, 1991 ). Treatment with proteasome inhibitors (PI), such as bortezomib, blocks p105 and p100 processing, and/or IkB degradation, thus inhibiting the NF-kB-mediated cancer promoting activity ( Adams, 2004a ; Johnson, 2015 ; Kaplan et al., 2017 ; Richardson et al., 2006 ). Indeed, NF-kB activation seems to play a major role in the antitumour effect of bortezomib, particularly in multiple myeloma and melanoma cells ( Amiri, Horton, LaFleur, Sosman, & Richmond, 2004 ; Hideshima et al., 2002 ). Among the numerous proteins regulated by NF-kB signalling, cyclin D1 plays a crucial role in cancer progression, since it is a key regulator of late G1 phase of cell cycle. The cyclin D1-Cdk4/6 complexes generate the phosphorylated form of the Rb protein, resulting in the release of EF2 transcription factors, inducing its activation. This is followed by the expression of cyclin E, which interacts with Cdk2 bringing about the hyper-phosphorylation of Rb, cyclin A and genes involved in DNA synthesis. These steps anticipate the S phase progression. Downregulation of NF-kB signalling, induced by proteasome inhibition, leads instead to a decrease in cyclin D1 level, impairing the phosphorylation of Rb and, in turn, the release of E2F, thus inhibiting G1/S transition ( Fig. 3 ) ( Diehl & Ponugoti, 2010 ; Harbour, Luo, Dei Santi, Postigo, & Dean, 1999 ; Masamha & Benbrook, 2009 ; Rastogi & Mishra, 2012 ).

p53

p53 is a nuclear transcription factor that regulates apoptosis, DNA repair, angiogenesis, cell growth and senescence ( Gupta et al., 2018 ; Vogelstein, Lane, & Levine, 2000 ); thus, regulation of its level is fundamental to guarantee cell homeostasis. This protein is characterized by a very rich functional spectrum that is the consequence of a structural complexity which renders it able to interact with a myriad of partners. p53 exists as a dynamic ensemble of different “proteoforms”, and this structural plasticity is due to the presence of intrinsically disordered regions, as well as to several modifications at transcriptional and post-translational level. Several p53 mutants form amyloid structures that aggregate in the cell through a "prion-like" fashion with a gain of function effect ( Rangel et al., 2019 ; Rangel, Costa, Vieira, & Silva, 2014 ; Silva, De Moura Gallo, Costa, & Rangel, 2014 ). It is noteworthy that p53 unfolded mutant forms are shared in cancer and in Alzheimer's disease (AD) tissues, actually entering in the list of biomarkers that can be used for their diagnosis ( Amor-Gutiérrez et al., 2020 ).

Under normal conditions, p53 degradation is a complex and finely regulated process, which is predominately orchestrated by the MDM2 protein, a RING-finger E3-ligase that promotes the poly-ubiquitination of p53, and, thus, its degradation by the 26S ( Fig. 3 ) ( Brown, Lain, Verma, Fersht, & Lane, 2009 ; Devine & Dai, 2013 ; Haupt, Maya, Kazaz, & Oren, 1997 ; Momand, Wu, & Dasgupta, 2000 ; Poyurovsky et al., 2007 ). P53 pro-apoptotic function covers a prominent role in tumour suppression, and mutations of p53 gene are among the most frequent genetic events in human tumours ( Gupta et al., 2019 ; Kandoth et al., 2013 ; Niazi, Purohit, & Niazi, 2018 ; Walerych et al., 2016 ; Walerych, Lisek, & Del Sal, 2015 ). Additionally, tumours expressing wt- p53 often have different mechanisms to bypass its activity, such as the overexpression of MDM2 ( Chène, 2003 ; Gupta et al., 2019 ; Quesnel et al., 1994 ). A series of studies, performed in different cancer cell models, including melanoma, head and neck and colon cancer, reveal that one of the main mechanisms of cell death induction by proteasome inhibition, is the p53 pathway stabilization ( An, Hwang, Trepel, & Blagosklonny, 2000 ; Concannon et al., 2007 ; Fernández et al., 2005 ; Gomez-Bougie et al., 2007 ; Li & Johnson, 2013 ; Li, Li, Grandis, & Johnson, 2008 ; Lopes, Erhardt, Yao, & Cooper, 1997 ; MacLaren, Chapman, Wyllie, & Watson, 2001 ; Morsi, Hage-Sleiman, Kobeissy, & Dbaibo, 2018 ; Qin et al., 2005 ; Yu, Carroll, & Thomas-Tikhonenko, 2007 ; Zhu et al., 2005 ). Accordingly, pro-apoptotic factors, such as Noxa and Bax, are primary p53-responsive elements ( Fig. 3 ) ( Albert, Brinkmann, & Kashkar, 2014 ; Oda et al., 2000 ). However, controversial results are still reported, since the killing of some cancer cells was shown to involve a p53-independent mechanism of Noxa induction, providing evidence for a novel strategy to bypass the apoptotic resistance of tumour cells (Perez-Galán et al., 2006; Qin et al., 2005 ; Strauss et al., 2007 ; Devine & Dai, 2013 ; Yerlikaya, Okur, & Ulukaya, 2012 ; Xue et al., 2019 ).

p21 and p27 Cdk inhibitors

One of the main hallmarks of carcinogenesis is the loss of cell division control. Proteasome is involved in the regulation of the cell cycle, since it degrades cyclin dependent kinases (Cdk) and Cdk inhibitors (CdkIs) ( Diehl & Ponugoti, 2010 ; Glickman & Ciechanover, 2002 ). Generally, the main function of CdkIs consists in the inhibition of cyclin/Cdk complexes, blocking cell division; p21 and p27 CdkIs expression is frequently suppressed in cancer, favouring the dysregulation of cell proliferation ( Abbas & Dutta, 2009 ; Chu, Hengst, & Slingerland, 2008 ).

27 is a well-known negative regulator of cell cycle progression in mammalian cells which binds and suppresses the activity of two crucial complexes ( i.e., Cdk2/cyclin E and Cdk2/cyclin A), mediating G1 progression and G1/S transition ( Sherr & Roberts, 1999 ; Slingerland & Pagano, 2000 ). P27 is ubiquitinated by the E3-ligase complex SCF Skp2 and then degraded by the 26S ( Abbas & Dutta, 2009 ; Chu et al., 2008 ; Rastogi & Mishra, 2012 ; Slingerland & Pagano, 2000 ). Low levels of p27 are reported in different cancers (including prostate, breast, and colorectal), as a consequence of an increased UPS activity, which leads to its accelerated degradation ( Glickman & Ciechanover, 2002 ; Loda et al., 1997 ; Tsihlias, Kapusta, & Slingerland, 1999 ). Moreover, consistent with the oncogenic role of SCF Skp2 , its overexpression is associated with low levels of p27, and thus with the deregulation of cell cycle progression ( Gstaiger et al., 2001 ; Lee & Lim, 2016 ).

p21, whose stability is essential for cell fate decision, binds Cdk2/cyclin E complex (blocking the onset of the cell S phase) and cyclin B/Cdk1 complex (leading to a growth arrest in the G2 phase) ( Abbas & Dutta, 2009 ; Rastogi & Mishra, 2012 ) ( Fig. 3 ). Moreover, p21 binds the proliferating cell nuclear antigen (PCNA), interfering with PCNA-dependent DNA polymerase activity, inhibiting DNA replication and modulating PCNA-dependent DNA repair processes ( Abbas & Dutta, 2009 ; Moldovan, Pfander, & Jentsch, 2007 ; Mortusewicz, Schermelleh, Walter, Cardoso, & Leonhardt, 2005 ; Walsh & Xu, 2006 ). Under normal conditions, p21 levels are controlled at a transcriptional level mainly by p53. In several cancer types, proteasome inhibition brings about accumulation of p53, enhancing its nuclear export, and thereby the expression of transcriptional target genes, including p21, counteracting the proliferation stimulus associated to low p21 levels ( Abbas & Dutta, 2009 ; Brugarolas et al., 1995 ; Deng, Zhang, Harper, Elledge, & Leder, 1995 ; Eastman, 2004 ; Roninson, 2002 ). Beside a transcriptional control by p53, p21 levels are modulated through either ( i ) a ubiquitin-dependent degradation by the 26S, and ( ii ) a ubiquitin -independent pathway of degradation by the uncapped 20S, which has been proposed for the free form of p21 ( Fig. 3 ) ( Chen et al., 2007 ; Deng et al., 2018 ; Li et al., 2007 ; Sheaff et al., 2000 ; Touitou et al., 2001 ). In particular, case ( i ) requires p21 ubiquitination by three E3 ligases ( i.e., SCFS KP2 , L4 CDT2 and APC/ C CDC20 ) at specific stages in an unperturbed cell cycle, which occurs only when p21 is bound to cyclin/Cdk complexes and PCNA. Consistent with these studies, proteasome inhibition has been reported to considerably increase the intracellular level of p27 and p21 in many cancers, favouring cell cycle arrest ( Huang et al., 2011 ; Li et al., 2018 ; Mi, Gan, & Chung, 2011 ; Rastogi & Mishra, 2012 ; Sterz et al., 2010 ).

Proteasome inhibitors for cancer therapy

Although more than one thousand proteins belong to the UPS function in the ubiquitination and recognition of ubiquitinated protein substrates, the vast majority of currently available inhibitors, which have been designed and synthesized to block this pathway, target the proteolytic core of 20S. These proteasome inhibitors are broadly categorized into different groups, according to the origin ( e.g., synthetic or natural products), the kinetic mechanism of inhibition ( e.g., competitive or non-competitive) or else the chemical structure/reactivity. This chapter and the following one will deal with the discussion on the most promising and clinically available inhibitors, pointing out, wherever possible, their molecular action as well as their pharmacological profile and therapeutic outcome of their usage in clinic.

Proteasome inhibitors were initially developed to prevent cancer-related cachexia, in view of proteasome role in protein turnover ( Manasanch & Orlowski, 2017 ). To date, UPS is universally considered a “ bona fide ” target for the development of anti-cancer drugs ( Adams, 2004a , Adams, 2004b ; Bullova, Cougnoux, Marzouca, Kopacek, & Pacak, 2017 ; Cloos et al., 2017 ; Gandolfi et al., 2017 ; King, Deshaies, Peters, & Kirschner, 1996 ; Landis-Piwowar et al., 2006 ; Narayanan et al., 2020 ; Niewerth et al., 2015 ; Roeten et al., 2018 ). Indeed, PIs represent the reference treatment of multiple myeloma (MM), in view of its high sensitivity to this class of anticancer agents ( Chauhan et al., 2005 ; Fricker, 2020 ; Gandolfi et al., 2017 ; Narayanan et al., 2020 ; Roccaro et al., 2006 ). It is important to recall that MM is an aggressive and often incurable plasma cell dyscrasia characterized by uncontrolled proliferation of abnormal plasma cells, which invade the bone marrow, producing abnormal monoclonal immunoglobulins, which circulate in the blood. The poor prognosis of MM, which reflects the genomic complexity of the disease, has dramatically improved after the introduction of PIs in disease management, mainly for patients displaying a refractory MM (RMM) and relapsed and refractory MM (RRMM), as discussed in the next section ( Leleu et al., 2018 ). As reported previously (see Section 3.2 ), proteasome inhibition results in multiple deleterious downstream effects in cancer cells, including down-regulation of NF-κB signalling, stabilization of p53, cell cycle arrest, which all lead to apoptosis. Moreover, PIs downregulate adhesion molecules and secretion of cytokines ( Chauhan, Hideshima, & Anderson, 2005 ; Read et al., 1995 ), inhibit angiogenesis ( Sunwoo et al., 2001 ) and induce DNA-damage ( Łuczkowska, Rogińska, Ulańczyk, & Machaliński, 2020 ).

The effort to develop PIs has a long history and many different approaches have been adopted, ranging from the use of endogenous and/or natural compounds to the synthesis of new ones ( Buac et al., 2013 ). Initially, proteasome targeting for cancer therapy has been viewed with scepticism, mainly because of the fundamental and crucial roles of proteasome in regulating cell homeostasis in all living cells ( Park, Miller, Jun, Lee, & Kim, 2018 ). Although the reason for the increased cytotoxicity of PIs on proliferating tumour cells is not completely understood, it is widely reported ( Chauhan, Catley, et al., 2005 ) that cancer cells are more dependent on proteasomal activity, likely because of the higher protein turnover they experience, thus being also more sensitive to its blockage ( Almond & Cohen, 2002 ). Tumour cells have a proteasome pathway more active than normal cells, since an increased capability for synthesis and modification of proteins is necessary to preserve their uncontrolled cell proliferation and their high metastatic capacity ( Chen et al., 2011 ; Chen & Madura, 2005 ).

Currently, three clinically approved PIs are available, namely: ( i ) bortezomib (Velcade, recently introduced in the market also in its generic version) (approved in 2003 and 2004, by FDA and EMA, respectively), ( ii ) carfilzomib (Kyprolis) (approved in 2012 and 2015, by FDA and EMA, respectively), and ( iii ) the first oral PI, ixazomib (Ninlaro) (approved in 2015 and 2016 by FDA and EMA, respectively) ( Feling et al., 2003 ; Fricker, 2020 ; Gandolfi et al., 2017 ; Narayanan et al., 2020 ). Although the availability of PIs has led to an improvement of patients'survival rate, the therapeutic potentiality of these drugs is limited by several drawbacks, including the low potency and specificity of approved molecules, adverse effects and development of drug resistance ( Assaraf et al., 2019 ; Cree & Charlton, 2017 ; Gacche & Assaraf, 2018 ; Gonen & Assaraf, 2012 ; Li, Wu, & Cheng, 2016 ; Wijdeven, Pang, Assaraf, & Neefjes, 2016 ; Zhitomirsky & Assaraf, 2016 ). Furthermore, the therapeutic potential of bortezomib is negatively affected by pharmacokinetic issues and by the very limited distribution to solid tumours which require exceedingly high and toxic doses ( Grigoreva, Tribulovich, Garabadzhiu, Melino, & Barlev, 2015 ; Huang et al., 2014 ). The use of more recently approved carfilzomib and ixazomib has only partially allowed to overcome these issues (see Section 3.3.2.1 ). Therefore, there is a growing demand of novel inhibitors with different mechanisms of action and more favourable pharmacological profiles. Additionally, it emerges that the antitumour activity of PIs is markedly improved in combination with conventional therapeutic strategies or with other molecularly targeting agents, such as cell surface death receptor, autophagy, STAT3 and Histone deacetylase (HDAC) inhibitors ( Li et al., 2009 ; Li et al., 2010 ; Li et al., 2012 ; Li & Johnson, 2012 ; Li, Zhou, & Chen, 2008 ; Pei, Dai, & Grant, 2004 ; Seki et al., 2010 ; Yoshiba et al., 2011 ). Accordingly, a number of preclinical and clinical studies are ongoing to evaluate further new drug combinations, and to optimize administration schedules of therapeutic protocols already used ( Berenson et al., 2007 ; Chen, Frezza, Schmitt, Kanwar, & Dou, 2011 ; Chen, Retzlaff, Roos, & Frydman, 2011 ; Johnson, 2015 ; Wallington‐Beddoe, Sobieraj‐Teague, Kuss, & Pitson, 2018 ).

Chemical structure and mechanism of action

Generally, PIs are electrophilic molecular species that react covalently with the threonine residues of the proteasome active sites ( Harer, Bhatia, & Bhatia, 2012 ). The first PIs were analogs of serine protease inhibitors, characterized by hydrophobic peptide aldehydes, mimicking substrates of the proteasome β5 active site and reacting with the nucleophilic hydroxyl group of threonine to form reversible hemiacetal adducts. However, first aldehyde inhibitors turned out to have additional targets in the cell besides the proteasome, also inhibiting cathepsin B and calpains ( Kisselev & Goldberg, 2001 ). For this reason, other molecular scaffolds have been investigated and peptide boronates (such as bortezomib and ixazomib) as well as epoxyketones ( i.e., carfilzomib and oprozomib) have been synthesized. These new and more specific PIs have experienced great success in clinics (as extensively discussed in Section 3.3.2 ), and most of them are currently used as therapeutic drugs, even though some of them still retain activity toward non-proteasome targets. In fact, bortezomib and most second-generation boronates also co-inhibit caspase-like sites ( Kisselev, van der Linden, & Overkleeft, 2012 ). In the next section, the main chemical properties of PIs approved and/or ongoing in clinical trials are discussed.

First-generation proteasome inhibitors: Bortezomib

Bortezomib is a dipeptide containing phenylalanine and leucine with a boronic acid instead of a carboxylic acid, and a pyrazinoic acid moiety to protect the N-terminus. The structure of bortezomib bound to the 20S has been solved, elucidating the binding mode and mechanism of action at the molecular level (Jung et al., 2004; Groll, Berkers, Ploegh, & Ovaa, 2006 ) ( Fig. 4 B). Bortezomib binds reversibly to the chymotryptic-like (CT-L) β5 subunit of the proteasome, even though it has also been reported to bind the caspase-like (C-L) β1 and trypsin-like (T-L) β2 subunits with lower affinity ( Buac et al., 2013 ); however, a good selectivity of bortezomib toward specific proteasome subunits is dictated by the composition of their substrate binding pockets, which differs in the three catalytic β-subunits. In the presence of bortezomib, an anti-parallel β sheet conformation is adopted by domains in the catalytic clefts, and direct hydrogen bonds are formed between the conserved residues ( i.e., Gly47N, Thr21N, Thr21O, and Ala49O) of the proteasome β-type subunits and the main chain atoms of the drug, stabilizing the complex ( Fig. 4 B). The boronic acid is responsible for the actual inhibition, ensuring an increased specificity for the proteasome. Indeed, the boron atom covalently binds the oxygen of Thr1Oγ (the electrophilic functional group that normally reacts with peptide bonds of substrates, see Section 2.2.1 ), while the acidic boronate hydroxyl groups are bound to Gly47N, bringing about a stabilization of the oxyanion hole. Further stabilization of the tetrahedral boronate adduct comes from a second acidic boronate hydroxyl moiety, which works as a catalytic proton acceptor and is H-bridged to the N-terminal threonine amine atom. A wide range of specific inhibitors has been developed, but usually peptide boron esters and acids are powerful inhibitors of serine proteases, as they interact covalently but reversibly with the active hydroxyl site of this class of the enzymes ( Harer et al., 2012 ). Furthermore, these peptide boron esters are less reactive toward circulating nucleophiles in aqueous solutions than their aldehyde counterparts ( Adams et al., 1998 ). Fig. 4 Proteasome binding structures of PIs. The structures of proteasome binding to carfilzomib ( panel A ), bortezomib ( panel B ), salinosporamide (marizomib) ( panel C ) and ixazomib ( panel D ); are reported. The β-5 subunit is represented as turquoise ribbon, the β-1 subunit is represented as purple ribbon, the inhibitors are represented as orange sticks and the protein residues interacting with the inhibitors as grey sticks. Fig. 4

Bortezomib induces toxicity in cancer cells through different mechanisms, including ( i ) inhibition of the NF-kB pathway, which has been envisaged as the main target of bortezomib clinical efficacy; ( ii ) stabilization of p53 pathway, which leads to apoptosis mainly by increasing the level of pro-apoptotic factors, such as NOXA and Bcl-2; ( iii ) modulation of CdkIs levels. Moreover, bortezomib inhibits tumour angiogenesis probably as a result of reduced vascular endothelial growth factor receptor (VEGFRs), which seems to be linked to the inhibition of NF-kB ( Hideshima et al., 2001 ; Hideshima et al., 2003 ; Nunes & Annunziata, 2017 ; Pandit & Gartel, 2011 ; Sunwoo et al., 2001 ). Despite the plethora of mechanisms of actions responsible for the high toxicity of bortezomib toward cancer cells and the high specificity toward serine proteases, peptide boron esters containing acids, such as bortezomib, can become bio-activated to chemically reactive imine amide metabolites inducing drug toxicity ( Li, Yu, Ring, & Chovan, 2013 ). As a matter of fact, carbinolamides metabolites have been detected after incubation with human liver proteins and the formation of GSH conjugates was also observed, both likely stemming from electrophilic reactions of the imine amides with the nucleophilic GSH. The observed metabolites seem to be produced via oxidative de‑boronation, catalyzed by hepatic cytochrome P450 enzyme, and bortezomib toxicity has been ascribed to their formation and high reactivity (see also Section 3.3.2.1 ). A way to reduce these adverse effects of bortezomib treatment is the use of appropriate methods for administering this agent, such as early-dose reduction and once-weekly and subcutaneous administration.

A crucial drawback, encountered when bortezomib is used as a therapeutic drug, is the rapid development of resistance in response to the treatment ( Barrio et al., 2019 ; Lee et al., 2015 ). Many studies have described a plethora of strategies the cancer cells may evolve to acquire bortezomib resistance. In this regard, selective down-regulation of specific 19S subunits and the consequent reduced flux of substrates through proteasome has been reported to be a major strategy that several cancer cells may adopt to cope with proteasome inhibition ( Tsvetkov et al., 2015 ; Tsvetkov et al., 2017 ). Somatic mutations in the catalytic cleft of β5 and involved in the binding to bortezomib have been further described in patients with MM who underwent prolonged therapy with PIs: in this case, resistance induction was acquired through missense mutations and resistance was effective, though at a variable extent, also to next generation PIs (see next paragraphs) ( Barrio et al., 2019 ).

Further, a selective overexpression (up to 60-fold) of a mutant β5 protein has been proposed at the origin of the bortezomib resistance, whereas marked changes in CT-L proteasome activity are not found ( Oerlemans et al., 2008 ). On the other hand, other studies have reported a significant decrease of the CT-L proteasome activity after 1 h in four different cell lines, maintaining such an inhibitory activity for as long as 24 h (Bonvini et al., 2007). Furthermore, an increase in the accumulation of the β5 precursor form was observed, even though no significant alteration in the expression profile of the mature form was detected (Yerlikaya & Okur, 2019). Analogously, it has been also reported that α5 promotes the tumourigenic process of prostate cancer cells and is linked to bortezomib resistance ( Fu et al., 2018 ). Other changes in bortezomib resistant cell lines, such as increased expression of β1 and β5 proteasome subunits, upregulation of pro-apoptotic proteins of the Bcl-2 protein family, Bax and Noxa have been also reported ( Wu, Yang, & Saitsu, 2016 ). Moreover, the lack of some proteins, such as XBP1, which decreases the endoplasmic reticulum burden and affects the unfolded protein response, has also been proposed as a possible cause of bortezomib resistance (Fall et al., 2014). Interestingly, certain factors have been proposed as predictive markers of response to bortezomib treatment. Among others, KLF9, CDK5, Nampt and accumulation of unfolded proteins in the endoplasmic reticulum (ER stress) and UPR-associated markers (XBP1, ATF3, and AFT4) have been identified to play an important role in bortezomib sensitivity. It is therefore clear that further studies are demanded in order to better understand the underlying mechanisms which limit the use of this compound for cancer treatment.

Second-generation proteasome inhibitors

The socalled second generation of PIs includes drugs with different chemical features that reflect their different pharmacological profile (see Section 3.3.2.2 ); they can be either epoxyketones ( e.g., carfilzomib and oprozomib), peptide boronate ( e.g., ixazomib and delazomib) or else nonpeptide proteasome inhibitor, such as marizomib.

In general, epoxyketone PIs are characterized by a short peptide core, and a terminal α,β-epoxyketone dual electrophilic reactive warhead, which determines their activity ( Schrader et al., 2016 ). The most important representative of this PI class is carfilzomib, a tetrapeptide with a terminal epoxyketone group, which seems to be highly specific for the proteasome (see 3.3.1.1 , 3.3.2.2.2 ) ( Muz et al., 2016 ). It displays an inhibitory power equivalent to that of bortezomib for CT-L subunits of the proteasome (IC 50 = 6 nM), whereas C-L and T-L sites are only very weakly inhibited by carfilzomib (IC 50 = 2400 and 3600 nM, respectively); thus it is considered a selective inhibitor of CT-L activity ( Demo et al., 2007 ; Boccon-Gibod et al., 2020). Carfilzomib forms a covalent adduct between its C-terminal ketone moiety and Thr1O of each inhibited subunit ( Fig. 4 A). Additionally, unlike peptide boronates, such as bortezomib and ixazomib, carfilzomib forms a stable morpholine ring between Thr1 N-terminal amino group and epoxide α carbon. These further covalent interactions dramatically enhance the specificity of epoxyketones for proteasome with respect to other proteases, making this PI class essentially irreversible under physiological-treatment conditions ( Groll et al., 2000 ). Crystallographic studies on human 20S proteasome in complex with carfilzomib clarify the structural basis for the high in vivo drug's selectivity for CT-L activity. The high specificity for CT-L activity can be attributed to van der Waals interactions between carfilzomib and S1, S3, and S4 pockets of β5 subunits, whereas in the T-L β2 subunit, carfilzomib forms favourable van der Waals interactions only with S3 and S4 pockets, but not with S1 ( Fig. 4 A). In fact, the main differences between CT-L and T-L sites mostly reside in the S1 pocket size, which is much wider in the T-L subunit than in the CT-L one, rendering less effective the van der Waals contacts between the P1 leucyl group of carfilzomib and the S1 pocket of the T-L subunit. In addition, His116 of β7 subunit sterically blocks the entry of carfilzomib P4 phenyl group into the S4 pocket of the T-L subunit, shifting P4 up to 3.7 Ǻ away from the S4. Furthermore, the polarity of the C-L S1 pocket impairs an interaction with the carfilzomib hydrophobic P1 leucyl group ( Fig. 4 A). As a whole, these differences lead to a disordered N-terminus of carfilzomib, likely contributing to the higher IC 50 value of C-L activity ( Harshbarger, Miller, Diedrich, & Sacchettini, 2015 ).

The mechanisms through which carfilzomib induces cell death are less known than for bortezomib, even though several studies demonstrated that it elicits programmed cell death acting in different ways, such as ( i ) activating c-Jun-N-terminal kinase, ( ii ) promoting mitochondrial membrane depolarization and favouring cytochrome c release, ( iii ) increasing the levels of pro-apoptotic factor Noxa, and activating caspase-3 and caspase-7 ( Narayanan et al., 2020 ; Parlati et al., 2009 ). The introduction of carfilzomib in clinical therapy has allowed to overcome some criticisms related to bortezomib administration, like a reduced incidence of adverse effects ( e.g., severe peripheral neuropathy) and acquired resistance; therefore, it has become a key option for the treatment of RMM patients (see Section 3.3.2.2.1 ). However, a number of patients also display intrinsic resistance or develop resistance to carfilzomib treatment ( Shah et al., 2018 ). The reasons for this might stem from mutations or overexpression of proteasome catalytic subunits, but a likely contributor to carfilzomib resistance could also be the overexpression of the efflux pump P-glycoprotein (P-gp), reducing the drug intracellular concentration, since carfilzomib is a recognized substrate of this enzyme ( Ao et al., 2012 ; Besse et al., 2018 ; Lee et al., 2019 ; Zang, Kirk, & Johnson, 2014 ; Zheng, Liu, Zheng, & Hu, 2017 ).

Oprozomib is an orally bioavailable peptide epoxyketone, currently tested in ongoing clinical trials. It is a tripeptide structural analogue of carfilzomib, which was synthesized to improve drug absorption. In fact, it is thought that smaller peptides are absorbed more effectively across the small intestine wall ( Hamman, Enslin, & Kotzé, 2005 ; Zhou et al., 2009 ). Like carfilzomib, oprozomib primarily exhibits irreversible binding kinetics to CT-L subunit ( Rajan & Kumar, 2016 ), and the co-crystal structure of human 20S and oprozomib enhanced the knowledge of how proteasome active sites interact with peptide epoxyketone inhibitors ( Schrader et al., 2016 ). Oprozomib, like other epoxyketone inhibitors, forms a seven-membered, 1,4-oxazepano adduct with the catalytic Thr within the β5 active site, whereas (as also reported for carfilzomib) previous findings reported the formation of a 1,4-morpholino adduct. Therefore, these novel solved structures have indicated that, during the second step of the inhibitory reaction, the Thr N-terminal amino group attacks the β carbon rather than the α carbon of the inhibitor's epoxide ( Carmony, Lee, & Kim, 2016 ; Schrader et al., 2016 ). Concerning the mechanism through which oprozomib mediates cancer cell death, it has been demonstrated that oprozomib induces apoptosis through the activation of caspase 3, 8 and 9 ( Chauhan et al., 2010 ), PARP cleavage, and, interestingly, it seems to block angiogenesis that it is known to play a key role in MM progression ( Chauhan et al., 2010a; Giuliani, Storti, Bolzoni, Palma, & Bonomini, 2011 ; Podar et al., 2001 ; Zhu et al., 2019 ).

Ixazomib and delazomib are both orally available structural analogues of bortezomib with a boronic acid as pharmacofore. In particular, ixazomib is a dipeptidyl leucine boronic acid, that was developed through a large-scale screening of boron-containing PIs in the search of compounds with an increased efficacy and reduced side effects with respect to bortezomib ( Chauhan et al., 2011 ; Kupperman et al., 2010 ; Offidani et al., 2014 ). Since it belongs to the same chemical class of bortezomib, it is not surprising that its acts through a similar mechanism of action. In fact, proteasome subunit inhibition occurs when boric acid group forms a covalent bond with the hydroxyl group of the catalytic N -terminal threonine residue ( Muz et al., 2016 ). Like bortezomib, ixazomib reversibly blocks the CT-L of the β5 subunit (IC 50 = 3.4 nmol/L for ixazomib vs 2.7 nmol/L for bortezomib) ( Chauhan et al., 2011 ; Lee, De la Mota-Peynado, & Roelofs, 2011 ) ( Fig. 4 D). Noteworthy, proteasome dissociation half-life for ixazomib is relatively shorter than for bortezomib (18 min for ixazomib and 110 for bortezomib), improving its general tissue distribution and rendering this drug more “re-available” (see Section 3.3.2.2.2 ) ( Kupperman et al., 2010 ; Narayanan et al., 2020 ). Biochemical analysis and in vitro studies showed that at high concentrations ixazomib (like bortezomib) inhibits also other proteolytic sites of 20S proteasome ( Chauhan et al., 2011 ). However, the most important advancement of ixazomib with respect to bortezomib is the possibility of an oral administration; thus, ixazomib can be formulated as an ester citrate prodrug (MLN2238), which is rapidly hydrolyzed in aqueous solution ( e.g., plasma) to the pharmacologically active metabolite MLN2238 with free boric acid ( Chauhan et al., 2011 ; Gupta et al., 2019 ; Kupperman et al., 2010 ; Okazuka & Ishida, 2018 ). Like bortezomib, the apoptotic activity of ixazomib is mediated by caspase 3–8-9 activation through a stabilization of the p53 pathway (see 3.2.2 , 3.2.3 ) ( Muz et al., 2016 ). Interestingly, microRNA studies in MM cells revealed that ixazomib induces upregulation of the tumour suppresso miR33b , leading to apoptosis by blocking proto-oncogene PIM-1 ( Tian et al., 2012 ).

Another peptide boronate is delazomib, which reversibly inhibits CT-L subunit with a magnitude and a mechanism of action similar to bortezomib and ixazomib ( Dorsey et al., 2008 ; Piva et al., 2008 ). Moreover, like bortezomib, in vitro studies have revealed that delazomib primary target is the inhibition of NF-kB pathway, with a consequent alteration of the expression of several NF-kB downstream effectors ( Kubiczkova, Pour, Sedlarikova, Hajek, & Sevcikova, 2014 ; Piva et al., 2008 ).

The main representative of the PI third class is marizomib, also named salinosporamide A, which is a natural product deriving from a sediment obligate marine actinomycete identified as Salinospora tropica (strain CNB-392) ( Potts & Lam, 2010 ; Pereira et al., 2019 ) . It is characterized by a different non-peptide-based structure with respect to other PIs so far described. Its unique structure displays a fused γ-lactam- β -lactone ring system containing a cyclohexenyl carbinol and chloroethyl functional groups. Marizomib is an irreversible inhibitor of all catalytic subunits of 20S, with IC 50 values ranging from the low pM to mid nM range ( Fenical et al., 2009 ), and it produces a prolonged 20S inhibition (≥72 h) ( Potts et al., 2011 ). Crystallographic structure of the complex between Salinosporamide A and yeast 20S showed that the drug perfectly occupies the active sites of all three pairs of catalytic subunits of 20S ( Fenical et al., 2009 ; Groll et al., 2006 ) ( Fig. 4 C). These findings provided a detailed understanding of the proteasome-ligand interactions at the molecular level, revealing a unique mechanism of action that renders the inhibitor irreversible. The first step of interaction is represented by the formation of a covalent ester bound between the catalytic N-terminal Thr1Oγ of each 20S subunit and the carbonyl of the β -lactone ring of the inhibitor ( Fig. 4 C). β-lactone ring opening is followed by chlorine elimination, giving rise to a stable 5-membered cyclic ether ( Groll et al., 2006 ). One of the main downstream effect observed after proteasome inactivation by marizomib is the inhibition of NF- κ B activation, in a fashion similar to other PIs ( Narayanan et al., 2020 ). Remarkably, as also discussed in Section 3.3.2.2.3 , the main advantage of marizomib is the capability to overcome the blood-brain barrier, which opened a novel therapeutic potential for this inhibitor, eliciting the research in attempt to improve its pharmacological profile ( Park et al., 2018 ; Singh et al., 2010 ).

An intriguing point on proteasome biology, which reflects on the identification of more clinically effective PIs and/or novel combination therapy, concerns the individual role and the functional meaning of different proteasome subunits. Although these crucial aspects are poorly understood, β5 subunit was initially identified as the rate limiting protease for proteasome-dependent protein turnover ( Heinemeyer et al., 1997 ; Arendt & Hochstrasser, 1999 ; Groll et al., 1999 ). Consequently, PIs were designed to target β5 subunits, as it comes from the above reported chemical features of main PIs ( Kubiczkova et al., 2014 ; Kisselev et al., 2012 ). However, advanced chemical manipulations, which allowed to monitor the activity of each individual proteolytic subunit, have pointed out that, at higher concentrations, all β5-targeted PIs lose their subunit selectivity and inhibit also the β1 and/or β2 types of proteasome subunits ( Kraus et al., 2015 ; Bruin et al., 2016). These co-inhibition patterns differ among individual PIs and seem to be responsible for the overcoming of drug resistance observed at higher concentrations. In this respect, recent investigations have revealed that β5 and β2 co-inhibition, exclusively achieved by high levels of carfilzomib, is the most effective proteasome inhibition profile in MM ( Besse et al., 2019 ). Therefore, it has been proposed that a better comprehension of the significance of different coinhibitory patterns should help to understand the differential activity and toxicity observed during treatment with different PIs as well as with different doses of the same drug ( Besse et al., 2019 ). Moreover, these findings could provide the rationale for preclinical and clinical investigations of a novel treatment schedules. These results showed that differences on the functional proteasome-interacting groups of the PI ( i.e., epoxyketone, β-lactone, or boronate) cannot account for the observed differences in the clinical efficacy of the various drugs (reversible versus irreversible proteasome binding); differences in the PIs affinity toward the various proteasome subunits should be considered instead. The observed differences in PIs affinities are mainly due to different interactions of the specific PI side chains with each of the proteasome subunits. The chemical interpretation of the different PIs inhibition capability for the various proteasome subunits ( Gozzetti et al., 2017 ) also explains why drugs combination is more effective than monotherapy.

Clinical pharmacology

First-generation proteasome inhibitors: bortezomib

In the 1990s, the reversible PI bortezomib (formerly named as PS-341) PI, was initially developed as anti-inflammatory and anti-cachectic agent. However, preclinical studies soon unravelled that bortezomib was highly effective against different tumours, in particular MM, inducing growth arrest and apoptosis and inhibiting angiogenesis ( Mitch & Goldberg, 1996 ; Adams et al., 1998 ; Adams et al., 1999 ; Teicher, Ara, Herbst, Palombella, & Adams, 1999 ; Hideshima et al., 2001 ; LeBlanc et al., 2002 ; Ma et al., 2003 ; Mitsiades et al., 2003 ; Sánchez-Serrano, 2006 ; Caravita, de Fabritiis, Palumbo, Amadori, & Boccadoro, 2006 ). Additionally, in vitro studies revealed that bortezomib increased in vitro tumour chemosensitivity and overcame chemoresistance to dexamethasone, doxorubicin, and melphalan, suggesting also its use in combination therapies ( Hideshima et al., 2001 ; Ma et al., 2003 ; Mitsiades et al., 2003 ). The overall bulk of in vitro and in vivo studies supported clinical investigations of bortezomib in patients with MM, who had received at least two prior therapies and have demonstrated disease progression after the last therapy ( Caravita et al., 2006 ; Park et al., 2018 ; Robak & Robak, 2019 ), leading to the first global approval of PI for cancer treatment.

A pivotal early phase I study investigated the maximum-tolerated dose, dose-limiting toxicity, and pharmacodynamics of bortezomib in patients with refractory haematological malignancies, showing activity against RMM ( Orlowski et al., 2002 ). Subsequently, a phase 2 study (CREST) showed the efficacy of the PI, as single agent or in combination with dexamethasone, in patients with relapsed MM after frontline therapy ( Jagannath et al., 2004 ). These observations provided the rationale for a phase 2 open-label, single-arm (SUMMIT) trial, which included 202 patients with RRMM receiving at least two prior therapies, in which bortezomib (1.3 mg/m 2 ) was administered by intravenous bolus twice weekly for 2 weeks, followed by 1 week without treatment, for up to eight cycles (24 weeks). This study reported 27.7% complete or partial response rate, a median response of 12 months and manageable adverse effects ( Richardson et al., 2003 ). Moreover, bortezomib increased the time to progression to a higher extent (2–4 folds) compared to the last treatment patients received before entering the clinical trial. These impressive results led to the accelerated approval of bortezomib for the treatment of patients with RRMM who had received at least two prior therapies, a particularly difficult-to-treat patient population. An extended follow-up of the SUMMIT study reported a median time to progression of 13.9 months for responding patients, whereas of 1.3 months for those with progressive disease or not evaluable ( Richardson, Mitsiades, Hideshima, & Anderson, 2006 ). The phase 3 trial, APEX, comparing bortezomib with high-dose dexamethasone for RRMM after one to three previous lines of treatment, showed a significant increased survival in patients treated with the PI (one-year survival rates of 80% versus 66%, P = .003; the hazard ratio for overall survival (OS) = was 0.57, P = .001) ( Richardson et al., 2005 ; Cengiz Seval & Beksac, 2018 ) and these results led in 2005 to the regular approval to bortezomib. The superiority of bortezomib was further confirmed after an extended follow-up, in which the reported median OS was 29.8 months for bortezomib and 23.7 months for high-dose dexamethasone, despite crossover from dexamethasone to bortezomib arm ( Richardson et al., 2007 ). Thereafter, in 2007 FDA approved label expansion of bortezomib to include patients with impaired kidney function, without the requirement of dose adjustment).

Bortezomib was also tested in multidrug regimens, since the shift in clinical practice to a more aggressive approach, including PIs (as also discussed in Section 3.2 ), has improved survival outcomes ( Leleu et al., 2019 ). In the DOXIL-MMY-3001 phase 3 study the safety and efficacy of bortezomib in combination with pegylated liposomal doxorubicin were compared to those of bortezomib as a single agent in patients with RRMM who had received at least one prior treatment. The doublet therapy was more effective than monotherapy, even though associated with a higher incidence of grade 3/4 (80 vs 64%, respectively) myelosuppression, gastrointestinal events, and hand-foot syndrome ( Orlowski et al., 2007 ). However, despite the significant increase in time to progression observed in the group treated with the drug combination, the final results of OS analysis after a median follow-up of 103 months indicated no significant differences between the two treatments ( Orlowski et al., 2016 ). The triple combination bortezomib-thalidomide-dexamethasone resulted in increased median time to progression (19.5 versus 13.8 months; hazard ratio, 0.59; P < .001) compared to the dual combination of the immunomodulatory agent thalidomide plus dexamethasone, in patients with MM progressing or relapsing after autologous stem-cell transplantation (ASCT), as demonstrated in a phase 3 study (MMVAR/IFM 2005–04) ( Garderet et al., 2012 ). Although a direct comparison between trials is not possible, the observed time to progression was higher than that observed in the APEX trial where bortezomib was tested as single agent (6.2 months) or in the DOXIL-MMY-3001 trial where the PI was combined with liposomal doxorubicin (9.3 month). The addition of bortezomib to thalidomide-dexamethasone was associated with a substantial increase of cumulative, dose-related grade 3 peripheral sensory neuropathy ( Garderet et al., 2012 ). Since thalidomide is also neurotoxic, in the triple combination this agent was replaced by lenalidomide, which is a better-tolerated analogue. Indeed, a phase 2 trial demonstrated a similar median time to progression of 19.5 months, but a markedly lower rate of grade 3 peripheral neuropathy compared to the triple combination including thalidomide (2% versus 29%) ( Richardson et al., 2014 ). The current guidelines recommend the triple combination of bortezomib, lenalidomide and dexamethasone as a preferred salvage regimen for previously treated multiple myeloma and as first-line therapy irrespective of transplantation eligibility (National Cancer Comprehensive Network, NCCN Clinical Practice Guidelines–Multiple-Myeloma.Version3.2020 https://www.nccn.org/professionals/physician_gls/pdf/myeloma.pdf . Accessed April 4, 2020). For RRMM patients who previously received lenalidomide, bortezomib and dexamethasone have also been evaluated in combination with the other immunomodulatory agent pomalidomide. In particular, the phase 3 clinical trial OPTIMISMM showed a significant increase in median progression-free survival (PFS) compared to the doublet bortezomib-dexamethasone (11.2 months versus 7.1 months, HR = 0.61, P 〈0,0001) ( Richardson et al., 2019 ). Two other interesting therapeutic triplet regimen in RRMM, which included bortezomib, are: (i) bortezomib-dexamethasone and the anti-CD38 monoclonal antibody daratumumab approved in 2016 by FDA and EMA for the treatment of patients who had received at least one prior therapy; (ii) bortezomib-dexamethasone and HDAC inhibitor panobinostat approved in 2015 by FDA and EMA for the treatment of patients who had received at least two prior therapies, including bortezomib and an immunomodulatory drug.

In the phase 3 CASTOR trial, the addition of darutumumab resulted in significantly longer PFS (median PFS 16.7 vs. 7.1 months, HR = 0.31) compared to bortezomib plus dexamethasone, but it was associated to infusion-related reactions and higher rates of thrombocytopenia and neutropenia ( Palumbo et al., 2016 ; Spencer et al., 2018 ).

Panabinostat was the first pan-HDAC inhibitor approved to treat MM, which acts via epigenetic modification and inhibition of the aggresome pathway ( i.e. a proteasome-independent pathway that eliminates misfolded proteins). The approval for RRMM was based on the results from the pivotal phase 3 PANORAMA-1 clinical trial, which demonstrated an improvement in PFS of 7.8 months for the three-drug combination compared with placebo plus bortezomib and dexamethasone in this patient population (11.99 vs. 8.08, P < .0001), even though several adverse events were more frequently observed in the panobinostat group ( San et al., 2014 ; San-Miguel et al., 2014 ; San et al., 2016 ).

A phase 2 study has also investigated the combination of bortezomib plus dexamethasone with the immunomodulatory agent elotuzumab, a monoclonal antibody against SLAMF7 (signalling lymphocytic activation molecule F7), reporting encouraging results (median PFS 9.7 vs. 6.9 months) ( Jakubowiak et al., 2016 ). Based on the results of this study, the National Comprehensive Cancer Network (NCCN) panel guidelines have included also this triple combination among the therapeutic option for RRMM who have previously received at least one prior therapy (National Cancer Comprehensive Network, NCCN Clinical Practice Guidelines – Multiple Myeloma. Version 3.2020. https://www.nccn.org/professionals/physician_gls/pdf/myeloma.pdf . Accessed April 4, 2020).

Once bortezomib efficacy was established for RRMM in the early 2000s, attention turned to patients with newly diagnosed disease (NDMM), in whom its efficacy was tested with or without dexamethasone, showing that the combined treatment was associated with improved response rate without additional severe toxicities compared to PI monotherapy ( Mateos et al., 2006 ; Mateos et al., 2008 ; Harousseau et al., 2010 ; Jagannath et al., 2005 ; Okazuka & Ishida, 2018 ). In 2008, the key phase 3 trial VISTA led to the approval of bortezomib, in combination with melphalan and prednisone, by FDA for previously untreated MM and by EMA for previously untreated MM not eligible for high dose chemotherapy and stem-cell transplantation (SCT) ( San Miguel et al., 2008 ). Melphalan plus prednisone, was the standard of care for NDMM patients over 65 years old, being instead high-dose chemotherapy followed by SCT the preferred treatment for patients under the age of 65 years ( Barlogie et al., 1997 ; Alexanian et al., 1969 ; San-Miguel et al., 2016). In the VISTA trial, 682 patients were randomized to receive either melphalan and prednisone or the same schedule with the addition of bortezomib (at the dose of 1.3 mg/ml). At the initial analysis, the time to disease progression was 24.0 months in the bortezomib group and 16.6 months in the control group (HR for the bortezomib group = 0.48; P < .001) (San-Miguel et al., 2008). The interim and final analyses confirmed the efficacy of the triplet regimen: after median follow-ups of 36.7 months and 60.1 months, 35% and 31% risk of death reduction were reported, respectively, in the bortezomib-containing group versus the control group (HR = 0.653; P < .001 and 0.695; P < .001, respectively) ( Mateos et al., 2010 ; San Miguel et al., 2013 ). The final reported median OS was 56.4 vs 43.1 months ( San Miguel et al., 2013 ). Even in this case, peripheral neuropathy events were more frequently documented in the bortezomib group. Interestingly, an Italian phase 3 study introduced thalidomide in the triplet regimen of the VISTA trial: 511 patients were randomly assigned to receive nine cycles of the four-drug combination bortezomib-melphalan-prednisone-thalidomide followed by continuous bortezomib-thalidomide as maintenance (VMPT-VT), or bortezomib-melphalan-prednisone (VMP, control group) at the same doses. In the initial analysis, the median PFS was not reached in the VMPT-VT arm and was 27.3 months in the VMP group; the 3-year PFS was 56% in patients receiving VMPT-VT and 41% in those receiving VMP (HR = 0.67; P = .008) ( Palumbo et al., 2010 ). A longer follow up confirmed the higher survival benefit of the VMPT-VT protocol compared to the triplet combination (median PFS 35.3 months versus 24.8 months; HR = 0.58; P < .001) ( Palumbo et al., 2014 ). Thereafter, several randomized, open label, phase 3 clinical trials (IFM 2005–01; GIMEMA; PETHEMA/GEM and MMY-3006) tested bortezomib-dexamethasone doublet regimen or bortezomib-dexamethasone-thalidomide triplet regimen in patients with previously untreated MM as induction therapy before SCT. These and other clinical trials demonstrated a statistically significant improvement in the post-transplantation complete response rate as a consequence of bortezomib inclusion in the induction regimens, even though the incidence of peripheral neuropathy was increased (Horousseau et al., 2010; Cavo et al., 2010 ; Rosiñol et al., 2012 ; Sonneveld et al., 2013 ). The results of these studies led to EMA approval in 2013 of bortezomib with dexamethasone or with dexamethasone plus thalidomide for the induction treatment of patients with previously untreated MM, eligible for high dose chemotherapy followed by SCT. Thereafter, the bortezomib-lenalinomide-dexamethasone triplet regimen has become one of the standard induction therapies before SCT ( Okazuka & Ishida, 2018 ). In two different phase 2 studies ( i.e., IFM and IFM/DFCI 2009) this therapeutic regimen was tested in patients with NDMM eligible for SCT as induction therapy, and as induction and consolidation therapy, respectively, with encouraging results in terms of PFS ( Richardson et al., 2010 ; Roussel et al., 2014 ). Additionally, results from a phase 3 clinical trial demonstrated that the lenalidomide-containing triplet therapy (RVD) followed by high-dose chemotherapy plus SCT was associated with significantly longer PFS than the RVD therapy alone, even though OS did not differ significantly between the two approaches ( Attal et al., 2017 ). A more recent study also confirmed the efficacy of the RVD regimen in the pre-transplant induction therapy so that it has to be considered as a standard of care in this clinical setting ( Rosiñol et al., 2019 ). Recently, a phase 3 trial has evaluated this regimen with respect to lenalidomide-dexamethasone in patients with previously untreated MM, who were not planned for immediate SCT, demonstrating a significant improvement in terms of PFS (43 months vs 30 months, HR = 0.712, P = .0018) and OS (75 months vs 64 months, HR = 0,709, P = .025) with an acceptable risk-benefit profile ( Durie et al., 2017 ). Therefore, although, as above mentioned, high-dose chemotherapy plus autologous SCT is the standard treatment for NDMM in adults up to 65 years of age, the use of combination therapy with lenalidomide, bortezomib and dexamethasone has raised questions about the role and timing of transplantation ( Attal et al., 2017 ; Okazuka & Ishida, 2018 ).

As in the case of RRMM treatment, bortezomib is always the backbone for therapeutic regimens for NDMM in combination with other novel targeted agents (Seval and Beksac, 2018). Accordingly, two therapeutic regimens, including the monoclonal antibody daratumumab, have been approved by both FDA and EMA:

1) in 2018, bortezomib, daratumumab, melphalan and prednisone combination for NDMM patients, who are ineligible for ASCT. This approval was based on the results of the open-label, multicentre phase 3 ALCYONE (MMY3007) study, in which patients ineligible for high-dose chemotherapy followed by ASCT (age > 65 years or comorbidities), were randomized to receive either daratumumab-bortezomib-melphalan-prednisone (D-VMP) or bortezomib-melphalan-prednisone (VMP, control group). The 3-year OS was significantly higher in the D-VMP group than in the VMP group (78% vs 67.9%); the PFS was also improved in the D-VMP arm (HR = 0.42 for daratumumab group; P < 0·0001) ( Mateos et al., 2020 );

2) in 2019 (FDA) and 2020 (EMA), daratumumab, bortezomib, thalidomide and dexamethasone for NDMM patients, who are eligible for ASCT, based on the results of the open-label, phase 3 CASSIOPEIA trial. In this study, patients were randomly assigned to receive four pre-transplantation induction and two post-transplantation consolidation cycles of bortezomib, thalidomide and dexamethasone alone or the same regimen plus daratumumab. At day 100 after transplantation, 39% patients in the daratumumab group achieved a complete response or better versus 26% in the control group ( p < .0001), with acceptable safety ( Moreau et al., 2019 ).

Initially, intravenous injection was the standard administration route for bortezomib. Thereafter, a large randomized phase 3 clinical trial, compared the efficacy and safety of subcutaneous versus intravenous treatment, at the approved 1.3 mg/ml dose and twice per week schedule in patients with RRMM, showing that subcutaneous administration induced similar effects in terms of overall response rate, but with improved tolerability and reduction of the incidence of peripheral neuropathy. Thus, currently subcutaneous injection is the preferred method of bortezomib administration, since this route is also more convenient for patients ( Arnulf et al., 2012 ; Moreau et al., 2011 ).

Common adverse effects associated with bortezomib administration are fatigue, gastrointestinal toxicity, trombocytopenia, anorexia, and peripheral neuropathy ( i.e., hyperesthesia, hypoesthesia, neuropathic pain, weakness), which is one of the most important complications that negatively affects the patient's quality life and daily activity (Seval and Beksac, 2018). Peripheral neuropathy has been regarded as an off-target effect, since it is due to inhibition of HtrA2/Omi, a serine protease involved in neuronal survival with potency near or equivalent to that for the proteasome ( Arastu-Kapur et al., 2011 ; Park et al., 2018 ).

Moreover, other adverse events described so far are: ( i ) tumour lysis syndrome ( Sanagawa et al., 2020 ); ( ii ) cardiovascular toxicities ( Enrico et al., 2007 ; Grandin, Ky, Cornell, Carver, & Lenihan, 2015 ), ( iii ) acute interstitial nephritis and rarely ( iv ) a severe syndrome of inappropriate anti-diuresis (SIAD) ( Cheungpasitporn et al., 2015 ; Peng, Chen, & Lou, 2017 ). Furthermore, treatment with bortezomib is associated with an increased risk of Varicella Zoster Virus (VZV) infection, and a continuous prophylaxis with antiviral agents, such as acyclovir and valacyclovir, is recommended ( Aoki, Nishiyama, Imahashi, & Kitamura, 2011 ; Chanan-Khan et al., 2008 ; Robak & Robak, 2019 ; Teh, Harrison, Worth, & Slavin, 2016 ).

The pharmacokinetics of bortezomib is poorly documented mostly due to analytical difficulties (Leveque et al., 2007). Two different studies on patients with prostate cancer and MM suggested that its kinetic profile is characterized by a large distribution volume (Vd), 721-1270 L, high systemic clearance, ranging from 1095 mL/min to 1866 mL/min, and terminal half-life ranging between 10 h and 31 h (calculated over a 24 h period) ( Papandreou et al., 2004 ; Levêque, Carvalho, & Maloisel, 2007 ).

For what concerns bortezomib clearance, the drug is converted into inactive de‑boronated metabolites by different cytocrome P450 enzymes (CYPs) ( e.g., 1A2, 2C9, 2C19, 2D6, and 3A4) ( Pekol et al., 2005 ; Uttamsingh, Lu, Miwa, & Gan, 2005 ), as also confirmed by studies in which the co-administration of ketoconazole, a CYP3A4 inhibitor, and rifampicin, a CYP3A4 inducer, increased and decreased patients' exposure to bortezomib, respectively ( Hellmann et al., 2011 ; Venkatakrishnan et al., 2009 ). Since bortezomib undergoes oxidative metabolism in the liver, a study was carried out on whether patients with a reduced hepatic functionality require dose adjustment ( LoRusso et al., 2012 ; Robak & Robak, 2019 ). In a first phase 1 clinical trial, the pharmacokinetics and safety of bortezomib in patients with varying degrees of hepatic impairment were evaluated, revealing that patients with mild hepatic impairment did not require a starting dose adjustment, whereas patients with moderate or severe hepatic impairment required a reduced dose of 0.7 mg/m 2 , and constantly monitoring during treatment ( LoRusso et al., 2012 ). These data were also confirmed by the VISTA trial, and specific dosing recommendations for patients with hepatic impairment are inserted into the drug label.

Bortezomib efficacy has been also investigated in other haematological malignancies, and several trials are currently ongoing, including Light Chain Amyloidosis (ALA), Waldenstrom Macroglobulinemia, Acute Lymphoblastic and Myeloid Leukemia, Indolent B-cell non-Hodgkin Lymphoma, Diffuse Large B-cell Lymphoma, T-cell lymphomas, as also recently reviewed elsewhere ( Robak & Robak, 2019 ). Moreover, bortezomib is used off-label in refractory or relapsed T-cell lymphomas, Waldenström Macroglobulinemia and ALA ( Du, Yang, & Zhang, 2016 ; Robak & Robak, 2019 ). The most promising results have been reported for ALA, in which bortezomib, dexamethasone and cyclophosphamide, or bortezomib, dexamethasone and melphalan represent the most commonly used first-line treatments, although several opened questions demand further investigation ( Venner et al., 2012 ; Mikhael et al., 2012 ; Palladini et al., 2004 ; Kastritis et al., 2019 ; Robak & Robak, 2019 ). In recent years, the introduction of bortezomib has had a great impact in the cure of Mantle cell Lymphoma (MCL), a non-Hodgkin lymphoma with a short remission duration to standard therapies, and a median OS of approximately 6–7 years ( Banks et al., 1992 ; Fisher et al., 1995 ; Teodorovic et al., 1995 ; Weisenburger et al., 2000 ; Vose, 2017 ). Therefore, there is a great need of therapeutic strategies directed against novel molecular targets. The chromosomal translocation t(11;14)-(q13;q32) is the molecular hallmark of MCL, resulting in overexpression of cyclin D1, which is not typically expressed in normal lymphocytes ( Vose, 2017 ), and the constitutive activation of NF-kB, which also plays a key role in MCL growth and survival (by controlling cyclin D1 expression, as reported in a previous section (see Section 3.2.1 ) ( Pham, Tamayo, Yoshimura, Lo, & Ford, 2003 ; Rosenberg et al., 1991 ). Therefore, proteasome inhibition has been envisaged as an achievable therapeutic strategy, which was confirmed by in vitro studies showing that NF-kB inhibition mediated by bortezomib leads to cell cycle arrest and apoptosis in MCL cells ( Pham et al., 2003 ).

Based on preclinical studies and a phase 1 trial in patients with refractory hematologic malignancies, including, besides MM (see above), MCL and follicular lymphomas ( Orlowski et al., 2002 ), the clinical efficacy of bortezomib as single agent was investigated in patients with relapsed and refractory MCL ( Goy et al., 2005 ; O'Connor et al., 2005 ; Goy et al., 2009 ; O'Connor et al., 2009 ). In 2006, the results of a pivotal open-label, single arm, multicentre phase 2 trials (PINNACLE) led to bortezomib approval by FDA for the treatment of MCL in patients who had received at least one prior treatment. In this trial, 155 patients with progressive MCL, who had undergone at least one prior therapy received, 1.3 mg/m 2 of bortezomib on day 1, 2, 5, and 11 of each 3-weeks cycle. The results showed an OR rate of 31% (median duration, 9.3 months), a complete response rate (CR + unconfirmed CR) of 8% (median duration, 15.4 months), a median time to response of 40 days (range, 31 to 204 days), and a median time to progression of 6.2 months. Adverse events were similar to those reported in other studies with bortezomib, such as peripheral neuropathies and gastrointestinal symptoms ( Fisher et al., 2006 ). An extended follow-up confirmed the positive trend, reporting a median OS of 35.4 months in responding patients ( Goy et al., 2009 ). Combined regimens, including bortezomib, have been evaluated in phase 2 clinical trials with a small number of patients, showing improved efficacy that needs to be confirmed by further investigation: bendamustine-bortezomib-rituximab (of 29 patients evaluable for efficacy, 83% achieved an objective response), and bortezomib plus cyclophosphamide-doxorubicin-vincristine-prednisone (CHOP) (OS = 36.6 for patients treated with bortezomib plus CHOP and 11.6 months for those treated with CHOP alone) ( Weigert et al., 2009 ; Orciuolo, Buda, Pelosini, & Petrini, 2010 ; Agathocleous et al., 2010 ; Furtado, Johnson, Kruger, Turner, & Rule, 2015 ; Kouroukis et al., 2011 ; Lamm et al., 2011 ; Friedrberg et al., 2011).

Bortezomib as single agent was tested also in previously untreated MCL, demonstrating clinical activity ( Belch et al., 2007 ; Robak & Robak, 2019 ). However, more promising results were obtained when bortezomib was combined with rituximab-CHOP (R-CHOP), as demonstrated by: (i) phase 1/2 studies on previously untreated patients with MCL ( Ruan et al., 2011 ; Till et al., 2016 ; Vose, 2017 ); (ii) a phase 2 study on patients with newly diagnosed MCL, who received also bortezomib as maintenance therapy ( Ruan et al., 2011 ; Till et al., 2016 ). These trials demonstrated that the combination of R-CHOP with bortezomib followed by bortezomib maintenance improves PFS, as compared to R-CHOP alone, with acceptable toxicity, suggesting further investigation ( Ruan et al., 2011 ; Till et al., 2016 ). In a large, randomized phase 3 trial, 487 patients with untreated, newly diagnosed MCL, who were not eligible for transplantation, were randomly assigned to two groups, one receiving R-CHOP and a modified R-CHOP regimen with bortezomib in place of vincristine (VR-CAP). After a median follow-up of 40 months, PFS was 14.4 months in the R-CHOP group versus 24.7 months in the VR-CAP group (HR = 0.63; P < .001). Moreover, the final analysis after a median follow-up of 82 months revealed a significantly longer OS in the VR-CAP group than in the R-CHOP group (90.7 months vs 55.7 months; HR =0.66, P = .001), with a manageable toxicity profile. These data led to FDA and EMA approval of bortezomib in combination with rituximab, cyclophosphamide, doxorubicin and prednisone for the treatment of adult patients with previously untreated MCL, who are unsuitable for SCT ( Drach et al., 2018 ; Robak et al., 2015 ; Robak et al., 2018 ; Robak et al., 2019 ). Another strategy, combining bortezomib or lenalidomide with bendamustine and rituximab, has shown efficacy in both first-line and salvage therapy for MCL ( Albertsson-Lindblad et al., 2016 ; Campo & Rule, 2015 ). Accordingly, in a prospective, multicentre phase 2 study evaluating rituximab, bendamustine, bortezomib and dexamethasone as first-line treatment for patients with MCL aged 65 years or older, at median follow-up of 52 months, the 2-year PFS was 70%, clearly demonstrating that this regimen is active and demands further evaluation ( Gressin et al., 2019 ). Despite other investigations have reported promising data, the results of a recent phase 2 trial for newly diagnosed MCL, in which bortezomib was administered as maintenance treatment after induction therapy with three cycles of R-CHOP, two cycles of high-dose cytarabine, BEAM (carmustine, etoposide, cytarabine, melphalan) and ASCT, demonstrated no positive effects of bortezomib as maintenance therapy ( Doorduijn et al., 2020 ).

On the basis of bortezomib success in haematological malignancies, its potential application in the treatment of solid tumours has been explored ( Roeten et al., 2018 ). A large amount of data has been collected in vitro and in vivo aiming at characterizing the possible activity of bortezomib in different models, such as pancreatic and breast cancers, hepatocellular and anaplastic thyroid carcinoma, with contradictory results ( Chen et al., 2016 ; Huang et al., 2019 ; Roeten et al., 2018 ). One of the most promising strategy is the combination of bortezomib with radiotherapy, which results in synergistic effects as a consequence of the bortezomib-induced cell accumulation at the G2/M radiosensitive phase of cell cycle and modulation of radio-resistance mechanisms ( i.e., NF-kB activation, loss of p53 and DNA double-strand break repair) ( Cron et al., 2013 ; Zhu et al., 2015 ; Roeten et al., 2018 ). Additionally, bortezomib might act as chemosensitizer in combination with standard chemotherapy as demonstrated in models of chemo-resistant small cell lung cancer xenografts ( Taromi et al., 2017 ). An impressive number of clinical trials on bortezomib (897) are reported in ClinicalTrial.gov. A number of these trials have been also performed in different solid tumours (see Table 1 ), revealing, as for preclinical studies, conflicting results. Due to their poor prognosis, two types of solid tumours have been mainly investigated, namely: Table 1 Clinical trials with bortezomib in non-haematological cancers. a Table 1 Combined agent NCT identifier Phase Status Setting Carboplatin NCT00028912 1 Terminated Recurrent or progressive ovarian epithelial, primary peritoneal, or fallopian tube cancer Tanespimycin NCT00096005 1 Terminated Advanced solid tumours Docetaxel NCT00064636 1 Terminated Solid tumours / NCT00004002 1 Completed Advanced solid tumours / NCT00091117 1 Completed Advanced malignancies / NCT00021216 1 Completed Pediatric advanced solid tumours / NCT02220049 1 Completed Solid tumours / NCT00054483 1 Completed Advanced cancers Vorinostat NCT01132911 1 Completed Refractory and recurrent solid tumours Dacarbazine NCT00580320 1 Completed Melanoma and sarcoma Temozolomide NCT00544284 1 Completed Solid tumours Celecoxib NCT00290680 1 Completed Solid tumours Chemotherapy NCT00424840 1 Terminated Lung cancer Lapatinib NCT01497626 1 Terminated Solid tumours / NCT02220049 1 Completed Solid tumours Topotecan NCT00388089 1 Completed Solid tumours Carboplatin NCT00059618 1 Completed Ovarian, abdominal, or fallopian tube cancer. 5-Fluorouracil, leucovorin, oxaliplatin NCT00098982 1 Completed Advanced or metastatic colorectal cancer Cetuximab, radiation with or without cisplatin NCT00629226 1 Completed Head and neck cancer 5-Fluorouracil, external-beam radiation therapy NCT00280176 1 Completed Rectal cancer Chemoradiation NCT00329589 1 Completed Brain, head and neck, and cervix cancer Erlotinib NCT00895687 1 Completed Advanced cancer Mitoxantrone NCT00059631 1 Completed Prostate cancer Vorinostat NCT00731952 1 Completed NSCLC Cetuximab with or without cisplatinum NCT01445405 1 Completed Head and neck cancer Placlitaxel NCT00030368 1 Completed Advanced or metastatic solid tumours Trastuzumab NCT00199212 1 Completed Overexpressing Her-2 breast cancer Bevacizumab NCT00428545 1 Completed Advanced malignancies Gemcitabine, carboplatin NCT00052338 1 Completed NSCLC Carboplatin, etoposide NCT00027898 1 Completed Advanced solid tumours 5-Fluorouracil, leucovorin NCT00007878 1 Completed Metastatic solid tumour Radiotherapy NCT00011778 1 Completed Head and neck cancer Topotecan NCT00068484 1 Completed Advanced malignancies Topotecan NCT00541359 1 Completed Advanced solid tumours Pegylated liposomal doxorubicin, gemcitabine NCT00500422 1 Completed Advanced solid tumours Omeprazole NCT00298779 1 Completed Advanced solid tumours or Non-Hodgkin's lymphoma Cetuximab NCT00622674 1 Completed Advanced solid tumours Gemcitabine NCT00620295 1 Completed Advanced solid tumours Paclitaxel NCT00667641 1 Completed Metastatic or unresectable malignant solid tumours Varinostat NCT00227513 1 Completed Metastatic or unresectable solid tumours Paclitaxel, carboplatin NCT00028587 1 Completed Advanced solid tumours Carboplatin NCT01074411 1 Completed Ovarian epithelial, fallopian tube or primary peritoneal cancer Belinostat NCT00348985 1 Completed Advanced solid tumours Vorinostat NCT00994500 1 Completed Advanced solid tumours Doxorubicin NCT00023855 1 Completed Advanced solid tumours Irinotecan NCT00644696 1 Completed Neuroblastoma Interferon α-2b NCT01462773 1 Completed Melanoma Sorafenib NCT01078961 1 Completed Melanoma Temozolomide, bevacizumab NCT01435395 1 Completed Recurrent glioblastoma Clofarabine NCT02211755 1 Recruiting Relapsed solid tumours NK cells NCT00720785 1 Recruiting Advanced cancers Gemcitabine, doxorubicin NCT00479128 1 Active, not recruiting Solid tumours DFMO NCT02139397 1 Active, not yet recruiting Relapsed and refractory neuroblastoma Pembrolizumab, cisplatin NCT04265872 1 Not yet recruiting Breast cancer Panitumumab NCT01504477 1/2 Terminated Advanced colorectal cancer Carboplatin, docetaxel NCT00714246 1/2 Terminated Non small cell lung cancer (NSCLC) Temozolomide NCT00512798 1/2 Terminated Solid tumours or melanoma Vandetanib NCT00923247 1/2 Terminated Medullary Thyroid Carcinoma Docetaxel NCT00064610 1/2 Completed Androgen-indipendent prostate cancer Carboplatin, paclitaxel NCT00093756 1/2 Completed NSCLC Temozolomide NCT03643549 1/2 Recruiting Glioblastoma Cannabidiol, leucovorin, oxaliplatin, bevacizumab, irinotecan, gemcitabine, temozolomide NCT03607643 1/2 Not yet recruiting Glioblastoma, gastrointestinal malignancies, MM Bevacizumab NCT00411593 1/2 Withdrawn NSCLC Docetaxel, cisplatin NCT00313690 1/2 Withdrawn NSCLC / NCT00200382 2 Terminated NSCLC / NCT00346645 2 Terminated NSCLC / NCT00085410 2 Terminated Bile duct or gallbladder carcinoma / NCT00346645 2 Terminated NSCLC / NCT00117351 2 Terminated Bronchioloalveolar carcinoma and adenocarcinoma Panobinostat NCT01056601 2 Terminated Pancreatic cancer progressing after gemcitabine therapy LH-RH agonist drug and androgen receptor antagonists NCT00103376 2 Terminated Relapsed prostate cancer Irinotecan NCT00106262 2 Terminated Progressive, recurrent or metastatic cervical, vulvar, or vaginal cancer Sorafenib NCT01100242 2 Terminated Renal carcinoma Carboplatin NCT00416793 2 Terminated Metastatic pancreatic cancer Doxorubicin NCT00574236 2 Terminated Metastatic breast cancer Fluorouracil, leucovorin NCT00103103 2 Terminated Metastatic or unresectable gastric or gastroesophageal junction adenocarcinoma Erlotinib NCT00283634 2 Terminated RRNSCLC / NCT00025584 2 Completed Metastatic breast cancer / NCT00028639 2 Completed Breast cancer / NCT00068289 2 Completed NSCLC / NCT00023712 2 Completed Recurrent ovarian epithelial or primary peritoneal cancer / NCT00425503 2 Completed Prostate cancer / NCT00017329 2 Completed Metastatic kidney cancer / NCT00051987 2 Completed Relapsed and refractory colorectal cancer / NCT00513877 2 Completed Mesothelioma / NCT00024011 2 Completed Metastatic melanoma / NCT00027716 2 Completed Metastatic sarcoma / NCT00051987 2 Completed Relapsed and refractory colorectal cancer / NCT00077441 2 Completed Liver cancer Docetaxel NCT00183937 2 Completed Hormoine refractory prostate cancer Docetaxel NCT00064012 2 Completed RRNSCLC Docetaxel NCT00193232 2 Completed Advanced hormone, refractory prostate cancer Acyclovir NCT01833143 2 Completed KRAS mutant NSCLC Gemcitabine, cisplatin NCT01633645 2 Completed NSCLC Carboplatin, paclitaxel NCT00107341 2 Completed Unresectable, metastatic esophagus or gastroesophageal junction cancer Gemcitabine, carboplatin NCT00075751 2 Completed NSCLC Pemetrexed NCT00343720 2 Completed NSCLC Doxorubicin NCT03509246 2 Recruiting Ovarian cancer Gemcitabine NCT00305734 2 Completed Recurrent or metastatic nasopharyngeal cancer Docetaxel NCT00425750 2 Completed Head and neck cancer Docetaxel NCT00362882 2 Completed Recurrent NSCLC With or without docetaxel NCT00051974 2 Completed NSCLC Doxorubicin NCT00610792 2 Withdrawn Ovarian cancer Vorinostat NCT00798720 2 Completed NSCLC With or without gemcitabine NCT00052689 2 Completed Metastatic pancreatic cancer Doxorubicin NCT00083226 2 Completed Liver cancer Temozolomide NCT00990652 2 Completed Glioma Cetuximab, docetaxel NCT00118183 2 Completed NSCLC / NCT00104871 2 Completed Metastatic thyroid cancer Irinotecan NCT00061932 2 Completed Gastroesophageal junction or stomach cancer / NCT00118144 2 Completed Lung Cancer Doxorubicin NCT00077428 2 Completed Recurrent, or metastatic adenoid cystic carcinoma Temozolomide, radiotherapy NCT00998010 2 Completed Glioblastoma Tamoxifen NCT00108069 2 Completed Brain cancers / NCT00072150 2 Completed Urothelial transitional cell carcinoma Cisplatin NCT00458913 2 Completed Mesothelioma. Irinotecan NCT00103259 2 Completed Head and neck cancer. Vorinostat NCT00641706 2 Completed Glioblastoma Linsitinib, erlotinib, paclitaxel, dexamethasone NCT02057380 2 Completed Advanced solid tumours Vorinostat NCT00937495 2 Completed Advanced sarcoma Avastin NCT00611325 2 Completed Recurrent glioma Carboplatin, placlitaxel NCT00288041 2 Completed Metastatic melanoma Doxorubicin NCT03509246 2 Recruiting Ovarian cancer / NCT00367718 2 Not yet recruiting Recurrent nasopharyngeal carcinoma / NCT03345303 3 Recruiting Intrahepatic cholangiocarcinoma patients a See https://clinicaltrials.gov/ , accessed April 27, 2020. (/ no drug)

( a ) Small cell lung cancer and non-small lung cancer, in which bortezomib as single agent has revealed limited efficacy, whereas combination therapy with paclitaxel, carboplatin, and concurrent thoracic radiation seemed more encouraging ( Gatti, Zuco, Zaffaroni, & Perego, 2013 ; Zhao, Zhai, Gygi, & Goldberg, 2015 );

( b ) Head and neck squamous cell carcinoma, in which, despite promising preclinical studies, clinical trials revealed poor results ( Lin, Chen, Chen, Cheng, & Chen, 2012 ; Li et al., 2013 ; Gilbert et al., 2013 );

Different mechanisms have been proposed to justify the different bortezomib activity between solid tumours and haematological malignancies, including: (i) alterations and mutations of the proteasome subunit composition; (ii) drug penetration; (iii) activation of compensatory mechanisms, such as autophagy; (iv) resistance to apoptosis induction, even though the exact role of each one of these mechanisms needs to be further investigated. Interestingly, to overcome the poor penetration of bortezomib in solid tumours, an alternative strategy currently investigated consists in a delivery system based on nanoparticles or micelle formulation, as also studied for other PIs ( Ao et al., 2015 ; Coelho, Almeida, Santos-Silva, Pereira, & Coelho, 2016 ; Shen et al., 2014 ).

Second-generation proteasome inhibitors

The strategy of proteasome activity inhibition and the introduction of bortezomib in clinical practice have dramatically changed the battle against MM, even though it was immediately evident that this drug suffers from several drawbacks that needed to be overcome. For example, ( i ) mutation in the β5 subunits, ( ii ) induction of drug efflux from cells and ( iii ) activation of signalling cascades promoting cell survival are all resistance mechanisms that have been identified in bortezomib-resistant cell lines ( Bringhen, De Wit, & Dimopoulos, 2017 ; Oerlemans et al., 2008 ; Sherman & Li, 2020 ). Therefore, second-generation PIs have been designed and tested, including carfilzomib, ixazomib, delanozomib, oprozomib and marizomib, whose properties will be discussed in the next sections. Particular emphasis will be given to carfilzomib and ixazomib, which are both FDA- and EMA-approved.

Carfilzomib

As mentioned in the previous section, two main limitation in the clinical use of bortezomib are (i ) the extent of proteasome inhibition and (ii) proteasome recovery after inhibition ( Deshaies, 2014 ). These issues stimulated an intense research, which culminated with the discovery of the natural compound epoxomicin, a covalent and irreversible inhibitor of the β5 subunit ( Hanada et al., 1992 ; Meng et al., 1999 ; Myung, Kim, Lindsten, Dantuma, & Crews, 2001 ; Deshaies, 2014 ), which was isolated by an unidentified Actinomycetes strain (see also Section 3.3.1.2 ). Importantly, epoxomicin owned unprecedented and exceptional selectivity for proteasome with respect to PI already available ( Kim & Crews, 2013 ; Sin et al., 1999 ). This high selectivity was supposed to guarantee more physiological tolerability, and a more favourable pharmacological profile than bortezomib. Moreover, the ability to irreversibly bind the β5 subunit implies that the only way to recover proteasome activity is the induction of novel synthesis of functional proteasome particles ( Demo et al., 2007 ; Deshaies, 2014 ; O'Connor et al., 2009 ). Thus, through a medicinal chemistry approach, epoxomicin became the scaffold for the synthesis of a more potent tetrapeptide epoxyketone, YU-101 (the parent lead compound of PR-171 or carfilzomib), an injectable drug, which is quickly cleared from plasma ( Demo et al., 2007 ; Myung et al., 2001 ; Kim & Crews, 2013 ; Deshaies, 2014 ). As a matter of fact, for carfilzomib an antitumor activity greater than that of bortezomib was reported in a human tumour xenograft model ( Demo et al., 2007 ).

Thereafter, based on the results of in vitro and in vivo preclinical studies, carfilzomib (Kyprolis) safety profile and clinical efficacy were evaluated in two pioneering studies, in which two different schedules were tested in patients with relapsed or refractory haematological malignancies. In the first phase 1 dose-escalating clinical trial (PX-171–001), carfilzomib doses ranging from 1.2 to 20 mg/m 2 were administered intravenously on five consecutive days following 9 days of rest in 14-day cycles, until occurrence of unacceptable toxicity or disease progression. This study revealed that the drug was well tolerated (a dose of 15 mg/m 2 was established as the maximum tolerated dose [MTD]), and was active in patients with MM or Waldenström macroglobulinemia that were treatment-refractory or relapsed after at least two lines of treatment (Connor et al., 2009b). In the second phase 1 trial (PX-171-002), carfilzomib (1.2–27 mg/m 2 ) was administered twice a week on two consecutive days for 3 weeks in a 4-week cycle; the results showed that this treatment schedule was well-tolerated, being the majority of adverse events manageable and low-grade in severity, and showed activity in particular against RRMM ( Alsina et al., 2012 ). This regimen was then selected for the subsequent clinical studies, including the PX-171-003A1 and ASPIRE trials. In 2012, in consideration of the results of an open-label, single-arm phase 2 study (PX-171-003-A1), which enrolled 266 patients, carfilzomib received accelerated approval by FDA as single agent for the treatment of MM in patients with clinical evidence of disease progression after at least two prior therapies, including bortezomib and an immunomodulatory agent. In this study, bortezomib was administered at the dose of 20 mg/m 2 to a patient population in the majority (89%) resistant to bortezomib and the reported overall response rate was 23.7% with median duration of response of 7.8 months ( Herndon et al., 2013 ). Over the last years, several phase 1 and 2 studies were run both on NDMM and, mainly on RRMM, by using different schedules of carfilzomib as monotherapy, or in combination with other agents, such as low-dose of dexamethasone and lenalidomide (being lenalidomide plus dexamethasone a reference treatment for RRMM). These trials generated a bulk of clinical data which demonstrated the efficacy and tolerability of this PI (see ClinicalTrials.gov ) ( Vij et al., 2012 ; Wang et al., 2013 ; Siegel et al., 2013 ; Moreau et al., 2015 ; Papadopoulos et al., 2015 ; Berenson et al., 2016 ). The results of these phase 2 studies supported the progression into phase 3 trials, which have further led in 2015 to the EMA approval of carfilzomib. In particular, based on the results of the open label ASPIRE phase 3 study ( NCT01080391 ), EMA approved carfilzomib in combination with lenalidomide and dexamethasone, for the treatment of adult patients with MM, who have received at least one prior therapy. Furthermore, in 2015, FDA approved carfilzomib in combination with lenalidomide and dexamethasone for the treatment of patients with MM, who had received one to three prior lines of therapy ( Stewart et al., 2015 ; Stewart et al., 2016 ). In the ASPIRE trial, 792 patients with RRMM were randomly assigned in a 1:1 ratio to the carfilzomib group, in which the drug was part of a triple combination therapy with lenalidomide and dexamethasone, or to the lenalidomide plus dexamethasone control group ( Stewart et al., 2015 ). Carfilzomib was administrated for 18 cycles, at a starting dose of 20 mg/m 2 on the first cycle with subsequent escalation to reach the target dose of 27 mg/m 2 in the following cycles. The addition of carfilzomib significantly increased the median PFS as compared with lenalidomide and dexamethasone alone (26.3 months vs. 17.6 months; HR for progression or death, 0.69; P = .0001). Furthermore, the triplet therapy showed a favourable risk-benefit profile and improved the health-related quality of life of RRMM patients ( Stewart et al., 2015 ; Stewart et al., 2016 ). Moreover, in 2016 FDA and EMA extended carfilzomib approval, in combination with dexamethasone, to patients with RRMM, who had received one to three lines of therapy (FDA) or at least one prior therapy (EMA), on the basis of the results of the phase 3 randomized, open label, ENDEAVOR study ( NCT01568866 ). In this head-to head comparative study of bortezomib and carfilzomib, 929 patients with RMM were randomly assigned to receive carfilzomib plus low-dose dexamethasone or bortezomib plus low-dose dexamethasone ( Dimopoulos et al., 2016 ). In this study, carfilzomib regimen was 27 mg/m 2 in the first cycle and 56 mg/m 2 thereafter, infused over 30 min, which is the maximum tolerated dose of carfilzomib tested in combination with dexamethasone in phase 1/2 clinical trials (Papadopulos et al., 2015; Dimopoulos et al., 2016 ). The primary endpoint of the trial was PFS that was reported to be longer for the carfilzomib group, as compared to the bortezomib one (18.7 versus 9.4 months, HR = 0.53; P < .0001) ( Dimopoulos et al., 2016 ). In another interim analysis aimed at comparing the OS between the two PI, it has been shown that patients treated with carfilzomib had a statistically significant and clinically meaningful improvement in OS than those treated with bortezomib (median 47.6 months versus 40.0 months; HR 0.791; 95%, P = .010) ( Dimopoulos et al., 2017 ; Ludwig et al., 2019 ; Dimopoulos et al., 2019 ; O'Connor et al., 2009). In 2018, FDA extended the use of carfilzomib to include a once-weekly dosing option in combination with dexamethasone (once-weekly Kd70) for patients with RRMM. This approval was based on data from the randomized, open-label, phase 3 A.R.R.O.W. trial, in which 478 patients with RRMM, who had received at least two (but no more than three) prior therapies (including bortezomib and an immunomodulatory drug), were assigned to receive a 30-min infusion of once-weekly (70 mg/m 2 ) carfilzomib vs a 10-min infusion of twice-weekly (27 mg/m 2 ). All patients also received the same dose of dexamethasone ( Moreau et al., 2018 ). The primary endpoint of the trial, PFS, was 11.2 months for the once-weekly regimen versus 7.6 months for the twice-weekly one (HR = 0.69; P = .0014). The ORR in patients treated with the once-weekly regimen was 62.9% versus 40.8% for those treated with twice-weekly ( p < .0001) ( Moreau et al., 2018 ; Moreau et al., 2019 ; Moreau et al., 2020 ). Thus, the once-weekly carfilzomib was safe and more effective as compared to the twice weekly schedule.

As mentioned above, early phase 1/2 trials suggested that carfilzomib in combination with other agents, such as melphalan and prednisone ( Moreau et al., 2015 ), lenalidomide and low-dose dexamethasone ( Jakubowiak et al., 2013 ), or thalidomide and low-dose dexamethasone ( Wester et al., 2019 ), could be a therapeutic opportunity also for NDMM patients, although the outcomes and the related adverse events are still not convincing. Carfilzomib seems to have a distinct pattern of adverse effects with respect to bortezomib. In fact, the rate of peripheral neuropathy is lower than for bortezomib, whereas some patients are affected by cardiovascular complication, such as hypertension and heart failure, rendering the ecocardiography assessment advisable before the onset of treatment. Additionally, unlike bortezomib, carfilzomib can lead to renal failure ( Siegel et al., 2013 ; Chari & Hajje, 2014 ; Korde et al., 2015 ; Manasanch & Orlowski, 2017 ). Adverse events in common with bortezomib are fatigue, anemia, nausea and thrombocytopenia ( Siegel et al., 2012 ). Currently, a great number of clinical studies on carfilzomib are listed in ClinicalTrials.gov. Besides MM, carfilzomib is evaluated in clinical trials for solid tumours, including lung, refractory renal, and metastatic prostate cancers ( Table 2 ). However, like bortezomib, its therapeutic potential is limited by the low distribution within the tumour mass, thus requiring very high and toxic doses to elicit a response ( Huang et al., 2014 ; Grigoreva et al., 2015 ; Johnson, 2015 ). Table 2 Clinical trials with carflizomb and ixazomib in non-haematological cancers. a Table 2 Drug Combined agent NCT identifier Phase Status Setting Carfilzomib Dexamethasone NCT02257476 1 Completed Advanced solid tumours Cyclophosphamide, etoposide NCT02512926 1 Recruiting Pediatric relapèsed and refractory solid tumours INCB052793, gemcitabine, nab-placlitaxel, dexamethasone, bortezomib, lenalidomide, azacitidine, pomalidomide, INCB050465, INCB039110 NCT02265510 1/2 Terminated Advanced solid tumours Carboplatin, etoposide NCT01987232 1/2 Completed Small cell lung cancer Dexamethasone NCT00531284 1/2 Completed Solid tumours, lymphoma, RRMM / NCT01775930 2 Completed Refractory renal cell carcinoma / NCT00884312 2 Completed Solid tumours and MM Dexamethasone, acyclovir NCT02047253 2 Completed Prostate cancer / NCT02318784 2 Active non recruiting Neuroendocrine cancers Ixazomib Fulvestrant NCT02384746 1 Terminated Breast cancer / NCT00830869 1 Completed Advanced non haematological malignancies / NCT01830816 1 Completed Advanced solid tumours and RRMM / NCT01912222 1 Completed Solid tumours and haematological malignancies / NCT02630030 1 Completed Glioblastoma Ketoconazole, rifampin, clarithromycin NCT01454076 1 Completed Advanced non haematological malignancies lymphoma Vorinostat NCT02042989 1 Active, non yet recruiting Advanced cancers Erlotinib NCT02942095 1 Active, non yet recruiting Solid tumours Selinexor NCT03880123 1 Recruiting Advanced sarcoma Nelfinavir NCT03422874 1 Withdrawn Advanced solid tumours and lymphoma Pegylated (IFN)α2b NCT02447887 1/2 Terminated Renal cell carcinoma Gemcitabine, doxorubicin NCT02420847 1/2 Active, non yet recruiting Urothelial cancer Carboplatin NCT02993094 1/2 Recruiting Triple negative breast cancer Doxorubicin, gemcitabine NCT03587662 2 Recruiting Advanced kidney cancer a See https://clinicaltrials.gov/ , accessed April 27, 2020. (/ no drug).

For what concerns pharmacokinetics, preclinical and clinical studies have shown that carfilzomib has an extremely short half-life of about 12–40 min ( Demo et al., 2007 ; Park et al., 2018 ; Yang et al., 2011 ). Furthermore, it displays a rapid systemic clearance (116–123 lLh) and a large distribution volume at steady state (9–28 L) ( Alsina et al., 2012 ; Wang et al., 2013 ; Papadopoulos et al., 2013 ). At all doses tested, carfilzomib clearance exceeded hepatic blood flow, envisaging a contribution of extra-hepatic mechanisms, via peptidase cleavage and epoxide hydrolysis, to its overall elimination. Cytochrome P450-mediated metabolism plays a minor role, suggesting that carfilzomib pharmacokinetic profile is poorly altered by administration of CYP inducers or inhibitors. Furthermore, no meaningful differences in carfilzomib pharmacokinetics are detected between patients with normal renal function and those with renal impairment ( Badros et al., 2013 ; Wang, Martin, et al., 2013 ; Quach et al., 2017 ). This is of particular relevance, taking into account that renal insufficiency is a common and often severe complication occurring in MM patients.

Ixazomib

Both bortezomib and carfilzomib require parental administration, and are associated to the development of specific toxic effects (see 3.3.1.1 , 3.3.2 .1a), mainly peripheral neuropathy and cardiovascular adverse events (carfilzomib) ( Moreau et al., 2016 ; Richardson et al., 2006 ; Dimopoulus et al., 2010; Richardson et al., 2012 ; Richardson et al., 2013 ; Dimopoulos et al., 2017 ; Waxman et al., 2018 ). In fact, although the weekly dosing and subcutaneous administration of bortezomib have attenuated the risk of peripheral neuropathy with bortezomib, this adverse effect is still an important concern. Moreover, bortezomib has a limited tissue distribution, due to its slow dissociation rate from red blood cells. Thus, development of an orally available PIs with improved pharmacokinetics and better tolerability profile was required ( Gupta et al., 2019 ). Accordingly, ixazomib (Ninlaro) represents the first orally administered PI approved for clinical use by FDA and EMA. This agent is a reversible inhibitor that preferentially binds the β5 site of the 20S proteasome. In vitro and in vivo preclinical studies reported for ixazomib a therapeutic efficacy greater than for bortezomib in different cancer models, including MM, as well as improved pharmacokinetics, pharmacodynamics, and antitumour activity in xenograft models. In particular, in MM xenograft models, mice treated with ixazomib presented a significant longer survival time than those treated with bortezomib ( Chauhan, Catley, et al., 2005 ; Kupperman et al., 2010 ; Chauhan et al., 2011 ; Lee, De la Mota-Peynado, & Roelofs, 2011 ). In addition, ixazomib induced apoptosis in MM cells resistant to bortezomib without affecting the viability of normal cells, suggesting a potential efficacy in patients with disease relapse after treatment with bortezomib-containing regimens ( Chauhan et al., 2011 ). Therefore, based on the encouraging results observed in these preclincal studies, together with its more convenient oral administration, ixazomib rapidly advanced in phase 1 clinical trials on RRMM to evaluate its safety and tolerability, as single-agent administered once-weekly ( NCT00963820 ) or twice-weekly ( NCT00932698 ) ( Kumar et al., 2014 ; Richardson, Baz, et al., 2014 ). In the once-weekly dosing study, the maximum tolerated dose was determined to be 2.97 mg/m 2 , whereas in the twice-weekly dosing was 2 mg/m 2 . Overall, ixazomib was generally well tolerated; no severe neuropathy was reported and most of the observed toxicities were manageable. These studies also indicated that ixazomib absorption is rapid, with a maximum plasma concentration at approximatively 1 h post-dose. After multiple dosing, the terminal half-life was 3.3–7.4 days and 3.6–11.3 days in the once-weekly and twice-weekly regimes, respectively. The ORR were 18% and 15% for once weekly and twice-weekly treatments, respectively, supporting the use of both schedules ( Kumar, Bensinger, et al., 2014 ; Richardson, Baz, et al., 2014 ). The efficacy of ixazomib as a single agent (5.5 mg/m 2 weekly for 3–4 weeks) was confirmed in the first part of a phase 2 trial recruiting 33 patients with relapsed MM who were PI naïve or previously exposed to bortezomib but were not refractory to this agent ( NCT01415882 ) ( Kumar et al., 2015 ). Moreover, in a second phase of this trial the efficacy and tolerability of ixazomib were evaluated in combination with dexamethasone in patients showing lack of adequate response or disease progression. The ORR was 34% and the main toxic effects observed were nausea, thrombocytopenia and fatigue that were in line with the ixazomib toxicity profile ( Kumar et al., 2015 ). The efficacy of the ixazomib (weekly doses of 4 or 5.5 mg/m 2 ) and dexamethasone combination was further investigated in another phase 2 trial recruiting patients with MM that had relapsed after at least 1 previous therapy but not refractory to bortezomib. The results of this study revealed that the ixazomib-dexamethasone doublet had promising efficacy and acceptable tolerability ( Richardson et al., 2018 ) (ORR = 31% and 51% with 4.0 mg and 5.5 mg, respectively). However, the combination with the higher dose of ixazomib was more toxic albeit, indicating the potential requirement of dose reductions to attenuate adverse effects ( Kumar et al., 2016 ).

In preclinical studies ixazomib was shown to exert synergistic effects with lenalidomide, and the results of these studies provided the rationale for the clinical testing of the PI with lenalidomide plus desamethasone ( Chauhan et al., 2011 ). The clinical efficacy and manageability of adverse events reported in early trials ( Richardson et al., 2014 ), were confirmed in the phase 3, randomized, double-blind, placebo-controlled TOURMALINE-MM1 trial ( NCT01564537 ), whose results led to FDA (2015) and EMA (2016) approval of the triplet regimen combining ixazomib with lenalidomide and dexamethasone in MM patients, who had received at least one prior therapy ( Moreau et al., 2016 ). In this trial, 722 patients, who had RMM or RRMM, were randomly assigned to receive ixazomib plus lenalidomide–dexamethasone (ixazomib group) or placebo plus lenalidomide–dexamethasone (placebo group). Interim results demonstrated that the addition of ixazomib significantly prolonged PFS compared to the control group (median 20.6 vs. 14.7 months; HR = 0.74; P = .01). Importantly, a benefit in terms of PFS was observed with ixazomib regimen in all patient subgroups, including subjects with high-risk cytogenetic abnormalities (del(17p), t(4;14), and/or t(14;16)), who are known to be burdened by a very severe prognosis ( Avet-Loiseau et al., 2016 ; Moreau et al., 2016 ). An arm of the TOURMALINE-MM1 trial also included patients previously treated with PI therapy and thalidomide/ lenalidomide combination; results demonstrated a substantial clinical benefit in terms of prolonged PFS with the ixazomib-lenalidomide/dexamethasone triplet regardless of prior administered therapy ( Mateos et al., 2017 ). A regional expansion of TOURMALINE-MM1 study in China population supported the clinical benefit of the ixazomib-containing triplet therapy, further reporting a significantly increase in OS (ixazomib-lenalidomide-dexamethasone vs placebo-lenalidomide-dexamethasone: median OS 25.8 vs 15.8 months, after median follow-up of 20.2 and 19.1 months, respectively; HR = 0.419; P = .001) ( Hou et al., 2017 ). Importantly, the combination of ixazomib with the lenalidomide-dexamethasone regimen was associated with a limited additional toxicity, and had no adverse impact on patient-reported quality of life. Commonly reported grade ≥ 3 adverse events with ixazomib include gastrointestinal symptoms, rash, thrombocytopenia, and arrhythmia ( Hari et al., 2018 ; Hou et al., 2017 ; Leleu et al., 2018 ; Moreau et al., 2016 ). For what concerns the peripheral neuropathy, this adverse event never exceeded 3% in all studies ( Bonnet & Moreau, 2017 ; Richardson et al., 2018 ). The low risk of peripheral neuropathy associated with ixazomib use is probably due to the high specificity of ixazomib in inhibiting the chymotrypsin-like (CT-L) site of the proteasome ( Muz et al., 2016 ).

The triplet regimen ixazomib-lenalidomide-dexamethasone followed, when feasible, by single-agent ixazomib as maintenance therapy, was investigated also in patients with NDMM in different trials. In in a phase 1/2 study ( NCT01217957 ), the combination therapy was well tolerated and associated with high ORR (92%) ( Kumar et al., 2014 ). Furthermore, analysis of the long-term efficacy and safety of this regimen, confirmed that ixazomib-lenalidomide-dexamethasone followed by ixazomib maintenance was highly active and caused manageable toxicity in this clinical setting. In particular, out of 65 enrolled patients, 23 patients discontinued induction for SCT, whereas in the remaining 42 patients, the ORR was 80%, including 63% very good partial response and 32% complete responses; these data underscore the feasibility of long-term maintenance treatment with single-agent ixazomib ( Kumar et al., 2019 ). Furthermore, in NDMM patients, a phase 1/2 dose-escalation study investigated the all-oral ixazomib-melphalan-prednisone regimen, followed by single-agent ixazomib maintenance, in transplant ineligible patients with encouraging results ( San-Miguel et al., 2018 ). Recently, the TOURMALINE-MM3 trial ( NCT02181413 ) investigated the ixazomib suitability versus placebo as a maintenance therapy in NDMM to delay disease progression and prolong patients' survival following ASCT. The results of this study revealed that ixazomib induced a 28% reduction in the risk of progression or death compared to placebo (median PFS 26.5 months vs 21.3 months; HR = 0.72; P = .0023), thus representing an additional therapeutic option for these patients (Dimopoulos et al., 2019b). Promising results come also from an ongoing phase 3 trial where ixazomib is administered in patients with RMM as post-ASCT maintenance strategy ( Striha et al., 2018 ).

Several studies are currently investigating the activity of ixazomib in patients with immunoglobulin light chain (AL) amyloidosis, Waldestrom Macroglobulinemia, bone plasmocytome and other non-haematological malignancies ( Smith et al., 2015 ; Smolewski & Rydygier, 2019 ; ClinicalTrials.gov). Until now the best results were obtained in a phase 1/2 study which evaluated the safety, tolerability, and preliminary efficacy of ixazomib in patients with relapsed/refractory AL amyloidosis, paving the road to a phase 3 study which is currently ongoing ( NCT01659658 ) ( Sanchorawala et al., 2017 ; Smolewski & Rydygier, 2019 ).

Using all collected clinical data, ixazomib pharmacokinetics was characterized by an absolute oral bioavailability of 58%, terminal long half-life of 9.5 days, large distribution volume of 543 L, and systemic clearance of approximately 1.86 L/h ( Gupta, Hanley, et al., 2018 ; Park et al., 2018 ; Richardson, Hofmeister, et al., 2018 ; Gupta et al., 2019 ). The faster dissociation rate of ixazomib compared to bortezomib, which allows it to associate and dissociate consecutively with more than one proteasome particle, likely contributes to the improved drug distribution into tissues ( Kupperman et al., 2010 ). Moreover, it has been shown that plasma exposure increases linearly with higher administered dose, and no dose adjustment is required on the basis of race, age, sex, body weight, mild-moderate renal impairment, and mild hepatic impairment ( Gupta, Zhao, Hui, Esseltine, & Venkatakrishnan, 2015 ; Gupta et al., 2016 ; Gupta et al., 2019 ). At clinical doses, ixazomib is mainly metabolized by non-CYP enzymes; in fact, no significant effect on its pharmacokinetics has been reported after the concomitant administration of CYP3A inhibitors, such as ketoconazole and clarithromycin, in patients with advanced solid tumours and lymphoma ( NCT01454076 ) ( Gupta, Hanley, et al., 2018 ; Gupta et al., 2018 ). However, the concomitant administration of the CYP3A-inducer rifampin causes a clinically relevant reduction in ixazomib activity, supporting the advice to avoid this combined treatment schedule, and underlying the complexity of ixazomib metabolism ( Gupta, Singh, Varshney, & Khan, 2018 a; Gupta et al., 2019 ). In all clinical trials so far described, ixazomib was administered on an empty stomach ( Gupta et al., 2019 ). However, since the absorption and metabolism of an oral drug can change with food, according to the US regulatory guidance ( Singh & Malhotra, 2004 ; United States Food and Drug Administration), a phase 1 study in adult patients with advanced solid tumours or lymphoma was carried out to evaluate whether pharmacokinetics of ixazomib might be altered when administered after a high-calorie, high-fat meal ( Gupta, Herzlich, Sauer, & Chan, 2016 ; Gupta et al., 2019 ). The results of this study showed that a high-fat meal reduces the rate and extent of absorption of ixazomib, supporting its administration on empty stomach, at least 1 h before or at least 2 h after food intake. These recommendations are inserted in the ixazomib prescribing information ( Gupta, Herzlich, et al., 2016 ; Gupta et al., 2019 ). Currently, 139 clinical studies on ixazomib are reported in ClinicalTrials.gov, also including the studies on solid tumurs ( Table 2 ).

Investigational PIs: oprozomib, marizomib and delanzomib

To overcome the clinical limitations of FDA/EMA approved PIs, a number of novel compounds have been identified over the last years. However, the only three drugs currently under evaluation in clinical trials are: oprozomib (PER-047 and ONX 0912); marizomib (NPI-0052, salinosporamide A) and delanzomib (CEP-18770) ( Table 3 ). Table 3 Clinical trials with oprozomib, marizomib and delazomib. a Table 3 Drug Combined agent NCT identifier Phase Status Setting Oprozomib / NCT01129349 1 Completed Advanced solid tumours Midazolam NCT02244112 1 Completed Advanced malignancies Dexamethasone, pomalidomide NCT01999335 1 Completed MM Dexamethasone, pomalidomide NCT02939183 1 Active, not yet recruiting RRMM / NCT01416428 1/2 Completed Haematological malignancies Dexamethasone NCT01832727 1/2 Completed RRMM Lenalidomide, dexamethasone, cyclophosphamide NCT01881789 1/2 Completed NDMM Melphalan, prednisone NCT02072863 1/2 Completed NDMM Sorafenib NCT02227914 1/2 Withdrawn Haepatocellular carcinoma Marizomib / NCT00396864 1 Completed Advanced solid tumours, lymphoma Dexamethasone NCT00629473 1 Completed Advanced tumours Vorinostat NCT00629473 1 Completed Pancreatic cancer, melanoma, lymphoma, NSCLC Pomalidomide, dexamethasone NCT02103335 1 Completed RRMM Panobinostat NCT04341311 1 Not yet recruiting Pediatric diffuse intrinsic pontine glioma Temozolomide, radiotherapy NCT02903069 1 Active, not yet recruiting Brain cancer Bevacizumab NCT02330562 1/2 Active, not yet recruiting Glioma, glioblastoma / NCT00461045 2 Completed RRMM / NCT03727841 2 Not yet recruiting Ependymoma Temozolomide, radiotherapy, bevacizumab, lomustine, ABI-009 NCT03463265 2 Recruiting Glioblastoma Temozolomide, radiotherapy NCT03345095 3 Recruiting Glioblastoma Delanzomib / NCT00572637 1 Completed Solid tumours and NHL / NCT01023880 1/2 Terminated RRMM Dexamethasone, lenalidomide NCT01348919 1/2 Terminated RRMM a See https://clinicaltrials.gov/ , accessed April 27, 2020. (no drug)

Oprozomib is an oral drug designed to improve the absorption rate, dosing flexibility, and to overcome two established bortezomib-resistance mechanisms, such as mutations in the proteasome β5 subunit, and drug efflux mediated by ATP-binding cassette transporters ( Verbrugge et al., 2012 ; Zhou et al., 2009 ). Preclinical studies showed that oprozomib has an antitumour activity comparable to that of carfilzomib, stimulating early clinical investigation ( Chauhan et al., 2010 ; Park et al., 2018 ). Some phase 1b/2 trials have evaluated oprozomib efficacy and safety profile as single agent or as a component of combined regimens in patients with RRMM. In particular, its combination with dexamethasone provided encouraging results. However, grade ≥ 3 adverse events occurred in approximately 80% of patients and the most common adverse events of any grade were gastrointestinal disorders (up to 84.8%), underscoring the requirement of novel oprozomib formulations to improve its gastrointestinal tolerability ( Hari et al., 2019 ). Recently, based on preclinical studies that have indicated greater efficacy of oprozomib in combination with dexamethasone and pomalidomide compared to oprozomib monotherapy, a phase 1b trial has evaluated this triplet regimen in 31 patients with RRMM ( Sanchez et al., 2017 ; Shah et al., 2019 ). The results of this study showed encouraging results in terms of efficacy but also confirmed the toxicity profile and the high pharmacokinetic variability of the original bortezomib formulation tested ( Shah et al., 2019 ), stimulating further clinical trials in order to evaluate novel drug formulations with improved gastrointestinal tolerability. Moreover, a phase 1b/2 study where oprozomib was tested as single agent revealed promising results in terms of ORR and tolerability also in patients with Waldenström Macroglobulinemia ( Ghobrial et al., 2019 ). Recently, two multicentre open-label 1b/2 phase studies evaluated three oprozomib-based regimens in transplant-ineligible NDMM patients: oprozomib-dexamethasone plus lenalidomide or cyclophosphamide (oprozomib003) and oprozomib-melphalan-prednisone (oprozomib006) ( Hari, Matous, et al., 2019 ). Although anti-myeloma activity was reported also in NDMM patients, gastrointestinal toxicities and the inter-individual pharmacokinetic variability limit oprozomib efficacy and clinical use, reinforcing the need of novel formulations ( Hari et al., 2019 ). Accordingly, a phase 1b dose-exploration study ( NCT02939183 ) in RRMM is currently ongoing in the attempt to evaluate two new oprozomib formulations ( Hari, Matous, et al., 2019 ; ClinicalTrials.gov ). Despite encouraging clinical results in MM and promising data obtained in preclinical models of solid tumours, in a first dose-escalation study in patients with advanced refractory or recurrent NSCLC and colorectal cancer, oprozomib as single agent showed minimal antitumour activity, with clinically relevant gastrointestinal toxicity ( Infante et al., 2016 ; Zang et al., 2012 ).

The pharmacokinetic profile of oprozomib was first investigated in preclinical models: the drug was rapidly absorbed (2–3 min) in duodenum and jejunum with an estimated absolute oral bioavailability of approximately 39% ( Park et al., 2018 ; Zhou et al., 2009 ). Moreover, phase 1 studies revealed a plasma half-life of about 1 h and a clearance that exceeded the hepatic blood flow, indicating extra-hepatic contribution to its metabolism. Accordingly, the epoxide hydrolase, which seems to be the primary enzyme involved in oprozomib metabolism, is expressed in many other tissues beyond the liver (Fang et al., 2015; Wang, Chemmama, et al., 2017 ). Therefore, though developed to improve carfilzomib pharmacokinetics properties, oprozomib still displays a high systemic clearance and a short half-life ( Wang, Martin, et al., 2013 , Wang, Yang, et al., 2013 ; Fang et al., 2015).

Marizomib is different from the structural point of view with respect to other PIs, and this translates into a different mechanism of proteasome inhibition, efficacy and toxicity profile ( Gozzetti et al., 2017 ). In vitro and in vivo studies demonstrated that marizomib induces apoptosis in MM and other haematological and solid malignancies with a lower toxicity compared to bortezomib ( Ruiz et al., 2006 ; Potts et al., 2011 ). Importantly, marizomib induced apoptosis even in tumour cells from MM patients relapsing after various prior therapies including bortezomib and/or thalidomid ( Potts et al., 2011 ; Chauhan, Catley, et al., 2005 ; Singh et al., 2010 ). Early phase clinical trials testing different treatment schedules of the PI as single agent in patients with advanced malignancies, reported marizomib activity mainly in patients with RRMM. Remarkably, marizomib did not exhibit severe peripheral neuropathy, warranting further evaluation ( Harrison et al., 2016 ; Levin et al., 2016 ). The most important adverse events observed in the phase 1 trials were fatigue, nausea, diarrhoea, and infusion site pain ( Harrison et al., 2016 ). In accordance with preclinical models in which marizomib was found to synergistically act with immunomodulatory agents ( Chauhan, Singh, Aujay, et al., 2010 ; Das et al., 2015 ), a phase 1 clinical trial demonstrated that the triplet combination of marizomib, pomalidomide and low-dose dexamethasone was well tolerated and endowed with promising activity in heavily pre-treated, high-risk RRMM patients, without increasing the incidence of adverse events ( Spencer et al., 2018 ). As mentioned in previous sections, PIs are relatively ineffective in treating solid tumours. Thanks to its more lipophilic structure, an additional differential feature of marizomib compared to other PIs is the ability to cross the blood-brain barrier in different species. Accordingly, preclinical studies demonstrated that oral administration of marizomib inhibits proteasome activity in the brain, and displays a greater activity than bortezomib in a range of solid tumour xenograft models ( Potts et al., 2011 ; Shabaneh et al., 2013 ; Di et al., 2016 ). In fact, marizomib was found to induce apoptosis in glioma cells, with minimal toxic effect on normal neurons ( Di et al., 2016 ; Manton et al., 2016 ). Based on these studies, this PI is currently evaluated in a clinical trial ( NCT03345095 ) for treating newly diagnosed glioblastoma, the most common aggressive malignant primary brain tumour in adults, having a median survival of about 12 months, after debulking surgery and radiotherapy ( Weller, Le Rhun, Preusser, Tonn, & Roth, 2019 ).

The pharmacokinetic profile of intravenously administered marizomib was investigated in a phase 1 clinical trial on patients with advanced solid malignancies, indicating a short half-life ( Chauhan, Singh, Ciccarelli., 2010 ) (lower than 30 min), rapid clearance (0.9–22 L/min), and a large volume of distribution (15–416 L) ( Harrison et al., 2016 ; ). Although the involvement of extra-hepatic clearance in the overall marizomib elimination has been proposed, detailed studies on excretion, metabolism, and in general pharmacokinetic-pharmacodinamic profiles are not available ( Potts et al., 2011 ; Harrison et al., 2016 ; Park et al., 2018 ).

Delanzomib is an oral PI that in vitro has shown significant activity on MM and a panel of solid tumours. Furthermore, both intravenous and oral administration resulted in complete tumour regression in MM xenograft models, and increased mice survival in a systemic model of human MM ( Piva et al., 2008 ). Moreover, administration of delanzomib in combination with other conventional anti-MM therapies, such as melphalan plus bortezomib and dexamethasone plus lenalidomide, was more effective than treatment with either agent alone ( Sanchez et al., 2010 ; Sanchez et al., 2012 ). Nevertheless, the results of early phase trials were not so encouraging ( Gallerani et al., 2013 ; Vogl et al., 2017 ). In a phase 1 trial, delanzomib showed a linear plasma pharmacokinetic profile, lack of peripheral neuropathy, but a very high incidence of severe skin toxicity in (53% of patients) ( Gallerani et al., 2013 ). In a second multicentre phase 1/2 study delanzomib as single-agent was investigated in patients with RRMM, but no efficacy was reported, whereas severe adverse events, such as rash and thrombocytopenia were reported. Thus, development of delanzomib for myeloma was discontinued ( Vogl et al., 2017 ). To date, it is not clear the rationale of different clinical and preclinical investigations ( Park et al., 2018 ).

New concepts for proteasome inhibitors

Beside the inhibitors, described in Section 3.3 , which switch off directly the enzymatic activity of the proteasome by locking down the active site(s) through a chemical bond with specific residues, new class(es) of molecules is/are emerging, which act(s) instead as modulator(s) of the proteasome activity rather than directly inhibiting it. Obviously, they are not alternative to other inhibitors, which often inhibit more efficiently the proteasome enzymatic action, but they rather affect the UPS activity to a variable extent and fashion, interfering at a different level, such as the interaction with 19S RP and/or conformational changes of the 20S. Their investigation is very recent and obviously almost no pharmacological studies have been carried out on these new class(es), but nonetheless it is very important to report on them because they might represent in the near future a relevant implementation of present therapeutic approaches.

Porphyrins

Porphyrins are an old class of antitumour agents which is now again at the center of renewed scientific interests for the possible role as multifunctional (anticancer) drugs. They are organic heterocyclic macrocycles with an extended π system that on one hand makes them highly hydrophobic and, on the other hand, provides porphyrins with a remarkably high extinction coefficient together with additional photo-physical properties. The latter properties make them well-suited to accomplish, for example, both clinical phototherapy (PDT) and cancer imaging, rendering them suitable to be employed in a multi-tasking role as theranostic tools ( Tsolekile, Nelana, & Oluwafemi, 2019 ). Apparently, their poor aqueous solubility might represent a major restriction to their application in the clinical use. Yet, their very versatile chemistry has allowed to get formulations for topical and systemic treatments thanks to the possibility to synthesize an almost unlimited number of water-soluble derivatives. Easy functionalization of the core (in particular in the meso - position) allows to tune their solubility, aggregation tendency and electronic properties by only choosing the nature, number and reciprocal topology of substituents. In addition to the chemistry related to the periphery, the central core also has a manifold role in determining the steric and physicochemical behavior ( e.g., absorption and emission properties) of porphyrins.

It is difficult to overstate the physiological relevance of porphyrins, since their biological role (such as oxygen carrier or storage and redox balance among others) indeed depends on the correct cell localization and on the matching between their structural features and those of the hosting cavity/compartment. For example, anionic porphyrins ( e.g., Uro, Copro, PP-IX, with their carboxylates) tend to localize in the acidic compartment of the lysosome. The role of porphyrins (natural or synthetic) and their relevance in affecting biological processes is underlined by the evidence that uncontrolled endogenous porphyrins exposure, caused by hemoproteins release ( i.e., ectopic porphyrins) and their subsequent binding to essential proteins may impair protein function, inducing an oxidative damage and altering cellular functions.

In addition to the well-known cellular damage, resulting by reactive oxygen species (ROS), photogenerated by porphyrins catalyzed reactions, several experimental observations have recently remarked porphyrin toxicity in the dark, that seems to be correlated to inhibition phenomena involving HSP90, ( Lee, Lee, Lim, Kim, & Kim, 2013 ) telomerase (Masood et al., 2003), (Cory et al., 2002) and proteasome. Other observations ( Szokalska et al., 2009 ) demonstrate that the cytotoxic effects, caused by a porphyrin photosensitizer in PDT, could be potentiated through inhibition of proteasome, encouraging to screen the ability of porphyrins to inhibit the proteasome activity (Chauhan, et al., 2005; Chauhan et al., 2010).

The first evidence that porphyrins, in particular hemin, could reduce “the protein degradation ATP dependent”, not yet known as proteasome, dates back to 40 years ago ( Etlinger & Goldberg, 1980 ). Afterwards, many reports showed that hemin effect on ubiquitin-dependent proteolysis is not restricted to erythroid cells but hemin is a negative UPS modulator in all eukaryotic cells ( Haas & Rose, 1981 ; Vierstra & Sullivan, 1988 ).

The first molecular investigation on purified 20S has demonstrated that micromolar amounts of cationic porphyrins inhibit reversibly all three main protease activities of proteasome ( Santoro et al., 2012 ). Quite interestingly, porphyrins activity is finely modulated (tuned) both by the nature and reciprocal topology of peripheral substituents and by the stereochemistry of the macrocyclic ring center. Thus, the inhibitory efficiency of the cationic macrocycles increases with the number of positive substituents in the meso position. As far as the porphyrin core is concerned, it is evident from the experimental data that among the various metallo-derivatives, the most active ones are those with no axial ligands, the activity decreasing going from penta- to hexacoordinated metals. In particular, the naked cationic porphyrins are the most active ones, indicating that the molecule should be flat in order to interact effectively with the proteasome. Interestingly, thanks to their high extinction coefficient these molecules are “visible inhibitors”, and in this sense they behave as very efficient spectroscopic “probes” for UV–Vis stopped-flow kinetic analysis. The latter studies, combined with NMR and computational study, helped in defining the tetra-cationic H 2 T 4 as a competitive inhibitor which binds the gate area on the α-ring, hindering the substrate access to catalytic chamber ( Santoro et al., 2016 ).

Starting from the first evidence, an accurate kinetic and computational analysis of the surface of the α-subunit ring revealed then that the positive charges play a critical role in the inhibition of the 20S, showing that cationic porphyrins may act as tuneable gatekeepers of the 20S (see Fig. 9 in Santoro, Cunsolo, et al., 2016 ). Indeed, the α-ring represents a receptor-like region physiologically involved in ionic interactions with canonical RPs; Ss a matter of fact, the regular arrangement of aminoacidic residues in these surfaces has been found to represent a sort of electrostatic code, exploited by the 19S, and regulating the gating phenomena (see 2.2.1 , 2.3.1 ). The charges of porphyrins represent the keys able to interfere with this “electrostatic code”, and, depending on their spatial distribution, a high variety of binding modes and inhibition mechanisms have been observed ( Di Dato et al., 2017 ). Furthermore, some functional effects, characterized by cooperative phenomena, are the resulting of conformational rearrangements that can reverberate onto the β5 subunit ( Arba et al., 2018 ). Finally, additional binding modes involve interactions with both the α- and β-rings regions, acting directly on the β5 catalytic subunit.

In conclusion, porphyrins are excellent candidates for multi-tasking biological active molecules. As an example, quite recently it was shown ( Vallelian et al., 2015 ) that high levels of intracellular heme disrupt cellular homeostasis through the combined activities of oxidative damage and proteasome inhibition, thus resulting in the accumulation of damaged proteins that contribute to the triggering of cell death. As far as it concerns their interactions with proteasome, porphyrins can either induce a partial competitive occlusion by hindering the substrate access into the catalytic chamber or else behave as an allosteric modulator, regulating the occurrence of conformational change(s) that affect(s) the dynamic equilibrium between the open and the closed state of the proteasome gates.

Metal complexes

The use of metal complexes as anticancer drugs has been adopted since early 1960s with the discovery and development of cisplatin and its derivatives ( Alderden, Hall, & Hambley, 2006 ). The first metal complexes used as anticancer agents were designed to interact with the cancerous cell DNA, inducing apoptosis of cancer cells, but it has been also demonstrated that they can alter the cellular redox chemistry through binding either the metal or other ligand redox centers of biomolecules involved in cellular redox pathways. Since tumour cells have a more reducing environment than normal cells, due to the accelerated metabolic activity, high rates of cell growth and proliferation, selectivity can be reached by using metal complexes which contain redox active metal ions. These are then reduced in the reducing environment of the cancer cell and metal complex drugs become activated. Therefore, the reduced metal ion (Co 2+ , Pt 2+ , Fe 2+ , Cu + , etc. ) exerts its anticancer activity with a marked selectivity for tumour cells, as the unique ability of metal complexes to undergo redox activation processes involve both metal and ligand redox centers and it can be tuned to specific potentials ( Zhang & Sadler, 2017 ). However, the idea of using metal complexes to disrupt proteasome activity in order to have an anticancer effect is relatively recent ( Shagufta & Ahmad, 2020 ). Disulfiram was the first metal complex containing copper capable of inducing apoptosis in cancer cells by inhibiting proteasome activity ( Chen, Cui, Yang, & Dou, 2006 ). It was later demonstrated that disulfiram rapidly converts in vivo to its reduced metabolite diethyldithiocarbamate (DDTC) before exerting its anticancer activity ( Pang, Chen, Cui, & Ping Dou, 2007 ). Interestingly, in most cases the presence of the metal ion turned out to be fundamental to have proteasome activity inhibition, since the non-metallated ligand has been demonstrated to be ineffective for this purpose. For example, the asymmetric ligands, containing pyridine and 4,6-substituted phenol moieties alone, do not have any influence on proteasome activity ( Shakya, Peng, Liu, Heeg, & Verani, 2006 ); conversely, the copper (II) chloride salt of this compound inhibits the proteasome activity in cell free conditions and, for this reason, it has been proposed that the copper complex works as a carrier to cross the cell membrane. Such an assumption would imply that the effectiveness of the metal complex should strongly depend on the metal-ligand species formed and, once inside the cell, the copper shuttling complex should induce proteasome inhibition by releasing the copper ions, which become available to coordinate with proteasome, involving amino acids capable of forming Cu—N, Cu—S, or Cu—O bonds ( Hindo et al., 2009 ). However, elucidating the molecular mechanisms by which copper ions are able to inhibit proteasome activity is a very challenging task, due to the very complex cellular environment and the difficulty in monitoring the fate of intracellular copper ions ( Satriano et al., 2013 ). Indeed, it has been reported in cell-free conditions that Cu (II) ions promote conformational changes associated to an impaired channel gating, without catalyzing redox reactions nor disrupting the assembly of the 20S proteasome ( Santoro et al., 2016 ). On the contrary, HeLa cells, grown in a Cu (II)-supplemented medium, exhibit a decreased proteasome activity, which was then restored in the presence of an antioxidant. For this reason, it has been proposed that, although the Cu(II)-inhibited 20S activities may be associated to proteasome conformational changes, favouring the closed state of the core particle, other effects may occur, such as ROS-mediated proteasome flooding and disassembly of the 26S proteasome into 20S and 19S.

Beside copper, other metal ions have been used in complex with various ligands to inhibit proteasome activity. For example, cadmium, though carcinogenic for humans, has been tested in complex with several organic ligands, such as indole-3-butyrric acid and indole-3-propionic acid. Strikingly, proteasomal inhibition, as well as accumulation of ubiquitinated proteins and induction of apoptosis, were observed in MDA MB 231 breast cancer cells, whereas non-tumourigenic breast MCF10A cells were much less sensitive to the cadmium complexes, indicating cell-specific apoptotic death ( Zhang et al., 2013 ). Cadmium complexes with heterocycle-L-tryptophan Schiff base ligands such as 2-acetylpyrazine-L-tryptophan,5-methylfurfural-L-tryptophan and 5-bromo-2-thiophenecarbaldehyde-L-tryptophan have also been synthesized and tested for cancer specific proteasome inhibitory and apoptosis -inducing activities. Results show that the inhibition of the proteasomal CT-L activity is strongly depending on the ligand; thus, while the Cd complex with 2-acetylpyrazine-L-tryptophan and 5-methylfurfural-L-tryptophan were powerful inhibitors, the use of 5-bromo-2-thiophenecarbaldehyde-L-tryptophan as ligand produced an inactive complex ( Zhang, Li, Huang, Guan, & Zhu, 2017 ).

Manganese and gold complexes have also been investigated for their inhibitory activity on proteasome in cancer. In particular, the cefepime-Mn complex has been demonstrated to inhibit the proteasomal CT-L activity and to induce the apoptosis of breast cancer cells in a dose-and time-dependent manner ( Zhang, Schulz, et al., 2015 ).

Gold(III) and gold(I) dithiocarbamate complexes have also been reported to be strong proteasomal CT-L activity inhibitors with IC 50 values ~1.1 μM. Interestingly, the different oxidation states of the gold ion seem to affect the mechanism of inhibition, as only the higher gold oxidation state has been reported to produce significant levels of ROS ( Zhang et al., 2010 ). Several additional gold complexes have been reported to have IC 50 in the μM range toward all three main proteasome catalytic activities, such as mononuclear gold, dinuclear(III) complexes and gold(I) phosphine complexes, whereas aurofin, an established gold(I) drug (currently in clinical use for the treatment of rheumatoid arthritis), has been demonstrated to be completely inactive in the modulation of proteasome ( Micale et al., 2014 ). The gold(III) complex AuL12 (dibromo[ethyl-N-(dithiocarboxy-kS,kS’)- N -methylglycinate])has shown attractive properties in terms of anticancer activity and toxicity. AuL12 was found to inhibit proteasome activity in living cells with an efficiency comparable to that of bortezomib ( Tomasello et al., 2017 ). Furthermore, AuL12 also inhibits Lys48-linked poly-ubiquitination in vitro at a concentration of about 7 μM, interfering with Ub activation reactions catalyzed by E1 enzymes. Another approach, based on the use of metal complexes to inhibit the function of the UPS system in cancerous cells, is to target deubiquitinases instead of the proteasome. As an example, nickel, as well as gold(I), pyrithione complexes were tested for their anticancer activity and it was found that both complexes are able to inhibit the UPS by targeting the 19S-associated deubiquitinases without directly affecting proteasome activity ( Li et al., 2019 ; Zhao et al., 2016 ).

Finally, it is worth mentioning that metal ions, such as copper, have a strong inhibitory effect also on other different enzymes ( Grasso et al., 2011 , Grasso et al., 2012 ), which have been found to be associated with the proteasome and to be able to modulate its activity ( Sbardella et al., 2015 ; Sbardella et al., 2018 ). For this reason, the use of metal complexes to modulate proteasome activity should consider all possible mechanisms and actors involved in the UPS system, including the interaction of the metal complexes with regulatory proteins, such as IDE ( Tundo et al., 2017 ).

Proteasome alteration in neurodegeneration

Although proteins are generally found in the right folded and functional state in healthy cells, unfolded configurations are present, mostly occurring upon exposure to environmental stressors; furthermore, they may originate from multi-faceted alterations in translation, folding and intracellular trafficking. Under physiological conditions, the misfolded variants of proteins are either: (i) tagged for degradation via UPS or autophagy pathways, or (ii) correctly refolded back to the native state by chaperones, or else (iii) sequestered into intracellular compartments, such as aggresomes, which preserve them for the following refolding or degradation ( Hartl & Hayer-Hartl, 2009 ; Chen, Retzlaff, et al., 2011 ; Escusa-Toret, Vonk, & Frydman, 2013 ; Hipp, Park, & Hartl, 2014 ; Sontag, Vonk, & Frydman, 2014 ; Sweeney et al., 2017 ). Misfolded proteins can aggregate to form high-molecular weight species of different nature, such as soluble oligomers, prefibrillar species and highly ordered amyloid structure, which often consist of different aggregated-prone and normally folded proteins ( Olzscha et al., 2011 ; Hong, Han, Fink, & Uversky, 2011 ; Brettschneider, Del Tredici, Lee, & Trojanowski, 2015 ; Sweeney et al., 2017 ; Olzscha, 2019 ). Although there is compelling evidence that many proteins (if not all) can form amyloid-like structures under stressful conditions, nonetheless disease-associated amyloidogenic proteins are characterized by intrinsic structurally disordered elements in their free soluble form ( Guijarro, Sunde, Jones, Campbell, & Dobson, 1998 ; Tzotzos & Doig, 2010 ). Thus, a common hallmark of neurodegenerative diseases is the accumulation of misfolded protein aggregates into affected tissues, leading to a derangement of PN, and, ultimately, to progressive death of neurons ( Bredesen, Rao, & Mehlen, 2006 ; Goedert, Clavaguera, & Tolnay, 2010 ; McAlary, Plotkin, & Cashman, 2019 ; Chiti & Dobson, 2017). In general, the major component of insoluble deposits is a specific-disease related protein, such as β-amyloid and tau in Alzheimer's disease (AD), α-synuclein in Parkinson's disease (PD), and huntingtin in Huntington Disease (HD), even though overlapping similarities between syndromes are reported ( Jellinger, 2012 ; McAlary et al., 2019 ).

Neurons, as well as each post-mitotic cell, are very susceptible to proteostasis unbalance mainly due to their long lifespan, morphology and enhanced metabolism ( Tai & Schuman, 2008 ). In particular, UPS is crucial in synaptic protein turnover, calcium flux dynamics, long-terminal plasticity and memory ( Bingol & Schuman, 2004 ; Bingol & Schuman, 2006 ; Colledge et al., 2003 ; Djakovic et al., 2012 ; Djakovic, Schwarz, Barylko, DeMartino, & Patrick, 2009 ; Fonseca, Vabulas, Hartl, Bonhoeffer, & Nägerl, 2006 ; Guo & Wang, 2007 ; Lopez-Salon, Alonso, Vianna, Viola, & Mello e Souza, Izquierdo, Pasquini, & Medina, 2001 ; Tai & Schuman, 2008 ; Wu et al., 2009 ). Furthermore, besides the intracellular proteasome, a membrane-associated proteasome complex, specific for the nervous system, has been recently discovered ( Ramachandran & Margolis, 2017 ). This complex seems to be involved in the modulation of neuronal function by degrading intracellular proteins into peptides that are then released into synaptic cleft, where they stimulate postsynaptic N -methyl- d -aspartate neuronal signalling ( Ramachandran & Margolis, 2017 ). Impaired proteasome activity is reported in idiopathic neurodegenerative diseases, and some hereditary form of neurodegeneration is due to mutations in UPS components, such as PARK1 and PINK (see next paragraphs) ( Ciechanover & Brundin, 2003 ; McKinnon & Tabrizi, 2014 ; Ortega, Díaz-Hernández, & Lucas, 2007 ; Ortega & Lucas, 2014 ; Thibaudeau & Smith, 2019 ). A wide range of studies support the notion that a decrease of proteasome activity with age positively correlates with misfolded protein accumulation. This culminates then, in the presence of other pathological stressors at which aged people are exposed, with the progressive development of neurodegeneration ( Smith, 2018 ; Vigouroux, Briand, & Briand, 2004 ; Mattson & Magnus, 2006 ; Chondrogianni & Gonos, 2010 ; Zabel et al., 2010 ; Tomaru et al., 2012 ). Accordingly, targeted proteasome inhibition in brain of animal models of neurodegeneration reproduces some clinical and neuropathological signatures of human diseases ( McNaught et al., 2002 ; McNaught, Perl, Brownell, & Olanow, 2004 ; Ciechanover & Brundin, 2003 ; Bedford et al., 2008 ; Li et al., 2010 ). Despite this evidence, it is still unclear whether reduced proteasome functionality is a primary event in neurodegeneration onset or the consequence of misfolded protein aggregation ( Ciechanover & Brundin, 2003 ; Dantuma & Bott, 2014 ; Ortega & Lucas, 2014 ; Thibaudeau & Smith, 2019 ). Soluble oligomers, which are believed to be the most toxic and pathologically significant species among the different forms of aggregates, have been shown to impair proteasome activity ( Cecarini et al., 2008 ; Tseng, Green, Chan, Blurton-Jones, & LaFerla, 2008 ; Díaz-Hernández et al., 2006 ; Bence, Sampat, & Kopito, 2001 ; Deriziotis et al., 2011 ; Dantuma & Bott, 2014 ). Recently, a common mechanism has been proposed, according to which oligomers of different diseases-related proteins inhibit the 20S proteasome activity through an allosteric-driven interaction ( Smith, 2018 ; Thibaudeau, Anderson, & Smith, 2018 ; Ciehanover, Hod, & Hershko, 1978 ). Specifically, the soluble oligomers, which adopt a similar three-dimensional conformation, were reported to bind the 20S, stabilizing the gate in the closed configuration. Oligomers-induced proteasome impairment seems then to be counteracted by HbYX peptides, which mimic the gate physiological opening induced by HbYX motifs of 19S ATPase (see also Section 2.3.2 ) ( Thibaudeau et al., 2018 ). Accordingly, α3Δn-HEK293 cells exhibit increased degradation of proteasome substrates, including neurodegenerative disease-related proteins (see also Section 2.3.2 ) ( Choi et al., 2016 ). These results support the scientific hypothesis that drugs, which directly open the 20S gate, might have a relevant therapeutic potential in the management of neurodegenerations ( Smith, 2018 ; Thibaudeau et al., 2018 ; Thibaudeau & Smith, 2019 ; VerPlank, Lokireddy, Zhao, & Goldberg, 2019 ). In this respect, it should be pointed out that the action of porphyrins (see Section 3.4.1 ) looks very promising (Di Di Dato et al., 2017 ).

However, although it was generally accepted that, once formed, amyloid aggregates are resistant to proteasome degradation ( Sweeney et al., 2017 ), it has been recently reported that proteasome holoenzyme seems to possess a “fibril-fragmenting activity”, being able to reduce the size of large tau and α-synuclein fibrils into smaller entities in vitro , thus opening a novel perspective in understanding proteasome role in neurodegeneration ( Cliffe et al., 2019 ; Ye, Klenerman, & Finley, 2020 ). In the rest of this chapter, the contribution of proteasome to the onset and progression of main neurodegenerative diseases is reported. Additionally, we will focus on strategies developed so far to enhance proteasome activity.

Alzheimer's disease

In the late 1901, the German neuropathologist Alois Alzheimer reported about the presence of amyloid plaques and neurofibrillary tangles (NFTs) in the brain of a woman suffering from progressive cognitive decline ( Stelzma, Schnitzlein, & Murllagh, 1995 ). This was the very first paper reporting a case of senile dementia, a neurodegenerative disease that will be later commonly recognized as Alzheimer's Disease (AD). Almost 80 years after this ground-breaking report, protein aggregates present in amyloid senile plaques ( i.e. amyloid peptides) ( Glenner & Wong, 1984 ) and NFTs ( i.e., hyperphosphorylated tau) ( Lee, Balin, Otvos, & Trojanowski, 1991 ) were fully characterized. However, only over the last two decades, attention in the area of protein aggregation has increased considerably, transforming it into a key subject of study in diverse research areas ranging from chemistry to biology and medicine. The most important reason for the rising attention in this field is that most of the disorders are associated with amyloid aggregation ( Chiti & Dobson, 2006 ) and neurodegeneration, which are becoming more and more expensive in terms of health care and social cost worldwide (Alzheimer's Disease International, 2010; Chiti & Dobson, 2017 ; Ciehanover et al., 1978 ). After an initial enthusiasm in targeting amyloid protein aggregation as a possible therapeutic approach to treat AD, other protective mechanisms associated with properties of the cellular environment, such as the existence of molecular chaperones and degradation mechanisms have attracted increasing attention ( Hartl, Bracher, & Hayer-Hartl, 2011 ; Morimoto, 2008 ). Substantial shreds of evidence point to UPS malfunction as an important factor playing a key role in Aβ amyloid growth and AD pathogenesis. This is not surprising if one bears in mind that UPS surveillance is needed for a tightly regulated maintenance of all proteome components involved in memory formation, as well as synaptic plasticity and functioning ( Djakovic et al., 2012 ; Lopez-Salon et al., 2001 ; Tai & Schuman, 2008 ).

However, a deeper understanding of all components of the proteome quality control network is needed to allow us to envisage a successful regulation of all pathogenic pathways. As an example, it is critical to single out all the key components of the UPS, including the upstream processes, involved in AD pathogenesis to allow the design of small molecules with higher efficacy and less severe side effects ( Cao, Zhong, Toro, Zhang, & Cai, 2019 ). In particular, proteasome function (if compared to age-matched controls) declines in AD brains, whereas other proteasome isoforms, such as immunoproteasome, are overexpressed in astrocytes ( Keller, Hanni, & Markesbery, 2000 ; Nijholt et al., 2011 ). As a matter of fact, several studies underscored that immunoproteasome is upregulated in glial cells surrounding Aβ plaques present in affected brains ( Yeo et al., 2019 ). The same work reported that YU102, a specific PI, abolished the production of inflammation cytokines from glial cells and improved cognitive performance in AD mice without any evident effect on Aβ plaques deposition. Hence, the proteasome is now emerging as a major target in the treatment of memory loss and cognitive impairment in AD ( Al Mamun et al., 2020 ; Hegde, Smith, Duke, Pourquoi, & Vaz, 2019 ). A very recent study, involving 48 CEpatients and 50 healthy volunteers, has clearly shown that proteasome levels are significantly decreased in erythrocytes of patients affected by AD. Moreover, the same study revealed that ubiquitin is overexpressed in red blood cells of AD individuals, thus suggesting that ( i ) the UPS is heavily involved in the pathogenesis of the disease and ( ii ) both Ub and proteasome may be investigated as AD biomarkers ( Lv et al., 2020 ). The role of proteasome inactivation in AD development, was addressed in vivo by studies using transgenic mice. In particular, APPswe/PS1de9 AD mice were crossed with mice expressing a green fluorescent protein (GFP) fused to a degradation signal (CL-1) targeted by the proteasome; these studies revealed that GFP protein-linked proteasome substrates build up in the hippocampus and cortex of AD mice at 4 weeks of age, and they were also confirmed by a concomitant accumulation of p53, an endogenous proteasome substrate, and of poly-ubiquitinated proteins. Altogether, these results suggest that the proteasome function is altered in AD mice even at a very young age, well before cognitive impairment and amyloid fibril deposition ( Liu, Fung, Chong, Shukla, & Hilgenfeld, 2014 ).

Although it is known that Aβ is a proteasome substrate, it may also, in turn, inhibit 20S peptidase activity; in this respect, scanning transmission electron microscopy (SEM) experiments have shown that Aβ peptide binds the 20S and inhibits its proteolytic activity ( Gregori, Hainfeld, Simon, & Goldgaber, 1997 ). These findings also reconcile with experiments outlining that neuronal cells incubated with amyloid Aβ peptides do exhibit inhibitory effect on proteasome activity ( Cecarini et al., 2008 ; Lopez Salon, Pasquini, Besio Moreno, Pasquini, & Soto, 2003 ; Tseng et al., 2008 ). Impaired tau metabolism has been also associated with abnormal UPS activity; thus, tau may be ubiquitinated at diverse sites and UPS impairment is involved in tauopathies ( Cripps et al., 2006 ; David et al., 2002 ; Han et al., 2014 ; Keck, Nitsch, Grune, & Ullrich, 2003 ; Lee, Lee, & Rubinsztein, 2013 ; Metcalfe, Huang, & Figueiredo-Pereira, 2012 ; Morris et al., 2015 ; Tai et al., 2012 ; Thomas, Cripps, & Yang, 2009 ). Hyperphosphorylated tau oligomers build up at synaptic and postsynaptic junctions in AD, ( Tai et al., 2012 ) and tau insoluble assemblies inhibit proteasome activity, leading to an accumulation of poly-ubiquitinated proteins ( Myeku et al., 2016 ). It has also been shown that CHIP (C-terminus of Hsp70-interacting proteins) E3 ligase, is overexpressed in AD patients and its downregulation brings about the accumulation of ubiquitinated tau proteins ( Dickey et al., 2007 ; Petrucelli et al., 2004 ; Shimura, Schwartz, Gygi, & Kosik, 2004 ). Recently, it has been demonstrated that toxic Aβ oligomeric assemblies may bind proteasome, impairing its activity by interfering with gating phenomena. Moreover, as mentioned previously, other different proteins ( e.g., α-synuclein and huntingtin), known to self-assemble into similar 3D structures, have the potential to inhibit 20S activity by a similar mechanism, thus pointing to a general oligomer-driven model of proteasome inhibition ( Thibaudeau et al., 2018 ). Notably, clearance of full-length monomeric tau was ATP-independent, whereas on the contrary, fibrillar tau hydrolysis was strictly related to the ATPase activity of the proteasome ( Cliffe et al., 2019 ).

Neurotoxic 30-mer Aβ assemblies (termed amylo-spheroids) are present in AD brains, showing to be the main responsible of neuronal cells death. Although it is still unclear how amylo-spheroids form in the brain and activate neurodegenerative processes, proteasome inhibition was shown to dramatically promote their accumulation in the trans-Golgi network of excitatory neurons, altering dendritic transport ( Komura et al., 2019 ).

Besides Aβ/tau aggregation in toxic oligomers, other causes are known to come into play for proteasome impairment in AD, including abnormal generation of ROS ( de Vrij, Fischer, van Leeuwen, & Hol, 2004 ). Indeed, high intracellular levels of the redox-active metal ions Cu(II) are known to be associated to both Aβ and tau pathologies, even though it is known that only Cu(I) is internalized into the cells by the plasma transporter Ctr1( Maryon, Molloy, Zimnicka, & Kaplan, 2007 ). In this respect, it should be outlined that Cu(II) ions are present at relatively high concentration in the brain and their levels are known to increase with age ( Morita, Kimura, & Itokawa, 1994 ; Tarohda, Yamamoto, & Amamo, 2004 ). As a matter of fact, Cu(II) ions are also found in amyloid plaques present in the AD brain ( Lovell, Robertson, Teesdale, Campbell, & Markesbery, 1998 ; Suh et al., 2000 ) co-purifying with Aβ from AD brain tissues ( Opazo et al., 2002 ). Intriguingly, several Cu(II) complexes exhibit remarkable proteasome inhibitory capacities ( Daniel, Gupta, Harbach, Guida, & Dou, 2004 ; Marzano, Pellei, Tisato, & Santini, 2009 ). All these findings reconcile with recent reports that demonstrate that Cu (II) ions inhibit all proteolytic activities of the 20S ( Grasso et al., 2017 ; Santoro, Monaco, et al., 2016 ; Bellia et al., 2019 ).

Human neuroblastoma cell lines SH-SY5Y, differentiated with retinoic acid, reproduce the neuronal morphology and function and are for this reason widely employed to mimic primary neurons. Low doses of MG-115, a known PI, brought about tau hyperphosphorylation, microtubule destabilization, and an impaired dendritic development in SH-SY5Y cell lines. Moreover, an inhibited proteasome activity increased the levels of signal proteins involved in AD pathogenesis, such as c-Jun N-terminal kinase, c-Jun and extracellular signal-regulated protein kinase (ERK). Notably, in the same study it came out evident that an inhibition of c-Jun and ERK was able to nullify the effects of proteasome inhibition. These results suggest that a reduced proteasome activity may induce an activation of c-jun/ERK signalling and, in turn, a cascade of adverse events leading to neuronal death ( Agholme et al., 2014 ).

Indirect proteasome inactivation may also play a significant role in AD development. For example, ubiquitin hydrolase (Uch-L1) is known to raise cytosolic mono-ubiquitin levels, thus enhancing protein degradation by the proteasome; not surprisingly, low Uch-L1 levels are correlated with amyloid accumulation. Furthermore, a neuron F-box protein Fbx2 may act as an E3 ligase, facilitating the ubiquitination of the β-secretase and its degradation by the proteasome, leading to a decrease of amyloid Aβ generation; notably, both Fbx2 and Uch-L1 increase synaptic plasticity and memory function in AD mice ( Gong, Radulovic, Figueiredo-Pereira, & Cardozo, 2016 ). It was also reported that UBB +1 , a ubiquitin form arising from a pathogenic mutation in the Ub gene through addiction of 19 residues to the C-terminus of the protein, inhibits proteasome activity and is present in neurofibrillary tangles of AD patients ( Fischer et al., 2003 ). Proteasome activation by exogenous agents may represent a promising strategy for AD therapy; in fact, feeding wild-type Caenorhabditis elegans with 18α-glycyrrhetinic acid (18α-GA), a known proteasome activator, resulted in an increased proteasome activitiy and increased lifespan of worms. Moreover, higher proteasome activity was related to lower paralysis rates in C. elegans AD models. Notably, analogous promising results were confirmed also when murine and human neuronal cells were treated with 18α-GA ( Papaevgeniou et al., 2016 ).

A defective proteasome activity has been also related to increased levels of the APP-derived intracellular C-terminal membrane fragment β (CTFβ), a neurotoxic peptide with significant roles in AD pathogenesis ( Bustamante et al., 2018 ).

Parkinson's disease

Parkinson's Disease (PD) is a multi-factorial neurodegenerative disease, which primarily affects the nigro-striatal dopaminergic motor neurons. The pathognomonic histological lesion of PD, though not shared by all disease variants, is the formation of peri-nuclear intracellular aggregates, called Lewy Bodies (LB) (extremely rich in a proto-typical amyloidogenic protein, i.e. α-synuclein, and ubiquitinated proteins) through a process referred to as aggresome formation. It is still debated whether aggresome formation is a pathogenic event in PD progression or it rather represents a protective pathway activated to limit the high toxicity of the soluble amyloidogenic oligomers, which chronologically precedes aggregates formation ( Raiss et al., 2016 ; Wakabayashi et al., 2013 ; Wakabayashi, Tanji, Mori, & Takahashi, 2007 ). In support of this second hypothesis, which is the prevalent one, aggresomes formation is a phenomenon orchestrated by several proteins (including HSP10, p62, HDAC6, GRB1, NUB1, c-Abl among the others), which either selectively bind α-synuclein or modulate its solubility and interacting properties through insertion of post-translational modifications, such as phosphorylation ( further discussed below ) ( Anisimov et al., 2019 ; Mahul-Mellier et al., 2014 ; Takahashi et al., 2018 ; Tanji et al., 2010 ). Finally, under healthy conditions an extensive crosstalk between the UPS and autophagy is expected to tightly balance aggresome growth and to avoid the protein imbalance of neurons ( Yang et al., 2013 ). According to this PD pathogenesis framework, inherited and acquired conditions which predispose to the disease onset would impair this sophisticated mechanism of proteostasis maintenance.

Although very recently it has been reported that Ub-positive inclusions in LB mostly deal with K63 ( i . e., autophagy-related) rather than UPS-specific K48 linkages, extensive studies on the genetically inherited familiar variants and idiopathic forms of the disease indicate that PD is probably the neurodegenerative disorder displaying the highest degree of association with a multi-faceted dysregulation of E3-ligases and of proteasome proteolytic activities early at disease onset ( Bentea, Verbruggen, & Massie, 2017 ; Krebiehl et al., 2010 ; Savolainen, Albert, Airavaara, & Myöhänen, 2017 ; Ugras et al., 2018 ).

Remarkably, although α-synuclein is a natural substrate of 20S proteasome, widely used to monitor its proteolytic activity under different experimental conditions, structural conformational changes of α-synuclein have been reported to transform it in an inhibitor of the 20S proteolytic activity ( Dächsel et al., 2005 ; Zondler et al., 2017 ). On the other hand, since IDE was shown to halt in vitro the oligomerisation of α-synuclein, as well as that of other amyloidogenic substrates ( Sharma et al., 2015 ; Sharma, Chorell, & Wittung-Stafshede, 2015 ; de Tullio et al., 2013 ), an important still unexplored aspect is the role of IDE and, possibly, of IDE::20S complexes; thus, interaction of the enzyme with the 20S might stimulate further information on the functionality of these pathways in fighting the proteotoxic stress in PD.

By analogy with other neurodegenerative disorders, the metabolic dysregulation, which stimulates neuron loss in PD, deals with redox imbalance, proteotoxicity and metalions dys-homeostasis ( Bentea et al., 2017 ; Zondler et al., 2017 ; Kumar et al., 2018 ; Le, 2014 ; Janda, Isidoro, Carresi, & Mollace, 2012 ). Nonetheless, extensive redox imbalance chemically induced by rotenone and paraquat ( i.e, mitochondrial respiratory chain inhibitors), leads to the development of specific forms of parkinsonism.

Interestingly, α-synuclein in LB is often phosphorylated at Ser129, a post-translational modification which reduces the aggregating propensity of the protein; impairment of the phosphorylation at Ser129, brough about by redox nitrosylation of neighbouring Tyr residues together with di-tyrosine cross-linking, especially at the C-terminus of the protein, induces pathological structural alterations of the protein ( Kleinknecht et al., 2016 ).

Furthermore, exceedingly high concentrations of metal ions, such as Zn 2+ , Cu 2+ and iron have been long detected in post-mortem brain biopsies ( Le, 2014 ); in particular, in murine models it has been observed that, besides metal-related toxicity, Zn 2+ ions also trigger the expression and oligomerization-propensity of α-synuclein in nigrostriatal tissues and the selective loss of specific proteasome subunits, such as β5 and Rpt6 ( Kumar et al., 2018 ). Additionally, copper metabolism appears to be implicated in inducing cell death, since over-expression of α-synuclein and copper transporters stimulated loss of proteasome function, regardless of the tendency to form aggregates ( Anandhan et al., 2015 ; Lan, Chen, Chai, & Hu, 2016 ).

A bulk of molecular evidences on PD pathogenesis comes from studies on genetic inheritance of the disease. Mutations of at least six genes have been linked with hereditary PD, namely α-synuclein (SNCA or PARK1), Parkin (PARK2), ubiquitin carboxyhydroxylase L1 (UCH-L1 or PARK5), PTEN-induced putative kinase 1 (PINK-1 or PARK6), DJ-1 (PARK7), and leucine-rich repeat kinase 2 (LRRK2 or PARK8) ( Janda et al., 2012 ; Nuytemans, Theuns, Cruts, & Van Broeckhoven, 2010 ). Interestingly, all proteins, encoded by these genes, were found to interact at some level with proteasome or ubiquitin-conjugating enzymes.

In particular, DJ-1 is an intracellular protein with pleiotropic activities which encompass cell morphology, functionality of mitochondria and ROS balance ( Irrcher et al., 2010 ); most notably, it has been shown to positively regulate the transcriptional activity of Nrf-2 upon inactivation of PTEN and activation of PI3kinase/Akt/mTOR pathway. Nrf2 is a master regulator of anti-oxidant defense systems including transcription of proteasome genes ( Niki et al., 2020 ). A tight link between DJ-1 and Akt/mTOR pathway has been reported also in Drosophila melanogaster , underscoring the degree of conservation across evolution of such a relevant pathway for neuron homeostasis ( Yang et al., 2005 ). Interestingly, detection of hyper-stimulated autophagy in DJ-1 deficient neurons is another indirect proof of the pivotal role of DJ-1 in stimulating the Akt/mTOR signalling, since this kinase is the major autophagy inhibitor. In this framework, it looks relevant to clarify the potentially controversial inhibitory activity of DJ-1 on the 20S catalytic activity both on synthetic and natural substrates, recently described ( Moscovitz et al., 2015 ). However, the authors propose that DJ-1 up-regulates the expression of proteasome subunits whilst repressing the catalytic activity of 20S assemblies, thus finely tuning the 20S proteasome bulk proteolytic activity. This activity would be necessary to address the cell need in balancing the clearance of oxidatively damaged proteins and that of native intrinsically unstructured proteins, which coordinate regulatory and signalling events ( Moscovitz et al., 2015 ).

Another gene, critically involved in PD, is LKKR2 and the clinical features of LRRK2-PD are often indistinguishable from idiopathic PD, with accumulation of α-synuclein and/or tau and/or ubiquitin in intraneuronal aggregates ( Lichtenberg, Mansilla, Zecchini, Fleming, & Rubinsztein, 2011 ). Although the mechanisms through which LKKR2 mediates toxicity are unknown, its mutation brings about a gain of functional mechanisms by means of an increased kinase activity, which was shown to stimulate α-synuclein aggregation and cytotoxicity ( Lin et al., 2009 ). Furthermore, LRRK2 overexpression in cells and in vivo down-regulates UPS activity which turns out into the accumulation of intracellular substrates ( Lichtenberg et al., 2011 ).

Probably, the most studied protein in PD is the ring-finger E2-dependent E3 ubiquitin–protein ligase parkin, which plays a plethora of intracellular functions linked to PD pathogenesis. Mutations in the UBL domain of parkin, as well as those observed in inherited PD cases, render the protein highly unstable, making it possible parkin detection in the cell lines only in the presence of PI. However, it is debated whether parkin genetic mutations impair the ability of carrying out ubiquitin conjugation of substrates or else disruption of the UBL domain enhances auto-catalytic self-ubiquitination of parkin promoting its self-clearance ( Chaugule et al., 2011 ). As a matter of fact, integrity of UBL is crucial for the regulation of parkin function in cells, and it has been proposed that parkin mutations lead to its altered sub-cellular localization and may further increase the aggregating-propensity of parkin ( Santos, Morais, Pereira, Sequeiros, & Alonso, 2019 ).

Parkin binds concurrently substrates and proteasome through interaction with 19S subunits, especially Rpn1, even though additional receptors have been identified ( Chaugule et al., 2011 ; Kabayama et al., 2017 ; Um et al., 2010 ; Um et al., 2006 ). Specifically, parkin was found to further bind Rpn13 through the UBL domain. On its turn, Rpn13 may regulate protein turnover of parkin, since Rpn13 silencing increases parkin bio-availability in cell ( Aguileta et al., 2015 ).

Although the affinity of parkin for 19S subunits is not very high, being likely in the μM range, it has been proposed that this interaction regulates an allosteric mechanism which activates parkin ubiquitin ligase activity ( Aguileta et al., 2015 ; Chaugule et al., 2011 ). Conversely, it looks still controversial whether parkin binds also the 20S, and the divergent findings are probably dependent on the experimental models used (Dachsel et al., 2005).

Biological function of parkin deals with turn-over of a plethora of intracellular substrates.

A direct clue between parkin and α-synuclein was originally provided by identifying the selective ubiquitination of a specialized form of α-synuclein expressed in neurons, α22SYn ( Shimura et al., 2001 ).

Additional substrates have been then identified, supporting a broader relevance of parkin activity in neuron homeostasis. A non-exhaustive list of substrates includes ( i ) CDCrel-1, which is the synaptic vesicle associated protein, ( ii ) p62, which is ubiquitinated at K13 by parkin, and ( iii ) p62 intracellular abundance, which follows an inverse linearity with parkin expression ( Okatsu et al., 2010 ; Song et al., 2016 ). Being p62 involved in aggresome formation, this finding elicits additional considerations on the cross-talk between the contribution of the key players of proteostasis in PD.

Another substrate of parkin is STEP6, which builds up in the striatum of PD subjects and also in murine models of the disease. The increase in STEP6, which follows parkin loss, is associated with a decrease in the phosphorylation of ERK1/2 and its downstream target, pCREB [phospho-CREB (cAMP response element-binding protein)] ( Kurup et al., 2015 ). Interestingly, dopamine signalling affects the de-phosphorylation of STEP6 and its catalytic activity; thus, in dopamine-deficient neurons STEP6 would be more active, depressing the ERK1/2 signalling pathway mentioned above, and grossly impacting on neuron homeostasis. Furthermore, STEP6 and BDNF are regulated through a reciprocal feedback mechanism and this may outline the loss of the neuron growth factor in PD.

Remarkably, one of the most relevant items about parkin biological role is its interaction with PINK, a Ser/Thr protein kinase encoded by the PINK1 gene, a major surveyor of mitochondria quality control, by targeting damaged mitochondria to autophagy-mediated clearance ( i.e., mitophagy) ( Greene et al., 2012 ; Gao et al., 2016 ; Fedorowicz et al., 2014 ; Rosen et al., 2010 ; Sun & Büeler, 2019 ; Gao et al., 2016 ).

In healthy mitochondria, PINK1 is quickly degraded by several mitochondrial peptidase, whereas in de-polarized mitochondria it is no longer cleaved and becomes exposed to the mitochondrial membrane, recruiting parkin for ubiquitination of various mitochondrial proteins thereby labelling the organelles for mitophagy ( Greene et al., 2012 ). Thus, down-regulation in parkin bioavailability turns out into a reduced clearance of damaged mitochondria which become a major source of ROS, dramatically contributing to the redox imbalance.

Furthermore, upon enzymatic shedding, PINK1 is further released as a soluble cytosolic form, called PINK1-s, which assists recruitment of parkin to the mitochondrial membrane and further contributes to the delivery of aggregating-prone proteins to the forming aggresomes during proteasome inhibition. Thus, PINK1-s works as a sensor that links the proteasomal deficiency signal to the aggresome formation process ( Fedorowicz et al., 2014 ; Gao et al., 2016 ).

As a whole, molecular insights on PD strongly support that tailored strategies of UPS modulation would provide a significant therapeutic efficacy in delaying disease progression.

Huntigton's disease

Huntington disease (HD) is a neuro-degenerative disorder with autosomal dominant inheritance caused by a triplet expansion ( i.e., CAG) in the huntingtin gene (Htt) which induces the protein to hold an exceedingly long poly-glutamine (polyQ) stretch at the N-terminus. A polyQ length > 35 residues render pathogenic Htt (hereafter referred to as mHtt), which then acquires aggregating-prone properties (Di Figlia et al., 1997; Boland et al., 2018 ); the removal of the poly-Q stretch appears to be sufficient to rescue neuron homeostasis and to milder neuro-degeneration in murine models of HD ( Zheng et al., 2010 ). From the clinical point of view, the disease is characterized by the anticipation phenomenon, that is an increase in the CAG triplet repetitions, and thus of polyQ length, over generations, bringing about an early onset of the disease through a more severe phenotype which further strengthens the pathogenic relevance of the polyQ stretch length ( Labbadia & Morimoto, 2013 ).

Although several neuronal cells are affected during disease progression, medium-spiny neurons of striatum are those displaying the highest degree of alteration. This feature might be due to the glutamergic cytotoxicity through an exceedingly high N -methyl d -aspartate receptor (NMDAR) activity ( Labbadia & Morimoto, 2013 ). However, differences in synaptic and extra-synaptic activities of these receptors have suggested that dysfunction of neuronal circuits might derive from a primary dysregulation of intracellular pathways, including those regulating proteostasis, through largely unknown mechanisms.

Molecular research on Htt has clarified that the toxicity of the mutated protein is given by the release of N-terminal fragments, which hold the polyQ stretch, upon cleavage of the full-length protein by intracellular proteases, most likely caspase 6 ( Soares, Reis, Pinho, Duchen, & Oliveira, 2019 ; Tebbenkamp et al., 2011 ).

From the early research on HD, multi-faceted interactions between mHtt fragments and intracellular proteolytic pathways have been sketched and therapeutic strategies proposed to improve the clearance of the proteins ( Harding & Tong, 2018 ; Zhang et al., 2019 ). Scherzinger first reported that poly-glutamine expansion had an amyloidogenic-like behaviour ( Scherzinger et al., 1997 ). As a matter of fact, cellular aggregates in cell-based models and, most notably, in post-mortem CNS explants of human subjects display positivity to either ubiquitin, proteasome subunits and several E3 ligases staining, thus envisaging a specific recruitment of proteasome complexes in growing aggregates ( DiFiglia et al., 1997 ; Juenemann, Wiemhoefer, & Reits, 2015 ; Dasgupta et al., 2015 ). However, soluble mHtt is not efficiently targeted by the 26S proteasome and the lack of efficient ubiquitination for proteasomal degradation leads to intracellular aggregation driven by the intrinsic disordered structure of mHtt ( Juenemann et al., 2015 ; Schipper-Krom, Juenemann, & Reits, 2012 ; Juenemann et al., 2013). Conversely, mHtt fragments are extensively targeted by E3 ligases and aggregates formation lead to the progressive sequestration of them with severe consequences for cell metabolism. More recently, the isolated mutated exon-1 of Htt appears to be a ubiquitin-independent substrate of free 20S, even though a direct proteasome inhibition by exon1-Htt, as occurring for other amyloidogenic proteins, is not expected to be a major mechanism of proteo-toxicity in HD ( Juenemann et al., 2018 ). It is instead more likely that Htt perturbs UPS functionality through the modulation of metabolic pathways and through transcriptional and post-translational events mostly involving the Ubiquitin-Conjugating Enzymes which are discussed below. In fact, it is well known that expression of mHtt stimulates a deep re-arrangement of the genes being transcribed, which are, among others, mitochondrial membranes, chromatin remodelling, lipid binding proteins, protein folding and a plethora of E3 ligases ( Tang et al., 2011 ). Furthermore, mHtt expression was reported to affect cell cycle, which obviously is not an issue for neurons, but it may affect glial cells and their mechanisms of preservation of CNS homeostasis ( van Hagen et al., 2017 ).

Abnormal synaptic transmission was reported to induce proteasome impairment in murine models of HD and a molecular mechanism through an increased cAMP signalling and the concomitant decreased of PKA activity was proposed to explain this feature in neurons. Nonetheless, the cAMP/PKA pathway has been long considered central to HD pathogenesis by virtue of the pivotal role played by the two molecular pathways in neuron homeostasis and plasticity. The PKA holoenzyme, which is catalytically inactive in the absence of cAMP, is made up of two PKA-Rs and two catalytic subunits (See section 4.5 and Box 3) ( Lin et al., 2013 ); in the presence of cAMP, cAMP-bound PKA-Rs dissociate from the catalytic subunits, which are then degraded by the proteasome. As a consequence, an impaired proteasome activity would allow the PKA-Rs to gather up reducing the amounts of free PKA catalytic subunits and thereby impairing PKA activity. On the other hand, PKA carries out the phosphorylation of Rpt6 at Ser120, a post-translational modification which enhances the activity of the proteasome and the constitution of capped particles ( Lin et al., 2013 ). Furthermore, during synaptic sprouting Rpt6 can also be phosphorylated by Ca 2+ /calmodulin-dependent protein kinase IIα (CaMKIIα) in a neuronal activity-dependent manner ( Jarome, Kwapis, Ruenzel, & Helmstetter, 2013 ). Therefore, the HD onset might be correlated to a vicious cycle wherefore proteasome content drops down, due to its seizure, and the PKA-mediated stimulation of proteolysis fades out.

A significant contribution to HD pathogenesis further comes from the iPSC model of HD. Remarkably, HD-iPSCs display constitutive increased proteasome activity, which was found to regulate the levels of both normal and mutated Htt, contributing to suppress polyQ-expanded Htt aggregation ( Koyuncu et al., 2018 ; Liu et al., 2017 ). As a matter of fact, HD iPSCs do not accumulate polyQ-expanded Htt aggregates even after multiple passages. Accordingly, a dysfunction in proteasome activity results in impaired Htt levels and aggregation of mHtt also in HD-iPSCs.

In iPSC, a major role in facilitating Htt clearance seems to be played by the E3 ligase UBR5. Although loss of UBR5 did not impair pluripotency markers in human control iPSCs, it induced instead the formation of misfolded protein aggregates ( i.e., aggresomes) ( Koyuncu et al., 2018 ).

The iPSCs experimental model has allowed to cast further light on the transcriptional activity of FOXOs, a subgroup of the Forkhead transcription factors (human cells encode four FOXO proteins, FOXO1, FOXO3a, FOXO4 and FOXO6), which are central regulators of cell metabolism ( Liu et al., 2017 ). Remarkably, HD iPSCs start lacking proteasome activity and expression upon differentiation into neural cells, an occurrence which appears to be dependent on FOXO4, which is downregulated during differentiation. Such a downregulation appeared to be dependent on the enhanced activation of Akt, a serine-threonine kinase which acts as an upstream regulator of FOXOs by mediating their clearance, in HD iPSC-derived neurons ( Liu et al., 2017 ). However, activated and total Akt levels strongly vary among different HD experimental models adopted and they were reported to be reduced in a rat HD model and in post-mortem brain extracts from patients with HD (Colin et al., 2005). Thus, further studies are demanded to address the relevance of this key metabolic pathway in preserving the PN in HD.

As mentioned above, several E3-ligases tag mutant Htt and are sequestered in aggregates. In different cell lines, mHtt clearance is usually carried out by canonical E3 ligases, such as UBR1, UBE3A, HSP and also non-canonical E3 ligases, such as Herp which, however, contain(s) a UBL domain ( Luo et al., 2018 ). Conversely, a role of de-ubiquitinase ataxin-3, involved in the pathogenesis of other neurodegenerative disorders (such as spino-cerebellar ataxia type 3), has not been confirmed in HTT progression by studies in murine models ( Zeng, Tallaksen-Greene, Wang, Albin, & Paulson, 2013 ). Moreover, atypical ubiquitination of mHtt by some E3 ligases, such as WWP1, may favour disease progression ( Lin et al., 2016 ).

Conversely, ubiquilin-1, a highly conserved family of proteins which facilitate protein disposal through autophagy and UPS and which is down-regulated in early HD, improves the clearance of Htt ( Safren et al., 2014 ).

Similarly, Usp14 has a favourable effect in cells expressing mutant Htt cells by decreasing the aggregate load and by enhancing cell viability ( Hyrskyluoto et al., 2014 ).

Finally, although UBE3A overexpression is known to promote UPS-mediated degradation of transfected mHtt in cultured cells, it is still unclear how UBE3A expression levels impact HD pathology. Remarkably, when the E3-ligase was up-regulated a drop in K63 ubiquitination of mHtt was observed ( Bhat, Yan, Wang, Li, & Li, 2014 ). In this study, the presence of the pathological polyQ stretch was proposed to alter the overall folding of mHtt favouring the formation of K63 Ub linkages, also through cooperation of p62/SQSTM1, which are more prone to aggregation ( Lim et al., 2015 ). Thus, to stimulate the UBE3A activity would be relevant to limit the toxicity of mHtt.

Involvement of UPS in retinal diseases

By virtue of its anatomical localization retina is also called the “window to the brain” ( London, Benhar, & Schwartz, 2013 ). Clinical and molecular investigation into this highly specialized nervous tissue has been long considered promising to search for early diagnostic and prognostic bio-markers of CNS disorders, such as AD, through non-invasive approaches. The identification of either quantifiable metabolic by-products, released in main eye fluids ( e.g., vitreous, humour aqueous, tears), or of ultra-structural alterations pathognomonic of a given diseases would fulfil this clinical opportunity. With respect to the second point, non-invasive imaging approaches, such as ( i ) retinal fiber layer imaging with spectral domain optical coherence tomography (OCT), ( ii ) Visual Evoked Potentials (VEPs) and ( iii ) Doppler haemodynamic parameters of retinal veins ( Berisha, Feke, Trempe, McMeel, & Schepens, 2007 ; Dehabadi, Davis, Wong, & Cordeiro, 2014 ; Iseri, Altinaş, Tokay, & Yüksel, 2006 ; Parisi et al., 2001 ; Nag & Wadhwa, 2012 ), have been already used to detect early morphological alterations in AD subjects and the advancement in imaging techniques will certainly allow to improve the sensibility and specificity of the proposed diagnostic tools.

Conversely, the identification of biomarkers in eye fluids is still devoid of solid evidences and a much greater knowledge of the retina metabolism is required to address this task. However, research on this topic is limited by technical and histological issues, namely ( a ) the difficulty in isolating vital retina tissue from human post-mortem explants, and, mostly, ( b ) the technical inability in obtaining homogeneous primary cultures of individual retina cells. In fact, retina is arranged into complex three-dimensional layers in the posterior segment of the eye populated by ( i ) photoreceptor cells ( i.e., rod and cones), which carry out from the physical to chemical transduction of the light stimulus during vision, ( ii ) bi-polar cells, which transduce rod and cones signal to ( iii ) Retinal Ganglion Cells (RGCs) which, finally, convey this information to the visual cortex through the optic nerve their axons generate ( Belenky, Smeraski, Provencio, Sollars, & Pickard, 2003 ; Lobanova et al., 2018 ). In addition, optimization of the light stimulus perception and transduction along with cells nourishment and survey of tissue homeostasis is carried out by a multitude of different cell lineages ( i.e., amacrine cells, horizontal cells, retinal pigment epithelium, muller glia cells), which lie in the retina layers. An extensive ultrastructural organization of the retina and other segments of the eye is provided elsewhere ( Gupta, Hanley, et al., 2016 ; Nag & Wadhwa, 2012 ).

Being terminally differentiated, photoreceptors, bi-polar cells and RGC, like any other post-mitotic cell, are vulnerable to proteostasis unbalance which may originate from either inherited or acquired disorders. A clearcut example are the metabolic complications of glaucoma and diabetic retinopathy (DR), discussed below to a greater detail, and the inheritance of mutated alleles of proteins, either tissue specific ( e.g., rhodopsin, a protein which senses light in photoreceptors) or not ( i.e., optineurin), which pose an amyloidogenic threat in retinal degenerative diseases and genetic variants of glaucoma, respectively ( Lobanova et al., 2018 ; Swarup & Sayyad, 2018 ; Minegishi et al., 2016 ; Shen et al., 2011 ; Yao et al., 2018 ; Fernandez-Godino & Pierce, 2018 ; Piippo et al., 2018 ; Felszeghy et al., 2019 ; Shen et al., 2019 ; Sirohi & Swarup, 2016 ; Ying et al., 2015 ; Shen, Li, Chen, Chern, & Tu, 2015 ; Li et al., 2015 ; Caballero, Liton, Challa, Epstein, & Gonzalez, 2004 ; Caballero, Liton, Epstein, & Gonzalez, 2003 ). Furthermore, the physiological enhanced metabolism of this tissue is sustained through the maintenance of a lipid profile extremely rich in long-chain poly-unsaturated fatty acids and very long chain poly-unsaturated fatty acids for membrane stability, and a high O 2 tension which are both sources of ROS production, proteo-toxicity, lipid peroxidation and membrane damage ( Gorusupudi, Liu, Hageman, & Bernstein, 2016 ) .

Whilst autophagy activation shows controversial issues, being also an apoptosis inducer in photoreceptors after prolonged proteo-toxicity, such as in models of inherited retinal degeneration, UPS activation appears to bring about only metabolic benefits ( Blasiak, Pawlowska, Szczepanska, & Kaarniranta, 2019 ; Yao et al., 2018 ) inasmuch proteasome loss leads to pathogenic events. Accordingly, proteasome pharmacological inhibition in rodents quickly turns out in retina degeneration ( Kageyama, Ota, Sasaoka, Katsuta, & Shinomiya, 2019 ). Nonetheless, a deepening on the molecular insights of UPS into the retina would help addressing several unresolved issues regarding UPS biology. In fact, highly specific mechanisms of regulation of protein turn-over are supposed to have evolved since metabolic activity follows a circadian rhythm with alternance of light and dark hours. Such an alternance is mirrored by a specular overall proteolytic burden and the molecular clock(s), which are finalized to turn the UPS off once light hours are over are unknown. No evidence for recruitment of specific PIPs and/or post-translational modification of UPS members is reported, but their identification might pave the road to novel mechanisms of regulation of proteasome proteolytic activity ( Fan et al., 2013 ; Fukuhara, Dirden, & Tosini, 2001 ; Knowles et al., 2009 ; Naash, Al-Ubaidi, & Anderson, 1997 ).

Moreover, surprisingly, murine transgenic models and human ex-vivo models of eye diseases point toward a major role of non-canonical proteasome assemblies in retina development and homeostasis. A predominant contribution seems to be played by the immunoproteasome, by means of either PA28 expression and incorporation of inducible proteasome subunits in 20S assemblies (See Box 1), which is over-expressed to an exceptionally high degree in synaptic terminals and in photoreceptors ( Hussong, Kapphahn, Phillips, Maldonado, & Ferrington, 2010 ; Hussong et al., 2011 ; Shang & Taylor, 2012 ; Lobanova et al., 2018 ; Aghdam and Mahmoudpour, 2016; Basler et al., 2015). In detail, differentiation of murine retinal progenitor cells into their mature lineages requires the mTORC1-dependent STAT1 activation, which triggers the transcriptional up-regulation of PSMB9 gene (which encodes for a catalytic subunit of immunoproteasome) but not of PSMB6 or PSMB7 (which encode for canonical subunits with trypsin-like and caspase-like proteolytic activities) ( Choi et al., 2018 ). Assembly of functional immunoproteasome is supposed to assist the 26S in clearing the bulk of short half-life proteins that accumulate in highly replicating cells ( Choi et al., 2018 ). Nonetheless, this finding envisages that the immunoproteasome might have a higher affinity than 26S for cell cycle substrates and for oxidized and unfolded proteins which may accumulate during replication. This last possibility is consistent with biochemical properties reported to date for the immunoproteasome and with the finding on a murine model of an inherited retinal degeneration, wherefore photoreceptors carry a rhodopsin allele mutation ( i.e., P23H). This mutation renders the protein unfolded and aggregating-prone, while the overexpression of PA28 counteracts the degeneration improving photoreceptor survival in such a murine model ( Raule et al., 2014 ; Raule, Cerruti, & Cascio, 2014 ; Lobanova et al., 2018 ).

The role of immunoproteasome and its substrate specificities gain further relevance when we consider that retina is an immune-privileged organ so that the processing of antigenic peptides, as well as their presentation, follow highly specific dynamics to regulate local immune response and immune-surveillance of this tissue, mostly concerning the maintenance of immune tolerance versus retinal self-antigens ( Lipski et al., 2017 ; McPherson, Heuss, Pierson, & Gregerson, 2014 ; Schuld et al., 2015 ; Voigt et al., 2017 ).

Although indirectly, the Rett Syndrome (RTT) case (see also Section 2.2.2 ) might further offer a clue for studying the specific regulation of proteasome biogenesis in retinal cells. RTT is a neurodevelopmental disorder classified as rare X-linked genetic disease ( Amir et al., 1999 ). In >95% of cases, girls, affected by the syndrome harbour a de-novo mutation in the MeCP2 gene, which encodes for an epigenetic transcriptional regulator with largely unknown biological functions ( Chahrour et al., 2008 ).

Whilst several brain areas display neuroanatomical abnormalities, retina, as well as vision, appear to be unaffected during RTT onset and progression ( Jain et al., 2010 ; Rose, Wass, Jankowski, Feldman, & Djukic, 2019 ). Interestingly, whilst MeCP2 is ubiquitous in human tissues and maximally abundant in the CNS, it is poorly expressed in the retina and this may outline the absence of morphological and functional alterations of the retina and the visual pathways in this disease. It has been recently unveiled that primary fibroblasts, isolated from skin biopsies of RTT subjects harbouring non-sense MeCP2 mutations, suffer from a defective proteasome biogenesis (see Section 2.2.2 ) due to the MeCP2 dependent down-regulation of PAC1 and PAC2 along with the α7 subunit of 20S ( Sbardella et al., 2020 ). Upon MeCP2 silencing, this defect shows up in human neuron-derived cells envisaging a general contribution of this transcription regulator in proteasome biogenesis. Thus, molecular investigation into retina might help addressing unresolved issues about the contribution of individual proteasome assemblies in PN and in immune responses and further clarify the transcriptional and molecular dynamics of proteasome biogenesis.

Among the neuro-degenerative eye disorders leading to irreversible blindness, which display the highest prevalence in western countries, there are ( a ) diabetic retinopathy (DR), that is a micro-vascular complication of diabetes, and ( b ) glaucoma. The clinical and epidemiological features underscoring onset and progression of these diseases are extensively discussed elsewhere and will not be discussed herein ( Lombardo et al., 2013 ; Parravano et al., 2013 ; Picconi et al., 2018 ; Tarr, Kaul, Chopra, Kohner, & Chibber, 2013 ). However, they represent two eye disorders wherefore UPS and alteration of the PN appear to follow a specular pattern and for which pharmacological strategies targeting the UPS might provide a valid therapeutic opportunity.

Contribution of the UPS in DR onset is poorly studied, even though several lines of research support a pivotal role played by the proteasome in regulating the nuclear activity of key transcription factors and the release of cytokines in the early vascular response to the hyper-glycemic insult ( Aghdam, Gurel, Ghaffarieh, Sorenson, & Sheibani, 2013 ; Campello, Esteve-Rudd, Cuenca, & Martín-Nieto, 2013 ; Rahimi, 2012 ).

Although choroidal endothelial cells seem to contribute only marginally to this phenomenon, an aberrant proliferation of retinal endothelial cells (which ultimately lead to micro-haemorragic lesion, vessel leakage and irreversible fibrosis) follows the increased secretion and bio-availability of VEGFs. Several independent research teams suggest that the retinal cell type, which first senses the hyper-glycemia is the Muller glia, and indeed metabolism of this cell type appears to be sensitive, through unknown mechanisms, to fluctuations in glucose concentration, as those occurring in vivo in diabetic subjects ( Wang, Xu, Elliott, Zhu, & Le, 2010 ; Le, 2017 ; Picconi et al., 2019 ; Picconi et al., 2017 ; Voigt et al., 2017 ; Matteucci et al., 2014 ; Sbardella et al. 2017 ). This insult stimulates the secretion of VEGFs as well as of other pro-inflammatory cytokines. VEGFs synthesis is mostly regulated by the transcriptional activity of NF-kB and HIF-1α whose nuclear translocation is regulated by proteasome proteolytic activity (see Section 3.2.1 ) ( Alkalay et al., 1995 ; Traenckner, Wilk, & Baeuerle, 1994 ; Chen et al., 1995 ; Ferrara, 2004 ).

Therefore, a better understanding of the molecular mechanisms of enhanced protein turn-over upon hyper-glycemia in Muller glia and, possibly, in additional retina cell types would allow to envisage selective therapeutic approaches to target VEGF synthesis rather than its biological cascade once secreted. In fact, clinical regimens for DR treatment mostly deal with VEGF inhibitors which however do not distinguish between the physiological and pathological angiogenesis with the former being as much relevant for retina homeostasis as blocking the latter one would be for DR progression ( Ferrara, 2004 ; Lacal & Graziani, 2018 ). Thus, selective targeting of factors, that regulate the proteasome-mediated turn-over of players in DR progression, would help to overcome the limitation of traditional proteasome inhibition strategies, which stop bulky proteolytic burden thereby compromising PN.

The involvement of proteasome in the onset of glaucoma appears to follow an opposite path to that observed in Muller glia cells in DR. In fact, there is a compelling evidence that PN might be dys-regulated in both main clinical forms of glaucoma, namely ( i ) primary open-angle glaucoma (POAG) and ( ii ) normal-tension glaucoma ( Oddone et al., 2016 ; Quaranta et al., 2016 ; Agarwal, Gupta, Agarwal, Saxena, & Agrawal, 2009 ; Weinreb, Aung, & Medeiros, 2014 ; Wunderlich, Golubnitschaja, Pache, Eberle, & Flammer, 2002 ; Caballero et al., 2003 ).

In this regard, the term glaucoma encompasses an heterogeneous group of neurodegenerative disorders characterized by the loss of retinal ganglion cells (RGC) and atrophy of the optic nerve their axons generate ( Minegishi et al., 2016 ; Oddone et al., 2016 ; Swarup & Sayyad, 2018 ).

Disease etiology displays a multi-factorial profile, wherefore genetic and acquired factors concur in determining its onset and progression. Among the acquired factors, redox imbalance and increased Intraocular Pressure (IOP) are likely the most relevant ones. In POAG, IOP increase is sustained especially through a pathogenic mechanism linked to the altered metabolism of an endothelial-like cell histo-type called Trabecular Meshwork Cell (TMC). TMCs synthesize and secrete the trabecular meshwork (TM), a specialized form of extracellular matrix localized in the anterior segment of the eye (at the sclero-corneal limit), which drains the aqueous humour, a fluid which shapes the eye-globe and nourishes the lining cells (Agarwal et al., 2009; Weinreb et al., 2014 ). Upon metabolic dysregulation, TMs acquire a senescent-like phenotype and display enhanced apoptosis which leads to a pathological remodelling of TM associated to an obstruction for the outflow of aqueous humour (Agarwal et al., 2009; Weinreb et al., 2014 ; Micera et al., 2016 ; Vernazza et al., 2019 ). This brings about the increase in (IOP) thereby exerting a mechanical compression of the retina and optic nerve, localized in the posterior eye, ultimately leading to RGCs loss, optic nerve degeneration and visual decline. Therapy with glucocorticoids (GC) has been long known to induce acute iatrogenic form of glaucoma by affecting the TMC metabolism, but the primary alteration of such a cell after GC administration is unknown ( Roberti et al., 2020 ). Interestingly, TMCs express a ubiquitous protein, called myocilin ( i.e. , from the MYOC gene), whose expression is up-regulated in TMCs when exposed to GC, but also oxidative stress and cytokines ( Qiu, Shen, Shyam, Yue, & Ying, 2014 ; Wang et al., 2019 ; Resch & Fautsch, 2009 ; Micera et al., 2016 ); when over-expressed, this protein is supposed to pose a metabolic threat to TMCs through unexplored gain of function mechanisms ( Jain et al., 2017 ; Kim et al., 2001 ). This pathogenic effect may occur either in the intracellular or extracellular compartments, in accordance with the broad localization of the protein. Furthermore, mutations in MYOC gene are the most studied cause of the juvenile form of glaucoma and, among the different point mutations described so far, the most prevalent ones ( e.g., Pro370Leu) render myocilin amyloidogenic and aggregating-prone ( Wang et al., 2019 ; Yam, Gaplovska-Kysela, Zuber, & Roth, 2007 ). Myocilin is a proteasome substrate, and, in the absence of GC therapy, proteasome activity appears to decline in TMCs culture isolated from patients suffering from glaucoma in an age-dependent manner; further, myocilin expression in HeLA cells was found to decrease the bioavailability of some 20S subunits ( Qiu et al., 2014 ). Conversely, the GC effect on MYOC processing and proteasome regulation is unknown. Nonetheless, the role of proteasome impairment in driving cell senescence and the role of proteasome re-activation in delaying this phenomenon is well studied in several cell types but not in TMCs ( Chondrogianni & Gonos, 2004 ; Deschênes-Simard, Lessard, Gaumont-Leclerc, Bardeesy, & Ferbeyre, 2014 ).

Even though transgenic models do not always support an unequivocal role of MYOC in glaucoma onset, the interactome of this protein is worth being studied to address the metabolism of TMC ( Jain et al., 2017 ; Joe, Nakaya, Abu-Asab, & Tomarev, 2015 ; Kim et al., 2001 ; Senatorov et al., 2006 ; Zhou, Grinchuk, & Tomarev, 2008 ). Therefore, to study the dynamics of myocilin digestion in TMCs and how proteasome might undergo dysregulation under metabolic conditions, that are commonly seen in glaucoma subjects, might help to explore the molecular insights of pharmacological strategies based on UPS rescue.

Differently from POAG, normal-tension glaucoma is not supported by an increased intra-ocular pressure and the degeneration of the optic nerve likely depends on a primary insult on RGCs. Even in this case, a tight involvement of intracellular proteolytic pathways is largely envisaged. Optineurin gene encodes for a protein involved in intracellular vesicle trafficking, and expression of mutated forms of optineurin induces a severe dysregulation of the UPS and of autophagy ( Shen et al., 2015 ; Shen et al., 2011 ; Sirohi & Swarup, 2016 ).

Besides the molecular findings discussed above, a major suggestion for the relevance of proteasome in handling the PN in retina comes indirectly from biochemical pharmacology ( Sbardella, Tundo, et al., 2020 ). Several clinical trials worldwide support the therapeutic efficacy of citicoline for glaucoma treatment ( Carnevale et al., 2019 ; Parisi et al., 2008 ; Parisi et al., 2018 ; Parisi et al., 2019 ; Roberti et al., 2015 ). Citicoline, also known as CDP choline, is a drug made up by choline and cytidine diphosphate which displays optimal bioavailability and easily crosses the blood brain barrier ( Faiq, Wollstein, Schuman, & Chan, 2019 ). Although the mechanisms of action of citicoline have never been identified at molecular detail, its wide usage in clinical regimens is based on its outstanding safety profile and on the efficacy also in neurological disorders, including neurodegenerative disease, such as the early phases of AD and PD onset, though the trials having been run are still limited ( Eberhardt, Birbamer, Gerstenbrand, Rainer, & Traegner, 1990 ).

Our group has very recently reported that citicoline is an allosteric modulator of proteasome in vitro and in vivo , wherefore citicoline binds the 20S with a very high affinity ( i.e., in the low nanomolar range), stimulating the clearance of synthetic substrates as well as of α-synuclein ( Sbardella, Tundo, et al., 2020 ). Surprisingly, in neuron-derived cells, citicoline was found to both stimulate 20S activity and to promote the assembly of proteolytic active capped assemblies ( i.e., 26S and 30S, see Section 2.3.2 ) ( Sbardella, Tundo, et al., 2020 ). As a matter of fact, cells stimulated with citicoline experience a very significant increase in the overall proteolytic burden by the UPS. Therefore, although it is reasonable that the proteasome stimulation is not the only therapeutic effect, the neuro-protective role of citicoline highlights the relevance of proteasome functionality in maintaining the post-mitotic cells homeostasis. Nonetheless, citicoline experience in clinical trials might be looked as a proof of concept that the activation of UPS is a valid strategy to delay the progression of pathologies sustained by proteo-toxicity.

Targeting proteasome as novel tool against proteotoxic diseases

Neurodegenerative diseases are clinically heterogeneous proteinopathies sustained by the accumulation of aggregates of misfolded disease associated proteins (see also Section 4.1 ). The prevalence and incidence of neurodegeneration increases dramatically with age, and, since people life expectancy rises worldwide, also the number of individuals suffering from these pathologies is expected to dramatically increase in the next years ( Jones & Tepe, 2019 ; McAlary et al., 2019 ). Despite social and economic relevance, no effective cure still exists; therefore, the development of novel therapeutic approaches is essential.

As discussed above (see Section 4.1 ), UPS alteration contributes to disease onset and progression, leading to an intense research effort with the purpose of identifying therapeutic strategies targeting UPS. Different approaches to enhance UPS functionality have been proposed, spanning from stimulation of ubiquitination and/or inhibition of de-ubiquitination (See Box 2), or inhibition of protein aggregation, being this last strategy founded on evidences that monomeric proteins are better degraded by proteasome than oligomers ( Dantuma & Bott, 2014 ; Wertz & Murray, 2019 ). In addition, modern strategies envisage the direct proteasome stimulation by either (i) drugs which specifically target proteasome particles increasing their bulk proteolytic activities (see Section 2.2.1 ) or (ii) phosphorylation of proteasome subunits ( Ottobelli et al., 2013 ; Parisi et al., 2015 ; Myeku & Duff, 2018 ).

According to the first point, the identification of “drug-like” molecules, which directly activate proteasome, is challenging. Notably, a chemical genetics screening of over 2750 compounds using a proteasome activity probe as a readout in a high-throughput live-cell fluorescence-activated cell sorting-based assay has led to the identification of more than ten compounds that increase proteasome activity ( Leestemaker et al., 2017 ). A promising, but still poorly explored strategy, is the development of therapeutic peptides and/or peptidomimetics, designed on the basis of specific binding regions of natural proteasome regulators ( Wilk & Chen, 1997 ; Fosgerau & Hoffmann, 2015 ; Jones & Tepe, 2019 ). In general, the advantage of peptide usage seems to deal with the higher specificity and selectivity with respect to molecular target; however, beside the complex synthesis, they suffer from low metabolic stability and poor membrane permeability ( Fosgerau and Hoffmann, 2015 , Lau and Dunn, 2018 ). The most common class of synthetic peptides acting as proteasome activators are based on the HbYX motif (see Section 2.3.2 ) ( Lau & Dunn, 2018 ). In this context, it has been shown that peptides derived from C-termini Rpt2 and Rp5, and a 14-mer peptide, based on the C-terminal fragment of Blm10 (the yeast ortholog of PA200, see Box 1) containing the HbYX motif, efficiently stimulated proteasome activity in vitro ( Sadre-Bazzaz, Whitby, Robinson, Formosa, & Hill, 2010 ; Smith et al., 2007 ; Karpowicz et al., 2015 ). Recently, it has been reported that upon introduction of the HbYX sequence, the proline- and arginine-rich peptide (PR11), which is a 20S allosteric inhibitor, turned to be a proteasome activator in vitro and in cell model ( Giżyńska et al., 2019 ; Osmulski et al., 2020 ) . Another example of a “drug-like” molecule with a different unknown mechanism of action is the proteasome-activating peptide 1 (PAP1), which increases the chymotrypsin-like proteasomal catalytic activity in vitro and in cell models and is further able to halt protein aggregation ( Dal Vechio, Cerqueira, Augusto, Lopes, & Demasi, 2014 ). Among proteasome activators, natural compounds, such as oleuropein, betulinic acid and fatty acids deserve particular attention, and their features will be discussed in the next sections.

Concerning the phosphorylation strategy, a bulk of studies has shown that reversible phosphorylation of proteasome subunits positively regulates its function ( Myeku & Duff, 2018 ; VerPlank et al., 2019 ; VerPlank & Goldberg, 2017 ; VerPlank & Goldberg, 2018 ). Accordingly, protein kinase A (PKA)-dependent phosphorylation is involved in the regulation of multiple aspects of proteasome functionality, such as: (i) enhancement of Rpt6 ATPase activity through phosphorylation which further stimulates the association of 20S and 19S in vitro (See Box 3), (ii) increase in the proteasome capacity to clear out ubiquitinated proteins, peptides and ATP as well as the degradation of aggregation-prone proteins in cells upon phosphorylation of Rpn6 at serine14 (Box 3) ( Asai et al., 2009 ; Jarome et al., 2013 ; Lokireddy, Kukushkin, & Goldberg, 2015 ; Lu et al., 2008 ; Zhang et al., 2007 ). Moreover, phosphorylation of Rpt6 by kinase CaMKIIα induces 26S translocation into dendritic spines in primary neurons, promoting local protein breakdown and driving the formation of synaptic connections ( Bingol et al., 2010 ; Djakovic et al., 2009 ; Hamilton et al., 2012 ; Jarome et al., 2013 ). Therefore, proteasome subunit phosphorylation has been suggested to rescue proteasome function and it could represent a promising strategy to treat neurodegeneration ( Myeku & Duff, 2018 ; VerPlank & Goldberg, 2017 ). Recently, it has been reported that, in hyppocampal neurons, only 20% of proteasome seems to be in an (see Section 2.4 ) “active” substrate-engaged state, whereas the remaining part is in an “inactive” substrate-accepting ground state. Therefore, it has been speculated that phosphorylation increases the percentage of active forms of proteasome, recruiting “idle” particles as well as directly stimulating their activities ( Asano et al., 2015 ; Myeku & Duff, 2018 ). As a matter of the fact, a promising strategy should be the stimulation of PKA activity through the modulation of the amplitude of cAMP signal. The cAMP level is curtailed by cyclic nucleotide phosphor-di-esterases (PDE), which act negatively by regulating PKA signals. Inhibition of PDE stimulates cAMP/PKA axis and activates proteasome, opening to a novel potential use of PDE inhibitors in the CNS diseases treatment ( Myeku & Duff, 2018 ; VerPlank & Goldberg, 2017 ). Accordingly, it has been shown that PDE4 selective inhibition by rolipram induces phosphorylation of several subunits of 26S, leading to an increase in mouse models of UPS-mediated clearance of tau and amyloid aggregate, accompanied by a reduced cognitive impairment ( Myeku et al., 2016 ; Smith, Pozueta, Gong, Arancio, & Shelanski, 2009 ; Vitolo et al., 2002 ). Moreover, cAMP/PKA axis activation, which follows PDE10 inhibition, reduces Htt aggregation through a proteasome-dependent mechanism, and ameliorates motor and cognitive deficit in Htt mouse model ( Beaumont et al., 2016 ; Giampà et al., 2010 ; Harada, Suzuki, & Kimura, 2017 ; Lin et al., 2013 ). Furthermore, the administration of the FDA approved PDE3 inhibitor, cilostazol, to a mouse model of tauopathy enhanced proteasome function and attenuated the tauopathy and cognitive decline in rTg4510 mice, suggesting that this drug could be potentially repurposed for the treatment of patients with early-stage tauopathy ( Schaler & Myeku, 2018 ). As a whole, despite some early encouraging results in mouse model, it seems clear that the broad range of biological functions, mediated by cAMP, can reduce the clinical efficacy of PDE inhibitors, due to their adverse effects ( Heckman, Blokland, Bollen, & Prickaerts, 2018 ; Myeku & Duff, 2018 ). It is worth recalling that cAMP/PKA pathway transduces the intracellular signalling of a number of hormones: thus, in such a way, proteasome function can be regulated by hormonal and metabolic stimuli (see Box 3) (VerPlank and Goldberg, 2015).

Direct enhancing proteasome activity by natural compounds

Many organisms have developed a large number of small molecules, which modulate the activity of UPS components ( Rousseau & Bertolotti, 2016 ). Those of nutraceutical origin (contained in fruits, in vegetables and their extracts) are very attractive for their positive effects as antiaging and in the treatment and the prevention of a wide range of pathologies. In fact, dietary phytochemicals exhibit broad and different biological activities, including antioxidant action, free radical scavenging, anti-inflammatory and metal-chelating properties, that represent the evolutive result of the vegetable system defense. All of them are secondary metabolites that plants produce to counteract against various stresses ( Murakami, 2013 ) thus, they can be considered “multifunctional drug-like molecules” (see Section 4.5 ), and it is not surprising such a high variety of targets, since they are small molecules with very simple chemical structures.

Concerning the activity of these compounds on proteasome, those able to activate/enhance proteasome activity are rare ( Bonfili et al., 2008 ; Dahlmann et al., 1993 ; Huang & Chen, 2009 ) and they are often characterized by an ambivalent behaviour, acting alternatively as inhibitors and/or activators according to diverse conditions. As an example, the effects of curcumin ( Cuanalo-Contreras & Moreno-Gonzalez, 2019 ) (1 E ,6 E )-1,7-bis-(4-idrossi-3-metossifenil)-epta-1,6-dien-3,5-dione) on the UPS reflects the hormesis principle ( i.e., the biphasic dose-response to an environmental agent characterized by a low dose stimulation or beneficial effect and a high dose inhibitory or toxic effect), being characterized by an inverted U shape dose-response ( Ali & Rattan, 2006 ); thus, curcumin treatment (up 1 μM for 24 h) increases proteasome activity in keratinocytes, but it displays an inhibitory effect at 10 μM ( Murakami, 2013 ). In particular, curcumin induces 26S perturbation, leading to an impairment of cell proliferation in various cancer cells and reduction of cancer burden in mice ( Banerjee et al., 2018 ) .

By analogy, quercetin (3,3′,4′,5,7-pentahydroxyflavonethe), the most abundant flavonoid found in fruits and vegetables, which has been initially reported to be a 20S inhibitor ( Chen et al., 2005 ) (IC 50 = 3.5 μM), it has been shown to enhance the proteasome activity in vivo and to reduce the Aβ-induced toxicity in a dose-dependent manner when administered to a Caenorhabditis elegans AD model ( Chondrogianni et al., 2010 ). Likewise, the polyphenol resveratrol, that was previously described as a natural direct PI ( Yang, Landis-Piwowar, Chen, Milacic, & Dou, 2008 ; Qureshi et al., 2012 ), recently has been reported to enhance proteasome activity recovering the impaired proteostasis in a C. elegans AD model, and in AD transgenic mice ( Regitz, Fitzenberger, Mahn, Dußling, & Wenzel, 2016 ); in addition, resveratrol has been shown to enhance cognitive activity by increasing 20S proteasome subunits levels and stimulating proteasome activity ( Corpas, Griñán-Ferré, Rodríguez-Farré, Pallàs, & Sanfeliu, 2019 ).

Hereafter, we focus only on bioactive compounds that directly target the naked catalytic particle, 20S, thus enhancing the ubiquitin-ATP-independent proteolysis, the main pathway degrading the oxidatively damaged and intrinsically disordered proteins ( Ben-Nissan & Sharon, 2014 ).

Oleuropein, the most abundant phenolic compound extracted from Olea europaea (leaf and olives), enhances all three proteasome activities in vitro and promotes cellular resistance to oxidants, prolonging human fibroblasts lifespan ( Katsiki, Chondrogianni, Chinou, Rivett, & Gonos, 2007 ). Systemic administration of oleuropein in pigs increased 20S activity in the subcortical white matter, reducing the damaged proteins accumulation after hypoxia and hypothermia and protecting the myelin.

The triterpene betulinic acid, extracted from the lipid fraction of the algae Phaeodactylum tricornutum and of many other medicinal plants, activates preferentially the CT-L activity ( Huang, Ho, & Chen, 2007 ) with minor effects on T-L and C-L activities. Some neuroprotective effects are reported in the transgenic C. elegans PD model, where betulinic acid decreased α-synuclein accumulation and the 6-hydroxydopamine-induced dopaminergic neuron degeneration ( Mullauer, Kessler, & Medema, 2010 ). Furthermore, it indirectly promoted the enhancement of proteasome activity by regulating rpn1 expression and downregulation of the apoptosis pathway gene, egl-1 ( Tsai et al., 2017 ) . Interestingly, betulinic acid has shown a neuroprotective effect in vascular dementia rat models, re-establishing the cerebral blood flow, restoring behaviour parameters and significantly improving the BDNF levels, with a restrain of the oxidative stress and of inflammatory parameters ( Kaundal, Zameer, Najmi, Parvez, & Akhtar, 2018 ).

Besides the proteasomal effect, oleuropein and betulinic acid could be also considered as pleiotropic small molecules, for their anti-HIV ( Mayaux et al., 1994 ; Yang, Gong, Zhang, & Lu, 2016 ) and anti-tumour activity toward some cancer cell lines ( Pisha et al., 1995 ; Saeed, Mahmoud, Sugimoto, Efferth, & Abdel-Aziz, 2018 ). Although both oleuropein and betulinic acid have been previously reported to be natural proteasome activators ( Katsiki et al., 2007 ), some authors recently clarified that the stimulatory activity is restricted to fluorogenic substrates, and no effect has been reported on the turnover of mis-folded proteins in vitro or in living cells.

Among naturally occurring activators of 20S proteasome there are some physiological cellular components, such as mucopolisaccarides ( e.g., heparin), glycolipids ( e.g., ceramides, lysophosphatidyl-inositol and cardiolipin) ( Matsumura & Aketa, 1991 ; Ruiz de Mena, Mahillo, Arribas, & Castaño, 1993 ), and some proteins (not included in the physiological UPS control), such as the arginine-rich histone H3, a chromatin binding protein able to selectively enhance protein degradation by the proteasome ( Orlowski, 2001 ).

The detailed mechanism by which these compounds regulate 20S degradation is still largely undetermined, but it might be related to the gate opening, mimicking the RP interaction (see 2.2.1 , 2.3.1 ).

Fatty acids

Fatty acids are likely the first small molecules that were first described as 20S proteasome modulators. Indeed, our basic knowledge of proteasome function is rooted on the pioneering studies conducted throughout the 80s and 90s ( Orlowski & Wilk, 1981 ; Ishiura et al., 1986; Folco, Busconi, Martone, & Sanchez, 1988 ), using fatty acids and SDS as activators. The first systematic study on proteasome peptidase activity reported the effects of several saturated and unsaturated fatty acids examining various carbon-chain lengths and underlining that the optimal 20S activation potency may be achieved by using fatty acids with a C18-C20 chain carbon; thus, the oleic acid resulted the most active compound, with an activating effect 50-fold higher than SDS ( Dahlmann, Rutschmann, Kuehn, & Reinauer, 1985 ). Although our knowledge of proteasome structure and function was still in its infancy, in those early reports it was proposed that activation mechanism could be basically related to conformational changes occurring in the enzyme. Furthermore, they also proposed that fatty acids, abundant in muscle, could participate in the physiological regulation of proteasome-mediated protein degradation. Later on, Orlowski et al. performed a detailed kinetic analysis, reporting that lauric acid activates the C-L activity like SDS; increasing concentrations of lauric acid caused a shift in the apparent K m toward lower substrate concentrations with a concomitant increase in V max ( Orlowski, Cardozo, Hidalgo, & Michaud, 1991 ). However, unlike the SDS-mediated one, this activation occurred with a sigmoidal shape of the velocity curve, suggesting the presence of two (or more) substrate binding sites interacting cooperatively. In other words, in the absence of an external activator only part of this activity is manifested, thus underlining the cooperative control of allosteric sites and the concepts of “latency” and of “multi-proteasic complex”; however, there is still some controversy concerning the ability of most fatty acids to enhance the three main 20S proteolytic activities. Actually, fatty acids often exhibit a double-faced nature, behaving as activators and/or inhibitors, according to the type of activity measured. In ostrich liver the C-L proteasome activity was found to be activated by all (with the exception of decanoic acid) types of fatty acids in a concentration-dependent fashion, whereas the CT- and T-L activities were differentially inhibited ( Klinkradt, Naudé, Muramoto, & Oelofsen, 1997 ). An attempt to clarify the intricate mechanism by which the three peptidasic activities of 20S proteasome are regulated by fatty acids was made by Yamada and co-authors, who reported that the pattern of activation of the T-L peptidase is distinctly different from those of CT-L and PHPH-L; thus, linoleic and oleic acids strongly activated both CT-L and the C-L hydrolase-type activities in a biphasic activation pattern ( Yamada et al., 1998 ). Conversely, the activation pattern of tryptic-type peptidase occurs in a tri-phasic manner through an inhibition over the low concentration range, activation in a middle concentration range and inhibition again over a higher concentration range. These apparently conflicting results were explained by hypothesizing the existence of two classes of binding sites, namely “latency sites” and “activation sites”, and the fatty acid activation or inhibition phenomena have been interpreted as the result of binding to these different sites.

Over last decades, the studies on isolated/purified proteasome have been replaced by investigations performed through cellular or clinical studies. Some of these reports focused on the role of polyunsaturated fatty acids on protein- breakdown in muscle mass of cachectic cancer patients. As an example, the neuroprotective effects of Long-Chain Polyunsaturated Fatty Acids (LCPUFA) have been ascribed to some modulatory effects on the UPS, albeit no evidences on direct interaction with 20S proteasome have been reported (Undurti, 2006). Docosahexaenoic acid, the most unsaturated omega-3 fatty acid, displaying pro-apoptotic activity against tumour cells, was reported to exert its anti-cancer activity acting on UPS, even though no evidence of a direct interaction with 20S has been reported ( Jing et al., 2014 ). There is also evidence describing the inhibitory ( Hamel, 2009 ) effect on proteasome by the saturated fatty acid palmitate; this fatty acid is believed to contribute to type-2 diabetes, blocking UPS activity with a consequent lipotoxic effect on pancreatic beta cells. Recently, fatty acid derivatives have been designed to obtain gate-opening 20S stimulators with drug-like properties ( Coleman et al., 2019 ). Using an arachidonic acid derivative AM-404, a very potent but toxic molecule, a series of molecules containing the aminophenol head group linked to aliphatic chains of varying length and degree of unsaturation have been synthesized and characterized. Their effect on the 20S activity indicates that, beside the chain length, saturated chains are generally not able to stimulate the activity of the 20S, while the most important structural feature useful to induce the stimulator activity on 20S, is the cis -double bond in the carbon chain. Furthermore, a greatly diminished capacity to stimulate the 20S was observed when the phenolic amide was substituted by aryl groups. These derivatives still need a careful pharmacological evaluation, also taking into account that, beyond their role as a nutritional energy source, fatty acids have several molecular targets, such as enzymes, receptors, and they are increasingly considered as important signalling molecules that can induce several physiological and pathophysiological effects.

Repositioning “old drugs” to activate the proteasome: the case of aminopyrine

Although over the last decade our knowledge of human diseases has greatly increased, its translation into new drugs and therapeutic benefits has been much slower than expected ( Ashburn & Thor, 2004 ; Scannell, Blanckley, Boldon, & Warrington, 2012 ) The reasons that may explain this apparent incongruity are multi-faceted and include the increased time needed to pipeline new drugs to the market, a high attrition rate of drug candidates in clinical trials ( Pammolli, Magazzini, & Riccaboni, 2011 ), and rapidly changing regulatory requirements. Some reports estimate that, on average, for every dollar invested by the pharmaceutical industry in research and development (R&D) less than a dollar is returned, thus suggesting that investments on R&D will rapidly decline in the very next future ( Pushpakom et al., 2019 ).

Therefore, drug repositioning (or repurposing), an approach to identify new medical applications for drugs, already approved for different therapeutic uses ( Nosengo, 2016 ), offers a number of advantages over the development of entirely new drugs, such as ( i ) the reduced costs in the case of failure, and ( ii ) the shorter time interval for the transfer to the market because safety assessment has been already completed. Thus, given the urgent need to find a treatment for neurodegenerative diseases (such as AD, PD and HD), it is not surprising that an increasingly large number of existing drugs are tested for these disorders; in this respect, an important example is the repositioning of galantamine, one of the drugs now available on the market for the treatment of AD ( Durães, Pinto, & Sousa, 2018 ).

Despite many examples of drug repositioning have been based on serendipity, a rational development of a repurposed drug implies a detailed knowledge of the pathways involved. As an example, there is evidence that AD development is associated to aggregation of the neurotoxic amyloid β (Aβ) peptide ( Kang et al., 1987 ), being the consequence of a failure of proteasome function and a consequent accumulation of poly-ubiquitinated substrates, as detected in AD neuronal tissues ( Perry, Friedman, Shaw, & Chau, 1987 ). In particular, in response to the increased oxidative and proteotoxic stress, the percentage of uncapped 20S proteasome is significantly increased in AD neuronal tissues in comparison to healthy cells ( Wang, Yen, Kaiser, & Huang, 2010 ). Moreover, the Aβ peptide, as well as other intrinsically disordered proteins (IDPs), is a substrate of the 20S proteasome.

On these premises, it has been reported that pyrazolones, a class of small molecules extensively used in the past as painkillers and antipyretic drugs, induce proteasome activation in mice moels of Amyotrophic lateral Sclerosis ( Trippier et al., 2014 ). In a recent report, it was demonstrated that some members of this class of molecules ( i.e., aminopyrine, 4-aminoantiypirine and nifenazone) may enhance CT-L proteasome activity in tube tests ( Santos et al., 2019 ). Proteasome activity assays, carried out in parallel in the presence of an excess of reducing agents ( i.e., ascorbic acid or glutathione), underscored that proteasome activation by pyrazolones is not directly related to their antioxidant properties, thus suggesting that an alternative mechanism of action should be proposed. In the case of aminopyrine, the evidence that it is able to activate native 20S, but it is ineffective on a mutant ( i.e., α3ΔN, which has a permanently opened gate, see Section 2.2.2 ), envisaged that its effect on proteasome activity is mainly related to enhanced dynamics of the outer α-rings, which is a common mechanism of proteasome activation by small molecules (see Section 2.2.2 ) ( Njomen & Tepe, 2019 ). Furthermore, the effects on the different cellular proteasome forms ( i.e., 20S, 26S and 30S, see 2.2.2 , 2.3.2 ) were also assayed in neuroblastoma SH-SY5Y cells by separating proteasome assemblies in non-denaturing gels. It was observed that proteasome assemblies resulted significantly stimulated two hours after the treatment with the drug, even though this effect vanishes over 24 h after stimulation, reflecting the pharmacokinetic properties of aminopyrine (with a half-life in blood serum of approximately 2 h). Docking simulations, performed using aminopyrine, antipyrine, 4-aminoantipyrine and nifenazone as ligands of human 20S, have outlined that they interact with α-rings, involving the α1/α2 and α5/α6 binding pockets; the most active molecules display a binding free energy ~30 Kcal/mol more favourable than the less active ones. In particular, it was observed that aminopyrine bridges α1and α2 subunits since its phenyl ring is involved in hydrophobic interactions with residues L22, Y25, E26, A126, and A157 of the α1 subunit; the oxygen atom of aminopyrine is also H-bonded with Y159 and the N -methyl group with residue A32 of the same subunit. Moreover, residues G30, G31 and A32 of the α2 subunit are linked by non-polar interactions to the 4-(dimethylamino) group of aminopyrine. Next, T-shaped stacking interactions bridge the residue F162 of the α5 subunit with the phenyl ring of aminopyrine, and the residue Q60 of the α6 subunit turns out to be H-bonded to the oxygen atom of the ligand. It is important to remind here that the α1/α2 grooves are the preferential anchoring sites of the HbYX motif which binds the 20S with the Rpt3 subunit of the regulatory particle 19S (see 2.2.2 , 2.3.2 ) ( Smith et al., 2007 ); furthermore, small molecules, known to mobilize 20S gating dynamics, bind the α5/α6 grooves (Di Di Dato et al., 2017 ). Notably, the inactive compound antipyrine mainly interacts through T-shaped aromatic interactions and H-bonds with two residues of α2 subunit Y159 and Y160, respectively, but no bridging interactions with any other subunit of the α-ring are observed (see Figs. 5A and 6A in Santos et al., 2019 ). MALDI-MS experiments, performed using as a substrate Aβ1–28, a water-soluble fragment of the amyloid peptide, further demonstrated that aminopyrine may enhance the rate of peptide degradation. Cell viability assays, carried out on differentiated SH-SY5Y neuroblastoma cells, underscored the neuroprotective properties of aminopyrine, and further experiments, conducted in the presence of the proteasome inhibitor bortezomib, demonstrated that aminopyrine rescues neuron-like cells from Aβ proteotoxicity with a mechanism of protection mostly related to proteasome activation. These results are likely to stimulate further studies focusing on proteasome activation by repurposed drugs and, ultimately, to relaunch investments from pharmaceutical industries in this risky area.

Insights on additional potential applications of proteasome inhibition: a role in SARS-Covid19 therapy?

Although not deeply investigated yet, proteasome inhibitors discussed so far display a known anti-inflammatory activity, envisaging a therapeutic efficacy in combined regimen in subjects with acute severe inflammatory processes, such as in viral infections. This potentiality assumes a particularly updated importance nowadays during the recent pandemia due to SARS-Cov-2, which is likely to infect millions of people worldwide with a significant lethality rate. Besides the public health and victim tolls, which are by far the most urgent topics, disease spread is dramatically impacting on world social and economic activities, making even more urgent the identification of a specific therapy or a vaccine ( Andersen, Rambaut, Lipkin, Holmes, & Garry, 2020 ; Sheeren et al., 2020; Baden & Rubin, 2020 ; Lipsitch, Swerdlow, & Finelli, 2020 ).

The limited clinical and laboratory data available so far suggest that in most cases SARS-Cov-2 infection evolves through symptoms overlapping those of canonical flu, even though a very large number of subjects do not develop symptoms ( Baden & Rubin, 2020 ; Zhang et al., 2020 ). In a limited, but significant, especially for the health assistance burden, number of cases, infection progresses toward a clinical picture of interstitial pneumonia sustained by the massive stimulation of the immune system the virus appears to be able to elicit ( Baden & Rubin, 2020 ; Zhang, Lin, et al., 2020 ).

The cytokine storm which underscores this disease progression often induces Acute Respiratory Distress Syndrome (ARDS) and diffuse thrombotic angiopathy which are, to date, the prevalent cause of death of ill patients.

Nonetheless, the massive inflammatory response rather than virus replication is gaining increasingly relevance as the real target of therapy. This is emphasized by the apparent efficacy of therapies based on biological drugs which target the pro-inflammatory cytokines ( Baden & Rubin, 2020 ). In any case, specific trials will definitively address the therapeutic efficacy of these approaches, hopefully allowing to identify a therapeutic regimen which can be early undertaken to prevent disease complication and sanitarian costs.

The most studied models for SARS-CoV infection, spread in 2003; encompass: (i) original strains of SARS-CoV isolated from human subjects for infection of cell cultures in vitro ; (ii) the Coronavirus Mouse Hepatitis virus (MHV) infection in murine models. MHV belongs to the same coronavirus genus of SARS-CoV and displays significant similarity concerning both structural features and pathogenesis, including the marked innate immune inflammatory cytokine release ( Ma et al., 2010 ).

Among studies, reported to date, there is a compelling evidence that the UPS could regulate the virus infectious cycle at multiple levels, with the exception of virus internalization which occurs through endocytosis following recognition of Angiotensin Converting Enzyme-2 (ACE2) receptor on host cells by the spike protein (S) ( Mathewson et al., 2008 ; Li et al., 2007 ; Li et al., 2003 ). Interestingly, chemical inhibitors of proteasome induced the virus particles to accumulate in late endosomes and lysosomes, suggesting a UPS role in virus release from endosomes ( Yu & Lai, 2005 ). Once released in the cytoplasm, the nucleic acid of SARS-Cov (a positively single stranded RNA) encodes four structural proteins, nucleocapsid (N), envelope (E), membrane (M) and spike (S) proteins and about 16 non-structural proteins which are translated as a single poly-protein ( Yu & Lai, 2005 ; Li, Sui, et al., 2007 ; Schneider et al., 2012 ). The highly antigenic protein N of SARS-CoV, which is an extensively glycosylated and positively charged protein of the nucleocapsid, forms a helical ribonucleoprotein complex with the viral RNA, and it was found to interact with ATPase 6 ( i.e., a 19S subunit) in lung fibroblasts infected with the virus ( Wang, Xu, et al., 2010 ). Therefore, although further studies are required to confirm this hypothesis, a direct down-regulation of proteasome complexes by SARS-CoV can be envisaged, since in the intracellular space the ATPase 6 subunit (because of its position in the multi-subunits complex, see Fig. 2 and Lander et al., 2012 ) could be accessible for binding also when assembled in 19S particles; as a consequence, proteasome inhibition would appear as a strategy to halt the antigenic processing of virus proteins. If so, this process should be properly balanced, since proteasome activity is further relevant for advancement of cell cycle, especially G1-S transition which allows the virus to replicate ( Wang, Xu, et al., 2010 ).

Studies on non-structural proteins, which intervene either in the virus replication and in the interaction with the host machineries, besides confirming that the UPS does not affect virus internalization, envisaged that virus-UPS interaction may account for the mechanisms of immune system evasion ( Wong et al., 2018 ; Liu et al., 2014 ; Hu, Yen, Singh, Kao, & Wu-Hsieh, 2012 ). In this respect, the SARS-CoV Papain-like proteasome (PL pro ), which, along with the main protease M pro (also called 3CL pro ), is essential for the cleavage and processing of the viral poly-protein, was found to repress IFNγ synthesis and secretion in lung cell cultures by targeting IRF3 phosphorylation and nuclear translocation ( Devaraj et al., 2007 ; Zhang et al., 2020 ). Interestingly, PL pro was reported to have DUB activity in human cells by recognizing the LXGG consensus de-ubiquitination motif and by directly binding to the proteasome through the N-terminal Ub-like domain which is present in its structure ( Ma et al., 2010 ). Although the DUB activity was shown to be ineffective in altering the repertoire of host poly-Ub proteins, since Ub-conjugation is a post-translational mechanism of regulation of intracellular trafficking and receptor signalling, it would be relevant to uncover the link between IRF3 signalling and Ub-labelling by PL pro .

In this framework, viral accessory protein 3a was reported to promote IFN-typeI receptor ubiquitination and proteasome degradation ( Minakshi et al., 2009 ). Along with this, accessory proteins 8a and 8b or 8ab, which were found to be expressed either as a single protein or spliced protein in different viruses at different stages of SARS-CoV infection, were found to bind to intracellular ubiquitinated proteins through an extensively glycosylated functional domain which account for ubiquitin binding and ubiquitin conjugation ( Le et al., 2007 ; Keng et al., 2011; Li & Johnson, 2012 ). Interestingly, 8b and 8ab appear to stimulate the UPS-mediated degradation of IRF3 at later stages of virus replication than PL pro was supposed to do ( Wong et al., 2018 ). Remarkably, proteasome inhibition was found to be ineffective in the assembly of virus ( Raaben, Grinwis, Rottier, & de Haan, 2010 ; Raaben et al., 2010 ). Although proteasome inhibition appears ineffective in the assembly of virus ( Raaben, Grinwis, et al., 2010 ; Raaben, Posthuma, et al., 2010 ), the role of the UPS in the virus infectious cycle is a relevant topic, and its putative role was suggested to rely in M-calpain rather than proteasome inhibition ( Schneider et al., 2012 ). What looks more convincing is that the UPS plays a key role in handling the immune response to the pathogen, but also in the aberrant inflammatory response the virus may elicit. From the first point of view, bortezomib treatment increased disease progression in the liver of mice infected by MHV, underscoring the relevance of the protective role UPS in inflammatory response against the viral agent ( Raaben, Posthuma, et al., 2010 ). Several independent investigations on different microbial pathogens reveal that a major role for the UPS is to drive antigenic processing and inflammation activation especially in monocytes/macrophages ( Forget, Gregory, & Olivier, 2005 ; Horan et al., 2013 ; Silswal, Reis, Qureshi, Papasian, & Qureshi, 2017 ). Furthermore, proteasome dysfunction in alveolar type II epithelial cells is associated with ARDS in the alveolar space of Rpt3-KO mice ( Sitaraman et al., 2019 ). From the second point of view, besides the known contribution in modulating the transcriptional activity of nuclear factors, such as NF-kB (see Section 3.2.1 ), additional tissue-specific mechanisms which should intervene in coronavirus infection are expected to take part to the pathogenesis. In this regard, the clearance of specific proteins, such as elafin by the exceedingly activated UPS might be a relevant factor in sustaining inflammation. In fact, elafin is a serine-proteasome inhibitor and its inhibitory activity resides within the C-terminal domain which has specificity for NE and proteinase 3. Notably, transglutaminase substrate binding motif (GQDPVK) is present at the N-terminus which allows it to cross-link extracellular matrix proteins ( Kerrin et al., 2013 ).

In addition, clinical studies highlight that the 20S proteasome is released in the alveolar space during ARDS in an active configuration, and that immune proteasome subunits are increased in the alveolar space envisaging a prognostic relevance of its quantification ( Sixt et al., 2009 ; Sixt et al., 2012 ; de Bruin et al., 2016 ). As a whole, the disastrous SARS-Cov-2 experience suggests that multi-faceted efforts by the scientific community are demanded to clarify the pathogenesis of coronaviruses especially in view of the concrete possibility that additional spill-overs in the next future might come up posing new pandemic threats. The central role of the UPS in regulating the complex dynamic of interactions between the pathogen and the host, along with the growing interest in the development of UPS modulators, could provide further clues for the identification of valid approaches which allow to limit the sanitarian, social and economic costs of similar pandemia.

Box 1: Elements in proteasome heterogeneity

Proteasome is a highly dynamic complex as demonstrated by the existence of alternative forms of proteasome which deal with specific biological roles. The immun-oproteasome is the most studied alternative form of proteasome and its proteolytic activity has been long linked only to generation of antigenic peptides for MHC class I presentation ( Murata, Takahama, Kasahara, & Tanaka, 2018 ; Rousseau & Bertolotti, 2018 ). However, a number of studies have reported a role for immunoproteasome in B and T cell differentiation, monocytes and dendritic cells activation, in the maintenance of pluripotency of stem cells ( Atkinson et al., 2012 ), in the differentiation of non-immune cells, such as skeletal muscle ones, and also in the homeostasis of nervous cells ( Kaur & Batra, 2016 ; Kimura, Caturegli, Takahashi, & Suzuki, 2015 ). Along with this, immunoproteasome dys-regulation has also been associated with various human diseases, including cancer, immune and inflammatory disorders: in fact, either hyper-activation or hypo-activation may turn out into a hyper-immune or hypo-immune phenotype ( Eskandari, Seelen, Lin, & Azzi, 2017 ). Therefore, there has been a great effort to develop specific immunoproteasome inhibitors, which showed minimal cross-reactivity with constitutive proteasome. From the structural point of view, the immunoproteasome differs from the canonical 20S for the replacement of catalytic subunits with its immune highly homolog counterparts β1i (LMP2), β2i (MECL1), and b5i (LMP7) ( Sherman & Li, 2020 ; Sijts & Kloetzel, 2011 ). Remarkably, these immunoproteasome subunits are constitutively expressed in different tissues, such as thymus and spleen. Moreover, non-immune cells preferentially incorporate them during the assembly de novo of 20S particles following exposure to proinflammatory stimuli ( e.g., IFN-γ, TNF-α, and lipopolysaccharide) or cytokine-independent stressors ( e.g., oxidative stress) ( Griffin et al., 1998 ; Heink, Ludwig, Kloetzel, & Krüger, 2005 ; Ferrington & Gregerson, 2012 ; Murata et al., 2018 ). Immune-subunits incorporation has been proposed to proceed cooperatively, since the direct binding of β5i to chaperone POMP is followed by the quick recruitment of b1i and b2i. Therefore, the rate of their assembly is about four times faster than that of canonical subunits in the forming 20S, a finding consistent with the primary biological role of immunoproteasome, which is demanded to cope with patho-physiological challenges in a dynamic and highly efficient manner ( Griffin et al., 1998 ; Groettrup, Standera, Stohwasser, & Kloetzel, 1997 ; Murata et al., 2018 ; Murata et al., 2001 ). The subunit substitution accounts for a shift in the catalytic preferences and activity; in fact, immunoproteasome exhibits elevated level of CT-L and T-L activities which favour the production of peptides with terminal basic or hydrophobic residues that fit better into the cleft of the MHC class I molecule ( Murata et al., 2018 ; Rousseau & Bertolotti, 2018 ).

IFN-γ also induces the expression of another important regulator, besides 19S, of 20S activity, the 11S regulator (PA28) ( Sherman & Li, 2020 ; Cascio, Hilton, Kisselev, Rock, & Goldberg, 2001 ; Cascio, Call, Petre, Walz, & Goldberg, 2002 ; Cascio, 2014 ).

Mammalian cells express three different subunits of 11S regulator: PA28α, PA28β, and PA28γ. PA28α and PA28β assemble into a hetero-heptameric complex, primarily located in the cytoplasm, while homo-heptameric PA28γ is mainly present inside the nucleus ( Wójcik, Tanaka, Paweletz, Naab, & Wilk, 1998 ; Cascio, 2014). Although the role of these regulators is not clear, it has been reported that both forms increase after oxidative stress, suggesting their involvement in the degradation of damaged proteins ( Pickering et al., 2010 ; Pickering et al., 2012; Kors, Geijtenbeek, Reits, & Schipper-Krom, 2019 ; Thibaudeau and Smith, 2018 ). Accordingly, PA28α-β association with the 20S does not enhance the degradation of poly-ubiquitinated protein/peptide substrates in vitro ( Li, Powell, & Wang, 2011 ; Cascio, 2014; Lobanova et al., 2018 ). In addition to its role in preserving cellular homeostasis after oxidative stress, the PA28 role in the regulation of the immune response has been extensively studied ( Tanahashi et al., 1997 ; Früh & Yang, 1999 ; Preckel et al., 1999 ; Cascio, 2014). In fact, following INFγ stimulation, the level of PA28α-β binding to inducible 20S increases, enhancing its proteolytic activity and mediating the generation of antigenic peptides (Groettrup et al., 1996; Sijts et al., 2002 ; Sijts et al., 2011; Fort, Kajava, Delsuc, & Coux, 2015 ). Unlike PA28α-β, PA28γ is not induced by IFNγ, suggesting a different biological role for PA28γ-20S complex. Although the role of this complex remains elusive, a number of studies imply an involvement in the cell cycle progression ( Kors et al., 2019 ).

In addition to immunoproteasome, other tissue-specialized forms of proteasome are thymo-proteasome and testis-proteasome (spermato-proteasome). The first one is expressed by cortical thymic epithelial cells and contains two immune catalytic subunits, β1i and β2i, and a thymus specific subunit (β5t), that, unlike β5 and β5i, is characterized by a number of hydrophilic amino acids in its catalytic pocket. Thus, thymo-proteasome produces a distinct spectrum of peptide fragments, and cells expressing it display a unique set of peptides associated with MHC-I molecules ( Florea et al., 2010 ; Murata et al., 2007 ; Sasaki et al., 2015 ). Thymo-proteasome role accounts for the positive selection of developing T cells, since it is essential to optimize the release of the repertoire of peptides for CD8 + T cell ( Murata et al., 2018 ; Nitta et al., 2010 ; Takada et al., 2015 ; Xing, Jameson, & Hogquist, 2013 ).

Spermato-proteasome is a testis-specific form of proteasome (described exclusively in spermatocytes, spermatids and sperm), and it is characterized by a chronologically defined expression. It contains a specific α4 subunit (α4s) (PMSA8 gene) in the place of corresponding constitutive α-subunit, whose incorporation into a newly formed 20S is mutually exclusive and does not alter the catalytic preferences of the constitutive 20S ( Morozov & Karpov, 2019 ; Qian et al., 2013 ; Uechi et al., 2014 ). Elevated expression of PSMA8 has been identified in different tumours, such as large B-cell lymphoma, thymoma, and testicular germ cell tumours. However, the biological significance and the possibility to make α4s a therapeutic target candidate has not been elucidated yet ( Bruggeman, Koster, Lodder, Repping, & Hamer, 2018 ; Morozov & Karpov, 2019 ). The incorporation of α4s in spermato-proteasome seems to favour the 20S association with another gate-activating RP, namely PA200 (Blm10 in yeast) ( Qian et al., 2013 ). PA200 is a nuclear-specific proteasome activator and it is expressed in all mammalian tissues, but it is particularly abundant in the testis, where it plays a crucial role in spermatogenesis ( Khor et al., 2006 ; Ustrell, Pratt, Gorbea, & Rechsteiner, 2005 ). Accordingly, PA200 deletion markedly reduces fertility of male mice ( Khor et al., 2006 ), and the PA200/20S spermato-proteasome complex catalyses ubiquitin-independent degradation of acetylated core histones during DNA repair and spermatogenesis ( Qian et al., 2013 ). PA200 also binds constitutive proteasome and the amount of PA200/constitutive 20S, as well as of PA28/20S complex, increases upon 26S inhibition, contributing to adapt the pool of different proteasome populations to the cell condition ( Welk et al., 2016 ). The crystal structure of this complex has revealed that C-terminal HbYX motif in PA200 fits between α5 and α6 inter-subunit pocket, mediating 20S gate opening ( Sadre-Bazzaz et al., 2010 ; Witkowska et al., 2017 ). Interestingly, it strongly stimulates the rate of C-L activity, although its biological significance is poorly clear yet ( Ustrell, Hoffman, Pratt, & Rechsteiner, 2002 ). Recently, it has been reported the identification of a non-canonical variant of constitutive 20S in mammalian cells, previously identified in yeast. It is known as alternative ‘α4-α4’ proteasome, which assembles upon replacement of α3 with an additional α4 subunit in the position normally occupied by the former ( Kusmierczyk, Kunjappu, Funakoshi, & Hochstrasser, 2008 ; Padmanabhan, Vuong, & Hochstrasser, 2016 ; Velichutina, Connerly, Arendt, Li, & Hochstrasser, 2004 ). Importantly, mammalian cells, primed to assemble these alternative proteasomes, exhibit enhanced resistance to cellular stress induced by metal ions ( Padmanabhan et al., 2016 ).

The existence of interchangeable subunits, and thus, of alternative proteasome forms above described, as well as the identification of hybrid proteasome particles ( i.e., 19S–20S-11Sα-β, 19S–20S-11Sγ,19S–20S-PA200) whose biological function is poorly known, underlie how cells modify proteasome repertoire in relation to its specific needs ( Cascio et al., 2002 ; Morozov & Karpov, 2019 ; Thibaudeau & Smith, 2019 ).

Box 2: Inhibition of UPS targetting deubiquitinases: Usp14 at a glance

De-ubiquitinases (DUBs), which catalyse ubiquitin moieties removal from target proteins, are key components of the UPS, being involved in ubiquitin recycling and editing ( Yuan et al., 2018 ). Three DUBs are associated with the proteasome: Rpn11, a Zn 2+ metallo-protease, which is part of the lid, USP14 and Uch37 which are two cysteine proteases, extrinsically associated with the base (see Section 2.3.1 ) ( D'Arcy, Wang, & Linder, 2015 ). Several studies have proposed that de-ubiquitination by Rpn11 stimulates the substrate degradation by removing bulky ubiquitin chains that otherwise might impair further substrate translocation into the proteasome ( de Poot et al., 2017 ) on the other hand, the ATP/independent de-ubiquitination by Usp14 and Uch37 is envisaged to suppress substrate degradation, promoting its premature dissociation from the proteasome ( Lam et al., 1997 ; Hanna et al., 2006 ; Lee et al., 2010 ; Lee et al., 2016b). Among the three DUBs, Usp14 is the most attractive way of intervention to regulate proteasome activity ( Chakraborty et al., 2018 ; Wertz & Murray, 2019 ). Human USP14 consists of two domains, namely (i) a N-terminal ubiquitin-like domain (UBL) and (ii) a C-terminal DUB domain, which contains the catalytic triad, Cys114, His435, and Asp451. In the free unbound state, the catalytic domain of Usp14 is characterized by a low level of de-ubiquitinase activity, whereas when it binds proteasome its activity is increased by about 800-fold and shows a preference for substrates ubiquitinated at more than one site ( Koulich, Li, & DeMartino, 2008 ; Hu et al., 2005 ; Lee et al., 2010 ; Lee et al., 2016b). It has been shown that Usp14 inhibition stimulates the degradation of some specific proteasome substrate in mammalian cells, such as cancer and neurodegeneration related proteins ( Lee et al., 2010 ; Homma et al., 2015 ; Zhu, Zhang, et al., 2016a ; McKinnon et al., 2016 ; Boselli et al., 2017 ; Liao et al., 2017 ; de Poot et al., 2017 ). Additionally, Usp14 activation through phosphorylation of Ser432 residue by AKT results in the suppression of the degradation of short-lived proteins, that may in turn promote tumour cell proliferation ( Kim & Goldberg, 2017 ; Wei et al., 2017 ; Xu et al., 2015 ). Some evidences also suggest that Usp14 expression is closely related to the onset of different tumours, including breast, gastric and lung cancer ( Wu et al., 2013 ; Zhu, Zhang, et al., 2016b ; Zhang et al., 2017 ; Fu et al., 2018 ) and its activity is required for nervous system development and functioning ( Chen, Retzlaff, et al., 2011 ; Kiprowska et al., 2017 ; Vaden et al., 2015 ; Wilson et al., 2002 ). Thus, considerable efforts have been dedicated to the discovery of small molecules that functionally inhibit Usp14 and several ones have been identified, such as b-AP15, auranofin, WP1130 and curcumin analogue AC17 ( Kapuria et al., 2010 ; Zhou et al., 2013 ; Wang et al., 2014 ; Coughlin et al., 2014 ; Liu et al., 2014 ; Tian et al., 2014 ; D'Arcy et al., 2015 ; Wang et al., 2015; Wang et al., 2016; Wang et al., 2018 ; Xia et al., 2019 ; Wertz & Murray, 2019 ; Ma et al., 2020 ). A common feature of most of these compounds is the presence of α,β-unsaturated carbonyl groups which can form covalent adducts with free thiols in the active site by Michael addition ( D'Arcy et al., 2015 ; Wang et al., 2015 ). Unfortunately, these compounds usually have poor selectivity across the DUB family, since most of them are cysteine enzymes which are easily “druggable” by compounds containing Michael acceptors ( D'Arcy et al., 2015 ). The small molecule IU1 was the first specific inhibitor identified, exhibiting excellent selectivity for USP14 over other DUBs ( Lee et al., 2010 ; Lee et al., 2016 ). Co-crystal studies reveal a unique mechanism of action of IU1 that exerts its inhibitory activity by binding to the thumb-palm cleft region of Usp14 catalytic domain, sterically preventing ubiquitin binding to the C-terminal of Usp14 ( Wang et al., 2018 ). Interestingly, the prion protein shows accelerated degradation upon IU1 treatment, and a more powerful analogue (IU1–47) enhances tau degradation in neurons ( Boselli et al., 2017 ; Homma et al., 2015 ; McKinnon et al., 2016 ), rendering it an intriguing target also for neurodegenerative diseases. VLX1570 is an analogue of b-AP15, being characterized by a higher potency and an improved solubility, which shows consistent anti-tumour activity in orthotopic and xenograft models of MM, lymphoma, Ewing's sarcoma, and other malignancies (D'Arcy et al., 2014; Chitta et al., 2015 ; Wang et al., 2016 ; Shukla et al., 2016 ). Moreover, VLX1570 retains prominent activity in bortezomib-resistant MM cells ( Rowinsky et al., 2020 ; Shukla et al., 2016 ). These studies together with the good tolerability profile, reported in preclinical models, provided the rationale for investigating this drug in clinical trials on patients with RRMM ( Rowinsky et al., 2020 ). Thus, in a phase 1 study fourteen patients with RRMM were enrolled and treated with escalating doses of intravenous infusion of VLX1570 ranging from 0.05 to 1.2 mg/kg and anti-myeloma effects were observed at dose of 0.6 mg/kg or more. Unfortunately, two patients treated with 1.2 mg/kg dose experienced severe and progressive respiratory insufficiency, culminating in death; thus, due to severity of the toxicity, the study was discontinued ( Rowinsky et al., 2020 ). Beside this molecule, no other inhibitors targeting DUBs have entered into clinical trial so far. However, since they are abnormally expressed in a variety of tumours and/or in tumour microenvironment ( Yuan et al., 2018 ), making them ideal anticancer target candidates, the identification of selective small-molecule inhibitors for Usp14 and in general for other specific DUBs remains an active and extremely challenging task.

Box 3: Metabolic control of proteasome function

The regulation of metabolic control of proteasome function is a challenging point in proteasome biology, which deserves particular attention, although many aspects are still obscures. The coordinated balance of the two post-translational modifications ( i.e., O-linked N -acetylglucosamine (O-GlcNAc) and phosphorylation) appears to be crucial in this process ( Rousseau & Bertolotti, 2018 ; Zhang et al., 2003 ; Zhang et al., 2007 ; Fardini et al., 2015). Recent studies have shown that different kinases, such as CaMKII, PKG, DYRK2 and PKA, induce the phosphorylation of proteasome subunits. In particular, the activation of PKA, which follows the increase of cAMP level, results in the phosphorylation of Ser14 of Rpn6, leading to proteasome activity enhancement ( VerPlank and Goldberg, 2018 , VerPlank and Goldberg, 2017 ). In vitro and in vivo studies have shown that a series of hormones with different biological functions, spanning from energy control ( i.e., glucagon and epinephrine) to water resorption ( i.e., antidiuretic hormone), stimulate adenylate cyclase, thus raising cAMP level and enhancing, through the Rpn6 phosphorylation, degradation by 26S proteasome of short-lived regulatory proteins and/or damaged proteins ( VerPlank et al., 2019 ). These findings suggest that proteasome activation, mediated by CAMKII and cAMP-PKA, could be a common cellular mechanism to answer a series of endocrine stimuli which, inducing a quick destruction of regulatory and/or damaged proteins, should help cells to adapt their proteome to the novel conditions determined by the hormones exposure ( VerPlank et al., 2019 ). O-GlcNAc is a post-translational modification that occurs at serine or threonine residues, which are also a target site of phosphorylation. It is catalyzed by the enzyme O-linked N -acetylglucosamine transferase, which modulates the addition of a GlcNAc moiety to target proteins, by using as substrate a product of hexosamine biosynthesis, the UDP-GlcNAc, whose availability is influenced by nutritional conditions. In fact, an increase in glucose availability raises UDP-GlcNAc levels and consequently promotes protein O-GlcNAc ( Comer & Hart, 2000 ; Zachara & Hart, 2004 ). It has been reported that 26S proteasome function is inhibited by the addition of sugar moieties to the Rpt2 subunit ( Zhang et al., 2003 ), a finding which provides a link between glucose metabolism and protein turnover ( Zhang et al., 2007 ). In the liver and muscle tissues of mouse models, when blood glucose drops ( e.g., under fasting conditions or during exercise) the PKA increase stimulates 26S activity selectively toward the clearance of short-lived regulatory proteins, with no alterations in proteasome content, confirming that proteasome activation occurs through post-synthetic modification of already existing particles ( Rousseau & Bertolotti, 2018 ; VerPlank et al., 2019 ). The low level of glucose also reduces the entry of this mono-saccharide into the hexosamine pathway, limiting the availability of UDP-GlcNAc; as a consequence, the O-GlcNAc modification of Rpt2 decreases thus removing the signal that inhibits proteasome function ( Zhang et al., 2003 ; Zhang et al., 2007 ). Moreover, it has been shown that lack of nutrient rapidly inhibits the stress and nutrient response of the mTOR complex. It results into the enhancement of autophagic process and of the rate of long-lived protein ubiquitination and, therefore, of their degradation by UPS ( Zhao et al., 2015 ). All these events bring to a general activation of proteolysis, promoting cellular adaptation, facilitating damaged and potentially toxic protein clearance, and providing essential amino acids for the synthesis of proteins necessary for cell survival and energy production (Zhao et al., 2015; VerPlank and Goldberg, 2015; Rosseau and Bertolotti, 2016).

Declaration of Competing Interest

The authors declare that there are no conflicts of interest.

References

Abbas T. Dutta A. p21 in cancer: intricate networks and multiple activities Nature Reviews. Cancer 9 6 2009 400 414 10.1038/nrc2657 19440234 PMC2722839

Adams J. The proteasome: structure, function, and role in the cell Cancer Treatment Reviews 29 Suppl. 1 2003 3 9 10.1016/s0305-7372(03)00081-1 12738238

Adams J. The development of proteasome inhibitors as anticancer drugs Cancer Cell 5 5 2004 417 421 10.1016/s1535-6108(04)00120-5 15144949

Adams J. The proteasome: a suitable antineoplastic target Nature Reviews. Cancer 4 5 2004 349 360 10.1038/nrc1361 15122206

Adams J. Behnke M. Chen S. Cruickshank A.A. Dick L.R. Grenier L. … Stein R.L. Potent and selective inhibitors of the proteasome: Dipeptidyl boronic acids Bioorganic & Medicinal Chemistry Letters 8 4 1998 333 338 10.1016/s0960-894x(98)00029-8 9871680

Adams J. Palombella V.J. Sausville E.A. Johnson J. Destree A. Lazarus D.D. … Elliott P.J. Proteasome inhibitors: A novel class of potent and effective antitumor agents Cancer Research 59 11 1999 2615 2622 10363983

Agarwal R. Gupta S.K. Agarwal P. Saxena R. Agrawal S.S. Current concepts in the pathophysiology of glaucoma Indian Journal of Ophthalmology 57 4 2009 257 266 10.4103/0301-4738.53049 19574692 PMC2712693

Agathocleous A. Rohatiner A. Rule S. Hunter H. Kerr J.P. Neeson S.M. … Lister A. Weekly versus twice weekly bortezomib given in conjunction with rituximab, in patients with recurrent follicular lymphoma, mantle cell lymphoma and Waldenström macroglobulinaemia British Journal of Haematology 151 4 2010 346 353 10.1111/j.1365-2141.2010.08340.x 20880120

Aggarwal B.B. Nuclear factor-kappaB: The enemy within Cancer Cell 6 3 2004 203 208 10.1016/j.ccr.2004.09.003 15380510

Aghdam S.Y. Gurel Z. Ghaffarieh A. Sorenson C.M. Sheibani N. High glucose and diabetes modulate cellular proteasome function: Implications in the pathogenesis of diabetes complications Biochemical and Biophysical Research Communications 432 2 2013 339 344 10.1016/j.bbrc.2013.01.101 23391566 PMC3595335

Agholme L. Nath S. Domert J. Marcusson J. Kågedal K. Hallbeck M. Proteasome inhibition induces stress kinase dependent transport deficits - implications for Alzheimer's disease Molecular and Cellular Neuroscience 58 2014 29 39 Scopus 10.1016/j.mcn.2013.11.001 24270002

Aguileta M.A. Korac J. Durcan T.M. Trempe J.-F. Haber M. Gehring K. … Husnjak K. The E3 ubiquitin ligase parkin is recruited to the 26 S proteasome via the proteasomal ubiquitin receptor Rpn13 The Journal of Biological Chemistry 290 12 2015 7492 7505 10.1074/jbc.M114.614925 25666615 PMC4367258

Akopian T.N. Kisselev A.F. Goldberg A.L. Processive degradation of proteins and other catalytic properties of the proteasome from Thermoplasma acidophilum The Journal of Biological Chemistry 272 3 1997 1791 1798 10.1074/jbc.272.3.1791 8999862

Al Mamun A. Uddin M.S. Kabir M.T. Khanum S. Sarwar M.S. Mathew B. … Ashraf G.M. Exploring the promise of targeting ubiquitin-proteasome system to combat Alzheimer's disease Neurotoxicity Research. Scopus. 2020 10.1007/s12640-020-00185-1 32157628

Albert M.-C. Brinkmann K. Kashkar H. Noxa and cancer therapy Molecular & Cellular Oncology 1 1 2014 10.4161/mco.29906 PMC4905168 27308315

Albertsson-Lindblad A. Kolstad A. Laurell A. Räty R. Grønbæk K. Sundberg J. … Jerkeman M. Lenalidomide-bendamustine-rituximab in patients older than 65 years with untreated mantle cell lymphoma Blood 128 14 2016 1814 1820 10.1182/blood-2016-03-704023 27354719

Alderden R.A. Hall M.D. Hambley T.W. The discovery and development of cisplatin Journal of Chemical Education 83 5 2006 728 10.1021/ed083p728

Alexanian R. Haut A. Khan A.U. Lane M. McKelvey E.M. Migliore P.J. … Wilson H.E. Treatment for multiple myeloma. Combination chemotherapy with different melphalan dose regimens JAMA 208 9 1969 1680 1685 10.1001/jama.208.9.1680 5818682

Ali R.E. Rattan S.I. Curcumin's biphasic hormetic response on proteasome activity and heat-shock protein synthesis in human keratinocytes Annals of the New York Academy of Sciences 1067 2006 394 399 10.1196/annals.1354.05 16804017

Alkalay I. Yaron A. Hatzubai A. Orian A. Ciechanover A. Ben-Neriah Y. Stimulation-dependent I kappa B alpha phosphorylation marks the NF-kappa B inhibitor for degradation via the ubiquitin-proteasome pathway Proceedings of the National Academy of Sciences of the United States of America 92 23 1995 10599 10603 7479848 10.1073/pnas.92.23.10599 PMC40659

Almond J.B. Cohen G.M. The proteasome: A novel target for cancer chemotherapy Leukemia 16 4 2002 433 443 10.1038/sj.leu.2402417 11960320

Alsina M. Trudel S. Furman R.R. Rosen P.J. O'Connor O.A. Comenzo R.L. … Goy A. A phase I single-agent study of twice-weekly consecutive-day dosing of the proteasome inhibitor carfilzomib in patients with relapsed or refractory multiple myeloma or lymphoma Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 18 17 2012 4830 4840 10.1158/1078-0432.CCR-11-3007 22761464

Amir R.E. Van den Veyver I.B. Wan M. Tran C.Q. Francke U. Zoghbi H.Y. Rett syndrome is caused by mutations in X-linked MECP2, encoding methyl-CpG-binding protein 2 Nature Genetics 23 2 1999 185 188 10.1038/13810 10508514

Amiri K.I. Horton L.W. LaFleur B.J. Sosman J.A. Richmond A. Augmenting chemosensitivity of malignant melanoma tumors via proteasome inhibition: implication for bortezomib (VELCADE, PS-341) as a therapeutic agent for malignant melanoma Cancer Research 64 14 2004 4912 4918 10.1158/0008-5472.CAN-04-0673 15256463

Amor-Gutiérrez O. Costa-Rama E. Arce-Varas N. Martínez-Rodríguez C. Novelli A. Fernández-Sánchez M.T. Costa-García A. Competitive electrochemical immunosensor for the detection of unfolded p53 protein in blood as biomarker for Alzheimer's disease Analytica Chimica Acta 1093 2020 28 34 10.1016/j.aca.2019.09.042 31735212

An W.G. Hwang S.-G. Trepel J.B. Blagosklonny M.V. Protease inhibitor-induced apoptosis: accumulation of wt p53, p21 WAF1/CIP1 , and induction of apoptosis are independent markers of proteasome inhibition Leukemia 14 7 2000 1276 1283 10.1038/sj.leu.2401812 10914553

Anandhan A. Rodriguez-Rocha H. Bohovych I. Griggs A.M. Zavala-Flores L. Reyes-Reyes E.M. … Franco R. Overexpression of alpha-synuclein at non-toxic levels increases dopaminergic cell death induced by copper exposure via modulation of protein degradation pathways Neurobiology of Disease 81 2015 76 92 10.1016/j.nbd.2014.11.018 25497688 PMC4459946

Andersen K.G. Rambaut A. Lipkin W.I. Holmes E.C. Garry R.F. The proximal origin of SARS-CoV-2 Nature Medicine 26 4 2020 450 452 10.1038/s41591-020-0820-9 PMC7095063 32284615

Anisimov S. Takahashi M. Kakihana T. Katsuragi Y. Kitaura H. Zhang L. … Fujii M. G3BP1 inhibits ubiquitinated protein aggregations induced by p62 and USP10 Scientific Reports 9 1 2019 12896 10.1038/s41598-019-46237-1 31501480 PMC6733845

Ao L. Reichel D. Hu D. Jeong H. Kim K.B. Bae Y. Lee W. Polymer micelle formulations of proteasome inhibitor carfilzomib for improved metabolic stability and anticancer efficacy in human multiple myeloma and lung cancer cell lines The Journal of Pharmacology and Experimental Therapeutics 355 2 2015 168 173 10.1124/jpet.115.226993 26311812 PMC4613964

Ao L. Wu Y. Kim D. Jang E.R. Kim K. Lee D.-M. … Lee W. Development of peptide-based reversing agents for p-glycoprotein-mediated resistance to carfilzomib Molecular Pharmaceutics 9 8 2012 2197 2205 10.1021/mp300044b 22734651 PMC3473138

Aoki T. Nishiyama T. Imahashi N. Kitamura K. Efficacy of continuous, daily, oral, ultra-low-dose 200 mg acyclovir to prevent herpes zoster events among bortezomib-treated patients: A report from retrospective study Japanese Journal of Clinical Oncology 41 7 2011 876 881 10.1093/jjco/hyr063 21616919

Arastu-Kapur S. Anderl J.L. Kraus M. Parlati F. Shenk K.D. Lee S.J. … Kirk C.J. Nonproteasomal targets of the proteasome inhibitors bortezomib and carfilzomib: A link to clinical adverse events Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 17 9 2011 2734 2743 10.1158/1078-0432.CCR-10-1950 21364033

Arba, M., Nur-Hidayat, A., Ruslin, Yusuf, M., Sumarlin, Hertadi, R., Wahyudi, S. T., Surantaadmaja, S. I., & Tjahjono, D. H. (2018). Molecular modeling on porphyrin derivatives as β5 subunit inhibitor of 20S proteasome. Computational Biology and Chemistry, 74, 230–238. 10.1016/j.compbiolchem.2018.03.002 . 29674291

Arendt C.S. Hochstrasser M. Eukaryotic 20S proteasome catalytic subunit propeptides prevent active site inactivation by N-terminal acetylation and promote particle assembly The EMBO Journal 18 13 1999 3575 3585 10.1093/emboj/18.13.3575 10393174 PMC1171436

Arlt A. Bauer I. Schafmayer C. Tepel J. Müerköster S.S. Brosch M. … Schäfer H. Increased proteasome subunit protein expression and proteasome activity in colon cancer relate to an enhanced activation of nuclear factor E2-related factor 2 (Nrf2) Oncogene 28 45 2009 3983 3996 10.1038/onc.2009.264 19734940

Armon T. Ganoth D. Hershko A. Assembly of the 26 S complex that degrades proteins ligated to ubiquitin is accompanied by the formation of ATPase activity The Journal of Biological Chemistry 265 34 1990 20723 20726 2174423

Arnsburg K. Kirstein-Miles J. Interrelation between protein synthesis, proteostasis and life span Current Genomics 15 1 2014 66 75 10.2174/1389202915666140210210542 24653664 PMC3958960

Arnulf B. Pylypenko H. Grosicki S. Karamanesht I. Leleu X. van de Velde H. … Moreau P. Updated survival analysis of a randomized phase III study of subcutaneous versus intravenous bortezomib in patients with relapsed multiple myeloma Haematologica 97 12 2012 1925 1928 10.3324/haematol.2012.067793 22689676 PMC3685287

Arrigo A.P. Tanaka K. Goldberg A.L. Welch W.J. Identity of the 19S “prosome” particle with the large multifunctional protease complex of mammalian cells (the proteasome) Nature 331 6152 1988 192 194 10.1038/331192a0 3277060

Asai M. Tsukamoto O. Minamino T. Asanuma H. Fujita M. Asano Y. … Kitakaze M. PKA rapidly enhances proteasome assembly and activity in in vivo canine hearts Journal of Molecular and Cellular Cardiology 46 4 2009 452 462 10.1016/j.yjmcc.2008.11.001 19059265

Asano S. Fukuda Y. Beck F. Aufderheide A. Förster F. Danev R. Baumeister W. Proteasomes. A molecular census of 26S proteasomes in intact neurons Science (New York, N.Y.) 347 6220 2015 439 442 10.1126/science.1261197 25613890

Ashburn T.T. Thor K.B. Drug repositioning: Identifying and developing new uses for existing drugs Nature Reviews Drug Discovery 3 8 2004 673 683 10.1038/nrd1468 15286734

Asher G. Tsvetkov P. Kahana C. Shaul Y. A mechanism of ubiquitin-independent proteasomal degradation of the tumor suppressors p53 and p73 Genes & Development 19 3 2005 316 321 10.1101/gad.319905 15687255 PMC546509

Assaraf Y.G. Brozovic A. Gonçalves A.C. Jurkovicova D. Linē A. Machuqueiro M. … Vasconcelos M.H. The multi-factorial nature of clinical multidrug resistance in cancer Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 46 2019 100645 10.1016/j.drup.2019.100645 31585396

Atkinson S.P. Collin J. Irina N. Anyfantis G. Kyung B.K. Lako M. Armstrong L. A putative role for the immunoproteasome in the maintenance of pluripotency in human embryonic stem cells Stem Cells (Dayton, Ohio) 30 7 2012 1373 1384 10.1002/stem.1113 22532526

Attal M. Lauwers-Cances V. Hulin C. Leleu X. Caillot D. Escoffre M. Arnulf B. Macro M. Belhadj K. Garderet L. Roussel M. Payen C. Mathiot C. Fermand J.P. Meuleman N. Rollet S. Maglio M.E. Zeytoonjian A.A. Weller E.A. … Moreau P. Lenalidomide, bortezomib, and dexamethasone with transplantation for myeloma New England Journal of Medicine 376 14 2017 1311 1320 10.1056/NEJMoa1611750 28379796 PMC6201242

Aubin-Tam M.-E. Olivares A.O. Sauer R.T. Baker T.A. Lang M.J. Single-molecule protein unfolding and translocation by an ATP-fueled proteolytic machine Cell 145 2 2011 257 267 10.1016/j.cell.2011.03.036 21496645 PMC3108460

Avet-Loiseau H. Fonseca R. Siegel D. Dimopoulos M.A. Špička I. Masszi T. Hájek R. Rosiñol L. Goranova-Marinova V. Mihaylov G. Maisnar V. Mateos M.-V. Wang M. Niesvizky R. Oriol A. Jakubowiak A. Minarik J. Palumbo A. Bensinger W. … Moreau P. Carfilzomib significantly improves the progression-free survival of high-risk patients in multiple myeloma Blood 128 9 2016 1174 1180 10.1182/blood-2016-03-707596 27439911 PMC5009511

Baden L.R. Rubin E.J. SARS-Cov-2 — The search for effective therapy New England Journal of Medicine 2020 10.1056/NEJMe2005477 PMC7121446 32187463

Badros A.Z. Vij R. Martin T. Zonder J.A. Kunkel L. Wang Z. … Niesvizky R. Carfilzomib in multiple myeloma patients with renal impairment: Pharmacokinetics and safety Leukemia 27 8 2013 1707 1714 10.1038/leu.2013.29 23364621 PMC3740399

Bajorek M. Finley D. Glickman M.H. Proteasome disassembly and downregulation is correlated with viability during stationary phase Current Biology: CB 13 13 2003 1140 1144 10.1016/s0960-9822(03)00417-2 12842014

Bajorek M. Glickman M.H. Keepers at the final gates: Regulatory complexes and gating of the proteasome channel Cellular and Molecular Life Sciences: CMLS 61 13 2004 1579 1588 10.1007/s00018-004-4131-y 15224182 PMC11138563

Balch W.E. Morimoto R.I. Dillin A. Kelly J.W. Adapting proteostasis for disease intervention Science (New York, N.Y.) 319 5865 2008 916 919 10.1126/science.1141448 18276881

Baldwin A.S. The NF-kappa B and I kappa B proteins: new discoveries and insights Annual Review of Immunology 14 1996 649 683 10.1146/annurev.immunol.14.1.649 8717528

Baldwin A.S. Control of oncogenesis and cancer therapy resistance by the transcription factor NF-kappaB The Journal of Clinical Investigation 107 3 2001 241 246 10.1172/JCI11991 11160144 PMC199203

Banerjee S. Ji C. Mayfield J.E. Goel A. Xiao J. Dixon J.E. Guo X. Ancient drug curcumin impedes 26S proteasome activity by direct inhibition of dual-specificity tyrosine-regulated kinase 2 Proceedings of the National Academy of Sciences of the United States of America 115 32 2018 8155 8160 10.1073/pnas.1806797115 29987021 PMC6094102

Banks P.M. Chan J. Cleary M.L. Delsol G. De Wolf-Peeters C. Gatter K. … Jaffe E.S. Mantle cell lymphoma. A proposal for unification of morphologic, immunologic, and molecular data The American Journal of Surgical Pathology 16 7 1992 637 640 10.1097/00000478-199207000-00001 1530105

Banno A. Garcia D.A. van Baarsel E.D. Metz P.J. Fisch K. Widjaja C.E. … Chang J.T. Downregulation of 26S proteasome catalytic activity promotes epithelial-mesenchymal transition Oncotarget 7 16 2016 21527 21541 10.18632/oncotarget.7596 26930717 PMC5008303

Bard J.A.M. Goodall E.A. Greene E.R. Jonsson E. Dong K.C. Martin A. Structure and function of the 26S proteasome Annual Review of Biochemistry 87 2018 697 724 10.1146/annurev-biochem-062917-011931 PMC6422034 29652515

Barlogie B. Jagannath S. Vesole D.H. Naucke S. Cheson B. Mattox S. … Tricot G. Superiority of tandem autologous transplantation over standard therapy for previously untreated multiple myeloma Blood 89 3 1997 789 793 9028309

Barrault M.-B. Richet N. Godard C. Murciano B. Le Tallec B. Rousseau E. … Peyroche A. Dual functions of the Hsm3 protein in chaperoning and scaffolding regulatory particle subunits during the proteasome assembly Proceedings of the National Academy of Sciences of the United States of America 109 17 2012 E1001 E1010 10.1073/pnas.1116538109 22460800 PMC3340050

Barrio S. Stühmer T. Da-Viá M. Barrio-Garcia C. Lehners N. Besse A. Cuenca I. Garitano-Trojaola A. Fink S. Leich E. Chatterjee M. Driessen C. Martinez-Lopez J. Rosenwald A. Beckmann R. Bargou R.C. Braggio E. Stewart A.K. Raab M.S. … Kortüm K.M. Spectrum and functional validation of PSMB5 mutations in multiple myeloma Leukemia 33 2 2019 447 456 10.1038/s41375-018-0216-8 30026573

Basu Ray S. PNPLA3-I148M: A problem of plenty in non-alcoholic fatty liver disease Adipocyte 8 1 2019 201 208 10.1080/21623945.2019.1607423 31062641 PMC6768214

Baugh J.M. Viktorova E.G. Pilipenko E.V. Proteasomes can degrade a significant proportion of cellular proteins independent of ubiquitination Journal of Molecular Biology 386 3 2009 814 827 10.1016/j.jmb.2008.12.081 19162040 PMC2649715

Baumeister W. Dahlmann B. Hegerl R. Kopp F. Kuehn L. Pfeifer G. Electron microscopy and image analysis of the multicatalytic proteinase FEBS Letters 241 1–2 1988 239 245 10.1016/0014-5793(88)81069-x 2461878

Beaumont V. Zhong S. Lin H. Xu W. Bradaia A. Steidl E. Gleyzes M. Wadel K. Buisson B. Padovan-Neto F.E. Chakroborty S. Ward K.M. Harms J.F. Beltran J. Kwan M. Ghavami A. Häggkvist J. Tóth M. Halldin C. … Munoz-Sanjuan I. Phosphodiesterase 10A inhibition improves cortico-basal ganglia function in Huntington's disease models Neuron 92 6 2016 1220 1237 10.1016/j.neuron.2016.10.064 27916455

Beck F. Unverdorben P. Bohn S. Schweitzer A. Pfeifer G. Sakata E. … Förster F. Near-atomic resolution structural model of the yeast 26S proteasome Proceedings of the National Academy of Sciences of the United States of America 109 37 2012 14870 14875 10.1073/pnas.1213333109 22927375 PMC3443124

Beckwith R. Estrin E. Worden E.J. Martin A. Reconstitution of the 26S proteasome reveals functional asymmetries in its AAA+ unfoldase Nature Structural & Molecular Biology 20 10 2013 1164 1172 10.1038/nsmb.2659 PMC3869383 24013205

Bedford L. Hay D. Devoy A. Paine S. Powe D.G. Seth R. … Mayer R.J. Depletion of 26S proteasomes in mouse brain neurons causes neurodegeneration and Lewy-like inclusions resembling human pale bodies The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 28 33 2008 8189 8198 10.1523/JNEUROSCI.2218-08.2008 18701681 PMC6670564

Beinke S. Ley S.C. Functions of NF-kappaB1 and NF-kappaB2 in immune cell biology The Biochemical Journal 382 2004 393 409 10.1042/BJ20040544 Pt 2 15214841 PMC1133795

Belch A. Kouroukis C.T. Crump M. Sehn L. Gascoyne R.D. Klasa R. … Eisenhauer E.A. A phase II study of bortezomib in mantle cell lymphoma: The National Cancer Institute of Canada Clinical Trials Group trial IND.150 Annals of Oncology: Official Journal of the European Society for Medical Oncology 18 1 2007 116 121 10.1093/annonc/mdl316 16971665

Belenky M.A. Smeraski C.A. Provencio I. Sollars P.J. Pickard G.E. Melanopsin retinal ganglion cells receive bipolar and amacrine cell synapses The Journal of Comparative Neurology 460 3 2003 380 393 10.1002/cne.10652 12692856

Bellia F. Lanza V. García-Viñuales S. Ahmed I.M.M. Pietropaolo A. Iacobucci C. … Milardi D. Ubiquitin binds the amyloid β peptide and interferes with its clearance pathways Chemical Science 10 9 2019 2732 2742 10.1039/c8sc03394c 30996991 PMC6419943

Benbrook D.M. Long A. Integration of autophagy, proteasomal degradation, unfolded protein response and apoptosis Experimental Oncology 34 3 2012 286 297 23070014

Bence N.F. Sampat R.M. Kopito R.R. Impairment of the ubiquitin-proteasome system by protein aggregation Science (New York, N.Y.) 292 5521 2001 1552 1555 10.1126/science.292.5521.1552 11375494

Ben-Nissan G. Sharon M. Regulating the 20S proteasome ubiquitin-independent degradation pathway Biomolecules 4 3 2014 862 884 10.3390/biom4030862 25250704 PMC4192676

Bentea E. Verbruggen L. Massie A. The proteasome inhibition model of Parkinson's disease Journal of Parkinson's Disease 7 1 2017 31 63 10.3233/JPD-160921 PMC5302045 27802243

Bercovich Z. Rosenberg-Hasson Y. Ciechanover A. Kahana C. Degradation of ornithine decarboxylase in reticulocyte lysate is ATP-dependent but ubiquitin-independent Journal of Biological Chemistry 264 27 1989 15949 15952 2550429

Berenson J.R. Cartmell A. Bessudo A. Lyons R.M. Harb W. Tzachanis D. … Berdeja J.G. CHAMPION-1: a phase 1/2 study of once-weekly carfilzomib and dexamethasone for relapsed or refractory multiple myeloma Blood 127 26 2016 3360 3368 10.1182/blood-2015-11-683854 27207788 PMC4929927

Berenson J.R. Matous J. Swift R.A. Mapes R. Morrison B. Yeh H.S. A phase I/II study of arsenic trioxide/bortezomib/ascorbic acid combination therapy for the treatment of relapsed or refractory multiple myeloma Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 13 6 2007 1762 1768 10.1158/1078-0432.CCR-06-1812 17363530

Berisha F. Feke G.T. Trempe C.L. McMeel J.W. Schepens C.L. Retinal abnormalities in early Alzheimer's disease Investigative Ophthalmology & Visual Science 48 5 2007 2285 2289 10.1167/iovs.06-1029 17460292

Bersuker K. Olzmann J.A. Establishing the lipid droplet proteome: Mechanisms of lipid droplet protein targeting and degradation Biochimica Et Biophysica Acta. Molecular and Cell Biology of Lipids 1862 10 Pt B 2017 1166 1177 10.1016/j.bbalip.2017.06.006 28627435 PMC5595636

Besse A. Besse L. Kraus M. Mendez-Lopez M. Bader J. Xin B.-T. … Driessen C. Proteasome Inhibition in Multiple Myeloma: Head-to-Head Comparison of Currently Available Proteasome Inhibitors Cell Chemical Biology 26 3 2019 340 351.e3 10.1016/j.chembiol.2018.11.007 30612952

Besse A. Stolze S.C. Rasche L. Weinhold N. Morgan G.J. Kraus M. … Driessen C. Carfilzomib resistance due to ABCB1/MDR1 overexpression is overcome by nelfinavir and lopinavir in multiple myeloma Leukemia 32 2 2018 391 401 10.1038/leu.2017.212 28676669 PMC5808083

Bhat K.P. Yan S. Wang C.-E. Li S. Li X.-J. Differential ubiquitination and degradation of huntingtin fragments modulated by ubiquitin-protein ligase E3A Proceedings of the National Academy of Sciences of the United States of America 111 15 2014 5706 5711 10.1073/pnas.1402215111 24706802 PMC3992696

Bigelow S. Hough R. Rechsteiner M. The selective degradation of injected proteins occurs principally in the cytosol rather than in lysosomes Cell 25 1 1981 83 93 10.1016/0092-8674(81)90233-6 7273138

Bingol B. Schuman E.M. A proteasome-sensitive connection between PSD-95 and GluR1 endocytosis Neuropharmacology 47 5 2004 755 763 10.1016/j.neuropharm.2004.07.028 15458847

Bingol B. Schuman E.M. Activity-dependent dynamics and sequestration of proteasomes in dendritic spines Nature 441 7097 2006 1144 1148 10.1038/nature04769 16810255

Bingol B. Wang C.-F. Arnott D. Cheng D. Peng J. Sheng M. Autophosphorylated CaMKIIalpha acts as a scaffold to recruit proteasomes to dendritic spines Cell 140 4 2010 567 578 10.1016/j.cell.2010.01.024 20178748

Blasiak J. Pawlowska E. Szczepanska J. Kaarniranta K. Interplay between autophagy and the ubiquitin-proteasome system and its role in the pathogenesis of age-related macular degeneration International Journal of Molecular Sciences 20 1 2019 10.3390/ijms20010210 PMC6337628 30626110

Bochtler M. Ditzel L. Groll M. Hartmann C. Huber R. The proteasome Annual Review of Biophysics and Biomolecular Structure 28 1999 295 317 10.1146/annurev.biophys.28.1.295 10410804

Bogyo M. Shin S. McMaster J.S. Ploegh H.L. Substrate binding and sequence preference of the proteasome revealed by active-site-directed affinity probes Chemistry & Biology 5 6 1998 307 320 10.1016/s1074-5521(98)90169-7 9653549

Bohn S. Sakata E. Beck F. Pathare G.R. Schnitger J. Nágy I. … Förster F. Localization of the regulatory particle subunit Sem1 in the 26S proteasome Biochemical and Biophysical Research Communications 435 2 2013 250 254 10.1016/j.bbrc.2013.04.069 23643786

Boland B. Yu W.H. Corti O. Mollereau B. Henriques A. Bezard E. … Millan M.J. Promoting the clearance of neurotoxic proteins in neurodegenerative disorders of ageing Nature Reviews. Drug Discovery 17 9 2018 660 688 10.1038/nrd.2018.109 30116051 PMC6456907

Bonfili L. Cecarini V. Amici M. Cuccioloni M. Angeletti M. Keller J.N. Eleuteri A.M. Natural polyphenols as proteasome modulators and their role as anti-cancer compounds FEBS Journal 275 22 2008 5512 5526 10.1111/j.1742-4658.2008.06696.x 18959740

Bonnet A. Moreau P. Safety of ixazomib for the treatment of multiple myeloma Expert Opinion on Drug Safety 16 8 2017 973 980 10.1080/14740338.2017.1344212 28661711

Borissenko L. Groll M. 20S proteasome and its inhibitors: Crystallographic knowledge for drug development Chemical Reviews 107 3 2007 687 717 10.1021/cr0502504 17316053

Boselli M. Lee B.-H. Robert J. Prado M.A. Min S.-W. Cheng C. … Finley D. An inhibitor of the proteasomal deubiquitinating enzyme USP14 induces tau elimination in cultured neurons The Journal of Biological Chemistry 292 47 2017 19209 19225 10.1074/jbc.M117.815126 28972160 PMC5702663

Brannigan J.A. Dodson G. Duggleby H.J. Moody P.C. Smith J.L. Tomchick D.R. Murzin A.G. A protein catalytic framework with an N-terminal nucleophile is capable of self-activation Nature 378 6555 1995 416 419 10.1038/378416a0 7477383

Braun T. Carvalho G. Fabre C. Grosjean J. Fenaux P. Kroemer G. Targeting NF-kappaB in hematologic malignancies Cell Death and Differentiation 13 5 2006 748 758 10.1038/sj.cdd.4401874 16498458

Bredesen D.E. Rao R.V. Mehlen P. Cell death in the nervous system Nature 443 7113 2006 796 802 10.1038/nature05293 17051206 PMC3970704

Brehm A. Liu Y. Sheikh A. Marrero B. Omoyinmi E. Zhou Q. Montealegre G. Biancotto A. Reinhardt A. Almeida de Jesus A. Pelletier M. Tsai W.L. Remmers E.F. Kardava L. Hill S. Kim H. Lachmann H.J. Megarbane A. Chae J.J. … Goldbach-Mansky R. Additive loss-of-function proteasome subunit mutations in CANDLE/PRAAS patients promote type I IFN production The Journal of Clinical Investigation 125 11 2015 4196 4211 10.1172/JCI81260 26524591 PMC4639987

Brettschneider J. Del Tredici K. Lee V.M.-Y. Trojanowski J.Q. Spreading of pathology in neurodegenerative diseases: a focus on human studies Nature Reviews. Neuroscience 16 2 2015 109 120 10.1038/nrn3887 25588378 PMC4312418

Bringhen S. De Wit E. Dimopoulos M.-A. New Agents in Multiple Myeloma: An Examination of Safety Profiles Clinical Lymphoma, Myeloma & Leukemia 17 7 2017 391 407.e5 10.1016/j.clml.2017.05.003 28601492

Brown C.J. Lain S. Verma C.S. Fersht A.R. Lane D.P. Awakening guardian angels: drugging the p53 pathway Nature Reviews. Cancer 9 12 2009 862 873 10.1038/nrc2763 19935675

Brugarolas J. Chandrasekaran C. Gordon J.I. Beach D. Jacks T. Hannon G.J. Radiation-induced cell cycle arrest compromised by p21 deficiency Nature 377 6549 1995 552 557 10.1038/377552a0 7566157

Bruggeman J.W. Koster J. Lodder P. Repping S. Hamer G. Massive expression of germ cell-specific genes is a hallmark of cancer and a potential target for novel treatment development Oncogene 37 42 2018 5694 5700 10.1038/s41388-018-0357-2 29907769 PMC6193945

de Bruin G. Xin B.T. Kraus M. van der Stelt M. van der Marel G.A. Kisselev A.F. … Overkleeft H.S. A set of activity-based probes to visualize human (immuno)proteasome activities Angewandte Chemie (International Ed. in. English) 55 13 2016 4199 4203 10.1002/anie.201509092 26511210

Buac D. Shen M. Schmitt S. Kona F.R. Deshmukh R. Zhang Z. … Dou Q.P. From bortezomib to other inhibitors of the proteasome and beyond Current Pharmaceutical Design 19 22 2013 4025 4038 23181572 10.2174/1381612811319220012 PMC3657018

Budenholzer L. Cheng C.L. Li Y. Hochstrasser M. Proteasome structure and assembly Journal of Molecular Biology 429 22 2017 3500 3524 10.1016/j.jmb.2017.05.027 28583440 PMC5675778

Bullova P. Cougnoux A. Marzouca G. Kopacek J. Pacak K. Bortezomib Alone and in Combination With Salinosporamid A Induces Apoptosis and Promotes Pheochromocytoma Cell Death In Vitro and in Female Nude Mice Endocrinology 158 10 2017 3097 3108 10.1210/en.2017-00592 28938421 PMC5659682

Burri L. Höckendorff J. Boehm U. Klamp T. Dohmen R.J. Lévy F. Identification and characterization of a mammalian protein interacting with 20S proteasome precursors Proceedings of the National Academy of Sciences of the United States of America 97 19 2000 10348 10353 10.1073/pnas.190268597 10973495 PMC27027

Bustamante H.A. González A.E. Cerda-Troncoso C. Shaughnessy R. Otth C. Soza A. Burgos P.V. Interplay between the autophagy-lysosomal pathway and the ubiquitin-proteasome system: A target for therapeutic development in alzheimer's disease Frontiers in Cellular Neuroscience 12 2018 10.3389/fncel.2018.00126 Scopus PMC5954036 29867359

Caballero M. Liton P.B. Challa P. Epstein D.L. Gonzalez P. Effects of donor age on proteasome activity and senescence in trabecular meshwork cells Biochemical and Biophysical Research Communications 323 3 2004 1048 1054 10.1016/j.bbrc.2004.08.195 15381105

Caballero M. Liton P.B. Epstein D.L. Gonzalez P. Proteasome inhibition by chronic oxidative stress in human trabecular meshwork cells Biochemical and Biophysical Research Communications 308 2 2003 346 352 10.1016/s0006-291x(03)01385-8 12901875

Calderwood S.K. Khaleque M.A. Sawyer D.B. Ciocca D.R. Heat shock proteins in cancer: Chaperones of tumorigenesis Trends in Biochemical Sciences 31 3 2006 164 172 10.1016/j.tibs.2006.01.006 16483782

Campello L. Esteve-Rudd J. Cuenca N. Martín-Nieto J. The ubiquitin-proteasome system in retinal health and disease Molecular Neurobiology 47 2 2013 790 810 10.1007/s12035-012-8391-5 23339020

Campo E. Rule S. Mantle cell lymphoma: Evolving management strategies Blood 125 1 2015 48 55 10.1182/blood-2014-05-521898 25499451

Cao J. Zhong M.B. Toro C.A. Zhang L. Cai D. Endo-lysosomal pathway and ubiquitin-proteasome system dysfunction in Alzheimer's disease pathogenesis Neuroscience Letters 703 2019 68 78 Scopus 10.1016/j.neulet.2019.03.016 30890471 PMC6760990

Caravita T. de Fabritiis P. Palumbo A. Amadori S. Boccadoro M. Bortezomib: Efficacy comparisons in solid tumors and hematologic malignancies Nature Clinical Practice. Oncology 3 7 2006 374 387 10.1038/ncponc0555 16826218

Cardozo C. Vinitsky A. Michaud C. Orlowski M. Evidence that the nature of amino acid residues in the P3 position directs substrates to distinct catalytic sites of the pituitary multicatalytic proteinase complex (proteasome) Biochemistry 33 21 1994 6483 6489 10.1021/bi00187a014 8204582

Carmony K.C. Lee W. Kim K.B. High-resolution snapshots of proteasome inhibitors in action revise inhibition paradigms and inspire next-generation inhibitor design Chembiochem: A European Journal of Chemical Biology. 2016 10.1002/cbic.201600488 PMC5192039 27605113

Carnevale C. Manni G. Roberti G. Micera A. Bruno L. Cacciamani A. … Oddone F. Human vitreous concentrations of citicoline following topical application of citicoline 2% ophthalmic solution PLoS One 14 11 2019 e0224982 10.1371/journal.pone.0224982 PMC6855484 31725734

Carrell R.W. Lomas D.A. Conformational disease The Lancet 350 9071 1997 134 138 10.1016/S0140-6736(97)02073-4 9228977

Carvalho A.S. Rodríguez M.S. Matthiesen R. Review and literature mining on proteostasis factors and cancer Methods in Molecular Biology (Clifton, N.J.) 1449 2016 71 84 10.1007/978-1-4939-3756-1_2 27613028

Cascio P. PA28αβ: The enigmatic magic ring of the proteasome? Biomolecules 4 2 2014 566 584 10.3390/biom4020566 24970231 PMC4101498

Cascio P. Call M. Petre B.M. Walz T. Goldberg A.L. Properties of the hybrid form of the 26S proteasome containing both 19S and PA28 complexes The EMBO Journal 21 11 2002 2636 2645 10.1093/emboj/21.11.2636 12032076 PMC126027

Cascio P. Hilton C. Kisselev A.F. Rock K.L. Goldberg A.L. 26S proteasomes and immunoproteasomes produce mainly N-extended versions of an antigenic peptide The EMBO Journal 20 10 2001 2357 2366 10.1093/emboj/20.10.2357 11350924 PMC125470

Cavo M. Tacchetti P. Patriarca F. Petrucci M.T. Pantani L. Galli M. Di Raimondo F. Crippa C. Zamagni E. Palumbo A. Offidani M. Corradini P. Narni F. Spadano A. Pescosta N. Deliliers G.L. Ledda A. Cellini C. Caravita T. … GIMEMA Italian Myeloma Network Bortezomib with thalidomide plus dexamethasone compared with thalidomide plus dexamethasone as induction therapy before, and consolidation therapy after, double autologous stem-cell transplantation in newly diagnosed multiple myeloma: A randomised phase 3 study Lancet (London, England) 376 9758 2010 2075 2085 10.1016/S0140-6736(10)61424-9 21146205

Cecarini V. Bonfili L. Amici M. Angeletti M. Keller J.N. Eleuteri A.M. Amyloid peptides in different assembly states and related effects on isolated and cellular proteasomes Brain Research 2008 1209 10.1016/j.brainres.2008.03.003 18400214

Cengiz Seval G. Beksac M. The safety of bortezomib for the treatment of multiple myeloma Expert Opinion on Drug Safety 17 9 2018 953 962 10.1080/14740338.2018.1513487 30118610

Chahrour M. Jung S.Y. Shaw C. Zhou X. Wong S.T.C. Qin J. Zoghbi H.Y. MeCP2, a key contributor to neurological disease, activates and represses transcription Science (New York, N.Y.) 320 5880 2008 1224 1229 10.1126/science.1153252 PMC2443785 18511691

Chakraborty J. von Stockum S. Marchesan E. Caicci F. Ferrari V. Rakovic A. … Ziviani E. USP14 inhibition corrects an in vivo model of impaired mitophagy EMBO Molecular Medicine 10 11 2018 e9014 10.15252/emmm.201809014 PMC6220287 30249595

Chanan-Khan A. Sonneveld P. Schuster M.W. Stadtmauer E.A. Facon T. Harousseau J.-L. … Richardson P.G. Analysis of herpes zoster events among bortezomib-treated patients in the phase III APEX study Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 26 29 2008 4784 4790 10.1200/JCO.2007.14.9641 18711175

Chang S.C. Ding J.L. Ubiquitination and SUMOylation in the chronic inflammatory tumor microenvironment Biochimica Et Biophysica Acta. Reviews on Cancer 1870 2 2018 165 175 10.1016/j.bbcan.2018.08.002 30318471

Chari A. Hajje D. Case series discussion of cardiac and vascular events following carfilzomib treatment: Possible mechanism, screening, and monitoring BMC Cancer 14 2014 915 10.1186/1471-2407-14-915 25471129 PMC4289164

Chaugule V.K. Burchell L. Barber K.R. Sidhu A. Leslie S.J. Shaw G.S. Walden H. Autoregulation of Parkin activity through its ubiquitin-like domain The EMBO Journal 30 14 2011 2853 2867 10.1038/emboj.2011.204 21694720 PMC3160258

Chauhan D. Catley L. Li G. Podar K. Hideshima T. Velankar M. … Anderson K.C. A novel orally active proteasome inhibitor induces apoptosis in multiple myeloma cells with mechanisms distinct from bortezomib Cancer Cell 8 5 2005 407 419 10.1016/j.ccr.2005.10.013 16286248

Chauhan D. Hideshima T. Anderson K.C. Proteasome inhibition in multiple myeloma: therapeutic implication Annual Review of Pharmacology and Toxicology 45 2005 465 476 10.1146/annurev.pharmtox.45.120403.100037 15822185

Chauhan D. Singh A.V. Aujay M. Kirk C.J. Bandi M. Ciccarelli B. … Anderson K.C. A novel orally active proteasome inhibitor ONX 0912 triggers in vitro and in vivo cytotoxicity in multiple myeloma Blood 116 23 2010 4906 4915 10.1182/blood-2010-04-276626 20805366 PMC3321748

Chauhan D. Singh A.V. Ciccarelli B. Richardson P.G. Palladino M.A. Anderson K.C. Combination of novel proteasome inhibitor NPI-0052 and lenalidomide trigger in vitro and in vivo synergistic cytotoxicity in multiple myeloma Blood 115 4 2010 834 845 10.1182/blood-2009-03-213009 19965674 PMC2815518

Chauhan D. Tian Z. Zhou B. Kuhn D. Orlowski R. Raje N. … Anderson K.C. In vitro and in vivo selective antitumor activity of a novel orally bioavailable proteasome inhibitor MLN9708 against multiple myeloma cells Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 17 16 2011 5311 5321 10.1158/1078-0432.CCR-11-0476 21724551 PMC3156932

Chen X. Barton L.F. Chi Y. Clurman B.E. Roberts J.M. Ubiquitin-independent degradation of cell-cycle inhibitors by the REGgamma proteasome Molecular Cell 26 6 2007 843 852 10.1016/j.molcel.2007.05.022 17588519 PMC2031223

Chen P.-C. Bhattacharyya B.J. Hanna J. Minkel H. Wilson J.A. Finley D. … Wilson S.M. Ubiquitin homeostasis is critical for synaptic development and function The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 31 48 2011 17505 17513 10.1523/JNEUROSCI.2922-11.2011 22131412 PMC3253363

Chen D. Cui Q.C. Yang H. Dou Q.P. Disulfiram, a clinically used anti-alcoholism drug and copper-binding agent, induces apoptotic cell death in breast cancer cultures and xenografts via inhibition of the proteasome activity Cancer Research 66 21 2006 10425 10433 Scopus 10.1158/0008-5472.CAN-06-2126 17079463

Chen D. Daniel K.J. Chen M.S. Kuhn D.J. Landis-Piwowar K.R. Dou Q.P. Dietary flavonoids as proteasome inhibitors and apoptosis inducers in human leukemia cells Biochemical Pharmacology 69 10 2005 1421 1432 15857606 10.1016/j.bcp.2005.02.022

Chen D. Frezza M. Schmitt S. Kanwar J. Dou Q.P. Bortezomib as the first proteasome inhibitor anticancer drug: current status and future perspectives Current Cancer Drug Targets 11 3 2011 239 253 10.2174/156800911794519752 21247388 PMC3306611

Chen Z. Hagler J. Palombella V.J. Melandri F. Scherer D. Ballard D. Maniatis T. Signal-induced site-specific phosphorylation targets I kappa B alpha to the ubiquitin-proteasome pathway Genes & Development 9 13 1995 1586 1597 10.1101/gad.9.13.1586 7628694

Chen P. Hochstrasser M. Biogenesis, structure and function of the yeast 20S proteasome The EMBO Journal 14 11 1995 2620 2630 7781614 10.1002/j.1460-2075.1995.tb07260.x PMC398376

Chen P. Hochstrasser M. Autocatalytic subunit processing couples active site formation in the 20S proteasome to completion of assembly Cell 86 6 1996 961 972 10.1016/s0092-8674(00)80171-3 8808631

Chen L. Madura K. Increased proteasome activity, ubiquitin-conjugating enzymes, and eEF1A translation factor detected in breast cancer tissue Cancer Research 65 13 2005 5599 5606 10.1158/0008-5472.CAN-05-0201 15994932

Chen B. Retzlaff M. Roos T. Frydman J. Cellular strategies of protein quality control Cold Spring Harbor Perspectives in Biology 3 8 2011 a004374 10.1101/cshperspect.a004374 21746797 PMC3140689

Chen Y.-J. Wu H. Shen X.-Z. The ubiquitin–proteasome system and its potential application in hepatocellular carcinoma therapy Cancer Letters 379 2 2016 245 252 10.1016/j.canlet.2015.06.023 26193663

Chène P. Inhibiting the p53-MDM2 interaction: an important target for cancer therapy Nature Reviews. Cancer 3 2 2003 102 109 10.1038/nrc991 12563309

Cheungpasitporn W. Leung N. Rajkumar S.V. Cornell L.D. Sethi S. Angioi A. Fervenza F.C. Bortezomib-induced acute interstitial nephritis Nephrology, Dialysis, Transplantation: Official Publication of the European Dialysis and Transplant Association - European Renal Association 30 7 2015 1225 1229 10.1093/ndt/gfv222 26109684

Chiti F. Dobson C.M. Protein misfolding, functional amyloid, and human disease Annual Review of Biochemistry 75 1 2006 333 366 10.1146/annurev.biochem.75.101304.123901 16756495

Chiti F. Dobson C.M. Protein Misfolding, Amyloid Formation, and Human Disease: A Summary of Progress Over the Last Decade Annual Review of Biochemistry 86 2017 27 68 10.1146/annurev-biochem-061516-045115 28498720

Chitta K. Paulus A. Akhtar S. Blake M.K.K. Caulfield T.R. Novak A.J. … Chanan-Khan A. Targeted inhibition of the deubiquitinating enzymes, USP14 and UCHL5, induces proteotoxic stress and apoptosis in Waldenström macroglobulinaemia tumour cells British Journal of Haematology 169 3 2015 377 390 10.1111/bjh.13304 25691154 PMC4846423

Choi W.H. de Poot S.A.H. Lee J.H. Kim J.H. Han D.H. Kim Y.K. … Lee M.J. Open-gate mutants of the mammalian proteasome show enhanced ubiquitin-conjugate degradation Nature Communications 7 2016 10963 10.1038/ncomms10963 PMC4786872 26957043

Choi J.-H. Jo H.S. Lim S. Kim H.-T. Lee K.W. Moon K.H. … Kim J.W. mTORC1 accelerates retinal development via the immunoproteasome Nature Communications 9 1 2018 2502 10.1038/s41467-018-04774-9 PMC6021445 29950673

Chondrogianni N. Gonos E.S. Proteasome inhibition induces a senescence-like phenotype in primary human fibroblasts cultures Biogerontology 5 1 2004 55 61 10.1023/B:BGEN.0000017687.55667.42 15138382

Chondrogianni N. Gonos E.S. Overexpression of hUMP1/POMP proteasome accessory protein enhances proteasome-mediated antioxidant defence Experimental Gerontology 42 9 2007 899 903 10.1016/j.exger.2007.01.012 17349762

Chondrogianni N. Gonos E.S. Proteasome function determines cellular homeostasis and the rate of aging Tavernarakis N. Protein Metabolism and Homeostasis in Aging 2010 Springer US 38 46 10.1007/978-1-4419-7002-2_4 20886755

Chondrogianni N. Kapeta S. Chinou I. Vassilatou K. Papassideri I. Gonos E.S. Anti-ageing and rejuvenating effects of quercetin Experimental Gerontology 45 10 2010 763 771 10.1016/j.exger.2010.07.001 20619334

Chu I.M. Hengst L. Slingerland J.M. The Cdk inhibitor p27 in human cancer: prognostic potential and relevance to anticancer therapy Nature Reviews Cancer 8 4 2008 253 267 10.1038/nrc2347 18354415

Ciechanover A. Proteolysis: From the lysosome to ubiquitin and the proteasome Nature Reviews. Molecular Cell Biology 6 1 2005 79 87 10.1038/nrm1552 15688069

Ciechanover A. Intracellular protein degradation: From a vague idea through the lysosome and the ubiquitin-proteasome system and onto human diseases and drug targeting Bioorganic & Medicinal Chemistry 21 12 2013 3400 3410 10.1016/j.bmc.2013.01.056 23485445

Ciechanover A. Brundin P. The ubiquitin proteasome system in neurodegenerative diseases: Sometimes the chicken, sometimes the egg Neuron 40 2 2003 427 446 10.1016/s0896-6273(03)00606-8 14556719

Ciechanover A. Finley D. Varshavsky A. Ubiquitin dependence of selective protein degradation demonstrated in the mammalian cell cycle mutant ts85 Cell 37 1 1984 57 66 10.1016/0092-8674(84)90300-3 6327060

Ciechanover A. Gonen H. Bercovich B. Cohen S. Fajerman I. Israël A. … Orian A. Mechanisms of ubiquitin-mediated, limited processing of the NF-kappaB1 precursor protein p105 Biochimie 83 3–4 2001 341 349 10.1016/s0300-9084(01)01239-1 11295495

Ciechanover A. Heller H. Elias S. Haas A.L. Hershko A. ATP-dependent conjugation of reticulocyte proteins with the polypeptide required for protein degradation Proceedings of the National Academy of Sciences of the United States of America 77 3 1980 1365 1368 10.1073/pnas.77.3.1365 6769112 PMC348495

Ciechanover A. Hod Y. Hershko A. A heat-stable polypeptide component of an ATP-dependent proteolytic system from reticulocytes. 1978 Biochemical and Biophysical Research Communications 425 3 2012 565 570 10.1016/j.bbrc.2012.08.025 22925675

Ciechanover A. Kwon Y.T. Protein quality control by molecular chaperones in neurodegeneration Frontiers in Neuroscience 11 2017 185 10.3389/fnins.2017.00185 28428740 PMC5382173

Ciehanover A. Hod Y. Hershko A. A heat-stable polypeptide component of an ATP-dependent proteolytic system from reticulocytes Biochemical and Biophysical Research Communications 81 4 1978 1100 1105 10.1016/0006-291x(78)91249-4 666810

Cliffe R. Sang J.C. Kundel F. Finley D. Klenerman D. Ye Y. Filamentous aggregates are fragmented by the proteasome holoenzyme Cell Reports 26 8 2019 2140 2149 e3 10.1016/j.celrep.2019.01.096 30784595 PMC6381791

Cloos J. Roeten M.S. Franke N.E. van Meerloo J. Zweegman S. Kaspers G.J. Jansen G. (Immuno)proteasomes as therapeutic target in acute leukemia Cancer Metastasis Reviews 36 4 2017 599 615 10.1007/s10555-017-9699-4 29071527 PMC5721123

Coelho S.C. Almeida G.M. Santos-Silva F. Pereira M.C. Coelho M.A.N. Enhancing the efficiency of bortezomib conjugated to pegylated gold nanoparticles: An in vitro study on human pancreatic cancer cells and adenocarcinoma human lung alveolar basal epithelial cells Expert Opinion on Drug Delivery 13 8 2016 1075 1081 10.1080/17425247.2016.1178234 27087021

Coleman R.A. Muli C.S. Zhao Y. Bhardwaj A. Newhouse T.R. Trader D.J. Analysis of chain length, substitution patterns, and unsaturation of AM-404 derivatives as 20S proteasome stimulators Bioorganic and Medicinal Chemistry Letters 29 3 2019 420 423 10.1016/j.bmcl.2018.12.030 30587447 PMC6348054

Colledge M. Snyder E.M. Crozier R.A. Soderling J.A. Jin Y. Langeberg L.K. … Scott J.D. Ubiquitination regulates PSD-95 degradation and AMPA receptor surface expression Neuron 40 3 2003 595 607 10.1016/s0896-6273(03)00687-1 14642282 PMC3963808

Collins G.A. Goldberg A.L. The logic of the 26S proteasome Cell 169 5 2017 792 806 10.1016/j.cell.2017.04.023 28525752 PMC5609836

Comer F.I. Hart G.W. O-glycosylation of nuclear and cytosolic proteins dynamic interplay between O-GlcNAc ando-phosphate Journal of Biological Chemistry 275 38 2000 29179 29182 10.1074/jbc.R000010200 10924527

Concannon C.G. Koehler B.F. Reimertz C. Murphy B.M. Bonner C. Thurow N. … Prehn J.H.M. Apoptosis induced by proteasome inhibition in cancer cells: predominant role of the p53/PUMA pathway Oncogene 26 12 2007 1681 1692 10.1038/sj.onc.1209974 16983338

Corpas R. Griñán-Ferré C. Rodríguez-Farré E. Pallàs M. Sanfeliu C. Resveratrol induces brain resilience against Alzheimer neurodegeneration through proteostasis enhancement Molecular Neurobiology 56 2 2019 1502 1516 10.1007/s12035-018-1157-y 29948950

Coughlin K. Anchoori R. Iizuka Y. Meints J. MacNeill L. Vogel R.I. … Bazzaro M. Small-molecule RA-9 inhibits proteasome-associated DUBs and ovarian cancer in vitro and in vivo via exacerbating unfolded protein responses Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 20 12 2014 3174 3186 10.1158/1078-0432.CCR-13-2658 24727327 PMC4269153

Coux O. Tanaka K. Goldberg A.L. Structure and functions of the 20S and 26S proteasomes Annual Review of Biochemistry 65 1996 801 847 10.1146/annurev.bi.65.070196.004101 8811196

Cree I.A. Charlton P. Molecular chess? Hallmarks of anti-cancer drug resistance BMC Cancer 17 1 2017 10 10.1186/s12885-016-2999-1 PMC5214767 28056859

Cripps D. Thomas S.N. Jeng Y. Yang F. Davies P. Yang A.J. Alzheimer disease-specific conformation of hyperphosphorylated paired helical filament-Tau is polyubiquitinated through Lys-48, Lys-11, and Lys-6 ubiquitin conjugation The Journal of Biological Chemistry 281 16 2006 10825 10838 10.1074/jbc.M512786200 16443603

Cron K.R. Zhu K. Kushwaha D.S. Hsieh G. Merzon D. Rameseder J. … Kozono D. Proteasome inhibitors block DNA repair and radiosensitize non-small cell lung cancer PLoS One 8 9 2013 e73710 10.1371/journal.pone.0073710 PMC3764058 24040035

Cuanalo-Contreras K. Moreno-Gonzalez I. Natural products as modulators of the proteostasis machinery: Implications in neurodegenerative diseases International Journal of Molecular Sciences 20 19 2019 10.3390/ijms20194666 PMC6801507 31547084

Cundiff M.D. Hurley C.M. Wong J.D. Boscia J.A. Bashyal A. Rosenberg J. … Kraut D.A. Ubiquitin receptors are required for substrate-mediated activation of the proteasome's unfolding ability Scientific Reports 9 1 2019 14506 10.1038/s41598-019-50857-y 31601863 PMC6787058

Dächsel J.C. Lücking C.B. Deeg S. Schultz E. Lalowski M. Casademunt E. … Gasser T. Parkin interacts with the proteasome subunit alpha4 FEBS Letters 579 18 2005 3913 3919 10.1016/j.febslet.2005.06.003 15987638

Dahlmann B. Mammalian proteasome subtypes: Their diversity in structure and function Archives of Biochemistry and Biophysics 591 2016 132 140 10.1016/j.abb.2015.12.012 26724758

Dahlmann B. Becher B. Sobek A. Ehlers C. Kopp F. Kuehn L. In vitro activation of the 20S proteasome Enzyme & Protein 47 4–6 1993 274 284 10.1159/000468685 7697125

Dahlmann B. Rutschmann M. Kuehn L. Reinauer H. Activation of the multicatalytic proteinase from rat skeletal muscle by fatty acids or sodium dodecyl sulphate Biochemical Journal 228 1 1985 171 177 10.1042/bj2280171 3890840 PMC1144966

Dahlqvist J. Klar J. Tiwari N. Schuster J. Törmä H. Badhai J. … Dahl N. A single-nucleotide deletion in the POMP 5’ UTR causes a transcriptional switch and altered epidermal proteasome distribution in KLICK genodermatosis American Journal of Human Genetics 86 4 2010 596 603 10.1016/j.ajhg.2010.02.018 20226437 PMC2850438

Dahlqvist J. Törmä H. Badhai J. Dahl N. siRNA silencing of proteasome maturation protein (POMP) activates the unfolded protein response and constitutes a model for KLICK genodermatosis PLoS One 7 1 2012 e29471 10.1371/journal.pone.0029471 22235297 PMC3250448

Dai C. Dai S. Cao J. Proteotoxic stress of cancer: Implication of the heat-shock response in oncogenesis Journal of Cellular Physiology 227 8 2012 2982 2987 10.1002/jcp.24017 22105155 PMC3431153

Dal Vechio F.H. Cerqueira F. Augusto O. Lopes R. Demasi M. Peptides that activate the 20S proteasome by gate opening increased oxidized protein removal and reduced protein aggregation Free Radical Biology & Medicine 67 2014 304 313 10.1016/j.freeradbiomed.2013.11.017 24291399

Dambacher C.M. Worden E.J. Herzik M.A. Martin A. Lander G.C. Atomic structure of the 26S proteasome lid reveals the mechanism of deubiquitinase inhibition ELife 5 2016 e13027 10.7554/eLife.13027 PMC4749569 26744777

Daniel K.G. Gupta P. Harbach R.H. Guida W.C. Dou Q.P. Organic copper complexes as a new class of proteasome inhibitors and apoptosis inducers in human cancer cells Biochemical Pharmacology 67 6 2004 1139 1151 10.1016/j.bcp.2003.10.031 15006550

Dantuma N.P. Bott L.C. The ubiquitin-proteasome system in neurodegenerative diseases: Precipitating factor, yet part of the solution Frontiers in Molecular Neuroscience 7 2014 70 10.3389/fnmol.2014.00070 25132814 PMC4117186

Das D.S. Ray A. Song Y. Richardson P. Trikha M. Chauhan D. Anderson K.C. Synergistic anti-myeloma activity of the proteasome inhibitor marizomib and the IMiD® immunomodulatory drug pomalidomide British Journal of Haematology 171 5 2015 798 812 10.1111/bjh.13780 26456076 PMC4715649

D'Arcy P. Wang X. Linder S. Deubiquitinase inhibition as a cancer therapeutic strategy Pharmacology & Therapeutics 147 2015 32 54 10.1016/j.pharmthera.2014.11.002 25444757

Dasgupta S. Fishman M.A. Mahallati H. Castro L.M. Tashima A.K. Ferro E.S. Fricker L.D. Reduced levels of proteasome products in a mouse striatal cell model of Huntington's disease PLoS One 10 12 2015 e0145333 10.1371/journal.pone.0145333 PMC4686214 26691307

Di Dato A.D. Cunsolo A. Persico M. Santoro A.M. D'Urso A. Milardi D. … Coletta M. Electrostatic map of proteasome α-rings encodes the design of allosteric porphyrin-based inhibitors able to affect 20S conformation by cooperative binding Scientific Reports 7 1 2017 17098 10.1038/s41598-017-17008-7 29213119 PMC5719074

David D.C. Layfield R. Serpell L. Narain Y. Goedert M. Spillantini M.G. Proteasomal degradation of tau protein Journal of Neurochemistry 83 1 2002 176 185 10.1046/j.1471-4159.2002.01137.x 12358741

Davies K.J. Protein modification by oxidants and the role of proteolytic enzymes Biochemical Society Transactions 21 2 1993 346 353 10.1042/bst0210346 8359494

Davies K.J. Degradation of oxidized proteins by the 20S proteasome Biochimie 83 3–4 2001 301 310 10.1016/s0300-9084(01)01250-0 11295490

De Duve C. Gianetto R. Appelmans F. Wattiaux R. Enzymic content of the mitochondria fraction Nature 172 4390 1953 1143 1144 10.1038/1721143a0 13111270

De Duve C. Wattiaux R. Functions of lysosomes Annual Review of Physiology 28 1966 435 492 10.1146/annurev.ph.28.030166.002251 5322983

De La Mota-Peynado A. Lee S.Y.-C. Pierce B.M. Wani P. Singh C.R. Roelofs J. The proteasome-associated protein Ecm29 inhibits proteasomal ATPase activity and in vivo protein degradation by the proteasome The Journal of Biological Chemistry 288 41 2013 29467 29481 10.1074/jbc.M113.491662 23995839 PMC3795246

Dehabadi M.H. Davis B.M. Wong T.K. Cordeiro M.F. Retinal manifestations of Alzheimer's disease Neurodegenerative Disease Management 4 3 2014 241 252 10.2217/nmt.14.19 25095818

Demo S.D. Kirk C.J. Aujay M.A. Buchholz T.J. Dajee M. Ho M.N. … Bennett M.K. Antitumor activity of PR-171, a novel irreversible inhibitor of the proteasome Cancer Research 67 13 2007 6383 6391 10.1158/0008-5472.CAN-06-4086 17616698

Deng T. Yan G. Song X. Xie L. Zhou Y. Li J. … Ye M. Deubiquitylation and stabilization of p21 by USP11 is critical for cell-cycle progression and DNA damage responses Proceedings of the National Academy of Sciences 115 18 2018 4678 4683 10.1073/pnas.1714938115 PMC5939064 29666278

Deng C. Zhang P. Harper J.W. Elledge S.J. Leder P. Mice lacking p21CIP1/WAF1 undergo normal development, but are defective in G1 checkpoint control Cell 82 4 1995 675 684 10.1016/0092-8674(95)90039-x 7664346

Deriziotis P. André R. Smith D.M. Goold R. Kinghorn K.J. Kristiansen M. … Tabrizi S.J. Misfolded PrP impairs the UPS by interaction with the 20S proteasome and inhibition of substrate entry The EMBO Journal 30 15 2011 3065 3077 10.1038/emboj.2011.224 21743439 PMC3160194

Deschênes-Simard X. Lessard F. Gaumont-Leclerc M.-F. Bardeesy N. Ferbeyre G. Cellular senescence and protein degradation Cell Cycle 13 12 2014 1840 1858 10.4161/cc.29335 24866342 PMC4111748

Deshaies R.J. Proteotoxic crisis, the ubiquitin-proteasome system, and cancer therapy BMC Biology 12 2014 94 10.1186/s12915-014-0094-0 25385277 PMC4226866

Devaraj S.G. Wang N. Chen Z. Chen Z. Tseng M. Barretto N. … Li K. Regulation of IRF-3-dependent innate immunity by the papain-like protease domain of the severe acute respiratory syndrome coronavirus Journal of Biological Chemistry 282 44 2007 32208 32221 10.1074/jbc.M704870200 17761676 PMC2756044

Deveraux Q. Ustrell V. Pickart C. Rechsteiner M. A 26 S protease subunit that binds ubiquitin conjugates The Journal of Biological Chemistry 269 10 1994 7059 7061 8125911

Devine T. Dai M.-S. Targeting the ubiquitin-mediated proteasome degradation of p53 for cancer therapy Current Pharmaceutical Design 19 18 2013 3248 3262 10.2174/1381612811319180009 23151129 PMC3637405

Di K. Lloyd G.K. Abraham V. MacLaren A. Burrows F.J. Desjardins A. … Bota D.A. Marizomib activity as a single agent in malignant gliomas: Ability to cross the blood-brain barrier Neuro-Oncology 18 6 2016 840 848 10.1093/neuonc/nov299 26681765 PMC4864261

Díaz-Hernández M. Valera A.G. Morán M.A. Gómez-Ramos P. Alvarez-Castelao B. Castaño J.G. … Lucas J.J. Inhibition of 26S proteasome activity by huntingtin filaments but not inclusion bodies isolated from mouse and human brain Journal of Neurochemistry 98 5 2006 1585 1596 10.1111/j.1471-4159.2006.03968.x 16787406

Dick T.P. Nussbaum A.K. Deeg M. Heinemeyer W. Groll M. Schirle M. … Schild H. Contribution of proteasomal beta-subunits to the cleavage of peptide substrates analyzed with yeast mutants The Journal of Biological Chemistry 273 40 1998 25637 25646 10.1074/jbc.273.40.25637 9748229

Dickey C.A. Kamal A. Lundgren K. Klosak N. Bailey R.M. Dunmore J. … Petrucelli L. The high-affinity HSP90-CHIP complex recognizes and selectively degrades phosphorylated tau client proteins The Journal of Clinical Investigation 117 3 2007 648 658 10.1172/JCI29715 17304350 PMC1794119

Diehl J.A. Ponugoti B. Ubiquitin-dependent proteolysis in G1/S phase control and its relationship with tumor susceptibility Genes & Cancer 1 7 2010 717 724 10.1177/1947601910382902 21113395 PMC2991141

DiFiglia M. Sapp E. Chase K.O. Davies S.W. Bates G.P. Vonsattel J.P. Aronin N. Aggregation of huntingtin in neuronal intranuclear inclusions and dystrophic neurites in brain Science (New York, N.Y.) 277 5334 1997 1990 1993 10.1126/science.277.5334.1990 9302293

Dimopoulos M.A. Goldschmidt H. Niesvizky R. Joshua D. Chng W.-J. Oriol A. … Moreau P. Carfilzomib or bortezomib in relapsed or refractory multiple myeloma (ENDEAVOR): An interim overall survival analysis of an open-label, randomised, phase 3 trial The Lancet. Oncology 18 10 2017 1327 1337 10.1016/S1470-2045(17)30578-8 28843768

Dimopoulos M.A. Moreau P. Palumbo A. Joshua D. Pour L. Hájek R. Facon T. Ludwig H. Oriol A. Goldschmidt H. Rosiñol L. Straub J. Suvorov A. Araujo C. Rimashevskaya E. Pika T. Gaidano G. Weisel K. Goranova-Marinova V. … Investigators, E.N.D.E.A.V.O.R Carfilzomib and dexamethasone versus bortezomib and dexamethasone for patients with relapsed or refractory multiple myeloma (ENDEAVOR): A randomised, phase 3, open-label, multicentre study The Lancet. Oncology 17 1 2016 27 38 10.1016/S1470-2045(15)00464-7 26671818

Dimopoulos M.A. Roussou M. Gavriatopoulou M. Psimenou E. Ziogas D. Eleutherakis-Papaiakovou E. … Kastritis E. Cardiac and renal complications of carfilzomib in patients with multiple myeloma Blood Advances 1 7 2017 449 454 10.1182/bloodadvances.2016003269 29296960 PMC5738981

Dimopoulos M. Siegel D. White D.J. Boccia R. Iskander K.S. Yang Z. … Niesvizky R. Carfilzomib vs bortezomib in patients with multiple myeloma and renal failure: A subgroup analysis of ENDEAVOR Blood 133 2 2019 147 155 10.1182/blood-2018-06-860015 30478094 PMC6346374

Ditzel L. Huber R. Mann K. Heinemeyer W. Wolf D.H. Groll M. Conformational constraints for protein self-cleavage in the proteasome Journal of Molecular Biology 279 5 1998 1187 1191 10.1006/jmbi.1998.1818 9642094

Djakovic S.N. Marquez-Lona E.M. Jakawich S.K. Wright R. Chu C. Sutton M.A. Patrick G.N. Phosphorylation of Rpt6 regulates synaptic strength in hippocampal neurons The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 32 15 2012 5126 5131 10.1523/JNEUROSCI.4427-11.2012 22496558 PMC3348785

Djakovic S.N. Schwarz L.A. Barylko B. DeMartino G.N. Patrick G.N. Regulation of the proteasome by neuronal activity and calcium/calmodulin-dependent protein kinase II The Journal of Biological Chemistry 284 39 2009 26655 26665 10.1074/jbc.M109.021956 19638347 PMC2785353

Djuranovic S. Hartmann M.D. Habeck M. Ursinus A. Zwickl P. Martin J. … Zeth K. Structure and activity of the N-terminal substrate recognition domains in proteasomal ATPases Molecular Cell 34 5 2009 580 590 10.1016/j.molcel.2009.04.030 19481487

Dong Z. Cui H. The autophagy-lysosomal pathways and their emerging roles in modulating proteostasis in tumors Cells 8 1 2018 10.3390/cells8010004 PMC6356230 30577555

Dong Y. Zhang S. Wu Z. Li X. Wang W.L. Zhu Y. … Mao Y. Cryo-EM structures and dynamics of substrate-engaged human 26S proteasome Nature 565 7737 2019 49 55 10.1038/s41586-018-0736-4 30479383 PMC6370054

Doorduijn J.K. Zijlstra J.M. Lugtenburg P.J. Kersten M.J. Böhmer L.H. Minnema M.C. … Kluin-Nelemans H.C. Bortezomib maintenance after R-CHOP, cytarabine and autologous stem cell transplantation in newly diagnosed patients with mantle cell lymphoma, results of a randomised phase II HOVON trial British Journal of Haematology 2020 10.1111/bjh.16567 PMC7496560 32150297

Dorsey B.D. Iqbal M. Chatterjee S. Menta E. Bernardini R. Bernareggi A. … Mallamo J.P. Discovery of a potent, selective, and orally active proteasome inhibitor for the treatment of cancer Journal of Medicinal Chemistry 51 4 2008 1068 1072 10.1021/jm7010589 18247547

Drach J. Huang H. Samoilova O. Belch A. Farber C. Bosly A. … Cavalli F. Efficacy and safety of frontline rituximab, cyclophosphamide, doxorubicin and prednisone plus bortezomib (VR-CAP) or vincristine (R-CHOP) in a subset of newly diagnosed mantle cell lymphoma patients medically eligible for transplantation in the randomized, phase 3 LYM-3002 study Leukemia & Lymphoma 59 4 2018 896 903 10.1080/10428194.2017.1365855 29338540

Du H.-P. Yang Q.-Q. Zhang Y. Bortezomib-based chemotherapy to treat refractory angioimmunoblastic T-cell lymphoma: A case report and review of the literature Oncology Letters 11 3 2016 2310 2314 10.3892/ol.2016.4213 26998168 PMC4774619

Dufey E. Urra H. Hetz C. ER proteostasis addiction in cancer biology: Novel concepts Seminars in Cancer Biology 33 2015 40 47 10.1016/j.semcancer.2015.04.003 25931388

Durães F. Pinto M. Sousa E. Old drugs as new treatments for neurodegenerative diseases Pharmaceuticals 11 2 2018 44 10.3390/ph11020044 PMC6027455 29751602

Durie B.G.M. Hoering A. Abidi M.H. Rajkumar S.V. Epstein J. Kahanic S.P. … Dispenzieri A. Bortezomib with lenalidomide and dexamethasone versus lenalidomide and dexamethasone alone in patients with newly diagnosed myeloma without intent for immediate autologous stem-cell transplant (SWOG S0777): A randomised, open-label, phase 3 trial Lancet (London, England) 389 10068 2017 519 527 10.1016/S0140-6736(16)31594-X PMC5546834 28017406

de Duve C. Pressman B.C. Gianetto R. Wattiaux R. Appelmans F. Tissue fractionation studies. 6. Intracellular distribution patterns of enzymes in rat-liver tissue Biochemical Journal 60 4 1955 604 617 13249955 10.1042/bj0600604 PMC1216159

Dyson H.J. Wright P.E. Intrinsically unstructured proteins and their functions Nature Reviews. Molecular Cell Biology 6 3 2005 197 208 10.1038/nrm1589 15738986

Eastman A. Cell cycle checkpoints and their impact on anticancer therapeutic strategies Journal of Cellular Biochemistry 91 2 2004 223 231 10.1002/jcb.10699 14743382

Eberhardt R. Birbamer G. Gerstenbrand F. Rainer E. Traegner H. Citicoline in the treatment of Parkinson's disease Clinical Therapeutics 12 6 1990 489 495 2289218

Ebstein F. Poli Harlowe M.C. Studencka-Turski M. Krüger E. Contribution of the unfolded protein response (UPR) to the pathogenesis of proteasome-associated autoinflammatory syndromes (PRAAS) Frontiers in Immunology 10 2019 2756 10.3389/fimmu.2019.02756 31827472 PMC6890838

Ehlinger, A., Park, S., Fahmy, A., Lary, J. W., Cole, J. L., Finley, D., & Walters, K. J. (2013). Conformational dynamics of the Rpt6 ATPase in proteasome assembly and Rpn14 binding. Structure (London, England: 1993), 21(5), 753–765. 10.1016/j.str.2013.02.021 . PMC3670613 23562395

Eisele M.R. Reed R.G. Rudack T. Schweitzer A. Beck F. Nagy I. … Sakata E. Expanded coverage of the 26S proteasome conformational landscape reveals mechanisms of peptidase gating Cell Reports 24 5 2018 1301 1315 e5 10.1016/j.celrep.2018.07.004 30067984 PMC6140342

Elsasser S. Chandler-Militello D. Müller B. Hanna J. Finley D. Rad23 and Rpn10 serve as alternative ubiquitin receptors for the proteasome The Journal of Biological Chemistry 279 26 2004 26817 26822 10.1074/jbc.M404020200 15117949

Enrico O. Gabriele B. Nadia C. Sara G. Daniele V. Giulia C. … Mario P. Unexpected cardiotoxicity in haematological bortezomib treated patients British Journal of Haematology 138 3 2007 396 397 10.1111/j.1365-2141.2007.06659.x 17561972

Erales J. Coffino P. Ubiquitin-independent proteasomal degradation Biochimica et Biophysica Acta (BBA) - Molecular Cell Research 1843 1 2014 216 221 10.1016/j.bbamcr.2013.05.008 23684952 PMC3770795

Erales J. Hoyt M.A. Troll F. Coffino P. Functional asymmetries of proteasome translocase pore The Journal of Biological Chemistry 287 22 2012 18535 18543 10.1074/jbc.M112.357327 22493437 PMC3365715

Escusa-Toret S. Vonk W.I.M. Frydman J. Spatial sequestration of misfolded proteins by a dynamic chaperone pathway enhances cellular fitness during stress Nature Cell Biology 15 10 2013 1231 1243 10.1038/ncb2838 24036477 PMC4121856

Eskandari S.K. Seelen M.a.J. Lin G. Azzi J.R. The immunoproteasome: An old player with a novel and emerging role in alloimmunity American Journal of Transplantation : Official Journal of the American Society of Transplantation and the American Society of Transplant Surgeons 17 12 2017 3033 3039 10.1111/ajt.14435 28719024

Estrin E. Lopez-Blanco J.R. Chacón P. Martin A. Formation of an intricate helical bundle dictates the assembly of the 26S proteasome lid Structure (London, England: 1993) 21 9 2013 1624 1635 10.1016/j.str.2013.06.023 23911091

Etlinger J.D. Goldberg A.L. A soluble ATP-dependent proteolytic system responsible for the degradation of abnormal proteins in reticulocytes Proceedings of the National Academy of Sciences of the United States of America 74 1 1977 54 58 10.1073/pnas.74.1.54 264694 PMC393195

Etlinger J.D. Goldberg A.L. Control of protein degradation in reticulocytes and reticulocyte extracts by hemin Journal of Biological Chemistry 255 10 1980 4563 4568 7372594

Evan G.I. Vousden K.H. Proliferation, cell cycle and apoptosis in cancer Nature 411 6835 2001 342 348 10.1038/35077213 11357141

Eytan E. Ganoth D. Armon T. Hershko A. ATP-dependent incorporation of 20S protease into the 26S complex that degrades proteins conjugated to ubiquitin Proceedings of the National Academy of Sciences of the United States of America 86 20 1989 7751 7755 10.1073/pnas.86.20.7751 2554287 PMC298148

Fabre B. Lambour T. Garrigues L. Ducoux-Petit M. Amalric F. Monsarrat B. … Bousquet-Dubouch M.-P. Label-free quantitative proteomics reveals the dynamics of proteasome complexes composition and stoichiometry in a wide range of human cell lines Journal of Proteome Research 13 6 2014 3027 3037 10.1021/pr500193k 24804812

Faiq M.A. Wollstein G. Schuman J.S. Chan K.C. Cholinergic nervous system and glaucoma: From basic science to clinical applications Progress in Retinal and Eye Research 72 2019 100767 10.1016/j.preteyeres.2019.06.003 31242454 PMC6739176

Fan J.-Y. Agyekum B. Venkatesan A. Hall D.R. Keightley A. Bjes E.S. … Price J.L. Noncanonical FK506-binding protein BDBT binds DBT to enhance its circadian function and forms foci at night Neuron 80 4 2013 984 996 10.1016/j.neuron.2013.08.004 24210908 PMC3869642

Fan C.M. Maniatis T. Generation of p50 subunit of NF-kappa B by processing of p105 through an ATP-dependent pathway Nature 354 6352 1991 395 398 10.1038/354395a0 1956402

Fedorowicz M.A. de Vries-Schneider R.L.A. Rüb C. Becker D. Huang Y. Zhou C. … Przedborski S. Cytosolic cleaved PINK1 represses Parkin translocation to mitochondria and mitophagy EMBO Reports 15 1 2014 86 93 10.1002/embr.201337294 24357652 PMC4303452

Feling R.H. Buchanan G.O. Mincer T.J. Kauffman C.A. Jensen P.R. Fenical W. Salinosporamide A: a highly cytotoxic proteasome inhibitor from a novel microbial source, a marine bacterium of the new genus salinospora Angewandte Chemie ( International Ed. in English) 42 3 2003 355 357 10.1002/anie.200390115 12548698

Felszeghy S. Viiri J. Paterno J.J. Hyttinen J.M.T. Koskela A. Chen M. Leinonen H. Tanila H. Kivinen N. Koistinen A. Toropainen E. Amadio M. Smedowski A. Reinisalo M. Winiarczyk M. Mackiewicz J. Mutikainen M. Ruotsalainen A.-K. Kettunen M. … Kaarniranta K. Loss of NRF-2 and PGC-1α genes leads to retinal pigment epithelium damage resembling dry age-related macular degeneration Redox Biology 20 2019 1 12 10.1016/j.redox.2018.09.011 30253279 PMC6156745

Fenical W. Jensen P.R. Palladino M.A. Lam K.S. Lloyd G.K. Potts B.C. Discovery and development of the anticancer agent salinosporamide A (NPI-0052) Bioorganic & Medicinal Chemistry 17 6 2009 2175 2180 10.1016/j.bmc.2008.10.075 19022674 PMC2814440

Fernández Y. Verhaegen M. Miller T.P. Rush J.L. Steiner P. Opipari A.W. … Soengas M.S. Differential regulation of noxa in normal melanocytes and melanoma cells by proteasome inhibition: therapeutic implications Cancer Research 65 14 2005 6294 6304 10.1158/0008-5472.CAN-05-0686 16024631

Fernandez-Godino R. Pierce E.A. C3a triggers formation of sub-retinal pigment epithelium deposits via the ubiquitin proteasome pathway Scientific Reports 8 1 2018 9679 10.1038/s41598-018-28143-0 29946065 PMC6018664

Ferrara N. Vascular endothelial growth factor: Basic science and clinical progress Endocrine Reviews 25 4 2004 581 611 10.1210/er.2003-0027 15294883

Ferrington D.A. Gregerson D.S. Immunoproteasomes: Structure, function, and antigen presentation Progress in Molecular Biology and Translational Science 109 2012 75 112 10.1016/B978-0-12-397863-9.00003-1 22727420 PMC4405001

Finley D. Recognition and processing of ubiquitin-protein conjugates by the proteasome Annual Review of Biochemistry 78 2009 477 513 10.1146/annurev.biochem.78.081507.101607 PMC3431160 19489727

Finley D. Chen X. Walters K.J. Gates, channels, and switches: Elements of the proteasome machine Trends in Biochemical Sciences 41 1 2016 77 93 10.1016/j.tibs.2015.10.009 26643069 PMC4706478

Finley D. Prado M.A. The proteasome and its network: Engineering for adaptability Cold Spring Harbor Perspectives in Biology 2019 a033985 10.1101/cshperspect.a033985 PMC6829053 30833452

Fischer D.F. De Vos R.a.I. Van Dijk R. De Vrij F.M.S. Proper E.A. Sonnemans M.a.F. … Van Leeuwen F.W. Disease-specific accumulation of mutant ubiquitin as a marker for proteasomal dysfunction in the brain The FASEB Journal 17 14 2003 2014 2024 10.1096/fj.03-0205com 14597671

Fisher R.I. Bernstein S.H. Kahl B.S. Djulbegovic B. Robertson M.J. de Vos S. … Goy A. Multicenter phase II study of bortezomib in patients with relapsed or refractory mantle cell lymphoma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 24 30 2006 4867 4874 10.1200/JCO.2006.07.9665 17001068

Fisher R.I. Dahlberg S. Nathwani B.N. Banks P.M. Miller T.P. Grogan T.M. A clinical analysis of two indolent lymphoma entities: Mantle cell lymphoma and marginal zone lymphoma (including the mucosa-associated lymphoid tissue and monocytoid B-cell subcategories): a Southwest Oncology Group study Blood 85 4 1995 1075 1082 7849295

Florea B.I. Verdoes M. Li N. van der Linden W.A. Geurink P.P. van den Elst H. … Overkleeft H.S. Activity-based profiling reveals reactivity of the murine thymoproteasome-specific subunit beta5t Chemistry & Biology 17 8 2010 795 801 10.1016/j.chembiol.2010.05.027 20797608 PMC3039300

Folco E.J. Busconi L. Martone C.B. Sanchez J.J. Multicatalytic proteinase in fish muscle Archives of Biochemistry and Biophysics 267 2 1988 599 605 10.1016/0003-9861(88)90067-7 3214172

Fonseca R. Vabulas R.M. Hartl F.U. Bonhoeffer T. Nägerl U.V. A balance of protein synthesis and proteasome-dependent degradation determines the maintenance of LTP Neuron 52 2 2006 239 245 10.1016/j.neuron.2006.08.015 17046687

Forget G. Gregory D.J. Olivier M. Proteasome-mediated degradation of STAT1alpha following infection of macrophages with Leishmania donovani The Journal of Biological Chemistry 280 34 2005 30542 30549 10.1074/jbc.M414126200 15983048

Förster A. Whitby F.G. Hill C.P. The pore of activated 20S proteasomes has an ordered 7-fold symmetric conformation The EMBO Journal 22 17 2003 4356 4364 10.1093/emboj/cdg436 12941688 PMC202378

Forsthoefel A.M. Peña M.M.O. Xing Y.Y. Rafique Z. Berger F.G. Structural determinants for the intracellular degradation of human thymidylate synthase Biochemistry 43 7 2004 1972 1979 10.1021/bi035894p 14967037

Fort P. Kajava A.V. Delsuc F. Coux O. Evolution of proteasome regulators in eukaryotes Genome Biology and Evolution 7 5 2015 1363 1379 10.1093/gbe/evv068 25943340 PMC4453063

Fosgerau K. Hoffmann T. Peptide therapeutics: Current status and future directions Drug Discovery Today 20 1 2015 122 128 10.1016/j.drudis.2014.10.003 25450771

Fricke B. Heink S. Steffen J. Kloetzel P.-M. Krüger E. The proteasome maturation protein POMP facilitates major steps of 20S proteasome formation at the endoplasmic reticulum EMBO Reports 8 12 2007 1170 1175 10.1038/sj.embor.7401091 17948026 PMC2267243

Fricker L.D. Proteasome Inhibitor Drugs Annual Review of Pharmacology and Toxicology 60 2020 457 476 10.1146/annurev-pharmtox-010919-023603 31479618

Früh K. Yang Y. Antigen presentation by MHC class I and its regulation by interferon gamma Current Opinion in Immunology 11 1 1999 76 81 10.1016/s0952-7915(99)80014-4 10047537

Fu Y. Ma G. Liu G. Li B. Li H. Hao X. Liu L. USP14 as a novel prognostic marker promotes cisplatin resistance via Akt/ERK signaling pathways in gastric cancer Cancer Medicine 7 11 2018 5577 5588 10.1002/cam4.1770 30296012 PMC6246950

Fukuhara C. Dirden J.C. Tosini G. Photic regulation of melatonin in rat retina and the role of proteasomal proteolysis Neuroreport 12 17 2001 3833 3837 10.1097/00001756-200112040-00046 11726804

Fukunaga K. Kudo T. Toh-e A. Tanaka K. Saeki Y. Dissection of the assembly pathway of the proteasome lid in Saccharomyces cerevisiae Biochemical and Biophysical Research Communications 396 4 2010 1048 1053 10.1016/j.bbrc.2010.05.061 20471955

Funakoshi M. Tomko R.J. Kobayashi H. Hochstrasser M. Multiple assembly chaperones govern biogenesis of the proteasome regulatory particle base Cell 137 5 2009 887 899 10.1016/j.cell.2009.04.061 19446322 PMC2718848

Furtado M. Johnson R. Kruger A. Turner D. Rule S. Addition of bortezomib to standard dose chop chemotherapy improves response and survival in relapsed mantle cell lymphoma British Journal of Haematology 168 1 2015 55 62 10.1111/bjh.13101 25146720

Gacche R.N. Assaraf Y.G. Redundant angiogenic signaling and tumor drug resistance Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 36 2018 47 76 10.1016/j.drup.2018.01.002 29499837

Gaczynska M. Osmulski P.A. Harnessing proteasome dynamics and allostery in drug design Antioxidants & Redox Signaling 21 17 2014 2286 2301 10.1089/ars.2013.5816 24410482 PMC4241894

Gallerani E. Zucchetti M. Brunelli D. Marangon E. Noberasco C. Hess D. … Sessa C. A first in human phase I study of the proteasome inhibitor CEP-18770 in patients with advanced solid tumours and multiple myeloma European Journal of Cancer 49 2 2013 290 296 10.1016/j.ejca.2012.09.009 23058787

Gandolfi S. Laubach J.P. Hideshima T. Chauhan D. Anderson K.C. Richardson P.G. The proteasome and proteasome inhibitors in multiple myeloma Cancer Metastasis Reviews 36 4 2017 561 584 10.1007/s10555-017-9707-8 29196868

Gao J. Li M. Qin S. Zhang T. Jiang S. Hu Y. … Jiang C. Cytosolic PINK1 promotes the targeting of ubiquitinated proteins to the aggresome-autophagy pathway during proteasomal stress Autophagy 12 4 2016 632 647 10.1080/15548627.2016.1147667 27050454 PMC4835960

Garderet L. Iacobelli S. Moreau P. Dib M. Lafon I. Niederwieser D. Masszi T. Fontan J. Michallet M. Gratwohl A. Milone G. Doyen C. Pegourie B. Hajek R. Casassus P. Kolb B. Chaleteix C. Hertenstein B. Onida F. … Gahrton G. Superiority of the triple combination of bortezomib-thalidomide-dexamethasone over the dual combination of thalidomide-dexamethasone in patients with multiple myeloma progressing or relapsing after autologous transplantation: The MMVAR/IFM 2005-04 Randomized Phase III Trial from the Chronic Leukemia Working Party of the European Group for Blood and Marrow Transplantation Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 30 20 2012 2475 2482 10.1200/JCO.2011.37.4918 22585692

Gatti L. Zuco V. Zaffaroni N. Perego P. Drug combinations with proteasome inhibitors in antitumor therapy Current Pharmaceutical Design 19 22 2013 4094 4114 10.2174/1381612811319220015 23181571

Ghobrial I.M. Vij R. Siegel D. Badros A. Kaufman J. Raje N. … Berdeja J.G. A phase Ib/II study of oprozomib in patients with advanced multiple myeloma and Waldenström macroglobulinemia Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 25 16 2019 4907 4916 10.1158/1078-0432.CCR-18-3728 31142508

Giampà C. Laurenti D. Anzilotti S. Bernardi G. Menniti F.S. Fusco F.R. Inhibition of the striatal specific phosphodiesterase PDE10A ameliorates striatal and cortical pathology in R6/2 mouse model of Huntington's disease PLoS One 5 10 2010 e13417 10.1371/journal.pone.0013417 PMC2955524 20976216

Gilbert J. Lee J.W. Argiris A. Haigentz M. Feldman L.E. Jang M. … Forastiere A.A. Phase II 2-arm trial of the proteasome inhibitor, PS-341 (bortezomib) in combination with irinotecan or PS-341 alone followed by the addition of irinotecan at time of progression in patients with locally recurrent or metastatic squamous cell carcinoma of the head and neck (E1304): A trial of the Eastern Cooperative Oncology Group Head & Neck 35 7 2013 942 948 10.1002/hed.23046 22791234 PMC3689864

Giuliani N. Storti P. Bolzoni M. Palma B.D. Bonomini S. Angiogenesis and multiple myeloma Cancer Microenvironment 4 3 2011 325 337 10.1007/s12307-011-0072-9 21735169 PMC3234322

Giulivi C. Pacifici R.E. Davies K.J. Exposure of hydrophobic moieties promotes the selective degradation of hydrogen peroxide-modified hemoglobin by the multicatalytic proteinase complex, proteasome Archives of Biochemistry and Biophysics 311 2 1994 329 341 10.1006/abbi.1994.1245 8203895

Giżyńska M. Witkowska J. Karpowicz P. Rostankowski R. Chocron E.S. Pickering A.M. … Jankowska E. Proline- and arginine-rich peptides as flexible allosteric modulators of human proteasome activity Journal of Medicinal Chemistry 62 1 2019 359 370 10.1021/acs.jmedchem.8b01025 30452262 PMC6796967

Glenner G.G. Wong C.W. Alzheimer's disease: Initial report of the purification and characterization of a novel cerebrovascular amyloid protein Biochemical and Biophysical Research Communications 120 3 1984 885 890 6375662 10.1016/s0006-291x(84)80190-4

Glickman M.H. Ciechanover A. The ubiquitin-proteasome proteolytic pathway: Destruction for the sake of construction Physiological Reviews 82 2 2002 373 428 10.1152/physrev.00027.2001 11917093

Goedert M. Clavaguera F. Tolnay M. The propagation of prion-like protein inclusions in neurodegenerative diseases Trends in Neurosciences 33 7 2010 317 325 10.1016/j.tins.2010.04.003 20493564

Goldberg A.L. Degradation of abnormal proteins in Escherichia coli (protein breakdown-protein structure-mistranslation-amino acid analogs-puromycin) Proceedings of the National Academy of Sciences of the United States of America 69 2 1972 422 426 10.1073/pnas.69.2.422 4551144 PMC426471

Goldberg A.L. St John A.C. Intracellular protein degradation in mammalian and bacterial cells: Part 2 Annual Review of Biochemistry 45 1976 747 803 10.1146/annurev.bi.45.070176.003531 786161

Goldberg A.L. Zhao J. Collins G.A. Blocking cancer growth with less POMP or proteasomes Molecular Cell 59 2 2015 143 145 10.1016/j.molcel.2015.07.005 26186288 PMC4787267

Gomez-Bougie P. Wuillème-Toumi S. Ménoret E. Trichet V. Robillard N. Philippe M. … Amiot M. Noxa up-regulation and Mcl-1 cleavage are associated to apoptosis induction by bortezomib in multiple myeloma Cancer Research 67 11 2007 5418 5424 10.1158/0008-5472.CAN-06-4322 17545623

Gonen N. Assaraf Y.G. Antifolates in cancer therapy: structure, activity and mechanisms of drug resistance Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 15 4 2012 183 210 10.1016/j.drup.2012.07.002 22921318

Gong B. Radulovic M. Figueiredo-Pereira M.E. Cardozo C. The ubiquitin-proteasome system: Potential therapeutic targets for alzheimer's disease and spinal cord injury Frontiers in Molecular Neuroscience 9 JAN 2016 10.3389/fnmol.2016.00004 Scopus PMC4727241 26858599

Gorusupudi A. Liu A. Hageman G.S. Bernstein P.S. Associations of human retinal very long-chain polyunsaturated fatty acids with dietary lipid biomarkers Journal of Lipid Research 57 3 2016 499 508 10.1194/jlr.P065540 26764040 PMC4766999

Goy A. Bernstein S.H. Kahl B.S. Djulbegovic B. Robertson M.J. de Vos S. … Fisher R.I. Bortezomib in patients with relapsed or refractory mantle cell lymphoma: Updated time-to-event analyses of the multicenter phase 2 PINNACLE study Annals of Oncology 20 3 2009 520 525 10.1093/annonc/mdn656 19074748 PMC4592328

Goy A. Younes A. McLaughlin P. Pro B. Romaguera J.E. Hagemeister F. … Rodriguez A.M. Phase II study of proteasome inhibitor bortezomib in relapsed or refractory B-cell non-Hodgkin's lymphoma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 23 4 2005 667 675 10.1200/JCO.2005.03.108 15613697

Gozzetti A. Papini G. Candi V. Brambilla C.Z. Sirianni S. Bocchia M. Second generation proteasome inhibitors in multiple myeloma Anti-Cancer Agents in Medicinal Chemistry 17 7 2017 920 926 10.2174/1871520616666160902101622 27592543

Grandin E.W. Ky B. Cornell R.F. Carver J. Lenihan D.J. Patterns of cardiac toxicity associated with irreversible proteasome inhibition in the treatment of multiple myeloma Journal of Cardiac Failure 21 2 2015 138 144 10.1016/j.cardfail.2014.11.008 25433360

Grasso G. Pietropaolo A. Spoto G. Pappalardo G. Tundo G.R. Ciaccio C. … Rizzarelli E. Copper(I) and copper(II) inhibit Aβ peptides proteolysis by insulin-degrading enzyme differently: Implications for metallostasis alteration in Alzheimer's disease Chemistry - A European Journal 17 9 2011 2752 2762 10.1002/chem.201002809 21274957

Grasso G. Salomone F. Tundo G.R. Pappalardo G. Ciaccio C. Spoto G. … Rizzarelli E. Metal ions affect insulin-degrading enzyme activity Journal of Inorganic Biochemistry 117 2012 351 358 22819648 10.1016/j.jinorgbio.2012.06.010

Grasso G. Santoro A.M. Lanza V. Sbardella D. Tundo G.R. Ciaccio C. … Milardi D. The double faced role of copper in A? Homeostasis: A survey on the interrelationship between metal dyshomeostasis, UPS functioning and autophagy in neurodegeneration Coordination Chemistry Reviews 347 2017 1 22

Greene A.W. Grenier K. Aguileta M.A. Muise S. Farazifard R. Haque M.E. … Fon E.A. Mitochondrial processing peptidase regulates PINK1 processing, import and Parkin recruitment EMBO Reports 13 4 2012 378 385 10.1038/embor.2012.14 22354088 PMC3321149

Gregori L. Hainfeld J.F. Simon M.N. Goldgaber D. Binding of amyloid beta protein to the 20 S proteasome The Journal of Biological Chemistry 272 1 1997 58 62 8995227 10.1074/jbc.272.1.58

Gressin R. Daguindau N. Tempescul A. Moreau A. Carras S. Tchernonog E. Schmitt A. Houot R. Dartigeas C. Pignon J.M. Corm S. Banos A. Mounier C. Dupuis J. Macro M. Fleury J. Jardin F. Sarkozy C. Damaj G. … Lymphoma Study Association A phase 2 study of rituximab, bendamustine, bortezomib and dexamethasone for first-line treatment of older patients with mantle cell lymphoma Haematologica 104 1 2019 138 146 10.3324/haematol.2018.191429 30171024 PMC6312036

Griffin T.A. Nandi D. Cruz M. Fehling H.J. Kaer L.V. Monaco J.J. Colbert R.A. Immunoproteasome assembly: Cooperative incorporation of interferon gamma (IFN-gamma)-inducible subunits The Journal of Experimental Medicine 187 1 1998 97 104 10.1084/jem.187.1.97 9419215 PMC2199179

Grigoreva T.A. Tribulovich V.G. Garabadzhiu A.V. Melino G. Barlev N.A. The 26S proteasome is a multifaceted target for anti-cancer therapies Oncotarget 6 28 2015 24733 24749 26295307 10.18632/oncotarget.4619 PMC4694792

Groettrup M. Standera S. Stohwasser R. Kloetzel P.M. The subunits MECL-1 and LMP2 are mutually required for incorporation into the 20S proteasome Proceedings of the National Academy of Sciences of the United States of America 94 17 1997 8970 8975 9256419 10.1073/pnas.94.17.8970 PMC22989

Groll M. Bajorek M. Köhler A. Moroder L. Rubin D.M. Huber R. … Finley D. A gated channel into the proteasome core particle Nature Structural Biology 7 11 2000 1062 1067 10.1038/80992 11062564

Groll M. Berkers C.R. Ploegh H.L. Ovaa H. Crystal structure of the boronic acid-based proteasome inhibitor bortezomib in complex with the yeast 20S proteasome Structure (London, England: 1993) 14 3 2006 451 456 10.1016/j.str.2005.11.019 16531229

Groll M. Ditzel L. Löwe J. Stock D. Bochtler M. Bartunik H.D. Huber R. Structure of 20S proteasome from yeast at 2.4 A resolution Nature 386 6624 1997 463 471 10.1038/386463a0 9087403

Groll M. Heinemeyer W. Jäger S. Ullrich T. Bochtler M. Wolf D.H. Huber R. The catalytic sites of 20S proteasomes and their role in subunit maturation: A mutational and crystallographic study Proceedings of the National Academy of Sciences of the United States of America 96 20 1999 10976 10983 10.1073/pnas.96.20.10976 10500111 PMC34229

Groll M. Huber R. Substrate access and processing by the 20S proteasome core particle The International Journal of Biochemistry & Cell Biology 35 5 2003 606 616 10.1016/s1357-2725(02)00390-4 12672453

Groll M. Nazif T. Huber R. Bogyo M. Probing structural determinants distal to the site of hydrolysis that control substrate specificity of the 20S proteasome Chemistry & Biology 9 5 2002 655 662 10.1016/s1074-5521(02)00144-8 12031672

Grune T. Catalgol B. Licht A. Ermak G. Pickering A.M. Ngo J.K. Davies K.J.A. HSP70 mediates dissociation and reassociation of the 26S proteasome during adaptation to oxidative stress Free Radical Biology & Medicine 51 7 2011 1355 1364 10.1016/j.freeradbiomed.2011.06.015 21767633 PMC3172204

Grune T. Reinheckel T. Davies K.J. Degradation of oxidized proteins in K562 human hematopoietic cells by proteasome The Journal of Biological Chemistry 271 26 1996 15504 15509 10.1074/jbc.271.26.15504 8663134

Gstaiger M. Jordan R. Lim M. Catzavelos C. Mestan J. Slingerland J. Krek W. Skp2 is oncogenic and overexpressed in human cancers Proceedings of the National Academy of Sciences of the United States of America 98 9 2001 5043 5048 10.1073/pnas.081474898 11309491 PMC33160

Guijarro J.I. Sunde M. Jones J.A. Campbell I.D. Dobson C.M. Amyloid fibril formation by an SH3 domain Proceedings of the National Academy of Sciences 95 8 1998 4224 4228 10.1073/pnas.95.8.4224 PMC22470 9539718

Guo F. He X.-B. Li S. Le W. A Central Role for Phosphorylated p38α in Linking Proteasome Inhibition-Induced Apoptosis and Autophagy Molecular Neurobiology 54 10 2017 7597 7609 10.1007/s12035-016-0260-1 27832521

Guo Q. Lehmer C. Martínez-Sánchez A. Rudack T. Beck F. Hartmann H. … Fernández-Busnadiego R. In Situ Structure of Neuronal C9orf72 Poly-GA Aggregates Reveals Proteasome Recruitment Cell 172 4 2018 696 705.e12 10.1016/j.cell.2017.12.030 29398115 PMC6035389

Guo L. Wang Y. Glutamate stimulates glutamate receptor interacting protein 1 degradation by ubiquitin-proteasome system to regulate surface expression of GluR2 Neuroscience 145 1 2007 100 109 10.1016/j.neuroscience.2006.11.042 17207582

Gupta N. Hanley M.J. Venkatakrishnan K. Bessudo A. Rasco D.W. Sharma S. … Nemunaitis J. Effects of strong CYP3A inhibition and induction on the pharmacokinetics of ixazomib, an oral proteasome inhibitor: Results of drug-drug interaction studies in patients with advanced solid tumors or lymphoma and a physiologically based pharmacokinetic analysis Journal of Clinical Pharmacology 58 2 2018 180 192 10.1002/jcph.988 28800141 PMC5811830

Gupta N. Hanley M.J. Venkatakrishnan K. Wang B. Sharma S. Bessudo A. … Nemunaitis J. The effect of a high-fat meal on the pharmacokinetics of ixazomib, an oral proteasome inhibitor, in patients with advanced solid tumors or lymphoma The Journal of Clinical Pharmacology 56 10 2016 1288 1295 10.1002/jcph.719 26872892 PMC5069578

Gupta N. Hanley M.J. Xia C. Labotka R. Harvey R.D. Venkatakrishnan K. Clinical pharmacology of ixazomib: The first oral proteasome inhibitor Clinical Pharmacokinetics 58 4 2019 431 449 10.1007/s40262-018-0702-1 30117017 PMC6397141

Gupta M.P. Herzlich A.A. Sauer T. Chan C.-C. Retinal anatomy and pathology Developments in Ophthalmology 55 2016 7 17 10.1159/000431128 26502225

Gupta I. Singh K. Varshney N.K. Khan S. Delineating Crosstalk Mechanisms of the Ubiquitin Proteasome System That Regulate Apoptosis Frontiers in Cell and Development Biology 6 2018 11 10.3389/fcell.2018.00011 PMC5811474 29479529

Gupta N. Zhang S. Pusalkar S. Plesescu M. Chowdhury S. Hanley M.J. … Shepard D.R. A phase I study to assess the mass balance, excretion, and pharmacokinetics of [14C]-ixazomib, an oral proteasome inhibitor, in patients with advanced solid tumors Investigational New Drugs 36 3 2018 407 415 10.1007/s10637-017-0509-1 28932928 PMC5948259

Gupta N. Zhao Y. Hui A.-M. Esseltine D.-L. Venkatakrishnan K. Switching from body surface area-based to fixed dosing for the investigational proteasome inhibitor ixazomib: A population pharmacokinetic analysis British Journal of Clinical Pharmacology 79 5 2015 789 800 10.1111/bcp.12542 25377318 PMC4415715

Guterman A. Glickman M.H. Complementary roles for Rpn11 and Ubp6 in deubiquitination and proteolysis by the proteasome The Journal of Biological Chemistry 279 3 2004 1729 1738 10.1074/jbc.M307050200 14581483

Haas A.L. Rose I.A. Hemin inhibits ATP-dependent ubiquitin-dependent proteolysis: Role of hemin in regulating ubiquitin conjugate degradation Proceedings of the National Academy of Sciences of the United States of America 78 11 II 1981 6845 6848 10.1073/pnas.78.11.6845 6273891 PMC349148

van Hagen M. Piebes D.G.E. de Leeuw W.C. Vuist I.M. van Roon-Mom W.M.C. Moerland P.D. Verschure P.J. The dynamics of early-state transcriptional changes and aggregate formation in a Huntington's disease cell model BMC Genomics 18 1 2017 373 10.1186/s12864-017-3745-z 28499347 PMC5429582

Hamazaki J. Hirayama S. Murata S. Redundant roles of Rpn10 and Rpn13 in recognition of ubiquitinated proteins and cellular homeostasis PLoS Genetics 11 7 2015 e1005401 10.1371/journal.pgen.1005401 PMC4519129 26222436

Hamazaki J. Iemura S.-I. Natsume T. Yashiroda H. Tanaka K. Murata S. A novel proteasome interacting protein recruits the deubiquitinating enzyme UCH37 to 26S proteasomes The EMBO Journal 25 19 2006 4524 4536 10.1038/sj.emboj.7601338 16990800 PMC1589993

Hamel F.G. Preliminary report: Inhibition of cellular proteasome activity by free fatty acids Metabolism, Clinical and Experimental 58 8 2009 1047 1049 10.1016/j.metabol.2009.04.005 19423142

Hamilton A.M. Oh W.C. Vega-Ramirez H. Stein I.S. Hell J.W. Patrick G.N. Zito K. Activity-dependent growth of new dendritic spines is regulated by the proteasome Neuron 74 6 2012 1023 1030 10.1016/j.neuron.2012.04.031 22726833 PMC3500563

Hamman J.H. Enslin G.M. Kotzé A.F. Oral delivery of peptide drugs: Barriers and developments BioDrugs: Clinical Immunotherapeutics, Biopharmaceuticals and Gene Therapy 19 3 2005 165 177 10.2165/00063030-200519030-00003 15984901

Han D.H. Na H.-K. Choi W.H. Lee J.H. Kim Y.K. Won C. … Lee M.J. Direct cellular delivery of human proteasomes to delay tau aggregation Nature Communications 5 2014 5633 10.1038/ncomms6633 25476420

Hanada M. Sugawara K. Kaneta K. Toda S. Nishiyama Y. Tomita K. … Oki T. Epoxomicin, a new antitumor agent of microbial origin The Journal of Antibiotics 45 11 1992 1746 1752 10.7164/antibiotics.45.1746 1468981

Hanahan D. Weinberg R.A. The hallmarks of cancer Cell 100 1 2000 57 70 10.1016/s0092-8674(00)81683-9 10647931

Hanahan D. Weinberg R.A. Hallmarks of cancer: the next generation Cell 144 5 2011 646 674 10.1016/j.cell.2011.02.013 21376230

Hanna J. Hathaway N.A. Tone Y. Crosas B. Elsasser S. Kirkpatrick D.S. … Finley D. Deubiquitinating enzyme Ubp6 functions noncatalytically to delay proteasomal degradation Cell 127 1 2006 99 111 10.1016/j.cell.2006.07.038 17018280

Hanna J. Meides A. Zhang D.P. Finley D. A ubiquitin stress response induces altered proteasome composition Cell 129 4 2007 747 759 10.1016/j.cell.2007.03.042 17512408

Hanssum A. Zhong Z. Rousseau A. Krzyzosiak A. Sigurdardottir A. Bertolotti A. An inducible chaperone adapts proteasome assembly to stress Molecular Cell 55 4 2014 566 577 10.1016/j.molcel.2014.06.017 25042801 PMC4148588

Harada A. Suzuki K. Kimura H. TAK-063, a novel phosphodiesterase 10A inhibitor, protects from striatal neurodegeneration and ameliorates behavioral deficits in the R6/2 mouse model of Huntington's disease The Journal of Pharmacology and Experimental Therapeutics 360 1 2017 75 83 10.1124/jpet.116.237388 27811172

Harbour J.W. Luo R.X. Dei Santi A. Postigo A.A. Dean D.C. Cdk phosphorylation triggers sequential intramolecular interactions that progressively block Rb functions as cells move through G1 Cell 98 6 1999 859 869 10.1016/s0092-8674(00)81519-6 10499802

Harding R.J. Tong Y.-F. Proteostasis in Huntington's disease: Disease mechanisms and therapeutic opportunities Acta Pharmacologica Sinica 39 5 2018 754 769 10.1038/aps.2018.11 29620053 PMC5943907

Harer S.L. Bhatia M.S. Bhatia N.M. Proteasome inhibitors mechanism; source for design of newer therapeutic agents The Journal of Antibiotics 65 6 2012 279 288 10.1038/ja.2011.84 22511225

Hari P. Lin H.M. Zhu Y. Berg D. Richardson P.G. Moreau P. Healthcare resource utilization with ixazomib or placebo plus lenalidomide-dexamethasone in the randomized, double-blind, phase 3 TOURMALINE-MM1 study in relapsed/refractory multiple myeloma Journal of Medical Economics 21 8 2018 793 798 10.1080/13696998.2018.1474745 29741409

Hari P. Matous J.V. Voorhees P.M. Shain K.H. Obreja M. Frye J. … Sonneveld P. Oprozomib in patients with newly diagnosed multiple myeloma Blood Cancer Journal 9 9 2019 10.1038/s41408-019-0232-6 PMC6697695 31420532

Hari P. Paba-Prada C.E. Voorhees P.M. Frye J. Chang Y.-L. Moreau P. … Shain K.H. Efficacy and safety results from a phase 1b/2, multicenter, open-label study of oprozomib and dexamethasone in patients with relapsed and/or refractory multiple myeloma Leukemia Research 83 2019 106172 10.1016/j.leukres.2019.106172 31229804 PMC6871647

Harousseau J.-L. Attal M. Avet-Loiseau H. Marit G. Caillot D. Mohty M. … Moreau P. Bortezomib plus dexamethasone is superior to vincristine plus doxorubicin plus dexamethasone as induction treatment prior to autologous stem-cell transplantation in newly diagnosed multiple myeloma: Results of the IFM 2005-01 phase III trial Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 28 30 2010 4621 4629 10.1200/JCO.2009.27.9158 20823406

Harris J.R. Release of a macromolecular protein component from human erythrocyte ghosts Biochimica et Biophysica Acta 150 3 1968 534 537 10.1016/0005-2736(68)90157-0 5650402

Harrison S.J. Mainwaring P. Price T. Millward M.J. Padrik P. Underhill C.R. … Spencer A. Phase I clinical trial of marizomib (NPI-0052) in patients with advanced malignancies including multiple myeloma: Study NPI-0052-102 final results Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 22 18 2016 4559 4566 10.1158/1078-0432.CCR-15-2616 27117181

Harshbarger W. Miller C. Diedrich C. Sacchettini J. Crystal structure of the human 20S proteasome in complex with carfilzomib Structure (London, England: 1993) 23 2 2015 418 424 10.1016/j.str.2014.11.017 25599644

Hartl F.U. Bracher A. Hayer-Hartl M. Molecular chaperones in protein folding and proteostasis Nature 475 7356 2011 324 332 10.1038/nature10317 21776078

Hartl F.U. Hayer-Hartl M. Converging concepts of protein folding in vitro and in vivo Nature Structural & Molecular Biology 16 6 2009 574 581 10.1038/nsmb.1591 19491934

Haupt Y. Maya R. Kazaz A. Oren M. Mdm2 promotes the rapid degradation of p53 Nature 387 6630 1997 296 299 10.1038/387296a0 9153395

He J. Kulkarni K. da Fonseca P.C.A. Krutauz D. Glickman M.H. Barford D. Morris E.P. The structure of the 26S proteasome subunit Rpn2 reveals its PC repeat domain as a closed toroid of two concentric α-helical rings Structure (London, England: 1993) 20 3 2012 513 521 10.1016/j.str.2011.12.015 22405010

Heckman P.R.A. Blokland A. Bollen E.P.P. Prickaerts J. Phosphodiesterase inhibition and modulation of corticostriatal and hippocampal circuits: Clinical overview and translational considerations Neuroscience & Biobehavioral Reviews 87 2018 233 254 10.1016/j.neubiorev.2018.02.007 29454746

Hegde A.N. Smith S.G. Duke L.M. Pourquoi A. Vaz S. Perturbations of ubiquitin-proteasome-mediated proteolysis in aging and Alzheimer's disease Frontiers in Aging Neuroscience 11 2019 10.3389/fnagi.2019.00324 Scopus PMC6910070 31866849

Heinemeyer W. Fischer M. Krimmer T. Stachon U. Wolf D.H. The active sites of the eukaryotic 20 S proteasome and their involvement in subunit precursor processing The Journal of Biological Chemistry 272 40 1997 25200 25209 10.1074/jbc.272.40.25200 9312134

Heink S. Ludwig D. Kloetzel P.-M. Krüger E. IFN-gamma-induced immune adaptation of the proteasome system is an accelerated and transient response Proceedings of the National Academy of Sciences of the United States of America 102 26 2005 9241 9246 10.1073/pnas.0501711102 15944226 PMC1166598

Hellmann A. Rule S. Walewski J. Shpilberg O. Feng H. van de Velde H. … Louw V.J. Effect of cytochrome P450 3A4 inducers on the pharmacokinetic, pharmacodynamic and safety profiles of bortezomib in patients with multiple myeloma or non-Hodgkin's lymphoma Clinical Pharmacokinetics 50 12 2011 781 791 10.2165/11594410-000000000-00000 22087865

Hemmis C.W. Heard S.C. Hill C.P. Phosphorylation of Tyr-950 in the proteasome scaffolding protein RPN2 modulates its interaction with the ubiquitin receptor RPN13 The Journal of Biological Chemistry 294 25 2019 9659 9665 10.1074/jbc.AC119.008881 31064842 PMC6597823

Herndon T.M. Deisseroth A. Kaminskas E. Kane R.C. Koti K.M. Rothmann M.D. … Pazdur R. U.s. Food and Drug Administration approval: Carfilzomib for the treatment of multiple myeloma Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 19 17 2013 4559 4563 10.1158/1078-0432.CCR-13-0755 23775332

Hershko A. Ciechanover A. Heller H. Haas A.L. Rose I.A. Proposed role of ATP in protein breakdown: Conjugation of protein with multiple chains of the polypeptide of ATP-dependent proteolysis Proceedings of the National Academy of Sciences of the United States of America 77 4 1980 1783 1786 10.1073/pnas.77.4.1783 6990414 PMC348591

Hershko A. Eytan E. Ciechanover A. Haas A.L. Immunochemical analysis of the turnover of ubiquitin-protein conjugates in intact cells. Relationship to the breakdown of abnormal proteins The Journal of Biological Chemistry 257 23 1982 13964 13970 6292216

Hideshima T. Chauhan D. Richardson P. Mitsiades C. Mitsiades N. Hayashi T. … Anderson K.C. NF-kappa B as a therapeutic target in multiple myeloma The Journal of Biological Chemistry 277 19 2002 16639 16647 10.1074/jbc.M200360200 11872748

Hideshima T. Mitsiades C. Akiyama M. Hayashi T. Chauhan D. Richardson P. … Anderson K.C. Molecular mechanisms mediating antimyeloma activity of proteasome inhibitor PS-341 Blood 101 4 2003 1530 1534 10.1182/blood-2002-08-2543 12393500

Hideshima T. Richardson P. Chauhan D. Palombella V.J. Elliott P.J. Adams J. Anderson K.C. The proteasome inhibitor PS-341 inhibits growth, induces apoptosis, and overcomes drug resistance in human multiple myeloma cells Cancer Research 61 7 2001 3071 3076 11306489

Hindo S.S. Frezza M. Tomco D. Heeg M.J. Hryhorczuk L. McGarvey B.R. … Verani C.N. Metals in anticancer therapy: Copper(II) complexes as inhibitors of the 20S proteasome European Journal of Medicinal Chemistry 44 11 2009 4353 4361 10.1016/j.ejmech.2009.05.019 19559507 PMC2759842

Hipp M.S. Park S.-H. Hartl F.U. Proteostasis impairment in protein-misfolding and -aggregation diseases Trends in Cell Biology 24 9 2014 506 514 10.1016/j.tcb.2014.05.003 24946960

Hirano Y. Hayashi H. Iemura S.-I. Hendil K.B. Niwa S.-I. Kishimoto T. … Murata S. Cooperation of multiple chaperones required for the assembly of mammalian 20S proteasomes Molecular Cell 24 6 2006 977 984 10.1016/j.molcel.2006.11.015 17189198

Hirano Y. Hendil K.B. Yashiroda H. Iemura S. Nagane R. Hioki Y. … Murata S. A heterodimeric complex that promotes the assembly of mammalian 20S proteasomes Nature 437 7063 2005 1381 1385 10.1038/nature04106 16251969

Hirano H. Kimura Y. Kimura A. Biological significance of co- and post-translational modifications of the yeast 26S proteasome Journal of Proteomics 134 2016 37 46 10.1016/j.jprot.2015.11.016 26642761

Hoefer M.M. Boneberg E.-M. Grotegut S. Kusch J. Illges H. Possible tetramerisation of the proteasome maturation factor POMP/proteassemblin/hUmp1 and its subcellular localisation International Journal of Biological Macromolecules 38 3–5 2006 259 267 10.1016/j.ijbiomac.2006.03.015 16624403

Homma T. Ishibashi D. Nakagaki T. Fuse T. Mori T. Satoh K. … Nishida N. Ubiquitin-specific protease 14 modulates degradation of cellular prion protein Scientific Reports 5 2015 10.1038/srep11028 PMC4462021 26061634

Hong D.P. Han S. Fink A.L. Uversky V.N. Characterization of the non-fibrillar α-synuclein oligomers Protein and Peptide Letters 18 3 2011 230 240 10.2174/092986611794578332 20858207

Horan K.A. Hansen K. Jakobsen M.R. Holm C.K. Søby S. Unterholzner L. … Paludan S.R. Proteasomal degradation of herpes simplex virus capsids in macrophages releases DNA to the cytosol for recognition by DNA sensors Journal of Immunology (Baltimore, MD.: 1950) 190 5 2013 2311 2319 10.4049/jimmunol.1202749 PMC3578088 23345332

Hou J. Jin J. Xu Y. Wu D. Ke X. Zhou D. Lu J. Du X. Chen X. Li J. Liu J. Gupta N. Hanley M.J. Li H. Hua Z. Wang B. Zhang X. Wang H. van de Velde H. … Moreau P. Randomized, double-blind, placebo-controlled phase III study of ixazomib plus lenalidomide-dexamethasone in patients with relapsed/refractory multiple myeloma: China Continuation study Journal of Hematology & Oncology 10 1 2017 137 10.1186/s13045-017-0501-4 28683766 PMC5500972

Hough R. Pratt G. Rechsteiner M. Ubiquitin-lysozyme conjugates. Identification and characterization of an ATP-dependent protease from rabbit reticulocyte lysates The Journal of Biological Chemistry 261 5 1986 2400 2408 3003114

Hough R. Pratt G. Rechsteiner M. Purification of two high molecular weight proteases from rabbit reticulocyte lysate The Journal of Biological Chemistry 262 17 1987 8303 8313 3298229

Hu M. Li P. Song L. Jeffrey P.D. Chenova T.A. Wilkinson K.D. … Shi Y. Structure and mechanisms of the proteasome-associated deubiquitinating enzyme USP14 The EMBO Journal 24 21 2005 3747 3756 10.1038/sj.emboj.7600832 16211010 PMC1276716

Hu W. Yen Y.-T. Singh S. Kao C.-L. Wu-Hsieh B.A. SARS-CoV regulates immune function-related gene expression in human monocytic cells Viral Immunology 25 4 2012 277 288 10.1089/vim.2011.0099 22876772 PMC3413073

Huang L. Chen C. Proteasome regulators: Activators and inhibitors Current Medicinal Chemistry 16 8 2009 931 939 10.2174/092986709787581860 19275603 PMC3608511

Huang I.T. Dhungel B. Shrestha R. Bridle K.R. Crawford D.H.G. Jayachandran A. Steel J.C. Spotlight on bortezomib: Potential in the treatment of hepatocellular carcinoma Expert Opinion on Investigational Drugs 28 1 2019 7 18 10.1080/13543784.2019.1551359 30474444

Huang L. Ho P. Chen C.H. Activation and inhibition of the proteasome by betulinic acid and its derivatives FEBS Letters 581 25 2007 4955 4959 10.1016/j.febslet.2007.09.031 17904555 PMC2083647

Huang H. Liu N. Zhao K. Zhu C. Lu X. Li S. … Liu J. Sanggenon C decreases tumor cell viability associated with proteasome inhibition Frontiers in Bioscience (Elite Edition) 3 2011 1315 1325 21622138 10.2741/e335 PMC3303154

Huang Z. Wu Y. Zhou X. Xu J. Zhu W. Shu Y. Liu P. Efficacy of therapy with bortezomib in solid tumors: A review based on 32 clinical trials Future Oncology (London, England) 10 10 2014 1795 1807 10.2217/fon.14.30 25303058

Huber E.M. Heinemeyer W. Li X. Arendt C.S. Hochstrasser M. Groll M. A unified mechanism for proteolysis and autocatalytic activation in the 20S proteasome Nature Communications 7 1 2016 1 10 10.1038/ncomms10900 PMC4792962 26964885

Husnjak K. Elsasser S. Zhang N. Chen X. Randles L. Shi Y. … Dikic I. Proteasome subunit Rpn13 is a novel ubiquitin receptor Nature 453 7194 2008 481 488 10.1038/nature06926 18497817 PMC2839886

Hussong S.A. Kapphahn R.J. Phillips S.L. Maldonado M. Ferrington D.A. Immunoproteasome deficiency alters retinal proteasome's response to stress Journal of Neurochemistry 113 6 2010 1481 1490 10.1111/j.1471-4159.2010.06688.x 20345760 PMC2909641

Hussong S.A. Roehrich H. Kapphahn R.J. Maldonado M. Pardue M.T. Ferrington D.A. A novel role for the immunoproteasome in retinal function Investigative Ophthalmology & Visual Science 52 2 2011 714 723 10.1167/iovs.10-6032 20881299 PMC3053103

Hyrskyluoto A. Bruelle C. Lundh S.H. Do H.T. Kivinen J. Rappou E. … Korhonen L. Ubiquitin-specific protease-14 reduces cellular aggregates and protects against mutant huntingtin-induced cell degeneration: Involvement of the proteasome and ER stress-activated kinase IRE1α Human Molecular Genetics 23 22 2014 5928 5939 10.1093/hmg/ddu317 24951540

Imai J. Maruya M. Yashiroda H. Yahara I. Tanaka K. The molecular chaperone Hsp90 plays a role in the assembly and maintenance of the 26S proteasome The EMBO Journal 22 14 2003 3557 3567 10.1093/emboj/cdg349 12853471 PMC165619

Infante J.R. Mendelson D.S. Burris H.A. Bendell J.C. Tolcher A.W. Gordon M.S. … Papadopoulos K.P. A first-in-human dose-escalation study of the oral proteasome inhibitor oprozomib in patients with advanced solid tumors Investigational New Drugs 34 2 2016 216 224 10.1007/s10637-016-0327-x 26924128

Irrcher I. Aleyasin H. Seifert E.L. Hewitt S.J. Chhabra S. Phillips M. Lutz A.K. Rousseaux M.W.C. Bevilacqua L. Jahani-Asl A. Callaghan S. MacLaurin J.G. Winklhofer K.F. Rizzu P. Rippstein P. Kim R.H. Chen C.X. Fon E.A. Slack R.S. … Park D.S. Loss of the Parkinson's disease-linked gene DJ-1 perturbs mitochondrial dynamics Human Molecular Genetics 19 19 2010 3734 3746 10.1093/hmg/ddq288 20639397

Isasa M. Katz E.J. Kim W. Yugo V. González S. Kirkpatrick D.S. … Crosas B. Monoubiquitination of RPN10 regulates substrate recruitment to the proteasome Molecular Cell 38 5 2010 733 745 10.1016/j.molcel.2010.05.001 20542005 PMC3282119

Iseri P.K. Altinaş O. Tokay T. Yüksel N. Relationship between cognitive impairment and retinal morphological and visual functional abnormalities in Alzheimer disease Journal of Neuro-Ophthalmology: The Official Journal of the North American Neuro-Ophthalmology Society 26 1 2006 18 24 10.1097/01.wno.0000204645.56873.26 16518161

Jagannath S. Barlogie B. Berenson J. Siegel D. Irwin D. Richardson P.G. … Anderson K.C. A phase 2 study of two doses of bortezomib in relapsed or refractory myeloma British Journal of Haematology 127 2 2004 165 172 10.1111/j.1365-2141.2004.05188.x 15461622

Jagannath S. Durie B.G.M. Wolf J. Camacho E. Irwin D. Lutzky J. … Crowley J. Bortezomib therapy alone and in combination with dexamethasone for previously untreated symptomatic multiple myeloma British Journal of Haematology 129 6 2005 776 783 10.1111/j.1365-2141.2005.05540.x 15953004

Jain D. Singh K. Chirumamilla S. Bibat G.M. Blue M.E. Naidu S.R. Eberhart C.G. Ocular MECP2 protein expression in patients with and without Rett syndrome Pediatric Neurology 43 1 2010 35 40 10.1016/j.pediatrneurol.2010.02.018 20682201 PMC2916867

Jain A. Zode G. Kasetti R.B. Ran F.A. Yan W. Sharma T.P. … Sheffield V.C. CRISPR-Cas9–based treatment of myocilin-associated glaucoma Proceedings of the National Academy of Sciences 114 42 2017 11199 11204 10.1073/pnas.1706193114 PMC5651749 28973933

Jakubowiak A.J. Dytfeld D. Griffith K.A. Jasielec J. McDonnell K. Lebovic D. … Kaminski M.S. Treatment outcome with the combination of carfilzomib, lenalidomide, and low-dose dexamethasone (CRd) for newly diagnosed multiple myeloma (NDMM) after extended follow-up Journal of Clinical Oncology 31 15_suppl 2013 8543 10.1200/jco.2013.31.15_suppl.8543

Jakubowiak A. Offidani M. Pégourie B. De La Rubia J. Garderet L. Laribi K. … Palumbo A. Randomized phase 2 study: Elotuzumab plus bortezomib/dexamethasone vs bortezomib/dexamethasone for relapsed/refractory MM Blood 127 23 2016 2833 2840 10.1182/blood-2016-01-694604 27091875 PMC4900953

Janda E. Isidoro C. Carresi C. Mollace V. Defective autophagy in Parkinson's disease: Role of oxidative stress Molecular Neurobiology 46 3 2012 639 661 10.1007/s12035-012-8318-1 22899187

Jang H.H. Regulation of Protein Degradation by Proteasomes in Cancer Journal of Cancer Prevention 23 4 2018 153 161 10.15430/JCP.2018.23.4.153 30671397 PMC6330989

Jarome T.J. Kwapis J.L. Ruenzel W.L. Helmstetter F.J. CaMKII, but not protein kinase A, regulates Rpt6 phosphorylation and proteasome activity during the formation of long-term memories Frontiers in Behavioral Neuroscience 7 2013 10.3389/fnbeh.2013.00115 PMC3757295 24009566

Jellinger K.A. Interaction between pathogenic proteins in neurodegenerative disorders Journal of Cellular and Molecular Medicine 16 6 2012 1166 1183 10.1111/j.1582-4934.2011.01507.x 22176890 PMC3823071

Jiang T.-X. Zhao M. Qiu X.-B. Substrate receptors of proteasomes Biological Reviews of the Cambridge Philosophical Society 93 4 2018 1765 1777 10.1111/brv.12419 29732666

Jing K. Shin S. Jeong S. Kim S. Song K.S. Park J.H. … Lim K. Docosahexaenoic acid induces the degradation of HPV E6/E7 oncoproteins by activating the ubiquitin-proteasome system Cell Death & Disease 5 11 2014 10.1038/cddis.2014.477 PMC4260735 25393480

Joe M.K. Nakaya N. Abu-Asab M. Tomarev S.I. Mutated myocilin and heterozygous Sod2 deficiency act synergistically in a mouse model of open-angle glaucoma Human Molecular Genetics 24 12 2015 3322 3334 10.1093/hmg/ddv082 25740847 PMC4447889

Johnson D.E. The ubiquitin-proteasome system: Opportunities for therapeutic intervention in solid tumors Endocrine-Related Cancer 22 1 2015 T1 17 10.1530/ERC-14-0005 24659480 PMC4170053

Jones C.L. Tepe J.J. Proteasome activation to combat proteotoxicity Molecules 24 15 2019 10.3390/molecules24152841 PMC6696185 31387243

Ju D. Xie Y. Proteasomal degradation of RPN4 via two distinct mechanisms, ubiquitin-dependent and -independent Journal of Biological Chemistry 279 23 2004 23851 23854 10.1074/jbc.C400111200 15090546

Juenemann K. Jansen A.H.P. van Riel L. Merkx R. Mulder M.P.C. An H. … Reits E.A. Dynamic recruitment of ubiquitin to mutant huntingtin inclusion bodies Scientific Reports 8 2018 10.1038/s41598-018-19538-0 PMC5780509 29362455

Juenemann K. Wiemhoefer A. Reits E.A. Detection of ubiquitinated huntingtin species in intracellular aggregates Frontiers in Molecular Neuroscience 2015 8 10.3389/fnmol.2015.00001 25674046 PMC4309157

Kabayama H. Tokushige N. Takeuchi M. Kabayama M. Fukuda M. Mikoshiba K. Parkin promotes proteasomal degradation of synaptotagmin IV by accelerating polyubiquitination Molecular and Cellular Neurosciences 80 2017 89 99 10.1016/j.mcn.2017.02.006 28254618

Kageyama M. Ota T. Sasaoka M. Katsuta O. Shinomiya K. Chemical proteasome inhibition as a novel animal model of inner retinal degeneration in rats PLoS One 14 5 2019 e0217945 10.1371/journal.pone.0217945 PMC6544319 31150519

Kandoth C. McLellan M.D. Vandin F. Ye K. Niu B. Lu C. … Ding L. Mutational landscape and significance across 12 major cancer types Nature 502 7471 2013 333 339 10.1038/nature12634 24132290 PMC3927368

Kang J. Lemaire H.G. Unterbeck A. Salbaum J.M. Masters C.L. Grzeschik K.H. … Müller-Hill B. The precursor of Alzheimer's disease amyloid A4 protein resembles a cell-surface receptor Nature 325 6106 1987 733 736 10.1038/325733a0 2881207

Kaplan G.S. Torcun C.C. Grune T. Ozer N.K. Karademir B. Proteasome inhibitors in cancer therapy: Treatment regimen and peripheral neuropathy as a side effect Free Radical Biology & Medicine 103 2017 1 13 10.1016/j.freeradbiomed.2016.12.007 27940347

Kapuria V. Peterson L.F. Fang D. Bornmann W.G. Talpaz M. Donato N.J. Deubiquitinase inhibition by small-molecule WP1130 triggers aggresome formation and tumor cell apoptosis Cancer Research 70 22 2010 9265 9276 10.1158/0008-5472.CAN-10-1530 21045142

Karin M. Cao Y. Greten F.R. Li Z.-W. NF-kappaB in cancer: from innocent bystander to major culprit Nature Reviews. Cancer 2 4 2002 301 310 10.1038/nrc780 12001991

Karin M. Greten F.R. NF-kappaB: linking inflammation and immunity to cancer development and progression Nature Reviews. Immunology 5 10 2005 749 759 10.1038/nri1703 16175180

Karpowicz P. Osmulski P.A. Witkowska J. Sikorska E. Giżyńska M. Belczyk-Ciesielska A. … Jankowska E. Interplay between structure and charge as a key to allosteric modulation of human 20S proteasome by the basic fragment of HIV-1 tat protein PLoS One 10 11 2015 e0143038 10.1371/journal.pone.0143038 PMC4648528 26575189

Kastritis E. Dialoupi I. Gavriatopoulou M. Roussou M. Kanellias N. Fotiou D. Ntanasis-Stathopoulos I. Papadopoulou E. Ziogas D.C. Stamatelopoulos K. Manios E. Ntalianis A. Eleutherakis-Papaiakovou E. Papanikolaou A. Migkou M. Papanota A.-M. Gakiopoulou H. Psimenou E. Tselegkidi M.I. … Dimopoulos M.A. Primary treatment of light-chain amyloidosis with bortezomib, lenalidomide, and dexamethasone Blood Advances 3 20 2019 3002 3009 10.1182/bloodadvances.2019000147 31648323 PMC6849948

Katsiki M. Chondrogianni N. Chinou I. Rivett A.J. Gonos E.S. The olive constituent oleuropein exhibits proteasome stimulatory properties in vitro and confers life span extension of human embryonic fibroblasts Rejuvenation Research 10 2 2007 157 172 10.1089/rej.2006.0513 17518699

Kaundal M. Zameer S. Najmi A.K. Parvez S. Akhtar M. Betulinic acid, a natural PDE inhibitor restores hippocampal cAMP/cGMP and BDNF, improve cerebral blood flow and recover memory deficits in permanent BCCAO induced vascular dementia in rats European Journal of Pharmacology 832 2018 56 66 10.1016/j.ejphar.2018.05.015 29778746

Kaur G. Batra S. Emerging role of immunoproteasomes in pathophysiology Immunology & Cell Biology 94 9 2016 812 820 10.1038/icb.2016.50 27192937 PMC13360420

Keck S. Nitsch R. Grune T. Ullrich O. Proteasome inhibition by paired helical filament-tau in brains of patients with Alzheimer's disease Journal of Neurochemistry 85 1 2003 115 122 12641733 10.1046/j.1471-4159.2003.01642.x

Keller J.N. Hanni K.B. Markesbery W.R. Impaired proteasome function in Alzheimer's disease Journal of Neurochemistry 75 1 2000 436 439 10854289 10.1046/j.1471-4159.2000.0750436.x

Keren-Kaplan T. Zeev Peters L. Levin-Kravets O. Attali I. Kleifeld O. Shohat N. … Prag G. Structure of ubiquitylated-Rpn10 provides insight into its autoregulation mechanism Nature Communications 7 1 2016 1 12 10.1038/ncomms12960 PMC5059453 27698474

Kerrin A. Weldon S. Chung A.H.-K. Craig T. Simpson A.J. O'Kane C.M. … Taggart C.C. Proteolytic cleavage of elafin by 20S proteasome may contribute to inflammation in acute lung injury Thorax 68 4 2013 315 321 10.1136/thoraxjnl-2012-202536 23242946

Khor B. Bredemeyer A.L. Huang C.-Y. Turnbull I.R. Evans R. Maggi L.B. … Sleckman B.P. Proteasome activator PA200 is required for normal spermatogenesis Molecular and Cellular Biology 26 8 2006 2999 3007 10.1128/MCB.26.8.2999-3007.2006 16581775 PMC1446934

Kikis E.A. Gidalevitz T. Morimoto R.I. Protein homeostasis in models of aging and age-related conformational disease Advances in Experimental Medicine and Biology 694 2010 138 159 20886762 10.1007/978-1-4419-7002-2_11 PMC3402352

Kim K.B. Crews C.M. From epoxomicin to carfilzomib: Chemistry, biology, and medical outcomes Natural Product Reports 30 5 2013 600 604 10.1039/c3np20126k 23575525 PMC3815659

Kim H.T. Goldberg A.L. The deubiquitinating enzyme Usp14 allosterically inhibits multiple proteasomal activities and ubiquitin-independent proteolysis The Journal of Biological Chemistry 292 23 2017 9830 9839 10.1074/jbc.M116.763128 28416611 PMC5465503

Kim H.J. Hawke N. Baldwin A.S. NF-kappaB and IKK as therapeutic targets in cancer Cell Death and Differentiation 13 5 2006 738 747 10.1038/sj.cdd.4401877 16485028

Kim B.S. Savinova O.V. Reedy M.V. Martin J. Lun Y. Gan L. … Johnson R.L. Targeted disruption of the myocilin gene (Myoc) suggests that human glaucoma-causing mutations are gain of function Molecular and Cellular Biology 21 22 2001 7707 7713 10.1128/MCB.21.22.7707-7713.2001 11604506 PMC99941

Kim J. Zaret K.S. Reprogramming of human cancer cells to pluripotency for models of cancer progression The EMBO Journal 34 6 2015 739 747 10.15252/embj.201490736 25712212 PMC4369311

Kimura H. Caturegli P. Takahashi M. Suzuki K. New insights into the function of the immunoproteasome in immune and nonimmune cells Journal of Immunology Research 2015 2015 541984 10.1155/2015/541984 26636107 PMC4617869

King R.W. Deshaies R.J. Peters J.M. Kirschner M.W. How proteolysis drives the cell cycle Science (New York, N.Y.) 274 5293 1996 1652 1659 10.1126/science.274.5293.1652 8939846

Kiprowska M.J. Stepanova A. Todaro D.R. Galkin A. Haas A. Wilson S.M. Figueiredo-Pereira M.E. Neurotoxic mechanisms by which the USP14 inhibitor IU1 depletes ubiquitinated proteins and Tau in rat cerebral cortical neurons: relevance to Alzheimer's disease Biochimica et Biophysica Acta 1863 6 2017 1157 1170 10.1016/j.bbadis.2017.03.017 28372990 PMC5549686

Kish-Trier E. Hill C.P. Structural biology of the proteasome Annual Review of Biophysics 42 2013 29 49 10.1146/annurev-biophys-083012-130417 PMC4878838 23414347

Kisselev A.F. Akopian T.N. Castillo V. Goldberg A.L. Proteasome active sites allosterically regulate each other, suggesting a cyclical bite-chew mechanism for protein breakdown Molecular Cell 4 3 1999 395 402 10.1016/s1097-2765(00)80341-x 10518220

Kisselev A.F. Goldberg A.L. Proteasome inhibitors: From research tools to drug candidates Chemistry & Biology 8 8 2001 739 758 10.1016/s1074-5521(01)00056-4 11514224

Kisselev A.F. Kaganovich D. Goldberg A.L. Binding of hydrophobic peptides to several non-catalytic sites promotes peptide hydrolysis by all active sites of 20 S proteasomes. Evidence for peptide-induced channel opening in the alpha-rings The Journal of Biological Chemistry 277 25 2002 22260 22270 10.1074/jbc.M112360200 11927581

Kisselev A.F. Songyang Z. Goldberg A.L. Why does threonine, and not serine, function as the active site nucleophile in proteasomes? The Journal of Biological Chemistry 275 20 2000 14831 14837 10.1074/jbc.275.20.14831 10809725

Kisselev A.F. van der Linden W.A. Overkleeft H.S. Proteasome inhibitors: An expanding army attacking a unique target Chemistry & Biology 19 1 2012 99 115 10.1016/j.chembiol.2012.01.003 22284358 PMC3503453

Klaips C.L. Jayaraj G.G. Hartl F.U. Pathways of cellular proteostasis in aging and disease The Journal of Cell Biology 217 1 2018 51 63 10.1083/jcb.201709072 29127110 PMC5748993

Kleinknecht A. Popova B. Lázaro D.F. Pinho R. Valerius O. Outeiro T.F. Braus G.H. C-terminal tyrosine residue modifications modulate the protective phosphorylation of serine 129 of α-synuclein in a yeast model of Parkinson's disease PLoS Genetics 12 6 2016 e1006098 10.1371/journal.pgen.1006098 PMC4920419 27341336

Klinkradt S. Naudé R.J. Muramoto K. Oelofsen W. Purification and characterization of proteasome from ostrich liver International Journal of Biochemistry & Cell Biology 29 4 1997 611 622 10.1016/s1357-2725(96)00143-4 9363639

Kniepert A. Groettrup M. The unique functions of tissue-specific proteasomes Trends in Biochemical Sciences 39 1 2014 17 24 10.1016/j.tibs.2013.10.004 24286712

Knowles A. Koh K. Wu J.-T. Chien C.-T. Chamovitz D.A. Blau J. The COP9 signalosome is required for light-dependent timeless degradation and Drosophila clock resetting The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 29 4 2009 1152 1162 10.1523/JNEUROSCI.0429-08.2009 19176824 PMC2648809

Köhler A. Bajorek M. Groll M. Moroder L. Rubin D.M. Huber R. … Finley D. The substrate translocation channel of the proteasome Biochimie 83 3–4 2001 325 332 10.1016/s0300-9084(01)01242-1 11295493

Köhler A. Cascio P. Leggett D.S. Woo K.M. Goldberg A.L. Finley D. The axial channel of the proteasome core particle is gated by the Rpt2 ATPase and controls both substrate entry and product release Molecular Cell 7 6 2001 1143 1152 10.1016/s1097-2765(01)00274-x 11430818

Komura H. Kakio S. Sasahara T. Arai Y. Takino N. Sato M. … Hoshi M. Alzheimer Aβ assemblies accumulate in excitatory neurons upon proteasome inhibition and kill nearby NAKα3 neurons by secretion IScience 13 2019 452 477 Scopus 10.1016/j.isci.2019.01.018 30827871 PMC6443839

Korde N. Roschewski M. Zingone A. Kwok M. Manasanch E.E. Bhutani M. Tageja N. Kazandjian D. Mailankody S. Wu P. Morrison C. Costello R. Zhang Y. Burton D. Mulquin M. Zuchlinski D. Lamping L. Carpenter A. Wall Y. … Landgren O. Treatment with carfilzomib-lenalidomide-dexamethasone with lenalidomide extension in patients with smoldering or newly diagnosed multiple myeloma JAMA Oncology 1 6 2015 746 754 10.1001/jamaoncol.2015.2010 26181891 PMC6662597

Kors S. Geijtenbeek K. Reits E. Schipper-Krom S. Regulation of proteasome activity by (post-)transcriptional mechanisms Frontiers in Molecular Biosciences 6 2019 10.3389/fmolb.2019.00048 PMC6646590 31380390

Koulich E. Li X. DeMartino G.N. Relative structural and functional roles of multiple deubiquitylating proteins associated with mammalian 26S proteasome Molecular Biology of the Cell 19 3 2008 1072 1082 10.1091/mbc.E07-10-1040 18162577 PMC2262970

Kouroukis C.T. Fernandez L.A.V. Crump M. Gascoyne R.D. Chua N.S. Buckstein R. … Eisenhauer E. A phase II study of bortezomib and gemcitabine in relapsed mantle cell lymphoma from the National Cancer Institute of Canada Clinical Trials Group (IND 172) Leukemia & Lymphoma 52 3 2011 394 399 10.3109/10428194.2010.546015 21323520

Koyuncu S. Saez I. Lee H.J. Gutierrez-Garcia R. Pokrzywa W. Fatima A. … Vilchez D. The ubiquitin ligase UBR5 suppresses proteostasis collapse in pluripotent stem cells from Huntington's disease patients Nature Communications 9 1 2018 2886 10.1038/s41467-018-05320-3 PMC6056416 30038412

Kozai T. Sekiguchi T. Satoh T. Yagi H. Kato K. Uchihashi T. Two-step process for disassembly mechanism of proteasome α7 homo-tetradecamer by α6 revealed by high-speed atomic force microscopy Scientific Reports 7 1 2017 1 9 10.1038/s41598-017-15708-8 29133893 PMC5684232

Kozlitina J. Smagris E. Stender S. Nordestgaard B.G. Zhou H.H. Tybjærg-Hansen A. … Cohen J.C. Exome-wide association study identifies a TM6SF2 variant that confers susceptibility to nonalcoholic fatty liver disease Nature Genetics 46 4 2014 352 356 10.1038/ng.2901 24531328 PMC3969786

Kraus M. Bader J. Geurink P.P. Weyburne E.S. Mirabella A.C. Silzle T. … Driessen C. The novel β2-selective proteasome inhibitor LU-102 synergizes with bortezomib and carfilzomib to overcome proteasome inhibitor resistance of myeloma cells Haematologica 100 10 2015 1350 1360 10.3324/haematol.2014.109421 26069288 PMC4591768

Kravtsova-Ivantsiv Y. Cohen S. Ciechanover A. Modification by single ubiquitin moieties rather than polyubiquitination is sufficient for proteasomal processing of the p105 NF-kappaB precursor Molecular Cell 33 4 2009 496 504 10.1016/j.molcel.2009.01.023 19250910

Krebiehl G. Ruckerbauer S. Burbulla L.F. Kieper N. Maurer B. Waak J. … Krüger R. Reduced basal autophagy and impaired mitochondrial dynamics due to loss of Parkinson's disease-associated protein DJ-1 PLoS One 5 2 2010 e9367 10.1371/journal.pone.0009367 PMC2826413 20186336

Krüger E. Kloetzel P.M. Enenkel C. 20S proteasome biogenesis Biochimie 83 3–4 2001 289 293 10.1016/s0300-9084(01)01241-x 11295488

Kubiczkova L. Pour L. Sedlarikova L. Hajek R. Sevcikova S. Proteasome inhibitors - molecular basis and current perspectives in multiple myeloma Journal of Cellular and Molecular Medicine 18 6 2014 947 961 10.1111/jcmm.12279 24712303 PMC4508135

Kudriaeva A.A. Belogurov A.A. Proteasome: A nanomachinery of creative destruction Biochemistry. Biokhimiia 84 Suppl. 1 2019 S159 S192 10.1134/S0006297919140104 31213201

Kudriaeva A. Kuzina E.S. Zubenko O. Smirnov I.V. Belogurov A. Charge-mediated proteasome targeting FASEB Journal : Official Publication of the Federation of American Societies for Experimental Biology 33 6 2019 6852 6866 10.1096/fj.201802237R 30811957

Kulak N.A. Geyer P.E. Mann M. Loss-less nano-fractionator for high sensitivity, high coverage proteomics Molecular & Cellular Proteomics: MCP 16 4 2017 694 705 10.1074/mcp.O116.065136 28126900 PMC5383787

Kumar S.K. Bensinger W.I. Zimmerman T.M. Reeder C.B. Berenson J.R. Berg D. … Niesvizky R. Phase 1 study of weekly dosing with the investigational oral proteasome inhibitor ixazomib in relapsed/refractory multiple myeloma Blood 124 7 2014 1047 1055 10.1182/blood-2014-01-548941 24904120 PMC4468583

Kumar S.K. Berdeja J.G. Niesvizky R. Lonial S. Laubach J.P. Hamadani M. … Richardson P.G. Safety and tolerability of ixazomib, an oral proteasome inhibitor, in combination with lenalidomide and dexamethasone in patients with previously untreated multiple myeloma: An open-label phase 1/2 study The Lancet. Oncology 15 13 2014 1503 1512 10.1016/S1470-2045(14)71125-8 25456369

Kumar S.K. Berdeja J.G. Niesvizky R. Lonial S. Laubach J.P. Hamadani M. … Richardson P.G. Ixazomib, lenalidomide, and dexamethasone in patients with newly diagnosed multiple myeloma: Long-term follow-up including ixazomib maintenance Leukemia 33 7 2019 1736 1746 10.1038/s41375-019-0384-1 30696949 PMC6755968

Kumar S.K. LaPlant B.R. Reeder C.B. Roy V. Halvorson A.E. Buadi F. Gertz M.A. Bergsagel P.L. Dispenzieri A. Thompson M.A. Crawley J. Kapoor P. Mikhael J. Stewart K. Hayman S.R. Hwa Y.L. Gonsalves W. Witzig T.E. Ailawadhi S. … Lacy M.Q. Randomized phase 2 trial of ixazomib and dexamethasone in relapsed multiple myeloma not refractory to bortezomib Blood 128 20 2016 2415 2422 10.1182/blood-2016-05-717769 27702799 PMC5114487

Kumar S.K. LaPlant B. Roy V. Reeder C.B. Lacy M.Q. Gertz M.A. … Dispenzieri A. Phase 2 trial of ixazomib in patients with relapsed multiple myeloma not refractory to bortezomib Blood Cancer Journal 5 2015 e338 10.1038/bcj.2015.60 PMC4558585 26275080

Kumar V. Singh D. Singh B.K. Singh S. Mittra N. Jha R.R. … Singh C. Alpha-synuclein aggregation, ubiquitin proteasome system impairment, and L-Dopa response in zinc-induced Parkinsonism: Resemblance to sporadic Parkinson's disease Molecular and Cellular Biochemistry 444 1–2 2018 149 160 10.1007/s11010-017-3239-y 29198021

Kumatori A. Tanaka K. Inamura N. Sone S. Ogura T. Matsumoto T. … Ichihara A. Abnormally high expression of proteasomes in human leukemic cells Proceedings of the National Academy of Sciences of the United States of America 87 18 1990 7071 7075 10.1073/pnas.87.18.7071 2205851 PMC54685

Kunjappu M.J. Hochstrasser M. Assembly of the 20S proteasome Biochimica et Biophysica Acta 1843 1 2014 2 12 10.1016/j.bbamcr.2013.03.008 23507199 PMC3752329

Kupperman E. Lee E.C. Cao Y. Bannerman B. Fitzgerald M. Berger A. … Bolen J. Evaluation of the proteasome inhibitor MLN9708 in preclinical models of human cancer Cancer Research 70 5 2010 1970 1980 10.1158/0008-5472.CAN-09-2766 20160034

Kurimoto E. Satoh T. Ito Y. Ishihara E. Okamoto K. Yagi-Utsumi M. … Kato K. Crystal structure of human proteasome assembly chaperone PAC4 involved in proteasome formation Protein Science: A Publication of the Protein Society 26 5 2017 1080 1085 10.1002/pro.3153 28263418 PMC5405420

Kurup P.K. Xu J. Videira R.A. Ononenyi C. Baltazar G. Lombroso P.J. Nairn A.C. STEP61 is a substrate of the E3 ligase parkin and is upregulated in Parkinson's disease Proceedings of the National Academy of Sciences 112 4 2015 1202 1207 10.1073/pnas.1417423112 PMC4313846 25583483

Kusmierczyk A.R. Kunjappu M.J. Funakoshi M. Hochstrasser M. A multimeric assembly factor controls the formation of alternative 20S proteasomes Nature Structural & Molecular Biology 15 3 2008 237 244 10.1038/nsmb.1389 18278055

Labbadia J. Morimoto R.I. Huntington's disease: Underlying molecular mechanisms and emerging concepts Trends in Biochemical Sciences 38 8 2013 378 385 10.1016/j.tibs.2013.05.003 23768628 PMC3955166

Labbadia J. Morimoto R.I. The biology of proteostasis in aging and disease Annual Review of Biochemistry 84 2015 435 464 10.1146/annurev-biochem-060614-033955 PMC4539002 25784053

Lacal P.M. Graziani G. Therapeutic implication of vascular endothelial growth factor receptor-1 (VEGFR-1) targeting in cancer cells and tumor microenvironment by competitive and non-competitive inhibitors Pharmacological Research 136 2018 97 107 10.1016/j.phrs.2018.08.023 30170190

Lam Y.A. Xu W. DeMartino G.N. Cohen R.E. Editing of ubiquitin conjugates by an isopeptidase in the 26S proteasome Nature 385 6618 1997 737 740 10.1038/385737a0 9034192

Lamm W. Kaufmann H. Raderer M. Hoffmann M. Chott A. Zielinski C. Drach J. Bortezomib combined with rituximab and dexamethasone is an active regimen for patients with relapsed and chemotherapy-refractory mantle cell lymphoma Haematologica 96 7 2011 1008 1014 10.3324/haematol.2011.041392 21486866 PMC3128220

Lan A.P. Chen J. Chai Z.F. Hu Y. The neurotoxicity of iron, copper and cobalt in Parkinson's disease through ROS-mediated mechanisms BioMetals 29 4 2016 665 678 10.1007/s10534-016-9942-4 27349232

Lander G.C. Estrin E. Matyskiela M.E. Bashore C. Nogales E. Martin A. Complete subunit architecture of the proteasome regulatory particle Nature 482 7384 2012 186 191 10.1038/nature10774 22237024 PMC3285539

Landis-Piwowar K.R. Milacic V. Chen D. Yang H. Zhao Y. Chan T.H. … Dou Q.P. The proteasome as a potential target for novel anticancer drugs and chemosensitizers Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 9 6 2006 263 273 10.1016/j.drup.2006.11.001 17197231

Lau J.L. Dunn M.K. Therapeutic peptides: Historical perspectives, current development trends, and future directions Bioorganic & Medicinal Chemistry 26 10 2018 2700 2707 10.1016/j.bmc.2017.06.052 28720325

Le W. Role of iron in UPS impairment model of Parkinson's disease Parkinsonism & Related Disorders 20 Suppl. 1 2014 S158 S161 10.1016/S1353-8020(13)70038-5 24262171

Le Y.-Z. VEGF production and signaling in Müller glia are critical to modulating vascular function and neuronal integrity in diabetic retinopathy and hypoxic retinal vascular diseases Vision Research 139 2017 108 114 10.1016/j.visres.2017.05.005 28601428 PMC5723217

Le Tallec B. Barrault M.-B. Courbeyrette R. Guérois R. Marsolier-Kergoat M.-C. Peyroche A. 20S proteasome assembly is orchestrated by two distinct pairs of chaperones in yeast and in mammals Molecular Cell 27 4 2007 660 674 10.1016/j.molcel.2007.06.025 17707236

Le T.M. Wong H.H. Tay F.P.L. Fang S. Keng C.-T. Tan Y.J. Liu D.X. Expression, post-translational modification and biochemical characterization of proteins encoded by subgenomic mRNA8 of the severe acute respiratory syndrome coronavirus The FEBS Journal 274 16 2007 4211 4222 10.1111/j.1742-4658.2007.05947.x 17645546 PMC7164070

LeBlanc R. Catley L.P. Hideshima T. Lentzsch S. Mitsiades C.S. Mitsiades N. … Anderson K.C. Proteasome inhibitor PS-341 inhibits human myeloma cell growth in vivo and prolongs survival in a murine model Cancer Research 62 17 2002 4996 5000 12208752

Lecker S.H. Goldberg A.L. Mitch W.E. Protein degradation by the ubiquitin-proteasome pathway in normal and disease states Journal of the American Society of Nephrology: JASN 17 7 2006 1807 1819 10.1681/ASN.2006010083 16738015

Lee V.M. Balin B.J. Otvos L. Trojanowski J.Q. A68: a major subunit of paired helical filaments and derivatized forms of normal Tau Science (New York, N.Y.) 251 4994 1991 675 678 10.1126/science.1899488 1899488

Lee S.Y.-C. De la Mota-Peynado A. Roelofs J. Loss of Rpt5 protein interactions with the core particle and Nas2 protein causes the formation of faulty proteasomes that are inhibited by Ecm29 protein The Journal of Biological Chemistry 286 42 2011 36641 36651 10.1074/jbc.M111.280875 21878651 PMC3196109

Lee E.C. Fitzgerald M. Bannerman B. Donelan J. Bano K. Terkelsen J. Bradley D.P. Subakan O. Silva M.D. Liu R. Pickard M. Li Z. Tayber O. Li P. Hales P. Carsillo M. Neppalli V.T. Berger A.J. Kupperman E. … Janz S. Antitumor activity of the investigational proteasome inhibitor MLN9708 in mouse models of B-cell and plasma cell malignancies Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 17 23 2011 7313 7323 10.1158/1078-0432.CCR-11-0636 21903769 PMC3443972

Lee W. Kim Y. Park J. Shim S. Lee J. Hong S. … Jung Y.-K. iRhom1 regulates proteasome activity via PAC1/2 under ER stress Scientific Reports 5 2015 11559 10.1038/srep11559 26109405 PMC4479803

Lee W.H. Lee J.M. Lim C. Kim S. Kim S.G. Structural requirements within protoporphyrin IX in the inhibition of heat shock protein 90 Chemico-Biological Interactions 204 1 2013 49 57 10.1016/j.cbi.2013.04.006 23624237

Lee B.-H. Lee M.J. Park S. Oh D.-C. Elsasser S. Chen P.-C. … Finley D. Enhancement of proteasome activity by a small-molecule inhibitor of USP14 Nature 467 7312 2010 179 184 10.1038/nature09299 20829789 PMC2939003

Lee M.J. Lee J.H. Rubinsztein D.C. Tau degradation: The ubiquitin-proteasome system versus the autophagy-lysosome system Progress in Neurobiology 105 2013 49 59 10.1016/j.pneurobio.2013.03.001 23528736

Lee Y. Lim H.-S. Skp2 Inhibitors: Novel Anticancer Strategies Current Medicinal Chemistry 23 22 2016 2363 2379 10.2174/0929867323666160510122624 27160538

Lee B.-H. Lu Y. Prado M.A. Shi Y. Tian G. Sun S. … Finley D. USP14 deubiquitinates proteasome-bound substrates that are ubiquitinated at multiple sites Nature 532 7599 2016 398 401 10.1038/nature17433 27074503 PMC4844788

Lee M.J. Miller Z. Park J.E. Bhattarai D. Lee W. Kim K.B. H727 cells are inherently resistant to the proteasome inhibitor carfilzomib, yet require proteasome activity for cell survival and growth Scientific Reports 9 1 2019 1 9 10.1038/s41598-019-40635-1 30858500 PMC6411724

Leestemaker Y. de Jong A. Witting K.F. Penning R. Schuurman K. Rodenko B. … Ovaa H. Proteasome activation by small molecules Cell Chemical Biology 24 6 2017 725 736 e7 10.1016/j.chembiol.2017.05.010 28552582

Leestemaker Y. Ovaa H. Tools to investigate the ubiquitin proteasome system Drug Discovery Today: Technologies 26 2017 25 31 10.1016/j.ddtec.2017.11.006 29249239

Leggett D.S. Hanna J. Borodovsky A. Crosas B. Schmidt M. Baker R.T. … Finley D. Multiple associated proteins regulate proteasome structure and function Molecular Cell 10 3 2002 495 507 10.1016/s1097-2765(02)00638-x 12408819

Lehmann A. Niewienda A. Jechow K. Janek K. Enenkel C. Ecm29 fulfils quality control functions in proteasome assembly Molecular Cell 38 6 2010 879 888 10.1016/j.molcel.2010.06.016 20620957

Leleu X. Martin T.G. Einsele H. Lyons R.M. Durie B.G.M. Iskander K.S. Ailawadhi S. Role of proteasome inhibitors in relapsed and/or refractory multiple myeloma Clinical Lymphoma, Myeloma & Leukemia 19 1 2019 9 22 10.1016/j.clml.2018.08.016 30287200

Leleu X. Masszi T. Bahlis N.J. Viterbo L. Baker B. Gimsing P. … Richardson P.G. Patient-reported health-related quality of life from the phase III TOURMALINE-MM1 study of ixazomib-lenalidomide-dexamethasone versus placebo-lenalidomide-dexamethasone in relapsed/refractory multiple myeloma American Journal of Hematology 2018 10.1002/ajh.25134 29726031

Levêque D. Carvalho M.C.M. Maloisel F. Review. Clinical pharmacokinetics of bortezomib In Vivo (Athens, Greece) 21 2 2007 273 278 17436576

Levin N. Spencer A. Harrison S.J. Chauhan D. Burrows F.J. Anderson K.C. … Trikha M. Marizomib irreversibly inhibits proteasome to overcome compensatory hyperactivation in multiple myeloma and solid tumour patients British Journal of Haematology 174 5 2016 711 720 10.1111/bjh.14113 27161872 PMC5084787

Li X. Amazit L. Long W. Lonard D.M. Monaco J.J. O'Malley B.W. Ubiquitin- and ATP-independent proteolytic turnover of p21 by the REGgamma-proteasome pathway Molecular Cell 26 6 2007 831 842 10.1016/j.molcel.2007.05.028 17588518

Li B. Fu J. Chen P. Ge X. Li Y. Kuiatse I. … Orlowski R.Z. The nuclear factor (erythroid-derived 2)-like 2 and proteasome maturation protein axis mediate bortezomib resistance in multiple myeloma The Journal of Biological Chemistry 290 50 2015 29854 29868 10.1074/jbc.M115.664953 26483548 PMC4705994

Li T. Ho L. Piperdi B. Elrafei T. Camacho F.J. Rigas J.R. … Gucalp R. Phase II study of the proteasome inhibitor bortezomib (PS-341, Velcade) in chemotherapy-naïve patients with advanced stage non-small cell lung cancer (NSCLC) Lung Cancer (Amsterdam, Netherlands) 68 1 2010 89 93 10.1016/j.lungcan.2009.05.009 19524318

Li X. Huang Q. Long H. Zhang P. Su H. Liu J. A new gold(I) complex-au(PPh3)PT is a deubiquitinase inhibitor and inhibits tumor growth EBioMedicine 39 2019 159 172 10.1016/j.ebiom.2018.11.047 30527624 PMC6354570

Li Y. Huang J. Zeng B. Yang D. Sun J. Yin X. … Ren G. PSMD2 regulates breast cancer cell proliferation and cell cycle progression by modulating p21 and p27 proteasomal degradation Cancer Letters 430 2018 109 122 10.1016/j.canlet.2018.05.018 29777785

Li C. Johnson D.E. Bortezomib induces autophagy in head and neck squamous cell carcinoma cells via JNK activation Cancer Letters 314 1 2012 102 107 10.1016/j.canlet.2011.09.020 21993018 PMC3225600

Li C. Johnson D.E. Liberation of functional p53 by proteasome inhibition in human papilloma virus-positive head and neck squamous cell carcinoma cells promotes apoptosis and cell cycle arrest Cell Cycle (Georgetown, Texas) 12 6 2013 923 934 10.4161/cc.23882 PMC3637351 23421999

Li C. Li R. Grandis J.R. Johnson D.E. Bortezomib induces apoptosis via Bim and Bik up-regulation and synergizes with cisplatin in the killing of head and neck squamous cell carcinoma cells Molecular Cancer Therapeutics 7 6 2008 1647 1655 10.1158/1535-7163.MCT-07-2444 18566236 PMC2534142

Li W. Moore M.J. Vasilieva N. Sui J. Wong S.K. Berne M.A. … Farzan M. Angiotensin-converting enzyme 2 is a functional receptor for the SARS coronavirus Nature 426 6965 2003 450 454 10.1038/nature02145 14647384 PMC7095016

Li J. Powell S.R. Wang X. Enhancement of proteasome function by PA28α overexpression protects against oxidative stress The FASEB Journal 25 3 2011 883 893 10.1096/fj.10-160895 21098724 PMC3042837

Li W. Sui J. Huang I.-C. Kuhn J.H. Radoshitzky S.R. Marasco W.A. … Farzan M. The S proteins of human coronavirus NL63 and severe acute respiratory syndrome coronavirus bind overlapping regions of ACE2 Virology 367 2 2007 367 374 10.1016/j.virol.2007.04.035 17631932 PMC2693060

Li F. Tian G. Langager D. Sokolova V. Finley D. Park S. Nucleotide-dependent switch in proteasome assembly mediated by the Nas6 chaperone Proceedings of the National Academy of Sciences of the United States of America 114 7 2017 1548 1553 10.1073/pnas.1612922114 28137839 PMC5321026

Li H. Wu X. Cheng X. Advances in diagnosis and treatment of metastatic cervical cancer Journal of Gynecologic Oncology 27 4 2016 e43 10.3802/jgo.2016.27.e43 27171673 PMC4864519

Li S.-W. Yang T.-C. Wan L. Lin Y.-J. Tsai F.-J. Lai C.-C. Lin C.-W. Correlation between TGF-β1 expression and proteomic profiling induced by severe acute respiratory syndrome coronavirus papain-like protease Proteomics 12 21 2012 3193 3205 10.1002/pmic.201200225 22936401 PMC7168038

Li A.C. Yu E. Ring S.C. Chovan J.P. Boronic acid-containing proteasome inhibitors: Alert to potential pharmaceutical bioactivation Chemical Research in Toxicology 26 4 2013 608 615 10.1021/tx400032n 23514361

Li G. Yuan S. Pan Y. Liu Y. Huang G. Binding states of protein-metal complexes in cells Analytical Chemistry 88 22 2016 10860 10866 10.1021/acs.analchem.6b00032 27748592

Li C. Zang Y. Sen M. Leeman-Neill R.J. Man D.S.K. Grandis J.R. Johnson D.E. Bortezomib up-regulates activated signal transducer and activator of transcription-3 and synergizes with inhibitors of signal transducer and activator of transcription-3 to promote head and neck squamous cell carcinoma cell death Molecular Cancer Therapeutics 8 8 2009 2211 2220 10.1158/1535-7163.MCT-09-0327 19638453 PMC2768047

Li Y. Zhou Z. Chen C. WW domain-containing E3 ubiquitin protein ligase 1 targets p63 transcription factor for ubiquitin-mediated proteasomal degradation and regulates apoptosis Cell Death and Differentiation 15 12 2008 1941 1951 10.1038/cdd.2008.134 18806757

Liao Y. Liu N. Hua X. Cai J. Xia X. Wang X. … Liu J. Proteasome-associated deubiquitinase ubiquitin-specific protease 14 regulates prostate cancer proliferation by deubiquitinating and stabilizing androgen receptor Cell Death & Disease 8 2 2017 e2585 10.1038/cddis.2016.477 28151478 PMC5386460

Lichtenberg M. Mansilla A. Zecchini V.R. Fleming A. Rubinsztein D.C. The Parkinson's disease protein LRRK2 impairs proteasome substrate clearance without affecting proteasome catalytic activity Cell Death & Disease 2 2011 e196 10.1038/cddis.2011.81 PMC3181424 21866175

Lim J. Lachenmayer M.L. Wu S. Liu W. Kundu M. Wang R. … Yue Z. Proteotoxic stress induces phosphorylation of p62/SQSTM1 by ULK1 to regulate selective autophagic clearance of protein aggregates PLoS Genetics 11 2 2015 e1004987 10.1371/journal.pgen.1004987 PMC4344198 25723488

Limanaqi F. Biagioni F. Gaglione A. Busceti C.L. Fornai F. A sentinel in the crosstalk between the nervous and immune system: The (Immuno)-proteasome Frontiers in Immunology 10 2019 10.3389/fimmu.2019.00628 PMC6450179 30984192

Lin J.-T. Chang W.-C. Chen H.-M. Lai H.-L. Chen C.-Y. Tao M.-H. Chern Y. Regulation of feedback between protein kinase A and the proteasome system worsens Huntington's disease Molecular and Cellular Biology 33 5 2013 1073 1084 10.1128/MCB.01434-12 23275441 PMC3623082

Lin Y.-C. Chen K.-C. Chen C.-C. Cheng A.-L. Chen K.-F. CIP2A-mediated Akt activation plays a role in bortezomib-induced apoptosis in head and neck squamous cell carcinoma cells Oral Oncology 48 7 2012 585 593 10.1016/j.oraloncology.2012.01.012 22342571

Lin L. Jin Z. Tan H. Xu Q. Peng T. Li H. Atypical ubiquitination by E3 ligase WWP1 inhibits the proteasome-mediated degradation of mutant huntingtin Brain Research 1643 2016 103 112 10.1016/j.brainres.2016.03.027 27107943

Lin X. Parisiadou L. Gu X.-L. Wang L. Shim H. Sun L. … Cai H. Leucine-rich repeat kinase 2 regulates the progression of neuropathology induced by Parkinson's-disease-related mutant alpha-synuclein Neuron 64 6 2009 807 827 10.1016/j.neuron.2009.11.006 20064389 PMC2807409

Lipsitch M. Swerdlow D.L. Finelli L. Defining the epidemiology of SARS-Cov-2 — Studies needed New England Journal of Medicine 382 13 2020 1194 1196 10.1056/NEJMp2002125 32074416

Lipski D.A. Dewispelaere R. Foucart V. Caspers L.E. Defrance M. Bruyns C. Willermain F. MHC class II expression and potential antigen-presenting cells in the retina during experimental autoimmune uveitis Journal of Neuroinflammation 14 1 2017 136 10.1186/s12974-017-0915-5 28720143 PMC5516361

Liu D.X. Fung T.S. Chong K.K.-L. Shukla A. Hilgenfeld R. Accessory proteins of SARS-CoV and other coronaviruses Antiviral Research 109 2014 97 109 10.1016/j.antiviral.2014.06.013 24995382 PMC7113789

Liu Y. Hettinger C.L. Zhang D. Rezvani K. Wang X. Wang H. The proteasome function reporter GFPu accumulates in young brains of the APPswe/PS1dE9 Alzheimer's disease mouse models Cellular and Molecular Neurobiology 34 3 2014 315 322 Scopus 10.1007/s10571-013-0022-9 24363091 PMC3954921

Liu N. Li X. Huang H. Zhao C. Liao S. Yang C. Liu S. Song W. Lu X. Lan X. Chen X. Yi S. Xu L. Jiang L. Zhao C. Dong X. Zhou P. Li S. Wang S. … Liu J. Clinically used antirheumatic agent auranofin is a proteasomal deubiquitinase inhibitor and inhibits tumor growth Oncotarget 5 14 2014 5453 5471 10.18632/oncotarget.2113 24977961 PMC4170648

Liu C.-W. Li X. Thompson D. Wooding K. Chang T. Tang Z. … DeMartino G.N. ATP binding and ATP hydrolysis play distinct roles in the function of 26S proteasome Molecular Cell 24 1 2006 39 50 10.1016/j.molcel.2006.08.025 17018291 PMC3951175

Liu Y. Qiao F. Leiferman P.C. Ross A. Schlenker E.H. Wang H. FOXOs modulate proteasome activity in human-induced pluripotent stem cells of Huntington's disease and their derived neural cells Human Molecular Genetics 26 22 2017 4416 4428 10.1093/hmg/ddx327 28973411 PMC6075623

Lobanova E.S. Finkelstein S. Li J. Travis A.M. Hao Y. Klingeborn M. … Arshavsky V.Y. Increased proteasomal activity supports photoreceptor survival in inherited retinal degeneration Nature Communications 9 1 2018 1738 10.1038/s41467-018-04117-8 PMC5928105 29712894

Loda M. Cukor B. Tam S.W. Lavin P. Fiorentino M. Draetta G.F. … Pagano M. Increased proteasome-dependent degradation of the cyclin-dependent kinase inhibitor p27 in aggressive colorectal carcinomas Nature Medicine 3 2 1997 231 234 10.1038/nm0297-231 9018245

Lokireddy S. Kukushkin N.V. Goldberg A.L. cAMP-induced phosphorylation of 26S proteasomes on Rpn6/PSMD11 enhances their activity and the degradation of misfolded proteins Proceedings of the National Academy of Sciences of the United States of America 112 52 2015 E7176 E7185 10.1073/pnas.1522332112 26669444 PMC4702992

Lombardo M. Parravano M. Serrao S. Ducoli P. Stirpe M. Lombardo G. Analysis of retinal capillaries in patients with type 1 diabetes and nonproliferative diabetic retinopathy using adaptive optics imaging Retina (Philadelphia, PA.) 33 8 2013 1630 1639 10.1097/IAE.0b013e3182899326 23492950

London A. Benhar I. Schwartz M. The retina as a window to the brain-from eye research to CNS disorders Nature Reviews. Neurology 9 1 2013 44 53 10.1038/nrneurol.2012.227 23165340

Lopes U.G. Erhardt P. Yao R. Cooper G.M. p53-dependent induction of apoptosis by proteasome inhibitors The Journal of Biological Chemistry 272 20 1997 12893 12896 10.1074/jbc.272.20.12893 9148891

Lopez Salon M. Pasquini L. Besio Moreno M. Pasquini J.M. Soto E. Relationship between beta-amyloid degradation and the 26S proteasome in neural cells Experimental Neurology 180 2 2003 131 143 12684027 10.1016/s0014-4886(02)00060-2

Lopez-Salon M. Alonso M. Vianna M.R. Viola H. Mello e Souza, T., Izquierdo, I., Pasquini, J. M., & Medina, J. H. The ubiquitin-proteasome cascade is required for mammalian long-term memory formation The European Journal of Neuroscience 14 11 2001 1820 1826 10.1046/j.0953-816x.2001.01806.x 11860477

LoRusso P.M. Venkatakrishnan K. Ramanathan R.K. Sarantopoulos J. Mulkerin D. Shibata S.I. … Ivy P. Pharmacokinetics and safety of bortezomib in patients with advanced malignancies and varying degrees of liver dysfunction: Phase I NCI Organ Dysfunction Working Group Study NCI-6432 Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 18 10 2012 2954 2963 10.1158/1078-0432.CCR-11-2873 22394984 PMC3834726

Lovell M.A. Robertson J.D. Teesdale W.J. Campbell J.L. Markesbery W.R. Copper, iron and zinc in Alzheimer's disease senile plaques Journal of the Neurological Sciences 158 1 1998 47 52 9667777 10.1016/s0022-510x(98)00092-6

Löwe J. Stock D. Jap B. Zwickl P. Baumeister W. Huber R. Crystal structure of the 20S proteasome from the archaeon T. acidophilum at 3.4 A resolution Science (New York, N.Y.) 268 5210 1995 533 539 10.1126/science.7725097 7725097

Lu H. Zong C. Wang Y. Young G.W. Deng N. Souda P. … Ping P. Revealing the dynamics of the 20 S proteasome phosphoproteome: A combined CID and electron transfer dissociation approach Molecular & Cellular Proteomics: MCP 7 11 2008 2073 2089 10.1074/mcp.M800064-MCP200 18579562 PMC2577207

Łuczkowska K. Rogińska D. Ulańczyk Z. Machaliński B. Effect of Bortezomib on Global Gene Expression in PC12-Derived Nerve Cells International Journal of Molecular Sciences 21 3 2020 10.3390/ijms21030751 PMC7038167 31979371

Ludwig H. Moreau P. Dimopoulos M.A. Mateos M.-V. Kaiser M. Hajek R. … Weisel K. Health-related quality of life in the ENDEAVOR study: Carfilzomib-dexamethasone vs bortezomib-dexamethasone in relapsed/refractory multiple myeloma Blood Cancer Journal 9 3 2019 23 10.1038/s41408-019-0181-0 30796199 PMC6386751

Luo H. Cao L. Liang X. Du A. Peng T. Li H. Herp promotes degradation of mutant Huntingtin: Involvement of the proteasome and molecular chaperones Molecular Neurobiology 55 10 2018 7652 7668 10.1007/s12035-018-0900-8 29430620

Lv H. Wei G.-Y. Guo C.-S. Deng Y.-F. Jiang Y.-M. Gao C. Jian C.-D. 20S proteasome and glyoxalase 1 activities decrease in erythrocytes derived from Alzheimer's disease patients Neural Regeneration Research 15 1 2020 178 183 Scopus 10.4103/1673-5374.264473 31535667 PMC6862418

Ma X.-Z. Bartczak A. Zhang J. Khattar R. Chen L. Liu M.F. … McGilvray I.D. Proteasome inhibition in vivo promotes survival in a lethal murine model of severe acute respiratory syndrome Journal of Virology 84 23 2010 12419 12428 10.1128/JVI.01219-10 20861244 PMC2976395

Ma Y.-S. Wang X.-F. Zhang Y.-J. Luo P. Long H.-D. Li L. … Fu D. Inhibition of USP14 Deubiquitinating Activity as a Potential Therapy for Tumors with p53 Deficiency Molecular Therapy - Oncolytics 16 2020 147 157 10.1016/j.omto.2019.12.013 32055678 PMC7005481

Ma M.H. Yang H.H. Parker K. Manyak S. Friedman J.M. Altamirano C. … Berenson J.R. The proteasome inhibitor PS-341 markedly enhances sensitivity of multiple myeloma tumor cells to chemotherapeutic agents Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 9 3 2003 1136 1144 12631619

MacLaren A.P. Chapman R.S. Wyllie A.H. Watson C.J. p53-dependent apoptosis induced by proteasome inhibition in mammary epithelial cells Cell Death and Differentiation 8 3 2001 210 218 10.1038/sj.cdd.4400801 11319603

Mahul-Mellier A.-L. Fauvet B. Gysbers A. Dikiy I. Oueslati A. Georgeon S. … Lashuel H.A. C-Abl phosphorylates α-synuclein and regulates its degradation: Implication for α-synuclein clearance and contribution to the pathogenesis of Parkinson's disease Human Molecular Genetics 23 11 2014 2858 2879 10.1093/hmg/ddt674 24412932 PMC4014189

Maillard R.A. Chistol G. Sen M. Righini M. Tan J. Kaiser C.M. … Bustamante C. ClpX(P) generates mechanical force to unfold and translocate its protein substrates Cell 145 3 2011 459 469 10.1016/j.cell.2011.04.010 21529717 PMC3686100

Majumder P. Rudack T. Beck F. Danev R. Pfeifer G. Nagy I. Baumeister W. Cryo-EM structures of the archaeal PAN-proteasome reveal an around-the-ring ATPase cycle Proceedings of the National Academy of Sciences of the United States of America 116 2 2019 534 539 10.1073/pnas.1817752116 30559193 PMC6329974

Manasanch E.E. Orlowski R.Z. Proteasome inhibitors in cancer therapy Nature Reviews. Clinical Oncology 14 7 2017 417 433 10.1038/nrclinonc.2016.206 PMC5828026 28117417

Manton C.A. Johnson B. Singh M. Bailey C.P. Bouchier-Hayes L. Chandra J. Induction of cell death by the novel proteasome inhibitor marizomib in glioblastoma in vitro and in vivo Scientific Reports 6 2016 18953 10.1038/srep18953 26804704 PMC4726202

Marques A.J. Palanimurugan R. Matias A.C. Ramos P.C. Dohmen R.J. Catalytic mechanism and assembly of the proteasome Chemical Reviews 109 4 2009 1509 1536 10.1021/cr8004857 19265443

Marshall R.S. Vierstra R.D. Dynamic regulation of the 26S proteasome: From synthesis to degradation Frontiers in Molecular Biosciences 6 2019 40 10.3389/fmolb.2019.00040 31231659 PMC6568242

Martin A. Baker T.A. Sauer R.T. Pore loops of the AAA+ ClpX machine grip substrates to drive translocation and unfolding Nature Structural & Molecular Biology 15 11 2008 1147 1151 10.1038/nsmb.1503 PMC2610342 18931677

Maryon, E. B., Molloy, S. A., Zimnicka, A. M., & Kaplan, J. H. (2007). Copper entry into human cells: Progress and unanswered questions. BioMetals, 20(3–4), 355–364. 10.1007/s10534-006-9066-3 . 17211679

Marzano C. Pellei M. Tisato F. Santini C. Copper complexes as anticancer agents Anti-Cancer Agents in Medicinal Chemistry 9 2 2009 185 211 19199864 10.2174/187152009787313837

Masamha C.P. Benbrook D.M. Cyclin D1 degradation is sufficient to induce G1 cell cycle arrest despite constitutive expression of cyclin E2 in ovarian cancer cells Cancer Research 69 16 2009 6565 6572 10.1158/0008-5472.CAN-09-0913 19638577

Mateos M.-V. Cavo M. Blade J. Dimopoulos M.A. Suzuki K. Jakubowiak A. Knop S. Doyen C. Lucio P. Nagy Z. Pour L. Cook M. Grosicki S. Crepaldi A. Liberati A.M. Campbell P. Shelekhova T. Yoon S.-S. Iosava G. … San-Miguel J. Overall survival with daratumumab, bortezomib, melphalan, and prednisone in newly diagnosed multiple myeloma (ALCYONE): a randomised, open-label, phase 3 trial Lancet (London, England) 395 10218 2020 132 141 10.1016/S0140-6736(19)32956-3 31836199

Mateos M.-V. Hernández J.-M. Hernández M.-T. Gutiérrez N.-C. Palomera L. Fuertes M. Díaz-Mediavilla J. Lahuerta J.-J. de la Rubia J. Terol M.-J. Sureda A. Bargay J. Ribas P. de Arriba F. Alegre A. Oriol A. Carrera D. García-Laraña J. García-Sanz R. … San Miguel J.-F. Bortezomib plus melphalan and prednisone in elderly untreated patients with multiple myeloma: Results of a multicenter phase 1/2 study Blood 108 7 2006 2165 2172 10.1182/blood-2006-04-019778 16772605

Mateos M.-V. Hernández J.M. Hernández M.T. Gutiérrez N.C. Palomera L. Fuertes M. Garcia-Sanchez P. Lahuerta J.J. de la Rubia J. Terol M.-J. Sureda A. Bargay J. Ribas P. Alegre A. de Arriba F. Oriol A. Carrera D. García-Laraña J. García-Sanz R. … San Miguel J.F. Bortezomib plus melphalan and prednisone in elderly untreated patients with multiple myeloma: Updated time-to-events results and prognostic factors for time to progression Haematologica 93 4 2008 560 565 10.3324/haematol.12106 18322252

Mateos M.-V. Masszi T. Grzasko N. Hansson M. Sandhu I. Pour L. … Moreau P. Impact of prior therapy on the efficacy and safety of oral ixazomib-lenalidomide-dexamethasone vs. placebo-lenalidomide-dexamethasone in patients with relapsed/refractory multiple myeloma in TOURMALINE-MM1 Haematologica 102 10 2017 1767 1775 10.3324/haematol.2017.170118 28751562 PMC5622861

Mateos M.-V. Richardson P.G. Schlag R. Khuageva N.K. Dimopoulos M.A. Shpilberg O. … San Miguel J.F. Bortezomib plus melphalan and prednisone compared with melphalan and prednisone in previously untreated multiple myeloma: Updated follow-up and impact of subsequent therapy in the phase III VISTA trial Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 28 13 2010 2259 2266 10.1200/JCO.2009.26.0638 20368561

Mathewson A.C. Bishop A. Yao Y. Kemp F. Ren J. Chen H. … Jones I.M. Interaction of severe acute respiratory syndrome-coronavirus and NL63 coronavirus spike proteins with angiotensin converting enzyme-2 The Journal of General Virology 89 2008 2741 2745 10.1099/vir.0.2008/003962-0 Pt 11 18931070 PMC2886958

Matias A.C. Ramos P.C. Dohmen R.J. Chaperone-assisted assembly of the proteasome core particle Biochemical Society Transactions 38 Pt 1 2010 29 33 10.1042/BST0380029 20074030

Matsumura K. Aketa K. Proteasome (multicatalytic proteinase) of sea urchin sperm and its possible participation in the acrosome reaction Molecular Reproduction and Development 29 2 1991 189 199 10.1002/mrd.1080290215 1878226

Matteucci A. Gaddini L. Villa M. Varano M. Parravano M. Monteleone V. … Pricci F. Neuroprotection by rat Müller glia against high glucose-induced neurodegeneration through a mechanism involving ERK1/2 activation Experimental Eye Research 125 2014 20 29 10.1016/j.exer.2014.05.011 24877742

Mattson M.P. Magnus T. Aging and neuronal vulnerabilitys Nature Review. Neuroscience 7 4 2006 278 294 10.1038/nrn1886 16552414 PMC3710114

Matyskiela M.E. Lander G.C. Martin A. Conformational switching of the 26S proteasome enables substrate degradation Nature Structural & Molecular Biology 20 7 2013 781 788 10.1038/nsmb.2616 PMC3712289 23770819

Mayaux J.F. Bousseau A. Pauwels R. Huet T. Hénin Y. Dereu N. … Le Pecq J.B. Triterpene derivatives that block entry of human immunodeficiency virus type 1 into cells Proceedings of the National Academy of Sciences of the United States of America 91 9 1994 3564 3568 10.1073/pnas.91.9.3564 8170948 PMC43620

McAlary L. Plotkin S.S. Cashman N.R. Emerging developments in targeting proteotoxicity in neurodegenerative diseases CNS Drugs 33 9 2019 883 904 10.1007/s40263-019-00657-9 31414322 PMC6776490

McKinnon C. Goold R. Andre R. Devoy A. Ortega Z. Moonga J. … Tabrizi S.J. Prion-mediated neurodegeneration is associated with early impairment of the ubiquitin-proteasome system Acta Neuropathologica 131 3 2016 411 425 10.1007/s00401-015-1508-y 26646779 PMC4752964

McKinnon C. Tabrizi S.J. The ubiquitin-proteasome system in neurodegeneration Antioxidants & Redox Signaling 21 17 2014 2302 2321 10.1089/ars.2013.5802 24437518

McNaught K.S.P. Mytilineou C. Jnobaptiste R. Yabut J. Shashidharan P. Jennert P. Olanow C.W. Impairment of the ubiquitin-proteasome system causes dopaminergic cell death and inclusion body formation in ventral mesencephalic cultures Journal of Neurochemistry 81 2 2002 301 306 10.1046/j.1471-4159.2002.00821.x 12064477

McNaught K.S.P. Perl D.P. Brownell A.-L. Olanow C.W. Systemic exposure to proteasome inhibitors causes a progressive model of Parkinson's disease Annals of Neurology 56 1 2004 149 162 10.1002/ana.20186 15236415

McPherson S.W. Heuss N.D. Pierson M.J. Gregerson D.S. Retinal antigen-specific regulatory T cells protect against spontaneous and induced autoimmunity and require local dendritic cells Journal of Neuroinflammation 2014 11 10.1186/s12974-014-0205-4 25498509 PMC4268905

Meng L. Mohan R. Kwok B.H. Elofsson M. Sin N. Crews C.M. Epoxomicin, a potent and selective proteasome inhibitor, exhibits in vivo antiinflammatory activity Proceedings of the National Academy of Sciences of the United States of America 96 18 1999 10403 10408 10.1073/pnas.96.18.10403 10468620 PMC17900

Metcalfe M.J. Huang Q. Figueiredo-Pereira M.E. Coordination between proteasome impairment and caspase activation leading to TAU pathology: Neuroprotection by cAMP Cell Death & Disease 3 6 2012 e326 10.1038/cddis.2012.70 22717581 PMC3388240

Mi L. Gan N. Chung F.-L. Isothiocyanates inhibit proteasome activity and proliferation of multiple myeloma cells Carcinogenesis 32 2 2011 216 223 10.1093/carcin/bgq242 21109604 PMC3026846

Micale N. Schirmeister T. Ettari R. Cinellu M.A. Maiore L. Serratrice M. … Messori L. Selected cytotoxic gold compounds cause significant inhibition of 20S proteasome catalytic activities Journal of Inorganic Biochemistry 141 2014 79 82 10.1016/j.jinorgbio.2014.08.001 25217719

Micera A. Quaranta L. Esposito G. Floriani I. Pocobelli A. Saccà S.C. … Oddone F. Differential protein expression profiles in glaucomatous trabecular meshwork: An evaluation study on a small primary open angle glaucoma population Advances in Therapy 33 2 2016 252 267 10.1007/s12325-016-0285-x 26820987 PMC4769730

Mikhael J.R. Schuster S.R. Jimenez-Zepeda V.H. Bello N. Spong J. Reeder C.B. … Fonseca R. Cyclophosphamide-bortezomib-dexamethasone (CyBorD) produces rapid and complete hematologic response in patients with AL amyloidosis Blood 119 19 2012 4391 4394 10.1182/blood-2011-11-390930 22331188 PMC3557400

Miller B.F. Drake J.C. Naylor B. Price J.C. Hamilton K.L. The measurement of protein synthesis for assessing proteostasis in studies of slowed aging Ageing Research Reviews 18 2014 106 111 10.1016/j.arr.2014.09.005 25283966 PMC4258117

Minakshi R. Padhan K. Rani M. Khan N. Ahmad F. Jameel S. The SARS Coronavirus 3a protein causes endoplasmic reticulum stress and induces ligand-independent downregulation of the type 1 interferon receptor PLoS One 4 12 2009 e8342 10.1371/journal.pone.0008342 PMC2791231 20020050

Minegishi Y. Sheng X. Yoshitake K. Sergeev Y. Iejima D. Shibagaki Y. … Iwata T. CCT2 mutations evoke Leber congenital amaurosis due to chaperone complex instability Scientific Reports 6 2016 33742 10.1038/srep33742 27645772 PMC5028737

Mitch W.E. Goldberg A.L. Mechanisms of muscle wasting. The role of the ubiquitin-proteasome pathway The New England Journal of Medicine 335 25 1996 1897 1905 10.1056/NEJM199612193352507 8948566

Mitsiades N. Mitsiades C.S. Richardson P.G. Poulaki V. Tai Y.-T. Chauhan D. … Anderson K.C. The proteasome inhibitor PS-341 potentiates sensitivity of multiple myeloma cells to conventional chemotherapeutic agents: Therapeutic applications Blood 101 6 2003 2377 2380 10.1182/blood-2002-06-1768 12424198

Moldovan G.-L. Pfander B. Jentsch S. PCNA, the maestro of the replication fork Cell 129 4 2007 665 679 10.1016/j.cell.2007.05.003 17512402

Momand J. Wu H.H. Dasgupta G. MDM2--master regulator of the p53 tumor suppressor protein Gene 242 1–2 2000 15 29 10.1016/s0378-1119(99)00487-4 10721693

Moreau P. Attal M. Hulin C. Arnulf B. Belhadj K. Benboubker L. Béné M.C. Broijl A. Caillon H.L. Caillot D. Corre J. Delforge M. Dejoie T. Doyen C. Facon T. Sonntag C.C. Fontan J. Garderet L. Jie K.-S. … Sonneveld P. Bortezomib, thalidomide, and dexamethasone with or without daratumumab before and after autologous stem-cell transplantation for newly diagnosed multiple myeloma (CASSIOPEIA): A randomised, open-label, phase 3 study The Lancet 394 10192 2019 29 38 10.1016/S0140-6736(19)31240-1 31171419

Moreau P. Kolb B. Attal M. Caillot D. Benboubker L. Tiab M. … Facon T. Phase 1/2 study of carfilzomib plus melphalan and prednisone in patients aged over 65 years with newly diagnosed multiple myeloma Blood 125 20 2015 3100 3104 10.1182/blood-2015-02-626168 25784682

Moreau P. Kumar S. Boccia R. Iida S. Goldschmidt H. Cocks K. … Dimopoulos M. Convenience, satisfaction, health-related quality of life of once-weekly 70 mg/m2 vs. twice-weekly 27 mg/m2 carfilzomib (randomized A.R.R.O.W. study) Leukemia 33 12 2019 2934 2946 10.1038/s41375-019-0480-2 31092895

Moreau P. Masszi T. Grzasko N. Bahlis N.J. Hansson M. Pour L. Sandhu I. Ganly P. Baker B.W. Jackson S.R. Stoppa A.-M. Simpson D.R. Gimsing P. Palumbo A. Garderet L. Cavo M. Kumar S. Touzeau C. Buadi F.K. … Richardson P.G. Oral ixazomib, lenalidomide, and dexamethasone for multiple myeloma New England Journal of Medicine 374 17 2016 1621 1634 10.1056/NEJMoa1516282 27119237

Moreau P. Mateos M.-V. Berenson J.R. Weisel K. Lazzaro A. Song K. … Stewart A.K. Once weekly versus twice weekly carfilzomib dosing in patients with relapsed and refractory multiple myeloma (A.R.R.O.W.): Interim analysis results of a randomised, phase 3 study The Lancet. Oncology 19 7 2018 953 964 10.1016/S1470-2045(18)30354-1 29866475

Moreau P. Pylypenko H. Grosicki S. Karamanesht I. Leleu X. Grishunina M. … Harousseau J.-L. Subcutaneous versus intravenous administration of bortezomib in patients with relapsed multiple myeloma: A randomised, phase 3, non-inferiority study The Lancet. Oncology 12 5 2011 431 440 10.1016/S1470-2045(11)70081-X 21507715

Moreau P. Stewart K.A. Dimopoulos M. Siegel D. Facon T. Berenson J. … Mateos M.-V. Once-weekly (70 mg/m2) vs twice-weekly (56 mg/m2) dosing of carfilzomib in patients with relapsed or refractory multiple myeloma: A post hoc analysis of the ENDEAVOR, A.R.R.O.W., and CHAMPION-1 trials Cancer Medicine 2020 10.1002/cam4.2945 PMC7196059 32108443

Morice-Picard F. Jonca N. Pichery M. Mermin D. Leauté-Labrèze C. Taïeb A. … Boralevi F. KLICK syndrome: Recognizable phenotype and hot-spot POMP mutation Journal of the European Academy of Dermatology and Venereology: JEADV 31 3 2017 e154 e156 10.1111/jdv.13898 27503413

Morimoto R.I. Proteotoxic stress and inducible chaperone networks in neurodegenerative disease and aging Genes & Development 22 11 2008 1427 1438 10.1101/gad.1657108 18519635 PMC2732416

Morita A. Kimura M. Itokawa Y. The effect of aging on the mineral status of female mice Biological Trace Element Research 42 2 1994 165 177 10.1007/BF02785387 7981006

Morozov A.V. Karpov V.L. Proteasomes and several aspects of their heterogeneity relevant to cancer Frontiers in Oncology 9 2019 761 10.3389/fonc.2019.00761 31456945 PMC6700291

Morris M. Knudsen G.M. Maeda S. Trinidad J.C. Ioanoviciu A. Burlingame A.L. Mucke L. Tau post-translational modifications in wild-type and human amyloid precursor protein transgenic mice Nature Neuroscience 18 8 2015 1183 1189 10.1038/nn.4067 26192747 PMC8049446

Morsi R.Z. Hage-Sleiman R. Kobeissy H. Dbaibo G. Noxa: Role in Cancer Pathogenesis and Treatment Current Cancer Drug Targets 18 10 2018 914 928 10.2174/1568009618666180308105048 29521234

Mortusewicz O. Schermelleh L. Walter J. Cardoso M.C. Leonhardt H. Recruitment of DNA methyltransferase I to DNA repair sites Proceedings of the National Academy of Sciences of the United States of America 102 25 2005 8905 8909 10.1073/pnas.0501034102 15956212 PMC1157029

Moscovitz O. Ben-Nissan G. Fainer I. Pollack D. Mizrachi L. Sharon M. The Parkinson's-associated protein DJ-1 regulates the 20S proteasome Nature Communications 6 2015 6609 10.1038/ncomms7609 25833141

Mullauer F.B. Kessler J.H. Medema J.P. Betulinic acid, a natural compound with potent anticancer effects Anti-Cancer Drugs 21 3 2010 215 227 10.1097/CAD.0b013e3283357c62 20075711

Murakami A. Modulation of protein quality control systems by food phytochemicals Journal of Clinical Biochemistry and Nutrition 52 3 2013 215 227 10.3164/jcbn.12-126 23704811 PMC3652296

Murakami Y. Matsufuji S. Kameji T. Hayashi S. Igarashi K. Tamura T. … Ichihara A. Ornithine decarboxylase is degraded by the 26S proteasome without ubiquitination Nature 360 6404 1992 597 599 10.1038/360597a0 1334232

Murata S. Sasaki K. Kishimoto T. Niwa S.-I. Hayashi H. Takahama Y. Tanaka K. Regulation of CD8+ T cell development by thymus-specific proteasomes Science (New York, N.Y.) 316 5829 2007 1349 1353 10.1126/science.1141915 17540904

Murata S. Takahama Y. Kasahara M. Tanaka K. The immunoproteasome and thymoproteasome: Functions, evolution and human disease Nature Immunology 19 9 2018 923 931 10.1038/s41590-018-0186-z 30104634

Murata S. Udono H. Tanahashi N. Hamada N. Watanabe K. Adachi K. … Chiba T. Immunoproteasome assembly and antigen presentation in mice lacking both PA28α and PA28β The EMBO Journal 20 21 2001 5898 5907 10.1093/emboj/20.21.5898 11689430 PMC125708

Muz B. Ghazarian R.N. Ou M. Luderer M.J. Kusdono H.D. Azab A.K. Spotlight on ixazomib: Potential in the treatment of multiple myeloma Drug Design, Development and Therapy 10 2016 217 226 10.2147/DDDT.S93602 PMC4714737 26811670

Myeku, N., Clelland, C. L., Emrani, S., Kukushkin, N. V, Yu, W. H., Goldberg, A. L., & Duff, K. E. (2016). Tau-driven 26S proteasome impairment and cognitive dysfunction can be prevented early in disease by activating cAMP-PKA signaling. Nature Medicine, 22(1), 46–53. 10.1038/nm.4011 . PMC4787271 26692334

Myeku N. Duff K.E. Targeting the 26S proteasome to protect against proteotoxic diseases Trends in Molecular Medicine 24 1 2018 18 29 10.1016/j.molmed.2017.11.006 29233753 PMC5905406

Myung J. Kim K.B. Lindsten K. Dantuma N.P. Crews C.M. Lack of proteasome active site allostery as revealed by subunit-specific inhibitors Molecular Cell 7 2 2001 411 420 10.1016/s1097-2765(01)00188-5 11239469

Naash M.I. Al-Ubaidi M.R. Anderson R.E. Light exposure induces ubiquitin conjugation and degradation activities in the rat retina Investigative Ophthalmology & Visual Science 38 11 1997 2344 2354 9344358

Nag T.C. Wadhwa S. Ultrastructure of the human retina in aging and various pathological states Micron (Oxford, England: 1993) 43 7 2012 759 781 10.1016/j.micron.2012.01.011 22445096

Nakanishi C. Toi M. Nuclear factor-kappaB inhibitors as sensitizers to anticancer drugs Nature Reviews. Cancer 5 4 2005 297 309 10.1038/nrc1588 15803156

Narayanan S. Cai C.-Y. Assaraf Y.G. Guo H.-Q. Cui Q. Wei L. … Chen Z.-S. Targeting the ubiquitin-proteasome pathway to overcome anti-cancer drug resistance Drug Resistance Updates 48 2020 100663 10.1016/j.drup.2019.100663 31785545

Niazi S. Purohit M. Niazi J.H. Role of p53 circuitry in tumorigenesis: A brief review European Journal of Medicinal Chemistry 158 2018 7 24 10.1016/j.ejmech.2018.08.099 30199707

Niewerth D. Jansen G. Assaraf Y.G. Zweegman S. Kaspers G.J.L. Cloos J. Molecular basis of resistance to proteasome inhibitors in hematological malignancies Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 18 2015 18 35 10.1016/j.drup.2014.12.001 25670156

Nijholt D.A.T. de Graaf T.R. van Haastert E.S. Oliveira A.O. Berkers C.R. Zwart R. … Scheper W. Endoplasmic reticulum stress activates autophagy but not the proteasome in neuronal cells: Implications for Alzheimer's disease Cell Death and Differentiation 18 6 2011 1071 1081 10.1038/cdd.2010.176 21252911 PMC3131935

Niki T. Endo J. Takahashi-Niki K. Yasuda T. Okamoto A. Saito Y. … Iguchi-Ariga S.M.M. DJ-1-binding compound B enhances Nrf2 activity through the PI3-kinase-Akt pathway by DJ-1-dependent inactivation of PTEN Brain Research 1729 2020 146641 10.1016/j.brainres.2019.146641 31891690

Nitta T. Murata S. Sasaki K. Fujii H. Ripen A.M. Ishimaru N. … Takahama Y. Thymoproteasome shapes immunocompetent repertoire of CD8+ T cells Immunity 32 1 2010 29 40 10.1016/j.immuni.2009.10.009 20045355

Njomen E. Tepe J.J. Proteasome activation as a new therapeutic approach to target proteotoxic disorders Journal of Medicinal Chemistry 2019 10.1021/acs.jmedchem.9b00101 PMC6745699 30839208

Nosengo N. Can you teach old drugs new tricks? Nature 534 7607 2016 314 316 10.1038/534314a 27306171

Nunes A.T. Annunziata C.M. Proteasome inhibitors: Structure and function Seminars in Oncology 44 6 2017 377 380 10.1053/j.seminoncol.2018.01.004 29935898 PMC6020165

Nuytemans K. Theuns J. Cruts M. Van Broeckhoven C. Genetic etiology of Parkinson disease associated with mutations in the SNCA, PARK2, PINK1, PARK7, and LRRK2 genes: A mutation update Human Mutation 31 7 2010 763 780 10.1002/humu.21277 20506312 PMC3056147

O'Connor O.A. Moskowitz C. Portlock C. Hamlin P. Straus D. Dumitrescu O. … Winter J. Patients with chemotherapy-refractory mantle cell lymphoma experience high response rates and identical progression-free survivals compared with patients with relapsed disease following treatment with single agent bortezomib: Results of a multicentre phase 2 clinical trial British Journal of Haematology 145 1 2009 34 39 10.1111/j.1365-2141.2008.07466.x 19220284

O'Connor O.A. Stewart A.K. Vallone M. Molineaux C.J. Kunkel L.A. Gerecitano J.F. Orlowski R.Z. A phase 1 dose escalation study of the safety and pharmacokinetics of the novel proteasome inhibitor carfilzomib (PR-171) in patients with hematologic malignancies Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 15 22 2009 7085 7091 10.1158/1078-0432.CCR-09-0822 19903785 PMC4019989

O'Connor O.A. Wright J. Moskowitz C. Muzzy J. MacGregor-Cortelli B. Stubblefield M. … Zelenetz A.D. Phase II clinical experience with the novel proteasome inhibitor bortezomib in patients with indolent non-Hodgkin's lymphoma and mantle cell lymphoma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 23 4 2005 676 684 10.1200/JCO.2005.02.050 15613699

Oda E. Ohki R. Murasawa H. Nemoto J. Shibue T. Yamashita T. … Tanaka N. Noxa, a BH3-only member of the Bcl-2 family and candidate mediator of p53-induced apoptosis Science (New York, N.Y.) 288 5468 2000 1053 1058 10.1126/science.288.5468.1053 10807576

Oddone F. Lucenteforte E. Michelessi M. Rizzo S. Donati S. Parravano M. Virgili G. Macular versus retinal nerve fiber layer parameters for diagnosing manifest glaucoma: A systematic review of diagnostic accuracy studies Ophthalmology 123 5 2016 939 949 10.1016/j.ophtha.2015.12.041 26891880

Oerlemans R. Franke N.E. Assaraf Y.G. Cloos J. van Zantwijk I. Berkers C.R. … Jansen G. Molecular basis of bortezomib resistance: Proteasome subunit beta5 (PSMB5) gene mutation and overexpression of PSMB5 protein Blood 112 6 2008 2489 2499 10.1182/blood-2007-08-104950 18565852

Offidani M. Corvatta L. Caraffa P. Gentili S. Maracci L. Leoni P. An evidence-based review of ixazomib citrate and its potential in the treatment of newly diagnosed multiple myeloma Oncotargets and Therapy 7 2014 1793 1800 10.2147/OTT.S49187 25302026 PMC4189713

Ogiso Y. Tomida A. Kim H.D. Tsuruo T. Glucose starvation and hypoxia induce nuclear accumulation of proteasome in cancer cells Biochemical and Biophysical Research Communications 258 2 1999 448 452 10.1006/bbrc.1999.0635 10329407

Ohsawa K. Yoshida M. Izumikawa M. Takagi M. Shin-Ya K. Goshima N. … Doi T. Synthesis and biological evaluation of thielocin B1 analogues as protein-protein interaction inhibitors of PAC3 homodimer Bioorganic & Medicinal Chemistry 26 23–24 2018 6023 6034 10.1016/j.bmc.2018.11.001 30455074

Okatsu K. Saisho K. Shimanuki M. Nakada K. Shitara H. Sou Y. … Matsuda N. p62/SQSTM1 cooperates with Parkin for perinuclear clustering of depolarized mitochondria Genes to Cells 15 8 2010 887 900 10.1111/j.1365-2443.2010.01426.x 20604804 PMC2970908

Okazuka K. Ishida T. Proteasome inhibitors for multiple myeloma Japanese Journal of Clinical Oncology 48 9 2018 785 793 10.1093/jjco/hyy108 30102324

Olzscha H. Posttranslational modifications and proteinopathies: How guardians of the proteome are defeated Biological Chemistry 400 7 2019 895 915 10.1515/hsz-2018-0458 30998500

Olzscha H. Schermann S.M. Woerner A.C. Pinkert S. Hecht M.H. Tartaglia G.G. … Vabulas R.M. Amyloid-like aggregates sequester numerous metastable proteins with essential cellular functions Cell 144 1 2011 67 78 10.1016/j.cell.2010.11.050 21215370

Opazo C. Huang X. Cherny R.A. Moir R.D. Roher A.E. White A.R. … Bush A.I. Metalloenzyme-like activity of Alzheimer's disease beta-amyloid. Cu-dependent catalytic conversion of dopamine, cholesterol, and biological reducing agents to neurotoxic H(2)O(2) The Journal of Biological Chemistry 277 43 2002 40302 40308 10.1074/jbc.M206428200 12192006

Orciuolo E. Buda G. Pelosini M. Petrini M. Fludarabine, Bortezomib, Myocet and rituximab chemotherapy in relapsed and refractory mantle cell lymphoma British Journal of Haematology 148 5 2010 810 812 10.1111/j.1365-2141.2009.07998.x 19919649

Orlowski M. Selective activation of the 20 S proteasome (multicatalytic proteinase complex) by histone h3 Biochemistry 40 50 2001 15318 15326 10.1021/bi0116240 11735414

Orlowski M. Cardozo C. Hidalgo M.C. Michaud C. Regulation of the peptidylglutamyl-peptide hydrolyzing activity of the pituitary multicatalytic proteinase complex Biochemistry 30 24 1991 5999 6005 10.1021/bi00238a025 2043637

Orlowski R.Z. Nagler A. Sonneveld P. Bladé J. Hajek R. Spencer A. … San-Miguel J.F. Final overall survival results of a randomized trial comparing bortezomib plus pegylated liposomal doxorubicin with bortezomib alone in patients with relapsed or refractory multiple myeloma Cancer 122 13 2016 2050 2056 10.1002/cncr.30026 27191689 PMC5701574

Orlowski R.Z. Nagler A. Sonneveld P. Bladé J. Hajek R. Spencer A. … Harousseau J.-L. Randomized phase III study of pegylated liposomal doxorubicin plus bortezomib compared with bortezomib alone in relapsed or refractory multiple myeloma: Combination therapy improves time to progression Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 25 25 2007 3892 3901 10.1200/JCO.2006.10.5460 17679727

Orlowski R.Z. Stinchcombe T.E. Mitchell B.S. Shea T.C. Baldwin A.S. Stahl S. … Soignet S.L. Phase I trial of the proteasome inhibitor PS-341 in patients with refractory hematologic malignancies Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 20 22 2002 4420 4427 10.1200/JCO.2002.01.133 12431963

Orlowski M. Wilk S. A multicatalytic protease complex from pituitary that forms enkephalin and enkephalin containing peptides Biochemical and Biophysical Research Communications 101 3 1981 814 822 10.1016/0006-291x(81)91823-4 7030330

Oromendia A.B. Amon A. Aneuploidy: Implications for protein homeostasis and disease Disease Models & Mechanisms 7 1 2014 15 20 10.1242/dmm.013391 24396150 PMC3882044

Ortega Z. Díaz-Hernández M. Lucas J.J. Is the ubiquitin-proteasome system impaired in Huntington's disease? Cellular and Molecular Life Sciences: CMLS 64 17 2007 2245 2257 10.1007/s00018-007-7222-8 17604996 PMC11136357

Ortega Z. Lucas J.J. Ubiquitin-proteasome system involvement in Huntington's disease Frontiers in Molecular Neuroscience 7 2014 77 10.3389/fnmol.2014.00077 25324717 PMC4179678

Osmulski P.A. Karpowicz P. Jankowska E. Bohmann J. Pickering A.M. Gaczyńska M. New peptide-based pharmacophore activates 20S proteasome Molecules (Basel, Switzerland) 25 6 2020 10.3390/molecules25061439 PMC7145288 32235805

Ottobelli L. Citicoline oral solution in glaucoma: is there a role in slowing disease progression? Ophthalmologica 2013 PMID 23615390 10.1159/000350496 23615390

Pack C.-G. Yukii H. Toh-e A. Kudo T. Tsuchiya H. Kaiho A. … Saeki Y. Quantitative live-cell imaging reveals spatio-temporal dynamics and cytoplasmic assembly of the 26S proteasome Nature Communications 5 2014 3396 10.1038/ncomms4396 24598877

Padmanabhan A. Vuong S.A.-T. Hochstrasser M. Assembly of an evolutionarily conserved alternative proteasome isoform in human cells Cell Reports 14 12 2016 2962 2974 10.1016/j.celrep.2016.02.068 26997268 PMC4828729

Palladini G. Perfetti V. Obici L. Caccialanza R. Semino A. Adami F. … Merlini G. Association of melphalan and high-dose dexamethasone is effective and well tolerated in patients with AL (primary) amyloidosis who are ineligible for stem cell transplantation Blood 103 8 2004 2936 2938 10.1182/blood-2003-08-2788 15070667

Palumbo A. Bringhen S. Larocca A. Rossi D. Di Raimondo F. Magarotto V. Patriarca F. Levi A. Benevolo G. Vincelli I.D. Grasso M. Franceschini L. Gottardi D. Zambello R. Montefusco V. Falcone A.P. Omedé P. Marasca R. Morabito F. … Cavo M. Bortezomib-melphalan-prednisone-thalidomide followed by maintenance with bortezomib-thalidomide compared with bortezomib-melphalan-prednisone for initial treatment of multiple myeloma: Updated follow-up and improved survival Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 32 7 2014 634 640 10.1200/JCO.2013.52.0023 24449241

Palumbo A. Bringhen S. Rossi D. Cavalli M. Larocca A. Ria R. Offidani M. Patriarca F. Nozzoli C. Guglielmelli T. Benevolo G. Callea V. Baldini L. Morabito F. Grasso M. Leonardi G. Rizzo M. Falcone A.P. Gottardi D. … Boccadoro M. Bortezomib-melphalan-prednisone-thalidomide followed by maintenance with bortezomib-thalidomide compared with bortezomib-melphalan-prednisone for initial treatment of multiple myeloma: a randomized controlled trial Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 28 34 2010 5101 5109 10.1200/JCO.2010.29.8216 20940200

Palumbo A. Chanan-Khan A. Weisel K. Nooka A.K. Masszi T. Beksac M. … Sonneveld P. Daratumumab, bortezomib, and dexamethasone for multiple myeloma New England Journal of Medicine 375 8 2016 754 766 10.1056/NEJMoa1606038 27557302

Pammolli F. Magazzini L. Riccaboni M. The productivity crisis in pharmaceutical R&D Nature Reviews. Drug Discovery 10 6 2011 428 438 10.1038/nrd3405 21629293

Panasenko O.O. Collart M.A. Not4 E3 ligase contributes to proteasome assembly and functional integrity in part through Ecm29 Molecular and Cellular Biology 31 8 2011 1610 1623 10.1128/MCB.01210-10 21321079 PMC3126335

Pandit B. Gartel A.L. Proteasome inhibitors induce p53-independent apoptosis in human cancer cells The American Journal of Pathology 178 1 2011 355 360 10.1016/j.ajpath.2010.11.010 21224072 PMC3069903

Pang H. Chen D. Cui Q.C. Ping Dou Q. Sodium diethyldithiocarbamate, an AIDS progression inhibitor and a copper-binding compound, has proteasome-inhibitory and apoptosis-inducing activities in cancer cells International Journal of Molecular Medicine 19 5 2007 809 816 Scopus 17390087

Pao K.-C. Wood N.T. Knebel A. Rafie K. Stanley M. Mabbitt P.D. … Virdee S. Activity-based E3 ligase profiling uncovers an E3 ligase with esterification activity Nature 556 7701 2018 381 385 10.1038/s41586-018-0026-1 29643511

Papadopoulos K.P. Burris H.A. Gordon M. Lee P. Sausville E.A. Rosen P.J. … Infante J.R. A phase I/II study of carfilzomib 2-10-min infusion in patients with advanced solid tumors Cancer Chemotherapy and Pharmacology 72 4 2013 861 868 10.1007/s00280-013-2267-x 23975329 PMC3784064

Papadopoulos K.P. Siegel D.S. Vesole D.H. Lee P. Rosen S.T. Zojwalla N. … Badros A. Phase I study of 30-minute infusion of carfilzomib as single agent or in combination with low-dose dexamethasone in patients with relapsed and/or refractory multiple myeloma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 33 7 2015 732 739 10.1200/JCO.2013.52.3522 25225420

Papaevgeniou N. Sakellari M. Jha S. Tavernarakis N. Holmberg C.I. Gonos E.S. Chondrogianni N. 18α-Glycyrrhetinic acid proteasome activator decelerates aging and Alzheimer's disease progression in Caenorhabditis elegans and neuronal cultures Antioxidants and Redox Signaling 25 16 2016 855 869 Scopus 10.1089/ars.2015.6494 26886723 PMC5124744

Papandreou C.N. Daliani D.D. Nix D. Yang H. Madden T. Wang X. … Logothetis C.J. Phase I trial of the proteasome inhibitor bortezomib in patients with advanced solid tumors with observations in androgen-independent prostate cancer Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 22 11 2004 2108 2121 10.1200/JCO.2004.02.106 15169797

Parisi V. Coppola G. Centofanti M. Oddone F. Angrisani A.M. Ziccardi L. … Manni G. Evidence of the neuroprotective role of citicoline in glaucoma patients Progress in Brain Research 173 2008 541 554 10.1016/S0079-6123(08)01137-0 18929133

Parisi V. Oddone F. Roberti G. Tanga L. Carnevale C. Ziccardi L. Manni G. Enhancement of retinal function and of neural conduction along the visual pathway induced by treatment with citicoline eye drops in liposomal formulation in open angle glaucoma: A pilot electrofunctional study Advances in Therapy 36 4 2019 987 996 10.1007/s12325-019-0897-z 30790180

Parisi V. Oddone F. Ziccardi L. Roberti G. Coppola G. Manni G. Citicoline and retinal ganglion cells: Effects on morphology and function Current Neuropharmacology 16 7 2018 919 932 10.2174/1570159X15666170703111729 28676014 PMC6120106

Parisi V. Restuccia R. Fattapposta F. Mina C. Bucci M.G. Pierelli F. Morphological and functional retinal impairment in Alzheimer's disease patients Clinical Neurophysiology: Official Journal of the International Federation of Clinical Neurophysiology 112 10 2001 1860 1867 10.1016/s1388-2457(01)00620-4 11595144

Parisi V et al. (2015) Treatment with citicoline eye drops enhances retinal function and neural conduction along the visual pathways in open angle glaucoma. Graefes Arch Clin Exp Ophthalmol . PMDI 26004075 10.1007/s00417-015-3044-9 26004075

Park S. Kim W. Tian G. Gygi S.P. Finley D. Structural defects in the regulatory particle-core particle interface of the proteasome induce a novel proteasome stress response The Journal of Biological Chemistry 286 42 2011 36652 36666 10.1074/jbc.M111.285924 21878652 PMC3196138

Park S. Li X. Kim H.M. Singh C.R. Tian G. Hoyt M.A. … Finley D. Reconfiguration of the proteasome during chaperone-mediated assembly Nature 497 7450 2013 512 516 10.1038/nature12123 23644457 PMC3687086

Park J.E. Miller Z. Jun Y. Lee W. Kim K.B. Next-generation proteasome inhibitors for cancer therapy Translational Research: The Journal of Laboratory and Clinical Medicine 198 2018 1 16 10.1016/j.trsl.2018.03.002 29654740 PMC6151281

Parlati F. Lee S.J. Aujay M. Suzuki E. Levitsky K. Lorens J.B. … Bennett M.K. Carfilzomib can induce tumor cell death through selective inhibition of the chymotrypsin-like activity of the proteasome Blood 114 16 2009 3439 3447 10.1182/blood-2009-05-223677 19671918

Parravano M. Oddone F. Boccassini B. Centofanti M. Tanga L. Cacciamani A. … Varano M. Functional retinal impairment in type 1 diabetic patients without any signs of retinopathy Ophthalmic Research 50 2 2013 108 112 10.1159/000350412 23838724

Pathare G.R. Nagy I. Bohn S. Unverdorben P. Hubert A. Körner R. … Bracher A. The proteasomal subunit Rpn6 is a molecular clamp holding the core and regulatory subcomplexes together Proceedings of the National Academy of Sciences of the United States of America 109 1 2012 149 154 10.1073/pnas.1117648108 22187461 PMC3252951

Pathare G.R. Nagy I. Śledź P. Anderson D.J. Zhou H.-J. Pardon E. … Baumeister W. Crystal structure of the proteasomal deubiquitylation module Rpn8-Rpn11 Proceedings of the National Academy of Sciences 2014 10.1073/pnas.1400546111 PMC3939901 24516147

Pei X.-Y. Dai Y. Grant S. Synergistic induction of oxidative injury and apoptosis in human multiple myeloma cells by the proteasome inhibitor bortezomib and histone deacetylase inhibitors Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 10 11 2004 3839 3852 10.1158/1078-0432.CCR-03-0561 15173093

Pekol T. Daniels J.S. Labutti J. Parsons I. Nix D. Baronas E. … Miwa G. Human metabolism of the proteasome inhibitor bortezomib: Identification of circulating metabolites Drug Metabolism and Disposition: The Biological Fate of Chemicals 33 6 2005 771 777 10.1124/dmd.104.002956 15764713

de la Peña A.H. Goodall E.A. Gates S.N. Lander G.C. Martin A. Substrate-engaged 26S proteasome structures reveal mechanisms for ATP-hydrolysis-driven translocation Science (New York, N.Y.) 362 6418 2018 10.1126/science.aav0725 PMC6519459 30309908

Peng B. Chen H. Lou X. Bortezomib-induced syndrome of inappropriate antidiuresis in a patient with multiple myeloma: A case report and literature review International Journal of Clinical Pharmacology and Therapeutics 55 12 2017 910 914 10.5414/CP203109 29092733

Pereira R.B. Evdokimov N.M. Lefranc F. Valentão P. Kornienko A. Pereira D.M. … Gomes N.G.M. Marine-Derived Anticancer Agents: Clinical Benefits, Innovative Mechanisms, and New Targets Marine Drugs 17 6 2019 10.3390/md17060329 PMC6627313 31159480

Perkins N.D. The diverse and complex roles of NF-κB subunits in cancer Nature Reviews. Cancer 12 2 2012 121 132 10.1038/nrc3204 22257950

Perry G. Friedman R. Shaw G. Chau V. Ubiquitin is detected in neurofibrillary tangles and senile plaque neurites of Alzheimer disease brains Proceedings of the National Academy of Sciences of the United States of America 84 9 1987 3033 3036 10.1073/pnas.84.9.3033 3033674 PMC304795

Petrucelli L. Dickson D. Kehoe K. Taylor J. Snyder H. Grover A. … Hutton M. CHIP and Hsp70 regulate tau ubiquitination, degradation and aggregation Human Molecular Genetics 13 7 2004 703 714 10.1093/hmg/ddh083 14962978

Pham L.V. Tamayo A.T. Yoshimura L.C. Lo P. Ford R.J. Inhibition of constitutive NF-kappa B activation in mantle cell lymphoma B cells leads to induction of cell cycle arrest and apoptosis Journal of Immunology (Baltimore, MD: 1950) 171 1 2003 88 95 10.4049/jimmunol.171.1.88 12816986

Picconi F. Mataluni G. Ziccardi L. Parravano M. Di Renzo A. Ylli D. … Frontoni S. Association between early neuroretinal dysfunction and peripheral motor unit loss in patients with type 1 diabetes mellitus Journal of Diabetes Research 2018 2018 9763507 10.1155/2018/9763507 30402503 PMC6193343

Picconi F. Parravano M. Sciarretta F. Fulci C. Nali M. Frontoni S. … Caccuri A.M. Activation of retinal Müller cells in response to glucose variability Endocrine 65 3 2019 542 549 10.1007/s12020-019-02017-5 31327157

Picconi F. Parravano M. Ylli D. Pasqualetti P. Coluzzi S. Giordani I. … Frontoni S. Retinal neurodegeneration in patients with type 1 diabetes mellitus: The role of glycemic variability Acta Diabetologica 54 5 2017 489 497 10.1007/s00592-017-0971-4 28238189 PMC5385321

Pickart C.M. Mechanisms underlying ubiquitination Annual Review of Biochemistry 70 2001 503 533 10.1146/annurev.biochem.70.1.503 11395416

Pickering A.M. Davies K.J.A. Differential roles of proteasome and immunoproteasome regulators Pa28αβ, Pa28γ and Pa200 in the degradation of oxidized proteins Archives of Biochemistry and Biophysics 523 2 2012 181 190 10.1016/j.abb.2012.04.018 22564544 PMC3384713

Pickering A.M. Koop A.L. Teoh C.Y. Ermak G. Grune T. Davies K.J.A. The immunoproteasome, the 20S proteasome, and the PA28αβ proteasome regulator are oxidative-stress-adaptive proteolytic complexes The Biochemical Journal 432 3 2010 585 594 10.1042/BJ20100878 20919990 PMC3133595

Piippo N. Korhonen E. Hytti M. Kinnunen K. Kaarniranta K. Kauppinen A. Oxidative stress is the principal contributor to inflammasome activation in retinal pigment epithelium cells with defunct proteasomes and autophagy Cellular Physiology and Biochemistry: International Journal of Experimental Cellular Physiology, Biochemistry, and Pharmacology 49 1 2018 359 367 10.1159/000492886 30138927

Pisha E. Chai H. Lee I.S. Chagwedera T.E. Farnsworth N.R. Cordell G.A. … Brown D.M. Discovery of betulinic acid as a selective inhibitor of human melanoma that functions by induction of apoptosis Nature Medicine 1 10 1995 1046 1051 10.1038/nm1095-1046 7489361

Piva R. Ruggeri B. Williams M. Costa G. Tamagno I. Ferrero D. Giai V. Coscia M. Peola S. Massaia M. Pezzoni G. Allievi C. Pescalli N. Cassin M. di Giovine S. Nicoli P. de Feudis P. Strepponi I. Roato I. … Inghirami G. CEP-18770: A novel, orally active proteasome inhibitor with a tumor-selective pharmacologic profile competitive with bortezomib Blood 111 5 2008 2765 2775 10.1182/blood-2007-07-100651 18057228

Podar K. Tai Y.T. Davies F.E. Lentzsch S. Sattler M. Hideshima T. … Anderson K.C. Vascular endothelial growth factor triggers signaling cascades mediating multiple myeloma cell growth and migration Blood 98 2 2001 428 435 10.1182/blood.v98.2.428 11435313

Poli M.C. Ebstein F. Nicholas S.K. de Guzman M.M. Forbes L.R. Chinn I.K. Mace E.M. Vogel T.P. Carisey A.F. Benavides F. Coban-Akdemir Z.H. Gibbs R.A. Jhangiani S.N. Muzny D.M. Carvalho C.M.B. Schady D.A. Jain M. Rosenfeld J.A. Emrick L. … Orange J.S. Heterozygous truncating variants in POMP escape nonsense-mediated decay and cause a unique immune dysregulatory syndrome American Journal of Human Genetics 102 6 2018 1126 1142 10.1016/j.ajhg.2018.04.010 29805043 PMC5992134

de Poot S.A.H. Tian G. Finley D. Meddling with fate: The proteasomal deubiquitinating enzymes Journal of Molecular Biology 429 22 2017 3525 3545 10.1016/j.jmb.2017.09.015 28988953 PMC5675770

Potts B.C. Albitar M.X. Anderson K.C. Baritaki S. Berkers C. Bonavida B. Chandra J. Chauhan D. Cusack J.C. Fenical W. Ghobrial I.M. Groll M. Jensen P.R. Lam K.S. Lloyd G.K. McBride W. McConkey D.J. Miller C.P. Neuteboom S.T.C. … Palladino M.A. Marizomib, a proteasome inhibitor for all seasons: Preclinical profile and a framework for clinical trials Current Cancer Drug Targets 11 3 2011 254 284 10.2174/156800911794519716 21247382 PMC3712795

Potts B.C. Lam K.S. Generating a generation of proteasome inhibitors: from microbial fermentation to total synthesis of salinosporamide a (marizomib) and other salinosporamides Marine Drugs 8 4 2010 835 880 10.3390/md8040835 20479958 PMC2866466

Powers E.T. Morimoto R.I. Dillin A. Kelly J.W. Balch W.E. Biological and chemical approaches to diseases of proteostasis deficiency Annual Review of Biochemistry 78 2009 959 991 10.1146/annurev.biochem.052308.114844 19298183

Poyurovsky M.V. Priest C. Kentsis A. Borden K.L.B. Pan Z.-Q. Pavletich N. Prives C. The Mdm2 RING domain C-terminus is required for supramolecular assembly and ubiquitin ligase activity The EMBO Journal 26 1 2007 90 101 10.1038/sj.emboj.7601465 17170710 PMC1782380

Preckel T. Fung-Leung W.P. Cai Z. Vitiello A. Salter-Cid L. Winqvist O. … Yang Y. Impaired immunoproteasome assembly and immune responses in PA28−/− mice Science (New York, N.Y.) 286 5447 1999 2162 2165 10.1126/science.286.5447.2162 10591649

Pushpakom S. Iorio F. Eyers P.A. Escott K.J. Hopper S. Wells A. … Pirmohamed M. Drug repurposing: Progress, challenges and recommendations Nature Reviews. Drug Discovery 18 1 2019 41 58 10.1038/nrd.2018.168 30310233

Qian M.-X. Pang Y. Liu C.H. Haratake K. Du B.-Y. Ji D.-Y. Wang G.-F. Zhu Q.-Q. Song W. Yu Y. Zhang X.-X. Huang H.-T. Miao S. Chen L.-B. Zhang Z.-H. Liang Y.-N. Liu S. Cha H. Yang D. … Qiu X.-B. Acetylation-mediated proteasomal degradation of core histones during DNA repair and spermatogenesis Cell 153 5 2013 1012 1024 10.1016/j.cell.2013.04.032 23706739 PMC3983474

Qin J.-Z. Ziffra J. Stennett L. Bodner B. Bonish B.K. Chaturvedi V. … Nickoloff B.J. Proteasome Inhibitors Trigger NOXA-Mediated Apoptosis in Melanoma and Myeloma Cells Cancer Research 65 14 2005 6282 6293 10.1158/0008-5472.CAN-05-0676 16024630

Qiu X.-B. Ouyang S.-Y. Li C.-J. Miao S. Wang L. Goldberg A.L. hRpn13/ADRM1/GP110 is a novel proteasome subunit that binds the deubiquitinating enzyme, UCH37 The EMBO Journal 25 24 2006 5742 5753 10.1038/sj.emboj.7601450 17139257 PMC1698896

Qiu Y. Shen X. Shyam R. Yue B.Y.J.T. Ying H. Cellular processing of myocilin PLoS One 9 4 2014 e92845 10.1371/journal.pone.0092845 PMC3986080 24732711

Quach H. White D. Spencer A. Ho P.J. Bhutani D. White M. … Gyger M. Pharmacokinetics and safety of carfilzomib in patients with relapsed multiple myeloma and end-stage renal disease (ESRD): An open-label, single-arm, phase I study Cancer Chemotherapy and Pharmacology 79 6 2017 1067 1076 10.1007/s00280-017-3287-8 28424963 PMC5438822

Quaranta L. Riva I. Gerardi C. Oddone F. Floriano I. Konstas A.G.P. Quality of life in glaucoma: A review of the literature Advances in Therapy 33 6 2016 959 981 10.1007/s12325-016-0333-6 27138604 PMC4920851

Quesnel B. Preudhomme C. Oscier D. Lepelley P. Hooghe M.C. Facon T. … Fenaux P. Over-expression of the MDM2 gene is found in some cases of haematological malignancies British Journal of Haematology 88 2 1994 415 418 10.1111/j.1365-2141.1994.tb05044.x 7803295

Qureshi A.A. Guan X.Q. Reis J.C. Papasian C.J. Jabre S. Morrison D.C. Qureshi N. Inhibition of nitric oxide and inflammatory cytokines in LPS-stimulated murine macrophages by resveratrol, a potent proteasome inhibitor Lipids in Health and Disease 11 1 2012 1 10.1186/1476-511X-11-76 22698256 PMC3393619

Qureshi A.A. Zuvanich E.G. Khan D.A. Mushtaq S. Silswal N. Qureshi N. Proteasome inhibitors modulate anticancer and anti-proliferative properties via NF-kB signaling, and ubiquitin-proteasome pathways in cancer cell lines of different organs Lipids in Health and Disease 17 1 2018 62 10.1186/s12944-018-0697-5 PMC5879737 29606130

Raaben M. Grinwis G.C.M. Rottier P.J.M. de Haan C.A.M. The proteasome inhibitor Velcade enhances rather than reduces disease in mouse hepatitis coronavirus-infected mice Journal of Virology 84 15 2010 7880 7885 10.1128/JVI.00486-10 20484516 PMC2897637

Raaben M. Posthuma C.C. Verheije M.H. te Lintelo E.G. Kikkert M. Drijfhout J.W. … de Haan C.A.M. The ubiquitin-proteasome system plays an important role during various stages of the coronavirus infection cycle Journal of Virology 84 15 2010 7869 7879 10.1128/JVI.00485-10 20484504 PMC2897594

Raasi S. Varadan R. Fushman D. Pickart C.M. Diverse polyubiquitin interaction properties of ubiquitin-associated domains Nature Structural & Molecular Biology 12 8 2005 708 714 10.1038/nsmb962 16007098

Rabl J. Smith D.M. Yu Y. Chang S.-C. Goldberg A.L. Cheng Y. Mechanism of gate opening in the 20S proteasome by the proteasomal ATPases Molecular Cell 30 3 2008 360 368 10.1016/j.molcel.2008.03.004 18471981 PMC4141531

Rahimi N. The ubiquitin-proteasome system meets angiogenesis Molecular Cancer Therapeutics 11 3 2012 538 548 10.1158/1535-7163.MCT-11-0555 22357635 PMC3297735

Raiss C.C. Braun T.S. Konings I.B.M. Grabmayr H. Hassink G.C. Sidhu A. … Claessens M.M.A.E. Functionally different α-synuclein inclusions yield insight into Parkinson's disease pathology Scientific Reports 6 2016 23116 10.1038/srep23116 26984067 PMC4794800

Rajan A.M. Kumar S. New investigational drugs with single-agent activity in multiple myeloma Blood Cancer Journal 6 7 2016 e451 10.1038/bcj.2016.53 PMC5030378 27471867

Ramachandran K.V. Margolis S.S. A mammalian nervous system-specific plasma membrane proteasome complex that modulates neuronal function Nature Structural & Molecular Biology 24 4 2017 419 430 10.1038/nsmb.3389 PMC5383508 28287632

Ramos P.C. Dohmen R.J. PACemakers of proteasome core particle assembly Structure (London, England: 1993) 16 9 2008 1296 1304 10.1016/j.str.2008.07.001 18786393

Rangel L.P. Costa D.C.F. Vieira T.C.R.G. Silva J.L. The aggregation of mutant p53 produces prion-like properties in cancer Prion 8 1 2014 75 84 10.4161/pri.27776 24509441 PMC7030899

Rangel L.P. Ferretti G.D.S. Costa C.L. Andrade S.M.M.V. Carvalho R.S. Costa D.C.F. Silva J.L. p53 reactivation with induction of massive apoptosis-1 (PRIMA-1) inhibits amyloid aggregation of mutant p53 in cancer cells The Journal of Biological Chemistry 294 10 2019 3670 3682 10.1074/jbc.RA118.004671 30602570 PMC6416452

Rastogi N. Mishra D.P. Therapeutic targeting of cancer cell cycle using proteasome inhibitors Cell Division 7 1 2012 26 10.1186/1747-1028-7-26 23268747 PMC3584802

Raule M. Cerruti F. Benaroudj N. Migotti R. Kikuchi J. Bachi A. … Cascio P. PA28αβ reduces size and increases hydrophilicity of 20S immunoproteasome peptide products Chemistry & Biology 21 4 2014 470 480 10.1016/j.chembiol.2014.02.006 24631123

Raule M. Cerruti F. Cascio P. Enhanced rate of degradation of basic proteins by 26S immunoproteasomes Biochimica et Biophysica Acta 1843 9 2014 1942 1947 10.1016/j.bbamcr.2014.05.005 24851840

Raynes R. Pomatto L.C.D. Davies K.J.A. Degradation of oxidized proteins by the proteasome: Distinguishing between the 20S, 26S, and immunoproteasome proteolytic pathways Molecular Aspects of Medicine 50 2016 41 55 10.1016/j.mam.2016.05.001 27155164 PMC4967006

Read M.A. Neish A.S. Luscinskas F.W. Palombella V.J. Maniatis T. Collins T. The proteasome pathway is required for cytokine-induced endothelial-leukocyte adhesion molecule expression Immunity 2 5 1995 493 506 10.1016/1074-7613(95)90030-6 7538441

Rechsteiner M. Hill C.P. Mobilizing the proteolytic machine: Cell biological roles of proteasome activators and inhibitors Trends in Cell Biology 15 1 2005 27 33 10.1016/j.tcb.2004.11.003 15653075

Regitz C. Fitzenberger E. Mahn F.L. Dußling L.M. Wenzel U. Resveratrol reduces amyloid-beta (Aβ₁₋₄₂)-induced paralysis through targeting proteostasis in an Alzheimer model of Caenorhabditis elegans European Journal of Nutrition 55 2 2016 741 747 10.1007/s00394-015-0894-1 25851110

Reinheckel T. Sitte N. Ullrich O. Kuckelkorn U. Davies K.J. Grune T. Comparative resistance of the 20S and 26S proteasome to oxidative stress The Biochemical Journal 335 Pt 3 1998 637 642 10.1042/bj3350637 9794805 PMC1219826

Reinheckel T. Ullrich O. Sitte N. Grune T. Differential impairment of 20S and 26S proteasome activities in human hematopoietic K562 cells during oxidative stress Archives of Biochemistry and Biophysics 377 1 2000 65 68 10.1006/abbi.2000.1717 10775442

Resch Z.T. Fautsch M.P. Glaucoma-associated myocilin: A better understanding but much more to learn Experimental Eye Research 88 4 2009 704 712 10.1016/j.exer.2008.08.011 18804106 PMC2682697

Richardson P.G. Barlogie B. Berenson J. Singhal S. Jagannath S. Irwin D. Rajkumar S.V. Srkalovic G. Alsina M. Alexanian R. Siegel D. Orlowski R.Z. Kuter D. Limentani S.A. Lee S. Hideshima T. Esseltine D.-L. Kauffman M. Adams J. … Anderson K.C. A phase 2 study of bortezomib in relapsed, refractory myeloma The New England Journal of Medicine 348 26 2003 2609 2617 10.1056/NEJMoa030288 12826635

Richardson P.G. Barlogie B. Berenson J. Singhal S. Jagannath S. Irwin D.H. … Anderson K.C. Extended follow-up of a phase II trial in relapsed, refractory multiple myeloma: Final time-to-event results from the SUMMIT trial Cancer 106 6 2006 1316 1319 10.1002/cncr.21740 16470606

Richardson P.G. Baz R. Wang M. Jakubowiak A.J. Laubach J.P. Harvey R.D. … Lonial S. Phase 1 study of twice-weekly ixazomib, an oral proteasome inhibitor, in relapsed/refractory multiple myeloma patients Blood 124 7 2014 1038 1046 10.1182/blood-2014-01-548826 24920586 PMC4574453

Richardson P.G. Briemberg H. Jagannath S. Wen P.Y. Barlogie B. Berenson J. … Amato A.A. Frequency, characteristics, and reversibility of peripheral neuropathy during treatment of advanced multiple myeloma with bortezomib Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 24 19 2006 3113 3120 10.1200/JCO.2005.04.7779 16754936

Richardson P.G. Delforge M. Beksac M. Wen P. Jongen J.L. Sezer O. Terpos E. Munshi N. Palumbo A. Rajkumar S.V. Harousseau J.L. Moreau P. Avet-Loiseau H. Lee J.H. Cavo M. Merlini G. Voorhees P. Chng W.J. Mazumder A. … Sonneveld P. Management of treatment-emergent peripheral neuropathy in multiple myeloma Leukemia 26 4 2012 595 608 10.1038/leu.2011.346 22193964

Richardson P.G. Hofmeister C.C. Rosenbaum C.A. Htut M. Vesole D.H. Berdeja J. … Baz R. Twice-weekly Oral MLN9708 (ixazomib citrate), An investigational proteasome inhibitor, in combination with lenalidomide (Len) and dexamethasone (Dex) in patients (Pts) with newly diagnosed multiple myeloma (MM): Final phase 1 results and phase 2 data Blood 122 21 2013 535 10.1182/blood.V122.21.535.535

Richardson P.G. Hofmeister C.C. Rosenbaum C.A. Htut M. Vesole D.H. Berdeja J.G. … Baz R. Twice-weekly ixazomib in combination with lenalidomide-dexamethasone in patients with newly diagnosed multiple myeloma British Journal of Haematology 182 2 2018 231 244 10.1111/bjh.15394 29938772 PMC6055619

Richardson P.G. Mitsiades C. Hideshima T. Anderson K.C. Bortezomib: proteasome inhibition as an effective anticancer therapy Annual Review of Medicine 57 2006 33 47 10.1146/annurev.med.57.042905.122625 16409135

Richardson P.G. Oriol A. Beksac M. Liberati A.M. Galli M. Schjesvold F. Lindsay J. Weisel K. White D. Facon T. Miguel J.S. Sunami K. O'Gorman P. Sonneveld P. Robak P. Semochkin S. Schey S. Yu X. Doerr T. … Yee A. Pomalidomide, bortezomib, and dexamethasone for patients with relapsed or refractory multiple myeloma previously treated with lenalidomide (OPTIMISMM): a randomised, open-label, phase 3 trial The Lancet Oncology 20 6 2019 781 794 10.1016/S1470-2045(19)30152-4 31097405

Richardson P.G. Sonneveld P. Schuster M. Irwin D. Stadtmauer E. Facon T. Harousseau J.-L. Ben-Yehuda D. Lonial S. Goldschmidt H. Reece D. Miguel J.S. Bladé J. Boccadoro M. Cavenagh J. Alsina M. Rajkumar S.V. Lacy M. Jakubowiak A. … Anderson K.C. Extended follow-up of a phase 3 trial in relapsed multiple myeloma: Final time-to-event results of the APEX trial Blood 110 10 2007 3557 3560 10.1182/blood-2006-08-036947 17690257

Richardson P.G. Sonneveld P. Schuster M.W. Irwin D. Stadtmauer E.A. Facon T. Harousseau J.-L. Ben-Yehuda D. Lonial S. Goldschmidt H. Reece D. San-Miguel J.F. Bladé J. Boccadoro M. Cavenagh J. Dalton W.S. Boral A.L. Esseltine D.L. Porter J.B. … Assessment of Proteasome Inhibition for Extending Remissions (APEX) Investigators Bortezomib or high-dose dexamethasone for relapsed multiple myeloma The New England Journal of Medicine 352 24 2005 2487 2498 10.1056/NEJMoa043445 15958804

Richardson P.G. Weller E. Lonial S. Jakubowiak A.J. Jagannath S. Raje N.S. Avigan D.E. Xie W. Ghobrial I.M. Schlossman R.L. Mazumder A. Munshi N.C. Vesole D.H. Joyce R. Kaufman J.L. Doss D. Warren D.L. Lunde L.E. Kaster S. … Anderson K.C. Lenalidomide, bortezomib, and dexamethasone combination therapy in patients with newly diagnosed multiple myeloma Blood 116 5 2010 679 686 10.1182/blood-2010-02-268862 20385792 PMC3324254

Richardson P.G. Xie W. Jagannath S. Jakubowiak A. Lonial S. Raje N.S. Alsina M. Ghobrial I.M. Schlossman R.L. Munshi N.C. Mazumder A. Vesole D.H. Kaufman J.L. Colson K. McKenney M. Lunde L.E. Feather J. Maglio M.E. Warren D. … Anderson K.C. A phase 2 trial of lenalidomide, bortezomib, and dexamethasone in patients with relapsed and relapsed/refractory myeloma Blood 123 10 2014 1461 1469 10.1182/blood-2013-07-517276 24429336 PMC4123434

Richardson P.G. Zweegman S. O'Donnell E.K. Laubach J.P. Raje N. Voorhees P. … Lonial S. Ixazomib for the treatment of multiple myeloma Expert Opinion on Pharmacotherapy 19 17 2018 1949 1968 10.1080/14656566.2018.1528229 30422008

Robak T. Huang H. Jin J. Zhu J. Liu T. Samoilova O. … Cavalli F. Bortezomib-based therapy for newly diagnosed mantle-cell lymphoma New England Journal of Medicine 372 10 2015 944 953 10.1056/NEJMoa1412096 25738670

Robak T. Huang H. Jin J. Zhu J. Liu T. Samoilova O. … Cavalli F. Association between bortezomib dose intensity and overall survival in mantle cell lymphoma patients on frontline VR-CAP in the phase 3 LYM-3002 study Leukemia & Lymphoma 60 1 2019 172 179 10.1080/10428194.2017.1321750 28583031

Robak T. Jin J. Pylypenko H. Verhoef G. Siritanaratkul N. Drach J. … LYM-3002 investigators Frontline bortezomib, rituximab, cyclophosphamide, doxorubicin, and prednisone (VR-CAP) versus rituximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (R-CHOP) in transplantation-ineligible patients with newly diagnosed mantle cell lymphoma: final overall survival results of a randomised, open-label, phase 3 study The Lancet. Oncology 19 11 2018 1449 1458 10.1016/S1470-2045(18)30685-5 30348538

Robak P. Robak T. Bortezomib for the treatment of hematologic malignancies: 15 years later Drugs in R&D 19 2 2019 73 92 10.1007/s40268-019-0269-9 30993606 PMC6544598

Roberti G. Oddone F. Agnifili L. Katsanos A. Michelessi M. Mastropasqua L. … Manni G. Steroid-induced glaucoma: Epidemiology, pathophysiology, and clinical management Survey of Ophthalmology 2020 10.1016/j.survophthal.2020.01.002 32057761

Roberti G. Tanga L. Michelessi M. Quaranta L. Parisi V. Manni G. Oddone F. Cytidine 5′-diphosphocholine (citicoline) in glaucoma: Rationale of its use, current evidence and future perspectives International Journal of Molecular Sciences 16 12 2015 28401 28417 10.3390/ijms161226099 26633368 PMC4691046

Roccaro A.M. Hideshima T. Raje N. Kumar S. Ishitsuka K. Yasui H. … Anderson K.C. Bortezomib mediates antiangiogenesis in multiple myeloma via direct and indirect effects on endothelial cells Cancer Research 66 1 2006 184 191 10.1158/0008-5472.CAN-05-1195 16397231

Roelofs J. Park S. Haas W. Tian G. McAllister F.E. Huo Y. … Finley D. Chaperone-mediated pathway of proteasome regulatory particle assembly Nature 459 7248 2009 861 865 10.1038/nature08063 19412159 PMC2727592

Roeten M.S.F. Cloos J. Jansen G. Positioning of proteasome inhibitors in therapy of solid malignancies Cancer Chemotherapy and Pharmacology 81 2 2018 227 243 10.1007/s00280-017-3489-0 29184971 PMC5778165

Roninson I.B. Oncogenic functions of tumour suppressor p21(Waf1/Cip1/Sdi1): association with cell senescence and tumour-promoting activities of stromal fibroblasts Cancer Letters 179 1 2002 1 14 10.1016/s0304-3835(01)00847-3 11880176

Rose S.A. Wass S. Jankowski J.J. Feldman J.F. Djukic A. Impaired visual search in children with Rett syndrome Pediatric Neurology 92 2019 26 31 10.1016/j.pediatrneurol.2018.10.002 30573328 PMC6397679

Rosen K.M. Moussa C.E.-H. Lee H.-K. Kumar P. Kitada T. Qin G. … Querfurth H.W. Parkin reverses intracellular beta-amyloid accumulation and its negative effects on proteasome function Journal of Neuroscience Research 88 1 2010 167 178 10.1002/jnr.22178 19610108 PMC2844439

Rosenberg C.L. Wong E. Petty E.M. Bale A.E. Tsujimoto Y. Harris N.L. Arnold A. PRAD1, a candidate BCL1 oncogene: Mapping and expression in centrocytic lymphoma Proceedings of the National Academy of Sciences of the United States of America 88 21 1991 9638 9642 10.1073/pnas.88.21.9638 1682919 PMC52773

Rosenberg-Hasson Y. Bercovich Z. Ciechanover A. Kahana C. Degradation of ornithine decarboxylase in mammalian cells is ATP dependent but ubiquitin independent European Journal of Biochemistry 185 2 1989 469 474 10.1111/j.1432-1033.1989.tb15138.x 2555193

Rosiñol L. Oriol A. Rios R. Sureda A. Blanchard M.J. Hernández M.T. Martínez-Martínez R. Moraleda J.M. Jarque I. Bargay J. Gironella M. de Arriba F. Palomera L. González-Montes Y. Martí J.M. Krsnik I. Arguiñano J.M. González M.E. González A.P. … Bladé J. Bortezomib, lenalidomide, and dexamethasone as induction therapy prior to autologous transplant in multiple myeloma Blood 134 16 2019 1337 1345 10.1182/blood.2019000241 31484647 PMC6888142

Rosiñol L. Oriol A. Teruel A.I. Hernández D. López-Jiménez J. de la Rubia J. Granell M. Besalduch J. Palomera L. González Y. Etxebeste M.A. Díaz-Mediavilla J. Hernández M.T. de Arriba F. Gutiérrez N.C. Martín-Ramos M.L. Cibeira M.T. Mateos M.V. Martínez J. … Programa para el Estudio y la Terapéutica de las Hemopatías Malignas/Grupo Español de Mieloma (PETHEMA/GEM) group Superiority of bortezomib, thalidomide, and dexamethasone (VTD) as induction pretransplantation therapy in multiple myeloma: A randomized phase 3 PETHEMA/GEM study Blood 120 8 2012 1589 1596 10.1182/blood-2012-02-408922 22791289

Rousseau A. Bertolotti A. An evolutionarily conserved pathway controls proteasome homeostasis Nature 536 7615 2016 184 189 10.1038/nature18943 27462806 PMC4990136

Rousseau A. Bertolotti A. Regulation of proteasome assembly and activity in health and disease Nature Reviews Molecular Cell Biology 19 11 2018 697 712 10.1038/s41580-018-0040-z 30065390

Roussel M. Lauwers-Cances V. Robillard N. Hulin C. Leleu X. Benboubker L. Marit G. Moreau P. Pegourie B. Caillot D. Fruchart C. Stoppa A.-M. Gentil C. Wuilleme S. Huynh A. Hebraud B. Corre J. Chretien M.-L. Facon T. … Attal M. Front-line transplantation program with lenalidomide, bortezomib, and dexamethasone combination as induction and consolidation followed by lenalidomide maintenance in patients with multiple myeloma: A phase II study by the Intergroupe francophone du Myélome Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 32 25 2014 2712 2717 10.1200/JCO.2013.54.8164 25024076

Rowinsky E.K. Paner A. Berdeja J.G. Paba-Prada C. Venugopal P. Porkka K. … Landgren O. Phase 1 study of the protein deubiquitinase inhibitor VLX1570 in patients with relapsed and/or refractory multiple myeloma Investigational New Drugs 2020 10.1007/s10637-020-00915-4 PMC7497669 32125598

Ruan J. Martin P. Furman R.R. Lee S.M. Cheung K. Vose J.M. … Leonard J.P. Bortezomib plus CHOP-rituximab for previously untreated diffuse large B-cell lymphoma and mantle cell lymphoma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 29 6 2011 690 697 10.1200/JCO.2010.31.1142 21189393

Rubin D.M. Glickman M.H. Larsen C.N. Dhruvakumar S. Finley D. Active site mutants in the six regulatory particle ATPases reveal multiple roles for ATP in the proteasome The EMBO Journal 17 17 1998 4909 4919 10.1093/emboj/17.17.4909 9724628 PMC1170820

Ruiz de Mena I. Mahillo E. Arribas J. Castaño J.G. Kinetic mechanism of activation by cardiolipin (diphosphatidylglycerol) of the rat liver multicatalytic proteinase Biochemical Journal 296 1 1993 93 97 10.1042/bj2960093 8250860 PMC1137659

Ruiz S. Krupnik Y. Keating M. Chandra J. Palladino M. McConkey D. The proteasome inhibitor NPI-0052 is a more effective inducer of apoptosis than bortezomib in lymphocytes from patients with chronic lymphocytic leukemia Molecular Cancer Therapeutics 5 7 2006 1836 1843 10.1158/1535-7163.MCT-06-0066 16891470

Ruschak A.M. Religa T.L. Breuer S. Witt S. Kay L.E. The proteasome antechamber maintains substrates in an unfolded state Nature 467 7317 2010 868 871 10.1038/nature09444 20944750

Sadre-Bazzaz K. Whitby F.G. Robinson H. Formosa T. Hill C.P. Structure of a Blm10 complex reveals common mechanisms for proteasome binding and gate opening Molecular Cell 37 5 2010 728 735 10.1016/j.molcel.2010.02.002 20227375 PMC2859072

Saeed M.E.M. Mahmoud N. Sugimoto Y. Efferth T. Abdel-Aziz H. Betulinic Acid Exerts Cytotoxic Activity Against Multidrug-Resistant Tumor Cells via Targeting Autocrine Motility Factor Receptor (AMFR) Frontiers in Pharmacology 9 2018 481 10.3389/fphar.2018.00481 29867487 PMC5962668

Saeki Y. Saitoh A. Toh-e A. Yokosawa H. Ubiquitin-like proteins and Rpn10 play cooperative roles in ubiquitin-dependent proteolysis Biochemical and Biophysical Research Communications 293 3 2002 986 992 10.1016/S0006-291X(02)00340-6 12051757

Saeki Y. Tanaka K. Assembly and function of the proteasome Methods in Molecular Biology (Clifton, N.J.) 832 2012 315 337 10.1007/978-1-61779-474-2_22 22350895

Saeki Y. Toh-e A. Kudo T. Kawamura H. Tanaka K. Multiple proteasome-interacting proteins assist the assembly of the yeast 19S regulatory particle Cell 137 5 2009 900 913 10.1016/j.cell.2009.05.005 19446323

Safren N. El Ayadi A. Chang L. Terrillion C.E. Gould T.D. Boehning D.F. Monteiro M.J. Ubiquilin-1 overexpression increases the lifespan and delays accumulation of Huntingtin aggregates in the R6/2 mouse model of Huntington's disease PLoS One 9 1 2014 e87513 10.1371/journal.pone.0087513 PMC3903676 24475300

Sala A.J. Bott L.C. Morimoto R.I. Shaping proteostasis at the cellular, tissue, and organismal level The Journal of Cell Biology 216 5 2017 1231 1241 10.1083/jcb.201612111 28400444 PMC5412572

San Miguel J.F. Schlag R. Khuageva N.K. Dimopoulos M.A. Shpilberg O. Kropff M. Spicka I. Petrucci M.T. Palumbo A. Samoilova O.S. Dmoszynska A. Abdulkadyrov K.M. Delforge M. Jiang B. Mateos M.-V. Anderson K.C. Esseltine D.-L. Liu K. Deraedt W. … Richardson P.G. Persistent overall survival benefit and no increased risk of second malignancies with bortezomib-melphalan-prednisone versus melphalan-prednisone in patients with previously untreated multiple myeloma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 31 4 2013 448 455 10.1200/JCO.2012.41.6180 23233713

San Miguel J.F. Schlag R. Khuageva N.K. Dimopoulos M.A. Shpilberg O. Kropff M. Spicka I. Petrucci M.T. Palumbo A. Samoilova O.S. Dmoszynska A. Abdulkadyrov K.M. Schots R. Jiang B. Mateos M.-V. Anderson K.C. Esseltine D.L. Liu K. Cakana A. … Trial Investigators V.I.S.T.A. Bortezomib plus melphalan and prednisone for initial treatment of multiple myeloma The New England Journal of Medicine 359 9 2008 906 917 10.1056/NEJMoa0801479 18753647

Sanagawa A. Hotta Y. Kondo M. Nishikawa R. Tohkin M. Kimura K. Tumor lysis syndrome associated with bortezomib: A post-hoc analysis after signal detection using the US Food and Drug Administration Adverse Event Reporting System Anti-Cancer Drugs 31 2 2020 183 189 10.1097/CAD.0000000000000862 31789626

Sanchez E. Li M. Li J. Wang C. Chen H. Jones-Bolin S. … Berenson J.R. CEP-18770 (delanzomib) in combination with dexamethasone and lenalidomide inhibits the growth of multiple myeloma Leukemia Research 36 11 2012 1422 1427 10.1016/j.leukres.2012.07.018 22906694

Sanchez E. Li M. Steinberg J.A. Wang C. Shen J. Bonavida B. … Berenson J.R. The proteasome inhibitor CEP-18770 enhances the anti-myeloma activity of bortezomib and melphalan British Journal of Haematology 148 4 2010 569 581 10.1111/j.1365-2141.2009.08008.x 19958357

Sanchez E. Li M. Wang C.S. Tang G. Gillespie A. Chen H. Berenson J.R. Anti-angiogenic and anti-multiple myeloma effects of oprozomib (OPZ) alone and in combination with pomalidomide (Pom) and/or dexamethasone (Dex) Leukemia Research 57 2017 45 54 10.1016/j.leukres.2017.03.002 28288323

Sánchez-Serrano I. Success in translational research: Lessons from the development of bortezomib Nature Reviews. Drug Discovery 5 2 2006 107 114 10.1038/nrd1959 16518378

Sanchorawala V. Palladini G. Kukreti V. Zonder J.A. Cohen A.D. Seldin D.C. … Merlini G. A phase 1/2 study of the oral proteasome inhibitor ixazomib in relapsed or refractory AL amyloidosis Blood 130 5 2017 597 605 10.1182/blood-2017-03-771220 28550039 PMC6911836

San-Miguel J. Bladé J. Shpilberg O. Grosicki S. Maloisel F. Min C.-K. … Z R. Phase 2 randomized study of bortezomib-melphalan-prednisone with or without siltuximab (anti-IL-6) in multiple myeloma Blood 123 26 2014 4136 4142 10.1182/blood-2013-12-546374 24833354 PMC4123433

San-Miguel J.F. Echeveste Gutierrez M.-A. Špicka I. Mateos M.-V. Song K. Craig M.D. … Lonial S. A phase I/II dose-escalation study investigating all-oral ixazomib-melphalan-prednisone induction followed by single-agent ixazomib maintenance in transplant-ineligible newly diagnosed multiple myeloma Haematologica 103 9 2018 1518 1526 10.3324/haematol.2017.185991 29954932 PMC6119151

San-Miguel J.F. Hungria V.T.M. Yoon S.-S. Beksac M. Dimopoulos M.A. Elghandour A. Jedrzejczak W.W. Günther A. Nakorn T.N. Siritanaratkul N. Corradini P. Chuncharunee S. Lee J.-J. Schlossman R.L. Shelekhova T. Yong K. Tan D. Numbenjapon T. Cavenagh J.D. … Richardson P.G. Panobinostat plus bortezomib and dexamethasone versus placebo plus bortezomib and dexamethasone in patients with relapsed or relapsed and refractory multiple myeloma: A multicentre, randomised, double-blind phase 3 trial The Lancet. Oncology 15 11 2014 1195 1206 10.1016/S1470-2045(14)70440-1 25242045

San-Miguel J.F. Hungria V.T.M. Yoon S.-S. Beksac M. Dimopoulos M.A. Elghandour A. … Richardson P.G. Overall survival of patients with relapsed multiple myeloma treated with panobinostat or placebo plus bortezomib and dexamethasone (the PANORAMA 1 trial): a randomised, placebo-controlled, phase 3 trial The Lancet. Haematology 3 11 2016 e506 e515 10.1016/S2352-3026(16)30147-8 27751707

Santoro A.M. Cunsolo A. D'Urso A. Sbardella D. Tundo G.R. Ciaccio C. … Purrello R. Cationic porphyrins are tunable gatekeepers of the 20S proteasome Chemical Science 7 2 2016 1286 1297 10.1039/c5sc03312h 29910886 PMC5975898

Santoro A.M. Lo Giudice M.C. D'Urso A. Lauceri R. Purrello R. Milardi D. Cationic porphyrins are reversible proteasome inhibitors Journal of the American Chemical Society 134 25 2012 10451 10457 10.1021/ja300781u 22642538

Santoro A.M. Monaco I. Attanasio F. Lanza V. Pappalardo G. Tomasello M.F. … Milardi D. Copper(II) ions affect the gating dynamics of the 20S proteasome: A molecular and in cell study Scientific Reports 6 1 2016 1 10 10.1038/srep33444 27633879 PMC5025780

Santos M. Morais S. Pereira C. Sequeiros J. Alonso I. Parkin truncating variants result in a loss-of-function phenotype Scientific Reports 9 1 2019 16150 10.1038/s41598-019-52534-6 31695088 PMC6834586

Sasaki K. Hamazaki J. Koike M. Hirano Y. Komatsu M. Uchiyama Y. … Murata S. PAC1 gene knockout reveals an essential role of chaperone-mediated 20S proteasome biogenesis and latent 20S proteasomes in cellular homeostasis Molecular and Cellular Biology 30 15 2010 3864 3874 10.1128/MCB.00216-10 20498273 PMC2916404

Sasaki K. Takada K. Ohte Y. Kondo H. Sorimachi H. Tanaka K. … Murata S. Thymoproteasomes produce unique peptide motifs for positive selection of CD8 + T cells Nature Communications 6 1 2015 1 10 10.1038/ncomms8484 PMC4557289 26099460

Satoh T. Yagi-Utsumi M. Okamoto K. Kurimoto E. Tanaka K. Kato K. Molecular and structural basis of the proteasome α subunit assembly mechanism mediated by the proteasome-assembling chaperone PAC3-PAC4 heterodimer International Journal of Molecular Sciences 20 9 2019 10.3390/ijms20092231 PMC6539346 31067643

Satriano C. Sfrazzetto G.T. Amato M.E. Ballistreri F.P. Copani A. Giuffrida M.L. … Toscano R.M. A ratiometric naphthalimide sensor for live cell imaging of copper(i) Chemical Communications 49 49 2013 5565 5567 23673488 10.1039/c3cc42069h

Savolainen M.H. Albert K. Airavaara M. Myöhänen T.T. Nigral injection of a proteasomal inhibitor, lactacystin, induces widespread glial cell activation and shows various phenotypes of Parkinson's disease in young and adult mouse Experimental Brain Research 235 7 2017 2189 2202 10.1007/s00221-017-4962-z 28439627

Sbardella D. Coletta A. Tundo G.R. Ahmed I.M.M. Bellia F. Oddone F. … Coletta M. Structural and functional evidence for citicoline binding and modulation of 20S proteasome activity: Novel insights into its pro-proteostatic effect Biochemical Pharmacology 177 2020 113977 10.1016/j.bcp.2020.113977 32298691

Sbardella D. Tundo G.R. Coletta A. Marcoux J. Koufogeorgou E.I. Ciaccio C. … Coletta M. The insulin-degrading enzyme is an allosteric modulator of the 20S proteasome and a potential competitor of the 19S Cellular and Molecular Life Sciences: CMLS 75 18 2018 3441 3456 10.1007/s00018-018-2807-y 29594388 PMC11105570

Sbardella D. Tundo G.R. Cunsolo V. Grasso G. Cascella R. Caputo V. Santoro A.M. Milardi D. Pecorelli A. Ciaccio C. Di Pierro D. Leoncini S. Campagnolo L. Pironi V. Oddone F. Manni P. Foti S. Giardina E. De Felice C. … Marini S. Defective proteasome biogenesis into skin fibroblasts isolated from Rett syndrome subjects with MeCP2 non-sense mutations Biochimica Et Biophysica Acta. Molecular Basis of Disease 1866 7 2020 165793 10.1016/j.bbadis.2020.165793 32275946

Sbardella D., Tundo G.R., Campagnolo L., Valacchi G., Orlandi A., Curatolo P., Borsellino G., D'Esposito M., Ciaccio C., Di Cesare S., Di Pierro D., Galasso C., Santarone M.E., Hayek J., Coletta M., (2017). Stefano Marini Retention of Mitochondria in Mature Human Red Blood Cells as the Result of Autophagy Impairment in Rett Syndrome Sci Rep . 7(1):12297. http://doi.org/10.1038/s41598-017-12069-0. 10.1038/s41598-017-12069-0 PMC5614985 28951555

Sbardella D. Tundo G.R. Sciandra F. Bozzi M. Gioia M. Ciaccio C. … Marini S. Proteasome activity is affected by fluctuations in insulin-degrading enzyme distribution PLoS One 10 7 2015 10.1371/journal.pone.0132455 PMC4506093 26186340

Scannell J.W. Blanckley A. Boldon H. Warrington B. Diagnosing the decline in pharmaceutical R&D efficiency Nature Reviews Drug Discovery 11 3 2012 191 200 10.1038/nrd3681 22378269

Schaler A.W. Myeku N. Cilostazol, a phosphodiesterase 3 inhibitor, activates proteasome-mediated proteolysis and attenuates tauopathy and cognitive decline Translational Research: The Journal of Laboratory and Clinical Medicine 193 2018 31 41 10.1016/j.trsl.2017.11.004 29232559 PMC10075247

Scheffner M. Nuber U. Huibregtse J.M. Protein ubiquitination involving an E1-E2-E3 enzyme ubiquitin thioester cascade Nature 373 6509 1995 81 83 10.1038/373081a0 7800044

Scherzinger E. Lurz R. Turmaine M. Mangiarini L. Hollenbach B. Hasenbank R. … Wanker E.E. Huntingtin-encoded polyglutamine expansions form amyloid-like protein aggregates in vitro and in vivo Cell 90 3 1997 549 558 10.1016/S0092-8674(00)80514-0 9267034

Schipper-Krom S. Juenemann K. Reits E.A.J. The ubiquitin-proteasome system in Huntington's disease: Are proteasomes impaired, initiators of disease, or coming to the rescue? Biochemistry Research International 2012 2012 10.1155/2012/837015 PMC3462393 23050151

Schmidt M. Finley D. Regulation of proteasome activity in health and disease Biochimica et Biophysica Acta 1843 1 2014 13 25 10.1016/j.bbamcr.2013.08.012 23994620 PMC3858528

Schmidtke G. Kraft R. Kostka S. Henklein P. Frömmel C. Löwe J. … Schmidt M. Analysis of mammalian 20S proteasome biogenesis: The maturation of beta-subunits is an ordered two-step mechanism involving autocatalysis The EMBO Journal 15 24 1996 6887 6898 9003765 PMC452515

Schneider M. Ackermann K. Stuart M. Wex C. Protzer U. Schätzl H.M. Gilch S. Severe acute respiratory syndrome coronavirus replication is severely impaired by MG132 due to proteasome-independent inhibition of M-calpain Journal of Virology 86 18 2012 10112 10122 10.1128/JVI.01001-12 22787216 PMC3446591

Schoenheimer R. The Dynamic State of Body Constituents 2nd ed. 1946 Mass Harvard University Press Cambridge https://trove.nla.gov.au/work/14374148

Schoenheimer R. Ratner S. Rittenberg D. Studies in protein metabolism vii. The metabolism of tyrosine Journal of Biological Chemistry 127 1 1939 333 344

Schrader J. Henneberg F. Mata R.A. Tittmann K. Schneider T.R. Stark H. … Chari A. The inhibition mechanism of human 20S proteasomes enables next-generation inhibitor design Science 353 6299 2016 594 598 10.1126/science.aaf8993 27493187

Schuld N.J. Hussong S.A. Kapphahn R.J. Lehmann U. Roehrich H. Rageh A.A. … Ferrington D.A. Immunoproteasome deficiency protects in the retina after optic nerve crush PLoS One 10 5 2015 e0126768 10.1371/journal.pone.0126768 PMC4433222 25978061

Schwartz S.A. Hernandez A. Mark Evers B. The role of NF-kappaB/IkappaB proteins in cancer: implications for novel treatment strategies Surgical Oncology 8 3 1999 143 153 10.1016/s0960-7404(00)00012-8 11113665

Seemuller E. Lupas A. Baumeister W. Autocatalytic processing of the 20S proteasome Nature 382 6590 1996 468 471 10.1038/382468a0 8684489

Seemüller E. Lupas A. Stock D. Löwe J. Huber R. Baumeister W. Proteasome from Thermoplasma acidophilum : A threonine protease Science (New York, N.Y.) 268 5210 1995 579 582 10.1126/science.7725107 7725107

Seki N. Toh U. Sayers T.J. Fujii T. Miyagi M. Akagi Y. … Yamana H. Bortezomib sensitizes human esophageal squamous cell carcinoma cells to TRAIL-mediated apoptosis via activation of both extrinsic and intrinsic apoptosis pathways Molecular Cancer Therapeutics 9 6 2010 1842 1851 10.1158/1535-7163.MCT-09-0918 20515944 PMC2884061

Senatorov V. Malyukova I. Fariss R. Wawrousek E.F. Swaminathan S. Sharan S.K. Tomarev S. Expression of mutated mouse myocilin induces open-angle glaucoma in transgenic mice The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 26 46 2006 11903 11914 10.1523/JNEUROSCI.3020-06.2006 17108164 PMC6674879

Shabaneh T.B. Downey S.L. Goddard A.L. Screen M. Lucas M.M. Eastman A. Kisselev A.F. Molecular basis of differential sensitivity of myeloma cells to clinically relevant bolus treatment with bortezomib PLoS One 8 2 2013 e56132 10.1371/journal.pone.0056132 PMC3584083 23460792

Shabek N. Herman-Bachinsky Y. Buchsbaum S. Lewinson O. Haj-Yahya M. Hejjaoui M. … Ciechanover A. The size of the proteasomal substrate determines whether its degradation will be mediated by mono- or polyubiquitylation Molecular Cell 48 1 2012 87 97 10.1016/j.molcel.2012.07.011 22902562

Shagufta Ahmad I. Transition metal complexes as proteasome inhibitors for cancer treatment Inorganica Chimica Acta 506 2020 119521 10.1016/j.ica.2020.119521

Shah C. Bishnoi R. Wang Y. Zou F. Bejjanki H. Master S. Moreb J.S. Efficacy and safety of carfilzomib in relapsed and/or refractory multiple myeloma: Systematic review and meta-analysis of 14 trials Oncotarget 9 34 2018 23704 23717 10.18632/oncotarget.25281 29805768 PMC5955098

Shah J. Usmani S. Stadtmauer E.A. Rifkin R.M. Berenson J.R. Berdeja J.G. … Niesvizky R. Oprozomib, pomalidomide, and dexamethasone in patients with relapsed and/or refractory multiple myeloma Clinical Lymphoma, Myeloma & Leukemia 19 9 2019 570 578 e1 10.1016/j.clml.2019.05.017 31326409

Shakya R. Peng F. Liu J. Heeg M.J. Verani C.N. Synthesis, structure, and anticancer activity of gallium(III) complexes with asymmetric tridentate ligands: Growth inhibition and apoptosis induction of cisplatin-resistant neuroblastoma cells Inorganic Chemistry 45 16 2006 6263 6268 10.1021/ic060106g 16878935

Shang F. Taylor A. Role of the ubiquitin-proteasome in protein quality control and signaling: Implication in the pathogenesis of eye diseases Progress in Molecular Biology and Translational Science 109 2012 347 396 10.1016/B978-0-12-397863-9.00010-9 22727427

Sharma S.K. Chorell E. Steneberg P. Vernersson-Lindahl E. Edlund H. Wittung-Stafshede P. Insulin-degrading enzyme prevents α-synuclein fibril formation in a nonproteolytical manner Scientific Reports 5 1 2015 1 10 10.1038/srep12531 PMC4521159 26228656

Sharma S.K. Chorell E. Wittung-Stafshede P. Insulin-degrading enzyme is activated by the C-terminus of α-synuclein Biochemical and Biophysical Research Communications 466 2 2015 192 195 10.1016/j.bbrc.2015.09.002 26343304

Sharon M. Taverner T. Ambroggio X.I. Deshaies R.J. Robinson C.V. Structural organization of the 19S proteasome lid: Insights from MS of intact complexes PLoS Biology 4 8 2006 e267 10.1371/journal.pbio.0040267 PMC1523230 16869714

Sheaff R.J. Singer J.D. Swanger J. Smitherman M. Roberts J.M. Clurman B.E. Proteasomal turnover of p21Cip1 does not require p21Cip1 ubiquitination Molecular Cell 5 2 2000 403 410 10.1016/s1097-2765(00)80435-9 10882081

Shen W.-C. Li H.-Y. Chen G.-C. Chern Y. Tu P.-H. Mutations in the ubiquitin-binding domain of OPTN/optineurin interfere with autophagy-mediated degradation of misfolded proteins by a dominant-negative mechanism Autophagy 11 4 2015 685 700 10.4161/auto.36098 25484089 PMC4502753

Shen Y. Luo X. Liu S. Shen Y. Nawy S. Shen Y. Rod bipolar cells dysfunction occurs before ganglion cells loss in excitotoxin-damaged mouse retina Cell Death & Disease 10 12 2019 905 10.1038/s41419-019-2140-x 31787761 PMC6885518

Shen J. Song G. An M. Li X. Wu N. Ruan K. … Hu R. The use of hollow mesoporous silica nanospheres to encapsulate bortezomib and improve efficacy for non-small cell lung cancer therapy Biomaterials 35 1 2014 316 326 10.1016/j.biomaterials.2013.09.098 24125776

Shen X. Ying H. Qiu Y. Park J.-S. Shyam R. Chi Z.-L. … Yue B.Y.J.T. Processing of optineurin in neuronal cells The Journal of Biological Chemistry 286 5 2011 3618 3629 10.1074/jbc.M110.175810 21059646 PMC3030366

Shereen M.A. Khan S. Kazmi A. Bashir N. Siddique R. COVID-19 infection: Origin, transmission, and characteristics of human coronaviruses Journal of Advanced Research 24 2020 91 98 10.1016/j.jare.2020.03.005 32257431 PMC7113610

Sherman D.J. Li J. Proteasome inhibitors: Harnessing proteostasis to combat disease Molecules 25 3 2020 671 10.3390/molecules25030671 PMC7037493 32033280

Sherr C.J. Roberts J.M. CDK inhibitors: positive and negative regulators of G1-phase progression Genes & Development 13 12 1999 1501 1512 10.1101/gad.13.12.1501 10385618

Shi Y. Chen X. Elsasser S. Stocks B.B. Tian G. Lee B.-H. … Walters K.J. Rpn1 provides adjacent receptor sites for substrate binding and deubiquitination by the proteasome Science (New York, N.Y.) 351 6275 2016 10.1126/science.aad9421 PMC4980823 26912900

Shimura H. Schlossmacher M.G. Hattori N. Frosch M.P. Trockenbacher A. Schneider R. … Selkoe D.J. Ubiquitination of a new form of alpha-synuclein by parkin from human brain: Implications for Parkinson's disease Science (New York, N.Y.) 293 5528 2001 263 269 10.1126/science.1060627 11431533

Shimura H. Schwartz D. Gygi S.P. Kosik K.S. CHIP-Hsc70 complex ubiquitinates phosphorylated tau and enhances cell survival The Journal of Biological Chemistry 279 6 2004 4869 4876 10.1074/jbc.M305838200 14612456

Shringarpure R. Grune T. Mehlhase J. Davies K.J.A. Ubiquitin conjugation is not required for the degradation of oxidized proteins by proteasome The Journal of Biological Chemistry 278 1 2003 311 318 10.1074/jbc.M206279200 12401807

Shukla N. Somwar R. Smith R.S. Ambati S. Munoz S. Merchant M. D'Arcy P. Wang X. Kobos R. Antczak C. Bhinder B. Shum D. Radu C. Yang G. Taylor B.S. Ng C.K.Y. Weigelt B. Khodos I. de Stanchina E. … Ladanyi M. Proteasome Addiction Defined in Ewing Sarcoma Is Effectively Targeted by a Novel Class of 19S Proteasome Inhibitors Cancer Research 76 15 2016 4525 4534 10.1158/0008-5472.CAN-16-1040 27256563 PMC5484002

Siegel D. Martin T. Nooka A. Harvey R.D. Vij R. Niesvizky R. … Lonial S. Integrated safety profile of single-agent carfilzomib: Experience from 526 patients enrolled in 4 phase II clinical studies Haematologica 98 11 2013 1753 1761 10.3324/haematol.2013.089334 23935022 PMC3815177

Siegel D.S. Martin T. Wang M. Vij R. Jakubowiak A.J. Lonial S. Trudel S. Kukreti V. Bahlis N. Alsina M. Chanan-Khan A. Buadi F. Reu F.J. Somlo G. Zonder J. Song K. Stewart A.K. Stadtmauer E. Kunkel L. … Jagannath S. A phase 2 study of single-agent carfilzomib (PX-171-003-A1) in patients with relapsed and refractory multiple myeloma Blood 120 14 2012 2817 2825 10.1182/blood-2012-05-425934 22833546 PMC4123387

Sijts E.J.A.M. Kloetzel P.M. The role of the proteasome in the generation of MHC class I ligands and immune responses Cellular and Molecular Life Sciences: CMLS 68 9 2011 1491 1502 10.1007/s00018-011-0657-y 21387144 PMC3071949

Sijts A. Sun Y. Janek K. Kral S. Paschen A. Schadendorf D. Kloetzel P.-M. The role of the proteasome activator PA28 in MHC class I antigen processing Molecular Immunology 39 3–4 2002 165 169 10.1016/s0161-5890(02)00099-8 12200048

Silswal N. Reis J. Qureshi A.A. Papasian C. Qureshi N. Of mice and men: Proteasome's role in LPS-induced inflammation and tolerance Shock (Augusta, GA.) 47 4 2017 445 454 10.1097/SHK.0000000000000743 PMC5842815 27648699

Silva J.L. De Moura Gallo C.V. Costa D.C.F. Rangel L.P. Prion-like aggregation of mutant p53 in cancer Trends in Biochemical Sciences 39 6 2014 260 267 10.1016/j.tibs.2014.04.001 24775734

Simons B.D. Clevers H. Strategies for homeostatic stem cell self-renewal in adult tissues Cell 145 6 2011 851 862 10.1016/j.cell.2011.05.033 21663791

Simpson M.V. The release of labeled amino acids from the proteins of rat liver slices The Journal of Biological Chemistry 201 1 1953 143 154 13044783

Sin N. Kim K.B. Elofsson M. Meng L. Auth H. Kwok B.H. Crews C.M. Total synthesis of the potent proteasome inhibitor epoxomicin: A useful tool for understanding proteasome biology Bioorganic & Medicinal Chemistry Letters 9 15 1999 2283 2288 10.1016/s0960-894x(99)00376-5 10465562

Singh B.N. Malhotra B.K. Effects of food on the clinical pharmacokinetics of anticancer agents: Underlying mechanisms and implications for oral chemotherapy Clinical Pharmacokinetics 43 15 2004 1127 1156 10.2165/00003088-200443150-00005 15568891

Singh A.V. Palladino M.A. Lloyd G.K. Potts B.C. Chauhan D. Anderson K.C. Pharmacodynamic and efficacy studies of the novel proteasome inhibitor NPI-0052 (marizomib) in a human plasmacytoma xenograft murine model British Journal of Haematology 149 4 2010 550 559 10.1111/j.1365-2141.2010.08144.x 20331453 PMC4427841

Sirohi K. Swarup G. Defects in autophagy caused by glaucoma-associated mutations in optineurin Experimental Eye Research 144 2016 54 63 10.1016/j.exer.2015.08.020 26302410

Sitaraman S. Na C.-L. Yang L. Filuta A. Bridges J.P. Weaver T.E. Proteasome dysfunction in alveolar type 2 epithelial cells is associated with acute respiratory distress syndrome Scientific Reports 9 1 2019 1 15 10.1038/s41598-019-49020-4 31467330 PMC6715642

Sixt S.U. Adamzik M. Spyrka D. Saul B. Hakenbeck J. Wohlschlaeger J. … Peters J. Alveolar extracellular 20S proteasome in patients with acute respiratory distress syndrome American Journal of Respiratory and Critical Care Medicine 179 12 2009 1098 1106 10.1164/rccm.200802-199OC 19286628

Sixt S.U. Alami R. Hakenbeck J. Adamzik M. Kloß A. Costabel U. … Peters J. Distinct proteasome subpopulations in the alveolar space of patients with the acute respiratory distress syndrome Mediators of Inflammation 2012 2012 10.1155/2012/204250 PMC3272875 22363101

Sklirou A. Papanagnou E.-D. Fokialakis N. Trougakos I.P. Cancer chemoprevention via activation of proteostatic modules Cancer Letters 413 2018 110 121 10.1016/j.canlet.2017.10.034 29107114

Śledź P. Unverdorben P. Beck F. Pfeifer G. Schweitzer A. Förster F. Baumeister W. Structure of the 26S proteasome with ATP-γS bound provides insights into the mechanism of nucleotide-dependent substrate translocation Proceedings of the National Academy of Sciences of the United States of America 110 18 2013 7264 7269 10.1073/pnas.1305782110 23589842 PMC3645540

Slingerland J. Pagano M. Regulation of the cdk inhibitor p27 and its deregulation in cancer Journal of Cellular Physiology 183 1 2000 10 17 10.1002/(SICI)1097-4652(200004)183:1<10::AID-JCP2>3.0.CO;2-I 10699961

Smith D.M. Could a common mechanism of protein degradation impairment underlie many neurodegenerative diseases? Journal of Experimental Neuroscience 12 2018 10.1177/1179069518794675 PMC6102758 30147359

Smith D.M. Chang S.-C. Park S. Finley D. Cheng Y. Goldberg A. Docking of the proteasomal ATPases' C-termini in the 20S proteasomes alpha ring opens the gate for substrate entry Molecular Cell 27 5 2007 731 744 10.1016/j.molcel.2007.06.033 17803938 PMC2083707

Smith D.M. Kafri G. Cheng Y. Ng D. Walz T. Goldberg A.L. ATP binding to PAN or the 26S ATPases causes association with the 20S proteasome, gate opening, and translocation of unfolded proteins Molecular Cell 20 5 2005 687 698 10.1016/j.molcel.2005.10.019 16337593

Smith D.C. Kalebic T. Infante J.R. Siu L.L. Sullivan D. Vlahovic G. … Thompson J.A. Phase 1 study of ixazomib, an investigational proteasome inhibitor, in advanced non-hematologic malignancies Investigational New Drugs 33 3 2015 652 663 10.1007/s10637-015-0230-x 25777468 PMC4435632

Smith D.L. Pozueta J. Gong B. Arancio O. Shelanski M. Reversal of long-term dendritic spine alterations in Alzheimer disease models Proceedings of the National Academy of Sciences 106 39 2009 16877 16882 10.1073/pnas.0908706106 PMC2743726 19805389

Smolewski P. Rydygier D. Ixazomib: An investigational drug for the treatment of lymphoproliferative disorders Expert Opinion on Investigational Drugs 28 5 2019 421 433 10.1080/13543784.2019.1596258 30907163

Soares T.R. Reis S.D. Pinho B.R. Duchen M.R. Oliveira J.M.A. Targeting the proteostasis network in Huntington's disease Ageing Research Reviews 49 2019 92 103 10.1016/j.arr.2018.11.006 30502498 PMC6320389

Soave C.L. Guerin T. Liu J. Dou Q.P. Targeting the ubiquitin-proteasome system for cancer treatment: discovering novel inhibitors from nature and drug repurposing Cancer Metastasis Reviews 36 4 2017 717 736 10.1007/s10555-017-9705-x 29047025 PMC5722705

Sokolova V. Li F. Polovin G. Park S. Proteasome activation is mediated via a functional switch of the Rpt6 C-terminal tail following chaperone-dependent assembly Scientific Reports 5 1 2015 1 15 10.1038/srep14909 PMC4598862 26449534

Sone T. Saeki Y. Toh-e A. Yokosawa H. Sem1p is a novel subunit of the 26 S proteasome from Saccharomyces cerevisiae The Journal of Biological Chemistry 279 27 2004 28807 28816 10.1074/jbc.M403165200 15117943

Song P. Li S. Wu H. Gao R. Rao G. Wang D. … Chen Q. Parkin promotes proteasomal degradation of p62: Implication of selective vulnerability of neuronal cells in the pathogenesis of Parkinson's disease Protein & Cell 7 2 2016 114 129 10.1007/s13238-015-0230-9 26746706 PMC4742389

Sonneveld P. Goldschmidt H. Rosiñol L. Bladé J. Lahuerta J.J. Cavo M. … Moreau P. Bortezomib-based versus nonbortezomib-based induction treatment before autologous stem-cell transplantation in patients with previously untreated multiple myeloma: A meta-analysis of phase III randomized, controlled trials Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 31 26 2013 3279 3287 10.1200/JCO.2012.48.4626 23897961

Sontag E.M. Vonk W.I.M. Frydman J. Sorting out the trash: The spatial nature of eukaryotic protein quality control Current Opinion in Cell Biology 26 2014 139 146 10.1016/j.ceb.2013.12.006 24463332 PMC4204729

Speliotes E.K. Yerges-Armstrong L.M. Wu J. Hernaez R. Kim L.J. Palmer C.D. Gudnason V. Eiriksdottir G. Garcia M.E. Launer L.J. Nalls M.A. Clark J.M. Mitchell B.D. Shuldiner A.R. Butler J.L. Tomas M. Hoffmann U. Hwang S.-J. Massaro J.M. … Consortium G.O.L.D. Genome-wide association analysis identifies variants associated with nonalcoholic fatty liver disease that have distinct effects on metabolic traits PLoS Genetics 7 3 2011 e1001324 10.1371/journal.pgen.1001324 PMC3053321 21423719

Spencer A. Harrison S. Zonder J. Badros A. Laubach J. Bergin K. … Richardson P. A phase 1 clinical trial evaluating marizomib, pomalidomide and low-dose dexamethasone in relapsed and refractory multiple myeloma (NPI-0052-107): Final study results British Journal of Haematology 180 1 2018 41 51 10.1111/bjh.14987 29076150 PMC6860364

Spencer A. Lentzsch S. Weisel K. Avet-Loiseau H. Mark T.M. Spicka I. Masszi T. Lauri B. Levin M.-D. Bosi A. Hungria V. Cavo M. Lee J.-J. Nooka A.K. Quach H. Lee C. Barreto W. Corradini P. Min C.-K. … Mateos M.-V. Daratumumab plus bortezomib and dexamethasone versus bortezomib and dexamethasone in relapsed or refractory multiple myeloma: Updated analysis of CASTOR Haematologica 103 12 2018 2079 2087 10.3324/haematol.2018.194118 30237264 PMC6269293

Spyracopoulos L. The proteasome: More than a means to an end Structure (London, England: 1993) 24 8 2016 1221 1223 10.1016/j.str.2016.07.005 27486912

Stelzma R.A. Schnitzlein H.N. Murllagh F.R. VIEWPOINT An english translation of Alzheimer's 1907 Paper, "ijber eine eigenartige Erlranliung der Hirnrinde" Clinical Anatomy 8 1995 429 443 8713166 10.1002/ca.980080612

Sterz J. Jakob C. Kuckelkorn U. Heider U. Mieth M. Kleeberg L. … von Metzler I. BSc2118 is a novel proteasome inhibitor with activity against multiple myeloma European Journal of Haematology 85 2 2010 99 107 10.1111/j.1600-0609.2010.01450.x 20374272

Stewart A.K. Dimopoulos M.A. Masszi T. Špička I. Oriol A. Hájek R. … Palumbo A. Health-related quality-of-life results from the open-label, randomized, phase III ASPIRE trial evaluating carfilzomib, lenalidomide, and dexamethasone versus lenalidomide and dexamethasone in patients with relapsed multiple myeloma Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 34 32 2016 3921 3930 10.1200/JCO.2016.66.9648 27601539 PMC5791840

Stewart A.K. Rajkumar S.V. Dimopoulos M.A. Masszi T. Špička I. Oriol A. Hájek R. Rosiñol L. Siegel D.S. Mihaylov G.G. Goranova-Marinova V. Rajnics P. Suvorov A. Niesvizky R. Jakubowiak A.J. San-Miguel J.F. Ludwig H. Wang M. Maisnar V. … ASPIRE Investigators Carfilzomib, lenalidomide, and dexamethasone for relapsed multiple myeloma The New England Journal of Medicine 372 2 2015 142 152 10.1056/NEJMoa1411321 25482145

Strauss S.J. Higginbottom K. Jüliger S. Maharaj L. Allen P. Schenkein D. … Joel S.P. The proteasome inhibitor bortezomib acts independently of p53 and induces cell death via apoptosis and mitotic catastrophe in B-cell lymphoma cell lines Cancer Research 67 6 2007 2783 2790 10.1158/0008-5472.CAN-06-3254 17363600

Striha A. Ashcroft A.J. Hockaday A. Cairns D.A. Boardman K. Jacques G. … Cook G. The role of ixazomib as an augmented conditioning therapy in salvage autologous stem cell transplant (ASCT) and as a post-ASCT consolidation and maintenance strategy in patients with relapsed multiple myeloma (ACCoRd [UK-MRA Myeloma XII] trial): study protocol for a Phase III randomised controlled trial Trials 19 1 2018 169 10.1186/s13063-018-2524-8 29514706 PMC5842589

Suh S.W. Jensen K.B. Jensen M.S. Silva D.S. Kesslak P.J. Danscher G. Frederickson C.J. Histochemically-reactive zinc in amyloid plaques, angiopathy, and degenerating neurons of Alzheimer's diseased brains Brain Research 852 2 2000 274 278 10678753 10.1016/s0006-8993(99)02096-x

Sun L. Büeler H. Proteasome inhibition promotes mono-ubiquitination and nuclear translocation of mature (52 kDa) PINK1 Biochemical and Biophysical Research Communications 517 2 2019 376 382 10.1016/j.bbrc.2019.07.051 31362890

Sunwoo J.B. Chen Z. Dong G. Yeh N. Crowl Bancroft C. Sausville E. … Van Waes C. Novel proteasome inhibitor PS-341 inhibits activation of nuclear factor-kappa B, cell survival, tumor growth, and angiogenesis in squamous cell carcinoma Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 7 5 2001 1419 1428 11350913

Swarup G. Sayyad Z. Altered functions and interactions of glaucoma-associated mutants of optineurin Frontiers in Immunology 2018 9 10.3389/fimmu.2018.01287 29951055 PMC6008547

Sweeney P. Park H. Baumann M. Dunlop J. Frydman J. Kopito R. … Hodgson R. Protein misfolding in neurodegenerative diseases: Implications and strategies Translational Neurodegeneration 6 2017 10.1186/s40035-017-0077-5 PMC5348787 28293421

Szokalska A. Makowski M. Nowis D. Wilczyński G.M. Kujawa M. Wójcik C. Młynarczuk-Biały I. Salwa P. Bil J. Janowska S. Agostinis P. Verfaillie T. Bugajski M. Gietka J. Issat T. Głodkowska E. Mrówka P. Stoklosa T. Hamblin M.R. … Golab J. Proteasome inhibition potentiates antitumor effects of photodynamic therapy in mice through induction of endoplasmic reticulum stress and unfolded protein response Cancer Research 69 10 2009 4235 4243 10.1158/0008-5472.CAN-08-3439 19435917 PMC2785802

Tai H.-C. Schuman E.M. Ubiquitin, the proteasome and protein degradation in neuronal function and dysfunction Nature Reviews. Neuroscience 9 11 2008 826 838 10.1038/nrn2499 18931696

Tai H.-C. Serrano-Pozo A. Hashimoto T. Frosch M.P. Spires-Jones T.L. Hyman B.T. The synaptic accumulation of hyperphosphorylated tau oligomers in Alzheimer disease is associated with dysfunction of the ubiquitin-proteasome system The American Journal of Pathology 181 4 2012 1426 1435 10.1016/j.ajpath.2012.06.033 22867711 PMC3463637

Takada K. Van Laethem F. Xing Y. Akane K. Suzuki H. Murata S. … Takahama Y. TCR affinity for thymoproteasome-dependent positively selecting peptides conditions antigen responsiveness in CD8+ T cells Nature Immunology 16 10 2015 1069 1076 10.1038/ni.3237 26301566 PMC4810782

Takahashi M. Kitaura H. Kakita A. Kakihana T. Katsuragi Y. Nameta M. … Fujii M. USP10 is a driver of ubiquitinated protein aggregation and aggresome formation to inhibit apoptosis IScience 9 2018 433 450 10.1016/j.isci.2018.11.006 30469013 PMC6249355

Tanahashi N. Yokota K. Ahn J.Y. Chung C.H. Fujiwara T. Takahashi E. … Tanaka K. Molecular properties of the proteasome activator PA28 family proteins and γ-interferon regulation Genes to Cells 2 3 1997 195 211 10.1046/j.1365-2443.1997.d01-308.x 9189757

Tanaka K. The proteasome: Overview of structure and functions Proceedings of the Japan Academy. Series B, Physical and Biological Sciences 85 1 2009 12 36 10.2183/pjab.85.12 PMC3524306 19145068

Tanaka K. Waxman L. Goldberg A.L. ATP serves two distinct roles in protein degradation in reticulocytes, one requiring and one independent of ubiquitin The Journal of Cell Biology 96 6 1983 1580 1585 10.1083/jcb.96.6.1580 6304111 PMC2112434

Tang B. Seredenina T. Coppola G. Kuhn A. Geschwind D.H. Luthi-Carter R. Thomas E.A. Gene expression profiling of R6/2 transgenic mice with different CAG repeat lengths reveals genes associated with disease onset and progression in Huntington's disease Neurobiology of Disease 42 3 2011 459 467 10.1016/j.nbd.2011.02.008 21334439 PMC3079804

Tanji K. Mori F. Mimura J. Itoh K. Kakita A. Takahashi H. Wakabayashi K. Proteinase K-resistant alpha-synuclein is deposited in presynapses in human Lewy body disease and A53T alpha-synuclein transgenic mice Acta Neuropathologica 120 2 2010 145 154 10.1007/s00401-010-0676-z 20339856

Tarohda T. Yamamoto M. Amamo R. Regional distribution of manganese, iron, copper, and zinc in the rat brain during development Analytical and Bioanalytical Chemistry 380 2 2004 240 246 10.1007/s00216-004-2697-8 15322797

Taromi S. Lewens F. Arsenic R. Sedding D. Sänger J. Kunze A. Möbs M. Benecke J. Freitag H. Christen F. Kaemmerer D. Lupp A. Heilmann M. Lammert H. Schneider C.-P. Richter K. Hummel M. Siegmund B. Burger M. … Grabowski P. Proteasome inhibitor bortezomib enhances the effect of standard chemotherapy in small cell lung cancer Oncotarget 8 57 2017 97061 97078 10.18632/oncotarget.21221 29228593 PMC5722545

Tarr J.M. Kaul K. Chopra M. Kohner E.M. Chibber R. Pathophysiology of diabetic retinopathy ISRN Ophthalmology 2013 2013 343560 10.1155/2013/343560 24563789 PMC3914226

Tebbenkamp A.T.N. Green C. Xu G. Denovan-Wright E.M. Rising A.C. Fromholt S.E. … Borchelt D.R. Transgenic mice expressing caspase-6-derived N-terminal fragments of mutant huntingtin develop neurologic abnormalities with predominant cytoplasmic inclusion pathology composed largely of a smaller proteolytic derivative Human Molecular Genetics 20 14 2011 2770 2782 10.1093/hmg/ddr176 21515588 PMC3118758

Teh B.W. Harrison S.J. Worth L.J. Slavin M.A. Antiviral prophylaxis for varicella zoster virus infections in patients with myeloma in the era of novel therapies Leukemia & Lymphoma 57 7 2016 1719 1722 10.3109/10428194.2015.1106538 26727243

Teicher B.A. Ara G. Herbst R. Palombella V.J. Adams J. The proteasome inhibitor PS-341 in cancer therapy Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 5 9 1999 2638 2645 10499643

Teodorovic I. Pittaluga S. Kluin-Nelemans J.C. Meerwaldt J.H. Hagenbeek A. van Glabbeke M. … Peeters C.D. Efficacy of four different regimens in 64 mantle-cell lymphoma cases: clinicopathologic comparison with 498 other non-Hodgkin's lymphoma subtypes. European Organization for the Research and Treatment of Cancer Lymphoma Cooperative Group Journal of Clinical Oncology: Official Journal of the American Society of Clinical Oncology 13 11 1995 2819 2826 10.1200/JCO.1995.13.11.2819 7595744

Thibaudeau T.A. Anderson R.T. Smith D.M. A common mechanism of proteasome impairment by neurodegenerative disease-associated oligomers Nature Communications 9 1 2018 1097 10.1038/s41467-018-03509-0 PMC5854577 29545515

Thibaudeau T.A. Smith D.M. A practical review of proteasome pharmacology Pharmacological Reviews 71 2 2019 170 197 10.1124/pr.117.015370 30867233 PMC6423620

Thomas S.N. Cripps D. Yang A.J. Proteomic analysis of protein phosphorylation and ubiquitination in Alzheimer's disease Methods in Molecular Biology (Clifton, N.J.) 566 2009 109 121 10.1007/978-1-59745-562-6_8 PMC4708882 20058168

Tian Z. D'Arcy P. Wang X. Ray A. Tai Y.-T. Hu Y. … Anderson K.C. A novel small molecule inhibitor of deubiquitylating enzyme USP14 and UCHL5 induces apoptosis in multiple myeloma and overcomes bortezomib resistance Blood 123 5 2014 706 716 10.1182/blood-2013-05-500033 24319254 PMC3907756

Tian G. Park S. Lee M.J. Huck B. McAllister F. Hill C.P. … Finley D. An asymmetric interface between the regulatory and core particles of the proteasome Nature Structural & Molecular Biology 18 11 2011 1259 1267 10.1038/nsmb.2147 PMC3210322 22037170

Tian Z. Zhao J. Tai Y.-T. Amin S.B. Hu Y. Berger A.J. … Anderson K.C. Investigational agent MLN9708/2238 targets tumor-suppressor miR33b in MM cells Blood 120 19 2012 3958 3967 10.1182/blood-2012-01-401794 22983447 PMC3496955

Till B.G. Li H. Bernstein S.H. Fisher R.I. Burack W.R. Rimsza L.M. … Friedberg J.W. Phase II trial of R-CHOP plus bortezomib induction therapy followed by bortezomib maintenance for newly diagnosed mantle cell lymphoma: SWOG S0601 British Journal of Haematology 172 2 2016 208 218 10.1111/bjh.13818 26492567 PMC4710555

Tofaris G.K. Layfield R. Spillantini M.G. Alpha-synuclein metabolism and aggregation is linked to ubiquitin-independent degradation by the proteasome FEBS Letters 509 1 2001 22 26 10.1016/s0014-5793(01)03115-5 11734199

Tomaru U. Takahashi S. Ishizu A. Miyatake Y. Gohda A. Suzuki S. … Kasahara M. Decreased proteasomal activity causes age-related phenotypes and promotes the development of metabolic abnormalities The American Journal of Pathology 180 3 2012 963 972 10.1016/j.ajpath.2011.11.012 22210478

Tomasello M.F. Nardon C. Lanza V. Di Natale G. Pettenuzzo N. Salmaso S. … Fregona D. New comprehensive studies of a gold(III) Dithiocarbamate complex with proven anticancer properties: Aqueous dissolution with cyclodextrins, pharmacokinetics and upstream inhibition of the ubiquitin-proteasome pathway European Journal of Medicinal Chemistry 138 2017 115 127 10.1016/j.ejmech.2017.06.013 28651154

Tomko R.J. Funakoshi M. Schneider K. Wang J. Hochstrasser M. Heterohexameric ring arrangement of the eukaryotic proteasomal ATPases: Implications for proteasome structure and assembly Molecular Cell 38 3 2010 393 403 10.1016/j.molcel.2010.02.035 20471945 PMC2879271

Tomko R.J. Hochstrasser M. Incorporation of the Rpn12 subunit couples completion of proteasome regulatory particle lid assembly to lid-base joining Molecular Cell 44 6 2011 907 917 10.1016/j.molcel.2011.11.020 22195964 PMC3251515

Tomko R.J. Hochstrasser M. The intrinsically disordered Sem1 protein functions as a molecular tether during proteasome lid biogenesis Molecular Cell 53 3 2014 433 443 10.1016/j.molcel.2013.12.009 24412063 PMC3947554

Tomko R.J. Taylor D.W. Chen Z.A. Wang H.-W. Rappsilber J. Hochstrasser M. A single α helix drives extensive remodeling of the proteasome lid and completion of regulatory particle assembly Cell 163 2 2015 432 444 10.1016/j.cell.2015.09.022 26451487 PMC4601081

Torres E.M. Dephoure N. Panneerselvam A. Tucker C.M. Whittaker C.A. Gygi S.P. … Amon A. Identification of aneuploidy-tolerating mutations Cell 143 1 2010 71 83 10.1016/j.cell.2010.08.038 20850176 PMC2993244

Torres E.M. Sokolsky T. Tucker C.M. Chan L.Y. Boselli M. Dunham M.J. Amon A. Effects of aneuploidy on cellular physiology and cell division in haploid yeast Science (New York, N.Y.) 317 5840 2007 916 924 10.1126/science.1142210 17702937

Torres E.M. Williams B.R. Amon A. Aneuploidy: Cells losing their balance Genetics 179 2 2008 737 746 10.1534/genetics.108.090878 18558649 PMC2429870

Touitou R. Richardson J. Bose S. Nakanishi M. Rivett J. Allday M.J. A degradation signal located in the C-terminus of p21WAF1/CIP1 is a binding site for the C8 alpha-subunit of the 20S proteasome The EMBO Journal 20 10 2001 2367 2375 10.1093/emboj/20.10.2367 11350925 PMC125454

Traenckner E.B. Pahl H.L. Henkel T. Schmidt K.N. Wilk S. Baeuerle P.A. Phosphorylation of human I kappa B-alpha on serines 32 and 36 controls I kappa B-alpha proteolysis and NF-kappa B activation in response to diverse stimuli The EMBO Journal 14 12 1995 2876 2883 7796813 10.1002/j.1460-2075.1995.tb07287.x PMC398406

Traenckner E.B. Wilk S. Baeuerle P.A. A proteasome inhibitor prevents activation of NF-kappa B and stabilizes a newly phosphorylated form of I kappa B-alpha that is still bound to NF-kappa B The EMBO Journal 13 22 1994 5433 5441 7957109 10.1002/j.1460-2075.1994.tb06878.x PMC395500

Trippier P.C. Zhao K.T. Fox S.G. Schiefer I.T. Benmohamed R. Moran J. … Silverman R.B. Proteasome activation is a mechanism for pyrazolone small molecules displaying therapeutic potential in amyotrophic lateral sclerosis ACS Chemical Neuroscience 5 9 2014 823 829 10.1021/cn500147v 25001311 PMC4176317

Tsai C.W. Tsai R.T. Liu S.P. Chen C.S. Tsai M.C. Chien S.H. … Fu R.H. Neuroprotective effects of betulin in pharmacological and transgenic Caenorhabditis elegans models of Parkinson's disease Cell Transplantation 26 12 2017 1903 1918 10.1177/0963689717738785 29390878 PMC5802634

Tseng B.P. Green K.N. Chan J.L. Blurton-Jones M. LaFerla F.M. Abeta inhibits the proteasome and enhances amyloid and tau accumulation Neurobiology of Aging 29 11 2008 1607 1618 10.1016/j.neurobiolaging.2007.04.014 17544172 PMC2664168

Tsihlias J. Kapusta L. Slingerland J. The prognostic significance of altered cyclin-dependent kinase inhibitors in human cancer Annual Review of Medicine 50 1999 401 423 10.1146/annurev.med.50.1.401 10073286

Tsolekile N. Nelana S. Oluwafemi O.S. Porphyrin as diagnostic and therapeutic agent Molecules 24 14 2019 2669 10.3390/molecules24142669 PMC6680575 31340553

Tsvetkov P. Mendillo M.L. Zhao J. Carette J.E. Merrill P.H. Cikes D. … Lindquist S. Compromising the 19S proteasome complex protects cells from reduced flux through the proteasome ELife 4 2015 10.7554/eLife.08467 PMC4551903 26327695

Tsvetkov P. Sokol E. Jin D. Brune Z. Thiru P. Ghandi M. … Lindquist S. Suppression of 19S proteasome subunits marks emergence of an altered cell state in diverse cancers Proceedings of the National Academy of Sciences 114 2 2017 382 387 10.1073/pnas.1619067114 PMC5240730 28028240

de Tullio M.B. Castelletto V. Hamley I.W. Martino Adami P.V. Morelli L. Castaño E.M. Proteolytically inactive insulin-degrading enzyme inhibits amyloid formation yielding non-neurotoxic Aβ peptide aggregates PLoS One 8 4 2013 10.1371/journal.pone.0059113 PMC3623905 23593132

Tundo G.R. Sbardella D. Ciaccio C. Bianculli A. Orlandi A. Desimio M.G. … Marini S. Insulin-degrading enzyme (IDE): A novel heat shock-like protein The Journal of Biological Chemistry 288 4 2013 2281 2289 10.1074/jbc.M112.393108 23188819 PMC3554900

Tundo G.R. Sbardella D. Ciaccio C. Grasso G. Gioia M. Coletta A. … Coletta M. Multiple functions of insulin-degrading enzyme: A metabolic crosslight? Critical Reviews in Biochemistry and Molecular Biology 52 5 2017 554 582 10.1080/10409238.2017.1337707 28635330

Tundo G.R. Sbardella D. Coletta M. Insights into proteasome conformation dynamics and intersubunit communication Trends in Biochemical Sciences 43 11 2018 852 853 10.1016/j.tibs.2018.08.002 30145017

Tundo G.R. Sbardella D. Lacal P.M. Graziani G. Marini S. On the horizon: Targeting next-generation immune checkpoints for cancer treatment Chemotherapy 64 2 2019 62 80 10.1159/000500902 31387102

Tzotzos S. Doig A.J. Amyloidogenic sequences in native protein structures Protein Science: A Publication of the Protein Society 19 2 2010 327 348 10.1002/pro.314 20027621 PMC2865711

Uechi H. Hamazaki J. Murata S. Characterization of the testis-specific proteasome subunit α4s in mammals The Journal of Biological Chemistry 289 18 2014 12365 12374 10.1074/jbc.M114.558866 24668818 PMC4007433

Ugras S. Daniels M.J. Fazelinia H. Gould N.S. Yocum A.K. Luk K.C. … Ischiropoulos H. Induction of the immunoproteasome subunit Lmp7 links proteostasis and immunity in α-synuclein aggregation disorders EBioMedicine 31 2018 307 319 10.1016/j.ebiom.2018.05.007 29759483 PMC6014061

Um J.W. Im E. Lee H.J. Min B. Yoo L. Yoo J. … Chung K.C. Parkin directly modulates 26S proteasome activity The Journal of Neuroscience: The Official Journal of the Society for Neuroscience 30 35 2010 11805 11814 10.1523/JNEUROSCI.2862-09.2010 20810900 PMC6633428

Um J.W. Min D.S. Rhim H. Kim J. Paik S.R. Chung K.C. Parkin ubiquitinates and promotes the degradation of RanBP2 The Journal of Biological Chemistry 281 6 2006 3595 3603 10.1074/jbc.M504994200 16332688

Unno M. Mizushima T. Morimoto Y. Tomisugi Y. Tanaka K. Yasuoka N. Tsukihara T. The structure of the mammalian 20S proteasome at 2.75 A resolution Structure (London, England: 1993) 10 5 2002 609 618 10.1016/s0969-2126(02)00748-7 12015144

Unverdorben P. Beck F. Śledź P. Schweitzer A. Pfeifer G. Plitzko J.M. … Förster F. Deep classification of a large cryo-EM dataset defines the conformational landscape of the 26S proteasome Proceedings of the National Academy of Sciences 111 15 2014 5544 5549 10.1073/pnas.1403409111 PMC3992697 24706844

Ustrell V. Hoffman L. Pratt G. Rechsteiner M. PA200, a nuclear proteasome activator involved in DNA repair The EMBO Journal 21 13 2002 3516 3525 10.1093/emboj/cdf333 12093752 PMC126083

Ustrell V. Pratt G. Gorbea C. Rechsteiner M. Purification and assay of proteasome activator PA200 Methods in Enzymology 398 2005 321 329 10.1016/S0076-6879(05)98026-9 16275339

Uttamsingh V. Lu C. Miwa G. Gan L.-S. Relative contributions of the five major human cytochromes P450, 1A2, 2C9, 2C19, 2D6, and 3A4, to the hepatic metabolism of the proteasome inhibitor bortezomib Drug Metabolism and Disposition: The Biological Fate of Chemicals 33 11 2005 1723 1728 10.1124/dmd.105.005710 16103134

Vaden J.H. Watson J.A. Howard A.D. Chen P.-C. Wilson J.A. Wilson S.M. Distinct effects of ubiquitin overexpression on NMJ structure and motor performance in mice expressing catalytically inactive USP14 Frontiers in Molecular Neuroscience 8 2015 10.3389/fnmol.2015.00011 PMC4407586 25954152

Vahid S. Thaper D. Zoubeidi A. Chaperoning the cancer: The proteostatic functions of the heat shock proteins in cancer Recent Patents on Anti-Cancer Drug Discovery 12 1 2017 35 47 10.2174/1574892811666161102125252 27809750

Vallelian F. Deuel J.W. Opitz L. Schaer C.A. Puglia M. Lönn M. … Schaer D.J. Proteasome inhibition and oxidative reactions disrupt cellular homeostasis during heme stress Cell Death and Differentiation 22 4 2015 597 611 10.1038/cdd.2014.154 25301065 PMC4356336

Van Waes C. Nuclear factor-kappaB in development, prevention, and therapy of cancer Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 13 4 2007 1076 1082 10.1158/1078-0432.CCR-06-2221 17317814

VanderLinden R.T. Hemmis C.W. Yao T. Robinson H. Hill C.P. Structure and energetics of pairwise interactions between proteasome subunits RPN2, RPN13, and ubiquitin clarify a substrate recruitment mechanism The Journal of Biological Chemistry 292 23 2017 9493 9504 10.1074/jbc.M117.785287 28442575 PMC5465478

Varshavsky A. The early history of the ubiquitin field Protein Science: A Publication of the Protein Society 15 3 2006 647 654 10.1110/ps.052012306 16501229 PMC2249785

Velichutina I. Connerly P.L. Arendt C.S. Li X. Hochstrasser M. Plasticity in eucaryotic 20S proteasome ring assembly revealed by a subunit deletion in yeast The EMBO Journal 23 3 2004 500 510 10.1038/sj.emboj.7600059 14739934 PMC1271798

Venkatakrishnan K. Rader M. Ramanathan R.K. Ramalingam S. Chen E. Riordan W. … Chatta G. Effect of the CYP3A inhibitor ketoconazole on the pharmacokinetics and pharmacodynamics of bortezomib in patients with advanced solid tumors: a prospective, multicenter, open-label, randomized, two-way crossover drug-drug interaction study Clinical Therapeutics 31 Pt 2 2009 2444 2458 10.1016/j.clinthera.2009.11.012 20110052

Venner C.P. Lane T. Foard D. Rannigan L. Gibbs S.D.J. Pinney J.H. … Wechalekar A.D. Cyclophosphamide, bortezomib, and dexamethasone therapy in AL amyloidosis is associated with high clonal response rates and prolonged progression-free survival Blood 119 19 2012 4387 4390 10.1182/blood-2011-10-388462 22331187

Verbrugge S.E. Assaraf Y.G. Dijkmans B.A.C. Scheffer G.L. Al M. den Uyl D. … Jansen G. Inactivating PSMB5 mutations and P-glycoprotein (multidrug resistance-associated protein/ATP-binding cassette B1) mediate resistance to proteasome inhibitors: Ex vivo efficacy of (immuno)proteasome inhibitors in mononuclear blood cells from patients with rheumatoid arthritis The Journal of Pharmacology and Experimental Therapeutics 341 1 2012 174 182 10.1124/jpet.111.187542 22235146

Verma R. Aravind L. Oania R. McDonald W.H. Yates J.R. Koonin E.V. Deshaies R.J. Role of Rpn11 metalloprotease in deubiquitination and degradation by the 26S proteasome Science (New York, N.Y.) 298 5593 2002 611 615 10.1126/science.1075898 12183636

Vernazza S. Tirendi S. Scarfì S. Passalacqua M. Oddone F. Traverso C.E. … Saccà S.C. 2D- and 3D-cultures of human trabecular meshwork cells: A preliminary assessment of an in vitro model for glaucoma study PLoS One 14 9 2019 e0221942 10.1371/journal.pone.0221942 PMC6731014 31490976

VerPlank J.J.S. Goldberg A.L. Regulating protein breakdown through proteasome phosphorylation The Biochemical Journal 474 19 2017 3355 3371 10.1042/BCJ20160809 28947610 PMC5831125

VerPlank J.J.S. Goldberg A.L. Exploring the regulation of proteasome function by subunit phosphorylation Methods in Molecular Biology (Clifton, N.J.) 1844 2018 309 319 10.1007/978-1-4939-8706-1_20 PMC6382073 30242718

VerPlank J.J.S. Lokireddy S. Zhao J. Goldberg A.L. 26S proteasomes are rapidly activated by diverse hormones and physiological states that raise cAMP and cause Rpn6 phosphorylation Proceedings of the National Academy of Sciences 116 10 2019 4228 4237 10.1073/pnas.1809254116 PMC6410790 30782827

Vielberg M.-T. Bauer V.C. Groll M. On the trails of the proteasome fold: Structural and functional analysis of the ancestral β-subunit protein anbu Journal of Molecular Biology 430 5 2018 628 640 10.1016/j.jmb.2018.01.004 29355501

Vierstra R.D. Sullivan M.L. Hemin inhibits ubiquitin-dependent proteolysis in both a higher plant and yeast Biochemistry 27 9 1988 3290 3295 10.1021/bi00409a025 2839230

Vigouroux S. Briand M. Briand Y. Linkage between the proteasome pathway and neurodegenerative diseases and aging Molecular Neurobiology 30 2 2004 201 221 10.1385/MN:30:2:201 15475627

Vij R. Wang M. Kaufman J.L. Lonial S. Jakubowiak A.J. Stewart A.K. Kukreti V. Jagannath S. McDonagh K.T. Alsina M. Bahlis N.J. Reu F.J. Gabrail N.Y. Belch A. Matous J.V. Lee P. Rosen P. Sebag M. Vesole D.H. … Siegel D.S. An open-label, single-arm, phase 2 (PX-171-004) study of single-agent carfilzomib in bortezomib-naive patients with relapsed and/or refractory multiple myeloma Blood 119 24 2012 5661 5670 10.1182/blood-2012-03-414359 22555973 PMC4123327

Vitolo O.V. Sant'Angelo A. Costanzo V. Battaglia F. Arancio O. Shelanski M. Amyloid beta -peptide inhibition of the PKA/CREB pathway and long-term potentiation: Reversibility by drugs that enhance cAMP signaling Proceedings of the National Academy of Sciences of the United States of America 99 20 2002 13217 13221 10.1073/pnas.172504199 12244210 PMC130613

Vogelstein B. Lane D. Levine A.J. Surfing the p53 network Nature 408 6810 2000 307 310 10.1038/35042675 11099028

Vogelstein B. Papadopoulos N. Velculescu V.E. Zhou S. Diaz L.A. Kinzler K.W. Cancer genome landscapes Science (New York, N.Y.) 339 6127 2013 1546 1558 10.1126/science.1235122 PMC3749880 23539594

Vogl D.T. Martin T.G. Vij R. Hari P. Mikhael J.R. Siegel D. … Gasparetto C. Phase I/II study of the novel proteasome inhibitor delanzomib (CEP-18770) for relapsed and refractory multiple myeloma Leukemia & Lymphoma 58 8 2017 1872 1879 10.1080/10428194.2016.1263842 28140719

Voigt V. Wikstrom M.E. Kezic J.M. Schuster I.S. Fleming P. Makinen K. … Forrester J.V. Ocular antigen does not cause disease unless presented in the context of inflammation Scientific Reports 7 1 2017 14226 10.1038/s41598-017-14618-z 29079770 PMC5660195

Vose J.M. Mantle cell lymphoma: 2017 update on diagnosis, risk-stratification, and clinical management American Journal of Hematology 92 8 2017 806 813 10.1002/ajh.24797 28699667

Voutsadakis I.A. Proteasome expression and activity in cancer and cancer stem cells Tumour Biology: The Journal of the International Society for Oncodevelopmental Biology and Medicine 39 3 2017 1010428317692248 10.1177/1010428317692248 28345458

de Vrij F.M.S. Fischer D.F. van Leeuwen F.W. Hol E.M. Protein quality control in Alzheimer's disease by the ubiquitin proteasome system Progress in Neurobiology 74 5 2004 249 270 10.1016/j.pneurobio.2004.10.001 15582222

Wakabayashi K. Tanji K. Mori F. Takahashi H. The Lewy body in Parkinson's disease: Molecules implicated in the formation and degradation of alpha-synuclein aggregates Neuropathology: Official Journal of the Japanese Society of Neuropathology 27 5 2007 494 506 10.1111/j.1440-1789.2007.00803.x 18018486

Wakabayashi K. Tanji K. Odagiri S. Miki Y. Mori F. Takahashi H. The Lewy body in Parkinson's disease and related neurodegenerative disorders Molecular Neurobiology 47 2 2013 495 508 10.1007/s12035-012-8280-y 22622968

Walerych D. Lisek K. Del Sal G. Mutant p53: One, No One, and One Hundred Thousand Frontiers in Oncology 5 2015 289 10.3389/fonc.2015.00289 26734571 PMC4685664

Walerych D. Lisek K. Sommaggio R. Piazza S. Ciani Y. Dalla E. … Del Sal G. Proteasome machinery is instrumental in a common gain-of-function program of the p53 missense mutants in cancer Nature Cell Biology 18 8 2016 897 909 10.1038/ncb3380 27347849

Wallington‐Beddoe C.T. Sobieraj‐Teague M. Kuss B.J. Pitson S.M. Resistance to proteasome inhibitors and other targeted therapies in myeloma British Journal of Haematology 182 1 2018 11 28 10.1111/bjh.15210 29676460

Walsh C.P. Xu G.L. Cytosine methylation and DNA repair Current Topics in Microbiology and Immunology 301 2006 283 315 10.1007/3-540-31390-7_11 16570853

Wang X. Chemmama I.E. Yu C. Huszagh A. Xu Y. Viner R. … Huang L. The proteasome-interacting Ecm29 protein disassembles the 26S proteasome in response to oxidative stress The Journal of Biological Chemistry 292 39 2017 16310 16320 10.1074/jbc.M117.803619 28821611 PMC5625060

Wang X. D'Arcy P. Caulfield T.R. Paulus A. Chitta K. Mohanty C. … Linder S. Synthesis and evaluation of derivatives of the proteasome deubiquitinase inhibitor b-AP15 Chemical Biology & Drug Design 86 5 2015 1036 1048 10.1111/cbdd.12571 25854145 PMC4846425

Wang Z. Fang Y. Teague J. Wong H. Morisseau C. Hammock B.D. … Wang Z. In vitro metabolism of oprozomib, an oral proteasome inhibitor: Role of epoxide hydrolases and cytochrome P450s Drug Metabolism and Disposition 45 7 2017 712 720 10.1124/dmd.117.075226 28428366 PMC5452678

Wang Y. Jiang Y. Ding S. Li J. Song N. Ren Y. … Mei Z. Small molecule inhibitors reveal allosteric regulation of USP14 via steric blockade Cell Research 28 12 2018 1186 1194 10.1038/s41422-018-0091-x 30254335 PMC6274642

Wang Q. Li C. Zhang Q. Wang T. Li J. Guan W. … Li D. Interactions of SARS coronavirus nucleocapsid protein with the host cell proteasome subunit p42 Virology Journal 7 2010 99 10.1186/1743-422X-7-99 20478047 PMC2894783

Wang H. Li M. Zhang Z. Xue H. Chen X. Ji Y. Physiological function of myocilin and its role in the pathogenesis of glaucoma in the trabecular meshwork (review) International Journal of Molecular Medicine 43 2 2019 671 681 10.3892/ijmm.2018.3992 30483726 PMC6317685

Wang M. Martin T. Bensinger W. Alsina M. Siegel D.S. Kavalerchik E. … Niesvizky R. Phase 2 dose-expansion study (PX-171-006) of carfilzomib, lenalidomide, and low-dose dexamethasone in relapsed or progressive multiple myeloma Blood 122 18 2013 3122 3128 10.1182/blood-2013-07-511170 24014245 PMC3814729

Wang X. Mazurkiewicz M. Hillert E.-K. Olofsson M.H. Pierrou S. Hillertz P. … D'Arcy P. The proteasome deubiquitinase inhibitor VLX1570 shows selectivity for ubiquitin-specific protease-14 and induces apoptosis of multiple myeloma cells Scientific Reports 6 2016 26979 10.1038/srep26979 PMC4893612 27264969

Wang X. Stafford W. Mazurkiewicz M. Fryknäs M. Brjnic S. Zhang X. … Linder S. The 19S Deubiquitinase inhibitor b-AP15 is enriched in cells and elicits rapid commitment to cell death Molecular Pharmacology 85 6 2014 932 945 10.1124/mol.113.091322 24714215

Wang J. Xu X. Elliott M.H. Zhu M. Le Y.-Z. Müller cell-derived VEGF is essential for diabetes-induced retinal inflammation and vascular leakage Diabetes 59 9 2010 2297 2305 10.2337/db09-1420 20530741 PMC2927953

Wang Z. Yang J. Kirk C. Fang Y. Alsina M. Badros A. … Infante J.R. Clinical pharmacokinetics, metabolism, and drug-drug interaction of carfilzomib Drug Metabolism and Disposition: The Biological Fate of Chemicals 41 1 2013 230 237 10.1124/dmd.112.047662 23118326

Wang X. Yen J. Kaiser P. Huang L. Regulation of the 26S proteasome complex during oxidative stress Science Signaling 3 151 2010 ra88 10.1126/scisignal.2001232 21139140 PMC3140957

Waxman A.J. Clasen S. Hwang W.-T. Garfall A. Vogl D.T. Carver J. … Weiss B.M. Carfilzomib-associated cardiovascular adverse events: A systematic review and meta-analysis JAMA Oncology 4 3 2018 e174519 10.1001/jamaoncol.2017.4519 PMC5885859 29285538

Weaver B.A.A. Cleveland D.W. Does aneuploidy cause cancer? Current Opinion in Cell Biology 18 6 2006 658 667 10.1016/j.ceb.2006.10.002 17046232

Wehmer M. Rudack T. Beck F. Aufderheide A. Pfeifer G. Plitzko J.M. … Sakata E. Structural insights into the functional cycle of the ATPase module of the 26S proteasome Proceedings of the National Academy of Sciences 114 6 2017 1305 1310 10.1073/pnas.1621129114 PMC5307450 28115689

Wehmer M. Sakata E. Recent advances in the structural biology of the 26S proteasome The International Journal of Biochemistry & Cell Biology 79 2016 437 442 10.1016/j.biocel.2016.08.008 27498189

Wei J. Dong S. Bowser R.K. Khoo A. Zhang L. Jacko A.M. … Zhao J. Regulation of the ubiquitylation and deubiquitylation of CREB-binding protein modulates histone acetylation and lung inflammation Science Signaling 10 483 2017 10.1126/scisignal.aak9660 PMC5863726 28611184

Weigert O. Weidmann E. Mueck R. Bentz M. von Schilling C. Rohrberg R. … Dreyling M. A novel regimen combining high dose cytarabine and bortezomib has activity in multiply relapsed and refractory mantle cell lymphoma - long-term results of a multicenter observation study Leukemia & Lymphoma 50 5 2009 716 722 10.1080/10428190902856790 19347767

Weinreb R.N. Aung T. Medeiros F.A. The pathophysiology and treatment of glaucoma JAMA 311 18 2014 1901 1911 10.1001/jama.2014.3192 24825645 PMC4523637

Weisenburger D.D. Vose J.M. Greiner T.C. Lynch J.C. Chan W.C. Bierman P.J. … Armitage J.O. Mantle cell lymphoma. A clinicopathologic study of 68 cases from the Nebraska Lymphoma Study Group American Journal of Hematology 64 3 2000 190 196 10.1002/1096-8652(200007)64:3<190::AID-AJH9>3.0.CO;2-B 10861815

Welk V. Coux O. Kleene V. Abeza C. Trümbach D. Eickelberg O. Meiners S. Inhibition of proteasome activity induces formation of alternative proteasome complexes The Journal of Biological Chemistry 291 25 2016 13147 13159 10.1074/jbc.M116.717652 27129254 PMC4933230

Weller M. Le Rhun E. Preusser M. Tonn J.-C. Roth P. How we treat glioblastoma ESMO Open 4 Suppl. 2 2019 10.1136/esmoopen-2019-000520 PMC6586206 31297242

Wertz I.E. Murray J.M. Structurally-defined deubiquitinase inhibitors provide opportunities to investigate disease mechanisms Drug Discovery Today: Technologies 31 2019 109 123 10.1016/j.ddtec.2019.02.003 31200854

Wester R. van der Holt B. Asselbergs E. Zweegman S. Kersten M.J. Vellenga E. … Sonneveld P. Phase 2 study of carfilzomib, thalidomide, and low-dose dexamethasone as induction and consolidation in newly diagnosed, transplant eligible patients with multiple myeloma, the carthadex trial Haematologica. 2019 10.3324/haematol.2018.205476 PMC6821616 30948492

Wijdeven R.H. Pang B. Assaraf Y.G. Neefjes J. Old drugs, novel ways out: Drug resistance toward cytotoxic chemotherapeutics Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 28 2016 65 81 10.1016/j.drup.2016.07.001 27620955

Wilk S. Chen W.E. Synthetic peptide-based activators of the proteasome Molecular Biology Reports 24 1–2 1997 119 124 10.1023/a:1006851428691 9228292

Wilk S. Orlowski M. Cation-sensitive neutral endopeptidase: Isolation and specificity of the bovine pituitary enzyme Journal of Neurochemistry 35 5 1980 1172 1182 10.1111/j.1471-4159.1980.tb07873.x 6778972

Wilk S. Orlowski M. Evidence that pituitary cation-sensitive neutral endopeptidase is a multicatalytic protease complex Journal of Neurochemistry 40 3 1983 842 849 10.1111/j.1471-4159.1983.tb08056.x 6338156

Wilkinson K.D. The discovery of ubiquitin-dependent proteolysis Proceedings of the National Academy of Sciences of the United States of America 102 43 2005 15280 15282 10.1073/pnas.0504842102 16230621 PMC1266097

Williams B.R. Amon A. Aneuploidy –Cancer's fatal flaw? Cancer Research 69 13 2009 5289 5291 10.1158/0008-5472.CAN-09-0944 19549887 PMC2917070

van der Willik K.D. Schagen S.B. Ikram M.A. Cancer and dementia: Two sides of the same coin? European Journal of Clinical Investigation 48 11 2018 10.1111/eci.13019 PMC6220770 30112764

Wilson S.M. Bhattacharyya B. Rachel R.A. Coppola V. Tessarollo L. Householder D.B. … Jenkins N.A. Synaptic defects in ataxia mice result from a mutation in Usp14, encoding a ubiquitin-specific protease Nature Genetics 32 3 2002 420 425 10.1038/ng1006 12368914

Windheim M. Peggie M. Cohen P. Two different classes of E2 ubiquitin-conjugating enzymes are required for the mono-ubiquitination of proteins and elongation by polyubiquitin chains with a specific topology The Biochemical Journal 409 3 2008 723 729 10.1042/BJ20071338 18042044

Witkowska J. Giżyńska M. Grudnik P. Golik P. Karpowicz P. Giełdoń A. … Jankowska E. Crystal structure of a low molecular weight activator Blm-pep with yeast 20S proteasome – Insights into the enzyme activation mechanism Scientific Reports 7 1 2017 1 11 10.1038/s41598-017-05997-4 28733623 PMC5522460

Witt E. Zantopf D. Schmidt M. Kraft R. Kloetzel P.M. Krüger E. Characterisation of the newly identified human Ump1 homologue POMP and analysis of LMP7(beta 5i) incorporation into 20 S proteasomes Journal of Molecular Biology 301 1 2000 1 9 10.1006/jmbi.2000.3959 10926487

Wójcik C. Tanaka K. Paweletz N. Naab U. Wilk S. Proteasome activator (PA28) subunits, alpha, beta and gamma (Ki antigen) in NT2 neuronal precursor cells and HeLa S3 cells European Journal of Cell Biology 77 2 1998 151 160 10.1016/s0171-9335(98)80083-6 9840465

Wong H.H. Fung T.S. Fang S. Huang M. Le M.T. Liu D.X. Accessory proteins 8b and 8ab of severe acute respiratory syndrome coronavirus suppress the interferon signaling pathway by mediating ubiquitin-dependent rapid degradation of interferon regulatory factor 3 Virology 515 2018 165 175 10.1016/j.virol.2017.12.028 29294448 PMC7112132

Worden E.J. Dong K.C. Martin A. An AAA motor-driven mechanical switch in Rpn11 controls deubiquitination at the 26S proteasome Molecular Cell 67 5 2017 799 811 e8 10.1016/j.molcel.2017.07.023 28844860

Worden E.J. Padovani C. Martin A. Structure of the Rpn11-Rpn8 dimer reveals mechanisms of substrate deubiquitination during proteasomal degradation Nature Structural & Molecular Biology 21 3 2014 220 227 10.1038/nsmb.2771 24463465

Wu S. Hyrc K.L. Moulder K.L. Lin Y. Warmke T. Snider B.J. Cellular calcium deficiency plays a role in neuronal death caused by proteasome inhibitors Journal of Neurochemistry 109 5 2009 1225 1236 10.1111/j.1471-4159.2009.06037.x 19476541 PMC2690718

Wu N. Liu C. Bai C. Han Y.-P. Cho W.C.S. Li Q. Over-Expression of Deubiquitinating Enzyme USP14 in Lung Adenocarcinoma Promotes Proliferation through the Accumulation of β-Catenin International Journal of Molecular Sciences 14 6 2013 10749 10760 10.3390/ijms140610749 23702845 PMC3709700

Wu W. Sahara K. Hirayama S. Zhao X. Watanabe A. Hamazaki J. … Murata S. PAC1-PAC2 proteasome assembly chaperone retains the core α4-α7 assembly intermediates in the cytoplasm Genes to Cells: Devoted to Molecular & Cellular Mechanisms 23 10 2018 839 848 10.1111/gtc.12631 30133132

Wu Z.-H. Shi Y. When ubiquitin meets NF-κB: a trove for anti-cancer drug development Current Pharmaceutical Design 19 18 2013 3263 3275 10.2174/1381612811319180010 23151140 PMC3759360

Wu Y.-X. Yang J.-H. Saitsu H. Bortezomib-resistance is associated with increased levels of proteasome subunits and apoptosis-avoidance Oncotarget 7 47 2016 77622 77634 10.18632/oncotarget.12731 27769058 PMC5363609

Wunderlich K. Golubnitschaja O. Pache M. Eberle A.-N. Flammer J. Increased plasma levels of 20S proteasome alpha-subunit in glaucoma patients: An observational pilot study Molecular Vision 8 2002 431 435 12447166

Xia X. Huang C. Liao Y. Liu Y. He J. Guo Z. … Huang H. Inhibition of USP14 enhances the sensitivity of breast cancer to enzalutamide Journal of Experimental & Clinical Cancer Research 38 1 2019 220 10.1186/s13046-019-1227-7 31126320 PMC6534920

Xing Y. Jameson S.C. Hogquist K.A. Thymoproteasome subunit-β5T generates peptide-MHC complexes specialized for positive selection Proceedings of the National Academy of Sciences of the United States of America 110 17 2013 6979 6984 10.1073/pnas.1222244110 23569244 PMC3637736

Xu D. Shan B. Lee B.-H. Zhu K. Zhang T. Sun H. … Yuan J. Phosphorylation and activation of ubiquitin-specific protease-14 by Akt regulates the ubiquitin-proteasome system ELife 4 2015 e10510 10.7554/eLife.10510 PMC4733041 26523394

Xue Y. Barker N. Hoon S. He P. Thakur T. Abdeen S.R. … Lane D.P. Bortezomib Stabilizes and Activates p53 in Proliferative Compartments of Both Normal and Tumor Tissues In Vivo Cancer Research 79 14 2019 3595 3607 10.1158/0008-5472.CAN-18-3744 31138526

Yam G.H.-F. Gaplovska-Kysela K. Zuber C. Roth J. Aggregated myocilin induces russell bodies and causes apoptosis The American Journal of Pathology 170 1 2007 100 109 10.2353/ajpath.2007.060806 17200186 PMC1762699

Yamada S. Yamada J. Sato K. Tokumoto T. Yasutomi M. Ishikawa K. Irreversible potent activation and reversible inhibition of trypsin-like activity of 20S proteasome purified from Xenopus oocytes by fatty acid Zoological Science 15 1 1998 43 49 10.2108/zsj.15.43 9615617

Yamano T. Mizukami S. Murata S. Chiba T. Tanaka K. Udono H. Hsp90-mediated assembly of the 26 S proteasome is involved in major histocompatibility complex class I antigen processing The Journal of Biological Chemistry 283 42 2008 28060 28065 10.1074/jbc.M803077200 18703510 PMC2661382

Yang Y. Gehrke S. Haque M.E. Imai Y. Kosek J. Yang L. … Lu B. Inactivation of Drosophila DJ-1 leads to impairments of oxidative stress response and phosphatidylinositol 3-kinase/Akt signaling Proceedings of the National Academy of Sciences of the United States of America 102 38 2005 13670 13675 10.1073/pnas.0504610102 16155123 PMC1224636

Yang D. Gong N. Zhang L. Lu Y. Isostructurality among 5 solvatomorphs of betulin: X-ray structure and characterization Journal of Pharmaceutical Science 105 6 2016 1867 1873 10.1016/j.xphs.2016.03.015 27129904

Yang H. Landis-Piwowar K.R. Chen D. Milacic V. Dou Q.P. Natural compounds with proteasome inhibitory activity for cancer prevention and treatment Current Protein & Peptide Science 9 3 2008 227 239 10.2174/138920308784533998 18537678 PMC3303152

Yang J. Wang Z. Fang Y. Jiang J. Zhao F. Wong H. … Kirk C.J. Pharmacokinetics, pharmacodynamics, metabolism, distribution, and excretion of carfilzomib in rats Drug Metabolism and Disposition: The Biological Fate of Chemicals 39 10 2011 1873 1882 10.1124/dmd.111.039164 21752943

Yang F. Yang Y. Mao C. Liu L. Zheng H. Hu L. Liu C. Crosstalk between the proteasome system and autophagy in the clearance of α-synuclein Acta Pharmacologica Sinica 34 5 2013 674 680 10.1038/aps.2013.29 23603979 PMC4002879

Yao T. Cohen R.E. A cryptic protease couples deubiquitination and degradation by the proteasome Nature 419 6905 2002 403 407 10.1038/nature01071 12353037

Yao J. Qiu Y. Frontera E. Jia L. Khan N.W. Klionsky D.J. … Zacks D.N. Inhibiting autophagy reduces retinal degeneration caused by protein misfolding Autophagy 14 7 2018 1226 1238 10.1080/15548627.2018.1463121 29940785 PMC6103695

Yao T. Song L. Xu W. DeMartino G.N. Florens L. Swanson S.K. … Cohen R.E. Proteasome recruitment and activation of the Uch37 deubiquitinating enzyme by Adrm1 Nature Cell Biology 8 9 2006 994 1002 10.1038/ncb1460 16906146

Ye Y. Klenerman D. Finley D. N-terminal ubiquitination of amyloidogenic proteins triggers removal of their oligomers by the proteasome holoenzyme Journal of Molecular Biology 432 2 2020 585 596 10.1016/j.jmb.2019.08.021 31518613 PMC6990400

Yeo I.J. Lee M.J. Baek A. Miller Z. Bhattarai D. Baek Y.M. … Kim K.B. A dual inhibitor of the proteasome catalytic subunits LMP2 and Y attenuates disease progression in mouse models of Alzheimer's disease Scientific Reports 9 1 2019 10.1038/s41598-019-54846-z Scopus PMC6895163 31804556

Yerlikaya A. Okur E. Ulukaya E. The p53-independent induction of apoptosis in breast cancer cells in response to proteasome inhibitor bortezomib Tumour Biology: The Journal of the International Society for Oncodevelopmental Biology and Medicine 33 5 2012 1385 1392 10.1007/s13277-012-0386-3 22477712

Ying H. Turturro S. Nguyen T. Shen X. Zelkha R. Johnson E.C. … Yue B.Y.J.T. Induction of autophagy in rats upon overexpression of wild-type and mutant optineurin gene BMC Cell Biology 16 2015 14 10.1186/s12860-015-0060-x 25943884 PMC4429416

Yoshiba S. Iwase M. Kurihara S. Uchida M. Kurihara Y. Watanabe H. Shintani S. Proteasome inhibitor sensitizes oral squamous cell carcinoma cells to TRAIL-mediated apoptosis Oncology Reports 25 3 2011 645 652 10.3892/or.2010.1127 21206980

Yu D. Carroll M. Thomas-Tikhonenko A. p53 status dictates responses of B lymphomas to monotherapy with proteasome inhibitors Blood 109 11 2007 4936 4943 10.1182/blood-2006-10-050294 17284530 PMC1885530

Yu G.-Y. Lai M.M.C. The ubiquitin-proteasome system facilitates the transfer of murine coronavirus from endosome to cytoplasm during virus entry Journal of Virology 79 1 2005 644 648 10.1128/JVI.79.1.644-648.2005 15596861 PMC538694

Yu Z. Livnat-Levanon N. Kleifeld O. Mansour W. Nakasone M.A. Castaneda C.A. … Glickman M.H. Base-CP proteasome can serve as a platform for stepwise lid formation Bioscience Reports 35 3 2015 10.1042/BSR20140173 PMC4438304 26182356

Yuan T. Yan F. Ying M. Cao J. He Q. Zhu H. Yang B. Inhibition of Ubiquitin-Specific Proteases as a Novel Anticancer Therapeutic Strategy Frontiers in Pharmacology 9 2018 10.3389/fphar.2018.01080 PMC6171565 30319415

Zabel C. Nguyen H.P. Hin S.C. Hartl D. Mao L. Klose J. Proteasome and oxidative phoshorylation changes may explain why aging is a risk factor for neurodegenerative disorders Journal of Proteomics 73 11 2010 2230 2238 10.1016/j.jprot.2010.08.008 20813214

Zachara N.E. Hart G.W. O-GlcNAc a sensor of cellular state: The role of nucleocytoplasmic glycosylation in modulating cellular function in response to nutrition and stress Biochimica et Biophysica Acta (BBA) - General Subjects 1673 1 2004 13 28 10.1016/j.bbagen.2004.03.016 15238246

Zang Y. Kirk C.J. Johnson D.E. Carfilzomib and oprozomib synergize with histone deacetylase inhibitors in head and neck squamous cell carcinoma models of acquired resistance to proteasome inhibitors Cancer Biology & Therapy 15 9 2014 1142 1152 10.4161/cbt.29452 24915039 PMC4128857

Zang Y. Thomas S.M. Chan E.T. Kirk C.J. Freilino M.L. DeLancey H.M. … Johnson D.E. Carfilzomib and ONX 0912 inhibit cell survival and tumor growth of head and neck cancer and their activities are enhanced by suppression of Mcl-1 or autophagy Clinical Cancer Research: An Official Journal of the American Association for Cancer Research 18 20 2012 5639 5649 10.1158/1078-0432.CCR-12-1213 22929803 PMC3473099

Zeng L. Tallaksen-Greene S.J. Wang B. Albin R.L. Paulson H.L. The de-ubiquitinating enzyme ataxin-3 does not modulate disease progression in a knock-in mouse model of Huntington disease Journal of Huntington's Disease 2 2 2013 201 215 10.3233/JHD-130058 PMC3966139 24683430

Zhang Z. Bi C. Buac D. Fan Y. Zhang X. Zuo J. … Dou Q.P. Organic cadmium complexes as proteasome inhibitors and apoptosis inducers in human breast cancer cells Journal of Inorganic Biochemistry 123 2013 1 10 10.1016/j.jinorgbio.2013.02.004 23499788 PMC3648619

Zhang Z. Bi C. Fan Y. Wang H. Bao Y. Cefepime, a fourth-generation cephalosporin, in complex with manganese, inhibits proteasome activity and induces the apoptosis of human breast cancer cells International Journal of Molecular Medicine 36 4 2015 1143 1150 10.3892/ijmm.2015.2297 26239216

Zhang N. Fan Y. Huang G. Buac D. Bi C. Ma Y. … Dou Q.P. L-tryptophan Schiff base cadmium(II) complexes as a new class of proteasome inhibitors and apoptosis inducers in human breast cancer cells Inorganica Chimica Acta 466 2017 478 485 10.1016/j.ica.2017.07.006

Zhang X. Frezza M. Milacic V. Ronconi L. Fan Y. Bi C. … Dou Q.P. Inhibition of tumor proteasome activity by gold–dithiocarbamato complexes via both redox-dependent and -independent processes Journal of Cellular Biochemistry 109 1 2010 162 172 10.1002/jcb.22394 19911377 PMC3767936

Zhang F. Hu Y. Huang P. Toleman C.A. Paterson A.J. Kudlow J.E. Proteasome function is regulated by cyclic AMP-dependent protein kinase through phosphorylation of Rpt6 The Journal of Biological Chemistry 282 31 2007 22460 22471 10.1074/jbc.M702439200 17565987

Zhang F. Hu M. Tian G. Zhang P. Finley D. Jeffrey P.D. Shi Y. Structural insights into the regulatory particle of the proteasome from Methanocaldococcus jannaschii Molecular Cell 34 4 2009 473 484 10.1016/j.molcel.2009.04.021 19481527 PMC3268689

Zhang B. Li M. Huang P. Guan X. Zhu Y. Overexpression of ubiquitin specific peptidase 14 predicts unfavorable prognosis in esophageal squamous cell carcinoma Thoracic Cancer 8 4 2017 344 349 10.1111/1759-7714.12453 28509417 PMC5494452

Zhang L. Lin D. Sun X. Curth U. Drosten C. Sauerhering L. … Hilgenfeld R. Crystal structure of SARS-CoV-2 main protease provides a basis for design of improved α-ketoamide inhibitors Science. 2020 10.1126/science.abb3405 PMC7164518 32198291

Zhang P. Sadler P.J. Redox-active metal complexes for anticancer therapy European Journal of Inorganic Chemistry 2017 12 2017 1541 1548 10.1002/ejic.201600908

Zhang X. Schulz R. Edmunds S. Krüger E. Markert E. Gaedcke J. … Dobbelstein M. MicroRNA-101 suppresses tumor cell proliferation by acting as an endogenous proteasome inhibitor via targeting the proteasome assembly factor POMP Molecular Cell 59 2 2015 243 257 10.1016/j.molcel.2015.05.036 26145175

Zhang F. Su K. Yang X. Bowe D.B. Paterson A.J. Kudlow J.E. O-GlcNAc modification is an endogenous inhibitor of the proteasome Cell 115 6 2003 715 725 10.1016/S0092-8674(03)00974-7 14675536

Zhang L. Wei P.-F. Song Y.-H. Dong L. Wu Y.-D. Hao Z.-Y. … Wen L.-P. MnFe2O4 nanoparticles accelerate the clearance of mutant huntingtin selectively through ubiquitin-proteasome system Biomaterials 216 2019 119248 10.1016/j.biomaterials.2019.119248 31226569

Zhang Y. Xiao M. Zhang S. Xia P. Cao W. Jiang W. Chen H. Ding X. Zhao H. Zhang H. Wang C. Zhao J. Sun X. Tian R. Wu W. Wu D. Ma J. Chen Y. Zhang D. … Zhang S. Coagulopathy and Antiphospholipid antibodies in patients with SARS-Cov-2 New England Journal of Medicine 2020 10.1056/NEJMc2007575 PMC7161262 32268022

Zhao C. Chen X. Zang D. Lan X. Liao S. Yang C. … Liu J. A novel nickel complex works as a proteasomal deubiquitinase inhibitor for cancer therapy Oncogene 35 45 2016 5916 5927 10.1038/onc.2016.114 27086925 PMC5497060

Zhao Y. Foster N.R. Meyers J.P. Thomas S.P. Northfelt D.W. Rowland K.M. … Adjei A.A. A phase I/II study of bortezomib in combination with paclitaxel, carboplatin, and concurrent thoracic radiation therapy for non-small-cell lung cancer: North Central Cancer Treatment Group (NCCTG)-N0321 Journal of Thoracic Oncology: Official Publication of the International Association for the Study of Lung Cancer 10 1 2015 172 180 10.1097/JTO.0000000000000383 PMC4320011 25247339

Zhao J. Zhai B. Gygi S.P. Goldberg A.L. mTOR inhibition activates overall protein degradation by the ubiquitin proteasome system as well as by autophagy Proceedings of the National Academy of Sciences of the United States of America 112 52 2015 15790 15797 10.1073/pnas.1521919112 26669439 PMC4703015

Zheng S. Clabough E.B.D. Sarkar S. Futter M. Rubinsztein D.C. Zeitlin S.O. Deletion of the huntingtin polyglutamine stretch enhances neuronal autophagy and longevity in mice PLoS Genetics 6 2 2010 e1000838 10.1371/journal.pgen.1000838 PMC2816686 20140187

Zheng Z. Liu T. Zheng J. Hu J. Clarifying the molecular mechanism associated with carfilzomib resistance in human multiple myeloma using microarray gene expression profile and genetic interaction network Oncotargets and Therapy 10 2017 1327 1334 10.2147/OTT.S130742 28280367 PMC5338971

Zhitomirsky B. Assaraf Y.G. Lysosomes as mediators of drug resistance in cancer Drug Resistance Updates: Reviews and Commentaries in Antimicrobial and Anticancer Chemotherapy 24 2016 23 33 10.1016/j.drup.2015.11.004 26830313

Zhou H.-J. Aujay M.A. Bennett M.K. Dajee M. Demo S.D. Fang Y. Ho M.N. Jiang J. Kirk C.J. Laidig G.J. Lewis E.R. Lu Y. Muchamuel T. Parlati F. Ring E. Shenk K.D. Shields J. Shwonek P.J. Stanton T. … Yang J. Design and synthesis of an orally bioavailable and selective peptide Epoxyketone proteasome inhibitor (PR-047) Journal of Medicinal Chemistry 52 9 2009 3028 3038 10.1021/jm801329v 19348473

Zhou Y. Grinchuk O. Tomarev S.I. Transgenic mice expressing the Tyr437His mutant of human myocilin protein develop glaucoma Investigative Ophthalmology & Visual Science 49 5 2008 1932 1939 10.1167/iovs.07-1339 18436825 PMC2809368

Zhou B. Zuo Y. Li B. Wang H. Liu H. Wang X. … Bu X. Deubiquitinase Inhibition of 19S Regulatory Particles by 4-Arylidene Curcumin Analog AC17 Causes NF-κB Inhibition and p53 Reactivation in Human Lung Cancer Cells Molecular Cancer Therapeutics 12 8 2013 1381 1392 10.1158/1535-7163.MCT-12-1057 23696216

Zhu W. Liu J. Nie J. Sheng W. Cao H. Shen W. … Cao J. MG132 enhances the radiosensitivity of lung cancer cells in vitro and in vivo Oncology Reports 34 4 2015 2083 2089 10.3892/or.2015.4169 26238156

Zhu Z. Ramos J. Kampa K. Adimoolam S. Sirisawad M. Yu Z. … Lopez C.D. Control of ASPP2/53BP2L Protein Levels by Proteasomal Degradation Modulates p53 Apoptotic Function Journal of Biological Chemistry 280 41 2005 34473 34480 10.1074/jbc.M503736200 16091363

Zhu H. Wang T. Xin Z. Zhan Y. Gu G. Li X. … Liu C. An oral second-generation proteasome inhibitor oprozomib significantly inhibits lung cancer in a p53 independent manner in vitro Acta Biochimica et Biophysica Sinica 51 10 2019 1034 1040 10.1093/abbs/gmz093 31518420

Zhu Y. Wang W.L. Yu D. Ouyang Q. Lu Y. Mao Y. Nucleotide-Driven Triple-State Remodeling of the AAA-ATPase Channel in the Activated Human 26S Proteasome BioRxiv 2017 132613 10.1101/132613 PMC5893597 29636472

Zhu Y. Zhang C. Gu C. Li Q. Wu N. Function of Deubiquitinating Enzyme USP14 as Oncogene in Different Types of Cancer Cellular Physiology and Biochemistry 38 3 2016 993 1002 10.1159/000443051 26938858

Zhu Y. Zhang Y. Sui Z. Zhang Y. Liu M. Tang H. USP14 de-ubiquitinates vimentin and miR-320a modulates USP14 and vimentin to contribute to malignancy in gastric cancer cells Oncotarget 8 30 2016 48725 48736 10.18632/oncotarget.10706 PMC5564720 27448976

Zieba B.A. Henry L. Lacroix M. Jemaà M. Lavabre-Bertrand T. Meunier L. … Stoebner P.-E. The proteasome maturation protein POMP increases proteasome assembly and activity in psoriatic lesional skin Journal of Dermatological Science 88 1 2017 10 19 10.1016/j.jdermsci.2017.04.009 28728908

Zondler L. Kostka M. Garidel P. Heinzelmann U. Hengerer B. Mayer B. … Danzer K.M. Proteasome impairment by α-synuclein PLoS One 12 9 2017 e0184040 10.1371/journal.pone.0184040 PMC5612461 28945746

Zwickl P. Kleinz J. Baumeister W. Critical elements in proteasome assembly Nature Structural Biology 1 11 1994 765 770 10.1038/nsb1194-765 7634086

Acknowledgment

The authors would like to thank Prof. G. Manni for his fruitful discussion.

This work was supported by the Italian Ministery of University and Research (PRIN 2017SNRXH3), the 10.13039/501100005010 Italian Association for Cancer Research (AIRC) under IG 2017 - ID. 20353 project-PI_Grazia Graziani, the Italian Ministry of Health RC18-2638151 to P.M.L. Special thanks are due to Omikron Italia SrL for a liberal donation.