Myeloma Bone Disease: The Osteoblast in the Spotlight.

1. Introduction

Multiple myeloma (MM) is the second most common haematological malignancy and accounts for approximately 1% of all cancer diagnoses worldwide [ 1 , 2 ]. This B-cell malignancy is characterised by the clonal expansion of malignant plasma cells (MPCs) (>10%) within the bone marrow (BM) (or confirmation plasmacytoma) and is normally associated with a measurable monoclonal immunoglobulin secreted from the MPCs [ 3 ]. MM causes hypercalcaemia, renal failure, anaemia, and bone disease (hence its diagnostic acronym, “CRAB criteria”). Predictive modelling suggests that MM incidence will continue to rise, in part due to the effects of an aging population [ 4 ], with a predicted increase of 11% from 2014 to 2035 [ 5 ]. Of all patients diagnosed with MM, 80–90% will develop the associated bone disease during their cancer journey [ 6 , 7 ]. This leads to losses in trabecular bone, reduced bone mineral density (BMD), and phenotypic osteolytic lesions. The risk of fracture for these patients increases nine-fold [ 8 ], and those who do suffer a pathological fracture have an increased mortality of 20% compared to those without a fracture [ 9 ]. The clinical impact of myeloma bone disease (MBD) is significant, leading to bone pain, poor mobility, and, subsequently, decreased quality of life.

2. Pathophysiology of Myeloma Bone Disease

Bone health is maintained throughout one’s life by the process of bone remodelling, a continuous process in which osteoclasts (OCs) remove old bone, and osteoblasts (OBs) lay down new bone. Bone remodelling is orchestrated by osteocytes (OCYs) in biomechanical response to loading forces or microdamage to the bone. Many signalling pathways are in place to maintain the bone remodelling equilibrium. The most important identified pathway is that of the receptor activator of nuclear factor kappa B (RANK)/RANK ligand (RANKL)/Osteoprotegerin (OPG) signalling ( Figure 1 ). When RANKL (produced by both OBs and OCYs) binds to RANK on the surface of immature OCs, it leads to OC differentiation to mitigate mature OCs to resorb bone. This is moderated by the release of OPG, also produced by OBs, which acts as a decoy receptor to RANKL to reduce OC formation/activity. In the case of MM, the presence of MPCs and their interaction with bone marrow stromal cells (BMSCs) causes bone remodelling to become dysregulated. In brief, MBD is driven by (a) an upregulation of OC activity, (b) an inhibition of OB activity, and (c) positive feedback, which accelerates tumour growth ( Figure 1 ). This results in net bone loss, profound osteolysis, losses in trabecular structure, and compromised skeletal strength and function [ 10 ]. The change from healthy bone to MBD is driven by MPCs in the bone marrow microenvironment (BMME) and subsequent alterations in cytokines, extracellular vesicles, and cellular signalling/interactions. Factors driving osteolysis are often categorised as either OC-activating factors (OAFs) or OB-inhibiting factors (OIFs; Table 1 ).

OAFs (e.g., RANKL, macrophage inflammatory protein-1 α (MIP-1α), and Activin A) are directly expressed by MPCs or indirectly increased by MPC signalling to other cells in the BMME. The upregulation of bone resorption results in a release of growth factors from the resorbed bone matrix (e.g., transforming growth factorβ (TGF-β) and Interleukin-6 (IL-6)), which themselves act as OAFs, OIFs, or can directly promote further tumour cell proliferation and survival. For this reason, osteolytic bone disease is often described as a “vicious cycle” because the presence of tumour cells promotes osteolysis, and this further promotes tumour cell proliferation and survival. A number of adhesion molecules responsible for homing of MPCs to the bone matrix have been shown to directly support osteolysis (e.g., syndecan-1 [ 37 ] or vascular cell adhesion molecule-1 (VCAM1)) [ 38 ]. One such example is Notch, which is released when MPCs adhere to bone, and subsequent interactions with Jagged result in increased RANKL, driving OC differentiation [ 39 , 40 ]. MPCs also drive the release of OIFs (e.g., TGFβ, Dickopf-1 (DKK-1) and soluble Frizzled transmembrane receptors (sFRP2)), resulting in halted bone formation and therefore impaired bone repair. This is further perpetuated by sclerostin, another OIF, which is produced from OCYs and released at higher levels in the presence of MPCs. Ultimately, the catatonic state of MBD leads to net bone loss and reduced bone integrity and strength.

Current management strategies for MBD include: (1) pain relief, (2) interventional radiology (e.g., vertebroplasty), (3) orthopaedic interventions, and (4) bone targeted pharmacological treatments. Effective analgesia can be a challenge to optimise due to underlying renal impairment and susceptibility to bowel disturbances. Improving underlying bone damage, in partnership with pain management, is a preferable approach. Radiological procedures, such as vertebroplasty, can be helpful in the treatment of vertebral compression fractures. Orthopaedic interventions can either be prophylactic (for severe and/or unstable bone disease) or for fracture repair. The two approved pharmacological options for the treatment of MBD, bisphosphonates or denosumab (a monoclonal antibody inhibiting RANKL) [ 41 ], are both anti-resorptives and do little to drive the repair of existing bone lesions ( Table 2 ). Improving our understanding of the impact of MPCs on OB physiology will allow us to explore novel targeted therapies to promote OB activity and the repair of cancer-induced bone damage. Here, we review the role of OBs in MBD and how this knowledge may drive future preclinical and clinical research, and, ultimately, disease management.

3. Osteoblast Dysfunction

As the only bone forming cell, OBs have a crucial role to play and account for approximately 5% of all bone cells in the BMME. In a physiological state, OBs would respond appropriately to OCY signalling, ensuring adequate bone formation in response to external stimuli. Having completed their physiological role, OBs either undergo apoptosis, differentiate into OCYs within the bone matrix, or differentiate into bone lining cells. OBs are mesenchymal stem cells (MSC) derived and committed to the OB lineage in the presence of transcription factors such as Runt-related transcription factor 2 (Runx2)/Cbfa1, β-catenin, and Osterix. MPCs both directly or indirectly oppose OB differentiation, function, and survival ( Figure 2 ). In the presence of MPCs, OB precursors have depleted Runx2 expression [ 20 ] and overall reduced Runx2-positive OB numbers [ 23 ], leading to osteogenic suppression and reduced bone formation. Xu et al. have produced work suggesting that the presence of these Runx2 deficient immature OBs observed in MM can both attract and promote MPC progression within the BMME [ 42 ]. MPCs can also express Runx2, and when this has been observed clinically, it correlates to more aggressive disease phenotypes [ 23 , 43 ]. Trotter et al. demonstrated that Runx2 expressing MPCs develop some bone-cell phenotypes, which may allow them to reside in the BMME more effectively [ 43 ], thus perpetuating their impact on the bone. MM derived exosomes also exaggerate osteolysis, driving the release of IL-6 (to promote tumour growth) and inhibiting OB differentiation via reduced Runx2, Osterix, and Osteocalcin [ 44 ].

Adipocytes are also MSC derived, and work by Ruan et al. suggests that heparanase from MPCs may influence OB differentiation by high jacking OB lineage in favour of adipocytes, and that the mechanisms involve enhanced Peroxisome proliferator-activated receptor γ (PPAR-γ) expression, decreased Runx2, and increased DKK1 secretion [ 20 ]. This is supported by Liu et al., who show that MPC–MSC interactions (via integrin α4 and VCAM1) activate protein kinase Cβ1 to stabilise PPAR-γ2, which drives adipogenesis [ 45 ]. MPCs may also cause epigenetic changes in OBs, resulting in long-term dysregulation in bone remodelling [ 46 ]. One such pathway appears to be via growth factor independence 1 (Gfi1), a transcription repressor, which appears to be expressed by OB precursors in the presence of MPCs. Gfi1 binds to Runx2, causing a reduction in osteoblastic development. The inhibition of cell differentiation persists despite removal of MPCs from the BMME, explaining why persistent and new osteolytic disease is observed clinically for some patients [ 47 ]. Gfi1-induced OB inhibition has been shown in vitro to be reversible with anti-TNFα or anti-IL-7 treatments [ 17 ]. OBs also appear to be particularly susceptible to MM-induced apoptosis, mediated by Fas/Fas Ligand, TNF-α, and tumour necrosis factor-related apoptosis-inducing ligand (TRAIL), which is thought to be, in part, due to functional exhaustion in response to inflammatory cytokines [ 48 ]. Depleted OB numbers in MM (whether due to hindered osteogenesis, dysregulated function, or induced apoptosis) result in reduced total OPG, further exaggerating a bias towards bone loss via unopposed RANKL-driven osteoclastogenesis [ 49 , 50 , 51 , 52 ]. Here, we focus our review on some of the key factors inhibiting osteogenesis ( Table 1 ).

4. Osteoblast Inhibiting Factors

4.1. Wnt/B-Catenin; DKK1, Frizzled Transmembrane Receptors and Sclerostin

Wnt/β-catenin signalling is the key regulator of OB differentiation and function, with multiple identified pathways (broadly categorised as canonical or non-canonical). Canonical Wnt signalling is β-catenin dependent, and when this pathway is activated, there is an expression of OB transcription factors. This canonical Wnt pathway is often inhibited in MM, leading to downregulated osteoblastogenesis ( Figure 3 ) [ 53 , 54 ]. Non-canonical signalling occurs via the Frizzled (Fzd) receptor (in the absence of a co-receptor) and is β-catenin independent. Two key non-canonical cascades are the calcium-dependent pathway and the planar cell polarity pathway. Although non-canonical cascades are less characterised in MBD, Bolzoni et al. have recognised the importance of the non-canonical Wnt5a/ROR2 pathway, and that MPCs have the ability to inhibit ROR2 expression and downstream Wnt signalling. Additionally, the reactivation of this pathway supports osteogenic potential [ 55 ]. Many of the acknowledged OIFs identified in MM are thought to antagonise Wnt signalling ( Table 1 ) [ 56 , 57 ]. Five key groups of secreted Wnt antagonist proteins are reported, including sFRPs, Dkk-1, Wnt inhibitory factor-1 (Wif-1), Wise, and Cerberus. Whereas sFRPs, Wif-1, and Cerberus are believed to bind to Wnt (thus preventing agonist-receptor interaction), Dkk-1 and Wise directly bind to low-density lipoprotein receptor related proteins (LRP5/6) to antagonise downstream signalling [ 56 , 58 , 59 , 60 ].

Dkk-1 antagonises Wnt/β-catenin via the binding of LRP-6 (co-receptors to Wnt) [ 61 ] and Kremen transmembrane proteins [ 62 ], inhibiting osteoblastogenesis by prevention of OB precursor differentiation into mature OBs. In 2007, MacDonald et al. demonstrated, using a hypomorphic DKK-1 mouse model, that lowering the expression of DKK-1 significantly altered the skeletal phenotype, resulting in thicker trabecular and cortical bone [ 63 ]. Dkk-1 is expressed from isolated human MM cells. Higher levels are seen in the BM and peripheral serum of patients with MM, with particularly elevated levels in MBD sufferers [ 14 , 15 ]. However, not all cases of advanced MBD are associated with the same upregulation of DKK-1 [ 15 ]. The expression of DKK-1 from MM cells has been shown to be particularly raised in some, but not all, subgroups of patients with specific MPC genetic profiles, indicating a link between DKK-1 and some genetic abnormalities [ 64 ]. In vitro exposure of OB cell lines to either recombinant Dkk-1 or co-cultures of MPCs (known to secrete Dkk-1) demonstrated altered OPG/RANKL secretion ratios favouring osteoclastogenesis, as well as the inhibition of Wnt [ 65 ]. Yaccoby et al. used a SCID-rab mouse model, implanting patient myeloma cells into bone chips treated with anti-DKK-1. Their findings showed an increase in OB numbers, a decrease in OC numbers, and an increase in BMD [ 66 ]. These preclinical studies suggest not only a benefit for patients with MBD, but also a reduction in MM cell growth [ 66 , 67 ]. An anti-Dkk-1 neutralising antibody, BHQ880, also yielded promising results preclinically and in early phase clinical trials. The 5T2MM murine myeloma model was treated with BHQ880, which prevented the development of osteolytic lesions [ 68 ]. Similar results were also seen in the INA-6 SCID-Hu murine myeloma model with increased OB numbers and trabecular bone numbers [ 69 ]. A phase Ib clinical study to assess BHQ880 in MBD resulted in increased BMD [ 70 ], but this study treated MBD concomitantly with zoledronic acid and anti-myeloma therapies. Assessment of BHQ880 as monotherapy in high-risk smouldering myeloma in a phase II single-arm study presented preliminary findings of bone anabolic activity radiologically [ 71 ]. Interestingly, some in vitro studies of anti-DKK-1 treatments suggested an antitumour effect, possibly mediated by a reduction in the MM growth factor IL-6 [ 66 ].

sFRPs are glycoproteins, of which sFRP-2 and sFRP-3 are proposed to have a role in antagonising Wnt signalling in MBD (although sFRP-1 and sFRP-4 may also be involved) [ 58 ]. sFRPs act as decoy receptors to Wnt to prevent binding to Fzd-LRP5/6, with sFRP2 previously shown to be expressed by MPCs [ 33 ]. In vitro studies demonstrated inhibition of BMP-2-induced OB differentiation when cells were cultured in media from a variety of different myeloma cell lines known to express sFRP-2. When sFRP-2 was immunosuppressed, an increase in OB matrix mineralisation was also seen, suggesting sFRP-2 as a potential target for MBD treatment [ 33 ], viz., that it is possible to block and reverse its effects. Periostin, a cell-adhesion protein, also inhibits Wnt pathways, with elevated levels in MM associated with osteolysis [ 73 ].

Sclerostin is produced by OCYs and inhibits osteoblastic bone formation by antagonising canonical Wnt signalling [ 74 ], as well as preventing BMP-mediated OB mineralisation [ 75 ]. Increased levels of sclerostin are seen in BM samples from MM patients [ 31 , 76 ]. An extensive study of MPCs from 630 MM patients, and 54 MM cell lines, concluded that sclerostin is not expressed directly from the MM cells, but from OCYs [ 14 ]. Levels of sclerostin appear to be higher in more active MM disease states and fall post-chemotherapy during disease plateau phases [ 32 ], suggesting further clinical importance of the OCY in the context of MBD. In vivo studies assessing sclerostin as a therapeutic target for bone disease receive significant attention in the osteoporosis field. When assessing the treatment of anti-sclerostin antibodies in murine myeloma models, there was a subsequent increase in OB numbers, improved fracture resistance, and prevention of MBD development (including osteolytic lesions). In addition, there was an additive effect when treated concurrently with Zoledronic acid. [ 14 ]. Romosozumab is a monoclonal antibody against sclerostin, and is approved for the treatment of osteoporosis to improve BMD [ 77 ]. Phase III studies ( NCT01796301 and NCT02186171 ) demonstrated that Romosozumab not only increased total hip and spine BMD [ 78 , 79 ], but also reduced new vertebral fracture rates by 48% when compared to the bisphosphonate Alendronic acid [ 80 ]. Unfortunately, Romosozumab was also associated with higher cardiovascular adverse events (2.5% vs. 1.9%) compared to Alendronic acid ( NCT01631214 ) [ 80 ], but despite this, Romosozumab has now been approved for treating severe osteoporosis if patients do not have any cardiovascular risk factors. Other anti-sclerostin agents such as Blosozumab (LY2541546) are also in the clinical trial stages [ 81 ].

4.2. TGFβ, ACTIVIN A, BMPs, and HGF

Members of the TGFβ family control different processes of cell proliferation, differentiation, and apoptosis, as well as production of the extracellular matrix. TGFβ is a potent inhibitor of OB differentiation and function [ 82 ], and is activated and released from the bone matrix during osteoclastic resorption, as well as directly from MPCs [ 83 ]. Higher levels of TGFβ are observed in BM extracellular fluid samples from MM patients [ 84 ]. TGFβ knockout has been shown to have reduced bone mass and elasticity [ 85 ]. Paton-Hough et al. demonstrated in MM mouse models (JJN3 and U266) that inhibition of TGFβ with 1D11 (a monoclonal antibody) increased trabecular bone volume, BMD, vertebral strength, and repaired osteolytic lesions [ 86 ], with additional benefits when combined with anti-resorptive therapy. Nyman et al. have also assessed 1D11 in both immune-competent and immunocompromised murine models of MM and found improvements in bone volume, architecture, BMD, and vertebral bone strength [ 86 , 87 ]. A small molecule inhibitor to TGFβ receptor 1, SD208, has also shown in the non-MM C57BL/6 mouse model to have bone anabolic effects (increased trabecular bone volume, BMD, and OB activity), and these effects are thought to be driven by increases in cytokines such as Runx2 [ 88 ]. Green et al. demonstrated in NOD scid gamma (NSG) mice inoculated with human JJN3 myeloma cells that early treatment with SD208 prevented lytic lesion development. Subsequently, the group developed a low-tumour MBD mouse model (NSG mice inoculated with human U266-GFP-luc myeloma) and treated established lytic disease with chemotherapy with or without SD208. Treatment with SD208 improved bone structure, lesion repair, and fracture resistance when compared to chemotherapy alone in this established MBD model [ 89 ]. In vitro treatment of human myeloma BMSC samples with SD208 also enhanced OB differentiations [ 89 ]. Some TGFβ antagonists have also shown preclinical evidence of antitumour effects, in addition to their bone anabolic effects [ 36 , 90 ].

Activin A is a member of the TGFβ superfamily, and is also known to inhibit OB mineralisation, as well as drive osteoclastogenesis [ 67 ]. There is an association between increased Activin A levels and the presence of osteolytic lesions in MM patients [ 11 ]. Antagonising Activin A in vivo, in a myeloma murine model, prevented osteolytic lesion formation, upregulated OB activity, and increased BMD [ 12 ]. This was also confirmed in another study using Activin A inhibitor (RAP-011) [ 13 ]. Sotatercept (a recombinant activin type IIa receptor ligand trap (previously named ACE-011)) was developed as a bone anabolic. Early phase clinical studies in osteoporosis noted that Sotatercept increased haemoglobin levels and BMD compared to controls [ 71 ]. These studies directed trials assessing efficacy and safety in the treatment of anaemia, particularly in the context of renal failure and myelodysplasia [ 91 , 92 ]. A phase IIa trial, combining treatment of Sotatercept with melphalan, prednisolone, and thalidomide in MM patients, observed an increase in haemoglobin, BMD, and bone-specific alkaline phosphatase in all patients receiving Sotatercept (in the absence of bisphosphonates) [ 93 ]. Importantly, all patients receiving Sotatercept reported improved perceptions of MBD pain [ 93 ]. The increased haemoglobin and haematocrit observed in some studies may exclude the use of Sotatercept for patients with previous thrombotic events or polycythaemia.

Other members of the TGF-β family relevant in MBD are BMPs, with BMP-2 and BMP-7 having a role in the differentiation of OBs, and they are therefore of potential value as bone anabolic agents. BMP-2 has also been shown to have anti-proliferation effects on MPCs [ 94 , 95 ]. BMP signalling is dysregulated in MBD. In a 5TGM1 myeloma-bearing mouse model, blockade of BMP with a small molecule inhibitor to BMP type 1 receptor improved MBD outcomes (increased trabecular and cortical bone mass and decreased lytic lesions) [ 96 ]. Seher et al. treated MM cell lines in vitro with custom-designed BMP-2 variants and confirmed that there was an antagonistic effect on Activin A, as well as a BMP-2 agonistic effect, which could lead to overall net bone gain, as well as potential antitumour effects [ 95 ]. PIM kinases have been identified as potential targets for haematological malignancies, and also inhibit BMP-mediated osteoblastogenesis [ 18 ]. In vitro treatment of primary OC and OB cells from healthy donors with a pan-PIM kinase inhibitor can decrease OC formation and activity, as well as increase OB formation and function [ 97 ]. PIM inhibition in human MM murine models has resulted in reduced MBD [ 30 , 97 ]. A pan-PIM kinase inhibitor was assessed in early phase clinical trials in MM patients, but with a focus on anti-myeloma effects [ 98 ]. Hepatocyte growth factor (HGF) is expressed at higher levels in patients with MM compared to those with MGUS or no haematological pathology, with particularly elevated levels associated with lytic bone lesions [ 19 ]. HGF is released by BMSCs [ 99 ], OCs [ 100 ], and some MPCs [ 18 , 101 ], and has been implicated in MPC homing and survival in the BMME [ 102 ], as well as bone remodelling. Expression of the HGF receptor (cMet) on both OBs and OCs suggests a coupling regulation of this cytokine in bone remodelling, with HGF reducing osteoblastogenesis via BMP signalling pathways [ 18 ]. Treatment of human MSCs with HGF in vitro inhibited the BMP-2-induced expression of OB transcription factors Runx2 and Osterix, as well as downregulating Smad signalling (within the Wnt pathway), suggesting significant interference of osteoblastogenesis. This is supported by observations that increased HGF levels seen clinically in MM patients are associated with lower levels of serum markers of bone formation (bone-specific alkaline phosphatase and procollagen type 1 N-terminal propeptide) [ 18 ]. There is also some evidence to suggest that HGF has an indirect role as an OAF, as HGF appears to upregulate IL-11 production by OBs [ 26 , 103 , 104 ].

4.3. Interleukins

IL-3, IL-7, and IL-11 are expressed at increased levels within BM plasma in a high proportion of MM patients [ 21 , 26 , 105 ]. In vitro assessment has shown inhibition of early OB differentiation, with the treatment of both recombinant IL-3 and human BM plasma containing high levels of IL-3 [ 22 ]. IL-3 is thought to indirectly inhibit BMP-2-driven osteoblastogenesis via CD45+/CD11b+ haemopoietic cells [ 22 ]. An IL-3 receptor-targeting therapy, SL-401 (a recombinant diphtheria toxin and IL-3 drug [ 106 ]), was shown to promote OB formation in preclinical MBD studies [ 107 ]. IL-11 is frequently quoted as a known OIF in myeloma, but few research studies have focussed on its role. In non-myeloma in vivo studies, treatment with IL-11 has reduced bone formation [ 108 ], but also has a described role in promoting osteoclastogenesis [ 109 ]. In an oestrogen-deficient mouse, IL-7 has been shown to have anti-anabolic effects [ 24 ]. In the context of MBD, in vitro investigation has suggested that IL-7 drives the downregulation of Runx2-mediated osteoblastogenesis [ 23 ], and that immortalised BM MSCs, co-cultured with MM patient plasma, had a reduction in Runx2-positive OBs seen. Treatment with anti-IL-7 limited this inhibitory effect [ 110 ].

4.4. Other Factors

TNF-α can supress OB precursor cells as well as reduce Runx2, the result being depleted OB differentiation [ 34 ], thus downregulating OB formation in MM patients [ 35 ]. TNF-α also has pro-osteoclastic synergistic effects with very small levels of RANKL, and, therefore, may well also be an OAF in the context of MBD [ 111 , 112 ]. Another member of the TNF superfamily, LIGHT, also appears to antagonise OB differentiation, by reducing OB precursor formation and may also lead to sclerostin expression from monocytes [ 113 ]. MIP-1α, also known as chemokine cytokine ligand 3, is expressed at higher levels in MM patients with MBD. MIP-1α is a known OAF but is also responsible for inhibiting osteogenesis [ 28 ]. Depleted levels of Runx2, Osterix, and Osteocalcin observed with MIP-1α stimulation are partially reversed with an MIP-1α antibody [ 27 , 28 ].

5. Conclusions

The presence and growth of MPCs in the BMME frequently lead to the inhibition of osteogenesis, the promotion of OB apoptosis, and the dysregulation of OB function. As a result, the majority of MM patients develop MBD. Here, we have reviewed the impact of MM on OBs, with a focus on MM-induced factors known to inhibit and/or disrupt normal OB differentiation and function. OIFs have the potential to be targeted to promote osteoblastogenesis in MM patients, with the ultimate goal to repair bone, increase BMD, and improve quality of life. In particular, Romosozumab (which targets sclerostin) is now an approved therapy for the treatment of osteoporosis and could potentially provide a promising option for MBD treatment, pending further investigation in early phase clinical trials. Given the persistent suppression of OBs, despite successful cancer treatment, and the significant impact of disease, the field is long overdue therapeutic advances to support improved outcomes for patients with MBD [ 114 ].

Author Contributions

Conceptualization, R.E.A.; writing—original draft preparation, R.E.A.; writing—review and editing, R.E.A., J.E.B., M.A.L. and A.D.C.; visualization, R.E.A.; supervision, J.E.B., M.A.L. and A.D.C. All authors have read and agreed to the published version of the manuscript.

Funding

R.E.A. receives research fellowship funding from Weston Park Cancer Charity, UK (RCN 509803-2).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Conflicts of Interest

J.E.B. reports personal fees from Novartis, Amgen, BMS, Ipsen, MSD, Sandoz and Bayer, with research grants from Amgen and Bayer. A.D.C. has received honoraria from Amgen, Takeda and Celgene.

Footnotes

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

1. Kazandjian D. Multiple myeloma epidemiology and survival: A unique malignancy. Semin. Oncol. 2016;43:676–681. doi: 10.1053/j.seminoncol.2016.11.004.

2. Siegel R.L., Miller K.D., Jemal A. Cancer statistics, 2016. CA Cancer J. Clin. 2016;66:7–30. doi: 10.3322/caac.21332.

3. Cook L., Macdonald D.H. Management of paraproteinaemia. Postgrad. Med. J. 2007;83:217–223. doi: 10.1136/pgmj.2006.054627.

4. Cowan A.J., Allen C., Barac A., Basaleem H., Bensenor I., Curado M.P., Foreman K., Gupta R., Harvey J., Hosgood D., et al. Global Burden of Multiple Myeloma: A Systematic Analysis for the Global Burden of Disease Study 2016. JAMA Oncol. 2018;4:1221–1227. doi: 10.1001/jamaoncol.2018.2128.

5. Smittenaar C.R., Petersen K.A., Stewart K., Moitt N. Cancer incidence and mortality projections in the UK until 2035. Br. J. Cancer. 2016;115:1147–1155. doi: 10.1038/bjc.2016.304.

6. Kyle R.A., Gertz M.A., Witzig T.E., Lust J.A., Lacy M.Q., Dispenzieri A., Fonseca R., Rajkumar S.V., Offord J.R., Larson D.R., et al. Review of 1027 patients with newly diagnosed multiple myeloma. Mayo Clin. Proc. 2003;78:21–33. doi: 10.4065/78.1.21.

7. Coleman R.E. Skeletal complications of malignancy. Cancer. 1997;80(Suppl. 8):1588–1594. doi: 10.1002/(SICI)1097-0142(19971015)80:8+<1588::AID-CNCR9>3.0.CO;2-G.

8. Melton L.J., 3rd, Kyle R.A., Achenbach S.J., Oberg A.L., Rajkumar S.V. Fracture risk with multiple myeloma: A population-based study. J. Bone Miner. Res. 2005;20:487–493. doi: 10.1359/JBMR.041131.

9. Saad F., Lipton A., Cook R., Chen Y.M., Smith M., Coleman R. Pathologic fractures correlate with reduced survival in patients with malignant bone disease. Cancer. 2007;110:1860–1867. doi: 10.1002/cncr.22991.

10. Novack D.V., Mbalaviele G. Osteoclasts-Key Players in Skeletal Health and Disease. Microbiol Spectr. 2016;4 doi: 10.1128/microbiolspec.MCHD-0011-2015.

11. Terpos E., Kastritis E., Christoulas D., Gkotzamanidou M., Eleutherakis-Papaiakovou E., Kanellias N., Papatheodorou A., Dimopoulos M.A. Circulating activin-A is elevated in patients with advanced multiple myeloma and correlates with extensive bone involvement and inferior survival; no alterations post-lenalidomide and dexamethasone therapy. Ann. Oncol. 2012;23:2681–2686. doi: 10.1093/annonc/mds068.

12. Chantry A.D., Heath D., Mulivor A.W., Pearsall S., Baud’huin M., Coulton L., Evans H., Abdul N., Werner E.D., Bouxsein M.L., et al. Inhibiting activin-A signaling stimulates bone formation and prevents cancer-induced bone destruction in vivo. J. Bone Miner. Res. 2010;25:2633–2646. doi: 10.1002/jbmr.142.

13. Vallet S., Mukherjee S., Vaghela N., Hideshima T., Fulciniti M., Pozzi S., Santo L., Cirstea D., Patel K., Sohani A.R., et al. Activin A promotes multiple myeloma-induced osteolysis and is a promising target for myeloma bone disease. Proc. Natl. Acad. Sci. USA. 2010;107:5124–5129. doi: 10.1073/pnas.0911929107.

14. McDonald M.M., Reagan M.R., Youlten S.E., Mohanty S.T., Seckinger A., Terry R.L., Pettitt J.A., Simic M.K., Cheng T.L., Morse A., et al. Inhibiting the osteocyte-specific protein sclerostin increases bone mass and fracture resistance in multiple myeloma. Blood. 2017;129:3452–3464. doi: 10.1182/blood-2017-03-773341.

15. Tian E., Zhan F., Walker R., Rasmussen E., Ma Y., Barlogie B., Shaughnessy J.D., Jr. The role of the Wnt-signaling antagonist DKK1 in the development of osteolytic lesions in multiple myeloma. N. Engl. J. Med. 2003;349:2483–2494. doi: 10.1056/NEJMoa030847.

16. Politou M.C., Heath D.J., Rahemtulla A., Szydlo R., Anagnostopoulos A., Dimopoulos M.A., Croucher P.I., Terpos E. Serum concentrations of Dickkopf-1 protein are increased in patients with multiple myeloma and reduced after autologous stem cell transplantation. Int. J. Cancer. 2006;119:1728–1731. doi: 10.1002/ijc.22033.

17. D’Souza S., del Prete D., Jin S., Sun Q., Huston A.J., Kostov F.E., Sammut B., Hong C.-S., Anderson J.L., Patrene K.D., et al. Gfi1 expressed in bone marrow stromal cells is a novel osteoblast suppressor in patients with multiple myeloma bone disease. Blood. 2011;118:6871–6880. doi: 10.1182/blood-2011-04-346775.

18. Standal T., Abildgaard N., Fagerli U.M., Stordal B., Hjertner O., Borset M., Algarín E.M., Martín-Sánchez M., Corbacho-González D., Ortiz-De-Solorzano C., et al. HGF inhibits BMP-induced osteoblastogenesis: Possible implications for the bone disease of multiple myeloma. Blood. 2007;109:3024–3030. doi: 10.1182/blood-2006-07-034884.

19. Kristensen I.B., Christensen J.H., Lyng M.B., Moller M.B., Pedersen L., Rasmussen L.M., Ditzel H.J., Abildgaard N. Hepatocyte growth factor pathway upregulation in the bone marrow microenvironment in multiple myeloma is associated with lytic bone disease. Br. J. Haematol. 2013;161:373–382. doi: 10.1111/bjh.12270.

20. Ruan J., Trotter T.N., Nan L., Luo R., Javed A., Sanderson R.D., Suva L.J., Yang Y. Heparanase inhibits osteoblastogenesis and shifts bone marrow progenitor cell fate in myeloma bone disease. Bone. 2013;57:10–17. doi: 10.1016/j.bone.2013.07.024.

21. Lee J.W., Chung H.Y., Ehrlich L.A., Jelinek D.F., Callander N.S., Roodman G.D., Choi S.J. IL-3 expression by myeloma cells increases both osteoclast formation and growth of myeloma cells. Blood. 2004;103:2308–2315. doi: 10.1182/blood-2003-06-1992.

22. Ehrlich L.A., Chung H.Y., Ghobrial I., Choi S.J., Morandi F., Colla S., Rizzoli V., David Roodman G., Giuliani N. IL-3 is a potential inhibitor of osteoblast differentiation in multiple myeloma. Blood. 2005;106:1407–1414. doi: 10.1182/blood-2005-03-1080.

23. Giuliani N., Colla S., Morandi F., Lazzaretti M., Sala R., Bonomini S., Grano M., Colucci S., Svaldi M., Rizzoli V. Myeloma cells block RUNX2/CBFA1 activity in human bone marrow osteoblast progenitors and inhibit osteoblast formation and differentiation. Blood. 2005;106:2472–2483. doi: 10.1182/blood-2004-12-4986.

24. Weitzmann M.N., Roggia C., Toraldo G., Weitzmann L., Pacifici R. Increased production of IL-7 uncouples bone formation from bone resorption during estrogen deficiency. J. Clin. Investig. 2002;110:1643–1650. doi: 10.1172/JCI0215687.

25. Giuliani N., Rizzoli V. Myeloma cells and bone marrow osteoblast interactions: Role in the development of osteolytic lesions in multiple myeloma. Leuk. Lymphoma. 2007;48:2323–2329. doi: 10.1080/10428190701648281.

26. Hjertner O., Torgersen M.L., Seidel C., Hjorth-Hansen H., Waage A., Borset M., Sundan A. Hepatocyte growth factor (HGF) induces interleukin-11 secretion from osteoblasts: A possible role for HGF in myeloma-associated osteolytic bone disease. Blood. 1999;94:3883–3888. doi: 10.1182/blood.V94.11.3883.

27. Vallet S., Pozzi S., Patel K., Vaghela N., Fulciniti M.T., Veiby P., Hideshima T., Santo L., Cirstea D., Scadden D.T., et al. A novel role for CCL3 (MIP-1alpha) in myeloma-induced bone disease via osteocalcin downregulation and inhibition of osteoblast function. Leukemia. 2011;25:1174–1181. doi: 10.1038/leu.2011.43.

28. Fu R., Liu H., Zhao S., Wang Y., Li L., Gao S., Ruan E., Wang G., Wang H.Q., Song J., et al. Osteoblast inhibition by chemokine cytokine ligand3 in myeloma-induced bone disease. Cancer Cell Int. 2014;14:132. doi: 10.1186/s12935-014-0132-6.

29. Groen R.W., de Rooij M.F., Kocemba K.A., Reijmers R.M., de Haan-Kramer A., Overdijk M.B., Aalders L., Rozemuller H., Martens A.C., Bergsagel P.L., et al. N-cadherin-mediated interaction with multiple myeloma cells inhibits osteoblast differentiation. Haematologica. 2011;96:1653–1661. doi: 10.3324/haematol.2010.038133.

30. Hiasa M., Teramachi J., Oda A., Amachi R., Harada T., Nakamura S., Miki H., Fujii S., Kagawa K., Watanabe K., et al. Pim-2 kinase is an important target of treatment for tumor progression and bone loss in myeloma. Leukemia. 2015;29:207–217. doi: 10.1038/leu.2014.147.

31. Brunetti G., Oranger A., Mori G., Specchia G., Rinaldi E., Curci P., Zallone A., Rizzi R., Grano M., Colucci S. Sclerostin is overexpressed by plasma cells from multiple myeloma patients. Ann. N. Y. Acad. Sci. 2011;1237:19–23. doi: 10.1111/j.1749-6632.2011.06196.x.

32. Terpos E., Christoulas D., Katodritou E., Bratengeier C., Gkotzamanidou M., Michalis E., Delimpasi S., Pouli A., Meletis J., Kastritis E., et al. Elevated circulating sclerostin correlates with advanced disease features and abnormal bone remodeling in symptomatic myeloma: Reduction post-bortezomib monotherapy. Int. J. Cancer. 2012;131:1466–1471. doi: 10.1002/ijc.27342.

33. Oshima T., Abe M., Asano J., Hara T., Kitazoe K., Sekimoto E., Tanaka Y., Shibata H., Hashimoto T., Ozaki S., et al. Myeloma cells suppress bone formation by secreting a soluble Wnt inhibitor, sFRP-2. Blood. 2005;106:3160–3165. doi: 10.1182/blood-2004-12-4940.

34. Mukai T., Otsuka F., Otani H., Yamashita M., Takasugi K., Inagaki K., Yamamura M., Makino H. TNF-alpha inhibits BMP-induced osteoblast differentiation through activating SAPK/JNK signaling. Biochem. Biophys. Res. Commun. 2007;356:1004–1010. doi: 10.1016/j.bbrc.2007.03.099.

35. Adamik J., Jin S., Sun Q., Zhang P., Weiss K.R., Anderson J.L., Silbermann R., Roodman G.D., Galson D.L. EZH2 or HDAC1 Inhibition Reverses Multiple Myeloma-Induced Epigenetic Suppression of Osteoblast Differentiation. Mol. Cancer Res. 2017;15:405–417. doi: 10.1158/1541-7786.MCR-16-0242-T.

36. Takeuchi K., Abe M., Hiasa M., Oda A., Amou H., Kido S., Harada T., Tanaka O., Miki H., Nakamura S., et al. Tgf-Beta inhibition restores terminal osteoblast differentiation to suppress myeloma growth. PLoS ONE. 2010;5:e9870. doi: 10.1371/journal.pone.0009870.

37. Sanderson R.D., Yang Y. Syndecan-1: A dynamic regulator of the myeloma microenvironment. Clin. Exp. Metastasis. 2008;25:149–159. doi: 10.1007/s10585-007-9125-3.

38. Katz B.Z. Adhesion molecules--The lifelines of multiple myeloma cells. Semin. Cancer Biol. 2010;20:186–195. doi: 10.1016/j.semcancer.2010.04.003.

39. Terpos E., Ntanasis-Stathopoulos I., Gavriatopoulou M., Dimopoulos M.A. Pathogenesis of bone disease in multiple myeloma: From bench to bedside. Blood Cancer J. 2018;8:7. doi: 10.1038/s41408-017-0037-4.

40. Schwarzer R., Nickel N., Godau J., Willie B.M., Duda G.N., Schwarzer R., Cirovic B., Leutz A., Manz R., Bogen B., et al. Notch pathway inhibition controls myeloma bone disease in the murine MOPC315.BM model. Blood Cancer J. 2014;4:e217. doi: 10.1038/bcj.2014.37.

41. Terpos E., Zamagni E., Lentzsch S., Drake M.T., Garcia-Sanz R., Abildgaard N., Ntanasis-Stathopoulos I., Schjesvold F., de la Rubia J., Kyriakou C., et al. Treatment of multiple myeloma-related bone disease: Recommendations from the Bone Working Group of the International Myeloma Working Group. Lancet Oncol. 2021;22:e119–e130. doi: 10.1016/S1470-2045(20)30559-3.

42. Xu X., Zhang C., Trotter T.N., Gowda P.S., Lu Y., Ponnazhagan S., Javed A., Li J., Yang Y. Runx2 Deficiency in Osteoblasts Promotes Myeloma Progression by Altering the Bone Microenvironment at New Bone Sites. Cancer Res. 2020;80:1036–1048. doi: 10.1158/0008-5472.CAN-19-0284.

43. Trotter T.N., Li M., Pan Q., Peker D., Rowan P.D., Li J., Zhan F., Suva L.J., Javed A., Yang Y. Myeloma cell-derived Runx2 promotes myeloma progression in bone. Blood. 2015;125:3598–3608. doi: 10.1182/blood-2014-12-613968.

44. Liu Z., Liu H., Li Y., Shao Q., Chen J., Song J., Fu R. Multiple myeloma-derived exosomes inhibit osteoblastic differentiation and improve IL-6 secretion of BMSCs from multiple myeloma. J. Investig. Med. 2020;68:45–51. doi: 10.1136/jim-2019-001010.

45. Liu Z., Liu H., He J., Lin P., Tong Q., Yang J. Myeloma cells shift osteoblastogenesis to adipogenesis by inhibiting the ubiquitin ligase MURF1 in mesenchymal stem cells. Sci. Signal. 2020;13 doi: 10.1126/scisignal.aay8203.

46. Adamik J., Roodman G.D., Galson D.L. Epigenetic-Based Mechanisms of Osteoblast Suppression in Multiple Myeloma Bone Disease. JBMR Plus. 2019;3:e10183. doi: 10.1002/jbm4.10183.

47. Adamik J., Galson D.L., Roodman G.D. Osteoblast suppression in multiple myeloma bone disease. J. Bone Oncol. 2018;13:62–70. doi: 10.1016/j.jbo.2018.09.001.

48. Silvestris F., Cafforio P., Tucci M., Grinello D., Dammacco F. Upregulation of osteoblast apoptosis by malignant plasma cells: A role in myeloma bone disease. Br. J. Haematol. 2003;122:39–52. doi: 10.1046/j.1365-2141.2003.04374.x.

49. Croucher P.I., Shipman C.M., Lippitt J., Perry M., Asosingh K., Hijzen A., Brabbs A.C., van Beek E.J.R., Holen I., Skerry T.M., et al. Osteoprotegerin inhibits the development of osteolytic bone disease in multiple myeloma. Blood. 2001;98:3534–3540. doi: 10.1182/blood.V98.13.3534.

50. Vanderkerken K., De Leenheer E., Shipman C., Asosingh K., Willems A., Van Camp B., Croucher P. Recombinant osteoprotegerin decreases tumor burden and increases survival in a murine model of multiple myeloma. Cancer Res. 2003;63:287–289.

51. Seidel C., Hjertner O., Abildgaard N., Heickendorff L., Hjorth M., Westin J., Lanng Nielsen J. Serum osteoprotegerin levels are reduced in patients with multiple myeloma with lytic bone disease. Blood. 2001;98:2269–2271. doi: 10.1182/blood.V98.7.2269.

52. Albers J., Keller J., Baranowsky A., Beil F.T., Catala-Lehnen P., Schulze J., Amling M., Schinke T. Canonical Wnt signaling inhibits osteoclastogenesis independent of osteoprotegerin. J. Cell Biol. 2013;200:537–549. doi: 10.1083/jcb.201207142.

53. Spaan I., Raymakers R.A., van de Stolpe A., Peperzak V. Wnt signaling in multiple myeloma: A central player in disease with therapeutic potential. J. Hematol. Oncol. 2018;11:67. doi: 10.1186/s13045-018-0615-3.

54. Van Andel H., Kocemba K.A., Spaargaren M., Pals S.T. Aberrant Wnt signaling in multiple myeloma: Molecular mechanisms and targeting options. Leukemia. 2019;33:1063–1075. doi: 10.1038/s41375-019-0404-1.

55. Bolzoni M., Donofrio G., Storti P., Guasco D., Toscani D., Lazzaretti M., Bonomini S., Agnelli L., Capocefalo A., Dalla Palma B., et al. Myeloma cells inhibit non-canonical wnt co-receptor ror2 expression in human bone marrow osteoprogenitor cells: Effect of wnt5a/ror2 pathway activation on the osteogenic differentiation impairment induced by myeloma cells. Leukemia. 2013;27:451–463. doi: 10.1038/leu.2012.190.

56. Baron R., Kneissel M. WNT signaling in bone homeostasis and disease: From human mutations to treatments. Nat. Med. 2013;19:179–192. doi: 10.1038/nm.3074.

57. Kenkre J.S., Bassett J. The bone remodelling cycle. Ann. Clin. Biochem. 2018;55:308–327. doi: 10.1177/0004563218759371.

58. Westendorf J.J., Kahler R.A., Schroeder T.M. Wnt signaling in osteoblasts and bone diseases. Gene. 2004;341:19–39. doi: 10.1016/j.gene.2004.06.044.

59. Kawano Y., Kypta R. Secreted antagonists of the Wnt signalling pathway. J. Cell Sci. 2003;116 Pt 13:2627–2634. doi: 10.1242/jcs.00623.

60. Itasaki N., Jones C.M., Mercurio S., Rowe A., Domingos P.M., Smith J.C., Krumlauf B. Wise, a context-dependent activator and inhibitor of Wnt signalling. Development. 2003;130:4295–4305. doi: 10.1242/dev.00674.

61. Zhou F., Meng S., Song H., Claret F.X. Dickkopf-1 is a key regulator of myeloma bone disease: Opportunities and challenges for therapeutic intervention. Blood Rev. 2013;27:261–267. doi: 10.1016/j.blre.2013.08.002.

62. Mao B., Wu W., Davidson G., Marhold J., Li M., Mechler B.M., Delius H., Hoppe D., Stannek P., Walter C., et al. Kremen proteins are Dickkopf receptors that regulate Wnt/beta-catenin signalling. Nature. 2002;417:664–667. doi: 10.1038/nature756.

63. MacDonald B.T., Joiner D.M., Oyserman S.M., Sharma P., Goldstein S.A., He X., Hauschka P.V. Bone mass is inversely proportional to Dkk1 levels in mice. Bone. 2007;41:331–339. doi: 10.1016/j.bone.2007.05.009.

64. Robbiani D.F., Chesi M., Bergsagel P.L. Bone lesions in molecular subtypes of multiple myeloma. N. Engl. J. Med. 2004;351:197–198. doi: 10.1056/NEJM200407083510223.

65. Qiang Y.W., Chen Y., Stephens O., Brown N., Chen B., Epstein J., Barlogie B., Shaughnessy J.D., Jr. Myeloma-derived Dickkopf-1 disrupts Wnt-regulated osteoprotegerin and RANKL production by osteoblasts: A potential mechanism underlying osteolytic bone lesions in multiple myeloma. Blood. 2008;112:196–207. doi: 10.1182/blood-2008-01-132134.

66. Yaccoby S., Ling W., Zhan F., Walker R., Barlogie B., Shaughnessy J.D., Jr. Antibody-based inhibition of DKK1 suppresses tumor-induced bone resorption and multiple myeloma growth in vivo. Blood. 2007;109:2106–2111. doi: 10.1182/blood-2006-09-047712.

67. Yaccoby S., Wezeman M.J., Zangari M., Walker R., Cottler-Fox M., Gaddy D., Ling W., Saha R., Barlogie B., Tricot G., et al. Inhibitory effects of osteoblasts and increased bone formation on myeloma in novel culture systems and a myelomatous mouse model. Haematologica. 2006;91:192–199.

68. Heath D.J., Chantry A.D., Buckle C.H., Coulton L., Shaughnessy J.D., Jr., Evans H.R., Snowden J.A., Stover D.R., Vanderkerken K., Croucher P.I. Inhibiting Dickkopf-1 (Dkk1) removes suppression of bone formation and prevents the development of osteolytic bone disease in multiple myeloma. J. Bone Miner. Res. 2009;24:425–436. doi: 10.1359/jbmr.081104.

69. Fulciniti M., Tassone P., Hideshima T., Vallet S., Nanjappa P., Ettenberg S.A., Shen Z.X., Patel N., Tai Y.-T., Chauhan D., et al. Anti-DKK1 mAb (BHQ880) as a potential therapeutic agent for multiple myeloma. Blood. 2009;114:371–379. doi: 10.1182/blood-2008-11-191577.

70. Iyer S.P., Beck J.T., Stewart A.K., Shah J., Kelly K.R., Isaacs R., Bilic S., Sen S., Munshi N.C. A Phase IB multicentre dose-determination study of BHQ880 in combination with anti-myeloma therapy and zoledronic acid in patients with relapsed or refractory multiple myeloma and prior skeletal-related events. Br. J. Haematol. 2014;167:366–375. doi: 10.1111/bjh.13056.

71. Munshi N. Early Evidence of Anabolic Bone Activity of BHQ880, a Fully Human Anti-DKK1 Neutralizing Antibody: Results of a Phase 2 Study in Previously Untreated Patients with Smoldering Multiple Myeloma At Risk for Progression. Blood. 2012;120:331. doi: 10.1182/blood.V120.21.331.331.

72. Ring E.S., Lawson M.A., Snowden J.A., Jolley I., Chantry A.D. New agents in the Treatment of Myeloma Bone Disease. Calcif. Tissue Int. 2018;102:196–209. doi: 10.1007/s00223-017-0351-7.

73. Terpos E., Christoulas D., Kastritis E., Bagratuni T., Gavriatopoulou M., Roussou M., Papatheodorou A., Eleutherakis-Papaiakovou E., Kanellias N., Liakou C., et al. High levels of periostin correlate with increased fracture rate, diffuse MRI pattern, abnormal bone remodeling and advanced disease stage in patients with newly diagnosed symptomatic multiple myeloma. Blood Cancer J. 2016;6:e482. doi: 10.1038/bcj.2016.90.

74. Spatz J.M., Wein M.N., Gooi J.H., Qu Y., Garr J.L., Liu S., Barry K.J., Uda Y., Lai F., Dedic C., et al. The Wnt Inhibitor Sclerostin Is Up-regulated by Mechanical Unloading in Osteocytes in Vitro. J. Biol. Chem. 2015;290:16744–16758. doi: 10.1074/jbc.M114.628313.

75. Winkler D.G., Sutherland M.K., Geoghegan J.C., Yu C., Hayes T., Skonier J.E., Shpektor D., Jonas M., Kovacevich B.R., Staehling-Hampton K., et al. Osteocyte control of bone formation via sclerostin, a novel BMP antagonist. EMBO J. 2003;22:6267–6276. doi: 10.1093/emboj/cdg599.

76. Colucci S., Brunetti G., Oranger A., Mori G., Sardone F., Specchia G., Rinaldi E., Curci P., Liso V., Passeri G., et al. Myeloma cells suppress osteoblasts through sclerostin secretion. Blood Cancer J. 2011;1:e27. doi: 10.1038/bcj.2011.22.

77. McClung M.R., Grauer A., Boonen S., Bolognese M.A., Brown J.P., Diez-Perez A., Langdahl B.L., Reginster J.-Y., Zanchetta J.R., Wasserman S.M., et al. Romosozumab in postmenopausal women with low bone mineral density. N. Engl. J. Med. 2014;370:412–420. doi: 10.1056/NEJMoa1305224.

78. Langdahl B.L., Libanati C., Crittenden D.B., Bolognese M.A., Brown J.P., Daizadeh N.S., Dokoupilova E., Engelke K., Finkelstein J.S., Genant H.K., et al. Romosozumab (sclerostin monoclonal antibody) versus teriparatide in postmenopausal women with osteoporosis transitioning from oral bisphosphonate therapy: A randomised, open-label, phase 3 trial. Lancet. 2017;390:1585–1594. doi: 10.1016/S0140-6736(17)31613-6.

79. Lewiecki E.M., Blicharski T., Goemaere S., Lippuner K., Meisner P.D., Miller P.D., Miyauchi A., Maddox J., Chen L., Horlait S. A Phase 3 Randomized Placebo-controlled Trial to Evaluate Efficacy and Safety of Romosozumab in Men with Osteoporosis. J. Clin. Endocrinol. Metab. 2018;103:3183–3193. doi: 10.1210/jc.2017-02163.

80. Saag K.G., Petersen J., Brandi M.L., Karaplis A.C., Lorentzon M., Thomas T., Maddox J., Fan M., Meisner P.D., Grauer A. Romosozumab or Alendronate for Fracture Prevention in Women with Osteoporosis. N. Engl. J. Med. 2017;377:1417–1427. doi: 10.1056/NEJMoa1708322.

81. McColm J., Hu L., Womack T., Tang C.C., Chiang A.Y. Single- and multiple-dose randomized studies of blosozumab, a monoclonal antibody against sclerostin, in healthy postmenopausal women. J. Bone Miner. Res. 2014;29:935–943. doi: 10.1002/jbmr.2092.

82. Matsumoto T., Abe M. TGF-beta-related mechanisms of bone destruction in multiple myeloma. Bone. 2011;48:129–134. doi: 10.1016/j.bone.2010.05.036.

83. Urashima M., Ogata A., Chauhan D., Hatziyanni M., Vidriales M.B., Dedera D.A., Schlossman R.L., Anderson K.C. Transforming growth factor-beta1: Differential effects on multiple myeloma versus normal B cells. Blood. 1996;87:1928–1938. doi: 10.1182/blood.V87.5.1928.1928.

84. Bruns I., Cadeddu R.P., Brueckmann I., Frobel J., Geyh S., Bust S., Fischer J.C., Roels F., Wilk C.M., Schildberg F.A., et al. Multiple myeloma-related deregulation of bone marrow-derived CD34(+) hematopoietic stem and progenitor cells. Blood. 2012;120:2620–2630. doi: 10.1182/blood-2011-04-347484.

85. Geiser A.G., Zeng Q.Q., Sato M., Helvering L.M., Hirano T., Turner C.H. Decreased bone mass and bone elasticity in mice lacking the transforming growth factor-beta1 gene. Bone. 1998;23:87–93. doi: 10.1016/S8756-3282(98)00078-7.

86. Paton-Hough J., Tazzyman S., Evans H., Lath D., Down J.M., Green A.C., Snowden J.A., Chantry A., Lawson M.A. Preventing and Repairing Myeloma Bone Disease by Combining Conventional Antiresorptive Treatment with a Bone Anabolic Agent in Murine Models. J. Bone Miner. Res. 2019;34:783–796. doi: 10.1002/jbmr.3606.

87. Nyman J.S., Merkel A.R., Uppuganti S., Nayak B., Rowland B., Makowski A.J., Oyajobi B.O., Sterling J.A. Combined treatment with a transforming growth factor beta inhibitor (1D11) and bortezomib improves bone architecture in a mouse model of myeloma-induced bone disease. Bone. 2016;91:81–91. doi: 10.1016/j.bone.2016.07.007.

88. Mohammad K.S., Chen C.G., Balooch G., Stebbins E., McKenna C.R., Davis H., Niewolna M., Peng X.H., Nguyen D.H.N., Ionova-Martin S.S., et al. Pharmacologic inhibition of the TGF-beta type I receptor kinase has anabolic and anti-catabolic effects on bone. PLoS ONE. 2009;4:e5275. doi: 10.1371/journal.pone.0005275.

89. Green A.C., Lath D., Hudson K., Walkley B., Down J.M., Owen R., Evans H.R., Paton-Hough J., Reilly G.C., Lawson M.A., et al. TGFbeta Inhibition Stimulates Collagen Maturation to Enhance Bone Repair and Fracture Resistance in a Murine Myeloma Model. J. Bone Miner. Res. 2019;34:2311–2326. doi: 10.1002/jbmr.3859.

90. Lu A., Pallero M.A., Lei W., Hong H., Yang Y., Suto M.J., Murphy-Ullrich J.E. Inhibition of Transforming Growth Factor-beta Activation Diminishes Tumor Progression and Osteolytic Bone Disease in Mouse Models of Multiple Myeloma. Am. J. Pathol. 2016;186:678–690. doi: 10.1016/j.ajpath.2015.11.003.

91. Raje N., Vallet S. Sotatercept, a soluble activin receptor type 2A IgG-Fc fusion protein for the treatment of anemia and bone loss. Curr. Opin. Mol. Ther. 2010;12:586–597.

92. Komrokji R., Garcia-Manero G., Ades L., Prebet T., Steensma D.P., Jurcic J.G., Sekeres M., Berdeja J., Savona M.R., Beyne-Rauzy O., et al. Sotatercept with long-term extension for the treatment of anaemia in patients with lower-risk myelodysplastic syndromes: A phase 2, dose-ranging trial. Lancet Haematol. 2018;5:e63–e72. doi: 10.1016/S2352-3026(18)30002-4.

93. Abdulkadyrov K.M., Salogub G.N., Khuazheva N.K., Sherman M.L., Laadem A., Barger R., Knight R., Srinivasan S., Terpos T. Sotatercept in patients with osteolytic lesions of multiple myeloma. Br. J. Haematol. 2014;165:814–823. doi: 10.1111/bjh.12835.

94. Kawamura C., Kizaki M., Ikeda Y. Bone morphogenetic protein (BMP)-2 induces apoptosis in human myeloma cells. Leuk. Lymphoma. 2002;43:635–639. doi: 10.1080/10428190290012182.

95. Seher A., Lagler C., Stuhmer T., Muller-Richter U.D.A., Kubler A.C., Sebald W., Müller T.D., Nickel J. Utilizing BMP-2 muteins for treatment of multiple myeloma. PLoS ONE. 2017;12:e0174884. doi: 10.1371/journal.pone.0174884.

96. Gooding S., Olechnowicz S.W.Z., Morris E.V., Armitage A.E., Arezes J., Frost J., Repapi E., Edwards J.R., Ashley N., Waugh C., et al. Transcriptomic profiling of the myeloma bone-lining niche reveals BMP signalling inhibition to improve bone disease. Nat. Commun. 2019;10:4533. doi: 10.1038/s41467-019-12296-1.

97. Paino T., Garcia-Gomez A., Gonzalez-Mendez L., San-Segundo L., Hernandez-Garcia S., Lopez-Iglesias A.A., Algarín E.M., Martín-Sánchez M., Corbacho D., Ortiz-de-Solorzano C., et al. The Novel Pan-PIM Kinase Inhibitor, PIM447, Displays Dual Antimyeloma and Bone-Protective Effects, and Potently Synergizes with Current Standards of Care. Clin. Cancer Res. 2017;23:225–238. doi: 10.1158/1078-0432.CCR-16-0230.

98. Raab M.S., Thomas S.K., Ocio E.M., Guenther A., Goh Y.T., Talpaz M., Hohmann N., Zhao S., Xiang F., Simon C., et al. The first-in-human study of the pan-PIM kinase inhibitor PIM447 in patients with relapsed and/or refractory multiple myeloma. Leukemia. 2019;33:2924–2933. doi: 10.1038/s41375-019-0482-0.

99. Takai K., Hara J., Matsumoto K., Hosoi G., Osugi Y., Tawa A., Okada S., Nakamura T. Hepatocyte growth factor is constitutively produced by human bone marrow stromal cells and indirectly promotes hematopoiesis. Blood. 1997;89:1560–1565. doi: 10.1182/blood.V89.5.1560.

100. Grano M., Galimi F., Zambonin G., Colucci S., Cottone E., Zallone A.Z., Comoglio P.M. Hepatocyte growth factor is a coupling factor for osteoclasts and osteoblasts in vitro. Proc. Natl. Acad. Sci. USA. 1996;93:7644–7648. doi: 10.1073/pnas.93.15.7644.

101. Seidel C., Borset M., Turesson I., Abildgaard N., Sundan A., Waage A. Elevated serum concentrations of hepatocyte growth factor in patients with multiple myeloma. Nord. Myeloma Study Group Blood. 1998;91:806–812.

102. Derksen P.W., de Gorter D.J., Meijer H.P., Bende R.J., van Dijk M., Lokhorst H.M., Bloem A.C., Spaargaren M., Pals S.T. The hepatocyte growth factor/Met pathway controls proliferation and apoptosis in multiple myeloma. Leukemia. 2003;17:764–774. doi: 10.1038/sj.leu.2402875.

103. Yasuda H., Shima N., Nakagawa N., Yamaguchi K., Kinosaki M., Mochizuki S., Tomoyasu A., Yano K., Goto M., Murakami A., et al. Osteoclast differentiation factor is a ligand for osteoprotegerin/osteoclastogenesis-inhibitory factor and is identical to TRANCE/RANKL. Proc. Natl. Acad. Sci. USA. 1998;95:3597–3602. doi: 10.1073/pnas.95.7.3597.

104. Kudo O., Sabokbar A., Pocock A., Itonaga I., Fujikawa Y., Athanasou N.A. Interleukin-6 and interleukin-11 support human osteoclast formation by a RANKL-independent mechanism. Bone. 2003;32:1–7. doi: 10.1016/S8756-3282(02)00915-8.

105. Giuliani N., Colla S., Sala R., Moroni M., Lazzaretti M., La Monica S., Bonomini S., Hojden M., Sammarelli G., Barillè S., et al. Human myeloma cells stimulate the receptor activator of nuclear factor-kappa B ligand (RANKL) in T lymphocytes: A potential role in multiple myeloma bone disease. Blood. 2002;100:4615–4621. doi: 10.1182/blood-2002-04-1121.

106. Alkharabsheh O., Frankel A.E. Clinical Activity and Tolerability of SL-401 (Tagraxofusp): Recombinant Diphtheria Toxin and Interleukin-3 in Hematologic Malignancies. Biomedicines. 2019;7:6. doi: 10.3390/biomedicines7010006.

107. Ray A., Das D.S., Song Y., Macri V., Richardson P., Brooks C.L., Chauhan D., Anderson K.C. A novel agent SL-401 induces anti-myeloma activity by targeting plasmacytoid dendritic cells, osteoclastogenesis and cancer stem-like cells. Leukemia. 2017;31:2652–2660. doi: 10.1038/leu.2017.135.

108. Hughes F.J., Howells G.L. Interleukin-11 inhibits bone formation in vitro. Calcif. Tissue Int. 1993;53:362–364. doi: 10.1007/BF01351844.

109. Stromme O., Psonka-Antonczyk K.M., Stokke B.T., Sundan A., Arum C.J., Brede G. Myeloma-derived extracellular vesicles mediate HGF/c-Met signaling in osteoblast-like cells. Exp. Cell Res. 2019;383:111490. doi: 10.1016/j.yexcr.2019.07.003.

110. Nierste B.A., Glackin C.A., Kirshner J. Dkk-1 and IL-7 in plasma of patients with multiple myeloma prevent differentiation of mesenchymal stem cells into osteoblasts. Am. J. Blood Res. 2014;4:73–85.

111. Brunetti G., Oranger A., Mori G., Centonze M., Colaianni G., Rizzi R., Liso V., Zallone A., Grano M., Colucci S. The formation of osteoclasts in multiple myeloma bone disease patients involves the secretion of soluble decoy receptor 3. Ann. N. Y. Acad. Sci. 2010;1192:298–302. doi: 10.1111/j.1749-6632.2009.05304.x.

112. Lam J., Takeshita S., Barker J.E., Kanagawa O., Ross F.P., Teitelbaum S.L. TNF-alpha induces osteoclastogenesis by direct stimulation of macrophages exposed to permissive levels of RANK ligand. J. Clin. Investig. 2000;106:1481–1488. doi: 10.1172/JCI11176.

113. Brunetti G., Rizzi R., Oranger A., Gigante I., Mori G., Taurino G., Mongelli T., Colaianni G., Di Benedetto A., Tamma R., et al. LIGHT/TNFSF14 increases osteoclastogenesis and decreases osteoblastogenesis in multiple myeloma-bone disease. Oncotarget. 2014;5:12950–12967. doi: 10.18632/oncotarget.2633.

114. Terpos E., Christoulas D., Gavriatopoulou M., Dimopoulos M.A. Mechanisms of bone destruction in multiple myeloma. Eur. J. Cancer Care. 2017;26:e12761. doi: 10.1111/ecc.12761.