The pivotal role of endoplasmic reticulum in cancer glucose metabolism.

The endoplasmic reticulum (ER) supports essential biosynthetic and quality control functions. These processes rely on sustained energy supply and precise redox control within the ER lumen. While ATP can be imported from mitochondria, pyridine nucleotides are impermeable to the ER membrane, necessitating compartment-specific mechanisms to regulate NAD(H) and NADP(H) pools. Here, we demonstrate that the ER-confined pentose phosphate pathway (ER-PPP), driven by hexose-6-phosphate dehydrogenase (H6PD), processes large amounts of glucose equivalents to preserve the local redox homeostasis in triple-negative breast cancer (TNBC) cells. H6PD silencing decreases the NADPH regeneration within the ER lumen. The consequent impairment of protein folding machinery accelerates lysosomes generation, up to disrupt the equivalence between the cell release of lactate and H+. Finally, the simultaneous impairment of glucose-6P (G6P) degradation (by H6PD silencing) and hydrolysis (by silencing glucose-6-phosphatase) eventually results in a measurable ER collapse documenting the high-rate nature of G6P flux across the reticular membrane.