Pivotal Advance: Protein synthesis modulates responsiveness of differentiating and malignant plasma cells to proteasome inhibitors

A previously unsuspected, considerable proportion of newly synthesized polypeptides are rapidly hydrolyzed by proteasomes, possibly competing with endogenous substrates and altering proteostasis. In view of the anti-cancer effects of proteasome inhibitors (PI), we set out to achieve a quantitative assessment of proteasome workload in cells hallmarked by different PI sensitivity, namely a panel of multiple myeloma (MM) cells, and in a dynamic model of plasma cell differentiation, a process that confers exquisite PI sensitivity. Our results suggest that protein synthesis is a key determinant of proteasomal proteolytic burden and PI sensitivity. In different MM cells and in differentiating plasma cells, the average proteolytic work accomplished per proteasome ranges over different orders of magnitude, an unexpected degree of variability, with increased workload invariably associated to increased PI sensitivity. The unfavorable load vs. capacity balance found in highly PI-sensitive MM lines is accounted for by a decreased total number of immunoproteasomes per cell coupled to enhanced generation of rapidly degraded polypeptides (RDPs). Moreover, indicative of cause-effect relationships, attenuating general protein synthesis by the otherwise toxic agent cycloheximide (CHX) reduces PI sensitivity both in activated B and in MM cells. Our data support the view that in plasma cells protein synthesis contributes to determine PI sensitivity by saturating the proteasomal degradative capacity. Quantitating protein synthesis and proteasome workload may thus prove crucial to design novel negative proteostasis regulators against cancer.