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

Abstract

A previously unsuspected, considerable proportion of newly synthesized polypeptides are hydrolyzed rapidly by proteasomes, possibly competing with endogenous substrates and altering proteostasis. In view of the anti-cancer effects of PIs, we set out to achieve a quantitative assessment of proteasome workload in cells hallmarked by different PI sensitivity, namely, a panel of 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-versus-capacity balance found in highly PI-sensitive MM lines is accounted for by a decreased total number of immunoproteasomes/cell coupled to enhanced generation of RDPs. Moreover, indicative of cause-effect relationships, attenuating general protein synthesis by the otherwise toxic agent CHX reduces PI sensitivity 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.

A mouse model recapitulating human monoclonal heavy chain deposition disease evidences the relevance of proteasome inhibitor therapy

“…Accumulating evidence has linked protein synthesis to the responsiveness of proteasome inhibitors in multiple myeloma. [55][56][57][58] Moreover, proteasome inhibitors were shown to trigger the protein kinase RNA-like endoplasmic reticulum kinase-dependent branch of the unfolded protein response and CHOP, leading to a terminal ER stress response due to accumulation of unfolded proteins. [57][58][59] Here, we demonstrated that isolated truncated gHCs are poorly secreted by plasma cells, which in turn, are intrinsically and basically stressed as observed by overexpression of CHOP and XBP1s.…”

A plastic SQSTM1/p62-dependent autophagic reserve maintains proteostasis and determines proteasome inhibitor susceptibility in multiple myeloma cells

“…5,6 Briefly, cells were sonicated in icecold buffer (50 mM Tris-HCl [pH 7.5], 1 mM DTT, 0.25 M sucrose, 5 mM MgCl 2 , 0.5 mM EDTA, 2 mM ATP), and extracts were prepared by centrifugation (30 min 10,000 g, and 15 min 100,000 g). Proteasome-specific chymotryptic peptidase activity was assayed by monitoring the production of 7-amino-4-methylcoumarin (amc) from 100 mM Suc-LLVY-amc (Bachem, I-1395) in 20 mM Tris-HCl (pH 7.5), 1 mM ATP, 2 mM MgCl 2 , 0.2% bovine serum albumin.…”

“…4 This load-versus-capacity model contributes to explain PI sensitivity in malignant PCs, as myelomas with high proteasome capacity and low proteasome workload show relative primary resistance. 5 Indicative of cause-effect relationships, both increasing proteasome expression 5 and attenuating protein synthesis 6 specifically reduced vulnerability of myeloma cells to the first-in-class PI bortezomib.…”

VR23: A Quinoline–Sulfonyl Hybrid Proteasome Inhibitor That Selectively Kills Cancer via Cyclin E–Mediated Centrosome Amplification

“…High levels of proteasomes were also observed in acute lymphocytic leukemia, T-cell leukemia, and acute myelocytic leukemia cells (13). It is thought that the high level of proteasome activity in cancer cells provides survival benefits under the stress of continued cell proliferation (14,15).…”