Constitutive protein degradation induces acute cell death via proteolysis products

Abstract

Modulation of proteolysis is an emerging therapeutic mainstay. The clinical success of thalidomide and analogs has inspired development of rationally-designed therapeutics that repurpose endogenous degradation machinery to target pathogenic proteins. However, it is unknown whether target removal is the critical effect that drives degrader-induced efficacy. Here we report that proteasome-generated peptides actively initiate degrader-induced cell death. Utilizing BET family degraders as exemplars, we find that induced proteasomal degradation of the BRD4-long isoform (BRD4-L) generates neo-amino-terminal peptides that neutralize Inhibitor of Apoptosis (IAP) proteins to precipitate cell death. Depletion of BRD4-L paradoxically suppresses caspase activation induced by numerous BET degraders. An unbiased screen revealed that other degrader compounds, including clinical CELMoDs, rely on the same mechanism to potentiate caspase activation and apoptosis. Finally, in the context of constitutive immunoglobulin proteostasis within multiple myeloma cells, we report that therapeutic proteasomal protease inhibition alters the peptide repertoire to neutralize IAPs, thus contributing to the clinical efficacy of bortezomib. Together, these findings clarify the counterintuitive clinical benefit achieved by combining thalidomide analogs with proteasome inhibitors. Our study reveals a previously unrealized pro-apoptotic function of the peptides generated by a variety of proteolysis-modulating compounds, that provide design considerations to maximize therapeutic benefit.

The Molecular Architecture of Variable Lifespan in Diversity Outbred Mice

“…The death of cells with high proteosomal factors could result from cells being overwhelmed by protein aggregates which interfere with proteosomal function 79 . Alternatively, continued activation of the proteasome may induce apoptosis through neutralization of IAP (inhibitor of apoptosis) in a stressed cell population 80 . While this mechanism may guard against sporadic failures of individual cells who can "pass the baton" to healthy cells, it could cascade into organism failure when a population of cells makes a concerted commitment to apoptosis.…”