Paradoxical role of the 19 S proteasomal regulator 1 Paradoxical resistance of multiple myeloma to proteasome inhibitors 1 by decreased levels of 19 S proteasomal subunits 2 3

29 30 Paradoxical role of the 19S proteasomal regulator 3 ABSTRACT 31 Hallmarks of cancer, including rapid growth and aneuploidy, can result in non-32 oncogene addiction to the proteostasis network that can be exploited clinically. The 33 defining example is the exquisite sensitivity of multiple myeloma (MM) to 20S 34 proteasome inhibitors, such as carfilzomib. However, MM patients invariably acquire 35 resistance to these drugs. Using a next-generation shRNA platform, we found that 36 proteostasis factors, including chaperones and stress-response regulators, controlled the 37 response to carfilzomib. Paradoxically, 19S proteasome regulator knockdown induced 38 resistance to carfilzomib in MM and non-MM cells. 19S subunit knockdown did not 39 affect the activity of the 20S subunits targeted by carfilzomib nor their inhibition by the 40 drug, suggesting an alternative mechanism, such as the selective accumulation of 41 protective factors. In MM patients, lower 19S levels predicted a diminished response to 42 carfilzomib-based therapies.