Abstract A43: Opposing roles of the 19S regulatory- and 20S core-proteasomal subunits in controlling sensitivity of multiple myeloma cells to proteasome inhibition.

Multiple myeloma (MM) remains a devastating incurable disease. Because of their secretory nature, MM cells heavily rely on proteostasis networks to maintain homeostasis. Indeed, paraprotein production by MM cells imposes a protein-folding burden that provides an opportunity for therapeutic intervention: because protein production in MM cells can exceed the buffering capacity of the cell for protein degradation, they are exquisitely sensitive to proteasome inhibitors. While successful, standard-of-care proteasome inhibitors, like bortezomib, remain not curative, as patients ultimately become refractory, suggesting the selection of genetic escape routes. Employing an ultra-complex shRNA library, we performed pooled shRNA screens in RPMI-8226, U-266, and JJN-3 MM cells, and in K-562 leukemia cells, and identified genes whose diminished function impact the cell9s response to proteasome inhibition. Unexpectedly, we found that, contrary to knockdown of subunits in the 20S proteasome core-particle, the genetic ablation of subunits within the 19S regulatory-particle led to partial desensitization towards bortezomib. Proteome-wide analyses of in vivo ubiquitylated sites in K562 cells, in which we constitutively ablated a component of the 19S regulatory-particle, revealed potential candidate substrates that were differentially degraded in knockdown cells. We are investigating the mechanistic basis for this differential degradation to pinpoint key substrates mediating proteasome inhibitor resistance. Our results reveal potential escape routes in MM tumor cells to proteasome inhibitors, suggesting new promising targets to pre-empt resistance. Citation Format: Diego Acosta-Alvear, Martin Kampmann, Min Y. Cho, Thomas Wild, Chunaram Choudhary, Marc A. Shuman, Jonathan S. Weissman, Peter Walter. Opposing roles of the 19S regulatory- and 20S core-proteasomal subunits in controlling sensitivity of multiple myeloma cells to proteasome inhibition. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr A43.