Ubiquitin receptor PSMD4/Rpn10 is a novel therapeutic target in multiple myeloma

PSMD4/Rpn10 is a subunit of the 19S proteasome unit that is involved with feeding target proteins into the catalytic machinery of the 26S proteasome. Since proteasome inhibition is a common therapeutic strategy in multiple myeloma (MM), we investigated Rpn10 and found that it is highly expressed in MM cells versus normal plasma cells. Rpn10 levels inversely correlated with overall survival in MM patients. Inducible knockout or knockdown of Rpn10 decreased MM cell viability both in vitro and in vivo by triggering the accumulation of polyubiquitinated proteins, cell cycle arrest, and apoptosis associated with activation of caspases and unfolded protein response-related pathways. Proteomic analysis revealed that inhibiting Rpn10 increased autophagy, antigen presentation and the activation of CD4+ T and NK cells. We developed an in vitro AlphaScreen binding assay for high-throughput screening and identified a novel Rpn10 inhibitor, SB699551 (SB). Treating MM cell lines, leukemic cell lines, and primary MM patient cells with SB decreased cell viability without affecting the viability of normal PBMCs. SB inhibited the proliferation of MM cells even in the presence of the tumor-promoting bone marrow milieu and overcame proteasome inhibitor (PI)-resistance without blocking the 20S proteasome catalytic function or the 19S deubiquitinating activity. Rpn10 blockade by SB triggered MM cell death via similar pathways as the genetic strategy. In MM xenograft models, SB was well-tolerated, inhibited tumor growth, and prolonged survival. Our data suggests that inhibiting Rpn10 will enhance cytotoxicity and overcome PI-resistance in MM, providing the basis for further optimization studies of Rpn10 inhibitors for clinical application.