Multiple myeloma is a plasma cell malignancy affecting more than 50,000 people in the United States. The disease remains incurable despite significant recent advances in treatment, including the immunomodulatory agents, thalidomide and lenalidomide, and the proteasome inhibitor, bortezomib. Thus the search for additional agents is warranted. We have previously shown that WP1130, a tyrphostin-like compound, has activity in a number of hematological malignancies including CML, mantle cell lymphoma and multiple myeloma. In this study, we investigated the activity and mechanism of action of WP1130 on multiple myeloma cells and compared these activities to bortezomib. Both WP1130 and bortezomib demonstrated significant cytotoxicity in MM1S cells with IC50s of 1 mM and 2 nM, respectively and both compounds induced similar levels of apoptosis (67% vs 73% cell death with WP1130 and bortezomib, respectively). Both WP1130 and bortezomib increased cellular ubiquitination levels in distinct cellular compartments but we were unable to detect 20S proteasome inhibition by WP1130. The effect of WP1130 on signaling modules in MM cells was also investigated. WP1130 altered the subcellular distribution of a subset of tyrosine kinases, including Jak2 and Syk which were irreversibly displaced from the cytoplasm to detergent insoluble complexes within the cell. Although Jak2 remained stable in this fraction, Syk was subject to proteolysis and destruction in this compartment. WP1130-induced translocation of these kinases through association with the cytoskeletal network and was associated with loss of cytokine and B-cell receptor signal transduction. WP1130 did not alter the cellular distribution of other tyrosine kinase or phosphoproteins including Lyn, STAT3, STAT5, PI3K, AKT, ERK, suggesting some selective protein translocation and signal modulation by WP1130. These results support a novel mechanism of protein trafficking, selective kinase destruction and apoptosis by WP1130 in MM cells.

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