Expression of Heat Shock Protein 90 and the Effects of HSP90 Inhibitor (17-AAG) in Multiple Myeloma.

Heat Shock Protein 90 (HSP90) is required for structural folding and maintenance of conformational integrity of various proteins, including several associated with cellular signaling. Recent studies utilizing 17-allylamino-17-demethoxygeldanamycin (17-AAG), an inhibitor of HSP90, demonstrated antitumor effect in solid tumors. In order to test whether HSP90 could be targeted in multiple myeloma (MM) patients, we first investigated expression of HSP90 by immunofluorescence and flow cytometric analysis in a myeloma cell line (U266) and primary myeloma cells. Following demonstration of HSP90 expression in myeloma cells, archival samples of 32 MM patients were analyzed by immunoperoxidase staining. Myeloma cells in all patients showed strong cytoplasmic expression of HSP90 in all samples and 55 % also demonstrated concurrent nuclear immunopositivity. Treatment of U266 and primary MM cells with 17AAG resulted in significantly increased apoptosis compared to untreated control cells. Analysis of anti-apoptotic BCL2 family proteins in MM cells incubated with 17-AAG revealed down-regulation of BCL-2, BCL-X_L and MCL-1. Furthermore, while low concentration of bortezomib had no cell death, combination of 17AAG and bortezomib treatment revealed a synergistic apoptotic effect on the U266 cell line. These data suggest that targeted inhibition of HSP90 may prove to be a valid and innovative strategy for the development of future therapeutic options for MM patients.