Inhibition of Heat Shock Factor 1 (HSF1) Is More Effective At Sensitizing Myeloma Cells to Bortezomib Than Inhibition of Individual HSF1 Targets.

Abstract 2953

Multiple myeloma is a plasma cell disorder with an average incidence of 21,000 new cases per year in the United States. Recent advances in therapeutic approaches such as the use of proteasome inhibitors have resulted in a significant increase in the overall survival of myeloma patients. Myeloma cells maintain many of the characteristics of normal plasma cells, including constitutive immunoglobulin production and secretion, therefore management of ER stress plays a role in myeloma cell sensitivity to proteasome inhibition. However, myeloma cells also upregulate protective genes in response to the proteotoxic stress that can limit the therapeutic response. Previous groups have published on the importance of the heat shock response and the heat shock protein (HSP) family, supporting preclinical and clinical exploration of HSP inhibition in myeloma.

Our group had interest in regulation of the HSP response and has evaluated the master regulator HSF1 as a potential therapeutic target. We found that siRNA-mediated silencing of HSF1 enhances bortezomib-induced apoptosis in a myeloma cell line. To define the effectors of the heat shock response important in regulating bortezomib response, we determined which heat shock response genes are induced by bortezomib in an HSF1-dependent manner. From a realtime PCR array of 84 HSP family genes, we found 21 genes that were induced greater than 2-fold by bortezomib. Of these 21 genes, 10 genes showed >50% reduction in HSF1-silenced cells. 7/10 genes were confirmed by independent qRT-PCR and western blot analysis. These genes include: CRYAB (alpha-crystallin B chain), DNAJB1 (HSP40 subfamily B), HSPA1A (HSP70-1A), HSPA1B (HSP70-1B), HSPB1 (HSP27), HSPH1 (HSP105/110), and HSP90AB1 (HSP90b1).

To begin to determine which of these genes was important for the HSF1-dependent protective response we silenced the 7 genes individually and subsequently treated the cells with bortezomib. Surprisingly only 1 of the 7 genes silenced individually, DNAJB1, had an observable effect on bortezomib-induced death. However DNAJB1 silencing does not account for all the HSF1 activity as the increase in cell death due to bortezomib is only 48% of that observed with HSF1 silencing. Thus targeting HSF1 is more effective at sensitizing multiple myeloma cells to bortezomib-induced apoptosis than targeting individual HSPs. Moreover these data suggest that HSP90 inhibitors are functioning by inhibiting at least two members of this family to be effective as single agents. Therefore, while clinical trials for individual HSP and HSP in combination with bortezomib are being conducted, a more effective strategy for apoptosis induction is achieved through inhibition of HSP regulators such as HSF1 in combination with bortezomib. These results provide support for investigating HSP regulation in response to PI to increase the efficacy of myeloma therapy.