Abstract LB-109: Therapeutic targeting of Heat shock factor 1 in chronic lymphocytic leukemia

Abstract

Abstract Heat shock factor 1 (HSF1) is a stress-activated transcription factor, that up-regulates the transcription of heat shock proteins (Hsps) and a growing list of tumor-promoting genes. HSF1-induced genes promote protein folding and maturation (through Hsps and associated co-chaperones) as well as orchestrate the transcription of tumor-promoting genes (for e.g., VEFGR, HIF1-alpha, GLUT-1, etc). In this study we propose to evaluate the efficacy of inhibiting HSF1 in Chronic Lymphocytic Leukemia (CLL), a disease that invariably relapses after patients show complete or partial response to first line chemotherapy. Consistent with the established tumor-promoting roles of HSF1, we report that CD19+ B cells isolated from relapsed/refractory CLL patients show high levels of immunofluorescent, nuclear HSF1 staining compared to normal CD19+ B cells, where it is weakly expressed and pre-dominantly cytosolic. CD19+ B primary CLL cells also show enhanced expression of HSF1 as assessed by immunoblot analysis compared to normal CD19+ cells. Treatment of cultured and primary CLL CD19+ B cells with the HSF1 inhibitor triptolide causes a dose-dependent increase in apoptosis, while relatively sparing normal CD19+ B cells. Treatment with triptolide results in the reduced expression of Hsp40, Hsp70 and Hsp27 proteins and a concomitant increase in the PARP cleavage and Caspase 3 cleavage in cultured and primary CLL cells. Lentiviral shRNA-mediated knockdown of HSF1 or triptolide treatment decreased the levels of Bcl-XL and pro-survival signaling molecules, phosphorylated STAT3 (Y-705) and AKT (S473). Based on the established role of Hsps in cytoprotection and resistance to chemotherapy, we hypothesized that inhibition of HSF1 would sensitize CLL cells to agents that promote the accumulation of misfolded proteins (for e.g. Hsp90 inhibitors) or inhibit their disposal (for e.g. treatment with autophagy inhibitors) by inducing proteotoxicity. Consistent with this hypothesis, knockdown of HSF1 or co-treatment with triptolide and the autophagy inhibitor chloroquine resulted in superior anti-CLL activity compared to treatment with either agent alone. Similarly, knockdown of HSF1 or its inhibition with triptolide also sensitized CLL cells to the Hsp90 inhibitor, 17-Dimethylaminoethylamino-17-demethoxygeldanamycin (17-DMAG). These pre-clinical studies provide a strong rationale to test the in vitro and in vivo activity of HSF1 inhibitors in combination with agents that promote proteotoxic stress against CLL. Citation Format: Rekha M. Rao, Yacoub Abdulraheem, Suman Kambhampati, Siddhartha Ganguly. Therapeutic targeting of Heat shock factor 1 in chronic lymphocytic leukemia. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr LB-109. doi:10.1158/1538-7445.AM2014-LB-109