Abstract 4516: Dual inhibition of CDK1 and HSP90 destabilizes HIF1α and synergistically induces cancer cell death (2017)

Abstract Solid tumors are often characterized by intratumoral hypoxia. Hypoxia-inducible factor 1α (HIF1α) has been implicated in cell proliferation, survival, angiogenesis, invasion and migration. The expression of HIF-1α is increased in a variety of cancers but this is not restricted to hypoxic regions. We have previously shown that cyclin-dependent kinase 1 (CDK1) stabilizes HIF1α through direct phosphorylation of its Ser668 residue in a Von Hippel-Lindau (VHL)-independent manner both under hypoxia and at G2/M under normoxia (Warfel et al., Cell Cycle, 2013). Heat shock protein 90 (HSP90) is also an acknowledged VHL-independent HIF1α stabilizer. We sought to explore the link between CDK1-mediated and HSP90-mediated HIF1α stabilization. Administration of the CDK1 inhibitor, Ro-3306, reverses the heat shock (40°C)-increased level of HIF1α in normoxia suggesting that CDK1 activity may contribute to HSP90-mediated HIF1α stabilization. Under hypoxia, combination treatment with HSP90 inhibitor geldanamycin and CDK1 inhibition/knockdown decreases HIF1α levels more robustly than either treatment alone. This led us to investigate whether combined CDK1 and HSP90 inhibition results in synergistic anti-cancer effects. Indeed, dual inhibition of CDK1 and HSP90 synergistically decreases HCT116 colon cancer cell viability. PARP cleavage and sub-G1 analysis indicate the combinational treatment triggers cancer cell death through apoptosis. Consistent with these findings, we observed that inhibition of cell viability by the combination is impaired in HCT116 Bax-/- cells. Furthermore, the dual inhibition also suppresses cell migration in vitro. We further validated our results with CDK1 and HSP90 inhibitors that have entered clinical trials. Second-generation HSP90 inhibitor ganetespib and Ro3306 synergistically inhibit cell viability in HCT116 cells. We have previously demonstrated that CDK4 also contributes to the stabilization of HIF1α under hypoxia (Warfel et al., Cell Cycle, 2013). Interestingly, the combination of CDK4 and HSP90 inhibitors is also able to improve downregulation of HIF1α as compared to either agent alone in various cancer cell lines (eg. colon cancer, gliobastoma, etc). The FDA-approved CDK4 inhibitor palbociclib along with ganetespib synergistically inhibits cell viability in HCT116 cells under hypoxia. Such effect is not observed in WI38 normal cells under normoxia at similar concentrations. Our findings demonstrate that CDK1 inhibition disrupts the interaction between HSP90 and HIF1α providing a rationale for therapeutic targeting of HIF1α through the combination of CDK1/4 and HSP90 inhibitors in cancer. Ongoing studies are examining the effects of CDK1/4 plus HSP90 dual inhibition in other tumor types and in vivo. Citation Format: Shuai Zhao, David T. Dicker, Wafik S. El-Deiry. Dual inhibition of CDK1 and HSP90 destabilizes HIF1α and synergistically induces cancer cell death [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4516. doi:10.1158/1538-7445.AM2017-4516