As solid tumors outgrow their oxygen supply, regions with abnormal vasculature become poorly oxygenated, known as intratumoral hypoxia. Hypoxia-inducible factor 1α (HIF1α) is the main mediator to hypoxia and promotes angiogenesis, cell survival as well as cancer metastases. Overexpression of HIF1α is correlated with poor prognosis in many malignancies such as colorectal cancers. Targeting HIF1α is a potential strategy for cancer therapy. 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., 2013). It has been acknowledged that the heat shock protein 90 (HSP90) is also a VHL-independent HIF1α stabilizer (Isaacs JS et al., 2002). We found that CDK1 may contribute to HSP90-mediated HIF1α stabilization. Combination treatment of HSP90 inhibition and CDK1 knockdown reduces HIF1α expression in hypoxia more robustly than either single treatment. Dual inhibition of CDK1 and HSP90 synergistically decreases cell viability in colorectal cancer cells and suppresses colony formation.

We have observed that CDK4 plays a role in HIF1α stabilization. Using the FDA-approved CDK4/6 inhibitors (palbociclib & abemaciclib), we observed that dual inhibition of CDK4 and HSP90 synergistically inhibits cancer viability in colorectal cancer cell lines (eg. HCT116, SW480, DLD1) under both normoxia and hypoxia. Multiple HSP90 inhibitors have been tested, including ganetespib, onalespib, XL888 and TAS116, which indicates such combinational inhibition as a class effect. As expected, the combination treatment of CDK4 knockdown/inhibition and HSP90 inhibition is able to reduce the level of HIF1α and suppress cell viability in various cancer cell lines (eg. colorectal, glioblastoma, breast, and prostate cancers). The combination treatment induces PARP cleavage, indicating activated apoptosis. Interestingly, we found that knockdown of RB does not affect the response of HCT116 or SW480 cells to the combination treatment with abemaciclib and TAS116, although RB deficiency contributes to a relative resistance to CDK4/6 inhibitor monotherapy. Combination of abemaciclib plus TAS116 inhibits cell viability in Rb-deficient cells (eg. H1048 and Saos2). Hypoxia sensitizes the Rb-deficient MDA-MB-468 breast cancer cell line to abemaciclib treatment. Our findings suggest a therapeutic potential for utilizing the combination of CDK4 and HSP90 inhibitors in cancer treatment.

Citation Format: Shuai Zhao, Lanlan Zhou, David T. Dicker, Wafik S. El-Deiry. Anti-tumor effect and HIF1α inhibition by combining CDK4 inhibitor with HSP90 inhibitor in various cancer types including Rb-deficient tumor cells [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3564.