Abstract 5088: Targeting the mTORC2/HIF-2alpha/p53 pathway in clear cell renal cell carcinoma.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Inactivating mutations in the von Hippel Lindau (VHL) tumour suppressor gene leads to both hereditary and sporadic clear cell renal cell carcinoma (ccRCC). Loss of VHL function results in stabilisation of hypoxia inducible factors 1 and 2 (HIF-1α and HIF-2α) with HIF-2α critical for renal tumour progression, angiogenesis and resistance to radiotherapy and chemotherapy. Previous work in VHL defective RCC cell lines that express HIF-2α has shown that loss of HIF-2α, either with siRNA or by reintroduction of VHL, increases phosphorylation and stabilisation of p53 which restored sensitivity to radiation and chemotherapy. Furthermore, as recent evidence suggests that mTORC2 is required for HIF-2α protein synthesis in VHL defective cells, we investigated the effects of pharmacological inhibition of HIF-2α with the mTORC1/2 kinase inhibitor pp242 alone or in combination with DNA damaging agents. Phosphorylated p53 and total p53 levels were significantly elevated in VHL competent RCC cells lacking HIF-2α as compared with VHL defective cells, confirming a role for HIF-2α mediated inhibition of p53. We analysed the effects of the mTORC1/2 kinase inhibitor pp242 on HIF-2α protein levels by Western blot. pp242 inhibited phosphorylation of both mTOR and its substrate p70S6K and significantly reduced protein levels of HIF-2α as compared with the mTORC1 inhibitor rapamycin, which had no effect on HIF-2α. Since the DNA damaging agent camptothecin (CPT) is a potent inhibitor of HIF-1α we also determined its effects on HIF-2α expression. CPT reduced HIF-2α protein levels in VHL defective RCC cells and increased p53 stabilisation, which was significantly augmented in VHL competent cells following complete loss of HIF-2α. Consistent with its inhibition of the p53 pathway, HIF-2α prevented both spontaneous and CPT induced apoptosis of RCC cells as demonstrated by FACS analysis of cells in sub G1 and Western blot analysis of cleaved PARP. Taken together these results confirm that HIF-2α negatively regulates p53, confers resistance to DNA damage induced apoptosis in VHL defective RCC cell lines and that further studies investigating the pharmacological inhibition of HIF-2α mediated by mTORC1/2 kinase inhibitors alone or in combination with DNA damaging agents for the treatment of ccRCC are warranted. Citation Format: Jogitha Selvarajah, Abdeladim Moumen, Veronica A. Carroll. Targeting the mTORC2/HIF-2alpha/p53 pathway in clear cell renal cell carcinoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5088. doi:10.1158/1538-7445.AM2013-5088