Inhibitors of mTORC1 have proven clinical activity in renal cell carcinoma (RCC), indicating that mTOR is an important signaling hub in this tumor type. mTOR exists in two functionally distinct protein complexes: mTORC1 and mTORC2. mTORC1 regulates multiple cellular functions including protein translation and tumor metabolism While this complex binds rapamycin some aspects of mTORC1 function are rapamycin insensitive. mTORC2 controls distinct cellular functions including survival and is not directly inhibited by rapalogs. In order to evaluate the relative contribution of mTORC1 and mTORC2 to the survival of RCC cells, we compared the effects of rapamycin, an allosteric inhibitor of mTORC1, to OSI-027, a selective catalytic-site inhibitor of mTORC1/mTORC2.

In RCC cell lines, OSI-027 provided greater maximal inhibition of proliferation than rapamycin, with reduction of cell viability to less than baseline in 5 of 12 cell lines, however caspase 3/7-dependent apoptosis was observed only in one cell line. We identified a subset of RCC cell lines which were sensitive to OSI-027 but not rapamycin, and another set which was relatively sensitive to both. Comparison of gene expression profiles of these groups identified MAP1LC3, encoding LC3, a critical effector of autophagy, as significantly upregulated in cell lines which were preferentially sensitive to OSI-027. We therefore sought to further define the role of autophagy in response to OSI-027 treatment.

Autophagy is a catabolic process by which cells consume proteins and organelles to promote survival under adverse conditions, and which can result in cell death. Both mTORC1 and mTORC2 have been shown to negatively regulate autophagy, and inhibition of mTOR has been shown to induce formation of autophagic vesicles. Treatment in vitro with OSI-027 resulted in the conversion of LC3 to the lipidated form LC3-II as well as accumulation of LC3 in autophagosomes, robust indicators of autophagy. Increased autophagosome content occurred in a dose-dependent manner and is inversely proportional to viable cell number although induction of apoptosis was not observed, implying that in these cells OSI-027-induced autophagy leads to complete growth arrest or apoptosis-independent cell death. OSI-027-mediated induction of autophagy occurred in the majority of RCC cell lines tested, whereas little or no autophagy resulted from rapamycin treatment. In xenograft tumors sensitive to OSI-027 but not rapamycin, we analyzed induction of LC3-II and apoptosis in tumor lysates. Consistent with our observations in vitro, we observed that treatment with OSI-027 but not rapamycin resulted in induction of autophagy in vivo, while neither compound induced apoptosis. Together these data indicate that autophagy may play a key role in mediating OSI-027 but not rapamycin efficacy and supports further exploration of the utility of dual mTORC1/mTORC2 inhibitors for the treatment of renal cell carcinoma.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4463. doi:10.1158/1538-7445.AM2011-4463