Exploiting the Regulation of Apoptosis to Improve Therapeutic Responses of Tumors to mTOR Inhibitors

Abstract

The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) is constitutively activated in most human cancers and in the genetic tumor syndrome tuberous sclerosis complex (TSC). Due to its role in driving anabolic processes to support proliferation, mTORC1 should be an effective pharmacological target for the elimination of tumors in these settings. However, current strategies using mTORC1 inhibitors, such as the allosteric inhibitor rapamycin (sirolimus), have been met with limited clinical success. While rapamycin can induce partial tumor shrinkage, there is no evidence of cell death within tumors and rapid regrowth occurs upon treatment withdrawal. Although mTORC1's role in anabolic metabolism has been well characterized in the context of human malignancies, its control of cell death and survival mechanisms is still not fully understood. Since the balance of pro-apoptotic and pro-survival proteins in a cell is a critical determinant of cell death decisions, the goals of this dissertation were to explore the regulation of the apoptotic machinery in tumors with chronic mTORC1 activation and identify an improved therapeutic approach that induces tumor cell death in TSC and cancer. Using cell culture models, I found that several BCL-2 family members are regulated by mTORC1 activation and inhibition. The downregulation of MCL-1 and upregulation of BCL-2 upon mTORC1 inhibition was consistently observed across cell models of TSC, leading to the exploration of reciprocal therapeutic sensitivities to BH3 mimetics that directly inhibit these pro-survival proteins. I demonstrated that genetic activation of mTORC1 through loss of the TSC tumor suppressors, TSC1 or TSC2, created a susceptibility to MCL-1 inhibitors, while the inhibition of mTORC1 with rapamycin reciprocally led to a susceptibility to BCL-2/BCL-XL inhibitors. I found that the combination of rapamycin and the BCL-2/BCL-XL inhibitor ABT-263 could induce selective apoptosis in cell models of TSC and TSC1/2-deficient cancers. Furthermore, I demonstrate that the downregulation of MCL-1 is a critical determinant of this response. Finally, I found that ABT-263 enhances the efficacy of rapamycin in a syngeneic mouse tumor model of TSC, leading to more pronounced tumor regression and more durable response upon treatment cessation.Together, these data provide insight into the control of cell death and survival by mTORC1 and improve our understanding of the mechanisms supporting cell survival in the presence of rapamycin and its analogs, which are in widespread clinical use. Finally, this dissertation identified a targetable vulnerability in the intrinsic apoptosis pathway, which has broad implications for more effective anti-tumor therapy in patients with TSC and cancer.