The mechanistic target of rapamycin complex 1 (mTORC1) is a central regulator of cell growth and proliferation that is often aberrantly activated in cancer, as it is downstream of several commonly mutated factors such as PTEN, PI3K, and Ras. Although there is interest in using mTORC1 inhibitors, such as rapamycin, to treat cancers with high mTORC1 signaling, their effects are mostly cytostatic. Patient responses to rapamycin and its analogs (rapalogs) vary, and there is not a good understanding of the mechanistic basis for sensitivity. Increasing evidence suggest that feedback mechanisms and downstream effectors of mTORC1, which in healthy cells maintain a homeostatic balance, act in cancer to override the anti-tumor effects of mTORC1 inhibition. To target effectively the mTOR signaling network in cancer, the action of kinases regulated downstream of mTORC1 must be fully characterized. A recent screen completed by the lab to identify downstream transcriptional targets of mTORC1 has identified an interesting class of six protein or lipid kinases whose expression are repressed by mTORC1 and strongly induced by rapamycin treatment. One of these kinases, PIM3, is of particular interest due to its classification as a proto-oncogene and it role in cell survival through inhibition of the pro-apoptotic protein BAD. Studying PIM3 and other kinases inhibited downstream of mTORC1 will allow identification of new downstream effectors of mTORC1 and characterize targetable cell survival mechanisms that are activated upon treatment with mTOR inhibitors. These findings will be useful for developing combination therapies for cancer patients with cytostatic responses to mTOR inhibitors, since it is likely that aberrant regulation of kinases downstream of mTORC1 is acting to drive tumorigenesis by activating cell survival pathways upon mTORC1 inhibition. The goals of this research proposal are: (1) To define the mechanism of PIM3 repression by mTORC1, by characterizing the transcriptional regulation of PIM3 by mTORC1 and then determining the mechanism of regulation of candidate transcription factors by mTORC1;(2) To determine the effectiveness of combining mTORC1 and PIM3 inhibition in cancer using cell culture and mouse xenograft models;(3) To characterize additional kinases induced by mTORC1 inhibition by investigating the regulation of HIPK3, GRK6, NEK7, PIK3CA, and RYK by mTORC1, and determining their role in cell survival mechanisms in cancer. The completion of this proposal will provide insights into new downstream targets of mTORC1 and characterize a class of kinases inhibited, rather than activated, by mTORC1, a relatively unexplored area. Furthermore, the characterization of PIM3 and other kinases as potential targets for combination therapies with mTOR inhibitors will indicate further avenues for study, potentially increasing the effectiveness of mTOR inhibitors in a variety of cancers.
The protein mTOR (mechanistic target of rapamycin) is highly activated in many types of cancer, but mTOR inhibitors such as rapamycin do not usually kill cancer cells;they typically only stop cell growth. It is therefore imperative to understand cll survival mechanisms activated by these drugs in order to develop better treatments for cancers with high mTOR activity. The experiments set forth in this proposal will characterize proteins such as PIM3 that are activated when cancer cells are treated with rapamycin or other mTOR inhibitors, in order to identify targets for combination drug therapies that will more effectively ill cancer cells.