Growth factor signaling pathways converge on the TSC complex to control mTOR

Abstract

The mechanistic target of rapamycin complex 1 (mTORC1) serves as a molecular link between cellular growth conditions and anabolic processes that are fundamental to cell growth and proliferation. However, while it is established that the PI3K-Akt pathway regulate mTORC1 through a mechanism that involves spatial control of the TSC complex, it is unknown whether other growth factor signaling pathways do so. The goals of this dissertation were to characterize the mechanisms by which upstream growth factor signaling pathways converge on the TSC complex to activate mTORC1, and to determine the significance of these mechanisms in disease. I identify that both the Ras-ERK and PKC pathways stimulate mTORC1 signaling through spatial regulation of the TSC complex. Growth stimulus specific to each pathway induces the dissociation of TSC2 from the lysosome, where it interacts with Rheb, and in serum-starved conditions, keeps Rheb in its GDP-bound form, unable to activate mTORC1. Using antibodies to specific TSC complex components, I further demonstrate that it is the intact TSC complex that dissociates from the lysosome upon growth stimuli treatment. Furthermore, suppression of both ERK and PKC signaling using multiple small molecule inhibitors specific to each pathway, reverses the growth stimuli-mediated TSC2 dissociation. Given previous models on ERK-mediated TSC complex regulation of mTORC1 activity, I sought to gain more mechanistic details surrounding this regulation. I specifically show that signaling through EGF and PMA does not affect TSC complex stability, but rather regulate the complex through effects on lysosomal localization and Rheb binding. Finally, I discuss how the TSC complex spatial regulation mechanism may be useful in predicting sensitivity or resistance of cell models to targeted therapeutics. Collectively these studies identify a possible general mechanism by which upstream growth factor-regulated kinases activate mTORC1 through the stimulated release of the TSC complex from lysosomal Rheb. Our results advance our understanding of how diverse upstream molecular inputs can affect the physiological and oncogenic activation of mTORC1, and will hopefully guide future work in studying resistance to targeted therapeutics.