A Parallel Path to TOR

Abstract

The mammalian target of rapamycin (mTOR) signaling pathway, which regulates protein synthesis and thereby cell growth and proliferation, is modulated both by amino acid availability and by insulin. Insulin regulates mTOR activity through a pathway that involves activation of class 1 phosphatidylinositol 3-kinase (PI3K) and the subsequent activation of PKB (protein kinase B), which phosphorylates and inactivates the tuberous sclerosis complex proteins 1 and 2 complex (TSC1/2), enabling the small GTPase (guanosine triphosphatase) Rheb to stimulate TOR signaling. It has not been clear, however, how information about nutrient supply regulates mTOR signaling. Nobukuni et al. found that, whereas amino acids, unlike insulin, had no effect on TSC2 phosphorylation on serine 939 (TSC2 S939), TSC2 abundance, or PKB activation, they stimulated phosphorylation of S6K1 (S6 kinase 1, a substrate downstream of mTOR) T389. Further, amino acid depletion inhibited S6K1 phosphorylation even in TSC2-deficient cells with enhanced Rheb activation. Although the PI3K inhibitor wortmannin inhibited amino acid-stimulated S6K1 phosphorylation, as did rapamycin, siRNA against class 1 PI3Kα did not. Amino acids stimulated the in vitro activity of immunoprecipitated class 3 PI3K hVps34 and the abundance in mouse embryo fibroblasts of its product, phosphatidylinositol 3-phosphate (PI3P). HEK293 cells transfected with GST-epitope-tagged hVps34 showed a dose-dependent increase in amino acid-mediated S6K1 phosphorylation, whereas hVps34 knockdown decreased this response, as did knockdown of hVps15 (which associates with and stimulates hVps34) or expression of a construct that competes for PI3P docking sites. Thus, the authors conclude that amino acids regulate TOR signaling through a pathway involving hVps34 that is parallel to that activated by insulin. T. Nobukuni, M. Joaquin, M. Roccio, S. G. Dann, S. Y. Kim, P. Gulati, M. P. Byfield, J. M. Backer, F. Natt, J. L. Bos, F. J. T. Zwartkruis, G. Thomas, Amino acids mediate mTOR/raptor signaling through activation of class 3 phosphatidylinositol 3OH-kinase. Proc. Natl. Acad. Sci. U.S.A. 102 , 14238-14243 (2005). [Abstract] [Full Text]