SUMMARY mTORC1 promotes cell growth in response to nutrients and growth factors. Insulin activates mTORC1 through the PI3K-Akt pathway, which inhibits the TSC1-TSC2-TBC1D7 complex (the TSC complex) to turn on Rheb, an essential activator of mTORC1. However, the mechanistic basis of how this pathway integrates with nutrient-sensing pathways is unknown. We demonstrate that insulin stimulates acute dissociation of the TSC complex from the lysosomal surface, where subpopulations of Rheb and mTORC1 reside. The TSC complex associates with the lysosome in a Rheb-dependent manner, and its dissociation in response to insulin requires Akt-mediated TSC2 phosphorylation. Loss of the PTEN tumor suppressor results in constitutive activation of mTORC1 through the Akt-dependent dissociation of the TSC complex from the lysosome. These findings provide a unifying mechanism by which independent pathways affecting the spatial recruitment of mTORC1 and the TSC complex to Rheb at the lysosomal surface serve to integrate diverse growth signals.

The class I phosphoinositide 3-kinase (PI3K) - mechanistic target of rapamycin complex 1 (mTORC1) signaling network directs cellular metabolism and growth. Activation of mTORC1, which is composed of mTOR, Raptor, mLST8, PRAS40, and DEPTOR, depends on the Rag and Rheb GTPases, and requires signals from amino acids, glucose, oxygen, energy (ATP), and growth factors (including cytokines and hormones such as insulin). Here we discuss the signal transduction mechanisms through which growth factor-responsive PI3K signaling activates mTORC1. We focus on how PI3K-dependent activation of Akt and spatial regulation of the TSC complex (composed of TSC1, TSC2, and TBC1D7) switches on Rheb at the lysosome, where mTORC1 is activated. Integration of PI3K- and amino acid-dependent signals upstream of mTORC1 at the lysosome is detailed in a working model. A coherent understanding of the PI3K-mTORC1 network is imperative as its dysregulation has been implicated in diverse pathologies including cancer, diabetes, autism, and aging.

Flux through metabolic pathways is inherently sensitive to the levels of specific substrates and products, but cellular metabolismis also managed by integrated control mechanisms that sense the nutrient and energy status of a cell or organism. The mechanistic target of rapamycin (mTOR) complex 1 (mTORC1), a protein kinase complex ubiquitous to eukaryotic cells, has emerged as a critical signaling node that links nutrient sensing to the coordinated regulation of cellular metabolism. Here, the role of mTORC1 as a conduit between cellular growth conditions and the anabolic processes that promote cell growth is discussed. The emerging network of signaling pathways by which mTORC1 integrates systemic signals, in the form of secreted growth factors, with local signals, in the form of cellular nutrients (amino acids, glucose, and oxygen) and energy (ATP) is detailed. Our expanding understanding of the exquisite regulatory network upstream of mTORC1 provides molecular insights into the integrated sensing mechanisms by which diverse cellular signals converge to control cell physiology.