Signal integration by mTORC1 coordinates nutrient input with biosynthetic output

Abstract

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

Figure 1. mTORC1 signaling links cellular growth conditions with metabolic processes underlying anabolic cell growth and proliferation

Many physiological and pathological signals affect the activation status of mTORC1, including cellular nutrients and energy, growth factors, oncogenes and tumor suppressors, and a variety of intracellular pathogens (i.e., infectious agents). When activated, mTORC1 regulates a number of cellular processes, with those affecting the metabolic state of the cell emphasized in this model. Through various downstream mechanisms, mTORC1 signaling inhibits autophagy, while stimulating mRNA translation, glycolysis, lipid synthesis, the pentose phosphate pathway, and de novo pyrimidine synthesis, thereby promoting the production of energy (ATP), reducing equivalents (NADPH), and the major macromolecules required for cell growth.

Figure 2. Secreted growth factors stimulate mTORC1 activity through the PI3K-Akt and Ras-Erk pathways

Through receptor tyrosine kinases (RTKs), a variety of secreted growth factors stimulate the recruitment and activation of PI3K, via binding to RTKs or scaffolding adaptor proteins. PI3K activity generates phosphoinositide-3,4,5-trisphosphate (PIP3), which recruits Akt to the plasma membrane, where it is activated by upstream kinases (not pictured). Ras is also activated downstream of RTKs and stimulates a kinase cascade leading to the activation of ERK and RSK. In response to growth factors, the Akt, ERK, and RSK protein kinases phosphorylate specific residues on TSC2 (inset) within the TSC complex, thereby negatively regulating the ability of this complex to act as a GAP for Rheb. Consequently, GTP-bound Rheb accumulates and activates mTORC1. Parallel inputs into mTORC1 from these kinases also exist, with Akt phosphorylating PRAS40 and ERK and RSK both phosphorylating residues on Raptor. Oncogenes and tumor suppressors mutated in human cancers and tumor syndromes are indicated (*).

Figure 3. Model of the Rag-dependent recruitment of mTORC1 to the lysosome in response to amino acids and the integration with Rheb-dependent growth factor signaling

(A) Amino acid-stimulated recruitment of mTORC1 to the lysosome. Under amino acid deplete conditions, the Ragulator is not active as a RagA/B GEF and RagA/B^GDP-RagC/D^GTP heterodimers that accumulate are unable to recruit mTORC1 to the lysosome. In the presence of amino acids, which enter the lysosome, an unknown signal or sensor within the lysosomal lumen triggers a conformational change within the V-ATPase that, through direct interactions, promotes the Ragulator GEF activity. In this manner, amino acids stimulate the formation of RagA/B^GTP-RagC/D^GDP heterodimers, which bind directly to mTORC1 and recruit the inactive kinase complex to the lysosomal surface. (B) The amino acid signal allows mTORC1 to come in contact with its essential upstream activator Rheb. In the absence of growth factors, Rheb is maintained in its GDP-bound state through the GAP activity of the TSC complex, and mTORC1 remains inactive. Growth factor signaling inhibits the TSC complex, allowing formation of Rheb^GTP, which binds to and activates mTORC1.

Figure 4. Transcriptional and post-translational regulation of mTORC1 by glucose, oxygen, and cellular energy

Sufficient glucose and oxygen levels are required for mTORC1 activation, and many sensing mechanisms have been identified. Glucose, glutamine, and oxygen are utilized for ATP production via glycolysis, the TCA cycle, and oxidative phosphorylation. Decreased availability of these nutrients can lower ATP levels, with a subsequent rise in AMP levels, conditions that stimulate the activation of AMPK. AMPK inhibits mTORC1 through activation of the TSC complex, which inhibits Rheb, and phosphorylation of Raptor within mTORC1. Glucose or oxygen deprivation, as well as other forms of energy stress, also stimulates the transcription of REDD1 through the action of either the HIF1, ATF4, or p53 transcription factors. REDD1 somehow cooperates with the TSC complex to inhibit Rheb and mTORC1. Through their sensing of AMP and oxygen, respectively, AMPK and the PHD proteins (in yellow) represent the only known direct sensors of cellular metabolic status within this network. In addition to energy stress, glucose and oxygen can also be sensed through ER homeostasis, as they are required for proper protein glycosylation and disulfide bond formation, respectively. Disrupting these processes results in activation of PERK and inhibition of eIF2a, which results in the selective translation of ATF4. Glucose starvation and energy stress also appear to signal to mTORC1 via the Rag GTPases, albeit through unknown mechanisms, and through a pathway involving the p38 and PRAK kinases leading to direct phosphorylation of Rheb. Severe states of ATP depletion inhibit the ability of the TTT-RUVBL1/2 complex to promote formation of functional mTORC1 dimers. Note: mTORC1 is depicted as a single unit at the lysosome for simplicity. Compounds such as 2-deoxyglucose (2-DG), AICAR, and the biguanides metformin and phenformin also have inputs into these different mechanisms of mTORC1 inhibition. Dashed lines denote unknown molecular mechanisms.