Unexpected role for AMPK and mTORC1 in cellular adaptation to nutrient stress

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Project Summary Signaling networks centered on the conserved protein kinases AMPK (AMP-activated protein kinase) and mTOR (mechanistic target of rapamycin) enable cells to sense and respond appropriately to dynamic fluctuations in a variety of systemic and local cues. Energetic stress activates AMPK to promote catabolic and suppress anabolic pathways to balance energy supply and demand. AMPK also promotes glucose and lipid homeostasis. In fact, the AMPK-activating drug metformin (aka, GlucoPhage) represents the most widely prescribed treatment for type II diabetes. Curiously, AMPK paradoxically functions as a tumor suppressor or tumor promoter depending on cellular context. Nutrients (i.e. amino acids) and growth factors (i.e. IGF/insulin) cooperate to activate mTOR complex 1 (mTORC1), which drives anabolic cell metabolism (i.e. protein and lipid synthesis) and suppresses autophagy. Physiologically, aberrant mTORC1 signaling contributes to diverse disorders including cancer and type II diabetes, and mTOR inhibitors are FDA-approved for renal cell carcinoma, tuberous sclerosis complex (TSC), and stent restenosis following angioplasty. Despite the physiologic and therapeutic importance of AMPK and mTORC1, major gaps exist in our molecular-level understanding of these cell signaling networks. In prior work we showed that phosphorylation of mTOR (on S1261) promotes mTORC1 signaling and cell growth/size. Unexpectedly, a kinome screen identified AMPK as an mTOR S1261 kinase. Finding that AMPK phosphorylates mTOR on an mTORC1-activating site presented an apparent paradox, however, as AMPK canonically inhibits mTORC1 in response to severe energetic stress. Our preliminary results reconcile this paradoxical finding and expose a major gap in our understanding of AMPK and its relationship with mTORC1. We find that after induction of nutrient stress, specifically amino acid starvation, amino acids provided either by re-feeding or autophagy activate AMPK and increase mTOR S1261 phosphorylation, which re-activates mTORC1 signaling and promotes cell survival in an AMPK dependent manner. Moreover, mTOR S1261 phosphorylation requires Rheb, an upstream mTORC1 activator under negative control by TSC. In this proposal we investigate an unexpected role for AMPK and mTORC1 in cellular adaptation to nutrient stress. We hypothesize that the AMPK-mTORC1 axis functions to maintain a survival level of metabolism during prolonged nutrient stress or facilitates metabolic recovery when the stress subsides. This research, which employs cultured cells and a CRISPR-engineered mouse model lacking mTOR S1261 phosphorylation, will provide conceptual advance as it will shift how we think about AMPK in metabolic control and its relationship with mTORC1. Elucidating paradoxical activation of mTORC1 by AMPK will advance our understanding of mTOR and AMPK in health and disease and may identify new therapeutic strategies for treatment of linked disorders, such as cancer and metabolic diseases.