Dietary protein governs the role of insulin signaling in the postprandial regulation of hepatic mTORC1

Abstract

The nutrient-sensing mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway controls cellular and organismal growth and metabolism, and aberrant activation is linked to human disease, including metabolic disease. Cellular studies have established regulatory mechanisms influencing mTORC1 activation, but the physiological signals that control mTORC1 at the organismal and tissue levels are less well-defined. mTORC1 is dynamically regulated by fasting and feeding in metabolic tissues, with both nutrients and insulin proposed to activate mTORC1 in response to feeding. Here, studies employing a liver-specific genetic mouse model demonstrate that AKT-mediated TSC2 phosphorylation is the predominant mechanism of hepatic mTORC1 induction by insulin but is dispensable for activation by feeding. Furthermore, postprandial activation of hepatic mTORC1 requires dietary protein, which dictates the insulin-responsiveness of the pathway. Contrary to dogma, hepatic mTORC1 signaling was not elevated in response to diet-induced obesity, despite overt impairments in insulin and glucose homeostasis, and blocking hepatic AKT-TSC-mTORC1 signaling did not affect these metabolic phenotypes. Evidence is also provided supporting a role for glucagon in hepatic mTORC1 suppression during fasting. This study reveals a hierarchy of physiological signals regulating hepatic mTORC1. The nutrient-sensing kinase complex mTORC1 regulates metabolism, yet the physiological signals controlling its activity in the liver are incompletely defined. Here, the authors show that dietary protein is required for postprandial hepatic mTORC1 activation and determines its responsiveness to insulin.