Amino Acid–Induced Activation of mTORC1 in Rat Liver Is Attenuated by Short-Term Consumption of a High-Fat Diet

Background: The chronic activation of the mechanistic (mammalian) target of rapamycin in complex 1 (mTORC1) in response to excess nutrients contributes to obesity-associated pathologies. Objective: To understand the initial events that ultimately lead to obesity-associated pathologies, the present study assessed mTORC1 responses in the liver after a relatively short exposure to a high-fat diet (HFD). Methods: Male, obesity-prone rats were meal-trained to consume either a control (CON; 10% of energy from fat) diet or an HFD (60% of energy from fat) for 2 wk. Livers were collected and analyzed for mTORC1 signaling [assessed by changes in phosphorylation of 70-kDa ribosomal protein S6 kinase 1 (p70S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1)] and potential regulatory mechanisms, including changes in the association of Ras-related GTP binding (Rag) A and RagC with mechanistic target of rapamycin (mTOR) and expression of Sestrin1, Sestrin2, and Sestrin3. Results: Feeding-induced activation of mTORC1 was blunted in the livers of rats fed the HFD compared with those fed the CON diet (p70S6K1 phosphorylation, 19% of CON; 4E-BP1 phosphorylation, 61% of CON). The attenuated response was not due to a change in a kinase also referred to as protein kinase B (Akt) signaling but rather to resistance to amino acid–induced activation of mTORC1, as evidenced by a reduction in the interaction of RagA (69% of CON) and RagC (66% of CON) with mTOR and enhanced expression of the mTORC1 repressors Sestrin2 (132% of CON) and Sestrin3 (143% of CON). The consumption of an HFD led to impaired amino acid–induced activation of mTORC1 as assessed in livers perfused in situ with medium containing various concentrations of amino acids. Conclusions: These results in rats support a model in which the initial response of the liver to an HFD is an attenuation of, rather than the expected activation of, mTORC1. The initial response likely represents a counterregulatory mechanism to handle the onset of excess nutrients and is caused by enhanced expression of Sestrin2 and Sestrin3, which, in turn, leads to impaired Rag signaling, resulting in resistance to amino acid–induced activation of mTORC1.