Inositol-requiring enzyme 1α links palmitate-induced mTOR activation and lipotoxicity in hepatocytes

Hepatic lipotoxicity, hepatocyte dysfunction/cell death induced by saturated fatty acids (SFA), plays a central role in the pathogenesis of nonalcoholic fatty liver disease (NAFLD); however, the underlying mechanisms remain unclear. Palmitate is the most abundant SFA in the circulation. In this study, via a small-scale screening of chemical inhibitors using AML12 hepatocytes, we identified mechanistic target of rapamycin (mTOR) complex 1 (mTORC1) to be a culprit in palmitate-induced cell death in hepatocytes in that mTOR inhibition is protective against palmitate-induced cell death. The protective effect of mTORC1 inhibition is independent of autophagy induction, as autophagy inhibition failed to ablate the mTORC1 inhibitor-conferred protection. We have previously reported that the endonuclease activity of inositol-requiring enzyme 1α (IRE1α), one of three canonical signaling pathways of endoplasmic reticulum (ER) stress, was implicated in palmitate-induced cell death in hepatocytes. The continuous mechanistic investigation in this study uncovered that IRE1α is a downstream target of mTORC1 activation upon palmitate exposure and the inhibition of either its endonuclease activity or kinase activity protects against the lipotoxic effect of palmitate. Our research further revealed that protein palmitoylation is potentially involved in palmitate-induced mTORC1 activation and lipotoxicity in hepatocytes. 2-Bromopalmitate, a protein palmitoylation inhibitor, ameliorated palmitate-triggered mTORC1 activation, concomitant with the protection of lipotoxicity in hepatocytes. Collectively, our data have identified that mTORC1 and ER stress are coordinately implicated in hepatocyte cell death in response to palmitate exposure and suggest that this pathway may potentially serve as a therapeutic target for the treatment of NAFLD as well as other metabolic disorders involving lipotoxicity.