Saturated phosphatidic acids induce mTORC1-driven integrated stress response contributing to glucolipotoxicity in hepatocytes.

Hepatic glucolipotoxicity, characterized by the synergistic detrimental effects of elevated glucose levels combined with excessive lipid accumulation in hepatocytes, plays a central role in the pathogenesis of various metabolic liver diseases. Despite recent advancements, the precise mechanisms underlying this process remain unclear. Using cultured AML12 and HepG2 cells exposed to excess palmitate, with and without high glucose, as an in vitro model, we aimed to elucidate the cellular and molecular mechanisms underlying hepatic glucolipotoxicity. Our data showed that palmitate exposure induced the integrated stress response (ISR) in hepatocytes, evidenced by increased eukaryotic translation initiation factor 2 alpha (eIF2α) phosphorylation (serine 51) and upregulated activating transcription factor 4 (ATF4) expression. Moreover, we identified mammalian target of rapamycin complex 1 (mTORC1) as a novel upstream kinase responsible for palmitate-triggered ISR induction. Furthermore, we showed that either mTORC1 inhibitors, ISRIB (an ISR inhibitor), or ATF4 knockdown abolished palmitate-induced cell death, indicating that the mTORC1-eIF2α-ATF4 pathway activation plays a mechanistic role in mediating palmitate-induced hepatocyte cell death. Our continuous investigations revealed that glycerol-3-phosphate acyltransferase (GPAT4)-mediated metabolic flux of palmitate into the glycerolipid synthesis pathway is required for palmitate-induced mTORC1 activation and subsequent ISR induction. Specifically, we uncovered that saturated phosphatidic acid production contributes to palmitate-triggered mTORC1 activation. Our study provides the first evidence that high glucose enhances palmitate-induced activation of the mTORC1-eIF2α-ATF4 pathway, thereby exacerbating palmitate-induced hepatotoxicity. This effect is mediated by the increased availability of glycerol-3-phosphate, a substrate essential for phosphatidic acid synthesis. In conclusion, our study highlights that the activation of the mTORC1-eIF2α-ATF4 pathway, driven by saturated phosphatidic acid overproduction, plays a mechanistic role in hepatic glucolipotoxicity.