Increasing cellular NAD ⁺ content protects hepatocytes against palmitate-induced lipotoxicity by preventing PARP-1 inhibition and the mTORC1-p300 pathway activation

Hepatic lipotoxicity, resulting from excessive lipid accumulation in hepatocytes, plays a central role in the pathogenesis of various metabolic liver diseases. Despite recent progress, the precise mechanisms remain incompletely understood. Employing excessive exposure to palmitate in hepatocytes as our primary experimental model and mice studies, we aimed to uncover the mechanisms behind hepatic lipotoxicity, thereby developing potential treatments. Our data reveal for the first time that exposure to palmitate leads to downregulated expression of poly(ADP-ribose) polymerase 1 (PARP-1) in hepatocytes, inhibiting its enzymatic activity. While inhibiting PARP-1 worsens palmitate-induced hepatotoxicity, preventing PARP-1 suppression, using NAD + precursors, nicotinamide N-methyltransferase (NNMT) inhibitors, or a poly(ADP-ribose) glycohydrolase (PARG) inhibitor, prevents it. Moreover, we uncover that PARP-1 suppression contributes to palmitate-triggered mTORC1 activation, which has been previously reported by us to contribute to palmitate-induced hepatocyte cell death. Furthermore, our results identify p300 as a downstream target of mTORC1 activation upon palmitate exposure. Importantly, p300 inhibition via either pharmacological or genetic approaches protects against palmitate hepatotoxicity. Additionally, we provide evidence that the TLR4-NF-κB pathway activation in response to palmitate plays a mechanistic role in mediating palmitate-induced PARP-1 downregulation in that both TLR4 antagonist and NF-kB inhibitors prevent palmitate-induced PARP-1 reduction and protect against hepatocyte cell death. In conclusion, our study provides new evidence supporting that the PARP-1-mTORC1-p300 pathway is a novel molecular mechanism underlying palmitate-induced hepatic lipotoxicity and enhancing cellular NAD + levels through NAD + precursor supplementation or NNMT inhibitors administration represents a potential therapeutic choice for the prevention and treatment of metabolic liver diseases. the US NIH Grant NIAAA R01AA026603 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.