Nicotinamide N-methyltransferase upregulation via the mTORC1-ATF4 pathway activation contributes to palmitate-induced lipotoxicity in hepatocytes

Am J Physiol Cell Physiol. 2021 Sep 1;321(3):C585-C595. doi: 10.1152/ajpcell.00195.2021. Epub 2021 Aug 11.


Defined as the dysfunction and/or cell death caused by toxic lipids accumulation in hepatocytes, hepatic lipotoxicity plays a pathological role in nonalcoholic fatty liver disease. The cellular and molecular mechanisms underlying lipotoxicity remain to be elucidated. In this study, using AML12 cells, a nontransformed murine hepatocyte cell line, exposed to palmitate (a 16-C saturated fatty acid) as an experimental model, we investigated the role and mechanisms of nicotinamide N-methyltransferase (NNMT), a methyltransferase catalyzing nicotinamide methylation and degradation, in hepatic lipotoxicity. We initially identified activating transcription factor 4 (ATF4) as a major transcription factor for hepatic NNMT expression. Here, we demonstrated that palmitate upregulates NNMT expression via activating ATF4 in a mechanistic target of rapamycin complex 1 (mTORC1)-dependent mechanism in that mTORC1 inhibition by both Torin1 and rapamycin attenuated ATF4 activation and NNMT upregulation. We further demonstrated that the mTORC1-dependent ATF4 activation is an integral signaling event of unfolded protein response (UPR) as both ATF4 activation and NNMT upregulation by tunicamycin, a well-documented endoplasmic reticulum (ER) stress inducer, are blunted when hepatocytes were pretreated with Torin1. Importantly, our data uncovered that NNMT upregulation contributes to palmitate-induced hepatotoxicity as NNMT inhibition, via either pharmacological (NNMT inhibitors) or genetic approach (siRNA transfection), provided protection against palmitate lipotoxicity. Our further mechanistic exploration identified protein kinase A (PKA) activation to contribute, at least, partially to the protective effect of NNMT inhibition against lipotoxicity. Collectively, our data demonstrated that NNMT upregulation by the mTORC1-ATF4 pathway activation contributes to the development of lipotoxicity in hepatocytes.

Keywords: ATF4; NNMT; PKA; lipotoxicity; mTORC1; palmitate.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Activating Transcription Factor 4 / genetics
  • Activating Transcription Factor 4 / metabolism*
  • Animals
  • Cell Death / drug effects
  • Cell Line
  • Cyclic AMP-Dependent Protein Kinases / metabolism
  • Endoplasmic Reticulum Stress / drug effects
  • Hepatocytes / drug effects*
  • Hepatocytes / enzymology
  • Hepatocytes / pathology
  • Male
  • Mechanistic Target of Rapamycin Complex 1 / metabolism*
  • Mice
  • Mice, Inbred C57BL
  • Nicotinamide N-Methyltransferase / genetics
  • Nicotinamide N-Methyltransferase / metabolism*
  • Palmitates / toxicity*
  • Signal Transduction
  • Unfolded Protein Response / drug effects
  • Up-Regulation
  • X-Box Binding Protein 1 / genetics
  • X-Box Binding Protein 1 / metabolism


  • Atf4 protein, mouse
  • Palmitates
  • X-Box Binding Protein 1
  • Xbp1 protein, mouse
  • Activating Transcription Factor 4
  • Nicotinamide N-Methyltransferase
  • Nnmt protein, mouse
  • Mechanistic Target of Rapamycin Complex 1
  • Cyclic AMP-Dependent Protein Kinases