PNPLA3 I148M mediates the regulatory effect of NF-kB on inflammation in PA-treated HepG2 cells

Abstract

Both PNPLA3 I148M and hepatic inflammation are associated with nonalcoholic fatty liver disease (NAFLD) progression. This study aimed to elucidate whether PNPLA3 I148M is involved in NF-kB-related inflammation regulation in NAFLD. HepG2 cells homozygous for the PNPLA3 I148M mutation were used. The human PNPLA3 promoter sequence was screened for NF-kB binding sites using the MATCH and PATCH tools. NF-kB-mediated transcriptional regulation of the PNPLA3 gene was assessed by luciferase reporter assay, EMSA and ChIP-qPCR. Wild-type (I148I) and mutant (M148M) PNPLA3 were overexpressed using stable lentivirus-mediated transfection. The pCMV vector and siRNA were transiently transfected into cells to direct NF-kB overexpression and PNPLA3 silencing, respectively. A putative NF-kB binding site in the human PNPLA3 promoter was shown to be necessary for basal and NF-kB-driven transcriptional activation of PNPLA3 and protein/DNA complex formation. Supershift analysis demonstrated a protein/DNA complex specifically containing the NF-kB p65 and p50 subunits. ChIP-qPCR confirmed the endogenous binding of NF-kB to the human PNPLA3 promoter in response to NF-kB overexpression and palmitic acid (PA) challenge. The silencing of PNPLA3 blocked the overexpression of NF-kB or PA-induced TNF-α up-regulation. Moreover, mutant PNPLA3 overexpression prevented NF-kB inhibitor-induced down-regulation of TNF-α expression in PA-treated HepG2 cells. Finally, the overexpression of mutant but not wild-type PNPLA3 increased TNF-α expression and activated the ER stress-mediated and NF-kB-independent inflammatory IRE-1α/JNK/c-Jun pathway. Human PNPLA3 was shown to be a target of NF-kB, and PNPLA3 I148M mediated the regulatory effect of NF-kB on inflammation in PA-treated HepG2 cells, most likely via the IRE-1α/JNK/c-Jun ER stress pathway.

Keywords: ER stress; NAFLD; NF-kB; PNPLA3; inflammation.