Role of the Nuclear Receptors HNF4α, PPARα, and LXRs in the TNFα-Mediated Inhibition of Human Apolipoprotein A-I Gene Expression in HepG2 Cells

摘要

The expression of the apolipoprotein A-I gene (apoA-I) in hepatocytes is repressed by pro-ninflammatory cytokines such as IL-1β and TNFR. In this work, we have demonstrated that treatment ofnHepG2 human hepatoma cellswith chemical inhibitors for JNK, p38 protein kinases, andNFκB transcriptionnfactor abolishes the TNFR-mediated inhibition of human apoA-I gene expression inHepG2 cells. In addition,nwe have shown that TNFR decreases also the rate of secretion of apoA-I protein by HepG2 cells, and thisneffect depends on JNK and p38, but not on NFκB andMEK1/2 signaling pathways. The inhibitory effect ofnTNFR has been found to be mediated by the hepatic enhancer of the apoA-I gene. The decrease in the level ofnhuman apoA-I gene expression under the impact of TNFR appears to be partly mediated by the inhibition ofnHNF4R and PPARR gene expression. Treatment of HepG2 cells with PPARR antagonist (MK886) or LXRnagonist (TO901317) abolishes the TNFR-mediated decrease in the level of apoA-I gene expression. PPARRnagonist (WY-14643) abolishes the negative effect of TNFR on apoA-I gene expression in the case ofnsimultaneous inhibition ofMEK1/2, although neither inhibition ofMEK1/2 nor addition ofWY-14643 leadsnto the blocking of the TNFR-mediated decrease in the level of apoA-I gene expression individually. Thenligand-dependent regulation of apoA-I gene expression by PPARR appears to be affected by the TNFR-nmediated activation of MEK1/2 kinases, probably through PPARR phosphorylation. Treatment of HepG2ncellswith PPARR and LXRsynthetic agonists also blocks the inhibition of apoA-I protein secretion inHepG2ncells under the impact of TNFR. A chromatin immunoprecipitation assay demonstrates that TNFR leads to an2-fold decrease in the level of PPARR binding with the apoA-I gene hepatic enhancer. At the same time, thenlevel of LXRβ binding with the apoA-I gene hepatic enhancer is increased 3-fold under the impact of TNFR.nThese results suggest that nuclear receptors HNF4R, PPARR, and LXRs are involved in the TNFR-mediatedndownregulation of human apoA-I gene expression and apoA-I protein secretion in HepG2 cells.