Abstract
HNF-4alpha (hepatocyte nuclear factor-4alpha) is a key regulator of liver-specific gene expression. To understand the mechanisms governing the regulation of HNF-4alpha function during the APR (acute-phase response), the effects of transcription co-activators, including p300, PGC-1alpha (peroxisome-proliferator-activated receptor-gamma co-activator-1alpha) and SRC (steroid receptor co-activator)-1alpha were investigated in an injury cell model. We have shown previously that the HNF-4alpha-sensitive APR genes ApoB (apolipoprotein B), TTR (transthyretin) and alpha1-AT (alpha1-antitrypsin) were regulated at the DNA binding and transcriptional levels after cytokine stimulation. We now show that co-activators have a differential impact on the transactivation of HNF-4alpha-sensitive genes via HNF-4alpha-binding sites in ApoB, TTR or alpha1-AT promoters. PGC-1alpha strongly enhances the transactivation of ApoB and alpha1-AT and, to a lesser extent, of TTR, whereas SRC-1alpha and p300 only have a weak or no effect on these three genes. More importantly, it was found that PGC-1alpha has a novel role in the modulation of the binding ability of HNF-4alpha in response to cytokine treatment. Using in vitro and in vivo approaches, electrophoretic mobility-shift and chromatin immunoprecipitation assays, we demonstrate that the reduced HNF-4alpha-DNA binding ability induced by cytokines is eliminated by overexpression of PGC-1alpha. Cytokine treatment does not significantly alter the protein levels of HNF-4alpha and PGC-1alpha, but it does reduce the recruitment of PGC-1alpha to HNF-4alpha-binding sites and thereby decreases transcriptional activity. These results establish the importance of PGC-1alpha for HNF-4alpha function and describe a new HNF-4alpha-dependent regulatory mechanism that is involved in the response to injury.