Ligand-activated pregnane X receptor interferes with HNF-4 signaling by targeting a common coactivator PGC-1alpha. Functional implications in hepatic cholesterol and glucose metabolism.

Abstract	Previous studies show that feedback inhibition of bile acid production by bile acids is mediated by multiple mechanisms, including activation of pregnane X receptor (PXR). Consistent with these studies, the antibiotic rifampicin, a ligand for human PXR, reduces hepatic bile acid levels in cholestasis patients. To delineate the mechanisms underlying PXR-mediated suppression of bile acid biosynthesis, we examined the functional cross-talk between human PXR and HNF-4, a key hepatic activator of genes involved in bile acid biosynthesis including the cholesterol 7-alpha hydroxylase (CYP7A1) and sterol 12-alpha hydroxylase (CYP8B1) genes. Treatment with rifampicin resulted in repression of endogenous human CYP7A1 expression in HepG2 cells that was reversed by PXR small interfering RNA. The coactivator PGC-1 enhanced transcriptional activity of HNF-4, and this enhancement was suppressed by rifampicin-activated PXR. Endogenous PGC-1 from mouse liver extracts bound to PXR, and recombinant PGC-1 directly interacted with both PXR and HNF-4 in vitro. Rifampicin-dependent interaction of PXR with PGC-1 was shown in cells by coimmunoprecipitation, and intranuclear localization studies using confocal microscopy provided further evidence for this interaction. In chromatin immunoprecipitation studies, rifampicin treatment did not inhibit HNF-4 binding to the native promoters of CYP7A1 and CYP8B1 but resulted in dissociation of PGC-1 and concomitant gene repression. Most interestingly, these rifampicin effects were also observed in the phosphoenolpyruvate carboxykinase gene that contains a functional HNF-4-binding site and is central to hepatic gluconeogenesis. Our study suggests that ligand-activated PXR interferes with HNF-4 signaling by targeting the common coactivator PGC-1, which underlies physiologically relevant inhibitory cross-talk between drug metabolism and cholesterol/glucose metabolism.
Authors	Sonali Bhalla, Cengiz Ozalp, Sungsoon Fang, Lingjin Xiang, Jongsook Kim Kemper
Journal	The Journal of biological chemistry (J Biol Chem) Vol. 279 Issue 43 Pg. 45139-47 (Oct 22 2004) ISSN: 0021-9258 [Print] United States
PMID	15322103 (Publication Type: Journal Article)
Chemical References	Bile Acids and Salts Chromatin DNA-Binding Proteins Enzyme Inhibitors Heat-Shock Proteins Hepatocyte Nuclear Factor 4 Ligands Oligonucleotides PPARGC1A protein, human Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha Phosphoproteins Pregnane X Receptor RNA, Messenger RNA, Small Interfering Receptors, Cytoplasmic and Nuclear Receptors, Steroid Recombinant Proteins Transcription Factors nuclear receptor subfamily 0, group B, member 2 Green Fluorescent Proteins Sepharose Cytochrome P-450 Enzyme System Cholesterol CYP3A protein, human Cytochrome P-450 CYP3A Cholesterol 7-alpha-Hydroxylase Steroid 12-alpha-Hydroxylase Glutathione Transferase Phosphoenolpyruvate Carboxykinase (GTP) Glutathione Glucose Rifampin
Topics	Animals Bile Acids and Salts (metabolism) Binding Sites Blotting, Western COS Cells Cell Line Cell Nucleus (metabolism) Cholesterol (metabolism) Cholesterol 7-alpha-Hydroxylase (metabolism) Chromatin (chemistry, metabolism) Cytochrome P-450 CYP3A Cytochrome P-450 Enzyme System (metabolism) DNA-Binding Proteins (chemistry, metabolism) Enzyme Inhibitors (pharmacology) Gene Expression Regulation Genes, Reporter Genetic Vectors Glucose (metabolism) Glutathione (metabolism) Glutathione Transferase (metabolism) Green Fluorescent Proteins (chemistry) Heat-Shock Proteins Hepatocyte Nuclear Factor 4 Humans Immunoprecipitation Ligands Liver (metabolism) Mice Oligonucleotides (chemistry) Peroxisome Proliferator-Activated Receptor Gamma Coactivator 1-alpha Phosphoenolpyruvate Carboxykinase (GTP) (metabolism) Phosphoproteins (chemistry, metabolism) Plasmids (metabolism) Pregnane X Receptor Promoter Regions, Genetic Protein Binding RNA, Messenger (metabolism) RNA, Small Interfering (metabolism) Receptors, Cytoplasmic and Nuclear (chemistry, metabolism) Receptors, Steroid (chemistry) Recombinant Proteins (chemistry) Reverse Transcriptase Polymerase Chain Reaction Rifampin (pharmacology) Sepharose (chemistry) Signal Transduction Steroid 12-alpha-Hydroxylase (metabolism) Transcription Factors (chemistry, metabolism) Transcription, Genetic Transcriptional Activation Transfection

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