Jung Diana, Kullak-Ublick Gerd A
Laboratory of Molecular Gastroenterology and Hepatology, University Hospital, Zurich, Switzerland.
Hepatology. 2003 Mar;37(3):622-31. doi: 10.1053/jhep.2003.50100.
Bile acids regulate the expression of genes involved in cholesterol homeostasis. They are ligands of the farnesoid X receptor, which induces small heterodimer partner (SHP)-1, a transcriptional repressor of bile acid synthetic enzymes. In cholestatic liver disease, hepatic bile acid concentrations are elevated and expression of the major Na+-independent bile acid uptake system, organic anion transporting polypeptide (OATP)-C (solute carrier gene family SLC21A6), is markedly decreased. Because the OATP-C gene is transcriptionally dependent on the hepatocyte nuclear factor (HNF) 1 alpha, we hypothesized that bile acids decrease OATP-C expression through direct repression of HNF1 alpha. To test this hypothesis, we studied the regulation of the human HNF1 alpha gene by bile acids. HNF1 alpha expression in cultured hepatoma cells was decreased approximately 50% after 12 hours' exposure to 100 micromol/L chenodeoxycholic acid (CDCA). Characterization of the human HNF1 alpha gene promoter identified a consensus bile acid response element that binds and is activated by HNF4 alpha. Mutagenesis of the HNF4 alpha site abolished baseline HNF1 alpha promoter activity. The central mechanism by which bile acids repress HNF1 alpha is decreased activation by HNF4 alpha. SHP directly inhibits HNF4 alpha-mediated transactivation of the HNF1 alpha promoter in cotransfection assays. In addition, HNF4 alpha nuclear binding activity is decreased by CDCA and the human HNF4 alpha gene promoter is repressed by CDCA through an SHP-independent mechanism. In conclusion, we show that repression of HNF1 alpha is an important new mechanism by which bile acids regulate the expression of HNF1 alpha-dependent genes in man. This explains the suppressive effect of bile acids on the OATP-C gene promoter, leading to decreased expression in cholestatic liver disease.
胆汁酸调节参与胆固醇稳态的基因表达。它们是法尼酯X受体的配体，该受体可诱导小异源二聚体伴侣（SHP）-1，后者是胆汁酸合成酶的转录抑制因子。在胆汁淤积性肝病中，肝脏胆汁酸浓度升高，主要的非Na⁺依赖性胆汁酸摄取系统——有机阴离子转运多肽（OATP）-C（溶质载体基因家族SLC21A6）的表达则显著降低。由于OATP-C基因的转录依赖于肝细胞核因子（HNF）1α，我们推测胆汁酸通过直接抑制HNF1α来降低OATP-C的表达。为验证这一假设，我们研究了胆汁酸对人HNF1α基因的调控。培养的肝癌细胞在暴露于100 μmol/L鹅去氧胆酸（CDCA）12小时后，HNF1α的表达降低了约50%。对人HNF1α基因启动子的特性分析确定了一个共有胆汁酸反应元件，该元件可与HNF4α结合并被其激活。HNF4α位点的诱变消除了HNF1α启动子的基础活性。胆汁酸抑制HNF1α的核心机制是HNF4α的激活减少。在共转染实验中，SHP直接抑制HNF4α介导的HNF1α启动子的反式激活。此外，CDCA可降低HNF4α的核结合活性，并且人HNF4α基因启动子通过一种不依赖SHP的机制被CDCA抑制。总之，我们表明抑制HNF1α是胆汁酸调节人类HNF1α依赖性基因表达的一种重要新机制。这解释了胆汁酸对OATP-C基因启动子的抑制作用，导致其在胆汁淤积性肝病中的表达降低。
