Regulation of Bile Acid Synthesis by Nuclear Receptors in Vivo

胆汁酸是肠道吸收和胆汁溶解胆固醇和脂肪所必需的。此外, 胆汁酸在胆固醇动态平衡中起着关键作用。胆汁酸中有两种关键酶 生物合成途径，胆固醇7-羟基酶/细胞色素P7A1(7-羟基酶)，它是限速和 经典途径的调控步骤，以及固醇12-羟基酶/细胞色素P8B1(12-羟基酶)，这是 胆酸合成的特异酶。7-羟基酶和12-羟基酶基因的高表达 受反馈调节机制的调节，主要在转录水平上。分子机制 参与这一调控的似乎是相似的，并涉及法尼醇X的协调活动 受体(FXR)，小异二聚体伴侣(SHP)和1-甲胎蛋白转录因子(FTF)，也被称为 作为肝脏受体同源物-1(LRH-1)和肝细胞核因子-4(HNF-4)。到目前为止所做的所有研究 已经确定FTF是参与7-和12-羟基酶反馈调节的因素 转录已经在组织培养中进行了。考虑到它们DNA中核受体的混杂 因此，确定FTF和HNF-4在体内胆汁酸生物合成中的作用是至关重要的。这个 本提案的总体目标是了解FTF和HNF-4在反馈中的作用和调节 体内胆汁酸生物合成的调控。私家侦探的假设是胆汁酸调节血压下降 通过两步机制调节7-羟基酶和12-羟基酶的转录：首先， 胆汁酸通过p38 MAP激酶途径(一种新的)介导的过程抑制HNF-4 最近在P.I.S实验室观察到)，并增加FTF的表达，这导致FTF的增加 结合在7-和12-羟基酶启动子中发现的重叠的FTF/HNF-4识别位点 第二，FTF通过与SHP的异二聚化而变得不活跃。 初步研究表明，这些过程中涉及的一些机制是胆汁酸特有的。 其中一些机制在12-羟基酶的调节中发挥了更大的作用。建议数 具体研究内容如下：1)研究p38蛋白在p38蛋白表达中的作用和作用机制 2)为了阐明p38途径、FTF和HNF-4在肿瘤发生发展中的作用。 胆汁酸介导的体内胆汁酸合成的调节，利用p38活性被抑制的小鼠，以及 FTF/-和SHP-/-小鼠；以及3)表征FTF从7-和12- 羟基酶启动子，参与胆汁酸调节胆汁酸的合成。 同样的老鼠模型。这项研究的成功完成将为我们提供对 胆汁酸生物合成调控的分子机制。

Bile acids are required for intestinal absorption and biliary solubilization of cholesterol and lipids. In addition, bile acids play a crucial role in cholesterol homeostasis. There are two key enzymes in the bile acid biosynthetic pathways, cholesterol 7-hydroxylase/CYP7A1 (7-hydroxylase), which is the rate limiting and regulatory step of the "classic" pathway, and sterol 12-hydroxylase/CYP8B1 (12-hydroxylase), which is the specific enzyme for cholic acid synthesis. Expression of the 7- and 12-hydroxylase genes is highly regulated by feedback regulatory mechanisms, mainly at the transcriptional level. The molecular mechanisms involved in this regulation appear to be similar, and involve the coordinated activity of the farnesoid X receptor (FXR), the small heterodimer partner (SHP) and 1-fetoprotein transcription factor (FTF), also known as liver receptor homolog-1 (LRH-1), and hepatocyte nuclear factor-4 (HNF-4). All the studies done to date that have identified FTF as a factor involved in the feedback regulation of 7- and 12-hydroxylase transcription have been performed in tissue culture. Given the promiscuity of nuclear receptors in their DNA binding abilities, it is crucial to determine the role of FTF and HNF-4 in bile acid biosynthesis in vivo. The overall goal of this proposal is to understand the role and regulation of FTF and HNF-4 in the feedback regulation of bile acid biosynthesis in vivo. The P.I. hypothesizes that bile acids mediate the down regulation of the transcription of the 7- and 12-hydroxylases, by a two-step mechanism: first, bile acids suppress HNF-4 through a process mediated by the p38 MAP kinase pathway (a novel observation recently made in the P.I.'s lab), and increase FTF expression, which results in an increase in FTF binding to the overlapping FTF/HNF-4 recognition site found in the 7- and 12-hydroxylase promoters displacing HNF-4 from that site, and second, FTF becomes inactive through its heterodimerization with SHP. Preliminary studies suggest that some of the mechanisms involved in these processes are bile acid-specific and some of these mechanisms play a greater role in the regulation of 12-hydroxylase. The proposed specific studies are: 1) to characterize the role and mechanism of action of p38 kinase in the expression of the 7- and 12-hydroxylase genes; 2) to elucidate the role of the p38 pathway, FTF and HNF-4 in the bile acid-mediated regulation of bile acid synthesis in vivo, utilizing mice with suppressed p38 activity, and FTF+/- and SHP-/- mice; and 3) to characterize the displacement of HNF-4 by FTF from the 7- and 12- hydroxylase promoters, that is involved in the bile acid-mediated regulation of bile acid synthesis using the same mouse models. The successful completion of this study will provide us with new insights into the molecular mechanisms involved in the regulation of bile acid biosynthesis.