del Castillo-Olivares A, Gil G
Department of Biochemistry and Molecular Biophysics, Medical College of Virginia, PO Box 980614, Richmond, VA 23298-0614, USA.
Nucleic Acids Res. 2000 Sep 15;28(18):3587-93. doi: 10.1093/nar/28.18.3587.
Bile acid biosynthesis is subjected to feedback regulation whereby bile acids down-regulate their own synthesis. The major point of this regulation is at the level of cholesterol 7alpha-hydroxylase (7alpha-hydroxylase), which controls bile acid output from the classic pathway. This regulation is at the level of transcription of the gene. Two bile acid response elements have been localized within the 5'-flanking region of the rat gene and these elements overlap three nuclear receptor binding sites for hepatocyte nuclear factor (HNF-4), liver X receptor (LXR) and alpha(1)-fetoprotein transcription factor (FTF). Recently it has been shown that bile acids are physiological ligands for the farnesyl X receptor (FXR), which suggested that FXR could function by binding to one of the three nuclear receptor sites to mediate regulation of 7alpha-hydroxylase transcription by bile acids. In this study we show that FXR is indeed a crucial factor for bile acid-mediated regulation, but that it functions without binding to DNA. Furthermore, we also demonstrate that neither the LXR nor the HNF-4 sites are involved in bile acid-mediated regulation of 7alpha-hydroxylase transcription. Most importantly, we show that the FTF site is essential for regulation of 7alpha-hydroxylase by bile acids, similar to what we have recently demonstrated for another gene of the bile acid biosynthetic pathway, the sterol 12alpha-hydroxylase gene. These studies demonstrate the crucial role of FTF in the expression and regulation of a critical gene in the bile acid biosynthetic pathways.
胆汁酸生物合成受到反馈调节，即胆汁酸会下调自身的合成。这种调节的主要作用点是在胆固醇7α-羟化酶（7α-羟化酶）水平，该酶控制着经典途径中胆汁酸的输出。这种调节发生在基因转录水平。在大鼠基因的5'侧翼区域内已定位到两个胆汁酸反应元件，这些元件与肝细胞核因子（HNF-4）、肝脏X受体（LXR）和α1-甲胎蛋白转录因子（FTF）的三个核受体结合位点重叠。最近有研究表明，胆汁酸是法尼酯X受体（FXR）的生理配体，这表明FXR可能通过与三个核受体位点之一结合来介导胆汁酸对7α-羟化酶转录的调节。在本研究中，我们表明FXR确实是胆汁酸介导调节中的关键因子，但它的作用并不依赖于与DNA的结合。此外，我们还证明LXR和HNF-4位点均不参与胆汁酸介导的7α-羟化酶转录调节。最重要的是，我们表明FTF位点对于胆汁酸对7α-羟化酶的调节至关重要，这与我们最近对胆汁酸生物合成途径中的另一个基因——固醇12α-羟化酶基因所证明的情况类似。这些研究证明了FTF在胆汁酸生物合成途径中关键基因的表达和调节中的关键作用。
