Shang Quan, Pan Luxing, Saumoy Monica, Chiang John Y L, Tint G Stephen, Salen Gerald, Xu Guorong
Department of Medicine, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, 07103, USA.
J Lipid Res. 2006 May;47(5):997-1004. doi: 10.1194/jlr.M500449-JLR200. Epub 2006 Feb 17.
The transcription of the cholesterol 7alpha-hydroxylase gene (CYP7A1) is greatly decreased in cholesterol-fed rabbits. To determine whether the molecular structure of the promoter is responsible for this downregulation, we cloned the rabbit CYP7A1 promoter, identified the binding sites for alpha-fetoprotein transcription factor (FTF) and liver X receptor (LXRalpha), and studied the effects of FTF, LXRalpha, and SHP on its transcription. Adding LXRalpha/retinoid X receptor together with their ligands (L/R) to the promoter/reporter construct transfected into HepG2 cells greatly increased its activity. FTF did not increase promoter activity, nor did it enhance the stimulatory effect of L/R. Mutating the FTF binding site abolished the promoter baseline activity. Increasing amounts of SHP abolished the effect of L/R, and FTF enhanced the ability of SHP to decrease promoter activity below baseline levels. Thus, downregulation of CYP7A1 in cholesterol-fed rabbits is attributable secondarily to the activation of farnesoid X receptor, which increases SHP expression to override the positive effects of LXRalpha. Although FTF is a competent factor for maintaining baseline activity, it does not further enhance and may suppress CYP7A1 transcription.
在喂食胆固醇的兔子中，胆固醇7α-羟化酶基因（CYP7A1）的转录大幅下降。为了确定启动子的分子结构是否是这种下调的原因，我们克隆了兔子CYP7A1启动子，鉴定了甲胎蛋白转录因子（FTF）和肝脏X受体（LXRα）的结合位点，并研究了FTF、LXRα和小异二聚体蛋白（SHP）对其转录的影响。将LXRα/视黄酸X受体与其配体（L/R）一起添加到转染到HepG2细胞中的启动子/报告基因构建体中，可大大增加其活性。FTF不会增加启动子活性，也不会增强L/R的刺激作用。突变FTF结合位点会消除启动子的基线活性。增加SHP的量可消除L/R的作用，并且FTF增强了SHP将启动子活性降低至基线水平以下的能力。因此，喂食胆固醇的兔子中CYP7A1的下调继发于法尼醇X受体的激活，后者增加SHP的表达以抵消LXRα的积极作用。虽然FTF是维持基线活性的一个有效因子，但它不会进一步增强，并且可能会抑制CYP7A1的转录。
