Stravitz R T, Hylemon P B, Heuman D M, Hagey L R, Schteingart C D, Ton-Nu H T, Hofmann A F, Vlahcevic Z R
Section of Gastroenterology, McGuire Veterans Administration Medical Center, Richmond, Virginia.
J Biol Chem. 1993 Jul 5;268(19):13987-93.
The role of bile acids in the regulation of cholesterol 7 alpha-monooxygenase (EC 1.14.13.17) was characterized using primary cultures of rat hepatocytes supplemented with dexamethasone and thyroxine. Taurocholate and taurodeoxycholate (50 microM) repressed cholesterol 7 alpha-hydroxylase mRNA to 44 +/- 9 and 52 +/- 4%, respectively, of control values. Repression by these natural, relatively hydrophobic bile acids was concentration dependent, with an IC50 of about 50 microM, and time dependent with a t1/2 for repression of 22 h. In contrast, two natural hydrophilic bile acids, tauroursodeoxycholate and taurohyodeoxycholate, had no effect on cholesterol 7 alpha-hydroxylase mRNA levels. Taurochenodeoxycholate and taurolithocholate also had no effect, but these hydrophobic bile acids were rapidly hydroxylated to more hydrophilic bile acids. Hydrophilic bile acid analogues (nor (C23) bile acids and beta-hydroxy epimers) repressed cholesterol 7 alpha-hydroxylase mRNA less potently than their corresponding and more hydrophobic C24 or alpha-hydroxy derivatives. Cholesterol 7 alpha-hydroxylase specific activity was decreased by taurocholate or taurodeoxycholate (50 microM) to 26 +/- 9 and 56 +/- 3% of control, respectively; its transcriptional activity was repressed to 52 +/- 5% of control by taurocholate (50 microM). The addition of cholesterol or the induction of cholesterol biosynthesis did not influence repression of cholesterol 7 alpha-hydroxylase mRNA levels by taurocholate. Based on several lines of evidence, cAMP was not involved in bile acid-induced repression. In rat hepatocytes cultured under conditions in which cholesterol 7 alpha-hydroxylase gene expression is maintained at in vivo levels, hydrophobic bile acids repress this enzyme at the level of gene transcription independently of cholesterol availability.
利用补充了地塞米松和甲状腺素的大鼠肝细胞原代培养物，对胆汁酸在胆固醇7α-单加氧酶（EC 1.14.13.17）调节中的作用进行了表征。牛磺胆酸盐和牛磺去氧胆酸盐（50微摩尔）分别将胆固醇7α-羟化酶mRNA抑制至对照值的44±9%和52±4%。这些天然的、相对疏水的胆汁酸的抑制作用呈浓度依赖性，半数抑制浓度约为50微摩尔，且呈时间依赖性，抑制的半衰期为22小时。相比之下，两种天然亲水性胆汁酸，牛磺熊去氧胆酸盐和牛磺猪去氧胆酸盐，对胆固醇7α-羟化酶mRNA水平没有影响。牛磺鹅去氧胆酸盐和牛磺石胆酸盐也没有影响，但这些疏水胆汁酸会迅速羟基化为更亲水的胆汁酸。亲水性胆汁酸类似物（去甲（C23）胆汁酸和β-羟基差向异构体）对胆固醇7α-羟化酶mRNA的抑制作用比其相应的、疏水性更强的C24或α-羟基衍生物弱。牛磺胆酸盐或牛磺去氧胆酸盐（50微摩尔）分别将胆固醇7α-羟化酶的比活性降低至对照的26±9%和56±3%；其转录活性被牛磺胆酸盐（50微摩尔）抑制至对照的52±5%。添加胆固醇或诱导胆固醇生物合成并不影响牛磺胆酸盐对胆固醇7α-羟化酶mRNA水平的抑制。基于多条证据，环磷酸腺苷不参与胆汁酸诱导的抑制作用。在胆固醇7α-羟化酶基因表达维持在体内水平的条件下培养的大鼠肝细胞中，疏水胆汁酸在基因转录水平上抑制该酶，且与胆固醇的可利用性无关。
