Pandak W M, Vlahcevic Z R, Chiang J Y, Heuman D M, Hylemon P B
Department of Medicine, Medical College of Virginia-VCU, Richmond.
J Lipid Res. 1992 May;33(5):659-68.
Taurocholate, a relatively hydrophobic bile salt, is a potent down-regulator of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase (C7 alpha H), the rate-determining enzymes of the cholesterol and bile acid biosynthetic pathways, respectively. Inhibition of cholesterol synthesis with a bolus dose of mevinolin (lovastatin) a competitive inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, profoundly decreases the specific activity of cholesterol 7 alpha-hydroxylase and rate of bile acid synthesis in rats with complete biliary diversion. It is therefore conceivable that taurocholate may suppress cholesterol 7 alpha-hydroxylase primarily by down-regulating the activity of HMG-CoA reductase. To test this hypothesis, taurocholate was coinfused simultaneously to rats with chronic bile fistula with mevalonate (administered as mevalonolactone), an intermediate in the cholesterol biosynthetic pathway. Mevalonolactone was administered to provide a constant supply of newly synthesized cholesterol to cholesterol 7 alpha-hydroxylase, in order to overcome any inhibitory effect of taurocholate on HMG-Coa reductase. Infusions were started 72 h after biliary diversion, and carried out for an additional 48 h. Complete biliary diversion resulted in an increase in C7 alpha H specific activity (510%), protein mass (550%), steady-state mRNA levels (1430%), and transcriptional activities (330%) as compared to control rats with intact enterohepatic circulations. When rats with biliary diversion were infused intraduodenally with taurocholate, the specific activities of HMG-CoA reductase and cholesterol 7 alpha-hydroxylase activities decreased by 75% (P less than 0.001) and 73% (P less than 0.001), respectively. Cholesterol 7 alpha-hydroxylase mass, mRNA, and transcriptional activity decreased after intraduodenal infusion of taurocholate to levels similar to those of rats with an intact enterohepatic circulation. The combination of constant infusion of mevalonate and taurocholate failed to reverse the inhibitory effects of taurocholate on cholesterol 7 alpha-hydroxylase activity, mRNA levels, and in vitro transcriptional rates. These data provide evidence that taurocholate represses cholesterol 7 alpha-hydroxylase at the level of gene transcription, and not via down-regulation of HMG-CoA reductase. Infusion of mevalonate alone to biliary diverted rats did not alter cholesterol 7 alpha-hydroxylase activity or mRNA levels, while leading to a 57% decrease in C7 alpha H gene transcription. This latter finding suggests that mevalonate or its metabolites may be capable of stabilizing C7 alpha H mRNA levels while down-regulating transcriptional activity.
牛磺胆酸盐是一种相对疏水的胆汁盐，是HMG-CoA还原酶和胆固醇7α-羟化酶（C7αH）的有效下调剂，分别是胆固醇和胆汁酸生物合成途径的限速酶。用大剂量美伐他汀（洛伐他汀）（一种3-羟基-3-甲基戊二酰辅酶A（HMG-CoA）还原酶的竞争性抑制剂）抑制胆固醇合成，可显著降低完全胆道分流大鼠中胆固醇7α-羟化酶的比活性和胆汁酸合成速率。因此，可以想象牛磺胆酸盐可能主要通过下调HMG-CoA还原酶的活性来抑制胆固醇7α-羟化酶。为了验证这一假设，将牛磺胆酸盐与胆固醇生物合成途径中的中间体甲羟戊酸（以甲羟戊酸内酯形式给药）同时静脉输注给患有慢性胆瘘的大鼠。给予甲羟戊酸内酯以向胆固醇7α-羟化酶持续供应新合成的胆固醇，以克服牛磺胆酸盐对HMG-Coa还原酶的任何抑制作用。在胆道分流72小时后开始输注，并再持续进行48小时。与具有完整肠肝循环的对照大鼠相比，完全胆道分流导致C7αH比活性增加（510%）、蛋白量增加（550%）、稳态mRNA水平增加（1430%）和转录活性增加（330%）。当向胆道分流的大鼠十二指肠内输注牛磺胆酸盐时，HMG-CoA还原酶和胆固醇7α-羟化酶的活性分别降低了75%（P<0.001）和73%（P<0.001）。十二指肠内输注牛磺胆酸盐后，胆固醇7α-羟化酶的量、mRNA和转录活性降低至与具有完整肠肝循环的大鼠相似的水平。持续输注甲羟戊酸和牛磺胆酸盐的组合未能逆转牛磺胆酸盐对胆固醇7α-羟化酶活性、mRNA水平和体外转录速率的抑制作用。这些数据提供了证据，表明牛磺胆酸盐在基因转录水平上抑制胆固醇7α-羟化酶，而不是通过下调HMG-CoA还原酶。单独向胆道分流的大鼠输注甲羟戊酸不会改变胆固醇7α-羟化酶的活性或mRNA水平，而导致C7αH基因转录降低57%。后一发现表明甲羟戊酸或其代谢产物可能能够稳定C7αH mRNA水平，同时下调转录活性。
