Abstract
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.
Highlights
Immunology, Medical Collegeof Virginia, Virginia Commonwealth University, Richmond,Virginia 23298 and )IDivision of Gastroenterology, University of California at SunDiego, La Jolla California 92093
In rat hepatocytes cultured under conditions in which cholesterol 7a-hydroxylase gene expression is maintained at in vivo levels, hydrophobic bile acids repress this enzyme at thelevel of gene transcription independently of cholesterol availability
Preparation of Primary Cultures of Adult Rat Hepatocytes-Primary cultures of rat hepatocytes were prepared as originally described by Bissell and Guzelian [23].Parenchymal cells (3.4 X lo6) were plated on 60-mm plastic Petri dishes coated with rat tail collagen and were ~ 9 0 %viable, as determined by Trypan blue exclusion
Summary
Immunology, Medical Collegeof Virginia, Virginia Commonwealth University, Richmond,Virginia 23298 and )IDivision of Gastroenterology, University of California at SunDiego, La Jolla California 92093. In rat hepatocytes cultured under conditions in which cholesterol 7a-hydroxylase gene expression is maintained at in vivo levels, hydrophobic bile acids repress this enzyme at thelevel of gene transcription independently of cholesterol availability. Repression of Cholesterol 7a-Hydroxylase mRNA,Specific bile acids were synthesized in tracer amountsusing [I4C]taurine[20]; Activity, and Transcriptional Activity by Naturally Occurring purification was bypreparative TLC.
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