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GASTROINTESTINAL, HEPATIC, PULMONARY, AND RENAL

Role of Farnesoid X Receptor in the Enhancement of Canalicular Bile Acid Output and Excretion of Unconjugated Bile Acids: A Mechanism for Protection against Cholic Acid-Induced Liver Toxicity

Division of Drug Metabolism and Molecular Toxicology, Graduate School of Pharmaceutical Sciences, Tohoku University, Sendai, Japan (M.M., A.T., T.N., H.O., K.N., Y.Y.); and Laboratory of Metabolism, National Cancer Institute, National Institutes of Health, Bethesda, Maryland (F.J.G.)

Mice lacking the farnesoid X receptor (FXR) involved in the maintenance of hepatic bile acid levels are highly sensitive to cholic acid-induced liver toxicity. Serum aspartate aminotransferase (AST) activity was elevated 15.7-fold after feeding a 0.25% cholic acid diet, whereas only slight increases in serum AST (1.7- and 2.5-fold) were observed in wild-type mice fed 0.25 and 1% cholic acid diet, respectively. Bile salt export pump mRNA and protein levels were increased in wild-type mice fed 1% cholic acid diet (2.1- and 3.0-fold) but were decreased in FXR-null mice fed 0.25% cholic acid diet. The bile acid output rate was 2.0- and 3.7-fold higher after feeding of 0.25 and 1.0% cholic acid diet in wild-type mice, respectively. On the other hand, no significant increase in bile acid output rate was observed in FXR-null mice fed 0.25% cholic acid diet in contrast to a significant decrease observed in mice fed a 1.0% cholic acid diet in spite of the markedly higher levels of hepatic tauro-conjugated bile acids. Unconjugated cholic acid was not detected in the bile of wild-type mice fed a control diet, but it was readily detected in wild-type mice fed 1% cholic acid diet. The ratio of biliary unconjugated cholic acid to total cholic acid (unconjugated cholic acid and tauro-conjugated cholic acid) reached 30% under conditions of hepatic taurine depletion. These results suggest that the cholic acid-induced enhancement of canalicular bile acid output rates and excretion of unconjugated bile acids are involved in adaptive responses for prevention of cholic acid-induced toxicity.

Address correspondence to: Dr. Masaaki Miyata, Division of Drug Metabolism and Molecular Toxicology, Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai, 980-8578, Japan. E-mail: miyata{at}mail.pharm.tohoku.ac.jp

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