Effects of Ursodeoxycholic and Cholic Acid Feeding on Hepatocellular Transporter Expression in Mouse Liver PETER FICKERT,* GERNOT ZOLLNER,* ANDREA FUCHSBICHLER, ‡ CONNY STUMPTNER, ‡ CHRISTINE POJER,* RAINER ZENZ, ‡ FRANK LAMMERT, § BRUNO STIEGER,  PETER J. MEIER,  KURT ZATLOUKAL, ‡ HELMUT DENK, ‡ and MICHAEL TRAUNER* *Departments of Medicine and ‡ Pathology, Karl-Franzens University, Graz, Austria; § Department of Medicine, University of Aachen, Aachen, Germany;  Division of Clinical Pharmacology and Toxicology, Department of Medicine, University Hospital Zurich, Zurich, Switzerland Background & Aims: Cholestasis is associated with re- tention of potentially toxic bile acids and alterations in hepatocellular transporter expression. Conversely, non- toxic ursodeoxycholic acid (UDCA) stimulates bile secre- tion and counteracts cholestasis. This study aimed to determine the effects of UDCA and cholic acid (CA) on the expression of hepatocellular transporters for bile acids (Ntcp, Bsep), organic anions (Oatp1, Mrp2), or- ganic cations (Mdr1a/b), and phospholipids (Mdr2) in mouse liver. Methods: Bile flow/composition was ana- lyzed in UDCA- or CA-fed mice. Transporter expression was studied by reverse-transcription polymerase chain reaction, Western blotting, and immunofluorescence mi- croscopy. Results: UDCA had no effect on basolateral Ntcp and down-regulated Oatp1, whereas canalicular Bsep and Mrp2 were up-regulated. CA down-regulated basolateral Ntcp and Oatp1, whereas canalicular Bsep, Mrp2, and Mdr1a/b were up-regulated. Neither UDCA nor CA affected Mdr2 expression. Both UDCA and CA stimulated biliary bile acid and glutathione excretion, although only CA increased phospholipid and choles- terol excretion. Conclusions: Down-regulation of basolat- eral and up-regulation of canalicular transporters in re- sponse to CA may represent a defense mechanism, in an attempt to prevent hepatocellular accumulation of potentially toxic bile acids. The therapeutic effects of UDCA may be caused in part by stimulation of canalic- ular transporter expression in the absence of hepatocel- lular toxicity. C holestasis results in systemic and intrahepatic reten- tion of potentially toxic bile acids that can cause liver injury, ultimately leading to biliary fibrosis and cirrhosis. 1,2 Down-regulation of hepatocellular transport systems may contribute to impaired hepatobiliary excre- tion of bile acids and other biliary constituents (e.g., conjugated bilirubin, glutathione) during cholestasis. 3,4 Bile acids regulate the expression of various genes, in- cluding those involved in the synthesis and intestinal transport of bile acids such as cholesterol 7-hydroxy- lase, intestinal bile acid– binding protein, and ileal so- dium-dependent bile acid transporter. 5–8 However, it is unknown whether bile acids accumulating during cho- lestasis contribute to the changes in hepatocellular trans- porter expression. Such information may be relevant for the development of new rational strategies for the treat- ment of cholestatic liver disorders. In contrast to potentially toxic bile acids, nontoxic, hydrophilic ursodeoxycholic acid (UDCA) is used to treat a variety of cholestatic liver diseases, including primary biliary cirrhosis (PBC), primary sclerosing cholangitis, cystic fibrosis, and intrahepatic cholestasis of preg- nancy. 2,9,10 The therapeutic benefit of UDCA in the treatment of cholestasis may result from a combination of cytoprotective, antiapoptotic, immunomodulatory, and choleretic effects. 1,2,9,10 UDCA stimulates hepatobiliary excretory function and, thereby, counteracts cholestasis in several ways, including stimulation of vesicular exo- cytosis and insertion of hepatobiliary transporters into the canalicular membrane. 2,10 –14 In addition, the anti- cholestatic effects of UDCA could in part be mediated by stimulation of gene expression of transport systems that are down-regulated in cholestatic liver diseases, as sug- gested by stimulation of Cl - /HCO 3 - exchanger expres- sion and function in UDCA-treated PBC patients. 15–17 However, the influence of UDCA on the expression of other hepatocellular transporters remains unclear. Abbreviations used in this paper: AP, alkaline phosphatase; Bsep, bile salt export pump; CA, cholic acid; FITC, fluorescein isothiocyanate; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GSH, glutathi- one; Lst-1, liver-specific organic anion transporter; Mdr1, multidrug export pump; Mdr2, phospholipid flippase; Mrp2, conjugate export pump; Ntcp, Na  /taurocholate cotransporter; Oatp, organic anion transporting polypeptide; PBC, primary biliary cirrhosis; PCR, polymer- ase chain reaction; Pgp, P-glycoprotein; RT-PCR, reverse-transcription polymerase chain reaction; TRITC, tetramethylrhodamine isothiocyanate. © 2001 by the American Gastroenterological Association 0016-5085/01/$35.00 doi:10.1053/gast.2001.25542 GASTROENTEROLOGY 2001;121:170 –183