Characterization of conjugated and unconjugated bile acid transport via human organic solute transporter α/β

Abstract Organic solute transporter (OST) α/β is a key bile acid transporter expressed in various organs, including the liver under cholestatic conditions. However, little is known about the involvement of OSTα/β in bile acid-mediated drug-induced liver injury (DILI), a major safety concern in drug development. This study investigated whether OSTα/β preferentially transports more hepatotoxic, conjugated, primary bile acids and to what extent xenobiotics inhibit this transport. Kinetic studies with OSTα/β-overexpressing cells revealed that OSTα/β preferentially transported bile acids in the following order: taurochenodeoxycholate > glycochenodeoxycholate > taurocholate > glycocholate. The apparent half-maximal inhibitory concentrations for OSTα/β-mediated bile acid (5 µM) transport inhibition by fidaxomicin, troglitazone sulfate, and ethinyl estradiol were: 210, 334, and 1050 µM, respectively, for taurochenodeoxycholate; 97.6, 333, and 337 µM, respectively, for glycochenodeoxycholate; 140, 265, and 527 µM, respectively, for taurocholate; 59.8, 102, and 117 µM, respectively, for glycocholate. The potential role of OSTα/β in hepatocellular glycine-conjugated bile acid accumulation and cholestatic DILI was evaluated using sandwich-cultured human hepatocytes (SCHH). Treatment of SCHH with the farnesoid X receptor agonist chenodeoxycholate (100 µM) resulted in substantial OSTα/β induction, among other proteomic alterations, reducing glycochenodeoxycholate and glycocholate accumulation in cells+bile 4.0- and 4.5-fold, respectively. Treatment of SCHH with troglitazone and fidaxomicin together under cholestatic conditions resulted in increased hepatocellular toxicity compared with either compound alone, suggesting that OSTα/β inhibition may accentuate DILI. In conclusion, this study provides insights into the role of OSTα/β in preferential disposition of bile acids associated with hepatotoxicity, the impact of xenobiotics on OSTα/β-mediated bile acid transport, and the role of this transporter in SCHH and cholestatic DILI.

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The nuclear receptor for bile acids, FXR, transactivates human organic solute transporter-α and -β genes

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.00430.2005 ◽

2006 ◽

Vol 290 (3) ◽

pp. G476-G485 ◽

Cited By ~ 136

Author(s):

Jean-François Landrier ◽

Jyrki J. Eloranta ◽

Stephan R. Vavricka ◽

Gerd A. Kullak-Ublick

Keyword(s):

Bile Acid ◽

Bile Acids ◽

Cultured Cells ◽

Farnesoid X Receptor ◽

Mrna Levels ◽

Small Interfering Rnas ◽

Organic Solute Transporter ◽

Genes Encoding ◽

Organic Solute ◽

Solute Transporter

Bile acids are synthesized from cholesterol in the liver and are excreted into bile via the hepatocyte canalicular bile salt export pump. After their passage into the intestine, bile acids are reabsorbed in the ileum by sodium-dependent uptake across the apical membrane of enterocytes. At the basolateral domain of ileal enterocytes, bile acids are extruded into portal blood by the heterodimeric organic solute transporter OSTα/OSTβ. Although the transport function of OSTα/OSTβ has been characterized, little is known about the regulation of its expression. We show here that human OSTα/OSTβ expression is induced by bile acids through ligand-dependent transactivation of both OST genes by the nuclear bile acid receptor/farnesoid X receptor (FXR). FXR agonists induced endogenous mRNA levels of OSTα and OSTβ in cultured cells, an effect that was not discernible upon inhibition of FXR expression by small interfering RNAs. Furthermore, OST mRNAs were induced in human ileal biopsies exposed to the bile acid chenodeoxycholic acid. Reporter constructs containing OSTα or OSTβ promoters were transactivated by FXR in the presence of its ligand. Two functional FXR binding motifs were identified in the OSTα gene and one in the OSTβ gene. Targeted mutation of these elements led to reduced inducibility of both OST promoters by FXR. In conclusion, the genes encoding the human OSTα/OSTβ complex are induced by bile acids and FXR. By coordinated control of OSTα/OSTβ expression, bile acids may adjust the rate of their own efflux from enterocytes in response to changes in intracellular bile acid levels.

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Identification and functional characterization of the bile acid transport proteins in non-mammalian ileum and mammalian liver

Proteins Structure Function and Bioinformatics ◽

10.1002/prot.21518 ◽

2007 ◽

Vol 70 (2) ◽

pp. 462-472 ◽

Cited By ~ 23

Author(s):

Mara Guariento ◽

Domenico Raimondo ◽

Michael Assfalg ◽

Serena Zanzoni ◽

Patrizia Pesente ◽

...

Keyword(s):

Bile Acid ◽

Functional Characterization ◽

Transport Proteins ◽

Acid Transport ◽

Bile Acid Transport ◽

Mammalian Liver

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Regulation of the mouse organic solute transporter α-β, Ostα-Ostβ, by bile acids

AJP Gastrointestinal and Liver Physiology ◽

10.1152/ajpgi.00479.2005 ◽

2006 ◽

Vol 290 (5) ◽

pp. G912-G922 ◽

Cited By ~ 75

Author(s):

Tamara Frankenberg ◽

Anuradha Rao ◽

Frank Chen ◽

Jamie Haywood ◽

Benjamin L. Shneider ◽

...

Keyword(s):

Bile Acid ◽

Bile Acids ◽

Promoter Activity ◽

Colon Adenocarcinoma ◽

Dominant Negative ◽

Farnesoid X Receptor ◽

Negative Feedback Regulation ◽

Organic Solute Transporter ◽

Organic Solute ◽

Solute Transporter

The mechanisms responsible for bile acid regulation of mouse intestinal organic solute transporter α-β (Ostα-Ostβ) expression were investigated. Expression of Ostα-Ostβ mRNA was increased in cecum and proximal colon of cholic acid-fed mice and in chenodeoxycholate-treated mouse CT26 colon adenocarcinoma cells. Sequence analysis revealed potential cis-acting elements for farnesoid X receptor (FXR) and liver receptor homolog-1 (LRH-1) in the mouse Ostα and Ostβ promoters and reporter constructs containing Ostα and Ostβ 5′-flanking sequences were positively regulated by bile acids. Expression of a dominant-negative FXR, reduction of FXR with interfering small RNA (siRNA), or mutation of the potential FXR elements decreased Ostα and Ostβ promoter activity and abolished the induction by chenodeoxycolic acid. Negative regulation of the Ostα and Ostβ promoters by bile acids was mediated through LRH-1 elements. Ostα and Ostβ promoter activities were increased by coexpression of LRH-1 and decreased by coexpression of SHP. Mutation of the potential LRH-1 elements and siRNA-mediated reduction of LRH-1 expression decreased basal promoter activity. As predicted from the promoter analyses, ileal Ostα and Ostβ mRNA expressions were increased in wild-type mice administered the FXR agonist GW4064 and decreased in FXR-null mice. Immunoblotting analysis revealed that Ostα and Ostβ intestinal protein expressions correlated with mRNA expression. The mouse Ostα and Ostβ promoters are unusual in that they contain functional FXR and LRH elements, which mediate, respectively, positive and negative feedback regulation by bile acids. Although the positive regulatory pathway appears to be dominant, this arrangement provides a mechanism to finely titrate Ostα-Ostβ expression to the bile acid flux.

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