Bile acids are transported across the ileal enterocyte brush border membrane by the well characterized apical sodium-dependent bile acid transporter (Asbt) Slc10a2; however, the carrier(s) responsible for transporting bile acids across the ileocyte basolateral membrane into the portal circulation have not been fully identified. Transcriptional profiling of wild type and Slc10a2 null mice was employed to identify a new candidate basolateral bile acid carrier, the heteromeric organic solute transporter (Ost)␣-Ost␤. By Northern blot analysis, Ost␣ and Ost␤ mRNA was detected only in mouse kidney and intestine, mirroring the horizontal gradient of expression of Asbt in the gastrointestinal tract. Analysis of Ost␣ and Ost␤ protein expression by immunohistochemistry localized both subunits to the basolateral surface of the mouse ileal enterocyte. The transport properties of Ost␣-Ost␤ were analyzed in stably transfected Madin-Darby canine kidney cells. Coexpression of mouse Ost␣-Ost␤, but not the individual subunits, stimulated Na ؉ -independent bile acid uptake and the apical-to-basolateral transport of taurocholate. In contrast, basolateral-to-apical transport was not affected by Ost␣-Ost␤ expression. Co-expression of Ost␣ and Ost␤ was required to convert the Ost␣ subunit to a mature glycosylated endoglycosidase H-resistant form, suggesting that co-expression facilitates the trafficking of Ost␣ through the Golgi apparatus. Immunolocalization studies showed that co-expression was necessary for plasma membrane expression of both Ost␣ and Ost␤. These results demonstrate that the mouse Ost␣-Ost␤ heteromeric transporter is a basolateral bile acid carrier and may be responsible for bile acid efflux in ileum and other ASBT-expressing tissues.

The ileal apical sodium bile acid cotransporter participates in the enterohepatic circulation of bile acids. In patients with primary bile acid malabsorption, mutations in the ileal bile acid transporter gene (Slc10a2) lead to congenital diarrhea, steatorrhea, and reduced plasma cholesterol levels. To elucidate the quantitative role of Slc10a2 in intestinal bile acid absorption, the Slc10a2 gene was disrupted by homologous recombination in mice. Animals heterozygous (Slc10a2+/-) and homozygous (Slc10a2-/-) for this mutation were physically indistinguishable from wild type mice. In the Slc10a2-/- mice, fecal bile acid excretion was elevated 10- to 20-fold and was not further increased by feeding a bile acid binding resin. Despite increased bile acid synthesis, the bile acid pool size was decreased by 80% and selectively enriched in cholic acid in the Slc10a2-/- mice. On a low fat diet, the Slc10a2-/- mice did not have steatorrhea. Fecal neutral sterol excretion was increased only 3-fold, and intestinal cholesterol absorption was reduced only 20%, indicating that the smaller cholic acid-enriched bile acid pool was sufficient to facilitate intestinal lipid absorption. Liver cholesteryl ester content was reduced by 50% in Slc10a2-/- mice, and unexpectedly plasma high density lipoprotein cholesterol levels were slightly elevated. These data indicate that Slc10a2 is essential for efficient intestinal absorption of bile acids and that alternative absorptive mechanisms are unable to compensate for loss of Slc10a2 function.

The apical sodium-dependent bile acid transporter (Asbt) is responsible for transport across the intestinal brush border membrane; however, the carrier(s) responsible for basolateral bile acid export into the portal circulation remains to be determined. Although the heteromeric organic solute transporter Ost␣-Ost␤ exhibits many properties predicted for a candidate intestinal basolateral bile acid transporter, the in vivo functions of Ost␣-Ost␤ have not been investigated. To determine the role of Ost␣-Ost␤ in intestinal bile acid absorption, the Ost␣ gene was disrupted by homologous recombination in mice. Ost␣ ؊/؊ mice were physically indistinguishable from wild-type mice. In everted gut sac experiments, transileal transport of taurocholate was reduced by >80% in Ost␣ ؊/؊ vs. wild-type mice; the residual taurocholate transport was further reduced to near-background levels in gut sacs prepared from Ost␣ ؊/؊ Mrp3 ؊/؊ mice. The bile acid pool size was significantly reduced (>65%) in Ost␣ ؊/؊ mice, but fecal bile acid excretion was not elevated. The decreased pool size in Ost␣ ؊/؊ mice resulted from reduced hepatic Cyp7a1 expression that was inversely correlated with ileal expression of fibroblast growth factor 15 (FGF15). These data indicate that Ost␣-Ost␤ is essential for intestinal bile acid transport in mice. Unlike a block in intestinal apical bile acid uptake, genetic ablation of basolateral bile acid export disrupts the classical homeostatic control of hepatic bile acid biosynthesis.cholesterol ͉ liver disease ͉ mouse model ͉ nuclear receptor

Regulation of the mouse organic solute transporter ␣-␤, Ost␣-Ost␤, by bile acids. Am J Physiol Gastrointest Liver Physiol 290: G912-G922, 2006. First published December 15, 2005 doi:10.1152/ajpgi.00479.2005.-The mechanisms responsible for bile acid regulation of mouse intestinal organic solute transporter ␣-␤ (Ost␣-Ost␤) expression were investigated. Expression of Ost␣-Ost␤ mRNA was increased in cecum and proximal colon of cholic acid-fed mice and in chenodeoxycholatetreated 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 Ost␣ and Ost␤ promoters and reporter constructs containing Ost␣ and Ost␤ 5Ј-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 Ost␣ and Ost␤ promoter activity and abolished the induction by chenodeoxycolic acid. Negative regulation of the Ost␣ and Ost␤ promoters by bile acids was mediated through LRH-1 elements. Ost␣ and Ost␤ 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 Ost␣ and Ost␤ mRNA expressions were increased in wild-type mice administered the FXR agonist GW4064 and decreased in FXR-null mice. Immunoblotting analysis revealed that Ost␣ and Ost␤ intestinal protein expressions correlated with mRNA expression. The mouse Ost␣ and Ost␤ 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 Ost␣-Ost␤ expression to the bile acid flux.

Background & Aims Blocking intestinal bile acid (BA) absorption by inhibiting or inactivating the apical sodium-dependent BA transporter (Asbt) classically induces hepatic BA synthesis. In contrast, blocking intestinal BA absorption by inactivating the basolateral BA transporter, organic solute transporter alpha–beta (Ostα–Ostβ) is associated with an altered homeostatic response and decreased hepatic BA synthesis. The aim of this study was to determine the mechanisms underlying this phenotype, including the role of the farnesoid X receptor (FXR) and fibroblast growth factor 15 (FGF15). Methods BA and cholesterol metabolism, intestinal phenotype, expression of genes important for BA metabolism, and intestinal FGF15 expression were examined in wild type, Ostα−/−, Fxr−/−, and Ostα−/−Fxr−/− mice. Results Inactivation of Ostα was associated with decreases in hepatic cholesterol 7α-hydroxylase (Cyp7a1) expression, BA pool size, and intestinal cholesterol absorption. Ostα−/− mice exhibited significant small intestinal changes, including altered ileal villus morphology, and increases in intestinal length and mass. Total ileal FGF15 expression was elevated almost 20-fold in Ostα−/− mice as a result of increased villus epithelial cell number and ileocyte FGF15 protein expression. Ostα−/−Fxr−/− mice exhibited decreased ileal FGF15 expression, restoration of intestinal cholesterol absorption, and increases in hepatic Cyp7a1 expression, fecal BA excretion, and BA pool size. FXR deficiency did not reverse the intestinal morphological changes or compensatory decrease for ileal Asbt expression in Ostα−/− mice. Conclusions These results indicate that signaling via FXR is required for the paradoxical repression of hepatic BA synthesis but not the complex intestinal adaptive changes in Ostα−/− mice.