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 enterohepatic circulation of bile acids is maintained by Na+-dependent transport mechanisms. To better understand these processes, a full-length human ileal Na+-bile acid cotransporter cDNA was identified using rapid amplification of cDNA ends and genomic cloning techniques. Using Northern blot analysis to determine its tissue expression, we readily detected the ileal Na+-bile acid cotransporter mRNA in terminal ileum and kidney. Direct cloning and mapping of the transcriptional start sites confirmed that the kidney cDNA was identical to the ileal Na+-bile acid cotransporter. In transiently transfected COS cells, ileal Na+-bile acid cotransporter-mediated taurocholate uptake was strictly Na+ dependent and chloride independent. Analysis of the substrate specificity in transfected COS or CHO cells showed that both conjugated and unconjugated bile acids are efficiently transported. When the inhibition constants for other potential substrates such as estrone-3-sulfate were determined, the ileal Na+-bile acid cotransporter exhibited a narrower substrate specificity than the related liver Na+-bile acid cotransporter. Whereas the multispecific liver Na+-bile acid cotransporter may participate in hepatic clearance of organic anion metabolites and xenobiotics, the ileal and renal Na+-bile acid cotransporter retains a narrow specificity for reclamation of bile acids.

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