The organic solute transporter α-β, Ostα-Ostβ, is essential for intestinal bile acid transport and homeostasis

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 alpha-Ost beta exhibits many properties predicted for a candidate intestinal basolateral bile acid transporter, the in vivo functions of Ost alpha-Ost beta have not been investigated. To determine the role of Ost alpha-Ost beta in intestinal bile acid absorption, the Ost alpha gene was disrupted by homologous recombination in mice. Ost alpha(-/-) mice were physically indistinguishable from wild-type mice. In everted gut sac experiments, transileal transport of taurocholate was reduced by >80% in Ost alpha(-/-) vs. wild-type mice; the residual taurocholate transport was further reduced to near-background levels in gut sacs prepared from Ost alpha(-/-)Mrp3(-/-) mice. The bile acid pool size was significantly reduced (>65%) in Ost alpha(-/-) mice, but fecal bile acid excretion was not elevated. The decreased pool size in Ost alpha(-/-) 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 alpha-Ost beta 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.