Effects of bile salt flux variations on the expression of hepatic bile salt transporters in vivo in mice

Background/Aims: Expression of hepatic bile salt transporters is partly regulated by bile salts via activation of nuclear farnesoid X-activated receptor (Fxr). We investigated the physiological relevance of this regulation by evaluating transporter expression in mice experiencing different transhepatic bile salt fluxes. Methods: Bile salt flux was manipulated by dietary supplementation with taurocholate (0.5% w/w) or cholestyramine (2% w/w) or by disruption of the cholesterol 7α-hydroxylase-gene (Cyp7A−/− mice) leading to reduced bile salt pool size. Expression of hepatic transporters was assessed (polymerase chain reaction (PCR), immunoblotting, and immunohistochemistry). Results: Biliary bile salt secretion was increased (+350%) or decreased (−50%) after taurocholate or cholestyramine feeding, respectively, but plasma bile salt concentrations and hepatic Fxr expression were not affected. The bile salt uptake system Na+-taurocholate co-transporting polypeptide (Ntcp) and organic anion transporting polypeptide-1 (Oatp1) were down-regulated by taurocholate and not affected by cholestyramine feeding. Cyp7A−/− mice did not show altered Ntcp or Oatp1 expression. Canalicular bile salt export pump (Bsep) was up-regulated by 65% in taurocholate-fed mice, and slightly down-regulated in Cyp7A−/− mice. Conclusions: Large variations in hepatic bile salt flux have minor effects on expression of murine Ntcp and Bsep in vivo, suggesting that these transporters are abundantly expressed and able to accommodate a wide range of ‘physiological’ bile salt fluxes.