showing bile acid–dependent liver growth and regeneration.18 Serum ALT and AST levels were higher in both cholic acid–fed groups, but not different between WT and KO mice (data not shown).

However, serum total bile acid levels in cholic acid–fed KO mice were approximately five-fold higher and hepatic bile acid levels three-fold higher than in WT mice, suggesting defective ability to regulate serum and liver bile acid levels (Fig. 6C,D). Expression of the sinusoidal bile acid uptake transporters Ntcp, Slco1a1, and Slco1b2 were lower in both cholic acid–fed groups (Fig. 7A). Expression of the latter two genes was also lower in chow-fed KO mice. Expression of the basolateral efflux transporters Abcc3 (Mrp3) and Abcc4 (Mrp4) was not different, Wnt inhibitor although there was a trend toward higher 5-Fluoracil in vitro Abcc4 levels in KO mice. Among the canalicular transporters, expression of Bsep (Abcb11) was lower in cholic acid–fed KO mice, whereas Abcb4 (Mdr2) was up-regulated in both cholic acid–fed

groups (Fig. 7B). Expression levels of other sinusoidal transporters were similar between the cholic acid–fed groups. Of the bile acid regulatory genes, expression of farnesoid X receptor (FXR) was lower in both cholic acid–fed groups. However, Shp-1 expression, a downstream target of FXR, was up-regulated in both WT and KO mice on a cholic acid diet (Fig. 7C). Fgf4r levels were similar in all four groups. Expression of constitutive androstane receptor (CAR) was lower in both cholic acid–fed groups and in chow-fed KO mice. However, despite lower expression levels of CAR, KO mice had higher expression of the downstream target of CAR, Cyp2b10. This result suggests that activation of CAR, possibly by a post-transcriptional mechanism, occurs in KO mice and may represent

a Methane monooxygenase protective mechanism to down-regulate bile acid biosynthetic genes. PPARα expression was higher in both cholic acid groups whereas pregnane X receptor expression was lower in cholic acid–fed KO mice. After H&E staining, histology samples from WT mice showed mildly increased lobular inflammation on cholic acid diet (Fig. 8). On the other hand, cholic acid–fed KO mice had increased ductular reaction and greater lobular inflammation on cholic acid diet along with increased pericellular fibrosis on reticulin staining. Staining for F-actin showed that WT mice on cholic acid diet had dilated bile canaliculi, likely reflecting the choleretic effect of bile acids (Fig. 8, bottom panel). However, the interconnected lattice-like structure of canaliculi was preserved in WT livers on cholic acid diet. On the other hand, KO mice on cholic acid diet had canaliculi that were not only dilated but also tortuous and distorted (Fig. 8P). Formation and excretion of bile is one of the important functions of the liver. This task is achieved by the coordinated action of multiple proteins and cellular organelles.