Ntcp has a high capacity for transporting T- and G-conjugated bile acids,16, 17 whereas hydrophobic bile acids are thought to pass the cell membrane by passive diffusion.18, 19 Oatp1a1, Oatp1a4, and Oatp1b2 are all able to transport in vitro both conjugated and unconjugated BAs.16 In Oatp1b2-null mice, the hepatic expression MG-132 in vitro of Oatp1a1 remains unchanged, whereas that of
Ntcp, Oatp1a4, and Oatp2b1 tends to be higher (Fig. 5), similar to previous studies.6, 20 Thus, the marked accumulation of unconjugated BAs in the plasma of Oatp1b2-null mice is unlikely due to secondary changes in other BA transporters. Decrease of BA-conjugating enzymes could also contribute to the observed elevation of serum-unconjugated BAs. However, the possibility of decreased activity of conjugating enzymes is very low, because there are no significant differences in either mRNA expression of bile acid–coenzyme A ligase and bile acid coenzyme A:amino acid:N-acyltransferase
(Supporting Information Fig. 1) or the concentrations of conjugated and unconjugated BAs in livers of WT and Oatp1b2-null mice. The concentration of total serum BAs SB203580 order is approximately seven-fold higher in Oatp1b2-null mice than in WT mice, which is due to the marked accumulation (10- to 45-fold) of αMCA, βMCA, CA, HDCA, and UDCA in plasma of Oatp1b2-null mice. However, absence of the Oatp1b2 transporter does not increase the plasma concentration of conjugated bile acids, except for T-DCA. This indicates that Oatp1b2 is essential for the hepatic uptake of unconjugated hydrophilic bile acids. Recently, Xiang et al.21 reported that humans carrying low-activity OATP1B1 polymorphisms have higher blood levels of BAs. Therefore, concentrations of BAs in 12-month-old male Oatp1b2-null, heterozygous, and WT mice were Rolziracetam quantified. The 12-month-old Oatp1b2-heterozygous mice have blood levels of α-MCA, β-MCA, and CA that are intermediate between WT and Oatp1b2-null mice (Supporting Information Fig. 2). The clear gene-dosage effects of
Oatp1b2 on blood levels of BAs is consistent with the many changes in the pharmacokinetics of drugs and blood levels of endogenous molecules found in humans with low-activity OATP1B1 polymorphisms.22-24 Surprisingly, the increase in plasma concentrations of BAs in Oatp1b2-null mice is not reflected by decreases in hepatic concentrations of BAs. Interestingly, in livers of Oatp1b2-null mice, the mRNA expression of the basolateral efflux transporters, Mrp4 and Ost-α, is 40% and 50% lower, respectively, which might help to retain the BAs in the liver. The biliary excretion of BAs by Oatp1b2-null mice is about the same as in WT mice, except for less αMCA and DCA in the null mice. In Oatp1b2-null mice, there are no changes in the mRNA expression of canalicular efflux transporters, which are responsible for maintaining bile flow and the biliary excretion of BAs.