Regulation of Drug Transporters by the Farnesoid X Receptor in Mice

The farnesoid X receptor (FXR, NR1H4) regulates bile acid and lipid homeostasis by acting as an intracellular bile acid-sensing transcription factor, resulting in altered expression of enzymes and transporters involved in bile acid synthesis and transport. Here, we quantitatively analyzed the alterations in expression levels of drug transporters, mainly organic anion-transporting polypeptides (oatp), in wild-type and FXR-null mice to evaluate the role of FXR in their expression and regulation by cholic acid. Changes in the mRNA amounts in liver, kidney, small intestine, and testis in FXR-null mice fed with or without a supplement of 0.5% cholic acid in the diet were analyzed by semiquantitative RT-PCR. In FXR-null mice, the mRNA levels of oatp1, oatp2, oatp3, and octn1 were lower than those of wild-type mice in kidney and testis, while there was no difference in liver or small intestine. Cholic acid feeding led to significantly decreased levels of expression of oatp1 and oct1 and an increased level of expression of oatp2 in wild-type mouse liver. In FXR-null mice, oatp1 and other transporters were downregulated in liver, kidney, and testis, whereas small intestine ASBT, octn2, and pept1 were upregulated. Our results suggested that FXR is involved in the transcriptional regulation of oatp and other transporters in a tissue-specific manner. Furthermore, the effect of cholic acid treatment indicates the involvement of regulatory mechanism(s) other than FXR. Keywords: Drug transporter; farnesoid X receptor; FXR-null mice; regulation