Abstract
We investigated the roles of hydrophobic deoxycholic acid (DCA) and hydrophilic ursocholic acid (UCA) in the regulation of the orphan nuclear farnesoid X receptor (FXR) in vivo. Rabbits with bile fistula drainage (removal of the endogenous bile acid pool), rabbits with bile fistula drainage and replacement with either DCA or UCA, and intact rabbits fed 0.5% cholic acid (CA) (enlarged endogenous bile acid pool) were studied. After bile fistula drainage, cholesterol 7alpha-hydroxylase (CYP7A1) mRNA and activity levels increased, FXR-mediated transcription was decreased, and FXR mRNA and nuclear protein levels declined. Replacing the enterohepatic bile acid pool with DCA restored FXR mRNA and nuclear protein levels and activated FXR-mediated transcription as evidenced by the increased expression of its target genes, SHP and BSEP, and decreased CYP7A1 mRNA level and activity. Replacing the bile acid pool with UCA also restored FXR mRNA and nuclear protein levels but did not activate FXR-mediated transcription, because the SHP mRNA level and CYP7A1 mRNA level and activity were unchanged. Feeding CA to intact rabbits expanded the bile acid pool enriched with the FXR high affinity ligand, DCA. FXR-mediated transcription became activated as shown by increased SHP and BSEP mRNA levels and decreased CYP7A1 mRNA level and activity but did not change FXR mRNA or nuclear protein levels. Thus, both hydrophobic and hydrophilic bile acids are effective in maintaining FXR mRNA and nuclear protein levels. However, the activating ligand (DCA) in the enterohepatic flux is necessary for FXR-mediated transcriptional regulation, which leads to down-regulation of CYP7A1.
Highlights
We investigated the roles of hydrophobic deoxycholic acid (DCA) and hydrophilic ursocholic acid (UCA) in the regulation of the orphan nuclear farnesoid X receptor (FXR) in vivo
Bile Fistula Drainage—As we have described previously [12], after 7 days of bile fistula drainage the secondary bile acid, DCA, disappeared from the hepatic bile, which indicated that the enterohepatic bile acid pool/flux was totally depleted and that no further intestinal bile acids had returned to the liver
We attempted to clarify: 1) the critical role of the FXR-activating ligand (DCA) in the enterohepatic bile acid flux for FXR activation; 2) whether bile acids play any role in the regulation of FXR transcription aside from a ligand role; and 3) whether FXR is the dominant regulator for bile acid synthesis (CYP7A1) and the canalicular transporter (BSEP)
Summary
Replacing the enterohepatic bile acid pool with DCA restored FXR mRNA and nuclear protein levels and activated FXR-mediated transcription as evidenced by the increased expression of its target genes, SHP and BSEP, and decreased CYP7A1 mRNA level and activity. To examine the proposed theories of the role of bile acids in FXR activation and the regulation of CYP7A1 under in vivo conditions, this study was carried out in rabbits with depleted bile acid pool/flux, which was replaced with either hydrophobic DCA or hydrophilic ursocholic acid (UCA), and intact rabbits acid); CDCA, chenodeoxycholic acid (3␣,7␣-dihydroxy-5-cholanoic acid); CYP7A1, cholesterol 7␣-hydroxylase; DCA, deoxycholic acid (3␣,12␣ dihydroxy-5-cholanoic acid); FTF, ␣-fetoprotein transcription factor; LA, lithocholic acid (3␣-hydroxy-5-cholanoic acid); LRH-1, liver receptor homolog-1; RXR, 9-cis-retinoic acid receptor; SHP, short heterodimer partner; UCA, ursocholic acid (3␣,7,12␣-trihydroxy-5-cholanoic acid)
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