FXR induces the UGT2B4 enzyme in hepatocytes: a potential mechanism of negative feedback control of FXR activity.
OBJECTIVE
Bile acids are essential for bile formation and intestinal absorption of lipids and fat-soluble vitamins. However, the intrinsic toxicity of hydrophobic bile acids demands a tight control of their intracellular concentrations. Bile acids are ligands for the farnesoid X receptor (FXR) that regulates the expression of genes controlling bile acid synthesis and transport. The human uridine 5'-diphosphate-glucuronosyltransferase 2B4 (UGT2B4) converts hydrophobic bile acids into more hydrophilic glucuronide derivatives. In this study, we identify UGT2B4 as an FXR target gene.


METHODS
Human hepatocytes or hepatoblastoma HepG2 cells were treated with chenodeoxycholic acid or the synthetic FXR agonist GW4064, and the levels of UGT2B4 messenger RNA, protein, and activity were determined by using real-time polymerase chain reaction, Western blot, and glucuronidation assays.


RESULTS
Treatment of hepatocytes and HepG2 cells with FXR agonists resulted in an increase of UGT2B4 messenger RNA, protein, and activity. A bile acid response element in the UGT2B4 promoter (B4-BARE) to which FXR, but not retinoid X receptor, binds, was identified by site-directed mutagenesis, electromobility shift, and chromatin immunoprecipitation assays. Retinoid X receptor activation abolished the induction of UGT2B4 expression and inhibited binding of FXR to the B4-BARE, suggesting that retinoid X receptor modulates FXR target gene activation. Overexpression of UGT2B4 in HepG2 cells resulted in the attenuation of bile acid induction of the FXR target gene small heterodimeric partner.


CONCLUSIONS
These data suggest that UGT2B4 gene induction by bile acids contributes to a feed-forward reduction of bile acid toxicity and a decrease of the activity of these biological FXR activators.
Barbier
Olivier
O
U545 INSERM, Department of Atherosclerosis, Faculty of Pharmacy, Lille Pasteur Institute and University of Lille II, 1 Rue du Pr Calmette, BP 245, 59019 Lille, France.
Torra
Ines Pineda
IP
Sirvent
Audrey
A
Claudel
Thierry
T
Blanquart
Christophe
C
Duran-Sandoval
Daniel
D
Kuipers
Folkert
F
Kosykh
Vladimir
V
Fruchart
Jean-Charles
JC
Staels
Bart
B
D001647:Bile Acids and Salts / Q000494:pharmacology; D002478:Cells, Cultured; D002635:Chenodeoxycholic Acid / Q000494:pharmacology; D004268:DNA-Binding Proteins; D004790:Enzyme Induction; D005246:Feedback; D014453:Glucuronosyltransferase / Q000096:biosynthesis* / Q000235:genetics; D022781:Hepatocytes / Q000201:enzymology*; D006801:Humans; D011401:Promoter Regions, Genetic; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology*; D020218:Response Elements; D014157:Transcription Factors; D000099150:Receptor, Farnesoid X-Activated
D001647:Bile Acids and Salts; D002635:Chenodeoxycholic Acid; D004268:DNA-Binding Proteins; D014453:Glucuronosyltransferase; D018160:Receptors, Cytoplasmic and Nuclear; D014157:Transcription Factors; D000099150:Receptor, Farnesoid X-Activated; C486404:UGT2B4 protein, human