Ciprofibrate regulation of rat hepatic bilirubin glucuronidation and UDP-glucuronosyltransferases expression

Synthetic fibrates are hypolipidemic drugs known to stimulate hepatic peroxisome proliferation and bilirubin glucuronidation. This study was designed to estimate the effects of ciprofibrate simultaneously on rat hepatic bilirubin glucuronoconjugation and on hepatic expression of UGT1A1, UGT1A2 and UGT1A5, all of which belong to the bilirubin cluster. Hepatic bilirubin glucuronidation activity and UDP-glucuronosyltransferase expression (RT-PCR and Western blotting) were measured after a single-dose ciprofibrate treatment (5 mg/kg by gastric intubation) in 36-h time course experiments. Ciprofibrate regulation of PPARα and UGT1A5 mRNA expression was also investigated in rat hepatocytes. Bilirubin conjugation activity was induced by ciprofibrate, reaching a maximum level (2.4×) 24 h after the treatment. UGT1A1 and UGT1A5 mRNA expression was induced 1.5 times by ciprofibrate, with UGT1A5 reaching the basal level of UGT1A1. Although UGT1A2 mRNA was induced approximately threefold by ciprofibrate, its expression level remained low in comparison with basal or induced levels of UGT1A1 and UGT1A5 mRNA. In the 36-h time course experiment, bilirubin conjugation activity as well as UGT1A5 and PPARα mRNA expression presented a biphasic induction profile. Although a similar level of induction was observed in primary cultured hepatocyte experiments, such biphasic variation was not observed for both UGT1A5 and PPARα, and the induction of UGT1A5 mRNA expression by ciprofibrate required de novo protein synthesis. A single dose of ciprofibrate significantly induces rat liver bilirubin conjugation as well as UGT1A1, UGT1A5 and PPARα expression. The induction mechanism may involve PPARα, at least regarding UGT1A5 regulation.