Ursodeoxycholate reduces ethinylestradiol glucuronidation in the rat: role in prevention of estrogen-induced...

Ethinylestradiol (EE) administration (5 mg/kg, s.c., daily for 5 days) to rats leads to cholestasis, and its derivative EE 17beta-glucuronide is a likely mediator of this effect. Coadministration of ursodeoxycholate (UDC) was shown to prevent ethinylestradiol-induced cholestasis. The aim of this study was to evaluate the inhibitory effect of UDC on EE glucuronidation in vivo and in vitro as a potential mechanism to explain UDC protection. UDC treatment (25 mg/kg, i.p., daily for 5 days) decreased the biliary excretion of EE 17beta-glucuronide in bile after administration of a trace dose of [3H]EE and reduced microsomal EE 17beta-glucuronidation activity by 20% and expression of UGT2B1, one of the enzymes involved in EE conjugation, by 30%. Glucuronidation kinetic studies were performed in vitro using normal microsomes and isolated hepatocytes in the presence of tauroursodeoxycholate (TUDC), the major endogenous derivative of UDC in the rat. Kinetic enzymatic studies in microsomes showed a noncompetitive inhibition of EE 17beta-glucuronidation by TUDC, which was unique for this bile salt since other endogenous bile salts such as taurocholate, taurochenodeoxycholate, or taurodeoxycholate did not affect the enzyme activity. Studies in isolated hepatocytes confirmed the inhibitory effect of TUDC on EE glucuronidation and indicated that TUDC can reach the enzyme active site in intact cells. In conclusion, both in vivo and in vitro experiments indicate that UDC decreased the metabolic pathways involved in EE glucuronidation, hence decreasing the formation of the cholestatic derivative EE 17beta-glucuronide.