Ursodeoxycholate reduces ethinylestradiol glucuronidation in the rat: Role of prevention in estrogen-induced cholestasis

Ethinylestradiol (EE) administration (5 mg/kg, s.c., daily for 5 days) to rats leads to cholestasis, and its derivative EE 17-glucuronide is a likely mediator of this effect. Coadministration of ursodeoxycholate (UDC) was shown to prevent ethinylestra-diol-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 17-glucuronide in bile after administration of a trace dose of [3H]EE and reduced microsomal EE 17-glucu-ronidation activity by 20 % and expression of UGT2B1, one of the enzymes involved in EE conjugation, by 30%. Glucuronida-tion kinetic studies were performed in vitro using normal micro-somes and isolated hepatocytes in the presence of taurour-sodeoxycholate (TUDC), the major endogenous derivative of UDC in the rat. Kinetic enzymatic studies in microsomes showed a noncompetitive inhibition of EE 17-glucuronidation by TUDC, which was unique for this bile salt since other en-dogenous bile salts such as taurocholate, taurochenodeoxy-cholate, or taurodeoxycholate did not affect the enzyme activ-ity. 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 de-creased the metabolic pathways involved in EE glucuronida-tion, hence decreasing the formation of the cholestatic deriva-tive EE 17-glucuronide. Administration of ethinylestradiol (EE), a synthetic estro-gen, is known to induce intrahepatic cholestasis in experi