Ethinyl estradiol cholestasis involves alterations in expression of liver sinusoidal transporters

The mechanisms involved in ethinyl estradiol-induced cholestasis are controversial. Basal bile flow was reduced by ethinyl estradiol administration, with a half time (t( 1/4 )) of 12.5 ± 0.6 h. In contrast, initial taurocholate uptake was not significantly reduced until 3 days to 59% of control and to 13 and 10% of control at 5 and 7 days, respectively. The t( 1/4 ) was 4.3 ± 0.1 days. These physiological changes were correlated with measurement of protein mass and steady-state mRNA for Na⁺-K⁺- adenosinetriphosphatase (Na⁺-K⁺-ATPase), Na⁺-dependent taurocholate transporter, organic anion transporters, and membrane lipid fluidity. Ethinyl estradiol significantly decreased Na⁺-K⁺-ATPase activity and membrane fluidity. However, neither Na⁺-K⁺-ATPase α-subunit nor β-subunit mass was altered by ethinyl estradiol administration. In contrast, protein content of the Na⁺-dependent taurocholate transporter was significantly reduced to 21% of control (P < 0.001) at 5 days. The Na⁺-dependent taurocholate transporter was identified in sinusoidal membrane fractions as a doublet with a molecular size estimated to be 51 and 56 kDa. Although both bands were reduced with ethinyl estradiol treatment, the 56-kDa band was decreased more rapidly and to a greater extent than the 51-kDa band. The estimated t( 1/4 ) of 4.8 ± 0.6 days for the doublet was similar to that for Na⁺-dependent taurocholate uptake. The organic anion transporter protein mass was similarly reduced with time of ethinyl estradiol administration to 21% of control (P < 0.01) at 5 days. Ethinyl estradiol also rapidly decreased the steady-state mRNA levels of Na⁺-dependent and organic anion transporters to ~50% and 15% of control at 5 days, respectively. These studies indicate early generalized abnormalities of the sinusoidal membrane lipid fluidity, Na⁺-K⁺-ATPase activity, and bile acid transport protein content.