Regulation of basolateral organic anion transporters in ethinylestradiol-induced cholestasis in the rat

BACKGROUND/AIMS: Estrogen-mediated cholestasis is an important clinical entity, but its molecular pathophysiology is still not fully understood. Impaired sodium-dependent uptake of bile acids has been associated with diminished expression of a basolateral Na(+)/bile acid cotransporter (Ntcp), whereas sodium-independent uptake is maintained despite a down-regulation of the organic anion transporter Oatp1. Thus, expression of the two other rat Oatps (Oatps2 and -4) was determined in estrogen-induced cholestasis. In addition, known transactivators of Oatp2 and Ntcp were studied to further characterize transcriptional regulation of these transporter genes. METHODS: Hepatic protein and mRNA expression of various Oatps (1, 2, 4) in comparison to Ntcp were analyzed after 0.5, 1, 3 and 5 days of ethinylestradiol (EE) treatment (5 mg/kg) in rats. Binding activities of Oatp2 and Ntcp transactivators were assessed by electrophoretic mobility shift assays. RESULTS: All basolateral Oatps (1, 2 and 4) were specifically down-regulated at the protein level by 30-40% of controls, but less pronounced than Ntcp (minus 70-80%). In contrast to unaltered Oatp4 mRNA levels, Oatp1 and Oatp2 mRNAs were reduced to various extents (minus 40-90% of controls). Binding activity of known transactivators of Ntcp and Oatp2 such as hepatocyte nuclear factor 1 (HNF1), CAAT enhancer binding protein alpha (C/EBPalpha) and pregnane X receptor (PXR) were also diminished during the time of cholestasis. CONCLUSIONS: Estrogen-induced cholestasis results in a down-regulation of all basolateral organic anion transporters. The moderate decline in expression of Oatp1, -2 and -4 may explain the unchanged sodium-independent transport of bile acids due to overlapping substrate specificity. Reduction in transporter gene expression seems to be mediated by a diminished nuclear binding activity of transactivators such as HNF1, C/EBP and PXR by estrogens.