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Extrahepatic cholestasis downregulates Oatp1 by TNF-α signalling without affecting Oatp2 and Oatp4 expression and sodium-independent bile salt uptake in rat liver


Andreas Geier, MD, Department of Internal Medicine III, Division of Gastroenterology and Hepatology, University Hospital Aachen (UKA), Aachen University (RWTH), Pauwelsstrasse 30, D-52074 Aachen, Germany.
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Hepatic uptake of bile salts is mediated by sodium-dependent and sodium-independent transport systems. During extrahepatic cholestasis, both the function and the expression of the Na+/taurocholate cotransporting polypeptide (Ntcp) are downregulated. To test whether sodium-independent organic anion-transporting polypeptides are also affected by extrahepatic cholestasis, the function and expression of all three Oatps have been determined in common bile duct-ligated (CBDL) rats. Oatp1/Oatp1a1 protein mass remained unchanged after CBDL for 1 day, but then declined by 75±7% and 90±17%, respectively, after 3 and 7 days. In contrast, Oatp2/Oatp1a4 and Oatp4/Oatp1b2 protein expression was not affected by CBDL as compared with controls. After CBDL, Oatp1 mRNA was rapidly downregulated by 68±21% of untreated controls (P<0.05) within 24 h, and remained at similar levels at 3 and 7 days. Cytokine-inactivation studies with etanercept pretreatment demonstrated that TNF-α-dependent signals mediated the down-regulation of this transporter gene at both protein and mRNA levels during obstructive cholestasis. Sodium-independent uptake of taurocholate and cholate into freshly isolated hepatocyte suspensions showed neither significant differences in Km nor Vmax values. These results indicate that sodium-independent transport of bile salts may be mediated by Oatp2 and 4 during biliary obstruction, because its expression remains unaffected and may compensate for loss of Oatp1 expression and function in cholestatic hepatocytes.

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