Cholestatic properties and hepatic transport of steroid glucuronides.

In summary, the data suggest that E217G is transported by both MOAT and P-glycoprotein into bile, but that P-glycoprotein serves as the target site for cholestasis. We postulate that this target site may be accessed from either the intracellular compartment or the canaliculus, and that MOAT serves as the major delivery route for E217G to the canaliculus. At low, physiologic concentrations of E217G, MOAT-mediated excretion into bile is a detoxification mechanism, serving to prevent intracellular accumulation of a toxic metabolite. However, following administration of high, cholestatic doses, MOAT-mediated excretion into bile results in very high concentrations in bile, on the other of 2-3 mM (see Fig. 4). It is likely that the hydrophobic nature of E217G allows it to partition from bile into the canalicular membrane, from which it can access P-glycoprotein and thus induce cholestasis. Much work is still needed to validate this model of E217G cholestasis. Definitive evidence of P-glycoprotein-mediated transport of E217G must be obtained in cell lines transfected with P-glycoprotein where MRP is absent. More importantly, the mechanism by which interaction of E217G with P-glycoprotein influences bile flow is unknown. Higgins and colleagues have provided evidence that P-glycoprotein regulates a Cl- channel in a manner analogous to that of CFTR, the cystic fibrosis transmembrane conductance regulator. While Cl- channels have been shown to be important in the regulation of the volume of the hepatocyte in the presence of altered osmotic conditions, a role for this channel in bile flow has not been demonstrated. Nevertheless, these studies implicate a role of P-glycoprotein in the regulation of bile secretion by the liver.