Expression and localization of the conjugate export pump encoded by the MRP2 (cMRP/cMOAT) gene in liver

The liver converts endogenous and xenobiotic lipophilic compounds into anionic conjugates with glutathione, glucuronate, or sulfate. These conjugates are transported across the canalicular (apical) membrane into bile by a 190 kDa membrane glycoprotein that has been cloned recently. This apical conjugate-transporting ATPase has been termed canalicular multidrug resistance protein (cMRP) because of the similarity in substrate specificity and sequence resistance protein canalicular multispecific organic anion transporter (cMOAT), or multidrug resistance protein 2 (MRP2). The amino acid sequence identity of human MRP2 and MRP1 is 49%. MRP2 is predominantly expressed in hepatocytes and localized to apical membrane domains, MRP2 is not expressed in the human Dubin-Johnson syndrome, which is therefore associated with an inherited deficiency in the secretion of amphiphilic anionic conjugates into the bile. The rat homolog Mrp2 is absent in two mutant strains of rats with different point mutations in the corresponding gene. These mutant rats are hyperbilirubinemic and deficient in the ATP-dependent transport of conjugates from hepatocytes into bile. Impairment of bile flow (cholestasis) can be associated with a down-regulation of the expression of the conjugate export pump, and MRP2 contributes to bile flow as an important driving force.