Hepatic secretion of conjugated drugs and endogenous substances.

Conjugate export pumps of the multidrug resistance protein (MRP) family mediate the ATP-dependent secretion of anionic conjugates across the canalicular and the basolateral hepatocyte membrane into bile and sinusoidal blood, respectively. Xenobiotic and endogenous lipophilic substances may be conjugated with glutathione, glucuronate, sulfate, or other negatively charged groups and thus become substrates for export pumps of the MRP family. The apical isoform, MRP2 (gene symbol ABCC2), has been localized to the apical membrane of several polarized epithelia and particularly to the canalicular membrane of hepatocytes. Absence of functionally active MRP2 glycoprotein from this membrane domain prevents the secretion of many anionic conjugates into bile. Prototypic endogenous substrates of high affinity for recombinant human MRP2 include bisglucuronosyl bilirubin, monoglucuronosyl bilirubin, and the glutathione S-conjugate leukotriene C4. Several mutations in the human MRP2 gene have been identified that lead to the absence of MRP2 from the canalicular membrane and to the conjugated hyperbilirubinemia of Dubin-Johnson syndrome. MRP2-mediated conjugate export represents a decisive final step in the detoxification of drugs, toxins, and endogenous substances. The basolateral isoform, MRP3 (gene symbol ABCC3), is upregulated in MRP2 deficiency and in extrahepatic cholestasis. MRP3 mediates the ATP-dependent transport of anionic conjugates, particularly of glucuronides and sulfoconjugates, across the basolateral hepatocyte membrane into sinusoidal blood. The inverse regulation of MRP3 and MRP2 expression under many conditions is consistent with their distinct localization and with a compensatory role of MRP3 in the hepatic secretion of anionic conjugates during impaired transport into bile.