EXPRESSION OF THE MRP GENE-ENCODED CONJUGATE EXPORT PUMP IN LIVER ANDITS SELECTIVE ABSENCE FROM THE CANALICULAR MEMBRANE IN TRANSPORT-DEFICIENT MUTANT HEPATOCYTES

We have previously shown that the multidrug resistance protein (MRP) m ediates the ATP-dependent membrane transport of glutathione S-conjugat es and additional amphiphilic organic anions, In the present study we demonstrate the expression of MRP in hepatocytes where it functions in hepatobiliary excretion, Analysis by reverse transcription-PCR of hum an and normal rat liver mRNA resulted in two expected cDNA fragments o f MRP, Four different antibodies against MRP reacted on immunoblots wi th the glycoprotein of about 190 kD from human canalicular as well as basolateral hepatocyte membrane preparations, A polyclonal antibody di rected against the carboxy-terminal sequence of MRP detected the rat h omolog of MRP in liver. Double immunofluorescence microscopy and confo cal laser scanning microscopy showed the presence of human MRP and rat Mrp in the canalicular as well as in the lateral membrane domains of hepatocytes. The transport function of the mrp gene-encoded conjugate export pump was assayed in plasma membrane vesicles with leukotriene C -4 as a high-affinity glutathione S-conjugate substrate, The deficient ATP dependent conjugate transport in canalicular membranes from TR(-) mutant rat hepatocytes was associated with a lack of amplification of one of the mrp cDNA fragments and with a selective loss of Mrp on imm unoblots of canalicular membranes. Double immunofluorescence microscop y of livers from transport-deficient TR(-) mutant rats localized Mrp o nly to the lateral but not to the canalicular membrane. Our results in dicate that the absence of Mrp or an isoform of Mrp from the canalicul ar membrane is the basis for the hereditary defect of the hepatobiliar y excretion of anionic conjugates by the transport-deficient hepatocyt e.