Good news on the conjugate export pump cMRP in human liver and the absence of cMRP or cMoat in canalicular membranes of TR- TR-rat hepatocytes
✍ Scribed by M Mohler; W Stremmel
- Publisher
- John Wiley and Sons
- Year
- 1996
- Tongue
- English
- Weight
- 157 KB
- Volume
- 24
- Category
- Article
- ISSN
- 0270-9139
No coin nor oath required. For personal study only.
✦ Synopsis
cMRP IN HUMAN LIVER AND THE ABSENCE OF cMrp The human Dubin-Johnson syndrome and its animal OR cMoat IN CANALICULAR MEMBRANES OF TR 0 TRmodel, the TR 0 rat, are characterized by a chronic conju-RAT HEPATOCYTES gated hyperbilirubinemia. TR 0 rats are defective in the Mayer R, Kartenbeck J, Bu ¨chler M, Jedlitschky G, Leier I, canalicular multispecific organic anion transporter Keppler D. Expression of the MRP gene-encoded conjugate (cMOAT), which mediates hepatobiliary excretion of nuexport pump in liver and its selective absence from the canamerous organic anions. The complementary DNA for rat licular membrane in transport-deficient mutant hepatocytes. cmoat, a homologue of the human multidrug-resistance J Cell Biol 1995;131:137-150. gene (hMRP1), was isolated and shown to be expressed in the canalicular membrane of hepatocytes. In the TR 0 rat, a single-nucleotide deletion in this gene resulted in ABSTRACT a reduced messenger RNA level and absence of the protein. It is likely that this mutation accounts for the TR 0 We have previously shown that the multidrug-rephenotype. sistance protein (MRP) mediates the adenosine triphosphate (ATP)-dependent membrane transport of glutathione S-conjugates and additional amphi-COMMENTS philic organic anions. We demonstrate the expres-Excretion into bile is a major pathway for the elimination sion of MRP in hepatocytes where it functions in of endogenous and xenobiotic lipophilic compounds from the hepatobiliary excretion. Analysis by reverse-tranmammalian organism. Hepatocytes are the primary secrescription polymerase chain reaction (RT-PCR) of hutory units of bile formation. They are equipped with different man and normal rat liver messenger RNA resulted in carrier proteins of the basolateral and canalicular memtwo expected complementary DNA (cDNA) fragments brane. 1 Some of these carrier proteins have a wide substrate of MRP. Four different antibodies against MRP respecificity and transport endogenous and exogenous comacted on immunoblots with the glycoprotein of about pounds. For the mamalian organism, toxic lipophilic com-190 kd from human canalicular as well as basolateral pounds are metabolized by transferases into amphiphilic anhepatocyte membrane preparations. A polyclonal ionic conjugates with glucuronate, glutathione, or sulfate. antibody directed against the carboxy-terminal se-
Excretion of these conjugates across the canalicular memquence of MRP detected the rat homologue of MRP in brane into bile is mediated by a primary-active ATP-depenliver. Double immunofluorescence microscopy and dent transport system, which has been characterized funcconfocal laser scanning microscopy showed the prestionally and termed non-bile acid organic anion transporter, 2 ence of human MRP and rat Mrp in the canalicular multispecific organic anion transporter cMOAT, 3 glutathione as well as in the lateral membrane domains of hepa-S-conjugate export pump, 4 or, leukotriene export pump. This tocytes. The transport function of the mrp gene -enlast term refers to the endogenous glutathione S-conjugate coded conjugate export pump was assayed in plasma leukotriene C 4 (LTC 4 ), which is the substrate with the highmembrane vesicles with leukotriene C 4 as a high-afest affinity for this transporter. Neither the protein nor the finity glutathione S-conjugate substrate. The defigene encoding this ATP-dependent transporter in the canaliccient ATP-dependent conjugate transport in canalicular membrane of human hepatocytes were previously identiular membranes from TR 0 mutant rat hepatocytes fied.
was associated with a lack of amplification of one of
Recently, a new transport protein has been cloned and the mrp cDNA fragments and with a selective loss characterized, MRP, which mediates multidrug-resistance of Mrp on immunoblots of canalicular membranes.
against chemotherapeutic drugs in tumor cells of the lung. 8,9
Double immunofluorescence microscopy of livers
Overexpression of this membrane glycophosphoprotein with from transport-deficient TR 0 mutant rats localized a molecular mass ranging from 170 to 190 kd results in the Mrp only to the lateral membrane but not to the canaresistance of human tumor cells to a number of cytotoxic licular membrane. Our results indicate that the abdrugs, including anthracycline, vinca alkaloids and epidophysence of Mrp or an isoform of Mrp from the canalicuilotoxins. 10-12 MRP bears a limited amino acid similarity to lar membrane is the basis for the hereditary defect MDR-1-p-glycoprotein (gp170), MDR-2-p-glycoprotein, and of the hepatobiliary excretion of anionic conjugates the cystic fibrosis transmembrane conductance regulator by the transport-deficient hepatocyte.
(CFTR). Although all are members of the superfamily of the ATP-binding cassette membrane transport proteins (ABC transporters), these transporters represent different func-