Multidrug Resistance-associated Protein (MRP) Family
MRP transporters belong to sub family C of the ATP-binding cassette (ABC) transporter family (ABCC). Within the ABCC family, there are 9 human MRP genes (MRPJ to MRP9); a channel gated by ATP binding and hydrolysis, the Cystic Fibrosis Transmembrane conductance Regulator (CFTR/ABCC7); and two ATP-dependent potassium channel regulators, sulfonylurea receptors 1 and 2 (SUR1/ABCC8 and SUR2/ABCC9).
MRPs are widely distributed throughout normal and malignant tissues. MRP1 and MRP5 are expressed in all tissues, whereas other members of the MRP family are confined to specific tissues. The MRP transporters are located at the plasma membranes and are also found on intracellular vesicles where they may traffic to the plasma membrane when required. MRP2 is located on apical membranes of polarized cells, whereas other MRPs are predominantly localized to basolateral membranes. MRP4 and MRP5 have been detected in both basolateral and apical membranes, depending on the cell type.
The predicted topology of many of the MRPs including MRP4, MRP5, MRP8 and MRP9, reveals two membrane spanning domains (MSD) each containing six transmembrane TM domains. Interestingly, MRP1, MRP2, MRP3 MRP6 and MRP7 have an additional MSD, usually called MSD0, at the NEb-terminus. The function of MSD0 is still not fully understood, but it may be important for the binding and transport of leukotriene C4 (LTC4), one of the best characterized substrates of MRP1 and MRP2. Another study suggested that MSD0 is important for correct trafficking of MRP2 to the apical membrane in polarized cells, whereas the linker 0 region, which connects MSD0 to MSDl, appears to be required by MRP1 for proper localization to the basolateral membrane.
The MRPs facilitate transport of both endogenous compounds such as peptides, ions, steroids, bile acids, folates and eicosanoids, in addition to exogenous drugs and chemicals. It is because of this latter function that they gain their name, multidrug resistance-associated proteins. Some members of the MRP family are also implicated in GSH transport, including MRP1, MRP2, MRP4, MRP5 and CFTR. MRP1 and MRP2 are described in more detail below, as these MRPs appear to have important roles in GSH transport.
MRP1 was the first member of this family to be identified. It was cloned in 1992 from the small cell lung cancer cell line H69 that had been repeatedly exposed to doxorubicin, an anthracycline. Analysis of this clone, which was resistant to doxorubicin and drugs structurally unrelated to doxorubicin, revealed a 1531 amino acid protein amplified approximately 100 times compared to wild type cells. LTC4 and other glutathione S-conjugates were identified as substrates for MRP1. Further analysis revealed that in addition to glutathione S-conjugates, MRP1 is capable of transporting glucuronide conjugates and sulfate conjugates, antineoplastics, antivirals, antibiotics, nonsteroidal anti-inflammatory and antiepileptic drugs, flavanoids, natural folates, fluorescent probes, and peptides. The high affinity transport of LTC4, together with other glutathione S-conjugates, prompted speculation that MRP1 may also transport GSH.
The second member of the ABCC family, MRP2 was identified in normal rat liver using probes against evolutionarily conserved regions of MRP 1. MRP2 has approximately 48% amino acid identity to MRP1, and has similar substrate specificity. Many functional characteristics of MRP2 were already characterized prior to its molecular identification because it was discovered that MRP2 is the protein responsible for transporting organic anions into the bile, a functional entity previously identified as the canalicular multispecific organic anion transporter (cMOAT). The characterization of cMOAT, including its substrate specificity, was done by performing transport studies using vesicles isolated from wild type rats and rats deficient in biliary transport. These rats contain natural mutations in MRP2, and include the Wistar transport deficient (TR-), the Groningen Yellow (GY), and the Sprague-Dawley Eisai hyperbilirubinuric rat (EHBR). The observed phenotypes in these rats, including the defect in transport of endogenous and exogenous conjugated anions from hepatocytes to bile leading to hyperbilirubinemia, were similar to those observed in patients diagnosed with Dubin-Johnson syndrome (DJS). Different mutations in the MRP2 gene were eventually recognized as the underlying cause of DJS in humans.