HEPATIC EXPRESSION OF MULTIDRUG RESISTANCE-ASSOCIATED PROTEIN-LIKE PROTEINS MAINTAINED IN EISAI HYPERBILIRUBINEMIC RATS

The biliary excretion of several organic anions is mediated by the can alicular multispecific organic anion transporter (cMOAT), which is her editarily defective in mutant rats such as Eisai hyperbilirubinemic ra ts (EHBR). In addition, using a kinetic study with isolated canalicula r membrane vesicles, we recently suggested the presence of ATP-depende nt organic anion transporter(s) other than cMOAT in EHBR [Pharm Res (N Y) 12:1746-1755 (1995); J Pharmacol Exp Ther 282:866-872 (1997)]. The aim of this study is to provide a molecular basis for the presence of multiplicity in the biliary excretion of organic anions in rats. Based on the homology with human multidrug resistance-associated protein (h MRP), two cDNA fragments encoding the carboxyl-terminal ATP-binding ca ssette region were amplified by reverse transcription-polymerase chain reaction from EHBR liver. These fragments exhibited approximately 70% amino acid identity with hMRP and rat cMOAT; therefore, they were des ignated MRP-like proteins (MLP-1 and MLP-2). The cloned full length cD NA of MLP-1 and -2 from the Sprague-Dawley (SD) rat liver and colon cD NA library was composed of 1502 and 1523 amino acids, respectively, ha d the characteristics of ATP-binding cassette transporters, and exhibi ted homology with hMRP and rat cMOAT. Northern blot analysis indicated that MLP-1 is expressed predominantly in the liver in both SD rats an d EHBR, whereas hepatic expression of MLP-2 was observed only in EHBR. In addition, MLP-2 was markedly induced by ligation of the bile duct in SD rat liver. In both SD rats and EHBR, MLP-2 was expressed predomi nantly in the duodenum, jejunum, and colon. These findings suggest tha t MLP-1 and MLP-2 might be novel members of the MRP family responsible for the excretion of organic anions from these epithelial cells, and that MLP-2 is an inducible one.