Expression of a putative ATP-binding cassette region, homologous to that in multidrug resistance-associated protein (MRP), is hereditarily defective in Eisai hyperbilirubinemic rats (EHBR)

Abstract

Abstract It is well established that several organic anions such as leukotriene C4 and S-(2,4-dinitrophenyl)-glutathione are excreted into the bile via an ATP-dependent primary active transporter located on the bile canalicular membrane. Although the molecular features of this transporter still remain to be clarified, this transporter might be a member of the ATP-binding cassette (ABC) transmembrane transporter superfamily which has a common ABC region. In the present study, a cDNA fragment was amplified from Sprague-Dawley (SD) rat liver by PCR using degenerate primers prepared from the conserved sequence in the COOH-terminal ABC region of human multidrug resistance-associated protein (MRP), a primary active transporter. The amplified 421 bp fragment exhibited homology with a human MRP and the human MRP-like fragment (yp75a11) with homology score of 66.3% and 83.0% at the cDNA level, and 73.3% and 84.7% deduced from the amino acid level, respectively. Northern blot analysis of poly(A)+ RNA prepared from SD rat liver revealed the presence of ~5 kb and 8.5 kb mRNA species which hybridized to this fragment. In contrast, poly(A)+ RNA from Eisai hyperbilirubinemic rats (EHBR), whose primary active transporter on the bile canalicular membrane is hereditarily defective, did not hybridize to this fragment. These results suggest (1) that the impaired expression of this particular region might be related to the pathogenesis of hyperbilirubinemia in EHBR and (2) that this region might encode part of the primary active transporter on the bile canalicular membrane.