Decreased Hepatic Breast Cancer Resistance Protein Expression and Function in Multidrug Resistance-Associated Protein 2-Deficient (TR-) Rats

Multidrug resistance-associated protein (Mrp) 2-deficient (TR-) Wistar rats have been used to elucidate the role of Mrp2 in drug disposition. Decreased breast cancer resistance protein (Bcrp) levels were reported in sandwich-cultured hepatocytes (SCH) from TR- rats compared with those from wild-type (WT) rats. This study was designed to characterize hepatic Bcrp expression and function in TR- rats, using nitrofurantoin and pitavastatin as substrates. Bcrp was knocked down by RNA interference in rat SCH. Antibody BXP53, but not BXP21, specifically detected Bcrp knockdown in SCH. Bcrp protein levels were decreased markedly in TR- but not Mrp2-deficient Sprague-Dawley [Eisai hyperbilirubinemic rats (EHBR)] rats. Bcrp mRNA levels were decreased significantly in TR- livers as determined by TaqMan real-time reverse transcriptase-polymerase chain reaction. Biliary excretion of nitrofurantoin, a specific Bcrp substrate, was decreased significantly in SCH and isolated perfused livers from TR- rats compared with those from WT controls, indicating that hepatic Bcrp function is decreased in TR- rats. In Bcrp knockdown SCH, the biliary excretion index and in vitro biliary clearance of pitavastatin were decreased significantly to similar to 58 and similar to 52% of control, respectively, indicating that Bcrp plays a role in pitavastatin biliary excretion. Pitavastatin biliary excretion was decreased significantly in perfused livers from TR- compared with those from WT rats. In conclusion, expression and function of hepatic Bcrp are decreased significantly in TR- rats. The potential role of both Bcrp and Mrp2 should be considered when data generated in TR- rats are interpreted. TR- and EHBR rats in combination may be useful in differentiating the role of Mrp2 and Bcrp in drug/metabolite disposition.