The multidrug resistance-associated protein (MRP) is over-expressed and functional in rat hepatoma cells

Expression of multidrug-resistance-associated protein (MRP), a drug efflux pump transporting a wide range of xenobiotics, including anti-cancer drugs and chemical carcinogens, and present at low levels in normal hepatocytes, was investigated in rat hepatoma cells. Northern-blot analysis allowed dete ction of high levels of MRP mRNA in rat diethylnitrosamine-induced hepatoca rcinomas when compared with normal liver. Similarly, elevated expression of MRP transcripts were evidenced in 6 rat hepatoma cell lines of different o rigins, especially in HTC cells, that, in contrast, failed to express mRNA of the canalicular multispecific organic anion transporter (cMOAT), an effl ux pump sharing numerous substrates with MRP, HTC cells were also found by Western blotting to display much higher amounts of MRP than those observed in normal hepatocytes, In contrast, the MRP gene copy number was similar bo th in hepatoma HTC cells and in hepatocytes, as assessed by Southern blotti ng. Analysis of MRP-related transport using 3 types of MRP substrates, name ly, the fluorescent glutathione-bimane, the anionic dye calcein and the cat ionic anti-cancer drug vincristine, demonstrated that HTC cells displayed c ellular efflux of these 3 compounds, an efflux strongly inhibited by MRP mo dulators such as indomethacin. These results indicate that MRP is over-expr essed and functional in rat hepatoma cells and may therefore be included in the de-toxifying pathways that are altered during hepatocarcinogenesis and are thus thought to contribute to the known multidrug resistance of liver tumors. (C) 1999 Wiley-Liss, Inc.