UP-REGULATION OF P-GLYCOPROTEIN EXPRESSION IN RAT-LIVER CELLS BY ACUTE DOXORUBICIN TREATMENT

Expression of P-glycoprotein, a plasma-membrane glycoprotein involved in multidrug resistance and encoded by mdr genes, was investigated in cultured rat liver cells acutely exposed to doxorubicin. This anticanc er drug was shown to increase mdr mRNA levels in a dose-dependent mann er in both rat liver epithelial (RLE) cells and primary rat hepatocyte s. This induction of mdr transcripts was detected as early as a 4-h ex posure to doxorubicin used at 0.5 mu g/ml. It occurred through increas ed expression of the mdr] gene as assessed by northern blot analysis u sing rat mdr-gene-specific probes. In addition, RLE cells exposed to d oxorubicin displayed an overexpression of a 140-kDa P-glycoprotein as demonstrated by western blotting. Moreover, doxorubicin-treated RLE ce lls displayed enhanced cellular efflux of the P-glycoprotein substrate rhodamine 123 that was inhibited by the P-glycoprotein blocker verapa mil, thus providing evidence that doxorubicin-induced P-glycoprotein w as functional in liver cells. Doxorubicin-mediated mdr mRNA induction was found to be fully inhibited by actinomycin D, thus indicating its dependence on RNA synthesis; it was demonstrated to be not associated with alteration of protein synthesis, suggesting it differed from the known mdr mRNA overexpression occurring in response to cycloheximide. In contrast to P-glycoprotein, other liver detoxification pathways suc h as cytochromes P-450 1A were not induced by doxorubicin treatment. T hese data indicate that doxorubicin can act as a potent acute inducer of functional P-glycoprotein in rat liver cells and therefore may modu late both chemosensitivity of hepatic cells and P-glycoprotein-mediate d biliary secretion of xenobiotics.