P-GLYCOPROTEIN INDUCTION IN RAT-LIVER EPITHELIAL-CELLS IN RESPONSE TOACUTE 3-METHYLCHOLANTHRENE TREATMENT

Expression of P-glycoprotein (P-gp), a plasma membrane glycoprotein in volved in multidrug resistance and encoded by mdr genes, was investiga ted in nonparenchymal rat liver epithelial (RLE) cells in response to acute exposure to carcinogenic polycyclic aromatic hydrocarbons (PAHs) . High levels of mdr mRNAs were evidenced by Northern blotting in two independent RLE cell lines after treatment by either 3-methylcholanthr ene (MC) or benzo-(a)pyrene. MC-mediated mdr mRNA induction was demons trated to be dose-dependent; it occurred through enhanced expression o f the mdr 1 gene, as indicated by reverse transcriptase polymerase cha in reaction analysis using rat mdr gene-specific primers and parallele d an induction of a 140 kDa P-gp as demonstrated by Western blotting. In addition, MC-induced P-gp appeared to be fully functional because R LE cells exposed to MC displayed enhanced cellular efflux of rhodamine 123, a known P-gp substrate, compared to their untreated counterparts . Analysis of time-course induction revealed that mdr mRNA levels were maximally increased when RLE cells were treated for 48 to 96 hr and r eturned to low levels after the PAH was removed. In contrast to P-gp, both cytochrome P-450 1A1 and cytochrome P-450 1A2 were not detected a fter exposure to MC, thus indicating that these liver detoxification p athways are not coordinately regulated with P-gp in RLE cells. In addi tion, MC-mediated P-gp regulation was not associated with major cellul ar disturbances such as alteration of protein synthesis and, thereby, differed from the known mdr mRNA induction occurring in response to cy cloheximide. Moreover, cotreatment with MC and cycloheximide led to a superinduction of mdr mRNAs, thus suggesting that the effects of the t wo xenobiotics were, at least partly, additive. In contrast to MC and benzo(a)pyrene, 2,3,7,8-tetrachlorodibenzo-p-dioxin and benzo(e)pyrene were unable to increase P-gp expression. These results indicate that some PAHs can act as potent inducers of P-gp in RLE cells and may be i nterpreted as an adaptive reaction of these cells in lowering cellular accumulation of toxic drugs, including carcinogens transported by P-g p and, therefore, conferring protection on these compounds.