CONSTITUTIVE EXPRESSION OF FUNCTIONAL P-GLYCOPROTEIN IN RAT HEPATOMA-CELLS

P-glycoprotein is a plasma-membrane glycoprotein involved in multidrug resistance. P-glycoprotein overexpression has been demonstrated to oc cur in tumor cells after cytotoxic drug exposure, but also in some can cers including hepatocellular carcinomas before any chemotherapeutic t reatment. In order to better analyze this constitutive type of tumoral drug resistance, we have investigated P-glycoprotein expression and f unction in rat liver tumors induced experimentally by administration o f diethylnitrosamine and in two cell clones derived from one of these tumors designated as RHC1 and RHC2. High levels of P-glycoprotein mRNA s were found in both liver tumor samples and the two hepatoma cell clo nes as assessed by Northern blotting; both RHC1 and RHC2 cells display ed altered liver functions commonly observed in rat hepatoma cells, pa rticularly the decreased expression of albumin and overexpression of t he fetal glutathione S-transferase 7-7. The use of specific multidrug resistance (mdr) probes revealed a major induction of the mdi-l gene i n liver tumor samples while RHC1 and RHC2 cells expressed both mdr1 an d mdr3 genes without displaying a major alteration in the number of md r gene copies as assessed by Southern blotting. High amounts of P-glyc oprotein were also demonstrated in RHC1 and RHC2 cells by Western blot ting. These cells were strongly resistant to doxorubicin and vinblasti ne, two anticancer drugs transported by P-glycoprotein. Doxorrbicin in tracellular retention was low in RHC1 and RHC2 cells, but was strongly enhanced in the presence of verapamil, a known modulator agent of P-g lycoprotein; low retention appeared to occur via a drug efflux mechani sm, indicating that P-glycoprotein was fully active. These results sho w that rat hepatoma cells can display elevated levels of functional P- glycoprotein without any prior cytotoxic drug selection and suggest th at these cells represent a useful model for analyzing P-glycoprotein r egulation in intrinsically clinical drug-resistant cancers.