PHARMACOLOGICAL AND MOLECULAR CHARACTERIZATION OF INTRINSIC AND ACQUIRED DOXORUBICIN RESISTANCE IN MURINE TUMOR-CELL LINES

We have studied the pharmacological parameters of doxorubicin resistan ce in three lines of murine cells selected by long-term culture in the presence of this drug or vincristine. A line originating from rat hep atoma spontaneously presented an intrinsic doxorubicin resistance as c ompared to the other lines, originating from a rat glioblastoma and fr om simian-virus-40-transformed mouse hepatocytes. This intrinsic resis tance, as well as the doxorubicin resistance exhibited by the vincrist ine-selected glioblastoma variant, could be entirely attributed to dec reased drug accumulation due to drug efflux. In contrast, the doxorubi cin-selected variants of the three lines exhibited an intracellular to lerance to this drug. Despite a reduction in drug accumulation when ex posed to the same amount of doxorubicin, they accumulated 6-12 times m ore doxorubicin than wild lines when submitted to equitoxic exposures. Verapamil could restore in these lines the doxorubicin accumulation o bserved in sensitive lines but could not restore doxorubicin cytotoxic ity. Quantitative evaluation of P-glycoprotein expression by Western b lotting with the C219 antibody indicated that the wild hepatoma line o verexpressed P-glycoprotein by a factor of 5 in comparison with the ot her wild lines, and that the vincristine-selected glioblastoma variant overexpressed this protein almost as much as the doxorubicin-selected variants. These observations favor the existence of P-glycoprotein-in dependent mechanisms of doxorubicin resistance, which are added to the classical multidrug-resistant phenotype in doxorubicin-selected highl y resistant variant cell lines.