Topoisomerase II-dependent and -independent mechanisms of etoposide resistance in Chinese hamster cell lines

Resistance to etoposide (VP-16), amsacrine (mAMSA), and doxorubicin (Adriamycin) was studied in two Chinese hamster cell lines primarily selected for resistance to the epipodophyllotoxin. Both lines demonstrated profound resistance to VP-16, and mAMSA stimulated DNA breakage. However, the resistance to mAMSA cytotoxicity in both lines was less than expected from the level of resistance to the effects of topoisomerase II inhibition. Similarly, resistance to the cytotoxicity of high VP-16 concentrations in one of the lines was less than expected from the resistance to inhibition of topoisomerase II. An analysis of the relation of DNA breaks to drug cytotoxicity suggests that cross-resistance to mAMSA was mainly conferred through loss of mAMSA-stimulated, topoisomerase II-mediated DNA breaks. This mechanism also contributed towards reduced VP-16 cytotoxicity. However, our studies suggest that additional mechanisms, independent of resistance to VP-16-mediated topoisomerase II effects, greatly increased the resistance to this agent. Resistance to VP-16 cytotoxicity, not dependent on resistance to drug-mediated DNA cleavage, could be overcome at high drug concentrations in one of the resistant lines and might be responsible for the greater relative resistance to VP-16 than to mAMSA. These findings suggest the presence of two distinct mechanisms of resistance to VP-16 cytotoxicity, one presumably mediated by topoisomerase II and dependent on resistance to drug-mediated DNA scission, and a second mechanism independent of the effects of the drug on topoisomerase II.