DNA topoisomerase II alpha expression is associated with alkylating agent resistance

Increased expression of DNA topoisomerase II alpha has been associated with resistance to certain DNA-damaging alkylating agents, but no causal relationship or mechanism has been established. To investigate this observation, we developed a model of topoisomerase II overexpression by transfecting a full-length Chinese hamster ovary topoisomerase II alpha into EMT6 mouse mammary carcinoma. Topoisomerase II alpha-transfected cell lines demonstrated continued topoisomerase II alpha mRNA and protein expression, which were undetectable in vector-only lines, in stationary phase (G0-G1). The topoisomerase II transfectants were approximately 5-10-fold resistant to the alkylating agents cisplatin and mechlorethamine. Upon release from G0-G1, the topoisomerase II transfectants demonstrated more rapid thymidine incorporation and shorter cell-doubling times than control cells. Purified topoisomerase II and nuclear extracts with topoisomerase II-decatenating activity bound to cisplatin-treated DNA with significantly greater affinity than to untreated DNA in a cisplatin concentration-dependent manner. These observations suggest that expression of topoisomerase II alpha may have a role in cellular resistance to antineoplastic alkylating agents. The mechanism for this may involve increased binding of topoisomerase II alpha to alkylating agent-damaged DNA.