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 cau sal relationship or mechanism has been established, To investigate thi s 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-transf ected cell lines demonstrated continued topoisomerase II alpha mRNA an d protein expression, which were undetectable in vector-only lines, in stationary phase (G(0)-G(1)). The topoisomerase II transfectants were similar to 5-10-fold resistant to the alkylating agents cisplatin and mechlorethamine, Upon release from G(0)-G(1), the topoisomerase II tr ansfectants demonstrated more rapid thymidine incorporation and shorte r cell-doubling times than control cells, Purified topoisomerase II an d nuclear extracts with topoisomerase II-decatenating activity bound t o cisplatin-treated DNA with significantly greater affinity than to un treated DNA in a cisplatin concentration-dependent manner, These obser vations suggest that expression of topoisomerase II alpha may have a r ole in cellular resistance to antineoplastic alkylating agents, The me chanism for this may involve increased binding of topoisomerase II alp ha to alkylating agent-damaged DNA.