ROLE OF TOPOISOMERASE II-BETA IN THE RESISTANCE OF 9-OH-ELLIPTICINE-RESISTANT CHINESE-HAMSTER FIBROBLASTS TO TOPOISOMERASE-II INHIBITORS

In the Chinese hamster lung cell line DC-3F/9-OH-E, made resistant to 9-OH-ellipticine and cross-resistant to other topoisomerase II inhibit ors, the amount of topoisomerase II alpha is 4-5-fold lower than in th e parental DC-3F cells, A mutation in position 1710 of topoisomerase I I beta cDNA, generating a stop codon, completely abolishes the express ion of this isoform in DC-3F/9-OH-E cells, To analyze the contribution of the loss of topoisomerase II beta to the resistance phenotype, DC- 3F/9-OH-E cells mere cotransfected with two plasmids, one conferring t he resistance to G418, the other carrying the topoisomerase II beta cD NA, Among 200 G418-resistant clones, one was found to contain a topois omerase II beta activity similar to that in the parental cells, These cells constitute an in vivo mammalian model to study the pharmacologic al role of topoisomerase II beta. In the transfected cells, different levels of cleavable complex formation and resistance reversion were ob served with each topoisomerase II inhibitor examined, This work demons trates that topoisomerase II beta is a pharmacological target for 9-OH -ellipticine, etoposide, or 4'-(9-acridinylamino) methanesulfon-m-anis idide and plays a role in the cytotoxicity of these agents, Furthermor e, topoisomerase II beta is the preferential target for 4'-(9-acridiny lamino)methanesulfon-m-anisidide. The loss of topoisomerase II beta ac tivity in the DC-3F/9-OH-E cells is then in part responsible for their resistance to topoisomerase II inhibitors.