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 inhibitors, the amount of topoisomerase IIα is 4–5-fold lower than in the parental DC-3F cells. A mutation in position 1710 of topoisomerase IIβ cDNA, generating a stop codon, completely abolishes the expression of this isoform in DC-3F/9-OH-E cells. To analyze the contribution of the loss of topoisomerase IIβ to the resistance phenotype, DC-3F/9-OH-E cells were cotransfected with two plasmids, one conferring the resistance to G418, the other carrying the topoisomerase IIβ cDNA. Among 200 G418-resistant clones, one was found to contain a topoisomerase IIβ activity similar to that in the parental cells. These cells constitute an in vivo mammalian model to study the pharmacological role of topoisomerase IIβ. In the transfected cells, different levels of cleavable complex formation and resistance reversion were observed with each topoisomerase II inhibitor examined. This work demonstrates that topoisomerase IIβ is a pharmacological target for 9-OH-ellipticine, etoposide, or 4′-(9-acridinylamino) methanesulfon-m-anisidide and plays a role in the cytotoxicity of these agents. Furthermore, topoisomerase IIβ is the preferential target for 4′-(9-acridinylamino)methanesulfon-m-anisidide. The loss of topoisomerase IIβ activity in the DC-3F/9-OH-E cells is then in part responsible for their resistance to topoisomerase II inhibitors.

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Supported in part by Association pour la Recherche sur le Cancer, Villejuif, France, and by Ligue Nationale Française contre le Cancer, Paris, France.

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