The H209/V6 cell line was derived from the H209 small cell lung cancer cell line by selection in etoposide (VP-16). Cytogenetic analysis indicates that the sensitive and resistant cell lines share 20 marker chromosomes and thus are clearly related. However, the H209/V6 cell line has four additional structurally altered chromosomes and a 2 N-modal chromosome number, while the H209 cell line is hypotetraploid (4 N-). H209/V6 cells are cross-resistant to some drugs that interact with topoisomerase II but not mitoxantrone. H209/V6 cells are also not cross-resistant to vincristine, trimetrexate, or cisplatin. The rates of VP-16 efflux are the same in the resistant and sensitive cell lines, which is consistent with the observation that P-glycoprotein mRNA is not detectable in either cell line. Fewer VP-16-induced DNA-protein complexes are observed in H209/V6 cells, and immunoblot analysis shows that levels of topoisomerase IIα are reduced in H209/V6 cells compared to the sensitive H209 cells. Furthermore, the topoisomerase IIα-related protein in H209/V6 cells has an increased electrophoretic mobility, with an apparent Mr of 160,000. The levels of the topoisomerase IIα 6.1-kilobase mRNA in H209/V6 cells are reduced >10-fold. In addition, a second topoisomerase IIα-related mRNA of approximately 4.8 kilobases is observed in H209/V6 cells but not in H209 cells. The quantity and electrophoretic mobility of the Mr 180,000 topoisomerase IIβ protein and its 6.1-kilobase mRNA are the same in the sensitive and resistant cell lines. The topoisomerase II strand-passing activity in H209/V6 nuclear extracts is reduced about 2-fold, but this activity is not more resistant to inhibition by VP-16 than the activity in H209 cells. However, band depletion immunoblot experiments show that the topoisomerase IIα-related Mr 160,000 protein in H209/V6 cells is not bound to DNA in the presence of concentrations of VP-16 that deplete the Mr 170,000 topoisomerase IIα in H209 cells and the Mr 180,000 topoisomerase IIβ in both the resistant and sensitive cells. We conclude that quantitative and qualitative alterations in topoisomerase IIα have occurred in H209/V6 cells and are likely to contribute to its resistance phenotype.

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Supported by Grant 2049 from the National Cancer Institute of Canada (S. P. C. C.). M. L. S. is a member of the City of Hope Cancer Research Center supported by Grant CA33572 from the National Cancer Institute. C. D. E. was supported in part by a Queen's University graduate student award. S. P. C. C. is a Career Scientist of the Ontario Cancer Treatment and Research Foundation.

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