TOPOISOMERASE-I ALTERATION IN A CAMPTOTHECIN-RESISTANT CELL-LINE DERIVED FROM CHINESE-HAMSTER DC3F CELLS IN CULTURE

Camptothecin-resistant DC3F Chinese hamster lung fibroblast cell lines were obtained after mutagenic treatment with ethylmethanesulfonate an d subsequent exposure to 1-mu-M camptothecin (CPT). The most resistant cell line, which was obtained after exposure to CPT for 10 days, was designated DC3F/C-10. Comparison of 50% inhibitory concentration value s after 8-h CPT treatments showed that DC3F/C-10 cells were 134-fold r esistant to CPT. Resistance was associated with marked reduction of CP T-induced DNA single-strand breaks and DNA-protein cross-links. This r eduction was not due to reduced amounts of immunoreactive DNA topoisom erase I protein, although nuclear extracts from DC3F/C-10 cells had le ss enzyme catalytic activity than those from DC3F cells. Also, fast pr otein liquid chromatography-purified DNA topoisomerase I from DC3F/C-1 0 had lower specific catalytic activity than that from DC3F cells. DNA topoisomerase I from DC3F/C-10 was resistant to inhibition of catalyt ic activity and induction of DNA cleavage by CPT. These results sugges t that CPT resistance in DC3F/C-10 cells is due to qualitative alterat ion of DNA topoisomerase I.