MECHANISMS OF RESISTANCE IN A HUMAN CELL-LINE EXPOSED TO SEQUENTIAL TOPOISOMERASE POISONING

Camptothecins are a new class of anticancer drugs that target DNA topo isomerase I; current efforts are directed toward elucidating optimal c ombinations of these drugs with other antineoplastic agents, A rationa le for the use of sequential therapy involving the combination of camp tothecins with topoisomerase II-targeting drugs, such as etoposide, ha s arisen from observations of increased topoisomerase II protein level s in cell lines resistant to camptothecin. In an effort to understand potential mechanisms of resistance to this strategy, we developed a U- 937 cell subline, denoted RERC, that is capable of surviving exposure to sequential topoisomerase poisoning, The RERC cells are 200-fold res istant to camptothecin, ZI-fold resistant to etoposide, and 10-fold hy persensitive to cisplatin compared to the parental U-937 cells. Bioche mical analyses indicate that the resistant phenotype involves alterati ons in both topoisomerase I and topoisomerase II alpha. Topoisomerase I catalytic activity in the resistant cells is similar to that of the parental line but is resistant to camptothecin. Moreover, the resistan t cells express a single mRNA species of topoisomerase I that codes fo r a mutation in codon 533. In addition, topoisomerase II alpha protein levels are decreased 10-fold in the resistant line, coincident with a two-fold decrease in the expression of topoisomerase II alpha mRNA, C ollectively, these results indicate that resistance to sequential topo isomerase poisoning may involve a reduction in total cellular topoisom erase activity.