title: Mechanisms of resistance in a human cell line exposed to sequential topoisomerase poisoning.
abstract: Camptothecins are a new class of anticancer drugs that target DNA topoisomerase I; current efforts are directed toward elucidating optimal combinations of these drugs with other antineoplastic agents. A rationale for the use of sequential therapy involving the combination of camptothecins with topoisomerase II-targeting drugs, such as etoposide, has arisen from observations of increased topoisomerase II protein levels 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 resistant to camptothecin, 8-fold resistant to etoposide, and 10-fold hypersensitive to cisplatin compared to the parental U-937 cells. Biochemical analyses indicate that the resistant phenotype involves alterations in both topoisomerase I and topoisomerase IIalpha. Topoisomerase I catalytic activity in the resistant cells is similar to that of the parental line but is resistant to camptothecin. Moreover, the resistant cells express a single mRNA species of topoisomerase I that codes for a mutation in codon 533. In addition, topoisomerase IIalpha protein levels are decreased 10-fold in the resistant line, coincident with a two-fold decrease in the expression of topoisomerase IIalpha mRNA. Collectively, these results indicate that resistance to sequential topoisomerase poisoning may involve a reduction in total cellular topoisomerase activity.
authors.affiliation: Department of Pharmacology, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick 08901, USA.
authors.forename: A
authors.initials: A
authors.lastname: Saleem
authors.forename: N
authors.initials: N
authors.lastname: Ibrahim
authors.forename: M
authors.initials: M
authors.lastname: Patel
authors.forename: X G
authors.initials: XG
authors.lastname: Li
authors.forename: E
authors.initials: E
authors.lastname: Gupta
authors.forename: J
authors.initials: J
authors.lastname: Mendoza
authors.forename: P
authors.initials: P
authors.lastname: Pantazis
authors.forename: E H
authors.initials: EH
authors.lastname: Rubin
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000494:pharmacology; D002467:Cell Nucleus / Q000201:enzymology; D004264:DNA Topoisomerases, Type I / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D004250:DNA Topoisomerases, Type II / Q000187:drug effects* / Q000235:genetics / Q000378:metabolism; D019008:Drug Resistance, Neoplasm; D005047:Etoposide / Q000494:pharmacology*; D006801:Humans; D007938:Leukemia / Q000201:enzymology / Q000473:pathology; D010641:Phenotype; D017354:Point Mutation; D012333:RNA, Messenger / Q000235:genetics; D014407:Tumor Cells, Cultured / Q000187:drug effects
chemical_list: D000970:Antineoplastic Agents; D012333:RNA, Messenger; D005047:Etoposide; D004264:DNA Topoisomerases, Type I; D004250:DNA Topoisomerases, Type II; C079905:rubitecan; D002166:Camptothecin