title: Low-level resistance to camptothecin in a human small-cell lung cancer cell line without reduction in DNA topoisomerase I or drug-induced cleavable complex formation.
abstract: To study the evolution of camptothecin (CPT) resistance, we have established two small-cell lung cancer cell lines with low (3.2-fold, NYH/CAM15) and high (18-fold, NYH/CAM50) resistance to CPT by stepwise drug exposure. NYH/CAM50 cells had reduced topoisomerase I (topo I) content and activity, and consequently CPT-induced DNA single strand breaks (SSBs) were reduced, as measured by alkaline elution. In contrast, NYH/CAM15 cells had identical topo I content and activity as compared with wild-type (wt) cells. CPT-mediated SSBs and the rate of their reversal after drug removal were also equal in wt and NYH/CAM15 cells, as were doubling time, the fraction of cells in S-phase and DNA synthesis rate in response to CPT. As the conversion of DNA SSBs to DNA double strand breaks (DSBs) is thought to represent a critical event leading to cell death, we measured DNA DSBs by neutral elution. In contrast to DNA SSBs, CPT induced fewer DNA DSBs in NYH/CAM15 than in wt cells. DNA flow cytometry showed that, in CPT-treated cells, the G1 phase was emptied as cells accumulated in late S- and G2M phase. A Spearman rank correlation showed that depletion of G1 and accumulation in late S and G2M correlated to CPT sensitivity in these three cell lines. In conclusion, acquired resistance to CPT can occur without a reduction in either topo I enzyme or CPT-induced cleavable complex formation, while a decrease in the level of CPT-induced DNA DSBs may be of major importance in the early stages of CPT resistance.
authors.affiliation: Laboratory of Experimental Medical Oncology, The Finsen Center, Rigshospitalet, Copenhagen, Denmark.
authors.forename: M
authors.initials: M
authors.lastname: Sorensen
authors.forename: M
authors.initials: M
authors.lastname: Sehested
authors.forename: I J
authors.initials: IJ
authors.lastname: Christensen
authors.forename: J K
authors.initials: JK
authors.lastname: Larsen
authors.forename: P B
authors.initials: PB
authors.lastname: Jensen
mesh_terms: D000972:Antineoplastic Agents, Phytogenic / Q000493:pharmacokinetics / Q000494:pharmacology*; D017209:Apoptosis / Q000187:drug effects; D002166:Camptothecin / Q000493:pharmacokinetics / Q000494:pharmacology*; D018288:Carcinoma, Small Cell / Q000188:drug therapy* / Q000201:enzymology* / Q000473:pathology; D002453:Cell Cycle / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects; D004249:DNA Damage; D004264:DNA Topoisomerases, Type I / Q000378:metabolism*; D004273:DNA, Neoplasm / Q000096:biosynthesis / Q000187:drug effects / Q000378:metabolism*; D019008:Drug Resistance, Neoplasm; D004354:Drug Screening Assays, Antitumor; D006801:Humans; D008175:Lung Neoplasms / Q000188:drug therapy* / Q000201:enzymology* / Q000473:pathology; D014407:Tumor Cells, Cultured
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D004273:DNA, Neoplasm; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin