title: Ku86 modulates DNA topoisomerase I-mediated radiosensitization, but not cytotoxicity, in mammalian cells.
abstract: Ku86 is an integral component of the nonhomologous end-joining (NHEJ) pathway of cellular double-strand break repair. In the current study, we investigated the role of Ku86 in DNA topoisomerase I-mediated radiosensitization induced by camptothecin in mammalian cells. Interestingly, as examined by clonogenic survival assay, a 30-minute camptothecin treatment induced significantly higher levels of radiosensitization in the Ku86-deficient Chinese hamster ovary xrs-6 cells than in the hamster Ku86-complemented xrs-6+hamKu86 cells, albeit exhibiting similar drug toxicity in these two cell lines. To confirm these findings, similar studies were conducted in two pairs of transfectant sublines established from the Ku86-deficient Chinese hamster lung fibroblast XR-V15B cells. Compared with the vector-alone sublines, radiation resistance was restored in the human Ku86-complemented sublines without alteration of cell cycle distributions. Again, significantly higher levels of camptothecin-induced radiosensitization were observed in the vector-alone sublines than in the Ku86-complemented XR-V15B sublines. In contrast, camptothecin treatments, ranging from 0.5 to 24 hours, induced similar cytotoxicities in both vector-alone and Ku86-complemented sublines. Because neither the DNA-damaging etoposide and cisplatin nor the tubulin-binder vinblastine induced enhanced levels of radiosensitization in the Ku86-deficient cells, Ku86 seems to uniquely affect topoisomerase I-mediated radiosensitization induced by camptothecin. Furthermore, cotreatment with DNA replication inhibitor aphidicolin abolished both camptothecin-induced cytotoxicity and radiosensitization in the vector-alone, as well as the Ku86-complemented subline cells, indicating both events are initiated by replication-dependent topoisomerase I-mediated DNA damages. Taken together, our data show a novel role of Ku86 in modulating topoisomerase I-mediated radiosensitization, but not cytotoxicity, in mammalian cells.
authors.affiliation: Department of Radiation Oncology, University of California Davis Medical Center, Sacramento, CA 95817, USA.
authors.forename: Shyh-Jen
authors.initials: SJ
authors.lastname: Shih
authors.forename: Tara
authors.initials: T
authors.lastname: Erbele
authors.forename: Allan Y
authors.initials: AY
authors.lastname: Chen
mesh_terms: D000818:Animals; D034961:Antigens, Nuclear / Q000235:genetics / Q000502:physiology*; D000970:Antineoplastic Agents / Q000494:pharmacology*; D016590:Aphidicolin / Q000494:pharmacology; D016466:CHO Cells; D002166:Camptothecin / Q000494:pharmacology*; D006224:Cricetinae; D004247:DNA / Q000096:biosynthesis / Q000235:genetics; D004261:DNA Replication / Q000187:drug effects; D004264:DNA Topoisomerases, Type I / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000172:deficiency / Q000235:genetics / Q000502:physiology*; D005347:Fibroblasts / Q000187:drug effects / Q000502:physiology; D006801:Humans; D000072200:Ku Autoantigen; D011836:Radiation Tolerance / Q000187:drug effects / Q000502:physiology*; D014162:Transfection
chemical_list: D034961:Antigens, Nuclear; D000970:Antineoplastic Agents; D004268:DNA-Binding Proteins; D016590:Aphidicolin; D004247:DNA; C000606509:Xrcc6 protein, human; D000072200:Ku Autoantigen; D004264:DNA Topoisomerases, Type I; D002166:Camptothecin