Ku86 Modulates DNA Topoisomerase I–Mediated Radiosensitization, but not Cytotoxicity, in Mammalian Cells

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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.