DNA-PK is a serine threonine kinase that contains a Ku heterodimer (Ku70 and Ku80, Ku80 also named Ku86) and a DNA-PK catalytic subunit (DNA-PKcs), and is crucial to the NHEJ process and in the maintenance of telomere stability

DNA-dependent protein kinase (DNA-PK), including Ku80, Ku70 and DNA-PK catalytic subunit (DNA-PKcs), is the key protein in non-homologous end-joining (NHEJ) after DNA double-strand breaks (DSBs) appear. In this study, small hairpin interfering RNAs (siRNAs) targeting Ku80 and DNAPKcs were used both individually and in combination, to explore the effects of these DSB proteins on HeLa cell functional changes after X-ray irradiation. HeLa cells co-transfected with Ku80-siRNA and DNA-PKcs-siRNA were more radiosensitive than the ones transfected individually. HeLa in the absence of Ku80 and pretreated with LY294002, a chemically specific PI 3-kinase inhibitor, resulted in cells that were even more sensitive to X-rays than HeLa/Ku80-siRNA transfected with DNAPKcs-siRNA. The cells inhibited by Ku80 either individually or in combination with DNA-PKcs showed cell accumulation in the G2/M phase 48 h post-irradiation, similarly to control cells. However, cells transfected with DNA-PKcs-siRNA or pretreated with LY294002 had a prolonged G2/M delay, suggesting the accumulation of significant un-repaired DNA damage following inhibition of DSB repair proteins. In conclusion, these data indicate that the role of Ku80 in DSB repair could be compensated by other DSB repair proteins; co-inhibition would be a suitable strategy to enhance the radiosensitivity of cancer cells.