Establishment of hamster cell lines with EGFP-tagged human XRCC4 and protection from low-dose X-ray radiation.

In clinical settings, cellular resistance to chemotherapy and radiotherapy is a significant component of tumor treatment failure. The mechanisms underlying the control of localization of DNA repair proteins play a key role in the regulation of DNA repair activity. The DNA repair protein XRCC4, which is a regulator of DNA ligase IV activity, might be a key contributor to not only chemoresistance to anticancer agents, e.g., etoposide, but also radioresistance. However, it remains unclear whether XRCC4, which is a key player in nonhomologous DNA-end-joining (NHEJ), plays a role in low-dose radioresistance. In this study, we confirmed that human XRCC4 tagged with the enhanced green fluorescent protein (EGFP-XRCC4), as well as the DNA damage sensor Ku80 tagged with EGFP, mainly localized in the nuclei and its accumulation at DNA damaged sites began immediately after microirradiation. Moreover, we generated and characterized cell lines expressing EGFP-XRCC4 in XRCC4-deficient cells, i.e., XR-1 cells derived from the Chinese hamster ovary. Our findings showed that XR-1 cells were more sensitive than controls (CHO-K1) to low-dose X-irradiation (<0.5 Gy), whereas the radiosensitive phenotype of XR-1 cells was rescued by the expression of EGFP-XRCC4. We also confirmed that EGFP-XRCC4 expressed stably in XR-1 cells stabilizes DNA ligase IV. Altogether, these cell lines might be useful for the study of not only the dynamics and function of XRCC4, but also the molecular mechanism underlying the cellular resistance via the NHEJ pathway to low-dose radiation in mammalian cells.