Fractionated radiation exposure enhances the DNA repair capacity to acquire radioresistance in HCT8 human colorectal cancer cells

Background: Fractionated radiotherapy is widely used for cancer treatment because of its advantages in the preservation of normal tissues; however, it may amplify the radioresistance of cancer cells. In this study, we aimed to understand whether and how fractionated radiation exposure induces radioresistance. Materials and Methods: HCT8 human colorectal cancer cells received a total X-ray dose of 5 Gy in either a single treatment (5 Gy administered once) or via fractionated exposure (1 Gy/day treatment for 5 consecutive days). We then examined the radioresistance of cancer cells exposed an additional 2 Gy X-ray by clonogenic assay and Western blot analysis. Results: Cells receiving fractionated exposure showed significantly greater proliferation and clonogenicity than those that received a single dose. Compared with the levels in the intact cells without radiation exposure, the expression levels of gamma H2AX, phospho-ATM and PARP were significantly enhanced only in the cells exposed to fractionated radiation. In contrast, the expression of cyclin D1 and cyclin E1 was enhanced only in the cells that received a single dose. In addition, the expression of SOD1 and SOD2 was slightly increased in the cells that received either the fractionated exposure or single exposure treatment. Conclusions: Fractionated radiation exposure facilitates radioresistance in HCT8 human colorectal cancer cells predominantly by enhancing their DNA repair capacity.