Determination of Clonogenic Radiosensitivity of HeLa Cells in Early and Late G1 Phases

Background/Aim: Using a fluorescent ubiquitinationbased cell cycle indicator (Fucci), we recently reported that post irradiation of HeLa cells, micronuclei frequency increased in the early G1 phase in comparison with that in the late G1 phase. This is inconsistent with the results of well-recognized studies that used clonogenic assays. In this study, we determined radiosensitivity of the cells using a clonogenic assay by making the best use of the Fucci property, while simultaneously characterizing cell cycle kinetics and DNA damage responses. Materials and Methods: Early and late G1 phase cell fractions were isolated using a cell sorter by exploiting the different red fluorescence intensities of Fucci. Radiosensitivity was determined by the colony formation assay. Time-lapse imaging and immunostaining were performed to analyze cell cycle kinetics and DNA damage. Results: Late G1 cells were more radioresistant than early G1 cells. Cells irradiated in the early and late G1 phases induced G2 arrest, while the latter demonstrated a significantly longer duration of G2 arrest. This difference became more evident as the radiation dose increased. Furthermore, 16 h after irradiation, a greater number of gamma H2AX foci remained in cells irradiated in the early G1 phase than in those irradiated in the late G1 phase. Conclusion: HeLa cells in the late G1 phase are more radioresistant than those in the early G1 phase, presumably because DNA damage is efficiently repaired during a longer G2 arrest in late G1 cells.