Objective To investigate the low-dose hyper-radiosensitivity (HRS)/induced radioresistance (IRR) in A549 cells synchronized at G2 phase and the role of ATM kinase in the process.Methods Human lung adenocarcinoma cell line A549 was synchronized at G2 phase by aphidicolin.The ATM-specific activator and inhibitor,chloroquine and KU55933,were used to regulate the activity of ATM.The colony formation assay was used to evaluate cell survival.Flow cytometry was used to determine the cell cycle of radiation-exposed A549 cells synchronized at G2 phase.Immunofluorescence was used to observe the dynamics of γ-H2AX fluorescence and evaluate the efficiency of DNA repair in A549 cells synchronized at G2 phase.Western blot was used to detect the expression of phosphorylated ATM (Ser1981) and ATM.Results A549 cells synchronized at G2 phase had substantially enhanced HRS than non-synchronized cells.The dose-induced transition from HRS to 1RR was in accordance with the dose-response pattern of early G2/M checkpoint.However,with the same threshold dose,the activation of early checkpoint occurred earlier and lasted longer than normal.The activation of ATM kinase inhibited HRS and enhanced DNA repair,while the inhibition of ATM kinase enhanced HRS and hindered DNA repair.Conclusions ATM kinase-mediated early G2+M checkpoint is a molecular switch for HRS in synchronized A549 cells.Low-dose irradiation with G2-phase synchronization and ATM inhibitor can enhance the low-dose radiosensitivity.