Low dose hyper-radiosensitivity in human lung cancer cell line A549 and its possible mechanisms

Objective To study the low dose hyper-radiosensitivity in human lung cancer cell line A549,and its possible mechanisms.Methods Exponentially growing A549 cells were irradiated with 60Co γ-rays at doses of 0-2 Gy.Together with flow cytometry for precise cell sorting,cell survival fraction was measured by mean of conventional colony-formation assay.ATM1981 Ser-P protein expression was examined by Western blot.Apoptosis was identified by Hoechst 33258 fluorescent staining,and Annexin V-FITC and propidium iodide staining flow cytometry.Cell cycle distribution was observed by flow cytometry.Results There was an excessive cell killing per unit dose when the doses were below about 0.3 Gy,and the cells exhibited more resistant response at the doses between 0.3 and 0.5 Gy,the cell survival fraction was decreased as the doses over 0.5 Gy.The expression of ATM1981Ser-P protein was first observed at 0.2 Gy,followed by an increase over 0.2 Gy,and reached the peak at 0.5 Gy(compared with 0.2 Gy group,t=7.96,P＜0.05),with no further increase as the doses at 1.0 and 2.0 Gy(t=0.69,0.55,P＞0.05).24 hours after irradiation,part cells presented the characteristic morpholos4cal change of apoptosis,and the apoptosis curve was coincident with the dose-survival curve.Compared with the control group,the cell cycle had no change post-irradiation to 0.1 and 0.2 Gy.G2/M phase arrest was manifested at 6 and 12 hours post-irradiation to 0.3,0.4 and 0.5 Gy(t=2.87,2.88,4.92 and 3.70,3.12,8.11,P＜0.05),and the ratio of G2/M phase was decreased at 24 hours post-irradiation(t=3.87,4.77,3.01,P＜0.05).Conclusions A549 cells displays the phenomenon of hyper-radiosensitivity(HRS)/induced radioresistance(IRR).The model of cell death induced by low dose irradiation is mainly apoptosis.The activity of ATM and cell cycle change might play an important role in HRS/IRR. Key words: Low dose hyper-radiosensitivity; ATM kinase; Apoptosis; Cell cycle arrest; A549 cell line