The role of DNA damage repair and Chk2 protein in hyper-radiosensitivity of lung adenocarcinoma A549 cells.

To explore the role of the Chk2 protein expression and DNA double strand breaks (DSBs) repair in low dose hyper-radiosensitivity (HRS)/increased radioresistance (IRR) of non-small cell lung cancer, A549 cells were subjected to irradiation at the dosage ranging from 0.05-2 Gy. Clonogenic survival was measured by using fluorescence-activated cell sorting (FACS) plating technique. Percentage of cells in M-phase after low doses of X-irradiation was evaluated by phospho-histone H3-FITC/PI and Western blotting was used to detect protein expression of Chk2 and phospo-Chk2. DNA DSBs repair efficiency was also measured by induction and persistence of γ-H2AX. The results showed that the killing ability of irradiation with A549 cells increased at low conditioning dose below 0.3 Gy. Within the dose of 0.3 to 0.5 Gy, A549 cells showed a certain extent of radiation resistance. And when the dose was more than 0.5 Gy, survival fraction exhibited a negative correlation with the dosage. There was no difference between the 0.1 or 0.2 Gy dosage groups and the un-irradiated group in terms of the percentage of cells in M phase. But in the high dosage group (0.3-1.0 Gy), the percentage of cells in M phase was decreased markedly. In addition, the percentage of cells in M phase began to decrease two hours after irradiation. One hour after irradiation, there was no conspicuous activation of Chk2 kinase in 0.1 or 0.2 Gy group, but when the irradiation dose reached 0.3 Gy or higher, Chk2 kinase started to be activated and the activation level showed no significant difference among high dosage groups (0.4, 0.5, 1.0 Gy). Within 1 to 6 h, the DNA DSBs repair efficiency was decreased at 0.2 Gy but increased at 0.5 Gy and 1.0 Gy, which was in line with Chk2 activation. We are led to conclude that the mechanism of HRS/IRR in A549 cell line was probably due to early G(2)/M checkpoint arrest and enhanced DNA DSBs repair. In this regard, Chk2 activation plays a key role in G(2)/M checkpoint activation.