The effect of ataxia-telangiectasia mutated kinase-dependent hyperphosphorylation of checkpoint kinase-2 on oligodeoxynucleotide 7909 containing CpG motifs-enhanced sensitivity to X-rays in human lung adenocarcinoma A549 cells

OBJECTIVE The aim of the study reported here was to further investigate the potential effect of ataxia-telangiectasia mutated (ATM) kinase-dependent hyperphosphorylation of checkpoint kinase-2 (Chk2) on radiosensitivity enhanced by oligodeoxynucleotide 7909 containing CpG motifs (CpG ODN7909) in human lung adenocarcinoma A549 cells. METHODS In vitro A549 cells were randomly separated into control, CpG, X-ray, CpG+ X-ray, ATM kinase-small interfering RNA (siRNA)+CpG+X-ray (ATM-siRNA), and Chk2-siRNA+CpG+X-ray (Chk2-siRNA) groups. siRNAs were adopted to silence the ATM and Chk2 genes. Expression and phosphorylation of ATM kinase and Chk2 were detected by Western blot assay. Cell colonies were observed under inverted phase-contrast microscopy. Cellular survival curves were fitted using a multi-target single-hitting model. Cell cycle and apoptosis were analyzed by flow cytometry. RESULTS Expression of ATM kinase and Chk2 was similar among the control, CpG, X-ray, and CpG+X-ray groups. Phosphorylated ATM kinase and Chk2 were significantly increased in the CpG+X-ray group compared with in the X-ray group (t=6.00, P<0.01 and t=3.13, P<0.05, respectively), though these were hardly detected in the control and CpG groups. However, expression of ATM kinase and Chk2 was clearly downregulated in the ATM-siRNA and Chk2-siRNA groups, respectively. Similarly, their phosphorylation levels were also significantly decreased in the ATM-siRNA group (t=14.35, P<0.01 and t=8.46, P<0.01, respectively) and a significant decrease in phosphorylated Chk2 was observed in the Chk2-siRNA group (t=7.28, P<0.01) when compared with the CpG+X-ray group. Further, the number of A549 cells at Gap 2/mitotic phase and the apoptosis rate were clearly increased in the CpG+X-ray group compared with in the other groups (all P<0.05). The multi-target single-hitting model showed that the sensitization enhancement ratio calculated by mean death dose was 1.39 in CpG+X-ray group (vs 1.04 and 1.03 in the ATM-siRNA and Chk2-siRNA groups, respectively). CONCLUSION This study provides the first evidence, as far as we are aware, that CpG ODN7909 can potentiate A549 cell radiosensitivity via increasing ATM kinase-dependent phosphorylation of Chk2, suggesting activation of the ATM kinase/Chk2 signal pathway. However, the mechanism of ATM kinase activation is worth further exploration.