Impact of the ATM/Chk2 pathway and cell cycle phase on radiation‑induced senescence in A549 human lung cancer cells

Cell senescence is a state of stable proliferation arrest characterized by morphological changes and high senescence-associated β-galactosidase (SA-β-gal) activity. Inducing senescence in cancer cells is beneficial for cancer therapy due to proliferation arrest, however, the mechanisms underlying this process remain insufficiently understood. Therefore, the present study investigated the mechanisms of radiation-induced cellular senescence in A549 human lung cancer cells, focusing on the DNA damage response and cell cycle regulation. Cellular senescence was estimated by activity of SA-β-gal, and cell cycle was analyzed by propidium iodide staining using a flow cytometer. Cell cycle synchronization was performed by the double thymine block method. First, the roles of ataxia telangiectasia mutated (ATM) and ataxia telangiectasia mutated and Rad3-related (ATR), which are important factors for DNA damages response, in radiation-induced cellular senescence were investigated. ATM/ATR inhibitors suppressed radiation-induced G2/M phase arrest and decreased the percentage of senescent cells with high SA-β-gal activity, implying that G2/M arrest was associated with radiation-induced senescence. However, an analysis using inhibitors of checkpoint kinase 1 (Chk1) and Chk2, which function downstream of ATR and ATM, respectively, revealed that the Chk2, but not the Chk1, pathway was involved in radiation-induced senescence. To enhance radiation-induced senescence, radiation was combined with olaparib treatment, an inhibitor of DNA single-strand break repair. Olaparib increased the number of radiation-induced senescent cells. Additionally, cell cycle synchronization experiments revealed that irradiation of cells in S or G2/M phase resulted in higher senescent cell counts than irradiation in G1 phase. Taken together, the present results demonstrated that the ATM/Chk2 pathway and the DNA content are involved in the radiation-induced senescence of A549 cells.