Changes in cell cycle progression of HeLaS3 cells synchronized after X-ray irradiation

Objective: To observe the effect of different doses of X-rays on cell cycle progression of synchronized HeLaS3 cells. Methods: Using a double block method with thymidine and flow cytometric analysis, the changes in cell cycle progression of synchronized HeLaS3 cells were examined after 75 mGy and 2.0 Gy X-irradiation in G0/G1, S and G2 + M phases respectively, and the dose-response relationship given in G2/M phase was analyzed after irradiation. Results: The S and G2 phases occurred late 9-15 h after the releasing point regardless of whether HeLaS3 cells were irradiated with 2 Gy in G0/G1, S or G2 + M phases. HeLaS3 cells underwent a G2 arrest 9 and 12 h after the releasing point when 75 mGy irradiation was administered in G0/G1 and G2 + M phases, and this delay skipped completely at 12 and 15 h, respectively. Moreover, the cell cycle progression was accelerated. However, there was no G2 delay at 9 and 11 h when the cells were irradiated with 75 mGy in S phase, and in this case the cell cycle progression from G2/M to G0/G1 phase was accelerated. A study on the dose-effect relationship showed that the number of G0/G1 phase cells decreased, and G2 delayed after 0.025-2.0 Gy irradiation given in G2/M phase, and the delay was dose-dependent. However, the changes in the number of S phase cells were different between low (0.025-0.1 Gy) and higher (0.5-2.0 Gy) doses. Conclusion: G2 delay is a very radiosensitive parameter, which occurs in HeLaS3 cells after X-ray irradiation with a dose as low as 25 mGy when administered in G0/G1 and G2 + M phases, but this delay only occurs with doses above 0.1 Gy when the cells are irradiated in S phase