Induction of radio-adaptive response in colony formation by low dose X-ray irradiation

In this study, we examined the induction of a radio-adaptive response to cell death using a colony formation test in m5S, G401.2/6TG.1 and HeLa cells. When m5S cells were subjected to priming irradiation of 0.05 to approximately 0.15 Gy 4 hr before being irradiated with 4.5 Gy, the survival ratios increased significantly to 39 to approximately 42%. The priming irradiation effect was also observed when G401.2/6TG.1 cells were subjected to priming irradiation of 0.025 to approximately 0.1 Gy 4 hr before being irradiated with 0.8 Gy. This effect showed a two-phasic characteristic, where the first peak was reached at 0.025 Gy, and the second peak was reached at 0.075 Gy. The first peak showed a survival ratio of 56%, while the second peak was at 55%. However, in HeLa cells, this priming irradiation effect was not observed. These results indicated that induction of the radio-adaptive response did not depend on whether cells are normal or cancerous. One of the differences in these cells is that m5S and G401.2/6TG.1 cells have gap-junctional intercellular communication, but HeLa cells do not. Induction of the radio-adaptive response may be related to gap-junctional intercellular communication.