Relationship between radiation-induced low-dose hypersensitivity and the bystander effect.

Recent advances in our knowledge of the biological effects of low doses of ionizing radiation have shown two unexpected phenomena: a "bystander effect" that can be demonstrated at low doses as a transferable factor(s) causing radiobiological effects in unexposed cells, and low-dose hyper-radiosensitivity and increased radioresistance that can be demonstrated collectively as a change in the dose-effect relationship, occurring around 0.5-1 Gy of low-LET radiation. In both cases, the effect of very low doses is greater than would be predicted by conventional DNA strand break/repair-based radiobiology. This paper addresses the question of whether the two phenomena have similar or exclusive mechanisms. Cells of 13 cell lines were tested using established protocols for expression of both hyper-radiosensitivity/increased radioresistance and a bystander response. Both were measured using clonogenicity as an end point. The results showed considerable variation in the expression of both phenomena and suggested that cell lines with a large bystander effect do not show hyper-radiosensitivity. The reverse was also true. This inverse relationship was not clearly related to the TP53 status or malignancy of the cell line. There was an indication that cell lines that have a radiation dose-response curve with a wide shoulder show hyper-radiosensitivity/increased radioresistance and no bystander effect. The results may suggest new approaches to understanding the factors that control cell death or the sectoring of survival at low radiation doses.