Keywords: bystander effects, adaptive response, low radiation, ionising radiation
Radiation-induced bystander effects: do they provide evidence for an adaptive response?
In this paper, our current knowledge of the mechanisms underlying the induction of bystander effects by low dose low LET ionising radiation is reviewed and the question of how bystander effects may be related to observed adaptive responses or other protective effects of low doses exposures is considered. Bystander effects appear to be the result of a generalised stress response in tissues or cells. The signals may be produced by all exposed cells but the response may require a quoram in order to be expressed. The major response involving low LET radiation exposure discussed in the existing literature is a death response. This has many characteristics of apoptosis but is p53 independent. While a death response might appear to be adverse, the position is argued in this paper that it is in fact protective and removes damaged cells from the population. Since many cell populations carry damaged cells without being exposed to radiation (so called 'background damage'), it is possible that low doses exposures cause removal of cells damaged by agents other than the test dose of radiation. This mechanism would lead to the production of 'U-shaped' dose response curves. In this scenario, the level of 'adaptive' or beneficial response will be related to the background damage carried by the cell population. This model may be important when attempting to predict the consequences of mixed exposures involving radiation and other environmental stressors.