Adaptive and bystander effects induced in human and rodent cell populations exposed to low dose/fluence ionizing radiation

In an effort to understand low-dose effects and their potential impact on risk from ionizing radiation, we have investigated the modulation of gene expression and induction of DNA damage in human and rodent cells exposed to low doses of γ-rays or very low fluences of α-particles. Cells pre-exposed to a low γ-ray dose were protected from the DNA damaging and killing effects induced by subsequent acute challenge exposure to γrays. Furthermore, a low dose chronic exposure to γ-rays decreased the frequency of neoplastic transformation to a level below the spontaneous rate in rodent cells. In contrast, in cell cultures exposed to fluences of α-particles by which a small fraction of the nuclei were traversed by a particle track, stressful effects were transmitted from irradiated to adjoining non-irradiated bystander cells. The mechanisms underlying these effects and their relative contribution to the overall risk to ionizing radiation will be discussed.