Host: The Japan Radiation Research Society
Co-host: City of Kitakyushu, University of Occupational and Environmental Health, Japan
There is increasing evidence that cellular responses to low dose / low dose rate ionizing radiation are fundamentally different from responses occurring at high doses. Two low dose responses that appear to be specific to low dose / low dose rate exposures are bystander effects and adaptive responses. Bystander effects refer to those effects occurring in cells that were not directly hit by radiation but were either neighbors of irradiated cells or received soluble secreted signals from irradiated cells. These bystander cells manifest many of the same endpoints as irradiated cells and suggest that the target for irradiation is larger than the target volume actually irradiated. Adaptive responses are induced by low dose/low dose rate irradiation and can make irradiated cells refractory to a second high dose challenge by radiation exposure. Both bystander effects and adaptive responses can be induced by low doses of radiation, have been described both in vivo and in vitro and appear to be strongly influenced by genetic factors. However the biological, cellular and molecular mechanism(s) underlying these effects are unknown. The biological impact of bystander effects is likely detrimental to an organism. However, they can be interpreted as both being detrimental and, by a stretch, beneficial. Mutations and/or micronuclei induction in non-targeted bystander cells are likely to be detrimental because of increased genomic damage out side the irradiated field. In contrast, apoptosis might well be beneficial in that it could eliminate damaged cells that could otherwise accumulate genomic change leading to transformation, as has been suggested in anecdotal literature from clinical radiation oncology. Adaptive responses on the other hand appear to stimulate a cells ability to deal with a subsequent genomic insult and consequently could be considered a beneficial effect of low dose irradiation - provided cells are exposed to a second challenge. This presentation will consider the biological significance of these two phenomena, and their implications for low dose radiation effects.
