Vanguards of paradigm shift in radiation biology; Radiation-induced adaptive and bystander responses

Matsumoto, Hideki*; Hamada, Nobuyuki*; Takahashi, Akihisa*; Kobayashi, Yasuhiko; Onishi, Takeo*

The risks of low dose ionizing irradiation ( 100 mSv) have been estimated using a linear no-threshold model by extrapolating from data obtained after high dose irradiation. The validity of this dose-response model is, however, controversial because evidence accumulated for over the past decade has shown that response of living organisms including humans to low dose/low dose-rate radiation is different from that to high dose/high dose-rate radiation. In another words, findings that cannot be explained by the classical "target theory" of radiobiology have accumulated. The radiation-induced adaptive response, bystander effects, low-dose radio-hypersensitivity, and genomic instability are specifically observed in response to low dose/low dose-rate radiation, and underpinnings of these responses often involve biochemical/molecular signals responding to targeted and non-targeted events. Recently, interrelations between the radioadaptive and bystander responses have been increasingly reported. We review observations supporting the existence of the latter two phenomena, and discuss the possible link between them from the aspect of production of reactive oxygen and nitrogen species.