Adaptive response and induced resistance

Cellular stress responses are upregulated following exposure to radiation and other DNA-damaging agents. Therefore radiation response can be dose dependent so that small acute exposures (and possibly exposures at very low dose rates?) are more lethal per unit dose than larger exposures above a threshold (typically 10–40 cGy) where induced radioprotection is triggered. We have termed these interlinked phenomena low-dose hypersensitivity (HRS) and induced radioresistance (IRR) as the dose increases. HRS/IRR has been recorded in cell-survival studies with yeast, bacteria, protozoa, algae, higher plant cells, insect cells, mammalian and human cells in vitro, and in studies on animal normal-tissue models in vivo. There is indirect evidence that cell survival-related HRS/IRR in response to single doses is a manifestation of the same underlying mechanism that determines the well-known adaptive response in the two-dose case and that it can be triggered by high- and low-LET radiations as well as a variety of other stress-inducing agents such as hydrogen peroxide and chemotherapeutic agents. Little is currently known about the precise nature of this underlying mechanism, but there is evidence that it operates by increasing the amount and rate of DNA repair, rather than by indirect mechanisms such as modulation of cell-cycle progression or apoptosis. Changed expression of some genes, only in response to low and not high doses, may occur within a few hours of irradiation and this would be rapid enough to explain the phenomenon of induced radioresistance although its specific molecular components have yet to be identified. Net cancer risk is a balance between cell transformation and cell kill. Our known low-dose cell-survival responses suggest that lethality may more than compensate for transformation at low radiation doses. However, adaptive reduction in sensitivity to radio-mutation has also been reported, which implies the existence also of enhanced mutation following very low single doses. So far this has not been confirmed, but provided the trigger dose for mutational protection was lower than the trigger dose for protection against cytotoxicity, cell killing would still dominate over at least the first 10 cGy of low-LET exposure. This would lead to a non-linear, threshold, dose-risk relationship and even provide some explanation for anecdotal reports of apparent ‘health promoting’ effects and lowered cancer risk from very low exposure to ionising radiation.