Low-dose radiation sensitivity and induced radioresistance to cell killing in HT-29 cells is distinct from the "adaptive response" and cannot be explained by a subpopulation of sensitive cells.

Several reports using two different improved assays of clonogenicity have indicated the presence of a hypersensitive region in the radiation survival response at low doses, followed by an increase in radioresistance, in many mammalian cell lines. Mathematical modeling of these responses has suggested that it is unlikely that this effect can be explained by the presence of a small subpopulation of sensitive cells; however, this possibility cannot be excluded solely on the basis of those results. A second explanation has been offered which hypothesizes that a radiation-induced mechanism causes an increase in cellular radioresistance. This proposal has led to speculation that the substructure observed at low doses in these cell lines is related to the adaptive response, which hypothesizes the induction of a repair mechanism after a small priming dose of radiation which can protect cells against a larger second dose given several hours later. We have investigated these proposals with a study of priming doses using human tumor HT-29 cells, which we have previously shown to exhibit low-dose hyper-radiosensitivity. Our results provide significant evidence that this effect cannot be explained by a subpopulation of sensitive cells. However, the results also suggest that the radiation-induced increase in radioresistance observed in this cell line is distinct from the adaptive response.