INDUCTION OF AN ADAPTIVE RESPONSE AGAINST SPONTANEOUS NEOPLASTIC TRANSFORMATION IN-VITRO BY LOW-DOSE GAMMA-RADIATION

An adaptive response against spontaneous neoplastic transformation in vitro induced by low-dose gamma radiation has recently been reported u sing a clone of C3H 10T1/2 cells with a predisposition toward spontane ous transformation (Azzam et al., Radiat. Res. 146, 369-370, 1996). To test the generality of this observation, the HeLa X skin fibroblast h uman hybrid cell system was used to look for such an adaptive response using a similar experimental protocol. In the experimental protocol u sed, the frequency of neoplastic transformation of unirradiated cultur es (Arm A) was compared to that of cultures which had been irradiated with 1 cGy of gamma radiation and either plated immediately (Arm B) or held for 24 h at 37 degrees C prior to plating (Arm C) for assay of n eoplastic transformation. The pooled data from four separate experimen ts demonstrated a significantly reduced transformation frequency for A rm C compared to Arms A and B. This is indicative of an adaptive respo nse in the case of postirradiation holding, in agreement with the earl ier study using C3H 10T1/2 cells. However, with the exception of one e xperiment, the existence of the adaptive response was not clear on the basis of analysis of individual experiments. This points out the impo rtance of sample size when looking at low-dose effects that are close in magnitude to the inherent variations in spontaneous transformation frequency. (C) 1998 by Radiation Research Society.