Iyer, R. and Lehnert, B. E. Alpha-Particle-Induced Increases in the Radioresistance of Normal Human Bystander Cells. Radiat. Res. 157, 3–7 (2002).

Numerous investigators have reported that direct exposure of cells to a low dose of ionizing radiation can induce a condition of enhanced radioresistance, i.e. a “radioadaptive” response. In this report, we investigated the hypothesis that a radioadaptive bystander effect may be induced in unirradiated cells by a transmissible factor(s) present in the supernatants of cells exposed to a low dose of α particles. Normal human lung fibroblasts (HFL-1) were irradiated with 1 cGy of α particles and their supernatants were transferred to unirradiated HFL-1 cells as a bystander cell model. Compared to directly irradiated cells that were not treated with supernatants from HFL-1 cells exposed to low-dose radiation, such treatment resulted in increased clonogenic survival after subsequent exposure to 10 and 19 cGy of α particles. Increases in protein levels of AP-endonuclease, a redox and DNA base excision repair protein, were found in the bystander cells, but not in directly irradiated cells. Supernatants from α-particle-irradiated cells were also found to increase the clonogenicity of unirradiated cells. These results, in conjunction with our earlier findings that supernatants from cells exposed to a low dose of α particles contain growth-promoting activity, suggest that this new bystander effect may be related to an increase in DNA repair and cell growth/cell cycle regulation.