Adaptive and bystander responses in human and rodent cell cultures exposed to low level ionizing radiation: the impact of linear energy transfer.

To understand the potential impact on risk from exposure to low-level ionizing radiation, we have investigated the modulation of gene expression, induction of DNA damage and of neoplastic transformation in human or rodent cells derived from cultures exposed in vitro to low dose gamma-rays (a low linear energy transfer radiation) or very low fluences of alpha-particles (a high linear energy transfer radiation). Pre-exposure of cells to a low gamma-ray dose protected cells from the DNA damaging and killing effects induced by a subsequent acute challenge exposure to gamma-rays. Furthermore, a low dose chronic exposure to gamma-rays decreased the frequency of micronucleus formation and neoplastic transformation to a level below the spontaneous rate in human and rodent cells respectively. In contrast, when cell cultures were exposed to low fluences of alpha-particles, wherein a small fraction of cells were irradiated, stressful effects were transmitted from the irradiated to adjoining nonirradiated bystander cells. The mechanisms underlying these effects and their relative contribution to the overall risk to ionizing radiation is discussed.