Cellular exposure to ionizing radiation leads to oxidizing events that alter atomic structure through direct interactions of radiation with target macromolecules or via products of water radiolysis. Further, the oxidative damage may spread from the targeted to neighboring, non-targeted bystander cells through redox-modulated intercellular communication mechanisms. To cope with the induced stress and the changes in the redox environment, organisms elicit transient responses at the molecular, cellular and tissue levels to counteract toxic effects of radiation. Metabolic pathways are induced during and shortly after the exposure. Depending on radiation dose, dose-rate and quality, these protective mechanisms may or may not be sufficient to cope with the stress. When the harmful effects exceed those of homeostatic biochemical processes, induced biological changes persist and may be propagated to progeny cells. Physiological levels of reactive oxygen and nitrogen species play critical roles in many cellular functions. In irradiated cells, levels of these reactive species may be increased due to perturbations in oxidative metabolism and chronic inflammatory responses, thereby contributing to the long-term effects of exposure to ionizing radiation on genomic stability. Here, in addition to immediate biological effects of water radiolysis on DNA damage, we also discuss the role of mitochondria in the delayed outcomes of ionization radiation. Defects in mitochondrial functions lead to accelerated aging and numerous pathological conditions. Different types of radiation vary in their linear energy transfer (LET) properties, and we discuss their effects on various aspects of mitochondrial physiology. These include short and long-term in vitro and in vivo effects on mitochondrial DNA, mitochondrial protein import and metabolic and antioxidant enzymes.

Exposure of eukaryotic cells to ionizing radiation (IR) results in the immediate formation of free radicals that last a matter of milliseconds. It has been assumed that the subsequent alterations in multiple intracellular processes following irradiation is due to the initial oxidative damage caused by these free radicals. However, it is becoming increasingly clear that intracellular metabolic oxidation/reduction (redox) reactions can be affected by this initial IR-induced free radical insult and may remain perturbed for minutes, hours, or days. It would seem logical that these cellular redox reactions might contribute to the activation of protective or damaging processes that could impact upon the damaging effects of IR. These processes include redox sensitive signaling pathways, transcription factor activation, gene expression, and metabolic activities that govern the formation of intracellular oxidants and reductants. The physiological manifestations of these radiation-induced alterations in redox sensitive processes have been suggested to contribute to adaptive responses, bystander effects, cell cycle perturbations, cytotoxicity, heat-induced radiosensitization, genomic instability, inflammation, and fibrosis. While a great deal is known about the molecular changes associated with the initial production of free radicals at the time of irradiation, the contribution of perturbations in redox sensitive metabolic processes to biological outcomes following exposure to IR is only recently becoming established. This review will focus on evidence supporting the concept that perturbations in intracellular metabolic oxidation/reduction reactions contribute to the biological effects of radiation exposure as well as new concepts emerging from the field of free radical biology that may be relevant to future studies in radiobiology.

We demonstrate by western analysis that the expression levels of TP53 (formerly known as p53), CDKN1A (formerly known as p21Waf1), CDC2 (formerly known as p34cdc2), CCNB1 (cyclin B1) and RAD51 are significantly modulated in confluent, density-inhibited human diploid cell populations exposed to doses where only a small fraction of the nuclei are actually traversed by an alpha-particle track. The extent of modulation of TP53 and CDKN1A is significantly reduced in the presence of the gap junction inhibitor lindane and in irradiated low-density cell populations. In situ immunofluorescence studies show that at doses where about 2% of the nuclei would be traversed by an alpha particle, induction of CDKN1A occurs in more cells than predicted. Furthermore, the induced cells are present in isolated aggregates of neighboring cells. Therefore, our studies at the gene expression level indicate that similar signaling pathways are induced in bystander cells that are not traversed by an alpha particle as in traversed cells, and that biological effects in cell populations are not restricted to the response of individual cells to the DNA damage they receive.

It has generally been considered that important biological effects of ionizing radiation arise as a direct consequence of DNA damage occurring in irradiated cells. We have examined this hypothesis by exposing cells to very low fluences of ␣-particles, similar to those emitted by radon gas, such that as few as 1% of the cells in a population are traversed by a particle and thus receive any radiation exposure. By using the endpoints of changes in gene expression and induction of DNA damage, we show that nonirradiated ''bystander'' cells participate in the overall response of confluent density-inhibited populations of cultured fibroblast and epithelial cells. I t has long been thought that the important biological effects of radiation in a cell population are a direct consequence of DNA damage occurring in the irradiated cells: unrepaired or misrepaired DNA damage in these cells is responsible for the genetic effects of radiation. Presumably, no effect would be expected in cells in the population that received no direct radiation exposure. Recently, however, evidence has been presented indicating that genetic changes such as increased levels of sister chromatid exchanges (1, 2), mutations (3, 4), micronuclei (5), and DNA damage-inducible proteins (6, 7) occur in a greater-thanexpected number of cells in cultures exposed to very low fluences of ␣-particles, fluences in which only a small fraction of the cells are actually traversed by a particle track and thus directly exposed to radiation. Finally, it has been shown that when some cells were labeled with tritiated thymidine in a three-dimensional multicellular cluster model, a cytotoxic effect was transmitted to adjoining nonlabeled cells present in the same cluster (8).Overall, these studies indicate that radiation traversal through the nucleus of a cell is not a necessary prerequisite to producing genetic damage or a biological response; cells in a population that are in the vicinity of directly hit cells can also respond to the radiation exposure. These nonirradiated cells that express genetic damage or changes in the expression of stress-induced genes have been termed ''bystander cells.'' The present investigation was designed to determine the mechanisms by which damage signals may be transmitted from irradiated to nonirradiated bystander cells.We previously presented preliminary evidence for the involvement of gap junction-mediated intercellular communication (GJIC) in the molecular events leading to the modulation of gene expression in bystander cells (7). In these studies, confluent density-inhibited cultures of normal human fibroblasts were exposed to low fluences of ␣-particles in the presence or absence of lindane, a chemical inhibitor of GJIC. Changes in gene expression were measured by Western blotting. The participation of bystander cells in the overall cellular response to the radiation stress was inferred from the observations that, the effect was significantly greater than expected based on the fraction of directly irradiated cells in the population...

We have previously shown that chronic exposure of plateau-phase C3H 10 T1/2 cells to (60)Co gamma radiation at doses as low as 10 cGy protected the cells against neoplastic transformation by a subsequent large acute radiation exposure. We have also shown that this induced resistance to neoplastic transformation correlated with an increased ability to repair radiation-induced chromosome breaks. We now show that a single exposure of quiescent cells to doses as low as 0.1 cGy also reduces the risk of neoplastic transformation, from the spontaneous level to a rate three- to fourfold below that level. Higher doses, up to 10 cGy at the same dose rate (0.24 cGy/min), did not reduce the neoplastic transformation frequency further. This protective effect was seen only in irradiated cells that were allowed to incubate at 37 degrees C before release from contact inhibition. Cells released into low-density subcultures immediately after irradiation had unchanged neoplastic transformation frequencies. These results demonstrate that low or chronic exposure to radiation can induce processes which protect the cell against naturally occurring as well as radiation-induced alterations that lead to cell transformation. If similar processes are induced in human cells, the results also suggest that a single low dose, at background or occupational exposure levels, may in some circumstances reduce rather than increase cancer risk, a conclusion inconsistent with the linear no-threshold model of cancer risk from radiation.