Radiation-induced bystander effects

Ionizing radiation is an important part of cancer therapy and at the molecular and cellular level it is generally believed that damage to the DNA within the cell nucleus is the major cause of reproductive cell death. This classical view also assumes that the cell killing effect is closely related to the amount of energy deposited in the DNA of the cell traversed by the radiation. In the last decade several newly recognized findings have suggested so-called nontargeted responses where unirradiated cells may respond when their neighbors are irradiated. This phenomenon is generally referred to as the bystander effect and the response in unirradiated cells may include cell killing, genomic instability or delayed death, DNA damage, chromosomal aberrations, altered gene expression or mutations [1]. The term bystander has previously been used in gene therapy where bystander effects are described as when the gene product of a transfected DNA travels from the transfected cell into neighboring cells. Upon radiation-exposure the bystander effect is so far mainly identified using in vitro cell cultures. Mothersill and Seymour [2] found that medium from low-LET irradiated cell cultures could reduce the survival of non-irradiated cells. The effect was cell-type dependent and recognized already at low radiation doses with no further effect at higher doses. Earlier studies with different experimental design showed that high-LET alpha particles induced chromosomal damage in 30% of the cells even if only 1% of the cell population was traversed by an alpha particle [3]. Several other studies have confirmed these results and more recent studies also involve irradiation of tissue explants with a charged particle microbeam [4]. Clearly, these findings challenge the conventional notion that damage to the DNA/genome is only induced by ionizing events in the irradiated cell. A critical question is of course how the bystander effect influences the therapeutic outcome, e.g. its impact on tumor cell killing, normal tissue response, and radiation-induced cancer. Studies on both early and late responses in normal and tumor cells, preferably in vivo or in more complex in vitro models could help to resolve these questions. In this issue of Acta Oncologica, Djordjevic and Lange [5] present experimental indications of a bystander cell killing effect identified when threedimensional spheroids of different constitutions of cell populations are used. In this system, primarily developed to evaluate radiation response of fresh tumor biopsy material without interference from overgrowing normal fibroblasts, they recognize bystander-like responses when mixing test cells with heavily irradiated feeder cells. Thus the presence of irradiated cells in a 3D spheroid co-culture system brings the clonogenicity down. Somewhat surprising, this bystander response was obtained at relatively high doses and it is possible that other factors could influence the response. However, this study pinpoints critical aspects on the use of appropriate in vitro tests of radiation sensitivity, and ultimately, predictive assays for use in tumor therapy. What do we know about the mechanism transferring the signals from an irradiated cell to an unirradiated bystander cells? The results so far implicate extracellular factors that are transferred to bystander cells, either directly via gap junctions between adjacent cells, or indirectly via autocrine or paracrine factors (reviewed in [6]). Clearly the signals and responses are cell-type dependent and irradiation of cells have been found to initiate a bystander response involving cytokines (e.g. tumor necrosis factor a or interleukin 8), reactive oxygen