Abstract
Radiation-induced bystander response is defined as a response in cells that have not been directly targeted by radiation but is in the neighborhood of cells that have been directly exposed. To elucidate the bystander response is important to evaluate the risk of low dose radiation. It has already reported many results of bystander response induced by high-LET charged particles using microbeam irradiation systems. On the other hand, it has not yet fully recognized bystander response to low-LET photons. Here, it has shown that bystander cell killing induced by synchrotron radiation (SR) X-ray microbeam irradiation was mainly mediated by nitric oxide (NO). Cell nucleus of confluent normal human lung fibroblast WI-38 cell was irradiated with a 5.35 keV monochromatic SR X-ray microbeam cut in 5 μmx5 μm square. All of the cells on dish were harvested and plated 24 h after irradiation. Surviving fractions were determined by colony formation assay. Surviving fractions were decreased between 0.25-1.5 Gy and were 0.85 at 1.4 Gy in the case of 5 cells in the center of dish were irradiated. Above 2.0 Gy, surviving fractions were recovered approximately 1.0, suggesting that the induction of bystander cell killing may need surviving activities in targeted cells, because the dose resulting in 37% cell survival was about 2.0 Gy. Bystander cell killing was significantly suppressed by pretreatment with aminoguanidine (an inhibitor of inducible NO synthase) or carboxy-PTIO (a scavenger of NO), but not by DMSO (a scavenger of reactive oxygen species) or lindane (an inhibitor of gap junction). These results suggest that NO is the chief initiator/mediator of bystander cell killing induced by SR X-ray microbeam irradiation.
