Constitutive nitric oxide acting as a possible intercellular signaling molecule in the initiation of radiation-induced DNA double strand breaks in non-irradiated bystander cells.

The initiation and propagation of the early processes of bystander signaling induced by low-dose alpha-particle irradiation are very important for understanding the underlying mechanism of the bystander process. Our previous investigation showed that the medium collected from cell culture exposed to low-dose alpha-particle rapidly induced phosphorylated form of H2AX protein foci formation among the non-irradiated medium receptor cells in a time-dependent manner. Using N(G)-methyl-L-arginine, 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate and N(omega)-nitro-L-arginine (L-NNA) treatment before exposure to 1 cGy alpha-particle, we showed in the present study that nitric oxide (NO(*)) produced in the irradiated cells was important and necessary for the DNA double strand break inducing activity (DIA) of conditioned medium and the generation of NO(*) in irradiated confluent AG1522 cells is in a time-dependent manner and that almost all NO(*) was generated within 15 min post-irradiation. Concurrently, the kinetics of NO(*) production in the medium of irradiated cells after irradiation was rapid and in a time-dependent manner as well, with a maximum yield observed at 10 min after irradiation with electron spin resonance analysis. Furthermore, our results that 7-Nitroindazole and L-NNA, but not aminoguanidine hemisulfate, treatment before exposure to 1 cGy alpha-particle significantly decrease the DIA of the conditioned medium suggested that constitutive NO(*) from the irradiated cells possibly acted as an intercellular signaling molecule to initiate and activate the early process (<or=30 min) of bystander response after low-dose irradiation.