Although evidence suggests that ionizing radiation can induce the bystander effect (radiation-induced bystander effect: RIBE) in cultured cells or mouse models, it is unclear whether the effect occurs in cells of wild animals. We investigated medium-mediated bystander micronucleus (MN) formation and DNA damage in un-irradiated cells from a large Japanese field mouse (Apodemus speciosus). We isolated four clones of A. speciosus embryonic fibroblasts (A603-1, A603-2, A603-3, and A603-4) derived from the same mother, and examined their radiation sensitivity using the colony-forming assay. A603-3 and A603-4 were similar, and A603-1 and A603-2 were highly sensitive compared with A603-3 and A603-4. We examined RIBE in the four clones in autologous medium from cell cultures exposed to 2 Gy X-ray radiation (irradiated cell conditioned medium: ICCM). We only observed increased MN prevalence and induction of DNA damage foci in A603-1 and A603-3 cells after ICCM transfer. The ICCM of A603-3 (RIBE-induced) was able to induce MN in A603-4 (not RIBE-induced). To assess the possible contribution of reactive oxygen species (ROS) or nitric oxide (NO) in medium-mediated RIBE, dimethyl sulfoxide (DMSO; a ROS scavenger) or 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO; an NO scavenger) were added to the medium. A suppressive effect was observed after adding DMSO, but there was no effect after treatment with c-PTIO. These results suggest that an enhanced radiosensitivity may not be directly related to the induction of medium-mediated RIBE. Moreover, ROS are involved in the transduction of the RIBE signal in A. speciosus cells, but NO is not. In conclusion, our results suggest that RIBE may be conserved in wild animals. The results contribute to better knowledge of radiation effects on wild, non-human species.
Keywords: Apodemus speciosus; Large Japanese field mouse; Radiation-induced bystander effect; Wild animal.