Bystander effect and genomic instability in human cells and their progeny after irradiation with X rays, protons or carbon ions: role of gap junction communication

Purpose: Ionizing Radiation (IR)-induced bystander effects and genomic instability have important implication in radiotherapy and radioprotection. Their persistence in the progeny of normal cells may contribute to risk of long-termradiation-related health effect in human, including cancer. Hence, this study investigates the role of gap junction intercellular communication (GJIC) and the quality of radiation in the propagation of stressful effects in the unirradiatedbystander cells and their progeny. Material and methods: Human skin fibroblasts in the confluent state were exposed to microbeam irradiations with different linear energy transfer (LET) at mean absorbed dose of 0.4 Gy, in the presence or absence of GJIC inhibitor (AGA) by which 0.036-0.4% of cells were directly targeted by IR. After 4 h irradiation or following 20 population doublings, the cells were harvested and assayed for micronucleus (MN) formation, gene mutation and protein oxidation.Results: Our results showed that high-LET carbon microbeams (LET ~103 keV/μm) and high-LET proton microbeams (LET~11 keV/μm) were more effective than low-LET X ray microbeam (LET ~6 keV/μm) in the induction of DNA damage (MN formation) in bystander cells. Interestingly, significant attenuation of MN formation occurred in bystander cells in the presence of AGA after proton and carbon microbeams. In contrast, incubation of the cells with AGA did not significantly affect the induction of MN formation in bystander cells during confluent holding after X irradiation. Further, the progeny of bystander cells exposed to X rays or protons showed persistent oxidative stress which correlated MN formation and mutation frequency. Such effects were not observed after irradiation by carbon ions. Importantly, the progeny of bystander cells from cultures exposed to protons or carbon ions under conditions where GJIC was inhibited harbored reduced oxidative and genetic damage. This mitigating effect was not detected when the cultured were exposed to X rays. Taken together, the overall results show the expression of stressful effects in the bystander cells and their progeny are dependent on the radiation quality or LET.Conclusions: Our findings suggest that the involvement of GJIC-dependent of radiation quality in the propagation of radiation induces stressful effects to bystander cells and their progeny. In addition, this work provides a strong support to the fact that carbon ions can significantly reduce the risk of cancer and have potential implications in the therapeutic outcome of radiotherapy compared to X rays or protons.American Association for Cancer Research Annual Meeting 201