Abstract
Understanding the mechanism (s) that underlie the bystander effects of low- or high-linear energy transfer (LET) radiations has been considered important for cancer therapy and a better understanding of the health risks associated with space exploration. We investigated the role of gap junction intercellular communication (GJIC) in propagation of events leading to amplification of stressful/bystander effects in confluent normal human fibroblasts (NB1RGB) cultures exposed to low doses/low fluences of low-or high-LET radiations using cell death and induction of DNA damage as endpoints.
Using microbeam irradiation, confluent NB1RBG cells were exposed to 5.35 keV X rays (LET 6 keV/µm), 220 MeV carbon ions (LET 103 keV/µm), 260 MeV neon ions (LET 380 keV/µm), or 460 MeV argon ions (LET 1060 keV/µm) under condition by which only 0.04% of cells in the population was targeted in the presence or absence of a gap junction inhibitor (AGA). As expected, carbon-ions, argon-ions and neon-ions were more effective than X-rays at inducing cell killing. However, inhibiting of GJIC with AGA promoted increase survival following exposure to high, but not low LET radiation. The percent cell survival for carbon-ions after 3 h irradiation was around 90% in the absence of the inhibitor and almost 100% in the presence of the inhibitor. On the other hands, the cell survivals for neon- and argon ions were almost 100% irrespective of the presence or absence of the inhibitor. Although the incidence of micronuclei formation induced by X-rays showed no difference irrespective of irradiation with or/ without a gap junction inhibition, the frequency of micronuclei induced by carbon-ions was significantly higher in the absence of AGA than in its presence. Collectively, the results show that the propagation from irradiated to neighboring bystander cells, of clastogenic promoting effects depends on the LET and dose of the radiation, and that GJIC promotes the propagation of stressful bystander effects in high-LET-irradiated cell cultures.
In conclusion, the expression of the above low dose/low fluence radiation-induced stressful/bystander effects are consistent with our earlier observation of a similar propagation of toxic effect among cells in cultures wherein all cells were targeted by densely ionizing radiations. Characterizing the nature of the propagated signaling molecules would have translational implications in radiotherapy and the formulation of countermeasures against toxic effects sustained by humans during space exploration.
Supported by NASA grant NNJ06HD91G and NIH grant CA049062 in the USA, and the National Institute of Radiological Sciences in Japan.
