Abstract
Comparison of same biological endpoints by exposing non-irradiated cells to the medium irradiated with cells and directly irradiated cells is important for investigating the efficiency of secret bystander factors inducing the biological effects. We have proposed that slow releasing long-lived radicals (SRLLRs) produced in irradiated cells are likely to responsible for inducing mutation or transformation. Very recently we have also found that level of SRLLRs in non-irradiated cells exposed to the medium, which was supernatant fluid of culture medium irradiated with other cells (bystander medium), increased in significant. Point mutation frequency also increased in either case although the levels are different. In this study we have compared the role of SRLLRs in directly irradiated cells and bystander cells (exposed to the medium) by using antioxidants for reducing the levels of SRLLRs and those of point mutation. Ascorbate (AA) and N-acetylcysteine (NAC) were used as antioxidants. When Syrian golden Hamster Embryo (SHE) cells were irradiated with γ-ray (4 Gy), levels of SRLLRs were increased in 21% and kept for 20 h. Post treatment of both AA or NAC (5 mM)at 20 min after irradiation for 2 h scavenged SRLLRs induced in the cells.. It has been reported that AA and RibCys (as an analog of NAC) can reduce the levels of mutation by the post treatment (Mutat. Res. 421 45 (1998); Mutat. Res. 551 (2004)). When Chinese Hamster Ovary (CHO) cells were exposed to the bystander medium for 24 h, which was the supernatant fluid of the medium irradiated with CHO cells (γ-ray, 1 Gy) and kept for 24 h after irradiation, AA (1 mM) can reduce the levels of SRLLRs but NAC (5 mM) could not when the antioxidants were added at the medium transfer and treated for 24 h. Similarly, the treatment of AA suppressed the mutation induction in the normal cells but NAC could not. Because both AA and NAC have similar antioxidant activity with similar molecular weights, their diffusion rates in cytoplasm should be almost the same. We speculate that SRLLRs in irradiated cells are mainly located in cytoplasm to make possible for AA and NAC to reach to the SRLLRs, while SRLLRs induced by the bystander medium are in some organelles having a function of importing AA selectivly (e.g. mitochondria with the GLUT membrane protein).
