Low- and high-LET radiation induced chronic stress responses in mice: implications for carcinogenesis Free

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Exposure to radiation either accidental or intentional induces a multitude of responses both at the cellular and the tissue level in an organism. Apart from the total dose, radiation quality can also have a major role in the short term as well as in the long-term consequences of exposure. We sought to determine whether γ-rays being low-LET (linear energy transfer) and iron ion (56-Fe) radiation, a type of high-LET radiation encountered during manned spaceflight, would produce different long-term stress responses. The data presented here describe the long-term stress responses in mice following γ-ray and 1000MeV Fe radiation; the latter was carried out at the National Space Research Laboratory at Brookhaven National Laboratory. Equitoxic doses of the two types of radiation, the LD50/30 (50% lethality in 30 days) for γ-ray and 56-Fe radiation was determined in 6-8 week old female C57BL/6J mice and was found to be 7.5 Gy for γ-ray and 5.8 Gy for Fe radiation. Subsequently, mice were irradiated with 0, 2, 3, 4, 5, and 6 Gy of Fe particles and 0, 2, 3, 5, and 7 Gy of γ-rays. Mice were followed for 5 months and stress responses were studied in serum and in tissue samples. At two months mice from 0, 2 and 4 Gy of Fe radiation and 0, 2, and 7 Gy of γ-ray radiation group were sacrificed and tissues collected for analysis. Intracellular reactive oxygen species (ROS) were measured by flow cytometry in intestinal cells using the fluorescent probe H2DCFDA. Total antioxidant status was measured in serum, liver, spleen and intestinal cell extracts. Effect on the hematopoetic system was measured in Giemsa stained blood smear. Presence of any sustained inflammatory response in these mice was studied by measuring proinflammatory cytokines IL6 and TNFα. Persistence of damage response pathway activation and antioxidant response was studied in tissue samples by western blot analysis for p53, γH2AX, and superoxide dismutase (SOD). Phenotypically the Fe irradiated mice showed more signs of premature aging. Premature aging will be further investigated by MRI image scanning of the brain in these mice. Our results show that Fe radiation leads to increase intracellular ROS in intestinal cells obtained from Fe-irradiated mice. Although increased antioxidant response was observed both at the organism level and selectively at the tissue level in Fe-irradiated mice, the increase in ROS level was higher than the antioxidant response resulting in a net pro-oxidant environment in the organism. No significant alteration either in antioxidant or in ROS level was observed in γ-irradiated mice. Significant differences in proinflammatory cytokines IL-6 and TNF-α level were observed for the two types of radiation. In conclusion our study shows that compared to γ-ray, exposure to high-LET Fe radiation leads to a sustained pro-oxidant environment in the organism, which may make it susceptible to premature aging and could potentially initiate higher tumor incidence.