High dietary iron increases oxidative stress and radiosensitivity in the rat retina and vasculature after exposure to fractionated gamma radiation

Abstract

Radiation exposure in combination with other space environmental factors including microgravity, nutritional status, and deconditioning is a concern for long-duration space exploration missions. Astronauts experience altered iron homeostasis due to adaptations to microgravity and an iron-rich food system. Iron intake reaches three to six times the recommended daily allowance due to the use of fortified foods on the International Space Station. Iron is associated with certain optic neuropathies and can potentiate oxidative stress. This study examined the response of eye and vascular tissue to gamma radiation exposure (3 Gy fractionated at 37.5 cGy per day every other day for 8 fractions) in rats fed an adequate-iron diet or a high-iron diet. Twelve-week-old Sprague-Dawley rats were assigned to one of four experimental groups: adequate-iron diet/no radiation (CON), high-iron diet/no radiation (IRON), adequate-iron diet/radiation (RAD), and high-iron diet/radiation (IRON+RAD). Animals were maintained on the corresponding iron diet for 2 weeks before radiation exposure. As previously published, the high-iron diet resulted in elevated blood and liver iron levels. Dietary iron overload altered the radiation response observed in serum analytes, as evidenced by a significant increase in catalase levels and smaller decrease in glutathione peroxidase and total antioxidant capacity levels. 8-OHdG immunostaining, showed increased intensity in the retina after radiation exposure. Gene expression profiles of retinal and aortic vascular samples suggested an interaction between the response to radiation and high dietary iron. This study suggests that the combination of gamma radiation and high dietary iron has deleterious effects on retinal and vascular health and physiology.

Figure 1

Effects of radiation and dietary iron on serum analyte levels. The white bar for each analyte is the percentage change in its level in the RAD group relative to its level in the CON group. The black bar for each analyte is the percentage change in its level in the IRON+RAD group relative to its level in the IRON group. *Main effect of radiation. ^†Pairwise comparison between columns is different P<0.05. Raw data for these analytes have been previously reported. CON, adequate-iron diet/no radiation; CRP, C-reactive protein; GPx, glutathione peroxidase; IRON, high-iron diet/no radiation; RAD, adequate-iron diet/radiation; TAC, total antioxidant capacity.

Figure 2

Effects of dietary iron and radiation on 8-OHdG immunoreactivity in retina sections. 8-OHdG immunoreactivity was determined in individual retinal neuronal layers (a–c) and in all retina layers combined (d) for the four groups of rats. Horizontal lines represent the mean immunoreactivity for each rat group (n=3 rats per group) and vertical bars indicate the s.d. Eight individual images from each biological sample were analyzed and their averages are represented by the symbols (circles, squares, up triangles, and down triangles) in the plots.

Figure 3

Radiation effects on gene expression in retina samples from rats on normal-iron or high-iron diets. Relative gene expression in retina samples, calculated using the ^ΔΔCt method, is presented as fold change associated with radiation (RAD versus CON groups for normal diet, IRON+RAD versus IRON groups for high-iron diet). All genes were considered to be upregulated by radiation if the relative value was above 1 and downregulated if the relative value was below 1. CON, adequate-iron diet/no radiation; IRON, high-iron diet/no radiation; RAD, adequate-iron diet/radiation.

Figure 4

Radiation effects on gene expression in the vasculature of rats on normal-iron or high-iron diets. Relative gene expression in aortic samples, calculated using the ^ΔΔCt method, is presented as fold change associated with radiation (RAD versus CON groups for normal diet, IRON+RAD versus IRON groups for high-iron diet). All genes were considered to be upregulated if the relative value was above 1 and downregulated if the relative value was below 1. CON, adequate-iron diet/no radiation; IRON, high-iron diet/no radiation; RAD, adequate-iron diet/radiation.