Effect of age on the sensitivity of the rat thyroid gland to ionizing radiation

Exposure to ionizing radiation during childhood is a well-known risk factor for thyroid cancer. our study evaluated the effect of age on the radiosensitivity of rat thyroid glands. four-week-old (4w),7 -week-old (7w),and 8-month-old (8m) male wistar rats were exposed to 8 gy of whole-body x-ray irradiation. thyroids were removed 3-72 h after irradiation,and non-irradiated thyroids served as controls. ki67-positivity and p53 binding protein 1 (53bp1) focus formation (a dna damage response) were evaluated via immunohistochemistry. amounts of proteins involved in dna damage response (p53,p53 phosphorylated at serine 15,p21),apoptosis (cleaved caspase-3),and autophagy (lc3,p62) were determined via western blotting. mrna levels of 84 key autophagy-related genes were quantified using polymerase chain reaction arrays. ki67-positive cells in 4w (with high proliferative activity) and 7w thyroids significantly decreased in number post-irradiation. the number of 53bp1 foci and amount of p53 phosphorylated at serine 15 increased 3 h after irradiation,regardless of age. no increase in apoptosis or in the levels of p53,p21 or cleaved caspase-3 was detected for any ages. levels of lc3-ii and p62 increased in irradiated 4w but not 8m thyroids,whereas expression of several autophagy-related genes was higher in 4w than 8m irradiated thyroids. irradiation increased the expression of genes encoding pro-apoptotic proteins in both 4w and 8m thyroids. in summary,no apoptosis or p53 accumulation was noted,despite the expression of some pro-apoptotic genes in immature and adult thyroids. irradiation induced autophagy in immature,but not in adult,rat thyroids. © 2015 the author.