Abstract
Exposure to ionizing radiation (IR), in particular during childhood, is a well-known risk factor for thyroid cancers. Among a-bomb survivors, a vast majority of thyroid cancers are developed in survivors who have been adulthood at the bombing. There was little evidence of any effect among persons exposed as adult. In this meeting of the last year, we had reported that the level of phosphorylate p53 at Ser15 was increased, while no increase was found in the level of p21 and cleaved caspase-3 in immature (4 week-old rat) and adult (7 week-old rat) thyroid up to 24 h after IR. This result suggested that neither p21-dependent cell cycle arrest nor apoptosis was induced in spite of DNA damage response was occurred in thyroid epithelial cells after IR. This study aimed to investigate the participation of autophagy and the difference of age in thyroid follicular cells after IR. Immature and adult male wistar rats were exposed to X-ray at 8 Gy, then thyroid tissues were removed from 0 to 72 h after 8Gy IR. Histological examination revealed expansional alteration of follicular cells after IR. The level of alteration cells was peaked 24 h and increased significantly by 72 h in immature. In contrast, it was increased at 48 h and then decreased at 72 h in adult. The rate of Ki-67-positive cycling cells in immature and adult follicular cells was significantly decreased, and Cleaved caspase 3 positive cells were not detected until 72 h. Electron microscopy demonstrated the expansion of endoplasmic reticulum and the pool of secretion in the expansional immature follicular cells, and autophagosome was also observed at 6 h after IR. The expansion and collapse of mitochondria, stack of golgi, unusual form of lysosome and microautophagy were observed in adult. These observations were remarkable at 48 h. The level of LC3-II, which is a reliable marker for autophagic activity, increased until 24 h in immature and adult thyroid after IR. The present study revealed that IR induced autophagy but not apoptosis in immature and adult thyroid. Further study is required to examine the role of authophagy in radiation-induced thyroid cancer.
