Low- and high-dose-rate radiation exposure alters the cellular composition and dynamics of the rat mammary epithelium for weeks preceding carcinogenesis.

In animals, low-dose-rate radiation induces cancer at a reduced rate compared with a high-dose-rate at an identical cumulative dose, although the underlying mechanism is not well understood. The immediate responses of cells to irradiation are well established, including DNA double-strand break repair, cell-cycle arrest and cell death; conversely, the changes in tissues weeks after irradiation are not well understood. We therefore analysed cellular dynamics in rat mammary tissue weeks after high- or low-dose-rate irradiation. We irradiated 5-week-old rats with 2 Gy (30 Gy/h) or 3- to 5-week-old rats with continuous 2 Gy (6 mGy/h). For histological analysis, luminal cells were identified with anti-cytokeratin (CK) 8 + 18; CK8 + 18Low cells are luminal progenitor cells, and CK8 + 18High cells are luminal mature cells. To evaluate cell composition by flow cytometry, epithelial cells were isolated from mammary tissue. The proliferative potential of luminal progenitor cells-as measured by Ki-67 on paraffin sections-decreased 2 weeks after irradiation at either the high- or low-dose rate but recovered to the control level by 4 weeks. No significant difference was observed in the S phase and total cell-cycle length identified by 5-ethynyl-2'-deoxyuridine and 5-bromo-2'-deoxyuridine or cell death marked by cleaved caspase-3 among the dose-rates. Furthermore, the composition of luminal mature cells changed 2-6 weeks after completing the high- and, to a lesser extent, low-dose-rate radiation exposure, indicating potential proliferative stimulation of luminal progenitor cells related to susceptibility to carcinogenesis. These findings suggest that the altered cell composition and dynamics of luminal cells for several weeks contribute to carcinogenesis.