Abstract
Radiation carcinogenesis in human is considered as a result of the combined effect of radiation and environment factors. We previously showed that smaller doses of X-rays reduced the incidence of TL induced by N-ethyl-N-nitrosourea (ENU), while large doses of X-rays increased and accelerated the TL development in a synergistic manner. This indicates the mode of combined effect is dependent upon the dose of X-rays. In this study, we aimed to determine the mode of combined effect on tumors other than TL and life-span (LS) shortening.
B6C3F1 mice (4-week-old female) were irradiated with X-rays (0.2 - 2.0 Gy a week x 4 times) or treated with ENU for 4weeks (50 - 400 ppm in drinking water) from 8-week-old. For combined treatment, the mice were irradiated with X-rays (0.2 - 1.0 Gy a week x 4 times) and then treated with ENU (50 - 200 ppm).
The LS after combined exposure (0.2/0.4 Gy X-rays with 200 ppm ENU) was much longer than that of ENU treatment alone, suggesting small dose of X-rays has antagonistic function. Contrary to the results of TL, incidence of tumors of lung, liver and ovaries increased more than additively after the combination of ENU with X-rays even at small doses. Thus, the role of X-rays in combined exposure was dependent not only the dose used but also the tissues examined.
