Low-dose radiation does not induce proliferation in tumor cells in vitro and in vivo

We have demonstrated that exposure of mice to low-dose radiation (75 mGy) stimulated bone marrow cell proliferation and peripheral mobilization (Li et al., Exp. Hematol. 32, 1088-1096, 2004). It is unclear whether such stimulating effects induced by low-dose radiation can also occur in tumor cells. In the present study, cells of two leukemia cell lines and five solid tumor cell lines together with four normal human cell lines were used to determine whether exposure to low-dose radiation (25 to 200 mGy X rays) can cause a stimulating effect on cell proliferation. A stimulating effect was found in the normal cell lines but not in the two leukemia and five solid tumor cell lines in response to low-dose radiation exposure in vitro. Examination of cell cycling changes and cell death for these cells by flow cytometry at different times after low-dose irradiation did not identify any changes attributable to the distinct effects of low-dose radiation on cell proliferation between tumor and normal cells. To provide further evidence for the absence of low-dose radiation-induced stimulating effects in tumor cells in vivo, cells of two solid tumor cell lines were implanted in nude mice. Exposure of tumor cells in vitro before implantation in nude mice or of tumor-bearing mice to low-dose radiation (75 mGy X rays) did not stimulate tumor growth compared to the tumor-bearing mice without low-dose radiation exposure. These results suggest that low-dose radiation stimulates growth of normal cells but not of leukemia and solid tumor cells in vitro and also does not stimulate growth of solid tumor cells in vivo.