The biological effect of 125I seed continuous low dose rate irradiation in CL187 cells

BACKGROUND To investigate the effectiveness and mechanism of 125I seed continuous low-dose-rate irradiation on colonic cell line CL187 in vitro. METHODS The CL187 cell line was exposed to radiation of 60Cogamma ray at high dose rate of 2 Gy/min and 125I seed at low dose rate of 2.77 cGy/h. Radiation responses to different doses and dose rates were evaluated by colony-forming assay. Under 125I seed low dose rate irradiation, a total of 12 culture dishes were randomly divided into 4 groups: Control group, and 2, 5, and 10 Gy irradiation groups. At 48 h after irradiation, apoptosis was detected by Annexin and Propidium iodide (PI) staining. Cell cycle arrests were detected by PI staining. In order to investigate the influence of low dose rate irradiation on the MAPK signal transduction, the expression changes of epidermal growth factor receptor (EGFR) and Raf under continuous low dose rate irradiation (CLDR) and/or EGFR monoclonal antibodies were determined by indirect immunofluorescence. RESULTS The relative biological effect (RBE) for 125I seeds compared with 60Co gamma ray was 1.41. Apoptosis rates of CL187 cancer cells were 13.74% +/- 1.63%, 32.58% +/- 3.61%, and 46.27% +/- 3.82% after 2 Gy, 5 Gy, and 10 Gy irradiation, respectively; however, the control group apoptosis rate was 1.67% +/- 0.19%. G2/M cell cycle arrests of CL187 cancer cells were 42.59% +/- 3.21%, 59.84% +/- 4.96%, and 34.61% +/- 2.79% after 2 Gy, 5 Gy, and 10 Gy irradiation, respectively; however, the control group apoptosis rate was 26.44% +/- 2.53%. P < 0.05 vs. control groups by Student's t-test were found in every treated group both in apoptosis and in G2/M cell cycle arrest. After low dose rate irradiation, EGFR and Raf expression increased, but when EGFR was blocked by a monoclonal antibody, EGFR and Raf expression did not change. CONCLUSION 125I seeds resulted in more effective inhibition than 60Co gamma ray high dose rate irradiation in CL187 cells. Apoptosis following G2/M cell cycle arrest was the main mechanism of cell-killing effects under low dose rate irradiation. CLDR could influence the proliferation of cells via MAPK signal transduction.