Objective To detect the effects of melatonin combined with γ-ray ionizing radiation on the proliferation of human colon cancer HCT 116 cells in vitro and in vivo and to explore the role of melatonin in regulating the radiosensitivity of HCT 116 cells.

Methods The cohorts were divided into blank control group (HCT 116 cells were not given any treatment), melatonin group (HCT 116 cells were treated with 1 mmol/L melatonin for 2 h), radiation group (HCT 116 cells were exposed to 6 Gy γ-ray radiation), and melatonin+radiation group (HCT 116 cells were treated with 1 mmol/L melatonin for 2 h and then exposed to 6 Gy γ-ray radiation). In in vitro experiments, colony formation assay was used to detect cell proliferation after exposure to 0, 2, 4, 6, or 8 Gy radiation. Flow cytometry was applied to detect the cell cycle distribution at 24 h and cell apoptosis at 24 and 48 h after exposure to 6 Gy radiation. Comet assay was performed to detect DNA damage to cells 2 h after exposure to 6 Gy radiation. In in vivo experiments, the tumor-bearing nude mouse model was built by inoculating HCT 116 cells. The volume and inhibition ratio of tumor xenografts were examined. T-test was used for comparison between groups.

Results ① In in vitro experiments, the colony number of HCT 116 cells treated with melatonin prior to radiation was significantly less than that of control cells (t=3.83, P=0.005). HCT 116 cells that were arrested at the G2 phase in the melatonin+radiation group (53.04%±4.67%) were increased, and significant differences were noted between the melatonin+radiation group and the radiation or melatonin group (t=2.940 and 20.660, P=0.017 and P<0.01, respectively). The cell apoptosis rate of HCT 116 cells in the melatonin+radiation group at 24 and 48 h after treatment was increased and reached (12.15±0.41)% and (30.57±1.91)%, respectively, which were markedly higher than those of the radiation (9.00%±0.70%, 8.69%±0.71%) or melatonin group (3.03%±0.42%, 12.56%±0.89%) (t=7.46, 17.75, 29.12, and 14.80, all P<0.01). The value of tail DNA, tail length, tail moment, and Olive tail moment in HCT 116 cells in the melatonin+radiation group were significantly higher than that in the radiation (t=4.72, 4.16, 4.74, 4.50, all P<0.01) or melatonin group (t=20.27, 22.80, 13.81, and 18.85, all P<0.01). ② In in vivo experiments, tumor xenografts in nude mice of the melatonin+radiation group grew slowly. The volume of tumor xenografts in the melatonin+radiation group at day 15 was significantly decreased compared with that in the radiation or melatonin group (t=3.51 and 2.72, P=0.006 and P=0.021, respectively). The inhibition ratio of xenografts in the melatonin+radiation group (54.7%±8.0%) was significantly higher than that in the radiation or melatonin group (t=7.50, 4.12, all P<0.01).

Conclusion Melatonin combined with γ-ray radiation had obvious inhibition effect on colon cancer cells and increased the radiosensitivity of colon cancer cells.