Modulation of the length of the cell cycle time of MCF-7 human breast cancer cells by melatonin.

It has been shown that melatonin has a direct inhibitory effect on the proliferation of MCF-7 human breast cancer cells in culture. In the present work, we studied whether the length of the cell cycle of MCF-7 cells in increased by melatonin. In MCF-7 cells partially synchronized and labelled with [3H]thymidine, melatonin (10(-9)M), added to the culture medium, shifted the period of the labeling index rhythm from 20.36 hours to 23.48 hours. The fact that melatonin significantly increased (p<0.005) the duration of the cell cycle of human breast cancer cells, support the notion that this hormone exerts part of its antitumor effect through a cell-cycle-specific mechanism by delaying the entry of MCF-7 cells into mitosis.