Melatonin inhibition of MCF-7 human breast-cancer cells growth: influence of cell proliferation rate

We have studied whether the cell proliferation rate modifies the inhibitory actions of melatonin on MCF-7 cell growth. The proliferative rate of cells was altered by plating them at different densities (5 × 104 to 100 × 104 cells/dish) in media with low charcoal-stripped serum concentrations. In this way, population doubling time ranged from 33 h (for density = 100 × 104 cells/dish) to 75 h (for density = 5 × 104 cells/dish). Melatonin (10−9M) only inhibited fast proliferating MCF-7 cells, increasing their cell doubling time, and did not significantly modify the length of doubling time in the cultures with low proliferation rate, in which doubling time was already long. These data clearly show that there is a direct relation between proliferative rate of cells and melatonin inhibitory actions on MCF-7 cells.