Combined effects of RU486 and tamoxifen on the growth and cell cycle phases of the MCF-7 cell line.

The antiproliferative effect of RU486 and its effect combined with tamoxifen on the growth and cell cycle kinetics parameters on the MCF-7 human carcinoma cells were investigated. When MCF-7 cells in the exponential growth phase were treated with RU486 (10(2) nmol/L), a time-dependent cell growth inhibition was observed which was significant 5 days after the beginning of treatment. This inhibition was accompanied by a time- and dose-dependent decrease in the percentage of S and G2-M phase cells and a concomitant increase in the percentage of cells in the G0/G1 phase of the cell cycle. With tamoxifen (10(5) pmol/L), growth inhibition was obtained after 4 days of treatment of cells, and the blockage of the cell cycle occurred in the G0/G1 phase. In the case of simultaneous treatment of MCF-7 cultures with RU486 (10(2) nmol/L) and tamoxifen (10(5) pmol/L), we observed a synergistic inhibitory effect on the proliferative rate for short treatment (less than or equal to 3 days), whereas RU486 or tamoxifen alone had no effect. For the intermediate treatment (4 days), the combined effect of RU486 and tamoxifen was not significant compared to the effect of tamoxifen alone. For the long treatment (greater than 4 days), there were no differences between the number of cells in the treated cultures under different experimental conditions, but all were inhibited compared to those in control cell cultures. This simultaneous treatment of cells does not induce any change in the distribution of cells in the different cell cycle phases compared to tamoxifen percentages. These results suggest that RU486 is a cell cycle phase-specific growth inhibitory agent, and a combination of antiestrogen/antiprogestin should be considered as a possible improvement in breast cancer endocrine therapy.