Cell Cycle Phase Dependence of Estrogen and Epidermal Growth Factor (EGF) Receptor Expression in MCF-7 Cells: Implications in Antiestrogen and EGF Cell Responsiveness*

In this report the effect of epidermal growth factor (EGF) and the antiestrogen hydroxytamoxifen (OHTAM) on the cell cycle of the breast cancer cell line MCF-7 was investigated as a function of the presence of their respective receptors. For this study synchronized cells were obtained by cell incubation in the presence of 2 mM thymidine for 24 h at 37 C. The treatment led to a partial synchronization, since at the end of thymidine treatment, 80% of cells were accumulated in the G1 phase. The removal of thymidine allowed the cells to progress through the cell cycle, since between 6–9 h after the arrest of the treatment, about 50% of cells were found in the S phase. By 9–12 h, most of the cells entered the G2 phase, and by 24 h, the cells returned to the G1 phase. When MCF-7 cells were incubated in the presence of OH-TAM for various periods of time before thymidine exposure, the progression of the cells through the cell cycle was dramatically inhibited. Also, a short term antiestrogen treatment (2 h) before...