Effects of tamoxifen on human breast cancer cell cycle kinetics: accumulation of cells in early G1 phase.

We have studied the effects of tamoxifen on the cell cycle kinetics of the endocrine-responsive MCF-7 human breast cancer cells. Tamoxifen inhibits proliferation of MCF-7 cells. The tritiated thymidine labeling index is markedly reduced by tamoxifen, indicating a reduction in the fraction of cells in S phase. Flow cytometry of mithramycin-stained cells reveals that cells accumulate in G1 phase, with a concomitant depletion of S- and G2-M-phase cells with tamoxifen. Mapping of G1-phase cells by morphology of prematurely condensed chromosomes demonstrated that tamoxifen-treated cells accumulate in early G1. These studies indicate that tamoxifen inhibits proliferation of MCF-7 human breast cancer cells by invoking a transition delay early in the G1 phase of the cell cycle.