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Human Breast Cancer Cell Cycle Synchronization by Estrogens and Antiestrogens in Culture¹

Department of Medicine, University of Texas Health Science Center, San Antonio, Texas 78284

The mechanisms by which estrogens and antiestrogens modulate breast cancer growth have not been totally defined. We have examined the cell cycle kinetic effects of estrogens and antiestrogens in cultured human breast cancer cell lines. In a previous study, we showed that tamoxifen induces a transition delay in early to mid-G₁ phase of the cell cycle. In the present study, we show that this cell kinetic alteration by tamoxifen is dose dependent and that other antiestrogens have identical effects. As little as 0.01 to 0.1 µM tamoxifen reduces the S and G₂+M fractions and increases the G₁ fraction of MCF-7 cells growing in medium with 5% charcoal-stripped bovine serum. More than 90% of cells are in G₁ 72 to 96 hr after the addition of 1 µM tamoxifen, a concentration achieved in patients treated with the drug. Nafoxidine and trioxifene have identical activity. Partial reversal of tamoxifen growth inhibition is observed with a simple change to tamoxifen-free medium. Complete reversal of the tamoxifen effect is observed with the addition of 17ß-estradiol. By 24 hr after the addition of estrogen, 60 to 70% of tamoxifen-inhibited cells have progressed through G₁ and into S phase, indicating that tamoxifen-treated cells remain viable. This estrogen "rescue" effect is observed even in the absence of a change to tamoxifen-free medium. A 100-fold-lower concentration of estradiol can totally reverse the inhibitory effects of 1.0 µM tamoxifen. Stimulation of the progression of G₁ cells to enter S phase is also observed when estradiol is added to cells maintained for four days in medium with stripped serum, even in the absence of tamoxifen. Similar effects are observed in the estrogen receptor-positive ZR75-1 breast cancer cells. No effects of antiestrogens or estrogens are observed in the receptor-negative MDA-231 cells, suggesting that these effects are mediated through the estrogen receptor. In summary, antiestrogens and estrogens have prominent effects on the cell cycle kinetics of endocrine-dependent human breast cancer cells. Antiestrogens cause an accumulation of cells in G₁ phase. Estrogen reverses this block with a synchronous cohort of cells progressing through the cell cycle. These data have important implications for the design of rational clinical trials of combined chemoendocrine therapy.

¹ This work was supported by NIH Grant CA 30251.

² To whom requests for reprints should be addressed.

Received 7/22/83. Accepted 1/ 3/84.

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