Effects of tamoxifen and 4-hydroxytamoxifen on synchronized cultures of the human breast cancer cell line MCF-7.

The effect of antiestrogens on cultures of synchronized MCF-7 human breast cancer cells was studied. Cultures were synchronized by an eighteen hour block with 1.5 mM hydroxyurea. Tamoxifen or 4-hydroxytamoxifen was added at various times following removal of the hydroxyurea block and the cell number and rate of incorporation of 3H-thymidine by the cultures were followed for 48 hours through two waves of DNA synthesis. Antiestrogens added when hydroxyurea was removed inhibited only the second wave of DNA synthesis. Inhibition of the second wave only occurred if the antiestrogen was added before or at about the time of mitosis. To determine whether antiestrogen had to be present before mitosis in order to block the next round of DNA synthesis, cells in mitosis were selected by shaking them loose. The subsequent division of these cells was followed to determine the effect of treating them with antiestrogens before, after, or both before and after mitosis. The addition of antiestrogens at the time of mitosis proved sufficient to block the next round of cell division. Thus antiestrogens are cell cycle specific agents and appear to act at a point early in G1 to block the next round of DNA synthesis and cell division.