Hormone dependency and the action of tamoxifen in human mammary carcinoma cells.

Conditions are described which allow to demonstrate estradiol dependency of proliferation in MCF-7 human mammary carcinoma cells in a reproducible manner. Our results concerning the action of Tamoxifen on the growth of MCF-7 cells differ in two aspects from previous reports: (a) The inhibitory effect of pharmacological doses (10(-7) M) is only obvious if the cells are stimulated by estradiol. (b) In the absence of estradiol, Tamoxifen acts as a weak estrogenic agonist, by stimulating the growth of MCF-7 cells. This finding could imply that premenopausal women should not be treated with Tamoxifen after ovariectomy.