The effects of 17β estradiol and tamoxifen on the ZR-75-1 human breast cancer cell line in serum free hormone supplemented medium are described. Addition of 17β estradiol leads to a marked stimulation of the breast cancer cells manifested by increases in cell number, precursor incorporation into macromolecules, liquid synthesis, and thymidine kinase activity. Concentrations of 17β estradiol as low as 10-11 M stimulate growth; 10-10 M 17β estradiol is maximally effective. Addition of the antiestrogen tamoxifen in cells devoid of estradiol leads to a period of estrogen reversible inhibition followed by cell death. 10-8 M tamoxifen has little effect on growth whereas 10-6 M tamoxifen is maximally effective. Hundred-fold lower concentrations of 17β estradiol when added simultaneously blocks antiestrogenic inhibition almost completely. These data obtained in a totally defined serum free system provide strong evidence for the absolute and direct growth dependence of some human breast cancer cells on estrogens.
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