Dehydroepiandrosterone (DHEA) is a mitogen for estrogen-dependent MCF-7 breast cancer cells. Our aims were to determine whether DHEA required conversion to estrogens in order to stimulate cell proliferation and estrogen-dependent gene expression. After incubation of cells with 100 nM DHEA for 4 days, estradiol was present in the medium at a concentration of approximately 200 pM. Other compounds identified were testosterone ( approximately 300 pM) and estrone. Significant stimulation of cell proliferation by 1 nM estradiol and 100 nM DHEA was observed after 38 h and 4 days of incubation, respectively, indicating the necessity of DHEA conversion. DHEA doses > or = 10 nM induced estrogen-dependent reporter gene expression in MCF-7 cells transfected with a luciferase reporter gene under the control of the estrogen response element. DHEA-dependent stimulation of proliferation and luciferase induction could be inhibited by the anti-estrogens ICI182,780 and tamoxifen, respectively, and by the aromatase inhibitor 4-hydroxyandrostenedione. An androgenic effect of DHEA on proliferation and gene expression of MCF-7 cells was not observed. We conclude that conversion of DHEA to estrogens, particularly estradiol, is required to exert a mitogenic response.