Molecular Mechanism of Action of Genistein and Related Phytoestrogens in Estrogen-Receptor Dependent and Independent Growth of Breast Cancer Cells

Investigations during the past year focused on growth inhibitory effects of phytoestrogens and their role as weak estrogens in breast cancer cell growth. Genistein, biochanin A, kaempferol and daidzein had growth stimulatory effects in ER-positive MCF-7 breast cancer cells at 10 micronmeter. To understand the estrogenic effects of genistein, we measured its interaction with human recombinant ER alpha and Beta. ER alpha or Beta bound to 3H-E2 showed a 6S dimeric form in sucrose gradients, whereas both receptors labeled with 14C-genistein sedimented as the 4S monomeric form. In EMSA experiments, we found that genistein facilitated ER 13 binding to ERE. In contrast, genistein did not have a significant effect on ER alpha-ERE binding. Northern blot analysis showed a slight decrease in the level of ornithine decarboxylase mRNA at 8 h with 100 micronmeter genistein treatment. Genistein and quercetin inhibited ER-negative MDA-MB-468 breast cancer cell growth with 1050 values of 8.8 and 18.1 Micronmeter, respectively. The other compounds were less effective. The mechanism of growth inhibition by genistein and quercetin involved G2M cell cycle arrest, changes in cyclin B1 levels and apoptosis. Our results indicate that genistein and quercetin may be useful in the treatment of ER-negative tumors. Results of our studies on MDA-MB-468 cells have been documented and submitted for publication.