Glyceollins, a Novel Class of Antiestrogenic Phytoalexins

Glyceollins, a group of novel phytoalexins isolated from activated soy, have demonstrated unique in vitro and in vivo antiestrogenic activity. The soybean plant under stress produces a mixture of glyceollins I, II, and III that bind to the estrogen receptor (ER) and inhibit estrogen-induced tumor progression. In further in vitro studies, the glyceollin mixture exhibits potential antiestrogenic, therapeutic activity preventing estrogen-stimulated tumorigenesis and displaying a differential pattern of gene expression from tamoxifen. By isolating the individual glyceollin isomers, we identified the active antiestrogenic component using competition binding assays with human ERa and estrogen-responsive element-based luciferase reporter assays. We identified glyceollin I as the active antiestrogenic component of the mixture. Ligand-receptor modeling (docking) of the isomers within the ERa ligand binding cavity demonstrated a unique type II antiestrogenic confirmation adopted by glyceollin I, but not isomers II and III. Glyceollin I treatment in 17β- estradiol-stimulated MCF-7 breast cancer cells and BG-1 ovarian cancer cells resulted in a novel inhibition of ER-mediated gene expression and cell proliferation/ survival. Glyceollin I may represent an important component of a phytoalexin-enriched food (activated) diet in terms of chemoprevention as well as a novel therapeutic agent for hormone dependent tumors.