Genistein and daidzein induce cell proliferation and their metabolites cause oxidative DNA damage in relation to isoflavone-induced cancer of estrogen-sensitive organs

The soy isoflavones, genistein (5,7,4'-trihydroxyisoflavone) and daidzein (7,4'-dihydroxyisoflavone), are representative phytoestrogens that function as chemopreventive agents against cancers, cardiovascular disease, and osteoporosis. However, recent studies indicated that genistein and/or daidzein induced cancers of reproductive organs in rodents, such as the uterus and vulva. To clarify the molecular mechanisms underlying the induction of carcinogenesis by soy isoflavones, we examined the ability of genistein, daidzein, and their metabolites, 5,7,3',4'-tetrahydroxyisoflavone (orobol), 7,3',4'-trihydroxyisoflavone (7,3',4'-OH-IF), and 6,7,4'-trihydroxyisoflavone (6,7,4'-OH-IF), to cause DNA damage and cell proliferation. An E-screen assay revealed that genistein and daidzein enhanced proliferation of estrogen-sensitive breast cancer MCF-7 cells, while their metabolites had little or no effect. A surface plasmon resonance sensor showed that binding of isoflavone-liganded estrogen receptors (ER) to estrogen response elements (ERE) was largely consistent with cell proliferative activity of isoflavones. Orobol and 7,3',4'-OH-IF significantly increased 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) formation in human mammary epithelial MCF-10A cells, while genistein, daidzein, and 6,7,4'-OH-IF did not. Experiments using isolated DNA revealed a metal-dependent mechanism of oxidative DNA damage induced by orobol and 7.3'.4'-OH-IF. DNA damage was enhanced by the addition of endogenous reductant NADH, formed via the redox cycle. These findings suggest that oxidative DNA damage by isoflavone metabolites plays a role in tumor initiation and that cell proliferation by isoflavones via ER-ERE binding induces tumor promotion and/or progression, resulting in cancer of estrogen-sensitive organs.