Estrogen receptor ␣ (ER␣)3 is a member of the steroid/nuclear receptor family of transcription regulators and mediates cell growth and metastasis and resistance to apoptosis and immunosurveillance (1-5). ER␣ is activated by binding of 17-estradiol (E 2 ), or by the epidermal growth factor-activated extracellular signal-regulated kinase pathway and other signal transduction pathways (6). ER␣-mediated gene transcription contributes to the development and spread of breast, uterine, and liver cancer (5,7,8). A role for ER action in ovarian cancer is supported by the recent finding that endocrine therapy is effective against relapsed ER-containing ovarian cancers (9, 10). Aromatase inhibitors that inhibit estrogen production and tamoxifen (Tam) and other selective estrogen receptor modulators (SERMs) are mainstays in treatment of estrogen-dependent cancers and have played an important role in developing our understanding of ER action (5,7,11,12). Tam and other SERMs work by competing with estrogens for binding in the ligand binding pocket of ER. Over time, tumors usually become resistant to tamoxifen and other SERMs (13-15), requiring new strategies to inhibit ER␣ action.In the best characterized model for ER action, ER␣ activates gene transcription by binding to palindromic estrogen response element (ERE) DNA and ERE half sites (4,16, 17). Thus, an alternative to current approaches that primarily target ER action at the level of ligand binding is to target ER␣ at the level of its interaction with ERE DNA. Although targeting protein binding to DNA is attractive, until recently this approach was questioned, because small molecules may not disrupt the large interaction surfaces of protein⅐DNA and protein⅐protein complexes (18). However, several recent studies support the feasi-
The risks and benefits of diets and supplements containing the estrogenic soy isoflavone genistein are not well established. We report that 10 nm genistein potently induces the granzyme B inhibitor, proteinase inhibitor 9 (PI-9) in MCF-7 human breast cancer cells. By inducing PI-9, genistein inhibits the ability of human natural killer (NK) cells to lyse the target breast cancer cells. In ERalphaHA cells, stably transfected MCF-7 cells, which contain elevated levels of estrogen receptor-alpha (ERalpha), 100 pm genistein or 17beta-estradiol potently induce PI-9 and prevent NK cells from killing the target breast cancer cells. The concentrations of genistein that fully induce PI-9 in MCF-7 cells, and in ERalphaHA cells, are far lower than those previously reported to elicit estrogenic responses through ERalpha. Because 4-hydroxytamoxifen, raloxifene, and ICI 182,780/Faslodex all block genistein induction of PI-9 and elevated levels of ERalpha enhance induction of PI-9, genistein acts via ERalpha to induce PI-9. Increasing levels of ERalpha in breast cancer cells results in a progressive increase in induction of PI-9 by genistein and in the cell's ability to evade killing by NK cells. Moderate levels of dietary genistein and soy flour effectively induce PI-9 in human breast cancers grown in ovariectomized athymic mice. A significant population consumes levels of genistein in soy products that may be high enough to induce PI-9, perhaps potentiating the survival of some preexisting breast cancers by enabling them to evade immunosurveillance.
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