Protection against breast cancer with genistein: a component of soy.

Breast cancer is the most common cancer in women. Because genetics is believed to account for only 10-15% of breast cancer cases, the environment, including nutrition, is thought to play a significant role in predisposing women to this cancer. Studies of Asian women suggest that those who consume a traditional diet high in soy products have a low incidence of breast cancer, but that among emigrants to the United States, the second generation, but not the first, loses this protection. These findings suggest a possible common mechanism of action for breast cancer protection from early, specific nutritional exposure. Genistein, an isoflavone found in soy, has been reported to have weak estrogenic and antiestrogenic properties, to be an antioxidant, to inhibit topoisomerase II and angiogenesis, and to induce cell differentiation. In studies of the mammary glands of immature rats, we showed that genistein up-regulates the expression of the epidermal growth factor receptor shortly after treatment, which may be responsible for the increased cell proliferation seen at that age. We hypothesize that the early genistein action promotes cell differentiation that results in a less active epidermal growth factor signaling pathway in adulthood that, in turn, suppresses the development of mammary cancer. We speculate that breast cancer protection in Asian women consuming a traditional soy-containing diet is derived from early exposure to soybean products containing genistein. We believe that early events are essential for the benefits of cancer protection.