Effects of mammalian and plant estrogens on mammary glands and uteri of macaques

Because of their possible beneficial effects on atherosclerosis and cancer risk, soy-derived isoflavone phytoestrogens may be useful as a dietary alternative or supplement to postmenopausal hormone replacement therapy. We examined this possibility in a well- characterized primate model of postmenopause. Adult, surgically postmenopausal female macaques (Macaca fascicularis) were treated continuously with either estradiol (E2), an isoflavone-rich soy protein isolate (SPI), or both (E2+SPI). Doses were equivalent on an energy basis to 1 and 148 mg/d per woman for E2 and SPI, respectively. After 6 mo, histopathologic, morphometric, and immunohistochemical measurements of the endometrium and mammary glands were taken. Increases in endometrial thickness, gland area, and epithelial proliferation were induced by E2 and E2+SPI. Morphometric changes were accompanied by increased epithelial staining of the proliferation marker Ki-67 in the E2-treated treated group. The effects of E2 were partially antagonized by SPI (manifested as decreased Ki-67 staining). Mammary gland proliferation was induced by E2 and E2+SPI. The effects of E2 were also antagonized by SPI in the mammary gland. Morphometric and immunohistochemical measures of proliferation were in agreement in endometrium. In this nonhuman primate model, treatment with SPI did not induce proliferation in endometrial and mammary tissue. SPI may have antiproliferative effects in the endometrium and mammary gland when given along with exogenous estrogen.