Epidemiological and animal studies have shown a negative correlation between breast cancer incidence and intake of soy-rich foods. Our laboratory has used soy protein isolate (SPI), the primary component of soy infant formula, as a paradigm to evaluate diet as a risk factor in mammary cancer. We previously demonstrated that lifetime exposure to dietary SPI reduced the incidence of NMU-induced mammary tumors in young adult rats relative to those fed the control Casein diet (CAS). This protection was associated with increased differentiation status of mammary epithelial cells prior to carcinogen insult, suggesting enhanced differentiation as a mechanism for mammary tumor protection. To identify early events contributing to diet-induced mammary differentiation, we used Affymetrix RAE230A GeneChips containing 14280 probes and GeneSpring Gx to analyze genomic profiles of mammary glands of prepubertal [postnatal day (PND) 21] rats which were lifetime exposed to dietary SPI or CAS. Of the 8 genes down-regulated by SPI, 4 are involved in lipid homeostasis: malic enzyme 1, fatty acid synthase (Fasn), stearoyl CoA desaturase-1 (Scd1), and insulin-induced gene 1. One of five SPI-induced genes is 11β-hydroxysteroid dehydrogenase 1 (Hsd1), which converts 11β-dehydrocorticosterone to corticosterone. Quantitative RT-PCR confirmed that Fasn and Scd1 transcript levels were down-regulated and Hsd1 was up-regulated, in PND21 mammary tissues of SPI vs CAS group. We evaluated potential paracrine effects of soy-mediated alterations in adipocyte lipid metabolism on mammary epithelial cell phenotype. In these studies, we used the mouse fibroblast 3T3-L1 cell line treated with the bioactive soy isoflavone genistein (GEN) and mouse mammary epithelial cell line HC11 as in vitro systems to recapitulate in vivo stromal/epithelial interactions. GEN (0.5 μM) regulated lipid homeostasis in fully differentiated 3T3-L1 adipocytes. GEN decreased Fasn and Scd1 and increased Hsd1 transcript levels in the absence of effects on leptin and PPARγ2 gene expression. To evaluate whether factors produced by GEN-treated adipocytes regulate mammary epithelial differentiation, mature adipocytes were incubated in medium with or without GEN (0.5 μM) for 48 h. Cells were refreshed with maintenance medium without GEN and 24 h later, conditioned medium (CM) was harvested, supplemented with prolactin (5 ng/ml), and added full-strength to HC11 cells. Cells incubated for 72 h in CM from GEN-exposed adipocytes had higher differentiation-associated β-casein transcript levels than those grown in non-GEN-treated CM. Our data support the hypothesis that GEN modulation of mammary fat pad metabolism and secretion constitutes one mechanism by which mammary epithelial differentiation is enhanced by diet to confer increased resistance to breast cancer. Studies are in progress to identify adipocyte-secreted factors regulated by this dietary isoflavone. USDA-CRIS-6251-5100-002-06S.
Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5082.
Thirty-first San Antonio Breast Cancer Symposium Dec 10-14, 2008; San Antonio, TX