Tumor suppressor PTEN signaling is up-regulated in mammary epithelial cells by soy isoflavone genistein: implications for breast cancer protection.

Abstract

Abstract Abstract #4069 Breast cancer is the second leading cause of death in the Western hemisphere affecting one of eight women in their lifetime. In addition to chromosomal and genetic alterations, nutrition is considered a risk determinant for breast cancer. Prevailing evidence suggests a negative correlation between intake of soy-rich foods and breast cancer incidence. Epidemiological and case control studies have shown a two-to eight fold lower occurrence of the disease in Asian women whose early intake of soy products is 20-25 times higher than their American counterparts. Previous work from our group has shown diet-induced protection against DMBA- and NMU-induced mammary tumors in vivo, with decreased tumor incidence and increased tumor latency in rats exposed to dietary soy protein isolate (SPI) or casein (CAS) supplemented with the major soy isoflavone genistein (GEN) when compared to those fed the control casein (CAS) diet. We also have shown that GEN enhanced apoptosis in mammary epithelial cells in vivo and in vitro (MCF-7 cells), attendant with increased expression of the tumor suppressor PTEN and several pro-apoptotic genes Bax, Bok, and p21. Here, we test the hypothesis that GEN induction of PTEN expression and nuclear localization in mammary epithelial cells result in decreased cellular proliferation and enhanced cellular differentiation, to confer protection from mammary tumorigenesis. Using the non-tumor human mammary epithelial cell line MCF-10A, we show that GEN at physiologically relevant concentrations (40 nM, 2 µM) increased PTEN mRNA and protein levels and enhanced nuclear accumulation of active (non-phosphorylated) PTEN. GEN induction of PTEN transcript levels was lost with the inclusion of the transcription inhibitor actinomycin D, suggesting PTEN is a direct GEN target. Further, cells treated with GEN for 6 days had lower cell viability when compared to control cells, concomitant with decreased expression of cyclin D1 and of the PTEN regulated gene pleiotrophin. Interestingly, GEN up-regulation of PTEN expression was accompanied by similar effects on another tumor suppressor p53, which co-localized with PTEN in the nuclei of GEN-treated cells. PTEN siRNA targeting, which decreased basal PTEN expression by 60%, significantly increased cyclin D1 but not p53 expression. Our findings provide support to the hypothesis that the protective effects of GEN in the mammary epithelium may be mediated by PTEN to enhance differentiation and inhibit cellular proliferation, and suggest the participation of another key tumor suppressor p53. Funding by USDA-CRIS 6251-5100002-06S. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4069.