Induction of PTEN-p53 Crosstalk in Mammary Epithelial Cells: A Novel Mechanism of Breast Cancer Prevention by the Dietary Factor Genistein.

Abstract Consumption of soy foods either at an early age or for lifetime has been associated with reduced risk for developing breast cancer in humans and in animal models. However, this association continues to be controversial and the precise mechanisms for protection remain elusive. Among the soy products, the isoflavone genistein (GEN) has been widely suggested to confer mammary tumor protection. Previously we demonstrated the increased expression of tumor suppressors PTEN and p53 in mammary epithelial cells (MECs) isolated from young adult female rats fed dietary soy protein isolate (SPI) or casein (CAS) supplemented with GEN, when compared to MECs from rats fed the control (CAS) diet. Since NMU-administered rats fed SPI had reduced tumor incidence and increased tumor latency than those fed CAS, PTEN and p53 likely mediate the observed tumor resistance with SPI in vivo. We hypothesized that GEN induction of PTEN and p53 in MECs results in the formation of a PTEN/p53 functional complex to negatively regulate breast cancer development. Here, we used the human non-tumorigenic, ER-negative mammary epithelial cell line, MCF-10A, as an in vitro system to mechanistically dissect ER-independent actions of GEN involving PTEN and p53. GEN (40 nM, 2μM) augmented PTEN and p53 expression in treated relative to control cells. GEN also induced nuclear co-localization and physical association of PTEN and p53. To test a functional consequence of GEN-induced PTEN/p53 cross-talk on mammary epithelial phenotype, we analyzed GEN effects on cell cycle progression and acini formation in 3D cultures. Our results showed attenuated cell proliferation and lower cyclin D1 and pleiotrophin transcript levels in GEN-treated cells, which were abrogated by small interfering RNA to PTEN, indicating PTEN-dependence. Using FACS analysis, we showed that GEN induced cell cycle arrest at G0-G1 phase. Treatment with GEN promoted early acini formation of MECs grown in Matrigel, which temporally coincided with PTEN-dependent suppression of p21 and p27 transcript levels. Further analyses of GEN effects on MECs demonstrated induction by GEN of PTEN promoter-luc reporter activity as measured by dual-luciferase assay. Interestingly, treatment with siRNA to either PTEN or p53 reduced basal and GEN-induced PTEN promoter activity. Given that p53 binds to the PTEN promoter, our results suggest a feed-forward cycle in which dietary factor (GEN) induction of nuclear PTEN leads to PTEN promotion of its own signaling. By maintaining a stable pool of nuclear p53 to boost its transcription, PTEN ensures its continuous expression in MECs to favor cell differentiation. These data elucidate a novel mechanism by which dietary factors with PTEN-inducing activity may attenuate breast cancer risk and development. Funding by USDA-CRIS 6251-5100002-06S and the Department of Defense Breast Cancer Program (0810548). Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 3149.