Abstract A50: Soy isoflavones and vitamin D in breast cancer prevention.

A50 Background Breast cancer incidence is higher in Western women compared to Asian women. Since Asian women have a 10-25-fold higher soya isoflavone intake, soy products may be useful in breast cancer chemoprevention. The dietary soya isoflavones daidzein and genistein possess structures similar to estrogen, and may mimic or antagonize estrogen effects in different tissues. Daidzein and genistein may be also be involved in gene regulation by modulating epigenetic events such as DNA methylation, directly or through an estrogen receptor dependent process. We also hypothesized that cellular differentiating vitamin D3 (VTD) may improve the isoflavone sensitivisty to the breast cancer cell line. Methods We treated three breast cancer cell lines (MCF7, 21PT and T47D) with daidzein and genistein alone and in combination with VTD for 96 hours. Effects were then compared to those seen with the known methyltransferase inhibitor 5’deoxy-azacytidine (AZA). Results Daidzein and genistein inhibited the growth of all three the breast cancer cell lines in a dose dependent manner. The combination of VTD with isoflavones showed an additive effect. Isoflavone treatment reversed methylation of several breast cancer candidate genes, including Hin-1, RAR beta and Rass-F1, associated with an induction of their expression levels. Combinations of VTD with daidzein or genistein showed more demethylation and mRNA re-expression than isoflavones alone. Furthermore, daidzein or genistein treatment resulted in a significant decrease of estrogen receptor alpha (ERA) level as well as an increase in ER beta (ERB) levels. These results imply that isoflavones may also protect from breast cancer by lowering ERA levels while inducing ERB levels. Indeed, recent studies indicate that high ERB levels are associated with a reduced breast cancer risk and that ERB is an important modulator of proliferation and invasion of breast cancer cells. ERB has several splice variants: ERB1, ERB2 (ERBcx) and ERB5. Among these variants, ERB1 forms a functionally active heterodimer with ERA, while ERB2 and ERB5 form non-functional heterodimers. High ratios of ERB2 to ERB1 or ERB5 to ERB1 block ERA downstream signaling. In our study, we observed induction of ERB2 > ERB1 levels isoflavone treatments, and ERA downstream signaling was decreased compared to the untreated control cells. Combination therapy of these isoflavones with VTD also resulted in improved induction of erb2 > ERB1 levels than isoflavone treatments alone. Conclusion Isoflavones can inhibit the growth of breast cancer cell lines MCF7, 21PT and T47D in a dose dependent manner. These isoflavones can both demethylate and re-express critical differentiating genes, and combination treatment with VTD shows more pronounced effects. Inhibition of ERA, ERA downstream genes and activation of ERB along with an increase of the ERB2 to ERB1 ratio by isoflavones in breast cancer cell lines imply the use of isoflavones may be an effective chemopreventive agent that will be well tolerated and may have similar benefits to more toxic agents such as AZA. Citation Information: Cancer Prev Res 2008;1(7 Suppl):A50.