Background: Isoflavones are one type of phytoestrogens found in soy. Isoflavones have been touted to protect against breast cancer and alleviate menopausal symptoms such as hot flashes; however, clinical and laboratory studies have generated both enthusiasm and concern. Tamoxifen (TAM) has been used both as treatment and a chemoprevention agent for breast cancer. Breast cancer patients taking TAM may use soy isoflavones for the purposes of promoting breast health as well as relieving TAM-induced hot flashes. Therefore, the effect of concurrent consumption of TAM and isoflavones in both preventive and therapeutic settings needs to be clarified.

Our laboratory previously reported that overexpression of protein kinase C alpha (PKCα) in the hormone dependent breast cancer cell line T47D results in TAM resistance and autonomous growth(Tonetti et. al., Br. J. Cancer 2000 6:782-91). Moreover, the T47D/PKCα tumors were found to regress rapidly after administration of 17β-estradiol (E2) (Chisamore et. al., Clin. Cancer Res. 2001 7:3156-3165). To determine the effect of isoflavones on our TAM-resistant T47D/PKCα breast cancer model, we tested the growth effects of genistein and daidzein both in vitro and in vivo.

Methods: In cell proliferation assay, the empty vector transfected cell line T47D/neo and T47D/PKCα cells were treated with E2 (10-9M), genistein (10-7M), daidzein (10-7M) and the daidzein metabolite equol (10-7M). Both cells were transfected with ERE-tk-luc plasmid treated with isoflavones, and ERE-luciferase activity was determined. In animal studies genistein (0.206 g/Kg diet) and daidzein (0.144 g/Kg diet) were administrated to athymic ovariectomized mice bearing T47D or T47D/PKCα tumors. Combination of TAM (1.5mg/mouse/day) with daidzein or genistein was also included in the animal studies.

Results: Cell proliferation studies indicated that only equol stimulated T47D/neo cells to grow, whereas T47D/PKCαcell proliferation was not affected by any of the isoflavones. ERE-luciferase activation is consistent with cell proliferation results since only equol induced luciferase activity. The tumor growth studies showed that genistein stimulated T47D/PKCα tumor growth while daidzein was partially inhibitory, whereas neither of the isoflavones stimulated T47D tumors to grow. Most important was the finding that the combination of TAM with isoflavones produced tumors of larger size as compared with either TAM or isoflavones alone.

Conclusion: Our work is the first report of the efficacy of isoflavones and combined isoflavones, genistein or daidzein, with TAM in a TAM-resistant breast cancer model. These findings suggest co-administration of genistein or daidzein with TAM may not be safe for those patients who may have occult breast tumor cells overexpressing PKCα. However, to fully determine the significance of combining isoflavones with TAM, more studies on other breast tumor models need to be performed.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA