Dietary Genistein Negates the Inhibitory Effect of Tamoxifen on Growth of Estrogen-dependent Human Breast Cancer (MCF-7) Cells Implanted in Athymic Mice. Ju YH, Doerge DR, Allred KF, Allred CD, Helferich WG Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 [Y. H. J., K. F. A., C. D. A., W. G. H.], and National Center for Toxicological Research, Jefferson, Arkansas 72079 [D. R. D.]. [Medline record in process] The use of dietary isoflavone supplements by postmenopausal women withbreast cancer is increasing. We investigated interactions between the soy isoflavone, genistein, and an antiestrogen, tamoxifen (TAM), on the growth of estrogen (E)-dependent breast cancer (MCF-7) cells implanted in ovariectomized athymic mice. We hypothesized that weakly estrogenic genistein negate/overwhelm the inhibitory effect of TAM on the growth of E-dependent breast tumors. Six treatment groups were used: control (C); 0.25 mg estradiol (E(2)) implant (E); E(2) implant + 2.5 mg TAM implant (2.5 TE); E(2) implant + 2.5 mg TAM implant + 1000 ppm genistein (2.5 TEG); E(2) implant + 5 mg TAM implant (5 TE), and E(2) implant +5 mg TAM implant +1000 ppm genistein (5 TEG). Treatment with TAM (2.5 TE and 5 TE) suppressed E(2)-stimulated MCF-7 tumor growth in ovariectomized athymic mice. Dietary genistein negated/overwhelmed the inhibitory effect of TAM on MCF-7 tumor growth, lowered E(2) level in plasma, and increased expression of E-responsive genes (e.g., pS2, PR, and cyclin D1). Therefore, caution is warranted for postmenopausal women consuming dietary genistein while on TAM therapy for E-responsive breast cancer. Cancer Res 2002 May 1;62(9):2474-7 PMID: 11980635, UI: 21977046 Remember we are NOT Doctors and have NO medical training. This site is like an Encylopedia - there are many pages, many links on many topics. Support our work with any size DONATION - see left side of any page - for how to donate. You can help raise awareness of CAM.