Emerging evidence suggests that the transcription factor Forkhead Box M1 (FoxM1) is associated with aggressive human carcinomas, including breast cancer. Because elevated expression of FoxM1 has been observed in human breast cancers, FoxM1 has attracted much attention in recent years as a potential target for the prevention and/or therapeutic intervention in breast cancer. However, no information is currently available regarding how downregulation of FoxM1 could be achieved for breast cancer prevention and therapy. Here, we report for the first time that 3,3'-diindolylmethane (DIM), a nontoxic dietary chemopreventive agent could effectively downregulate FoxM1 in various breast cancer cell lines. Using gene transfection, real-time reverse transcription-PCR, Western blotting, invasion and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assays, we found that DIM could enhance Taxotere-induced growth inhibition of breast cancer cells, and decreased invasive capacity of breast cancer cells was observed after either treatment alone or the combination. These effects were associated with downregulation of FoxM1. We also found that knock down of FoxM1 expression by small interfering RNA (siRNA) transfection increased DIM-induced cell growth inhibition, whereas over-expression of FoxM1 by cDNA transfection attenuated DIM-induced cell growth inhibition, suggesting the mechanistic role of FoxM1. Most importantly, the combination treatment significantly inhibited tumor growth in severe combined immunodeficiency (SCID) mice, and the results were correlated with the downregulation of FoxM1 in tumor remnants. We conclude that inactivation of FoxM1 and its target genes by DIM could enhance the therapeutic efficacy of Taxotere in breast cancer, which could be a useful strategy for the prevention and/or treatment of breast cancer.
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