1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)), a steroid hormone derived from Vitamin D(3), is a negative growth regulator of breast cancer cells, and Vitamin D(3) analogs represent a novel treatment approach for human cancer. Elucidation of Vitamin D(3) receptor (VDR) regulation may reveal strategies to sensitize cancer cells to the effects of 1,25-dihydroxyvitamin D(3) and Vitamin D(3) analogs. We have previously characterized an estrogen responsive promoter region (800 bp upstream of exon 1c) in the human VDR gene, and the present studies examined regulation of this VDR promoter region by two phytoestrogens, resveratrol (present in red wine) and genistein (present in soy). We transiently transfected a VDR promoter luciferase construct into the estrogen receptor (ER) positive human breast cancer cell lines T47D and MCF-7, and treated with 0.4-4 microM resveratrol or 5-500 nM genistein. Both phytoestrogens up-regulated the transcription of the VDR promoter, as measured by reporter gene activity, approximately two-fold compared to vehicle treated cells. Co-treatment with the anti-estrogen tamoxifen (TAM) in T47D cells and transfection in an estrogen receptor negative breast cancer cell line demonstrated that the effects of phytoestrogens on the VDR promoter are dependent on estrogen receptor. Resveratrol and genistein also increased VDR protein expression as detected by Western blotting. Treatment with resveratrol had no effect on cell number or cell cycle profile, while treatment with genistein increased cell number. Because resveratrol could up-regulate VDR without increasing breast cancer cell growth, we hypothesized that resveratrol mediated increase in VDR expression would sensitize breast cancer cells to the effects of 1,25-dihydroxyvitamin D(3) and Vitamin D(3) analogs. In support of this hypothesis, both T47D and MCF-7 cells pre-treated with resveratrol exhibited increased VDR mediated transactivation of a Vitamin D(3) responsive promoter compared to cells pre-treated with vehicle. In addition, co-treatment with resveratrol enhanced the growth inhibitory effects of 1,25-dihydroxyvitamin D(3) and the Vitamin D(3) analog EB1089. These data support the concept that dietary factors, such as phytoestrogens, may impact on breast cancer cell sensitivity to Vitamin D(3) analogs through regulation of the VDR promoter.