Tumor growth requires the development and remodeling of the vascular system, involving paracrine signaling between various growth factors and endothelial receptors. Vascular endothelial growth factor (VEGF) is a key regulator of developmental, physiological and pathological neovascularization, especially involved in tumor growth. Recent studies indicate that 17beta-estradiol (E2) modulates VEGF expression in breast cancer cells through transcriptional activation. We have investigated both the molecular mechanisms of E2-induction of VEGF expression and of VEGF control of breast cancer angiogenesis. In transient transfection assays using the VEGF promoter-luciferase construct, E2 increased VEGF transcriptional activity in MCF-7 cells and in MDA-MB-231 cotransfected with estrogen receptor (ERalpha or ERbeta). The positive effect was abolished when MCF-7 cells were treated with the pure antiestrogen ICI 182,780 or the agonist/antagonist tamoxifen. We further identified an imperfect estrogen responsive element (ERE1520) in the VEGF promoter, which formed a complex with ERalpha or ERbeta proteins in gel shift assay using MCF-7 or MDA-MB-231 nuclear extracts; the ERE sequence is involved in the transcriptional regulation of VEGF in our experimental conditions. These results demonstrate that in breast cancer (BC) cells VEGF is a target gene for ERalpha or ERbeta. To determine the role of VEGF in the progression of human breast carcinoma, we generated stable human breast carcinoma cells (MCF-7) overexpressing VEGF165 (V165 clones). Cells or control vector clones were implanted subcutaneously in athymic mice. Our in vivo findings show that overexpression of VEGF significantly decreased tumor uptake and increased tumor growth and angiogenesis in a murine model of BC.