Estrogen and IGF-I are potent mitogens for most breast cancer cell lines, and although their signaling pathways contrast, there is considerable interaction between them. Recent evidence indicating that IGF-I can alter estrogen receptor (ER) action led us to investigate whether an inhibitor of IGF-I action, IGF-binding protein-1 (IGFBP-1), could affect transcriptional activation of ER. First, we confirmed that tamoxifen (TAM) could inhibit IGF-I-mediated proliferation of MCF-7 cells. Although TAM can increase IGFBP-3 expression in MCF-7 cells, and this binding protein has been shown to be able to inhibit IGF action, TAM had no effect on IGF-I-stimulated tyrosine phosphorylation of IGF-I receptor or the downstream signaling molecule, insulin receptor substrate-1. Therefore, to confirm that IGF-I was affecting transcriptional activation of ER, we utilized a gene reporter assay using a single consensus estrogen response element (ERE-tk-luc) upstream ofluciferase. As expected, estradiol (E2; 1 nm) increased transcriptional activation three- to fivefold from the ERE in three ER-positive breast cancer cell lines (MCF-7, ZR-75 and T47D). A 2·5- to 4-fold increase was also seen with IGF-I (5 nm). TAM (1 μm) effectively blocked activation by E2 and IGF-I, indicating disruption of ER-mediated transcription. As expected, human recombinant IGFBP-1 (80 nm) completely inhibited IGF-I-mediated activation of ER, however, IGFBP-1 also caused a significant decrease in E2-mediated activation. We also noticed that IGF-I increased the activity of all plasmids that we cotransfected including TATA-luc, SV40-luc and pGLBasic. This effect was post-transcriptional, as it was not affected by actinomycin D (2 μg/ml), while we were able to completely inhibit E2-mediated transcriptional activation of ERE-tk-luc. Unlike the complete inhibition of ER-mediated transcriptional activation by actinomycin D, IGF-I-mediated transactivation was reduced by only 50%, indicating that the activation by IGF-I represented both transcriptional and post-transcriptional effects. This study confirmed that IGF-I can cause transcriptional activation of endogenous ER in human breast cancer cells, and inhibition of ER action by IGFBP-1 suggests that IGF-I signaling may be necessary for maximal ER activation.