The insulin-like growth factors (IGFs) are potent mitogens for malignant cell proliferation. The majority of secreted IGFs are bound to specific IGF-binding proteins (IGFBPs) that are secreted by a large number of cells. These proteins may either inhibit or enhance IGF actions. Breast carcinoma cells secrete a variety of IGFBPs. We have previously demonstrated that retinoic acid (RA) inhibition of IGF-I-stimulated MCF-7 cell proliferation is associated with increased IGFBP-3 levels in the conditioned media. We therefore investigated the effect of recombinant IGFBP-3 as well as IGFBP-2, -4 and -5 on IGF-I stimulation of DNA synthesis and IGF-I binding in the MCF-7 human breast carcinoma cell line. IGFBP-2 and -3 enhanced IGF-I stimulation of DNA synthesis in MCF-7 cells while IGFBP-4 and -5 had no effect. Transfection of MCF-7 cells with an IGFBP-3 expression vector resulted in the enhanced secretion of IGFBP-3 with an accompanying increase in IGF-I binding as well as increased cell proliferation upon treatment of the cells with IGF-I. IGF-I preincubation of MCF-7 cells transfected with control pSVneo plasmids results in cells refractory to further IGF-I stimulation of thymidine incorporation while IGF-I continues to stimulate [3H]-thymidine incorporation in IGFBP-3-transfected MCF-7 cells, suggesting that IGFBP-3 protects the cells from IGF-I-mediated down regulation of its receptor. Therefore, IGFBP-3 secreted by MCF-7 cells can enhance IGF-I stimulation of DNA synthesis, increase IGF-I binding to these cells, and prevent IGF-I-induced desensitization of its own receptor, suggesting that IGFBP-3 plays a significant role in IGF-I-mediated breast carcinoma proliferation.