Tumor necrosis factor-alpha (TNF-alpha) is a multifunctional cytokine involved in host response to neoplasia. TNF-alpha has been shown to inhibit proliferation and induce apoptosis of MCF-7 breast carcinoma cells. Insulin-like growth factors I and II (IGF-I and IGF-II) are potent mitogens involved in growth regulation of breast epithelial cells and are implicated in the pathophysiology of breast cancer. Their bioactivity is strongly influenced by specific IGF-binding proteins (IGFBPs). We report that accumulation of IGFBP-3 in the conditioned media of MCF-7 cells is increased over control values in the presence of TNF-alpha. The increased IGFBP-3 accumulation induced by TNF-alpha is correlated with increased IGFBP-3 mRNA abundance. TNF-alpha also decreases IGF-I receptor levels in MCF-7 cells. Estradiol-stimulated MCF-7 cell proliferation is associated with reduced IGFBP-3 accumulation, and we show that TNF-alpha attenuation of estradiol-stimulated proliferation is associated with increased IGFBP-3 accumulation. Finally, we demonstrate that an IGFBP-3 antisense oligodeoxynucleotide antagonizes TNF-alpha-induced inhibition of cell proliferation and TNF-alpha-induced IGFBP-3 accumulation. These data strongly suggest that IGFBP-3 plays a role in modulation of breast cancer cell proliferation by TNF-alpha.