Recombinant insulin-like growth factor binding protein-1 inhibits IGF-I, serum, and estrogen-dependent growth of MCF-7 human breast cancer cells

The insulin-like growth factors (IGFs) are potent mitogens for breast cancer cells and their activity is modulated by high affinity binding proteins (IGFBPs). We have recently shown that IGFBP-1 purified from human amniotic fluid neutralizes IGF-I-dependent growth of MCF-7 cells. In this study we examined the effects of recombinant IGFBP-1 (rBP-1) on IGF-I, estradiol (E₂), and serum-induced monolayer and anchorage independent growth (AIG) of MCF-7 cells. Under serum-free conditions, rBP-1 had no effect on MCF-7 basal monolayer growth. However, 40 nM rBP-1 completely blocked the mitogenic action of both IGF-I and 5% charcoal stripped serum (CSS). This concentration of rBP-1 partially inhibited E₂-induced growth, while 80 nM rBP-1 completely abolished E₂ mitogenicity. The addition of either excess IGF-I or 5 nM [Arg³]IGF-I, a species that does not bind IGFBPs, neutralized rBP-1 inhibitory effects. In AIG assays, 80 nM rBP-1 reduced colony number by at least 70% and decreased colony size in all treatment groups compared to control. We examined rBP-1 effects on both IGF-I binding to MCF-7 membranes and activation of type I IGF receptor (IGFR₁) and found that 80 nM rBP-1 reduced IGF-I receptor binding to levels of nonspecific binding and completely abolished ligand-dependent IGFR, phosphorylation. However, neither treatment with 5% CSS nor exposure to E₂ resulted in IGFR₁ phosphorylation suggesting that different mechanism(s) are responsible for rBP-1 inhibitory action under this condition. Our data suggest rBP-1 may serve as an antagonist of human breast cancer growth by interfering with growth factor-mediated cell proliferation. © 1993 Wiley-Liss, Inc.