Insulin-like growth factor (IGF) binding protein-3 inhibits type 1 IGF receptor activation independently of its IGF binding affinity

Insulin-like growth factor binding proteins (IGFBPs) regulate the cellular actions of the IGFs owing to their strong affinities, which are equal to or stronger than the affinity of the type 1 IGF receptor (IGF-IR), the mediat or of IGF signal transduction. We recently found that IGFBP-3 modulates IGF -I binding to its receptor via a different mechanism possibly involving con formational alteration of the receptor. We have now investigated the effect s of IGFBP-3 on the initial steps in the IGF signaling pathway. MCF-7 breas t carcinoma cells were preincubated with increasing concentrations of IGFBP -3 and then stimulated with IGF-I, des(1-3)IGF-I, or [Q(3)A(4)Y(15)L(16)]-I GF-I, the latter two being IGF-I analogs with intact affinity for the type 1 IGF receptor, but weak or virtually no affinity for IGFBPs. Stimulation o f autophosphorylation of the receptor and its tyrosine kinase activity was dose-dependently depressed. At 2.5 nM, IGFBP-3 provoked more than 50% inhib ition of the stimulation induced by 3 nM des(1-3)IGF-1 and, at 10 nM, more than 80% inhibition. Similar results were obtained with [Q(3)A(4)Y(15)L(16) ]_IGF-I. Cross-linking experiments using iodinated or unlabeled IGFBP-3 and anti-IGF-IR antibodies indicated that the inhibitory effects do not involv e direct interaction between IGFBP-3 and IGF-IR. The inhibition appeared to be specific to IGFBP-3, because IGFBP-1 and IGFBP-5 at 10 nM had no signif icant effect. Also, inhibition was restricted to the IGF receptor, because IGFBP-3 failed to inhibit the tyrosine kinase activity of the insulin recep tor stimulated by physiological concentrations of insulin. Our results prov ide the first demonstration that IGFBP-3 can specifically modulate the IGF- I signaling pathway independently of its IGF-I-binding ability. They also r eveal a regulatory mechanism specific to the type 1 IGF receptor, with no e ffect on insulin receptor activation.