Demonstration of Type I Insulin-Like Growth Factor Receptors on Human Platelets

Abstract

Human platelets, freshly isolated from healthy human adults, express receptors for insulin-like growth factor I. The IC₅₀ for displacement of ¹²⁵I-IGF-I binding by unlabeled IGF-I was 0.2 nM, by IGF-II 32 nM and by insulin 160 nM. Scatchard analysis of IGF-I binding demonstrates dissociation constants of 0.14 ± 0.08 nM for high affinity binding site and 54 ± 18 nM for low affinty binding site. The presence of the α-subunit of type I IGF receptor, as high affinity binding site, was verified by affinity crosslinking of ¹²⁵I-IGF-I to platelet surface membranes. Under reducing con-conditions a M_r= 135,000 band was preferentially labeled. The complete type I IGF receptor complex, which revealed under nonreducing conditions, has an approximately molecular mass of M_r > 400,000. The immunoprecipitation of the ¹²⁵I-IGF-I cross-linked type I receptor with αIR-3 confirmed the results achieved by affinity crosslinking.