Subchapter 28D - Insulin-like growth factor binding proteins

Abstract

IGFBP-1 to -6 form a family of cysteine-rich proteins that specifically binds IGF-1 and IGF-2 with high affinity. IGFBPs consist of three domains: conserved N- and C-terminal domains, and a variable L (linker) domain. The structure of IGFBPs is not related to the IGF receptors. Twelve and six cysteine residues in the N- and C-terminal domains, respectively, are important for high-affinity binding to IGFs, and are conserved among IGFBP-1 to -5. IGFBPs prolong the half-lives of IGFs in the circulation, target them to certain tissues, and either potentiate or inhibit the availability of IGFs to the receptors. The most abundant IGFBP, IGFBP-3, carrying 75%–80% of circulating IGFs, forms a ternary complex with IGF and an acid-labile subunit (ALS). As well as the IGF-modulating action, some IGFBPs exert IGF-independent actions on cell growth through translocating into the nucleus or interacting with cell surface receptors.