Insulinlike growth factor binding proteins

The insulinlike growth factor system is an increasingly complex network, consisting of the two insulinlike growth factor peptides (IGF-I and IGF-II), two IGF receptors, at least seven IGF-binding proteins (IGFBPs), IGFBP cell surface receptor proteins, and IGFBP proteases. The IGFBPs constitute a family of seven structurally homologous proteins that bind IGF-I and IGF-II peptides with high affinity. IGFBPs modulate the mitogenic and metabolic actions of IGFs, enhancing or inhibiting the presentation of IGFs to their receptors. By binding to cell membranes, they may also directly regulate cell growth, exerting an IGF-independent action. Despite their structural homology and their common property of binding IGFs, each of the IGFBPs has specific tissue production, different regulatory systems, and specific functions, suggesting that the variability of IGF actions might be related to specific IGFBP regulation.