The insulin-like growth factor binding protein (IGFBP) family consists of six structurally related proteins that bind IGF with high affinity. These proteins share conserved cysteine-rich N-and C-terminal regions that participate in IGF binding. IGFBPs regulate the bioavailability of IGFs and modulate their biological activities, both positively and negatively. Some IGFBPs also have intrinsic bioactivity that is IGF-independent. Post-transitional modifications of the IGFBPs, including glycosylation, phosphorylation and proteolysis, influence IGF binding affinities and tissue localization, affecting both the IGF-dependent and independent functions.
IGF-binding proteins prolong the half-life of the IGFs and have been shown to either inhibit or stimulate the growth promoting effects of the IGFs on cell culture. They alter the interaction of IGFs with their cell surface receptors. Also exhibits IGF-independent antiproliferative and apoptotic effects mediated by its receptor TMEM219/IGFBP-3R. Promotes testicular germ cell apoptosis.
Insulin-like growth factor binding protein 6 (IGFBP6) is a 24-kDa protein that binds insulin-like growth factor 1 (IGF-1) and IGF-2 with high affinity and inhibits IGF action in vitro. The Insulin-like growth factor-binding protein also known as IGFBP serves as a carrier protein for Insulin-like growth factor 1. IGFBPs are clearly distinct but are sharing regions with strong homology. All members of the IGFBP family bind IGF-I and IGF-II with about equal affinity. Insulin-like growth factor (IGF) binding proteins (IGFBPs) have been shown to either inhibit or enhance the action of IGF, or act in an IGF-independent manner in the prostate. IGF-binding protein-4 (IGFBP-4) inhibits IGF-I action in vitro and is the most abundant IGFBP in the rodent arterial wall. IGFBP6 is directly downregulated by the beta-catenin/TCF complex in desmoid tumors, and imply a role for the IGF axis in the proliferation of desmoid tumors. There is mounting evidence that the structure of the IGFBP proteins plays a key role in the regulation of IGF bioavailability, by modulating its molecular size, capillary membrane permeability, target tissue specificity, cell membrane adherence and IGF affinity.
The insulin-like growth factors, isolated from plasma, are structurally and functionally related to insulin but have a much higher growth-promoting activity. May be a physiological regulator of [1-14C]-2-deoxy-D-glucose (2DG) transport and glycogen synthesis in osteoblasts. Stimulates glucose transport in bone-derived osteoblastic (PyMS) cells and is effective at much lower concentrations than insulin, not only regarding glycogen and DNA synthesis but also with regard to enhancing glucose uptake. May play a role in synapse maturation. Ca2+-dependent exocytosis of IGF1 is required for sensory perception of smell in the olfactory bulb. Acts as a ligand for IGF1R. Binds to the alpha subunit of IGF1R, leading to the activation of the intrinsic tyrosine kinase activity which autophosphorylates tyrosine residues in the beta subunit thus initiatiating a cascade of down-stream signaling events leading to activation of the PI3K-AKT/PKB and the Ras-MAPK pathways. Binds to integrins ITGAV:ITGB3 and ITGA6:ITGB4. Its binding to integrins and subsequent ternary complex formation with integrins and IGFR1 are essential for IGF1 signaling. Induces the phosphorylation and activation of IGFR1, MAPK3/ERK1, MAPK1/ERK2 and AKT1 (By similarity).
Immunoglobulin G (IgG) is a type of antibody composed of four peptide chains—two identical heavy chains and two identical light chains arranged in a Y-shape typical of antibody monomers.There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum. The IgG2a isotype was able to interact very efficiently with FcgammaR, and share approximately 64-78% amino acid sequence identity with IgG1 and IgG2b in the heavy chain constant region domains, CH1, CH2 and CH3, which cause the diffrence in the interaction with complement, NK and mast cells, induction or inhibition by specific cytokines, and ability to cross the placenta.
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