Insulin-like growth factor binding protein 3 (IGFBP-3)
Type: Family

Insulin is found in many animals, and is involved in the regulation of normal glucose homeostasis. It also has other specific physiological effects, such as increasing the permeability of cells to monosaccharides, amino acids and fatty acids, and accelerating glycolysis and glycogen synthesis in the liver . Insulin exerts its effects by interaction with a cell-surface receptor, which may also result in the promotion of cell growth .

Insulin-like Growth Factor Binding Proteins (IGFBP) are a group of vertebrate secreted proteins, which bind to IGF-I and IGF-II with high affinity and modulate the biological actions of IGFs. The IGFBP family has six distinct subgroups, IGFBP-1 through 6, based on conservation of gene (intron-exon) organisation, structural similarity, and binding affinity for IGFs. Across species, IGFBP-5 exhibits the most sequence conservation, while IGFBP-6 exhibits the least sequence conservation. The IGFBPs contain inhibitor domain homologues, which are related to MEROPS protease inhibitor family I31 (equistatin, clan IX).

All IGFBPs share a common domain architecture (

:

). While the N-terminal (

, IGF binding protein domain), and the C-terminal (

, thyroglobulin type-1 repeat) domains are conserved across vertebrate species, the mid-region is highly variable with respect to protease cleavage sites and phosphorylation and glycosylation sites. IGFBPs contain 16-18 conserved cysteines located in the N-terminal and the C-terminal regions, which form 8-9 disulphide bonds .

As demonstrated for human IGFBP-5, the N terminus is the primary binding site for IGF. This region, comprised of Val49, Tyr50, Pro62 and Lys68-Leu75, forms a hydrophobic patch on the surface of the protein . The C terminus is also required for high affinity IGF binding, as well as for binding to the extracellular matrix and for nuclear translocation of IGFBP-3 and -5.

IGFBPs are unusually pleiotropic molecules. Like other binding proteins, IGFBP can prolong the half-life of IGFs via high affinity binding of the ligands. In addition to functioning as simple carrier proteins, serum IGFBPs also serve to regulate the endocrine and paracrine/autocrine actions of IGF by modulating the IGF available to bind to signalling IGF-I receptors . Furthermore, IGFBPs can function as growth modulators independent of IGFs. For example, IGFBP-5 stimulates markers of bone formation in osteoblasts lacking functional IGFs . The binding of IGFBP to its putative receptor on the cell membrane may stimulate the signalling pathway independent of an IGF receptor, to mediate the effects of IGFBPs in certain target cell types. IGFBP-1 and -2, but not other IGFBPs, contain a C-terminal Arg-Gly-Asp integrin-binding motif. Thus, IGFBP-1 can also stimulate cell migration of CHO and human trophoblast cells through an action mediated by alpha 5 beta 1 integrin . Finally, IGFBPs transported into the nucleus (via the nuclear localisation signal) may also exert IGF-independent effects by transcriptional activation of genes.

This family represents IGFBP-3, which is the major IGF-binding protein in serum and is expressed in many tissues, including normal and malignant breast epithelium . IGFBP-3 has a well-characterised role in modulating the mitogenic and anti-apoptotic effects of IGFs by regulating their access to the IGF-I receptor . There is now accumulating evidence to suggest that IGFBP-3 may have intrinsic anti-proliferative and pro-apoptotic effects on the growth of human cancer cells. Similar to IGFBP-5, IGFBP-3 can accumulate in the nucleus via its C-terminal nuclear translocation signal , where it may exert its growth-modulating effects. IGFBP-3, via its C-terminal end, can bind to the acid-labile subunit (ALS), which together with IGF forms the 150kDa ternary complex found in serum. In addition, IGFBP-3 can function via a TGFbeta-related pathway, probably by binding to a type V TGF-beta receptor .