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Abstract
The insulin-like growth factor binding proteins (IGFBPs) are a family of six proteins
that bind to insulin-like growth factor-I and -II with very high affinity. Because
their affinity constants are between two- and 50-fold greater than the IGF-I receptor,
they control the distribution of the IGFs among soluble IGFBPs in interstitial fluids,
IGFBPs bound to cell surfaces or extracellular matrix (ECM) and cell surface receptors.
Although there are six forms of insulin-like growth factor binding proteins, most
interstitial fluids contain only three or four forms, and usually only one or two
predominate. The proteins differ significantly in their biochemical characteristics,
and this accounts for many of the differences that have been observed in their biological
actions. Several different types of protease cleave these binding proteins. Proteolytic
cleavage generally inactivates the binding proteins or reduces their ability to bind
to IGF-I or -II substantially. Several cell types have been shown to secrete these
proteases; therefore, the factors that regulate protease activity can control binding
protein actions indirectly. Other post-translational modifications, such as glycosylation
and phosphorylation, have been shown to alter IGF binding protein activity. While
binding protein actions have been studied extensively in vitro, many of the in vivo
activities of these proteins remain to be defined.