Insulin-like growth factor binding proteins in the human circulation: a review.

The actions and bioavailability of the insulin-like growth factors (IGFs) are regulated by a family of six IGF binding proteins. IGFBP-3, the major circulating IGFBP, is unique in combining with a glycoprotein, the acid-labile subunit (ALS), to form a ternary complex with IGF-I or IGF-II. Each component of this trimer is growth hormone-dependent, and the hypoglycemic potential of circulating IGFs appears neutralized in this form. IGFs in the complex have a greatly extended circulating half-life, their stability being conferred by the presence of ALS, which is itself very stable in the circulation. IGFBP-1 does not appear to be a carrier of IGFs, but to act as an acute modulator of their metabolic activities. In this role it can be viewed as an insulin counter-regulator, blocking 'free' insulin-like activity during fasting or hypoglycemia. IGF-I administration causes complex changes in circulating IGFBPs, so that a detailed knowledge of their regulation is essential if the therapeutic potential of IGF-I is to be optimised.