Circulating insulin-like growth factor binding proteins in fish: Their identities and physiological regulation

Insulin-like growth factor binding proteins (IGFBPs) play crucial roles in regulating the availability of IGFs to receptors and prolong the half-lives of IGFs. There are six IGFBPs present in the mammalian circulation with IGFBP-3 being most abundant. In mammals IGFBP-3 is the major carrier of circulating IGFs, facilitated by forming a ternary complex with IGF and an acid-labile subunit (ALS). IGFBP-1 is generally inhibitory to IGF action by preventing it from interacting with its receptors. In teleosts, the third-round of vertebrate whole genome duplication created paralogs of each IGFBP, except IGFBP-4. In the fish circulation, three major IGFBPs are typically detected at molecular ranges of 20-25, 28-32 and 40-50 kDa. However, their identities are not well established. Three major circulating IGFBPs in Chinook salmon have been identified through protein purification and cDNA cloning. Salmon 28- and 22-kDa IGFBPs are co-orthologs of IGFBP-1, termed IGFBP-1a and -1b, respectively. They are induced under catabolic conditions such as stress and fasting but their responses are somewhat different, with IGFBP-1b being the most sensitive of the two. Cortisol stimulates production and secretion of these IGFBP-1 subtypes while, unlike in mammals, insulin may not be a primary suppressor. Salmon 41-kDa IGFBP, a major carrier of IGF-I, is not IGFBP-3, as might be expected extrapolating from mammals, but is in fact IGFBP-2b. Salmon IGFBP-2b levels in plasma are high when fish are fed, and GH treatment increases its circulating levels similar to mammalian IGFBP-3. These findings suggest that salmon IGFBP-2b acquired the role and regulation similar to mammalian IGFBP-3. Multiple replications of fish IGFBPs offer a unique opportunity to investigate molecular evolution of IGFBPs.