Circulating salmon 41-kDa insulin-like growth factor binding protein (IGFBP) is not IGFBP-3 but an IGFBP-2 subtype

Abstract

In vertebrates, most circulating insulin-like growth factor (IGF) is bound to multiple forms of IGF-binding proteins (IGFBPs) that differ both structurally and functionally. In mammals, the largest reservoir of IGF in the circulation comes from a large (150kDa) ternary complex comprised of IGF bound to IGFBP-3, which is bound to an acid label subunit (ALS), and this variant of IGFBP is regulated by growth hormone (GH) and feed intake. Although multiple variants of IGFBPs ranging from 20 to 50 kDa have been found in fishes, no ternary complex is present and it has been assumed that the majority of circulating IGF is bound to fish IGFBP-3. Consistent with this assumption is previous work in salmon showing the presence of a 41-kDa IGFBP that is stimulated by GH, decreases with fasting and increases with feeding. However, the hypothesis that the salmon 41-kDa IGFBP is structurally homologous to mammalian IGFBP-3 has not been directly tested. To address this issue, we cloned cDNAs for several Chinook salmon IGFBPs, and found that the cDNA sequence of the 41-kDa IGFBP is most similar to that of mammalian IGFBP-2 and dissimilar to IGFBP-3. We found an additional IGFBP (termed IGFBP-2a) with high homology to mammalian IGFBP-2. These results demonstrate that salmon 41-kDa IGFBP is not IGFBP-3, but a paralog of IGFBP-2 (termed IGFBP-2b). Salmon IGFBP-2s are also unique in terms of having potential N-glycosylation sites and splice variants. Additional research on non-mammalian IGFBPs is needed to fully understand the molecular/functional evolution of the IGFBP family and the significance of the ternary complex in vertebrates

Dynamic transcriptional regulation of autocrine/paracrine igfbp1, 2, 3, 4, 5, and 6 in the skeletal muscle of the fine flounder during different nutritional statuses

“…On the other hand, igfbp2 is highly expressed in the liver of fine flounder. Recently, in Chinook salmon, Igfbp2 was shown to be the most abundant Igfbp into circulation system (Shimizu et al 2011); this coincides with our results pointing out that liver is the major organ for igfbp2 synthesis. Likewise, igfbp3 is highly expressed in the liver, but it is also detected in all the tissues assessed, according to the previous reports on tilapia (Cheng et al 2002) and zebra fish (Chen et al 2004).…”

Role of the GH-IGF-1 system in Atlantic salmon and rainbow trout postsmolts at elevated water temperature

“…IGFBP-1b has been suggested to have a major role in regulating the metabolic action of IGF-1, showing higher levels under catabolic conditions induced by starvation and stress in Chinook salmon (Shimizu et al, 2006(Shimizu et al, , 2011a(Shimizu et al, , 2011b, during starvation in Atlantic salmon (Hevrøy et al, 2011) and by cortisol treatment of rainbow trout (Shimizu et al, 2011b). In the present study we quantified for the first time the IGFBP-1b in Atlantic salmon and rainbow trout.…”

“…Our earlier examinations in fish held at elevated temperatures in sea water have shown that plasma IGF-1 is regulated by three major circulating IGFBPs at 23, 32 and 43 kDa in Atlantic salmon (Hevrøy et al, 2011, corresponding to salmonid IGFBP-1b, -1a and -2b, respectively (Shimizu et al, 2011a(Shimizu et al, , 2011b. IGFBP-1b has been suggested to have a major role in regulating the metabolic action of IGF-1, showing higher levels under catabolic conditions induced by starvation and stress in Chinook salmon (Shimizu et al, 2006(Shimizu et al, , 2011a(Shimizu et al, , 2011b, during starvation in Atlantic salmon (Hevrøy et al, 2011) and by cortisol treatment of rainbow trout (Shimizu et al, 2011b).…”

“…PCR cycles consisted of 1 cycle of 94°C for 3 min; 36 cycles of 94°C for 45 s, 50°C for 45 s, 72°C for 1 min 30 s; 1 cycle of 72°C for 5 min. PCR products were cloned into the pGEM-T Easy Vector Systems (Promega, Madison, WI) and positive clones were sequenced as described in Shimizu et al (2011a).…”

Evaluation of nucleic acids and plasma IGF1 levels for estimating short-term responses of postsmolt Atlantic salmon (Salmo salar) to food availability

“…The circulating level of IGF1 in the blood generally is determined by the most abundant binding protein (IGFBP2b), which itself is regulated by nutritional states (Shimizu et al, 2009;Kawaguchi et al, 2013). The establishment of methods to measure IGF binding proteins in fish blood is still on-going (Shimizu et al, 2011a(Shimizu et al, , 2011b and our understanding of the factors that regulate the abundance of different binding proteins and how they affect IGF1 levels is quite incomplete. We do not have the technical capacity to determine whether or not the current results (IGF1 and fasting, IGF1 and growth, IGF1 and refeeding) were due to the induction of different suites of binding proteins and how these proteins may have affected circulating IGF1 levels differently from those of other studies.…”