Characterization of a teleost insulin-like growth factor II (IGF-II) gene: evidence for promoter CCAAT/enhancer-binding protein (C/EBP) sites, and the presence of hepatic C/EBP

Insulin-like growth factor I (IGF-I) and IGF-II, acting under the regulatory control of growth hormone, are the principal mediators of vertebrate growth. We have previously demonstrated that like humans, but unlike rodents, rainbow trout maintain high hepatic IGF-II messenger RNA levels into adulthood. Here we describe a rainbow trout IGF-II gene with a proximal promoter that contains two CCAAT/enhancer-binding protein (C/EBP) binding sites (TCBS1 and TCBS2). Nuclear proteins corresponding in size to rat C/EBP alpha and C/EBP beta were detected on Western immunoblots of growth-hormone-treated and mock-treated trout liver extracts. Electrophoretic mobility shift assay of these nuclear extracts further suggests the presence of C/EBPs in trout liver and confirms the ability of TCBS1 to form a complex with trout liver nuclear proteins that is identical in mobility and specificity to that formed by a mammalian consensus CBS construct. In both Western blot and mobility assay results, the growth-hormone-treated trout livers appeared to have a greater accumulation of C/EBP, suggesting a molecular mechanism by which growth hormone can influence the level of serum IGF-II.