DEVELOPMENTAL REGULATION OF INSULIN-LIKE GROWTH-FACTOR BINDING PROTEIN-2 IN CHICK-EMBRYO SERUM AND VITREOUS-HUMOR

The chick embryo is a useful vertebrate model for studying development al embryogenesis. Insulin-like growth factor I (IGF-I), a potent mitog en, is thought to contribute to the general growth of the embryo as an endocrine factor, and as a paracrine factor to the development of the early embryo and of specific organs such as the eye. Recent data sugg est that a family of at least six IGF binding proteins (IGFBPs) comple x IGF-I and modulate its biological actions. In the present study, we examine the expression of IGFBPs in chicken serum and vitreous humor a t different stages of embryonic development, and compare it with that of IGF-I. As determined by ligand blotting, the predominant IGFBP in c hick serum and vitreous humor between embryonic days 4 and 22 (E4-E22) is a 30 kDa IGFBP. This IGFBP was specifically immunoprecipitated by a polyclonal antiserum raised against rat IGFBP-2, the predominant IGF BP in fetal human and rat serum. Although IGFBP-2 is present in both c hick fluids at all times examined, serum IGFBP-2 increased progressive ly between E10-E22, whereas vitreous IGFBP-2 was highest during eye or ganogenesis (E4-E8). This suggests that vitreous IGFBP-2 is synthesize d locally. Like serum IGFBP-2, levels of immunoreactive IGF-I in serum are higher in the second week of embryogenesis than the first. Despit e this correlation, changes in IGFBP-2 do not appear to be regulated b y IGF-1: (a) serum IGF-I decreases after day 15, whereas IGFBP-2 level s remain stable until hatching; (b) vitreous IGF-1, like serum IGF-I, is higher in the second week of embryogenesis, whereas vitreous IGFBP- 2 is highest in the first week; (c) embryos cultured ex ovo express IG FBP-2 at E15-E19, although they lack the normal mid-embryogenesis surg e in IGF-I. We conclude that vitreous IGFBP-2 is synthesized locally i n the eye, and that the expression of IGFBP-2 in chick embryos is not directly regulated by IGF-I.