FGFs stimulate IGF binding protein synthesis without affecting IGF synthesis in rat astroblasts in primary culture

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Abstract
The production of insulin‐like growth factor (IGF)‐I and ‐II and their binding proteins (BPs) has been studied in new‐born rat astroblasts at confluency in primary culture. Under the influence of fibroblast growth factors (FGFs) (acidic and basic), the morphology of the astroblasts was altered, 125I‐deoxyuridine incorporation was increased, and glutamine synthetase activity was stimulated. IGF production and IGF mRNA expression remained unchanged. Production of the 32 kDa BP (IGFBP‐2), the sole or predominant form under base‐line conditions, was enhanced and the 43–39 kDa form (IGFBP‐3) appeared or were increased. Epidermal growth factor (EGF) also stimulated production of these BPs, whereas thrombin and db‐cAMP had no effect. Our data suggest that a relationship exists between FGF‐induced maturation of astroblasts and the forms of BP they produce. The data also indicate that some factors may act specifically on BP synthesis, without affecting IGF synthesis, and in this way play a role in regulating the bioavailability of the IGFs.