Regulation of Insulin-Like Growth Factor-I Ribonucleic Acid Expression, Polypeptide Secretion, and Binding Protein Activity by Growth Hormone in Porcine Preadipocyte Cultures*

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Abstract
Insulin-like grwoth factor-I (IGF-I) is a mitogenic polypeptide postulated to mediate the effect of GH on adipose tissue development. To determine if the effect of GH could be mediated by the local production of IGF-I, we have characterized IGF-I RNA expression, polypeptide secretion, and binding protein activity in primary preadipocyte culture derived from porcine adipose tissues. GH acutely regulated the abundance of multiple IGF-I RNA transcripts and resulted in a 2-fold increase in secreted immunoreactive IGF-I (iIGF-I) polypetide in medium conditioned for 48 h preadipocyte cultures relative to those not receiving GH. Immunocytochemical data indicated that IGF-I is synthesized by presumptive and mature adipocytes. The effect of GH on iIGF-I secretion was observed in cultures derived from both fetal and postnatal animals, while secreted IGF-binding protein activity was increased due to GH only in cultures from fetal animals. The increase in local IGF-I production in response to GH was associated with a decrease in adipocyte development, suggesting that local IGF-I may contribute to suppression of differential phenotype.