Transcriptional regulation of insulin-like growth factor binding protein-1 expression by insulin and cyclic AMP.
In biological fluids, insulin-like growth factor binding proteins (IGFBPs) interact with the IGFs and modulate their effects. In this study, changes in IGFBP-1 expression were investigated under the influence of insulin and cAMP which may regulate expression of the IGFBP-1 gene in vivo during the perinatal period. Western ligand blot analysis of IGFBPs secreted by HepG2 human hepatoma cells showed that 24 h treatment with forskolin increased IGFBP-1 secretion by approximately 100%, whereas similar treatment with insulin resulted in a 50% reduction. After 24 h, the amounts of IGFBP-1 mRNA (measured by Northern blotting) were increased 2.5 times by forskolin and decreased by 65% by insulin. Transient transfection experiments showed that forskolin enhanced IGFBP-1 promoter activity by 70%, suggesting that stimulation of IGFBP-1 gene expression by cAMP is transcriptional, via a protein recognizing the cAMP responsive element (CRE) consensus sequence (nt -268 to -248). In contrast, modulation of gene expression by insulin is more complex, probably involving several levels of regulation. Complementary experiments (site-directed mutagenesis and/or use of a heterologous promoter) will be needed to confirm the functionality of the proteins interacting with the IRE (nt -285 and -276) and the CRE (between nt -268 and -248) described.
Babajko
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Unité de Recherches sur la régulation de la croissance, INSERM U.142, Hôpital Saint Antoine, Paris, France.
D015152:Blotting, Northern; D015153:Blotting, Western; D006528:Carcinoma, Hepatocellular; D002352:Carrier Proteins / Q000235:genetics*; D000242:Cyclic AMP / Q000494:pharmacology*; D005786:Gene Expression Regulation / Q000187:drug effects*; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D018970:Insulin-Like Growth Factor Binding Protein 1; D008113:Liver Neoplasms; D016297:Mutagenesis, Site-Directed; D011401:Promoter Regions, Genetic; D012333:RNA, Messenger / Q000378:metabolism; D014158:Transcription, Genetic; D014162:Transfection; D014407:Tumor Cells, Cultured
D002352:Carrier Proteins; D007328:Insulin; D018970:Insulin-Like Growth Factor Binding Protein 1; D012333:RNA, Messenger; D000242:Cyclic AMP