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.