We previously reported that cAMP inhibits autocrine IGF-I gene expression in rat C6 glioma cells. In this study we examined the influence of cAMP on IGF-binding protein gene expression in C6 cells. cAMP potently inhibited IGF-binding protein-3 mRNA and, to a lesser extent, IGF-binding protein-4 mRNA and transiently stimulated IGF-binding protein-5 mRNA. The changes in secreted IGF-binding proteins whose molecular weights were consistent with IGF-binding protein-3 and -5 correlated with those of mRNA levels. cAMP decreased the IGF-binding protein-3 mRNA half-life, but did not alter IGF-binding protein-4 and -5 mRNA half-lives. An IGF-binding protein-5 promoter/luciferase fusion construct containing 888 bp of 5′-flanking sequence and the first 114 bp of exon 1 sequence was stimulated by cAMP after 24 h by approximately 2-fold in transient transfection assays. 5′- or 3′-deletion to -33 or +10 (the transcription start site was designated as +1), respectively, did not alter the increase caused by cAMP. Site-directed mutagenesis of the region from -14 to -5 led to a loss of the ability of the IGF-binding protein-5 promoter to respond to cAMP. H89, a cell-permeable protein kinase A inhibitor, did not alter the regulation of IGF-binding protein mRNAs in response to cAMP.
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