cAMP inhibits growth and stimulates differentiation in glioma cells. We examined the effect of cAMP on insulin-like growth factor I (IGF-I) gene expression in the C6 cell line, a rat glioma cell line previously reported to grow in response to autocrine IGF-I. cAMP potently inhibited IGF-I messenger RNA (mRNA) and peptide secretion in C6 cells, associated with an attenuation of DNA synthesis. Exogenous IGF-I peptide at least partially prevented the inhibition of DNA synthesis, suggesting that the reduction in IGF-I biosynthesis may contribute to the inhibitory effect of cAMP on C6 cell growth. cAMP also inhibited IGF-I mRNA in rat RG2 glioma cells, but not in three other nonglioma tumor cell lines. The nuclear IGF-I pre-mRNA level and the half-life of mature IGF-I mRNA were both reduced by cAMP in C6 cells, suggesting effects on gene transcription and mRNA stability. However, cAMP had no effect on the activities of IGF-I exon 1 promoter-luciferase constructs. Protein synthesis inhibition partially reduced the inhibition of IGF-I mRNA by cAMP. Inhibition of cAMP-activated protein kinase A activity by H89 did not alter the inhibition of IGF-I gene expression in response to cAMP, suggesting that protein kinase A does not mediate the cAMP inhibitory effect on IGF-I gene expression.
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