MOLECULAR MECHANISMS FOR SOMATOSTATIN INHIBITION OF C-FOS GENE-EXPRESSION

We reported previously that somatostatin inhibits the expression of th e immediate early gene c-fos. Accordingly we characterized the molecul ar mechanisms by which somatostatin inhibits c-fos gene expression. Be cause growth factors activate c-fos through a region of its promoter k nown as the serum response element [SRE; base pairs (bp) -357 to -276] we transfected rat pituitary adenoma cells (GH(3)) with plasmids cont aining the SRE or the SRE core fragment (bp -320 to -298) upstream of the luciferase reporter gene. Epidermal growth factor (EGF) stimulated SRE-luciferase activity, and this effect was inhibited by somatostati n and by the analog MK-678. Identical results were obtained with the S RE core plasmid, demonstrating that the sequence between bp -320 and - 298 of the c-fos promoter is a somatostatin response element. Because the extracellular signal-regulated protein kinases (ERKs) induce the S RE via phosphorylation of transcription factors such as Elk-1, we exam ined the effect of somatostatin on ERK phosphorylation and activation. EGF stimulated tyrosine phosphorylation of ERK2, and MK-678 attenuate d this effect. In experiments using in-gel kinase assays, MK-678 also inhibited EGF-stimulated ERK activity via a pertussis toxin sensitive pathway, and this effect resulted in inhibition of Elk-1 transcription al activity. Our data suggest that one mechanism of somatostatin actio n involves inhibition of ERK activity, Elk-1 phosphorylation and trans criptional activation, and ultimately c-fos gene transcription.