Molecular mechanisms for somatostatin inhibition of c-fos gene expression.
We reported previously that somatostatin inhibits the expression of the immediate early gene c-fos. Accordingly, we characterized the molecular mechanisms by which somatostatin inhibits c-fos gene expression. Because growth factors activate c-fos through a region of its promoter known as the serum response element [SRE; base pairs (bp) -357 to -276] we transfected rat pituitary adenoma cells (GH3) with plasmids containing 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 somatostatin and by the analog MK-678. Identical results were obtained with the SRE 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 SRE via phosphorylation of transcription factors such as Elk-1, we examined the effect of somatostatin on ERK phosphorylation and activation. EGF stimulated tyrosine phosphorylation of ERK2, and MK-678 attenuated 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 transcriptional activity. Our data suggest that one mechanism of somatostatin action involves inhibition of ERK activity, Elk-1 phosphorylation and transcriptional activation, and ultimately c-fos gene transcription.
Todisco
A
A
Department of Internal Medicine, University of Michigan Medical Center, Ann Arbor 48109-0682, USA.
Takeuchi
Y
Y
Yamada
J
J
Sadoshima
J I
JI
Yamada
T
T
D000818:Animals; D015153:Blotting, Western; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000037:antagonists & inhibitors; D004268:DNA-Binding Proteins; D004338:Drug Combinations; D004815:Epidermal Growth Factor / Q000494:pharmacology; D015870:Gene Expression / Q000187:drug effects*; D016762:Genes, fos; D006727:Hormone Antagonists / Q000494:pharmacology; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases; D010456:Peptides, Cyclic / Q000494:pharmacology; D010766:Phosphorylation / Q000187:drug effects; D011233:Precipitin Tests; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins / Q000378:metabolism; D051381:Rats; D013004:Somatostatin / Q000494:pharmacology*; D014157:Transcription Factors; D014158:Transcription, Genetic / Q000187:drug effects; D014407:Tumor Cells, Cultured; D050807:ets-Domain Protein Elk-1
D004268:DNA-Binding Proteins; D004338:Drug Combinations; C494114:Elk1 protein, rat; D006727:Hormone Antagonists; D010456:Peptides, Cyclic; D011518:Proto-Oncogene Proteins; D014157:Transcription Factors; D050807:ets-Domain Protein Elk-1; D013004:Somatostatin; C040968:seglitide; D004815:Epidermal Growth Factor; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020928:Mitogen-Activated Protein Kinases