Inhibition of Egr1 expression underlies the anti-mitogenic effects of cAMP in vascular smooth muscle cells

• Published in: Journal of molecular and cellular cardiology Vol. 72; no. 100; pp. 9 - 19
• Main Authors: Kimura, Tomomi E, Duggirala, Aparna, Hindmarch, Charles C.T, Hewer, Richard C, Cui, Mei-Zhen, Newby, Andrew C, Bond, Mark
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.07.2014; Academic Press
• Subjects: 3′-5′-Cyclic adenosine monophosphate; Adenosine - pharmacology; Aminopyridines - pharmacology; Animals; Cardiovascular; Cell Proliferation - drug effects; Colforsin - pharmacology; Cyclic AMP - pharmacology; Cyclic AMP Response Element-Binding Protein - genetics; Cyclic AMP Response Element-Binding Protein - metabolism; Cyclic AMP-Dependent Protein Kinases - genetics; Cyclic AMP-Dependent Protein Kinases - metabolism; Early growth response gene 1; Early Growth Response Protein 1 - antagonists & inhibitors; Early Growth Response Protein 1 - genetics; Early Growth Response Protein 1 - metabolism; Epoprostenol - analogs & derivatives; Epoprostenol - pharmacology; ets-Domain Protein Elk-1 - genetics; ets-Domain Protein Elk-1 - metabolism; Exchange protein activated by cAMP; Gene Expression Regulation; Guanine Nucleotide Exchange Factors - genetics; Guanine Nucleotide Exchange Factors - metabolism; Human Umbilical Vein Endothelial Cells - cytology; Human Umbilical Vein Endothelial Cells - drug effects; Human Umbilical Vein Endothelial Cells - metabolism; Male; Muscle, Smooth, Vascular - cytology; Muscle, Smooth, Vascular - drug effects; Muscle, Smooth, Vascular - metabolism; Myocytes, Smooth Muscle - cytology; Myocytes, Smooth Muscle - drug effects; Myocytes, Smooth Muscle - metabolism; Organ Specificity; Original; Primary Cell Culture; Protein Binding; rac1 GTP-Binding Protein - genetics; rac1 GTP-Binding Protein - metabolism; Rats; Rats, Sprague-Dawley; Serum response factor; Serum Response Factor - genetics; Serum Response Factor - metabolism; Signal Transduction; Zif268; Serum response factor; Early growth response gene 1; 3′-5′-Cyclic adenosine monophosphate; Exchange protein activated by cAMP; Zif268
• ISSN: 0022-2828; 1095-8584
• DOI: 10.1016/j.yjmcc.2014.02.001

Abstract Aims Cyclic AMP inhibits vascular smooth muscle cell (VSMC) proliferation which is important in the aetiology of numerous vascular diseases. The anti-mitogenic properties of cAMP in VSMC are dependent on activation of protein kinase A (PKA) and exchange protein activated by cAMP (EPAC), but the mechanisms are unclear. Methods and results Selective agonists of PKA and EPAC synergistically inhibited Egr1 expression, which was essential for VSMC proliferation. Forskolin, adenosine, A2B receptor agonist BAY60-6583 and Cicaprost also inhibited Egr1 expression in VSMC but not in endothelial cells. Inhibition of Egr1 by cAMP was independent of cAMP response element binding protein (CREB) activity but dependent on inhibition of serum response element (SRE) activity. SRF binding to the Egr1 promoter was not modulated by cAMP stimulation. However, Egr1 expression was dependent on the SRF co-factors Elk1 and 4 but independent of MAL. Inhibition of SRE-dependent Egr1 expression was due to synergistic inhibition of Rac1 activity by PKA and EPAC, resulting in rapid cytoskeleton remodelling and nuclear export of ERK1/2. This was associated with de-phosphorylation of the SRF co-factor Elk1. Conclusion cAMP inhibits VSMC proliferation by rapidly inhibiting Egr1 expression. This occurs, at least in part, via inhibition of Rac1 activity leading to rapid actin-cytoskeleton remodelling, nuclear export of ERK1/2, impaired Elk1-phosphorylation and inhibition of SRE activity. This identifies one of the earliest mechanisms underlying the anti-mitogenic effects of cAMP in VSMC but not in endothelial cells, making it an attractive target for selective inhibition of VSMC proliferation.