title: Inhibition of Egr1 expression underlies the anti-mitogenic effects of cAMP in vascular smooth muscle cells.
abstract: OBJECTIVE
  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.
  
  
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Bristol Heart Institute, University of Bristol, Bristol BS2 8HW, UK.
authors.forename: Tomomi E
authors.initials: TE
authors.lastname: Kimura
authors.affiliation: Bristol Heart Institute, University of Bristol, Bristol BS2 8HW, UK.
authors.forename: Aparna
authors.initials: A
authors.lastname: Duggirala
authors.affiliation: Laboratory for Integrative Neuroscience & Endocrinology, University of Bristol, Bristol BS2 8HW, UK; University of Malaya, Department of Physiology, Faculty of Medicine, Kuala Lumpur, Malaysia.
authors.forename: Charles C T
authors.initials: CC
authors.lastname: Hindmarch
authors.affiliation: Bristol Heart Institute, University of Bristol, Bristol BS2 8HW, UK.
authors.forename: Richard C
authors.initials: RC
authors.lastname: Hewer
authors.affiliation: Department of Pathobiology, College of Veterinary Medicine, The University of Tennessee, USA.
authors.forename: Mei-Zhen
authors.initials: MZ
authors.lastname: Cui
authors.affiliation: Bristol Heart Institute, University of Bristol, Bristol BS2 8HW, UK.
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Newby
authors.affiliation: Bristol Heart Institute, University of Bristol, Bristol BS2 8HW, UK. Electronic address: mark.bond@bris.ac.uk.
authors.forename: Mark
authors.initials: M
authors.lastname: Bond
mesh_terms: D000241:Adenosine / Q000494:pharmacology; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D049109:Cell Proliferation / Q000187:drug effects; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology*; D017362:Cyclic AMP Response Element-Binding Protein / Q000235:genetics / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism; D051766:Early Growth Response Protein 1 / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D011464:Epoprostenol / Q000031:analogs & derivatives / Q000494:pharmacology; D005786:Gene Expression Regulation; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism; D061307:Human Umbilical Vein Endothelial Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D008297:Male; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D032389:Myocytes, Smooth Muscle / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D009928:Organ Specificity; D061251:Primary Cell Culture; D011485:Protein Binding; D051381:Rats; D017207:Rats, Sprague-Dawley; D026362:Serum Response Factor / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D050807:ets-Domain Protein Elk-1 / Q000235:genetics / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D000631:Aminopyridines; C518875:BAY 60-6583; D017362:Cyclic AMP Response Element-Binding Protein; C497325:EGR1 protein, human; D051766:Early Growth Response Protein 1; C486110:Egr1 protein, rat; C494114:Elk1 protein, rat; D020662:Guanine Nucleotide Exchange Factors; C116122:RAPGEF3 protein, human; C526973:Rapgef3 protein, rat; C476858:SRF protein, human; D026362:Serum Response Factor; D050807:ets-Domain Protein Elk-1; D005576:Colforsin; D011464:Epoprostenol; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D020830:rac1 GTP-Binding Protein; D000241:Adenosine; C043867:cicaprost
keywords: 3′-5′-Cyclic adenosine monophosphate; Early growth response gene 1; Exchange protein activated by cAMP; Serum response factor; Zif268