title: Mechanisms underlying maintenance of smooth muscle cell quiescence in rat aorta: role of the cyclin dependent kinases and their inhibitors.
abstract: OBJECTIVE
  We sought to understand why smooth muscle cell proliferation is effectively repressed in intact rat aortic tissue.
  
  
  METHODS
  Quiescent isolated rat aortic smooth muscle cells and segments of intact rat aorta were stimulated with 10% serum and the time course of expression and activity of proteins involved in cell cycle control were determined.
  
  
  RESULTS
  After serum stimulation, smooth muscle cells in intact aortic tissue exhibit no proliferation, whereas isolated cells entered S phase 14-16 h later. Activation of ERKs 1 and 2, and induction of cyclin D1 occurred both in isolated cells and aortic tissue. Regulation of Cdk4, cyclin E and Cdk2 protein levels was also not different. Levels of the cyclin-dependent kinase inhibitors (CKIs), p16 and p27, were initially high in quiescent isolated cells and tissue; levels were downregulated by serum in isolated cells but not in aortic tissue. Cyclin D1/Cdk4, and cyclin E/Cdk2 kinases were active before S phase entry in isolated cells, but remained inactive in aortic tissue.
  
  
  CONCLUSIONS
  Cell cycle entry is prevented in aortic tissue, and this is associated with an inability to downregulate p16 and p27 CKIs, and therefore to activate cyclin D1 and cyclin E associated kinase activities.
authors.affiliation: Bristol Heart Institute, Bristol Royal Infirmary, Bristol BS2 8HW, UK.
authors.forename: Tanya D
authors.initials: TD
authors.lastname: Izzard
authors.forename: Christine
authors.initials: C
authors.lastname: Taylor
authors.forename: Sonia D
authors.initials: SD
authors.lastname: Birkett
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Jackson
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Newby
mesh_terms: D000818:Animals; D001011:Aorta; D015153:Blotting, Western; D042846:CDC2-CDC28 Kinases; D002455:Cell Division; D002478:Cells, Cultured; D019938:Cyclin D1 / Q000378:metabolism; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000378:metabolism; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D004789:Enzyme Activation; D007150:Immunohistochemistry / Q000379:methods; D008297:Male; D008840:Microfilament Proteins / Q000378:metabolism; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D009124:Muscle Proteins; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000378:metabolism*; D009924:Organ Culture Techniques; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D051381:Rats; D017208:Rats, Wistar; D013936:Thymidine / Q000378:metabolism
chemical_list: D019927:Cyclin E; D019941:Cyclin-Dependent Kinase Inhibitor p16; D008840:Microfilament Proteins; D009124:Muscle Proteins; D011518:Proto-Oncogene Proteins; C058992:Tagln protein, mouse; D019938:Cyclin D1; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495892:Cdk2 protein, rat; C495903:Cdk4 protein, rat; D051357:Cyclin-Dependent Kinase 2; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases; D019950:Mitogen-Activated Protein Kinase 1; D013936:Thymidine