title: Differential association of p21Cip1 and p27Kip1 with cyclin E-CDK2 during rat liver regeneration.
abstract: OBJECTIVE
  The cell cycle inhibitors p21Cip1 and p27Kip1 regulate liver regeneration by modulating the activity of cyclin-dependent kinases (CDKs). However, the specific role of these inhibitors in the regulation of CDK2 activity during liver regeneration remains unknown. The aim of this study was to examine the association of p21Cip1 and p27Kip1 with cyclin E-CDK2 and cyclin A-CDK2 complexes during rat liver regeneration and to correlate the association of both inhibitors with CDK2 activity.
  
  
  METHODS
  The association of p21Cip1 or p27Kip1 with cyclin E-CDK2 or cyclin A-CDK2 and the activities of these complexes were analyzed by immunoprecipitation of rat liver homogenates obtained at different times after a partial hepatectomy (PH), followed by Western blotting or kinase assays.
  
  
  RESULTS
  High amounts of p27Kip1 bound to cyclin E-CDK2 were observed during the first 13 h after PH, when CDK2 activity was very low. At 24 h, when CDK2 activity was maximal, the amount of bound-p27Kip1 decreased strongly. The amount of p21Cip1 bound to these complexes was low during the first 13 h but subsequently increased. No cyclin A-CDK2 complexes were found during the first 13 h after PH. At 24 h, complexes containing low levels of both inhibitors were detected and at 28 h, a significant increase in p21Cip1 and p27Kip1 associated with cyclin A-CDK2 was observed.
  
  
  CONCLUSIONS
  p27Kip1 acts as a brake on cyclin E-CDK2 activity during the first 13 h after a PH. Both p21Cip1 and p27Kip1 down-regulate cyclin A-CDK2 activity at 28 h after PH, after its maximal activation.
authors.affiliation: Department of Cell Biology, Faculty of Medicine, Institut d'Investigacions Biomèdiques August Pi Sunyer (IDIBAPS), University of Barcelona, Spain.
authors.forename: M J
authors.initials: MJ
authors.lastname: Pujol
authors.forename: M
authors.initials: M
authors.lastname: Jaime
authors.forename: J
authors.initials: J
authors.lastname: Serratosa
authors.forename: M
authors.initials: M
authors.lastname: Jaumot
authors.forename: N
authors.initials: N
authors.lastname: Agell
authors.forename: O
authors.initials: O
authors.lastname: Bachs
mesh_terms: D000818:Animals; D042846:CDC2-CDC28 Kinases; D018797:Cell Cycle Proteins; D019927:Cyclin E / Q000502:physiology*; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D050760:Cyclin-Dependent Kinase Inhibitor p27; D018844:Cyclin-Dependent Kinases / Q000502:physiology*; D016213:Cyclins / Q000502:physiology*; D016147:Genes, Tumor Suppressor; D008115:Liver Regeneration / Q000502:physiology*; D008297:Male; D008869:Microtubule-Associated Proteins / Q000502:physiology*; D017346:Protein Serine-Threonine Kinases / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D025521:Tumor Suppressor Proteins
chemical_list: C493958:Cdkn1a protein, rat; C493962:Cdkn1b protein, rat; D018797:Cell Cycle Proteins; D019927:Cyclin E; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D008869:Microtubule-Associated Proteins; D025521:Tumor Suppressor Proteins; D050760:Cyclin-Dependent Kinase Inhibitor p27; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495892:Cdk2 protein, rat; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases