Regulation of G(1) cyclin-dependent kinases in the liver: role of nuclear localization and p27 sequestration

Am J Physiol. 1999 Dec;277(6):G1207-16. doi: 10.1152/ajpgi.1999.277.6.G1207.


Recent studies suggest that cyclin D1 mediates progression of hepatocytes through G(1) phase of the cell cycle. The present study further examines the regulation of cyclin D1-dependent kinase activity and the interplay between cyclin D1 and other G(1) phase regulatory proteins during liver regeneration. After 70% partial hepatectomy in rats, there was upregulation of kinase activity associated with cyclins (A, D1, D3, and E), cyclin-dependent kinases (Cdk2 and Cdk4), and Cdk-inhibitory proteins (p27, p107, and p130). Although cyclin D1/Cdk4 complexes were more abundant in the cytoplasmic fraction after partial hepatectomy, kinase activity was detected primarily in the nuclear fraction. Cytoplasmic cyclin D1/Cdk4 complexes were activated by recombinant cyclin H/Cdk7. Because endogenous Cdk7 activity was found in the nucleus, this suggests that activation of cyclin D1/Cdk4 requires nuclear importation and subsequent phosphorylation by cyclin H/Cdk7. Recombinant cyclin E/Cdk2 was inhibited by extracts from quiescent liver, and cyclin D1 could titrate out this inhibitory activity. Induction of cyclin D1 was accompanied by increased abundance of cyclin D1/p27 complexes, and most p27 was sequestered by cyclin D1 after partial hepatectomy. Thus cyclin D1 appears to play two roles during G(1) phase progression in the regenerating liver: it forms a nuclear kinase complex, and it promotes activation of Cdk2 by sequestering inhibitory proteins such as p27. These experiments underscore the complexity of cyclin/Cdk regulatory networks in the regenerating liver.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • CDC2-CDC28 Kinases*
  • Cell Cycle Proteins*
  • Cell Nucleus / enzymology
  • Cyclin D1 / metabolism
  • Cyclin E / metabolism
  • Cyclin G
  • Cyclin G1
  • Cyclin-Dependent Kinase 2
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinase Inhibitor p27
  • Cyclin-Dependent Kinases / metabolism*
  • Cyclins / metabolism*
  • Cytoplasm / enzymology
  • Enzyme Inhibitors / metabolism
  • Liver / enzymology*
  • Liver Regeneration / physiology*
  • Male
  • Microtubule-Associated Proteins / metabolism*
  • Phosphorylation
  • Protein Serine-Threonine Kinases / metabolism
  • Proto-Oncogene Proteins*
  • Rats
  • Rats, Sprague-Dawley
  • S Phase / physiology
  • Tumor Suppressor Proteins*


  • Ccng1 protein, rat
  • Cdkn1b protein, rat
  • Cell Cycle Proteins
  • Cyclin E
  • Cyclin G
  • Cyclin G1
  • Cyclins
  • Enzyme Inhibitors
  • Microtubule-Associated Proteins
  • Proto-Oncogene Proteins
  • Tumor Suppressor Proteins
  • Cyclin D1
  • Cyclin-Dependent Kinase Inhibitor p27
  • Protein Serine-Threonine Kinases
  • CDC2-CDC28 Kinases
  • Cdk2 protein, rat
  • Cdk4 protein, rat
  • Cyclin-Dependent Kinase 2
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinases