C/EBPalpha Triggers Proteasome-Dependent Degradation of cdk4 During Growth Arrest

EMBO J. 2002 Mar 1;21(5):930-41. doi: 10.1093/emboj/21.5.930.


CCAAT/enhancer binding protein alpha (C/EBPalpha) causes growth arrest via direct interaction with the cyclin-dependent kinases cdk2 and cdk4. In this paper, we present evidence showing that C/EBPalpha enhances a proteasome-dependent degradation of cdk4 during growth arrest in liver of newborn mice and in cultured cells. Overexpression of C/EBPalpha in several biological systems leads to a reduction of cdk4 protein levels, but not mRNA levels. Experiments with several tissue culture models reveal that C/EBPalpha enhances the formation of cdk4-ubiquitin conjugates and induces degradation of cdk4 through a proteasome-dependent pathway. As a result, the half-life of cdk4 is shorter and protein levels of cdk4 are reduced in cells expressing C/EBPalpha. Gel filtration analysis of cdk4 complexes shows that a chaperone complex cdk4-cdc37-Hsp90, which protects cdk4 from degradation, is abundant in proliferating livers that lack C/EBPalpha, but this complex is weak or undetectable in livers expressing C/EBPalpha. Our studies show that C/EBPalpha disrupts the cdk4-cdc37-Hsp90 complex via direct interaction with cdk4 and reduces protein levels of cdk4 by increasing proteasome-dependent degradation of cdk4.

Publication types

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

MeSH terms

  • Animals
  • Animals, Newborn
  • CCAAT-Enhancer-Binding Protein-alpha / deficiency
  • CCAAT-Enhancer-Binding Protein-alpha / genetics
  • CCAAT-Enhancer-Binding Protein-alpha / physiology*
  • COS Cells
  • Cell Cycle / physiology*
  • Cell Cycle Proteins / metabolism
  • Cells, Cultured / metabolism
  • Chlorocebus aethiops
  • Chromatography, Gel
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinases / genetics
  • Cyclin-Dependent Kinases / metabolism*
  • Cysteine Endopeptidases / metabolism*
  • Drosophila Proteins*
  • Fetal Proteins / metabolism
  • HSP90 Heat-Shock Proteins / metabolism
  • Half-Life
  • Hepatocytes / metabolism
  • Isopropyl Thiogalactoside / pharmacology
  • Lac Operon / drug effects
  • Liver / embryology
  • Liver / metabolism*
  • Macromolecular Substances
  • Mice
  • Mice, Knockout
  • Molecular Chaperones / metabolism
  • Multienzyme Complexes / metabolism*
  • Proteasome Endopeptidase Complex
  • Proto-Oncogene Proteins*
  • RNA, Messenger / biosynthesis
  • Recombinant Fusion Proteins / metabolism
  • Transfection
  • Ubiquitin / metabolism


  • CCAAT-Enhancer-Binding Protein-alpha
  • Cdc37 protein, mouse
  • Cell Cycle Proteins
  • Drosophila Proteins
  • Fetal Proteins
  • HSP90 Heat-Shock Proteins
  • Macromolecular Substances
  • Molecular Chaperones
  • Multienzyme Complexes
  • Proto-Oncogene Proteins
  • RNA, Messenger
  • Recombinant Fusion Proteins
  • Ubiquitin
  • Isopropyl Thiogalactoside
  • Cdk4 protein, mouse
  • Cyclin-Dependent Kinase 4
  • Cyclin-Dependent Kinases
  • Cysteine Endopeptidases
  • Proteasome Endopeptidase Complex