Abstract
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
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Research Support, U.S. Gov't, P.H.S.
MeSH terms
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Animals
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Animals, Newborn
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CCAAT-Enhancer-Binding Protein-alpha / deficiency
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CCAAT-Enhancer-Binding Protein-alpha / genetics
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CCAAT-Enhancer-Binding Protein-alpha / physiology*
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COS Cells
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Cell Cycle / physiology*
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Cell Cycle Proteins / metabolism
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Cells, Cultured / metabolism
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Chlorocebus aethiops
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Chromatography, Gel
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Cyclin-Dependent Kinase 4
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Cyclin-Dependent Kinases / genetics
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Cyclin-Dependent Kinases / metabolism*
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Cysteine Endopeptidases / metabolism*
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Drosophila Proteins*
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Fetal Proteins / metabolism
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HSP90 Heat-Shock Proteins / metabolism
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Half-Life
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Hepatocytes / metabolism
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Isopropyl Thiogalactoside / pharmacology
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Lac Operon / drug effects
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Liver / embryology
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Liver / metabolism*
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Macromolecular Substances
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Mice
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Mice, Knockout
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Molecular Chaperones / metabolism
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Multienzyme Complexes / metabolism*
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Proteasome Endopeptidase Complex
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Proto-Oncogene Proteins*
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RNA, Messenger / biosynthesis
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Recombinant Fusion Proteins / metabolism
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Transfection
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Ubiquitin / metabolism
Substances
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CCAAT-Enhancer-Binding Protein-alpha
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Cdc37 protein, mouse
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Cell Cycle Proteins
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Drosophila Proteins
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Fetal Proteins
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HSP90 Heat-Shock Proteins
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Macromolecular Substances
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Molecular Chaperones
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Multienzyme Complexes
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Proto-Oncogene Proteins
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RNA, Messenger
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Recombinant Fusion Proteins
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Ubiquitin
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Isopropyl Thiogalactoside
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Cdk4 protein, mouse
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Cyclin-Dependent Kinase 4
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Cyclin-Dependent Kinases
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Cysteine Endopeptidases
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Proteasome Endopeptidase Complex