title: C/EBPalpha triggers proteasome-dependent degradation of cdk4 during growth arrest.
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.
authors.affiliation: Department of Pathology and Huffington Center on Aging, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX 77030, USA.
authors.forename: Hongmei
authors.initials: H
authors.lastname: Wang
authors.forename: Triona
authors.initials: T
authors.lastname: Goode
authors.forename: Polina
authors.initials: P
authors.lastname: Iakova
authors.forename: Jeffrey H
authors.initials: JH
authors.lastname: Albrecht
authors.forename: Nikolai A
authors.initials: NA
authors.lastname: Timchenko
mesh_terms: D000818:Animals; D000831:Animals, Newborn; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000172:deficiency / Q000235:genetics / Q000502:physiology*; D019556:COS Cells; D002453:Cell Cycle / Q000502:physiology*; D018797:Cell Cycle Proteins / Q000378:metabolism; D002478:Cells, Cultured / Q000378:metabolism; D002522:Chlorocebus aethiops; D002850:Chromatography, Gel; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases / Q000235:genetics / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D029721:Drosophila Proteins; D005326:Fetal Proteins / Q000378:metabolism; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism; D006207:Half-Life; D022781:Hepatocytes / Q000378:metabolism; D007544:Isopropyl Thiogalactoside / Q000494:pharmacology; D007763:Lac Operon / Q000187:drug effects; D008099:Liver / Q000196:embryology / Q000378:metabolism*; D046911:Macromolecular Substances; D051379:Mice; D018345:Mice, Knockout; D018832:Molecular Chaperones / Q000378:metabolism; D009097:Multienzyme Complexes / Q000378:metabolism*; D046988:Proteasome Endopeptidase Complex; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger / Q000096:biosynthesis; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism
chemical_list: D022763:CCAAT-Enhancer-Binding Protein-alpha; C513271:Cdc37 protein, mouse; D018797:Cell Cycle Proteins; D029721:Drosophila Proteins; D005326:Fetal Proteins; D018841:HSP90 Heat-Shock Proteins; D046911:Macromolecular Substances; D018832:Molecular Chaperones; D009097:Multienzyme Complexes; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D025801:Ubiquitin; D007544:Isopropyl Thiogalactoside; C495902:Cdk4 protein, mouse; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex