title: Cyclin E-cdk2 phosphorylation promotes late G1-phase degradation of MyoD in muscle cells.
abstract: Proliferating myoblasts already express MyoD before the induction of differentiation. Overexpression of MyoD in normal and transformed cell lines was shown to block cells from entering S phase, suggesting that the MyoD growth suppressive effect must be tightly controlled in growing myoblasts. Here we show that during G1 phase, but not in G2, MyoD abundance is down-regulated by the ubiquitin-proteasome pathway through phosphorylation of serine 200. Roscovitine, a specific inhibitor of cyclin-Cdk2 complexes, prevents both phosphorylation and degradation of MyoD in G1. Inhibition of the ubiquitin-dependent proteasome pathway by MG132 results in stabilization of MyoD-wt, with little effect on a MyoD mutant where serine 200 is replaced by an alanine. Our results show that MyoD Ser200 is the substrate for phosphorylation by cyclin E-Cdk2 stimulating its degradation by the ubiquitin-proteasome system which controls MyoD levels in G1. Phosphorylation/degradation of MyoD at the end of G1 thus represents the regulatory checkpoint in growing myoblasts allowing progression into S phase in a manner similar to the recently examplified cdk2-phosphorylation/degradation of p27(Kip1).
authors.affiliation: Laboratoire de Génétique Oncologique, Institut Gustave Roussy, Villejuif, 94805, France.
authors.forename: L A
authors.initials: LA
authors.lastname: Tintignac
authors.forename: M P
authors.initials: MP
authors.lastname: Leibovitch
authors.forename: M
authors.initials: M
authors.lastname: Kitzmann
authors.forename: A
authors.initials: A
authors.lastname: Fernandez
authors.forename: B
authors.initials: B
authors.lastname: Ducommun
authors.forename: L
authors.initials: L
authors.lastname: Meijer
authors.forename: S A
authors.initials: SA
authors.lastname: Leibovitch
mesh_terms: D000818:Animals; D042846:CDC2-CDC28 Kinases; D019927:Cyclin E / Q000378:metabolism*; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D003546:Cysteine Endopeptidases / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000166:cytology; D016193:G1 Phase / Q000187:drug effects / Q000502:physiology*; D051379:Mice; D009097:Multienzyme Complexes / Q000378:metabolism; D018485:Muscle Fibers, Skeletal / Q000166:cytology* / Q000201:enzymology; D018482:Muscle, Skeletal / Q000166:cytology; D017570:MyoD Protein / Q000378:metabolism*; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D011687:Purines / Q000494:pharmacology; D000077546:Roscovitine; D016196:S Phase / Q000187:drug effects / Q000502:physiology; D012694:Serine; D013379:Substrate Specificity; D014452:Ubiquitins / Q000378:metabolism
chemical_list: D019927:Cyclin E; D004791:Enzyme Inhibitors; D009097:Multienzyme Complexes; D017570:MyoD Protein; D011687:Purines; D014452:Ubiquitins; D000077546:Roscovitine; D012694:Serine; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495893:Cdk2 protein, mouse; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex