title: Loss of p21 CDKN1A impairs entry to quiescence and activates a DNA damage response in normal fibroblasts induced to quiescence.
abstract: The cell cycle inhibitor p21(CDKN1A) induces cell cycle arrest under different conditions, including senescence and terminal differentiation. Still debated is its involvement in the reversible transition from proliferation to a non-dividing quiescent state (G(0)), in which a significant role has been attributed to cell cycle inhibitor p27(CDKN1B). Here we provide evidence showing that high p21 protein levels are necessary to enter and maintain the quiescence state following contact inhibition and growth factor withdrawal. In fact, entry into quiescence was impaired, both in human fibroblasts in which p21 gene has been deleted, or protein expression knocked-down by RNA interference. Importantly, in the absence of p21, human fibroblasts activate a DNA damage-like signalling pathway, as shown by phosphorylation of histone H2AX and Chk1 proteins. In addition, we show that in the absence of p21, checkpoint is activated by an unscheduled entry into S phase, with a reduced efficiency in DNA maturation, in the presence of high c-myc protein levels. These results highlight the role of p21 in counteracting inappropriate proliferation stimuli for genome stability maintenance.
authors.affiliation: Dipartimento di Medicina Sperimentale, Sezione Patologia Generale C. Golgi, Piazza Botta 10, Pavia, Italy.
authors.forename: Paola
authors.initials: P
authors.lastname: Perucca
authors.forename: Ornella
authors.initials: O
authors.lastname: Cazzalini
authors.forename: Mark
authors.initials: M
authors.lastname: Madine
authors.forename: Monica
authors.initials: M
authors.lastname: Savio
authors.forename: Ronal Alfred
authors.initials: RA
authors.lastname: Laskey
authors.forename: Vanio
authors.initials: V
authors.lastname: Vannini
authors.forename: Ennio
authors.initials: E
authors.lastname: Prosperi
authors.forename: Lucia Anna
authors.initials: LA
authors.lastname: Stivala
mesh_terms: D002453:Cell Cycle; D018797:Cell Cycle Proteins / Q000378:metabolism; D019925:Cyclin A / Q000378:metabolism; D019927:Cyclin E / Q000378:metabolism; D051357:Cyclin-Dependent Kinase 2 / Q000378:metabolism; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000172:deficiency* / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000378:metabolism; D004247:DNA / Q000096:biosynthesis; D004249:DNA Damage; D004261:DNA Replication; D005347:Fibroblasts / Q000166:cytology* / Q000201:enzymology; D006801:Humans; D064111:Minichromosome Maintenance Complex Component 2; D009687:Nuclear Proteins / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000378:metabolism; D044967:Serum; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism; D015854:Up-Regulation
chemical_list: D018797:Cell Cycle Proteins; D019925:Cyclin A; D019927:Cyclin E; D050759:Cyclin-Dependent Kinase Inhibitor p21; D009687:Nuclear Proteins; D016271:Proto-Oncogene Proteins c-myc; D016159:Tumor Suppressor Protein p53; D050760:Cyclin-Dependent Kinase Inhibitor p27; D004247:DNA; D051357:Cyclin-Dependent Kinase 2; C084859:MCM2 protein, human; D064111:Minichromosome Maintenance Complex Component 2