title: A role for a p21-E2F interaction during senescence arrest of normal human fibroblasts.
abstract: The family of E2F transcription factors forms different multiprotein complexes with cell cycle regulatory proteins to control the expression of genes important in cell proliferation. In this study, we identified four distinct E2F complexes present in aged and senescent normal, human diploid fibroblasts. Two appeared to be identical to the previously described G1-specific p130 and Rb-E2F complexes present in young G0-arrested cells. The other two were novel E2F complexes that contained the cyclin-dependent kinase inhibitor p21 (cip1/WAF1/Sdi1/CAP20/PIC1) complexed with Rb/CDK2/cyclin E or with the Rb-related p107/CDK2/ cyclin D. These p21-E2F complexes, while present in young G1 cells at very low levels, were elevated in senescent cells. The p21 containing E2F complexes were not detected during the S-phase in young cells. The DNA-binding stability of the p21 complexes was approximately 10 times greater than the stability of any other E2F complex or uncomplexed E2F. Addition of purified p21 protein to the S-phase-specific cyclin A/ CDK2-p107-E2F complex from young cells dissociated cyclin A and CDK2 from p107/E2F, suggesting an additional novel function for p21. Finally, expression of p21 specifically inhibited transcription from an E2F-dependent promoter but had no effect on a mutant E2 promoter. In addition to its inhibition of CDK enzymes and proliferating cell nuclear antigen function in DNA replication, these studies reveal a novel mechanism by which p21 mediates growth arrest: direct interaction with E2F complexes and negative regulation of E2F transcription factor activity.
authors.affiliation: Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710, USA.
authors.forename: C A
authors.initials: CA
authors.lastname: Afshari
authors.forename: M A
authors.initials: MA
authors.lastname: Nichols
authors.forename: Y
authors.initials: Y
authors.lastname: Xiong
authors.forename: M
authors.initials: M
authors.lastname: Mudryj
mesh_terms: D042846:CDC2-CDC28 Kinases; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D016922:Cellular Senescence / Q000502:physiology*; D051357:Cyclin-Dependent Kinase 2; D050759:Cyclin-Dependent Kinase Inhibitor p21; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D016213:Cyclins / Q000378:metabolism / Q000502:physiology*; D004268:DNA-Binding Proteins / Q000502:physiology; D050684:E2F Transcription Factors; D005347:Fibroblasts / Q000378:metabolism*; D016193:G1 Phase; D005786:Gene Expression Regulation; D006801:Humans; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D056465:Retinoblastoma-Binding Protein 1; D016196:S Phase; D050680:Transcription Factor DP1; D014157:Transcription Factors / Q000502:physiology*; D014158:Transcription, Genetic; D014162:Transfection
chemical_list: C493956:CDKN1A protein, human; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004268:DNA-Binding Proteins; D050684:E2F Transcription Factors; D056465:Retinoblastoma-Binding Protein 1; D050680:Transcription Factor DP1; D014157:Transcription Factors; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases