title: Nucleophosmin (B23) targets ARF to nucleoli and inhibits its function.
abstract: The ARF tumor suppressor is a nucleolar protein that activates p53-dependent checkpoints by binding Mdm2, a p53 antagonist. Despite persuasive evidence that ARF can bind and inactivate Mdm2 in the nucleoplasm, the prevailing view is that ARF exerts its growth-inhibitory activities from within the nucleolus. We suggest ARF primarily functions outside the nucleolus and provide evidence that it is sequestered and held inactive in that compartment by a nucleolar phosphoprotein, nucleophosmin (NPM). Most cellular ARF is bound to NPM regardless of whether cells are proliferating or growth arrested, indicating that ARF-NPM association does not correlate with growth suppression. Notably, ARF binds NPM through the same domains that mediate nucleolar localization and Mdm2 binding, suggesting that NPM could control ARF localization and compete with Mdm2 for ARF association. Indeed, NPM knockdown markedly enhanced ARF-Mdm2 association and diminished ARF nucleolar localization. Those events correlated with greater ARF-mediated growth suppression and p53 activation. Conversely, NPM overexpression antagonized ARF function while increasing its nucleolar localization. These data suggest that NPM inhibits ARF's p53-dependent activity by targeting it to nucleoli and impairing ARF-Mdm2 association.
authors.affiliation: Department of Pharmacology, The University of Iowa, College of Medicine, Iowa City, IA 52242, USA.
authors.forename: Chandrashekhar
authors.initials: C
authors.lastname: Korgaonkar
authors.forename: Jussara
authors.initials: J
authors.lastname: Hagen
authors.forename: Van
authors.initials: V
authors.lastname: Tompkins
authors.forename: April A
authors.initials: AA
authors.lastname: Frazier
authors.forename: Chantal
authors.initials: C
authors.lastname: Allamargot
authors.forename: Frederick W
authors.initials: FW
authors.lastname: Quelle
authors.forename: Dawn E
authors.initials: DE
authors.lastname: Quelle
mesh_terms: D000818:Animals; D019556:COS Cells; D002466:Cell Nucleolus / Q000235:genetics / Q000378:metabolism*; D049109:Cell Proliferation; D002522:Chlorocebus aethiops; D019941:Cyclin-Dependent Kinase Inhibitor p16; D006801:Humans; D051379:Mice; D041681:NIH 3T3 Cells; D009687:Nuclear Proteins / Q000378:metabolism*; D000090243:Nucleophosmin; D011485:Protein Binding; D021381:Protein Transport / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000378:metabolism*; D051736:Proto-Oncogene Proteins c-mdm2; D014407:Tumor Cells, Cultured; D025803:Tumor Suppressor Protein p14ARF / Q000235:genetics / Q000378:metabolism*; D016159:Tumor Suppressor Protein p53 / Q000378:metabolism*
chemical_list: C417743:Cdkn2a protein, mouse; D019941:Cyclin-Dependent Kinase Inhibitor p16; C000716967:NPM1 protein, human; C000715570:Npm1 protein, mouse; D009687:Nuclear Proteins; D011518:Proto-Oncogene Proteins; D025803:Tumor Suppressor Protein p14ARF; D016159:Tumor Suppressor Protein p53; D000090243:Nucleophosmin; C497206:MDM2 protein, human; C497207:Mdm2 protein, mouse; D051736:Proto-Oncogene Proteins c-mdm2