title: Bimodal regulation of FoxO3 by AKT and 14-3-3.
abstract: FoxO3 is a member of FoxO family transcription factors that mediate cellular functions downstream of AKT. FoxO3 phosphorylation by AKT generates binding sites for 14-3-3, which in-turn regulates FoxO3 transcriptional activity and localization. We examine here the functional significance of AKT-FoxO3 interaction and further detail the mechanistic aspects of FoxO3 regulation by AKT and 14-3-3. Our data show that AKT overexpression increases the steady-state levels of FoxO3 protein in a manner dependent on AKT activity and its ability to bind FoxO3. Characterization of the AKT-FoxO3 interaction shows that the three AKT phosphorylation-site-recognition motifs (RxRxxS/T) present on FoxO3, which are required for FoxO3 phosphorylation, are dispensable for AKT binding, suggesting that AKT has a docking point on FoxO3 distinct from the phosphorylation-recognition motifs. Development of a FoxO3 mutant deficient in 14-3-3 binding (P34A), which can be phosphorylated by AKT, established that 14-3-3 binding and not AKT phosphorylation per se controls FoxO3 transcriptional activity. Intriguingly, 14-3-3 binding was found to stabilize FoxO3 by inhibiting its dephosphorylation and degradation rates. Collectively, our data support a model where both AKT and 14-3-3 positively regulate FoxO3 in addition to their established negative roles and that 14-3-3 availability could dictate the fate of phosphorylated FoxO3 toward degradation or recycling.
authors.affiliation: Karmanos Cancer Institute and Department of Pathology, Wayne State University, Detroit, MI 48201, USA.
authors.forename: Melissa
authors.initials: M
authors.lastname: Dobson
authors.forename: Gopalakrishnan
authors.initials: G
authors.lastname: Ramakrishnan
authors.forename: Stephanie
authors.initials: S
authors.lastname: Ma
authors.forename: Ludmila
authors.initials: L
authors.lastname: Kaplun
authors.forename: Vitaly
authors.initials: V
authors.lastname: Balan
authors.forename: Rafael
authors.initials: R
authors.lastname: Fridman
authors.forename: Guri
authors.initials: G
authors.lastname: Tzivion
mesh_terms: D048948:14-3-3 Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D002460:Cell Line; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D056945:Hep G2 Cells; D006801:Humans; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D054730:Protein Interaction Domains and Motifs; D051057:Proto-Oncogene Proteins c-akt / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism
chemical_list: D048948:14-3-3 Proteins; C499517:FOXO3 protein, human; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D011994:Recombinant Proteins; D051057:Proto-Oncogene Proteins c-akt