Transcriptional activator that recognizes and binds to the DNA sequence 5'-[AG]TAAA[TC]A-3' and regulates different processes, such as apoptosis and autophagy (PubMed:10102273, PubMed:16751106, PubMed:21329882, PubMed:30513302). Acts as a positive regulator of autophagy in skeletal muscle: in starved cells, enters the nucleus following dephosphorylation and binds the promoters of autophagy genes, such as GABARAP1L, MAP1LC3B and ATG12, thereby activating their expression, resulting in proteolysis of skeletal muscle proteins (By similarity). Triggers apoptosis in the absence of survival factors, including neuronal cell death upon oxidative stress (PubMed:10102273, PubMed:16751106). Participates in post-transcriptional regulation of MYC: following phosphorylation by MAPKAPK5, promotes induction of miR-34b and miR-34c expression, 2 post-transcriptional regulators of MYC that bind to the 3'UTR of MYC transcript and prevent its translation (PubMed:21329882). In response to metabolic stress, translocates into the mitochondria where it promotes mtDNA transcription (PubMed:23283301). In response to metabolic stress, translocates into the mitochondria where it promotes mtDNA transcription. Also acts as a key regulator of chondrogenic commitment of skeletal progenitor cells in response to lipid availability: when lipids levels are low, translocates to the nucleus and promotes expression of SOX9, which induces chondrogenic commitment and suppresses fatty acid oxidation (By similarity). Also acts as a key regulator of regulatory T-cells (Treg) differentiation by activating expression of FOXP3 (PubMed:30513302). Upon metabolic stress, forms a complex composed of FOXO3, SIRT3 and mitochondrial RNA polymerase POLRMT; the complex is recruited to mtDNA in a SIRT3-dependent manner (PubMed:23283301). Also forms a complex composed of FOXO3, SIRT3, TFAM and POLRMT (PubMed:29445193). Interacts with SIRT2; the interaction occurs independently of SIRT2 deacetylase activity (By similarity). Interacts with YWHAB/14-3-3-beta and YWHAZ/14-3-3-zeta, which are required for cytosolic sequestration (PubMed:16751106). Upon oxidative stress, interacts with STK4/MST1, which disrupts interaction with YWHAB/14-3-3-beta and leads to nuclear translocation (PubMed:16751106). Interacts with PIM1 (PubMed:18593906). Interacts with DDIT3/CHOP (PubMed:22761832). Interacts (deacetylated form) with SKP2 (PubMed:21841822). Interacts with CHUK and IKBKB (PubMed:15084260, PubMed:22313691). Interacts with CAMK2A, CAMK2B and calcineurin A (By similarity). Interacts with NUPR1; this interaction represses FOXO3 transactivation (PubMed:20181828). Retention in the cytoplasm contributes to its inactivation (PubMed:10102273, PubMed:15084260, PubMed:16751106). Translocates to the nucleus upon oxidative stress and in the absence of survival factors (PubMed:10102273, PubMed:16751106). Translocates from the cytosol to the nucleus following dephosphorylation in response to autophagy-inducing stimuli (By similarity). Translocates in a AMPK-dependent manner into the mitochondrion in response to metabolic stress (PubMed:23283301, PubMed:29445193). Serum deprivation increases localization to the nucleus, leading to activate expression of SOX9 and subsequent chondrogenesis (By similarity). Ubiquitous. In the presence of survival factors such as IGF-1, phosphorylated on Thr-32 and Ser-253 by AKT1/PKB (PubMed:10102273). This phosphorylated form then interacts with 14-3-3 proteins and is retained in the cytoplasm (PubMed:10102273). Survival factor withdrawal induces dephosphorylation and promotes translocation to the nucleus where the dephosphorylated protein induces transcription of target genes and triggers apoptosis (PubMed:10102273). Although AKT1/PKB doesn't appear to phosphorylate Ser-315 directly, it may activate other kinases that trigger phosphorylation at this residue (PubMed:10102273, PubMed:11154281). Phosphorylated by STK4/MST1 on Ser-209 upon oxidative stress, which leads to dissociation from YWHAB/14-3-3-beta and nuclear translocation (PubMed:16751106). Phosphorylated by PIM1 (PubMed:18593906). Phosphorylation by AMPK leads to the activation of transcriptional activity without affecting subcellular localization (PubMed:17711846). In response to metabolic stress, phosphorylated by AMPK on Ser-30 which mediates FOXO3 mitochondrial translocation (PubMed:29445193). Phosphorylation by MAPKAPK5 promotes nuclear localization and DNA-binding, leading to induction of miR-34b and miR-34c expression, 2 post-transcriptional regulators of MYC that bind to the 3'UTR of MYC transcript and prevent its translation (PubMed:21329882). Phosphorylated by CHUK/IKKA and IKBKB/IKKB (PubMed:15084260). TNF-induced inactivation of FOXO3 requires its phosphorylation at Ser-644 by IKBKB/IKKB which promotes FOXO3 retention in the cytoplasm, polyubiquitination and ubiquitin-mediated proteasomal degradation (PubMed:15084260). May be dephosphorylated by calcineurin A on Ser-299 which abolishes FOXO3 transcriptional activity (By similarity). In cancer cells, ERK mediated-phosphorylation of Ser-12 is required for mitochondrial translocation of FOXO3 in response to metabolic stress or chemotherapeutic agents (PubMed:29445193). Phosphorylation at Ser-253 promotes its degradation by the proteasome (PubMed:30513302). Dephosphorylation at Ser-253 by protein phosphatase 2A (PPP2CA) promotes its stabilization; interaction with PPP2CA is enhanced by AMBRA1 (PubMed:30513302). Deacetylation by SIRT1 or SIRT2 stimulates interaction of FOXO3 with SKP2 and facilitates SCF(SKP2)-mediated FOXO3 ubiquitination and proteasomal degradation (PubMed:21841822). Deacetylation by SIRT2 stimulates FOXO3-mediated transcriptional activity in response to oxidative stress (By similarity). Deacetylated by SIRT3 (PubMed:23283301). Deacetylation by SIRT3 stimulates FOXO3-mediated mtDNA transcriptional activity in response to metabolic stress (PubMed:23283301). Heavily methylated by SET9 which decreases stability, while moderately increasing transcriptional activity. The main methylation site is Lys-271. Methylation doesn't affect subcellular location. Polyubiquitinated. Ubiquitinated by a SCF complex containing SKP2, leading to proteasomal degradation. The N-terminus is cleaved following import into the mitochondrion. A chromosomal aberration involving FOXO3 is found in secondary acute leukemias. Translocation t(6;11)(q21;q23) with KMT2A/MLL1.