FOXO4 is a member of the forkhead family transciption factors O subclass, which is characterized by a winged helix domain used for DNA binding. There are 4 members of the FOXO family, including FOXO1, FOXO3, and FOXO6. Their activity is modified by many post translational activities, such as phosphorylation, ubiquitination, and acetylation. Depending on this modified state, FOXO4 binding affinity for DNA is altered, allowing for FOXO4 to regulate many cellular pathways including oxidative stress signaling, longevity, insulin signaling, cell cycle progression, and apoptosis. Two of the main upstream regulators of FOXO4 activity are phosphoinositide 3- kinase (PI3K) and serine/threonine kinase AKT/PKB. Both PI3K and AKT modify FOXO4 and prevent it from translocating to the nucleus, effectively preventing the transcription of the downstream FOXO targets.
FOXO transcription factors have been shown to be the down downstream effector molecules of insulin-like growth factor (IGF) signaling pathway. In the absence of insulin, PI3K is inactive, so the FOXO homologdaf-16 is able to translocate to the nucleus and turn on many genetic pathways associated with longevity in the roundworm Caenorhabditis elegans. FOXO’s ability to restrict this pathway produces an increase in lifespan for worms, flies, mice; similar variants of FOXO3a have been associated with longer human lives as well.
Many different kinds of cancers have been observed to contain mutations that promote AKT phosphorylation, and thus the inactivation of FOXOs, effectively preventing proper cell cycle regulation. FOXO4 activates the cell cycle dependent kinase inhibitor, P27, which in turn prevents tumors from progressing into G1. In HER-2 positive tumor cells, increasing FOXO4 activity reduces tumor size. Chromosomal translocations of FOXO4 have been shown to be a cause of acute leukemia. The fusion proteins formed by these translocations lack the DNA-binding domain, causing the protein to lose function.
In gastric cancers (GC), it has been observed that there were lower levels of FOXO4 mRNA in cancers that had already progressed to invading lymph nodes compared to cancers that remained in situ. When compared to normal tissue, all GC epithelia had lower levels of FOXO4 located in the nucleus, consistent with less FOXO4 effector activity and FOXO4’s function as a suppressor of carcinogenic properties. It does this by causing cell cycle arrest between the Go and S phases, preventing cell proliferation, as well as by inhibiting metastasis by downregulating vimentin. These results are consistent with FOXO4 providing a role in inhibiting the epithelia to mesenchymal transition (EMT).
In non-small cell lung carcinoma, there are varying levels of FOXO4 expressed that correspond to how the cancer was staged; worse cases had the lowest amount of FOXO4 while less severe cases had higher levels of FOXO4. As with gastric cancer, these cancers with the lowest levels of FOXO4 also had the lowest levels of E-cadherin and highest levels of vimentin, consistent with FOXO4 acting as a suppressor of the EMT phenotype.
^Parry P, Wei Y, Evans G (Feb 1995). "Cloning and characterization of the t(X;11) breakpoint from a leukemic cell line identify a new member of the forkhead gene family". Genes Chromosomes Cancer11 (2): 79–84. doi:10.1002/gcc.2870110203. PMID7529552.
^Boura E, Silhan J, Herman P, Vecer J, Sulc M, Teisinger J, Obsilova V, Obsil T (Mar 2007). "Both the N-terminal loop and wing W2 of the forkhead domain of transcription factor Foxo4 are important for DNA binding". The Journal of Biological Chemistry282 (11): 8265–8275. doi:10.1074/jbc.M605682200. PMID17244620.
^Tsai KL, Sun YJ, Huang CY, Yang JY, Hung MC, Hsiao CD (2007). "Crystal structure of the human FOXO3a-DBD/DNA complex suggests the effects of post-translational modification". Nucleic Acids Research35 (20): 6984–6994. doi:10.1093/nar/gkm703. PMID17940099.
^Brent MM, Anand R, Marmorstein R (Sep 2008). "Structural basis for DNA recognition by FoxO1 and its regulation by posttranslational modification". Structure16 (9): 1407–16. doi:10.1016/j.str.2008.06.013. PMID18786403.
^Li J, Yen C, Liaw D, Podsypanina K, Bose S, Wang SI, Puc J, Miliaresis C, Rodgers L, McCombie R, Bigner SH, Giovanella BC, Ittmann M, Tycko B, Hibshoosh H, Wigler MH, Parsons R (Mar 1997). "PTEN, a putative protein tyrosine phosphatase gene mutated in human brain, breast, and prostate cancer". Science275 (5308): 1943–1947. doi:10.1126/science.275.5308.1943. PMID9072974.
^Samuels Y, Wang Z, Bardelli A, Silliman N, Ptak J, Szabo S, Yan H, Gazdar A, Powell SM, Riggins GJ, Willson JK, Markowitz S, Kinzler KW, Vogelstein B, Velculescu VE (Apr 2004). "High frequency of mutations of the PIK3CA gene in human cancers". Science304 (5670): 554. doi:10.1126/science.1096502. PMID15016963.
^Saal LH, Holm K, Maurer M, Memeo L, Su T, Wang X, Yu JS, Malmström PO, Mansukhani M, Enoksson J, Hibshoosh H, Borg A, Parsons R (Apr 2005). "PIK3CA mutations correlate with hormone receptors, node metastasis, and ERBB2, and are mutually exclusive with PTEN loss in human breast carcinoma". Cancer Research65 (7): 2554– 2559. doi:10.1158/0008-5472-CAN-04-3913. PMID15805248.
^ abYang H, Zhao R, Yang HY, Lee MH (Mar 2005). "Constitutively active FOXO4 inhibits Akt activity, regulates p27 Kip1 stability, and suppresses HER2-mediated tumorigenicity". Oncogene24 (11): 1924–35. doi:10.1038/sj.onc.1208352. PMID15688030.
^ abPaik JH, Kollipara R, Chu G, Ji H, Xiao Y, Ding Z, Miao L, Tothova Z, Horner JW, Carrasco DR, Jiang S, Gilliland DG, Chin L, Wong WH, Castrillon DH, DePinho RA (Jan 2007). "FoxOs are lineage-restricted redundant tumor suppressors and regulate endothelial cell homeostasis". Cell128 (2): 309–323. doi:10.1016/j.cell.2006.12.029. PMID17254969.
^Liu X, Zhang Z, Sun L, Chai N, Tang S, Jin J, Hu H, Nie Y, Wang X, Wu K, Jin H, Fan D (Dec 2011). "MicroRNA-499-5p promotes cellular invasion and tumor metastasis in colorectal cancer by targeting FOXO4 and PDCD4". Carcinogenesis32 (12): 1798–1805. doi:10.1093/carcin/bgr213. PMID21934092.
^Su L, Liu X, Chai N, Lv L, Wang R, Li X, Nie Y, Shi Y, Fan D (2014). "The transcription factor FOXO4 is down-regulated and inhibits tumor proliferation and metastasis in gastric cancer". BMC Cancer14: 378. doi:10.1186/1471-2407-14-378. PMID24886657.
^ abXu MM, Mao GX, Liu J, Li JC, Huang H, Liu YF, Liu JH (2014). "Low expression of the FoxO4 gene may contribute to the phenomenon of EMT in non-small cell lung cancer". Asian Pacific Journal of Cancer Prevention15 (9): 4013–4018. doi:10.7314/apjcp.2014.15.9.4013. PMID24935588.
^Brenkman AB, de Keizer PL, van den Broek NJ, van der Groep P, van Diest PJ, van der Horst A, Smits AM, Burgering BM (Sep 2008). "The peptidyl-isomerase Pin1 regulates p27kip1 expression through inhibition of Forkhead box O tumor suppressors". Cancer Res.68 (18): 7597–605. doi:10.1158/0008-5472.CAN-08-1059. PMID18794148.
Borkhardt A, Repp R, Haas OA, Leis T, Harbott J, Kreuder J, Hammermann J, Henn T, Lampert F (1997). "Cloning and characterization of AFX, the gene that fuses to MLL in acute leukemias with a t(X;11)(q13;q23)". Oncogene14 (2): 195–202. doi:10.1038/sj.onc.1200814. PMID9010221.
Peters U, Haberhausen G, Kostrzewa M, Nolte D, Müller U (1997). "AFX1 and p54nrb: fine mapping, genomic structure, and exclusion as candidate genes of X-linked dystonia parkinsonism". Hum. Genet.100 (5-6): 569–72. doi:10.1007/s004390050553. PMID9341872.
Kops GJ, de Ruiter ND, De Vries-Smits AM, Powell DR, Bos JL, Burgering BM (1999). "Direct control of the Forkhead transcription factor AFX by protein kinase B". Nature398 (6728): 630–4. doi:10.1038/19328. PMID10217147.
Weigelt J, Climent I, Dahlman-Wright K, Wikström M (2000). "1H, 13C and 15N resonance assignments of the DNA binding domain of the human forkhead transcription factor AFX". J. Biomol. NMR17 (2): 181–2. doi:10.1023/A:1008358816478. PMID10921784.
Weigelt J, Climent I, Dahlman-Wright K, Wikström M (2001). "Solution structure of the DNA binding domain of the human forkhead transcription factor AFX (FOXO4)". Biochemistry40 (20): 5861–9. doi:10.1021/bi001663w. PMID11352721.
Schuur ER, Loktev AV, Sharma M, Sun Z, Roth RA, Weigel RJ (2001). "Ligand-dependent interaction of estrogen receptor-alpha with members of the forkhead transcription factor family". J. Biol. Chem.276 (36): 33554–60. doi:10.1074/jbc.M105555200. PMID11435445.
Tang TT, Dowbenko D, Jackson A, Toney L, Lewin DA, Dent AL, Lasky LA (2002). "The forkhead transcription factor AFX activates apoptosis by induction of the BCL-6 transcriptional repressor". J. Biol. Chem.277 (16): 14255–65. doi:10.1074/jbc.M110901200. PMID11777915.
Yang Z, Whelan J, Babb R, Bowen BR (2002). "An mRNA splice variant of the AFX gene with altered transcriptional activity". J. Biol. Chem.277 (10): 8068–75. doi:10.1074/jbc.M106091200. PMID11779849.
Tang TT, Lasky LA (2003). "The forkhead transcription factor FOXO4 induces the down-regulation of hypoxia-inducible factor 1 alpha by a von Hippel-Lindau protein-independent mechanism". J. Biol. Chem.278 (32): 30125–35. doi:10.1074/jbc.M302042200. PMID12761217.
Crossley LJ (2003). "Neutrophil activation by fMLP regulates FOXO (forkhead) transcription factors by multiple pathways, one of which includes the binding of FOXO to the survival factor Mcl-1". J. Leukoc. Biol.74 (4): 583–92. doi:10.1189/jlb.0103020. PMID12960271.