DISCOVERIES (2013, Oct-Dec), 1(1): e5 DOI: 10.15190/d.2013.5 FOXO proteins control critical cellular decisions and cellular fate REVIEW Article Critical physiological and pathological functions of Forkhead Box O tumor suppressors Georgiana R. Dumitrascu1 and Octavian Bucur2,* 1“Victor Babes” National Institute of Pathology and Biomedical Sciences, Bucharest, Romania; 2Department of Pathology, Harvard Medical School and Beth Israel Deaconess Medical Center, Boston, MA, USA; Correspondence to: Dr. Octavian Bucur, Department of Pathology, Harvard Medical School and Beth Israel Deaconess Medical Center, 330 Brookline Ave, Boston, MA, 02215, USA. Phone: +1 617 667 4314; E-mail: [email protected]. Citation: Dumitrascu GR, Bucur O. Critical physiological and pathological functions of Forkhead Box O tumor suppressors. Discoveries 2013, Oct-Dec; 1(1): e5. DOI: 10.15190/d.2013.5 ABSTRACT death (Puma); Bcl-2/adenovirus E1B 19 kDa-interacting The Forkhead box, subclass O (FOXO) proteins protein (bNIP3); PTEN – induced kinase 1 (Pink1); are critical transcription factors, ubiquitously peroxisome proliferative activated receptor-gama expressed in the human body. These proteins are (PGC1); glucose-6-phosphatase (G6Pase); phosphor- characterized by a remarkable functional diversity, enol-pyruvate carboxykinase (PEPCK); Agouti-related protein (Agrp); neuropeptide Y (NpY); pro-opiomelan- being involved in cell cycle arrest, apoptosis, ocortin (POMC); glycogenolytic gene glucose-6- oxidative detoxification, DNA damage repair, stem phosphatase (G6pc); manganese superoxide dismutase cell maintenance, cell differentiation, cell (MnSOD); CBP/p300-interacting trans-activator with metabolism, angiogenesis, cardiac development, ED-rich tail 2 (Cited 2); mixed lineage leukemia aging and others. In addition, FOXO have critical (MLL); L-selectin (CD62L); activation-induced cytidine implications in both normal and cancer stem cell deaminase (AID); biology. New strategies to modulate FOXO expression and activity may now be developed SUMMARY 1. Introduction since the discovery of novel FOXO regulators and 2. Structure of FOXO family members non-coding RNAs (such as microRNAs) targeting 3. Physiological functions of FOXO proteins FOXO transcription factors. This review focuses 4. Implications of FOXO proteins in pathological on physiological and pathological functions of processes and associated diseases FOXO proteins and on their action as fine 5. Critical functions of FOXO transcription regulators of cell fate and context-dependent cell factors in stem cells and their role in cancer decisions. A better understanding of the structure stem cells and critical functions of FOXO transcription 6. Expression of FOXO family members in factors and tumor suppressors may contribute to various tissues the development of novel therapies for cancer and 7. FOXOs and microRNAs other diseases. 8. Conclusions and future perspectives Keywords: FOXO, FOX, apoptosis, cancer, stem 1. Introduction cells, cancer stem cells, microRNAs The Forkhead box (FOX) proteins represent a wide Abbreviations family of transcription factors that display an B-cell lymphoma 2 (Bcl-2); bcl-2 interacting mediator extraordinary functional diversity, regulating a of cell death (Bim); p53 upregulated modulator of cell variety of critical biological processes1,2. FOX www.discoveriesjournals.org/discoveries 1 FOXO proteins control critical cellular decisions and cellular fate FOXO proteins are characterized by a changed the initial terms for the forkhead proteins (e.g. FREAC - Forkhead RElated Activator; fkh - remarkable functional diversity, controlling Drosophila gene fork head) 7. Currently, based on many critical cellular processes: phylogenetic analysis, FOXO genes are classified - cycle arrest, apoptosis, oxidative detoxification, in subclasses that range from FOXA to FOXS to DNA damage repair, stem cell maintenance, cell yield 23 subclasses in total10, 11, 12, 13. Each differentiation, cell metabolism, angiogenesis, subclass has several members noted with an cardiac development, fertility, immune response, arabic number. While for human forkhead neuronal survival, aging and others. proteins abbreviations have all letters in uppercase (e.g., FOXD3), for mouse only the first proteins are well known to control the following letter is capitalized (e.g., Foxd3) and for all other physiological procceses: apoptosis, cell-cycle, chordates the first and subclass letter are in cellular metabolism, immune response, uppercase (e.g., FoxD3)7. differentiation, development (such as cardiac One of the largest and the most important development) or aging. FOX proteins are also subclass of FOX family is represented by FOXO involved in various pathologies, such as cancer, (Forkhead box, subclass O) transcription factors. diabetes and neurodegenerative diseases3, 4. FOXO transcription factors (FOXOs) are The first forkhead transcription factor was first characterized by the same conserved DNA binding identified in 1989, and its function was related to domain that define the family of forkhead the development of the anterior and posterior gut proteins1. However, FOXOs share an additional of the Drosophila embryo. The “forkhead” name unique 5 amino acid sequence insertion within the was given because of the two spiked-head DNA binding domain that is not present in other structures found in the embryos of the Drosophila forkhead proteins14. Melanogaster forkhead mutant presenting Only one FOXO transcription factor is known modifications of the gut formation5. One year later, in invertebrates (named abnormal dauer formation the sequence comparison between forkhead and protein 16/DAF-16 in the nematode worm mammalian hepatocyte-enriched transcription Caenorhabditis elegans and dFOXO in the fruit fly factor HNF-3A showed a conserved 110-amino- Drosophila melanogaster), while in mammals four acid DNA binding domain, bringing evidence that FOXO proteins encoded by four different genes forkhead proteins are a new class of transcription were identified: FOXO1, FOXO3, FOXO4 and factors6. FOXO615. Hence, forkhead transcription factors are Initially, FOXO1 transcription factor defined by their winged-helix DNA binding (previously known as FKHR-forkhead in domain, a conserved structure called „Forkhead rhabdomyosarcoma) was identified through its box”, the symbol FOX being assigned to all involvement in chromosomal translocations t(2;13) vertebrate forkhead transcription factors, according and t(1;13) in alveolar rhabdomyosarcoma tumors to the revised nomenclature7. due to PAX3/7-FKHR fusion transcript16,17. A few Since its discovery, numerous forkhead genes years later, FOXO3 (previously known as have been identified in a broad range of organisms, FKHRL1 - forkhead in rhabdomyosarcoma like from yeast and worms to humans 2. Interestingly, protein 1) was characterized and named, based on up to now, FOX genes have not been identified in similarities to FKHR18. The gene for FOXO4 was plants8. However, an update published in 2010 described as being fused to MLL transcription reported that are 50 FOX genes identified and factor as a result of the t(X; 11) chromosomal classified in the human genome and 44 in the translocation in acute lymphoblastic leukemia, mouse genome9. Therefore, a standard therefore, the initial term for FOXO4 was AFX nomenclature system was required for this (The acute leukemia fusion gene located on extended number of discovered factors. In 2000, chromosome X)19. Daniel E. Martinez and his team established Fox FOXO proteins are considered unique cellular Nomenclature Committee, the first step towards an targets, regulating a wide variety of critical cellular unified nomenclature for the winged-helix / processes, such as: apoptosis, oxidative stress, forkhead transcription factors. The Committee has DNA damage repair, cell cycle, stem cell www.discoveriesjournals.org/discoveries 2 FOXO proteins control critical cellular decisions and cellular fate proliferation and maintenance, metabolism, binding domain among all the members of the angiogenesis, vascular tone, cardiovascular broader group of forkhead transcription factors, but development, fertility, immune response and also among species, in eukaryotic organisms, from neuronal survival4,20. The aim of this review is to yeast to humans3,27. Moreover, several studies point discuss the most recent advances in elucidating the out that forkhead DBD is not the only region in the functions, structure and transcriptional regulation FOXO molecule that is highly conserved. of Forkhead Box O transcription factors, both in Similarities were also observed in the N-terminal physiological and pathological conditions. The region near the first AKT/protein kinase B (PKB) advances in understating the mechanism of FOXOs phosphorylation site (Thr24 for FOXO1 and Thr32 regulation of stem cells, cancer stem cells and how for FOXO3), the region containing NLS, and non-coding RNAs are regulated and regulate the sequences from C-terminal transactivation 25 function of FOXO genes/protein are presented. domain . As mentioned above, the forkhead domains in 2. Structure of FOXO family members FOXO subclass of FOX family are similar to the forkhead regions of other subclasses. For example, In humans, the primary structure of the FOXO structural similarities have been proven in the core proteins is characterized by a length of region of FOXO3 compared to FOXA3, FOXK1 approximately 655-675 amino acid residues (aa) and FOXP228. The forkhead/winged helix motif is for FOXO1 (655