FOX (forkhead box)proteins are a family oftranscription factors that play important roles in regulating the expression ofgenes involved incell growth, proliferation,differentiation, andlongevity. Many FOXproteins are important to embryonic development.[1][2] FOX proteins also havepioneering transcription activity by being able to bind condensedchromatin during cell differentiation processes.[3]
There are 50 different FOX genes encoding FOX proteins in humans that are further divided into 19 subdivisions based on conserved sequence similarity.[4] The defining feature of FOX proteins is theforkhead box, a sequence of 80 to 100amino acids forming amotif that binds toDNA. This forkhead motif is also known as thewinged helix, due to the butterfly-like appearance of the loops in the protein structure of the domain.[5] FOX proteins are a subgroup of thehelix-turn-helix class of proteins.
FOX genes are key elements in many developmental andbiological processes, including regulating thecell cycle, metabolism,apoptosis, immune control, and pluripotency ofembryonic stem cells. Beginning inunicellulareukaryotes, FOX genes developed by means of duplication andevolutionary divergence to acquire specialized roles.[6] By binding to particularDNA sequences, these proteins controlgene expression and so affect cellular differentiation andorganogenesis.[4]
Many genes encoding FOX proteins have been identified. There are 50 FOX genes in humans, divided into 19 different subclasses from FOXA to FOXS, based on conserved sequences. These subdivisions have diverse functions across different tissues and biological processes and genes within a given subunit often exhibit functional similarities. For example, the FOXM genes encode proteins that are involved incell cycle progression.[7] FOXC genes encode proteins that ensure normal embryonic development and play a key role in the growth and function of different organs.[8]
FOX proteins play an important role in apoptosis and function astumor suppressors, removing damaged cells. This is done via a mitochondria-dependent pathway or a mitochondria-independent pathway. In the mitochondria-independent pathway, FOX proteins increase the expression of death receptor ligands such asFas ligand (FasL) andTNF-related apoptosis-inducing ligand (TRAIL). In the mitochondria-dependent pathway, FOX proteins activate pro-apoptoticBcl-2 family proteins.[9]
FOXM1 is a well-defined transcription factor controlling genes linked to cell cycle development, preserving cellularhomeostasis.[10] FOXM1 promotes the entry of acell into theS phase and ensures the cell undergoesmitosis properly. FOXM1 activity is regulated by proliferation and anti-proliferation signals.[11] FOXM1 is a highly expressed tumourrepressor in growing cells and contributes to tumorigenesis when dysregulated.[citation needed] Phosphorylation events regulate FOXM1 activity by influencing its localization and transcriptional action.[4]
TheFOXO1 gene is involved in maintaining the pluripotency ofembryonic stem cells. FOXO1 regulates critical pluripotency associated genes such asOCT4,NANOG andSOX2 (Oct4 and Sox2 areYamanaka factors) by occupying and activating their promoters. This function can be inhibited by the ATK protein kinase.[12] The FOXO genes also play a role in the regulation ofmetabolism. FOXO proteins translateinsulin and growth factor signaling into physiological responses, including suppressinggene expression. FOXO1 is involved in promoting gluconeogenesis in theliver by interacting withPGC-1α. This interaction can be inhibited by phosphorylation events, where FOXO1 is removed from the nucleus.[13]
Some FOX genes are downstream targets of thehedgehog signaling pathway, which plays a role in the development ofbasal cellcarcinomas. Members of class O (FOXO- proteins) regulate metabolism, cellular proliferation, stress tolerance and possibly lifespan. The activity of FoxO is controlled bypost-translational modifications, includingphosphorylation,acetylation andubiquitination.[14]
Control of FOX protein activity, localization, and stability depends critically onpost-translational modifications (PTMs). These modifications, including phosphorylation,methylation and acetylation, help FOX proteins respond to various cellular signals, thereby enabling them to mediate essential biological processes such asapoptosis, cell survival, and cell cycle progression.[10]
Among the primary PTMs influencing FOX proteins isphosphorylation. For instance, phosphorylation of FOXOproteins can drive their nuclear translocation in response to stress signals, which is necessary for starting apoptoticgene expression. This change allows FOXO proteins to mediate the stress reaction and control cell survival.[10] Additionally, under control by phosphorylation isFOXM1, a necessary component of cell cycle progression. Specifically, phosphorylation increases the transcriptional activity of FOXM1, so advancing cell cycle progression duringDNA replication andmitosis, which is a process essential for appropriate cellular growth.[4]
Particularly in relation to the development ofcancer and the way cells traverse their growth cycle,acetylation is particularly crucial in determining the function of FOXM1. Enzymes such asp300/CBP addacetyl groups to specific sites on the FOXM1 protein, thus this process occurs. Particularly, this happens at particular lysine residues, including K63, K422, K440, K603, and K614. FOXM1 is able to greatly increase its capacity to activate genes linked with DNA copying andcell division by means of acetylation. Remarkably, the degree of acetylation of FOXM1 varies during the cell cycle rather than being constant. It peaks in the S, G2, and M phases of thecell cycle—the times when cells are actively getting ready for division. During these phases, the acetylated form of FOXM1 can more readily attach to its target genes, helping the cell to move through the cell cycle. On the other hand, FOXM1 becomes less active in theG1 phase, and the degree of acetylation also falls here. This variation in acetylation serves as a timing mechanism to guarantee that FOXM1 only acts when the cell needs it. Regarding cancer, the stakes are even higher. Acetylation improves FOXM1's capacity to activate genes, helping cancercells grow, survive, and repair their DNA. When FOXM1 cannot be acetylated, as is the case whenmutations stop the process from happening, its capacity to activate genes reduces, as well as its capacity to cause the development of tumours. This is whyscientists are looking at several approaches to interfere with FOXM1's acetylation in order to either stop or slow down cancer's spread. The focus of this process could create fresh paths for the evolution of the next treatments.[15]
Another post translational modification of the FOX protein involves addingmethyl groups to specific amino acids.[16] These modifications play a crucial role inimmune response, cancer progression, and aging by altering FOX protein functions through protein-level changes.[16]
The founding member and namesake of the FOX family is thefork head transcription factor inDrosophila, discovered by German biologistsDetlef Weigel and Herbert Jäckle.[17][18] Since then a large number of family members have been discovered, especially invertebrates. Originally, they were given vastly different names (such as HFH, FREAC, and fkh), but in 2000 a unified nomenclature was introduced that grouped the FOX proteins into subclasses (FOXA-FOXS) based on sequence conservation.[19]
The discovery of the FOX gene family and its evolutionary significance was outlined in a 2009 study by Hannenhalli and Kaestner.[6] The researchers detailed how FOX genes, originating in unicellulareukaryotes, evolved throughgene duplication and loss events to form a complex family in mammals. This study also highlighted the diverse biological roles of FOX genes, including contributions to developmental processes such asorganogenesis andspeech acquisition, and their association with various diseases, includingcancer andlanguage disorders.[6]
FOXgenes must be under extreme evolutionary supervision in a genomic sequence orcis-acting elements. If not, they can lead to the development of many different types of cancer, includingcarcinoma,breast cancer,prostate cancer, andacute lymphocytic leukemia.[6]
Depending on the subfamily, the deregulation of FOX proteins is often associated with tumorigenesis and can act as atumor suppressor or anoncogene.[22] Changes in post-translational modifications, genetic events, oroncoviruses are known causes for this deregulation.[22]
FOX proteins play critical roles in cellularhomeostasis as they act as bothtumor suppressors andoncogenes depending on the context. Dysregulation of FOX proteins may also contribute to diseases such asneurodevelopmental disorders andmetabolic syndromes.[23] For example, FOXM1 is essential for cell cycle progression and is frequently over-expressed in tumors while FOXO proteins regulateapoptosis and stress responses indicating they often act as tumor suppressors.[23]
A member of the FOX family,FOXD2, has been detected progressively over-expressed inhuman-papillomavirus-positiveneoplastic keratinocytes derived from uterine cervicalpreneoplastic lesions at different levels of malignancy.[24] For this reason, this gene is likely to be associated with tumorigenesis and may be a potential prognostic marker for uterine cervicalpreneoplastic lesions progression.[24]
Additional FOX family members have also been implicated in cancer progression andmetastasis. FOXP1 acts as a tumor suppressor inbreast andprostate cancers while showing oncogenic traits in certainlymphomas andleukemias.[25] FOXP3 which is crucial for regulatoryT cell function has been shown to repress oncogenic pathways and exhibits tumor suppressive behaviour in breast and prostate cancers.[26]
FOXD2-AS1 is a longnon-coding RNA related to the FOXD2 gene which serves as a potentialbiomarker in cancer. It is over-expressed in severalmalignancies includingcolorectal andgastric cancers and has been associated with poorprognosis and increased proliferation, invasion, and migration of cancer cells.[26][27]
FOXQ1 is found to promoteepithelial-mesenchymal transition which is a process that promotes invasion and metastasis through the repression ofepithelial markers such asE-cadherin and increases expression of mesenchymal genes. Over-expression of FOXQ1 has been linked to colorectal, gastric, and lung cancers, where it contributes to the tumor progression.[28][29]
FOXK2 has been linked to cancer and can function differently depending on the tissue type and molecular pathway it interacts with. Innon-small-cell lung cancer, FOXK2 suppresses tumor progression by down-regulatingcyclin D1 and CDKs allowing for the inhibition of cell proliferation and invasion.[30]
FOXO3a is another member that exhibits tissue-specific behaviour in cancer. In gastric cancer, its over-expression promotes invasion and migration by up-regulatingcathepsin L which promotes epithelial-mesenchymal transition.[31] FOXO3a also acts as a tumor suppressor innephroblastoma by inhibiting proliferation and invasion while inducing apoptosis.[32] In breast cancer, FOXO3a suppresses metastasis by down-regulatingTWIST-1.[33]
