Research advancements regarding the relationship between FBXO2 and malignant tumors (Review)

Introduction

At present, cancer ranks as either the first or second leading cause of premature mortality among most populations worldwide, and the global number of individuals affected by cancer is expected to continue to rise in the years ahead (1). Cancer is a complex disorder with multiple contributing factors and developmental stages, involving processes such as the activation of proto-oncogenes and the inactivation of tumor suppressor genes. Despite the current advancements in comprehensive cancer treatment, the complex pathogenesis of the disease still merits in-depth exploration to provide a novel direction for personalized cancer therapy (2). Proteins, such as organic macromolecules, are not only the fundamental organic matter constituting cells and the material basis of life, but are also associated with life phenomena (such as maintaining cell survival, proliferation, differentiation and homeostasis, and activating and regulating intracellular signaling pathways and material metabolic balance). Notably, the ubiquitin-proteasome system (UPS) is responsible for catalyzing the repair and degradation of a large number of key protein substrates. Dysregulation of the UPS may thus be implicated in the onset and progression of numerous diseases, including cancer. During the process of ubiquitination, three enzymes are essential for attaching ubiquitin proteins to their target proteins: i) Ubiquitin-activating enzyme (E1); ii) ubiquitin-conjugating enzyme (E2); and iii) ubiquitin ligase (E3) (3–5). Specifically, E3 binds to the substrate protein, and this interaction ultimately results in the degradation of the substrate through the 26S proteasome (3). Research has shown that the modulation of E3 activity constitutes one of the key factors influencing the development and progression of cancer (4). Among the various types of E3 ligases, Cullin-RING ligases (CRL) are the ones with the largest number of members and the most diverse types, and they play a role in cancer development (5,6). One of these CRL1 ligases, also referred to as the S-phase kinase-associated protein 1 (SKP1)/Cullin 1 (CUL1)/F-box protein (FBP) complex (SCF complex), has been extensively investigated (7–9). A total of 80–90% of proteins within cells undergo degradation via the UPS (2). The SCF complex consists of three invariant core components and variable FBPs. RING-box 1 is responsible for the recruitment of E2; CUL1, a scaffolding protein, also serves as the catalytic core; and SKP1 links the SCF complex to variable FBPs and their corresponding target proteins to recruit substrates for ubiquitination (9). To date, 69 FBPs have been identified in humans. These proteins can be further categorized into the following three groups: i) Those containing leucine-rich repeats (FBXL); ii) those with WD-40 repeats (FBXW); and iii) those featuring only uncharacterized structural domains (FBXO) (10,11). A growing body of evidence indicates that abnormal expression of FBPs is linked to the occurrence, proliferation, angiogenesis and metastasis of various malignant tumors. For instance, FABP5 can alter the fatty acid metabolism pathways and products of tumor cells, providing them with more energy and nutritional support, thereby promoting the growth and spread of tumors (12). FABP4 is induced in endothelial cells by the NOTCH1 signaling pathway and plays a significant role in the formation of the tumor vascular system. In in situ mouse ovarian tumor models, FABP4 is essential for angiogenesis and is an important target in tumor angiogenesis, especially in tumors of low-grade, interstitial and free fatty acid-rich tissues (13). Within the FBP family, F-box protein 2 (FBXO2) is abnormally expressed in multiple types of cancer. It shows abnormal expression in various cancers, such as gastric cancer (GC) (14), colorectal cancer (CRC) (15), ovarian cancer (OV) (16), endometrial cancer (EC) (17), osteosarcoma (OS) (18), glioblastoma (GB) (19), papillary thyroid carcinoma (PTC) (20) and oral squamous cell carcinoma (OSCC) (21). Specifically, it regulates EMT, mediates substrate degradation through the UPS, activates related signaling pathways (such as STAT3, PI3K-Akt), affects cell cycle and autophagy, thereby promoting the malignant processes of corresponding cancers such as cell proliferation, migration and invasion, and chemotherapy resistance. Furthermore, its expression level is often associated with clinical features such as cancer prognosis and metastasis (14–21).

Composition and role of the UPS

It is widely acknowledged that protein homeostasis is essential for maintaining normal cellular physiological activities, among which the timely degradation or repair of misfolded proteins is particularly crucial (22,23). The degradation pathways mainly encompass autophagy (such as lysosomal degradation pathway) and the UPS (24). The relationship between ubiquitin and protein regulation was initially investigated by Hershko et al (25) in the 1980s. Subsequently, to gain a deeper understanding of ubiquitin and its associated protein hydrolysis, Hershko et al (22) elaborated on ubiquitinating enzymes further. This allowed the clarification of the biological characteristics of the UPS, highlighting its essential role in protein degradation, as well as its specific physiological functions (including roles in the cell cycle, DNA repair, protein synthesis, transcription and stress responses). The UPS selective basis (such as short-term signals involved in protein-specific degradation) and key mechanistic traits such as polyubiquitin chains and the subunit selectivity of protein degradation were also identified. These findings triggered a substantial expansion of research in the ubiquitin field during the 1990s (26,27). The UPS acts as a major system for the intracellular non-lysosomal degradation of proteins, and regulates and eliminates aberrant proteins in a highly specific manner. The UPS has been demonstrated to be aberrantly activated in a variety of biologically notable processes, including cell viability, proliferation and invasion, colonization and metastasis, recurrence, vascular invasion, immunomodulation, and chemotherapy resistance, in a wide range of cancer types (28,29). In OV, the UPS is abnormally activated (16). On one hand, it promotes cell proliferation and inhibits apoptosis through the SOX6-FBXO2-Sad1 and UNC84 domain-containing protein 2 (SUN2) axis. On the other hand, it is associated with chemotherapy resistance through FBXO2. In PTC, UPS promotes the ubiquitination and degradation of p53 through abnormal expression of FBXO2, promoting cell proliferation and being related to tumor size and metastasis (20). In OSCC, UPS regulates the transformation of tumor cells to highly malignant clones through FBXO2, promoting malignant processes such as proliferation and invasion (21). The basic components of the UPS comprise ubiquitin, E1, E2, E3, the 26 S proteasome and deubiquitinating enzyme (DUB) (3). The process by which ubiquitin binds to a substrate and causes it to be degraded is referred to as ubiquitination (3). Ubiquitination is a protein degradation process involving a multi-step cascade reaction, primarily mediated by three types of ubiquitinating enzymes: E1, E2 and E3. Initially, E1 utilizes the energy released from ATP hydrolysis to form a thioester bond between the C-terminal of ubiquitin and the cysteine residue at the active site of E1, thereby activating ubiquitin, which constitutes the first step of ubiquitination. Subsequently, the activated ubiquitin is transferred to the cysteine residue located at the active site of E2. After which, the activated ubiquitin is transferred to a cysteine residue located at the active site of the E2, leading to the formation of a thioester bond between the E2 enzyme and ubiquitin. Then, in a two-step reaction, the E2 enzyme facilitates the attachment of ubiquitin to the substrate with the assistance of E3 that recognizes a specific target. In this process, the specificity for substrates is guaranteed by 500–1,000 specific E3 enzymes encoded in the human genome (3,4,7,29). Eventually, the substrate proteins marked with ubiquitin are broken down by the 26S proteasome, which in turn regulates DNA repair, influences stress responses and modifies cell proliferation (30) (Fig. 1). When mutated or overexpressed in various malignancies, ubiquitinating enzymes affect and regulate various cellular pathways, such as protein transport, chromatin remodeling, the cell cycle and apoptosis. Overexpression of NEDD4-1 in liver cancer can affect protein transport-related signals by regulating AKT ubiquitination, promoting the cell cycle and inhibiting apoptosis (31). In multiple myeloma, it can also mediate AKT degradation to accelerate apoptosis (32). MDM2 overexpression in cancer can degrade p53 and interfere with apoptosis and the cell cycle (33). Mutation will cause loss of substrate degradation ability, leading to uncontrolled cell cycle and abnormal proliferation (34). Therefore, the UPS serves a vital role in oncogenic signaling and the development of malignant tumors (7). Furthermore, it has been demonstrated that the UPS also exerts a notable role in neurodegenerative diseases and various inflammatory reactions (23).

Figure 1. Schematic diagram of the ubiquitination process. E1, ubiquitin-activating enzyme; E2, ubiquitin-conjugating enzyme; E3, ubiquitin ligase; TPP II, tripeptidyl peptidase II; RBX1, ubiquitin ligase with zinc finger structure; CUL1, Cullin-1 proteins; SKP1, S-phase kinase-related protein 1; F-box, F-box protein; Ub, ubiquitin; PPi, pyrophosphate.

Among ubiquitinating enzymes, the selectivity of the UPS is determined by the E3 ligase (7,28). A number of E3 ligases, when dysfunctional, can impair DNA damage repair, cause cell cycle disturbances, lead to abnormal gene expression regulation and disrupt or continuously interrupt signal transduction, ultimately exerting carcinogenic or tumor-promoting effects. For instance, anaphase-promoting complex (APC/C) is a representative E3 ligase that is indispensable for mitotic progression (35). Research has revealed that the abnormal regulation of cell division cycle 20 (CDC20) and cadherin 1 (CDH1) is linked to cancer (36). Inhibition of CDC20 can disrupt mitosis and subsequently induce cell apoptosis, which suggests that CDC20 possesses oncogenic characteristics (36,37). The depletion of CDC20 can suppress the Wnt signaling pathway, in turn reducing the proliferation of cancer cells in colon cancer (38,39). CDH1 facilitates the ubiquitination-mediated hydrolysis process initiated by BRAF either in an APC-dependent or APC-independent manner, thereby contributing to tumor formation (35). Neurologically expressed developmentally downregulated 4-1 (NEDD4-1) has been demonstrated to exert a vital function in modulating cancer progression. Specifically, NEDD4-1 stabilizes MDM2 via Lys63-linked polyubiquitination, a process that facilitates p53 degradation and thereby enhances cell proliferation (31). Furthermore, NEDD4-1 binds to N-Myc and strengthens the polyubiquitination of this protein, thereby promoting the proliferation of neuroblastoma cells (40). Silencing of NEDD4-1 results in reduced AKT phosphorylation and increased PTEN expression, which in turn inhibits the proliferation and migration of hepatocellular carcinoma cells (41). In addition, NEDD4-1 mediates the ubiquitination of AKT at the Ser473 site, which triggers phosphorylation at this position. This process promotes the proteasomal degradation of AKT, thereby inhibiting AKT signaling and accelerating apoptosis in multiple myeloma cells (32). MDM2, a key regulator of the tumor suppressor p53, mediates the ubiquitination of p53 and triggers its degradation via the proteasome. Mutations or deletions of p53 are present in approximately half of all types of cancer (33). MDM2 expression is upregulated in numerous cancer types, such as lung cancer and liver cancer, and the chemotherapeutic drug cisplatin induces apoptosis in cancer cells by phosphorylating and activating the p53 protein (42). However, increased MDM2 expression, combined with p53 mutations or downregulation, leads to the development of cisplatin resistance in epidermoid carcinoma (42).

The E3 ligases that regulate the ubiquitination-mediated degradation of proteins can be mainly classified into two types (30). HECT ligases are characterized by a C-terminal domain. They first form a thioester bond to receive ubiquitin molecules from E2 ligases, after which the ubiquitin undergoes refolding and is transferred to substrate proteins (43). RING ligase, which consists of RING and RING-like ligases and their accompanying proteins, has a zinc finger that transfers ubiquitin directly to the substrate via the ubiquitin conjugating enzyme E2 (44). CRL is the most extensive E3 ligase and serves a role in cancer. CRL1 ligase, otherwise referred to as the SCF complex, comprises FBPs and FBXW7 as key components. These elements serve a role in regulating the stability of diverse cellular factors, including cell cycle regulators, oncogenic transcription factors, cell surface receptors and signaling molecules (34,45). Consequently, impairment of the SCF-FBXW7 complex leads to unregulated cell proliferation and survival, genomic instability and disordered signaling pathways that promote cancer invasion and metastasis (46–48).

In summary, the substrate specificity of the SCF complex is governed by FBPs, with each type of FBP capable of recognizing and binding to a unique range of substrates (49). In addition to being components of the SCF complex, FBPs are involved in DNA replication, transcription, cell differentiation and cell death (50).

Composition and role of the FBP family

According to their specific structural domains, FBPs can be further divided into three subfamilies: i) FBXL; ii) FBXW; and iii) FBXO (10,11). FBPs directly interact with post-translationally modified substrates and mediate the ubiquitination and degradation of target proteins in various cellular biological processes, such as the cell cycle, epithelial-mesenchymal transition (EMT), apoptosis and multiple tumor-related signaling pathways (such as the PI3K-AKT-mTOR, p53 and nuclear factor erythroid 2-related factor 2 pathways), thus affecting tumor progression (51).

Numerous members of the FBP family serve notable roles in tumor development (Table SI). For instance, SKP2 engages with multiple signaling cascades, including but not limited to the PI3K/Akt (52), ERK (53), peroxisome proliferator-activated receptor γ (54), insulin-like growth factor 1 (55) and mTOR (10) signaling pathways. Through the aforementioned signaling pathways and their potential interactions, SKP2 exhibits high expression in breast cancer, melanoma, pancreatic cancer, GC, lymphoma, prostate cancer and nasopharyngeal cancer; notably, its high expression is associated with poor prognosis in these malignancies (56). The dysregulation of the SKP2/mH2A1/CDK8 pathway and the hydrolysis of mH2A1 by SKP2-targeted proteins are crucial for breast cancer progression and prognosis (57,58). In gliomas, FBXL18 exerts oncogenic effects by curbing apoptosis, which is achieved through promotion of K63-linked ubiquitination of AKT (59). In OV, FBXO6 (60) and FBXO16 (61) function as proto-oncogenes through ribonuclease T2 and heterogeneous nuclear ribonucleoprotein L ubiquitination and degradation, respectively. Additionally, FBXO32 promotes lung adenocarcinoma progression by targeting PTEN and promoting its degradation to regulate the cell cycle, promote the PI3K/AKT/mTOR pathway and ultimately accelerate EMT (62). FBXO32 acts as a target gene for melanocyte inducing transcription factor (a major transcription factor in melanocytes), facilitating melanoma progression in vivo, and the knockdown of FBXO32 induces global changes in melanoma gene expression profiles (63).

On the other hand, FBXW7, as an oncogene, mediates tumor suppression by negatively regulating a number of oncogenic proteins (50). The inactivation of this protein in lung cancer, hepatocellular carcinoma, CRC, breast cancer and hematopoietic system tumors holds importance for the initiation and progression of these malignancies (64). Among the five types of tumors mentioned above [lung cancer, hepatocellular carcinoma, CRC, breast cancer and hematopoietic system tumors], microRNA-223 influences the proliferation and apoptosis of CRC cells by activating the NOTCH and AKT/mTOR pathways (65). Thus, it can be inferred that the loss of FBXW7 may serve as an independent prognostic marker for tumors (64). It has also been shown that FBXO4 acts as another tumor suppressor within the FBP family, and it facilitates ubiquitin-mediated degradation of cyclin D1 through phosphorylation at the Thr286 site (8). Consequently, dysfunction of FBXO4 causes cyclin D1 to accumulate in vivo and drives the progression of malignant tumors, including melanoma (66) and esophageal cancer (67,68). The absence of FBXO4 expression in mice caused cyclin D1 accumulation, and thus, malignant transformation of BrafV600E melanoma, suggesting that FBXO4 deficiency serves a role in melanoma development (66). These studies have substantiated the tumor-suppressive effects of FBXO4 (8,66–68). FBXO11 and FBXO31 are also recognized tumor suppressors, and both target a variety of oncogenic substrates. For instance, FBXO11 mediates the ubiquitination-dependent degradation of the proto-oncogene product B-cell lymphoma 6 protein, and it is often absent or mutated in diffuse large B-cell lymphomas (69). FBXO31 orchestrates its regulatory role in tumor-related processes by triggering the ubiquitination and degradation of distinct substrates, with the underlying mechanism exhibiting substrate-specific divergence. For cyclinD1, FBXO31 exerts its function by recognizing phosphorylation modifications (e.g., at the Thr286 site) of cyclinD1; it then associates with the SCF (SKP1-Cullin-F-box) E3 ubiquitin ligase complex, thereby facilitating the ubiquitination and subsequent proteasomal degradation of cyclinD1 to preclude aberrant cell cycle progression (70). In the case of SNAIL, a key transcription factor governing epithelial-mesenchymal transition (EMT), FBXO31 binds to specific structural domains of SNAIL, which disrupts the interaction between SNAIL and its stabilizing factors (e.g., SIP1) and induces conformational changes in SNAIL. These events collectively promote the ubiquitination and degradation of SNAIL, ultimately suppressing tumor cell invasion and metastasis (71). Regarding MDM2, the E3 ubiquitin ligase of the tumor suppressor p53, FBXO31 interacts with the RING domain of MDM2 to trigger MDM2 self-ubiquitination. This self-ubiquitination leads to MDM2 degradation, which alleviates the inhibitory effect of MDM2 on p53 and restores the tumor-suppressive functions of p53 (e.g., cell cycle arrest and apoptosis induction) (72). For mitogen-activated protein kinase kinase 6 (MAPKK6), a pivotal upstream kinase in the MAPK signaling pathway, FBXO31 is capable of recognizing the unique conformational state of hyperactivated MAPKK6. It then mediates the ubiquitination and degradation of hyperactivated MAPKK6 via the SCF complex, preventing excessive activation of the MAPK pathway and the consequent abnormal cell proliferation (73). Collectively, FBXO31 achieves the goal of inhibiting the occurrence and development of various malignant tumors such as breast cancer (74), OV (75), liver cancer (76) and prostate cancer (77) by triggering the ubiquitination degradation of specific substrates. Additionally, FBXL14 is capable of inducing the ubiquitination-mediated degradation of the key oncogene c-Myc. However, in glioma stem cells, this ubiquitination process can be reversed by the DUB ubiquitin specific peptidase 13 (USP13), which suggests that an antagonistic relationship exists between FBXL14 and USP13 (78). Furthermore, FBXL14 can target and degrade CUB domain-containing protein 1 (CDCP1), thereby reducing the stability of CDCP1 and inhibiting breast cancer metastasis (79).

Taken together, all of the aforementioned FBP family members influence various cellular signaling pathways via UPS, and thereby affect the progression of malignant tumors.

Introduction of FBXO2

FBXO2, alternatively referred to as Fbx2, Fbg1, Fbs1, neural F-box 42 kDa (NFB42) and organ of Corti protein 1, is an FBP. It is highly abundant in the cytoplasm of eukaryotic cells and functions as a subunit of the E3 ligase (80,81). Research on FBXO2 began with the proteomic analysis of rat neural tissues. As early as 1998, researchers identified an FBP referred to as NFB42, with a molecular weight of ~42 kDa, in rat brain tissues. This protein was highly expressed in neurons and served a role in sustaining the non-dividing state of cells (82). Subsequently, through homology sequence cloning technology, it was revealed that human FBXO2 (Online Mendelian Inheritance in Man 607112) is situated on chromosome 1p36.22 and comprises six exons encoding a cytoplasmic protein containing 296 amino acids (83) (Fig. 2). The human FBXO2 consists of an F-box domain (FBA) and a glycoprotein-binding domain (SBD). The FBA domain is situated at the N-terminal and exhibits a triple-helical bundle conformation (84). It binds to the SKP1 protein within the SCF complex and serves as the core module for E3 ligase activity (83). The SBD is located at the C-terminal and contains multiple protein interaction modules, such as hydrophobic pockets and sugar recognition motifs (84,85). For instance, FBXO2 recognizes the Epstein-Barr virus glycoprotein B (gB) through glycosylation sites and mediates its endoplasmic reticulum-associated degradation (86). Research into the functionality of FBXO2 has indicated that it functions as the substrate recognition subunit of the SCF ubiquitin ligase complex, facilitating the ubiquitination and degradation of a variety of proteins (87,88). Further research has shown that FBXO2 is abnormally highly expressed in various cancer types (such as OV and thyroid cancer), and promotes tumor progression by regulating key signaling molecules such as p53 (16,20).

Figure 2. Schematic diagram of FBXO2 generation. FBXO2, F-box protein 2; MHC, major histocompatibility complex.

Comparative genomics studies have shown that FBXO2 has similar domain composition and substrate recognition abilities in species such as mammals, birds and fish, but its regulatory network shows certain adaptive differentiation among different species (89–91). FBXO2 is specifically expressed in tissues such as the brain and adrenal glands of mice, and its absence leads to the accumulation of amyloid precursor protein in neurons, suggesting its potential role in Alzheimer's disease (AD) (92). Additionally, FBXO2 participates in the mechanism of liver fibrosis induced by excessive exercise in mice by regulating the formation of muscle lactate vesicles (89). In fish, the homologous genes may be involved in ubiquitination regulation during liver development. For instance, the abnormal expression of FBXO32 in fish skeletal muscle tissues is associated with muscle atrophy (90), suggesting the conserved function of F-box family genes in organ development. FBXO2 exhibits conservation during the evolution of insect sex chromosomes, and its biological function may be associated with the dosage compensation effect of insect sex chromosomes (i.e., balancing the expression dosage of genes on sex chromosomes between male and female individuals through specific mechanisms to maintain normal physiological functions) (91). Current studies have not yet identified the homologous gene of FBXO2 in fruit flies, but FBPs serve a key role in the asymmetric division of neural stem cells, indicating the cross-species conservation of ubiquitination regulation in stem cell biology (93).

FBXO2, as an important member of the FBP family, exhibits both evolutionary conservation and functional diversity, making it a hot topic in interdisciplinary research. From neural development to disease regulation, FBXO2 precisely regulates protein homeostasis through the ubiquitin network. Future studies need to further analyze the regulatory differences of FBXO2 in various diseases and explore its potential as a therapeutic target for diseases.

Role and mechanisms of FBXO2 in non-tumorigenic diseases

FBXO2 holds considerable importance in metabolic disorders. FBXO2 is a functional E3 ligase for insulin receptors (IRs) in the liver, and FBXO2 protein and mRNA levels are increased in the livers of obese individuals. FBXO2 mediates the ubiquitin-dependent degradation of IRs, thereby inhibiting the PI3K-AKT pathway and impairing the integrity of insulin signaling (87). This leads to insulin resistance and abnormal glucose metabolism, offering a novel therapeutic target for type 2 diabetes and associated metabolic disorders (87,94). The expression levels of FBXO2 are elevated in the liver tissues of patients suffering from non-alcoholic fatty liver disease (NAFLD). The upregulated FBXO2 aggravates NAFLD by targeting the α subunit of hydroxy-COA dehydrogenase and promoting its proteasome degradation in HepG2 and 293T cells. This suggests that FBXO2 may serve as a potential therapeutic target for NAFLD (88). Additionally, FBXO2 exerts considerable effects in neurological diseases. A study has shown that there is an association between FBXO2 and the occurrence and development of Parkinson's disease (PD), and variations in FBXO2 may reduce the risk of PD among Han individuals in mainland China and serve as a biomarker for risk assessment of PD (95). Cognitive dysfunction is one of the notable features of AD (96). The pathogenesis of AD is associated with multiple mechanisms such as transcriptional dysregulation, erroneous protein degradation and synaptic dysfunction (97). A previous study has shown that the upregulation of FBXO2 induced by histone methyltransferase Smyd3 in AD mouse models (P301S Tau mice) affected the function of the N-methyl-D-aspartate receptor (NMDAR) and the cognitive behavior of mice, which provides a potential therapeutic strategy for AD (97). Niemann-Pick C (NPC) disease is an autosomal recessive genetic disorder that ultimately leads to neurodegeneration and premature mortality. In the central nervous system, FBXO2 localizes to damaged lysosomes and facilitates their degradation. Deficiency of FBXO2 delays the clearance of impaired lysosomes, thereby exacerbating motor function impairments in NPC, intensifying neurodegeneration, and ultimately reducing survival (98). In osteoarthritis (OA), Tang et al (99) demonstrated that RNA binding motif protein 47 stabilized FBXO2 mRNA by activating the STAT3 signaling pathway, thereby promoting the inflammatory, apoptotic and extracellular matrix (ECM) degradation of chondrocytes treated with IL-1β, ultimately progressing OA. Another study (100) employed single-cell transcriptomic analysis to pinpoint novel specific biomarkers for nucleus pulposus and intrafibrous annulus cells, while also defining the cell populations in non-degenerative and degenerative human intervertebral discs (IVD) from the same individual. According to gene expression profiles at the single-cell resolution and Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses, FBXO2 was identified as one of the genes that could predict IVD degeneration, which offers novel perspectives on the biomarkers of IVD degeneration, and thus, enhances diagnostic and therapeutic strategies. Furthermore, previous research (101) has demonstrated that inner ear organoids are derived from embryonic stem cells, by targeting the lineage-specific ear gene FBXO2 with the multi-allelic reporter cassette (Venus/Hygro/CreER), researchers revealed the gradient characteristics exhibited by cochlear tissue during development (these characteristics are associated with cell differentiation or structural formation) through the expression of Venus and the activity of CreER. This approach also marked the sensory lineage, confirmed the enrichment of the type I vestibular hair cell subpopulation and revealed strong expression in adult cerebellar granule cells. Subsequently, McGovern et al (102) described a novel tamoxifen-induced CreER mouse strain, referred to as FBXO2CreERT2 mice. In adult mice, the induction of FBXO2CreERT2 occurred more frequently in type I hair cells than in type II hair cells. Therefore, FBXO2CreERT2 mice were considered a novel tool for specifically manipulating inner ear epithelial cells and targeting the type I hair cells of the vestibule. The FBXO2/SCF ubiquitin ligase complex enhances ubiquitin-mediated phagocytosis of group A streptococcus (GAS) by recognizing the N-acetylglucosamine on GAS side chains, promoting ubiquitin-mediated ligase activity (103). Additionally, gB in the Epstein-Barr virus binds to the sugar-binding structural domain of FBXO2 through its glycosylation site. Through the ubiquitin-proteasome pathway, gB is degraded to limit viral infectivity (86).

Role and mechanisms of FBXO2 in tumorigenic diseases

FBXO2 exerts a cancer-promoting effect in multiple types of malignancies, such as GC, OV, OS and thyroid cancer, with its mechanism of action differing according to the specific cancer type (Table SII). In GC, it enhances cell proliferation, migration and invasion through the regulation of EMT (14). In CRC, it may inhibit tumor suppressor proteins through EMT and UPS degradation, affecting the Wnt signaling pathway (104). Specifically, FBXO2 regulates key EMT transcription factors such as SNAIL and TWIST, initiating the EMT process in CRC cells. Subsequently, these key EMT transcription factors can bind to the promoter of tumor suppressor genes to inhibit their transcription, or affect the subcellular localization and function of tumor suppressor proteins by altering the cell state. At the same time, EMT may also collaborate with the UPS, changing the kinase activity to phosphorylate tumor suppressor proteins, which are then recognized by the UPS and degraded. Eventually, the inhibition of tumor suppressor proteins will release the negative regulation on Wnt and other pathways, promoting the progression of CRC (43). In OV, it promotes cancer development through the SOX6-FBXO2-SUN2 axis (among them, SUN2 is fully known as Sad1 and UNC84 domain-containing protein 2, which is a transmembrane protein located on the nuclear membrane. Its core function is related to the maintenance of the nuclear membrane structure, nuclear localization, and the transport of nuclear and cytoplasmic substances. Additionally, it plays a role in the connection between the cytoskeleton and the nuclear skeleton) and related pathways, and is associated with chemotherapy resistance (16). In addition, in EC it degrades fibrillin 1 (FBN1) to regulate the cell cycle and autophagy. In OS, it stabilizes IL-6R to activate the STAT3 signaling pathway (17). In GB, it participates in tumor-microenvironment interaction (19), and in PTC, it degrades p53 to promote proliferation (20). Finally, in oral OSCC, it promotes the transformation of cells to malignant clones (21). Therefore, FBXO2 has the potential to act as a biomarker and therapeutic target in a variety of cancer types.

Effect of FBXO2 on GC and its mechanisms

GC is a common malignant tumor globally and stands as the fourth primary cause of cancer-associated deaths. The incidence of GC rises steadily with age, and the median survival time for patients with advanced GC is <12 months (105,106). Therefore, it is of great importance to explore approaches for the management of this disease (106). Sun et al (14) used four GC cell lines (MGC-80-3, AGS, SGC-7901 and MKN-28) to conduct comparisons in terms of their transcription and translation levels, as well as migratory and invasive abilities, using reverse transcription-quantitative PCR, western blotting, Transwell assays and wound healing assays. To further substantiate the experimental conclusions, MGC 80-3 cells were transfected with FBXO2 small interfering RNA (siRNA), and the aforementioned experiments were repeated. The findings showed that in siRNA-FBXO2 MGC 80-3 cells, the mRNA expression of FBXO2 was downregulated, and the ability of cells to proliferate, migrate and invade was notably suppressed. Additionally, the expression of epithelial markers (E-cadherin protein) was elevated, and the expression of mesenchymal markers (N-cadherin and vimentin protein) was notably reduced. All these results suggested that low FBXO2 expression could inhibit the proliferation, migration and invasion of GC cells by reducing EMT. In summary, FBXO2 has the ability to promote the proliferation, migration and invasion of GC cells, and FBXO2-mediated EMT serves a notable role in the migration and invasion of GC, and may emerge as a novel target for the diagnosis and treatment of this malignancy.

Effect of FBXO2 on CRC and its mechanisms

CRC is the second and third most prevalent cancer type in women and men, respectively, accounting for 9.2% of global deaths, with incidence and mortality rates being 25% higher in men than in women (107). Wei et al (15) observed that FBXO2 was mainly expressed in the cytoplasm of CRC cells, and that the overexpression of FBXO2 increased the expression of N-cadherin. This indicates that FBXO2 is implicated in CRC metastasis through EMT. In this article (15), the team further investigated and found that the mechanism by which FBXO2 promotes the development of CRC may be through the degradation of N-cadherin by UPS, which regulates the tumor proliferative activity of CRC. In addition, the excessive expression of N-cadherin in CRC can further impact the expression and localization of β-catenin. Furthermore, Zhao et al (104) demonstrated that β-catenin serves a notable role in the Wnt signaling pathway and colon cancer development. These studies further suggested that the upregulation of FBXO2 expression is associated with proliferation, infiltration and distant metastasis of CRC, and that the inhibition of FBXO2 expression may constitute a therapeutic strategy to improve the prognosis of patients with CRC. Therefore, expression levels of FBXO2 can serve as a potential biomarker for CRC metastasis.

Effect of FBXO2 on OV and its mechanisms

OV is the fifth most lethal cancer among women globally, with an incidence and mortality rate of 3.4 and 4.7%, respectively. This indicates that it poses a serious threat to the health and survival of women (108). Ji et al (16) found, through analysis of multiple genetic databases, that FBXO2 was highly expressed in OV tissues and cells. The underlying mechanisms may involve the transcription factor SOX6 binding to FBXO2, which leads to the abnormal upregulation of FBXO2 expression. Subsequently, FBXO2 binds to the glycosylated SUN2 protein and functions as an E3 ligase to mediate the ubiquitination-dependent degradation of SUN2. This process inhibits apoptosis, promotes cell proliferation and ultimately accelerates the progression of OV. Further silencing of FBXO2 expression using short hairpin RNA revealed that cells with FBXO2 silenced showed reduced proliferative, migratory and invasive abilities compared with previously. Furthermore, in in vivo experiments, the knockdown of FBXO2 led to a notable reduction in the volume, size and weight of subcutaneous tumors. Thus, the aforementioned findings revealed a novel SOX6-FBXO2-SUN2 axis that serves a role in OV development, and targeting this axis could serve as an effective therapeutic approach for OV. Recent research (109) has indicated that FBXO2 is capable of impacting chemotherapy resistance. It achieves this by affecting the PI3K-Akt signaling pathway, as well as the interactions between focal adhesions and ECM receptors, while also regulating tumorigenesis. In vitro experiments showed that, in A2780 and SKOV3 OV cell lines where FBXO2 was silenced, the 50% maximum inhibitory concentration of cisplatin was reduced. This suggests that FBXO2 is a potential biomarker linked to chemotherapy resistance in high-grade serous OV (HGSOC), and it can act as a valuable prognostic indicator as well as a potential target in HGSOC (109).

Effect of FBXO2 on EC and its mechanisms

EC accounts for >90% of all uterine malignancies, and its incidence is increasing. The majority of patients diagnosed with EC are postmenopausal, with the median age at diagnosis being 60 years (110). Therefore, improving the early diagnosis and prognostic assessment of patients with EC is of critical importance (110). Che et al (17) revealed that FBXO2 binds to and causes the degradation of FBN1 via polyubiquitination, which is further enhanced by the regulation of cell cycle proteins (CDK4, CyclinD1, CyclinD2 and CyclinA1) and the inhibition of autophagy signaling pathways [autophagy related 4A cysteine peptidase (ATG4A) and ATG4D], to promote EC proliferation. Subsequently, a knockdown mouse model, verification via immunohistochemistry and other techniques confirmed that FBXO2 knockdown led to elevated FBN1 expression and reduced Ki67 expression. Additionally, either FBN1 knockdown alone or the combined knockdown of both FBXO2 and FBN1 resulted in decreased Ki67 expression. These results implied that the deletion of FBN1 blocks the proliferative effect of FBXO2. In conclusion, the aforementioned experimental study demonstrated that targeting FBXO2 may treat EC through the regulation of cell cycle and autophagy signaling pathways, and that patients with high FBXO2 expression may be potential candidates for treatment with CDK4/6 inhibitors in EC (17).

Effect of FBXO2 on OS and its mechanisms

OS is the most common primary bone malignancy, characterized by strong local invasiveness and metastatic potential, with the 5-year survival rate of patients with metastases being <20% (111). Research into improving the survival of patients with OS has proved challenging. Zhao et al (18) confirmed that the persistent activation of the IL-6 signaling pathway is harmful to bone tissue and hypothesized that this ultimately contributes to the development of OS. Through further research, it was identified that upregulated FBXO2 in OS cells binds to the C-terminal SBD of IL-6R. This binding gives rise to a stable complex. Once the stable IL-6R binds to IL-6, it recruits Janus kinase, which in turn causes the continuous phosphorylation of the Y705 site on STAT3. This, in turn, activates the STAT3 signaling pathway. A luciferase reporter gene assay showed that the overexpression of FBXO2 tripled the transcriptional activity of STAT3, and the mRNA levels of its downstream target genes [such as the anti-apoptotic proteins Mcl-1 and X-linked inhibitor of apoptosis (XIAP)] were notably upregulated. Chromatin immunoprecipitation further confirmed that FBXO2 strengthened the binding of STAT3 to the promoters of Mcl-1 and XIAP, promoting their transcription and ultimately promoting the multiplication of OS cells. In a nude mouse xenograft model, the expression levels of IL-6R and phosphorylated STAT3 in the tumors formed by U2OS cells with FBXO2 knockout were notably reduced, and the protein levels of Mcl-1 and XIAP decreased by ~50%. This directly demonstrated the core role of the FBXO2-IL-6R-STAT3 axis in tumor growth. Treatment of cells overexpressing FBXO2 (such as MG63 cells) with STAT3 inhibitors (such as Stattic) could reverse their promoting effect, and the cell count returned to the level of the control group. Furthermore, the addition of IL-6R neutralizing antibodies (such as CNTO328) also notably inhibited STAT3 activation mediated by FBXO2 and cell proliferation. According to the aforementioned research, in OS, the function of FBXO2 undergoes a functional reversal, whereby its glycoprotein recognition domain specifically binds to IL-6R, inhibits its degradation to stabilize the receptor and thereby activates the STAT3 signaling pathway, instead of following the traditional ubiquitination degradation pathway. Therefore, inhibiting the expression or activity of FBXO2 may represent an effective therapeutic strategy for OS (112). At present, biotherapies aimed at the IL-6/STAT3 signaling pathway primarily center on monoclonal antibodies. Based on the aforementioned study, the application of IL-6 monoclonal antibodies may prove to be an effective approach for the treatment of OS.

Effect of FBXO2 on GB and its mechanisms

GB is the most common and highly malignant primary brain tumor among adults. The median survival is 14–24 months and surgical treatment is the main therapeutic modality (113). However, the recurrence rate remains high and the prognosis remains poor (113). Buehler et al (19) revealed that FBXO2 expression was increased in recurrent GB, and that knockdown of FBXO2 enhanced in vivo survival and mitigated the invasive growth of glioma cells in brain sections. FBXO2 was also found to be concentrated in the infiltration zone, with FBXO2-positive cancer cells being linked to synaptic signaling processes. In addition, Atkin et al (114) found that FBXO2 in the brain governs the abundance and localization of specific NMDAR subunits, GluN1 and GluN2A, and affects synapse formation and maintenance to a certain degree. The aforementioned research illustrates the potential function of FBXO2-dependent interactions within the glioma microenvironment in facilitating tumor growth. Thus, inhibition of FBXO2 may be used to treat recurrent GB.

Effect of FBXO2 on PTC and its mechanisms

PTC accounts for ~84% of all thyroid cancers. It is categorized as a well-differentiated thyroid cancer, with the 5-year relative survival rate reaching 98.5%. The majority of cases of well-differentiated thyroid cancer are asymptomatic and are identified during physical examinations or incidentally during diagnostic imaging studies. Surgery is effective for the majority of cases of well-differentiated thyroid cancer, and post-surgical radioactive iodine treatment is capable of enhancing the overall survival rate in patients at a high risk of recurrence (115). Anti-angiogenic tyrosine kinase inhibitors and targeted therapies against the genetic mutations that cause thyroid cancer are increasingly being used in the treatment of metastatic diseases (115). Guo et al (20) performed in vitro experiments to examine FBXO2 expression in PTC tissues. The findings indicated that FBXO2 expression was upregulated in both PTC tissues and cell lines. The expression levels of FBXO2 were positively associated with the tumor size of PTC, as well as lymph node metastasis and extracapsular invasion. Furthermore, silencing of FBXO2 suppressed the proliferation of PTC cells and induced cell apoptosis, while the overexpression of FBXO2 notably boosted the proliferation of PTC cells. A mechanistic study demonstrated that FBXO2 is capable of directly binding to p53 and facilitating its ubiquitination and degradation (20). Knocking down the p53 gene to a certain extent weakened the inhibitory effect of FBXO2 knockdown on the progression of PTC cells, allowing the proliferation, invasion, and metastasis processes of the originally blocked tumor cells to recover. Knockdown of FBXO2 suppressed the proliferation of PTC cells and promoted their apoptosis by targeting p53 for ubiquitination and degradation (20). The aforementioned research findings lay the groundwork for the diagnosis and treatment of PTC.

Effect of FBXO2 on OSCC and its mechanisms

OSCC accounts for >90% of all malignant tumors in the oral cavity (116). Due to the extensive genetic susceptibility of the oral mucosa of patients with OSCC to carcinogenic factors, the risk of developing concurrent tumors in the oral cavity notably increases (116). Cheng et al (21) established the evolutionary trajectory of tumor cells by relying on single-cell RNA sequencing data, identified the dynamic clonal characteristics of OSCC and further clarified that FBXO2 is a key gene that determines the specific transcriptional state leading to the transformation of tumor cells into clones with notably high malignant characteristics. The expression levels of FBXO2 in OSCC are notably higher than those in normal samples, particularly in patients with more advanced clinical stages. In OSCC cells, knockdown of FBXO2 leads to a corresponding inhibition of cell proliferation, G1-S phase transition, migration, invasion, EMT and anti-apoptotic capacity, whereas its overexpression results in the promotion of these processes (117). These findings offer novel perspectives on clonal heterogeneity and lay the foundation for more effective therapeutic approaches against OSCC, as well as for overcoming the resistance of OSCC to treatment.

Role of FBXO2 in clinical tumor treatment

FBXO2 is involved in the specific recognition and degradation of substrate proteins within the UPS. Impairment of its function is closely linked to the onset and progression of various tumors such as GC (14), OV (16), OS (18) and thyroid cancer (20), thereby endowing it with the potential to act as a diagnostic or prognostic biomarker. In OV, Ji et al (16) demonstrated that the transcription factor SOX6 promotes the expression of FBXO2 by recognizing the potential response elements located in the promoter region of FBXO2., Abnormally highly expressed FBXO2 targeted and degraded the tumor suppressor gene SUN2, inhibited cell apoptosis and activated the proliferation-promoting signaling pathway, thereby promoting the progression of OV. Knockdown of FBXO2 notably inhibited the proliferation, metastasis and apoptosis of OV cells, suggesting that its high expression may serve as an independent indicator of poor prognosis in OV. In related research on GB (19), it was found that FBXO2 was continuously upregulated in recurrent GB, and its expression levels exhibited a negative association with patient survival rates. In addition, it was demonstrated that knockout of FBXO2 could prolong the survival period of mice in an orthotopic xenograft model and reduce the invasive growth of tumors in organoid brain slices. Further analysis showed that high FBXO2 expression was related to the enrichment of tumor infiltration areas and the interaction between glioma and the microenvironment, and may promote malignant progression by regulating the interaction between tumor cells and surrounding tissues (19). Through immunohistochemical studies on CRC, it was revealed that high expression of FBXO2 is closely related to distant metastasis and the American Joint Committee on Cancer clinical stage of CRC, and is an independent influencing factor for poor prognosis of patients (15,104). In addition, its carcinogenic mechanisms may be related to regulating tumor proliferation activity (positively associated with Ki-67 expression) and angiogenesis formation (positively associated with VEGF expression) (104). When exploring the potential of FBXO2 as a diagnostic biomarker, a new patented study (Chinese patent no. CN111381047A) proposed that detecting the autoantibodies of FBXO2 in serum could be used for lung cancer screening. Clinical data indicated that the levels of FBXO2 autoantibodies in the serum of patients with lung cancer were notably lower compared with those of healthy controls, with a specificity of 96.6% but a sensitivity of 20.0%. It is hypothesized that combination with other markers may further improve the diagnostic efficacy. Currently, this method still needs to be verified by a larger-scale cohort study.

At present, the development of specific small-molecule compounds targeting FBXO2 remains in the early stages. Some proteasome inhibitors (such as bortezomib and MG-132) (32,118) are not specifically targeted at FBXO2, but can inhibit the degradation function of its substrates by blocking the UPS. For example, treatment with MG-132 can increase the accumulation of FBXO2 substrate proteins, thereby indirectly inhibiting its oncogenic activity (118). Salidroside (a metabolic regulator) can reduce the secretion of lactate vesicles carrying FBXO2 by muscles in an over-exercise -induced liver fibrosis model, thereby alleviating liver cell apoptosis and fibrosis (89). This suggests that metabolic intervention may affect the intercellular transfer of FBXO2, providing novel ideas for cancer treatment.

Summary and future prospects

In conclusion, the relationship between the FBP family and tumors has attracted significant attention from researchers. As research progresses, a more comprehensive understanding of the interactions among the FBP family members is required. Future drug development should focus on the complex interactions and regulatory mechanisms of the FBP family to target drugs at the FBP family. Among them, FBXO2 is a key component of the ubiquitin ligase SCF complex. Besides serving a notable role in hepatic glucose metabolism and cerebral degenerative diseases, abnormally expressed FBXO2 is closely linked to the progression of diverse tumors, such as GC, CRC, OV, EC, GB, thyroid cancer and OSCC, and is also regarded to be one of the proto-oncogenes. In the present review, the association between FBXO2 and tumorigenesis and development was analyzed by reviewing the composition of the UPS and FBP families, the roles they serve in malignant tumors, and the progress of research on the relationship between FBXO2 and tumors. Nevertheless, the complex mechanisms through which upregulated FBXO2 promotes the growth, migration and invasion of various tumors needs to be further studied in depth, which will help to more clearly elucidate the pathogenic mechanisms of tumors and offer novel avenues for the treatment of human malignant neoplasms.

Supplementary Material

Supporting Data

Acknowledgements

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Funding

The present study was supported by The Human Resources and Social Security Department System of Shanxi Province, the Fund Program for the Scientific Activities of Selected Returned Overseas Professionals in Shanxi Province (grant no. 20210001), the Research Project Supported by Shanxi Scholarship Council of China (grant no. 2021-116), Shanxi ‘136’ Leading Clinical Key Specialty (grant no. 2019XY002) and Shanxi Provincial Key Laboratory of Hepatobiliary and Pancreatic Diseases (under construction).

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Authors' contributions

JZ was responsible for drafting and revising the manuscript; JY, XZ, YY, SS and SZ collected relevant literature and assisted in manuscript revision. JZ and JY designed the tables and figures. JH revised the manuscript. Data authentication is not applicable. All authors have read and approved the final version of the manuscript.

Ethics approval and consent to participate

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Patient consent for publication

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Competing interests

The authors declare that they have no competing interests.

Glossary

Abbreviations

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References