Research progress of circular RNA FOXO3 in diseases (review).

The transcription factor FOXO3, part of the O subclass of the Forkhead box family, features a highly conserved forkhead DNA binding domain and is universally expressed. The mammalian FOXO proteins consist of four members: FOXO1 (FKHR), FOXO3 (FKHRL1 or FOXO3a), FOXO4 (AFX), and FOXO6. These are involved in numerous physiopathological processes and play a key role in maintaining homeostatic balance in the body, including developmental processes, metabolism, oxidative stress, etc. [19] , [20] . They fulfill a wide range of important and complex roles in higher organisms. Like FOXO3 mRNA, circFOXO3 is encoded by the FOXO3 gene [21] . Specifically, circFOXO3 is derived from exon 2 of the FOXO3 gene ( Fig. 1 ), consists of 1435 nucleotides (according to circBase) [22] , and is primarily found in the cytoplasm [23] . Fig. 1 Biogenesis diagram of circFOXO3. FOXO3 is located on chromosome 6q21, and has 10 exons, whereas circFOXO3 is derived from the back splicing of exon 2 of the FOXO3 gene and contains 1435 nucleotides. Fig. 1 Biogenesis diagram of circFOXO3. FOXO3 is located on chromosome 6q21, and has 10 exons, whereas circFOXO3 is derived from the back splicing of exon 2 of the FOXO3 gene and contains 1435 nucleotides. According to the literature, circFOXO3 is specifically expressed in different tissues and cells. It can act as sponges of miRNAs to promote or inhibit cancer progression and affect drug or radiosensitivity by regulating the transduction of various signaling pathways through the mechanism of competing endogenous RNAs (ceRNAs) [24] , [25] , [26] , [27] , [28] , [29] . Additionally, it can play a protective role by inducing or inhibiting autophagy [30] , [31] , [32] , regulating cell cycle progression [21] , or delaying senescence [23] by interaction with certain proteins, and so on. In conclusion, studies indicate that circFOXO3 is a key gene with diverse regulatory roles and significant biological functions. It is closely related to cancer, cardiovascular disease, neurological disease, senescence, and inflammation, making it a promising biomarker and potential therapeutic target. Given the inconsistent trends of circFOXO3 action across different diseases, Fig. 2 illustrates the involvement of circFOXO3 in a variety of diseases through different pathways. Table 1 summarizes the biological functions and potential molecular mechanisms of circFOXO3 in senescence, inflammation, nervous system diseases, and cardiovascular diseases, and Table 2 summarizes the biological functions and potential molecular mechanisms of circFOXO3 in various cancers. Fig. 2 circFOXO3 is involved in various diseases through different pathways. Standard-shaped arrow: promotion; T-shaped arrow: inhibition. Fig. 2 Table 1 Biological functions and potential mechanisms of circFOXO3 in senescence, inflammation, neurological diseases, and cardiovascular diseases. Table 1 Disease Types Pathology circFOXO3 Expression Effect On Disease Associated Pathway/Process References Senescence / ↓（cancer cells） Suppression circFOXO3-p21-CDK2 [21] / ↑ Promotion circFOXO3/ID1; circFOXO3 /E2F1; circFOXO3 /HIF1α; circFOXO3 /FAK [23] Inflammation Chronic obstructive pulmonary disease ↑ Promotion circFOXO3/miR−214 −3p/IKK-β/NF-κB [37] Osteoarthritis ↓ Suppression circFOXO3/FOXO3/PI3K/AKT [30] Neurological diseases Blood-brain barrier damage ↑ Suppression circFOXO3/mTOR/mTORC1 circFOXO3/E2F1/mTORC1 [31] Neurodegenerative diseases ↑ Promotion circFOXO3/FOXO3/Bim EL [41] Intracranial aneurysm ↑ Promotion circFOXO3/miR−122 −5p/KLF6 [42] Cardiovascular disease Ischemia-reperfusion injury in heart transplantation ↑ Promotion circFOXO3/FOXO3 [45] Myocardial infarction ↓ Suppression circFOXO3/KAT7 / HMGB1 [32] ↑upregulated, ↓downregulated. Table 2 Biological function and potential mechanisms of circFOXO3 in cancer. Table 2 Cancer types circFOXO3 Expression Effect On Cancer Associated Pathway/Process Function roles References Prostate cancer ↓ Suppression circFOXO3/FOXO3/EMT Migration, invasion, apoptosis and chemoresistance [27] ↑ Promotion circFOXO3/miR−29a−3p/SLC25A15 Proliferation and apoptosis [24] ↑ Promotion circFOXO3/miR−1299/CFL2 Proliferation, migration, invasion, apoptosis, and tumorigenesis [49] Breast cancer ↓ Suppression circFOXO3-p53-MDM2 circFOXO3/MDM2/FOXO3/PUMA Viability, apoptosis and tumorigenesis [51] ↓ Suppression circFOXO3/WHSC1/H3K36me2/Zeb2 Proliferation, migration, invasion, and tumorigenesis [52] ↓ Suppression / Apoptosis [29] Hepatocellular carcinoma ↑ Promotion circFOXO3/miR−199a−5p/ABCC1 Invasion, migration, tumorigenesis and chemoresistance [28] ↑ Promotion circFOXO3/miR−624 Proliferation and migration [25] ↓ Suppression circFOXO3/PI3K/AKT Proliferation, migration, invasion and apoptosis [54] Glioblastoma ↑ Promotion circFOXO3/miR−138 −5p/NFAT5, circFOXO3/miR−432 −5p/NFAT5 Proliferation, migration, invasion and tumorigenesis [55] Gastric carcinoma ↑ Promotion circFOXO3/miR−143 −3p/USP44 Proliferation, migration and tumorigenesis [56] Endometrial carcinoma ↑ Promotion circFOXO3/miR−29a−3p/HDAC4 Proliferation, migration, invasion, apoptosis and tumorigenesis [57] Oral squamous cell carcinoma ↑ Promotion circFOXO3/miR−214/KDM2A Proliferation and invasion [58] Non-small cell lung cancer ↑ Promotion circFOXO3/miR−545 −3p/HMGB3, circFOXO3/miR−506 −3p/HMGB3 Proliferation, migration, invasion and tumorigenesis [59] Colorectal cancer ↓ Suppression circFOXO3/miR−543/LATS1 Proliferation, migration and invasion [26] Esophageal squamous cell cancer ↓ Suppression circFOXO3/miR−23a/PTEN Proliferation, migration, invasion apoptosis and tumorigenesis [60] Bladder cancer ↓ Suppression circFOXO3/miR−9 −5p/TGFBR2 Proliferation, migration and invasion [61] Clear cell Renal cell carcinoma ↑ Suppression KLF16/circFOXO3/miR−29a−3p, KLF16/circFOXO3/miR−122 −5p Proliferation and apoptosis [62] ↑ upregulated, ↓ downregulated. circFOXO3 is involved in various diseases through different pathways. Standard-shaped arrow: promotion; T-shaped arrow: inhibition. Biological functions and potential mechanisms of circFOXO3 in senescence, inflammation, neurological diseases, and cardiovascular diseases. ↑upregulated, ↓downregulated. Biological function and potential mechanisms of circFOXO3 in cancer. ↑ upregulated, ↓ downregulated. Senescence is a multifactorial process involving the gradual deterioration of function at the cellular, tissue, and organ levels, with cellular senescence being the physiological basis of organismal senescence [23] . Cellular senescence constitutes a permanent state of cell cycle arrest that promotes tissue remodeling during development and after injury; however, it can also lead to a decline in the regenerative potential and functionality of tissues, as well as inflammation and tumourigenesis in organisms [33] . Cell senescence may result from telomere shortening, DNA damage, oxidative stress, physiological stress, and other factors [34] . Du WW et al. [21] explored the potential role of circFOXO3 in regulating cell cycle progression and observed that circFOXO3 was upregulated in non-cancer cells. The overexpression of circFOXO3 was found to exert inhibitory effects on cell proliferation and cell cycle progression. Moreover, it was demonstrated that circFOXO3 combines with p21 and CDK2 to form a circFOXO3-P21-CDK2 ternary complex, which blocks the cell transition from the G1 phase to the S phase, thereby delaying the cell cycle process [21] . The researchers further discovered that circFOXO3 is highly expressed in the hearts of aged mice and patients and correlated with extensive senescence [23] . Ectopic expression of circFOXO3 induces senescence and exacerbates adriamycin-induced cardiomyopathy, whereas silencing endogenous circFOXO3 inhibits senescence and attenuates cardiomyopathy [23] . This effect is mediated by interactions with antiaging-associated proteins ID1 and E2F1, as well as antistress-associated proteins HIF1α and FAK. This interaction blocks the nuclear translocation of these transcription factors, retaining them in the cytoplasm, where they cannot exert their anti-aging and anti-stress effects, thereby promoting cellular senescence [23] . Furthermore, it has been reported that circFOXO3, the profile of expressed circRNAs in human peripheral blood, exhibits differential expression in one or more types of human senescent cells and is negatively correlated with measures of human parental longevity [35] . In conclusion, senescence represents one of the most intricate phenotypes. These few studies above suggest that circFOXO3 plays an important regulatory role in cellular senescence, underscoring the promise of circFOXO3 as a potential biomarker and target for drug development to inhibit tissue senescence. Inflammation represents an adaptive response elicited by injurious stimuli and conditions, such as infection or tissue injury, and underlies a variety of physiopathological processes [36] . The inflammatory response is intended to adapt the host to abnormal conditions, restore tissue function and homeostasis [36] . In general, it is beneficial to control the inflammatory response. The biological role and therapeutic potential of circRNAs in inflammation have received increasing attention in recent years. Chronic obstructive pulmonary disease (COPD) is a prevalent chronic airway inflammatory disease. ZHOU L et al. [37] investigated the impact of circFOXO3 on the inflammatory processes associated with COPD pneumonia, observing a significant upregulation of circFOXO3 expression in murine lungs exposed to cigarette smoke (CS) and in murine alveolar epithelial cells treated with cigarette smoke extract (CSE). Knockdown of circFOXO3 has a protective effect on CS-induced inflammation, and this effect is mediated by downregulating the output of circFOXO3 - miR-214–3p - IKK-β axis and inhibiting the NF-κB pathway [37] . Osteoarthritis (OA) is a common and disabling degenerative joint disease characterized by cartilage degeneration and joint space narrowing, leading to pain and dysfunction [38] . As a sterile inflammatory condition, OA involves complex biological processes that contribute to its progression and symptomatology. ZHAO C et al. [30] investigated the role of circFOXO3 in the progression of OA and found that circFOXO3 is highly expressed in normal articular cartilage but lowly expressed in OA tissues. Additionally, a negative correlation was observed between the expression level of circFOXO3 and the severity of OA. Moreover, this study revealed that circFOXO3 regulates cell proliferation, apoptosis, and extracellular matrix(ECM) metabolic homeostasis by targeting its parental gene FOXO3 and activating PI3K/AKT-mediated autophagy, thus exerting a chondroprotective effect [30] . Numerous circRNAs have been reported to be abundantly present, dynamically expressed, and spatially regulated in the brain, significantly influencing central nervous system development [39] , [40] . Furthermore, alterations in circRNAs levels mediate brain diseases and degeneration. circFOXO3 has been found to play an important role in neurological disorders by mediating autophagy, oxidative stress, and other pathways. Yang, Z et al. [31] revealed an up-regulation of circFOXO3 expression in the Blood-brain barrier (BBB) following ischemia/reperfusion (I/R). Furthermore, circFOXO3 was found to suppress mTORC1 activity by interacting with mTOR and E2F1, thereby inducing autophagy to mitigate BBB damage [31] . Lin, SP et al. [41] investigated the role of circFOXO3 in neurodegenerative diseases(ND) and observed a significant upregulation of circFOXO3 in glutamate-induced neuronal cells HT22. The high expression of circFOXO3 resulted in the upregulation and nuclear localization of FOXO3, thereby contributing to mitochondria-mediated apoptosis through activation of the circFOXO3/FOXO3/Bim EL pathway [41] . Consequently, blocking circFOXO3 can protect neuronal cells from glutamate-induced oxidative damage. Moreover, a study has demonstrated that circFOXO3 exhibits high expression levels in intracranial aneurysm(IA) tissues and is involved in intracranial aneurysm progression through the circFOXO3/miR-122–5p/KLF6 axis [42] . Overall, the existing studies provide new insights and directions for the regulatory role of circFOXO3 in neurological diseases, highlighting its potential as a novel therapeutic target. However, further research is necessary to explore and validate the specific effects of circFOXO3 on neurological disorders and to understand the complex mechanisms underlying its actions. Cardiovascular disease (CVD) remains a leading cause of global death and the world's disease burden [43] . Emerging research supports that circRNAs play an important role in cardiovascular disease as a potential new biomarker and therapeutic target for CVD [44] . Su, Y et al. [45] investigated the role of circFOXO3 in I/R injury in heart transplantation(HT) and observed a significant upregulation of circFOXO3 in HT I/R-injured hearts and as well as in hypoxia/reoxygenation-injured cardiomyocytes. Knockdown of circFOXO3 mitigated the I/R injury in HT mice, leading to improved cardiac transplantation function. Furthermore, it was found that circFOXO3 promotes I/R injury in HT by inhibiting FOXO3 phosphorylation through interaction with FOXO3 [45] . In contrast to the findings regarding circFOXO3 in this study, Sun, G et al. [32] reported that circFOXO3 exhibited cardioprotective effects in both in vivo and in vitro myocardial infarction (MI) models. In a rat model of myocardial infarction where circFOXO3 expression was downregulated, it was observed to inhibit autophagy by targeting the KAT7/HMGB1 axis, thereby mitigating myocardial I/R injury and modulating MI-associated cardiac dysfunction [32] . Furthermore, Zhou, Y et al. [46] discovered that the rs12196996 polymorphism on the flanking intron of circFOXO3 was associated with the susceptibility to developing coronary artery disease(CAD) in the Chinese Han population, particularly prominent among younger individuals and non-smokers carrying the G allele. Further studies showed that the rs12196996 GG genotype is correlated with circFOXO3 expression, thereby modulating individual predisposition to CAD [46] . These findings indicate that circFOXO3 holds significant potential for clinical application in CVD. Moreover, the expression of circFOXO3 may be subject to regulation by gene polymorphism, presenting a promising potential target for future CVD diagnosis and therapy. Prostate cancer(PCa) has the fourth highest incidence of cancer worldwide and is the fifth leading cause of cancer deaths in men [47] . As a highly heterogeneous and complex cancer, PCa with high morbidity and mortality has become an important health problem [48] . SHEN Z et al. [24] observed that circFOXO3 exhibited low expression levels in high-grade PCa and androgen-independent PCa cell lines. Furthermore, they discovered that ectopic expression of circFOXO3 inhibited PCa cell survival, migration, invasion, and chemoresistance to docetaxel. Additionally, it was further explored that circFOXO3 inhibited PCa progression and chemoresistance to docetaxel by enhancing FOXO3 expression and inhibiting epithelial-mesenchymal transition(EMT), suggesting that targeting the circFOXO3 /FOXO3/ EMT axis may represent a viable strategy for exploring potential prognostic and therapeutic approaches for PCa [24] . On the contrary, KONG Z et al. [27] reported an upregulation of circFOXO3 in both PCa tissue and serum samples, which promoted PCa cell proliferation and cell cycle while inhibiting apoptosis. Furthermore, they found that circFOXO3 promoted PCa progression by acting as the miR-29a-3p sponge, leading to the upregulation of solute carrier family 25 member 15(SLC25A15) [27] . In addition, Li, P et al. [49] reported that circFOXO3 expression was upregulated in PCa tissues and cells, and circFOXO3 promoted PCa cell viability, migration, invasion, and proliferation, inhibited apoptosis, and thus promotes PCa progression by regulating the miR-1299/CFL2 axis. This is consistent with the report of KONG Z et al. [27] . Overall, although the role of circFOXO3 in PCa is currently controversial and needs to be further validated, these findings indicate that circFOXO3 has the potential to function as a valuable biomarker and therapeutic target for PCa. Breast cancer(BC), an aggressive malignant tumor, is the most common cancer in women and one of the leading causes of cancer death among women worldwide [47] . BC is highly heterogeneous and clinically divided into three main subtypes based on hormone receptor(ER and PR) and human epidermal receptor 2 {HER2 (ERBB2)} status: ER+ and PR+ , HER2 + and triple-negative breast cancer(TNBC) [50] . Du WW et al. [51] discovered the expression of circFOXO3 was downregulated in BC tissues and cells. High levels of circFOXO3 expression inhibited cell viability, promoted apoptosis, and inhibited tumor xenograft growth [51] . Mechanistically, circFOXO3 enhances the activity of FOXO3 by: circFOXO3 interacts with p53-MDM2 to promote MDM2-induced ubiquitination and degradation of p53. This interaction reduces the MDM2-mediated ubiquitination of FOXO3, thereby elevating its expression, which in turn increases the expression of the downstream target PUMA, promoting apoptosis [51] . Consistently, Chen, D et al. [52] reported a significant downregulation of circFOXO3 in TNBC tissues and blood samples from patients, which correlated with lymph node metastasis and poor prognosis in TNBC patients. Overexpression of circFOXO3 inhibited the proliferation, invasion, and metastasis of TNBC cells both in vivo and in vitro. Additionally, circFOXO3 inhibited TNBC progression by interacting with WHSC1 and inhibiting the WHSC1/H3K36me2/Zeb2 signaling pathway [52] . Abdollahi, E et al. [26] reported that circFOXO3 expression is downregulated in peripheral blood mononuclear cells (PBMCs) of BC patients and may serve as a potential predictor for cellular radiosensitivity in BC patients. Notably, another study reported that circFOXO3 expression was downregulated in breast cancer stem-like cells (BCSCs), suggesting that BCSCs evade apoptosis by downregulating circFOXO3 [53] . The above findings suggest that circFOXO3, which is lowly expressed in BC and plays a role as a tumor suppressor, has the potential to serve as both a biomarker and therapeutic target for BC. Additionally, it shows promise as a potential biomarker for predicting radiosensitivity in BC patients. However, further studies are needed to explore and validate these possibilities in the future. Hepatocellular carcinoma (HCC) is a malignant tumor characterized by a high incidence and the third leading cause of cancer-related death worldwide, has become a major global health challenge [47] . Huang, W et al. [25] reported an upregulation of circFOXO3 expression in HCC tissues and cells, particularly in adriamycin (ADM)-resistant HCC. Further investigations revealed that circFOXO3 promotes ADM resistance by targeting miR-199a-5p, thereby positively regulating the expression of ATP-binding cassette subfamily C member 1 (ABCC1) protein and inducing EMT [25] . Zhang, L et al. [28] reported that circFOXO3 is overexpressed in HCC tissues and cells, promoting HCC progression through negative regulation of miR-624 to induce HCC cell growth, cell cycle, and migration. In contrast to their findings, Liu, S et al. [54] reported that circFOXO3 expression was downregulated in HCC cells. Additionally, the overexpression of circFOXO3 markedly inhibited the proliferation, migration, and invasion of HCC cells by modulating the PI3K/Akt pathway while promoting apoptosis [54] . The above findings indicate that circFOXO3 is closely related to the development of HCC and may serve as both a novel biomarker and therapeutic target for HCC. However, based on current study reports, the regulatory role of circFOXO3 in HCC remains controversial and requires further validation through additional studies in the future. In addition to the above-mentioned cancers, several studies have reported a significant association between circFOXO3 and the development of other cancers, including glioblastoma(GBM) [55] , gastric cancer(GC) [56] , endometrial cancer(EC) [57] , oral squamous cell carcinoma(OSCC) [58] and non-small cell lung cancer(NSCLC) [59] , which play a cancer-promoting role, as well as colorectal cancer(CRC) [29] , esophageal squamous cell carcinoma(ESCC) [60] , bladder cancer [61] and renal cell carcinoma [62] , which play a cancer-suppressing role. Among the studies reporting that circFOXO3 plays a cancer-promoting role, Zhang, S et al. [55] discovered high expression of circFOXO3 in GBM tissues and cells, which significantly correlated with tumor size, histological grade, wild-type IDH expression, and MGMT methylation status. They also found that upregulation of circFOXO3 expression promoted cell proliferation, migration, and invasion while conversely suppressing these processes [55] . Additionally, circFOXO3 acts as a sponge for miR-138–5p/miR-432–5p and regulates NFAT5 expression through a ceRNA mechanism, thereby promoting GBM progression in vivo and in vitro [55] . Xiang, T et al. [56] found that circFOXO3 promotes GC progression by interacting with miR-143–3p to upregulate ubiquitin-specific peptidase 44(USP44) expression. Yang, L et al. [57] found a significant upregulation of circFOXO3 in EC tissues and cells and regulates EC progression through the miR-29a-3p/HDAC4 axis. In addition, circFOXO3 has been reported to promote OSCC progression by targeting miR-214 to upregulate KDM2A expression [58] . It also promotes NSCLC progression by sponging miR-545–3p and miR-506–3p to upregulate HMGB3 [59] . Among the studies reporting that circFOXO3 plays a cancer-suppressive effect, Deng, YY et al. [29] discovered a downregulation of circFOXO3 expression in CRC tissues and cells, which correlated with poor overall survival in CRC patients. Additionally, circFOXO3 was also associated with tumor size, distant metastasis, differentiation, lymph node metastasis, and TMN stage in CRC patients [29] . Overexpression of circFOXO3 can inhibit CRC cell proliferation, migration, and invasion, while inhibit CRC metastasis and progression by acting as an miR-543 sponge to upregulate large tumor suppressor kinase 1 (LATS1) expression [29] . XING Y et al. [60] found that circFOXO3 can inhibit the progression of ESCC by regulating the miR-23a/PTEN pathway through the ceRNA mechanism. LI Y et al. [61] found that circFOXO3 can inhibit the progression of bladder cancer by regulating the miR-9–5p/TGFBR2 signaling pathway. Furthermore, it has been reported that circFOXO3 modulates the progression of clear cell renal cell carcinoma (ccRCC) and the cytotoxic activity of natural killer cells via the KLF16/circFOXO3/miR-29a-3p/miR-122–5p signaling pathway, under the positive transcriptional regulation of the upstream regulator KLF16 [62] .

Linhu Ye is responsible for the conceptualization and design of the work, Min Zhao conducts a comprehensive literature review and drafts the manuscript, Minting Lin is involved in editing the manuscript, and Zhibo Zhang contributes to reviewing and revising the manuscript.

Not applicable.

This work is supported by the Foshan “Fourteen Five” Key Medical Specialty Construction Project ( FSZD145035 ).

circRNAs are a class of RNAs characterized by covalent closed-loop structures formed through the reverse splicing of pre-mRNAs. circRNAs are endogenous, abundant, stable, conserved, and tissue-specific in mammalian cells [3] , [5] , [6] , [63] . They not only play an important regulatory role in the occurrence and development of human diseases but also have the potential to serve as targets for disease diagnosis and treatment. As one of the most important circRNAs, circFOXO3 is specifically expressed in different tissues and cells, and it’s closely associated with several diseases, including cancer, cardiovascular diseases, neurological diseases, senescence, and inflammation. circFOXO3 functions as a sponge for certain miRNAs, regulating cell proliferation, apoptosis, migration, and invasion, while also influencing cancer progression by modulating the transduction of multiple signaling pathways through a ceRNA mechanism. circFOXO3 may also be involved in the regulation of neurological disorders, participate in myocardial I/R injury, mediate inflammatory responses, interact with certain proteins to regulate cell cycle progression, delay senescence, etc. Moreover, circFOXO3 has been reported to enhance the resistance of HCC to doxorubicin [25] , inhibit the chemotherapy resistance of PCa to docetaxel [24] , and influence the response of ovarian cancer to docetaxel therapy by regulating the expression of p-GP [64] . Interestingly, circFOXO3 also protects cardiomyocytes from radiation-induced cardiotoxicity by mitigating DNA damage and apoptosis, suggesting that it may serve as a potential therapeutic target for radiation-induced cardiotoxicity [65] . In addition to this, recent studies have reported that circFOXO3 can promote granulosa cell apoptosis under oxidative stress by modulating FOXO3 proteins, resulting in diminished ovarian function [66] . Furthermore, it has been demonstrated that circFOXO3 can mediate hypoxia-induced autophagy of endometrial stromal cells, playing a role in the development of endometriosis [67] . Additionally, circFOXO3 can contribute to the progression of intervertebral disc degeneration (IVDD) through the mediation of oxeiptosis [68] . Notably, phase II clinical studies have been completed for drugs(RG-101, carbohydrate-acetylgalactosamine-conjugated oligonucleotide) that inhibit miR-122, highlighting circFOXO3 as a promising new target for drug development [69] , [70] . As a miR-122–5p sponge, circFOXO3 may play a significant role in mediating therapeutic effects, making it an attractive candidate for further research [42] , [62] , [70] . It is noteworthy that circFOXO3 exhibits stable expression in human plasma, suggesting its potential utility as a biomarker for predicting GBM [71] . The expression of circFOXO3 has also been detected in human peripheral blood, and its association with human aging phenotypes indicates its potential as a biomarker for age-related diseases [35] . circFOXO3 is downregulated in peripheral blood mononuclear cells of BC patients, highlighting its potential as a biomarker for predicting BC and the radiosensitivity of BC patients [26] . Furthermore, a study has reported that circFOXO3 is downregulated in the bone marrow of patients with de novo acute myeloid leukemia(AML), indicating certain diagnostic values for AML [72] . However, the current literature reveals inconsistencies in the findings regarding the expression and role of circFOXO3 in disease, which remain a subject of debate. In the context of CVD, Su, Y et al. [45] found that circFOXO3 overexpression promotes I/R injury in HT, while Sun, G et al. [32] reported that upregulation of circFOXO3 expression ameliorates MI-induced cardiac dysfunction and mitigates myocardial I/R injury. In the same type of cancer study, SHEN Z et al. [24] reported that circFOXO3 exhibited low expression in PCa and showed cancer-suppressive activity; however, both KONG Z et al. [27] and Li, P et al. [49] reported an upregulation of circFOXO3 expression in PCa with carcinogenic activity. In addition, both Huang, W et al. [25] and Zhang, L et al. [28] reported an upregulation of circFOXO3 expression in HCC tissues and cells, which promoted HCC progression. Conversely, Liu, S et al. [54] found that circFOXO3 was low expression in HCC cells and exhibited anticancer activity. Based on a comprehensive analysis of the available studies, there are several potential explanations for the divergent roles and contradictory findings of circFOXO3, both within the same category of disease and even in identical diseases: (1) disparities in experimental conditions; (2) variations in both experimental cell status and disease status may influence research outcomes; (3) inadequate sample sizes and biased experimental results leading to instability in findings; (4) the effect of tumor subtypes; (5) the heterogeneity of tumors and differences in response result in diverse effects; (6) the influence of epigenetics; (7) single nucleotide polymorphisms may also contribute to causation; (8) the mechanism of circFOXO3 action is complex, encompassing multiple signaling pathways and molecular interactions, and different research teams focus on different downstream pathways or molecular events, leading to inconsistent or even contradictory results, etc. In conclusion, future studies should expand the sample size to further elucidate the exact role of circFOXO3 in the disease and validate the feasibility and reliability of circFOXO3 as a biomarker and therapeutic target. Although much progress has been made in the study of circRNAs, the biological functions of circRNAs are still largely unknown, and their exact roles and complex regulatory mechanisms are still unclear. Therefore, we need to further study the intricate network of circRNAs interactions in diseases in the future, including a comprehensive exploration of the detailed upstream and downstream regulatory mechanisms, miRNAs, lncRNAs, and roles in the DNA genome, etc. Additionally, elucidating the production and degradation pathways, biological functions, interaction pathways and target receptors of circRNAs is essential for a more thorough understanding. This will provide valuable insights into human disease pathogenesis, early prediction, diagnosis, and therapeutic strategies. Future research on circRNAs may need to consider the following issues: (1) Given the increasing number of identified circRNAs and the ongoing advancement of research techniques, a standardized nomenclature system is necessaryl for naming circRNAs. (2) Exploring and elucidating the generation pathway, degradation process, secondary structure, and function of circRNAs and their roles in the DNA genome is an essential aspect of the research. (3) The biological functions of circRNAs and their involvement in disease development are shaped by the intricate network of gene interactions and pathway regulation, and unraveling this complex network is a key point for future research. (4) Despite many reports on the potential of circRNAs as promising new biomarkers and therapeutic targets, there is still a lack of studies validating circRNAs in these roles. Future research should be to confirm the clinical significance of circRNAs in diseases. (5) Further preclinical and in vivo studies are warranted to elucidate the impact of circRNAs on diseases. Subsequent research endeavors should expand the sample size, incorporate clinical samples, and enhance methodological rigor to ensure the robustness of findings. (6) Disease states may affect the results of circRNAs studies, and therefore disease progression and state must be considered in the development of drugs and diagnostic reagents. (7) Due to the wide distribution of circRNAs in different cells and tissues, diagnostic specificity is also a challenge. (8) The use of tissue samples to detect circRNAs may cause some harm to patients. Therefore, there is a need to determine the expression level and stability of circRNAs based on blood or body fluid assays and look forward to the minimally invasive screening of patients in the future. (9) Another aspect that cannot be ignored is the cost of synthesis, preparation, and detection of circRNAs. In this paper, we present a comprehensive review of recent research advancements regarding circFOXO3 in various diseases. We outline the important biological functions and potential molecular mechanisms of circFOXO3 in senescence, inflammation, neurological disorders, cardiovascular diseases, and cancer. This provides a theoretical foundation and novel insights for future in-depth research on circFOXO3. With advancements in technology and extensive research, circFOXO3 holds promise as a clinical biomarker and a target for drug development. We expect that further studies will enable circFOXO3 to enter into clinical practice soon, offering patients with individualized, precise, and effective diagnosis, prevention, and treatment through genetic testing and other means, bringing patients the gospel.

Circular RNAs (circRNAs) are a type of non-coding RNA (ncRNA) first reported in 1976, initially believed to be a result of RNA splicing errors [1] , [2] . With the development and application of high-throughput sequencing technology and various bioinformatics methods, more circRNAs and their functional characteristics have been discovered. Generally, circRNAs are mainly found in the cytoplasm, with a few in the nucleus [3] . Unlike linear RNA, circRNAs are generated through the back-splicing of precursor mRNA (pre-mRNA). Currently, three primary circularization patterns have been reported: direct back-splicing, RNA-binding protein-mediated circularization, and lariat-driven circularization [4] . circRNAs possess a covalently closed loop structure with neither 5′−3′ polarity nor a polyadenylated (poly-A) tail. This unique configuration prevents degradation by RNA exonucleases, resulting in greater stability and abundance compared to typical linear transcripts of the same genes within the cell [4] , [5] . Studies have shown that there are a large number of circRNAs in mammals, which have the characteristics of endogenous, high abundance, stability, evolutionary conservation, and tissue/developmental stage-specific expression in species [3] , [5] , [6] . These attributes enable circRNAs to play an important regulatory role in biological functions, gene expression, and pathophysiological processes. Existing studies have shown that circRNAs can be primarily categorized into four types according to different occurrence patterns in genomic regions: exonic circRNA (ecircRNAs), intronic circRNA (ciRNAs), exonic-intronic circRNA (EIciRNAs), and intergenic circRNA [7] , [8] . The primary roles of circRNAs identified in current research encompass: (1) Acting as a microRNA(miRNA) sponge. circRNAs can share miRNA response elements (MRE) with other RNAs such as mRNA, pseudogenes, and long noncoding RNAs (lncRNAs), etc. They competitively bind to miRNA sites and act as miRNA sponges to influence miRNA activity against other target genes [9] , [10] . (2) Interaction with proteins. circRNAs can modulate protein function by serving as decoys, scaffolds, and recruiters, thereby enhancing, reducing, or influencing various protein functions such as nuclear entry and exit or translocation [11] . (3) Regulation of gene transcription. circRNAs can compete with pre-mRNA splicing to exert gene regulation [12] . Furthermore, circRNAs can interact with the RNA Pol II machinery and enhance its transcriptional activity [13] . (4) Translation into proteins. Although circRNAs are categorized as ncRNAs, they can be translated into proteins driven by internal ribosome entry site (IRES) or N6-methyladenosine (m6A)-containing short sequences [14] . Furthermore, research has indicated that circRNAs may undergo translation into proteins via the mechanism of rolling circle amplification (RCA) [15] , [16] . (5) Transporting substances and information. circRNAs are highly abundant and stably expressed in exosomes and participate in the process of exosome function, mediating the transport of substances and information [17] , [18] . Overall, circRNAs play a wide and diverse range of functions in biological processes and in the onset and development of disease, which has attracted the attention of more and more researchers and has become one of the hotspots of current research. As one of the most extensively investigated and important circRNAs, circular RNA FOXO3 (circFOXO3, Hsa_circ_0006404) is involved in many physiopathological processes, with specific expression in different tissues and cells. It also has been closely associated with cancer, cardiovascular disease, neurological disease, senescence, and inflammation. Given the significance of circFOXO3 in the pathogenesis of human diseases, this paper presents a systematic review of existing studies on circFOXO3 to elucidate its biological functions and potential molecular mechanisms, as well as explore the future directions and challenges in the study of, circRNAs, to provide valuable insights and references for further explorations in this field.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.