Foxp3+ Treg cells predominantly secrete Foxp3, a subset of T cells that regulate the body’s immune system by exerting immunosuppressive effects. Foxp3+ Treg can be acquired in two ways: the first is through maturation in the thymus and release into the periphery, at which point Foxp3+ Treg are also known as natural regulatory T cells (nTregs); another method of production is through Transforming Growth Factor-β (TGF-β) Induced regulatory T cells (iTreg). These cells protect their own reactive lymphocytes from immunological reactions, thereby maintaining their immunity and tissue stability (7) ( Table 1 ).

The transcription factor Foxp3 belongs to the multi-domain Fox family of forkhead box proteins. Proline and other amino acids make up the majority of the N-terminal region, which binds to different protein molecules to carry out transcriptional inhibition functions. For example, it can inhibit the production of NF-AT-mediated transcription-activating factor IL-2 (36, 37); the stable C-terminal forked head domain (FKH) can recognize and bind specific DNA sequences. Central zinc finger transcriptional regulation and oligomerization modification with leucine domain (38). Foxp3 is primarily produced by T cells; however, non-Treg cells can also generate some Foxp3. Foxp3, a crucial transcription factor in regulatory T cells (Treg cells), regulates various functions and processes, such as development and maturation. The function of Treg cells may be inhibited by a lack of or low expression. Significantly, human X-linked and autoimmune diseases, including immune system disorders, several endocrine disorders, intestinal diseases, X-linked syndrome (IPEX syndrome), and others, can result from the functional loss or mutation of the Foxp3 gene (39, 40). The primary features include various autoimmune disorders affecting organs, significant allergies, and excessive inflammation. T cell differentiation and function may be influenced by the interactions of Foxp3 with other factors. Through its antagonistic effects on Toll-like receptors (TLRs), Foxp3 contributes to the reprogramming of T cell metabolism, enhancing T cell oxidative phosphorylation (OXPHOS) and fatty acid oxidation capacity. Additionally, Foxp3 collaborates with the original glycolytic pathway to produce energy, ensuring that Treg cells have the essential resources for proliferation and preventing cell apoptosis (41, 42). Conversely, Foxp3 suppresses the glycolysis and Myc signaling pathways when it binds to the oncogene Myc promoter, which impacts T cell metabolism (43).

Additionally, Foxp3 can control the body’s immunity by directly or indirectly influencing downstream components in tumor cells. In their investigation of non-small cell lung cancer cells, Peng et al. (44) discovered that Foxp3 overexpression in the tumor microenvironment can suppress anti-tumor immunity and promote the proliferation of cancer cells. Direct binding of Foxp3 to the LINC00885 promoter can upregulate the production of proteins associated with the epithelial mesenchymal transition (EMT), thereby promoting the growth and invasion of cervical cancer cells (45). Furthermore, numerous clinical investigations have established that overexpression of Foxp3 is associated with a poor prognosis and low survival rate in cancer patients (46, 47). For instance, Foxp3 expression is higher and survival is lower in individuals with oral squamous cell carcinoma (48). On the other hand, patients with specific malignancies have been shown to benefit from elevated Foxp3 expression. According to published research, patients with high levels of VEGF and CD44 expression in breast cancer have a comparatively short survival period. Additionally, there is a negative correlation between Foxp3 expression and VEGF and CD44 expression. On one hand, Foxp3 directly inhibits the activity of the VEGF promoter through a specific forkhead binding motif, which leads to the inhibition of angiogenesis in breast cancer by suppressing the expression of VEGF, consequently downregulating VEGF (49); On the other hand, Foxp3 binds to the promoter of CD44 coding gene to suppress breast cancer metastasis, thereby hindering the progress of breast cancer (50). In addition, Gal-1 regulates the anti-tumor properties of Foxp3 by binding with the FKH domain of Foxp3 in Foxp3-positive breast cancer cells, thereby maintaining the stability of cancer cells (51). According to reports, Foxp3 is a downstream target of p53 mediated cell aging, promoting the aging of epithelial cancer cells by inducing the expression and generation of p21 and ROS (52). It Collaborates with microRNA-155 to regulate the transcription process of Zinc finger E-box binding homology box 2 (ZEB2), inhibiting the expression of ZEB2 in colon cancer. This inhibition leads to the suppression of cancer cell proliferation and metastasis, enhancing anti-tumor immunity (53) ( Figure 1 ). Therefore, due to the widespread expression of Foxp3 in tumor cells, it has a dual role in tumor induced proliferation or inhibition. Studying the regulatory mechanisms of Foxp3+ Treg cells and Foxp3 involvement in malignant tumors is of great significance for understanding and developing treatments for each disease.

Expression of Foxp3 in relation to T cells and cancer cells in the CRC tumor microenvironment. In colon cancer cells, Foxp3 and microRNA-155 work together to synergistically regulate the transcription of Zinc finger E-box binding homology box 2 (ZEB2), which suppresses ZEB2 expression and boosts anti-tumor immunity, reducing cancer cell growth and metastasis; In addition, Overexpression of Foxp3 also promotes MMP9 expression through the SAM cycle; The decrease of TCF-1 leads to an increase in the binding of Foxp3 to downstream gene regulatory elements, thereby promoting the expression of Treg, inhibiting the proliferation of effector T cells, and promoting the progression of CRC. ZEB2,the Zinc finger E-box binding homology box 2; MMP9,Matrix Metallo Protein 9; TCF-1,T Cell Transcription Factor 1.

It has been discovered that the conserved non-coding sequence CNS0–3 includes the Cage1 site, which is situated 1500 bp upstream of the core promoter, and the Cage2 site, which is situated 2000 bp upstream, as well as other required regulatory regions for Foxp3 gene expression (54). The TCR on T cells surface activates Foxp3 transcription negatively by binding to NF-κB. The conserved CNS3 regulatory region of Foxp3 is then joined with the c-Rel of the family. Conversely, acetylated c-Rel on the site inhibits Foxp3 production by adversely regulating the activation of the Foxp3 promoter (8). Alternatively, Foxp3 expression can be inhibited by the specific recruitment of enolase-1 in the glycolytic pathway to the Foxp3 promoter and its CNS2 regulatory region; however, this study also has shown that TCR activation leads to an increase in the cell’s ability to uptake glucose and glycolysis, which, in turn, leads to an increase in the expression of Foxp3 (9). Reports state that long non-coding RNA (lncRNA) fricr is comparable to the Foxp3 genome and has the ability to act cis on Foxp3’s transcription process, decreasing its stability (12); additionally, miR-125b has been shown in thyroid cancer research to target Foxp3 and suppress its mRNA and protein production while increasing cancer cells’ susceptibility to cisplatin treatments (14). Reduced Foxp3 expression is advantageous for enhancing the therapeutic benefit of patients on anti-cancer medications, as miR-4281 directly binds to the Foxp3 promoter, upregulates the expression of Foxp3 protein, and promotes the immunosuppressive role of Foxp3+Treg (15). To better guide anti-cancer treatment, it is imperative to have a thorough understanding of the mechanisms behind the activity of non-coding RNAs (ncRNAs), as they are also crucial in regulating the Foxp3 process.

The TCR signaling pathway has a distinct regulatory mechanism that manifests a twofold regulatory effect throughout the transcriptional activation process of Foxp3. Deoxyribonucleoside triphosphate triphosphate hydrolase (SAMHD1) co-localizes with ODNps25 in the cytoplasm and interacts directly with Foxp3 through phosphorylation of T592 in the TCR/CDK2/SAMHD1 pathway, which increases the stability of Foxp3 mRNA by inhibiting the activity of the dNTPase (10). The DNA demethylating Tet2 enzyme (a member of the TETase family) is increased by high-intensity TCR signaling stimulation (55) or TGF–induced phosphorylation of Uhrf1, which enables it to act on the CpG island of Foxp3 CNS2 and preserves the stability of the Foxp3+ Treg (11). However, Stephan Sauer et al. (13) has shown that stimulation of the PI3K/AKT/mTOR pathway reduces Foxp3 production and persistent TCR signaling prevents naive CD4+T cells from differentiating into Foxp3+CD4+ T cells. In summary, further research is necessary to fully understand the effects of Foxp3 involvement on cancer cells and Foxp3+Treg function in particular tumor microenvironments, as well as to demonstrate the advantageous role of Foxp3 therapeutic blockade in future immunotherapy strategies, which will offer fresh concepts for clinical targeted therapy. This is even though we have clarified the specific regulatory mechanisms of some Foxp3 transcription processes in different tumor types.

According to the study’s findings, Foxp3+Treg expression increased in the majority of tumors, and while the tumor microenvironment was anaerobic, HIF-1 aggregation was present (57). In this situation, HIF-1can bind to the Foxp3 protein and can inhibit HIF-1 degradation, and at the same time, Foxp3 can increase HIF-1’s downstream target genes, including VEGF and the glucose transporter protein (GLUT) (58). Lung cancer cell growth is aided by the MIF/NF-ƘB/HIF-1 pathway of macrophage migration inhibitory factor, which keeps the Warburg effect-related factors stable (25). In conditions with high lactate and low sugar, oxidizing lactate to pyruvate facilitates nicotinamide adenine dinucleotide (NAD) regeneration and oxidative phosphorylation. Foxp3+Treg benefits from changes in tumor glycolysis because it strengthens its immunosuppressive capacity in the tumor microenvironment and encourages tumor cell survival (59). Nonetheless, other researchers have also noted that Foxp3 protein’s ubiquitination is disrupted when HIF-1 α binds to it, which in turn impairs T cell development and, ultimately, the production and functionality of Foxp3+Treg (60).

In conclusion, changes in glucose metabolism enable tumor cells to maintain their energy source, promote growth, and adapt better to hypoxic environments. Still, there are additional elements in the glucose metabolism pathway that are regulated by Foxp3 and HIF-1α. We must conduct more research to better understand the connection between Foxp3 and its function.

Esophageal Carcinoma (ESCC) is a cancerous tumor that primarily affects the esophageal epithelium. According to research, the upstream regulatory factor p53 of miR-149–3p (61) is inhibited from being ubiquitinated when miR-5b-1p binds to the molecular sponge rcRUNX3 (62) or lncRNA MEG3. This helps to upregulate Foxp3 expression in ESCC. By increasing Foxp3 expression through STAT3 modification of Foxp3+Treg transcription process, activating the STAT3/Foxp3 signaling pathway within ESCC tissue hinders macrophage phagocytosis and increases Foxp3+Treg immunosuppressive activity, which helps ESCC evade the body’s immune response (26). Furthermore, upon recognition of its receptor ST2, IL-33 attracts a significant number of Foxp3+Tregs to congregate in the stroma of ESCC, thereby initiating the IL-33/ST2/Foxp3+Treg/COX-2/PGs regulatory pathway, encouraging Foxp3+Treg secretion of cyclooxygenase-2 (COX-2) and enabling the conversion and production of prostaglandins (PGs). These actions work in concert to preserve the immune system’s stability and suppress the tumor microenvironment’s effect, encouraging the proliferation and growth of ESCC cells (27).

The most prevalent type of pancreatic cancer is pancreatic ductal adenocarcinoma (PDAC), and an increase in Foxp3 in the tumor microenvironment of pancreatic cancer affects immune cells (DCs), which are antigen-presenting cells, impairing the body’s ability to mount an immune defense (63). It has been shown that Foxp3 works synergistically to control the recruitment of chemokine CCL5 to Treg cells in the tumor microenvironment to promote the immune escape of PDAC. Foxp3 also directly interacts with the promoter region of programmed cell death-ligand (PD-L1) to promote PD-L1 inhibition of effector T cell activation (3, 28, 64). Foxp3 also confers a suppressive phenotype on Tregs in pancreatic cancer cells by repressing the transcriptional activation of several T cell-stimulated target genes, such as Interleukin-2 (IL-2) (65). It is hypothesized that CTLA-4 could be linked to Foxp3+ T cell suppression of effector T cell function and mediate PDAC immune escape because it is another cytotoxic T lymphocyte-associated antigen with immunosuppressive effects that is expressed more frequently (66). The tissues of pancreatic tumors have elevated expression of the Ceramide synthase (CerSs) isoform CERS6. AKT1-mediated phosphorylation of Foxp3 at the S418 site aids in maintaining the production of CERS6, and elevated levels of CERS6 mRNA predict a worse prognosis for PDAC patients, according to ex vivo research. In addition, CERS6 promotes the p53 mutation that causes PDAC. As a result, inhibiting the AKT1/FOXP3/CERS6 axis can be a possible tactic to prevent the growth of pancreatic tumors (29).

Genetic variations and epigenetic changes are linked to an increased risk of gastric cancer (GC) (67). In GC tumor tissues, Foxp3 protein is expressed more intracellularly (68). When compared to GC tumor tissues, the CD8/Foxp3 ratio in GC paracancerous tissues is much greater, and the elevated expression of PD-L1 could signal a bad prognosis for GC patients (69). The number of Foxp3 + Treg cells is significantly higher than that of paraneoplastic tissues, and the release of COX-2 by Foxp3 + Treg leads to the induction of Prostaglandin E2 (PGE2) expression. These factors together inhibits the anti-cancer effects of effector T cells (6), allowing GC to avoid immune attack and promoting its immune escape and proliferation. Additionally, using COX-2 inhibitors to treat GC offers the chance to reduce Foxp3+ Treg activity. Foxp3 interacts to the PSMD7 promoter and stimulates PSMD7 expression, both of which are up-regulated in gastric cancer tissues, boosting GC cell proliferation and blocking apoptosis (70). With extensive research, Li et al. (30) discovered that FoxM1, along with an increase in the number of Foxp3+Treg, increased significantly in gastric cancer tissues, that overexpression of FoxM1 and Foxp3+Treg favored GC infiltration and invasion, and that inhibition of the regulatory pathways of FoxM1 and Foxp3 could block GC proliferation, implying that the combination of FoxM1 and Foxp3+Treg could be used as a biomarker for diagnosis and prognostic survival of clinical gastric cancer patients. Foxp3+Treg cells share the functional characteristics of Foxp3+Treg cells and RORt-expressing TH17 cells, respectively, for having enhanced immunosuppressive and pro-inflammatory effects, while inhibiting the production of anticancer factors, such as IFN-γ, granzyme B, and other anticancer factors by effector T cells, resulting in an imbalance in the body’s immunity to achieve anticancer immune escape, which is detrimental to the prognosis of patients (71).

Furthermore, investigations have demonstrated that Foxp3 plays an oncogenic function in GC. Foxp3 promotes p21 protein expression by binding to the p21 promoter region, which inhibits cancer cell proliferation; however, in the presence of a large amount of inflammatory infiltration in the GC, Foxp3 has an enhanced interaction with p65, which reduces binding to the p21 promoter, thus contributing to cancer cell proliferation (72); S-phase kinase-associated protein 2 (SKP2) is associated with a double negative feedback loop of p21 and p27, which can lead to the hypothesis that overexpression of Foxp3 in GC inhibits the oncogenic effects of SKP2 (73); the MKL1/miR34a/Foxp3 pathway inhibits the expression of Foxp3 and promotes the proliferation of GC (31). Foxp3 inhibits GC cell proliferation by activating the apoptotic signaling pathway, and increasing Foxp3 expression can increase the expression of pro-apoptotic genes such as PARP, caspase-3, and Casp9, effectively inducing apoptosis in GC cells, and vice versa (74). Foxp3 suppression or binding to miR-133a-3p can limit Foxp3 expression, which stimulates GC cell proliferation and autophagy, helping to deplete and remove damaged cells and maintain cancer cell homeostasis (32). Meanwhile, Foxp3 reduces COX2 expression and cell metastasis as a negative regulator of NF-ƘB activity, presenting new therapeutic and diagnostic alternatives for gastric cancer (75, 76).

The study of CRC markers and their associated signaling pathways may offer suggestions for targeted therapy treatment of colon cancer because colorectal cancer (CRC) is a common cancer of the digestive system, the incidence rate is rising annually in China, and the survival rate of patients is poor (77). Treg cells have different subtypes, and Treg accumulates at the tumor site. On the one hand, it may be due to the co expression of chemokines and other products such as Foxp3 and Helios produced by tumor cells and stroma in the TME of CRC, which can enhance the inhibitory characteristics of Treg more strongly than other molecules acting alone. In addition, The expression levels of PD-1/CTLA-4 and PD-1/CD39 in the subgroup of Treg are elevated. They have a synergistic effect in inhibiting T cell activation and function, as well as inhibiting tumor specific immune responses, thereby helping tumor cells evade T cell immune attacks and promoting cancer cell progression (33). On the other hand, in the presence of TGF - β, it can also drive the expression of tumor infiltrating Treg cells. In addition, The Treg density of FOXP3 in tumor tissue is higher than that in normal colon mucosa, and its increased expression is associated with poor CRC survival rate. Overexpression of Foxp3 promotes MMP9 expression through the SAM cycle, promoting liver metastasis in CRC (78–82) ( Figure 1 ). TCF-1 and Foxp3 can bind to regulate the same genes. TCF-1 can control T cell development in the thymus (34). As TCF-1 levels drop in CRC, more Foxp3 can bind to downstream gene regulatory elements, promoting the development of Treg, inhibiting the growth of effector T cells, and advancing CRC (83) ( Figure 1 ). While Foxp3+Treg is not positively correlated with Foxp3+ cancer cells, and purely high expression of Foxp3+Treg has no correlation with CRC prognosis, some researchers have discovered, contrary to the majority of findings, that high expression of Foxp3 in Foxp3+ cancer cells is correlated with poor prognosis (84). Additionally, Liu et al.’s research (85) demonstrated that Foxp3 expression was markedly down-regulated in colon cancer stem cells, that activating the Foxp3/NF-κB/COX2 pathway could prevent COX2’s transcriptional activation and impede the proliferation of colon cancer stem cells, and that Foxp3 was able to inhibit the growth of cancer cells. In summary, the research on Foxp3 expression in CRC is valuable and has predictive implications, but further investigation is required to comprehend its role in different cell types. This knowledge will be beneficial for improving CRC treatment and prevention strategies.

Hepatocellular carcinoma (HCC) refers to malignant tumors of the liver, including primary and metastatic hepatocellular carcinoma, and HCC patients are often detected in peripheral mononuclear lymphocytes with a significant increase in the expression of the factors Foxp3 and RORt, which are closely related to the development of HCC, compared to normal cells (86, 87). Yong Huang (88) has discovered that in HBV-infected people, IL-17-producing Th17 cells congregate with Foxp3+ Treg cells, facilitating HCC progression. MMP12 might promote Foxp3+Treg infiltration in tumor tissue; TLR4 might obliquely attract Foxp3+Treg to the tumor location by interacting with TGF-β and macrophages (89, 90). Through promoting Treg cell polarization, it facilitates HCC immunological escape. High basal levels of lnc-EGFR specifically bind to EGFR in HCC patients, blocking its interaction and ubiquitination with c-CBL, stabilizing it and enhancing its own and downstream activation of the AP-1/NF-AT1 axis. This prolongs the lifespan of EGFR, drives Treg differentiation, stifles CTL activity, and encourages the growth of HCC (91). Furthermore, it was discovered that TLR ligands, particularly polyIC (a synthetic double-stranded RNA polyinosinic polycytidylic acid), stimulated an increased capacity of hepatocellular carcinoma cell line (CCL-9.1) in mice to release Hepatoma Derived Growth Factor (HDGF), as well as Foxp3+ Treg cells to proliferate, which together inhibit the release of perforin and granzyme B from effector CD8+ T cells into the tumor microenvironment for the purpose of assisting cancer cell immune escape (35). The binding of the lncRNA NEAT1 to the Foxp3 binding site results in high expression of its downstream target gene, pyruvate kinase PKM2, and over-expression of NEAT1/Foxp3 promotes PKM2 transcriptional activation (92), which promotes HCC proliferation by enhancing the aerobic glycolysis pathway.

Furthermore, Foxp3 are found to have an oncogenic influence in both HCC and CRC. Mutations in the FKH structural domain of Foxp3 has been found at the transcriptional level in HCC tumor tissues, affecting the function of controlling the expression of target genes (93). Foxp3 expression is down-regulated in HCC tissues, but P62 expression is up-regulated. Low Foxp3 expression and P62 over-expression are found to be closely connected to a decrease in overall survival in HCC patients (94). Liu et al. (95) has discovered that the Foxp3 promoter and CpG region are hypermethylated by NA (cytosine-5)-methyl transferase 1 (DNMT1), which can limit Foxp3+Treg function and negatively controls HCC progression. Moreover, Gong et al. (96) also can confirme that Foxp3 exhibits oncogenic effects in HCC through in vitro and in vivo experiments, and Foxp3 regulates the TGF-β/smad3/4 pathway to recognize and directly or indirectly act on the Myc promoter region of oncogenes to inhibit oncogene expression; at the same time, the over-expression of Foxp3 could promote the increase of apoptotic marker Bax and expression of apoptosis inhibitor p53, which promote cancer cell apoptosis (97, 98).