Since 2003, researchers have gained insight into the characteristics of ANG1 as a member of the angiopoietin family, and its activation mechanism and subsequent biological effects have become a hot research topic. Specifically, the core of ANG1 activation lies in its precise targeting and activation of the Tie-2 signaling pathway, which is like a precise molecular key to unlock life. Firstly, ANG1 shows its power in multiple dimensions of inflammation with its excellent inhibitory ability, especially in the regulation of vascular permeability and inhibition of leukocyte adhesion on activated endothelial cells, which opens up new avenues for the treatment of sepsis, acute lung injury and other vascular permeability-related diseases (7). Under physiological conditions, in order to guard vascular homeostasis, ANG1 is maintained at a high concentration in the circulation, ensuring the stability and integrity of the vascular structure (14). In contrast, the serum of cancer patients shows a different picture, with a rise in ANG2 levels, a disruption of the ANG1 to ANG2 balance, and a significant decrease in the ratio, signaling vascular abnormaliTies in the disease state (15, 16). The activation of Tie-2 by ANG1 promotes the generation and survival of vascular endothelial cells. The cornerstone of this effect lies in its strengthening of the endothelial cell-extracellular connectivity (EC-EC) and the actin cytoskeleton, and especially in its stabilizing effect on vascular endothelial cadherin (VE-cadherin), which is an impenetrable defense for vascular stabilization, as it effectively protects against VEGF- or inflammatory cytokine-induced protein loss (17). Activation of Tie-2 also triggers an exquisite displacement of the receptor from the intercellular junction to the tail end of the cell, where it embraces the extracellular matrix (ECM), further deepening the connection between endothelial cells and the external environment (18). Delving deeper, ANG1 activates and cares for vascular endothelial cells through its dual strategy of anti-apoptosis and pro-proliferation, paving the way for vascular growth and repair. In this process, the Tie-2 receptor becomes the stage for ANG1 to work its magic, and the two work hand in hand to regulate the integrity and stability of blood vessels ( Figure 2 ). ANG1 is essential for cardiovascular development during embryogenesis and for the maintenance and repair of the vascular network in adults. Its critical role in vascular biology is well-established (19). Notably, although ANG1 and ANG2 are structurally similar, differing only in a subtle binding displacement ( Figure 1 ), this does not diminish their binding to Tie-2 or alter the rate of dissociation, and does not fully account for the differences in signaling between them (20). However, it is this subtle difference that confers ANG1 with unique physiological functions, such as promoting the migration of vascular endothelial cells and actively participating in the weaving and remodeling of neovascular networks. In the field of oncology, the overexpression of ANG1 has even demonstrated its potential to inhibit the invasion and metastasis of tumor cells, providing a new perspective to reduce the risk of distal tumor metastasis (21). In conclusion, ANG1, with its unique activation mode, plays a crucial role in angiogenesis, stability maintenance, inflammatory response regulation, and tumor metastasis inhibition in various physiological and pathological aspects, and its study not only deepens our understanding of vascular biology, but also opens up new avenues for the prevention and treatment of related diseases (22) ( Figure 2 ).

ANG2, as an environmentally adaptable agonist, is unique in that it mainly resides in the Weibel-Palade vesicles of endothelial cells, waiting for the right moment. Its activation may be induced by inflammatory factors: when the cells are provoked by inflammatory factors, ANG2 is awakened and breaks out from these secretive vesicles to bravely take up the important task of counteracting the ANG1—TIE-2 signaling axis (23) ( Figure 2 ). In stark contrast to the facilitating role of ANG1, ANG2 functions as an antagonist within the angiopoietin family, primarily regulated by endothelial cells during vascular remodeling. (ii) Signaling through integrin heterodimers (24), which in turn activates a series of downstream pathways that not only promote endotheli ANG2 disrupts the connection between endothelial cells and perivascular cells, promoting cell death and vascular degenerational cell proliferation and migration, but also quietly accelerate angiogenesis and tumor expansion (25, 26).

Deeper dissection of the molecular structures of ANG1 and ANG2 revealed that the two are like twins, sharing the c-terminal fibrinogen-like structural domain (FLD) that is in intimate contact with the Tie-2 receptor, but remaining detached from Tie-1 ( Figure 3A ).

They also cleverly weave the coiled-coiled structural domain (CCD) in the middle, which is not only a key enabler for them to form asymmetric dimers, but also ensures that only one FLD can be tightly embraced with Tie-2. In contrast, the supercluster domain (SCD), this brief n-terminal region, is their secret weapon for polymerization into complex multimers ( Figure 3B ) (27). In this delicate layout, the binding of FLD to Tie-2, the shaping of dimer asymmetry by CCD, and the catalysis of multimer formation by SCD combine to promote Tie-2 phosphorylation activation (28) ( Figure 4 ).

ANG2 co-localizes with pro-angiogenic factors including VEGF in perivascular niches of malignant tumors. Endothelial-derived ANG2 induces vascular destabilization through Tie2 receptor modulation, while VEGF orchestrates three key pathophysiological processes: enhanced vascular permeability, neoangiogenesis, and lymphangiogenesis (6, 29, 30). Tumor-derived VEGF has been found to be a potent inducer of ANG2 expression in endothelial cells, thereby destabilizing the host vascular system. VEGF regulates vascular permeability, angiogenesis and lymphangiogenesis (31). And breast cancer VEGFR, as the faithful messenger of VEGF, is driving tumors such as breast cancer and malignant melanoma to the point of no return (26, 32–34). The potential impact of ANG2 on vascular destruction is well demonstrated in experiments with transgenic mice, in which impaired angiogenesis significantly leads to embryonic death when ANG2 expression levels are abnormally elevated. In particular, ANG2 plays a central role in tumor-induced angiogenesis during pathological angiogenesis. It contributes to tumor enlargement and increased risk of metastasis. Therefore, ANG2 has also been proposed as a biomarker for different cancer types. The expression levels of ANG2 are all proportional to cancer stage. However, further studies are needed to determine its exact role and expression levels in various types and subtypes of lung cancer. In a population-based study of patients with hepatocellular carcinoma, ANG2 was negatively associated with overall survival. ANG2 levels were upregulated in the liver compared to normal tissues, and these patients typically had higher ANG2 levels than the healthy population (35). Remarkably, in certain contexts, ANG2 can also swing into a direct agonist of the Tie-2 receptor, activating the Tie-2 signaling pathway. However, this ability is not an unconditional gift; in an arena where VEGF is absent, ANG2 turns to inhibit Tie-2 signaling, leading the vasculature towards degeneration (36).

ANG3, a member of the angiopoietin family, uniquely inhibits ANG1-induced Tie-2 phosphorylation. It also acts as a strong activator of the Tie-2 receptor, which is critical for vascular endothelial cell survival and migration. As a bioactive protein, ANG3 enhances the survival of human umbilical vein endothelial cells (HUVECs) but has minimal impact on their migratory behavior. The activation mechanism may proceed as follows: in mouse lung tissue, ANG3 is fully activated, significantly enhancing the phosphorylation of Tie-2 and Akt, demonstrating a stronger activation effect. In this context, ANG3 exhibits greater efficacy than ANG4 in promoting both survival and migration of mouse lung endothelial cells (37). Investigating the anticancer mechanism of ANG3, we observed that it functions as an inhibitor by suppressing the excessive proliferation and survival of endothelial cells, effectively blocking tumor neovascularization, and thereby impeding tumor growth and metastasis. This mechanism is mediated by ANG3’s specific targeting of ANG1 and the VEGF-driven Erk1/2 and Akt kinase activation pathways. Through this signaling intervention, ANG3 significantly reduces tumor blood supply, leading to nutrient deprivation and growth inhibition in tumors (38).

ANG4, another member of the angiopoietin family, is known for its high-profile expression in the human lung. It inherits the qualities of ANG1 and becomes a great fan of the Tie-2 receptor, activating its potential ability to (39). ANG4 is even more unique in the field of angiogenesis, where it plays an indispensable role as a counterbalance to the angiogenic wave set off by recombinant growth factors with its remarkable inhibitory talents. Unlike some simplistic anti-cancer methods, ANG4’s inhibitory action does not rely on a direct toxic attack on tumor cells, but rather takes a smarter path - hitting the lifeblood of angiogenesis head-on. In the precision-designed experiments, despite the ubiquitous presence of ANG4, the proliferation rate of the tumor cells did not seem to be affected in the slightest, and they continued to multiply normally. This result clearly reveals that ANG4 is characterized by the network of blood vessels that power tumor growth, rather than the tumor cells themselves.

This discovery has undoubtedly opened up a whole new line of therapeutic thinking. ANG4 inhibits tumor angiogenesis via dual mechanisms: Tie2 receptor blockade and pericyte-mediated vascular pruning, reducing microvessel density and interstitial pressure in preclinical models. Its selective suppression of pathological neovascularization preserves physiological angiogenesis in chronic inflammation. Current research focuses on ANG4-VEGF crosstalk and spatiotemporal therapeutic optimization, advancing precision anti-angiogenic strategies (40). The study of ANG4 has been conducted in a number of countries.

Tie-1 (tyrosine kinase with immunoglobulin-like and EGF-like domains-1), as a member of the tyrosine kinase family with immunoglobulin-like and EGF-like domains, occupies an indispensable position in the biological activities of vascular endothelial cells. It is activated in the following 2 ways: (a) in close collaboration with integrins: the mechanism of activation is subtle in its close collaboration with integrins to promote angiogenesis. (b) Tight binding with ANG1 ligand: When Tie-1 binds tightly with ANG1 ligand, a story about the fate of blood vessels will quietly appear. This binding is not just a simple touch between molecules, but also activates a series of downstream signaling pathways, which work in concert to finely regulate the formation and stability of blood vessels, ensuring that each pathway is unobstructed (41, 42).

The role of Tie-1 is much more than a bystander or helper; it is the master regulator of vascular endothelial cell function and a key driver in the process of vascular development. Under its command, endothelial cells seem to gain unlimited vitality, promoting proliferation and migration as well as angiogenesis. At the same time, Tie-1 also strengthens the defense of the blood vessel wall with its unique charisma, making it more impregnable. Tie-1’s ability is far more than that. It is also a regulator, able to acutely sense and respond to subtle changes in vascular permeability, adjusting its strategy in time to ensure that the fluid balance is not disrupted. In the storm of inflammation, Tie-1 stands up to the storm and builds a solid defense for the vascular system with its robust regulatory ability, protecting the body from unnecessary damage (43, 44).

Tie-2, as another important member of the tyrosine kinase receptor family, has its life’s music closely revolving around the growth and differentiation of vascular endothelial cells. It acts as a bridge between the world of angiogenic factors and the fate of endothelial cells, and ensures the harmony and stability of the vascular system through a delicate regulatory mechanism. Its activation mechanism is similar to that of Tie-1: (a) Tight binding to ANG1 ligand: ANG1, the companion of Tie-2, with its strong agonistic effect, provides a solid backbone for the survival of endothelial cells, the stability of blood vessels, and the maintenance of endothelial barrier function (45). When ANG1 binds to Tie-2, a complex intracellular signaling process is initiated; the interaction between ANG1 and Tie-2 energizes the serine kinase AKT, which in turn leads to a series of downstream signals. This river of signals ultimately inhibits the growth of Forkhead box protein - Tie1 (FOXO1) and indirectly stifles FOXO1’s tendency to repress the ANG2 gene (46, 47) ( Figure 5 ). (b) Through activation of the AKT pathway: activation of the AKT pathway, like a fuse, triggers phosphorylation of Foxo1. In this process, like a precise mechanical bonding, Foxo1 is led to the edge of the cytoplasm and its function is inhibited, while ANG2 expression is silently reduced in this delicate balance. However, when PI3K is inhibited, the path of AKT activation is blocked and the phosphorylation of Foxo1 comes to an abrupt end. Lost in bondage, Foxo1, as if breaking free of gravity, returned to the interior of the cell nucleus. Here, it found its strength and activated its potential, which in turn drove a significant increase in ANG2 mRNA levels (48, 49) ( Figure 5 ). (c) Through interactions between the structural domains of Fn3: it was found to signal its binding to ANG1 (50) and its activation was also achieved through interactions between Fn3 structural domains. These interactions form a dimeric structure that promotes the activation and phosphorylation of Tie-2 (51) More interestingly, Tie-1 and Tie-2 are not independent of each other. They interact with each other and jointly regulate the effect of the Ang family on Tie-2 phosphorylation (52) ( Figure 5 ). This finding not only e1-nriches our understanding of the relationship between the Tie receptor families, but also provides new clues for us to explore the deeper mechanisms of angiogenesis and stabilization.

When the tumor continues to develop with active angiogenesis, the tumor cells may secrete pro-angiogenic factors, and when the pro-angiogenic factor signals are dominant, blood vessel formation will be induced, a process known as the “angiogenic switch”. This process is called the “angiogenic switch”, which is the initiating mark of tumor vascularization. The angiogenic switch can make the tumor active from the dormant state, and when the tumor vascularization is initiated, an abnormal vascular network will be formed, leading to impaired tumor perfusion, creating a hypoxic microenvironment, which can promote the invasiveness of tumor cells, and at the same time, it can hinder the killing effect of the immune cells on the tumor cells, which can further promote the tumorigenesis (53, 54).

The formation of new capillaries, termed “angiogenesis”, is one of the most widespread and fundamental biological processes in mammals. Angiogenesis is an important event in a variety of physiological environments, such as embryonic development, chronic inflammation, and wound repair, and angiogenesis is a feature of a limited number of physiological processes; the etiology and pathogenesis of a growing number of pathological conditions have been shown to be the result of angiogenic responses. Ocular diseases, vascular malformations, and tumors are a few examples where angiogenesis plays an important role (55–57).

The ANG family (ANG1, ANG2, and ANG4) primarily mediates its biological functions via Tie-2 receptor interactions on endothelial cells (58). The absence or overexpression of ANG can lead to impaired interaction between endothelial cells and perivascular cells, which in turn affects the structural integrity of the vasculature (59–61). The ANG have been shown to have a significant impact on the structural integrity of the vascular system. Studies have shown that ANG1, ANG2, and ANG4 activate PI3-AKT signaling pathways, which promote cell growth, differentiation, and proliferation, thereby promoting angiogenesis (62, 63). ANG2 and ANG4 activate PI3-AKT signaling pathways, which promote cell growth, differentiation and proliferation, thereby promoting angiogenesis (57, 58). In recent years, ANG2 has been intensively investigated as a second-generation anti-angiogenic candidate molecule, as it plays a key role in both agonism and antagonism as an important ligand for the maintenance of the resting state of the endothelial cell ANGPT1/Tie-2 signaling axis. In preclinical studies, deletion of the Angpt2 gene results in a transient delay in primary tumor growth (64). In postoperative adjuvant therapy, the combination of ANGPT2 neutralizing antibody and low-dose beat-to-beat chemotherapy not only inhibits the angiogenic response of endothelial cells within the metastatic site, but also suppresses their inflammatory response, thus limiting metastasis (65). These findings suggest that ANG have broad and far-reaching effects on the vascular and lymphatic systems, and that there are both complementary and potentially contradictory points between them and other important growth factors such as VEGF. Through these complex interactions, ANG play an integral role in several aspects of angiogenesis, permeability regulation, and lymphatic system development (66).

In addition to this, findings suggest that in addition to tumor growth and angiogenesis, systemic ANG2 overexpression promotes tumor lymphangiogenesis, lymph node and lung metastasis (67). Another study found that a higher proportion of tumor cells expressed ANG2 in patients with metastatic melanoma compared to patients with primary tumors and benign nevi (68). Further experimental analyses showed that ANG2 secreted by tumor cells does not affect the function of blood vessels or immune cells, but rather promotes the metastatic and colonization ability of tumor cells by controlling their metabolic functions. ANG2 secreted by tumor cells promotes melanoma metastasis and colonization ability by regulating the metabolic function and mitochondrial function of tumor cells (65). In various cancers, including hepatocellular carcinoma, researchers have identified a link between angiogenic stimulation and the epithelial-mesenchymal transition process (EMT). Studies by Dong et al, Ribatti et al. have demonstrated that EMT plays a key role in driving tumor progression by increasing invasion and metastasis (69). In some in vitro cancer models, such as melanoma, cervical and breast cancers, EMT markers (i.e., e.g., N-cadherin, Vimentin, Snail, and Twist) act as facilitators of cell motility and increase resistance to anticancer therapy (70). In addition, recent studies have highlighted the clinical significance of EMT in HCC and CCA. This includes its potential as a therapeutic target for cancer, as well as its role as a prognostic marker, the presence of which provides valuable insights into cancer therapy. In conclusion, ANG is important in tumor anti-angiogenic strategies in head and neck tumor therapy, and inhibition of the activity of molecules such as ANG2 shows potential therapeutic benefits (71).

ANG2 disrupts endothelial-perivascular interactions, inducing cell death and vascular degeneration. However, together with VEGF, ANG2 promotes neointimal formation. Thus, angiopoietins play a crucial role in the angiogenic switch during tumor progression, and elevated expression of ANG2 relative to ANG1 in tumors correlates with poor prognosis. The central role of the angiopoietin/Tie signaling pathway in the regulation of physiological and pathological angiogenesis makes it an attractive target for the treatment of vascular disease and cancer (72). In addition, bone marrow (bm)-derived pro-angiogenic and/or tumor-infiltrating cells, including monocytes, macrophages, granulocytes and neutrophils, as well as specific cell subpopulations such as tumor-associated macrophages (TAMs), TIE-2-expressing monocytes (TEMs), and myeloid-derived suppressor cells (MDSCs) have been shown to provide high levels of angiogenic factors by locally promoting tumor angiogenesis in a paracrine manner (72). In the complex network of tumor angiogenesis, tam is recognized as a key driver of tumor angiogenesis (73). Whereas TEM, as a specific population of cells present in the blood, once recruited to the tumor microenvironment, they undergo a rapid process of differentiation and polarization, transforming into functional tam (74). Further, when these TEMs are eliminated within the tumor, their contribution to tam generation is blocked, thereby inhibiting the process of tumor angiogenesis (75). Compared to Tie-2-negative monocytes, circulating TEMs exhibit a higher angiogenic potential before they have even entered the tumor microenvironment. These cells have been pre-programmed in the circulatory system to express higher levels of pro-angiogenic genes, thereby promoting the angiogenic process within the tumor more efficiently ( Figure 6 ).

In summary, ANG may play a two-fold role in tumor angiogenesis or inhibition, while the study of ANG function is dominated by ANG1 and ANG2. More unknown functions of ANG3 and ANG4 remain to be discovered by subsequent scholars.

In recent years, progress has been made in anti-angiogenic therapy for prostate cancer and other types of solid tumors (89–92), ANG is not only involved in tumor angiogenesis, but in addition to its angiogenic activity, angiopoietin also plays a role in cancer cell proliferation, and thus may be an effective molecular target for cancer treatment (93). This has aroused great interest in the therapeutic role of angiogenic factors in head and neck tumors.

The secretion of pro-angiogenic factors by tumor cells leads to the formation of abnormal vascular networks, which affects tumor perfusion and in turn creates a hypoxic environment that enhances tumor cell invasiveness and reduces the tumor-killing effect of immune cells. Abnormal tumor perfusion also reduces the efficacy of chemotherapy and radiotherapy. Therefore, vascular normalization has emerged as a new therapeutic strategy, and treatments that inhibit neoangiogenesis by interfering with signal transduction pathways that regulate angiogenesis and growth or by directly targeting tumor-associated endothelial cells have been proposed as promising strategies (114). In many studies, tissue expression levels of angiogenic factors have been correlated with the likelihood of tumor spread; therefore, they are considered predictive indicators for identifying patients at high risk of poor prognosis. The use of ANG2 as a biomarker helps in the diagnosis and therapeutic monitoring of Kasibo sarcoma. By measuring ANG2 levels in serum, physicians can assess a patient’s disease status and response to treatment. High levels of ANG2 are strongly associated with the presence of Kassipo sarcoma and Kassipo lymphangioma, and can therefore be used as one of the indicators for the diagnosis of these diseases. In addition, changes in ANG2 levels can be used to monitor a patient’s response to treatment. If treatment is effective, ANG2 levels may decrease; conversely, if treatment is ineffective or the disease progresses, ANG2 levels may remain elevated (115). Disease progression disease-free survival is significantly shorter in CLL patients with high expression of ANG2 (median 21 months) and significantly longer in patients expressing low levels of ANG2 (median 146 months). In addition, patients with high ANG2 expression typically have a more advanced clinical stage, a higher proportion of CD38-positive cells, an unmutated immunoglobulin status, and unfavorable cytogenetics, all of which support the involvement of ANG2 in the disease mechanism of CLL, and therefore, the expression level of ANG2 may serve as a poor predictor of disease prognosis (116).

Angiogenic treatment with bevacizumab not only enhanced the binding of ANG1 to Tie-2, but also triggered a significant decrease in circulating Tie-2 levels. It is worth mentioning that Tie-2, as a promising prognostic biomarker, is regarded as an early warning signal for disease recurrence if the increase in its circulating levels exceeds 50%, showing its promise in predicting disease progression (99, 117). The researchers’ findings are also relevant for osteosarcoma, which is a cancerous tumor. In the case of osteosarcoma, researchers have found that inhibiting VEGF gene expression using ribonucleic acid disruption significantly inhibits tumor cell proliferation and promotes apoptosis. Meanwhile, targeted therapeutic strategies against VEGF are being actively investigated in the clinic for osteosarcoma. By monitoring the expression levels of VEGF and ANG2 in the serum of osteosarcoma patients, we are able to more accurately assess the biological characteristics of the disease and provide an important reference for the adjustment of treatment plans. This approach is of great significance for improving the clinical outcome and prognosis of osteosarcoma (118, 119). The results of this approach are very important for improving the clinical outcome and prognosis of osteosarcoma (103, 104). In contrast, there was no significant difference in angiopoietin expression between basal-like cell carcinoma and squamous cell carcinoma in the peripheral vessels of basal-like cell carcinoma and squamous cell carcinoma in the head and neck region. In cancer cells, angiopoietin expression was lower in basal-like cell carcinoma and higher in squamous cell carcinoma. In addition, angiopoietin expression in basal-like cell carcinoma was associated with the stage of the tumor, whereas angiopoietin expression in squamous cell carcinoma was not associated with the stage of the tumor. These results suggest that basal-like cell carcinoma and squamous cell carcinoma differ in their angiopoietin expression patterns, which may have implications for the prognosis of both types of head and neck tumors (120).

What’s more, transient receptor potential vanilloid-like protein 4 (TRPV4) was found to be expressed at enhanced protein levels in oral squamous cell carcinoma lesion tissues, with significant differences compared to normal tissues (121). mRNA levels of TRPV4 were also upregulated and expressed in animals treated with GSK1016790A(TRPV4 agonist) (122). VEGF immunoassays showed a significant increase in protein concentration, while cisplatin or its combination with TRPV4 agonist reduced VEGF levels. These results suggest that TRPV4 agonist combined with cisplatin may modulate angiogenesis in oral squamous cell carcinoma via the Angiopoietin/Tie pathway. Moreover, cisplatin alone or with TRPV4 agonists reduced tumor volume and malignant lesion incidence. Vascular normalization, facilitated by modulating the ANG1/Tie-2 pathway, may restore structural and functional abnormalities, improving the tumor microenvironment (123). In turn, the interaction between Tie-2 and ANG1 may also enhance the intercellular and cell-extracellular matrix adhesion capacity of OSCC cells, thereby promoting tumor metastasis and invasion. Meanwhile, Tie-2 and ANG1 play an important role in cancer metastasis and may be potential biomarkers and therapeutic targets for OSCC metastasis (124). Even ANG1-mediated vascular lymphangiogenesis can be inhibited by vasopressors (125). The researchers also found that ANG is a potential biomarker and therapeutic target for OSCC metastasis (107). Meanwhile, researchers have also found that the combination of ANG with immune checkpoint inhibitors (e.g., anti-PD-L1 antibodies) can also be used for anti-tumor therapy (126). This combination therapeutic strategy can inhibit tumor growth and metastasis and activate the host immune response, thereby increasing the anti-tumor killing capacity. At the same time, this combination therapeutic strategy can also improve the immunosuppressive state in the tumor microenvironment and enhance the anti-tumor killing capacity of infiltrating cytotoxic T cells (127).

In summary, the ANG family plays a critical role in anti-angiogenic strategies for head and neck tumors, with ANG2 inhibition demonstrating significant therapeutic potential. Also, the correlation of TRPV4 with angiogenesis and tumor metastasis provides new potential biomarkers and therapeutic targets for head and neck tumor therapy. The important concept of anti-angiogenesis is significant in head and neck tumor therapy, especially for molecules such as ANG, which provide new therapeutic ideas and potential targets for the regulation of tumor angiogenesis and metastasis mechanisms.