Vascular endothelial growth factor (VEGF) biology and its proposed role in rheumatic diseases pathogenesis

Abstract

Vascular endothelial growth factor (VEGF) VEGF is a multifunctional cytokine with pleiotropic roles in human physiology. This potent angiogenic factor modulates cell proliferation, survival, migration, and permeability. However, in several pathological conditions, such as autoimmune diseases (ADs), VEGF plays an important role in disease pathophysiology. Furthermore, VEGF exerts direct effects on various innate and adaptive immune cells, which are important in autoimmunity. Increasing evidence suggests a correlation between the VEGF system and autoimmune diseases. Research indicates that VEGF levels are related to the severity of ADs in certain patients. However, the role of VEGF in some diseases has been controversial. This review summarizes the current knowledge regarding VEGF’s role in rheumatic diseases, how VEGF bioavailability contributes to their pathogenesis and the potential effects of VEGF-targeted treatments on these conditions.

Introduction

Vascular endothelial growth factor (VEGF) is an extremely powerful molecule that promotes angiogenesis and increases the permeability of blood vessel walls [1]. The VEGF family comprises five mammalian factors: VEGFA, VEGFB, VEGFC, VEGFD, and placenta growth factor (PlGF). Furthermore, VEGF-165 is a heparin-binding homodimeric glycoprotein that contributes to embryonic vasculogenesis and angiogenesis and represents a major secreted form from cells through an unspecified pathway. VEGF-165 is a specific variant of VEGF-A that consists of 165 amino acids. The different isoforms arise due to alternative splicing of the VEGF-A gene, resulting in proteins with varying lengths [2].

VEGF-A has been recognized as a crucial regulator of angiogenesis [3]. Furthermore, VEGF-C, VEGF-D, and VEGF-A regulate lymphangiogenesis [4, 5] VEGF-B has been recognized as a survival factor for various cells, including vascular cells, neurons, and cardiac myocytes, by inhibiting apoptosis through the suppression of BH3-only protein and other apoptosis-related genes [6]. Numerous cell types produce VEGF, including macrophages, platelets, keratinocytes, renal mesangial cells [7], cardiac myofibroblasts [8], and B cells [9]. VEGF is not limited to vascular functions; it also exerts a notable influence on bone formation [10], hematopoiesis [11], wound healing, and development [12]. Nevertheless, several pathological conditions, including autoimmune diseases (ADs), stimulate the expression of VEGF, diverting it from its physiological role. This alteration leads to various pathological manifestations such as angiogenesis, impaired endothelial cell function, reduced expression of pro-apoptotic proteins, and activation of numerous other inflammatory mediators [13, 14]. Increasing evidence suggests that angiogenesis and inflammation are interdependent and there is a close relationship between circulating VEGF level and ADs’ severity. In patients with AD, circulating VEGF levels are associated with disease manifestations and activity [15]. Indeed, angiogenesis is a major contributing factor in immune-mediated tissue damage in ADs [15]. As inflammation progresses, immune cells synthesize and secrete pro-angiogenic factors that encourage neovascularization, which has several steps, and new vessel formation is almost entirely mediated by VEGF [5]. Enhanced comprehension of VEGF’s involvement in the pathogenesis of rheumatic diseases could pave the way for the development of more efficacious therapies.

VEGF signaling pathway

Three classes of VEGF receptors have been identified : (VEGFR1 (Flt-1), VEGFR2 also known as KDR (kinase insert domain receptor) or Flk-1 in humans, and VEGFR3) each with different physical and biological properties [1]. VEGF-A interacts with both R1 and R2 receptors, whereas VEGF-B and PlGF selectively bind to R1. Conversely, VEGF-C and VEGF-D engage with the R3 receptor [6].

VEGF is commonly referred to as VEGF-A. VEGFR2 is the most important effector in angiogenesis. This receptor induces many signaling pathways. When VEGF-A binds to VEGFR2, it prompts the formation of a receptor dimer, initiating PLCγ (phospholipase Cγ) and the PKC-Raf kinase-MEK-mitogen-activated protein kinase (MAPK) pathway. This cascade stimulates DNA synthesis, fostering endothelial cell proliferation and angiogenesis. Simultaneously, the Activation of Src signaling promotes the activation of the phosphatidylinositol 3’-kinase (PI3K)-Akt pathway, thereby enhancing endothelial cell viability. Src signaling activation additionally triggers systemic degradation of the basement membrane and extracellular matrix, heightening vascular permeability. Moreover, following VEGFR2 activation, there is activation of the Cell division control protein 42 homolog (CDC42) and p38 mitogen-activated protein kinase (MAPK) signaling pathways. Activation of p38 MAPK induces actin reorganization, facilitating endothelial cell migration. Thus, these pathways collectively regulate the proliferation, survival, migration, and permeability of cells expressing VEGFR2 [14].VEGFR-1 primarily interacts with VEGF-A, VEGF-B, and placental growth factor (PlGF), initiating signaling pathways crucial for various cellular processes. These include promoting cell proliferation and survival through the MAPK/ERK Pathway, regulating cell survival and migration via the PI3K/Akt Pathway, and influencing endothelial cell permeability and cytoskeletal rearrangement by modulating intracellular calcium levels through the PLCγ pathway.

Moreover, VEGFR-3 is fundamental in facilitating the effects of VEGF-C and VEGF-D, which are vital for the process of lymphangiogenesis. Upon binding to its ligands, VEGFR-3 triggers several signaling pathways. For example, activation of the PI3K/Akt pathway promotes both cell viability and proliferation. Similarly, the MAPK/ERK pathway aids in cell proliferation and migration. Moreover, VEGFR-3 regulates lymphatic endothelial cell survival and function by activating the JAK/STAT pathway [16]. (Fig. 1)

Fig. 1.

VEGF signaling pathway. VEGF receptors are classified into three categories: VEGFR1 (Flt-1), VEGFR2 (KDR or Flk-1 in humans), and VEGFR3, each exhibiting unique physical and biological characteristics. VEGF-A binding to VEGFR2 induces receptor dimerization and activates the PLCγ (phospholipase Cγ) pathway as well as the PKC-Raf-MEK mitogen-activated protein kinase (MAPK) pathway. This cascade induces DNA synthesis, enhances endothelial cell proliferation, and facilitates angiogenesis. VEGFR-1 predominantly interacts with VEGF-A, VEGF-B, and placental growth factor (PlGF), hence initiating signaling pathways essential for numerous vital cellular activities. This encompasses the promotion of cell proliferation and survival through the MAPK/ERK pathway, the regulation of cell survival and migration via the PI3K/Akt route, and the modulation of endothelial cell permeability and cytoskeletal rearrangement by altering intracellular calcium levels through the PLCγ system. VEGFR-3 is crucial for mediating the effects of VEGF-C and VEGF-D, which are vital for lymphangiogenesis. VEGFR-3 activates multiple signaling pathways upon ligand binding. For example, stimulation of the PI3K/Akt pathway improves cell survival and proliferation

VEGF effects on endothelial cells

VEGF is crucial in the formation of capillaries from existing blood vessels, regulating angiogenesis, and shielding endothelial cells from apoptosis. Factors such as hypoxia, cytokines, and growth factors tightly control the expression of VEGF [17]. Flk-1 regulates the growth and viability of endothelial cells, and Flt-1 regulates chemotaxis and vascular permeability. Following the activation of the Flk-1 signaling pathway, protein kinase B (or Akt) and MAPK pathways will be activated. Angiogenesis is dependent on VEGF, Flk-1, and Flt-1, and their absence causes embryonic lethality, overgrowth of endothelial cells, or incomplete development of endothelial cells [18]. Under physiological conditions, VEGF promotes the survival of endothelial cells. However, in pathological contexts, the presence of TGF-β can induce endothelial cell apoptosis and hinder neovascularization [19].

Various effects of VEGF on immune system cells

Autoimmune diseases are characterized by a breakdown of immunological self-tolerance, primarily mediated by the adaptive immune system, resulting in immune responses directed against self-antigens. This loss of tolerance leads to sustained activation of immune effector mechanisms and chronic inflammation, ultimately causing progressive tissue injury and organ dysfunction. The mechanisms of tissue damage in autoimmune diseases closely resemble those seen in protective immunity, and hypersensitivity diseases can impact a variety of cells within the innate as well as the adaptive immune systems, thereby influencing their function, which is summarized in Table 1.

Table 1.

The influence of VEGF on the immune cells

Cell Type		The influence of VEGF on the cell
Dendritic Cells (DCs)		Inhibition of migration Impairment of antigen-presenting function Promotion of angiogenic properties and differentiation into endothelial-like cells
Macrophages		Recruitment and polarization towards M2 phenotype Immunomodulation Promotion of immunosuppressive and angiogenic properties
T Cells		Regulation of inflammatory responses Contribution to chronic inflammation Promotion of tissue repair and inflammation resolution
B Cells		Promotion of survival and function Inhibition of apoptosis Enhancement of AKT phosphorylation

Dendritic cells (DCs)

A common feature of autoimmune diseases is an alteration of the specialized dendritic cell (DC) system, which is pivotal in maintaining the equilibrium between immunity and tolerance [20].

DCs express VEGFR-1 and VEGFR-2, and VEGF inhibits DC migration via the RhoA-cofilin1 pathway [21]. The VEGFR1 signaling pathway harms DC maturation directly by restraining the nuclear factor of κ-light-chain-enhancer of activated B cells (NF-κB) activity and affects the expression of NF-κB subunits, such as rel-B. Hence, antigen-presenting and secretory functions of DCs can be impaired. Several findings have shown that blocking NF-κB activation by the VEGFR1 signaling pathway can impair DC’s differentiation [22].

The RhoA/ROCK signaling pathway is activated upon VEGF binding to VEGFR2. RhoA belongs to the Rho GTP subfamily of GTPases. RhoA is also involved in various aspects of DC biology, encompassing their antigen-presenting functions, adhesion, IL-12 production, T-cell activation, and morphology. According to studies, The Rho family of small GTP-binding proteins, comprising Rho, Rac, and Cdc42, govern the cytoskeleton and orchestrate a range of cellular functions. Active RhoA accounts for activating serine/threonine protein kinase Rho-associated kinase (ROCK). The active ROCK can prevent dephosphorylation of myosin light chains (MLCs) by inactivating downstream myosin phosphatase, leading to increased interaction between actin and myosin. Therefore, cell contraction and migration are affected [23]. In the presence of high VEGF levels, the phosphorylation of the RhoA/ROCK pathway is increased. Normally, when DCs undergo maturation, this pathway is inhibited. Therefore, abnormally high levels of RhoA/ROCK pathway phosphorylation inhibit DC maturation [24]. Furthermore, VEGF can increase the expression of the checkpoint inhibitor PD-L1 on the surface of DCs. This upregulation serves to shield DCs from elimination by cytotoxic T lymphocytes [25].

VEGFR3 suppresses the trafficking of DCs to draining lymph nodes to mediate delayed-type hypersensitivity (DTH) by preventing TLR4 (Toll-like receptor)/NF-κB activation [26]. Furthermore, VEGF can change DCs’ nature and, for illustration, enable DCs to develop angiogenic properties and differentiate into endothelial-like cells [27, 28]. Research has unveiled that when exposed to angiogenic growth factors and certain cytokines, dendritic cells (DCs) originating from peripheral monocytes can differentiate into endothelial-like cells (ELCs) [29]. This phenomenon suggests a potential link between the immune and vascular systems, highlighting the dynamic interactions between immune and endothelial cells. ELCs may participate in angiogenic processes, facilitating neoangiogenesis, an indispensable element for tissue repair and regeneration. This process is important in the wound healing process, ischemic conditions, and tissue injury [29].

Macrophages

Macrophages, as an important component of the immune system, are functionally plastic based on their phenotype. Two types of macrophages, M1 and M2, were identified. The former produces pro-inflammatory cytokines, while the latter releases anti‐inflammatory mediators and stimulating tissue repair in the late inflammatory response [30]. VEGF significantly contributes to macrophage recruitment and the polarization of M2 macrophages [31]. Furthermore, M2 migration is induced by VEGF at least partially through fms-related receptor tyrosine kinase 1 (Flt-1) expression [31]. Flt-1 holds a central role in facilitating cellular responses to vascular endothelial growth factors (VEGFs), essential regulators of angiogenesis [32].

M2 macrophages are generally considered to be more prevalent in diseases characterized by chronic inflammation and tissue remodeling. M2 macrophages are often observed in rheumatoid arthritis, systemic sclerosis, systemic lupus erythematosus, and ankylosing spondylitis [33]. VEGF acts not only as a paracrine but also as an autocrine for M2 macrophages to regulate PD-L1 expression and mediates immunomodulating effect on T cells [34]. PD-L1-Positive macrophages have immunosuppressive and angiogenic properties [35], thus, the role of VEGF in the context of ADs is intricate and contingent on the specific context.

T cells

VEGF regulates T-cell responses contributing to both inflammatory associated with autoimmune diseases and anti-inflammatory responses associated with tissue repair and wound healing. It may promote the expansion of pro-inflammatory T-cell subsets, such as Th17, and contribute to chronic inflammation. Additionally, it may have a role in recruiting and activating T cells within the inflamed gastrointestinal mucosa, thus influencing the inflammatory response. Conversely, VEGF can modulate the immune response by fostering the recruitment of T cells to areas of tissue repair, aiding in the resolution of inflammation and the reinstatement of tissue integrity [14, 36].

In T cell development, VEGF can decrease the count of CD4 + and CD8 + T cells by reducing the number of early hematopoietic progenitors and decreasing CD3 + T cell proliferation and cytotoxicity. VEGF-A can trigger the expression of programmed cell death 1 (PD-1), cytotoxic T-lymphocyte associated protein 4 (CTLA-4), and T cell immunoglobulin and mucin-domain-containing-3 (TIM-3), which result in T cell exhaustion [21].

VEGF orchestrates the initiation of both the extracellular signal-regulated kinase (ERK) and Akt signaling pathways in human T cells and it is a powerful chemoattractant for T cells [37]. VEGF can affect T cell inflammatory response through augmenting of IFN-γ and inhibition of IL-10 secretion; thus, VEGF can enhance a Th1 phenotype [38].

Moreover, it heightens the cytotoxic function of CD4 cytotoxic T lymphocytes (CTLs), which in pathological circumstances, can foster the onset of autoimmune diseases or viral infections via the VEGF-R1/VEGF-R2/AKT/mTOR pathway [39]. As well, through the upregulation of ERK and AKT, VEGF-A enhances the immune reactions of CD4 + CD45RO+ memory T cells [40].

All in all, VEGF may enhance the activation and survival of T cells, potentially contributing to the persistence of inflammation in rheumatic diseases. Furthermore, VEGF may influence the balance between effector T cells and Tregs, potentially impacting the autoimmune response in rheumatic diseases.

B cells

In autoimmune diseases, B cells critically contribute to the regulation of lymphoid tissue structure, antigen presentation, and co-stimulation. They further modulate the function of dendritic cells and the pathways of T helper cell differentiation. Additionally, B cells release inflammatory cytokines such as IL-8, IL-6, LT-α, and TNF-α [41].

VEGF promotes the development of bone marrow B cells and increases total splenic B cells through VEGFR1, while VEGFR2 inhibits this process [42].

VEGF-A derived from B cells, along with its receptor VEGFR2, holds significance in lymphangiogenesis, particularly in the enlargement of lymphatic vessels. However, they further exert suppressive effects on certain aspects of the ensuing immune responses [4]. So, B cells can be an important source of VEGF in some conditions.

VEGF is mentioned as an important survival factor for B cells by engaging PKCβǁ and increasing PKC mRNA levels in CLL cells, however, VEGF does not have an impact on PKC mRNA levels in normal B cells [43]. Furthermore, by contacting mesenchymal stromal cells and B cells, MSCs produce an increased amount of VEGF inhibit caspase-3-mediated apoptosis, and promote the phosphorylation of AKT within B cells, thereby promoting B cell survival and protecting them from apoptosis. Consequently, direct cell contact between mesenchymal stem cells (MSCs) and B cells fosters B cell viability and function. This suggests that MSCs might not be a suitable therapy for B cell-related diseases [44]. Thus, an increased level of VEGF can contribute to B cell activation and proliferation, and it may enhance the survival, and expansion of B cells, and modulate antibody production by B cells. (Fig. 2)

Fig. 2.

Various effects of VEGF on immune system cells. The VEGFR1 signaling pathway directly hinders DC development by restricting NF-kB activity, hence compromising the antigen-presenting and secretory functions of dendritic cells. VEGF plays a crucial role in the recruitment of anti-inflammatory macrophages, the polarization of M2 macrophages, and the enhancement of PD-L1 expression in these cells. VEGF facilitates the maturation of bone marrow B cells and enhances the overall population of splenic B cells via VEGFR1. VEGF has a complex role in multiple facets of osteoblast function. throughout the earliest stages of osteoblast differentiation, VEGF expression is very low; nevertheless, it markedly increases throughout terminal differentiation. VEGF is essential in regulating bone remodeling by promoting osteoblast development via VEGFR1

VEGF role in osteoclast differentiation and survival

A complex interaction occurs between bone-resorbing osteoclasts and bone-forming osteoblasts to maintain skeletal growth. During bone formation, two distinct mechanisms are involved in bone formation: The processes of endochondral ossification and intramembranous ossification. Throughout bone growth, osteogenesis and angiogenesis are closely intertwined processes [45].

VEGF exerts multifaceted effects on osteoblast function. Research findings suggest that VEGF elicits an attractive response from primary human osteoblasts and mesenchymal progenitor cells, with the intensity of this response varying according to the dosage [46]. At the beginning of osteoblast differentiation, VEGF expression was initially low; however, it significantly increased during terminal differentiation. VEGF is pivotal in regulating bone remodeling by fostering osteoblast differentiation through VEGFR1 (Flt-1) [47].

Osteoblasts and osteoclasts directly interact, with osteoblasts fulfilling a crucial role in osteoclast differentiation through the expression of RANKL (receptor activator of nuclear factor kappa beta (NF-kB) ligand) as a membrane-associated factor. RANK-expressing osteoclast precursors recognize RANKL through cell-cell interaction and differentiate into osteoclasts. Treating purified bone marrow osteoclast precursors with VEGF results in enhanced survival, differentiation into mature osteoclasts, and increased resorptive activity. Two distinct VEGF receptors, KDR/Flk-1 and Flt-1, were detectable in osteoclasts at the gene and protein levels. VEGF’s effects on mature osteoclasts predominantly engage the receptor VEGFR2 (Flk1, KDR), with receptor signaling involving both the PI3K/Akt and MEK/ERK pathways. VEGF-dependent phosphorylation of multiple downstream targets of activated Akt is associated with an increase in osteoclast survival and resorption, with the involvement of the PI3K/Akt and MEK/ERK pathways [48].

VEGF role in fibrosis

VEGF is involved in extracellular matrix (ECM) metabolism and has a pivotal function in angiogenesis. VEGF stimulates vascular permeability by increasing matrix metalloproteinase expression, VEGF is necessary for the modification of ECM. VEGF can directly induce collagen synthesis in dermal fibroblasts. Furthermore, it directly regulates numerous genes associated with fibrosis and significantly enhances the expression of collagen and smooth muscle actin (α-SMA) [49]. VEGF induces a profibrotic phenotype in perivascular cells when it induces myofibroblast differentiation. Following VEGF binding to VEGFR‐2 it can activate focal adhesion kinase (FAK)‐related non‐kinase (FRNK) directly. Consequently, the increased fibroblast mobilization, coupled with enhanced myofibroblast differentiation and migration, results in a cascade of events fostering proliferation and fibrotic responses within fibroblasts. Additionally, VEGF has been observed to promote myofibroblast proliferation, migration, and collagen production by activating transforming growth factor‐β1 (TGF‐β1) and ERK signaling [50].

VEGF in rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune disease with destroyed synovial membranes in the joints. This systemic inflammatory disorder affects roughly 1% of the global population [51]. The pathogenesis of RA involves a multitude of cells including immune cells, fibroblasts, chondrocytes, and dendritic cells. A hallmark of RA is the progressive destruction of synovial-lined joints and degradation is primarily mediated by the fibroblast-like synoviocytes (FLSs) [52].

VEGF and its receptors are overexpressed in RA patients, with elevated levels of VEGF observed in both synovial fluid (SF) and serum compared to healthy controls. Studies have demonstrated that serum VEGF levels correlate with erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and disease activity score 28 (DAS28), parameters associated with disease activity. Additionally, serum VEGF levels have been suggested to offer a more accurate representation of treatment response compared to ESR and CRP [53].

Rheumatoid arthritis (RA) is associated with increased angiogenesis and tissue neovascularization [54]. Joint damage likely results from synovial angiogenesis when inflammatory cells infiltrate, pannus develops, and fibrovascular granulation tissues become invasive [55]. Numerous factors, including VEGF, TGF-β, and FGF stimulate angiogenesis in the early stage of RA. VEGF can strongly induce endothelial cell activation and proliferation. Moreover, through the upregulation of Bcl-2 expression and translocation of Bax, VEGF-165 confers apoptotic resistance to synoviocytes. This implicates abnormal synovial growth in the form of “pannus,” contributing to synovial hyperplasia [40]. Beyond conferring apoptotic resistance to synoviocytes, VEGF-165 stimulates the production of TNF-α, IL-6, and substantial amounts of VEGF-165 itself, creating an autocrine amplification loop. Moreover, VEGF-165 triggers endothelial cell activation, resulting in the release of chemokines such as MCP-1 and IL-8. These chemokines can draw monocytes into the synovial membranes, facilitating the recruitment of new macrophages. Upon stimulation by VEGF or upon direct interaction with activated endothelial cells, newly recruited macrophages produce TNF-α and IL-6. TNF-α and IL-6, in turn, further increase macrophages and synoviocyte’s capacity to secrete VEGF, contributing to a vicious cycle of inflammation. Another crucial characteristic of RA is bone erosion. The effect of VEGF on osteoclast differentiation is both direct and indirect, as VEGF directly regulates the differentiation of osteoclasts from monocytes and stimulates the release of RANKL, a cytokine implicated in osteoclast differentiation from fibroblasts [56].

Considering the role of VEGF in the pathogenesis of RA as outlined earlier, chronic synovitis can be retarded by anti-VEGF treatment in several ways, including (1) nutrient depletion in the tumor-like synovium, (2) inhibition of leukocyte adhesion and mobility by reducing endothelial cell surface area, (3) decreasing the production of chemokines and cytokines by activated endothelial cells, (4) decreasing TNF-α and IL6 expression by monocytes/macrophages, and (5) impairing the pro-survival effects of VEGF. Hence, blocking agents targeting VEGF have emerged as potential candidates for RA treatment. In a study, Seung-Ah Yoo and her colleagues used dRK6 as a small peptide, which is relatively secure, affordable, and readily manufacturable, to block VEGF and they observed that dRK6 suppressed the destructive circle of inflammation in mice and blocked the VEGF-induced production of pro-inflammatory cytokines. Thus, dRK6 may be an effective strategy in decreasing manifestations of RA [57]. The initial anti-angiogenic drug approved by the FDA was bevacizumab, a humanized form of anti-VEGF antibody. Studies have suggested that anti-VEGF antibodies can thwart collagen-induced arthritis in mice [58]. Therefore, VEGF contributes to multiple aspects of RA pathogenesis, and its inhibition represents a potential therapeutic strategy for RA. (Fig. 3)

Fig. 3.

VEGF biology and its Role in Rheumatic Diseases Pathogenesis. In patients with RA, there is an overexpression of VEGF and its receptors, correlating with tissue neovascularization. Joint damage is probable due to synovial angiogenesis resulting from the infiltration of inflammatory cells, the development of pannus, and the invasiveness of fibrovascular granulation tissue. Multiple factors, such as VEGF and TGF-β, as well as the early stage, can activate the VEGF gene and its expression. These factors significantly promote endothelial cell activation and proliferation. VEGF stimulates the synthesis of TNF-α, IL-6, and significant quantities of VEGF-165. Newly recruited macrophages produce TNF-α and IL-6 in response to stimulation by VEGF or direct interaction with activated endothelial cells. TNF-α and IL-6 enhance the ability of macrophages and synoviocytes to produce VEGF, thereby perpetuating the inflammatory cycle that significantly contributes to bone erosion in RA. B cells are essential in SLE through both antibody-dependent and antibody-independent mechanisms. These mechanisms significantly contribute to the complex pathogenesis of SLE. B cells in individuals with systemic lupus erythematosus (SLE) exhibit abnormal activation and differentiation, resulting in the production of autoantibodies against self-antigens, which is elevated in the peripheral blood of patients with active SLE. VEGF has shown the capacity to improve the survival and activation of B cells by functioning as a survival factor in particular microenvironments. It inhibits apoptosis and can enhance the synthesis of antiapoptotic proteins, such as Bcl-2. Therefore, under these conditions, VEGF contributes to the maintenance of B-cell functionality throughout disease pathogenesis.This results in heightened autoantibody production and the release of self-antigens, thereby disrupting tolerance within the pathogenic process. VEGF-mediated angiogenesis may significantly contribute to the progression of Sjögren’s Disease by facilitating inflammation and tissue damage, exhibiting both local effects at the tissue level and involvement in the NF-kB inflammatory signaling pathway

VEGF in SSc

Systemic sclerosis (SSc), scleroderma, is an uncommon autoimmune disease marked by abnormal connective tissue growth, which supports the skin and internal organs. Overproducing collagen can lead to the skin and other tissues experiencing increased thickness and rigidity. Systemic sclerosis can affect various organs and systems in the body, and its clinical presentation can vary widely from person to person [59]. Fibrosis and vasculopathy are closely related and result in extremely heterogeneous clinical manifestations with widely varying prognoses. Inflammatory processes are essential for initiating and advancing fibrosis as well as structural vasculopathy in response to disruptions in equilibrium [60].

From a clinical perspective, Raynaud’s phenomenon begins in most cases months or years before the appearance of other clinical manifestations, including fibrosis of the skin and organs, and it has been believed that vasculopathy is the initiating event. When the endothelium malfunctions, it leads to an imbalance in vasoactive factors. This is marked by overproduction of the vasoconstrictor endothelin-1 (ET-1) in the skin, lung tissue, and serum, alongside diminished production of the vasodilators nitric oxide (NO) and prostacyclin in individuals with SSc. This imbalance ultimately results in tissue hypoxia [61]. Tissue fibrosis develops due to various events, such as myofibroblast differentiation and activation, endothelial dysfunction, inflammation, elevated vascular permeability, and platelet aggregation [62].

The response of endothelial cells to hypoxic stress can vary depending on the duration of hypoxia. Hypoxia can induce several genes, including platelet-derived growth factor B (PDGF-B), insulin-like growth factor (IGF), basic fibroblast growth factor (bFGF), ET-1, and VEGF [63]. These gene products mediate irreversible structural remodeling of blood vessels and surrounding tissues acting as mitogens to endothelial cells, smooth muscle cells, and fibroblasts, causing increased fibrosis and hypoxia. Additionally, TGF-ß serves as another trigger capable of stimulating VEGF production in multiple cell types. There is a noted positive association between serum levels of VEGF and TGF-ß in patients diagnosed with SSc [63]. VEGF is markedly elevated in the skin and serum of SSc patients, including its anti-angiogenic VEGF-165 isoform [64]. As a potential biomarker for SSc lung interstitial involvement, serum VEGF levels are measured [65]. Additionally, VEGF, among other indicators, serves as a gauge to anticipate disease activity in SSc [66], although, there is no observed correlation between VEGF levels and intrarenal stiffness or renal function in patients with SSc [67].

Macrophages are implicated in the pathogenesis of SSc with evidence suggesting their involvement in the inflammatory and fibrotic processes. M2 macrophages are known for their anti-inflammatory and tissue-repair functions. Their dysregulation, however, has been linked to the development of autoimmune diseases, such as SSc. M2 macrophages may contribute to fibrosis by promoting tissue repair responses that, when dysregulated, lead to excessive collagen deposition and fibrotic changes in affected organs. VEGF influences M2 macrophage polarization through multiple signaling pathways, including VEGFR-1, VEGFR-2, TIE2 (Tyrosine Kinase with Immunoglobulin and Epidermal Growth Factor Homology Domain 2), and HIF signaling. These pathways collectively regulate the expression of M2-associated genes and promote the immunosuppressive and pro-angiogenic functions of M2 macrophages in various physiological and pathological contexts [31].

Within the diverse array of immune system contributors in SSc, the nearly ubiquitous existence of autoantibodies has long suggested a significant role for B cells in driving the disease’s development. B cells contribute to the development of SSc through various mechanisms, namely the synthesis of autoantibodies, the production of proinflammatory or profibrotic cytokines, the downregulation of immunomodulatory cytokines, and/or direct interaction with other cell types [68]. It is important to note that, the increased concentration of VEGF in the serum of individuals with SSc may be regarded as a contributing factor that supports the survival and proliferation of B cells.

In a recent investigation conducted by Sanges et al., the levels of various markers indicating B-cell activation in the bloodstream were examined within a substantial and comprehensively characterized population of individuals with SSc. Intriguingly, prior research has underscored the ability of B cells to release both pro- and anti-angiogenic substances, including angiopoietins, VEGFs, PDGFs, MMPs, and TIMPs, in other conditions. However, this finding suggests that B cells may contribute to SSc-associated vasculopathy through the production of angiogenic mediators [9].

The persistent and uncontrolled overexpression of proangiogenic mediators at different stages of the disease could potentially disrupt vessel structure and endothelial performance, rather than facilitating new vessel formation. Consequently, the implications of B cell production of angiogenic mediators for SSc microangiopathy remain uncertain. Further mechanistic investigations are necessary to unravel the pathophysiological significance of this discovery [9]. Furthermore, VEGF activates the enzyme protein kinase C, which leads to the B cells’ survival, as seen in chronic lymphocytic leukemia and multiple myeloma [43, 69].

Therefore, concerning VEGF’s role in various mechanisms and pathways including stimulating the function and proliferation of fibroblasts, endothelial cell dysfunction, M2 polarization, and induction of B cell survival and based on the results of Huang et al.. study that analyzed the effects of nintedanib as an inhibitor targeting VEGF in the fos-related antigen-2 (Fra2) mouse model of SSc [70], inhibition of VEGF can be the new treatment strategy for SSc. (Fig. 3)

VEGF in SLE

Systemic lupus erythematosus (SLE) is a persistent autoimmune condition marked by a diverse range of clinical features and a fluctuating pattern of remission and relapse, the underlying cause of which remains elusive [71]. Based on global estimates of SLE incidence, the disease varied regionally. It is also known that SLE occurs more frequently in women than in men, with a ratio of approximately 9:1 for women [72] More than 40 SLE susceptibility loci have been identified in exons and introns through genome-wide association studies (GWASs) in the past decade [73].

Given the diverse manifestations of lupus, identifying markers for diagnosing and monitoring the disease has proven challenging. Considering that high levels of anti-phospholipid antibodies (aPLs) can persist for years in individuals who show no symptoms, it is likely that vascular injury or endothelial cell activation precedes the development of thrombosis in individuals with the antibody. Researchers postulated that there is a correlation between serum concentration of VEGF and laboratory markers of disease activity such as ESR and CRP. This angiogenic marker can reflect the extent of microvascular damage in SLE patients [74]. According to studies, there was a notable increase in VEGF levels among SLE patients with or without antiphospholipid syndrome (APS) compared to the control [74, 75]. In addition, VEGF predicts disease activity independently [14]. VEGF is one of the key indicators of endothelial dysregulation in pediatric-onset SLE, hinting at its potential as a valuable indicator for assessing disease activity and organ involvement associated with the condition for clinicians. Additionally, it is valuable for studying vascular abnormalities and monitoring disease progression [76]. Mouth sores are a prevalent symptom in individuals with SLE, and there has been an observed association with VEGF [76]. Moreover, VEGF may contribute to the increased occurrence of neuropsychiatric manifestations in individuals with SLE [77].

VEGF levels were significantly higher in those with mucocutaneous and articular manifestations or nephritis than those without [78]. Additionally, SLE patients with high angiogenic activity are more likely to have renal complications as well as anti-dsDNA antibodies [78]. SLE patients taking mycophenolate mofetil had a decreased level of VEGF-A compared to those not taking it [79]. Elevated VEGF serum levels indicate that glomerular endothelial cells receive sufficient levels of VEGF and ensure their integrity. However, lupus nephritis is associated with significant endothelial cell damage. There is a possibility that patients with lupus nephritis exhibit increased serum levels of soluble VEGF receptor-1 (sFlt1) as well as VEGF [80]. Given that this receptor, in both its membrane form (Flt1) and its soluble form (sFlt1), exhibits a tenfold higher affinity for VEGF compared to VEGFR2/Flk1, an elevation in serum sFlt1 could effectively bind circulating VEGF. Consequently, this could diminish VEGF availability to glomerular endothelial cells, elucidating endothelial injury and apoptosis in patients with lupus nephritis. In lupus patients, altered VEGF signaling is associated with increased expression of adhesion molecules like E-selectin, vascular cell adhesion molecule-1 (VCAM-1), and intercellular adhesion molecule-1 (ICAM-1). A damaged glomerular endothelial cell exposes the glomerular basement membrane to circulation, allowing circulating DNA to directly bind to this membrane. Consequently, autoantibodies may bind to this membrane-bound DNA. Subsequently, these immune complexes could attract monocytes, which in turn bind to endothelial cells expressing these adhesion molecules. This cascade of events could potentially elucidate the development of vasculitis despite high levels of circulating VEGF [80, 81].

B cells play a crucial role through antibody-dependent and -independent mechanisms in SLE. These mechanisms are significant contributors to the complex pathogenesis of SLE. B cells in SLE individuals undergo abnormal activation and differentiation, leading to autoantibody production against self-antigens which is increased in the peripheral blood of patients with active SLE [82]. These autoantibodies, including anti-nuclear antibodies (ANA) and anti-double-stranded DNA (anti-dsDNA) antibodies, contribute to immune complex formation and deposition, triggering inflammation and tissue damage. B cells commonly exhibit deficiencies in immune tolerance mechanisms in individuals with SLE. This breakdown in tolerance enables autoreactive B cells to evade standard regulatory processes, producing antibodies that target the body’s tissues [83]. VEGF has demonstrated the ability to enhance the survival and activation of B cells by acting as a survival factor in specific microenvironments. It prevents apoptosis and can stimulate the production of antiapoptotic proteins, including Bcl-2 [84]. Hence, in this condition, VEGF contributes to the persistence of B cell functionality during the disease’s pathogenesis. This leads to an increased production of autoantibodies and the release of self-antigens, disrupting tolerance in the pathogenic process. Therefore, concerning the increased expression of VEGF and its soluble receptor (sFlt1) which can contribute to endothelial cell damage and increased survival of B cells that may be related to elevated levels of VEGF, inhibition of VEGF can be considered as a new approach for SLE treatment. (Fig. 3)

VEGF in Sjögren Disease (SD)

Sjögren Disease (SD) is an autoimmune disorder that affects the salivary and lacrimal glands, characterized by the infiltration of mononuclear cells around the ducts, accompanied by autoimmune responses, endothelial cell impairment, and eventual cell death [85]. Angiogenesis is stimulated by growth factors, proteases, and cytokines secreted by inflammatory cells. These mediators trigger the breakdown of the extracellular matrix ECM, endothelial cell growth and migration, promote angiogenesis, and recruit inflammatory cells to the site of inflammation, perpetuating the inflammatory process. Angiogenic factors stimulate inflammation by expressing adhesive molecules, cytokines, and chemokines, which can cause chronic inflammation [86]. Angiogenesis emerges as a promising new indicator for primary SD and may be implicated in its development [87]. VEGF, functioning as a key regulator of proangiogenesis, promotes the migration of inflammatory cells into the extracellular matrix by enhancing vascular permeability and increasing endothelial cell expression of adhesion molecules [88]. Yet, the involvement of VEGF in SD patients remains a subject of debate, as some studies have reported its elevation while others have not observed a difference between the patient and control groups [14]. Some researchers have shown a close relationship between the amount of VEGF and the severity of SD. For instance, J. Błochowiak et al. observed significant amounts of VEGF in the glandular epithelium as well as inflammatory cells within inflamed glands of SD patients when compared to healthy subjects. In addition, an immunohistochemical examination showed strongly positive staining for VEGF and VEGFR-2 proteins in SD biopsy specimens. In this study, VEGF levels were found to be higher in parotid saliva compared to serum levels. This suggests that elevated VEGF levels observed in salivary gland samples are not mirrored by corresponding elevations in serum levels [88]. Since vasculitis and related alterations in blood vessels are more pressing concerns in SD, Manetti et al. demonstrated that increased plasma levels of the VEGF-165 isoform correlate with the extent of capillary architectural abnormalities in SD patients. So, it has been revealed that VEGF-165 might actively participate in the loss of microvessels [89]. According to other studies, anti-Ro/SS-A antibodies stimulated VEGF expression through TACE metalloproteinase (tumor necrosis factor alpha-converting enzyme expression) activation in SD salivary glands biopsy specimens [87, 90]. Additionally, Huang et al. confirmed the positive correlations between VEGF-A levels and inflammatory markers such as CRP, WBC, NEUT, C3, and C4. In contrast with other studies, Huang et al. found that serum VEGF-A levels in SD patients were not different from those in healthy individuals [91]. Furthermore, activated NF-κB can regulate VEGF-A because its genes have a NF-κB binding site in the promoter region and based on studies, NF-κB transcriptional activity in SD exceeds that observed in healthy individuals, indicating the significance of NF-κB signaling in regulating immune responses. Also, in human salivary gland epithelial cells treated with anti-Ro/SSA Abs, NF-κB translocation was correlated with increased VEGF-A expression. By inhibiting VEGF-A molecules by anti-Ro/SSA Abs, anti-VEGFA mAb significantly reduced NF-κB activation, whereas treatment with exogenous VEGF-A reversed the effect of VEGF-A neutralization [90] (Fig. 3).

Conclusion

VEGF is consistently elevated in the serum of multiple rheumatic diseases and emerges as a shared pathogenic nexus linking immune activation, angiogenesis, and tissue remodeling. The positive association between circulating VEGF levels and disease activity underscores its potential value as a biomarker for disease monitoring and progression. Despite this strong biological relevance, therapeutic inhibition of VEGF in rheumatological diseases has proven challenging. VEGF plays essential physiological roles in vascular homeostasis and tissue repair, and its blockade may result in paradoxical or compensatory effects due to pathway redundancy and complex immune–vascular interactions. Consequently, VEGF inhibition alone is unlikely to provide sustained clinical benefit. Future translational efforts should therefore focus on validating VEGF-related biomarkers, identifying disease- and context-specific VEGF signaling patterns, and developing more selective or combinatorial therapeutic strategies. Such approaches may enable safer and more effective targeting of VEGF-associated pathways in rheumatology. The roles of VEGF across autoimmune diseases are summarized in Table 2.

Table 2.

The role of VEGF in rheumatic diseases

Rheumatic Diseases	Mechanisms	Clinical Results
Rheumatoid Arthritis	Overexpression of VEGF and its receptors, promotion of synovial angiogenesis, induction of synoviocyte hyperplasia and resistance to apoptosis, stimulation of TNF-α and IL-6 production, facilitation of bone erosion	Elevated serum VEGF levels correlate with disease activity markers (ESR, CRP, DAS28), VEGF inhibition retards chronic synovitis, potential treatment strategy using anti-VEGF agents like bevacizumab and dRK6 peptide
Systemic Sclerosis	Dysregulated angiogenesis, promotion of fibrosis and vasculopathy, increased VEGF production in response to hypoxic stress and TGF-ß stimulation, contribution to M2 macrophage polarization, survival and activation of B cells	Elevated serum VEGF levels associated with SSc lung interstitial involvement and disease activity, potential biomarker for disease activity and organ involvement, inhibition of VEGF as a treatment strategy
Systemic Lupus Erythematosus	Elevated serum VEGF levels, correlation with disease activity and organ involvement, association with oral ulceration and neuropsychiatric disorders, potential involvement in vasculitis development	Serum VEGF levels predict disease activity, potential biomarker for disease activity and organ involvement, inhibition of VEGF as a treatment strategy
Sjögren Disease	Controversial role, potential promotion of angiogenesis and vasculitic changes, association with anti-Ro/SS-A antibodies, stimulation of NF-κB signaling	VEGF levels vary between studies, potential involvement in microvessel loss and severity of capillary architectural derangement, potential role in inflammation and immune modulation

Author contributions

MV contributed to the acquisition and interpretation of data‎ and the drafting of the article. EF, AJ, and MM contributed to the interpretation of data, conception and design of the study, and critical revision of the article for important intellectual content. All authors contributed to the revision of the manuscript, read it, and gave final approval for the submitted version of the article.

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Data availability

Not applicable.

Declarations

Ethical approval

Not applicable.

Consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.