1.1. Angiogenesis
Angiogenesis is a complex biological process involving the formation of new blood vessels from preexisting vessels [
1]. It plays a crucial role in various physiological and pathological conditions, including embryogenesis, wound healing, and tumor growth [
2]. The process of angiogenesis is tightly regulated by multiple signaling pathways and factors. Matrix metalloproteinases are primarily responsible for degrading the basement membrane surrounding existing blood vessels, allowing endothelial cells to migrate and proliferate toward the angiogenic stimulus [
3]. One of the key proangiogenic factors is vascular endothelial growth factor (VEGF), which is essential for the formation of new blood vessels during embryonic development and is produced by various cell types, including tumors [
4]. VEGF binds to specific receptors on endothelial cells, promoting their survival, migration, and differentiation [
5,
6]. Other factors involved in angiogenesis regulation include fibroblast growth factors (FGFs), platelet-derived growth factor (PDGF), and angiopoietins [
3]. These factors act synergistically to ensure the proper formation and remodeling of blood vessels. The regulation of angiogenesis is maintained through a dynamic balance between proangiogenic and antiangiogenic factors. This balance can be disrupted in various diseases, leading to either excessive or insufficient blood vessel formation [
1]. Insufficient angiogenesis, on the other hand, can lead to tissue ischemia, impaired wound healing, and various cardiovascular diseases [
2]. Peripheral artery disease (PAD) is an example where the narrowing or blockage of blood vessels reduces blood flow to the legs or arms, resulting in pain, skin ulcers, and an increased risk of amputation [
2]. Therapeutic angiogenesis, which aims to promote blood vessel formation, has been investigated as a potential treatment approach for PAD, with the administration of VEGF or the use of gene therapy [
1].
It is important to emphasize that integrins, a group of transmembrane proteins that play a crucial role in cell adhesion and communication between cells and the extracellular environment, are the focus of interest, as they play an important role in angiogenesis [
7].
1.2. Integrins
Integrins play an essential role in the regulation of biological processes such as cell migration, adhesion, proliferation, differentiation and signaling [
8]. Integrins are heterodimeric glycoproteins consisting of alpha and beta subunits, which together form a complex transmembrane receptor [
9]. A total of 18 alpha subunits and 8 beta subunits have been identified in mammalian cells, allowing the formation of 24 different heterodimers. The α subunit determines affinity to extracellular matrix component (ECM), while the β subunit associates with cytoplasmic structural and regulatory proteins. These subunits have a long transmembrane domain and a short cytoplasmic domain associated with cytoskeletal proteins [
10]. Although integrins are constitutively expressed on the cell surface, they need to be activated to interact with their ligands [
11]. Such activation can occur in the presence of chemokines and cytokines and is characterized by a conformational change at the extracellular integrin domain that exposes their binding sites on the α and β subunits, allowing them to interact with their ligands on the ECM or with proteins on the membranes of neighboring cells. This interaction is primarily controlled by a conserved tripeptide pattern of arginine, glycine, and aspartate, commonly known as the Arg-Gly-Asp motif (RGD) [
12,
13,
14]. They are crucial for cell adhesion, migration, and signal transduction by mediating interactions between cells and ECM proteins. The main role of these molecules is to provide a link between the cytoskeleton of the cell and certain ECM components, such as fibronectin, vitronectin, laminin, and collagen. In addition, they are responsible for triggering intracellular signal transduction pathways upon interaction with the ECM. Integrins can undergo conformational changes that influence their ligand binding properties and downstream signaling events [
15]. They possess a unique arrangement of cysteine residues that enables them to adopt a compact and stable structure consisting of a well-defined loop disulfide array [
16]. This structural motif is critical for their high-affinity binding to integrins, as it allows them to bind to specific integrin subunits and block their ligand binding sites. In addition, integrins also interact with growth factor receptors to regulate cell migration, blood vessel development, and angiogenesis. Importantly, the understanding of the mechanism of action of integrins paralleled the discovery of proteins from snake venoms, called disintegrins, which act as potent inhibitors of platelet aggregation and integrin receptor-dependent cell adhesion [
17,
18].
1.3. Snake Venom Metalloproteinases and Snake Venom Disintegrins
Snake venom metalloproteinases (SVMPs) are a major component of most crotalid and viperid venoms [
19]. SVMPs are known to be a class of key toxins involved in the pathophysiology associated with viperid venoms and are classified into classes and subclasses from P-I to P-III according to the organization of their domains, as shown in
Figure 1 [
20]. In general, class PI includes metalloproteinases that have only a catalytic domain containing zinc; class PII, metalloproteinases that have a catalytic domain followed by a disintegrin domain containing the tripeptide RGD (arginine-glycine-aspartic acid); class PIII, metalloproteinases that have a catalytic domain, a disintegrin-like domain, and a cysteine-rich domain [
21].
Snake venom disintegrins (SVDs) are a class of proteins derived from SVMPs. The disintegrins present in snake venoms can be formed in the venom gland I n two distinct ways: (1) By proteolysis of SVMPs of class P-II, where cleavage occurs between the catalytic domain and the disintegrin domain, leaving only the disintegrin domain. These disintegrins are known as RGD-dependent disintegrins, which are abundant in viperid venoms and contain the sequence XGD (X-Gly-Asp), MLD (Met-Leu-Asp), or K/RTS (Lys/Arg-Thr-Ser) on the exposed surface of the loop that specifically binds to integrins on the surface of different cell types [
22,
23]. The amino acid sequences immediately adjacent to the RGD site of disintegrins could form an extended RGD locus that, in conjunction with the conformational representation of the RGD sequence, could be involved in determining integrin selectivity and affinity [
13]. The family of disintegrins containing the RGD motif is widely recognized as the most extensive and well-studied group. Most of these units function as monomers (small, medium, or large), but a subgroup has the ability to combine into dimers and form homo or heterodimers [
24]. (2) Proteolysis of class P-III SVMPs results in fragments that covalently link the disintegrin-like and cysteine-rich domains and are referred to as ECD-disintegrin-like/cysteine-rich domains (
Figure 2). This disintegrin-like domain has a sequence of non-RGD tripeptides in its binding site [
21,
25].
Figure 1.
Disintegrins are categorized according to their structural composition. The number of disulfide bonds and the length of the polypeptide chain determine this categorization.
Figure 1.
Disintegrins are categorized according to their structural composition. The number of disulfide bonds and the length of the polypeptide chain determine this categorization.
Figure 2.
SVMP classification into three classes: P–I (20–30 kDa), which in its mature form contains a metalloproteinase domain; P-II (30–60 kDa), which contains a disintegrin domain linked to the C-terminus of the metalloproteinase domain; and P-III (60–100 kDa), which consists of a metalloproteinase domain, a disintegrin-like domain and a cysteine-rich domain.
Figure 2.
SVMP classification into three classes: P–I (20–30 kDa), which in its mature form contains a metalloproteinase domain; P-II (30–60 kDa), which contains a disintegrin domain linked to the C-terminus of the metalloproteinase domain; and P-III (60–100 kDa), which consists of a metalloproteinase domain, a disintegrin-like domain and a cysteine-rich domain.
SVDs are known for their ability to bind to integrin receptors and modulate various cellular functions, such as inflammation, apoptosis in endothelial cells and inhibition of platelet aggregation [
26]. Jararhagin-C, an SVD containing the ECD-disintegrin-like/cysteine-rich domains produced by the proteolytic cleavage of Jararhagin (SVMP P-III), is present in Bothrops jararaca venom and specifically interacts with α2β1 integrin, inhibiting collagen and ADP-induced platelet aggregation [
8]. Jararhagin-C is also capable of triggering the local release of cytokines [
27] and induces changes in leukocyte-endothelium interactions through the expression of the adhesion molecules ICAM-1, CD11a and CD11b [
28,
29]. Alternagin-C, a toxin isolated from B. alternatus venom, is a protein composed of the ECD-disintegrin-like/cysteine-rich domain, with 92% homology to Jar-C. It is able to interfere with α2β1 integrin functions and may contribute to apoptosis by interfering with cell adhesion [
30].
One of the notable characteristics of SVDs is their ability to inhibit platelet aggregation, an important step in blood clot formation [
31]. This property has led to the exploration of disintegrins as potential antithrombotic drugs for the prevention and treatment of conditions such as deep vein thrombosis and stroke. For example, Insularin, a monomeric RGD-disintegrin isolated from the venom of Bothrops insularis, strongly inhibits human platelet aggregation and fibrinogen-adhesion of endothelial cells [
32]. Echistatin, an RGD disintegrin from the venom of the saw-scaled viper (Echis carinatus), effectively inhibits platelet aggregation by binding to integrin αIIbβ3 [
33]. Upon binding to integrins, SVDs interfere with various cellular processes. Disintegrins have gained attention in the field of medicine due to their unique properties and potential therapeutic applications.
SVDs have also shown promise in cancer research [34-36]. Integrins play a crucial role in tumor growth, invasion, and metastasis [
37]. By targeting specific integrins expressed on cancer cells, SVD can potentially inhibit the proliferation and migration of these cells. Contortrostatin, a disintegrin from the venom of the Agkistrodon contortrix contortrix, has been found to have anticancer properties by interacting with integrins αvβ3 and α5β1 [
38]. Vicrostatin, a recombinant disintegrin developed by fusing 62 N-terminal amino acids of the disintegrin Contortrostatin with 6 C-terminal amino acids of Echistatin, is the best-characterized and most preclinically advanced disintegrin shown to target multiple tumor-associated integrins and to exhibit potent antitumor and antiangiogenic activity in in vitro and in vivo models without appreciable toxicity [
23].
Since the 1987 report on the isolation of trigramin, the first disintegrin isolated from the venom of Trimeresurus gramineus, approximately 100 other disintegrins have become known from snake venoms with potential applications in cancer research and therapy (see the list in [
17]).
1.4. The role and properties of SVDs in angiogenesis
One of the most well-known effects of SVD is its ability to prevent cell adhesion to extracellular matrix proteins [
16]. By occupying the ligand binding sites on integrins, disintegrins disrupt the attachment of cells to their surrounding microenvironment. This disruption can have profound effects on cell migration, invasion and angiogenesis, processes that are essential for tumor progression and metastasis [
18]. Dysregulation of integrin activity has been implicated in numerous pathological conditions, such as cancer, inflammation, and thrombosis [
39,
40]. Consequently, inhibiting integrin binding has emerged as a promising therapeutic strategy [
41,
42].
Several studies highlight the dual role of SVD in angiogenesis, with some disintegrins acting as inhibitors and others acting as enhancers of the process [
43,
44,
45,
46]. The interaction between disintegrins and integrin receptors on endothelial cells plays a critical role in modulating angiogenesis. Therefore, inhibitors of α1β1 and α2β1 integrins alone or in combination with antagonists of other integrins involved in angiogenesis (eg. αvβ3, αvβ5, αIIbβ3 and α6β4) may prove beneficial in controlling neovascularization, making SVDs valuable tools for the study and potential manipulation of this complex biological process.
Some SVDs have been identified as potent inhibitors of angiogenesis
(Table 1). For instance, obtustatin, a disintegrin isolated from the venom of Vipera lebetina obtusa, has been shown to selectively inhibit α1β1 integrin, leading to the suppression of angiogenesis in vitro and in vivo [
47]. A similar effect was also observed with viperistatin isolated from the venom of Vipera paleastinae [
48]. Furthermore, jerdostatin, isolated from the venom of Trimeresurus jerdonii [
49], and lebestatin, isolated from the venom of Macrovipera lebetina [
43], two other small monomeric disintegrins that antagonize the function of the α1β1 integrin, have also been described as inhibitors of angiogenesis. Rhodostomin, a medium disintegrin from the venom of Calloselasma rhodostoma, has been reported to inhibit angiogenesis by binding to integrins and inhibiting bFGF-induced proliferation of endothelial cells [
50].
Table 1.
List of disintegrins found in snake venom that have pro- and/or anti-angiogenic properties.
Table 1.
List of disintegrins found in snake venom that have pro- and/or anti-angiogenic properties.
Name (Source) |
Recognizing Motif |
Physiological Target |
Angiogenic factors |
References |
Jararhagin-C |
ECD-disintegrin-like/cysteine-rich domains |
Interferes with α2β1 integrin functions |
Pro-angiogenic |
[8] |
Alternagina-C |
Exhibits both pro- and anti-angiogenic effects |
[30,51,52] |
Leberagin-C |
Disintegrin-like |
Interferes with αvβ3, αvβ6, and α5β1 integrins |
Anti-angiogenesis |
[53](63) |
Echistatin |
RGD-dependent disintegrins |
Binds to integrin αIIbβ3, GPIIb/IIIa and interacts with αvβ3 integrin |
Anti-angiogenesis |
[14,33,54] |
Rhodostomin |
[50] |
Contortrostatin |
Interacts with the integrins αvβ3 and α5β1 |
[38] |
Vicrostatin |
antagonize the function of the αIIbβ3, αvβ3, αvβ5 and α5β1 integrins |
[23] |
DisBa-01 |
Binds to integrin αvβ3 |
[55] |
Aggretin |
Binds to integrin α2β1 |
Pro-angiogenic |
[56] |
Trigramin |
Binds to αIIbβ3, α8β1, αvβ3, αvβ5and/or α5β1 integrins |
[57] |
Obtustatin |
KTS-disintegrin |
Selectively inhibit α1β1-integrin |
Anti-angiogenesis |
[47] |
Viperistatin |
Inhibitory activity against collagen receptors, α1β1 and α2β1-integrins |
[48] |
Lebestatin |
Inhibis binding of α1β1 integrin to type IV and type I collagen |
[43] |
Jerdostatin |
RTS-disintegrin |
Antagonizes the function of the α1β1 integrin |
Anti-angiogenesis |
[49] |
Agkistin-s |
InteractS with GPIB |
Anti-angiogenesis |
[58] |
On the other hand, certain SVDs have been found to promote angiogenesis. For instance, Jararhagin-C has been shown to promote angiogenesis by activating integrin receptors and stimulating endothelial cell migration, increasing the density of blood vessels and the synthesis of proangiogenic cytokines (VEGF and FGF) [
45].
Alternagin-C exhibits both pro- and antiangiogenic effects depending on the concentration. Concentrations less than 50 nM were found to be proangiogenic, whereas concentrations greater than 100 nM were found to be antiangiogenic both in vitro and in vivo [
51,
52]. Alternagin-C inhibits VEGF/VEGFR2 signaling after binding to α2β1 integrin, resulting in impaired angiogenesis [
51].
1.4.1. Interference of SVD with VEGF
VGEF is a key molecule for regulating and promoting the angiogenesis process through the stimulation of proliferation, migration and organization of endothelial cells, triggering signaling cascades such as Notch, angiopoietin/Tie, MAPK, FAK, PI3K/AKT, ERK1/2, Src and PLCγ. While VEGFs stimulate the proliferation of endothelial cells, integrins help anchor these cells to the extracellular matrix to ensure that vessel formation occurs in an organized manner. SVDs can affect vascular VEGF, which plays a crucial role in regulating blood vessel formation and vascular permeability [
59,
60,
61].
There are several ways in which SVDs can interfere with VEGF function. Disintegrins can inhibit angiogenesis, the process of forming new blood vessels, which is essential for tumor growth and metastasis [
7,
62]. By interfering with VEGF-induced signaling, disintegrins hinder the formation of new blood vessels and thus impair tumor progression [
63]. Moreover, disintegrins can interfere with the signal transduction pathways initiated by VEGF [
64,
65]. This interference can affect various cellular processes, including gene expression and protein synthesis, which are crucial for the regulation of blood vessel formation and permeability [
59]. Overall, SVD exerts its effects on VEGFs through the inhibition of angiogenesis and interference with signal transduction pathways, ultimately influencing the regulation of blood vessel formation and vascular permeability.
1.5. Anti-angiogenic effects of SVDs
Recent studies have demonstrated the antiangiogenic effects of SVD, suggesting their potential as therapeutic agents for various human diseases, including cancer. In vitro experiments have shown that contortrostatin exhibits antiangiogenic activity [
38]. Similarly, DisBa-01, a disintegrin from Bothrops alternatus snake venom, has been found to inhibit the proliferation, migration, and tube formation of human umbilical vein endothelial cells, which play crucial roles in angiogenesis [
55]. Echistatin has also been shown to inhibit the proliferation and migration of human microvascular endothelial cells [
64]. Animal model studies have further supported the antiangiogenic effects of SVD. For instance, contortrostatin has been found to inhibit tumor growth and angiogenesis in a mouse model of melanoma [
38,
63]. Similarly, Echistatin has demonstrated the ability to inhibit tumor growth and angiogenesis in a mouse model of glioma [
66]. Agkistin is a P-II class SVMP containing a metalloproteinase and a disintegrin domain purified from crude venom of Formosan Agkistrodon acutus. Agkistin-s is the disintegrin domain of Agkistin and induces endothelial cell apoptosis, exhibiting profound antiangiogenic activity [
58].
Additionally, Leberagin-C (Leb-C), a disintegrin from Macrovipera lebetina transmediterrannea snakes, has been shown to disrupt the adhesion, migration, and invasion capabilities of MDA-MB-231 breast cancer cells and its highly metastatic D3H2LN subpopulation [
53].