Biological aspects in controlling angiogenesis: current progress

Abstract

All living beings continue their life by receiving energy and by excreting waste products. In animals, the arteries are the pathways of these transfers to the cells. Angiogenesis, the formation of the arteries by the development of pre-existed parental blood vessels, is a phenomenon that occurs naturally during puberty due to certain physiological processes such as menstruation, wound healing, or the adaptation of athletes’ bodies during exercise. Nonetheless, the same life-giving process also occurs frequently in some patients and, conversely, occurs slowly in some physiological problems, such as cancer and diabetes, so inhibiting angiogenesis has been considered to be one of the important strategies to fight these diseases. Accordingly, in tissue engineering and regenerative medicine, the highly controlled process of angiogenesis is very important in tissue repairing. Excessive angiogenesis can promote tumor progression and lack of enough angiogensis can hinder tissue repair. Thereby, both excessive and deficient angiogenesis can be problematic, this review article introduces and describes the types of factors involved in controlling angiogenesis. Considering all of the existing strategies, we will try to lay out the latest knowledge that deals with stimulating/inhibiting the angiogenesis. At the end of the article, owing to the early-reviewed mechanical aspects that overshadow angiogenesis, the strategies of angiogenesis in tissue engineering will be discussed.

Introduction

Almost all tissues need a constant source of oxygen, nutrients, and minerals, as well as a means of removing waste products produced in the tissues. In vertebrates, this need is met by the vascular system, which requires the process of angiogenesis. Angiogenesis is the process of growing new capillaries from existing capillaries in response to a change in cell’s mechanism and/or metabolic environment that takes place in the form of sprouting and longitudinal division of endothelial cells [1]. Angiogenesis in adults is present in the post-exercise regeneration process (recovery period) and other conditions such as limb development, wound healing, fetal development, post-ovarian corpus luteum formation, and new postmenstrual endometrium formation [2]. This phenomenon is a complex multi-step process and involves several signal pathways coordinated by angiogenesis inhibitors and angiogenic stimuli (pro- and anti-angiogenesis). Also, angiogenesis can be a challenge in various pathological conditions, such as tumor growth and metastasis, arthritis, and diabetic retinopathy. Inhibition of angiogenesis in various pathological conditions is essential. As the best example of this, as stated in early 1971, tumors never grow beyond a certain size unless their arteries expand [3]. Therefore, many researchers have studied the effects of angiogenic stimulants and inhibitors due to their importance in preventing and treating this type of disease in a variety of laboratory models. In general, due to the importance of the formation and/or non-formation of blood vessels in different conditions, the study of inhibitors or stimulants of angiogenesis is of great importance.

In this review article, we will first refer to how the vessel is formed and what types of angiogenic pathways are excited in the body. Then, relying on the role of angiogenesis in diseases and natural processes of the body, some explanations of the importance of this process in different conditions would be given. In the next part of this study, the types of anti-angiogenic and pro-angiogenic factors will be introduced. Due to the importance of exogenous factors that inhibit and/or stimulate angiogenesis in leading studies in the field of nanotechnology, biochemistry and synthetic chemistry, these factors will be followed up by the explanation and mention of more references. In the final parts of this study, the types of physical anti-angiogenic and pro-angiogenic will be cited. These factors can provide worthy solutions to tissue engineering experts to design systems with optimal shape and architecture. Finally, this review article will be summarized by bringing up the challenges facing researchers in the field of angiogenesis and the future perspective of this research area. Studying this review article will be a great help to provide a research line and coherence of knowledge of researchers in the field of synthetic chemistry (to suggest new anti-angiogenic and pro-angiogenic factors) tissue engineering (to control the process of angiogenesis), biochemistry, nanotechnology, cancer research and will be a benefit for other angiogenesis enthusiasts.

The process of angiogenesis

Depending on how the vessels are formed, the time of vessel formation and their morphology, different types of pathways for angiogenesis have been suggested. In general, the process of angiogenesis has been categorized into the following sectors.

Vasculogenesis

In the earliest stages of the development of the embryo, in the absence of arteries, it receives the absorption of nutrients through physical diffusion. Then, in a systematic and sequential process, the embryo quickly becomes a more developed creature with many arteries. The initial organization of the endothelial cells that lead to the formation of blood vessels is called vasculogenesis, and before that, there is no other vascular system [4]. Therefore, when new tissue is formed, blood vessels must be formed along with the tissue to guarantee the rest of the growth process. Vasculogenesis begins with the differentiation of mesoderm cells into hemangioblasts, which underlie hematopoietic and endothelial cells. With more differentiation, hemangioblasts turn into angioblasts and with the accumulation of angioblasts, primary islets of blood would be formed. These islets of blood then merge to form the primary retinal vascular network, which consists of thin capillaries formed by endothelial cells. Vasculogenesis continues with the formation of the primary vascular network and its development during the angiogenic process [5].

Angiogenesis

Angiogenesis is the development of new arteries from existing arteries, which occurs in two forms: non-sprouting and sprouting. The sprouting form is considered the main mechanism of angiogenesis during the stages of natural development and cancer [6]. The branching and protrusion of a new capillary from the previous capillary indicate that excessive proliferation of endothelial cells is required [7]. In contrast, the non-sprouting system of angiogenesis refers to the halving of an evolved vessel by capillary fission from within the capillary (a longitudinal division of the capillary, intussusception) and the transformation of one capillary into two capillaries. In this way, because there is less need for endothelial cell proliferation, it is a more efficient process compared to sprouting [8].

Arteriogenesis

There is another angiogenic process called arteriogenesis. Arteriogenesis means the enlargement of the arteries both in terms of diameter and vascular wall thickness [9]. This form occurs mainly in large arterioles and small arthritis. The process requires the proliferation of smooth muscle cells and endothelial cells. The most important stimulus involved in the process of arteriogenesis is the hemodynamic stimulus of shear stress [9].

Figure 1 summarizes the process associated with angiogenesis, vasculogenesis and arteriogenesis.

Fig. 1.

The processes of angiogenesis, vasculogenesis and arteriogenesis. In the classical form of angiogenesis, the main inducing factor is oxygen depletion, followed by the Hypoxia-inducible Factors (HIFs), which leads to the activation of other factors such as Vascular Endothelial Growth factor (VEGF), Fibroblast Growth Factors (FGF) and Angiopoietin 1 (Ang 1). In this type of angiogenesis, helped by these factors, new vessels grow from existing vessels. Unlike the angiogenesis, in the process of vasculogenesis, new blood vessels are formed by the division of endothelial progenitor cells. In this process, the cells grow through factors such as VEGF, FGF, Ang 1 and Transforming Growth Factor (TGF) (without the need for stimulation by hypoxia). In the arteriogenesis process, most of the lateral connection occurs between the arteries, which requires the division of two types of smooth muscle cells and endothelial cells

An overview of the molecular mechanism of angiogenesis

Researchers believe that reducing oxygen pressure (hypoxia) in the tissue is very important for inducing angiogenesis in physiological or pathological conditions that involve several stages. Under such conditions, the tissue makes and releases pro-angiogenic factors such as VEGF [10]. These factors activate endothelial cells when they bind to their receptors. Signs of activation of high mitotic endothelial cells are an increase in the capacity for invasion and proteolysis of the extracellular matrix [11]. Active endothelial cells are capable of invading the basement membrane and the matrix outside the cell, disrupting integrin-mediated interactions. In this way, with the onset of endothelial cell activity, certain types of metalloproteases are secreted from the tip cells and decompose the basement membrane in the mentioned region [12]. By digesting the basement membrane, endothelial cells migrate and proliferate. In addition, binding molecules such as αvβ5 and αvβ3 integrins contribute to the process of pulling and advancing growing blood vessel buds [13]. In the later stages of the angiogenesis process, metalloproteinases in the matrix are produced to degrade the extracellular matrix and begin its regeneration. Tie2 angiopoietin is then initiated to begin the process of tube formation. In the next stage, the EphB–ephrinB system also regulates the process of tubular formation, and finally, pericytes and smooth muscle cells are provided to stabilize the newly formed blood vessel [14]. All steps are pictured in Fig. 2.

Fig. 2.

Different steps in physiological angiogenesis. Initially, the endothelial cells of a stable vessel begin to increase permeability. At this stage, the endothelial cells are still firmly held together by the pericytes. Following increased permeability, the plasma protein content, which also contains different pro-angiogenics, would be secreted into the extracellular matrix space. At the same time, due to the activation of proteases, the connection between pericytes and the basement membrane would be weakened and pro-angiogenics-derived from the basement membrane would be secreted into the extracellular matrix space. These factors gradually lead to the onset of endothelial cell proliferation and the formation of new blood vessels

Natural angiogenesis depends on the coordination of several independent processes. To form new blood vessels, the mural cells first move from the existing vessel branch. Angiopoietin-2 mediated vascular instability changes endothelial cells from a stable, stunted state to a multifocal phenotype. VEGF then increases vascular permeability. At this stage, proteases and matrix compounds leak from the vessel wall and endothelial cells begin to proliferate. Following proliferation, endothelial cell migration occurs, then tube-like structures are formed and blood can flow. Mesenchymal cells proliferate and migrate along new arteries and then differentiate into mature pericardial cells. Enhancing cell–cell interactions and precise fabrication of the new matrix will stabilize the new vessel [15].

Vascular germination requires co-operation between stem cell migration and stem cell proliferation. Initially, the endothelial cells at the leading edge of the vascular sprout expand themselves toward angiogenic stimulus signals [16]. They migrate to the anterior part where the VEGF level is highest, the VEGF activates the VEGF receptor 2 (VEGFR2) receptor presented at the surface of the tip cell to stimulate the cell migration. In addition, VEGFR2 signals are amplified through a cursor called Nrp1, which improves the performance of the primary cell [17]. But the important point is that if all the endothelial cells are equally exposed to angiogenic stimuli, then part of the vascular network breaks down and the blood supply to that part of the tissue is disrupted [18]. As a result, to prevent this from happening, mechanisms have been put in place to select only one endothelial cell within the capillary to initiate angiogenesis of the apex. These mechanisms include the Notch family receptors and their ligands, delta-like 4 (Dll4). Endothelial cells either become progressive and invasive apical cells or proliferate stem cells, which regulate this phenotype. By activating endothelial cells with VEGFR2 signals, they increase Dll4 expression and bind to the Notch receptor on neighboring endothelial cells. After binding Dll4 to the Notch receptor, a transcription factor called notch intracellular domain (NICD) will be released, which will decrease VEGFR2 and Neuropilin 1 (Nrp1) expression. The tendency of VEGFR1 to catch VEGF is higher than VEGFR2; its signaling power is ten times faster than VEGFR2. Therefore, endothelial cells with the highest VEGFR2 and the lowest VEGFR1 migrate to the crest position [19, 20].

As the budding sprouts move along the higher concentration of VEGF, the tip cells attach to the extracellular matrix (ECM) via integrin and migrate to the signal molecules such as semaphorin and ephrin. Stalk cells are located behind the apical cell and proliferate to elongate sprouts and form lumens. While Notch signals prevent stem cell proliferation, notch regulated ankyrin repeat protein (Nrarp) expression in the branching locus allows Wnt signals to support the stalk cell proliferation. Lumen formation and subsequent tube formation is one of the characteristics of angiogenesis and relatively specific behavior of endothelial cells after the mentioned signaling cascades. As the stalk cells proliferate, the lumen lengthens to the point where the two tip cells from the two adjacent arteries meet and merge. The mechanism of fusion is mediated by macrophages around this micro-location. The attachment of tip cells to each other is enhanced by connections containing vascular endothelial cadherin (VE-cadherin), and macrophages around the arteries stimulate the budding by the production of angiogenic factors. The stalk cells also stabilize the developing vessel by depositing the basement membrane and absorbing pericytes. Mesenchymal progenitor cells are adsorbed by endothelial cells through the expression of platelet-derived growth factor (PDGF) and differentiate into pericytes in response to TGFβ. The pericytes lead to the stabilization of emerging arteries by reducing migration, proliferating endothelial cells, and reducing vascular leakage [21].

Pericytes and their relationship with angiogenesis

In most cases, small vessels and capillaries are covered with protective-adult cells. For many years, the definition of these cells, today is known as pericytes, was controversial. Pending, based on complementary observations of past information, the researchers finally concluded that the definition of pericytes is any type of adult cell that surrounds capillaries. From another perspective, pericytes are identified based on information such as location, the markers they express, and morphology. It has been strongly approved that the physical connection of these cells with the endothelial cells plays a decisive role in maintaining the structure and integrity of the vascular cell membrane [22]. It has also been shown that this physical connection allows pericytes to regulate blood flow in the vessels [23]. The interesting thing that has been seen in different parts of the body is that the ratio of the number of pericytes to endothelial cells in different organs of the body is different and in general it can be said that it depends on the local blood pressure; in the lungs the ratio of is ~ 1:10, while in the striated muscle tissue this ratio reaches 1:100. In organs such as the retina and central nervous system, this ratio is equal, leading to the blood–retinal barrier and blood–brain barrier formation [24]. Perhaps the most important way of signaling communication between the endothelial cells and the pericytes is through the paracrine, through which the cells are able to force the last cells to divide and mature, and wherever there is a need for vascular buds, by using pro-angiogenic factors, vessel sprouting is employed. In the opposite direction, pericytes can inhibit angiogenesis by suppressing these factors [25]. The connection between these two types of cells has many secrets behind. It has been observed that in addition to the usual receptor–ligand bonding, there is a state of mechanical connection between them. Many studies have shown by changing the micromechanical space around these cells such as blood pressure, cyclic strain and fluid shear stress, the communication function of the cells undergoes new changes [26]. It has also been experimented that the diffusion of cells and proteins from the wall of the arteries to the perivascular space is also controlled by the pericytes. These processes are particularly evident in the maturation of immune cells and the use of these cells in inflamed tissues [22, 27, 28].

Mechanical factors in angiogenesis

As explained, angiogenesis is a phenomenon that requires coordination between several different processes. One of these important processes is to change the behavior of mechanical forces in the tissue to create angiogenesis. In addition to being in contact with themselves, endothelial cells are also in contact with the extracellular matrix via protein connector molecules, which are transformed during angiogenesis. The purpose of this section is to present the forces involved during angiogenesis from a biomechanical perspective [29].

Cell-generated forces in angiogenesis

The inside of the cells is surrounded by an organized and dynamic network of cytoskeletons. This well-architectured structure plays a major role in important processes such as cell division, cell movement, overall cell shape, and cell attachment to the environment. In general, the cytoskeleton consists of three main types of filaments: microtubule filaments, actin filaments, and intermediate filaments [30]. The importance of cell communication with the environment in the overall formation of cells is noticeable. With a more detailed look, when mammalian cells are suspended in the environment, most of them take on a spherical shape while attaching to the extracellular matrix environment (through the intracellular skeleton and the binding proteins in the membrane and finally the extracellular environment) they would be transformed to flattened shape. Maintaining this particular shape depends on the relationship between actin and myosin. These stress strands eventually bind to integrin proteins anchored to the membrane, transmitting intercellular forces to the cell membrane. As a result, the cells attached to the surface are always under stress to maintain their final shape [31, 32]. This stress changes during processes such as migration and cell division. In the case of angiogenesis, these cellular stresses are seen in the three processes of matrix remodeling, cell migration, and tube formation. During the migration of endothelial cells to form new vessels, the tensile forces at the beginning and end of the migration path are maximal and in the middle part of the cell, where the nucleus is located, are the lowest. These forces are ultimately driving the cell to the right place and cause the cell to migrate to a specific point. These stresses are such that they affect even the final shape of the new tubes during angiogenesis. It has been observed that endothelial tubes formed in environments with weak cell–matrix strength had a longer length with an expanded lumen. Also, it was seen that the cells in this condition were less connected to each other [33].

Physical properties of ECM on angiogenesis

The degree of hardness of the extracellular environment plays an important role in the angiogenesis process. It has been observed that when the artificial extracellular environment became stronger and stiffer (using chemical linkers), the rate of angiogenesis in them decreased sharply [34–36]. The stiffness of the matrix is subsequently important that it can even affect angiogenic factors. Pro-angiogenic factors are effective when endothelial cells have easy access to vascular growth factors, which are associated with extracellular matrix stiffness. A more malleable extracellular matrix is very important for angiogenesis. But this property should not be too weak and/or strong [37]. It was observed that new vessels do not form in very soft and loose environments [33]. In very soft environments, due to the lack of tension between the cell and the environment, the cell may enter apoptosis [38]. It has been observed that there is an approximate inverse relationship between matrix tissue stiffness and the potential of endothelial cells to expand toward vascular network formation. As seen in the two-dimensional models of angiogenesis, when the rigidity of the matrix increased with strategies such as increasing gelatin and/or collagen, the rate of development of planar networks in them is decreased [34, 36, 39]. It is believed that one of the reasons behind this evidence could be the accessibility of endothelial cells to soluble pro-angiogenic factors in the extracellular matrix [40].

These connections, all of which play a major role in the overall behavior of endothelial cells to form new vessels, are depicted in Fig. 3.

Fig. 3.

Effect of different extracellular and intracellular forces on endothelial cells. The relationship that the endothelial cell has with the environment and the extracellular matrix may directly affect gene expression or, by interacting with the cytoskeleton, mediate the change in genome expression and ultimately different cellular responses

Externally applied mechanical stresses in angiogenesis

The endothelial cells are constantly bearing the environmental forces caused by the blood flow in them or the surrounding environment such as the growing tissue or the skeletal muscle cells. Numerous observations indicate the effect of these forces on the change in gene expression of endothelial cells to increase and/or decrease angiogenesis [41, 42]. The most important force that can be applied to them is shear stress. Shear stress due to blood flow in the arteries has been shown to increase angiogenesis, while as the stress decreases, angiogenic buds gradually shrink and fade [43]. Increased fluid shear stress has also been shown to increase VEGF expression [44] and matrix metallopeptidase 9 (MMP-9) activity [45], which in turn leads to angiogenesis. The same responses have been observed in endothelial cells during increased mechanical forces [44, 46, 47]. Figure 4 shows these forces.

Fig. 4.

Two main forces that affect angiogenesis. The force exerted by the skeletal muscles is called the mechanical stress force and the force originated by the blood flow is called the shear force

Physiological and pathological processes related to angiogenesis

As mentioned, angiogenesis is a biological process involved in wound healing, fetal growth and development, and endometrial proliferation. Angiogenesis is also a major process in the proliferation and metastasis of cancer cells [48]. In general, when the tumor size is less than 2–3 mm, the nutrients and oxygen required by the cancer cells are provided through diffusion (avascular stage), nonetheless when the tumor size is more than 2–3 mm, the nutrients and oxygen from the diffusion to the cancer cells do not reach enough size, at which point the cells begin to produce angiogenic factors, and the tumor remains in the dormancy stage until it can grow new blood vessels by secreting angiogenic processes [49].

Angiogenesis is very important in the processes mentioned below.

The role of angiogenesis in fetal development

Fetal blood vessels are formed through both vasculogenesis and angiogenesis. In the fetus, endothelial cells that make up blood vessels and hematopoietic tissues, develop together. In the early stages of embryonic development, angioblasts are derived from the lateral mesoderm layer and the crescent of the heart. Some of these cells migrate into the brain. Several cells also accumulate inside the endocardium of the primary myocardium. Other angioblasts form networks of endothelial cells at the base of the heart tube that fuses with the vitelline, allowing blood cells to flow from the yolk sac into the fetus's body. They directly surround the mesenchyme by the angiogenic invasion of tissues, forming visceral vessels, and finally, angiogenesis is stimulated by both endoderm and ectoderm, and eventually causes the development of various embryonic organs [50].

Angiogenesis in the wound healing process

Skin abnormalities caused by various factors such as burns, wounds, various diseases such as diabetes, skin diseases, etc., would bring many problems in the individual and social life of people [51]. Today, the introduction of new, effective, and efficient treatments for such disorders is of great importance. Wound healing is a vital biological process that is very important in tissue homeostasis [51, 52]. This process is disrupted in some diseases and causes many pathological problems for the person. Many people around the world suffer from burn wounds, bed sores and diabetic wounds. Statistical studies have shown that 50% of people with diabetes develop diabetic foot ulcers during their lifetime [53]. All these prove that studying the process of wound healing and helping to accelerate this process will be of great value to the scientific community. In terms of angiogenesis, blood vessels play a very important role in wound healing.

The first stage in the wound healing process is the initial vascular response to damaged tissue. This response time is about 10 min from the moment of injury to the stop of the bleeding [51, 52]. At this stage, the damaged cells at the wound site secrete factors that lead to the narrowing of the arteries, thus preventing bleeding. Tissue damage and skin lesions lead to rupture of arteries and bleeding at the site, which continues with the activation of the coagulation response and the formation of fibrin filaments, and eventually, the formed clot prevents the progression of bleeding [54, 55]. The second stage of the wound healing process is the inflammatory response. This response lasts up to a week. The formation of a clot is a repair signal that causes the monocytes to be called to the wound site 1 day after the onset of damage and then turn into macrophages. Platelets at the wound site also begin to secrete TGFs, fibroblast growth factors (FGFs), and PDGFs, which stimulate cells to grow and multiply. Furthermore, the role of macrophages is to clear the site of injury through microspores and the secretion of cytokines and growth factors. Studies have shown that macrophages are effective in wound healing by secreting interleukins, including interleukins 1 and 2. At the end of the third day and the end of the inflammatory response, the leukocytes migrate from the wound site. If infection occurs, the leukocytes are called back to the site and resume cleansing. This prolongs the inflammatory response and wound healing process. One of the manifestations of this stage is swelling and redness. This is due to the release of histamine, serotonin, and bradykinin at the site. These factors cause the small blood vessels to dilate, resulting in more blood flow to the affected area and eventually swelling and redness [56, 57]. The cell proliferation phase begins on the fourth day of injury. At this stage, blood vessels form at the site of injury, and cell proliferation and collagen production take place to repair skin tissue. Fibroblasts migrate into the wound through the dermis after macrophages clear the site of injury and begin to proliferate at the site of injury. Fibroblasts are found closer to the edges of the wound and are exposed to a suitable growth environment and oxygen pressure of about 40 mmHg. In cell culture, this oxygen pressure is suitable for the proliferation of fibroblasts. Fibroplasia (proliferation of fibroblasts) is stimulated by some mechanisms that begin with the secretion of factors such as insulin-like growth factor PDGF and TGFβ [Growth Factor 1; insulin-like growth factor 1G (IGF-1G) from platelets] and continue to be secreted by cytokines. The epidermal growth factor is also transported through the blood to the site of injury, where type III collagen is first made and then replaced by type I collagen. For the connection between type I collagens, Fe ions as a cofactor, oxygen, and vitamin C are required. The presence of collagen in the extracellular matrix leads to tissue strengthening [58]. New epidermal cells begin through cell division and cell migration around the wound, which eventually leads to wound healing [59].

Angiogenesis and endometriosis

Endometriosis, which is described by the incidence of endometrial-like tissue outside the uterine cavity, especially in the ovaries and peritoneal cavity, is a disease that often affects women of childbearing age only. Inflammation, fibrosis, pelvic pain, painful menstrual cycles (dysmenorrhea) and infertility are the symptoms of this disease [60]. According to evidence of new vessel formation by laparoscopic results, it has been shown that abnormal angiogenesis plays a vital role in endometriosis, as a feature of endometriosis [61]. There are several theories about the pathogenesis of endometriosis; the most accepted of which is that endometrial cells invade the mesenteric layers by enzymatic degradation and ultimately increase angiogenesis, which eventually leads to endometriosis [62, 63]. Therefore, angiogenesis plays a key role in the growth and stabilization of endometrial lesions. Evidence suggests that the balance of pro- and anti-angiogenic factors in lesions determines their growth or non-growth. As mentioned, one of the main features of this disease is its inflammatory nature and cytokines released from immune cells that play an important role in the pathogenesis and angiogenesis related to the condition. For example, interleukin-1 is released by active macrophages, leading to increased expression of VEGF and interleukin 6 (IL-6). Scientists in this field believe that VEGF is a major potential risk in this disease. Studies have shown that VEGF is expressed in endometrial lesions and activated macrophages and neutrophils. It has been observed that increased levels of VEGF in the peritoneal fluid of endometriosis patients are controlled compared with healthy women; however, the level of VEGF in urine and blood serum of the patients was not different from that of control women [60, 64, 65]. As a result, it can be said that there is a positive relationship between the stages of the disease and the concentration of VEGF. On the other hand, estradiol, which is found in high concentrations in endometrial lesions, is a potent stimulant of angiogenesis by directly increasing VEGF expression. Therefore, it has been suggested that controlling VEGF secretion and its products can be a new and interesting treatment for endometriosis [66]. Hull et al. showed that in mice with endometriosis, systemic treatment with antibodies against VEGF, and its sflt-1 receptor inhibited the growth of endometriosis lesions [67].

Angiogenesis in cancer

In 1971, Folkman noted that "tumors never grow beyond a certain size unless their arteries enlarge." In those days, he also theorized that tumors have new blood vessels that use a diffuse factor. He referred to these factors as tumor angiogenesis. Finally, he stated that theoretically, if it could inhibit angiogenesis, the tumors would remain small in size and would not eventually be damaged. Therefore, according to Folkman's theory, inhibition of angiogenesis and subsequent inhibition of cell metastasis is a good way to fight cancer and recognizing the factors involved in normal and/or abnormal angiogenesis is very important and vital [68].

In cancer, the molecular mechanism of angiogenesis is almost similar to what occurs naturally, however, differences have been observed in certain aspects. Angiogenic factors are released into the environment by tumor cells and stimulate different types of normal cells. This stimulation especially involves endothelial cells adjacent to the tumor. These cells break down their basement membrane and migrate to the tumor mass by separating from adjacent cells and entering the extracellular matrix. Nonetheless, cell division also occurs in the sprout, and with increasing migration, fibrous endothelial cells are formed from these cells and develop the basement membrane, forming a tubular structure. These tubes then connect to form new blood vessels that eventually connect to the circulatory system. Thus, a capillary network is formed in the tumor mass and can continue to expand. Oncogenic changes and hypoxia in tumor cells may be involved in the induction and spread of angiogenesis through angiogenic factors [69, 70]. As mentioned, one of the important factors that induce angiogenesis is hypoxia. Several pro-angiogenic factors such as bFGF, IL-1β, TNF and VEGF are induced by hypoxia. For example, in human cervical cancer and soft tissue sarcoma, the presence of hypoxia before treatment increases the risk of distant metastases. In addition, the presence of high-density vascular tumor areas in breast carcinoma and prostate cancer has a poor prognosis. These reports indicate that hypoxia activates angiogenesis and causes metastasis. In the presence of oxygen around the cell, HIF is hydroxylated and finally degraded. However, in the absence of oxygen, this factor remains un-hydroxylated and rests stably from degradation and migrates to the nucleus and induces effective factors in angiogenesis. Oncogenic changes in tumor cells may be involved in the induction and spread of angiogenesis through angiogenic factors. Mutations in some oncogenes such as K-rat sarcoma virus (K-ras), H-ras, V-raf and V-src genes induce VEGF expression. Also, mutations in the tumor protein P53 suppressor gene lead to decreased thrombospondin (TSP) production and increased VEGF expression, resulting in activated angiogenesis. Other angiogenic growth factors such as β and α TGFs are enhanced by ras gene mutants and lead to the activation of angiogenic growth factor promoter regions [69, 71]. Tumors can provide the blood source they need in several ways. In a process comparable to usual angiogenesis, the tumor may deform the blood vessels normally. In addition, tumor cells can grow around existing arteries and do not initially require new angiogenesis; known as vessel co-option which is a process in which tumor cells use pre-existing tissue blood vessels to support tumor survival, growth and metastasis [72]. Although tumor-induced vessels form a tubular structure for the transfer of metabolites, they are ultrastructurally abnormal. In a wide range of cancers, many inactive pericytes shrink and expand and become permeable due to the presence of intercellular pores and the lack of a complete basement membrane [73]. Tumor vessel walls could be made up of both tumor and endothelial cell types. These abnormal structures indicate the pathological nature of tumor stimulation, however, their ability to support cell growth depends on physiological mechanisms of angiogenesis.

Angiogenesis is a decidedly regulated process under physiological conditions. The primary vascular network normally undergoes new formation, and not only chemical factors such as cytokines and various proteins are necessary for this process, but also physical forces such as the important blood flow are parts of this process. The dependence of the formation of new vessels on blood flow causes only functional vessels that contain sufficient blood flow to become part of the vascular network [74, 75]. Adequate blood circulation is essential for the rapid growth of tumor-developing tissue (Fig. 5). Tumor cells can receive the nutrients and oxygen they need through simple diffusion until they reach a size of 1–2 mm. But for more growth, they need to receive more blood vessels [76]. Angiogenesis, both in tumors and under physiological conditions, is influenced by activating factors, such as bFGF, VEGF, nitric oxide, and MMPs. All of these activating factors can be produced by the tumors themselves or their surrounding tissue and/or by macrophages and fibroblasts [76], and all of them are located at the level of the endothelial cells that line the endocrine glands [77, 78]. The expression of pro- and anti-angiogenic factors by cancer cells is regulated by oncogenes, tumor suppressor genes, and multiple transcription factors. In addition, the main effective environmental factors are the rate of oxygen and glucose uptake [78]. The endothelial cells involved in tumor arteries, in addition to being more active, also show some surface indicators of endothelial cells, such as Tie2, Tie1, VEGF, and most of the integrins of α5β1 and αvβ3. In addition, the expression of surface adhesive molecules such as E-selectin is much higher in them [79]. St. Croix et al. compared the expression pattern of normal endothelial cell genes derived from colorectal cancer tissue and showed a significant difference in the expression of 79 factors. Most of these genes were similar to the factors affecting angiogenesis in wound healing and ovulation stages, suggesting that tumor signaling pathways of angiogenesis are similar to those of physiological angiogenesis signaling pathways. In addition, these findings indicate that the tumor environment is very effective in changing the expression pattern of genes in the endothelial cell and inducing angiogenic phenotype [80–82]. In short, angiogenesis in tumors is performed by the two main mechanisms of hypoxia and inflammation. Chronic hypoxia due to the rapid division of tumor cells leads to increased expression of HIF and its downstream genes, which are, in fact, the main genes for oxygen-induced homeostasis through the mechanism of glomerulus. Another effective factor is the infiltration of inflammatory cells such as lymphocytes, neutrophils, and macrophages. Extensive networks of adhesive molecules, growth factors, cytokines, and proteases resulting from the secretion of these cells are involved in tumor angiogenic growth [81]. In addition, the macrophages, which are trapped in the hypoxic and necrotic regions of the tumor, begin to secrete large amounts of matrix metalloproteinase, all of which stimulate proliferation, migration, and finally the metastasis of cancerous cells. Thus, the more vessels are formed in the tumor tissue, the more tumor cells pass through the permeable wall of the arteries and enter the bloodstream [83, 84]. In a study by Hirakawa et al., it was shown that increasing angiogenic markers dramatically increased tumor growth and progression and decreased apoptosis of tumor cells [85]. There is also a strong direct relationship between increased capillary density, metastasis, and mortality in people with breast, colorectal, lung, melanoma, ovarian, and bladder progenitor cells; so with increasing capillary density, the number of tumor cells in the bloodstream increases sharply [86]. The proliferation of some angiogenic features, such as VEGF-A, not only increases the formation of new blood vessels but also increases the lymphatic vessels within the tumor [87], a process that facilitated metastasis. Therefore, anti-angiogenic treatments to prevent the growth and metastasis of tumors have recently received much attention. For example, a synthetic molecule that blocks VEGF activity and its receptor is able to reduce melanoma tumor growth in animal models [88].

Fig. 5.

Angiogenesis in a tumor. Tumor cells by the inflammation they created and by increasing metabolism and subsequently reducing oxygen provide two important signals for angiogenesis. These signals lead to the formation of a vessel by chemotaxis towards the tumor mass (left panel). As it moves away from the blood vessels and closer to the site of cancer (cancer cell accumulation), the oxygen concentration decreases (red triangle), which is one of the strongest stimulants of angiogenesis. Cancer cells also secrete other angiogenic factors, such as VEGF, MMPs, and different ILs by initiating signaling due to a decrease in oxygen pressure, which gradually decreases as they move away from these masses (blue triangle). When the vessels form around the cancerous mass, the vessels are eventually used to allow the cancer cells to migrate to other tissues and trigger metastasis (right panel)

Exosomes: a smart cancer-derived tool for progressing metastases by increasing angiogenesis

The progression of cancer cells depends on angiogenesis. One of the clever solutions that cancer cells use to accelerate the process of angiogenesis is the production of exosomes [89]. Exosomes are smaller lipid-enclosed particles that act like secretory vesicles and contain factors such as proteins, deoxyribonucleic acid (DNA) fragments, microRNAs, and can even contain drug particles that are transported from one cell to another. The content of exosomes can vary depending on the type of cell from which they originate [90]. For example, pro-angiogenic proteins such as VEGF, FGF, interleukin 6 and 8 have been observed in exosomes extracted from glioblastoma cells [91]. Exosomes vary in size from 30 to 100 nm and vary in density from 1.13 to 1.19 g/mL. As far as being observed, these small vesicles can be produced and secreted from both healthy cells and in pathological conditions from cells involved in the disease. The presence of these objects has been confirmed in various parts of the body such as nasal secretions, tears, cerebrospinal fluid, milk, saliva, urine and plasma [92, 93]. VEGF-rich exosomes are believed to cause some cancer cells to become resistant to anti-angiogenic therapies [94]. The communication pathway is seen through the exosomes eventually leads to angiogenesis, the interaction of these bodies with the endothelial cells and immune cells. In vitro and in vivo studies have shown that exosomes containing pro-angiogenics are able to activate angiogenesis signaling pathways upon contact with endothelial cells, and thus promise the angiogenic process [95]. On the other hand, with a different mechanism, exosomes are able to enter these cells by communicating with T cells (via direct ligand-acceptor communication) and stimulating the angiogenic process. It has been observed that the rate of absorption of these objects by normal cells is between 2 and 4 h [96]. Exosome bodies also have the ability to develop drug-resistant cells in a variety of cancers. For example, recent work has shown that long non-coding RNA lncARSR can make resistance to the anti-cancer drug sunitinib in renal cell carcinoma. This hereditary nature RNA can be transferred from drug-resistant cells to other cells and make them resistant to the drug [97]. Despite all the negatives that have been said about exosomes, a view has recently emerged that these objects can be used as drug carriers to fight cancer cells. In the meantime, attempts have been made to use mesenchymal stem cells as the most important cells that produce exosomes [98]. A recent interesting study found that the exosomes secreted by human embryonic stem cells were able to not only delay the aging process in laboratory mice but also increase the rejuvenation process mediated by nuclear factor-erythroid factor 2-related factor 2 (Nrf2) factor. Another piece of evidence shows that these exosomes are rich in miR-200a factors and eventually increase Nrf2 expression through negative regulation of Kelch-like ECH-associated protein 1(Keap1) [99].

Angiogenesis in diabetic patients

Diabetes mellitus is a chronic metabolic disease characterized by some features such as hyperinsulinemia, hyperglycemia, insulin resistance, and major metabolic changes. The disease is also associated with advanced atherosclerosis of the large arteries, which is a major risk factor for acute myocardial infarction. In addition, diabetes abnormalities in neovascularization associated with defects in wound healing increased the risk of graft rejection, fetal malformations in diabetic mothers, and the formation of defective vascular adverse coronary. All of these characteristics in other words are associated with impaired angiogenesis [100, 101]. Diabetes is generally a paradoxical disease in terms of vascularity and angiogenesis; because on the one hand, it causes neovascularization to increase in organs such as the kidneys and eyes, and on the other hand, it stops angiogenesis in the coronary arteries of the heart and also peripheral arteries. Therefore, the paradox refers to the concurrent presence of pro- and anti-angiogenic conditions [102, 103]. Diabetes is also associated with incomplete arteriogenesis [104], a complication that reduces the growth and maturation of lateral arteries, decreases the capillary density, especially following myocardial infarction, and ultimately reduces the number of small vessels in the heart. The result of all these factors is a decrease in myocardial perfusion and ultimately an increase in mortality [103]. Abaci et al., in a study of 410 people with coronary artery disease (205 diabetics and 205 non-diabetics), found that diabetics had a lower density of coronary artery [105]. Studies by Werner et al. also support the finding that the development of lateral coronary arteries in diabetic patients is significantly reduced [106]. In addition, it has been suggested that diabetes causes a decrease in capillary diameter, a reduction in the ratio of capillaries to fibers, a decrease in capillary diffusion capacity, and a disturbance in the hemodynamic regulation of muscle vessels [107, 108]. On the other hand, increased pathological angiogenesis is associated with diabetic retinopathy, nephropathy, and hemorrhage in atherosclerotic plaques and their instability [109]. Although there is a growing awareness of the anti-diabetic effects of angiogenesis, the molecular mechanisms involved in this phenomenon are not well understood. Inhibited diabetic angiogenesis is thought to be associated with inadequate degradation of the basement membrane, changes in the balance of growth factors, and cytokines that regulate vascular stability. Alternatively, increased angiogenesis in diabetes with up-regulation of FGF and VEGF, up-regulation of integrins (which facilitates the migration and mobilization of growth factors necessary for angiogenesis and glaucoma), metabolic disorders, and non-enzymatic glycosylation of proteins are associated. Recently, an interesting and new hypothesis has been proposed to explain the paradox of angiogenesis in diabetes by considering the following three facts: (1) VEGF is a suitable stimulant for arteriogenesis and angiogenesis. (2) In diabetes mellitus, angiogenic VEGF levels increase. (3) In diabetes mellitus, the angiogenic and arteriogenic responses to VEGF are reduced. This hypothesis states that, in diabetes mellitus, there is resistance to VEGF and a defective response to it. However, this response must be studied to decide whether it is due to a defect in the signal transmission path downstream of VEGF receptors or due to a decrease in VEGFR2 expression in endothelial cells [104]. Although studies of Waltenberger et al. on monocytes extracted from diabetics showed a deficiency in VEGF-related signal transduction [100], the question that may arise is how to induce angiogenesis in retinopathy. Did diabetes and atherosclerotic plaque justify VEGF receptors despite the defective response? The best possible response may be to pathologically increase the level of VEGF, which is a compensatory mechanism for the elimination of its signal path defects. Contrariwise, high levels of VEGF induce a local inflammatory response by increasing the permeability of vascular structures throughout the body, especially in the eye, and eventually lead to the emergence of new arteries.

Angiogenesis in hypertension

Hypertension is a disease characterized by features such as increased vasoconstriction, decreased vasodilation, and structural and functional changes in small and large vascular networks. Structurally, hypertension increases the wall thickness of arterioles, increases the wall-to-lumen ratio of arteries, and alters their components. Hypertension also reduces the number of capillaries and arteries. Microvascular shrinkage has also been observed in animal and human models with hypertension, which may be due in part to defects in angiogenesis and the formation of the microvascular network [110, 111]. There is growing evidence that hypertension is associated with inadequate, incomplete, and inappropriate responses to angiogenic growth factors; it is also possible that angiogenesis and arteriogenesis are suppressed during the progression of hypertension so that patients with hypertension have reduced capillary density [110]. In a study, it was found that long-term antihypertensive treatment in non-diabetic patients amplified capillary density compared to the control group. In a clinical model using Bevacizumab (which is a monoclonal antibody against VEGF and is used as an angiogenesis inhibitor) for the treatment of renal cell carcinoma, hypertension was observed as a complication [112]. A contradictory finding that has been demonstrated in recent studies on hypertension is that there are high levels of angiogenic growth factors in this disease; for example, in an analysis of 248 patients with hypertension and other cardiovascular risk factors, the presence of a positive correlation between hypertension and high levels of VEGF was demonstrated in the patients. In the patients, a low level of sflt-1 factor was also reported in comparison with individuals with normal blood pressure [113, 114]. Possible mechanisms involved in the increase of angiogenic factors in hypertension include tissue ischemia, increased vascular traction, endothelial damage due to high blood pressure, and decreased renal responses. This begs the question: why do these factors not promote angiogenesis? The endothelium in patients with hypertension at the cellular level (receptors) may be resistant to angiogenic factors and not respond adequately to these factors. Defects in VEGF-related signal cascades have also been reported, and animal models support these findings [115]. Therefore, it seems that antihypertensive therapies normalize the angiogenic markers that are out of regulation and restore the normal ability for angiogenesis [114].

Relationship between angiogenesis and atherosclerosis

For the reason that abnormal deposition in the extracellular matrix interferes with oxygen uptake, atherosclerotic plaques have the potential to cause hypoxia, which can be an important driving force for blood vessels to propagate. As shown in human studies, there is a direct relationship between the development of atherosclerotic lesions and the development of small vascular networks derived from vasa vasorum. This suggests that angiogenesis is the main process involved in atherosclerosis, which can be through the growth of plaque or its augmentation. These newly formed capillaries (vasa vasorum) may be prone to bleeding into the plaque, thereby activating platelets and stimulating the migration and proliferation of vascular smooth muscle cells [116].

One diagnostic approach for direct intervention in atherosclerosis is to stabilize the atheroma plaques (strengthen the fibrous cap, reduce the lipid pool) instead of reducing the size of the lesions or their destruction. Stabilization of these structures should inhibit vascular events such as myocardial infarction and stroke through non-invasive therapies versus invasive strategies (invasive strategies include angioplasty, endarterectomy, and bypass surgery) [117, 118].

Enhancing therapeutic angiogenesis

Most research is focused on curbing angiogenesis (mainly to fight cancer). However, due to the close relationship between this phenomenon and some conditions such as wound healing, it is necessary to pay attention to the factors that lead to increased angiogenesis. For example, the biggest challenge in tissue engineering of three-dimensional structures is the limitations with the transfer of nutrients, consequently, angiogenesis is of great relevance. Regardless of what the target tissue is, there are several limitations in the transport of nutrients and oxygen to the cell and metabolic products outside the tissue. In addition, angiogenesis participates connective tissue with the surrounding environment and affects other appropriate cellular behaviors [119]. The branches of the vascular network are regularly organized in three dimensions to deliver enough nutrients to all tissue cells and to allow heat to distribute. Changes in metabolic activity lead to proportional changes in capillary formation. Oxygen plays a fundamental role in this regulation, and hemodynamic factors are critical to the survival of the capillary network and the structural adaptation of the vascular wall. Angiogenesis is a completely dynamic process; depending on external demand in the tissue, capillaries can grow and develop. Exercise, for example, stimulates angiogenesis in the muscles and heart, while stopping physical activity stimulates capillary relapse. Capillaries also grow in adipose tissue during weight gain and are lost with weight loss. Angiogenesis can be a treatment for ischemic heart disease and peripheral artery disease or wound healing. Reducing and preventing angina is also very effective in treating cancer, eye diseases, and rheumatoid arthritis [120]. Scottish anatomist John Hunter recorded the first scientific view of angiography based on his studies. His observations showed that, in health or disease, there is a correlation between metabolic needs and the rate of angiogenesis [119]. One type of angiogenesis is therapeutic angiogenesis; this process can be simulated in the patient's tissue for therapeutic purposes. For example, during a heart attack, due to plaque formation, the blood supply to the left ventricle is stopped, and the cell would die due to a lack of nutrients. If a method is designed to increase blood flow to these areas, the function of the heart muscle would be restored, and cell death would be prevented. This treatment is known as angiogenesis therapy. This treatment is a sub-branch of regenerative medicine [121]. In this mechanism, new vessels are formed by transferring angiogenic factors (VEGF, FGF) to the desired location. The different methods used to transmit these agents include direct protein transfer and transfer of protein-expressing genes. These agents can be injected into the coronary arteries through a catheter and/or directly into the heart muscle [122].

Endogenous pro-angiogenic factors

Naturally, there are a variety of factors in the body that are essential for angiogenesis. The types of these endogenous factors are listed in the table below (Table 1).

Table 1.

The list of important endogenous pro-angiogenic factors

Full name	Biological effects
Vascular endothelial growth factor (VEGF)	Increasing migration and proliferation of endothelial cells and vascular network formation
Fibroblast growth factor (FGF)	Migration, proliferation and differentiation of endothelial cells, extracellular proteolysis
Hepatocyte growth factor (HGF)	The decreasing of endothelial cell apoptosis, extracellular matrix repair
Angiopoietin 1	Inhibition of endothelial cell apoptosis, vascular network formation and stability
Interleukin 8 (IL-8)	Proliferation and migration of endothelial cells, capillary tube formation
Tumor necrosis factor-alpha (TNF-α)	Increasing the expression of VEGF and its receptors, endothelial differentiation
Angiogenin	Migration and differentiation of endothelial cells, induction of plasminogen activator
Transforming growth factor-alpha (TGF-α)	Proliferation, migration, and differentiation of endothelial cells
Transforming growth factor-beta (TGF-β)	Differentiation of endothelial cells
Heparin	Facilitates binding of FGF, VEGF to cellular receptors increases NO level
Nitric oxide (ON)	Migration and proliferation of endothelial cells and inhibition of apoptosis
Estrogens	Proliferate, migrate, and differentiate endothelial cells
Matrix metalloprotease (MMP)	Degradation of the anterior vascular basement membrane, invasion and migration of endothelial cells
E-selectin	Migration and differentiation of endothelial cells
Leptin	Endothelial cell proliferation increased the expression of MMP
microRNA	As small non-coding RNAs, these factors can negatively regulate gene expression of anti-angiogenic factors at post-transcriptional level [124]

The contents were extracted from Refs. [7, 123]

Apart from the above factors, the role of VEGF factor is more effective than other factors, and according to the focus of the rest on this factor, in this comparison, this angiogenic factor is further interpreted.

Vascular endothelial growth factor (VEGF)

VEGF is a heparin-binding homodimer protein with a molecular weight of 45 kDa, which has the potential for pro-angiogenic activity in vivo and in vitro [125]. VEGF has seven isoforms including A–F and PIGF, which are made as a result of different splicing of the VEGF gene. These isoforms differ from each other in terms of molecular weight and biological properties. Owing to different observations, VEGF-A seems to be the most important isoform. Over-expression of this isoform produces potent angiogenic effects in a variety of tissues. It also increases the permeability and dilation of blood vessels. VEGF-A itself has subtypes 145, 121, 189, 183, 167, 165 and 205, which differ in the number of amino acids. The most common type is 165 subtype, sometimes abbreviated to VEGF. VEGF class is one of the most important factors, which acts as mitogenic stimulation of angiogenesis by their two tyrosine kinase receptors, VEGFR1 and VEGFR2 on endothelial cells [125, 126].

Studies have shown that VEGF-A and VEGF-D have the highest angiogenic potency, while VEGF-C and VEGF-D are effective in lymphogenesis [125, 127]. Hypoxia, hypoglycemia, and some cytokines are the most important factors stimulating the expression of VEGF and its receptors. Whereas cytokines such as IL-13 and IL-10 can hinder the release of VEGF [128]. VEGF is involved in cell migration, proliferation, matrix endothelial cell degradation, vascular network formation, as well as nitric oxide production. In addition, it has an anti-apoptotic effect on endothelial cells through the expression of B-cell lymphoma (BCL2) and A1 anti-apoptotic proteins [104]. In vivo, it also regulates vascular permeability, which is essential for the initiation of angiogenesis; this is why it is called the vascular permeability factor, and in this case, it acts 50,000 times stronger than histamine [129]. The specificity of this factor for endothelial cells has played an effective role in angiogenesis therapy; for this reason, most studies and efforts have progressed towards the therapeutic gene and gene application of this protein. For example, intracardiac myocardial gene therapy using the VEGF gene in patients undergoing coronary artery bypass grafting showed that cardiac function improved in 28% of patients [130]. On the other hand, the unregulated expression of this factor along with the increase of angiogenesis leads to the development of solid tumors and several diseases. For example, in a 2006 study by Zhao et al. on 67 patients with gastric cancer (54 males and 13 females), the results showed an increase of approximately 76% in VEGF expression in gastric tumor tissues compared to healthy adjacent tissues. The possible role of VEGF in tumor progression is through increased angiogenesis [131]. Other studies examining the expression of VEGF in 73 patients with breast cancer showed that in approximately 61% of patients with malignancies and 19% of patients with benign cancers, the expression of VEGF increased significantly [132]. Therefore, the use of antibodies against this angiogenic factor can be considered a therapeutic target for cancers.

The role of VEGF in diabetic angiogenesis

Abnormal VEGF levels have been reported in abnormal angiogenesis in diabetes. Previous reports have revealed that patients with diabetic retinopathy have elevated levels of this factor. Another study showed that the serum level of VEGF-A was significantly (1.5 times) higher in patients with diabetic retinopathy and coronary artery disease than in patients without retinopathy but with coronary artery disease [133]. Hyperglycemia and ischemia have also been shown to activate protein kinase C, which triggers VEGF expression in the retinal pigment epithelium. In animal models, intracranial injection of recombinant human VEGF induces vascular pathological symptoms (such as capillary obstruction, capillary abnormalities such as micro aneurysms, etc.). It is similar to what is seen in people with diabetic retinopathy. In addition, preclinical studies show double expression of the VEGFR2 and VEGFR1 receptors in the retina of insulin-resistant diabetic rats [134]. Other studies have shown an amplifying of VEGF-A and VEGFR2 in the myocardium of diabetics compared with non-diabetics [135]. These opposing results designate a difference in the regulation of the VEGF system in the eye and heart. Angiogenic growth factors are also associated with diabetic nephropathy, which is characterized by an increase in serum creatinine and a decrease in glomerular filtration rate [136]. The number of glomerular capillaries in diabetics is higher than in healthy individuals, which indicates an angiogenic function in diabetic patients. In the kidney, podocytes can secrete VEGF [136] and overexpression of this factor has been observed in all individuals with nephropathy [137]. It has recently been shown that blocking the VEGF signaling pathway leads to a reduction in diabetic albuminuria [136].

Exogenous pro-angiogenic agents

In normal conditions, the body does not need external induction of angiogenesis, however, in some diseases and/or special conditions, such as repairing damaged tissue, the angiogenesis process needs to be induced using exogenous factors. For example, insufficient growth or incomplete expansion of blood vessels is associated with diseases such as stroke, myocardial infarction, and neurodegenerative diseases [138, 139].

Some scientists have used minerals to speed up angiogenesis, while others are using angiogenesis-inducing peptides. In several cases, nucleotides, such as aptamers [140, 141] have been used, and eventually, some researchers have focused on compounds extracted from plants. Each of these strategies has its advantages and disadvantages, and this is why some researchers believe that a combination of several angiogenic inducers can be used to induce effective angiogenesis. In this section, we will move on to introduce exogenous pro-angiogenic factors.

Pro-angiogenic peptides in biomedicine

Some angiogenesis-inducing peptides are new in themselves, but others have mimicked the functional parts of angiogenesis growth factors [142]. The following table (Table 2) will introduce the types of peptides that have been used to induce angiogenesis.

Table 2.

The introduction of several pro-angiogenic peptides

Peptide name	Source	Aim	References
DSC127	Angiotensin (1–7)	Wound healing	[143, 144]
GHK	Cu2+ binding region of SPARC	Wound healing/cosmetic	[145]
AF12198	Phage display	Anti-inflammatory	[146, 147]
Endostatin peptide fragment I (180–199)	Collagen XVIII	Chemotherapeutic	[148]
BA058	Parathyroid hormone receptor	Osteoporosis	[149]
Oritavancin	Semisynthetic lipoglycopeptide	Anti-bacterial	[150]
QK	α-helix region of VEGF	Wound healing	[151]
T7 vasculotide	Tie-2-binding region of Ang1	Diabetic wound healing	[152]
Comb1	Combination of the epidermal growth factor-like domains of fibrillin 1 and tenascin X	Wound healing	[153, 154]
KRX-725	Second intercellular loop of sphingosine 1-phosphate (S1P) 3	Vascularization	[155]
Pep-12	Ig-like domain II of VEGF receptor 1	Angiogenesis	[156]
AcSDKP	The endogenous peptide in bone marrow bone marrow	Cell migration and tube formation during angiogenesis	[157]

Inorganic pro-angiogenic factors

The advantages of using pro-angiogenic minerals in tissue engineering are due to factors such as resistance to various synthetic methods in tissue engineering, temperature, and enzymatic stability. However, in some cases, these minerals can generate free radicals, and their long half-lives may prolong the angiogenesis process more than what is needed. In the use of minerals, the concentration and kinetics of release in the target tissue must be taken with the utmost care, and also how it is excreted from the body must be studied properly. The following table (Table 3) summarizes the types of inorganic pro-angiogenic agents. The interesting thing about minerals is that some of them, like titanium, have dual effects (pro and anti-angiogenic).

Table 3.

The list of pro-angiogenic inorganic materials

Name	Proposed mechanism of action	References
Vanadium	It is believed that there is a strong relationship between vanadate and the expression of HIF-1 and VEGF	[158]
Nickel	Like vanadium, Ni can induce the expression of several angiogenesis-related proteins such as VEGF, phosphoinositide-3 kinase, extracellular signal-regulated kinase and Cap43	[159, 160]
Arsenic	As pro-angiogenic, it was observed that by the use of the metal, activation of the ERK pathway is a consequence	[161]
Titanium	Increasing the expression of VEGF-A	[162]
silica nanoparticles	Increasing NOx-mediated generation of ROS	[163, 164]
Cerium	Pro-angiogenic effect by stabilizing HIF-1α and increasing VEGF expression	[165]
Cobalt	Play a pro-angiogenic role similar to cerium	[166, 167]
Europium	Through increasing the expression of MMP-9, CD31, PDGFRa/b and VEGFR1/2 of human umbilical vein endothelial cells (HUVECs) can promote the angiogenesis	[168, 169]
Sulfur	Pro-angiogenic by an unknown mechanism	[170, 171]
Lithium	Encourage the secretion of VEGF	[172]
Terbium	By the activation of the PI3K/Akt/MAPK signaling cascade and NOx-mediated generation of reactive oxygen species (ROS) can play as a pro-angiogenic factor	[173]
Yttrium	stimulate EGFR and VEGF secretions	[174]
Zinc	ROS generation and up-regulation of bFGF and VEGF	[175, 176]
Iron	Through activation of several proteins such as prolyl hydroxylase domain (PHD), HIF and heme oxygenase 1 can indirectly activate angiogenesis	[177]
Copper	The activation of E1 prostaglandin, ceruloplasmin, glycyl-histidyl-lysine (GHK) and heparin is strongly dependent on copper ion as a pro-angiogenic	[178, 179]
Calcium	Through regulation of FGF2-induced phosphorylation events, calcium can regulate, manly as pro-angiogenic, angiogenesis	[180]
Boron	Boron can induce the expression of pro-angiogenic factors such as VEGF and TGF-b1	[181]
Magnesium	Increases the proliferation and migration of microvascular cells	[182]
Niobium	enhancing VEGF secretion	[183]
Phosphate	Phosphate ions can stimulate migration and tube formation in the HUVEC model	[184]

Plant-derived pro-angiogenic compounds

From time immemorial, when a wound was inflicted, the wound dressing by plants was used, according to local tradition, to heal wounds more quickly. Today, it is proven that some plant derivatives can accelerate the angiogenesis process and, thus, help wound healing. Some of these plant extracts can play a role by increasing the expression of angiogenic inducers such as VEGF, and others accelerate cell division or cell migration, which is vital for the angiogenesis process. Finally, some extracts have been shown to increase angiogenesis by increasing free radicals. Table 4 lists the types of materials extracted from plants with angiogenic properties along with their proposed mechanism.

Table 4.

Pro-angiogenic plants and proposed mechanism of action

Name	Source	The mechanism of action	References
Rg1	Panax ginseng	Increases cell proliferation, migration and tube formation/the expression of endothelial nitric oxide synthase (eNOS)/overexpression of VEGF	[185–187]
Total saponins	Panax notoginseng	Over-expression of VEGF and KDR/Flk-1 mRNA	[188]
Re and Rg2	Several species of Panax	enhancing proliferation, migration and tube formation of HUVECs	[186, 189, 190]
Beta-sitosterol	Aloe vera	Enhances the expression of Von Willebrand factor (vWF), VEGF, VEGF receptor Flk-1, and laminin	[191]
Leaf extract	Hippophae rhamnoides	Up-regulation of VEGF, MMP-2 and MMP-9 expression	[192]
Total extracts	Angelica and ChuanXiong	VEGF expression	[193]
Aqueous extract	Angelica sinensis	Enhancing VEGF expression and stimulating c-Jun N-terminal kinases 1 and 2 (JNK1/2) and p38 phosphorylation	[194]
Total extracts	Radix Astragali	Enhancing VEGF mRNA expression	[195]
Formononetin, ononin, calycosin, and calycosin-7-O-β-d-glucoside	Radix Astragali	Increase the level of HIF-1α	[196]
Ethanol extract	Cinnamomum cassia	Increasing the production of VEGF and up-regulation of fetal liver kinase 1 (Flk)-1/KDR receptor expression	[197]
Ethanol extraction	Geum japonicum	The possible involvement of janus kinase-signal transducer and activator of transcription (JAK–STAT) signaling	[198]

Pro-angiogenic aptamers and mechanisms of action

Similar to what was reviewed to use pro-angiogenic peptides, minerals, and plants, the main purpose of using aptamers is to increase angiogenesis in the healing of wounds, bones, and severe tissue damage. Therefore, in this section, we will refer to the types of aptamers that have accelerated this process in the angiogenic process. The use of these materials has received much attention, so a technique called ‘framework nucleic acid’ has recently been included in the research list of some researchers. This technique is an attempt to make one- to three-dimensional nanostructures of nucleic acid origin. For example, Tetrahedral framework nucleic acid (tFNA) is a three-dimensional structure of four types of single-stranded DNA that, in addition to being highly biocompatible, has been shown to have many sites for different chemical modifications to apply new properties. This structure has been shown to increase angiogenesis in areas of wound healing and/or severe injury [199–201]. In the case of aptamers, the main reports and efforts have been made in the field of suppression of angiogenesis and providing aptamers with anti-angiogenic properties. However, the table below (Table 5) presents the types of aptamers and their characteristics along with the proposed mechanism of action.

Table 5.

Pro-angiogenic aptamers and the proposed mechanism of action

Name	Nucleic acid	Mechanism of action	References
Apt02	DNA	VEGFA-mimicking activity can bind to VEGFR-1 and VEGFR-2	[202]
Apt01	DNA	The action is similar to Apt02 but with lower effectiveness	[202]
AptVEGF	DNA	The aptamers can stabilize the VEGF in a medium by binding to it	[141, 203]
tFNA	DNA	Activation of the Notch signaling pathway	[204, 205]
3R02	DNA	The aptamers can stabilize the VEGF in a medium by binding to it	[141]
Aptamer-functionalized fibrin hydrogels (AFH)	DNA	Binding to VEGF, prolongs the release of the growth factor	[206]

Tissue engineering with angiogenic approach

All that has been reviewed herein about angiogenesis can be a great help to tissue engineering. Tissue engineering is the use of material engineering to design an appropriate scaffold for damaged tissue in a laboratory setting in the hope of repairing severed lost tissue. In recent years, the use of this science to treat tissue and organ lesions and even complete replacement of an organ has received much attention. The use of tissue engineering and the design of smart structures similar to damaged tissues require the use of advanced tools such as scaffold architecture, selecting appropriate cell(s), and bioactive molecules (messenger molecules) in vitro. Success in tissue engineering is due to the best possible imitation of the cells' in-body environment to exchange messages for natural tissue to grow [207]. The biggest challenge in tissue engineering of three-dimensional structures is angiogenesis and consequently mass transfer constraints. Regardless of what the target tissue is, researchers see limitations in the transport of nutrients and oxygen to the cell and metabolic products outside it. In addition, angiogenesis integrates connective tissue with surrounding tissues and affects other appropriate cellular behaviors. If this integration, i.e., unfavorable angiogenesis, is not formed, tissue engineering will not be successful [208]. Over the years, several innovations have been proposed to vein implanted tissues in the body, all based on the control and monitoring of cell implantation in 3D scaffolds in the laboratory. However, even with the benefits of these innovations, it has been proven that the rate of migration of endothelial cells and the physical growth of new vessels is not optimal due to the limited distance of oxygen distribution, so that in these conditions, implanted cells, especially those located at the center of the scaffold struggle to survive [209].

There are a variety of approaches to induce angiogenesis in engineered tissue: strategies such as scaffold functionalization (including engineered tissue with angiogenic factors), cell-based technique (the addition of pro-angiogenic expressing cells to artificial tissues), bioreactors (containing rotating and perfusion bioreactors that are responsible for transferring angiogenic factors and other nutrients to artificial tissue), microfluidic systems (artificial design of vascular networks), modular assembly (an evolved form of cell-based techniques with the difference that a layer of endothelial cells is embedded around a prefabricated polymer, and eventually the system is added to the engineered tissue as artificial vessels), and the latest strategy, in vivo systems. In the latter system, layers of capillary-free cells are gradually placed in the vicinity of blood vessels in the living system, and due to lack of oxygen and the presence of other pro-angiogenic factors, small capillaries will be generated in the cell layer, and then another layer of capillary-free cells will be added to the previous layer. This cycle continues until a set of layers with a capillary system will finally obtain) [210].Vascular growth in engineered tissue is a very complex and dynamic process; the first step in the process is to stimulate local arteries at the desired site using angiogenic factors. In general, the role of the scaffold is to create a temporary three-dimensional structure for adhesion, penetration, and cell proliferation [209]. Scaffolds have interconnected porosity for cell feeding, cell leaching out of the scaffold, extracellular matrix formation, and angiogenesis. Percentage of porosity, suitable pore size, sterilizability, mechanical properties similar to natural tissue, facilitating cell infiltration, stimulation of new tissue formation, and integration with host tissues are important characteristics of the scaffolds [211]. Despite many efforts in this field and regardless of the numerous scaffolds that have been made through various tissue engineering methods, the construction of blood vessels that are completely biomimetic has not yet been achieved, and this requires further research. Different materials, such as polymers, are used to make scaffolding. The choice of a suitable polymer depends on its characteristics such as good adhesion to cells and tensile expansion. The polymeric materials can be natural, synthetic, or a combination of these two types of polymers. In the case of natural polymers, it should be noted that these polymers are obtained from the general source of living organisms, plants, or animals, and all together, it is said that they have desirable properties such as biocompatibility and proper cell interaction. Examples of this category include collagen, elastin, chitosan, gelatin and hyaluronan. In some cases, using a series of buffers and enzymes, decellularized tissue can also be used as a natural polymeric scaffold. These scaffolds contain substances such as elastin and collagen but are free of cells and genetic materials. In the synthesis of synthetic polymers, efforts are made to make the final material biocompatible, non-immunogenic, and ultimately degradable. However, due to the control of the synthesis process, it has been seen that the mechanical properties of these types of polymers are better than those natural polymers. Polyethyl terephthalate, polyglycolic acid, polytetrafluoroethylene, polylactic acid are some of the important examples of these polymers. Degradability is a very important property for this group of polymers because the blood vessels and the tissue being repaired must destroy their substrate during growth and the implanted substrate as a whole should not disturb the overall structure of the repaired tissue. Consequently, the residues from the decomposition of the implanted scaffold should have no immunological disturbing properties—either used by the cells in the metabolic cycle or easily excreted by the body [212–214].

Inhibiting pathologic angiogenesis

Extensive strategies for intervening in angiogenesis have been introduced. Angiogenesis depends primarily on the proper activation, proliferation, binding, migration, and maturation of endothelial cells. To date, the most successful way to inhibit angiogenesis is to use agents that specifically inhibit the growth and proliferation of endothelial cells. In a closer look at the studies that have been done to control angiogenesis, it can be emphasized that the main purpose of such research is to find a way to fight cancer disease. Chemotherapy has been used for many years to treat cancer in medical centers. However, this form of treatment has been successful in treating some tumors, including testicular cancer and some types of leukemia, and has been somewhat effective in treating common epithelial tumors, such as the breast, colon, and lung tumors. Ideally, chemotherapeutic drugs should only target the cancer cells and reduce the size of the tumor by inducing cytotoxic effects with minimal "side effects" to normal cells. The effectiveness of chemotherapy has been reduced by a wide range of problems, including toxicity induced to natural cells due to non-specific targeting, rapid drug metabolism and elimination, and intrinsic and acquired drug resistance [215]. Various anti-angiogenics have been proposed to target endothelial cells and prevent tumor angiogenesis. Targeting these cells that support tumor growth, is a relatively promising line of attack against cancer because endothelial cells are genetically very stable and less likely to mutate. Also, these cells are less expected to develop drug resistance quickly [216]. The advantages of angiogenesis therapy over direct treatment of tumor cells are: (1) As mentioned before, endothelial cells are genetically stable, diploid and homogeneous, and spontaneous mutations are less common in them; (2) Tumor endothelial cells divide 50–100 times faster than normal endothelial cells and have special markers that are not expressed or are rarely expressed in silent endothelial cells; (3) normal angiogenesis occurs in adults-only under certain conditions, such as wound healing or the reproductive cycle in women. Therefore, anti-angiogenic treatment is a very selective option with very few serious side effects; (4) Unlike tumor cells, endothelial cells are easily accessible from the bloodstream; (5) Many tumor cells are dependent on a single capillary, and the destruction of a very small number of capillaries would bring many antitumor effects. Anti-angiogenic drugs are not limited to a specific tumor type, because almost all tumors are angiogenic. In addition, endothelial cells have a genetically stable expression of MHC on their surface that is not seen in tumor cells. Therefore, endothelial cells are much less exposed to drug resistance as a proposed target [217]. It is commonly said that in abnormal conditions, the balance of pro-angiogenic and anti-angiogenic factors is reversed in comparison with normal conditions, so that the strength of pro-angiogenics increases and consequently not only leads to the proliferation of endothelial cells but also the shape and permeability of newly formed vessels and even in some cases the pattern of blood flow in them will be different from normal counterparts. One of the hypotheses that has attracted the attention of researchers in this field is the vascular normalization hypothesis. In this hypothesis, it is more promising to deal with normalizing the balance of pro-angiogenic and anti-angiogenic factors in order to enable cancer therapy to function more efficiently. Under these conditions, assuming the balance of the factors in the tumor masses, the vessels in the tumor will return to normal and the results of cancer treatment will be more predictable [218].

As mentioned, angiogenesis interferes with many pathological conditions and causes many problems. Consequently, depending on the purpose of the research, inhibiting this process can be beneficial. In general, in this study, the types of angiogenic inhibitors are classified into two categories: endogenous and exogenous factors. Figure 6 summarizes the information in this section by listing the types of angiogenic inhibitors, which will be explained in more detail later.

Fig. 6.

Overall view on all available anti-angiogenic factors. Between these groups, plant-derived compounds have been wildly studied and there are different classes of them in literature which will be discussed in the corresponded section

Endogenous anti-angiogenic factors

To date, a large number of endogenous angiogenesis inhibitory factors have been identified, many of which originate naturally from the extracellular matrix, and some are essentially basement membrane proteins. Accordingly, endogenous anti-angiogenic factors are divided into two main classes, including matrix-derived inhibitors and non-matrix-derived inhibitors [219].

From another point of view, more than 40 endogenous anti-angiogenics are generally classified into the following four general categories:

Interferons

Interferons (INF)-α, -β and -ẟ are members of a group of glycoproteins. These compounds were initially considered for their antiviral effects [220]. Endogenous angiogenesis inhibitors were first identified by identifying INF-α with the ability to inhibit the chemotaxis of endothelial cells in vitro [221]. Several studies have shown the anti-angiogenic effects of INF-α in vitro [222]. Also, the ability of interferons to reduce the regulation of FGF mRNA levels in bladder, kidney, breast and prostate cancer cells has been demonstrated [223, 224]. Researchers have confirmed the use of INF-α in the treatment of melanoma, when the lymph nodes are involved, or the disease is in advanced stages, in the adolescent environment. Because it is possible that INF-α also has antiangiogenic activity, its combination with thalidomide and/or anti-VEGF antibody may have synergistic activity [225].

Interleukins

Interleukins are leukocyte-secreting proteins that mediate a range of cell activity, from lymphocyte proliferation and activity to stimulating the release of IgE immunoglobulin from B cells [226]. The inhibitory role of these proteins in tumor growth has also been well established [227]. Interleukins directly inhibit the proliferation of some tumor cells and induce immune responses against the tumor [228]. Inhibition of bFGF-induced angiogenesis in the rat cornea is another anti-angiogenic effect of IL-4 from the interleukin family. Interestingly, interleukins such as IL-8, which at their amino terminus have the Glu–Leu–Arg sequence, potentiate the angiogenesis process, and interleukins without this sequence, such as IL-4, inhibit angiogenesis. This can be an important guide for protein engineering researchers to design new peptide compounds against angiogenesis.

Matrix metalloproteinase inhibitors

The role of matrix metalloproteinase enzymes in the angiogenesis process is very prominent. For the migration of proliferating endothelial cells, the extracellular matrix and basement membrane must be digested. In addition, intercellular connections must be detached. All these are done by the large family of matrix metalloproteases. Inhibition of their secretion and/or activity can lead to tumor control in angiogenesis. Matrix metalloproteinase inhibitors inhibit both active and inactive forms of metalloproteinase. These inhibitors have been shown to inhibit the migration of endothelial cells well into the gelatin substrate [229].

Proteolytic fractions

Many anti-angiogenic compounds are parts of the digestion of larger proteins. Some of these fractions are consequential from extracellular matrix compounds such as collagen and fibronectin, but others are the result of the activity of enzymes such as plasminogen and MMP-2. Angiostatin and endostatin are both in this group.

Angiostatin

Angiostatin is a 38-kDa plasminogen-derived fraction that strongly inhibits the growth of capillary endothelial cells [230]. It has been shown that intraperitoneal administration of angiostatin in mice can inhibit angiogenesis and suppress tumor metastasis [231].

Endostatin

As mentioned, many endogenous angiogenesis inhibitors are hidden components of larger extracellular matrix molecules [232]. Endostatin is an endogenous inhibitor that has been identified and purified from the hemangioendothelioma cell line in Murine mice. This small protein is a 21-kDa fragment derived from the carboxylic end and the NC1 domain of type 18 collagen. Recombinant endostatin effectively inhibited angiogenesis and suppressed early tumor metastasis in studied animal models without any obvious side effects, drug resistance and toxicity [6]. Different enzymes can produce endostatin-containing fragments of type 18 collagen. Among the enzymes, cathepsins B and L and elastase are more efficient [233]. The production of endostatin from collagen is a two-step process: an initial metal-dependent step that results in the formation of larger fragments of endostatin, which is then transformed by the elastase activity into an amino-terminated component called functional endostatin.

In general, all these angiogenesis inhibitors can prevent the development of new blood vessels through the following mechanisms:

(1) Growth factor inhibitors, which include tyrosine kinases receptor inhibitors. (2) Intracellular kinase inhibitors, (3) Cytokines and chemokines, (4) The inhibition of the HIF pathway, which includes (A) inhibition by binding to the target DNA sequence and (B) The inhibition of HIF by blocking the cofactor binding site. And the final inhibitors categorizes as (5) Integrin antagonists [234, 235].

Furthermore, several other angiogenic inhibitors have been approved for use in certain physiological and pathophysiological conditions, including PDGF for wound healing and Visudyne for the treatment of muscular dystrophy. Theoretically, the use of anti-angiogenic agents to treat cancer has several potential benefits. These factors may be compared with drugs that act directly on tumor cells that have easy access to endothelial cells. Anti-angiogenic drugs may not cause many of the unwanted toxicities of standard chemotherapy agents. They may also inhibit tumor resistance mechanisms. If the use of antiangiogenic agents would proceed successfully, they may be useful for many types of tumors [236]. However, there are several major barriers to the use of antiangiogenic drugs in clinical trials. These barriers include: determining the appropriate dose of first phase trials to progress to subsequent phases. Also, the timing of drugs, biological correlations, proper application of these agents in the clinical environment, how to combine these treatments with chemotherapy, radiation therapy, or other biological treatments in the best possible way and finally the need to determine the angiogenesis index are other considerations that should be considered. Therefore, it is very critical to pay attention to them when the researchers are using angiogenic inhibitors.

Exogenous anti-angiogenic agents

Other factors of extracellular and non-somatic origins have also been considered to deal with the pathological process of angiogenesis. To date, various categories of these agents have been discovered and can be classified as anti-angiogenic peptides, inorganic inhibitors, sugars, aptamers, antibodies, and plant origin inhibitors.

Anti-angiogenic peptides

Peptides can be respectable alternatives to chemical drugs today. The reason for the preference of using peptides over industrial-synthetic drugs is alternatives, such as ease of excretion from the body, their high specificity to their receptors, and their less toxicity in the body. It has been estimated that peptides are 2–3 times more likely to succeed in clinical trials than synthesized chemical drugs [237, 238]. Also, the optimization of peptide synthesis methods, such as the development of the solid phase of peptide synthesis, has caused the number of pharmaceutical peptides to increase every day. In this huge group of drug peptides, one of them is peptides with angiogenic inhibitory properties. In addition to the above, other features such as ease of tissue uptake due to their small size, ability to modify peptides to improve their activity or stability, as well as their use in combination therapies have led to special attention to therapeutic peptides [239]. The origin of angiogenic inhibitory peptides can be endogenous or exogenous. It should be noted, however, that most endogenous peptides, due to their large size, will have little ability to penetrate target tissues. Therefore, efforts are being made to engineer these peptides on a smaller scale to maintain their activity and increase their permeability in the tissues in question. Since, in most available sources, peptides are referred to as small amino acid strands limited to less than 50 subunits, this study will also refer to peptides that are less than this number of amino acids. The multiplicity of anti-angiogenic peptides is very high, but according to their origin, which is all part of known proteins, they can be classified into several groups. The schematic diagram below (Fig. 7) refers to different types of anti-angiogenic peptides [240].

Fig. 7.

Different proteinous sources of anti-angiogenic peptides

Since almost all anti-angiogenic peptides are derived from a protein source, looking to the considerations to select the protein interest would be helpful. Meanwhile, the angiogenic phenomenon depends on the co-operation of several factors; it can provide researchers with many opportunities to choose a path to control the whole process. For example, it has been shown that some ions can accelerate the process of angiogenesis. As a new peptide, it is possible to study domains of proteins to design peptides that can trap angiogenesis-accelerating ions. Alternatively, by sequencing most proteins and studying the structural similarities between existing proteins and proteins that play an important role in angiogenesis, such as VEGF, peptides can be obtained from proteins that have a high percentage of similarity to angiogenic activators to choose them and eventually used them as a complementary therapy as a competitor for antagonistic uses.

But not everything is positive about peptides. The use of peptides to inhibit angiogenesis also has limitations. For example, peptides have a short half-life in the body due to degradation by proteases. Additionally, due to their short chain length, they also have short thermodynamic instability. Besides, the use of certain peptides has been shown to elicit immune responses. For this reason, some researchers are trying to reduce the dark spots of peptides by chemical changes and the addition of groups to peptides. The table below (Table 6) lists the types of important changes in peptides, along with an example and purpose of the study [240].

Table 6.

The list of different anti-angiogenic peptides

Modification	Example (peptide name/sequence)	Aim	References
Cyclization	Tumstatin fragment/YSNSG	Increasing the stability (half-life)	[241]
Conjugation	Targeting RGD/cRGD-HL	Evaluating inhibition effects	[242]
Acetylation	ATN-161/Ac-PHSCN-NH2	Increasing the stability (half-life), prolonged the circulation time in blood	[243]
Introducing Non-natural amino acid	Cilengitide/c-[RGD-DFNMEV]	Target selecting (Cilengitide designed to target αv integrins)	[244]
Mutation	C16Y/DFKLFAVTIKYR	More potent in promoting endothelial cell	[245]
Retro-inversion	VEGF derived peptide/D(LPR)	Increasing activity (inhibits neovascularization)	[246]
Multimerization	HPRG derived/(HHPHG)4	Increasing activity	[247, 248]
Capping	Å6/KPSSPPE	Increasing activity	[249, 250]
End modifications	ABT-510/ Ac-Sar-GV-DalloI-TNorV-IRP-NHEt	Increasing the stability (half-life)	[251]

Aptamers: a new class of anti-angiogenic ligands

Aptamers are single-stranded oligonucleotide ligands (including DNA and RNA) that are between 30 and 70 nucleotides in length. These sequences can twist and can bind specifically to different targets [252]. Target binding occurs through hydrogen bonds, electrostatic reactions, weak van der Waals forces, and/or a combination of these forces [253]. Aptamers can bind to specific ligands such as proteins, peptides, enzymes, cell surface receptors, microorganisms, etc. with very high selectivity [254]. One of the most important advantages of aptamers over antigen/antibody can be the ease of modification and stabilization methods [255], the specificity of their binding, stability to above temperatures, ultra-high stability to pH changes and different salt concentrations, very small size, very easy synthesis, and the possibility of long-term storage. These salient features of aptamers have made them useful in various therapeutic, diagnostic, and biosensors fields [256]. Also, unlike monoclonal antibodies, DNA aptamers do not stimulate the immune response. This feature makes them more effective in treating diseases and their long-term medical applications [257].

VEGF as the most important angiogenic activator has been studied by many researchers to design aptamers. Table 7 deals with past studies in this field.

Table 7.

The introduction of different anti-angiogenic aptamers

Aptamer	Nature	Protein target	Modification	FDA approve	Study	References
NX1838	RNA	VEGF-165	PEGylated	Yes	Neovascular age-related macular degeneration	[258, 259]
Anti-PDGF-B	DNA	PDGF-B			KAT-4 cell line in human anaplastic thyroid carcinoma	[260]
EYE001	DNA	VEGF	Poly (lactic-coglycolic) acid microspheres	No	Human umbilical vein endothelial cells	[258]
E10030		PDGF-B	PEGylated	Yes	Vascular Age-Related Macular Degeneration	[261]
RBM-007	RNA	FGF2	No	No	Vascular age-related macular degeneration	[262]
AX102	DNA	PDGF	No	No	Ovarian cancer	[263]
Apt-3 ssDNA	DNA	C-repeat-binding factor (CBF) 1	No	No	Human umbilical vein endothelial cells	[264]
ND	RNA	bFGF	No	No	ND	[265]
ND	RNA	VEGF-165	2′-Fluoropyrimidine/pegylated	No	ND	[266]
ND	RNA	Angiopoietin-2	3′ Deoxythymidine cap	No	Rat Corneal Pocket Assay	[267]
Anti-VEGFR2	DNA	VEGFR2	The aptamer incorporated into a tHA-PEGDA hydrogel	No	Human umbilical vein endothelial cells	[268]

Antibodies

Antibodies are found in many body fluids, including tears, respiratory secretions, salivary glands, intestinal secretions, and urine. However, they are most concentrated in the blood serum, which is why serum antibodies are used to perform various tests. Antibodies, like other proteins, can be tested for properties such as solubility in concentrated saline solutions, electrical charge, molecular weight, and antigenicity. Antibodies are a class of proteins with a large structure called immunoglobulins. They are secreted by B-lymphocytes. These immunoglobulins are divided into nine distinct chemical classes. These include four classes of IgG, two classes of IgA, and one class in each of IgM, IgE, and IgD. Conventional antibodies consist of two similar heavy chains and two similar light chains that are joined together by disulfide bonds. In the most abundant type of circulating antibody, immunoglobulin G, the hinge region of the antibody is sensitive to proteolytic attack due to its high proline content. In some species of organisms, there are only heavy chain antibodies. For example, the serum of members of the species Camelidae contains heavy chain antibodies (HcAbs). HcAbs have a unique structure, including a variable region of VHH (or nanobody), a hinge region, and two fixed regions (CH2 and CH3). So far, many antibodies have been used to inhibit angiogenesis. For this purpose, there are many target proteins that, like what was reviewed about aptamers, the selected antibody must be able to inhibit the target protein in such a way that the angiogenic process is affected and consequently disrupted. Although different target proteins have been used to inhibit angiogenesis, VEGF is one of the most important. Due to the important role of VEGF in the proliferation of vascular endothelial cells, anti-VEGF antibodies (such as Bevacizumab) are one of the best candidates to stop angiogenesis in diseases [269, 270]. Several mechanisms have been suggested for VEGF inhibitors in cancer patients. The anti-VEGF therapeutic effects occur because VEGF receptors are expressed not only on the surface of tumor endothelial cells, but also on the surface of hematopoietic and stromal cells, as well as malignant cells in some cancers (brain, colorectal, and pancreas). VEGF antibodies are also designed to bind to VEGF receptors at the cell surface (such as Sorafenib and Subitinib). The binding of the inhibitors prevents the binding of VEGF to its receptor and, thus, stops the series of reactions leading to angiogenesis. Eventually, the overgrown tumor vessels return to normal. For example, Bevacizumab, a VEGF inhibitor, reduces tumor vascular density by up to 50%, thereby reducing blood flow to the tumor as well as the number of circulating endothelial cells. Reduction of tumor vessels leads to apoptosis of cancer cells [269]. As mentioned at the beginning of this review, different types of VEGFs have receptors called VEGFR1 (flt1), VEGFR2 (KDR), and VEGFR3. VEGFR1 and VEGFR2 have seven immunoglobulin-like domains in their extracellular section, a transmembrane domain, and an intracellular tyrosine kinase domain [271]. These receptors lead to molecular changes in the cell through specific pathways and are very important. Numerous antibodies have also been made against these two types of receptors to treat cancer.

In 1992, Ferrara et al. used VEGF-165 as an immunogen and produced four types of mouse anti-VEGF monoclonal antibodies: A4.6.1, B2.6.2, A3.12.1 and B4.3.1. These antibodies were IgG1 and had a high affinity for human VEGF. A3.12.1 and B2.6.2 antibodies identified the same type of epitope, and A4.6.1 and B4.3.1 antibodies identified different epitopes. Of all the antibodies, A4.6.1 had a high affinity for binding to all types of VEGF isoforms. It also inhibited VEGF activity more effectively. This antibody binds tightly to VEGF and, thus, inhibits the activity of the receptor [125]. Because mouse antibodies are not used to treat humans, the A4.6.1 antibody was humanized in later stages of research. The antibody made was called Bevacizumab, which is now a familiar name [272]. Bevacizumab was the first anti-angiogenic drug to be approved by the US Food and Drug Administration in 2000 and entered treatment. It is now approved along with chemotherapy for colorectal cancer and lung cancer and is the most widely used anti-VEGF drug to date. The other two antibodies (Sorafenib and Sunitinib) inhibitors of tyrosine kinases are involved in angiogenesis, which in addition to inhibiting multiple receptors and kinases in cancer cells, target VEGF receptors in endothelial cells. These two antibodies are used in metastatic carcinoma of the kidney cells, and Sunitinib is also used to treat gastrointestinal tumors.

In addition to the current anti-VEGF drugs such as Vatalanib, Vandetanib, and Cediranib, which are in the final stages of commercial production, it appears that anti-VEGF drugs will soon be used for various types of tumors.

Another type of antibody that has been used to suppress the activity of angiogenic activators is nanobodies. Weighing in at 15 kDa, nanobodies are the smallest antigen-binding fragment in nature. They have a diameter of 2.5 nm and a length of 4 nm. Nanobodies have many advantages and have many applications in medicine and biotechnology. They are well expressed in microorganisms and have high stability and solubility. They are also suitable for making larger molecules and selective systems such as phage, yeast, or ribosomal display. Nanobodies have other biophysical advantages in addition to the common properties of antibodies, such as their high affinity and selectivity for the target molecule and low toxicity. They are small in size (only one-tenth of a typical antibody), consequently they can enter tissues more effectively and detect hidden or abnormal epitopes. Second, these nanobodies do not have spontaneous dimerization compared to some antibodies such as single-chain fragment variable (scFv), which is often dimerized as scFv2. Also, the single-stranded nanobodies make them the best choice for the production of dual-property antibodies. Nanobodies are naturally soluble in aqueous media and do not tend to form aggregate. This is probably due to the replacement of hydrophobic amino acids with hydrophilic ones. Nanobodies are also very resistant to high temperatures so that after a week of incubation at 37 °C, it has 90% ability to bind to their receptor. This indicates that nanobodies have a long half-life. The melting point of nanobodies is between 67 and 79 °C. In addition to high temperatures stability, they are stable to the effects of chaotropic salts, proteases and extreme pHs. Therefore, nanobodies maintain their stability in harsh conditions, such as the stomach. Therefore, they can be used as an oral drug to treat gastrointestinal diseases [273]. To refer to the types of nanobodies in the field of angiogenesis inhibition, it is useful to read the review article [274].

Inorganic anti-angiogenic agents

Many efforts in the field of angiogenesis and inhibition of angiogenesis have relied mainly on the use of the factors mentioned so far. However, the role of some other factors remains hidden. In recent years, some scientists have pointed to the important role of mineral factors in angiogenesis. Some metals, such as vanadium, titanium, nickel, and cadmium, have been shown to affect angiogenesis through an unknown mechanism, while others have been bonded to important proteins in the angiogenesis process. In general, Table 8 refers to the types of inorganic anti-angiogenic candidates.

Table 8.

Different anti-angiogenic with the source of inorganic

Name	Proposed mechanism of action	References
Cadmium	Cd has a strong binding affinity to proteins such as metallothionein and, low affinity, other as albumin. Through binding to vascular endothelium (VE), the metal can disrupt cadherin-dependent cell–cell junctions, a critical process during angiogenesis	[275, 276]
Vanadium	Vanadium inhibites the proliferation and migration of the endothelial cells (HUVECs and EA.hy926) and disrupted the blood vasculature in a chick embryo model, indicating their anti-angiogenic properties	[277]
Arsenic	Via inhibition of endothelial cell migration, proliferation and tube formation arsenic can inhibit the angiogenesis	[278]
selenium-derived compounds	It is considered that the action of these compounds is the opposite of the arsenic, inhibition of the ERK pathway	[159]
Titanium	Phosphorylation-mediated inactivation of VEGFR-2 has been suggested	[279]
Gold, silver, and silica nanoparticles	The chemicals inhibit vascular endothelium growth factor (VEGF)-induced proliferation of vascular endothelial cells. Also, the inactivation of VEGFR-2 was confirmed by the gold and silica nanoparticles	[280, 281]
Cerium	It has been shown that cerium can decreased ROS level which correlate with decreasing VEGF expression	[282]
Zinc	The anti-angiogenic activity of endostatin strongly depends on Zn ions, so the mutation on Zn-binding domain of the protein drastically affects the action of the endostatin. Also, there are several Zn-binding transcription factors, such as serum amyloid A activating factor-1, which govern the angiogenesis process	[176]
Iron	Increased level of irons in endothelia cells inhibits VEGF-induced vascular endothelial cell proliferation, migration, tube formation and sprouting	[283] [177]
Calcium	Calcium inhibites VEGF-induced effects on vascular endothelial cell proliferation	[284]
Magnesium	High concentrations of magnesium modulate vascular endothelial cell behavior	[285]

The use of these inorganics can provide a promising path for researchers in the field of tissue engineering and biomaterials.

Plant-derived anti-angiogenic molecules

From ancient times plants have been used for the prevention or treatment of many diseases. Although no attempt was made to separate pure compounds from plants around 1800 AD, which paved the way for modern pharmacy, many compounds of medicinal plants were discovered and isolated during and after that decade [286]. Alternatively, in recent decades, following the identification of the process of angiogenesis and the indisputable role of this phenomenon in the occurrence and spread of various chronic diseases, including cancer, researchers have tried to find various compounds with inhibitory effects on angiogenesis, including compounds derived from natural resources. Due to the consumption of natural plant resources in different communities and due to their anti-angiogenic effect, their preventive and therapeutic role in the incidence of these diseases is very important. Finally, the study of plants to identify and discover their anti-angiogenic compounds in the manufacture of drugs is of great importance due to their lesser side effects.

Many research models are currently used to screen plants that have anti-angiogenic activity [287]. The outcomes of studies show that foods of plant origin can prevent about one-third of cancers [288]. Therefore, consuming a plant-based diet can prevent the spread and progression of chronic diseases such as malignant tumors, which are associated with angiogenesis [289]. Evidence has also shown that anti-angiogenic agents alone have limited effectiveness. Beneficial natural products comprise a range of multipart organic chemicals that may have a synergistic action. They may interact with multiple angiogenic pathways or other pathways such as cell messaging and the apoptotic pathways, affecting the response of cancer cells to the immune system and, thus, inhibiting angiogenesis [290]. Some anti-angiogenic agents also have anticoagulant activity, which may reduce tumor metastasis [291]. In addition, other anti-angiogenic compounds have been known and extracted over the years through extensive research, such as the antitumor agent camptothecin from Camptotheca acuminate in 1966, identification and isolation of taxol from Taxus baccata in 1971 and Combretastatin from Combretum caffrum in 1987, and many other studies in the field of identifying the angiogenic properties of various plants and their effective compounds. Plants are multifarious chemical cocktails with medicinal properties that modern pharmacies cannot produce. Some of the plant-derived anti-cancer drugs that also have anti-angiogenic properties include [292].

Taxol

In 1962, some scientists recognized an extract from the bark of the Pacific yew tree, scientifically known as Taxus brevifolia, which had anti-cancer properties. In 1971, the active ingredient in this cocktail was recognized by Wani and colleagues as Taxol, which is a multipart polyoxygenated diterpene. Later, a synthetic compound of taxol was made and used. This compound kills dividing cancer cells by breaking up their microtubule cytoskeleton. Scientists have revealed that taxol inhibits angiogenesis at low concentrations (Pico-molar) by inhibiting VEGF production and inhibiting the protein expression of HIF-1. Due to the anti-angiogenic properties shown by Taxol, its anti-tumor properties are further enhanced. Now, years have passed since the clinical use of taxol for the approved treatment of cancer and is still used today [293–295].

Camptothecin

Using bioactivity-directed fractionation technique, camptothecin was isolated from the Chinese tree Camptotheca acuminate. The use of this compound as an anti-cancer agent has been studied for almost 15 years, pending its unique mode of action for killing tumor cells was identified. This compound traps the topoisomerase enzyme in DNA-complexing complexes, thus preventing DNA replication and resulting in cancer cell death. The discovery sparked a resurgence of researchers' desire to produce water-soluble, camptothecin-like compounds that also retain their anti-cancer activity. Finally, in the mid-1990s, two similar compounds, topotecan and irinotecan, were permitted by the Food and Drug Administration for use to fight ovarian, lung, and colon cancers [296, 297]. In a study, it was shown that camptothecin and topotecan inhibited the growth of human endothelial cells in vitro without toxic effects, and this inhibition continued for up to 96 h. The researchers also showed that these compounds, contrasting the non-specific toxic agent cisplatin and TNP-470, are effective in inhibiting angiogenesis in the in vivo model of the angiogenic disc. For this reason, in addition to activities such as killing tumors, camptothecin may also have an indirect anti-tumor effect in vivo by inhibiting angiogenesis [298].

Combretastatin

Combretastatin is an antimicrobial agent that originates from the bark of the African plant Combretum caffrum, which was recognized by Pettit et al. in 1987. Further research led to the production of water-soluble phosphate derivatives of these compounds with greater bioavailability. Vincent et al. also showed that combretastatin A4 (CA4P is also known as fosbretabulin tromethamine) selectively targets endothelial cells and vanishes new tumor vessels emerging in mice by disrupting the action of endothelial cell adhesion molecules. The chemical also increases the permeability of the endothelial cell and prevents migration and capillary tube formation mainly by disrupting the signaling pathway of beta-catenin and protein kinase B (PKB). The compound has a synergistic effect with a monoclonal antibody against the most potent vascular endothelial. When the antibody is administered in low doses, it stops the accumulation of new blood vessels and therefore hinders tumor growth. These findings indicate that CA4P selectively destroys new unstable vascular tumors [299–301].

Today, the important role of foods of plant origin in the prevention of various diseases, including cancer, which is associated with angiogenesis, is well established. However, more studies are needed to screen and identify more plants that have anti-angiogenic therapeutic properties.

Soybeans and their effective compounds

Soybeans have been a staple food in some cultures for hundreds of years. This plant is rich in protein, unsaturated fats, fiber and minerals with many uses. What is more important than the nutritional value of these seeds is that regular use of soy and its products reduces the risk of some dangerous and deadly diseases, such as cardiovascular disease, cancer, and osteoporosis. The beneficial properties of soy seem to be related to a phytochemical called flavonoids that are present in almost all soy products.

The prevalence of breast and prostate cancers in some countries such as China and Japan is much lower than in western countries and the US. Results show that this significant difference in the incidence of cancer among different racial groups depends on some factors in their eating habits. In truth, one of the main differences in the nutrition of these populations is that the Chinese and Japanese use foods in their diet that typically contain plant products such as soy. Approving this, Fotsis et al. examined the urine of those who had consumed large amounts of plant foods in their diets (to identify anti-mitogenic and anti-angiogenic compounds) and found that their urine contained specific isoflavones of soybean genes and metabolites that can inhibit the proliferation of cerebral myocardial endothelial cells stimulated by bFGF. Other observations have shown that pure genistein, a class of isoflavones, has a robust dose-dependent inhibitory effect on endothelial cell proliferation. Genistein also prevents the proliferation of other vascular endothelial cells, such as cells derived from the bovine adrenal cortex and aorta [302–304]. Researchers have shown that Genistein inhibits VEGF-induced endothelial cell proliferation and the activation of protein kinase C (PKC) and tyrosine phosphorylation signaling molecules in the micromolar range. In addition, Genistein effectively inhibits mast cell chemotaxis (as a facilitator of new vascular formation) in response to certain angiogenic stimuli such as VEGF, PDGF, and bFGF. This compound may also prevent angiogenesis through a variety of means, such as modulating the activity of proteolytic enzymes. Studies have also shown that proteolytic digestion of the extracellular matrix by endothelial cells is governed by angiogenic factors such as bFGF, which increases plasminogen activator (PA)I-1 and urokinase plasminogen activator (uPA) production. Genistein also causes a sharp decrease in PAI-1 and PA in vitro [305, 306]. In addition, Genistein suppresses the expression of VEGF and FGF2 and inhibits tyrosine kinase receptor phosphorylation and Akt activation. In addition, the compound inhibits NF-κB activation, leading to apoptosis even in apoptotic-resistant cancer cells. Genistein, as a phytoestrogen, also targets the androgen and estrogen signaling paths in carcinogenesis and has antioxidant properties. These observations and epidemiological and experimental observations have shown that consuming a diet rich in soy products is associated with a reduced risk of cancer. Flavonoids such as 4, 3 dihydroxy flavones, 1, 3 dihydroxy flavones, lutelin, apigenin, and fisetin can inhibit angiogenesis in vitro in the micromolar concentration range. Also, bFGF-induced endothelial cell proliferation and bFGF and VEGF-induced vasculogenesis are inhibited by flavonoids in the 3D model of collagen gel [306]. In general, in vivo and in vitro studies have shown that this isoflavone is a promising factor in the control or treatment of cancer.

Green tea

In 1999, some researchers described that green tea, specifically one of its constituents, epigallocatechin gallate (EGCG), specifically inhibited angiogenesis. Further research has shown that EGCG suppresses cytokine-induced IL-8 production and inhibits Akt activation (activated by VEGF) as well as phosphorylation of vascular endothelial cadherin at physiological concentrations. The verdicts also show that green tea can be useful in preventing and treating diseases related to angiogenesis such as diabetic retinopathy and cancer. However, appropriate pharmacokinetic studies on humans should be performed to confirm this [307–310].

Red grape and its resveratrol

Grapes are rich in a substance called resveratrol. However, it is also found in ground almonds, berries, and medicinal plants such as Polygonum. Research has shown that resveratrol prevents angiogenesis when given orally, without causing significant side effects. The compound, as one of the most encouraging chemical inhibitors of cancer, has attracted increasing attention in recent years. Fukuda et al. described that resveratrol improves myocardial injury by prompting angiogenesis by VEGF and the Flk-1 tyrosine kinase receptor [311–313].

Previously, the inhibitory role of resveratrol in the oxidation of low-density lipoprotein and, thus, the anti-deposing effect of the compound on the vessel wall were reported [314, 315]. It was also shown that resveratrol has a significant effect on the prevention of low-density lipoprotein deposition in the aortic arch endothelium and its improvement process [316, 317]. The results of a study by Jean-Louis 2016 reported that taking resveratrol at a dose of 400 mg daily can play a very strong role in cardiovascular protection and reduce cardiovascular risk [318]. In recent years, many studies have reported the potential effects of resveratrol supplementation in animal models, though in human models, limited studies have been performed, especially in cardiovascular patients.

Curcumin

Curcumin has almost all the beneficial properties that a compound has. Regarding its special role in the field of cancer and angiogenesis, it can be said that it can interact with cancer cells at different levels and increase the tumor-killing effects of radiation and chemical drugs. Its anti-metastatic effects, to some extent, reduce the expression of the enzyme metalloproteinase matrix MMP-2 and increase the tissue inhibitory expression of a TIMP1 metalloproteinase inhibitor. It should be noted that these enzymes play an important role in regulating angiogenesis and invasion of tumor cells. Research has also revealed that this compound hinders the transcription of VEGF and bFGF angiogenic factors and, in addition, reduces the production of nitric oxide in endothelial cells, which has an important role in angiogenesis and growth. Other activities of this compound such as binding to CD13 antibody expressed by vascular cells and inhibiting its activity, reducing the expression of MMP-9, VEGF genes and inhibiting VEGF and EGF receptors, as well as inhibiting intestinal inflammation [319–322].

Nowadays, because increasing the resistance of cancers to conventional therapies has become a problem, researchers’ efforts to discover and identify new anti-cancer mediators that upturn the sensitivity of cancer cells are expanding. Resistance of cancer cells to chemical drugs reduces the level of response of these cells to the drug and, thus, the failure of treatment. Therefore, research and development of more effective drugs with fewer side effects is of great importance. At present, many chemical drugs are the result of products of natural origin, including plants or derivatives. Some of the cytotoxic agents of chemotherapy that are used in doses below normal will inhibit angiogenesis and have minimal toxicity. This strategy may enable patients with advanced cancer to live longer and have a better lifestyle. The metronometric model of old-style chemotherapy displays that it would be also possible that the administration of herbal compounds that interact with the multiple processes of angiogenesis will enhance the positive effects of conventional chemotherapy. In other words, targeting vascular endothelial using low-dose, non-toxic therapeutic agents may control tumor spread without leading to additional toxicity. The potential role of such therapies in increasing patient survival and improving their quality of life needs to be investigated in future clinical trials. Therefore, clinicians are also attracted to compounds that, when used in low doses, specifically respond to and counteract the angiogenesis process. These agents may be somewhat less toxic at low doses and are more likely to lead to better therapeutic results. Increasing research on plants for the treatment of various cancers due to their therapeutic and long-term effects, including anti-cancer properties, using various in vivo and in vitro studies has been significantly growing and new research is needed. It is hoped that research on new plants with anti-cancer properties in the future will lead to the identification and discovery of new anti-cancer drugs of plant origin.

FDA-approved anti-angiogenic drugs

The volume of studies on the control of angiogenesis, which mainly leads to overcoming the challenges of different cancers, is increasing every day. Many treatment candidates have been suggested, some of them are in the clinical trials but others have long been used in the field of treatment. Table 9 presents a list of US Food and Drug Administration (FDA)-approved drugs. As can be seen in this table, among all drug factors, protein drugs (monoclonal antibodies) have been more successful than others.

Table 9.

FDA-approved anti-angiogenic drugs [234, 235, 323, 324]

Anti-angiogenic agent	Target molecules	Use in diseases
Bevacizumab	Binds to VEGF-A and stops it from binding to its receptor	Solid tumors such as, lung cancer, colorectal cancer, glioblastoma, ovarian cancer, cervical cancer and breast cancer
Ziv-aflibercept (Aflibercept)	Inhibits VEGF-A, VEGF-B and PlGF by direct interaction to them	Metastatic colorectal cancer
Axitinib	Tyrosine kinases receptor inhibitor	Renal cell carcinoma
Nintedanib	Tyrosine kinases receptor inhibitor	Idiopathic pulmonary fibrosis (IPF)
Regorafenib	Tyrosine kinases receptor inhibitor	Gastrointestinal stromal tumor, colorectal cancer and hepatocellular carcinoma
Pazobanib	Tyrosine kinases receptor inhibitor	Soft tissue sarcoma
Cabozantinib	Tyrosine kinases receptor inhibitor	Thyroid cancer
Vandetanib	Tyrosine kinases receptor inhibitor	Thyroid cancer
Thalidomide	Inhibits Akt phosphorylation	Myeloma
Imatinib	Tyrosine kinases receptor inhibitor	Chronic myelogenous leukemia (CML) and acute lymphocytic leukemia (ALL)
Erlotinib	Tyrosine kinases receptor inhibitor	Different cancers such as pancreatic cancer and non-small cell lung cancer
Semaxanib	Tyrosine kinases receptor inhibitor	Colorectal cancer
Sorafenib	Tyrosine kinases receptor inhibitor	Primary kidney cancer
Sunitinib	Multi-targeted receptor tyrosine kinase inhibitor	Gastrointestinal stromal tumor and primary kidney cancer
Perifosine	Intracellular kinase inhibitors; inhibits Akt	Colorectal cancer and multiple myeloma
Rapamycin	Intracellular kinase inhibitors; inhibits mTOR	Perivascular epithelioid cell tumor
Vitaxin formerly as Etaracizumab	Integrin antagonists	It has little effect on advanced several cancers

In diseases such as heart problems and sudden strokes, numerous angiogenic studies at the clinical level have been performed to develop new vessels [325]. On the other hand, other diseases such as various cancers (especially colon cancer), and similar clinical studies have been performed to suppress angiogenesis [326, 327]. A variety of drug-mediated pathways have been suggested to combat the process of angiogenesis (in cancer) or its progression (in strokes), which generally fall into three categories. (1) Drugs that act directly on endothelial cells and lead to the proliferation and/or death of these cells. (2) Drugs that indirectly affect the process of angiogenesis, which mainly affect the expression of intracellular processes through the microenvironment. (3) Drugs that affect the process of angiogenesis simultaneously from paths 1 and 2 [328]. Previous studies have suggested that the use of anti-angiogenics along with other anti-cancer strategies has a much better response [329]. It has even been suggested that the use of these agents alone in the later stages of the disease will be almost hopeless [112, 329]. For example, anti-VEGF monoclonal antibody bevacizumab drug (rhuMAB VEGF; Avastin; Genentech Inc., South San Francisco, CA) has been widely used in combination with other drugs in previous studies [327]. In one study, this antibody was used in combination with chemotherapy to fight metastatic colorectal cancer. Together, two phase III studies and the pooled study examined the effect of this antibody [330, 331]. The use of this drug in IFL (irinotecan/5-fluoruracil/leucovorin) treatment regimen showed a 10% increase in response to treatment (44.8% in combination mode compared to 34.8% in chemotherapy alone) and was able to increase the overall survival 15.6–20.3 months [330]. The pooled study, which focused on therapeutic responses from 5-fluoruracil/leucovorin/bevacizumab and 5-fluoruracil/leucovorin, also promised a significant increase in response to treatment (34.1% in the combined case compared with 24.5% in the chemotherapy case alone) [331]. Although, looking at previous studies, one can realize that bevacizumab antibody has been the focus of more trial studies, but efforts have been made on other anti-angiogenic agents, as well. For example, BAY 43-9006 (BAY) as oral multi-kinase suppressor agent with inhibiting effect on protein intermediates such as c-raf and b-raf has been studied in different attempts [332, 333]. In a study on 202 patients with advanced renal cell carcinoma (RCC), BAY (400 mg) was given for 12 weeks. Compared with the placebo group, progression-free survival increased for 17 weeks [332]. In another objective study performed on 769 patients with the advanced RCC, progression-free survival was reported twice as often in placebo patients as in placebo [333].

Challenges and future perspectives

Angiogenesis is an important process in physiological conditions, such as ovulation and wound healing, and pathophysiological conditions, such as tumor growth, diabetes, endometriosis and ischemic heart disease [334]. Many attempts have been made to identify the mechanisms and factors involved in this process. A major challenge is to elucidate the relationship between the pathophysiology of various diseases and the imbalance between peripheral and anti-angiogenic factors, the identification of which can pave the way for the development of these pathogenic pathways [335]. Nowadays, due to the increasing resistance of cancers to common treatments, researchers are trying to discover and identify new anti-cancer agents. Resistance of cancer cells to chemical drugs reduces the level of response of these cells to the drug and, thus, the failure of treatment [336]. Considering the importance of angiogenesis in several types of cancers attempts to discover more novel anti-angiogenic factors would be a promising path for cancer treatment. Accordingly, the development and use of different angiogenesis models for this purpose are becoming more important, so many researchers around the world use different angiogenesis models to study this important phenomenon and the factors affecting it. In this regard, researchers using angiogenesis models have succeeded in studying, identifying and investigating a variety of angiogenesis inhibiting compounds [337].

As our understanding of homeostasis, cell differentiation, and tissue organization increases, in vitro models also provide a defined and appropriate environment for cancer research compared to the complex environment of an in vivo model. Due to the huge potential of three-dimensional tumor cultures, today these models have been used by many branches of biology and medicine. Angiogenesis can also be studied ex vivo by culturing aortic ring grafts in biological gels [338]. Ex vivo models fill the gap between in vivo and in vitro models and include the advantages of both systems so that the angiogenic and anti-angiogenic effects of various soluble factors or matrix factors can be easily assessed using this model. In comparison, in vivo models are more specific in identifying anti-angiogenic activity and provide a more complete evaluation of the angiogenesis process, although they usually require more time and money. Today, the most important angiogenic challenges are included in the two major areas of cancer and tissue engineering. When designing artificial tissue to create a suitable environment for the growth of damaged cells, one of the most important goals is to distribute nutrients and oxygen to the cells that are dividing and regenerating the tissue. To achieve this important goal, a regular and highly predictable system of blood vessels must be formed in the artificial tissue. Today, efforts are focused on how the vascular system should be such that it guarantees cell growth on the one hand and does not lead to cancer on the other. Considerations such as the selection of growth factor, its rate of release in the artificial tissue environment, the half-life of growth factor in the body, and how it is excreted in tissue should be considered. It was also reviewed how stiffness and considering the structure, and even the shape of artificial tissue and the environment around endothelial cells can affect angiogenesis.

Author contributions

All authors have contributed equally.

Availability of data and materials

All data generated or analyzed during this study are included in this published article.

Declarations

Conflict of interest

There are no conflicts to declare.

Ethics approval and consent to participate

Not applicable.

Consent for publication

The authors give their consents for the publication of contents of this paper to be published in the Cellular and Molecular Life Sciences.