Introduction of Angiogenesis
Angiogenesis refers to the formation of new blood vessels due to the development of existing capillaries and postcapillary veins. Accurately speaking, angiogenesis is not the same as vasculogenesis, which is the de novo formation of endothelial cells from mesoderm cell precursors, and neovascularization. The first vessels in the developing embryo form through vasculogenesis, after which angiogenesis is responsible for most, if not all, blood vessel growth during development and in disease.
Angiogenesis is a normal and vital process in growth and development, as well as in wound healing and in the formation of granulation tissue. However, it is also a fundamental step in the transition of tumors from a benign state to a malignant one, leading to the use of angiogenesis inhibitors in the treatment of cancer. Tumor angiogenesis is an extremely complex process that generally includes steps such as degradation of the vascular endothelial matrix, migration of endothelial cells, proliferation of endothelial cells, formation of vascular loops by the branching of endothelial cells, and the formation of a new basement membrane. Because the neovascular structure and function of the tumor tissue are abnormal, and the vascular matrix is incomplete, the microvessels are prone to leakage. Therefore, the tumor cells do not need to go through a complicated invasion process and penetrate directly into the bloodstream and metastasis. A growing number of studies have shown that benign tumors have sparse angiogenesis and slow blood vessel growth, while most malignant tumors have intensive angiogenesis and rapid growth. Therefore, angiogenesis plays an important part in the development and metastasis of tumors, and it is a marker for most malignant tumors. Inhibiting this process will significantly prevent the development and spread of tumor tissue.
Angiogenesis is performed by various angiogenic proteins e.g integrins and prostaglandins, including several growth factors, such as VEGF, FGF.
The fibroblast growth factor (FGF) family with its prototype members FGF-1 (acidic FGF) and FGF-2 (basic FGF) consists to date of at least 22 known members. In general, FGFs stimulate a variety of cellular functions by binding to cell surface FGF-receptors in the presence of heparin proteoglycans. The FGF-receptor family has seven members, and all the receptor proteins are single-chain receptor tyrosine kinases that become activated through autophosphorylation induced by a mechanism of FGF-mediated receptor dimerization. Receptor activation gives rise to a signal transduction cascade that leads to gene activation and diverse biological responses, including cell differentiation, proliferation, and matrix dissolution, thus initiating a process of mitogenic activity critical for the growth of endothelial cells, fibroblasts, and smooth muscle cells. FGF-1 can bind to all seven FGF-receptor subtypes, making it the broadest-acting member of the FGF family, and a potent mitogen for the diverse cell types needed to mount an angiogenic response in damaged tissues, where upregulation of FGF-receptors occurs. FGF-1 stimulates the proliferation and differentiation of all cell types necessary for building an arterial vessel, including endothelial cells and smooth muscle cells; this fact distinguishes FGF-1 from other pro-angiogenic growth factors, such as vascular endothelial growth factor (VEGF), which primarily drives the formation of new capillaries.
Besides FGF-1, one of the most important functions of fibroblast growth factor-2 (FGF-2 or bFGF) is the promotion of endothelial cell proliferation and the physical organization of endothelial cells into tube-like structures, thus promoting angiogenesis. FGF-2 is a more potent angiogenic factor than VEGF or PDGF (platelet-derived growth factor); however, it is less potent than FGF-1. As well as stimulating blood vessel growth, aFGF (FGF-1) and bFGF (FGF-2) are critical players in wound-healing. They stimulate the proliferation of fibroblasts and endothelial cells that give rise to angiogenesis and developing granulation tissue; both increase blood supply and fill up a wound space early in the wound healing process.
Vascular endothelial growth factor (VEGF), a homodimer glycoprotein encoded by a single gene, can directly stimulate the migration, proliferation and division of vascular endothelial cells, and increase microvascular permeability. It is aimed at the highest specificity of endothelial cells and the strongest mitogenic effect of mitogens. In vitro studies clearly demonstrate that VEGF is a potent stimulator of angiogenesis because, in the presence of this growth factor, plated endothelial cells will proliferate and migrate, eventually forming tube structures resembling capillaries. VEGF causes a massive signaling cascade in endothelial cells. Binding to VEGF receptor-2 (VEGFR-2) starts a tyrosine kinase signaling cascade that stimulates the production of factors that variously stimulate vessel permeability (eNOS, producing NO), proliferation/survival (bFGF), migration (ICAMs/VCAMs/MMPs) and finally differentiation into mature blood vessels. In this process, VEGF binds with high affinity to two receptors KDR and Flt-1 on endothelial cells, directly stimulates the proliferation of vascular endothelial cells, and induces their migration and formation of lumen-like structures; at the same time, it also increases microvascular permeability and induces plasma proteins (mainly fibrinogen) is extravasated and promotes neovascularization in vivo by inducing interstitial production. VEGF plays a central regulatory role in angiogenesis and formation and is a key angiogenesis stimulator.
The angiogenesis process requires interactions between vascular endothelial cells (EC) and the extracellular matrix, between EC and EC, and between EC and other surrounding cells. This role is accomplished by adhesion factors, in which matrix metalloproteinase (MMP) initiates the activation and migration of endothelial cells by degrading the basement membrane glycoprotein and extracellular matrix components, and the integrin family mediated by binding to different ligands. The migration and adhesion of vascular endothelial cells contribute to the maturation and stability of neovascularization, and cell adhesion factor (ICAM-1) can produce immunosuppression and reduce the cytotoxicity of natural killer cells, helping ectopic tissue to escape the immune system of the body. The killing of natural killer cells promotes angiogenesis after invading ectopic tissue.
The angiogenesis mechanism is complex, and many factors involved in and promote angiogenesis. The number of macrophages in the peritoneal fluid of epithelial mesenchymal transitions (EMT) is significantly increased. Macrophages secrete TNF-α and IL-8, and then they can promote the proliferation of vascular endothelial cells. Besides, transforming growth factor-β ( TGF-β), platelet-derived endothelial cell growth factor (PD-ECGF), heparanase, angiogenin (angs), osteogenin (OPN), cyclooxygenase (COX-2), hypoxia-inducible factor -1, Laminin (LN), placenta growth factor (PLGF), Survivin, erythropoietin (Epo) are involved in the formation of EMT blood vessels.
Endostatin (ENS) is a C-terminal fragment of XVIII collagen that specifically inhibits endothelial cell proliferation and promotes apoptosis. It inhibits angiogenic factors such as VEGF and bFGF and their biological effects, and can also interact with MMPs and integrin ανβ3, Ανβ5. In addition, ENS can inhibit the migration and adhesion of endothelial cells and macrophages. It has a strong ability to inhibit neovascularization, and plays an important role in the regulation of tumor angiogenesis.
In angiogenesis, there is a substance that inhibits the growth of new blood vessels, is called angiogenesis inhibitor. They include: Angiostain, it can selectively inhibit endothelial cell proliferation; Thrombospondin-1 (TSP-1), it inhibits angiogenesis induced by VEGF or bFGF by interacting with the cell matrix and is concentration-dependent; Tissue inhibitors of metalloproteinases (TIMPs), it can inhibit angiogenesis through the formation of complexes with MMPs, thereby inhibiting angiogenesis. In addition, platelet factor-4 (PF-4), interferon-α (IFN-α), interleukin-13, interleukin-4, interleukin-10, and plasminogen activator inhibitor all inhibit the process of blood vessel formation.
Cancer cells are cells that have lost their ability to divide in a controlled fashion. A malignant tumor consists of a population of rapidly dividing and growing cancer cells that progressively accrues mutations. However, tumors need a dedicated blood supply to provide the oxygen and other essential nutrients they require in order to grow beyond a certain size. So angiogenesis is essential for tumor growth and metastasis, controlling tumor-associated angiogenesis is a promising tactic in limiting cancer progression. Tumor angiogenesis is an extremely complex process that generally includes steps such as degradation of the vascular endothelial matrix, migration of endothelial cells, proliferation of endothelial cells, formation of vascular loops by the branching of endothelial cells, and the formation of a new basement membrane. Tumor angiogenesis occurrence depends on an interaction effect. On the one hand, tumor cells release angiogenic factors to activate vascular endothelial cells and promote endothelial cell proliferation and migration. On the other hand, endothelial cells also secrete certain angiogenic growth factors to stimulate the growth of tumor cells. The interaction between tumor cells and endothelial cells runs through the whole process of tumor angiogenesis from beginning to end. In general, tumor-derived capillaries are formed by extending and expanding on the basis of the original blood vessels, and the process is similar to the typical process of wound healing and embryogenesis. These neovascularizations provide nutrients for the continuous infiltration of the growing primary tumor. In turn, the tumor cells secrete a variety of substances during growth to accelerate the formation of tumor-derived capillaries.
Similar with the common angiogenesis, FGF, VEGF and other angiogenic stimulator is also necessity for the associated-tumor angiogenesis. For instance, VEGF can promote endothelial cell proliferation, increase vascular permeability, and promote the expression of plasminogen activator(PA) and plasminogen activator inhibitor (PAI), interstitial collagenase and thrombin in endothelial cells, and extravasation of plasma fibrin, leading to deposition of cellulose in tumor stroma and promotion of macrophage Cells, fibroblasts, and endothelial cells grow, leading to tumor angiogenesis and play an important role in tumor growth. Most of the VEGF secreted by tumor cells is concentrated around tumor blood vessels. The response of tumor blood vessels to VEGF was higher than that of normal blood vessels, indicating that VEGF is closely related to tumor angiogenesis. In cell transfection experiments, Me157 melanoma cells transfected with VEGF gene can secrete a large amount of VEGF. After inoculated subcutaneously in nude mice, a large number of blood vessels appear in the tumor tissue, and they pass radially through the tumor by radial, suggesting that it not only affects the number of angiogenesis of the tumor, but also affects the structure of angiogenesis. Therefore, clinical trials with tumor angiogenesis as a target have attracted much attention. Finding a way that can cure the tumor by inhibiting tumor angiogenesis is a hot research direction in cancer therapy.