Angiogenesis is the physiological process through which new blood vessels form from pre-existing vessels. This is distinct from vasculogenesis, which is the de novo formation of endothelial cells from mesoderm cell precursors.
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. The essential role of angiogenesis in tumor growth was first proposed in 1971 by Judah Folkman, who described tumors as “hot and bloody.”
Sprouting angiogenesis was the first identified form of angiogenesis. It occurs in several well-characterized stages.
First, biological signals known as angiogenic growth factors activate receptors on endothelial cells present in pre-existing blood vessels. Second, the activated endothelial cells begin to release enzymes called proteases that degrade the basement membrane to allow endothelial cells to escape from the original (parent) vessel walls.
The endothelial cells then proliferate into the surrounding matrix and form solid sprouts connecting neighboring vessels. As sprouts extend toward the source of the angiogenic stimulus, endothelial cells migrate in tandem, using adhesion molecules called integrins.
These sprouts then form loops to become a full-fledged vessel lumen as cells migrate to the site of angiogenesis. Sprouting occurs at a rate of several millimeters per day, and enables new vessels to grow across gaps in the vasculature.
It is markedly different from splitting angiogenesis because it forms entirely new vessels as opposed to splitting existing vessels.
By Intussusception, also known as splitting angiogenesis, a new blood vessel is created by splitting of an existing blood vessel in two.
Intussusception was first observed in neonatal rats. In this type of vessel formation, the capillary wall extends into the lumen to split a single vessel in two.
There are four phases of intussusceptive angiogenesis.
First, the two opposing capillary walls establish a zone of contact. Second, the endothelial cell junctions are reorganized and the vessel bilayer is perforated to allow growth factors and cells to penetrate into the lumen. Third, a core is formed between the 2 new vessels at the zone of contact that is filled with pericytes and myofibroblasts. These cells begin laying collagen fibers into the core to provide an extracellular matrix for growth of the vessel lumen. Finally, the core is fleshed out with no alterations to the basic structure.
Intussusception is important because it is a reorganization of existing cells. It allows a vast increase in the number of capillaries without a corresponding increase in the number of endothelial cells.
This is especially important in embryonic development as there are not enough resources to create a rich microvasculature with new cells every time a new vessel develops.
Mechanical stimulation of angiogenesis is not well characterized. There is a significant amount of controversy with regard to shear stress acting on capillaries to cause angiogenesis, although current knowledge suggests that increased muscle contractions may increase angiogenesis. This may be due to an increase in the production of nitric oxide during exercise. Nitric oxide results in vasodilation of blood vessels.
Fibroblast Growth Factor
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. Most are single-chain peptides of 16-18 kDa and display high affinity to heparin and heparan sulfate. 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 is composed of 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, unique among all 22 members of the FGF family, 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 (hypoxic) 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.
Until 2007, three human clinical trials have been successfully completed with FGF-1, in which the angiogenic protein was injected directly into the damaged heart muscle. Also, one additional human FGF-1 trial has been completed to promote wound healing in diabetics with chronic wounds.
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 important 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/cavity early in the wound-healing process.
Vascular endothelial growth factor (VEGF) has been demonstrated to be a major contributor to angiogenesis, increasing the number of capillaries in a given network. Initial in vitro studies demonstrated bovine capillary endothelial cells will proliferate and show signs of tube structures upon stimulation by VEGF and bFGF, although the results were more pronounced with VEGF.
Upregulation of VEGF is a major component of the physiological response to exercise and its role in angiogenesis is suspected to be a possible treatment in vascular injuries. 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. Mechanically, VEGF is upregulated with muscle contractions as a result of increased blood flow to affected areas. The increased flow also causes a large increase in the mRNA production of VEGF receptors 1 and 2.
The increase in receptor production means muscle contractions could cause upregulation of the signaling cascade relating to angiogenesis. As part of the angiogenic signaling cascade, NO is widely considered to be a major contributor to the angiogenic response because inhibition of NO significantly reduces the effects of angiogenic growth factors. However, inhibition of NO during exercise does not inhibit angiogenesis, indicating there are other factors involved in the angiogenic response.
The angiopoietins, Ang1 and Ang2, are required for the formation of mature blood vessels, as demonstrated by mouse knock out studies. Ang1 and Ang2 are protein growth factors which act by binding their receptors, Tie-1 and Tie-2; while this is somewhat controversial, it seems that cell signals are transmitted mostly by Tie-2; though some papers show physiologic signaling via Tie-1 as well.
These receptors are tyrosine kinases. Thus, they can initiate cell signaling when ligand binding causes a dimerization that initiates phosphorylation on key tyrosines.
Application in Medicine
Angiogenesis as a Therapeutic Target
Angiogenesis may be a target for combating diseases characterized by either poor vascularisation or abnormal vasculature. Application of specific compounds that may inhibit or induce the creation of new blood vessels in the body may help combat such diseases.
The presence of blood vessels where there should be none may affect the mechanical properties of a tissue, increasing the likelihood of failure. The absence of blood vessels in a repairing or otherwise metabolically active tissue may inhibit repair or other essential functions.
Several diseases, such as ischemic chronic wounds, are the result of failure or insufficient blood vessel formation and may be treated by a local expansion of blood vessels, thus bringing new nutrients to the site, facilitating repair. Other diseases, such as age-related macular degeneration, may be created by a local expansion of blood vessels, interfering with normal physiological processes.
The modern clinical application of the principle of angiogenesis can be divided into two main areas: anti-angiogenic therapies, which angiogenic research began with, and pro-angiogenic therapies. Whereas anti-angiogenic therapies are being employed to fight cancer and malignancies, which require an abundance of oxygen and nutrients to proliferate, pro-angiogenic therapies are being explored as options to treat cardiovascular diseases, the number one cause of death in the Western world.
One of the first applications of pro-angiogenic methods in humans was a German trial using fibroblast growth factor 1 (FGF-1) for the treatment of coronary artery disease. Clinical research in therapeutic angiogenesis is ongoing for a variety of atherosclerotic diseases, like coronary heart disease, peripheral arterial disease, wound healing disorders, etc.
Also, regarding the mechanism of action, pro-angiogenic methods can be differentiated into three main categories: gene-therapy, targeting genes of interest for amplification or inhibition; protein-therapy, which primarily manipulates angiogenic growth factors like FGF-1 or vascular endothelial growth factor, VEGF; and cell-based therapies, which involve the implantation of specific cell types.
There are still serious, unsolved problems related to gene therapy. Difficulties include effective integration of the therapeutic genes into the genome of target cells, reducing the risk of an undesired immune response, potential toxicity, immunogenicity, inflammatory responses, and oncogenesis related to the viral vectors used in implanting genes and the sheer complexity of the genetic basis of angiogenesis.
The most commonly occurring disorders in humans, such as heart disease, high blood pressure, diabetes and Alzheimer’s disease, are most likely caused by the combined effects of variations in many genes, and, thus, injecting a single gene may not be significantly beneficial in such diseases.
Oral, intravenous, intra-arterial, or intramuscular routes of protein administration are not always as effective, as the therapeutic protein may be metabolized or cleared before it can enter the target tissue. Cell-based pro-angiogenic therapies are still early stages of research, with many open questions regarding best cell types and dosages to use.
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 (generally 1–2 mm3).
Tumors induce blood vessel growth (angiogenesis) by secreting various growth factors (e.g. VEGF). Growth factors such as bFGF and VEGF can induce capillary growth into the tumor, which some researchers suspect supply required nutrients, allowing for tumor expansion.
Unlike normal blood vessels, tumor blood vessels are dilated with an irregular shape. In 2007, it was discovered that cancerous cells stop producing the anti-VEGF enzyme PKG. In normal cells (but not in cancerous ones), PKG apparently limits beta-catenin, which solicits angiogenesis.
Other clinicians believe angiogenesis really serves as a waste pathway, taking away the biological end products secreted by rapidly dividing cancer cells. In either case, angiogenesis is a necessary and required step for transition from a small harmless cluster of cells, often said to be about the size of the metal ball at the end of a ball-point pen, to a large tumor.
Angiogenesis is also required for the spread of a tumor, or metastasis. Single cancer cells can break away from an established solid tumor, enter the blood vessel, and be carried to a distant site, where they can implant and begin the growth of a secondary tumor.
Evidence now suggests the blood vessel in a given solid tumor may, in fact, be mosaic vessels, composed of endothelial cells and tumor cells. This mosaicity allows for substantial shedding of tumor cells into the vasculature, possibly contributing to the appearance of circulating tumor cells in the peripheral blood of patients with malignancies. The subsequent growth of such metastases will also require a supply of nutrients and oxygen and a waste disposal pathway.
Endothelial cells have long been considered genetically more stable than cancer cells. This genomic stability confers an advantage to targeting endothelial cells using antiangiogenic therapy, compared to chemotherapy directed at cancer cells, which rapidly mutate and acquire ‘drug resistance’ to treatment. For this reason, endothelial cells are thought to be an ideal target for therapies directed against them.
Recent studies by Klagsbrun, et al. have shown, however, that endothelial cells growing within tumors do carry genetic abnormalities. Thus, tumor vessels have the theoretical potential for developing acquired resistance to drugs. This is a new area of angiogenesis research being actively pursued.
Two independent studies published in the journal Nature in 2010 November confirmed the ability of tumors to make their own blood vessels. When one group found that tumor stem cells could make their own blood vessels and avastin could not inhibit their early differentiation, the other group showed that selective targeting of endothelial cells generated by tumor-derived stem cells in mouse xenografts resulted in tumour reduction. These studies done in glioblastoma model may have implications in other tumors.