Angiogenesis and Arteriogenesis - the Basics

Eric Wahlberg, MD

The formation of true new blood vessels angiogenesis as well as the development of collaterals from preexisting vessels arteriogenesis - is of importance in the pathophysiology of vascular disease. By stimulating arteriogenesis in particular, we might be able to provide a new treatment strategy for patients with lower limb ischemia. This article discusses the basic mechanisms underlying these two processes.

Angiogenesis is a physiological process required for embryonic development, the menstrual cycle and wound healing. It also plays a role in pathological processes such as tumor growth, rheumatoid arthritis, diabetes and cardiovascular diseases. The angiogenic process is complex and not yet fully understood, but a number of growth promoting factors regulate the induction of angiogenesis (Table 1). The majority stimulate proliferation and migration of cells in the vascular wall and inhibit apoptosis.

Table 1. Examples of cytokines believed to be involved in angio- and arteriogenesis

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The initial step during angiogenesis is vasodilatation mediated by nitric oxide (NO). NO upregulates vascular endothelial cell growth factor (VEGF) transcription. Hypoxia also induces VEGF synthesis. This is initiated by binding of hypoxia inducible factor -1alpha (HIF-1 alpha) to the hypoxia response area in the VEGF gene promoter region. The VEGF family of proteins is probably the most important mediator of angiogenesis. It consists of six members; VEGF-A is the original (also called simply VEGF). Find out more detailed information about the VEGF family. Splicing of the VEGF-A gene results in five variants, each differing in total amino-acid number. It is probable that each of these isotypes has a different role in the angiogenic process. For example, VEGF-A189 may exclusively stimulate endothelial cell (EC) proliferation whereas VEGF-A165, the predominant form, may also stimulate ECs to coalesce and form larger vessels.

The next step in the angiogenic process is a VEGF mediated increase in vascular permeability. VEGF produces alterations in cell membrane structure and a redistribution of intracellular adhesion molecules, PECAM-1 and vascular endothelial-cadherin. Vascular permeability is also controlled and downregulated by Angiopoetin-1. Extravasation of plasma proteins stimulated by VEGF, creates a milieu supporting EC migration. Degradation of extracellular matrix, by Angiopoetin-2 and a large number of proteinases, such as matrix metalloproteinases (MMPs) also contributes to this process.

EC proliferation and migration, two important effects of VEGFs are mediated via three cell surface VEGF receptors. VEGFR-1 (Flt-1) and VEGFR -2 (KDR/Flk-1) are mainly expressed on ECs. VEGFR-3 is found in lymphatic endothelium. Receptor expression in endothelial cells is also upregulated by hypoxia. The signaling pathways activated by the VEGF receptor have not been completely elucidated. It is possible that VEGFR-1 may act as a negative regulator of VEGFR-2. Find out more about VEGF ligand binding to its receptors and VEGF receptor signaling.

When ECs reach the extracellular matrix they form cords and subsequently a lumen. This is accomplished by thinning of the ECs and fusion with existing vessels. Different VEGF isoforms (as mentioned), angiopoetins and integrins regulate lumen diameter, and transforming growth factor (TGF)-beta stimulates extracellular matrix production in order to stabilize the structure of the new vessel.

Arteriogenesis occurs when the lumen of a pre-existing vessel increases to form a collateral. After myocardial infarction or limb ischemia arterioles become large conductance vessels which maintain blood flow in the face of major arterial occlusion. Not all vessels can become collaterals and there are large differences in this capacity between species, vascular beds and probably also individuals.

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Once a stenosis in a main artery becomes hemodynamically significant, blood flow is directed toward low resistance in the periphery via pre-existing arterioles. The initial step in arteriogenesis is the development of elevated shear stress against the wall of the arteriole. Shear stress is the frictional wall pressure at the cell surface caused by blood flow and the compensatory forces striving to counteract this pressure. ECs become activated in response to increased shear stress. There may be specific shear stress receptors on the endothelial cell surface. Candidates for shear stress receptors are caveole, integrins and tyrosine kinase receptors (one is VEGFR-1). Multiple intracellular signaling pathways are then activated by these receptors. These signaling pathways converge to activate transcription factors called shear stress response elements (SSREs). Find out more about these pathways.

ECs react (through SSREs) by activating endothelial NO synthetase (eNOS) and genes for cytokines, the most important being monocyte chemoattractant protein-1 (MCP-1), granulocyte-macrophage colony stimulating factor (GM-CSF), tumor necrosis factor-alpha (TNF-alpha) and cell adhesion molecules. Aided by MCP-1 and adhesion molecules, circulating monocytes adhere to and invade the vascular wall. Platelets also adhere and produce interleukin-4, further stimulating adhesion molecule synthesis. GM-CSF has its main role in providing an environment for a stable monocyte function. After transformation to macrophages, monocytes produce fibronectin and proteoglycans as well as proteases that remodel the extracellular matrix. These inflammatory cells then produce numerous vascular growth factors (Table 1), many from the fibroblast growth factor (FGF) family. FGF-2 and basic FGF are the most prevalent but many different FGF isoforms are known. There are four different receptor types, FGFR-1 to 4. After FGF-2 has bound to it’s receptors, EC and smooth muscle cell (SMC) proliferation is stimulated. Read more about FGF, its receptors and their biological function.

After the initiation of EC and SMC replication, the vascular wall of the arteriole is remodeled by MMPs. MMPs also help to create the space needed for vessel enlargement. The newly formed collaterals are initially tortuous in order to compensate for the still increased shear forces, but the collateral eventually becomes indistinguishable from a normal artery. It has both a medial layer and normal reactivity.

The mechanisms described above are based on a number of studies published during the last few years. When the concept of therapeutic angiogenesis – stimulating angiogensis to treat patients with ischemia in the heart or leg - was launched, the complexity of these processes was underestimated. Today most researchers believe that simply adding vascular growth factors to an ischemic area is not sufficient for effective therapeutic angiogenesis. Presently, more interest and efforts are being devoted to understanding the basic mechanisms of these processes and attention is being directed toward stimulating arteriogenesis or both processes, rather than angiogenesis, alone.