Controlling the angiogenic switch in developing atherosclerotic plaques: Possible targets for therapeutic intervention
Journal of Angiogenesis Research. 2009;
Received: 25 June 2009 | Accepted: 21 September 2009 | Published: 21 September 2009
Vascular Cell ISSN: 2045-824X
Plaque angiogenesis may have an important role in the development of atherosclerosis. Vasa vasorum angiogenesis and medial infiltration provides nutrients to the developing and expanding intima and therefore, may prevent cellular death and contribute to plaque growth and stabilization in early lesions. However in more advanced plaques, inflammatory cell infiltration, and concomitant production of numerous pro-angiogenic cytokines may be responsible for induction of uncontrolled neointimal microvessel proliferation resulting in production of immature and fragile neovessels similar to that seen in tumour development. These could contribute to development of an unstable haemorrhagic rupture-prone environment. Increasing evidence has suggested that the expression of intimal neovessels is directly related to the stage of plaque development, the risk of plaque rupture, and subsequently, the presence of symptomatic disease, the timing of ischemic neurological events and myocardial/cerebral infarction. Despite this, there is conflicting evidence regarding the causal relationship between neovessel expression and plaque thrombosis with some in vivo experimental models suggesting the contrary and as yet, few direct mediators of angiogenesis have been identified and associated with plaque instability in vivo.
In recent years, an increasing number of angiogenic therapeutic targets have been proposed in order to facilitate modulation of neovascularization and its consequences in diseases such as cancer and macular degeneration. A complete knowledge of the mechanisms responsible for initiation of adventitial vessel proliferation, their extension into the intimal regions and possible de-novo synthesis of neovessels following differentiation of bone-marrow-derived stem cells is required in order to contemplate potential single or combinational anti-angiogenic therapies. In this review, we will examine the importance of angiogenesis in complicated plaque development, describe the current knowledge of molecular mechanisms of its initiation and maintenance, and discuss possible future anti-angiogenic therapies to control plaque stability.
According to a World Health Organization Fact Sheet (EURO/03/06) cardiovascular disease (CVD) is the number one killer in Europe, with heart disease and stroke being the major cause of death in all 53 Member States. Figures show that 34,421 (23% of all non-communicable diseases) of Europeans died from CVD in 2005. The report also highlighted the fact that there is approximately a 10-fold difference in premature CVD mortality between Western Europe and countries in Central and Eastern Europe with a higher occurrence of CVD amongst the poor and vulnerable. Although improvements in understanding have helped to reduce the number of Western European dying from CVD and related diseases further advances will require a clearer understanding of the pathobiological mechanisms responsible for the development of stroke, atherosclerosis and myocardial infarction. Approximately 75% of acute coronary events and 60% of symptomatic carotid artery disease are associated with disruption of atherosclerotic plaques .
Angiogenesis in cardio- and cerebrovascular disease: Evidence for it as a major determinant of plaque growth, instability and rupture
Relationship between inflammation and angiogenesis in promotion of plaque development?
The atherosclerotic process is often initiated before adulthood, when cholesterol-containing low-density lipoproteins accumulate in the intima and activate the endothelium. Leukocyte adhesion molecules and chemokines promote recruitment of monocytes and T cells. Monocytes can differentiate into macrophages and up-regulate pattern recognition receptors, including scavenger receptors and toll-like receptors. Scavenger receptors mediate lipoprotein internalization, which leads to foam-cell formation. Toll-like receptors transmit activating signals that lead to the release of cytokines, proteases, and vasoactive molecules, and are considered to be an important link between inflammation and cardiovascular disease . Deficiency of toll-like receptor 4 (TLR-4) protein leads to a reduction in macrophage recruitment in association with reduced cytokine and chemokine levels . T cells in lesions recognize local antigens and mount T helper-1 responses with secretion of pro-inflammatory cytokines that contribute to local inflammation and growth of the plaque .
VV density, their proliferation and medial-intimal infiltration, and concurrent adventitial inflammation are strongly associated with advanced lesions . Plaque neovascularization correlated with the extent of inflammation in hypercholesterolemic apoE mice and inhibition of vessel formation reduced macrophage accumulation and plaque progression . Similarly, transfection with murine soluble VEGF-R1 inhibited early inflammation and late neointimal formation, again suggesting that as neovessels are generated, so the inflammatory response is perpetuated in a continuous cycle . Inflammatory infiltrates enhance recruitment of monocytes, secrete matrix metalloproteinases and increase the expression of γ-interferon (from t-lymphocytes) which may weaken the fibrous cap; similarly, they can induce synthesis of angiogenic tissue angiotensin-converting enzyme, growth factors, interleukin-8 and tissue factor . The importance of the inflammatory response was demonstrated in vivo where oral treatment of apoE-/LDL-double knockout mice with the anti-inflammatory compound 3-deazaadenosine prevented lesion formation . A strong correlation has been shown between macrophage infiltration, intraplaque haemorrhage and rupture-prone thin-cap lesions with high microvessel density, whilst these features are not common in calcified or hyalinized human arterial plaques, suggesting a strong link between neovascularization, inflammation and thrombosis [15, 31]. The phenotype of plaque neovessels could also be important in determining plaque stability, for example, new vascular networks, and immature vessels with poor integrity and no smooth muscle cell/pericyte coverage would likely act as sites for inflammatory cell infiltration, inflammatory cell leakage and intraplaque haemorrhage respectively . The correlation of focal collections of inflammatory cells with areas of intraplaque neovascularization and haemorrhage, suggests that release of growth factors and cytokines by macrophages and leukocytes may have a key role in modulating the vascularization process . Evidence for the existence of hotspots or "neovascular milieu" was found in lesions from ApoE-/- mice where the density of VV was highly correlated with the presence of inflammatory cells rather than plaque size, whilst deposition of RBC membranes within the necrotic core of plaques also leads to an increase in macrophage infiltration and therefore may further potentiate the inflammatory response [22, 23].
Hypercholesterolaemia may be associated with proliferation of VV in coronary and carotid vessels at early stages of plaque development. Williams JK  first demonstrated that the presence of atherosclerosis in hypercholesterolemic monkeys induced an increase in blood flow through the VV and plaque regression caused by removal of the high lipid diet reduced the VV concentration and blood flow to the coronary media and intima. High cholesterol levels are associated with increased serum VEGF expression and may cause up-regulation of growth factor receptors on both endothelial and smooth muscle cells . Furthermore, oxidized LDL (ox-LDL) generated in response to pro-oxidative cellular changes can exacerbate the inflammatory response, since engulfment of intact apoptotic cells was reduced in the presence of ox-LDL and in its absence, rapid phagocytosis suppressed macrophage inflammatory cytokine release, suggesting a link between high lipid levels, inflammation and possibly angiogenesis . In vitro studies have demonstrated that stimulation of HUVEC with ox-LDL up-regulates adhesion molecules (including ICAM-1, E-selectin and P-selectin), inflammatory proteins including Il-6, thrombotic factors including tissue factor and remodeling proteins such as MMP-2 and MMP-9, many of which are also stimulators of angiogenesis .
Medial and intimal thickening induced by hypercholesterolemia may result in a limited supply of oxygen and nutrients reaching these areas from either the lumen and/or VV, resulting in a hypoxic environment. Since the major outcome of hypoxia is increased vascularization, intraplaque vessels may proliferate in association with this potent stimulus. Hypoxia-inducible factor (HIF-1) is expressed in hypoxic regions of expanding and developing plaques, and directs migration of EC towards the hypoxic environment via direct HIF-1 binding the regulatory gene of VEGF and subsequent induction of VEGF transcription . VEGF is a potent angiogenic growth factor, stimulating EC mitogenesis and blood vessel formation via activation of intracellular signalling intermediates including mitogen-activated protein kinase 1/2 (MAPK1/2) and src. Increased expression of VEGF and its receptors in hypoxic areas, in association with interaction with cell membrane integrins including α5β3, is one of the main causes of vessel leakiness . Leaky plaque VV have been identified by ultrastructural visualization of defects between endothelial tight, gap and adherens junctions, and VEGF is known to affect junctional adhesion molecule expression, block gap junctional communication between adjacent endothelial cells and disrupt tight junctional communication through a src-dependent pathway .
Oxidized phospholipids such as 1-palmitoyl-2-arachidonoyl-
Pharmacological inhibition of VV angiogenesis inhibits plaque development and instability in vivo models of atherosclerosis
Current imaging methodologies may be used in conjunction with emerging therapies based on the use of angiogenesis-targeted nanoparticles which will allow both the identification and tracking of drugs and also prove to be more effective for the delivery of drugs to target sites. The preferential aim would be to prevent initial development of neointimal vascular sites by blocking adventitial blood vessel activation in patients with low grade plaques might represent 1) future imaging targets able to identify patients developing unstable plaque regions susceptible to rupture, and 2) prevent or slow down development of atheroma thereby improving treatment and survival rates of patients with a history of development of myocardial infarction or ischaemic stroke.
We would like to thank Mr Mick Hoult for help in production of the Figures.
Authors’ original submitted files for images
Below are the links to the authors’ original submitted files for images.
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