Current Vascular Pharmacology - Volume 3, Issue 4, 2005

Tumours require oxygenation, nutrition and a route for dissemination. This necessitates the development of new vessels or angiogenesis. High levels of new vessel development are indicators of poor prognosis in cancer; they also provide new avenues of anti-tumour therapy. Angiogenesis in cancer produces structurally different vessels from angiogenesis in wound healing and inflammation. This article reviews the differences between vessels in tumour angiogenesis and 'normal angiogenesis. The main focus of the article is the role of the vasoactive peptide endothelin-1 (ET-1) in tumour angiogenesis. The role of ET-1 in tumour development is reviewed, before the direct and indirect effects of ET-1 in angiogenesis are examined. ET-1 has a direct angiogenic effect on endothelial and peri-vascular cells. It also has an indirect action through the increased release of the potent pro-angiogenic substance vascular endothelial growth factor (VEGF), via hypoxia inducible factor-1. ET-1 also indirectly stimulates angiogenesis by stimulating fibroblasts and cancer cells to produce pro-angiogenic proteases. ET-1 is a novel stimulator of tumour angiogenesis and warrants further examination as an anti-angiogenic treatment target.

Despite the exploration of a large number of disparate drugs in animal models and clinical trials, no pharmacological intervention, with the exception of aggressive lipid lowering therapy has reduced late vein graft failure in man. The importance of devising more effective strategies is exemplified by the considerable economic consequences of vein graft failure. Worldwide, there are currently more than 1,000,000 coronary artery bypass graft surgery (CABG) operations a year, the same number of patients undergoing infrainguinal bypass (IIBS) for vascular diseases of the lower limb. The pathophysiology of vein graft failure is complex, involving disparate factors that include adhesion of platelets and leukocytes, rheological forces, metalloproteinase expression, proliferation and migration of vascular smooth muscle cells, neointima formation, oxidative stress, hypoxia and neural re-organisation. Although this diverse aetiology may seem to preclude any single drug type as being effective in preventing vein graft failure, one factor that is involved in every facet of vein graft pathobiology is endothelin-1 (ET-1). Thus, in this review, we will consider the diverse aetiology of vein graft disease in relation to ET-1 and will then present an argument (with evidence) that ET-1A (ETA) receptor antagonists constitute a potentially effective means of preventing vein graft failure.

Peripheral vascular disease can compromise the blood supply to the lower limb with amputation being necessary in severe cases. Reduced blood flow may be due to arterial occlusive disease or constriction of skeletal microvessels with the resultant ischaemia causing pain, tissue damage, ulceration and gangrene. These events are associated with endothelial damage or dysfunction: endothelin-1 is implicated as a mediator via its constrictor, proinflammatory and proliferative actions. Raised plasma and tissue levels of this peptide have been described in various ischaemic conditions, including peripheral vascular disease. Here, the possible role of endothelin-1 in peripheral vascular disease is discussed and potential therapeutic tools are considered.

Soon after its identification as a powerful vasoconstrictor peptide, endothelin (ET-1) was implicated as a detrimental agent involved in determining the outcome of myocardial ischaemia and reperfusion. Early experimental studies demonstrated that ETA selective and mixed ETA/ETB receptor antagonists can reduce infarct size and prevent ischaemiainduced ventricular arrhythmias in models of ischaemia/reperfusion, implying that ET-1 acts through the ETA receptor to contribute to injury and arrhythmogenesis. However, as our understanding of the physiology of ET-1 has expanded, the role of ET-1 in the ischaemic heart appears ever more complex. Recent evidence suggests that ET-1 exerts actions on the heart that are not only detrimental (vasoconstriction, inhibition of NO production, activation of inflammatory cells), but which may also contribute to tissue repair, such as inhibition of cardiomyocyte apoptosis. In addition, ET-1-induced mast cell degranulation has been linked to a homeostatic mechanism that controls endogenous ET-1 levels, which may have important implications for the ischaemic heart. Furthermore the mechanism by which ET-1 promotes arrhythmogenesis remains controversial. Some studies imply a direct electrophysiological effect of ET-1, via ETA receptors, to increase monophasic action potential duration (MAPD) and induce early after-depolarisations (EADs), while other studies support the view that coronary constriction resulting in ischaemia is the basis for the generation of arrhythmias. Moreover, ET-1 can induce cardioprotection (precondition) against infarct size and ventricular arrhythmias, through as yet incompletely understood mechanisms. To enable us to identify the most appropriate means of targeting this system in a therapeutically meaningful way we need to continue to explore the physiology of ET-1, both in the normal and the ischaemic heart.

Endothelin A (ETA) transmembrane receptors predominate in rat cardiac myocytes. These are G proteincoupled receptors whose actions are mediated by the Gq heterotrimeric G proteins. Through these, ET-1 binding to ETAreceptors stimulates the hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate to diacylglycerol and inositol 1,4,5-trisphosphate. Diacylglycerol remains in the membrane whereas inositol 1,4,5-trisphosphate is soluble (though its importance in the cardiac myocyte is still debated). Isoforms of the phospholipid-dependent protein kinase, protein kinase C (PKC), are intracellular receptors for diacylglycerol. Cytoplasmic nPKCδ and nPKCε detect increases in membrane diacylglycerols and translocate to the membrane. This brings about PKC activation, though modifications additional to binding to phospholipids and diacylglycerol are involved. The next event (probably associated with PKC activation) is the activation of the membrane-bound small G protein Ras by exchange of GTP for GDP. Ras.GTP loading translocates Raf family mitogen-activated protein kinase (MAPK) kinase kinases to the membrane, initiates the activation of Raf, and thus activates the extracellular signal-regulated kinase 1/2 (ERK1/2) cascade. Over longer times, two analogous protein kinase cascades, the c-Jun N-terminal kinase and p38-mitogen-activated protein kinase cascades, become activated. As the signals originating from the ETA receptor are transmitted through these protein kinase pathways, other signalling molecules become phosphorylated, thus changing their biological activities. For example, ET-1 increases the expression of the c-jun transcription factor gene, and increases abundance and phosphorylation of c-Jun protein. These changes in c-Jun expression and phosphorylation are likely to be important in the regulation of gene transcription.

The aortic valve is a complex structure, the function of which is fundamental to sustain life. Previously believed to be an inert structure that merely opens in response to the forward flow of blood out of the left ventricle, it is now established that it is a sophisticated structure with specific biological properties. However, little is known about the mechanisms that regulate its function. In this respect, endothelin is of particular interest due to its range of biological actions within the cardiovascular system that suggest it may be capable of stimulating the cells that reside in valve cusps. Endothelin can be detected in the endothelial cells that cover valve cusps and it has been demonstrated that it is has the ability to stimulate contractile responses of cusp tissue in vitro. These contractions vary with different regions of the aortic valve cusp and occur preferentially in the circumferential direction. In addition, evidence exists that suggests endothelin may also have a role in the morphogenesis of the aortic valve. Further studies are required to determine the significance of the effects mediated by endothelin on cusp tissue to the function of the aortic valve in health and disease.

There is increasing evidence to suggest that chronic activation of the endothelin-1 system can lead to heterologous desensitization of the glucose-regulatory and mitogenic actions of insulin with subsequent development of glucose intolerance, hyperinsulinemia, impaired endothelial function and exacerbation of cardiovascular disease. Effects are mediated through a variety of mechanisms that include attenuation of key insulin signalling pathways and decreased tyrosine phosphorylation of insulin receptor substrates IRS-1, SHC and G alpha q/11. Other actions involve hemodynamic changes leading to reduced delivery of insulin and glucose to peripheral tissues as well as enhanced hepatic glycogenolysis, decreased glucose-transporter translocation and modulation of various adipokines that regulate insulin action. Overall the data suggest that ET-1 antagonists may provide an effective means of improving cardiac dysfunction and favourably influencing glucose tolerance in obese humans and patients with early insulin sensitivity where there is clear evidence for activation of the ET-1 system. Although most effects of ET-1 that modulate mechanisms leading to glucose intolerance appear to involve the ETA receptor subtype recent data indicates that combined ETA/ETB receptor antagonists may function as effectively as selective ETA blockers. Prospective trials are needed to assess whether ET-1 antagonists, either alone or in combination, are superior to other more conventional therapies such as insulin sensitizers and to evaluate effects of combined treatments on the development of insulin resistance and the progression of diabetes. Early screening of patients at risk for evidence of ET-1 activation would help to identify subjects who may benefit most from such treatment.

Both endothelin(ET)-1 and oxidative stress have been the subjects of intense investigation within the cardiovascular field over the past decade and a half, yet little is known about the precise relationship between these important modulators of vascular function. There is a firm evidence that ET-1 can stimulate the production of superoxide via NADPH oxidase activation, and at the same time, reactive oxygen species appear to stimulate ET-1 production. What is less clear is how these changes participate in the pathogenesis of vascular dysfunction. There is mixed evidence on whether oxidative stress plays a role in ET-dependent hypertension, however, a specific influence of ET-induced oxidative stress to reduce vascular reactivity is more convincing. The current review summarizes recent investigations into the relationship between ET-1 and oxidative stress and highlights several areas that require further investigation.

Connective tissue remodeling is achieved by a complex process involving several cell types, a plethora of growth factors, cytokines, chemokines and turnover of extracellular matrix (ECM). The main enzymes that degrade ECM molecules are matrix metalloproteinases (MMPs) and their activities are regulated by endogenous inhibitors, the tissue inhibitors of metalloproteinases (TIMPs). Recent studies have indicated that endothelins and their receptor expression affects tissue remodeling and repair. Endothelins are rapidly produced by endothelial cells in response to tissue injury and they have potent vasoconstrictive properties. They also promote tissue remodeling through activation of resident connective tissue cells and controlling the production of MMPs and TIMPs by the activated cells. In this review we present the cross-talk between the endothelins and the MMP-TIMP system and their implications in controlling the normal and abnormal tissue remodeling.

Since the discovery of endothelin in vascular endothelial cells and its pivotal role in vascular physiology (Yanagisawa and colleagues [1]), a number of studies have focused on the localisation of this vasoconstrictor peptide in human and animal vascular tissue, largely in endothelial cells. Various vascular beds have been the subject of research in normal and pathophysiological conditions, for example in neonates, during ageing, pregnancy, hypertension, diabetes, heart failure, experimental metastases and neurological disorders. These studies have revealed the presence of endothelin in the blood vessel wall, suggesting the involvement of this peptide in vascular physiology in health and disease. This chapter reviews studies on the distribution of endothelin-1 (ET-1) and its receptors (ETA and ETB) in vascular tissue with emphasis on their ultrastructural localisation.

There is conflicting evidence regarding the effect of endothelin-1 (ET-1) on platelets. Some studies show that ET-1 activates platelets, others show platelet inhibition with ET-1 and some studies did not detect an effect of ET-1. These conflicting results may be due to complex interactions between platelet ETA and ETB receptors. ET-1 antagonism may emerge as an important therapeutic strategy in the management of several vascular disorders. However, to date the only prescribed ET-1 antagonist is bosentan for pulmonary arterial hypertension. Bosentan is a 'dual' ET- 1 antagonist (i.e. it acts on both ETA and ETB receptors). Whether this action involves an effect on platelets remains to be established. In this review some of the studies describing the effect of ET-1 on human platelets are discussed. Vascular diseases where ET-1 is implicated are also considered.