Current Pharmaceutical Design - Volume 13, Issue 17, 2007

Endothelial injury and dysfunction are early alterations in vessel wall biology preceding atherosclerotic plaque formation. In the presence of established cardiovascular risk factors, endothelial cells are constantly injured and repaired by the proliferation of resident cells and circulating endothelial progenitor cells. The maintenance of the endothelial layer physical continuity and function represents a major target for the prevention of vascular disease. This special issue covers key aspects of endothelial cell biology and potential therapeutic approaches that may restore the function of the endothelium. Basic and clinical researchers have reviewed the current state of art in endothelial dysfunction, endothelium-monocyte cross-talk, angiogenesis, arteriogenesis and the potentiality of bone-marrow derived progenitor cells to achieve a successful re-endothelization of arterial segments. The vascular endothelium is a continuous monolayer of thin, flat cells that lines the interior surface of small and large blood vessels, forming an interface between circulating blood and the subnendothelial matrix [1] (Fig. 1). Endothelial cells line the entire circulatory system, from the heart to the smallest capillary. In small blood vessels and capillaries, endothelial cells (ECs) are often the only cell-type present. For many years ECs were seen as a mere barrier that passively participated in the transport of substances from the blood to the rest of the arterial wall [1]. However, nowadays there are no doubts that the vascular endothelium acts as a key integrator and modulator of many important functions of the arterial wall (Fig. 1) [2;3]. Endothelial dysfunction is a commonly used phrase but that encompasses many different biological processes or diagnostic measures of vascular disease. Expression of selectins and integrins and subsequent enhanced monocyte adhesion, disturbed vasodilating responses and increased permeability of the endothelial layer are all features that can be observed in atherosclerotic disease and are used as a measure for endothelial dysfunction. Early in atherogenesis measures of endothelial dysfunction are detectable before other structural and/or compositional changes in the blood vessels [4]. Subsequent steps following vascular injury imply recruitment of inflammatory cells, accumulation of lipid into foam cells, oxidation of LDL, intimal growth, atherosclerotic plaque expansion and/or remodeling [5;6]. Endothelial dysfunction is a common feature in subjects suffering from diabetes mellitus, hypertension or other vascular disorders [4]. It is independently related with adverse cardiovascular events, including myocardial infarction, coronary death, and the need for revascularization. One of the main mechanisms of endothelial dysfunction is the diminishing of actions of nitric oxide (NO) [7]. The importance of NO is here documented by Braam et al. [23] who summarized several aspects regarding NO biology with a special emphasis in its numerous actions and different pharmacologic approaches leading to increase the production of endothelial-derived NO. The clinical significance and the limitations of the current methods to test endothelial functionality in human beings will be critically reviewed by Frick et al. [24]. Dysfunctional endothelial cells express more adhesion molecules and as a consequence circulating monocytes are captured by activated endothelium promoting an inflammatory reaction [8]. Monocyte adhesion to activated endothelial cells is a multistep process [5,6]. First, L-selectin and the P-selectin glycoprotein ligand-1 (PSGL-1) expressed on monocytes [7,8] and E-selectin and P-selectin expressed on activated endothelial cells [9,10] mediate the initial tethering of leukocytes, also called rolling adhesion. When a rolling monocyte encounters chemokines presented by the activated endothelial cells, integrins get activated, a process called inside-out signalling [11]. Not only chemokines, but also other stimuli like growth factors, cytokines, and bacterial-derived products such as lipopolysaccharide (LPS)3, a Toll-like receptor 4 ligand [12], and R-848, a Toll-like receptor 7 ligand [13], are able to activate integrins on monocytes. In this issue, Martin et al. [25], discuss potential pharmacological targets for the modulation of endothelial cell-monocyte cross-talk.......

Knowledge about the function of endothelial nitric oxide synthase (eNOS), and its regulation in pathophysiological states has tremendously increased. It is now clear that diminished activity of nitric oxide (NO) contributes to endothelial dysfunction, which is a characteristic of impeding atherosclerosis. This review aims to summarize the available knowledge about the impact of important cardiovascular risk factors on NO production by eNOS. There are 4 principle causes of diminished NO bio-activity: decreased expression and/or activity of the eNOS enzyme, eNOS uncoupling, enhanced breakdown or scavenging of NO and impaired transmission of NOmediated signaling events (failure of the effector mechanisms). From the analysis, it becomes clear, that several aspects of eNOS functionality have only scarcely been tested under conditions of increased (experimental) cardiovascular risk. These aspects include palmitoylation, myristoylation and phosphorylation of the eNOS enzyme. Clear is that enhanced production of reactive oxygen species (ROS) and eNOS uncoupling are relatively important causes of reduced NO-bioactivity in cardiovascular disease states . Ideally, eNOS is sufficiently expressed, produces NO sufficiently and not abundantly, does not produce superoxide and is not scavenged by ROS; the produced NO then reaches its signaling target, mainly soluble guanylyl cyclase (sGC) and elicits a cellular response. Considering which aspects of eNOS are now assessable in a clinical setting and which therapeutic measures are available, there is a great challenge ahead.

Endothelial dysfunction is a well documented early phenomenon in atherosclerosis. Because it may precede structural changes and clinical manifestations, major research efforts have focused on the detection of endothelial dysfunction in humans. The utility of such tests in clinical practice critically depends on the proof of their prognostic value, their safety and reproducibility. First data supporting the prognostic impact of endothelial function have come from studies using intracoronary infusion of acetylcholine, a test clearly too invasive to be performed in asymptomatic subjects. Therefore, non-invasive techniques such as flow-mediated vasodilation of the brachial artery and strain-gauge venous plethysmography of the forearm have been developed. Numerous studies in a variety of patient populations have been performed to evaluate the prognostic value of these methods. This review summarizes the current status of endothelial dysfunction as an early parameter of atherosclerosis and its potential use in the clinical arena. The value of endothelial function as a surrogate endpoint in cardiovascular studies is critically reviewed.

The aim of this chapter is to present and identify potential pharmacological targets in endothelial cell-monocyte interactions leading to vascular syndrome and involving inflammation, coagulation, vascular remodelling and thrombosis. Increasing evidence is indicating that endothelial cells play a key role in atherothombosis by their capacity to attract, bind and allow the extravasation of monocytes to sites of inflammation. Surface expression and/or activation of constituent cell adhesion molecules (for e.g. P-selectin, E-selectin, ICAM-1, and VCAM-1) on endothelial cells together with chemokines such as CXCL8 (IL-8), Platelet-activating factor (PAF), CCL2 and CCL5 (Table 1) allow the rolling, adhesion and extravasation of monocytes. This review focuses on pharmacological targets implicated in endothelial cells interactions with monocytes/macrophages in vascular disease states and on cutting edge genomic tools for the identification and characterization of such targets.

Caveolae are 50-100 nm cell surface plasma membrane invaginations that are highly enriched in cholesterol and sphingolipids and are characterized by the protein marker caveolin-1. Caveolin-1 is highly expressed in terminally differentiated cells. Among these cells, endothelial cells, smooth muscle cells, and macrophages have all been shown to play key roles in the development of vascular disease. Atherosclerosis and neointimal formation are two major processes that have been associated with arterial occlusion. In both cases, caveolin-1 has been shown to play an important role. However, depending on the cell type and the metabolic pathways regulated by this protein, caveolin-1 may positively or negatively influence the development of vascular disease. Both of these aspects will be discussed in this review.

Endothelial dysfunction has been shown to be a prognostic factor for cardiovascular disease and improvement of endothelial dysfunction prevents cardiovascular event presentation. Endothelial dysfunction is associated to a reduced nitric oxide (NO) bioactivity, as a result of the impairment of NO synthesis/release by the endothelial NO synthase (eNOS) or by inactivation of NO. Endothelial dysfunction measurements are valuable surrogate markers to assess the effectiveness of interventions addressed to prevent o treat coronary heart disease (CHD). Dyslipemia and other cardiovascular risk factors promote endothelial dysfunction and life style changes and pharmacological treatment, particularly HMG-CoA reductase inhibitors (statins), have shown early improve of endothelial-dependent vasomotion. Statins efficiently reduce plasma LDL cholesterol, an effect that may account for their beneficial effect on endothelial function, but they also reduce cellular levels of isoprenoid compounds relevant for the bioavailability of NO. Statins restore NO production by several mechanisms, including up-regulation of eNOS mRNA and protein levels and preservation of NO inactivation by reactive oxygen species (ROS). These effects are mediated, at least in a part, through mechanisms independent of their lipid lowering effect (pleiotropic effects). In this article we discuss the relevance of endothelium-dependent effects on the early and delayed clinical benefit of statins, as well as the multiple ways by which statins may restore endothelial function acting not only on the endothelium but also on endothelial progenitor cells (EPC), which likely could contribute to both ischemia-induced neovascularization and endothelial regeneration after injury.

Angiogenesis, the formation of new vessels from pre-existing capillaries, is a fundamental physiological process which is also critical for the development of several pathological conditions; thus a diminished angiogenic response is related to ischemic disorders, whereas increased angiogenesis is associated with tumorigenesis and chronic inflammatory diseases. New ways of modulating angiogenesis therefore have potential in the treatment of these diseases. During angiogenesis, normally quiescent endothelial cells (ECs) become migratory and invade the surrounding tissue. To do this, they require a specific enzyme machinery to degrade the tissue barriers presented by the basement membranes and the interstitial matrix. This function is supplied by matrix metalloproteinase (MMP) proteins, a large family of enzymes responsible for degrading a variety of extracellular matrix (ECM) components and for modulating the bioactivity of transmembrane receptors and soluble factors. In this review we examine the participation of MMPs - in particular membrane type 1-matrix metalloproteinase (MT1-MMP) - in the different steps of angiogenesis, and discuss the mechanisms of regulation of MT1-MMP in ECs. Finally, we explore the potential use of MMP inhibitors (MMPI) in the treatment of angiogenesis-related disease, with especial emphasis on novel approaches to the inhibition of MT1-MMP activity in ECs.

Stimulation of neovascularization (angiogenesis, arteriogenesis) has emerged as a promising new strategy to treat patients with coronary disease. These strategies aim to improve cardiac function by ensuring myocardial perfusion and to reduce the risk of myocardial infarction. While angiogenesis describes a de-novo formation of small caliber capillary vessels, arteriogenesis leads to the outgrowth of pre-existing arterioles into large conductance collateral arteries. Inflammatory cells (e.g. monocytes), which can produce and secrete growth factors and cytokines, mediate both processes. Several trials have shown that intra-coronary infusion of growth factors or progenitor cells can improve left ventricular function after arterial occlusion. Despite these encouraging results, potential unfavorable effects on plaque progression and stability should not be neglected. Destabilization of atherosclerotic plaques leads to plaque rupture, intravascular thrombosis and tissue infarction. Increased neovascularization of the plaque (e.g. by angiogenesis) is thought to arise from the adventitial vasa vasorum, leading to an abnormal vascular development. This network of immature vessels is a viable source of invading inflammatory cells that can contribute to plaque instability. Furthermore, intra-plaque hemorrhages can lead to accumulation of erythrocyte membranes in the plaque that are rich in phospholipids and free cholesterol, promoting lesion instability through necrotic core expansion. Future angiogenic and arteriogenic approaches need to take these pitfalls into account and should focus on stimulation of vessel growth in combination with neutral or even beneficial effects on plaque formation and composition. This review discusses the delicate balance between the benefits and the drawbacks of therapeutic strategies to influence angiogenesis and arteriogenesis.

Atherosclerosis is still the principal cause of morbidity and mortality in Western countries and although a significant progress has been made in the understanding of its pathophysiology, the determinants of atherosclerotic plaque instability are still poorly understood. The endothelium plays a pivotal role for the development, progression, and complication of atherosclerosis. Endothelial dysfunction is widely recognized as one of the early alteration in the vessel wall preceding the development of the plaque. However, considering the plethora of vascular functions which are regulated by endothelium, it plays a pivotal role throughout the atherosclerotic process and indeed the loss of endothelial cells, leading to plaque denudation, is one of the main causes of plaque complication. It is therefore conceivable that the maintenance of the endothelial layer physical continuity and function is crucial for the prevention of atherosclerosis. In the presence of cardiovascular risk factors, endothelial cells are continuously injured and repaired by the proliferation of resident cells and circulating endothelial progenitor cells. Indeed the number of circulating endothelial progenitor cells has been identified as an predictor of cardiovascular events. The increase in bone marrow release of endogenous progenitor cells or the enhancement of their homing in arterial denuded sites or in intravascular stent surface, are currently pursued to reduce atherosclerosis development/complication and intrastent restenosis, respectively. However, some challenges may arise from procedures enhancing endothelialization, including unwanted angiogenesis which may favor neoplasia progression and paradoxically atherosclerotic plaque expansion and complication.