Oxidative stress and cardiovascular disease: Novel tools give (free) radical insight

Abstract

Cardiovascular disease is the most common cause of mortality in the Western world and accounts for up to a third of all deaths worldwide. Cardiovascular disease is multifactorial and involves complex interplay between lifestyle (diet, smoking, exercise, ethanol consumption) and fixed (genotype, age, menopausal status, gender) causative factors. The initiating step in cardiovascular disease is endothelial damage, which exposes these cells and the underlying cell layers to a deleterious inflammatory process which ultimately leads to the formation of atherosclerotic lesions. Intrinsic to lesion formation is cellular oxidative stress, due to the production of damaging free radicals (reactive oxygen and nitrogen species) by many cell types including endothelial cells, vascular smooth muscle cells and monocytes/macrophages. Exogenous factors such as smoking and the existence of other disease states such as diabetes also contribute to oxidative stress and are strong risk factors for cardiovascular disease. In this review we describe this role of free radicals in atherosclerosis and discuss the mechanisms and cellular systems by which these radicals are produced. We also highlight recent technological advances which have added to the vascular biologist's armoury and which promise to provide new insight into the role of reactive oxygen species in cardiovascular disease.

Introduction

Cardiovascular disease is the major cause of mortality in the Western world. According to American Heart Association figures, in the USA alone greater than 80 million people suffer from some form of cardiovascular disease leading to the loss of around 870,000 lives in 2004. A large proportion of these deaths occurred prematurely, i.e. before average life expectancy [1]. Cardiovascular disease is also a rising cause of death in the developing world [2], [3], and in both the developed and undeveloped world the economic burden is said to be high and increasing [2], [3], [4]. Materialising in many forms, cardiovascular disease may be broadly classified into coronary heart disease, cerebrovascular disease and peripheral vascular disease, in which the blood supplies to the heart, the brain and the peripheral vasculature, respectively, are compromised. Common to each of these classifications is the formation of an atherosclerotic plaque or lesion (an atheroma), which can occlude small blood vessels and disrupt blood flow. This leads to acute manifestations such as myocardial infarction and stroke in which tissue oxygen and nutrient supply are severely compromised. A complication of atherosclerotic plaques is their vulnerability to rupture, giving rise to a thrombus which has the ability to occlude vessels away from the initial plaque site [5], [6], [7].

Although atherosclerosis was first considered to be a simple disease involving arterial lipid accumulation, it is now known to involve a defined cascade of inflammatory processes [6], [7], [8]. The initiating step in the development of an atherosclerotic lesion is damage to the endothelium [9], a monolayer of cells lining blood vessels which is a master regulator of vascular function. In a healthy individual and prior to the onset of cardiovascular disease the endothelium plays a homeostatic role in maintaining vascular tone and blood flow [9]. In the early stages of cardiovascular disease progression endothelial dysfunction triggers a chronic inflammatory process in the vessel wall. Induction of adhesion molecules in endothelial cells facilitates the attachment of monocytic leukocytes, trapping them and allowing their transmigration through the endothelial layer into the underlying intima. Here, they become tissue macrophages and subsequently foam cells following the uptake of oxidatively-modified lipids [7], [10]. Other cell types become involved in the enlargement of the atherosclerotic lesion, including vascular smooth muscle cells (VSMC). Whether already residing in the intimal layer or following their migration into this layer from the medial layer these cells begin proliferating to enhance plaque growth [7], [11]. Platelets are also recruited to the atherosclerotic lesion, again due to the exposure of chemical attractant molecules within the injured intimal layer [12], [13]. Altered vasoreactivity and enhanced propensity for clot formation, due to the loss of and an increase in anti- and pro-thrombotic factors respectively, contribute to atherothrombus formation and disease progression.

Since endothelial dysfunction and injury play a central role in atherosclerosis initiation, many studies have examined the cellular factors involved in abrogating endothelial function. Reactive oxygen species (ROS), also termed oxygen free radicals, are molecules containing unpaired electrons and include singlet oxygen, superoxide, peroxides, hydroxyl radicals and hypochlorous acid. ROS are an integral part of numerous physiological cellular signalling pathways in many cell types within the cardiovascular system and elsewhere. However, due to their unpaired shell electrons these entities are also highly reactive and damaging to cells. A substantial body of evidence now implicates oxygen free radicals in endothelial injury, dysfunction and cardiovascular disease progression; these have been reviewed extensively elsewhere [14], [15], [16], [17], [18]. In brief, oxidative stress occurs when an imbalance develops between the production of ROS and the efficacy of the cell's antioxidant defence, leading to an altered redox status which can contribute to endothelial dysfunction and/or cell death [19], [20]. The endothelium is not the sole cell type whose function is modified by oxidative stress. Many types of ROS have been implicated in the migration of smooth muscle cells into the intimal layer [11]. This may be facilitated by ROS-dependent alterations in the expression and activation of matrix metalloproteinases [21], [22], which degrade intimal extracellular matrices and promote smooth muscle migration. Proliferative activity of vascular smooth muscle is also ROS-regulated, although the exact type of ROS involved and the direction of the proliferative response (increased or decreased) are complex and unclear [11]. Monocytes/macrophages also possess the ability to produce ROS, which plays a role in lesion progression and inflammation and has been shown to cause oxidative modification of the low density lipoprotein [23], [24]. Platelets too can produce and be activated by superoxide and other radicals, promoting aggregation and thrombogenesis [13], [25], [26].

Given the large number of cell types and cellular processes in which oxidative stress plays a role in cardiovascular disease initiation and progression, the literature concerning the mechanisms underlying the contribution of oxidative stress is vast and outside the scope of this review. Detailed mechanistic reviews have been published elsewhere by others [e.g. [27], [28], [29]]. The aim of this current review is to provide an insight into novel tools and techniques with which to examine the role of oxidative stress in cardiovascular disease. We highlight recent advances in in vitro and in vivo model systems and the development of powerful tools which have the potential to better equip the cardiovascular biologist's armoury and aid the advancement of our understanding of oxidative stress in cardiovascular disease.