Mitochondrial Reactive Oxygen Species and Risk of Atherosclerosis

High levels of reactive oxygen species (ROS) are observed in chronic human diseases such as obesity, type 2 diabetes, atherosclerosis, and cardiovascular diseases. In addition to the presence of oxidative stress, these diseases are also characterized by deregulated inflammatory responses. Our first aim is to discuss distinct molecular pathways that determine the rate of mitochondrial ROS (mtROS) production and identify agents and enzymes that disrupt the balance between ROS generation and ROS elimination. Recent studies exploring the mechanisms linking ROS and inflammation found that ROS derived from mitochondria act as signal-transducing molecules that provoke endothelial dysfunction associated with uncoupling of nitric oxide synthase, induce the infiltration and activation of inflammatory cells, and increase apoptosis of endothelial and vascular smooth muscle cells. Therefore, our second aim is to give a comprehensive overview of the role of mtROS in all these processes contributing to atherosclerotic lesion progression and causing plaque erosion and rupture. Our third aim is to emphasize the role of the inflammatory toll-like receptor 2/NF-κB signaling pathway in the induction of pro-inflammatory cytokines and mtROS production in relation to insulin resistance, type 2 diabetes, and atherosclerosis. Because mtROS play an active role in several pathogenic mechanisms there is need for mitochondria-targeted antioxidants. Preliminary experiments in cell and animal models of cardiovascular diseases showed that some mitochondria-targeted antioxidants indeed reduce ROS production. However, wide-spread use in humans requires the development of specific and sensitive assays to evaluate mitochondrial oxidative stress and the development of orally active compounds.     

Oxidative Stress and Vascular Function: Implications for Pharmacologic Treatments

Production of considerable amounts of reactive oxygen species (ROS) eventually leads to oxidative stress. A key role of oxidative stress is evident in the pathologic mechanisms of endothelial dysfunction and associated cardiovascular diseases. Vascular enzymes such as NADPH oxidases, xanthine oxidase, and uncoupled endothelial nitric oxide synthase are involved in the production of ROS. The question remains whether pharmacologic approaches can effectively combat the excessive ROS production in the vasculature. Interestingly, existing registered cardiovascular drugs can directly or indirectly act as antioxidants, thereby preventing the damaging effects of ROS. Moreover, new compounds targeting NADPH oxidases have been developed. Finally, food-derived compounds appear to be effective inhibitors of oxidative stress and preserve vascular function.     

Molecular mechanisms of hypertension--reactive oxygen species and antioxidants: a basic science update for the clinician

Many factors have been implicated in the pathophysiology of hypertension such as upregulation of the renin-angiotensin-aldosterone system, activation of the sympathetic nervous system, perturbed G protein-coupled receptor signalling, inflammation, and altered T-cell function. Common to these processes is increased bioavailability of reactive oxygen species (ROS) (termed oxidative stress) due to excess ROS generation, decreased nitric oxide (NO) levels, and reduced antioxidant capacity in the cardiovascular, renal, and nervous systems. Although oxidative stress may not be the sole etiology of hypertension, it amplifies blood pressure elevation in the presence of other prohypertensive factors. In the cardiovascular system ROS play a physiological role in controlling endothelial function, vascular tone, and cardiac function, and a pathophysiological role in inflammation, hypertrophy, proliferation, apoptosis, migration, fibrosis, angiogenesis, and rarefaction, all of which are important processes contributing to endothelial dysfunction and cardiovascular remodelling in hypertension. A major source for cardiovascular ROS is a family of nonphagocytic nicotinamide adenine dinucleotide phosphate (NADPH) oxidases (Nox1, Nox2, Nox4, and Nox5). Other sources include mitochondrial enzymes, xanthine oxidase, and uncoupled NO synthase (NOS). Although convincing data from animal studies support a causative role for oxidative stress in the pathogenesis of hypertension, there is still no solid evidence that oxidative stress causes hypertension in humans. However, biomarkers of excess ROS are increased in patients with hypertension and oxidative damage is important in the molecular mechanisms associated with cardiovascular and renal injury in hypertension. Although clinical trials failed to show beneficial antihypertensive effects of antioxidants, strategies that combat oxidative stress by targeting Noxs in an isoform-specific manner may have therapeutic potential.     

Oxidative stress and hypertension

Mammalian cells are capable of generating metabolites of oxygen, referred to as reactive oxygen species (ROS) via the action of several enzymes. In vascular cells, ROS are predominantly produced by the NADPH oxidases, uncoupled nitric oxide synthase, xanthine oxidase and by mitochondrial sources. In hypertension, ROS production by these sources is increased, and this not only contributes to hypertension, but also causes vascular disease and dysfunction. ROS production in other organs, particularly the kidney and the centers within the brain, likely participate in blood pressure regulation. Despite the wealth of data supporting a role of ROS in hypertension and other cardiovascular diseases, treatment with commonly employed antioxidants have failed, and in some cases have proven harmful, prompting a reconsideration of the concept of oxidative stress. Within the cell, ROS are produced locally and have important signaling roles, such that scavenging of these species by exogenous antioxidants is difficult and could produce untoward effects. In this article, we consider these tissues and discuss potential new approaches to treatment of “oxidative stress”.     

Oxidative Stress and Cardiovascular Disease: Antioxidants and Unresolved Issues

Experimental and clinical studies suggest that oxidative stress contributes to the development and progression of cardiovascular disease. However, clinical trials with classic vitamin antioxidants failed to demonstrate any benefit in cardiovascular outcomes. Recent advances in our understanding of mechanisms involved in free radical generation reinstate that a more comprehensive approach targeting the prevention of reactive oxygen species (ROS) formation early in the disease process may prove beneficial. Experimental studies and reviews in oxidative stress were selected to provide a better understanding of the roles of the reactive species in the initiation and progression of cardiovascular disease (CVD). Clinical studies that evaluated the efficacy of several classes of antioxidants in CVD were included in the second part of this review to discuss future therapeutic guidelines based on currently available evidence. In conclusion, before a potential role for antioxidants in the treatment of CVD is eliminated, more carefully designed studies with classic as well as new antioxidants in well‐defined patient populations are warranted to provide a definitive answer.     

Insulin resistance and postprandial hyperglycemia the bad companions in natural history of diabetes: effects on health of vascular tree

In diabetic patients the incidence of cardiovascular diseases (CVD) is higher compared with those without diabetes. This elevated incidence may be due to an increased prevalence of established risk factors, such as obesity, dyslipidemia and hypertension. However, several other determinants must be considered. Attention must be paid to the role that specific factors strictly related to diabetes, insulin-resistance and post-prandial hyperglycemia, play in the etiopathogenesis of CVD, as for example atherosclerosis. This review acknowledges the incidence of diabetes on cardiovascular diseases and atherosclerosis from endothelial dysfunction to plaque destabilization, suggesting that insulin resistance and postprandial hyperglycemia should be considered keys in the generation of these worst diabetic cardiovascular outcomes. It finds in hyperglycemia the primum movens that mediates the cascade of vascular damaging events from the beginning of ROS formation to plaque rupture, through increased inflammation. It also adds insights of why diverse therapeutic interventions, which have in common the ability to reduce oxidative stress and inflammation, can impede or delay the onset of complication of atherosclerosis in diabetic patients.     

Nuclear Factor (Erythroid-Derived 2)-Like-2 Factor (Nrf2), a Key Regulator of the Antioxidant Response to Protect Against Atherosclerosis and Nonalcoholic Steatohepatitis

Tissue oxidative stress is a common hallmark of atherosclerosis and non-alcoholic steatohepatitis (NASH), 2 conditions linked epidemiologically and pathophysiologically. Nuclear factor (erythroid-derived 2)-like 2 (Nrf2) is the master regulator of inducible antioxidant responses, that can attenuate cellular injury from oxidative stress induced by obesity and other redox insults. Nrf2 expression and activation is reduced in mouse and human vessels that harbor accelerated atherosclerosis and in livers with histologic criteria of NASH. Systemic antioxidants have thus been attractive therapeutic targets, but clinical trials have been largely unsuccessful in improving cardiovascular health. Macrophage-selective Nrf2 activation may, however, provide an approach to reduce vascular and hepatocyte injury without the complications of systemic antioxidants, since macrophages play key roles in the development and progression of both atherosclerosis and NASH. In this article, we review the common mechanisms of oxidative stress and inflammation in atherosclerosis and NASH, and discuss the role of Nrf2 in vascular and hepatocyte protection.     

Prevention of diabetes-induced cardiovascular complications upon treatment with antioxidants

Oxidative stress is considered to play an important role in the pathogenesis of diabetes-induced cardiovascular disease (CVD), which is invariably associated with abnormal blood lipid profile, insulin resistance and metabolic syndrome. Stress, smoking, high saturated fat intake as well as low fruit and vegetable intakes have been shown to increase oxidative stress and hyperlipidemia, which increase the predisposition of diabetic subjects to atherosclerosis, stroke and coronary heart disease. The oxidation of low-density lipoprotein by oxidative stress is essential for the development of atherosclerosis, and the reduction in oxidative stress as well as blood glucose and cholesterol is considered critical for the prevention of diabetes-induced CVD. Although epidemiological studies have demonstrated that vitamin C and vitamin E decrease the incidence of coronary heart disease, different clinical trials have failed to support the beneficial effect of these antioxidants. Nonetheless, it has been suggested that natural forms of these vitamins may be more efficacious than synthetic vitamins, and this may explain the inconsistencies in results. Antioxidants, N-acetyl-l-cysteine and resveratrol, have also been shown to attenuate the diabetes-induced cardiovascular complications. It has been indicated that the antioxidant therapy may be effective in a prevention strategy rather than as a treatment for CVD. The evidence presented here supports the view that cardiovascular complications in diabetes may be induced by oxidative stress and appropriate antioxidant therapy may be promising for attenuating the progression of diabetes-induced CVD.     

Role of secreted oxidative stress-induced factors (SOXFs) in the pathogenesis of atherosclerosis

Oxidative stress, a state of excessive reactive oxygen species (ROS) activity, has been implicated in the pathogenesis of cardiovascular disease, including atherosclerosis, hypertension, and restenosis. Recently we have identified several SOXFs (Secreted OXidative stress-induced Factors) from vascular smooth muscle cells (VSMC). Our studies have demonstrated that SOXF function as redox mediators to regulate growth of VSMC, inflammation and apoptosis of endothelial cells, and are involved in the processes of vascular lesion formation after vascular injury and in apolipoprotein E deficient (ApoE-/-) mice. Understanding the mechanisms by which ROS stimulate secretion of SOXF and the role of SOXF in vascular cells should provide important insight into the cellular response to oxidative stress and new therapeutic targets for vascular diseases.     

Reactive oxygen species, vascular oxidative stress, and redox signaling in hypertension: what is the clinical significance?

Metabolism of oxygen by cells generates potentially deleterious reactive oxygen species (ROS). Under normal conditions the rate and magnitude of oxidant formation is balanced by the rate of oxidant elimination. However, an imbalance between prooxidants and antioxidants results in oxidative stress, which is the pathogenic outcome of oxidant overproduction that overwhelms the cellular antioxidant capacity. The kidney and vasculature are rich sources of NADPH oxidase-derived ROS, which under pathological conditions play an important role in renal dysfunction and vascular damage. Strong experimental evidence indicates that increased oxidative stress and associated oxidative damage are mediators of renovascular injury in cardiovascular pathologies. Increased production of superoxide anion and hydrogen peroxide, reduced nitric oxide synthesis, and decreased bioavailability of antioxidants have been demonstrated in experimental and human hypertension. These findings have evoked considerable interest because of the possibilities that therapies targeted against free radicals by decreasing ROS generation or by increasing nitric oxide availability and antioxidants may be useful in minimizing vascular injury and renal dysfunction and thereby prevent or regress hypertensive end-organ damage. This article highlights current developments in the field of ROS and hypertension, focusing specifically on the role of oxidative stress in hypertension-associated vascular damage. In addition, recent clinical trials investigating cardiovascular benefits of antioxidants are discussed, and some explanations for the rather disappointing results from these studies are addressed. Finally, important avenues for future research in the field of ROS, oxidative stress, and redox signaling in hypertension are considered.     

Effects of interventions on oxidative stress and inflammation of cardiovascular diseases

Excessive oxidative stress and low-grade chronic inflammation are major pathophysiological factors contributing to the development of cardiovascular diseases (CVD) such as hypertension, diabetes and atherosclerosis. Accumulating evidence suggests that a compromised antioxidant system can lead to excessive oxidative stress in cardiovascular related organs, resulting in cell damage and death. In addition, increased circulating levels of pro-inflammatory cytokines, such as tumor necrosis factor α, interleukin-6 and C-reactive protein, are closely related to morbidity and mortality of cardiovascular complications. Emerging evidence suggests that interventions including nutrition, pharmacology and exercise may activate expression of cellular anti-oxidant systems via the nuclear factor erythroid 2-related factor 2-Kelchlike ECH-associated protein 1 signaling pathway and play a role in preventing inflammatory processes in CVD. The focus of the present review is to summarize recent evidence showing the role of these anti-oxidant and anti-inflammatory interventions in cardiovascular disease. We believe that these findings may prompt new effective pathogenesis-oriented interventions, based on the exercise-induced protection from disease in the cardiovascular system, aimed at targeting oxidant stress and inflammation.     

Modulation of oxidant and antioxidant enzyme expression and function in vascular cells

Pathological conditions that predispose to cardiovascular events, such as hypertension, hypercholesterolemia, and diabetes, are associated with oxidative stress. These observations and further data derived from a plethora of investigations provided accumulating evidence that oxidative stress is decisively involved in the pathogenesis of endothelial dysfunction and atherosclerosis. Several enzymes expressed in vascular tissue contribute to production and efficient degradation of reactive oxygen species, and enhanced activity of oxidant enzymes and/or reduced activity of antioxidant enzymes may cause oxidative stress. Various agonists, pathological conditions, and therapeutic interventions lead to modulated expression and function of oxidant and antioxidant enzymes, including NAD(P)H oxidase, endothelial nitric oxide synthase, xanthine oxidase, myeloperoxidase, superoxide dismutases, catalase, thioredoxin reductase, and glutathione peroxidase. Data from numerous studies underline the importance of dysregulated oxidant and antioxidant enzymes for the development and progression of atherosclerotic disease in animal models and humans. Specific pharmacological modulation of key enzymes involved in the propagation of oxidative stress rather than using direct antioxidants may be an approach to reduce oxygen radical load in the vasculature and subsequent disease progression in humans. This review focuses on the modulation of expression and activity of major antioxidant and oxidant enzymes expressed in vascular cells.     

Role of oxidative stress in cardiovascular diseases

OBJECTIVES: In view of the critical role of intracellular Ca2 overload in the genesis of myocyte dysfunction and the ability of reactive oxygen species (ROS) to induce the intracellular Ca2+-overload, this article is concerned with analysis of the existing literature with respect to the role of oxidative stress in different types of cardiovascular diseases. OBSERVATIONS: Oxidative stress in cardiac and vascular myocytes describes the injury caused to cells resulting from increased formation of ROS and/or decreased antioxidant reserve. The increase in the generation of ROS seems to be due to impaired mitochondrial reduction of molecular oxygen, secretion of ROS by white blood cells, endothelial dysfunction, auto-oxidation of catecholamines, as well as exposure to radiation or air pollution. On the other hand, depression in the antioxidant reserve, which serves as a defense mechanism in cardiac and vascular myocytes, appears to be due to the exhaustion and/or changes in gene expression. The deleterious effects of ROS are mainly due to abilities of ROS to produce changes in subcellular organelles, and induce intracellular Ca2+-overload. Although the cause-effect relationship of oxidative stress with any of the cardiovascular diseases still remains to be established, increased formation of ROS indicating the presence of oxidative stress has been observed in a wide variety of experimental and clinical conditions. Furthermore, antioxidant therapy has been shown to exert beneficial effects in hypertension, atherosclerosis, ischemic heart disease, cardiomyopathies and congestive heart failure. CONCLUSIONS: The existing evidence support the view that oxidative stress may play a crucial role in cardiac and vascular abnormalities in different types of cardiovascular diseases and that the antioxidant therapy may prove beneficial in combating these problems.     

Roles of adiponectin and oxidative stress in obesity-associated metabolic and cardiovascular diseases

The recent increase in populations with obesity is a worldwide social problem, and the enhanced susceptibility of obese people to metabolic and cardiovascular diseases has become a growing health threat. An understanding of the molecular basis for obesity-associated disease development is required to prevent these diseases. Many studies have revealed that the mechanism involves various bioactive molecules that are released from adipose tissues and designated as adipocytokines/adipokines. Adiponectin is an adipocytokine that exerts insulin-sensitizing effects in the liver and skeletal muscle via adenosine monophosphate-activated protein kinase and proliferator-activated receptor α activation. Additionally, adiponectin can suppress atherosclerosis development in vascular walls via various anti-inflammatory effects. In contrast, oxidative stress is a harmful factor that systemically increases during obesity and promotes the development of diabetes, atherosclerosis, and various other diseases. In obese mice, oxidative stress is enhanced in adipose tissue before diabetes development, but not in the liver, skeletal muscle, and aorta, suggesting that in obesity, adipose tissue may be a major source of reactive oxygen species (ROS). ROS suppress adiponectin production in adipocytes. Treatment of obese mice with anti-oxidative agents improves insulin resistance and restores adiponectin production. Recent studies have demonstrated that adiponectin protects against oxidative stress-induced damage in the vascular endothelium and myocardium. Thus, decreased circulating adiponectin levels and increased oxidative stress, which are closely linked to each other, should be deeply involved in obesity-associated metabolic and cardiovascular disease pathogenesis.     

Oxidative stress and vascular damage in hypertension

Metabolism of oxygen by cells generates potentially deleterious reactive oxygen species, including superoxide anion radical, hydrogen peroxide, and hydroxyl radical. Under normal physiologic conditions the rate and magnitude o oxidant formation is balanced by the rate of oxidant elimination. However, an imbalance between prooxidants and antioxidants results in oxidative stress, which is the pathogenic outcome of the overproduction of oxidants that overwhelms the cellular antioxidant capacity. There is increasing evidence that an elevation of oxidative stress and associated oxidative damages are mediators of vascular injury in various cardiovascular pathologies, including hypertension, atherosclerosis, and ischemia-reperfusion. This review focuses on the vascular effects of reactive oxygen species and the role of oxidative stress in vascular damage in hypertension.     

The molecular sources of reactive oxygen species in hypertension

In both animal models and humans, increased blood pressure has been associated with oxidative stress in the vasculature, i.e. an excessive endothelial production of reactive oxygen species (ROS), which may be both a cause and an effect of hypertension. In addition to NADPH oxidase, the best characterized source of ROS, several other enzymes may contribute to ROS generation, including nitric oxide synthase, lipoxygenases, cyclo-oxygenases, xanthine oxidase and cytochrome P450 enzymes. It has been suggested that also mitochondria could be considered a major source of ROS: in situations of metabolic perturbation, increased mitochondrial ROS generation might trigger endothelial dysfunction, possibly contributing to the development of hypertension. However, the use of antioxidants in the clinical setting induced only limited effects on human hypertension or cardiovascular endpoints. More clinical studies are needed to fully elucidate this so called "oxidative paradox" of hypertension.     

Mitochondria and cardiovascular aging

Old age is a major risk factor for cardiovascular diseases. Several lines of evidence in experimental animal models have indicated the central role of mitochondria both in lifespan determination and in cardiovascular aging. In this article we review the evidence supporting the role of mitochondrial oxidative stress, mitochondrial damage and biogenesis as well as the crosstalk between mitochondria and cellular signaling in cardiac and vascular aging. Intrinsic cardiac aging in the murine model closely recapitulates age-related cardiac changes in humans (left ventricular hypertrophy, fibrosis and diastolic dysfunction), while the phenotype of vascular aging include endothelial dysfunction, reduced vascular elasticity, and chronic vascular inflammation. Both cardiac and vascular aging involve neurohormonal signaling (eg, renin-angiotensin, adrenergic, insulin-IGF1 signaling) and cell-autonomous mechanisms. The potential therapeutic strategies to improve mitochondrial function in aging and cardiovascular diseases are also discussed, with a focus on mitochondrial-targeted antioxidants, calorie restriction, calorie restriction mimetics, and exercise training.     

Endothelial dysfunction in hyperglycemia as a trigger of atherosclerosis

Type 2 diabetes is associated with a two to fourfold increased risk of both coronary heart disease and stroke. Dysfunction of endothelial cells (EC) is known to promote abnormal vascular growth such as that in atherosclerosis and arteriosclerosis and has been postulated as an initial trigger of the progression of atherosclerosis in patients with diabetes mellitus, and hyperglycemia is an independent risk factor for the development of cardiovascular disease. We and others have previously demonstrated that high D-glucose induced apoptosis through activation of the bax-caspase proteases pathway in human EC and the potential contribution of hepatocyte growth factor, as an anti-apoptotic factor, to the pathogenesis of endothelial dysfunction. The anti-apoptotic action of HGF was due to bcl-2-upregulation and the phosphatidylinositol 3-kinase pathway, which is involved in Akt activation. Although it has been known for years that cardiovascular tissues can release a large amount ROS, including superoxide, hydrogen peroxide, and nitric oxide, the role of oxidative stress in atherogenesis has received increasing attention in recent years. Recent work strongly suggests that NADPH oxidase is a major source of superoxide in cardiovascular cells, and oxidative stress can be involved in the process of endothelial dysfunction. NADPH oxidase can be activated in hyperglycemia through the protein kinase C pathway. From the viewpoint of these molecular mechanisms, HMG-CoA reductase inhibitors (statins) might inhibit the high glucose-induced NADPH oxidase activation through inhibition of Rac activity and finally prevent the increase in ROS production in diabetes. A recent clinical trial suggested that statins prevent several vascular events in patients with type 2 diabetes without a high concentration of LDL-cholesterol. These pleiotropic effects of statins can be expected to improve endothelial dysfunction through nitric oxide production and/or an anti-oxidant effect in diabetic patients.     

Oxidative enzymopathies and vascular disease

In the vasculature, reactive oxygen species (ROS) generated by both mitochondrial respiration and enzymatic sources serve as integral components of cellular signaling and homeostatic mechanisms. Because ROS are highly reactive biomolecules, the cellular redox milieu is carefully maintained by small-molecule antioxidants and antioxidant enzymes to prevent the deleterious consequences of ROS excess. When this redox balance is perturbed, because of either increased ROS production or decreased antioxidant capacity, oxidant stress is increased in the vessel wall and, if not offset, vascular dysfunction ensues. A number of heritable polymorphisms of pro-oxidant enzymes, including 5-lipoxygenase, cyclooxygenase-2, nitric oxide synthase-3, and NAD(P)H oxidase, have been identified and found to modulate ROS production and, thereby, the risk of atherothrombotic cardiovascular disease in individuals with these genetic polymorphisms. Similarly, heritable deficiency of the antioxidant enzymes catalase, glutathione peroxidases, glutathione-S-transferases, heme oxygenase, and glucose-6-phosphate dehydrogenase favors ROS accumulation, and has been associated with an increased risk of vascular disease. Individually, each of these polymorphisms imposes a state of uncompensated oxidant stress on the vasculature and collectively comprise the oxidative enzymopathies.     

The biological relevance and measurement of plasma markers of oxidative stress in diabetes and cardiovascular disease

Oxidative stress is associated with many chronic diseases. In this review, we look at the role that oxidative stress may play in diabetes and related cardiovascular disease (CVD) and how oxidative damage may be measured in the plasma. Increased production of reactive oxygen species (ROS) has been implicated in the initiation and progression of both of these conditions and it may be that oxidative stress accounts for the unexplained increase in cardiovascular risk observed in diabetes. Plasma measurements are difficult because of the highly reactive nature of these molecules. Several studies have focused on measuring the total antioxidant buffering capacity of plasma or alternatively specific measures of free radical-mediated damage such as F(2)-isoprostane or oxidised-LDL (Ox-LDL). Perhaps, in the future, the discovery of an 'easy to measure marker' of oxidative stress might be incorporated into risk prediction in diabetes and cardiovascular disease.