The role of peroxidases in the pathogenesis of atherosclerosis

Reactive oxygen species (ROS), which include superoxide anions and peroxides, induce oxidative stress, contributing to the initiation and progression of cardiovascular diseases involving atherosclerosis. The endogenous and exogenous factors hypercholesterolemia, hyperglycemia, hypertension, and shear stress induce various enzyme systems such as nicotinamide adenine dinucleotide (phosphate) oxidase, xanthine oxidase, and lipoxygenase in vascular and immune cells, which generate ROS. Besides inducing oxidative stress, ROS mediate signaling pathways involved in monocyte adhesion and infiltration, platelet activation, and smooth muscle cell migration. A number of antioxidant enzymes (e.g., superoxide dismutases, catalase, glutathione peroxidases, and peroxiredoxins) regulate ROS in vascular and immune cells. Atherosclerosis results from a local imbalance between ROS production and these antioxidant enzymes. In this review, we will discuss 1) oxidative stress and atherosclerosis, 2) ROS-dependent atherogenic signaling in endothelial cells, macrophages, and vascular smooth muscle cells, 3) roles of peroxidases in atherosclerosis, and 4) antioxidant drugs and therapeutic perspectives.     

Contribution of cytochrome P450 1B1 to hypertension and associated pathophysiology: a novel target for antihypertensive agents

The aim of this review is to discuss the contribution of cytochrome P450 (CYP) 1B1 in vascular smooth muscle cell growth, hypertension, and associated pathophysiology. CYP1B1 is expressed in cardiovascular and renal tissues, and mediates angiotensin II (Ang II)-induced activation of NADPH oxidase and generation of reactive oxygen species (ROS), and vascular smooth muscle cell migration, proliferation, and hypertrophy. Moreover, CYP1B1 contributes to the development and/or maintenance of hypertension produced by Ang II-, deoxycorticosterone (DOCA)-salt-, and N(ω)-nitro-L-arginine methyl ester-induced hypertension and in spontaneously hypertensive rats. The pathophysiological changes, including cardiovascular hypertrophy, increased vascular reactivity, endothelial and renal dysfunction, injury and inflammation associated with Ang II- and/or DOCA-salt induced hypertension in rats, and Ang II-induced hypertension in mice are minimized by inhibition of CYP1B1 activity with 2,4,3',5'-tetramethoxystilbene or by Cyp1b1 gene disruption in mice. These pathophysiological changes appear to be mediated by increased production of ROS via CYP1B1-dependent NADPH oxidase activity and extracellular signal-regulated kinase 1/2, p38 mitogen-activated protein kinase, and c-Src.     

Reactive oxygen species in vascular biology: implications in hypertension

Reactive oxygen species (ROS), including superoxide (·O₂^–), hydrogen peroxide (H₂O₂), and hydroxyl anion (OH-), and reactive nitrogen species, such as nitric oxide (NO) and peroxynitrite (ONOO^–), are biologically important O₂ derivatives that are increasingly recognized to be important in vascular biology through their oxidation/reduction (redox) potential. All vascular cell types (endothelial cells, vascular smooth muscle cells, and adventitial fibroblasts) produce ROS, primarily via cell membrane-associated NAD(P)H oxidase. Reactive oxygen species regulate vascular function by modulating cell growth, apoptosis/anoikis, migration, inflammation, secretion, and extracellular matrix protein production. An imbalance in redox state where pro-oxidants overwhelm anti-oxidant capacity results in oxidative stress. Oxidative stress and associated oxidative damage are mediators of vascular injury and inflammation in many cardiovascular diseases, including hypertension, hyperlipidemia, and diabetes. Increased generation of ROS has been demonstrated in experimental and human hypertension. Anti-oxidants and agents that interrupt NAD(P)H oxidase-driven ·O₂^– production regress vascular remodeling, improve endothelial function, reduce inflammation, and decrease blood pressure in hypertensive models. This experimental evidence has evoked considerable interest because of the possibilities that therapies targeted against reactive oxygen intermediates, by decreasing generation of ROS and/or by increasing availability of antioxidants, may be useful in minimizing vascular injury and hypertensive end organ damage. The present chapter focuses on the importance of ROS in vascular biology and discusses the role of oxidative stress in vascular damage in hypertension.     

Iron homeostasis and oxidative stress: An intimate relationship

Iron is a transition metal and essential constituent of almost all living cells and organisms. As component of various metalloproteins it is involved in critical biochemical processes such as transport of oxygen in tissues, electron transfer reactions during respiration in mitochondria, synthesis and repair of DNA, metabolism of xenobiotics, etc. However, when present in excess within cells and tissues, iron disrupts redox homeostasis and catalyzes the propagation of reactive oxygen species (ROS), leading to oxidative stress. ROS are critical for physiological signaling pathways, but oxidative stress is associated with tissue injury and disease. At the cellular level, oxidative stress may lead to ferroptosis, an iron-dependent form of cell death. In this review, we focus on the intimate relationship between iron metabolism and oxidative stress in health and disease. We discuss aspects of redox- and iron-mediated signaling, toxicity, ferroptotic cell death, homeostatic pathways and pathophysiological implications.     

Reactive oxygen species as mediators of angiogenesis signaling. Role of NAD(P)H oxidase

Angiogenesis, a process of new blood vessel growth, contributes to various pathophysiologies such as cancer, diabetic retinopathy and atherosclerosis. Accumulating evidence suggests that cardiovascular diseases are associated with increased oxidative stress in blood vessels. Reactive oxygen species (ROS) such as superoxide and H2O2 cause blood vessels to thicken, produce inflammation in the vessel wall, and thus are regarded as "risk factors" for vascular disease, whereas ROS also act as signaling molecules in many aspects of growth factor-mediated physiological responses. Recent reports suggest that ROS play an important role in angiogenesis; however, its underlying molecular mechanisms remain unknown. Vascular endothelial growth factor (VEGF) induces angiogenesis by stimulating endothelial cell (EC) proliferation and migration primarily through the receptor tyrosine kinase VEGF receptor2 (Flk1/KDR). VEGF binding initiates tyrosine phosphorylation of KDR, which results in activation of downstream signaling enzymes including ERK1/2, Akt and eNOS, which contribute to angiogenic-related responses in EC. Importantly, the major source of ROS in EC is a NAD(P)H oxidase and EC express all the components of phagocytic NAD(P)H oxidase including gp91phox, p22phox, p47phox, p67phox and the small G protein Rac1. We have recently demonstrated that ROS derived from NAD(P)H oxidase are critically important for VEGF signaling in vitro and angiogenesis in vivo. Furthermore, a peptide hormone, angiotensin II, a major stimulus for vascular NAD(P)H oxidase, also plays an important role in angiogenesis. Because EC migration and proliferation are primary features of the process of myocardial angiogenesis, we would like to focus on the recent progress that has been made in the emerging area of NAD(P)H oxidase-derived ROS-dependent signaling in ECs, and discuss the possible roles in angiogenesis. Understanding these mechanisms may provide insight into the components of NAD(P)H oxidase as potential therapeutic targets for treatment of angiogenesis-dependent diseases such as cancer and atherosclerosis and for promoting myocardial angiogenesis in ischemic heart diseases.     

Phosphoinositide 3-kinase signaling in the cellular response to oxidative stress

Oxidative stress is linked to the pathogenesis and pathobiochemistry of various diseases, including cancer, diabetes and cardiovascular disorders. The non-specific damaging effect of reactive oxygen species (ROS) generated during oxidative stress is involved in the development of diseases, as well as the activation of specific signaling cascades in cells exposed to the higher oxidant load. A cellular signaling cascade that is activated by several types of reactive oxygen species is the phosphoinositide 3'-kinase (PI 3-kinase)/protein kinase B (PKB) pathway, which regulates cellular survival and fuel metabolism, thus establishing a link between oxidative stress and signaling in neoplastic, metabolic or degenerative diseases. Several links of PI 3-kinase/PKB signaling to ROS are discussed in this review, with particular focus on the molecular mechanisms involved in the regulation of PI 3-kinase signaling by oxidative stress and important players such as (i) the glutathione and glutaredoxin system, (ii) the thioredoxin system and (iii) Ser/Thr- and Tyr phosphatases.     

Expression of NOX-I, gp91phox, p47phox and P67phox in the aorta segments above and below coarctation

Aorta coarctation results in hypertension (HTN) in the arterial tree proximal to stenosis and, as such, provides an ideal model to discern the effects of different levels of blood pressure on the vascular tissue in the same animal. Compelling evidence has emerged supporting the role of oxidative stress as a cause of HTN. However, whether or not HTN (independent of the circulating humoral factors) can cause oxidative stress is less certain. NAD(P)H oxidase isoforms are the main source of reactive oxygen species (ROS) in the vascular tissues. We therefore compared the expressions of NOX-I, gp91phox and the regulatory subunits of the enzyme in the aorta segments residing above and below coarctation in rats with abdominal aorta banding. Rats were studied 4 weeks after aorta banding above the renal arteries or sham operation. Subunits of NAD(P)H oxidase and its NOX-I isoform as well as endothelial NO synthase (eNOS) and nitrotyrosine (footprint of NO oxidation by superoxide) were measured in the aorta segments above and below coarctation. The gp91phox, p47phox, and p67phox subunits of NAD(P)H oxidase, NOX-I isoform, eNOS and nitrotyrosine were markedly increased in the aorta segment above coarctation (hypertensive zone), but were virtually unchanged in the segment below coarctation. Since, excepting blood pressure, all other conditions were constant, the upregulation of NAD(P)H oxidase isoforms and the increased NO oxidation in the aorta segment above, but not below, coarctation prove that HTN, per se, independent of circulating mediators can cause oxidative/nitrosative stress in the arterial wall. These observations suggest that HTN control may represent a specific form of antioxidant therapy for hypertensive disorders.     

Involvement of Kv1.5 Protein in Oxidative Vascular Endothelial Cell Injury

Endothelial injury related to oxidative stress is a key event in cardiovascular diseases, such as hypertension and atherosclerosis. The activation of the redox-sensitive Kv1.5 potassium channel mediates mitochondrial reactive oxygen species (ROS)-induced apoptosis in vascular smooth muscle cells and some cancer cells. Kv1.5 channel is therefore taken as a new potential therapeutic target for pulmonary hypertension and cancers. Although Kv1.5 is abundantly expressed in vascular endothelium, there is little knowledge of its role in endothelial injury related to oxidative stress. We found that DPO-1, a specific inhibitor of Kv1.5, attenuated H₂O₂-evoked endothelial cell apoptosis in an in vivo rat carotid arterial model. In human umbilical vein endothelial cells (HUVECs) and human pulmonary arterial endothelial cells (HPAECs), angiotensin II and oxLDL time- or concentration-dependently enhanced Kv1.5 protein expression in parallel with the production of intracellular ROS and endothelial cell injury. Moreover, siRNA-mediated knockdown of Kv1.5 attenuated, whereas adenovirus-mediated Kv1.5 cDNA overexpression enhanced oxLDL–induced cellular damage, NADPH oxidase and mitochondria-derived ROS production and restored the decrease in protein expression of mitochondria uncoupling protein 2 (UCP2). Collectively, these data suggest that Kv1.5 may play an important role in oxidative vascular endothelial injury.     

Adverse effects of the classic antioxidant uric acid in adipocytes: NADPH oxidase-mediated oxidative/nitrosative stress

Uric acid is considered a major antioxidant in human blood that may protect against aging and oxidative stress. Despite its proposed protective properties, elevated levels of uric acid are commonly associated with increased risk for cardiovascular disease and mortality. Furthermore, recent experimental studies suggest that uric acid may have a causal role in hypertension and metabolic syndrome. All these conditions are thought to be mediated by oxidative stress. In this study we demonstrate that differentiation of cultured mouse adipocytes is associated with increased production of reactive oxygen species (ROS) and uptake of uric acid. Soluble uric acid stimulated an increase in NADPH oxidase activity and ROS production in mature adipocytes but not in preadipocytes. The stimulation of NADPH oxidase-dependent ROS by uric acid resulted in activation of MAP kinases p38 and ERK1/2, a decrease in nitric oxide bioavailability, and an increase in protein nitrosylation and lipid oxidation. Collectively, our results suggest that hyperuricemia induces redox-dependent signaling and oxidative stress in adipocytes. Since oxidative stress in the adipose tissue has recently been recognized as a major cause of insulin resistance and cardiovascular disease, hyperuricemia-induced alterations in oxidative homeostasis in the adipose tissue might play an important role in these derangements.     

Janus-faced role of endothelial NO synthase in vascular disease: uncoupling of oxygen reduction from NO synthesis and its pharmacological reversal

Endothelial NO synthase (eNOS) is the predominant enzyme responsible for vascular NO synthesis. A functional eNOS transfers electrons from NADPH to its heme center, where L-arginine is oxidized to L-citrulline and NO. Common conditions predisposing to atherosclerosis, such as hypertension, hypercholesterolemia, diabetes mellitus and smoking, are associated with enhanced production of reactive oxygen species (ROS) and reduced amounts of bioactive NO in the vessel wall. NADPH oxidases represent major sources of ROS in cardiovascular pathophysiology. NADPH oxidase-derived superoxide avidly interacts with eNOS-derived NO to form peroxynitrite (ONOO(-)), which oxidizes the essential NOS cofactor (6R-)5,6,7,8-tetrahydrobiopterin (BH(4)). As a consequence, oxygen reduction uncouples from NO synthesis, thereby rendering NOS to a superoxide-producing pro-atherosclerotic enzyme. Supplementation with BH(4) corrects eNOS dysfunction in several animal models and in patients. Administration of high local doses of the antioxidant L-ascorbic acid (vitamin C) improves endothelial function, whereas large-scale clinical trials do not support a strong role for oral vitamin C and/or E in reducing cardiovascular disease. Statins, angiotensin-converting enzyme inhibitors and AT1 receptor blockers have the potential of reducing vascular oxidative stress. Finally, novel approaches are being tested to block pathways leading to oxidative stress (e.g. protein kinase C) or to upregulate antioxidant enzymes.     

Vascular oxidant stress: Molecular mechanisms and pathophysiological implications

The term oxidative stress refers to a situation in which cells are exposed to excessive levels of either molecular oxygen or chemical derivatives of oxygen (ie, reactive oxygen species). Three enzyme systems produce reactive oxygen species in the vascular wall: NADH/NADPH oxidase, xanthine oxidoreductase, and endothelial nitric oxide synthase. Among vascular reactive oxygen species superoxide anion plays a critical role in vascular biology because it is the source for many other reactive oxygen species and various vascular cell functions. It is currently thought that increases in oxidant stress, namely excessive production of superoxide anion, are involved in the pathophysiology of endothelial dysfunction that accompanies a number of cardiovascular risk factors including hypercholesterolemia, hypertension and cigarette smoking. On the other hand, vascular oxidant stress plays a pivotal role in the evolution of clinical conditions such as atherosclerosis, diabetes and heart failure.     

Cadmium exposure induces vascular injury due to endothelial oxidative stress: the role of local angiotensin II and COX-2

Cadmium is an environmental pollutant that is closely linked with cardiovascular diseases, such as atherosclerosis and hypertension. Moreover, cadmium can induce an increase in oxidative stress. One of the main sites affected by oxidative stress is the aorta, which consequently develops atherosclerosis. However, there are few reports demonstrating aortic effects induced by small concentrations of cadmium that are similar to those found in the blood resulting from occupational exposure. Furthermore, several studies have reported on chronic cadmium exposure, and the results of these studies may have been influenced by the secondary effects induced by this metal, such as hypertension. Therefore, we investigated the effects of acute cadmium exposure on the vascular reactivity to phenylephrine of aortic rings isolated from male Wistar rats. Cadmium increased phenylephrine reactivity without changing the vasorelaxation induced by acetylcholine and sodium nitroprusside. Endothelial damage or incubation with L-NAME shifted the phenylephrine concentration–response curves leftward in arteries incubated with or without cadmium, but the curves were shifted to a lesser degree after cadmium incubation. Enalapril, losartan, the nonselective COX inhibitor indomethacin, the TXA(2) synthase inhibitor furegrelate, the selective COX-2 inhibitor NS 398, the TP receptor antagonist SQ 29.548, the EP1 receptor antagonist SC 19.220, superoxide dismutase, and the NADPH oxidase inhibitor apocynin partially reverted the cadmium-induced effects on the reactivity to phenylephrine. Cadmium exposure increased vasoconstrictor activity by reducing NO bioavailability owing to the increased production of ROS by NADPH oxidase. The results of the tested cadmium concentration, which is below the reference values, suggest that acute cadmium exposure may induce vascular injury through endothelial oxidative stress. These data contribute to the evidence indicating that cadmium is a high risk to public health.     

Reactive oxygen species as mediators of angiogenesis signaling. Role of NAD(P)H oxidase

Angiogenesis, a process of new blood vessel growth, contributes to various pathophysiologies such as cancer, diabetic retinopathy and atherosclerosis. Accumulating evidence suggests that cardiovascular diseases are associated with increased oxidative stress in blood vessels. Reactive oxygen species (ROS) such as superoxide and H₂O₂ cause blood vessels to thicken, produce inflammation in the vessel wall, and thus are regarded as “risk factors” for vascular disease, whereas ROS also act as signaling molecules in many aspects of growth factor-mediated physiological responses. Recent reports suggest that ROS play an important role in angiogenesis; however, its underlying molecular mechanisms remain unknown. Vascular endothelial growth factor (VEGF) induces angiogenesis by stimulating endothelial cell (EC) proliferation and migration primarily through the receptor tyrosine kinase VEGF receptor2 (Flk1/KDR). VEGF binding initiates tyrosine phosphorylation of KDR, which results in activation of downstream signaling enzymes including ERK1/2, Akt and eNOS, which contribute to angiogenic-related responses in EC. Importantly, the major source of ROS in EC is a NAD(P)H oxidase and EC express all the components of phagocytic NAD(P)H oxidase including gp91phox, p22phox, p47phox, p67phox and the small G protein Rac1. We have recently demonstrated that ROS derived from NAD(P)H oxidase are critically important for VEGF signaling in vitro and angiogenesis in vivo. Furthermore, a peptide hormone, angiotensin II, a major stimulus for vascular NAD(P)H oxidase, also plays an important role in angiogenesis. Because EC migration and proliferation are primary features of the process of myocardial angiogenesis, we would like to focus on the recent progress that has been made in the emerging area of NAD(P)H oxidase-derived ROS-dependent signaling in ECs, and discuss the possible roles in angiogenesis. Understanding these mechanisms may provide insight into the components of NAD(P)H oxidase as potential therapeutic targets for treatment of angiogenesis-dependent diseases such as cancer and atherosclerosis and for promoting myocardial angiogenesis in ischemic heart diseases. (Mol Cell Biochem 264: 85–97, 2004)     

Cross talk between mitochondria and NADPH oxidases

Reactive oxygen species (ROS) play an important role in physiological and pathological processes. In recent years, a feed-forward regulation of the ROS sources has been reported. The interactions between the main cellular sources of ROS, such as mitochondria and NADPH oxidases, however, remain obscure. This work summarizes the latest findings on the role of cross talk between mitochondria and NADPH oxidases in pathophysiological processes. Mitochondria have the highest levels of antioxidants in the cell and play an important role in the maintenance of cellular redox status, thereby acting as an ROS and redox sink and limiting NADPH oxidase activity. Mitochondria, however, are not only a target for ROS produced by NADPH oxidase but also a significant source of ROS, which under certain conditions may stimulate NADPH oxidases. This cross talk between mitochondria and NADPH oxidases, therefore, may represent a feed-forward vicious cycle of ROS production, which can be pharmacologically targeted under conditions of oxidative stress. It has been demonstrated that mitochondria-targeted antioxidants break this vicious cycle, inhibiting ROS production by mitochondria and reducing NADPH oxidase activity. This may provide a novel strategy for treatment of many pathological conditions including aging, atherosclerosis, diabetes, hypertension, and degenerative neurological disorders in which mitochondrial oxidative stress seems to play a role. It is conceivable that the use of mitochondria-targeted treatments would be effective in these conditions.     

Regulation of smooth muscle by inducible nitric oxide synthase and NADPH oxidase in vascular proliferative diseases

Inflammation plays a critical role in promoting smooth muscle migration and proliferation during vascular diseases such as postangioplasty restenosis and atherosclerosis. Another common feature of many vascular diseases is the contribution of reactive oxygen (ROS) and reactive nitrogen (RNS) species to vascular injury. Primary sources of ROS and RNS in smooth muscle are several isoforms of NADPH oxidase (Nox) and the cytokine-regulated inducible nitric oxide (NO) synthase (iNOS). One important example of the interaction between NO and ROS is the reaction of NO with superoxide to yield peroxynitrite, which may contribute to the pathogenesis of hypertension. In this review, we discuss the literature that supports an alternate possibility: Nox-derived ROS modulate NO bioavailability by altering the expression of iNOS. We highlight data showing coexpression of iNOS and Nox in vascular smooth muscle demonstrating the functional consequences of iNOS and Nox during vascular injury. We describe the relevant literature demonstrating that the mitogen-activated protein kinases are important modulators of proinflammatory cytokine-dependent expression of iNOS. A central hypothesis discussed is that ROS-dependent regulation of the serine/threonine kinase protein kinase Cdelta is essential to understanding how Nox may regulate signaling pathways leading to iNOS expression. Overall, the integration of nonphagocytic NADPH oxidase with cytokine signaling in general and in vascular smooth muscle in particular is poorly understood and merits further investigation.     

Prenatal lipopolysaccharide exposure causes mesenteric vascular dysfunction through the nitric oxide and cyclic guanosine monophosphate pathway in offspring

Cardiovascular diseases, such as hypertension, could be programmed in fetal life. Prenatal lipopolysaccharide (LPS) exposure in utero results in increased blood pressure in offspring, but the vascular mechanisms involved are unclear. Pregnant Sprague–Dawley rats were intraperitoneally injected with LPS (0.79 mg/kg) or saline (0.5 ml) on gestation days 8, 10, and 12. The offspring of LPS-treated dams had higher blood pressure and decreased acetylcholine (ACh)-induced relaxation and increased phenylephrine (PE)-induced contraction in endothelium-intact mesenteric arteries. Endothelium removal significantly enhanced the PE-induced contraction in offspring of control but not LPS-treated dams. The arteries pretreated with l-NAME to inhibit nitric oxide synthase (eNOS) in the endothelium or ODQ to inhibit cGMP production in the vascular smooth muscle had attenuated ACh-induced relaxation but augmented PE-induced contraction to a larger extent in arteries from offspring of control than those from LPS-treated dams. In addition, the endothelium-independent relaxation caused by sodium nitroprusside was also decreased in arteries from offspring of LPS-treated dams. The functional results were accompanied by a reduction in the expressions of eNOS and soluble guanylate cyclase (sGC) and production of NO and cGMP in arteries from offspring of LPS-treated dams. Furthermore, LPS-treated dam’s offspring arteries had increased oxidative stress and decreased antioxidant capacity. Three-week treatment with TEMPOL, a reactive oxygen species (ROS) scavenger, normalized the alterations in the levels of ROS, eNOS, and sGC, as well as in the production of NO and cGMP and vascular function in the arteries of the offspring of LPS-treated dams. In conclusion, prenatal LPS exposure programs vascular dysfunction of mesenteric arteries through increased oxidative stress and impaired NO–cGMP signaling pathway.     

Differential involvement of various sources of reactive oxygen species in thyroxin-induced hemodynamic changes and contractile dysfunction of the heart and diaphragm muscles

Thyroid hormones are key regulators of basal metabolic state and oxidative metabolism. Hyperthyroidism has been reported to cause significant alterations in hemodynamics, and in cardiac and diaphragm muscle functions, all of which have been linked to increased oxidative stress. However, the definite source of increased reactive oxygen species (ROS) in each of these phenotypes is still unknown. The goal of the current study was to test the hypothesis that thyroxin (T4) may produce distinct hemodynamic, cardiac, and diaphragm muscle abnormalities by differentially affecting various sources of ROS. Wild-type and T4 mice with and without 2-week treatments with allopurinol (xanthine oxidase inhibitor), apocynin (NADPH oxidase inhibitor), L-NIO (nitric oxide synthase inhibitor), or MitoTEMPO (mitochondria-targeted antioxidant) were studied. Blood pressure and echocardiography were noninvasively evaluated, followed by ex vivo assessments of isolated heart and diaphragm muscle functions. Treatment with L-NIO attenuated the T4-induced hypertension in mice. However, apocynin improved the left-ventricular (LV) dysfunction without preventing the cardiac hypertrophy in these mice. Both allopurinol and MitoTEMPO reduced the T4-induced fatigability of the diaphragm muscles. In conclusion, we show here for the first time that T4 exerts differential effects on various sources of ROS to induce distinct cardiovascular and skeletal muscle phenotypes. Additionally, we find that T4-induced LV dysfunction is independent of cardiac hypertrophy and NADPH oxidase is a key player in this process. Furthermore, we prove the significance of both xanthine oxidase and mitochondrial ROS pathways in T4-induced fatigability of diaphragm muscles. Finally, we confirm the importance of the nitric oxide pathway in T4-induced hypertension.     

Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling

Reactive oxygen species (ROS) are generated during mitochondrial oxidative metabolism as well as in cellular response to xenobiotics, cytokines, and bacterial invasion. Oxidative stress refers to the imbalance due to excess ROS or oxidants over the capability of the cell to mount an effective antioxidant response. Oxidative stress results in macromolecular damage and is implicated in various disease states such as atherosclerosis, diabetes, cancer, neurodegeneration, and aging. Paradoxically, accumulating evidence indicates that ROS also serve as critical signaling molecules in cell proliferation and survival. While there is a large body of research demonstrating the general effect of oxidative stress on signaling pathways, less is known about the initial and direct regulation of signaling molecules by ROS, or what we term the "oxidative interface." Cellular ROS sensing and metabolism are tightly regulated by a variety of proteins involved in the redox (reduction/oxidation) mechanism. This review focuses on the molecular mechanisms through which ROS directly interact with critical signaling molecules to initiate signaling in a broad variety of cellular processes, such as proliferation and survival (MAP kinases, PI3 kinase, PTEN, and protein tyrosine phosphatases), ROS homeostasis and antioxidant gene regulation (thioredoxin, peroxiredoxin, Ref-1, and Nrf-2), mitochondrial oxidative stress, apoptosis, and aging (p66Shc), iron homeostasis through iron-sulfur cluster proteins (IRE-IRP), and ATM-regulated DNA damage response.