Hyperhomocysteinemia, endothelial dysfunction, and cardiovascular risk: the potential role of ADMA

Hyperhomocysteinemia is an emerging risk factor for cardiovascular disease and stroke. The mechanisms underlying the pathophysiology of hyperhomocysteinemia are not completely defined, but endothelial dysfunction resulting from impaired bioavailability of nitric oxide is a consistent finding in experimental models. One potential mechanism for decreased nitric oxide bioavailability is inhibition of endothelial nitric oxide synthase by its endogenous inhibitor, asymmetric dimethylarginine (ADMA). Elevated plasma levels of ADMA have been found in association with hyperhomocysteinemia and endothelial dysfunction in both animals and humans. Additional studies are required to determine the mechanisms by which ADMA accumulates in hyperhomocysteinemia and to define the importance of ADMA in the endothelial dysfunction of hyperhomocysteinemia in vivo.     

Fluvastatin ameliorates the hyperhomocysteinemia-induced endothelial dysfunction: the antioxidative properties of fluvastatin.

BACKGROUND: Hyperhomocysteinemia induces vascular endothelial dysfunction, contributing to a predisposition to the onset and/or progression of atherosclerosis. The major mechanism suggested for the adverse effect of homocysteine on vascular function seems to involve oxidative stress. Thus, we hypothesized that the administration of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitor fluvastatin, which is experimentally demonstrated to have antioxidative properties as one of its pleiotropic effects, is a useful strategy for eliminating the detrimental events induced by hyperhomocysteinemia. METHODS AND RESULTS: In diet-induced hyperhomocysteinemic rats, we estimated oxidative stress and assessed endothelium-dependent vasodilatation. Hyperhomocysteinemia induced significant increases in urinary 8-isoprostaglandin F2alpha-III excretion and vascular superoxide generation, and impaired endothelium-dependent vasodilatation. Additional oral administration of the antioxidant fluvastatin or vitamin E, which normalized increased oxidative stress induced by hyperhomocysteinemia, ameliorated endothelial dysfunction. CONCLUSIONS: Hyperhomocysteinemia, even mild to moderate, induces endothelial dysfunction through its oxidative effect. The antioxidant fluvastatin was able to cancel out the oxidative stress induced by hyperhomocysteinemia and ameliorate endothelial dysfunction. Clinical use of fluvastatin might be a potent strategy for eliminating the detrimental events induced by hyperhomocysteinemia as well as hyperlipidemia. In addition to lowering homocysteine by means of folate supplementation, administration of the antioxidants is expected to be a potentially effective anti-homocysteine therapy.     

ADMA and hyperhomocysteinemia

Hyperhomocysteinemia is a risk factor for cardiovascular disease and stroke. Like many other cardiovascular risk factors, hyperhomocysteinemia produces endothelial dysfunction due to impaired bioavailability of endothelium-derived nitric oxide (NO). The molecular mechanisms responsible for decreased NO bioavailability in hyperhomocysteinemia are incompletely understood, but emerging evidence suggests that asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, may be a key mediator. Homocysteine is produced during the synthesis of ADMA and can alter ADMA metabolism by inhibiting dimethylarginine dimethylaminohydrolase (DDAH). Several animal and clinical studies have demonstrated a strong association between plasma total homocysteine, plasma ADMA, and endothelial dysfunction. These observations suggest a model in which elevation of ADMA may be a unifying mechanism for endothelial dysfunction during hyperhomocysteinemia. The recent development of transgenic mice with altered ADMA metabolism should provide further mechanistic insights into the role of ADMA in hyperhomocysteinemia.     

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.     

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.     

Hyperhomocysteinemia, vascular pathology, and endothelial dysfunction

Hyperhomocysteinemia has been associated with premature atherothrombotic vascular disease. It is not known whether hyperhomocysteinemia induces a distinct type of vascular disease. Its interaction, if any, with traditional risk factors also remains unclear. The pathophysiological mechanisms linking hyperhomocysteinemia to vascular disease have been extensively studied in vitro and in animals. From these studies, it has been suggested that homocysteine limits the bioavailability of nitric oxide (NO), increases oxidative stress, stimulates smooth cell proliferation, and alters elastic wall properties. The relevance of these proposed mechanisms in vivo is unclear, because clinical studies have yielded controversial results with regard to the relation between plasma homocysteine levels and indices of endothelial function, such as brachial artery flow-mediated vasodilatation and plasma levels of endothelium-derived marker proteins. Up till now, there have been no controlled data on the effects of homocysteine-lowering treatment on vascular function or clinical end points. The precise mechanisms (if any) by which homocysteine mediates its adverse vascular effects are in fact unknown but may relate to impaired endothelial and smooth muscle cell function.     

Homocysteine, a risk factor for cardiovascular disease

Fasting hyperhomocysteinemia is an independent risk factor for coronary artery disease, stroke, peripheral vascular atherosclerosis, and for arterial and venous thromboembolism. The risk for cardiovascular disease with homocysteine is similar to conventional risk factors. The interaction of hyperhomocysteinemia with hypertension and smoking is strong and the combined effect is more than multiplicative. The combined effect of homocysteine and cholesterol is additive. Homocysteine produces atherosclerosis, thromboembolism, and vascular endothelial cell injury. Vascular dysfunction produced by homocysteine may be due to endothelial cell damage. Homocysteinemia-induced atherosclerosis is probably due to various factors including endothelial cell injury, inability to sustain S-nitroso-homocysteine formation because of imbalance between production of nitric oxide by dysfunctional endothelium and homocysteine, smooth muscle cell proliferation, and thromboembolism. There is strong evidence that endothelial cell injury is associated with oxidative stress produced by homocysteine. Hyperhomocysteinemia is associated with numerous conditions, including coronary disease, stroke, peripheral vascular disease (carotid artery and cerebrovascular atherosclerosis), venous thrombosis, renal disease, diabetes mellitus, and organ transplant. Folic acid, vitamin B₁₂ and B₆ have been shown to be beneficial in reducing plasma homocysteine levels. Folic acid is specifically very effective, safe and inexpensive.     

Enhanced susceptibility to arterial thrombosis in a murine model of hyperhomocysteinemia

Hyperhomocysteinemia is a risk factor for thrombosis, but the mechanisms are not well defined. We tested the hypothesis that hyperhomocysteinemia accelerates arterial thrombosis in mice. Mice heterozygous for a targeted disruption of the cystathionine beta-synthase gene (Cbs+/-) and wild-type littermates (Cbs+/+) were fed either a control diet or a high methionine/low folate (HM/LF) diet for 6 to 8 months to produce graded hyperhomocysteinemia. The time to occlusion of the carotid artery after photochemical injury was shortened by more than 50% in Cbs+/+ or Cbs+/- mice fed the HM/LF diet (P < .001 versus control diet). Carotid artery thrombosis was not accelerated in mice deficient in endothelial nitric oxide synthase (Nos3), which suggests that decreased endothelium-derived nitric oxide is not a sufficient mechanism for enhancement of thrombosis. Cbs+/+ and Cbs+/- mice fed the HM/LF diet had elevated levels of reactive oxygen species in the carotid artery, increased aortic expression of the NADPH oxidase catalytic subunit, Nox4, and decreased activation of anticoagulant protein C in the aorta (P < .05 versus control diet). We conclude that hyperhomocysteinemia enhances susceptibility to arterial thrombosis through a mechanism that is not caused by loss of endothelium-derived nitric oxide but may involve oxidative stress and impairment of the protein C anticoagulant pathway.     

ADMA and oxidative stress

Elevated plasma concentrations of the endogenous nitric oxide synthase (eNOS) inhibitor asymmetric dimethylarginine (ADMA) represent a novel risk factor for the development of endothelial dysfunction and a predictor for all-cause and cardiovascular mortality. However, it is unknown whether elevated ADMA plasma concentrations may be considered simply as a marker for cardiovascular disease or whether increased ADMA levels per se may predispose to the development of vascular disease. There is experimental and clinical evidence linking endothelial dysfunction to increased production of oxygen-derived free radicals such as superoxide anion. Oxidative stress has been shown to increase the activity of arginine methylating and ADMA degrading enzymes leading to increased ADMA concentrations. Interestingly, the endothelial nitric oxide synthase may become uncoupled in the presence of high ADMA levels further contributing to the vascular oxidative stress burden. It remains to be established to what extent ADMA is able to interact with eNOS in vivo. Possible mechanisms underlying increased oxidative stress in the setting of elevated ADMA concentrations and therapeutic implications will be discussed.     

Tetrahydrobiopterin: regulator of endothelial nitric oxide synthase in vascular disease

In vascular disease states such as atherosclerosis and diabetes, endothelial nitric oxide (NO) bioactivity is reduced and oxidative stress is increased, resulting in endothelial dysfunction. Recent studies suggest that changes in the activity and regulation of endothelial NO synthase by its cofactor tetrahydrobiopterin (BH4) is an important contributor to endothelial dysfunction. Pharmacologic studies and more recent insights from genetically modified mouse models have improved the understanding of the mechanistic role and importance of BH4 in vascular disease pathogenesis. Targeting BH4 may provide new therapeutic strategies in vascular disease.     

Might erectile dysfunction be due to the thermolabile variant of methylenetetrahydrofolate reductase?

Hyperhomocysteinemia is considered one of the most important cardiovascular risk factors increasing considerably the risk of stroke and myocardial infarction. With respect to endothelial function, direct effects of hyperhomocysteinemia on vascular endothelial cells have been demonstrated through the reduction of endothelial nitric oxide production. In this paper, we report the case of a young man with homozygote genotype mutated with 5-methylenetetrahydrofolate reductase (MTHFR) thermolabile variant who, in the absence of relational stress, developed an erectile dysfunction (ED) refractory to the vasoactive type-V phosphodiesterase (PDE5) inhibitor therapy. After one month of treatment with 5 mg/day folic acid and 1000 microg/day cyanocobalamin, the patient restarted the assumption of 50 mg sildenafil, obtaining satisfying erections during sexual intercourse. We suggest that hyperhomocysteinemia may interfere with penile blood supply and, thus, be responsible for ED. If this relationship is confirmed, plasma levels and urinary homocysteine (HCy) should be evaluated in selected young patients with vascular ED. Furthermore, careful attention should be given to the risk of ED when dealing with this metabolic disturbance.     

高尿酸血症与内皮功能障碍

高尿酸血症是常见的与代谢综合征相关的疾病,其与内皮细胞功能障碍的发生密切相关.尿酸既可以通过增强细胞内氧化应激、上调内皮细胞内的Ras-丝裂原活化蛋白激酶信号转导通路、引起线粒体钙离子显著超负荷和活性氧簇产生增加、降低一氧化氮和内皮型一氧化氮合酶(eNOS)的产生等直接损害血管内皮细胞的功能,又能够通过其常伴随的低脂联素血症、高瘦素血症和与高尿酸血症互为相关的胰岛素抵抗等共同影响磷脂酰肌醇3激酶-蛋白激酶B-eNOS和AMP活化蛋白激酶-eNOS等信号通路,间接损害内皮功能.虽然目前对于无症状高尿酸血症是否应该给予临床干预尚存在争议,但许多证据支持高尿酸血症可损害内皮功能,应当适当时机给无症状高尿酸血症患者予以干预.     

Secondary endothelial dysfunction: hypertension and heart failure

The endothelium is a major regulator of vascular tone, releasing vasoactive substances such as endothelium-derived nitric oxide (EDRF), endothelium-derived hyperpolarizing factor(s), cycloxygenase metabolites, endothelin and other endothelium-derived contracting factors (EDCF). In a number of cardiovascular pathologies, such as hypertension or heart failure, the balance in the endothelial production of vasodilating and vasoconstricting mediators is altered. The resulting apparent decrease in endothelium-dependent relaxations is termed 'endothelial dysfunction'. In hypertensive patients and in animal models of hypertension, endothelium-dependent relaxations are impaired. However, this endothelial dysfunction presents different characteristics depending on the model studied. In Dahl-salt-sensitive rats, the decrease in endothelium-dependent relaxations is associated with impaired constitutive nitric oxide synthase activity. The presence of an endogenous nitric oxide synthase inhibitor and a decreased response of vascular smooth muscle to the mediator may contribute also to the dysfunction observed in this model. In other animal models of hypertension (such as spontaneous hypertension). the contribution of the L-arginine nitric oxide pathway to endothelium-dependent responses appears normal or impaired despite reports of increased nitric oxide synthase activity or expression. In large arteries from SHR, endothelium-dependent relaxations are impaired mainly because of the concomitant augmented release of endoperoxides activating thromboxane-endoperoxide receptors. Superoxide anions may also play a role in some models, but only in the early phase of the disease: whether or not these species contribute to further development of endothelial dysfunction or to increases in blood pressure remains to be examined. The endothelial dysfunction observed in hypertension is likely to be a consequence of high blood pressure. but it could facilitate the maintenance of elevated peripheral resistance at a later stage in the disease and favour the occurrence of complications, such as atherosclerosis.     

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.     

Mechanisms Underlying Endothelial Dysfunction in Diabetes Mellitus

Hyperglycemia is the major causal factor in the development of endothelial dysfunction in patients with diabetes mellitus. Although the mechanisms underlying this phenomenon are likely to be multifactorial, recent in vivo and in vitro studies have indicated a crucial role of the diacylglycerol (DAG)-protein kinase C (PKC) pathway in mediating this phenomenon. PKC may have multiple adverse effects on vascular function, including the activation of superoxide-producing enzymes such as the nicotinamide adenine dinicleotide phosphate (NADPH) oxidase as well as increased expression of a dysfunctional, superoxide-producing, uncoupled endothelial nitric oxide synthase (NOS III). PKC-mediated superoxide production may inactivate nitric oxide (NO) derived from endothelial NOS III, but also may inhibit the activity and/or expression of the NO downstream target, the soluble guanylyl cyclase. Among the different isoforms of PKC, mainly the beta-isoforms have been shown to be activated. Recent studies with selective (isoform-specific) and non-selective PKC inhibitors show that they are able to beneficially influence glucose-induced endothelial dysfunction in experimental animal models as well as in patients, pointing to the therapeutic potential of these compounds in the prevention and treatment of vascular complications of diabetes.     

Vascular consequences of endothelial nitric oxide synthase uncoupling for the activity and expression of the soluble guanylyl cyclase and the cGMP-dependent protein kinase

Endothelial dysfunction in the setting of cardiovascular risk factors, such as hypercholesterolemia, hypertension, diabetes mellitus, chronic smoking, as well as in the setting of heart failure, has been shown to be at least partly dependent on the production of reactive oxygen species (ROS), such as the superoxide radical, and the subsequent decrease in vascular bioavailability of nitric oxide (NO). Superoxide-producing enzymes involved in increased oxidative stress within vascular tissue include the NAD(P)H oxidase, the xanthine oxidase, and mitochondrial superoxide-producing enzymes. Superoxide produced by the NADPH oxidase may react with NO released by endothelial nitric oxide synthase (eNOS), thereby generating peroxynitrite. Peroxynitrite in turn has been shown to uncouple eNOS, thereby switching an antiatherosclerotic NO-producing enzyme to an enzyme that may initiate or even accelerate the atherosclerotic process by producing superoxide. Increased oxidative stress in the vasculature, however, is not restricted to the endothelium and has also been demonstrated to occur within the smooth muscle cell layer in the setting of hypercholesterolemia, diabetes mellitus, hypertension, congestive heart failure, and nitrate tolerance. Increased superoxide production by the endothelial and/or smooth muscle cells has important consequences with respect to signaling by the soluble guanylyl cyclase (sGC) and the cGMP-dependent protein kinase I (cGK-I), the activity and expression of which has been shown to be regulated in a redox-sensitive fashion. The present review summarizes current concepts concerning eNOS uncoupling and also focuses on the consequences for downstream signaling with respect to activity and expression of the sGC and cGK-I in various diseases.     

The preventive anti-oxidant action of thiazolidinediones: a new therapeutic prospect in diabetes and insulin resistance.

BACKGROUND: Hyperglycaemia-derived oxygen free radicals may be mediators of diabetic complications. METHODS: Recent studies show that hyperglycaemia-induced overproduction of superoxide seems to be the first and key event in activation of pathways involved in the pathogenesis of diabetic complications. Superoxide overproduction is accompanied by increased nitric oxide generation and consequently formation of the powerful oxidant peroxynitrite and by poly(ADP-ribose) polymerase activation. This results in acute endothelial dysfunction and activation of inflammation in blood vessels that contribute to the development of diabetic complications. RESULTS: Thiazolidinediones are a new class of insulin-sensitizing agents. They inhibit intracellular free radical overproduction. In particular, they inhibit the same pathways involved in hyperglycaemia-derived oxidative stress, particularly iNOS and NF-kappaB. Studies in animal models suggest that thiazolidinediones can reduce oxidative stress, independent of their ability to reduce hyperglycaemia. CONCLUSIONS: The availability of compounds that simultaneously decrease hyperglycaemia, restore insulin resistance and inhibit pathways activated by high glucose producing oxidative stress signals a promising approach.     

Endothelial dysfunction and atherothrombosis in mild hyperhomocysteinemia

Mildly elevated plasma homocysteine levels are an independent risk factor for atherothrombotic vascular disease in the coronary, cerebrovascular, and peripheral arterial circulation. Endothelial dysfunction as manifested by impaired endothelium-dependent regulation of vascular tone and blood flow, by increased recruitment and adhesion of circulating inflammatory cells to the endothelium, and by a loss of endothelial cell antithrombotic function contributes to the vascular disorders linked to hyperhomocysteinemia. Increased vascular oxidant stress through imbalanced thiol redox status and inhibition of important antioxidant enzymes by homocysteine results in decreased bioavailability of the endothelium-derived signaling molecule nitric oxide via oxidative inactivation. This plays a central role in the molecular mechanisms underlying the effects of homocysteine on endothelial function. Supplementation of folic acid and vitamin B12 has been demonstrated to be efficient in lowering mildly elevated plasma homocysteine levels and in reversing homocysteine-induced impairment of endothelium-dependent vasoreactivity. Results from ongoing intervention trials will determine whether homocysteine-lowering therapies contribute to the prevention and reduction of atherothrombotic vascular disease and may thereby provide support for the causal relationship between hyperhomocysteinemia and atherothrombosis.     

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 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.