Selenium, selenoproteins and the thyroid gland: interactions in health and disease

The trace element selenium is an essential micronutrient that is required for the biosynthesis of selenocysteine-containing selenoproteins. Most of the known selenoproteins are expressed in the thyroid gland, including some with still unknown functions. Among the well-characterized selenoproteins are the iodothyronine deiodinases, glutathione peroxidases and thioredoxin reductases, enzymes involved in thyroid hormone metabolism, regulation of redox state and protection from oxidative damage. Selenium content in selenium-sensitive tissues such as the liver, kidney or muscle and expression of nonessential selenoproteins, such as the glutathione peroxidases GPx1 and GPx3, is controlled by nutritional supply. The thyroid gland is, however, largely independent from dietary selenium intake and thyroid selenoproteins are preferentially expressed. As a consequence, no explicit effects on thyroid hormone profiles are observed in healthy individuals undergoing selenium supplementation. However, low selenium status correlates with risk of goiter and multiple nodules in European women. Some clinical studies have demonstrated that selenium-deficient patients with autoimmune thyroid disease benefit from selenium supplementation, although the data are conflicting and many parameters must still be defined. The baseline selenium status of an individual could constitute the most important parameter modifying the outcome of selenium supplementation, which might primarily disrupt self-amplifying cycles of the endocrine-immune system interface rectifying the interaction of lymphocytes with thyroid autoantigens. Selenium deficiency is likely to constitute a risk factor for a feedforward derangement of the immune system-thyroid interaction, while selenium supplementation appears to dampen the self-amplifying nature of this derailed interaction.     

Reduced selenium concentrations and glutathione peroxidase activity in preeclamptic pregnancies

Preeclampsia is pregnancy-specific, affecting 2% to 7% of women, and is a leading cause of perinatal and maternal morbidity and mortality. Preeclampsia may also predispose the fetus to increased risks of adult cardiovascular disease. Selenium, acting through the selenoprotein glutathione peroxidases, has critical roles in regulating antioxidant status. Recent reports implicate poor maternal selenium status as a nutritional factor predisposing the mother to preeclampsia but the fetus and placenta have not been studied in tandem. Measurement of selenium concentrations, expression, and activity levels of glutathione peroxidase and markers of oxidative stress were performed on maternal and umbilical venous blood samples or the placenta from 27 normal pregnant, 25 preeclamptic, and 22 healthy age-matched nonpregnant women. The results of this study revealed highly significant reductions in serum selenium concentrations and plasma glutathione peroxidase activity in pregnancy per se compared to nonpregnant controls. Moreover, these levels were further decreased in the preeclamptic mothers and babies compared to normal pregnancies. Umbilical venous selenium was particularly low (42.1+/-11.8 and 29.0+/-9.9 microg/L; mean+/-SD; P<0.05). Both mother and baby had significantly increased levels of markers for oxidative stress in the preeclamptic group. The placental glutathione peroxidase activity and immunohistochemical staining were also reduced in the preeclampsia placentae. Oxidative stress associated with preeclampsia may be a consequence of reduced antioxidant defense pathways specifically involving glutathione peroxidases, perhaps linked to reduced selenium availability. Reduced glutathione peroxidases could be associated with increased generation of toxic lipid peroxides contributing to the endothelial dysfunction and hypertension of preeclampsia.     

硒与自身免疫性甲状腺疾病

自身免疫性甲状腺疾病(AITD)的发病机制十分复杂,可能与自由基诱发的氧化应激和凋亡有关.硒是哺乳动物维持正常功能必需的微量元素之一,其在甲状腺激素的合成、活化、代谢过程及甲状腺抗氧化系统和免疫系统中发挥重要作用.血清硒水平低可以引起或加重甲状腺组织的免疫紊乱与慢性炎性反应,引起AITD.补充一定剂量的硒可能改善AITD患者的炎性反应,为AITD的治疗开辟一条新的途径.     

Dietary and blood antioxidants in patients with chronic heart failure. Insights into the potential importance of selenium in heart failure.

BACKGROUND: Chronic heart failure (CHF) seems to be associated with increased oxidative stress. However, the hypothesis that antioxidant nutrients may contribute to the clinical severity of the disease has never been investigated. AIMS: To examine whether antioxidant nutrients influence the exercise capacity and left ventricular function in patients with CHF. METHODS: Dietary intake and blood levels of major antioxidant nutrients were evaluated in 21 consecutive CHF patients and in healthy age- and sex-matched controls. Two indexes of the severity of CHF, peak exercise oxygen consumption (peak VO2) and left ventricular ejection fraction (LVEF), were measured and their relations with antioxidants were analysed. RESULTS: Whereas plasma alpha-tocopherol and retinol were in the normal range, vitamin C (P=0.005) and beta-carotene (P=0.01) were lower in CHF. However, there was no significant association between vitamins and either peak VO2 or LVEF. Dietary intake (P<0.05) and blood levels of selenium (P<0.0005) were lower in CHF. Peak VO2 (but not LVEF) was strongly correlated with blood selenium: r=0.76 by univariate analysis (polynomial regression) and r=0.87 (P<0.0005) after adjustment for age, sex and LVEF. CONCLUSIONS: Antioxidant defences are altered in patients with CHF. Selenium may play a role in the clinical severity of the disease, rather than in the degree of left ventricular dysfunction. Further studies are warranted to confirm the data in a large sample size and to investigate the mechanisms by which selenium and other antioxidant nutrients are involved in CHF.     

Hypothyroidism as a risk factor for cardiovascular disease

The cardiovascular risk in patients with hypothyroidism is related to an increased risk of functional cardiovascular abnormalities and to an increased risk of atherosclerosis. The pattern of cardiovascular abnormalities is similar in subclinical and overt hypothyroidism, suggesting that a lesser degree of thyroid hormone deficiency may also affect the cardiovascular system. Hypothyroid patients, even those with subclinical hypothyroidism, have impaired endothelial function, normal/depressed systolic function, left ventricular diastolic dysfunction at rest, and systolic and diastolic dysfunction on effort, which may result in poor physical exercise capacity. There is also a tendency to increase diastolic blood pressure as a result of increased systemic vascular resistance. All these abnormalities regress with L-T₄ replacement therapy. An increased risk for atherosclerosis is supported by autopsy and epidemiological studies in patients with thyroid hormone deficiency. The “traditional” risk factors are hypertension in conjunction with an atherogenic lipid profile; the latter is more often observed in patients with TSH>10 mU/L. More recently, C-reactive protein, homocysteine, increased arterial stiffness, endothelial dysfunction, and altered coagulation parameters have been recognized as risk factors for atherosclerosis in patients with thyroid hormone deficiency. This constellation of reversible cardiovascular abnormalities in patient with TSH levels<10 mU/L indicate that the benefits of treatment of mild thyroid failure with appropriate doses of l-thyroxine outweigh the risk.     

硒与心脏疾病

硒被认为是与人类健康息息相关的一种微量元素。越来越多的证据表明,硒与包括心血管疾病在内的多种疾病的发生、发展有关。在临床上,硒具有促进冠心病及心力衰竭恢复的特性,在发生心血管疾病的患者中常常可发现硒缺乏。研究表明,硒缺乏可通过破坏体内氧化-还原平衡、影响某些重要硒化物的含量及某些硒蛋白（如谷胱甘肽过氧化物酶）的合成、修饰与活性等参与心血管疾病的发生发展。补充硒可以重建体内氧化-还原平衡,降低体内氧化反应造成的心肌应激损伤。另外,硒在心律失常、心肌病以及心肌炎的发病过程中也起着重要的作用,但机制还尚不清楚。相信对于硒和心脏疾病研究的不断深入可最终在促进心血管疾病基础研究、药物开发以及患者营养治疗等方面起到重要作用。     

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.     

Oxidative stress induces myeloperoxidase expression in endocardial endothelial cells from patients with chronic heart failure

Increased oxidative stress has been implicated in the pathogenesis of a number of cardiovascular diseases. Recent findings suggest that myeloperoxidase (MPO) may play a key role in the initiation and maintenance of chronic heart failure (CHF) by contributing to the depletion of the intracellular reservoir of nitric oxide (NO). NO consumption through MPO activity may lead to protein chlorination or nitration, leading to tissue damage. Primary cultures of human endocardial endothelial cells (EEC) obtained at heart transplantation of patients with CHF and human umbilical vein endothelial cells (HUVEC) were subjected to oxidative stress by incubation with hydrogen peroxide at non lethal (60 μM) dose for different exposure times (3 and 6 h). Treated and control cells were tested by immunohistochemistry and RT-PCR for MPO and 3-chlorotyrosine expression. Both endothelial cell types expressed myeloperoxidase following oxidative stress, with higher levels in EEC. Moreover, 3-chlorotyrosine accumulation in treated cells alone indicated the presence of MPO-derived hypochlorous acid. Immunohistochemistry on sections from post-infarcted heart confirmed in vivo the endothelial positivity to MPO, 3-chlorotyrosine and, to a minor extent, nitrotyrosine. Immunohistochemical observations were confirmed by detection of MPO mRNA in both stimulated EEC and HUVEC cells. This study demonstrates for the first time that EEC can express MPO after oxidative stress, both in vitro and in vivo, followed by accumulation of 3-chlorotyrosine, an end product of oxidative stress. Deregulation of endothelial functions may contribute to the development of a number of cardiovascular diseases, including CHF. The results also highlight the notion that endothelium is not only a target but also a key player in oxidative-driven cardiovascular stress. Electronic supplementary material  The online version of this article (doi: ) contains supplementary material, which is available to authorized users. G. La Rocca and A. Di Stefano contributed equally to the current work. Returned for 1. Revision: 7 January 2008 1. Revision received: 18 June 2008 Returned for 2. Revision: 17 July 2008 2. Revision received: 17 October 2008     

Selenium supplementation prevents the increase in atherogenic electronegative LDL (LDL minus) in the postprandial phase

Evidence is accumulating that postprandial phenomena play a role in atherogenesis. Dietary lipid hydroperoxides that escape from the gastrointestinal barrier can be incorporated into plasma lipoproteins, leading to a modified form of LDL (LDL minus).The present human study was designed to investigate the effect of selenium supplementation on the formation of LDL minus in the postprandial phase. Fourteen healthy subjects ate the same test meal, high in lipid hydroperoxides, at baseline and after 10-day selenium supplementation (110 μg/day). Plasma selenium, LDL minus, LDL resistance to oxidative modification, plasma antioxidants (ascorbic acid, GSH and GPx activity) and MDA were measured in preprandial (time 0) and postprandial (3 h) phases. Supplementation did not induce changes in the concentration of selenium in fasting plasma, but, at the same time, it induced a significant decrease in preprandial plasma GPx activity and inhibited the meal-induced increase in GPx activity. Selenium supplementation fully prevented the meal-induced increase in both LDL minus level and LDL susceptibility to oxidation.This study demonstrated the efficacy of selenium in preventing postprandial oxidative stress. The results, obtained on subjects adequately supplied with selenium, suggest that a non-limiting selenium availability counteracts the postprandial formation of the atherogenic form of LDL and provide a rationale for the epidemiological evidence of the inverse correlation between selenium intake and the incidence of chronic and degenerative diseases.     

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.     

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.     

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.     

Oxidative stress and hyperuricaemia: pathophysiology,clinical relevance,and therapeutic implications in chronic heart failure

Heart failure (HF) is a state of chronic deterioration of oxidative mechanisms due to enhanced oxidative stress and consequent subcellular alterations. In this condition, oxidant‐producing enzymes, in particular xanthine oxidase (XO), the major cardiovascular source of reactive oxygen species (ROS), are up‐regulated. Growing evidence shows that this impaired oxidative metabolism due to enhanced ROS release is implicated in the development of cardiac hypertrophy, myocardial fibrosis, left ventricular remodelling, and contractility impairment responsible for worsening of cardiac function in CHF. Uric acid (UA) has long been linked with cardiovascular diseases, and hyperuricaemia is a common finding in patients with CHF. Hyperuricaemia is associated with impairment of peripheral blood flow and reduced vasodilator capacity, which relate closely to clinical status and reduced exercise capacity. Recent studies also suggest an association between UA levels and parameters of diastolic function; more importantly, UA has emerged as a strong independent prognostic factor in patients with CHF. In this review, we describe the up‐to‐date experimental and clinical studies that have begun to test whether the inhibition of XO translates into meaningful beneficial pathophysiological changes. This treatment gives evidence that myocardial energy, endothelial dysfunction, and vasodilator reactivity to exercise are improved by reducing markers of oxidative stress responsible for vascular dysfunction, so it represents an interesting therapeutic alternative for better outcome in CHF patients.     

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.     

The importance of selenium to human health

The essential trace mineral, selenium, is of fundamental importance to human health. As a constituent of selenoproteins, selenium has structural and enzymic roles, in the latter context being best-known as an antioxidant and catalyst for the production of active thyroid hormone. Selenium is needed for the proper functioning of the immune system, and appears to be a key nutrient in counteracting the development of virulence and inhibiting HIV progression to AIDS. It is required for sperm motility and may reduce the risk of miscarriage. Deficiency has been linked to adverse mood states. Findings have been equivocal in linking selenium to cardiovascular disease risk although other conditions involving oxidative stress and inflammation have shown benefits of a higher selenium status. An elevated selenium intake may be associated with reduced cancer risk. Large clinical trials are now planned to confirm or refute this hypothesis. In the context of these health effects, low or diminishing selenium status in some parts of the world, notably in some European countries, is giving cause for concern.     

Neutrophil superoxide anion--generating capacity, endothelial function and oxidative stress in chronic heart failure: effects of short- and long-term vitamin C therapy

OBJECTIVES: First, we sought to study the effects of short- and long-term vitamin C therapy on oxidative stress and endothelial dysfunction in chronic heart failure (CHF), and second, we sought to investigate the role of neutrophils as a cause of oxidative stress in CHF. BACKGROUND: Oxidative stress may contribute to endothelial dysfunction in CHF. Vitamin C ameliorates endothelial dysfunction in CHF, presumably by reducing oxidative stress, but this is unproven. METHODS: We studied 55 patients with CHF (ischemic and nonischemic etiologies) and 15 control subjects. Flow-mediated dilation (FMD) in the brachial artery was measured by ultrasound wall-tracking, neutrophil superoxide anion (O2-) generation by lucigenin-enhanced chemiluminescence and oxidative stress by measurement of free radicals (FRs) in venous blood using electron paramagnetic resonance (EPR) spectroscopy and plasma thiobarbituric acid reactive substances (TBARS). Measurements were performed at baseline in all subjects. The effects of short-term (intravenous) and long-term (oral) vitamin C therapy versus placebo were tested in patients with nonischemic CHF. RESULTS: At baseline, FRs were higher in patients with CHF than in control subjects (p < 0.01), TBARS were greater (p < 0.005), neutrophil O2- -generating capacity was enhanced (p < 0.005) and FMD was lower (p < 0.0001). Compared with placebo, short-term vitamin C therapy reduced FR levels (p < 0.05), tended to reduce TBARS and increased FMD (p < 0.05), but did not affect neutrophil O2- -generating capacity. Long-term vitamin C therapy reduced FR levels (p < 0.05), reduced TBARS (p < 0.05) and improved FMD (p < 0.05), but also reduced neutrophil O2- -generating capacity (p < 0.05). Endothelial dysfunction was not related to oxidative stress, and improvements in FMD with vitamin C therapy did not relate to reductions in oxidative stress. CONCLUSIONS: Oxidative stress is increased in ischemic and nonischemic CHF, and neutrophils may be an important cause. Vitamin C reduces oxidative stress, increases FMD and, when given long term, decreases neutrophil O2- generation, but the lack of a correlation between changes in endothelial function and oxidative stress with vitamin C implies possible additional non-antioxidant benefits of vitamin C.     

The role of selenium in thyroid autoimmunity and cancer.

The essential micronutrient selenium (Se) occurs in the form of the amino acid selenocysteine in selenoproteins which exert various effects, while maintaining the cell reduction-oxidation balance. The discovery that all three deiodinases that convert thyroxine (T4) into triiodothyronine (T3) contain selenocysteine illustrates how the production of the active thyroid hormone is dependent on Se status. The selenoenzyme families of glutathione peroxidases (GPx) and thioredoxin reductases (TRx) possess powerful antioxidant properties and form a complex defense system that protects thyrocytes from oxidative damage. Se supplementation in patients with autoimmune thyroiditis seems to modify the immune response, probably by enhancing plasma GPx activity and decreasing excess levels of hydrogen peroxide. However, the enhancement of immunocompetence may also be the result of the synergistic action of various selenoproteins and not exclusively of GPx. There is evidence supporting considerable oxidative stress in Graves' disease where Se supplementation, because of its free radical scavenging properties, may increase the enzymatic antioxidant activity. TRx has been found significantly elevated in GD revealing its involvement in the pathogenesis of this condition and representing a potential future target for therapeutical intervention. Low Se serum levels have also been associated with increased risk of thyroid cancer and may play a role in carcinogenesis. It is noteworthy, that the Food and Drug Administration has recently determined that there is sufficient evidence to warrant a qualified health claim for Se and cancer. Furthermore, the recent discovery that defects in the SECIS-binding protein 2 (SBP2), which is an indispensable protein for the incorporation of Se into the selenoproteins, result in thyroid dysfunction, together with the recognition of the many roles of selenoprotein P in Se distribution and storage in the human body, reveal not only the indispensability of Se and the selenoproteins as essential factors in thyroid metabolism and pathogenesis, but open up new prospects for enhanced treatment.     

硒与甲状腺疾病

硒是人体所必需的一种微量元素,在机体中发挥多种生物学作用,如抗氧化、提高免疫功能、抗肿瘤等。硒和碘一样,参与甲状腺激素的合成、活化及代谢过程,从而影响甲状腺功能。在甲状腺肿、自身免疫性甲状腺疾病、甲状腺癌等患者中,血清硒的水平都存在异常。后者可能通过影响甲状腺滤泡上皮细胞内多种含硒基蛋白酶和(或)影响机体的免疫功能而导致甲状腺疾病的发生与发展。给与某些甲状腺疾病患者含硒制剂可以改善甲状腺功能,这为甲状腺疾病的治疗另辟蹊径。     

Mechanisms of the atherogenic effects of elevated homocysteine in experimental models

Hyperhomocysteinemia is a risk factor for cardiovascular disease and stroke. During the last decade, considerable progress in delineating the mechanisms that underlie the atherogenic effects of hyperhomocysteinemia has been achieved through the use of experimental animal models. Among the most informative animal models are those that use genetic and dietary approaches to produce hyperhomocysteinemia in mice. Recent findings demonstrate that hyperhomocysteinemia can accelerate the development of atherosclerosis in susceptible models such as the apolipoprotein E-deficient mouse. Hyperhomocysteinemia also is a potent inducer of endothelial dysfunction, particularly in small vessels such as cerebral arterioles. Mechanisms of endothelial dysfunction may include inhibition of endothelial nitric oxide synthase by its endogenous inhibitor, asymmetric dimethylarginine, and oxidative inactivation of nitric oxide mediated by upregulation of prooxidant enzymes and downregulation of antioxidant enzymes. There also is good evidence from animal models that hyperhomocysteinemia produces endoplasmic reticulum stress, which may contribute to atherosclerosis and endothelial dysfunction by activating signal transduction pathways leading to inflammation, oxidative stress, and apoptosis.