Homocysteine decreases endothelin-1 production by cultured human endothelial cells

The circadian rhythm: A new target of natural products that can protect against diseases of the metabolic system, cardiovascular system, and nervous system

BACKGROUND

The treatment of metabolic system, cardiovascular system, and nervous system diseases remains to be explored. In the internal environment of organisms, the metabolism of substances such as carbohydrates, lipids and proteins (including biohormones and enzymes) exhibit a certain circadian rhythm to maintain the energy supply and material cycle needed for the normal activities of organisms. As a key factor for the health of organisms, the circadian rhythm can be disrupted by pathological conditions, and this disruption accelerates the progression of diseases and results in a vicious cycle. The current treatments targeting the circadian rhythm for the treatment of metabolic system, cardiovascular system, and nervous system diseases have certain limitations, and the identification of safer and more effective circadian rhythm regulators is needed.

AIM OF THE REVIEW

To systematically assess the possibility of using the biological clock as a natural product target for disease intervention, this work reviews a range of evidence on the potential effectiveness of natural products targeting the circadian rhythm to protect against diseases of the metabolic system, cardiovascular system, and nervous system. This manuscript focuses on how natural products restore normal function by affecting the amplitude of the expression of circadian factors, sleep/wake cycles and the structure of the gut microbiota.

KEY SCIENTIFIC CONCEPTS OF THE REVIEW

This work proposes that the circadian rhythm, which is regulated by the amplitude of the expression of circadian rhythm-related factors and the sleep/wake cycle, is crucial for diseases of the metabolic system, cardiovascular system and nervous system and is a new target for slowing the progression of diseases through the use of natural products. This manuscript provides a reference for the molecular modeling of natural products that target the circadian rhythm and provides a new perspective for the time-targeted action of drugs.

The effects of homocysteine-related compounds on cardiac contractility, coronary flow, and oxidative stress markers in isolated rat heart

Research on the effects of homocysteine on the vascular wall, especially in endothelial and smooth muscle cells, has indicated that increased homocysteine levels lead to cellular stress and cell damage. Considering the adverse effects of homocysteine on vascular function and the role of oxidative stress in these mechanisms, the aim of this study was to estimate the influence of different homocysteine isoforms on cardiac contractility, coronary flow, and oxidative stress markers in isolated rat heart. The hearts of male Wistar albino rats (n = 36, age 8 weeks, body mass 180-200 g), were excised and retrogradely perfused according to the Langendorff technique at a constant perfusion pressure (70 cmH(2)O) and administered with three isoforms of 10 μM homocysteine [DL-Hcy, DL-Hcy thiolactone-hydrochloride (TLHC) and L-Hcy TLHC). After the insertion and placement of the sensor in the left ventricle, the parameters of heart function: maximum rate of pressure development in the left ventricle (dP/dt max), minimum rate of pressure development in the left ventricle (dP/dt min), systolic left ventricular pressure (SLVP), diastolic left ventricular pressure (DLVP), mean blood pressure (MBP) and heart rate (HR)] were continuously registered. Flowmetry was used to evaluate the coronary flow. Markers of oxidative stress: index of lipid peroxidation measured as TBARS, nitric oxide measured through nitrites (NO(2)(-)), superoxide anion radical (O(2)(-)), and hydrogen peroxide (H(2)O(2)) in the coronary venous effluent were assessed spectrophotometrically. Our results showed that administration of Hcy compounds in concentration of 10 μM induced depression of cardiac contractility, manifested by a decrease in dp/dt max after administration of any Hcy compound, decrease in dp/dt min after administration of L-Hcy TLHC, decrease in SLVP after administration of DL-Hcy TLHC and DL-Hcy, and the drop in CF after administration of any Hcy compound. Regarding the effects of Hcy on oxidative stress parameters, only L-Hcy TLHC significantly affected O(2)(-) release. L-Hcy TLHC showed a cardiotoxic effect by affecting heart contractility, but surprisingly, it decreased the release of O(2)(-).

Association between homocysteine and endothelial dysfunction markers in stroke disease

BACKGROUND

Evidence shows that there is an increase in concentrations of markers of endothelial dysfunction immediately following acute ischaemic stroke. Several studies suggest that endothelial dysfunction may be partly caused by oxidation related to the effects of raised total plasma homocysteine.

OBJECTIVE

The aim of this study was to measure changes in total plasma homocysteine and markers of endothelial dysfunction in stroke disease within a known period of time post infarct.

SUBJECTS AND METHODS

We studied 40 acute ischaemic stroke patients (mean age +/- SD, 50.2 +/- 9.5 years) and 42 hospitalised non-stroke patients (mean age, 44.3 +/- 14.9 years). Fasting venous blood was obtained within 24 h, 3 days and 7 days after the stroke onset and hospitalisation for non-stroke patients for measurements of total plasma homocysteine, markers of endothelial dysfunction including intracellular adhesion molecule (i-CAM), vascular cell adhesion molecule-1 (v-CAM) and leukocyte adhesion molecule-1 (E-selectin) and C-reactive proteins (CRPs).

RESULTS

We found no significant differences in baseline total plasma homocysteine, E-selectin, v-CAM, vitamin B(12), and folate concentrations between ischaemic stroke patients and non-stroke controls. i-CAM concentrations were significantly higher and CRPs non-significantly lower at baseline in stroke patients compared with controls. Although all endothelial dysfunction markers increased significantly during the study period, the rise in E-selectin levels was less than that seen in i-CAM, and v-CAM. Total plasma homocysteine concentrations showed positive correlations with creatinine (r = 0.537; P < 0.02), and inverse correlations with both vitamin B(12) (r = -0.560; P < 0.001) and folate (r = -0.469; P < 0.002); however, there were no significant correlations between total plasma homocysteine or B-vitamins and markers of endothelial dysfunction in ischaemic stroke patients or controls.

CONCLUSIONS

We found evidence of an increase in markers of endothelial dysfunction following acute ischaemic stroke but this had no relationship with total plasma homocysteine concentrations.

Homocysteine and Raynaud's phenomenon: a review

Raynaud's phenomenon, categorized as primary and secondary when occurring isolated or in association with an underlying disease, respectively, is a paroxysmal and recurrent acral ischemia resulting from an abnormal arterial vasospastic response to cold or emotional stress. The key issue in the pathogenesis of Raynaud's phenomenon is presumed to be a dysregulation in the mechanisms of vascular motility resulting in an imbalance between vasodilatation and vasoconstriction. Homocysteine, a non-protein forming sulphured amino acid proposed as an independent risk factor for atherothrombosis in the general population, clearly demonstrated to produce vascular damage through mechanisms also including endothelial injury and modifications in circulating mediators of vasomotion. The rationale for homocysteine involvement in the pathogenesis of Raynaud's phenomenon led some authors to investigate the possible association between mild hyperhomocysteinemia and such a vascular disturbance, particularly in the course of connective tissue disease. Here we review data regarding this putative association and the supposed mechanisms involved, also discussing the emblematic case of a patient with new-onset severe Raynaud's phenomenon and markedly elevated homocysteinemia.

Hyperhomocysteinemia and Endothelial Dysfunction

Hyperhomocysteinemia (HHcy) is a significant and independent risk factor for cardiovascular diseases. Endothelial dysfunction (ED) is the earliest indicator of atherosclerosis and vascular diseases. We and others have shown that HHcy induced ED in human and in animal models of HHcy induced by either high-methionine load or genetic deficiency. Six mechanisms have been suggested explaining HHcy-induced ED. These include 1) nitric oxide inhibition, 2) prostanoids regulation, 3) endothelium-derived hyperpolarizing factors suppression, 4) angiotensin II receptor-1 activation, 5) endothelin-1 induction, and 6) oxidative stress. The goal of this review is to elaborate these mechanisms and to discuss biological and molecular events related to HHcy-induced ED.

Increased endothelin-1 reactivity and endothelial dysfunction in carotid arteries from rats with hyperhomocysteinemia

BACKGROUND AND PURPOSE

There are interactions between endothelin-1 (ET-1) and endothelial vascular injury in hyperhomocysteinemia (HHcy), but the underlying mechanisms are poorly understood. Here we evaluated the effects of HHcy on the endothelin system in rat carotid arteries.

EXPERIMENTAL APPROACH

Vascular reactivity to ET-1 and ET(A) and ET(B) receptor antagonists was assessed in rings of carotid arteries from normal rats and those with HHcy. ET(A) and ET(B) receptor expression was assessed by mRNA (RT-PCR), immunohistochemistry and binding of [(125)I]-ET-1.

KEY RESULTS

HHcy enhanced ET-1-induced contractions of carotid rings with intact endothelium. Selective antagonism of ET(A) or ET(B) receptors produced concentration-dependent rightward displacements of ET-1 concentration response curves. Antagonism of ET(A) but not of ET(B) receptors abolished enhancement in HHcy tissues. ET(A) and ET(B) receptor gene expressions were not up-regulated. ET(A) receptor expression in the arterial media was higher in HHcy arteries. Contractions to big ET-1 served as indicators of endothelin-converting enzyme activity, which was decreased by HHcy, without reduction of ET-1 levels. ET-1-induced Rho-kinase activity, calcium release and influx were increased by HHcy. Pre-treatment with indomethacin reversed enhanced responses to ET-1 in HHcy tissues, which were reduced also by a thromboxane A(2) receptor antagonist. Induced relaxation was reduced by BQ788, absent in endothelium-denuded arteries and was decreased in HHcy due to reduced bioavailability of NO.

CONCLUSIONS AND IMPLICATIONS

Increased ET(A) receptor density plays a fundamental role in endothelial injury induced by HHcy. ET-1 activation of ET(A) receptors in HHcy changed the balance between endothelium-derived relaxing and contracting factors, favouring enhanced contractility.

Endothelin-1 induces alveolar epithelial-mesenchymal transition through endothelin type A receptor-mediated production of TGF-beta1

Endothelin-1 (ET-1) is implicated in the pathogenesis of idiopathic pulmonary fibrosis (IPF), but the cellular mechanisms underlying the role it plays in this disease are not well characterized. Epithelial-mesenchymal transition (EMT), which was recently demonstrated in alveolar epithelial cells (AEC), may play an important role in the pathogenesis of IPF and other forms of pulmonary fibrosis. Whether ET-1 contributes to the induction of EMT in AEC is unknown. The aims of this study were to evaluate AEC production of ET-1 and to determine if ET-1 induces EMT in AEC. We demonstrate that ET-1 is produced at physiologically relevant levels by primary AEC and is secreted preferentially toward the basolateral surface. We also demonstrate that AEC express high levels of endothelin type A receptors (ET-A) and, to a lesser extent, type B receptors (ET-B), suggesting autocrine or paracrine function for alveolar ET-1. In addition, ET-1 induces EMT through ET-A activation. Furthermore, TGF-beta1 synthesis is increased by ET-1, ET-1 induces Smad3 phosphorylation, and ET-1-induced EMT is attenuated by a TGF-beta1-neutralizing antibody. Thus, ET-1 is an important mediator of EMT in AEC, acting through ET-A-mediated TGF-beta1 production. These findings increase our basic understanding of the role of ET-1 in pulmonary fibrosis and suggest potential roles for AEC-derived ET-1 in the pathogenesis of other alveolar epithelial-mediated lung diseases.

5-methyltetrahydrofolic acid stimulates endothelin-1 production in low density lipoprotein-treated human endothelial cells

BACKGROUND AND AIMS

Several studies have shown the beneficial effects of folate treatment in improving cardiovascular function. However, the mechanisms involved have not been clearly identified. The aim of this study is to determine the effect of folates and vitamin B12 on endothelial vasoconstriction/vasodilatation parameters in cultured human endothelial cells incubated with human low density lipoproteins (LDL).

METHODS AND RESULTS

Human umbilical vein endothelial cells (HUVEC) were extracted from recently delivered umbilical cords, cultured until confluence was achieved, and then incubated for 24h with folic acid (FA), 5-methyltetrahydrofolic acid (5-MTHF) or vitamin B12 (B12) in the presence or absence of LDL that was isolated from healthy volunteers. Total nitrites (as a measure of nitric oxide production), thiobarbituric acid reactive species (TBARS, a parameter of lipid peroxidation), and endothelin-1 (ET-1) were determined in the incubation media. None of the vitamins, either in the presence or absence of LDL, was able to modify nitric oxide production by HUVEC. A significant reduction of ET-1 production was observed in LDL-treated cells. This effect was not modified by FA or B12; however, 5-MTHF caused a concentration-dependent increase on ET-1 production, an effect coincidental with reduced TBARS production.

CONCLUSIONS

This study demonstrates for the first time that 5-MTHF, but not FA or B12, increases ET-1 production in LDL-treated endothelial cells. Although this effect was associated with the antioxidant properties of this folate, our results show that additional specific mechanisms involving 5-MTHF-LDL interactions may be operating to regulate endothelial function.

Mechanisms of homocysteine toxicity in humans

Homocysteine, a non-protein amino acid, is an important risk factor for ischemic heart disease and stroke in humans. This review provides an overview of homocysteine influence on endothelium function as well as on protein metabolism with a special respect to posttranslational modification of protein with homocysteine thiolactone. Homocysteine is a pro-thrombotic factor, vasodilation impairing agent, pro-inflammatory factor and endoplasmatic reticulum-stress inducer. Incorporation of Hcy into protein via disulfide or amide linkages (S-homocysteinylation or N-homocysteinylation) affects protein structure and function. Protein N-homocysteinylation causes cellular toxicity and elicits autoimmune response, which may contribute to atherogenesis.

Antioxidant supplementation with or without B-group vitamins after acute ischemic stroke: a randomized controlled trial

BACKGROUND

Evidence shows that there is a rapid increase in the production of markers of oxidative damage immediately after acute ischemic stroke and that endogenous antioxidant defenses are rapidly depleted, thus permitting further tissue damage. Several studies point to an antioxidant effect of B-group vitamins and a pro-oxidant effect of elevated total plasma homocysteine (tHcy).

METHODS

To test whether supplementary antioxidants with or without B-group vitamins during this critical period enhance antioxidant capacity or mitigate oxidative damage, ninety-six acute ischemic stroke patients within 12 hours of symptom onset were randomly assigned to receive either daily oral 800 IU (727 mg) vitamin E and 500 mg vitamin C (n = 24), or B-group vitamins (5 mg folic acid, 5 mg vitamin B(2), 50 mg vitamin B(6), and 0.4 mg of vitamin B(12); n = 24), both vitamins together (n = 24), or no supplementation (n = 24) for 14 days. Treatment groups and controls were matched for stroke subtype and age. Blood was obtained before treatment, at day 7, and day 14 for measurements of plasma or blood vitamin status, plasma total antioxidant capacity (TAOC), malondialdehyde (MDA), tHcy and C-reactive protein (CRP).

RESULTS

Supplementation with antioxidant vitamins and B-group vitamins separately or together significantly increased the plasma concentration of vitamin C, E, pyridoxal phosphate (B(6) status), red blood cell folate, and improved a measure of B(2) status (red cell glutathione reductase activation coefficient [EGRAC]), compared with the control group. Plasma TAOC increased significantly in the antioxidant treatment groups compared with the nonsignificant decline seen in the control group. tHcy concentrations decreased in subjects who received B-group vitamins and the control group compared with the rise seen in those who received antioxidants alone. There was a significant reduction in plasma MDA concentration in the 3 treatment groups, in contrast to the increase seen in the control group; however, the changes were most evident in antioxidant groups. CRP concentrations (a marker of tissue inflammation) were significantly lower in the 3 treatment groups compared with the control group. There were no additive or synergistic effects of antioxidants and B-group vitamins together on any outcome measure.

CONCLUSIONS

Antioxidants supplementation with or without B-group vitamins enhances antioxidant capacity, mitigates oxidative damage, and may have an anti-inflammatory effect immediately postinfarct in stroke disease.

Thiol compounds metabolism in mice, rats and humans: comparative study and potential explanation of rodents protection against vascular diseases

BACKGROUND

Rodents are often used as animal models to dissect mechanisms underlying hyperhomocysteinemia atherogenicity in humans. However, neither wild-type rodents nor cystathionine beta-synthase deficient mice develop spontaneous atherosclerosis. We investigated whether species-specific differences in thiols metabolism may explain the respective sensitivity of rodents and humans to hyperhomocysteinemia.

METHODS

Thiols and vitamins B levels were determined in normohomocysteinemic humans and rodents, and in hyperhomocysteinemic mice.

RESULTS

In basal status, although plasma homocysteine, cysteine and cysteinylglycine levels were lower, glutathione levels were higher in mice than in humans (4.0+/-1.6 vs. 7.9+/-2.2, P<0.0005; 147.4+/-40.3 vs. 278.5+/-50.0, P<0.0001; 2.3+/-0.7 vs. 36.6+/-7.3, P<0.0001; and 70.9+/-20.1 vs. 4.6+/-1.6, P<0.0001). Serum vitamin B12 and folate levels were 2.5- and 7.7-fold higher in rats than in humans. In wild-type mice, the increase in plasma Hcy levels induced by methionine-enriched diet was accompanied by a proportional increase in GSH levels.

CONCLUSION

GSH levels are enough to modulate Hcy effects in normo- and hyperhomocysteinemic mice but not in humans. This rodents characteristic, likely supported by species differences in the relative contribution of remethylation and transsulfuration pathways, may partly explain their protection against atherosclerosis.

Homocysteine-induced endothelin-1 release is dependent on hyperglycaemia and reactive oxygen species production in bovine aortic endothelial cells

BACKGROUND

Elevated plasma homocysteine (Hcy) is a risk factor for coronary disease. The objective of this study was to investigate whether Hcy either alone or in high glucose conditions induces endothelin-1 (ET-1) synthesis via the production of reactive oxygen species (ROS).

METHODS

Bovine aortic endothelial cells were grown in high (25 mmol/l) and low (5 mmol/l) glucose medium.

RESULTS

In high glucose, Hcy caused a time-dependent increase in ET-1 release, which was greatest with 50 micromol/l Hcy at 24 h (p < 0.01). This effect was not seen in low glucose conditions. In high glucose and 50 micromol/l Hcy, ET-1 mRNA levels were maximal after 1 h (p < 0.05). Tissue factor mRNA levels were raised at 4 h (p < 0.05) and functional activity was raised at 6 h (p < 0.01). Intracellular ROS production was increased by 50 micromol/l Hcy after 24 h (p < 0.05) but only in high glucose. To investigate the role of mitochondrial metabolism in ROS production, cells were incubated with thenoyltrifluoroacetone (inhibitor of complex II) or carbonyl cyanide m-chlorophenylhydrazone (uncoupler of oxidative phosphorylation). Both compounds abolished the Hcy-induced increase in ROS production and ET-1 release. There was an alteration in intracellular glutathione (GSH) levels with Hcy treatment with more oxidised GSH present.

CONCLUSION

The combined metabolic burden of Hcy and high glucose stimulates ET-1 synthesis in bovine aortic endothelial cells via a mechanism dependent on the production of mitochondrial ROS, but may not be generalisable to all types of endothelial cells.

Homocysteine at pathophysiologic concentrations activates human monocyte and induces cytokine expression and inhibits macrophage migration inhibitory factor expression

OBJECTIVE

Homocystinemia is an important independent risk factor for atherosclerosis. Inflammatory cytokines play key roles in the development of atherogenesis. This study investigated the effect of homocysteine on inflammatory cytokine expression.

METHODS

Human monocytes were treated in vitro with a variety of DL-homocysteine concentrations that ranged from physiologic concentration to higher than pathophysiologic concentration, and we analyzed their expressions of inflammatory cytokines, including tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, interleukin-8, interleukin-12, and migration inhibitory factor.

RESULTS

DL-homocysteine at a marginal physiologic concentration of 2 microg/mL (15 microM) activated monocytes. In addition, DL-homocysteine at the pathophysiologic dose of 25 microg/mL (185 microM) induced mRNA and protein expressions of inflammatory cytokines tumor necrosis factor-alpha, IL-1beta, interleukin-6, interleukin-8, and interleukin-12. Moreover, at the larger dose of 50 microg/mL (370 microM) DL-homocysteine decreased expression of migration inhibitory factor at the mRNA and protein levels.

CONCLUSION

These findings suggest that homocysteine may contribute to the initiation and progression of vascular disease by activating monocytes, resulting in the secretion of cytokines that amplify the inflammatory response.

Hyperhomocyst(e)inemia impairs angiogenesis in a murine model of limb ischemia

Hyperhomocyst(e)inemia (HH) is an established independent risk factor for coronary, cerebral and peripheral vascular diseases. Recent studies have indicated that certain cardiovascular risk factors, including diabetes and hypercholesterolemia, impair expression of vascular endothelial growth factor (VEGF) and endogenous angiogenesis. In this study, we investigate the impact of moderate HH on angiogenesis and VEGF pathway in a mouse model of hindlimb ischemia. Upon induction of unilateral hindlimb ischemia, endogenous angiogenesis, expression of VEGF, and phosphorylation of the VEGF receptor Flk-1 were evaluated in mice heterozygous for a deletion of the cystathionine beta-synthase gene (CBS) and compared with those observed in CBS+/+ mice. CBS+/- mice exhibit moderate HH, as demonstrated by measuring plasma total homocyst(e)ine (tHcy) levels, which were significantly higher in these animals compared with CBS+/+ mice (4.77 +/- 0.82 vs 2.10 +/- 0.28, p < 0.01). Twenty-eight days after induction of ischemia, hindlimb blood flow was significantly reduced in CBS+/- mice compared with CBS+/+ animals (0.49 +/- 0.03, n = 12 vs 0.71 +/- 0.09, n = 10; p < 0.05). In addition, there was a significant negative correlation between plasma homocyst(e)ine levels and the laser Doppler perfusion ratio in CBS+/- mice (p = 0.0087, r = -0.7171). While VEGF expression and Flk-1 phosphorylation were not impaired in the ischemic muscles of CBS+/- mice, phosphorylation of the endothelial cell survival factor Akt was significantly inhibited by homocyst(e)ine in a dose-dependent manner in human umbilical vein endothelial cell (HUVECs) in vitro. In conclusion, our findings demonstrate that endogenous angiogenesis is inversely related to plasma levels of homocyst(e)ine in genetically engineered, heterozygous mice with moderate HH. This impairment, however, is not dependent on reduced expression of VEGF or impaired phosphorylation of its receptor Flk-1. In contrast, our data suggest that impaired Akt phosphorylation mediates the impairment of angiogenesis associated with HH.

High levels of homocysteine inhibit lysyl oxidase (LOX) and downregulate LOX expression in vascular endothelial cells

BACKGROUND

Hyperhomocysteinemia, an independent risk factor for cardiovascular disease and atherothrombosis, alters endothelial function through a mechanism not fully understood. Downregulation of lysyl oxidase (LOX), an enzyme involved in extracellular matrix maturation, impairs the endothelial barrier function and could be involved in homocysteine (HC)-induced endothelial dysfunction.

OBJECTIVE

The aim of this study was to analyze the effect of HC on LOX regulation in vascular endothelial cells.

RESULTS

HC at pathophysiological concentrations (35 microM) inhibited LOX activity in porcine aortic endothelial cells. Homocysteine thiolactone and related molecules containing sulfhydryl groups (cysteine), but not methionine or homocystine (non-containing thiol-group) inhibited LOX. In addition, the blockade of HC-sulfhydryl group by N-ethylmaleimide abrogated HC-induced LOX downregulation. This process was triggered by oxidative stress since superoxide dismutase and vitamin C reverted LOX inhibition caused by HC. On the contrary, the effect was not mediated through the induction of endoplasmic reticulum stress. Finally, higher doses of HC (200 microM), common in severe hyperhomocysteinemia, decreased LOX mRNA levels ( approximately 2-fold) and LOX promoter activity in transient transfection experiments.

CONCLUSIONS

These findings suggest that LOX inhibition contributes to the endothelial dysfunction associated with hyperhomocysteinemia. This effect was dependent on a mechanism involving both an inhibition of LOX activity and a reduction of LOX expression.

Effect of homocysteine, folates, and cobalamin on endothelial cell- and copper-induced LDL oxidation

Oxidation of LDL contributes to endothelial dysfunction and atherosclerosis. This process could be associated with hyperhomocysteinemia, a condition that can be reduced after folic acid treatment. Because a reduction in LDL oxidation may improve endothelial function, we studied the effect of some vitamins (folic acid, 5-methyltetrahydrofolic acid, and vitamin B-12) on LDL oxidation, either in the presence or absence of homocysteine. For this purpose, two in vitro systems were used: an endothelial cell-catalyzed LDL oxidation system and a cell-free copper-initiated LDL oxidation system. The kinetics of copper-catalyzed LDL oxidation was determined by continuous monitoring of the production of conjugated dienes in the reaction medium. TBARS production, a parameter of lipid peroxidation, was also evaluated. In both in vitro systems, only 5-methyltetrahydrofolic acid was able to decrease TBARS production in a concentration-dependent manner, independently of the presence or absence of homocysteine. In the copper-induced LDL oxidation system, vitamin B-12 and 5-methyltetrahydrofolic acid increased the lag time of conjugated diene production by 25 and 47%, respectively, suggesting that both vitamins in this system had antioxidant properties. Folic acid was unable to show antioxidant properties when included in either in vitro system. The results demonstrate that 5-methyltetrahydrofolic acid and vitamin B-12 are important protective agents against LDL oxidative modifications.

Homocysteine and cardiovascular disease: biological mechanisms, observational epidemiology, and the need for randomized trials

Basic research indicates that homocysteine causes endothelial dysfunction and damage, accelerates thrombin formation, inhibits native thrombolysis, promotes lipid peroxidation through free radical formation, and induces vascular smooth muscle proliferation and monocyte chemotaxis. Most, but not all, observational epidemiological studies indicate that individuals with higher homocysteine levels have increased risks of cardiovascular disease. The magnitude ranges from approximately 20% in prospective studies to approximately 80% in retrospective case-control studies. In all observational epidemiological studies, however, the amount of uncontrolled and uncontrollable confounding is as large as the postulated small to moderate effect size. Thus, the totality of evidence should include randomized trials of sufficient sample size and duration with clinical end points. Folic acid reduces levels of homocysteine, but at present, despite several plausible biological mechanisms and a large body of observational epidemiological data, it is unclear whether supplementation will reduce risks of cardiovascular disease. It is also unclear whether any benefit of folic acid is attributable to lowering homocysteine levels. The current evidence is necessary, but not sufficient to judge causality. Such judgments await the availability of data from large-scale randomized trials. The availability of such data would permit rational clinical decision-making for individual patients and policy decisions for the health of the general public.

Homocysteine Induces Endothelial Cell Detachment and Vessel Wall Thickening During Chick Embryonic Development

Homocysteine affects the migration and differentiation of neural crest cells in vitro and can result in neural tube defects in vivo. Furthermore, homocysteine has been described as an important determinant in vascular disease in human adults. However, little is known about the effects of homocysteine on the development of embryonic vessels. In this study, we injected homocysteine (30 μmol/L) into the neural tube lumen of chick embryos at the time point of neural crest cell emigration, and analyzed the effects on the neural crest–derived pharyngeal arch arteries, like the brachiocephalic arteries, and the mesoderm-derived arteries, such as the dorsal aorta. By stage HH35, we observed detachment of the endothelium, decreased expression of the extracellular matrix proteins fibrillin-2, and fibronectin in the pharyngeal arch arteries, whereas the dorsal aorta was identical in homocysteine-neural tube–injected and control embryos. No effect of homocysteine on endothelin-1 mRNA expression was observed. By stage HH40, the brachiocephalic arteries of homocysteine-neural tube–injected embryos displayed a decreased lumen diameter, an increased intima- and media-thickness, and an increased number of actin layers compared with the brachiocephalic arteries in control embryos. We propose that homocysteine affects the neural crest–derived smooth muscle cells and their extracellular matrix proteins in the pharyngeal arch arteries, resulting in an abnormal smooth muscle to endothelial cell interaction, leading to endothelial cell detachment. We suggest that, as in adult life, increased homocysteine concentrations lead to vascular damage in the embryo. This prenatal damage might increase the susceptibility to develop vessel pathology later in life.

Homocysteine modulates the proteolytic potential of human vascular endothelial cells

Pathological levels of homocysteine induce a metalloproteinase-dependent degradation of the elastic structures in arterial wall. This elastolytic process is preferentially localized toward the internal elastic laminae and in the first layers of the media, suggesting endothelium could participate in extracellular matrix degradation induced by homocysteine. Therefore, we studied the effects of homocysteine on proteolytic potential of endothelial cells. Human umbilical vein endothelial cells were cultured with concentrations of homocysteine matching human physiological (10 microM) and pathological (50, 100, and 250 microM) plasma homocysteine levels. Pathological levels of homocysteine increased the secretion of elastolytic metalloproteinase-2 and -9, but not of metalloproteinase-3 and -7. Homocysteine also increased the expression of human tissue kallikrein, a potential activator of matrix metalloproteinase-2 and -9, while the expression of urokinase plasminogen activator was not altered. These results suggest vascular endothelial cells could participate in the subendothelial degradation of the arterial elastic structures occurring in hyperhomocysteinemia.

Endothelin-1 and vascular tone in subjects with atherogenic risk factors

Endothelin-1 (ET-1) is a potent vasoconstrictor that increases vascular tone in the resistance vessels of subjects with hypertension. It is unclear whether endogenous ET-1 affects resistance-vessel function equally in patients with other cardiovascular risk factors. Vasoconstriction to ET-1 is mediated principally via the endothelin-A (ETA) receptor on vascular smooth muscle cells. Accordingly, we used an ETA-specific antagonist, BQ-123, to test the hypothesis that endogenous ET-1 increases vascular resistance selectively in subjects with hypertension compared with other risk factors. BQ-123 was infused at 100 nmol/min for 80 minutes into the brachial artery of 10 subjects with hypertension (mean+/-SEM arterial pressure, 106+/-5 mm Hg), 12 subjects with hypercholesterolemia (mean+/-SEM total cholesterol, 7.1+/-0.2 mmol/L), 10 active smokers (mean+/-SEM, 42+/-11 pack-years), and 11 healthy, age-matched individuals. Forearm blood flow (FBF) was measured by venous occlusion plethysmography. BQ-123 dilated resistance arterioles in hypertensive subjects, with FBF's increasing by 46+/-7% from baseline (P<0.001). BQ-123 increased FBF to a lesser extent in hypercholesterolemic (24+/-5%, P<0.001) and healthy (20+/-8%, P=0.007) individuals but did not affect FBF significantly in smokers (10+/-8%, P=0.185). The vasodilator response in hypertensive subjects, but not in hypercholesterolemic patients or smokers, was significantly greater than that in healthy individuals (P=0.012). Endogenous ET-1, acting via the ETA receptor, increases resistance-vessel tone in subjects with hypertension more than in subjects with hypercholesterolemia or in smokers. These results indicate that ET-1 contributes more to the pathophysiology of hypertension than of other risk factors in subjects without overt atherosclerosis.

Association of methylenetetrahydrofolate reductase gene polymorphism with carotid atherosclerosis depending on smoking status in a Japanese general population

BACKGROUND AND PURPOSE

The association of the methylenetetrahydrofolate reductase gene (MTHFR) with carotid atherosclerosis remains inconsistent. This may be due to small sample size and inappropriate analysis. We investigated the association of C677T/MTHFR with blood pressure and carotid atherosclerosis in a Japanese general population.

METHODS

Subjects (30 to 89 years of age; 1693 women, 1554 men) who gave informed consent were randomly selected from a general population in Suita, Japan. MTHFR genotypes were determined by TaqMan polymerase chain reaction. Carotid atherosclerosis was evaluated by high-resolution ultrasonography with atherosclerotic indexes of intimal-medial thickness (IMT), maximum IMT in the common carotid artery (CCA), plaque score, and stenosis (>50%).

RESULTS

Age-adjusted diastolic blood pressure was significantly higher in women with the TT genotype than in those with the CC genotype. In a recessive model (CC+CT versus TT), all adjusted odds ratios for hypertension and >50% stenosis in women were 1.42 and 3.42 (95% confidence intervals, 1.01 to 1.99 and 1.23 to 9.53), respectively. In women, maximum IMT in CCA for smokers with the TT genotype was significantly higher than for smokers with the CC genotype and nonsmokers with the TT genotype (P<0.05).

CONCLUSIONS

Our study suggests that the MTHFR TT genotype is a risk factor for hypertension and carotid stenosis in women. Significant interactions between C677T/MTHFR and smoking on maximum IMT in CCA were observed in women but not in men. Smoking cessation for subjects with the TT genotype is important in the prevention of cerebrovascular disease.

Endothelial markers and homocysteine in patients with classic Fabry disease

AIM

Fabry disease is an X-linked inborn error of glycosphingolipid metabolism due to the deficient activity of alpha-galactosidase A, a lysosomal enzyme. It is a multisystem disorder characterized by progressive renal insufficiency, with added morbidity from cardio- and cerebrovascular involvement. The recent availability of genetically engineered enzyme offers an effective targeted treatment approach, but also emphasizes the need for surrogate markers to delineate organ damage and monitor the efficacy of enzyme replacement therapy (ERT).

METHODS

Multiple endothelial factors and plasma homocysteine concentrations were investigated in 12 consecutive hemizygous males with classic Fabry disease and 15 controls as part of an exhaustive baseline evaluation prior to ERT.

RESULTS

Compared with the controls, plasma concentrations of homocysteine were significantly (p < 0.01) higher in patients with Fabry disease in the absence of chronic renal failure or vitamin deficiency. Plasma concentrations of vascular cell adhesion molecule-1 were also significantly (p < 0.05) higher in the patients, and there was a trend for decreased endothelin-1 levels. No difference was found in serum intercellular adhesion molecule-1, plasma P-selectin, serum E-selectin and plasma thrombomodulin between the patients and controls.

CONCLUSIONS

The results do not reveal measurable evidence for endothelial and leukocyte activation that could reliably serve as surrogate markers for routine monitoring of the efficacy of ERT in patients with Fabry disease. While the exact origin and clinical significance of hyperhomocysteinaemia in Fabry disease remains to be studied in a larger cohort of patients carefully monitored for their concurrent medications, especially carbamazepine, we suggest that patients may benefit from folic acid or multivitamin therapy to treat this additional vascular risk factor, when present.

Homocysteine is the only plasma thiol associated with carotid artery remodeling

Several authors have reported that moderate hyperhomocysteinemia is related to asymptomatic carotid arterial wall remodeling, but few data are available on other thiol compounds with potential vascular toxicity. We, therefore, investigated the relationships between major plasma thiol compounds (homocysteine, cysteine and glutathione) and the structural phenotype of the common carotid artery in a cohort of 123 subjects with no evidence of cardiovascular disease. Fasting levels of thiol compounds were measured by high-performance liquid chromatography, and arterial geometry was evaluated using high-resolution echotracking devices. In univariate regression analysis, plasma homocysteine and plasma cysteine concentrations were positively associated with carotid artery internal diameter (P=0.0001 and 0.002, respectively) and intima media thickness (P=0.003 and 0.004), but the plasma glutathione concentration was not. In multivariate analysis, plasma homocysteine was independently and positively associated with carotid artery internal diameter (P<0.005) and intima media thickness (P<0.05), but plasma cysteine was not. These data suggest that homocysteine is the only plasma thiol compound that may be considered as a risk factor for preclinical cardiovascular disease.

Homocysteine decreases endothelin-1 expression by interfering with the AP-1 signaling pathway

The increased cardiovascular risk associated with hyperhomocysteinemia has been linked to homocysteine-induced endothelial cell (EC) dysfunction. Endothelin-1 is a vasoactive peptide, synthesized mainly by vascular ECs. We have previously shown that homocysteine decreases endothelin-1 biosynthesis. Here we addressed the molecular mechanism of endothelin-1 regulation by homocysteine. Experiments with the transcription inhibitor actinomycin D indicated that the decrease in preproendothelin-1 mRNA content in homocysteine-treated cells did not result from transcript destabilization. Transient transfection assays demonstrated that homocysteine downregulated endothelin-1 at the transcriptional level by decreasing preproendothelin-1 promoter activity. Mutation of the activator protein-1 (AP-1) site of the promoter eliminated the repression induced by homocysteine. Western blot analysis showed that the homocysteine-induced decrease in promoter activity was not associated with reduced expression of the AP-1 components c-Fos and c-Jun. The inhibitory action of homocysteine on preproendothelin-1 mRNA expression was not prevented by cycloheximide. Electrophoretic mobility shift assays demonstrated that homocysteine reduced the binding activity of ECs nuclear extracts to an AP-1 consensus site. These results indicate that homocysteine downregulates endothelin-1 synthesis by inhibiting AP-1 activity, and that the AP-1 signaling pathway may be of major importance in homocysteine-induced endothelial dysfunction.

Homocysteine-induced decrease in endothelin-1 production is initiated at the extracellular level and involves oxidative products

The increased cardiovascular risk associated with hyperhomocysteinemia has been partly related to homocysteine (Hcy)-induced endothelial cell dysfunction. However, the intra or extracellular starting point of the interaction between Hcy and endothelial cells, leading to cellular dysfunction, has not yet been identified. We investigated the effects of both intracellular and extracellular Hcy accumulation on endothelin-1 (ET-1) synthesis by cultured human endothelial cells. Incubation of cultures with methionine (1.0 mmol x L(-1)) for 2 h induced a slight increase in cellular Hcy content but no change in ET-1 production. Incubation of cells with Hcy (0.2 mmol x L(-1)) led to a significant fall in ET-1 generation, accompanied by a significant increase in cellular Hcy content. Addition of the amino-acid transport system L substrate 2-amino-2-norbornane carboxylic acid had no effect on the Hcy-induced decrease in ET-1 production but significantly inhibited the Hcy-induced increase in the cellular Hcy content. Incubation of cells with a lower Hcy concentration (0.05 mmol x L(-1)) also reduced ET-1 production without increasing the cellular Hcy content. Co-incubation with extracellular free-radical inhibitors (superoxide dismutase, catalase and mannitol) markedly reduced the effect of Hcy on ET-1 production. Thus, it is extracellular Hcy accumulation that triggers the decrease in ET-1 production by endothelial cells through oxidative products.

Homocysteine inhibits TNF-alpha-induced endothelial adhesion molecule expression and monocyte adhesion via nuclear factor-kappaB dependent pathway

Cellular adhesion molecules play a pivotal role in the pathogenesis of atherosclerosis by mediating the adherence of blood leukocytes. Since hyperhomocysteinemia appears to be an independent risk factor for the development of atherosclerosis, in this study we investigated the effect of homocysteine on basal and TNF-alpha-induced expression of intercellular adhesion molecule-1 (ICAM-1), vascular cell-adhesion molecule-1 (VCAM-1), and endothelial leukocyte adhesion molecule-1 (E-selectin) on human umbilical-vein endothelial cells. Incubation of endothelial cells with homocysteine resulted in dose-dependent reduction in TNF-alpha-induced (5 ng/ml) expression of VCAM-1, E-selectin, and ICAM-1 (the latter less pronounced). This effect was found to be specific since other thiol compounds-cysteine and glutathione-did not mimic homocysteine activity. Homocysteine attenuated TNF-alpha-stimulated U-937 adhesion to the endothelial monolayer and reduced TNF-alpha-induced activation of the transcription factor NF-kappaB, indicating that NF-kappaB inhibition may play a role in inhibiting expression of adhesion molecules in endothelial cells.

Pathogenesis of Hyperhomocysteinemia-New Insights

Mild to moderately elevated levels of homocysteine (Hey) in plasma, denoted as hyperhomocysteinemia, is emerging as a prevalent and strong risk factor for atherosclerotic vascular disease in coronary, cerebral and peripheral vessels, as well as for arterial and venous thromboembolism. Despite its clinical significance, the molecular mechanism of homocysteine's effects is not yet clearly defined. Most of the effects of homocysteine that have been demonstrated in vitro, affecting endothelial function have been attributed to the oxidant reactivity of this molecule, which is shown to affect the vasoregulatory and thrombotic/fibrinolytic function of endothelium. However, the relevance of these observations to the clinical situations is questionable, since excessively high concentrations of homocysteine are used in most of the experiments. We have observed that homocysteine, at physiologically relevant concentrations, specifically induces the expression of tissue factor by monocytes, and a non-specific redox effect is not involved. Tissue factor expression by monocytes is mediated by increased intracellular concentrations of the metabolic intermediate, S-adenosylhomocysteine, which is a potent inhibitor of methyl transferases. These studies suggest that tissue factor expression by circulating monocytes by intracellular perturbations may be a plausible mechanism by which homocysteine may induce thrombosis.