Homocysteine and hypomethylation. A novel link to vascular disease

A mild elevation in serum homocysteine levels is an independent risk factor for arteriosclerosis and venous thrombosis. Despite the clinical significance of homocysteine, however, the molecular mechanisms of homocysteine-induced arteriosclerosis have not been completely elucidated. This lack of understanding is due in large part to the excessively high concentrations of homocysteine (greater than 1 mM) used in experiments. Many of homocysteine's effects have been attributed to its prooxidant activity, which is implicated as the mechanism through which it inhibits production of endothelium-derived relaxing factor and activates quiescent vascular smooth muscle cells. We have found that homocysteine at 10 to 50 microM (but not cysteine) inhibits progression of the vascular endothelial cell cycle at or before the G1-S junction. This inhibition appears to be mediated by decreases in the carboxyl methylation, membrane association, and activity of p21ras--a major G1 regulator. Homocysteine may play an important role in promoting arteriosclerosis by inducing endothelial dysfunction, by inhibiting endothelial cell regeneration, and by directly activating quiescent vascular smooth muscle cells.     

Homocysteine: a novel risk factor in vascular disease

Homocysteine is a sulphydryl-containing aminoacid derived from the metabolic demethylation of methionine. Moderately raised concentrations of total homocysteine (tHcy) have been correlated with an increased risk of atherothrombotic vascular events. The prevalence of hyperhomocysteinaemia has been estimated to be about 5% in the general population, and 13–47% among patients with symptomatic atherosclerotic vascular disease. Nutritional deficiencies in the vitamin cofactors (folate, vitamin B12, and vitamin B6) required for homocysteine metabolism may promote hyperhomocysteinaemia. Clinical and experimental studies suggest that high homocysteine concentrations may cause the atherogenic and thrombotic tendencies of homocystinuric and hyperhomocysteinaemic patients. Experimental evidence suggests that the atherogenic propensity associated with hyperhomocysteinaemia results from endothelial dysfunction and injury followed by platelet activation and thrombus formation. The treatment of hyperhomocysteinaemia varies with the underlying cause; however, vitamin supplementation (with folic acid, pyridoxine [vitamin B₆], and vitamin B₁₂) is generally effective in reducing homocysteine concentrations. Before advocating widespread screening of patients with atherosclerotic vascular disease, we must have a clearer understanding of the clinical efficacy of potential therapeutic interventions. Prospective, randomized clinical trials, however, will be necessary to determine the effect of vitamin supplementation on cardiovascular morbidity and mortality.     

Phosphatidylserine-related adhesion of human erythrocytes to vascular endothelium

In pathological conditions such as sickle cell disease, falciparum malaria and diabetes, an abnormal adherence of erythrocytes to endothelium is concomitant with loss of phospholipid asymmetry resulting in phosphatidylserine (PS) exposure. We have investigated the involvement of PS in this interaction by studying adhesion of human erythrocytes, treated with Ca2+-ionophore A23187 in combination with N-ethylmaleimide, to human umbilical vein endothelial cells in a flow-based assay. Results showed that erythrocytes which exposed PS, massively adhered to HUVEC in a Ca2+-dependent manner. This adhesion was inhibited by PS liposomes and by annexin V, giving clear evidence of the PS dependence of these interactions.     

Diminished adherence of sickle erythrocytes to cultured vascular endothelium by piracetam

Erythrocytes from both normal and sickle cell donors were incubated on a bed of cultured rat vascular endothelial cells. Although the adherence of the erythrocytes to the endothelial cells was equivalent for both classes of donor, addition of 1.5 mM piracetam significantly decreased this adherence regardless of donor.     

Clinical relevance of vascular endothelial growth factor levels in sickle cell disease

In recent years, vascular inflammation, marked by activated monocytes and endothelium, has been proven to play a significant role in the pathogenesis of vaso-occlusive events (VOEs) in sickle cell disease (SCD). Vascular endothelial growth factor (VEGF), an endothelial cell mitogen, has been shown to contribute to the increased endothelial cell adhesivity by increasing the expression of cell adhesion molecules ICAM-1 (intercellular adhesion molecule-1) and VCAM-1 (vascular cell adhesion molecule-1) on the endothelium. We have investigated VEGF alterations in 37 patients with SCD during VOEs and/or steady state. VEGF levels (mean±SEM) were found to be significantly elevated during VOEs (703.1±119.0 pg/ml) when compared with those at steady state (258.0±57.8 pg/ml) and healthy controls (196.6±21.9 pg/ml) (p<0.001). However, we did not find a difference between VEGF concentrations in sickle patients at steady state and the normal subjects (p>0.05). We suggest that considering a possible stimulatory role of tissue hypoxia in VEGF production during VOEs, VEGF levels in sickle patients might be helpful in monitoring disease severity.     

Homocysteine metabolism, hyperhomocysteinaemia and vascular disease: An overview

Summary Hyperhomocysteinaemia has been regarded as a new modifiable risk factor for atherosclerosis and vascular disease. Homocysteine is a branch-point intermediate of methionine metabolism, which can be further metabolised via two alternative pathways: degraded irreversibly through the transsulphuration pathway or remethylated to methionine by the remethylation pathway. Both pathways are B-vitamin-dependent. Plasma homocysteine concentrations are determined by nongenetic and genetic factors. The metabolism of homocysteine, the role of B vitamins and the contribution of nongenetic and genetic determinants of homocysteine concentrations are reviewed. The mechanisms whereby homocysteine causes endothelial damage and vascular disease are not fully understood. Recently, a link has been postulated between homocysteine, or its intermediates, and an alterated DNA methylation pattern. The involvement of epigenetic mechanisms in the context of homocysteine and atherosclerosis, due to inhibition of transmethylation reactions, is briefly overviewed. Communicating editor: Guy Besley Competing interests: None declared     

Sodium phenyl butyrate downregulates endothelin-1 expression in cultured human endothelial cells: relevance to sickle-cell disease

As hydroxyurea (HU), sodium phenyl butyrate (SPB) is known to induce fetal hemoglobin (HbF) expression and thus shows potentials for sickle-cell disease (SCD) treatment. More recently, few studies suggested that endothelial cells (ECs), a major pathophysiological actor of SCD, are also a target of SPB. Here, we show that SPB, as HU, reduces endothelin-1 mRNA expression and peptide release by human ECs in culture. SPB increases VCAM-1 and ICAM-1 mRNAs and soluble ICAM-1 release. Both drugs have a cumulative effect on ICAM-1 expression. We conclude that SPB, as HU, also affects the expression of molecules important to the pathophysiology of SCD, in addition to its effect on HbF. Its potential as an alternative or adjuvant drug in SCD treatment warrants further investigations.     

Homocysteine. The association with atherosclerotic vascular disease in older persons

Plasma homocysteine is a risk factor for coronary artery disease, stroke, peripheral arterial disease, extracranial carotid arterial disease, aortic atherosclerosis, deep vein thrombosis, and possibly dementia and Alzheimer's disease in older persons. Randomized trials are in progress investigating whether multivitamin therapy with folic acid, vitamin B12, and vitamin B6 to reduce plasma homocysteine levels will reduce the risk for atherosclerotic vascular disease.     

Animal models of spontaneous autoimmune diabetes: Notes on their relevance to the human disease

The spontaneous autoimmune diabetes of the nonobese diabetic mouse and the BioBreeding diabetes-prone rat bears a striking similarity to human type 1 diabetes. For this reason it comes as a disappointment that interventions known to prevent the disease in the animal models proved ineffective in human trials. For an explanation I propose that, although the mechanism of b-cell destruction is the same in the three species, this common mechanism may be the end point of different loss-of-tolerance pathways in different human patients, only two of which are represented by the two animal models. Research strategies to address this problem will require more attention to immune subphenotypes or their corresponding subgenotypes in the human and, possibly, the generation of additional mouse models of spontaneous disease.     

Recombinant tumor necrosis factor induces procoagulant activity in cultured human vascular endothelium: characterization and comparison with the actions of interleukin 1.

Human recombinant tumor necrosis factor (rTNF) was found to act directly on cultured human vascular endothelium to induce a tissue factor-like procoagulant activity (PCA). After a 4-hr incubation in rTNF (100 units/ml), serially passaged endothelial cells isolated from umbilical veins, saphenous veins, iliac arteries, and thoracic aortae demonstrated a dramatic increase (4- to 15-fold, 21 experiments) in total cellular PCA as measured with a one-stage clotting assay. rTNF-induced PCA was also expressed at the surface of intact viable endothelial monolayers. Induction of PCA by rTNF was concentration dependent (maximum, 500 units/ml), time dependent, reversible, and blocked by cycloheximide and actinomycin D, and it occurred without detectable endothelial cell damage. Actions of rTNF were compared with those of natural human interleukin 1 (IL-1) derived from stimulated monocytes and two distinct species of recombinant IL-1, each of which also induced endothelial PCA. The use of recombinant polypeptides and specific neutralizing antisera established the distinct natures of the mediators. The kinetics of the endothelial PCA responses to TNF and IL-1 were similar, demonstrating a rapid rise to peak activity at approximately equal to 4 hr, and a decline toward basal levels by 24 hr. This characteristic decline in PCA after prolonged incubation with TNF or IL-1 was accompanied by selective endothelial hyporesponsiveness to the initially stimulating monokine. Interestingly, the effects of TNF and IL-1 were found to be additive even at apparent maximal doses of the individual monokines. Endothelial-directed actions of TNF, alone or in combination with other monokines, may be important in the initiation of coagulation and inflammatory responses in vivo.     

Animal models of sinusitis: relevance to human disease

Experimental animal models are necessary to assess the efficacy of both medical and surgical procedures. The researcher who wishes to confirm and publish his results must overcome controversies concerning the validity of his model and, therefore, needs guidelines for standardization. Various models are offered in the literature. In this article, the rabbit and murine models of experimental sinusitis and their relevance to human disease with anatomical, histopathologic, and microbiologic aspects are discussed.     

Antioxidant properties of N-acetylcysteine: their relevance in relation to chronic obstructive pulmonary disease.

Oxidative stress has been implicated in the pathogenesis and progression of chronic obstructive pulmonary disease. Both reactive oxidant species from inhaled cigarette smoke and those endogenously formed by inflammatory cells constitute an increased intrapulmonary oxidant burden. Structural changes to essential components of the lung are caused by oxidative stress, contributing to irreversible damage of both parenchyma and airway walls. In addition, oxidative stress results in alterations in the local immune response, increasing the risk of infections and exacerbations, which, in turn, may accelerate lung function decline. The antioxidant N-acetylcysteine, a glutathione precursor, has been applied in these patients in order to reduce symptoms, exacerbations and the accelerated lung function decline. This article reviews the presently available experimental and clinical data on the antioxidative effects of N-acetylcysteine in chronic obstructive pulmonary disease.     

Arterial endothelium and atherothrombogenesis. I--Intact endothelium in vascular and blood homeostasis.

Normal endothelium constitutes a physical and biological barrier between the blood and the vascular wall, and also acts as a sensor and transducer of various endogenous and exogenous factors that modulate the blood circulation. Endothelial activity in a given individual at any particular moment reflects the balance between cardiovascular risk factors, genetic predisposition and vascular protection mechanisms. The availability and activity of endothelium-derived nitric oxide (NO) is a major factor in these mechanisms. Further, vasoactive substances synthesized by the vascular wall and/or by blood cells may affect the behavior of the blood-endothelium interface. Vasomotricity is dependent on the balance between vasodilator substances (particularly prostacylin) and vasoconstrictor products (mainly endothelin-1 and angiotensin II). The coagulation-anticoagulation or fibrinolysis balance is also affected by various different proteins. The mechanisms of these factors, on which blood fluidity depends under normal conditions and with intact endothelium, are discussed, along with mention of potential abnormalities, which will be examined in the second part of this review.     

Adenosine-receptor subtypes: their relevance to adenosine-mediated responses in asthma and chronic obstructive pulmonary disease.

Adenosine administration by inhalation elicits concentration-related bronchoconstriction in subjects with asthma and chronic obstructive pulmonary disease (COPD). The mechanisms of adenosine-induced bronchoconstriction appear to involve a selective interaction with activated mast cells with subsequent release of preformed and newly-formed mediators. Further evidence linking adenosine signalling to asthma and COPD comes from the finding that many cell types that play important roles in the exacerbation of these conditions express adenosine receptors and demonstrate relevant effects through stimulation of these receptors. Therefore, blockade of these receptors may be a valuable approach to the treatment of asthma and chronic obstructive pulmonary disease. Promising adenosine-receptor targets for novel therapeutics of asthma and chronic obstructive pulmonary disease have recently been identified in a number of inflammatory cell types, including mast cells, eosinophils, lymphocytes, neutrophils, and macrophages. The recent characterisation of the A2B receptors indicates the human lung mast cell as one of the most strategic cellular targets.     

同型半胱氨酸对血管内皮细胞E-选择素、IL-6分泌的影响

目的探讨同型半胱氨酸（homocysteine,Hcy）对人脐静脉内皮细胞E-选择素、白细胞介素6（IL6）分泌的影响。方法在人脐静脉内皮细胞ECV304培养基中加入不同浓度Hcy,孵育不同时间,用ELISA法分别测定培养上清液中E-选择素及IL6的含量。结果终浓度为250μmol/L的Hcy孵育内皮细胞,E选择素的分泌在4h显著上调,6h达峰值;IL6的分泌在6h开始升高,24h达峰值,之后随着时间的延长逐渐下降。Hcy呈浓度依赖性（50μmol/L～500μmol/L）促进内皮细胞E选择素及IL6的分泌。结论Hcy能刺激血管内皮细胞分泌E选择素及IL6。