Biochemical evidence for a link between elevated levels of homocysteine and lipid peroxidation in vivo

Decreased autophagy of vascular smooth muscle cells was involved in hyperhomocysteinemia-induced vascular ageing

Ageing and hyperhomocysteinemia (HHcy) are important risk factors for cardiovascular diseases (CVDs). HHcy affects the occurrence of vascular diseases in the elderly. So far, the mechanism of HHcy-induced vascular ageing remains largely unknown. Autophagy level is significantly reduced in the ageing process, and restoring impaired autophagy to a normal state may be one of the possible ways to extend the expected longevity and lifespan in the future. In this study, we established the HHcy rat model by feeding a 2.5% methionine diet. Small animal ultrasound and the tail-cuff method indicated that the vascular pulse wave velocity (PWV) and pulse pressure (PP) of HHcy rats were increased significantly compared with the control group. Vascular morphology and structure have been changed in HHcy rats, including lumen dilation, increased collagen fibre deposition and increased p53/p21/p16 expression. In vitro, under the stimulation of homocysteine (500 μmol/L, 24 hours), the rat vascular smooth muscle cells (VSMCs) presented senescence, which was characterized by the increased expression of ageing-related markers, such as p16, p21 and p53 as well as increased senescence-associated beta-galactosidase (SA-β-gal) activity. Meanwhile, the autophagy level was decreased both in vivo and in vitro, shown as the increased level of autophagy substrate p62 and the reduced level of autophagy marker LC3 II/I in the thoracic aorta of HHcy rats and in Hcy-treated VSMCs, respectively. The senescence phenotype of VSMCs was reversed by increased autophagy levels induced by rapamycin. Our findings indicate that decreased autophagy of VSMCs is involved in hyperhomocysteinemia-induced vascular ageing.

Implication of Hyperhomocysteinemia in Blood Retinal Barrier (BRB) Dysfunction

Elevated plasma homocysteine (Hcy) level, known as hyperhomocysteinemia (HHcy) has been linked to different systemic and neurological diseases, well-known as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and has been identified as a risk factor for several ocular disorders, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). Different mechanisms have been proposed to explain HHcy-induced visual dysfunction, including oxidative stress, upregulation of inflammatory mediators, retinal ganglion cell apoptosis, and extracellular matrix remodeling. Our previous studies using in vivo and in vitro models of HHcy have demonstrated that Hcy impairs the function of both inner and outer blood retinal barrier (BRB). Dysfunction of BRB is a hallmark of vision loss in DR and AMD. Our findings highlighted oxidative stress, ER stress, inflammation, and epigenetic modifications as possible mechanisms of HHcy-induced BRB dysfunction. In addition, we recently reported HHcy-induced brain inflammation as a mechanism of blood–brain barrier (BBB) dysfunction and pathogenesis of Alzheimer’s disease (AD). Moreover, we are currently investigating the activation of glutamate receptor N-methyl-d-aspartate receptor (NMDAR) as the molecular mechanism for HHcy-induced BRB dysfunction. This review focuses on the studied effects of HHcy on BRB and the controversial role of HHcy in the pathogenesis of aging neurological diseases such as DR, AMD, and AD. We also highlight the possible mechanisms for such deleterious effects of HHcy.

Hyperhomocysteinemia and neurologic disorders: a review

Homocysteine (Hcy) is a sulfur-containing amino acid that is generated during methionine metabolism. It has a physiologic role in DNA metabolism via methylation, a process governed by the presentation of folate, and vitamins B6 and B12. Physiologic Hcy levels are determined primarily by dietary intake and vitamin status. Elevated plasma levels of Hcy (eHcy) can be caused by deficiency of either vitamin B12 or folate, or a combination thereof. Certain genetic factors also cause eHcy, such as C667T substitution of the gene encoding methylenetetrahydrofolate reductase. eHcy has been observed in several medical conditions, such as cardiovascular disorders, atherosclerosis, myocardial infarction, stroke, minimal cognitive impairment, dementia, Parkinson's disease, multiple sclerosis, epilepsy, and eclampsia. There is evidence from laboratory and clinical studies that Hcy, and especially eHcy, exerts direct toxic effects on both the vascular and nervous systems. This article provides a review of the current literature on the possible roles of eHcy relevant to various neurologic disorders.

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.

Genetically Determined Procoagulant States and Heparin Use

The propensity for both arterial and venous thrombotic disorders involves a genetic predetermination that operates In concert with environmental factors or triggers. Appropriate clinical assessment and therapeutic recommendations for patients with thrombosis requires a thorough knowledge of genetic variables that influence this propensity. This review focuses on the pathophysiology, natural history, and molecular biology of defined thrombophilic risk factors relevant to the care of patients with thrombotic disorders.

Folic acid enhances endothelial function and reduces blood pressure in smokers: a randomized controlled trial

OBJECTIVE

Cigarette smoking is associated with increased plasma homocysteine concentrations, endothelial dysfunction and arterial stiffening. Homocysteine per se induces endothelial dysfunction and arterial stiffening and might account, at least partly, for the vascular abnormalities observed in smokers. We sought to determine whether folic acid supplementation, by reducing plasma homocysteine concentrations, enhanced endothelial function and reduced arterial stiffness in smokers.

DESIGN

Double-blind, randomized controlled, parallel-group, trial.

SETTING

Academic medical centre.

SUBJECTS

A consecutive sample of 24 healthy cigarette smokers (age 37.8 +/- 2.5 years, mean +/- SEM).

INTERVENTION

Subjects were randomly assigned to 4-week folic acid 5 mg day-1 or placebo.

MAIN OUTCOME MEASURES

The following were measured before and after treatment: (i) peripheral vasoreactivity (forearm arterial blood flow, FABF) during intra-arterial administration of endothelium-dependent (acetylcholine 1.5, 4.5 and 15 microg min-1) and endothelium-independent (sodium nitroprusside 1, 2 and 4 microg min-1) vasodilators; (ii) carotid-femoral pulse-wave velocity (PWV); (iii) blood pressure (BP).

RESULTS

Folic acid reduced homocysteine concentrations (10.8 +/- 0.6 vs. 8.2 +/- 0.5 micromol L-1, P < 0.001) and enhanced endothelium-dependent vasodilatation during each acetylcholine infusion rate (ratio between the FABF in the infused and control arm during increasing infusion rates at baseline 1.09 +/- 0.03 vs. 1.41 +/- 0.09 after treatment, P < 0.01; 1.39 +/- 0.07 vs. 1.83 +/- 0.12, P < 0.01; 1.65 +/- 0.16 vs. 2.72 +/- 0.36, P < 0.05) whilst endothelium-independent vasodilatation was unaffected. A significant fall in BP was also observed (mean BP 88 +/- 2 vs. 83 +/- 1 mmHg, P < 0.01). By contrast, PWV did not significantly change (8.4 +/- 0.3 vs. 7.8 +/- 0.4 m s-1). No significant changes in plasma homocysteine concentrations, FABF, BP, and PWV were observed in the placebo group. A multiple regression analysis showed that changes in folic acid plasma concentrations independently predicted both FABF changes during maximal acetylcholine-mediated vasodilatation (P < 0.01) and BP changes (P = 0.01).

CONCLUSIONS

Short-term folic acid supplementation significantly enhanced endothelial function and reduced BP in young chronic smokers. These effects were largely independent from the homocysteine lowering effects. Thus, a simple, nontoxic, and relatively inexpensive vitamin intervention might be useful in primary cardiovascular prevention in this high-risk group.

Homocysteine and cardiovascular disease: current evidence and future prospects

Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. Despite the well-known effectiveness of vitamin supplementation in reducing homocysteine levels, it is not known whether lowering of homocysteine levels is associated with a reduction in cardiovascular morbidity and mortality. The aim of this review is to discuss the epidemiologic evidence about the relation between homocysteine and cardiovascular disease, the pathophysiologic mechanisms responsible for the deleterious vascular and hemostatic effects of homocysteine, and studies of the potential benefits of homocysteine-lowering therapy.