Diet-induced hyperhomocysteinemia exacerbates neointima formation in rat carotid arteries after balloon injury

Rosiglitazone reduces serum homocysteine levels, smooth muscle proliferation, and intimal hyperplasia in Sprague-Dawley rats fed a high methionine diet

Homocysteine (Hcy) is a metabolite of the essential amino acid methionine. Hyperhomocysteinemia is associated with vascular disease, particularly carotid stenosis. Rosiglitazone, a ligand of the peroxisome proliferator-activated receptor gamma , attenuates balloon catheter-induced carotid intimal hyperplasia in type 2 diabetic rats. We studied 4 groups (n = 7 per group) of adult female Sprague-Dawley rats fed (a) powdered laboratory chow (control), (b) control diet with rosiglitazone (3.0 mg/kg/d), (c) diet containing 1.0% l -methionine, and (d) diet containing methionine and rosiglitazone. After 1 week on high methionine diet, the rats were administered an aqueous preparation of rosiglitazone by oral gavage. One week after initiation of rosiglitazone, balloon catheter injury of the carotid artery was carried out using established methods, and the animals continued on their respective dietary and drug regimens for another 21 days. At the end of the experimental period, blood samples were collected, and carotid arteries and liver were harvested. Serum Hcy increased significantly on methionine diet compared with controls (28.9 +/- 3.2 vs 6.3 +/- 0.04 micromol/L). Development of intimal hyperplasia was 4-fold higher in methionine-fed rats; this augmentation was significantly reduced ( P < .018) in rosiglitazone-treated animals. Rosiglitazone treatment significantly ( P < .001) suppressed Hcy levels and increased the activity of the Hcy metabolizing enzyme, cystathionine-beta-synthase in the liver samples. Hcy (100 micromol/L) produced a 3-fold increase in proliferation of rat aortic vascular smooth muscle cells; this augmentation was inhibited by incorporating rosiglitazone (10 micromol/L). After balloon catheter injury to the carotid artery of animals on a high methionine diet, there was an increase in the rate of development of intimal hyperplasia consistent with the known effects of Hcy. It is demonstrated for the first time that the peroxisome proliferator-activated receptor gamma agonist rosiglitazone can attenuate the Hcy-stimulated increase in the rate of development of intimal hyperplasia indirectly by increasing the rate of catabolism of Hcy by cystathionine-beta-synthase and directly by inhibiting vascular smooth muscle cell proliferation. These findings may have important implications for the prevention of cardiovascular disease and events in patients with hyperhomocysteinemia (HHcy).

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.

Homocysteine and carotid intima-media thickness: a critical appraisal of the evidence

This review examines the relationship between hyperhomocysteinemia, a risk factor for vascular disease, and carotid intima-media thickness (CIMT), a valid marker of generalized atherosclerosis and future vascular disease risk. The relationship between two important determinants of hyperhomocysteinemia in the general population-folate status and the 677C --> T methylenetetrahydrofolate reductase (MTHFR) polymorphism-and CIMT is also covered.

METHODS

We searched literature databases for articles examining homocysteine and CIMT published before September 2003.

RESULTS

We identified 54 studies. Observational studies generally failed to demonstrate a relationship between homocysteine and CIMT in homocystinuric, uremic, hypercholesterolemic or non-insulin-dependent diabetes mellitus patients or in subjects with insulin insensitivity. Weak associations, but usually only in certain sub-populations were found in vascular disease patients and in population-based studies. B vitamins reduce the progression of CIMT in renal transplant recipients and vascular disease patients as demonstrated by two trials. The majority of studies demonstrated increased CIMT in individuals with the MTHFR 677TT genotype. Folate status showed no relation to CIMT.

DISCUSSION

In non-patient populations, hyperhomocysteinemia is weakly associated with CIMT. The association of the 677 C--> T MTHFR polymorphism with CIMT further supports this finding. Lastly, folate levels may need to reach a critically low status before an association can be found between folate and CIMT. Larger trials in various population types are needed to determine whether folate alone or in combination with Vitamins B6 and B12 will slow down or even reverse atherosclerotic progression.

C677T polymorphism of the methylenetetrahydrofolate reductase gene is a risk factor of adverse events after coronary revascularization

BACKGROUND

A common point mutation (C677T) in the gene for 5,10-methylenetetrahydrofolate reductase (MTHFR) is associated with hyperhomocysteinemia, an independent risk factor and a strong predictor of mortality in patients with coronary artery disease (CAD). The aim of this study was to investigate whether C677T polymorphism can be a predictor of major adverse cardiac events after myocardial revascularization.

METHODS

We determined MTHFR genotype in 159 patients with CAD undergoing myocardial revascularization [72 percutaneous transluminal coronary angioplasty (PTCA) and 87 coronary artery bypass graft (CABG)]. Recurrent angina, nonfatal myocardial infarction (MI), target vessel revascularization, heart failure and cardiac death were considered major adverse cardiac events that occurred after discharge from index hospitalization.

RESULTS

During the follow-up (6.9+/-0.3 months, mean+/-S.E.M.), the composite endpoint accounted for 25.9%, 11.4% and 4.3% for TT, CT and CC genotype (log-rank statistic 5.2, p=0.02), respectively. Subjects with mutant TT genotype had a threefold increase of any cardiac event (hazard ratio [HR]=3.0; 95% [CI], 1.1-8.1). In multiple-variable regression Cox, predictors of events were TT genotype (HR=2.8; 95% CI, 1.01-7.62, p=0.047), low-ejection fraction<40% (HR=4.5; 95% CI, 1.62-12.6, p=0.004) and revascularization procedure (HR=6.1; 95% CI, 1.86-20.34, p=0.003).

CONCLUSIONS

These data indicate that the TT genotype seems to be significantly associated with major adverse cardiac events after myocardial revascularization in CAD patients, suggesting a potential pathological influence of homocysteine in the clinical outcome.

Folate therapy and in-stent restenosis after coronary stenting

BACKGROUND

Vitamin therapy to lower homocysteine levels has recently been recommended for the prevention of restenosis after coronary angioplasty. We tested the effect of a combination of folic acid, vitamin B6, and vitamin B12 (referred to as folate therapy) on the risk of angiographic restenosis after coronary-stent placement in a double-blind, multicenter trial.

METHODS

A total of 636 patients who had undergone successful coronary stenting were randomly assigned to receive 1 mg of folic acid, 5 mg of vitamin B6, and 1 mg of vitamin B12 intravenously, followed by daily oral doses of 1.2 mg of folic acid, 48 mg of vitamin B6, and 60 microg of vitamin B12 for six months, or to receive placebo. The angiographic end points (minimal luminal diameter, late loss, and restenosis rate) were assessed at six months by means of quantitative coronary angiography.

RESULTS

At follow-up, the mean (+/-SD) minimal luminal diameter was significantly smaller in the folate group than in the placebo group (1.59+/-0.62 mm vs. 1.74+/-0.64 mm, P=0.008), and the extent of late luminal loss was greater (0.90+/-0.55 mm vs. 0.76+/-0.58 mm, P=0.004). The restenosis rate was higher in the folate group than in the placebo group (34.5 percent vs. 26.5 percent, P=0.05), and a higher percentage of patients in the folate group required repeated target-vessel revascularization (15.8 percent vs. 10.6 percent, P=0.05). Folate therapy had adverse effects on the risk of restenosis in all subgroups except for women, patients with diabetes, and patients with markedly elevated homocysteine levels (15 micromol per liter or more) at baseline.

CONCLUSIONS

Contrary to previous findings, the administration of folate, vitamin B6, and vitamin B12 after coronary stenting may increase the risk of in-stent restenosis and the need for target-vessel revascularization.

Mild hyperhomocysteinemia induced by feeding rats diets rich in methionine or deficient in folate promotes early atherosclerotic inflammatory processes

High homocysteine levels in vitro promote the expression of inflammatory agents responsible for atherogenesis. We investigated the long-term effects of elevated plasma homocysteine on the expression of inflammatory molecules and attempted to elucidate their mechanisms. Male Sprague-Dawley rats (n = 36) were randomly divided into 3 groups, which received the control AIN-93G diet, the control diet plus 10 g/kg of L-methionine, or that diet without folate (0 m/kg) for 14 wk. Mild hyperhomocysteinemia was then induced in both experimental groups. The mildly hyperhomocysteinemic rats had markedly increased expression of intracellular adhesion molecule-1 (ICAM-1) in the aorta and elevated serum levels of monocyte chemoattractant protein-1 (MCP-1), compared to the control rats. The activation of nuclear factor kappaB and formation of nitrotyrosine in the aorta were greater in rats with mild hyperhomocysteinemia than in control rats. Serum levels of malonyldialdehyde (MDA) were higher in mildly hyperhomocysteinemic rats than in control rats. These results suggest that the oxidative stress resulting from elevated plasma homocysteine stimulates the activation of nuclear factor kappaB, and consequently increases the expression of the inflammatory factors in vivo. Such an effect may contribute to atherogenesis by enhancing the inflammatory response of the vascular endothelium.

Effects of dietary folic acid supplementation on cerebrovascular endothelial dysfunction in rats with induced hyperhomocysteinemia

This study shows, for the first time, that hyperhomocysteinemia induces endothelial dysfunction in a rat brain, and that this can be alleviated by dietary folic acid supplementation. Our experiments examined the effects of folic acid supplementation on the endothelial nitric oxide synthase (eNOS) expression in the hyperhomocysteinemic rat brain, and related the observed changes in eNOS expression to the expression of the cell adhesion molecule and the glucose transporter protein. The animals were raised on an experimental diet containing 0.3% homocystine for 2 weeks and then they were placed either on a 0.3% homocystine, 0.3% homocystine with 8 mg/kg folic acid, or folic acid (8 mg/kg) diet for 2 weeks. The cerebrovascular eNOS activity was examined immunohistochemically. Cerebral levels of eNOS, glucose transporter-1 (GLUT-1), and the vascular cell adhesion molecule-1 (VCAM-1) proteins were evaluated by Western blot analysis. At 4 weeks, the homocystine diet induced a fourfold increase in plasma homocysteine (control: 6.5+/-0.4 micromol/l, homocystine: 26.2+/-2.5 micromol/l), and a reduction in the cerebral eNOS and GLUT-1 expression levels with a concomitant increase in the level of VCAM-1 expression. Dietary folic acid supplementation caused a significant decrease in the plasma homocysteine levels, a concomitant increase in the hyperhomocysteinemia-induced reduction in the cerebral eNOS and GLUT-1 expression levels, and a decrease in the hyperhomocysteinemia-induced VCAM-1 expression levels.

The atherogenic effect of excess methionine intake

Methionine is the precursor of homocysteine, a sulfur amino acid intermediate in the methylation and transsulfuration pathways. Elevated plasma homocysteine (hyperhomocysteinemia) is associated with occlusive vascular disease. Whether homocysteine per se or a coincident metabolic abnormality causes vascular disease is still an open question. Animals with genetic hyperhomocysteinemia have so far not displayed atheromatous lesions. However, when methionine-rich diets are used to induce hyperhomocysteinemia, vascular pathology is often observed. Such studies have not distinguished the effects of excess dietary methionine from those of hyperhomocysteinemia. We fed apolipoprotein E-deficient mice with experimental diets designed to achieve three conditions: (i) high methionine intake with normal blood homocysteine; (ii) high methionine intake with B vitamin deficiency and hyperhomocysteinemia; and (iii) normal methionine intake with B vitamin deficiency and hyperhomocysteinemia. Mice fed methionine-rich diets had significant atheromatous pathology in the aortic arch even with normal plasma homocysteine levels, whereas mice fed B vitamin-deficient diets developed severe hyperhomocysteinemia without any increase in vascular pathology. Our findings suggest that moderate increases in methionine intake are atherogenic in susceptible mice. Although homocysteine may contribute to the effect of methionine, high plasma homocysteine was not independently atherogenic in this model. Some product of excess methionine metabolism rather than high plasma homocysteine per se may underlie the association of homocysteine with vascular disease.

Effect of metabolic control on homocysteine levels in type 2 diabetic patients: a 3-year follow-up

OBJECTIVES

Hyperhomocysteinaemia has emerged as a novel risk factor for cardiovascular disease. The determinants of total homocysteine (tHcy) levels in type 2 diabetic patients (D2p) have not been studied in detail. We examined prospectively the effect of different degrees of metabolic control on plasma tHcy in D2p with preserved kidney function. SUBJECTS AND MAIN OUTCOME MEASUREMENTS: Ninety-five D2p were studied. Clinical parameters, fasting plasma glucose, HbA1c, serum lipids, blood urea nitrogen (BUN) and creatinine, vitamin B12 and folate and tHcy were measured at the baseline and after 36 months. The methylentetrahydrofolate reductase (MTHFR) C677T polymorphism was also determined. Subjects were categorized according to deltaHbA1c into group A (+/-1 point), B (>1 point increase) or C (>1 point decrease).

RESULTS

Total homocysteine was reduced in subjects whose HbA1c decreased with time, whilst patients showing a worsened metabolic control had an increased tHcy in respect to baseline. A larger response to the improved metabolic control in terms of tHcy reduction was noted in wild type patients versus those homozygous for the mutation. A multivariate analysis revealed MTHFR polymorphism and HbA1c as strong determinants of changes in tHcy with time.

CONCLUSIONS

The findings suggest that in D2p tHcy decreases even with modest improvement of glycaemic control; moreover patients homozygous for the MTHFR C677T mutation show the largest changes in tHcy levels with concomitant changing of HbA1c. These results define a further mechanism through which hyperglycaemia might promote cardiovascular damage in diabetic patients.

Effects of long-term administration of methionine on vascular endothelium in rabbits

BACKGROUND AND AIM

We studied the effects of long-term methionine administration on the vascular endothelium of Japanese white rabbits.

METHODS AND RESULTS

Eleven rabbits were divided into a control group (n = 6) and a methionine-fed group (n = 5), and reared for 22 weeks. Blood samples were collected at baseline and after 22 weeks for the measurement of serum homocysteine and cysteine, serum lipids and serum superoxide dismutase activity. At the end of experiments, the animals were sacrificed, and the thoracic aorta was removed for the measurement of isometric tension and histopathological examination. The blood samples taken from the methionine group in the 22nd week showed slight but significant increases in serum homocysteine and cysteine levels (Hcy: 13.7 +/- 1.4 vs 21.0 +/- 4.9, p < 0.01; Cys: 241.6 +/- 37.8 vs 342.6 +/- 35.0, p < 0.01). In the isometric tension experiments, the methionine group had a significantly decreased (p < 0.01) vasodilatation reaction induced by acetylcholine, an endothelium-dependent vasodilator. The histopathological examination (immunostaining in response to eNOS and tissue factor) showed significant increases in endothelium expression in the methionine group before atherosclerotic changes appeared.

CONCLUSIONS

The above results suggest that vascular endothelial dysfunction played an important role in the atherosclerosis occurring after excess methionine feeding.

Homocisteína y progresión de la aterosclerosis de la arteria carótida en pacientes con enfermedad coronaria

BACKGROUND AND OBJECTIVE

Carotid intima-media thickness (IMT) is a marker of generalized atherosclerosis. Sequential evaluation of carotid IMT has permitted to know the factors involved in its progression. However, there are few studies about the influence of homocysteine in such progression. The aim of this work was to know the effect of homocysteine values on the evolution of carotid IMT in patients with coronary disease.

PATIENTS AND METHOD

Carotid IMT (baseline and after 4 years of follow-up) was evaluated by a B-mode ultrasonography in 187 patients with coronary disease (166 males and 21 females; age: mean [standard deviation], 60 [7] years); 185 patients were treated with statins from the beginning of the study.

RESULTS

Carotid IMT progression was confirmed in 59 patients (31.6%; 95% confidence interval [CI], 25.0-38.7%). Cardiovascular risk factors, basal biochemical parameters and methylenetetrahydrofolate reductase-C677T polymorphism were similar in patients with and without progression except for homocysteine values which were higher in the former (13.3 [5.3]; 95% CI, 12.0-14.6 vs 11.1 [3.5]; 95% CI, 10.5-11.7 (mol/l; p = 0.001). Biochemical changes at the end of the study were similar in both groups. In the multivariate analysis, IMT progression was associated with basal values of homocysteine (odds ratio [OR] 1.19; 95% CI, 1.07-1.31; p = 0.0008), female gender (OR 3.50; 95% CI, 1.17-10.50; p = 0.02), hypertension (OR 2.52; 95% CI, 1.14-5.59; p = 0.02) and basal high-density lipoprotein (HDL)-cholesterol values (OR, 0.94; 95% CI, 0.90-0.98; p = 0.009).

CONCLUSIONS

The concentration of homocysteine is associated with the progression of carotid atherosclerosis in patients with coronary heart disease treated with statins.

Total plasma homocysteine and restenosis after percutaneous coronary angioplasty: current evidence

BACKGROUND

Restenosis after percutaneous coronary angioplasty (PTCA) remains an important limitation of this procedure.

AIM

To assess the relationship between homocysteine levels and restenosis after PTCA, and discuss the potential benefit of homocysteine-lowering therapy.

METHOD

MEDLINE-based literature review.

RESULTS

The conflicting literature on the association between homocysteine levels and restenosis after PTCA can partially be explained by differences in methodology. Depending on the type of studies considered, a pooling of data resulted in a 22%-36% risk reduction of restenosis in lesions exposed to low homocysteine levels. The strongest reduction was found in balloon-only treated lesions (42%), while only a trend (14%) was seen in stented lesions. Based on the only available trial, homocysteine-lowering therapy yielded a 54% restenosis rate reduction, 76% in balloon-only treated lesions and 31% in stented lesions. Furthermore, homocysteine-lowering therapy provided a significant clinical benefit with a 40% relative reduction in major adverse events at 6 months' follow-up.

CONCLUSIONS

This review suggests that plasma homocysteine is a modifiable risk factor for restenosis, which when lowered improves outcome after PTCA. This inexpensive treatment with virtually no side-effects could therefore be considered as adjunctive therapy for patients undergoing PTCA, while awaiting results from further studies.

Platelet activity and in vivo arterial thrombus formation in rats with mild hyperhomocysteinaemia

Elevated plasma levels of total homocysteine (hcy) have been associated with an increased occurrence of arterial thrombosis. In the present study, we investigated the influence of hyperhomocysteinaemia on platelet aggregation and arterial thrombus formation in vivo. Fifty-one rats were included in the study, of which 29 received hcy in the drinking water for 4 weeks. Blood samples were withdrawn for measurement of platelet count and mean platelet volume. Platelet aggregation response in platelet-rich plasma following adenosine diphosphate or collagen stimulation were examined. In vivo thrombus formation was investigated by transillumination and videotape recording of the rat femoral artery after a thrombogenic injury was established. Off-line videotape analysis using computer-assisted planimetry permitted quantification of the thrombus area, and area versus time curves were obtained. In the intervention group receiving hcy, total hcy in plasma increased two-fold to 14.3 micromol/l, as compared with 7.3 micromol/l in the control group (P < 0.001). The platelet count and mean platelet volume did not differ between the two groups. In vivo thrombus formation expressed as the area under the curve or maximum thrombus area was not found to be altered in the presence of an increased homocysteine level, neither was adenosine diphosphate-induced platelet aggregation. However, collagen-induced platelet aggregation significantly decreased in the hcy group (P = 0.02). Pro-thrombotic effects of isolated mild hyperhomocysteinaemia are not supported by the present study in rats.

Plasma homocysteine levels and late outcome after coronary angioplasty

OBJECTIVES

The aim of this study was to evaluate a possible relationship between homocysteine levels on admission and late outcome after successful percutaneous coronary intervention (PCI).

BACKGROUND

Increasing evidence suggests that mild to moderate elevation of total plasma homocysteine is a graded and potentially modifiable risk factor for cardiovascular disease and death that appears to be largely independent of other traditional risk factors.

METHODS

A total of 549 patients were included after successful PCI of at least one coronary stenosis (> or =50%). End points were cardiac death, nonfatal myocardial infarction (MI), target lesion revascularization (TLR), and a composite of major adverse cardiac events (MACE). The relationship between homocysteine levels and study endpoints was assessed.

RESULTS

After a median (+/- SD) follow-up of 58 +/- 20 weeks, 6 patients died of cardiac death, 14 were diagnosed with a new MI, and 71 underwent repeat TLR. A graded relationship between homocysteine levels (quartiles) and freedom from MACE was found (p = 0.01). Homocysteine levels (+/- SD) were associated with cardiac death (14.9 +/- 1.7 micromol/l vs. 9.6 +/- 4.3 micromol/l, p < 0.005), TLR (10.7 +/- 4.4 micromol/l vs. 9.5 +/- 4.3 micromol/l, p < 0.05), and overall MACE (11.0 +/- 4.4 micromol/l vs. 9.4 +/- 4.3 micromol/l, p < 0.005). These findings remained unchanged after adjustment for potential confounders.

CONCLUSIONS

Plasma homocysteine is an independent predictor of mortality, nonfatal MI, TLR, and overall adverse late outcome after successful coronary angioplasty.

Renal tubulointerstitial injury in weanling rats with hyperhomocysteinemia

BACKGROUND

While hyperhomocysteinemia is associated with an increased risk of atherosclerosis and the related cardiovascular diseases, the effect of hyperhomocysteinemia on the kidney has not been clearly demonstrated. The purpose of this study was to investigate whether long-term hyperhomocysteinemia develops atherosclerotic lesions in the kidney.

METHODS

The effects of various dietary combinations, including folate deficiency, choline deficiency and methionine loading, on the plasma homocysteine concentration, renal function and renal histopathology were examined for 12 weeks in male weanling Fisher rats.

RESULTS

Folate deficiency, choline deficiency and methionine loading synergistically induced hyperhomocysteinemia up to 69.7 +/- 23.1 micromol/L (control, 11.6 +/- 3.9 micromol/L, P < 0.01) without any change in blood pressure. Creatinine clearance was negatively correlated with the plasma homocysteine concentration (r = -0.55, P < 0.01). Arterial and arteriolar wall thickening, and focal tubulointerstitial fibrosis were found in the kidneys of the hyperhomocysteinemic rat. The lesions of tubulointerstitial fibrosis appeared striped or wedge-shaped at the subcapsular cortex of the kidney. In addition, the expression of vascular endothelial growth factor, an indicator of hypoxia, was increased in the adjacent more intact area of the cortex. These findings suggest that the renal tubulointerstitial lesions were likely to be mediated by severe ischemia due to regional circulatory disturbance. Folate supplementation diminished these vascular and tubulointerstitial changes.

CONCLUSION

These results indicate that diet-induced chronic hyperhomocysteinemia could induce vascular remodeling and tubulointerstitial injury in the kidney, and that these changes were ameliorated by folate supplementation.

Homocysteine and atherothrombosis

Atherosclerosis with or without thrombosis superimposed is the most frequent cause of ischemic heart disease (IHD), peripheral arterial disease, and a main cause of stroke. Conflicting results have been reported in genetic, observational, and experimental studies on the relationship between homocysteine and these atherothrombotic diseases. Although cardiovascular complications are common in homocystinuric patients (severe hyperhomocysteinemia), IHD, the most frequent manifestation of atherothrombosis in the general population, appears to be rare. On the basis of findings in individuals with hyperhomocysteinemia of genetic origin, there is in fact no clear evidence for a causal role of homocysteine in the pathogenesis of atherothrombotic disease, and the positive association between plasma homocysteine and IHD observed in many, but not all epidemiologic studies does not prove causality. To infer causality from observational studies, there should be a temporal, consistent, strong, independent, graded (dose-response effect), and duration-dependent relationship between exposure and outcomes, and a biologically plausible mechanism should exist. The relationship between plasma homocysteine levels and IHD does not fulfill these criteria beyond reasonable doubt. In the general population, plasma homocysteine levels are to a great extent determined by dietary habits, and plasma homocysteine could be a marker, or a consequence, of atherothrombosis and/or risk-associated behavior (e.g., a diet low in fruits and vegetables) rather than a cause of atherothrombosis. Experimentally, hyperhomocysteinemia is not in itself atherogenic in normal animals with relatively low plasma cholesterol levels. The homocysteine theory of atherosclerosis should be tested more thoroughly in hypercholesterolemic animals that develop atherosclerosis spontaneously to determine whether elevated plasma homocysteine levels are harmful under atherogenic conditions. A causal role of homocysteine in atherothrombotic disease remains to be established.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Diet-induced mild hyperhomocysteinemia and increased salt intake diminish vascular endothelial function in a synergistic manner

OBJECTIVES

We investigated the influence of hyperhomocysteinemia and high salt intake on sodium handling, oxidative state, vascular endothelial function and blood pressure in a rat model.

METHODS

Eight-week-old male Sprague-Dawley rats were divided into subgroups and maintained for 4 weeks prior to experimentation on either control chow containing 0.36% methionine and 0.5% NaCl; or one of the following modified diets containing either 0.7% methionine, 8% NaCl or 0.7% methionine + 8% NaCl. Sodium handling, homocysteine metabolism, lipid profile, NO synthesis, oxidative state, blood pressure and relaxation to acetylcholine of carotid rings were evaluated and compared.

RESULTS

Diet-induced mild hyperhomocysteinemia (plasma homocysteine levels 1.4-fold higher than control), by itself, had no significant influence on sodium excretion, vascular endothelial function and blood pressure. Increased salt intake had no influence on homocysteine metabolism, vascular endothelial function and blood pressure. The coexistence of mild hyperhomocysteinemia and high salt intake significantly diminished vascular endothelial function (rmax to acetylcholine; control chow 83.2 +/- 6.2%, 0.7% methionine diet 74.7 +/- 3.9%, 8% NaCl diet 85.1 +/- 4.6%, 0.7% methionine + 8% NaCl diet 57.9 +/- 6.6%) but manifested no rise in blood pressure. No significant difference in oxidative state was observed in this analysis.

CONCLUSIONS

Diet-induced mild hyperhomocysteinemia, the extent of which is comparable with the levels that are associated with a predisposition to common atherosclerotic diseases, was found to induce vascular endothelial dysfunction only when accompanied by high salt intake.