Optimization of dietary folate or low-dose folic acid supplements lower homocysteine but do not enhance endothelial function in healthy adults, irrespective of the methylenetetrahydrofolate reductase (C677T) genotype

OBJECTIVES

We sought to study the effect of low-dose folic acid supplementation or optimization of dietary folate intake on plasma homocysteine and endothelial function in healthy adults.

BACKGROUND

Elevated homocysteine is associated with cardiovascular disease, but it is not known whether this relationship is causal. Individuals homozygous (TT) for the C677T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene ( approximately 12% of the population) have increased homocysteine levels, particularly in association with suboptimal folate intake.

METHODS

Healthy subjects (n = 126; 42 of each MTHFR genotype) were included in this cross-over study of three interventions of four months each: 1) placebo plus natural diet; 2) daily 400-microg folic acid supplement plus natural diet; and 3) increased dietary folate intake to 400 microg/day.

RESULTS

At baseline, homocysteine was inversely related to plasma folate and was higher in TT homozygotes. For the whole group, plasma folate increased by 46% after dietary folate and by 79% after supplementation, with reductions of homocysteine of 14% and 16%, respectively. Within the genotype, TT homozygotes exhibited the most marked changes in these variables. Brachial artery endothelial function, as determined by a change in end-diastolic diameter in response to increased flow, was not changed by increased folate intake (98 +/- 73 microm at baseline, 110 +/- 69 microm after a high-folate diet, 114 +/- 59 microm after supplementation and 118 +/- 68 microm after placebo). Plasma von Willebrand factor antigen was unaltered.

CONCLUSIONS

Optimization of dietary folate or low-dose folic acid supplementation reduces plasma homocysteine but does not enhance endothelial function, irrespective of the MTHFR (C667T) genotype.

Folate, homocysteine, endothelial function and cardiovascular disease. What is the link?

Elevated plasma homocysteine concentrations are associated with an increased risk of cardiovascular disease, but the relationship has not been proven to be causal. Folate is the strongest nutritional and pharmacological determinant of plasma homocysteine concentrations, which also interact with the genetic variation in methylenetetrahydrofolate reductase (MTHFR). Endothelial dysfunction due to reduced nitric oxide bioavailability is an early feature of vascular pathology. This can be assessed noninvasively by measurement of flow-mediated dilatation. Human studies on folic acid, homocysteine and endothelial function are reported. It is proposed that folic acid in high doses may have beneficial effects on endothelial function, which are independent of homocysteine lowering.

Folate improves endothelial function in coronary artery disease: an effect mediated by reduction of intracellular superoxide?

Homocysteine is a risk factor for coronary artery disease (CAD). Folic acid lowers homocysteine and may improve endothelial function in CAD, although the mechanism is unclear. We investigated the effect of folic acid on endothelial function, homocysteine, and oxidative stress in patients with CAD. We also examined the acute effect of 5-methyltetrahydrofolate (5-MTHF), the principal circulating folate, on endothelial function in vivo and on intracellular superoxide in cultured endothelial cells. A randomized crossover study of folic acid (5 mg daily) for 6 weeks was undertaken in 52 patients with CAD. Ten further patients were given intra-arterial 5-MTHF. Endothelial function was assessed by flow-mediated dilatation (FMD). Folic acid increased plasma folate (P<0.001), lowered homocysteine by 19% (P<0.001), and improved FMD (P<0.001). FMD improvement did not correlate with homocysteine reduction. Malondialdehyde and total plasma antioxidant capacity, markers of oxidative stress, were unchanged. 5-MTHF acutely improved FMD (P<0.001) without altering homocysteine (P=0.47). In vitro, 5-MTHF abolished homocysteine-induced intracellular superoxide increase (P<0.001); this effect was also observed with folic acid and tetrahydrobiopterin. Our data support the beneficial effect of folic acid on endothelial function in CAD but suggest that the mechanism is independent of homocysteine. Reduction of intracellular endothelial superoxide may have contributed to the effect.

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

OBJECTIVES

First, we sought to study the effects of short- and long-term vitamin C therapy on oxidative stress and endothelial dysfunction in chronic heart failure (CHF), and second, we sought to investigate the role of neutrophils as a cause of oxidative stress in CHF.

BACKGROUND

Oxidative stress may contribute to endothelial dysfunction in CHF. Vitamin C ameliorates endothelial dysfunction in CHF, presumably by reducing oxidative stress, but this is unproven.

METHODS

We studied 55 patients with CHF (ischemic and nonischemic etiologies) and 15 control subjects. Flow-mediated dilation (FMD) in the brachial artery was measured by ultrasound wall-tracking, neutrophil superoxide anion (O2-) generation by lucigenin-enhanced chemiluminescence and oxidative stress by measurement of free radicals (FRs) in venous blood using electron paramagnetic resonance (EPR) spectroscopy and plasma thiobarbituric acid reactive substances (TBARS). Measurements were performed at baseline in all subjects. The effects of short-term (intravenous) and long-term (oral) vitamin C therapy versus placebo were tested in patients with nonischemic CHF.

RESULTS

At baseline, FRs were higher in patients with CHF than in control subjects (p < 0.01), TBARS were greater (p < 0.005), neutrophil O2- -generating capacity was enhanced (p < 0.005) and FMD was lower (p < 0.0001). Compared with placebo, short-term vitamin C therapy reduced FR levels (p < 0.05), tended to reduce TBARS and increased FMD (p < 0.05), but did not affect neutrophil O2- -generating capacity. Long-term vitamin C therapy reduced FR levels (p < 0.05), reduced TBARS (p < 0.05) and improved FMD (p < 0.05), but also reduced neutrophil O2- -generating capacity (p < 0.05). Endothelial dysfunction was not related to oxidative stress, and improvements in FMD with vitamin C therapy did not relate to reductions in oxidative stress.

CONCLUSIONS

Oxidative stress is increased in ischemic and nonischemic CHF, and neutrophils may be an important cause. Vitamin C reduces oxidative stress, increases FMD and, when given long term, decreases neutrophil O2- generation, but the lack of a correlation between changes in endothelial function and oxidative stress with vitamin C implies possible additional non-antioxidant benefits of vitamin C.

Homocysteine and endothelial dysfunction: a link with cardiovascular disease

The nature of the link between homocysteine and cardiovascular disease has not yet been clearly established. Impaired endothelium-independent vasodilatation is an early feature of vascular disease. In human studies, methionine loading, which acutely elevates plasma homocysteine, induces endothelial dysfunction. Folate therapy, which lowers homocysteine, enhances endothelial function. This is consistent with, but not proof of, homocysteine toxicity to endothelium in vivo. Homocysteine, in high concentration, can induce endothelial dysfunction in vitro. This is accompanied by increased superoxide production, which when inhibited, restores normal endothelial function. These observations suggest that homocysteine may induce vascular endothelial dysfunction by a mechanism involving reactive oxygen species.

Oral folate enhances endothelial function in hyperhomocysteinaemic subjects

BACKGROUND

Elevated plasma homocysteine (Hcy) is a risk factor for vascular disease. A postulated mechanism is vascular endothelial damage by homocysteine. Hcy levels are inversely related to blood concentrations of folate and can be lowered by folate supplements. The effect of oral folic acid on endothelial function was investigated in healthy adults with mild hyperhomocysteinaemia.

PATIENTS AND METHODS

Eighteen healthy subjects (Hcy > 13 micromol L-1 at entry), from a screening population of 890 volunteers, were entered into a randomised double-blind placebo-controlled crossover study of oral folic acid (5 mg daily for six weeks) with a six week interval between treatments. Flow-mediated (endothelium-dependent) and (endothelial-independent) glyceryl trinitrate (GTN)-mediated brachial artery dilatation were measured by high resolution wall tracking.

RESULTS

Folate supplementation enhanced endothelium-dependent responses (+0.08 +/- 0.05 vs. +0.04 +/- 0.04 mm, P = 0.015) but endothelium-independent responses (GTN) were unchanged. Folate reduced Hcy (8.7 +/- 2.5 vs. 12.1 +/- 3.6 micromol L-1).

CONCLUSION

High dose folic acid supplementation enhances endothelium-dependent vascular function and lowers plasma Hcy. This provides preliminary evidence that folate may have beneficial cardiovascular effects in adults with mild hyperhomocysteinaemia.

Hyperhomocysteinemia after an oral methionine load acutely impairs endothelial function in healthy adults

BACKGROUND

Elevated plasma homocysteine is a risk factor for arteriosclerosis, but a cause-and-effect relationship remains to be fully established. Endothelial dysfunction, an early event in the atherogenic process, has been shown to be associated with hyperhomocysteinemia in experimental and human studies. To further establish a direct relationship between changes in plasma homocysteine and endothelial dysfunction, we investigated whether moderate hyperhomocysteinemia induced by an oral methionine load would acutely impair flow-mediated endothelium-dependent vasodilatation in healthy adults.

METHODS AND RESULTS

Twenty-four healthy volunteers completed a randomized crossover study in which an oral methionine load (0.1 g/kg) was administered on 1 of 2 study days, 7 days apart. At each visit, plasma homocysteine and brachial artery endothelium-dependent and -independent dilatation were measured at baseline and at 4 hours. To further elucidate the temporal relationship between methionine, homocysteine, and endothelial function, an oral methionine load was administered in 10 subjects on a separate visit, and the time courses of plasma methionine, homocysteine, and flow-mediated brachial artery dilatation were measured at baseline and after 1, 2, 3, 4, and 8 hours. After oral methionine, plasma homocysteine increased from 7. 9+/-2.0 micromol/L at baseline to 23.1+/-5.4 micromol/L at 4 hours (P<0.0001, n=24) and was associated with a decrease in flow-mediated brachial artery dilatation from 0.12+/-0.09 to 0.06+/-0.09 mm (P<0. 05). The time course of the impairment of flow-mediated vasodilatation mirrored the time course of the increase in homocysteine concentration.

CONCLUSIONS

Oral methionine loading raises plasma homocysteine and impairs flow-mediated endothelium-dependent vasodilatation. This supports the view that homocysteine may promote vascular disease by inducing endothelial dysfunction.

ACP Broadsheet No 152: March 1998. Clinical implications of plasma homocysteine measurement in cardiovascular disease

Plasma homocysteine is emerging as an important risk factor for cardiovascular disease. Measurement in clinical laboratories is feasible by several techniques, including HPLC. Of particular importance is the potential for treatment by dietary modification and vitamin supplements, particularly folic acid. The outcome of vitamin intervention on cardiovascular events is awaited with interest. If the role of homocysteine is confirmed then it may rival cholesterol in importance as a cardiovascular risk factor.

Putative mechanisms for vascular damage by homocysteine

In homocystinuria homocysteine appears to be directly toxic to the vasculature, but in mild hyperhomocysteinaemia a cause and effect relationship remains unproven. Evidence for a causal role is derived from recent primate and human studies in which endothelial dysfunction was produced by modestly elevated blood homocysteine concentrations. Endothelial dysfunction would account for an increased risk of both arterial and venous disease. A key abnormality may be impaired release and/or action of nitric oxide in response to flow. Other possible mechanisms include smooth muscle cell proliferation, extracellular matrix modification and lipoprotein oxidation. Although demonstrated in vitro, a role for lipoprotein oxidation in man has not been substantiated. However an effect of homocysteine on cellular redox status remains a possible mechanism. Homocysteine does not appear to alter circulating coagulation factors consistently, but may promote enhanced thrombin production indirectly by its effects on endothelium. Further studies are required to elucidate the pathological actions of homocysteine, concentrating on the effects of mild hyper-homocysteinaemia on endothelial function in man.

A rapid method for measurement of the susceptibility to oxidation of low-density lipoprotein

Oxidation of low-density lipoprotein (LDL) may be important in the pathogenesis of atherosclerosis. We describe a method which measures the oxidation resistance of LDL isolated by a rapid procedure without added antioxidants. LDL was isolated from heparinized plasma by density gradient ultracentrifugation and desalted by gel filtration. The protein concentration was standardized to 50 mg/L and oxidation was promoted by copper (2 mumol/L) at 37 degrees C. The total sample preparation time was 2.5 h. Conjugated diene production was monitored at lambda = 234 nm with computation of the lag time. LDL oxidation was inhibited by EDTA but not heparin. Albumin inhibited LDL oxidation but only in concentrations greater than 50 mg/L. LDL was stable in frozen plasma (-70 degrees C) for 10 weeks, but unstable in the isolated and desalted state. The lag time for LDL from patients treated with the antioxidant probucol was markedly prolonged compared to normal subjects.

The effect of probucol and vitamin E treatment on the oxidation of low-density lipoprotein and forearm vascular responses in humans

This study investigates the hypothesis that lipid soluble antioxidants may increase the resistance of low-density lipoprotein (LDL) to oxidation and also enhance vascular endothelial responses in humans. In a double-blind parallel group study, 24 hypercholesterolaemic patients already on treatment with simvastatin (20 mg day-1), were randomized to supplementary treatment with probucol (500 mg bd), vitamin E (400 IU daily) or placebo for 8 weeks. Mean serum cholesterol before antioxidant treatment was 7.00 mmol l-1. Resistance of LDL to oxidation by copper was increased by 830% in the probucol group and by 30% in the vitamin E group. However, thiobarbituric acid reacting substances in whole serum were not altered by either antioxidant. Probucol lowered HDL- and LDL-cholesterol levels and increased the QT interval. Forearm vascular responses, as measured by venous occlusion plethysmography, to acetylcholine, glyceryl trinitrate and NG-monomethyl-L-arginine, were not significantly changed by antioxidant treatment. Probucol has a major, and vitamin E a minor, effect on LDL resistance to oxidation but neither compound appears to alter forearm vascular responses in vivo.

Oxidation of low density lipoprotein in patients on regular haemodialysis

Patients undergoing chronic haemodialysis are known to have a high incidence of premature atherosclerosis for reasons which have not been fully elucidated. The susceptibility of low density lipoprotein (LDL) to oxidation by copper ions in vitro is widely used as a measure of its atherogenicity in vivo. We measured the susceptibility of LDL to oxidation using copper ions in haemodialysis patients and found, surprisingly, a markedly increased resistance to oxidation. The experiment was therefore repeated using an alternative free radical generator, AAPH [2,2'-azobis-(2-amidinopropane hydrochloride)], to promote LDL oxidation; using AAPH, the susceptibility to oxidation was similar in the dialysis group compared to controls. Abnormal LDL composition in the dialysis patients was also demonstrated. We suggest that, in such situations, susceptibility of LDL to oxidation in vitro may be highly dependent on the biochemical method employed and therefore may not accurately reflect atherogenic risk.

Rare apolipoprotein E variant identified in a patient with type III hyperlipidaemia

We report a rare apolipoprotein E variant in an Irish female with Type III hyperlipidaemia who has the phenotype E2E1 as determined by isoelectric focusing. Sequence analysis of the apolipoprotein E gene from the proband and from four other family members, using DNA amplified by the polymerase chain reaction, demonstrated the presence of a point mutation in the common epsilon 2 allele with a G-->A transition at nucleotide 3791. This was confirmed by digestion with the restriction endonuclease TaqI, which cuts at a new site within the apolipoprotein E gene, created by the base change. This mutation results in a substitution of aspartic acid for glycine at position 127 of the mature protein. We believe this to be the first description of this apolipoprotein E variant in a family from the British Isles. The mutation appears to be 'recessive' with respect to the expression of Type III hyperlipidaemia, although it may be somewhat more potent in this regard than the parent epsilon 2 allele. The Type III hyperlipidaemia is responsive to treatment with diet and gemfibrozil.

Simvastatin in severe hypercholesterolaemia: a placebo controlled trial

The effect of simvastatin in 27 patients with severe primary hypercholesterolaemia was assessed by a double-blind placebo controlled parallel group trial. Total serum cholesterol, LDL-cholesterol and apoprotein B (ApoB) were significantly reduced by simvastatin 40 mg daily. Reductions in triglyceride and VLDL-cholesterol and an increase in HDL-cholesterol levels were only significant when calculated as a percentage of baseline, because of wide inter-individual variability. No changes in apoprotein A1, lipoprotein (a), fibrinogen, viscosity or blood pressure were observed. Leucocyte HMG-CoA reductase activity was unchanged after 4 weeks of active treatment but increased by 87% after 3 months (n = 21, P less than 0.05). No severe adverse effects or changes in CK or AST levels were noted. We conclude that simvastatin is effective in the treatment of severe and resistant hypercholesterolaemia, and well tolerated in the short term.

Apolipoprotein E phenotype determined by agarose gel electrofocusing and immunoblotting

Polymorphism at the apolipoprotein E (ApoE) locus is an important factor in the development of remnant (Type III) hyperlipidemia and also influences the distribution of cholesterol concentrations in the population. The new method for ApoE phenotyping described here gives good results with simple apparatus. Serum (10 microL) is digested with sialidase (EC 3.2.1.18), delipidated, and redissolved in 6 mol/L urea. Electrofocusing is carried out in agarose, followed by immunoblotting with a monoclonal antibody to ApoE and an anti-immunoglobulin-peroxidase conjugate. Sialidase-catalyzed digestion effectively removes sialated forms of ApoE, which eases interpretation. This method can be used in nonspecialist laboratories and is particularly suited for assay of large numbers of samples.

Apolipoprotein E phenotype determined by agarose gel electrofocusing and immunoblotting

Polymorphism at the apolipoprotein E (ApoE) locus is an important factor in the development of remnant (Type III) hyperlipidemia and also influences the distribution of cholesterol concentrations in the population. The new method for ApoE phenotyping described here gives good results with simple apparatus. Serum (10 microL) is digested with sialidase (EC 3.2.1.18), delipidated, and redissolved in 6 mol/L urea. Electrofocusing is carried out in agarose, followed by immunoblotting with a monoclonal antibody to ApoE and an anti-immunoglobulin-peroxidase conjugate. Sialidase-catalyzed digestion effectively removes sialated forms of ApoE, which eases interpretation. This method can be used in nonspecialist laboratories and is particularly suited for assay of large numbers of samples.

Systemic amyloidosis--three illustrative cases

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Testicular function after renal transplantation: comparison of Cyclosporin A with azathioprine and prednisone combination regimes

Testicular function was assessed in 24 men after renal transplantation who were on either Cyclosporin A (CSA) (8 men) or a combination of azathioprine and prednisone (AZP) (16 men) as immunosuppressive therapy. The different regimes were not associated with any differences in clinical or hormonal (LH, FSH, prolactin, testosterone, estradiol) indices of testicular function although adrenal androgen (dehydroepiandrosterone sulphate) was suppressed in prednisone-treated men. Overall, however, poor graft function was associated with abnormal testicular function. Renal allograft function rather than immunosuppressive regime was the major determinant of gonadal function.

Ovarian function after renal transplantation: comparison of cyclosporin A with azathioprine and prednisone combination regimens

Ovarian function was assessed in 24 women after renal transplantation who were treated either with cyclosporin A (10 patients) or with a combination of azathioprine and prednisone (14 patients) as immunosuppressive therapy. The different regimens were not associated with any differences in clinical or endocrine indices of ovarian function (LH, FSH, prolactin, testosterone, oestradiol, dehydroepiandrosterone sulphate). Excessive hair growth was common in both treatment groups. Levels of testosterone and dehydroepiandrosterone were higher in cyclosporin-treated women but this was due to prednisone-induced suppression of adrenal androgen output in the azathioprine- and prednisone-treated women. Excessive hair growth was present in postmenopausal women on both treatments suggesting that hypertrichosis is a consequence of renal transplantation and is not a specific side-effect of cyclosporin A therapy.

Characterization of the pituitary gonadotroph cells of hypogonadal (hpg) male mice: comparison with normal mice

Hypogonadal (hpg) mice lack hypothalamic luteinizing hormone releasing hormone (LH-RH). Gonadotrophs from male hpg mice have been identified by immunocytochemistry for LH and quantitative electron microscopy allied with radioimmunoassay of the gonadotrophins. In comparison with those of normal mice, gonadotrophs in male hpg animals are less numerous and smaller, with less cytoplasm and rough endoplasmic reticulum and a smaller Golgi apparatus. They have fewer granules, especially those of larger (greater than 210 nm) diameter. Thus, in the absence of LH-RH, gonadotrophs can differentiate but remain relatively inactive.

Effects of luteinizing hormone releasing hormone on the gonadotrophs of hypogonadal (hpg) mice

Hypogonadal (hpg) mice lack hypothalamic luteinizing hormone releasing hormone (LH-RH); consequently the pituitary gonadotrophs, though differentiated, remain inactive. The acute and chronic responses of gonadotrophs in hpg mice to LH-RH to male hpg mice produced an eightfold increase in plasma LH after 5 min, but granule depletion was not detectable ultrastructurally. Vacuoles (300-600 nm) were present more frequently in gonadotrophs from hpg mice given LH-RH compared with control mice. Daily s.c. administration of 2 micrograms LH-RH to male hpg mice for 20 days stimulated a 63-fold increase in the pituitary content of FSH but only a twofold increase in the pituitary content of LH. Testicular weight increased fivefold but the weight of the seminal vesicles did not change. The frequency of cells immunoreactive for LH increased (x 1.6), the gonadotrophs hypertrophied (x 1.9) and the cell content of granules increased (x 2.3) to values close to the normal range. The rough endoplasmic reticulum and Golgi apparatus became more prominent. A striking result of long-term daily administration of LH-RH was the accumulation of large (1-2 micrometers) lipid droplets in about 40% of gonadotrophs. All these changes induced by LH-RH regressed towards the untreated hpg state when LH-RH administration wad discontinued for 5 and 10 days. These results show that the gonadotrophs of hpg mice can be stimulated trophically by LH-RH, and that long-term administration stimulates the synthesis and release of the gonadotrophins but the effect on FSH is much greater than on LH.