Membrane protein damage and methylation reactions in chronic renal failure

Methyl Donor Nutrients in Chronic Kidney Disease: Impact on the Epigenetic Landscape

Epigenetic alterations, such as those linked to DNA methylation, may potentially provide molecular explanations for complications associated with altered gene expression in illnesses, such as chronic kidney disease (CKD). Although both DNA hypo- and hypermethylation have been observed in the uremic milieu, this remains only a single aspect of the epigenetic landscape and, thus, of any biochemical dysregulation associated with CKD. Nevertheless, the role of uremia-promoting alterations on the epigenetic landscape regulating gene expression is still a novel and scarcely studied field. Although few studies have actually reported alterations of DNA methylation via methyl donor nutrient intake, emerging evidence indicates that nutritional modification of the microbiome can affect one-carbon metabolism and the capacity to methylate the genome in CKD. In this review, we discuss the nutritional modifications that may affect one-carbon metabolism and the possible impact of methyl donor nutrients on the microbiome, CKD, and its phenotype.

Chronic kidney disease in children and the role of epigenetics: Future therapeutic trajectories

Global differences in the observed causes of chronic kidney disease (CKD) in children are well documented and are attributed to dissimilarities in clime, race, hereditary, and ancestry. Thus, familial clustering and disparities in CKD prevalence rates across ethnic and racial groups indicate that the progression of renal disease has a strong genetic component. Mammalian studies have demonstrated a feasible nexus between nutrition and non-genetic exposure (around the time of conception and in epigenetic changes) in the expression of major genes identified in renal organogenesis. The major consequence is a reduction in the number of nephrons, with subsequent predisposition to hypertension and CKD. Identifying these epigenetic changes is crucial (due to their potentially reversible nature), as they may serve as future therapeutic targets to prevent kidney fibrosis and CKD. Despite progress in the field of epigenetics in oncology, research in other subspecialties of medicine is largely experimental with few existing studies regarding the clinical implication of epigenetics in renal disease. Therapeutic trajectories for CKD in children based on the influence of epigenetics may eventually revolutionize the management of this disease. The aim of the current narrative review is to appraise the role of epigenetics in CKD, and highlight the potential future therapeutic pathways.

Endothelial dysfunction and oxidative stress in polycystic kidney disease

Cardiovascular disease (CVD) is the leading cause of premature mortality in ADPKD patients. The aim was to identify potential serum biomarkers associated with the severity of ADPKD. Serum samples from a homogenous group of 61 HALT study A ADPKD patients [early disease group with estimated glomerular filtration rate (eGFR) >60 ml·min(-1)·1.73 m(-2)] were compared with samples from 49 patients from the HALT study B group with moderately advanced disease (eGFR 25-60 ml·min(-1)·1.73 m(-2)). Targeted tandem-mass spectrometry analysis of markers of endothelial dysfunction and oxidative stress was performed and correlated with eGFR and total kidney volume normalized to the body surface area (TKV/BSA). ADPKD patients with eGFR >60 ml·min(-1)·1.73 m(-2) showed higher levels of CVD risk markers asymmetric and symmetric dimethylarginine (ADMA and SDMA), homocysteine, and S-adenosylhomocysteine (SAH) compared with the healthy controls. Upon adjustments for age, sex, systolic blood pressure, and creatinine, SDMA, homocysteine, and SAH remained negatively correlated with eGFR. Resulting cellular methylation power [S-adenosylmethionine (SAM)/SAH ratio] correlated with the reduction of renal function and increase in TKV. Concentrations of prostaglandins (PGs), including oxidative stress marker 8-isoprostane, as well as PGF2α, PGD₂, and PGE₂, were markedly elevated in patients with ADPKD compared with healthy controls. Upon adjustments for age, sex, systolic blood pressure, and creatinine, increased PGD₂ and PGF₂α were associated with reduced eGFR, whereas 8-isoprostane and again PGF₂α were associated with an increase in TKV/BSA. Endothelial dysfunction and oxidative stress are evident early in ADPKD patients, even in those with preserved kidney function. The identified pathways may provide potential therapeutic targets for slowing down the disease progression.

Epigenetics of progression of chronic kidney disease: fact or fantasy?

Epigenetic modifications are important in the normal functioning of the cell, from regulating dynamic expression of essential genes and associated proteins to repressing those that are unneeded. Epigenetic changes are essential for development and functioning of the kidney, and aberrant methylation, histone modifications, and expression of microRNA could lead to chronic kidney disease (CKD). Here, epigenetic modifications modulate transforming growth factor β signaling, inflammation, profibrotic genes, and the epithelial-to-mesenchymal transition, promoting renal fibrosis and progression of CKD. Identification of these epigenetic changes is important because they are potentially reversible and may serve as therapeutic targets in the future to prevent subsequent renal fibrosis and CKD. In this review we discuss the different types of epigenetic control, methods to study epigenetic modifications, and how epigenetics promotes progression of CKD.

Development of an animal model for chronic mild hyperhomocysteinemia and its response to oxidative damage

The purpose of this study was to develop a chronic chemically induced model of mild hyperhomocysteinemia in adult rats. We produced levels of Hcy in the blood (30μM), comparable to those considered a risk factor for the development of neurological and cardiovascular diseases, by injecting homocysteine subcutaneously (0.03μmol/g of body weight) twice a day, from the 30th to the 60th postpartum day. Controls received saline in the same volumes. Using this model, we evaluated the effect of chronic administration of homocysteine on redox status in the blood and cerebral cortex of adult rats. Reactive oxygen species and thiobarbituric acid reactive substances were significantly increased in the plasma and cerebral cortex, while nitrite levels were reduced in the cerebral cortex, but not in the plasma, of rats subjected to chronic mild hyperhomocysteinemia. Homocysteine was also seen to disrupt enzymatic and non-enzymatic antioxidant defenses in the blood and cerebral cortex of rats. Since experimental animal models are useful for understanding the pathophysiology of human diseases, the present model of mild hyperhomocysteinemia may be useful for the investigation of additional mechanisms involved in tissue alterations caused by homocysteine.

Impaired transmethylation potential in Parkinson's disease patients treated with L-Dopa

Hyperhomocysteinaemia was reported in patients with Parkinson's disease (PD) treated with l-Dopa. The increase in plasma concentration of this sulfur compound arises from the massive methylation of the drug operated by the enzyme catechol-O-methyltransferase (COMT), which acts as a powerful sink of methyl groups. The contemporary occurrence of C677T polymorphism in homozygosity, leading to a temperature-labile variant of the MTHFR enzyme, induces an even more marked increase in tHcy. Here we show that l-Dopa administration in hyperhomocysteinemic PD patients is able to lower intracellular concentration of S-Adenosylmethionine (AdoMet) in erythrocytes (RBC), while the occurrence of hyperhomocysteinaemia causes a significant increase in S-Adenosylhomocysteine (AdoHcy) level. In patients with PD treated with l-Dopa and hyperhomocysteinemic, the remarkable decrease in AdoMet and the concurrent increase in AdoHcy concentration both contribute to significantly lower the transmethylation potential ([AdoMet]/[AdoHcy]), a useful index of the effectiveness of methyl group transfer by methyltransferases. This decrease could indeed contribute to partly attenuate, through a self-limiting kinetic mechanism, the tendency of developing drug resistance, partly mediated in these patients by COMT upregulation. Our results also support the conclusion that COMT inhibitors (entacapone or tolcapone), when administered in PD patients treated with l-Dopa, may potentiate the endogenous AdoHcy-dependent COMT inhibition mechanism already operative in a variable fashion.

The arginine-creatine pathway is disturbed in children and adolescents with renal transplants

Cardiovascular disease is an important cause of morbidity in recipients of renal transplants. The aim of the present study was to analyze the status of the arginine-creatine pathway in such patients, given the relationship between the arginine metabolism and both renal function and the methionine-homocysteine cycle. Twenty-nine children and adolescents (median age 13, range 6-18 years), who had received a renal allograft 14.5-82.0 months before, were recruited for the study. On immunosuppressive therapy, all patients evidenced an adequate level of renal function. Plasma concentrations of homocysteine and glycine were significantly higher, whereas urinary excretions of guanidinoacetate and creatine were significantly lower than controls. Urinary excretions of guanidinoacetate and creatine correlated positively with creatinine clearance. Urinary excretion of creatine was negatively correlated with plasma concentration of homocysteine. The demonstration of disturbances in the arginine-creatine pathway in patients with well-functioning renal transplants and in absence of chronic renal failure represents a novel finding. We speculate that the low urinary excretion of guanidinoacetate and creatine is probably related to the nephrotoxic effect of immunosuppressive therapy and to defective methylation associated with the presence of hyperhomocysteinemia.

Individualizing Therapy in Patients With Chronic Kidney Disease

Patients with chronic kidney diseases have multiple clinical abnormalities that may affect disposition of drugs, including alterations in glomerular filtration rate, excretion of plasma proteins, reductions in serum albumin, and reductions in drug metabolizing enzyme activity. Inflammation may also influence the previous factors. Concomitant drug therapies can lead to drug— drug interactions that may affect the pharmacokinetics of administered drugs. Pharmacogenomics has begun to be evaluated for effects of genotype and haplotype of drug metabolizing enzymes and transporters on drug disposition. Because of the multiple potential etiologies for alterations in drug disposition in patients with chronic kidney diseases, they require appropriate evaluation for implementation of individualized strategies in therapies to enhance efficacy and reduce toxicities. This review will highlight the disease- and patient-specific variables that are targets for patient-centered approaches to therapeutic interventions. The field of pharmacogenomics will be reviewed with reference to common therapies for transplantation and glomerular diseases.

Homocysteine as a risk factor for cardiovascular and related disease: nutritional implications

The objectives were firstly to assess the evidence that homocysteine is a significant and independent risk factor for vascular disease with special reference to cardiovascular disease, and secondly to evaluate the evidence that a food staple fortified with folic acid will reduce this problem on a population basis.

The structure of plasma homocysteine (tHcy) is described. Homocysteine, a highly reactive compound, is synthesized from the amino acid, methionine, and is metabolized by two pathways, the catabolic transsulphuration routeviacystathionine β-synthase (EC 4.2.1.22) and the remethylation path using 5-methyltetrahy-drofolate polyglutamate, the product of 5,10-methylenetetrahydrofolate reductase (MTHFR; EC 1.1.1.171),viathe cobalamin dependent enzyme, methionine synthase (MS; EC 2.1.1.13).

The mechanisms whereby hyper-tHcy is produced include both increased rates of synthesis and decreased metabolism. The latter may occur owing to nutritional deficiency of the vitamin cofactors which are necessary for the normal function of the metabolic enzymes. In particular, folate is required for methylene reductase, pyridoxal phosphate for cystathionine synthase and cobalamin for methionine synthase. When these vitamins are deficient hyper-tHcy is induced and this occurs especially in the elderly. Alternatively, a variant form of methylene reductase has recently been described which occurs in nearly 10% of the normal population. This variant is associated with hyper-tHcy, especially in situations associated with a low folate nutritional status.

Meta-analysis of both retrospective case-control studies, nested prospective case-control surveys and a secondary trial of mortality in postmyocardial infarct patients have shown that the association of hyper-tHcy with vascular disease is beyond doubt. This has been further supported by direct assessments of the degree of vascular disease in the carotid brachial and aortic arteries in relation to tHcy levels. Furthermore, treatment with a cocktail of the vitamin cofactors has produced lowering of tHcy levels and regression of the vascular disease in the carotid arteries of affected individuals.

Suggested pathogenic mechanisms in vascular disease induced by hyper-tHcy include vascular endothelial cell dysfunction, smooth muscle proliferation and derangements of normal intravascular regulation mechanisms. A variety of clinical conditions are known to be associated with a high incidence of thromboembolic complications. Some of these are associated with hyper-tHcy.

Low physiological doses of folic acid, as well as pharmocological doses, lower tHcy. However, because of the poor bioavailability of food folate (50%) and the considerable chemical instability of the naturally occurring reduced forms of folate, in most people it would require unacceptably high consumption of green vegetables to accomplish the necessary increase in intracellular folate and reduction in tHcy. Accordingly, folic acid, the nonreduced synthetic form of the vitamin, which is 100% bioavailable and chemically extremely stable, should be added to a food staple such as flour to ensure maximum protection for most of the population.

The use of an in vivo model to study the effects of hyperhomocysteinaemia on vascular function

BACKGROUND/AIMS

To use an in vivo rat model of hyperhomocysteinaemia (HHCy) to study its impact on vascular function.

METHODS

Twenty rats were fed either a control or HHCy-inducing diet for 10 wk. The response of aortic rings to contraction with phenylephrine, and relaxation to acetylcholine (endothelium-dependant relaxation) or sodium nitroprusside (endothelium-independent relaxation) was analyzed. The results were compared using an analysis of variance (ANOVA).

RESULTS

There was a significant elevation of HCy in the treated group (20.5 versus 1.6 micromol/L, P = 0.004). There was no significant difference between the two groups in blood pressure measurements (ANOVA, P = 0.152). In a dose-dependant manner, phenylephrine elicited significantly greater contraction in aorta taken from HHCy rats than that taken from controls (ANOVA, P < 0.001), acetylcholine elicited significantly less percentage relaxation in aorta taken from HHCy rats than from controls (ANOVA, P = 0.003) and though sodium nitroprusside stimulated less percentage relaxation in aorta taken from HHCy rats than controls, this did not reach significance (ANOVA, P = 0.051).

CONCLUSIONS

In diet induced hyperhomocysteinaemic rats, there is enhanced vascular contraction in response to phenylephrine and impaired endothelium-dependant relaxation in response to acetylcholine.

Drug dosing in chronic kidney disease

Patients with chronic kidney disease (CKD) are at high risk for adverse drug reactions and drug-drug interactions. Drug dosing in these patients often proves to be a difficult task. Renal dysfunction-induced changes in human pathophysiology regularly results may alter medication pharmacodynamics and handling. Several pharmacokinetic parameters are adversely affected by CKD, secondary to a reduced oral absorption and glomerular filtration; altered tubular secretion; and reabsorption and changes in intestinal, hepatic, and renal metabolism. In general, drug dosing can be accomplished by multiple methods; however, the most common recommendations are often to reduce the dose or expand the dosing interval, or use both methods simultaneously. Some medications need to be avoided all together in CKD either because of lack of efficacy or increased risk of toxicity. Nevertheless, specific recommendations are available for dosing of certain medications and are an important resource, because most are based on clinical or pharmacokinetic trials.

Hyperhomocysteinemia and macromolecule modifications in uremic patients

Hyperhomocysteinemia is present in the majority of well-nourished chronic renal failure and uremic patients. Most observations reported in the literature come from studies carried out in end-stage renal disease patients treated with hemodialysis. The underlying mechanisms of the toxic effects of homocysteine in uremia related to cardiovascular disease and other disturbances are still under scrutiny. As a consequence, macromolecules (i.e., proteins and DNA) have been found to be altered to various extents. One of the mechanisms of homocysteine toxicity is related to the action of its metabolic precursor, S-adenosylhomocysteine, a powerful methyltransferase competitive inhibitor. Disruption of DNA methylation has been demonstrated to occur as a result of hyperhomocysteinemia, and/or is associated with vascular damage. DNA hypomethylation has been found in the mononuclear cell fraction of uremic patients with hyperhomocysteinemia. Proteins are also targets of homocysteine-dependent molecular damage. The formation of oxidative products with free cysteinyl residue thiol groups has been demonstrated to occur in blood. The latter also represents a mechanism for the transport of homocysteine in plasma. In addition, homocysteine thiolactone has been shown to react with free amino groups in proteins to form isopeptide bonds, in particular at the lysine residue level. Another type of isopeptide bond in proteins may result from the deamidation and isomerization of asparaginyl residues, yielding abnormal isoaspartyl residues, which have been demonstrated to be increased in uremic patients. Folate treatment exerts a partial, but significant, homocysteine-lowering effect in uremic patients and has been shown to improve the changes in macromolecules induced by high homocysteine levels. In conclusion, both DNA and proteins are structurally modified in uremia as a consequence of high homocysteine levels. The role of these macromolecule changes in inducing the clinical complications of hyperhomocysteinemia in these patients, although still conjectural in some respects, is at present sustained by several pieces of evidence.

Hepatic drug metabolism and transport in patients with kidney disease

The disposition of many drugs is altered in patients with acute (AKD) and chronic kidney disease (CKD). A decline in renal clearance of several drugs has been correlated significantly with residual renal function (ie, creatinine clearance) of subjects. Reductions in nonrenal clearance of some compounds also have been reported and associated with clearance of markers of oxidative and/or conjugative metabolism or P-glycoprotein-mediated transport. Although initial accounts of reduced hepatic microsomal cytochrome P-450 (CYP) content and activity in animal models of AKD and CKD were published almost 25 years ago, it is only in the last decade that technical advances in molecular biology and clinical pharmacology have enabled researchers to begin to characterize the phenotypic expression of individual enzymes and, importantly, distinguish the molecular and/or genetic basis for these changes. The selective modulation of hepatic CYP enzyme activity observed in kidney disease is caused, at least in part, by differentially altered expression of several CYP isoforms. This review summarizes data available through June 2003 regarding the effect of AKD and CKD on drug metabolism. Knowledge of the impact and nature of these alterations associated with kidney disease may facilitate the individualization of medication management in this patient population.

Folate treatment and unbalanced methylation and changes of allelic expression induced by hyperhomocysteinaemia in patients with uraemia

BACKGROUND

Hyperhomocysteinaemia occurs in several genetically determined and acquired disorders and is highly prevalent in patients with uraemia. In these disorders, homocysteine precursor S-adenosylhomocysteine, a powerful competitive inhibitor of S-adenosylmethionine-dependent methyltransferases, is increased, suggesting unbalanced methylation. We aimed to investigate whether DNA hypomethylation is present in patients with uraemia who also have hyperhomocysteinaemia and whether regulation of specific classes of genes, dependent on DNA methylation, is compromised.

METHODS

We selected men with hyperhomocysteinaemia and uraemia who were having standard haemodialysis treatment, and compared them with healthy male controls. We measured the homocysteine concentration from plasma samples and obtained DNA and RNA samples from peripheral mononuclear cells. DNA methylation was assessed by cytosine extension assay and by Southern blotting. Allelic expression of pseudoautosomal and imprinted genes was investigated by analysis of suitable restriction fragment length polymorphisms.

FINDINGS

Total DNA hypomethylation was higher in patients than in controls (z score -4.593, p=0.0006) and allelic expression was changed in both sex-linked and imprinted genes. The shift from monoallelic to biallelic expression was dependent on homocysteine concentrations. Folate therapy, a common method to reduce hyperhomocysteinaemia, restored DNA methylation to normal levels and corrected the patterns of gene expression.

INTERPRETATION

Our results suggest that hyperhomocysteinaemia affects epigenetic control of gene expression, which can be reverted by folate treatment. Our data support the hypothesis that the toxic action of homocysteine can be mediated by macromolecule hypomethylation.

Possible mechanisms of homocysteine toxicity

Hyperhomocysteinemia is a risk factor for cardiovascular disease in the general population. In chronic renal failure (CRF), plasma homocysteine levels rise when the glomerular filtration rate (GFR) is reduced 50%, and in uremia the majority of patients are hyperhomocysteinemic. The purpose of this study was to review possible mechanisms of homocysteine toxicity. Homocysteine, a sulfur amino acid found in blood in micromolar concentrations, can have toxic effects through a handful of general possible mechanisms. These mechanisms include oxidative stress (through the production of reactive oxygen species), binding to nitric oxide, production of homocysteinylated/acylated proteins, and accumulation of its precursor, S-adenosyl-homocysteine, a potent inhibitor of transmethylation reactions. Methyltransferase inhibition actually occurs in CRF and in uremia, and can have several functional consequences.

Homocysteine in uremia

Hyperhomocysteinemia is an independent cardiovascular risk factor that possibly accounts for about one of 5 cardiovascular deaths. It is conceivable that the importance of hyperhomocysteinemia will increase when other risk factors, such as hypertension or hypercholesterolemia, will become less prevalent in the general population. In chronic renal failure (CRF), high plasma homocysteine levels are a common finding and in uremia almost the rule. However, a small subset of patients remains normohomocysteinemic. The cause of hyperhomocysteinemia in CRF, whether it lies in an impaired renal or extrarenal metabolism or through uremic retention toxins, is still under intensive scrutiny. As for the consequences of high homocysteine levels in the general population and in patients with CRF, these are many-fold and linked to the mechanism of homocysteine toxic action. In fact, homocysteine can be harmful to cells because (1) it evokes oxidative stress (through the production of reactive oxygen species), (2) binds to nitric oxide, (3) produces homocysteinylated proteins, or (4) leads to the accumulation of its precursor, S-adenosylhomocysteine, a potent inhibitor of biological transmethylations. Macromolecule hypomethylation is a common feature in CRF and uremia with possible functional consequences. Nutritional or pharmacologic interventions have been proposed in the treatment of hyperhomocysteinemia, while the results of large clinical trials designed to assess if lowering homocysteine levels is effective in reducing cardiovascular risk, are pending.

Tissue levels of S-adenosylhomocysteine in the rat kidney: effects of ischemia and homocysteine

Most S-adenosylmethionine (AdoMet)-dependent methyltransferases are regulated in vivo by the AdoMet/S-adenosylhomocysteine (AdoHcy) ratio, also termed as "methylation potential." Since adenosine inhibits in vitro AdoHcy hydrolysis and since adenosine tissue levels increase during hypoxia, it can be predicted that AdoHcy levels may increase in the rat kidney in parallel of those of adenosine. Therefore, the present investigation was performed to assess changes of renal AdoHcy and AdoMet tissue contents during ischemia and after administration of adenosine and homocysteine or both in the ischemic rat kidney. In anesthetized rats ischemia of the kidney was induced by renal artery occlusion for various time intervals. Adenosine and homocysteine were infused into the renal artery of the ischemic kidney. To induce a hyperhomocysteinemia homocysteine was continuously infused. The kidneys were removed and immediately snap-frozen. Tissue contents of AdoHcy, AdoMet, adenosine and adenine nucleotides were analyzed by means of HPLC. Under normoxic condition the tissue contents of AdoHcy, AdoMet and adenosine were 0.7+/-0.05, 44.1+/-1.0 and 3.8+/-0.1nmol/g wet weight, respectively. Renal ischemia for 30min resulted in an increase of AdoHcy levels from 0.7+/-0.05 to 9.1+/-0.6nmol/g wet weight and in a dramatic decrease of the AdoMet/AdoHcy ratio and energy charge from 65.1+/-5.6 to 2.8+/-0.2 and from 0.87+/-0.01 to 0.25+/-0.01, respectively. Application of exogenous adenosine into the ischemic kidney did not result in further AdoHcy accumulation. However, when homocysteine was infused into the ischemic kidney, AdoHcy increased five-fold above control levels, during 5min ischemia. Systemic infusion of homocysteine leads to a reduction of the methylation potential also in the normoxic kidney. We conclude that (i) the methylation potential in the kidney is markedly reduced during ischemia, mainly due to accumulation of AdoHcy; (ii) elevation of AdoHcy tissue content during ischemia is the result of the inhibition of AdoHcy hydrolysis; (iii) homocysteine is rate limiting for AdoHcy synthesis in the ischemic kidney; (iv) under normoxic conditions hyperhomocysteinemia can affect the methylation potential in the renal tissue.

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.

Hyperhomocysteinemia predicts cardiovascular outcomes in hemodialysis patients

BACKGROUND

We prospectively tested the prediction power of homocysteinemia for all-cause and cardiovascular outcomes in a cohort of 175 hemodialysis patients followed for 29 +/- 12 months.

METHODS

Survival analysis was performed by the Cox's proportional hazard model and data were expressed as hazard ratio and 95% confidence interval (CI).

RESULTS

During the follow-up period 51 patients died, 31 of them (61%) of cardiovascular causes and 16 patients developed non-fatal atherothrombotic complications. Plasma total homocysteine was an independent predictor of cardiovascular mortality (P=0.01). Combined analysis of fatal and non-fatal atherothrombotic events showed that homocysteine was a strong and independent predictor of these outcomes because the risk of these events was 8.2 times higher (95% CI 1.9 to 32.2) in patients in the third homocysteine tertile than in those in the first tertile (P=0.005).

CONCLUSIONS

There is a clear association between hyperhomocysteinemia and incident cardiovascular mortality and atherothrombotic events in hemodialysis patients. Intervention studies are needed to determine whether the accumulation of this substance has a causal role in the pathogenesis of cardiovascular damage in patients undergoing hemodialysis.

Metabolic consequences of hyperhomocysteinemia in uremia

An elevated blood level of homocysteine (Hcy), a sulfur amino acid, is associated with increased cardiovascular risk. Hcy is generated from S-adenosylhomocysteine (AdoHcy), the demethylated product of S-adenosylmethionine (AdoMet) in transmethylation reactions. AdoHcy is a competitive inhibitor of AdoMet-dependent methyltransferases. AdoHcy accumulation is prevented by rapid metabolism of its products. Chronic renal failure (CRF) is almost constantly associated with hyperhomocysteinemia. It has been shown that: (1) AdoHcy concentration is significantly increased and the AdoMet-AdoHcy ratio is reduced in erythrocytes of patients with CRF; (2) erythrocyte membrane protein methyl esterification, catalyzed by the enzyme protein L-isoaspartyl O-methyltransferase (PCMT; EC 2.1.1.77), is reduced in CRF; PCMT catalyzes a repair reaction involved in the conversion of an isopeptide bond (detrimental to protein structure and function) into a normal peptide bond; (3) D-aspartate residues, a side product of protein methylation and repair, are significantly reduced in erythrocyte membrane proteins of patients with CRF; and (4) folate treatment significantly reduces plasma Hcy levels and improves AdoMet-AdoHcy ratios. Stable isotope studies recently confirmed that the rate of methyl transfer reactions is significantly reduced in uremia. Additional evidence, obtained by independent groups, is consistent with this interpretation. We recently found increased isoaspartyl content of circulating plasma protein levels, particularly albumin, which was only partially reduced after folate treatment, in uremia. This kind of molecular damage possibly is caused by protein increased intrinsic instability as a result of interference with the uremic milieu. In conclusion, Hcy is an uremic toxin involved in protein molecular damage through the inhibition of methylation reactions and protein PCMT-mediated repair.

Heterogeneity of the association between plasma homocysteine and atherothrombotic disease: insights from studies of vascular structure and function

Among individuals with severe hyperhomocysteinaemia, there is a striking heterogeneity in the severity of the clinical features. This observation demonstrates that factors must exist that modulate the relationship between hyperhomocysteinaemia and clinical disease. Investigations of the association between mild-to-moderate hyperhomocysteinaemia and atherothrombotic disease also suggest heterogeneity in the association between plasma homocysteine levels and 1) clinical disease; 2) angiographic and echographic estimates of the extent of atherosclerosis; 3) arterial stiffness; 4) endothelial function; and 5) procoagulant status. The commonly held view that homocysteine is a vasculotoxic substance that promotes atherogenesis by causing endothelial damage is incomplete, because it cannot explain this heterogeneity. I suggest that homocysteine may have both prothrombotic and proatherogenic properties, but that there are strong, as yet unidentified enhancing and protective factors, the prevalence of which may differ among populations. This concept could account for some of the observed heterogeneity. Identifying these factors would be of major clinical importance and would provide crucial mechanistic insights.

Plasma proteins containing damaged L-isoaspartyl residues are increased in uremia: implications for mechanism

BACKGROUND

Several alterations of protein structure and function have been reported in uremia. Impairment of a transmethylation-dependent protein repair mechanism possibly related to a derangement in homocysteine metabolism is also present in this condition, causing erythrocyte membrane protein damage. Homocysteine may affect proteins via the accumulation of its parent compound S-adenosylhomocysteine (AdoHcy), a powerful in vivo methyltransferase inhibitor. However, since plasma homocysteine is mostly protein bound, a direct influence on protein structures cannot be ruled out. We measured the levels of L-isoaspartyl residues in plasma proteins of uremic patients on hemodialysis. These damaged residues are markers of molecular age, which accumulate when transmethylation-dependent protein repair is inhibited and/or protein instability is increased.

METHODS

L-isoaspartyl residues in plasma proteins were quantitated using human recombinant protein carboxyl methyl transferase (PCMT). Plasma concentrations of homocysteine metabolites were also measured under different experimental conditions in hemodialysis patients.

RESULTS

The concentration of damaged plasma proteins was increased almost twofold compared to control (controls 147.83 +/- 17.75, uremics 282.80 +/- 26.40 pmol of incorporated methyl groups/mg protein, P < 0.003). The major protein involved comigrated with serum albumin. Although hyperhomocysteinemia caused a redistribution of thiols bound to plasma proteins, this mechanism did not significantly contribute to the increase in isoaspartyl residues. The S-adenosylmethionine (AdoMet)/AdoHcy concentration ratio, an indicator of the flux of methyl group transfer, was altered. This ratio was partially corrected by folate treatment (0.385 +/- 0.046 vs. 0.682 +/- 0.115, P < 0.01), but protein L-isoaspartate content was not.

CONCLUSIONS

Plasma protein damage, as determined by protein L-isoaspartyl content, is increased in uremia. This alteration is to be ascribed to an increased protein structural instability, rather than the effect of hyperhomocysteinemia.

Use of bioimpedance spectroscopy to estimate body water distribution in rats fed high dietary sulfur amino acids

The effect of dietary sulfur amino acids on bioelectric properties was studied in rats by using bioimpedance spectroscopy. Weanling rats were assigned to one of 12 groups in a factorially arranged experiment with dietary variables of supplemental sulfur amino acid (none, 10 g DL-methionine/kg or 10 g DL-homocystine/kg), pyridoxine hydrochloride (0 or 7.5 mg/kg) and nickel (0 or 1 mg/kg). After 9 wk of feeding, 20-h urine specimens were collected from food-deprived rats for measurements of creatinine, and then bioimpedance was measured with multifrequency (Hydra ECF/ICF 4200) and single-frequency (RJL Systems model 101) analyzers. Urinary creatinine excretion was measured by intracellular water (ICW), total body solid and urinary volume (R2 = 0.675). Extracellular water (ECW) did not add significantly to the model. Rats fed methionine had significantly lower total body water, ICW and ECW than rats fed no supplemental sulfur amino acid. Rats fed homocystine had significantly lower ECW and a significantly higher ratio of ICW to ECW. Rats fed methionine or homocystine had significantly lower capacitance corrected for body length and ICW than those fed no supplemental sulfur amino acids. These results suggest that dietary homocystine changes the distribution of body water and that sulfur amino acids can affect membrane porosity and/or membrane thickness.

Homocysteine and transmethylations in uremia

Homocysteine is regarded as a cardiovascular risk factor in both the general population and chronic renal failure patients. Among the mechanisms for homocysteine toxicity, its interference with transmethylation reactions, through its precursor/derivative S-adenosylhomocysteine, plays a multifarious role. In uremia, inhibition of S-adenosylmethionine methyl transfer reactions has been reported by independent investigators, using multiple approaches. This has several possible consequences, which can ultimately affect the patient's relative state of health.

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.

Pathophysiology and Treatment of Hyperhomocysteinemia in End-Stage Renal Disease Patients

The pathophysiology of hyperhomocysteinemia in end-stage renal disease (ESRD) patients includes impaired remethylation of homocysteine (Hcy) to methionine, inhibition of extrarenal Hcy metabolism by uremic solutes, a block in decarboxylation of cysteinesulfinic acid, impaired [adenosylmethionine]/[adenosylhomocysteine] ratio, and a probable impairment of renal Hcy metabolism and excretion. Treatment of hyperhomocysteinemia in ESRD patients includes administration of folic acid (1 - 15 mg per day). No additional effects have been observed with higher folic acid doses, folinic acid, or 5-methyltetrahydrofolate. Oral supplementation with vitamin B ₆ and vitamin B ₁₂ has no effect, but some studies reported a decrease of plasma Hcy with high intravenous vitamin doses. Effective reduction of plasma total Hcy (tHcy) in patients treated with super-flux hemodialyzers suggests the removal of uremic toxins with inhibitory activities against enzymes involved in the extrarenal Hcy metabolism.

The removal of uremic toxins

Three major groups of uremic solutes can be characterized: the small water-soluble compounds, the middle molecules, and the protein-bound compounds. Whereas small water-soluble compounds are quite easily removed by conventional hemodialysis, this is not the case for many other molecules with different physicochemical characteristics. Continuous ambulatory peritoneal dialysis (CAPD) is often characterized by better removal of those compounds. Urea and creatinine are small water-soluble compounds and the most current markers of retention and removal, but they do not exert much toxicity. This is also the case for many other small water-soluble compounds. Removal pattern by dialysis of urea and creatinine is markedly different from that of many other uremic solutes with proven toxicity. Whereas middle molecules are removed better by dialyzers containing membranes with a larger pore size, it is not clear whether this removal is sufficient to prevent the related complications. Larger pore size has virtually no effect on the removal of protein-bound toxins. Therefore, at present, the current dialytic methods do not offer many possibilities to remove protein-bound compounds. Nutritional and environmental factors as well as the residual renal function may influence the concentration of uremic toxins in the body fluids.

Interrelations between plasma homocysteine and intracellular S-adenosylhomocysteine

S-Adenosylhomocysteine, a potent intracellular methylation inhibitor, is suggested as a potential mediator for hyperhomocysteinemia-related vascular changes. We investigated the effect of acute and chronic hyperhomocysteinemia on intracellular S-adenosylhomocysteine and S-adenosylmethionine in rats and humans. Elevated plasma homocysteine in rats infused with homocysteine produced an increase in S-adenosylhomocysteine (P < 0.001) but not S-adenosylmethionine levels (P > 0.05) in various rat tissues. However intraerythrocyte S-adenosylhomocysteine and S-adenosylmethionine levels were not changed in homocysteine-infused rats and human subjects with experimentally acute hyperhomocysteinemia by methionine loading test. In contrast, erythrocyte S-adenosylhomocysteine levels were significantly higher in chronic renal failure patients, who had chronically elevated plasma homocysteine levels, than in either vascular disease patients or healthy controls (P < 0.05). In conclusion, acute hyperhomocysteinemia can increase intracellular S-adenosylhomocysteine levels in tissues actively involved in homocysteine metabolism. The findings are relevant to homocysteine-related endothelial dysfunction since S-adenosylhomocysteine modulates endothelial cell apoptosis.

Hyperhomocysteinemia, nutritional status, and cardiovascular disease in hemodialysis patients

BACKGROUND

Hyperhomocysteinemia, cardiovascular disease (CVD), and malnutrition are common in patients with end-stage renal disease (ESRD). This study was designed to assess possible relationships between total plasma homocysteine (tHcy), nutritional status, and ischemic CVD.

METHODS

We performed a cross-sectional study in 117 unselected patients on maintenance hemodialysis (HD) treatment, among whom there was a high prevalence of malnutrition (56%), as assessed by the subjective global nutritional assessment (SGNA), and a high prevalence of CVD (60%), and prospectively, we followed-up the overall mortality for four years.

RESULTS

The level of tHcy was elevated in 95% of the HD patients, and that of total plasma cysteine (tCys) was also significantly elevated, while the plasma concentrations of methionine (Met), serine (Ser), and taurine (Tau) were significantly lower than those in healthy controls. The 65 patients who were malnourished according to the SGNA score had significantly lower levels of serum albumin (SAlb), plasma IFG-1 (p-IGF-1), tHcy, tCys, and Met than the 52 patients with normal nutritional status, whereas the levels of Ser, Tau, plasma folate, and vitamin B12 were similar in the two groups. The prevalence of malnutrition was 30% in the 47 patients without CVD and was significantly higher (70%, P < 0.001) in the 70 patients with CVD, who also had lower tHcy, SAlb, plasma IGF-1, serum creatinine (SCr), and blood hemoglobin. The tHcy levels were positively correlated with SAlb, Met, tCys, and SCr. Stepwise, multiple-regression analysis showed that tCys, SAlb, and normalized protein equivalent of nitrogen appearance (nPNA), an indicator of protein intake, were independent predictors of tHcy. The patients with tHcy <24 micromol/L (median value) had a significantly worse four-year survival than those with a higher tHcy (> or =24 micromol/L).

CONCLUSIONS

Our results demonstrate that most of HD patients have grossly elevated tHcy levels, but that the absolute level appears to be dependent on nutritional status, protein intake, and SAlb. The results also suggest that the lower tHcy levels in patients with CVD than in those without CVD may be related to the higher prevalence of malnutrition and hypoalbuminemia in the CVD patients. This is also in accordance with our observation that the patients with lower tHcy had a worse survival rate than those with higher tHcy, considering that malnutrition is a strong risk factor for mortality and that CVD is the most common cause of death in ESRD patients.

Clinical relevance of hyperhomocysteinaemia in atherothrombotic disease

High fasting plasma homocysteine levels (> 12 to 15 mumol/L) are commonly encountered in clinical practice and are associated with increased risk of atherothrombotic disease. Treatment with folic acid (1 to 5 mg/day) is inexpensive and effective in normalising plasma homocysteine levels. High plasma homocysteine levels after methionine loading (> 40 to 50 mumol/L) are also common and can be treated with pyridoxine-based regimens (50 to 250 mg/day). As compared with fasting plasma homocysteine levels, the association between high postmethionine loading plasma homocysteine levels and atherothrombotic disease has been less extensively studied. There is reasonable, but not clearly definitive, evidence that high plasma homocysteine levels are causally related to atherothrombotic disease. Results of randomised trials of homocysteine-lowering treatment with clinical end-points will be available in 4 to 6 years. At present, a reasonable policy for the practising clinician would be to consider homocysteine-lowering treatment in individuals at very high risk of atherothrombotic disease, such as patients with clinically manifest atherothrombotic disease with onset before 55 years of age, patients with end stage renal disease, and healthy subjects with a strong family history of early-onset atherothrombotic disease. Such a policy should be reassessed as the results of randomised trials become available.

Homocysteine and methionine metabolism in ESRD: A stable isotope study

BACKGROUND

Hyperhomocysteinemia has a high prevalence in the end-stage renal disease (ESRD) population, which may contribute to the high cardiovascular risk in these patients. The cause of hyperhomocysteinemia in renal failure is unknown, and therapies have not been able to normalize plasma homocysteine levels. Insight into methionine-homocysteine metabolism in ESRD is therefore necessary.

METHODS

Using a primed, continuous infusion of [2H3-methyl-1-13C]methionine, we measured whole body rates of methionine and homocysteine metabolism in the fasting state in four hyperhomocysteinemic hemodialysis patients and six healthy control subjects.

RESULTS

Remethylation of homocysteine was significantly decreased in the hemodialysis patients: 2.6+/-0.2 (SEM) vs. 3.8+/-0.3 micromol. kg(-1)x hr(-1) in the control subjects (P = 0.03), whereas transsulfuration was not 2.5+/-0.3 vs. 3.0+/-0.1 micromol. kg(-1) x hr(-1) (P = 0.11). The transmethylation rate was proportionally and significantly lower in the ESRD patients as compared with controls: 5.2+/-0.4 vs. 6.8+/-0.3 micromol. kg(-1) x hr(-1) (P = 0.02). Methionine fluxes to and from body protein were similar.

CONCLUSIONS

The conversion of homocysteine to methionine is substantially (approximately 30%) decreased in hemodialysis patients, whereas transsulfuration is not. Decreased remethylation may explain hyperhomocysteinemia in ESRD. This stable isotope technique is applicable for developing new and effective homocysteine-lowering treatment regimens in ESRD based on pathophysiological mechanisms.

Homocysteine

Homocysteine is a sulfur-containing amino acid generated through the demethylation of methionine. It is largely catabolized by trans-sulfuration to cysteine, but it may also be remethylated to methionine. Regulation of homocysteine is dependent on nutrient intake, especially folate, vitamins B6 and B12. It is also controlled by individual genetic differences in how vitamins are utilized as cofactors in the reactions controlling homocysteine metabolism. In excess quantities, homocysteine is thought to be thrombophilic and to damage the vascular endothelium. Total plasma homocysteine (tHcy) is now established as a clinical risk factor for coronary artery disease, as well as other arterial and venous occlusive disease in adult populations. These effects are probably related to its role as a teratogen in the pathogenesis of neural tube defects--genetic variants causing hyperhomocysteinemia are associated with both neural tube defects in susceptible pregnancies and with risks for vaso-occlusive disease in later years. Considerable care must be taken in assaying tHcy. Plasma should be separated shortly after collection to avoid artifactual increases due to synthesis by blood cells in vitro. tHcy concentrations must be interpreted in light of the fact that serum albumin, urate, creatinine, and vitamin concentrations may be important analytical covariates. Moreover, concentrations are age- and sex-dependent and are altered by renal function, hormonal status, drug intake, and a variety of other common clinical factors. Why then is homocysteine now of such great clinical and scientific interest? If the homocysteine moiety itself is important in the pathogenesis of vaso-occlusive disease, then simple treatment of hyperhomocysteinemia with vitamins should lead to a significant reduction in disease risk. Such a possibility lies behind the growing momentum to recommend increased supplements of folate and B vitamins to at-risk populations and patient groups today.

Influence of 5,10-methylenetetrahydrofolate reductase gene polymorphism on plasma homocysteine concentration in patients with end-stage renal disease

The purpose of this study is to observe the influence of the methylenetetrahydrofolate reductase (MTHFR) gene (677C-->T substitution) on plasma homocysteine levels in end-stage renal disease (ESRD) patients who received a relatively large amount of folate (2 mg/d) and are undergoing hemodialysis. A cross-sectional study of plasma homocysteine, vitamin B(12), and folate was performed in patients with ESRD. The study population for the MTHFR gene study included 312 healthy subjects and 106 patients with ESRD undergoing hemodialysis. The C677T transition in the MTHFR gene was detected by HinF 1 restriction enzyme analysis and subsequent electrophoresis in a 3% agarose gel. The genotype of the MTHFR gene in 106 patients with ESRD was homozygous C677T mutation (VV) in 17 patients (16.1%) and heterozygous (AV) in 63 patients (58.4%); 26 patients (24.5%) did not carry this mutation (AA). The mean levels of homocysteine, vitamin B(12), and folate in the patients with ESRD were 23.3 +/- 14.0 mmol/L, 620.2 +/- 98.5 pmol/L, and 138.6 +/- 55.6 nmol/L, respectively. There was no significant difference in homocysteine levels among the three genotypes: 28.2 +/- 19.4 mmol/L for VV, 22.7 +/- 14.9 mmol/L for AV, and 23.4 +/- 11.1 mmol/L for AA genotype (P > 0.05). There was no difference in genotype distribution between the patient groups of less than 25th and greater than 75th percentiles, classified according to plasma homocysteine levels (P = 0.47). In conclusion, with high-dose folate supplementation, the hyperhomocysteinemia in patients with ESRD does not seem to be caused by the 677C-->T mutation in the MTHFR gene.

Effects of high-dose folic acid and pyridoxine on plasma and erythrocyte sulfur amino acids in hemodialysis patients

In this investigation, sulfur amino acids (sAA) and sulfhydryls were determined in the plasma and erythrocytes (RBC) of 10 uremic patients on regular hemodialysis (HD) treatment and 10 healthy subjects, before and after supplementation with 15 mg/d of folic acid and 200 mg/d of pyridoxine for 4 wk. The basal total plasma concentrations of homocysteine (Hcy), cysteine (Cys), cysteinylglycine (Cys-Gly), gamma-glutamylcysteine (gamma-Glu-Cys), glutathione (GSH), and free cysteinesulfinic acid (CSA) were significantly higher in HD patients when compared to healthy subjects, whereas methionine (Met) and taurine (Tau) concentrations were the same in the two groups. HD patients showed significantly higher RBC levels of Hcy and Cys-Gly, whereas the RBC concentrations of Met, Cys, Tau, and GSH were not different from those in the healthy subjects. The plasma concentrations of sAA and sulfhydryls differed compared with RBC levels in the healthy subjects and HD patients. In both groups, supplementation with high doses of folic acid and pyridoxine reduced the plasma Hcy concentration. In addition, increased plasma concentrations of Cys-Gly and GSH were found in the HD patients and of CSA in the healthy subjects. After vitamin supplementation, the RBC concentrations of Hcy, Cys, and GSH increased and that of Tau decreased in healthy subjects. The only significant finding in RBC of HD patients was an increase in GSH levels after supplementation. This study shows several RBC and plasma sAA and sulfhydryl abnormalities in HD patients, which confirms earlier findings that RBC and plasma pools play independent roles in interorgan amino acid transport and metabolism. Moreover, high-dose supplementation with folic acid and pyridoxine significantly reduced Hcy levels, but did not restore the sAA and sulfhydryl abnormalities to normal levels. The increase that was observed in GSH after vitamin supplementation may have a beneficial effect in improving blood antioxidant status in uremic patients. Finally, the findings of elevated plasma Cys levels correlating to the elevated plasma Hcy levels in the presence of elevated plasma CSA levels, both before and after vitamin supplementation, led to the hypothesis that a block in decarboxylation of CSA is linked to hyperhomocysteinemia in end-stage renal failure.

D-amino acids in aging erythrocytes

Mature human erythrocytes are highly differentiated cells which have lost the ability to biosynthesize proteins de novo. During cell aging in circulation, erythrocyte proteins undergo spontaneous postbiosynthetic modifications, regarded as "protein fatigue" damage, which include formation of isomerized and/or racemized aspartyl residues. These damaged proteins cannot be replaced by new molecules; nevertheless, data support the notion that they can be repaired to a significant extent, through an enzymatic transmethylation reaction. This repair reaction has therefore been used as a means to monitor the increase of altered aspartyl residues in erythrocyte membrane proteins during cell aging. The relationship between protein repair and aspartyl racemization in red blood cell stress and disease is discussed.

Plasma homocysteine concentration predicts mortality in non-insulin-dependent diabetic patients with and without albuminuria

BACKGROUND

A high plasma total homocysteine (tHcy) concentration is a risk factor for cardiovascular disease in the nondiabetic population and in nondiabetic patients with end-stage renal disease.

METHODS

We prospectively evaluated the impact of tHcy concentrations on mortality in 211 white non-insulin-dependent diabetic (NIDDM) patients of less than 70 years of age at entry (61 with microalbuminuria and 44 with macroalbuminuria). They were followed for a median of 6.4 (range 0.2 to 7.1) years.

RESULTS

At the end of the follow-up period, 49 of 211 (23%) patients had died, 30 (61%) from cardiovascular disease. Univariate Cox survival analysis revealed that baseline tHcy level (1 micromol/liter) was associated with an increased all-cause mortality risk of 1.11 [95% confidence interval (CI) 1.08 to 1.15, P < 0.0001], and a cardiovascular mortality risk of 1.09 (CI 1.03 to 1.16, P < 0.01). The six-year cumulative all-cause mortality hazard was 44%, 14%, and 15% in the high (tHcy >/= 8.2 micromol/liter), the middle (tHcy 6. 2-8.1 micromol/liter), and the low (tHcy </= 6.1 micromol/liter) tertile of tHcy levels, respectively (P < 0.001 high vs. middle; P < 0.001 high vs. low; and P = 0.88 middle vs. low). Cox proportional hazards regression analysis revealed significant predictors of all-cause mortality to be tHcy level (per 1 micromol/liter), relative risk 1.09 (1.03 to 1.14); pre-existing coronary heart disease (yes vs. no), relative risk 1.98 (1.09 to 3.61); log10 albumin excretion rate (AER; factor 10), relative risk 1.89 (1.31 to 2.74); and age (per 1 year), relative risk 1.08 (1.03 to 1.13). Predictors of cardiovascular mortality were pre-existing coronary heart disease, log10 AER, and age. tHcy level did not predict cardiovascular mortality independently of these risk factors.

CONCLUSION

Plasma tHcy concentration is a significant predictor of mortality in NIDDM patients with or without albuminuria.

Major determinants of hyperhomocysteinemia in peritoneal dialysis patients

The mechanisms leading to elevated total homocysteine concentrations in peritoneal dialysis patients are only partially understood. We show that a common polymorphism in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene (C677T transition) results in increased total homocysteine levels in peritoneal dialysis patients compared to age- and sex-matched healthy individuals. The allelic frequency of the C677T transition in the MTHFR gene in peritoneal dialysis patients (0.29) was comparable to the frequency in healthy individuals (0.34). Separate comparison of the total homocysteine plasma levels between non-carriers of the MTHFR polymorphism (C/C), heterozygous (C/T) and homozygous (T/T) subjects was performed by analysis of covariance in the patient and the control group. In the patient group the mean total homocysteine level was 61.7 +/- 40.1 mumol/liter in individuals with the (T/T) genotype, which was significantly higher than the total homocysteine concentration of 23.1 +/- 15.8 mumol/liter in (C/T) patients and 22.2 +/- 11.1 mumol/liter for non-carriers (P = 0.0001). Vitamin B12 (P = 0.0001), folate (P = 0.0005), serum creatinine (P = 0.016), albumin (P = 0.0157) and dialysis center (P = 0.0173) significantly influenced total homocysteine plasma levels in peritoneal dialysis patients, whereas this was not the case for age, gender, weekly Kt/V, weekly creatinine clearance, residual renal function, duration of dialysis, mode of peritoneal dialysis and vitamin intake. Folate levels in peritoneal dialysis patients were significantly affected by the MTHFR genotype (P = 0.016). Elevated total homocysteine levels in diabetic patients with cardiovascular disease were associated with increased cardiovascular morbidity. In summary, the present study provides evidence that homozygosity for the C677T transition in the MTHFR gene, low vitamin B12 and low folate levels result in elevated total homocysteine levels in peritoneal dialysis patients.

The relationship between plasma homocysteine and amino acid concentrations in patients with end-stage renal disease

OBJECTIVE

Observe the relationship between homocysteine and other amino acids in patients with end-stage renal disease (ESRD).

DESIGN

A cross-sectional study of amino acids and homocysteine and comparison of the correlations between ESRD and control group.

SETTING

Nephrology unit of Soonchunhyang University hospital in Chunan, Korea.

PARTICIPANTS

Forty-five ESRD patients and 30 control volunteers.

MAIN OUTCOME MEASURES

Plasma amino acids and homocysteine.

RESULTS

Concentrations of asparate, proline and cysteine were higher and serine, tyrosine, valine, isoleuline, leucine, and lysine levels were lower in the ESRD group than in control group. The branched chain amino acids (BCAAs) and essential amino acid were lower in the ESRD group than in the control group, but there was no difference in non-essential amino acid and total amino acid between the two groups. The mean plasma total homocysteine concentrations were 6 +/- 1 mmol/L in the control group and 14 +/- 4 mmol/L in the ESRD group (P < .001). In the ESRD group, homocysteine concentrations showed a direct correlation with the concentration of histidine (R2: .403, P < .001), valine (R2: .324, P < .01), leucine (R2: .400, P < .01), isoleucine (R2: .351, P < .005), cysteine (R2: .287, P < .001), methionine (R2: .256, P < .01), BCAA (R2: .50, P < .01), and essential amino acid (R2: .416, P < .01). In the control group, no correlation between the homocysteine and amino acid concentrations was found.

CONCLUSIONS

Contrary to the control group, the homocysteine concentrations showed a direct correlation with the concentration of valine, leucine, isoleucine, methionine, and histidine levels in the ESRD group. Altered essential amino acid metabolism, specifically BCAAs and histidine, influence hyperhomocysteinemia in ESRD. Further study is needed to confirm this theory.