Renal uptake and excretion of homocysteine in rats with acute hyperhomocysteinemia

Tangshen formula improves diabetic nephropathy in STZ-induced diabetes rats fed with hyper-methionine by regulating the methylation status of kidney

BACKGROUND

The objective of this study was to examine and analyze differential methylation profiles in order to investigate the influence of hyper-methioninemia (HM) on the development of diabetic nephropathy (DN). Male Wistar rats, aged eight weeks and weighing 250-300 g, were randomly assigned into four groups: a control group (Healthy, n = 8), streptozocin-induced rats (STZ group, n = 8), HM + STZ group (n = 8), and the Tangshen Formula (TSF) treatment group (TSF group, n = 8). Blood glucose levels and other metabolic indicators were monitored before treatment and at four-week intervals until 12 weeks. Total DNA was extracted from the aforementioned groups, and DNA methylation landscapes were analyzed via reduced representative bisulfite sequencing.

RESULTS

Both the STZ group and HM + STZ group exhibited increased blood glucose levels and urinary albumin/creatinine ratios in comparison with the control group. Notably, the HM + STZ group exhibited a markedly elevated urinary albumin/creatinine ratio (411.90 ± 88.86 mg/g) compared to the STZ group (238.41 ± 62.52 mg/g). TSF-treated rats demonstrated substantial reductions in both blood glucose levels and urinary albumin/creatinine ratios in comparison with the HM + STZ group. In-depth analysis of DNA methylation profiles revealed 797 genes with potential therapeutic effects related to TSF, among which approximately 2.3% had been previously reported as homologous genes.

CONCLUSION

While HM exacerbates DN through altered methylation patterns at specific CpG sites, TSF holds promise as a viable treatment for DN by restoring abnormal methylation levels. The identification of specific genes provides valuable insights into the underlying mechanisms of DN pathogenesis and offers potential therapeutic targets for further investigation.

Protective Effects and Metabolic Regulatory Mechanisms of Shenyan Fangshuai Recipe on Chronic Kidney Disease in Rats

Background

Chronic kidney disease (CKD) is one of the major causes of renal damage. Shenyan Fangshuai Recipe (SFR), a modified prescription of traditional medicine in China, showed potent effects in alleviating edema, proteinuria, and hematuria of CKD in clinical practices. In this study, we aimed to investigate scientific evidence-based efficacy as well as metabolic regulations of SFR in CKD treatment.

Materials and Methods

The effect of SFR on CKD was observed in a rat model which is established with oral administration of adenine-ethambutol mixture for 21 days. Further, metabolites in serum were detected and identified with ultra-performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS). Metabolomics study was performed using Ingenuity Pathway Analysis (IPA) software.

Results

With H&E staining and Masson's trichrome, the results showed that chronic kidney damage is significantly rescued with SFR treatment and recovered to an approximately normal condition. Along with 44 differential metabolites discovered, the regulation of SFR on CKD was enriched in glycine biosynthesis I, mitochondrial L-carnitine shuttle pathway, phosphatidylethanolamine biosynthesis III, sphingosine-1-phosphate signaling, L-serine degradation, folate transformations I, noradrenaline and adrenaline degradation, salvage pathways of pyrimidine ribonucleotides, cysteine biosynthesis III (Mammalia), glycine betaine degradation, and cysteine biosynthesis/homocysteine degradation. Further, TGFβ-1 and MMP-9 were observed playing roles in this regulatory process by performing immunohistochemical staining.

Conclusion

SFR exerts potent effects of alleviating glomerular sclerosis and interstitial fibrosis in the kidney, mainly via integrated regulations on metabolism and production of homocysteine, L-carnitine, and epinephrine, as well as the expression of TGFβ-1. This study provides evidence for SFR's protective effects on CKD and reveals the metabolic mechanism behind these benefits for the first time.

Hierarchical Surface Architecture of Hemodialysis Membranes for Eliminating Homocysteine Based on the Multifunctional Role of Pyridoxal 5'-phosphate

Patients with end-stage renal disease are prone to developing a complication of hyperhomocysteinemia, manifesting as an elevation of the homocysteine (Hcy) concentration in human plasma. However, Hcy as a protein-bound toxin is barely removed by conventional hemodialysis membranes. Here, we report a novel hemodialysis membrane by preparing a bioactive coating of pyridoxal 5'-phosphate (PLP) and adding biocompatible hyperbranched polyglycerol (HPG) brushes to achieve Hcy removal. The dip-applied PLP coating, a coenzyme with a role in Hcy metabolism, dramatically promoted a decrease in the Hcy concentration in human plasma. Moreover, the aldehyde group of PLP had an intrinsic chemical reactivity toward the terminal amino group to immobilize the HPG brushes on the hemodialysis membrane surface. The hierarchical PLP-HPG layer-functionalized membranes had a high efficacy for eliminating Hcy, with a concentration from the initial stage of 150 μmol/L reduced to a nearly normal level of 20 μmol/L in simulated dialysis. By analyzing the impact of HPG brushes with various chain lengths, we found that HPG brushes with a medium length enabled the PLP coating with the bioactive function of Hcy conversion to additionally protect Hcy-attacked target cells by providing excellent hydrophilicity and a dense enough chain volume overlap of the hyperbranched architecture. Simultaneously, the densely packed HPG brushes generated a maximal steric and hydration barrier that significantly improved biofouling resistance against blood proteins. The optimally functionalized membranes showed a clearance of 83.1% urea and 49.6% lysozyme and a rejection of 96.0% bovine serum albumin. The diversely functionalized PLP-HPG layers demonstrate a potential route for a more integrated hemodialysis membrane that can cope with the urgent issue of hyperhomocysteinemia in clinical hemodialysis therapy.

Assessing Plasma Total Homocysteine in Patients with End-Stage Renal Disease

Elevated plasma total homocysteine (tHcy) is a risk factor for cardiovascular disease; however, in light of several recent randomized trials, the issue of causality has been cast into doubt. Patients with end-stage renal disease are particularly interesting as they consistently have elevated tHcy and their leading causes of morbidity and mortality are related to cardiovascular disease. In the present article, we review the early evidence for the homocysteine theory of atherosclerosis, homocysteine metabolism, mechanisms of toxicity, and pertinent available clinical investigations. Where appropriate, the sparse evidence of homocysteine in peritoneal dialysis is reviewed. We conclude by addressing the difficulties associated with lowering plasma tHcy in patients with end-stage renal disease and suggest some novel methods for lowering tHcy in this resistant population. Finally, to address the issue of causality, we recommend that clinicians and scientists await the results of the FAVORIT trial before abandoning homocysteine as a modifiable risk factor for cardiovascular disease, as this study has recruited patients from a population with consistently elevated plasma tHcy who are known to respond to vitamin therapy.

Methionine restriction leads to hyperhomocysteinemia and alters hepatic H2S production capacity in Fischer-344 rats

Dietary methionine restriction (MR) increases lifespan in several animal models. Despite low dietary intake of sulphur amino acids, rodents on MR develop hyperhomocysteinemia. On the contrary, MR has been reported to increase H₂S production in mice. Enzymes involved in homocysteine metabolism also take part in H₂S production and hence, in this study, the impact of MR on hyperhomocysteinemia and H₂S production capacity were investigated using Fischer-344 rats assigned either a control or a MR diet for 8 weeks. The MR animals showed elevated plasma homocysteine accompanied with a reduction in liver cysteine content and methylation potential. It was further found that MR decreased cystathionine-β-synthase (CBS) activity in the liver, however, MR increased hepatic cystathionine-γ-lyase (CGL) activity which is the second enzyme in the transsulfuration pathway and also participates in regulating H₂S production. The relative contribution of CGL in H₂S production increased concomitantly with the increased CGL activity. Additionally, hepatic mercaptopyruvate-sulphur-transferase (MPST) activity also increased in response to MR. Taken together, our results suggest that reduced CBS activity and S-Adenosylmethionine availability contributes to hyperhomocysteinimia in MR animals. Elevated CGL and MPST activities may provide a compensatory mechanism for maintaining hepatic H₂S production capacity in response to the decreased CBS activity.

Clinical Parameters and Gut Microbiome Changes Before and After Surgery in Thoracic Aortic Dissection in Patients with Gastrointestinal Complications

Thoracic aortic dissection (TAAD) is one of the most common types of aortic diseases. Although surgery remains the main method of treatment, the high rate of postoperative gastrointestinal complications significantly influences the effects of surgery and the recovery process. Moreover, the mechanisms underlying this disease remain unclear. To address these problems, we examined changes in the gut microbiota in 40 thoracic aortic dissection patients with abdominal complications after surgery. Levels of white blood cells (WBC), neutrophile granulocytes (NE), alanine aminotransferase (ALT), and aspartate aminotransferase (AST) were higher in all patients after surgery. Levels of inflammatory cytokines, including interleukin (IL)-2, IL-6, IL-8, and IL-10, were also higher after surgery. A metagenome analysis revealed that levels of Oscillibacter, Anaerotruncus, Alistipes, and Clostridium difficile were higher after the operation. The abundance of functional genes, such as the spermidine/putrescine transport system permease protein, the flagellar motor switch protein, and branched-chain amino acid transport system proteins, was also higher post-surgery. These changes likely contribute to diarrhea, bloating, gastrointestinal bleeding, and other abdominal complications after surgery, and our research opens up new treatment possibilities for patients suffering from abdominal complications after surgical treatment.

Lean Body Mass Harbors Sensing Mechanisms that Allow Safeguarding of Methionine Homeostasis

Protein-depleted states generate allosteric inhibition of liver cystathionine β-synthase (CBS), which governs the first enzymatic step of the transsulfuration cascade, resulting in upstream accretion of homocysteine (Hcy) in body fluids. A similar Hcy increase may arise from normal hepatocytes undergoing experimentally-induced impairment of betaine-homocysteine methyltransferase (BHTM) activity or from components of lean body mass (LBM) submitted to any inflammatory disorder. LBM comprises a composite agglomeration of extrarenal tissues characterized by naturally occurring BHTM inactivity. As a result of cellular injury, LBM releases high concentrations of Hcy into the extracellular space, contrasting with the disruption of normal remethylation pathways. Hyperhomocysteinemia acts as a biomarker, reflecting the severity of insult and operating as an alarm signal. Elevated Hcy levels constitute a precursor pool recognized by a CBS coding region that reacts to meet increased methionine requirements in LBM tissues, using its enhanced production in hepatocytes. Preservation of methionine homeostasis benefits from its high metabolic priority and survival value.

Homocysteine and hydrogen sulfide in epigenetic, metabolic and microbiota related renovascular hypertension

Over the past several years, hydrogen sulfide (H₂S) has been shown to be an important player in a variety of physiological functions, including neuromodulation, vasodilation, oxidant regulation, inflammation, and angiogenesis. H₂S is synthesized primarily through metabolic processes from the amino acid cysteine and homocysteine in various organ systems including neuronal, cardiovascular, gastrointestinal, and kidney. Derangement of cysteine and homocysteine metabolism and clearance, particularly in the renal vasculature, leads to H₂S biosynthesis deregulation causing or contributing to existing high blood pressure. While a variety of environmental influences, such as diet can have an effect on H₂S regulation and function, genetic factors, and more recently epigenetics, also have a vital role in H₂S regulation and function, and therefore disease initiation and progression. In addition, new research into the role of gut microbiota in the development of hypertension has highlighted the need to further explore these microorganisms and how they influence the levels of H₂S throughout the body and possibly exploiting microbiota for use of hypertension treatment. In this review, we summarize recent advances in the field of hypertension research emphasizing renal contribution and how H₂S physiology can be exploited as a possible therapeutic strategy to ameliorate kidney dysfunction as well as to control blood pressure.

Homocysteine as a predictive biomarker in early diagnosis of renal failure susceptibility and prognostic diagnosis for end stages renal disease

Glomerular filtration rate and/or creatinine are not accurate methods for renal failure prediction. This study tested homocysteine (Hcy) as a predictive and prognostic marker for end stage renal disease (ESRD). In total, 176 subjects were recruited and divided into: healthy normal group (108 subjects); mild-to-moderate impaired renal function group (21 patients); severe impaired renal function group (7 patients); and chronic renal failure group (40 patients) who were on regular hemodialysis. Blood samples were collected, and serum was separated for analysis of total Hcy, creatinine, high sensitive C-reactive protein (CRP), serum albumin, and calcium. Data showed that Hcy level was significantly increased from normal-to-mild impairment then significantly decreases from mild impairment until the patient reaches severe impairment while showing significant elevation in the last stage of chronic renal disease. Creatinine level was increased in all stages of kidney impairment in comparison with control. CRP level was showing significant elevation in the last stage. A significant decrease in both albumin and calcium was occurred in all stages of renal impairment. We conclude Hcy in combination with CRP, creatinine, albumin, and calcium can be used as a prognostic marker for ESRD and an early diagnostic marker for the risk of renal failure.

Homocysteine in Chronic Kidney Disease

Hyperhomocysteinemia occurs in chronic- and end-stage kidney disease at the time when dialysis or transplant becomes indispensable for survival. Excessive accumulation of homocysteine (Hcy) aggravates conditions associated with imbalanced homeostasis and cellular redox thereby resulting in severe oxidative stress leading to oxidation of reduced free and protein-bound thiols. Thiol modifications such as N-homocysteinylation, sulfination, cysteinylation, glutathionylation, and sulfhydration control cellular responses that direct complex metabolic pathways. Although cysteinyl modifications are kept low, under Hcy-induced stress, thiol modifications persist thus surpassing cellular proteostasis. Here, we review mechanisms of redox regulation and show how cysteinyl modifications triggered by excess Hcy contribute development and progression of chronic kidney disease. We discuss different signaling events resulting from aberrant cysteinyl modification with a focus on transsulfuration.

Molecular characterization of betaine-homocysteine methyltransferase 1 from the liver, and effects of aestivation on its expressions and homocysteine concentrations in the liver, kidney and muscle, of the African lungfish, Protopterus annectens

Homocysteine accumulation has numerous deleterious effects, and betaine-homocysteine S-methyltransferase (BHMT) catalyses the synthesis of methionine from homocysteine and betaine. This study aimed to determine homocysteine concentrations, and mRNA expression levels and protein abundances of bhmt1/Bhmt1 in the liver, kidney and muscle of the African lungfish, Protopterus annectens, during the induction (6 days), maintenance (6 months) or arousal (3 days after arousal) phase of aestivation. The homocysteine concentration decreased significantly in the liver of P. annectens after 6 days or 6 months of aestivation, but it returned to the control level upon arousal. By contrast, homocysteine concentrations in the kidney and muscle remained unchanged during the three phases of aestivation. The complete coding cDNA sequence of bhmt1 from P. annectens consisted of 1236 bp, coding for 412 amino acids. The Bhmt1 from P. annectens had a close phylogenetic relationship with those from tetrapods and Callorhinchus milii. The expression of bhmt1 was detected in multiple organs/tissues of P. annectens, and this is the first report on the expression of bhmt1/Bhmt1 in animal skeletal muscle. The mRNA and protein expression levels of bhmt1/Bhmt1 were up-regulated in the liver of P. annectens during the induction and maintenance phases of aestivation, possibly to regulate the hepatic homocysteine concentration. The significant increase in hepatic Bhmt1 protein abundance during the arousal phase could be a response to increased cellular methylation for the purpose of tissue reconstruction. Unlike the liver, Bhmt1 expression in the kidney and muscle of P. annectens was regulated translationally, and its up-regulation could be crucial to prevent homocysteine accumulation.

Neutral aminoaciduria in cystathionine β-synthase-deficient mice, an animal model of homocystinuria

The kidney is one of the major loci for the expression of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CTH). While CBS-deficient ( Cbs−/−) mice display homocysteinemia/methioninemia and severe growth retardation, and rarely survive beyond the first 4 wk, CTH-deficient ( Cth−/−) mice show homocysteinemia/cystathioninemia but develop with no apparent abnormality. This study examined renal amino acid reabsorption in those mice. Although both 2-wk-old Cbs−/− and Cth−/− mice had normal renal architecture, their serum/urinary amino acid profiles largely differed from wild-type mice. The most striking feature was marked accumulation of Met and cystathionine in serum/urine/kidney samples of Cbs−/− and Cth−/− mice, respectively. Levels of some neutral amino acids (Val, Leu, Ile, and Tyr) that were not elevated in Cbs−/− serum were highly elevated in Cbs−/− urine, and urinary excretion of other neutral amino acids (except Met) was much higher than expected from their serum levels, demonstrating neutral aminoaciduria in Cbs−/− (not Cth−/−) mice. Because the bulk of neutral amino acids is absorbed via a B0AT1 transporter and Met has the highest substrate affinity for B0AT1 than other neutral amino acids, hypermethioninemia may cause hyperexcretion of neutral amino acids.

Association between homocysteine status and the risk of nephropathy in type 2 diabetes mellitus

BACKGROUND

It is well documented that hyperhomocysteinemia induces renal injury. However, the association between homocysteine level and type 2 diabetic nephropathy (T2DN) remains elusive.

METHODS

We evaluated the alteration of plasma level of homocysteine in T2DN patients with macroalbuminuria or microalbuminuria compared with type 2 diabetes mellitus (T2DM) controls without albuminuria by performing a meta-analysis. We searched the PubMed, Embase and Cochrane databases from January 1990 to October 2013 to identify studies that met predefined criteria.

RESULTS

Seven studies were included in this investigation. T2DN patients with macroalbuminuria demonstrated a significantly higher level of plasma homocysteine than T2DM without albuminuria (4 studies, random effects SMD: 1.66, 95% CI: 0.46 to 2.87, P=0.007) and T2DN with microalbuminuria (3 studies, random effects SMD: 0.99, 95% CI: 0.62 to 1.36, P<0.001). T2DN patients with microalbuminuria demonstrated significantly higher level of plasma homocysteine than T2DM without albuminuria (6 studies, random effects SMD: 1.29, 95%. CI: 0.59 to 2, P<0.001). Exclusion of any single study had little impact on the pooled SMDs. No evidence of publication bias was observed.

CONCLUSIONS

Our findings indicate that the status of plasma homocysteine is associated with both the risk and severity of nephropathy in T2DM. Frequent monitoring and early intervention should be recommended.

The contribution of portal drained viscera to circadian homocysteinemia in pigs

Homocysteine (Hcy) is an intermediary S-containing amino acid produced by the methylation process within all cells. It is known as a powerful pro-oxidant with multiple deleterious effects on immune and physiological functions. Blood plasma total Hcy (tHcy), the most common indicator of Hcy status, can be reduced by dietary folates or vitamin B(12) in pigs as in most mammalians. In humans, homocysteinemia is routinely assessed after an overnight fast (≥ 12 h) although information is not available on circadian tHcy changes. Using a subgroup of pigs from a study on portal appearance of vitamin B(12) after a single meal containing 0, 25 or 250 μg of cyanocobalamin, the present study aimed to report the circadian profile of postmeal blood plasma tHcy and estimate the contribution of portal drained viscera (PDV) to the systemic tHcy. Four pigs (39.7 ± 1.1 kg BW) were surgically equipped at 101.0 ± 8.2 d of age with catheters in the portal vein and carotid artery; an ultrasonic flow probe was also fitted around the portal vein for blood flow recordings. Blood samples were collected simultaneously from the 2 catheters once before meal and at least every hour during 24 h after ingestion of 1.2 kg of a vitamin-free semipurified diet. Arterial tHcy changed considerably during the 24-h postmeal period (P < 0.001; SE = 0.8). In fact, from 12.3 μM 10 min before meal, tHcy gradually reached a maximum of 23.4 μM 13 h postmeal and returned to 15.5 μM 23 h after the meal. Net fluxes of tHcy across PDV were not influenced by levels of dietary vitamin B(12), postprandial time, or their interaction (P > 0.25); average net flux did not differ from zero (P > 0.08). These results suggest that systemic Hcy following a meal originates from metabolic pools other than PDV. It appears that an overnight fast of 12 h will reflect the peak rather than the basal value for tHcy. The duration of the fasting period is therefore a critical factor for a reliable interpretation of tHcy homeostasis in pigs. Such information may be also relevant for human health and nutrition because pig is recognized as a reliable model for Hcy metabolism.

Tissue-specific alterations of methyl group metabolism with DNA hypermethylation in the Zucker (type 2) diabetic fatty rat

BACKGROUND

Altered methyl group and homocysteine metabolism were tissue-specific, persistent, and preceded hepatic DNA hypomethylation in type 1 diabetic rats. Similar metabolic perturbations have been shown in the Zucker (type 2) diabetic fatty (ZDF) rat in the pre-diabetic and early diabetic stages, but tissue specificity and potential impact on epigenetic marks are unknown, particularly during pathogenesis.

METHODS

ZDF (fa/fa) and lean (+/?) control rats were killed at 12 and 21 weeks of age, representing early and advanced diabetic conditions. Blood and tissues were analysed with respect to methyl group and homocysteine metabolism, including DNA methylation.

RESULTS

At 12 weeks, hepatic glycine N-methyltransferase (GNMT), methionine synthase, and cystathionine β-synthase (CBS) activity and/or abundance were increased in ZDF rats. At 21 weeks, GNMT activity was increased in liver and kidney; however, only hepatic CBS protein abundance (12 weeks) and betaine-homocysteine S-methyltransferase mRNA expression (21 weeks) were significantly elevated (78 and 100%, respectively). Hepatic phosphatidylethanolamine N-methyltransferase expression was also elevated in the ZDF rat. Homocysteine concentrations were decreased in plasma and kidney, but not in liver, at 12 and 21 weeks. In contrast to hepatic DNA hypomethylation in the type 1 diabetic rat, genomic DNA was hypermethylated at 12 and 21 weeks in the liver of ZDF rats, concomitant with an increase in DNA methyltransferase 1 expression at 21 weeks.

CONCLUSIONS

The pathogenesis of type 2 diabetes in the ZDF rat was associated with tissue and disease stage-specific aberrations of methyl group and homocysteine metabolism, with persistent hepatic global DNA hypermethylation.

Homocysteine and Hypertension in Diabetes: Does PPARgamma Have a Regulatory Role?

Dysfunction of macro- and microvessels is a major cause of morbidity and mortality in patients with cardio-renovascular diseases such as atherosclerosis, hypertension, and diabetes. Renal failure and impairment of renal function due to vasoconstriction of the glomerular arteriole in diabetic nephropathy leads to renal volume retention and increase in plasma homocysteine level. Homocysteine, which is a nonprotein amino acid, at elevated levels is an independent cardio-renovascular risk factor. Homocysteine induces oxidative injury of vascular endothelial cells, involved in matrix remodeling through modulation of the matrix metalloproteinase (MMP)/tissue inhibitor of metalloproteinase (TIMP) axis, and increased formation and accumulation of extracellular matrix protein, such as collagen. In heart this leads to increased endothelial-myocyte uncoupling resulting in diastolic dysfunction and hypertension. In the kidney, increased matrix accumulation in the glomerulus causes glomerulosclerosis resulting in hypofiltration, increased renal volume retention, and hypertension. PPARγ agonist reduces tissue homocysteine levels and is reported to ameliorate homocysteine-induced deleterious vascular effects in diabetes. This review, in light of current information, focuses on the beneficial effects of PPARγ agonist in homocysteine-associated hypertension and vascular remodeling in diabetes.

Tissue methionine cycle activity and homocysteine metabolism in female rats: impact of dietary methionine and folate plus choline

Impaired transfer of methyl groups via the methionine cycle leads to plasma hyperhomocysteinemia. The tissue sources of plasma homocysteine in vivo have not been quantified nor whether hyperhomocysteinemia is due to increased entry or decreased removal. These issues were addressed in female rats offered diets with either adequate or excess methionine (additional methyl groups) with or without folate and choline (impaired methyl group transfer) for 5 wk. Whole body and tissue metabolism was measured based on isotopomer analysis following infusion with either [1-(13)C,methyl-(2)H3]methionine or [U-(13)C]methionine plus [1-(13)C]homocysteine. Although the fraction of intracellular methionine derived from methylation of homocysteine was highest in liver (0.18-0.21), most was retained. In contrast, the pancreas exported to plasma more of methionine synthesized de novo. The pancreas also exported homocysteine to plasma, and this matched the contribution from liver. Synthesis of methionine from homocysteine was reduced in most tissues with excess methionine supply and was also lowered in liver (P<0.01) with diets devoid of folate and choline. Plasma homocysteine concentration (P<0.001) and flux (P=0.001) increased with folate plus choline deficiency, although the latter still represented <12% of estimated tissue production. Hyperhomocysteinemia also increased (P<0.01) the inflow of homocysteine into most tissues, including heart. These findings indicate that a full understanding of hyperhomocysteinemia needs to include metabolism in a variety of organs, rather than an exclusive focus on the liver. Furthermore, the high influx of homocysteine into cardiac tissue may relate to the known association between homocysteinemia and hypertension.

Hyperhomocysteinemia: association with renal transsulfuration and redox signaling in rats

Despite substantial evidence indicating the association of hyperhomocysteinemia (hHcys) and end-stage renal disease (ESRD), the pathogenic role of increased plasma homocysteine (Hcys) levels in the progression of ESRD remains unclear. This review will briefly summarize recent findings regarding the role of hHcys in the development of glomerulosclerosis, the association of hHcys with reduced renal transsulfuration and Hcys-induced changes of redox signaling in the development of glomerulosclerosis in rat kidneys. Based on these results, it is concluded that hHcys is implicated in glomerular sclerosis in hypertension, elevated plasma Hcys in Dahl salt-sensitive (SS) hypertensive rats is due to downregulation of cystathionine beta-synthase (CBS) expression and consequent abnormality of transsulfuration in the kidney compared with normotensive rats. Hcys-induced superoxide (O(2)(*-)) production by activation of NADPH oxidase as a triggering mechanism contributes to the effects of Hcys on the homeostasis of extracellular matrix and consequent sclerosis in the glomeruli, and NADPH oxidase activation by Hcys is associated with enhanced Rac GTPase activity.

Homocysteine metabolism in diabetes

An increase in the plasma level of Hcy (homocysteine), an intermediate in the catabolism of methionine, has been identified as a risk factor for many diseases including CVD (cardiovascular disease). CVD is the major cause of death in patients with diabetes mellitus. Therefore the study of Hcy metabolism in diabetes mellitus has been a major focus of current research. Studies conducted in our laboratory were able to show that in both Type 1 and Type 2 diabetes with no renal complications, the plasma Hcy levels were lower than in controls. In Type 1 diabetes, increased activities of the trans-sulfuration enzymes were the major cause for the reduction in plasma Hcy. In Type 2 diabetes, BHMT (betaine:homocysteine methyltransferase) was also observed to play a major role in the increased catabolism of Hcy in addition to the trans-sulfuration enzymes. We were also able to demonstrate the direct effect of insulin and the counter-regulatory hormones on the regulation of cystathionine beta-synthase and BHMT, which accounts for the changes in the activities of these two enzymes seen in diabetes mellitus.

Testosterone regulation of renal cystathionine β-synthase: implications for sex-dependent differences in plasma homocysteine levels

Elevated plasma total homocysteine (tHcy) is an independent risk factor for ischemic heart disease and stroke. Epidemiological studies reveal that men have higher tHcy levels than women, but the mechanism underlying this sex-dependent difference is unknown. One route for intracellular disposal of homocysteine is catalyzed by cystathionine β-synthase (CBS). Renal function is known to be an important determinant of tHcy, and, in this study, we demonstrate that renal CBS expression and activity in mice diminished approximately twofold after castration, whereas ovariectomization was without effect. The higher renal CBS activity in males (22.7 ± 3.1 mmol cystathionine·h−1·kg kidney−1) vs. females (8.4 ± 3.4 mmol cystathionine·h−1·kg kidney−1, P ≤ 10−6) in C57Bl/6J mice was associated with lower plasma tHcy levels in males vs. females, and this difference was exacerbated in Cbs+/− mice (7.7 ± 1.9 μmol/l in males vs. 13.8 ± 6.4 μmol/l in females, P = 0.005). Surprisingly, mammals exhibit a diversity of regulatory patterns for kidney CBS, with females exhibiting lower CBS activity in mice, higher in rats and humans, and being indistinguishable from males in rabbit, hamster, and guinea pig. Our data suggest that testosterone-dependent regulation of human CBS in kidney may contribute to sex-dependent differences in homocysteine transsulfuration.

The influence of estimated creatinine clearance on plasma homocysteine in hypertensive patients with normal serum creatinine

OBJECTIVES

To examine the relation of estimated creatinine clearance (eCrCl) and plasma total homocysteine (tHcy) in hypertensive patients with a normal serum creatinine level.

DESIGN AND METHODS

A total of 137 hypertensive patients (mean age 66.6 years, 69 men) with serum creatinine level </=1.5 mg/dL gave 10-h fasting blood samples for measurement of tHcy, serum folic acid, and serum vitamin B(12).

RESULTS

95 patients fell into a chronic renal insufficiency (CRI) group (eCrCl< or =60 mL/min/1.73 m(2)) and 42 into a normal renal function (NRF) group (eCrCl>60 mL/min/1.73 m(2)). The CRI group was older (p<0.001), had higher tHcy (p<0.001), higher serum urea nitrogen (p<0.001), higher serum creatinine (p<0.001), lower eCrCl (p<0.001), and lower diastolic blood pressure (p=0.001). In univariate analysis, eCrCl had the strongest correlation with tHcy (r=-0.453, p<0.001). Significant correlations, ranging in decreasing order from r=-0.418, p<0.001 to r=-0.170, p=0.047, were also noted between tHcy and twelve other variables. In multivariate analysis, only eCrCl (p<0.001), usage of fibrate (p<0.001), serum level of vitamin B(12) (p=0.002), serum level of folic acid (p=0.009), and smoking (p=0.027) were independent predictors of tHcy.

CONCLUSION

eCrCl is a strong independent predictor of tHcy in hypertensive patients with normal serum creatinine.

Hyperhomocysteinemia associated with decreased renal transsulfuration activity in Dahl S rats

Elevated plasma homocysteine (Hcys) has been reported to participate in the development of arterial and glomerular sclerosis in Dahl salt-sensitive hypertensive (SS) rats. The mechanism resulting in hyperhomocysteinemia in these animals remains unknown. Disposal of Hcys in the kidneys plays an important role in regulating the plasma Hcys level. We, therefore, examined the activities and expressions of the enzymes involved in the metabolism of Hcys in the kidneys of SS rats, compared with that of Brown Norway rats and SSBN13 rats, a consomic subcolony of SS rats that carries a substituted chromosome 13 from Brown Norway rats. High-performance liquid chromatography analysis demonstrated that plasma Hcys levels were significantly higher in SS rats. The conversion of S-adenosylhomocysteine into Hcys via S-adenosylhomocysteine hydrolase by renal tissue was not different among these 3 rat strains. However, the metabolic rate of Hcys into cysteine was markedly reduced in the SS rat kidneys. The mRNA and protein levels of cystathionine beta-synthase (CBS), one of the key enzymes in the transsulfuration pathway in the kidneys, were significantly lower in SS rats. In microdissected nephron segments, CBS mRNA was shown to be mainly present in renal proximal tubules (PTs). The mRNA levels of CBS in the PTs were also significantly decreased in SS rats, accompanied by a reduced CBS activity in PTs. We conclude that hyperhomocysteinemia is associated with a decreased activity and expression of CBS in renal PTs because of the defect of chromosome 13 in SS rats.

Effect of acute hyperhomocysteinemia on methylation potential of erythrocytes and on DNA methylation of lymphocytes in healthy male volunteers

Homocysteine is a precursor of S-adenosylmethionine (AdoMet) and a metabolite of S-adenosylhomocysteine (AdoHcy). The ratio of AdoMet to AdoHcy, defined as the methylation potential (MP), indicates the flow of methyl groups within the cells. Chronic elevations of total homocysteine (tHcy) in plasma correlate with increased AdoHcy concentrations, decreased MP, and impaired DNA methylation. However, the influence of acute hyperhomocysteinemia on MP is unknown. We induced acute hyperhomocysteinemia in 14 healthy volunteers by oral administration of l-homocysteine (65.1 μmol/kg body wt) in an open, randomized, placebo-controlled two-period crossover study. The kinetics of tHcy in blood and urine, MP in blood, and global DNA methylation in lymphocytes were studied systematically during 48 h. Plasma tHcy concentrations reached a peak at 34 ± 11 min after an oral load with l-homocysteine and decreased with a half-life of 257 ± 41 min (means ± SD). Only 2.3% of the homocysteine dose were recovered in urine. AdoHcy concentrations and MP in whole blood and erythrocytes were not affected by the oral homocysteine load. Furthermore, global DNA methylation in lymphocytes did not change under these conditions. We found no difference between the genotypes of 5,10-methylenetetrahydrofolate reductase in response to the homocysteine load. However, AdoMet content in erythrocytes was significantly higher in the C677T carriers (CT; n = 7) compared with the CC genotype ( n = 7). Although chronic elevation of tHcy has been shown to affect MP and DNA methylation, acute elevation of plasma tHcy above 20 μmol/l for 8 h is not sufficient to change MP and to induce DNA hypomethylation in lymphocytes.

Handling of the HomocysteineS-Conjugate of Methylmercury by Renal Epithelial Cells: Role of Organic Anion Transporter 1 and Amino Acid Transporters

Recently, the activity of the organic anion transporter 1 (OAT1) protein has been implicated in the basolateral uptake of inorganic mercuric species in renal proximal tubular cells. Unfortunately, very little is known about the role of OAT1 in the renal epithelial transport of organic forms of mercury, such as methylmercury (CH(3)Hg(+)). Homocysteine (Hcy) S-conjugates of methylmercury [(S)-(3-amino-3-carboxypropylthio)(methyl)mercury (CH(3)Hg-Hcy)] have been identified recently as being potentially important biologically relevant forms of mercury. Thus, the present study was designed to characterize the transport of CH(3)Hg-Hcy in Madin-Darby canine kidney (MDCK) cells (which are derived from the distal nephron) that were transfected stably with the human isoform of OAT1 (hOAT1). Data on saturation kinetics, time dependence, substrate specificity, and temperature dependence demonstrated that CH(3)Hg-Hcy is a transportable substrate of hOAT1. However, substrate-specificity data from the control MDCK cells also showed that CH(3)Hg-Hcy is a substrate of one or more transporter(s) that is/are not hOAT1. Additional findings indicated that at least one amino acid transport system was probably responsible for this transport. It is noteworthy that the activity of amino acid transporters accounted for the greatest level of uptake of CH(3)Hg-Hcy in the hOAT1-expressing cells. Furthermore, rates of survival of the hOAT1-transfected MDCK cells were significantly lower than those of corresponding control MDCK cells when they were exposed to cytotoxic concentrations of CH(3)Hg-Hcy. Collectively, the present data indicate that CH(3)Hg-Hcy is a transportable substrate of OAT1 and amino acid transporters and, thus, is probably a transportable mercuric species taken up in vivo by proximal tubular epithelial cells.

Creatine supplementation does not decrease total plasma homocysteine in chronic hemodialysis patients

BACKGROUND

Hyperhomocysteinemia is present in the majority of chronic hemodialysis patients. Treatment with folic acid, vitamin B12, and vitamin B6 cannot fully normalize plasma homocysteine concentrations (tHcy). Previously we have demonstrated the tHcy-lowering effect of creatine supplementation in an animal model of uremia (Kidney Int 64:1331-1337, 2003). The present study investigates the effects of creatine supplementation on tHcy in a vitamin-repleted chronic hemodialysis population.

METHODS

Forty-five hemodialysis patients receiving folic acid and vitamin B6 and B12 were included. Patients were treated with creatine (2 g/day) or placebo during 2 treatment periods of 4 weeks, separated by a washout of 4 weeks. Plasma tHcy, creatine, Kt/V(urea), folic acid, vitamin B12, and routine biochemistry were determined, as well as the prognostic inflammatory and nutritional index.

RESULTS

All patients had elevated tHcy concentrations (21.2 +/- 5.6 micromol/L). Creatine treatment resulted in increased plasma and red blood cell creatine levels, documenting uptake of creatine. Creatine did not affect tHcy concentrations. There was no relationship between plasma creatine concentrations and tHcy concentrations. No changes in body weight, routine biochemistry, nutritional status, folic acid, or vitamin B12 were observed during the study.

CONCLUSION

Creatine supplementation at a rate of 2 g/day does not further decrease tHcy concentrations in chronic dialysis patients already treated with high dose folic acid, vitamin B6, and B12 supplementation.

Homocysteine, system b0,+ and the renal epithelial transport and toxicity of inorganic mercury

Proximal tubular epithelial cells are major sites of homocysteine (Hcy) metabolism and are the primary sites for the accumulation and intoxication of inorganic mercury (Hg(2+)). Previous in vivo data from our laboratory have demonstrated that mercuric conjugates of Hcy are transported into these cells by unknown mechanisms. Recently, we established that the mercuric conjugate of cysteine [2-amino-3-(2-amino-2-carboxy-ethylsulfanylmercuricsulfanyl)propionic acid; Cys-S-Hg-S-Cys], is transported by the luminal, amino acid transporter, system b(0,+). As Cys-S-Hg-S-Cys and the mercuric conjugate of Hcy (2-amino-4-(3-amino-3-carboxy-propylsulfanylmercuricsulfanyl)butyric acid; Hcy-S-Hg-S-Hcy) are similar structurally, we hypothesized that Hcy-S-Hg-S-Hcy is a substrate for system b(0,+). To test this hypothesis, we analyzed the saturation kinetics, time dependence, temperature dependence, and substrate specificity of Hcy-S-Hg-S-Hcy transport in Madin-Darby canine kidney (MDCK) cells stably transfected with system b(0,+). MDCK cells are good models in which to study this transport because they do not express system b(0,+). Uptake of Hg(2+) was twofold greater in the transfectants than in wild-type cells. Moreover, the transfectants were more susceptible to the toxic effects of Hcy-S-Hg-S-Hcy than wild-type cells. Accordingly, our data indicate that Hcy-S-Hg-S-Hcy is transported by system b(0,+) and that this transporter likely plays a role in the nephropathy induced after exposure to Hg(2+). These data are the first to implicate a specific, luminal membrane transporter in the uptake and toxicity of mercuric conjugates of Hcy in any epithelial cell.

Homocysteine and cardiovascular disease in renal disease. Review Article

Elevated homocysteine (hyperhomocysteinaemia) in renal patients is a major concern for physicians. Although cause and effect between homocysteine and cardiovascular disease (CVD) has not been established in either the general population or renal patients, there is much evidence that this relationship does exist. Purported mechanisms that may explain this effect include increases in endothelial injury, smooth muscle cell proliferation, low-density lipoprotein oxidation and changes in haemostatic balance. Renal patients have a much greater incidence of hyperhomocysteinaemia and this may be explained by decreases in either the renal or extrarenal metabolism of the compound. We conclude that data from long-term placebo-controlled trials are urgently required to determine whether hyperhomocysteinaemia in renal patients is a cause of CVD events and requires therapeutic targeting.

Effect of dialyser membrane pore size on plasma homocysteine levels in haemodialysis patients

BACKGROUND

Hyperhomocysteinaemia is a putative risk factor for atherothrombotic cardiovascular disease in the haemodialysis population. High-dose vitamin B therapy does not entirely normalize elevated plasma total homocysteine (tHcy) levels in haemodialysis patients. Alternative therapies to reduce tHcy further are therefore required. Modifications of the dialysis regimen may result in a better removal of Hcy. We examined the effect of dialyser membrane pore size on tHcy levels in vitamin-replete chronic haemodialysis patients.

METHODS

Forty-five haemodialysis patients were dialysed during 4 weeks with a low-flux, a high-flux and a super-flux membrane, in random order. Pre-dialysis tHcy was determined at baseline and every 4 weeks. In 18 patients, plasma tHcy before and after dialysis and dialysate tHcy concentrations were measured.

RESULTS

Pre-dialysis tHcy decreased significantly during 4 weeks super-flux dialysis (-14.6 +/- 2.8%), whereas it remained stable during high-flux (+0.5 +/- 2.4%) and low-flux dialysis (+1.7 +/- 3.2%). The homocysteine reduction ratio was not different for the three membranes: 0.39 +/- 0.03 for the super-flux, 0.47 +/- 0.02 for the high-flux and 0.39 +/- 0.02 for the low-flux dialyser. The amount of Hcy recovered in the dialysate during a single dialysis session was also similar: 117.5 +/- 3.6 micro mol during super-flux, 95.3 +/- 11.5 micro mol during high-flux and 116.5 +/- 11.6 micro mol during low-flux dialysis.

CONCLUSION

Super-flux dialysis significantly lowers tHcy in chronic haemodialysis patients. Improved removal of middle-molecule uraemic toxins with inhibitory effects on Hcy-metabolizing enzymes, rather than better dialytic clearance of Hcy itself, may explain the beneficial effect of the super-flux membrane.

Potential strategies to normalize the levels of homocysteine in chronic renal failure patients

Recently published evidence suggests that, either than folate therapy, the enhancement of homocysteine remethylation in tissues by correcting the multiple abnormalities of the remethylation pathway in chronic renal failure that extend beyond folate-related disturbances, or else the improved removal of uremic toxins and/or Hcy through intensified dialysis procedures may represent two strategies to normalize total homocysteine in uremic patients.

Interorgan exchange of aminothiols in humans

In the present study, we used organ balance across the kidney, splanchnic organs, and lower limb in subjects undergoing diagnostic central venous catheterizations to gain insight into the renal and extrarenal exchange of aminothiols in humans. Although Hcy was released only in low amounts from leg tissues, Cys-Gly (a peptide derived from GSH hydrolysis) was released by both the leg and splanchnic organs, whereas Cys was released by the kidney and taken up by splanchnic organs. The kidney removed approximately 90% of the Cys-Gly released into the circulation. Removal of Cys-Gly by the kidney depended on Cys-Gly arterial levels and showed a high fractional extraction ( approximately 26%), with clearance rates slightly higher than the glomerular filtration rate (GFR). Although the average kidney removal of Hcy was not statistically significant, the fractional extraction of Hcy across the kidney varied directly with renal plasma flow. Our data show that thiol metabolism in humans is a compartmentalized interorgan process involving fluxes of individual aminothiols that are parallel and of opposite sign among peripheral tissues, splanchnic organs, and kidney. Cys-Gly is released by peripheral tissue and splanchnic organs from GSH hydrolysis and is taken up by the kidney by GFR; the kidney returns Cys to the circulation to preserve substrate availability for GSH synthesis. On the other hand, Hcy is released by peripheral tissues in low amounts, and its removal by the kidney seems to depend on blood supply. These findings may help explain several alterations in aminothiol metabolism observed in patients with chronic diseases.

Uremia-related metabolic cardiac risk factors in chronic kidney disease

Growing evidence has been gathered over the last 15 years regarding the role of nontraditional or uremia-related risk factors in the pathogenesis of atherosclerosis in subjects with renal failure. Among those factors, dyslipidemia, inflammation, hyperhomocysteinemia, and oxidant stress have been extensively studied. However, the clinical significance of many of these factors remains controversial in light of reported studies. In this article, the existing evidence regarding the role of uremia-related risk factors in the pathogenesis of atherosclerosis is reviewed, with special emphasis on prevalence, cardiac risk, and management in patients with chronic kidney disease (CKD). Consensus treatment recommendations are provided for risk factors for which there is evidence to support preventive or therapeutic interventions.

Selenium deficiency in Fisher-344 rats decreases plasma and tissue homocysteine concentrations and alters plasma homocysteine and cysteine redox status

The purpose of the present study was to determine the effect of graded amounts of dietary selenium on plasma and tissue parameters of methionine metabolism including homocysteine. Male weanling Fisher-344 rats (n = 7-8/group) were fed a selenium-deficient, torula yeast-based diet, supplemented with 0 (selenium deficient), 0.02, 0.05 or 0.1 microg (adequate) selenium (as selenite)/g diet. After 61 d, plasma total homocysteine and cysteine were decreased (P < 0.0001) and glutathione increased (P < 0.0001) by selenium deficiency. The concentrations of homocysteine in kidney and heart were decreased (P = 0.02) by selenium deficiency. The activities of liver betaine homocysteine methyltransferase, methionine synthase, S-adenosylmethionine synthase, cystathionine synthase and cystathionase were determined; selenium deficiency affected only betaine homocysteine methyltransferase, which was decreased (P < 0.0001). The ratios of plasma free reduced homocysteine (or cysteine) to free oxidized homocysteine (or cysteine) or to total homocysteine (or cysteine) were increased by selenium deficiency, suggesting that selenium status affects the normally tightly controlled redox status of these thiols. Most differences due to dietary selenium were between rats fed 0 or 0.02 microg selenium/g diet and those fed 0.05 or 0.1 microg selenium/g diet. The metabolic consequences of a marked decrease in plasma homocysteine and smaller but significant decreases in tissue homocysteine are not known.

A study on the relationship between homocysteine and diabetic nephropathy in rats

Hyperhomocysteinemia is known to be associated with many of the occlusive vascular diseases including ischemic heart disease. Elevated plasma total homocysteine (t-Hcy) is also remarkably common among patients with moderate to severe renal failure. The purpose of this study was to investigate the role of homocysteine (Hcy) in the pathogenesis of diabetic nephropathy in the rat. Additionally, any effect of aminoguanidine (AG), an inhibitor of advanced glycation end product (AGE) formation, on the onset of nephropathic symptoms and on the concentrations of Hcy was searched for. Diabetes was induced in male Wistar albino rats (6 months old) by a single injection of 50 mg x kg (-1)streptozotocin (STZ) into the penile vein. Animals with blood glucose levels higher than 350 mg x dl (-1)72 h after STZ injection were included in the study. Age-matched rats receiving a single dose of citrate buffer served as controls. One half of the control and diabetic groups received AG via drinking water (1 g l (-1)). The experimental period lasted for ten weeks. Animals were killed by cardiac venipuncture after 24 hour urine samples were collected. Serum t-Hcy was quantified using HPLC, and urinary GAGs using the spectrophotometric 1,9-dimethyl methylene blue dye method. Serum glucose, protein, creatinine and total sulfydryl (t-SH) measurements, and urinary protein determinations were carried out spectrophotometrically. In diabetic rats, serum t-Hcy levels were significantly decreased (P< 0.001), and were negatively correlated with the urinary protein concentration (r= -0.67, P< 0.05). Urinary GAG levels were also increased in diabetic rats (P< 0.001). AG neither affected the t-Hcy levels, nor ameliorated the nephropathic symptoms. These results indicate that diabetic nephropathy is not linked to homocysteinemia in the rat.

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.

Implications of hyperhomocysteinemia in glomerular sclerosis in hypertension

Hyperhomocysteinemia (hHcys) has been recognized as a new risk factor for cardiovascular diseases independent of plasma lipid levels or other factors. However, it remains unknown whether hHcys is implicated in the target organ damages associated with hypertension. The present study first examined the possible role of hHcys in the development of glomerulosclerosis in Dahl salt-sensitive (DS) hypertensive rats. High-performance liquid chromatography showed that plasma total homocysteine (tHcys) concentration was 7.64 +/- 0.29 micromol/L in conscious DS rats on a low salt (0.4% NaCl) diet, which was higher than 5.23 +/- 0.25 micromol/L in Dahl salt-resistant normotensive rats. When these rats were exposed to a high salt (4% NaCl) diet, plasma tHcys markedly increased in DS rats (14.7 +/-1.31 micromol/L) but not in Dahl salt-resistant rats (5.34 +/- 0.54 micromol/L). An iron chelater, desferrioxamine (0.3 mg/kg IV per day), completely normalized high salt--induced elevations of plasma tHcys and significantly attenuated the sclerotic changes in the glomeruli in DS rats. To further determine whether hHcys has an independent effect in the development of glomerulosclerosis, Sprague-Dawley rats were fed drinking water containing methionine (1 g/kg per day) for 6 weeks to produce hHcys. In these rats, plasma tHcys increased to 12.5 +/- 1.9 micromol/L (versus 6.1 +/- 2.6 micromol/L in control rats), and the aorta exhibited typical sclerotic changes, but arterial pressure was not altered. Urinary protein excretion increased to 52 +/- 2 mg/24 hours (versus 17 +/- 2 mg/24 hours in control rats), and the glomerular mesangium was expanded with glomerular hypercellularity, capillary collapse, and fibrous deposition in the rats with hHcys. These results suggest that elevated plasma homocysteine may be an important pathogenic factor for glomerular damage in hypertension independent of arterial pressure.

Intraperitoneal infusion of homocysteine increases intimal hyperplasia in balloon-injured rat carotid arteries

Hyperhomocysteinemia is a significant risk factor in atherosclerosis and thrombosis. However, its role in the development of intimal hyperplasia after arterial reconstructive procedures remains uncertain. We therefore studied the effect of homocysteine on intimal hyperplasia in a rat model of carotid artery balloon injury. Twenty-four Sprague-Dawley rats were divided into three groups: control (saline infusion), and low dose (0.14 mg/day) and high dose (0.71 mg/day) homocysteine delivered continuously via osmotic pumps implanted intraperitoneally. All animals underwent left common carotid artery balloon denudation with sacrifice after 14 days. Plasma homocysteine levels, intimal hyperplasia, and cell proliferation of rat carotid arteries were determined. In vitro rat smooth muscle cell (SMC) proliferation with homocysteine treatment was also performed. Plasma homocysteine levels at sacrifice were 1.80+/-0.35, 2.65+/-0.05 and 3.50+/-0.22 microM in three groups, respectively. Intimal hyperplasia developed in all balloon-injured arteries in both control and homocysteine-treated animals. The intimal area and intima/media area ratio were increased by 92% (P<0.05) and 105% (P<0.05), respectively, in the high dose-homocysteine-treated animals as compared to the control animals. Homocysteine (high dose) also significantly promoted the intimal cell proliferation (bromodeoxyuridine incorporation) by 2.2-fold as compared to controls. Furthermore, homocysteine treatment in the cell culture study showed a concentration-dependent increase of rat SMC proliferation. These data demonstrate that the continuous intraperitoneal administration of homocysteine significantly increases intimal hyperplasia and SMC proliferation after carotid artery balloon injury in the rat as well as in vitro SMC proliferation. This study suggests that, following arterial reconstructive procedures, elevated plasma homocysteine may increase the complications of clinical restenoses that are associated with intimal hyperplasia.

Intracellular S-adenosylhomocysteine concentrations predict global DNA hypomethylation in tissues of methyl-deficient cystathionine beta-synthase heterozygous mice

Because S-adenosylmethionine (SAM) and S-adenosylhomocysteine (SAH) are the substrate and product of essential methyltransferase reactions; the ratio of SAM:SAH is frequently used as an indicator of cellular methylation potential. However, it is not clear from the ratio whether substrate insufficiency, product inhibition or both are required to negatively affect cellular methylation capacity. A combined genetic and dietary approach was used to modulate intracellular concentrations of SAM and SAH. Wild-type (WT) or heterozygous cystathionine beta-synthase (CBS +/-) mice consumed a control or methyl-deficient diet for 24 wk. The independent and combined effect of genotype and diet on SAM, SAH and the SAM:SAH ratio were assessed in liver, kidney, brain and testes and were correlated with relative changes in tissue-specific global DNA methylation. The combined results from the different tissues indicated that a decrease in SAM alone was not sufficient to affect DNA methylation in this model, whereas an increase in SAH, either alone or associated with a decrease in SAM, was most consistently associated with DNA hypomethylation. A decrease in SAM:SAH ratio was predictive of reduced methylation capacity only when associated with an increase in SAH; a decrease in the SAM:SAH ratio due to SAM depletion alone was not sufficient to affect DNA methylation in this model. Plasma homocysteine levels were positively correlated with intracellular SAH levels in all tissues except kidney. These results support the possibility that plasma SAH concentrations may provide a sensitive biomarker for cellular methylation status.

Proteinuria and plasma total homocysteine levels in chronic renal disease patients with a normal range serum creatinine: critical impact of true glomerular filtration rate

Conflicting data have been reported concerning the independent association between proteinuria and plasma total homocysteine (tHcy) levels, particularly among chronic renal disease (CRD) patients with a normal range serum creatinine. Studies of this potential relationship have been limited by failure to assess true GFR, failure to assess proteinuria in a quantitative manner, or arbitrary restriction of the range of proteinuria examined. We examined the potential independent relationship between plasma tHcy levels and a wide range of quantitatively determined proteinuria (i.e., 0.000-8.340 g/day), among 109 CRD patients with a normal range serum creatinine (range; 0.8-1.5 mg/dl; median=1.2 mg/dl). Glomerular filtration rate (GFR) was directly assessed by iohexol clearance, and plasma status of folate, pyridoxal 5'-phosphate, and B12, along with serum albumin, were also determined. Linear modeling with ANCOVA revealed that proteinuria was not independently associated with tHcy levels (partial R=0.127; P=0.201), after adjustment for potential confounding by GFR (partial R=0.408; P<0.001), age, sex, plasma B-vitamin status, and serum albumin. Moreover, descending across quartiles (Q) [from Q4 to Q1] of GFR, ANCOVA-adjusted (i.e., for age, sex, and folate status) geometric mean tHcy levels (micromol/l) were significantly increased: tHcy Q4 GFR=9.6; tHcy Q3 GFR=10.5; tHcy Q2 GFR=11.9; tHcy Q4 GFR=14.5; P<0.001 for overall Q difference. We conclude that across a broad spectrum of quantitatively determined proteinuria, after adjustment for true GFR, in particular, there is no independent relationship between proteinuria and tHcy levels among CRD patients with a normal range serum creatinine.

The kidney and homocysteine metabolism

Homocysteine (Hcy) is an intermediate of methionine metabolism that, at elevated levels, is an independent risk factor for vascular disease and atherothrombosis. Patients with renal disease, who exhibit unusually high rates of cardiovascular morbidity and death, tend to be hyperhomocysteinemic, particularly as renal function declines. This observation and the inverse relationship between Hcy levels and GFR implicate the kidney as an important participant in Hcy handling. The normal kidney plays a major role in plasma amino acid clearance and metabolism. The existence in the kidney of specific Hcy uptake mechanisms and Hcy-metabolizing enzymes suggests that this role extends to Hcy. Dietary protein intake may affect renal Hcy handling and should be considered when measuring Hcy plasma flux and renal clearance. The underlying cause of hyperhomocysteinemia in renal disease is not entirely understood but seems to involve reduced clearance of plasma Hcy. This reduction may be attributable to defective renal clearance and/or extrarenal clearance and metabolism, the latter possibly resulting from retained uremic inhibitory substances. Although the currently available evidence is not conclusive, it seems more likely that a reduction in renal Hcy clearance and metabolism is the cause of the hyperhomocysteinemic state. Efforts to resolve this important issue will advance the search for effective Hcy-lowering therapies in patients with renal disease.

Treatment of hyperhomocysteinemia in hemodialysis patients and renal transplant recipients

BACKGROUND

Hyperhomocysteinemia, a putative atherothrombotic risk factor, is observed in at least 85% of patients undergoing maintenance hemodialysis (HD), as well as 65 to 70% of renal transplant recipients (RTRs). The hyperhomocysteinemia regularly found in HD patients is largely refractory to combined oral vitamin B supplementation featuring supraphysiological doses of folic acid (FA). Relative to their HD counterparts, the hyperhomocysteinemia of RTRs appears to be considerably less refractory to treatment with high-dose FA-based vitamin B supplementation regimens, although controlled comparison data are lacking. We evaluated whether improved total homocysteine (tHcy)-lowering efficacy could be achieved in chronic HD patients with a high-dose L-5-methyltetrahydrofolate (MTHF)-based regimen, as suggested by recent uncontrolled findings, and compared the relative responsiveness of RTRs and HD patients with equivalent baseline tHcy levels, to 12 weeks of tHcy lowering with combined folate-based vitamin B treatment.

METHODS

First, we blocked randomized 50 chronic, stable HD patients based on their screening predialysis tHcy levels, sex, and dialysis center into two groups of 25 subjects treated for 12 weeks with oral FA at 15 mg/day, or an equimolar amount (17 mg/day) of oral MTHF. All 50 subjects also received 50 mg/day of oral vitamin B6 and 1.0 mg/day of oral vitamin B12.

RESULTS

The mean percentage (%) reductions (+/- 95% confidence intervals) in predialysis tHcy were not significantly different [MTHF 17.0% (12.0 to 22.0%), FA 14.8% (9.6 to 20.1%), P = 0.444 by matched analysis of covariance adjusted for pretreatment tHcy]. Final on-treatment values (mean with 95% confidence interval) were: MTHF, 20.0 micromol/L (18.8 to 21.2); and FA, 19.5 micromol/L (18.3 to 20.7). Moreover, neither treatment resulted in "normalization" of tHcy levels (that is, final on-treatment values <12 micromol/L) among a significantly different or clinically meaningful number of patients [MTHF, 2 out of 25 (8%); FA, 0 out of 25 (0%); Fisher's exact test of between groups difference, P = 0.490]. Second, we compared the relative responsiveness of (N = 10) RTRs and (N = 39) HD patients with equivalent baseline tHcy levels (RTR range of 14.2 to 23.6 micromol/L, and HD range of 14.4 to 24.9 micromol/L) to 12 weeks of tHcy-lowering treatment. The RTRs received 2.4 mg/day of FA, 50.0 mg/day of vitamin B6, and 0.4 mg/day of vitamin B12, while the HD patients received 15 mg/day of FA or an equimolar amount (17 mg/day) of the reduced folate, MTHF, in addition to 50.0 mg/day of vitamin B6 and 1.0 mg/day of vitamin B12. The mean percentage (%) reductions (+/- 95% confidence interval) in tHcy were as follows: RTR 28.1% (16.2 to 40.0%); HD 12.1% (6.6 to 17.7%, P = 0.027 for comparison of between groups differences by analysis of covariance adjusted for baseline tHcy levels). Moreover, 5 out of 10 (50.0%) of the RTR versus only 2 out of 39 (5.1%) of the HD patients had final on-treatment tHcy levels <12 micromol/L (P = 0.002 for comparison of between groups differences by Fisher's exact test).

CONCLUSIONS

First, in comparison to high-dose FA, high-dose oral MTHF-based supplementation does not afford improved tHcy-lowering efficacy among HD patients. The preponderance of HD patients (that is,> 90%) exhibits mild hyperhomocysteinemia refractory to treatment with either regimen. This treatment refractoriness is not related to defects in folate absorption or circulating plasma and tissue distribution. Second, relative to RTR with comparable baseline tHcy levels, the mild hyperhomocysteinemia of maintenance HD patients is much more refractory to tHcy-lowering vitamin B treatment regimens featuring supraphysiological amounts of FA or the reduced folate MTHF. Accordingly, RTRs are a preferable target population for controlled clinical trials testing the hypothesis that tHcy-lowering vitamin B intervention may reduce arteriosclerotic cardiovascular disease event rates in patients with chronic renal disease.

Rapid communication: L-folinic acid versus folic acid for the treatment of hyperhomocysteinemia in hemodialysis patients

BACKGROUND

The hyperhomocysteinemia found in most hemodialysis patients is refractory to combined oral B-vitamin supplementation featuring supraphysiological doses of folic acid (FA). We evaluated whether a high-dose L-folinic acid-based regimen provided improved total homocysteine (tHcy)-lowering efficacy in chronic hemodialysis patients, as suggested by a recent uncontrolled report.

METHODS

We block-randomized 48 chronic, stable hemodialysis patients based on their screening predialysis tHcy levels, sex, and dialysis center into two groups of 24 subjects treated for 12 weeks with oral FA at 15 mg/day or an equimolar amount (20 mg/day) of oral L-folinic acid (FNA) [L-5-formyltetrahydrofolate]. All 48 subjects also received 50 mg/day of oral vitamin B6 and 1.0 mg/day of oral vitamin B12.

RESULTS

The mean percentage (%) reductions (with 95% CIs) in predialysis tHcy were not significantly different [FNA = 22.1% (11.8 to 31.4%), FA = 20.7% (11.7 to 30.5%), P = 0.950 by paired t test]. Final on-treatment values (mean with 95% CI) were as follows: FNA, 15.9 micromol/L (14.0 to 18.0); FA, 16.9 micromol/L (14.8 to 18.8). Moreover, in those subjects with baseline tHcy levels >/=14 micromol/L, neither treatment resulted in "normalization" of tHcy levels (that is, final on-treatment values <12 micromol/L) among a significantly different or clinically meaningful number of patients [FNA = 2 out of 22 (9.1%); FA = 2 out of 24 (8.3%); Fisher's exact test of between groups difference, P = 1.000].

CONCLUSIONS

Relative to high-dose FA, high-dose oral L-folinic acid-based supplementation does not afford improved tHcy-lowering efficacy in hemodialysis patients. The preponderance of hemodialysis patients (that is,> 90%) exhibits mild hyperhomocysteinemia refractory to treatment with either regimen.

Hyperhomocyst(e)inemia in chronic stable renal transplant patients

PURPOSE

Hyperhomocyst(e)inaemia is an important risk factor for atherosclerosis, which is currently a major cause of death in renal transplant patients. The aim of this study was to assess the influence of immunosuppressive therapy on homocyst(e)inemia in renal transplant recipients.

METHODS

Total serum homocysteine (by high performance liquid chromatography), creatinine, lipid profile, folic acid (by radioimmunoassay-RIA) and vitamin B12 (by RIA) concentrations were measured in 3 groups. Group I patients (n=20) were under treatment with cyclosporine, azathioprine, and prednisone; group II (n=9) were under treatment with azathioprine and prednisone; and group III (n=7) were composed of renal graft donors for groups I and II. Creatinine, estimated creatinine clearance, cyclosporine trough level, lipid profile, folic acid, and vitamin B12 concentrations and clinical characteristics of patients were assessed with the aim of ascertaining determinants of hyperhomocyst(e)inemia.

RESULTS

Patient ages were 48.8 +/- 15.1 yr (group I), 43.3 +/- 11.3 yr (group II); and 46.5 +/- 14.8 yr (group III). Mean serum homocyst(e)ine (tHcy) concentrations were 18.07 +/- 8.29 mmol/l in renal transplant recipients; 16.55 +/- 5.6 mmol/l and 21.44 +/- 12.1 mmol/l respectively for group I (with cyclosporine) and group II (without cyclosporine) (NS). In renal donors, tHcy was significantly lower (9.07 +/- 3.06 mmol/l; group I + group II vs. group III, p<0.008). There was an unadjusted correlation (p<0.10) between age (r=0.427; p<0.005) body weight (r=0.412; p<0.05), serum creatinine (r=0.427; p<0.05), estimated creatinine clearance (r=0.316; p<0.10), and tHcy in renal recipients (group I +II). Independent regressors (r2=0.46) identified in the multiple regression model were age (coefficient= 0.253; p=0.009) and serum creatinine (coefficient=8.07; p=0.045). We found no cases of hyperhomocyst(e)inemia in the control group. In contrast, 38% of renal recipients had hyperhomocyst(e)inemia: 7 cases (35%) on cyclosporine and 4 (45%) without cyclosporine, based on serum normal levels.

CONCLUSIONS

Renal transplant recipients frequently have hyperhomocyst(e)inemia. Hyperhomocyst(e)inemia in renal transplant patients is independent of the scheme of immunosuppression they are taking. The older the patients are and the higher are their serum creatinine levels, the more susceptible they are to hyperhomocyst(e)inemia following renal transplantation.

Non-traditional cardiovascular disease risk factors in end-stage renal disease: oxidate stress and hyperhomocysteinemia

Studies in experimental animals have shown that oxidative stress and hyperhomocyst(e)inemia culminate in abnormal vascular and endothelial regulation, functional nitric oxide deficiency, vascular hypertrophy, and atherosclerosis. Oxidative stress is accompanied by increased advanced glycation endproducts and oxidized low density lipoproteins. Studies of patients with end-stage renal disease provide extensive indirect, evidence of increased oxidative stress and more than ninety percent are hyperhomocyt(e)inemic. Presently, only uncontrolled or observational studies are available to assess the effects of anti-oxidant therapy for oxidative stress or folate therapy for hyperhomocyst(e)inemia in these patients. Promising developments include the reports of beneficial effects of a vitamin E coated dialyzer, and the reduction in homocyst(e)ine levels in patients with end-stage renal disease given an intravenous folate metabolite. However, there is presently no therapy available to reverse fully oxidative stress or hyperhomocyst(e)inemia. Therefore, the causative role of these nontraditional risk factors of cardiovascular disease remains untested.