Adverse effects of supplemental L-arginine in atherosclerosis: consequences of methylation stress in a complex catabolism?

Arginine recycling in endothelial cells is regulated BY HSP90 and the ubiquitin proteasome system

Despite the saturating concentrations of intracellular l-arginine, nitric oxide (NO) production in endothelial cells (EC) can be stimulated by exogenous arginine. This phenomenon, termed the "arginine paradox" led to the discovery of an arginine recycling pathway in which l-citrulline is recycled to l-arginine by utilizing two important urea cycle enzymes argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL). Prior work has shown that ASL is present in a NO synthetic complex containing hsp90 and endothelial NO synthase (eNOS). However, it is unclear whether hsp90 forms functional complexes with ASS and ASL and if it is involved regulating their activity. Thus, elucidating the role of hsp90 in the arginine recycling pathway was the goal of this study. Our data indicate that both ASS and ASL are chaperoned by hsp90. Inhibiting hsp90 activity with geldanamycin (GA), decreased the activity of both ASS and ASL and decreased cellular l-arginine levels in bovine aortic endothelial cells (BAEC). hsp90 inhibition led to a time-dependent decrease in ASS and ASL protein, despite no changes in mRNA levels. We further linked this protein loss to a proteasome dependent degradation of ASS and ASL via the E3 ubiquitin ligase, C-terminus of Hsc70-interacting protein (CHIP) and the heat shock protein, hsp70. Transient over-expression of CHIP was sufficient to stimulate ASS and ASL degradation while the over-expression of CHIP mutant proteins identified both TPR- and U-box-domain as essential for ASS and ASL degradation. This study provides a novel insight into the molecular regulation l-arginine recycling in EC and implicates the proteasome pathway as a possible therapeutic target to stimulate NO signaling.

Atherosclerosis Linked to Aberrant Amino Acid Metabolism and Immunosuppressive Amino Acid Catabolizing Enzymes

Cardiovascular disease is the leading global health concern and responsible for more deaths worldwide than any other type of disorder. Atherosclerosis is a chronic inflammatory disease in the arterial wall, which underpins several types of cardiovascular disease. It has emerged that a strong relationship exists between alterations in amino acid (AA) metabolism and the development of atherosclerosis. Recent studies have reported positive correlations between levels of branched-chain amino acids (BCAAs) such as leucine, valine, and isoleucine in plasma and the occurrence of metabolic disturbances. Elevated serum levels of BCAAs indicate a high cardiometabolic risk. Thus, BCAAs may also impact atherosclerosis prevention and offer a novel therapeutic strategy for specific individuals at risk of coronary events. The metabolism of AAs, such as L-arginine, homoarginine, and L-tryptophan, is recognized as a critical regulator of vascular homeostasis. Dietary intake of homoarginine, taurine, and glycine can improve atherosclerosis by endothelium remodeling. Available data also suggest that the regulation of AA metabolism by indoleamine 2,3-dioxygenase (IDO) and arginases 1 and 2 are mediated through various immunological signals and that immunosuppressive AA metabolizing enzymes are promising therapeutic targets against atherosclerosis. Further clinical studies and basic studies that make use of animal models are required. Here we review recent data examining links between AA metabolism and the development of atherosclerosis.

Metabolic Pathways Fueling the Endothelial Cell Drive

Endothelial cell (EC) metabolism is important for health and disease. Metabolic pathways, such as glycolysis, fatty acid oxidation, and amino acid metabolism, determine vasculature formation. These metabolic pathways have different roles in securing the production of energy and biomass and the maintenance of redox homeostasis in vascular migratory tip cells, proliferating stalk cells, and quiescent phalanx cells, respectively. Emerging evidence demonstrates that perturbation of EC metabolism results in EC dysfunction and vascular pathologies. Here, we summarize recent insights into EC metabolic pathways and their deregulation in vascular diseases. We further discuss the therapeutic implications of targeting EC metabolism in various pathologies.

Role of the eNOS Uncoupling and the Nitric Oxide Metabolic Pathway in the Pathogenesis of Autoimmune Rheumatic Diseases

Atherosclerosis and its clinical complications constitute the major healthcare problems of the world population. Due to the central role of endothelium throughout the atherosclerotic disease process, endothelial dysfunction is regarded as a common mechanism for various cardiovascular (CV) disorders. It is well established that patients with rheumatic autoimmune diseases are characterized by significantly increased prevalence of cardiovascular morbidity and mortality compared with the general population. The current European guidelines on cardiovascular disease (CVD) prevention in clinical practice recommend to use a 1,5-factor multiplier for CV risk in rheumatoid arthritis as well as in other autoimmune inflammatory diseases. However, mechanisms of accelerated atherosclerosis in these diseases, especially in the absence of traditional risk factors, still remain unclear. Oxidative stress plays the major role in the endothelial dysfunction and recently is strongly attributed to endothelial NO synthase dysfunction (eNOS uncoupling). Converted to a superoxide-producing enzyme, uncoupled eNOS not only leads to reduction of the nitric oxide (NO) generation but also potentiates the preexisting oxidative stress, which contributes significantly to atherogenesis. However, to date, there are no systemic analyses on the role of eNOS uncoupling in the excess CV mortality linked with autoimmune rheumatic diseases. The current review paper addresses this issue.

Urine metabolomics signatures in reversible cerebral vasoconstriction syndrome

Background

The pathophysiology of reversible cerebral vasoconstriction syndrome is unclear. An unbiased systems-based approach might help to illustrate the metabolite profiling and underlying pathophysiology.

Methods

Urine samples were collected from reversible cerebral vasoconstriction syndrome patients and matched controls recruited in Taipei Veterans General Hospital. 1H-Nuclear magnetic resonance was used to initially explore the metabolic profile, and liquid chromatography tandem mass spectrometry was then used to identify metabolic alterations in reversible cerebral vasoconstriction syndrome. Untargeted metabolite screening was randomly performed on 10 reversible cerebral vasoconstriction syndrome patients and 10 control subjects in the discovery phase. The selected untargeted metabolites were further validated on 47 reversible cerebral vasoconstriction syndrome patients during their ictal stage (with 40 of them having remission samples) and 47 controls in the replication phase.

Results and conclusion

Six metabolites-hippurate, citrate, 1,3,7-trimethyluric acid, ascorbic acid, D-glucurono-6,3-lactone, and D- threo-isocitric acid-with t-test derived p-value < 0.05 and VIP score >1, were identified as potential urine signatures that can well distinguish reversible cerebral vasoconstriction syndrome subjects at ictal stage from controls. Among them, citrate, hippurate, ascorbic acid, and D-glucurono-6,3-lactone were significantly lower, and 1,3,7-trimethyluric acid and D- threo-isocitric acid were higher in reversible cerebral vasoconstriction syndrome patients. Of these, four selected metabolites, citrate, D-glucurono-6,3-lactone, ascorbic acid, and 1,3,7-trimethyluric acid, returned to normal levels in remission. These metabolites are related to pathways associated with free radical scavenging, with the hub molecules being associated with endothelial dysfunction or sympathetic overactivity. Whether these metabolites and their implicated networks play a role in the pathogenesis of reversible cerebral vasoconstriction syndrome remains to be confirmed.

Central Role of Metabolism in Endothelial Cell Function and Vascular Disease

The importance of endothelial cell (EC) metabolism and its regulatory role in the angiogenic behavior of ECs during vessel formation and in the function of different EC subtypes determined by different vascular beds has been recognized only in the last few years. Even more importantly, apart from a role of nitric oxide and reactive oxygen species in EC dysfunction, deregulations of EC metabolism in disease only recently received increasing attention. Although comprehensive metabolic characterization of ECs still needs further investigation, the concept of targeting EC metabolism to treat vascular disease is emerging. In this overview, we summarize EC-specific metabolic pathways, describe the current knowledge on their deregulation in vascular diseases, and give an outlook on how vascular endothelial metabolism can serve as a target to normalize deregulated endothelium.

Arginine metabolism in vascular biology and disease

Arginine metabolism plays a major role in cardiovascular physiology and pathophysiology, largely via nitric oxide (NO)-dependent processes. It is becoming increasingly apparent, however, that arginine metabolic enzymes other than the NO synthases can also play important roles via both NO-dependent and -independent processes. There are three sources of arginine in vivo and at least five mammalian enzymes or enzyme families that utilize arginine as substrate. Changes in arginine availability or in production of the different end products of the various arginine metabolic pathways can have distinct and profound physiologic consequences. However, our knowledge regarding the complex interplay between these pathways at the level of the whole body, specific tissues, and even individual cells, is incomplete. This review will highlight recent findings in this area that may suggest additional avenues of investigation that will allow a fuller understanding of cardiovascular physiology in health and disease.

Clinical phenotype, biochemical profile, and treatment in 19 patients with arginase 1 deficiency

BACKGROUND

Arginase 1 (ARG1) deficiency is a rare urea cycle disorder (UCD). This hypothesis-generating study explored clinical phenotypes, metabolic profiles, molecular genetics, and treatment approaches in a cohort of children and adults with ARG1 deficiency to add to our understanding of the underlying pathophysiology.

METHODS

Clinical data were retrieved retrospectively from physicians using a questionnaire survey. Plasma aminoacids, guanidinoacetate (GAA), parameters indicating oxidative stress and nitric oxide (NO) synthesis as well as asymmetric dimethylarginine (ADMA) were measured at a single study site.

RESULTS

Nineteen individuals with ARG1 deficiency and 19 matched controls were included in the study. In patients, paraparesis, cognitive impairment, and seizures were significantly associated suggesting a shared underlying pathophysiology. In patients plasma GAA exceeded normal ranges and plasma ADMA was significantly elevated. Compared to controls, nitrate was significantly higher, and the nitrite:nitrate ratio significantly lower in subjects with ARG1 deficiency suggesting an advantage for NO synthesis by inducible NO synthase (iNOS) over endothelial NOS (eNOS). Logistic regression revealed no significant impact of any of the biochemical parameters (including arginine, nitrates, ADMA, GAA, oxidative stress) or protein restriction on long-term outcome.

CONCLUSION

Three main hypotheses which must be evaluated in a hypothesis driven confirmatory study are delineated from this study: 1) clinical manifestations in ARG1 deficiency are not correlated with arginine, protein intake, ADMA, nitrates or oxidative stress. 2) GAA is elevated and may be a marker or an active part of the pathophysiology of ARG1 deficiency. 3) Perturbations of NO metabolism merit future attention in ARG1 deficiency.

Redox signaling in cardiovascular health and disease

Spatiotemporal regulation of the activity of a vast array of intracellular proteins and signaling pathways by reactive oxygen species (ROS) governs normal cardiovascular function. However, data from experimental and animal studies strongly support that dysregulated redox signaling, resulting from hyperactivation of various cellular oxidases or mitochondrial dysfunction, is integral to the pathogenesis and progression of cardiovascular disease (CVD). In this review, we address how redox signaling modulates the protein function, the various sources of increased oxidative stress in CVD, and the labyrinth of redox-sensitive molecular mechanisms involved in the development of atherosclerosis, hypertension, cardiac hypertrophy and heart failure, and ischemia-reperfusion injury. Advances in redox biology and pharmacology for inhibiting ROS production in specific cell types and subcellular organelles combined with the development of nanotechnology-based new in vivo imaging systems and targeted drug delivery mechanisms may enable fine-tuning of redox signaling for the treatment and prevention of CVD.

The antihypertensive effect of arginine

Hypertension is a leading cause of morbidity and mortality worldwide. Individuals with hypertension are at increased risk of stroke, heart disease and kidney failure. Although the etiology of essential hypertension has a genetic component, lifestyle factors such as diet play an important role. Reducing dietary salt is effective in lowering blood pressure in salt-sensitive individuals. Insulin resistance and altered glucose metabolism are common features of hypertension in humans and animal models, with or without salt sensitivity. Altered glucose metabolism leads to increased formation of advanced glycation end products. Insulin resistance is also linked to oxidative stress, and alterations in the nitric oxide pathway and renin angiotensin system. A diet rich in protein containing the semiessential amino acid, arginine, and arginine treatment, lowers blood pressure in humans and in animal models. This may be due to the ability of arginine to improve insulin resistance, decrease advanced glycation end products formation, increase nitric oxide, and decrease levels of angiotensin II and oxidative stress, with improved endothelial cell function and decreased peripheral vascular resistance. The Dietary Approaches to Stop Hypertension (DASH) study demonstrated that the DASH diet, rich in vegetables, fruits and low-fat dairy products; low in fat; and including whole grains, poultry, fish and nuts, lowered blood pressures even more than a typical North American diet with similar reduced sodium content. The DASH diet is rich in protein; the blood pressure-lowering effect of the DASH diet may be due to its higher arginine-containing protein, higher antioxidants and low salt content.

Effect of L-arginine and selenium added to a hypocaloric diet enriched with legumes on cardiovascular disease risk factors in women with central obesity: a randomized, double-blind, placebo-controlled trial

BACKGROUND/AIMS

We aimed to discover if L-arginine and selenium alone or together can increase the effect of a hypocaloric diet enriched in legumes (HDEL) on central obesity and cardiovascular risk factors in women with central obesity.

METHODS

This randomized, double-blind, placebo-controlled trial was undertaken in 84 premenopausal women with central obesity. After a 2-week run-in period on an isocaloric diet, participants were randomly assigned to a control diet (HDEL), L-arginine (5 g/day) and HDEL, selenium (200 μg/day) and HDEL or L-arginine, selenium and HDEL for 6 weeks. Cardiovascular risk factors were assessed before intervention and 3 and 6 weeks afterwards.

RESULTS

After 6 weeks, L-arginine had significantly reduced waist circumference (WC); selenium had significantly lowered fasting concentrations of serum insulin and the homeostasis model assessment of insulin resistance index; the interaction between L-arginine and selenium significantly reduced the fasting concentration of nitric oxides (NO(x)), and HDEL lowered triglycerides (TG) and WC and significantly increased the fasting concentration of NO(x). HDEL reduced high-sensitivity C-reactive protein levels in the first half of the study and returned them to basal levels in the second half.

CONCLUSION

These data indicate the beneficial effects of L-arginine on central obesity, selenium on insulin resistance and HDEL on serum concentrations of NO(x) and TG.

The effect of L-arginine or L-citrulline supplementation on biochemical parameters and the vascular aortic wall in high-fat and high-cholesterol-fed rats

The aim of the present study is to investigate the potential role of L-arginine or L-citrulline in rats fed high-fat and high-cholesterol (HFC) diet. HFC feeding increased significantly serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities, urea and all lipid profiles and decreased significantly serum high-density lipoprotein-cholesterol (HDL-c) and non significantly serum nitric oxide levels. L-arginine or L-citrulline administration reversed the increase in serum AST and ALT activities, urea and all lipid profiles. These effects were associated with a concomitant increase in HDL-c and nitric oxide levels. In general, rats fed HFC diet and orally treated with L-arginine or L-citrulline had higher relative percentage of 18:0, 20:0 and 22:6 and lower 16:0 fatty acids than rats fed HFC diet. Light and transmission electron microscopic findings of the thoracic aorta confirmed the biochemical results and demonstrated structural changes in the endothelial cells of the intimal layer, medial smooth muscle cells as well as in the adventitial layer in HFC fed-animals. However, these findings indicate little structural alterations in animals supplemented with L-arginine or L-citrulline along with HFC feeding. In the present study, L-arginine or L-citrulline was effective hypocholesterolemic and hypolipidemic agents in rats.

Natural antioxidants and hypertension: promise and challenges

Hypertension reigns as a leading cause of cardiovascular morbidity and mortality worldwide. Excessive reactive oxygen species (ROS) have emerged as a central common pathway by which disparate influences may induce and exacerbate hypertension. Potential sources of excessive ROS in hypertension include nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, mitochondria, xanthine oxidase, endothelium-derived NO synthase, cyclooxygenase 1 and 2, cytochrome P450 epoxygenase, and transition metals. While a significant body of epidemiological and clinical data suggests that antioxidant-rich diets reduce blood pressure and cardiovascular risk, randomized trials and population studies using natural antioxidants have yielded disappointing results. The reasons behind this lack of efficacy are not completely clear, but likely include a combination of (1) ineffective dosing regimens, (2) the potential pro-oxidant capacity of some of these agents, (3) selection of subjects less likely to benefit from antioxidant therapy (too healthy or too sick), and (4) inefficiency of nonspecific quenching of prevalent ROS versus prevention of excessive ROS production. Commonly used antioxidants include Vitamins A, C and E, L-arginine, flavanoids, and mitochondria-targeted agents (Coenzyme Q10, acetyl-L-carnitine, and alpha-lipoic acid). Various reasons, including incomplete knowledge of the mechanisms of action of these agents, lack of target specificity, and potential interindividual differences in therapeutic efficacy preclude us from recommending any specific natural antioxidant for antihypertensive therapy at this time. This review focuses on recent literature evaluating naturally occurring antioxidants with respect to their impact on hypertension.

Novel associations of CPS1, MUT, NOX4, and DPEP1 with plasma homocysteine in a healthy population: a genome-wide evaluation of 13 974 participants in the Women's Genome Health Study

BACKGROUND

Homocysteine is a sulfur amino acid whose plasma concentration has been associated with the risk of cardiovascular diseases, neural tube defects, and loss of cognitive function in epidemiological studies. Although genetic variants of MTHFR and CBS are known to influence homocysteine concentration, common genetic determinants of homocysteine remain largely unknown.

METHODS AND RESULTS

To address this issue comprehensively, we performed a genome-wide association analysis, testing 336 469 single-nucleotide polymorphisms in 13 974 healthy white women. Although we confirm association with MTHFR (1p36.22; rs1801133; P=8.1 x 10(-35)) and CBS (21q22.3; rs6586282; P=3.2 x 10(-10)), we found novel associations with CPS1 (2q34; rs7422339; P=1.9 x 10(-11)), MUT (6p12.3; rs4267943; P=2.0 x 10(-9)), NOX4 (11q14.3; rs11018628; P=9.6 x 10(-12)), and DPEP1 (16q24.3; rs1126464; P=1.2 x 10(-12)). The associations at MTHFR, DPEP1, and CBS were replicated in an independent sample from the PROCARDIS study, whereas the association at CPS1 was only replicated among the women.

CONCLUSIONS

These associations offer new insight into the biochemical pathways involved in homocysteine metabolism and provide opportunities to better delineate the role of homocysteine in health and disease.

Effect of L-arginine oral supplementation on response to myocardial infarction in hypercholesterolemic and hypertensive rats

The well known metabolic functions of L-arginine have been recently increased with the discovery of its role as the substrate for the synthesis of nitric oxide (NO), which has emerged as an endogenous signaling molecule with potential therapeutic implications for cardiovascular disease. Steady-state levels of NO are derived in part from dietary sources. It has been reported that supplementation of L-arginine reduces atherosclerosis in rabbits and reduces the arterial pressure in hypertensive rats. Therefore, we investigated the effect of L-arginine supplementation using a group of induced hypercholesterolemic rats and a group of spontaneously hypertensive rats; the infarcted area in cardiac tissue of both groups was measured during the response to myocardial infarction in the ischemia-reperfusion model. Hypercholesterolemic rats supplemented with 170 mg kg(-1) of L-arginine showed a significant (P <or= 0.05) reduction in total cholesterol (25.2%) and LDL (27.8%). Spontaneously hypertensive rats supplemented with L-arginine presented a significant reduction (20.3%) in mean blood pressure (P <or= 0.05). The index infarcted area/total heart area, in both: hypercholesterolemic and hypertensive rats supplemented with L-arginine, showed a significant 36% and 29% of cardio protection (P <or= 0.05) effect, respectively. Dietary supplementation with L-arginine may represent a potentially novel nutritional strategy for the treatment of cardiovascular disease.

The effect of L-arginine and creatine on vascular function and homocysteine metabolism

Studies with L-arginine supplementation have shown inconsistent effects on endothelial function. The generation of guanidinoacetate (GAA) from L-arginine with subsequent formation of creatine and homocysteine and consumption of methionine may reduce the pool of L-arginine available for nitric oxide generation. Experimental studies suggest that creatine supplementation might block this pathway. We sought to determine the effects of L-arginine, creatine, or the combination on endothelium-dependent vasodilation and homocysteine metabolism in patients with coronary artery disease. Patients with coronary artery disease were randomized to L-arginine (9 g/day), creatine (21 g/day), L-arginine plus creatine, or placebo for 4 days (n = 26-29/group). Brachial artery flow-mediated dilation and plasma levels of L-arginine, creatine, homocysteine, methionine, and GAA were measured at baseline and follow-up. L-arginine and creatine supplementation had no effects on vascular function. L-arginine alone increased GAA (p < 0.01) and the ratio of homocysteine to methionine (p < 0.01), suggesting increased methylation demand. The combination of creatinine and L-arginine did not suppress GAA production or prevent the increase in homocysteine-to-methionine ratio. Unexpectedly, creatine supplementation (alone or in combination with L-arginine) was associated with an 11-20% increase in homocysteine concentration (p < 0.05), which was not attributable to worsened renal function, providing evidence against an effect of creatine on decreasing methylation demand. In conclusion, the present study provides no evidence that L-arginine supplementation improves endothelial function and suggests that l-arginine may increase methylation demand. Creatine supplementation failed to alter the actions of L-arginine on vascular function or suppress methylation demand. The unexpected increase in homocysteine levels following creatine supplementation could have adverse effects and merits further study, since creatine is a commonly used dietary supplement.

Early prevention by L-Arginine attenuates coronary atherosclerosis in a model of hypercholesterolemic animals; no positive results for treatment

Background

Endothelial dysfunction (ED) is an independent predictor of cardiovascular events. ED is also a reversible disorder, and nitric oxide donors like L-arginine may promote this process. Despite the positive results from several studies, there are some studies that have shown that L-arginine administration did not improve endothelium-dependent dilation or the inflammatory state of patients. In this study the early and the late effects of L-arginine on coronary fatty streak formation and ED biomarkers were considered in hypercholesterolemic rabbits.

Methods

36 white male rabbits randomly assigned in 3 groups. Rabbits were fed 1% high-cholesterol diet (LP group, n = 15), or high-cholesterol diet with oral L-arginine (3% in drinking water) (EP group, n = 15) or standard diet (control group, n = 6) for 4 weeks (phase I). Afterward, all animals were fed normal diet for 4 weeks (phase II). In the second phase, L-arginine was discontinued for EP group and was begun for LP group. The plasma levels of lipids, von Willebrand factor (vWF), and nitrite were compared before and after 4 and 8 weeks of experiment. Coronary fatty streak formation was measure after 4 and 8 weeks of experiment.

Results

The plasma levels of lipids were increased significantly in both groups of LP and EP after phase I. The hypercholesterolemia induced significant increased vWF release in LP group. The L-arginine supplementation led to significant plasma nitrite increment in EP group. The vWF in LP group was higher than other groups (p < 0.05). By the end of phase II, despite of start of L-arginine supplementation for LP group and L-arginine discontinuation in EP group, there were significantly more fatty streaks lesions in LP group coronary arteries than EP group. Furthermore, L-arginine supplementation did not result in significant nitrite increment in LP group.

Conclusion

Early prevention by L-arginine may be helpful to prevent the ED, but our study did not suggest the treatment. It seems reasonable to consider ED-aside from control the cardiovascular risk factors in primary prevention of atherosclerosis and its clinical outcomes before development of irreversible vascular damage.

L-arginine fails to prevent ventricular remodeling and heart failure in the spontaneously hypertensive rat

BACKGROUND

The effects of long-term oral administration of L-arginine, a substrate for nitric oxide (NO) production, on left ventricular (LV) remodeling, myocardial function and the prevention of heart failure (HF) was compared to the angiotensin-converting enzyme (ACE) inhibitor captopril in a rat model of hypertensive HF (aged spontaneously hypertensive rat (SHR)).

METHODS

SHRs and age-matched normotensive Wistar-Kyoto (WKY) rats were assigned to either no treatment, treatment with L-arginine (7.5 g/l in drinking water) or captopril (1 g/l in drinking water) beginning at 14 months of age, a time when SHRs exhibit stable compensated hypertrophy with no hemodynamic impairment; animals were studied at 23 months of age or at the time of HF.

RESULTS

In untreated SHR, relative to WKY, there was significant LV hypertrophy, myocardial fibrosis, and isolated LV muscle performance and response to isoproterenol (ISO) were depressed; and, 7 of 10 SHRs developed HF. Captopril administration to six SHRs attenuated hypertrophy and prevented impaired inotropic responsiveness to ISO, contractile dysfunction, fibrosis, increased passive stiffness, and HF. In contrast, L-arginine administration to SHR increased LV hypertrophy and myocardial fibrosis while cardiac performance was depressed; and 7 of 9 SHRs developed HF. In WKY, L-arginine treatment but not captopril resulted in increased LV weight and the contractile response to ISO was blunted. Neither L-arginine nor captopril treatment of WKY changed fibrosis and HF did not occur.

CONCLUSION

These data demonstrate that in contrast to captopril, long-term treatment with L-arginine exacerbates age-related cardiac hypertrophy, fibrosis, and did not prevent contractile dysfunction or the development of HF in aging SHR.

Insulin reduces plasma arginase activity in type 2 diabetic patients

OBJECTIVE

We sought to determine whether dysregulation of arginine metabolism is related to insulin resistance and underlies impaired nitric oxide (NO) generation in type 2 diabetic patients.

RESEARCH DESIGN AND METHODS

We measured plasma arginase activity, arginine metabolites, and skeletal muscle NO synthase (NOS) activity in 12 type 2 diabetic and 10 age-/BMI-matched nondiabetic subjects before and following a 4-h euglycemic-hyperinsulinemic clamp with muscle biopsies. Arginine metabolites were determined by tandem mass spectroscopy. Arginase activity was determined by conversion of [(14)C] guanidoinoarginine to [(14)C] urea.

RESULTS

Glucose disposal (R(d)) was reduced by 50% in diabetic versus control subjects. NOS activity was fourfold reduced in the diabetic group (107 +/- 45 vs. 459 +/- 100 pmol x min(-1) x mg protein(-1); P < 0.05) and failed to increase with insulin. Plasma arginase activity was increased by 50% in the diabetic versus control group (0.48 +/- 0.11 vs. 0.32 +/- 0.12 micromol x ml(-1) x h(-1); P < 0.05) and markedly declined in diabetic subjects with 4-h insulin infusion (to 0.13 +/- 0.04 micromol x ml(-1) x h(-1) vs. basal; P < 0.05). In both groups collectively, plasma arginase activity correlated positively with fasting plasma glucose (R = 0.46, P < 0.05) and A1C levels (R = 0.51, P < 0.02) but not with R(d).

CONCLUSIONS

Plasma arginase activity is increased in type 2 diabetic subjects with impaired NOS activity, correlates with the degree of hyperglycemia, and is reduced by physiologic hyperinsulinemia. Elevated arginase activity may contribute to impaired NO generation in type 2 diabetes, and insulin may ameliorate this defect via reducing arginase activity.

Asymmetric dimethylarginine (ADMA) in vascular, renal and hepatic disease and the regulatory role of L-arginine on its metabolism

Asymmetric dimethylarginine (ADMA), an inhibitor of nitric oxide synthase (NOS), has been identified as a new and emerging contributor to, or marker for, cardiovascular risk. The ADMA-mediated regulation of nitric oxide (NO) production is determined by the quantitative bioavailability of intracellular and extracellular ADMA. Dimethylarginine dimethylaminohydrolase (DDAH), which is ubiquitously expressed in tissues, especially liver and kidney, converts the majority of the ADMA to citrulline. In this review, we discuss a new regulatory mechanism for the metabolism of ADMA in which L-arginine acts as a competitive inhibitor of DDAH activity. This novel regulatory pathway is consistent with ADMA contributing to cardiovascular risk when levels are increased but not when levels are within the normal range. The pathway then has a physiological role in the regulation of NO production by preventing overproduction of NO. The regulatory role of L-arginine on ADMA may explain the unexpected outcomes in some L-arginine supplementation studies. This paper also reviews associations between the metabolism of ADMA and insulin resistance, smoking and homocysteine which are all associated with an increased risk of vascular disease.

Mechanisms of disease: L-arginine in coronary atherosclerosis--a clinical perspective

L-arginine is the substrate of endothelial nitric oxide synthase and the main precursor of nitric oxide in the vascular endothelium, thus its effects are mediated largely by increases in nitric oxide production. L-arginine has antioxidant and antiapoptotic properties, increases smooth muscle relaxation, inhibits the expression of adhesion molecules and chemotactic peptides, decreases endothelin-1 expression, and inhibits platelet aggregation. This amino acid also improves endothelial function in patients with coronary artery disease and dilates human epicardial atheromatous coronary arteries. Despite the positive results from small case-control studies, it is still unclear whether chronic administration of L-arginine has any effect on clinical outcome in patients with coronary artery disease. In addition, other indirect strategies, such as the inhibition of arginase, could prove more effective at improving intracellular L-arginine bioavailability than exogenous L-arginine administration. The potential clinical usefulness of L-arginine, therefore, needs further evaluation in large, prospective clinical trials. Here, we present a critique of the existing literature about the role of L-arginine in the prevention of atherosclerosis.

l-Arginine, the substrate for NO synthesis: An alternative treatment for premature atherosclerosis?

L-Arginine is the substrate of endothelial nitric oxide synthase (eNOS) and the main precursor of nitric oxide (NO) in the vascular endothelium. L-Arginine improves endothelial function in patients with hypercholesterolemia, hypertension and smokers, while its role in diabetes remains unclear. Oral supplementation of L-arginine leads to a significant improvement of endothelium-dependent forearm vasodilation in hypercholesterolemic patients, while intravenous infusion of L-arginine improves endothelial function in healthy smokers. L-Arginine has anti-hypertensive properties, although its effects on endothelial function in hypertensive patients needs further evaluation. In conclusion, L-arginine administration may be useful in patients with premature atherosclerosis.

L-arginine plus atorvastatin for prevention of atheroma formation in genetically hypercholesterolaemic rabbits

We investigated the combined effect of dietary supplementation with L-arginine, which is the precursor of NO, and pharmacological treatment with atorvastatin, which is a 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase inhibitor, on the development of atherosclerosis in homozygous Watanabe heritable hyperlipidaemic rabbits. Rabbits were fed either standard rabbit chow (group C; n 9) as control, a 1.5 % L-arginine diet (group A; n 9), standard rabbit chow plus atorvastatin (2.5 mg/kg per d) in drinking water (group S; n 8), or standard rabbit chow plus a 1.5 % L-arginine diet with atorvastatin (group SA; n 8). Blood was sampled at 2-week intervals. After 8 weeks (T8), the aorta was harvested for topographic and histological analysis. Only the SA group showed decreases in total area of lesions (21 %) and the area of abdominal lesions (44 %) compared with the control group (P = 0.019). Furthermore, plaques in the SA group were smaller and less thick than those observed in the S group. Unexpectedly, plasma nitrite + nitrate levels were not modified under either the L-arginine diet alone or under L-arginine plus atorvastatin. The present study is the first to demonstrate that diet supplementation with L-arginine associated with a statin (atorvastatin) is more efficient in reducing lesion size than treatment with L-arginine or a statin alone. This is a relatively novel therapeutic approach associating a macronutrient and a drug.

Endothelial arginase: a new target in atherosclerosis

Decreased endothelial nitric oxide (NO) bioavailability as it relates to endothelial dysfunction plays an important role in various cardiovascular disorders, including athero-sclerosis. Recent research has provided evidence that endothelial dysfunction in atherosclerosis is not primarily caused by decreased endothelial NO synthase (eNOS) gene expression, but rather deregulation of eNOS enzymatic activity, which contributes to the increased oxidative stress in atherosclerosis. Among other mechanisms, the substrate L-arginine is an important limiting factor for NO production. Emerging evidence demonstrates that L-arginine is not only converted to NO via eNOS, but also metabolized to urea and l-ornithine via arginase in endothelial cells. Hence, arginase competes with eNOS for the substrate L-arginine, resulting in deceased NO production. There are an increasing number of studies showing that enhanced arginase gene expression and/or activity contribute to endothelial dysfunction in various cardiovascular disorders, including atherosclerosis. Thus, endothelial arginase may represent a new therapeutic target in atherosclerosis.

Arginine metabolism in vascular biology and disease

Arginine metabolism plays a major role in cardiovascular physiology and pathophysiology, largely via nitric oxide (NO)-dependent processes. It is becoming increasingly apparent, however, that arginine metabolic enzymes other than the NO synthases can also play important roles via both NO-dependent and -independent processes. There are three sources of arginine in vivo and at least five mammalian enzymes or enzyme families that utilize arginine as substrate. Changes in arginine availability or in production of the different end products of the various arginine metabolic pathways can have distinct and profound physiologic consequences. However, our knowledge regarding the complex interplay between these pathways at the level of the whole body, specific tissues, and even individual cells, is incomplete. This review will highlight recent findings in this area that may suggest additional avenues of investigation that will allow a fuller understanding of cardiovascular physiology in health and disease.

Recent advances in understanding endothelial dysfunction in atherosclerosis

Over the last two decades, it has become evident that decreased bioavailability of endothelial nitric oxide (NO) produced from endothelial NO synthase (eNOS), referred to as endothelial dysfunction, plays a crucial role in the development and progression of atherosclerosis. Much progress has been made in understanding the mechanisms of decreased endothelial NO bioavailability at the levels of regulation of eNOS gene expression, eNOS enzymatic activity and NO inactivation. Initial studies suggest that increasing eNOS gene expression would improve endothelial NO release in the hope of inhibiting the progression of atherosclerosis. Recent experimental studies, however, do not always support this therapeutic concept and show some evidence that overexpression of eNOS in atherosclerosis may be even harmful for the disease progression.Thus, recent research to improve endothelial function in atherosclerosis has focused on regulation of eNOS enzymatic activity and prevention of NO inactivation by oxidative stress. Since the role of oxidative stress in endothelial NO bioavailability has been reviewed in a large number of comprehensive articles, this article focuses on the relevant regulatory mechanisms of eNOS enzymatic activity that are emerging to play a role in endothelial dysfunction in atherosclerosis.

Endothelial Nitric Oxide Synthase

This article explores the physiology of superoxide generation by endothelial nitric oxide synthase (eNOS), the so-called "uncoupled" state of the enzyme. The fact that this alternative chemistry of the eNOS enzyme is evolutionary strongly conserved, suggests that it may play a physiological role. It is proposed that this uncoupled state may contribute to defense against infections. As the switch from NO production to reactive oxygen species by eNOS is also the final common pathway in atherogenesis, the uncoupling of eNOS further builds on the hypothesis that atherogenesis is driven by cellular mechanisms that originally serve host defense. The central role of uncoupled eNOS in redox signaling in the endothelium may open up new avenues for therapy to prevent atherosclerosis.

The impact of metabolism on DNA methylation

Methylation of genomic cytosines is one of the best characterized epigenetic mechanisms, and investigation of its relationship with other biochemical pathways represents a critical stage in the elucidation of biological information processing. The field also has immense potential for the development of medical treatments for any number of conditions ranging from aging to neurological disorders. The DNA methylation status of genes is responsible for many heritable traits and varies more or less independently of the genetic code. This variation is often a result of cellular environmental factors including metabolism. A key metabolite in this regard is homocysteine. Knowledge of homocysteine metabolism continues to be amassed, yet the part played by aberrant DNA methylation in homocysteine-related pathologies is often, at best, conjectural. In this analysis, we will review recent insights and attempt to speculate meaningfully concerning the dynamics of the methionine cycle in relation to DNA methylation and disease.

Endothelial cell dysfunction: can't live with it, how to live without it

Endothelial cell dysfunction is emerging as an ultimate culprit for diverse cardiovascular diseases and cardiovascular complications of chronic renal diseases, yet the definition of this new syndrome, its pathophysiology, and therapy remain poorly defined. Here, I summarize some molecular mechanisms leading from hyperhomocystinemia, elevated asymmetric dimethylarginine, and advanced glycolation end product-modified protein level to the proatherogenic, prothrombogenic, and proinflammatory endothelial phenotype and offer a model of endothelial dysfunction based on the interconnectedness of diverse functions. Finally, several therapeutic strategies to prevent and correct endothelial dysfunction are discussed in the light of uncertainty of their action modulated by the endothelial dysfunction per se.

Oral L-arginine improves hemodynamic responses to stress and reduces plasma homocysteine in hypercholesterolemic men

When administered intravenously, L-arginine substantially reduces blood pressure (BP) and peripheral vascular resistance in healthy adults and in patients with vascular disease. Oral L-arginine has been shown to improve endothelial function; however, it is not clear whether oral administration has significant effects on systemic hemodynamics. In a randomized, placebo-controlled, crossover study we tested whether oral L-arginine (12 g/d for 3 wk) affected hemodynamics, glucose, insulin, or C-reactive protein in 16 middle-age men with hypercholesterolemia. After each treatment, hemodynamic variables were measured at rest and during 2 standardized stressor tasks (a simulated public-speaking task and the cold pressor). Regardless of treatment, the stressor tasks increased BP and heart rate (P < or = 0.02). Relative to placebo, L-arginine changed cardiac output (-0.4 L/m), diastolic BP (-1.9 mm Hg), pre-ejection period (+3.4 ms), and plasma homocysteine (-2.0 micromol/L) (P < or = 0.03). The change in plasma L-arginine was inversely correlated with the change in plasma homocysteine (r = -0.57, P = 0.03). Contrary to the results of previous studies of L-arginine administered intravenously, oral administration did not affect total peripheral resistance or plasma insulin. Oral L-arginine also did not affect plasma glucose, C-reactive protein, or lipids. This pattern of findings is consistent with the hypothesis that oral L-arginine reduces BP. This study is the first to describe a hemodynamic mechanism for the hypotensive effect of oral L-arginine and the first to show substantial reductions in homocysteine with oral administration.

The procyanidin-induced pseudo laminar shear stress response: a new concept for the reversal of endothelial dysfunction

Reduced endothelium-dependent vasodilator responses with increased synthesis of ET-1 (endothelin-1) are characteristics of endothelial dysfunction in heart failure and are predictive of mortality. Identification of treatments that correct these abnormalities may have particular benefit for patients who become refractory to current regimens. Hawthorn preparations have a long history in the treatment of heart failure. Therefore we tested their inhibitory effects on ET-1 synthesis by cultured endothelial cells. These actions were compared with that of GSE (grape seed extract), as the vasoactive components of both these herbal remedies are mainly oligomeric flavan-3-ols called procyanidins. This showed extracts of hawthorn and grape seed were equipotent as inhibitors of ET-1 synthesis. GSE also produced a potent endothelium-dependent vasodilator response on preparations of isolated aorta. Suppression of ET-1 synthesis at the same time as induction of endothelium-dependent vasodilation is a similar response to that triggered by laminar shear stress. Based on these results and previous findings, we hypothesize that through their pharmacological properties procyanidins stimulate a pseudo laminar shear stress response in endothelial cells, which helps restore endothelial function and underlies the benefit from treatment with hawthorn extract in heart failure.

Arginine and endothelial and vascular health

The vascular endothelium is a crucial regulator of vascular function and homeostasis. Nitric oxide (NO) is an important paracrine substance released by the endothelium to regulate vasomotor tone. Risk factors for atherosclerosis, as well as atherosclerosis per se, are associated with endothelial dysfunction and decreased bioavailablilty of NO. Indeed, endothelial dysfunction is integral to the pathogenesis of atherosclerosis and other cardiovascular diseases. Moreover, endothelial dysfunction relates to an increased risk of adverse cardiovascular outcomes. L-Arginine is an essential amino acid required by the constitutive enzyme, endothelial NO oxide synthase (eNOS), to produce NO. Administration of L-arginine improves endothelial function in animal models and in humans with hypercholesterolemia and with atherosclerosis. Clinical trials to date support potential clinical applications of L-arginine in the treatment of coronary artery disease and peripheral arterial disease, as well as in the prevention of in-stent restenosis. The mechanism of benefit of L-arginine on endothelial function is unclear, because intracellular concentrations of L-arginine far exceed that required by eNOS. One potential explanation of this "arginine paradox" is that L-arginine restores endothelial function in atherosclerotic patients, in whom there are elevated levels of asymmetric dimethylarginine, an endogenous inhibitor of eNOS. Given the promising findings of early studies of L-arginine as a potential therapy for cardiovascular disorders, large-scale clinical trials are warranted.

L-arginine and atherothrombosis

L-arginine, the principal substrate for endothelial nitric oxide synthase, is oxidized to L-citrulline and nitric oxide. Endothelial dysfunction is associated with decreased bioactive nitric oxide production, an abnormality observed in atherothrombosis. Acute or chronic administration of supplemental L-arginine enhances endothelial nitric oxide production and improves endothelial function in the setting of atherothrombosis. The mechanisms by which L-arginine improves endothelial nitric oxide bioactivity include increased intracellular uptake via the high-affinity cationic transporter; substrate competition with asymmetric dimethylarginine, a naturally occurring inhibitor of nitric oxide synthase; direct antioxidant activity; stimulated release of histamine from mast cells, which produces a vasodilator response; decreased activity of norepinephrine, which promotes the effect of endogenous vasodilators including nitric oxide; and increased insulin secretion, which causes vasodilation. By virtue of its link to methyl group metabolism, supplemental L-arginine can, however, also increase the production of S-adenosylhomocysteine from S-adenosylmethionine through the methylation-dependent generation of creatine from guanidinoacetate. This reaction can theoretically lead to increased homocysteine synthesis from its S-adenosyl derivative, which itself can have adverse effects on endothelial function. The interrelationships among these effects of L-arginine are reviewed here, and the potential benefits and risks of L-arginine supplementation are discussed.

Argininosuccinate synthase expression is required to maintain nitric oxide production and cell viability in aortic endothelial cells

Although cellular levels of arginine greatly exceed the apparent K(m) for endothelial nitric-oxide synthase, current evidence suggests that the bulk of this arginine may not be available for nitric oxide (NO) production. We propose that arginine regeneration, that is the recycling of citrulline back to arginine, defines the essential source of arginine for NO production. To support this proposal, RNA interference analysis was used to selectively reduce the expression of argininosuccinate synthase (AS), because the only known metabolic role for AS in endothelial cells is in the regeneration of l-arginine from l-citrulline. Western blot analysis demonstrated a significant and dose-dependent reduction of AS protein as a result of AS small interfering RNA treatment with a corresponding diminished capacity to produce basal or stimulated levels of NO, despite saturating levels of arginine in the medium. Unanticipated, however, was the finding that the viability of AS small interfering RNA-treated endothelial cells was significantly decreased when compared with control cells. Trypan blue exclusion analysis suggested that the loss of viability was not because of necrosis. Two indicators, reduced expression of Bcl-2 and an increase in caspase activity, which correlated directly with reduced expression of AS, suggested that the loss of viability was because of apoptosis. The exposure of cells to an NO donor prevented apoptosis associated with reduced AS expression. Overall, these results demonstrate the essential role of AS for endothelial NO production and cell viability.

Introduction to the 2nd Amino Acid Assessment Workshop

The proportions of amino acids in diets typical of human populations usually differ from the proportions in which they are required, although adverse effects due to such differences are not common. However, there is little systematic information about the adverse effects and the pathophysiological mechanisms of excessive intakes of single or mixtures of amino acids in human subjects. To promote the safe and effective application of amino acids in clinical nutrition and for health promotion it is necessary to establish a sound scientific basis for evaluating their efficacy and safety under various conditions of use. Hence, a series of Amino Acid Assessment Workshops (AAAW) are being organized to bring together experts in amino acid nutrition, metabolism, cell and molecular biology, toxicology and regulation/policy with the eventual purpose of establishing a paradigm for the characterization of risks associated with the ingestion of specific intakes of amino acids by humans. In this introductory paper I summarize the major issues arising at the 1st AAAW, held in Tokyo June, 2001, and provide an introductory context to the present, 2nd AAAW.

The caveolar nitric oxide synthase/arginine regeneration system for NO production in endothelial cells

The enzyme endothelial nitric oxide synthase (eNOS) catalyzes the conversion of arginine, oxygen and NADPH to NO and citrulline. Previous results suggest an efficient, compartmentalized system for recycling of citrulline to arginine utilized for NO production. In support of this hypothesis, the recycling enzymes, argininosuccinate synthase (AS) and argininosuccinate lyase (AL), have been shown to colocalize with eNOS in caveolae, a subcompartment of the plasma membrane. Under unstimulated conditions, the degree of recycling is minimal. Upon stimulation of NO production by bradykinin, however, recycling is co-stimulated to the extent that more than 80% of the citrulline produced is recycled to arginine. These results suggest an efficient caveolar recycling complex that supports the receptor-mediated stimulation of endothelial NO production. To investigate the molecular basis for the unique location and function of endothelial AS and AL, endothelial AS mRNA was compared with liver AS mRNA. No differences were found in the coding region of the mRNA species, but significant differences were found in the 5'-untranslated region (5'-UTR). The results of these studies suggest that sequence in the endothelial AS-encoding gene, represented by position -92 nt to -43 nt from the translation start site in the extended AS mRNA 5'-UTRs, plays an important role in differential and tissue-specific expression. Overall, a strong evidential case has been developed supporting the proposal that arginine availability, governed by a caveolar-localized arginine regeneration system, plays a key role in receptor-mediated endothelial NO production.