Modulating role of estradiol on arginase II expression in hyperlipidemic rabbits as an atheroprotective mechanism

Peroxisome proliferator-activated receptor δ improves the features of atherosclerotic plaque vulnerability by regulating smooth muscle cell phenotypic switching

BACKGROUND AND PURPOSE

Vascular smooth muscle cells (SMCs) undergo phenotypic switching during sustained inflammation, contributing to an unfavourable atherosclerotic plaque phenotype. PPARδ plays an important role in regulating SMC functions; however, its role in atherosclerotic plaque vulnerability remains unclear. Here, we explored the pathological roles of PPARδ in atherosclerotic plaque vulnerability in severe atherosclerosis and elucidated the underlying mechanisms.

EXPERIMENTAL APPROACH

Plasma levels of PPARδ were measured in patients with acute coronary syndrome (ACS) and stable angina (SA). SMC contractile and synthetic phenotypic markers, endoplasmic reticulum (ER) stress, and features of atherosclerotic plaque vulnerability were analysed for the brachiocephalic artery of apolipoprotein E-knockout (ApoE-/- ) mice, fed a high-cholesterol diet (HCD) and treated with or without the PPARδ agonist GW501516. In vitro, the role of PPARδ was elucidated using human aortic SMCs (HASMCs).

KEY RESULTS

Patients with ACS had significantly lower plasma PPARδ levels than those with SA. GW501516 reduced atherosclerotic plaque vulnerability, a synthetic SMC phenotype, ER stress markers, and NLRP3 inflammasome expression in HCD-fed ApoE-/- mice. ER stress suppressed PPARδ expression in HASMCs. PPARδ activation inhibited ER stress-induced synthetic phenotype development, ER stress-NLRP3 inflammasome axis activation and matrix metalloproteinase 2 (MMP2) expression in HASMCs. PPARδ inhibited NFκB signalling and alleviated ER stress-induced SMC phenotypic switching.

CONCLUSIONS AND IMPLICATIONS

Low plasma PPARδ levels may be associated with atherosclerotic plaque vulnerability. Our findings provide new insights into the mechanisms underlying the protective effect of PPARδ on SMC phenotypic switching and improvement the features of atherosclerotic plaque vulnerability.

Elevation of Arginase-II in Podocytes Contributes to Age-Associated Albuminuria in Male Mice

One of the manifestations of renal aging is podocyte dysfunction and loss, which are associated with proteinuria and glomerulosclerosis. Studies show a male bias in glomerular dysfunction and chronic kidney diseases, and the underlying mechanisms remain obscure. Recent studies demonstrate the role of an age-associated increase in arginase-II (Arg-II) in proximal tubules of both male and female mice. However, it is unclear whether Arg-II is also involved in aging glomeruli. The current study investigates the role of the sex-specific elevation of Arg-II in podocytes in age-associated increased albuminuria. Young (3-4 months) and old (20-22 months) male and female mice of wt and arginase-II knockout (arg-ii-/-) were used. Albuminuria was employed as a readout of glomerular function. Cellular localization and expression of Arg-II in glomeruli were analyzed using an immunofluorescence confocal microscope. A more pronounced age-associated increase in albuminuria was found in male than in female mice. An age-associated induction of Arg-II in glomeruli and podocytes (as demonstrated by co-localization of Arg-II with the podocyte marker synaptopodin) was also observed in males but not in females. Ablation of the arg-ii gene in mice significantly reduces age-associated albuminuria in males. Also, age-associated decreases in podocyte density and glomerulus hypertrophy are significantly prevented in male arg-ii-/- but not in female mice. However, age-associated glomerulosclerosis is not affected by arg-ii ablation in both sexes. These results demonstrate a role of Arg-II in sex-specific podocyte injury in aging. They may explain the sex-specific differences in the development of renal disease in humans during aging.

Single Nucleotide Polymorphisms in the Arginase 1 and 2 Genes Are Differentially Associated with Circulating l-Arginine Concentration in Unsupplemented and l-Arginine-Supplemented Adults

BACKGROUND

Genetic variation in arginase may underlie variability in whole blood l-arginine concentrations in unsupplemented and l-arginine-supplemented adults.

OBJECTIVES

We aimed to study whether single nucleotide polymorphisms (SNPs) in the arginase 1 (ARG1) and arginase 2 (ARG2) genes are associated with blood l-arginine concentrations in unsupplemented and l-arginine-supplemented individuals.

METHODS

In 374 adults (mean ± SD age: 59.6 ± 14.6 y; 180 males), we analyzed SNPs in the ARG1 (rs2246012 and rs2781667) and ARG2 genes (rs3742879 and rs2759757) and their associations with blood l-arginine concentrations. We analyzed associations of haplotypes for the ARG1 gene and for the ARG1 and ARG2 genes combined with blood l-arginine concentrations in supplement users and unsupplemented participants.

RESULTS

Of study participants, 120 had low (<42 μmol/L), 133 had medium (42-114 μmol/L), and 121 had high blood l-arginine concentrations (>114 μmol/L); 58 individuals were current l-arginine supplement users. We found a significantly higher prevalence of the minor allele of ARG1 rs2246012 in supplement users with higher blood l-arginine concentrations (P = 0.03). Mean ± SEM l-arginine concentration was 263 ± 9.76 μmol/L in supplement users homozygous for the minor allele of ARG1 rs2246012 (P = 0.004); it was 70.4 ± 25.6 μmol/L in unsupplemented participants homozygous for the minor allele of ARG2 rs3759757 (P = 0.03). The ARG1 haplotype was significantly associated with blood l-arginine concentrations in supplement users (P = 0.046), whereas the combined ARG1/ARG2 haplotype was significantly associated with blood l-arginine concentrations in the cohort as a whole (P = 0.012).

CONCLUSIONS

Genetic variability in the ARG1 and ARG2 genes is associated with blood l-arginine concentrations in humans: ARG1 is associated with blood l-arginine concentrations in l-arginine supplement users, whereas ARG2 is associated with blood l-arginine concentrations in unsupplemented participants. Our study is the first to describe a possible functional relation between ARG1 and ARG2 SNPs and blood l-arginine concentrations; genetic variability in ARG1 may explain variation in blood l-arginine concentrations during supplement use and discrepant study results.

Influence of cardiometabolic comorbidities on myocardial function, infarction, and cardioprotection: Role of cardiac redox signaling

The morbidity and mortality from cardiovascular diseases (CVD) remain high. Metabolic diseases such as obesity, hyperlipidemia, diabetes mellitus (DM), non-alcoholic fatty liver disease (NAFLD) and non-alcoholic steatohepatitis (NASH) as well as hypertension are the most common comorbidities in patients with CVD. These comorbidities result in increased myocardial oxidative stress, mainly from increased activity of nicotinamide adenine dinucleotide phosphate oxidases, uncoupled endothelial nitric oxide synthase, mitochondria as well as downregulation of antioxidant defense systems. Oxidative and nitrosative stress play an important role in ischemia/reperfusion injury and may account for increased susceptibility of the myocardium to infarction and myocardial dysfunction in the presence of the comorbidities. Thus, while early reperfusion represents the most favorable therapeutic strategy to prevent ischemia/reperfusion injury, redox therapeutic strategies may provide additive benefits, especially in patients with heart failure. While oxidative and nitrosative stress are harmful, controlled release of reactive oxygen species is however important for cardioprotective signaling. In this review we summarize the current data on the effect of hypertension and major cardiometabolic comorbidities such as obesity, hyperlipidemia, DM, NAFLD/NASH on cardiac redox homeostasis as well as on ischemia/reperfusion injury and cardioprotection. We also review and discuss the therapeutic interventions that may restore the redox imbalance in the diseased myocardium in the presence of these comorbidities.

Role of tubular epithelial arginase-II in renal inflammaging

The aging kidney undergoes complex changes and is vulnerable to injury and development of chronic kidney disease (CKD) with preponderance affecting more women than men. Evidence has been presented that the type-II L-arginine:ureohydrolase, arginase-II (Arg-II) plays a role in the acceleration of aging. Arg-II is highly expressed in the kidney. However, the role of Arg-II in renal aging is not known. This study is to investigate whether Arg-II is involved in the kidney aging process dependently on sex. Arg-II level in the kidney of wild type (WT) mice is significantly elevated with aging, which is accompanied by an increase in expression of the inflammatory cytokines/chemokines, tissue macrophages, factors involved in fibrosis, and tubulointestitial fibrosis in both males and females. This renal aging phenotype is significantly suppressed in arg-II^−/− mice, mainly in the females in which Arg-II level is higher than in the males. Importantly, numerous factors such as IL-1β, MCP1, VCAM-1, and TGFβ1 are mainly localized in the proximal tubular S3 segment cells expressing Arg-II in the aging kidney. In human proximal tubular cells (HK-2), TNF-α enhances adhesion molecule expression dependently on Arg-II upregulation. Overexpression of Arg-II in the cells enhances TGFβ1 levels which is prevented by mitochondrial ROS inhibition. In summary, our study reveals that renal proximal tubular Arg-II plays an important role in the kidney aging process in females. Arg-II could be a promising therapeutic target for the treatment and prevention of aging-associated kidney diseases.

Detrimental Effects of Chronic L-Arginine Rich Food on Aging Kidney

The impaired L-arginine/nitric oxide pathway is a well-recognized mechanism for cardiovascular and renal diseases with aging. Therefore, supplementation of L-arginine is widely proposed to boost health or as adjunct therapy for the patients. However, clinical data, show adverse effects and even enhanced mortality in patients receiving long-term L-arginine supplementation. The effects of long-term L-arginine supplementation on kidney aging and the underlying mechanisms remain elusive. Moreover, high protein and high amino acid diet has been thought detrimental for kidney. We therefore investigated effects of chronic dietary L-arginine supplementation on kidney aging. In both young (4 months) and old (18-24 months) mice, animals either receive standard chow containing 0.65% L-arginine or diet supplemented with L-arginine to 2.46% for 16 weeks. Inflammation and fibrosis markers and albuminuria are then analyzed. Age-associated increases in tnf-α, il-1β, and il-6, vcam-1, icam-1, mcp1, inos, and macrophage infiltration, collagen expression, and S6K1 activation are observed, which is not favorably affected, but rather further enhanced, by L-arginine supplementation. Importantly, L-arginine supplementation further enhances age-associated albuminuria and mortality particularly in females, accompanied by elevated renal arginase-II (Arg-II) levels. The enhanced albuminuria by L-arginine supplementation in aging is not protected in Arg-II^-/- mice. In contrast, L-arginine supplementation increases ROS and decreases nitric oxide production in old mouse aortas, which is reduced in Arg-II^-/- mice. The results do not support benefits of long-term L-arginine supplementation. It rather accelerates functional decline of kidney and vasculature in aging. Thus, the long-term dietary L-arginine supplementation should be avoided particularly in elderly population.

[Prevention of the progression of atherosclerosis and aging through nitric oxide (NO)]

The EDRF discovered in 1986 by Furchgott was later identified as NO by Ignarro. NO was a widely noted gas with diverse functions, having arginine (L-Arg) as a substrate for the NO synthase (NOS). L-Arg and L-citrulline (L-Cit) have long been associated with the urea cycle. L-Cit was produced with NO by the reaction of L-Arg and oxygen. It was shown that administration of L-Arg in animals and humans caused vasodilation and anti-arteriosclerosis effects. Despite the arginine paradox ratio of intracellular arginine concentration to the Km value of NOS gaining widespread attention, advanced arteriosclerosis is known to reduce vascular reactivity towards L-Arg. In recent years, the anti-arteriosclerosis and anti-cell aging effects of the reactive substance citrulline (L-Cit) have been studied. L-Cit and L-Arg combination therapy are starting to be considered in various clinical applications as well.

Where Metabolism Meets Senescence: Focus on Endothelial Cells

Despite the decline in their proliferative potential, senescent cells display a high metabolic activity. Senescent cells have been shown to acquire a more glycolytic state even in presence of high oxygen levels, in a way similar to cancer cells. The diversion of pyruvate, the final product of glycolysis, away from oxidative phosphorylation results in an altered bioenergetic state and may occur as a response to the enhanced oxidative stress caused by the accumulation of dysfunctional mitochondria. This metabolic shift leads to increased AMP/ATP and ADP/ATP ratios, to the subsequent AMPK activation, and ultimately to p53-mediated growth arrest. Mounting evidences suggest that metabolic reprogramming is critical to direct considerable amounts of energy toward specific activities related to the senescent state, including the senescence-associated secretory phenotype (SASP) and the modulation of immune responses within senescent cell tissue microenvironment. Interestingly, despite the relative abundance of oxygen in the vascular compartment, healthy endothelial cells (ECs) produce most of their ATP content from the anaerobic conversion of glucose to lactate. Their high glycolytic rate further increases during senescence. Alterations in EC metabolism have been identified in age-related diseases (ARDs) associated with a dysfunctional vasculature, including atherosclerosis, type 2 diabetes and cardiovascular diseases. In particular, higher production of reactive oxygen species deriving from a variety of enzymatic sources, including uncoupled endothelial nitric oxide synthase and the electron transport chain, causes DNA damage and activates the NAD+-consuming enzymes polyADP-ribose polymerase 1 (PARP1). These non-physiological mechanisms drive the impairment of the glycolytic flux and the diversion of glycolytic intermediates into many pathological pathways. Of note, accumulation of senescent ECs has been reported in the context of ARDs. Through their pro-oxidant, pro-inflammatory, vasoconstrictor, and prothrombotic activities, they negatively impact on vascular physiology, promoting both the onset and development of ARDs. Here, we review the current knowledge on the cellular senescence-related metabolic changes and their contribution to the mechanisms underlying the pathogenesis of ARDs, with a particular focus on ECs. Moreover, current and potential interventions aimed at modulating EC metabolism, in order to prevent or delay ARD onset, will be discussed.

A relative L-arginine deficiency contributes to endothelial dysfunction across the stages of the menopausal transition

Vascular endothelial function declines across the menopause transition in women. We tested the hypothesis that reduced availability of the endothelial nitric oxide synthase [eNOS] substrate L‐arginine is an underlying mechanism to vascular endothelial dysfunction across menopause stages. Endothelial function (brachial artery flow‐mediated dilation [FMD]) and plasma markers of L‐arginine metabolism (citrulline, N^G‐mono‐methyl‐ւ‐arginine [L‐NMMA] asymmetric dimethylarginine [ADMA] and N^G‐N^′G‐dimethyl‐l‐arginine [SDMA]), were measured in 129 women: 36 premenopausal (33 ± 7 years), 16 early‐ (49 ± 3 years) or 21 late‐ (50 ± 4 years) perimenopausal, and 21 early‐ (55 ± 3 years) or 35 late‐ (61 ± 4 years) postmenopausal. FMD was progressively reduced across menopause stages (P < 0.001). Menopause stage was associated with L‐arginine concentrations (P = 0.012), with higher levels in early postmenopausal compared to early and late perimenopausal women (P < 0.05). The methylarginine and eNOS inhibitor L‐NMMA was higher in early and late postmenopausal women compared to premenopausal and early and late perimenopausal women (all P < 0.001), and was inversely correlated with FMD (r = −0.30, P = 0.001). The L‐arginine/L‐NMMA ratio, a potential biomarker of relative L‐arginine levels, was lower in postmenopausal compared to either premenopausal or perimenopausal women (both P < 0.001), and was positively correlated with FMD (r = 0.33, P < 0.001). There were no differences in plasma citrulline, ADMA or SDMA across groups. These data suggest that a relative L‐arginine deficiency may be a mechanism underlying the decline in endothelial function with the menopause transition in women. The relative L‐arginine deficiency may be related to elevated levels of the methylarginine L‐NMMA, which would compete with L‐arginine for eNOS and for intracellular transport, reducing NO biosynthesis.

Administration of L-arginine plus L-citrulline or L-citrulline alone successfully retarded endothelial senescence

L-citrulline and L-arginine supplementation has been shown to have several beneficial effects on the cardiovascular system. Nitric oxide (NO) protects against the progression of atherosclerosis and is synthesized by nitric oxide synthase (NOS), which converts L-arginine (L-Arg) into L-citrulline (L-Cit). Our previous study revealed that chronic administration of a combination of L-Cit and L- Arg has a better therapeutic effect on high cholesterol-induced atherosclerosis in rabbits. We investigated how L-Arg and L-Cit affect endothelial function, aging and atherosclerosis. Following a 3-day stimulation of human umbilical venous endothelial cells (HUVECs) with high glucose (HG: 22 mM) and L-Arg (300 μM), L-Cit (300 μM) or L-Arg plus L-Cit (LALC: each 150 μM) supplementation, endothelial senescence and function were evaluated. These amino acids were also administered to dyslipidemic type 2 diabetic (ZDFM) rats fed a high cholesterol diet. They were fed L-Arg or L-Cit or LALC for four weeks. Aortic senescence was investigated by measuring senescence-associated ß-galactosidase (SA-ß-gal), telomerase activity, DNA damage and p16^INK4a protein expression. Only L-Cit and LALC supplementation retarded the HG-induced endothelial senescence, as evaluated by SA-ß-gal activity, a widely used marker of cellular senescence, p16^INK4a expression, a senescence-related protein, and DNA damage. Under HG conditions, L-Cit and LCLA restored telomerase activity to levels observed under normal glucose (NG) conditions. Under HG conditions, L-Cit decreased ROS production, as measured by CM-H₂DCFDA and the expression of p67^phox, a major component of NADPH oxidase. Under HG conditions, L-Cit and LALC increased NO production, as measured by DAF-2AM. Endothelial NO synthase (eNOS) and phosphorylated eNOS were decreased under HG conditions and L-Cit and LALC significantly increased these levels. Arginase 2 protein expression increased under the HG conditions, and L-Cit and LALC significantly attenuated this effect. In ZDFM rats, SA-ß-gal activity was detected on the aortic endothelial surface; however, L-Cit and LALC reduced these levels. L-Cit and LALC both decreased the proportion of senescent cells. Furthermore, treatment with LALC for 4 weeks increased plasma NO production. Therefore conclusively, L-citrulline supplementation rescued NO levels better than L-arginine supplementation by inhibiting ROS production and arginase 2 protein expression. Consequently, L-Cit and LCLA supplementation retaeded HG-induced endothelial senescence.

Does l-citrulline supplementation improve exercise blood flow in older adults?

NEW FINDINGS

What is the central question of this study? Does short-term supplementation with l-citrulline in order to increase l-arginine improve exercise blood flow and peripheral dilatation responses to exercise in older adults? What is the main finding and its importance? l-Citrulline increased femoral blood flow by 11% and vascular conductance by 14% during lower-limb exercise in older men, whereas no changes were observed in older women. This modest improvement in bulk muscle blood flow in older men has implications for altering muscle metabolism that may result in enhanced exercise tolerance in older adults. l-Citrulline (Cit) increases l-arginine (Arg), the primary substrate for nitric oxide biosynthesis. We tested the hypothesis that muscle blood flow during exercise would be enhanced by Cit supplementation in older adults. Femoral artery blood flow was measured during calf exercise using Doppler ultrasound, and vascular conductance (FVC) was calculated in 25 older adults (13 women and 12 men) before and after 14 days of Cit (6 g day^-1 ) and placebo (maltodextrin) in a randomized, double-blind, crossover study. Plasma [Arg] and resting blood pressure were also measured before and after each condition. Women and men were analysed separately because of significant sex-by-condition interactions for the change in exercise blood flow and FVC. Plasma [Arg] was increased by 30 and 35% after Cit (P < 0.01) in women and men, respectively, with no change after placebo. Citrulline lowered diastolic blood pressure in men (75 ± 9 versus 71 ± 6 mmHg, P = 0.02), but this variable remained unchanged in women. Blood flow and FVC during exercise at higher workloads were increased after Cit in men (flow, 521 ± 134 versus 584 ± 166 ml min^-1 , P = 0.04; FVC, 5.0 ± 1.5 versus 5.8 ± 1.7 m, min mmHg^-1 , P = 0.01) but were not different after placebo. These variables were not altered by Cit in women. Adjusting for baseline diastolic blood pressure removed (P = 0.10) the difference in FBF and FVC following Cit in men. These results indicate that l-citrulline has a modest effect of improving muscle blood flow during submaximal exercise in older men.

Arginase-II Promotes Tumor Necrosis Factor-α Release From Pancreatic Acinar Cells Causing β-Cell Apoptosis in Aging

Aging is associated with glucose intolerance. Arginase-II (Arg-II), the type-II L-arginine-ureahydrolase, is highly expressed in pancreas. However, its role in regulation of pancreatic β-cell function is not known. Here we show that female (not male) mice deficient in Arg-II (Arg-II^-/-) are protected from age-associated glucose intolerance and reveal greater glucose induced-insulin release, larger islet size and β-cell mass, and more proliferative and less apoptotic β-cells compared with the age-matched wild-type (WT) controls. Moreover, Arg-II is mainly expressed in acinar cells and is upregulated with aging, which enhances p38 mitogen-activated protein kinase (p38 MAPK) activation and release of tumor necrosis factor-α (TNF-α). Accordingly, conditioned medium of isolated acinar cells from old WT (not Arg-II^-/-) mice contains higher TNF-α levels than the young mice and stimulates β-cell apoptosis and dysfunction, which are prevented by a neutralizing anti-TNF-α antibody. In acinar cells, our study demonstrates an age-associated Arg-II upregulation, which promotes TNF-α release through p38 MAPK leading to β-cell apoptosis, insufficient insulin secretion, and glucose intolerance in female rather than male mice.

Roles of Vascular Oxidative Stress and Nitric Oxide in the Pathogenesis of Atherosclerosis

Major reactive oxygen species (ROS)–producing systems in vascular wall include NADPH (reduced form of nicotinamide adenine dinucleotide phosphate) oxidase, xanthine oxidase, the mitochondrial electron transport chain, and uncoupled endothelial nitric oxide (NO) synthase. ROS at moderate concentrations have important signaling roles under physiological conditions. Excessive or sustained ROS production, however, when exceeding the available antioxidant defense systems, leads to oxidative stress. Animal studies have provided compelling evidence demonstrating the roles of vascular oxidative stress and NO in atherosclerosis. All established cardiovascular risk factors such as hypercholesterolemia, hypertension, diabetes mellitus, and smoking enhance ROS generation and decrease endothelial NO production. Key molecular events in atherogenesis such as oxidative modification of lipoproteins and phospholipids, endothelial cell activation, and macrophage infiltration/activation are facilitated by vascular oxidative stress and inhibited by endothelial NO. Atherosclerosis develops preferentially in vascular regions with disturbed blood flow (arches, branches, and bifurcations). The fact that these sites are associated with enhanced oxidative stress and reduced endothelial NO production is a further indication for the roles of ROS and NO in atherosclerosis. Therefore, prevention of vascular oxidative stress and improvement of endothelial NO production represent reasonable therapeutic strategies in addition to the treatment of established risk factors (hypercholesterolemia, hypertension, and diabetes mellitus).

Oral L-citrulline supplementation enhances cycling time trial performance in healthy trained men: Double-blind randomized placebo-controlled 2-way crossover study

Background

Many human studies report that nitric oxide (NO) improves sport performance. This is because NO is a potential modulator of blood flow, muscle energy metabolism, and mitochondrial respiration during exercise. L-Citrulline is an amino acid present in the body and is a potent endogenous precursor of L-arginine, which is a substrate for NO synthase. Here, we investigated the effect of oral L-citrulline supplementation on cycling time trial performance in humans.

Methods

A double-blind randomized placebo-controlled 2-way crossover study was employed. Twenty-two trained males consumed 2.4 g/day of L-citrulline or placebo orally for 7 days. On Day 8 they took 2.4 g of L-citrulline or placebo 1 h before a 4-km cycling time trial. Time taken to complete the 4 km cycle, along with power output/VO₂ ratio (PO/VO₂), plasma nitrite and nitrate (NOx) and amino acid levels, and visual analog scale (VAS) scores, was evaluated.

Results

L-Citrulline supplementation significantly increased plasma L-arginine levels and reduced completion time by 1.5 % (p < 0.05) compared with placebo. Moreover, L-citrulline significantly improved subjective feelings of muscle fatigue and concentration immediately after exercise.

Conclusions

Oral L-citrulline supplementation reduced the time take to complete a cycle ergometer exercise trial.

Trial registration

Current Controlled Trials UMIN000014278.

Arginase inhibition augments nitric oxide production and facilitates left ventricular systolic function in doxorubicin-induced cardiomyopathy in mice

A metabolizing enzyme arginase can decrease nitric oxide (NO) production by competing with NO synthase for arginine as a substrate, but its pathophysiological role in heart failure remains unknown. We aimed to investigate the effect of pharmacological inhibition of arginase on left ventricular function in doxorubicin‐induced cardiomyopathy in mice. Doxorubicin administration for 5 weeks significantly increased protein expression levels or activity of arginase in the lungs and liver, and caused moderate increase in arginase 2 expression in the aorta. In the lungs, accumulated interstitial cells strongly expressed both arginase 1 and arginase 2 by doxorubicin administration. Echocardiography revealed that administration of a potent, reversible arginase inhibitor N‐omega‐hydroxy‐nor‐l‐arginine completely reversed doxorubicin‐induced decrease in the ejection fraction, in parallel with expression levels of BNP mRNA, without affecting apoptosis, hypertrophy, fibrosis, or macrophage infiltration in the left ventricle. Arginase inhibition reversibly lowered systolic blood pressure, and importantly, it recovered doxorubicin‐induced decline in NO concentration in the serum, lungs, and aorta. Furthermore, arginase inhibition stimulated NO secretion from aortic endothelial cells and peritoneal macrophages in vitro. In conclusion, pharmacological inhibition of arginase augmented NO concentration in the serum, lungs, and aorta, promoted NO‐mediated decrease in afterload for left ventricle, and facilitated left ventricular systolic function in doxorubicin‐induced cardiomyopathy in mice.

e12130

This figure shows that administration of an arginase inhibitor nor‐ NOHA facilitates LV systolic function in murine Dox‐induced cardiomyopathy.

Regulation of endothelial nitric oxide synthase and high-density lipoprotein quality by estradiol in cardiovascular pathology

Estrogens have been recognized, in the last 3 decades, as important hormones in direct and indirect modulation of vascular health. In addition to their direct benefit on cardiovascular health, the presence of esterified estrogen in the lipid core of high-density lipoprotein (HDL) particles indirectly contributes to atheroprotection by significantly improving HDL quality and functionality. Estrogens modulate their physiological activity via genomic and nongenomic mechanisms. Genomic mechanisms are thought to be mediated directly by interaction of the hormone receptor complex with the hormone response elements that regulate gene expression. Nongenomic mechanisms are thought to occur via interaction of the estrogen with membrane-bound receptors, which rapidly activate intracellular signaling without binding of the hormone receptor complex to its hormone response elements. Estradiol in particular mediates early and late endothelial nitric oxide synthase (eNOS) activation via interaction with estrogen receptors through both nongenomic and genomic mechanisms. In the vascular system, the primary endogenous source of nitric oxide (NO) generation is eNOS. Nitric oxide primarily influences blood vessel relaxation, the heart rate, and myocyte contractility. The abnormalities in expression and/or functions of eNOS lead to the development of cardiovascular diseases, both in animals and in humans. Although considerable research efforts have been dedicated to understanding the mechanisms of action of estradiol in regulating cardiac eNOS, more research is needed to fully understand the details of such mechanisms. This review focuses on recent findings from animal and human studies on the regulation of eNOS and HDL quality by estradiol in cardiovascular pathology.

Vasomotor regulation of coronary microcirculation by oxidative stress: role of arginase

Overproduction of reactive oxygen species, i.e., oxidative stress, is associated with the activation of redox signaling pathways linking to inflammatory insults and cardiovascular diseases by impairing endothelial function and consequently blood flow dysregulation due to microvascular dysfunction. This review focuses on the regulation of vasomotor function in the coronary microcirculation by endothelial nitric oxide (NO) during oxidative stress and inflammation related to the activation of L-arginine consuming enzyme arginase. Superoxide produced in the vascular wall compromises vasomotor function by not only scavenging endothelium-derived NO but also inhibiting prostacyclin synthesis due to formation of peroxynitrite. The upregulation of arginase contributes to the deficiency of endothelial NO and microvascular dysfunction in various vascular diseases by initiating or following oxidative stress and inflammation. Hydrogen peroxide, a diffusible and stable oxidizing agent, exerts vasodilator function and plays important roles in the physiological regulation of coronary blood flow. In occlusive coronary ischemia, the release of hydrogen peroxide from the microvasculature helps to restore vasomotor function of coronary collateral microvessels with exercise training. However, excessive production and prolonged exposure of microvessels to hydrogen peroxide impairs NO-mediated endothelial function by reducing L-arginine availability through hydroxyl radical-dependent upregulation of arginase. The redox signaling can be a double-edged sword in the microcirculation, which helps tissue survival in one way by improving vasomotor regulation and elicits oxidative stress and tissue injury in the other way by causing vascular dysfunction. The impact of vascular arginase on the development of vasomotor dysfunction associated with angiotensin II receptor activation, hypertension, ischemia-reperfusion, hypercholesterolemia, and inflammatory insults is discussed.

Role of arginase in vessel wall remodeling

Arginase metabolizes the semi-essential amino acid l-arginine to l-ornithine and urea. There are two distinct isoforms of arginase, arginase I and II, which are encoded by separate genes and display differences in tissue distribution, subcellular localization, and molecular regulation. Blood vessels express both arginase I and II but their distribution appears to be cell-, vessel-, and species-specific. Both isoforms of arginase are induced by numerous pathologic stimuli and contribute to vascular cell dysfunction and vessel wall remodeling in several diseases. Clinical and experimental studies have documented increases in the expression and/or activity of arginase I or II in blood vessels following arterial injury and in pulmonary and arterial hypertension, aging, and atherosclerosis. Significantly, pharmacological inhibition or genetic ablation of arginase in animals ameliorates abnormalities in vascular cells and normalizes blood vessel architecture and function in all of these pathological states. The detrimental effect of arginase in vascular remodeling is attributable to its ability to stimulate vascular smooth muscle cell and endothelial cell proliferation, and collagen deposition by promoting the synthesis of polyamines and l-proline, respectively. In addition, arginase adversely impacts arterial remodeling by directing macrophages toward an inflammatory phenotype. Moreover, the proliferative, fibrotic, and inflammatory actions of arginase in the vasculature are further amplified by its capacity to inhibit nitric oxide (NO) synthesis by competing with NO synthase for substrate, l-arginine. Pharmacologic or molecular approaches targeting specific isoforms of arginase represent a promising strategy in treating obstructive fibroproliferative vascular disease.

The role of arginase I in diabetes-induced retinal vascular dysfunction in mouse and rat models of diabetes

AIMS/HYPOTHESIS

A reduction in retinal blood flow occurs early in diabetes and is likely to be involved in the development of diabetic retinopathy. We hypothesise that activation of the arginase pathway could have a role in the vascular dysfunction of diabetic retinopathy.

METHODS

Experiments were performed using a mouse and rat model of streptozotocin (STZ)-induced diabetes for in vivo and ex vivo analysis of retinal vascular function. For in vivo studies, mice were infused with the endothelial-dependent vasodilator acetylcholine (ACh) or the endothelial-independent vasodilator sodium nitroprusside (SNP), and vasodilation was assessed using a fundus microscope. Ex vivo assays included pressurised vessel myography, western blotting and arginase activity measurements.

RESULTS

ACh-induced retinal vasodilation was markedly impaired in diabetic mice (40% of control values), whereas SNP-induced dilation was not altered. The diabetes-induced vascular dysfunction was markedly blunted in mice lacking one copy of the gene encoding arginase I and in mice treated with the arginase inhibitor 2(S)-amino-6-boronohexanoic acid. Ex vivo studies performed using pressure myography and central retinal arteries isolated from rats with STZ-induced diabetes showed a similar impairment of endothelial-dependent vasodilation that was partially blunted by pretreatment of the isolated vessels with another arginase inhibitor, (S)-2-boronoethyl-L-cysteine. The diabetes-induced vascular alterations were associated with significant increases in both arginase I protein levels and total arginase activity.

CONCLUSIONS/INTERPRETATION

These results indicate that, in the mouse and rat model, diabetes-induced increases in arginase I were involved in the diabetes-induced impairment of retinal blood flow by a mechanism involving vascular endothelial cell dysfunction.

Sexual dimorphism in rodent models of hypertension and atherosclerosis

Approximately one third of all deaths are attributed to cardiovascular disease (CVD), making it the biggest killer worldwide. Despite a number of therapeutic options available, the burden of CVD morbidity continues to grow indicating the need for continued research to address this unmet need. In this respect, investigation of the mechanisms underlying the protection that premenopausal females enjoy from cardiovascular-related disease and mortality is of interest. In this review, we discuss the essential role that rodent animal models play in enabling this field of research. In particular, we focus our discussion on models of hypertension and atherosclerosis.

Altered neuronal nitric oxide synthase in the aging vascular system: implications for estrogens therapy

Ovarian dysfunction at any age is associated with increased cardiovascular risk in women; however, therapeutic effects of exogenous estrogens are age dependent. Estradiol (E2) activates neuronal nitric oxide synthase (nNOS) in vascular cells. Because nNOS is prone to uncoupling under unfavorable biochemical conditions (as seen in aging), E2 stimulation of nNOS may lack vascular benefits in aging. Small mesenteric arteries were isolated from female Sprague Dawley rats, 3 or 12 months old, who were ovariectomized (Ovx) and treated with placebo or E2 for 4 wk. Vascular relaxation to exogenous E2 (0.001-100 μmol/liter) ± selective nNOS inhibitor (N-propyl-l-arginine, 2 μmol/liter) or pan-NOS inhibitor [Nω-nitro-l-arginine methyl ester (l-NAME), 100 μmol/liter] was examined on wire myograph. NOS expression was measured by Western blotting in thoracic aortas, in which superoxide generation was detected as dihydroethidium (DHE) fluorescence. E2 relaxations were impaired in Ovx conditions. E2 treatment (4 wk) normalized vascular function in young rats only. Both l-N-propyl-l-arginine and l-NAME blunted E2 relaxation in young controls, but only l-NAME did so in aging controls. NOS inhibition had no effect on acute E2 relaxation in Ovx rats, regardless of age or treatment. nNOS expression was similar in all animal groups. However, nNOS inhibition increased DHE fluorescence in young controls, whereas it reduced it in aging or Ovx animals. In E2-treated animals of either age, superoxide production was NOS independent. In conclusion, nNOS contributed to vascular relaxation in young, but not aging rats, where its enzymatic function shifted toward superoxide production. Thus, nNOS dysfunction may explain a mechanism of impaired E2 signaling in aging conditions.

Endothelial arginase II and atherosclerosis

Atherosclerotic vascular disease is the leading cause of morbidity and mortality in developed countries. While it is a complex condition resulting from numerous genetic and environmental factors, it is well recognized that oxidized low-density lipoprotein produces pro-atherogenic effects in endothelial cells (ECs) by inducing the expression of adhesion molecules, stimulating EC apoptosis, inducing superoxide anion formation and impairing protective endothelial nitric oxide (NO) formation. Emerging evidence suggests that the enzyme arginase reciprocally regulates NO synthase and NO production by competing for the common substrate L-arginine. As oxidized LDL (OxLDL) results in arginase activation/upregulation, it appears to be an important contributor to endothelial dysfunction by a mechanism that involves substrate limitation for endothelial NO synthase (eNOS) and NO synthesis. Additionally, arginase enhances production of reactive oxygen species by eNOS. Arginase inhibition in hypercholesterolemic (ApoE^-/-) mice or arginase II deletion (ArgII^-/-) mice restores endothelial vasorelaxant function, reduces vascular stiffness and markedly reduces atherosclerotic plaque burden. Furthermore, arginase activation contributes to vascular changes including polyamine-dependent vascular smooth muscle cell proliferation and collagen synthesis. Collectively, arginase may play a key role in the prevention and treatment of atherosclerotic vascular disease.

Oxidized low-density lipoprotein inhibits nitric oxide-mediated coronary arteriolar dilation by up-regulating endothelial arginase I

Oxidized low-density lipoprotein (OxLDL) causes impairment of endothelium-dependent, nitric oxide (NO)-mediated vasodilation involving l-arginine deficiency. However, the underlying mechanism remains elusive. Since arginase and endothelial NO synthase (eNOS) share the substrate l-arginine, we hypothesized that OxLDL may reduce l-arginine availability to eNOS for NO production, and thus vasodilation, by up-regulating arginase. To test this hypothesis, porcine subepicardial arterioles (70-130 μm) were isolated for vasomotor study and for immunohistochemical detection of arginase and eNOS expressions. The coronary arterioles dilated dose-dependently to the endothelium-dependent NO-mediated vasodilator serotonin. This vasodilation was inhibited in the same manner by NOS inhibitor N(G)-nitro-l-arginine methyl ester and by lumenal OxLDL (0.5 mg protein/mL). The inhibitory effect of OxLDL was reversed after treating the vessels with either l-arginine (3 mM) or arginase inhibitor difluoromethylornithine (DFMO; 0.4 mM). Consistent with vasomotor alterations, OxLDL inhibited serotonin-induced NO release from coronary arterioles and this inhibition was reversed by DFMO. Vascular arginase activity was significantly elevated by OxLDL. Immunohistochemical analysis indicated that OxLDL increased arginase I expression in the vascular wall without altering eNOS expression. Taken together, these results suggest that OxLDL up-regulates arginase I, which contributes to endothelial dysfunction by reducing l-arginine availability to eNOS for NO production and thus vasodilation.

Short-term effects of L-citrulline supplementation on arterial stiffness in middle-aged men

BACKGROUND

Nitric oxide (NO) plays a key role in the maintenance of vascular tone, contributing to the functional regulation of arterial stiffness. Although oral L-citrulline could become the effective precursor of L-arginine (substrate for endothelial NO synthase) via the L-citrulline/ L-arginine pathway, little is known about the efficacy of L-citrulline application on arterial stiffness.

OBJECTIVE

We examined the short-term effects of L-citrulline supplementation on arterial stiffness in humans.

METHODS

In a double-blind, randomized, placebo-controlled parallel-group trial, 15 healthy male subjects (age: 58.3 ± 4.4 years) with brachial-ankle pulse wave velocity (baPWV; index of arterial stiffness >1400 cm/sec) were given 5.6g/day of L-citrulline (n=8) or placebo (n=7) for 7 days. baPWV and various clinical parameters were measured before (baseline) and after oral supplementation of L-citrulline or placebo.

RESULTS

Compared with the placebo group, baPWV was significantly reduced in the L-citrulline group (p<0.01). No significant differences in blood pressure (BP) were found between the two groups, and no correlation was observed between BP and baPWV. The serum nitrogen oxide (NOx, the sum of nitrite plus nitrate) and NO metabolic products were significantly increased only in the L-citrulline group (p<0.05). Plasma citrulline, arginine and the ratio of arginine/asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase (arginine/ADMA ratio) were significantly increased in the L-citrulline group compared with the placebo group (p<0.05, p<0.01, p<0.05, respectively). Moreover, there was a correlation between the increase of plasma arginine and the reduction of baPWV (r=-0.553, p<0.05).

CONCLUSION

These findings suggest that short-term L-citrulline supplementation may functionally improve arterial stiffness, independent of blood pressure, in humans.

Possibility of the regression of atherosclerosis through the prevention of endothelial senescence by the regulation of nitric oxide and free radical scavengers

In the elderly, atherosclerotic diseases such as stroke and myocardial infarction occupy a major part of their causes of death and care. The elderly always have atherosclerosis in their aorta and other arteries and are exposed to risk of attacks. It is the elderly who should receive its safe, harmless and advanced treatment. Advanced stage of atherosclerosis in the elderly is progressed by complicated risk factors such as dyslipidemia and diabetes mellitus and specific risk factors for the elderly, aging (and menopause). Treatment of atherosclerotic disease may need special ones targeted for the elderly. Recent studies reported that frequencies of dyslipidemia were not decreased in the older oldest. In the elderly, impaired glucose tolerance occurs and it progresses atherosclerosis. Endothelial dysfunction like impairment of nitric oxide (NO) bioavailability also progresses atherosclerosis. Although we tried to regress the high cholesterol diet-induced atherosclerosis in rabbit aorta with a normal diet with or without statin, regression could not be achieved. NO targeting gene therapy (adenovirus endothelial nitric oxide synthase [eNOS] gene vector) regressed 20% of atherosclerotic lesions through reduction of lipid contents, however, a more integrated strategy is important for complete regression. We paid attention to NO bioavailability and developed two ways of increasing it in atherosclerosis: citrulline therapy and arginase II inhibition by estrogen. Further, we found a close relation between atherosclerosis and endothelial senescence and that NO can prevent it, especially in a diabetic model. Taken together, regression of atherosclerosis can be achieved by not only regulation of various risk factors but regulation of the cross-talk of NO and free radicals.

Need for research on estrogen receptor function: importance for postmenopausal hormone therapy and atherosclerosis

BACKGROUND

Cardiovascular disease is the leading cause of morbidity and mortality in men and women worldwide. Although rare in premenopausal women, its incidence rises sharply after menopause, indicating atheroprotective effects of endogenous estrogens.

OBJECTIVE

This review discusses the differential effects of estrogen receptor function on atherosclerosis progression in pre- and postmenopausal women, including aspects of gender differences in vascular physiology of estrogens and androgens.

METHODS

Recent advances in the understanding of the pathogenesis of atherosclerosis, estrogen receptor function, and hormone therapy are reviewed, with particular emphasis on clinical and molecular issues.

RESULTS

Whether hormone therapy can improve cardiovascular health in postmenopausal women remains controversial. Current evidence suggests that the vascular effects of estrogen are affected by the stage of reproductive life, the time since menopause, and the extent of subclinical atherosclerosis. The mechanisms of vascular responsiveness to sex steroids during different stages of atherosclerosis development remain poorly understood in women and men.

CONCLUSION

In view of the expected increase in the prevalence of atherosclerotic vascular disease worldwide due to population aging, research is needed to determine the vascular mechanism of endogenous and exogenous sex steroids in patients with atherosclerosis. Such research may help to define new strategies to improve cardiovascular health in women and possibly also in men.

Arginase I induction by modified lipoproteins in macrophages: a peroxisome proliferator-activated receptor-gamma/delta-mediated effect that links lipid metabolism and immunity

Macrophages are phagocytic cells that play essential roles in innate immunity and lipid homeostasis. The uptake of modified lipoproteins is an important early event in the development of atherosclerosis. We analyzed the ability of modified low-density lipoprotein (LDL) (oxidized and acetylated) to alter the expression and activity of arginases (ArgI and ArgII) in macrophages. We show that ArgI expression is potently induced by both oxidized and acetylated LDL in macrophages. We further show that this effect is mediated by peroxisome proliferator-activated receptors (PPAR). ArgI expression is highly responsive to agonists for PPARgamma and PPARdelta but not PPARalpha. Moreover, the induction of ArgI by both PPAR agonists and IL-4 is blocked in macrophages from PPARgamma- and PPARdelta-deficient mice. Functionally, PPAR activity induces macrophage activation toward a more Th2 immune phenotype in a model of Leishmania major infection. We show that PPARgamma and -delta ligands promote intracellular amastigote growth in infected macrophages, and this effect is dependent on both PPAR expression and Arg activity. Collectively, our results strongly suggest that ArgI is a key marker of the alternative program triggered by PPAR in macrophages.

Models of Gender Differences in Cardiovascular Disease

Clinical observations made over several decades support the existence of gender differences in cardiovascular disease prevalence and severity. For example, women exhibit a delay in the onset of vascular disease compared to men and the temporal link between menopause and the rise in vascular events in women suggests that ovarian hormones may be important in reducing the risk of vascular disease in women. Gender differences have also been observed in the severity and outcome of myocardial diseases such that women with heart failure have a better prognosis than men coupled with gender-specific patterns of ventricular remodeling. These clinical observations have fostered great interest in understanding the mechanisms of gender differences in cardiovascular diseases with the goal being to identify novel therapeutic targets. The purpose of this review is to describe animal models of cardiovascular disease that have demonstrated clear gender differences in the pathophysiologic responses to a given stimulus. Animal models from two broad areas of cardiovascular investigation will be highlighted: vascular disease and heart failure.

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.