Mitochondrial arginase II constrains endothelial NOS-3 activity

Endothelial Cell-Specific Prolyl Hydroxylase-2 Deficiency Augments Angiotensin II-Induced Arterial Stiffness and Cardiac Pericyte Recruitment in Mice

BACKGROUND

Endothelial prolyl hydroxylase-2 (PHD2) is essential for pulmonary remodeling and hypertension. In the present study, we investigated the role of endothelial PHD2 in angiotensin II-mediated arterial stiffness, pericyte recruitment, and cardiac fibrosis.

METHODS AND RESULTS

Chondroitin sulfate proteoglycan 4 tracing reporter chondroitin sulfate proteoglycan 4- red fluorescent protein (DsRed) transgenic mice were crossed with PHD2flox/flox (PHD2f/f) mice and endothelial-specific knockout of PHD2 (PHD2ECKO) mice. Transgenic PHD2f/f (TgPHD2f/f) mice and TgPHD2ECKO mice were infused with angiotensin II for 4 weeks. Arterial thickness, stiffness, and histological and immunofluorescence of pericytes and fibrosis were measured. Infusion of TgPHD2f/f mice with angiotensin II resulted in a time-dependent increase in pulse-wave velocity. Angiotensin II-induced pulse-wave velocity was further elevated in the TgPHD2ECKO mice. TgPHD2ECKO also reduced coronary flow reserve compared with TgPHD2f/f mice infused with angiotensin II. Mechanistically, knockout of endothelial PHD2 promoted aortic arginase activity and angiotensin II-induced aortic thickness together with increased transforming growth factor-β1 and ICAM-1/VCAM-1 expression in coronary arteries. TgPHD2f/f mice infused with angiotensin II for 4 weeks exhibited a significant increase in cardiac fibrosis and hypertrophy, which was further developed in the TgPHD2ECKO mice. Chondroitin sulfate proteoglycan 4 pericyte was traced by DsRed+ staining and angiotensin II infusion displayed a significant increase of DsRed+ pericytes in the heart, as well as a deficiency of endothelial PHD2, which further promoted angiotensin II-induced pericyte increase. DsRed+ pericytes were costained with fibroblast-specific protein 1 and α-smooth muscle actin for measuring pericyte-myofibroblast cell transition. The knockout of endothelial PHD2 increased the amount of DsRed+/fibroblast-specific protein 1+ and DsRed+/α-smooth muscle actin+ cells induced by angiotensin II infusion.

CONCLUSIONS

Knockout of endothelial PHD2 enhanced angiotensin II-induced cardiac fibrosis by mechanisms involving increasing arterial stiffness and pericyte-myofibroblast cell transitions.

Methylglyoxal-Modified Albumin Effects on Endothelial Arginase Enzyme and Vascular Function

Advanced glycation end products (AGEs) contribute significantly to vascular dysfunction (VD) in diabetes. Decreased nitric oxide (NO) is a hallmark in VD. In endothelial cells, NO is produced by endothelial NO synthase (eNOS) from L-arginine. Arginase competes with NOS for L-arginine to produce urea and ornithine, limiting NO production. Arginase upregulation was reported in hyperglycemia; however, AGEs' role in arginase regulation is unknown. Here, we investigated the effects of methylglyoxal-modified albumin (MGA) on arginase activity and protein expression in mouse aortic endothelial cells (MAEC) and on vascular function in mice aortas. Exposure of MAEC to MGA increased arginase activity, which was abrogated by MEK/ERK1/2 inhibitor, p38 MAPK inhibitor, and ABH (arginase inhibitor). Immunodetection of arginase revealed MGA-induced protein expression for arginase I. In aortic rings, MGA pretreatment impaired acetylcholine (ACh)-induced vasorelaxation, which was reversed by ABH. Intracellular NO detection by DAF-2DA revealed blunted ACh-induced NO production with MGA treatment that was reversed by ABH. In conclusion, AGEs increase arginase activity probably through the ERK1/2/p38 MAPK pathway due to increased arginase I expression. Furthermore, AGEs impair vascular function that can be reversed by arginase inhibition. Therefore, AGEs may be pivotal in arginase deleterious effects in diabetic VD, providing a novel therapeutic target.

L-citrulline increases arginase II protein levels and arginase activity in hypoxic piglet pulmonary artery endothelial cells

The L-arginine precursor, L-citrulline, re-couples endothelial nitric oxide synthase, increases nitric oxide production, and ameliorates chronic hypoxia-induced pulmonary hypertension in newborn pigs. L-arginine can induce arginase, which, in turn, may diminish nitric oxide production. Our major purpose was to determine if L-citrulline increases arginase activity in hypoxic piglet pulmonary arterial endothelial cells, and if so, concomitantly impacts the ability to increase endothelial nitric oxide synthase re-coupling and nitric oxide production. Piglet pulmonary arterial endothelial cells were cultured in hypoxic conditions with L-citrulline (0-3 mM) and/or the arginase inhibitor S-(2-boronoethyl)-L-cysteine. We measured arginase activity and nitric oxide production. We assessed endothelial nitric oxide synthase coupling by measuring endothelial nitric oxide synthase dimers and monomers. L-citrulline concentrations ≥0.5 mM increased arginase activity in hypoxic pulmonary arterial endothelial cells. L-citrulline concentrations ≥0.1 mM increased nitric oxide production and concentrations ≥0.5 mM elevated endothelial nitric oxide synthase dimer-to-monomer ratios. Co-treatment with L-citrulline and S-(2-boronoethyl)-L-cysteine elevated endothelial nitric oxide synthase dimer-to-monomer ratios more than sole treatment. Despite inducing arginase, L-citrulline increased nitric oxide production and endothelial nitric oxide synthase coupling in hypoxic piglet pulmonary arterial endothelial cells. However, these dose-dependent findings raise the possibility that there could be L-citrulline concentrations that elevate arginase to levels that negate improvements in endothelial nitric oxide synthase dysfunction. Moreover, our findings suggest that combining an arginase inhibitor with L-citrulline merits evaluation as a treatment for chronic hypoxia-induced pulmonary hypertension.

Transporters at the Interface between Cytosolic and Mitochondrial Amino Acid Metabolism

Mitochondria are central organelles that coordinate a vast array of metabolic and biologic functions important for cellular health. Amino acids are intricately linked to the bioenergetic, biosynthetic, and homeostatic function of the mitochondrion and require specific transporters to facilitate their import, export, and exchange across the inner mitochondrial membrane. Here we review key cellular metabolic outputs of eukaryotic mitochondrial amino acid metabolism and discuss both known and unknown transporters involved. Furthermore, we discuss how utilization of compartmentalized amino acid metabolism functions in disease and physiological contexts. We examine how improved methods to study mitochondrial metabolism, define organelle metabolite composition, and visualize cellular gradients allow for a more comprehensive understanding of how transporters facilitate compartmentalized metabolism.

Selective inhibition of arginase-2 in endothelial cells but not proximal tubules reduces renal fibrosis

Fibrosis is the final common pathway in the pathophysiology of most forms of chronic kidney disease (CKD). As treatment of renal fibrosis still remains largely supportive, a refined understanding of the cellular and molecular mechanisms of kidney fibrosis and the development of novel compounds are urgently needed. Whether arginases play a role in the development of fibrosis in CKD is unclear. We hypothesized that endothelial arginase-2 (Arg2) promotes the development of kidney fibrosis induced by unilateral ureteral obstruction (UUO). Arg2 expression and arginase activity significantly increased following renal fibrosis. Pharmacologic blockade or genetic deficiency of Arg2 conferred kidney protection following renal fibrosis, as reflected by a reduction in kidney interstitial fibrosis and fibrotic markers. Selective deletion of Arg2 in endothelial cells (Tie2Cre/Arg2fl/fl) reduced the level of fibrosis after UUO. In contrast, selective deletion of Arg2 specifically in proximal tubular cells (Ggt1Cre/Arg2fl/fl) failed to reduce renal fibrosis after UUO. Furthermore, arginase inhibition restored kidney nitric oxide (NO) levels, oxidative stress, and mitochondrial function following UUO. These findings indicate that endothelial Arg2 plays a major role in renal fibrosis via its action on NO and mitochondrial function. Blocking Arg2 activity or expression could be a novel therapeutic approach for prevention of CKD.

The Arginase Pathway in Neonatal Brain Hypoxia-Ischemia

Brain damage after hypoxia-ischemia (HI) occurs in an age-dependent manner. Neuroprotective strategies assumed to be effective in adults might have deleterious effects in the immature brain. In order to create effective therapies, the complex pathophysiology of HI in the developing brain requires exploring new mechanisms. Critical determinants of neuronal survival after HI are the extent of vascular dysfunction, inflammation, and oxidative stress, followed later by tissue repair. The key enzyme of these processes in the human body is arginase (ARG) that acts via the bioavailability of nitric oxide, and the synthesis of polyamines and proline. ARG is expressed throughout the brain in different cells. However, little is known about the effect of ARG in pathophysiological states of the brain, especially hypoxia-ischemia. Here, we summarize the role of ARG during neurodevelopment as well as in various brain pathologies.

Arginase II inhibition prevents interleukin-8 production through regulation of p38 MAPK phosphorylation activated by loss of mitochondrial membrane potential in nLDL-stimulated hAoSMCs

Arginase inhibition exhibits beneficial effects in vascular endothelial and smooth muscle cells. In human aortic smooth muscle cells (hAoSMCs), native low-density lipoprotein (nLDL) induced the production of interleukin-8 (IL-8) that is involved in the pathogenesis of cardiovascular diseases. Therefore, we examined the effect of arginase inhibition on IL-8 production and the underlying mechanism. In hAoSMCs, reverse transcription–PCR, western blotting and immunocytochemistry with MitoTracker confirmed that arginase II was confined predominantly to mitochondria. The mitochondrial membrane potential (MMP) was assessed using tetramethylrhodamine ethyl ester. The MMP decreased upon nLDL stimulation but was restored upon arginase inhibition. MMP loss caused by nLDL was prevented by treatment with the intracellular Ca²⁺ chelator BAPTA-AM. In mitochondrial Ca²⁺ measurements using Rhod-2 AM, increased mitochondrial Ca²⁺ levels by nLDL were inhibited upon preincubation with an arginase inhibitor. Among the polyamines, spermine, an arginase activity-dependent product, caused mitochondrial Ca²⁺ movement. The nLDL-induced MMP change resulted in p38 mitogen-activated protein kinase (MAPK) phosphorylation and IL-8 production and was prevented by the arginase inhibitors BAPTA and ruthenium 360. In isolated AoSMCs from ApoE^−/− mice fed a high-cholesterol diet, arginase activity, p38 MAPK phosphorylation, spermine and mitochondrial Ca²⁺ levels and keratinocyte-derived chemokine (KC) production were increased compared with wild-type (WT) mice. However, in AoSMCs isolated from arginase II-null mice, increases in MMP and decreases in mitochondrial Ca²⁺ levels were noted compared with WT and were associated with p38 MAPK activation and IL-8 production. These data suggest that arginase activity regulates the change in MMP through Ca²⁺ uptake that is essential for p38 MAPK phosphorylation and IL-8 production.

miR-190a-5p participates in the regulation of hypoxia-induced pulmonary hypertension by targeting KLF15 and can serve as a biomarker of diagnosis and prognosis in chronic obstructive pulmonary disease complicated with pulmonary hypertension

PURPOSE

miR-190a-5p expression alters dynamically in response to hypoxia. However, the role of miR-190a-5p expression in hypoxia-induced pulmonary hypertension (PH) remains unclear. We sought to correlate the miR-190a-5p expression levels with the severity, diagnosis, and prognosis of PH in relation to chronic obstructive pulmonary disease (COPD-PH). Additionally, we evaluated the effect of miR-190a-5p through in vitro experiments on human pulmonary endothelial cells (HPECs) that were exposed to hypoxia and in vivo experiments using an animal model of hypoxia-induced PH.

METHODS

Circulating miR-190a-5p levels were measured from 73 patients with PH and 32 healthy controls through quantitative real-time PCR. The levels of miR-190a-5p and the expression of Krüppel-like factor 15 (KLF15) were analyzed in HPECs that were exposed to hypoxia, and the effects of antagomir-190a-5p in mice with chronic hypoxia-induced PH were tested. Target gene analysis was performed by Western blot and luciferase assay.

RESULTS

The miR-190a-5p level was significantly higher in patients with COPD-PH than in the healthy controls. Higher miR-190a-5p levels were associated with a greater severity of COPD-PH. In vitro experiments on HPECs showed that exposure to hypoxia increased the miR-190a-5p levels significantly. KLF15 was validated as a target of miR-190a-5p. Transfection with miR-190a-5p mimicked inhibition of KLF15 expression in HPECs. In the mouse model of PH, antagomir-190a-5p reduced right ventricular systolic pressure and enhanced the KLF15 expression levels in lung tissue.

CONCLUSION

miR-190a-5p regulates hypoxia-induced PH by targeting KLF15. The circulating levels of miR-190a-5p correlate with the severity of COPD-PH, thereby confirming the diagnostic and prognostic value of this parameter in COPD-PH.

Arginase-2 mediates renal ischemia-reperfusion injury

Novel therapeutic interventions for preventing or attenuating kidney injury following ischemia-reperfusion injury (IRI) remain a focus of significant interest. Currently, there are no definitive therapeutic or preventive approaches available for ischemic acute kidney injury (AKI). Our objective is to determine 1) whether renal arginase activity or expression is increased in renal IRI, and 2) whether arginase plays a role in development of renal IRI. The impact of arginase activity and expression on renal damage was evaluated in male C57BL/6J (wild type) and arginase-2 (ARG2)-deficient (Arg2^-/- ) mice subjected to bilateral renal ischemia for 28 min, followed by reperfusion for 24 h. ARG2 expression and arginase activity significantly increased following renal IRI, paralleling the increase in kidney injury. Pharmacological blockade or genetic deficiency of Arg2 conferred kidney protection in renal IRI. Arg2^-/- mice had significantly attenuated kidney injury and lower plasma creatinine and blood urea nitrogen levels after renal IRI. Blocking arginases using S-(2-boronoethyl)-l-cysteine (BEC) 18 h before ischemia mimicked arginase deficiency by reducing kidney injury, histopathological changes and kidney injury marker-1 expression, renal apoptosis, kidney inflammatory cell recruitment and inflammatory cytokines, and kidney oxidative stress; increasing kidney nitric oxide (NO) production and endothelial NO synthase (eNOS) phosphorylation, kidney peroxisome proliferator-activated receptor-γ coactivator-1α expression, and mitochondrial ATP; and preserving kidney mitochondrial ultrastructure compared with vehicle-treated IRI mice. Importantly, BEC-treated eNOS-knockout mice failed to reduce blood urea nitrogen and creatinine following renal IRI. These findings indicate that ARG2 plays a major role in renal IRI, via an eNOS-dependent mechanism, and that blocking ARG2 activity or expression could be a novel therapeutic approach for prevention of AKI.

Cloning and comparative analysis the proximal promoter activities of arginase and agmatinase genes in Apostichopus japonicus

Our previous work demonstrated that Apostichopus japonicus arginase and agmatinase from l-arginine metabolism synergistically compete with NOS under pathogens challenge. Here we conducted a study to further investigate the mechanism in the regulation of arginase and agmatinase genes in l-arginine metabolism using EPC cell system. Luciferase analysis and progressive 5' deletion analysis suggested that Ajagmatinase promoter was a very robust promoter for its transcription, and the core region of Ajarginase promoter was located within -277 bp to -157 bp. Besides, their promoter activities were significantly activated by LPS and l-arginine challenge both in a time- and dose-dependent manners in EPC cells. When different truncated reporter vector and expression vector co-transfection experiment revealed transcription factor NF-κB/Rel and STAT5 could significantly inhibited Ajarginase promoter activity, but not Ajagmatinase. Our findings were provided novel insights into the transcriptional regulation of Ajarginase and Ajagmatinase, and selectively change their expressions might prevent pathogens infection.

A Slow- Compared with a Fast-Release Form of Oral Arginine Increases Its Utilization for Nitric Oxide Synthesis in Overweight Adults with Cardiometabolic Risk Factors in a Randomized Controlled Study

BACKGROUND

Oral l-arginine supplements can have a beneficial effect on nitric oxide (NO)-related functions when subjects have cardiovascular disease risk factors.

OBJECTIVE

The study was designed to determine the utilization for NO synthesis of oral l-arginine as a function of the cardiometabolic risk and the speed of absorption by comparing immediate-release arginine (IR-Arg), as in supplements, and sustained-release arginine (SR-Arg), which mimics the slow release of dietary arginine.

METHODS

In a randomized, single-blind, 2-period crossover, controlled trial (1 wk of treatment, >2 wk of washout), using [(15)N-(15)N-(guanidino)]-arginine for the first morning dose, we compared the bioavailability (secondary outcome) and utilization for NO synthesis (primary outcome) of 1.5 g IR- and SR-Arg 3 times/d in 12 healthy overweight [body mass index (BMI; in kg/m(2)): 25-30] adults with the hypertriglyceridemic waist phenotype [HTW; plasma triglycerides (TGs): >150 mg/dL; waist circumference: >94 cm (men) or >80 cm (women)] and 15 healthy control adults (CON; BMI: 18.5-25; no elevated TGs and waist circumference).

RESULTS

Plasma oral arginine areas under the curve were lower after supplementation with SR-Arg than with IR-Arg (112 ± 52.3 and 142 ± 50.8 μmol ⋅ h/L; P < 0.01). The utilization of oral arginine for NO synthesis was 58% higher in HTW subjects than in CON subjects and higher with SR-Arg than with IR-Arg (P < 0.05 both), particularly in HTW subjects (group-by-treatment interaction, P < 0.05). In HTW subjects administered the SR form, utilization for NO synthesis was 32% higher than with the IR form and 87% higher than in CON subjects who were administered the SR form.

CONCLUSION

In overweight adults with the HTW phenotype, a slow- compared with a fast-release form of oral arginine markedly favors the utilization of arginine for NO synthesis. The utilization of low-dose, slow-release arginine for NO synthesis is higher in overweight adults with the HTW phenotype than in healthy controls, suggesting that the sensitivity of NO synthesis to the dietary arginine supply increases with cardiometabolic risk. The trial was registered at clinicaltrials.gov as NCT02352740.

Protective Role of Arginase II in Cerebral Ischemia and Excitotoxicity

BACKGROUND

Arginase (Arg), one of the enzymes involved in the urea cycle, provides an essential route for the disposal of excess nitrogen resulting from amino acid and nucleotide metabolism. Two reported subtypes of Arg (ArgI and II) compete with nitric oxide synthase (NOS) to use L-arginine as a substrate, and subsequently regulate NOS activity. It has been reported that Arg has significant effects on circulation that suggest the potential role of this enzyme in regulating vascular function. However, the role of Arg following brain damage has not been elucidated. In this study, we hypothesize that the deletion of ArgII will lead to aggravated brain injury following cerebral ischemia and excitotoxicity.

METHODS AND FINDINGS

To test our hypothesis, male C57BL/6 wildtype (WT) and ArgII^-/- mice were subjected to permanent distal middle cerebral artery occlusion and survived for 7 d. Cerebral blood flow (CBF) data revealed a statistically non-significant decrease in CBF in ArgII^-/- mice. However, ArgII^-/- mice had significantly higher neurologic deficit scores and brain infarctions. The hypothesis was further tested in a more specific N-methyl-D-aspartate (NMDA)-induced acute excitotoxic model. WT and ArgII^-/- mice were given a single intrastriatal injection of 15 nmol NMDA. Forty-eight hours later, the excitotoxic brain damage was significantly worse in ArgII^-/- mice. The data from both models confirm the neuroprotective effect of ArgII.

CONCLUSION

Targeting ArgII could be considered an integrative part of a multi-modal approach to fight acute brain damage excitotoxicity, ischemic brain injury, and other forms of brain trauma.

The first description of complete invertebrate arginine metabolism pathways implies dose-dependent pathogen regulation in Apostichopus japonicus

In this study, three typical members representative of different arginine metabolic pathways were firstly identified from Apostichopus japonicus, including nitric oxide synthase (NOS), arginase, and agmatinase. Spatial expression analysis revealed that the AjNOS transcript presented negative expression patterns relative to those of Ajarginase or Ajagmatinase in most detected tissues. Furthermore, Vibrio splendidus-challenged coelomocytes and intestine, and LPS-exposed primary coelomocytes could significantly induce AjNOS expression, followed by obviously inhibited Arginase and AjAgmatinase transcripts at the most detected time points. Silencing the three members with two specific siRNAs in vivo and in vitro collectively indicated that AjNOS not only compete with Ajarginase but also with Ajagmatinase in arginine metabolism. Interestingly, Ajarginase and Ajagmatinase displayed cooperative expression profiles in arginine utilization. More importantly, live pathogens of V. splendidus and Vibrio parahaemolyticus co-incubated with primary cells also induced NO production and suppressed arginase activity in a time-dependent at an appropriate multiplicity of infection (MOI) of 10, without non-pathogen Escherichia coli. When increasing the pathogen dose (MOI = 100), arginase activity was significantly elevated, and NO production was depressed, with a larger magnitude in V. splendidus co-incubation. The present study expands our understanding of the connection between arginine's metabolic and immune responses in non-model invertebrates.

Arginase-2 is cooperatively up-regulated by nitric oxide and histone deacetylase inhibition in human umbilical artery endothelial cells

Arginase-2 counteracts endothelial nitric oxide synthase (eNOS) activity in human endothelium, and its expression is negatively controlled by histone deacetylase (HDAC2). Conversely NO inhibits HDAC and previous studies suggest that arginase-2 is up-regulated by NO. We studied whether NO regulates arginase-2 expression in umbilical artery endothelial cells (HUAEC) increasing ARG2 promoter accessibility. HUAEC exposed to NOC-18 (NO donor, 1-100 μM, 0-24 h) showed an increase in arginase-2 but a decrease in eNOS mRNA levels in a time-dependent manner, with a maximal effect at 100 μM (24 h). Conversely NOS inhibition with L-NAME (100 μM) reduced arginase-2 mRNA and protein levels, an effect reverted by co-incubation with NOC-18. Treatment with TSA paralleled the effects of NO on arginase-2 and eNOS at mRNA and protein levels, with maximal effect at 10 μM. Co-incubation of NOC-18 (100 μM) with a sub-maximal concentration of TSA (1 μM) potentiated the increase in arginase-2 mRNA levels, whilst L-NAME prevented TSA-dependent arginase-2 induction. The effects on arginase-2 mRNA were paralleled by changes in chromatin accessibility, as well as increased levels of H3K9 and H4K12 acetylation, at ARG2 proximal (-579 to -367 and -280 to -73 bp from TSS) and core (-121 to +126 bp from TSS) promoter. Finally NO-dependent arginase-2 induction was prevented by pre-incubation for 10 min with the cysteine blocker MMTS (10 mM). These data showed for the first time that NO up-regulates arginase-2 expression in primary cultured human endothelial cells by an epigenetic-mediated mechanism increasing ARG2 promoter accessibility suggesting a negative regulatory loop for eNOS activity.

Mitochondrial transporters for ornithine and related amino acids: a review

Among the members of the mitochondrial carrier family, there are transporters that catalyze the translocation of ornithine and related substrates, such as arginine, homoarginine, lysine, histidine, and citrulline, across the inner mitochondrial membrane. The mitochondrial carriers ORC1, ORC2, and SLC25A29 from Homo sapiens, BAC1 and BAC2 from Arabidopsis thaliana, and Ort1p from Saccharomyces cerevisiae have been biochemically characterized by transport assays in liposomes. All of them transport ornithine and amino acids with side chains terminating at least with one amine. There are, however, marked differences in their substrate specificities including their affinity for ornithine (KM values in the mM to μM range). These differences are most likely reflected by minor differences in the substrate binding sites of these carriers. The physiological role of the above-mentioned mitochondrial carriers is to link several metabolic pathways that take place partly in the cytosol and partly in the mitochondrial matrix and to provide basic amino acids for mitochondrial translation. In the liver, human ORC1 catalyzes the citrulline/ornithine exchange across the mitochondrial inner membrane, which is required for the urea cycle. Human ORC1, ORC2, and SLC25A29 are likely to be involved in the biosynthesis and transport of arginine, which can be used as a precursor for the synthesis of NO, agmatine, polyamines, creatine, glutamine, glutamate, and proline, as well as in the degradation of basic amino acids. BAC1 and BAC2 are implicated in some processes similar to those of their human counterparts and in nitrogen and amino acid metabolism linked to stress conditions and the development of plants. Ort1p is involved in the biosynthesis of arginine and polyamines in yeast.

OxLDL triggers retrograde translocation of arginase2 in aortic endothelial cells via ROCK and mitochondrial processing peptidase

RATIONALE

Increased arginase activity contributes to endothelial dysfunction by competition for l-arginine substrate and reciprocal regulation of nitric oxide synthase (NOS). The rapid increase in arginase activity in human aortic endothelial cells exposed to oxidized low-density lipoprotein (OxLDL) is consistent with post-translational modification or subcellular trafficking.

OBJECTIVE

To test the hypotheses that OxLDL triggers reverse translocation of mitochondrial arginase 2 (Arg2) to cytosol and Arg2 activation, and that this process is dependent on mitochondrial processing peptidase, lectin-like OxLDL receptor-1 receptor, and rho kinase.

METHODS AND RESULTS

OxLDL-triggered translocation of Arg2 from mitochondria to cytosol in human aortic endothelial cells and in murine aortic intima with a concomitant rise in arginase activity. All of these changes were abolished by inhibition of mitochondrial processing peptidase or by its siRNA-mediated knockdown. Rho kinase inhibition and the absence of the lectin-like OxLDL receptor-1 in knockout mice also ablated translocation. Aminoterminal sequencing of Arg2 revealed 2 candidate mitochondrial targeting sequences, and deletion of either of these confined Arg2 to the cytoplasm. Inhibitors of mitochondrial processing peptidase or lectin-like OxLDL receptor-1 knockout attenuated OxLDL-mediated decrements in endothelial-specific NO production and increases in superoxide generation. Finally, Arg2(-/-) mice bred on an ApoE(-/-) background showed reduced plaque load, reduced reactive oxygen species production, enhanced NO, and improved endothelial function when compared with ApoE(-/-) controls.

CONCLUSIONS

These data demonstrate dual distribution of Arg2, a protein with an unambiguous mitochondrial targeting sequence, in mammalian cells, and its reverse translocation to cytoplasm by alterations in the extracellular milieu. This novel molecular mechanism drives OxLDL-mediated arginase activation, endothelial NOS uncoupling, endothelial dysfunction, and atherogenesis.

Transcriptional regulation of endothelial arginase 2 by histone deacetylase 2

OBJECTIVE

Arginase 2 (Arg2) is a critical target in atherosclerosis because it controls endothelial nitric oxide, proliferation, fibrosis, and inflammation. Regulators of Arg2 transcription in the endothelium have not been characterized. The goal of the current study is to determine the role of specific histone deacetylases (HDACs) in the regulation of endothelial Arg2 transcription and endothelial function.

APPROACH AND RESULTS

The HDAC inhibitor trichostatin A increased levels of Arg2 mRNA, protein, and activity in both human aortic endothelial cells and mouse aortic rings. These changes occurred in both time- and dose-dependent patterns and resulted in Arg2-dependent endothelial dysfunction. Trichostatin A and the atherogenic stimulus oxidized low-density lipoprotein enhanced the activity of common promoter regions of Arg2. HDAC inhibition with trichostatin A also decreased endothelial nitric oxide, and these effects were blunted by arginase inhibition. Nonselective class I HDAC inhibitors enhanced Arg2 expression, whereas the only selective inhibitor that increased Arg2 expression was mocetinostat, a selective inhibitor of HDACs 1 and 2. Additionally, mouse aortic rings preincubated with mocetinostat exhibited dysfunctional relaxation. Overexpression of HDAC2 (but not HDAC 1, 3, or 8) cDNA in human aortic endothelial cells suppressed Arg2 expression in a concentration-dependent manner, and siRNA knockdown of HDAC2 enhanced Arg2 expression. Chromatin immunoprecipitation indicated direct binding of HDAC2 to the Arg2 promoter, and HDAC2 overexpression in human aortic endothelial cells blocked oxidized low-density lipoprotein-mediated activation of the Arg2 promoter. Finally, overexpression of HDAC2 blocked oxidized low-density lipoprotein-mediated vascular dysfunction.

CONCLUSIONS

HDAC2 is a critical regulator of Arg2 expression and thereby endothelial nitric oxide and endothelial function. Overexpression or activation of HDAC2 represents a novel therapy for endothelial dysfunction and atherosclerosis.

The human gene SLC25A29, of solute carrier family 25, encodes a mitochondrial transporter of basic amino acids

The human genome encodes 53 members of the solute carrier family 25 (SLC25), also called the mitochondrial carrier family, many of which have been shown to transport carboxylates, amino acids, nucleotides, and cofactors across the inner mitochondrial membrane, thereby connecting cytosolic and matrix functions. In this work, a member of this family, SLC25A29, previously reported to be a mitochondrial carnitine/acylcarnitine- or ornithine-like carrier, has been thoroughly characterized biochemically. The SLC25A29 gene was overexpressed in Escherichia coli, and the gene product was purified and reconstituted in phospholipid vesicles. Its transport properties and kinetic parameters demonstrate that SLC25A29 transports arginine, lysine, homoarginine, methylarginine and, to a much lesser extent, ornithine and histidine. Carnitine and acylcarnitines were not transported by SLC25A29. This carrier catalyzed substantial uniport besides a counter-exchange transport, exhibited a high transport affinity for arginine and lysine, and was saturable and inhibited by mercurial compounds and other inhibitors of mitochondrial carriers to various degrees. The main physiological role of SLC25A29 is to import basic amino acids into mitochondria for mitochondrial protein synthesis and amino acid degradation.

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.

Arginase inhibition mediates renal tissue protection in diabetic nephropathy by a nitric oxide synthase 3-dependent mechanism

Recently we showed that pharmacological blockade or genetic deficiency of arginase-2 confers kidney protection in diabetic mouse models. Here we tested whether the protective effect of arginase inhibition is nitric oxide synthase-3 (eNOS)-dependent in diabetic nephropathy. Experiments were conducted in eNOS knockout and their wild type littermate mice using multiple low doses of vehicle or streptozotocin and treated with continuous subcutaneous infusion of vehicle or the arginase inhibitor S-(2-Boronoethyl)-L-cysteine by an osmotic pump. Inhibition of arginases for 6 weeks in diabetic wild type mice significantly attenuated albuminuria, the increase in plasma creatinine and blood urea nitrogen, histopathological changes, kidney fibronectin and TNF-α expression, kidney macrophage recruitment, and oxidative stress compared to vehicle-treated diabetic wild type mice. Arginase inhibition in diabetic eNOS knockout mice failed to affect any of these parameters but reduced kidney macrophage recruitment and kidney TNF-α expression compared to vehicle-treated diabetic eNOS knockout mice. Furthermore, diabetic wild type and eNOS knockout mice exhibited increased kidney arginase-2 protein, arginase activity and ornithine levels. Thus, arginase inhibition mediates renal tissue protection in diabetic nephropathy by an eNOS-dependent mechanism and has an eNOS-independent effect on kidney macrophage recruitment.

Increased arginase II activity contributes to endothelial dysfunction through endothelial nitric oxide synthase uncoupling in aged mice

The incidence of cardiovascular disease is predicted to increase as the population ages. There is accumulating evidence that arginase upregulation is associated with impaired endothelial function. Here, we demonstrate that arginase II (ArgII) is upregulated in aortic vessels of aged mice and contributes to decreased nitric oxide (NO) generation and increased reactive oxygen species (ROS) production via endothelial nitric oxide synthase (eNOS) uncoupling. Inhibiting ArgII with small interfering RNA technique restored eNOS coupling to that observed in young mice and increased NO generation and decreased ROS production. Furthermore, enhanced vasoconstrictor responses to U46619 and attenuated vasorelaxation responses to acetylcholine in aged vasculature were markedly improved following siRNA treatment against ArgII. These results might be associated with increased L-arginine bioavailability. Collectively, these results suggest that ArgII may be a valuable target in age-dependent vascular diseases.

Kinetics of the utilization of dietary arginine for nitric oxide and urea synthesis: insight into the arginine-nitric oxide metabolic system in humans

BACKGROUND

The systemic availability of oral/dietary arginine and its utilization for nitric oxide (NO) synthesis remains unknown and may be related to a competitive hydrolysis of arginine into urea in the splanchnic area and systemic circulation.

OBJECTIVES

We investigated the kinetics and dose-dependency of dietary arginine utilization for NO compared with urea synthesis and studied the characteristics of the arginine-NO metabolic system in healthy humans.

DESIGN

We traced the metabolic fate and analyzed the utilization dynamics of dietary arginine after its ingestion at 2 nutritional amounts in healthy humans (n = 9) in a crossover design by using [(15)N-(15)N-(guanido)]-arginine, isotope ratio mass spectrometry techniques, and data analysis with a compartmental modeling approach.

RESULTS

Whatever the amount of dietary arginine, 60 ± 3% (±SEM) was converted to urea, with kinetics indicative of a first-pass splanchnic phenomenon. Despite this dramatic extraction, intact dietary arginine made a major contribution to the postprandial increase in plasma arginine. However, the model identified that the plasma compartment was a very minor (~2%) precursor for the conversion of dietary arginine into NO, which, in any case, was small (<0.1% of the dose). The whole-body and plasma kinetics of arginine metabolism were consistent with the suggested competitive metabolism by the arginase and NO synthase pathways.

CONCLUSIONS

The conversion of oral/dietary arginine into NO is not limited by the systemic availability of arginine but by a tight metabolic compartmentation at the systemic level. We propose an organization of the arginine metabolic system that explains the daily maintenance of NO homeostasis in healthy humans.

Competitive metabolism of L-arginine: arginase as a therapeutic target in asthma

Exhaled breath nitric oxide (NO) is an accepted asthma biomarker. Lung concentrations of NO and its amino acid precursor, L-arginine, are regulated by the relative expressions of the NO synthase (NOS) and arginase isoforms. Increased expression of arginase I and NOS2 occurs in murine models of allergic asthma and in biopsies of asthmatic airways. Although clinical trials involving the inhibition of NO-producing enzymes have shown mixed results, small molecule arginase inhibitors have shown potential as a therapeutic intervention in animal and cell culture models. Their transition to clinical trials is hampered by concerns regarding their safety and potential toxicity. In this review, we discuss the paradigm of arginase and NOS competition for their substrate L-arginine in the asthmatic airway. We address the functional role of L-arginine in inflammation and the potential role of arginase inhibitors as therapeutics.

Arginase II inhibition prevents nitrate tolerance

BACKGROUND AND PURPOSE

Nitrate tolerance, the loss of vascular responsiveness with continued use of nitrates, remains incompletely understood and is a limitation of these therapeutic agents. Vascular superoxide, generated by uncoupled endothelial NOS (eNOS), may play a role. As arginase competes with eNOS for L-arginine and may exacerbate the production of reactive oxygen species (ROS), we hypothesized that arginase inhibition might reduce nitrate tolerance.

EXPERIMENTAL APPROACH

Vasodilator responses were measured in aorta from C57Bl/6 and arginase II knockout (argII -/-) mice using myography. Uncoupling of eNOS, determined as eNOS monomer : dimer ratio, was assessed using low-temperature SDS-PAGE and ROS levels were measured using L-012 and lucigenin-enhanced chemiluminescence.

KEY RESULTS

Repeated application of glyceryl trinitrate (GTN) on aorta isolated from C57Bl/6 mice produced a 32-fold rightward shift of the concentration-response curve. However this rightward shift (or resultant tolerance) was not observed in the presence of the arginase inhibitor (s)-(2-boronethyl)-L-cysteine HCl (BEC; 100 µM) nor in aorta isolated from argII -/- mice. Similar findings were obtained after inducing nitrate tolerance in vivo. Repeated administration of GTN in human umbilical vein endothelial cells induced uncoupling of eNOS from its dimeric state and increased ROS levels, which were reduced with arginase inhibition and exogenous L-arginine. Aortae from GTN tolerant C57Bl/6 mice exhibited increased arginase activity and ROS production, whereas vessels from argII -/- mice did not.

CONCLUSION AND IMPLICATIONS

Arginase II removal prevents nitrate tolerance. This may be due to decreased uncoupling of eNOS and consequent ROS production.

Selective endothelial overexpression of arginase II induces endothelial dysfunction and hypertension and enhances atherosclerosis in mice

Background

Cardiovascular disorders associated with endothelial dysfunction, such as atherosclerosis, have decreased nitric oxide (NO) bioavailability. Arginase in the vasculature can compete with eNOS for L-arginine and has been implicated in atherosclerosis. The aim of this study was to evaluate the effect of endothelial-specific elevation of arginase II expression on endothelial function and the development of atherosclerosis.

Methodology/Principal Findings

Transgenic mice on a C57BL/6 background with endothelial-specific overexpression of human arginase II (hArgII) gene under the control of the Tie2 promoter were produced. The hArgII mice had elevated tissue arginase activity except in liver and in resident peritoneal macrophages, confirming endothelial specificity of the transgene. Using small-vessel myography, aorta from these mice exhibited endothelial dysfunction when compared to their non-transgenic littermate controls. The blood pressure of the hArgII mice was 17% higher than their littermate controls and, when crossed with apoE −/− mice, hArgII mice had increased aortic atherosclerotic lesions.

Conclusion

We conclude that overexpression of arginase II in the endothelium is detrimental to the cardiovascular system.

Treatment with the arginase inhibitor Nw-hydroxy-nor-L-arginine restores endothelial function in rat adjuvant-induced arthritis

Introduction

Endothelial dysfunction (ED) participates to atherogenesis associated to rheumatoid arthritis. We recently reported increased arginase activity/expression in vessels from adjuvant-induced arthritis (AIA) rats. In the present study, we investigated the effects of a curative treatment with the arginase inhibitor N_w-hydroxy-nor-L-arginine (nor-NOHA) on vascular dysfunction in AIA rats.

Methods

AIA rats were treated with nor-NOHA (40 mg/kg/d, ip) for 21 days after the onset of arthritis. A group of untreated AIA rats and a group of healthy rats served as controls. ED was assessed by the vasodilatory effect of acetylcholine (Ach) on aortic rings. The role of superoxide anions, prostanoids, endothelium-derived hyperpolarizing factor (EDHF) and nitric oxide synthase (NOS) pathway was studied. Plasma levels of IL-6 and vascular endothelial growth factor (VEGF) were determined by ELISA kits. Arthritis severity was estimated by a clinical, radiological and histological analysis.

Results

Nor-NOHA treatment fully restored the aortic response to Ach to that of healthy controls. The results showed that this beneficial effect is mediated by an increase in NOS activity and EDHF and reduced superoxide anion production as well as a decrease in the activity of cyclooxygenase (COX)-2, thromboxane and prostacyclins synthases. In addition, nor-NOHA decreased IL-6 and VEGF plasma levels in AIA rats. By contrast, the treatment did not modify arthritis severity in AIA rats.

Conclusions

The treatment with an arginase inhibitor has a potent effect on ED in AIA independently of the severity of the disease. Our results suggest that this new pharmacological approach has the potential as a novel add-on therapy in the treatment of RA.

Arginase-2 mediates diabetic renal injury

OBJECTIVE

To determine 1) whether renal arginase activity or expression is increased in diabetes and 2) whether arginase plays a role in development of diabetic nephropathy (DN).

RESEARCH DESIGN AND METHODS

The impact of arginase activity and expression on renal damage was evaluated in spontaneously diabetic Ins2(Akita) mice and in streptozotocin (STZ)-induced diabetic Dilute Brown Agouti (DBA) and arginase-2-deficient mice (Arg2(-/-)).

RESULTS

Pharmacological blockade or genetic deficiency of arginase-2 conferred kidney protection in Ins2(Akita) mice or STZ-induced diabetic renal injury. Blocking arginases using S-(2-boronoethyl)-L-cysteine for 9 weeks in Ins2(Akita) mice or 6 weeks in STZ-induced diabetic DBA mice significantly attenuated albuminuria, the increase in blood urea nitrogen, histopathological changes, and kidney macrophage recruitment compared with vehicle-treated Ins2(Akita) mice. Furthermore, kidney arginase-2 expression increased in Ins2(Akita) mice compared with control. In contrast, arginase-1 expression was undetectable in kidneys under normal or diabetes conditions. Arg2(-/-) mice mimicked arginase blockade by reducing albuminuria after 6 and 18 weeks of STZ-induced diabetes. In wild-type mice, kidney arginase activity increased significantly after 6 and 18 weeks of STZ-induced diabetes but remained very low in STZ-diabetic Arg2(-/-) mice. The increase in kidney arginase activity was associated with a reduction in renal medullary blood flow in wild-type mice after 6 weeks of STZ-induced diabetes, an effect significantly attenuated in diabetic Arg2(-/-) mice.

CONCLUSIONS

These findings indicate that arginase-2 plays a major role in induction of diabetic renal injury and that blocking arginase-2 activity or expression could be a novel therapeutic approach for treatment of DN.

Arginase inhibition promotes wound healing in mice

OBJECTIVE

Arginase plays important regulatory roles in polyamine, ornithine, and nitric oxide syntheses. However, its role in the healing process has not been delineated. In this study, we used a highly potent and specific inhibitor of arginase, namely 2(S)-amino-6-boronohexanoic acid NH4 (ABH) to evaluate the role of arginase function in wound healing.

MATERIALS AND METHODS

ABH or saline was applied topically to full thickness, dorsal, excisional wounds in C57BL/6 mice every 8 hours for 14 days post surgery and the rate of wound closure was estimated planimetrically. Wound tissue was harvested from mice sacrificed on postoperative days 3 and 7 and examined histologically. The extent of epithelial, connective, and granulation tissue present within the wound area was estimated histomorphometrically. The effect of ABH on wound arginase activity, production of nitric oxide metabolites (NO(x)), and presence of smooth muscle actin positive cells (myofibroblasts) was evaluated.

RESULTS

While arginase activity was inhibited in vivo, the rate of wound closure significantly increased 7 days post-surgery, (21 ± 4%: P < .01; Student t test) in ABH treated animals. This was accompanied by an early increase in wound granulation tissue and accumulation of NO(x) followed by enhanced re-epithelialization and localization of myofibroblasts beneath the wound epithelium.

CONCLUSION

Arginase inhibition improves excisional wound healing and may be used to develop therapeutics for early wound closure.

Acute localized administration of tetrahydrobiopterin and chronic systemic atorvastatin treatment restore cutaneous microvascular function in hypercholesterolaemic humans

Elevated oxidized low-density lipoproteins (LDL) are associated with vascular dysfunction in the cutaneous microvasculature, induced in part by upregulated arginase activity and increased globalized oxidant stress. Since tetrahydrobiopterin (BH(4)) is an essential cofactor for endothelial nitric oxide synthase (NOS3), decreased bioavailability of the substrate l-arginine and/or BH(4) may contribute to decreased NO production with hypercholesterolaemia. We hypothesized that (1) localized administration of BH(4) would augment NO-dependent vasodilatation in hypercholesterolaemic human skin, which would be further increased when combined with arginase inhibition and (2) the improvement induced by localized BH(4) would be attenuated after a 3 month oral atorvastatin intervention (10 mg). Four microdialysis fibres were placed in the skin of nine normocholesterolaemic (NC: LDL = 95 ± 4 mg dl(-1)) and nine hypercholesterolaemic (HC: LDL = 177 ± 6 mg dl(-1)) men and women before and after 3 months of systemic atorvastatin. Sites served as control, NOS inhibited, BH(4), and arginase inhibited + BH(4) (combo). Skin blood flow was measured while local skin heating (42°C) induced NO-dependent vasodilatation. After the established plateau l-NAME was perfused in all sites to quantify NO-dependent vasodilatation (NO). Data were normalized to maximum cutaneous vascular conductance (CVC). Vasodilatation at the plateau and NO-dependent vasodilatation were reduced in HC subjects (plateau HC: 70 ± 5% CVC(max) vs. NC: 95 ± 2% CVC(max); NO HC: 45 ± 5% CVC(max) vs. NC: 64 ± 5% CVC(max); both P < 0.001). Localized BH(4) alone or combo augmented the plateau (BH(4): 93 ± 3% CVC(max); combo 89 ± 3% CVC(max), both P < 0.001) and NO-dependent vasodilatation in HC (BH(4): 74 ± 3% CVC(max); combo 76 ± 3% CVC(max), both P < 0.001), but there was no effect in NC subjects (plateau BH(4): 90 ± 2% CVC(max); combo 95 ± 3% CVC(max); NO-dependent vasodilatation BH(4): 68 ± 3% CVC(max); combo 58 ± 4% CVC(max), all P > 0.05 vs. control site). After the atorvastatin intervention (LDL = 98 ± mg * dl(-1)) there was an increase in the plateau in HC (96 ± 4% CVC(max), P < 0.001) and NO-dependent vasodilatation (68 ± 3% CVC(max), P < 0.001). Localized BH(4) alone or combo was less effective at increasing NO-dependent vasodilatation after the drug intervention (BH(4): 60 ± 5% CVC(max); combo 58 ± 2% CVC(max), both P < 0.001). These data suggest that decreased BH(4) bioavailability contributes in part to cutaneous microvascular dysfunction in hypercholesterolaemic humans and that atorvastatin is an effective systemic treatment for improving NOS coupling mechanisms in the microvasculature.

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.

S-nitrosation of arginase 1 requires direct interaction with inducible nitric oxide synthase

Arginase constrains endothelial nitric oxide synthase activity by competing for the common substrate, L -Arginine. We have recently shown that inducible nitric oxide synthase (NOS2) S-nitrosates and activates arginase 1 (Arg1) leading to age-associated vascular dysfunction. Here, we demonstrate that a direct interaction of Arg1 with NOS2 is necessary for its S-nitrosation. The specific domain of NOS2 that mediates this interaction is identified. Disruption of this interaction in human aortic endothelial cells prevents Arg1 S-nitrosation and activation. Thus, disruption of NOS2-Arg1 interaction may represent a therapeutic strategy to attenuate age related vascular endothelial dysfunction.

Oral atorvastatin therapy restores cutaneous microvascular function by decreasing arginase activity in hypercholesterolaemic humans

Elevated low-density lipoproteins (LDLs) are associated with vascular dysfunction evident in the cutaneous microvasculature. We hypothesized that uncoupled endothelial nitric oxide synthase (NOS3) through upregulated arginase contributes to cutaneous microvascular dysfunction in hyperocholesterolaemic (HC) humans and that a statin intervention would decrease arginase activity. Five microdialysis fibres were placed in the skin of nine normocholesterolaemic (NC: LDL level 95±4 mg dl⁻¹) and nine hypercholesterolaemic (HC: LDL: 177±6 mg dl⁻¹) men and women before and after 3 months of systemic atrovastatin. Sites served as control, NOS inhibited, arginase inhibited, L-arginine supplemented and arginase inhibited plus L-arginine supplemented. Skin blood flow was measured while local skin heating (42°C) induced NO-dependent vasodilatation. L-NAME was infused after the established plateau in all sites to quantify NO-dependent vasodilatation. Data were normalized to maximum cutaneous vascular conductance (CVC(max)). Skin samples were obtained to measure total arginase activity and arginase I and arginase II protein. Vasodilatation was reduced in hyperocholesterolaemic subjects (HC: 76±2 vs. NC: 94±3%CVC(max), P < 0.001) as was NO-dependent vasodilatation (HC: 43±5 vs. NC: 62±4%CVC(max), P < 0.001). The plateau and NO-dependent vasodilatation were augmented in HC with arginase inhibition (92±2, 67±2%CVC(max), P < 0.001), L-arginine (93±2, 71±5%CVC(max), P < 0.001) and combined treatments (94±4, 65±5%CVC(max), P < 0.001) but not in NC. After statin intervention (LDL: 98±5 mg dl⁻¹) there was no longer a difference between control sites (88±4, 61±5%CVC(max)) and localized microdialysis treatment sites (all P > 0.05). Arginase activity and protein were increased in HC skin (P < 0.05 vs. NC) and activity decreased with atrovastatin treatment (P < 0.05). Reduced NOS3 substrate availability through upregulated arginase contributes to cutaneous microvascular dysfunction in hyperocholesterolaemic humans, which is corrected with atorvastatin therapy.

Arginase II deletion increases corpora cavernosa relaxation in diabetic mice

INTRODUCTION

Diabetes-induced erectile dysfunction involves elevated arginase (Arg) activity and expression. Because nitric oxide (NO) synthase and Arg share and compete for their substrate L-arginine, NO production is likely linked to regulation of Arg. Arg is highly expressed and implicated in erectile dysfunction.

AIM

It was hypothesized that Arg-II isoform deletion enhances relaxation function of corpora cavernosal (CC) smooth muscle in a streptozotocin (STZ) diabetic model.

METHODS

Eight weeks after STZ-induced diabetes, vascular functional studies, Arg activity assay, and protein expression levels of Arg and constitutive NOS (using Western blots) were assessed in CC tissues from nondiabetic wild type (WT), diabetic (D) WT (WT + D), Arg-II knockout (KO), and Arg-II KO+D mice (N = 8-10 per group).

MAIN OUTCOME MEASURES

Inhibition or lack of arginase results in facilitation of CC relaxation in diabetic CC.

RESULTS

Strips of CC from Arg-II KO mice exhibited an enhanced maximum endothelium-dependent relaxation (from 70 + 3% to 84 + 4%) and increased nitrergic relaxation (by 55%, 71%, 42%, 42%, and 24% for 1, 2, 4, 8 and 16 Hz, respectively) compared with WT mice. WT + D mice showed a significant reduction of endothelium-dependent maximum relaxation (44 + 8%), but this impairment of relaxation was significantly prevented in Arg-II KO+D mice (69 + 4%). Sympathetic-mediated and alpha-adrenergic agent-induced contractile responses also were increased in CC strips from D compared with non-D controls. Contractile responses were significantly lower in Arg-II KO control and D versus the WT groups. WT + D mice increased Arg activity (1.5-fold) and Arg-II protein expression and decreased total and phospho-eNOS at Ser-1177, and nNOS levels. These alterations were not seen in Arg-II KO mice. Additionally, the Arg inhibitor BEC (50 µM) enhanced nitrergic and endothelium-dependent relaxation in CC of WT + D mice.

CONCLUSION

These studies show for the first time that Arg-II deletion improves CC relaxation in type 1 diabetes.

Impact of arginase II on CBF in experimental cortical impact injury in mice using MRI

Traumatic brain injury (TBI) results in reduced cerebral blood flow (CBF) and low levels of the vasodilator nitric oxide (NO) may be involved. Arginase II negatively regulates NO production through competition for the substrate L-arginine. We determined whether arginase II-deficient (ArgII(-/-)) mice would show improved CBF after TBI through arterial spin-labeling magnetic resonance imaging (MRI). The ArgII(-/-) mice exhibit a significantly improved CBF recovery after trauma in the perilesional brain (P=0.0015) and in various other brain regions. In conclusion, arginase II deficiency leads to a better CBF recovery after TBI and implicates arginase II in hemodynamic processes.

Effects of angiotensin type 1 receptor blockade on arginine and ADMA synthesis and metabolic pathways in fawn-hooded hypertensive rats

BACKGROUND

The fawn-hooded hypertensive (FHH) rat develops spontaneous glomerulosclerosis that is ameliorated by inhibition of the angiotensin II type 1 receptor (AT-1). Since kidney damage is associated with nitric oxide (NO) deficiency, we investigated how AT-1 antagonism influenced nitric oxide synthase (NOS), as well as NOS substrate [L-arginine (L-Arg)] and inhibitor [asymmetric dimethylarginine (ADMA)]. L-Arg is synthesized by renal argininosuccinate synthase/argininosuccinate lyase (ASS/ASL) and then either consumed within the kidney by arginase II or NOS or released into the circulation. L-Arg is then taken up from plasma into cells where it can be utilized by NOS and other pathways. The competitive inhibitor of NOS, ADMA, is degraded by dimethylarginine dimethylaminohydrolase (DDAH).

METHODS AND RESULTS

Male FHH rats were put on a 40% casein diet for 13 weeks, and some received AT-1 antagonist which reduced blood pressure and kidney weight and prevented glomerulosclerosis and hyperfiltration. The AT-1 antagonist reduced the expression of DDAH2, increased DDAH1 and increased total DDAH activity in the kidney cortex, although there was no change in plasma or kidney cortex ADMA levels. The AT-1 antagonist caused no change in the expression of renal ASS/ASL, but reduced renal and aortic arginase expression and renal arginase activity, which could explain the increased plasma L-Arg. In separate studies, 1 week of AT-1 blockade in young FHH rats had no effect on any of these parameters.

CONCLUSION

Thus, the net result of AT-1 antagonist was an improved L-Arg to ADMA ratio due to the prevention of renal and vascular arginase activation which favours increased NO production. Since 1 week of AT-1 blockade in the absence of kidney damage was without effect on arginases, this suggests that the reduction in arginase activity is secondary to the prevention of structural damage rather than a direct immediate effect of AT-1 antagonism.

Arginase II knockout mouse displays a hypertensive phenotype despite a decreased vasoconstrictory profile

Arginase upregulation is associated with aging and cardiovascular diseases. In this study we report on the cardiovascular phenotype of the arginase II knockout (KO) mouse. We demonstrate that vascular sensitivity and reactivity altered over time in these animals such that no influence on responses to vasoconstrictor activity was observed in 7-week-old KO mice, but dampened responses to norepinephrine and phenylephrine were observed by 10 and 15 weeks with Rho kinase influencing these effects in the 15-week-old animals. Despite these dampened vasoconstrictory responses, KO mice demonstrated increased mean arterial pressure from 8 weeks old. This hypertensive phenotype was associated with an increase in left ventricular weight, left ventricular systolic pressure, and diminished diastolic function. KO mice also show enhanced plasma norepinephrine turnover, suggesting an increased sympathetic outflow. In conclusion, our data suggest that global loss of arginase II activity results in hypertension. We suggest that this strain of mouse warrants further investigation as a potentially novel model of hypertension.

Recent advances in arginine metabolism: roles and regulation of the arginases

As arginine can serve as precursor to a wide range of compounds, including nitric oxide, creatine, urea, polyamines, proline, glutamate and agmatine, there is considerable interest in elucidating mechanisms underlying regulation of its metabolism. It is now becoming apparent that the two isoforms of arginase in mammals play key roles in regulation of most aspects of arginine metabolism in health and disease. In particular, work over the past several years has focused on the roles and regulation of the arginases in vascular disease, pulmonary disease, infectious disease, immune cell function and cancer. As most of these topics have been considered in recent review articles, this review will focus more closely on results of recent studies on expression of the arginases in endothelial and vascular smooth muscle cells, post-translational modulation of arginase activity and applications of arginase inhibitors in vivo.

Arginase promotes neointima formation in rat injured carotid arteries

OBJECTIVE

Arginase stimulates the proliferation of cultured vascular smooth muscle cells (VSMCs); however, the influence of arginase on VSMC growth in vivo is not known. This study investigated the impact of arginase on cell cycle progression and neointima formation after experimental arterial injury.

METHODS AND RESULTS

Balloon injury of rat carotid arteries resulted in a sustained increase in arginase activity in the vessel wall and the induction of arginase I protein in both the media and neointima of injured vessels. Furthermore, local perivascular application of the potent and selective arginase inhibitors S-(2-boronoethyl)-L-cysteine (BEC) or N(G)-hydroxy-nor-L-arginine (L-OHNA) immediately after injury markedly attenuated medial and neointimal DNA synthesis and neointima formation. Substantial arginase I protein and arginase activity was also detected in rat cultured aortic VSMCs. Moreover, treatment of VSMCs with BEC or L-OHNA, or knockdown of arginase I protein, arrested cells in the G(0)/G(1) phase of the cell cycle and induced the expression of the cyclin-dependent protein kinase inhibitor, p21.

CONCLUSIONS

This study demonstrates that arginase is essential for VSMCs to enter the cell cycle and that arginase I contributes to the remodeling response after arterial injury. Arginase I represents a potentially new therapeutic target for the treatment of vasculoproliferative disorders.

Uric acid decreases NO production and increases arginase activity in cultured pulmonary artery endothelial cells

Elevated levels of serum uric acid (UA) are commonly associated with primary pulmonary hypertension but have generally not been thought to have any causal role. Recent experimental studies, however, have suggested that UA may affect various vasoactive mediators. We therefore tested the hypothesis that UA might alter nitric oxide (NO) levels in pulmonary arterial endothelial cells (PAEC). In isolated porcine pulmonary artery segments (PAS), UA (7.5 mg/dl) inhibits acetylcholine-induced vasodilation. The incubation of PAEC with UA caused a dose-dependent decrease in NO and cGMP production stimulated by bradykinin or Ca(2+)-ionophore A23187. We explored cellular mechanisms by which UA might cause reduced NO production focusing on the effects of UA on the l-arginine-endothelial NO synthase (eNOS) and l-arginine-arginase pathways. Incubation of PAEC with different concentrations of UA (2.5-15 mg/dl) for 24 h did not affect l-[(3)H]arginine uptake or activity/expression of eNOS. However, PAEC incubated with UA (7.5 mg/dl; 24 h) released more urea in culture media than control PAEC, suggesting that arginase activation might be involved in the UA effect. Kinetic analysis of arginase activity in PAEC lysates and rat liver and kidney homogenates demonstrated that UA activated arginase by increasing its affinity for l-arginine. An inhibitor of arginase (S)-(2-boronoethyl)-l-cysteine prevented UA-induced reduction of A23187-stimulated cGMP production by PAEC and abolished UA-induced inhibition of acetylcholine-stimulated vasodilation in PAS. We conclude that UA-induced arginase activation is a potential mechanism for reduction of NO production in PAEC.

Endothelial arginase II: a novel target for the treatment of atherosclerosis

Oxidized low-density lipoproteins increase arginase activity and reciprocally decrease endothelial NO in human aortic endothelial cells. Here, we demonstrate that vascular endothelial arginase activity is increased in atherogenic-prone apolipoprotein E-null (ApoE(-/-)) and wild-type mice fed a high cholesterol diet. In ApoE(-/-) mice, selective arginase II inhibition or deletion of the arginase II gene (Arg II(-/-) mice) prevents high-cholesterol diet-dependent decreases in vascular NO production, decreases endothelial reactive oxygen species production, restores endothelial function, and prevents oxidized low-density lipoprotein-dependent increases in vascular stiffness. Furthermore, arginase inhibition significantly decreases plaque burden. These data indicate that arginase II plays a critical role in the pathophysiology of cholesterol-mediated endothelial dysfunction and represents a novel target for therapy in atherosclerosis.