Effects of the new arginase inhibitor N(omega)-hydroxy-nor-L-arginine on NO synthase activity in murine macrophages

Effect of the Macromolecular Crowding Agents on the Structure and Function of Human Arginase-I, a Therapeutically Important Enzyme

Macromolecular crowding has been known to influence the structure and function of many enzymes through excluded volume effects and/or soft interactions. Here, we employed two synthetic macromolecular crowders, Dextrans and poly(ethylene glycol)s (PEGs) with varying molecular masses, to examine how they affected the structure and function of a therapeutically important enzyme, human arginase-I that catalyzes the conversion of l-arginine to l-ornithine and urea. Except at greater concentrations of Dextran 200, Dextrans were observed to slightly reduce the enzymatic activity, indicating that they exert their influence mainly through the excluded volume effects. Similar outcomes were seen with PEGs, with the exception of PEG 1000, where the activity decreased with increasing PEG concentrations, showing the maximum effect at a 20 g/L concentration. This finding suggests that the enzyme function is reduced by the soft interactions of this macromolecule with the enzyme, supported by the binding measurement. Secondary and local tertiary structures and thermodynamic stability were also affected, suggesting that PEG 1000 has an impact on the protein's structure. Furthermore, molecular dynamics simulation studies suggest that the catalytic pocket is disturbed, presumably by the unwinding of neighboring helix 9. As a result, the positioning of nearby Glu277 is altered, which prevents His141 and Glu277 from making contact. This hampers the proton transfer from the catalytic His141 to the intermediate species to form ornithine, a crucial step for the substrate hydrolysis reaction by this arginase. Overall, the knowledge gained from this study might be helpful for understanding how different enzymes work in a crowded/cellular environment.

Arginase 1 Expression by Macrophages Promotes Cryptococcus neoformans Proliferation and Invasion into Brain Microvascular Endothelial Cells

Cryptococcal meningoencephalitis caused by Cryptococcus neoformans infection is the most common cause of death in HIV/AIDS patients. Macrophages are pivotal for the regulation of immune responses to cryptococcal infection by either playing protective function or facilitating fungal dissemination. However, the mechanisms underlying macrophage responses to C. neoformans remain unclear. To analyze the transcriptomic changes and identify the pathogenic factors of macrophages, we performed a comparative transcriptomic analysis of alveolar macrophage responses during C. neoformans infection. Alveolar macrophages isolated from C. neoformans-infected mice showed dynamic gene expression patterns, with expression change from a protective M1 (classically activated)-like to a pathogenic M2 (alternatively activated)-like phenotype. Arg1, the gene encoding the enzyme arginase 1, was found as the most upregulated gene in alveolar macrophages during the chronic infection phase. The in vitro inhibition of arginase activity resulted in a reduction of cryptococcal phagocytosis, intracellular growth, and proliferation, coupled with an altered macrophage response from pathogenic M2 to a protective M1 phenotype. In an in vitro model of the blood-brain barrier, macrophage-derived arginase was found to be required for C. neoformans invasion of brain microvascular endothelium. Further analysis of the degree of virulence indicated a positive correlation between arginase 1 expression in macrophages and cryptococcal brain dissemination in vivo. Thus, our data suggest that a dynamic macrophage activation that involves arginase expression may contribute to the cryptococcal disease by promoting cryptococcal growth, proliferation, and the invasion to the brain endothelium.

Arginine metabolism governs microcycle conidiation by changing nitric oxide content in Metarhizium acridum

Microcycle conidiation commonly exists in filamentous fungi and has great potential for mass production of mycoinsecticides. L-Arginine metabolism is essential for conidiation and conditional growth and virulence, but its role in microcycle conidiation has not been explored. Here, a unique putative arginase (MaAGA) was characterized in the entomopathogenic fungus Metarhizium acridum. Conidial germination and thermotolerance were facilitated by the disruption of MaAGA. Despite little impact on fungal growth and virulence, the disruption resulted in normal conidiation after a 60-h incubation on microcycle conidiation medium (SYA) under normal culture conditions. In the MaAGA-disruption mutant (ΔMaAGA), intracellular arginine accumulation was sharply increased. Replenishment of the direct metabolites of arginase, namely ornithine and/or urea, was unable to restore the disruption mutant's microcycle conidiation on SYA. Interestingly, nitric oxide synthase (NOS) activity and nitric oxide (NO) levels of the ΔMaAGA strain were markedly decreased in the 60-h-old SYA cultures. Finally, adding N^ω-nitro-L-arginine, an inhibitor of NOS, into the SYA converted the microcycle conidiation of the wild-type strain to normal conidiation. In contrast, adding sodium nitroprusside, an NO donor, into the SYA recovered the mutant's microcycle conidiation. The results indicate that arginine metabolism controls microcycle conidiation by changing the content of NO. KEY POINTS: • The MaAGA-disruption led to normal conidiation on microcycle conidiation medium SYA. • Nitric oxide (NO) level of the ΔMaAGA strain was markedly decreased. • Adding an NO donor into the SYA recovered the microcycle conidiation of ΔMaAGA.

Arginase: shedding light on the mechanisms and opportunities in cardiovascular diseases

Arginase, a binuclear manganese metalloenzyme in the urea, catalyzes the hydrolysis of L-arginine to urea and L-ornithine. Both isoforms, arginase 1 and arginase 2 perform significant roles in the regulation of cellular functions in cardiovascular system, such as senescence, apoptosis, proliferation, inflammation, and autophagy, via a variety of mechanisms, including regulating L-arginine metabolism and activating multiple signal pathways. Furthermore, abnormal arginase activity contributes to the initiation and progression of a variety of CVDs. Therefore, targeting arginase may be a novel and promising approach for CVDs treatment. In this review, we give a comprehensive overview of the physiological and biological roles of arginase in a variety of CVDs, revealing the underlying mechanisms of arginase mediating vascular and cardiac function, as well as shedding light on the novel and promising therapeutic approaches for CVDs therapy in individuals.

There is no direct competition between arginase and nitric oxide synthase for the common substrate l-arginine

AIMS

Extrahepatic arginases are postulated to be involved in cardiovascular-related pathologies by competing with nitric oxide synthase (NOS) for the common substrate l-arginine, subsequently decreasing nitric oxide production. However, previous models used to study arginase and NOS competition did not account for steady state level of l-arginine pool, which is dependent on conditions of l-arginine supply and utilization pathways. This work aimed at revisiting the concept of NOS and arginase competition while considering different conditions of l-arginine supply and l-arginine utilization pathways.

METHODS AND RESULTS

Mouse macrophage-like RAW cells and human vascular endothelial cells co-expressing NOS and arginase were used to reevaluate the concept of substrate competition between arginase and NOS under conditions of l-arginine supply that mimicked either a continuous (similar to in vivo conditions) or a limited supply (similar to previous in vitro models). Enzyme kinetics simulation models were used to gain mechanistic insight and to evaluate the tenability of a substrate competition between the two enzymes. In addition to arginase and NOS, other l-arginine pathways such as transporters and utilization towards protein synthesis were considered to understand the intricacies of l-arginine metabolism. Our results indicate that when there is a continuous supply of l-arginine, as is the case for most cells in vivo, arginase does not affect NOS activity by a substrate competition. Furthermore, we demonstrate that l-arginine pathways such as transporters and protein synthesis are more likely to affect NOS activity than arginase.

CONCLUSIONS

Arginase does not outcompete NOS for the common substrate l-arginine. Findings from this study should be considered to better understand the role of arginase in certain pathologies and for the interpretation of in vivo studies with arginase inhibitors.

The chemical constituents of Sterculia comosa (wall) Roxb woods for arginase inhibitory, antioxidant activity, and molecular docking against SARS CoV-2 protein

Flavonoids and phenols have an arginase inhibitory and antioxidant activity. The Sterculia genus has phenols and flavonoids content. This study aimed to investigate the arginase inhibitory and antioxidant activity of the chemical constituent of Sterculia comosa (wall) Roxb and also their binding affinities to arginase. The most active extract was methanol extract. This active extract was determined for its arginase inhibitory and antioxidant activity, determined the total phenols and total flavonoids, and identified chemical compound. The methanol extract has IC₅₀ 2.787 μg/ml for arginase inhibitory activity and IC₅₀ 4,199 μg/ml for DPPH scavenging activity. The total phenols 723.61 mg GAE/gr, total flavonoids content 28.96 mg QE/gr extract. The chemical constituent: KC4.4.6 ((-)-2-(E)-caffeoyl-D-glyceric acid) and KC4.4.5.1 (trans-isoferulic acid) have an arginase inhibitory activity KC4.4.6: 98,03 μg/ml and KC4.4.5.1: 292,58 μg/ml. Antioxidant activity with DPPH methods KC4.4.6: 48,77 μg/ml and KC4.4.5.1: 88,08 μg/ml. Antioxidant by FRAP methods KC4.4.6: 16,4 FeEAC mol/g and KC4.4.5.1: 15,79 FeEAC mol/g. The isolate trans-isoferulic acid predicted has good interaction to arginase. Isolate KC4.4.6. Predicted has good interaction to PLPro of SARS CoV-2 PLpro. However, both isolates did not show good interaction to 3CLPro, nsp12, and Spike protein of SARS CoV-2.

An Update on Arginase Inhibitors and Inhibitory Assays

Arginase, which converts arginine into ornithine and urea, is a promising therapeutic target. Arginase is involved in cardiovascular diseases, parasitic infections and, through a critical role in immunity, in some cancers. There is a need to develop effective arginase inhibitors and therefore efforts to identify and optimize new inhibitors are increasing. Several methods of evaluating arginase activity are available, but few directly measure the product. Radiometric assays need to separate urea and dying reactions require acidic conditions and sometimes heating. Hence, there are a variety of different approaches available, and each approach has its own limits and benefits. In this review, we provide an update on arginase inhibitors, followed by a discussion on available arginase assays and alternative methods, with a focus on the intrinsic biases and parameters that are likely to impact results.

Arginase Signalling as a Key Player in Chronic Wound Pathophysiology and Healing

Arginase (ARG) represents an important evolutionarily conserved enzyme that is expressed by multiple cell types in the skin. Arg acts as the mediator of the last step of the urea cycle, thus providing protection against excessive ammonia under homeostatic conditions through the production of L-ornithine and urea. L-ornithine represents the intersection point between the ARG-dependent pathways and the urea cycle, therefore contributing to cell detoxification, proliferation and collagen production. The ARG pathways help balance pro- and anti-inflammatory responses in the context of wound healing. However, local and systemic dysfunctionalities of the ARG pathways have been shown to contribute to the hindrance of the healing process and the occurrence of chronic wounds. This review discusses the functions of ARG in macrophages and fibroblasts while detailing the deleterious implications of a malfunctioning ARG enzyme in chronic skin conditions such as leg ulcers. The review also highlights how ARG links with the microbiota and how this impacts on infected chronic wounds. Lastly, the review depicts chronic wound treatments targeting the ARG pathway, alongside future diagnosis and treatment perspectives.

Monocytic MDSCs skew Th17 cells toward a pro-osteoclastogenic phenotype and potentiate bone erosion in rheumatoid arthritis

OBJECTIVES

While myeloid-derived suppressor cells (MDSCs) were previously shown to promote a proinflammatory T helper (Th) 17 response in autoimmune conditions, a potential impact of the MDSC-Th17 immune axis on abnormal bone destruction in RA remains largely unknown.

METHODS

We investigated the correlation between the frequency of MDSCs or its subsets and joint destruction in RA patients. The reciprocal actions of patient-derived MDSCs and Th17 cells were studied using osteoclast (OC) differentiation and bone resorption assays in vitro, which were further validated using mouse models of RA. Contribution of MDSCs to osteoclastogenesis and bone erosion in vivo was determined by depletion or transfer of MDSCs.

RESULTS

Human MDSCs, particularly monocytic MDSCs (M-MDSCs), exhibit inherent OC-differentiating capacity and positively correlate with clinical bone erosion in RA patients. Strikingly, patient-derived M-MDSCs can program Th17 cells towards a pro-osteoclastogenic phenotype, which in return potentiates OC differentiation via the receptor activator of nuclear factor κΒ ligand (RANK-L)-RANK signalling. This enhanced osteolysis driven by the reciprocal actions of M-MDSCs and Th17 cells is further confirmed using mouse models of RA. Selective depletion of M-MDSCs significantly ameliorates osteoclastogenesis and disease severity in arthritic mice, whereas transfer of M-MDSCs aggravates bone erosion associated with increased OCs in recipient mice.

CONCLUSION

Our findings highlight the functional plasticity of MDSCs and identify a novel pro-osteoclastogenic pathway governed by interplay between myeloid cells and T lymphocytes in autoimmune RA.

A Study of Arginase Expression in Chronic Non-healing Wounds

Arginase expression has been recently shown to increase in numerous disease states like neurodegeneration, inflammation, and malignancies. Although it has been found to be functionally important in various disease pathologies, little is known about its role in wound healing. Here, we look at the expression of arginase and its isoforms in chronic non-healing wounds and also study the expression of nitric oxide synthase (NOS) and oxidative stress enzymes in them. Wound tissues and blood samples were collected at the time of index presentation and follow-up from 61 chronic non-healing wound cases. The expression patterns of arginase isoenzymes, NOS, superoxide dismutases (SOD), lactic acid dehydrogenase (LDH), and catalase were examined by using enzyme-linked immunosorbent assay, immunohistochemistry, and western blot analysis at the transcript and protein level. We reported a significant decrease of serum arginase levels in chronic nonhealing wounds in the progress of wound healing. Interestingly, tissue arginase levels were found to be increased with improved wound condition at follow-up. Tissue NOS, LDH, and catalase activity were also found to be increased with the progress of healing, whereas SOD levels were downregulated. Our findings reported increased expression at the transcript level of arginase-I and arginase-II in chronic non-healing wounds for the first time. In conclusion, we observed decreased serum arginase levels in completely healed patients as compared to non-healed cases. Our study findings support the hypothesis that inhibition of the activity of arginase delays wound healing. Arginase and iNOS may also find their place in the future as possible biomarkers for wound healing.

Regulation of macrophage subsets and cytokine production in leishmaniasis

Macrophages are host cells for parasites of the genus Leishmania where they multiply inside parasitophorous vacuoles. Paradoxically, macrophages are also the cells responsible for killing or controlling parasite growth, if appropriately activated. In this review, we will cover the patterns of macrophage activation and the mechanisms used by the parasite to circumvent being killed. We will highlight the impacts of the vector bite on macrophage activation. Finally, we will discuss the ontogeny of macrophages that are infected by Leishmania spp.

Antunes I, Dömling A, H. Elsinga P. Arginase as a Potential Biomarker of Disease Progression: A Molecular Imaging Perspective

Arginase is a widely known enzyme of the urea cycle that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. The action of arginase goes beyond the boundaries of hepatic ureogenic function, being widespread through most tissues. Two arginase isoforms coexist, the type I (Arg1) predominantly expressed in the liver and the type II (Arg2) expressed throughout extrahepatic tissues. By producing L-ornithine while competing with nitric oxide synthase (NOS) for the same substrate (L-arginine), arginase can influence the endogenous levels of polyamines, proline, and NO•. Several pathophysiological processes may deregulate arginase/NOS balance, disturbing the homeostasis and functionality of the organism. Upregulated arginase expression is associated with several pathological processes that can range from cardiovascular, immune-mediated, and tumorigenic conditions to neurodegenerative disorders. Thus, arginase is a potential biomarker of disease progression and severity and has recently been the subject of research studies regarding the therapeutic efficacy of arginase inhibitors. This review gives a comprehensive overview of the pathophysiological role of arginase and the current state of development of arginase inhibitors, discussing the potential of arginase as a molecular imaging biomarker and stimulating the development of novel specific and high-affinity arginase imaging probes.

Correction of arginine metabolism with sepiapterin-the precursor of nitric oxide synthase cofactor BH4-induces immunostimulatory-shift of breast cancer

Immunotherapy is a first-line treatment for many tumor types. However, most breast tumors are immuno-suppressive and only modestly respond to immunotherapy. We hypothesized that correcting arginine metabolism might improve the immunogenicity of breast tumors. We tested whether supplementing sepiapterin, the precursor of tetrahydrobiopterin (BH4)-the nitric oxide synthase (NOS) cofactor-redirects arginine metabolism from the pathway synthesizing polyamines to that of synthesizing nitric oxide (NO) and make breast tumors more immunogenic. We showed that sepiapterin elevated NO but lowered polyamine levels in tumor cells, as well as in tumor-associated macrophages (TAMs). This not only suppressed tumor cell proliferation, but also induced the conversion of TAMs from the immuno-suppressive M2-type to immuno-stimulatory M1-type. Furthermore, sepiapterin abrogated the expression of a checkpoint ligand, PD-L1, in tumors in a STAT3-dependent manner. This is the first study which reveals that supplementing sepiapterin normalizes arginine metabolism, improves the immunogenicity and inhibits the growth of breast tumor cells.

A possible role for inducible arginase isoform (AI) in the pathogenesis of chronic venous leg ulcer

Chronic venous ulcer (CVU) is a major cause of chronic wounds of lower extremities and presents a significant financial and resource burden to health care systems worldwide. Defects in the vasculature, matrix deposition, and re-epithelialization are the main histopathological changes believed to impede healing. Supplementation of the amino acid arginine that plays a crucial role in the interactions that occur during inflammation and wound healing was proven clinically to improve acute wound healing probably through enhancing activity of inducible arginase (AI) locally in the wounds. However, the possible mechanism of arginine action and the potential beneficial effects of AI/arginine in human chronic wounds remain unclear. In the present study, using biopsies, taken under local anesthesia, from adult patients (n = 12, mean age 55 years old) with CVUs in lower extremities, we investigated the correlation between AI distribution in CVUs and the histopathological changes, mainly proliferative and vascular changes. Our results show a distinct spatial distribution of AI along the ulcer in the epidermis and in the dermis with the highest level of expression being at the ulcer edge and the least expression towards the ulcer base. The AI cellular immunoreactivity, enzymatic activity, and protein levels were significantly increased towards the ulcer edge. Interestingly, a similar pattern of expression was encountered in the proliferative and the vascular changes with strong correlations between AI and the proliferative activity and vascular changes. Furthermore, AI cellular distribution was associated with increased proliferative activity, inflammation, and vascular changes. Our findings of differential expression of AI along the CVU base, edge, and nearby surrounding skin and its associations with increased proliferative activity and vascular changes provide further support to the AI implication in CVU pathogenesis. The presence of high levels of AI in the epidermis of chronic wounds may serve as a molecular marker of impaired healing and may provide future targets for therapeutic intervention.

The arginase inhibitor Nω-hydroxy-nor-arginine (nor-NOHA) induces apoptosis in leukemic cells specifically under hypoxic conditions but CRISPR/Cas9 excludes arginase 2 (ARG2) as the functional target

Cancer cells, including in chronic myeloid leukemia (CML), depend on the hypoxic response to persist in hosts and evade therapy. Accordingly, there is significant interest in drugging cancer-specific hypoxic responses. However, a major challenge in leukemia is identifying differential and druggable hypoxic responses between leukemic and normal cells. Previously, we found that arginase 2 (ARG2), an enzyme of the urea cycle, is overexpressed in CML but not normal progenitors. ARG2 is a target of the hypoxia inducible factors (HIF1−α and HIF2−α), and is required for the generation of polyamines which are required for cell growth. We therefore explored if the clinically-tested arginase inhibitor N^ω−hydroxy−nor−arginine (nor−NOHA) would be effective against leukemic cells under hypoxic conditions. Remarkably, nor−NOHA effectively induced apoptosis in ARG2-expressing cells under hypoxia but not normoxia. Co-treatment with nor−NOHA overcame hypoxia-mediated resistance towards BCR−ABL1 kinase inhibitors. While nor−NOHA itself is promising in targeting the leukemia hypoxic response, we unexpectedly found that its anti-leukemic activity was independent of ARG2 inhibition. Genetic ablation of ARG2 using CRISPR/Cas9 had no effect on the viability of leukemic cells and their sensitivity towards nor−NOHA. This discrepancy was further evidenced by the distinct effects of ARG2 knockouts and nor−NOHA on cellular respiration. In conclusion, we show that nor−NOHA has significant but off-target anti-leukemic activity among ARG2-expressing hypoxic cells. Since nor−NOHA has been employed in clinical trials, and is widely used in studies on endothelial dysfunction, immunosuppression and metabolism, the diverse biological effects of nor−NOHA must be cautiously evaluated before attributing its activity to ARG inhibition.

Pharmacokinetics and Pharmacodynamics of Promising Arginase Inhibitors

Up-regulation of arginase activity in several chronic disease conditions, including cancer and hypertension, may suggest new targets for treatment. Recently, the number of new arginase inhibitors with promising therapeutic effects for asthma, cancer, hypertension, diabetes mellitus, and erectile dysfunction has shown a remarkable increase. Arginase inhibitors may be chemical substances, such as boron-based amino acid derivatives, α-difluoromethylornithine (DMFO), and Nω-hydroxy-nor-L-arginine (nor-NOHA) or, of plant origin such as sauchinone, salvianolic acid B (SAB), piceatannol-3-O-β-D-glucopyranoside (PG) and obacunone. Despite their promising therapeutic potential, little is known about pharmacokinetics and pharmacodynamics of some of these agents. Several studies were conducted in different animal species and in vitro systems and reported significant differences in pharmacokinetics and pharmacodynamics of arginase inhibitors. Therefore, extra caution should be considered before extrapolating these studies to human. Physicochemical and pharmacokinetic profiles of some effective arginase inhibitors make it challenging to formulate stable and effective formulation. In this article, existing literature on the pharmacokinetics and pharmacodynamics of arginase inhibitors were reviewed and compared together with emphasis on possible drug interactions and solutions to overcome pharmacokinetics challenges and shortage of arginase inhibitors in clinical practice.

Utilization of Host Polyamines in Alternatively Activated Macrophages Promotes Chronic Infection by Brucella abortus

Treatment of intracellular bacterial pathogens with antibiotic therapy often requires a long course of multiple drugs. A barrier to developing strategies that enhance antibiotic efficacy against these pathogens is our poor understanding of the intracellular nutritional environment that maintains bacterial persistence. The intracellular pathogen Brucella abortus survives and replicates preferentially in alternatively activated macrophages (AAMs); however, knowledge of the metabolic adaptations promoting exploitation of this niche is limited. Here we show that one mechanism promoting enhanced survival in AAMs is a shift in macrophage arginine utilization from production of nitric oxide (NO) to biosynthesis of polyamines, induced by interleukin 4 (IL-4)/IL-13 treatment. Production of polyamines by infected AAMs promoted both intracellular survival of B. abortus and chronic infection in mice, as inhibition of macrophage polyamine synthesis or inactivation of the putative putrescine transporter encoded by potIHGF reduced both intracellular survival in AAMs and persistence in mice. These results demonstrate that increased intracellular availability of polyamines induced by arginase-1 expression in IL-4/IL-13-induced AAMs promotes chronic persistence of B. abortus within this niche and suggest that targeting of this pathway may aid in eradicating chronic infection.

Candida albicans Chitin Increases Arginase-1 Activity in Human Macrophages, with an Impact on Macrophage Antimicrobial Functions

The opportunistic human fungal pathogen Candida albicans can cause a variety of diseases, ranging from superficial mucosal infections to life-threatening systemic infections. Phagocytic cells of the innate immune response, such as neutrophils and macrophages, are important first-line responders to an infection and generate reactive oxygen and nitrogen species as part of their protective antimicrobial response. During an infection, host cells generate nitric oxide through the enzyme inducible nitric oxide synthase (iNOS) to kill the invading pathogen. Inside the phagocyte, iNOS competes with the enzyme arginase-1 for a common substrate, the amino acid l-arginine. Several pathogenic species, including bacteria and parasitic protozoans, actively modulate the production of nitric oxide by inducing their own arginases or the host’s arginase activity to prevent the conversion of l-arginine to nitric oxide. We report here that C. albicans blocks nitric oxide production in human-monocyte-derived macrophages by induction of host arginase activity. We further determined that purified chitin (a fungal cell wall polysaccharide) and increased chitin exposure at the fungal cell wall surface induces this host arginase activity. Blocking the C. albicans-induced arginase activity with the arginase-specific substrate inhibitor Nω-hydroxy-nor-arginine (nor-NOHA) or the chitinase inhibitor bisdionin F restored nitric oxide production and increased the efficiency of fungal killing. Moreover, we determined that C. albicans influences macrophage polarization from a classically activated phenotype toward an alternatively activated phenotype, thereby reducing antimicrobial functions and mediating fungal survival. Therefore, C. albicans modulates l-arginine metabolism in macrophages during an infection, potentiating its own survival.

The availability and metabolism of amino acids are increasingly recognized as crucial regulators of immune functions. In acute infections, the conversion of the “conditionally essential” amino acid l-arginine by the inducible nitric oxide synthase to nitric oxide is a resistance factor that is produced by the host to fight pathogens. Manipulation of these host defense mechanisms by the pathogen can be key to successful host invasion. We show here that the human opportunistic fungal pathogen Candida albicans influences l-arginine availability for nitric oxide production by induction of the substrate-competing host enzyme arginase-1. This led to a reduced production of nitric oxide and, moreover, reduced eradication of the fungus by human macrophages. We demonstrate that blocking of host arginase-1 activity restored nitric oxide production and increased the killing potential of macrophages. These results highlight the therapeutic potential of l-arginine metabolism in fungal diseases.

Assessing the impact of minimizing arginine conversion in fully defined SILAC culture medium in human embryonic stem cells

We present a fully defined culture system (adapted Essential8^TM [E8^TM] medium in combination with vitronectin) for human embryonic stem cells that can be used for SILAC purposes. Although a complete incorporation of the labels was observed after 4 days in culture, over 90% of precursors showed at least 10% conversion. To reduce this arginine conversion, E8^TM medium was modified by adding (1) l‐proline, (2) l‐ornithine, (3) N^ω‐hydroxy‐nor‐l‐arginine acetate, or by (4) lowering the arginine concentration. Reduction of arginine conversion was best obtained by adding 5 mM l‐ornithine, followed by 3.5 mM l‐proline and by lowering the arginine concentration in the medium to 99.5 μM. No major changes in pluripotency and cell amount could be observed for the adapted E8^TM media with ornithine and proline. However, our subsequent ion mobility assisted data‐independent acquisition (high‐definition MS) proteome analysis cautions for ongoing changes in the proteome when aiming at longer term suppression of arginine conversion.

Peroxiredoxin 1 Contributes to Host Defenses against Mycobacterium tuberculosis

Peroxiredoxin (PRDX)1 is an antioxidant that detoxifies hydrogen peroxide and peroxinitrite. Compared with wild-type (WT) mice, Prdx1-deficient (Prdx1^-/-) mice showed increased susceptibility to Mycobacterium tuberculosis and lower levels of IFN-γ and IFN-γ-producing CD4⁺ T cells in the lungs after M. tuberculosis infection. IL-12 production, c-Rel induction, and p38 MAPK activation levels were lower in Prdx1^-/- than in WT bone marrow-derived macrophages (BMDMs). IFN-γ-activated Prdx1^-/- BMDMs did not kill M. tubercuosis effectively. NO production levels were lower, and arginase activity and arginase 1 (Arg1) expression levels were higher, in IFN-γ-activated Prdx1^-/- than in WT BMDMs after M. tuberculosis infection. An arginase inhibitor, N^ω-hydroxy-nor-arginine, restored antimicrobial activity and NO production in IFN-γ-activated Prdx1^-/- BMDMs after M. tuberculosis infection. These results suggest that PRDX1 contributes to host defenses against M. tuberculosis PRDX1 positively regulates IL-12 production by inducing c-Rel and activating p38 MAPK, and it positively regulates NO production by suppressing Arg1 expression in macrophages infected with M. tuberculosis.

Deregulation of arginase induces bone complications in high-fat/high-sucrose diet diabetic mouse model

A balanced diet is crucial for healthy development and prevention of musculoskeletal related diseases. Diets high in fat content are known to cause obesity, diabetes and a number of other disease states. Our group and others have previously reported that activity of the urea cycle enzyme arginase is involved in diabetes-induced dysregulation of vascular function due to decreases in nitric oxide formation. We hypothesized that diabetes may also elevate arginase activity in bone and bone marrow, which could lead to bone-related complications. To test this we determined the effects of diabetes on expression and activity of arginase, in bone and bone marrow stromal cells (BMSCs). We demonstrated that arginase 1 is abundantly present in the bone and BMSCs. We also demonstrated that arginase activity and expression in bone and bone marrow is up-regulated in models of diabetes induced by HFHS diet and streptozotocin (STZ). HFHS diet down-regulated expression of healthy bone metabolism markers (BMP2, COL-1, ALP, and RUNX2) and reduced bone mineral density, bone volume and trabecular thickness. However, treatment with an arginase inhibitor (ABH) prevented these bone-related complications of diabetes. In-vitro study of BMSCs showed that high glucose treatment increased arginase activity and decreased nitric oxide production. These effects were reversed by treatment with an arginase inhibitor (ABH). Our study provides evidence that deregulation of l-arginine metabolism plays a vital role in HFHS diet-induced diabetic complications and that these complications can be prevented by treatment with arginase inhibitors. The modulation of l-arginine metabolism in disease could offer a novel therapeutic approach for osteoporosis and other musculoskeletal related diseases.

CD11b(+)Ly6C(++)Ly6G(-) Cells with Suppressive Activity Towards T Cells Accumulate in Lungs of Influenza a Virus-Infected Mice

Influenza A virus (IAV) infection causes an acute respiratory disease characterized by a strong inflammatory immune response and severe immunopathology. Proinflammatory mechanisms are well described in the murine IAV infection model, but less is known about the mechanisms leading to the resolution of inflammation. Here, we analyzed the contribution of CD11b⁺Ly6C⁺⁺Ly6G^– cells to this process. An accumulation of CD11b⁺Ly6C⁺⁺Ly6G^– cells within the lungs was observed during the course of IAV infection. Phenotypic characterization of these CD11b⁺Ly6C⁺⁺Ly6G^– cells by flow cytometry and RNA-Seq revealed an activated phenotype showing both pro- and anti-inflammatory features, including the expression of inducible nitric oxide synthase (iNOS) by a fraction of cells in an IFN-γ-dependent manner. Moreover, CD11b⁺Ly6C⁺⁺Ly6G^– cells isolated from lungs of IAV-infected animals displayed suppressive activity when tested in vitro, and iNOS inhibitors could abrogate this suppressive activity. Collectively, our data suggest that during IAV infection, CD11b⁺Ly6C⁺⁺Ly6G^– cells acquire immunoregulatory function, which might contribute to the prevention of pathology during this life-threatening disease.

Arginine deprivation and immune suppression in a mouse model of Alzheimer's disease

The pathogenesis of Alzheimer's disease (AD) is a critical unsolved question; and although recent studies have demonstrated a strong association between altered brain immune responses and disease progression, the mechanistic cause of neuronal dysfunction and death is unknown. We have previously described the unique CVN-AD mouse model of AD, in which immune-mediated nitric oxide is lowered to mimic human levels, resulting in a mouse model that demonstrates the cardinal features of AD, including amyloid deposition, hyperphosphorylated and aggregated tau, behavioral changes, and age-dependent hippocampal neuronal loss. Using this mouse model, we studied longitudinal changes in brain immunity in relation to neuronal loss and, contrary to the predominant view that AD pathology is driven by proinflammatory factors, we find that the pathology in CVN-AD mice is driven by local immune suppression. Areas of hippocampal neuronal death are associated with the presence of immunosuppressive CD11c(+) microglia and extracellular arginase, resulting in arginine catabolism and reduced levels of total brain arginine. Pharmacologic disruption of the arginine utilization pathway by an inhibitor of arginase and ornithine decarboxylase protected the mice from AD-like pathology and significantly decreased CD11c expression. Our findings strongly implicate local immune-mediated amino acid catabolism as a novel and potentially critical mechanism mediating the age-dependent and regional loss of neurons in humans with AD.

A natural compound macelignan protects midbrain dopaminergic neurons from inflammatory degeneration via microglial arginase-1 expression

Inflammatory events involving activated microglia have been recognized to play an important role in pathogenesis of various neurodegenerative disorders including Parkinson disease. Compounds regulating activation profiles of microglia may provide therapeutic benefits for Parkinson disease characterized by degeneration of midbrain dopaminergic neurons. Here we examined the effect of macelignan, a compound derived from nutmeg, on inflammatory degeneration of midbrain dopaminergic neurons. Treatment of midbrain slice cultures with interferon (IFN)-γ and lipopolysaccharide (LPS) caused a substantial decrease in viable dopaminergic neurons and an increase in nitric oxide (NO) production indicated by extracellular nitrite accumulation. Application of macelignan (10 μM) concomitantly with LPS prevented the loss of dopaminergic neurons. Besides nitrite accumulation, up-regulation of inducible NO synthase protein expression in response to IFN-γ/LPS was confirmed by Western blotting, and immunohistochemical examination revealed expression of inducible NO synthase in a subpopulation of Iba-1-poitive microglia. However, macelignan did not affect any of these NO-related parameters. On the other hand, macelignan promoted expression of arginase-1 in midbrain slice cultures irrespective of the presence or the absence of IFN-γ/LPS treatment. Arginase-1 expression was mainly localized in a subpopulation of Iba-1-positive cells. Importantly, the neuroprotective effect of macelignan was antagonized by N(ω)-hydroxy-nor-L-arginine, a specific arginase inhibitor. The neuroprotective effect of macelignan was also prevented by GW9662, a peroxisome proliferator-activated receptor γ (PPARγ) antagonist. Overall, these results indicate that macelignan, a compound with PPARγ agonist activity, can provide neuroprotective effect on dopaminergic neurons in an arginase-dependent but NO-independent manner.

Reprogramming the tumor microenvironment: tumor-induced immunosuppressive factors paralyze T cells

It has become evident that tumor-induced immuno-suppressive factors in the tumor microenvironment play a major role in suppressing normal functions of effector T cells. These factors serve as hurdles that limit the therapeutic potential of cancer immunotherapies. This review focuses on illustrating the molecular mechanisms of immunosuppression in the tumor microenvironment, including evasion of T-cell recognition, interference with T-cell trafficking, metabolism, and functions, induction of resistance to T-cell killing, and apoptosis of T cells. A better understanding of these mechanisms may help in the development of strategies to enhance the effectiveness of cancer immunotherapies.

Expression quantitative trait loci and receptor pharmacology implicate Arg1 and the GABA-A receptor as therapeutic targets in neuroblastoma

The development of targeted therapeutics for neuroblastoma, the third most common tumor in children, has been limited by a poor understanding of growth signaling mechanisms unique to the peripheral nerve precursors from which tumors arise. In this study, we combined genetics with gene-expression analysis in the peripheral sympathetic nervous system to implicate arginase 1 and GABA signaling in tumor formation in vivo. In human neuroblastoma cells, either blockade of ARG1 or benzodiazepine-mediated activation of GABA-A receptors induced apoptosis and inhibited mitogenic signaling through AKT and MAPK. These results suggest that ARG1 and GABA influence both neural development and neuroblastoma and that benzodiazepines in clinical use may have potential applications for neuroblastoma therapy.

Nitric oxide synthase deficiency and the pathophysiology of muscular dystrophy

The secondary loss of neuronal nitric oxide synthase (nNOS) that occurs in dystrophic muscle is the basis of numerous, complex and interacting features of the dystrophic pathology that affect not only muscle itself, but also influence the interaction of muscle with other tissues. Many mechanisms through which nNOS deficiency contributes to misregulation of muscle development, blood flow, fatigue, inflammation and fibrosis in dystrophic muscle have been identified, suggesting that normalization in NO production could greatly attenuate diverse aspects of the pathology of muscular dystrophy through multiple regulatory pathways. However, the relative importance of the loss of nNOS from the sarcolemma versus the importance of loss of total nNOS from dystrophic muscle remains unknown. Although most current evidence indicates that nNOS localization at the sarcolemma is not required to achieve NO-mediated reductions of pathology in muscular dystrophy, the question remains open concerning whether membrane localization would provide a more efficient rescue from features of the dystrophic phenotype.

Myeloid-derived suppressor cells as a potential therapy for experimental autoimmune myasthenia gravis

We recently demonstrated that hepatic stellate cells induce the differentiation of myeloid-derived suppressor cells (MDSCs) from myeloid progenitors. In this study, we found that adoptive transfer of these MDSCs effectively reversed disease progression in experimental autoimmune myasthenia gravis (EAMG), a T cell-dependent and B cell-mediated model for myasthenia gravis. In addition to ameliorated disease severity, MDSC-treated EAMG mice showed suppressed acetylcholine receptor (AChR)-specific T cell responses, decreased levels of serum anti-AChR IgGs, and reduced complement activation at the neuromuscular junctions. Incubating MDSCs with B cells activated by anti-IgM or anti-CD40 Abs inhibited the proliferation of these in vitro-activated B cells. Administering MDSCs into mice immunized with a T cell-independent Ag inhibited the Ag-specific Ab production in vivo. MDSCs directly inhibit B cells through multiple mechanisms, including PGE2, inducible NO synthase, and arginase. Interestingly, MDSC treatment in EAMG mice does not appear to significantly inhibit their immune response to a nonrelevant Ag, OVA. These results demonstrated that hepatic stellate cell-induced MDSCs concurrently suppress both T and B cell autoimmunity, leading to effective treatment of established EAMG, and that the MDSCs inhibit AChR-specific immune responses at least partially in an Ag-specific manner. These data suggest that MDSCs could be further developed as a novel approach to treating myasthenia gravis and, even more broadly, other diseases in which T and B cells are involved in pathogenesis.

Human papillomavirus e7 oncoprotein transgenic skin develops an enhanced inflammatory response to 2,4-dinitrochlorobenzene by an arginase-1-dependent mechanism

We have shown that the expression of human papillomavirus type 16 E7 (HPV16.E7) protein within epithelial cells results in local immune suppression and a weak and ineffective immune response to E7 similar to that occuring in HPV-associated premalignancy and cancers. However, a robust acute inflammatory stimulus can overcome this to enable immune elimination of HPV16.E7-transformed epithelial cells. 2,4-Dinitrochlorobenzene (DNCB) can elicit acute inflammation and it has been shown to initiate the regression of HPV-associated genital warts. Although the clinical use of DNCB is discouraged owing to its mutagenic potential, understanding how DNCB-induced acute inflammation alters local HPV16.E7-mediated immune suppression might lead to better treatments. Here, we show that topical DNCB application to skin expressing HPV16.E7 as a transgene induces a hyperinflammatory response, which is not seen in nontransgenic control animals. The E7-associated inflammatory response is characterized by enhanced expression of Th2 cytokines and increased infiltration of CD11b(+)Gr1(int)F4/80(+)Ly6C(hi)Ly6G(low) myeloid cells, producing arginase-1. Inhibition of arginase with an arginase-specific inhibitor, N(omega)-hydroxy-nor-L-arginine, ameliorates the DNCB-induced inflammatory response. Our results demonstrate that HPV16.E7 protein enhances DNCB-associated production of arginase-1 by myeloid cells and consequent inflammatory cellular infiltration of skin.

Alternatively Activated Macrophages Revisited: New Insights into the Regulation of Immunity, Inflammation and Metabolic Function following Parasite Infection

The role of macrophages in homeostatic conditions and the immune system range from clearing debris to recognizing and killing pathogens. While classically activated macrophages (CAMacs) are induced by T helper type 1 (Th1) cytokines and exhibit microbicidal properties, Th2 cytokines promote alternative activation of macrophages (AAMacs). AAMacs contribute to the killing of helminth parasites and mediate additional host-protective processes such as regulating inflammation and wound healing. Yet, other parasites susceptible to Th1 type responses can exploit alternative activation of macrophages to diminish Th1 immune responses and prolong infection. In this review, we will delineate the factors that mediate alternative activation (e.g. Th2 cytokines and chitin) and the resulting downstream signaling events (e.g. STAT6 signaling). Next, the specific AAMac-derived factors (e.g. Arginase1) that contribute to resistance or susceptibility to parasitic infections will be summarized. Finally, we will conclude with the discussion of additional AAMac functions beyond immunity to parasites, including the regulation of inflammation, wound healing and the regulation of metabolic disorders.

The Concise Guide to PHARMACOLOGY 2013/14: enzymes

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Enzymes are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, ion channels, nuclear hormone receptors, catalytic receptors and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Proteomic identification of mitochondrial targets of arginase in human breast cancer

We have previously reported arginase expression in human breast cancer cells and demonstrated that the inhibition of arginase by N^ω hydroxy L-arginine (NOHA) in MDA-MB-468 cells induces apoptosis. However, arginase expression and its possible molecular targets in human breast tumor samples and potential clinical implications have not been fully elucidated. Here, we demonstrate arginase expression in human breast tumor samples, and several established breast cancer cell lines, in which NOHA treatment selectively inhibits cell proliferation. The over-expression of Bcl2 in MDA-MB-468 cells abolished NOHA-induced apoptosis, suggesting that the mitochondria may be the main site of NOHA’s action. We, therefore, undertook a proteomics approach to identify key mitochondrial targets of arginase in MDA-MB-468 cells. We identified 54 non-mitochondrial and 13 mitochondrial proteins that were differentially expressed in control and NOHA treated groups. Mitochondrial serine hydroxymethyltransferase (mSHMT) was identified as one of the most promising targets of arginase. Both arginase II (Arg II) and mSHMT expressions were higher in human breast tumor tissues compared to the matched normal and there was a strong correlation between Arg II and mSHMT protein expression. MDA-MB-468 xenografts had significant upregulation of Arg II expression that preceded the induction of mSHMT expression. Small inhibitory RNA (siRNA)-mediated inhibition of Arg II in MDA-MB-468 and HCC-1806 cells led to significant inhibition of both the mSHMT gene and protein expression. As mSHMT is a key player in folate metabolism, our data provides a novel link between arginine and folate metabolism in human breast cancer, both of which are critical for tumor cell proliferation.

Development of novel arginase inhibitors for therapy of endothelial dysfunction

Endothelial dysfunction and resulting vascular pathology have been identified as an early hallmark of multiple diseases, including diabetes mellitus. One of the major contributors to endothelial dysfunction is a decrease in nitric oxide (NO) bioavailability, impaired NO signaling, and an increase in the amount of reactive oxygen species (ROS). In the endothelium NO is produced by endothelial nitric oxide synthase (eNOS), for which l-arginine is a substrate. Arginase, an enzyme critical in the urea cycle also metabolizes l-arginine, thereby directly competing with eNOS for their common substrate and constraining its bioavailability for eNOS, thereby compromising NO production. Arginase expression and activity is upregulated in many cardiovascular diseases including ischemia reperfusion injury, hypertension, atherosclerosis, and diabetes mellitus. More importantly, since the 1990s, specific arginase inhibitors such as N-hydroxy-guanidinium or N-hydroxy-nor-l-arginine, and boronic acid derivatives, such as, 2(S)-amino-6-boronohexanoic acid, and S-(2-boronoethyl)-l-cysteine, that can bridge the binuclear manganese cluster of arginase have been developed. These highly potent and specific inhibitors can now be used to probe arginase function and thereby modulate the redox milieu of the cell by changing the balance between NO and ROS. Inspired by this success, drug discovery programs have recently led to the identification of α–α-disubstituted amino acid based arginase inhibitors [such as (R)-2-amino-6-borono-2-(2-(piperidin-1-yl)ethyl)hexanoic acid], that are currently under early investigation as therapeutics. Finally, some investigators concentrate on identification of plant derived compounds with arginase inhibitory capability, such as piceatannol-3′-O-β-d-glucopyranoside (PG). All of these synthesized or naturally derived small molecules may represent novel therapeutics for vascular disease particularly that associated with diabetes.

Nitric oxide acts as a positive regulator to induce metamorphosis of the ascidian Herdmania momus

Marine invertebrates commonly have a biphasic life cycle in which the metamorphic transition from a pelagic larva to a benthic post-larva is mediated by the nitric oxide signalling pathway. Nitric oxide (NO) is synthesised by nitric oxide synthase (NOS), which is a client protein of the molecular chaperon heat shock protein 90 (HSP90). It is notable, then, that both NO and HSP90 have been implicated in regulating metamorphosis in marine invertebrates as diverse as urochordates, echinoderms, molluscs, annelids, and crustaceans. Specifically, the suppression of NOS activity by the application of either NOS- or HSP90-inhibiting pharmacological agents has been shown consistently to induce the initiation of metamorphosis, leading to the hypothesis that a negative regulatory role of NO is widely conserved in biphasic life cycles. Further, the induction of metamorphosis by heat-shock has been demonstrated for multiple species. Here, we investigate the regulatory role of NO in induction of metamorphosis of the solitary tropical ascidian, Herdmania momus. By coupling pharmacological treatments with analysis of HmNOS and HmHSP90 gene expression, we present compelling evidence of a positive regulatory role for NO in metamorphosis of this species, in contrast to all existing ascidian data that supports the hypothesis of NO as a conserved negative regulator of metamorphosis. The exposure of competent H. momus larvae to a NOS inhibitor or an NO donor results in an up-regulation of NOS and HSP90 genes. Heat shock of competent larvae induces metamorphosis in a temperature dependent manner, up to a thermal tolerance that approaches 35°C. Both larval/post-larval survival and the appearance of abnormal morphologies in H. momus post-larvae reflect the magnitude of up-regulation of the HSP90 gene in response to heat-shock. The demonstrated role of NO as a positive metamorphic regulator in H. momus suggests the existence of inter-specific adaptations of NO regulation in ascidian metamorphosis.

Local arginase 1 activity is required for cutaneous wound healing

Chronic nonhealing wounds in the elderly population are associated with a prolonged and excessive inflammatory response, which is widely hypothesized to impede healing. Previous studies have linked alterations in local L-arginine metabolism, principally mediated by the enzymes arginase (Arg) and inducible nitric oxide synthase (iNOS), to pathological wound healing. Over subsequent years, interest in Arg/iNOS has focused on the classical versus alternatively activated (M1/M2) macrophage paradigm. Although the role of iNOS during healing has been studied, Arg contribution to healing remains unclear. Here, we report that Arg is dynamically regulated during acute wound healing. Pharmacological inhibition of local Arg activity directly perturbed healing, as did Tie2-cre-mediated deletion of Arg1, revealing the importance of Arg1 during healing. Inhibition or depletion of Arg did not alter alternatively activated macrophage numbers but instead was associated with increased inflammation, including increased influx of iNOS(+) cells and defects in matrix deposition. Finally, we reveal that in preclinical murine models reduced Arg expression directly correlates with delayed healing, and as such may represent an important future therapeutic target.

Comparative pharmacokinetics of N(ω)-hydroxy-nor-L-arginine, an arginase inhibitor, after single-dose intravenous, intraperitoneal and intratracheal administration to brown Norway rats

1.  Rodent studies have documented that N(ω)-hydroxy-nor-L-arginine (nor-NOHA), an arginase inhibitor, has therapeutic potential in the treatment of cardiovascular and obstructive airway diseases. However, its bioavailability and pharmacokinetics have not been described so far. 2.  Anesthetized brown Norway rats were administered single doses of nor-NOHA (10, 30 or 90 mg/kg) intravenously (i.v.), intraperitonealy (i.p.) or via intratracheal (i.t.) instillation of aerosol. Plasma nor-NOHA was assayed using a validated HPLC method. 3.  Upon i.v. administration, the mean concentration showed a biphasic decline and its value dropped below 10% of the maximum after 20 min. The pharmacokinetics were linear with the total and inter-compartmental clearances of 33 and 17 mL/min/kg, central and peripheral volumes of distribution of 0.19 and 0.43 L/kg and terminal half-life of 30 min. 4.  The average absolute bioavailability of nor-NOHA after i.p. and i.t. delivery was 98% and 53%, respectively. The absorption from the airways was rate-limiting and its extent decreased with the dose. 5.  In conclusion, nor-NOHA is rapidly cleared from the plasma in concordance with the short time window of its in vivo inhibitory activity reported in the literature. I.t. instillation of aerosol for topical effects of nor-NOHA in the airways is characterized with significant systemic availability.

Inhibition of arginase ameliorates experimental ulcerative colitis in mice

Nitric oxide (NO) is produced from the conversion of L-arginine by NO synthase (NOS) and regulates a variety of processes in the gastrointestinal tract. Considering the increased activity of arginase in colitis tissue, it is speculated that arginase could inhibit NO synthesis by competing for the same L-arginine substrate, resulting in the exacerbation of colitis. We examined the role of arginase and its relationship to NO metabolism in dextran sulfate sodium (DSS)-induced colitis. Experimental colitis was induced in mice by administration of 2.5% DSS in drinking water for 8 days. Treatment for arginase inhibition was done by once daily intraperitoneal injection of N(ω)-hydroxy-nor- arginine (nor-NOHA). On day 8, we evaluated clinical parameters (body weight, disease activity index, and colon length), histological features, the activity and expression of arginase, L-arginine content, the expression of NO synthase (NOS), and the concentration of NO end-product (NOx: nitrite + nitrate). Administration of nor-NOHA improved the worsened clinical parameters and histological features in DSS-induced colitis. Treatment with nor-NOHA attenuated the increased activity of arginase, upregulation of arginase Ι at both mRNA and protein levels, and decreased the content of L-arginine in colonic tissue in the DSS-treated mice. Conversely, despite the decreased expression of NOS2 mRNA, the decreased concentration of NOx in colonic tissues was restored to almost normal levels. The consumption of L-arginine by arginase could lead to decreased production of NO from NOS, contributing to the pathogenesis of the colonic inflammation; thus, arginase inhibition might be effective for improving colitis.

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.

Oxygen-dependent regulation of nitric oxide production by inducible nitric oxide synthase

Inducible nitric oxide synthase (iNOS) catalyzes the reaction that converts the substrates O(2) and l-arginine to the products nitric oxide (NO) and l-citrulline. Macrophages, and many other cell types, upregulate and express iNOS primarily in response to inflammatory stimuli. Physiological and pathophysiological oxygen tension can regulate NO production by iNOS at multiple levels, including transcriptional, translational, posttranslational, enzyme dimerization, cofactor availability, and substrate dependence. Cell culture techniques that emphasize control of cellular PO(2), and measurement of NO or its stable products, have been used by several investigators for in vitro study of the O(2) dependence of NO production at one or more of these levels. In most cell types, prior or concurrent exposure to cytokines or other inflammatory stimuli is required for the upregulation of iNOS mRNA and protein by hypoxia. Important transcription factors that target the iNOS promoter in hypoxia include hypoxia-inducible factor 1 and/or nuclear factor κB. In contrast to the upregulation of iNOS by hypoxia, in most cell types NO production is reduced by hypoxia. Recent work suggests a prominent role for O(2) substrate dependence in the short-term regulation of iNOS-mediated NO production.

HPLC determination of arginases inhibitor N-(ω)-hydroxy-nor-L-arginine using core-shell particle column and LC-MS/MS identification of principal metabolite in rat plasma

For the purpose of in vivo pharmacokinetic studies, an HPLC method was developed and validated for the quantification of N-(ω)-hydroxy-nor-L-arginine, L-arginine and N-(ω)-ethyl-L-arginine (internal standard) in rat plasma. Sample processing involved a solid-phase extraction on the Waters MCX cartridges and on-line pre-column derivatization of the analytes with o-phthaldialdehyde and 3-mercaptopropionic acid. Separation of the derivatives was carried out on a core-shell Kinetex C18 column in a gradient elution mode with a mobile phase consisting of methanol and water (pH=3.00 adjusted with formic acid). Fluorimetric detection with the excitation/emission wavelengths of 235/450 nm was used. The method was validated according to the FDA guidelines and applied to pilot pharmacokinetic experiments. An unknown metabolite was extracted from the plasma of Wistar rats after a single bolus of N-(ω)-hydroxy-nor-L-arginine (i.v. 10 mg kg(-1)). The metabolite was identified as nor-L-arginine using mass spectrometry. Validated method was successfully used for pilot pharmacokinetic experiment on rats.

In vitro evidence of involvement of the epithelial y+ transporter in β-defensin production on the ocular surface

To analyse the hypothesis as to whether there is a functional relationship between human cationic amino acid transporters (hCATs, y(+) transporter, the main transporter of L-arginine and L-lysine) and human β-defensin (important components of immune function) production on the ocular surface, arginase and nitrate monoxide synthase (NOS), enzymes that compete for L-arginine, were inhibited by norNOHA (N(omega)-hydroxy-nor-L-arginine) and/or L-NAME (NG-nitro-L-arginine methyl ester) in cultured human corneal epithelial cells. In addition, the transport activity of hCAT proteins was inhibited or activated through α-tocopherol or PMA (phorbol myristate acetate), respectively. Concentrations of the human inducible β-defensins (hBD) 2 and 3 were determined by ELISA experiments. The basic expression of hBD3 in non-stimulated HCE cells significantly exceeded that of hBD2. Both β-defensins also differed as to how readily their excretion could be stimulated. HBD2 excretion rate was 3.5 time more by L-NAME, whereas norNOHA had no effect. In contrast, hBD3 excretion was increased by norNOHA by a factor of 1.5 but L-NAME alone had no effect. The excretion of both β-defensins was increased 3- and 6-fold by combined administration of L-NAME, norNOHA and interleukin (IL)-1β. Administration of α-tocopherol increased hBD2 excretion twofold. No effect was observed for hBD3. With PMA, on the other hand, a reduction in secretion for both β-defensins was observed. These in vitro findings provide evidence of a functional association between CAT proteins and β-defensins 2 and 3 opening up a new field of research with pharmacological perspectives for treatment of inflammatory diseases such as keratitis or dry eye disease.

Overexpression of ornithine decarboxylase decreases ventricular systolic function during induction of cardiac hypertrophy

Ornithine decarboxylase (ODC), the first enzyme of polyamine metabolism, is rapidly upregulated in response to agents that induce a pathological cardiac hypertrophy. Transgenic mice overexpressing ODC in the heart (MHC-ODC mice) experience a much more dramatic left ventricular hypertrophy in response to β-adrenergic stimulation with isoproterenol (ISO) compared to wild-type (WT) controls. ISO also induced arginase activity in transgenic hearts but not in controls. The current work studies the cooperation between the cardiac polyamines and L-arginine (L-Arg) availability in MHC-ODC mice. Although ISO-induced hypertrophy is well-compensated, MHC-ODC mice administered L-Arg along with ISO showed a rapid onset of systolic dysfunction and died within 48 h. Myocytes isolated from MHC-ODC mice administered L-Arg/ISO exhibited reduced contractility and altered calcium transients, suggesting an alteration in [Ca(2+)] homeostasis, and abbreviated action potential duration, which may contribute to arrhythmogenesis. The already elevated levels of spermidine and spermine were not further altered in MHC-ODC hearts by L-Arg/ISO treatment, suggesting alternative L-Arg utilization pathways lead to dysregulation of intracellular calcium. MHC-ODC mice administered an arginase inhibitor (Nor-NOHA) along with ISO died almost as rapidly as L-Arg/ISO-treated mice, while the iNOS inhibitor S-methyl-isothiourea (SMT) was strongly protective against L-Arg/ISO. These results point to the induction of arginase as a protective response to β-adrenergic stimulation in the setting of high polyamines. Further, NO generated by exogenously supplied L-Arg may contribute to the lethal consequences of L-Arg/ISO treatment. Since considerable variations in human cardiac polyamine and L-Arg content are likely, it is possible that alterations in these factors may influence myocyte contractility.

TGF-β2 reduces nitric oxide synthase mRNA through a ROCK-dependent pathway in airway epithelial cells

Exhaled NO (eNO) is a potential noninvasive biomarker of inflammation in asthma. The significant intersubject variability of eNO within clinically similar patients has contributed to its limited clinical application. Arginase and NO synthase (NOS) utilize the same substrate (l-arginine) and contribute to the fibrotic and inflammatory features of asthma, respectively. Interestingly, TGF-β(2) can increase the expression of arginase, stimulates fibrosis, and is overexpressed in asthma. We hypothesized that TGF-β(2)-enhanced arginase activity would decrease gas phase NO release from lung epithelial cells by limiting l-arginine availability for NOS. Our results show that TGF-β(2) (5 ng/ml) significantly enhances total arginase activity up to two- to threefold in both primary small airway epithelial cells (SAECs) and the A549 cell line. Preincubation with TGF-β(2) prior to cytokine (IL-1β, TNF-α, and IFN-γ, 10 ng/ml each) stimulation decreases gas phase NO release to baseline levels (from 1.66 ± 0.52 to 0.30 ± 0.12 pl·s(-1)·cm(-2) and from 0.27 ± 0.03 pl·s(-1)·cm(-2) to near zero in SAEC and A549 cells, respectively). Addition of arginase inhibitor (N(ω)-hydroxy-nor-l-arginine) or small interfering RNA only partly reverses the reduction. In contrast, Rho-kinase (ROCK) pathway inhibitor (Y-27632) completely recovers the cytokine-induced NO flux in the present of TGF-β(2). Inducible NO synthase (iNOS) mRNA and protein levels change in a similar trend as NO release from the cells. We conclude that TGF-β(2) impacts cytokine-induced NO production in airway epithelial cells by reducing iNOS mRNA and protein levels through a ROCK-dependent pathway.

Arginine bioavailability ratios are associated with cardiovascular mortality in patients referred to coronary angiography

OBJECTIVES

Arginine is the only source for nitric oxide (NO) synthesis. The bioavailability of NO plays a pivotal role in endothelial function and consequently in cardiovascular disease. The aim of the current study is to investigate the association of arginine bioavailability ratios with markers of endothelial function and cardiovascular mortality in patients referred to coronary angiography.

METHODS

We investigated 2236 patients recruited within the LUdwigshafen RIsk and Cardiovascular Health (LURIC) study that were followed up for a median of 7.7 years. Arginine, ornithine and citrulline were chromatographically determined after precolumn-derivatisation followed by postcolumn continuous reaction with ninhydrin. Global arginine bioavailability (GABR) was calculated by arginine divided by the sum of ornithine plus citrulline.

RESULTS

We observed a significant rise in cardiovascular mortality with decreasing GABR and arginine to ornithine ratio quartiles. The adjusted Cox proportional HRs for GABR were 1.27 (0.88-1.83), 1.27 (0.89-1.80) and 1.75 (1.24-2.45) for the 3rd, the 2nd and the 1st quartile respectively in comparison to the 4th quartile. The HRs for the quartiles of the arginine to ornithine ratio were 1.83 (1.25-2.67), 2.17 (1.50-3.20) and 2.02 (1.39-2.92) respectively. Patients with type 2 diabetes mellitus had a significantly lower GABR than persons without diabetes (0.88 ± 0.23 vs. 0.94 ± 0.24, p<0.001). GABR was found to be inversely correlated with endothelial markers as VCAM-1 (r=-0.301, p<0.001) or ICAM-1 (r=-0.136, p<0.001).

CONCLUSIONS

GABR and the arginine to ornithine ratio are associated with markers of endothelial dysfunction and increased risk of cardiovascular mortality. Further studies are warranted to elucidate the pathobiology and clinical relevance of the arginine bioavailability ratios in cardio-metabolic diseases.

Modeling gas phase nitric oxide release in lung epithelial cells

Nitric oxide (NO) is present in exhaled breath and is generally considered to be a noninvasive marker of airway inflammation, and is thus of particular relevance to monitoring asthma. NO is produced when L-arginine is converted to L-citrulline by NO synthase (NOS); however, L-arginine is also the substrate for arginase and both enzymes are upregulated in asthma. Recent reports have speculated that enhanced expression of one or both enzymes could lead to a limitation in substrate availability, and hence impact downstream targets or markers such as exhaled NO. The non-linear nature and vastly different kinetics of the enzymes make predictions difficult, particularly over the wide range of enzyme activity between baseline and inflammation. In this study, we developed a steady state model of L-arginine transmembrane transport, NO production, diffusion, and gas phase NO release from lung epithelial cells. We validated our model with experimental results of gas phase NO release and intracellular l-arginine concentration in A549 cells, and then performed a sensitivity analysis to determine relative impact of each enzyme on NO production. Our model predicts intracellular L-arginine and gas phase NO release over a wide range of initial extracellular L-arginine concentrations following stimulation with cytomix (10ng/ml TNF-α, IL-1β, and INF-γ). Relative sensitivity analysis demonstrates that enhanced arginase activity has little impact on l-arginine bioavailability for NOS. In addition, NOS activity is the dominant parameter which impacts gas phase NO release.

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.