Elevated asymmetric dimethylarginine alters lung function and induces collagen deposition in mice

Asymmetric dimethylarginine is increased in asthma

RATIONALE

Asymmetric dimethylarginine (ADMA) is an endogenous nitric oxide synthase (NOS) inhibitor that competes with L-arginine for binding to NOS. It has been suggested that ADMA contributes to inflammation, collagen deposition, nitrosative stress, and lung function in murine models.

OBJECTIVES

To test the hypothesis that ADMA is increased in asthma and that NOS inhibition by ADMA contributes to airways obstruction.

METHODS

We assessed alterations of L-arginine, ADMA, and symmetric dimethylarginine (SDMA) levels in a murine model of allergic airways inflammation using LC-tandem mass spectrometry. Based on the levels of ADMA observed in the murine model, we further tested the direct effects of nebulized inhaled ADMA on airways responsiveness in naive control mice. We also assessed alterations of L-arginine, ADMA, and SDMA in humans in adult lung specimens and sputum samples from pediatric patients with asthma.

MEASUREMENTS AND MAIN RESULTS

ADMA was increased in lungs from the murine model of allergic airways inflammation. Exogenous administration of ADMA to naive mice, at doses consistent with the levels observed in the allergically inflamed lungs, resulted in augmentation of the airways responsiveness to methacholine. ADMA levels were also increased in human asthma lungs and sputum samples.

CONCLUSIONS

ADMA levels are increased in asthma and contribute to NOS-related pathophysiology.

Acute lung injury-induced collagen deposition is associated with elevated asymmetric dimethylarginine and arginase activity

Evidence suggests that lung structure and function are partly maintained by a balance between the competing arginine-metabolizing enzymes arginase and nitric oxide (NO) synthase. Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of NO synthase. It is metabolized by dimethylarginine dimethylaminohydrolase 2 (DDAH-2), which is oxidant-sensitive. The mechanism that induces excess lung collagen deposition in burned patients has not yet been explored. Our objective was to investigate the role of ADMA and the arginase pathway in acute lung injury. An ovine model for burn and smoke inhalation injury was used to assess excess lung collagen deposition. Sheep were deeply anesthetized during the injury, mechanically ventilated, resuscitated with fluid, and killed after either 2 or 3 weeks. Lungs were assessed histologically and biochemically for collagen content, arginase activity, lipid peroxidation product and antioxidant concentration, and protein concentrations. Plasma was assessed for amino acid and nitrate/nitrite concentrations. Burn and inhalation injury resulted in significantly reduced pulmonary function and increased lung collagen deposition. These physiological changes were associated with significantly increased lung arginase activity, collagen synthesis precursor ornithine aminotransferase, and ornithine decarboxylase, which is associated with cell proliferation. Significant decreases in plasma nitrate/nitrite after injury were associated with increased lung ADMA concentrations and decreased DDAH-2 expression. The decreased DDAH-2 expression was associated with significantly increased lipid peroxidation product and decreased antioxidant content in the lung. These data support that excess lung collagen deposition and reduced pulmonary function in acute lung injury after burn and inhalation injury are mediated through the arginase pathway.

Potential role of the inflammasome-derived inflammatory cytokines in pulmonary fibrosis

Pulmonary fibrosis is a progressive, disabling disease with mortality rates that appear to be increasing in the western population, including the USA. There are over 140 known causes of pulmonary fibrosis as well as many unknown causes. Treatment options for this disease are limited due to poor understanding of the molecular mechanisms of the disease progression. However, recent progress in inflammasome research has greatly contributed to our understanding of its role in inflammation and fibrosis development. The inflammasome is a multiprotein complex that is an important component of both the innate and adaptive immune systems. Activation of proinflammatory cytokines following inflammasome assembly, such as IL-1β and IL-18, has been associated with development of PF. In addition, components of the inflammasome complex itself, such as the adaptor protein ASC have been associated with PF development. Recent evidence suggesting that the fibrotic process can be reversed via blockade of pathways associated with inflammasome activity may provide hope for future drug strategies. In this paper we will give an introduction to pulmonary fibrosis and its known causes. In addition, we will discuss the importance of the inflammasome in the development of pulmonary fibrosis as well as discuss potential future treatment options.

Asymmetric dimethylarginine potentiates lung inflammation in a mouse model of allergic asthma

Nitric oxide (NO), formed by nitric oxide synthase (NOS), is an important mediator of lung inflammation in allergic asthma. Asymmetric dimethylarginine (ADMA), a competitive endogenous inhibitor of NOS, is metabolized by the enzyme dimethylarginine dimethylaminohydrolase (DDAH). Elevated ADMA has been shown to affect lung function in mice, and by inhibiting NOS it alters NO and reactive oxygen species production in mouse lung epithelial cells. However, the effects of altered ADMA levels during lung inflammation have not been explored. A model of allergen-induced airway inflammation was utilized in combination with the modulation of endogenous circulating ADMA levels in mice. Airway inflammation was assessed by quantifying inflammatory cell infiltrates in lung lavage and by histology. Lung DDAH expression was assessed by quantitative PCR and immunohistochemistry. Nitrite levels were determined in lung lavage fluid as a measure of NO production. iNOS expression was determined by immunohistochemistry, immunofluorescence, Western blot, and quantitative PCR. NF-κB binding activity was assessed by a transcription factor binding assay. Allergen-induced lung inflammation was potentiated in mice with elevated circulating ADMA and was reduced in mice overexpressing DDAH. Elevated ADMA reduced nitrite levels in lung lavage fluid in both allergen-challenged and control animals. ADMA increased iNOS expression in airway epithelial cells in vivo following allergen challenge and in vitro in stimulated mouse lung epithelial cells. ADMA also increased NF-κB binding activity in airway epithelial cells in vitro. These data support that ADMA may play a role in inflammatory airway diseases such as asthma through modulation of iNOS expression in lung epithelial cells.

Emerging interface between metabolic syndrome and asthma

There is growing epidemiological evidence that obesity increases the risk of developing asthma. In some studies, insulin resistance or metabolic syndrome is a stronger risk factor than body mass. The obese-asthma subphenotype is marked by a paucity of inflammation but also by marked symptoms, poor response to glucocorticoids, and peripheral airway dysfunction. Although obesity may predispose to increased Th2 inflammation or atopic tendencies, other mechanisms that are independent of inflammatory cells need to be considered. There is growing evidence of the influence of hyperglycemia, hyperinsulinemia, and insulin-like growth factors on airway structure and function. Also, studies from mouse models of asthma have highlighted the importance of nitric oxide-arginine metabolism abnormalities and oxonitrosative stress in lungs. Such changes are well established features of the metabolic syndrome and may represent an interface between these diseases that can be therapeutically targeted. Such therapies, including administration of l-arginine or statins, increasing endothelial nitric oxide synthase, or the use of arginase inhibitors, have been successful in experimental models but have not yet translated to the clinical arena. We review the current understanding of the potential mechanistic links between obesity and asthma, emphasizing the potential influence of metabolic abnormalities on asthmatic processes, therapeutic implications, and expected challenges.

Simvastatin improves epithelial dysfunction and airway hyperresponsiveness: from asymmetric dimethyl-arginine to asthma

Altered arginine metabolism, the uncoupling of nitric oxide synthase (NOS) by asymmetric dimethyl-arginine (ADMA), increased oxo-nitrosative stress, and cellular injury were reported in airway epithelial cells in asthma. Statins improve vascular endothelial dysfunction by reducing ADMA and increasing endothelial NOS (eNOS), thereby reducing oxo-nitrosative stress in cardiovascular diseases. Whether statin therapy leads to similar beneficial effects in lung epithelium in asthma is unknown. The effects of simvastatin therapy after sensitization (40 mg/kg, intraperitoneally) on markers of arginine and NO metabolism and features of asthma were ascertained in a murine model of allergic asthma. The effects of simvastatin on the expression of NOS in A549 lung epithelial cells were studied in vitro. Simvastatin induced eNOS in lung epithelial cells in vitro. In acute and chronic models of asthma, simvastatin therapy was associated with significantly reduced airway inflammation, airway hyperresponsiveness, and airway remodeling. ADMA and inducible nitric oxide synthase were reduced by simvastatin, but eNOS was increased. A marked reduction of nitrotyrosine, a marker of oxo-nitrosative stress, was evident in airway epithelium. Cell injury markers such as cytosolic cytochrome c, caspases 3 and 9 and apoptotic protease activating factor 1 (Apaf-1) were also reduced. Simvastatin improves dysfunctional nitric oxide metabolism in allergically inflamed lungs. Important pleiotropic mechanisms may be responsible for the statin-induced reduction of airway inflammation, epithelial injury, and airway hyperresponsiveness.

Arginase 1 and arginase 2 variations associate with asthma, asthma severity and beta2 agonist and steroid response

RATIONALE

Arginase probably plays an important role in asthma development, severity and progression. Polymorphisms in arginase 1 and arginase 2 genes have been associated with childhood asthma and FEV1 reversibility to beta2 agonists.

OBJECTIVES

We investigated the association between arginase 1 and arginase 2 polymorphisms and adult asthma, asthma severity and treatment response in a longitudinal cohort of 200 asthma patients.

METHODS

Patients were studied during 1962-1975 and reexamined during 1990-1999, together with their families. Longitudinal data on lung function and treatment were extracted from medical records. Associations between haplotype-tagging polymorphisms in arginase 1 (n=3) and arginase 2 (n=8) and asthma, asthma severity, acute response to bronchodilators and chronic response to inhaled corticosteroids were analyzed.

MEASUREMENTS AND MAIN RESULTS

Two polymorphisms in arginase 2 (rs17249437 and rs3742879) were associated with asthma and with more severe airway obstruction. Increased airway hyperresponsiveness and lower beta2 agonist reversibility, but not anticholinergic reversibility, were associated with both arginase 1 and arginase 2. Inhaled corticosteroids slowed down the annual FEV1 decline, which was significantly less effective in homozygote carriers of the C-allele of the arginase 1 polymorphism, rs2781667.

CONCLUSION

We show that previously reported associations between arginase polymorphisms and childhood asthma are also present in adult asthma and the previously found associations with lower reversibility are specific for beta2 agonists. Furthermore, we identified associations of arginase 1 and arginase 2 genes with asthma severity, as reflected by a lower lung function, more severe airway hyperresponsiveness, and less long-term response to inhaled corticosteroids. Studies on the functionality of the polymorphisms are warranted to further unravel the complex mechanisms underlying these observations.

Direct inhibition of arginase attenuated airway allergic reactions and inflammation in a Dermatophagoides farinae-induced NC/Nga mouse model

The expression of arginase I has been a focus of research into the pathogenesis of experimental asthma, because arginase deprives nitric oxide synthase (NOS) of arginine and therefore participates in the attenuation of bronchodilators such as nitric oxide (NO). The present study used an intranasal mite-induced NC/Nga mouse model of asthma to investigate the contribution of arginase to the asthma pathogenesis, using an arginase inhibitor, N(omega)-hydroxy-nor-l-arginine (nor-NOHA). The treatment with nor-NOHA inhibited the increase in airway hyperresponsiveness (AHR) and the number of eosinophils in bronchoalveolar lavage fluid. NOx levels in the lung were elevated despite suppressed NOS2 mRNA expression. Accompanied by the attenuated activity of arginase, the expression of arginase I at both the mRNA and protein level was downregulated. The levels of mRNA for T helper 2 cytokines such as IL-4, IL-5, and IL-13, and for chemotactants such as eotaxin-1 and eotaxin-2, were reduced. Moreover, the accumulation of inflammatory cells and the ratio of goblet cells in the bronchiole were decreased. The study concluded that the depletion of NO caused by arginase contributes to AHR and inflammation, and direct administration of an arginase inhibitor to the airway may be beneficial and could be of use in treating asthma due to its anti-inflammatory and airway-relaxing effects, although it is not clear whether the anti-inflammatory effect is direct or indirect.

Beneficial effects of high dose of L-arginine on airway hyperresponsiveness and airway inflammation in a murine model of asthma

BACKGROUND

Disturbance in the delicate balance between L-arginine-metabolizing enzymes such as nitric oxide synthase (NOS) and arginase may lead to decreased L-arginine availability to constitutive forms of NOS (endothelial NOS), thereby increasing the nitro-oxidative stress and airway hyperresponsiveness (AHR).

OBJECTIVE

In this study, we investigated the effects of high doses of L-arginine on L-arginine-metabolizing enzymes and subsequent biological effects such as cyclic guanosine monophosphate production, lipid peroxidation, peroxynitrite, AHR, and airway inflammation in a murine model of asthma.

METHODS

Different doses of L-arginine were administered to ovalbumin-sensitized and challenged mice. Exhaled nitric oxide, AHR, airway inflammation, T(H)2 cytokines, goblet cell metaplasia, nitro-oxidative stress, and expressions of arginase 1, endothelial NOS, and inducible NOS in lung were determined.

RESULTS

L-arginine significantly reduced AHR and airway inflammation including bronchoalveolar lavage fluid eosinophilia, T(H)2 cytokines, TGF-beta1, goblet cell metaplasia, and subepithelial fibrosis. Further, L-arginine increased ENO levels and cyclic guanosine monophosphate in lung and reduced the markers of nitro-oxidative stress such as nitrotyrosine, 8-isoprostane, and 8-hydroxy-2'-deoxyguanosine. This was associated with reduced activity and expression of arginase 1, increased expression of endothelial NOS, and reduction of inducible NOS in bronchial epithelia.

CONCLUSION

We conclude that L-arginine administration may improve disordered nitric oxide metabolism associated with allergic airway inflammation, and alleviates some features of asthma.

Arginase: a key enzyme in the pathophysiology of allergic asthma opening novel therapeutic perspectives

Allergic asthma is a chronic inflammatory airways' disease, characterized by allergen-induced early and late bronchial obstructive reactions, airway hyperresponsiveness (AHR), airway inflammation and airway remodelling. Recent ex vivo and in vivo studies in animal models and asthmatic patients have indicated that arginase may play a central role in all these features. Thus, increased arginase activity in the airways induces reduced bioavailability of L-arginine to constitutive (cNOS) and inducible (iNOS) nitric oxide synthases, causing a deficiency of bronchodilating and anti-inflammatory NO, as well as increased formation of peroxynitrite, which may be involved in allergen-induced airways obstruction, AHR and inflammation. In addition, both via reduced NO production and enhanced synthesis of L-ornithine, increased arginase activity may be involved in airway remodelling by promoting cell proliferation and collagen deposition in the airway wall. Therefore, arginase inhibitors may have therapeutic potential in the treatment of acute and chronic asthma. This review focuses on the pathophysiological role of arginase in allergic asthma and the emerging effectiveness of arginase inhibitors in the treatment of this disease.

ADMA and the role of the genes: lessons from genetically modified animals and human gene polymorphisms

In large population-based cohorts, elevated plasma levels of asymmetric dimethylarginine (ADMA) were found to be associated with cardiovascular events and mortality. Impairment of nitric oxide (NO) synthesis from l-arginine has been postulated as underlying mechanism. In the present review, we compare different experimental models of NOS deficiency or overexpression with corresponding models of altered metabolism of ADMA by dimethylarginine dimethylaminohydrolase (DDAH). The latter models show a considerable overlap with the pathophysiological features of impaired NO synthesis, such as impaired endothelial function, elevation of blood pressure, and microvascular fibrosis. In line with these findings, first data regarding genetic variation of DDAH-metabolism in humans are reminiscent of the (rather modest) effects previously observed with polymorphisms of the eNOS gene. However, several peculiar observations suggest that ADMA- or DDAH-related pathology may extend beyond impairment of NO-mediated signalling. Notably, the complete knock out of DDAH1 appears to be lethal while triple NOS(-/-) mice are viable. Moreover, some ADMA-mediated pathology appears to respond rather to ACE-inhibition than to l-arginine. Here, a further investigation of alternative target enzymes for ADMA and other endogenous DDAH substrates is warranted.Taken together, the current data suggest that ADMA-related pathology can largely but not completely be explained by impaired NO metabolism.

Altered asymmetric dimethyl arginine metabolism in allergically inflamed mouse lungs

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of nitric oxide synthase (NOS), causes uncoupling of NOS leading to generation of reactive nitrogen species, such as peroxynitrite. The lung generates a significant amount of ADMA, potentially contributing to plasma ADMA levels that have been related to endothelial dysfunction. ADMA infusion causes increased collagen deposition in lungs, suggesting that it could influence the development of chronic lung diseases such as fibrosis, chronic obstructive pulmonary disease, and asthma. To explore the link between endogenous ADMA and asthma, we determined the levels of ADMA, enzymes implicated in its metabolism, and peroxynitrite in murine models of allergic airway inflammation (AAI) resembling asthma. ADMA levels and nitrosative stress were found to be positively correlated in cytosol and mitochondria during AAI. This was associated with increased expression of protein-arginine methyltransferase-2, an ADMA-synthesizing enzyme, and reduced expression of dimethylarginine dimethylaminohydrolase-2, an ADMA-degrading enzyme, in bronchial epithelia. Increased nitrotyrosine similarly localized to the bronchial epithelium, as well as in infiltrated inflammatory cells. Administration of L-arginine, which was expected to compete with ADMA and reverse the uncoupling/inhibition of NOS, restored normal ADMA metabolism, along with the expected reduction of nitrosative stress in lung. Because dimethylarginine dimethylaminohydrolase-2 function is known to be negatively related to oxidative stress, this may represent a feed-forward loop effect. We conclude that a delicate balance between ADMA-metabolizing enzymes is disturbed in bronchial epithelium during AAI, potentially causing increased nitrosative stress in a self-propagating cycle. This represents a potential therapeutic target in asthma.

From arginine methylation to ADMA: a novel mechanism with therapeutic potential in chronic lung diseases

Protein arginine methylation is a novel posttranslational modification regulating a diversity of cellular processes, including protein-protein interaction, signal transduction, or histone function. It has recently been shown to be dysregulated in chronic renal, vascular, and pulmonary diseases, and metabolic products originating from protein arginine methylation have been suggested to serve as biomarkers in cardiovascular and pulmonary diseases.

Protein arginine methylation is performed by a class of enzymes called protein arginine methyltransferases (PRMT), which specifically methylate protein-incorporated arginine residues to generate protein-incorporated monomethylarginine (MMA), symmetric dimethylarginine (SDMA), or asymmetric dimethylarginine (ADMA). Upon proteolytic cleavage of arginine-methylated proteins, free intracellular MMA, SDMA, or ADMA is generated, which, upon secretion into the extracellular space (including plasma), directly affects the methylarginine concentration in the plasma. Free methylarginines are cleared from the body by renal excretion or hepatic metabolism. In addition, MMA and ADMA, but not SDMA, can be degraded via a class of intracellular enzymes called dimethylarginine dimethylaminohydrolases (DDAH).

ADMA and MMA are endogenous inhibitors of nitric oxide synthases (NOS) and ADMA has been suggested to serve as a biomarker of endothelial dysfunction in cardiovascular diseases. This view has now been extended to the idea that, in addition to serum ADMA, the amount of free, as well as protein-incorporated, intracellular ADMA influences pulmonary cell function and determines the development of chronic lung diseases, including pulmonary arterial hypertension (PAH) or pulmonary fibrosis. This review will present and discuss the recent findings of dysregulated arginine methylation in chronic lung disease. We will highlight novel directions for future investigations evaluating the functional contribution of arginine methylation in lung homeostasis and disease with the outlook that modifying PRMT or DDAH activity presents a novel therapeutic option for the treatment of chronic lung disease.