Role of L-arginine in the deficiency of nitric oxide and airway hyperreactivity after the allergen-induced early asthmatic reaction in guinea-pigs

Pharmacological Screening Identifies SHK242 and SHK277 as Novel Arginase Inhibitors with Efficacy against Allergen-Induced Airway Narrowing In Vitro and In Vivo

Arginase is a potential target for asthma treatment. However, there are currently no arginase inhibitors available for clinical use. Here, a novel class of arginase inhibitors was synthesized, and their efficacy was pharmacologically evaluated. The reference compound 2(S)-amino-6-boronohexanoic acid (ABH) and >200 novel arginase inhibitors were tested for their ability to inhibit recombinant human arginase 1 and 2 in vitro. The most promising compounds were separated as enantiomers. Enantiomer pairs SHK242 and SHK243, and SHK277 and SHK278 were tested for functional efficacy by measuring their effect on allergen-induced airway narrowing in lung slices of ovalbumin-sensitized guinea pigs ex vivo. A guinea pig model of acute allergic asthma was used to examine the effect of the most efficacious enantiopure arginase inhibitors on allergen-induced airway hyper-responsiveness (AHR), early and late asthmatic reactions (EAR and LAR), and airway inflammation in vivo. The novel compounds were efficacious in inhibiting arginase 1 and 2 in vitro. The enantiopure SHK242 and SHK277 fully inhibited arginase activity, with IC₅₀ values of 3.4 and 10.5 μM for arginase 1 and 2.9 and 4.0 µM for arginase 2, respectively. Treatment of slices with ABH or novel compounds resulted in decreased ovalbumin-induced airway narrowing compared with control, explained by increased local nitric oxide production in the airway. In vivo, ABH, SHK242, and SHK277 protected against allergen-induced EAR and LAR but not against AHR or lung inflammation. We have identified promising novel arginase inhibitors for the potential treatment of allergic asthma that were able to protect against allergen-induced early and late asthmatic reactions. SIGNIFICANCE STATEMENT: Arginase is a potential drug target for asthma treatment, but currently there are no arginase inhibitors available for clinical use. We have identified promising novel arginase inhibitors for the potential treatment of allergic asthma that were able to protect against allergen-induced early and late asthmatic reactions. Our new inhibitors show protective effects in reducing airway narrowing in response to allergens and reductions in the early and late asthmatic response.

Arginase 1 deletion in myeloid cells affects the inflammatory response in allergic asthma, but not lung mechanics, in female mice

BACKGROUND

(Over-)expression of arginase may limit local availability of arginine for nitric oxide synthesis. We investigated the significance of arginase1 (ARG1) for the development of airway hyperresponsiveness (AHR) and lung inflammation in female mice with ovalbumin (OVA)-induced allergic asthma.

METHODS

Arg1 was ablated in the lung by crossing Arg1 fl/fl and Tie2Cre tg/- mice. OVA sensitization and challenge were conducted, and AHR to methacholine was determined using the Flexivent system. Changes in gene expression, chemokine and cytokine secretion, plasma IgE, and lung histology were quantified using RT-qPCR, ELISA, and immunohistochemistry, respectively.

RESULTS

Arg1 ablation had no influence on the development of OVA-induced AHR, but attenuated OVA-induced increases in expression of Arg2 and Nos2, Slc7a1, Slc7a2, and Slc7a7 (arginine transporters), Il4, Il5 and Il13 (TH2-type cytokines), Ccl2 and Ccl11 (chemokines), Ifng (TH1-type cytokine), Clca3 and Muc5ac (goblet cell markers), and OVA-specific IgE. Pulmonary IL-10 protein content increased, but IL-4, IL-5, IL-13, TNFα and IFNγ content, and lung histopathology, were not affected. Arg1 elimination also decreased number and tightness of correlations between adaptive changes in lung function and inflammatory parameters in OVA/OVA-treated female mice. OVA/OVA-treated female mice mounted a higher OVA-IgE response than males, but the correlation between lung function and inflammation was lower. Arg1-deficient OVA/OVA-treated females differed from males in a more pronounced decline of arginine-metabolizing and -transporting genes, higher plasma arginine levels, a smaller OVA-specific IgE response, and no improvement of peripheral lung function.

CONCLUSION

Complete ablation of Arg1 in the lung affects mRNA abundance of arginine-transporting and -metabolizing genes, and pro-inflammatory genes, but not methacholine responsiveness or accumulation of inflammatory cells.

New strategies for the treatment of pulmonary hypertension in sickle cell disease : the rationale for arginine therapy

Nitric oxide (NO) is inactivated in sickle cell disease (SCD), while bioavailability of arginine, the substrate for NO synthesis, is diminished. Impaired NO bioavailability represents the central feature of endothelial dysfunction, and is a key factor in the pathophysiology of SCD. Inactivation of NO correlates with the hemolytic rate and is associated with erythrocyte release of cell-free hemoglobin and arginase during hemolysis. Accelerated consumption of NO is enhanced further by the inflammatory environment of oxidative stress that exists in SCD. Based upon its critical role in mediating vasodilation and cell growth, decreased NO bioavailability has also been implicated in the pathogenesis of pulmonary arterial hypertension (PHT). Secondary PHT is a common life-threatening complication of SCD that also occurs in most hereditary and chronic hemolytic disorders. Aberrant arginine metabolism contributes to endothelial dysfunction and PHT in SCD, and is strongly associated with prospective patient mortality. The central mechanism responsible for this metabolic disorder is enhanced arginine turnover, occurring secondary to enhanced plasma arginase activity. This is consistent with a growing appreciation of the role of excessive arginase activity in human diseases, including asthma and PHT. Decompartmentalization of hemoglobin into plasma consumes endothelial NO and thus drives a metabolic requirement for arginine, whose bioavailability is further limited by arginase activity. New treatments aimed at maximizing both arginine and NO bioavailability through arginase inhibition, suppression of hemolytic rate, or oral arginine supplementation may represent novel therapeutic strategies.

l-Arginine administration attenuates airway inflammation by altering l-arginine metabolism in an NC/Nga mouse model of asthma

Changes in l-arginine metabolism, including increased arginase levels and decreased nitric oxide production, are involved in the pathophysiology of asthma. In this study, using an intranasal mite-induced NC/Nga mouse model of asthma, we examined whether administration of l-arginine ameliorated airway hyperresponsiveness and inflammation by altering l-arginine metabolism. Experimental asthma was induced in NC/Nga mice via intranasal administration of mite crude extract (50 µg/day) on 5 consecutive days (days 0-4, sensitization) and on day 11 (challenge). Oral administration of l-arginine (250 mg/kg) was performed twice daily on days 5-10 for prevention or on days 11-13 for therapy. On day 14, we evaluated the inflammatory airway response (airway hyperresponsiveness, the number of cells in the bronchoalveolar lavage fluid, and the changes in pathological inflammation of the lung), arginase expression and activity, l-arginine bioavailability, and the concentration of NOx, the end products of nitric oxide. Treatment with l-arginine ameliorated the mite-induced inflammatory airway response. Furthermore, l-arginine administration attenuated the increases in arginase expression and activity and elevated the NOx levels by enhancing l-arginine bioavailability. These findings indicate that l-arginine administration may contribute to the improvement of asthmatic symptoms by altering l-arginine metabolism.

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

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

Arginase and arginine dysregulation in asthma

In recent years, evidence has accumulated indicating that the enzyme arginase, which converts L-arginine into L-ornithine and urea, plays a key role in the pathogenesis of pulmonary disorders such as asthma through dysregulation of L-arginine metabolism and modulation of nitric oxide (NO) homeostasis. Allergic asthma is characterized by airway hyperresponsiveness, inflammation, and remodeling. Through substrate competition, arginase decreases bioavailability of L-arginine for nitric oxide synthase (NOS), thereby limiting NO production with subsequent effects on airway tone and inflammation. By decreasing L-arginine bioavailability, arginase may also contribute to the uncoupling of NOS and the formation of the proinflammatory oxidant peroxynitrite in the airways. Finally, arginase may play a role in the development of chronic airway remodeling through formation of L-ornithine with downstream production of polyamines and L-proline, which are involved in processes of cellular proliferation and collagen deposition. Further research on modulation of arginase activity and L-arginine bioavailability may reveal promising novel therapeutic strategies for asthma.

Nitric oxide in asthma physiopathology

Asthma is a chronic inflammatory airway disease characterized by allergen-induced airway hyperresponsiveness, airway inflammation, and remodeling. Nitric oxide (NO) derived from constitutive and inducible enzymes affects many aspects of asthma physiopathology. Animal in vivo studies have indicated that inhibition of iNOS may play a central role in the modulation of these features, particularly extracellular matrix remodeling. Additionally, increases in iNOS-derived NO, observed in asthmatic patients, may lead to an increase in peroxynitrite and an imbalance of oxidant and antioxidant pathways. In addition, endogenous nitric oxide produced by constitutive enzymes may protect against the remodeling of the lung. Therefore, nitric oxide donors and/or iNOS inhibitors may have therapeutic potential in asthma treatment and can also be used with corticosteroids to counteract airway remodeling. This paper focuses on the pathophysiological role of nitric oxide, mainly derived from inducible isoforms, in the various pathologic mechanisms of allergic asthma and the importance of nitric oxide and/or arginase inhibitors in asthma treatment.

Inhaled fluoride, magnesium salt and L-arginine reverse bronchospasma

In vitro studies showed that fluoride and magnesium salts relax bronchial smooth muscle cells. Their combined administration could have potential interest. Magnesium fluoride salt (MgF₂) is nearly insoluble. A soluble derivate can be obtained by introducing L-arginineinine (L-arginine) between the ions. L-arginine, being the substrate leading to the release of NO, might add another relaxing effect to this derivate. Relaxing effects of NaF, MgSO₄, L-arginine, NaF+MgSO₄ and MgF₂+L-arginine given via the inhaled route were studied on rats challenged with acetylmethylcholine (ACMCH) following eight successive doses. Tested salts were given at the fourth dose of ACMCH. Changes in bronchial resistances (R) were measured and compared to results obtained in a control group, receiving ACMCH alone. NaF, MgSO₄, and L-arginine led to significant bronchorelaxing effects (p < 0.05). The association NaF+MgSO₄ gave a greater decrease in bronchial resistance compared to that obtained with each salt (fluoride and magnesium) separately. (MgF₂, 2L-arginine) induced a significant fall in R at the fourth dose of ACMCH just after its inhalation. R values obtained with (MgF₂, 2L-arginine) were significantly lower than L-arginine alone at the sixth dose of ACMCH (p < 0.05). (MgF₂, 2L-arginine) is a triple combination able to induce a significant and constant bronchodilating effect through three different pathways. The effect looked partly additive.

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.

Asthma management: reinventing the wheel in sickle cell disease

Asthma is a common comorbidity in sickle cell disease (SCD) with a reported prevalence of 30-70%. The high frequency of asthma in this population cannot be attributed to genetic predisposition alone, and likely reflects in part, the contribution of overlapping mechanisms shared between these otherwise distinct disorders. There is accumulating evidence that dysregulated arginine metabolism and in particular, elevated arginase activity contributes to pulmonary complications in SCD. Derangements of arginine metabolism are also emerging as newly appreciated mechanism in both asthma and pulmonary hypertension independent of SCD. Patients with SCD may potentially be at risk for an asthma-like condition triggered or worsened by hemolysis-driven release of erythrocyte arginase and low nitric oxide bioavailability, in addition to classic familial asthma. Mechanisms that contributed to asthma are complex and multifactorial, influenced by genetic polymorphisms as well as environmental and infectious triggers. Given the association of asthma with inflammation, oxidative stress and hypoxemia, factors known to contribute to a vasculopathy in SCD, and the consequences of these factors on sickle erythrocytes, comorbid asthma would likely contribute to a vicious cycle of sickling and subsequent complications of SCD. Indeed a growing body of evidence documents what should come as no surprise: Asthma in SCD is associated with acute chest syndrome, stroke, pulmonary hypertension, and early mortality, and should therefore be aggressively managed based on established National Institutes of Health Guidelines for asthma management. Barriers to appropriate asthma management in SCD are discussed as well as strategies to overcome these obstacles in order to provide optimal care.

Nitric oxide and arginine dysregulation: a novel pathway to pulmonary hypertension in hemolytic disorders

Secondary pulmonary hypertension (PH) is emerging as one of the leading causes of mortality and morbidity in patients with hemolytic anemias such as sickle cell disease (SCD) and thalassemia. Impaired nitric oxide (NO) bioavailability represents the central feature of endothelial dysfunction, and is a major factor in the pathophysiology of PH. Inactivation of NO correlates with hemolytic rate and is associated with the erythrocyte release of cell-free hemoglobin, which consumes NO directly, and the simultaneous release of the arginine-metabolizing enzyme arginase, which limits bioavailability of the NO synthase substrate arginine during the process of intravascular hemolysis. Rapid consumption of NO is accelerated by oxygen radicals that exists in both SCD and thalassemia. A dysregulation of arginine metabolism contributes to endothelial dysfunction and PH in SCD, and is strongly associated with prospective patient mortality. The central mechanism responsible for this metabolic disorder is enhanced arginine turnover, occurring secondary to enhanced plasma arginase activity. This is consistent with a growing appreciation of the role of excessive arginase activity in human diseases, including asthma and pulmonary arterial hypertension. New treatments aimed at improving arginine and NO bioavailability through arginase inhibition, suppression of hemolytic rate, oral arginine supplementation, or use of NO donors represent potential therapeutic strategies for this common pulmonary complication of hemolytic disorders.

Inducible nitric oxide synthase inhibition attenuates lung tissue responsiveness and remodeling in a model of chronic pulmonary inflammation in guinea pigs

We evaluated the influence of iNOS-derived NO on the mechanics, inflammatory, and remodeling process in peripheral lung parenchyma of guinea pigs with chronic pulmonary allergic inflammation. Animals treated or not with 1400 W were submitted to seven exposures of ovalbumin in increasing doses. Seventy-two hours after the 7th inhalation, lung strips were suspended in a Krebs organ bath, and tissue resistance and elastance measured at baseline and after ovalbumin challenge. The strips were submitted to histopathological measurements. The ovalbumin-exposed animals showed increased maximal responses of resistance and elastance (p<0.05), eosinophils counting (p<0.001), iNOS-positive cells (p<0.001), collagen and elastic fiber deposition (p<0.05), actin density (p<0.05) and 8-iso-PGF2alpha expression (p<0.001) in alveolar septa compared to saline-exposed ones. Ovalbumin-exposed animals treated with 1400 W had a significant reduction in lung functional and histopathological findings (p<0.05). We showed that iNOS-specific inhibition attenuates lung parenchyma constriction, inflammation, and remodeling, suggesting NO-participation in the modulation of the oxidative stress pathway.

Arginase inhibition protects against allergen-induced airway obstruction, hyperresponsiveness, and inflammation

RATIONALE

In a guinea pig model of allergic asthma, using perfused tracheal preparations ex vivo, we demonstrated that L-arginine limitation due to increased arginase activity underlies a deficiency of bronchodilating nitric oxide (NO) and airway hyperresponsiveness (AHR) after the allergen-induced early and late asthmatic reaction.

OBJECTIVES

Using the same animal model, we investigated the acute effects of the specific arginase inhibitor 2(S)-amino-6-boronohexanoic acid (ABH) and of L-arginine on AHR after the early and late reaction in vivo. In addition, we investigated the protection of allergen-induced asthmatic reactions, AHR, and airway inflammation by pretreatment with the drug.

METHODS

Airway responsiveness to inhaled histamine was measured in permanently instrumented, freely moving guinea pigs sensitized to ovalbumin at 24 hours before allergen challenge and after the allergen-induced early and late asthmatic reactions by assessing histamine PC(100) (provocative concentration causing a 100% increase of pleural pressure) values.

MEASUREMENTS AND MAIN RESULTS

Inhaled ABH acutely reversed AHR to histamine after the early reaction from 4.77 +/- 0.56-fold to 2.04 +/- 0.34-fold (P < 0.001), and a tendency to inhibition was observed after the late reaction (from 1.95 +/- 0.56-fold to 1.56 +/- 0.47-fold, P < 0.10). Quantitatively similar results were obtained with inhaled l-arginine. Remarkably, after pretreatment with ABH a 33-fold higher dose of allergen was needed to induce airway obstruction (P < 0.01). Consequently, ABH inhalation 0.5 hour before and 8 hours after allergen challenge protected against the allergen-induced early and late asthmatic reactions, AHR and inflammatory cell infiltration.

CONCLUSIONS

Inhalation of ABH or l-arginine acutely reverses allergen-induced AHR after the early and late asthmatic reaction, presumably by attenuating arginase-induced substrate deficiency to NO synthase in the airways. Moreover, ABH considerably reduces the airway sensitivity to inhaled allergen and protects against allergen-induced bronchial obstructive reactions, AHR, and airway inflammation. This is the first in vivo study indicating that arginase inhibitors may have therapeutic potential in allergic asthma.

Arginase and pulmonary diseases

Recent studies have indicated that arginase, which converts L-arginine into L-ornithine and urea, may play an important role in the pathogenesis of various pulmonary disorders. In asthma, chronic obstructive pulmonary disease (COPD) and cystic fibrosis, increased arginase activity in the airways may contribute to obstruction and hyperresponsiveness of the airways by inducing a reduction in the production of bronchodilatory nitric oxide (NO) that results from its competition with constitutive (cNOS) and inducible (iNOS) NO synthases for their common substrate. In addition, reduced L-arginine availability to iNOS induced by arginase may result in the synthesis of both NO and the superoxide anion by this enzyme, thereby enhancing the production of peroxynitrite, which has procontractile and pro-inflammatory actions. Increased synthesis of L-ornithine by arginase may also contribute to airway remodelling in these diseases. L-Ornithine is a precursor of polyamines and L-proline, and these metabolic products may promote cell proliferation and collagen production, respectively. Increased arginase activity may also be involved in other fibrotic disorders of the lung, including idiopathic pulmonary fibrosis. Finally, through its action of inducing reduced levels of vasodilating NO, increased arginase activity has been associated with primary and secondary forms of pulmonary hypertension. Drugs targeting the arginase pathway could have therapeutic potential in these diseases.

Arginine homeostasis in allergic asthma

Allergic asthma is a chronic disease characterized by early and late asthmatic reactions, airway hyperresponsiveness, airway inflammation and airway remodelling. Changes in l-arginine homeostasis may contribute to all these features of asthma by decreased nitric oxide (NO) production and increased formation of peroxynitrite, polyamines and l-proline. Intracellular l-arginine levels are regulated by at least three distinct mechanisms: (i) cellular uptake by cationic amino acid (CAT) transporters, (ii) metabolism by NO-synthase (NOS) and arginase, and (iii) recycling from l-citrulline. Ex vivo studies using animal models of allergic asthma have indicated that attenuated l-arginine bioavailability to NOS causes deficiency of bronchodilating NO and increased production of procontractile peroxynitrite, which importantly contribute to allergen-induced airway hyperresponsiveness after the early and late asthmatic reaction, respectively. Decreased cellular uptake of l-arginine, due to (eosinophil-derived) polycations inhibiting CATs, as well as increased consumption by increased arginase activity are major causes of substrate limitation to NOS. Increasing substrate availability to NOS by administration of l-arginine, l-citrulline, the polycation scavenger heparin, or an arginase inhibitor alleviates allergen-induced airway hyperresponsiveness by restoring the production of bronchodilating NO. In addition, reduced l-arginine levels may contribute to the airway inflammation associated with the development of airway hyperresponsiveness, which similarly may involve decreased NO synthesis and increased peroxynitrite formation. Increased arginase activity could also contribute to airway remodelling and persistent airway hyperresponsiveness in chronic asthma via increased synthesis of l-ornithine, the precursor of polyamines and l-proline. Drugs that increase the bioavailability of l-arginine in the airways - particularly arginase inhibitors - may have therapeutic potential in allergic asthma.

Pharmacology of airway smooth muscle proliferation

Airway smooth muscle thickening is a pathological feature that contributes significantly to airflow limitation and airway hyperresponsiveness in asthma. Ongoing research efforts aimed at identifying the mechanisms responsible for the increased airway smooth muscle mass have indicated that hyperplasia of airway smooth muscle, due in part to airway myocyte proliferation, is likely a major factor. Airway smooth muscle proliferation has been studied extensively in culture and in animal models of asthma, and these studies have revealed that a variety of receptors and mediators contributes to this response. This review aims to provide an overview of the receptors and mediators that control airway smooth muscle cell proliferation, with emphasis on the intracellular signalling mechanisms involved.

Clinical differences between children and adults with pulmonary hypertension and sickle cell disease

Pulmonary hypertension (PHT) is an important co-morbidity in sickle cell disease (SCD). Despite increasing research in adults, the prevalence and implication of this condition in children is unknown. Charts of 362 SCD patients followed at the Children's Hospital & Research Center Oakland were reviewed to determine clinical variables associated with obtaining echocardiographic screening for PHT, clinical associations of PHT, and associated mortality following diagnosis in adults and children with SCD. In this cohort, patients with underlying lung abnormalities or those on chronic transfusions were more likely to have echocardiograms, however the diagnosis of PHT was often unrecognized. A different clinical phenotype for PHT in adults versus children was identified. Associations with PHT for adults included age, renal and lung disease, hepatitis C, chronic transfusions, and a history of acute chest syndrome (ACS), with ACS being protective. Surprisingly, for children, a history of sepsis, along with a history of ACS, or obstructive lung disease were associated with PHT. Survival analysis found significant mortality for PHT, with a hazard ratio of 17.3 (95% confidence interval 4.9-60.4). The divergent clinical spectrum for PHT between adults and children may point to different age-specific mechanisms or biological expression of PHT.

Disruption of NO-cGMP signaling by neonatal hyperoxia impairs relaxation of lung parenchyma

Exposure of immature lungs to hyperoxia for prolonged periods contributes to neonatal lung injury and airway hyperreactivity. We studied the role of disrupted nitric oxide-guanosine 3',5'-cyclic monophosphate (NO-cGMP) signaling in impairing the relaxant responses of lung tissue from hyperoxia-exposed rat pups. Pups were exposed to >/=95% O(2) or room air for 7 days starting from days 1, 5, or 14. The animals were killed, lungs were removed, and 1-mm-thick lung parenchymal strips were prepared. Lung parenchymal strips of room air or hyperoxic pups were preconstricted using bethanechol and then graded electrical field stimulation (EFS) was applied to induce relaxation. EFS-induced relaxation of lung parenchymal strips was greater at 7 and 12 days than at 21 days in room air-exposed rat pups. Hyperoxic exposure significantly reduced relaxation at 7 and 12 days but not 21 days compared with room air exposure. NO synthase blockade with N(omega)-nitro-l-arginine methyl ester diminished relaxant responses in room air but not in hyperoxic pups at 12 days. After incubation with supplemental l-arginine, the relaxation response of hyperoxic strips was restored. cGMP, a key mediator of the NO signaling pathway, also decreased in strips from hyperoxic vs. room air pups and cGMP levels were restored after incubation with supplemental l-arginine. In addition, arginase activity was significantly increased in hyperoxic lung parenchymal strips compared with room air lung parenchymal strips. These data demonstrate disruption of NO-cGMP signaling in neonatal rat pups exposed to hyperoxia and show that bioavailability of the substrate l-arginine is implicated in the predisposition of this model to airway hyperreactivity.

Clinical hemoglobinopathies: iron, lungs and new blood

PURPOSE OF REVIEW

Sickle cell disease and beta-thalassemia major are clinically significant hereditary anemias that elicit worldwide attention due to the frequency and severity of these disorders. Historically, most children who inherited these disorders died in the first decade of life. Recently, however, supportive care has extended lifespan through the fifth decade of life and beyond, with survival through early adulthood now indistinguishable from those unaffected by these disorders. As a result, chronic health impairments that significantly reduce the quality of life such as pulmonary hypertension and the consequences of transfusional iron overload have become principal challenges.

RECENT FINDINGS

We focus on important recent advances that are very likely to alter the nature of supportive care of these disorders or make it possible to identify prospectively high-risk patients who might benefit from novel therapies or even curative treatment in the form of hematopoietic cell transplantation. The availability of the latter, traditionally constrained by the requirement of a human leukocyte antigen-identical sibling donor, is very likely to be broadened as results after unrelated donor hematopoietic cell transplantation improve.

SUMMARY

In this review, several areas that are very likely to have a significant impact in the management of patients who inherit these disorders are discussed.

Asymmetric dimethylarginine induces oxidative and nitrosative stress in murine lung epithelial cells

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) produced by epithelial and inflammatory cells are key mediators of the chronic airway inflammation of asthma. Low L-arginine levels can result in the uncoupling of nitric oxide synthase (NOS) leading to production of both ROS and RNS. Asymmetric dimethylarginine (ADMA) is a competitive endogenous inhibitor of all NOS isoforms and has been demonstrated to inhibit NO formation and increase oxidative stress in vascular endothelial and smooth muscle cells. The effect of ADMA on inducible NOS (iNOS) activity in epithelial cells has not been explored. In this study, we investigated whether addition of exogenous ADMA alters the generation of NO and superoxide anion (O2-), leading to peroxynitrite (ONOO-) formation in a mouse epithelial cell line. In stimulated LA-4 cells, ADMA dose-dependently inhibited nitrite accumulation after 24 h of treatment. In addition, ADMA concentrations as low as 10 microM induced rapid increases in O2- production as measured by dihydroethidium oxidation. Furthermore, using dihydrorhodamine to monitor ONOO- formation, ADMA caused a dose-dependent increase in ONOO- after treatment for 24 h. Similar effects of ADMA were seen using purified iNOS protein in a cell-free system. Together, these data indicate that elevated ADMA may contribute to the production of ROS and RNS in airway inflammation.

Role of the l-citrulline/l-arginine cycle in iNANC nerve-mediated nitric oxide production and airway smooth muscle relaxation in allergic asthma

Nitric oxide synthase (NOS) converts L-arginine into nitric oxide (NO) and L-citrulline. In NO-producing cells, L-citrulline can be recycled to L-arginine in a two-step reaction involving argininosuccinate synthase (ASS) and -lyase (ASL). In guinea pig trachea, L-arginine is a limiting factor in neuronal nNOS-mediated airway smooth muscle relaxation upon inhibitory nonadrenergic noncholinergic (iNANC) nerve stimulation. Moreover, in a guinea pig model of asthma iNANC nerve-induced NO production and airway smooth muscle relaxation are impaired after the allergen-induced early asthmatic reaction, due to limitation of L-arginine. Using guinea pig tracheal preparations, we now investigated whether (i) the L-citrulline/L-arginine cycle is active in airway iNANC nerves and (ii) the NO deficiency after the early asthmatic reaction involves impaired L-citrulline recycling. Electrical field stimulation-induced relaxation was measured in tracheal open-rings precontracted with histamine. L-citrulline as well as the ASL inhibitor succinate did not affect electrical field stimulation-induced relaxation under basal conditions. However, reduced relaxation induced by a submaximal concentration of the NOS inhibitor N(omega)-nitro-L-arginine was restored by L-citrulline, which was prevented by the additional presence of succinate or the ASS inhibitor alpha-methyl-D,L-aspartate. Remarkably, the impaired iNANC relaxation after the early asthmatic reaction was restored by L-citrulline. In conclusion, the L-citrulline/L-arginine cycle is operative in guinea pig iNANC nerves in the airways and may be effective under conditions of low L-arginine utilization by nNOS (caused by NOS inhibitors), and during reduced L-arginine availability after allergen challenge. Enzymatic dysfunction in the L-citrulline/L-arginine cycle appears not to be involved in the L-arginine limitation and reduced iNANC activity after the early asthmatic reaction.

The therapeutic potential of drugs targeting the arginase pathway in asthma

Arginine metabolism by arginases may be of importance in health and disease, either by competing with nitric oxide synthases for the common substrate or by the production of L-ornithine. L-ornithine serves as a precursor for L-proline and polyamines, which may be involved in tissue remodelling by promoting collagen synthesis and cell proliferation. Arginase activity potentiates airway reactivity by reducing the production of bronchodilatory nitric oxide. Increased arginase activity has been implicated in the development of allergen-induced airway hyper-responsiveness in experimental asthma. In addition, reduced L-arginine availability to inducible nitric oxide synthase by arginase may lead to an increased production of peroxynitrite, contributing to increased airway smooth muscle contractility, airway inflammation and cell damage in this disease. Recent studies demonstrate that the upregulation of arginase by T helper type 2 cytokines in lung tissue as well as in cultured airway fibroblasts indicates a possible role of the enzyme in airway re-modelling. These findings, in conjunction with human studies showing a role for arginase in acute asthma, open a new horizon for the therapeutic potential of drugs targeting the arginase pathway in asthma.

Modulation of nitric oxide pathways: therapeutic potential in asthma and chronic obstructive pulmonary disease

Nitric oxide (NO) is present in the exhaled breath of humans and other mammalian species. It is generated in the lower airways by enzymes of the nitric oxide synthase (NOS) family, although nonenzymatic synthesis and consumptive processes may also influence levels of NO in exhaled breath. The biological properties of NO in the airways are multiple, complex, and bidirectional. Under physiological conditions, NO appears to play a homeostatic bronchoprotective role. However, its proinflammatory properties could also potentially cause tissue injury and contribute to airway dysfunction in disease states such as asthma and chronic obstructive pulmonary disease (COPD). This article will review the physiological and pathophysiological roles of NO in the airways, discuss the rationale for the use of drugs that modulate NO pathways--nitric oxide synthase inhibitors and NO donors--to treat inflammatory airway diseases, and attempt to predict the likely therapeutic benefit of such agents.

Arginase strongly impairs neuronal nitric oxide-mediated airway smooth muscle relaxation in allergic asthma

BACKGROUND

Using guinea pig tracheal preparations, we have recently shown that endogenous arginase activity attenuates inhibitory nonadrenergic noncholinergic (iNANC) nerve-mediated airway smooth muscle relaxation by reducing nitric oxide (NO) production--due to competition with neuronal NO-synthase (nNOS) for the common substrate, L-arginine. Furthermore, in a guinea pig model of allergic asthma, airway arginase activity is markedly increased after the early asthmatic reaction (EAR), leading to deficiency of agonist-induced, epithelium-derived NO and subsequent airway hyperreactivity. In this study, we investigated whether increased arginase activity after the EAR affects iNANC nerve-derived NO production and airway smooth muscle relaxation.

METHODS

Electrical field stimulation (EFS; 150 mA, 4 ms, 4 s, 0.5-16 Hz)-induced relaxation was measured in tracheal open-ring preparations precontracted to 30% with histamine in the presence of 1 microM atropine and 3 microM indomethacin. The contribution of NO to EFS-induced relaxation was assessed by the nonselective NOS inhibitor Nomega-nitro-L-arginine (L-NNA, 100 microM), while the involvement of arginase activity in the regulation of EFS-induced NO production and relaxation was investigated by the effect of the specific arginase inhibitor Nomega-hydroxy-nor-L-arginine (nor-NOHA, 10 microM). Furthermore, the role of substrate availability to nNOS was measured in the presence of exogenous L-arginine (5.0 mM).

RESULTS

At 6 h after ovalbumin-challenge (after the EAR), EFS-induced relaxation (ranging from 3.2 +/- 1.1% at 0.5 Hz to 58.5 +/- 2.2% at 16 Hz) was significantly decreased compared to unchallenged controls (7.1 +/- 0.8% to 75.8 +/- 0.7%; P < 0.05 all). In contrast to unchallenged controls, the NOS inhibitor L-NNA did not affect EFS-induced relaxation after allergen challenge, indicating that NO deficiency underlies the impaired relaxation. Remarkably, the specific arginase inhibitor nor-NOHA normalized the impaired relaxation to unchallenged control (P < 0.05 all), which effect was inhibited by L-NNA (P < 0.01 all). Moreover, the effect of nor-NOHA was mimicked by exogenous L-arginine.

CONCLUSION

The results clearly demonstrate that increased arginase activity after the allergen-induced EAR contributes to a deficiency of iNANC nerve-derived NO and decreased airway smooth muscle relaxation, presumably via increased substrate competition with nNOS.

Genetic polymorphisms in arginase I and II and childhood asthma and atopy

BACKGROUND

A recent microarray study implicated arginase I (ARG1) and arginase II (ARG2) in mouse allergic asthma models and human asthma.

OBJECTIVES

To examine the association between genetic variation in ARG1 and ARG2 and childhood asthma and atopy risk.

METHODS

We enrolled 433 case-parent triads, consisting of patients with asthma 4 to 17 years old and their biologic parents, from the allergy clinic of a public hospital in Mexico City between 1998 and 2003. Atopy to 24 aeroallergens was determined by skin prick tests. We genotyped 4 single nucleotide polymorphisms (SNPs) of ARG1 and 4 SNPs of ARG2 with minor allele frequencies higher than 10% by using the TaqMan assay (Roche Molecular Systems, Pleasanton, Calif).

RESULTS

ARG1 SNPs and haplotypes were not associated with asthma, but all 4 ARG1 SNPs were associated with the number of positive skin tests (P = .007-.018). Carrying 2 copies of minor alleles for either of 2 highly associated ARG2 SNPs was associated with a statistically significant increased relative risk (RR) of asthma (1.5, 95% CI = 1.1-2.1 for arg2s1; RR = 1.6, 95% CI = 1.1-2.3 for arg2s2). The association was slightly stronger among children with a smoking parent (arg2s1 RR = 2.1, 95% CI = 1.2 - 3.9 with a smoking parent; RR = 1.2, 95% CI = 0.8-1.9 without; interaction P = .025). Haplotype analyses reduced the sample size but supported the single SNP results. One ARG2 SNP was related to the number of positive skin tests (P = .027).

CONCLUSION

Variation in arginase genes may contribute to asthma and atopy in children.

Arginase attenuates inhibitory nonadrenergic noncholinergic nerve-induced nitric oxide generation and airway smooth muscle relaxation

Background

Recent evidence suggests that endogenous arginase activity potentiates airway responsiveness to methacholine by attenuation of agonist-induced nitric oxide (NO) production, presumably by competition with epithelial constitutive NO synthase for the common substrate, L-arginine. Using guinea pig tracheal open-ring preparations, we now investigated the involvement of arginase in the modulation of neuronal nitric oxide synthase (nNOS)-mediated relaxation induced by inhibitory nonadrenergic noncholinergic (iNANC) nerve stimulation.

Methods

Electrical field stimulation (EFS; 150 mA, 4 ms, 4 s, 0.5 – 16 Hz)-induced relaxation was measured in tracheal preparations precontracted to 30% with histamine, in the presence of 1 μM atropine and 3 μM indomethacin. The contribution of NO to the EFS-induced relaxation was assessed by the nonselective NOS inhibitor L-NNA (0.1 mM), while the involvement of arginase activity in the regulation of EFS-induced NO production and relaxation was investigated by the effect of the specific arginase inhibitor nor-NOHA (10 μM). Furthermore, the role of substrate availability to nNOS in EFS-induced relaxation was measured in the presence of various concentrations of exogenous L-arginine.

Results

EFS induced a frequency-dependent relaxation, ranging from 6.6 ± 0.8% at 0.5 Hz to 74.6 ± 1.2% at 16 Hz, which was inhibited with the NOS inhibitor L-NNA by 78.0 ± 10.5% at 0.5 Hz to 26.7 ± 7.7% at 8 Hz (P < 0.01 all). In contrast, the arginase inhibitor nor-NOHA increased EFS-induced relaxation by 3.3 ± 1.2-fold at 0.5 Hz to 1.2 ± 0.1-fold at 4 Hz (P < 0.05 all), which was reversed by L-NNA to the level of control airways in the presence of L-NNA (P < 0.01 all). Similar to nor-NOHA, exogenous L-arginine increased EFS-induced airway relaxation (P < 0.05 all).

Conclusion

The results indicate that endogenous arginase activity attenuates iNANC nerve-mediated airway relaxation by inhibition of NO generation, presumably by limiting L-arginine availability to nNOS.

Neurokinins and inflammatory cell iNOS expression in guinea pigs with chronic allergic airway inflammation

In the present study we evaluated the role of neurokinins in the modulation of inducible nitric oxide synthase (iNOS) inflammatory cell expression in guinea pigs with chronic allergic airway inflammation. In addition, we studied the acute effects of nitric oxide inhibition on this response. Animals were anesthetized and pretreated with capsaicin (50 mg/kg sc) or vehicle 10 days before receiving aerosolized ovalbumin or normal saline twice weekly for 4 wk. Animals were then anesthetized, mechanically ventilated, given normal saline or N(G)-nitro-l-arginine methyl ester (l-NAME, 50 mg/kg ic), and challenged with ovalbumin. Prechallenge exhaled NO increased in ovalbumin-exposed guinea pigs (P < 0.05 compared with controls), and capsaicin reduced this response (P < 0.001). Compared with animals inhaled with normal saline, ovalbumin-exposed animals presented increases in respiratory system resistance and elastance and numbers of total mononuclear cells and eosinophils, including those expressing iNOS (P < 0.001). Capsaicin reduced all these responses (P < 0.05) except for iNOS expression in eosinophils. Treatment with l-NAME increased postantigen challenge elastance and restored both resistance and elastance previously attenuated by capsaicin treatment. Isolated l-NAME administration also reduced total eosinophils and mononuclear cells, as well as those cells expressing iNOS (P < 0.05 compared with ovalbumin alone). Because l-NAME treatment restored lung mechanical alterations previously attenuated by capsaicin, NO and neurokinins may interact in controlling airway tone. In this experimental model, NO and neurokinins modulate eosinophil and lymphocyte infiltration in the airways.

Nitric oxide in health and disease of the respiratory system

During the past decade a plethora of studies have unravelled the multiple roles of nitric oxide (NO) in airway physiology and pathophysiology. In the respiratory tract, NO is produced by a wide variety of cell types and is generated via oxidation of l-arginine that is catalyzed by the enzyme NO synthase (NOS). NOS exists in three distinct isoforms: neuronal NOS (nNOS), inducible NOS (iNOS), and endothelial NOS (eNOS). NO derived from the constitutive isoforms of NOS (nNOS and eNOS) and other NO-adduct molecules (nitrosothiols) have been shown to be modulators of bronchomotor tone. On the other hand, NO derived from iNOS seems to be a proinflammatory mediator with immunomodulatory effects. The concentration of this molecule in exhaled air is abnormal in activated states of different inflammatory airway diseases, and its monitoring is potentially a major advance in the management of, e.g., asthma. Finally, the production of NO under oxidative stress conditions secondarily generates strong oxidizing agents (reactive nitrogen species) that may modulate the development of chronic inflammatory airway diseases and/or amplify the inflammatory response. The fundamental mechanisms driving the altered NO bioactivity under pathological conditions still need to be fully clarified, because their regulation provides a novel target in the prevention and treatment of chronic inflammatory diseases of the airways.

Heparin normalizes allergen-induced nitric oxide deficiency and airway hyperresponsiveness

It has been established that polycations cause airway hyperresponsiveness (AHR) to methacholine by inducing a deficiency of constitutive nitric oxide synthase (cNOS)-derived bronchodilating nitric oxide (NO). Since a deficiency of cNOS-derived NO also contributes to allergen-induced AHR after the early asthmatic reaction (EAR) and since this AHR is associated with the release of polycationic proteins from infiltrated eosinophils in the airways, we hypothesized that endogenous polycations underlie or at least contribute to the allergen-induced NO deficiency and AHR. Using a guinea-pig model of allergic asthma, we addressed this hypothesis by examining the effect of the polyanion heparin, acting as a polycation antagonist, on the responsiveness to methacholine of isolated perfused tracheae from unchallenged control animals and from animals 6 h after ovalbumin challenge, that is, after the EAR. A 2.0-fold AHR (P<0.001) to intraluminal administration of methacholine was observed in airways from allergen-challenged animals compared to control. Incubation of these airways with 250 U ml(-1) heparin completely normalized the observed hyperresponsiveness (P<0.001), whereas the responsiveness to methacholine of airways from unchallenged control animals was not affected. The effect of heparin on airways from allergen-challenged guinea-pigs was dose-dependently (0.1 and 1.0 mM) reversed by the NOS inhibitor L-NAME (P<0.01). These results indicate that endogenous (presumably eosinophil-derived) polycations are involved in allergen-induced NO deficiency and AHR after the EAR, probably by inhibition of l-arginine transport.

Nitric oxide synthase inhibition: therapeutic potential in asthma

Nitric oxide (NO) is synthesized from L-arginine in the human respiratory tract by enzymes of the NO synthase (NOS) family. Levels of NO in exhaled air are increased in asthma, and measurement of exhaled NO has been advocated as a noninvasive tool to monitor the underlying inflammatory process. However, the relation of NO to disease pathophysiology is uncertain, and in particular the fundamental question of whether it should be viewed primarily as beneficial or harmful remains unanswered. Exogenously administered NO has both bronchodilator and bronchoprotective properties. Although it is unlikely that NO is an important regulator of basal airway tone, there is good evidence that endogenous NO release exerts a protective effect against various bronchoconstrictor stimuli. This response is thought to involve one or both of the constitutive NOS isoforms, endothelial NOS (eNOS) and neuronal NOS (nNOS). Therefore, inhibition of these enzymes is unlikely to be therapeutically useful in asthma and indeed may worsen disease control. On the other hand, the high concentrations of NO in asthma, which are believed to reflect upregulation of inducible NOS (iNOS) by proinflammatory cytokines, may produce various deleterious effects. These include increased vascular permeability, damage to the airway epithelium, and promotion of inflammatory cell infiltration. However, the possible effects of iNOS inhibition on allergic inflammation in asthma have not yet been described and studies in animal models have yielded inconsistent findings. Thus, the evidence to suggest that inhibition of iNOS would be a useful therapeutic strategy in asthma is limited at present. More definitive information will require studies combining agents that potently and specifically target individual NOS isoforms with direct measurement of inflammatory markers.

Decreased arginine bioavailability and increased serum arginase activity in asthma

Recent studies suggest that a nitric oxide (NO) deficiency and elevated arginase activity may play a role in the pathogenesis of asthma. Although much attention has been directed toward measurements of exhaled NO in asthma, no studies to date have evaluated levels of plasma arginase or arginine, the substrate for NO production, in patients with asthma. This study, therefore, measured amino acid levels, arginase activity, and nitric oxide metabolites in the blood of patients with asthma, as well as NO in exhaled breath. Although levels of virtually all amino acids were reduced, patients with asthma exhibited a striking reduction in plasma arginine levels compared with normal control subjects without asthma (45 +/- 22 vs. 94 +/- 29 microM, p < 0.0001), and serum arginase activity was elevated (1.6 +/- 0.8 vs. 0.5 +/- 0.3 micromol/ml/hour, asthma vs. control, p < 0.0001). High arginase activity in patients with asthma may contribute to low circulating arginine levels, thereby limiting arginine bioavailability and creating a NO deficiency that induces hyperreactive airways. Addressing the alterations in arginine metabolism may result in new strategies for treatment of asthma.

Arginase and asthma: novel insights into nitric oxide homeostasis and airway hyperresponsiveness

For many years it has been supposed that the production of an excess of nitric oxide (NO) by inducible NO synthase (iNOS) plays a major role in inflammatory diseases, including asthma. However, recent studies indicate that a deficiency of beneficial, bronchodilating constitutive NOS (cNOS)-derived NO is important in allergen-induced airway hyperresponsiveness. Although several mechanisms are proposed to explain the reduction of cNOS activity, reduced substrate availability, caused by a combination of increased arginase activity and decreased cellular uptake of L-arginine, appears to play a key role. Recent evidence also indicates that iNOS-induced pathophysiological effects involve substrate deficiency. Thus, at low concentrations of L-arginine iNOS produces both NO and superoxide anions, which results in the increased synthesis of the highly reactive, detrimental oxidant peroxynitrite. Based on these observations, we propose that a relative deficiency of NO caused by increased arginase activity and altered L-arginine homeostasis is a major factor in the pathology of asthma.

Nitric oxide in respiratory diseases

The formation and modulation of nitric oxide (NO) in the lungs is reviewed. Its beneficial and deleterious roles in airways diseases, including asthma, chronic obstructive pulmonary disease, and cystic fibrosis, and in animal models is discussed. The pharmacological effects of agents that modulate NO production or act as NO donors are described. The clinical pharmacology of these agents is described and the therapeutic potential for their use in airways disease is considered.

Type 2 nitric oxide synthase and protein nitration in chronic lung infection

Inflammation in the lung can lead to increased expression of inducible nitric oxide synthase (iNOS) and enhanced NO production. It has been postulated that the resultant highly reactive NO metabolites may have an important role in host defence, although they might also contribute to tissue damage. However, in a number of inflammatory lung diseases, including bronchiectasis, iNOS expression is increased but no elevation of airway NO can be detected. A potential explanation for this finding is that NO is rapidly scavenged by reaction with superoxide radicals, forming peroxynitrite, which is preferentially metabolized via nitration and nitrosation reactions. To test this hypothesis, anaesthetized, specific pathogen-free rats were inoculated with Pseudomonas aeruginosa incorporated into agar beads (chronically infected group) or sterile agar beads (control group). Ten to 15 days later, the lungs were isolated and fixed. Pseudomonas organisms were isolated from the lungs of the chronically infected group. These lungs showed extensive inflammatory cell infiltration and tissue damage, which were not observed in control lungs. Expression of iNOS was increased in the chronically infected group when compared with the control group. However, the mean number of cells staining for nitrotyrosine in the chronically infected group was not significantly different from that in the controls, nor was there an excess of nitrotyrosine, nitrate, nitrite or nitrosothiol concentrations in the infected lungs. Thus, no evidence was found of increased NO metabolites in chronically infected lungs, including products of the peroxynitrite pathway. These findings suggest that chronic infection does not cause increased iNOS activity in the lung, despite increased expression of iNOS.

Increased arginase activity underlies allergen-induced deficiency of cNOS-derived nitric oxide and airway hyperresponsiveness

1. A deficiency of constitutive nitric oxide synthase (cNOS)-derived nitric oxide (NO), due to reduced availability of L-arginine, importantly contributes to allergen-induced airway hyperresponsiveness (AHR) after the early asthmatic reaction (EAR). Since cNOS and arginase use L-arginine as a common substrate, we hypothesized that increased arginase activity is involved in the allergen-induced NO deficiency and AHR. 2. Using a guinea-pig model of allergic asthma, we addressed this hypothesis by examining the effects of the specific arginase inhibitor N(omega)-hydroxy-nor-L-arginine (nor-NOHA) on the responsiveness to methacholine of isolated perfused tracheae from unchallenged control animals and from animals 6 h after ovalbumin challenge. Arginase activity in these preparations was investigated by measuring the conversion of L-[14C]arginine to [14C]urea. 3. Airways from allergen-challenged animals showed a 2 fold (P<0.001) increase in responsiveness to intraluminal (IL) administration of methacholine compared to controls. A similar hyperresponsiveness (1.8 fold, P<0.01) was observed in control airways incubated with the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 0.1 mM, IL), while L-NAME had no further effect on the airways from challenged animals. 4. Remarkably, 5 microM nor-NOHA (IL) normalized the hyperresponsiveness of challenged airways to basal control (P<0.001), and this effect was fully reversed again by 0.1 mM L-NAME (P<0.05). Moreover, arginase activity in homogenates of the hyperresponsive airways was 3.5 fold (P<0.001) enhanced compared to controls. 5. The results indicate that enhanced arginase activity contributes to allergen-induced deficiency of cNOS-derived NO and AHR after the EAR, presumably by competition with cNOS for the common substrate, L-arginine. This is the first demonstration that arginase is involved in the pathophysiology of asthma.

Nitric oxide is generated in smooth muscle layer by neurokinin A and counteracts constriction in guinea pig airway

It has been reported that several bronchoconstrictors generate nitric oxide (NO), counteracting bronchoconstriction, and removal of bronchial epithelia reduces NO production. However, it has not been elucidated whether neurokinin A (NKA), a potent bronchoconstrictor liberated from nerve terminals, generates NO. Specific questions in this study were (1) does NKA also generate NO, (2) does NO counteract NKA-induced bronchoconstriction, and (3) does the NO generation require bronchial epithelial cells? In an in vivo study exogenous as well as endogenous (capsaicin-induced) NKA increased airway opening pressure (P(ao)) and the exhaled NO level, and both were inhibited by an antagonist selective for NK(2) receptor (a receptor for NKA), SR48968. The exhaled NO level became negligible with an inhibitor of NO synthase (NOS) type 1-3 (N(G)-nitro-L-arginine methyl ester, L-NAME) with increased P(ao), but not with a NOS type 2 inhibitor. In an in vitro study, NKA increased the nitrite/nitrate level in superfused fluid of tracheal segments. Removing smooth muscle reduced nitrite/nitrate in the fluid to negligible levels, while the level was unchanged with removal of the epithelia. Pretreatment with l-NAME enhanced the tension of epithelia-removed tracheal segments. These findings indicate that (1) NKA generates NO, (2) NO counteracts NKA-induced bronchoconstriction, and (3) NKA activates NOS in the muscle layer, independently of bronchial epithelia.

Nerve growth factor increases airway responses and decreases levels of exhaled nitric oxide during histamine challenge in an in vivo guinea-pig model

There is a growing body of evidence supporting the idea that nerve growth factor (NGF) may be involved in the development of asthma-associated symptoms, such as airway hyper-responsiveness. Increased levels of NGF have recently been described in serum and in the airways of asthmatics. We have examined whether exhaled nitric oxide (NO) levels might be altered during the increased airway responses upon NGF treatment in guinea-pigs in vivo. Intravenous (i.v.) administration of histamine normally elicits a rapid peak in insufflation pressure (IP) and in exhaled NO, followed by a period of decreased concentrations of exhaled NO. Anaesthetized guinea-pigs were pre-treated intravenously with either saline, 4 or 80 ng x kg(-1) NGF 30 min before i.v. challenge with 16 microg x kg(-1) histamine. At 80 ng x kg(-1) NGF significantly enhanced the airway obstruction caused by histamine, whereas the peak acute increase in exhaled NO was not enhanced. Following the increase, came a rapid drop, an effect enforced in the NGF treated animals. Subsequently, the time to return to 90% of resting exhaled NO was increased, from 12 min in saline-treated animals to 48 min in NGF-treated animals. Our data confirm that NGF can enhance airway responses to histamine. Moreover, our study shows a decrease in exhaled NO following a histamine challenge, an effect enhanced by NGF. A reduced ability to release exhaled NO may be a mechanism for increased airway responses during elevated NGF levels. The interaction between NGF and airway NO formation, and its relation to airway responses, merit further investigation.

Role of nitric oxide and superoxide in allergen-induced airway hyperreactivity after the late asthmatic reaction in guinea-pigs

1. In the present study, the roles of nitric oxide (NO) and superoxide anions (O2(-)) in allergen-induced airway hyperreactivity (AHR) after the late asthmatic reaction (LAR) were investigated ex vivo, by examining the effects of the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) and superoxide dismutase (SOD) on the responsiveness to methacholine of isolated perfused guinea-pig tracheae from unchallenged (control) animals and from animals 24 h after ovalbumin challenge. 2. At 24 h after allergen challenge, the animals developed AHR in vivo, as indicated by a mean 2.63 +/- 0.54 fold (P < 0.05) increase in sensitivity to histamine inhalation. 3. Compared to unchallenged controls, tracheal preparations from the ovalbumin-challenged guinea-pigs displayed a significant 1.8 fold (P < 0.01) increase in the maximal response (E(max)) to methacholine, both after intraluminal (IL) and extraluminal (EL) administration of the agonist. No changes were observed in the sensitivity (pEC(50)) to the agonist. Consequently, the DeltapEC(50) (EL-IL), as a measure of epithelial integrity, was unchanged. 4. In the presence of L-NAME (100 microM, IL), tracheae from control guinea-pigs showed a 1.6 fold (P < 0.05) increase in the E(max) of IL methacholine. By contrast, the E(max) of IL methacholine was significantly decreased in the presence of 100 u ml(-1) EL SOD (54% of control, P < 0.01). 5. Remarkably, the increased responsiveness to IL methacholine at 24 h after allergen challenge was reversed by L-NAME to control (P < 0.01), and a similar effect was observed with SOD (P < 0.01). 6. The results indicate that both NO and O2(-) are involved in the tracheal hyperreactivity to methacholine after the LAR, possibly by promoting airway smooth muscle contraction through the formation of peroxynitrite.

Inflammatory cell distribution in guinea pig airways and its relationship to airway reactivity

Although airway inflammation and airway hyperreactivity are observed after allergen inhalation both in allergic humans and animals, little is known about the mechanisms by which inflammatory cells can contribute to allergen-induced airway hyperreactivity. To understand how inflammatory cell infiltration can contribute to airway hyperreactivity, the location of these cells within the airways may be crucial Using a guinea pig model of acute allergic asthma, we investigated the inflammatory cell infiltration in different airway compartments at 6 and 24 h (i.e. after the early and the late asthmatic reaction, respectively) after allergen or saline challenge in relation to changes in airway reactivity (AR) to histamine. At 6 h after allergen challenge, a threefold (p < 0.01) increase in the AR to histamine was observed. At 24 h after challenge, the AR to histamine was lower, but still significantly enhanced (1.6-fold, p < 0.05). Adventitial eosinophil and neutrophil numbers in both bronchi and bronchioli were significantly increased at 6 h post-allergen provocation as compared with saline (p < 0.01 for all), while there was a strong tendency to enhanced eosinophils in the bronchial submucosa at this time point (p = 0.08). At 24h after allergen challenge, the eosinophilic and neutrophilic cell infiltration was reduced. CD3+ T lymphocytes were increased in the adventitial compartment of the large airways (p < 0.05) and in the parenchyma (p < 0.05) at 24h post-allergen, while numbers of CD8+ cells did not differ from saline treatment at any time point post-provocation. The results indicate that, after allergen provocation, inflammatory cell numbers in the airways are mainly elevated in the adventitial compartment. The adventitial inflammation could be important for the development of allergen-induced airway hyperreactivity.

Airway reactivity, inflammatory cell influx and nitric oxide in guinea-pig airways after lipopolysaccharide inhalation

1. The aim of this study was to investigate the relationship between airway reactivity, leukocyte influx and nitric oxide (NO), in conscious guinea-pigs after aerosolized lipopolysaccharide (LPS) exposure. 2. Inhaled histamine (1 mM, 20 s), causing no bronchoconstriction before LPS exposure (30 microg ml(-1), 1 h), caused bronchoconstriction at 0.5 and 1 h (P:<0.02) after LPS exposure. This airway hyperreactivity (AHR) recovered by 2 h. In contrast, 48 h after LPS exposure, the response from a previously bronchoconstrictor dose of histamine (3 mM, 20 s) was attenuated (P:<0.01) i.e. airway hyporeactivity (AHOR). 3. Investigation of the cellular content of bronchoalveolar lavage fluid (BALF) from these animals revealed a rapid (0.5 h: 691 fold increase) and progressive neutrophil influx after LPS exposure (24 h: 36.3+/-2.3x10(6) cells per sample), that subsided 48 h later. Macrophages and eosinophils also time-dependently increased (0.5 h: 4.6+/-0.4 and 0.1+/-0.05; 48 h: 31.0+/-6.0 and 1.8+/-0.3x10(6) cells per sample, respectively) after LPS, compared to vehicle exposure (24 h: neutrophils, eosinophils and macrophages: 0.28+/-0.19, 0.31+/-0.04 and 4.96+/-0. 43x10(6) cells per sample, respectively). 4. The combined NO metabolites in BALF, after vehicle (1 h), or LPS (1 h: AHR and 48 h: AHOR) exposure, were respectively increased (41%, P:<0.01), decreased (47%, P:<0.01) and further increased (80%, P:<0.001), compared with naïve animals. 5. Inhaled N(o)-nitro-L-arginine methyl ester (L-NAME: 1.2 and 12 mM, 15 min), reduced BALF NO metabolites 2 h later, but did not cause AHR to histamine (P:>0.05). When L-NAME inhalation followed LPS, AHR was prolonged from 1 h to at least 4 h (P:<0.01). 6. In summary, aerosolized LPS inhalation caused neutrophil and macrophage airways infiltration, and an early development of AHR followed 48 h later by AHOR to histamine. AHR and AHOR coincided with a respective reduction and elevation in airways NO (metabolites). Thus, NO may aid recovery from AHR, as inhibition of its production prolongs AHR. However, NO deficiency alone is not responsible for LPS-induced AHR.

Modulation of cholinergic airway reactivity and nitric oxide production by endogenous arginase activity

Cholinergic airway constriction is functionally antagonized by agonist-induced constitutive nitric oxide synthase (cNOS)-derived nitric oxide (NO). Since cNOS and arginase, which hydrolyzes L-arginine to L-ornithine and urea, use L-arginine as a common substrate, competition between both enzymes for the substrate could be involved in the regulation of cholinergic airway reactivity. Using a perfused guinea-pig tracheal tube preparation, we investigated the modulation of methacholine-induced airway constriction by the recently developed, potent and specific arginase inhibitor N(Omega)-hydroxy-nor-L-arginine (nor-NOHA). Intraluminal (IL) administration of nor-NOHA caused a concentration-dependent inhibition of the maximal effect (E(max)) in response to IL methacholine, which was maximal in the presence of 5 microM nor-NOHA (E(max)=31.2+/-1.6% of extraluminal (EL) 40 mM KCl-induced constriction versus 51.6+/-2.1% in controls, P<0.001). In addition, the pEC(50) (-log(10) EC(50)) was slightly but significantly reduced in the presence of 5 microM nor-NOHA. The inhibition of E(max) by 5 microM nor-NOHA was concentration-dependently reversed by the NOS inhibitor N(Omega)-nitro-L-arginine methyl ester (L-NAME), reaching an E(max) of 89.4+/-7.7% in the presence of 0.5 mM L-NAME (P<0.01). A similar E(max) in the presence of 0.5 mM L-NAME was obtained in control preparations (85.2+/-9.7%, n.s.). In the presence of excess of exogenously applied L-arginine (5 mM), 5 microM nor-NOHA was ineffective (E(max)=33.1+/-5.8 versus 31.1+/-7.5% in controls, n.s.). The results indicate that endogenous arginase activity potentiates methacholine-induced airway constriction by inhibition of NO production, presumably by competition with cNOS for the common substrate, L-arginine. This finding may represent an important novel regulation mechanism of airway reactivity.