S-nitrosothiol repletion by an inhaled gas regulates pulmonary function

Regulation of macrophage activation by S-Nitrosothiols following ozone-induced lung injury

Acute exposure to ozone causes oxidative stress, characterized by increases in nitric oxide (NO) and other reactive nitrogen species in the lung. NO has been shown to modify thiols generating S-nitrosothiols (SNOs); this results in altered protein function. In macrophages this can lead to changes in inflammatory activity which impact the resolution of inflammation. As SNO formation is dependent on the redox state of both the NO donor and the recipient thiol, the local microenvironment plays a key role in its regulation. This dictates not only the chemical feasibility of SNO formation but also mechanisms by which they may form. In these studies, we compared the ability of the SNO donors, ethyl nitrite (ENO), which targets both hydrophobic and hydrophilic thiols, SNO-propanamide (SNOPPM) which targets hydrophobic thiols, and S-nitroso-N-acetylcysteine. (SNAC) which targets hydrophilic thiols. to modify macrophage activation following ozone exposure. Mice were treated with air or ozone (0.8 ppm, 3 h) followed 1 h later by intranasal administration of ENO, SNOPPM or SNAC (1-500 μM) or appropriate controls. Mice were euthanized 48 h later. Each of the SNO donors reduced ozone-induced inflammation and modified the phenotype of macrophages both within the lung lining fluid and the tissue. ENO and SNOPPM were more effective than SNAC. These findings suggest that the hydrophobic SNO thiol pool targeted by SNOPPM and ENO plays a major role in regulating macrophage phenotype following ozone induced injury.

Bronchopulmonary Dysplasia and Pulmonary Hypertension. The Role of Smooth Muscle adh5

Bronchopulmonary dysplasia (BPD) is characterized by alveolar simplification, airway hyperreactivity, and pulmonary hypertension. In our BPD model, we have investigated the metabolism of the bronchodilator and pulmonary vasodilator GSNO (S-nitrosoglutathione). We have shown the GSNO catabolic enzyme encoded by adh5 (alcohol dehydrogenase-5), GSNO reductase, is epigenetically upregulated in hyperoxia. Here, we investigated the distribution of GSNO reductase expression in human BPD and created an animal model that recapitulates the human data. Blinded comparisons of GSNO reductase protein expression were performed in human lung tissues from infants and children with and without BPD. BPD phenotypes were evaluated in global (adh5^-/-) and conditional smooth muscle (smooth muscle/adh5^-/-) adh5 knockout mice. GSNO reductase was prominently expressed in the airways and vessels of human BPD subjects. Compared with controls, expression was greater in BPD smooth muscle, particularly in vascular smooth muscle (2.4-fold; P = 0.003). The BPD mouse model of neonatal hyperoxia caused significant alveolar simplification, airway hyperreactivity, and right ventricular and vessel hypertrophy. Global adh5^-/- mice were protected from all three aspects of BPD, whereas smooth muscle/adh5^-/- mice were only protected from pulmonary hypertensive changes. These data suggest adh5 is required for the development of BPD. Expression in the pulmonary vasculature is relevant to the pathophysiology of BPD-associated pulmonary hypertension. GSNO-mimetic agents or GSNO reductase inhibitors, both of which are currently in clinical trials for other conditions, could be considered for further study in BPD.

Pulmonary Vasodilation by Intravenous Infusion of Organic Mononitrites Of 1,2-Propanediol in Acute Pulmonary Hypertension Induced by Aortic Cross Clamping and Reperfusion: A Comparison With Nitroglycerin in Anesthetized Pigs

INTRODUCTION

Suprarenal aortic cross clamping (SRACC) and reperfusion may cause acute pulmonary hypertension and multiple organ failure.

HYPOTHESIS

The organic mononitrites of 1,2-propanediol (PDNO), an nitric oxide donor with a very short half-life, are a more efficient pulmonary vasodilator and attenuator of end-organ damage and inflammation without significant side effects compared with nitroglycerin and inorganic nitrite in a porcine SRACC model.

METHODS

Anesthetized and instrumented domestic pigs were randomized to either of four IV infusions until the end of the experiment (n = 10 per group): saline (control), PDNO (45 nmol kg min), nitroglycerin (44 nmol kg min), or inorganic nitrite (a dose corresponding to PDNO). Thereafter, all animals were subjected to 90 min of SRACC and 10 h of reperfusion and protocolized resuscitation. Hemodynamic and respiratory variables as well as blood samples were collected and analysed.

RESULTS

During reperfusion, mean pulmonary arterial pressure and pulmonary vascular resistance were significantly lower, and stroke volume was significantly higher in the PDNO group compared with the control, nitroglycerin, and inorganic nitrite groups. In parallel, mean arterial pressure, arterial oxygenation, and fraction of methaemoglobin were similar in all groups. The serum concentration of creatinine and tumor necrosis factor alpha were lower in the PDNO group compared with the control group during reperfusion.

CONCLUSIONS

PDNO was an effective pulmonary vasodilator and appeared superior to nitroglycerin and inorganic nitrite, without causing significant systemic hypotension, impaired arterial oxygenation, or methaemoglobin formation in an animal model of SRACC and reperfusion. Also, PDNO may have kidney-protective effects and anti-inflammatory properties.

Tracing the path of inhaled nitric oxide: Biological consequences of protein nitrosylation

Nitric oxide (NO) is a comprehensive regulator of vascular and airway tone. Endogenous NO produced by nitric oxide synthases regulates multiple signaling cascades, including activation of soluble guanylate cyclase to generate cGMP, relaxing smooth muscle cells. Inhaled NO is an established therapy for pulmonary hypertension in neonates, and has been recently proposed for the treatment of hypoxic respiratory failure and acute respiratory distress syndrome due to COVID-19. In this review, we summarize the effects of endogenous and exogenous NO on protein S-nitrosylation, which is the selective and reversible covalent attachment of a nitrogen monoxide group to the thiol side chain of cysteine. This posttranslational modification targets specific cysteines based on the acid/base sequence of surrounding residues, with significant impacts on protein interactions and function. S-nitrosothiol (SNO) formation is tightly compartmentalized and enzymatically controlled, but also propagated by nonenzymatic transnitrosylation of downstream protein targets. Redox-based nitrosylation and denitrosylation pathways dynamically regulate the equilibrium of SNO-proteins. We review the physiological roles of SNO proteins, including nitrosohemoglobin and autoregulation of blood flow through hypoxic vasodilation, and pathological effects of nitrosylation including inhibition of critical vasodilator enzymes; and discuss the intersection of NO source and dose with redox environment, in determining the effects of protein nitrosylation.

S-Nitroso-L-Cysteine Ameliorated Pulmonary Hypertension in the MCT-Induced Rats through Anti-ROS and Anti-Inflammatory Pathways

Pulmonary hypertension (PH) is a progressive and life-threatening chronic disease in which increased pulmonary artery pressure (PAP) and pulmonary vasculature remodeling are prevalent. Inhaled nitric oxide (NO) has been used in newborns to decrease PAP in the clinic; however, the effects of NO endogenous derivatives, S-nitrosothiols (SNO), on PH are still unknown. We have reported that S-nitroso-L-cysteine (CSNO), one of the endogenous derivatives of NO, inhibited RhoA activity through oxidative nitrosation of its C16/20 residues, which may be beneficial for both vasodilation and remodeling. In this study, we presented data to show that inhaled CSNO attenuated PAP in the monocrotaline- (MCT-) induced PH rats and, moreover, improved right ventricular (RV) hypertrophy and fibrosis induced by RV overloaded pressure. In addition, aerosolized CSNO significantly inhibited the hyperactivation of signal transducers and activators of transduction 3 (STAT3) and extracellular regulated protein kinases (ERK) pathways in the lung of MCT-induced rats. CSNO also regulated the expression of smooth muscle contractile protein and improved aberrant endoplasmic reticulum (ER) stress and mitophagy in lung tissues following MCT induction. On the other hand, CSNO inhibited reactive oxygen species (ROS) production in vitro, which is induced by angiotensin II (AngII) as well as interleukin 6 (IL-6). In addition, CSNO inhibited excessive ER stress and mitophagy induced by AngII and IL-6 in vitro; finally, STAT3 and ERK phosphorylation was inhibited by CSNO in a concentration-dependent manner. Taken together, CSNO led to pulmonary artery relaxation and regulated pulmonary circulation remodeling through anti-ROS and anti-inflammatory pathways and may be used as a therapeutic option for PH treatment.

Voltage-gated potassium channel proteins and stereoselective S-nitroso-l-cysteine signaling

S-nitroso-l-cysteine (L-CSNO) behaves as a ligand. Its soluble guanylate cyclase–independent (sGC-independent) effects are stereoselective — that is, not recapitulated by S-nitroso-d-cysteine (D-CSNO) — and are inhibited by chemical congeners. However, candidate L-CSNO receptors have not been identified. Here, we have used 2 complementary affinity chromatography assays — followed by unbiased proteomic analysis — to identify voltage-gated K⁺ channel (Kv) proteins as binding partners for L-CSNO. Stereoselective L-CSNO–Kv interaction was confirmed structurally and functionally using surface plasmon resonance spectroscopy; hydrogen deuterium exchange; and, in Kv1.1/Kv1.2/Kvβ2-overexpressing cells, patch clamp assays. Remarkably, these sGC-independent L-CSNO effects did not involve S-nitrosylation of Kv proteins. In isolated rat and mouse respiratory control (petrosyl) ganglia, L-CSNO stereoselectively inhibited Kv channel function. Genetic ablation of Kv1.1 prevented this effect. In intact animals, L-CSNO injection at the level of the carotid body dramatically and stereoselectively increased minute ventilation while having no effect on blood pressure; this effect was inhibited by the L-CSNO congener S-methyl-l-cysteine. Kv proteins are physiologically relevant targets of endogenous L-CSNO. This may be a signaling pathway of broad relevance.

Two complementary affinity chromatography assays, followed by unbiased proteomic analysis, identified voltage-gated K⁺ channel (Kv) proteins as binding partners for S-nitroso-l-cysteine (L-CSNO).

Role of Nitric Oxide Carried by Hemoglobin in Cardiovascular Physiology: Developments on a Three-Gas Respiratory Cycle

A continuous supply of oxygen is essential for the survival of multicellular organisms. The understanding of how this supply is regulated in the microvasculature has evolved from viewing erythrocytes (red blood cells [RBCs]) as passive carriers of oxygen to recognizing the complex interplay between Hb (hemoglobin) and oxygen, carbon dioxide, and nitric oxide-the three-gas respiratory cycle-that insures adequate oxygen and nutrient delivery to meet local metabolic demand. In this context, it is blood flow and not blood oxygen content that is the main driver of tissue oxygenation by RBCs. Herein, we review the lines of experimentation that led to this understanding of RBC function; from the foundational understanding of allosteric regulation of oxygen binding in Hb in the stereochemical model of Perutz, to blood flow autoregulation (hypoxic vasodilation governing oxygen delivery) observed by Guyton, to current understanding that centers on S-nitrosylation of Hb (ie, S-nitrosohemoglobin; SNO-Hb) as a purveyor of oxygen-dependent vasodilatory activity. Notably, hypoxic vasodilation is recapitulated by native S-nitrosothiol (SNO)-replete RBCs and by SNO-Hb itself, whereby SNO is released from Hb and RBCs during deoxygenation, in proportion to the degree of Hb deoxygenation, to regulate vessels directly. In addition, we discuss how dysregulation of this system through genetic mutation in Hb or through disease is a common factor in oxygenation pathologies resulting from microcirculatory impairment, including sickle cell disease, ischemic heart disease, and heart failure. We then conclude by identifying potential therapeutic interventions to correct deficits in RBC-mediated vasodilation to improve oxygen delivery-steps toward effective microvasculature-targeted therapies. To the extent that diseases of the heart, lungs, and blood are associated with impaired tissue oxygenation, the development of new therapies based on the three-gas respiratory system have the potential to improve the well-being of millions of patients.

A pilot study on the kinetics of metabolites and microvascular cutaneous effects of nitric oxide inhalation in healthy volunteers

RATIONALE

Inhaled nitric oxide (NO) exerts a variety of effects through metabolites and these play an important role in regulation of hemodynamics in the body. A detailed investigation into the generation of these metabolites has been overlooked.

OBJECTIVES

We investigated the kinetics of nitrite and S-nitrosothiol-hemoglobin (SNO-Hb) in plasma derived from inhaled NO subjects and how this modifies the cutaneous microvascular response.

FINDINGS

We enrolled 15 healthy volunteers. Plasma nitrite levels at baseline and during NO inhalation (15 minutes at 40 ppm) were 102 (86-118) and 114 (87-129) nM, respectively. The nitrite peak occurred at 5 minutes of discontinuing NO (131 (104-170) nM). Plasma nitrate levels were not significantly different during the study. SNO-Hb molar ratio levels at baseline and during NO inhalation were 4.7E-3 (2.5E-3-5.8E-3) and 7.8E-3 (4.1E-3-13.0E-3), respectively. Levels of SNO-Hb continued to climb up to the last study time point (30 min: 10.6E-3 (5.3E-3-15.5E-3)). The response to acetylcholine iontophoresis both before and during NO inhalation was inversely associated with the SNO-Hb level (r: -0.57, p = 0.03, and r: -0.54, p = 0.04, respectively).

CONCLUSIONS

Both nitrite and SNO-Hb increase during NO inhalation. Nitrite increases first, followed by a more sustained increase in Hb-SNO. Nitrite and Hb-SNO could be a mobile reservoir of NO with potential implications on the systemic microvasculature.

Organic mononitrites of 1,2-propanediol act as an effective NO-releasing vasodilator in pulmonary hypertension and exhibit no cross-tolerance with nitroglycerin in anesthetized pigs

Purpose

Clinically available intravenous (IV) nitric oxide (NO) donor drugs such as nitroglycerin (GTN) cause systemic hypotension and/or tolerance development. In a porcine model, novel NO donor compounds – the organic mononitrites of 1,2-propanediol (PDNO) – were compared to GTN with regard to pulmonary selectivity and tolerance development. The vasodilatory effects of inorganic nitrite were investigated.

Materials and methods

In anesthetized piglets, central hemodynamics were monitored. At normal pulmonary vascular resistance (PVR), IV infusions of PDNO (15–60 nmol kg⁻¹ min⁻¹), GTN (13–132 nmol kg⁻¹ min⁻¹), and inorganic nitrite (dosed as PDNO) were administered. At increased PVR (by U46619 IV), IV infusions of PDNO (60–240 nmol kg⁻¹ min⁻¹) and GTN (75–300 nmol kg⁻¹ min⁻¹) before and after a 5 h infusion of GTN (45 nmol kg⁻¹ min⁻¹) were given.

Results

At normal PVR, PDNO (n=12) and GTN (n=7) caused significant dose-dependent decreases in mean systemic and pulmonary arterial pressures, whereas inorganic nitrite (n=13) had no significant effect. At increased PVR, PDNO (n=6) and GTN (n=6) significantly decreased mean systemic and pulmonary pressures and resistances, but only PDNO reduced the ratio between pulmonary and systemic vascular resistances significantly. After the 5 h GTN infusion, the hemodynamic response to GTN infusions (n=6) was significantly suppressed, whereas PDNO (n=6) produced similar hemodynamic effects to those observed before the GTN infusion.

Conclusion

PDNO is a vasodilator with selectivity for pulmonary circulation exhibiting no cross-tolerance to GTN, but GTN causes non selective vasodilatation with substantial tolerance development in the pulmonary and systemic circulations. Inorganic nitrite has no vasodilatory properties at relevant doses.

Pharmacologic Targeting of Red Blood Cells to Improve Tissue Oxygenation

Disruption of microvascular blood flow is a common cause of tissue hypoxia in disease, yet no therapies are available that directly target the microvasculature to improve tissue oxygenation. Red blood cells (RBCs) autoregulate blood flow through S-nitroso-hemoglobin (SNO-Hb)-mediated export of nitric oxide (NO) bioactivity. We therefore tested the idea that pharmacological enhancement of RBCs using the S-nitrosylating agent ethyl nitrite (ENO) may provide a novel approach to improve tissue oxygenation. Serial ENO dosing was carried out in sheep (1-400 ppm) and humans (1-100 ppm) at normoxia and at reduced fraction of inspired oxygen (FiO₂ ). ENO increased RBC SNO-Hb levels, corrected hypoxia-induced deficits in tissue oxygenation, and improved measures of oxygen utilization in both species. No adverse effects or safety concerns were identified. Inasmuch as impaired oxygenation is a major cause of morbidity and mortality, ENO may have widespread therapeutic utility, providing a first-in-class agent targeting the microvasculature.

Alcohol drives S-nitrosylation and redox activation of protein phosphatase 1, causing bovine airway cilia dysfunction

Individuals with alcohol (ethanol)-use disorders are at increased risk for lung infections, in part, due to defective mucociliary clearance driven by motile cilia in the airways. We recently reported that isolated, demembranated bovine cilia (axonemes) are capable of producing nitric oxide (^∙NO) when exposed to biologically relevant concentrations of alcohol. This increased presence of ^∙NO can lead to protein S-nitrosylation, a posttranslational modification signaling mechanism involving reversible adduction of nitrosonium cations or ^∙NO to thiolate or thiyl radicals, respectively, of proteins forming S-nitrosothiols (SNOs). We quantified and compared SNO content between isolated, demembranated axonemes extracted from bovine tracheae, with or without in situ alcohol exposure (100 mM × 24 h). We demonstrate that relevant concentrations of alcohol exposure shift the S-nitrosylation status of key cilia regulatory proteins, including 20-fold increases in S-nitrosylation of proteins that include protein phosphatase 1 (PP1). With the use of an ATP-reactivated axoneme motility system, we demonstrate that alcohol-driven S-nitrosylation of PP1 is associated with PP1 activation and dysfunction of axoneme motility. These new data demonstrate that alcohol can shift the S-nitrothiol balance at the level of the cilia organelle and highlight S-nitrosylation as a novel signaling mechanism to regulate PP1 and cilia motility.

S-Nitrosoglutathione Attenuates Airway Hyperresponsiveness in Murine Bronchopulmonary Dysplasia

Bronchopulmonary dysplasia (BPD) is characterized by lifelong obstructive lung disease and profound, refractory bronchospasm. It is observed among survivors of premature birth who have been treated with prolonged supplemental oxygen. Therapeutic options are limited. Using a neonatal mouse model of BPD, we show that hyperoxia increases activity and expression of a mediator of endogenous bronchoconstriction, S-nitrosoglutathione (GSNO) reductase. MicroRNA-342-3p, predicted in silico and shown in this study in vitro to suppress expression of GSNO reductase, was decreased in hyperoxia-exposed pups. Both pretreatment with aerosolized GSNO and inhibition of GSNO reductase attenuated airway hyperresponsiveness in vivo among juvenile and adult mice exposed to neonatal hyperoxia. Our data suggest that neonatal hyperoxia exposure causes detrimental effects on airway hyperreactivity through microRNA-342-3p–mediated upregulation of GSNO reductase expression. Furthermore, our data demonstrate that this adverse effect can be overcome by supplementing its substrate, GSNO, or by inhibiting the enzyme itself. Rates of BPD have not improved over the past two decades; nor have new therapies been developed. GSNO-based therapies are a novel treatment of the respiratory problems that patients with BPD experience.

Nitrogen chemistry and lung physiology

The versatile chemistry of nitrogen is important to pulmonary physiology. Indeed, almost all redox forms of nitrogen are relevant to pulmonary physiology and to pathophysiology. Here we review the relevance to pulmonary biology of (a) elemental nitrogen; (b) reduced forms of nitrogen such as amines, ammonia, and hydroxylamine; and (c) oxidized forms of nitrogen such as the nitroxyl anion, the nitric oxide free radical, and S-nitrosothiols. Our focus is on oxidized nitrogen in the form of S-nitrosothiol bond-containing species, which are now appreciated to be important to every type of cell-signaling process in the lung. We also review potential clinical applications of nitrogen oxide biochemistry. These principles are being translated into clinical practice as diagnostic techniques and therapies for a range of pulmonary diseases including asthma, cystic fibrosis, adult respiratory distress syndrome, primary ciliary dyskinesia, and pulmonary hypertension.

Nitric oxide transport in blood: a third gas in the respiratory cycle

The trapping, processing, and delivery of nitric oxide (NO) bioactivity by red blood cells (RBCs) have emerged as a conserved mechanism through which regional blood flow is linked to biochemical cues of perfusion sufficiency. We present here an expanded paradigm for the human respiratory cycle based on the coordinated transport of three gases: NO, O₂, and CO₂. By linking O₂ and NO flux, RBCs couple vessel caliber (and thus blood flow) to O₂ availability in the lung and to O₂ need in the periphery. The elements required for regulated O₂-based signal transduction via controlled NO processing within RBCs are presented herein, including S-nitrosothiol (SNO) synthesis by hemoglobin and O₂-regulated delivery of NO bioactivity (capture, activation, and delivery of NO groups at sites remote from NO synthesis by NO synthase). The role of NO transport in the respiratory cycle at molecular, microcirculatory, and system levels is reviewed. We elucidate the mechanism through which regulated NO transport in blood supports O₂ homeostasis, not only through adaptive regulation of regional systemic blood flow but also by optimizing ventilation-perfusion matching in the lung. Furthermore, we discuss the role of NO transport in the central control of breathing and in baroreceptor control of blood pressure, which subserve O₂ supply to tissue. Additionally, malfunctions of this transport and signaling system that are implicated in a wide array of human pathophysiologies are described. Understanding the (dys)function of NO processing in blood is a prerequisite for the development of novel therapies that target the vasoactive capacities of RBCs.

Repletion of S-nitrosohemoglobin improves organ function and physiological status in swine after brain death

OBJECTIVE

To determine if reduction in nitric oxide bioactivity contributes to the physiological instability that occurs after brain death and, if so, to also determine in this setting whether administration of a renitrosylating agent could improve systemic physiological status.

BACKGROUND

Organ function after brain death is negatively impacted by reduced perfusion and increased inflammation; the magnitude of these responses can impact post-graft function. Perfusion and inflammation are normally regulated by protein S-nitrosylation but systemic assessments of nitric oxide bioactivity after brain death have not been performed.

METHODS

Brain death was induced in instrumented swine by inflation of a balloon catheter placed under the cranium. The subjects were then serially assigned to receive either standard supportive care or care augmented by 20 ppm of the nitrosylating agent, ethyl nitrite, blended into the ventilation circuit.

RESULTS

Circulating nitric oxide bioactivity (in the form of S-nitrosohemoglobin) was markedly diminished 10 hours after induction of brain death-a decline that was obviated by administration of ethyl nitrite. Maintenance of S-nitrosohemoglobin was associated with improvements in tissue blood flow and oxygenation, reductions in markers of immune activation and cellular injury, and preservation of organ function.

CONCLUSIONS

In humans, the parameters monitored in this study are predictive of post-graft function. As such, maintenance of endocrine nitric oxide bioactivity after brain death may provide a novel means to improve the quality of organs available for donation.

S-nitrosoglutathione

BACKGROUND

S-Nitrosoglutathione (GSNO) is the S-nitrosated derivative of glutathione and is thought to be a critical mediator of the down stream signaling effects of nitric oxide (NO). GSNO has also been implicated as a contributor to various disease states.

SCOPE OF REVIEW

This review focuses on the chemical nature of GSNO, its biological activities, the evidence that it is an endogenous mediator of NO action, and implications for therapeutic use.

MAJOR CONCLUSIONS

GSNO clearly exerts its cellular actions through both NO- and S-nitrosation-dependent mechanisms; however, the chemical and biological aspects of this compound should be placed in the context of S-nitrosation as a whole.

GENERAL SIGNIFICANCE

GSNO is a central intermediate in formation and degradation of cellular S-nitrosothiols with potential therapeutic applications; thus, it remains an important molecule of study. This article is part of a Special Issue entitled Cellular functions of glutathione.

Can nitric oxide-based therapy prevent bronchopulmonary dysplasia?

A growing understanding of endogenous nitric oxide (NO) biology is helping to explain how and when exogenous NO may confer benefit or harm; this knowledge is also helping to identify new better-targeted NO-based therapies. In this review, results of the bronchopulmonary dysplasia clinical trials that used inhaled NO in the preterm population are placed in context, the biologic basis for novel NO therapeutics is considered, and possible future directions for NO-focused clinical and basic research in developmental lung disease are identified.

S-nitrosoglutathione reductase in human lung cancer

S-Nitrosoglutathione (GSNO) reductase regulates cell signaling pathways relevant to asthma and protects cells from nitrosative stress. Recent evidence suggests that this enzyme may prevent human hepatocellular carcinoma arising in the setting of chronic hepatitis. We hypothesized that GSNO reductase may also protect the lung against potentially carcinogenic reactions associated with nitrosative stress. We report that wild-type Ras is S-nitrosylated and activated by nitrosative stress and that it is denitrosylated by GSNO reductase. In human lung cancer, the activity and expression of GSNO reductase are decreased. Further, the distribution of the enzyme (including its colocalization with wild-type Ras) is abnormal. We conclude that decreased activity of GSNO reductase could leave the human lung vulnerable to the oncogenic effects of nitrosative stress, as is the case in the liver. This potential should be considered when developing therapies that inhibit pulmonary GSNO reductase to treat asthma and other conditions.

Transport rather than diffusion-dependent route for nitric oxide gas activity in alveolar epithelium

The pathway by which inhaled NO gas enters pulmonary alveolar epithelial cells has not been directly tested. Although the expected mechanism is diffusion, another route is the formation of S-nitroso-L-cysteine, which then enters the cell through the L-type amino acid transporter (LAT). To determine if NO gas also enters alveolar epithelium this way, we exposed alveolar epithelial-rat type I, type II, L2, R3/1, and human A549-cells to NO gas at the air liquid interface in the presence of L- and D-cysteine+/-LAT competitors. NO gas exposure concentration dependently increased intracellular NO and S-nitrosothiol levels in the presence of L- but not D-cysteine, which was inhibited by LAT competitors, and was inversely proportional to diffusion distance. The effect of L-cysteine on NO uptake was also concentration dependent. Without preincubation with L-cysteine, NO uptake was significantly reduced. We found similar effects using ethyl nitrite gas in place of NO. Exposure to either gas induced activation of soluble guanylyl cylase in a parallel manner, consistent with LAT dependence. We conclude that NO gas uptake by alveolar epithelium achieves NO-based signaling predominantly by forming extracellular S-nitroso-L-cysteine that is taken up through LAT, rather than by diffusion. Augmenting extracellular S-nitroso-L-cysteine formation may augment pharmacological actions of inhaled NO gas.

Increased nitrosoglutathione reductase activity in hypoxic pulmonary hypertension in mice

Altered S-nitrosothiols (RSNO) signaling is linked to pulmonary hypertension. Recent studies have shown that S-nitrosoglutathione (GSNO) reductase (GSNOR) catalyzes the degradation of GSNO and indirectly regulates the level of RSNO in vivo. Our present study tested the hypothesis that chronic hypoxia causes pulmonary hypertension, in part, by the change of GSNOR activity that contributes to the depletion of RSNO. Male mice were exposed to normobaric hypoxia in a ventilated chamber for 1 to 21 days or normoxia for 21 days. Right ventricular systolic pressure, right ventricle hypertrophy, and the number and media thickness of muscular pulmonary vessels increased significantly after 21 days of hypoxic exposure. Hypoxia induced the overexpression of endothelial nitric oxide synthase and inducible nitric oxide synthase. The mRNA expression of GSNOR decreased on day 1 of hypoxic exposure, but increased significantly on day 7 compared with the normoxic group. The protein expression of GSNOR increased significantly in the lung tissue after 7 days of hypoxic exposure and its enzymatic activities also increased. Both the ratios of glutathione to glutathione disulfide and nitrate to nitrite were significantly lower in the hypoxic groups than in the normoxic controls. The results suggest an increased GSNOR activity interfered with the metabolism of RSNO in mice with hypoxic pulmonary hypertension. An imbalanced of redox status is associated with the pathogenesis of hypoxic pulmonary hypertension.

Protection from lipopolysaccharide-induced lung injury by augmentation of airway S-nitrosothiols

RATIONALE

S-Nitrosothiols (SNO) inhibit immune activation of the respiratory epithelium and airway SNO levels are decreased in inflammatory lung disease. Ethyl nitrite (ENO) is a gas with chemical properties favoring SNO formation. Augmentation of airway SNO by inhaled ENO treatment may decrease lung inflammation and subsequent injury by inhibiting activation of the airway epithelium.

OBJECTIVES

To determine the effect of inhaled ENO on airway SNO levels and LPS-induced lung inflammation/injury.

METHODS

Mice were treated overnight with inhaled ENO (10 ppm) or air, followed immediately by exposure to aerosolized LPS or saline. Parameters of inflammation and lung injury were quantified 1 hour after completion of the aerosol exposure and correlated to lung airway and tissue SNO levels.

MEASUREMENTS AND MAIN RESULTS

Aerosolized LPS induced a decrease in airway and lung tissue SNO levels including S-nitrosylated NF-kappaB. The decrease in lung SNO was associated with an increase in lung NF-kappaB activity, cytokine/chemokine expression (keratinocyte-derived chemokine, tumor necrosis factor-alpha, and IL-6), airway neutrophil influx, and worsened lung compliance. Pretreatment with inhaled ENO restored airway SNO levels and reduced LPS-mediated NF-kappaB activation thereby inhibiting the downstream inflammatory response and preserving lung compliance.

CONCLUSIONS

Airway SNO serves an antiinflammatory role in the lung. Inhaled ENO can be used to augment airway SNO and protect from LPS-induced acute lung injury.

Emerging drugs for the prevention of bronchopulmonary dysplasia

BACKGROUND

Bronchopulmonary dysplasia (BPD) is a common adverse outcome of very premature birth and is associated with chronic respiratory morbidity.

OBJECTIVE

To determine if there were preventative therapies proven safe and efficacious in appropriately powered randomised trials.

METHODS

A literature review was undertaken.

RESULTS

Systemically administered corticosteroids, if given in the first 2 weeks, do significantly reduce BPD but have serious side effects. Vitamin A also reduces BPD, but has side effects, and further investigation is needed to identify the safest dosage regimen. There are, however, promising therapies that include antioxidants, low-dose nitric oxide and methylxanthines.

CONCLUSION

Further work is necessary to identify safe and effective preventative drugs for BPD.

Brief periods of nitric oxide inhalation protect against myocardial ischemia-reperfusion injury

BACKGROUND

Prolonged breathing of nitric oxide reduces myocardial ischemia-reperfusion injury, but the precise mechanisms responsible for the cardioprotective effects of inhaled nitric oxide are incompletely understood.

METHODS

The authors investigated the fate of inhaled nitric oxide (80 parts per million) in mice and quantified the formation of nitric oxide metabolites in blood and tissues. The authors tested whether the accumulation of nitric oxide metabolites correlated with the ability of inhaled nitric oxide to protect against cardiac ischemia-reperfusion injury.

RESULTS

Mice absorbed nitric oxide in a nearly linear fashion (0.19 +/- 0.02 micromol/g x h). Breathing nitric oxide rapidly increased a broad spectrum of nitric oxide metabolites. Levels of erythrocytic S-nitrosothiols, N-nitrosamines, and nitrosyl-hemes increased dramatically within 30 s of commencing nitric oxide inhalation. Marked increases of lung S-nitrosothiol and liver N-nitrosamine levels were measured, as well as elevated cardiac and brain nitric oxide metabolite levels. Breathing low oxygen concentrations potentiated the ability of inhaled nitric oxide to increase cardiac nitric oxide metabolite levels. Concentrations of each nitric oxide metabolite, except nitrate, rapidly reached a plateau and were similar after 5 and 60 min. In a murine cardiac ischemia-reperfusion injury model, breathing nitric oxide for either 5 or 60 min before reperfusion decreased myocardial infarction size as a fraction of myocardial area at risk by 31% or 32%, respectively.

CONCLUSIONS

Breathing nitric oxide leads to the rapid accumulation of a variety of nitric oxide metabolites in blood and tissues, contributing to the ability of brief periods of nitric oxide inhalation to provide cardioprotection against ischemia-reperfusion injury. The nitric oxide metabolite concentrations achieved in a target tissue may be more important than the absolute amounts of nitric oxide absorbed.

Pulmonary alveolar epithelial uptake of S-nitrosothiols is regulated by L-type amino acid transporter

Nitric oxide (NO) effects are often mediated via S-nitrosothiol (SNO) formation; SNO uptake has recently been shown to be mediated in some cell types via system L-type amino acid transporters (LAT-1, 2). Inhaled NO therapy may exert some biological effects via SNO formation. We therefore sought to determine if pulmonary epithelial SNO uptake depended on LAT or peptide transporter 2 (PEPT2). Both LAT-1 and PEPT2 proteins were detected by immunoblot and immunocytochemistry in L2 cells and rat lung. We tested SNO uptake through the transporters by exposing rat alveolar epithelial cells (L2 and type II) to RSNOs: S-nitrosoglutathione, S-nitrosocysteinylglycine (SNO-Cys-Gly), S-nitrosocysteine (CSNO), and to NO donor diethylamine NONOate (DEA-NONOate). SNO was detected in cell lysates by ozone chemiluminescence. NO uptake was detected by fluorescence in alveolar epithelial cells loaded with 4-amino-5-methylamino-2',7'-difluorofluorescein (DAF-FM) diacetate cultured in submersion and exposed to RSNOs and DEA NONOate. Addition of L-Cys but not D-Cys to RSNOs or DEA NONOate increased SNO and DAF-FM signal that was inhibited by coincubation with LAT competitors. Incubation of cells with PEPT2 substrate SNO-Cys-Gly showed no increase in SNO or DAF-FM signal unless incubated with L-Cys. This was unaffected by PEPT2 inhibition. We conclude that RSNOs (thionitrites, S-nitrosothiols) and NO enter alveolar epithelial cells predominantly by S-nitrosation of L-Cys, which is then imported through LAT.

Selective inhibition of inducible NO synthase activity in vivo reverses inflammatory abnormalities in surfactant protein D-deficient mice

Surfactant protein D (SP-D)-deficient (SP-D-/-) mice exhibit early development of emphysema. Previously we have shown that SP-D deficiency results in increased production and activity of inducible NO synthase (iNOS). In this study, we examined whether treatment with the iNOS inhibitor 1400W could inhibit the inflammatory phenotype. Mice were treated with 1400W systemically for 7 wk from 3 wk of age. Treatment reduced total lung NO synthase activity to 14.7+/-6.1% of saline-treated 10-wk-old SP-D-/- littermates. Long-term administration of 1400W reduced lung inflammation and cellular infiltration; and significantly attenuated the increased levels of matrix metalloproteinases 2 and 9, chemokines (KC, TARC), and cytokines (IFN-gamma) seen in bronchoalveolar lavage (BAL) of SP-D-/- mice. Abrogation of these levels was associated with decreasing BAL chemotactic activity for RAW cells. Two weeks of treatment with 1400W reduced total lung NO synthase (NOS) activity to 12.7+/-6.3% of saline-treated SP-D-/- mice. Short-term iNOS inhibition resulted in attenuation of pulmonary inflammation within SP-D-/- mice as shown by decreases in total BAL cell count (63+/-6% of SP-D-/- control), macrophage size (>25 microm) within the BAL (62+/-10% of SP-D-/- control), and a percentage of BAL macrophages producing oxidants (76+/-9% of SP-D-/- control). These studies showed that s.c. delivery of 1400W can be achieved in vivo and can attenuate the inflammatory processes within SP-D deficiency. Our results represent the first report linking defects in the innate immune system in the lung with alterations in NO homeostasis.

Inhaled ethyl nitrite prevents hyperoxia-impaired postnatal alveolar development in newborn rats

RATIONALE

Inhaled nitric oxide (NO) has been used to prevent bronchopulmonary dysplasia, but with variable results. Ethyl nitrite (ENO) forms S-nitrosothiols more readily than does NO, and resists higher-order nitrogen oxide formation. Because S-nitrosylation is a key pathway mediating many NO biological effects, treatment with inhaled ENO may better protect postnatal lung development from oxidative stress than NO.

OBJECTIVES

To compare inhaled NO and ENO on hyperoxia-impaired postnatal lung development.

METHODS

We treated newborn rats beginning at birth to air or 95% O(2) +/- 0.2-20.0 ppm ENO for 8 days, or to 10 ppm NO for 8 days. Pups treated with the optimum ENO dose, 10 ppm, and pups treated with 10 ppm NO were recovered in room air for 6 more days.

MEASUREMENTS AND MAIN RESULTS

ENO and NO partly prevented 95% O(2)-induced airway neutrophil influx in lavage, but ENO had a greater effect than did NO in prevention of lung myeloperoxidase accumulation, and in expression of cytokine-induced neutrophil chemoattractant-1. Treatment with 10 ppm ENO, but not NO, for 8 days followed by recovery in air for 6 days prevented 95% O(2)-induced impairments of body weight, lung compliance, and alveolar development.

CONCLUSIONS

Inhaled ENO conferred protection superior to inhaled NO against hyperoxia-induced inflammation. ENO prevented hyperoxia impairments of lung compliance and postnatal alveolar development in newborn rats.

Inclusion of an S-nitrosylating agent in the insufflating gas does not alter gastric activity in rats following pneumoperitoneum

BACKGROUND

We have previously shown that mixing the S-nitrosylating agent ethyl nitrite with carbon dioxide can attenuate pneumoperitoneum-induced decreases in splanchnic blood flow, but it was unclear if this agent would alter gastric function. This question was answered using rats by assessing gastric emptying and gastrointestinal transit times following gavage with radioactive chromium.

METHODS

There were five experimental groups: absolute control, anesthesia control, and carbon dioxide alone or with 100 or 300 parts per million ethyl nitrite. The period of insufflation was 1 h, and all animals were euthanized 6.5 h after chromium administration.

RESULTS

The mean amount of radioactivity remaining in the stomach ranged between 16% and 27% of the total administered; these differences were not statistically significant (p > 0.05). Modest differences in chromium distribution were identified in the gastrointestinal tract, but for all treatments, the peak amount of radioactivity was located in the distal portion. Location of the peak, expressed as a percentage of total tract length, varied between 70% and 85% (p = 0.366).

CONCLUSIONS

This study found no adverse effect of ethyl nitrite on postoperative gastric emptying or gastrointestinal transit time following pneumoperitoneum. The findings support continued assessment of the clinical utility of ethyl nitrite in the setting of laparoscopic surgery.

Extrapulmonary effects of inhaled nitric oxide: role of reversible S-nitrosylation of erythrocytic hemoglobin

Early applications of inhaled nitric oxide (iNO), typically in the treatment of diseases marked by acute pulmonary hypertension, were met by great enthusiasm regarding the purported specificity of iNO: vasodilation by iNO was specific to the lung (without a change in systemic vascular resistance), and within the lung, NO activity was said to be confined spatially and temporally by Hb within the vascular lumen. Underlying these claims were classical views of NO as a short-lived paracrine hormone that acts largely through the heme groups of soluble guanylate cyclase, and whose potential activity is terminated on encountering the hemes of red blood cell (RBC) Hb. These classical views are yielding to a broader paradigm, in which NO-related signaling is achieved through redox-related NO adducts that endow NO synthase products with the ability to act at a distance in space and time from NO synthase itself. Evidence supporting the biological importance of such stable NO adducts is probably strongest for S-nitrosothiols (SNOs), in which NO binds to critical cysteine residues in proteins or peptides. The circulating RBC is a major SNO reservoir, and RBC Hb releases SNO-related bioactivity peripherally on O2 desaturation. These new paradigms describing NO transport also provide a plausible mechanistic understanding of the increasingly recognized peripheral effects of inhaled NO. An explanation for the peripheral actions of inhaled NO is discussed here, and the rationale and results of attempts to exploit the "NO delivery" function of the RBC are reviewed.

Is It Go or NO Go for S-Nitrosylation Modification-Based Therapies of Cystic Fibrosis Transmembrane Regulator Trafficking?: Fig. 1

Nitric-oxide synthases (NOS) are abundant in the respiratory epithelium and generate the NO radical, which can activate guanylate cyclase, react with superoxide, or modify proteins by S-nitrosylation (SNO) of Cys thiols. There is increasing appreciation that SNO modification is analogous to phosphorylation, because both signaling mechanisms modulate a wide range of cellular functions. Zaman et al. (p. 1435) in this issue report on the capability of S-nitrosoglutathione (GSNO) to increase the expression, trafficking, and function of mutant and wild-type cystic fibrosis transmembrane regulator (CFTR). The CFTR is a cAMP-regulated chloride channel that functions to regulate salt and water content in glands and ducts of secretory epithelia. GSNO is a low molecular weight SNO (S-nitrosothiol) formed during oxidation of NO. The authors use GSNO as a lead compound to restore mutant CFTR function. Earlier contradictory reports that GSNO decreased CFTR function by oxidative modification (glutathionylation) may now be explained by high concentrations of GSNO associated with decreased CFTR transcription and disruption of CFTR function. Zaman et al. show that at physiologic concentrations, GSNO and the constitutively active S-nitroso-glutathione diethyl ester stimulate CFTR transcription through SP1 and SP3 and promote normal trafficking. The mechanism behind rescue from the degradative pathway relies on increasing the expression of cysteine string proteins and SNO modification of chaperones involved in mediating CFTR transit through the endoplasmic reticulum and Golgi apparatus.

S-nitrosylating agents: a novel class of compounds that increase cystic fibrosis transmembrane conductance regulator expression and maturation in epithelial cells

The endogenous bronchodilator, S-nitrosoglutathione (GSNO), increases expression, maturation, and function of both the wild-type and the DeltaF508 mutant of the cystic fibrosis transmembrane conductance regulatory protein (CFTR). Though transcriptional mechanisms of action have been identified, GSNO seems also to have post-transcriptional effects on CFTR maturation. Here, we report that 1) GSNO is only one of a class of S-nitrosylating agents that, at low micromolar concentrations, increase DeltaF508 and wild-type CFTR expression and maturation; 2) NO itself (at these concentrations) and 8-bromocyclic GMP are minimally active on CFTR; 3) a novel agent, S-nitrosoglutathione diethyl ester, bypasses the need for GSNO bioactivation by gamma-glutamyl transpeptidase to increase CFTR maturation; 4) surprisingly, expression-but not S-nitrosylation-of cysteine string proteins (Csp) 1 and 2 is increased by GSNO; 5) the effect of GSNO to increase full maturation of wild-type CFTR is inhibited by Csp silencing (si)RNA; 6) proteins relevant to CFTR trafficking are SNO-modified, and SNO proteins traffic through the endoplasmic reticulum (ER) and Golgi after GSNO exposure; and 7) GSNO alters the interactions of DeltaF508 CFTR with Csp and Hsc70 in the ER and Golgi. These data suggest that GSNO is one of a class of S-nitrosylating agents that act independently of the classic NO radical/cyclic GMP pathway to increase CFTR expression and maturation. They also suggest that the effect of GSNO is dependent on Csp and on intracellular SNO trafficking. We speculate that these data will be of relevance to the development of NO donor-based therapies for CF.

EXPERIMENTAL THERAPIES FOR HYPOXIA-INDUCED PULMONARY HYPERTENSION DURING ACUTE LUNG INJURY

Hypoxic pulmonary vasoconstriction (HPV) and pulmonary hypertension present a common and formidable clinical problem for practicing thoracic, transplant, and trauma surgeons. The recent discovery of efficacious drugs that are selective for the pulmonary vasculature has brought about the potential for very powerful therapeutic agents. Inhaled nitric oxide (NO) therapy has already found broad clinical utility, yet its use is limited by potential toxicities. Rho kinase (ROK) has been discovered to play a very central role in the formation of hypoxia induced pulmonary hypertension, and the advent of very specific ROK inhibitors has shown positive clinical results. Finally, phosphodiesterase-5 inhibitors have been found to selectively vasodilate the pulmonary vasculature in the midst of HPV. The purposes of this review are to: 1) discuss the advantages and disadvantages of inhaled preparations of NO; 2) address experimental alternatives to inhaled preparations of NO to treat HPV; 3) explore potential therapeutic avenues associated with inhibition of Rho-kinase; and, 4) examine the use of phosphodiesterase-5 (PDE-5) inhibitors and combination therapy in the treatment of HPV.

A nitric oxide processing defect of red blood cells created by hypoxia: deficiency of S-nitrosohemoglobin in pulmonary hypertension

The mechanism by which hypoxia [low partial pressure of O(2) (pO(2))] elicits signaling to regulate pulmonary arterial pressure is incompletely understood. We considered the possibility that, in addition to its effects on smooth muscle, hypoxia may influence pulmonary vascular tone through an effect on RBCs. We report that exposure of native RBCs to sustained hypoxia is accompanied by a buildup of heme iron-nitrosyl (FeNO) species that are deficient in pO(2-)governed intramolecular transfer of NO to cysteine thiol, yielding a deficiency in the vasodilator S-nitrosohemoglobin (SNO-Hb). S-nitrosothiol (SNO)-deficient RBCs produce impaired vasodilator responses in vitro and exaggerated pulmonary vasoconstrictor responses in vivo and are defective in oxygenating the blood. RBCs from hypoxemic patients with elevated pulmonary arterial pressure (PAP) exhibit a similar FeNO/SNO imbalance and are thus deficient in pO(2)-coupled vasoregulation. Chemical restoration of SNO-Hb levels in both animals and patients restores the vasodilator activity of RBCs, and this activity is associated with improved oxygenation and lower PAPs.

A novel inhaled organic nitrate that affects pulmonary vascular tone in a piglet model of hypoxia-induced pulmonary hypertension

Persistent pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance after birth leading to right-to-left shunting and systemic arterial hypoxemia. Inhaled nitric oxide (NO) is effective in reducing the need for extracorporeal membrane oxygenation, but it has potential toxicities, especially in an oxygen-rich environment. A number of other NO-based molecules have been given by inhalation, but their structure-function relationships have not been established. Recent studies have raised the idea that toxic and beneficial properties can be separated. We synthesized a novel organic nitrate [ethyl nitrate (ENO2)], tested it in vitro, and administered it to hypoxic piglets. ENO2 lowered pulmonary artery pressure and raised the Po2 in arterial blood but did not alter systemic vascular resistance or methemoglobin levels. In addition, we tested the effect of ENO2 in the presence of the thiol glutathione, both in vivo and in vitro, and found its action to be enhanced. Although ENO2 is less potent than inhaled NO on a dose-equivalency basis, pretreatment of hypoxic animals with glutathione, which may be depleted in injured lungs, led to a markedly enhanced effect (largely mitigating the difference in potency). These results suggest that ENO2 may hold promise as a safe alternative to NO, particularly in hypoxemic conditions characterized by thiol depletion.

A method to attenuate pneumoperitoneum-induced reductions in splanchnic blood flow

OBJECTIVE

To determine if increasing nitric oxide bioactivity by inclusion of ethyl nitrite (ENO) in the insufflation admixture would attenuate pneumoperitoneum-induced decreases in splanchnic perfusion.

SUMMARY BACKGROUND DATA

Organ blood flow is reduced during pneumoperitoneum and can contribute to laparoscopy-associated morbidity and mortality. Previous attempts to control such decreases in flow have been ineffective.

METHODS

Laser-Doppler flow probes were placed on the liver and right kidney of anesthetized pigs. After a baseline recording period, animals were insufflated to a final intraperitoneal pressure of 15 mm Hg. Group one received CO2 (standard practice), whereas group 2 received CO2 plus 100 ppm ENO. Insufflation was maintained for 60 minutes and then the abdomen was manually deflated; monitoring was continued for another 60 minutes.

RESULTS

CO2 insufflation (n = 5) cut liver blood flow in half; liver flow remained at this level throughout the postinsufflation period. Inclusion of 100 ppm ENO (n = 6) attenuated both the acute and prolonged blood flow decreases. Statistical modeling of the data showed that, on average, liver blood flow was 14.3 U/min higher in the ENO pigs compared with the CO2 group (P = 0.0454). In contrast, neither treatment significantly altered kidney blood flow (P = 0.6215).

CONCLUSION

The data indicate that ENO can effectively attenuate pneumoperitoneum-induced blood flow decreases within the peritoneal cavity. The result suggests a novel therapeutic method of regulating hemodynamic changes during laparoscopic procedures.

Nitric oxide and redox signaling in allergic airway inflammation

A number of diseases of the respiratory tract, as exemplified in this review by asthma, are associated with increased amounts of nitric oxide (NO) in the expired breath. Asthma is furthermore characterized by increased production of reactive oxygen species that scavenge NO to form more reactive nitrogen species as demonstrated by the enhanced presence of nitrated proteins in the lungs of these patients. This increased oxidative metabolism leaves less bioavailable NO and coincides with lower amounts of S-nitrosothiols. In this review, we speculate on mechanisms responsible for the increased amounts of NO in inflammatory airway disease and discuss the apparent paradox of higher levels of NO as opposed to decreased amounts of S-nitrosothiols. We will furthermore give an overview of the regulation of NO production and biochemical events by which NO transduces signals into cellular responses, with a particular focus on modulation of inflammation by NO. Lastly, difficulties in studying NO signaling and possible therapeutic uses for NO will be highlighted.

Oxygen tension and inhaled nitric oxide modulate pulmonary levels of S-nitrosocysteine and 3-nitrotyrosine in rats

The oxidative environment within the lung generated upon administration of oxygen may be a critical regulator for the efficacy of inhaled nitric oxide therapy, possibly as a consequence of changes in nitrosative and nitrative chemistry. Changes in S-nitrosocysteine and 3-nitrotyrosine adducts were therefore evaluated after exposure of rats to 80% or >95% oxygen for 24 or 48 h with and without 20 ppm inhaled nitric oxide. Exposure to 80% oxygen led to increased formation of S-nitrosocysteine and 3-nitrotyrosine adducts in lung tissue that were also associated with increased expression of iNOS. The addition of inhaled nitric oxide in 80% oxygen exposure did not alter any of these adducts in the lung or in the bronchoalveolar lavage (BAL). Exposure to >95% oxygen led to a significant decrease in S-nitrosocysteine and an increase in 3-nitrotyrosine adducts in the lung. Co-administration of inhaled nitric oxide with >95% oxygen prevented the decrease in S-nitrosocysteine levels. The levels of S-nitrosocysteine and 3-nitrotyrosine returned to baseline in a time-dependent fashion after termination of exposure to >95% oxygen and inhaled nitric oxide. These data suggest the formation of S-nitrosating and tyrosine-nitrating species is regulated by oxygen tensions and co-administration of inhaled nitric oxide restores the nitrosative chemistry without a significant impact upon the nitrative pathway.

Biological significance of nitric oxide-mediated protein modifications

Nitric oxide (NO), despite an apparently simple diatomic structure, has a wide variety of functions in both physiology and pathology and within every major organ system. It has become an increasingly important scientific challenge to decipher how this wide range of activity is achieved. To this end a number of investigators have begun to explore how NO-mediated posttranslational modifications of proteins may represent mechanisms of cellular signaling. These modifications include: 1). binding to metal centers; 2). nitrosylation of thiol and amine groups; 3). nitration of tyrosine, tryptophan, amine, carboxylic acid, and phenylalanine groups; and 4). oxidation of thiols (both cysteine and methionine residues) and tyrosine. However, two particular modifications have recently received much attention, nitrosylation of thiols to produce S-nitrosothiol and nitration of tyrosine residues to produce nitrotyrosine. It is the purpose of this review to examine the possibility that these modifications may play a role in NO-mediated signaling.

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.

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.

Hypoxic pulmonary vasoconstriction in cardiothoracic surgery: basic mechanisms to potential therapies

Hypoxic pulmonary vasoconstriction is postulated to be an adaptive mechanism to match lung perfusion with ventilation; however, the consequences of the maladaptive effects of pulmonary vasoconstriction represent formidable therapeutic challenges. Understanding the basic mechanisms of hypoxic pulmonary vasoconstriction will enhance the assimilation of translational research into clinical practice. The purposes of this review are to (1) define basic mechanisms of pulmonary vasoconstriction and vasorelaxation; (2) delineate the biphasic contractile response to hypoxia; (3) critically examine data that support the mediator hypothesis versus the ion channel hypothesis; and (4) explore potential mechanistic-based therapies for hypoxic pulmonary vasoconstriction.

Surfactant protein-D, a mediator of innate lung immunity, alters the products of nitric oxide metabolism

Surfactant protein (SP)-D, a 43-kD multifunctional collagen-like lectin, is synthesized and secreted by the airway epithelium. SP-D knockout (SP-D [-/-]) mice exhibit an increase in the number and size of airway macrophages, peribronchiolar inflammation, increases in metalloproteinase activity, and development of emphysema. Nitric oxide (NO) is involved in a variety of signaling processes, and because altered NO metabolism has been observed in inflammation, we hypothesized that alterations in its metabolism would underlie the proinflammatory state observed in SP-D deficiency. Examination of the bronchial alveolar lavage (BAL) from SP-D (-/-) mice reveals a significant increase in protein and phospholipid content and total cell count. NO production and inducible NO synthase expression were increased in the BAL; however, there was a decline in S-nitrosothiol (SNO) content in the BAL and a loss of SNO immunoreactivity within the tissue. This decline in SNO was accompanied by an increase in nitrotyrosine staining. We conclude that inflammation that occurs in SP-D deficiency results in an increase in NO production and a shift in the chemistry and targets of NO. We speculate that the proinflammatory response due to SP-D deficiency results, in part, from a disruption of NO-mediated signaling within the innate immune system.

S-nitrosylation in health and disease

S-nitrosylation is a ubiquitous redox-related modification of cysteine thiol by nitric oxide (NO), which transduces NO bioactivity. Accumulating evidence suggests that the products of S-nitrosylation, S-nitrosothiols (SNOs), play key roles in human health and disease. In this review, we focus on the reaction mechanisms underlying the biological responses mediated by SNOs. We emphasize reactions that can be identified with complex (patho)physiological responses, and that best rationalize the observed increase or decrease in specific classes of SNOs across a spectrum of disease states. Thus, changes in the levels of various SNOs depend on specific defects in both enzymatic and non-enzymatic mechanisms of nitrosothiol formation, processing and degradation. An understanding of these mechanisms is crucial for the development of an integrated model of NO biology, and for effective treatment of diseases associated with dysregulation of NO homeostasis.

Nitric oxide and protein nitration in the cystic fibrosis airway

Cystic fibrosis (CF), characterized by chronic airway infection and inflammation, ultimately leads to respiratory failure. Exhaled nitric oxide (NO), elevated in most inflammatory airway diseases, is decreased in CF, suggesting either decreased production or accelerated metabolism of NO. The present studies performed on two groups of CF patients provide further support for a disordered NO airway metabolism in CF respiratory tract disease. Despite confirmation of subnormal NOS2 in the CF airway epithelium, alternative isoforms NOS1 and NOS3 were present, and inflammatory cells in the CF airway expressed abundant NOS2. Increased immunohistochemical staining for nitrotyrosine was demonstrated in lung tissues from patients with CF as compared to control. To our knowledge, this is the first report localizing nitrotyrosine in diseased CF lung tissue. While the relative NOS2 deficiency in CF respiratory tract epithelium may contribute to the lower expired NO levels, these results suggest that increased metabolism of NO is also present in advanced CF lung disease. The significance of altered NO metabolism and protein nitration in CF remains to be fully elucidated.

Inhaled ethyl nitrite gas for persistent pulmonary hypertension of the newborn

Inhaled nitric oxide is used to alleviate pulmonary hypertension and hypoxaemia, but generates toxic free radicals and oxides of nitrogen (NO(x)), which can cause rebound-hypoxia and additional pulmonary and other morbidity. To address these problems, we assessed the efficacy of inhaled O-nitrosoethanol gas (ENO) as a novel alternative means of providing nitric oxide bioactivity in the treatment of persistent pulmonary hypertension of newborns. We administered ENO over 4 h to seven neonates who required assisted ventilation, and who had an oxygenation index of 25 or more. ENO was then shut off for 15 min before start of treatment with inhaled nitric oxide. Our results show that ENO produced sustained improvements in postductal arterial oxygenation and systemic haemodynamics, which were maintained during the off-drug observation period. Increases in methaemoglobinaemia were modest and toxic NO(x) were not detected. Thus, ENO can improve oxygenation and systemic haemodynamics in neonates, and seems to reduce rebound hypoxaemia and production of toxic byproducts.