Glutathione peroxidase-1 as a novel therapeutic target for COPD

Abstract

Oxidative stress plays a role in a variety of diseases but it is even more pertinent in chronic obstructive pulmonary disease (COPD) given the increased oxidant burden in smokers. The increased oxidant burden results from the fact that cigarette smoke contains over 4700 different chemical compounds and more than 10¹⁵ oxidants/free radicals per puff. Other factors, such as air pollutants, infections, and occupational dusts that may exacerbate COPD, also have the potential to produce oxidative stress. These oxidants give rise to Reactive Oxygen Species (ROS) that are generated enzymatically by inflammatory and epithelial cells within the lung as part of an inflammatory immune response towards a pathogen or irritant. Thus, while ROS are necessary for host defence against invading pathogens, increased levels of ROS have been implicated in initiating inflammatory responses in the lungs through the activation of transcriptional factors, signal transduction pathways, chromatin remodelling and gene expression of pro-inflammatory mediators. However, the normal lung has developed defences to ROS-mediated damage, which include antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase. In this review we consider the therapeutic potential of the antioxidant enzyme glutathione peroxidase-1 for the treatment of cigarette smoke-induced lung inflammation and damage.

Keywords:

• Antioxidant
• COPD
• Cigarette smoke
• Ebselen
• Glutathione peroxidase-1
• Hydrogen peroxide
• Lung inflammation
• Reactive oxygen species

Introduction

Cigarette smoking is the major cause of chronic obstructive pulmonary disease (COPD) and accounts for more than 95% of cases in industrialized countries, but other environmental pollutants are important causes in developing countries.¹ COPD is ‘a disease state characterized by airflow limitation that is not fully reversible. The airflow limitation is usually progressive and associated with an abnormal inflammatory response of lungs to noxious particles and gases’.¹ COPD encompasses chronic obstructive bronchiolitis with fibrosis and obstruction of small airways, and emphysema with enlargement of airspaces and destruction of lung parenchyma, loss of lung elasticity, and closure of small airways. Most patients with COPD have all three pathological conditions (chronic obstructive bronchiolitis, emphysema, and mucus plugging), but the relative extent of emphysema and obstructive bronchiolitis within individual patients can vary.

It is well established that macrophages, neutrophils and lymphocytes are all involved in the pathogenesis of COPD.¹ However, several studies have highlighted that macrophages play a pivotal role in the pathophysiology of COPD and can account for most of the known features of the disease.² There is a marked increase in the numbers of macrophages in airways, lung parenchyma, bronchoalveolar lavage fluid (BALF) and sputum in patients with COPD.^3–5 There is a correlation between macrophage numbers in the airways and the severity of COPD.⁶ Macrophages are activated by cigarette smoke and other irritants to release inflammatory mediators including tumour necrosis factor (TNF), monocyte chemotactic peptide (MCP)-1, reactive oxygen species (ROS) and neutrophil chemotactic factors such as leukotriene B₄ and interleukin (IL)-8.¹ It is now clear from mouse models of lung inflammation that different macrophage subpopulations exist in the inflamed lung and that although the existence of such subpopulations is implicated in COPD, the importance of these subpopulations is unknown.⁷ Inflammatory macrophages recruited from circulating monocytes are phenotypically different from the resident population of cells and secrete many cytokines important in pro-inflammatory responses.⁷ Alveolar macrophages also secrete elastolytic enzymes, including matrix metalloprotease (MMP)-2, MMP-9 and MMP-12, and cathepsin K, L and S which together are responsible for destruction of lung parenchyma.¹ In patients with emphysema there is an increase in BALF concentrations and macrophage expression of MMP-1 and MMP-9⁸ and an increase in activity of MMP-9 in the lung parenchyma.⁹ Alveolar macrophages from normal smokers express more MMP-9 than those from normal subjects¹⁰ and there is an even greater increase in cells from patients with COPD, which have enhanced elastolytic activity.¹¹

Histological and bronchial biopsy studies show that patients with COPD have an increased number of neutrophils.^5,6 In addition, BALF and sputum of COPD patients have a marked increase in neutrophils.^3,4 Neutrophil numbers in bronchial biopsies and induced sputum are correlated with COPD disease severity and with the rate of decline in lung function.^4,6 Neutrophils secrete serine proteases, including neutrophil elastase, cathepsin G, and protease-3, as well as MMP-8 and MMP-9, which contribute to alveolar destruction and produce emphysema in laboratory animals.¹ Neutrophils migrate into the respiratory tract under the direction of neutrophil chemotactic factors.¹

Oxidative stress in the pathogenesis of COPD

Oxidative stress plays a role in a variety of diseases but it is even more pertinent in COPD given the increased oxidant burden in smokers. The increased oxidant burden results from the fact that cigarette smoke contains over 4700 different chemical compounds and more than 10¹⁵ oxidants/free radicals per puff.^12–15 Other factors, such as air pollutants, infections, and occupational dusts that may exacerbate COPD, also have the potential to produce oxidative stress. These oxidants give rise to ROS, which are a family of highly reactive molecules that are generated enzymatically by inflammatory and epithelial cells within the lung as part of an inflammatory immune response towards a pathogen or irritant. Activation of macrophages and neutrophils by cigarette smoke generates superoxide radicals (O₂^•−), which can then either react with nitric oxide (NO) to form reactive peroxynitrite molecules (ONOO⁻) or alternatively be rapidly converted to damaging hydrogen peroxide (H₂O₂) under the influence of superoxide dismutase (SOD). This in turn can result in the non-enzymatic production of the more damaging hydroxyl radical (^•OH) from H₂O₂ in the presence of Fe²⁺ through the Fenton reaction (Fig. 1). This leads to the oxidation of Fe²⁺ and Fe³⁺, which in turn can also generate ^•OH direct from O₂^•− and regenerate Fe²⁺ through the Haber-Weiss reaction. Thus, while ROS are necessary for host defence against invading pathogens, increased levels of ROS have been implicated in initiating inflammatory responses in the lungs through the activation of transcriptional factors such as nuclear factor kappaB (NFκB) and activator protein-1 (AP-1), signal transduction pathways, chromatin remodelling, and gene expression of pro-inflammatory mediators.^12,16 However, the normal lung has developed defences to ROS-mediated damage, which include the antioxidant enzymes SOD, catalase and glutathione peroxidase (Gpx).

Figure 1. Sources and production of ROS in the lung. Activation of macrophages, neutrophils, and epithelium by cigarette smoke generates superoxide radicals (O₂^•−) which can then either react with nitric oxide (NO) to form reactive peroxynitrite molecules (ONOO⁻) or alternatively be rapidly converted to damaging hydrogen peroxide (H₂O₂) under the influence of SOD. This in turn can result in the non-enzymatic production of damaging hydroxyl radical (^•OH) from H₂O₂ in the presence of Fe²⁺. H₂O₂ is subsequently enzymatically reduced by glutathione peroxidases (Gpxs), including Gpx-1, as well as catalase (CAT). Gpx-1 uses GSH as a cofactor to reduce H₂O₂, resulting in the formation of oxidized glutathione (GSSG), which can then be reduced to GSH by glutathione reductase (GR). The ROS O₂^•−, ONOO⁻, H₂O₂, and ^•OH can then cause lung inflammation, DNA damage, protein denaturation, lipid peroxidation, and emphysema.

Sources of ROS in the lung

The lungs are exposed to essentially two sources of ROS, environmental and cellular. Environmental-derived ROS consists of both gaseous and particulate air pollution. This ranges from cigarette smoke and oxidant gases, such as ozone, nitrogen dioxide and sulphur dioxide, to airborne particulate matter <10 µM (PM10) from diesel car exhaust fumes that can promote ROS production in situ. The nature of ROS found within cigarette smoke varies from short-lived oxidants, such as O₂^•− and NO, to long lived organic radicals, such as semiquinones that can undergo redox cycling within the epithelial lining fluid of smokers for some considerable period of time.^12,16

Cellular-derived ROS are enzymatically produced by inflammatory and epithelial cells within the lung as part of an inflammatory-immune response towards a pathogen or irritant. Several sources for ROS production exist within a cell and include mitochondrial respiration, NADPH oxidase, and the xanthine/xanthine oxidase system, of which the principal ROS generator is NADPH oxidase. Oxidants present in cigarette smoke can stimulate alveolar macrophages to produce ROS and to release a host of mediators, some of which attract neutrophils and other inflammatory cells into the lungs. Both macrophages and neutrophils, which are known to migrate in increased numbers into the lungs of people with COPD, can generate ROS via the NADPH oxidase system.¹⁷ Alveolar macrophages obtained in BAL fluid from the lungs of smokers are more activated to release ROS compared with those obtained from non-smokers.¹⁷ One manifestation of this is the release of increased amounts of ROS such as O₂^•− and H₂O₂.¹⁷ Exposure to cigarette smoke in vitro also has been shown to increase the oxidative metabolism of alveolar macrophages.¹⁸ Subpopulations of alveolar macrophages with a higher granular density appear to be more prevalent in the lungs of smokers and are responsible for the increased O₂^•− production associated with macrophages from smokers.^18,19 The generation of ROS in epithelial lining fluid may be further enhanced by the presence of increased amounts of free iron in the pulmonary airspaces in smokers.^20,21 This is relevant to COPD since the intracellular iron content of alveolar macrophages is augmented in cigarette smokers and is further increased in those who develop chronic bronchitis, compared with non-smokers.²² In addition, macrophages obtained from smokers release more free iron in vitro than those from non-smokers.²³ Not surprisingly, the net effect of all these ROS activities is that smokers and patients with COPD have higher levels of exhaled H₂O₂ than non-smokers,²⁴ and levels are even higher during exacerbations of COPD.²⁵ This increase in H₂O₂ is in part derived from increased release of O₂^•− from alveolar macrophages in smokers.²⁵ H₂O₂ can induce apoptosis in airway epithelial cells²⁶ and evidence from studies both in animals and man has shown that apoptosis occurs in smoke-exposed macrophages and airway epithelial cells.^27–29

Macrophages also employ other enzymes to produce ROS. This involves the activity of the heme peroxidase, myeloperoxidase, or eosinophil peroxidase, which are also found in neutrophils and eosinophils, respectively. These enzymes catalyse the formation of the potent and very damaging oxidants hypochlorous acid (HOCl) and hypobromous acid (HOBr) from H₂O₂ in the presence of chloride (Cl⁻) and bromide (Br⁻) ions, respectively. Circulating neutrophils from cigarette smokers and patients with exacerbations of COPD release more O₂^•−.³⁰ In addition, an association between O₂^•− release by peripheral blood neutrophils and bronchial hyperreactivity in patients with COPD has been shown, suggesting a role for systemic ROS in the pathogenesis of COPD.³¹

When generated close to cell membranes, ROS oxidize membrane phospholipids (lipid peroxidation), a process that may continue as a chain reaction, generating many lipid hydroperoxide molecules within the cell membrane.³² This can result in the oxidation of proteins, DNA, and lipids that may cause direct tissue injury or induce a variety of cellular responses, through the generation of secondary metabolic reactive species. This can impair membrane function, inactivate membrane-bound receptors and enzymes and increase tissue permeability. Lipid peroxidation can result in the formation of reactive aldehydes such as acrolein and 4-hydroxy-2-nonenal (4-HNE), which are highly diffusible and are able to induce various cellular events, such as proliferation, apoptosis, and activation of signalling pathways.^33,34 Increased levels of proteins modified by 4-HNE have been observed in airway and alveolar epithelial cells, endothelial cells, and neutrophils in smokers with airway obstruction compared with individuals without airway obstruction, suggesting a role for 4-HNE in the pathogenesis of COPD.³⁵ In addition, these reactive aldehydes can form adducts with both intracellular proteins, such as histone deacetylase (HDAC)-2, and extracellular proteins, such as collagen and fibronectin, altering their function that in turn can then impact on cell function.^36,37 Protein nitration by reactive peroxynitrite anions (ONOO⁻) can also affect enzyme activity by reducing HDAC activity.^37,38 This could explain why corticosteroids are not effective in resolving the inflammatory response in COPD patients, given that they have a much reduced HDAC-2 activity correlating with disease severity.^39,40 Levels of nitrotyrosine formation are elevated in COPD and levels of protein nitration have been found in induced sputum cells of COPD patients.⁴¹ ONOO⁻ is also capable of attacking sulphydryl groups forming nitrosothiols, a process called nitrosylation, which can also impact upon protein function.¹⁶

Endogenous antioxidant defences within the lung

In order to combat and neutralize the deleterious effects of ROS, the normal lung has various endogenous antioxidant strategies, which employ both enzymatic and non-enzymatic mechanisms. Within the lung lining fluid, several non-enzymatic antioxidant species exist, which include glutathione (GSH), vitamin C, uric acid, vitamin E, and albumin.^12,16 GSH is present in greater concentrations in the epithelial lining fluid compared with plasma and plays an important protective role in the airspaces and epithelial cells against oxidants in the extracellular milieu.⁴² Human studies have shown elevated levels of GSH in epithelial lining fluid in chronic cigarette smokers compared to non-smokers.^42,43 Enzymatic antioxidant defences include SOD, catalase, thioredoxin, Gpx, and glutathione-S-transferase. Bronchial epithelial cells of rats exposed to cigarette smoke have shown increased expression of the antioxidant genes manganese superoxide dismutase, metallothionein, and Gpx.⁴⁴ Increased activity of SOD in alveolar macrophages from young smokers also has been reported.⁴⁵ However, Kondo and co-workers found that the increased superoxide generation by alveolar macrophages in elderly smokers was associated with decreased antioxidant enzyme activities when compared to non-smokers.⁴⁶ In addition, this reduced activity was not associated with decreased gene expression, but was due to modification at the post-translational level.⁴⁶ The activities of SOD and Gpx have been shown to be higher in the lungs of rats exposed to cigarette smoke.⁴⁷ The mechanism(s) for the induction of antioxidant enzymes in erythrocytes,⁴⁸ alveolar macrophages,⁴⁵ and lungs⁴⁷ by cigarette smoke exposure are currently unknown.

Glutathione peroxidases

GPxs are a family of selenium-dependent and -independent antioxidant enzymes that catalyze the reduction of damaging H₂O₂ as well as a large variety of hydroperoxides (such as DNA peroxides and lipid peroxides) into water and alcohols, respectively, typically using GSH as reductant and thus protect biomembranes and cellular components against oxidative stress.⁴⁹ According to phylogeny, the Gpx family consists of three evolutionary groups arising from a Cys-containing ancestor: Gpx-1/Gpx-2, Gpx-3/Gpx-5/Gpx-6, and Gpx-4/Gpx-7/Gpx-8, the expression levels of each isoform varying depending on the tissue type.⁵⁰ Despite the well known ability of Gpxs to remove H₂O₂, the exact role of these enzymes under physiological and oxidative stress conditions is still not clearly defined. Recent studies using knockout mice have provided data about the function of the most abundant glutathione peroxidase, Gpx-1. Mice deficient in Gpx-1 are healthy and fertile and do not show any histopathologies (organs examined included lung, liver, brain, heart, and kidney) up to 15 months of age,⁵¹ consistent with a limited role for Gpx-1 during normal development and under physiological conditions.^52,53 In addition, these studies showed that Gpx-1 plays a protective role in the cell in situations of oxidative stress.⁵¹ This hypothesis is confirmed by our study showing that Gpx-1 knockout mice are susceptible to oxidative stressors such as cigarette smoke.⁵⁴ Moreover, Gpx-1 has been implicated in the development and prevention of many common and complex diseases, including cancer and cardiovascular disease.⁵⁵

Gpxs in normal physiology and lung diseases

While it is clear that there are various isoforms of Gpx, the lung literature has essentially divided them into two groups, cellular (classical) and extracellular. The alveolar epithelial lining fluid contains a very high amount of both extracellular Gpx and classical Gpx and these enzymes each contribute about half of the Gpx activity in epithelial lining fluid.⁴² Primary bronchial epithelial cells, alveolar macrophages and other lung cell lines can synthesize classical Gpx and extracellular Gpx and also secrete the extracellular enzyme.⁵⁶ Classical Gpx is induced by hyperoxia⁵⁷ and by the combination of hyperoxia and TNF.⁵⁸ The levels of both classical Gpx and extracellular Gpx are decreased after exposure to ozone.⁵⁹ Extracellular Gpx has been shown to protect alveolar epithelial cells against hyperoxia-induced injury in rats.⁶⁰ It has been shown that Gpx-2 is induced in the lungs of mice in response to cigarette smoke and that basal and CS-inducible expression of Gpx-2 are directly dependent on Nrf2.^61,62 In humans, Gpx-1 has been shown to be increased in COPD,⁶³ and Gpx-2 showed a three- to five-fold up-regulation in epithelial cells of smokers compared with nonsmokers.^64–66 Gpx-3 showed a two-fold up-regulation in epithelial cells of smokers compared with nonsmokers.^65,67 There was little evidence of differential regulation of Gpx-4, Gpx-5, or Gpx-7 by disease status.⁶⁵ Gpx activity is significantly reduced in smokers and subjects with COPD, highlighting its prominent role in lung antioxidant defense.⁶³ With respect to reduced Gpx activity in COPD patients and smokers, erythrocyte Gpx activity was significantly lower in patients with severe COPD compared with patients with moderate COPD.⁶⁸ In addition, Gpx activity was decreased in plasma from COPD patients⁶⁹ and total blood from smokers and ex-smokers.⁷⁰ It has been shown that COPD patients are deficient in selenium and that this could explain the observed reduction in Gpx activity.⁷⁰ Moreover, selenium is an important element in the Gpx catalysis of the reaction between GSH and ROS. There is a direct relationship between systemic Gpx activity and FEV1,⁶⁸ and oxidative stress correlates with both lung function and body mass index in COPD.⁶⁹ Elevated levels of H₂O₂ are measured in the exhaled breath condensate of COPD patients, particularly during exacerbations.²⁵

Therapeutic intervention with antioxidants

Given the role of oxidative stress in COPD, various approaches have been used to study the benefit of antioxidants in COPD. The strategies employed have included (1) increasing the endogenous antioxidant enzyme defences or (2) enhancing the non-enzymatic defences through dietary or pharmacological means.^12,16 Of these strategies, enzyme mimetics have shown the most promise. Enzyme mimetics are generally small compounds that posses catalytic activity that mimics the activity of larger enzyme-based molecules. A number of SOD mimetics based around organomanganese complexes have been developed, which retain their antioxidant properties in vivo. Within the various classes of SOD mimetics only the metalloporphyrin-based compounds AEOL10150 and AEOL10113 have been studied in models of airway inflammation. In one study, AEOL10150 was demonstrated to inhibit cigarette smoke-induced lung inflammation,⁷¹ suggestive of a potential therapeutic benefit in COPD. Another type of catalytic antioxidant is the Gpx mimetic ebselen. This is a selenium-based organic complex and has been shown to be a very powerful antioxidant against damaging H₂O₂ and the potentially destructive peroxynitrite molecule.⁷² It is able to prevent both NFκB/AP-1 activation and pro-inflammatory gene expression in human leukocytes exposed to peroxynitrite. Other studies have shown that Ebselen is also active in vivo in ameliorating airway inflammation induced by LPS,^73,74 ozone,⁷⁵ sephadex,⁷⁶ and influenza A virus.⁷⁷

Gpx-1 protects against cigarette smoke-induced lung inflammation and damage

We previously have shown that mice lacking the Gpx-1 gene are highly susceptible to oxidative stress but do not display an overt phenotype and thus proposed that Gpx-1 may be an attractive target for increasing the antioxidant capacity in ischemia/reperfusion brain injury where oxidative stress is involved.^51,78,79 Given that the known biology of Gpx-1 appears to be protection during oxidative stress, and evidence of a role in COPD, we proposed that Gpx-1 protects against cigarette smoke-induced lung inflammation and damage. Indeed, we have shown that Gpx-1-deficient mice exposed to short-term cigarette smoke had significantly elevated levels in BALF macrophages, neutrophils, proteolytic burden, and whole lung macrophage and neutrophil chemotactic factor gene expression.⁵⁴ Moreover, BALF from cigarette smoke-exposed Gpx-1 deficient mice had increased protease expression (MMP-9) and activity compared to wild-type cigarette smoke-exposed mice. Thus, our data provide new evidence suggesting that Gpx-1 is required to control cigarette smoke-induced lung inflammation and the potential therapeutic utility of targeting Gpx-1 in vivo. In addition, the fact that Gpx-1-deficient mice have normal BALF cell counts at baseline (i.e. no cigarette smoke exposure) strengthens the hypothesis that Gpx-1 may be protective in cigarette smoke-induced lung inflammation where there is an enhanced oxidant burden.

In our study, we did not explore the role of Gpx-1 in the development of emphysema. We and others have proposed that short-term responses to cigarette smoke exposure may be a useful predictor of the development of emphysema, and such models may be a useful screen by which to identify therapeutic targets.^80–85 Thus, given that Gpx-1 protects against cigarette smoke-induced lung inflammation in the present study, we would predict that Gpx-1 also protects against cigarette smoke-induced emphysema. This prediction is in accordance with work by Foronjy and colleagues showing that transgenic Gpx-1 mice exposed to cigarette smoke for 12 months were protected against the formation of emphysema and airway inflammation whereas Gpx-1 knockout mice exhibited an exaggerated emphysema phenotype.⁸⁶

Ebselen reduces cigarette smoke-induced lung inflammation in mice

Ebselen has been shown to be protective in vivo in disease situations hallmarked by oxidative stress such as diabetes-associated atherosclerosis and cerebral ischaemia–reperfusion injury.^87,88 In addition, ebselen has been used in clinical trials of acute ischaemic stroke.^89,90 Specifically, Yamaguchi et al.⁹⁰ explored the effects of ebselen on the outcome of acute ischaemic stroke in a multi-centre, placebo-controlled, double-blind clinical trial. They demonstrated that early treatment (i.e. patients who started ebselen within 24 h of stroke onset) with ebselen (150 mg bid) improved the outcome of acute ischaemic stroke.⁹⁰ Similarly, Ogawa et al.⁸⁹ showed in a randomized, double-blind, placebo-controlled trial of ebselen conducted in patients with complete occlusion of the middle cerebral artery that ebselen protected the brain from ischaemic damage in the acute stage. In this light, it is possible that ebselen may influence the key reactions involved in the inflammatory responses in COPD, but no studies have yet been reported on the protective role of ebselen in cigarette smoke-induced lung inflammation. We showed for the first time that ebselen, when administered prophylactically and during established inflammation, reduces cigarette smoke-induced BALF inflammation in mice.⁵⁴ Of interest was that the approximately two-fold increase in BALF inflammation of Gpx-1-deficient mice exposed to cigarette smoke was abolished by ebselen administration. This clearly shows that there is a link between ebselen and its direct effects on Gpx-1 in smoke-exposed mice. Moreover, this is in accord with our previous study showing that pretreatment of Gpx-1 deficient mice with ebselen restored microvascular perfusion, limited the induction and activation of MMP-9, and attenuated the increases in infarct size and vascular permeability.⁸⁸ It is likely that ebselen reduced cigarette smoke-induced BALF inflammation by inhibiting the gene expression of a variety of neutrophil (e.g. IL-17A) and macrophage (e.g. MIP-1α and MCP-1) chemotactic factors pertinent to lung inflammation. Suppression of GM-CSF by ebselen would also have contributed to the reduced BALF inflammation given that GM-CSF is a survival factor for neutrophils and macrophages and that GM-CSF inhibits neutrophil apoptosis.^91–93 In addition, it could be possible that ebselen's antioxidant properties of removing H₂O₂, scavenging ONOO⁻, and enhancing pulmonary expression of both copper/zinc and manganese SODs (which can contribute to a decrease in the formation of ONOO⁻ by lowering the concentration of available O₂^•−) may have also contributed to reduced inflammation. In addition, it is possible that ebselen reduced cigarette smoke-induced BALF inflammation by inducing cell death as previously described by Guerin and Gauthier.⁹⁴ Thus, targeting Gpx-1 with mimetics such as ebselen might exert anti-inflammatory and antioxidant effects in vivo.

Conclusion

COPD is a major health problem and a significant economic burden worldwide. It is believed that an exaggerated inflammatory response to cigarette smoke in which macrophages, neutrophils, and lymphocytes are prominent leads to oxidative stress, chronic inflammation, emphysema, small airway fibrosis, mucus hypersecretion, and progressive airflow limitation. Given the increased oxidant burden in smokers and patients with COPD, we propose that the antioxidant enzyme Gpx-1 may be a novel therapeutic target for the treatment of cigarette smoke-induced lung disease. Indeed, we and others have provided evidence that the antioxidant enzyme Gpx-1 protects the lung from cigarette smoke-induced inflammation and emphysema. In addition, we have shown that increasing the antioxidant capacity of the lungs by using a Gpx mimetic such as ebselen is beneficial in resolving inflammation induced by cigarette smoke. It is important to note that ebselen was effective when given prophylactically and, perhaps more importantly, when administered therapeutically (i.e. in established disease) as would be the case in clinical practice. Thus, the striking effect of ebselen in our model suggests that Gpx-1 may be a novel target that can be exploited therapeutically to slow or prevent cigarette smoke-induced emphysema and reduce the severity of inflammation in COPD.