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Keywords:

  • antioxidant;
  • lung;
  • oxidative stress;
  • reactive oxygen species;
  • signal transduction

ABSTRACT

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Reactive oxygen species (ROS) are products of normal cellular metabolism and are known to act as second messengers. Under physiological conditions, ROS participate in maintenance of cellular ‘redox homeostasis’ in order to protect cells against oxidative stress through various redox-regulatory mechanisms. Overproduction of ROS, most frequently due to excessive stimulation of either reduced nicotinamide adenine dinucleotide phosphate by cytokines or the mitochondrial electron transport chain and xanthine oxidase, results in oxidative stress. Oxidative stress is a deleterious process that leads to lung damage and consequently to various disease states. Knowledge of the mechanisms of ROS regulation could lead to the pharmacological manipulation of antioxidants in lung inflammation and injury.


INTRODUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Reactive oxygen species (ROS) is a collective term that includes a large variety of free oxygen radicals [e.g. superoxide anion (O2•−) and hydroxyl radicals (OH)], as well as derivatives of oxygen that do not contain unpaired electrons [e.g. hydrogen peroxide (H2O2), hypochlorous acid (HOCl), peroxynitrite (ONOO) and ozone (O3)].1 ROS are unstable molecules with unpaired electrons, capable of initiating oxidation.

Biological systems are continuously exposed to oxidants, either generated endogenously by metabolic reactions (e.g. from mitochondrial electron transport during respiration or during activation of phagocytes), or derived from exogenous sources (e.g. air pollutants and cigarette smoke).2–5 The lung is exposed to high levels of oxygen, which together with its large surface area and blood supply make it susceptible to injury mediated by ROS. Oxidative stress results from an oxidant/antioxidant imbalance in favour of oxidants,6–8 and causes oxidation of proteins, DNA and lipids, as well as inducing a variety of cellular responses through the generation of secondary metabolic ROS.9 There is now substantial evidence that oxidative stress plays an important role in the pathogenesis of various lung disorders such as asthma, COPD, acute lung injury (ALI), pulmonary fibrosis and lung cancer.1,10–12

This review describes: (i) the chemistry and biochemistry of ROS and sources of free radical generation; (ii) ROS-induced signalling pathways; (iii) the role of ROS in the pathogenesis of lung diseases such as asthma, COPD, ALI, pulmonary fibrosis and lung cancer; and (iv) the role of antioxidants in lung diseases.

CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Overproduction of ROS was first observed in phagocytic cells such as neutrophils and macrophages and was named ‘the respiratory burst’ due to the transient consumption of oxygen. The respiratory burst is initiated by the multi-component enzyme, reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Superoxide is formed upon one-electron reduction of oxygen, mediated by enzymes such as NADPH oxidase, xanthine oxidase or enzymes of the respiratory chain.

  • image
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H2O2 is produced from O2•− through dismutation catalysed by the superoxide dismutase (SOD) enzymes:

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The hydroxyl radical (OH), which is the most potent ROS, is formed in vivo from H2O2 via the Fenton reaction in a metal-catalysed process and upon exposure to high energy irradiation (e.g. X-rays) by haemolytic cleavage of water. UV energy is insufficient to split water, but does cleave H2O2 to yield OH. Due to its high reactivity, OH radical reacts immediately with surrounding target molecules at the site where it is generated.

Peroxyl radicals (ROO) are generated in the process of lipid peroxidation, which is initiated by the elimination of a hydrogen atom from polyunsaturated fatty acids (PUFA). ROO·are relatively long-lived species with considerable diffusion path lengths in biological systems.

Singlet molecular oxygen (1O2) is electronically excited oxygen, which is formed in biological systems by photosensitization reactions. This pathway is thought to be important in tissues exposed to light. 1O2 can interact with target molecules either by transferring its excitation energy or by chemical combination. Preferential targets for chemical reactions are double bonds in PUFA and guanine bases in DNA.

SOURCES OF REACTIVE OXYGEN SPECIES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Cell-derived reactive oxygen species

With the exception of unusual circumstances such as exposure to UV light, ionizing radiation and other high energy sources, ROS are generally produced in cells by electron transfer reactions, which can be enzymatic or non-enzymatic. Epithelial cells, resident macrophages, endothelial cells and recruited inflammatory cells, such as neutrophils, eosinophils, monocytes and lymphocytes, generate ROS in response to increased levels of secretagogue stimuli. Activation of macrophages, neutrophils and eosinophils results first in the formation of O2•−, which is rapidly converted to H2O2 by SOD, and OH is formed non-enzymatically in the presence of Fe2+ as a secondary reaction. ROS produced by phagocytes at sites of inflammation are a major cause of cell and tissue damage associated with various chronic inflammatory lung diseases.

Reactive oxygen species are generated intracellularly from several sources, including mitochondrial respiration, cytochrome P-450, the NADPH oxidase system, xanthine/xanthine oxidase and metabolism of arachidonic acid.13–19 The major ROS generating enzyme is NADPH oxidase, a membrane-bound multi-component enzyme complex that is present in phagocytes as well as non-phagocytic cells.13,15 In addition to NADPH oxidase, phagocytes contain other ROS generating enzymes, the heme peroxidases, myeloperoxidase (MPO) and eosinophil peroxidase. Activation of these peroxidases results in the formation of the potent oxidants HOCl and hypobromous acid (HOBr) from H2O2 in the presence of chloride (Cl-) and bromide (Br-) ions, respectively.

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The oxidant burden produced by eosinophils is substantial, since these cells possess several times greater capacity for generating O2•− and H2O2 than neutrophils, and the content of eosinophil peroxidase in eosinophils is several times higher than that of MPO in neutrophils. Therefore eosinophils, which are present in increased numbers in the airways of asthmatic patients, have a major role in ROS generation in airway inflammation.

Detailed studies have shown that metals, including iron (Fe), copper (Cu), cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni), vanadium (V) and several other metals, possess the ability to produce reactive radicals.20 While Fe, Cu, Cr, V and cobalt (Co) undergo redox-cycling reactions, the primary toxicity of a second group of metals, including Hg, Cd and Ni, is due to depletion of glutathione (GSH) and bonding to the sulphydryl groups of proteins. Arsenic (As) is thought to bind directly to critical thiols or to be involved in the formation of H2O2 under physiological conditions. The unifying factor in the toxicity of all these metals is the generation of ROS. The redox state of the cell is largely linked to an Fe (and Cu) redox couple and is maintained within strict physiological limits. It has been suggested that Fe regulation ensures that there is no free intracellular Fe. However, under conditions of stress, an excess of superoxide radical releases ‘free Fe’ from Fe-containing molecules. The released Fe2+ ion can participate in the generation of the highly reactive hydroxyl radical, catalysed by the Fenton reaction (Eqn 1).19,21–23

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The superoxide anion participates in the Haber–Weiss reaction (Eqn 3), which is a combination of a Fenton reaction and the reduction of Fe3+ by O2•−, yielding Fe2+ and O2 (Eqn 2).19

  • image(3)

Cigarette smoke and inhaled oxidants

Inhalation of volatile substances in cigarette smoke, as well as fine particulate matter, may increase ROS levels in the lungs.3–5 Inhalation of cigarette smoke and airborne pollutants, either oxidant gases such as O3 and sulphur dioxide (SO2), or particulate air pollution, results in direct lung damage as well as the activation of inflammatory responses in the lungs. Cigarette smoke is a complex mixture of over 4700 chemical compounds, including high concentrations of oxidants (1014 oxidant radicals/puff).24 The cellular mechanisms resulting in oxidative stress induced by smoking are complex and poorly understood. However, there is striking evidence for oxidative stress and an imbalance between oxidants and antioxidants in smokers.25 The gas phase of cigarette smoke contains largely short-lived oxidants such as O2•− and nitric oxide (NO), which react to form the highly reactive peroxynitrite (ONOO-) molecule.26 The tar phase of cigarette smoke contains long-lived radicals such as the semiquinone radicals, which can react with O2•− to form OH and H2O2.27,28 The tar phase of cigarette smoke also generates H2O2 continuously for long periods in aqueous media.27,28 Cigarette smoking is also correlated with increased concentrations of MPO.29

According to the USA Environmental Protection Agency, the burden of particulate matter in ambient air due to chemical (31%) and non-chemical (35%) industrial processes, as well as transportation (27%) and fuel combustion (35%), is increasing worldwide.30 Many types of inhaled particles have the ability to generate free radicals in biological systems and to activate oxidative stress-response signalling pathways in cells.31 Ambient particulate matter may also induce oxidative DNA damage in lung epithelial cells.32 In addition, a recent study revealed that inhalation of exogenous H2O2 increased lung vascular permeability in an animal model.33

Lipid peroxidation

The hydroxyl radical can eliminate a hydrogen atom from PUFA resulting in the formation of lipid radicals, which can interact further with oxygen to generate the lipid peroxyl radical. If the resulting lipid peroxyl radical is not reduced by antioxidants, lipid peroxidation occurs. In contrast to free radicals, products of lipid peroxidation are generally stable, can diffuse within, or even escape from the cell, and attack targets far from the site of the original free radical event. In addition to their cytotoxic properties, lipid peroxides are increasingly recognized as being important in signal transduction for a number of inflammatory responses.34

SIGNAL TRANSDUCTION

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Redox signalling requires that the steady state ‘redox balance’ is disturbed by either an increase in ROS formation or a decrease in the activities of antioxidant systems. Physiological redox regulation involves a temporary shift of the intracellular redox state towards more oxidizing conditions. Under pathological conditions, however, abnormally large concentrations of ROS in cells may lead to permanent changes in signal transduction and gene expression that are typical of disease states.

Signal transduction enables information to be transmitted from the outside of a cell to various functional elements within the cell. Most cells have been shown to undergo a small oxidative burst, generating low concentrations of ROS when stimulated by cytokines, growth factors and hormones such as IL-1, IL-6, IL-3, tumour necrosis factor (TNF)-α, angiotensin II, platelet-derived growth factor (PDGF), nerve growth factor, transforming growth factor (TGF)-β1, granulocyte/macrophage colony stimulating factor, and fibroblast growth factor-2.35 Signals to the transcription machinery responsible for the expression of certain genes are normally transmitted to the nucleus by a class of proteins called transcription factors. These signal transduction processes can induce various biological functions, such as muscle contraction, gene expression, cell growth and nerve transmission.35 The initiation and/or proper functioning of several signal transduction pathways rely on ROS as signalling molecules, which may act at different levels of the signal transduction cascade. ROS thus play a very important physiological role as second messengers.36,37

Cytokine and growth factor signalling

A variety of cytokines and growth factors that bind to receptors of different classes have been reported to generate ROS in non-phagocytic cells.38 Growth factor tyrosine kinases receptors play a key role in the transmission of information from outside the cell into the cytoplasm and nucleus.39 ROS production results from the activation of signalling through the epidermal growth factor (EGF), PDGF, and vascular endothelial growth factor (VEGF) receptors.39 Moreover, ROS induce VEGF expression in vitro and in vivo.33,40 We have demonstrated that antioxidants markedly reduce the increase in VEGF expression.41

Reactive oxygen species activate serine/threonine kinases

The mitogen-activated protein kinases (MAPK), including extracellular signal-regulated kinase, Jun NH2-terminal kinase and p38 MAPK, are serine/threonine kinases.42–44 MAPK relay signals generated by exogenous and endogenous stimuli into the cell by phosphorylation of proteins. MAPK-catalysed phosphorylation of substrate proteins functions as a switch to turn on or off the activity of the substrate protein. The substrates include other protein kinases, phospholipases, transcription factors and cytoskeletal proteins. During this process of intracellular communication, MAPK interact with upstream mediators, including growth factor receptors, G-proteins, tyrosine kinases and ROS.45 MAPK regulate many key functional and antimicrobial responses in neutrophils,46–48 and contribute to the production of pro-inflammatory cytokines.49

Akt is a serine/threonine kinase that is recruited to the cell membrane by phosphoinositide-3-kinase and activated by phosphorylation. The end result of Akt activation is stimulation of growth pathways and inhibition of apoptotic pathways. The antioxidant, L-2-oxothiazolidine-4-carboxylic acid (OTC), which is a precursor for GSH biosynthesis, inhibits Akt signalling pathways in an ovalbumin murine model of asthma.50

Protein kinase C (PKC) has been implicated in a wide variety of cellular responses to diverse stimuli, including hormones, growth factors, chemoattractants such as formyl-methionyl-leucyl-phenylalanine and tumour-promoting phorbol esters.51 Importantly, the activity of this cysteine-rich enzyme is regulated by oxidants. In vitro studies with purified PKC demonstrated that its activity is modulated by H2O2.52 As PKC participates in the regulation of many leukocyte responses, redox regulation of PKC by endogenous ROS has important functional implications.

Reactive oxygen species have also been implicated in phorbol ester-mediated phospholipase A2 (PLA2) activation.53,54 PLA2, which releases arachidonic acid from membrane phospholipids, is one of the key enzymes in the production of prostaglandins and leukotrienes. The exposure of macrophages to H2O2 in combination with vanadate markedly stimulated PLA2 activity, and inhibition of NADPH oxidase and ROS production by diphenyleneiodonium prevented PLA2 activation. Goldman et al. have shown good correlations among ROS formation, activation of PKC, suppression of protein tyrosine phosphatase, activation of protein tyrosine kinase, enhanced tyrosine phosphorylation, activation of myelin basic protein kinase and activation of PLA2, which are mediated by ROS.53,54

Oxidative stress and expression of inflammatory genes

Oxidative stress and nuclear factor-kappaB

Nuclear factor-kappaB (NF-κB) is a multi-protein complex that is known to activate a great number of genes involved in the early cellular defence reactions in higher organisms. NF-κB is composed of a heterodimer with one 50 kDa (p50) and one 65 kDa (p65) polypeptide. In non-stimulated cells, NF-κB is present in the cytosol in an inactive form through association with its inhibitory protein, inhibitory kappaB (IκB).55

NF-κB is known to play an important role in immune and inflammatory responses and is present in most cell types.56–60 The role of ROS in the activation of NF-κB signal transduction was initially observed in cells treated with H2O2.61 Exogenous H2O2 also activated NF-κB in a murine model of ROS-induced ALI.33

A recent study indicated that development of an oxidant/antioxidant imbalance in asthma leads to activation of the redox-sensitive transcription factor NF-κB.62 ROS have been directly implicated as second messengers in the activation of NF-κB, based upon their ability to oxidize its cysteine-SH group or regulate the ubiquitination and proteolysis of IκB.63–65 Under hyperoxic conditions, NF-κB activity is modulated through activation of the IκB kinase (IKK) pathway. Hyperoxic conditions enhance the activation of IKK, leading to prolonged degradation of IκB-α and sustained nuclear translocation and DNA binding of NF-κB.66

Activation of NF-κB induces a variety of inflammatory genes that are abnormally expressed in asthma. These include the genes for the cytokines, IL-4, IL-5, IL-9, IL-15 and TNF-α, the chemokines, regulated upon activation, normal T-cell expressed and secreted (RANTES), eotaxin and monocyte chemotactic protein-3, and the adhesion molecules, intercellular adhesion molecule-1 and vascular cell adhesion molecule-1.60,67–69 In a recent study, a murine model of asthma was used to evaluate the effect of the antioxidant, OTC, on inflammation of the airways.70 Administration of OTC resulted in significant reduction of NF-κB translocation into the nucleus and expression of adhesion molecules, chemokines and cytokines. These results showed that OTC inhibited NF-κB activity by preventing ROS-induced translocation of this transcription factor into the nucleus. Antioxidants have also been shown to inhibit NF-κB activation by preventing IκB degradation in response to various stimuli.71–75 Taken together, these findings indicate that activation of NF-κB by ROS is a critical signalling mechanism for evoking inflammatory responses.

Oxidative stress and hypoxia-inducible factor-1

Hypoxia-inducible factor (HIF)-1, a heterodimeric basic helix-loop-helix-PAS domain transcription factor, is known to activate hypoxia-responsive genes, including VEGF which is one of the major determinants of asthma.76,77 HIF-1 is composed of two subunits, HIF-1α and HIF-1β. Whereas the β-subunit protein is expressed constitutively, the stability of the α-subunit and its transcriptional activity are precisely controlled by the intracellular oxygen concentration.78,79 In addition to oxygen concentration, ROS have also been shown to stabilize HIF-1α under hypoxic or non-hypoxic conditions.80,81 Exposure to H2O2 increased HIF-1α protein levels in the lungs, but HIF-1α protein levels were significantly decreased by the administration of the antioxidants, OTC and α-lipoic acid, suggesting that exogenous H2O2 also regulates HIF-1α pathway.33

Oxidative stress and activator protein-1

Activator protein-1 (AP-1) is a sequence-specific transcriptional activator composed mainly of members of the Jun and Fos families. These proteins, which belong to the bZIP group of DNA binding proteins, associate to form a variety of homo- and heterodimers that bind to a common site.82 AP-1 is differentially regulated during cell cycle progression and in response to many diverse stimuli. AP-1 has been shown to be involved in oxidant signalling, immune responses, cellular differentiation and apoptosis.83,84 Oxidants also induce AP-1 and AP-1-dependent gene expression.85–88 A number of studies suggest that AP-1 is involved in the pathogenesis of hyperoxic inflammatory lung injury.89,90 However, the precise mechanisms by which ROS stimulate AP-1 activation remain unclear. Unlike NF-κB, AP-1 is also strongly induced by antioxidants such as pyrrolidine dithiocarbamate and N-acetyl cysteine (NAC).91,92 Binding of the Fos/Jun heterodimer to DNA is increased by the reduction of a single conserved cysteine in the DNA-binding domain of each of the proteins.93 A number of studies have demonstrated that both O2•− and H2O2 induce the expression of Jun and Fos proteins,94,95 and ROS have also been implicated in the activation of AP-1 by ionizing radiation.96 More recently, there has been speculation that intracellular redox status plays a role in the induction of AP-1 and NF-κB activities.97 Similar to NF-κB activation, high intracellular oxidized glutathione (GSSG) concentrations are associated with increased AP-1 activation.98

Oxidative stress and histone modifications

Histones are subject to an enormous number of post-translational modifications, including acetylation, methylation, ubiquitination, sumoylation and phosphorylation.99 Histone acetylation plays a particularly important role in the regulation of gene expression. In resting cells, DNA is tightly compacted to prevent access of transcription factors. Upon activation by stimuli, DNA is made less compact and available to transcription factors through histone acetylation.100 Histone acetylation is associated with the transcriptional regulation of genes and is controlled by the reversible covalent modifying enzymes, histone acetyltransferase (HAT) and histone deacetylase (HDAC).101 In addition, there are many nuclear receptor coactivators that possess intrinsic HAT activity, including steroid receptor coactivator 1, cyclic AMP response element binding binding protein (CBP)/adenoviral protein E1A (p300) protein, CBP/p300 associated factor and activator transcription factor-2.102–104 The acetylation of core histones, H3/H4 by CBP/p300 and/or ATF-2 has been reported to activate NF-κB/AP-1-mediated expression of genes for pro-inflammatory mediators. On the other hand, HDAC has been reported to function in corticosteroid-mediated anti-inflammatory mechanisms.105

Oxidative stress has been suggested to influence histone acetylation, phosphorylation and ADP-ribosylation by a mechanism dependent on activation of the MAPK pathway.106,107 ROS such as H2O2, as well as TNF-α, have been shown to increase histone acetylation in alveolar epithelial cells.108,109 Oxidants may also play an important role in the modulation of HDAC activity.110 However, the effect of ROS on histone acetylation/deacetylation remains controversial.

OXIDATIVE STRESS AND BRONCHIAL ASTHMA

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Bronchial asthma is a chronic inflammatory disease of the airways that is characterized by airway eosinophilia, goblet cell hyperplasia with mucus hypersecretion and hyperresponsiveness to inhaled allergens and non-specific stimuli, which usually induce increased vascular permeability resulting in plasma exudation.111,112 There is increasing evidence that inflammation, which is characteristic of asthma, results in increased oxidative stress in the airways.113 Alveolar macrophages from asthmatic subjects show increased release of O2•− and other ROS compared with those of healthy controls.114,115 Eosinophils, alveolar macrophages and neutrophils from asthmatic patients produce more ROS than those from normal subjects.116,117 ROS are involved in airway smooth muscle contraction, impairment of β-adrenergic receptor function, decreased numbers and function of epithelial cilia, increased mucus production, altered release of inflammatory mediators, influx of inflammatory cells and increased vascular permeability.1

Reactive oxygen species cause direct contraction of airway smooth muscle preparations and this effect is enhanced when the epithelium is injured or removed.118 Overproduction of ROS or depression of the protective system also results in bronchial hyperreactivity which is characteristic of asthma.118–120 Animal models have shown that ROS contribute to airway hyperresponsiveness by increasing vagal tone due to damage of oxidant-sensitive β-adrenergic receptors, as well as decreasing mucociliary clearance.121,122 H2O2 causes contraction of airway smooth muscle and has been implicated in airway hyperresponsiveness in animal models.33

Reactive oxygen species appear to directly stimulate histamine release from mast cells and mucus secretion from airway epithelial cells.123 Increased generation of ROS can result in direct oxidant damage and shedding of epithelial cells.116 Previous studies have demonstrated that ROS can contribute to endothelial barrier dysfunction and increased permeability to fluid, macromolecules and inflammatory cells.1

OXIDATIVE STRESS AND COPD

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

COPD is characterized by airflow limitation that is not fully reversible. Its major feature is chronic inflammation throughout the airways, parenchyma and pulmonary vasculature, with increased numbers of neutrophils, macrophages and T lymphocytes (especially CD8+). Markers of oxidative stress (e.g. H2O2, NO) have been found in the epithelial lining fluid, breath and urine of cigarette smokers and patients with COPD.124–127 There is accumulating evidence that ROS can react with a variety of biological molecules, including proteins, lipids and nucleic acids, and this can lead to cell dysfunction or death, as well as damage to the lung extracellular matrix. In addition to directly damaging the lung, oxidative stress contributes to a proteinase–antiproteinase imbalance, both by inactivating antiproteinases such as α1-antitrypsin and secretory leukocyte proteinase inhibitor, and by activating proteinases such as matrix metalloproteinases. Oxidants also promote inflammation by activating NF-κB, which orchestrates the expression of multiple inflammatory genes thought to be important in COPD, including IL-8 and TNF-α. Finally, oxidative stress may contribute to reversible airway narrowing. H2O2 constricts airway smooth muscle in vitro and isoprostane F2α-III, formed by free radical peroxidation of arachidonic acid, is a biomarker of lung oxidative stress in vivo, and a potent constrictor of human airways.128

In addition, circulating neutrophils from patients with COPD show increased O2•− production and upregulation of adhesion molecules.25,129 Lipid peroxidation products such as thiobarbituric acid reactive substances, conjugated dienes of linoleic acid and F2-isoprostane are significantly increased in the plasma of healthy smokers and patients with acute exacerbations of chronic bronchitis compared with healthy non-smokers.129–131 Moreover, plasma antioxidant capacity is significantly decreased in smokers.130,131 Thus, the products of lipid peroxidation formed by oxidative stress also activate signals that enhance airway inflammation.

OXIDATIVE STRESS AND ACUTE LUNG INJURY

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Acute lung injury is a disease process characterized by diffuse inflammation of the lung parenchyma. Oxidant-mediated tissue injury is likely to be important in the pathogenesis of ALI.33,132 Lung injury due to hyperoxia is a commonly used model for the study of ALI in animals. ROS are generated as a by-product of the activation of neutrophils and macrophages. In addition, the requirement by many patients with ALI for a high fraction of inspired oxygen (FiO2) may predispose to oxidative stress. Decreased levels of GSH, a major endogenous scavenger of ROS, have been observed in the alveolar fluid of patients with ARDS. In response to various inflammatory stimuli, lung endothelial cells, alveolar cells and airway epithelial cells, as well as activated alveolar macrophages, produce both NO and O2•−. These species form peroxynitrite, which nitrates and oxidizes key amino acids in various lung proteins such as surfactant protein A, resulting in the inhibition of their function. The nitration and oxidation of a variety of crucial proteins in the alveolar space have been shown to be associated with diminished function in vitro, and modified proteins have also been identified ex vivo in samples from patients with ALI.

OXIDATIVE STRESS AND PULMONARY FIBROSIS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Pulmonary fibrosis is the end result of a diverse group of lung disorders. Although there are multiple initiating agents for pulmonary fibrosis, including toxins, fibres/particles, autoimmune reactions, drugs and radiation, the aetiology of the majority of cases of pulmonary fibrosis is unknown and these cases are referred to as IPF. The immunopathogenic responses of lung tissue are complex, and the precise pathways leading from injury to fibrosis are not well established. Several studies have suggested that oxidant–antioxidant imbalance in the airways plays a critical role in the pathogenesis of IPF.133–138 Fibrotic stimuli of unknown origin are thought to create an imbalance between oxidant production and antioxidant protection, resulting in the accumulation of ROS.139 In addition, oxidants may contribute to the development of pulmonary fibrosis due to their effects on the production of cytokines and growth factors such as TGF-β, a key regulator of aberrant repair mechanisms that are characteristic of many fibrotic diseases including IPF. There are several potential interactions between TGF-β and oxidants/antioxidants in the lung. TGF-β not only induces ROS production by activation of NADPH oxidases and/or mitochondrial dysfunction, but also decreases natural cellular antioxidant production through decreased expression of both catalase and mitochondrial SOD.140–144 Increased levels of oxidized proteins have been reported in human subjects with IPF.145–147 In addition, some studies have reported that various antioxidant enzyme systems protect against lung fibrosis.148–150 IPF subjects also have lower antioxidant capacity than healthy subjects.151 Thus, oxidants and TGF-β seem to interact to enhance the fibrotic response in the lungs.

OXIDATIVE STRESS AND LUNG CANCER

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Reactive oxygen species have been suggested to stimulate oncogenes such as Jun and Fos. Overexpression of Jun is directly associated with lung cancer.152,153 In lung cancers, p53, which is associated with the production of ROS, is often mutated and defective in inducing apoptosis. When mutated, p53 accumulates in the cytoplasm and functions as an oncogene.154 Proteins and lipids are also major targets for oxidative attack, and modification of these molecules may increase the risk of mutagenesis, through formation of genotoxic lipid peroxidative by-products that react with DNA, oxidative modification of DNA polymerase or inhibition of DNA repair enzymes.155–157

ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

Reactive oxygen and nitrogen species can cause cell injury by various mechanisms, including: (i) direct damage to DNA resulting in strand breaks and point mutations; (ii) lipid peroxidation with formation of vasoactive and pro-inflammatory molecules such as thromboxane; (iii) oxidation of proteins (primarily at sulphydryl groups) that alters protein activity, leading to release of proteases and inactivation of antioxidant enzymes and antiproteases; and (iv) alteration in the activities of transcription factors such as AP-1 and NF-κB, leading to enhanced expression of pro-inflammatory genes. Thus, oxidative stress may affect remodelling of extracellular matrix, mitochondrial respiration, cell proliferation, alveolar repair and immune modulation, as well as protective mechanisms in the lungs, such as the surfactant and antiprotease screens. Oxidative stress is also thought to be a central event in inflammatory responses, through the activation of transcription factors such as NF-κB and AP-1, and thereby signal transduction and expression of genes for pro-inflammatory mediators.

Exposure to free radicals from a variety of sources has led organisms to develop a number of endogenous defence mechanisms.158 Defence mechanisms against free radical-induced oxidative stress involve: (i) preventative mechanisms; (ii) repair mechanisms; (iii) physical defences; and (iv) enzymatic and non-enzymatic antioxidant defences. Enzymatic antioxidant defences include SOD, glutathione peroxidase (GPX) and catalase. Non-enzymatic antioxidants are represented by ascorbic acid (vitamin C), α-tocopherol (vitamin E), GSH, carotenoids, flavonoids and other antioxidants.

Therapeutic approaches to antioxidants for oxidative stress-induced airway disorders may be divided into the use of exogenous and endogenous entities. One therapeutic approach is the simple administration of antioxidants. Several studies with antioxidants such as vitamins C and E have shown inconsistent effects on oxidative stress-induced airway disorders, although vitamin E has been shown to reduce oxidative stress in COPD.159 Attempts to supplement GSH in the lung, using GSH or its precursors, have also provided unconvincing results. Nebulized GSH increases bronchial hyperreactivity as an additional side-effect.160 Administration of the amino acid, cysteine, which is rate-limiting for GSH synthesis, was also ineffective, since it was oxidized to neurotoxic cystine.161 However, more positive results have been obtained using the cysteine-donor, NAC, or OTC, which has the potential to produce GSH. It is known that NAC increases plasma GSH levels dose-dependently.162 NAC also increases intracellular GSH concentrations in alveolar macrophages and inhibits H2O2 and O2•− release from circulating neutrophils of smokers and patients with COPD.163 OTC acts as a prodrug of cysteine and also raises plasma concentrations of cysteine and GSH.164–166 Several studies have demonstrated that OTC replenishes cellular GSH stores when GSH is acutely depleted,167,168 and that it is more effective than NAC in replenishing intracellular GSH stores.169,170 In addition, recent studies have shown that the administration of OTC reduces bronchial inflammation and airway hyperresponsiveness in allergic airway disease.41,50,70 Several studies have demonstrated that administration of compounds with antioxidant properties ameliorates pulmonary fibrosis in a number of model systems.171 In addition, GSH (600 mg, bd for 3 days) has been administered by inhalation to IPF patients and increased epithelial lining fluid GSH levels with decreased ROS production by airway macrophages.138

The other therapeutic approach is to enhance the endogenous lung antioxidant screen172 by the molecular manipulation of antioxidant genes, such as GPX, or genes involved in the synthesis of GSH, such as γ-glutamyl cysteine synthase, or by developing molecules with activities similar to those of antioxidant enzymes such as catalase and SOD, which may react directly with oxidants in areas of inflammation.173 Molecular engineering of antioxidant genes may be a future therapeutic option in genomic medicine. An animal study has demonstrated that gene therapy using recombinant SOD prevents inflammatory cell influx into air spaces and cytokine release by noxious stimuli.174

Based on these observations, antioxidants with good bioavailability or molecules that have antioxidant enzyme activity may be developed as therapies that not only protect against the direct injurious effects of oxidants, but also fundamentally alter inflammatory events associated with the pathogenesis of various airway diseases. However, well-designed clinical trials are required to demonstrate the clinical applicability, safety and efficacy of these promising therapeutic strategies.

CONCLUSION AND PERSPECTIVES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

There is now very good evidence implicating oxidative stress in lung diseases such as asthma, COPD, ALI, pulmonary fibrosis and lung cancer. Oxidative stress plays a critical role in inflammatory responses in lung diseases through the upregulation of redox-sensitive transcription factors and thereby pro-inflammatory gene expression. Inflammation itself results in oxidative stress in the lungs. ROS appear to be key regulatory factors in the molecular pathways leading to the induction of lung diseases (Fig. 1), and offer potential points for therapeutic intervention. Future work to understand the molecular mechanisms of ROS-mediated pathophysiological pathways and their control by various antioxidants may aid in the design of novel therapies that target the respective molecular pathways.

image

Figure 1. General overview of the role of reactive oxygen species (ROS) in lung diseases.

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ACKNOWLEDGEMENTS

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES

We thank Prof. Mie-Jae Im for critical reading of the manuscript. This work was supported by a grant from the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Program funded by the Ministry of Science and Technology (R0A-2005-000-10052-0(2008)), by a Korea Research Foundation Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund) (KRF-2005-201-E00014), by a grant from the Korea Health 21 R&D project, Ministry of Health & Welfare, Republic of Korea (A060169), and also by a grant from the Korea Health 21 R&D Project (0412-CR03-0704-0001).

REFERENCES

  1. Top of page
  2. ABSTRACT
  3. INTRODUCTION
  4. CHEMISTRY AND BIOCHEMISTRY OF REACTIVE OXYGEN SPECIES
  5. SOURCES OF REACTIVE OXYGEN SPECIES
  6. SIGNAL TRANSDUCTION
  7. OXIDATIVE STRESS AND BRONCHIAL ASTHMA
  8. OXIDATIVE STRESS AND COPD
  9. OXIDATIVE STRESS AND ACUTE LUNG INJURY
  10. OXIDATIVE STRESS AND PULMONARY FIBROSIS
  11. OXIDATIVE STRESS AND LUNG CANCER
  12. ROLE OF ANTIOXIDANTS IN THE TREATMENT OF LUNG DISEASES
  13. CONCLUSION AND PERSPECTIVES
  14. ACKNOWLEDGEMENTS
  15. REFERENCES