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SOD Inactivation in Asthma

2005; Elsevier BV; Volume: 166; Issue: 3 Linguagem: Inglês

10.1016/s0002-9440(10)62286-9

1525-2191

Yvonne M. W. Janssen‐Heininger, Karina Ckless, Niki L. Reynaert, Albert van der Vliet,

Chronic Obstructive Pulmonary Disease (COPD) Research

Changes in the oxidative milieu are well known to accompany inflammatory diseases, including asthma, and the severity of oxidative stress, measured through the accumulation of stable oxidation end products, often directly correlates with the severity of disease. As an example, tyrosine nitration has been reported to positively associate with cardiac disease,1Shishehbor MH Hazen SL Inflammatory and oxidative markers in atherosclerosis: relationship to outcome.Curr Atheroscler Rep. 2004; 6: 243-250Crossref PubMed Scopus (61) Google Scholar and a direct correlation between nitrotyrosine reactivity and functional abnormalities has been reported in patients with asthma.2Saleh D Ernst P Lim S Barnes PJ Giaid A Increased formation of the potent oxidant peroxynitrite in the airways of asthmatic patients is associated with induction of nitric oxide synthase: effect of inhaled glucocorticoid.FASEB J. 1998; 12: 929-937Crossref PubMed Scopus (395) Google Scholar These observations, coupled with animal studies demonstrating that administration of antioxidant compounds ameliorate various manifestations of inflammatory disorders3Chang LY Crapo JD Inhibition of airway inflammation and hyperreactivity by an antioxidant mimetic.Free Radic Biol Med. 2002; 33: 379-386Crossref PubMed Scopus (88) Google Scholar, 4Henderson Jr, WR Chi EY Teo JL Nguyen C Kahn M A small molecule inhibitor of redox-regulated NF-kappa B and activator protein-1 transcription blocks allergic airway inflammation in a mouse asthma model.J Immunol. 2002; 169: 5294-5299Crossref PubMed Scopus (109) Google Scholar and that transgenic mice overexpressing antioxidant enzymes display attenuated damage,5Folz RJ Abushamaa AM Suliman HB Extracellular superoxide dismutase in the airways of transgenic mice reduces inflammation and attenuates lung toxicity following hyperoxia.J Clin Invest. 1999; 103: 1055-1066Crossref PubMed Scopus (261) Google Scholar, 6Bowler RP Nicks M Tran K Tanner G Chang LY Young SK Worthen GS Extracellular superoxide dismutase attenuates lipopolysaccharide-induced neutrophilic inflammation.Am J Respir Cell Mol Biol. 2004; 31: 432-439Crossref PubMed Scopus (94) Google Scholar, 7White C Avraham K Shanley P Groner Y Transgenic mice with expression of elevated levels of copper-zinc superoxide dismutase in the lungs are resistant to pulmonary oxygen toxicity.J Clin Invest. 1991; 87: 2162-2168Crossref PubMed Scopus (115) Google Scholar have provided substantial evidence in support of a causal role of oxidative changes in the inflammatory disease process. Nonetheless, a mechanistic basis underlying such causality is generally lacking, due to an overall failure to pinpoint the critical oxidative targets in relation to functional changes that drive the disease process. In this regard, the study by Dr. Comhair and colleagues8Comhair SAA Xu W Ghosh S Thunnissen FBJM Almasan A Calhoun WJ Janocha AJ Zheng L Hazen SL Erzurum SC Superoxide dismutase inactivation in pathophysiology of asthmatic airway remodeling and reactivity.Am J Pathol. 2005; 166: 663-674Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar in this issue of The American Journal of Pathology has made significant progress in delineating critical oxidative events in patients with asthma and the role these oxidative alterations may play in the morphological and functional alterations seen in the asthmatic lung. The authors demonstrate in bronchial brush material obtained from patients with mild asthma that SOD is inactivated. Using sophisticated mass spectrometric analyses and immuno-approaches, they also reveal that the mitochondrially localized isoform of superoxide dismutase (SOD), MnSOD, contains multiple oxidations of phenylalanines and tyrosines, including nitrated tyrosine residues. To address the ramifications of such oxidative inactivation of MnSOD, the authors used a chemical inhibitor of SOD in a human bronchial epithelial cell line, or knocked down MnSOD using siRNA, and demonstrated that SOD inactivation or knockdown is sufficient to cause apoptosis. These results are consistent with their observations in epithelial brushings from patients which also revealed evidence of apoptosis, as evidenced by increases in TUNEL reactivity, caspase 3 and 9 cleavage and activation, and PARP cleavage. Lastly, decreases in SOD activity in asthmatics were found to correlate with decreased lung function.8Comhair SAA Xu W Ghosh S Thunnissen FBJM Almasan A Calhoun WJ Janocha AJ Zheng L Hazen SL Erzurum SC Superoxide dismutase inactivation in pathophysiology of asthmatic airway remodeling and reactivity.Am J Pathol. 2005; 166: 663-674Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar Thus, the scenario emerges that oxidative inactivation of SOD within cells of the conducting airways leads to enhanced apoptosis and a compromised epithelial barrier. These are considered potential contributors to airway hyper responsiveness in patients with asthma and can also fuel airway remodeling.9Davies DE Wicks J Powell RM Puddicombe SM Holgate ST Airway remodeling in asthma: new insights.J Allergy Clin Immunol. 2003; 111: 215-225quiz 226Abstract Full Text Full Text PDF PubMed Scopus (469) Google Scholar, 10Bousquet J Jeffery PK Busse WW Johnson M Vignola AM Asthma. From bronchoconstriction to airways inflammation and remodeling.Am J Respir Crit Care Med. 2000; 161: 1720-1745Crossref PubMed Scopus (1582) Google Scholar These findings are highly significant in that they not only highlight the strength of translational studies but also provide a much needed mechanistic framework toward elucidating the mechanism of action of oxidants in the pulmonary inflammatory disease process. Despite the importance of the present study by Comhair et al8Comhair SAA Xu W Ghosh S Thunnissen FBJM Almasan A Calhoun WJ Janocha AJ Zheng L Hazen SL Erzurum SC Superoxide dismutase inactivation in pathophysiology of asthmatic airway remodeling and reactivity.Am J Pathol. 2005; 166: 663-674Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar in providing evidence for mechanistic links between specific oxidative events, changes in antioxidant function, and decreased epithelial integrity or lung function, a number of unresolved questions remain that will provide a continued challenge for future investigations. First, it remains unclear to what extent the measured tyrosine modifications within MnSOD from samples of asthmatic patients actually contribute to inactivation of the enzyme and consequently drive the apoptotic process. While the efforts to relate changes in SOD activity to specific oxidative modifications are highly commendable, especially considering the challenges associated with analysis of patient specimens, only selected oxidative modifications in two amino acid residues were analyzed. The total number of oxidations measured ranged from 1.13–1.73 mmol oxidation product/mol precursor tyrosine or phenylalanine, reflecting attack by highly reactive nitrating species or oxidants with hydroxyl radical like activity, which are claimed to have affected up to 6% of total MnSOD. Nevertheless, it is difficult to envision how these quantitatively modest changes account for the extensive enzymatic inactivation of SOD seen in asthmatic patients.11De Raeve HR Thunnissen FB Kaneko FT Guo FH Lewis M Kavuru MS Secic M Thomassen MJ Erzurum SC Decreased Cu, Zn-SOD activity in asthmatic airway epithelium: correction by inhaled corticosteroid in vivo.Am J Physiol. 1997; 272: L148-L154PubMed Google Scholar, 12Smith LJ Shamsuddin M Sporn PH Denenberg M Anderson J Reduced superoxide dismutase in lung cells of patients with asthma.Free Radic Biol Med. 1997; 22: 1301-1307Crossref PubMed Scopus (140) Google Scholar, 13Comhair SA Bhathena PR Dweik RA Kavuru M Erzurum SC Rapid loss of superoxide dismutase activity during antigen-induced asthmatic response.Lancet. 2000; 355: 624Abstract Full Text Full Text PDF PubMed Scopus (166) Google Scholar, 14Adler V Yin Z Tew KD Ronai Z Role of redox potential and reactive oxygen species in stress signaling.Oncogene. 1999; 18: 6104-6111Crossref PubMed Scopus (604) Google Scholar Furthermore, the specific location of oxidized tyrosine or phenylalanine residues within the MnSOD protein are unknown, and consequences of modification of these amino acids for alterations in structure and function of MnSOD remain to be determined. In this regard, it is also important to consider oxidative modifications of other amino acids such as cysteine and methionine that have significant impact on the activity of many proteins, as evidenced by the multiple evolutionary conserved redox systems that exist to regulate or reverse such modifications, including thioredoxin, peroxyredoxins, glutaredoxins, and methionine sulfoxide reductase.15Stadtman ER Moskovitz J Levine RL Oxidation of methionine residues of proteins: biological consequences.Antioxid Redox Signal. 2003; 5: 577-582Crossref PubMed Scopus (297) Google Scholar, 16Fernandes AP Holmgren A Glutaredoxins: glutathione-dependent redox enzymes with functions far beyond a simple thioredoxin backup system.Antioxid Redox Signal. 2004; 6: 63-74Crossref PubMed Scopus (556) Google Scholar, 17Jones DP Go YM Anderson CL Ziegler TR Kinkade Jr, JM Kirlin WG Cysteine/cystine couple is a newly recognized node in the circuitry for biologic redox signaling and control.FASEB J. 2004; 18: 1246-1248Crossref PubMed Scopus (256) Google Scholar, 18Wood ZA Poole LB Karplus PA Peroxiredoxin evolution and the regulation of hydrogen peroxide signaling.Science. 2003; 300: 650-653Crossref PubMed Scopus (1167) Google Scholar, 19Toone WM Morgan BA Jones N Redox control of AP-1-like factors in yeast and beyond.Oncogene. 2001; 20: 2336-2346Crossref PubMed Scopus (139) Google Scholar, 20Wakabayashi N Dinkova-Kostova AT Holtzclaw WD Kang MI Kobayashi A Yamamoto M Kensler TW Talalay P Protection against electrophile and oxidant stress by induction of the phase 2 response: fate of cysteines of the Keap1 sensor modified by inducers.Proc Natl Acad Sci USA. 2004; 101: 2040-2045Crossref PubMed Scopus (854) Google Scholar Indeed, a wealth of evidence suggests that reversible sulfhydryl oxidations could play a prominent regulatory role in cell signaling and apoptosis.14Adler V Yin Z Tew KD Ronai Z Role of redox potential and reactive oxygen species in stress signaling.Oncogene. 1999; 18: 6104-6111Crossref PubMed Scopus (604) Google Scholar, 19Toone WM Morgan BA Jones N Redox control of AP-1-like factors in yeast and beyond.Oncogene. 2001; 20: 2336-2346Crossref PubMed Scopus (139) Google Scholar However, due to their reversible nature, and the lack of specific reagents, accurate detection of these oxidative modifications is an exceedingly difficult, if not impossible, task that limits our knowledge of the full spectrum of oxidations that could drive the disease process associated with chronic inflammation. Thus, the question that remains unanswered is whether the subset of oxidations measured in the present study are indeed the ones that are relevant to MnSOD inactivation and apoptosis. Clarification of this issue awaits studies with specific mutant proteins, in cell culture or animal models, and the development of much needed additional reagents to adequately probe sulfhydryl and other reversible oxidations. Another question that remains incompletely addressed is which isoform(s) of SOD is inactivated in asthmatic airways. Three distinct superoxide dismutases exist in humans: MnSOD (SOD2), which contains manganese and is expressed in the mitochondria, CuZn containing superoxide dismutase (SOD1), which is expressed in the cytosol, and extracellular (Ec) SOD (SOD3), which contains Cu and Zn, as well as a heparin binding domain, and is localized on the cell surface or in the extracellular matrix (21Fattman CL Schaefer LM Oury TD Extracellular superoxide dismutase in biology and medicine.Free Radic Biol Med. 2003; 35: 236-256Crossref PubMed Scopus (548) Google Scholar for review). While the present study convincingly demonstrates that MnSOD is oxidized in the airways of asthmatic subjects and that its knockdown is sufficient to induce apoptosis, previous studies by the same team of investigators have indicated that lowered SOD activity in asthmatic airways is primarily attributed to decreases in CuZnSOD activity.11De Raeve HR Thunnissen FB Kaneko FT Guo FH Lewis M Kavuru MS Secic M Thomassen MJ Erzurum SC Decreased Cu, Zn-SOD activity in asthmatic airway epithelium: correction by inhaled corticosteroid in vivo.Am J Physiol. 1997; 272: L148-L154PubMed Google Scholar In addition, SOD activity measured in the BAL fluid of asthmatics was also substantially lowered,13Comhair SA Bhathena PR Dweik RA Kavuru M Erzurum SC Rapid loss of superoxide dismutase activity during antigen-induced asthmatic response.Lancet. 2000; 355: 624Abstract Full Text Full Text PDF PubMed Scopus (166) Google Scholar which may in fact reflect changes in expression, localization, and/or activity of EcSOD. While decreases in MnSOD can easily be implicated in decreased mitochondrial function and increased apoptosis,22Van Remmen H Williams MD Guo Z Estlack L Yang H Carlson EJ Epstein CJ Huang TT Richardson A Knockout mice heterozygous for Sod2 show alterations in cardiac mitochondrial function and apoptosis.Am J Physiol Heart Circ Physiol. 2001; 281: H1422-H1432PubMed Google Scholar, 23Fujimura M Morita-Fujimura Y Kawase M Copin JC Calagui B Epstein CJ Chan PH Manganese superoxide dismutase mediates the early release of mitochondrial cytochrome C and subsequent DNA fragmentation after permanent focal cerebral ischemia in mice.J Neurosci. 1999; 19: 3414-3422Crossref PubMed Google Scholar such causal relations between apoptosis and changes in other SOD isozymes are much less apparent. Thus, further elucidation of the extent of oxidative modifications in all SOD isoforms, the ranges at which these occur, and specific assessment of inactivation of each isoform in control subjects and asthmatic patients will be needed to clarify these uncertainties. The importance of this issue may be best illustrated by the several studies that report the effects of genetic deletion of either SOD isoform. While genetic deficiency of MnSOD has significant consequences for mitochondrial integrity and survival,24Lebovitz RM Zhang H Vogel H Cartwright Jr, J Dionne L Lu N Huang S Matzuk MM Neurodegeneration, myocardial injury, and perinatal death in mitochondrial superoxide dismutase-deficient mice.Proc Natl Acad Sci USA. 1996; 93: 9782-9787Crossref PubMed Scopus (859) Google Scholar, 25Li Y Huang TT Carlson EJ Melov S Ursell PC Olson JL Noble LJ Yoshimura MP Berger C Chan PH et al.Dilated cardiomyopathy and neonatal lethality in mutant mice lacking manganese superoxide dismutase.Nat Genet. 1995; 11: 376-381Crossref PubMed Scopus (1480) Google Scholar phenotypic changes due to genetic deficiency of either CuZnSOD or EcSOD are more subtle and typically present at more advanced age or in the context of elevated inflammation or oxidative stress.6Bowler RP Nicks M Tran K Tanner G Chang LY Young SK Worthen GS Extracellular superoxide dismutase attenuates lipopolysaccharide-induced neutrophilic inflammation.Am J Respir Cell Mol Biol. 2004; 31: 432-439Crossref PubMed Scopus (94) Google Scholar, 26Ho YS Gargano M Cao J Bronson RT Heimler I Hutz RJ Reduced fertility in female mice lacking copper-zinc superoxide dismutase.J Biol Chem. 1998; 273: 7765-7769Crossref PubMed Scopus (281) Google Scholar, 27Elchuri S Oberley TD Qi W Eisenstein RS Jackson Roberts L Van Remmen H Epstein CJ Huang TT CuZnSOD deficiency leads to persistent and widespread oxidative damage and hepatocarcinogenesis later in life.Oncogene. 2005; 24: 367-380Crossref PubMed Scopus (517) Google Scholar Overexpression of CuZnSOD also failed to rescue neonatal lethality associated with the MnSOD knockout genotype,28Copin JC Gasche Y Chan PH Overexpression of copper/zinc superoxide dismutase does not prevent neonatal lethality in mutant mice that lack manganese superoxide dismutase.Free Radic Biol Med. 2000; 28: 1571-1576Crossref PubMed Scopus (80) Google Scholar clearly confirming that the SOD isoforms have non-redundant roles. Therefore, depending on the isoform of SOD that is targeted in asthma, several scenarios can be drawn to implicate alterations in cell signaling or function. What is it about SOD oxidation/inactivation that drives apoptosis in epithelial cells, and what are the critical redox changes in this process? These questions are not easily answered because of the complex and multifaceted nature of redox perturbations that will ensue on loss of functional SOD, and the uncertainties regarding the SOD isoform affected in asthmatic airways. The simplest scenario perhaps reflects the increased steady state concentrations of superoxide that will occur following SOD inactivation, which have been shown to inhibit aconitase in association with inhibition of the respiratory chain, destabilization of the mitochondrial membrane, opening of the pore complex, and initiation of the apoptotic cascade.29Powell CS Jackson RM Mitochondrial complex I, aconitase, and succinate dehydrogenase during hypoxia-reoxygenation: modulation of enzyme activities by MnSOD.Am J Physiol Lung Cell Mol Physiol. 2003; 285: L189-L198Crossref PubMed Scopus (103) Google Scholar, 30Melov S Coskun P Patel M Tuinstra R Cottrell B Jun AS Zastawny TH Dizdaroglu M Goodman SI Huang TT Miziorko H Epstein CJ Wallace DC Mitochondrial disease in superoxide dismutase 2 mutant mice.Proc Natl Acad Sci USA. 1999; 96: 846-851Crossref PubMed Scopus (501) Google Scholar, 31Cadenas E Mitochondrial free radical production and cell signaling.Mol Aspects Med. 2004; 25: 17-26Crossref PubMed Scopus (356) Google Scholar, 32Levonen AL Patel RP Brookes P Go YM Jo H Parthasarathy S Anderson PG Darley-Usmar VM Mechanisms of cell signaling by nitric oxide and peroxynitrite: from mitochondria to MAP kinases.Antioxid Redox Signal. 2001; 3: 215-229Crossref PubMed Scopus (98) Google Scholar Another well appreciated scenario through which SOD inactivation may promote redox changes is through the loss of nitric oxide as a consequence of the rapid reaction of superoxide with nitric oxide, causing the formation of the damaging nitrating species peroxynitrite.33Radi R Beckman JS Bush KM Freeman BA Peroxynitrite oxidation of sulfhydryls. The cytotoxic potential of superoxide and nitric oxide.J Biol Chem. 1991; 266: 4244-4250Abstract Full Text PDF PubMed Google Scholar, 34Beckman JS Koppenol WH Nitric oxide, superoxide, and peroxynitrite: the good, the bad, and ugly.Am J Physiol. 1996; 271: C1424-C1437PubMed Google Scholar Whereas the consequences of enhanced superoxide-dependent formation of peroxynitrite are easily implicated in the apoptotic process,35Shrivastava P Pantano C Watkin R McElhinney B Guala A Poynter ML Persinger RL Budd R Janssen-Heininger Y Reactive nitrogen species-induced cell death requires Fas-dependent activation of c-Jun N-terminal kinase.Mol Cell Biol. 2004; 24: 6763-6772Crossref PubMed Scopus (54) Google Scholar, 36Persinger RL Blay WM Heintz NH Hemenway DR Janssen-Heininger YM Nitrogen dioxide induces death in lung epithelial cells in a density-dependent manner.Am J Respir Cell Mol Biol. 2001; 24: 583-590Crossref PubMed Scopus (36) Google Scholar, 37Lang Jr, JD Chumley P Eiserich JP Estevez A Bamberg T Adhami A Crow J Freeman BA Hypercapnia induces injury to alveolar epithelial cells via a nitric oxide-dependent pathway.Am J Physiol Lung Cell Mol Physiol. 2000; 279: L994-L1002PubMed Google Scholar it is also important to consider the importance of loss of NO that results from ONOO− formation. Such regulation of NO bioactivity would strongly depend on the location of NO production, in association with the isoform of SOD that is inactivated, and could affect NO signaling either extracellularly, in cytoplasmic compartments and/or in mitochondria. NO can be stored in cells in the form of S-nitrosothiol, a chemical form of functional NO associated with protein thiol groups (referred to as S-nitrosation or S-nitrosylation).38Foster MW McMahon TJ Stamler JS S-nitrosylation in health and disease.Trends Mol Med. 2003; 9: 160-168Abstract Full Text Full Text PDF PubMed Scopus (484) Google Scholar, 39Hess DT Matsumoto A Nudelman R Stamler JS S-nitrosylation: spectrum and specificity.Nat Cell Biol. 2001; 3: E46-E49Crossref PubMed Scopus (219) Google Scholar A role for S-nitrosothiols in the regulation of the apoptotic process has been implicated based on elegant studies demonstrating that the activity of caspases, which are cysteine dependent proteases, is repressed by S-nitrosylation. During the sequelae of apoptosis, caspase denitrosylation occurs and in turn leads to their activation.40Liu L Stamler JS NO: an inhibitor of cell death.Cell Death Differ. 1999; 6: 937-942Crossref PubMed Scopus (169) Google Scholar, 41Mannick JB Hausladen A Liu L Hess DT Zeng M Miao QX Kane LS Gow AJ Stamler JS Fas-induced caspase denitrosylation.Science. 1999; 284: 651-654Crossref PubMed Scopus (711) Google Scholar S-nitrosylation of various caspases has been detected in multiple subcellular compartments,41Mannick JB Hausladen A Liu L Hess DT Zeng M Miao QX Kane LS Gow AJ Stamler JS Fas-induced caspase denitrosylation.Science. 1999; 284: 651-654Crossref PubMed Scopus (711) Google Scholar, 42Mannick JB Schonhoff C Papeta N Ghafourifar P Szibor M Fang K Gaston B S-Nitrosylation of mitochondrial caspases.J Cell Biol. 2001; 154: 1111-1116Crossref PubMed Scopus (334) Google Scholar pointing to an important role for NO in the prevention of apoptosis, and suggesting that the loss of functional NO through redox changes may be highly relevant in promoting the apoptotic process. Whether the loss of bioavailable NO, which results from SOD inactivation following the formation of ONOO− mentioned earlier or through other redox changes, plays a role in the causation of apoptosis in epithelial cells of asthmatic patients remains to be explored. Nonetheless, it is important to note that S-nitrosothiol levels are markedly suppressed in the asthmatic airways,43Gaston B Sears S Woods J Hunt J Ponaman M McMahon T Stamler JS Bronchodilator S-nitrosothiol deficiency in asthmatic respiratory failure.Lancet. 1998; 351: 1317-1319Abstract Full Text Full Text PDF PubMed Scopus (205) Google Scholar, 44Dweik RA Comhair SA Gaston B Thunnissen FB Farver C Thomassen MJ Kavuru M Hammel J Abu-Soud HM Erzurum SC NO chemical events in the human airway during the immediate and late antigen-induced asthmatic response.Proc Natl Acad Sci USA. 2001; 98: 2622-2627Crossref PubMed Scopus (218) Google Scholar consistent with this biochemical scenario. Is oxidatively inactivated SOD functionally silent, or could the inactive protein itself be redox active, thereby contributing to the apoptotic process? This possibility is exemplified by mutant SOD1 isoforms that constitute a fraction of patients with familiar amyotrophic lateral sclerosis (ALS),45Valentine JS Hart PJ Misfolded CuZnSOD and amyotrophic lateral sclerosis.Proc Natl Acad Sci USA. 2003; 100: 3617-3622Crossref PubMed Scopus (502) Google Scholar drawing an interesting parallel between inactivation of SOD1 in asthmatic airways and SOD1 mutations that mediate the clinical manifestations of ALS, which also encompass apoptosis.46Przedborski S Mitsumoto H Rowland LP Recent advances in amyotrophic lateral sclerosis research.Curr Neurol Neurosci Rep. 2003; 3: 70-77Crossref PubMed Scopus (26) Google Scholar These mutant SOD1 proteins are destabilized and are either Zn-deficient or more susceptible to Zn release from their active site, leading to catalysis of aberrant oxidations, tyrosine nitration, enhanced decomposition of S-nitrosothiols, and polymerization.47Crow JP Sampson JB Zhuang Y Thompson JA Beckman JS Decreased zinc affinity of amyotrophic lateral sclerosis-associated superoxide dismutase mutants leads to enhanced catalysis of tyrosine nitration by peroxynitrite.J Neurochem. 1997; 69: 1936-1944Crossref PubMed Scopus (429) Google Scholar, 48Johnson MA Macdonald TL Mannick JB Conaway MR Gaston B Accelerated s-nitrosothiol breakdown by amyotrophic lateral sclerosis mutant copper, zinc-superoxide dismutase.J Biol Chem. 2001; 276: 39872-39878Crossref PubMed Scopus (52) Google Scholar It would be intriguing if oxidatively inactivated SOD1 in asthmatic subjects similarly acquires such a “toxic” gain of function that could thereby promote apoptosis. Formal testing of this possibility will require assessment of the sites of oxidations, and the consequences of these events for enzyme structure and function. While it is easily envisioned that inactivation of SOD alters subcellular or extracellular oxidative events through processes described above, the source of oxidants and the course of events that lead to the reported oxidative changes in MnSOD remain unclear. It is often implied that oxidants derived from pro-inflammatory cells are responsible for the damage associated with inflammation, and result in certain “signature” oxidative modifications, including bromotyrosines and chlorotyrosines, reflecting the specific involvement of eosinophil peroxidase and myeloperoxidase, respectively.49Brennan ML Wu W Fu X Shen Z Song W Frost H Vadseth C Narine L Lenkiewicz E Borchers MT Lusis AJ Lee JJ Lee NA Abu-Soud HM Ischiropoulos H Hazen SL A tale of two controversies: defining both the role of peroxidases in nitrotyrosine formation in vivo using eosinophil peroxidase and myeloperoxidase-deficient mice, and the nature of peroxidase-generated reactive nitrogen species.J Biol Chem. 2002; 277: 17415-17427Crossref PubMed Scopus (462) Google Scholar Such oxidative modifications appear to primarily affect extracellular proteins.50Van Der Vliet A Nguyen MN Shigenaga MK Eiserich JP Marelich GP Cross CE Myeloperoxidase and protein oxidation in cystic fibrosis.Am J Physiol Lung Cell Mol Physiol. 2000; 279: L537-L546PubMed Google Scholar Therefore, it is not entirely clear how such oxidative events specifically affect intracellular or mitochondrial proteins. In this regard, the recent recognition of non-phagocytic NADPH dependent oxidases within the airway epithelium51Geiszt M Witta J Baffi J Lekstrom K Leto TL Dual oxidases represent novel hydrogen peroxide sources supporting mucosal surface host defense.FASEB J. 2003; 17: 1502-1504Crossref PubMed Scopus (426) Google Scholar warrants further investigation into their mechanisms of activation, especially within the context of activation inflammatory-immune processes, and their involvement in regulating cellular target enzymes. These endeavors will be facilitated through an amalgamation of both mechanistic and translational studies, exemplified by the study by Comhair et al8Comhair SAA Xu W Ghosh S Thunnissen FBJM Almasan A Calhoun WJ Janocha AJ Zheng L Hazen SL Erzurum SC Superoxide dismutase inactivation in pathophysiology of asthmatic airway remodeling and reactivity.Am J Pathol. 2005; 166: 663-674Abstract Full Text Full Text PDF PubMed Scopus (164) Google Scholar in the current issue of The American Journal of Pathology. Indeed, this important study paves the way for much needed additional translational studies to unequivocally demonstrate the causal role of specific oxidative processes in cellular dysfunction and the development or propagation of chronic inflammatory diseases. We apologize to the investigators whose work could not be acknowledged due to space constraints.