© 2002 American Thoracic Society
Chairman's SummaryOxidants and Antioxidants: Transatlantic Airway Conference 2002Institute for Environmental Medicine, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania Correspondence and requests for reprints should be addressed to Aron B. Fisher, M.D., Institute for Environmental Medicine, University of Pennsylvania Medical Center, 1 John Morgan Building, 36th & Hamilton Walk, Philadelphia, PA 19104-6068. E-mail: abf{at}mail.med.upenn.edu The pulmonary toxicity of oxygen has been recognized for more than 100 years since the classic description by J. Lorraine Smith in the Journal of Physiology. However, the basis for its toxic effects remained obscure until the 1960s when the importance of oxygen radicals (now called reactive oxygen species, ROS) was discovered. Since then, there has been burgeoning interest in this field, and a search of the literature indicates more than 7,000 publications related to lung oxidants and antioxidants. It is now recognized that oxygen toxicity represents only the tip of the iceberg and that lung injury due to oxidants can occur with varied pathologies and with a broad spectrum of etiologic agents including many drugs and toxicants. Both tissue inflammation and reperfusion after vascular occlusion manifest their effects primarily through oxidant mechanisms. Physiologically relevant ROS include superoxide anions, hydrogen peroxide (H2O2), and hydroxyl radicals. It has been recognized recently that nitrogen-based radicals and their products also have an important role in biology giving rise to the concept of reactive nitrogen species. The increased appreciation of oxidant mechanisms has kindled interest in antioxidants as a means of preventing oxidant-mediated injury. The Transatlantic Airway Conference (TAC) 2002 in Lucerne brought together experts in the field of oxidants/antioxidants to discuss recent developments as they relate to lung airways. PHYSIOLOGIC ROLE OF ROS Recent evidence has indicated that ROS are not only "bad guys" but that these molecules have physiologic functions. The first session of TAC 2002 examined some physiologic roles of ROS. Henry Forman proposed that ROS produced by alveolar macrophages have a signaling function as second messengers in addition to their bactericidal role. Alveolar macrophages generate superoxide anions by activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, a membrane bound enzyme complex, and signaling may occur by modification of enzymes through reversible oxidation of critical thiols. Due to the action of cellular antioxidants, the elevation of ROS is transient, thus fulfilling one of the important requirements for a signaling molecule. The end result of signaling is not clear but may result in production and release of cytokines and other important mediators of inflammation. Paul Kemp discussed the role of ROS in hypoxia sensing by airway neuroepithelial bodies. He has identified a cell line derived from a small cell carcinoma that appears to share a common lineage with neuroepithelial bodies. Using patch clamp techniques, he showed increased potassium current with exogenous H2O2 and decreased current with inhibitors of NADPH oxidase. He proposes that ROS produced by NADPH oxidase during normoxia activate a potassium channel in the neuroepithelial body cell membrane, whereas decreased ROS during hypoxia leads to channel closure and membrane depolarization. A critical question for this hypothesis is whether NADPH oxidase in the neuroepithelial bodies, unlike that in phagocytic cells, is constitutively active. Similar mechanisms have been proposed as the basis for oxygen sensing by other tissues such as carotid bodies and pulmonary vascular smooth muscle.
Yvonne Janssen-Heininger discussed the role of reactive nitrogen species in signaling for cell death via the regulated apoptotic pathway in lung epithelial cell lines. She has observed that reactive nitrogen species selectively kill these cells during the log phase of growth, whereas confluent cells are resistant and has ascribed this effect to nuclear factor OXIDATIVE STRESS AND PULMONARY INJURY The second session of TAC 2002 explored some recent developments related to ROS in oxidant-mediated lung injury. Although many lung insults are believed to exert their effects through oxidant mechanisms, this possibility has been difficult to evaluate because of the transient nature of the radical species and the lack of a specific signature for their effects. Jason Morrow presented his work on measurement of isoprostanes as an index of oxidant stress. These derivatives of arachidonic acid are produced by its direct interaction with ROS in contrast to the enzymatic activity required for generation of prostaglandins and other biologically active eicosanoids. Chemically, a broad spectrum of isoprostanes can be generated, but the F2 isoprostanes are the most stable and therefore afford the most accurate measure of oxidant stress. Increased levels of plasma or urinary isoprostanes have been demonstrated in groups of subjects with a spectrum of chronic lung diseases including chronic heavy cigarette smoking, possibly associated with an inflammatory component. Although blood and plasma are easily obtainable for patient evaluation, the expired air is even more readily available. Paolo Paredi discussed analysis of expired gas for evidence of oxidant-mediated processes. Carbon monoxide is generated in areas of inflammation through activity of heme oxygenase. Ethane is a breakdown product of lipid hydroperoxides and thus reflects oxidative modification of cellular lipids. Carbon monoxide and ethane in exhaled breath were increased in patients with asthma, chronic obstructive pulmonary disease, and cystic fibrosis. It is also possible to evaluate expired air for H2O2, nitric oxide, isoprostanes, and a variety of other potential oxidants markers. Although initial measurements of H2O2 showed promise as an index of oxidant stress, Dr. Paredi stated that their clinical utility has not been borne out by subsequent measurements. Both of these new methods for detection of ROS may be useful for analysis of groups, but their utility for identification of oxidant processes in individuals remains to be determined. An important caveat for interpretation is that the presence of these readily diffusible compounds in the expired air could reflect their generation either in lungs or in systemic organs, with transport to lungs by blood. For example, increased destruction of erythrocytes from whatever cause will result in increased carbon monoxide generation and its appearance in expired air. Constance Barazzone described the pattern of cell injury and death in hyperoxia. Oxygen toxicity represents a clinically relevant and prototypic example of oxidant-mediated lung injury. Although our understanding of the mechanisms and pathophysiology of oxygen-induced injury has expanded considerably over the past several decades, the precise mechanisms are still not fully understood. As studied in animal models, pulmonary oxygen toxicity is characterized by a relatively long latent period during which there is evidence for increased ROS generation and relatively subtle oxidation of cellular macromolecules. In rats breathing 100% oxygen, this phase lasts approximately 60 hours. Subsequently, there is relatively rapid deterioration of lung function accompanied by an acute inflammatory response of varying degree. A precise trigger for this second phase is not known. An intriguing possibility is that ROS-generated signals for cellular apoptosis may represent an important initiating factor, but opinion remains divided as to whether cell death at this stage represents primarily an apoptotic or necrotic event. James Crapo discussed oxidative stress in airways, with special emphasis on the role of extracellular superoxide dismutase (SOD). SODs are a family of enzymes that catalyze the dismutation of superoxide anions, serving to regulate its intracellular concentration and thereby limiting its potential toxicity. Another important function for SOD may be the generation of H2O2, a freely diffusible member of the ROS family that appears to be the major molecule involved in ROS-mediated signal transduction. Members of the SOD family found in the lungs include forms predominantly localized to mitochondria, cytosol, and the extracellular space. Dr. Crapo presented evidence that extracellular SOD is highly expressed in lungs and is abundant in both airway epithelium and vascular endothelium, especially associated with the surrounding connective tissue matrix. Overexpression of extracellular SOD in the lungs of mice was protective for lung injury associated with hyperoxia suggesting an antioxidant role for this enzyme. Carroll Cross reviewed the mechanisms for antioxidant defense in plants and demonstrated great similarity with mammalian systems. ANTIOXIDANT MECHANISMS The third session of the TAC 2002 Conference discussed the role of antioxidant mechanisms, continuing the dialog initiated by Dr. Crapo with his presentation of extracellular SOD. Ye Shih Ho described several mouse models of genetic alteration of antioxidant enzyme expression including overexpression of mitochondrial SOD and deficiency (knock-out) of copper/zinc (cytosolic) SOD and glutathione peroxidase. Each of these enzymes has a presumed important antioxidant role. However, mice overexpressing mitochondrial SOD were only slightly protected, and the knock-outs had no effect on sensitivity of mice to hyperoxic injury. Although compensatory changes in the expression of other proteins could account for these effects, no change in levels was observed for other antioxidant enzymes. These provocative results raise unresolved issues for our understanding of oxygen poisoning. Leopold Flohe described a newly discovered class of antioxidant enzymes, the peroxiredoxins, a widely distributed family that use cysteine as the redox active center, in contrast to glutathione peroxidase where the redox active center is a selenocysteine. For most members of the peroxiredoxin family, thioredoxin is the redox cofactor, although Type VI peroxiredoxin that is expressed at an especially high level in lung epithelium appears to use glutathione. Perhaps these enzymes play a central role in defense against hyperoxia although that remains to be determined. Gregory Conner discussed his investigation of pathways for scavenging H2O2 in airway secretions of sheep. He has shown the presence of lactoperoxidase, a heme peroxidase that has been identified previously in milk and saliva. This peroxidase uses H2O2 to oxidize thiocyanate, thereby generating an antimicrobial product, perhaps indicating another physiologic role for lung-generated ROS. Berndt Rüstow gave the final presentation of TAC 2002 on vitamin E as a lung antioxidant. Lung surfactant that is synthesized and secreted by alveolar Type II cells contains vitamin E that is delivered to Type II cells by high-density lipoproteins with intracellular transfer mediated by at least three high-density lipoprotein-specific receptors. In addition to increased sensitivity to oxidant stress, vitamin E deficiency may result in abnormalities of the inflammatory process including alterations in the secretion of cytokines and the expression of heme oxygenase and some adhesion molecules. A key feature that emerged from the discussion of antioxidants is that the old real estate axiom applies: three important issues are location, location, and location. Although we have learned quite a bit from molecular models (overexpression, knock-out), we still do not understand how these antioxidant enzymes and compounds interact to prevent oxidant injury. In summary, it is clear that the redox balance (oxidants versus antioxidants) of the lung plays a major role in its response to injury, including the pathophysiology associated with toxic and inflammatory insults. It is increasingly appreciated that an individual's longevity including lung "health" may represent one's success with dealing with the oxidant stresses of life. From that perspective, a 2-day conference could only scratch the surface of recent developments related to oxidant/antioxidant balance. We have learned about novel ROS/reactive nitrogen species-based signaling processes, about new approaches to detect increased oxidant stress, and about new antioxidant enzyme systems. However, much more needs to be learned including an answer to the original question concerning the basic molecular pathophysiology of pulmonary oxygen toxicity. Perhaps answers to some of these problems will be forthcoming at the next TAC on Oxidants/Antioxidants. Received in original form June 14, 2002; accepted in final form October 7, 2002 This article has been cited by other articles:
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B and c-JUN-N-terminal kinase. The overall balance between these two pathways appears to be critical for determining survival or programmed cell death. 
