© 2007 American Thoracic Society doi: 10.1164/rccm.200612-1821ED
Epithelial Responses to Oxidative Stress in Chronic Obstructive Pulmonary DiseaseLessons from Expression ProfilingUniversity of Modena and Reggio Emilia, Modena, Italy
Leiden University Medical Center, Leiden, The Netherlands Cigarette smoke is a complex mixture that contains over 4,500 chemical compounds, including free radicals and oxidants (1). Although cigarette smoking is the main environmental risk factor for chronic obstructive pulmonary disease (COPD), only about 15% of smokers develop clinically significant disease (2). Therefore, the role of cigarette smoke in the development of COPD is incompletely understood. The airway epithelium forms a primary interface with the outside world and is therefore a target of the numerous toxic particles and gases from tobacco smoke. A large number of studies have shown that exposure of airway epithelial cells to smoke results in marked changes in gene expression, resulting in the release of inflammatory mediators, but also in expression of molecules involved in defense against oxidative stress (3).
Oxidative stress develops when the balance between oxidants and antioxidants shifts in favor of oxidants (4). Oxidative stress contributes to the development and progression of COPD in the following various ways: (1) oxidants react with a variety of substances in the lung, including proteins, lipids, and nucleic acids (5); (2) oxidative stress contributes to proteinase/antiproteinase imbalance both by inactivating proteinase inhibitors (e.g.,
In this issue of the Journal (pp. 577–586) Pierrou and colleagues report on a study in which they used gene expression profiling to identify genes that are differentially expressed in bronchial epithelial cells obtained by bronchial brushings from well-characterized healthy nonsmoking and smoking subjects, and from current smokers with COPD (8). They observed that epithelial expression of genes involved in oxidant/antioxidant responses in smokers is markedly different from that in nonsmokers, and they discovered additional differences in smokers who had developed COPD. Altered gene expression was also found to be related to the severity of COPD and might thereby be a determinant of disease progression. These results are in line with the hypothesis that the oxidative stress response is a major feature of smoke-induced changes in epithelial function, and that a specific response is associated with the development of COPD. The authors observed that a number of transcription factor binding sites, such as NF- An important finding of this study was the observation that gene expression was not linear in relation to disease severity: the expression of some genes peaked in mild COPD and was lower in more severe disease. These results indicate that the impact of oxidative stress on gene expression may be different at various disease stages and may even decrease in more severe forms of the disease. This possibly reflects a failed protective response against oxidative stress in more severe COPD. The authors used a recently introduced bioinformatics approach for analysis of their microarray dataset. Instead of using clustering methods and other bioinformatics tools to analyze the DNA microarray data, Gene Set Enrichment Analysis (GSEA) (10) was used to detect changes in a predefined set of genes involved in the response to oxidative stress. These genes were partly selected based on the available literature on oxidant response genes, whereas the majority of genes (automated set) were selected based on the presence of sequences known to be related to oxidative stress responses. Clearly, only a very small part of the available expression data was used. However, by focusing on a particular set of genes that share a common biological function, GSEA helps in data interpretation and partly avoids the multiple testing problem in microarray analysis. Quantitative real-time reverse transcriptase–polymerase chain reaction was used to validate the observed differences in gene expression for a small selected set of genes. The validity of the GSEA approach was demonstrated by the observation that part of the gene expression profile could be reproduced using exposure of air–liquid interface cultures of well-differentiated epithelial cells obtained from a small subset of donors to an aqueous extract of cigarette smoke. This strongly suggests that the differential expression between smokers and nonsmokers of some genes observed in vivo may, at least in part, be due to direct oxidant effects of cigarette smoke and may not require the full inflammatory response that involves cells such as neutrophils and macrophages. One of the limitations of this study is that it does not consider interactions with cell types other than the epithelial cells that are involved in COPD pathogenesis, such as neutrophils, macrophages, and lymphocytes. Another limitation is that a relationship between protein and mRNA could not be demonstrated as shown by the results of an immunohistochemical analysis of expression of the most differentially expressed gene, CYP1B1. Further studies are clearly needed to confirm and further extend the conclusions from this dataset. Although this study is descriptive, it is the first to use genomewide gene expression profiles to study gene expression in bronchial epithelial brushings obtained from healthy nonsmokers, healthy smokers, and subjects with COPD, and it is unique in that it shows significant differences in the oxidative stress response that appear to be related to various aspects of COPD pathogenesis. So far, antioxidant therapy has not been a very successful approach in COPD treatment. In line with this, a recent study showed that treatment of patients with COPD with N-acetylcysteine does not affect the rate of functional decline, yearly exacerbation, or deterioration in health status (11). Studies such as the present one by Pierrou and colleagues may contribute to improved therapeutic interventions for the prevention and treatment of smoking-induced lung disease and particularly COPD by providing a better understanding of the molecular mechanisms involved in airway oxidative stress responses. FOOTNOTES Conflict of Interest Statement: F.L. has no financial relationship with a commercial entity that has an interest in the subject of this manuscript. P.S.H. has participated as a speaker in various meetings cofinanced by various pharmaceutical companies; the Department of Pulmonology, and thereby P.S.H. as staff member of the department, has received grants from AltanaPharma ($222,616), Novartis ($90,640), Bayer ($61,762), AstraZeneca ($113,155), Boehringer Ingelheim ($90,000), Roche ($120,000), and GlaxoSmithKline ($299,495) from 2001 until 2005. REFERENCES
Related articles in AJRCCM:
|
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1-antitrypsin and secretory leukocyte protease inhibitor) and by activating proteinases, such as matrix metalloproteases (
B, that orchestrate the expression of a range of genes encoding inflammatory mediators believed to be important in COPD, such as interleukin-8 and tumor necrosis factor (TNF)-
owski K, Virtala R, Lindqvist E, Richter A, Wilson SJ, Angco G, Möller S, et al. Expression of genes involved in oxidative stress responses in airway epithelial cells of smokers with chronic obstructive pulmonary disease. Am J Respir Crit Care Med 2007;175:577–586.