Cytokine-mediated xanthine oxidase upregulation in chronic obstructive pulmonary disease's airways
Introduction
Chronic obstructive pulmonary disease (COPD) is a condition characterized by airway inflammation and airflow limitation that is progressive and largely irreversible [1], [2], [3]. Although COPD is a major cause of morbidity and mortality in the world [4], the pathogenesis of this disease has not yet been fully elucidated.
There is increasing evidence that an oxidant/antioxidant imbalance occurs in COPD [5], [6], [7]. Oxidants including superoxide anion (), hydrogen peroxide and peroxynitrite cause airway inflammation by means of tissue injury [5], the activation of matrix metalloproteinases [8], the inactivation of α1-antitrypsin [9], and enhanced production of the potent neutrophil chemoattractant interleukin (IL)-8 [10]. Therefore, these oxidants appear to be involved in the pathophysiology of the airway inflammation in COPD patients. Recently, endogenously generated oxidants have been thought to be important in COPD [5], [6]. Among these, is the most important molecule since other oxidants are derived from this molecule.
Xanthine oxidoreductase (XOR) is a rate-limiting enzyme of purine catabolism that exists in two forms, as xanthine dehydrogenase (XD) and as xanthine oxidase (XO) [11], [12]. XOR, particularly in the XO form, generates reactive oxygen species such as , hydroxyl radicals and hydrogen peroxide. It has been reported that XO was enhanced in an animal model of asthma [13] and in virus-induced pneumonia in mice [14]. These studies showed that mediated by XO caused both airway and lung parenchymal inflammation. Previously, we have reported that the XO activity in sputum from patients with COPD was increased compared with that of healthy subjects [15]. Similarly, Pinamonti et al. [16] have reported that the XO activity was increased in bronchoalveolar lavage fluid from COPD patients. However, these methodologies using sputum and bronchoalveolar fluid samples are semiquantative.
Further, the mechanisms responsible for the upregulation of the XO activity are still unclear. Some proinflammatory cytokines such as tumor necrosis factor (TNF)-α, IL-1β, and interferon (IFN)-γ have been reported to upregulate XO gene expression in bovine renal epithelial cells [17], rat alveolar macrophages [18], and in human mammary epithelial cells [19]. In a rat model, IL-1 and IFN-γ intratracheal instillation cause enhanced XO activity in the lungs [20]. Therefore, it is possible that these proinflammatory cytokines are excessively produced in COPD airways and upregulated the XO gene expression.
The aim of this study was to quantify the XO activity in COPD airway epithelial lining fluid (ELF) using a new bronchoscopic microsampling technique [21], [22]. We also quantified proinflammatory cytokines in the ELF that are responsible for XO gene upregulation.
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Subjects
Thirteen stable Japanese COPD patients and 10 Japanese healthy subjects participated in the present study. Forced expiratory volume in 1 s (FEV1) was assessed with a dry rolling seal spirometer (Chestac 11, Chest Co., Tokyo, Japan). Table 1 shows the characteristics of the study subjects. None of the healthy subjects were atopic nor had abnormal lung function. They did not have clinical manifestations of bronchial asthma such as recurrent episodes of wheezing. COPD was diagnosed according to the
Results
Table 1 shows the characteristics of the subjects who participated in the present study. In the present study, the collection of ELF samples with the microsampling probe was accomplished without serious adverse events such as pneumonia or pulmonary hemorrhage.
The XO activity in the ELF is shown in Fig. 1. The XO activity in the patients with COPD was significantly increased compared with healthy subjects (114.1±16.2 vs 31.4±7.7 nmol isoxanthopterin/mg protein h−1, p<0.01). Furthermore, there was
Discussion
We have shown that the XO activity in the ELF of COPD patients was significantly higher than that in healthy subjects. In addition, the values of the XO activity were significantly correlated with the values of %FEV1, suggesting that this enzyme may be involved in the inflammatory process of COPD.
XOR including XO and XD is a rate-limiting enzyme of purine catabolism that catalyzes the oxidative hydroxylation of hypoxanthine to xanthine and xanthine to uric acid [11], [12]. In particular, the XO
Acknowledgements
We thank Mr Brent Bell for reading the manuscript and Dr Motohiko Miura and Dr Uichiro Katsumata for collecting the epithelial lining fluid samples.
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