Molecular Mechanisms of Increased Nitric Oxide (NO) in Asthma: Evidence for Transcriptional and Post-Translational Regulation of NO Synthesis

Abstract

Evidence supporting increased nitric oxide (NO) in asthma is substantial, although the cellular and molecular mechanisms leading to increased NO are not known. Here, we provide a clear picture of the events regulating NO synthesis in the human asthmatic airway in vivo. We show that human airway epithelium has abundant expression of NO synthase II (NOSII) due to continuous transcriptional activation of the gene in vivo. Individuals with asthma have higher than normal NO concentrations and increased NOSII mRNA and protein due to transcriptional regulation through activation of Stat1. NOSII mRNA expression decreases in asthmatics receiving inhaled corticosteroid, treatment effective in reducing inflammation in asthmatic airways. In addition to transcriptional mechanisms, post-translational events contribute to increased NO synthesis. Specifically, high output production of NO is fueled by a previously unsuspected increase in the NOS substrate, l-arginine, in airway epithelial cells of asthmatic individuals. Finally, nitration of proteins in airway epithelium provide evidence of functional consequences of increased NO. In conclusion, these studies define multiple mechanisms that function coordinately to support high level NO synthesis in the asthmatic airway. These findings represent a crucial cornerstone for future therapeutic strategies aimed at regulating NO synthesis in asthma.

Regulation of inducible nitric oxide synthase by rapid cellular turnover and cotranslational down-regulation by dimerization inhibitors

“…In human lungs, airway bronchial epithelial cells are a major source of iNOS (4,7,16). In airway inflammation of asthma, iNOS expression is markedly increased in these cells (7). Therefore, we sought to determine the half-life of iNOS in primary bronchial epithelial cells cultured at the air-liquid interface system (10).…”

“…The use of selective inhibitors of iNOS has been advocated as a novel therapeutic approach for several inflammatory diseases, e.g., asthma, rheumatoid arthritis, multiple sclerosis, and glaucoma (6)(7)(8). The catalytic activity of iNOS can be inhibited by using L-arginine analogues (3).…”

“…Calmodulin is tightly bound to iNOS even at basal Ca 2ϩ levels; therefore, iNOS is notably distinguished from the constitutive isoforms by its prolonged production of a relatively large amount of NO (5). iNOS has been implicated in the pathogenesis of many diseases including Alzheimer's disease, tuberculosis, asthma, transplant rejection, stroke, glaucoma, inflammatory bowel disease, arthritis, and septic shock (6,7). Such wide implication has produced a corresponding intense interest in understanding the regulation of NO synthesis by iNOS with the goal of developing therapeutic strategies aimed at selective modulation of iNOS activity (8).…”

Bacterial Lipopolysaccharide Signaling Through Toll-Like Receptor 4 Suppresses Asthma-Like Responses Via Nitric Oxide Synthase 2 Activity

“…In humans with normal airways, exhaled NO is derived from constitutive endothelial and neural nitric oxide synthases (NOS3 and NOS1, respectively), whereas the increased levels of NO detected in asthmatics appear to be derived from inducible NOS (or NOS2) expressed by the inflamed airways (17,18). Whether NO production has a beneficial or deleterious effect in asthma is still controversial.…”

Nitric oxide paradox in asthma

“…As such, in asthmatics or other inflammatory lung diseases the exhaled NO appears to derive from NOS2 expressed within bronchial epithelial cells or immuno-inflammatory cells (Kharitonov et al 1994, Barnes 1995, Guo et al 2000, Van Der Vliet et al 2000, Yates 2001. Indeed, the level of NO concentration in exhaled air is considered to be a marker of airway inflammation and the exhaled NO observed in asthmatics has been associated with airway eosinophilia and AHR (Saleh et al 1998, Ricciardolo 2003.…”