Serial Review: Redox Signaling in Immune Function and Cellular Responses in Lung Injury and DiseasesSerial Review Editors: Victor Darley-Usmar, Lin Mantell
Nitric oxide and reactive nitrogen species in airway epithelial signaling and inflammation

https://doi.org/10.1016/j.freeradbiomed.2006.05.011Get rights and content

Abstract

Nitric oxide (NOradical dot) is produced by many diverse cell types as a cellular or intracellular signaling molecule, by the activation of nitric oxide synthases (NOSs). All three known NOS isoforms are expressed within the respiratory tract and mediate various airway functional properties such as airway smooth muscle tone, ciliary function, epithelial electrolyte transport, and innate host defense. The respiratory epithelium is a major source of NOradical dot, in which it regulates normal epithelial cell function and signaling as well as signaling pathways involved in airway inflammation. In addition to its normal physiological properties, increased airway NOradical dot production in inflammatory respiratory tract diseases such as asthma may activate additional signaling mechanisms to regulate inflammatory-immune pathways, and epithelial barrier (dys)function or repair. The biological actions of NOradical dot are controlled at various levels, including mechanisms that regulate NOS localization and activation, and variable oxidative metabolism of NOradical dot, resulting in generation of bioactive reactive nitrogen species (RNS). Moreover, in addition to altered production of NOradical dot or RNS, the presence of various target enzymes and/or metabolic regulators of NOradical dot/RNS can be dramatically altered during airway inflammatory conditions, and contribute to alterations in NOradical dot-mediated signaling pathways in disease. This review summarizes current knowledge regarding NOradical dot-mediated epithelial signaling, as well as disease-related changes in airway NOS biology and target enzymes that affect NOradical dot/RNS signaling mechanisms. A detailed understanding of these various changes and their impact on NOradical dot signaling pathways are needed to fully appreciate the contributions of NOradical dot/RNS to airway inflammation and to develop suitable therapeutic approaches based on regulating NOradical dot function.

Introduction

Nitric oxide (NOradical dot) is a ubiquitous reactive signaling molecule, synthesized by oxidative conversion of the amino acid L-arginine by NOradical dot synthases (NOSs). The functional NOS protein is a homodimer, with each subunit comprising a reductase domain mediating electron transfer from NADPH to the oxygenase domain, where O2 binding and arginine oxidation occur. Between these domains, a calmodulin-binding region controls NOS activation in response to Ca2+ binding [1]. Three NOS isoforms have been identified, and all three are known to be expressed within the respiratory tract. NOS1 (nNOS) and NOS3 (eNOS) are primarily expressed in neuronal and endothelial cell types, respectively, and are highly dependent on increases in intracellular Ca2+ for enzyme activation [2]. In the airway, nonadrenergic, noncholinergic nerve fibers express NOS1 that generates NOradical dot as a major mediator of neural smooth muscle relaxation [3]. Lung NOS3 expression is primarily observed within pulmonary endothelial cells, and also in bronchial [4] and alveolar epithelial cells [5]. In the bronchial epithelium, NOS3 is localized at the basal membrane of ciliary microtubules, and mediates regulation of ciliary beat frequency [6], [7]. The third isoform, NOS2, is primarily present in cells of the inflammatory-immune system, where it is induced during inflammatory conditions [1]. However, NOS2 is continuously expressed in human airway epithelial cells under normal conditions [8], and is the primary source of exhaled NOradical dot in healthy subjects [9]. Moreover, the airway epithelium appears to be the primary site of induced NOS2 expression in asthmatic airways [8]. Because of the importance of the airway epithelium as a major regulator of airway inflammation, and epithelial functional defects in chronic inflammatory disorders such as asthma [10], the identification of the airway epithelium as a primary location for NOS and airway NOradical dot production urges increased understanding of the involvement of NOradical dot in epithelial cell signaling pathways and alterations during airway inflammation. These issues will form the central theme of the present review.

Section snippets

Regulation of NOS expression and activation

While NOS1 and NOS3 are typically considered to be constitutively expressed proteins (although their expression is subject to regulation under certain conditions [11]), expression of NOS2 is primarily controlled by complex transcriptional regulation in response to cells stimulation by proinflammatory cytokines such as tumor necrosis factor-α (TNF-α), interferon-γ (IFN-γ), and interleukin (IL)-1β [12]. Additionally, NOS2 expression is also regulated by cytosine methylation within the promoter

Signaling mechanisms of nitric oxide

In addition to complex regulation of NOS expression and activation, the biology of NOradical dot is further complicated by its interaction in various metabolic pathways due its chemical reactivity [34]. Since NOradical dot contains an unpaired electron, it is a free radical that reacts readily with other radical intermediates or paramagnetic species. The biological activities NOradical dot can in general be attributed to three separate signaling mechanisms, that are schematically illustrated in Fig. 1 and will be discussed

NOradical dot in airway epithelial physiology and pathophysiology

Being situated at the interface between the external environment and the underlying airway structures, the respiratory epithelium is a primary target for inhaled microbes, allergens, and particulates, and is therefore principally involved in airway responses to these environmental factors and the orchestration of inflammatory-immune processes [65], [66]. In addition to its more classically recognized functions as a physical barrier and in mucociliary clearance, the airway epithelium is also

NOradical dot signaling in the airway epithelium: Regulation of inflammation

The airway epithelium is critically involved in regulating inflammatory-immune processes in response to environmental stimuli, and appears to be a major site of induced NOS2 expression for chronic lung diseases such as asthma. Moreover, the airway epithelium of asthmatic subjects appears to possess altered intrinsic properties [66], [67], which may be related to altered NOradical dot-mediated epithelial signaling pathways. Indeed, it has become widely appreciated that NOradical dot is capable of regulating various

Alterations in NOradical dot-mediated signaling during airway inflammation

Although much is still to be learned about the mechanisms of NOradical dot-mediated signaling under normal physiological conditions, increased epithelial NOS2 expression and NOradical dot/RNS production during airway inflammation will undoubtedly result in additional and altered NOradical dot-dependent signaling pathways. Recent studies also indicate potential changes in other NOS isoforms during airway inflammation, illustrated by reduced airway epithelial NOS3 expression during endotoxemia, in conjunction with NOS2 induction

Final considerations and future perspectives

Based on the observed deficiency in S-nitrosothiols within airway secretions of patients with severe asthma or cystic fibrosis, and findings that SNOs are potentially useful as donors of NOradical dot to promote bronchodilation and airway function, inhalation of SNOs may present a suitable and effective strategy for therapeutic management. However, the diverse roles of NOradical dot in regulating important airway functional aspects, ranging from beneficial (attenuating airway hyperresponsiveness, downregulating Th1

Acknowledgments

The authors would like to thank Yvonne Janssen-Heininger and Wolfgang Dostmann for frequent research discussions, and are grateful for research support from NIH (Grants HL068865 and HL074295) and the Department of Pathology at UVM.

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    This article is part of a series of reviews on “Redox signaling in immune function and cellular responses in lung injury and diseases.” The full list of papers may be found on the home page of the journal.

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