Serial Review: Redox signaling in immune function and cellular responses in lung injury and diseases Serial Review Editors: Victor Darley–Usmar, Lin Mantell
Purinergic signaling and kinase activation for survival in pulmonary oxidative stress and disease

https://doi.org/10.1016/j.freeradbiomed.2006.03.004Get rights and content

Abstract

Stimulus-induced release of endogenous ATP into the extracellular milieu has been shown to occur in a variety of cells, tissues, and organs. Extracellular ATP can propagate signals via P2 receptors that are essential for growth and survival of cells. Abundance of P2 receptors, their multiple isoforms, and their ubiquitous distribution indicate that they transmit vital signals. Pulmonary epithelium and endothelium are rich in both P2X and P2Y receptors. ATP release from lung tissue and cells occurs upon stimulation both in vivo and in vitro. Extracellular ATP can activate signaling cascades composed of protein kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3K). Here we summarize progress related to release of endogenous ATP and nucleotide signaling in pulmonary tissues upon exposure to oxidant stress. Hypoxic, hyperoxic, and ozone exposures cause a rapid increase of extracellular ATP in primary pulmonary endothelial and epithelial cells. Extracellular ATP is critical for survival of these cells in high oxygen and ozone concentrations. The released ATP, upon binding to its specific receptors, triggers ERK and PI3K signaling and renders cells resistant to these stresses. Impairment of ATP release and transmission of such signals could limit cellular survival under oxidative stress. This may further contribute to disease pathogenesis or exacerbation.

Introduction

ATP is the principal intracellular energy source. Recent attention has been focused on ATP's functions outside the cell. Cytosolic ATP and that packed into secretory granules and vesicles can be released from cells either under basal conditions or upon stimulation by cellular stress via regulated nonlytic mechanisms [1], [2]. Released ATP and its metabolites ADP and adenosine can mediate a diverse spectrum of effects in many cell types upon binding to their distinct and specific receptors. P1 receptors are activated by adenosine, whereas ATP and its analogs activate P2 receptors. P2 receptors are of two types: the ionotropic P2X and the metabotropic P2Y. Cells express multiple isoforms of P2X and P2Y receptors (Fig. 1) [2], [3]. Once released, ATP interacts with these purinergic receptors and activates rapid signaling events that are both autocrine and paracrine [4]. Thus, ATP is an important signaling molecule that participates in intercellular communication and also regulates a broad range of physiological responses like (i) increases in intracellular calcium; (ii) activation of phospholipases; (iii) modulation of cell volume; (iv) inhibition of platelet aggregation; (v) alteration of vascular tone; (vi) neurotransmission; (vii) elevation of cardiac/skeletal muscle contractility; (viii) immune cell activation; (ix) increased ciliary beat frequency and mucus secretion; (x) pulmonary surfactant release from alveolar type II epithelial cells; (xi) bronchoconstriction in individuals with hyperreactive airways; and (xii) enhanced cell growth and proliferation [5]. Extracellular ATP and adenosine may also cause cell death/apoptosis depending on their extracellular concentration and cell type so exposed [6].

ATP release and ATP-mediated signaling during oxidative stress have been less thoroughly investigated in the lung than in the cardiovascular system. Several studies indicate that purinergic receptors are involved in the cellular response to oxidative stress [7], [8], [9], [10], [11], [12], [13]. However, the role of ATP release in oxidative stress has not been altogether clear. Severe oxidative stress can diminish intracellular ATP [14], [15]. Delayed catabolism of extracellular nucleotides due to diminished ecto-nucleotidase activity in the presence of reactive oxygen species (ROS) also has been suggested [15]. Further, extracellular ATP itself could lead to oxidative stress in some cell types [16], [17]. For example, extracellular ATP can induce H2O2 production by increasing Ca2+-mediated Duox, an NADPH oxidase, in airway epithelium [18]. Stimulation of P2X receptors also can induce superoxide production and contraction of mouse aorta [19]. Thus, ATP not only responds to redox changes but also can itself trigger redox changes.

Oxidative stress like that in hyperoxia also signals through various pathways. Previously we and others have shown that microvascular endothelial cells are primary targets of hyperoxic lung injury and in other forms of acute respiratory failure [20], [21]. Hypoxic preexposure of human lung microvascular endothelial cells (HLMVEC) attenuates cell death caused by subsequent hyperoxic exposure [21]. This is partially attributed to upregulation of PI 3-kinase (PI3K), a component of an established cell survival pathway, during the hyperoxic exposure. Hypoxic (1 or 3% O2) exposure also induces ATP release in HLMVEC and fetal lung fibroblasts [22], [23]. There, hypoxia and extracellular ATP acted synergistically to upregulate common proliferation-inducing pathways in lung adventitial fibroblasts. Extracellular ATP and purinergic receptor activation play a critical role in determining the intracellular effects of key growth-regulating factors [4], [24], [25]. In addition to ATP release, hypoxic conditions also favor proliferation in endothelial cells [22], [26]. In primary HLMVEC, we have also shown previously that ATP is released in hyperoxia and that the presence of extracellular ATP is essential for survival of these cells [27]. Enhanced glucose uptake and upregulation of hexokinase II (HKII) are other potential contributing survival factors in hyperoxia [28], [29]. A possible link between these events and ATP signaling remains under investigation. In sum, deviations from usual oxygen tensions, both high and low, can cause ATP release from endothelial and other lung cell types.

Another common pulmonary oxidant is ozone. It is an important component of ambient air pollution and exposure to ozone can cause exacerbation of asthma and other chronic lung diseases [30], [31], [32]. Extracellular ATP protects lung epithelial cells against ozone toxicity [33]. P2Y receptor-mediated increases in ERK and Akt phosphorylations were important signaling events in this protection. Therefore, identification of links between ATP release and activation of survival pathways will illuminate a novel pathway in cellular defense against oxidative stress. In this Review we highlight the unique role of extracellular ATP as a danger signal, an “SOS” for host defense during pulmonary disorders caused by oxidative stress such as bronchopulmonary dysplasia and cystic fibrosis.

Section snippets

Purinoreceptor expression in pulmonary cells

Both P2Y and P2X receptors are ubiquitous in lung. Airway epithelium has been one of the most heavily studied tissues for effects of purinergic agonists [34], [35], [36]. Airway epithelial cells release ATP and possess both P2X and P2Y receptors, as do endothelial cells [2], [5], [25], [37]. Localization and distribution of P2X receptor subtypes in lungs, along with methods of detection used, are summarized in Table 1. In general, the mammalian pulmonary vasculature contains most P2X receptor

Endogenous ATP release in lung and its effects

Stimuli which cause ATP release by regulated mechanisms include mechanical stress/trauma, shear stress (increased blood flow and or lung ventilation; volutrauma) [40], hypotonic medium [41], [42], inflammation, cAMP, and ATP itself [43]. The mechanisms responsible for the appearance of extracellular ATP are likely multiple and probably differ by cell type. First, it may exit the cell via ATP-permeable release channels. Second, ATP may be conducted down favorable concentration gradients by such

ATP-mediated protection against oxidative stress

As described above, in lungs, mechanical stress or introduction of foreign particles on the luminal surface can trigger nucleotide release. Subsequently, P2Y2 receptor-mediated secretion can be stimulated and larger amounts of fluid generated, potentially contributing to airway particle clearance [101]. In addition to clearing the epithelial surface and restoring alveolar homeostasis, extracellular ATP also could protect pulmonary epithelium and endothelium against oxidant stresses caused by

Signaling events associated with ATP-mediated protection

P2X receptors mediate “rapid: nonselective passage of cations (Na+, K+, Ca2+) across cell membranes resulting in increased intracellular Ca2+ and depolarization [123], [124]. Conductance of extracellular Ca2+ through such channels is a significant source of increased intracellular Ca2+. Such responses are very rapid and do not involve production of second messengers. These signals are important for rapid neuronal signaling and regulation of muscle contractility. P2Y receptors are

Acknowledgments

This publication was made possible by Grant ES014448-01 from the National Institute of Environmental Health Sciences (NIEHS), NIH. The authors are also grateful to Gabriele Cheatham for the preparation of the manuscript.

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