Identification of an ATP/P2X7/mast cell pathway mediating ozone-induced bronchial hyperresponsiveness

Ozone is a highly reactive environmental pollutant with well-recognized adverse effects on lung health. Bronchial hyperresponsiveness (BHR) is one consequence of ozone exposure, particularly for individuals with underlying lung disease. Our data demonstrated that ozone induced substantial ATP release from human airway epithelia in vitro and into the airways of mice in vivo and that ATP served as a potent inducer of mast cell degranulation and BHR, acting through P2X7 receptors on mast cells. Both mast cell–deficient and P2X7 receptor–deficient (P2X7^–/–) mice demonstrated markedly attenuated BHR to ozone. Reconstitution of mast cell–deficient mice with WT mast cells and P2X7^–/– mast cells restored ozone-induced BHR. Despite equal numbers of mast cells in reconstituted mouse lungs, mice reconstituted with P2X7^–/– mast cells demonstrated significantly less robust BHR than mice reconstituted with WT mast cells. These results support a model where P2X7 on mast cells and other cell types contribute to ozone-induced BHR.

Pannexin 1 channels control the hemodynamic response to hypoxia by regulating O2-sensitive extracellular ATP in blood

Pannexin 1 (Panx1) channels export ATP and may contribute to increased concentration of the vasodilator ATP in plasma during hypoxia in vivo. We hypothesized that Panx1 channels and associated ATP export contribute to hypoxic vasodilation, a mechanism that facilitates the matching of oxygen delivery to metabolic demand of tissue. Male and female mice devoid of Panx1 (Panx1-/-) and wild-type controls (WT) were anesthetized, mechanically ventilated, and instrumented with a carotid artery catheter or femoral artery flow transducer for hemodynamic and plasma ATP monitoring during inhalation of 21% (normoxia) or 10% oxygen (hypoxia). ATP export from WT vs. Panx1-/-erythrocytes (RBC) was determined ex vivo via tonometer experimentation across progressive deoxygenation. Mean arterial pressure (MAP) was similar in Panx1-/- (n = 6) and WT (n = 6) mice in normoxia, but the decrease in MAP in hypoxia seen in WT was attenuated in Panx1-/- mice (-16 ± 9% vs. -2 ± 8%; P < 0.05). Hindlimb blood flow (HBF) was significantly lower in Panx1-/- (n = 6) vs. WT (n = 6) basally, and increased in WT but not Panx1-/- mice during hypoxia (8 ± 6% vs. -10 ± 13%; P < 0.05). Estimation of hindlimb vascular conductance using data from the MAP and HBF experiments showed an average response of 28% for WT vs. -9% for Panx1-/- mice. Mean venous plasma ATP during hypoxia was 57% lower in Panx1-/- (n = 6) vs. WT mice (n = 6; P < 0.05). Mean hypoxia-induced ATP export from RBCs from Panx1-/- mice (n = 8) was 82% lower than that from WT (n = 8; P < 0.05). Panx1 channels participate in hemodynamic responses consistent with hypoxic vasodilation by regulating hypoxia-sensitive extracellular ATP levels in blood.NEW & NOTEWORTHY Export of vasodilator ATP from red blood cells requires pannexin 1. Blood plasma ATP elevations in response to hypoxia in mice require pannexin 1. Hemodynamic responses to hypoxia are accompanied by increased plasma ATP in mice in vivo and require pannexin 1.

Purinergic receptors in airway hydration

Airway epithelial purinergic receptors control key components of the mucociliary clearance (MCC), the dominant component of pulmonary host defense. In healthy airways, the periciliary liquid (PCL) is optimally hydrated, thus acting as an efficient lubricant layer over which the mucus layer moves by ciliary force. When the hydration of the airway surface decreases, the mucus becomes hyperconcentrated, the PCL collapses, and the "thickened" mucus layer adheres to cell surfaces, causing plaque/plug formation. Mucus accumulation is a major contributing factor to the progression of chronic obstructive lung diseases such as cystic fibrosis (CF) and chronic bronchitis (CB). Mucus hydration is regulated by finely tuned mechanisms of luminal Cl^- secretion and Na⁺ absorption with concomitant osmotically driven water flow. These activities are regulated by airway surface liquid (ASL) concentrations of adenosine and ATP, acting on airway epithelial A_2B and P2Y₂ receptors, respectively. The goal of this article is to provide an overview of our understanding of the role of purinergic receptors in the regulation of airway epithelial ion/fluid transport and the mechanisms of nucleotide release and metabolic activities that contribute to airway surface hydration in healthy and chronically obstructed airways.

Proinflammatory P2Y14 receptor inhibition protects against ischemic acute kidney injury in mice

Ischemic acute kidney injury (AKI), a complication that frequently occurs in hospital settings, is often associated with hemodynamic compromise, sepsis, cardiac surgery, or exposure to nephrotoxins. Here, using a murine renal ischemia/reperfusion injury (IRI) model, we show that intercalated cells (ICs) rapidly adopted a proinflammatory phenotype after IRI. Wwe demonstrate that during the early phase of AKI either blockade of the proinflammatory P2Y14 receptor located on the apical membrane of ICs or ablation of the gene encoding the P2Y14 receptor in ICs (a) inhibited IRI-induced increase of chemokine expression in ICs, (b) reduced neutrophil and monocyte renal infiltration, (c) reduced the extent of kidney dysfunction, and (d) attenuated proximal tubule damage. These observations indicate that the P2Y14 receptor participates in the very first inflammatory steps associated with ischemic AKI. In addition, we show that the concentration of the P2Y14 receptor ligand UDP-glucose (UDP-Glc) was higher in urine samples from intensive care unit patients who developed AKI compared with patients without AKI. In particular, we observed a strong correlation between UDP-Glc concentration and the development of AKI in cardiac surgery patients. Our study identifies the UDP-Glc/P2Y14 receptor axis as a potential target for the prevention and/or attenuation of ischemic AKI.

Inhibition of ATP hydrolysis restores airway surface liquid production in cystic fibrosis airway epithelia

Airway surface dehydration is a pathological feature of cystic fibrosis (CF) lung disease. CF is caused by mutations in the CF transmembrane conductance regulator (CFTR), a cyclic AMP-regulated Cl- channel controlled in part by the adenosine A2B receptor. An alternative CFTR-independent mechanism of fluid secretion is regulated by ATP via the P2Y2 receptor (P2Y2R) that activates Ca2+-regulated Cl- channels (CaCC/TMEM16) and inhibits Na+ absorption. However, due to rapid ATP hydrolysis, steady-state ATP levels in CF airway surface liquid (ASL) are inadequate to maintain P2Y2R-mediated fluid secretion. Therefore, inhibiting airway epithelial ecto-ATPases to increase ASL ATP levels constitutes a strategy to restore airway surface hydration in CF. Using [γ32P]ATP as radiotracer, we assessed the effect of a series of ATPase inhibitory compounds on the stability of physiologically occurring ATP concentrations. We identified the polyoxometalate [Co4(H2O)2(PW9O34)2]10- (POM-5) as the most potent and effective ecto-ATPase inhibitor in CF airway epithelial cells. POM-5 caused long-lasting inhibition of ATP hydrolysis in airway epithelia, which was reversible upon removal of the inhibitor. Importantly, POM-5 markedly enhanced steady-state levels of released ATP, promoting increased ASL volume in CF cell surfaces. These results provide proof of concept for ecto-ATPase inhibitors as therapeutic agents to restore hydration of CF airway surfaces. As a test of this notion, cell-free sputum supernatants from CF subjects were studied and found to have abnormally elevated ATPase activity, which was markedly inhibited by POM-5.

Mucus Hydration in Subjects with Stable Chronic Bronchitis: A Comparison of Spontaneous and Induced Sputum

Mucus hydration is important in mucus clearance and lung health. This study sought to test the relative utility of spontaneous sputum (SS) versus the reasonably noninvasive induced sputum (IS) samples for measurement of mucus hydration. SS and IS samples were collected over a 2-day study interval. Sputum was induced with escalating inhaled nebulized 3-5% hypertonic saline. Viscous portions of the samples ("plugs") were utilized for percent solids and total mucin analyses. Cytokines, nucleotides/nucleosides and cell differentials were measured in plugs diluted into 0.1% Sputolysin. Overall, 61.5% of chronic bronchitis (CB) subjects produced a SS sample and 95.2% an IS sample. Total expectorate sample weights were less for the SS (0.94 ± 0.98 g) than the IS (2.67 ± 2.33 g) samples. Percent solids for the SS samples (3.56% ± 1.95; n = 162) were significantly greater than the IS samples (3.08% ± 1.81; n = 121), p = 0.133. Total mucin concentrations also exhibited a dilution of the IS samples: SS = 4.15 ± 3.23 mg/ml (n = 62) versus IS= 3.34 ± 2.55 mg/ml (n = 71) (p = 0.371). Total mucins (combined SS and IS) but not percent solids, were inversely associated with FEV₁ percent predicted (p = 0.052) and FEV₁,/FVC % (p = 0.035). There were no significant differences between sample types in cytokine or differential cell counts. The probability of sample collections was less for SS than IS samples. Measurements of hydration revealed modest dilution of the IS samples compared to SS. Thus for measurements of mucus hydration, both SS and IS samples appear to be largely interchangeable.

Cinnamaldehyde in flavored e-cigarette liquids temporarily suppresses bronchial epithelial cell ciliary motility by dysregulation of mitochondrial function

Aldehydes in cigarette smoke (CS) impair mitochondrial function and reduce ciliary beat frequency (CBF), leading to diminished mucociliary clearance (MCC). However, the effects of aldehyde e-cigarette flavorings on CBF are unknown. The purpose of this study was to investigate whether cinnamaldehyde, a flavoring agent commonly used in e-cigarettes, disrupts mitochondrial function and impairs CBF on well-differentiated human bronchial epithelial (hBE) cells. To this end, hBE cells were exposed to diluted cinnamon-flavored e-liquids and vaped aerosol and assessed for changes in CBF. hBE cells were subsequently exposed to various concentrations of cinnamaldehyde to establish a dose-response relationship for effects on CBF. Changes in mitochondrial oxidative phosphorylation and glycolysis were evaluated by Seahorse Extracellular Flux Analyzer, and adenine nucleotide levels were quantified by HPLC. Both cinnamaldehyde-containing e-liquid and vaped aerosol rapidly yet transiently suppressed CBF, and exposure to cinnamaldehyde alone recapitulated this effect. Cinnamaldehyde impaired mitochondrial respiration and glycolysis in a dose-dependent manner, and intracellular ATP levels were significantly but temporarily reduced following exposure. Addition of nicotine had no effect on the cinnamaldehyde-induced suppression of CBF or mitochondrial function. These data indicate that cinnamaldehyde rapidly disrupts mitochondrial function, inhibits bioenergetic processes, and reduces ATP levels, which correlates with impaired CBF. Because normal ciliary motility and MCC are essential respiratory defenses, inhalation of cinnamaldehyde may increase the risk of respiratory infections in e-cigarette users.

P2Y6 Receptor Activation Promotes Inflammation and Tissue Remodeling in Pulmonary Fibrosis

Idiopathic pulmonary fibrosis (IPF) is a disease with a poor prognosis and very few available treatment options. The involvement of the purinergic receptor subtypes P2Y₂ and P2X₇ in fibrotic lung disease has been demonstrated recently. In this study, we investigated the role of P2Y₆ receptors in the pathogenesis of IPF in humans and in the animal model of bleomycin-induced lung injury. P2Y₆R expression was upregulated in lung structural cells but not in bronchoalveolar lavage (BAL) cells derived from IPF patients as well as in animals following bleomycin administration. Furthermore, BAL fluid levels of the P2Y₆R agonist uridine-5′-diphosphate were elevated in animals with bleomycin-induced pulmonary fibrosis. Inflammation and fibrosis following bleomycin administration were reduced in P2Y₆R-deficient compared to wild-type animals confirming the pathophysiological relevance of P2Y₆R subtypes for fibrotic lung diseases. Experiments with bone marrow chimeras revealed the importance of P2Y₆R expression on lung structural cells for pulmonary inflammation and fibrosis. Similar effects were obtained when animals were treated with the P2Y₆R antagonist MRS2578. In vitro studies demonstrated that proliferation and secretion of the pro-inflammatory/pro-fibrotic cytokine IL-6 by lung fibroblasts are P2Y₆R-mediated processes. In summary, our results clearly demonstrate the involvement of P2Y₆R subtypes in the pathogenesis of pulmonary fibrosis. Thus, blocking pulmonary P2Y₆ receptors might be a new target for the treatment of IPF.

UDP-glucose promotes neutrophil recruitment in the lung

In addition to their role in glycosylation reactions, UDP-sugars are released from cells and activate widely distributed cell surface P2Y₁₄ receptors (P2Y₁₄R). However, the physiological/pathophysiological consequences of UDP-sugar release are incompletely defined. Here, we report that UDP-glucose levels are abnormally elevated in lung secretions from patients with cystic fibrosis (CF) as well as in a mouse model of CF-like disease, the βENaC transgenic (Tg) mouse. Instillation of UDP-glucose into wild-type mouse tracheas resulted in enhanced neutrophil lung recruitment, and this effect was nearly abolished when UDP-glucose was co-instilled with the P2Y₁₄R antagonist PPTN [4-(piperidin-4-yl)-phenyl)-7-(4-(trifluoromethyl)-phenyl-2-naphthoic acid]. Importantly, administration of PPTN to βENaC-Tg mice reduced neutrophil lung inflammation. These results suggest that UDP-glucose released into the airways acts as a local mediator of neutrophil inflammation.

The Relationship of Mucus Concentration (Hydration) to Mucus Osmotic Pressure and Transport in Chronic Bronchitis

RATIONALE

Chronic bronchitis (CB) is characterized by persistent cough and sputum production. Studies were performed to test whether mucus hyperconcentration and increased partial osmotic pressure, in part caused by abnormal purine nucleotide regulation of ion transport, contribute to the pathogenesis of CB.

OBJECTIVES

We tested the hypothesis that CB is characterized by mucus hyperconcentration, increased mucus partial osmotic pressures, and reduced mucus clearance.

METHODS

We measured in subjects with CB as compared with normal and asymptomatic smoking control subjects indices of mucus concentration (hydration; i.e., percentage solids) and sputum adenine nucleotide/nucleoside concentrations. In addition, sputum partial osmotic pressures and mucus transport rates were measured in subjects with CB.

MEASUREMENTS AND RESULTS

CB secretions were hyperconcentrated as indexed by an increase in percentage solids and total mucins, in part reflecting decreased extracellular nucleotide/nucleoside concentrations. CB mucus generated concentration-dependent increases in partial osmotic pressures into ranges predicted to reduce mucus transport. Mucociliary clearance (MCC) in subjects with CB was negatively correlated with mucus concentration (percentage solids). As a test of relationships between mucus concentration and disease, mucus concentrations and MCC were compared with FEV1, and both were significantly correlated.

CONCLUSIONS

Abnormal regulation of airway surface hydration may slow MCC in CB and contribute to disease pathogenesis.

UDP-Sugars as Extracellular Signaling Molecules: Cellular and Physiologic Consequences of P2Y14 Receptor Activation

UDP-sugars, which are indispensable for protein glycosylation reactions in cellular secretory pathways, also act as important extracellular signaling molecules. We discuss here the broadly expressed P2Y14 receptor, a G-protein-coupled receptor targeted by UDP sugars, and the increasingly diverse set of physiologic responses discovered recently functioning downstream of this receptor in many epithelia as well as in immune, inflammatory, and other cells.

Computational model for the regulation of extracellular ATP and adenosine in airway epithelia

Extracellular nucleotides are key components of the signaling network regulating airway clearance. They are released by the epithelium into the airway surface liquid (ASL) to stimulate cilia beating activity, mucus secretion and airway hydration. Understanding the factors affecting their availability for purinoceptor activation is an important step toward the development of new therapies for obstructive lung diseases. This chapter presents a mathematical model developed to gain predictive insights into the regulation of ASL nucleotide concentrations on human airway epithelia. The parameters were estimated from experimental data collected on polarized primary cultures of human nasal and bronchial epithelial cells. This model reproduces major experimental observations: (1) the independence of steady-state nucleotide concentrations on ASL height, (2) the impact of selective ectonucleotidase inhibitors on their steady-state ASL concentrations, (3) the changes in ASL composition caused by mechanical stress mimicking normal breathing, (4) and the differences in steady-state concentrations existing between nasal and bronchial epithelia. In addition, this model launched the study of nucleotide release into uncharted territories, which led to the discovery that airway epithelia release, not only ATP, but also ADP and AMP. This study shows that computational modeling, coupled to experimental validation, provides a powerful approach for the identification of key therapeutic targets for the improvement of airway clearance in obstructive respiratory diseases.

Inflammation promotes airway epithelial ATP release via calcium-dependent vesicular pathways

ATP in airway surface liquid (ASL) controls mucociliary clearance functions via the activation of airway epithelial purinergic receptors. However, abnormally elevated ATP levels have been reported in inflamed airways, suggesting that excessive ATP in ASL contributes to airway inflammation. Despite these observations, little is known about the mechanisms of ATP accumulation in the ASL covering inflamed airways. In this study, links between cystic fibrosis (CF)-associated airway inflammation and airway epithelial ATP release were investigated. Primary human bronchial epithelial (HBE) cells isolated from CF lungs exhibited enhanced IL-8 secretion after 6 to 11 days, but not 28 to 35 days, in culture, compared with normal HBE cells. Hypotonic cell swelling-promoted ATP release was increased in 6- to 11-day-old CF HBE cells compared with non-CF HBE cells, but returned to normal values after 28 to 35 days in culture. The exposure of non-CF HBE cells to airway secretions isolated from CF lungs, namely, sterile supernatants of mucopurulent material (SMM), also caused enhanced IL-8 secretion and increased ATP release. The SMM-induced increase in ATP release was sensitive to Ca(2+) chelation and vesicle trafficking/exocytosis inhibitors, but not to pannexin inhibition. Transcript levels of the vesicular nucleotide transporter, but not pannexin 1, were up-regulated after SMM exposure. SMM-treated cultures displayed increased basal mucin secretion, but mucin secretion was not enhanced in response to hypotonic challenge after the exposure of cells to either vehicle or SMM. We propose that CF airway inflammation up-regulates the capacity of airway epithelia to release ATP via Ca(2+)-dependent vesicular mechanisms not associated with mucin granule secretion.

A selective high-affinity antagonist of the P2Y14 receptor inhibits UDP-glucose-stimulated chemotaxis of human neutrophils

The nucleotide-sugar-activated P2Y14 receptor (P2Y14-R) is highly expressed in hematopoietic cells. Although the physiologic functions of this receptor remain undefined, it has been strongly implicated recently in immune and inflammatory responses. Lack of availability of receptor-selective high-affinity antagonists has impeded progress in studies of this and most of the eight nucleotide-activated P2Y receptors. A series of molecules recently were identified by Gauthier et al. (Gauthier et al., 2011) that exhibited antagonist activity at the P2Y14-R. We synthesized one of these molecules, a 4,7-disubstituted 2-naphthoic acid derivative (PPTN), and studied its pharmacological properties in detail. The concentration-effect curve of UDP-glucose for promoting inhibition of adenylyl cyclase in C6 glioma cells stably expressing the P2Y14-R was shifted to the right in a concentration-dependent manner by PPTN. Schild analyses revealed that PPTN-mediated inhibition followed competitive kinetics, with a KB of 434 pM observed. In contrast, 1 μM PPTN exhibited no agonist or antagonist effect at the P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, or P2Y13 receptors. UDP-glucose-promoted chemotaxis of differentiated HL-60 human promyelocytic leukemia cells was blocked by PPTN with a concentration dependence consistent with the KB determined with recombinant P2Y14-R. In contrast, the chemotactic response evoked by the chemoattractant peptide fMetLeuPhe was unaffected by PPTN. UDP-glucose-promoted chemotaxis of freshly isolated human neutrophils also was blocked by PPTN. In summary, this work establishes PPTN as a highly selective high-affinity antagonist of the P2Y14-R that is useful for interrogating the action of this receptor in physiologic systems.

Kinetics of extracellular ATP in mastoparan 7-activated human erythrocytes

BACKGROUND

The peptide mastoparan 7 (MST7) stimulated ATP release in human erythrocytes. We explored intra- and extracellular processes governing the time-dependent accumulation of extracellular ATP (i.e., ATPe kinetics).

METHODS

Human erythrocytes were treated with MST7 in the presence or absence of two blockers of pannexin 1. ATPe concentration was monitored by luciferin-luciferase based real-time luminometry.

RESULTS

Exposure of human erythrocytes to MST7 led to an acute increase in [ATPe], followed by a slower increase phase. ATPe kinetics reflected a strong activation of ATP efflux and a low rate of ATPe hydrolysis by ectoATPase activity. Enhancement of [ATPe] by MST7 required adhesion of erythrocytes to poly-D-lysin-coated coverslips, and correlated with a 31% increase of cAMP and 10% cell swelling. However, when MST7 was dissolved in a hyperosmotic medium to block cell swelling, ATPe accumulation was inhibited by 49%. Erythrocytes pre-exposure to 10μM of either carbenoxolone or probenecid, two blockers of pannexin 1, exhibited a partial reduction of ATP efflux. Erythrocytes from pannexin 1 knockout mice exhibited similar ATPe kinetics as those of wild type mice erythrocytes exposed to pannexin 1 blockers.

CONCLUSIONS

MST7 induced release of ATP required either cell adhesion or strong activation of cAMP synthesis. Part of this release required cell swelling. Kinetic analysis and a data driven model suggested that ATP efflux is mediated by two ATP conduits displaying different kinetics, with one conduit being fully blocked by pannexin 1 blockers.

GENERAL SIGNIFICANCE

Kinetic analysis of extracellular ATP accumulation from human erythrocytes and potential effects on microcirculation.

Vesicular nucleotide transporter regulates the nucleotide content in airway epithelial mucin granules

Nucleotides within the airway surface liquid promote fluid secretion via activation of airway epithelial purinergic receptors. ATP is stored within and released from mucin granules as co-cargo with mucins, but the mechanism by which ATP, and potentially other nucleotides, enter the lumen of mucin granules is not known. We assessed the contribution of the recently identified SLC17A9 vesicle nucleotide transporter (VNUT) to the nucleotide availability within isolated mucin granules and further examined the involvement of VNUT in mucin granule secretion-associated nucleotide release. RT-PCR and Western blot analyses indicated that VNUT is abundantly expressed in airway epithelial goblet-like Calu-3 cells, migrating as a duplex with apparent mobility of 55 and 60 kDa. Subcellular fractionation studies indicated that VNUT55 was associated with high-density mucin granules, whereas VNUT60 was associated with low-density organelles. Immunofluorescence studies showed that recombinant VNUT localized to mucin granules and other organelles. Mucin granules isolated from VNUT short hairpin RNA-expressing cells exhibited a marked reduction of ATP, ADP, AMP, and UTP levels within granules. Ca(2+)-regulated vesicular ATP release was markedly reduced in these cells, but mucin secretion was not affected. These results suggest that VNUT is the relevant nucleotide transporter responsible for the uptake of cytosolic nucleotides into mucin granules. By controlling the entry of nucleotides into mucin granules, VNUT contributes to the release of purinergic signaling molecules necessary for the proper hydration of co-released mucins.

PGC-1α rescues Huntington's disease proteotoxicity by preventing oxidative stress and promoting TFEB function

Huntington's disease (HD) is caused by CAG repeat expansions in the huntingtin (htt) gene, yielding proteins containing polyglutamine repeats that become misfolded and resist degradation. Previous studies demonstrated that mutant htt interferes with transcriptional programs coordinated by the peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α (PGC-1α), a regulator of mitochondrial biogenesis and oxidative stress. We tested whether restoration of PGC-1α could ameliorate the symptoms of HD in a mouse model. We found that PGC-1α induction virtually eliminated htt protein aggregation and ameliorated HD neurodegeneration in part by attenuating oxidative stress. PGC-1α promoted htt turnover and the elimination of protein aggregates by activating transcription factor EB (TFEB), a master regulator of the autophagy-lysosome pathway. TFEB alone was capable of reducing htt aggregation and neurotoxicity, placing PGC-1α upstream of TFEB and identifying these two molecules as important therapeutic targets in HD and potentially other neurodegenerative disorders caused by protein misfolding.

The UDP-sugar-sensing P2Y(14) receptor promotes Rho-mediated signaling and chemotaxis in human neutrophils

The G(i)-coupled P2Y(14) receptor (P2Y(14)-R) is potently activated by UDP-sugars and UDP. Although P2Y(14)-R mRNA is prominently expressed in circulating neutrophils, the signaling pathways and functional responses associated with this receptor are undefined. In this study, we illustrate that incubation of isolated human neutrophils with UDP-glucose resulted in cytoskeleton rearrangement, change of cell shape, and enhanced cell migration. We also demonstrate that UDP-glucose promotes rapid, robust, and concentration-dependent activation of RhoA in these cells. Ecto-nucleotidases expressed on neutrophils rapidly hydrolyzed extracellular ATP, but incubation with UDP-glucose for up to 1 h resulted in negligible metabolism of the nucleotide-sugar. HL60 human promyelocytic leukemia cells do not express the P2Y(14)-R, but neutrophil differentiation of HL60 cells with DMSO resulted in markedly enhanced P2Y(14)-R expression. Accordingly, UDP-glucose, UDP-galactose, and UDP-N-acetylglucosamine promoted Rho activation in differentiated but not in undifferentiated HL60 cells. Stable expression of recombinant human P2Y(14)-R conferred UDP-sugar-promoted responses to undifferentiated HL60 cells. UDP-glucose-promoted RhoA activation also was accompanied by enhanced cell migration in differentiated HL60 cells, and these responses were blocked by Rho kinase inhibitors. These results support the notion that UDP-glucose is a stable and potent proinflammatory mediator that promotes P2Y(14)-R-mediated neutrophil motility via Rho/Rho kinase activation.

Toll-like receptor 4 engagement inhibits adenosine 5'-monophosphate-activated protein kinase activation through a high mobility group box 1 protein-dependent mechanism

Despite the potent antiinflammatory effects of pharmacologically induced adenosine 5'-monophosphate kinase (AMPK) activation on Toll-like receptor 4 (TLR4)-induced cellular activation, there is little evidence that AMPK is activated during inflammatory conditions. In the present studies, we examined mechanisms by which TLR4 engagement may affect the ability of AMPK to become activated in neutrophils and macrophages under in vitro conditions and in the lungs during lipopolysaccharide (LPS)-induced acute lung injury. We found that incubation of neutrophils or macrophages with LPS diminished the ability of 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) or hydrogen peroxide (H(2)O(2)) to activate AMPK. Although ratios of AMP to adenosine 5'-triphosphate (ATP) were increased in LPS-treated neutrophils and in the lungs of LPS exposed mice, a condition that should result in AMPK activation, no activation of AMPK was found. Immunocytochemistry and Western blot analysis revealed that nuclear to cytosolic translocation of the proinflammatory mediator high mobility group box 1 protein (HMGB1) correlated with inhibition of AMPK activation in LPS-stimulated macrophages. Moreover, while induced overexpression of HMGB1 resulted in inhibition of AMPK activation, Small interfering RNA (siRNA)-induced knockdown of HMGB1 was associated with enhanced activation of AMPK in macrophages incubated with AICAR. Increased interaction between liver kinase B1 (LKB1), an upstream activator of AMPK, and HMGB1 was found in LPS-stimulated macrophages and in the lungs of mice exposed to LPS. These results suggest that nuclear to cytoplasmic translocation of HMGB1 in TLR4-activated cells potentiates inflammatory responses by binding to LKB1, thereby inhibiting the antiinflammatory effects of AMPK activation.

Vesicular and conductive mechanisms of nucleotide release

Extracellular nucleotides and nucleosides promote a vast range of physiological responses, via activation of cell surface purinergic receptors. Virtually all tissues and cell types exhibit regulated release of ATP, which, in many cases, is accompanied by the release of uridine nucleotides. Given the relevance of extracellular nucleotide/nucleoside-evoked responses, understanding how ATP and other nucleotides are released from cells is an important physiological question. By facilitating the entry of cytosolic nucleotides into the secretory pathway, recently identified vesicular nucleotide and nucleotide-sugar transporters contribute to the exocytotic release of ATP and UDP-sugars not only from endocrine/exocrine tissues, but also from cell types in which secretory granules have not been biochemically characterized. In addition, plasma membrane connexin hemichannels, pannexin channels, and less-well molecularly defined ATP conducting anion channels have been shown to contribute to the release of ATP (and UTP) under a variety of conditions.

VAMP8 is a vesicle SNARE that regulates mucin secretion in airway goblet cells

Mucin secretion is an innate defence mechanism, which is noxiously upregulated in obstructive lung diseases (e.g. chronic obstructive pulmonary disease (COPD), cystic fibrosis and asthma). Mucin granule exocytosis is regulated by specific protein complexes, but the SNARE exocytotic core has not been defined in airway goblet cells. In this study, we identify VAMP8 as one of the SNAREs regulating mucin granule exocytosis. VAMP8 mRNA was present in human airway and lung epithelial cells, and deep-sequencing and expression analyses of airway epithelial cells revealed that VAMP8 transcripts were expressed at 10 times higher levels than other VAMP mRNAs. In human airway epithelial cell cultures and freshly excised tissues, VAMP8 immunolocalised mainly to goblet cell mucin granules. The function of VAMP8 in airway mucin secretion was tested by RNA interference techniques. Both VAMP8 short interfering RNAs (siRNAs) and short hairpin RNAs (shRNAs) reduced mucin secretion induced by PAR agonists, neutrophil elastase and ATP in two airway epithelial cell culture models. Notably, basal (non-agonist elicited) mucin secretion was also reduced in these experiments. VAMP8 knockdown was also effective in decreasing mucin secretion in airway epithelial cell cultures with induced mucous metaplasia/mucin hypersecretion. Unlike VAMP8 silencing, knockdown of VAMP2 or VAMP3 did not affect mucin secretion. Importantly, in VAMP8 knock-out (KO) mice with IL-13-induced mucous metaplasia, mucin content in the bronchoalveolar lavage (BAL) and ATP-stimulated mucin secretion in the trachea were reduced compared to WT-matched littermates. Our data indicate that VAMP8 is an essential SNARE in airway mucin granule exocytosis. Reduction of VAMP8 activity/expression may provide a novel therapeutic target to ameliorate airway mucus obstruction in lung diseases.

Homeostasis of extracellular ATP in human erythrocytes

We explored the intra- and extracellular processes governing the kinetics of extracellular ATP (ATPe) in human erythrocytes stimulated with agents that increase cAMP. Using the luciferin-luciferase reaction in off-line luminometry we found both direct adenylyl cyclase activation by forskolin and indirect activation through β-adrenergic stimulation with isoproterenol-enhanced [ATP]e in a concentration-dependent manner. A mixture (3V) containing a combination of these agents and the phosphodiesterase inhibitor papaverine activated ATP release, leading to a 3-fold increase in [ATP]e, and caused increases in cAMP concentration (3-fold for forskolin + papaverine, and 10-fold for 3V). The pannexin 1 inhibitor carbenoxolone and a pannexin 1 blocking peptide ((10)Panx1) decreased [ATP]e by 75-84%. The residual efflux of ATP resulted from unavoidable mechanical perturbations stimulating a novel, carbenoxolone-insensitive pathway. In real-time luminometry experiments using soluble luciferase, addition of 3V led to an acute increase in [ATP]e to a constant value of ∼1 pmol × (10(6) cells)(-1). A similar treatment using a surface attached luciferase (proA-luc) triggered a rapid accumulation of surface ATP levels to a peak concentration of 2.4 pmol × (10(6) cells)(-1), followed by a slower exponential decay (t(&frac12;) = 3.7 min) to a constant value of 1.3 pmol × (10(6) cells)(-1). Both for soluble luciferase and proA-luc, ATP efflux was fully blocked by carbenoxolone, pointing to a 3V-induced mechanism of ATP release mediated by pannexin 1. Ecto-ATPase activity was extremely low (∼28 fmol × (10(6) cells min)(-1)), but nevertheless physiologically relevant considering the high density of erythrocytes in human blood.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

Rho signaling regulates pannexin 1-mediated ATP release from airway epithelia

ATP released from airway epithelial cells promotes purinergic receptor-regulated mucociliary clearance activities necessary for innate lung defense. Cell swelling-induced membrane stretch/strain is a common stimulus that promotes airway epithelial ATP release, but the mechanisms transducing cell swelling into ATP release are incompletely understood. Using knockdown and knockout approaches, we tested the hypothesis that pannexin 1 mediates ATP release from hypotonically swollen airway epithelia and investigated mechanisms regulating this activity. Well differentiated primary cultures of human bronchial epithelial cells subjected to hypotonic challenge exhibited enhanced ATP release, which was paralleled by the uptake of the pannexin probe propidium iodide. Both responses were reduced by pannexin 1 inhibitors and by knocking down pannexin 1. Importantly, hypotonicity-evoked ATP release from freshly excised tracheas and dye uptake in primary tracheal epithelial cells were impaired in pannexin 1 knockout mice. Hypotonicity-promoted ATP release and dye uptake in primary well differentiated human bronchial epithelial cells was accompanied by RhoA activation and myosin light chain phosphorylation and was reduced by the RhoA dominant negative mutant RhoA(T19N) and Rho and myosin light chain kinase inhibitors. ATP release and Rho activation were reduced by highly selective inhibitors of transient receptor potential vanilloid 4 (TRPV4). Lastly, knocking down TRPV4 impaired hypotonicity-evoked airway epithelial ATP release. Our data suggest that TRPV4 and Rho transduce cell membrane stretch/strain into pannexin 1-mediated ATP release in airway epithelia.

Nucleotide release by airway epithelia

The purinergic events regulating the airways' innate defenses are initiated by the release of purines from the epithelium, which occurs constitutively and is enhanced by chemical or mechanical stimulation. While the external triggers have been reviewed exhaustively, this chapter focuses on current knowledge of the receptors and signaling cascades mediating nucleotide release. The list of secreted purines now includes ATP, ADP, AMP and nucleotide sugars, and involves at least three distinct mechanisms reflecting the complexity of airway epithelia. First, the constitutive mechanism involves ATP translocation to the ER/Golgi complex as energy source for protein folding, and fusion of Golgi-derived vesicles with the plasma membrane. Second, goblet cells package ATP with mucins into granules, which are discharged in response to P2Y(2)R activation and Ca(2+)-dependent signaling pathways. Finally, non-mucous cells support a regulated mechanism of ATP release involving protease activated receptor (PAR)-elicited G(12/13) activation, leading to the RhoGEF-mediated exchange of GDP for GTP on RhoA, and cytoskeleton rearrangement. Together, these pathways provide fine tuning of epithelial responses regulated by purinergic signaling events.

Coupled nucleotide and mucin hypersecretion from goblet-cell metaplastic human airway epithelium

Adenosine triphosphate (ATP) and its metabolite adenosine regulate airway mucociliary clearance via activation of purinoceptors. In this study, we investigated the contribution of goblet cells to airway epithelial ATP release. Primary human bronchial epithelial (HBE) cultures, typically dominated by ciliated cells, were induced to develop goblet cell metaplasia by infection with respiratory syncytial virus (RSV) or treatment with IL-13. Under resting conditions, goblet-cell metaplastic cultures displayed enhanced mucin secretion accompanied by increased rates of ATP release and mucosal surface adenosine accumulation as compared with nonmetaplastic control HBE cultures. Intracellular calcium chelation [1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester] or disruption of the secretory pathways (nocodazole, brefeldin A, and N-ethylmaleimide) decreased mucin secretion and ATP release in goblet-cell metaplastic HBE cultures. Conversely, stimuli that triggered calcium-regulated mucin secretion (e.g., ionomycin or UTP) increased luminal ATP release and adenyl purine accumulation in control and goblet-cell metaplastic HBE cultures. Goblet cell-associated ATP release was not blocked by the connexin/pannexin hemichannel inhibitor carbenoxolone, suggesting direct nucleotide release from goblet cell vesicles rather than the hemichannel insertion. Collectively, our data demonstrate that nucleotide release is increased by goblet cell metaplasia, reflecting, at least in part, a mechanism tightly associated with goblet cell mucin secretion. Increased goblet cell nucleotide release and resultant adenosine accumulation provide compensatory mechanisms to hydrate mucins by paracrine stimulation of ciliated cell ion and water secretion and maintain mucociliary clearance, and to modulate inflammatory responses.

Exposure to hydrogen peroxide induces oxidation and activation of AMP-activated protein kinase

Although metabolic conditions associated with an increased AMP/ATP ratio are primary factors in the activation of 5'-adenosine monophosphate-activated protein kinase (AMPK), a number of recent studies have shown that increased intracellular levels of reactive oxygen species can stimulate AMPK activity, even without a decrease in cellular levels of ATP. We found that exposure of recombinant AMPKαβγ complex or HEK 293 cells to H(2)O(2) was associated with increased kinase activity and also resulted in oxidative modification of AMPK, including S-glutathionylation of the AMPKα and AMPKβ subunits. In experiments using C-terminal truncation mutants of AMPKα (amino acids 1-312), we found that mutation of cysteine 299 to alanine diminished the ability of H(2)O(2) to induce kinase activation, and mutation of cysteine 304 to alanine totally abrogated the enhancing effect of H(2)O(2) on kinase activity. Similar to the results obtained with H(2)O(2)-treated HEK 293 cells, activation and S-glutathionylation of the AMPKα subunit were present in the lungs of acatalasemic mice or mice treated with the catalase inhibitor aminotriazole, conditions in which intracellular steady state levels of H(2)O(2) are increased. These results demonstrate that physiologically relevant concentrations of H(2)O(2) can activate AMPK through oxidative modification of the AMPKα subunit. The present findings also imply that AMPK activation, in addition to being a response to alterations in intracellular metabolic pathways, is directly influenced by cellular redox status.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.

Receptor-promoted exocytosis of airway epithelial mucin granules containing a spectrum of adenine nucleotides

Purinergic regulation of airway innate defence activities is in part achieved by the release of nucleotides from epithelial cells. However, the mechanisms of airway epithelial nucleotide release are poorly understood. We have previously demonstrated that ATP is released from ionomycin-stimulated airway epithelial goblet cells coordinately with mucin exocytosis, suggesting that ATP is released as a co-cargo molecule from mucin-containing granules. We now demonstrate that protease-activated-receptor (PAR) agonists also stimulate the simultaneous release of mucins and ATP from airway epithelial cells. PAR-mediated mucin and ATP release were dependent on intracellular Ca(2+) and actin cytoskeleton reorganization since BAPTA AM, cytochalasin D, and inhibitors of Rho and myosin light chain kinases blocked both responses. To test the hypothesis that ATP is co-released with mucin from mucin granules, we measured the nucleotide composition of isolated mucin granules purified based on their MUC5AC and VAMP-8 content by density gradients. Mucin granules contained ATP, but the levels of ADP and AMP within granules exceeded by nearly 10-fold that of ATP. Consistent with this finding, apical secretions from PAR-stimulated cells contained relatively high levels of ADP/AMP, which could not be accounted for solely based on ATP release and hydrolysis. Thus, mucin granules contribute to ATP release and also are a source of extracellular ADP and AMP. Direct release of ADP/AMP from mucin granules is likely to provide a major source of airway surface adenosine to signal in a paracrine faction ciliated cell A(2b) receptors to activate ion/water secretion and appropriately hydrate goblet cell-released mucins.

Nucleotides released by apoptotic cells act as a find-me signal to promote phagocytic clearance

Phagocytic removal of apoptotic cells occurs efficiently in vivo such that even in tissues with significant apoptosis, very few apoptotic cells are detectable. This is thought to be due to the release of 'find-me' signals by apoptotic cells that recruit motile phagocytes such as monocytes, macrophages and dendritic cells, leading to the prompt clearance of the dying cells. However, the identity and in vivo relevance of such find-me signals are not well understood. Here, through several lines of evidence, we identify extracellular nucleotides as a critical apoptotic cell find-me signal. We demonstrate the caspase-dependent release of ATP and UTP (in equimolar quantities) during the early stages of apoptosis by primary thymocytes and cell lines. Purified nucleotides at these concentrations were sufficient to induce monocyte recruitment comparable to that of apoptotic cell supernatants. Enzymatic removal of ATP and UTP (by apyrase or the expression of ectopic CD39) abrogated the ability of apoptotic cell supernatants to recruit monocytes in vitro and in vivo. We then identified the ATP/UTP receptor P2Y(2) as a critical sensor of nucleotides released by apoptotic cells using RNA interference-mediated depletion studies in monocytes, and macrophages from P2Y(2)-null mice. The relevance of nucleotides in apoptotic cell clearance in vivo was revealed by two approaches. First, in a murine air-pouch model, apoptotic cell supernatants induced a threefold greater recruitment of monocytes and macrophages than supernatants from healthy cells did; this recruitment was abolished by depletion of nucleotides and was significantly decreased in P2Y(2)(-/-) (also known as P2ry2(-/-)) mice. Second, clearance of apoptotic thymocytes was significantly impaired by either depletion of nucleotides or interference with P2Y receptor function (by pharmacological inhibition or in P2Y(2)(-/-) mice). These results identify nucleotides as a critical find-me cue released by apoptotic cells to promote P2Y(2)-dependent recruitment of phagocytes, and provide evidence for a clear relationship between a find-me signal and efficient corpse clearance in vivo.

Quantification of Gi-mediated inhibition of adenylyl cyclase activity reveals that UDP is a potent agonist of the human P2Y14 receptor

The P2Y14 receptor was initially identified as a G protein-coupled receptor activated by UDP-glucose and other nucleotide sugars. We have developed several cell lines that stably express the human P2Y14 receptor, allowing facile examination of its coupling to native Gi family G proteins and their associated downstream signaling pathways (J Pharmacol Exp Ther 330:162-168, 2009). In the current study, we examined P2Y14 receptor-dependent inhibition of cyclic AMP accumulation in human embryonic kidney (HEK) 293, C6 glioma, and Chinese hamster ovary (CHO) cells stably expressing this receptor. Not only was the human P2Y14 receptor activated by UDP-glucose, but it also was activated by UDP. The apparent efficacies of UDP and UDP-glucose were similar, and the EC50 values (74, 33, and 29 nM) for UDP-dependent activation of the P2Y14 receptor in HEK293, CHO, and C6 glioma cells, respectively, were similar to the EC50 values (323, 132, and 72 nM) observed for UDP-glucose. UDP and UDP-glucose also stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in P2Y14 receptor-expressing HEK293 cells but not in wild-type HEK293 cells. A series of analogs of UDP were potent P2Y14 receptor agonists, but the naturally occurring nucleoside diphosphates, CDP, GDP, and ADP exhibited agonist potencies over 100-fold less than that observed with UDP. Two UDP analogs were identified that selectively activate the P2Y14 receptor over the UDP-activated P2Y6 receptor, and these molecules stimulated phosphorylation of ERK1/2 in differentiated human HL-60 promyeloleukemia cells, which natively express the P2Y14 receptor but had no effect in wild-type HL-60 cells, which do not express the receptor. We conclude that UDP is an important cognate agonist of the human P2Y14 receptor.

Quantification of extracellular UDP-galactose

The human P2Y(14) receptor is potently activated by UDP-glucose (UDP-Glc), UDP-galactose (UDP-Gal), UDP-N-acetylglucosamine (UDP-GlcNAc), and UDP-glucuronic acid. Recently, cellular release of UDP-Glc and UDP-GlcNAc has been reported, but whether additional UDP-sugars are endogenous agonists for the P2Y(14) receptor remains poorly defined. In the present study, we describe an assay for the quantification of UDP-Gal with subnanomolar sensitivity. This assay is based on the enzymatic conversion of UDP-Gal to UDP, using 1-4-beta-galactosyltransferase. UDP is subsequently phosphorylated by nucleoside diphosphokinase in the presence of [gamma-(32)P]ATP and the formation of [gamma-(32)P]UTP is monitored by high-performance liquid chromatography. The overall conversion of UDP-Gal to [gamma-(32)P]UTP was linear between 0.5 and 30 nM UDP-Gal. Extracellular UDP-Gal was detected on resting cultures of various cell types, and increased release of UDP-Gal was observed in 1321N1 human astrocytoma cells stimulated with the protease-activated receptor agonist thrombin. The occurrence of regulated release of UDP-Gal suggests that, in addition to its role in glycosylation reactions, UDP-Gal is an important extracellular signaling molecule.

Neutrophilic inflammation is associated with altered airway hydration in stable asthmatics

BACKGROUND

Airway dehydration is a potential trigger of bronchoconstriction in exercise-induced asthma; however, its role in stable asthma has not been explored. Using sputum percent solids, as an indicator of airway hydration, we sought relationships between airway hydration and other known markers of neutrophilic (TH1) and allergic (TH2) inflammation in stable asthma.

METHODS

Thirty-seven atopic subjects with stable asthma and 15 healthy controls underwent sputum induction. Sputum was analyzed for percent solids, cell counts, cellular and biochemical markers of inflammation and purines.

RESULTS

Sputum percent solids was significantly elevated in stable asthmatics vs. controls and positively correlated with markers of neutrophilic/TH1-type inflammation (neutrophils, IL-8 and AMP). Sputum percent solids were not correlated with markers of allergic/TH2-type inflammation. These data suggest a direct relationship between neutrophil inflammation and airway hydration in stable asthmatics.

Thrombin promotes release of ATP from lung epithelial cells through coordinated activation of rho- and Ca2+-dependent signaling pathways

Extracellular ATP controls key aspects of lung function via activation of epithelial cell purinergic receptors, but how ATP is released from cells remains poorly understood. To identify mechanistic components upstream of ATP release, we examined the effect of selected G protein coupled-receptor activation on ATP release from lung epithelial cells. The protease-activated receptor (PAR) agonist thrombin elicited a rapid Ca(2+)-dependent release of ATP from A549 cells. In contrast, the P2Y(2) receptor agonist UTP caused negligible ATP release, despite promoting a robust Ca(2+) response. Agonist-elicited ATP release was associated with Rho activation and was reduced in cells transfected with dominant negative mutants of p115-Rho GEF or RhoA, and by inhibitors of Rho kinase (ROCK). However, RhoA activation alone did not promote ATP release if temporally separated from Ca(2+) mobilization. PAR3 was the only PAR subtype detected in A549 cells by reverse transcription-PCR. Transfection of cells with human PAR3 cDNA increased thrombin-promoted ATP release, inositol phosphate formation, and RhoA activation. Conversely, small interference RNA against PAR3 diminished thrombin-evoked responses. Thrombin-elicited ATP release was accompanied by an enhanced cellular uptake of propidium iodide in a Ca(2+)- and ROCK-dependent manner and was inhibited by connexin/pannexin hemichannel blockers. Our data suggest that thrombin promotes ATP release from A549 cells via Rho- and Ca(2+)-dependent activation of connexin/pannexin hemichannels. The relevance of these findings is highlighted by the observation that exposure of primary cultures of well differentiated human bronchial epithelial cells to thrombin resulted in robust ATP release, which was inhibited by ROCK inhibitors and by connexin/pannexin hemichannel blockers.

Endoplasmic reticulum/golgi nucleotide sugar transporters contribute to the cellular release of UDP-sugar signaling molecules

Extracellular UDP-sugars promote cellular responses by interacting with widely distributed P2Y(14) receptors, but the mechanisms by which these molecules are released from cells are poorly understood. Given the active role of UDP-sugars in glycosylation reactions within the secretory pathway, we hypothesized that UDP-sugar release includes an exocytotic component. This hypothesis was tested by assessing the contribution of endoplasmic reticulum (ER)/Golgi-resident UDP-GlcNAc transporters to the cellular release of their cognate substrates. A sensitive and highly selective assay for UDP-GlcNAc mass was developed using purified AGX2, an isoenzyme of human UDP-GlcNAc pyrophosphorylase. Robust constitutive release of UDP-GlcNAc was observed in yeast as well as in well differentiated human airway epithelial cells. The human UDP-GlcNAc transporter HFRC1 was overexpressed in human bronchial epithelial cells and was shown to localize in the Golgi and to enhance the surface expression of N-acetylglucosamine-rich glycans. HFRC1-overexpressing cells also displayed increased constitutive and hypotonic stress-stimulated release of UDP-GlcNAc. Yeast mutants lacking Yea4 (the ER UDP-GlcNAc transporter endogenously expressed in Saccharomyces cerevisiae) showed reduced UDP-GlcNAc release. Yea4-deficient cells complemented with Yea4 showed UDP-GlcNAc release rates at levels similar to or higher than wild type cells. Our results illustrate that ER/Golgi lumen constitutes a significant source of extracellular UDP-sugars and therefore plays a critical role in nucleotide sugar-promoted cell signaling.

Gi-dependent cell signaling responses of the human P2Y14 receptor in model cell systems

Eight G protein-coupled receptors comprise the P2Y receptor family of cell signaling proteins. The goal of the current study was to define native cell signaling pathways regulated by the uridine nucleotide sugar-activated P2Y(14) receptor (P2Y(14)-R). The P2Y(14)-R was stably expressed in human embryonic kidney (HEK) 293 and C6 rat glioma cells by retroviral infection. Nucleotide sugar-dependent P2Y(14)-R activation was examined by measuring inhibition of forskolin-stimulated cAMP accumulation. The effect of P2Y(14)-R activation on mitogen-activated protein kinase signaling also was studied in P2Y(14)-HEK293 cells and in differentiated HL-60 human myeloid leukemia cells. UDP-Glc, UDP-galactose, UDP-glucuronic acid, and UDP-N-acetylglucosamine promoted inhibition of forskolin-stimulated cAMP accumulation in P2Y(14)-HEK293 and P2Y(14)-C6 cells, and this signaling effect was abolished by pretreatment of cells with pertussis toxin. Inhibition of cAMP formation by nucleotide sugars also was observed in direct assays of adenylyl cyclase activity in membranes prepared from P2Y(14)-C6 cells. UDP-Glc promoted concentration-dependent and pertussis toxin-sensitive extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in P2Y(14)-HEK293 cells. P2Y(14)-R mRNA was not observed in wild-type HL-60 cells but was readily detected in dimethyl sulfoxide-differentiated cells. Consistent with this observation, no effect of UDP-Glc was observed in wild-type HL-60 cells, but UDP-Glc-promoted pertussis toxin-sensitive activation of ERK1/2 occurred after differentiation. These results illustrate that the human P2Y(14)-R signals through G(i) to inhibit adenylyl cyclase, and P2Y(14)-R activation also leads to ERK1/2 activation. This work also identifies two stable P2Y(14)-R-expressing cell lines and differentiated HL-60 cells as model systems for the study of P2Y(14)-R-dependent signal transduction.

Purinergic receptors in airway epithelia

Nucleotides and nucleosides within the airway surface liquid regulate mucociliary clearance (MCC) activities, the primary innate defense mechanism that removes foreign particles and pathogens from airway surfaces. Nucleotide and nucleoside actions in the airways are mediated mainly by two purinergic receptor subtypes, the Gq-coupled ATP/UTP-sensing P2Y2 receptor and the Gs-coupled A2b adenosine receptor. Activation of the A2b receptor results in cyclic AMP-dependent activation of the cystic fibrosis transmembrane regulator (CFTR) Cl- channel and stimulation of ciliary beat frequency. Agonist occupation of the P2Y2 receptor promotes inhibition of Na+ absorption as well as CFTR-dependent and CFTR-independent Cl- secretion, ciliary beating, and mucin secretion.

[Purinergic signals]

In the last decade evidence accumulated that nucleosides and nucleotides of both uridine and adenine can act as extracellular signaling factors. Their action is mediated by two main types of surface receptors commonly known as purinergic. P1 receptors are metabotropic and activated by adenosine, whereas receptors for nucleotides (ATP, ADP, UTP and UDP) and nucleotide-sugars (UDP-glucose and UDP-galactose) can be either metabotropic (P2Y) or ionotropic (P2X). The importance and complexity of this signaling system is evidenced by various mechanisms of nucleotide release, as well as by the ibiquitous distribution of various types of ectonucleotidases which catalyze and convert extracellular nucleotides. Up to now about twenty receptors have been cloned and found to modulate the nerve impulse, inflammatory response, insuline secretion, the regulation of the vascular tone and nociception, among other processes. In the present review we describe the main structural and pharmacological features of purinergic receptors, and analyze how the dynamic interaction between these receptors, nucleotides and nucleosides, and ectonucleotidases modulate several biological responses. Particular focus is given to platelet aggregation and thrombus formation, the immune response and the hydration of the mucosal linings of the respiratory tract.

Assessment of extracellular ATP concentrations

Most cells release ATP to the extracellular milieu. Extracellular ATP plays important signaling roles by activating a score of broadly distributed cell surface purinergic receptors (purinoceptors). Biological responses regulated by purinergic receptors include neurotransmission, smooth muscle relaxation and contraction, epithelial cell ion transport, inflammation, platelet activation, immune responses, cardiac function, endocrine and exocrine secretion, glucose transport, and cell proliferation. ATP concentrations at the cell surface, and consequently the magnitude of purinergic receptor stimulation, reflect a well-controlled balance between rates of ATP release and extracellular metabolism. Given the broad spectrum of responses triggered by extracellular ATP, there is a growing interest in accurately assessing the concentrations of this nucleotide at the cell surface. In this chapter, we discuss the use of the luciferin/luciferase-based reaction to measure extracellular ATP concentrations with high sensitivity. Protocols are adapted to assess ATP levels either in sampled extracellular fluids or in situ at the cell surface. Although our focus is on studies of ATP release from epithelial cells, protocols described here are applicable to practically all cell types.

Similarities between UDP-glucose and adenine nucleotide release in yeast: involvement of the secretory pathway

Extracellular UDP-glucose is a natural purinergic receptor agonist, but its mechanisms of cellular release remain unclear. We studied these mechanisms in Saccharomyces cerevisiae, a simple model organism that releases ATP, another purinergic agonist. Similar to ATP, UDP-glucose was released by S. cerevisiae at a rate that was linear over time. However, unlike ATP release, UDP-glucose release was not dependent on glucose stimulation. This discrepancy was resolved by demonstrating the apparent glucose stimulation of ATP release reflected glucose-dependent changes in the intracellular pattern of adenine nucleotides, with AMP release dominating in the absence of glucose. Indeed, total adenine nucleotide release, like UDP-glucose release, did not vary with glucose concentration over the short term. The genetic basis of UDP-glucose release was explored through analysis of deletion mutants, aided by development of a novel bioassay for UDP-glucose based on signaling through heterologously expressed human P2Y 14 receptors. Using this assay, an elevated rate of UDP-glucose release was demonstrated in mutants lacking the putative Golgi nucleotide sugar transporter YMD8. An increased rate of UDP-glucose release in ymd8Delta was reduced by deletion of the YEA4 UDP- N-acetylglucosamine or the HUT1 UDP-galactose transporters, and overexpression of YEA4 or HUT1 increased the rate of UDP-glucose release. These findings suggest an exocytotic release mechanism similar to that of ATP, a conclusion supported by decreased rates of ATP, AMP, and UDP-glucose release in response to the secretory inhibitor Brefeldin A. These studies demonstrate the involvement of the secretory pathway in nucleotide and nucleotide sugar efflux in yeast and offer a powerful model system for further investigation.

Influenza A virus inhibits alveolar fluid clearance in BALB/c mice

RATIONALE

Pulmonary infections can impair alveolar fluid clearance (AFC), contributing to formation of lung edema. Effects of influenza A virus (IAV) on AFC are unknown.

OBJECTIVES

To determine effects of IAV infection on AFC, and to identify intercellular signaling mechanisms underlying influenza-mediated inhibition of AFC.

METHODS

BALB/c mice were infected intranasally with influenza A/WSN/33 (10,000 or 2,500 focus-forming units per mouse). AFC was measured in anesthetized, ventilated mice by instilling 5% bovine serum albumin into the dependent lung.

MEASUREMENTS AND MAIN RESULTS

Infection with high-dose IAV resulted in a steady decline in arterial oxygen saturation and increased lung water content. AFC was significantly inhibited starting 1 hour after infection, and remained suppressed through Day 6. AFC inhibition at early time points (1-4 h after infection) did not require viral replication, whereas AFC inhibition later in infection was replication-dependent. Low-dose IAV infection impaired AFC for 10 days, but induced only mild hypoxemia. High-dose IAV infection increased bronchoalveolar lavage fluid ATP and UTP levels. Impaired AFC at Day 2 resulted primarily from reduced amiloride-sensitive AFC, mediated by increased activation of the pyrimidine-P2Y purinergic receptor axis. However, an additional component of AFC impairment was due to activation of A(1) adenosine receptors and stimulation of increased cystic fibrosis transmembrane regulator-mediated anion secretion. Finally, IAV-mediated inhibition of AFC at Day 2 could be reversed by addition of beta-adrenergic agonists to the AFC instillate.

CONCLUSIONS

AFC inhibition may be an important feature of early IAV infection. Its blockade may reduce the severity of pulmonary edema and hypoxemia associated with influenza pneumonia.

Mathematical model of nucleotide regulation on airway epithelia. Implications for airway homeostasis

In the airways, adenine nucleotides support a complex signaling network mediating host defenses. Released by the epithelium into the airway surface liquid (ASL) layer, they regulate mucus clearance through P2 (ATP) receptors, and following surface metabolism through P1 (adenosine; Ado) receptors. The complexity of ASL nucleotide regulation provides an ideal subject for biochemical network modeling. A mathematical model was developed to integrate nucleotide release, the ectoenzymes supporting the dephosphorylation of ATP into Ado, Ado deamination into inosine (Ino), and nucleoside uptake. The model also includes ecto-adenylate kinase activity and feed-forward inhibition of Ado production by ATP and ADP. The parameters were optimized by fitting the model to experimental data for the steady-state and transient concentration profiles generated by adding ATP to polarized primary cultures of human bronchial epithelial (HBE) cells. The model captures major aspects of ATP and Ado regulation, including their >4-fold increase in concentration induced by mechanical stress mimicking normal breathing. The model also confirmed the independence of steady-state nucleotide concentrations on the ASL volume, an important regulator of airway clearance. An interactive approach between simulations and assays revealed that feed-forward inhibition is mediated by selective inhibition of ecto-5'-nucleotidase. Importantly, the model identifies ecto-adenylate kinase as a key regulator of ASL ATP and proposes novel strategies for the treatment of airway diseases characterized by impaired nucleotide-mediated clearance. These new insights into the biochemical processes supporting ASL nucleotide regulation illustrate the potential of this mathematical model for fundamental and clinical research.

Thrombin-promoted release of UDP-glucose from human astrocytoma cells

BACKGROUND AND PURPOSE

The P2Y14 receptor is activated by UDP-sugars, most potently by UDP-glucose, but not by free nucleotides, suggesting that UDP-glucose is the cognate agonist for this receptor. However, evidence for regulated release of UDP-glucose is scarce. In the present study, the occurrence of receptor-promoted release of UDP-glucose was investigated, using 1321N1 human astrocytoma cells.

EXPERIMENTAL APPROACH

UDP-glucose release and hydrolysis were measured using HPLC-based techniques. Phospholipase C activation and actin cytoskeleton reorganization were assessed by measuring inositol phosphate formation and fluorescence confocal microscopy, respectively.

KEY RESULTS

Thrombin and the protease-activating receptor-1 (PAR1) peptide TFLLRNPNDK (PAR1-AP) evoked the release of UDP-glucose and ATP, which was accompanied by enhanced inositol phosphate formation. Although carbachol promoted fourfold greater inositol phosphate formation than thrombin, it failed to promote nucleotide release. Thrombin-promoted nucleotide release was inhibited by BAPTA-AM, brefeldin A and cytochalasin D, and was insensitive to Pertussis toxin and PI3-kinase inhibitors. Thrombin, but not carbachol, induced actin cytoskeleton reorganization, a hallmark of Rho activation in 1321N1 cells. However, PAR-promoted UDP-glucose release was not affected by Rho kinase inhibition.

CONCLUSIONS AND IMPLICATIONS

PAR1-evoked UDP-glucose release reflected a Ca2+-dependent mechanism, engaging additional signalling independently of G(i) and Rho kinase activation and requiring a functional actin cytoskeleton and Golgi structures. Our study demonstrates the occurrence of Ca2+ -dependent release of UDP-glucose from astrocytoma cells in response to a physiologically relevant stimulus, that is, a G-protein-coupled receptor agonist. Given the presence of P2Y14 receptors in astrocytes, UDP-glucose may have important autocrine/paracrine functions in the brain.

Extracellular purines are biomarkers of neutrophilic airway inflammation

Purinergic signalling regulates airway defence mechanisms, suggesting that extracellular purines could serve as airway inflammation biomarkers in cystic fibrosis (CF). The purines adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and adenosine were measured in sputum from 21 adults (spontaneously expectorated from seven CF patients, induced from 14 healthy controls) to assess normal values and CF-associated changes. Subsequently, purine levels were measured in bronchoalveolar lavage fluid (BALF) from 37 children (25 CF patients, 12 disease controls) and compared with neutrophil counts, presence of airway infection and lung function. To noninvasively assess airway purines, ATP levels were measured using luminometry in exhaled breath condensate (EBC) from 14 children with CF and 14 healthy controls, then 14 CF children during a pulmonary exacerbation. Both ATP and AMP were elevated in sputum and BALF from CF subjects compared with controls. In BALF, ATP and AMP levels were inversely related to lung function and strongly correlated with neutrophil counts. In EBC, ATP levels were increased in CF relative to controls and decreased after treatment of CF pulmonary exacerbation. The purines adenosine triphosphate and adenosine monophosphate are candidate biomarkers of neutrophilic airways inflammation. Measurement of purines in sputum or exhaled breath condensate may provide a relatively simple and noninvasive method to track this inflammation.

UDP is a competitive antagonist at the human P2Y14 receptor

G protein-coupled P2Y receptors (P2Y-R) are activated by adenine and uracil nucleotides. The P2Y(14) receptor (P2Y(14)-R) is activated by at least four naturally occurring UDP sugars, with UDP-glucose (UDP-Glc) being the most potent agonist. With the goal of identifying a competitive antagonist for the P2Y(14)-R, UDP was examined for antagonist activity in COS-7 cells transiently expressing the human P2Y(14)-R and a chimeric Galpha protein that couples Gi-coupled receptors to stimulation of phosphoinositide hydrolysis. UDP antagonized the agonist action of UDP-Glc, and Schild analysis confirmed that the antagonism was competitive (pK(B) = 7.28). Uridine 5'-O-thiodiphosphate also antagonized the human P2Y(14)-R (hP2Y(14)-R) with an apparent affinity similar to that of UDP. In contrast, no antagonist activity was observed with ADP, CDP, or GDP, and other uracil analogs also failed to exhibit antagonist activity. The antagonist activity of UDP was not observed at other hP2Y-R. In contrast to its antagonist action at the hP2Y(14)-R, UDP was a potent agonist (EC(50) = 0.35 muM) at the rat P2Y(14)-R. These results identify the first competitive antagonist of the P2Y(14)-R and demonstrate pharmacological differences between receptor orthologs.

Adenosine Deaminase 1 and Concentrative Nucleoside Transporters 2 and 3 Regulate Adenosine on the Apical Surface of Human Airway Epithelia: Implications for Inflammatory Lung Diseases

Adenosine is a multifaceted signaling molecule mediating key aspects of innate and immune lung defenses. However, abnormally high airway adenosine levels exacerbate inflammatory lung diseases. This study identifies the mechanisms regulating adenosine elimination from the apical surface of human airway epithelia. Experiments conducted on polarized primary cultures of nasal and bronchial epithelial cells showed that extracellular adenosine is eliminated by surface metabolism and cellular uptake. The conversion of adenosine to inosine was completely inhibited by the adenosine deaminase 1 (ADA1) inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA). The reaction exhibited Km and Vmax values of 24 microM and 0.14 nmol x min(-1) x cm(-2). ADA1 (not ADA2) mRNA was detected in human airway epithelia. The adenosine/mannitol permeability coefficient ratio (18/1) indicated a minor contribution of paracellular absorption. Adenosine uptake was Na+-dependent and was inhibited by the concentrative nucleoside transporter (CNT) blocker phloridzin but not by the equilibrative nucleoside transporter (ENT) blocker dipyridamole. Apparent Km and Vmax values were 17 microM and 7.2 nmol x min(-1) x cm(-2), and transport selectivity was adenosine = inosine = uridine > guanosine = cytidine > thymidine. CNT3 mRNA was detected throughout the airways, while CNT2 was restricted to nasal epithelia. Inhibition of adenosine elimination by EHNA or phloridzin raised apical adenosine levels by >3-fold and stimulated IL-13 and MCP-1 secretion by 6-fold. These responses were reproduced by the adenosine receptor agonist 5'-(N-ethylcarboxamido)adenosine (NECA) and blocked by the adenosine receptor antagonist, 8-(p-sulfophenyl) theophylline (8-SPT). This study shows that adenosine elimination on human airway epithelia is mediated by ADA1, CNT2, and CNT3, which constitute important regulators of adenosine-mediated inflammation.

Coordinated release of nucleotides and mucin from human airway epithelial Calu-3 cells

The efficiency of the mucociliary clearance (MCC) process that removes noxious materials from airway surfaces depends on the balance between mucin secretion, airway surface liquid (ASL) volume, and ciliary beating. Effective mucin dispersion into ASL requires salt and water secretion onto the mucosal surface, but how mucin secretion rate is coordinated with ion and, ultimately, water transport rates is poorly understood. Several components of MCC, including electrolyte and water transport, are regulated by nucleotides in the ASL interacting with purinergic receptors. Using polarized monolayers of airway epithelial Calu-3 cells, we investigated whether mucin secretion was accompanied by nucleotide release. Electron microscopic analyses of Calu-3 cells identified subapical granules that resembled goblet cell mucin granules. Real-time confocal microscopic analyses revealed that subapical granules, labelled with FM 1-43 or quinacrine, were competent for Ca(2+)-regulated exocytosis. Granules containing MUC5AC were apically secreted via Ca(2+)-regulated exocytosis as demonstrated by combined immunolocalization and slot blot analyses. In addition, Calu-3 cells exhibited Ca(2+)-regulated apical release of ATP and UDP-glucose, a substrate of glycosylation reactions within the secretory pathway. Neither mucin secretion nor ATP release from Calu-3 cells were affected by activation or inhibition of the cystic fibrosis transmembrane conductance regulator. In SPOC1 cells, an airway goblet cell model, purinergic P2Y(2) receptor-stimulated increase of cytosolic Ca(2+) concentration resulted in secretion of both mucins and nucleotides. Our data suggest that nucleotide release is a mechanism by which mucin-secreting goblet cells produce paracrine signals for mucin hydration within the ASL.

Post-infection A77-1726 blocks pathophysiologic sequelae of respiratory syncytial virus infection

Despite respiratory syncytial virus (RSV) bronchiolitis remaining the most common cause of lower respiratory tract disease in infants worldwide, treatment has progressed little in the past 30 years. The aim of our study was to determine whether post-infection administration of de novo pyrimidine synthesis inhibitors could prevent the reduction in alveolar fluid clearance (AFC) and hypoxemia that occurs at Day 2 after intranasal infection of BALB/c mice with RSV. BALB/c mice were infected intranasally with RSV strain A2. AFC was measured in anesthetized, ventilated mice after instillation of 5% bovine serum albumin into the dependent lung. Post-infection systemic treatment with leflunomide has no effect on AFC. However, when added to the AFC instillate, leflunomide's active metabolite, A77-1726, blocks RSV-mediated inhibition of AFC at Day 2. This block is reversed by uridine (which allows pyrimidine synthesis via the scavenger pathway) and not recapitulated by genistein (which mimics the tyrosine kinase inhibitor effects of A77-1726), indicating that the effect is specific for the de novo pyrimidine synthesis pathway. More importantly, when administered intranasally at Day 1, A77-1726, but not its vehicle dimethyl sulfoxide, maintains its beneficial effect on AFC and lung water content until Day 2. Intranasal instillation of A77-1726 at Day 1 also reduces bronchoalveolar lavage nucleotide levels, lung inflammation, and hypoxemia at Day 2 without impairing viral replication at Day 2 or viral clearance at Day 8. Post-infection intranasal or aerosolized treatment with pyrimidine synthesis inhibitors may provide symptomatic relief from the pathophysiologic sequelae of impaired AFC in children with RSV bronchiolitis.