O’Grady, Scott M.

Exposure of human bronchial epithelial (HBE) cells from normal and asthmatic subjects to Alternaria alternata extract evoked rapid and sustained ATP release with greater efficacy observed in cells from asthmatic subjects. Previously, Alternaria aeroallergens were shown to elicit sustained increases in intracellular [Ca 2+ ] i that depended on coordinated activation of P2Y 2 and P2X 7 receptors. In this study, pretreatment with BAPTA‐AM inhibited ATP release, indicating dependency on increases in [Ca 2+ ] i . Alternaria ‐evoked ATP release exhibited a greater peak response and higher EC 50 value in cells obtained from asthmatic subjects compared to normal controls. Furthermore, the maximum increase in [Ca 2+ ] i was greater in cells from asthmatic subjects. The vesicle transport inhibitor brefeldin A blocked Alternaria ‐stimulated incorporation of fluorescent lipid (FM1–43) labeled vesicles into the plasma membrane and ATP release. Additionally, inhibiting vesicular ATP uptake with bafilomycin also blocked ATP release similar to brefeldin A. These results indicated that a primary mechanism for Alternaria ‐induced ATP release involved Ca 2+ ‐dependent vesicular transport. Serine and cysteine protease inhibitors also reduced Alternaria ‐induced ATP release. This result was consistent with our previous data showing that Alternaria ‐evoked increases in [Ca 2+ ] i depend on purinoceptor signaling.

Alternaria aeroallergens have been previously shown to stimulate allergic inflammation and exacerbation of respiratory distress in patients with asthma. An early event following exposure to Alternaria extract is the sustained release of ATP and a persistent increase in intracellular [Ca 2+ ] ([Ca 2+ ] i ) resulting from uptake of Ca 2+ ions from the extracellular solution. In the present study we tested the hyopothesis that the release of ATP and the subsequent increase in [Ca 2+ ] i was due to oxidative stress induced by exposure to Alternaria . ATP release following apical stimulation with Alternaria was measured in human primary bronchial epithelial cells using a luciferase/luciferin ATP measurement kit in combination with a modified luminometer that allows for tracking ATP release in real time. Pretreatment with glutathione (GSH) produced a concentration‐dependent inhibition of ATP release with an EC 50 = 500 μM. Furthermore, GSH blocked the increase in [Ca 2+ ] i typically observed following apical exposure to Alternaria. Moreover an increase in the expression of specific antioxidant protein mRNAs was also observed after 12 hours of treatment with Alternaria extract. These proteins included peroxiredoxins 1‐6, keratin 1, cyclooxygenase 2 (COX2) and oxidative stress responsive 1 protein with COX2 having the highest level of expression (~20 fold). These results implicate oxidative stress as an important initiating event in Alternaria evoked airway inflammation. Grant Funding Source : Supported by NIH: HL110539

Allergens derived from the common fungus Alternaria alternata induce oxidative stress in human airway epithelial (HBE) cells that leads to the regulated release of ATP and subsequent activation of P2X 7 receptors that facilitate Ca 2+ uptake into the cell. Sustained increases in [Ca 2+ ] i are required for nuclear processing and secretion of IL‐33, which initiates allergic inflammation by activating ST2 receptors on immune cells to induce Th2 cytokine secretion. Pretreatment of HBE cells with the K2P channel blockers bupivacaine or quinidine produced a significant increase in ATP release following Alternaria stimulation. This response was blocked by inhibiting ATP loading into membrane vesicles by treatment with 200 nM bafilomycin A 1 before allergen exposure. Similarly, pretreatment with the PKC inhibitor Go6976 also blocked approximately 50% of Alternaria ‐induced ATP release and completely abolished the increase in ATP release produced by pretreatment with K2P channel blockers. The effects of bupivacaine and quinidine were comparable to the effect of phorbol 12‐myristate 13‐acetate (PMA, 2 mM), a well‐characterized PKC activating compound prior to Alternaria stimulation. These results support the conclusion that K2P channel inhibition is associated with PKC activation, which functions to potentiate the effects of Alternaria allergens on vesicular ATP release. We propose that enhanced PKC activity is dependent on membrane depolarization associated with inhibition of constitutive, K2P channel mediated K + efflux from these cells. Support or Funding Information This study was supported by grants from the NIH (R01‐AI128729), awarded to HK and SMO and from the Minnesota Agriculture Experiment Station (AES‐0016097) awarded to SMO.

Key points Alternaria aeroallergens induce the release of ATP from human bronchial epithelial (HBE) cells by activating a conductive pathway involving voltage‐dependent anion channel‐1 (VDAC‐1) and by exocytosis of ATP localized within membrane vesicles. Inhibition of VDAC‐1 blocked Alternaria‐evoked Ca2+ uptake across the plasma membrane of HBE cells and interleukin (IL)‐33 release into the extracellular media. Reducing cholesterol content with a cholesterol scavenger (β‐methylcyclodextrin) or statin compound (simvastatin) blocked ATP and IL‐33 release by lowering the expression of VDAC‐1 in the plasma membrane. Pretreatment with simvastatin for 24 h also inhibited the increase in tight junction macromolecule permeability that occurs following Alternaria exposure. These results establish a novel role for VDAC‐1 as a mechanism underlying ATP release induced by fungal allergens and suggests a possible therapeutic use for cholesterol lowering compounds in reducing Alternaria‐stimulated allergic inflammation. AbstractHuman bronchial epithelial (HBE) cells exposed to allergens derived from the common saprophytic fungus, Alternaria alternata release ATP, which in turn stimulates P2X7 receptor‐mediated Ca2+ uptake across the plasma membrane. The subsequent increase in intracellular calcium concentration induces proteolytic processing and secretion of interleukin (IL)‐33, a critical cytokine involved in the initiation of allergic airway inflammation. A major objective of the present study was to identify the mechanism responsible for conductive ATP release. The results show that pretreatment of HBE cells with inhibitors of the voltage‐dependent anion channel‐1 (VDAC‐1) or treatment with a VDAC‐1 selective blocking antibody or silencing mRNA expression of the channel by RNA interference, inhibit Alternaria‐evoked ATP release. Moreover, inhibition of VDAC‐1 channel activity or reducing protein expression blocked the secretion of IL‐33. Similarly, reducing the cholesterol content of HBE cells with simvastatin or the cholesterol scavenger β‐methylcyclodextrin also blocked ATP release and IL‐33 secretion by decreasing the level of VDAC‐1 expression in the plasma membrane. In addition, simvastatin inhibited the increase in tight junction macromolecule permeability that was previously observed after Alternaria exposure. These results demonstrate a novel function for VDAC‐1 as the conductive mechanism responsible for Alternaria‐induced ATP release, an essential early step in the processing, mobilization and secretion of IL‐33 by the airway epithelium. Furthermore, the simvastatin‐evoked reduction of VDAC‐1 expression in the plasma membrane, suggests the possibility that cholesterol lowering compounds may be beneficial in alleviating allergic airway inflammation induced by fungal allergens.

Aeroallergens produced by Alternaria alternata can elicit life-threatening exacerbations of asthma in patients sensitized to this fungus. In this study, the effect of Alternaria on ion transport mechanisms underlying mucociliary clearance and airway epithelial barrier function was investigated in human airway epithelial cells. Apical exposure to Alternaria induced an increase in anion secretion that was inhibited by blockers of CFTR and Ca2+-activated Cl− channels. Stimulation of anion secretion was dependent on Ca2+ uptake from the apical solution. Alternaria exposure also produced an increase in reactive oxygen species (ROS) that was blocked by pretreatment with the oxidant scavenger glutathione (GSH). GSH and the NADPH oxidase inhibitor/complex 1 electron transport inhibitor diphenylene iodonium chloride (DPI) blocked ATP release and the increase in intracellular [Ca2+] evoked by Alternaria. Alternaria also decreased transepithelial resistance, and a portion of this effect was dependent on the increase in ROS. However, the Alternaria-induced increase in unidirectional dextran (molecular mass = 4,000 Da) flux across the epithelium could not be accounted for by increased oxidative stress. These results support the conclusion that oxidative stress induced by Alternaria was responsible for regulating Ca2+-dependent anion secretion and tight junction electrical resistance that would be expected to affect mucociliary clearance.

AbstractBackgroundInterleukin (IL)‐33 is implicated in the pathophysiology of asthma and allergic diseases. However, our knowledge is limited regarding how IL‐33 release is controlled. The transcription factor nuclear factor‐erythroid‐2‐related factor 2 (Nrf2) plays a key role in antioxidant response regulation.ObjectiveThe goal of this project was to investigate the role of cellular oxidative stress in controlling IL‐33 release in airway epithelium.MethodsComplementary approaches were used that included human bronchial epithelial cells and mouse models of airway type‐2 immunity that were exposed to fungus Alternaria extract. The clinically available Nrf2 activator 2‐cyano‐3,12‐dioxooleana‐1,9‐dien‐28‐oic acid methyl ester (CDDO‐Me) was used to evaluate the role of Nrf2‐induced antioxidant molecules.ResultsHuman bronchial epithelial cells produced reactive oxygen species (ROS) when they were exposed to Alternaria extract. ROS scavengers, such as glutathione (GSH) and N‐acetyl cysteine, prevented extracellular secretion of ATP and increases in intracellular calcium concentrations that precede IL‐33 release. Administration of CDDO‐Me to mice enhanced expression of a number of antioxidant molecules in the lungs and elevated lung levels of endogenous GSH. Importantly, CDDO‐Me treatment reduced allergen‐induced ATP secretion and IL‐33 release by airway epithelial cells in vitro and protected mice from IL‐33 release and asthma‐like pathological changes in the lungs.ConclusionsThe balance between oxidative stress and antioxidant responses plays a key role in controlling IL‐33 release in airway epithelium. The therapeutic potential of Nrf2 activators needs to be considered for asthma and allergic airway diseases.

Loss of cystic fibrosis transmembrane conductance regulator (CFTR) function reduces chloride secretion and increases sodium uptake, but it is not clear why CFTR mutation also results in progressive lung inflammation and infection. We previously demonstrated that CFTR-silenced airway cells migrate more slowly during wound repair than CFTR-expressing controls. In addition, CFTR-deficient cells and mouse models have been reported to have altered sphingolipid levels. Here, we investigated the hypothesis that reduced migration in CFTR-deficient airway epithelial cells results from altered sphingolipid composition. We used cell lines derived from a human airway epithelial cell line (Calu-3) stably transfected with CFTR short hairpin RNA (CFTR-silenced) or nontargeting short hairpin RNA (controls). Cell migration was measured by electric cell substrate impedance sensing (ECIS). Lipid analyses, addition of exogenous glycosphingolipids, and immunoblotting were performed. We found that levels of the glycosphingolipid, GM1 ganglioside, were ∼60% lower in CFTR-silenced cells than in controls. CFTR-silenced cells exhibited reduced levels of activated β1-integrin, phosphorylated tyrosine 576 of focal adhesion kinase (pFAK), and phosphorylation of Crk-associated substrate (pCAS). Addition of GM1 (but not GM3) ganglioside to CFTR-silenced cells restored activated β1-integrin, pFAK, and pCAS to near control levels and partially restored (∼40%) cell migration. Our results suggest that decreased GM1 in CFTR-silenced cells depresses β1-integrin signaling, which contributes to the delayed wound repair observed in these cells. These findings have implications for the pathology of cystic fibrosis, where altered sphingolipid levels in airway epithelial cells could result in a diminished capacity for wound repair after injury.

Abstract The molecular mechanisms underlying the initiation of innate and adaptive proallergic Th2-type responses in the airways are not well understood. IL-33 is a new member of the IL-1 family of molecules that is implicated in Th2-type responses. Airway exposure of naive mice to a common environmental aeroallergen, the fungus Alternaria alternata, induces rapid release of IL-33 into the airway lumen, followed by innate Th2-type responses. Biologically active IL-33 is constitutively stored in the nuclei of human airway epithelial cells. Exposing these epithelial cells to A. alternata releases IL-33 extracellularly in vitro. Allergen exposure also induces acute extracellular accumulation of a danger signal, ATP; autocrine ATP sustains increases in intracellular Ca2+ concentration and releases IL-33 through activation of P2 purinergic receptors. Pharmacological inhibitors of purinergic receptors or deficiency in the P2Y2 gene abrogate IL-33 release and Th2-type responses in the Alternaria-induced airway inflammation model in naive mice, emphasizing the essential roles for ATP and the P2Y2 receptor. Thus, ATP and purinergic signaling in the respiratory epithelium are critical sensors for airway exposure to airborne allergens, and they may provide novel opportunities to dampen the hypersensitivity response in Th2-type airway diseases such as asthma.

Abstract Thymic stromal lymphopoietin (TSLP) is produced by epithelial cells and triggers dendritic cell-mediated Th2-type inflammation. Although TSLP is up-regulated in epithelium of patients with asthma, the factors that control TSLP production have not been studied extensively. Because mouse models suggest roles for protease(s) in Th2-type immune responses, we hypothesized that proteases from airborne allergens may induce TSLP production in a human airway epithelial cell line, BEAS-2B. TSLP mRNA and protein were induced when BEAS-2B cells were exposed to prototypic proteases, namely, trypsin and papain. TSLP induction by trypsin required intact protease activity and also a protease-sensing G protein-coupled receptor, protease-activated receptor (PAR)-2; TSLP induction by papain was partially dependent on PAR-2. In humans, exposure to ubiquitous airborne fungi, such as Alternaria, is implicated in the development and exacerbation of asthma. When BEAS-2B cells or normal human bronchial epithelial cells were exposed to Alternaria extract, TSLP was potently induced. The TSLP-inducing activity of Alternaria was partially blocked by treating the extract with a cysteine protease inhibitor, E-64, or by infecting BEAS-2B cells with small interfering RNA for PAR-2. Protease-induced TSLP production by BEAS-2B cells was enhanced synergistically by IL-4 and abolished by IFN-γ. These findings demonstrate that TSLP expression is induced in airway epithelial cells by exposure to allergen-derived proteases and that PAR-2 is involved in the process. By promoting TSLP production in the airways, proteases associated with airborne allergens may facilitate the development and/or exacerbation of Th2-type airway inflammation, particularly in allergic individuals.

The effects of platelet-activating factor (PAF) and IL-5 on intracellular pH were investigated in human eosinophils. Purified peripheral blood eosinophils were loaded with the ratiometric fluorescent pH indicator BCECF-AM ester. Stimulation of eosinophils with PAF produced time-dependent alkalinization of the cytoplasm from an initial pH of 7.1±0.04 to 7.5±0.05. A similar alkalinization response was produced by the calcium ionophore, ionomycin and by the calcium ATPase inhibitor, thapsigargin. These compounds as well as PAF produce significant increases in cytoplasmic calcium ([Ca2+]i). In contrast, IL-5 and the protein kinase C (PKC) activator phorbol myristate acetate (PMA) did not produce cytoplasmic alkalinization and had no effect on [Ca2+]i in eosinophils. PAF-stimulated alkalinization was not inhibited under conditions that blocked plasma membrane Na+-H+ exchange, proton channel or plasma membrane H+-ATPase activities. Measurements of intragranule pH with a cell permeant pH indicator (LysoSensor Yellow/Blue DND-160), which partitions into intracellular acidic compartments, revealed that PAF-stimulated cytosolic alkalinization correlated with intragranule acidification. These results suggest that the increase in [Ca2+]i after PAF stimulation activates a H+-ATPase present in the granule membranes, leading to enhanced granule acidification and cytoplasmic alkalinization. We propose that granule acidification is an important step in solubilization of major basic protein crystals, which are stored within the granule core, in preparation for degranulation and release of these proteins.