Nitric oxide inhibits heterologous CFTR expression in polarized epithelial cells

Targeting the phosphoinositide-3-kinase/protein kinase B pathway in airway innate immunity

The airway innate immune system maintains the first line of defense against respiratory infections. The airway epithelium and associated immune cells protect the respiratory system from inhaled foreign organisms. These cells sense pathogens via activation of receptors like toll-like receptors and taste family 2 receptors (T2Rs) and respond by producing antimicrobials, inflammatory cytokines, and chemokines. Coordinated regulation of fluid secretion and ciliary beating facilitates clearance of pathogens via mucociliary transport. Airway cells also secrete antimicrobial peptides and radicals to directly kill microorganisms and inactivate viruses. The phosphoinositide-3-kinase/protein kinase B (Akt) kinase pathway regulates multiple cellular targets that modulate cell survival and proliferation. Akt also regulates proteins involved in innate immune pathways. Akt phosphorylates endothelial nitric oxide synthase (eNOS) enzymes expressed in airway epithelial cells. Activation of eNOS can have anti-inflammatory, anti-bacterial, and anti-viral roles. Moreover, Akt can increase the activity of the transcription factor nuclear factor erythroid 2 related factor-2 that protects cells from oxidative stress and may limit inflammation. In this review, we summarize the recent findings of non-cancerous functions of Akt signaling in airway innate host defense mechanisms, including an overview of several known downstream targets of Akt involved in innate immunity.

Influenza virus infection alters ion channel function of airway and alveolar cells: mechanisms and physiological sequelae

The cystic fibrosis transmembrane conductance regulator (CFTR) and the amiloride-sensitive epithelial sodium channels (ENaC) are located in the apical membranes of airway and alveolar epithelial cells. These transporters play an important role in the regulation of lung fluid balance across airway and alveolar epithelia by being the conduits for chloride (Cl^-) and bicarbonate ([Formula: see text]) secretion and sodium (Na⁺) ion absorption, respectively. The functional role of these channels in the respiratory tract is to maintain the optimum volume and ionic composition of the bronchial periciliary fluid (PCL) and alveolar lining fluid (ALF) layers. The PCL is required for proper mucociliary clearance of pathogens and debris, and the ALF is necessary for surfactant homeostasis and optimum gas exchange. Dysregulation of ion transport may lead to mucus accumulation, bacterial infections, inflammation, pulmonary edema, and compromised respiratory function. Influenza (or flu) in mammals is caused by influenza A and B viruses. Symptoms include dry cough, sore throat, and is often followed by secondary bacterial infections, accumulation of fluid in the alveolar spaces and acute lung injury. The underlying mechanisms of flu symptoms are not fully understood. This review summarizes our present knowledge of how influenza virus infections alter airway and alveolar epithelial cell CFTR and ENaC function in vivo and in vitro and the role of these changes in influenza pathogenesis.

Augmentation of CFTR maturation by S-nitrosoglutathione reductase

S-nitrosoglutathione (GSNO) reductase regulates novel endogenous S-nitrosothiol signaling pathways, and mice deficient in GSNO reductase are protected from airways hyperreactivity. S-nitrosothiols are present in the airway, and patients with cystic fibrosis (CF) tend to have low S-nitrosothiol levels that may be attributed to upregulation of GSNO reductase activity. The present study demonstrates that 1) GSNO reductase activity is increased in the cystic fibrosis bronchial epithelial (CFBE41o(-)) cells expressing mutant F508del-cystic fibrosis transmembrane regulator (CFTR) compared with the wild-type CFBE41o(-) cells, 2) GSNO reductase expression level is increased in the primary human bronchial epithelial cells expressing mutant F508del-CFTR compared with the wild-type cells, 3) GSNO reductase colocalizes with cochaperone Hsp70/Hsp90 organizing protein (Hop; Stip1) in human airway epithelial cells, 4) GSNO reductase knockdown with siRNA increases the expression and maturation of CFTR and decreases Stip1 expression in human airway epithelial cells, 5) increased levels of GSNO reductase cause a decrease in maturation of CFTR, and 6) a GSNO reductase inhibitor effectively reverses the effects of GSNO reductase on CFTR maturation. These studies provide a novel approach to define the subcellular location of the interactions between Stip1 and GSNO reductase and the role of S-nitrosothiols in these interactions.

Oxidized glutathione (GSSG) inhibits epithelial sodium channel activity in primary alveolar epithelial cells

Amiloride-sensitive epithelial Na(+) channels (ENaC) regulate fluid balance in the alveoli and are regulated by oxidative stress. Since glutathione (GSH) is the predominant antioxidant in the lungs, we proposed that changes in glutathione redox potential (Eh) would alter cell signaling and have an effect on ENaC open probability (Po). In the present study, we used single channel patch-clamp recordings to examine the effect of oxidative stress, via direct application of glutathione disulfide (GSSG), on ENaC activity. We found a linear decrease in ENaC activity as the GSH/GSSG Eh became less negative (n = 21; P < 0.05). Treatment of 400 μM GSSG to the cell bath significantly decreased ENaC Po from 0.39 ± 0.06 to 0.13 ± 0.05 (n = 8; P < 0.05). Likewise, back-filling recording electrodes with 400 μM GSSG reduced ENaC Po from 0.32 ± 0.08 to 0.17 ± 0.05 (n = 10; P < 0.05), thus implicating GSSG as an important regulatory factor. Biochemical assays indicated that oxidizing potentials promote S-glutathionylation of ENaC and irreversible oxidation of cysteine residues with N-ethylmaleimide blocked the effects of GSSG on ENaC Po. Additionally, real-time imaging studies showed that GSSG impairs alveolar fluid clearance in vivo as opposed to GSH, which did not impair clearance. Taken together, these data show that glutathione Eh is an important determinant of alveolar fluid clearance in vivo.

Cigarette smoke and CFTR: implications in the pathogenesis of COPD

Chronic obstructive pulmonary disease (COPD) is a progressive respiratory disorder consisting of chronic bronchitis and/or emphysema. COPD patients suffer from chronic infections and display exaggerated inflammatory responses and a progressive decline in respiratory function. The respiratory symptoms of COPD are similar to those seen in cystic fibrosis (CF), although the molecular basis of the two disorders differs. CF is a genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene encoding a chloride and bicarbonate channel (CFTR), leading to CFTR dysfunction. The majority of COPD cases result from chronic oxidative insults such as cigarette smoke. Interestingly, environmental stresses including cigarette smoke, hypoxia, and chronic inflammation have also been implicated in reduced CFTR function, and this suggests a common mechanism that may contribute to both the CF and COPD. Therefore, improving CFTR function may offer an excellent opportunity for the development of a common treatment for CF and COPD. In this article, we review what is known about the CF respiratory phenotype and discuss how diminished CFTR expression-associated ion transport defects may contribute to some of the pathological changes seen in COPD.

Influenza matrix protein 2 alters CFTR expression and function through its ion channel activity

The human cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-activated chloride (Cl(-)) channel in the lung epithelium that helps regulate the thickness and composition of the lung epithelial lining fluid. We investigated whether influenza M2 protein, a pH-activated proton (H(+)) channel that traffics to the plasma membrane of infected cells, altered CFTR expression and function. M2 decreased CFTR activity in 1) Xenopus oocytes injected with human CFTR, 2) epithelial cells (HEK-293) stably transfected with CFTR, and 3) human bronchial epithelial cells (16HBE14o-) expressing native CFTR. This inhibition was partially reversed by an inhibitor of the ubiquitin-activating enzyme E1. Next we investigated whether the M2 inhibition of CFTR activity was due to an increase of secretory organelle pH by M2. Incubation of Xenopus oocytes expressing CFTR with ammonium chloride or concanamycin A, two agents that alkalinize the secretory pathway, inhibited CFTR activity in a dose-dependent manner. Treatment of M2- and CFTR-expressing oocytes with the M2 ion channel inhibitor amantadine prevented the loss in CFTR expression and activity; in addition, M2 mutants, lacking the ability to transport H(+), did not alter CFTR activity in Xenopus oocytes and HEK cells. Expression of an M2 mutant retained in the endoplasmic reticulum also failed to alter CFTR activity. In summary, our data show that M2 decreases CFTR activity by increasing secretory organelle pH, which targets CFTR for destruction by the ubiquitin system. Alteration of CFTR activity has important consequences for fluid regulation and may potentially modify the immune response to viral infection.

CFTR activity and mitochondrial function

Cystic Fibrosis (CF) is a frequent and lethal autosomal recessive disease, caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR). Before the discovery of the CFTR gene, several hypotheses attempted to explain the etiology of this disease, including the possible role of a chloride channel, diverse alterations in mitochondrial functions, the overexpression of the lysosomal enzyme α-glucosidase and a deficiency in the cytosolic enzyme glucose 6-phosphate dehydrogenase. Because of the diverse mitochondrial changes found, some authors proposed that the affected gene should codify for a mitochondrial protein. Later, the CFTR cloning and the demonstration of its chloride channel activity turned the mitochondrial, lysosomal and cytosolic hypotheses obsolete. However, in recent years, using new approaches, several investigators reported similar or new alterations of mitochondrial functions in Cystic Fibrosis, thus rediscovering a possible role of mitochondria in this disease. Here, we review these CFTR-driven mitochondrial defects, including differential gene expression, alterations in oxidative phosphorylation, calcium homeostasis, oxidative stress, apoptosis and innate immune response, which might explain some characteristics of the complex CF phenotype and reveals potential new targets for therapy.

Effect of nitric oxide on epithelial ion transports in noncystic fibrosis and cystic fibrosis human proximal and distal airways

The airways of patients with cystic fibrosis (CF) exhibit decreased nitric oxide (NO) concentrations, which might affect airway function. The aim of this study was to determine the effects of NO on ion transport in human airway epithelia. Primary cultures of non-CF and CF bronchial and bronchiolar epithelial cells were exposed to the NO donor sodium nitroprusside (SNP), and bioelectric variables were measured in Ussing chambers. Amiloride was added to inhibit the Na+channel ENaC, and forskolin and ATP were added successively to stimulate cAMP- and Ca2+-dependent Cl−secretions, respectively. The involvement of cGMP was assessed by measuring the intracellular cGMP concentration in bronchial cells exposed to SNP and the ion transports in cultures exposed to 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one, an inhibitor of the soluble guanylate cyclase (ODQ), or to 8Z, a cocktail of 8-bromo-cGMP and zaprinast (phosphodiesterase 5 inhibitor). SNP decreased the baseline short-circuit current ( Isc) and the changes in Iscinduced by amiloride, forskolin, and ATP in non-CF bronchial and bronchiolar cultures. The mechanism of this inhibition was studied in bronchial cells. SNP increased the intracellular cGMP concentration ([cGMP]i). The inhibitory effect of SNP was abolished by 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, an NO scavenger (PTIO) and ODQ and was partly mimicked by increasing [cGMP]i. In CF cultures, SNP did not significantly modify ion transport; in CF bronchial cells, 8Z had no effect; however, SNP increased the [cGMP]i. In conclusion, exogenous NO may reduce transepithelial Na+absorption and Cl−secretion in human non-CF airway epithelia through a cGMP-dependent pathway. In CF airways, the NO/cGMP pathway appears to exert no effect on transepithelial ion transport.

Hsp 70/Hsp 90 organizing protein as a nitrosylation target in cystic fibrosis therapy

The endogenous signaling molecule S-nitrosoglutathione (GSNO) and other S-nitrosylating agents can cause full maturation of the abnormal gene product DeltaF508 cystic fibrosis (CF) transmembrane conductance regulator (CFTR). However, the molecular mechanism of action is not known. Here we show that Hsp70/Hsp90 organizing protein (Hop) is a critical target of GSNO, and its S-nitrosylation results in DeltaF508 CFTR maturation and cell surface expression. S-nitrosylation by GSNO inhibited the association of Hop with CFTR in the endoplasmic reticulum. This effect was necessary and sufficient to mediate GSNO-induced cell-surface expression of DeltaF508 CFTR. Hop knockdown using siRNA recapitulated the effect of GSNO on DeltaF508 CFTR maturation and expression. Moreover, GSNO acted additively with decreased temperature, which promoted mutant CFTR maturation through a Hop-independent mechanism. We conclude that GSNO corrects DeltaF508 CFTR trafficking by inhibiting Hop expression, and that combination therapies--using differing mechanisms of action--may have additive benefits in treating CF.

IFN{gamma} regulates retinal pigment epithelial fluid transport

The present experiments show that IFNgamma receptors are mainly localized to the basolateral membrane of human retinal pigment epithelium (RPE). Activation of these receptors in primary cultures of human fetal RPE inhibited cell proliferation and migration, decreased RPE mitochondrial membrane potential, altered transepithelial potential and resistance, and significantly increased transepithelial fluid absorption. These effects are mediated through JAK-STAT and p38 MAPK signaling pathways. Second messenger signaling through cAMP-PKA pathway- and interferon regulatory factor-1-dependent production of nitric oxide/cGMP stimulated the CFTR at the basolateral membrane and increased transepithelial fluid absorption. In vivo experiments using a rat model of retinal reattachment showed that IFNgamma applied to the anterior surface of the eye can remove extra fluid deposited in the extracellular or subretinal space between the retinal photoreceptors and RPE. Removal of this extra fluid was blocked by a combination of PKA and JAK-STAT pathway inhibitors injected into the subretinal space. These results demonstrate a protective role for IFNgamma in regulating retinal hydration across the outer blood-retinal barrier in inflammatory disease processes and provide the basis for possible therapeutic interventions.

Reactive nitrogen species: molecular mechanisms and potential significance in health and disease

Reactive nitrogen species (RNS) are various nitric oxide-derived compounds, including nitroxyl anion, nitrosonium cation, higher oxides of nitrogen, S-nitrosothiols, and dinitrosyl iron complexes. RNS have been recognized as playing a crucial role in the physiologic regulation of many, if not all, living cells, such as smooth muscle cells, cardiomyocytes, platelets, and nervous and juxtaglomerular cells. They possess pleiotropic properties on cellular targets after both posttranslational modifications and interactions with reactive oxygen species. Elevated levels of RNS have been implicated in cell injury and death by inducing nitrosative stress. The aim of this comprehensive review is to address the mechanisms of formation and removal of RNS, highlighting their potential cellular targets: lipids, DNA, and proteins. The specific importance of RNS and their paradoxic effects, depending on their local concentration under physiologic conditions, is underscored. An increasing number of compounds that modulate RNS processing or targets are being identified. Such compounds are now undergoing preclinical and clinical evaluations in the treatment of pathologies associated with RNS-induced cellular damage. Future research should help to elucidate the involvement of RNS in the therapeutic effect of drugs used to treat neurodegenerative, cardiovascular, metabolic, and inflammatory diseases and cancer.

S-nitrosothiol signaling in respiratory biology

Genetic and biochemical data demonstrate a pivotal role for S-nitrosothiols (SNOs) in mediating the actions of nitric oxide synthases (NOSs). SNOs serve to convey NO bioactivity and to regulate protein function. This understanding is of immediate interest to the pulmonary clinical and research communities. This article reviews the following: (1) biochemical and cellular evidence that SNOs in amino acids, peptides, and proteins elicit NOS-dependent signaling in the respiratory system and (2) studies that link SNO signaling to pulmonary medicine. SNO-mediated signaling is involved in the regulation of minute ventilation, ventilation-perfusion matching, pulmonary arterial pressure, basal airway tone, and respiratory and peripheral muscle function. Derangements in SNO signaling are implicated in many disorders relevant to pulmonary and critical care medicine, including apnea, hypoxemia, pulmonary hypertension, asthma, cystic fibrosis, pneumonia, and septic shock.

Nitric oxide and zinc homeostasis in acute lung injury

Among putative small molecules that affect sensitivity to acute lung injury, zinc and nitric oxide are potentially unique by virtue of their interdependence and dual capacities to be cytoprotective or injurious. Nitric oxide and zinc appear to be linked via an intracellular signaling pathway involving S-nitrosation of metallothoinein--itself a small protein known to be an important inducible gene product that may modify lung injury. In the present article, we summarize recent efforts using genetic and fluorescence optical imaging techniques to: (1) demonstrate that S-nitrosation of metallothionein affects intracellular zinc homeostasis in intact pulmonary endothelial cells; and (2) reveal a protective role for this pathway in hyperoxic and LPS-induced injury.

Concentration-dependent effects of endogenous S-nitrosoglutathione on gene regulation by specificity proteins Sp3 and Sp1

The activities of certain nuclear regulatory proteins are modified by high concentrations of S-nitrosothiols associated with nitrosative stress. In the present study, we have studied the effect of physiological (low microM) concentrations of the endogenous S-nitrosothiol, GSNO (S-nitrosoglutathione), on the activities of nuclear regulatory proteins Sp3 and Sp1 (specificity proteins 3 and 1). Low concentrations of GSNO increased Sp3 binding, as well as Sp3-dependent transcription of the cystic fibrosis transmembrane conductance regulatory gene, cftr. However, higher GSNO levels prevented Sp3 binding, augmented Sp1 binding and prevented both cftr transcription and CFTR (cystic fibrosis transmembrane conductance regulator) expression. We conclude that low concentrations of GSNO favour Sp3 binding to 'housekeeping' genes such as cftr, whereas nitrosative stress-associated GSNO concentrations shut off Sp3-dependent transcription, possibly to redirect cellular resources. Since low micromolar concentrations of GSNO also increase the maturation and activity of a clinically common CFTR mutant, whereas higher concentrations have the opposite effect, these observations may have implications for dosing of S-nitrosylating agents used in cystic fibrosis clinical trials.

Mutations in the extracellular loop of alpha-rENaC alter sensitivity to amiloride and reactive species

We studied the effects of two mutations of the extracellular loop of the alpha-subunit of the (ENaC) on amiloride-sensitive current in Xenopus laevis oocytes and the inhibition of this current by 3-morpholinosydnonimine (SIN-1). Injection of oocytes with wild-type (wt) alpha-,beta-,gamma-rENaC cRNA (8.3 ng/subunit) resulted 48-72 h later in inward Na(+) currents (-5.5 +/- 0.8 microA; means +/- SE at -100 mV; n = 21), which were completely inhibited by amiloride. Oocytes injected with either alpha(Y279A)- or alpha(Y283A)- and beta-,gamma-rENaC cRNAs had significantly lower Na(+) currents. Furthermore, alpha(Y279A)-,beta-,gamma-rENaC-injected oocytes had a higher K(i) for amiloride (0.54 +/- 0.97 vs. 0.10 +/- 0.04 microM; P < 0.01). Exposure of oocytes to SIN-1 (1 mM) for 5 min decreased both total Na(+) and amiloride-sensitive currents across wt and alpha(Y279A)- but not alpha(Y283A)-,beta-,gamma-rENaC. Furthermore, exposure to SIN-1 increased the K(i) for amiloride across wt but not alpha(Y279A)-,beta-,gamma-rENaC-injected oocytes. These data indicate that both tyrosines are important for proper ENaC function and their oxidative modifications contribute to altered ENaC function.

Regulation of ion channel structure and function by reactive oxygen-nitrogen species

Ion channels subserve diverse cellular functions. Reactive oxygen and nitrogen species modulate ion channel function by a number of mechanisms including 1) transcriptional regulation of gene expression, 2) posttranslational modifications of channel proteins, i.e. nitrosylation, nitration, and oxidation of key amino acid residues, 3) by altering the gain in other signaling pathways that may in turn lead to changes in channel activity or channel gene expression, and 4) by modulating trafficking or turnover of channel proteins, as typified by oxygen radical activation of NF-kappa B, with subsequent changes in proteasomal degradation of channel degradation. Regardless of the mechanism, as was discussed in a symposium at the 2003 Experimental Biology Meeting in San Diego, CA, changes in the cellular level of reactive oxygen and nitrogen species can have profound effects on the activity of ion channels and cellular function.