Adenosine receptors and phosphodiesterase inhibitors stimulate Cl- secretion in Calu-3 cells

What role will ensifentrine play in the future treatment of chronic obstructive pulmonary disease patients? Implications from recent clinical trials

Data from the phase III ENHANCE clinical trials provide compelling evidence that ensifentrine, an inhaled 'bifunctional' dual phosphodiesterase 3/4 inhibitor, can provide additional benefit to existing treatments in patients with chronic obstructive pulmonary disease and represents a 'first-in-class' drug having bifunctional bronchodilator and anti-inflammatory activity in a single molecule. Ensifentrine, generally well tolerated, can provide additional bronchodilation when added to muscarinic receptor antagonists or β2-agonists and reduce the exacerbation risk. This information allows us to consider better the possible inclusion of ensifentrine in the future treatment of chronic obstructive pulmonary disease. However, there is less information on whether it provides additional benefit when added to inhaled corticosteroid or 'triple therapy' and, therefore, when this drug is best utilized in clinical practice.

Hyperinflammation and airway surface liquid dehydration in cystic fibrosis: purinergic system as therapeutic target

OBJECTIVE AND DESIGN

The exacerbate inflammatory response contributes to the progressive loss of lung function in cystic fibrosis (CF), a genetic disease that affects the osmotic balance of mucus and mucociliary clearance, resulting in a microenvironment that favors infection and inflammation. The purinergic system, an extracellular signaling pathway characterized by nucleotides, enzymes and receptors, may have a protective role in the disease, through its action in airway surface liquid (ASL) and anti-inflammatory response.

MATERIALS AND METHODS

To make up this review, studies covering topics of CF, inflammation, ASL and purinergic system were selected from the main medical databases, such as Pubmed and ScienceDirect.

CONCLUSION

We propose several ways to modulate the purinergic system as a potential therapy for CF, like inhibition of P2X7, activation of P2Y2, A2A and A2B receptors and blocking of adenosine deaminase. Among them, we postulate that the most suitable strategy is to block the action of adenosine deaminase, which culminates in the increase of Ado levels that presents anti-inflammatory actions and improves mucociliary clearance. Furthermore, it is possible to maintain the physiological levels of ATP to control the hydration of ASL. These therapies could correct the main mechanisms that contribute to the progression of CF.

Cyclic nucleotide phosphodiesterase inhibitors as therapeutic interventions for cystic fibrosis

Cystic Fibrosis (CF) lung disease results from mutations in the CFTR anion channel that reduce anion and fluid secretion by airway epithelia. Impaired secretion compromises airway innate defence mechanisms and leads to bacterial colonization, excessive inflammation and tissue damage; thus, restoration of CFTR function is the goal of many CF therapies. CFTR channels are activated by cyclic nucleotide-dependent protein kinases. The second messengers 3'5'-cAMP and 3'5'-cGMP are hydrolysed by a large family of cyclic nucleotide phosphodiesterases that provide subcellular spatial and temporal control of cyclic nucleotide-dependent signalling. Selective inhibition of these enzymes elevates cyclic nucleotide levels, leading to activation of CFTR and other downstream effectors. Here we examine members of the PDE family that are likely to regulate CFTR-dependent ion and fluid secretion in the airways and discuss other actions of PDE inhibitors that can influence cyclic nucleotide-regulated mucociliary transport, inflammation and bronchodilation. Finally, we review PDE inhibitors and the potential benefits they could provide as CF therapeutics.

Modulation of cAMP metabolism for CFTR potentiation in human airway epithelial cells

Cystic fibrosis (CF) is a genetic disease characterized by CF transmembrane regulator (CFTR) dysfunction. With over 2000 CFTR variants identified, in addition to known patient to patient variability, there is a need for personalized treatment. The discovery of CFTR modulators has shown efficacy in certain CF populations, however there are still CF populations without valid therapeutic options. With evidence suggesting that single drug therapeutics are insufficient for optimal management of CF disease, there has been an increased pursuit of combinatorial therapies. Our aim was to test cyclic AMP (cAMP) modulation, through ATP Binding Cassette Transporter C4 (ABCC4) and phosphodiesterase-4 (PDE-4) inhibition, as a potential add-on therapeutic to a clinically approved CFTR modulator, VX-770, as a method for increasing CFTR activity. Human airway epithelial cells (Calu-3) were used to test the efficacy of cAMP modulation by ABCC4 and PDE-4 inhibition through a series of concentration–response studies. Our results showed that cAMP modulation, in combination with VX-770, led to an increase in CFTR activity via an increase in sensitivity when compared to treatment of VX-770 alone. Our study suggests that cAMP modulation has potential to be pursued as an add-on therapy for the optimal management of CF disease.

The Phosphodiesterase Inhibitor Ensifentrine Reduces Production of Proinflammatory Mediators in Well Differentiated Bronchial Epithelial Cells by Inhibiting PDE4

Cystic fibrosis (CF) is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel that impair airway salt and fluid secretion. Excessive release of proinflammatory cytokines and chemokines by CF bronchial epithelium during airway infection leads to chronic inflammation and a slow decline in lung function; thus, there is much interest in finding safe and effective treatments that reduce inflammation in CF. We showed previously that the cyclic nucleotide phosphodiesterase (PDE) inhibitor ensifentrine (RPL554; Verona Pharma) stimulates the channel function of CFTR mutants with abnormal gating and also those with defective trafficking that are partially rescued using a clinically approved corrector drug. PDE inhibitors also have known anti-inflammatory effects; therefore, we examined whether ensifentrine alters the production of proinflammatory cytokines in CF bronchial epithelial cells. Ensifentrine reduced the production of monocyte chemoattractant protein-1 and granulocyte monocyte colony-stimulating factor (GM-CSF) during challenge with interleukin-1β Comparing the effect of ensifentrine with milrinone and roflumilast, selective PDE3 and PDE4 inhibitors, respectively, demonstrated that the anti-inflammatory effect of ensifentrine was mainly due to inhibition of PDE4. Beneficial modulation of GM-CSF was further enhanced when ensifentrine was combined with low concentrations of the β ₂-adrenergic agonist isoproterenol or the corticosteroid dexamethasone. The results indicate that ensifentrine may have beneficial anti-inflammatory effects in CF airways particularly when used in combination with β ₂-adrenergic agonists or corticosteroids. SIGNIFICANCE STATEMENT: Airway inflammation that is disproportionate to the burden of chronic airway infection causes much of the pathology in the cystic fibrosis (CF) lung. We show here that ensifentrine beneficially modulates the release of proinflammatory factors in well differentiated CF bronchial epithelial cells that is further enhanced when combined with β₂-adrenergic agonists or low-concentration corticosteroids. The results encourage further clinical testing of ensifentrine, alone and in combination with β₂-adrenergic agonists or low-concentration corticosteroids, as a novel anti-inflammatory therapy for CF.

The dual phosphodiesterase 3/4 inhibitor RPL554 stimulates rare class III and IV CFTR mutants

Over 2,000 mutations have been reported in the cystic fibrosis transmembrane conductance regulator (cftr) gene, many of which cause disease but are rare and have no effective treatment. Thus, there is an unmet need for new, mutation-agnostic therapies for cystic fibrosis (CF). Phosphodiesterase (PDE) inhibitors are one such class of therapeutics that have been shown to elevate intracellular cAMP levels and stimulate CFTR-dependent anion secretion in human airway epithelia; however, the number of people with CF that could be helped by PDE inhibitors remains to be determined. Here we used Fisher rat thyroid (FRT) cells stably transduced with rare human CFTR mutants and studied their responsiveness to the dual phosphodiesterase 3/4 inhibitor RPL554 (Verona Pharma). Through its inhibitory effect on PDE4D, we find that RPL554 can elevate intracellular cAMP leading to a potentiation of forskolin-stimulated current mediated by R334W, T338I, G551D, and S549R mutants of CFTR when used alone or in combination with CFTR modulators. We also were able to reproduce these effects of RPL554 on G551D-CFTR when it was expressed in primary human bronchial epithelial cells, indicating that RPL554 would have stimulatory effects on rare CFTR mutants in human airways and validating FRT cells as a model for PDE inhibitor studies. Furthermore, we provide biochemical evidence that VX-809 causes surprisingly robust correction of several class III and IV CFTR mutants. Together, our findings further support the therapeutic potential of RPL554 for patients with CF with class III/IV mutations and emphasize the potential of PDEs as potential drug targets that could benefit patients with CF.

Hypercapnia modulates cAMP signalling and cystic fibrosis transmembrane conductance regulator-dependent anion and fluid secretion in airway epithelia

Hypercapnia is clinically defined as an arterial blood partial pressure of CO2 of above 40 mmHg and is a feature of chronic lung disease. In previous studies we have demonstrated that hypercapnia modulates agonist-stimulated cAMP levels through effects on transmembrane adenylyl cyclase activity. In the airways, cAMP is known to regulate cystic fibrosis transmembrane conductance regulator (CFTR)-mediated anion and fluid secretion, which contributes to airway surface liquid homeostasis. The aim of the current work was to investigate if hypercapnia could modulate cAMP-regulated ion and fluid transport in human airway epithelial cells. We found that acute exposure to hypercapnia significantly reduced forskolin-stimulated elevations in intracellular cAMP as well as both adenosine- and forskolin-stimulated increases in CFTR-dependent transepithelial short-circuit current, in polarised cultures of Calu-3 human airway cells. This CO2 -induced reduction in anion secretion was not due to a decrease in HCO3 (-) transport given that neither a change in CFTR-dependent HCO3 (-) efflux nor Na(+) /HCO3 (-) cotransporter-dependent HCO3 (-) influx were CO2 -sensitive. Hypercapnia also reduced the volume of forskolin-stimulated fluid secretion over 24 h, yet had no effect on the HCO3 (-) content of the secreted fluid. Our data reveal that hypercapnia reduces CFTR-dependent, electrogenic Cl(-) and fluid secretion, but not CFTR-dependent HCO3 (-) secretion, which highlights a differential sensitivity of Cl(-) and HCO3 (-) transporters to raised CO2 in Calu-3 cells. Hypercapnia also reduced forskolin-stimulated CFTR-dependent anion secretion in primary human airway epithelia. Based on current models of airways biology, a reduction in fluid secretion, associated with hypercapnia, would be predicted to have important consequences for airways hydration and the innate defence mechanisms of the lungs.

The dual phosphodiesterase 3 and 4 inhibitor RPL554 stimulates CFTR and ciliary beating in primary cultures of bronchial epithelia

Cystic fibrosis (CF), a genetic disease caused by mutations in the CFTR gene, is a life-limiting disease characterized by chronic bacterial airway infection and severe inflammation. Some CFTR mutants have reduced responsiveness to cAMP/PKA signaling; hence, pharmacological agents that elevate intracellular cAMP are potentially useful for the treatment of CF. By inhibiting cAMP breakdown, phosphodiesterase (PDE) inhibitors stimulate CFTR in vitro and in vivo. Here, we demonstrate that PDE inhibition by RPL554, a drug that has been shown to cause bronchodilation in asthma and chronic obstructive pulmonary disease (COPD) patients, stimulates CFTR-dependent ion secretion across bronchial epithelial cells isolated from patients carrying the R117H/F508del CF genotype. RPL554-induced CFTR activity was further increased by the potentiator VX-770, suggesting an additional benefit by the drug combination. RPL554 also increased cilia beat frequency in primary human bronchial epithelial cells. The results indicate RPL554 may increase mucociliary clearance through stimulation of CFTR and increasing ciliary beat frequency and thus could provide a novel therapeutic option for CF.

Roflumilast partially reverses smoke-induced mucociliary dysfunction

BACKGROUND

Phosphodiesterases (PDEs) break down cAMP, thereby regulating intracellular cAMP concentrations and diffusion. Since PDE4 predominates in airway epithelial cells, PDE4 inhibitors can stimulate Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) by increasing cAMP. Tobacco smoking and COPD are associated with decreased CFTR function and impaired mucociliary clearance (MCC). However, the effects of the PDE4 inhibitor roflumilast on smoke-induced mucociliary dysfunction have not been fully explored.

METHODS

Primary normal human bronchial epithelial cells (NHBE) from non-smokers, cultured at the air-liquid interface (ALI) were used for most experiments. Cultures were exposed to cigarette smoke in a Vitrocell VC-10 smoking robot. To evaluate the effect of roflumilast on intracellular cAMP concentrations, fluorescence resonance energy transfer (FRET) between CFP- and YFP-tagged protein kinase A (PKA) subunits was recorded. Airway surface liquid (ASL) was measured using light refraction scanning and ciliary beat frequency (CBF) employing infrared differential interference contrast microscopy. Chloride conductance was measured in Ussing chambers and CFTR expression was quantified with qPCR.

RESULTS

While treatment with 100 nM roflumilast had little effect alone, it increased intracellular cAMP upon stimulation with forskolin and albuterol in cultures exposed to cigarette smoke and in control conditions. cAMP baselines were lower in smoke-exposed cells. Roflumilast prolonged cAMP increases in smoke-exposed and control cultures. Smoke-induced reduction in functional, albuterol-mediated chloride conductance through CFTR was improved by roflumilast. ASL volumes also increased in smoke-exposed cultures in the presence of roflumilast while it did not in its absence. Cigarette smoke exposure decreased CBF, an effect rescued with roflumilast, particularly when used together with the long-acting ß-mimetic formoterol. Roflumilast also enhanced forskolin-induced CBF stimulation in ASL volume supplemented smoked and control cells, confirming the direct stimulatory effect of rising cAMP on ciliary function. In active smokers, CFTR mRNA expression was increased compared to non-smokers and ex-smokers. Roflumilast also increased CFTR mRNA levels in cigarette-smoke exposed cell cultures.

CONCLUSIONS

Our results show that roflumilast can rescue smoke-induced mucociliary dysfunction by reversing decreased CFTR activity, augmenting ASL volume, and stimulating CBF, the latter particularly in combination with formoterol. As expected, CFTR mRNA expression was not indicative of apical CFTR function.

Cystic fibrosis transmembrane conductance regulator activation by roflumilast contributes to therapeutic benefit in chronic bronchitis

Cigarette smoking causes acquired cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction and is associated with delayed mucociliary clearance and chronic bronchitis. Roflumilast is a clinically approved phosphodiesterase 4 inhibitor that improves lung function in patients with chronic bronchitis. We hypothesized that its therapeutic benefit was related in part to activation of CFTR. Primary human bronchial epithelial (HBE) cells, Calu-3, and T84 monolayers were exposed to whole cigarette smoke (WCS) or air with or without roflumilast treatment. CFTR-dependent ion transport was measured in modified Ussing chambers. Airway surface liquid (ASL) was determined by confocal microscopy. Intestinal fluid secretion of ligated murine intestine was monitored ex vivo. Roflumilast activated CFTR-dependent anion transport in normal HBE cells with a half maximal effective concentration of 2.9 nM. Roflumilast partially restored CFTR activity in WCS-exposed HBE cells (5.3 ± 1.1 μA/cm(2) vs. 1.2 ± 0.2 μA/cm(2) [control]; P < 0.05) and was additive with ivacaftor, a specific CFTR potentiator approved for the treatment of CF. Roflumilast improved the depleted ASL depth of HBE monolayers exposed to WCS (9.0 ± 3.1 μm vs. 5.6 ± 2.0 μm [control]; P < 0.05), achieving 79% of that observed in air controls. CFTR activation by roflumilast also induced CFTR-dependent fluid secretion in murine intestine, increasing the wet:dry ratio and the diameter of ligated murine segments. Roflumilast activates CFTR-mediated anion transport in airway and intestinal epithelia via a cyclic adenosine monophosphate-dependent pathway and partially reverses the deleterious effects of WCS, resulting in augmented ASL depth. Roflumilast may benefit patients with chronic obstructive pulmonary disease with chronic bronchitis by activating CFTR, which may also underlie noninfectious diarrhea caused by roflumilast.

Dual PDE3/4 and PDE4 inhibitors: novel treatments for COPD and other inflammatory airway diseases

Selective phosphodiesterase (PDE) 4 and dual PDE3/4 inhibitors have attracted considerable interest as potential therapeutic agents for the treatment of respiratory diseases, largely by virtue of their anti-inflammatory (PDE4) and bifunctional bronchodilator/anti-inflammatory (PDE3/4) effects. Many of these agents have, however, failed in early development for various reasons, including dose-limiting side effects when administered orally and lack of sufficient activity when inhaled. Indeed, only one selective PDE4 inhibitor, the orally active roflumilast-n-oxide, has to date received marketing authorization. The majority of the compounds that have failed were, however, orally administered and non-selective for either PDE3 (A,B) or PDE4 (A,B,C,D) subtypes. Developing an inhaled dual PDE3/4 inhibitor that is rapidly cleared from the systemic circulation, potentially with subtype specificity, may represent one strategy to improve the therapeutic index and also exhibit enhanced efficacy versus inhibition of either PDE3 or PDE4 alone, given the potential positive interactions with regard to anti-inflammatory and bronchodilator effects that have been observed pre-clinically with dual inhibition of PDE3 and PDE4 compared with inhibition of either isozyme alone. This MiniReview will summarize recent clinical data obtained with PDE inhibitors and the potential for these drugs to treat COPD and other inflammatory airways diseases such as asthma and cystic fibrosis.

Correction of chloride transport and mislocalization of CFTR protein by vardenafil in the gastrointestinal tract of cystic fibrosis mice

Although lung disease is the major cause of mortality in cystic fibrosis (CF), gastrointestinal (GI) manifestations are the first hallmarks in 15–20% of affected newborns presenting with meconium ileus, and remain major causes of morbidity throughout life. We have previously shown that cGMP-dependent phosphodiesterase type 5 (PDE5) inhibitors rescue defective CF Transmembrane conductance Regulator (CFTR)-dependent chloride transport across the mouse CF nasal mucosa. Using F508del-CF mice, we examined the transrectal potential difference 1 hour after intraperitoneal injection of the PDE5 inhibitor vardenafil or saline to assess the amiloride-sensitive sodium transport and the chloride gradient and forskolin-dependent chloride transport across the GI tract. In the same conditions, we performed immunohistostaining studies in distal colon to investigate CFTR expression and localization. F508del-CF mice displayed increased sodium transport and reduced chloride transport compared to their wild-type littermates. Vardenafil, applied at a human therapeutic dose (0.14 mg/kg) used to treat erectile dysfunction, increased chloride transport in F508del-CF mice. No effect on sodium transport was detected. In crypt colonocytes of wild-type mice, the immunofluorescence CFTR signal was mostly detected in the apical cell compartment. In F508del-CF mice, a 25% reduced signal was observed, located mostly in the subapical region. Vardenafil increased the peak of intensity of the fluorescence CFTR signal in F508del-CF mice and displaced it towards the apical cell compartment. Our findings point out the intestinal mucosa as a valuable tissue to study CFTR transport function and localization and to evaluate efficacy of therapeutic strategies in CF. From our data we conclude that vardenafil mediates potentiation of the CFTR chloride channel and corrects mislocalization of the mutant protein. The study provides compelling support for targeting the cGMP signaling pathway in CF pharmacotherapy.

Nongenomic effects of fluticasone propionate and budesonide on human airway anion secretion

This study investigated the physiological effects of inhaled corticosteroids, which are used widely to treat asthma. The application of fluticasone propionate (FP, 100 μM) induced sustained increases in the short-circuit current (I(SC)) in human airway Calu-3 epithelial cells. The FP-induced I(SC) was prevented by the presence of H89 (10 μM, a protein kinase A inhibitor) and SQ22536 (100 μM, an adenylate cyclase inhibitor). The FP-induced responses involved bumetanide (a Na(+)-K(+)-2Cl(-) cotransporter inhibitor)-sensitive and 4,4'-dinitrostilbene-2,2'-disulfonic acid (an inhibitor of HCO(3)(-)-dependent anion transporters)-sensitive components, both of which reflect basolateral anion transport. Further, FP augmented apical membrane Cl(-) current (I(Cl)), reflecting cystic fibrosis transmembrane conductance regulator (CFTR)-mediated conductance, in the nystatin-permeabilized monolayer. In I(SC) and I(Cl) responses, FP failed to enhance the responses to forskolin (10 μM, an adenylate cyclase activator). Nevertheless, we found that FP synergistically increased cytosolic cAMP concentrations in combination with forskolin. All these effects of FP were reproduced with the use of budesonide. Collectively, inhaled corticosteroids such as FP and budesonide stimulate CFTR-mediated anion transport through adenylate cyclase-mediated mechanisms in a nongenomic fashion, thus sharing elements of a common pathway with forskolin. However, the corticosteroids cooperate with forskolin for synergistic cAMP production, suggesting that the corticosteroids and forskolin do not compete with each other to exert their effects on adenylate cyclase. Considering that such synergism was also observed in the FP/salmeterol combination, these nongenomic aspects may play therapeutic roles in mucus congestive airway diseases, in addition to genomic aspects that are generally recognized.

A pharmacologic approach to acquired cystic fibrosis transmembrane conductance regulator dysfunction in smoking related lung disease

Background

Mucus stasis in chronic obstructive pulmonary disease (COPD) is a significant contributor to morbidity and mortality. Potentiators of cystic fibrosis transmembrane conductance regulator (CFTR) activity pharmacologically enhance CFTR function; ivacaftor is one such agent approved to treat CF patients with the G551D-CFTR gating mutation. CFTR potentiators may also be useful for other diseases of mucus stasis, including COPD.

Methods and Findings

In primary human bronchial epithelial cells, exposure to cigarette smoke extract diminished CFTR-mediated anion transport (65.8±0.2% of control, P<0.005) and mucociliary transport (0.17±0.05 µm/sec vs. 2.4±0.47 µm/sec control, P<0.05) by reducing airway surface liquid depth (7.3±0.6 µm vs. 13.0±0.6 µm control, P<0.005) and augmenting mucus expression (by 64%, P<0.05) without altering transepithelial resistance. Smokers with or without COPD had reduced CFTR activity measured by nasal potential difference compared to age-matched non-smokers (−6.3±1.4 and −8.0±2.0 mV, respectively vs. −15.2±2.7 mV control, each P<0.005, n = 12–14/group); this CFTR decrement was associated with symptoms of chronic bronchitis as measured by the Breathlessness Cough and Sputum Score (r = 0.30, P<0.05) despite controlling for smoking (r = 0.31, P<0.05). Ivacaftor activated CFTR-dependent chloride transport in non-CF epithelia and ameliorated the functional CFTR defect induced by smoke to 185±36% of non-CF control (P<0.05), thereby increasing airway surface liquid (from 7.3±0.6 µm to 10.1±0.4 µm, P<0.005) and mucociliary transport (from 0.27±0.11 µm/s to 2.7±0.28 µm/s, P<0.005).

Conclusions

Cigarette smoking reduces CFTR activity and is causally related to reduced mucus transport in smokers due to inhibition of CFTR dependent fluid transport. These effects are reversible by the CFTR potentiator ivacaftor, representing a potential therapeutic strategy to augment mucociliary clearance in patients with smoking related lung disease.

Suppression of adenosine-activated chloride transport by ethanol in airway epithelia

Alcohol abuse is associated with increased lung infections. Molecular understanding of the underlying mechanisms is not complete. Airway epithelial ion transport regulates the homeostasis of airway surface liquid, essential for airway mucosal immunity and lung host defense. Here, air-liquid interface cultures of Calu-3 epithelial cells were basolaterally exposed to physiologically relevant concentrations of ethanol (0, 25, 50 and 100 mM) for 24 hours and adenosine-stimulated ion transport was measured by Ussing chamber. The ethanol exposure reduced the epithelial short-circuit currents (I(SC)) in a dose-dependent manner. The ion currents activated by adenosine were chloride conductance mediated by cystic fibrosis transmembrane conductance regulator (CFTR), a cAMP-activated chloride channel. Alloxazine, a specific inhibitor for A(2B) adenosine receptor (A(2B)AR), largely abolished the adenosine-stimulated chloride transport, suggesting that A(2B)AR is a major receptor responsible for regulating the chloride transport of the cells. Ethanol significantly reduced intracellular cAMP production upon adenosine stimulation. Moreover, ethanol-suppression of the chloride secretion was able to be restored by cAMP analogs or by inhibitors to block cAMP degradation. These results imply that ethanol exposure dysregulates CFTR-mediated chloride transport in airways by suppression of adenosine-A(2B)AR-cAMP signaling pathway, which might contribute to alcohol-associated lung infections.

The participation of plasma membrane hemichannels to purinergic signaling

The field of hemichannels is closely related to the purinergic signaling and both areas have been growing in parallel. Hemichannels open in response to a wide range of stressful conditions, such as ischemia, pressure or swelling. Hemichannels represent an important mechanism for the cellular release of adenosine 5'-triphosphate (ATP), which is an agonist of the P2Y and P2X family of purinergic receptors. Therefore, hemichannels are key molecules in the regulation of purinergic receptor activation, during physiological and pathophysiological conditions. Furthermore, purinergic receptor activation can also lead to the opening of hemichannels and the subsequent amplification of purinergic signaling via a positive signaling feedback loop, giving rise to the concept of ATP-induced ATP release. Purinergic receptor signaling is involved in regulating many physiological and pathophysiological processes. P2Y receptors activate inositol trisphosphate and transiently increase intracellular calcium. This signaling opens both connexin and pannexin channels, therefore contributing to the expansion of calcium waves across astrocytes and epithelial cells. In addition, several of the P2X receptor subtypes, including the P2X2, P2X4 and P2X7 receptors, activate select cellular permeation pathways to large molecules, including the pannexin-1 channels, which are involved in the initiation of inflammatory responses and cell death. Consequently, the interplay between purinergic receptors and hemichannels could represent a novel target with substantial therapeutic implications in areas such as chronic pain, inflammation or atherosclerosis. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Functional regulation of cystic fibrosis transmembrane conductance regulator-containing macromolecular complexes: a small-molecule inhibitor approach

CFTR (cystic fibrosis transmembrane conductance regulator) has been shown to form multiple protein macromolecular complexes with its interacting partners at discrete subcellular microdomains to modulate trafficking, transport and signalling in cells. Targeting protein-protein interactions within these macromolecular complexes would affect the expression or function of the CFTR channel. We specifically targeted the PDZ domain-based LPA2 (type 2 lysophosphatidic acid receptor)-NHERF2 (Na+/H+ exchanger regulatory factor-2) interaction within the CFTR-NHERF2-LPA2-containing macromolecular complexes in airway epithelia and tested its regulatory role on CFTR channel function. We identified a cell-permeable small-molecule compound that preferentially inhibits the LPA2-NHERF2 interaction. We show that this compound can disrupt the LPA2-NHERF2 interaction in cells and thus compromises the integrity of macromolecular complexes. Functionally, it elevates cAMP levels in proximity to CFTR and upregulates its channel activity. The results of the present study demonstrate that CFTR Cl- channel function can be finely tuned by modulating PDZ domain-based protein-protein interactions within the CFTR-containing macromolecular complexes. The present study might help to identify novel therapeutic targets to treat diseases associated with dysfunctional CFTR Cl- channels.

DeltaF508 CFTR processing correction and activity in polarized airway and non-airway cell monolayers

We examined the activity of DeltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) stably expressed in polarized cystic fibrosis bronchial epithelial cells (CFBE41o(-)) human airway cells and Fisher Rat Thyroid (FRT) cells following treatment with low temperature and a panel of small molecule correctors of DeltaF508 CFTR misprocessing. Corr-4a increased DeltaF508 CFTR-dependent Cl(-) conductance in both cell types, whereas treatment with VRT-325 or VRT-640 increased activity only in FRT cells. Total currents stimulated by forskolin and genistein demonstrated similar dose/response effects to Corr-4a treatment in each cell type. When examining the relative contribution of forskolin and genistein to total stimulated current, CFBE41o(-) cells had smaller forskolin-stimulated I(sc) following either low temperature or corr-4a treatment (10-30% of the total I(sc) produced by the combination of both CFTR agonists). In contrast, forskolin consistently contributed greater than 40% of total I(sc) in DeltaF508 CFTR-expressing FRT cells corrected with low temperature, and corr-4a treatment preferentially enhanced forskolin dependent currents only in FRT cells (60% of total I(sc)). DeltaF508 CFTR cDNA transcript levels, DeltaF508 CFTR C band levels, or cAMP signaling did not account for the reduced forskolin response in CFBE41o(-) cells. Treatment with non-specific inhibitors of phosphodiesterases (papaverine) or phosphatases (endothall) did not restore DeltaF508 CFTR activation by forskolin in CFBE41o(-) cells, indicating that the Cl(-) transport defect in airway cells is distal to cAMP or its metabolism. The results identify important differences in DeltaF508 CFTR activation in polarizing epithelial models of CF, and have important implications regarding detection of rescued of DeltaF508 CFTR in vivo.

Compartmentalized cyclic adenosine 3',5'-monophosphate at the plasma membrane clusters PDE3A and cystic fibrosis transmembrane conductance regulator into microdomains

Formation of multiple-protein macromolecular complexes at specialized subcellular microdomains increases the specificity and efficiency of signaling in cells. In this study, we demonstrate that phosphodiesterase type 3A (PDE3A) physically and functionally interacts with cystic fibrosis transmembrane conductance regulator (CFTR) channel. PDE3A inhibition generates compartmentalized cyclic adenosine 3',5'-monophosphate (cAMP), which further clusters PDE3A and CFTR into microdomains at the plasma membrane and potentiates CFTR channel function. Actin skeleton disruption reduces PDE3A-CFTR interaction and segregates PDE3A from its interacting partners, thus compromising the integrity of the CFTR-PDE3A-containing macromolecular complex. Consequently, compartmentalized cAMP signaling is lost. PDE3A inhibition no longer activates CFTR channel function in a compartmentalized manner. The physiological relevance of PDE3A-CFTR interaction was investigated using pig trachea submucosal gland secretion model. Our data show that PDE3A inhibition augments CFTR-dependent submucosal gland secretion and actin skeleton disruption decreases secretion.

Genistein stimulates electrogenic Cl− secretion via phosphodiesterase modulation in the mouse jejunum

Previously, we demonstrated that genistein stimulated Cl− secretion in the mouse jejunum (Baker MJ and Hamilton KL, Am J Physiol Cell Physiol 287: C1636–C1645, 2004); however, the mode of action of genistein still remains unclear. Here, we examined the activation of Cl− secretion by the modulation of phosphodiesterases (PDEs) by genistein (75 μM) in the mouse jejunum with the Ussing short-circuit current ( Isc) technique. Drugs tested included theophylline (10 mM), a nonspecific PDE inhibitor; 8-methoxymethyl-3-isobutyl-1-methylxanthine (8-MM-IBMX; 100 μM), erythro-9-(2-hydroxyl-3-nonyl)-adenine (EHNA; 40 μM), milrinone (100 μM), and rolipram (40 and 100 μM), which are specific inhibitors of PDE1–PDE4, respectively. Theophylline stimulated a bumetanide-sensitive Isc, indicative of Cl− secretion, and abolished genistein's stimulatory action on Isc. Neither 8-MM-IBMX nor EHNA altered the basal Isc nor did these PDE inhibitors affect the stimulatory action of genistein on the Isc of the mouse jejunum. Rolipram had no effect on basal Isc, but it reduced the genistein-stimulated Isc compared with time-matched control tissues. Milrinone stimulated a concentration-dependent increase in Isc. Bumetanide (10 μM) inhibited 60 ± 4% of milrinone-induced Isc. Pretreating tissues with milrinone prevented genistein from stimulating Isc, and pretreatment with genistein reduced the effect of milrinone on Isc. H89 (50 μM), a PKA inhibitor, reduced the milrinone-stimulated Isc. Likewise, H89 reduced the genistein-stimulated Isc. Here, we demonstrate, for the first time, that genistein activates Cl− secretion of the mouse jejunum via inhibition of a PDE3-dependent pathway.

Adenosine signaling and the regulation of chronic lung disease

Chronic lung diseases such as asthma, chronic obstructive pulmonary disease and interstitial lung disease are characterized by inflammation and tissue remodeling processes that compromise pulmonary function. Adenosine is produced in the inflamed and damaged lung where it plays numerous roles in the regulation of inflammation and tissue remodeling. Extracellular adenosine serves as an autocrine and paracrine signaling molecule by engaging cell surface adenosine receptors. Preclinical and cellular studies suggest that adenosine plays an anti-inflammatory role in processes associated with acute lung disease, where activation of the A(2A)R and A(2B)R has promising implications for the treatment of these disorders. In contrast, there is growing evidence that adenosine signaling through the A(1)R, A(2B)R and A(3)R may serve pro-inflammatory and tissue remodeling functions in chronic lung diseases. This review discusses the current progress of research efforts and clinical trials aimed at understanding the complexities of these signaling pathway as they pertain to the development of treatment strategies for chronic lung diseases.

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.

Adenosine promotes IL-6 release in airway epithelia

In the airway epithelia, extracellular adenosine modulates a number of biological processes. However, little is known about adenosine's role in the inflammatory responses of airway epithelial cells. Recent studies suggest that the chronic elevation of extracellular adenosine in mice leads to pulmonary inflammation and fibrosis. Yet, the underlying molecular mechanism has not been well understood and little attention has been paid to the role of airway epithelia in adenosine-triggered inflammation. In the present work, we examined the role of adenosine in releasing IL-6 from airway epithelia. In Calu-3 human airway epithelial cells, apical but not basolateral adenosine elicited robust, apically polarized release of IL-6, along with proinflammatory IL-8. Both protein kinase A and protein kinase C mediated the adenosine-induced IL-6 release, at least partly via phosphorylation of CREB. Protein kinase C appeared to phosphorylate CREB through activating ERK. In addition, A2A but not A2B adenosine receptors were specifically required for the adenosine-induced IL-6 release. Furthermore, in rat bronchoalveolar lavage fluid, adenosine triggered the release of IL-6 as well as proinflammatory IL-1beta. Adenosine also mediated the release of a considerable portion of the LPS-induced IL-6 in rat bronchoalveolar lavage fluid. Our findings provide a possible molecular link between extracellular adenosine elevation and lung inflammation and fibrosis.

Coupling of airway ciliary activity and mucin secretion to mechanical stresses by purinergic signaling

The mucociliary clearance system is comprised of three components, ion transport activities controlling the height of airway surface liquid (ASL), mucin secretion, and ciliary activity. These activities in humans are controlled principally by local agonists, extracellular nucleotides and nucleosides released from the epithelium. Importantly, mechanical stresses stimulate goblet cell mucin secretion, ciliary beating, and Cl- and fluid secretion through mechanically induced nucleotide release. Emerging evidence also implicates co-secretion of nucleotides and mucin from goblet cells as a source of extracellular agonist. At rest, ATP is released onto airway surfaces at approximately 370fmol/mincm2, but only approximately 3% of released ATP is recovered in ASL. Secreted UTP meets with a similar fate. A wide variety of hydrolytic and transphosphorylating ecto-enzymes convert the triphosphate nucleotides into ADP, AMP, and adenosine, UDP, UMP, and uridine. Of these, ATP, adenosine, UTP, and UDP act as agonists at apical P2Y2 (ATP, UTP), P2Y6 (UDP), and A2B (adenosine) receptors on ciliated and/or goblet cells to regulate mucociliary clearance.

Effects of Cordyceps sinensis, Cordyceps militaris and their isolated compounds on ion transport in Calu-3 human airway epithelial cells

AIM OF THE STUDY

The traditional Chinese medicine Cordyceps sinensis (CS) (Clavicipitaceae) improves pulmonary function and is used to treat respiratory disease. Here, we compare the efficacy and mechanisms of action of Cordyceps sinensis and Cordyceps militaris (CM) (Clavicipitaceae) in Calu-3 human airway epithelial monolayer model.

MATERIAL AND METHODS

The extracts of Cordyceps sinensis and Cordyceps militaris, as well as their isolated compounds, cordycepin and adenosine, stimulated ion transport in a dose-dependent manner in Calu-3 monolayers. In subsequent experiments, transport inhibitor bumetanide and carbonic anhydrase inhibitor acetazolamide were added after Cordyceps sinensis and Cordyceps militaris extracts to determine their effects on Cl- and HCO3- movement.

RESULTS

The results suggested that Cordyceps sinensis and Cordyceps militaris extracts may affect the anion movement from the basolateral to apical compartments in the airway epithelia.

CONCLUSIONS

Basolateral Na+-K+-2Cl- cotransporter and apical cAMP-dependent cystic fibrosis transmembrane conductance regulator Cl- channel are involved in the process. The results provide the first evidence for the pharmacological mechanism of Cordyceps sinensis and Cordyceps militaris on respiratory tract.

Apical adenosine regulates basolateral Ca2+-activated potassium channels in human airway Calu-3 epithelial cells

In airway epithelial cells, apical adenosine regulates transepithelial anion secretion by activation of apical cystic fibrosis transmembrane conductance regulator (CFTR) via adenosine receptors and cAMP/PKA signaling. However, the potent stimulation of anion secretion by adenosine is not correlated with its modest intracellular cAMP elevation, and these uncorrelated efficacies have led to the speculation that additional signaling pathways may be involved. Here, we showed that mucosal adenosine-induced anion secretion, measured by short-circuit current (Isc), was inhibited by the PLC-specific inhibitor U-73122 in the human airway submucosal cell line Calu-3. In addition, the Isc was suppressed by BAPTA-AM (a Ca2+ chelator) and 2-aminoethoxydiphenyl borate (2-APB; an inositol 1,4,5-trisphosphate receptor blocker), but not by PKC inhibitors, suggesting the involvement of PKC-independent PLC/Ca2+ signaling. Ussing chamber and patch-clamp studies indicated that the adenosine-induced PLC/Ca2+ signaling stimulated basolateral Ca2+-activated potassium (KCa) channels predominantly via A2B adenosine receptors and contributed substantially to the anion secretion. Thus, our data suggest that apical adenosine activates contralateral K+ channels via PLC/Ca2+ and thereby increases the driving force for transepithelial anion secretion, synergizing with its modulation of ipsilateral CFTR via cAMP/PKA. Furthermore, the dual activation of CFTR and KCa channels by apical adenosine resulted in a mixed secretion of chloride and bicarbonate, which may alter the anion composition in the secretion induced by secretagogues that elicit extracellular ATP/adenosine release. Our findings provide novel mechanistic insights into the regulation of anion section by adenosine, a key player in the airway surface liquid homeostasis and mucociliary clearance.

A2B adenosine receptors regulate the mucus clearance component of the lung's innate defense system

Adenosine (ADO) signaling is altered in both asthma and chronic obstructive pulmonary disease, and the A(2B) adenosine receptor (A(2B)-R) may drive pulmonary inflammation. Accordingly, it has been proposed that specific inhibition of the A(2B)-R could treat inflammatory lung diseases. However, stimulation of the cystic fibrosis transmembrane conductance regulator (CFTR) by ADO may be crucial in permitting the superficial epithelium to maintain airway surface liquid (ASL) volume, which is required to ensure hydrated and clearable mucus. Our goal was to determine which ADO receptor (ADO-R) underlies ASL volume regulation in bronchial epithelia. We used PCR techniques to determine ADO-R expression in bronchial epithelia and used nasal potential difference measurements, Ussing chambers studies, and XZ-confocal microscopy to look at Cl- secretion and ASL volume regulation. The A(2B)-R was the most highly expressed ADO-R in donor specimens of human bronchial epithelia, and inhibition of ADO-R in vivo prevented activation of CFTR. A(2B)-R was the only ADO-R detected in cultured human bronchial epithelial cells and inhibition of this receptor with specific A(2B)-R antagonists resulted in ASL height collapse and a failure to effect ASL height homeostasis. Removal of ADO with ADO deaminase and replacement with 5'N-ethylcarboxamide adenosine resulted in dose-dependent changes in ASL height, and suggested that the cell surface (ADO) may be in excess of 1 microM, which is sufficient to activate A(2B)-R. A(2B)-R are required for ASL volume homeostasis in human airways, and therapies directed at inhibiting A(2B)-R may lead to a cystic fibrosis-like phenotype with depleted ASL volume and mucus stasis.

Preclinical evidence that sildenafil and vardenafil activate chloride transport in cystic fibrosis

RATIONALE

Sildenafil has been implicated in the activation of cystic fibrosis transmembrane conductance regulator (CFTR) protein. The effect was observed in vitro and in the presence of doses roughly 300 times larger than those commonly used for treating erectile dysfunction.

OBJECTIVES

To evaluate in vivo the therapeutic efficacy of clinical doses of sildenafil and vardenafil, two clinically approved phosphodiesterase 5 inhibitors, for activating ion transport in cystic fibrosis.

METHODS

We used transepithelial potential difference in vivo across the nasal mucosa as a measure of sodium and chloride transport. The effect of a single intraperitoneal injection of sildenafil (0.7 mg/kg) or vardenafil (0.14 mg/kg) was investigated in F508del, cftr knockout and normal homozygous mice.

MEASUREMENTS AND MAIN RESULTS

In F508del mice, but not in cftr knockout mice, the chloride conductance, evaluated by perfusing the nasal mucosa with a chloride-free solution in the presence of amiloride and with forskolin, was corrected 1 hour after sildenafil administration. A more prolonged effect, persisting for at least 24 hours, was observed with vardenafil. The forskolin response was increased after sildenafil and vardenafil in both normal and F508del mutant animals. In F508del mice, the chloride conductance in the presence of 200 microM 4-4'-diisothiocyanostilbene-2,2'-disulphonic acid, an inhibitor of alternative chloride channels, was much higher after sildenafil injection than after placebo treatment. No effect on the sodium conductance was detected in any group of animals.

CONCLUSIONS

Our results provide preclinical evidence that both drugs stimulate chloride transport activity of F508del-CFTR protein.

Correctors of Protein Trafficking Defects Identified by a Novel High-Throughput Screening Assay

High-throughput small-molecule screens hold great promise for identifying compounds with potential therapeutic value in the treatment of protein-trafficking diseases such as cystic fibrosis (CF) and nephrogenic diabetes insipidus (NDI). The approach usually involves expressing the mutant form of the gene in cells and assaying function in a multiwell format when cells are exposed to libraries of compounds. Although such functional assays are useful, they do not directly test the ability of a compound to correct defective trafficking of the protein. To address this we have developed a novel corrector-screening assay for CF, in which the appearance of the mutant protein at the cell surface is measured. We used this assay to screen a library of 2000 compounds and have isolated several classes of trafficking correctors that had not previously been identified. This novel screening approach to protein-trafficking diseases is robust and general, and could enable the selection of molecules that could be translated rapidly to a clinical setting.

Liquid movement across the surface epithelium of large airways

The cystic fibrosis transmembrane conductance regulator CFTR gene is found on chromosome 7 [Kerem, B., Rommens, J.M., Buchanan, J.A., Markiewicz, D., Cox, T.K., Chakravarti, A., Buchwald, M., Tsui, L.C., 1989. Identification of the cystic fibrosis gene: genetic analysis. Science 245, 1073-1080; Riordan, J.R., Rommens, J.M., Kerem, B., Alon, N., Rozmahel, R., Grzelczak, Z., Zielenski, J., Lok, S., Plavsic, N., Chou, J.L., et al., 1989. Identification of the cystic fibrosis gene: cloning and characterization of complementary DNA. Science 245, 1066-1073] and encodes for a 1480 amino acid protein which is present in the plasma membrane of epithelial cells [Anderson, M.P., Sheppard, D.N., Berger, H.A., Welsh, M.J., 1992. Chloride channels in the apical membrane of normal and cystic fibrosis airway and intestinal epithelia. Am. J. Physiol. 263, L1-L14]. This protein appears to have many functions, but a unifying theme is that it acts as a protein kinase C- and cyclic AMP-regulated Cl(-) channel [Winpenny, J.P., McAlroy, H.L., Gray, M.A., Argent, B.E., 1995. Protein kinase C regulates the magnitude and stability of CFTR currents in pancreatic duct cells. Am. J. Physiol. 268, C823-C828; Jia, Y., Mathews, C.J., Hanrahan, J.W., 1997. Phosphorylation by protein kinase C is required for acute activation of cystic fibrosis transmembrane conductance regulator by protein kinase A. J. Biol. Chem. 272, 4978-4984]. In the superficial epithelium of the conducting airways, CFTR is involved in Cl(-) secretion [Boucher, R.C., 2003. Regulation of airway surface liquid volume by human airway epithelia. Pflugers Arch. 445, 495-498] and also acts as a regulator of the epithelial Na(+) channel (ENaC) and hence Na(+) absorption [Boucher, R.C., Stutts, M.J., Knowles, M.R., Cantley, L., Gatzy, J.T., 1986. Na(+) transport in cystic fibrosis respiratory epithelia. Abnormal basal rate and response to adenylate cyclase activation. J. Clin. Invest. 78, 1245-1252; Stutts, M.J., Canessa, C.M., Olsen, J.C., Hamrick, M., Cohn, J.A., Rossier, B.C., Boucher, R.C., 1995. CFTR as a cAMP-dependent regulator of sodium channels. Science 269, 847-850]. In this chapter, we will discuss the regulation of these two ion channels, and how they can influence liquid movement across the superficial airway epithelium.

Adenosine-evoked Na+ transport in human airway epithelial cells

BACKGROUND AND PURPOSE

Absorptive epithelia express apical receptors that allow nucleotides to inhibit Na(+) transport but ATP unexpectedly stimulated this process in an absorptive cell line derived from human bronchiolar epithelium (H441 cells) whilst UTP consistently caused inhibition. We have therefore examined the pharmacological basis of this anomalous effect of ATP.

EXPERIMENTAL APPROACH

H441 cells were grown on membranes and the short circuit current (I(SC)) measured in Ussing chambers. In some experiments, [Ca(2+)](i) was measured fluorimetrically using Fura -2. mRNAs for adenosine receptors were determined by the polymerase chain reaction (PCR).

KEY RESULTS

Cross desensitization experiments showed that the inhibitory response to UTP was abolished by prior exposure to ATP whilst the stimulatory response to ATP persisted in UTP-pre-stimulated cells. Apical adenosine evoked an increase in I(SC) and this response resembled the stimulatory component of the response to ATP, and could be mimicked by adenosine receptor agonists. Pre-stimulation with adenosine abolished the stimulatory component of the response to ATP. mRNA encoding A(1), A(2A) and A(2B) receptor subtypes, but not the A(3) subtype, was detected in H441 cells and adenosine receptor antagonists could abolish the ATP-evoked stimulation of Na(+) absorption.

CONCLUSIONS AND IMPLICATIONS

The ATP-induced stimulation of Na(+) absorption seems to be mediated via A(2A/B) receptors activated by adenosine produced from the extracellular hydrolysis of ATP. The present data thus provide the first description of adenosine-evoked Na(+) transport in airway epithelial cells and reveal a previously undocumented aspect of the control of this physiologically important ion transport process.

Protease-activated receptor regulation of Cl- secretion in Calu-3 cells requires prostaglandin release and CFTR activation

Human lung epithelial (Calu-3) cells were used to investigate the effects of protease-activated receptor (PAR) stimulation on Cl(-) secretion. Quantitative RT-PCR (QRT-PCR) showed that Calu-3 cells express PAR-1, -2, and -3 receptor mRNAs, with PAR-2 mRNA in greatest abundance. Addition of either thrombin or the PAR-2 agonist peptide SLIGRL to the basolateral solution of monolayers mounted in Ussing chambers produced a rapid increase in short-circuit current (I(sc): thrombin, 21 +/- 2 microA; SLIGRL, 83 +/- 22 microA), which returned to baseline within 5 min after stimulation. Pretreatment of monolayers with the cell-permeant Ca(2+)-chelating agent BAPTA-AM (50 microM) abolished the increase in I(sc) produced by SLIGRL. When monolayers were treated with the cyclooxygenase inhibitor indomethacin (10 microM), nearly complete inhibition of both the thrombin- and SLIGRL-stimulated I(sc) was observed. In addition, basolateral treatment with the PGE(2) receptor antagonist AH-6809 (25 microM) significantly inhibited the effects of SLIGRL on I(sc). QRT-PCR revealed that Calu-3 cells express mRNAs for CFTR, the Ca(2+)-activated KCNN4 K(+) channel, and the KCNQ1 K(+) channel subunit, which, in association with KCNE3, is known to be regulated by cAMP. Stimulation with SLIGRL produced an increase in apical Cl(-) conductance that was blocked in cells expressing short hairpin RNAs designed to target CFTR. These results support the conclusion that PAR stimulation of Cl(-) secretion occurs by an indirect mechanism involving the synthesis and release of prostaglandins. In addition, PAR-stimulated Cl(-) secretion requires activation of CFTR and at least two distinct K(+) channels located in the basolateral membrane.

Inducible NO synthase expression is low in airway epithelium from young children with cystic fibrosis

BACKGROUND

This is the first study to measure inducible nitric oxide synthase (iNOS) gene and protein expression quantitatively in primary epithelial cells from very young children with cystic fibrosis (CF). Low levels of exhaled nitric oxide (NO) in CF suggest dysregulation of NO production in the airway. Due to the importance of NO in cell homeostasis and innate immunity, any defect in the pathway associated with CF would be a potential target for treatment.

METHODS

Cells were obtained by tracheobronchial brushing from 40 children with CF of mean (SD) age 2.1 (1.5) years and from 12 healthy non-atopic children aged 3.4 (1.2) years. Expression of iNOS mRNA was measured using quantitative PCR and iNOS protein by immunofluorescence and Western blot analysis.

RESULTS

Inducible NOS mRNA expression was significantly lower in CF patients with and without bacterial infection than in healthy children (0.22 and 0.23 v 0.76; p=0.002 and p=0.01, respectively). Low levels of iNOS gene expression were accompanied by low levels of iNOS protein expression as detected by Western blot analysis.

CONCLUSIONS

These results support the findings of previous studies in adult patients with advanced disease, cell lines, and animal models. Our findings reflect the situation in children with mild lung disease. They indicate that low iNOS expression may be an innate defect in CF with potential consequences for local antimicrobial defence and epithelial cell function and provide evidence for an approach to treatment based on increasing epithelial NO production or the sensitivity of NO dependent cellular processes.

Adenosine regulation of cystic fibrosis transmembrane conductance regulator through prostenoids in airway epithelia

Cystic fibrosis is caused by dysfunction of the cystic fibrosis transmembrane conductance regulator (CFTR) protein, leading to altered ion transport, chronic infection, and excessive inflammation. Here we investigated regulation of CFTR in airway cell monolayers by adenosine, adenosine receptors, and arachidonic acid. Our studies demonstrate that the A2B adenosine receptor is expressed at high levels relative to the other adenosine receptor subtypes, with a characteristic low-affinity profile for adenosine-stimulated CFTR Cl- currents in both Calu-3 cells and CFBE41o- airway cell monolayers stably transduced with wild-type CFTR. The levels of adenosine found in sputum from patients with cystic fibrosis with moderate to severe lung disease stimulated apical prostaglandin release in Calu-3 and CFBE41o- cells, implicating adenosine regulation of phospholipase A2 (PLA2) activity. A2B adenosine receptor and arachidonic acid stimulation produced CFTR-dependent currents in airway monolayers and increased cAMP levels that were sensitive to cyclooxygenase inhibition. Arachidonic acid demonstrated dual regulation of CFTR, stimulating CFTR and Cl- currents in intact airway monolayers, and potently inhibiting PKA-activated Cl- currents in excised membrane patches. Cl- currents produced by arachidonic acid were sensitive to inhibition of PKA, cyclooxygenase, and 5-lipoxygenase. Together, the results provide a converging mechanism to link regulation of CFTR and airway cell inflammation through adenosine and adenosine receptors.

Failure of cAMP agonists to activate rescued deltaF508 CFTR in CFBE41o- airway epithelial monolayers

The cystic fibrosis transmembrane conductance regulator (CFTR) is a cyclic AMP-regulated chloride channel. Mutations in the CFTR gene result in cystic fibrosis (CF). The most common mutation, deltaF508, results in endoplasmic reticulum-associated degradation (ERAD) of CFTR. DeltaF508 CFTR has been described as a temperature-sensitive mutation that can be rescued following growth at 27 degrees C. In order to study the processing and function of wild-type and rescued deltaF508 CFTR at the cell surface under non-polarized and polarized conditions, we developed stable cell lines expressing deltaF508 or wild-type CFTR. CFBE41o- is a human airway epithelial cell line capable of forming high resistance, polarized monolayers when cultured on permeable supports, while HeLa cells are normally grown under non-polarizing conditions. Immunoprecipitation, cell surface biotinylation, immunofluorescence, and functional assays confirmed the presence of deltaF508 CFTR at the cell surface in both cell lines after incubating the cells for 48 h at 27 degrees C. However, stimulators of wild-type CFTR such as forskolin, beta2-adrenergic or A2B-adenosine receptor agonists failed to activate rescued deltaF508 CFTR in CFBE41o- monolayers. Rescued deltaF508 CFTR could be stimulated with genistein independent of pretreatment with cAMP signalling agonists. Interestingly, rescued deltaF508 CFTR in HeLa cells could be efficiently stimulated with either forskolin or genistein to promote Cl- transport. These results indicate that deltaF508 CFTR, when rescued in CFBE41o- human airway epithelial cells, is poorly responsive to signalling pathways known to regulate wild-type CFTR. Furthermore, the differences in rescue and activation of deltaF508 CFTR in the two cell lines suggest that cell-type specific differences in deltaF508 CFTR processing are likely to complicate efforts to identify potentiators and/or correctors of the deltaF508 defect.

Dynamic activation of cystic fibrosis transmembrane conductance regulator by type 3 and type 4D phosphodiesterase inhibitors

The diseases of cystic fibrosis, chronic obstructive pulmonary disease (COPD), and chronic bronchitis are characterized by mucus-congested and inflamed airways. Anti-inflammatory agents that can simultaneously restore or enhance mucociliary clearance through cystic fibrosis transmembrane conductance regulator (CFTR) activation may represent new therapeutics in their treatment. Herein, we report the activation of CFTR-mediated chloride secretion by phosphodiesterase (PDE) 4 inhibitors in T84 monolayer using (125)I anion as tracer. In the absence of forskolin, the iodide secretion was insensitive to PDE4 inhibitor L-826,141 [4-[2-(3,4-bis-difluoromethoxyphenyl)-2-[4-(1,1,1,3,3,3-hexafluoro-2-hydroxypropan-2-yl)phenyl]-ethyl]-3-methylpyridine-1-oxide], roflumilast, or to PDE3 inhibitor trequinsin. However, these inhibitors potently augmented iodide secretion after forskolin stimulation, with efficacy coupled to the activation states of adenylyl cyclase. The iodide secretion from PDE3 or PDE4 inhibition was characterized at first by a prolonged efflux duration, followed by progressively elevated peak efflux rates at higher inhibitor concentrations. Paralleled with an increased phosphor-cAMP response element-binding protein formation, the CFTR activation dissociated from a global cAMP elevation and was blocked by H89 [N-[2-((p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide]. 2-(4-Fluorophenoxy)-N-[(1S)-1-(4-methoxyphenyl)ethyl]nicotinamide, a stereoselective PDE4D inhibitor, augmented iodide efflux more efficiently than its less potent (R)-isomer. The peak efflux from maximal PDE4 and PDE3 inhibition matched that from full adenylyl cyclase activation. These data suggest that PDE3 and PDE4 (mainly PDE4D) form the major cAMP diffusion barrier in T84 cells to ensure a compartmentalized CFTR signaling. Together with their potent anti-inflammatory properties, the potentially enhanced airway mucociliary clearance from CFTR activation may have contributed to the efficacy of PDE4 inhibitors in COPD and asthmatic patients. PDE4 inhibitors may represent new opportunities to combat cystic fibrosis and other respiratory diseases in future.

Phosphodiesterase 4D Forms a cAMP Diffusion Barrier at the Apical Membrane of the Airway Epithelium

We demonstrated previously that Calu-3 airway epithelial cells sense adenosine on their luminal surface through adenosine A2B receptors coupled to adenylyl cyclase. Occupancy of these receptors leads to activation of the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel through protein kinase A (PKA) anchored at the apical membrane. Because luminal A2B receptor activation does not raise total cellular cAMP levels, we hypothesized that activation of phosphodiesterases (PDEs) confines cAMP generated by apical A2B receptors to a microdomain that includes the CFTR channel. Using reverse transcription-PCR, Western blotting, and activity measurements, PDE4D was identified as the major PDE species in airway epithelia. Consistent with these results, inhibitors of PDE4, but not PDE3, selectively abolished the lateral confinement of cAMP signaling in apical membrane patches during cell-attached recordings. Furthermore, stimulation of the CFTR in excised apical patches by rolipram and RS25344 indicated that PDE4 is localized in close proximity to the CFTR channel. Indeed, immunohistochemistry of human airway sections revealed that PDE4D is localized in the apical domain of the cell. PDE4 was activated after luminal adenosine exposure in a PKA-dependent manner. Because PDE4 activity is positively regulated by PKA, our results support a model whereby the PDE diffusion barrier is proportional to the degree of receptor stimulation. These findings underscore the concept that subcellular localization of individual PDE isozymes is an important mechanism for confining cAMP signaling to functional domains within cells.

Nucleotide release provides a mechanism for airway surface liquid homeostasis

Nucleotides within the airway surface liquid (ASL) regulate airway epithelial ion transport rates by Ca(2+) -and protein kinase C-dependent mechanisms via activation of specific P2Y receptors. Extracellular adenine nucleotides also serve as precursors for adenosine, which promotes cyclic AMP-mediated activation of the cystic fibrosis transmembrane regulator chloride channel via A(2b) adenosine receptors. A biological role for extracellular ATP in ASL volume homeostasis has been suggested by the demonstration of regulated ATP release from airway epithelia. However, nucleotide hydrolysis at the airway surface makes it difficult to assess the magnitude of ATP release and the relative abundance of adenyl purines and, hence, to define their biological functions. We have combined ASL microsampling and high performance liquid chromatography analysis of fluorescent 1,N(6)-ethenoadenine derivatives to measure adenyl purines in ASL. We found that adenosine, AMP, and ADP accumulated in high concentrations relative to ATP within the ASL covering polarized primary human normal or cystic fibrosis airway epithelial cells. By using immortalized epithelial cell monolndogenayers that eously express a luminal A(2b) adenosine receptor, we found that basal as well asforskolin-promoted cyclic AMP production was reduced by exogenous adenosine deaminase, suggesting that A(2b) receptors sense endogenous adenosine within the ASL. The physiological role of adenosine was further established by illustrating that adenosine removal or inhibition of adenosine receptors in primary cultures impaired ASL volume regulation. Our data reveal a complex pattern of nucleotides/nucleosides in ASL under resting conditions and suggest that adenosine may play a key role in regulating ASL volume homeostasis.

Activation of airway cl- secretion in human subjects by adenosine

We investigated cystic fibrosis (CF) transmembrane conductance regulator (CFTR) regulation by A2 adenosine (Ado) receptors and beta2 adrenergic receptors in CFTR-corrected CFBE41o- airway cells and human subjects. CFBE41o- cells stimulated with Ado (10 microM), isoproterenol (Iso, 10 microM), or Ado + Iso (10 microM each) elevated cyclic AMP (cAMP) above control conditions (P < 0.001), with the Iso conditions increasing cAMP approximately 10-fold above that produced by Ado alone (P < 0.001). All agonist conditions had similar effects on short circuit current at 10 and 25 microM, with no further currents produced by subsequent stimulation with forskolin (20 microM). CFTR dependence was demonstrated by glybenclamide block of agonist-stimulated currents. Nasal potential difference studies in normal (n = 50) subjects demonstrated that Ado (10 microM) and Ado + Iso (10 microM each) produced more polarization compared with Iso (10 microM Ado increase = 44%, 10 microM Ado + Iso increase = 52%, P < 0.05 for each condition compared with Iso alone). Studies completed in patients with CF (n = 10, "severe" genotypes) confirmed that Ado-stimulated polarization was CFTR-dependent. Together, these results indicate that Ado is a potent Cl- secretagogue in vivo, with relatively small effects on cAMP levels despite strong effects on CFTR-dependent short circuit current and nasal Cl- transport. These findings support growing evidence indicating a role for Ado regulation of CFTR-dependent Cl- secretion in vivo.

Purinergic regulation of epithelial transport

Purinergic receptors are a family of ubiquitous transmembrane receptors comprising two classes, P1 and P2 receptors, which are activated by adenosine and extracellular nucleotides (i.e. ATP, ADP, UTP and UDP), respectively. These receptors play a significant role in regulating ion transport in epithelial tissues through a variety of intracellular signalling pathways. Activation of these receptors is partially dependent on ATP (or UTP) release from cells and its subsequent metabolism, and this release can be triggered by a number of stimuli, often in the setting of cellular damage. The function of P2Y receptor stimulation is primarily via signalling through the G(q)/PLC-beta pathway and subsequent activation of Ca(2+)-dependent ion channels. P1 signalling is complex, with each of the four P1 receptors A(1), A(2A), A(2B), and A(3) having a unique role in different epithelial tissue types. In colonic epithelium the A(2B) receptor plays a prominent role in regulating Cl(-) and water secretion. In airway epithelium, A(2B) and A(1) receptors are implicated in the control of Cl(-) and other currents. In the renal tubular epithelium, A(1), A(2A), and A(3) receptors have all been identified as playing a role in controlling the ionic composition of the lumenal fluid. Here we discuss the intracellular signalling pathways for each of these receptors in various epithelial tissues and their roles in pathophysiological conditions such as cystic fibrosis.