Chloride transporting capability of Calu-3 epithelia following persistent knockdown of the cystic fibrosis transmembrane conductance regulator, CFTR

A Peptide Nucleic Acid (PNA) Masking the miR-145-5p Binding Site of the 3'UTR of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) mRNA Enhances CFTR Expression in Calu-3 Cells

Peptide nucleic acids (PNAs) have been demonstrated to be very useful tools for gene regulation at different levels and with different mechanisms of action. In the last few years the use of PNAs for targeting microRNAs (anti-miRNA PNAs) has provided impressive advancements. In particular, targeting of microRNAs involved in the repression of the expression of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which is defective in cystic fibrosis (CF), is a key step in the development of new types of treatment protocols. In addition to the anti-miRNA therapeutic strategy, inhibition of miRNA functions can be reached by masking the miRNA binding sites present within the 3′UTR region of the target mRNAs. The objective of this study was to design a PNA masking the binding site of the microRNA miR-145-5p present within the 3′UTR of the CFTR mRNA and to determine its activity in inhibiting miR-145-5p function, with particular focus on the expression of both CFTR mRNA and CFTR protein in Calu-3 cells. The results obtained support the concept that the PNA masking the miR-145-5p binding site of the CFTR mRNA is able to interfere with miR-145-5p biological functions, leading to both an increase of CFTR mRNA and CFTR protein content.

Most bicarbonate secretion by Calu-3 cells is mediated by CFTR and independent of pendrin

Bicarbonate plays an important role in airway host defense, however, its transport mechanisms remain uncertain. Here we examined the relative contributions of the anion channel CFTR (cystic fibrosis transmembrane conductance regulator, ABCC7) and the anion exchanger pendrin (SLC26A4) to HCO3- secretion by the human airway cell line Calu-3. Pendrin and CFTR were both detected in parental Calu-3 cells, although mRNA and protein expression appeared higher for CFTR than for pendrin. Targeting pendrin transcripts with lentiviral shRNA reduced pendrin detection by immunofluorescence staining but did not alter the rates of HCO3- or fluid secretion, HCO3- transport under pH-stat conditions, or net HCO3- flux across basolaterally permeabilized monolayers. Intracellular pH varied with step changes in apical Cl- and HCO3- concentrations in control and pendrin knockdown Calu-3 cells, but not in CFTR deficient cells. Exposure to the proinflammatory cytokine IL-4, which strongly upregulates pendrin expression in airway surface epithelia, had little effect on Calu-3 pendrin expression and did not alter fluid or HCO3- secretion. Similar results were obtained using air-liquid interface and submerged cultures, although CFTR and pendrin mRNA expression were both lower when cells were cultured under submerged conditions. While the conclusions cannot be extrapolated to other airway epithelia, the present results demonstrate that most HCO3- secretion by Calu-3 cells is mediated by CFTR.

Identification of the fatty acid activation site on human ClC-2

Fatty acids (including lubiprostone and cobiprostone) are human ClC-2 (hClC-2) Cl^- channel activators. Molecular and cellular mechanisms underlying this activation were examined. Role of a four-amino acid PKA activation site, RGET₆₉₁, of hClC-2 was investigated using wild-type (WT) and mutant (AGET, RGEA, and AGAA) hClC-2 expressed in 293EBNA cells as well as involvement of PKA, intracellular cAMP concentration ([cAMP]_i), EP₂, or EP₄ receptor agonist activity. All fatty acids [lubiprostone, cobiprostone, eicosatetraynoic acid (ETYA), oleic acid, and elaidic acid] caused significant rightward shifts in concentration-dependent Cl^- current activation (increasing EC₅₀s) with mutant compared with WT hClC-2 channels, without changing time and voltage dependence, current-voltage rectification, or methadone inhibition of the channel. As with lubiprostone, cobiprostone activation of hClC-2 occurred with PKA inhibitor (myristoylated protein kinase inhibitor) present or when using double PKA activation site (RRAA₆₅₅/RGEA₆₉₁) mutant. Cobiprostone did not activate human CFTR. Fatty acids did not increase [cAMP]_i in hClC-2/293EBNA or T84 cells. Using T84 CFTR knockdown cells, cobiprostone increased hClC-2 Cl^- currents without increasing [cAMP]_i, while PGE₂ and forskolin-IBMX increased both. Fatty acids were not agonists of EP₂ or EP₄ receptors. L-161,982, a supposed EP₄-selective inhibitor, had no effect on lubiprostone-activated hClC-2 Cl^- currents but significantly decreased T84 cell barrier function measured by transepithelial resistance and fluorescent dextran transepithelial movement. The present findings show that RGET₆₉₁ of hClC-2 (possible binding site) plays an important functional role in fatty acid activation of hClC-2. PKA, [cAMP]_i, and EP₂ or EP₄ receptors are not involved. These studies provide the molecular basis for fatty acid regulation of hClC-2.

Mechanosensitivity of wild-type and G551D cystic fibrosis transmembrane conductance regulator (CFTR) controls regulatory volume decrease in simple epithelia

Mutations of cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial ligand-gated anion channel, are associated with the lethal genetic disease cystic fibrosis. The CFTR G551D mutation impairs ATP hydrolysis and thereby makes CFTR refractory to cAMP stimulation. Both wild-type (WT) and G551D CFTR have been implicated in regulatory volume decrease (RVD), but the underlying mechanism remains incompletely understood. Here, we show that the channel activity of both WT and G551D CFTR is directly stimulated by mechanical perturbation induced by cell swelling at the single-channel, cellular, and tissue levels. Hypotonicity activated CFTR single channels in cell-attached membrane patches and WT-CFTR-mediated short-circuit current (Isc) in Calu-3 cells, and this was independent of Ca(2+)and cAMP/PKA signaling. Genetic suppression and ablation but not G551D mutation of CFTR suppressed the hypotonicity- and stretch-inducedIscin Calu-3 cells and mouse duodena. Moreover, ablation but not G551D mutation of the CFTR gene inhibited the RVD of crypts isolated from mouse intestine; more importantly, CFTR-specific blockers markedly suppressed RVD in both WT- and G551D CFTR mice, demonstrating for the first time that the channel activity of both WT and G551D CFTR is required for epithelial RVD. Our findings uncover a previously unrecognized mechanism underlying CFTR involvement in epithelial RVD and suggest that the mechanosensitivity of G551D CFTR might underlie the mild phenotypes resulting from this mutation.-Xie, C., Cao, X., Chen, X, Wang, D., Zhang, W. K., Sun, Y., Hu, W., Zhou, Z., Wang, Y., Huang, P. Mechanosensitivity of wild-type and G551D cystic fibrosis transmembrane conductance regulator (CFTR) controls regulatory volume decrease in simple epithelia.

ClC-2 regulation of intestinal barrier function: Translation of basic science to therapeutic target

The ClC-2 chloride channel is a member of the voltage-gated chloride channel family. ClC-2 is involved in various physiological processes, including fluid transport and secretion, regulation of cell volume and pH, maintaining the membrane potential of the cell, cell-to-cell communication, and tissue homeostasis. Recently, our laboratory has accumulated evidence indicating a critical role of ClC-2 in the regulation of intestinal barrier function by altering inter-epithelial tight junction composition. This review will detail the role of ClC-2 in intestinal barrier function during intestinal disorders, including experimental ischemia/reperfusion injury and dextran sodium sulfate (DSS)-induced inflammatory bowel disease. Details of pharmacological manipulation of ClC-2 via prostone agonists will also be provided in an effort to show the potential therapeutic relevance of ClC-2 regulation, particularly during intestinal barrier disruption.

Protein phosphatase 1 coordinates CFTR-dependent airway epithelial HCO3- secretion by reciprocal regulation of apical and basolateral membrane Cl(-)-HCO3- exchangers

BACKGROUND AND PURPOSE

Our recent studies on human airway serous-like Calu-3 cells showed that cAMP agonists stimulated a HCO3(-) rich secretion containing up to 80 mM HCO3(-). This alkaline secretion relied on a coordinated switch in the activity of distinct Cl(-)-HCO3(-) anion exchangers (AE) located at different regions of the cell. At the apical membrane, cAMP agonists activated the electroneutral AE pendrin (SLC26A4), together with cystic fibrosis transmembrane conductance regulator (CFTR), while at the basolateral membrane the agonists inhibited AE2 (SLC4A2). However, the underlying mechanism(s) that orchestrates this cAMP-dependent switch in AE activity has not been elucidated.

EXPERIMENTAL APPROACH

Apical and basolateral Cl(-)-HCO3(-) exchange was assessed by measuring Cl(-)-dependent changes in intracellular pH (pH(i)).

KEY RESULTS

We show that protein phosphatase 1 (PP1), together with CFTR, play central roles in this reciprocal regulation of AE activity. Activation of pendrin by cAMP agonists, but not inhibition of the basolateral exchanger, was protein kinase A-dependent. Knocking down CFTR expression, or blocking its activity with GlyH-101, led to incomplete inhibition of the basolateral AE by cAMP, supporting a role for CFTR in this process. Addition of the PP1/2A inhibitor, okadaic acid, but not the PP2A specific inhibitor fostreicin, mimicked the effect of cAMP stimulation. Furthermore, okadaic acid-treated Calu-3 monolayers produced a more alkaline fluid than untreated cells, which was comparable with that produced by cAMP stimulation.

CONCLUSIONS AND IMPLICATIONS

These results identify PP1 as a novel regulator of AE activity which, in concert with CFTR, coordinates events at both apical and basolateral membranes, crucial for efficient HCO3(-) secretion from Calu-3 cells.

A comparison of linaclotide and lubiprostone dosing regimens on ion transport responses in human colonic mucosa

Linaclotide, a synthetic guanylyl cyclase C (GC-C) agonist, and the prostone analog, Lubiprostone, are approved to manage chronic idiopathic constipation and constipation-predominant irritable bowel syndrome. Lubiprostone also protects intestinal mucosal barrier function in ischemia. GC-C signaling regulates local fluid balance and other components of intestinal mucosal homeostasis including epithelial barrier function. The aim of this study was to compare if select dosing regimens differentially affect linaclotide and lubiprostone modulation of ion transport and barrier properties of normal human colonic mucosa. Normal sigmoid colon biopsies from healthy subjects were mounted in Ussing chambers. Tissues were treated with linaclotide, lubiprostone, or vehicle to determine effects on short-circuit current (I_sc). Subsequent I_sc responses to the cAMP agonist, forskolin, and the calcium agonist, carbachol, were also measured to assess if either drug caused desensitization. Barrier properties were assessed by measuring transepithelial electrical resistance. I_sc responses to linaclotide and lubiprostone were significantly higher than vehicle control when administered bilaterally or to the mucosal side only. Single versus cumulative concentrations of linaclotide showed differences in efficacy while cumulative but not single dosing caused desensitization to forskolin. Lubiprostone reduced forskolin responses under all conditions. Linaclotide and lubiprostone exerted a positive effect on TER that was dependent on the dosing regimen. Linaclotide and lubiprostone increase ion transport responses across normal human colon but linaclotide displays increased sensitivity to the dosing regimen used. These findings may have implications for dosing protocols of these agents in patients with constipation.

Pulmonary epithelial barrier function: some new players and mechanisms

The pulmonary epithelium serves as a barrier to prevent access of the inspired luminal contents to the subepithelium. In addition, the epithelium dictates the initial responses of the lung to both infectious and noninfectious stimuli. One mechanism by which the epithelium does this is by coordinating transport of diffusible molecules across the epithelial barrier, both through the cell and between cells. In this review, we will discuss a few emerging paradigms of permeability changes through altered ion transport and paracellular regulation by which the epithelium gates its response to potentially detrimental luminal stimuli. This review is a summary of talks presented during a symposium in Experimental Biology geared toward novel and less recognized methods of epithelial barrier regulation. First, we will discuss mechanisms of dynamic regulation of cell-cell contacts in the context of repetitive exposure to inhaled infectious and noninfectious insults. In the second section, we will briefly discuss mechanisms of transcellular ion homeostasis specifically focused on the role of claudins and paracellular ion-channel regulation in chronic barrier dysfunction. In the next section, we will address transcellular ion transport and highlight the role of Trek-1 in epithelial responses to lung injury. In the final section, we will outline the role of epithelial growth receptor in barrier regulation in baseline, acute lung injury, and airway disease. We will then end with a summary of mechanisms of epithelial control as well as discuss emerging paradigms of the epithelium role in shifting between a structural element that maintains tight cell-cell adhesion to a cell that initiates and participates in immune responses.

Lubiprostone decreases mouse colonic inner mucus layer thickness and alters intestinal microbiota

BACKGROUND

Lubiprostone has been used to treat constipation through its effects to stimulate Cl(-) secretion, resulting in water and electrolyte secretion.

AIM

Potential associated changes in intestinal mucus and the colonizing bacteria (microbiome) have not been studied. As mucus obstructions may play a role in cystic fibrosis, the hypothesis that lubiprostone alters intestinal mucus and the microbiome was investigated.

METHODS

Ion transport studies were performed ex vivo. For mucus and microbiome studies, mice were gavaged daily with lubiprostone or vehicle. Mucin from intestinal sections was analyzed in Carnoy's fixed tissues stained with Alcian blue. Microbiome composition was analyzed by 16S rRNA gene-based sequencing.

RESULTS

Lubiprostone stimulated short circuit current in all mouse intestinal segments after both serosal and mucosal additions, albeit at lower concentrations in the latter. Current was Cl-dependent and blocked by mucosal diphenylcarboxylic acid, serosal bumetanide, and serosal Ba(++). The CFTR inhibitor CFTRinh172 had a marginal effect. Mucus near epithelial cells (inner layer mucus) was not present in the small intestine of any mice. Proximal colon inner mucus layer was thicker in ∆F/∆F compared with +/∆F and +/+ mice. Lubiprostone decreased inner mucus layer thickness in both proximal and distal colon of all mice. Furthermore, lubiprostone altered the intestinal microbiome by increasing abundance of Lactobacillus and Alistipes.

CONCLUSIONS

Lubiprostone activates non-CFTR Cl(-) secretion and alters the colonic inner mucus layer, which is associated with changes in the composition of the enteric microbiome.

The mitochondrial complex I activity is reduced in cells with impaired cystic fibrosis transmembrane conductance regulator (CFTR) function

Cystic fibrosis (CF) is a frequent and lethal autosomal recessive disease. It results from different possible mutations in the CFTR gene, which encodes the CFTR chloride channel. We have previously studied the differential expression of genes in CF and CF corrected cell lines, and found a reduced expression of MTND4 in CF cells. MTND4 is a mitochondrial gene encoding the MTND4 subunit of the mitochondrial Complex I (mCx-I). Since this subunit is essential for the assembly and activity of mCx-I, we have now studied whether the activity of this complex was also affected in CF cells. By using Blue Native-PAGE, the in-gel activity (IGA) of the mCx-I was found reduced in CFDE and IB3-1 cells (CF cell lines) compared with CFDE/6RepCFTR and S9 cells, respectively (CFDE and IB3-1 cells ectopically expressing wild-type CFTR). Moreover, colon carcinoma T84 and Caco-2 cells, which express wt-CFTR, either treated with CFTR inhibitors (glibenclamide, CFTR(inh)-172 or GlyH101) or transfected with a CFTR-specific shRNAi, showed a significant reduction on the IGA of mCx-I. The reduction of the mCx-I activity caused by CFTR inhibition under physiological or pathological conditions may have a profound impact on mitochondrial functions of CF and non-CF cells.

Lubiprostone activates CFTR, but not ClC-2, via the prostaglandin receptor (EP(4))

The goal of this study was to determine the mechanism of lubiprostone activation of epithelial chloride transport. Lubiprostone is a bicyclic fatty acid approved for the treatment of constipation [1]. There is uncertainty, however, as to how lubiprostone increases epithelial chloride transport. Direct stimulation of ClC-2 and CFTR chloride channels as well as stimulation of these channels via the EP(4) receptor has been described [2-5]. To better define this mechanism, two-electrode voltage clamp was used to assay Xenopus oocytes expressing ClC-2, with or without co-expression of the EP(4) receptor or β adrenergic receptor (βAR), for changes in conductance elicited by lubiprostone. Oocytes co-expressing CFTR and either βAR or the EP(4) receptor were also studied. In oocytes co-expressing ClC-2 and βAR conductance was stimulated by hyperpolarization and acidic pH (pH = 6), but there was no response to the β adrenergic agonist, isoproterenol. Oocytes expressing ClC-2 only or co-expressing ClC-2 and EP(4) did not respond to the presence of 0.1, 1, or 10 μM lubiprostone in the superperfusate. Oocytes co-expressing CFTR and βAR did not respond to hyperpolarization, acidic pH, or 1 μM lubiprostone. However, conductance was elevated by isoproterenol and inhibited by CFTR(inh)172. Co-expression of CFTR and EP(4) resulted in lubiprostone-stimulated conductance, which was also sensitive to CFTR(inh)172. The EC(50) for lubiprostone mediated CFTR activation was ~10 nM. These results demonstrate no direct action of lubiprostone on either ClC-2 or CFTR channels expressed in oocytes. However, the results confirm that CFTR can be activated by lubiprostone via the EP(4) receptor in oocytes.

Bicarbonate-dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu-3

Anion and fluid secretion are both defective in cystic fibrosis (CF); however, the transport mechanisms are not well understood. In this study, Cl(-) and HCO(3)(-) secretion was measured using genetically matched CF transmembrane conductance regulator (CFTR)-deficient and CFTR-expressing cell lines derived from the human airway epithelial cell line Calu-3. Forskolin stimulated the short-circuit current (I(sc)) across voltage-clamped monolayers, and also increased the equivalent short-circuit current (I(eq)) calculated under open-circuit conditions. I(sc) was equivalent to the HCO(3)(-) net flux measured using the pH-stat technique, whereas I(eq) was the sum of the Cl(-) and HCO(3)(-) net fluxes. I(eq) and HCO(3)(-) fluxes were increased by bafilomycin and ZnCl(2), suggesting that some secreted HCO(3)(-) is neutralized by parallel electrogenic H(+) secretion. I(eq) and fluid secretion were dependent on the presence of both Na(+) and HCO(3)(-). The carbonic anhydrase inhibitor acetazolamide abolished forskolin stimulation of I(eq) and HCO(3)(-) secretion, suggesting that HCO(3)(-) transport under these conditions requires catalysed synthesis of carbonic acid. Cl(-) was the predominant anion in secretions under all conditions studied and thus drives most of the fluid transport. Nevertheless, 50-70% of Cl(-) and fluid transport was bumetanide-insensitive, suggesting basolateral Cl(-) loading by a sodium-potassium-chloride cotransporter 1 (NKCC1)-independent mechanism. Imposing a transepithelial HCO(3)(-) gradient across basolaterally permeabilized Calu-3 cells sustained a forskolin-stimulated current, which was sensitive to CFTR inhibitors and drastically reduced in CFTR-deficient cells. Net HCO(3)(-) secretion was increased by bilateral Cl(-) removal and therefore did not require apical Cl(-)/HCO(3)(-) exchange. The results suggest a model in which most HCO(3)(-) is recycled basolaterally by exchange with Cl(-), and the resulting HCO(3)(-)-dependent Cl(-) transport provides an osmotic driving force for fluid secretion.

Mutation conferring apical-targeting motif on AE1 exchanger causes autosomal dominant distal RTA

Mutations in SLC4A1 that mislocalize its product, the chloride/bicarbonate exchanger AE1, away from its normal position on the basolateral membrane of the α-intercalated cell cause autosomal dominant distal renal tubular acidosis (dRTA). We studied a family exhibiting dominant inheritance and defined a mutation (AE1-M909T) that affects the C terminus of AE1, a region rich in potential targeting motifs that are incompletely characterized. Expression of AE1-M909T in Xenopus oocytes confirmed preservation of its anion exchange function. Wild-type GFP-tagged AE1 localized to the basolateral membrane of polarized MDCK cells, but AE1-M909T localized to both the apical and basolateral membranes. Wild-type AE1 trafficked directly to the basolateral membrane without apical passage, whereas AE1-M909T trafficked to both cell surfaces, implying the gain of an apical-targeting signal. We found that AE1-M909T acquired class 1 PDZ ligand activity that the wild type did not possess. In summary, the AE1-M909T mutation illustrates the role of abnormal targeting in dRTA and provides insight into C-terminal motifs that govern normal trafficking of AE1.

Novel role for pendrin in orchestrating bicarbonate secretion in cystic fibrosis transmembrane conductance regulator (CFTR)-expressing airway serous cells

In most HCO(3)(-)-secreting epithelial tissues, SLC26 Cl(-)/HCO(3)(-) transporters work in concert with the cystic fibrosis transmembrane conductance regulator (CFTR) to regulate the magnitude and composition of the secreted fluid, a process that is vital for normal tissue function. By contrast, CFTR is regarded as the only exit pathway for HCO(3)(-) in the airways. Here we show that Cl(-)/HCO(3)(-) anion exchange makes a major contribution to transcellular HCO(3)(-) transport in airway serous cells. Real-time measurement of intracellular pH from polarized cultures of human Calu-3 cells demonstrated cAMP/PKA-activated Cl(-)-dependent HCO(3)(-) transport across the luminal membrane via CFTR-dependent coupled Cl(-)/HCO(3)(-) anion exchange. The pharmacological and functional profile of the luminal anion exchanger was consistent with SLC26A4 (pendrin), which was shown to be expressed by quantitative RT-PCR, Western blot, and immunofluorescence. Pendrin-mediated anion exchange activity was confirmed by shRNA pendrin knockdown (KD), which markedly reduced cAMP-activated Cl(-)/HCO(3)(-) exchange. To establish the relative roles of CFTR and pendrin in net HCO(3)(-) secretion, transepithelial liquid secretion rate and liquid pH were measured in wild type, pendrin KD, and CFTR KD cells. cAMP/PKA increased the rate and pH of the secreted fluid. Inhibiting CFTR reduced the rate of liquid secretion but not the pH, whereas decreasing pendrin activity lowered pH with little effect on volume. These results establish that CFTR predominately controls the rate of liquid secretion, whereas pendrin regulates the composition of the secreted fluid and identifies a critical role for this anion exchanger in transcellular HCO(3)(-) secretion in airway serous cells.

Anion secretion by a model epithelium: more lessons from Calu-3

Anion transport drives fluid into the airways and is essential for humidifying inspired air and supplying surface liquid for mucociliary transport. Despite the importance of airway secretion in diseases such as cystic fibrosis, the cellular mechanisms remain poorly understood, in part due to the small size and complicated structure of the submucosal glands that produce most of the fluid. The Calu-3 human lung adenocarcinoma cell line has become a popular model for studying airway secretion because it can be cultured as a flat sheet, expresses the cystic fibrosis transmembrane conductance regulator and several acinar cell markers, forms polarized monolayers with tight junctions, has robust cAMP-stimulated anion transport, and responds to secretagogues that regulate the glands in vivo. However, some properties of Calu-3 cells are less consistent with those of native tissue. In particular, Calu-3 monolayers do not secrete chloride when stimulated by forskolin under short-circuit conditions. Bicarbonate ions are thought to carry the short-circuit current (I(sc)) and the drive secretion of alkaline fluid, in contrast to the neutral pH secretions that are produced by submucosal glands. Calu-3 cells also have abnormal chromosomes and characteristics of both serous and mucus cells. In this article, we discuss Calu-3 as a model in light of our ongoing studies, which suggest that Calu-3 monolayers resemble submucosal glands more closely than was previously thought. For example, we find that net HCO(3)(-) flux fully accounts for I(sc) as previously suggested but Cl(-) is the main anion transported under physiological conditions. A novel, HCO(3)(-) -dependent mechanism of Cl(-) transport is emerging which may explain secretion by Calu-3 and perhaps other epithelial cells.

Lubiprostone targets prostanoid EP₄ receptors in ovine airways

BACKGROUND AND PURPOSE

Lubiprostone, a prostaglandin E₁ derivative, is reported to activate ClC-2 chloride channels located in the apical membranes of a number of transporting epithelia. Lack of functioning CFTR chloride channels in epithelia is responsible for the genetic disease cystic fibrosis, therefore, surrogate channels that can operate independently of CFTR are of interest. This study explores the target receptor(s) for lubiprostone in airway epithelium.

EXPERIMENTAL APPROACH

All experiments were performed on the ventral tracheal epithelium of sheep. Epithelia were used to measure anion secretion from the apical surface as short circuit current or as fluid secretion from individual airway submucosal glands, using an optical method.

KEY RESULTS

The EP₄ antagonists L-161982 and GW627368 inhibited short circuit current responses to lubiprostone, while EP₁(,)₂(&)₃ receptor antagonists were without effect. Similarly, lubiprostone induced secretion in airway submucosal glands was inhibited by L-161982. L-161982 effectively competed with lubiprostone with a K(d) value of 0.058 µM, close to its value for binding to human EP₄ receptors (0.024 µM). The selective EP₄ agonist L-902688 and lubiprostone behaved similarly with respect to EP₄ receptor antagonists. Results of experiments with H89, a protein kinase A inhibitor, were consistent with lubiprostone acting through a G(s) -protein coupled EP₄ receptor/cAMP cascade.

CONCLUSIONS AND IMPLICATIONS

Lubiprostone-induced short-circuit currents and submucosal gland secretions were inhibited by selective EP₄ receptor antagonists. The results suggest EP₄ receptor activation by lubiprostone triggers cAMP production necessary for CFTR activation and the secretory responses, a possibility precluded in CF tissues.

Cardiac ion channel current modulation by the CFTR inhibitor GlyH-101

The role in the heart of the cardiac isoform of the cystic fibrosis transmembrane conductance regulator (CFTR), which underlies a protein kinase A-dependent Cl(-) current (I(Cl.PKA)) in cardiomyocytes, remains unclear. The identification of a CFTR-selective inhibitor would provide an important tool for the investigation of the contribution of CFTR to cardiac electrophysiology. GlyH-101 is a glycine hydrazide that has recently been shown to block CFTR channels but its effects on cardiomyocytes are unknown. Here the action of GlyH-101 on cardiac I(Cl.PKA) and on other ion currents has been established. Whole-cell patch-clamp recordings were made from rabbit isolated ventricular myocytes. GlyH-101 blocked I(Cl.PKA) in a concentration- and voltage-dependent fashion (IC(50) at +100 mV=0.3 ± 1.5 μM and at -100 mV=5.1 ± 1.3 μM). Woodhull analysis suggested that GlyH-101 blocks the open pore of cardiac CFTR channels at an electrical distance of 0.15 ± 0.03 from the external membrane surface. A concentration of GlyH-101 maximally effective against I(Cl.PKA) (30 μM) was tested on other cardiac ion currents. Inward current at -120 mV, comprised predominantly of the inward-rectifier background K(+) current, I(K1), was reduced by ∼43% (n=5). Under selective recording conditions, the Na(+) current (I(Na)) was markedly inhibited by GlyH-101 over the entire voltage range (with a fractional block at -40 mV of ∼82%; n=8). GlyH-101 also produced a voltage-dependent inhibition of L-type Ca(2+) channel current (I(Ca,L)); fractional block at +10 mV of ∼49% and of ∼28% at -10 mV; n=11, with a ∼-3 mV shift in the voltage-dependence of I(Ca,L) activation. Thus, this study demonstrates for the first time that GlyH-101 blocks cardiac I(Cl.PKA) channels in a similar fashion to that reported for recombinant CFTR. However, inhibition of other cardiac conductances may limit its use as a CFTR-selective blocker in the heart.

Lubiprostone activates Cl- secretion via cAMP signaling and increases membrane CFTR in the human colon carcinoma cell line, T84

BACKGROUND

Lubiprostone, used clinically (b.i.d.) to treat constipation, has been reported to increase transepithelial Cl(-) transport in T84 cells by activating ClC-2 channels.

AIM

To identify the underlying signaling pathway, we explored the effects of short-term and overnight lubiprostone treatment on second messenger signaling and Cl(-) transport.

METHODS

Cl(-) transport was assessed either as I(sc) across T84 monolayers grown on Transwells and mounted in Ussing chambers or by the iodide efflux assay. [cAMP](i) was measured by enzyme immunoassay, and [Ca(2+)](i) by Fluo-3 fluorescence. Quantitation of apical cell surface CFTR protein levels was assessed by Western blotting and biotinylation with the EZ-Link Sulfo-NHS-LC-LC-Biotin. ClC-2 mRNA level was studied by RT-PCR.

RESULTS

Lubiprostone and the cAMP stimulator, forskolin, caused comparable and maximal increases of I(sc) in T84 cells. The I(sc) effects of lubiprostone and forskolin were each suppressed if the tissue had previously been treated with the other agent. These responses were unaltered even if the monolayers were treated with lubiprostone overnight. Lubiprostone-induced increases in iodide efflux were ~80% of those obtained with forskolin. Lubiprostone increased [cAMP](i). H89, bumetanide, or CFTR(inh)-172 greatly attenuated lubiprostone-stimulated Cl(-) secretion, whereas the ClC-2 inhibitor CdCl(2) did not. Compared to controls, FSK-treatment increased membrane-associated CFTR by 1.9 fold, and lubiprostone caused a 2.6-fold increase in apical membrane CFTR as seen by immunoblotting following cell surface biotinylation.

CONCLUSIONS

Lubiprostone activates Cl(-) secretion in T84 cells via cAMP, protein kinase A, and by increasing apical membrane CFTR protein.

Lubiprostone ameliorates the cystic fibrosis mouse intestinal phenotype

Background

Cystic fibrosis (CF) is caused by mutations in the CFTR gene that impair the function of CFTR, a cAMP-regulated anion channel. In the small intestine loss of CFTR function creates a dehydrated, acidic luminal environment which is believed to cause an accumulation of mucus, a phenotype characteristic of CF. CF mice have small intestinal bacterial overgrowth, an altered innate immune response, and impaired intestinal transit. We investigated whether lubiprostone, which can activate the CLC2 Cl^- channel, would improve the intestinal phenotype in CF mice.

Methods

Cftr^tm1UNC (CF) and wildtype (WT) littermate mice on the C57BL/6J background were used. Lubiprostone (10 μg/kg-day) was administered by gavage for two weeks. Mucus accumulation was estimated from crypt lumen widths in periodic acid-Schiff base, Alcian blue stained sections. Luminal bacterial load was measured by qPCR for the bacterial 16S gene. Gastric emptying and small intestinal transit in fasted mice were assessed using gavaged rhodamine dextran. Gene expression was evaluated by Affymetrix Mouse430 2.0 microarray and qRT-PCR.

Results

Crypt width in control CF mice was 700% that of WT mice (P < 0.001). Lubiprostone did not affect WT crypt width but, unexpectedly, increased CF crypt width 22% (P = 0.001). Lubiprostone increased bacterial load in WT mice to 490% of WT control levels (P = 0.008). Conversely, lubiprostone decreased bacterial overgrowth in CF mice by 60% (P = 0.005). Lubiprostone increased gastric emptying at 20 min postgavage in both WT (P < 0.001) and CF mice (P < 0.001). Lubiprostone enhanced small intestinal transit in WT mice (P = 0.024) but not in CF mice (P = 0.377). Among other innate immune markers, expression of mast cell genes was elevated 4-to 40-fold in the CF intestine as compared to WT, and lubiprostone treatment of CF mice decreased expression to WT control levels.

Conclusions

These results indicate that lubiprostone has some benefits for the CF intestinal phenotype, especially on bacterial overgrowth and the innate immune response. The unexpected observation of increased mucus accumulation in the crypts of lubiprostone-treated CF mice suggests the possibility that lubiprostone increases mucus secretion.

Use of the chloride channel activator lubiprostone for constipation in adults with cystic fibrosis: a case series

OBJECTIVE

To describe the use of lubiprostone for constipation in 3 adults with cystic fibrosis (CF).

CASE SUMMARY

This case series describes the use of lubiprostone for the treatment of constipation in 3 adults with CF (mean +/- SD length of therapy 17.3 +/- 1.5 mo). All 3 patients were prescribed lubiprostone 24 microg twice daily after hospitalization for treatment of intestinal obstruction. Patient 1 continues on chronic polyethylene glycol (PEG) 3350 and lubiprostone and has not had a recurrence of obstruction. Patient 2 requires aggressive chronic therapy with PEG 3350, lubiprostone, and methylnaltrexone. She has had 1 recurrence of obstruction. Patient 3 continues with lubiprostone taken several times per week with good control of constipation and no recurrence of obstruction to date. The adverse effect profile has been tolerable in all 3 patients.

DISCUSSION

CF is caused by a genetic mutation resulting in a dysfunctional or absent CF transmembrane conductance regulator that normally functions as a chloride channel. This results in viscous secretions in multiple organ systems including the lungs and intestinal tract. Accumulation of viscous intestinal contents contributes to constipation, which is common among adults with CF and can sometimes lead to intestinal obstruction. Lubiprostone is indicated for chronic constipation and works by activating type 2 chloride channels (ClC-2) in the intestinal tract. Because it utilizes an alternate chloride channel, lubiprostone may be especially effective for constipation in patients with CF.

CONCLUSIONS

Lubiprostone provides an additional option for the treatment of constipation in adults with CF. Its use in the CF population deserves further study.

Lubiprostone stimulates secretion from tracheal submucosal glands of sheep, pigs, and humans

Lubiprostone, a putative ClC-2 chloride channel opener, has been investigated for its effects on airway epithelia (tracheas). Lubiprostone is shown to increase submucosal gland secretion in pigs, sheep, and humans and to increase short-circuit current (SCC) in the surface epithelium of pigs and sheep. Use of appropriate blocking agents and ion-substitution experiments shows anion secretion is the driving force for fluid formation in both glands and surface epithelium. From SCC concentration-response relations, it is shown that for apical lubiprostone K(d) = 10.5 nM with a Hill slope of 1.08, suggesting a single type of binding site and, from the speed of the response, close to the apical surface, confirmed the rapid blockade by Cd ions. Responses to lubiprostone were reversible and repeatable, responses being significantly larger with ventral compared with dorsal epithelium. Submucosal gland secretion rates following basolateral lubiprostone were, respectively, 0.2, 0.5, and 0.8 nl gl(-1) min(-1) in humans, sheep, and pigs. These rates dwarf any contribution surface secretion adds to the accumulation of surface liquid under the influence of lubiprostone. Lubiprostone stimulated gland secretion in two out of four human cystic fibrosis (CF) tissues and in two of three disease controls, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis (COPD/IPF), but in neither type of tissue was the increase significant. Lubiprostone was able to increase gland secretion rates in normal human tissue in the continuing presence of a high forskolin concentration. Lubiprostone had no spasmogenic activity on trachealis muscle, making it a potential agent for increasing airway secretion that may have therapeutic utility.

Salmeterol restores secretory functions in cystic fibrosis airway submucosal gland serous cells

The activity of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) can be mediated by surface G protein-coupled receptors such as the beta(2)-adrenergic receptor. In this study, we explored the effect of a long-acting beta(2)-adrenergic agonist, salmeterol, on the CFTR-dependent secretory capacity of a human CF tracheal gland serous cell line (CF-KM4), homozygous for the delF508 mutation. We showed that, compared with the untreated CF serous cells, a 24-hour pre-incubation period with 200 nM salmeterol induced an 83% increase in delF508-CFTR-mediated chloride efflux. The restoration of the bioelectric properties is associated with increased apical surface pool of delF508-CFTR. Salmeterol induced a decrease in ion concentration and an increase in the level of hydration of the mucus packaged inside the CF secretory granules. The effects of salmeterol are not associated with a persistent production of cAMP. Western blotting on isolated secretory granules demonstrated immunoreactivity for CFTR and lysozyme. In parallel, we measured by atomic force microscopy an increased size of secretory granules isolated from CF serous cells compared with non-CF serous cells (MM39 cell line) and showed that salmeterol was able to restore a CF cell granule size similar to that of non-CF cells. To demonstrate that the salmeterol effect was a CFTR-dependent mechanism, we showed that the incubation of salmeterol-treated CF serous cells with CFTR-inh172 suppressed the restoration of normal secretory functions. The capacity of salmeterol to restore the secretory capacity of glandular serous cells suggests that it could also improve the airway mucociliary clearance in patients with CF.

Lubiprostone activates non-CFTR-dependent respiratory epithelial chloride secretion in cystic fibrosis mice

Periciliary fluid balance is maintained by the coordination of sodium and chloride channels in the apical membranes of the airways. In the absence of the cystic fibrosis transmembrane regulator (CFTR), chloride secretion is diminished and sodium reabsorption exaggerated. ClC-2, a pH- and voltage-dependent chloride channel, is present on the apical membranes of airway epithelial cells. We hypothesized that ClC-2 agonists would provide a parallel pathway for chloride secretion. Using nasal potential difference (NPD) measurements, we quantified lubiprostone-mediated Cl(-) transport in sedated cystic fibrosis null (gut-corrected), C57Bl/6, and A/J mice during nasal perfusion of lubiprostone (a putative ClC-2 agonist). Baseline, amiloride-inhibited, chloride-free gluconate-substituted Ringer with amiloride and low-chloride Ringer plus lubiprostone (at increasing concentrations of lubiprostone) were perfused, and the NPD was continuously recorded. A clear dose-response relationship was detected in all murine strains. The magnitude of the NPD response to 20 muM lubiprostone was -5.8 +/- 2.1 mV (CF, n = 12), -8.1 +/- 2.6 mV (C57Bl/6 wild-type, n = 12), and -5.3 +/- 1.2 mV (AJ wild-type, n = 8). A cohort of ClC-2 knockout mice did not respond to 20 muM lubiprostone (n = 6, P = 0.27). In C57Bl/6 mice, inhibition of CFTR with topical application of CFTR inhibitor-172 did not abolish the lubiprostone response, thus confirming the response seen is independent of CFTR regulation. RT-PCR confirmed expression of ClC-2 mRNA in murine lung homogenate. The direct application of lubiprostone in the CF murine nasal airway restores nearly normal levels of chloride secretion in nasal epithelia.

Hypoxia inducible factor-1 (HIF-1)-mediated repression of cystic fibrosis transmembrane conductance regulator (CFTR) in the intestinal epithelium

Diarrhea is widespread in intestinal diseases involving ischemia and/or hypoxia. Since hypoxia alters stimulated Cl(-) and water flux, we investigated the influence of such a physiologically and pathophysiologically important signal on expression of the cystic fibrosis transmembrane conductance regulator (CFTR). Located on the apical membrane, this cAMP-activated Cl(-) channel determines salt and fluid transport across mucosal surfaces. Our studies revealed depression of CFTR mRNA, protein, and function in hypoxic epithelia. Chromatin immunoprecipitation identified a previously unappreciated binding site for the hypoxia inducible factor-1 (HIF-1), and promoter studies established its relevance by loss of repression following point mutation. Consequently, HIF-1 overexpressing cells exhibited significantly reduced transport capacity in colorimetric Cl(-) efflux studies, altered short circuit measurements, and changes in transepithelial fluid movement. Whole-body hypoxia in wild-type mice resulted in significantly reduced small intestinal fluid and HCO(3)(-) secretory responses to forskolin. Experiments performed in Cftr(-/-) and Nkcc1(-/-) mice underlined the role of altered CFTR expression for these functional changes, and work in conditional Hif1a mutant mice verified HIF-1-dependent CFTR regulation in vivo. In summary, our study clarifies CFTR regulation and introduces the concept of a HIF-1-orchestrated response designed to regulate ion and fluid movement across hypoxic intestinal epithelia.

Cystic Fibrosis: The Mechanisms of Pathogenesis of an Inherited Lung Disorder

Cystic fibrosis patients exhibit lung disease consistent with a failure of innate airway defense mechanisms. The link between abnormal ion transport and disease initiation and progression is not fully understood, but airway mucus dehydration seems paramount in the initiation of CF lung disease. New therapies are currently in development that target the ion transport defects in CF with the intention of rehydrating airway surfaces.