Differential regulations between adenosine triphosphate (ATP)- and uridine triphosphate-induced Cl(-) secretion in bovine tracheal epithelium. Direct stimulation of P1-like receptor by ATP

Temporal differentiation of bovine airway epithelial cells grown at an air-liquid interface

There is an urgent need to develop improved, physiologically-relevant in vitro models of airway epithelia with which to better understand the pathological processes associated with infection, allergies and toxicological insults of the respiratory tract of both humans and domesticated animals. In the present study, we have characterised the proliferation and differentiation of primary bovine bronchial epithelial cells (BBECs) grown at an air-liquid interface (ALI) at three-day intervals over a period of 42 days from the introduction of the ALI. The differentiated BBEC model was highly representative of the ex vivo epithelium from which the epithelial cells were derived; a columnar, pseudostratified epithelium that was highly reflective of native airway epithelium was formed which comprised ciliated, goblet and basal cells. The hallmark defences of the respiratory tract, namely barrier function and mucociliary clearance, were present, thus demonstrating that the model is an excellent mimic of bovine respiratory epithelium. The epithelium was fully differentiated by day 21 post-ALI and, crucially, remained healthy and stable for a further 21 days. Thus, the differentiated BBEC model has a three-week window which will allow wide-ranging and long-term experiments to be performed in the fields of infection, toxicology or general airway physiology.

Infection by Toxoplasma gondii, a severe parasite in neonates and AIDS patients, causes impaired anion secretion in airway epithelia

The airway epithelia initiate and modulate the inflammatory responses to various pathogens. The cystic fibrosis transmembrane conductance regulator-mediated Cl(-) secretion system plays a key role in mucociliary clearance of inhaled pathogens. We have explored the effects of Toxoplasma gondii, an opportunistic intracellular protozoan parasite, on Cl(-) secretion of the mouse tracheal epithelia. In this study, ATP-induced Cl(-) secretion indicated the presence of a biphasic short-circuit current (Isc) response, which was mediated by a Ca(2+)-activated Cl(-) channel (CaCC) and the cystic fibrosis transmembrane conductance regulator. However, the ATP-evoked Cl(-) secretion in T. gondii-infected mouse tracheal epithelia and the elevation of [Ca(2+)]i in T. gondii-infected human airway epithelial cells were suppressed. Quantitative reverse transcription-PCR revealed that the mRNA expression level of the P2Y2 receptor (P2Y2-R) increased significantly in T. gondii-infected mouse tracheal cells. This revealed the influence that pathological changes in P2Y2-R had on the downstream signal, suggesting that P2Y2-R was involved in the mechanism underlying T. gondii infection in airways. These results link T. gondii infection as well as other pathogen infections to Cl(-) secretion, via P2Y2-R, which may provide new insights for the treatment of pneumonia caused by pathogens including T. gondii.

Dapsone inhibits IL-8 secretion from human bronchial epithelial cells stimulated with lipopolysaccharide and resolves airway inflammation in the ferret

BACKGROUND

IL-8 is an important activator and chemoattractant for neutrophils that is produced by normal human bronchial epithelial (NHBE) cells through mitogen-activated protein kinase (MAPK) and nuclear factor-κB (NF-κB) p65 pathways. Dapsone, a synthetic sulfone, is widely used to treat chronic neutrophil dermatoses. We investigated the effects of dapsone on polarized IL-8 secretion from lipopolysaccharide (LPS)-stimulated NHBE cells and further evaluated its ability to decrease LPS-induced inflammation in the ferret airway.

METHODS

NHBE cells were grown at air-liquid interface (ALI) to ciliated differentiation. Baseline and endotoxin (LPS)-stimulated IL-8 secretion was measured by enzyme-linked immunosorbent assay at air and basal sides with and without dapsone. Western blotting was used to determine signaling pathways. In vivo, ferrets were exposed to intratracheal LPS over a period of 5 days. Once inflammation was established, oral or nebulized dapsone was administered for 5 days. Intraepithelial neutrophil accumulation was analyzed histologically, and mucociliary transport was measured on the excised trachea.

RESULTS

Dapsone, 1 μg/mL, did not influence unstimulated (basal) IL-8 secretion. Apical LPS stimulation induced both apical and basolateral IL-8, but basolateral LPS increased only basolateral IL-8. Dapsone inhibited polarized IL-8 secretion from ALI-conditioned cells. Dapsone also decreased LPS-induced IL-8 mRNA level. LPS led to phosphorylation of extracellular signal-regulated kinase 1/2, but not p38 MAPK or c-Jun NH(2)-terminal kinase. LPS also induced NF-κB p65 phosphorylation, an effect that was inhibited by dapsone. Both oral and aerosol dapsone decreased LPS-induced intraepithelial neutrophil accumulation, but only treatment with aerosol dapsone restored mucociliary transport to normal.

CONCLUSIONS

Dapsone, given either systemically or as an aerosol, may be useful in treating neutrophilic airway inflammation.

P2Y2 receptor polymorphisms and haplotypes in cystic fibrosis and their impact on Ca2+ influx

OBJECTIVE

Activation of P2Y2 receptors in airway epithelia by ATP and UTP stimulates a Ca2+-regulated Cl- channel, which regulates Cl- secretion in cystic fibrosis (CF). We hypothesized that genetic alterations in the P2Y2 receptor may act as disease modifiers in CF and thus analyzed the coding region of this gene for polymorphisms in 146 CF patients and 64 healthy controls. We also assessed the impact of the genetic variants on Ca2+-influx of P2Y2-null cells transfected with several P2Y2 receptor haplotypes.

RESULTS

We identified three frequent nonsynonymous P2Y2 receptor polymorphisms: Leu46Pro; Arg312Ser and Arg334Cys, of which only Arg312Ser was significantly more common in CF: Arg = 0.80, Ser = 0.20 (CF) vs. Arg = 0.72, Ser = 0.28 (controls), P < 0.05; for Leu46Pro, Leu = 0.92, Pro = 0.08 (CF) vs. Leu = 0.96, Pro = 0.04 (controls), P = 0.65 and for Arg334Cys, Arg = 0.79, Cys = 0.21 (CF) vs. Arg = 0.84, Cys = 0.16 (controls), P = 0.79. The most frequent haplotype was Leu46Leu/Arg312Arg/Arg334Arg (28% in CF, 31% in controls) but 6% of CF patients and none of the controls had Leu46Leu/Ser312Ser/Arg334Cys or Leu46Leu/Arg312Arg/Cys334Cys. To assess function of the receptor haplotypes, we stably transfected 1321N1 (P2Y-null) cells to similar levels of mRNA expression with Leu46Leu/Arg312Arg/Arg334Arg (wild-type), Leu46Leu/Ser312Ser/Arg334Arg and Leu46Leu/Arg312Arg/Cys334Cys and measured ATP-stimulated transient Ca2+-influx. Cells expressing the homozygous Cys334 variant had significantly increased Ca2+-influx compared to wild-type (P<0.01). The increase in Ca2+-influx was more pronounced in cells carrying the homozygous Ser312 variant than in cells with the other two genotypes (P<0.01).

CONCLUSIONS

These data indicate that P2Y2 receptor gene haplotypes influence intracellular Ca2+-release. Such genetic variants might therefore represent modifiers of Cl- secretion or of response to P2Y2 agonist therapy in CF.

Purinergic P2Y6 receptors induce Ca2+ and CFTR dependent Cl- secretion in mouse trachea

In airways Cl- secretion is activated and Na+ absorption is inhibited when P2Y2 receptors are stimulated by ATP or UTP. Both nucleotides are subject to degradation to ADP and UDP by ecto-nucleotidases. Here we show that these metabolites change electrolyte transport by stimulation of P2Y6 receptors in mouse trachea. Immunohistochemistry confirmed luminal and basolateral expression of P2Y6 receptors. In Ussing chamber experiments luminal ADP, UDP or the P2Y6 receptor agonist INS48823 induced both transient and persistent increase in short circuit currents (ISC). Activation of ISC was inhibited by the P2Y6 receptor blocker PPADS. The transient response was inhibited by DIDS, whereas the persistent ISC was inhibited by glibenclamide and by the protein kinase A (PKA) blocker H-89. Moreover, sustained activation of ISC by luminal UDP was inhibited by blocking basolateral K+ channels with 293B. Possible effects of diphosphates on P2Y1 or adenosine receptors were excluded by the inhibitors MRS2179 and 8-SPT, respectively. Inhibition of amiloride sensitive Na+ absorption was only seen after blocking basolateral K+ channels with 293B. In contrast, Cl- secretion activated by basolateral ADP or UDP was only transient and was blocked by the sk4 K+ channel blocker clotrimazole. In summary, activation of luminal P2Y6 receptors in the airways shifts electrolyte transport towards secretion by increasing intracellular Ca+ and activation of PKA.

Regulation of ion transport via apical purinergic receptors in intact rabbit airway epithelium

We investigated purinergic receptors involved in ion transport regulation in the intact rabbit nasal airway epithelium. Stimulation of apical membrane P2Y receptors with ATP or UTP (200 microM) induced transient increases in short-circuit current (Isc) of 13 and 6% followed by sustained inhibitions to 8 and 17% below control level, respectively. Serosal application of nucleotides had no effect. The ATP-induced response appeared to involve additional activation of apical adenosine (P1) and P2X receptors. The inhibitory effect of ATP and UTP on Isc was eliminated by pretreatment with amiloride (100 microM), while the stimulatory effect was potentiated, indicating that ATP and UTP inhibit Na+ and stimulate Cl- current. Ionomycin (1 microM) induced responses similar to UTP and ATP and desensitized the epithelium to the nucleotides, indicating involvement of intracellular Ca2+ (Ca2+ i. Furthermore, ATP, UTP and ionomycin induced 21, 24, and 21% decreases, respectively, in transepithelial conductance. Measurements of unidirectional isotope fluxes showed a 39% decrease in the dominant net Na+ absorption in response to ATP, while the smaller net Cl- secretion increased only insignificantly and unidirectional Cl- fluxes decreased significantly. The results suggest that nucleotides released to the airway surface liquid exert an autocrine regulation of epithelial NaCl absorption mainly by inhibiting the amiloride-sensitive epithelial Na+ channel (ENaC) and paracellular anion conductance via a P2Y receptor-dependent increase in Ca2+ i, while stimulation of Cl- secretion is of minor importance.

Hypotonicity-induced ATP release is potentiated by intracellular Ca2+ and cyclic AMP in cultured human bronchial cells

We have examined the cultured human bronchial epithelial cells (16HBE) to learn if changes in Cl(-) concentration or osmolality stimulate the cells to release ATP and to determine whether its release is cyclic AMP (cAMP)- and/or Ca(2+)-dependent by using the luciferin-luciferase luminometric assay. In a control solution (290 mosmol kg H(2)O(-1)), the external ATP concentration and the rate of ATP release were 0.52 +/- 0.20 nM and 0.036 +/- 0.034 pmol min(-1), respectively. Upon hypotonicity (205 mosmol kg H(2)O(-1)), they increased to 7.0 +/- 1.3 nM and 3.1 +/- 0.6 pmol min(-1), respectively, at 6 min, then decreased. At the peak, the rate of ATP release is estimated to be 6.2x10(4) ATP molecules s(-1) per cell. An accumulation of the released ATP for the initial 10 min increased significantly (p < 0.005) by 71.5% in the presence of forskolin (10 microM), adenylyl cyclase activator, however, it was abolished (p < 0.001) by pretreatment with BAPTA-AM (25 microM), a membrane permeable Ca(2+) chelator. On the other hand, neither low Cl(2-) (75 mM, isotonic) nor hypertonicity (+NaCl or +mannitol, 500 mosmol kg H(2)O(-1)) could significantly increase the ATP release. Further, forskolin or ionomycin (a Ca(2+) ionophore) or, both, failed to stimulate ATP release under the isotonic condition. In conclusion, first, hypertonicity and changes in Cl(-) concentrations are not effective signals for the ATP release; second, hypotonicity-induced ATP release is potentiated by the level of intracellular Ca(2+) and cAMP; and third, a biphasic increase in ATP release and its low rate at the peak support the hypothesis that ATP is released through a non-conducting pathway model, such as exocytosis, or through a volume-dependent, ATP-conductive anion channel.

Ion transport across Xenopus alveolar epithelium is regulated by extracellular ATP, UTP and adenosine

Native alveolar epithelium from Xenopus lung was used for electrophysiological Ussing chamber experiments to investigate ion transport regulation. The tissue exhibits a considerable absorption of Na(+) ions and this transepithelial transport is largely up-regulated after treatment of donor animals with ACTH. Extracellular ATP, UTP and adenosine were tested for their regulating effects and all three increased I(sc), which was mainly due to a stimulation of amiloride sensitive Na(+) transport (increase of I(ami) 32% for ATP, 21% for UTP, 25% for adenosine). Solely the effect of UTP was completely abolished in the presence of amiloride. In contrast, the effects of ATP or adenosine disappeared under Cl(-)-free conditions. ATP and UTP proved to have additive effects and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), an antagonist of purinergic receptors, inhibited selectively the effect of UTP on I(sc). Further, I(sc) was increased by the P2X selective agonist beta,gamma-meATP. We were able to demonstrate, that extracellular purines and pyrimidines play a possible role as auto/paracrine messengers for alveolar ion transport regulation in Xenopus lung.

ATP release triggered by activation of the Ca2+-activated K+ channel in human airway Calu-3 cells

Airway mucociliary clearance is subject to the autocrine/paracrine regulation of extracellular nucleotides released from the airway epithelial cells. The present study was performed in pursuit of effective modulators of ATP release under physiologic conditions in polarized human airway epithelial cells (Calu-3). Neither isoproterenol, forskolin, nor ionomycin augmented extracellular ATP release detected by luciferase assay. However, direct activation of the human intermediate conductance, Ca(2+)-activated K(+) channel (hIK-1) by 1-ethyl-2-benzimdazolinone (1-EBIO, 1 mM) and chlorzoxazone (CZ, 1 mM) increased ATP release predominantly in the apical compartment. Measurement of fluo-3 signals revealed that 1-EBIO- and CZ-stimulated cytosolic Ca(2+) mobilization was suppressed by the presence of MRS-2179, a specific P2Y(1) receptor antagonist. The hIK-1-mediated ATP release was inhibited by a hIK-1 blocker (charybdotoxin), and an Na(+)-K(+)-2Cl(-) cotransport blocker (bumetanide) without interruption by GdCl(3), an inhibitor of stretch-activated nonselective cation (SA) channels, or glybenclamide, a blocker of the cystic fibrosis transmembrane conductance regulator (CFTR). These results suggest that a cell volume decrease via the hIK-1-mediated KCl loss and the resultant induction of a regulatory volume increase via the Na(+)-K(+)-2Cl(-) transporter may trigger release of ATP, which causes P2Y(1)-mediated Ca(2+) mobilization, through mechanisms unrelated to the CFTR and SA channels.

Apical and basolateral ATP-induced anion secretion in polarized human airway epithelia

The present study investigated mechanisms underlying apical and basolateral P2Y(1)-mediated Cl(-) secretion in human airway epithelial cells. Apical and basolateral ATP induced short-circuit currents (I(sc)) with different properties via P2Y(1) receptors. The former comprised an immediate rise followed by a slow attenuation, whereas the latter was a transient rise with a higher peak and shorter duration (< 2 min). The actions of ATP were simulated by those of ADP, ADPbetaS, and ATPgammaS. Antagonists of phosphatidylinositol-phospholipase C (U73122, ET-18-OCH(3)) were without any effect on the bilateral ATP-induced I(sc), which were, in contrast, attenuated by a phosphatidylcholine-phospholipase C inhibitor (D609) and an adenylate cyclase inhibitor (SQ22536). The responses to ATP from either aspect were also sensitive to an intracellular Ca(2+) chelator, 1,2-bis (o-amino-phenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra-(acetoxymethyl)-ester, or a Ca(2+)-activated K(+) channel inhibitor, charybdotoxin, although differential Ca(2+) signals were concomitant with each reaction. Nystatin permeabilization studies revealed a good correlation between the I(sc) and the basolateral K(+) current rather than the apical Cl(-) current under ATP-stimulated conditions. In conclusion, apical and basolateral P2Y(1) receptors couple with both phosphatidylcholine-phospholipase C and adenylate cyclase, leading to Cl(-) secretion, whose rate is essentially regulated by the Ca(2+)-activated K(+) channel-mediated K(+) conductance. This suggests the importance of this channel in airway mucociliary clearance.