Na,K-ATPase gene transfer mitigates an oxidant-induced decrease of active sodium transport in rat fetal ATII cells

Mechanical properties of the premature lung: From tissue deformation under load to mechanosensitivity of alveolar cells

Many preterm infants require mechanical ventilation as life-saving therapy. However, ventilation-induced overpressure can result in lung diseases. Considering the lung as a viscoelastic material, positive pressure inside the lung results in increased hydrostatic pressure and tissue compression. To elucidate the effect of positive pressure on lung tissue mechanics and cell behavior, we mimic the effect of overpressure by employing an uniaxial load onto fetal and adult rat lungs with different deformation rates. Additionally, tissue expansion during tidal breathing due to a negative intrathoracic pressure was addressed by uniaxial tension. We found a hyperelastic deformation behavior of fetal tissues under compression and tension with a remarkable strain stiffening. In contrast, adult lungs exhibited a similar response only during compression. Young’s moduli were always larger during tension compared to compression, while only during compression a strong deformation-rate dependency was found. In fact, fetal lung tissue under compression showed clear viscoelastic features even for small strains. Thus, we propose that the fetal lung is much more vulnerable during inflation by mechanical ventilation compared to normal inspiration. Electrophysiological experiments with different hydrostatic pressure gradients acting on primary fetal distal lung epithelial cells revealed that the activity of the epithelial sodium channel (ENaC) and the sodium-potassium pump (Na,K-ATPase) dropped during pressures of 30 cmH₂O. Thus, pressures used during mechanical ventilation might impair alveolar fluid clearance important for normal lung function.

Paracrine stimulation of perinatal lung functional and structural maturation by mesenchymal stem cells

Background

Mesenchymal stem cells (MSCs) were shown to harbor therapeutic potential in models of respiratory diseases, such as bronchopulmonary dysplasia (BPD), the most common sequel of preterm birth. In these studies, cells or animals were challenged with hyperoxia or other injury-inducing agents. However, little is known about the effect of MSCs on immature fetal lungs and whether MSCs are able to improve lung maturity, which may alleviate lung developmental arrest in BPD.

Methods

We aimed to determine if the conditioned medium (CM) of MSCs stimulates functional and structural lung maturation. As a measure of functional maturation, Na⁺ transport in primary fetal distal lung epithelial cells (FDLE) was studied in Ussing chambers. Na⁺ transporter and surfactant protein mRNA expression was determined by qRT-PCR. Structural maturation was assessed by microscopy in fetal rat lung explants.

Results

MSC-CM strongly increased the activity of the epithelial Na⁺ channel (ENaC) and the Na,K-ATPase as well as their mRNA expression. Branching and growth of fetal lung explants and surfactant protein mRNA expression were enhanced by MSC-CM. Epithelial integrity and metabolic activity of FDLE cells were not influenced by MSC-CM. Since MSC’s actions are mainly attributed to paracrine signaling, prominent lung growth factors were blocked. None of the tested growth factors (VEGF, BMP, PDGF, EGF, TGF-β, FGF, HGF) contributed to the MSC-induced increase of Na⁺ transport. In contrast, inhibition of PI3-K/AKT and Rac1 signaling reduced MSC-CM efficacy, suggesting an involvement of these pathways in the MSC-CM-induced Na⁺ transport.

Conclusion

The results demonstrate that MSC-CM strongly stimulated functional and structural maturation of the fetal lungs. These effects were at least partially mediated by the PI3-K/AKT and Rac1 signaling pathway. Thus, MSCs not only repair a deleterious tissue environment, but also target lung cellular immaturity itself.

Reactive species generated by heme impair alveolar epithelial sodium channel function in acute respiratory distress syndrome

We previously reported that the highly reactive cell-free heme (CFH) is increased in the plasma of patients with chronic lung injury and causes pulmonary edema in animal model of acute respiratory distress syndrome (ARDS) post inhalation of halogen gas. However, the mechanisms by which CFH causes pulmonary edema are unclear. Herein we report for the first time that CFH and chlorinated lipids (formed by the interaction of halogen gas, Cl₂, with plasmalogens) are increased in the plasma of patients exposed to Cl₂ gas. Ex vivo incubation of red blood cells (RBC) with halogenated lipids caused oxidative damage to RBC cytoskeletal protein spectrin, resulting in hemolysis and release of CFH. Patch clamp and short circuit current measurements revealed that CFH inhibited the activity of amiloride-sensitive epithelial Na⁺ channel (ENaC) and cation sodium (Na⁺) channels in mouse alveolar cells and trans-epithelial Na⁺ transport across human airway cells with EC₅₀ of 125 nM and 500 nM, respectively. Molecular modeling identified 22 putative heme-docking sites on ENaC (energy of binding range: 86-1563 kJ/mol) with at least 2 sites within its narrow transmembrane pore, potentially capable of blocking Na⁺ transport across the channel. A single intramuscular injection of the heme-scavenging protein, hemopexin (4 μg/kg body weight), one hour post halogen gas exposure, decreased plasma CFH and improved lung ENaC activity in mice. In conclusion, results suggested that CFH mediated inhibition of ENaC activity may be responsible for pulmonary edema post inhalation injury.

Influenza-mediated reduction of lung epithelial ion channel activity leads to dysregulated pulmonary fluid homeostasis

Severe influenza (IAV) infection can develop into bronchopneumonia and edema, leading to acquired respiratory distress syndrome (ARDS) and pathophysiology. Underlying causes for pulmonary edema and aberrant fluid regulation largely remain unknown, particularly regarding the role of viral-mediated mechanisms. Herein, we show that distinct IAV strains reduced the functions of the epithelial sodium channel (ENaC) and the cystic fibrosis transmembrane regulator (CFTR) in murine respiratory and alveolar epithelia in vivo, as assessed by measurements of nasal potential differences and single-cell electrophysiology. Reduced ion channel activity was distinctly limited to virally infected cells in vivo and not bystander uninfected lung epithelium. Multiple lines of evidence indicated ENaC and CFTR dysfunction during the acute infection period; however, only CFTR dysfunction persisted beyond the infection period. ENaC, CFTR, and Na,K-ATPase activities and protein levels were also reduced in virally infected human airway epithelial cells. Reduced ENaC and CFTR led to changes in airway surface liquid morphology of human tracheobronchial cultures and airways of IAV-infected mice. Pharmacologic correction of CFTR function ameliorated IAV-induced physiologic changes. These changes are consistent with mucous stasis and pulmonary edema; furthermore, they indicate that repurposing therapeutic interventions correcting CFTR dysfunction may be efficacious for treatment of IAV lung pathophysiology.

Ion channels of the lung and their role in disease pathogenesis

Maintenance of normal epithelial ion and water transport in the lungs includes providing a thin layer of surface liquid that coats the conducting airways. This airway surface liquid is critical for normal lung function in a number of ways but, perhaps most importantly, is required for normal mucociliary clearance and bacterial removal. Preservation of the appropriate level of hydration, pH, and viscosity for the airway surface liquid requires the proper regulation and function of a battery of different types of ion channels and transporters. Here we discuss how alterations in ion channel/transporter function often lead to lung pathologies.

Dexmedetomidine enhances human lung fluid clearance through improving alveolar sodium transport

Accumulating evidence shows that dexmedetomidine can attenuate lung edema with acute lung injury in experimental mouse and rat models, but the mechanisms of dexmedetomidine on human alveolar fluid transport are still unknown. We measured the effects of dexmedetomidine on alveolar fluid clearance in human lung lobes ex vivo. Moreover, we measured the regulation of transepithelial Na⁺ transport by dexmedetomidine in H441 cells by electrophysiological technique and Western blot method. Our results showed that intratracheal instillation of dexmedetomidine markedly increased the reabsorption of 5% bovine serum albumin instillate (19.8 ± 1.4%, P < 0.01 vs. Control, n = 5). Further studies suggested that dexmedetomidine increased amiloride-sensitive short-circuit currents in permeabilized H441 monolayers and whole cell amiloride-sensitive Na⁺ currents in a dose-dependent fashion. Real-time PCR and Western blot results showed that dexmedetomidine could enhance the mRNA and protein expression of α-ENaC subunit, while inhibiting the phosphorylation of ERK_1/2 . These data demonstrate that dexmedetomidine could improve human lung fluid clearance and lung epithelial Na⁺ channel activity, and these effects may be mediated through the enhancement of α-ENaC expression and inhibition of ERK_1/2 pathway.

Sex-specific effects of sex steroids on alveolar epithelial Na+ transport

Alveolar fluid clearance mediates perinatal lung transition to air breathing in newborn infants, which is accomplished by epithelial Na⁺ channels (ENaC) and Na-K-ATPase. Male sex represents a major risk factor for developing respiratory distress, especially in preterm infants. We previously showed that male sex is associated with reduced epithelial Na⁺ transport, possibly contributing to the sexual dimorphism in newborn respiratory distress. This study aimed to determine sex-specific effects of sex steroids on epithelial Na⁺ transport. The effects of testosterone, 5α-dihydrotestosterone (DHT), estradiol, and progesterone on Na⁺ transport and Na⁺ channel expression were determined in fetal distal lung epithelial (FDLE) cells of male and female rat fetuses by Ussing chamber and mRNA expression analyses. DHT showed a minor effect only in male FDLE cells by decreasing epithelial Na⁺ transport. However, flutamide, an androgen receptor antagonist, did not abolish the gender imbalance, and testosterone lacked any effect on Na⁺ transport in male and female FDLE cells. In contrast, estradiol and progesterone increased Na⁺ transport and Na⁺ channel expression especially in females, and prevented the inhibiting effect of DHT in males. Estrogen receptor inhibition decreased Na⁺ channel expression and eliminated the sex differences. In conclusion, female sex steroids stimulate Na⁺ transport especially in females and prevent the inhibitory effect of DHT in males. The ineffectiveness of testosterone suggests that Na⁺ transport is largely unaffected by androgens. Thus, the higher responsiveness of female cells to female sex steroids explains the higher Na⁺ transport activity, possibly leading to a functional advantage in females.

Male Sex is Associated with a Reduced Alveolar Epithelial Sodium Transport

Respiratory distress syndrome (RDS) is the most frequent pulmonary complication in preterm infants. RDS incidence differs between genders, which has been called the male disadvantage. Besides maturation of the surfactant system, Na⁺ transport driven alveolar fluid clearance is crucial for the prevention of RDS. Na⁺ transport is mediated by the epithelial Na⁺ channel (ENaC) and the Na,K-ATPase, therefore potential differences in their expression or activity possibly contribute to the gender imbalance observed in RDS. Fetal distal lung epithelial (FDLE) cells of rat fetuses were separated by sex and analyzed regarding expression and activity of the Na⁺ transporters. Ussing chamber experiments showed a higher baseline short-circuit current (I_SC) and amiloride-sensitive ΔI_SC in FDLE cells of female origin. In addition, maximal amiloride-sensitive ΔI_SC and maximal ouabain-sensitive ΔI_SC of female cells were higher when measured in the presence of a permeabilized basolateral or apical membrane, respectively. The number of FDLE cells per fetus recoverable during cell isolation was also significantly higher in females. In addition, lung wet-to-dry weight ratio was lower in fetal and newborn female pups. Female derived FDLE cells had higher mRNA levels of the ENaC- and Na,K-ATPase subunits. Furthermore, estrogen (ER) and progesterone receptor (PR) mRNA levels were higher in female cells, which might render female cells more responsive, while concentrations of placenta-derived sex steroids do not differ between both genders during fetal life. Inhibition of ER-β abolished the sex differences in Na⁺ transport and female cells were more responsive to estradiol stimulation. In conclusion, a higher alveolar Na⁺ transport, possibly attributable to a higher expression of hormone receptors in female FDLE cells, provides an explanation for the well known sex-related difference in RDS occurrence and outcome.

Knockout mice reveal key roles for claudin 18 in alveolar barrier properties and fluid homeostasis

Claudin proteins are major constituents of epithelial and endothelial tight junctions (TJs) that regulate paracellular permeability to ions and solutes. Claudin 18, a member of the large claudin family, is highly expressed in lung alveolar epithelium. To elucidate the role of claudin 18 in alveolar epithelial barrier function, we generated claudin 18 knockout (C18 KO) mice. C18 KO mice exhibited increased solute permeability and alveolar fluid clearance (AFC) compared with wild-type control mice. Increased AFC in C18 KO mice was associated with increased β-adrenergic receptor signaling together with activation of cystic fibrosis transmembrane conductance regulator, higher epithelial sodium channel, and Na-K-ATPase (Na pump) activity and increased Na-K-ATPase β1 subunit expression. Consistent with in vivo findings, C18 KO alveolar epithelial cell (AEC) monolayers exhibited lower transepithelial electrical resistance and increased solute and ion permeability with unchanged ion selectivity. Claudin 3 and claudin 4 expression was markedly increased in C18 KO mice, whereas claudin 5 expression was unchanged and occludin significantly decreased. Microarray analysis revealed changes in cytoskeleton-associated gene expression in C18 KO mice, consistent with observed F-actin cytoskeletal rearrangement in AEC monolayers. These findings demonstrate a crucial nonredundant role for claudin 18 in the regulation of alveolar epithelial TJ composition and permeability properties. Increased AFC in C18 KO mice identifies a role for claudin 18 in alveolar fluid homeostasis beyond its direct contributions to barrier properties that may, at least in part, compensate for increased permeability.

Rapid elevation of sodium transport through insulin is mediated by AKT in alveolar cells

Abstract Alveolar fluid clearance is driven by vectorial Na(+) transport and promotes postnatal lung adaptation. The effect of insulin on alveolar epithelial Na(+) transport was studied in isolated alveolar cells from 18-19-day gestational age rat fetuses. Equivalent short-circuit currents (ISC) were measured in Ussing chambers and different kinase inhibitors were used to determine the pathway of insulin stimulation. In Western Blot measurements the activation of mediators stimulated by insulin was analyzed. The ISC showed a fast dose-dependent increase by insulin, which could be attributed to an increased ENaC (epithelial Na(+) channel) activity in experiments with permeabilized apical or basolateral membrane. 5-(N-Ethyl-N-isopropyl)amiloride inhibition of ISC was not affected, however, benzamil-sensitive ISC was increased in insulin-stimulated monolayers. The application of LY-294002 and Akti1/2 both completely blocked the stimulating effect of insulin on ISC. PP242 partly blocked the effect of insulin, whereas Rapamycin evoked no inhibition. Western Blot measurements revealed an increased phosphorylation of AKT after insulin stimulation. SGK1 activity was also increased by insulin as shown by Western Blot of pNDRG1. However, in Ussing chamber measurements, GSK650394, an inhibitor of SGK1 did not prevent the increase in ISC induced by insulin. The application of IGF-1 mimicked the effect of insulin and increased the ENaC activity. In addition, an increased autophosphorylation of the IGF-1R/IR was observed after insulin stimulation. We conclude that insulin rapidly increases epithelial Na(+) transport by enhancing the activity of endogenous ENaC through activation of PI3K/AKT in alveolar cells.

Von Hippel-Lindau protein and respiratory diseases

Von Hippel-Lindau protein (pVHL) was first identified as a tumor suppressor gene as mutations in the VHL gene predispose individuals to systemic benign or malignant tumors and cysts in many organs, including renal cell carcinoma of the clear-cell type and hemangioblastoma. Although pVHL is best known to act as a component of ubiquitin protein ligase for the proteasomal degradation of hypoxia inducible factor (HIF)-α, pVHL also interacts with extracellular matrix proteins and cytoskeleton, regulating extracellular matrix assembly, cell signaling, and many other cellular functions. Recent studies suggest that pVHL contributes to many lung diseases, including pulmonary arterial hypertension, lung cancer, pulmonary fibrosis, and acute respiratory distress syndrome. Mutation or loss of function of pVHL activates HIF and induced expression of vascular endothelial growth factor, endothelin-1, and FoxM1, leading to pulmonary arterial hypertension. Loss of pVHL in lung cancer cells promotes epithelial-mesenchymal transition and cancer migration and invasion while decreasing lung cancer cell proliferation and colonization. In patients of idiopathic pulmonary fibrosis, elevated expression of pVHL induces expression of fibronectin/integrin α5β1/focal adhesion kinase signaling, resulting in fibroproliferation and fibrosis. In alveolar epithelial cells, pVHL mediates Na-K-ATPase degradation in an HIF independent pathway, causing decreased edema clearance during hypoxia. These studies suggest that pVHL plays key roles in the pathogenesis of many lung diseases, and further investigations are warranted to elucidate the underlying molecular mechanisms.

Efficient gene delivery to primary alveolar epithelial cells by nucleofection

Primary alveolar epithelial cells play a pivotal role in lung research, particularly when focusing on gas exchange, barrier function, and transepithelial transport processes. However, efficient transfection of primary alveolar epithelial cells continues to be a major challenge. In the present study, we applied nucleofection, a novel method of gene and oligonucleotide delivery to the nucleus of cells by electroporation, to achieve highly efficient transfection of primary alveolar epithelial type II (ATII) cells. To quantify the amount of ATII cells effectively transfected, we applied a plasmid expressing GFP and assessed the amount of GFP-expressing cells by flow cytometry. Analysis of the nucleofected ATII cells revealed a concentration-dependent transfection efficiency of up to 50% when using 3-8 μg plasmid DNA without affecting cell viability. Nucleofection of cultured A549 and H441 cells yielded similar transfection rates. Importantly, nucleofection of ATII cells did not interfere with the integrity of ATII monolayers even with use of relatively high concentrations of plasmid DNA. In subsequent studies, we also efficiently delivered small interfering RNAs to ATII cells by nucleofection, thereby silencing Akt and the multiligand receptor megalin, which has been recently shown to play a key role in removal of excess protein from the alveolar space, and effectively inhibited megalin-driven uptake and transcellular transport of albumin in ATII cells. Thus we report successful transfection of primary rat alveolar epithelial cells with both plasmids and oligonucleotides via nucleofection with high viability and consistently good transfection rates without impairing key physiological properties of the cells.

Benzimidazolones enhance the function of epithelial Na⁺ transport

BACKGROUND AND PURPOSE

Pharmacological enhancement of vectorial Na⁺ transport may be useful to increase alveolar fluid clearance. Herein, we investigated the influence of the benzimidazolones 1-ethyl-1,3-dihydro-2-benzimidazolone (1-EBIO), 5,6-dichloro-1-EBIO (DC-EBIO) and chlorzoxazone on vectorial epithelial Na⁺ transport.

EXPERIMENTAL APPROACH

Effects on vectorial Na⁺ transport and amiloride-sensitive apical membrane Na⁺ permeability were determined by measuring short-circuit currents (I(SC)) in rat fetal distal lung epithelial (FDLE) monolayers. Furthermore, amiloride-sensitive membrane conductance and the open probability of epithelial Na⁺ channels (ENaC) were determined by patch clamp experiments using A549 cells.

KEY RESULTS

I(SC) was increased by approximately 50% after addition of 1-EBIO, DC-EBIO and chlorzoxazone. With permeabilized basolateral membranes in the presence of a 145:5 apical to basolateral Na⁺ gradient, the benzimidazolones markedly increased amiloride-sensitive I(SC). 5-(N-Ethyl-N-isopropyl)amiloride-induced inhibition of I(SC) was not affected. The benzamil-sensitive I(SC) was increased in benzimidazolone-stimulated monolayers. Pretreating the apical membrane with amiloride, which inhibits ENaC, completely prevented the stimulating effects of benzimidazolones on I(SC). Furthermore, 1-EBIO (1 mM) and DC-EBIO (0.1 mM) significantly increased (threefold) the open probability of ENaC without influencing current amplitude. Whole cell measurements showed that DC-EBIO (0.1 mM) induced an amiloride-sensitive increase in membrane conductance.

CONCLUSION AND IMPLICATIONS

Benzimidazolones have a stimulating effect on vectorial Na⁺ transport. The antagonist sensitivity of this effect suggests the benzimidazolones elicit this action by activating the highly selective ENaC currents. Thus, the results demonstrate a possible new strategy for directly enhancing epithelial Na⁺ transport.

Modulation of sodium transport in alveolar epithelial cells by estradiol and progesterone

The effects of estradiol (E2) and progesterone (P) on alveolar epithelial Na+ transport were studied in isolated alveolar epithelial cells from 18- to 19-d GA rat fetuses, grown to confluence in serum-free media supplemented with E2 (0-1 μM) and P (0-2.8 μM). Short-circuit currents (ISC) were measured, showing an increase by E2 and P in a dose-dependent manner. The Na,K-ATPase subunits -α1 and -β1 were detected by Western blotting, but total expression was not significantly altered. Furthermore, all three epithelial Na+ channel (ENaC) subunits -α, -β, and -γ were detected, with trends toward a higher expression in the presence of E2 and P. Real-time PCR revealed an increase of α- and β-ENaC expression but no alteration of γ-ENaC. In addition, the mRNA expression of cystic fibrosis transmembrane conductance regulator (CFTR) and Na,K-ATPase-β1 subunit were elevated in the presence of E2 and P. Single-channel patch clamp analysis demonstrated putative highly selective and nonselective cation channels in the analyzed cells, with a higher percentage of responsive patches under the influence of E2 and P. We conclude that E2 and P increased Na+ transport in alveolar epithelial cells by enhancing the expression and activity of ENaC and Na,K-ATPase.

Nasal potential difference to detect Na+ channel dysfunction in acute lung injury

Pulmonary fluid clearance is regulated by the active transport of Na(+) and Cl(-) through respiratory epithelial ion channels. Ion channel dysfunction contributes to the pathogenesis of various pulmonary fluid disorders including high-altitude pulmonary edema (HAPE) and neonatal respiratory distress syndrome (RDS). Nasal potential difference (NPD) measurement allows an in vivo investigation of the functionality of these channels. This technique has been used for the diagnosis of cystic fibrosis, the archetypal respiratory ion channel disorder, for over a quarter of a century. NPD measurements in HAPE and RDS suggest constitutive and acquired dysfunction of respiratory epithelial Na(+) channels. Acute lung injury (ALI) is characterized by pulmonary edema due to alveolar epithelial-interstitial-endothelial injury. NPD measurement may enable identification of critically ill ALI patients with a susceptible phenotype of dysfunctional respiratory Na(+) channels and allow targeted therapy toward Na(+) channel function.

K+ channel openers restore verapamil-inhibited lung fluid resolution and transepithelial ion transport

Background

Lung epithelial Na⁺ channels (ENaC) are regulated by cell Ca²⁺ signal, which may contribute to calcium antagonist-induced noncardiogenic lung edema. Although K⁺ channel modulators regulate ENaC activity in normal lungs, the therapeutical relevance and the underlying mechanisms have not been completely explored. We hypothesized that K⁺ channel openers may restore calcium channel blocker-inhibited alveolar fluid clearance (AFC) by up-regulating both apical and basolateral ion transport.

Methods

Verapamil-induced depression of heterologously expressed human αβγ ENaC in Xenopus oocytes, apical and basolateral ion transport in monolayers of human lung epithelial cells (H441), and in vivo alveolar fluid clearance were measured, respectively, using the two-electrode voltage clamp, Ussing chamber, and BSA protein assays. Ca²⁺ signal in H441 cells was analyzed using Fluo 4AM.

Results

The rate of in vivo AFC was reduced significantly (40.6 ± 6.3% of control, P < 0.05, n = 12) in mice intratracheally administrated verapamil. K_Ca3.1 (1-EBIO) and K_ATP (minoxidil) channel openers significantly recovered AFC. In addition to short-circuit current (Isc) in intact H441 monolayers, both apical and basolateral Isc levels were reduced by verapamil in permeabilized monolayers. Moreover, verapamil significantly altered Ca²⁺ signal evoked by ionomycin in H441 cells. Depletion of cytosolic Ca²⁺ in αβγ ENaC-expressing oocytes completely abolished verapamil-induced inhibition. Intriguingly, K_V (pyrithione-Na), K _Ca3.1 (1-EBIO), and K_ATP (minoxidil) channel openers almost completely restored the verapamil-induced decrease in Isc levels by diversely up-regulating apical and basolateral Na⁺ and K⁺ transport pathways.

Conclusions

Our observations demonstrate that K⁺ channel openers are capable of rescuing reduced vectorial Na⁺ transport across lung epithelial cells with impaired Ca²⁺ signal.

Triiodo-L-thyronine rapidly stimulates alveolar fluid clearance in normal and hyperoxia-injured lungs

RATIONALE

Edema fluid resorption is critical for gas exchange and requires active epithelial ion transport by Na, K-ATPase and other ion transport proteins.

OBJECTIVES

In this study, we sought to determine if alveolar fluid clearance (AFC) is stimulated by 3,3',5 triiodo-L-thyronine (T(3)).

METHODS

AFC was measured in in situ ventilated lungs and ex vivo isolated lungs by instilling isosmolar 5% bovine serum albumin solution with fluorescein-labeled albumin tracer and measuring the change in fluorescein isothiocyanate-albumin concentration over time.

MEASUREMENTS AND MAIN RESULTS

Systemic treatment with intraperitoneal injections of T(3) for 3 consecutive days increased AFC by 52.7% compared with phosphate-buffered saline-injected control rats. Membranes prepared from alveolar epithelial cells from T(3)-treated rats had higher Na, K-ATPase hydrolytic activity. T(3) (10(-6) M), but not reverse T(3) (3,3',5' triiodo-L-thyronine), applied to the alveolar space increased AFC by 31.8% within 1.5 hours. A 61.5% increase in AFC also occurred by airspace instillation of T(3) in ex vivo isolated lungs, suggesting a direct effect of T(3) on the alveolar epithelium. Exposure of rats to an oxygen concentration of greater than 95% for 60 hours increased wet-to-dry lung weights and decreased AFC, whereas the expression of thyroid receptor was not markedly changed. Airspace T(3) rapidly restored the AFC in rat lungs with hyperoxia-induced lung injury.

CONCLUSIONS

Airspace T(3) rapidly stimulates AFC by direct effects on the alveolar epithelium in rat lungs with and without lung injury.

Na,K-ATPase expression is increased in the lungs of alcohol-fed rats

BACKGROUND

Alcohol abuse independently increases the risk of developing the acute respiratory distress syndrome (ARDS), a disease characterized by diffuse alveolar epithelial damage, lung edema, and consequent severe hypoxemia. Chronic alcohol abuse increases alveolar epithelial permeability both in vitro and in vivo, in part due to altered tight junction formation. However, both alcohol-fed animals and otherwise healthy alcoholic humans do not have pulmonary edema at baseline, even though their lungs are highly susceptible to acute edematous injury in response to inflammatory stresses. This suggests that active fluid transport by the alveolar epithelium is preserved or even augmented in the alcoholic lung. Chronic alcohol ingestion increases expression of apical sodium channels in the alveolar epithelium; however, its effects on the Na,K-ATPase complex that drives sodium and fluid transport out of the alveolar space have not been examined.

METHODS

Age- and gender-matched Sprague-Dawley rats were fed the Lieber-DeCarli liquid diet containing either alcohol or an isocaloric substitution (control diet) for 6 weeks. Gene and protein expression of lung Na,K-ATPase alpha1, alpha2, and beta1 subunits were quantified via real-time PCR and immunobiological analyses, respectively. Alcohol-induced, Na,K-ATPase-dependent epithelial barrier dysfunction was determined by calculating lung tissue wet:dry ratios following an ex vivo buffer-perfused challenge for 2 hours in the presence of ouabain (10(-4) M), a Na,K-ATPase inhibitor.

RESULTS

Chronic alcohol ingestion significantly increased gene and protein expression of each Na,K-ATPase subunit in rat lungs. Immunohistochemical analyses of the alcoholic lung also revealed that protein expression of the Na,K-ATPase alpha1 subunit was increased throughout the alveolar epithelium. Additionally, lungs isolated from alcohol-fed rats developed more edema than comparably treated lungs from control-fed rats, as reflected by increased lung tissue wet:dry ratios.

CONCLUSIONS

These findings indicate that chronic alcohol ingestion, which is known to increase alveolar epithelial paracellular permeability, actually increases the expression of Na,K-ATPase in the lung as a compensatory mechanism. This provides a potential explanation as to why the otherwise healthy alcoholic does not have evidence of pulmonary edema at baseline.

Amiloride-sensitive nasal potential difference is not changed by estradiol and progesterone replacement but relates to BPD or death in a randomized trial on preterm infants

Postnatal replacement of placental estradiol (E2) and progesterone (P) in preterm infants may improve lung function, possibly mediated through enhanced epithelial Na(+) transport and alveolar fluid clearance. Preterm infants of <29 wk gestational age and <1000 g birth weight requiring mechanical ventilation within 12 h of birth were randomized to receive either 2.5 mg/kg E2 and 22.5 mg/kg P per day (E2/P), or vehicle placebo. Epithelial Na(+) transport was assessed in 29 infants by measuring total nasal potential difference (NPD) and amiloride-sensitive NPD (ASNPD) on postnatal days of life 1, 3, 5, and 7, and mean values of all four measurements were calculated. Bronchopulmonary dysplasia (BPD) was defined as need for supplemental oxygen (goal Sa(O2) 90%) or mechanical ventilation at 36 wk corrected postmenstrual age. Mean ASNPD was -6.5 +/- 2.8 mV in infants receiving E2/P and -6.1 +/- 2.6 mV in infants receiving placebo (not significant). NPD was -10.6 +/- 3.8 mV and -10.7 +/- 3.6 mV, respectively. The ASNPD was significantly higher in infants surviving without BPD (-7.1 +/- 2.5 mV) than in infants developing BPD or not surviving (-5.2 +/- 2.4 mV). In conclusion, ASNPD is not changed by postnatal replacement of E2 and P. Infants at high risk of developing BPD had lower ASNPD values in the immediate postnatal period.

Developmental acquisition of T3-sensitive Na-K-ATPase stimulation by rat alveolar epithelial cells

Late in gestation, the developing air space epithelium switches from chloride and fluid secretion to sodium and fluid absorption. Absorption requires Na-K-ATPase acting in combination with apical sodium entry mechanisms. Hypothyroidism inhibits perinatal fluid resorption, and thyroid hormone [triiodothyronine (T3)] stimulates adult alveolar epithelial cell (AEC) Na-K-ATPase. This study explored the developmental regulation of Na-K-ATPase by T3 in fetal rat distal lung epithelial (FDLE) cells. T3 increased Na-K-ATPase activity in primary FDLE cells from gestational day 19 [both primary FDLE cells at embryonic day 19 (E19) and the cell line FD19 derived from FDLE cells at E19]. However, T3 did not increase the Na-K-ATPase activity in less mature FDLE cells, including primary E17 and E18 FDLE cells and the cell line FD18 (derived from FDLE cells at E18). Subsequent experiments assessed the T3 signal pathway to define whether it was similar in the late FDLE and adult AEC and to determine the site of the switch in responsiveness to T3. As in adult AEC, in the FD19 cell line, the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin blocked the T3-induced increase in Na-K-ATPase activity and plasma membrane quantity. T3 caused a parallel increase in phosphorylation of Akt at Ser473 in FDLE cells from E19, but not from E17 or E18. In the FD18 cell line, transient expression of a constitutively active mutant of the PI3K catalytic p110 subunit significantly augmented the Na-K-ATPase activity and the cell surface expression of Na-K-ATPase alpha(1) protein. In conclusion, FDLE cells from E17 and E18 lacked T3-sensitive Na-K-ATPase activity but acquired this response at E19. The developmental stimulation of Na-K-ATPase by T3 in rat FDLE cells requires activation of PI3K, and the acquisition of T3 responsiveness may be at PI3K or upstream in the signaling pathway.

Dexamethasone inhibits the action of TNF on ENaC expression and activity

We have reported that TNF, a proinflammatory cytokine present in several lung pathologies, decreases the expression and activity of the epithelial Na(+) channel (ENaC) by approximately 70% in alveolar epithelial cells. Because dexamethasone has been shown to upregulate ENaC mRNA expression and is well known to downregulate proinflammatory genes, we tested if it could alleviate the effect of TNF on ENaC expression and activity. In cotreatment with TNF, we found that dexamethasone reversed the inhibitory effect of TNF and upregulated alpha, beta, and gammaENaC mRNA expression. When the cells were pretreated for 24 h with TNF before cotreatment, dexamethasone was still able to increase alphaENaC mRNA expression to 1.8-fold above control values. However, in these conditions, beta and gammaENaC mRNA expression was reduced to 47% and 14%, respectively. The potential role of TNF and dexamethasone on alphaENaC promoter activity was tested in A549 alveolar epithelial cells. TNF decreased luciferase (Luc) expression by approximately 25% in these cells, indicating that the strong diminution of alphaENaC mRNA must be related to posttranscriptional events. Dexamethasone raised Luc expression by fivefold in the cells and augmented promoter activity by 2.77-fold in cotreatment with TNF. In addition to its effect on alphaENaC gene expression, dexamethasone was able to maintain amiloride-sensitive current as well as the liquid clearance abilities of TNF-treated cells within the normal range. All these results suggest that dexamethasone alleviates the downregulation of ENaC expression and activity in TNF-treated alveolar epithelial cells.

Mechanisms of pulmonary edema clearance

The mechanisms of pulmonary edema resolution are different from those regulating edema formation. Absorption of excess alveolar fluid is an active process that involves vectorial transport of Na+out of alveolar air spaces with water following the Na+osmotic gradient. Active Na+transport across the alveolar epithelium is regulated via apical Na+and chloride channels and basolateral Na-K-ATPase in normal and injured lungs. During lung injury, mechanisms regulating alveolar fluid reabsorption are inhibited by yet unclear pathways and can be upregulated by pharmacological means. Better understanding of the mechanisms that regulate edema clearance may lead to therapeutic interventions to improve the ability of lungs to clear fluid, which is of clinical significance.

In vivo timing of onset of transgene expression following adenoviral-mediated gene transfer

Recombinant adenoviruses are efficient gene transfer vehicles that could be used for treatment of acute diseases. However, the time required for adenoviruses to produce physiologically relevant levels of transgene in vivo is unknown. To address this question rat lungs were infected with an E1a(-)/E3a(-) adenovirus that contains an hCMV-driven human beta(2)-adrenergic receptor (beta(2)AR) cDNA. Human beta(2)AR message and protein expression were noted 2-4 h postinfection without evidence of pseudotransduction. beta(2)AR function (cAMP production) was increased at 6 h postinfection. To determine when beta(2)AR gene transfer affects downstream catecholamine-sensitive pathways, we measured lung Na,K-ATPase expression and alveolar fluid clearance (AFC). beta(2)AR gene transfer increased Na,K-ATPase number by 80% at 6 h, and AFC by 20% at 8 h postinfection. These data indicate that recombinant adenoviruses can produce physiologically significant levels of transgene within hours of infection and that they may be suitable for gene therapies for acute, rapidly progressive diseases.

Alveolar type 1 cells express the alpha2 Na,K-ATPase, which contributes to lung liquid clearance

The alveolar epithelium is composed of alveolar type 1 (AT1) and alveolar type 2 (AT2) cells, which represent approximately 95% and approximately 5% of the alveolar surface area, respectively. Lung liquid clearance is driven by the osmotic gradient generated by the Na,K-ATPase. AT2 cells have been shown to express the alpha1 Na,K-ATPase. We postulated that AT1 cells, because of their larger surface area, should be important in the regulation of active Na+ transport. By immunofluorescence and electron microscopy, we determined that AT1 cells express both the alpha1 and alpha2 Na,K-ATPase isoforms. In isolated, ouabain-perfused rat lungs, the alpha2 Na,K-ATPase in AT1 cells mediated 60% of the basal lung liquid clearance. The beta-adrenergic agonist isoproterenol increased lung liquid clearance by preferentially upregulating the alpha2 Na,K-ATPase protein abundance in the plasma membrane and activity in alveolar epithelial cells (AECs). Rat AECs and human A549 cells were infected with an adenovirus containing the rat Na,K-ATPase alpha2 gene (Adalpha2), which resulted in the overexpression of the alpha2 Na,K-ATPase protein and caused a 2-fold increase in Na,K-ATPase activity. Spontaneously breathing rats were also infected with Adalpha2, which increased alpha2 protein abundance and resulted in a approximately 250% increase in lung liquid clearance. These studies provide the first evidence that alpha2 Na,K-ATPase in AT1 cells contributes to most of the active Na+ transport and lung liquid clearance, which can be further increased by stimulation of the beta-adrenergic receptor or by adenovirus-mediated overexpression of the alpha2 Na,K-ATPase.

Modulation of sodium transport in fetal alveolar epithelial cells by oxygen and corticosterone

Regulation of active Na(+) transport across fetal distal lung epithelial cells (FDLE) by corticosterone (CST), corticotropin-releasing hormone (CRH), and oxygen tension may be crucial for postnatal adaptation. FDLE isolated from 19-day rat fetuses (term: 22 days) were grown on permeable supports to confluent monolayers (duration 3 days) in 2.5, 5, 12, or 20% O(2) with 5% CO(2)-balance N(2) and mounted in Ussing chambers for measurement of short-circuit currents (I(sc)). FDLE monolayers grown in 20% O(2) had significantly higher levels of total I(sc) and of their amiloride-sensitive (I(amil)) and ouabain-sensitive (I(ouab)) components than hypoxic cells. Values (microA/cm(2) +/- SE) for 2.5-5% O(2) and 20% O(2) were, respectively, I(sc) 5.3 +/- 0.2 vs. 8.4 +/- 0.3 (P < 0.001), I(amil) 3.4 +/- 0.2 vs. 4.3 +/- 0.2 (P < 0.01), and I(ouab) 3.4 +/- 0.6 vs. 9.1 +/- 0.6 (P < 0.001). Addition of CST but not CRH to the culture medium at any O(2) concentration increased I(amil). FDLE cells grown at 5% O(2) expressed significantly lower levels of alpha-, beta-, and gamma-epithelial Na(+) channel (ENaC), and of the alpha(1)-Na(+)-K(+)-ATPase, as determined by Western blotting. We conclude that higher O(2) concentrations increased total vectorial Na(+) transport, and the function of Na(+)-K(+)-ATPase and apical amiloride-sensitive Na(+) conductance, whereas CST only increased ENaC function.

Effects of beta2-adrenergic receptor overexpression on alveolar epithelial active transport

beta-Adrenergic receptor (betaAR) agonists accelerate the clearance of edema from the alveolar airspace by increasing the function of epithelial transport proteins, including epithelial Na(+) channels and Na,K-adenosinetriphosphatases. To improve our understanding of the role of the beta(2)AR in regulating alveolar fluid clearance, we used an adenoviral-mediated gene transfer strategy to effect significant increases in membrane-bound beta(2)AR number and function in the alveolar epithelium of normal rats. Alveolar fluid clearance in beta(2)AR-overexpressing lungs, measured by means of an isolated lung model in the absence of catecholamine supplementation, was 100% greater than in controls. These findings were associated with significant increases of epithelial Na(+) channel function and Na,K-adenosine triphosphatase function in the peripheral lung. Experiments performed with adrenalectomized rats, a beta(2)-agonist (procaterol), and a nonspecific beta-antagonist (propranolol) indicate that overexpression maximally up-regulates beta(2)-adrenergic-responsive alveolar fluid clearance and improves responsiveness to endogenous catecholamines. Mechanistic studies in human lung epithelial cells (A549) indicate that receptor overexpression prevents homologous receptor desensitization, possibly by overwhelming endogenous regulatory pathways. Our studies demonstrate that overexpression of beta(2)AR in lung epithelial cells can be used to study the role and regulation of alveolar beta(2)ARs. They also suggest a therapeutic role for the beta(2)AR in the treatment of pulmonary edema.

Invited review: lung edema clearance: role of Na(+)-K(+)-ATPase

Acute hypoxemic respiratory failure is a consequence of edema accumulation due to elevation of pulmonary capillary pressures and/or increases in permeability of the alveolocapillary barrier. It has been recognized that lung edema clearance is distinct from edema accumulation and is largely effected by active Na(+) transport out of the alveoli rather than reversal of the Starling forces, which control liquid flux from the pulmonary circulation into the alveolus. The alveolar epithelial Na(+)-K(+)-ATPase has an important role in regulating cell integrity and homeostasis. In the last 15 yr, Na(+)-K(+)-ATPase has been localized to the alveolar epithelium and its contribution to lung edema clearance has been appreciated. The importance of the alveolar epithelial Na(+)-K(+)-ATPase function is reflected in the changes in the lung's ability to clear edema when the Na(+)-K(+)-ATPase is inhibited or increased. An important focus of the ongoing research is the study of the mechanisms of Na(+)-K(+)-ATPase regulation in the alveolar epithelium during lung injury and how to accelerate lung edema clearance by modulating Na(+)-K(+)-ATPase activity.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Na,K-ATPase overexpression improves alveolar fluid clearance in a rat model of elevated left atrial pressure

BACKGROUND

Acute elevation of left atrial pressure (LAP) increases extravascular water and impairs active Na(+) transport in rat lungs. We have reported that overexpression of Na,K-ATPase subunit genes in the alveolar epithelium increases alveolar fluid clearance (AFC) in normal and injured rat lungs with normal LAP. We reasoned that adenovirus-mediated transfer of an Na,K-ATPase beta-subunit gene to the alveolar epithelium could improve AFC in rat lungs in the presence of acutely elevated LAP.

METHODS AND RESULTS

Normal rats were infected with 4x10(9) plaque-forming units of E1a(-)/E3(-) recombinant adenoviruses that contained a cytomegalovirus promoter coupled to a rat Na,K-ATPase beta(1)-subunit cDNA (adbeta(1)) or no cDNA (adNull) 7 days before study. Na,K-ATPase alpha(1)- and beta(1)-subunit abundance in basolateral cell membranes isolated from the peripheral lung was significantly increased in adbeta(1)-infected lungs compared with sham and adNull-infected controls. In all groups, elevation of LAP reduced membrane-bound Na,K-ATPase abundance; however, abundance in adbeta(1)-infected lungs remained greater than in controls. AFC, measured with a fluid-filled isolated lung preparation in the presence of elevated LAP (15 cmH(2)O), in Na,K-ATPase beta(1)-subunit-overexpressing lungs was up to 100% greater than in controls and was not different from rats studied at normal LAP (0 cmH(2)O).

CONCLUSIONS

These data suggest that alveolar overexpression of an Na,K-ATPase beta(1)-subunit can counteract downregulation of membrane-bound solute transporters owing to elevated pulmonary vascular pressures and can restore active Na(+) transport and AFC in this rat model of acute hydrostatic pulmonary edema.

beta(2)-adrenergic receptor overexpression increases alveolar fluid clearance and responsiveness to endogenous catecholamines in rats

beta-Adrenergic agonists accelerate the clearance of alveolar fluid by increasing the expression and activity of epithelial solute transport proteins such as amiloride-sensitive epithelial Na(+) channels (ENaC) and Na,K-ATPases. Here we report that adenoviral-mediated overexpression of a human beta(2)-adrenergic receptor (beta(2)AR) cDNA increases beta(2)AR mRNA, membrane-bound receptor protein expression, and receptor function (procaterol-induced cAMP production) in human lung epithelial cells (A549). Receptor overexpression was associated with increased catecholamine (procaterol)-responsive active Na(+) transport and increased abundance of Na,K-ATPases in the basolateral cell membrane. beta(2)AR gene transfer to the alveolar epithelium of normal rats improved membrane-bound beta(2)AR expression and function and increased levels of ENaC (alpha subunit) abundance and Na,K-ATPases activity in apical and basolateral cell membrane fractions isolated from the peripheral lung, respectively. Alveolar fluid clearance (AFC), an index of active Na(+) transport, in beta(2)AR overexpressing rats was up to 100% greater than sham-infected controls and rats infected with an adenovirus that expresses no cDNA. The addition of the beta(2)AR-specific agonist procaterol to beta(2)AR overexpressing lungs did not increase AFC further. AFC in beta(2)AR overexpressing lungs from adrenalectomized or propranolol-treated rats revealed clearance rates that were the same or less than normal, untreated, sham-infected controls. These experiments indicate that alveolar beta(2)AR overexpression improves beta(2)AR function and maximally upregulates beta-agonist-responsive active Na(+) transport by improving responsiveness to endogenous catecholamines. These studies suggest that upregulation of beta(2)AR function may someday prove useful for the treatment of pulmonary edema.