Adenovirus-mediated transfer of an Na+/K+-ATPase beta1 subunit gene improves alveolar fluid clearance and survival in hyperoxic rats

Gene Therapy for Lung Diseases

Gene therapy is under development for a variety of lung disease, both those caused by single gene defects, such as cystic fibrosis and α₁-antitrypsin deficiency, and multifactorial diseases such as cancer, asthma, lung fibrosis, and ARDS. Both viral and nonviral approaches have been explored, the major limitation to the former being the inability to repeatedly administer, which renders this approach perhaps more applicable to conditions requiring single administration, such as cancer. Progress in development and clinical trials in each of these diseases is reviewed, together with some potential newer approaches for the future.

Gene expression changes during the development of acute lung injury: role of transforming growth factor beta

RATIONALE

Acute lung injury can occur from multiple causes, resulting in high mortality. The pathophysiology of nickel-induced acute lung injury in mice is remarkably complex, and the molecular mechanisms are uncertain.

OBJECTIVES

To integrate molecular pathways and investigate the role of transforming growth factor beta (TGF-beta) in acute lung injury in mice.

METHODS

cDNA microarray analyses were used to identify lung gene expression changes after nickel exposure. MAPPFinder analysis of the microarray data was used to determine significantly altered molecular pathways. TGF-beta1 protein in bronchoalveolar lavage fluid, as well as the effect of inhibition of TGF-beta, was assessed in nickel-exposed mice. The effect of TGF-beta on surfactant-associated protein B (Sftpb) promoter activity was measured in mouse lung epithelial cells.

MEASUREMENTS AND MAIN RESULTS

Genes that decreased the most after nickel exposure play important roles in lung fluid absorption or surfactant and phospholipid synthesis, and genes that increased the most were involved in TGF-beta signaling. MAPPFinder analysis further established TGF-beta signaling to be significantly altered. TGF-beta-inducible genes involved in the regulation of extracellular matrix function and fibrinolysis were significantly increased after nickel exposure, and TGF-beta1 protein was also increased in the lavage fluid. Pharmacologic inhibition of TGF-beta attenuated nickel-induced protein in bronchoalveolar lavage. In addition, treatment with TGF-beta1 dose-dependently repressed Sftpb promoter activity in vitro, and a novel TGF-beta-responsive region in the Sftpb promoter was identified.

CONCLUSIONS

These data suggest that TGF-beta acts as a central mediator of acute lung injury through the alteration of several different molecular pathways.

To live or die: a critical decision for the lung

Every cell in the body expresses a set of proteins designed to trigger permeabilization of the mitochondria and cell death. Inactivation or inappropriate triggering of these pathways is increasingly recognized as a contributor to human disease. A study in this issue of the JCI demonstrates that IL-6 exerts its protective effect against the development of lung injury following exposure of mice to 95% O(2) by increasing the expression of a Bcl-2-related protein, A1. This protein acts to prevent mitochondrial membrane permeabilization and cell death following exposure to hyperoxia. The data in this study lend support to the hypothesis that inappropriate triggering of cell-death pathways may contribute to the development of hyperoxic pulmonary edema, lung injury, and respiratory failure.

In vivo molecular imaging characterizes pulmonary gene expression during experimental lung transplantation

Experimental gene therapy is a promising strategy to prevent ischemia-reperfusion (I/R) injury and allograft rejection after lung transplantation, and methods will eventually be needed to characterize pulmonary transgene expression in vivo in humans. Therefore, we studied positron emission tomography (PET) as a means of performing in vivo molecular imaging in rodent models of lung transplantation. Rats were transfected endotracheally with adenovirus encoding a fusion gene of a mutant Herpes simplex virus-1 thymidine kinase and the green fluorescent protein gene (the former serving as an imaging reporter gene). Twenty-four hours after transfection, lungs were transplanted in groups representing normal transplantation, I/R injury and acute allograft rejection. Imaging was obtained either 24 h after transplantation to study reperfusion injury or 4 days after transplantation to study graft rejection. After imaging, lungs were excised and analyzed for thymidine kinase activity. Imaging detected transgene expression in transplanted lungs even in the presence of acute rejection or I/R injury. The PET imaging signal correlated with in vitro lung tissue assays of thymidine kinase activity (r(2) = 0.534). Thus, noninvasive molecular imaging with PET is a feasible, sensitive and quantitative method for characterizing pulmonary transgene expression in experimental lung transplantation.

Interdependency of beta-adrenergic receptors and CFTR in regulation of alveolar active Na+ transport

Beta-adrenergic receptors (betaAR) regulate active Na+ transport in the alveolar epithelium and accelerate clearance of excess airspace fluid. Accumulating data indicates that the cystic fibrosis transmembrane conductance regulator (CFTR) is important for upregulation of the active ion transport that is needed to maintain alveolar fluid homeostasis during pulmonary edema. We hypothesized that betaAR regulation of alveolar active transport may be mediated via a CFTR dependent pathway. To test this hypothesis we used a recombinant adenovirus that expresses a human CFTR cDNA (adCFTR) to increase CFTR function in the alveolar epithelium of normal rats and mice. Alveolar fluid clearance (AFC), an index of alveolar active Na+ transport, was 92% greater in CFTR overexpressing lungs than controls. Addition of the Cl- channel blockers NPPB, glibenclamide, or bumetanide and experiments using Cl- free alveolar instillate solutions indicate that the accelerated AFC in this model is due to increased Cl- channel function. Conversely, CFTR overexpression in mice with no beta1- or beta2-adrenergic receptors had no effect on AFC. Overexpression of a human beta2AR in the alveolar epithelium significantly increased AFC in normal mice but had no effect in mice with a non-functional human CFTR gene (Deltaphi508 mutation). These studies indicate that upregulation of alveolar CFTR function speeds clearance of excess fluid from the airspace and that CFTRs effect on active Na+ transport requires the betaAR. These studies reveal a previously undetected interdependency between CFTR and betaAR that is essential for upregulation of active Na+ transport and fluid clearance in the alveolus.

Decreased expression of both the alpha1- and alpha2-subunits of the Na-K-ATPase reduces maximal alveolar epithelial fluid clearance

Impaired epithelial sodium channel function predisposes to delayed resorption of pulmonary edema and more severe experimental lung injury, whereas even a small fraction of the normal Na-K-ATPase activity is thought to be sufficient to maintain normal ion transport. However, direct proof is lacking. Therefore, we studied baseline and cAMP stimulated alveolar fluid clearance (AFC) in mice with a 50% decrease in lung protein expression of the alpha(1)- and/or alpha(2)-subunit of the Na-K-ATPase. There was no difference in basal and stimulated AFC in alpha(1)(+/-) or alpha(2)(+/-) mice compared with wild-type littermates. Also, the compound heterozygous mice (alpha(1)(+/-)/alpha(2)(+/-)) had normal basal AFC. However, the combined alpha(1)(+/-)/alpha(2)(+/-) mice showed a significant decrease in cAMP-stimulated AFC compared with wild-type littermates (11.1 +/- 1.0 vs. 14.9 +/- 1.8%/30 min, P < 0.001). When exposed to 96 h of >95% hyperoxia, the decrease in stimulated AFC in the alpha(1)(+/-)/alpha(2)(+/-) mice was not associated with more lung edema compared with wild-type littermates (lung wet-to-dry weight ratio 6.6 +/- 0.9 vs. 5.9 +/- 1.1, respectively; P = not significant). Thus a 50% decrease in protein expression of the alpha(1)- or alpha(2)-subunits of the Na-K-ATPase does not impair basal or stimulated AFC. However, a 50% protein reduction in both the alpha(1)- and alpha(2)-subunits of the Na-K-ATPase produces a submaximal stimulated AFC, suggesting a synergistic role for alpha(1)- and alpha(2)-subunits in cAMP-dependent alveolar epithelial fluid clearance.

Gene transfer of the Na+,K+-ATPase beta1 subunit using electroporation increases lung liquid clearance

The development of nonviral methods for efficient gene transfer to the lung is highly desired for the treatment of several pulmonary diseases. We have developed a noninvasive procedure using electroporation to transfer genes to the lungs of rats. Purified plasmid (100-600 microg) was delivered to the lungs of anesthetized rats through an endotracheal tube, and a series of square-wave pulses were delivered via electrodes placed on the chest. Relatively uniform gene expression was observed in multiple cell types and layers throughout the lung, including airway and alveolar epithelial cells, airway smooth muscle cells, and vascular endothelial cells, and this finding was dose- and pulse length-dependent. Most important, no inflammatory response was detected. To demonstrate efficacy of this approach, the beta1 subunit of the Na(+),K(+)-ATPase was transferred to the lungs of rats with or without electroporation, and 3 days later, alveolar fluid clearance was measured. Animals electroporated with the beta1 subunit plasmid showed a twofold increase in alveolar fluid clearance and Na(+),K(+)-ATPase activity as compared with animals receiving all other plasmids, with or without electroporation. These results demonstrate that electroporation is an effective method to increase clearance by introducing therapeutic genes (Na(+),K(+)-ATPase) into the rat lung.

Bench-to-bedside review: the role of the alveolar epithelium in the resolution of pulmonary edema in acute lung injury

Clearance of pulmonary edema fluid is accomplished by active ion transport, predominantly by the alveolar epithelium. Various ion pumps and channels on the surface of the alveolar epithelial cell generate an osmotic gradient across the epithelium, which in turn drives the movement of water out of the airspaces. Here, the mechanisms of alveolar ion and fluid clearance are reviewed. In addition, many factors that regulate the rate of edema clearance, such as catecholamines, steroids, cytokines, and growth factors, are discussed. Finally, we address the changes to the alveolar epithelium and its transport processes during acute lung injury (ALI). Since relevant clinical outcomes correlate with rates of edema clearance in ALI, therapies based on our understanding of the mechanisms and regulation of fluid transport may be developed.

Oleic acid inhibits alveolar fluid reabsorption: a role in acute respiratory distress syndrome?

Levels of oleic acid (OA) are elevated in plasma and bronchoalveolar lavage fluids of patients with acute respiratory distress syndrome (ARDS). OA is also widely used to provoke edema, by unknown mechanisms, in experimental models of ARDS. We investigated the impact of intravascularly applied OA on epithelial lining fluid balance. OA (25 microM) dramatically blocked active transepithelial (22)Na(+) transport (by 92%) in an isolated, ventilated, and perfused rabbit lung model, provoking alveolar edema, assessed by increases in lung weight and epithelial lining fluid volume. OA did not alter epithelial permeability, measured by [(3)H]mannitol and fluorescently labeled albumin flux, but did increase endothelial permeability, assessed by capillary filtration coefficient. In A549 cells, OA completely blocked amiloride-sensitive sodium currents measured by patch clamp, and also largely abrogated ouabain-sensitive Na(+),K(+)-ATPase-mediated (86)Rb(+) uptake. Although OA did not alter epithelial sodium channel or Na(+),K(+)-ATPase surface expression, it covalently associated with both molecules and directly, dramatically, and dose-dependently inhibited the catalytic activity of purified Na(+),K(+)-ATPase. Therefore, OA impaired the two essential transepithelial active sodium transport mechanisms of the lung, and could thus promote alveolar edema formation and prevent edema resolution, thereby contributing to the development of ARDS.

Adenovirus-mediated transfer of the 1-cys peroxiredoxin gene to mouse lung protects against hyperoxic injury

1-Cys peroxiredoxin (1-cysPrx) is a novel antioxidant enzyme that has been shown to reduce a broad spectrum of peroxides including phospholipid hydroperoxides. We tested the hypothesis that adenovirus-mediated transfer of the 1-cysPrx gene can protect lungs of mice from oxidant injury. Mice infected with AdLacZ/AdNull were used as a control (AdCon). X-galactosidase staining revealed widespread expression of the LacZ gene in airways and lung alveoli. Compared with AdCon, 1-cysPrx expression was increased about twofold at 3 days after adenovirus infection. Mice with increased Prx expression showed less loss of body weight and longer survival during exposure to 100% O(2) or to 85% O(2) for 4 days followed by 100% O(2). At 72 h of 100% O(2) exposure, AdPrx infection protected mouse lungs from injury as indicated by less pleural effusion, lower lung wet/dry weight, less protein and fewer nucleated cells in bronchoalveolar lavage fluid, and lower content of thiobarbituric acid-reactive substances and protein carbonyls in lung homogenate. These findings show that increased expression of 1-cysPrx through adenovirus-mediated gene transfer protects mouse lungs from hyperoxic injury and delays death.

Upregulation of alveolar epithelial active Na+ transport is dependent on beta2-adrenergic receptor signaling

Alveolar epithelial beta-adrenergic receptor (betaAR) activation accelerates active Na+ transport in lung epithelial cells in vitro and speeds alveolar edema resolution in human lung tissue and normal and injured animal lungs. Whether these receptors are essential for alveolar fluid clearance (AFC) or if other mechanisms are sufficient to regulate active transport is unknown. In this study, we report that mice with no beta1- or beta2-adrenergic receptors (beta1AR-/-/beta2AR-/-) have reduced distal lung Na,K-ATPase function and diminished basal and amiloride-sensitive AFC. Total lung water content in these animals was not different from wild-type controls, suggesting that betaAR signaling may not be required for alveolar fluid homeostasis in uninjured lungs. Comparison of isoproterenol-sensitive AFC in mice with beta1- but not beta2-adrenergic receptors to beta1AR-/-/beta2AR-/- mice indicates that the beta2AR mediates the bulk of beta-adrenergic-sensitive alveolar active Na+ transport. To test the necessity of betaAR signaling in acute lung injury, beta1AR-/-/beta2AR-/-, beta1AR+/+/beta2AR-/-, and beta1AR+/+/beta2AR+/+ mice were exposed to 100% oxygen for up to 204 hours. beta1AR-/-/beta2AR-/- and beta1AR+/+/beta2AR-/- mice had more lung water and worse survival from this form of acute lung injury than wild-type controls. Adenoviral-mediated rescue of beta2-adrenergic receptor (beta2AR) function into the alveolar epithelium of beta1AR-/-/beta2AR-/- and beta1AR+/+/beta2AR-/- mice normalized distal lung beta2AR function, alveolar epithelial active Na+ transport, and survival from hyperoxia. These findings indicate that betaAR signaling may not be necessary for basal AFC, and that beta2AR is essential for the adaptive physiological response needed to clear excess fluid from the alveolar airspace of normal and injured lungs.

Transgenic overexpression of granulocyte macrophage-colony stimulating factor in the lung prevents hyperoxic lung injury

Granulocyte macrophage-colony stimulating factor (GM-CSF) plays an important role in pulmonary homeostasis, with effects on both alveolar macrophages and alveolar epithelial cells. We hypothesized that overexpression of GM-CSF in the lung would protect mice from hyperoxic lung injury by limiting alveolar epithelial cell injury. Wild-type C57BL/6 mice and mutant mice in which GM-CSF was overexpressed in the lung under control of the SP-C promoter (SP-C-GM mice) were placed in >95% oxygen. Within 6 days, 100% of the wild-type mice had died, while 70% of the SP-C-GM mice remained alive after 10 days in hyperoxia. Histological assessment of the lungs at day 4 revealed less disruption of the alveolar wall in SP-C-GM mice compared to wild-type mice. The concentration of albumin in bronchoalveolar lavage fluid after 4 days in hyperoxia was significantly lower in SP-C-GM mice than in wild-type mice, indicating preservation of alveolar epithelial barrier properties in the SP-C-GM mice. Alveolar fluid clearance was preserved in SP-C-GM mice in hyperoxia, but decreased significantly in hyperoxia-exposed wild-type mice. Staining of lung tissue for caspase 3 demonstrated increased apoptosis in alveolar wall cells in wild-type mice in hyperoxia compared to mice in room air. In contrast, SP-C-GM mice exposed to hyperoxia demonstrated only modest increase in alveolar wall apoptosis compared to room air. Systemic treatment with GM-CSF (9 micro g/kg/day) during 4 days of hyperoxic exposure resulted in decreased apoptosis in the lungs compared to placebo. In studies using isolated murine type II alveolar epithelial cells, treatment with GM-CSF greatly reduced apoptosis in response to suspension culture. In conclusion, overexpression of GM-CSF enhances survival of mice in hyperoxia; this effect may be explained by preservation of alveolar epithelial barrier function and fluid clearance, at least in part because of reduction in hyperoxia-induced apoptosis of cells in the alveolar wall.

Na,K-ATPase gene transfer increases liquid clearance during ventilation-induced lung injury

Mechanical ventilation with high tidal volumes (HVT) downregulates alveolar Na,K-ATPase function and impairs lung liquid clearance. We hypothesized that overexpression of Na,K-ATPase in the alveolar epithelium could counterbalance these changes and increase clearance in a rat model of mild ventilation-induced lung injury. We used a surfactant-based system to deliver 4 x 10(9) plaque-forming units of E1a-/E3- recombinant adenovirus containing either a rat beta1 Na,K-ATPase subunit cDNA (adbeta1) or no cDNA (adnull) to rat lungs 7 days before ventilation with a VT of approximately 40 ml/kg (peak airway pressure of less than 35 cm H2O) for 40 minutes. Lung liquid clearance and Na, K-ATPase activity and protein abundance were increased in HVT adbeta1-infected lungs as compared with sham and adnull-infected HVT lungs. These results suggest that Na,K-ATPase subunit gene overexpression in the alveolar epithelium increases Na,K-ATPase function and lung liquid clearance in a model of HVT. We provide here the first evidence that using a genetic approach improves active Na+ transport and thus liquid clearance in the setting of mild ventilation-induced lung injury.

Imaging the spatial distribution of transgene expression in the lungs with positron emission tomography

This study was designed to evaluate the utility of positron emission tomography (PET) to quantify the magnitude and spatial distribution of transgene expression after different methods of adenoviral vector delivery (with surfactant- and saline-based vehicles) within rat lungs. In all, 17 animals (eight in the surfactant group, nine in the saline group) were studied 3 days after intratracheal administration of a replication-incompetent adenovirus encoding a mutant Herpes simplex virus-1 thymidine kinase (mHSV1-TK)-enhanced green fluorescent protein fusion gene driven by a Cytomegalovirus promoter (Ad-CMV-mNLS-HSV1sr39tk-egfp). PET images were obtained 1 h after i.v. administration of 9-(4-[(18)F]-fluoro-3-hydroxymethylbutyl)guanine ([(18)F]-FHBG), an imaging substrate for mHSV1-TK. Overall, the average lung concentration of [(18)F]-FHBG was significantly greater in the surfactant group than in the saline group (0.24+/-0.06 versus 0.17+/-0.03% injected dose/ml lung, P< or =0.05). Lung [(18)F]-FHBG distribution was more peripheral and more homogeneous in the surfactant group than in the saline group (mean coefficient of variation=31+/-4 versus 36+/-3%, respectively, P< or =0.05). Regions of increased tracer concentration in the surfactant group compared to the saline group were evenly distributed throughout the lungs. We conclude that PET imaging provides useful and meaningful information about the effectiveness of different gene transfer delivery strategies within the lungs, and that surfactant-based vehicles may be a superior strategy for pulmonary gene transfer.

Repetitive Imaging of Reporter Gene Expression in the Lung

Positron emission tomographic imaging is emerging as a powerful technology to monitor reporter transgene expression in the lungs and other organs. However, little information is available about its usefulness for studying gene expression over time. Therefore, we infected 20 rats with a replication-deficient adenovirus containing a fusion gene encoding for a mutant Herpes simplex virus type-1 thymidine kinase and an enhanced green fluorescent protein. Five additional rats were infected with a control virus. Pulmonary gene transfer was performed via intratracheal administration of vector using a surfactant-based method. Imaging was performed 4-6 hr, and 4, 7, and 10 days after gene transfer, using 9-(4-[18F]-fluoro-3-hydroxymethylbutyl)guanine, an imaging substrate for the mutant kinase. Lung tracer uptake assessed with imaging was moderately but significantly increased 4-6 hr after gene transfer, was maximal after 4 days, and was no longer detectable by 10 days. The temporal pattern of transgene expression measured ex vivo with in vitro assays of thymidine kinase activity and green fluorescent protein was similar to imaging. In conclusion, positron emission tomography is a reliable new tool to evaluate the onset and duration of reporter gene expression noninvasively in the lungs of intact animals.

The effect of inhaled nitric oxide and oxygen on the hydroxylation of salicylate in rat lungs

Inhaled nitric oxide (iNO) is used as a selective pulmonary vasodilator, and often under conditions when a high fraction of inspired oxygen is indicated. However, little is known about the potential toxicity of iNO therapy with or without concomitant oxygen therapy. NO can combine with superoxide (O2-) to form peroxynitrite (ONOO-), which can in turn decompose to form hydroxyl radical (OH.). Both OH. and ONOO- are involved in various forms of lung injury. To begin evaluation of the effect of iNO under either normoxic or hyperoxic conditions on OH. and/or ONOO- formation, rats were exposed for 58 h to either 21% O2, 21% O2 + 10 parts per million (ppm) NO, 21% O2 + 100 ppm NO, 50% O2, 90% O2, 90% O2 + 10 ppm NO, or 90% O2 + 100 ppm NO. We used a salicylate hydroxylation assay to detect the effects of these exposures on lung OH. and/or ONOO- formation measured as the appearance of 2,3-dihydroxybenzoic acid (2,3-DHBA). Exposure to 90% O2 and 90% O2 + 100 ppm NO resulted in significantly (p < 0.05) greater lung wet weight (1.99 +/- 0.14 g and 3.14 +/- 0.30 g, respectively) compared with 21% O2 (1.23 +/- 0.01 g). Exposure to 21% O2 + 100 ppm NO led to 2.5 times the control (21% O2 alone) 2,3 DHBA formation (p < 0.05) and exposure to 90% O2 led to 2.4 times the control 2,3-DHBA formation (p < 0.05). However, with exposure to both 90% O2 and 100 ppm NO, the 2,3-DHBA formation was no greater than the control condition (21% O2). Thus, these results indicate that, individually, both the hyperoxia and the 100 ppm NO led to greater salicylate hydroxylation, but that the combination of hyperoxia and 100 ppm NO led to less salicylate hydroxylation than either did individually. The production of OH. and/or ONOO- in the lung during iNO therapy may depend on the ratio of NO to O2.

Fibroblast growth factor-10 upregulates Na,K-ATPase via the MAPK pathway

We studied the effects of fibroblast growth factor (FGF-10) on alveolar epithelial cell (AEC) Na,K-ATPase regulation. Within 30 min FGF-10 increased Na,K-ATPase activity and alpha1 protein abundance by 2.5-fold at the AEC plasma membrane. Pretreatment of AEC with the mitogen-activated protein kinase (MAPK) inhibitor U0126, a Grb2-SOS inhibitor (SH3-b-p peptide), or a Ras inhibitor (farnesyl transferase inhibitor (FTI 277)), as well as N17-AEC that express a Ras dominant negative protein each prevented FGF-10-mediated Na,K-ATPase recruitment to the AEC plasma membrane. Accordingly, we provide first evidence that FGF-10 upregulates (short-term) the Na,K-ATPase activity in AEC via the Grb2-SOS/Ras/MAPK pathway.

Alveolar liquid clearance and sodium channel expression are decreased in transplanted canine lungs

To determine the impact of transplantation-associated injury on the clearance mechanisms of pulmonary edema, we created a canine single lung transplant model. After 3 hours of preservation and 4 hours of reperfusion, right native lungs and left transplanted lungs were used to measure alveolar liquid clearance (ALC) in ex vivo liquid-filled lung preparations. We also examined the role of the pulmonary circulation in edema clearance in in vivo liquid-filled lungs between 4 and 8 hours of reperfusion. To study molecular modifications in ALC, we also measured expression levels of the epithelial sodium channel (ENaC) and sodium-potassium-adenosine triphosphatase (ATPase). We found that ALC was significantly lower in transplanted than in right native lungs ex vivo (p < 0.05) and that transplanted lungs did not respond to the beta-adrenergic agonist terbutaline. Our in vivo study confirmed the ex vivo results. Molecular analyses revealed that ENaC messenger RNA but not sodium-potassium-ATPase was significantly decreased in transplanted lungs (p < 0.01). Furthermore, there was a significant decrease in ENaC protein expression. Therefore, we conclude that the current investigation indicates that the lung injury caused by lung preservation and transplantation significantly reduces the edema clearance ability of transplanted lungs.

Imaging pulmonary gene expression with positron emission tomography

We evaluated positron emission tomographic imaging of pulmonary transgene expression, using an enhanced mutant herpes simplex virus-1 thymidine kinase as the reporter gene, in the lungs of normal rats. Sixteen rats were studied 3 days after an intratracheal administration of 5 x 10(9) to 1 x 10(11) viral particles of a replication-incompetent adenovirus containing a fusion gene of the mutant kinase and green fluorescent protein. Three rats infected with adenovirus containing no insert (null vector) served as control subjects. Images were obtained 1 hour after an intravenous injection of 9-(4-[18F]-fluoro-3-hydroxymethylbutyl)guanine, an imaging substrate for the viral kinase. After euthanasia, tissue radioactivity was determined in a gamma counter, and thymidine kinase activity and green fluorescent protein levels were measured in lung tissue samples. Imaging and gamma counting radioactivity measurements were strongly and linearly correlated (r2 = 0.96, p < 0.001). Imaging detected thymidine kinase expression above background (null vector) in 15 of 16 rats, even at low viral doses that produced little to no measurable green fluorescent protein expression. Lung 9-(4-[18F]-fluoro-3-hydroxymethylbutyl)guanine uptake (as assessed by imaging) correlated with in vitro assays of both kinase activity (r(2) = 0.48, p < 0.001) and fluorescent protein (r(2) = 0.46, p < 0.001). We conclude that positron emission tomographic imaging is a sensitive and quantitative method for detecting pulmonary reporter gene expression noninvasively.

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.

Induction of mRNAs and proteins for Na/K ATPase alpha1 and beta1 subunits following hypoxia/reoxygenation in astrocytes

Characteristics of the cellular response to oxygen deprivation and subsequent reoxygenation (hypoxia/reoxygenation) include redirection of energy metabolism, increased glucose utilization and expression of oxygen-regulated proteins. Inhibition of protein synthesis during early reoxygenation period prevented effective astrocyte adaptation to hypoxia/reoxygenation, resulting in eventual cell death. To elucidate the role of astrocytes in the central nervous system in response to hypoxia/reoxygenation, we analyzed the cDNA library derived from the cultured rat astrocytes subjected to 24 h of hypoxia followed by reoxygenation by differential display, and isolated a cDNA corresponding to Na/K ATPase alpha1 subunit. The expression of Na/K ATPase alpha1 subunit mRNA as well as beta1subunit mRNA was transiently increased after reoxygenation, whereas hypoxia itself did not induce any gene expression change. Na/K ATPase alpha1 subunit protein was transiently increased, whereas the protein expression for Na/K ATPase beta1 subunit showed sustained induction after reoxygenation. Overexpression of beta1 subunit in HEK 293 cells subjected to hypoxia/reoxygenation promoted survival of the cells. These findings suggest that Na/K ATPases may contribute to maintain the cellular environment of astrocytes subjected to hypoxia/reoxygenation.

Lung edema clearance: 20 years of progress: invited review: alveolar edema fluid clearance in the injured lung

Resolution of pulmonary edema involved active transepithelial sodium transport. Although several of the cellular and molecular mechanisms involved are relatively well understood, it is only recently that the regulation of these mechanisms in injured lung are being evaluated. Interestingly, in mild-to-moderate lung injury, alveolar edema fluid clearance is often preserved. This preserved or enhanced alveolar fluid clearance is mediated by catecholamine-dependent or -independent mechanisms. This stimulation of alveolar liquid clearance is related to activation or increased expression of sodium transport molecules such as the epithelial sodium channel or the Na(+)-K(+)-ATPase pump and may also involve the cystic fibrosis transmembrane conductance regulator. When severe lung injury occurs, the decrease in alveolar liquid clearance may be related to changes in alveolar permeability or to changes in activity or expression of sodium or chloride transport molecules. Multiple pharmacological tools such as beta-adrenergic agonists, vasoactive drugs, or gene therapy may prove effective in stimulating the resolution of alveolar edema in the injured lung.

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.

Na(+)-K(+)-ATPase activity in alveolar epithelial cells increases with cyclic stretch

Na(+)-K(+)-ATPase pumps (Na(+) pumps) in the alveolar epithelium create a transepithelial Na(+) gradient crucial to keeping fluid from the pulmonary air space. We hypothesized that alveolar epithelial stretch stimulates Na(+) pump trafficking to the basolateral membrane (BLM) and, thereby, increases overall Na(+) pump activity. Alveolar type II cells were isolated from Sprague-Dawley rats and seeded onto elastic membranes coated with fibronectin or 5-day-conditioned extracellular matrix. After 2 days in culture, cells were uniformly stretched for 1 h in a custom-made device. Na(+) pump activity was subsequently assessed by ouabain-inhibitable uptake of (86)Rb(+), a K(+) tracer, and BLM Na(+) pump abundance was measured. In support of our hypothesis, cells increased Na(+) pump activity in a "dose-dependent" manner when stretched to 12, 25, or 37% change in surface area (DeltaSA), and cells stretched to 25% DeltaSA more than doubled Na(+) pump abundance in the BLM. Cells on 5-day matrix tolerated higher strain than cells on fibronectin before the onset of Na(+) pump upregulation. Treatment with Gd(3+), a stretch-activated channel blocker, amiloride, a Na(+) channel blocker, or both reduced but did not abolish stretch-induced effects. Sustained tonic stretch, unlike cyclic stretch, elicited no significant Na(+) pump response.

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.

Acute hyperoxic lung injury does not impede adenoviral-mediated alveolar gene transfer

The transfer of protective genes to the alveolar epithelium can attenuate lung injury if accomplished before its onset. The pathobiology of acute lung injury (ALI) includes formidable hurdles to gene transfer, including alveoli filled with fluid, inflammatory cells, and cytokines, all of which may impair gene transfer after the onset of injury. We tested the hypothesis that adenovectors could efficiently transduce injured alveoli by exposing adult, male Sprague-Dawley rats to 100% oxygen for 48 or 60 h before endotracheal instillation of either 1 x 10(9) or 4 x 10(9) plaque-forming units of an adenovirus that expresses an Escherichia coli lac Z gene (adbeta-gal) in a surfactant-based vehicle (Survanta). X-gal staining 72 h postinfection revealed transgene expression in all segments of room air control and hyperoxic lungs infected with either dose of adbeta-gal. Net transgene expression in hyperoxic lungs was not different from room air controls despite the presence of pulmonary edema and severe histologic injury. These findings show that adenovectors can efficiently transduce the alveoli of acutely injured, edematous lungs. The data indicate that the pathophysiologic processes of ALI do not impair adenoviral-mediated alveolar gene transfer and provide support for the development of gene therapies for ALI.

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.

Gene therapy for acute diseases

The use of gene transfer systems to study cell function makes it apparent that overexpression of a transgene can restore or improve the function of a protein and positively influence cell function in a predetermined manner for purposes of counterbalancing cellular pathophysiology. The ability of some gene transfer vehicles to produce transgene product within hours of delivery positions gene transfer as a unique pharmaceutical administration system that can quickly affect production of biologic response modifiers in a highly compartmentalized fashion. This approach can be expected to overcome many of the adverse effects and high costs of systemic delivery of recombinant pharmaceuticals. This review highlights recent advances toward development of gene therapies for acute illnesses with particular emphasis on preclinical models of disease. In this context, a growing body of data suggests that gene therapies for polygenic and non-genetic diseases such as asthma, cardiogenic and non-cardiogenic pulmonary edema, stroke, subarachnoid hemorrhage, seizures, acute myocardial infarction, endovascular thrombosis, and infections may someday be options for the treatment of patients.

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.