Karen M. Ridge

Isoproterenol increases Na+-K+-ATPase activity by membrane insertion of α-subunits in lung alveolar cells

Catecholamines promote lung edema clearance via β-adrenergic-mediated stimulation of active Na+ transport across the alveolar epithelium. Because alveolar epithelial type II cell Na+-K+-ATPase contributes to vectorial Na+ flux, the present study was designed to investigate whether Na+-K+-ATPase undergoes acute changes in its catalytic activity in response to β-adrenergic-receptor stimulation. Na+-K+-ATPase activity increased threefold in cells incubated with 1 μM isoproterenol for 15 min, which also resulted in a fourfold increase in the cellular levels of cAMP. Forskolin (10 μM) also stimulated Na+-K+-ATPase activity as well as ouabain binding. The increase in Na+-K+-ATPase activity was abolished when cells were coincubated with a cAMP-dependent protein kinase inhibitor. This stimulation, however, was not due to protein kinase-dependent phosphorylation of the Na+-K+-ATPase α-subunit; rather, it was the result of an increased number of α-subunits recruited from the late endosomes into the plasma membrane. The recruitment of α-subunits to the plasma membrane was prevented by stabilizing the cortical actin cytoskeleton with phallacidin or by blocking anterograde transport with brefeldin A but was unaffected by coincubation with amiloride. In conclusion, isoproterenol increases Na+-K+-ATPase activity in alveolar type II epithelial cells by recruiting α-subunits into the plasma membrane from an intracellular compartment in an Na+-independent manner.

Augmentation of lung liquid clearance via adenovirus-mediated transfer of a Na,K-ATPase beta1 subunit gene.

Previous studies have suggested that alveolar Na,K-ATPases play an important role in active Na+ transport and lung edema clearance. We reasoned that overexpression of Na,K-ATPase subunit genes could increase Na,K-ATPase function in lung epithelial cells and edema clearance in rat lungs. To test this hypothesis we produced replication deficient human type 5 adenoviruses containing cDNAs for the rat alpha1 and beta1 Na,K-ATPase subunits (adMRCMValpha1 and adMRCMVbeta1, respectively). As compared to controls, adMRCMVbeta1 increased beta1 subunit expression and Na,K-ATPase function by 2. 5-fold in alveolar type 2 epithelial cells and rat airway epithelial cell monolayers. No change in Na,K-ATPase function was noted after infection with adMRCMValpha1. Rat lungs infected with adMRCMVbeta1, but not adMRCMValpha1, had increased beta1 protein levels and lung liquid clearance 7 d after tracheal instillation. Alveolar epithelial permeability to Na+ and mannitol was mildly increased in animals infected with adMRCMVbeta1 and a similar Escherichia coli lacZ-expressing virus. Our data shows, for the first time, that transfer of the beta1 Na,K-ATPase subunit gene augments Na,K-ATPase function in epithelial cells and liquid clearance in rat lungs. Conceivably, overexpression of Na,K-ATPases could be used as a strategy to augment lung liquid clearance in patients with pulmonary edema.

Modulation of lung liquid clearance by isoproterenol in rat lungs

beta-Adrenergic agonists have been reported to increase lung liquid clearance by stimulating active Na+ transport across the alveolar epithelium. We studied mechanisms by which beta-adrenergic isoproterenol (Iso) increases lung liquid clearance in isolated perfused fluid-filled rat lungs. Iso perfused through the pulmonary circulation at concentrations of 10(-4) to 10(-8) M increased lung liquid clearance compared with that of control lungs (P < 0.01). The increase in lung liquid clearance was inhibited by the beta-antagonist propranolol (10(-5) M), the Na(+)-channel blocker amiloride (10(-4) M), and the antagonist of Na-K-ATPase, ouabain (5 x 10(-4) M). Colchicine, which inhibits cell microtubular transport of ion-transporting proteins to the plasma membrane, blocked the stimulatory effects of Iso on active Na+ transport, whereas the isomer lumicolchicine, which does not affect cell microtubular transport, did not inhibit Na+ transport. In parallel with these changes, the Na-K-ATPase alpha 1-subunit protein abundance and activity increased in alveolar type II cells stimulated by 10(-6) M Iso. Colchicine blocked the stimulatory effect of Iso and the recruitment of Na-K-ATPase alpha 1-protein to the basolateral membrane of alveolar type II cells. Accordingly, Iso increased active Na+ transport and lung liquid clearance by stimulation of beta-adrenergic receptors and probably by upregulation of apical Na+ channels and basolateral Na-K-ATPase mechanisms. Recruitment from intracellular pools and microtubular transport of Na+ pumps to the plasma membrane participate in beta-adrenergic stimulation of lung liquid clearance in rat lungs.β-Adrenergic agonists have been reported to increase lung liquid clearance by stimulating active Na+ transport across the alveolar epithelium. We studied mechanisms by which β-adrenergic isoproterenol (Iso) increases lung liquid clearance in isolated perfused fluid-filled rat lungs. Iso perfused through the pulmonary circulation at concentrations of 10-4 to 10-8 M increased lung liquid clearance compared with that of control lungs ( P < 0.01). The increase in lung liquid clearance was inhibited by the β-antagonist propranolol (10-5 M), the Na+-channel blocker amiloride (10-4 M), and the antagonist of Na-K-ATPase, ouabain (5 × 10-4 M). Colchicine, which inhibits cell microtubular transport of ion-transporting proteins to the plasma membrane, blocked the stimulatory effects of Iso on active Na+ transport, whereas the isomer lumicolchicine, which does not affect cell microtubular transport, did not inhibit Na+ transport. In parallel with these changes, the Na-K-ATPase α1-subunit protein abundance and activity increased in alveolar type II cells stimulated by 10-6 M Iso. Colchicine blocked the stimulatory effect of Iso and the recruitment of Na-K-ATPase α1-protein to the basolateral membrane of alveolar type II cells. Accordingly, Iso increased active Na+ transport and lung liquid clearance by stimulation of β-adrenergic receptors and probably by upregulation of apical Na+ channels and basolateral Na-K-ATPase mechanisms. Recruitment from intracellular pools and microtubular transport of Na+pumps to the plasma membrane participate in β-adrenergic stimulation of lung liquid clearance in rat lungs.

Evidence for the role of alveolar epithelial gp60 in active transalveolar albumin transport in the rat lung

Transcytosis of albumin, involving the 60 kDa albumin‐binding glycoprotein, gp60, was studied in cultured type II alveolar epithelial cells obtained from rat lungs. Type II cells internalized the interfacial fluorescent dye RH 414, which marks for plasmalemma vesicles. Fluorescent forms of albumin and anti‐gp60 antibody colocalized in the same plasmalemma vesicles. Antibody (100 μg ml−1) cross‐linking of gp60 for brief periods (15 min) markedly stimulated vesicular uptake of fluorescently tagged albumin. The caveolar disrupting agent, filipin (10 nm), abolished the stimulated internalization of albumin. The vast majority of plasmalemmal vesicles carrying albumin also immunostained for caveolin‐1; however, lysosomes did not stain for caveolin‐1. Filipin depleted the epithelial cells of the caveolin‐1‐positive, albumin‐transporting plasmalemma vesicles. Prolonged (> 1 h) stimulation of type II cells with cross‐linking anti‐gp60 antibody produced loss of cell‐surface gp60 and abolished endocytic albumin uptake. Transalveolar transport of albumin was also studied in the isogravimetric rat lung preparation perfused at 37°C. 125I‐labelled albumin was instilled into distal airspaces of lungs, and the resulting 125I‐labelled albumin efflux into the vascular perfusate was determined. Unlabelled albumin (studied over a range of 0–10 g (100 instilled ml)−1) inhibited 40% of the transport of labelled albumin ((5.7 ± 0.4) × 105 counts (instilled ml)−1) with an IC50 value of 0.34 g (100 ml)−1. Filipin blocked the displacement‐sensitive component of 125I‐labelled albumin transport, but had no effect on the transport of the paracellular tracer 3[H]mannitol. Displacement‐sensitive 125I‐labelled albumin transport had a significantly greater Q10 (27–37 °C) than the non‐displaceable component. Cross‐linking of gp60 by antibody instillation stimulated only the displacement‐sensitive 125I‐labelled albumin transalveolar transport in intact rat lungs. To estimate the transport capacity of the displacement‐sensitive system, the percentage of instilled 125I‐labelled albumin counts remaining in lung tissue was compared in lungs treated with instillates containing either 0.05 g (100 ml)−1 unlabelled albumin or 5 g (100 ml)−1 unlabelled albumin. Approximately 25% of instilled 125I‐labelled albumin was cleared from the lung preparations per hour by the displacement‐sensitive transport pathway. This component was blocked by filipin.

Dexamethasone upregulates the Na-K-ATPase in rat alveolar epithelial cells

Previous studies in kidney, heart, and liver cells have demonstrated that dexamethasone regulates the expression of Na-K-ATPase. In the lungs, Na-K-ATPase has been reported in alveolar epithelial type II (ATII) cells and is thought to participate in active Na+ transport and lung edema clearance. The aim of this study was to determine whether Na-K-ATPase would be regulated by dexamethasone in cultured rat ATII cells. Regulation of the Na-K-ATPase by dexamethasone could lead to a greater understanding of its role in active Na+ transport and lung edema clearance. Rat ATII cells were isolated, plated for 24 h, and exposed to 10-7 and 10-8 M dexamethasone. These cells were harvested at 0, 3, 6, 12, and 24 h after dexamethasone exposure for determination of steady-state Na-K-ATPase mRNA transcript levels, protein expression, and function. The steady-state Na-K-ATPase β1-mRNA transcript levels increased in ATII cells 6, 12, and 24 h after dexamethasone exposure ( P < 0.05). However, the steady-state α1-mRNA transcript levels were unchanged. The protein expression for the α1- and β1-subunits increased in ATII cells exposed to dexamethasone compared with controls in association with a temporal increase in Na-K-ATPase function after dexamethasone exposure. These results suggest that dexamethasone regulates Na-K-ATPase in ATII cells possibly by transcriptional, translational, and posttranslational mechanisms.

β-Agonists regulate Na,K-ATPase via novel MAPK/ERK and rapamycin-sensitive pathways

We studied whether the β‐adrenergic agonist, isoproterenol (ISO), regulates Na,K‐ATPase in alveolar epithelial cells (AEC) via a mitogen‐activated protein kinase (MAPK)/extracellular signaling related kinase (ERK) dependent pathway. ISO increased ERK activity in AEC by 10 min via a β‐adrenergic receptor, protein kinase A (PKA)‐dependent mechanism. Activation of the MAPK pathway by ISO, resulted in increased Na,K‐ATPase β1 and α1 subunit protein abundance in whole cell lysates, which resulted in functional Na,K‐ATPases at the basolateral membranes. ISO did not change the α1 or β1 mRNA steady state levels, but rapamycin, the inhibitor of the mammalian target of rapamycin, also blocked the ISO‐mediated increase in Na,K‐ATPase total protein abundance, suggesting a posttranscriptional regulation. We conclude that ISO, regulates the Na,K‐ATPase in AEC via PKA, ERK and rapamycin‐sensitive mechanisms.

LUNG ALVEOLAR EPITHELIAL CELLS SYNTHESIZE INTERSTITIAL COLLAGENASE AND GELATINASES A AND B IN VITRO

Type II pneumocytes are multifunctional alveolar epithelial cells that play a major role in the maintenance of lung structure and function. Recent evidence supports that these cells can synthesize a variety of extracellular matrix components in vitro, suggesting an active participation in connective tissue remodeling. However, their possible role in extracellular matrix degradation is unknown. In this study the production of matrix metalloproteinases (MMPs) was examined in primary cultures of rat alveolar type II pneumocytes after 2 and 7 days in culture. Under basal conditions, at both periods type II cells expressed interstitial collagenase mRNA. The immunoreactive protein was detected both in the cells and in conditioned media, and collagenolytic activity was revealed after trypsin activation. Gelatinolytic activity was detected by zymography showing a relative molecular mass of approximately 72 and 92 kDa (gelatinases A and B). Phorbol treatment increased collagenase and gelatinase activities. In addition, three alveolar epithelial cell lines were analysed for MMP production: MLE-12 (mice), L2 (rat), and A549 (human). The cell lines A549 and MLE-12 revealed collagenase and gelatinase A and B activities whereas the L2 cell line only exhibited gelatinase A activity, even after PMA induction. These findings demonstrate that alveolar epithelial cells synthesize in vitro several MMPs that confer on them the ability to degrade extracellular matrix and basement membrane components, a capacity of considerable importance for the remodeling of the stromal/epithelial interface.

Hydrogen peroxide increases Na+/K+-ATPase function in alveolar type II cells

We have studied the regulation of Na+/K(+)-ATPase function in alveolar type II cells submitted to oxidative stress. Alveolar type II cells were isolated from Sprague Dawley rats and suspended in Dulbeccos modified Eagles medium. 500 muM xanthine plus 0.5 or 5 mU/ml xanthine oxidase (group 1 and 2, respectively) were added to the cell suspensions. Following various exposure times the reaction was stopped by adding allopurinol and cells were processed to assay H2O2 steady state concentrations, enzymatic activity of catalase and Na+/K(+)-ATPase function. Hydrogen peroxide production by the xanthine-xanthine oxidase system reached maximal values at 30 min of incubation in both groups. H2O2 steady state concentration increased 2- and 10-fold, respectively. Catalase activity was not changed after slight oxidative stress (group 1) but decreased in severe oxidative stress (group 2). Decreases in the Na+/K(+)-ATPase activity (10 and 60% for groups 1 and 2) were found during the first hour of exposure coinciding with the peak in H2O2 steady state concentration. This early inactivation was followed by progressive increases in the activity up to 70% over the control value in group 1, and to the control value in group 2. [3H]Ouabain binding studies showed that the increase in Na+/K(+)-ATPase activity after oxidative stress was due to an increase in the number of phosphorylated pump molecules in the plasma membrane of alveolar type II cells.

Proapoptotic Noxa is required for particulate matter-induced cell death and lung inflammation.

Elevated ambient levels of particulate matter air pollution are associated with excess daily mortality, largely attributable to increased rates of cardiovascular events. We have previously reported that particulate matter induces p53‐dependent apoptosis in primary human alveolar epithelial cells. Activation of the intrinsic apoptotic pathway by p53 often requires the transcription of the proapoptotic Bcl‐2 proteins Noxa, Puma, or both. In this study, we exposed alveolar epithelial cells in culture and mice to fine particulate matter <2.5 μm in diameter (PM25) collected from the ambient air in Washington, D. C. Exposure to PM25 induced apoptosis in primary alveolar epithelial cells from wild‐type but not Noxa_/_ mice. Twenty‐four hours after the intratracheal instillation of PM25, wildtype mice showed increased apoptosis in the lung and increased levels of mRNA encoding Noxa but not Puma. These changes were associated with increased permeability of the alveolar‐capillary membrane and inflammation. All of these findings were absent or attenuated in Noxa_/_ animals. We conclude that PM2 5‐induced cell death requires Noxa both in vitro and in vivo and that Noxa‐dependent cell death might contribute to PM‐induced alveolar epithelial dysfunction and the resulting inflammatory response.—Urich, D.,Soberanes, S., Burgess, Z., Chiarella, S. E., Ghio, A. J., Ridge, K. M., Kamp, D. W., Chandel, N. S., Mutlu, G. M., Budinger, G. R. S. Proapoptotic Noxa is required for particulate matter‐induced cell death and lung inflammation. FASEB J. 23, 2055–2064 (2009)

Hyperbaric oxygenation upregulates rat lung Na,K-ATPase

Exposure of rats to hyperoxia is associated with increased active Na+ transport in rat lungs and increased Na,K-adenosine triphosphate (ATPase) expression in alveolar epithelial cells. Hyperbaric oxygenation (HBO) has been reported to act as an accelerated model of hyperoxic cell damage. Sublethal and intermittent exposure to HBO, however, has been suggested to upregulate endogenous protective mechanisms. In the present study, we tested whether short-term HBO, prior to inducing lung injury, would upregulate lung Na,K-ATPase. The results show that HBO, either intermittent or single 2.5 h exposure, increased lung Na,K-ATPase alpha-1 and beta-1 messenger ribonucleic acid (mRNA) transcript levels up to fourfold. Na,K-ATPase activity in lungs of rats exposed to HBO increased twofold during the first 2 h following removal from the hyperbaric chamber, and remained elevated for up to 6 h following HBO. Conceivably, the increase in Na,K-ATPase activity following HBO is due to an increase in activity from a basal to a higher rate, or possibly due to recruitment/translocation of Na,K-ATPases from inner membranes to the plasma membrane.