Ubiquitination participates in the lysosomal degradation of Na,K-ATPase in steady-state conditions

The alveolar epithelial cell (AEC) Na,K-ATPase contributes to vectorial Na(+) transport and plays an important role in keeping the lungs free of edema. We determined, by cell surface labeling with biotin and immunofluorescence, that approximately 30% of total Na,K-ATPase is at the plasma membrane of AEC in steady-state conditions. The half-life of the plasma membrane Na,K-ATPase was about 4 hours, and the incorporation of new Na,K-ATPase to the plasma membrane was Brefeldin A sensitive. Both protein kinase C (PKC) inhibition with bisindolylmaleimide (10 microM) and infection with an adenovirus expressing dominant-negative PKCzeta prevented Na,K-ATPase degradation. In cells expressing the Na,K-ATPase alpha1-subunit lacking the PKC phosphorylation sites, the plasma membrane Na,K-ATPase had a moderate increase in half-life. We also found that the Na,K-ATPase was ubiquitinated in steady-state conditions and that proteasomal inhibitors prevented its degradation. Interestingly, mutation of the four lysines described to be necessary for ubiquitination and endocytosis of the Na,K-ATPase in injurious conditions did not have an effect on its half-life in steady-state conditions. Lysosomal inhibitors prevented Na,K-ATPase degradation, and co-localization of Na,K-ATPase and lysosomes was found after labeling and chasing the plasma membrane Na,K-ATPase for 4 hours. Accordingly, we provide evidence suggesting that phosphorylation and ubiquitination are necessary for the steady-state degradation of the plasma membrane Na,K-ATPase in the lysosomes in alveolar epithelial cells.

Endothelin-1 impairs alveolar epithelial function via endothelial ETB receptor

RATIONALE

Endothelin-1 (ET-1) is increased in patients with high-altitude pulmonary edema and acute respiratory distress syndrome, and these patients have decreased alveolar fluid reabsorption (AFR).

OBJECTIVES

To determine whether ET-1 impairs AFR via activation of endothelial cells and nitric oxide (NO) generation.

METHODS

Isolated perfused rat lung, transgenic rats deficient in ETB receptors, coincubation of lung human microvascular endothelial cells (HMVEC-L) with rat alveolar epithelial type II cells or A549 cells, ouabain-sensitive 86Rb+ uptake.

MEASUREMENTS AND MAIN RESULTS

The ET-1-induced decrease in AFR was prevented by blocking the endothelin receptor ETB, but not ETA. Endothelial-epithelial cell interaction is required, as direct exposure of alveolar epithelial cells (AECs) to ET-1 did not affect Na,K-ATPase function or protein abundance at the plasma membrane, whereas coincubation of HMVEC-L and AECs with ET-1 decreased Na,K-ATPase activity and protein abundance at the plasma membrane. Exposing transgenic rats deficient in ETB receptors in the pulmonary vasculature (ET-B(-/-)) to ET-1 did not decrease AFR or Na,K-ATPase protein abundance at the plasma membrane of AECs. Exposing HMVEC-L to ET-1 led to increased NO, and the ET-1-induced down-regulation of Na,K-ATPase was prevented by the NO synthase inhibitor l-NAME, but not by a guanylate cyclase inhibitor.

CONCLUSIONS

We provide the first evidence that ET-1, via an endothelial-epithelial interaction, leads to decreased AFR by a mechanism involving activation of endothelial ETB receptors and NO generation leading to alveolar epithelial Na,K-ATPase down-regulation in a cGMP-independent manner.

Regulation of alveolar epithelial function by hypoxia

Patients with acute respiratory distress syndrome and high-altitude pulmonary oedema build up excess lung fluid, which leads to alveolar hypoxia. In patients with acute respiratory distress syndrome and hypoxia, there is a decrease in oedema fluid clearance, due in part to the downregulation of plasma membrane sodium-potassium adenosine triphosphatase (Na,K-ATPase). In alveolar epithelial cells, acute hypoxia promotes Na,K-ATPase endocytosis from the plasma membrane to intracellular compartments, resulting in inhibition of Na,K-ATPase activity. Exposure to prolonged hypoxia leads to degradation of plasma membrane Na,K-ATPase. The downregulation of plasma membrane Na,K-ATPase reduces adenosine triphosphate demand, as part of a survival mechanism of cellular adaptation to hypoxia. Hypoxia has also been shown to disassemble and degrade the keratin intermediate filament network, a fundamental component of the cell cytoskeleton, affecting epithelial barrier function. Accordingly, better understanding of the mechanisms regulating cellular adaptation to hypoxia may lead to the development of novel therapeutic strategies for acute respiratory distress syndrome and high-altitude pulmonary oedema patients.

Overexpression of the Na-K-ATPase alpha2-subunit improves lung liquid clearance during ventilation-induced lung injury

Mechanical ventilation with high tidal volumes (HV(T)) impairs lung liquid clearance (LLC) and downregulates alveolar epithelial Na-K-ATPase. We have previously reported that the Na-K-ATPase alpha(2)-subunit contributes to LLC in normal rat lungs. Here we tested whether overexpression of Na-K-ATPase alpha(2)-subunit in the alveolar epithelium would increase clearance in a HV(T) model of lung injury. We infected rat lungs with a replication-incompetent adenovirus that expresses Na-K-ATPase alpha(2)-subunit gene (Adalpha(2)) 7 days before HV(T) mechanical ventilation. HV(T) ventilation decreased LLC by approximately 50% in untreated, sham, and Adnull-infected rats. Overexpression of Na-K-ATPase alpha(2)-subunit prevented the decrease in clearance caused by HV(T) and was associated with significant increases in Na-K-ATPase alpha(2) protein abundance and activity in peripheral lung basolateral membrane fractions. Ouabain at 10(-5) M, a concentration that inhibits the alpha(2) but not the Na-K-ATPase alpha(1), decreased LLC in Adalpha(2)-infected rats to the same level as sham and Adnull-infected lungs, suggesting that the increased clearance in Adalpha(2) lungs was due to Na-K-ATPase alpha(2) expression and activity. In summary, we provide evidence that augmentation of the Na-K-ATPase alpha(2)-subunit, via gene transfer, may accelerate LLC in the injured lung.

Regulation of Na,K-ATPase during acute lung injury

A hallmark of acute lung injury is the accumulation of a protein rich edema which impairs gas exchange and leads to hypoxemia. The resolution of lung edema is effected by active sodium transport, mostly contributed by apical Na(+) channels and the basolateral located Na,K-ATPase. It has been reported that the decrease of Na,K-ATPase function seen during lung injury is due to its endocytosis from the cell plasma membrane into intracellular pools. In alveolar epithelial cells exposed to severe hypoxia, we have reported that increased production of mitochondrial reactive oxygen species leads to Na,K-ATPase endocytosis and degradation. We found that this regulated process follows what is referred as the Phosphorylation-Ubiquitination-Recognition-Endocytosis-Degradation (PURED) pathway. Cells exposed to hypoxia generate reactive oxygen species which activate PKC zeta which in turn phosphorylates the Na,K-ATPase at the Ser18 residue in the N-terminus of the alpha1-subunit leading the ubiquitination of any of the four lysines (K16, K17, K19, K20) adjacent to the Ser18 residue. This process promotes the alpha1-subunit recognition by the mu2 subunit of the adaptor protein-2 and its endocytosis trough a clathrin dependent mechanism. Finally, the ubiquitinated Na,K-ATPase undergoes degradation via a lysosome/proteasome dependent mechanism.

AMP-activated protein kinase regulates CO2-induced alveolar epithelial dysfunction in rats and human cells by promoting Na,K-ATPase endocytosis

Hypercapnia (elevated CO(2) levels) occurs as a consequence of poor alveolar ventilation and impairs alveolar fluid reabsorption (AFR) by promoting Na,K-ATPase endocytosis. We studied the mechanisms regulating CO(2)-induced Na,K-ATPase endocytosis in alveolar epithelial cells (AECs) and alveolar epithelial dysfunction in rats. Elevated CO(2) levels caused a rapid activation of AMP-activated protein kinase (AMPK) in AECs, a key regulator of metabolic homeostasis. Activation of AMPK was mediated by a CO(2)-triggered increase in intracellular Ca(2+) concentration and Ca(2+)/calmodulin-dependent kinase kinase-beta (CaMKK-beta). Chelating intracellular Ca(2+) or abrogating CaMKK-beta function by gene silencing or chemical inhibition prevented the CO(2)-induced AMPK activation in AECs. Activation of AMPK or overexpression of constitutively active AMPK was sufficient to activate PKC-zeta and promote Na,K-ATPase endocytosis. Inhibition or downregulation of AMPK via adenoviral delivery of dominant-negative AMPK-alpha(1) prevented CO(2)-induced Na,K-ATPase endocytosis. The hypercapnia effects were independent of intracellular ROS. Exposure of rats to hypercapnia for up to 7 days caused a sustained decrease in AFR. Pretreatment with a beta-adrenergic agonist, isoproterenol, or a cAMP analog ameliorated the hypercapnia-induced impairment of AFR. Accordingly, we provide evidence that elevated CO(2) levels are sensed by AECs and that AMPK mediates CO(2)-induced Na,K-ATPase endocytosis and alveolar epithelial dysfunction, which can be prevented with beta-adrenergic agonists and cAMP.

Hypoxia-mediated Na-K-ATPase degradation requires von Hippel Lindau protein

Hypoxia inhibits Na-K-ATPase activity and leads to its degradation in mammalian cells. Von Hippel Lindau protein (pVHL) and hypoxia inducible factor (HIF) are key mediators in cellular adaptation to hypoxia; thus, we set out to investigate whether pVHL and HIF participate in the hypoxia-mediated degradation of plasma membrane Na-K-ATPase. We found that in the presence of pVHL hypoxia decreased Na-K-ATPase activity and promoted the degradation of plasma membrane Na-K-ATPase. In pVHL-deficient cells, hypoxia did not decrease the Na-K-ATPase activity and the degradation of plasma membrane Na-K-ATPase was prevented. pVHL-mediated degradation of Na-K-ATPase required the functional pVHL E3 ligase and Ubc5 since pVHL mutants and dominant-negative Ubc5 prevented Na-K-ATPase from degradation. The generation of reactive oxygen species was necessary for pVHL-mediated Na-K-ATPase degradation during hypoxia. Desferrioxamine, which stabilizes HIF1/2alpha, did not affect the half-life of plasma membrane Na-K-ATPase. In addition, stabilizing HIF1/2alpha by infecting mammalian cells with adenoviruses containing the oxygen-dependent degradation domain of HIF1alpha did not affect the plasma membrane Na-K-ATPase degradation. In cells with suppression of pVHL by short hairpin RNA, the Na-K-ATPase was not degraded during hypoxia, whereas cells with knockdown of HIF1/2alpha retained the ability to degrade plasma membrane Na-K-ATPase. These findings suggest that pVHL participates in the hypoxia-mediated degradation of plasma membrane Na-K-ATPase in a HIF-independent manner.

High CO2 levels impair alveolar epithelial function independently of pH

Background

In patients with acute respiratory failure, gas exchange is impaired due to the accumulation of fluid in the lung airspaces. This life-threatening syndrome is treated with mechanical ventilation, which is adjusted to maintain gas exchange, but can be associated with the accumulation of carbon dioxide in the lung. Carbon dioxide (CO₂) is a by-product of cellular energy utilization and its elimination is affected via alveolar epithelial cells. Signaling pathways sensitive to changes in CO₂ levels were described in plants and neuronal mammalian cells. However, it has not been fully elucidated whether non-neuronal cells sense and respond to CO₂. The Na,K-ATPase consumes ∼40% of the cellular metabolism to maintain cell homeostasis. Our study examines the effects of increased pCO₂ on the epithelial Na,K-ATPase a major contributor to alveolar fluid reabsorption which is a marker of alveolar epithelial function.

Principal Findings

We found that short-term increases in pCO₂ impaired alveolar fluid reabsorption in rats. Also, we provide evidence that non-excitable, alveolar epithelial cells sense and respond to high levels of CO₂, independently of extracellular and intracellular pH, by inhibiting Na,K-ATPase function, via activation of PKCζ which phosphorylates the Na,K-ATPase, causing it to endocytose from the plasma membrane into intracellular pools.

Conclusions

Our data suggest that alveolar epithelial cells, through which CO₂ is eliminated in mammals, are highly sensitive to hypercapnia. Elevated CO₂ levels impair alveolar epithelial function, independently of pH, which is relevant in patients with lung diseases and altered alveolar gas exchange.

Ambient particulate matter accelerates coagulation via an IL-6-dependent pathway

The mechanisms by which exposure to particulate matter increases the risk of cardiovascular events are not known. Recent human and animal data suggest that particulate matter may induce alterations in hemostatic factors. In this study we determined the mechanisms by which particulate matter might accelerate thrombosis. We found that mice treated with a dose of well characterized particulate matter of less than 10 microM in diameter exhibited a shortened bleeding time, decreased prothrombin and partial thromboplastin times (decreased plasma clotting times), increased levels of fibrinogen, and increased activity of factor II, VIII, and X. This prothrombotic tendency was associated with increased generation of intravascular thrombin, an acceleration of arterial thrombosis, and an increase in bronchoalveolar fluid concentration of the prothrombotic cytokine IL-6. Knockout mice lacking IL-6 were protected against particulate matter-induced intravascular thrombin formation and the acceleration of arterial thrombosis. Depletion of macrophages by the intratracheal administration of liposomal clodronate attenuated particulate matter-induced IL-6 production and the resultant prothrombotic tendency. Our findings suggest that exposure to particulate matter triggers IL-6 production by alveolar macrophages, resulting in reduced clotting times, intravascular thrombin formation, and accelerated arterial thrombosis. These results provide a potential mechanism linking ambient particulate matter exposure and thrombotic events.

Carbonic anhydrase II and alveolar fluid reabsorption during hypercapnia

Carbonic anhydrase II (CAII) plays an important role in carbon dioxide metabolism and intracellular pH regulation. In this study, we provide evidence that CAII is expressed in both type I (AECI) and type II (AECII) alveolar epithelial cells by RT-PCR and Western blotting in freshly isolated rat cells. These results were further confirmed by double immunostaining with CAII antibodies and AECI- or AECII-specific markers in freshly isolated alveolar epithelial cells and rat lung tissues. Inhibition of CAII by acetazolamide or methazolamide delayed the decrease in the intracellular pH observed during hypercapnia in cultured AECI, AECII, and AECI-like cells. In an isolated-perfused rat lung model, alveolar fluid reabsorption significantly decreased during high CO(2) exposure, which was not prevented by carbonic anhydrase inhibition. Thus, we provide evidence that CAII is expressed in rat alveolar epithelial cells and does not regulate lung alveolar fluid reabsorption during hypercapnia.

Role of the small GTPase RhoA in the hypoxia-induced decrease of plasma membrane Na,K-ATPase in A549 cells

Hypoxia impairs alveolar fluid reabsorption by promoting Na,K-ATPase endocytosis, from the plasma membrane of alveolar epithelial cells. The present study was designed to determine whether hypoxia induces Na,K-ATPase endocytosis via reactive oxygen species (ROS)-mediated RhoA activation. In A549 cells, RhoA activation occurred within 15 minutes of cells exposure to hypoxia. This activation was inhibited in cells infected with adenovirus coding for gluthatione peroxidase (an H2O2 scavenger), in mitochondria depleted (rho0) cells or cells expressing decreased levels of the Rieske iron-sulfur protein (inhibitor of mitochondrial complex III), which suggests a role for mitochondrial ROS. Moreover, exogenous H2O2 treatment during normoxia mimicked the effects of hypoxia on RhoA, further supporting a role for ROS. Cells expressing dominant negative RhoA failed to endocytose the Na,K-ATPase during hypoxia or after H2O2 treatment. Na,K-ATPase endocytosis was also prevented in cells treated with Y-27632, a Rho-associated kinase (ROCK) inhibitor, and in cells expressing dominant negative ROCK. In summary, we provide evidence that in human alveolar epithelial cells exposed to hypoxia, RhoA/ROCK activation is necessary for Na,K-ATPase endocytosis via a mechanism that requires mitochondrial ROS.

Alveolar epithelium and Na,K-ATPase in acute lung injury

Active transport of sodium across the alveolar epithelium, undertaken in part by the Na,K-adenosine triphosphatase (Na,K-ATPase), is critical for clearance of pulmonary edema fluid and thus the outcome of patients with acute lung injury. Acute lung injury results in disruption of the alveolar epithelial barrier and leads to impaired clearance of edema fluid and altered Na,K-ATPase function. There has been significant progress in the understanding of mechanisms regulating alveolar edema clearance and signaling pathways modulating Na,K-ATPase function during lung injury. The accompanying review by Morty et al. focuses on intact organ and animal models as well as clinical studies assessing alveolar fluid reabsorption in alveolar epithelial injury. Elucidation of the mechanisms underlying regulation of active Na⁺ transport, as well as the pathways by which the Na,K-ATPase regulates epithelial barrier function and edema clearance, are of significance to identify interventional targets to improve outcomes of patients with acute lung injury.

Phosphorylation and ubiquitination are necessary for Na,K-ATPase endocytosis during hypoxia

As a cellular adaptative response, hypoxia decreases Na,K-ATPase activity by triggering the endocytosis of its alpha(1) subunit in alveolar epithelial cells. Here, we present evidence that the ubiquitin conjugating system is important in the Na,K-ATPase endocytosis during hypoxia and that ubiquitination of Na,K-ATPase alpha(1) subunit occurs at the basolateral membrane. Endocytosis and ubiquitination were prevented when the Ser 18 in the PKC phosphorylation motif of the Na,K-ATPase alpha(1) subunit was mutated to an alanine, suggesting that phosphorylation at Ser-18 is required for ubiquitination. Mutation of the four lysines surrounding Ser 18 to arginine prevented Na,K-ATPase ubiquitination and endocytosis during hypoxia; however, only one of them was sufficient to restore hypoxia-induced endocytosis. We provide evidence that ubiquitination plays an important role in cellular adaptation to hypoxia by regulating Na,K-ATPase alpha(1)-subunit endocytosis.

The alveolar-epithelial barrier: a target for potential therapy

During acute lung injury (ALI), the alveolar-capillary barrier is damaged, resulting in the accumulation of fluid and protein in the alveolar space characteristic of the acute respiratory distress syndrome (ARDS). Disordered epithelial repair may contribute to the development of fibrosis and worsen outcomes in patients who have lung injury. This article discusses novel emerging therapies based on these mechanisms that are designed to preserve the function and promote the repair of the alveolar epithelium in patients who have ALI/ARDS.

[Effect of changes in airway pressure and the inspiratory volume on the fluid filtration rate and pulmonary artery pressure in isolated rabbit lungs perfused with blood and acellular solution]

It has been reported that ventilation with large tidal volumes causes pulmonary edema in rats by the stimulation and release of proinflammatory mediators. Our objective was to determine the level at which volutrauma induced by changes in Airway Pressure (PAW) and Inspiratory Volume (VI) produce significant changes on the Fluid Filtration Rate (FFR) and Pulmonary Artery Pressure (PAP) in lungs perfused with blood (cellular groups) or with a buffer-albumin solution (acellular groups), with a Positive End Expiratory Pressure (PEEP) 0 or 2 cmH2O and to study the effect of a vasodilator with antiinflammatory properties (fenoterol) in blood-perfused groups. Three experimental groups were used: the cellular groups studied the effect of increased PAW and IV in isolated lungs perfused with blood and PEEP 0 and 2; the acellular groups studied the increased PAW and IV in isolated lungs perfused with a buffer-albumin solution and PEEP 0 and 2; The fenoterol group studied the effect of increased PAW and IV in isolated lungs perfused with blood + fenoterol and PEEP 2. The results show that an increase of FFR is produced earlier in acellular groups than in cellular ones and that the damage in cellular groups is microscopically and macroscopically inferior when compared to acellular groups. Fenoterol did not inhibit edema formation, and that PEEP 2, both in the cellular and the acellular groups, has a protective effect. We propose the possible existence of mediators with protective effects against the formation of pulmonary edema in the blood. These data suggest that volutrauma induced pulmonary edema has a predominantly traumatic origin when the lungs are perfused with blood.

Alveolar fluid reabsorption is increased in rats with compensated heart failure

Alveolar fluid reabsorption (AFR) is important in keeping the air spaces free of edema. This process is accomplished via active transport of Na+across the alveolo-capillary barrier mostly by apical Na+channels and basolateral Na+-K+-ATPases. Recently, we have reported that acute elevation of left atrial pressures is associated with decreased AFR in isolated rat lungs. However, the effect of chronic elevation of pulmonary capillary pressure, such as seen in patients with congestive heart failure (CHF), on AFR is unknown. CHF was induced by creating an aorto-caval fistula (ACF) in Sprague-Dawley male rats. Seven days after the placement of the fistula, AFR was studied in the isolated perfused rat lung model. AFR in control rats was 0.49 ± 0.02 ml/h (all values are means ± SE) and increased by ∼40% (0.69 ± 0.03 ml/h) in rats with chronic CHF ( P < 0.001). The albumin flux from the pulmonary circulation into the air spaces did not increase in the experimental groups, indicating that lung permeability for large solutes was not increased. Na+-K+-ATPase activity and protein abundance at the plasma membrane of distal alveolar epithelial tissue were significantly increased in CHF rats compared with controls. These changes were associated with increased plasma norepinephrine levels in CHF rats compared with controls. We provide evidence that in a rat model of chronic compensated CHF, AFR is increased, possibly due to increased endogenous norepinephrine upregulating active sodium transport and protecting against alveolar flooding.

Na,K‐ATPase α1‐subunit dephosphorylation by protein phosphatase 2A is necessary for its recruitment to the plasma membrane

In alveolar epithelial cells, G-protein coupled-receptors agonists (GPCR) induce the recruitment of the Na,K-ATPase to the plasma membrane. Here we report that for the recruitment of the Na,K-ATPase to occur, dephosphorylation of its alpha1-subunit at serine 18 is necessary, as demonstrated by in vitro phosphorylation, mutation of the serine 18 to alanine, and use of a specific phospho-antibody. Several approaches strongly suggest dephosphorylation to be mediated by protein phosphatase 2A (PP2A): 1) Na,K-ATPase dephosphorylation and recruitment were prevented by okadaic acid (OA); 2) the Na,K-ATPase alpha1-subunit is an in vitro substrate for PP2A; and 3) glutathione S-transferase (GST)-fusion proteins binding assays demonstrate a direct interaction between the catalytic subunit of PP2A and the first 90 amino acids of the Na,K-ATPase alpha1-subunit. Finally, GPCR agonists induced a rapid translocation of PP2A from the cytosol to the membrane fraction, which corresponded with increased coimmunoprecipitation and colocalization of PP2A and the Na,K-ATPase. Accordingly, we provide evidence that GPCR agonists promote PP2A translocation to the membrane fraction, leading to the dephosphorylation of the Na,K-ATPase alpha1-subunit at the serine 18 residue and its recruitment to the cell plasma membrane, which is of biological and physiological importance.

Integrating acute lung injury and regulation of alveolar fluid clearance

The acute respiratory distress syndrome (ARDS) is characterized by non-cardiogenic pulmonary edema and flooding of the alveolar air spaces with proteinaceous fluid. ARDS develops in response to inflammatory stresses including sepsis, trauma, and severe pneumonia, and despite aggressive critical care management, it still has a mortality of 30-50%. At the time of its original description in 1967, relatively little was known about the specific mechanisms by which the alveolar epithelium regulated lung fluid balance. Over the last 20 years, substantial advances in our understanding of the alveolar epithelium have provided major new insights into how molecular and cellular mechanisms regulate the active transport of solutes and fluid across the alveolar epithelium under both normal and pathological conditions. Beginning with the elucidation of active sodium transport as a major driving force for the transport of water from the air space to the interstitium, elegant work by multiple investigators has revealed a complex and integrated network of membrane channels and pumps that coordinately regulates sodium, chloride, and water flux in both a cell- and condition-specific manner. At the Experimental Biology Meeting in San Francisco on April 4, 2006, a symposium was held to discuss some of the most recent advances. Although there is still much to learn about the mechanisms that impair normal alveolar fluid clearance under pathological conditions, the compelling experimental findings presented in this symposium raise the prospect that we are now poised to test and develop therapeutic strategies to improve outcome in patients with acute lung injury.

Hypoxia-mediated degradation of Na,K-ATPase via mitochondrial reactive oxygen species and the ubiquitin-conjugating system

We set out to determine whether cellular hypoxia, via mitochondrial reactive oxygen species, promotes Na,K-ATPase degradation via the ubiquitin-conjugating system. Cells exposed to 1.5% O2 had a decrease in Na,K-ATPase activity and oxygen consumption. The total cell pool of alpha1 Na,K-ATPase protein decreased on exposure to 1.5% O2 for 30 hours, whereas the plasma membrane Na,K-ATPase was 50% degraded after 2 hours of hypoxia, which was prevented by lysosome and proteasome inhibitors. When Chinese hamster ovary cells that exhibit a temperature-sensitive defect in E1 ubiquitin conjugation enzyme were incubated at 40 degrees C and 1.5% O2, the degradation of the alpha1 Na,K-ATPase was prevented. Exogenous reactive oxygen species increased the plasma membrane Na,K-ATPase degradation, whereas, in mitochondrial DNA deficient rho(0) cells and in cells transfected with small interfering RNA against Rieske iron sulfur protein, the hypoxia-mediated Na,K-ATPase degradation was prevented. The catalase/superoxide dismutase (SOD) mimetic (EUK-134) and glutathione peroxidase overexpression prevented the hypoxia-mediated Na,K-ATPase degradation and overexpression of SOD1, but not SOD2, partially inhibited the Na+ pump degradation. Accordingly, we provide evidence that during hypoxia, mitochondrial reactive oxygen species are necessary to degrade the plasma membrane Na,K-ATPase via the ubiquitin-conjugating system.

Phosphorylation of adaptor protein-2 mu2 is essential for Na+,K+-ATPase endocytosis in response to either G protein-coupled receptor or reactive oxygen species

Activation of G protein-coupled receptor by dopamine and hypoxia-generated reactive oxygen species promote Na+,K+-ATPase endocytosis. This effect is clathrin dependent and involves the activation of protein kinase C (PKC)-zeta and phosphorylation of the Na+,K+-ATPase alpha-subunit. Because the incorporation of cargo into clathrin vesicles requires association with adaptor proteins, we studied whether phosphorylation of adaptor protein (AP)-2 plays a role in its binding to the Na+,K+-ATPase alpha-subunit and thereby in its endocytosis. Dopamine induces a time-dependent phosphorylation of the AP-2 mu2 subunit. Using specific inhibitors and dominant-negative mutants, we establish that this effect was mediated by activation of the adaptor associated kinase 1/PKC-zeta isoform. Expression of the AP-2 mu2 bearing a mutation in its phosphorylation site (T156A) prevented Na+,K+-ATPase endocytosis and changes in activity induced by dopamine. Similarly, in lung alveolar epithelial cells, hypoxia-induced endocytosis of Na+,K+-ATPase requires the binding of AP-2 to the tyrosine-based motif (Tyr-537) located in the Na+,K+-ATPase alpha-subunit, and this effect requires phosphorylation of the AP-2 mu2 subunit. We conclude that phosphorylation of AP-2 mu2 subunit is essential for Na+,K+-ATPase endocytosis in response to a variety of signals, such as dopamine or reactive oxygen species.

Airborne particulate matter inhibits alveolar fluid reabsorption in mice via oxidant generation

Ambient particulate matter is increasingly recognized as a significant contributor to human cardiopulmonary morbidity and mortality in the United States and worldwide. We sought to determine whether exposure to ambient particulate matter would alter alveolar fluid clearance in mice. Mice were exposed to a range of doses of a well-characterized particulate matter collected from the ambient air in Düsseldorf, Germany through a single intratracheal instillation, and alveolar fluid clearance and measurements of lung injury were made. Exposure to even very low doses of particulate matter (10 microg) resulted in a significant reduction in alveolar fluid clearance that was maximal 24 h after the exposure, with complete resolution after 7 d. This was paralleled by a decrease in lung Na,K-ATPase activity. To investigate the mechanism of this effect, we measured plasma membrane Na,K-ATPase abundance in A549 cells and Na,K-ATPase activity in primary rat alveolar type II cells after exposure to particulate matter in the presence or absence of the combined superoxide dismutase and catalase mimetic EUK-134 (5 microM). Membrane but not total protein abundance of the Na,K-ATPase was decreased after exposure to particulate matter, as was Na,K-ATPase activity. This decrease was prevented by the combined superoxide dismutase/catalase mimetic EUK-134. The intratracheal instillation of particulate matter results in alveolar epithelial injury and decreased alveolar fluid clearance, conceivably due to downregulation of the Na,K-ATPase.