Protein kinase A-Iα regulates Na,K-ATPase endocytosis in alveolar epithelial cells exposed to high CO(2) concentrations.

Elevated concentrations of CO2 (hypercapnia) lead to alveolar epithelial dysfunction by promoting Na,K-ATPase endocytosis. In the present report, we investigated whether the CO2/HCO3(-) activated soluble adenylyl cyclase (sAC) regulates this process. We found that hypercapnia increased the production of cyclic adenosine monophosphate (cAMP) and stimulated protein kinase A (PKA) activity via sAC, which was necessary for Na,K-ATPase endocytosis. During hypercapnia, cAMP was mainly produced in specific microdomains in the proximity of the plasma membrane, leading to PKA Type Iα activation. In alveolar epithelial cells exposed to high CO2 concentrations, PKA Type Iα regulated the time-dependent phosphorylation of the actin cytoskeleton component α-adducin at serine 726. Cells expressing small hairpin RNA for PKAc, dominant-negative PKA Type Iα, small interfering RNA for α-adducin, and α-adducin with serine 726 mutated to alanine prevented Na,K-ATPase endocytosis. In conclusion, we provide evidence for a new mechanism by which hypercapnia via sAC, cAMP, PKA Type Iα, and α-adducin regulates Na,K-ATPase endocytosis in alveolar epithelial cells.

Hypoxia leads to Na,K-ATPase downregulation via Ca(2+) release-activated Ca(2+) channels and AMPK activation.

To maintain cellular ATP levels, hypoxia leads to Na,K-ATPase inhibition in a process dependent on reactive oxygen species (ROS) and the activation of AMP-activated kinase α1 (AMPK-α1). We report here that during hypoxia AMPK activation does not require the liver kinase B1 (LKB1) but requires the release of Ca(2+) from the endoplasmic reticulum (ER) and redistribution of STIM1 to ER-plasma membrane junctions, leading to calcium entry via Ca(2+) release-activated Ca(2+) (CRAC) channels. This increase in intracellular Ca(2+) induces Ca(2+)/calmodulin-dependent kinase kinase ß (CaMKKß)-mediated AMPK activation and Na,K-ATPase downregulation. Also, in cells unable to generate mitochondrial ROS, hypoxia failed to increase intracellular Ca(2+) concentration while a STIM1 mutant rescued the AMPK activation, suggesting that ROS act upstream of Ca(2+) signaling. Furthermore, inhibition of CRAC channel function in rat lungs prevented the impairment of alveolar fluid reabsorption caused by hypoxia. These data suggest that during hypoxia, calcium entry via CRAC channels leads to AMPK activation, Na,K-ATPase downregulation, and alveolar epithelial dysfunction.

Extracellular signal-regulated kinase (ERK) participates in the hypercapnia-induced Na,K-ATPase downregulation.

Hypercapnia has been shown to impair alveolar fluid reabsorption (AFR) by decreasing Na,K-ATPase activity. Extracellular signal-regulated kinase pathway (ERK) is activated under conditions of cellular stress and has been known to regulate the Na,K-ATPase. Here, we show that hypercapnia leads to ERK activation in a time-dependent manner in alveolar epithelial cells (AEC). Inhibition of ERK by U0126 or siRNA prevented both the hypercapnia-induced Na,K-ATPase endocytosis and impairment of AFR. Moreover, ERK inhibition prevented AMPK activation, a known modulator of hypercapnia-induced Na,K-ATPase endocytosis. Accordingly, these data suggest that hypercapnia-induced Na,K-ATPase endocytosis is dependent on ERK activation in AEC and that ERK plays an important role in hypercapnia-induced impairment of AFR in rat lungs.

Insulin regulates alveolar epithelial function by inducing Na+/K+-ATPase translocation to the plasma membrane in a process mediated by the action of Akt.

Stimulation of Na(+)/K(+)-ATPase translocation to the cell surface increases active Na(+) transport, which is the driving force of alveolar fluid reabsorption, a process necessary to keep the lungs free of edema and to allow normal gas exchange. Here, we provide evidence that insulin increases alveolar fluid reabsorption and Na(+)/K(+)-ATPase activity by increasing its translocation to the plasma membrane in alveolar epithelial cells. Insulin-induced Akt activation is necessary and sufficient to promote Na(+)/K(+)-ATPase translocation to the plasma membrane. Phosphorylation of AS160 by Akt is also required in this process, whereas inactivation of the Rab GTPase-activating protein domain of AS160 promotes partial Na(+)/K(+)-ATPase translocation in the absence of insulin. We found that Rab10 functions as a downstream target of AS160 in insulin-induced Na(+)/K(+)-ATPase translocation. Collectively, these results suggest that Akt plays a major role in Na(+)/K(+)-ATPase intracellular translocation and thus in alveolar fluid reabsorption.

Role of kinesin light chain-2 of kinesin-1 in the traffic of Na,K-ATPase-containing vesicles in alveolar epithelial cells.

Recruitment of the Na,K-ATPase to the plasma membrane of alveolar epithelial cells results in increased active Na(+) transport and fluid clearance in a process that requires an intact microtubule network. However, the microtubule motors involved in this process have not been identified. In the present report, we studied the role of kinesin-1, a plus-end microtubule molecular motor that has been implicated in the movement of organelles in the Na,K-ATPase traffic. We determined by confocal microscopy and biochemical assays that kinesin-1 and the Na,K-ATPase are present in the same membranous cellular compartment. Knockdown of kinesin-1 heavy chain (KHC) or the light chain-2 (KLC2), but not of the light chain-1 (KLC1), decreased the movement of Na,K-ATPase-containing vesicles when compared to sham siRNA-transfected cells (control group). Thus, a specific isoform of kinesin-1 is required for microtubule-dependent recruitment of Na,K-ATPase to the plasma membrane, which is of physiological significance.

Myosin-Va restrains the trafficking of Na+/K+-ATPase-containing vesicles in alveolar epithelial cells.

Stimulation of Na(+)/K(+)-ATPase activity in alveolar epithelial cells by cAMP involves its recruitment from intracellular compartments to the plasma membrane. Here, we studied the role of the actin molecular motor myosin-V in this process. We provide evidence that, in alveolar epithelial cells, cAMP promotes Na(+)/K(+)-ATPase recruitment to the plasma membrane by increasing the average speed of Na(+)/K(+)-ATPase-containing vesicles moving to the cell periphery. We found that three isoforms of myosin-V are expressed in alveolar epithelial cells; however, only myosin-Va and Vc colocalized with the Na(+)/K(+)-ATPase in intracellular membrane fractions. Overexpression of dominant-negative myosin-Va or knockdown with specific shRNA increased the average speed and distance traveled by the Na(+)/K(+)-ATPase-containing vesicles, as well as the Na(+)/K(+)-ATPase activity and protein abundance at the plasma membrane to similar levels as those observed with cAMP stimulation. These data show that myosin-Va has a role in restraining Na(+)/K(+)-ATPase-containing vesicles within intracellular pools and that this restrain is released after stimulation by cAMP allowing the recruitment of the Na(+)/K(+)-ATPase to the plasma membrane and thus increased activity.

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.

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.

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 approximately 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 PKCzeta 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.

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.

Beneficial effects of hydrocortisone and papaverine on a model of pulmonary embolism induced by autologous blood clots in isolated and perfused rabbit lungs.

BACKGROUND AND OBJECTIVE: Mechanical obstruction has been considered the prime determinant of haemodynamic changes after pulmonary embolism (PE); however, the function of vasoconstrictive and inflammatory mediators in the physiopathology of this disease is unclear. The aim of this investigation was to study the effect of an anti-inflammatory and a vasodilator in a setting of induced PE. METHODS: A prospective, laboratory study was undertaken using 30 New Zealand white rabbits. A model of isolated and perfused rabbit lungs was used; PE was induced using autologous blood clots. Six study groups were established (each n = 5): PE without any drug (PG); PE + papaverine (PpG); PE + hydrocortisone (HG); PE in West's Zone III (ZIIIG); PE using acellular perfusate (AG) and PE using acellular perfusate + papaverine (APpG). The pulmonary artery pressure (PAP) and fluid filtration rate (FFR) were continuously measured during the experiments. RESULTS: Increases in PAP and oedema formation were observed in the PG after embolization. The PpG and the APpG showed neither oedema nor significant PAP increases. The HG group developed less oedema and less increase in PAP compared with the PG. The ZIIIG developed oedema the fastest. The AG developed less oedema and increases in PAP compared with the PG. CONCLUSION: These findings suggest that vasoconstriction and inflammatory mediators play an important role in the physiopathology of PE, as neither PAP increases nor oedema were observed in the PpG and a reduction of oedema and PAP was seen in the HG group. The decrease in oedema and PAP in the acellular group strongly suggests a key role of circulating blood cells.