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

Effects of Hypoxia on Respiratory Diseases: Perspective View of Epithelial Ion Transport

The balance of gas exchange and lung ventilation is essential for the maintenance of body homeostasis. There are many ion channels and transporters in respiratory epithelial cells, including epithelial sodium channel, Na,K-ATPase, cystic fibrosis transmembrane conductance regulator, and some transporters. These ion channels/transporters maintain the capacity of liquid layer on the surface of respiratory epithelial cells, and provide an immune barrier for the respiratory system to clear off foreign pathogens. However, in some harmful external environment and/or pathological conditions, the respiratory epithelium is prone to hypoxia, which would destroy the ion transport function of the epithelium and unbalance the homeostasis of internal environment, triggering a series of pathological reactions. Many respiratory diseases associated with hypoxia manifest an increased expression of hypoxia-inducible factor-1, which mediates the integrity of the epithelial barrier and affects epithelial ion transport function. It is important to study the relationship between hypoxia and ion transport function, whereas the mechanism of hypoxia-induced ion transport dysfunction in respiratory diseases is not clear. This review focuses on the relationship of hypoxia and respiratory diseases, as well as dysfunction of ion transport and tight junctions in respiratory epithelial cells under hypoxia.

Cell maturation influences the ability of hESC-RPE to tolerate cellular stress

Background

Transplantation of human pluripotent stem cell-derived retinal pigment epithelium (RPE) is an urgently needed treatment for the cure of degenerative diseases of the retina. The transplanted cells must tolerate cellular stress caused by various sources such as retinal inflammation and regain their functions rapidly after the transplantation. We have previously shown the maturation level of the cultured human embryonic stem cell-derived RPE (hESC-RPE) cells to influence for example their calcium (Ca²⁺) signaling properties. Yet, no comparison of the ability of hESC-RPE at different maturity levels to tolerate cellular stress has been reported.

Methods

Here, we analyzed the ability of the hESC-RPE populations with early (3 weeks) and late (12 weeks) maturation status to tolerate cellular stress caused by chemical cell stressors protease inhibitor (MG132) or hydrogen peroxide (H₂O₂). After the treatments, the functionality of the RPE cells was studied by transepithelial resistance, immunostainings of key RPE proteins, phagocytosis, mitochondrial membrane potential, Ca²⁺ signaling, and cytokine secretion.

Results

The hESC-RPE population with late maturation status consistently showed improved tolerance to cellular stress in comparison to the population with early maturity. After the treatments, the early maturation status of hESC-RPE monolayer showed impaired barrier properties. The hESC-RPE with early maturity status also exhibited reduced phagocytic and Ca²⁺ signaling properties, especially after MG132 treatment.

Conclusions

Our results suggest that due to better tolerance to cellular stress, the late maturation status of hESC-RPE population is superior compared to monolayers with early maturation status in the transplantation therapy settings.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-02712-7.

Downregulation of RAB7 and Caveolin-1 increases MMP-2 activity in renal tubular epithelial cells under hypoxic conditions

Tubulointerstitial fibrosis leads to tubular basement membrane thickening and accumulation of interstitial extracellular matrix (ECM). Matrix metallopeptidase-2 (MMP-2) is involved in the breakdown of ECM. Chronic hypoxia often occurs in the kidney tissues of patients with chronic kidney disease. Our previous study reported the effect of autophagy and endocytosis on MMP-2 activity in hypoxia-treated HK-2 cells. In this study, the relationship between the Ras-related protein Rab-7a (RAB7) and MMP-2 activity was further investigated. RAB7 overexpression decreased MMP-2 activity. In contrast, the results for RAB7 knockdown displayed the opposite pattern. Short hairpin RNA technology was used to knockdown Caveolin-1 (Cav-1) or Beclin-1 (Bec-1) in HK-2 cells. The two genes displayed differential effects on MMP-2 activity. Cav-1 and RAB7 interference increased MMP-2 activity. This study suggested that autophagy and endocytosis, RAB7, Cav-1, and Bec-1 may serve as potential mediators for altered MMP-2 activity.

Regulatory effect of insulin on the structure, function and metabolism of Na+/K+-ATPase (Review)

Na⁺/K⁺-ATPase is an ancient enzyme, the role of which is to maintain Na⁺ and K⁺ gradients across cell membranes, thus preserving intracellular ion homeostasis. The regulation of Na⁺/K⁺-ATPase is affected by several regulatory factors through a number of pathways, with hormones serving important short-term and long-term regulatory functions. Na⁺/K⁺-ATPase can also be degraded through activation of the ubiquitin proteasome and autophagy-lysosomal pathways, thereby affecting its abundance and enzymatic activity. As regards the regulatory effect of insulin, it has been found to upregulate the relative abundance of Na⁺/K⁺-ATPase and restore the transport efficiency in multiple in vitro and in vivo experiments. Therefore, elucidating the role of insulin in the regulation Na⁺/K⁺-ATPase may help uncover new drug targets for the treatment of related diseases. The aim of the present study was to review the structure and function of Na⁺/K⁺-ATPase and to discuss the possible mechanisms through which it may be regulated by insulin, in order to investigate the possibility of designing new therapies for related diseases.

20-HETE downregulates Na/K-ATPase α1 expression via the ubiquitination pathway

In this article, we found that 20-Hydroxyeicosatetraenoic acid (20-HETE) reduced Na/K-ATPase α1 expression via the ubiquitin-proteasome pathway. The ubiquitination level of Na/K-ATPase α1 protein was increased in 20-HETE-treated mouse cortical collecting duct cells and the kidney tissues of CYP4F2 transgenic mice. We also demonstrated that 20-HETE-induced high level phosphorylation of Na/K-ATPase α1 was necessary for its ubiquitination.The protein kinase C inhibitor sotrastaurin significantly reduced the phosphorylation of Na/K-ATPase α1 and increased the expression of Na/K-ATPase α1 although 20-HETE stimulus being applied at the same time. Moreover, high level of 20-HETE increased the expression and neddylation of Cullin3,which is an important ubiquitin E3 ligase in kidney. MLN4924, an inhibitor of NEDD8-activating enzyme, inhibited neddylation of Cullin3 and reversed the reduction of Na/K-ATPase α1 expression caused by 20-HETE. Thus, 20-HETE downregulates Na/K-ATPase α1 via the ubiquitination pathway, and phosphorylation of Na/K-ATPase α1 is a prerequisite to ubiquitination. Additionally, 20-HETE regulates Cullin3 expression via neddylation.

Crosstalk between phosphorylation and ubiquitination is involved in high salt-induced WNK4 expression

With no lysine 4 (WNK4) is a serine/threonine kinase, which is expressed in the kidney and associated with salt-sensitive hypertension. However, how salt regulates WNK4 remains unclear. In the present study, the C57BL/6 mice and HEK293 cells were treated with high salt and the expression of WNK4 protein and its ubiquitination and phosphorylation levels were detected. Western blotting demonstrated that WNK4 expression was significantly increased in high salt-treated mice and cells. Meanwhile, co-immunoprecipitation analysis demonstrated that the ubiquitination of WNK4 was decreased under high-salt simulation. It was also identified that the Lys-1023 site was the most important ubiquitination site for WNK4, and it was found that phosphorylation at the Ser-1022 site was a prerequisite for ubiquitination. These results suggested that there was crosstalk between phosphorylation and ubiquitination in the WNK4 protein, and high salt may downregulate its phosphorylation and, in turn, decrease its ubiquitination, leading to a decrease in WNK4 degradation. This eventually resulted in an increase in the abundance of WNK4 protein.

Helicobacter pylori infection impairs chaperone-assisted maturation of Na-K-ATPase in gastric epithelium

Helicobacter pylori infection always induces gastritis, which may progress to ulcer disease or cancer. The mechanisms underlying mucosal injury by the bacteria are incompletely understood. Here, we identify a novel pathway for H. pylori-induced gastric injury, the impairment of maturation of the essential transport enzyme and cell adhesion molecule, Na-K-ATPase. Na-K-ATPase comprises α- and β-subunits that assemble in the endoplasmic reticulum (ER) before trafficking to the plasma membrane. Attachment of H. pylori to gastric epithelial cells increased Na-K-ATPase ubiquitylation, decreased its surface and total levels, and impaired ion balance. H. pylori did not alter degradation of plasmalemma-resident Na-K-ATPase subunits or their mRNA levels. Infection decreased association of α- and β-subunits with ER chaperone BiP and impaired assembly of α/β-heterodimers, as was revealed by quantitative mass spectrometry and immunoblotting of immunoprecipitated complexes. The total level of BiP was not altered, and the decrease in interaction with BiP was not observed for other BiP client proteins. The H. pylori-induced decrease in Na-K-ATPase was prevented by BiP overexpression, stopping protein synthesis, or inhibiting proteasomal, but not lysosomal, protein degradation. The results indicate that H. pylori impairs chaperone-assisted maturation of newly made Na-K-ATPase subunits in the ER independently of a generalized ER stress and induces their ubiquitylation and proteasomal degradation. The decrease in Na-K-ATPase levels is also seen in vivo in the stomachs of gerbils and chronically infected children. Further understanding of H. pylori-induced Na-K-ATPase degradation will provide insights for protection against advanced disease.NEW & NOTEWORTHY This work provides evidence that Helicobacter pylori decreases levels of Na-K-ATPase, a vital transport enzyme, in gastric epithelia, both in acutely infected cultured cells and in chronically infected patients and animals. The bacteria interfere with BiP-assisted folding of newly-made Na-K-ATPase subunits in the endoplasmic reticulum, accelerating their ubiquitylation and proteasomal degradation and decreasing efficiency of the assembly of native enzyme. Decreased Na-K-ATPase expression contributes to H. pylori-induced gastric injury.

Comparison of the adhesion and endocytosis of calcium oxalate dihydrate to HK-2 cells before and after repair by Astragalus polysaccharide

This work investigated the effects of repairing injured renal proximal tubular epithelial (HK-2) cells by using three Astragalus polysaccharides (APS) with different molecular weights and the adhesion and endocytosis of HK-2 cells to the calcium oxalate dihydrate (COD) nanocrystals before and after repair to develop new products that can protect against kidney stones. HK-2 cells cultured in vitro were injured with 2.6 mmol/L oxalic acid to establish a damaged cell model. Three kinds of APS (APS0, APS1, and APS2 with molecular weights of 11.03, 4.72, and 2.60 kDa, respectively) were used to repair the damaged cells. The changes in the adhesion and endocytosis of 100 nm COD crystals to cells before and after the repair were detected. After the repair of HK-2 cells by the APS, the speed of wound healing of the damaged HK-2 cells increased, and the amount of phosphatidylserine (PS) ectropion decreased. In addition, the proportion of cells with adhered COD crystals decreased, whereas the proportion of cells with internalized crystals increased. As a result of the repair activity, APS can inhibit the adhesion and promote the endocytosis of COD nanocrystals to damaged cells. APS1, which had a moderate molecular weight, displayed the strongest abilities to repair the cells, inhibit adhesion, and promote endocytosis. Thus, APS, particularly APS1, may serve as potential green drugs for preventing kidney stones.

DR region of Na+-K+-ATPase is a new target to protect heart against oxidative injury

Previous studies have shown that the activity and expression of Na⁺/K⁺-ATPase (NKA) are down-regulated in the failing hearts, and that an antibody against the DR-region of NKA (DR-Ab) can stimulate its activity. The present study was designed to investigate the beneficial effects of this antibody against cardiac injury and the underlying mechanisms. We found that DR-Ab improved cardiac function, alleviated cardiac hypertrophy and reduced oxidative stress in isoproterenol-treated mice. In AC16 human cardiomyocytes, DR-Ab increased cell viability and attenuated apoptosis under oxidative stress. Corresponding to the observation of reduced NKA activity, NKA abundance on plasma membrane was lowered during oxidative stress. Suppressed activity of protein phosphatase 2 A (PP2A) was responsible for the loss of membrane NKA due to the increased phosphorylation of key serine residues that trigger endocytosis. Incubation with DR-Ab restored PP2A activity and stabilized NKA expression on the plasma membrane. Inhibitors of PP2A abolished the protective effect of DR-Ab against oxidative stress. In summary, our data indicate that loss of membrane NKA may contribute to cardiac pathologies in heart failure. DR-Ab, by stabilizing membrane NKA, protects cardiomyocytes against oxidative injury and improves cardiac function in the failing hearts, suggesting a novel approach to treat heart failure.

Ubiquitin-proteasome signaling in lung injury

Cell homeostasis requires precise coordination of cellular proteins function. Ubiquitination is a post-translational modification that modulates protein half-life and function and is tightly regulated by ubiquitin E3 ligases and deubiquitinating enzymes. Lung injury can progress to acute respiratory distress syndrome that is characterized by an inflammatory response and disruption of the alveolocapillary barrier resulting in alveolar edema accumulation and hypoxemia. Ubiquitination plays an important role in the pathobiology of acute lung injury as it regulates the proteins modulating the alveolocapillary barrier and the inflammatory response. Better understanding of the signaling pathways regulated by ubiquitination may lead to novel therapeutic approaches by targeting specific elements of the ubiquitination pathways.

Hypoxia Activates Src and Promotes Endocytosis Which Decreases MMP-2 Activity and Aggravates Renal Interstitial Fibrosis

The aggravation of renal interstitial fibrosis in the advanced-stage of chronic kidney disease is related to decreased matrix metalloproteinase-2 (MMP-2) activity, which is induced by hypoxia in the kidney; however, the specific mechanism remains unclear. We previously demonstrated that inhibition of Caveolin-1, a key gene involved in endocytosis, increased MMP-2 activity in hypoxic HK-2 cells. It has been reported that activated Src (phospho-Src Tyr416) is a key molecule in multiple fibrotic pathways. However, whether Src functions on the regulation of Caveolin-1 and MMP-2 activity in hypoxic HK-2 cells remains poorly understood. To explore the underlying mechanism, a rat model of renal interstitial fibrosis was established, then we observed obvious hypoxia in fibrotic kidney tissue and the protein levels of phospho-Src and Caveolin-1 increased, while MMP-2 activity decreased. Next, we treated HK-2 cells with the phospho-Src inhibitor PP1. Compared with normal cells grown in hypoxia, in cells treated with PP1, the protein levels of phospho-Src and Caveolin-1 decreased, as did the protein levels of the MMP-2-activity-regulated molecules RECK (reversion-inducing-cysteine-rich protein with kazal motifs) and TIMP-2 (tissue inhibitor of metalloproteinase-2), while the protein level of MT1-MMP (membrane type 1-matrix metalloproteinase) increased and MMP-2 activity was enhanced. Therefore, hypoxia promotes the phosphorylation of Src and phospho-Src can enhance the endocytosis of HK-2 cells, which leads to decreased MMP-2 activity and aggravates renal interstitial fibrosis.

HIF and HOIL-1L-mediated PKCζ degradation stabilizes plasma membrane Na,K-ATPase to protect against hypoxia-induced lung injury

Organisms have evolved adaptive mechanisms in response to stress for cellular survival. During acute hypoxic stress, cells down-regulate energy-consuming enzymes such as Na,K-ATPase. Within minutes of alveolar epithelial cell (AEC) exposure to hypoxia, protein kinase C zeta (PKCζ) phosphorylates the α₁-Na,K-ATPase subunit and triggers it for endocytosis, independently of the hypoxia-inducible factor (HIF). However, the Na,K-ATPase activity is essential for cell homeostasis. HIF induces the heme-oxidized IRP2 ubiquitin ligase 1L (HOIL-1L), which leads to PKCζ degradation. Here we report a mechanism of prosurvival adaptation of AECs to prolonged hypoxia where PKCζ degradation allows plasma membrane Na,K-ATPase stabilization at ∼50% of normoxic levels, preventing its excessive down-regulation and cell death. Mice lacking HOIL-1L in lung epithelial cells (Cre^SPC/HOIL-1L^fl/fl ) were sensitized to hypoxia because they express higher levels of PKCζ and, consequently, lower plasma membrane Na,K-ATPase levels, which increased cell death and worsened lung injury. In AECs, expression of an α₁-Na,K-ATPase construct bearing an S18A (α₁-S18A) mutation, which precludes PKCζ phosphorylation, stabilized the Na,K-ATPase at the plasma membrane and prevented hypoxia-induced cell death even in the absence of HOIL-1L. Adenoviral overexpression of the α₁-S18A mutant Na,K-ATPase in vivo rescued the enhanced sensitivity of Cre^SPC/HOIL-1L^fl/fl mice to hypoxic lung injury. These data suggest that stabilization of Na,K-ATPase during severe hypoxia is a HIF-dependent process involving PKCζ degradation. Accordingly, we provide evidence of an important adaptive mechanism to severe hypoxia, whereby halting the exaggerated down-regulation of plasma membrane Na,K-ATPase prevents cell death and lung injury.

Cysteine residues 244 and 458-459 within the catalytic subunit of Na,K-ATPase control the enzyme's hydrolytic and signaling function under hypoxic conditions

Our previous findings suggested that reversible thiol modifications of cysteine residues within the actuator (AD) and nucleotide binding domain (NBD) of the Na,K-ATPase may represent a powerful regulatory mechanism conveying redox- and oxygen-sensitivity of this multifunctional enzyme. S-glutathionylation of Cys244 in the AD and Cys 454-458-459 in the NBD inhibited the enzyme and protected cysteines' thiol groups from irreversible oxidation under hypoxic conditions. In this study mutagenesis approach was used to assess the role these cysteines play in regulation of the Na,K-ATPase hydrolytic and signaling functions. Several constructs of mouse α1 subunit of the Na,K-ATPase were produced in which Cys244, Cys 454-458-459 or Cys 244-454-458-459 were replaced by alanine. These constructs were expressed in human HEK293 cells. Non-transfected cells and those expressing murine α1 subunit were exposed to hypoxia or treated with oxidized glutathione (GSSG). Both conditions induced inhibition of the wild type Na,K-ATPase. Enzymes containing mutated mouse α1 lacking Cys244 or all four cysteines (Cys 244-454-458-459) were insensitive to hypoxia. Inhibitory effect of GSSG was observed for wild type murine Na,K-ATPase, but was less pronounced in Cys454-458-459Ala mutant and completely absent in the Cys244Ala and Cys 244-454-458-459Ala mutants. In cells, expressing wild type enzyme, ouabain induced activation of Src and Erk kinases under normoxic conditions, whereas under hypoxic conditions this effect was inversed. Cys454-458-459Ala substitution abolished Src kinase activation in response to ouabain treatment, uncoupled Src from Erk signaling, and interfered with O2-sensitivity of Na,K-ATPase signaling function. Moreover, modeling predicted that S-glutathionylation of Cys 458 and 459 should prevent inhibitory binding of Src to NBD. Our data indicate for the first time that cysteine residues within the AD and NBD influence hydrolytic as well as receptor function of the Na,K-ATPase and alter responses of the enzyme to hypoxia or upon treatment with cardiotonic steroids.

Gas Exchange Disturbances Regulate Alveolar Fluid Clearance during Acute Lung Injury

Disruption of the alveolar-capillary barrier and accumulation of pulmonary edema, if not resolved, result in poor alveolar gas exchange leading to hypoxia and hypercapnia, which are hallmarks of acute lung injury and the acute respiratory distress syndrome (ARDS). Alveolar fluid clearance (AFC) is a major function of the alveolar epithelium and is mediated by the concerted action of apically-located Na⁺ channels [epithelial Na⁺ channel (ENaC)] and the basolateral Na,K-ATPase driving vectorial Na⁺ transport. Importantly, those patients with ARDS who cannot clear alveolar edema efficiently have worse outcomes. While hypoxia can be improved in most cases by O₂ supplementation and mechanical ventilation, the use of lung protective ventilation settings can lead to further CO₂ retention. Whether the increase in CO₂ concentrations has deleterious or beneficial effects have been a topic of significant controversy. Of note, both low O₂ and elevated CO₂ levels are sensed by the alveolar epithelium and by distinct and specific molecular mechanisms impair the function of the Na,K-ATPase and ENaC thereby inhibiting AFC and leading to persistence of alveolar edema. This review discusses recent discoveries on the sensing and signaling events initiated by hypoxia and hypercapnia and the relevance of these results in identification of potential novel therapeutic targets in the treatment of ARDS.

MMP-2 and 9 in Chronic Kidney Disease

Gelatinases are members of the matrix metalloproteinase (MMPs) family; they play an important role in the degradation of the extracellular matrix (ECM). This effect is also crucial in the development and progression of chronic kidney disease (CKD). Its expression, as well as its activity regulation are closely related to the cell signaling pathways, hypoxia and cell membrane structural change. Gelatinases also can affect the development and progression of CKD through the various interactions with tumor necrosis factors (TNFs), monocyte chemoattractant proteins (MCPs), growth factors (GFs), oxidative stress (OS), and so on. Currently, their non-proteolytic function is a hot topic of research, which may also be associated with the progression of CKD. Therefore, with the in-depth understanding about the function of gelatinases, we can have a more specific and accurate understanding of their role in the human body.

Effects of autophagy and endocytosis on the activity of matrix metalloproteinase‑2 in human renal proximal tubular cells under hypoxia

Tubulointerstitial fibrosis is characterized by tubular atrophy with basement membrane thickening and accumulation of interstitial extracellular matrix (ECM). A decrease in the activity of matrix metalloproteinase‑2 (MMP‑2) may promote this process. Although proximal tubular cells are sensitive to oxygen deprivation, whether cellular autophagy or endocytosis induced by hypoxia can alter the activity of MMP‑2 remains to be elucidated. The aim of the present study was to investigate whether autophagy and endocytosis induced by hypoxia can have an effect on the activity of MMP‑2 in HK‑2 cells. The investigations involved exposing the HK‑2 cell line to an autophagy inhibitor, 3‑MA, or an endocytotic inhibitor, filipin. The mRNA expression of MMP‑2 was elevated in the hypoxic milieu. Furthermore, it was found that filipin increased the activity of MMP‑2 under hypoxia. These results suggested that autophagy and endocytosis were potential mediators for the altered expression of MMP‑2, and endocytosis was a potential target for regulating the activity of MMP‑2. These data suggested that hypoxia may be an important pro‑fibrogenic stimulus, which acts in part via endocytosis.

Chronic insulin treatment phosphorylates the renal Na-K-ATPase α1-subunit at serine 16/23 and reduces its activity involving PI3-kinase-dependent PKC activation

The regulation of Na-K-ATPase in various tissues is under the control of a number of hormones and peptides that exert both short- and long-term control over its activity. The present study was performed to investigate the effect of chronic insulin treatment on Na-K-ATPase in renal proximal tubular cells. Incubation of opossum kidney (OK) cells, transfected with the rat Na-K-ATPase α₁-subunit, with 1 nmol/l insulin for 48 h decreased Na-K-ATPase activity. Insulin decreased α₁-protein content and increased α₁-serine phosphorylation and α₁-adaptor protein 2 (AP2) interaction. Removal of the 26 NH₂-terminal (-NT) amino acid from the α₁-subunit containing serine/threonine sites abolished the insulin-mediated serine phosphorylation and inhibition of Na-K-ATPase. Substitution of serine 16 and 23 with alanine showed a comparable effect on -NT. Insulin increased the activity of protein kinase C (PKC), which was blocked by the phosphatidylinositol 3-kinase (PI3K) inhibitor wortmannin. Both PI3K and PKC inhibitors abolished the insulin-mediated inhibition of Na-K-ATPase. Insulin increased the expression of PKC-β₁, -δ, -ξ, and-λ; however, only PKC-ξ/λ-specific inhibitors blocked insulin-induced phosphorylation and inhibition of Na-K-ATPase. Our data demonstrate that insulin activates the atypical PKC isoforms-ξ/λ via the PI3K pathway. PKC-ξ/λ-induced phosphorylation of the α₁-subunit at serine 16 and 23 leads to AP2 recruitment, degradation, and a decrease in Na-K-ATPase activity.

"Oxygen Sensing" by Na,K-ATPase: These Miraculous Thiols

Control over the Na,K-ATPase function plays a central role in adaptation of the organisms to hypoxic and anoxic conditions. As the enzyme itself does not possess O2 binding sites its "oxygen-sensitivity" is mediated by a variety of redox-sensitive modifications including S-glutathionylation, S-nitrosylation, and redox-sensitive phosphorylation. This is an overview of the current knowledge on the plethora of molecular mechanisms tuning the activity of the ATP-consuming Na,K-ATPase to the cellular metabolic activity. Recent findings suggest that oxygen-derived free radicals and H2O2, NO, and oxidized glutathione are the signaling messengers that make the Na,K-ATPase "oxygen-sensitive." This very ancient signaling pathway targeting thiols of all three subunits of the Na,K-ATPase as well as redox-sensitive kinases sustains the enzyme activity at the "optimal" level avoiding terminal ATP depletion and maintaining the transmembrane ion gradients in cells of anoxia-tolerant species. We acknowledge the complexity of the underlying processes as we characterize the sources of reactive oxygen and nitrogen species production in hypoxic cells, and identify their targets, the reactive thiol groups which, upon modification, impact the enzyme activity. Structured accordingly, this review presents a summary on (i) the sources of free radical production in hypoxic cells, (ii) localization of regulatory thiols within the Na,K-ATPase and the role reversible thiol modifications play in responses of the enzyme to a variety of stimuli (hypoxia, receptors' activation) (iii) redox-sensitive regulatory phosphorylation, and (iv) the role of fine modulation of the Na,K-ATPase function in survival success under hypoxic conditions. The co-authors attempted to cover all the contradictions and standing hypotheses in the field and propose the possible future developments in this dynamic area of research, the importance of which is hard to overestimate. Better understanding of the processes underlying successful adaptation strategies will make it possible to harness them and use for treatment of patients with stroke and myocardial infarction, sleep apnoea and high altitude pulmonary oedema, and those undergoing surgical interventions associated with the interruption of blood perfusion.

The acute respiratory distress syndrome: from mechanism to translation

The acute respiratory distress syndrome (ARDS) is a form of severe hypoxemic respiratory failure that is characterized by inflammatory injury to the alveolar capillary barrier, with extravasation of protein-rich edema fluid into the airspace. Although many modalities to treat ARDS have been investigated over the past several decades, supportive therapies remain the mainstay of treatment. In this article, we briefly review the definition, epidemiology, and pathophysiology of ARDS and present emerging aspects of ARDS pathophysiology that encompass modulators of the innate immune response, damage signals, and aberrant proteolysis that may serve as a foundation for future therapeutic targets.

The acute respiratory distress syndrome: from mechanism to translation

The acute respiratory distress syndrome (ARDS) is a form of severe hypoxemic respiratory failure that is characterized by inflammatory injury to the alveolar capillary barrier, with extravasation of protein-rich edema fluid into the airspace. Although many modalities to treat ARDS have been investigated over the past several decades, supportive therapies remain the mainstay of treatment. In this article, we briefly review the definition, epidemiology, and pathophysiology of ARDS and present emerging aspects of ARDS pathophysiology that encompass modulators of the innate immune response, damage signals, and aberrant proteolysis that may serve as a foundation for future therapeutic targets.

Dopamine D₁-like receptors regulate the α₁A-adrenergic receptor in human renal proximal tubule cells and D₁-like dopamine receptor knockout mice

The homeostatic control of blood pressure hinges upon the delicate balance between prohypertensinogenic and antihypertensinogenic systems. D₁-like dopamine receptors [dopamine D₁ and D₅ receptors (D₁Rs and D₅Rs, respectively)] and the α₁A-adrenergic receptor (α₁A-AR) are expressed in the renal proximal tubule and engender opposing effects on Na(+) transport, i.e., natriuresis (via D₁Rs and D5Rs) or antinatriuresis (via α₁A-ARs). We tested the hypothesis that the D₁R/D₅R regulates the α₁A-AR. D₁-like dopamine receptors coimmunoprecipitated, colocalized, and cofractionated with α₁A-ARs in lipid rafts in immortalized human renal proximal tubule cells. Long-term treatment with the D₁R/D₅R agonist fenoldopam resulted in decreased D₁R and D₅R expression but increased α₁A-AR abundance in the plasma membrane. Short-term fenoldopam treatment stimulated the translocation of Na(+)-K(+)-ATPase from the plasma membrane to the cytosol that was partially reversed by an α₁A-AR agonist, which by itself induced Na(+)-K(+)-ATPase translocation from the cytosol to the plasma membrane. The α₁A-AR-specific agonist A610603 also minimized the ability of fenoldopam to inhibit Na(+)-K(+)-ATPase activity. To determine the interaction among D₁Rs, D₅Rs, and α₁A-ARs in vivo, we used phenylephrine and A610603 to decrease Na(+) excretion in several D1-like dopamine receptor knockout mouse strains. Phenylephrine and A61603 treatment resulted in a partial reduction of urinary Na(+) excretion in wild-type mice and its abolition in D1R knockout, D₅R knockout, and D₁R-D₅R double-knockout mice. Our results demonstrate the ability of the D₁-like dopamine receptors to regulate the expression and activity of α₁A-AR. Elucidating the intricacies of the interaction among these receptors is crucial for a better understanding of the crosstalk between anti- and pro-hypertensive systems.

Overendocytosis of gold nanoparticles increases autophagy and apoptosis in hypoxic human renal proximal tubular cells

Background

Gold nanoparticles (GNPs) can potentially be used in biomedical fields ranging from therapeutics to diagnostics, and their use will result in increased human exposure. Many studies have demonstrated that GNPs can be deposited in the kidneys, particularly in renal tubular epithelial cells. Chronic hypoxic is inevitable in chronic kidney diseases, and it results in renal tubular epithelial cells that are susceptible to different types of injuries. However, the understanding of the interactions between GNPs and hypoxic renal tubular epithelial cells is still rudimentary. In the present study, we characterized the cytotoxic effects of GNPs in hypoxic renal tubular epithelial cells.

Results

Both 5 nm and 13 nm GNPs were synthesized and characterized using various biophysical methods, including transmission electron microscopy, dynamic light scattering, and ultraviolet–visible spectrophotometry. We detected the cytotoxicity of 5 and 13 nm GNPs (0, 1, 25, and 50 nM) to human renal proximal tubular cells (HK-2) by Cell Counting Kit-8 assay and lactate dehydrogenase release assay, but we just found the toxic effect in the 5 nm GNP-treated cells at 50 nM dose under hypoxic condition. Furthermore, the transmission electron microscopy images revealed that GNPs were either localized in vesicles or free in the lysosomes in 5 nm GNPs-treated HK-2 cells, and the cellular uptake of the GNPs in the hypoxic cells was significantly higher than that in normoxic cells. In normoxic HK-2 cells, 5 nm GNPs (50 nM) treatment could cause autophagy and cell survival. However, in hypoxic conditions, the GNP exposure at the same condition led to the production of reactive oxygen species, the loss of mitochondrial membrane potential (ΔΨM), and an increase in apoptosis and autophagic cell death.

Conclusion/significance

Our results demonstrate that renal tubular epithelial cells presented different responses under normoxic and hypoxic environments, which provide an important basis for understanding the risks associated with GNP use–especially for the potential GNP-related therapies in chronic kidney disease patients.

Reciprocal regulation of endocytosis and metabolism

The cellular uptake of many nutrients and micronutrients governs both their cellular availability and their systemic homeostasis. The cellular rate of nutrient or ion uptake (e.g., glucose, Fe(3+), K(+)) or efflux (e.g., Na(+)) is governed by a complement of membrane transporters and receptors that show dynamic localization at both the plasma membrane and defined intracellular membrane compartments. Regulation of the rate and mechanism of endocytosis controls the amounts of these proteins on the cell surface, which in many cases determines nutrient uptake or secretion. Moreover, the metabolic action of diverse hormones is initiated upon binding to surface receptors that then undergo regulated endocytosis and show distinct signaling patterns once internalized. Here, we examine how the endocytosis of nutrient transporters and carriers as well as signaling receptors governs cellular metabolism and thereby systemic (whole-body) metabolite homeostasis.

Functional genomic assessment of phosgene-induced acute lung injury in mice

In this study, a genetically diverse panel of 43 mouse strains was exposed to phosgene and genome-wide association mapping performed using a high-density single nucleotide polymorphism (SNP) assembly. Transcriptomic analysis was also used to improve the genetic resolution in the identification of genetic determinants of phosgene-induced acute lung injury (ALI). We prioritized the identified genes based on whether the encoded protein was previously associated with lung injury or contained a nonsynonymous SNP within a functional domain. Candidates were selected that contained a promoter SNP that could alter a putative transcription factor binding site and had variable expression by transcriptomic analyses. The latter two criteria also required that ≥10% of mice carried the minor allele and that this allele could account for ≥10% of the phenotypic difference noted between the strains at the phenotypic extremes. This integrative, functional approach revealed 14 candidate genes that included Atp1a1, Alox5, Galnt11, Hrh1, Mbd4, Phactr2, Plxnd1, Ptprt, Reln, and Zfand4, which had significant SNP associations, and Itga9, Man1a2, Mapk14, and Vwf, which had suggestive SNP associations. Of the genes with significant SNP associations, Atp1a1, Alox5, Plxnd1, Ptprt, and Zfand4 could be associated with ALI in several ways. Using a competitive electrophoretic mobility shift analysis, Atp1a1 promoter (rs215053185) oligonucleotide containing the minor G allele formed a major distinct faster-migrating complex. In addition, a gene with a suggestive SNP association, Itga9, is linked to transforming growth factor β1 signaling, which previously has been associated with the susceptibility to ALI in mice.

Na,K-ATPase is a target of cigarette smoke and reduced expression predicts poor patient outcome of smokers with lung cancer

Diminished Na,K-ATPase expression has been reported in several carcinomas and has been linked to tumor progression. However, few studies have determined whether Na,K-ATPase function and expression are altered in lung malignancies. Because cigarette smoke (CS) is a major factor underlying lung carcinogenesis and progression, we investigated whether CS affects Na,K-ATPase activity and expression in lung cell lines. Cells exposed to CS in vitro showed a reduction of Na,K-ATPase activity. We detected the presence of reactive oxygen species (ROS) in cells exposed to CS before Na,K-ATPase inhibition, and neutralization of ROS restored Na,K-ATPase activity. We further determined whether Na,K-ATPase expression correlated with increasing grades of lung adenocarcinoma and survival of patients with smoking history. Immunohistochemical analysis of lung adenocarcinoma tissues revealed reduced Na,K-ATPase expression with increasing tumor grade. Using tissue microarray containing lung adenocarcinomas of patients with known smoking status, we found that high expression of Na,K-ATPase correlated with better survival. For the first time, these data demonstrate that CS is associated with loss of Na,K-ATPase function and expression in lung carcinogenesis, which might contribute to disease progression.

Ubiquitination and proteolysis in acute lung injury

Ubiquitination is a posttranslational modification that regulates a variety of cellular functions depending on timing, subcellular localization, and type of tagging, as well as modulators of ubiquitin binding leading to proteasomal or lysosomal degradation or nonproteolytic modifications. Ubiquitination plays an important role in the pathogenesis of acute lung injury (ALI) and other lung diseases with pathologies secondary to inflammation, mechanical ventilation, and decreased physical mobility. Particularly, ubiquitination has been shown to affect alveolar epithelial barrier function and alveolar edema clearance by targeting the Na,K-ATPase and epithelial Na(+) channels upon lung injury. Notably, the proteasomal system also exhibits distinct functions in the extracellular space, which may contribute to the pathogenesis of ALI and other pulmonary diseases. Better understanding of these mechanisms may ultimately lead to novel therapeutic modalities by targeting elements of the ubiquitination pathway.

Alcohol worsens acute lung injury by inhibiting alveolar sodium transport through the adenosine A1 receptor

Objective

Alcohol intake increases the risk of acute lung injury (ALI) and the acute respiratory distress syndrome (ARDS) and is associated with poor outcomes in patients who develop these syndromes. No specific therapies are currently available to treat or decrease the risk of ARDS in patients with alcoholism. We have recently shown increased levels of lung adenosine inhibit alveolar fluid clearance, an important predictor of outcome in patients with ARDS. We hypothesized that alcohol might worsen lung injury by increasing lung adenosine levels, resulting in impaired active Na⁺ transport in the lung.

Methods

We treated wild-type mice with alcohol administered i.p. to achieve blood alcohol levels associated with moderate to severe intoxication and measured the rate of alveolar fluid clearance and Na,K-ATPase expression in peripheral lung tissue and assessed the effect of alcohol on survival during exposure to hyperoxia. We used primary rat alveolar type II cells to investigate the mechanisms by which alcohol regulates alveolar Na⁺ transport.

Results

Exposure to alcohol reduced alveolar fluid clearance, downregulated Na,K-ATPase in the lung tissue and worsened hyperoxia-induced lung injury. Alcohol caused an increase in BAL fluid adenosine levels. A similar increase in lung adenosine levels was observed after exposure to hyperoxia. In primary rat alveolar type II cells alcohol and adenosine decreased the abundance of the Na,K-ATPase at the basolateral membrane via a mechanism that required activation of the AMPK.

Conclusions

Alcohol decreases alveolar fluid clearance and impairs survival from acute lung injury. Alcohol induced increases in lung adenosine levels may be responsible for reduction in alveolar fluid clearance and associated worsening of lung injury.

Regulation of alpha1 Na/K-ATPase expression by cholesterol

We have reported that α1 Na/K-ATPase regulates the trafficking of caveolin-1 and consequently alters cholesterol distribution in the plasma membrane. Here, we report the reciprocal regulation of α1 Na/K-ATPase by cholesterol. Acute exposure of LLC-PK1 cells to methyl β-cyclodextrin led to parallel decreases in cellular cholesterol and the expression of α1 Na/K-ATPase. Cholesterol repletion fully reversed the effect of methyl β-cyclodextrin. Moreover, inhibition of intracellular cholesterol trafficking to the plasma membrane by compound U18666A had the same effect on α1 Na/K-ATPase. Similarly, the expression of α1, but not α2 and α3, Na/K-ATPase was significantly reduced in the target organs of Niemann-Pick type C mice where the intracellular cholesterol trafficking is blocked. Mechanistically, decreases in the plasma membrane cholesterol activated Src kinase and stimulated the endocytosis and degradation of α1 Na/K-ATPase through Src- and ubiquitination-dependent pathways. Thus, the new findings, taken together with what we have already reported, revealed a previously unrecognized feed-forward mechanism by which cells can utilize the Src-dependent interplay among Na/K-ATPase, caveolin-1, and cholesterol to effectively alter the structure and function of the plasma membrane.

Modulation of Na(+)-K(+)-ATPase cell surface abundance through structural determinants on the α1-subunit

Through their ion-pumping and non-ion-pumping functions, Na(+)-K(+)-ATPase protein complexes at the plasma membrane are critical to intracellular homeostasis and to the physiological and pharmacological actions of cardiotonic steroids. Alteration of the abundance of Na(+)-K(+)-ATPase units at the cell surface is one of the mechanisms for Na(+)-K(+)-ATPase regulation in health and diseases that has been closely examined over the past few decades. We here summarize these findings, with emphasis on studies that explicitly tested the involvement of defined regions or residues on the Na(+)-K(+)-ATPase α1 polypeptide. We also report new findings on the effect of manipulating Na(+)-K(+)-ATPase membrane abundance by targeting one of these defined regions: a dileucine motif of the form [D/E]XXXL[L/I]. In this study, opossum kidney cells stably expressing rat α1 Na(+)-K(+)-ATPase or a mutant where the motif was disrupted (α1-L499V) were exposed to 30 min of substrate/coverslip-induced-ischemia followed by reperfusion (I-R). Biotinylation studies suggested that I-R itself acted as an inducer of Na(+)-K(+)-ATPase internalization and that surface expression of the mutant was higher than the native Na(+)-K(+)-ATPase before and after ischemia. Annexin V/propidium iodide staining and lactate dehydrogenase release suggested that I-R injury was reduced in α1-L499V-expressing cells compared with α1-expressing cells. Hence, modulation of Na(+)-K(+)-ATPase cell surface abundance through structural determinants on the α-subunit is an important mechanism of regulation of cellular Na(+)-K(+)-ATPase in various physiological and pathophysiological conditions, with a significant impact on cell survival in face of an ischemic stress.

Antiviral activity of innate immune protein ISG15

The host innate immune response, including the production of type-I IFN, represents the primary line of defense against invading viral pathogens. Of the hundreds of IFN-stimulated genes (ISGs) discovered to date, ISG15 was one of the first identified and shown to encode a ubiquitin-like protein that functions, in part, as a modifier of protein function. Evidence implicating ISG15 as an innate immune protein with broad-spectrum antiviral activity continues to accumulate rapidly. This review will summarize recent findings on the innate antiviral activity of ISG15, with a focus on the interplay between ubiquitination and ISGylation pathways resulting in modulation of RNA virus assembly/budding. Indeed, ubiquitination is known to be proviral for some RNA viruses, whereas the parallel ISGylation pathway is known to be antiviral. A better understanding of the antiviral activities of ISG15 will enhance our fundamental knowledge of host innate responses to viral pathogens and may provide insight useful for the development of novel therapeutic approaches designed to enhance the immune response against such pathogens.

Role of ubiquitination in Na,K-ATPase regulation during lung injury

During acute lung injury edema accumulates in the alveolar space, resulting in hypoxemia due to intrapulmonary shunt. The alveolar Na,K-ATPase, by effecting active Na(+) transport, is essential for removing edema from the alveolar spaces. However, during hypoxia it is endocytosed and degraded, which results in decreased Na,K-ATPase function and impaired lung edema clearance. Na,K-ATPase endocytosis and degradation require the phosphorylation and subsequent ubiquitination of the Na,K-ATPase. These events are the results of cross-talk between post-translational modifications, and how ubiquitination of a specific protein can result from injurious extracellular stimuli. Here, we review current knowledge on the regulation of Na,K-ATPase activity during lung injury, focusing on the role of Na,K-ATPase ubiquitination during hypoxia. A better understanding of these signaling pathways can be of relevance for the design of novel treatments to ameliorate the deleterious effects of acute lung injury.

Lung fluid movements in hypoxia

Pulmonary edema is a problem of major clinical importance resulting from a persistent imbalance between forces that drive water into the airspace of the lung and the biological mechanisms for its removal. Here, we will first review the fundamental mechanisms implicated in the regulation of lung fluid homeostasis, namely, the Starling forces and the respiratory transepithelial sodium transport. Second, we will discuss the contribution of hypoxia to the perturbation of this fine balance and the role of such perturbations in the development of high-altitude pulmonary edema, a disease characterized by a very high morbidity and mortality. Finally, we will review possible interventions aimed to maintain/restore lung fluid homeostasis and their importance for the prevention/treatment of pulmonary edema.

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 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.