AMPK controls epithelial Na+ channels through Nedd4-2 and causes an epithelial phenotype when mutated

Signaling pathways and potential therapeutic targets in acute respiratory distress syndrome (ARDS)

Acute respiratory distress syndrome (ARDS) is a common condition associated with critically ill patients, characterized by bilateral chest radiographical opacities with refractory hypoxemia due to noncardiogenic pulmonary edema. Despite significant advances, the mortality of ARDS remains unacceptably high, and there are still no effective targeted pharmacotherapeutic agents. With the outbreak of coronavirus disease 19 worldwide, the mortality of ARDS has increased correspondingly. Comprehending the pathophysiology and the underlying molecular mechanisms of ARDS may thus be essential to developing effective therapeutic strategies and reducing mortality. To facilitate further understanding of its pathogenesis and exploring novel therapeutics, this review provides comprehensive information of ARDS from pathophysiology to molecular mechanisms and presents targeted therapeutics. We first describe the pathogenesis and pathophysiology of ARDS that involve dysregulated inflammation, alveolar-capillary barrier dysfunction, impaired alveolar fluid clearance and oxidative stress. Next, we summarize the molecular mechanisms and signaling pathways related to the above four aspects of ARDS pathophysiology, along with the latest research progress. Finally, we discuss the emerging therapeutic strategies that show exciting promise in ARDS, including several pharmacologic therapies, microRNA-based therapies and mesenchymal stromal cell therapies, highlighting the pathophysiological basis and the influences on signal transduction pathways for their use.

STIM1 in tumor cell death: angel or devil?

Stromal interaction molecule 1 (STIM1) is involved in mediating the store-operated Ca2+ entry (SOCE), driving the influx of the intracellular second messenger calcium ion (Ca2+), which is closely associated with tumor cell proliferation, metastasis, apoptosis, autophagy, metabolism and immune processes. STIM1 is not only regulated at the transcriptional level by NF-κB and HIF-1, but also post-transcriptionally modified by miRNAs and degraded by ubiquitination. Recent studies have shown that STIM1 or Ca2+ signaling can regulate apoptosis, autophagy, pyroptosis, and ferroptosis in tumor cells and act discrepantly in different cancers. Furthermore, STIM1 contributes to resistance against antitumor therapy by influencing tumor cell death. Further investigation into the mechanisms through which STIM1 controls other forms of tumor cell death could aid in the discovery of novel therapeutic targets. Moreover, STIM1 has the ability to regulate immune cells within the tumor microenvironment. Here, we review the basic structure, function and regulation of STIM1, summarize the signaling pathways through which STIM1 regulates tumor cell death, and propose the prospects of antitumor therapy by targeting STIM1.

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.

Chronic activation of AMP-activated protein kinase leads to early-onset polycystic kidney phenotype

AMP-activated protein kinase (AMPK) plays a key role in the cellular response to low energy stress and has emerged as an attractive therapeutic target for tackling metabolic diseases. Whilst significant progress has been made regarding the physiological role of AMPK, its function in the kidney remains only partially understood. We use a mouse model expressing a constitutively active mutant of AMPK to investigate the effect of AMPK activation on kidney function in vivo. Kidney morphology and changes in gene and protein expression were monitored and serum and urine markers were measured to assess kidney function in vivo. Global AMPK activation resulted in an early-onset polycystic kidney phenotype, featuring collecting duct cysts and compromised renal function in adult mice. Mechanistically, the cystic kidneys had increased cAMP levels and ERK activation, increased hexokinase I (Hk I) expression, glycogen accumulation and altered expression of proteins associated with autophagy. Kidney tubule-specific activation of AMPK also resulted in a polycystic phenotype, demonstrating that renal tubular AMPK activation caused the cystogenesis. Importantly, human autosomal dominant polycystic kidney disease (ADPKD) kidney sections revealed similar protein localisation patterns to that observed in the murine cystic kidneys. Our findings show that early-onset chronic AMPK activation leads to a polycystic kidney phenotype, suggesting dysregulated AMPK signalling is a contributing factor in cystogenesis.

Function and Regulation of the Epithelial Na+ Channel ENaC

The Epithelial Na⁺ Channel, ENaC, comprised of 3 subunits (αβγ, or sometimes δβγENaC), plays a critical role in regulating salt and fluid homeostasis in the body. It regulates fluid reabsorption into the blood stream from the kidney to control blood volume and pressure, fluid absorption in the lung to control alveolar fluid clearance at birth and maintenance of normal airway surface liquid throughout life, and fluid absorption in the distal colon and other epithelial tissues. Moreover, recent studies have also revealed a role for sodium movement via ENaC in nonepithelial cells/tissues, such as endothelial cells in blood vessels and neurons. Over the past 25 years, major advances have been made in our understanding of ENaC structure, function, regulation, and role in human disease. These include the recently solved three-dimensional structure of ENaC, ENaC function in various tissues, and mutations in ENaC that cause a hereditary form of hypertension (Liddle syndrome), salt-wasting hypotension (PHA1), or polymorphism in ENaC that contributes to other diseases (such as cystic fibrosis). Moreover, great strides have been made in deciphering the regulation of ENaC by hormones (e.g., the mineralocorticoid aldosterone, glucocorticoids, vasopressin), ions (e.g., Na⁺ ), proteins (e.g., the ubiquitin-protein ligase NEDD4-2, the kinases SGK1, AKT, AMPK, WNKs & mTORC2, and proteases), and posttranslational modifications [e.g., (de)ubiquitylation, glycosylation, phosphorylation, acetylation, palmitoylation]. Characterization of ENaC structure, function, regulation, and role in human disease, including using animal models, are described in this article, with a special emphasis on recent advances in the field. © 2021 American Physiological Society. Compr Physiol 11:1-29, 2021.

Mechanism and application of metformin in kidney diseases: An update

Metformin is an oral antihyperglycemic drug widely used to treat type 2 diabetes mellitus (T2DM), acting via indirect activation of 5' Adenosine monophosphate-activated Protein Kinase (AMPK). Beyond the anti-diabetic effect, accumulative pieces of evidence have revealed that metformin also everts a beneficial effect in diverse kidney diseases. In various acute kidney diseases (AKI) animal models, metformin protects renal tubular cells from inflammation, apoptosis, reactive oxygen stress (ROS), endoplasmic reticulum (ER) stress, epithelial-mesenchymal transition (EMT) via AMPK activation. In diabetic kidney disease (DKD), metformin also alleviates podocyte loss, mesangial cells apoptosis, and tubular cells senescence through AMPK-mediated signaling pathways. Besides, metformin inhibits cystic fibrosis transmembrane conductance regulator (CFTR)-mediated fluids secretion and the mammalian target of rapamycin (mTOR)-involved cyst formation negatively regulated by AMPK in autosomal dominant polycystic kidney disease (APDKD). Furthermore, metformin also contributes to the alleviation of urolithiasis and renal cell carcinoma (RCC). As the common pathway for chronic kidney disease (CKD) progressing towards end-stage renal disease (ESRD), renal fibrosis is ameliorated by metformin, to a great extent dependent on AMPK activation. However, clinical data are not always consistent with preclinical data, some clinical investigations showed the unmeaningful even detrimental effect of metformin on T2DM patients with kidney diseases. Most importantly, metformin-associated lactic acidosis (MALA) is a vital issue restricting the application of metformin. Thus, we conclude the application of metformin in kidney diseases and uncover the underlying molecular mechanisms in this review.

ENaC regulation by phospholipids and DGK explained through mathematical modeling

Cystic fibrosis is a condition caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR). It is also thought to increase the activity of epithelial sodium channels (ENaC). The altered function of these ion channels is one of the causes of the thick dehydrated mucus that characterizes the disease and is partially responsible for recurrent pulmonary infections and inflammation events that ultimately destroy the lungs of affected subjects. Phosphoinositides are signaling lipids that regulate numerous cellular processes and membrane proteins, including ENaC. Inhibition of diacylglycerol kinase (DGK), an enzyme of the phosphoinositide pathway, reduces ENaC function. We propose a computational analysis that is based on the combination of two existing mathematical models: one representing the dynamics of phosphoinositides and the other explaining how phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) influences ENaC activity and, consequently, airway surface liquid. This integrated model permits, for the first time, a detailed assessment of the intricate interactions between DGK and ENaC and is consistent with available literature data. In particular, the computational approach allows comparisons of two competing hypotheses regarding the regulation of ENaC. The results strongly suggest that the regulation of ENaC is primarily exerted through the control of PI(4,5)P2 production by type-I phosphatidylinositol-4-phosphate 5-kinase (PIP5KI), which in turn is controlled by phosphatidic acid (PA), the product of the DGK reaction.

AMPK phosphorylation of the β1Pix exchange factor regulates the assembly and function of an ENaC inhibitory complex in kidney epithelial cells

The metabolic sensor AMP-activated protein kinase (AMPK) inhibits the epithelial Na⁺ channel (ENaC), a key regulator of salt reabsorption by the kidney and thus total body volume and blood pressure. Recent studies have suggested that AMPK promotes the association of p21-activated kinase-interacting exchange factor-β₁ β₁Pix, 14-3-3 proteins, and the ubiquitin ligase neural precursor cell expressed developmentally downregulated protein (Nedd)4-2 into a complex that inhibits ENaC by enhancing Nedd4-2 binding to ENaC and ENaC degradation. Functional β₁Pix is required for ENaC inhibition by AMPK and promotes Nedd4-2 phosphorylation and stability in mouse kidney cortical collecting duct cells. Here, we report that AMPK directly phosphorylates β₁Pix in vitro. Among several AMPK phosphorylation sites on β₁Pix detected by mass spectrometry, Ser⁷¹ was validated as functionally significant. Compared with wild-type β₁Pix, overexpression of a phosphorylation-deficient β₁Pix-S71A mutant attenuated ENaC inhibition and the AMPK-activated interaction of both β₁Pix and Nedd4-2 to 14-3-3 proteins in cortical collecting duct cells. Similarly, overexpression of a β₁Pix-Δ602-611 deletion tract mutant unable to bind 14-3-3 proteins decreased the interaction between Nedd4-2 and 14-3-3 proteins, suggesting that 14-3-3 binding to β₁Pix is critical for the formation of a β₁Pix/Nedd4-2/14-3-3 complex. With expression of a general peptide inhibitor of 14-3-3-target protein interactions (R18), binding of both β₁Pix and Nedd4-2 to 14-3-3 proteins was reduced, and AMPK-dependent ENaC inhibition was also attenuated. Altogether, our results demonstrate the importance of AMPK-mediated phosphorylation of β₁Pix at Ser⁷¹, which promotes 14-3-3 interactions with β₁Pix and Nedd4-2 to form a tripartite ENaC inhibitory complex, in the mechanism of ENaC regulation by AMPK.

Estrogen negatively regulates the renal epithelial sodium channel (ENaC) by promoting Derlin-1 expression and AMPK activation

The main functions of the epithelial sodium channel (ENaC) in the kidney distal nephron are mediation of sodium and water balance and stabilization of blood pressure. Estrogen has important effects on sodium and water balance and on premenopausal blood pressure, but its role in the regulation of ENaC function is not fully understood. Female Sprague–Dawley rats were treated with 17β-estradiol for 6 weeks following bilateral ovariectomy. Plasma estrogen, aldosterone, creatinine, and electrolytes were analyzed, and α-ENaC and derlin-1 protein expression in the kidney was determined by immunohistochemistry and western blotting. The expression levels of α-ENaC, derlin-1, AMPK, and related molecules were also examined by western blotting and real-time PCR in cultured mouse renal collecting duct (mpkCCDc14) epithelial cells following estrogen treatment. Immunofluorescence and coimmunoprecipitation were performed to detect α-ENaC binding with derlin-1 and α-ENaC ubiquitination. The results demonstrated that the loss of estrogen elevated systolic blood pressure in ovariectomized (OVX) rats. OVX rat kidneys showed increased α-ENaC expression but decreased derlin-1 expression. In contrast, estrogen treatment decreased α-ENaC expression but increased derlin-1 expression in mpkCCDc14 cells. Moreover, estrogen induced α-ENaC ubiquitination by promoting the interaction of α-ENaC with derlin-1 and evoked phosphorylation of AMPK in mpkCCDc14 cells. Our study indicates that estrogen reduces ENaC expression and blood pressure in OVX rats through derlin-1 upregulation and AMPK activation.

Estrogen treatment could prove valuable in tackling high blood pressure (hypertension) in postmenopausal women. Long-term healthy blood pressure is linked to the correct regulation of sodium and water levels in the kidneys. The renal epithelial sodium channel (ENaC) is a cellular membrane channel responsible for mediating sodium reabsorption and fluid balance. Liang and co-workers at Nanjing Medical University in Nanjing, China, conducted experiments on postmenopausal rat models, and found that loss of estrogen elevates systolic blood pressure (the pressure during heart muscle contraction), and that the rats had high levels of ENaC expression. Further investigations showed that estrogen treatment restored blood pressure to normal levels by promoting two key proteins involved in cellular membrane health and energy metabolism. This in turn reinstated normal levels of ENaC breakdown in the kidneys, limiting hypertension.

AMP-Activated Protein Kinase (AMPK)-Dependent Regulation of Renal Transport

AMP-activated kinase (AMPK) is a serine/threonine kinase that is expressed in most cells and activated by a high cellular AMP/ATP ratio (indicating energy deficiency) or by Ca²⁺. In general, AMPK turns on energy-generating pathways (e.g., glucose uptake, glycolysis, fatty acid oxidation) and stops energy-consuming processes (e.g., lipogenesis, glycogenesis), thereby helping cells survive low energy states. The functional element of the kidney, the nephron, consists of the glomerulus, where the primary urine is filtered, and the proximal tubule, Henle's loop, the distal tubule, and the collecting duct. In the tubular system of the kidney, the composition of primary urine is modified by the reabsorption and secretion of ions and molecules to yield final excreted urine. The underlying membrane transport processes are mainly energy-consuming (active transport) and in some cases passive. Since active transport accounts for a large part of the cell's ATP demands, it is an important target for AMPK. Here, we review the AMPK-dependent regulation of membrane transport along nephron segments and discuss physiological and pathophysiological implications.

β1Pix exchange factor stabilizes the ubiquitin ligase Nedd4-2 and plays a critical role in ENaC regulation by AMPK in kidney epithelial cells

Our previous work has established that the metabolic sensor AMP-activated protein kinase (AMPK) inhibits the epithelial Na⁺ channel (ENaC) by promoting its binding to neural precursor cell-expressed, developmentally down-regulated 4-2, E3 ubiquitin protein ligase (Nedd4-2). Here, using MS analysis and in vitro phosphorylation, we show that AMPK phosphorylates Nedd4-2 at the Ser-444 (Xenopus Nedd4-2) site critical for Nedd4-2 stability. We further demonstrate that the Pak-interacting exchange factor β₁Pix is required for AMPK-mediated inhibition of ENaC-dependent currents in both CHO and murine kidney cortical collecting duct (CCD) cells. Short hairpin RNA-mediated knockdown of β₁Pix expression in CCD cells attenuated the inhibitory effect of AMPK activators on ENaC currents. Moreover, overexpression of a β₁Pix dimerization-deficient mutant unable to bind 14-3-3 proteins (Δ602-611) increased ENaC currents in CCD cells, whereas overexpression of WT β₁Pix had the opposite effect. Using additional immunoblotting and co-immunoprecipitation experiments, we found that treatment with AMPK activators promoted the binding of β₁Pix to 14-3-3 proteins in CCD cells. However, the association between Nedd4-2 and 14-3-3 proteins was not consistently affected by AMPK activation, β₁Pix knockdown, or overexpression of WT β₁Pix or the β₁Pix-Δ602-611 mutant. Moreover, we found that β₁Pix is important for phosphorylation of the aforementioned Nedd4-2 site critical for its stability. Overall, these findings elucidate novel molecular mechanisms by which AMPK regulates ENaC. Specifically, they indicate that AMPK promotes the assembly of β₁Pix, 14-3-3 proteins, and Nedd4-2 into a complex that inhibits ENaC by enhancing Nedd4-2 binding to ENaC and its degradation.

AMP-activated kinase is a regulator of fibroblast growth factor 23 production

Fibroblast growth factor 23 (FGF23) is a proteohormone regulating renal phosphate transport and vitamin D metabolism as well as inducing left heart hypertrophy. FGF23-deficient mice suffer from severe tissue calcification, accelerated aging and a myriad of aging-associated diseases. Bone cells produce FGF23 upon store-operated calcium ion entry (SOCE) through the calcium selective ion channel Orai1. AMP-activated kinase (AMPK) is a powerful energy sensor helping cells survive states of energy deficiency, and AMPK down-regulates Orai1. Here we investigated the role of AMPK in FGF23 production. Fgf23 gene transcription was analyzed by qRT-PCR and SOCE by fluorescence optics in UMR106 osteoblast-like cells while the serum FGF23 concentration and phosphate metabolism were assessed in AMPKα1-knockout and wild-type mice. The AMPK activator, 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) down-regulated, whereas the AMPK inhibitor, dorsomorphin dihydrochloride (compound C) and AMPK gene silencing induced Fgf23 transcription. AICAR decreased membrane abundance of Orai1 and SOCE. SOCE inhibitors lowered Fgf23 gene expression induced by AMPK inhibition. AMPKα1-knockout mice had a higher serum FGF23 concentration compared to wild-type mice. Thus, AMPK participates in the regulation of FGF23 production in vitro and in vivo. The inhibitory effect of AMPK on FGF23 production is at least in part mediated by Orai1-involving SOCE.

To die or not to die SGK1-sensitive ORAI/STIM in cell survival

The pore forming Ca²⁺ release activated Ca²⁺ channel (CRAC) isoforms ORAI1-3 and their regulators STIM1,2 accomplish store operated Ca²⁺ entry (SOCE). Activation of SOCE may lead to cytosolic Ca²⁺ oscillations, which in turn support cell proliferation and cell survival. ORAI/STIM and thus SOCE are upregulated by the serum and glucocorticoid inducible kinase SGK1, a kinase under powerful genomic regulation and activated by phosphorylation via the phosphoinositol-3-phosphate pathway. SGK1 enhances ORAI1 abundance partially by phosphorylation of Nedd4-2, an ubiquitin ligase priming the channel protein for degradation. The SGK1-phosphorylated Nedd4-2 binds to the protein 14-3-3 and is thus unable to ubiquinate ORAI1. SGK1 further increases the ORAI1 and STIM1 protein abundance by activating nuclear factor kappa B (NF-κB), a transcription factor upregulating the expression of STIM1 and ORAI1. SGK1-sensitive upregulation of ORAI/STIM and thus SOCE is triggered by a wide variety of hormones and growth factors, as well as several cell stressors including ischemia, radiation, and cell shrinkage. SGK1 dependent upregulation of ORAI/STIM confers survival of tumor cells and thus impacts on growth and therapy resistance of cancer. On the other hand, SGK1-dependent upregulation of ORAI1 and STIM1 may support survival of neurons and impairment of SGK1-dependent ORAI/STIM activity may foster neurodegeneration. Clearly, further experimental effort is needed to define the mechanisms linking SGK1-dependent upregulation of ORAI1 and STIM1 to cell survival and to define the impact of SGK1-dependent upregulation of ORAI1 and STIM1 on malignancy and neurodegenerative disease.

AMP-activated protein kinase: a therapeutic target in intestinal diseases

Adenosine monophosphate (AMP)-activated protein kinase (AMPK), a highly conserved energy sensor, has a crucial role in cardiovascular, neurodegenerative and inflammatory diseases, as well as in cancer and metabolic disorders. Accumulating studies have demonstrated that AMPK activation enhances paracellular junctions, nutrient transporters, autophagy and apoptosis, and suppresses inflammation and carcinogenesis in the intestine, indicating an essential role of AMPK in intestinal health. AMPK inactivation is an aetiological factor in intestinal dysfunctions. This review summarizes the favourable outcomes of AMPK activation on intestinal health, and discusses AMPK as a potential therapeutic target for intestinal diseases.

Hypercapnia Impairs ENaC Cell Surface Stability by Promoting Phosphorylation, Polyubiquitination and Endocytosis of β-ENaC in a Human Alveolar Epithelial Cell Line

Acute lung injury is associated with formation of pulmonary edema leading to impaired gas exchange. Patients with acute respiratory distress syndrome (ARDS) require mechanical ventilation to improve oxygenation; however, the use of relatively low tidal volumes (to minimize further injury of the lung) often leads to further accumulation of carbon dioxide (hypercapnia). Hypercapnia has been shown to impair alveolar fluid clearance (AFC), thereby causing retention of pulmonary edema, and may lead to worse outcomes; however, the underlying molecular mechanisms remain incompletely understood. AFC is critically dependent on the epithelial sodium channel (ENaC), which drives the vectorial transport of Na⁺ across the alveolar epithelium. Thus, in the current study, we investigated the mechanisms by which hypercapnia effects ENaC cell surface stability in alveolar epithelial cells (AECs). Elevated CO₂ levels led to polyubiquitination of β-ENaC and subsequent endocytosis of the α/β-ENaC complex in AECs, which were prevented by silencing the E3 ubiquitin ligase, Nedd4-2. Hypercapnia-induced ubiquitination and cell surface retrieval of ENaC were critically dependent on phosphorylation of the Thr615 residue of β-ENaC, which was mediated by the extracellular signal-regulated kinase (ERK)1/2. Furthermore, activation of ERK1/2 led to subsequent activation of AMP-activated protein kinase (AMPK) and c-Jun N-terminal kinase (JNK)1/2 that in turn phosphorylated Nedd4-2 at the Thr899 residue. Importantly, mutation of Thr899 to Ala markedly inhibited the CO₂-induced polyubiquitination of β-ENaC and restored cell surface stability of the ENaC complex, highlighting the critical role of Nedd4-2 phosphorylation status in targeting ENaC. Collectively, our data suggest that elevated CO₂ levels promote activation of the ERK/AMPK/JNK axis in a human AEC line, in which ERK1/2 phosphorylates β-ENaC whereas JNK mediates phosphorylation of Nedd4-2, thereby facilitating the channel-ligase interaction. The hypercapnia-induced ENaC dysfunction may contribute to impaired alveolar edema clearance and thus, interfering with these molecular mechanisms may improve alveolar fluid balance and lead to better outcomes in patients with ARDS.

Lack of Effects of Metformin and AICAR Chronic Infusion on the Development of Hypertension in Dahl Salt-Sensitive Rats

In the kidney, reabsorption via the epithelial sodium channel (ENaC) is involved in long-term blood pressure control. Previously we demonstrated that ENaC hyperactivity is associated with development of salt-sensitive (SS) hypertension in Dahl SS rats. AMP-activated kinase (AMPK), playing a role in cellular energy homeostasis, has been shown to decrease ENaC activity. Here, we tested whether metformin and AICAR, two drugs that activate AMPK, affect the development of salt-induced hypertension. High salt diet significantly increased mean arterial pressure (MAP) in Dahl SS rats. Blood pressure elevation was accompanied by a short-term decline of heart rate and increased circadian arterial pressure dipping. Metformin and AICAR were delivered intravenously at doses of 200 and 20 mg/kg/day, respectively. However, both control and drug-treated groups had similar development of high blood pressure within 3 weeks of 8% NaCl dietary salt intake. In the metformin-treated animals MAP reached 164.9 ± 9.1 mmHg, which was not significantly different from the control group (171.8 ± 5.6 mmHg). Patch clamp analysis revealed that the metformin-treated rats had no difference in the activity of ENaC. AICAR treatment also did not affect the development of hypertension and kidney injury. MAP reached 182.8 ± 4.8 and 178.0 ± 2.8 mmHg in AICAR and vehicle treated groups, respectively. Of note, we found that high-salt diet activated AMPK in the Dahl SS rats, and treatment with these AMPK activators had no significant further effect on AMPK activity. We conclude that AMPK activators, at least under these conditions, do not affect development of hypertension during high-salt diet in the Dahl SS rat model.

AMPK/α-Ketoglutarate Axis Regulates Intestinal Water and Ion Homeostasis in Young Pigs

Water and ion absorption via sensitive aquaporins (AQPs) and ion channels is of critical importance in intestinal health. However, whether α-ketoglutarate (AKG) could improve intestinal water and ion homeostasis in lipopolysaccharide (LPS)-challenged piglets and whether the AMP-activated protein kinase (AMPK) pathway is involved remains largely unknown. This study was conducted to investigate the effect of dietary AKG supplementation on the small intestinal water and ion homeostasis through modulating the AMPK pathway in a piglet diarrhea model. A total of 32 weaned piglets were used in a 2 × 2 factorial design; the major factors were diet (basal diet or 1% AKG diet) and challenge (Escherichia coli LPS or saline). The results showed that LPS challenge increased the diarrhea index and affected the concentrations of serum Na⁺, K⁺, Cl^-, glucose, and AKG and its metabolites in piglets fed the basal or AKG diet. However, the addition of AKG attenuated diarrhea incidence and reversed these serum parameter concentrations. Most AQPs (e.g., AQP1, AQP3, AQP4, AQP5, AQP8, AQP10, and AQP11) and ion transporters (NHE3, ENaC, and DRA/PAT1) were widely distributed in the duodenum and jejunum of piglets. We also found that AKG up-regulated the expression of intestinal epithelial AQPs while inhibiting the expression of ion transporters. LPS challenge decreased (P < 0.05) the gene and protein expression of the AMPK pathway (AMPKα1, AMPKα2, SIRT1, PGC-1α, ACC, and TORC2) in the jejunum and ileum. Notably, AKG supplementation enhanced the abundance of these proteins in the LPS-challenged piglets. Collectively, AKG plays an important role in increasing water and ion homeostasis through modulating the AMPK pathway. Our novel finding has important implications for the prevention and treatment of gut dysfunction in neonates.

Kidney-specific genetic deletion of both AMPK α-subunits causes salt and water wasting

AMP-activated kinase (AMPK) controls cell energy homeostasis by modulating ATP synthesis and expenditure. In vitro studies have suggested AMPK may also control key elements of renal epithelial electrolyte transport but in vivo physiological confirmation is still insufficient. We studied sodium renal handling and extracellular volume regulation in mice with genetic deletion of AMPK catalytic subunits. AMPKα1 knockout (KO) mice exhibit normal renal sodium handling and a moderate antidiuretic state. This is accompanied by higher urinary aldosterone excretion rates and reduced blood pressure. Plasma volume, however, was found to be increased compared with wild-type mice. Thus blood volume is preserved despite a significantly lower hematocrit. The lack of a defect in renal function in AMPKα1 KO mice could be explained by a compensatory upregulation in AMPK α2-subunit. Therefore, we used the Cre-loxP system to knock down AMPKα2 expression in renal epithelial cells. Combining this approach with the systemic deletion of AMPKα1 we achieved reduced renal AMPK activity, accompanied by a shift to a moderate water- and salt-wasting phenotype. Thus we confirm the physiologically relevant role of AMPK in the kidney. Furthermore, our results indicate that in vivo AMPK activity stimulates renal sodium and water reabsorption.

AMP-Activated Protein Kinase Attenuates High Salt-Induced Activation of Epithelial Sodium Channels (ENaC) in Human Umbilical Vein Endothelial Cells

Recent studies suggest that the epithelial sodium channel (ENaC) is expressed in the endothelial cells. To test whether high salt affects the NO production via regulation of endothelial ENaC, human umbilical vein endothelial cells (HUVECs) were incubated in solutions containing either normal or high sodium (additional 20 mM NaCl). Our data showed that high sodium treatment significantly increased α-, β-, and γ-ENaC expression levels in HUVECs. Using the cell-attached patch-clamp technique, we demonstrated that high sodium treatment significantly increased ENaC open probability (P_O). Moreover, nitric oxide synthase (eNOS) phosphorylation (Ser 1177) levels and NO production were significantly decreased by high sodium in HUVECs; the effects of high sodium on eNOS phosphorylation and NO production were inhibited by a specific ENaC blocker, amiloride. Our results showed that high sodium decreased AMP-activated kinase (AMPK) phosphorylation in endothelial cells. On the other hand, metformin, an AMPK activator, prevented high sodium-induced upregulation of ENaC expression and P_O. Moreover, metformin prevented high salt-induced decrease in NO production and eNOS phosphorylation. These results suggest that high sodium stimulates ENaC activation by negatively modulating AMPK activity, thereby leading to reduction in eNOS activity and NO production in endothelial cells.

Minireview: hey U(PS): metabolic and proteolytic homeostasis linked via AMPK and the ubiquitin proteasome system

One of the master regulators of both glucose and lipid cellular metabolism is 5'-AMP-activated protein kinase (AMPK). As a metabolic pivot that dynamically responds to shifts in nutrient availability and stress, AMPK dysregulation is implicated in the underlying molecular pathology of a variety of diseases, including cardiovascular diseases, diabetes, cancer, neurological diseases, and aging. Although the regulation of AMPK enzymatic activity by upstream kinases is an active area of research, less is known about regulation of AMPK protein stability and activity by components of the ubiquitin-proteasome system (UPS), the cellular machinery responsible for both the recognition and degradation of proteins. Furthermore, there is growing evidence that AMPK regulates overall proteasome activity and individual components of the UPS. This review serves to identify the current understanding of the interplay between AMPK and the UPS and to promote further exploration of the relationship between these regulators of energy use and amino acid availability within the cell.

Hypoxia-induced inhibition of epithelial Na(+) channels in the lung. Role of Nedd4-2 and the ubiquitin-proteasome pathway

Transepithelial sodium transport via alveolar epithelial Na(+) channels (ENaC) and Na(+),K(+)-ATPase constitutes the driving force for removal of alveolar edema fluid. Alveolar hypoxia associated with pulmonary edema may impair ENaC activity and alveolar Na(+) absorption through a decrease of ENaC subunit expression at the apical membrane of alveolar epithelial cells (AECs). Here, we investigated the mechanism(s) involved in this process in vivo in the β-Liddle mouse strain mice carrying a truncation of β-ENaC C-terminus abolishing the interaction between β-ENaC and the ubiquitin protein-ligase Nedd4-2 that targets the channel for endocytosis and degradation and in vitro in rat AECs. Hypoxia (8% O2 for 24 h) reduced amiloride-sensitive alveolar fluid clearance by 69% in wild-type mice but had no effect in homozygous mutated β-Liddle littermates. In vitro, acute exposure of AECs to hypoxia (0.5-3% O2 for 1-6 h) rapidly decreased transepithelial Na(+) transport as assessed by equivalent short-circuit current Ieq and the amiloride-sensitive component of Na(+) current across the apical membrane, reflecting ENaC activity. Hypoxia induced a decrease of ENaC subunit expression in the apical membrane of AECs with no change in intracellular expression and induced a 2-fold increase in α-ENaC polyubiquitination. Hypoxic inhibition of amiloride-sensitive Ieq was fully prevented by preincubation with the proteasome inhibitors MG132 and lactacystin or with the antioxidant N-acetyl-cysteine. Our data strongly suggest that Nedd4-2-mediated ubiquitination of ENaC leading to endocytosis and degradation of apical Na(+) channels is a key feature of hypoxia-induced inhibition of transepithelial alveolar Na(+) transport.

AMPK-sensitive cellular transport

The energy sensing AMP-activated protein kinase (AMPK) regulates cellular and whole-body energy balance through stimulating catabolic ATP-generating and suppressing anabolic ATP-consuming pathways thereby helping cells survive during energy depletion. The kinase has previously been reported to be either directly or indirectly involved in the regulation of several carriers, channels and pumps of high significance in cellular physiology. Thus AMPK provides a necessary link between cellular energy metabolism and cellular transport activity. Better understanding of the AMPK role in cellular transport offers a potential for improved therapies in various human diseases and disorders. In this review, we discuss recent advances in understanding the role and function of AMPK in transport regulation under physiological and pathological states.

Regulation of ion channels and transporters by AMP-activated kinase (AMPK)

The energy-sensing AMP-activated kinase AMPK ensures survival of energy-depleted cells by stimulating ATP production and limiting ATP utilization. Both energy production and energy consumption are profoundly influenced by transport processes across the cell membane including channels, carriers and pumps. Accordingly, AMPK is a powerful regulator of transport across the cell membrane. AMPK regulates diverse K(+) channels, Na(+) channels, Ca(2+) release activated Ca(2+) channels, Cl(-) channels, gap junctional channels, glucose carriers, Na(+)/H(+)-exchanger, monocarboxylate-, phosphate-, creatine-, amino acid-, peptide- and osmolyte-transporters, Na(+)/Ca(2+)-exchanger, H(+)-ATPase and Na(+)/K(+)-ATPase. AMPK activates ubiquitin ligase Nedd4-2, which labels several plasma membrane proteins for degradation. AMPK further regulates transport proteins by inhibition of Rab GTPase activating protein (GAP) TBC1D1. It stimulates phosphatidylinositol 3-phosphate 5-kinase PIKfyve and inhibits phosphatase and tensin homolog (PTEN) via glycogen synthase kinase 3β (GSK3β). Moreover, it stabilizes F-actin as well as downregulates transcription factor NF-κB. All those cellular effects serve to regulate transport proteins.

AMPK couples plasma renin to cellular metabolism by phosphorylation of ACC1

Salt reabsorption is the major energy-requiring process in the kidney, and AMP-activated protein kinase (AMPK) is an important regulator of cellular metabolism. Mice with targeted deletion of the β1-subunit of AMPK (AMPK-β1(-/-) mice) had significantly increased urinary Na(+) excretion on a normal salt diet. This was associated with reduced expression of the β-subunit of the epithelial Na(+) channel (ENaC) and increased subapical tubular expression of kidney-specific Na(+)-K(+)-2Cl(-) cotransporter 2 (NKCC2) in the medullary thick ascending limb of Henle. AMPK-β1(-/-) mice fed a salt-deficient diet were able to conserve Na(+), but renin secretion increased 180% compared with control mice. Cyclooxygenase-2 mRNA also increased in the kidney cortex, indicating greater signaling through the macula densa tubular salt-sensing pathway. To determine whether the increase in renin secretion was due to a change in regulation of fatty acid metabolism by AMPK, mice with a mutation of the inhibitory AMPK phosphosite in acetyl-CoA carboxylase 1 [ACC1-knockin (KI)(S79A) mice] were examined. ACC1-KI(S79A) mice on a normal salt diet had no increase in salt loss or renin secretion, and expression of NKCC2, Na(+)-Cl(-) cotransporter, and ENaC-β were similar to those in control mice. When mice were placed on a salt-deficient diet, however, renin secretion and cortical expression of cyclooxygenase-2 mRNA increased significantly in ACC1-KI(S79A) mice compared with control mice. In summary, our data suggest that renin synthesis and secretion are regulated by AMPK and coupled to metabolism by phosphorylation of ACC1.

Oestrogen promotes KCNQ1 potassium channel endocytosis and postendocytic trafficking in colonic epithelium

The cAMP-regulated potassium channel KCNQ1:KCNE3 plays an essential role in transepithelial Cl(-) secretion. Recycling of K(+) across the basolateral membrane provides the driving force necessary to maintain apical Cl(-) secretion. The steroid hormone oestrogen (17β-oestradiol; E2), produces a female-specific antisecretory response in rat distal colon through the inhibition of the KCNQ1:KCNE3 channel. It has previously been shown that rapid inhibition of the channel conductance results from E2-induced uncoupling of the KCNE3 regulatory subunit from the KCNQ1 channel pore complex. The purpose of this study was to determine the mechanism required for sustained inhibition of the channel function. We found that E2 plays a role in regulation of KCNQ1 cell membrane abundance by endocytosis. Ussing chamber experiments have shown that E2 inhibits both Cl(-) secretion and KCNQ1 current in a colonic cell line, HT29cl.19A, when cultured as a confluent epithelium. Following E2 treatment, KCNQ1 was retrieved from the plasma membrane by a clathrin-mediated endocytosis, which involved the association between KCNQ1 and the clathrin adaptor, AP-2. Following endocytosis, KCNQ1 was accumulated in early endosomes. Following E2-induced endocytosis, rather than being degraded, KCNQ1 was recycled by a biphasic mechanism involving Rab4 and Rab11. Protein kinase Cδ and AMP-dependent kinase were rapidly phosphorylated in response to E2 on their activating phosphorylation sites, Ser643 and Thr172, respectively (as previously shown). Both kinases are necessary for the E2-induced endocytosis, because E2 failed to induce KCNQ1 internalization following pretreatment with specific inhibitors of both protein kinase Cδ and AMP-dependent kinase. The ubiquitin ligase Nedd4.2 binds KCNQ1 in response to E2 to induce channel internalization. This study has provided the first demonstration of hormonal regulation of KCNQ1 trafficking. In conclusion, we propose that internalization of KCNQ1 is a key event in the sustained antisecretory response to oestrogen.

AMP-activated protein kinase regulates hERG potassium channel

Besides their role in cardiac repolarization, human ether-a-go-go-related gene potassium (hERG) channels are expressed in several tumor cells including rhabdomyosarcoma cells. The channels foster cell proliferation. Ubiquitously expressed AMP-dependent protein kinase (AMPK) is a serine-/threonine kinase, stimulating energy-generating and inhibiting energy-consuming processes thereby helping cells survive periods of energy depletion. AMPK has previously been shown to regulate Na⁺/K⁺ ATPase, Na⁺/Ca²⁺ exchangers, Ca²⁺ channels and K⁺ channels. The present study tested whether AMPK regulates hERG channel activity. Wild type AMPK (α1β1γ1), constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), or catalytically inactive (αK45R)AMPK (α1(K45R)β1γ1) were expressed in Xenopus oocytes with hERG. Tail currents were determined as a measure of hERG channel activity by two-electrode-voltage clamp. hERG membrane abundance was quantified by chemiluminescence and visualized by immunocytochemistry and confocal microscopy. Moreover, hERG currents were measured in RD rhabdomyosarcoma cells after pharmacological modification of AMPK activity using the patch clamp technique. Coexpression of wild-type AMPK and of constitutively active (γR70Q)AMPK significantly downregulated the tail currents in hERG-expressing Xenopus oocytes. Pharmacological activation of AMPK with AICAR or with phenformin inhibited hERG currents in Xenopus oocytes, an effect abrogated by AMPK inhibitor compound C. (γR70Q)AMPK enhanced the Nedd4-2-dependent downregulation of hERG currents. Coexpression of constitutively active (γR70Q)AMPK decreased membrane expression of hERG in Xenopus oocytes. Compound C significantly enhanced whereas AICAR tended to inhibit hERG currents in RD rhabdomyosarcoma cells. AMPK is a powerful regulator of hERG-mediated currents in both, Xenopus oocytes and RD rhabdomyosarcoma cells. AMPK-dependent regulation of hERG may be particularly relevant in cardiac hypertrophy and tumor growth.

AMP-activated protein kinase regulation of kidney tubular transport

PURPOSE OF REVIEW

The coupling of epithelial transport to underlying metabolic status is critical because solute transport processes normally consume a large proportion of total cellular energy. Recently, AMP-activated protein kinase (AMPK) has emerged as a critical transport regulator in tissues throughout the body. This review summarizes the role of AMPK in the regulation of renal epithelial transport, updates the growing list of AMPK transport protein targets and regulatory mechanisms, and discusses the potential clinical significance of this regulation in normal and disease states.

RECENT FINDINGS

Recent work has identified several new ion channels, transporters, and pumps that are regulated by AMPK in the kidney, and a better understanding of the mechanisms for the AMPK-dependent regulation of membrane transport proteins is emerging. Treatment with AMPK activators may be beneficial in preventing deleterious effects in the kidney in the setting of various diseases, including acute ischemia, diabetes mellitus and polycystic kidney disease, via mechanisms that depend at least partly on the regulatory effects of AMPK on solute transport.

SUMMARY

The energy-sensing kinase AMPK has a growing list of pleiotropic effects on cells and tissues, including its key role in the coupling of membrane transport to metabolic status in epithelial tissues like the kidney. AMPK is also involved in the coordination of hormonal, inflammatory, and other cellular stress pathway signals to produce an integrated effect on tubular transport. Identifying and characterizing new transport protein targets of AMPK should yield valuable new insights into various physiological and pathological processes in the kidney.

AMPK: A regulator of ion channels

Ion transport processes are highly energy consuming. It is therefore critical to couple ion transport processes to the metabolic state of the cell. An important player in this coupling appears to be the AMP-activated protein kinase (AMPK). This kinase becomes activated during conditions of cellular metabolic stress and is well-known for its role in promoting ATP-generating catabolic pathways while turning off ATP-utilizing anabolic pathways. Over the past decade AMPK has also emerged as a key regulator of ion channel activity as an increasing number of ion channels are reported to be either directly or indirectly regulated by the kinase. AMPK therefore provides a necessary link between cellular energy levels and ion channel activity.

Downregulation of the renal outer medullary K(+) channel ROMK by the AMP-activated protein kinase

The 5'-adenosine monophosphate-activated serine/threonine protein kinase (AMPK) is stimulated by energy depletion, increase in cytosolic Ca(2+) activity, oxidative stress, and nitric oxide. AMPK participates in the regulation of the epithelial Na(+) channel ENaC and the voltage-gated K(+) channel KCNE1/KCNQ1. It is partially effective by decreasing PIP(2) formation through the PI3K pathway. The present study explored whether AMPK regulates the renal outer medullary K(+) channel ROMK. To this end, cRNA encoding ROMK was injected into Xenopus oocytes with and without additional injection of constitutively active AMPK(γR70Q) (AMPK(α1)-HA+AMPK(β1)-Flag+AMPKγ1(R70Q)), or of inactive AMPK(αK45R) (AMPK(α1K45R)+AMPK(β1)-Flag+AMPK(γ1)-HA), and the current determined utilizing two-electrode voltage-clamp and single channel patch clamp. ROMK protein abundance was measured utilizing chemiluminescence in Xenopus oocytes and western blot in whole kidney tissue. Moreover, renal Na(+) and K(+) excretion were determined in AMPK(α1)-deficient mice (ampk ( -/- )) and wild-type mice (ampk ( +/+ )) prior to and following an acute K(+) load (111 mM KCl, 30 mM NaHCO(3), 4.7 mM NaCl, and 2.25 g/dl BSA) at a rate of 500 μl/h. As a result, coexpression of AMPK(γR70Q) but not of AMPK(αK45R) significantly decreased the current in ROMK1-expressing Xenopus oocytes. Injection of phosphatidylinositol PI((4,5))P(2) significantly increased the current in ROMK1-expressing Xenopus oocytes, an effect reversed in the presence of AMPK(γR70Q). Under control conditions, no significant differences between ampk ( -/- ) and ampk ( +/+ ) mice were observed in glomerular filtration rate (GFR), urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentrations as well as absolute and fractional Na(+) and K(+) excretion. Following an acute K(+) load, GFR, urinary flow rate, serum aldosterone, plasma Na(+), and K(+) concentration were again similar in both genotypes, but renal absolute and fractional Na(+) and K(+) excretion were higher in ampk ( -/- ) than in ampk ( +/+ ) mice. According to micropuncture following a K(+) load, delivery of Na(+) to the early distal tubule but not delivery of K(+) to late proximal and early distal tubules was increased in ampk (-/-) mice. The upregulation of renal ROMK1 protein expression by acute K(+) load was more pronounced in ampk (-/-) than in ampk ( +/+ ) mice. In conclusion, AMPK downregulates ROMK, an effect compromising the ability of the kidney to excrete K(+) following an acute K(+) load.

Downregulation of Kv1.5 K channels by the AMP-activated protein kinase

BACKGROUND

The voltage gated K(+) channel Kv1.5 participates in the repolarization of a wide variety of cell types. Kv1.5 is downregulated during hypoxia, which is known to stimulate the energy-sensing AMP-activated serine/threonine protein kinase (AMPK). AMPK is a powerful regulator of nutrient transport and metabolism. Moreover, AMPK is known to downregulate several ion channels, an effect at least in part due to stimulation of the ubiquitin ligase Nedd4- 2. The present study explored whether AMPK regulates Kv1.5.

METHODS

cRNA encoding Kv1.5 was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (α1 β 1γ1), of constitutively active (γR70Q)AMPK (α1 β 1γ1(R70Q)), of inactive mutant (αK45R)AMPK (α1(K45R)β1γ1), or of Nedd4-2. Kv1.5 activity was determined by two-electrode voltage-clamp. Moreover, Kv1.5 protein abundance in the cell membrane was determined by chemiluminescence and immunostaining with subsequent confocal microscopy.

RESULTS

Coexpression of wild-type AMPK(WT) and constitutively active AMPK(γR70Q), but not of inactive AMPK(αK45R) significantly reduced Kv1.5-mediated currents. Coexpression of constitutively active AMPKγR70Q further reduced Kv1.5 K(+) channel protein abundance in the cell membrane. Co-expression of Nedd4-2 similarly downregulated Kv1.5-mediated currents.

CONCLUSION

AMPK is a potent regulator of Kv1.5. AMPK inhibits Kv1.5 presumably in part by activation of Nedd4- 2 with subsequent clearance of channel protein from the cell membrane.

AMP-activated protein kinase in BK-channel regulation and protection against hearing loss following acoustic overstimulation

The energy-sensing AMP-activated serine/threonine protein kinase (AMPK) confers cell survival in part by stimulation of cellular energy production and limitation of cellular energy utilization. AMPK-sensitive functions further include activities of epithelial Na+ channel ENaC and voltage-gated K+ channel KCNE1/KCNQ1. AMPK is activated by an increased cytosolic Ca2+ concentration. The present study explored whether AMPK regulates the Ca2+-sensitive large conductance and voltage-gated potassium (BK) channel. cRNA encoding BK channel was injected into Xenopus oocytes with and without additional injection of wild-type AMPK (AMPKα1+AMPKβ1+AMPKγ1), constitutively active AMPKγR70Q, or inactive AMPKαK45R. BK-channel activity was determined utilizing the 2-electrode voltage-clamp. Moreover, BK-channel protein abundance in the cell membrane was determined by confocal immunomicroscopy. As BK channels are expressed in outer hair cells (OHC) of the inner ear and lack of BK channels increases noise vulnerability, OHC BK-channel expression was examined by immunohistochemistry and hearing function analyzed by auditory brain stem response measurements in AMPKα1-deficient mice (ampk-/-) and in wild-type mice (ampk+/+). As a result, coexpression of AMPK or AMPKγR70Q but not of AMPKαK45R significantly enhanced BK-channel-mediated currents and BK-channel protein abundance in the oocyte cell membrane. BK-channel expression in the inner ear was lower in ampk-/- mice than in ampk+/+ mice. The hearing thresholds prior to and immediately after an acoustic overexposure were similar in ampk-/- and ampk+/+ mice. However, the recovery from the acoustic trauma was significantly impaired in ampk-/- mice compared to ampk+/+ mice. In summary, AMPK is a potent regulator of BK channels. It may thus participate in the signaling cascades that protect the inner ear from damage following acoustic overstimulation.

Nedd4-2 and the regulation of epithelial sodium transport

Nedd4-2 is a ubiquitin ligase previously demonstrated to regulate endocytosis and lysosomal degradation of the epithelial Na(+) channel (ENaC) and other ion channels and transporters. Recent studies using Nedd4-2 knockout mice specifically in kidney or lung epithelia has revealed a critical role for this E3 ubiquitin ligase in regulating salt and fluid transport in these tissues/organs and in maintaining homeostasis of body blood pressure. Interestingly, the primary targets for Nedd4-2 may differ in these two organs: in the lung Nedd4-2 targets ENaC, and loss of Nedd4-2 leads to excessive ENaC function and to cystic fibrosis - like lung disease, whereas in the kidney, Nedd4-2 targets the Na(+)/Cl(-) cotransporter (NCC) in addition to targeting ENaC. In accord, loss of Nedd4-2 in the distal nephron leads to increased NCC abundance and function. The aldosterone-responsive kinase, Sgk1, appears to be involved in the regulation of NCC by Nedd4-2 in the kidney, similar to its regulation of ENaC. Collectively, these new findings underscore the physiological importance of Nedd4-2 in regulating epithelial salt and fluid transport and balance.

Regulation of Orai1/STIM1 by the kinases SGK1 and AMPK

STIM and Orai isoforms orchestrate store operated Ca2+ entry (SOCE) and thus cytosolic Ca2+ fluctuations following stimulation by hormones, growth factors and further mediators. Orai1 is a target of Nedd4-2, an ubiquitin ligase preparing several plasma membrane proteins for degradation. Phosphorylation of Nedd4-2 by the serum and glucocorticoid inducible kinase SGK1 leads to the binding of Nedd4-2 to the protein 14-3-3 thus preventing its interaction with Orai1. Nedd4-2 is activated by the energy sensing AMP activated kinase AMPK. Thus, SGK1 disrupts and AMPK fosters degradation of Orai1. New synthesis of both, Orai1 and STIM1, is stimulated by the transcription factor NF-κB (nuclear factor kappa B), which binds to the respective promoter regions of the genes encoding STIM1 and Orai1. SGK1 upregulates and AMPK presumably downregulates NF-κB and thus de novo synthesis of Orai1 and STIM1 proteins. The regulation by SGK1 links SOCE to the signaling of a wide variety of hormones and growth factors, the AMPK dependent regulation of Orai1 and STIM1 may serve to limit inadequate activation of SOCE following energy depletion, which is otherwise expected to activate SOCE by depletion of intracellular Ca2+ stores due to impairment of the ATP consuming sarco/endoplasmatic reticulum Ca2+ ATPase SERCA.

Activation of AMP-activated protein kinase stimulates Na+,K+-ATPase activity in skeletal muscle cells

Contraction stimulates Na(+),K(+)-ATPase and AMP-activated protein kinase (AMPK) activity in skeletal muscle. Whether AMPK activation affects Na(+),K(+)-ATPase activity in skeletal muscle remains to be determined. Short term stimulation of rat L6 myotubes with the AMPK activator 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR), activates AMPK and promotes translocation of the Na(+),K(+)-ATPase α(1)-subunit to the plasma membrane and increases Na(+),K(+)-ATPase activity as assessed by ouabain-sensitive (86)Rb(+) uptake. Cyanide-induced artificial anoxia, as well as a direct AMPK activator (A-769662) also increase AMPK phosphorylation and Na(+),K(+)-ATPase activity. Thus, different stimuli that target AMPK concomitantly increase Na(+),K(+)-ATPase activity. The effect of AICAR on Na(+),K(+)-ATPase in L6 myotubes was attenuated by Compound C, an AMPK inhibitor, as well as siRNA-mediated AMPK silencing. The effects of AICAR on Na(+),K(+)-ATPase were completely abolished in cultured primary mouse muscle cells lacking AMPK α-subunits. AMPK stimulation leads to Na(+),K(+)-ATPase α(1)-subunit dephosphorylation at Ser(18), which may prevent endocytosis of the sodium pump. AICAR stimulation leads to methylation and dephosphorylation of the catalytic subunit of the protein phosphatase (PP) 2A in L6 myotubes. Moreover, AICAR-triggered dephosphorylation of the Na(+),K(+)-ATPase was prevented in L6 myotubes deficient in PP2A-specific protein phosphatase methylesterase-1 (PME-1), indicating a role for the PP2A·PME-1 complex in AMPK-mediated regulation of Na(+),K(+)-ATPase. Thus contrary to the common paradigm, we report AMPK-dependent activation of an energy-consuming ion pumping process. This activation may be a potential mechanism by which exercise and metabolic stress activate the sodium pump in skeletal muscle.

Regulation of ENaC biogenesis by the stress response protein SERP1

Cystic fibrosis lung disease is caused by reduced Cl(-) secretion along with enhanced Na(+) absorption, leading to reduced airway surface liquid and compromised mucociliary clearance. Therapeutic strategies have been developed to activate cystic fibrosis transmembrane conductance regulator (CFTR) or to overcome enhanced Na(+) absorption by the epithelial Na(+) channel (ENaC). In a split-ubiquitin-based two-hybrid screening, we identified stress-associated ER protein 1 (SERP1)/ribosome-associated membrane protein 4 as a novel interacting partner for the ENaC β-subunit. SERP1 is induced during cell stress and interacts with the molecular chaperone calnexin, thus controlling early biogenesis of membrane proteins. ENaC activity was measured in the human airway epithelial cell lines H441 and A549 and in voltage clamp experiments with ENaC-overexpressing Xenopus oocytes. We found that expression of SERP1 strongly inhibits amiloride-sensitive Na(+) transport. SERP1 coimmunoprecipitated and colocalized with βENaC in the endoplasmic reticulum, together with the chaperone calnexin. In contrast to the inhibitory effects on ENaC, SERP1 appears to promote expression of CFTR. Taken together, SERP1 is a novel cochaperone and regulator of ENaC expression.

Enhanced Ca²⁺ entry and Na+/Ca²⁺ exchanger activity in dendritic cells from AMP-activated protein kinase-deficient mice

In dendritic cells (DCs), chemotactic chemokines, such as CXCL12, rapidly increase cytosolic Ca(2+)concentrations ([Ca(2+)](i)) by triggering Ca(2+) release from intracellular stores followed by store-operated Ca(2+) (SOC) entry. Increase of [Ca(2+)](i) is blunted and terminated by Ca(2+) extrusion, accomplished by K(+)-independent Na(+)/Ca(2+) exchangers (NCXs) and K(+)-dependent Na(+)/Ca(2+) exchangers (NCKXs). Increased [Ca(2+)](i) activates energy-sensing AMP-activated protein kinase (AMPK), which suppresses proinflammatory responses of DCs and macrophages. The present study explored whether AMPK participates in the regulation of DC [Ca(2+)](i) and migration. DCs were isolated from AMPKα1-deficient (ampk(-/-)) mice and, as control, from their wild-type (ampk(+/+)) littermates. AMPKα1, Orai1-2, STIM1-2, and mitochondrial calcium uniporter protein expression was determined by Western blotting, [Ca(2+)](i) by Fura-2 fluorescence, SOC entry by inhibition of endosomal Ca(2+) ATPase with thapsigargin (1 μM), Na(+)/Ca(2+) exchanger activity from increase of [Ca(2+)](i), and respective whole-cell current in patch clamp following removal of extracellular Na(+). Migration was quantified utilizing transwell chambers. AMPKα1 protein is expressed in ampk(+/+) DCs but not in ampk(-/-) DCs. CXCL12 (300 ng/ml)-induced increase of [Ca(2+)](i), SOC entry, Orai 1 protein abundance, NCX, and NCKX were all significantly higher in ampk(-/-) DCs than in ampk(+/+) DCs. NCX and NCKX currents were similarly increased in ampk(-/-) DCs. Moreover, CXCL12 (50 ng/ml)-induced DC migration was enhanced in ampk(-/-) DCs. AMPK thus inhibits SOC entry, Na(+)/Ca(2+) exchangers, and migration of DCs.

Role of angiotensin II-mediated AMPK inactivation on obesity-related salt-sensitive hypertension

Salt-sensitive hypertension is a characteristic of the metabolic syndrome. Given the links to cardiovascular events, the mechanisms underlying sodium metabolism may represent an important therapeutic target for this disorder. Angiotensin II (AII) is a key peptide underlying sodium retention. However, 5'AMP-activated protein kinase (AMPK) has also been reported to participate in the regulation of ion transport. In this study we examined the relationship between AII and AMPK on the development of hypertension in two salt-sensitive mouse models. In the first model, the mice were maintained on a high-fat diet (HFD) for 12 weeks, in order to develop features similar to the metabolic syndrome, including salt-sensitive hypertension. HFD-induced obese mice showed elevated systolic blood pressure and lower sodium excretion in response to salt loading, along with an increase in AII contents and inactivation of AMPK in the kidney, which were significantly improved by the treatment of an angiotensin II antagonist, losartan, for 2 weeks. To clarify the effects of AII, a second group of mice was infused with AII via an osmotic pump, which led to higher systolic blood pressure, and decreases in urinary sodium excretion and the expression of AMPK, in a manner similar to those observed in the HFD mice. However, treatment with an AMPK activator, metformin, improved the changes induced by the AII, suggesting that AII induced sodium retention works by acting on AMPK activity. Finally, we evaluated the changes in salt-sensitivity by performing 2-week salt loading experiments with or without metformin. AII infusion elevated blood pressure by salt loading but metformin prevented it. These findings indicate that AII suppresses AMPK activity in the kidney, leading to sodium retention and enhanced salt-sensitivity, and that AMPK activation may represent a new therapeutic target for obesity-related salt-sensitive hypertension.

Preactivation of AMPK by metformin may ameliorate the epithelial cell damage caused by renal ischemia

Alterations in epithelial cell polarity and in the subcellular distributions of epithelial ion transport proteins are key molecular consequences of acute kidney injury and intracellular energy depletion. AMP-activated protein kinase (AMPK), a cellular energy sensor, is rapidly activated in response to renal ischemia, and we demonstrate that its activity is upregulated by energy depletion in Madin-Darby canine kidney (MDCK) cells. We hypothesized that AMPK activity may influence the maintenance or recovery of epithelial cell organization in mammalian renal epithelial cells subjected to energy depletion. MDCK cells were ATP depleted through a 1-h incubation with antimycin A and 2-deoxyglucose. Immunofluoresence localization demonstrated that this regimen induces mislocalization of the Na-K-ATPase from its normal residence at the basolateral plasma membrane to intracellular vesicular compartments. When cells were pretreated with the AMPK activator metformin before energy depletion, basolateral localization of Na-K-ATPase was preserved. In MDCK cells in which AMPK expression was stably knocked down with short hairpin RNA, preactivation of AMPK with metformin did not prevent Na-K-ATPase redistribution in response to energy depletion. In vivo studies demonstrate that metformin activated renal AMPK and that treatment with metformin before renal ischemia preserved cellular integrity, preserved Na-K-ATPase localization, and led to reduced levels of neutrophil gelatinase-associated lipocalin, a biomarker of tubular injury. Thus AMPK may play a role in preserving the functional integrity of epithelial plasma membrane domains in the face of energy depletion. Furthermore, pretreatment with an AMPK activator before ischemia may attenuate the severity of renal tubular injury in the context of acute kidney injury.

AMP-activated protein kinase deficiency reduces ozone-induced lung injury and oxidative stress in mice

BACKGROUND

Acute ozone exposure causes lung oxidative stress and inflammation leading to lung injury. At least one mechanism underlying the lung toxicity of ozone involves excessive production of reactive oxygen and nitrogen intermediates such as peroxynitrite. In addition and beyond its major prooxidant properties, peroxynitrite may nitrate tyrosine residues altering phosphorylation of many protein kinases involved in cell signalling. It was recently proposed that peroxynitrite activates 5'-AMP-activated kinase (AMPK), which regulates metabolic pathways and the response to cell stress. AMPK activation as a consequence of ozone exposure has not been previously evaluated. First, we tested whether acute ozone exposure in mice would impair alveolar fluid clearance, increase lung tissue peroxynitrite production and activate AMPK. Second, we tested whether loss of AMP-activated protein kinase alpha1 subunit in mouse would prevent enhanced oxidative stress and lung injury induced by ozone exposure.

METHODS

Control and AMPKα1 deficient mice were exposed to ozone at a concentration of 2.0 ppm for 3 h in glass cages. Evaluation was performed 24 h after ozone exposure. Alveolar fluid clearance (AFC) was evaluated using fluorescein isothiocyanate tagged albumin. Differential cell counts, total protein levels, cytokine concentrations, myeloperoxidase activity and markers of oxidative stress, i.e. malondialdehyde and peroxynitrite, were determined in bronchoalveolar lavage (BAL) and lung homogenates (LH). Levels of AMPK-Thr172 phosphorylation and basolateral membrane Na(+)-K(+)-ATPase abundance were determined by Western blot.

RESULTS

In control mice, ozone exposure induced lung inflammation as evidence by increased leukocyte count, protein concentration in BAL and myeloperoxidase activity, pro-inflammatory cytokine levels in LH. Increases in peroxynitrite levels (3 vs 4.4 nM, p = 0.02) and malondialdehyde concentrations (110 vs 230 μmole/g wet tissue) were detected in LH obtained from ozone-exposed control mice. Ozone exposure consistently increased phosphorylated AMPK-Thr172 to total AMPK ratio by 80% in control mice. Ozone exposure causes increases in AFC and basolateral membrane Na(+)-K(+)-ATPase abundance in control mice which did not occur in AMPKα1 deficient mice.

CONCLUSIONS

Our results collectively suggest that AMPK activation participates in ozone-induced increases in AFC, inflammation and oxidative stress. Further studies are needed to understand how the AMPK pathway may provide a novel approach for the prevention of ozone-induced lung injury.

Inhibition of Kir2.1 (KCNJ2) by the AMP-activated protein kinase

The inward rectifier K(+) channel Kir2.1 participates in the maintenance of the cell membrane potential in a variety of cells including neurons and cardiac myocytes. Mutations of KCNJ2 encoding Kir2.1 underlie the Andersen-Tawil syndrome, a rare disorder clinically characterized by periodic paralysis, cardiac arrhythmia and skeletal abnormalities. The maintenance of the cardiac cell membrane potential is decreased in ischaemia, which is known to stimulate the AMP-activated serine/threonine protein kinase (AMPK). This energy-sensing kinase stimulates energy production and limits energy utilization. The present study explored whether AMPK regulates Kir2.1. To this end, cRNA encoding Kir2.1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1+AMPKβ1+AMPKγ1), of the constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), of the kinase dead mutant (αK45R)AMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. Kir2.1 activity was determined in two-electrode voltage-clamp experiments. Moreover, Kir2.1 protein abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced Kir2.1-mediated currents and Kir2.1 protein abundance in the cell membrane. Expression of wild type Nedd4-2 or of Nedd4-2(S795A) lacking an AMPK phosphorylation consensus sequence downregulated Kir2.1 currents. The effect of wild type Nedd4-2 but not of Nedd4-2(S795A) was significantly augmented by additional coexpression of AMPK. In conclusion, AMPK is a potent regulator of Kir2.1. AMPK is at least partially effective through phosphorylation of the ubiquitin ligase Nedd4-2.

Activating AMP-activated protein kinase (AMPK) slows renal cystogenesis

Renal cyst development and expansion in autosomal dominant polycystic kidney disease (ADPKD) involves both fluid secretion and abnormal proliferation of cyst-lining epithelial cells. The chloride channel of the cystic fibrosis transmembrane conductance regulator (CFTR) participates in secretion of cyst fluid, and the mammalian target of rapamycin (mTOR) pathway may drive proliferation of cyst epithelial cells. CFTR and mTOR are both negatively regulated by AMP-activated protein kinase (AMPK). Metformin, a drug in wide clinical use, is a pharmacological activator of AMPK. We find that metformin stimulates AMPK, resulting in inhibition of both CFTR and the mTOR pathways. Metformin induces significant arrest of cystic growth in both in vitro and ex vivo models of renal cystogenesis. In addition, metformin administration produces a significant decrease in the cystic index in two mouse models of ADPKD. Our results suggest a possible role for AMPK activation in slowing renal cystogenesis as well as the potential for therapeutic application of metformin in the context of ADPKD.

Inhibition of the heterotetrameric K+ channel KCNQ1/KCNE1 by the AMP-activated protein kinase

The heterotetrameric K(+)-channel KCNQ1/KCNE1 is expressed in heart, skeletal muscle, liver and several epithelia including the renal proximal tubule. In the heart, it contributes to the repolarization of cardiomyocytes. The repolarization is impaired in ischemia. Ischemia stimulates the AMP-activated protein kinase (AMPK), a serine/threonine kinase, sensing energy depletion and stimulating several cellular mechanisms to enhance energy production and to limit energy utilization. AMPK has previously been shown to downregulate the epithelial Na(+) channel ENaC, an effect mediated by the ubiquitin ligase Nedd4-2. The present study explored whether AMPK regulates KCNQ1/KCNE1. To this end, cRNA encoding KCNQ1/KCNE1 was injected into Xenopus oocytes with and without additional injection of wild type AMPK (AMPKα1 + AMPKβ1 + AMPKγ1), of the constitutively active (γR70Q)AMPK (α1β1γ1(R70Q)), of the kinase dead mutant (αK45R)AMPK (α1(K45R)β1γ1), or of the ubiquitin ligase Nedd4-2. KCNQ1/KCNE1 activity was determined in two electrode voltage clamp experiments. Moreover, KCNQ1 abundance in the cell membrane was determined by immunostaining and subsequent confocal imaging. As a result, wild type and constitutively active AMPK significantly reduced KCNQ1/KCNE1-mediated currents and reduced KCNQ1 abundance in the cell membrane. Similarly, Nedd4-2 decreased KCNQ1/KCNE1-mediated currents and KCNQ1 protein abundance in the cell membrane. Activation of AMPK in isolated perfused proximal renal tubules by AICAR (10 mM) was followed by significant depolarization. In conclusion, AMPK is a potent regulator of KCNQ1/KCNE1.

Molecular mechanisms of ubiquitin-dependent membrane traffic

Over the past 14 years, ubiquitination has emerged as a centrally important mechanism governing the subcellular trafficking of proteins. Ubiquitination, interaction with sorting factors that contain ubiquitin-binding domains, and deubiquitination govern the itineraries of cargo proteins that include yeast carboxypeptidase S, the epithelial sodium channel ENaC, and epidermal growth factor receptor. The molecular structures and mechanisms of the paradigmatic HECT and RING domain ubiquitin ligases, of JAMM- and USP-domain-deubiquitinating enzymes, and of numerous ubiquitin-binding domains involved in these pathways have been worked out in recent years and are described.

Effect of Annexin A5 on CFTR: regulated traffic or scaffolding?

Previous studies have implicated annexins in regulating ion channels and in particular annexin A5 (AnxA5) in the traffic of the cystic fibrosis transmembrane conductance regulator (CFTR). In the present study, we further investigated the role of AnxA5 in regulating CFTR function and intracellular trafficking in both Xenopus oocytes and mammalian cells. Although we could confirm the previously reported CFTR/AnnxA5 interaction, we found that in oocytes AnxA5 inhibits CFTR-mediated whole-cell membrane conductance presumably by a mechanism independent of PDZ-binding domain at the C-terminus of CFTR but protein kinase C (PKC)-dependent and results from either endocytosis activation and/or exocytosis block. In contrast, in human cells, co-expression of AnxA5 augmented CFTR whole-cell currents, an effect that was independent of CFTR PDZ-binding domain. We conclude that annexin A5 has multiple effects on CFTR, so that the net effect observed is cell system-dependent. Nevertheless, both effects observed here are consistent with the described role of annexins forming scaffolding platforms at cell membranes, thus contributing to a decrease in their dynamics. Finally, we could not confirm that AnxA5 overexpression rescues traffic/function of the most frequent disease-causing mutant F508del-CFTR, thus concluding that AnxA5 is not a promising tool for correction of the F508del-CFTR defect.