ERK1/2 Mediates Insulin Stimulation of Na,K-ATPase by Phosphorylation of the α-Subunit in Human Skeletal Muscle Cells

The Na+/K+-ATPase: A potential therapeutic target in cardiometabolic diseases

Cardiometabolic diseases (CMD) are a direct consequence of modern living and contribute to the development of multisystem diseases such as cardiovascular diseases and diabetes mellitus (DM). CMD has reached epidemic proportions worldwide. A sodium pump (Na⁺/K⁺-ATPase) is found in most eukaryotic cells' membrane and controls many essential cellular functions directly or indirectly. This ion transporter and its isoforms are important in the pathogenesis of some pathological processes, including CMD. The structure and function of Na⁺/K⁺-ATPase, its expression and distribution in tissues, and its interactions with known ligands such as cardiotonic steroids and other suspected endogenous regulators are discussed in this review. In addition, we reviewed recent literature data related to the involvement of Na⁺/K⁺-ATPase activity dysfunction in CMD, focusing on the Na⁺/K⁺-ATPase as a potential therapeutic target in CMD.

Gi/o protein-coupled receptor inhibition of beta-cell electrical excitability and insulin secretion depends on Na+/K+ ATPase activation

Gi/o-coupled somatostatin or α2-adrenergic receptor activation stimulated β-cell NKA activity, resulting in islet Ca2+ fluctuations. Furthermore, intra-islet paracrine activation of β-cell Gi/o-GPCRs and NKAs by δ-cell somatostatin secretion slowed Ca2+ oscillations, which decreased insulin secretion. β-cell membrane potential hyperpolarization resulting from Gi/o-GPCR activation was dependent on NKA phosphorylation by Src tyrosine kinases. Whereas, β-cell NKA function was inhibited by cAMP-dependent PKA activity. These data reveal that NKA-mediated β-cell membrane potential hyperpolarization is the primary and conserved mechanism for Gi/o-GPCR control of electrical excitability, Ca2+ handling, and insulin secretion.

Potassium Concentration in Initial Fluid Therapy and In-Hospital Mortality of Patients with Diabetic Ketoacidosis

CONTEXT

Guidelines worldwide recommend potassium replacement of 10 to 40 mmol/L in the initial fluid therapy for patients with diabetic ketoacidosis. However, evidence is lacking as to the association between infused potassium concentration and mortality.

OBJECTIVE

We aimed to determine the association between infused potassium concentration and in-hospital mortality.

METHODS

Using the Japanese Diagnosis Procedure Combination database, we retrospectively identified inpatients admitted for treatment of diabetic ketoacidosis from July 2010 to March 2018. Patients with kidney dysfunction or serum potassium abnormalities were excluded. We evaluated the association of the potassium concentration in the total infused solutions in the first 2 days of hospitalization with 28-day in-hospital mortality using multivariable regression analysis with a cubic spline model. We also assessed the association between potassium concentration and occurrence of hyperkalemia.

RESULTS

We identified 14 216 patients with diabetic ketoacidosis and observed 261 deaths. The quartile cut-points for potassium concentration were 7.7, 11.4, and 16.1 mmol/L. Within the range of approximately 10 to 40 mmol/L, potassium concentration was not associated with occurrence of hyperkalemia or death. Lower potassium concentrations were associated with higher 28-day in-hospital mortality; the odds ratio for patients receiving 8 mmol/L was 1.69 (95% CI, 1.03 to 2.78; reference: 20 mmol/L), and the odds ratio increased monotonically as potassium concentration decreased further.

CONCLUSION

Patients receiving potassium replacement at concentrations of 10 to 40 mmol/L had similar in-hospital mortality rates, whereas lower concentrations were associated with higher mortality.

Effect of differentiation, de novo innervation, and electrical pulse stimulation on mRNA and protein expression of Na+,K+-ATPase, FXYD1, and FXYD5 in cultured human skeletal muscle cells

Denervation reduces the abundance of Na⁺,K⁺-ATPase (NKA) in skeletal muscle, while reinnervation increases it. Primary human skeletal muscle cells, the most widely used model to study human skeletal muscle in vitro, are usually cultured as myoblasts or myotubes without neurons and typically do not contract spontaneously, which might affect their ability to express and regulate NKA. We determined how differentiation, de novo innervation, and electrical pulse stimulation affect expression of NKA (α and β) subunits and NKA regulators FXYD1 (phospholemman) and FXYD5 (dysadherin). Differentiation of myoblasts into myotubes under low serum conditions increased expression of myogenic markers CD56 (NCAM1), desmin, myosin heavy chains, dihydropyridine receptor subunit α_1S, and SERCA2 as well as NKAα2 and FXYD1, while it decreased expression of FXYD5 mRNA. Myotubes, which were innervated de novo by motor neurons in co-culture with the embryonic rat spinal cord explants, started to contract spontaneously within 7–10 days. A short-term co-culture (10–11 days) promoted mRNA expression of myokines, such as IL-6, IL-7, IL-8, and IL-15, but did not affect mRNA expression of NKA, FXYDs, or myokines, such as musclin, cathepsin B, meteorin-like protein, or SPARC. A long-term co-culture (21 days) increased the protein abundance of NKAα1, NKAα2, FXYD1, and phospho-FXYD1^Ser68 without attendant changes in mRNA levels. Suppression of neuromuscular transmission with α-bungarotoxin or tubocurarine for 24 h did not alter NKA or FXYD mRNA expression. Electrical pulse stimulation (48 h) of non-innervated myotubes promoted mRNA expression of NKAβ2, NKAβ3, FXYD1, and FXYD5. In conclusion, low serum concentration promotes NKAα2 and FXYD1 expression, while de novo innervation is not essential for upregulation of NKAα2 and FXYD1 mRNA in cultured myotubes. Finally, although innervation and EPS both stimulate contractions of myotubes, they exert distinct effects on the expression of NKA and FXYDs.

Measuring enzyme activities in crude homogenates: Na+/K+-ATPase as a case study in optimizing assays

In this review of assays of Na⁺/K⁺-ATPase (NKA), we explore the choices made by researchers assaying the enzyme to investigate its role in physiological regulation. We survey NKA structure and function in the context of how it is typically assayed, and how technical choices influence what can be said about the enzyme. In comparing different methods for extraction and assay of NKA, we identified a series of common pitfalls that compromise the veracity of results. We include experimental work to directly demonstrate how choices in detergents, salts and substrates influence NKA activities measured in crude homogenates. Our review of assay approaches integrates what is known from enzymology, biomedical physiology, cell biology and evolutionary biology, offering a more robust method for assaying the enzyme in meaningful ways, identifying caveats and future directions to explore its structure and function. The goal is to provide the sort of background on the enzyme that should be considered in exploring the function of the enzyme in comparative physiology.

The role of AMPK in regulation of Na+,K+-ATPase in skeletal muscle: does the gauge always plug the sink?

AMP-activated protein kinase (AMPK) is a cellular energy gauge and a major regulator of cellular energy homeostasis. Once activated, AMPK stimulates nutrient uptake and the ATP-producing catabolic pathways, while it suppresses the ATP-consuming anabolic pathways, thus helping to maintain the cellular energy balance under energy-deprived conditions. As much as ~ 20-25% of the whole-body ATP consumption occurs due to a reaction catalysed by Na⁺,K⁺-ATPase (NKA). Being the single most important sink of energy, NKA might seem to be an essential target of the AMPK-mediated energy saving measures, yet NKA is vital for maintenance of transmembrane Na⁺ and K⁺ gradients, water homeostasis, cellular excitability, and the Na⁺-coupled transport of nutrients and ions. Consistent with the model that AMPK regulates ATP consumption by NKA, activation of AMPK in the lung alveolar cells stimulates endocytosis of NKA, thus suppressing the transepithelial ion transport and the absorption of the alveolar fluid. In skeletal muscles, contractions activate NKA, which opposes a rundown of transmembrane ion gradients, as well as AMPK, which plays an important role in adaptations to exercise. Inhibition of NKA in contracting skeletal muscle accentuates perturbations in ion concentrations and accelerates development of fatigue. However, different models suggest that AMPK does not inhibit or even stimulates NKA in skeletal muscle, which appears to contradict the idea that AMPK maintains the cellular energy balance by always suppressing ATP-consuming processes. In this short review, we examine the role of AMPK in regulation of NKA in skeletal muscle and discuss the apparent paradox of AMPK-stimulated ATP consumption.

Ouabain Suppresses IL-6/STAT3 Signaling and Promotes Cytokine Secretion in Cultured Skeletal Muscle Cells

The cardiotonic steroids (CTS), such as ouabain and marinobufagenin, are thought to be adrenocortical hormones secreted during exercise and the stress response. The catalytic α-subunit of Na,K-ATPase (NKA) is a CTS receptor, whose largest pool is located in skeletal muscles, indicating that muscles are a major target for CTS. Skeletal muscles contribute to adaptations to exercise by secreting interleukin-6 (IL-6) and plethora of other cytokines, which exert paracrine and endocrine effects in muscles and non-muscle tissues. Here, we determined whether ouabain, a prototypical CTS, modulates IL-6 signaling and secretion in the cultured human skeletal muscle cells. Ouabain (2.5–50 nM) suppressed the abundance of STAT3, a key transcription factor downstream of the IL-6 receptor, as well as its basal and IL-6-stimulated phosphorylation. Conversely, ouabain (50 nM) increased the phosphorylation of ERK1/2, Akt, p70S6K, and S6 ribosomal protein, indicating activation of the ERK1/2 and the Akt-mTOR pathways. Proteasome inhibitor MG-132 blocked the ouabain-induced suppression of the total STAT3, but did not prevent the dephosphorylation of STAT3. Ouabain (50 nM) suppressed hypoxia-inducible factor-1α (HIF-1α), a modulator of STAT3 signaling, but gene silencing of HIF-1α and/or its partner protein HIF-1β did not mimic effects of ouabain on the phosphorylation of STAT3. Ouabain (50 nM) failed to suppress the phosphorylation of STAT3 and HIF-1α in rat L6 skeletal muscle cells, which express the ouabain-resistant α1-subunit of NKA. We also found that ouabain (100 nM) promoted the secretion of IL-6, IL-8, GM-CSF, and TNF-α from the skeletal muscle cells of healthy subjects, and the secretion of GM-CSF from cells of subjects with the type 2 diabetes. Marinobufagenin (10 nM), another important CTS, did not alter the secretion of these cytokines. In conclusion, our study shows that ouabain suppresses the IL-6 signaling via STAT3, but promotes the secretion of IL-6 and other cytokines, which might represent a negative feedback in the IL-6/STAT3 pathway. Collectively, our results implicate a role for CTS and NKA in regulation of the IL-6 signaling and secretion in skeletal muscle.

Another Consequence of the Warburg Effect? Metabolic Regulation of Na+/H+ Exchangers May Link Aerobic Glycolysis to Cell Growth

To adjust cell growth and proliferation to changing environmental conditions or developmental requirements, cells have evolved a remarkable network of signaling cascades that integrates cues from cellular metabolism, growth factor availability and a large variety of stresses. In these networks, cellular information flow is mostly mediated by posttranslational modifications, most notably phosphorylation, or signaling molecules such as GTPases. Yet, a large body of evidence also implicates cytosolic pH (pHc) as a highly conserved cellular signal driving cell growth and proliferation, suggesting that pH-dependent protonation of specific proteins also regulates cellular signaling. In mammalian cells, pHc is regulated by growth factor derived signals and responds to metabolic cues in response to glucose stimulation. Importantly, high pHc has also been identified as a hall mark of cancer, but mechanisms of pH regulation in cancer are only poorly understood. Here, we discuss potential mechanisms of pH regulation with emphasis on metabolic signals regulating pHc by Na⁺/H⁺-exchangers. We hypothesize that elevated NHE activity and pHc in cancer are a direct consequence of the metabolic adaptations in tumor cells including enhanced aerobic glycolysis, generally referred to as the Warburg effect. This hypothesis not only provides an explanation for the growth advantage conferred by a switch to aerobic glycolysis beyond providing precursors for accumulation of biomass, but also suggests that treatments targeting pH regulation as a potential anti-cancer therapy may effectively target the result of altered tumor cell metabolism.

Reduced Na+ K+ -ATPase activity may reduce amino acid uptake in intrauterine growth restricted fetal sheep muscle despite unchanged ex vivo amino acid transporter activity

KEY POINTS

Fetuses with intrauterine growth restriction (IUGR) have reduced muscle mass that persists postnatally, which may contribute to their increased risk for adult onset metabolic diseases, such as diabetes and obesity. Amino acid transporter-mediated histidine uptake and system L amino acid transporter activity were similar in sarcolemmal membranes isolated from control and IUGR hindlimb skeletal muscle. Activity of Na⁺ K⁺ -ATPase, which is responsible for establishing the sodium gradient necessary for system A and N amino acid transporter function, was significantly reduced in IUGR skeletal muscle sarcolemma compared to control. ATP content was lower in IUGR skeletal muscle. Expression and phosphorylation of proteins in the mechanistic target of rapamycin pathway were similar in control and IUGR skeletal muscle homogenate. Our data suggest that lower Na⁺ K⁺ -ATPase activity, which reduces the driving force for active amino acid transport, and lower ATP availability contribute to reduced amino acid uptake and protein synthesis in IUGR fetal skeletal muscle.

ABSTRACT

Fetuses with intrauterine growth restriction (IUGR) have lower muscle mass that persists postnatally. Using a sheep model of placental insufficiency and IUGR, we have previously demonstrated lower net total uptake of amino acids by the fetal hindlimb and lower skeletal muscle protein synthesis rates. To investigate the mechanisms underlying these changes, we tested the hypothesis that ex vivo amino acid transporter and Na⁺ K⁺ -ATPase activity is reduced, and ex vivo ATP levels are lower in hindlimb skeletal muscle of the IUGR fetus. We developed a novel protocol to measure transporter-mediated histidine uptake, system L amino acid transporter activity and Na⁺ K⁺ -ATPase activity using sarcolemmal membranes isolated from hindlimb muscle of control (CON, n = 11-12) and IUGR (n = 12) late gestation fetal sheep. We also determined ATP content and the activity of insulin and mechanistic target of rapamycin (mTOR) signalling, which are involved in regulating cellular amino acid uptake and protein synthesis, by measuring the expression and phosphorylation of AKT, 4E-BP1, eIF2α, AMPKα, p70 S6 kinase and rpS6 in muscle homogenates. Transporter-mediated histidine uptake and system L activity were similar in control and IUGR sarcolemma, although ex vivo Na⁺ K⁺ -ATPase activity was lower by 64% (P = 0.019) in IUGR sarcolemma. ATP content was lower by 25% (P = 0.007) in IUGR muscle. Insulin, AMPK, and mTOR signalling activity was similar in control and IUGR muscle. We speculate that reduced muscle sarcolemmal Na⁺ K⁺ -ATPase activity and lower ATP content diminishes the sodium gradient in vivo, resulting in a reduced driving force for sodium-dependent transporters and subsequently lower muscle amino acid uptake.

Anisodamine Ameliorates Hyperkalemia during Crush Syndrome through Estradiol-Induced Enhancement of Insulin Sensitivity

Hyperkalemia is a major cause of on-site death in crush syndrome (CS), which is more severe and common in male victims. Anisodamine is a belladonna alkaloid and widely used in China for treatment of shock through activation of α7 nicotinic acetylcholine receptor (α7nAChR). The present work was designed to study the protective effect of anisodamine in CS and the possible role of estradiol involved. Male and ovariectomized female CS mice exhibited lower serum estradiol and insulin sensitivity, and higher potassium compared to the relative female controls at 6 h after decompression. There was no gender difference in on-site mortality in CS mice within 24 h after decompression. Serum estradiol increased with similar values in CS mice of both gender compared to that in normal mice. Anisodamine decreased serum potassium and increased serum estradiol and insulin sensitivity in CS mice, and methyllycaconitine, selective antagonist of α7nAChR, counteracted such effects of anisodamine. Treatment with anisodamine or estradiol increased serum estradiol and insulin sensitivity, decreased serum potassium and on-site mortality, and eliminated the difference in these parameters between CS mice received ovariectomy or its sham operation. Anisodamine could also increase blood pressure in CS rats within 3.5 h after decompression, which could also be attenuated by methyllycaconitine, without influences on heart rate. These results suggest that activation of α7nAChR with anisodamine could decrease serum potassium and on-site mortality in CS through estradiol-induced enhancement of insulin sensitivity.

Unique Regulation of Na-K-ATPase during Growth and Maturation of Intestinal Epithelial Cells

Na-K-ATPase on the basolateral membrane provides the favorable transcellular Na gradient for the proper functioning of Na-dependent nutrient co-transporters on the brush border membrane (BBM) of enterocytes. As cells mature from crypts to villus, Na-K-ATPase activity doubles, to accommodate for the increased BBM Na-dependent nutrient absorption. However, the mechanism of increased Na-K-ATPase activity during the maturation of enterocytes is not known. Therefore, this study aimed to determine the mechanisms involved in the functional transition of Na-K-ATPase during the maturation of crypts to villus cells. Na-K-ATPase activity gradually increased as IEC-18 cells matured in vitro from day 0 (crypts) through day 4 (villus) of post-confluence. mRNA abundance and Western blot studies showed no change in the levels of Na-K-ATPase subunits α1 and β1 from 0 to 4 days post-confluent cells. However, Na-K-ATPase α1 phosphorylation levels on serine and tyrosine, but not threonine, residues gradually increased. These data indicate that as enterocytes mature from crypt-like to villus-like in culture, the functional activity of Na-K-ATPase increases secondary to altered affinity of the α1 subunit to extracellular K⁺, in order to accommodate the functional preference of the intestinal cell type. This altered affinity is likely due to increased phosphorylation of the α1 subunit, specifically at serine and tyrosine residues.

Hormonal regulation of Na+-K+-ATPase from the evolutionary perspective

Na⁺-K⁺-ATPase, an α/β heterodimer, is an ancient enzyme that maintains Na⁺ and K⁺ gradients, thus preserving cellular ion homeostasis. In multicellular organisms, this basic housekeeping function is integrated to fulfill the needs of specialized organs and preserve whole-body homeostasis. In vertebrates, Na⁺-K⁺-ATPase is essential for many fundamental physiological processes, such as nerve conduction, muscle contraction, nutrient absorption, and urine excretion. During vertebrate evolution, three key developments contributed to diversification and integration of Na⁺-K⁺-ATPase functions. Generation of novel α- and β-subunits led to formation of multiple Na⁺-K⁺-ATPase isoenyzmes with distinct functional characteristics. Development of a complex endocrine system enabled efficient coordination of diverse Na⁺-K⁺-ATPase functions. Emergence of FXYDs, small transmembrane proteins that regulate Na⁺-K⁺-ATPase, opened new ways to modulate its function. FXYDs are a vertebrate innovation and an important site of hormonal action, suggesting they played an especially prominent role in evolving interaction between Na⁺-K⁺-ATPase and the endocrine system in vertebrates.

Chemical genetic identification of GAK substrates reveals its role in regulating Na+/K+-ATPase

Novel GAK phosphorylation targets are identified using chemical genetic methods. One of the substrates is the α subunit of the Na⁺/K⁺-ATPase, phosphorylation of which is necessary for its surface trafficking from endosomes. Conserved functions of NAK family kinases are described.

Cyclin G–associated kinase (GAK) is a ubiquitous serine/threonine kinase that facilitates clathrin uncoating during vesicle trafficking. GAK phosphorylates a coat adaptor component, AP2M1, to help achieve this function. GAK is also implicated in Parkinson's disease through genome-wide association studies. However, GAK's role in mammalian neurons remains unclear, and insight may come from identification of further substrates. Employing a chemical genetics method, we show here that the sodium potassium pump (Na⁺/K⁺-ATPase) α-subunit Atp1a3 is a GAK target and that GAK regulates Na⁺/K⁺-ATPase trafficking to the plasma membrane. Whole-cell patch clamp recordings from CA1 pyramidal neurons in GAK conditional knockout mice show a larger change in resting membrane potential when exposed to the Na⁺/K⁺-ATPase blocker ouabain, indicating compromised Na⁺/K⁺-ATPase function in GAK knockouts. Our results suggest a modulatory role for GAK via phosphoregulation of substrates such as Atp1a3 during cargo trafficking.

Tubulin-Na+ , K + -ATPase interaction: Involvement in enzymatic regulation and cellular function

A new function for tubulin was described by our laboratory: acetylated tubulin forms a complex with Na⁺ ,K ⁺ -ATPase (NKA) and inhibits its activity. This process was shown to be a regulatory factor of physiological importance in cultured cells, human erythrocytes, and several rat tissues. Formation of the acetylated tubulin-NKA complex is reversible. We demonstrated that in cultured cells, high concentrations of glucose induce translocation of acetylated tubulin from cytoplasm to plasma membrane with a consequent inhibition of NKA activity. This effect is reversed by adding glutamate, which is coctransported to the cell with Na ⁺ . Another posttranslational modification of tubulin, detyrosinated tubulin, is also involved in the regulation of NKA activity: it enhances the NKA inhibition induced by acetylated tubulin. Manipulation of the content of these modifications of tubulin could work as a new strategy to maintain homeostasis of Na ⁺ and K ⁺ , and to regulate a variety of functions in which NKA is involved, such as osmotic fragility and deformability of human erythrocytes. The results summarized in this review show that the interaction between tubulin and NKA plays an important role in cellular physiology, both in the regulation of Na ⁺ /K ⁺ homeostasis and in the rheological properties of the cells, which is mechanically different from other roles reported up to now.

Src Family Kinase Links Insulin Signaling to Short Term Regulation of Na,K-ATPase in Nonpigmented Ciliary Epithelium

Insulin has been shown to elicit changes of Na,K-ATPase activity in various tissues. Na,K-ATPase in the nonpigmented ciliary epithelium (NPE) plays a role in aqueous humor secretion and changes of Na,K-ATPase activity impact the driving force. Because we detect a change of NPE Na,K-ATPase activity in response to insulin, studies were carried out to examine the response mechanism. Ouabain-sensitive rubidium (Rb) uptake by cultured NPE cells, measured as a functional index of Na,K-ATPase-mediated inward potassium transport, was found to increase in cells exposed for 5 min to insulin. The maximally effective concentration was 100 nM. An intrinsic increase of Na,K-ATPase activity evident as a >2-fold increase in the rate of ouabain-sensitive ATP hydrolysis in homogenates obtained from cells exposed to 100 nM insulin for 5 min was also observed. Insulin-treated cells exhibited Akt, Src family kinase (SFK), ERK1/2, and p38 activation, all of which were prevented by a pI3 kinase inhibitor LY294002. The Rb uptake and Na,K-ATPase activity response to insulin both were abolished by PP2, an SFK inhibitor which also prevented p38 and ERK1/2 but not Akt activation. The Akt inhibitor MK-2206 did not change the Na,K-ATPase response to insulin. The findings suggest insulin activates pI3K-dependent Akt and SFK signaling pathways that are separate. ERK1/2 and p38 activation is secondary to and dependent on SFK activation. The increase of Na,K-ATPase activity is dependent on activation of the SFK pathway. The findings are consistent with previous studies that indicate a link between Na,K-ATPase activity and SFK signaling. J. Cell. Physiol. 232: 1489-1500, 2017. © 2016 Wiley Periodicals, Inc.

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.

Nutrient Non-equivalence: Does Restricting High-Potassium Plant Foods Help to Prevent Hyperkalemia in Hemodialysis Patients?

Hemodialysis patients are often advised to limit their intake of high-potassium foods to help manage hyperkalemia. However, the benefits of this practice are entirely theoretical and not supported by rigorous randomized controlled trials. The hypothesis that potassium restriction is useful is based on the assumption that different sources of dietary potassium are therapeutically equivalent. In fact, animal and plant sources of potassium may differ in their potential to contribute to hyperkalemia. In this commentary, we summarize the historical research basis for limiting high-potassium foods. Ultimately, we conclude that this approach is not evidence-based and may actually present harm to patients. However, given the uncertainty arising from the paucity of conclusive data, we agree that until the appropriate intervention studies are conducted, practitioners should continue to advise restriction of high-potassium foods.

Na,K-ATPase regulation in skeletal muscle

Skeletal muscle contains one of the largest and the most dynamic pools of Na,K-ATPase (NKA) in the body. Under resting conditions, NKA in skeletal muscle operates at only a fraction of maximal pumping capacity, but it can be markedly activated when demands for ion transport increase, such as during exercise or following food intake. Given the size, capacity, and dynamic range of the NKA pool in skeletal muscle, its tight regulation is essential to maintain whole body homeostasis as well as muscle function. To reconcile functional needs of systemic homeostasis with those of skeletal muscle, NKA is regulated in a coordinated manner by extrinsic stimuli, such as hormones and nerve-derived factors, as well as by local stimuli arising in skeletal muscle fibers, such as contractions and muscle energy status. These stimuli regulate NKA acutely by controlling its enzymatic activity and/or its distribution between the plasma membrane and the intracellular storage compartment. They also regulate NKA chronically by controlling NKA gene expression, thus determining total NKA content in skeletal muscle and its maximal pumping capacity. This review focuses on molecular mechanisms that underlie regulation of NKA in skeletal muscle by major extrinsic and local stimuli. Special emphasis is given to stimuli and mechanisms linking regulation of NKA and energy metabolism in skeletal muscle, such as insulin and the energy-sensing AMP-activated protein kinase. Finally, the recently uncovered roles for glutathionylation, nitric oxide, and extracellular K(+) in the regulation of NKA in skeletal muscle are highlighted.

Activation of α7 Nicotinic Acetylcholine Receptor Decreases On-site Mortality in Crush Syndrome through Insulin Signaling-Na/K-ATPase Pathway

On-site mortality in crush syndrome remains high due to lack of effective drugs based on definite diagnosis. Anisodamine (Ani) is widely used in China for treatment of shock, and activation of α7 nicotinic acetylcholine receptor (α7nAChR) mediates such antishock effect. The present work was designed to test whether activation of α7nAChR with Ani decreased mortality in crush syndrome shortly after decompression. Sprague-Dawley rats and C57BL/6 mice with crush syndrome were injected with Ani (20 mg/kg and 28 mg/kg respectively, i.p.) 30 min before decompression. Survival time, serum potassium, insulin, and glucose levels were observed shortly after decompression. Involvement of α7nAChR was verified with methyllycaconitine (selective α7nAChR antagonist) and PNU282987 (selective α7nAChR agonist), or in α7nAChR knockout mice. Effect of Ani was also appraised in C2C12 myotubes. Ani reduced mortality and serum potassium and enhanced insulin sensitivity shortly after decompression in animals with crush syndrome, and PNU282987 exerted similar effects. Such effects were counteracted by methyllycaconitine or in α7nAChR knockout mice. Mortality and serum potassium in rats with hyperkalemia were also reduced by Ani. Phosphorylation of Na/K-ATPase was enhanced by Ani in C2C12 myotubes. Inhibition of tyrosine kinase on insulin receptor, phosphoinositide 3-kinase, mammalian target of rapamycin, signal transducer and activator of transcription 3, and Na/K-ATPase counteracted the effect of Ani on extracellular potassium. These findings demonstrated that activation of α7nAChR could decrease on-site mortality in crush syndrome, at least in part based on the decline of serum potassium through insulin signaling-Na/K-ATPase pathway.

Imbalanced insulin action in chronic over nutrition: Clinical harm, molecular mechanisms, and a way forward

The growing worldwide prevalence of overnutrition and underexertion threatens the gains that we have made against atherosclerotic cardiovascular disease and other maladies. Chronic overnutrition causes the atherometabolic syndrome, which is a cluster of seemingly unrelated health problems characterized by increased abdominal girth and body-mass index, high fasting and postprandial concentrations of cholesterol- and triglyceride-rich apoB-lipoproteins (C-TRLs), low plasma HDL levels, impaired regulation of plasma glucose concentrations, hypertension, and a significant risk of developing overt type 2 diabetes mellitus (T2DM). In addition, individuals with this syndrome exhibit fatty liver, hypercoagulability, sympathetic overactivity, a gradually rising set-point for body adiposity, a substantially increased risk of atherosclerotic cardiovascular morbidity and mortality, and--crucially--hyperinsulinemia. Many lines of evidence indicate that each component of the atherometabolic syndrome arises, or is worsened by, pathway-selective insulin resistance and responsiveness (SEIRR). Individuals with SEIRR require compensatory hyperinsulinemia to control plasma glucose levels. The result is overdrive of those pathways that remain insulin-responsive, particularly ERK activation and hepatic de-novo lipogenesis (DNL), while carbohydrate regulation deteriorates. The effects are easily summarized: if hyperinsulinemia does something bad in a tissue or organ, that effect remains responsive in the atherometabolic syndrome and T2DM; and if hyperinsulinemia might do something good, that effect becomes resistant. It is a deadly imbalance in insulin action. From the standpoint of human health, it is the worst possible combination of effects. In this review, we discuss the origins of the atherometabolic syndrome in our historically unprecedented environment that only recently has become full of poorly satiating calories and incessant enticements to sit. Data are examined that indicate the magnitude of daily caloric imbalance that causes obesity. We also cover key aspects of healthy, balanced insulin action in liver, endothelium, brain, and elsewhere. Recent insights into the molecular basis and pathophysiologic harm from SEIRR in these organs are discussed. Importantly, a newly discovered oxide transport chain functions as the master regulator of the balance amongst different limbs of the insulin signaling cascade. This oxide transport chain--abbreviated 'NSAPP' after its five major proteins--fails to function properly during chronic overnutrition, resulting in this harmful pattern of SEIRR. We also review the origins of widespread, chronic overnutrition. Despite its apparent complexity, one factor stands out. A sophisticated junk food industry, aided by subsidies from willing governments, has devoted years of careful effort to promote overeating through the creation of a new class of food and drink that is low- or no-cost to the consumer, convenient, savory, calorically dense, yet weakly satiating. It is past time for the rest of us to overcome these foes of good health and solve this man-made epidemic.

Effects of 17β-estradiol on cardiac Na(+)/K(+)-ATPase in high fat diet fed rats

The aim of this study was to investigate in vivo effects of estradiol on Na(+)/K(+)-ATPase activity/expression in high fat (HF) diet fed rats. Adult male Wistar rats were fed normally (Control, n = 7) or with a HF diet (Obese, n = 14) for 10 weeks. After 10 weeks, half of the obese rats were treated with estradiol (Obese + Estradiol, n = 7, 40 μg/kg, i.p.) as a bolus injection and 24 h after treatment all the rats were sacrificed. Estradiol in vivo in obese rats in comparison with obese non-treated rats led to a statistically significant increase in concentration of serum Na(+) (p < 0.05), Na(+)/K(+)-ATPase activity (p < 0.01), expression of α1 (p < 0.01) and α2 (p < 0.05) subunit of Na(+)/K(+)-ATPase, both PI3K subunits p85 (p < 0.01), p110 (p < 0.05), and association of IRS-1 with p85 (p < 0.05), while significantly decrease expression of AT1 (p < 0.05) and Rho A (p < 0.01) proteins. Our results suggest that estradiol in vivo in pathophysiological conditions, such as obesity accompanied with insulin resistance stimulates activity and expression of Na(+)/K(+)-ATPase by a mechanism that involves the participation of IRS-1/PI3K/Akt signaling. In addition, the decreasing level of AT1 and Rho A proteins estradiol probably attenuates the detrimental effect of obesity to decreased IRS-1/PI3K association and consequently reduce Na(+)/K(+)-ATPase activity/expression.

High CO2 Leads to Na,K-ATPase Endocytosis via c-Jun Amino-Terminal Kinase-Induced LMO7b Phosphorylation

The c-Jun amino-terminal kinase (JNK) plays a role in inflammation, proliferation, apoptosis, and cell adhesion and cell migration by phosphorylating paxillin and β-catenin. JNK phosphorylation downstream of AMP-activated protein kinase (AMPK) activation is required for high CO2 (hypercapnia)-induced Na,K-ATPase endocytosis in alveolar epithelial cells. Here, we provide evidence that during hypercapnia, JNK promotes the phosphorylation of LMO7b, a scaffolding protein, in vitro and in intact cells. LMO7b phosphorylation was blocked by exposing the cells to the JNK inhibitor SP600125 and by infecting cells with dominant-negative JNK or AMPK adenovirus. The knockdown of the endogenous LMO7b or overexpression of mutated LMO7b with alanine substitutions of five potential JNK phosphorylation sites (LMO7b-5SA) or only Ser-1295 rescued both LMO7b phosphorylation and the hypercapnia-induced Na,K-ATPase endocytosis. Moreover, high CO2 promoted the colocalization and interaction of LMO7b and the Na,K-ATPase α1 subunit at the plasma membrane, which were prevented by SP600125 or by transfecting cells with LMO7b-5SA. Collectively, our data suggest that hypercapnia leads to JNK-induced LMO7b phosphorylation at Ser-1295, which facilitates the interaction of LMO7b with Na,K-ATPase at the plasma membrane promoting the endocytosis of Na,K-ATPase in alveolar epithelial cells.

In vivo effects of 17β-estradiol on cardiac Na(+)/K(+)-ATPase expression and activity in rat heart

In this study the in vivo effects of estradiol in regulating Na(+)/K(+)-ATPase function in rat heart was studied. Adult male Wistar rats were treated with estradiol (40μg/kg, i.p.) and after 24h the animals were sacrificed and the heart excised. Following estradiol administration, cardiac Na(+)/K(+)-ATPase activity, expression of the α1 subunit, and phosphorylation of the α1 subunit were significantly increased. These animals also had significantly decreased levels of digoxin-like immunoreactive factor(s). Na(+) levels were also significantly reduced but to a level that was still within the normal physiological range, highlighting the ability of the Na(+)/K(+)-ATPase to balance the ionic composition following treatment with estradiol. Estradiol treated rats also showed increased phosphorylation of protein kinase B (Akt), and extracellular-signal-regulated kinase 1/2 (ERK1/2). We therefore suggest a role for Akt and/or ERK1/2 in estradiol-mediated regulation of cardiac Na(+)/K(+)-ATPase expression and activity in rat heart.

Nitric oxide regulation of Na, K-ATPase activity in ocular ciliary epithelium involves Src family kinase

The nitric oxide (NO) donor sodium nitroprusside (SNP) is known to reduce aqueous humor (AH) secretion in the isolated porcine eye. Previously, SNP was found to inhibit Na,K-ATPase activity in nonpigmented ciliary epithelium (NPE), AH-secreting cells, through a cGMP/protein kinase G (PKG)-mediated pathway. Here we show Src family kinase (SFK) activation in the Na,K-ATPase activity response to SNP. Ouabain-sensitive (86) Rb uptake was reduced by >35% in cultured NPE cells exposed to SNP (100 µM) or exogenously added cGMP (8-Br-cGMP) (100 µM) and the SFK inhibitor PP2 (10 µM) prevented the response. Ouabain-sensitive ATP hydrolysis was reduced by ~40% in samples detected in material obtained from SNP- and 8-Br-cGMP-treated cells following homogenization, pointing to an intrinsic change of Na,K-ATPase activity. Tyrosine-10 phosphorylation of Na,K-ATPase α1 subunit was detected in SNP and L-arginine-treated cells and the response prevented by PP2. SNP elicited an increase in cell cGMP. Cells exposed to 8-Br-cGMP displayed SFK activation (phosphorylation) and inhibition of both ouabain-sensitive (86) Rb uptake and Na,K-ATPase activity that was prevented by PP2. SFK activation, which also occurred in SNP-treated cells, was suppressed by inhibitors of soluble guanylate cyclase (ODQ; 10 µM) and PKG (KT5823; 1 µM). SNP and 8-Br-cGMP also increased phosphorylation of ERK1/2 and p38 MAPK and the response prevented by PP2. However, U0126 did not prevent SNP or 8-Br-cGMP-induced inhibition of Na,K-ATPase activity. Taken together, the results suggest that NO activates guanylate cyclase to cause a rise in cGMP and subsequent PKG-dependent SFK activation. Inhibition of Na,K-ATPase activity depends on SFK activation.

Effects of obesity and estradiol on Na+/K+-ATPase and their relevance to cardiovascular diseases

Obesity is associated with aberrant sodium/potassium-ATPase (Na(+)/K(+)-ATPase) activity, apparently linked to hyperglycemic hyperinsulinemia, which may repress or inactivate the enzyme. The reduction of Na(+)/K(+)-ATPase activity in cardiac tissue induces myocyte death and cardiac dysfunction, leading to the development of myocardial dilation in animal models; this has also been documented in patients with heart failure (HF). During several pathological situations (cardiac insufficiency and HF) and in experimental models (obesity), the heart becomes more sensitive to the effect of cardiac glycosides, due to a decrease in Na(+)/K(+)-ATPase levels. The primary female sex steroid estradiol has long been recognized to be important in a wide variety of physiological processes. Numerous studies, including ours, have shown that estradiol is one of the major factors controlling the activity and expression of Na(+)/K(+)-ATPase in the cardiovascular (CV) system. However, the effects of estradiol on Na(+)/K(+)-ATPase in both normal and pathological conditions, such as obesity, remain unclear. Increasing our understanding of the molecular mechanisms by which estradiol mediates its effects on Na(+)/K(+)-ATPase function may help to develop new strategies for the treatment of CV diseases. Herein, we discuss the latest data from animal and clinical studies that have examined how pathophysiological conditions such as obesity and the action of estradiol regulate Na(+)/K(+)-ATPase activity.

DIDS inhibits Na-K-ATPase activity in porcine nonpigmented ciliary epithelial cells by a Src family kinase-dependent mechanism

The anion transport inhibitor DIDS is known to reduce aqueous humor secretion but questions remain about anion dependence of the effect. In some tissues, DIDS is reported to cause Na-K-ATPase inhibition. Here, we report on the ability of DIDS to inhibit Na-K-ATPase activity in nonpigmented ciliary epithelium (NPE) and investigate the underlying mechanism. Porcine NPE cells were cultured to confluence on permeable supports, treated with drugs added to both sides of the membrane, and then used for (86)Rb uptake measurements or homogenized to measure Na-K-ATPase activity or to detect protein phosphorylation. DIDS inhibited ouabain-sensitive (86)Rb uptake, activated Src family kinase (SFK), and caused a reduction of Na-K-ATPase activity. PP2, an SFK inhibitor, prevented the DIDS responses. In BCECF-loaded NPE, DIDS was found to reduce cytoplasmic pH (pHi). PP2-sensitive Na-K-ATPase activity inhibition, (86)Rb uptake suppression, and SFK activation were observed when a similar reduction of pHi was imposed by low-pH medium or an ammonium chloride withdrawal maneuver. PP2 and the ERK inhibitor U0126 prevented robust ERK1/2 activation in cells exposed to DIDS or subjected to pHi reduction, but U0126 did not prevent SFK activation or the Na-K-ATPase activity response. The evidence points to an inhibitory influence of DIDS on NPE Na-K-ATPase activity by a mechanism that hinges on SFK activation associated with a reduction of cytoplasmic pH.

Elimination of autofluorescence background from fluorescence tissue images by use of time-gated detection and the AzaDiOxaTriAngulenium (ADOTA) fluorophore

Sample autofluorescence (fluorescence of inherent components of tissue and fixative-induced fluorescence) is a significant problem in direct imaging of molecular processes in biological samples. A large variety of naturally occurring fluorescent components in tissue results in broad emission that overlaps the emission of typical fluorescent dyes used for tissue labeling. In addition, autofluorescence is characterized by complex fluorescence intensity decay composed of multiple components whose lifetimes range from sub-nanoseconds to a few nanoseconds. For these reasons, the real fluorescence signal of the probe is difficult to separate from the unwanted autofluorescence. Here we present a method for reducing the autofluorescence problem by utilizing an azadioxatriangulenium (ADOTA) dye with a fluorescence lifetime of approximately 15 ns, much longer than those of most of the components of autofluorescence. A probe with such a long lifetime enables us to use time-gated intensity imaging to separate the signal of the targeting dye from the autofluorescence. We have shown experimentally that by discarding photons detected within the first 20 ns of the excitation pulse, the signal-to-background ratio is improved fivefold. This time-gating eliminates over 96 % of autofluorescence. Analysis using a variable time-gate may enable quantitative determination of the bound probe without the contributions from the background.

The role of pathway-selective insulin resistance and responsiveness in diabetic dyslipoproteinemia

PURPOSE OF REVIEW

Type 2 diabetes mellitus (T2DM) and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis, hyperglycemia, which causes microvascular disease, and hypertension. These features share a common, but unexplained, origin--namely, pathway-selective insulin resistance and responsiveness (SEIRR). Here, we review recent work on hepatic SEIRR indicating that deranged insulin signaling may have a remarkably simple molecular basis.

RECENT FINDINGS

Comprehensive examination of a set of 18 insulin targets revealed that T2DM liver in vivo exhibits a specific defect in the ability of the NAD(P)H oxidase 4 (NOX4) to inactivate protein tyrosine phosphatase gene family members after stimulation with insulin, and that impairment of this single molecule, NOX4, in cultured hepatocytes recapitulates all features of hepatic SEIRR in vivo. These features include insulin-stimulated generation of an unusual monophosphorylated form of AKT at Thr308 (pT308-AKT) with only weak phosphorylation at Ser473, impaired insulin-stimulated pathways for lowering plasma levels of lipids and glucose, but continued lipogenic pathways and robust extracellular signal-regulated kinase activation. This new study, in combination with important prior work, provides clues to several long-standing mysteries, such as how AKT might regulate lipid-lowering and glucose-lowering pathways that become insulin-resistant but also lipogenic pathways that remain insulin-responsive, as well as a potential role for NOX4 in insulin-stimulated generation of oxysterol ligands for LXR, a key lipogenic factor.

SUMMARY

These findings suggest a unified molecular explanation for fatty liver, atherogenic dyslipoproteinemia, hyperglycemia, and hence accelerated atherosclerosis and microvascular disease in T2DM, obesity, and related syndromes of positive caloric imbalance.

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.

Cyclic stretch stimulates recruitment of active Na⁺/K⁺-ATPase subunits to the plasma membrane of skeletal muscle cells

Cyclic stretch increases Na(+)/K(+)-ATPase activity and abundance in several tissues, including skeletal muscle cells. The present study was undertaken to investigate whether Na(+)/K(+)-ATPase undergoes acute changes in its catalytic activity in response to cyclic stretch. Na(+)/K(+)-ATPase activity increased after continuously stretched for 6 h, and reached the maximum at 24 h. The inhibition of gene transcription (actinomycin D) had no effect on stretch-induced Na(+)/K(+)-ATPase activity. Cyclic stretch also increases the plasma membrane content of α(1)- and α(2)-subunit of Na(+)/K(+)-ATPase. Brefeldin A could completely abolished the stretch-induced recruitment of α-subunits to the plasma membrane and Na(+)/K(+)-ATPase activity. In conclusion, cyclic stretch directly stimulates Na(+)/K(+)-ATPase activity in skeletal muscle cells through post-transcriptional activation, likely by increasing translocation of Na(+)/K(+)-ATPase molecules to plasma membrane.

Spermidine decreases Na⁺,K⁺-ATPase activity through NMDA receptor and protein kinase G activation in the hippocampus of rats

Spermidine is an endogenous polyamine with a polycationic structure present in the central nervous system of mammals. Spermidine regulates biological processes, such as Ca(2+) influx by glutamatergic N-methyl-d-aspartate receptor (NMDA receptor), which has been associated with nitric oxide synthase (NOS) and cGMP/PKG pathway activation and a decrease of Na(+),K(+)-ATPase activity in rats' cerebral cortex synaptosomes. Na(+),K(+)-ATPase establishes Na(+) and K(+) gradients across membranes of excitable cells and by this means maintains membrane potential and controls intracellular pH and volume. However, it has not been defined whether spermidine modulates Na(+),K(+)-ATPase activity in the hippocampus. In this study we investigated whether spermidine alters Na(+),K(+)-ATPase activity in slices of hippocampus from rats, and possible underlying mechanisms. Hippocampal slices and homogenates were incubated with spermidine (0.05-10 μM) for 30 min. Spermidine (0.5 and 1 μM) decreased Na(+),K(+)-ATPase activity in slices, but not in homogenates. MK-801 (100 and 10 μM), a non-competitive antagonist of NMDA receptor, arcaine (0.5μM), an antagonist of the polyamine binding site at the NMDA receptor, and L-NAME (100μM), a NOS inhibitor, prevented the inhibitory effect of spermidine (0.5 μM). ODQ (10 μM), a guanylate cyclase inhibitor, and KT5823 (2 μM), a protein kinase G inhibitor, also prevented the inhibitory effect of spermidine on Na(+),K(+)-ATPase activity. Spermidine (0.5 and 1.0 μM) increased NO(2) plus NO(3) (NOx) levels in slices, and MK-801 (100 μM) and arcaine (0.5 μM) prevented the effect of spermidine (0.5 μM) on the NOx content. These results suggest that spermidine-induced decrease of Na(+),K(+)-ATPase activity involves NMDA receptor/NOS/cGMP/PKG pathway.

NOX4 pathway as a source of selective insulin resistance and responsiveness

OBJECTIVE

Type 2 diabetes mellitus and related syndromes exhibit a deadly triad of dyslipoproteinemia, which leads to atherosclerosis; hyperglycemia, which causes microvascular disease; and hypertension. These features share a common, but unexplained, origin-namely, pathway-selective insulin resistance and responsiveness. Here, we undertook a comprehensive characterization of pathway-selective insulin resistance and responsiveness in liver and hepatocytes by examining 18 downstream targets of the insulin receptor, surveying the AKT, ERK, and NAD(P)H oxidase 4 pathways.

METHODS AND RESULTS

Injection of insulin into hyperphagic, obese type 2 diabetic db/db mice failed to inactivate hepatic protein tyrosine phosphatase gene family members, a crucial action of NAD(P)H oxidase 4 previously thought to be required for all signaling through AKT and ERK. Insulin-stimulated type 2 diabetic livers unexpectedly produced an unusual form of AKT that was phosphorylated at Thr308 (pT308), with only weak insulin-stimulated phosphorylation at Ser473. Remarkably, knockdown or inhibition of NAD(P)H oxidase 4 in cultured hepatocytes recapitulated the entire complicated pattern of pathway-selective insulin resistance and responsiveness seen in vivo-namely, monophosphorylated pT308-AKT, impaired insulin-stimulated pathways for lowering plasma lipids and glucose, but continued lipogenic pathways and robust ERK activation.

CONCLUSIONS

Functional disturbance of a single molecule, NAD(P)H oxidase 4, is sufficient to induce the key harmful features of deranged insulin signaling in type 2 diabetes mellitus, obesity, and other conditions associated with hyperinsulinemia and pathway-selective insulin resistance and responsiveness.

Ouabain and insulin induce sodium pump endocytosis in renal epithelium

Cardiotonic steroids signaling through the basolateral sodium pump (Na/K-ATPase) have been shown to alter renal salt handling in intact animals. Because the relationship between renal salt handling and blood pressure is a key determinant of hypertension, and patients with insulin resistance are frequently hypertensive, we chose to examine whether there might be competition for resources necessary for receptor-mediated endocytosis. In LLC-PK1 cells, the Na/K-ATPase-α1 and carcinoembryonic antigen cell adhesion molecule 1, a plasma membrane protein that promotes receptor-mediated endocytosis, colocalized in the plasma membranes and translocated to the intracellular region in response to ouabain. Either ouabain or insulin alone caused accumulation of and carcinoembryonic antigen cell adhesion molecule, as well as insulin receptor-β, and epidermal growth factor receptor in early endosomes, but no synergy was demonstrable. Like ouabain, insulin also caused c-Src activation. When caveolin or Na/K-ATPase-α1 expression was knocked down with small interfering RNA, insulin but not ouabain induced carcinoembryonic antigen cell adhesion molecule 1, insulin receptor-β, and epidermal growth factor receptor endocytosis. To determine whether this might be relevant to salt handling in vivo, we examined salt loading in mice with null renal carcinoembryonic antigen cell adhesion molecule 2 expression. The null renal carcinoembryonic antigen cell adhesion molecule 2 animals demonstrated greater increases in blood pressure with increases in dietary salt than control animals. These data demonstrate that cardiotonic steroids and insulin compete for cellular endocytosis resources and suggest that, under conditions where circulating insulin concentrations are high, cardiotonic steroid-mediated natriuresis could be impaired.

A novel domain of caveolin-2 that controls nuclear targeting: regulation of insulin-specific ERK activation and nuclear translocation by caveolin-2

Herein, we report that insulin-activated extracellular signal-regulated kinase (ERK) is translocated to the nuclear envelope by caveolin-2 (cav-2) and associates with lamin A/C in the inner nuclear membrane in response to insulin. We identified that the Ser¹⁵⁴ -Val¹⁵⁵ -Ser¹⁵⁶ domain on the C-terminal of cav-2 is essential for insulin-induced phosphorylation and nuclear targeting of ERK and cav-2. In human embryonic kidney 293T cells, ERK was not activated and translocated to the nucleus by insulin in comparison to insulin-like growth factor-1 (IGF-1). However, insulin-stimulated activation of ERK was induced by exogenous addition of cav-2. The activated ERK associated and translocated with the cav-2 to the nucleus. In turn, cav-2 promoted phospho-ERK interaction with lamin A/C in the inner nuclear membrane. In contrast, ERK, but not cav-2, was phosphorylated and translocated to the nucleus by IGF-1. The nuclear targeted phospho-ERK failed to localize in the nuclear envelope in response to IGF-1. Together, our data demonstrate that translocation of phospho-ERK to the nuclear envelope is mediated by Ser¹⁵⁴ -Val¹⁵⁵ -Ser¹⁵⁶ domain of cav-2 and this event is an insulin-specific action.

AS160 associates with the Na+,K+-ATPase and mediates the adenosine monophosphate-stimulated protein kinase-dependent regulation of sodium pump surface expression

The sodium pump interacts with AS160, a protein that regulates the trafficking of the GLUT4 glucose transporter. This interaction drives the internalization of the sodium pump from the cell surface, and this process is in turn controlled by the energy-sensing kinase adenosine monophosphate-stimulated protein kinase.

The Na⁺,K⁺-ATPase is the major active transport protein found in the plasma membranes of most epithelial cell types. The regulation of Na⁺,K⁺-ATPase activity involves a variety of mechanisms, including regulated endocytosis and recycling. Our efforts to identify novel Na⁺,K⁺-ATPase binding partners revealed a direct association between the Na⁺,K⁺-ATPase and AS160, a Rab-GTPase-activating protein. In COS cells, coexpression of AS160 and Na⁺,K⁺-ATPase led to the intracellular retention of the sodium pump. We find that AS160 interacts with the large cytoplasmic NP domain of the α-subunit of the Na⁺,K⁺-ATPase. Inhibition of the activity of the adenosine monophosphate-stimulated protein kinase (AMPK) in Madin-Darby canine kidney cells through treatment with Compound C induces Na⁺,K⁺-ATPase endocytosis. This effect of Compound C is prevented through the short hairpin RNA-mediated knockdown of AS160, demonstrating that AMPK and AS160 participate in a common pathway to modulate the cell surface expression of the Na⁺,K⁺-ATPase.

Adiponectin resistance and vascular dysfunction in the hyperlipidemic state

Insulin plays an important role in the stimulation of vascular nitric oxide production, with both short term (vasomotility and anti-thrombotic effects) and long term (smooth muscle cell growth and migration inhibition) benefits. Impaired vasodilatory response to insulin, the hallmark of vascular insulin resistance (IR), has important implications for circulatory pathophysiology. An association between adipokines and IR has been observed in both diabetic and nondiabetic states. Adiponectin (APN) is an insulin-sensitizing adipokine known to stimulate skeletal muscle fatty acid (FA) oxidation and reduce lipid accumulation. Recent demonstrations of potential cross-talk between APN and insulin in vascular function regulation are particularly interesting. The lipid accumulation observed after chronic high-fat (HF) diets and in the obese state may reduce vascular response to APN, a pathologic state termed as APN resistance. This review highlights the importance of insulin sensitivity and APN activity in the maintenance of endothelial function. It explores the relationships between vascular IR and APN resistance in the hyperlipidemic pathological condition, representative of the metabolic syndrome. The investigation of vascular insulin and APN resistance provides not only better understanding of vascular pathophysiology, but also an opportunity for therapeutic targeting in individuals affected by the metabolic syndrome.

Regulation and identification of Na,K-ATPase alpha1 subunit phosphorylation in rat parotid acinar cells

The stimulation of fluid and electrolyte secretion in salivary cells results in ionic changes that promote rapid increases in the activity of the Na,K-ATPase. In many cell systems, there are conflicting findings concerning the regulation of the phosphorylation of the Na,K-ATPase α subunit, which is the catalytic moiety. Initially, we investigated the phosphorylation sites on the α1 subunit in native rat parotid acinar cells using tandem mass spectrometry and identified two new phosphorylation sites (Ser(222), Ser(407)), three sites (Ser(217), Tyr(260), Ser(47)) previously found from large scale proteomic screens, and two sites (Ser(23), Ser(16)) known to be phosphorylated by PKC. Subsequently, we used phospho-specific antibodies to examine the regulation of phosphorylation on Ser(23) and Ser(16) and measured changes in ERK phosphorylation in parallel. The G-protein-coupled muscarinic receptor mimetic carbachol, the phorbol ester phorbol 12-myristate 13-acetate, the Ca(2+) ionophore ionomycin, and the serine/threonine phosphatase inhibitor calyculin A increased Ser(23) α1 phosphorylation. Inhibition of classical PKC proteins blocked carbachol-stimulated Ser(23) α1 subunit phosphorylation but not ERK phosphorylation, which was blocked by an inhibitor of novel PKC proteins. The carbachol-initiated phosphorylation of Ser(23) α1 subunit was not modified by ERK or PKA activity. The Na,K-ATPase inhibitor ouabain reduced and enhanced the carbachol-promoted phosphorylation of Ser(23) and Ser(16), respectively, the latter because ouabain itself increased Ser(16) phosphorylation; thus, both sites display conformational-dependent phosphorylation changes. Ouabain-initiated phosphorylation of Ser(16) α1 was not blocked by PKC inhibitors, unlike carbachol- or phorbol 12-myristate 13-acetate-initiated phosphorylations, suggesting that this site was also a substrate for a kinase other than PKC.

Phosphorylation of the Na+,K+-ATPase and the H+,K+-ATPase

Phosphorylation is a widely used, reversible means of regulating enzymatic activity. Among the important phosphorylation targets are the Na(+),K(+)- and H(+),K(+)-ATPases that pump ions against their chemical gradients to uphold ionic concentration differences over the plasma membrane. The two pumps are very homologous, and at least one of the phosphorylation sites is conserved, namely a cAMP activated protein kinase (PKA) site, which is important for regulating pumping activity, either by changing the cellular distribution of the ATPases or by directly altering the kinetic properties as supported by electrophysiological results presented here. We further review the other proposed pump phosphorylations.

Targeting of renal proximal tubule Na,K-ATPase by salt-inducible kinase

The renal proximal tubule (RPT) is a central locale for Na+ reabsorption, and blood pressure regulation. Na+ reabsorption in the RPT depends upon the Na,K-ATPase, which is controlled by a complex regulatory network, including Salt-Inducible Protein Kinase (SIK). SIKs are recently discovered members of the AMP-activated Protein Kinase (AMPK) family, which regulate salt homeostasis and metabolism in a number of tissues. In the RPT, SIK interacts with the Na,K-ATPase in the basolateral membrane (BM), regulating both the activity and level of Na,K-ATPase in the BM. Thus, Na,K-ATPase activity can be rapidly adjusted in response to changes in Na+ balance. Long-term changes in Na+ intake affect the state of SIK phosphorylation, and as a consequence the phosphorylation of TORCs, Transducers of Regulated CREB (cAMP Regulatory Element Binding Protein). Once phosphorylated, TORCs enter the nucleus, and activate transcription of the ATP1B1 gene encoding for the Na,K-ATPase beta subunit.

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Abstract Potassium (K(+)) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K(+) concentration ([K(+)](e)) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K(+)](e) increases progressively with increasing work intensity, reaching values higher than 10 mm. This increase in [K(+)](e) is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K(+) (K(v)) channels that generate the action potential (AP) it is suggested that the big-conductance Ca(2+)-dependent K(+) (K(Ca)1.1) channel contributes significantly to the K(+) release into the T-tubules. Also the ATP-dependent K(+) (K(ATP)) channel participates, but is suggested primarily to participate in K(+) release to the interstitium. Because there is restricted diffusion of K(+) to the interstitium, K(+) released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K(+) reuptake in the T-tubules is the Na(+),K(+)-ATPase alpha(2) dimers, but a significant contribution of the strong inward rectifier K(+) (Kir2.1) channel is also suggested. The Na(+), K(+), 2Cl(-) 1 (NKCC1) cotransporter also participates in K(+) reuptake but probably mainly from the interstitium. The relative content of the different K(+)-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K(+)](e) tolerance observed.

AMP-activated protein kinase activator A-769662 is an inhibitor of the Na(+)-K(+)-ATPase

Muscle contraction and metabolic stress are potent activators of AMP-activated protein kinase (AMPK). AMPK restores energy balance by activating processes that produce energy while inhibiting those that consume energy. The role of AMPK in the regulation of active ion transport is unclear. Our aim was to determine the effect of the AMPK activator A-769662 on Na(+)-K(+)-ATPase function in skeletal muscle cells. Short-term incubation of differentiated rat L6 myotubes with 100 microM A-769662 increased AMPK and acetyl-CoA carboxylase (ACC) phosphorylation in parallel with decreased Na(+)-K(+)-ATPase alpha(1)-subunit abundance at the plasma membrane and ouabain-sensitive (86)Rb(+) uptake. Notably, the effect of A-769662 on Na(+)-K(+)-ATPase was similar in muscle cells that do not express AMPK alpha(1)- and alpha(2)-catalytic subunits. A-769662 directly inhibits the alpha(1)-isoform of the Na(+)-K(+)-ATPase, purified from rat and human kidney cells in vitro with IC(50) 57 microM and 220 microM, respectively. Inhibition of the Na(+)-K(+)-ATPase by 100 microM ouabain decreases sodium pump activity and cell surface abundance, similar to the effect of A-769662, without affecting AMPK and ACC phosphorylation. In conclusion, the AMPK activator A-769662 inhibits Na(+)-K(+)-ATPase activity and decreases the sodium pump cell surface abundance in L6 skeletal muscle cells. The effect of A-769662 on sodium pump is due to direct inhibition of the Na(+)-K(+)-ATPase activity, rather than AMPK activation. This AMPK-independent effect on Na(+)-K(+)-ATPase calls into question the use of A-769662 as a specific AMPK activator for metabolic studies.

Altered expression and insulin-induced trafficking of Na+-K+-ATPase in rat skeletal muscle: effects of high-fat diet and exercise

Skeletal muscle Na(+)-K(+)-ATPase plays a central role in the clearance of K(+) from the extracellular fluid, therefore maintaining blood [K(+)]. Na(+)-K(+)-ATPase activity in peripheral tissue is impaired in insulin resistant states. We determined effects of high-fat diet (HFD) and exercise training (ET) on skeletal muscle Na(+)-K(+)-ATPase subunit expression and insulin-stimulated translocation. Skeletal muscle expression of Na(+)-K(+)-ATPase isoforms and transcription factor DNA binding was determined before or after 5 days of swim training in Wistar rats fed chow or HFD for 4 or 12 wk. Skeletal muscle insulin resistance was observed after 12 wk of HFD. Na(+)-K(+)-ATPase alpha(1)-subunit protein expression was increased 1.6-fold (P < 0.05), whereas alpha(2)- and beta(1)-subunits and protein expression were decreased twofold (P < 0.01) in parallel with decrease in plasma membrane Na(+)-K(+)-ATPase activity after 4 wk of HFD. Exercise training restored alpha(1)-, alpha(2)-, and beta(1)-subunit expression and Na(+)-K(+)-ATPase activity to control levels and reduced beta(2)-subunit expression 2.2-fold (P < 0.05). DNA binding activity of the alpha(1)-subunit-regulating transcription factor ZEB (AREB6) and alpha(1) mRNA expression were increased after HFD and restored by ET. DNA binding activity of Sp-1, a transcription factor involved in the regulation of alpha(2)- and beta(1)-subunit expression, was decreased after HFD. ET increased phosphorylation of the Na(+)-K(+)-ATPase regulatory protein phospholemman. Phospholemman mRNA and protein expression were increased after HFD and restored to control levels after ET. Insulin-stimulated translocation of the alpha(2)-subunit to plasma membrane was impaired by HFD, whereas alpha(1)-subunit translocation remained unchanged. Alterations in sodium pump function precede the development of skeletal muscle insulin resistance. Disturbances in skeletal muscle Na(+)-K(+)-ATPase regulation, particularly the alpha(2)-subunit, may contribute to impaired ion homeostasis in insulin-resistant states such as obesity and type 2 diabetes.

Na+/K+-ATPase-mediated signal transduction and Na+/K+-ATPase regulation

A number of studies suggest that Na(+)/K(+)-ATPase in caveolae interacts with neighboring membrane proteins and organizes cytosolic cascades of signaling proteins to send messages to intracellular organelles in different tissues, mostly in cardiac myocytes. Low concentration of ouabain binding to Na(+)/K(+)-ATPase activates Src/epidermal growth factor receptor complex to initiate multiple signal pathways, which include PLC/IP3/CICR, PI3K, reactive oxygen species (ROS), PLC/DG/PKC/Raf/MEK/ERK1/2, and Ras/Raf/MEK/ERK1/2 pathways. In cardiac myocytes, the resulting downstream events include the induction of some early response proto-oncogenes, activation of transcription factors, activator protein-1, and nuclear factor-kappaB, the regulation of a number of cardiac growth-related genes, and the stimulation of protein synthesis and myocyte hypertrophy and apoptosis. Conversely, several factors acting through signal pathways, such as protein kinases, Ca(2+), ROS, etc., can modulate the activity of the Na(+)/K(+)-ATPase.