Cardiotonic Steroids Stimulate Glycogen Synthesis in Human Skeletal Muscle Cells via a Src- and ERK1/2-dependent Mechanism

The Na/K-ATPase α1/Src Signaling Axis Regulates Mitochondrial Metabolic Function and Redox Signaling in Human iPSC-Derived Cardiomyocytes

Na/K-ATPase (NKA)-mediated regulation of Src kinase, which involves defined amino acid sequences of the NKA α1 polypeptide, has emerged as a novel regulatory mechanism of mitochondrial function in metazoans. Mitochondrial metabolism ensures adequate myocardial performance and adaptation to physiological demand. It is also a critical cellular determinant of cardiac repair and remodeling. To assess the impact of the proposed NKA/Src regulatory axis on cardiac mitochondrial metabolic function, we used a gene targeting approach in human cardiac myocytes. Human induced pluripotent stem cells (hiPSC) expressing an Src-signaling null mutant (A420P) form of the NKA α1 polypeptide were generated using CRISPR/Cas9-mediated genome editing. Total cellular Na/K-ATPase activity remained unchanged in A420P compared to the wild type (WT) hiPSC, but baseline phosphorylation levels of Src and ERK1/2 were drastically reduced. Both WT and A420P mutant hiPSC readily differentiated into cardiac myocytes (iCM), as evidenced by marker gene expression, spontaneous cell contraction, and subcellular striations. Total NKA α1-3 protein expression was comparable in WT and A420P iCM. However, live cell metabolism assessed functionally by Seahorse extracellular flux analysis revealed significant reductions in both basal and maximal rates of mitochondrial respiration, spare respiratory capacity, ATP production, and coupling efficiency. A significant reduction in ROS production was detected by fluorescence imaging in live cells, and confirmed by decreased cellular protein carbonylation levels in A420P iCM. Taken together, these data provide genetic evidence for a role of NKA α1/Src in the tonic stimulation of basal mitochondrial metabolism and ROS production in human cardiac myocytes. This signaling axis in cardiac myocytes may provide a new approach to counteract mitochondrial dysfunction in cardiometabolic diseases.

The vascular Na,K-ATPase: clinical implications in stroke, migraine, and hypertension

In the vascular wall, the Na,K-ATPase plays an important role in the control of arterial tone. Through cSrc signaling, it contributes to the modulation of Ca2+ sensitivity in vascular smooth muscle cells. This review focuses on the potential implication of Na,K-ATPase-dependent intracellular signaling pathways in severe vascular disorders; ischemic stroke, familial migraine, and arterial hypertension. We propose similarity in the detrimental Na,K-ATPase-dependent signaling seen in these pathological conditions. The review includes a retrospective proteomics analysis investigating temporal changes after ischemic stroke. The analysis revealed that the expression of Na,K-ATPase α isoforms is down-regulated in the days and weeks following reperfusion, while downstream Na,K-ATPase-dependent cSrc kinase is up-regulated. These results are important since previous studies have linked the Na,K-ATPase-dependent cSrc signaling to futile recanalization and vasospasm after stroke. The review also explores a link between the Na,K-ATPase and migraine with aura, as reduced expression or pharmacological inhibition of the Na,K-ATPase leads to cSrc kinase signaling up-regulation and cerebral hypoperfusion. The review discusses the role of an endogenous cardiotonic steroid-like compound, ouabain, which binds to the Na,K-ATPase and initiates the intracellular cSrc signaling, in the pathophysiology of arterial hypertension. Currently, our understanding of the precise control mechanisms governing the Na,K-ATPase/cSrc kinase regulation in the vascular wall is limited. Understanding the role of vascular Na,K-ATPase signaling is essential for developing targeted treatments for cerebrovascular disorders and hypertension, as the Na,K-ATPase is implicated in the pathogenesis of these conditions and may contribute to their comorbidity.

The microtubule network enables Src kinase interaction with the Na,K-ATPase to generate Ca2+ flashes in smooth muscle cells

Background: Several local Ca²⁺ events are characterized in smooth muscle cells. We have previously shown that an inhibitor of the Na,K-ATPase, ouabain induces spatially restricted intracellular Ca²⁺ transients near the plasma membrane, and suggested the importance of this signaling for regulation of intercellular coupling and smooth muscle cell contraction. The mechanism behind these Na,K-ATPase-dependent “Ca²⁺ flashes” remains to be elucidated. In addition to its conventional ion transport function, the Na,K-ATPase is proposed to contribute to intracellular pathways, including Src kinase activation. The microtubule network is important for intracellular signaling, but its role in the Na,K-ATPase-Src kinase interaction is not known. We hypothesized the microtubule network was responsible for maintaining the Na,K-ATPase-Src kinase interaction, which enables Ca²⁺ flashes.

Methods: We characterized Ca²⁺ flashes in cultured smooth muscle cells, A7r5, and freshly isolated smooth muscle cells from rat mesenteric artery. Cells were loaded with Ca²⁺-sensitive fluorescent dyes, Calcium Green-1/AM and Fura Red/AM, for ratiometric measurements of intracellular Ca²⁺. The Na,K-ATPase α2 isoform was knocked down with siRNA and the microtubule network was disrupted with nocodazole. An involvement of the Src signaling was tested pharmacologically and with Western blot. Protein interactions were validated with proximity ligation assays.

Results: The Ca²⁺ flashes were induced by micromolar concentrations of ouabain. Knockdown of the α2 isoform Na,K-ATPase abolished Ca²⁺ flashes, as did inhibition of tyrosine phosphorylation with genistein and PP2, and the inhibitor of the Na,K-ATPase-dependent Src activation, pNaKtide. Ouabain-induced Ca²⁺ flashes were associated with Src kinase activation by phosphorylation. The α2 isoform Na,K-ATPase and Src kinase colocalized in the cells. Disruption of microtubule with nocodazole inhibited Ca²⁺ flashes, reduced Na,K-ATPase/Src interaction and Src activation.

Conclusion: We demonstrate that the Na,K-ATPase-dependent Ca²⁺ flashes in smooth muscle cells require an interaction between the α2 isoform Na, K-ATPase and Src kinase, which is maintained by the microtubule network.

Phosphorylation of Na+,K+-ATPase at Tyr10 of the α1-Subunit is Suppressed by AMPK and Enhanced by Ouabain in Cultured Kidney Cells

Na⁺,K⁺-ATPase (NKA) is essential for maintenance of cellular and whole-body water and ion homeostasis. In the kidney, a major site of ion transport, NKA consumes ~ 50% of ATP, indicating a tight coordination of NKA and energy metabolism. AMP-activated protein kinase (AMPK), a cellular energy sensor, regulates NKA by modulating serine phosphorylation of the α1-subunit, but whether it modulates other important regulatory phosphosites, such as Tyr10, is unknown. Using human kidney (HK-2) cells, we determined that the phosphorylation of Tyr10 was stimulated by the epidermal growth factor (EGF), which was opposed by inhibitors of Src kinases (PP2), tyrosine kinases (genistein), and EGF receptor (EGFR, gefitinib). AMPK activators AICAR and A-769662 suppressed the EGF-stimulated phosphorylation of EGFR (Tyr1173) and NKAα1 at Tyr10. The phosphorylation of Src (Tyr416) was unaltered by AICAR and increased by A-769662. Conversely, ouabain (100 nM), a pharmacological NKA inhibitor and a putative adrenocortical hormone, enhanced the EGF-stimulated Tyr10 phosphorylation without altering the phosphorylation of EGFR (Tyr1173) or Src (Tyr416). Ouabain (100–1000 nM) increased the ADP:ATP ratio, while it suppressed the lactate production and the oxygen consumption rate in a dose-dependent manner. Treatment with ouabain or gene silencing of NKAα1 or NKAα3 subunit did not activate AMPK. In summary, AMPK activators and ouabain had antagonistic effects on the phosphorylation of NKAα1 at Tyr10 in cultured HK-2 cells, which implicates a role for Tyr10 in coordinated regulation of NKA-mediated ion transport and energy metabolism.

Targeting Na+ /K+ -ATPase by berbamine and ouabain synergizes with sorafenib to inhibit hepatocellular carcinoma

BACKGROUND AND PURPOSE

The multikinase inhibitor sorafenib is a first-line drug for advanced hepatocellular carcinoma. The response to sorafenib varies among hepatocellular carcinoma patients and many of the responders suffer from reduced sensitivity after long-term treatment. This study aims to explore a novel strategy to potentiate or maximize the anti-hepatocellular carcinoma effects of sorafenib.

EXPERIMENTAL APPROACH

We used hepatocellular carcinoma cell lines, western blotting, various antagonists, siRNA and tumour xenografts mouse model to determine the anti- hepatocellular carcinoma effects of sorafenib in combination with berbamine or other Na⁺ /K⁺ -ATPase ligands.

KEY RESULTS

Berbamine and the cardiotonic steroid, ouabain, synergize with sorafenib to inhibit hepatocellular carcinoma cells growth. Mechanistically, berbamine induces Src phosphorylation in Na⁺ /K⁺ -ATPase-dependent manner, leading to the activation of p38MAPK and EGFR-ERK pathways. The Na⁺ /K⁺ -ATPase ligand ouabain also induces Src, EGFR, type I insulin-like growth factor receptor, ERK1/2 and p38MAPK phosphorylation in hepatocellular carcinoma cells. Treatment of hepatocellular carcinoma cells with Src or EGFR inhibitor inhibits the induction of ERK1/2 phosphorylation by berbamine. Moreover, sorafenib inhibits the induction of Src, p38MAPK, EGFR and ERK1/2 phosphorylation by berbamine and ouabain. Importantly, combination of sorafenib with berbamine or ouabain synergistically inhibits both sorafenib-naïve and sorafenib-resistant hepatocellular carcinoma cells growth. Co-treatment of hepatocellular carcinoma cells with berbamine and sorafenib significantly induces cell death and significantly inhibits hepatocellular carcinoma xenografts growth in vivo.

CONCLUSION AND IMPLICATIONS

Berbamine or other Na⁺ /K⁺ -ATPase ligands have a potential for improving sorafenib responsiveness in hepatocellular carcinoma. Targeting Na⁺ /K⁺ -ATPase represents a novel strategy to potentiate the anti- hepatocellular carcinoma effects of sorafenib.

Chronic Ouabain Prevents Na,K-ATPase Dysfunction and Targets AMPK and IL-6 in Disused Rat Soleus Muscle

Sustained sarcolemma depolarization due to loss of the Na,K-ATPase function is characteristic for skeletal muscle motor dysfunction. Ouabain, a specific ligand of the Na,K-ATPase, has a circulating endogenous analogue. We hypothesized that the Na,K-ATPase targeted by the elevated level of circulating ouabain modulates skeletal muscle electrogenesis and prevents its disuse-induced disturbances. Isolated soleus muscles from rats intraperitoneally injected with ouabain alone or subsequently exposed to muscle disuse by 6-h hindlimb suspension (HS) were studied. Conventional electrophysiology, Western blotting, and confocal microscopy with cytochemistry were used. Acutely applied 10 nM ouabain hyperpolarized the membrane. However, a single injection of ouabain (1 µg/kg) prior HS was unable to prevent the HS-induced membrane depolarization. Chronic administration of ouabain for four days did not change the α1 and α2 Na,K-ATPase protein content, however it partially prevented the HS-induced loss of the Na,K-ATPase electrogenic activity and sarcolemma depolarization. These changes were associated with increased phosphorylation levels of AMP-activated protein kinase (AMPK), its substrate acetyl-CoA carboxylase and p70 protein, accompanied with increased mRNA expression of interleikin-6 (IL-6) and IL-6 receptor. Considering the role of AMPK in regulation of the Na,K-ATPase, we suggest an IL-6/AMPK contribution to prevent the effects of chronic ouabain under skeletal muscle disuse.

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.

Na,K-ATPase as a target for endogenous cardiotonic steroids: What's the evidence?

With an exception of few reports, the plasma concentration of ouabain and marinobufagenin, mostly studied cardiotonic steroids (CTS) assessed by immunoassay techniques, is less than 1 nM. During the last 3 decades, the implication of these endogenous CTS in the pathogenesis of hypertension and other volume-expanded disorders is widely disputed. The threshold for inhibition by CTS of human and rodent α1-Na,K-ATPase is ∼1 and 1000 nM, respectively, that rules out the functioning of endogenous CTS (ECTS) as natriuretic hormones and regulators of cell adhesion, cell-to-cell communication, gene transcription and translation, which are mediated by dissipation of the transmembrane gradients of monovalent cations. In several types of cells ouabain and marinobufagenin at concentrations corresponding to its plasma level activate Na,K-ATPase, decrease the [Na⁺]_i/[K⁺]_i-ratio and increase cell proliferation. Possible physiological significance and mechanism of non-canonical Na⁺_i/K⁺_i-dependent and Na⁺_i/K⁺_i-independent cell responses to CTS are discussed.

Low Ouabain Doses and AMP-Activated Protein Kinase as Factors Supporting Electrogenesis in Skeletal Muscle

Many motor disorders are associated with depolarization of the membrane of skeletal muscle fibers due to the impaired functioning of Na,K-ATPase. Here, we studied the role of ouabain (specific Na,K-ATPase ligand) and AMP-activated protein kinase (key regulator of muscle metabolism) in the maintenance of muscle electrogenesis; the levels of these endogenous factors are directly related to the motor activity. After 4-day intraperitoneal administration of ouabain (1 µg/kg daily), a hyperpolarization of sarcolemma was registered in isolated rat diaphragm muscles due to an increase in the electrogenic activity of Na,K-ATPase. In acute experiments, addition of nanomolar ouabain concentrations to the bathing solution resulted in the muscle membrane hyperpolarization within 15 min. The effect of ouabain reversed to membrane depolarization with the increase in the external potassium concentration. It is possible that Na,K-ATPase activation by ouabain may be regulated by such factors as specific subcellular location, interaction with molecular partners, and changes in the ionic balance. Preventive administration of the AMP-activated protein kinase activator AICAR (5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside; 400 mg/kg body weight daily for 7 days) in chronic experiments resulted in the stabilization of the endplate structure and abolishment of depolarization of the rat soleus muscle membrane caused by the motor activity cessation. The obtained data can be useful for creating approaches for correction of muscle dysfunction, especially at the early stages, prior to the development of muscle atrophy.

Monoclonal Antibody to Marinobufagenin Downregulates TGFβ Profibrotic Signaling in Left Ventricle and Kidney and Reduces Tissue Remodeling in Salt-Sensitive Hypertension

Background Elevated levels of an endogenous Na/K-ATPase inhibitor marinobufagenin accompany salt-sensitive hypertension and are implicated in cardiac fibrosis. Immunoneutralization of marinobufagenin reduces blood pressure in Dahl salt-sensitive (Dahl-S) rats. The effect of the anti-marinobufagenin monoclonal antibody on blood pressure, left ventricular (LV) and renal remodeling, and gene expression were investigated in hypertensive Dahl-S rats. Methods and Results Dahl-S rats were fed high NaCl (8%, HS; n=14) or low NaCl (0.1%, LS; n=14) diets for 8 weeks. Animals were administered control antibody (LS control antibody, LSC; HS control antibody, HSC; n=7 per group) or anti-marinobufagenin antibody once on week 7 of diet intervention (n=7 per group). Levels of marinobufagenin, LV, and kidney mRNAs and proteins implicated in profibrotic signaling were assessed. Systolic blood pressure was elevated (211±8 versus 133±3 mm Hg, P<0.01), marinobufagenin increased 2-fold in plasma (P<0.05) and 5-fold in urine (P<0.01), LV and kidney weights increased, and levels of LV collagen-1 rose 3.5-fold in HSC versus LSC. Anti-marinobufagenin antibody treatment decreased systolic blood pressure by 24 mm Hg (P<0.01) and reduced organ weights and level of LV collagen-1 (P<0.01) in hypertensive Dahl salt-sensitive rats with anti-marinobufagenin antibody versus HSC. The expression of genes related to transforming growth factor-β-dependent signaling was upregulated in the left ventricles and kidneys in HSC versus LSC groups and became downregulated following administration of anti-marinobufagenin antibody to hypertensive Dahl-S rats. Marinobufagenin also activated transforming growth factor-β signaling in cultured ventricular myocytes from Dahl-S rats. Conclusions Immunoneutralization of heightened marinobufagenin levels in hypertensive Dahl-S rats resulted in a downregulation of genes implicated in transforming growth factor-β pathway, which indicates that marinobufagenin is an activator of profibrotic transforming growth factor-β-dependent signaling in salt-sensitive hypertension.

The Na,K-ATPase in vascular smooth muscle cells

The Na,K-ATPase is an enzyme essential for ion homeostasis in all cells. Over the last decades, it has been well-established that in addition to the transport of Na⁺/K⁺ over the cell membrane, the Na,K-ATPase acts as a receptor transducing humoral signals intracellularly. It has been suggested that ouabain-like compounds serve as endogenous modulators of this Na,K-ATPase signal transduction. The molecular mechanisms underlying Na,K-ATPase signaling are complicated and suggest the confluence of divergent biological pathways. This review discusses recent updates on the Na,K-ATPase signaling pathways characterized or suggested in vascular smooth muscle cells. The conventional view on this signaling is based on a microdomain structure where the Na,K-ATPase controls the Na,Ca-exchanger activity via modulation of intracellular Na⁺ in the spatially restricted submembrane space. This, in turn, affects intracellular Ca²⁺ and Ca²⁺ load in the sarcoplasmic reticulum leading to modulation of contractility as well as gene expression. An ion-transport-independent signal transduction from the Na,K-ATPase is based on molecular interactions. This was primarily characterized in other cell types but recently also demonstrated in vascular smooth muscles. The downstream signaling from the Na,K-ATPase includes Src and phosphatidylinositol-4,5-bisphosphate 3 kinase signaling pathways and generation of reactive oxygen species. Moreover, in vascular smooth muscle cells the interaction between the Na,K-ATPase and proteins responsible for Ca²⁺ homeostasis, e.g., phospholipase C and inositol triphosphate receptors, contributes to an integration of the signaling pathways. Recent update on the Na,K-ATPase dependent intracellular signaling and the significance for physiological functions and pathophysiological changes are discussed in this review.

Autonomic activity and its relationship with the endogenous cardiotonic steroid marinobufagenin: the African-PREDICT study

Aim: Marinobufagenin (MBG), a cardiotonic steroid and a natriuretic hormone, is elevated in response to high salt diet consumption. In animal models salt intake stimulates adrenocortical MBG secretion via increased angiotensin II, sympathetic activity and aldosterone. No evidence in humans exists to suggest the involvement of the angiotensinergic-sympatho-excitatory pathway in MBG production. We investigated whether MBG is related to indices of autonomic activity in men and women. Methods: This cross-sectional study included 680 black and white, men and women from the African-PREDICT study (aged 20-30 years). Continuous 24 hr ECG recordings were used to obtain low and high frequency (LF, HF) heart rate variability (HRV). We measured 24 hr urinary MBG excretion and serum aldosterone. Results: We found a positive association of MBG excretion with estimated salt intake (P < 0.001) and aldosterone (P < 0.001) in women and men. In women only, a positive relationship was evident between MBG excretion and LF HRV in multivariate adjusted regression analyses (Adj. R ²= 0.33; β = 0.11; P = 0.030). In men, MBG excretion associated positively with HF HRV in similar regression analyses (R ² = 0.36; β = 0.12; P = 0.034). Sex-specific results were corroborated only in blacks, namely, a positive association of MBG excretion with LF HRV in black women (R ² = 0.38; β = 0.13; P = 0.036), and negative association with HF HRV in black men (R ² = 0.40; β = 0.18; P = 0.045). No relationships were evident in white women (P = 0.58) or men (P = 0.27). Conclusion: Our findings in this human cohort support suggested mechanisms whereby MBG is elevated as a result of increased salt intake, including autonomic activity, previously demonstrated in Dahl salt-sensitive hypertension.

Telocinobufagin and Marinobufagin Produce Different Effects in LLC-PK1 Cells: A Case of Functional Selectivity of Bufadienolides

Bufadienolides are cardiotonic steroids (CTS) identified in mammals. Besides Na⁺/K⁺-ATPase inhibition, they activate signal transduction via protein–protein interactions. Diversity of endogenous bufadienolides and mechanisms of action may indicate the presence of functional selectivity and unique cellular outcomes. We evaluated whether the bufadienolides telocinobufagin and marinobufagin induce changes in proliferation or viability of pig kidney (LLC-PK1) cells and the mechanisms involved in these changes. In some experiments, ouabain was used as a positive control. CTS exhibited an inhibitory IC₅₀ of 0.20 (telocinobufagin), 0.14 (ouabain), and 3.40 μM (marinobufagin) for pig kidney Na⁺/K⁺-ATPase activity and concentrations that barely inhibited it were tested in LLC-PK1 cells. CTS induced rapid ERK1/2 phosphorylation, but corresponding proliferative response was observed for marinobufagin and ouabain instead of telocinobufagin. Telocinobufagin increased Bax:Bcl-2 expression ratio, sub-G0 cell cycle phase and pyknotic nuclei, indicating apoptosis. Src and MEK1/2 inhibitors blunted marinobufagin but not telocinobufagin effect, which was also not mediated by p38, JNK1/2, and PI3K. However, BIO, a GSK-3β inhibitor, reduced proliferation and, as telocinobufagin, phosphorylated GSK-3β at inhibitory Ser9. Combination of both drugs resulted in synergistic antiproliferative effect. Wnt reporter activity assay showed that telocinobufagin impaired Wnt/β-catenin pathway by acting upstream to β-catenin stabilization. Our findings support that mammalian endogenous bufadienolides may exhibit functional selectivity.

The α2 isoform Na,K-ATPase modulates contraction of rat mesenteric small artery via cSrc-dependent Ca2+ sensitization

AIMS

The Na,K-ATPase is involved in a large number of regulatory activities including cSrc-dependent signalling. Upon inhibition of the Na,K-ATPase with ouabain, cSrc activation is shown to occur in many cell types. This study tests the hypothesis that acute potentiation of agonist-induced contraction by ouabain is mediated through Na,K-ATPase-cSrc signalling-dependent sensitization of vascular smooth muscle cells to Ca²⁺ .

METHODS

Agonist-induced rat mesenteric small artery contraction was examined in vitro under isometric conditions and in vivo in anaesthetized rats. Arterial wall tension and [Ca²⁺ ]_i in vascular smooth muscle cells were measured simultaneously. Changes in cSrc and myosin phosphatase targeting protein 1 (MYPT1) phosphorylation were analysed by Western blot. Protein expression was examined with immunohistochemistry. The α1 and α2 isoforms of the Na,K-ATPase were transiently downregulated by siRNA transfection in vivo.

RESULTS

Ten micromolar ouabain, but not digoxin, potentiated contraction to noradrenaline. This effect was not endothelium-dependent. Ouabain sensitized smooth muscle cells to Ca²⁺ , and this was associated with increased phosphorylation of cSrc and MYPT1. Inhibition of tyrosine kinase by genistein, PP2 or pNaKtide abolished the potentiating effect of ouabain on arterial contraction and Ca²⁺ sensitization. Downregulation of the Na,K-ATPase α2 isoform made arterial contraction insensitive to ouabain and tyrosine kinase inhibition.

CONCLUSION

Data suggest that micromolar ouabain potentiates agonist-induced contraction of rat mesenteric small artery via Na,K-ATPase-dependent cSrc activation, which increases Ca²⁺ sensitization of vascular smooth muscle cells by MYPT1 phosphorylation. This mechanism may be critical for acute control of vascular tone.

The Na,K-ATPase-Dependent Src Kinase Signaling Changes with Mesenteric Artery Diameter

Inhibition of the Na,K-ATPase by ouabain potentiates vascular tone and agonist-induced contraction. These effects of ouabain varies between different reports. In this study, we assessed whether the pro-contractile effect of ouabain changes with arterial diameter and the molecular mechanism behind it. Rat mesenteric small arteries of different diameters (150–350 µm) were studied for noradrenaline-induced changes of isometric force and intracellular Ca²⁺ in smooth muscle cells. These functional changes were correlated to total Src kinase and Src phosphorylation assessed immunohistochemically. High-affinity ouabain-binding sites were semi-quantified with fluorescent ouabain. We found that potentiation of noradrenaline-sensitivity by ouabain correlates positively with an increase in arterial diameter. This was not due to differences in intracellular Ca²⁺ responses but due to sensitization of smooth muscle cell contractile machinery to Ca²⁺. This was associated with ouabain-induced Src activation, which increases with increasing arterial diameter. Total Src expression was similar in arteries of different diameters but the density of high-affinity ouabain binding sites increased with increasing arterial diameters. We suggested that ouabain binding induces more Src kinase activity in mesenteric small arteries with larger diameter leading to enhanced sensitization of the contractile machinery to Ca²⁺.

Na/K-ATPase Signaling and Cardiac Pre/Postconditioning with Cardiotonic Steroids

The first reports of cardiac Na/K-ATPase signaling, published 20 years ago, have opened several major fields of investigations into the cardioprotective action of low/subinotropic concentrations of cardiotonic steroids (CTS). This review focuses on the protective cardiac Na/K-ATPase-mediated signaling triggered by low concentrations of ouabain and other CTS, in the context of the enduring debate over the use of CTS in the ischemic heart. Indeed, as basic and clinical research continues to support effectiveness and feasibility of conditioning interventions against ischemia/reperfusion injury in acute myocardial infarction (AMI), the mechanistic information available to date suggests that unique features of CTS-based conditioning could be highly suitable, alone /or as a combinatory approach.

Isoform-specific role of Na/K-ATPase α1 in skeletal muscle

The distribution of Na/K-ATPase α-isoforms in skeletal muscle is unique, with α1 as the minor (15%) isoform and α2 comprising the bulk of the Na/K-ATPase pool. The acute and isoform-specific role of α2 in muscle performance and resistance to fatigue is well known, but the isoform-specific role of α1 has not been as thoroughly investigated. In vitro, we reported that α1 has a role in promoting cell growth that is not supported by α2. To assess whether α1 serves this isoform-specific trophic role in the skeletal muscle, we used Na/K-ATPase α1-haploinsufficient (α1^+/-) mice. A 30% decrease of Na/K-ATPase α1 protein expression without change in α2 induced a modest yet significant decrease of 10% weight in the oxidative soleus muscle. In contrast, the mixed plantaris and glycolytic extensor digitorum longus weights were not significantly affected, likely because of their very low expression level of α1 compared with the soleus. The soleus mass reduction occurred without change in total Na/K-ATPase activity or glycogen metabolism. Serum analytes including K⁺, fat tissue mass, and exercise capacity were not altered in α1^+/- mice. The impact of α1 content on soleus muscle mass is consistent with a Na/K-ATPase α1-specific role in skeletal muscle growth that cannot be fulfilled by α2. The preserved running capacity in α1^+/- is in sharp contrast with previously reported consequences of genetic manipulation of α2. Taken together, these results lend further support to the concept of distinct isoform-specific functions of Na/K-ATPase α1 and α2 in skeletal muscle.

Protein Interaction and Na/K-ATPase-Mediated Signal Transduction

The Na/K-ATPase (NKA), or Na pump, is a member of the P-type ATPase superfamily. In addition to pumping ions across cell membrane, it is engaged in assembly of multiple protein complexes in the plasma membrane. This assembly allows NKA to perform many non-pumping functions including signal transduction that are important for animal physiology and disease progression. This article will focus on the role of protein interaction in NKA-mediated signal transduction, and its potential utility as target for developing new therapeutics.

Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc kinase-dependent connexin43 phosphorylation

Communication between vascular smooth muscle cells (VSMCs) is dependent on gap junctions and is regulated by the Na-K-ATPase. The Na-K-ATPase is therefore important for synchronized VSMC oscillatory activity, i.e., vasomotion. The signaling between the Na-K-ATPase and gap junctions is unknown. We tested here the hypothesis that this signaling involves cSrc kinase. Intercellular communication was assessed by membrane capacitance measurements of electrically coupled VSMCs. Vasomotion in isometric myograph, input resistance, and synchronized [Ca2+]i transients were used as readout for intercellular coupling in rat mesenteric small arteries in vitro. Phosphorylation of cSrc kinase and connexin43 (Cx43) were semiquantified by Western blotting. Micromole concentration of ouabain reduced the amplitude of norepinephrine-induced vasomotion and desynchronized Ca2+ transients in VSMC in the arterial wall. Ouabain also increased input resistance in the arterial wall. These effects of ouabain were antagonized by inhibition of tyrosine phosphorylation with genistein, PP2, and by an inhibitor of the Na-K-ATPase-dependent cSrc activation, pNaKtide. Moreover, inhibition of cSrc phosphorylation increased vasomotion amplitude and decreased the resistance between cells in the vascular wall. Ouabain inhibited the electrical coupling between A7r5 cells, but pNaKtide restored the electrical coupling. Ouabain increased cSrc autophosphorylation of tyrosine 418 (Y418) required for full catalytic activity whereas pNaKtide antagonized it. This cSrc activation was associated with Cx43 phosphorylation of tyrosine 265 (Y265). Our findings demonstrate that Na-K-ATPase regulates intercellular communication in the vascular wall via cSrc-dependent Cx43 tyrosine phosphorylation.

Membrane lipid rafts are disturbed in the response of rat skeletal muscle to short-term disuse

Marked loss of skeletal muscle mass occurs under various conditions of disuse, but the molecular and cellular mechanisms leading to atrophy are not completely understood. We investigate early molecular events that might play a role in skeletal muscle remodeling during mechanical unloading (disuse). The effects of acute (6-12 h) hindlimb suspension on the soleus muscles from adult rats were examined. The integrity of plasma membrane lipid rafts was tested utilizing cholera toxin B subunit or fluorescent sterols. In addition, resting intracellular Ca²⁺ level was analyzed. Acute disuse disturbed the plasma membrane lipid-ordered phase throughout the sarcolemma and was more pronounced in junctional membrane regions. Ouabain (1 µM), which specifically inhibits the Na-K-ATPase α2 isozyme in rodent skeletal muscles, produced similar lipid raft changes in control muscles but was ineffective in suspended muscles, which showed an initial loss of α2 Na-K-ATPase activity. Lipid rafts were able to recover with cholesterol supplementation, suggesting that disturbance results from cholesterol loss. Repetitive nerve stimulation also restores lipid rafts, specifically in the junctional sarcolemma region. Disuse locally lowered the resting intracellular Ca²⁺ concentration only near the neuromuscular junction of muscle fibers. Our results provide evidence to suggest that the ordering of lipid rafts strongly depends on motor nerve input and may involve interactions with the α2 Na-K-ATPase. Lipid raft disturbance, accompanied by intracellular Ca²⁺ dysregulation, is among the earliest remodeling events induced by skeletal muscle disuse.

The Role of Na/K-ATPase Signaling in Oxidative Stress Related to Obesity and Cardiovascular Disease

Na/K-ATPase has been extensively studied for its ion pumping function, but, in the past several decades, has been identified as a scaffolding and signaling protein. Initially it was found that cardiotonic steroids (CTS) mediate signal transduction through the Na/K-ATPase and result in the generation of reactive oxygen species (ROS), which are also capable of initiating the signal cascade. However, in recent years, this Na/K-ATPase/ROS amplification loop has demonstrated significance in oxidative stress related disease states, including obesity, atherosclerosis, heart failure, uremic cardiomyopathy, and hypertension. The discovery of this novel oxidative stress signaling pathway, holds significant therapeutic potential for the aforementioned conditions and others that are rooted in ROS.

Specialized Functional Diversity and Interactions of the Na,K-ATPase

Na,K-ATPase is a protein ubiquitously expressed in the plasma membrane of all animal cells and vitally essential for their functions. A specialized functional diversity of the Na,K-ATPase isozymes is provided by molecular heterogeneity, distinct subcellular localizations, and functional interactions with molecular environment. Studies over the last decades clearly demonstrated complex and isoform-specific reciprocal functional interactions between the Na,K-ATPase and neighboring proteins and lipids. These interactions are enabled by a spatially restricted ion homeostasis, direct protein-protein/lipid interactions, and protein kinase signaling pathways. In addition to its "classical" function in ion translocation, the Na,K-ATPase is now considered as one of the most important signaling molecules in neuronal, epithelial, skeletal, cardiac and vascular tissues. Accordingly, the Na,K-ATPase forms specialized sub-cellular multimolecular microdomains which act as receptors to circulating endogenous cardiotonic steroids (CTS) triggering a number of signaling pathways. Changes in these endogenous cardiotonic steroid levels and initiated signaling responses have significant adaptive values for tissues and whole organisms under numerous physiological and pathophysiological conditions. This review discusses recent progress in the studies of functional interactions between the Na,K-ATPase and molecular microenvironment, the Na,K-ATPase-dependent signaling pathways and their significance for diversity of cell function.

Expression of rat Na-K-ATPase α2 enables ion pumping but not ouabain-induced signaling in α1-deficient porcine renal epithelial cells

Na-K-ATPase is a fundamental component of ion transport. Four α isoforms of the Na-K-ATPase catalytic α subunit are expressed in human cells. The ubiquitous Na-K-ATPase α1 was recently discovered to also mediate signal transduction through Src kinase. In contrast, α2 expression is limited to a few cell types including myocytes, where it is coupled to the Na(+)/Ca(2+) exchanger. To test whether rat Na-K-ATPase α2 is capable of cellular signaling like its α1 counterpart in a recipient mammalian system, we used an α1 knockdown pig renal epithelial cell (PY-17) to create an α2-expressing cell line with no detectable level of α1 expression. These cells exhibited normal ouabain-sensitive ATPase, but failed to effectively regulate Src. In contrast to α1-expressing cells, ouabain did not stimulate Src kinase or downstream effectors such as ERK and Akt in α2 cells, although their signaling apparatus was intact as evidenced by EGF-mediated signal transduction. Additionally, α2 cells were unable to rescue caveolin-1. Unlike the NaKtide sequence derived from Na-K-ATPase α1, which downregulates basal Src activity, the corresponding α2 NaKtide was unable to inhibit Src in vitro. Finally, coimmunoprecipitation of cellular Src was diminished in α2 cells. These findings indicate that Na-K-ATPase α2 does not regulate Src and, therefore, may not serve the same role in signal transduction as α1. This further implies that the signaling mechanism of Na-K-ATPase is isoform specific, thereby supporting a model where α1 and α2 isoforms play distinct roles in mediating contraction and signaling in myocytes.

Involvement of Na/K-ATPase in hydrogen peroxide-induced activation of the Src/ERK pathway in LLC-PK1 cells

We have shown that Na/K-ATPase interacts with Src. Here, we test the role of this interaction in H2O2-induced activation of Src and ERK. We found that exposure of LLC-PK1 cells to H2O2 generated by the addition of glucose oxidase into the culture medium activated Src and ERK1/2. It also caused a modest reduction in the number of surface Na/K-ATPases and in ouabain-sensitive Rb(+) uptake. These effects of H2O2 seem similar to those induced by ouabain, a specific ligand of Na/K-ATPase, in LLC-PK1 cells. In accordance, we found that the effects of H2O2 on Src and ERK1/2 were inhibited in α1 Na/K-ATPase-knockdown PY-17 cells. Whereas expression of wild-type α1 or the A420P mutant α1 defective in Src regulation rescued the pumping activity in PY-17 cells, only α1, and not the A420P mutant, was able to restore the H2O2-induced activation of protein kinases. Consistent with this, disrupting the formation of the Na/K-ATPase/Src complex with pNaKtide attenuated the effects of H2O2 on the kinases. Moreover, a direct effect of H2O2 on Na/K-ATPase-mediated regulation of Src was demonstrated. Finally, H2O2 reduced the expression of E-cadherin through the Na/K-ATPase/Src-mediated signaling pathway. Taken together, the data suggest that the Na/K-ATPase/Src complex may serve as one of the receptor mechanisms for H2O2 to regulate Src/ERK protein kinases and consequently the phenotype of renal epithelial cells.

Regulation of renal function and structure by the signaling Na/K-ATPase

The Na/K-ATPase as an essential ion pump was discovered more than 50 years ago (Skou (1989) Biochim. Biophys. Acta 1000, 439-446; Feraille and Doucet (2001) Physiol. Rev. 81, 345-418). The signaling function of Na/K-ATPase has been gradually appreciated over the last 20 years, first from the studies of regulatory effects of ouabain on cardiac cell growth. Several reviews on this topic have been written during the last few years (Schoner and Scheiner-Bobis (2007) Am. J. Physiol. Cell. Physiol. 293, C509-C536; Xie and Cai (2003) Mol. Interv. 3, 157 - 168; Bagrov et al. (2009) Pharmacol. Rev. 61, 9-38; Tian and Xie (2008) Physiology 23, 205-211; Fontana et al. (2013) FEBS J. 280, 5450-5455; Blanco and Wallace (2013) Am. J. Physiol. Renal Physiol. 305, F797-F812). This article will focus on the molecular mechanism of Na/K-ATPase-mediated signal transduction and its potential regulatory role in renal physiology and diseases.

Ouabain-induced internalization and lysosomal degradation of the Na+/K+-ATPase

Internalization of the Na(+)/K(+)-ATPase (the Na(+) pump) has been studied in the human lung carcinoma cell line H1299 that expresses YFP-tagged α1 from its normal genomic localization. Both real-time imaging and surface biotinylation have demonstrated internalization of α1 induced by ≥100 nm ouabain which occurs in a time scale of hours. Unlike previous studies in other systems, the ouabain-induced internalization was insensitive to Src or PI3K inhibitors. Accumulation of α1 in the cells could be augmented by inhibition of lysosomal degradation but not by proteosomal inhibitors. In agreement, the internalized α1 could be colocalized with the lysosomal marker LAMP1 but not with Golgi or nuclear markers. In principle, internalization could be triggered by a conformational change of the ouabain-bound Na(+)/K(+)-ATPase molecule or more generally by the disruption of cation homeostasis (Na(+), K(+), Ca(2+)) due to the partial inhibition of active Na(+) and K(+) transport. Overexpression of ouabain-insensitive rat α1 failed to inhibit internalization of human α1 expressed in the same cells. In addition, incubating cells in a K(+)-free medium did not induce internalization of the pump or affect the response to ouabain. Thus, internalization is not the result of changes in the cellular cation balance but is likely to be triggered by a conformational change of the protein itself. In physiological conditions, internalization may serve to eliminate pumps that have been blocked by endogenous ouabain or other cardiac glycosides. This mechanism may be required due to the very slow dissociation of the ouabain·Na(+)/K(+)-ATPase complex.

Calcium oscillations triggered by cardiotonic steroids

Na(+), K(+)-ATPase (NKA) is well known for its function as an ion pump. Studies during the last decade have revealed an additional role for NKA as a signal transducer. In this brief review, we describe how cardiotonic steroids, which are highly specific NKA ligands, trigger slow Ca(2+) oscillations by promoting the interaction between NKA and the inositol trisphosphate receptor, and how this Ca(2+) signal activates the NF-κB subunit p65 and increases the expression of the antiapoptotic factor Bcl-xL. The potential tissue-protective effects of this signal are discussed.

Regulatory mechanisms for glycogenolysis and K+ uptake in brain astrocytes

Recent advances in brain energy metabolism support the notion that glycogen in astrocytes is necessary for the clearance of neuronally-released K(+) from the extracellular space. However, how the multiple metabolic pathways involved in K(+)-induced increase in glycogen turnover are regulated is only partly understood. Here we summarize the current knowledge about the mechanisms that control glycogen metabolism during enhanced K(+) uptake. We also describe the action of the ubiquitous Na(+)/K(+) ATPase for both ion transport and intracellular signaling cascades, and emphasize its importance in understanding the complex relation between glycogenolysis and K(+) uptake.

Rethinking Heart Failure

An increasing body of clinical observations and experimental evidence suggests that cardiac dysfunction results from autonomic dysregulation of the contractile output of the heart. Excessive activation of the sympathetic nervous system and a decrease in parasympathetic tone are associated with increased mortality. Elevated levels of circulating catecholamines closely correlate with the severity and poor prognosis in heart failure. Sympathetic over-stimulation causes increased levels of catecholamines, which induce excessive aerobic metabolism leading to excessive cardiac oxygen consumption. Resulting impaired mitochondrial function causes acidosis, which results in reduction in blood flow by impairment of contractility. To the extent that the excessive aerobic metabolism resulting from adrenergic stimulation comes to a halt the energy deficit has to be compensated for by anaerobic metabolism. Glucose and glycogen become the essential nutrients. Beta-adrenergic blockade is used successfully to decrease hyperadrenergic drive. Neurohumoral antagonists block adrenergic over-stimulation but do not provide the heart with fuel for compensatory anaerobic metabolism. The endogenous hormone ouabain reduces catecholamine levels in healthy volunteers, promotes the secretion of insulin, induces release of acetylcholine from synaptosomes and potentiates the stimulation of glucose metabolism by insulin and acetylcholine. Ouabain stimulates glycogen synthesis and increases lactate utilisation by the myocardium. Decades of clinical experience with ouabain confirm the cardioprotective effects of this endogenous hormone. The so far neglected sympatholytic and vagotonic effects of ouabain on myocardial metabolism clearly make a clinical re-evaluation of this endogenous hormone necessary. Clinical studies with ouabain that correspond to current standards are warranted.

Reactive Oxygen Species Modulation of Na/K-ATPase Regulates Fibrosis and Renal Proximal Tubular Sodium Handling

The Na/K-ATPase is the primary force regulating renal sodium handling and plays a key role in both ion homeostasis and blood pressure regulation. Recently, cardiotonic steroids (CTS)-mediated Na/K-ATPase signaling has been shown to regulate fibrosis, renal proximal tubule (RPT) sodium reabsorption, and experimental Dahl salt-sensitive hypertension in response to a high-salt diet. Reactive oxygen species (ROS) are an important modulator of nephron ion transport. As there is limited knowledge regarding the role of ROS-mediated fibrosis and RPT sodium reabsorption through the Na/K-ATPase, the focus of this review is to examine the possible role of ROS in the regulation of Na/K-ATPase activity, its signaling, fibrosis, and RPT sodium reabsorption.

Na,K-ATPase activity modulates Src activation: a role for ATP/ADP ratio

Digitalis-like compounds (DLCs), specific inhibitors of Na,K-ATPase, are implicated in cellular signaling. Exposure of cell cultures to ouabain, a well-known DLC, leads to up- or down regulation of various processes and involves activation of Src kinase. Since Na,K-ATPase is the only known target for DLC binding an in vitro experimental setup using highly purified Na,K-ATPase from pig kidney and commercially available recombinant Src was used to investigate the mechanism of coupling between the Na,K-ATPase and Src. Digoxin was used as a representative DLC for inhibition of Na,K-ATPase. The activation of Src kinase was measured as the degree of its autophosphorylation. It was observed that in addition to digoxin, Src activation was dependent on concentrations of other specific ligands of Na,K-ATPase: Na(+), K(+), vanadate, ATP and ADP. The magnitude of the steady-state ATPase activity therefore seemed to affect Src activation. Further experiments with an ATP regenerating system showed that the ATP/ADP ratio determined the extent of Src activation. Thus, our model system which represents the proposed very proximal part of the Na,K-ATPase-Src signaling cascade, shows that Src kinase activity is regulated by both ATP and ADP concentrations and provides no evidence for a direct interaction between Na,K-ATPase and Src.

Formation of new high density glycogen-microtubule structures is induced by cardiac steroids

Cardiac steroids (CS), an important class of naturally occurring compounds, are synthesized in plants and animals. The only established receptor for CS is the ubiquitous Na(+),K(+)-ATPase, a major plasma membrane transporter. The binding of CS to Na(+),K(+)-ATPase causes the inhibition of Na(+) and K(+) transport and elicits cell-specific activation of several intracellular signaling mechanisms. It is well documented that the interaction of CS with Na(+),K(+)-ATPase is responsible for numerous changes in basic cellular physiological properties, such as electrical plasma membrane potential, cell volume, intracellular [Ca(2+)] and pH, endocytosed membrane traffic, and the transport of other solutes. In the present study we show that CS induces the formation of dark structures adjacent to the nucleus in human NT2 and ACHN cells. These structures, which are not surrounded by membranes, are clusters of glycogen and a distorted microtubule network. Formation of these clusters results from a relocation of glycogen and microtubules in the cells, two processes that are independent of one another. The molecular mechanisms underlying the formation of the clusters are mediated by the Na(+),K(+)-ATPase, ERK1/2 signaling pathway, and an additional unknown factor. Similar glycogen clusters are induced by hypoxia, suggesting that the CS-induced structural change, described in this study, may be part of a new type of cellular stress response.

Ouabain stimulates Na-K-ATPase through a sodium/hydrogen exchanger-1 (NHE-1)-dependent mechanism in human kidney proximal tubule cells

Recent investigations demonstrate increased Na/H exchanger-1 (NHE-1) activity and plasma levels of ouabain-like factor in spontaneously hypertensive rats. At nanomolar concentrations, ouabain increases Na-K-ATPase activity, induces cell proliferation, and activates complex signaling cascades. We hypothesize that the activity of NHE-1 and Na-K-ATPase are interdependent. To test whether treatment with picomolar ouabain regulates Na-K-ATPase through an NHE-1-dependent mechanism, we examined the role of NHE-1 in ouabain-mediated stimulation of Na-K-ATPase in kidney proximal tubule cell lines [opossum kidney (OK), HK-2, HKC-5, and HKC-11] and rat kidney basolateral membranes. Ouabain stimulated Na-K-ATPase activity and tyrosine phosphorylation in cells that express NHE-1 (OK, HKC-5, and HKC-11) but not in HK-2 cells that express very low levels of NHE-1. Inhibition of NHE-1 with 5 microM EIPA, a NHE-1-specific inhibitor, prevented ouabain-mediated stimulation of (86)Rb uptake and Na-K-ATPase phosphorylation in OK, HKC-5, and HKC-11 cells. Expression of wild-type NHE-1 in HK2 cells restored regulation of Na-K-ATPase by picomolar ouabain. Treatment with picomolar ouabain increased membrane expression of Na-K-ATPase and enhanced NHE-1-Na-K-ATPase alpha1-subunit association. Treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) increased Na-K-ATPase activity, expression, phosphorylation, and association with NHE-1 increased in rat kidney cortical basolateral membranes. Eight days' treatment with ouabain (1 microg x kg body wt(-1) x day(-1)) resulted in increased blood pressure in these rats. These results suggest that the association of NHE-1 with Na-K-ATPase is critical for ouabain-mediated regulation of Na-K-ATPase and that these effects may play a role in cardioglycoside-stimulated hypertension.

The sodium pump and cardiotonic steroids-induced signal transduction protein kinases and calcium-signaling microdomain in regulation of transporter trafficking

The Na/K-ATPase was discovered as an energy transducing ion pump. A major difference between the Na/K-ATPase and other P-type ATPases is its ability to bind a group of chemicals called cardiotonic steroids (CTS). The plant-derived CTS such as digoxin are valuable drugs for the management of cardiac diseases, whereas ouabain and marinobufagenin (MBG) have been identified as a new class of endogenous hormones. Recent studies have demonstrated that the endogenous CTS are important regulators of renal Na(+) excretion and blood pressure. The Na/K-ATPase is not only an ion pump, but also an important receptor that can transduce the ligand-like effect of CTS on intracellular protein kinases and Ca(2+) signaling. Significantly, these CTS-provoked signaling events are capable of reducing the surface expression of apical NHE3 (Na/H exchanger isoform 3) and basolateral Na/K-ATPase in renal proximal tubular cells. These findings suggest that endogenous CTS may play an important role in regulation of tubular Na(+) excretion under physiological conditions; conversely, a defect at either the receptor level (Na/K-ATPase) or receptor-effector coupling would reduce the ability of renal proximal tubular cells to excrete Na(+), thus culminating/resulting in salt-sensitive hypertension.

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.

A-769662 activates AMPK β1-containing complexes but induces glucose uptake through a PI3-kinase-dependent pathway in mouse skeletal muscle

5′-AMP-activated protein kinase (AMPK) regulates several aspects of metabolism. Recently, A-769662 was shown to activate AMPK in skeletal muscle. However, no biological effects of AMPK activation by A-769662 in this tissue have been reported. We hypothesized that A-769662 would increase glucose uptake in skeletal muscle. We studied incubated soleus and extensor digitorum longus (EDL) muscles from 129S6/sv and C57BL/6 mice. Glucose uptake increased only in soleus from 129S6/sv when concentrations of A-769662 were 500 μM (∼15%, P < 0.05) and 1 mM (∼60%, P < 0.01). AMPK β1- but not β2-containing complexes were dose dependently activated by A-769662 in muscles from both genotypes (∼100% at 200 μM and 300–600% at 1 mM). The discrepancy between the A-769662-induced AMPK activation pattern and stimulation of glucose uptake suggested that these effects were unrelated. A-769662 increased phosphorylation of Akt in both muscles from both genotypes, with phosphorylation of T308 being significantly higher in soleus than in EDL in 129S6/sv mice ( P < 0.01). In soleus from 129S6/sv mice, insulin receptor substrate 1-associated phosphatidylinositol 3 (PI3)-kinase activity was markedly increased with A-769662, and Akt phosphorylation and glucose uptake were inhibited by wortmannin while phosphorylation of acetyl-CoA carboxylase (S227) was unaffected. Thus, A-769662 activates β1-containing AMPK complexes in skeletal muscle but induces glucose uptake through a PI3-kinase-dependent pathway. Although development of A-769662 has constituted a step forward in the search for AMPK activators targeting specific AMPK trimers, our data suggest that in intact muscle, A-769662 has off-target effects. This may limit use of A-769662 to study the role of AMPK in skeletal muscle metabolism.

Changes in sodium pump expression dictate the effects of ouabain on cell growth

Here we show that ouabain-induced cell growth regulation is intrinsically coupled to changes in the cellular amount of Na/K-ATPase via the phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway. Ouabain increases the endocytosis and degradation of Na/K-ATPase in LLC-PK1, human breast (BT20), and prostate (DU145) cancer cells. However, ouabain stimulates the PI3K/Akt/mTOR pathway and consequently up-regulates the expression of Na/K-ATPase in LLC-PK1 but not BT20 and DU145 cells. This up-regulation is sufficient to replete the plasma membrane pool of Na/K-ATPase and to stimulate cell proliferation in LLC-PK1 cells. On the other hand, ouabain causes a gradual depletion of Na/K-ATPase and an increased expression of cell cycle inhibitor p21(cip), which consequently inhibits cell proliferation in BT20 and DU145 cells. Consistently, we observe that small interfering RNA-mediated knockdown of Na/K-ATPase is sufficient to induce the expression of p21(cip) and slow the proliferation of LLC-PK1 cells. Moreover, this knockdown converts the growth stimulatory effect of ouabain to growth inhibition in LLC-PK1 cells. Mechanistically, both Src and caveolin-1 are required for ouabain-induced activation of Akt and up-regulation of Na/K-ATPase. Furthermore, inhibition of the PI3K/Akt/mTOR pathway by rapamycin completely blocks ouabain-induced expression of Na/K-ATPase and converts ouabain-induced growth stimulation to growth inhibition in LLC-PK1 cells. Taken together, we conclude that changes in the expression of Na/K-ATPase dictate the growth regulatory effects of ouabain on cells.

Roles of ERK, PI3 kinase, and PLC-gamma pathways induced by overexpression of translationally controlled tumor protein in HeLa cells

We reported previously that translationally controlled tumor protein (TCTP) is a cytoplasmic repressor of Na,K-ATPase in HeLa cells. In the current study, we showed that TCTP overexpression using adenovirus as vehicle, induced partial inhibition of Na,K-ATPase; phosphorylation of EGFR tyrosine residues 845, 992,1068, and 1148; activation of Ras/Raf/ERK pathway; activation of PI3K/Akt pathway; and phosphorylation of PLC-gamma in HeLa cells. Specific inhibition of PI3K/Akt pathway in contrast to the inhibition of ERK,significantly decreased TCTP overexpression-induced survival signal. Inhibition of PLC-gamma pathway significantly decreased TCTP overexpression-induced cell migration but inhibition of ERK had less effect. These results suggest that TCTP plays a key physiological role in cell survival through Akt pathway and migration through PLC-gamma pathway.

Signal pathways in ouabain-induced proliferation of leukemia cells

BACKGROUND

Cardiotonic steroids (CTSs) can bind to Na(+)/K(+)-ATPase and activate protein kinase cascades, resulting in changes in cell proliferation, differentiation or apoptosis in a cell-specific manner. We explored the participation of ouabain-activated signaling pathways in growth regulation of leukemia cells.

METHODS

Lymphocytic leukemia Jhhan cells and megakaryocytic leukemia M07e cells were incubated at different concentrations of ouabain (0, 1 and 10 nmol) for 24 hours. Cell proliferation was measured by methyl thiazolyl tetrazolium (MTT) assay. To probe the role of ouabain-induced signaling in control of cell growth, we employed Src kinase inhibitor PP2 and the MEK inhibitor PD98059, respectively. The expression of Na(+)/K(+)-ATPase alpha1 subunit of leukemia cells was evaluated by RT-PCR and Western blotting.

RESULTS

One nmol and 10 nmol ouabain promoted proliferation of both Jhhan and M07e cells. Ouabain also up-regulated the expression of Na(+)/K(+)-ATPase alpha1 subunit. Addition of either PP2 or PD98059 blocked the effects of ouabain on cell proliferation.

CONCLUSION

Ouabain activates Src and ERK1/2 pathways and regulates the proliferation of leukemia cells.

Endogenous cardiotonic steroids: physiology, pharmacology, and novel therapeutic targets

Endogenous cardiotonic steroids (CTS), also called digitalis-like factors, have been postulated to play important roles in health and disease for nearly half a century. Recent discoveries, which include the specific identification of endogenous cardenolide (endogenous ouabain) and bufadienolide (marinobufagenin) CTS in humans along with the delineation of an alternative mechanism by which CTS can signal through the Na(+)/K(+)-ATPase, have increased the interest in this field substantially. Although CTS were first considered important in the regulation of renal sodium transport and arterial pressure, more recent work implicates these hormones in the regulation of cell growth, differentiation, apoptosis, and fibrosis, the modulation of immunity and of carbohydrate metabolism, and the control of various central nervous functions and even behavior. This review focuses on the physiological interactions between CTS and other regulatory systems that may be important in the pathophysiology of essential hypertension, preeclampsia, end-stage renal disease, congestive heart failure, and diabetes mellitus. Based on our increasing understanding of the regulation of CTS as well as the molecular mechanisms of these hormone increases, we also discuss potential therapeutic strategies.

The cardiotonic steroid hormone marinobufagenin induces renal fibrosis: implication of epithelial-to-mesenchymal transition

We recently demonstrated that the cardiotonic steroid marinobufagenin (MBG) induced fibrosis in rat hearts through direct stimulation of collagen I secretion by cardiac fibroblasts. This stimulation was also responsible for the cardiac fibrosis seen in experimental renal failure. In this study, the effect of MBG on the development of renal fibrosis in rats was investigated. Four weeks of MBG infusion triggered mild periglomerular and peritubular fibrosis in the cortex and the appearance of fibrotic scars in the corticomedullary junction of the kidney. MBG also significantly increased the protein levels and nuclear localization of the transcription factor Snail in the tubular epithelia. It is known that activation of Snail is associated with epithelial-to-mesenchymal transition (EMT) during renal fibrosis. To examine whether MBG alone can trigger EMT, we used the porcine proximal tubular cell line LLC-PK1. MBG (100 nM) caused LLC-PK1 cells grown to confluence to acquire a fibroblast-like shape and have an invasive motility. The expressions of the mesenchymal proteins collagen I, fibronectin, and vimentin were increased twofold. However, the total level of E-cadherin remained unchanged. These alterations in LLC-PK1 cells in the presence of MBG were accompanied by elevated expression and nuclear translocation of Snail. During the time course of EMT, MBG did not have measurable inhibitory effects on the ion pumping activity of its natural ligand, Na(+)-K(+)-ATPase. Our data suggest that the MBG may be an important factor in inducing EMT and, through this mechanism, elevated levels of MBG in chronic renal failure may play a role in the progressive fibrosis.

Frontiers: skeletal muscle sodium pump regulation: a translocation paradigm

The skeletal muscle sodium pump plays a major role in the removal of K(+) ions from the circulation postprandial, or after a physical activity bout, thereby preventing the development of hyperkalemia and fatigue. Insulin and muscle contractions stimulate Na(+)-K(+)-ATPase activity in skeletal muscle, at least partially via translocation of sodium pump units to the plasma membrane from intracellular stores. The molecular mechanism of this phenomenon is poorly understood. Due to the contradictory reports in the literature, the very existence of the translocation of Na(+)-K(+)-ATPase to the skeletal muscle cell surface is questionable. This review summarizes more than 30 years work on the skeletal muscle sodium pump translocation paradigm. Furthermore, the methodological caveats of major approaches to study the sodium pump translocation in skeletal muscle are discussed. An understanding of the molecular regulation of Na(+)-K(+)-ATPase in skeletal muscle will have important clinical implications for the understanding of the development of complications associated with the metabolic syndrome, such as cardiovascular diseases or increased muscle fatigue in diabetic patients.

Ouabain-induced stimulation of sodium-hydrogen exchange in rat optic nerve astrocytes

Sodium-dependent transporters are inhibited indirectly by the Na-K-ATPase inhibitor ouabain. Here we report stimulation of sodium-hydrogen exchange (NHE) in ouabain-treated cells. BCECF was used to measure cytoplasmic pH in cultured rat optic nerve astrocytes. Ammonium chloride was applied to acid load the cells. On removal of ammonium chloride, cytoplasmic pH fell abruptly, then gradually recovered toward baseline. Ouabain (1 microM) did not change cell sodium content, but the rate of pH recovery increased by 68%. Ouabain speeded pH recovery both in the presence and absence of bicarbonate. In bicarbonate-free medium, dimethylamiloride, an NHE inhibitor, eliminated the effect of 1 microM ouabain on pH recovery. Western blot analysis showed an NHE1 immunoreactive band but not NHE2, NHE3, or NHE4. Immunoprecipitation studies showed phosphorylation of NHE1 in cells treated with 1 microM ouabain. Ouabain evoked an increase of cAMP, and the effect of 1 microM ouabain on pH recovery was abolished by H-89, a protein kinase A inhibitor. 8-Bromoadenosine-cAMP increased the pH recovery rate, and this recovery was not further increased by ouabain. Although 1 microM ouabain did not alter cytoplasmic calcium concentration, it stimulated calcium entry after store depletion, a response abolished by 2-APB. Ouabain-induced stimulation of pH recovery was suppressed by inhibitors of capacitative calcium entry, SKF-96365, and 2-APB, as well as the cytoplasmic calcium chelator BAPTA. The cAMP increase in ouabain-treated cells was abolished by BAPTA and 2-APB. Taken together, the results are consistent with increased capacitative calcium entry and subsequent cAMP-PKA-dependent stimulation of NHE1 in ouabain-treated cells.