Low dose ouabain stimulates NaK ATPase α1 subunit association with angiotensin II type 1 receptor in renal proximal tubule cells

Effects of Renal Denervation on Ouabain-Induced Hypertension in Rats

Background

Ouabain, a Na+, K+-ATPase inhibitor, is elevated in hypertensive patients. Evidence suggests ouabain contributes to hypertension mainly through activation of the sympathetic nervous system (SNS). Renal nerves play a vital role in the regulation of SNS activity, so we hypothesize that renal denervation may attenuate the development of ouabain-induced hypertension.

Methods and Results

Forty Sprague-Dawley rats were divided into following groups (n = 10 each): control group (sham surgery plus intraperitoneal saline injection), RDN group (renal denervation (RDN) plus intraperitoneal saline injection), ouabain group (sham surgery plus intraperitoneal ouabain injection), and ouabain + RDN group (RDN plus intraperitoneal ouabain injection). After eight weeks, compared with the control group, rats in the ouabain group exhibited elevated blood pressure (P < 0.05), increased plasma epinephrine, norepinephrine, angiotensin II, and aldosterone levels (P < 0.05). These indexes could be significantly ameliorated by RDN. RDN also reduced the thickening of aortic tunica media and downregulated the expression of proliferating cell nuclear antigen (PCNA) in the thoracic aorta induced by ouabain. Masson staining and echocardiography showed that myocardial fibrosis and increased left ventricular mass in the ouabain group could be attenuated by RDN.

Conclusions

The present study reveals that renal nerves play an important role in the development of ouabain-induced hypertension. RDN could inhibit the pressor effect and the myocardial remodeling induced by ouabain potentially via inhibiting catecholamine release and vascular smooth muscle cell proliferation. Clinical studies are needed to explore whether RDN may exhibit better antihypertensive effects on hypertensive patients with high plasma ouabain levels as compared to those with normal plasma ouabain levels.

Cyclooxygenase-2 is a critical determinant of angiotensin II-induced vascular remodeling and stiffness in resistance arteries of ouabain-treated rats

OBJECTIVE

To investigate the role of angiotensin II/AT 1 receptor signaling and/or cyclooxygenase-2 (COX-2) activation on vascular remodeling and stiffening of the mesenteric resistance arteries (MRA) of ouabain-treated rats.

METHODS

Ouabain-treated (OUA, 30 μg kg/day for 5 weeks) and vehicle (VEH)-treated Wistar rats were co-treated with losartan (LOS, AT 1 R antagonist), nimesulide (NIM, COX-2 inhibitor) or hydralazine hydrochloride plus hydrochlorothiazide. MRA structure and mechanics were assessed with pressure myography and histology. Picrosirius red staining was used to determine the total collagen content. Western blotting was used to detect the expression of collagen I/III, MMP-2, Src, NFκB, Bax, Bcl-2 and COX-2. Reactive oxygen species (ROS) and plasma angiotensin II levels were measured by fluorescence and ELISA, respectively.

RESULTS

Blockade of AT 1 R or inhibition of COX-2 prevented ouabain-induced blood pressure elevation. Plasma angiotensin II level was higher in OUA than in VEH. LOS, but not hydralazine hydrochloride with hydrochlorothiazide, prevented inward hypotrophic remodeling, increased collagen deposition and stiffness, and oxidative stress in OUA MRA. LOS prevented the reduction in the total number of nuclei in the media layer and the Bcl-2 expression induced by OUA in MRA. The higher pSrc/Src ratio, NFκB/IκB ratio, and COX-2 expression in OUA MRA were also prevented by LOS. Likewise, COX-2 inhibition prevented vascular remodeling, mechanical changes, oxidative stress and inflammation in OUA MRA.

CONCLUSION

The results suggest that, regardless of hemodynamic adjustments, the angiotensin II/AT 1 R/pSrc/ROS/NFκB/COX-2 pathway is involved in the development of MRA inward hypotrophic remodeling and stiffness in ouabain-treated rats.

Short-Term Mild Hypoxia Modulates Na,K-ATPase to Maintain Membrane Electrogenesis in Rat Skeletal Muscle

The Na,K-ATPase plays an important role in adaptation to hypoxia. Prolonged hypoxia results in loss of skeletal muscle mass, structure, and performance. However, hypoxic preconditioning is known to protect against a variety of functional impairments. In this study, we tested the possibility of mild hypoxia to modulate the Na,K-ATPase and to improve skeletal muscle electrogenesis. The rats were subjected to simulated high-altitude (3000 m above sea level) hypobaric hypoxia (HH) for 3 h using a hypobaric chamber. Isolated diaphragm and soleus muscles were tested. In the diaphragm muscle, HH increased the α2 Na,K-ATPase isozyme electrogenic activity and stably hyperpolarized the extrajunctional membrane for 24 h. These changes were accompanied by a steady increase in the production of thiobarbituric acid reactive substances as well as a decrease in the serum level of endogenous ouabain, a specific ligand of the Na,K-ATPase. HH also increased the α2 Na,K-ATPase membrane abundance without changing its total protein content; the plasma membrane lipid-ordered phase did not change. In the soleus muscle, HH protected against disuse (hindlimb suspension) induced sarcolemmal depolarization. Considering that the Na,K-ATPase is critical for maintaining skeletal muscle electrogenesis and performance, these findings may have implications for countermeasures in disuse-induced pathology and hypoxic therapy.

Chronic Ouabain Prevents Radiation-Induced Reduction in the α2 Na,K-ATPase Function in the Rat Diaphragm Muscle

The damaging effect of ionizing radiation (IR) on skeletal muscle Na,K-ATPase is an open field of research. Considering a therapeutic potential of ouabain, a specific ligand of the Na,K-ATPase, we tested its ability to protect against the IR-induced disturbances of Na,K-ATPase function in rat diaphragm muscle that co-expresses the α1 and α2 isozymes of this protein. Male Wistar rats (n = 26) were subjected to 6-day injections of vehicle (0.9% NaCl) or ouabain (1 µg/kg/day). On the fourth day of injections, rats were exposed to one-time total-body X-ray irradiation (10 Gy), or a sham irradiation. The isolated muscles were studied 72 h post-irradiation. IR decreased the electrogenic contribution of the α2 Na,K-ATPase without affecting its protein content, thereby causing sarcolemma depolarization. IR increased serum concentrations of ouabain, IL-6, and corticosterone, decreased lipid peroxidation, and changed cellular redox status. Chronic ouabain administration prevented IR-induced depolarization and loss of the α2 Na,K-ATPase electrogenic contribution without changing its protein content. This was accompanied with an elevation of ouabain concentration in circulation and with the lack of IR-induced suppression of lipid peroxidation. Given the crucial role of Na,K-ATPase in skeletal muscle performance, these findings may have therapeutic implications as countermeasures for IR-induced muscle pathology.

Sleep deprivation is associated with increased circulating levels of endogenous ouabain: Potential role in bipolar disorder

Endogenously produced cardiac glycosides, like endogenous ouabain (EO), are putative hormones that have been implicated in the pathophysiology of bipolar disorder. Individuals with bipolar disorder appear to be unable to sufficiently upregulate production of EO in situations of increased need. This study was performed to determine the effect of sleep deprivation on the circulating levels of EO. Plasma EO concentrations were measured by ouabain-radioimmunoassay in heterozygote Na,K-ATPase a2 knockout (KO) mice, which have been used as an animal model of mania, and wildtype siblings at baseline and after sleep fragmentation utilizing the moving bar method. a2 KO animals had elevated endogenous ouabain concentrations compared to wild type controls (0.82 ± SD 0.22 nM vs 0.26 ± 0.02, P = 0.03). Sleep fragmentation increased ouabain concentrations in wild type mice (0.53 ± 0.08 nM sleep fragmentation vs 0.26 ± 0.02 nM baseline, P = 0.04), but not in a2 KO mice (0.60 ± 0.07 nM sleep fragmentation vs 0.82 ± 0.22 nM baseline, P > 0.05). These studies demonstrate that sleep disturbance can increase EO in control mice but animals that exhibit some manic behaviors are unable to increase EO production.

Role of Na+/K+-ATPase in ischemic stroke: in-depth perspectives from physiology to pharmacology

Na⁺/K⁺-ATPase (NKA) is a large transmembrane protein expressed in all cells. It is well studied for its ion exchanging function, which is indispensable for the maintenance of electrochemical gradients across the plasma membrane and herein neuronal excitability. The widely recognized pump function of NKA closely depends on its unique structure features and conformational changes upon binding of specific ions. Various Na⁺-dependent secondary transport systems are rigorously controlled by the ionic gradients generated by NKA and are essential for multiple physiological processes. In addition, roles of NKA as a signal transducer have also been unveiled nowadays. Plethora of signaling cascades are defined including Src-Ras-MAPK signaling, IP3R-mediated calcium oscillation, inflammation, and autophagy though most underlying mechanisms remain elusive. Ischemic stroke occurs when the blood flow carrying nutrients and oxygen into the brain is disrupted by blood clots, which is manifested by excitotoxicity, oxidative stress, inflammation, etc. The protective effect of NKA against ischemic stress is emerging gradually with the application of specific NKA inhibitor. However, NKA-related research is limited due to the opposite effects caused by NKA inhibitor at lower doses. The present review focuses on the recent progression involving different aspects about NKA in cellular homeostasis to present an in-depth understanding of this unique protein. Moreover, essential roles of NKA in ischemic pathology are discussed to provide a platform and bright future for the improvement in clinical research on ischemic stroke.

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.

Transcription-based drug repurposing for COVID-19

We have utilised the transcriptional response of lung epithelial cells following infection by the original Severe Acute Respiratory Syndrome coronavirus (SARS) to identify repurposable drugs for COVID-19. Drugs best able to recapitulate the infection profile are highly enriched for antiviral activity. Nine of these have been tested against SARS-2 and found to potently antagonise SARS-2 infection/replication, with a number now being considered for clinical trials. It is hoped that this approach may serve to broaden the spectrum of approved drugs that should be further assessed as potential anti-COVID-19 agents and may help elucidate how this seemingly disparate collection of drugs are able to inhibit SARS-2 infection/replication.

Rapid Lipid Modification of Endothelial Cell Membranes in Cardiac Ischemia/Reperfusion Injury: a Novel Therapeutic Strategy to Reduce Infarct Size

PURPOSE

Plasma membranes constitute a gathering point for lipids and signaling proteins. Lipids are known to regulate the location and activity of signaling proteins under physiological and pathophysiological conditions. Membrane lipid therapies (MLTs) that gradually modify lipid content of plasma membranes have been developed to treat chronic disease; however, no MLTs have been developed to treat acute conditions such as reperfusion injury following myocardial infarction (MI) and percutaneous coronary intervention (PCI). A fusogenic nanoliposome (FNL) that rapidly incorporates exogenous unsaturated lipids into endothelial cell (EC) membranes was developed to attenuate reperfusion-induced protein signaling. We hypothesized that administration of intracoronary (IC) FNL-MLT interferes with EC membrane protein signaling, leading to reduced microvascular dysfunction and infarct size (IS).

METHODS

Using a myocardial ischemia/reperfusion swine model, the efficacy of FNL-MLT in reducing IS following a 60-min coronary artery occlusion was tested. Animals were randomized to receive IC Ringer's lactate solution with or without 10 mg/mL/min of FNLs for 10 min prior to reperfusion (n = 6 per group).

RESULTS

The IC FNL-MLT reduced IS (25.45 ± 16.4% vs. 49.7 ± 14.1%, P < 0.02) and enhanced regional myocardial blood flow (RMBF) in the ischemic zone at 15 min of reperfusion (2.13 ± 1.48 mL/min/g vs. 0.70 ± 0.43 mL/min/g, P < 0.001). The total cumulative plasma levels of the cardiac injury biomarker cardiac troponin I (cTnI) were trending downward but were not significant (999.3 ± 38.7 ng/mL vs. 1456.5 ± 64.8 ng/mL, P = 0.1867). However, plasma levels of heart-specific fatty acid binding protein (hFABP), another injury biomarker, were reduced at 2 h of reperfusion (70.3 ± 38.0 ng/mL vs. 137.3 ± 58.2 ng/mL, P = 0.0115).  CONCLUSION: The IC FNL-MLT reduced IS compared to vehicle in this swine model. The FNL-MLT maybe a promising adjuvant to PCI in the treatment of acute MI.

Circulating Ouabain Modulates Expression of Claudins in Rat Intestine and Cerebral Blood Vessels

The ability of exogenous low ouabain concentrations to affect claudin expression and therefore epithelial barrier properties was demonstrated previously in cultured cell studies. We hypothesized that chronic elevation of circulating ouabain in vivo can affect the expression of claudins and tight junction permeability in different tissues. We tested this hypothesis in rats intraperitoneally injected with ouabain (1 μg/kg) for 4 days. Rat jejunum, colon and brain frontal lobes, which are variable in the expressed claudins and tight junction permeability, were examined. Moreover, the porcine jejunum cell line IPEC-J2 was studied. In IPEC-J2-cells, ouabain (10 nM, 19 days of incubation) stimulated epithelial barrier formation, increased transepithelial resistance and the level of cSrc-kinase activation by phosphorylation, accompanied with an increased expression of claudin-1, -5 and down-regulation of claudin-12; the expression of claudin-3, -4, -8 and tricellulin was not changed. In the jejunum, chronic ouabain increased the expression of claudin-1, -3 and -5 without an effect on claudin-2 and -4 expression. In the colon, only down-regulation of claudin-3 was observed. Chronic ouabain protected the intestine transepithelial resistance against functional injury induced by lipopolysaccharide treatment or by modeled acute microgravity; this regulation was most pronounced in the jejunum. Claudin-1 was also up-regulated in cerebral blood vessels. This was associated with reduction of claudin-3 expression while the expression of claudin-5 and occludin was not affected. Altogether, our results confirm that circulating ouabain can functionally and tissue-specifically affect barrier properties of epithelial and endothelial tissues via Na,K-ATPase-mediated modulation of claudins expression.

Skeletal Muscle Na,K-ATPase as a Target for Circulating Ouabain

While the role of circulating ouabain-like compounds in the cardiovascular and central nervous systems, kidney and other tissues in health and disease is well documented, little is known about its effects in skeletal muscle. In this study, rats were intraperitoneally injected with ouabain (0.1-10 µg/kg for 4 days) alone or with subsequent injections of lipopolysaccharide (1 mg/kg). Some rats were also subjected to disuse for 6 h by hindlimb suspension. In the diaphragm muscle, chronic ouabain (1 µg/kg) hyperpolarized resting potential of extrajunctional membrane due to specific increase in electrogenic transport activity of the 2 Na,K-ATPase isozyme and without changes in 1 and 2 Na,K-ATPase protein content. Ouabain (10-20 nM), acutely applied to isolated intact diaphragm muscle from not injected rats, hyperpolarized the membrane to a similar extent. Chronic ouabain administration prevented lipopolysaccharide-induced (diaphragm muscle) or disuse-induced (soleus muscle) depolarization of the extrajunctional membrane. No stimulation of the 1 Na,K-ATPase activity in human red blood cells, purified lamb kidney and Torpedo membrane preparations by low ouabain concentrations was observed. Our results suggest that skeletal muscle electrogenesis is subjected to regulation by circulating ouabain via the 2 Na,K-ATPase isozyme that could be important for adaptation of this tissue to functional impairment.

21-Benzylidene digoxin decreases proliferation by inhibiting the EGFR/ERK signaling pathway and induces apoptosis in HeLa cells

Cardiotonic steroids (CTS) are agents traditionally known for their capacity to bind to the Na,K-ATPase (NKA), affecting the ion transport and the contraction of the heart. Natural CTS have been shown to also have effects on cell signaling pathways. With the goal of developing a new CTS derivative, we synthesized a new digoxin derivative, 21-benzylidene digoxin (21-BD). Previously, we have shown that this compound binds to NKA and has cytotoxic actions on cancer, but not on normal cells. Here, we further studied the mechanisms of actions of 21-BD. Working with HeLa cells, we found that 21-BD decreases the basal, as well as the insulin stimulated proliferation. 21-BD reduces phosphorylation of the epidermal growth factor receptor (EGFR) and extracellular-regulated kinase (ERK), which are involved in pathways that stimulate cell proliferation. In addition, 21-BD promotes apoptosis, which is mediated by the translocation of Bax from the cytosol to mitochondria and the release of mitochondrial cytochrome c to the cytosol. 21-BD also activated caspases-8, -9 and -3, and induced the cleavage of poly (ADP-ribose) polymerase-1 (PARP-1). Altogether, these results show that the new compound that we have synthesized exerts cytotoxic actions on HeLa cells by inhibition of cell proliferation and the activation of both the extrinsic and intrinsic apoptotic pathways. These results support the relevance of the cardiotonic steroid scaffold as modulators of cell signaling pathways and potential agents for their use in cancer.

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.

Cardiotonic Steroids Stimulate Macrophage Inflammatory Responses Through a Pathway Involving CD36, TLR4, and Na/K-ATPase

OBJECTIVE

Circulating levels of cardiotonic steroids (CTS) are elevated in various chronic inflammatory conditions, but the role of CTS in inflammation remains largely unknown. We have previously shown that the CTS ouabain stimulates proinflammatory responses in murine macrophages. In this study, we aim to explore the mechanism how CTS induce proinflammatory responses in primary murine and human macrophages.

APPROACH AND RESULTS

Using both murine peritoneal macrophages and human monocyte-derived macrophages, we demonstrated that ouabain activated NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells), leading to proinflammatory cytokine (eg, MCP-1 [monocyte chemotactic protein 1], TNF-α [tumor necrosis factor-α], IL-1β [interleukin-1β], and IL-6) production. By applying siRNA techniques and murine peritoneal macrophages isolated from genetically modified mice, we showed that macrophages partially deficient in Na/K-ATPase, the receptor for CTS, or fully deficient in the scavenger receptor CD36 or TLR4 (Toll-like receptor) were resistant to ouabain-induced NF-κB activation, suggesting an indispensable role of these 3 receptors in this pathway. Mechanistically, this effect of ouabain was independent of the ion transport function of the Na/K-ATPase. Instead, ouabain stimulated a signaling complex, including Na/K-ATPase, CD36, and TLR4. Subsequently, TLR4 recruited MyD88 adaptor protein for NF-κB activation. Furthermore, intraperitoneal injection of ouabain into mice specifically recruited Ly6C⁺CCR2⁺ monocyte subtypes to the peritoneal cavities, indicating that the CTS ouabain triggers inflammation in vivo.

CONCLUSIONS

CTS activate NF-κB leading to proinflammatory cytokine production in primary macrophages through a signaling complex, including CD36, TLR4, and Na/K-ATPase. These findings warrant further studies on endogenous CTS in chronic inflammatory diseases, such as atherosclerosis.

Na⁺i,K⁺i-Dependent and -Independent Signaling Triggered by Cardiotonic Steroids: Facts and Artifacts

Na⁺,K⁺-ATPase is the only known receptor of cardiotonic steroids (CTS) whose interaction with catalytic α-subunits leads to inhibition of this enzyme. As predicted, CTS affect numerous cellular functions related to the maintenance of the transmembrane gradient of monovalent cations, such as electrical membrane potential, cell volume, transepithelial movement of salt and osmotically-obliged water, symport of Na⁺ with inorganic phosphate, glucose, amino acids, nucleotides, etc. During the last two decades, it was shown that side-by-side with these canonical Na⁺_i/K⁺_i-dependent cellular responses, long-term exposure to CTS affects transcription, translation, tight junction, cell adhesion and exhibits tissue-specific impact on cell survival and death. It was also shown that CTS trigger diverse signaling cascades via conformational transitions of the Na⁺,K⁺-ATPase α-subunit that, in turn, results in the activation of membrane-associated non-receptor tyrosine kinase Src, phosphatidylinositol 3-kinase and the inositol 1,4,5-triphosphate receptor. These findings allowed researchers to propose that endogenous CTS might be considered as a novel class of steroid hormones. We focus our review on the analysis of the relative impact Na⁺_i,K⁺_i-mediated and -independent pathways in cellular responses evoked by CTS.