Ouabain- and Marinobufagenin-Induced Proliferation of Human Umbilical Vein Smooth Muscle Cells and a Rat Vascular Smooth Muscle Cell Line, A7r5

Bioassay-Guided Fractionation and Biological Activity of Cardenolides from Streptocaulon juventas

The discovery that Na/K-ATPase acts as a signal transducer led us to investigate the structural diversity of cardiotonic steroids and study their ligand effects. By applying Na/K-ATPase activity assay-guided fractionation, we isolated a total of 20 cardiotonic steroids from Streptocaulon juventas, including an undescribed juventasoside B (10: ) and 19 known cardiotonic steroids. Their structures have been elucidated. Using our platform of purified Na/K-ATPase and an LLC-PK1 cell model, we found that 10: , at a concentration that induces less than 10% Na/K-ATPase inhibition, can stimulate the Na/K-ATPase/Src receptor complex and selectively activate downstream pathways, ultimately altering prostate cancer cell growth. By assessing the ligand effect of the isolated cardiotonic steroids, we found that the regulation of cell viability by the isolated cardiotonic steroids was not associated with their inhibitory potencies against Na/K-ATPase activity but reflected their ligand-binding affinity to the Na/K-ATPase receptor. Based on this discovery, we identified a unique active cardiotonic steroid, digitoxigenin (1: ), and verified that it can protect LLC-PK1 cells from hypoxic injury, implicating its potential use in ischemia/reperfusion injury and inducing collagen synthesis in primary human dermal fibroblast cells, and implicating that compound 2: is the molecular basis of the wound healing activity of S. juventas.

Live cell screening identifies glycosides as enhancers of cardiomyocyte cell cycle activity

Promoting cardiomyocyte proliferation is a promising strategy to regenerate the heart. Yet, so far, it is poorly understood how cardiomyocyte proliferation is regulated, and no factor identified to promote mammalian cardiomyocyte proliferation has been translated into medical practice. Therefore, finding a novel factor will be vital. Here, we established a live cell screening based on mouse embryonic stem cell-derived cardiomyocytes expressing a non-functional human geminin deletion mutant fused to Azami Green (CM7/1-hgem-derived cardiomyocytes). We screened for a subset of compounds of the small molecule library Spectrum Collection and identified 19 potential inducers of stem cell-derived cardiomyocyte proliferation. Furthermore, the pro-proliferative potential of identified candidate compounds was validated in neonatal and adult rat cardiomyocytes as well as human induced pluripotent stem cell-derived cardiomyocytes. 18 of these compounds promoted mitosis and cytokinesis in neonatal rat cardiomyocytes. Among the top four candidates were two cardiac glycosides, peruvoside and convallatoxin, the flavonoid osajin, and the selective α-adrenoceptor antagonist and imidazoline I1 receptor ligand efaroxan hydrochloride. Inhibition of PTEN and GSK-3β enhanced cell cycle re-entry and progression upon stimulation with cardiac glycosides and osajin, while inhibition of IP3 receptors inhibited the cell cycle-promoting effect of cardiac glycosides. Collectively, we established a screening system and identified potential compounds to promote cardiomyocyte proliferation. Our data suggest that modulation of calcium handling and metabolism promotes cardiomyocyte proliferation, and cardiac glycosides might, besides increasing myocardial contraction force, contribute to cardiac repair by inducing cardiomyocyte proliferation.

Inhibitory Effects of Digoxin and Digitoxin on Cell Growth in Human Ovarian Cancer Cell Line SKOV-3

Background:

Cardiac glycosides (CGs) possess a chemical structure similar to steroids, and are inhibitors of the sodium potassium pump. An anti-tumor effect of CGs in breast and prostate cancers has been reported, but the effect of CGs on ovarian cancer is still unclear.

Aims:

In this study, the effects of CGs on proliferation, cytotoxicity and cell cycle of ovarian cancer cell line (SKOV-3) have been investigated.

Procedure:

The cell proliferation and cytotoxicity were detected by MTT assay and LDH activity assay, respectively. CGs, at concentrations higher than IC50, decreased cell proliferation and showed increased cytotoxicity toward SKOV-3 cells. The colony-formation ability was reduced after treatment with digoxin and digitoxin for 10 days. Furthermore, we explored the effect of digoxin and digitoxin on the distribution of cell cycle by flow cytometry.

Results:

Results revealed that both digoxin and digitoxin led to cell cycle arrest in G₀/G₁ phase with 24 or 48 hours, but the arrest of G₀/G₁ phase was not observed at 72 hours. We evaluated the percentage of hypodiploid cell population as an index of the cellular fragments through flow cytometry. The data indicated that cellular fragments were significantly increased by treating with digitoxin at the concentrations of IC50 and 10⁻⁶ M for 72 hours.

Conclusion:

Taken together, these data suggest that CGs decreased cell proliferation and increased cytotoxicity through cell cycle arrest at the G₀/G₁ phase. CGs have anti-tumor effect in SKOV-3 cells and might be a potential therapeutic drug for ovarian cancer. Since this study is a preliminary investigation of CGs on SKOV-3 cells, more experiments might be performed in the future. Furthermore, more ovarian cancer cell lines might also be employed in the future studies to confirm the effect of CGs in ovarian cancer.

Ouabain Promotes Gap Junctional Intercellular Communication in Cancer Cells

Gap junctions are molecular structures that allow communication between neighboring cells. It has been shown that gap junctional intercellular communication (GJIC) is notoriously reduced in cancer cells compared to their normal counterparts. Ouabain, a plant derived substance, widely known for its therapeutic properties on the heart, has been shown to play a role in several types of cancer, although its mechanism of action is not yet fully understood. Since we have previously shown that ouabain enhances GJIC in epithelial cells (MDCK), here we probed whether ouabain affects GJIC in a variety of cancer cell lines, including cervico-uterine (CasKi, SiHa and Hela), breast (MDA-MB-321 and MCF7), lung (A549), colon (SW480) and pancreas (HPAF-II). For this purpose, we conducted dye transfer assays to measure and compare GJIC in monolayers of cells with and without treatment with ouabain (0.1, 1, 10, 50 and 500 nM). We found that ouabain induces a statistically significant enhancement of GJIC in all of these cancer cell lines, albeit with distinct sensitivity. Additionally, we show that synthesis of new nucleotides or protein subunits is not required, and that Csrc, ErK1/2 and ROCK-Rho mediate the signaling mechanisms. These results may contribute to explaining how ouabain influences cancer.

Ouabain and Marinobufagenin: Physiological Effects on Human Epithelial and Endothelial Cells

Long-term study on the identification of Na,K-ATPase endogenous inhibitors in mammalian tissues has resulted in the discovery of ouabain, marinobufagenin (MBG), and other cardiotonic steroids (CTS) in the blood plasma. Production of ouabain and MBG is increased in essential hypertension and other diseases associated with hypervolemia. Here, we compared the effects of ouabain and MBG on the Na,K-ATPase activity (measured as the transport of Na+, K+, and Rb⁺ ions) and proliferation and death of human renal epithelial cells (HRECs) and human umbilical vein endothelial cells (HUVEC) expressing α1-Na,K-ATPase. Ouabain concentration that provided the half-maximal inhibition of the Rb⁺ influx (IC₅₀) into HRECs and HUVECs was 0.07 μM. In both types of cells, the IC₅₀ values for MBG were 10 times higher than for ouabain. Incubation of HREC and HUVEC with 0.001-0.01 μM ouabain for 30 h resulted in 40% increase in the [3H]thymidine incorporation into DNA; further elevation of ouabain concentration to 0.1 μM completely suppressed DNA synthesis. MBG at the concentration of 0.1 μM activated DNA synthesis by 25% in HRECs, but not in HUVECs; 1 μM MBG completely inhibited DNA synthesis in HRECs and by 50% in HUVECs. In contrast to HRECs, incubation of HUVECs in the serum-free medium induced apoptosis, which was almost completely suppressed by ouabain and MBG at the concentrations of 0.1 and 3 μM, respectively. Based on these data, we can conclude that (i) the effect of MBG at the concentrations detected in the blood plasma (<0.01 μM) on HRECs and HUVECs was not due to the changes in the [Na+]_i/[K+]_i ratio; (ii) the effect of physiological concentrations of ouabain on these cells might be mediated by the activation of Na,K-ATPase, leading to cell proliferation.

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.

Oleandrin induces apoptosis via activating endoplasmic reticulum stress in breast cancer cells

BACKGROUND

Breast cancer is the most common malignant tumor in women. Due to limited treatment outcome and high rate of metastasis, the prognosis is especially poor for triple-negative breast cancer. It is urgent to discover and develop novel agents for treatment of breast cancer. Herein, we investigated the potential mechanisms of Oleandrin's (a cardiac glycoside) cytotoxic activity against breast cancer cells.

METHODS

Cell proliferation was assessed by xCELLigence Real-Time Cell Analyzer (RTCA)-MP system. Apoptotic cells were detected by using Annexin V/PI staining and nuclear fragments observation. The effect of oleandrin on ATP1B3 expression and markers of ER stress were determined by western blot. A primary cell sensitivity assay was performed via a collagen gel droplet-embedded culture drug sensitivity method (CD-DST).

RESULTS

Oleandrin suppressed cell proliferation and colony formation in the three breast cancer cell lines but did not affect normal mammary epithelial cells. Additionally, the expression of ATP1B3 was higher in the three breast cancer cell lines compared to MCF10A cells. Treatment with oleandrin increased the number of apoptotic cells and led to nuclear pyknosis, fragmentation, and apoptotic body formation in breast cancer cells. Furthermore, oleandrin treatment increased expression of Bax and Bim but decreased that of Bcl-2. Treatment with oleandrin also upregulated the expression of endoplasmic reticulum stress associated proteins, including eIF2α, ATF4, and CHOP, but not PERK. oleandrin treatment also induced the phosphorylation of PERK and eIF2α. Of note, oleandrin exhibited antitumor effects on patient-derived breast cancer cells under three-dimensional culture conditions.

CONCLUSIONS

Taken together, our results suggest that oleandrin induces mitochondrial-mediated apoptosis by activating endoplasmic reticulum stress in breast cancer. Moreover, oleandrin may be an effective strategy for the treatment of breast cancer.

Influence of Endogenous Cardiac Glycosides, Digoxin, and Marinobufagenin in the Physiology of Epithelial Cells

Cardiac glycosides are a group of compounds widely known for their action in cardiac tissue, some of which have been found to be endogenously produced (ECG). We have previously studied the effect of ouabain, an endogenous cardiac glycoside, on the physiology of epithelial cells, and we have shown that in concentrations in the nanomolar range, it affects key properties of epithelial cells, such as tight junction, apical basolateral polarization, gap junctional intercellular communication (GJIC), and adherent junctions. In this work, we study the influence of digoxin and marinobufagenin, two other endogenously expressed cardiac glycosides, on GJIC as well as the degree of transepithelial tightness due to tight junction integrity (TJ). We evaluated GJIC by dye transfer assays and tight junction integrity by transepithelial electrical resistance (TER) measurements, as well as immunohistochemistry and western blot assays of expression of claudins 2 and 4. We found that both digoxin and marinobufagenin improve GJIC and significantly enhance the tightness of the tight junctions, as evaluated from TER measurements. Immunofluorescence assays show that both compounds promote enhanced basolateral localization of claudin-4 but not claudin 2, while densitometric analysis of western blot assays indicate a significantly increased expression of claudin 4. These changes, induced by digoxin and marinobufagenin on GJIC and TER, were not observed on MDCK-R, a modified MDCK cell line that has a genetically induced insensitive α1 subunit, indicating that Na-K-ATPase acts as a receptor mediating the actions of both ECG. Plus, the fact that the effect of both cardiac glycosides was suppressed by incubation with PP2, an inhibitor of c-Src kinase, PD98059, an inhibitor of mitogen extracellular kinase-1 and Y-27632, a selective inhibitor of ROCK, and a Rho-associated protein kinase, indicate altogether that the signaling pathways involved include c-Src and ERK1/2, as well as Rho-ROCK. These results widen and strengthen our general hypothesis that a very important physiological role of ECG is the control of the epithelial phenotype and the regulation of cell-cell contacts.

Ouabain-regulated phosphoproteome reveals molecular mechanisms for Na+, K+-ATPase control of cell adhesion, proliferation, and survival

The ion pump Na⁺, K⁺-ATPase (NKA) is a receptor for the cardiotonic steroid ouabain. Subsaturating concentration of ouabain triggers intracellular calcium oscillations, stimulates cell proliferation and adhesion, and protects from apoptosis. However, it is controversial whether ouabain-bound NKA is considered a signal transducer. To address this question, we performed a global analysis of protein phosphorylation in COS-7 cells, identifying 2580 regulated phosphorylation events on 1242 proteins upon 10- and 20-min treatment with ouabain. Regulated phosphorylated proteins include the inositol triphosphate receptor and stromal interaction molecule, which are essential for initiating calcium oscillations. Hierarchical clustering revealed that ouabain triggers a structured phosphorylation response that occurs in a well-defined, time-dependent manner and affects specific cellular processes, including cell proliferation and cell-cell junctions. We additionally identify regulation of the phosphorylation of several calcium and calmodulin-dependent protein kinases (CAMKs), including 2 sites of CAMK type II-γ (CAMK2G), a protein known to regulate apoptosis. To verify the significance of this result, CAMK2G was knocked down in primary kidney cells. CAMK2G knockdown impaired ouabain-dependent protection from apoptosis upon treatment with high glucose or serum deprivation. In conclusion, we establish NKA as the coordinator of a broad, tightly regulated phosphorylation response in cells and define CAMK2G as a downstream effector of NKA.-Panizza, E., Zhang, L., Fontana, J. M., Hamada, K., Svensson, D., Akkuratov, E. E., Scott, L., Mikoshiba, K., Brismar, H., Lehtiö, J., Aperia, A. Ouabain-regulated phosphoproteome reveals molecular mechanisms for Na⁺, K⁺-ATPase control of cell adhesion, proliferation, and survival.

Oleandrin and Its Derivative Odoroside A, Both Cardiac Glycosides, Exhibit Anticancer Effects by Inhibiting Invasion via Suppressing the STAT-3 Signaling Pathway

The cardiac glycosides oleandrin and odoroside A, polyphenolic monomer compounds extracted from Nerium oleander, have been found to have antitumor effects on various tumors at low doses. However, the mechanisms of anticancer effects of oleandrin and odoroside A are not well known. Therefore, in this study, we aimed to investigate the anticancer effects of oleandrin and odoroside A and their associated mechanisms in highly metastatic MDA-MB-231 breast cancer cells and radiotherapy-resistant (RT-R) MDA-MB-231 cells. Our results showed that oleandrin and odoroside A dose-dependently decreased the colony formation and the invasion of both cell lines at nanomolar ranges. Furthermore, oleandrin (50 nM) and odoroside A (100 nM) reduced octamer-binding transcription factor 3/4 (OCT3/4) and β-catenin levels and matrix metalloproteinase-9 (MMP-9) activity. Finally, we found that phospho-STAT-3 levels were increased in MDA-MB-231 and RT-R-MDA-MB-231, but not in endothelial cells (ECs), and that the levels were significantly decreased by oleandrin (50 nM) and odoroside A (100 nM). Inhibition of phospho-signal transducer and activator of transcription (STAT)-3 significantly reduced OCT3/4 and β-catenin levels and MMP-9 activity, ultimately resulting in reduced invasion. These results suggest that the anticancer effects of oleandrin and odoroside A might be due to the inhibition of invasion through of phospho-STAT-3-mediated pathways that are involved in the regulation of invasion-related molecules.

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.

Na+/K+ ATPase activity promotes invasion of endocrine resistant breast cancer cells

Background

The Na⁺/K⁺-ATPase (NKP) is an important ion transporter also involved in signal transduction. Its expression profile is altered in various tumours including that of the breast. We studied the effect of inhibiting NKP activity in non-tumorigenic breast cell line and in estrogen receptor positive and negative breast cancer cells.

Methods

Expression and localization of NKP and downstream signaling molecules were determined by RT-PCR, western blotting and immunofluorescence. Cell proliferation, apoptosis and cell cycle stage were determined using MTT, annexin V and flow cytometry. Cell motility and invasion were determined using wound healing and matrigel assays. Total matrix metalloproteinase (MMP) was determined by a fluorescence-based assay.

Results

NKP was mainly localized on the cell membrane. Its baseline expression and activity were enhanced in breast cancer compared to the non-tumorigenic breast cell line. Ouabain and 3,4,5,6-tetrahydroxyxanthone (TTX) treatment significantly inhibited NKP activity, which significantly reduced cell proliferation, motility, invasion and pH-induced membrane blebbing. EGF stimulation induced internalization of NKP from the cell membrane to the cytoplasm. Ouabain inhibited EGF-induced phosphorylation of Rac/cdc42, profillin, ERK1/2 and P70S6K.

Conclusions

The NKP may offer a novel therapeutic target in breast cancer patients who have developed metastasis, aiming to improve therapeutic outcomes and enhance survival rate.

The Na, K-ATPase β-Subunit Isoforms Expression in Glioblastoma Multiforme: Moonlighting Roles

Glioblastoma multiforme (GBM) is the most common form of malignant glioma. Recent studies point out that gliomas exploit ion channels and transporters, including Na, K-ATPase, to sustain their singular growth and invasion as they invade the brain parenchyma. Moreover, the different isoforms of the β-subunit of Na, K-ATPase have been implicated in regulating cellular dynamics, particularly during cancer progression. The aim of this study was to determine the Na, K-ATPase β subunit isoform subcellular expression patterns in all cell types responsible for microenvironment heterogeneity of GBM using immunohistochemical analysis. All three isoforms, β1, β2/AMOG (Adhesion Molecule On Glia) and β3, were found to be expressed in GBM samples. Generally, β1 isoform was not expressed by astrocytes, in both primary and secondary GBM, although other cell types (endothelial cells, pericytes, telocytes, macrophages) did express this isoform. β2/AMOG and β3 positive expression was observed in the cytoplasm, membrane and nuclear envelope of astrocytes and GFAP (Glial Fibrillary Acidic Protein) negative cells. Interestingly, differences in isoforms expression have been observed between primary and secondary GBM: in secondary GBM, β2 isoform expression in astrocytes was lower than that observed in primary GBM, while the expression of the β3 subunit was more intense. These changes in β subunit isoforms expression in GBM could be related to a different ionic handling, to a different relationship between astrocyte and neuron (β2/AMOG) and to changes in the moonlighting roles of Na, K-ATPase β subunits as adaptor proteins and transcription factors.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

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.

Reducing Cardiac Fibrosis: Na/K-ATPase Signaling Complex as a Novel Target

Cardiac fibrosis is a common pathological process in cardiac disease and may lead to heart failure. It can also cause sudden death even in those without cardiac symptoms. Tissue fibrosis can be categorized into two categories: replacement fibrosis (also called reparative fibrosis) and reactive fibrosis. In replacement fibrosis, infiltration of inflammatory cells and accumulation of Extracellular Matrix (ECM) proteins are the initial steps in forming scarlike fibrotic tissue after acute cardiac injury and cardiac cell necrosis. Reactive fibrosis can be formed in response to hormonal change and pressure or volume overload. Experimental studies in animals have identified important pathways such as the Renin-Angiotensin-Aldosterone System (RAAS) and the endothelin pathway that contribute to fibrosis formation. Despite the fact that clinical trials using RAAS inhibitors as therapies for reducing cardiac fibrosis and improving cardiac function have been promising, heart failure is still the leading cause of deaths in the United States. Intensive efforts have been made to find novel targets and to develop new treatments for cardiac fibrosis and heart failure in the past few decades. The Na/K-ATPase, a canonical ion transporter, has been shown to also function as a signal transducer and prolonged activation of Na/K-ATPase signaling has been found to promote the formation of cardiac fibrosis. Novel tools that block the activation of Na/K-ATPase signaling have been developed and have shown promise in reducing cardiac fibrosis. This review will discuss the recent development of novel molecular targets, focusing on the Na/K-ATPase signaling complex as a therapeutic target in treatment of cardiac fibrosis.

Downregulation of TGF-β Receptor-2 Expression and Signaling through Inhibition of Na/K-ATPase

Transforming growth factor-beta (TGF-β) is a multi-functional cytokine implicated in the control of cell growth and differentiation. TGF-β signals through a complex of TGF-β receptors 1 and 2 (TGFβR1 and TGFβR2) that phosphorylate and activate Smad2/3 transcription factors driving transcription of the Smad-target genes. The Na+/K+-ATPase is an integral plasma membrane protein critical for maintaining the electro-chemical gradient of Na+ and K+ in the cell. We found that inhibition of the Na+/K+ ATPase by ouabain results in a dramatic decrease in the expression of TGFβR2 in human lung fibrobalsts (HLF) at the mRNA and protein levels. This was accompanied by inhibition of TGF-β-induced Smad phosphorylation and the expression of TGF-β target genes, such as fibronectin and smooth muscle alpha-actin. Inhibition of Na+/K+ ATPase by an alternative approach (removal of extracellular potassium) had a similar effect in HLF. Finally, treatment of lung alveolar epithelial cells (A549) with ouabain also resulted in the downregulation of TGFβR2, the inhibition of TGF-β-induced Smad phosphorylation and of the expression of mesenchymal markers, vimentin and fibronectin. Together, these data demonstrate a critical role of Na+/K+-ATPase in the control of TGFβR2 expression, TGF-β signaling and cell responses to TGF-β.

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.

FXYD5 Protein Has a Pro-inflammatory Role in Epithelial Cells

The FXYD proteins are a family of small membrane proteins that share an invariant four amino acid signature motif F-X-Y-D and act as tissue-specific regulatory subunits of the Na,K-ATPase. FXYD5 (also termed dysadherin or RIC) is a structurally and functionally unique member of the FXYD family. As other FXYD proteins, FXYD5 specifically interacts with the Na,K-ATPase and alters its kinetics by increasing Vmax However, unlike other family members FXYD5 appears to have additional functions, which cannot be readily explained by modulation of transport kinetics. Knockdown of FXYD5 in MDA-MB-231 breast cancer cells largely decreases expression and secretion of the chemokine CCL2 (MCP-1). A related effect has also been observed in renal cell carcinoma cells. The current study aims to further characterize the relationship between the expression of FXYD5 and CCL2 secretion. We demonstrate that transfection of M1 epithelial cell line with FXYD5 largely increases lipopolysaccharide (LPS) stimulated CCL2 mRNA and secretion of the translated protein. We have completed a detailed analysis of the molecular events leading to the above response. Our key findings indicate that FXYD5 generates a late response by increasing the surface expression of the TNFα receptor, without affecting its total protein level, or mRNA transcription. LPS administration to mice demonstrates induced secretion of CCL2 and TNFα in FXYD5-expressing lung peripheral tissue, which suggests a possible role for FXYD5 in normal epithelia during inflammation.

Na,K-ATPase Isozymes in Colorectal Cancer and Liver Metastases

The goal of this study was to define Na,K-ATPase α and β subunit isoform expression and isozyme composition in colorectal cancer cells and liver metastases. The α1, α3, and β1 isoforms were the most highly expressed in tumor cells and metastases; in the plasma membrane of non-neoplastic cells and mainly in a cytoplasmic location in tumor cells. α1β1 and α3β1 isozymes found in tumor and metastatic cells exhibit the highest and lowest Na⁺ affinity respectively and the highest K⁺ affinity. Mesenchymal cell isozymes possess an intermediate Na⁺ affinity and a low K⁺ affinity. In cancer, these ions are likely to favor optimal conditions for the function of nuclear enzymes involved in mitosis, especially a high intra-nuclear K⁺ concentration. A major and striking finding of this study was that in liver, metastasized CRC cells express the α3β1 isozyme. Thus, the α3β1 isozyme could potentially serve as a novel exploratory biomarker of CRC metastatic cells in liver.

Na+-K+-ATPase, a new class of plasma membrane receptors

The Na(+)-K(+)-ATPase (NKA) differs from most other ion transporters, not only in its capacity to maintain a steep electrochemical gradient across the plasma membrane, but also as a receptor for a family of cardiotonic steroids, to which ouabain belongs. Studies from many groups, performed during the last 15 years, have demonstrated that ouabain, a member of the cardiotonic steroid family, can activate a network of signaling molecules, and that NKA will also serve as a signal transducer that can provide a feedback loop between NKA and the mitochondria. This brief review summarizes the current knowledge and controversies with regard to the understanding of NKA signaling.

Heat shock proteins and hormesis in the diagnosis and treatment of neurodegenerative diseases

Modulation of endogenous cellular defense mechanisms via the vitagene system represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. The possibility of high-throughoutput screening using proteomic techniques, particularly redox proteomics, provide more comprehensive overview of the interaction of proteins, as well as the interplay among processes involved in neuroprotection. Here by introducing the hormetic dose response concept, the mechanistic foundations and applications to the field of neuroprotection, we discuss the emerging role of heat shock protein as prominent member of vitagene network in neuroprotection and redox proteomics as a tool for investigating redox modulation of stress responsive vitagenes. Hormetic mechanisms are reviewed as possibility of targeted therapeutic manipulation in a cell-, tissue- and/or pathway-specific manner at appropriate points in the neurodegenerative disease process.

Digoxin inhibits PDGF-BB-induced VSMC proliferation and migration through an increase in ILK signaling and attenuates neointima formation following carotid injury

The increased proliferation and migration of vascular smooth muscle cells (VSMCs) are key events in the development of artery restenosis following percutaneous coronary intervention. Digoxin has long been used in the treatment of heart failure and has been shown to inhibit the proliferation of cancer cells through multiple pathways. However, the potential role of digoxin in the regulation of VSMC proliferation and migration and its effectiveness in the treatment of cardiovascular diseases, such as restenosis, remains unexplored. In the present study, we demonstrate that digoxin-induced growth inhibition is associated with the downregulation of CDK activation and the restoration of p27Kip1 levels in platelet-derived growth factor (PDGF)-stimulated VSMCs. In addition, we found that digoxin restored the PDGF‑BB-induced inhibition of integrin linked kinase (ILK) expression and prevented the PDGF‑BB-induced activation of glycogen synthase kinase (GSK)-3β. Furthermore, digoxin inhibited adhesion molecule and extracellular matrix relative protein expression. Finally, we found that digoxin significantly inhibited neointima formation, accompanied by a decrease in cell proliferation following vascular injury in rats. These effects of digoxin were shown to be mediated, at least in part, through an increase in ILK/Akt signaling and a decrease in GSK-3β signaling in PDGF‑BB-stimulated VSMCs. In conclusion, our data demonstrate that digoxin exerts an inhibitory effect on the PDGF‑BB-induced proliferation, migration and phenotypic modulation of VSMCs, and prevents neointima formation in rats. These observations indicate the potential therapeutic application of digoxin in the treatment of cardiovascular diseases, such as restenosis.

Advances in understanding the role of cardiac glycosides in control of sodium transport in renal tubules

Cardiotonic steroids have been used for the past 200 years in the treatment of congestive heart failure. As specific inhibitors of membrane-bound Na(+)/K(+) ATPase, they enhance cardiac contractility through increasing myocardial cell calcium concentration in response to the resulting increase in intracellular Na concentration. The half-minimal concentrations of cardiotonic steroids required to inhibit Na(+)/K(+) ATPase range from nanomolar to micromolar concentrations. In contrast, the circulating levels of cardiotonic steroids under physiological conditions are in the low picomolar concentration range in healthy subjects, increasing to high picomolar levels under pathophysiological conditions including chronic kidney disease and heart failure. Little is known about the physiological function of low picomolar concentrations of cardiotonic steroids. Recent studies have indicated that physiological concentrations of cardiotonic steroids acutely stimulate the activity of Na(+)/K(+) ATPase and activate an intracellular signaling pathway that regulates a variety of intracellular functions including cell growth and hypertrophy. The effects of circulating cardiotonic steroids on renal salt handling and total body sodium homeostasis are unknown. This review will focus on the role of low picomolar concentrations of cardiotonic steroids in renal Na(+)/K(+) ATPase activity, cell signaling, and blood pressure regulation.

Marinobufagenin in essential hypertension and primary aldosteronism: a cardiotonic steroid with clinical and diagnostic implications

BACKGROUND

The cardiotonic steroid marinobufagenin (MBG) is increasingly suggested to be responsible for some of the cardiovascular injury that has been previously attributed to aldosterone. We examined the clinical correlates of circulating MBG concentrations in hypertensive patients and tested the hypothesis that MBG serves as a reliable diagnostic tool for detecting primary aldosteronism (PA).

METHODS

Plasma MBG concentrations (mean: 0.51±0.25 nmol/l) were measured in the morning fasting samples in 20 patients with PA and 20 essential hypertensive (EH) controls matched for age, sex, body mass index, renal function, urinary sodium and intake of antihypertensive medication (mean age: 51.6 years; 52.2% women).

RESULTS

Overall, plasma MBG was directly correlated with plasma aldosterone, aldosterone to active renin ratio (AARR), diastolic blood pressure, mean carotid intima-media thickness, serum sodium, urinary protein to creatinine ratio and inversely with serum potassium levels. Plasma MBG levels were significantly higher in patients with PA compared to EH (mean: 0.68±0.12 versus 0.35±0.24 nmol/l; p<0.001). ROC analysis yielded a greater AUC for plasma MBG compared to the AARR, PAC and serum potassium levels for detecting PA. Youden's Index analyses yielded the optimal plasma MBG cut-off score for diagnosing PA at >0.49 nmol/l with specificity and sensitivity values of 0.85 and 0.95, respectively, which were higher than those at the optimum AARR cut-off at >3.32 ng/dl/µU/ml.

CONCLUSIONS

In a well-characterized cohort, values of plasma MBG were significantly related to clinical correlates of cardiovascular and renal disease. Plasma MBG emerged as a valuable alternative to the AARR for screening of PA.

Description of a utrophin associated protein complex in lipid raft domains of human artery smooth muscle cells

The dystrophin-associated protein complex (DAPC) is a multimeric complex that links the extracellular matrix to the actin cytoskeleton, and in some cases dystrophin can be substituted by its autosomal homologue utrophin to form the utrophin-associated protein complex (UAPC). Both complexes maintain the stability of plasma membrane during contraction process and play an important role in transmembrane signaling. Mutations in members of the DAPC are associated with muscular dystrophy and dilated cardiomyopathy. In a previous study with human umbilical cord vessels, we observed that utrophin colocalize with caveolin-1 (Cav-1) which proposed the presence of UAPC in the plasma membrane of vascular smooth muscle (VSM). In the current study, we demonstrated by immunofluorescence analysis, co-immunoprecipitation assays, and subcellular fractionation by sucrose gradients, the existence of an UAPC in lipid raft domains of human umbilical artery smooth muscle cells (HUASMC). This complex is constituted by utrophin, β-DG, ε-SG, α-smooth muscle actin, Cav-1, endothelial nitric oxide synthase (eNOS) and cavin-1. It was also observed the presence of dystrophin, utrophin Dp71, β-SG, δ-SG, δ-SG3 and sarcospan in non-lipid raft fractions. Furthermore, the knockdown of α/β-DG was associated with the decrease in both the synthesis of nitric oxide (NO) and the presence of the phosphorylated (active) form of eNOS; and with a reduction in the downstream activation of some cGMP signaling transduction pathway components. Together these results show the presence of an UAPC complex in HUASMC that may participate in the activity regulation of eNOS and in the vascular function.

Effect of ouabain on myocardial remodeling in rats

The aim of this study was to investigate the effect of ouabain (EO) on myocardial remodeling. Twenty-two adult male Sprague-Dawley rats were randomly divided into two groups: the rats in the EO group (n=12) were intraperitoneally injected with EO daily and those in the control group (n=10) were injected with physiological saline daily. After 8 weeks the rats were sacrificed. The ultrastructural changes in the myocardium were observed. The expression levels of voltage-gated potassium channel 4.2 (K_V4.2) were detected by real-time quantitative reverse transcription-polymerase chain reaction. The effects of EO on the myocardial action potential and transient potassium efflux (I_to) were measured by patch clamping. The systolic blood pressure (SBP) of 10 of the 12 rats in the EO group, designated as the EO-sensitive (OS) rats, began to increase from the fifth week of treatment and was significantly higher compared with that of the control group 6 weeks later (P<0.01). The remaining 2 rats in the EO group that presented no increase in SBP following 8 weeks of treatment (P>0.05) were designated as EO-resistant (OR) rats. Pathological ultrastructural changes were significant in the apical mid-myocardium of the OS rats. No significant differences in K_V4.2 expression were observed among the OS, OR and control rats. The patch clamp results revealed that EO prolongs the action potential duration, reduces I_to and triggers the electrical remodeling of the myocardium. EO induces a blood pressure increase and triggers structural and electrical remodeling.

Novel role of ouabain as a cystogenic factor in autosomal dominant polycystic kidney disease

The classic role of the Na-K-ATPase is that of a primary active transporter that utilizes cell energy to establish and maintain transmembrane Na(+) and K(+) gradients to preserve cell osmotic stability, support cell excitability, and drive secondary active transport. Recent studies have revealed that Na-K-ATPase located within cholesterol-containing lipid rafts serves as a receptor for cardiotonic steroids, including ouabain. Traditionally, ouabain was viewed as a toxin produced only in plants, and it was used in relatively high concentrations to experimentally block the pumping action of the Na-K-ATPase. However, the new and unexpected role of the Na-K-ATPase as a signal transducer revealed a novel facet for ouabain in the regulation of a myriad of cell functions, including cell proliferation, hypertrophy, apoptosis, mobility, and metabolism. The seminal discovery that ouabain is endogenously produced in mammals and circulates in plasma has fueled the interest in this endogenous molecule as a potentially important hormone in normal physiology and disease. In this article, we review the role of the Na-K-ATPase as an ion transporter in the kidney, the experimental evidence for ouabain as a circulating hormone, the function of the Na-K-ATPase as a signal transducer that mediates ouabain's effects, and novel results for ouabain-induced Na-K-ATPase signaling in cystogenesis of autosomal dominant polycystic kidney disease.

Pro-death and pro-survival properties of ouabain in U937 lymphoma derived cells

BACKGROUND

Epidemiological studies revealed significantly lower mortality rates in cancer patients receiving cardiac glycosides, which turned on interest in the anticancer properties of these drugs. However, cardiac glycosides have also been shown to stimulate cell growth in several cell types. In the present investigation we analyzed the pro-death and pro-survival properties of ouabain in the human lymphoma derived cell line U937.

METHODS

ROS, intracellular Ca++, cell cycle were evaluated by loading the cells with fluorescent probes under cytofluorimetry. Cell counts and evaluation of trypan blue-excluding cells were performed under optic microscope. Protein detection was done by specific antibodies after protein separation from cellular lysates by SDS-PAGE and transfer blot.

RESULTS

High doses of ouabain cause ROS generation, elevation of [Ca++]i and death of lymphoma derived U937 cells. Lower doses of OUA activate a survival pathway in which plays a role the Na+/Ca++-exchanger (NCX), active in the Ca++ influx mode rather than in the Ca++ efflux mode. Also p38 MAPK plays a pro-survival role. However, the activation of this MAPK does not appear to depend on NCX.

CONCLUSION

This investigation shows that the cardiac glycoside OUA is cytotoxic also for the lymphoma derived cell line U937 and that can activate a survival pathway in which are involved NCX and p38 MAPK. These molecules can represent potential targets of combined therapy.

Hormesis: its impact on medicine and health

This article offers a broad assessment of the hormetic dose response and its relevance to biomedical researchers, physicians, the pharmaceutical industry, and public health scientists. This article contains a series of 61 questions followed by relatively brief but referenced responses that provides support for the conclusion that hormesis is a reproducible phenomenon, commonly observed, with a frequency far greater than other dose-response models such as the threshold and linear nonthreshold dose-response models. The article provides a detailed background information on the historical foundations of hormesis, its quantitative features, mechanistic foundations, as well as how hormesis is currently being used within medicine and identifying how this concept could be further applied in the development of new therapeutic advances and in improved public health practices.

Na+/K+-ATPase α1 isoform mediates ouabain-induced expression of cyclin D1 and proliferation of rat sertoli cells

Novel roles for the interaction of cardiotonic steroids to Na(+)/K(+)-ATPase have been established in recent years. The aim of this study was to investigate the intracellular signaling events downstream the action of ouabain on Na(+)/K(+)-ATPase in Sertoli cell obtained from immature rats. Treatment of Sertoli cells with ouabain (1 μM) induced a rapid and transient increase in the extracellular signal-regulated kinase (ERK1/2 or MAPK3/1) and phosphatidylinositol 3-kinase (PI3K)/serine-threonine protein kinase (AKT) phosphorylation. Also, ouabain upregulated the expression of cyclin D1 and incorporation of [methyl-(3)H]thymidine, both of which were dependent on MAPK3/1 but not AKT intracellular cascade, as shown by pretreatment with MEK (MAP2K1/2) inhibitor U0126 and PI3K inhibitor wortmannin respectively. Moreover, the effect of ouabain on these proliferation parameters was completely prevented by phospho-cAMP response element-binding protein (CREB)/CREB-binding protein complex inhibitor KG501 and only partially by nuclear factor κB nuclear translocation inhibitor SN50. Pretreatment with estrogen receptor antagonist ICI 182780 showed that MAPK3/1 activation by ouabain does not involve this receptor. The Na(+)/K(+)-ATPase α1 isoform, but not α4, was detected in Sertoli cells, suggesting that ouabain effects in Sertoli cells are mediated via α1. Taken together, these results show a rapid ouabain action in the Sertoli cells, which in turn can modulate nuclear transcriptional events essential for Sertoli cell proliferation in a critical period of testicular development. Our findings are important to understand the role of ouabain in the testis and its possible implications in male infertility.

Cardiotonic steroids stabilize regulator of G protein signaling 2 protein levels

Regulator of G protein signaling 2 (RGS2), a G(q)-specific GTPase-activating protein, is strongly implicated in cardiovascular function. RGS2(-/-) mice are hypertensive and prone to heart failure, and several rare human mutations that accelerate RGS2 degradation have been identified among patients with hypertension. Therefore, pharmacological up-regulation of RGS2 protein levels might be beneficial. We used a β-galactosidase complementation method to screen several thousand compounds with known pharmacological functions for those that increased RGS2 protein levels. Several cardiotonic steroids (CTSs), including ouabain and digoxin, increased RGS2 but not RGS4 protein levels. CTSs increased RGS2 protein levels through a post-transcriptional mechanism, by slowing protein degradation. RGS2 mRNA levels in primary vascular smooth muscle cells were unaffected by CTS treatment, whereas protein levels were increased 2- to 3-fold. Na(+)/K(+)-ATPase was required for the increase in RGS2 protein levels, because the effect was lost in Na(+)/K(+)-ATPase-knockdown cells. Furthermore, we demonstrated that CTS-induced increases in RGS2 levels were functional and reduced receptor-stimulated, G(q)-dependent, extracellular signal-regulated kinase phosphorylation. Finally, we showed that in vivo treatment with digoxin led to increased RGS2 protein levels in heart and kidney. CTS-induced increases in RGS2 protein levels and function might modify several deleterious mechanisms in hypertension and heart failure. This novel CTS mechanism might contribute to the beneficial actions of low-dose digoxin treatment in heart failure. Our results support the concept of small-molecule modulation of RGS2 protein levels as a new strategy for cardiovascular therapy.

Cellular stress responses, hormetic phytochemicals and vitagenes in aging and longevity

Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This paper introduces the emerging role of exogenous molecules in hormetic-based neuroprotection and the mitochondrial redox signaling concept of hormesis and its applications to the field of neuroprotection and longevity. Maintenance of optimal long-term health conditions is accomplished by a complex network of longevity assurance processes that are controlled by vitagenes, a group of genes involved in preserving cellular homeostasis during stressful conditions. Vitagenes encode for heat shock proteins (Hsp) Hsp32, Hsp70, the thioredoxin and the sirtuin protein systems. Dietary antioxidants, such as polyphenols and L-carnitine/acetyl-L-carnitine, have recently been demonstrated to be neuroprotective through the activation of hormetic pathways, including vitagenes. Hormesis provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose response relationships, their mechanistic foundations, their relationship to the concept of biological plasticity as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This paper describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways including sirtuin, Nrfs and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. This article is part of a Special Issue entitled: Antioxidants and Antioxidant Treatment in Disease.

Endogenous ouabain regulates cell viability

The endogenous cardiac steroid-like compounds, endogenous ouabain (EO) in particular, are present in the human circulation and are considered putative ligands of the inhibitory binding site of the plasma membrane Na(+)-K(+)-ATPase. A vast amount of data shows that, when added to cell cultures, these steroids promote the growth of cardiac, vascular, and epithelial cells. However, the involvement of the endogenous compounds in the regulation of cell viability and proliferation has never been addressed experimentally. In this study, we show that EO is present in mammalian sera and cerebral spinal fluid, as well as in commercial bovine and horse sera. The lowering of serum EO concentration by the addition of specific anti-ouabain antibodies caused a decrease in the viability of several cultured cell lines. Among these, neuronal NT2 cells were mostly affected, whereas no reduction in viability was seen in rat neuroendocrine PC12 and monkey kidney COS-7 cells. The anti-ouabain antibody-induced reduction in NT2 cell viability was significantly attenuated by the addition of ouabain and was not observed in cells growing in serum-free media. Furthermore, the addition to the medium of low concentrations (nM) of the cardenolide ouabain, but not of the bufadienolide bufalin, increased NT2 and PC12 cell viability and proliferation. In addition, at these concentrations both ouabain and bufalin caused the activation of ERK1/2 in the NT2 cells. The specific ERK1/2 inhibitor U0126 inhibited both the ouabain-induced activation of the enzyme and the increase in cell viability. Furthermore, anti-ouabain antibodies attenuated serum-stimulated ERK1/2 activity in NT2 but not in PC12 cells. Cumulatively, our results suggest that EO plays a significant role in the regulation of cell viability. In addition, our findings support the notion that activation of the ERK1/2 signaling pathway is obligatory but not sufficient for the induction of cell viability by EO.

Distribution of Na/K-ATPase alpha 3 isoform, a sodium-potassium P-type pump associated with rapid-onset of dystonia parkinsonism (RDP) in the adult mouse brain

The Na(+)/K(+)-ATPase1 alpha subunit 3 (ATP1α(3)) is one of many essential components that maintain the sodium and potassium gradients across the plasma membrane in animal cells. Mutations in the ATP1A3 gene cause rapid-onset of dystonia parkinsonism (RDP), a rare movement disorder characterized by sudden onset of dystonic spasms and slowness of movement. To achieve a better understanding of the pathophysiology of the disease, we used immunohistochemical approaches to describe the regional and cellular distribution of ATP1α(3) in the adult mouse brain. Our results show that localization of ATP1α(3) is restricted to neurons, and it is expressed mostly in projections (fibers and punctuates), but cell body expression is also observed. We found high expression of ATP1α(3) in GABAergic neurons in all nuclei of the basal ganglia (striatum, globus pallidus, subthalamic nucleus, and substantia nigra), which is a key circuitry in the fine movement control. Several thalamic nuclei structures harboring connections to and from the cortex expressed high levels of the ATP1α(3) isoform. Other structures with high expression of ATP1α(3) included cerebellum, red nucleus, and several areas of the pons (reticulotegmental nucleus of pons). We also found high expression of ATP1α(3) in projections and cell bodies in hippocampus; most of these ATP1α(3)-positive cell bodies showed colocalization to GABAergic neurons. ATP1α(3) expression was not significant in the dopaminergic cells of substantia nigra. In conclusion, and based on our data, ATP1α(3) is widely expressed in neuronal populations but mainly in GABAergic neurons in areas and nuclei related to movement control, in agreement with RDP symptoms.

Cellular stress responses, the hormesis paradigm, and vitagenes: novel targets for therapeutic intervention in neurodegenerative disorders

Despite the capacity of chaperones and other homeostatic components to restore folding equilibrium, cells appear poorly adapted for chronic oxidative stress that increases in cancer and in metabolic and neurodegenerative diseases. Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This article introduces the concept of hormesis and its applications to the field of neuroprotection. It is argued that the hormetic dose response provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose-response relationships, their mechanistic foundations, and their relationship to the concept of biological plasticity, as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This article describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways, including sirtuin and Nrf2 and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. Particular attention is given to the emerging role of nitric oxide, carbon monoxide, and hydrogen sulfide gases in hormetic-based neuroprotection and their relationship to membrane radical dynamics and mitochondrial redox signaling.

Hormesis is central to toxicology, pharmacology and risk assessment

This paper summarizes numerous conceptual and experimental advances over the past two decades in the study of hormesis. Hormesis is now generally accepted as a real and reproducible biological phenomenon, being highly generalized and independent of biological model, endpoint measured and chemical class/physical stressor. The quantitative features of the hormetic dose response are generally highly consistent, regardless of the model and mechanism, and represent a quantitative index of biological plasticity at multiple levels of biological organization. The hormetic dose-response model has been demonstrated to make far more accurate predictions of responses in low dose zones than either the threshold or linear at low dose models. Numerous therapeutic agents widely used by humans are based on the hormetic dose response and its low dose stimulatory characteristics. It is expected that as low dose responses come to dominate toxicological research that risk assessment practices will incorporate hormetic concepts in the standard setting process.

Alterations of Na+/K+-ATPase function in caveolin-1 knockout cardiac fibroblasts

Recent studies have demonstrated that the Na(+)/K(+)-ATPase is not only an ion pump, but also a membrane receptor that confers the ligand-like effects of cardiotonic steroids (CTS) such as ouabain on protein kinases and cell growth. Because CTS have been implicated in cardiac fibrosis, this study examined the role of caveolae in the regulation of Na(+)/K(+)-ATPase function and CTS signaling in cardiac fibroblasts. In cardiac fibroblasts prepared from wild-type and caveolin-1 knockout [Cav-1(-/-)] mice, we found that the absence of caveolin-1 did not affect total cellular amount or surface expression of Na(+)/K(+)-ATPase alpha1 subunit. However, it did increase ouabain-sensitive (86)Rb(+) uptake. While knockout of caveolin-1 increased basal activities of Src and ERK1/2, it abolished the activation of these kinases induced by ouabain but not angiotensin II. Finally, ouabain stimulated collagen synthesis and cell proliferation in wild type but not Cav-1(-/-) cardiac fibroblasts. Thus, we conclude that caveolae are important for regulating both pumping and signal transducing functions of Na(+)/K(+)-ATPase. While depletion of caveolae increases the pumping function of Na(+)/K(+)-ATPase, it suppresses CTS-induced signal transduction, growth, and collagen production in cardiac fibroblasts.

The physiological significance of the cardiotonic steroid/ouabain-binding site of the Na,K-ATPase

The Na,K-ATPase is the membrane "pump" that generates the Na(+) and K(+) gradients across the plasma membrane that drives many physiological processes. This enzyme is highly sensitive to inhibition by cardiotonic steroids, most notably the digitalis/ouabain class of compounds, which have been used for centuries to treat congestive heart failure and arrhythmias. The amino acids that constitute the ouabain-binding site are highly conserved across the evolutionary spectrum. This could be fortuitous or could result from this site being conserved because it has an important biological function. New physiological approaches using genetically engineered mice are being used to define the biological significance of the "receptor function" of the Na,K-ATPase and its regulation by potential endogenous cardiotonic steroid-like compounds. These studies extend the reach of earlier studies involving the biochemical purification of endogenous regulatory ligands.

Left ventricular geometry and endogenous ouabain in a Flemish population

BACKGROUND

Endogenous ouabain, an endogenous digitalis-like steroid, is synthesized in the adrenal glands and possibly in the hypothalamus. In vitro and in vivo, endogenous ouabain triggers growth and proliferation of cardiomyocytes.

OBJECTIVE

We therefore investigated the association between left ventricular structure and function and plasma endogenous ouabain in a general population.

METHODS

We randomly recruited 536 individuals from a general population (50.7% women, mean age 53.1 years). Measurements included echocardiographic left ventricular structure and function, blood pressure, plasma endogenous ouabain, and the 24-h urinary excretion of sodium.

RESULTS

The geometric mean plasma endogenous ouabain was 95.5 pmol/l (interquartile range 79.4-120.2 pmol/l). We expressed effect sizes for a 1-SD increase in plasma endogenous ouabain (0.21 on the logarithmic scale), while accounting for important covariables. For a 1-SD increment in plasma endogenous ouabain, SBP, left ventricular posterior wall, the interventricular septum, and relative wall thickness increased by 1.59 mmHg (P = 0.009), 0.138 mm (P = 0.003), 0.152 mm (P = 0.013), and 0.71 x 10 (P = 0.008), respectively. In a sensitivity analysis, involving 431 individuals aged 29-71 years (10-90th percentile interval), a 1-SD increase in plasma endogenous ouabain was associated with opposite trends in ejection fraction (+0.90%, P = 0.005) and left ventricular systolic longitudinal strain (-0.48%, P = 0.011). Moreover, in individuals whose sodium excretion was above median (160 mmol/24 h), the aforementioned associations reached a higher level of statistical significance.

CONCLUSION

Our population-based study suggested that endogenous ouabain might have a trophic effect on the myocardium, independent of blood pressure and other covariables. The clinical implication of these findings remains to be elucidated.

Physiological roles of endogenous ouabain in normal rats

Endogenous ouabain (EO)-like compounds are synthesized in and released from the adrenal gland. Although EO has been implicated in several pathological states such as hypertension and heart and kidney failure, its physiological roles in normal animal have not been elucidated. To address this issue, we studied the effects of reduction in plasma EO resulting from antiouabain antibody administration. Normal rats were treated for 28 days with antiouabain antibodies or rabbit IgG as control. Infusions were delivered through a jugular vein cannula by osmotic pumps, and blood pressure was monitored by tail-cuff plethysmography. The animals were housed in metabolic cages to measure water and food consumption and urine excretion. After 28 days, the thoracic aorta was isolated and used to study phenylephrine-induced contraction and atrial natriuretic peptide (ANP)-induced vasorelaxation. The adrenal gland cortex was enlarged in the antiouabain antibody-treated rats. Moreover, on the second day of treatment, there was a significant transient reduction in natriuresis in the antiouabain antibody-treated rats, suggesting that EO is a natriuretic hormone. Reduction in natriuresis was also observed when EO levels were reduced by active immunization resulting from sequential injection of ouabain-albumin. Furthermore, following 28 days of treatment, the response to phenylephrine was significantly lowered and that to ANP was significantly increased in aortic rings from antiouabain antibody-treated rats. These findings show for the first time that circulatory ouabain plausibly originating in the adrenal has physiological roles controlling vasculature tone and sodium homeostasis in normal rats.

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.

Na,K-ATPase signal transduction triggers CREB activation and dendritic growth

Dendritic growth is pivotal in the neurogenesis of cortical neurons. The sodium pump, or Na,K-ATPase, is an evolutionarily conserved protein that, in addition to its central role in establishing the electrochemical gradient, has recently been reported to function as a receptor and signaling mediator. Although a large body of evidence points toward a dual function for the Na,K-ATPase, few biological implications of this signaling pathway have been described. Here we report that Na,K-ATPase signal transduction triggers dendritic growth as well as a transcriptional program dependent on cAMP response element binding protein (CREB) and cAMP response element (CRE)-mediated gene expression, primarily regulated via Ca(2+)/calmodulin-dependent protein (CaM) kinases. The signaling cascade mediating dendritic arbor growth also involves intracellular Ca(2+) oscillations and sustained phosphorylation of mitogen-activated protein (MAP) kinases. Thus, our results suggest a novel role for the Na,K-ATPase as a modulator of dendritic growth in developing neurons.

Hormesis and medicine

Evidence is presented which supports the conclusion that the hormetic dose-response model is the most common and fundamental in the biological and biomedical sciences, being highly generalizable across biological model, endpoint measured and chemical class and physical agent. The paper provides a broad spectrum of applications of the hormesis concept for clinical medicine including anxiety, seizure, memory, stroke, cancer chemotherapy, dermatological processes such as hair growth, osteoporosis, ocular diseases, including retinal detachment, statin effects on cardiovascular function and tumour development, benign prostate enlargement, male sexual behaviours/dysfunctions, and prion diseases.