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

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.

Identification of a mutant α1 Na/K-ATPase that pumps but is defective in signal transduction

BACKGROUND

It has not been possible to study the pumping and signaling functions of Na/K-ATPase independently in live cells.

RESULTS

Both cell-free and cell-based assays indicate that the A420P mutation abolishes the Src regulatory function of Na/K-ATPase.

CONCLUSION

A420P mutant has normal pumping but not signaling function.

SIGNIFICANCE

Identification of Src regulation-null mutants is crucial for addressing physiological role of Na/K-ATPase. The α1 Na/K-ATPase possesses both pumping and signaling functions. However, it has not been possible to study these functions independently in live cells. We have identified a 20-amino acid peptide (Ser-415 to Gln-434) (NaKtide) from the nucleotide binding domain of α1 Na/K-ATPase that binds and inhibits Src in vitro. The N terminus of NaKtide adapts a helical structure. In vitro kinase assays showed that replacement of residues that contain a bulky side chain in the helical structure of NaKtide by alanine abolished the inhibitory effect of the peptide on Src. Similarly, disruption of helical structure by proline replacement, either single or in combination, reduced the inhibitory potency of NaKtide on Src. To identify mutant α1 that retains normal pumping function but is defective in Src regulation, we transfected Na/K-ATPase α1 knockdown PY-17 cells with expression vectors of wild type or mutant α1 carrying Ala to Pro mutations in the region of NaKtide helical structure and generated several stable cell lines. We found that expression of either A416P or A420P or A425P mutant fully restored the α1 content and consequently the pumping capacity of cells. However, in contrast to A416P, either A420P or A425P mutant was incapable of interacting and regulating cellular Src. Consequently, expression of these two mutants caused significant inhibition of ouabain-activated signal transduction and cell growth. Thus we have identified α1 mutant that has normal pumping function but is defective in signal transduction.

Pathogenesis and promising non-invasive markers for preeclampsia

Preeclampsia is one of the leading causes of maternal mortality/morbidity and preterm delivery in the world, affecting 3% to 5% of pregnant women. The pathophysiology of preeclampsia likely involves both maternal and fetal/placental factors. Abnormalities in the development of placental vessels early in pregnancy may result in placental hypoperfusion, hypoxia, or ischemia. Hypoperfusion, hypoxia, and ischemia are critical components in the pathogenesis of preeclampsia because the hypoperfused placenta transfers many factors into maternal vessels that alter maternal endothelial cell function and lead to the systemic symptoms of preeclampsia. There are several hypotheses to explain the pathogenesis of preeclampsia, including altered angiogenic balance, circulating angiogenic factors (such as marinobufagenin, a bufadienolide trigger), and activation of the renin-angiotensin system. Epigenetically-modified cell-free nucleic acids that circulate in plasma and serum might be novel markers with promising non-invasive clinical applications in the diagnosis of preeclampsia.

Gender differences in the development of uremic cardiomyopathy following partial nephrectomy: Role of progesterone

Gender difference has been suggested as a risk factor for developing cardiovascular and renal diseases in humans and experimental animals. As a major sex hormone, progesterone was reported to compete with cardiotonic steroid binding to Na/K-ATPase. Our previous publication demonstrated that cardiotonic steroids (e.g., marinobufagenin) play an important role in the development of experimental uremic cardiomyopathy. We also observed that the putative mineralocorticoid antagonists, spironolactone and its major metabolite canrenone, antagonize binding of cardiotonic steroids to Na/K-ATPase in a competitive manner and also ameliorate experimental uremic cardiomyopathy induced by partial nephrectomy. In the following studies, we noted that progesterone displayed competitive inhibition of cardiotonic steroid binding to Na/K-ATPase and partially inhibited collagen synthesis induced by marinobufagenin in cultured cardiac fibroblasts. Therefore, we sought to examine whether female rats displayed less uremic cardiomyopathy than male rats when subjected to partial nephrectomy. Although partial nephrectomy caused the induction of smaller increases in blood pressure of female rats, they appeared to be similarly susceptible to cardiac remodeling induced by partial nephrectomy in terms of hypertrophy and fibrosis as age-matched male rats. The possible explanations for our findings are therefore discussed.

Age-associated increase in salt sensitivity is accompanied by a shift in the atrial natriuretic peptide modulation of the effect of marinobufagenin on renal and vascular sodium pump

BACKGROUND

Marinobufagenin (MBG) promotes natriuresis via inhibition of renotubular Na/K-ATPase (NKA) and causes vasoconstriction via inhibition of vascular NKA. Atrial natriuretic peptide (ANP), via cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG)-dependent mechanism, sensitizes renal NKA to MBG but reduces MBG-induced inhibition of vascular NKA. As aging is associated with a downregulation of cGMP/PKG signaling, we hypothesized that in older rats, ANP would not potentiate renal effects of MBG and would not oppose vascular effects of MBG.

METHODS

In younger (3-month-old) and older (12-month-old) Sprague-Dawley rats, we compared SBP, natriuresis, activity of NKA in aorta and renal medulla, and levels of MBG and α-ANP at baseline and following acute NaCl loading (20%, 2.5 ml/kg, intraperitoneally), and studied modulation of MBG-induced NKA inhibition by α-ANP in vitro.

RESULTS

As compared with younger rats, NaCl-loaded older rats exhibited a greater MBG response, greater SBP elevation (25 vs. 10 mmHg, P < 0.01) and greater inhibition of NKA in aorta (39 vs. 7%, P < 0.01), 30% less natriuresis, and less inhibition of renal NKA (25 vs. 42%, P < 0.05) in the presence of comparable responses of α-ANP and cGMP. In aorta and kidney of older rats, the levels of PKG were reduced, the levels of phosphodiesterase-5 were increased compared with that in young rats, and α-ANP failed to modulate MBG-induced NKA inhibition.

CONCLUSION

Age-associated downregulation of cGMP/PKG-dependent signaling impairs the ability of ANP to modulate the effects of MBG on the sodium pump, which contributes to salt sensitivity.

Digoxin and ouabain induce the efflux of cholesterol via liver X receptor signalling and the synthesis of ATP in cardiomyocytes

Cardioactive glycosides exert positive inotropic effects on cardiomyocytes through the inhibition of Na(+)/K(+)-ATPase. We showed previously that in human hepatoma cells, digoxin and ouabain increase the rate of the mevalonate cascade and therefore have Na(+)/K(+)-ATPase-independent effects. In the present study we found that they increase the expression and activity of 3-hydroxy-3 methylglutaryl-CoA reductase and the synthesis of cholesterol in cardiomyocytes, their main target cells. Surprisingly this did not promote intracellular cholesterol accumulation. The glycosides activated the liver X receptor transcription factor and increased the expression of ABCA1 (ATP-binding cassette protein A1) transporter, which mediates the efflux of cholesterol and its delivery to apolipoprotein A-I. By increasing the synthesis of ubiquinone, another derivative of the mevalonate cascade, digoxin and ouabain simultaneously enhanced the rate of electron transport in the mitochondrial respiratory chain and the synthesis of ATP. Mice treated with digoxin showed lower cholesterol and higher ubiquinone content in their hearts, and a small increase in their serum HDL (high-density lipoprotein) cholesterol. The results of the present study suggest that cardioactive glycosides may have a role in the reverse transport of cholesterol and in the energy metabolism of cardiomyocytes.

Activation of c-SRC underlies the differential effects of ouabain and digoxin on Ca(2+) signaling in arterial smooth muscle cells

Cardiotonic steroids (CTS) of the strophanthus and digitalis families have opposing effects on long-term blood pressure (BP). This implies hitherto unrecognized divergent signaling pathways for these CTS. Prolonged ouabain treatment upregulates Ca(2+) entry via Na(+)/Ca(2+) exchanger-1 (NCX1) and TRPC6 gene-encoded receptor-operated channels in mesenteric artery smooth muscle cells (ASMCs) in vivo and in vitro. Here, we test the effects of digoxin on Ca(2+) entry and signaling in ASMC. In contrast to ouabain treatment, the in vivo administration of digoxin (30 μg·kg(-1)·day(-1) for 3 wk) did not raise BP and had no effect on resting cytolic free Ca(2+) concentration ([Ca(2+)](cyt)) or phenylephrine-induced Ca(2+) signals in isolated ASMCs. Expression of transporters in the α2 Na(+) pump-NCX1-TRPC6 Ca(2+) signaling pathway was not altered in arteries from digoxin-treated rats. Upregulated α2 Na(+) pumps and a phosphorylated form of the c-SRC protein kinase (pY419-Src, ~4.5-fold) were observed in ASMCs from rats treated with ouabain but not digoxin. Moreover, in primary cultured ASMCs from normal rats, treatment with digoxin (100 nM, 72 h) did not upregulate NCX1 and TRPC6 but blocked the ouabain-induced upregulation of these transporters. Pretreatment of ASMCs with the c-Src inhibitor PP2 (1 μM; 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine) but not its inactive analog eliminated the effect of ouabain on NCX1 and TRPC6 expression and ATP-induced Ca(2+) entry. Thus, in contrast to ouabain, the interaction of digoxin with α2 Na(+) pumps is unable to activate c-Src phosphorylation and upregulate the downstream NCX1-TRPC6 Ca(2+) signaling pathway in ASMCs. The inability of digoxin to upregulate c-Src may underlie its inability to raise long-term BP.

Leptospira and inflammation

Leptospirosis is an important zoonosis and has a worldwide impact on public health. This paper will discuss both the role of immunogenic and pathogenic molecules during leptospirosis infection and possible new targets for immunotherapy against leptospira components. Leptospira, possess a wide variety of mechanisms that allow them to evade the host immune system and cause infection. Many molecules contribute to the ability of Leptospira to adhere, invade, and colonize. The recent sequencing of the Leptospira genome has increased our knowledge about this pathogen. Although the virulence factors, molecular targets, mechanisms of inflammation, and signaling pathways triggered by leptospiral antigens have been studied, some questions are still unanswered. Toll-like receptors (TLRs) are the primary sensors of invading pathogens. TLRs recognize conserved microbial pattern molecules and activate signaling pathways that are pivotal to innate and adaptive immune responses. Recently, a new molecular target has emerged—the Na/K-ATPase—which may contribute to inflammatory and metabolic alteration in this syndrome. Na/K-ATPase is a target for specific fatty acids of host origin and for bacterial components such as the glycolipoprotein fraction (GLP) that may lead to inflammasome activation. We propose that in addition to TLRs, Na/K-ATPase may play a role in the innate response to leptospirosis infection.

Effect of ouabain on metabolic oxidative state in living cardiomyocytes evaluated by time-resolved spectroscopy of endogenous NAD(P)H fluorescence

Time-resolved spectrometry of endogenous nicotinamide dinucleotide phosphate [NAD(P)H] fluorescence is a useful method to evaluate metabolic oxidative state in living cells. Ouabain is a well-known pharmaceutical drug used in the treatment of cardiovascular disease, the effects of which on myocardial metabolism were recently demonstrated. Mechanisms implicated in these actions are still poorly understood. We investigate the effect of ouabain on the metabolic oxidative state of living cardiac cells identified by time-resolved fluorescence spectroscopy of mitochondrial NAD(P)H. Spectral unmixing is used to resolve individual NAD(P)H fluorescence components. Ouabain decreased the integral intensity of NAD(P)H fluorescence, leading to a reduced component amplitudes ratio corresponding to a change in metabolic state. We also noted that lactate/pyruvate, affecting the cytosolic NADH gradient, increased the effect of ouabain on the component amplitudes ratio. Cell oxidation levels, evaluated as the percentage of oxidized NAD(P)H, decreased exponentially with rising concentrations of the cardiac glycoside. Ouabain also stimulated the mitochondrial NADH production. Our study sheds a new light on the role that ouabain plays in the regulation of metabolic state, and presents perspective on a noninvasive, pharmaceutical approach for testing the effect of drugs on the mitochondrial metabolism by means of time-resolved fluorescence spectroscopy in living cells.

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.

Interaction of atrial natriuretic peptide and ouabain in the myocardium

Natriuretic peptides and digitalis-like compounds serve as regulators of homeostasis, including control of volume expansion and blood pressure. The aim of the present study was to explore possible interactions between atrial natriuretic peptide (ANP) and ouabain in the heart. ANP (1 nmol/L) had no effect in papillary muscle preparations from guinea pigs. Ouabain (1 µmol/L) induced positive inotropic effect. The addition of ANP prior to ouabain resulted in a significant decrease in the ouabain-induced positive inotropic effect, manifested as an attenuated increase in twitch maximal upward force slope and resting muscular tension. In addition, ANP caused an increase in Na+–K+-ATPase activity in heart microsomal preparations. The effect of ouabain on Na+–K+-ATPase activity was shown in a biphasic manner. Ouabain (0.01–1 nmol/L) had a small but significant increase on pump activity, but higher doses of ouabain inhibited activity. ANP attenuated ouabain-induced Na+–K+-ATPase activity. Furthermore, ouabain (50 nmol/L) or ANP (10 nmol/L) alone induced Akt activation in cardiomyocytes. However, ANP blocked ouabain-induced Akt activation. These results point to the existence of interactions between ANP and ouabain on Na+–K+-ATPase signaling and function in the heart, which may be mediated by regulation of Na+–K+-ATPase activity and (or) signal transduction mechanisms.

Pathogenesis of pre-eclampsia: marinobufagenin and angiogenic imbalance as biomarkers of the syndrome

Pre-eclampsia (preE), a pregnancy disorder with the de novo onset of hypertension and proteinuria after 20 weeks of gestation, has multiple triggers that initiate pathophysiologic mechanisms. This review addresses translational aspects of preE by synthesizing information on preE pathogenesis, describing diagnostic biomarkers that predict disease, and suggesting strategies to lessen adverse outcomes. Key to this understanding is the role of cardiotonic bufodienolides, with marinobufagenin (MBG) as the prototype, and angiogenic factors in preE pathogenesis. Data from a rat model believed to mimic human preE show that urinary excretion of MBG increases before the onset of hypertension and proteinuria and that affected animals have an increased vascular leakage and blood brain barrier permeability. Angiogenic imbalance occurs with the onset of the syndrome in this model. Also, we report that MBG levels in preE patients exceed those in normal pregnancy and that angiogenic factors are altered in patients showing signs and symptoms of overt disease. In vitro administration of MBG inhibits cytotrophoblast function and triggers hyperpermeability in endothelial cell monolayers. We advance the hypotheses that MBG precedes preE; MBG causes disruption of tight junction proteins leading to vascular leak via activation of MAPK which triggers apoptotic mechanisms resulting in further endothelial dysfunction leading to edema with the release of angiogenic factors. This review provides new evidence about the role of MBG and vasoactive intermediates in preE pathogenesis including the neurologic sequela and may reveal new therapeutic targets for the prevention of preE complications.

Inhibitory effect of bufalin and cinobufagin on steroidogenesis via the activation of ERK in human adrenocortical cells

BACKGROUND AND PURPOSE

Bufalin and cinobufagin exhibit cardiotonic and natriuretic activities. The aim of this study was to evaluate the effects of bufalin and cinobufagin on aldosterone and cortisol secretion and their mechanisms of action in human adrenocortical cells (NCI-H295).

EXPERIMENTAL APPROACH

H295 cells were incubated with bufalin or cinobufagin in the presence or absence of angiotensin II (Ang II), forskolin, 8-Br-cAMP, corticosterone or deoxycortisol. The role of ERK1/2 was studied by use of the inhibitor of MEK (U0126). The binding of transcription factor steroidogenic factor 1 (SF-1) to steroidogenic acute regulatory (StAR) gene promoter was analysed by EMSA.

KEY RESULTS

Bufalin and cinobufagin markedly inhibited basal, Ang II-, forskolin- or 8-Br-cAMP-stimulated aldosterone and cortisol secretion, and the conversions of corticosterone to aldosterone and deoxycortisol to cortisol. Bufalin and cinobufagin also inhibited StAR protein expression and SF-1 binding to StAR gene promoter. They both increased phosphorylation of ERK1/2, and U0126 fully abolished these effects on ERK1/2 in H295 cells. Furthermore, U0126 reversed the inhibitory effects of bufalin and cinobufagin on StAR protein expression and the binding of SF-1 to StAR gene promoter. However, U0126 did not completely reverse their inhibitory effects on aldosterone and cortisol release.

CONCLUSIONS AND IMPLICATIONS

The inhibitory effects of bufalin and cinobufagin on steroidogenesis of aldosterone and cortisol were associated with inhibition of aldosterone synthase and 11β-hydroxylase, as well as the suppression of StAR protein expression and SF-1 binding to StAR promoter via the phosphorylation of ERK1/2 in H295 cells.

2011 Homer Smith Award: To serve and protect: classic and novel roles for Na+, K+ -adenosine triphosphatase

The ability of cells to maintain sharp ion gradients across their membranes is the foundation for the molecular transport and electrical excitability. Across animal species and cell types, Na(+),K(+)-adenosine triphosphatase (ATPase) is arguably the most powerful contributor to this phenomenon. By producing a steep concentration difference of sodium and potassium between the intracellular and extracellular milieu, Na(+),K(+)-ATPase in the tubules provides the driving force for renal sodium reabsorption. Pump activity is downregulated by natriuretic hormones, such as dopamine, and is upregulated by antinatriuretic hormones, such as angiotensin. In the past decade, studies have revealed a novel and surprising role: that Na(+),K(+)-ATPase is a transducer of signals from extracellular to intracellular compartments. The signaling function of Na(+),K(+)-ATPase is activated by ouabain, a mammalian steroid hormone, at far lower concentrations than those that inhibit pump activity. By promoting growth and inhibiting apoptosis, activation of Na(+),K(+)-ATPase exerts tissue-protective effects. Ouabain-stimulated Na(+),K(+)-ATPase signaling has recently shown clinical promise by protecting the malnourished embryonic kidney from adverse developmental programming. A deeper understanding of the tissue-protective role of Na(+),K(+)-ATPase signaling and the regulation of Na(+),K(+)-ATPase pumping activity is of fundamental importance for the understanding and treatment of kidney diseases and kidney-related hypertension.

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.

Pharmacological management of hypertensive emergencies and urgencies: focus on newer agents

INTRODUCTION

Hypertensive crises are categorized as hypertensive emergencies and urgencies depending on the presence of acute target-organ damage; the former are potentially life-threatening medical conditions, requiring urgent treatment under close monitoring. Although several short-acting intravenous antihypertensive agents are approved for this purpose, until recently little evidence from proper trials on the relative merits of different therapies was available.

AREAS COVERED

This article discusses in brief the pathophysiology, epidemiology and diagnostic approach of hypertensive crises and provides an extensive overview of established and emerging pharmacological agents for the treatment of patients with hypertensive emergencies and urgencies.

EXPERT OPINION

Agents such as sodium nitroprusside, nitroglycerin and hydralazine have been used for many years as first-line options for patients with hypertensive emergencies, although their potential adverse effects and difficulties in use were well known. With time, equally potent and less toxic alternatives, including nicardipine, fenoldopam, labetalol and esmolol are increasingly used worldwide. Recently, clevidipine, a third-generation dihydropyridine calcium-channel blocker with unique pharmacodynamic and pharmacokinetic properties was added to our therapeutic armamentarium and was shown in clinical trials to reduce mortality when compared with nitroprusside. In view of such evidence, a change in pharmacological treatment practices for hypertensive crises toward newer and safer agents is warranted.

DigiFab interacts with endogenous cardiotonic steroids and reverses preeclampsia-induced Na/K-ATPase inhibition

Elevated levels of endogenous Na/K-ATPase (NKA) inhibitors, cardiotonic steroids (CTSs) including marinobufagenin (MBG), contribute to pathogenesis of preeclampsia (PE) and represent a target for immunoneutralization by Digibind (Ovine Digoxin Immune Antibody, Glaxo-Smith Kline). Because Digibind is no longer commercially available, we studied whether DigiFab (BTG International Ltd, UK) can substitute Digibind for immunoneutralization of CTS in patients with PE. We compared DigiFab, Digibind, and anti-MBG monoclonal antibody (mAb) with respect to their ability to interact with CTS in PE plasma and to restore NKA activity in erythrocytes from patients with PE. Using immunoassays based on DigiFab, Digibind, and anti-MBG mAb, we studied the elution profile of CTS following high-performance liquid chromatography (HPLC) fractionation of PE plasma. Totally, 7 patients with mild PE (28 ± 2 years; gestational age, 39 ± 0.5 weeks; blood pressure 156 ± 5/94 ± 2 mm Hg) and 6 normotensive pregnant participants (28 ± 1 years; gestational age, 39 ± 0.4 weeks; blood pressure 111 ± 2/73 ± 2 mm Hg) were enrolled. Preeclampsia was associated with a substantial inhibition of erythrocyte NKA (1.47 ± 0.17 vs 2.65 ± 0.16 µmol Pi/mL per h in control group, P < .001). Ex vivo, at 10 µg/mL concentration, which is consistent with the clinical dosing of Digibind administered previously in PE, DigiFab and Digibind as well as anti-MBG mAb (0.5 µg/mL) restored erythrocyte NKA activity. Following HPLC fractionation of pooled PE and control plasma, PE-associated increase in CTS material was detected by Digibind (176 vs 75 pmoles), DigiFab (221 vs 70 pmoles), and anti-MBG mAb (1056 vs 421 pmoles). Therefore, because DigiFab interacts with CTS from PE plasma and reverses PE-induced NKA inhibition, it can substitute Digibind for immunoneutralization of CTS in patients with PE.

Ouabain-stimulated trafficking regulation of the Na/K-ATPase and NHE3 in renal proximal tubule cells

We have demonstrated that ouabain regulates protein trafficking of the Na/K-ATPase α1 subunit and NHE3 (Na/H exchanger, isoform 3) via ouabain-activated Na/K-ATPase signaling in porcine LLC-PK1 cells. To investigate whether this mechanism is species-specific, ouabain-induced regulation of the α1 subunit and NHE3 as well as transcellular (22)Na(+) transport were compared in three renal proximal tubular cell lines (human HK-2, porcine LLC-PK1, and AAC-19 originated from LLC-PK1 in which the pig α1 was replaced by ouabain-resistant rat α1). Ouabain-induced inhibition of transcellular (22)Na(+) transport is due to an ouabain-induced redistribution of the α1 subunit and NHE3. In LLC-PK1 cells, ouabain also inhibited the endocytic recycling of internalized NHE3, but has no significant effect on recycling of endocytosed α1 subunit. These data indicated that the ouabain-induced redistribution of the α1 subunit and NHE3 is not a species-specific phenomenon, and ouabain-activated Na/K-ATPase signaling influences NHE3 regulation.

Quiescence and γH2AX in neuroblastoma are regulated by ouabain/Na,K-ATPase

Background:

Cellular quiescence is a state of reversible proliferation arrest that is induced by anti-mitogenic signals. The endogenous cardiac glycoside ouabain is a specific ligand of the ubiquitous sodium pump, Na,K-ATPase, also known to regulate cell growth through unknown signalling pathways.

Methods:

To investigate the role of ouabain/Na,K-ATPase in uncontrolled neuroblastoma growth we used xenografts, flow cytometry, immunostaining, comet assay, real-time PCR, and electrophysiology after various treatment strategies.

Results:

The ouabain/Na,K-ATPase complex induced quiescence in malignant neuroblastoma. Tumour growth was reduced by >50% when neuroblastoma cells were xenografted into immune-deficient mice that were fed with ouabain. Ouabain-induced S-G2 phase arrest, activated the DNA-damage response (DDR) pathway marker γH2AX, increased the cell cycle regulator p21^Waf1/Cip1 and upregulated the quiescence-specific transcription factor hairy and enhancer of split1 (HES1), causing neuroblastoma cells to ultimately enter G0. Cells re-entered the cell cycle and resumed proliferation, without showing DNA damage, when ouabain was removed.

Conclusion:

These findings demonstrate a novel action of ouabain/Na,K-ATPase as a regulator of quiescence in neuroblastoma, suggesting that ouabain can be used in chemotherapies to suppress tumour growth and/or arrest cells to increase the therapeutic index in combination therapies.

DOCA-salt hypertension does not require the ouabain-sensitive binding site of the α2 Na,K-ATPase

BACKGROUND

We have shown that the ouabain-sensitive α2 Na,K-ATPase is required for adrenocorticotropic hormone (ACTH)-induced hypertension and gestational blood pressure regulation. It is therefore of interest to explore whether this binding site participates in the development of other forms of hypertension, such as deoxycorticosterone acetate (DOCA)-salt using mutant mice with altered sensitivity to ouabain.

METHODS

Wild-type (α1 ouabain-resistant, α2 ouabain-sensitive: α(R/R)α2(S/S)), α1-resistant, α2-resistant (α1(R/R)α2(R/R)) and α1-sensitive, α2-resistant (α1(S/S)α2(R/R)) mice were uninephrectomized and implanted with DOCA pellets. The animals were given either tap water or 1% NaCl, and blood pressure was measured before and after DOCA.

RESULTS

DOCA-salt-treated α1(R/R)α2(R/R) mice developed hypertension to the same extent as α1(R/R)α2(S/S) mice (wild type), and the α1(S/S)α2(R/R) mice given DOCA-salt also showed no difference from the other two genotypes. The expression of the α1 isoform was not changed by DOCA-salt treatment in either α1(R/R)α2(S/S) or α1(R/R)α2(R/R) mice. However, the α2 subunit was expressed at substantially higher levels in the hearts of α1(R/R)α2(R/R) than α1(R/R)α2(S/S) mice, regardless of treatment. Plasma levels of ouabain did not change consistently, but those of marinobufagenin were modestly higher in DOCA-salt treated mice relatively to those without salt.

CONCLUSIONS

The ouabain-binding site of either the α1 or α2 Na,K-ATPase subunit does not play an essential role in the development of DOCA-salt hypertension in this mouse model. These findings indicate that the underlying mechanisms of hypertension induced by DOCA-salt treatment are different from those of ACTH-induced hypertension.

High salt intake causes adverse fetal programming--vascular effects beyond blood pressure

BACKGROUND

High salt intake causes hypertension, adverse cardiovascular outcomes and potentially also blood pressure (BP)-independent target organ damage. Excess salt intake in pregnancy is known to affect BP in the offspring. The present study was designed to assess whether high salt intake in pregnancy affects BP and vascular morphology in the offspring.

METHODS

Sprague-Dawley rats were fed a standard rodent diet with low-normal (0.15%) or high (8.0%) salt content during pregnancy and lactation. After weaning at 4 weeks of age, offspring were maintained on the same diet or switched to a high- or low-salt diet, respectively. Vascular geometry was assessed in male offspring at 7 and 12 weeks postnatally.

RESULTS

Up to 12 weeks of age, there was no significant difference in telemetrically measured BP between the groups of offspring. At 12 weeks of age, wall thickness of central (aorta, carotid), muscular (mesenteric) and intrapulmonary arteries was significantly higher in offspring of mothers on a high-salt diet irrespective of the post-weaning diet. This correlated with increased fibrosis of the aortic wall, more intense nitrotyrosine staining as well as elevated levels of marinobufagenin (MBG) and asymmetric dimethyl arginine (ADMA).

CONCLUSIONS

High salt intake in pregnant rats has long-lasting effects on the modeling of central and muscular arteries in the offspring independent of postnatal salt intake and BP. Circulating MBG and ADMA and local oxidative stress correlate with the adverse vascular modeling.

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

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

Chemical synthesis of the cardiotonic steroid glycosides and related natural products

The active components from the extracts of Digitalis, cardiotonic steroid glycosides, have been ingested by humans for more than 200 years as a medicinal therapy for heart failure and abnormal heart rhythms. The positive inotropic activity of the cardiotonic steroids that mediates clinically useful physiological effects in patients has been attributed largely to a high affinity inhibitory interaction with the extracellular surface of the membrane-bound sodium pump (Na(+)/K(+)-ATPase). However, previously unrecognized intracellular signaling pathways continue to be uncovered. This Review examines both partial and de novo synthetic approaches to the medicinally important and structurally captivating cardenolide and bufadienolide steroid families, with an emphasis on the stereocontrolled construction of the pharmacophoric aglycone (genin) framework.

"The Lower Threshold" phenomenon in tumor cells toward endogenous digitalis-like compounds: Responsible for tumorigenesis?

Since their first discovery as potential anti-cancer drugs decades ago, there is increasing evidence that digitalis-like compounds (DLC) have anti-tumor effects. Less is known about endogenous DLC (EDLC) metabolism and regulation. As stress hormones synthesized in and secreted from the adrenal gland, they likely take part in the hypothalamo-pituitary-adrenal (HPA) axis. In a previous study, we revealed reduced EDLC concentrations in plasma and organs from immune-compromised animals and proposed that a similar situation of a deregulated HPA axis with "adrenal EDLF exhaustion" may contribute to tumorigenesis in chronic stress situations. Here, we put forward the hypothesis that a lowered EDLC response threshold of tumor cells as compared with normal cells increases the risk of tumorigenesis, especially in those individuals with reduced EDLC plasma concentrations after chronic stress exposure. We will evaluate this hypothesis by (a) summarizing the effects of different DLC concentrations on tumor as compared with normal cells and (b) reviewing some essential differences in the Na/K-ATPase of tumor as compared with normal cells (isoform pattern, pump activity, mutations of other signalosome receptors). We will conclude that (1) tumor cells, indeed, seem to have their individual "physiologic" EDLC response range that already starts at pmolar levels and (2) that individuals with markedly reduced (pmolar) EDLC plasma levels are predisposed to cancer because these EDLC concentrations will predominantly stimulate the proliferation of tumor cells. Finally, we will summarize preliminary results from our department supporting this hypothesis.

Ouabain and insulin induce sodium pump endocytosis in renal epithelium

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

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

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

Critical roles of transitional cells and Na/K-ATPase in the formation of vestibular endolymph

The mechanotransduction of vestibular sensory cells depends on the high endolymphatic potassium concentration ([K+]) maintained by a fine balance between K+ secretion and absorption by epithelial cells. Despite the crucial role of endolymph as an electrochemical motor for mechanotransduction, little is known about the processes that govern endolymph formation. To address these, we took advantage of an organotypic rodent model, which regenerates a genuine neonatal vestibular endolymphatic compartment, facilitating the determination of endolymphatic [K+] and transepithelial potential (Vt) during endolymph formation. While mature Vt levels are almost immediately achieved, K+ accumulates to reach a steady [K+] by day 5 in culture. Inhibition of sensory cell K+ efflux enhances [K+] regardless of the blocker used (FM1.43, amikacin, gentamicin, or gadolinium). Targeting K+ secretion with bumetanide partially and transiently reduces [K+], while ouabain application and Kcne1 deletion almost abolishes it. Immunofluorescence studies demonstrate that dark cells do not express Na-K-2Cl cotransporter 1 (the target of bumetanide) in cultured and young mouse utricles, while Na/K-ATPase (the target of ouabain) is found in dark cells and transitional cells. This global analysis of the involvement of endolymphatic homeostasis actors in the immature organ (1) confirms that KCNE1 channels are necessary for K+ secretion, (2) highlights Na/K-ATPase as the key endolymphatic K+ provider and shows that Na-K-2Cl cotransporter 1 has a limited impact on K+ influx, and (3) demonstrates that transitional cells are involved in K+ secretion in the early endolymphatic compartment.

Natriuretic effect of bufalin in isolated rat kidneys involves activation of the Na+-K+-ATPase-Src kinase pathway

Bufadienolides are structurally related to the clinically relevant cardenolides (e.g., digoxin) and are now considered as endogenous steroid hormones. Binding of ouabain to Na(+)-K(+)-ATPase has been associated, in kidney cells, to the activation of the Src kinase pathway and Na(+)-K(+)-ATPase internalization. Nevertheless, whether the activation of this cascade also occurs with other cardiotonic steroids and leads to diuresis and natriuresis in the isolated intact kidney is still unknown. In the present work, we perfused rat kidneys for 120 min with bufalin (1, 3, or 10 μM) and measured its vascular and tubular effects. Thereafter, we probed the effect of 10 μM 3-(4-chlorophenyl)1-(1,1-dimethylethyl)-1H-pyrazolo[3,4-d]pyrimidin-4amine (PP2), a Src family kinase inhibitor, and 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio] butadiene (UO126), a highly selective inhibitor of both MEK1 and MEK2, on bufalin-induced renal alterations. Bufalin at 3 and 10 μM profoundly increased several parameters of renal function in a time- and/or concentration-dependent fashion. At a concentration that produced similar inhibition of the rat kidney Na(+)-K(+)-ATPase, ouabain had a much smaller diuretic and natriuretic effect. Although bufalin fully inhibited the rat kidney Na(+)-K(+)-ATPase in vitro, its IC(50) (33 ± 1 μM) was threefold higher than the concentration used ex vivo and all its renal effects were blunted by PP2 and UO126. Furthermore, the phosphorylated (activated) ERK1/2 expression was increased after bufalin perfusion and this effect was totally prevented after PP2 pretreatment. The present study shows for the first time the direct diuretic, natriuretic, and kaliuretic effects of bufalin in isolated rat kidney and the relevance of Na(+)-K(+)-ATPase-mediated signal transduction.

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

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

How NaCl raises blood pressure: a new paradigm for the pathogenesis of salt-dependent hypertension

Excess dietary salt is a major cause of hypertension. Nevertheless, the specific mechanisms by which salt increases arterial constriction and peripheral vascular resistance, and thereby raises blood pressure (BP), are poorly understood. Here we summarize recent evidence that defines specific molecular links between Na(+) and the elevated vascular resistance that directly produces high BP. In this new paradigm, high dietary salt raises cerebrospinal fluid [Na(+)]. This leads, via the Na(+)-sensing circumventricular organs of the brain, to increased sympathetic nerve activity (SNA), a major trigger of vasoconstriction. Plasma levels of endogenous ouabain (EO), the Na(+) pump ligand, also become elevated. Remarkably, high cerebrospinal fluid [Na(+)]-evoked, locally secreted (hypothalamic) EO participates in a pathway that mediates the sustained increase in SNA. This hypothalamic signaling chain includes aldosterone, epithelial Na(+) channels, EO, ouabain-sensitive α(2) Na(+) pumps, and angiotensin II (ANG II). The EO increases (e.g.) hypothalamic ANG-II type-1 receptor and NADPH oxidase and decreases neuronal nitric oxide synthase protein expression. The aldosterone-epithelial Na(+) channel-EO-α(2) Na(+) pump-ANG-II pathway modulates the activity of brain cardiovascular control centers that regulate the BP set point and induce sustained changes in SNA. In the periphery, the EO secreted by the adrenal cortex directly enhances vasoconstriction via an EO-α(2) Na(+) pump-Na(+)/Ca(2+) exchanger-Ca(2+) signaling pathway. Circulating EO also activates an EO-α(2) Na(+) pump-Src kinase signaling cascade. This increases the expression of the Na(+)/Ca(2+) exchanger-transient receptor potential cation channel Ca(2+) signaling pathway in arterial smooth muscle but decreases the expression of endothelial vasodilator mechanisms. Additionally, EO is a growth factor and may directly participate in the arterial structural remodeling and lumen narrowing that is frequently observed in established hypertension. These several central and peripheral mechanisms are coordinated, in part by EO, to effect and maintain the salt-induced elevation of BP.

The Na+-K+-ATPase as self-adhesion molecule and hormone receptor

Thanks to the homeostasis of the internal milieu, metazoan cells can enormously simplify their housekeeping efforts and engage instead in differentiation and multiple forms of organization (tissues, organs, systems) that enable them to produce an astonishing diversity of mammals. The stability of the internal milieu despite drastic variations of the external environment (air, fresh or seawater, gastrointestinal fluids, glomerular filtrate, bile) is due to transporting epithelia that can adjust their specific permeability to H(2)O, H(+), Na(+), K(+), Ca(2+), and Cl(-) over several orders of magnitude and exchange substances with the outer milieu with exquisite precision. This exchange is due to the polarized expression of membrane proteins, among them Na(+)-K(+)-ATPase, an oligomeric enzyme that uses chemical energy from ATP molecules to translocate ions across the plasma membrane of epithelial cells. Na(+)-K(+)-ATPase presents two types of asymmetries: the arrangement of its subunits, and its expression in one pole of the epithelial cell ("polarity"). In most epithelia, polarity consists of the expression of Na(+)-K(+)-ATPase towards the intercellular space and arises in part from the interaction of the extracellular segment of the β-subunit with another β-subunit present in a Na(+)-K(+)-ATPase molecule expressed by a neighboring cell. In addition to enabling the Na(+)-K(+)-ATPase to transport ions and water vectorially, this position exposes its receptors to ouabain and analogous cardiotonic steroids, which are present in the internal milieu because these were secreted by endocrine cells.

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.

Influence of N-methyl-D-aspartate receptors on ouabain activation of nuclear factor-κB in the rat hippocampus

It has been shown that ouabain (OUA) can activate the Na,K-ATPase complex and mediate intracellular signaling in the central nervous system (CNS). Inflammatory stimulus increases glutamatergic transmission, especially at N-methyl-D-aspartate (NMDA) receptors, which are usually coupled to the activation of nitric oxide synthase (NOS). Nuclear factor-κB (NF-κB) activation modulates the expression of genes involved in development, plasticity, and inflammation. The present work investigated the effects of OUA on NF-κB binding activity in rat hippocampus and the influence of this OUA-Na,K-ATPase signaling cascade in NMDA-mediated NF-κB activation. The findings presented here are the first report indicating that intrahippocampal administration of OUA, in a concentration that did not alter Na,K-ATPase or NOS activity, induced an activation of NF-κB, leading to increases in brain-derived neurotrophic factor (Bdnf), inducible NOS (iNos), tumor necrosis factor-α (Tnf-α), and B-cell leukemia/lymphoma 2 (Bcl2) mRNA levels. This response was not linked to any significant signs of neurodegeneration as showed via Fluoro-Jade B and Nissl stain. Intrahippocampal administration of NMDA induced NF-κB activation and increased NOS and α(2/3) -Na,K-ATPase activities. NMDA treatment further increased OUA-induced NF-κB activation, which was partially blocked by MK-801, an antagonist of NMDA receptor. These results suggest that OUA-induced NF-κB activation is at least in part dependent on Na,K-ATPase modulatory action of NMDA receptor in hippocampus. The interaction of these signaling pathways could be associated with biological mechanisms that may underlie the basal homeostatic state linked to the inflammatory signaling cascade in the brain.

Marinobufagenin and cyclic strain may activate endothelial NADPH oxidase, contributing to the adverse impact of salty diets on vascular and cerebral health

Limited but provocative ecologic epidemiology suggests that dietary salt may play a central role in the genesis of not only of stroke, but also dementia, including Alzheimer's disease. Impairment of nitric oxide bioactivity in the cerebral microvasculature is a likely mediator of this effect. Salted diets evoke increased adrenal secretion of the natriuretic steroid marinobufagenin (MBG), which promotes natriuresis via inhibition of renal tubular Na+/K+-ATPase; this effect is notably robust in salt-sensitive rodent strains in which other compensatory natriuretic mechanisms are subnormally efficient. MBG-mediated inhibition of sodium pumps in vascular smooth muscle likely plays a role in the hypertension induced by salty diets in these rodents. However, salt sensitivity in humans is associated with increased vascular mortality and ventricular hypertrophy independent of blood pressure; this suggests that MBG may be pathogenic via mechanisms unrelated to blood pressure control. Indeed, recent evidence indicates that MBG, via interaction with alpha1 isoforms of the sodium pump, can activate various intracellular signaling pathways at physiological concentrations too low to notably inhibit pump activity. An overview of current evidence suggests the hypothesis that MBG - as well as the cyclic strain induced by hypertension per se - may induce endothelial oxidative stress by activating NADPH oxidase. If so, this could rationalize the increase in vascular and systemic oxidative stress observed in salt-sensitive rodents fed salty diets, or in rodents infused with MBG; moreover, if this effect is a particularly prominent determinant of oxidative stress in cerebrovascular endothelium, it might help to explain the virtual absence of stroke and dementia in low-salt societies. As a corollary of this hypothesis, it can be predicted that spirulina-derived phycobilins, which appear to mimic the physiological role of bilirubin as an inhibitor of NAPDH oxidase complexes, may have potential for ameliorating the adverse health impacts of MBG and of salty diets. Potassium-rich diets are also likely to be protective in this regard, as they should suppress MBG production via their natriuretic impact, while their stimulatory effect on sodium pump activity may exert a hyperpolarizing effect on plasma membranes that suppresses NADPH oxidase activity.

Bufadienolide compounds sensitize human breast cancer cells to TRAIL-induced apoptosis via inhibition of STAT3/Mcl-1 pathway

The death receptor ligand TRAIL is considered a promising candidate for cancer therapy because of its preferential toxicity to malignant cells. However its efficacy has been challenged by a number of resistance mechanisms. Therefore, agents that can overcome the resistance to enhance therapeutic efficacy of TRAIL are needed. In the current study, we found that bufalin, bufotalin and gamabufotalin, key members of bufadienolides isolated from a traditional Chinese medicine ChanSu, significantly potentiated human breast cancer cells with different status of ER-alpha to apoptosis induction of TRAIL, as evidenced by enhanced Annexin V/FITC positive cells (apoptotic cells), cytoplasmic histone-associated-DNA-fragments, membrane permeability transition (MPT), caspases activation and PARP cleavage. Further mechanistic investigation demonstrated that bufalin was able to significantly decrease Mcl-1 expression and modestly decrease Bcl-XL expression level. Down-regulations of these anti-apoptotic proteins were well correlated with inhibition of transcription factor STAT3 activation. The important consequence of down-regulation Mcl-1 in the enhancement action by combining bufalin with TRAIL was confirmed by either knockdown or overexpression of Mcl-1 approach. Our findings for the first time provided strong evidences that bufadienolide compounds have excellent potential to be developed as a novel class of sensitizers of TRAIL.