Organ hypertrophic signaling within caveolae membrane subdomains triggered by ouabain and antagonized by PST 2238

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

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

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

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

Small artery remodeling and stiffening in deoxycorticosterone acetate-salt hypertensive rats involves the interaction between endogenous ouabain/Na + K + -ATPase/cSrc signaling

OBJECTIVE

Endogenous ouabain (EO) increases in some patients with hypertension and in rats with volume-dependent hypertension. When ouabain binds to Na + K + -ATPase, cSrc is activated, which leads to multieffector signaling activation and high blood pressure (BP). In mesenteric resistance arteries (MRA) from deoxycorticosterone acetate (DOCA)-salt rats, we have demonstrated that the EO antagonist rostafuroxin blocks downstream cSrc activation, enhancing endothelial function and lowering oxidative stress and BP. Here, we examined the possibility that EO is involved in the structural and mechanical alterations that occur in MRA from DOCA-salt rats.

METHODS

MRA were taken from control, vehicle-treated DOCA-salt or rostafuroxin (1 mg/kg per day, for 3 weeks)-treated DOCA-salt rats. Pressure myography and histology were used to evaluate the mechanics and structure of the MRA, and western blotting to assess protein expression.

RESULTS

DOCA-salt MRA exhibited signs of inward hypertrophic remodeling and increased stiffness, with a higher wall:lumen ratio, which were reduced by rostafuroxin treatment. The enhanced type I collagen, TGFβ1, pSmad2/3 Ser465/457 /Smad2/3 ratio, CTGF, p-Src Tyr418 , EGFR, c-Raf, ERK1/2 and p38MAPK protein expression in DOCA-salt MRA were all recovered by rostafuroxin.

CONCLUSION

A process combining Na + K + -ATPase/cSrc/EGFR/Raf/ERK1/2/p38MAPK activation and a Na + K + -ATPase/cSrc/TGF-1/Smad2/3/CTGF-dependent mechanism explains how EO contributes to small artery inward hypertrophic remodeling and stiffening in DOCA-salt rats. This result supports the significance of EO as a key mediator for end-organ damage in volume-dependent hypertension and the efficacy of rostafuroxin in avoiding remodeling and stiffening of small arteries.

Different antihypertensive and metabolic responses to rostafuroxin in undernourished and normonourished male rats: Outcomes on bodily Na+ handling

Hypertension is a pandemic nowadays. We aimed to investigate whether chronic undernutrition modifies the response to the antihypertensive drug rostafuroxin in juvenile hypertensive rats. Chronic undernutrition was induced in male rats using a multideficient diet known as the Regional Basic Diet (RBD), mimicking alimentary habits in impoverished regions worldwide. Animals were given RBD-or a control/CTRL normal diet for rodents-from weaning to 90 days, and rostafuroxin (1 mg/kg body mass) was orally administered from day 60 onwards. For the last 2 days, the rats were hosted in metabolic cages to measure food/energy, water, Na+ ingestion, and urinary volume. Rostafuroxin increased food/energy/Na+ intake in CTRL and RBD rats but had opposite effects on Na+ balance (intake minus urinary excretion). The drug normalized the decreased plasma Na+ concentration in RBD rats, increased urinary volume in RBD but not in CTRL, and decreased and increased urinary Na+ concentration in the RBD and CTRL groups, respectively. Rostafuroxin decreased the ouabain-sensitive (Na+ +K+ )ATPase and increased the ouabain-resistant Na+ -ATPase from proximal tubule cells in both groups and normalized the systolic blood pressure in RBD without effect in CTRL rats. We conclude that chronic undernutrition modifies the response of blood pressure and metabolic responses to rostafuroxin.

Inhibition of endogenous ouabain by atrial natriuretic peptide is a guanylyl cyclase independent effect

Background

Endogenous ouabain (EO) and atrial natriuretic peptide (ANP) are important in regulation of sodium and fluid balance. There is indirect evidence that ANP may be involved in the regulation of endogenous cardenolides.

Methods

H295R are human adrenocortical cells known to release EO. Cells were treated with ANP at physiologic concentrations or vehicle (0.1% DMSO), with or without guanylyl cyclase inhibitor 1,2,4 oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). Cyclic guanosine monophosphate (cGMP), the intracellular second messenger of ANP, was measured by a chemiluminescent immunoassay and EO was measured by radioimmunoassay of C18 extracted samples.

Results

EO secretion is inhibited by ANP treatment, with the most prolonged inhibition (90 min vs ≤ 60 min) occurring at physiologic ANP concentrations (50 pg/mL). Inhibition of guanylyl cyclase with ODQ, also reduces EO secretion. The inhibitory effects on EO release in response to cotreatment with ANP and ODQ appeared to be additive.

Conclusions

ANP inhibits basal EO secretion, and it is unlikely that this is mediated through ANP-A or ANP-B receptors (the most common natriuretic peptide receptors) or their cGMP second messenger; the underlying mechanisms involved are not revealed in the current studies. The role of ANP in the control of EO synthesis and secretion in vivo requires further investigation.

Ouabain inhibitor rostafuroxin attenuates dextromethorphan-induced manic potential

Dextromethorphan (DM) abuse produces mania-like symptoms in humans. ERK/Akt signaling activation involved in manic potential can be attenuated by the inhibition of ouabain-like cardiac steroids. In this study, increased phosphorylations of ERK/Akt and hyperlocomotion induced by DM (30 mg/kg, i.p./day × 7) were significantly protected by the ouabain inhibitor rostafuroxin (ROSTA), suggesting that DM induces the manic potential. ROSTA significantly attenuated DM-induced protein kinase C δ (PKCδ) phosphorylation, GluN2B (i.e., MDA receptor subunit) expression, and phospho-PKCδ/GluN2B interaction. DM instantly upregulated the nuclear factor erythroid-2-related factor 2 (Nrf2)-dependent system. However, DM reduced Nrf2 nuclear translocation, Nrf2 DNA binding activity, γ-glutamylcysteine mRNA expression, and subsequent GSH/GSSG level and enhanced oxidative parameters following 1-h of administration. ROSTA, PKCδ inhibitor rottlerin, and GluN2B inhibitor traxoprodil significantly attenuated DM-induced alterations in Nrf2-related redox parameters and locomotor activity induced by DM in wild-type mice. Importantly, in PKCδ knockout mice, DM failed to alter the above parameters. Further, ROSTA and traxoprodil also failed to enhance PKCδ depletion effect, suggesting that PKCδ is a critical target for the anti-manic potential of ROSTA or GluN2B antagonism. Our results suggest that ROSTA inhibits DM-induced manic potential by attenuating ERK/Akt activation, GluN2B/PKCδ signalings, and Nrf2-dependent system.

The Na/K-ATPase Signaling and SGLT2 Inhibitor-Mediated Cardiorenal Protection: A Crossed Road?

In different large-scale clinic outcome trials, sodium (Na⁺)/glucose co-transporter 2 (SGLT2) inhibitors showed profound cardiac- and renal-protective effects, making them revolutionary treatments for heart failure and kidney disease. Different theories are proposed according to the emerging protective effects other than the original purpose of glucose-lowering in diabetic patients. As the ATP-dependent primary ion transporter providing the Na⁺ gradient to drive other Na⁺-dependent transporters, the possible role of the sodium–potassium adenosine triphosphatase (Na/K-ATPase) as the primary ion transporter and its signaling function is not explored.

Endogenous Mammalian Cardiotonic Steroids-A New Cardiovascular Risk Factor?-A Mini-Review

The role of endogenous mammalian cardiotonic steroids (CTS) in the physiology and pathophysiology of the cardiovascular system and the kidneys has interested researchers for more than 20 years. Cardiotonic steroids extracted from toads or plants, such as digitalis, have been used to treat heart disease since ancient times. CTS, also called endogenous digitalis-like factors, take part in the regulation of blood pressure and sodium homeostasis through their effects on the transport enzyme called sodium–potassium adenosine triphosphatase (Na/K-ATPase) in renal and cardiovascular tissue. In recent years, there has been increasing evidence showing deleterious effects of CTS on the structure and function of the heart, vasculature and kidneys. Understanding the role of CTS may be useful in the development of potential new therapeutic strategies.

Chronic cyclooxygenase-2 inhibition prevents the worsening of hypertension and endothelial dysfunction induced by ouabain in resistance arteries of spontaneously hypertensive rats

AIM

Previous studies raise cyclooxygenase (COX) activation as a possible mechanism involved in the pathophysiology of ouabain-induced hypertension. We hypothesized that inhibition of COX-2 activity might prevent ouabain-induced vascular dysfunction and worsening of hypertension in spontaneously hypertensive rats (SHR).

METHODS

SHR were exposed to ouabain or vehicle and treated or not with the selective COX-2 inhibitor nimesulide for 5 weeks. Systolic blood pressure was measured by plethysmography. Vascular reactivity by wire myograph and protein expression by Western-blot were assessed in mesenteric resistance arteries (MRA) of groups. Thromboxane A2 (TXA₂) production by ELISA was evaluated in MRA supernatants of groups.

RESULTS

Noradrenaline-induced maximal contraction (Emax) was greater in MRA from SHR receiving ouabain than those of vehicle group. In situ inhibition of COX-2, TXA₂ synthase, or TP receptor reduced the Emax to noradrenaline in MRA of ouabain to vehicle levels. TXA₂ production was higher in ouabain than in vehicle group. Ouabain enhanced expression of cytoplasmic tyrosine kinase Src (c-Src)/ERK1/2/COX-2/TXA₂ synthase/TP receptor in SHR MRA, but did not change NFkB/iKB ratio. Anticontractile effect of nitric oxide (NO) was smaller in MRA from ouabain- than vehicle-treated SHR, as well as eNOS and nNOS expression. Nimesulide co-treatment prevented the ouabain-induced worsening of hypertension and noradrenaline MRA hypercontractility in SHR.

CONCLUSION

Ouabain worsen hypertension and induce MRA hypercontractility in SHR associated with upregulated c-Src/ERK1/2/COX-2/TXA₂ synthase/TXA₂/TP receptor axis. These effects were prevented by COX-2 inhibition.

Quo vadis Cardiac Glycoside Research?

Cardiac glycosides (CGs), toxins well-known for numerous human and cattle poisoning, are natural compounds, the biosynthesis of which occurs in various plants and animals as a self-protective mechanism to prevent grazing and predation. Interestingly, some insect species can take advantage of the CG’s toxicity and by absorbing them, they are also protected from predation. The mechanism of action of CG’s toxicity is inhibition of Na⁺/K⁺-ATPase (the sodium-potassium pump, NKA), which disrupts the ionic homeostasis leading to elevated Ca²⁺ concentration resulting in cell death. Thus, NKA serves as a molecular target for CGs (although it is not the only one) and even though CGs are toxic for humans and some animals, they can also be used as remedies for various diseases, such as cardiovascular ones, and possibly cancer. Although the anticancer mechanism of CGs has not been fully elucidated, yet, it is thought to be connected with the second role of NKA being a receptor that can induce several cell signaling cascades and even serve as a growth factor and, thus, inhibit cancer cell proliferation at low nontoxic concentrations. These growth inhibitory effects are often observed only in cancer cells, thereby, offering a possibility for CGs to be repositioned for cancer treatment serving not only as chemotherapeutic agents but also as immunogenic cell death triggers. Therefore, here, we report on CG’s chemical structures, production optimization, and biological activity with possible use in cancer therapy, as well as, discuss their antiviral potential which was discovered quite recently. Special attention has been devoted to digitoxin, digoxin, and ouabain.

Cardiac Oxidative Signaling and Physiological Hypertrophy in the Na/K-ATPase α1s/sα2s/s Mouse Model of High Affinity for Cardiotonic Steroids

The Na/K-ATPase is the specific receptor for cardiotonic steroids (CTS) such as ouabain and digoxin. At pharmacological concentrations used in the treatment of cardiac conditions, CTS inhibit the ion-pumping function of Na/K-ATPase. At much lower concentrations, in the range of those reported for endogenous CTS in the blood, they stimulate hypertrophic growth of cultured cardiac myocytes through initiation of a Na/K-ATPase-mediated and reactive oxygen species (ROS)-dependent signaling. To examine a possible effect of endogenous concentrations of CTS on cardiac structure and function in vivo, we compared mice expressing the naturally resistant Na/K-ATPase α1 and age-matched mice genetically engineered to express a mutated Na/K-ATPase α1 with high affinity for CTS. In this model, total cardiac Na/K-ATPase activity, α1, α2, and β1 protein content remained unchanged, and the cardiac Na/K-ATPase dose–response curve to ouabain shifted to the left as expected. In males aged 3–6 months, increased α1 sensitivity to CTS resulted in a significant increase in cardiac carbonylated protein content, suggesting that ROS production was elevated. A moderate but significant increase of about 15% of the heart-weight-to-tibia-length ratio accompanied by an increase in the myocyte cross-sectional area was detected. Echocardiographic analyses did not reveal any change in cardiac function, and there was no fibrosis or re-expression of the fetal gene program. RNA sequencing analysis indicated that pathways related to energy metabolism were upregulated, while those related to extracellular matrix organization were downregulated. Consistent with a functional role of the latter, an angiotensin-II challenge that triggered fibrosis in the α1^r/rα2^s/s mouse failed to do so in the α1^s/sα2^s/s. Taken together, these results are indicative of a link between circulating CTS, Na/K-ATPase α1, ROS, and physiological cardiac hypertrophy in mice under baseline laboratory conditions.

Antihypertensive treatment guided by genetics: PEARL-HT, the randomized proof-of-concept trial comparing rostafuroxin with losartan

We compared a standard antihypertensive losartan treatment with a pharmacogenomics-guided rostafuroxin treatment in never-treated Caucasian and Chinese patients with primary hypertension. Rostafuroxin is a digitoxigenin derivative that selectively disrupts the binding to the cSrc-SH2 domain of mutant α-adducin and of the ouabain-activated Na-K pump at 10^-11 M. Of 902 patients screened, 172 were enrolled in Italy and 107 in Taiwan. After stratification for country and genetic background, patients were randomized to rostafuroxin or losartan, being the difference in the fall in office systolic blood pressure (OSBP) after 2-month treatment the primary endpoint. Three pharmacogenomic profiles (P) were examined, considering: P1, adding to the gene variants included in the subsequent P2, the variants detected by post-hoc analysis of a previous trial; P2, variants of genes encoding enzymes for endogenous ouabain (EO) synthesis (LSS and HSD3B1), EO transport (MDR1/ABCB1), adducin (ADD1 and ADD3); P3, variants of the LSS gene only. In Caucasians, the group differences (rostafuroxin 50 μg minus losartan 50 mg in OSBP mmHg) were significant both in P2 adjusted for genetic heterogeneity (P2a) and P3 LSS rs2254524 AA [9.8 (0.6-19.0), P = 0.038 and 13.4 (25.4-2.5), P = 0.031, respectively]. In human H295R cells transfected with LSS A and LSS C variants, the EO production was greater in the former (P = 0.038); this difference was abolished by rostafuroxin at 10^-11 M. Chinese patients had a similar drop in OSBP to Caucasians with losartan but no change in OSBP with rostafuroxin. These results show that genetics may guide drug treatment for primary hypertension in Caucasians.

The Na/K-ATPase α1 and c-Src form signaling complex under native condition: A crosslinking approach

The protein-protein interactions amongst the Na/K-ATPase α1 subunit, c-Src, and caveolin-1 (cav-1) are essential for the Na/K-ATPase signaling functions. However, there are arguments concerning the interaction model. The present study aims to clarify the interactions amongst the endogenous native proteins in live cells under native resting condition. Under native condition, Blue Native-PAGE and Blue Native-PAGE/SDS-PAGE 2D analyses demonstrated co-existence of the α1 subunit and c-Src in same protein complex, as well as a direct interaction between the α1 subunit and c-Src. By comparison of cleavable and non-cleavable cysteine-cysteine crosslinked samples, capillary immunoblotting analysis demonstrated that depletion of Src kinase family members (c-Src, Yes, and Fyn) or cav-1 clearly reduced the interactions of the α1 subunit with proteins, but depletion of cav-1 did not affect the interaction of c-Src with the α1 subunit. The data indicated that there are direct interactions between the α1 subunit and c-Src as well as between the α1 subunit and cav-1, but argued about the interaction between c-Src and cav-1 under the condition. Furthermore, the data also indicated the existence of different protein complexes containing the α1 subunit and c-Src, which might have different signaling functions.

The Redox-Sensitive Na/K-ATPase Signaling in Uremic Cardiomyopathy

In recent years, Na/K-ATPase signaling has been implicated in different physiological and pathophysiological conditions, including cardiac hypertrophy and uremic cardiomyopathy. Cardiotonic steroids (CTS), specific ligands of Na/K-ATPase, regulate its enzymatic activity (at higher concentrations) and signaling function (at lower concentrations without significantly affecting its enzymatic activity) and increase reactive oxygen species (ROS) generation. On the other hand, an increase in ROS alone also regulates the Na/K-ATPase enzymatic activity and signaling function. We termed this phenomenon the Na/K-ATPase-mediated oxidant-amplification loop, in which oxidative stress regulates both the Na/K-ATPase activity and signaling. Most recently, we also demonstrated that this amplification loop is involved in the development of uremic cardiomyopathy. This review aims to evaluate the redox-sensitive Na/K-ATPase-mediated oxidant amplification loop and uremic cardiomyopathy.

Endogenous cardiotonic steroids and cardiovascular disease, where to next?

Ever since British Physician William Withering first described the use of foxglove extract for treatment of patients with congestive heart failure in 1785, cardiotonic steroids have been used clinically to treat heart failure and more recently atrial fibrillation. Due to their ability to bind and inhibit the ubiquitous transport enzyme sodium potassium pump, thus regulating intracellular Na⁺ concentration in every living cell, they are also an essential tool for research into the sodium potassium pump structure and function. Exogenous CTS have been clearly demonstrated to affect cardiovascular system through modulation of vagal tone, cardiac contraction (via ionic changes) and altered natriuresis. Reports of a number of endogenous CTS, since the 1980s, have intensified research into their physiologic and pathophysiologic roles and opened up novel therapeutic targets. Substantive evidence pointing to the role of endogenous ouabain and marinobufagenin, the two most prominent CTS, in development of cardiovascular disease has accumulated. Nevertheless, their presence, structure, biosynthesis pathways and even mechanism of action remain unclear or controversial. In this review the current state-of-the-art, the controversies and the remaining questions surrounding the role of endogenous cardiotonic steroids in health and disease are discussed.

Cardiotonic Steroids-A Possible Link Between High-Salt Diet and Organ Damage

High dietary salt intake has been listed among the top ten risk factors for disability-adjusted life years. We discuss the role of endogenous cardiotonic steroids in mediating the dietary salt-induced hypertension and organ damage.

Cardiotonic Steroids and the Sodium Trade Balance: New Insights into Trade-Off Mechanisms Mediated by the Na⁺/K⁺-ATPase

In 1972 Neal Bricker presented the "trade-off" hypothesis in which he detailed the role of physiological adaptation processes in mediating some of the pathophysiology associated with declines in renal function. In the late 1990's Xie and Askari published seminal studies indicating that the Na⁺/K⁺-ATPase (NKA) was not only an ion pump, but also a signal transducer that interacts with several signaling partners. Since this discovery, numerous studies from multiple laboratories have shown that the NKA is a central player in mediating some of these long-term "trade-offs" of the physiological adaptation processes which Bricker originally proposed in the 1970's. In fact, NKA ligands such as cardiotonic steroids (CTS), have been shown to signal through NKA, and consequently been implicated in mediating both adaptive and maladaptive responses to volume overload such as fibrosis and oxidative stress. In this review we will emphasize the role the NKA plays in this "trade-off" with respect to CTS signaling and its implication in inflammation and fibrosis in target organs including the heart, kidney, and vasculature. As inflammation and fibrosis exhibit key roles in the pathogenesis of a number of clinical disorders such as chronic kidney disease, heart failure, atherosclerosis, obesity, preeclampsia, and aging, this review will also highlight the role of newly discovered NKA signaling partners in mediating some of these conditions.

Endogenous Ouabain and Related Genes in the Translation from Hypertension to Renal Diseases

The endogenous ouabain (EO) is a steroid hormone secreted by the adrenal gland with cardio-tonic effects. In this article, we have reviewed and summarized the most recent reports about EO, particularly with regard to how it may interact with specific genetic backgrounds. We have focused our attention on the EO’s potential pathogenic role in several diseases, including renal failure, essential hypertension and heart failure. Notably, these reports have demonstrated that EO acts as a pro-hypertrophic and growth-promoting hormone, which might lead to a cardiac remodeling affecting cardiovascular functions and structures. In addition, a possible role of EO in the development of acute kidney injury has been hypothesized. During the last decays, many important improvements permitted a deeper understanding of EO’s metabolisms and functions, including the characteristics of its receptor and the effects of its activation. Such progresses indicated that EO has significant implications in the pathogenesis of many common diseases. The patho-physiological role of EO in the development of hypertension and other cardiac and renal complications have laid the basis for the development of a new selective compound that could selectively modulate the genetic and molecular mechanisms involved in EO’s action. It is evident that the knowledge of EO has incredibly increased; however, many important areas remain to be further investigated.

Targeting Na/K-ATPase Signaling: A New Approach to Control Oxidative Stress

Renal and cardiac function are greatly affected by chronic oxidative stress which can cause many pathophysiological states. The Na/K-ATPase is well-described as an ion pumping enzyme involved in maintaining cellular ion homeostasis; however, in the past two decades, extensive research has been done to understand the signaling function of the Na/K-ATPase and determine its role in physiological and pathophysiological states. Our lab has shown that the Na/K-ATPase signaling cascade can function as an amplifier of reactive oxygen species (ROS) which can be initiated by cardiotonic steroids or increases in ROS. Regulation of systemic oxidative stress by targeting Na/K-ATPase signaling mediated oxidant amplification improves 5/6th partial nephrectomy (PNx) mediated uremic cardiomyopathy, renal sodium handling, as well as ameliorates adipogenesis. This review will present this new concept of Na/K-ATPase signaling mediated oxidant amplification loop and its clinic implication.

The pump, the exchanger, and the holy spirit: origins and 40-year evolution of ideas about the ouabain-Na+ pump endocrine system

Two prescient 1953 publications set the stage for the elucidation of a novel endocrine system: Schatzmann's report that cardiotonic steroids (CTSs) are all Na⁺ pump inhibitors, and Szent-Gyorgi's suggestion that there is an endogenous "missing screw" in heart failure that CTSs like digoxin may replace. In 1977 I postulated that an endogenous Na⁺ pump inhibitor acts as a natriuretic hormone and simultaneously elevates blood pressure (BP) in salt-dependent hypertension. This hypothesis was based on the idea that excess renal salt retention promoted the secretion of a CTS-like hormone that inhibits renal Na⁺ pumps and salt reabsorption. The hormone also inhibits arterial Na⁺ pumps, elevates myocyte Na⁺ and promotes Na/Ca exchanger-mediated Ca²⁺ gain. This enhances vasoconstriction and arterial tone-the hallmark of hypertension. Here I describe how those ideas led to the discovery that the CTS-like hormone is endogenous ouabain (EO), a key factor in the pathogenesis of hypertension and heart failure. Seminal observations that underlie the still-emerging picture of the EO-Na⁺ pump endocrine system in the physiology and pathophysiology of multiple organ systems are summarized. Milestones include: 1) cloning the Na⁺ pump isoforms and physiological studies of mutated pumps in mice; 2) discovery that Na⁺ pumps are also EO-triggered signaling molecules; 3) demonstration that ouabain, but not digoxin, is hypertensinogenic; 4) elucidation of EO's roles in kidney development and cardiovascular and renal physiology and pathophysiology; 5) discovery of "brain ouabain", a component of a novel hypothalamic neuromodulatory pathway; and 6) finding that EO and its brain receptors modulate behavior and learning.

How does pressure overload cause cardiac hypertrophy and dysfunction? High-ouabain affinity cardiac Na+ pumps are crucial

Left ventricular hypertrophy is frequently observed in hypertensive patients and is believed to be due to the pressure overload and cardiomyocyte stretch. Three recent reports on mice with genetically engineered Na⁺ pumps, however, have demonstrated that cardiac ouabain-sensitive α₂-Na⁺ pumps play a key role in the pathogenesis of transaortic constriction-induced hypertrophy. Hypertrophy was delayed/attenuated in mice with mutant, ouabain-resistant α₂-Na⁺ pumps and in mice with cardiac-selective knockout or transgenic overexpression of α₂-Na⁺ pumps. The latter, seemingly paradoxical, findings can be explained by comparing the numbers of available (ouabain-free) high-affinity (α₂) ouabain-binding sites in wild-type, knockout, and transgenic hearts. Conversely, hypertrophy was accelerated in α₂-ouabain-resistant (R) mice in which the normally ouabain-resistant α₁-Na⁺ pumps were mutated to an ouabain-sensitive (S) form (α₁^S/Sα₂^R/R or "SWAP" vs. wild-type or α₁^R/R α₂^S/S mice). Furthermore, transaortic constriction-induced hypertrophy in SWAP mice was prevented/reversed by immunoneutralizing circulating endogenous ouabain (EO). These findings show that EO and its receptor, ouabain-sensitive α₂, are critical factors in pressure overload-induced cardiac hypertrophy. This complements reports linking elevated plasma EO to hypertension, cardiac hypertrophy, and failure in humans and elucidates the underappreciated role of the EO-Na⁺ pump pathway in cardiovascular disease.

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

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

Sodium potassium adenosine triphosphatase (Na/K-ATPase) as a therapeutic target for uremic cardiomyopathy

INTRODUCTION

Clinically, patients with significant reductions in renal function present with cardiovascular dysfunction typically termed, uremic cardiomyopathy. It is a progressive series of cardiac pathophysiological changes, including left ventricular diastolic dysfunction and hypertrophy (LVH) which sometimes progress to left ventricular dilation (LVD) and systolic dysfunction in the setting of chronic kidney disease (CKD). Uremic cardiomyopathy is almost ubiquitous in patients afflicted with end stage renal disease (ESRD). Areas covered: This article reviews recent epidemiology, pathophysiology of uremic cardiomyopathy and provide a board overview of Na/K-ATPase research with detailed discussion on the mechanisms of Na/K-ATPase/Src/ROS amplification loop. We also present clinical and preclinical evidences as well as molecular mechanism of this amplification loop in the development of uremic cardiomyopathy. A potential therapeutic peptide that targets on this loop is discussed. Expert opinion: Current clinical treatment for uremic cardiomyopathy remains disappointing. Targeting the ROS amplification loop mediated by the Na/K-ATPase signaling function may provide a novel therapeutic target for uremic cardiomyopathy and related diseases. Additional studies of Na/K-ATPase and other strategies that regulate this loop will lead to new therapeutics.

Na/K-ATPase Signaling and Salt Sensitivity: The Role of Oxidative Stress

Other than genetic regulation of salt sensitivity of blood pressure, many factors have been shown to regulate renal sodium handling which contributes to long-term blood pressure regulation and have been extensively reviewed. Here we present our progress on the Na/K-ATPase signaling mediated sodium reabsorption in renal proximal tubules, from cardiotonic steroids-mediated to reactive oxygen species (ROS)-mediated Na/K-ATPase signaling that contributes to experimental salt sensitivity.

Regulation of Cardiac Remodeling by Cardiac Na(+)/K(+)-ATPase Isoforms

Cardiac remodeling occurs after cardiac pressure/volume overload or myocardial injury during the development of heart failure and is a determinant of heart failure. Preventing or reversing remodeling is a goal of heart failure therapy. Human cardiomyocyte Na⁺/K⁺-ATPase has multiple α isoforms (1–3). The expression of the α subunit of the Na⁺/K⁺-ATPase is often altered in hypertrophic and failing hearts. The mechanisms are unclear. There are limited data from human cardiomyocytes. Abundant evidences from rodents show that Na⁺/K⁺-ATPase regulates cardiac contractility, cell signaling, hypertrophy and fibrosis. The α1 isoform of the Na⁺/K⁺-ATPase is the ubiquitous isoform and possesses both pumping and signaling functions. The α2 isoform of the Na⁺/K⁺-ATPase regulates intracellular Ca²⁺ signaling, contractility and pathological hypertrophy. The α3 isoform of the Na⁺/K⁺-ATPase may also be a target for cardiac hypertrophy. Restoration of cardiac Na⁺/K⁺-ATPase expression may be an effective approach for prevention of cardiac remodeling. In this article, we will overview: (1) the distribution and function of isoform specific Na⁺/K⁺-ATPase in the cardiomyocytes. (2) the role of cardiac Na⁺/K⁺-ATPase in the regulation of cell signaling, contractility, cardiac hypertrophy and fibrosis in vitro and in vivo. Selective targeting of cardiac Na⁺/K⁺-ATPase isoform may offer a new target for the prevention of cardiac remodeling.

Modulation of the transducer function of Na+,K+-ATPase: new mechanism of heart remodeling

Endogenous digitalis-like factors were found in the mammalian and human blood. It was the starting point for the elucidation of the new non-pumping function of the Na⁺,K⁺-ATPase. It was previously well known that Na⁺,K⁺-ATPase is a pharmacological target receptor for cardiac glycosides (J.C. Skou. 1957. Biochim. Biophys. Acta, 23: 394-401). We have investigated the trophotropic effects of such agents as ouabain, epinephrine, norepinephrine, atenolol, and comenic acid using the organotypic tissue culture combined with the reconstruction of optical cross sections and confocal microscopy. It was shown that the growth zone of organotypic culture forms a multidimensional structure. Our results indicate that the cardiac glycoside ouabain applied in endogenous concentrations (10^-8, 10^-10 mol/L) can modulate transducer function of Na⁺,K⁺-ATPase and control the cell growth and proliferation. It was also shown that Src-kinase is involved in "endogenous" ouabain activated intracellular pathways as a series unit. Epinephrine (10^-9-10^-14 mol/L) and comenic acid (10^-6-10^-10 mol/L) were demonstrated to modulate the growth of 10- to 12-day-old chicken embryo cardiac tissue explants in a dose-dependent manner. Epinephrine and comenic acid regulate growth and proliferation of the cardiac tissue via receptor-mediated modulation Na⁺,K⁺-ATPase as a signal transducer. The trophotropic effects of the investigated agents specifically control the heart remodeling phenomenon.

Personalized Therapy of Hypertension: the Past and the Future

During the past 20 years, the studies on genetics or pharmacogenomics of primary hypertension provided interesting results supporting the role of genetics, but no actionable finding ready to be translated into personalized medicine. Two types of approaches have been applied: a "hypothesis-driven" approach on the candidate genes, coding for proteins involved in the biochemical machinery underlying the regulation of BP, and an "unbiased hypothesis-free" approach with GWAS, based on the randomness principles of frequentist statistics. During the past 10-15 years, the application of the latter has overtaken the application of the former leading to an enlargement of the number of previously unknown candidate loci or genes but without any actionable result for the therapy of hypertension. In the present review, we summarize the results of our hypothesis-driven approach based on studies carried out in rats with genetic hypertension and in humans with essential hypertension at the pre-hypertensive and early hypertensive stages. These studies led to the identification of mutant adducin and endogenous ouabain as candidate genetic-molecular mechanisms in both species. Rostafuroxin has been developed for its ability to selectively correct Na(+) pump abnormalities sustained by the two abovementioned mechanisms and to selectively reduce BP in rats and in humans carrying the gene variants underlying the mutant adducin and endogenous ouabain (EO) effects. A clinical trial is ongoing to substantiate these findings. Future studies should apply both the candidate gene and GWAS approaches to fully exploit the potential of genetics in optimizing the personalized therapy.

Carbonylation Modification Regulates Na/K-ATPase Signaling and Salt Sensitivity: A Review and a Hypothesis

Na/K-ATPase signaling has been implicated in different physiological and pathophysiological conditions. Accumulating evidence indicates that oxidative stress not only regulates the Na/K-ATPase enzymatic activity, but also regulates its signaling and other functions. While cardiotonic steroids (CTS)-induced increase in reactive oxygen species (ROS) generation is an intermediate step in CTS-mediated Na/K-ATPase signaling, increase in ROS alone also stimulates Na/K-ATPase signaling. Based on literature and our observations, we hypothesize that ROS have biphasic effects on Na/K-ATPase signaling, transcellular sodium transport, and urinary sodium excretion. Oxidative modulation, in particular site specific carbonylation of the Na/K-ATPase α1 subunit, is a critical step in proximal tubular Na/K-ATPase signaling and decreased transcellular sodium transport leading to increases in urinary sodium excretion. However, once this system is overstimulated, the signaling, and associated changes in sodium excretion are blunted. This review aims to evaluate ROS-mediated carbonylation of the Na/K-ATPase, and its potential role in the regulation of pump signaling and sodium reabsorption in the renal proximal tubule (RPT).

Do Src Kinase and Caveolin Interact Directly with Na,K-ATPase?

Much evidence points to a role of Na,K-ATPase in ouabain-dependent signal transduction. Based on experiments with different cell lines and native tissue membranes, a current hypothesis postulates direct interactions between the Na,K-ATPase and Src kinase (non-receptor tyrosine kinase). Na,K-ATPase is proposed to bind Src kinase and inhibit its activity, whereas ouabain, the specific Na,K-ATPase inhibitor, binds and stabilizes the E2 conformation, thus exposing the Src kinase domain and its active site Tyr-418 for activation. Ouabain-dependent signaling is thought to be mediated within caveolae by a complex consisting of Na,K-ATPase, caveolin, and Src kinase. In the current work, we have looked for direct interactions utilizing purified recombinant Na,K-ATPase (human α1β1FXYD1 or porcine α1D369Nβ1FXYD1) and purified human Src kinase and human caveolin 1 or interactions between these proteins in native membrane vesicles isolated from rabbit kidney. By several independent criteria and techniques, no stable interactions were detected between Na,K-ATPase and purified Src kinase. Na,K-ATPase was found to be a substrate for Src kinase phosphorylation at Tyr-144. Clear evidence for a direct interaction between purified human Na,K-ATPase and human caveolin was obtained, albeit with a low molar stoichiometry (1:15-30 caveolin 1/Na,K-ATPase). In native renal membranes, a specific caveolin 14-5 oligomer (95 kDa) was found to be in direct interaction with Na,K-ATPase. We inferred that a small fraction of the renal Na,K-ATPase molecules is in a ∼1:1 complex with a caveolin 14-5 oligomer. Thus, overall, whereas a direct caveolin 1/Na,K-ATPase interaction is confirmed, the lack of direct Src kinase/Na,K-ATPase binding requires reassessment of the mechanism of ouabain-dependent signaling.

Na+/Ca2+ exchange and Na+/K+-ATPase in the heart

This paper is the third in a series of reviews published in this issue resulting from the University of California Davis Cardiovascular Symposium 2014: Systems approach to understanding cardiac excitation–contraction coupling and arrhythmias: Na⁺ channel and Na⁺ transport. The goal of the symposium was to bring together experts in the field to discuss points of consensus and controversy on the topic of sodium in the heart. The present review focuses on cardiac Na⁺/Ca²⁺ exchange (NCX) and Na⁺/K⁺-ATPase (NKA). While the relevance of Ca²⁺ homeostasis in cardiac function has been extensively investigated, the role of Na⁺ regulation in shaping heart function is often overlooked. Small changes in the cytoplasmic Na⁺ content have multiple effects on the heart by influencing intracellular Ca²⁺ and pH levels thereby modulating heart contractility. Therefore it is essential for heart cells to maintain Na⁺ homeostasis. Among the proteins that accomplish this task are the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺/K⁺ pump (NKA). By transporting three Na⁺ ions into the cytoplasm in exchange for one Ca²⁺ moved out, NCX is one of the main Na⁺ influx mechanisms in cardiomyocytes. Acting in the opposite direction, NKA moves Na⁺ ions from the cytoplasm to the extracellular space against their gradient by utilizing the energy released from ATP hydrolysis. A fine balance between these two processes controls the net amount of intracellular Na⁺ and aberrations in either of these two systems can have a large impact on cardiac contractility. Due to the relevant role of these two proteins in Na⁺ homeostasis, the emphasis of this review is on recent developments regarding the cardiac Na⁺/Ca²⁺ exchanger (NCX1) and Na⁺/K⁺ pump and the controversies that still persist in the field.

Endogenous Ouabain: An Old Cardiotonic Steroid as a New Biomarker of Heart Failure and a Predictor of Mortality after Cardiac Surgery

Cardiovascular diseases remain the main cause of mortality and morbidity worldwide; primary prevention is a priority for physicians. Biomarkers are useful tools able to identify high-risk individuals, guide treatments, and determine prognosis. Our aim is to investigate Endogenous Ouabain (EO), an adrenal stress hormone with hemodynamic effects, as a valuable biomarker of heart failure. In a population of 845 patients undergoing elective cardiac surgery, we have investigated the relationships between EO and echocardiography parameters/plasmatic biomarker of cardiac function. EO was found to be correlated negatively with left ventricular EF (p = 0.001), positively with Cardiac End-Diastolic Diameter (p = 0.047), and positively with plasmatic NT-proBNP level (p = 0.02). Moreover, a different plasmatic EO level (both preoperative and postoperative) was found according to NYHA class (p = 0.013). All these results have been replicated on an independent cohort of patients (147 subjects from US). Finally, a higher EO level in the immediate postoperative time was indicative of a more severe cardiological condition and it was associated with increased perioperative mortality risk (p = 0.023 for 30-day morality). Our data suggest that preoperative and postoperative plasmatic EO level identifies patients with a more severe cardiovascular presentation at baseline. These patients have a higher risk of morbidity and mortality after cardiac surgery.

Influence of Sugar Amine Regiochemistry on Digitoxigenin Neoglycoside Anticancer Activity

The synthesis of a set of digitoxigenin neogluco/xylosides and corresponding study of their anticancer SAR revealed sugar amine regiochemistry has a dramatic effect upon activity. Specifically, this study noted sugar 3-amino followed by 4-amino-substitution to be most advantageous where the solvent accessibility of the appended amine within neoglycoside-Na(+),K(+)-ATPase docked models correlated with increased anticancer potency. This study presents a preliminary model for potential further warhead optimization in the context of antibody-directed steroidal glycosides and extends the demonstrated compatibility of aminosugars in the context of neoglycosylation.

Ouabain Regulates CFTR-Mediated Anion Secretion and Na,K-ATPase Transport in ADPKD Cells

Cyst enlargement in autosomal dominant polycystic kidney disease (ADPKD) requires the transepithelial secretion of fluid into the cyst lumen. We previously showed that physiological amounts of ouabain enhance cAMP-dependent fluid secretion and cyst growth of human ADPKD cyst epithelial cells in culture and formation of cyst-like dilations in metanephric kidneys from Pkd1 mutant mice. Here, we investigated the mechanisms by which ouabain promotes cAMP-dependent fluid secretion and cystogenesis. Ouabain (3 nM) enhanced cAMP-induced cyst-like dilations in embryonic kidneys from Pkd1 (m1Bei) mice, but had no effect on metanephroi from Pkd1 (m1Bei) mice that lack expression of the cystic fibrosis transmembrane conductance regulator (CFTR). Similarly, ouabain stimulation of cAMP-induced fluid secretion and in vitro cyst growth of ADPKD cells were abrogated by CFTR inhibition, showing that CFTR is required for ouabain effects on ADPKD fluid secretion. Moreover, ouabain directly enhanced the cAMP-dependent Cl(-) efflux mediated by CFTR in ADPKD monolayers. Ouabain increased the trafficking of CFTR to the plasma membrane and up-regulated the expression of the CFTR activator PDZK1. Finally, ouabain decreased plasma membrane expression and activity of the Na,K-ATPase in ADPKD cells. Altogether, these results show that ouabain enhances net fluid secretion and cyst formation by activating apical anion secretion via CFTR and decreasing basolateral Na(+) transport via Na,K-ATPase. These results provide new information on the mechanisms by which ouabain affects ADPKD cells and further highlight the importance of ouabain as a non-genomic stimulator of cystogenesis in ADPKD.

Endogenous cardiotonic steroids in kidney failure: a review and an hypothesis

In response to progressive nephron loss, volume and humoral signals in the circulation have increasing relevance. These signals, including plasma sodium, angiotensin II, and those related to volume status, activate a slow neuromodulatory pathway within the central nervous system (CNS). The slow CNS pathway includes specific receptors for angiotensin II, mineralocorticoids, and endogenous ouabain (EO). Stimulation of the pathway leads to elevated sympathetic nervous system activity (SNA) and increased circulating EO. The sustained elevation of circulating EO (or ouabain) stimulates central and peripheral mechanisms that amplify the impact of SNA on vascular tone. These include changes in synaptic plasticity in the brain and sympathetic ganglia that increase preganglionic tone and amplify ganglionic transmission, amplification of the impact of SNA on arterial tone in the vascular wall, and the reprogramming of calcium signaling proteins in arterial myocytes. These increase SNA, raise basal and evoked arterial tone, and elevate blood pressure (BP). In the setting of CKD, we suggest that sustained activation/elevation of the slow CNS pathway, plasma EO, and the cardiotonic steroid marinobufagenin, comprises a feed-forward system that raises BP and accelerates kidney and cardiac damage. Block of the slow CNS pathway and/or circulating EO and marinobufagenin may reduce BP and slow the progression to ESRD.

The role of cardiotonic steroids in the pathogenesis of cardiomyopathy in chronic kidney disease

Cardiotonic steroids (CTS) are a new class of hormones that circulate in the blood and are divided into two distinct groups, cardenolides, such as ouabain and digoxin, and bufadienolides, such as marinobufagenin, telocinobufagin and bufalin. They have the ability to bind and inhibit the ubiquitous transport enzyme sodium potassium pump, thus regulating intracellular Na(+) concentration in every living cell. Although digoxin has been prescribed to heart failure patients for at least 200 years, the realization that CTS are endogenously produced has intensified research into their physiological and pathophysiological roles. Over the last two decades, substantial evidence has accumulated demonstrating the effects of endogenously synthesised CTS on the kidneys, vasculature and the heart. In this review, the current state of art and the controversies surrounding the manner in which CTS mediate their pathophysiological effects are discussed. Several potential therapeutic strategies have emerged as a result of our increased understanding of the role CTS play in health and disease.

Rostafuroxin ameliorates endothelial dysfunction and oxidative stress in resistance arteries from deoxycorticosterone acetate-salt hypertensive rats: the role of Na+K+-ATPase/ cSRC pathway

AIMS

Endogenous ouabain is elevated in patients and experimental models of hypertension and is associated with elevated mortality. In this context, it is reasonable to assume that a new antihypertensive drug that inhibits the deleterious effects of endogenous ouabain may be a specific pharmacological tool for hypertension treatment. Here, we investigated the effects of rostafuroxin (ROSTA), an ouabain inhibitor, on SBP, endothelial dysfunction and oxidative stress in deoxycorticosterone acetate (DOCA)-salt rats.

METHODS AND RESULTS

A hypertensive model was established in uninephrectomized Wistar rats using DOCA-salt. After SBP stabilization, DOCA-salt rats were divided into two groups: DOCA-salt (control) and DOCA-salt treatment with ROSTA (1 mg/kg per day gavage, 3 weeks). The SBP was measured using the tail-cuff method, and vascular function was assessed in mesenteric-resistance arteries (MRAs) using a wire myograph. Nitric oxide and reactive oxygen species production were investigated. Western blot was performed to quantify protein expression. Our results indicated that ROSTA treatment decreased SBP, improved acetylcholine-induced relaxation via enhanced nitric oxide synthesis and bioavailability, decreased superoxide anion generation from NAD(P)H oxidase and cyclooxygenase-2 and reduced cytoplasmic tyrosine kinase Src phosphorylation without changes in NaKATPase activity in MRA from DOCA-salt rats.

CONCLUSION

This study reports the critical role of endogenous ouabain in volume-dependent hypertension. In MRA from DOCA-salt rats, the binding of endogenous ouabain to NaK-ATPase results in downstream c-SRC activation, oxidative stress and endothelial dysfunction. Endogenous ouabain is a putative target for the treatment of hypertension, and ROSTA may represent a novel therapeutic approach.

Rostafuroxin protects from podocyte injury and proteinuria induced by adducin genetic variants and ouabain

Glomerulopathies are important causes of morbidity and mortality. Selective therapies that address the underlying mechanisms are still lacking. Recently, two mechanisms, mutant β-adducin and ouabain, have been found to be involved in glomerular podocytopathies and proteinuria through nephrin downregulation. The main purpose of the present study was to investigate whether rostafuroxin, a novel antihypertensive agent developed as a selective inhibitor of Src-SH2 interaction with mutant adducin- and ouabain-activated Na,K-ATPase, may protect podocytes from adducin- and ouabain-induced effects, thus representing a novel pharmacologic approach for the therapy of podocytopathies and proteinuria caused by the aforementioned mechanisms. To study the effect of rostafuroxin on podocyte protein changes and proteinuria, mice carrying mutant β-adducin and ouabain hypertensive rats were orally treated with 100 μg/kg per day rostafuroxin. Primary podocytes from congenic rats carrying mutant α-adducin or β-adducin (NB) from Milan hypertensive rats and normal rat podocytes incubated with 10(-9) M ouabain were cultured with 10(-9) M rostafuroxin. The results indicated that mutant β-adducin and ouabain caused podocyte nephrin loss and proteinuria in animal models. These alterations were reproduced in primary podocytes from NB rats and normal rats incubated with ouabain. Treatment of animals, or incubation of cultured podocytes with rostafuroxin, reverted mutant β-adducin- and ouabain-induced effects on nephrin protein expression and proteinuria. We conclude that rostafuroxin prevented podocyte lesions and proteinuria due to mutant β-adducin and ouabain in animal models. This suggests a potential therapeutic effect of rostafuroxin in patients with glomerular disease progression associated with these two mechanisms.

A new clinical multivariable model that predicts postoperative acute kidney injury: impact of endogenous ouabain

BACKGROUND

Acute kidney injury (AKI) is an important complication of cardiac surgery. Recently, elevated levels of endogenous ouabain (EO), an adrenal stress hormone with haemodynamic and renal effects, have been associated with worse renal outcome after cardiac surgery. Our aim was to develop and evaluate a new risk model of AKI using simple preoperative clinical parameters and to investigate the utility of EO.

METHODS

The primary outcome was AKI according to Acute Kidney Injury Network stage II or III. We selected the Northern New England Cardiovascular Disease Study Group (NNECDSG) as a reference model. We built a new internal predictive risk model considering common clinical variables (CLIN-RISK), compared this model with the NNECDSG model and determined whether the addition of preoperative plasma EO improved prediction of AKI.

RESULTS

All models were tested on >800 patients admitted for elective cardiac surgery in our hospital. Seventy-nine patients developed AKI (9.9%). Preoperative EO levels were strongly associated with the incidence of AKI and clinical complication (total ICU stay and in-hospital mortality). The NNECDSG model was confirmed as a good predictor of AKI (AUC 0.74, comparable to the NNECDSG reference population). Our CLIN-RISK model had improved predictive power for AKI (AUC 0.79, CI 95% 0.73-0.84). Furthermore, addition of preoperative EO levels to both clinical models improved AUC to 0.79 and to 0.83, respectively (ΔAUC +0.05 and +0.04, respectively, P < 0.01).

CONCLUSION

In a population where the predictive power of the NNECDSG model was confirmed, CLIN-RISK was more powerful. Both clinical models were further improved by the addition of preoperative plasma EO levels. These new models provide improved predictability of the relative risk for the development of AKI following cardiac surgery and suggest that EO is a marker for renal vascular injury.

A functional common polymorphism of the ABCB1 gene is associated with chronic kidney disease and hypertension in Chinese

BACKGROUND

Permeability glycoprotein is encoded by the ATP-binding cassette B1 gene (ABCB1) and is an extruder of toxic metabolites in the kidney. A functional common polymorphism (C3435T, rs1045642) in the human ABCB1 gene has been found to be associated with allograft outcome in kidney transplant patients. In this study, we investigated the association of the C3435T polymorphism with renal function and blood pressure (BP) in 2 Chinese populations.

METHODS

The discovery and replication populations were recruited from a mountainous area (Zhejiang Province) and a newly urbanized suburban area (Shanghai), respectively. We genotyped all subjects using the ABI SNapShot method. Chronic kidney disease (CKD) was defined as an estimated glomerular filtration rate <60 ml/min × 1.73 m(2) or 24-hour urinary albumin excretion ≥30 mg.

RESULTS

In the discovery population of 1,987 subjects, after adjustment for covariables, TT homozygosity (n = 217) was associated with a higher risk of CKD (n = 369; odds ratio (OR) = 1.73; P = 0.003) and with higher systolic BP (+3.1 mm Hg; P = 0.03) and pulse pressure (+3.4 mm Hg; P = 0.001). These associations were dependent on age (Pint ≤ 0.05). In subjects aged ≥60 years (n = 374), the corresponding OR or difference was 2.40 for CKD, 15.1 mm Hg for systolic BP, and 12.4 mm Hg for pulse pressure (P < 0.001). In similar adjusted analyses in the replication population of 2,427 elderly (≥60 years) subjects, TT homozygosity was also associated with a higher risk of CKD (OR = 1.39; P = 0.02) and an enhanced association of hypertension with CKD (OR = 1.50; P = 0.04).

CONCLUSIONS

The ABCB1 C3435T polymorphism might predict CKD, especially in the elderly.

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

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

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

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

cGMP-dependent protein kinase 1 polymorphisms underlie renal sodium handling impairment

Defective pressure-natriuresis related to abnormalities in the natriuretic response has been associated with hypertension development. A major signaling pathway mediating pressure natriuresis involves the cGMP-dependent protein kinase 1 (PRKG1) that, once activated by Src kinase, inhibits renal Na(+) reabsorption via a direct action on basolateral Na-K ATPase and luminal Na-H exchanger type 3, as shown in renal tubuli of animals. Because a clear implication of PRKG1 in humans is still lacking, here we addressed whether PRKG1 polymorphisms affect pressure-natriuresis in patients. Naive hypertensive patients (n = 574), genotyped for PRKG1 rs1904694, rs7897633, and rs7905063 single nucleotide polymorphisms (SNPs), underwent an acute Na(+) loading, and the slope of the pressure-natriuresis relationship between blood pressure and Na(+) excretion was calculated. The underlying molecular mechanism was investigated by immunoblotting protein quantifications in human kidneys. The results demonstrate that the PRKG1 risk haplotype GAT (rs1904694, rs7897633, rs7905063, respectively) associates with a rightward shift of the pressure-natriuresis curve (0.017 ± 0.004 μEq/mm Hg per minute) compared with the ACC (0.0013 ± 0.003 μEq/mm Hg per minute; P = 0.001). In human kidneys, a positive correlation of protein expression levels between PRKG1 and Src (r = 0.83; P<0.001) or α1 Na-K ATPase (r = 0.557; P<0.01) and between α1 Na-K ATPase and Na-H exchanger type 3 (r = 0.584; P<0.01) or Src (r = 0.691; P<0.001) was observed in patients carrying PRKG1 risk GAT (n = 23) but not ACC (n = 14) variants. A functional signaling complex among PRKG1, α1 Na-K ATPase, and Src was shown by immunoprecipitation from human renal caveolae. These findings indicate that PRKG1 risk alleles associate with salt-sensitivity related to a loss of the inhibitory control of renal Na(+) reabsorption, suggestive of a blunt pressure-natriuresis response.

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.

Preoperative endogenous ouabain predicts acute kidney injury in cardiac surgery patients

OBJECTIVES

Acute kidney injury is a frequent complication of cardiac surgery and increases morbidity and mortality. As preoperative biomarkers predicting the development of acute kidney injury are not available, we have tested the hypothesis that preoperative plasma levels of endogenous ouabain may function as this type of biomarker.

RATIONALE AND DESIGN

Endogenous ouabain is an adrenal stress hormone associated with adverse cardiovascular outcomes. Its involvement in acute kidney injury is unknown. With studies in patients and animal settings, including isolated podocytes, we tested the above mentioned hypothesis.

PATIENTS

Preoperative endogenous ouabain was measured in 407 patients admitted for elective cardiac surgery and in a validation population of 219 other patients. We also studied the effect of prolonged elevations of circulating exogenous ouabain on renal parameters in rats and the influence of ouabain on podocyte proteins both "in vivo" and "in vitro."

MAIN RESULTS

In the first group of patients, acute kidney injury (2.8%, 8.3%, 20.3%, p < 0.001) and ICU stay (1.4±0.38, 1.7±0.41, 2.4±0.59 days, p = 0.014) increased with each incremental preoperative endogenous ouabain tertile. In a linear regression analysis, the circulating endogenous ouabain value before surgery was the strongest predictor of acute kidney injury. In the validation cohort, acute kidney injury (0%, 5.9%, 8.2%, p < 0.0001) and ICU stay (1.2±0.09, 1.4±0.23, 2.2±0.77 days, p = 0.003) increased with the preoperative endogenous ouabain tertile. Values for preoperative endogenous ouabain significantly improved (area under curve: 0.85) risk prediction over the clinical score alone as measured by integrate discrimination improvement and net reclassification improvement. Finally, in the rat model, elevated circulating ouabain reduced creatinine clearance (-18%, p < 0.05), increased urinary protein excretion (+ 54%, p < 0.05), and reduced expression of podocyte nephrin (-29%, p < 0.01). This last finding was replicated ex vivo by incubating podocyte primary cell cultures with low-dose ouabain.

CONCLUSIONS

Preoperative plasma endogenous ouabain levels are powerful biomarkers of acute kidney injury and postoperative complications and may be a direct cause of podocyte damage.