Negative regulation of thyroid adenoma-associated protein (THADA) in the cardiac glycoside-induced anti-cancer effect

Cardiac glycosides, known as inhibitors of Na+,K+-ATPase, have anti-cancer effects such as suppression of cancer cell proliferation and induction of cancer cell death. Here, we examined the signaling pathway elicited by cardiac glycosides in the human hepatocellular carcinoma HepG2 cells and human epidermoid carcinoma KB cells. Three kinds of cardiac glycosides (ouabain, oleandrin, and digoxin) inhibited the cancer cell proliferation and decreased the expression level of thyroid adenoma-associated protein (THADA). Interestingly, the knockdown of THADA inhibited cancer cell proliferation, and the proliferation was significantly rescued by re-expression of THADA in the THADA-knockdown cells. In addition, the THADA-knockdown markedly decreased the expression level of L-type amino acid transporter LAT1. Cardiac glycosides also reduced the LAT1 expression. The LAT1 inhibitor, JPH203, significantly weakened the cancer cell proliferation. These results suggest that the binding of cardiac glycosides to Na+,K+-ATPase negatively regulates the THADA-LAT1 pathway, exerting the anti-proliferative effect in cancer cells.

Cardiac glycosides inhibit early and late vaccinia virus protein expression

Cardiac glycosides (CGs) are natural steroid glycosides, which act as inhibitors of the cellular sodium-potassium ATPase pump. Although traditionally considered toxic to human cells, CGs are widely used as drugs for the treatment of cardiovascular-related medical conditions. More recently, CGs have been explored as potential anti-viral drugs and inhibit replication of a range of RNA and DNA viruses. Previously, a compound screen identified CGs that inhibited vaccinia virus (VACV) infection. However, no further investigation of the inhibitory potential of these compounds was performed, nor was there investigation of the stage(s) of the poxvirus lifecycle they impacted. Here, we investigated the anti-poxvirus activity of a broad panel of CGs. We found that all CGs tested were potent inhibitors of VACV replication. Our virological experiments showed that CGs did not impact virus infectivity, binding, or entry. Rather, experiments using recombinant viruses expressing reporter proteins controlled by VACV promoters and arabinoside release assays demonstrated that CGs inhibited early and late VACV protein expression at different concentrations. Lack of virus assembly in the presence of CGs was confirmed using electron microscopy. Thus, we expand our understanding of compounds with anti-poxvirus activity and highlight a yet unrecognized mechanism by which poxvirus replication can be inhibited.

Cardiac glycoside ouabain efficiently targets leukemic stem cell apoptotic machinery independent of cell differentiation status

BACKGROUND

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy characterized by an accumulation of immature leukemic myeloblasts initiating from leukemic stem cells (LSCs)-the subpopulation that is also considered the root cause of chemotherapy resistance. Repurposing cardiac glycosides to treat cancers has gained increasing attention and supporting evidence, but how cardiac glycosides effectively target LSCs, e.g., whether it involves cell differentiation, remains largely unexplored.

METHODS

Digoxin, a user-designed digitoxigenin-α-L-rhamnoside (D6-MA), and ouabain were tested against various human AML-derived cells with different maturation phenotypes. Herein, we established two study models to specifically determine the effects of cardiac glycosides on LSC death and differentiation-one allowed change in dynamics of LSCs and leukemic progenitor cells (LPCs), while another maintained their undifferentiated status. Regulatory mechanisms underlying cardiac glycoside-induced cytotoxicity were investigated and linked to cell cycle distribution and apoptotic machinery.

RESULTS

Primitive AML cells containing CD34+ LSCs/LPCs were very responsive to nanomolar concentrations of cardiac glycosides, with ouabain showing the greatest efficiency. Ouabain preferentially induces caspase-dependent apoptosis in LSCs, independent of its cell differentiation status, as evidenced by (i) the tremendous induction of apoptosis by ouabain in AML cells that acquired less than 15% differentiation and (ii) the higher rate of apoptosis in enriched LSCs than in LPCs. We sorted LSCs and LPCs according to their cell cycle distribution into G0/G1, S, and G2/M cells and revealed that G0/G1 cells in LSCs, which was its major subpopulation, were the top ouabain responders, indicating that the difference in ouabain sensitivity between LSCs and LPCs involved both distinct cell cycle distribution and intrinsic apoptosis regulatory mechanisms. Further, Mcl-1 and c-Myc, which were differentially expressed in LSCs and LPCs, were found to be the key apoptosis mediators that determined ouabain sensitivity in AML cells. Ouabain induces a more rapid loss of Mcl-1 and c-Myc in LSCs than in LPCs via the mechanisms that in part involve an inhibition of Mcl-1 protein synthesis and an induction of c-Myc degradation.

CONCLUSIONS

Our data provide new insight for repurposing cardiac glycosides for the treatment of relapsed/refractory AML through targeting LSCs via distinct cell cycle and apoptosis machinery. Video Abstract.

The Na+/K+-ATPase: A potential therapeutic target in cardiometabolic diseases

Cardiometabolic diseases (CMD) are a direct consequence of modern living and contribute to the development of multisystem diseases such as cardiovascular diseases and diabetes mellitus (DM). CMD has reached epidemic proportions worldwide. A sodium pump (Na⁺/K⁺-ATPase) is found in most eukaryotic cells' membrane and controls many essential cellular functions directly or indirectly. This ion transporter and its isoforms are important in the pathogenesis of some pathological processes, including CMD. The structure and function of Na⁺/K⁺-ATPase, its expression and distribution in tissues, and its interactions with known ligands such as cardiotonic steroids and other suspected endogenous regulators are discussed in this review. In addition, we reviewed recent literature data related to the involvement of Na⁺/K⁺-ATPase activity dysfunction in CMD, focusing on the Na⁺/K⁺-ATPase as a potential therapeutic target in CMD.

Evidence for Cardiac Glycosides in Foliage of Colorado Potato Beetle-Resistant Solanum okadae

Leptinotarsa decemlineata, the Colorado potato beetle (CPB), is a herbivore that primarily feeds on Solanum foliage and is a global pest of the potato agricultural industry. Potato breeding through cross-hybridization with CPB-resistant wild relatives is used for genetic improvement. The wild species Solanum okadae was demonstrated to deter CPB feeding in choice and no choice feeding assays. Liquid chromatography-mass spectrometry (LC-MS) was used for comparative metabolite profiling between S. okadae and CPB-susceptible domesticated potato variety, Solanum tuberosum cv. Shepody. Major foliar metabolites detected were steroidal glycoalkaloids (SGAs) with tomatine and dehydrotomatine produced in S. okadae and solanine and chaconine in S. tuberosum cv. Shepody. Cardiac glycosides were also detected in the foliar metabolite profile of S. okadae but not S. tuberosum cv. Shepody. This class of plant compounds have known insecticidal activity through inhibition of animal Na+/K+ ATPase. Thin-layer chromatography (TLC) separation of foliar extracts also provided evidence for cardiac glycosides in S. okadae. Cardiac glycosides are known inhibitors of Na+/K+ ATPase, and foliar extracts from S. okadae (OKA15), but not S. tuberosum cv. Shepody, were able to inhibit the Na+/K+ ATPase of CPB. These findings suggest a novel mechanism of plant resistance against CPB involving production of cardiac glycosides in S. okadae.

Cardiac glycosides cause cytotoxicity in human macrophages and ameliorate white adipose tissue homeostasis

BACKGROUND AND PURPOSE

Cardiac glycosides inhibit Na+ /K+ -ATPase and are used to treat heart failure and arrhythmias. They can induce inflammasome activation and pyroptosis in macrophages, suggesting cytotoxicity, which remains to be elucidated in human tissues.

EXPERIMENTAL APPROACH

To determine the cell-type specificity of this cytotoxicity, we used human monocyte-derived macrophages and non-adherent peripheral blood cells from healthy donors, plus omental white adipose tissue, stromal vascular fraction-derived pre-adipocytes and adipocytes from obese patients undergoing bariatric surgery. All these cells/tissues were treated with nanomolar concentrations of ouabain (50, 100, 500 nM) to investigate the level of cytotoxicity and the mechanisms leading to cell death. In white adipose tissue, we investigated ouabain-mediated cytotoxicity by measuring insulin sensitivity, adipose tissue function and extracellular matrix deposition ex vivo.

KEY RESULTS

Ouabain induced cell death through pyroptosis and apoptosis, and was more effective in monocyte-derived macrophages compared to non-adherent peripheral blood mononuclear cell populations. This cytotoxicity is dependent on K+ flux, as ouabain causes intracellular depletion of K+ and accumulation of Na+ and Ca2+ . Consistently, the cell death caused by these ion imbalances can be rescued by addition of potassium chloride to human monocyte-derived macrophages. Remarkably, when white adipose tissue explants from obese patients are cultured with nanomolar concentrations of ouabain, this causes depletion of macrophages, down-regulation of type VI collagen levels and amelioration of insulin sensitivity ex vivo.

CONCLUSION AND IMPLICATIONS

The use of nanomolar concentration of cardiac glycosides could be an attractive therapeutic treatment for metabolic syndrome, characterized by pathogenic infiltration and activation of macrophages.

LINKED ARTICLES

This article is part of a themed issue on Inflammation, Repair and Ageing. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.9/issuetoc.

Repurposing Cardiac Glycosides: Drugs for Heart Failure Surmounting Viruses

Drug repositioning is a successful approach in medicinal research. It significantly simplifies the long-term process of clinical drug evaluation, since the drug being tested has already been approved for another condition. One example of drug repositioning involves cardiac glycosides (CGs), which have, for a long time, been used in heart medicine. Moreover, it has been known for decades that CGs also have great potential in cancer treatment and, thus, many clinical trials now evaluate their anticancer potential. Interestingly, heart failure and cancer are not the only conditions for which CGs could be effectively used. In recent years, the antiviral potential of CGs has been extensively studied, and with the ongoing SARS-CoV-2 pandemic, this interest in CGs has increased even more. Therefore, here, we present CGs as potent and promising antiviral compounds, which can interfere with almost any steps of the viral life cycle, except for the viral attachment to a host cell. In this review article, we summarize the reported data on this hot topic and discuss the mechanisms of antiviral action of CGs, with reference to the particular viral life cycle phase they interfere with.

Long-Range Stereodirecting Participation across a Glycosidic Linkage in Glycosylation Reactions

The formation of an unprecedented 12-membered macrocyclic ketal through the long-range participation of a levulinoyl group across a glycosidic linkage was observed in glycosylation reactions. This finding indicated that stereodirecting participation is not limited to groups within the glycan ring being activated, thus broadening the scope of remote group participation in glycosylation.

[Different Membrane Environments Generate Multiple Functions of P-type Ion Pumps]

P-type ion pumps (P-type ATPases) are involved in various fundamental biological processes. For example, the gastric proton pump (H+,K+-ATPase) and sodium pump (Na+,K+-ATPase) are responsible for secretion of gastric acid and maintenance of cell membrane potential, respectively. In this review, we summarize three topics of our studies. The first topic is gastric H+,K+-ATPase associated with Cl--transporting proteins (Cl-/H+ exchanger ClC-5 and K+-Cl- cotransporter KCC4). In gastric parietal cells, we found that ClC-5 is predominantly expressed in intracellular tubulovesicles and that KCC4 is predominantly expressed in the apical membrane. Gastric acid (HCl) secretion may be accomplished by the two different complexes of H+,K+-ATPase and Cl--transporting protein. The second topic focuses on the Na+,K+-ATPase α1-isoform (α1NaK) associated with the volume-regulated anion channel (VRAC). In the cholesterol-enriched membrane microdomains of human cancer cells, we found that α1NaK has a receptor-like (non-pumping) function and that binding of low concentrations (nM level) of cardiac glycosides to α1NaK activates VRAC and exerts anti-cancer effects without affecting the pumping function of α1NaK. The third topic is the Na+,K+-ATPase α3-isoform (α3NaK) in human cancer cells. We found that α3NaK is abnormally expressed in the intracellular vesicles of attached cancer cells and that the plasma membrane translocation of α3NaK upon cell detachment contributes to the survival of metastatic cancer cells. Our results indicate that multiple functions of P-type ion pumps are generated by different membrane environments and their associated proteins.

Ouabain, endogenous ouabain and ouabain-like factors: The Na+ pump/ouabain receptor, its linkage to NCX, and its myriad functions

In this brief review we discuss some aspects of the Na⁺ pump and its roles in mediating the effects of ouabain and endogenous ouabain (EO): i) in regulating the cytosolic Ca²⁺ concentration ([Ca²⁺]_CYT) via Na/Ca exchange (NCX), and ii) in activating a number of protein kinase (PK) signaling cascades that control a myriad of cell functions. Importantly, [Ca²⁺]_CYT and the other signaling pathways intersect at numerous points because of the influence of Ca²⁺ and calmodulin in modulating some steps in those other pathways. While both mechanisms operate in virtually all cells and tissues, this article focuses primarily on their functions in the cardiovascular system, the central nervous system (CNS) and the kidneys.

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.

New ways to acquire resistance: imperfect convergence in insect adaptations to a potent plant toxin

Evolution of insensitivity to the toxic effects of cardiac glycosides has become a model in the study of convergent evolution, as five taxonomic orders of insects use the same few similar amino acid substitutions in the otherwise highly conserved Na,K-ATPase α. We show here that insensitivity in pyrgomorphid grasshoppers evolved along a slightly divergent path. As in other lineages, duplication of the Na,K-ATPase α gene paved the way for subfunctionalization: one copy maintains the ancestral, sensitive state, while the other copy is resistant. Nonetheless, in contrast with all other investigated insects, the grasshoppers' resistant copy shows length variation by two amino acids in the first extracellular loop, the main part of the cardiac glycoside-binding pocket. RT-qPCR analyses confirmed that this copy is predominantly expressed in tissues exposed to the toxins, while the ancestral copy predominates in the nervous tissue. Functional tests with genetically engineered Drosophila Na,K-ATPases bearing the first extracellular loop of the pyrgomorphid genes showed the derived form to be highly resistant, while the ancestral state is sensitive. Thus, we report convergence in gene duplication and in the gene targets for toxin insensitivity; however, the means to the phenotypic end have been novel in pyrgomorphid grasshoppers.

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.

Production of the Cytotoxic Cardenolide Glucoevatromonoside by Semisynthesis and Biotransformation of Evatromonoside by a Digitalis lanata Cell Culture

Recent studies demonstrate that cardiac glycosides, known to inhibit Na⁺/K⁺-ATPase in humans, have increased susceptibility to cancer cells that can be used in tumor therapy. One of the most promising candidates identified so far is glucoevatromonoside, which can be isolated from the endangered species Digitalis mariana ssp. heywoodii. Due to its complex structure, glucoevatromonoside cannot be obtained economically by total chemical synthesis. Here we describe two methods for glucoevatromonoside production, both using evatromonoside obtained by chemical degradation of digitoxin as the precursor. 1) Catalyst-controlled, regioselective glycosylation of evatromonoside to glucoevatromonoside using 2,3,4,6-tetra-O-acetyl-α-D-glucopyranosyl bromide as the sugar donor and 2-aminoethyldiphenylborinate as the catalyst resulted in an overall 30 % yield. 2) Biotransformation of evatromonoside using Digitalis lanata plant cell suspension cultures was less efficient and resulted only in overall 18 % pure product. Structural proof of products has been provided by extensive NMR data. Glucoevatromonoside and its non-natural 1-3 linked isomer neo-glucoevatromonoside obtained by semisynthesis were evaluated against renal cell carcinoma and prostate cancer cell lines.

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

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

[Na+, K(+)-ATPase, endogenous cardiotonic steroids and their transducing role]

Na+, K(+)-ATPase--a protein complex of plasmatic membrane, which performs the dual function: firstly, it supports the Na+ and K+ homeostasis, and also transmembrane potential gradient, secondly, it serves as the transducer of signals and as the regulator of the expression of many key genes. Endogenous cardiotonic steroids, which are synthesized in the adrenal glands and hypothalamus, serve as the signal molecules. New concepts about the mechanisms of the realization of the Na+, K(+)-ATPase signal function and their connection with cellular functions, apoptosis, and with pathologies of cardiovascular system and water-salt homeostasis are described in the survey.

Effect of administration of activated charcoal and fibre on absorption, excretion and steady state blood levels of digoxin and digitoxin. Evidence for intestinal secretion of the glycosides

The effects of activated charcoal and fibre on absorption and stationary plasma levels of digoxin and digitoxin have been studied. The effect of fibre alone is small but an interactive effect on absorption may occur if fibre and digoxin are ingested simultaneously. Charcoal effectively decreases glycoside absorption even when administered after the glycosides. During maintenance therapy with digoxin or digitoxin, charcoal administration decreased the glycoside plasma levels by 31.2% and 18.3% respectively. It is suggested that even digoxin may have a significant biliary excretion and enterohepatic circulation or that this glycoside has a significant intestinal secretion. The therapeutic implication of this study is that charcoal may be of value, not only in the management of acute glycoside poisoning, but also in some cases of more chronic intoxication.

Endogenous and exogenous cardiac glycosides: their roles in hypertension, salt metabolism, and cell growth

Cardiotonic steroids (CTS), long used to treat heart failure, are endogenously produced in mammals. Among them are the hydrophilic cardenolide ouabain and the more hydrophobic cardenolide digoxin, as well as the bufadienolides marinobufagenin and telecinobufagin. The physiological effects of endogenous ouabain on blood pressure and cardiac activity are consistent with the "Na(+)-lag" hypothesis. This hypothesis assumes that, in cardiac and arterial myocytes, a CTS-induced local increase of Na(+) concentration due to inhibition of Na(+)/K(+)-ATPase leads to an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) via a backward-running Na(+)/Ca(2+) exchanger. The increase in [Ca(2+)](i) then activates muscle contraction. The Na(+)-lag hypothesis may best explain short-term and inotropic actions of CTS. Yet all data on the CTS-induced alteration of gene expression are consistent with another hypothesis, based on the Na(+)/K(+)-ATPase "signalosome," that describes the interaction of cardiac glycosides with the Na(+) pump as machinery activating various signaling pathways via intramembrane and cytosolic protein-protein interactions. These pathways, which may be activated simultaneously or selectively, elevate [Ca(2+)](i), activate Src and the ERK1/2 kinase pathways, and activate phosphoinositide 3-kinase and protein kinase B (Akt), NF-kappaB, and reactive oxygen species. A recent development indicates that new pharmaceuticals with antihypertensive and anticancer activities may be found among CTS and their derivatives: the antihypertensive rostafuroxin suppresses Na(+) resorption and the Src-epidermal growth factor receptor-ERK pathway in kidney tubule cells. It may be the parent compound of a new principle of antihypertensive therapy. Bufalin and oleandrin or the cardenolide analog UNBS-1450 block tumor cell proliferation and induce apoptosis at low concentrations in tumors with constitutive activation of NF-kappaB.

Role of homologous ASP334 and GLU319 in human non-gastric H,K- and Na,K-ATPases in cardiac glycoside binding

Cardiac steroids inhibit Na,K-ATPase and the related non-gastric H,K-ATPase, while they do not interact with gastric H,K-ATPase. Introducing an arginine, the residue present in the gastric H,K-ATPase, in the second extracellular loop at the corresponding position 334 in the human non-gastric H,K-ATPase (D334R mutation) rendered it completely resistant to 2mM ouabain. The corresponding mutation (E319R) in alpha1 Na,K-ATPase produced a approximately 2-fold increase of the ouabain IC(50) in the ouabain-resistant rat alpha1 Na,K-ATPase and a large decrease of the ouabain affinity of human alpha1 Na,K-ATPase, on the other hand this mutation had no effect on the affinity for the aglycone ouabagenin. These results provide a strong support for the orientation of ouabain in its biding site with its sugar moiety interacting directly with the second extracellular loop.

The Na,K-ATPase receptor complex: its organization and membership

A major difference between the Na,K-ATPase ion pump and other P-type ATPases is its ability to bind cardiotonic steroids such as ouabain. Na,K-ATPase also interacts with many membrane and cytosolic proteins. In addition to their role in Na,K-ATPase regulation, it became apparent that some of the newly identified interactions are capable of organizing the Na,K-ATPase into various signaling complexes. This new function confers a ligand-like effect to cardiotonic steroids on cellular signal transduction. This article reviews these new developments and provides a comparison of Na,K-ATPase-mediated signal transduction with other receptors and ion transporters.

Endogenous and exogenous cardiac glycosides and their mechanisms of action

Cardiac glycosides have been used for decades to treat congestive heart failure. The recent identification of cardiotonic steroids such as ouabain, digoxin, marinobufagenin, and telocinobufagin in blood plasma, adrenal glands, and hypothalamus of mammals led to exciting new perspectives in the pathology of heart failure and arterial hypertension. Biosynthesis of ouabain and digoxin occurs in adrenal glands and is under the control of angiotensin II, endothelin, and epinephrine released from cells of the midbrain upon stimulation of brain areas sensing cerebrospinal Na(+) concentration and, apparently, the body's K(+) content. Rapid changes of endogenous ouabain upon physical exercise may favor the economy of the heart by a rise of intracellular Ca(2)(+) levels in cardiac and atrial muscle cells. According to the sodium pump lag hypothesis, this may be accomplished by partial inhibition of the sodium pump and Ca(2+) influx via the Na(+)/Ca(2+) exchanger working in reverse mode or via activation of the Na(+)/K(+)-ATPase signalosome complex, generating intracellular calcium oscillations, reactive oxygen species, and gene activation via nuclear factor-kappaB or extracellular signal-regulated kinases 1 and 2. Elevated concentrations of endogenous ouabain and marinobufagenin in the subnanomolar concentration range were found to stimulate proliferation and differentiation of cardiac and smooth muscle cells. They may have a primary role in the development of cardiac dysfunction and failure because (i) offspring of hypertensive patients evidently inherit elevated plasma concentrations of endogenous ouabain; (ii) such elevated concentrations correlate positively with cardiac dysfunction, hypertrophy, and arterial hypertension; (iii) about 40% of Europeans with uncomplicated essential hypertension show increased concentrations of endogenous ouabain associated with reduced heart rate and cardiac hypertrophy; (iv) in patients with advanced arterial hypertension, circulating levels of endogenous ouabain correlate with BP and total peripheral resistance; (v) among patients with idiopathic dilated cardiomyopathy, high circulating levels of endogenous ouabain and marinobufagenin identify those individuals who are predisposed to progressing more rapidly to heart failure, suggesting that endogenous ouabain (and marinobufagenin) may contribute to toxicity upon digoxin therapy. In contrast to endogenous ouabain, endogenous marinobufagenin may act as a natriuretic substance as well. It shows a higher affinity for the ouabain-insensitive alpha(1) isoform of Na(+)/K(+)-ATPase of rat kidney tubular cells and its levels are increased in volume expansion and pre-eclampsia. Digoxin, which is synthesized in adrenal glands, seems to counteract the hypertensinogenic action of ouabain in rats, as do antibodies against ouabain, for example, (Digibind) and rostafuroxin (PST 2238), a selective ouabain antagonist. It lowers BP in ouabain- and adducin-dependent hypertension in rats and is a promising new class of antihypertensive medication in humans.

Vascular endothelium as a target for endogenous ouabain: studies on the effect of ouabain on human endothelial cells

It has been suggested that an endogenous inhibitor of the sodium pump, identified as ouabain, contributes to the regulation of blood pressure and the pathogenesis of certain forms of hypertension. Vascular endothelial cells, whose functional integrity is crucial for the maintenance of blood flow and the antithrombotic activity, could be a target for endogenous ouabain. We studied the effect of ouabain on human umbilical vein endothelial cells (HUVEC) and found that nanomolar concentrations of the glycoside have an antiapoptotic activity that is dependent on the activation of phosphatidylinositol 3 kinase (PI-3K) and extracellular signal-regulated kinases (ERKs). At the same concentrations we found that ouabain affects the endocytosis of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) through the activation of signaling proteins such as Src kinase. This review sumarizes our findings on the effect of ouabain on HUVEC, the signal transduction pathways involved and the significance of these observations on the pathophysiology of endothelial function.

The rapid decline of MTT reduction is not a marker of death signaling in ouabain-treated cells

Decreased staining with dimethylthiazol diphenyltetrazolium (MTT) is widely used for cell death detection. This study examined MTT assay as a marker of the Na+i,K+i-independent mode of cell death revealed in ouabain-treated C7-MDCK cells derived from distal tubule of the Madin-Darby canine kidney. The action of 3-M ouabain on MTT reduction in C7-MDCK cells exhibited bipartite kinetics with a rapid ~2-fold decline occurring in 30-120 min and a delayed ~8-10-fold decrease after 10 hr of ouabain addition. Treatment with ouabain for 18 hr led to 6-fold activation of caspase-3, 4-fold elevation of chromatin fragmentation, and massive cell detachment. Caspase-3 activation, chromatin fragmentation and cell detachment were completely abolished by acidification of the incubation medium from pH 7.2 to 6.7. In contrast, the 2-fold inhibition of MTT reduction seen in 5 hr of ouabain addition was not affected by medium acidification. Within the 5-hr time window, we did not observe any significant impact of ouabain on the cellular redox state estimated by the autofluorescence ratio of reduced pyridine nucleotides and oxidized flavoproteins. In rat aortic endothelial cells and primary astrocytes, exposure to 5-mM ouabain attenuated MTT reduction but did not affect cell survival. Thus, our results show that diminished staining with MTT in ouabain-treated cells is not sufficient proof of triggering of the cell death machinery. We speculate that altered endo- and exocytoses evoked by cardiotonic steroids contribute to decreased MTT reduction.

Short-term effects of cardiac steroids on intracellular membrane traffic in neuronal NT2 cells

Cardiac steroids (CS) are specific inhibitors of Na+, K+-ATPase activity. Although the presence of CS-like compounds in animal tissues has been established, their physiological role is not clear. In a previous study we showed that in pulse-chase membrane-labeling experiments, long term (hours) interaction of CS at physiological concentrations (nM) with Na+, K+-ATPase, caused changes in endocytosed membrane traffic in human NT2 cells. This was associated with the accumulation of large vesicles adjacent to the nucleus. For this sequence of events to function in the physiological setting, however, CS would be expected to modify membrane traffic upon short term (min) exposure and membrane labeling. We now demonstrate that CS affects membrane traffic also following a short exposure. This was reflected by the CS-induced accumulation of FM1-43 and transferrin in the cells, as well as by changes in their colocalization with Na+, K+-ATPase. We also show that the CS-induced changes in membrane traffic following up to 2 hrs exposure are reversible, whereas longer treatment induces irreversible effects. Based on these observations, we propose that endogenous CS-like compounds are physiological regulators of the recycling of endocytosed membrane proteins and cargo in neuronal cells, and may affect basic mechanisms such as neurotransmitter release and reuptake.

Metabolic and signaling events mediated by cardiotonic steroid ouabain in rat skeletal muscle

The cardiac glycoside ouabain initiates a cascade of signaling events through Na+,K+-ATPase, leading to an increase in cell growth and proliferation in different cell types. We explored the effects of ouabain on glucose metabolism in skeletal muscle and clarified the mechanisms of ouabain signal transduction. In rat soleus muscle 200 microM ouabain decreased basal glucose uptake without effect on insulin-stimulated glucose uptake. Ouabain increased glycogen synthesis additively to insulin and this effect was abolished in the presence of a MEK1/2 inhibitor (PD98059) or a c-Src inhibitor (PP2). Ouabain exposure reduced glucose oxidation, and this effect was reversed in the presence of PP2. Incubation with ouabain did not affect intramuscular ATP and its metabolites; however acetyl-CoA carboxylase phosphorylation was reduced, with no effect on AMPK phosphorylation. Insulin-stimulated Akt phosphorylation was not affected by ouabain. Ouabain reduced basal and insulin-stimulated phosphorylation of PKC alpha/beta and delta isoforms, whereas phosphorylation of PKCzeta was unchanged. Ouabain exposure increased interaction of 1- and 2-subunits of Na-pump with c-Src, as assessed by co-immunoprecipitation with c-Src. Phosphorylation of ERK1/2, GSK 3 / and p90rsk activity was increased in response to ouabain, and these effects were prevented in the presence of PD98059 and PP2. In conclusion, the cardiac glycoside ouabain stimulates glycogen synthesis additively to insulin in rat skeletal muscle. This effect is mediated by activation of c-Src-, ERK1/2- p90rsk- and GSK3-dependent signaling pathway.

Molecular insights into uremic cardiomyopathy: cardiotonic steroids and Na/K ATPase signaling

Patients with chronic renal failure develop a "uremic" cardiomyopathy characterized by diastolic dysfunction, left ventricular hypertrophy, fibrosis, and systemic oxidant stress. Patients with chronic renal failure also are known to have increases in the circulating concentrations of endogenous cardiotonic steroids (also referred to as endogenous digitalis-like substances.) Endogenous cardiotonic steroids produce reactive oxygen species as part of the signal cascade induced by binding to the plasmalemmal Na/K-ATPase in patients, and this signal cascade appears capable of inducing several key pathophysiologic features of uremic cardiomyopathy. In addition, these patients develop both fibrosis and oxidant stress without a known mechanism. In this review we highlight data supporting the hypothesis that endogenous cardiotonic steroids are a key molecular component involved in the diastolic dysfunction, left ventricular hypertrophy, fibrosis, and systemic oxidant stress associated with chronic kidney disease.

Na+,K+-ATPase and hormone ouabain:new roles for an old enzyme and an old inhibitor

Na+,K+-ATPase and its specific inhibitor ouabain entered the 21st century with an entirely new set of properties, that are the focuses of the present review. (i) The adhesive property of the beta-subunit explains why is Na+,K+-ATPase expressed polarizedly on one side of epithelial cells, a crucial property to explain the exchange of substances between higher organisms and the environment; (ii) Ouabain was recently recognized to be a hormone. (iii) Na+,K+-ATPase is known to act as a receptor for hormone ouabain, (iv) binding of ouabain to the Na+,K+-ATPase modifies adhesion: at high concentrations the outcome is total detachment. (v) Ouabain-resistant cells and ouabain-sensitive ones establish a special type of cell-cell interaction, so that sensitive cells withstand the presence of otherwise lethal levels of ouabain. (vi) Hormone ouabain provokes relocalization of specific molecules from the submembrane scaffold to the nucleus, where these bind to promoters of genes involved in proliferation, differentiation, migration, etc. (vii) Finally, ouabain causes a retrieval of Na+,K+-ATPase from the plasma membrane. We speculate that this would reduce the driving force that operates co- and counter-transporters, which are responsible for the exchange of substances across epithelia.

Signalling pathways involving sodium pump stimulate endothelin-1 secretion and nitric oxide production in endothelial cells

The cardiac steroid ouabain, a known inhibitor of the sodium pump, or Na+,K+-ATPase, has been shown to induce a variety of signaling cascades in various cells. The present study addresses the question of which signaling pathways are activated by ouabain in endothelial cells. Our findings indicate that ouabain, applied to human umbilical artery endothelial cells (HUAECs) in culture at low concentrations that do not cause global sodium pump inhibition, induces a reaction cascade that leads to the release of the vasoactive peptide endothelin-1 (ET-1). While ouabain-induced ET-1 release seems to be accomplished within 10 min, ouabain also stimulates a second signaling cascade that involves activation of Akt (also known as protein kinase B, or PKB), activation of endothelial nitric oxide synthase (eNOS) and increased NO production in HUAECs. This reaction cascade reaches its maximum approximately 30 min after exposure to the steroid. The results indicate that ouabain or similar compounds might actively participate in the regulation of vascular tone.

Hypertension, Na+/Ca2+ exchanger, and Na+, K+-ATPase

Hypertension is the most prevalent risk factor for stroke, myocardial infarction, or end-stage renal failure. The critical importance of excess salt intake in the pathogenesis of hypertension is widely recognized, but the mechanisms whereby salt intake elevates blood pressure have puzzled researchers. Recent studies using Na+/Ca2+ exchange inhibitors and genetically engineered mice provide evidence that vascular Na+/Ca2+ exchanger type 1 (NCX1) is involved in the development of salt-dependent hypertension. Endogenous cardiac glycosides, which may contribute to salt-dependent hypertension, seem to be necessary for NCX1-mediated hypertension. Intriguingly, studies using knock-in mice with modified cardiac glycoside binding affinity of Na+,K+-ATPases provide a clear demonstration that this cardiac glycoside-binding site plays an important role in blood pressure regulation. Taken all together: (1) endogenous cardiac glycosides are secreted after high salt intake; (2) these cardiac glycosides inhibit Na+,K+-ATPase in vascular smooth muscle cells; (3) this inhibition results in the elevation of local Na+ on the submembrane area; and (4) this elevation of local Na+ facilitates Ca2+ entry through NCX1, resulting in vasoconstriction. This proposed pathway may have enabled us to explain how to link dietary salt to hypertension.

Physiological role of the α1- and α2-isoforms of the Na+-K+-ATPase and biological significance of their cardiac glycoside binding site

An interesting feature of Na+-K+-ATPase is that it contains four isoforms of the catalytic α-subunit, each with a tissue-specific distribution. Our laboratory has used gene targeting to define the functional role of the α1- and α2-isoforms. While knockout mice demonstrated the importance of the α1- and α2-isoforms for survival, the knockin mice, in which each isoform can be individually inhibited by ouabain and its function determined, demonstrated that both isoforms are regulators of cardiac muscle contractility. Another intriguing aspect of the Na+-K+-ATPase is that it contains a binding site for cardiac glycosides, such as digoxin. Conservation of this site suggests that it may have an in vivo role and that a natural ligand must exist to interact with this site. In fact, cardiac glycoside-like compounds have been observed in mammals. Our recent study demonstrates that the cardiac glycoside binding site of the Na+-K+-ATPase plays a role in the regulation of blood pressure and that it mediates both ouabain-induced and ACTH-induced hypertension in mice. Whereas chronic administration of ouabain or ACTH caused hypertension in wild-type mice, it had no effect on blood pressure in mice with a ouabain-resistant α2-isoform of Na+-K+-ATPase. Interestingly, animals with the ouabain-sensitive α1-isoform and a ouabain-resistant α2-isoform develop ACTH-induced hypertension to a greater extent than wild-type animals. Taken together, these results demonstrate that the cardiac glycoside binding of the Na+-K+-ATPase has a physiological role and suggests a function for a naturally occurring ligand that is stimulated by administration of ACTH.

The highly conserved cardiac glycoside binding site of Na,K-ATPase plays a role in blood pressure regulation

The Na,K-ATPase contains a binding site for cardiac glycosides, such as ouabain, digoxin, and digitoxin, which is highly conserved among species ranging from Drosophila to humans. Although advantage has been taken of this site to treat congestive heart failure with drugs such as digoxin, it is unknown whether this site has a natural function in vivo. Here we show that this site plays an important role in the regulation of blood pressure, and it specifically mediates adrenocorticotropic hormone (ACTH)-induced hypertension in mice. We used genetically engineered mice in which the Na,K-ATPase alpha2 isoform, which is normally sensitive to cardiac glycosides, was made resistant to these compounds. Chronic administration of ACTH caused hypertension in WT mice but not in mice with an ouabain-resistant alpha2 isoform of Na,K-ATPase. This finding demonstrates that the cardiac glycoside binding site of the Na,K-ATPase plays an important role in blood pressure regulation, most likely by responding to a naturally occurring ligand. Because the alpha1 isoform is sensitive to cardiac glycosides in humans, we developed mice in which the naturally occurring ouabain-resistant alpha1 isoform was made ouabain-sensitive. Mice with the ouabain-sensitive "human-like" alpha1 isoform and an ouabain-resistant alpha2 isoform developed ACTH-induced hypertension to greater extent than WT animals. This result indicates that the cardiac glycoside binding site of the alpha1 isoform can also mediate ACTH-induced hypertension. Taken together these results demonstrate that the cardiac glycoside binding site of the alpha isoforms of the Na,K-ATPase have a physiological function and supports the hypothesis for a role of the endogenous cardiac glycosides.

Regulation of adrenocortical cardiotonic steroid production by dopamine and PKA signaling

There is growing evidence that the adrenal cortex is the source of cardiotonic steroid (CS) regulators of sodium, potassium-ATPase (NKA). The control of adrenocortical production CS may play a critical role in mediating renal and vascular responses involved in arterial hypertension. Dopamine (DA) controls renal NKA by direct regulatory phosphorylation and indirectly by modification of aldosterone release. In the present studies, Y-1 adrenocortical cell cultures which have been shown to produce a cardiotonic steroid indistinguishable from the known vertebrate steroid, marinobufagenin (MBG), were treated with various agents to stimulate or antagonize dopamine signaling pathways. We demonstrate that Y-1 cells express both pharmacological types of dopamine receptor (DA1 and DA2). Treatment of Y-1 cells with DA stimulated MBG production in a dose range similar to that shown to inhibit aldosterone production by the adrenal cortex. Experiments with specific DA1 and DA2 receptor agonists and antagonists were performed and allowed us to attribute the DA stimulatory effect to a DA1 type receptor. The DA stimulatory effect on MBG depended on protein kinase A (PKA) and could be blocked by Rp-cAMPS. Although both basal and forskolin-stimulated progesterone production by Y-1 cells were profoundly inhibited in Y-1 cell lines expressing the dominant negative type I regulatory subunit of PKA, both basal and forskolin-stimulated MBG production were demonstrated in these lines. This evidence suggests a possible role of DA1 signaling through camp-mediated activation of the type II PKA holoenzyme in the adrenal cortex.

Endogenous Cardiac Glycosides: Hormones Using the Sodium Pump as Signal Transducer

The search for an endogenous digitalis has led to the identification of the cardenolides ouabain and digoxin and the bufadienolide marinobufagenin in mammalian tissues and biological fluids. Ouabain's release from adrenal glands is under the control of epinephrine and angiotensin II; hence, its blood concentration changes rapidly on physical exercise. It also is controlled by brain areas sensing cerebrospinal Na+ concentration and apparently the body's K+ content because urinary K+ loss leads to an increase in its plasma concentration as well. Long-term treatment of rats with ouabain results in arterial hypertension, and 50% of Caucasians with low-renin hypertension have increased plasma concentrations of this cardenolide. Levels of digoxin, which is synthesized from acetate in adrenal glands, increase slightly in blood on prolonged exercise. It counteracts the hypertensinogenic action of ouabain in rats, as does the ouabain antagonist PST 2238. The plasma concentration of the bufadienolide marinobufagenin is increased after cardiac infarction. It may show natriuretic properties because it inhibits the alpha1 isoform of Na+/K+-adenosine triphosphatase (ATPase), the main sodium pump isoform of the kidney, much better than other sodium pump isoforms. These effects of endogenous cardiac glycosides are observed at concentrations that do not inhibit the sodium pump. Apparently, Na+/K+-ATPase is used by these steroids as a signal transducer to activate tissue proliferation, heart contractility, arterial hypertension, and natriuresis via various intracellular signaling pathways.

The Na/K-ATPase-mediated signal transduction as a target for new drug development

The Na/K-ATPase, or Na+ pump, is a member of the P-type ATPase superfamily. In addition to pumping ions, the Na/K-ATPase is a receptor that not only regulates the function of protein kinases, but also acts as a scaffold, capable of tethering different proteins into a signalplex. The signaling Na/K-ATPase resides in caveolae and forms a "binary receptor" with the tyrosine kinase Src. Endogenous cardiotonic steroids and digitalis drugs such as ouabain act as agonists and provoke this binary receptor, resulting in tyrosine phosphorylation of the proteins that are either associated with, or in close proximity to, the signaling Na/K-ATPase. Subsequently, this initiates protein kinase cascades including ERKs and PKC isozymes. It also increases mitochondrial production of reactive oxygen species (ROS) and regulates intracellular calcium concentration. Like other receptors, activation of the Na/K-ATPase/Src by ouabain induces the endocytosis of the plasma membrane Na/K-ATPase. Significantly, this newly appreciated signaling function of the Na/K-ATPase appears to play an important role in the pathogenesis of many cardiovascular diseases, therefore serving as an important target for development of novel therapeutic agents.

Studies on the cardenolide sequestration in African milkweed butterflies (Danaidae)

Butterflies of the Danaidae family are considered to be toxic or distasteful due to the presence of cardiac glycosides sequestered from their larval food plants. Alcoholic extracts of specimens of Danaus chrysippus aegyptius and Amauris ochlea ochlea from southern Africa (Namibia, S.-Africa, Mozambique) were analyzed by thin-layer chromatography for these cardenolides. But only 4 of 75 specimens of D. chrysippus aegyptius contained trace amounts, all others including 13 specimens of A. ochlea ochlea were negative. Genetic analysis of the ouabain binding site of the Na(+), K(+)-ATPase revealed that both species do not present an amino acid replacement at the position 122, which otherwise makes the enzyme insensitive to cardenolides suggesting that other strategies of toxin tolerance must have been developed.

[Na,K-ATPase and cardiac glycosides: new functions of the known protein]

Review of hormonal function of endogenous cardiac steroids. Special attention is paid to recently discovered mechanism of signal transduction from Na,K-ATPase that is due to not a change of ionic gradients but to ouabain-induced alteration of enzyme conformation, that, in turn, results in interaction of the enzyme with intracellular proteins. The data concerning discovery and identification of endogenous cardiac steroids and different isoforms of Na,K-ATPase that have various sensitivity to cardiac steroid, are also considered.

Interactions between Cardiac Glycosides and Sodium/Potassium-ATPase: Three-Dimensional Structure−Activity Relationship Models for Ligand Binding to the E2-Pi Form of the Enzyme versus Activity Inhibition

Sodium/potassium-ATPase (Na/K-ATPase) is a transmembrane enzyme that utilizes energy gained from ATP hydrolysis to transport sodium and potassium ions across cell membranes in opposite directions against their chemical and electrical gradients. Its transport activity is effectively inhibited by cardiac glycosides, which bind to the extracellular side of the enzyme and are of significant therapeutic value in the treatment of congestive heart failure. To determine the extent to which high-affinity binding of cardiac glycosides correlates with their potency in inhibiting pump activity, we determined experimentally both the binding affinities and inhibitory potencies of a series of 37 cardiac glycosides using radioligand binding and ATPase activity assays. The observed variations in key structural elements of these compounds correlating with binding and inhibition were analyzed by comparative molecular similarity index analysis (CoMSIA), which allowed a molecular level characterization and comparison of drug-Na/K-ATPase interactions that are important for ligand binding and activity inhibition. In agreement with our earlier comparative molecular field analysis studies [Farr, C. D., et al. (2002) Biochemistry 41, 1137-1148], the CoMSIA models predicted favorable inhibitor interactions primarily at the alpha-sugar and lactone ring moieties of the cardiac glycosides. Unfavorable interactions were located about the gamma-sugar group and at several positions about the steroid ring system. Whereas for most compounds a correlation between binding affinity and inhibitory potency was found, some notable exceptions were identified. Substitution of the five-membered lactone of cardenolides with the six-membered lactone of bufadienolides caused binding affinity to decline but inhibitory potency to increase. Furthermore, while the removal of ouabain's rhamnose moiety had little effect on inhibitory potency, it caused a dramatic decline in ligand binding affinity.

Clinical aspects of the MDR1 (ABCB1) gene polymorphism

Transporter proteins, in particular P-glycoprotein (Pgp), are important determinants in absorption, tissue targeting, and elimination of drugs. In addition to physiological and environmental factors, its expression and function are modified by genetic polymorphisms of the MDR1 gene. So far, several MDR1 SNPs have been identified, and mutations at positions 2677 and 3435 were associated with alteration of Pgp expression and/or function. In contrast to drug-metabolizing enzymes (eg, CYP2D6), for which loss of function mutations or gene amplification manifests as distinct phenotypes in the population, the impact of MDR1 polymorphisms on pharmacokinetics and pharmacodynamics of Pgp substrates is moderate. Clinical studies on the effects of the C3435T polymorphism and drug treatment with cardiac glycosides, the immunosuppressants cyclosporine and tacrolimus, HIV protease inhibitors, and tricyclic antidepressants are discussed.

[New steroid hormone family: endogenous cardiac glycosides and their role in physiologic and pathologic conditions]

In the last two decades extensive study has been carried out on the isolation, identification and biosynthesis of the "endogenous digitalis-like compounds" whose physiological and pathophysiological functions are only starting to be understood. Besides ouabain (strophanthin) and digoxin, four further endogenous cardiac glycosides were isolated and identified so far. These compounds are found in almost all mammalian tissues, including blood plasma and urine, but with the highest concentrations in the adrenal gland, pituitary and hypothalamus. De novo biosynthesis of these glycosides occurs in zona fasciculata cells of adrenal glands, precursors such as progesterone, pregnenolone, and rhamnose increase the synthesis of the ouabain-like immunoreactive material. The secretion of these compounds from the adrenocortical cells are controlled by adrenerg mechanisms, as well as via the renin-angiotensin system. The hydrophobic cardiac glycosides are transported in blood as complexes bound to specific binding globulins. The identified endogenous cardiac glycosides fulfill all the postulated criterions of the hormones, so they represent a new class of steroid hormones. The cardiac glycosides influence the active sodium pump, indirectly the intracellular free calcium concentration and therefore exert a positive inotropic effect on cardiac muscle. Furthermore, in physiological concentrations they can regulate the cell growth and protein synthesis inducing activation of intracellular signal pathways. Under pathological conditions, however, when the concentration of these steroids are high, they play a crucial role in the development of different serious illnesses such as essential hypertension as well as congestive heart failure. Further intensive investigations are needed to clarify some contradictory details accumulated during the last few years in this field.

Strategies for developing biomarkers of heart failure

BACKGROUND

Heart failure (HF) is a devastating disease with increasing prevalence in elderly populations. One-half of all patients die within 5 years of diagnosis. The annual cost of treating patients with HF in the US is more than $20 billion, which is estimated to be greater than that of myocardial infarction and all cancers combined. Given the complex pathophysiology and varied manifestations of HF, interest has intensified in developing biological markers to predict susceptibility and aid in the early diagnosis and management of this disease.

METHODS

We searched Medline via Ovid for studies published during the period 1966-2003 regarding various biomarkers suggested for HF. Our review focused on developing strategies for discovering and using new biomarkers, particularly those potentially linked to pathophysiologic mechanisms. We also point out strategic advantages, limitations, and methods available for measuring each of the currently proposed markers.

RESULTS

Biomarkers reviewed include those released from the heart during normal homeostasis (natriuretic peptides), those produced elsewhere that act on the heart (endogenous cardiotonic steroids and other hormones), and those released in response to tissue damage (inflammatory cytokines). The concept of using a combination of multiple markers based on diagnosis, prognosis, and acute vs chronic disease is also discussed. In view of recent advances in our understanding of molecular biochemical derangements observed during cardiac failure, we consider the concept of myocardial remodeling and the heart as part of an endocrine system as strategies.

CONCLUSION

Strategically, biomarkers linked to mechanisms involved in the etiology of HF, such as dysregulation of ion transport, seem best suited for serving as early biological markers to predict and diagnose disease, select therapy, or assess progression.

11-hydroxylation in the biosynthesis of endogenous ouabain: multiple implications

Accumulating evidence indicates that mammals use steroidal glycosides with "digitalis-like" activity. An endogenous ouabain (EO) has been described and is linked with long-term changes in sodium balance and cardiovascular structure and function. In the adrenal gland, the biosynthesis of EO and similar compounds appears to involve cholesterol side-chain cleavage with sequential metabolism of pregnenolone and progesterone. The more distal events in the biosynthesis have not been elucidated. Preliminary work using primary cell cultures from the bovine adrenal cortex suggests that the biosynthesis of EO is affected by inhibitors of 11beta-hydroxylase. Direct participation of 11-hydoxylase in EO synthesis would lead to an 11beta isomer of ouabain in mammals and, in vivo, an 11beta-oriented hydroxyl group would spontaneously form a mixture of two 11-19 hemiketal isomers. The latter isomers would likely be converted back to a single 11beta isomer of ouabain during isolation. The existence of an additional ring in the hemiketals, along with reduced flexion of the steroidal A, B, and C rings, raises the possibility that their in vivo physiological targets and actions differ from the isolated form of EO.

Ouabain as a mammalian hormone

Endogenous ouabain changes rapidly in humans and dogs upon physical exercise and is under the control of epinephrine and angiotensin II. Hence, the steroid acts as a rapidly acting hormone. A search for a specific binding globulin for cardiac glycosides in bovine plasma resulted in the identification of the d allotype of the micro chain of IgM whose hydrophobic surfaces interact with cardiotonic steroids and cholesterol. Such IgM complexes might be involved in the hepatic elimination of cardiotonic steroids. Thus, differences in the signaling cascade starting at Na(+),K(+)-ATPase must explain any differences in the action of ouabain and digoxin in the genesis of arterial hypertension.

Interactions of cationic drugs and cardiac glycosides at the hepatic uptake level: studies in the rat in vivo, isolated perfused rat liver, isolated rat hepatocytes and oocytes expressing oatp2

This paper deals with a crucial mechanism for interaction of basic drugs and cardiac glycosides at the hepatic uptake level. Available literature data is provided and new material is presented to picture the differential transport inhibition of bulky (type2) cationic drugs by a number of cardiac glycosides in rat liver. It is shown that the so called organic anion transporting peptide 2 (oatp2) is the likely interaction site: differential inhibition patterns as observed in oocytes expressing oatp2, could be clearly identified also in isolated rat hepatocytes, isolated perfused rat liver and the rat in vivo. The anticipation of transport interactions at the hepatic clearance level should be based on data on the relative affinities of interacting substrates for the transport systems involved along with knowledge on the pharmacokinetics of these agents as well as the chosen dose regimen in the studied species. This review highlights the importance of multispecific tranporter systems such as OATP, accommodating a broad spectrum of organic compounds of various charge, implying potential transport interactions that can affect body distribution and organ clearance.

[Diabetes mellitus as a general membrane disease and its consequences]

The metabolic disturbances and their consequences in diabetes mellitus are well known more or less in details too. However, our knowledge on the diabetic disorders in membrane functions are limited. These damages are connected mostly with the disregulation of the membrane protein syntheses due to deficiency of insulin. In this review the impairments of the Na(+)-pump and the Ca(2+)-transport mechanisms as well as the insulin-dependent glucose transporter GLUT4 will be discussed in diabetes. The capacity of these transporters could be decreased even more than 50 percent in diabetes. This is the reason why using the same dose of cardioactive steroids as if in not diabetic subjects--can cause toxic alterations on the heart in diabetic patients. Insulin regulates not only the expression of some membrane proteins but it can initiate the translocation of the Na(+)-pump and GLUT4 from the intracellular membrane compartments to the plasma membrane in muscle, heart and adipose tissue. Therefore the uptake of K+ and glucose into these tissues will increase significantly under the acute influence of insulin. Untreated diabetic patients generally show hyperkalemia. Forceful treatment with insulin of these subjects often causes severe hypokalemia as a consequence of sudden translocation of the Na(+)-pump. Different Ca(2+)-transport systems are also impaired in diabetes. These changes may result significantly higher free Ca2+ concentration in the cytoplasma of cardiomyocytes. This is one of the most important reason for the Ca2+ overloading and ultimately for heart death. According to authors opinion, beside the dangerous metabolic disorders, general membrane damage and extended disturbances in membrane functions are also very characteristic for diabetes. The acknowledgement of these alterations are very important for the exact planning of the up to date treatment of diabetes.

Comparison of "type I" and "type II" organic cation transport by organic cation transporters and organic anion-transporting polypeptides

Previous inhibition studies with taurocholate and cardiac glycosides suggested the presence of separate uptake systems for small "type I" (system1) and for bulky "type II" (system2) organic cations in rat hepatocytes. To identify the transport systems involved in type I and type II organic cation uptake, we compared the organic cation transport properties of the rat and human organic cation transporter 1 (rOCT1; hOCT1) and of the organic anion-transporting polypeptides 2 and A (rat Oatp2; human OATP-A) in cRNA-injected Xenopus laevis oocytes. Based on characteristic cis-inhibition patterns of rOCT1-mediated tributylmethylammonium and Oatp2-mediated rocuronium uptake, rOCT1 and Oatp2 could be identified as the organic cation uptake systems1 and 2, respectively, in rat liver. While hOCT1 exhibited similar transport properties as rOCT1, OATP-A- but not Oatp2-mediated rocuronium uptake was inhibited by the OATP-A substrate N-methyl-quinidine. The latter substrate was also transported by rOCT1 and hOCT1, demonstrating distinct organic cation transport activities for rOCT1 and Oatp2 and overlapping organic cation transport activities for hOCT1 and OATP-A. Finally, the data demonstrate that unmethylated quinidine is transported by rOCT1, hOCT1, and OATP-A at pH 6.0, but not at pH 7.5, indicating that quinidine requires a positive charge for carrier-mediated uptake into hepatocytes. In conclusion, the studies demonstrate that in rat liver the suggested organic cation uptake systems1 and 2 correspond to rOCT1 and Oatp2, respectively. However, the rat-based type I and II organic cation transporter classification cannot be extended without modification from rat to human.

Endogenous cardiotonic steroids

The search for endogenous digitalis led to the isolation of ouabain from blood adrenals and hypothalamus. Additional cardiotonic steroids of the cardenolid and bufadienolide type seem to circulate in blood. Adrenal cortical cells in tissue culture release ouabain upon addition of angiotensin 11. Ouabain in blood is increased in 50% of Caucasians with low renin hypertension. Analogous to other steroid hormones, cardiotonic steroid hormones in blood are bound to a specific cardiac glycoside binding globulin. Since ouabain induced growth of myocytes in tissue culture, this effect probably mediates by partial inhibition of the sodium pump and consecutive rise of intracellular Ca2+ the thickening of the wall of arteries and myocardium. PST 2238, an antagonist of cardiac glycoside function at the sodium pump, leads in rats under prolonged therapy to a decrease of hypertension. The finding of ouabain as a new adrenal hormone of the Na+ metabolism and of ouabain antagonists opens new possibilities of therapy of hypertension and congestive heart failure.

Abnormalities of sodium pump function in hypertension and the role of endogenous cardiotonic steroids

Elevated arterial blood pressure is a common heritable susceptibility in the human population. The high penetrance of this trait in industrialized societies may be influenced by the interactions of environmental factors and common genetic variants. This review examines the role of the renal sodium pump (sodium, potassium-ATPase, NKA) in hypertension and its integration into mechanisms of body sodium balance. In particular, renal NKA provides an appealing target by which inherited factors caninfluence renal sodium reabsorption. Recent work has indicated how some such genetic mechanisms may function. In this paper, the capacity of renal NKA to integrate environmental and heritable factors to increase blood pressure are examined.