Search for intermediates of Na+,K+-ATPase-mediated [Na+]i/[K+]i-independent death signaling triggered by cardiotonic steroids

Ouabain-Induced Cell Death and Survival. Role of α1-Na,K-ATPase-Mediated Signaling and [Na+]i/[K+]i-Dependent Gene Expression

Ouabain is of cardiotonic steroids (CTS) family that is plant-derived compounds and is known for many years as therapeutic and cytotoxic agents. They are specific inhibitors of Na,K-ATPase, the enzyme, which pumps Na+ and K+ across plasma membrane of animal cells. Treatment of cells by CTS affects various cellular functions connected with the maintenance of the transmembrane gradient of Na+ and K+. Numerous studies demonstrated that binding of CTS to Na,K-ATPase not only suppresses its activity but also induces some signal pathways. This review is focused on different mechanisms of two ouabain effects: their ability (1) to protect rodent cells from apoptosis through the expression of [Na+]i-sensitive genes and (2) to trigger death of non-rodents cells (so-called «oncosis»), possessing combined markers of «classic» necrosis and «classic» apoptosis. Detailed study of oncosis demonstrated that the elevation of the [Na+]i/[K+]i ratio is not a sufficient for its triggering. Non-rodent cell death is determined by the characteristic property of "sensitive" to ouabain α1-subunit of Na,K-ATPase. In this case, ouabain binding leads to enzyme conformational changes triggering the activation of p38 mitogen-activated protein kinases (MAPK) signaling. The survival of rodent cells with ouabain-«resistant» α1-subunit is connected with another conformational transition induced by ouabain binding that results in the activation of ERK 1/2 signaling pathway.

Identification of Proteins Whose Interaction with Na+,K+-ATPase Is Triggered by Ouabain

Prolonged exposure of different epithelial cells (canine renal epithelial cells (MDCK), vascular endothelial cells from porcine aorta (PAEC), human umbilical vein endothelial cells (HUVEC), cervical adenocarcinoma (HeLa), as well as epithelial cells from colon carcinoma (Caco-2)) with ouabain or with other cardiotonic steroids was shown earlier to result in the death of these cells. Intermediates in the cell death signal cascade remain unknown. In the present study, we used proteomics methods for identification of proteins whose interaction with Na+,K+-ATPase is triggered by ouabain. After exposure of Caco-2 human colorectal adenocarcinoma cells with 3 µM of ouabain for 3 h, the protein interacting in complex with Na+,K+-ATPase was coimmunoprecipitated using antibodies against the enzyme α1-subunit. Proteins of coimmunoprecipitates were separated by 2D electrophoresis in polyacrylamide gel. A number of proteins in the coimmunoprecipitates with molecular masses of 71-74, 46, 40-43, 38, and 33-35 kDa was revealed whose binding to Na+,K+-ATPase was activated by ouabain. Analyses conducted by mass spectroscopy allowed us to identify some of them, including seven signal proteins from superfamilies of glucocorticoid receptors, serine/threonine protein kinases, and protein phosphatases 2C, Src-, and Rho-GTPases. The possible participation of these proteins in activation of cell signaling terminated by cell death is discussed.

Cardiac Glycosides Activate the Tumor Suppressor and Viral Restriction Factor Promyelocytic Leukemia Protein (PML)

Cardiac glycosides (CGs), inhibitors of Na+/K+-ATPase (NKA), used clinically to treat heart failure, have garnered recent attention as potential anti-cancer and anti-viral agents. A high-throughput phenotypic screen designed to identify modulators of promyelocytic leukemia protein (PML) nuclear body (NB) formation revealed the CG gitoxigenin as a potent activator of PML. We demonstrate that multiple structurally distinct CGs activate the formation of PML NBs and induce PML protein SUMOylation in an NKA-dependent fashion. CG effects on PML occur at the post-transcriptional level, mechanistically distinct from previously described PML activators and are mediated through signaling events downstream of NKA. Curiously, genomic deletion of PML in human cancer cells failed to abrogate the cytotoxic effects of CGs and other apoptotic stimuli such as ceramide and arsenic trioxide that were previously shown to function through PML in mice. These findings suggest that alternative pathways can compensate for PML loss to mediate apoptosis in response to CGs and other apoptotic stimuli.

The death of ouabain-treated renal epithelial C11-MDCK cells is not mediated by swelling-induced plasma membrane rupture

This study examined the role of cell volume modulation in plasma membrane rupture and death documented in ouabain-treated renal epithelial cells. Long-term exposure to ouabain caused massive death of C11-MDCK (Madin-Darby canine kidney) epithelial cells, documented by their detachment, chromatin cleavage and complete loss of lactate dehydrogenase (LDH), but did not affect the survival of vascular smooth muscle cells (VSMCs) from the rat aorta. Unlike the distinct impact on cell survival, 2-h exposure to ouabain led to sharp elevation of the [Na⁺](i)/[K⁺](i) ratio in both cell types. A similar increment of Na⁺(i) content was evoked by sustained inhibition of Na⁺,K⁺-ATPase in K⁺-free medium. However, in contrast to ouabain, C11-MDCK cells survived perfectly during 24-h exposure to K⁺-free medium. At 3 h, the volume of ouabain-treated C11-MDCK cells and VSMCs, measured by the recently developed dual-image surface reconstruction technique, was increased by 16 and 12%, respectively, whereas 5-10 min before the detachment of ouabain-treated C11-MDCK cells, their volume was augmented by ~30-40%. To examine the role of modest swelling in the plasma membrane rupture of ouabain-treated cells, we compared actions of hypotonic medium on volume and LDH release. We observed that LDH release from hypoosmotically swollen C11-MDCK cells was triggered when their volume was increased by approximately fivefold. Thus, our results showed that the rupture of plasma membranes in ouabain-treated C11-MDCK cells was not directly caused by cell volume modulation evoked by Na⁺,K⁺-ATPase inhibition and inversion of the [Na⁺](i)/[K⁺](i) ratio.

Investigation of mechanism of p38 MAPK activation in renal epithelial cell from distal tubules triggered by cardiotonic steroids

Ouabain and other cardiotonic steroids (CTS) kill renal epithelial cells from distal tubules (C7-MDCK) via interaction with Na,K-ATPase but independently of inhibition of Na,K-ATPase-mediated ion fluxes. Recently, we demonstrated that modest intracellular acidification and inhibition of p38 MAPK suppress death of C7-MDCK cells triggered by ouabain. In the present study we investigate the mechanism of p38 MAPK activation in renal epithelial cell from distal tubules evoked by cardiotonic steroids. Using Na+/K+ ionophores (monensin, nigericin) and media with different content of monovalent cations, we revealed that p38 MAPK phosphorylation in ouabain-treated renal epithelial cells is not caused by Na,K-ATPase inhibition and inversion of the [Na+](i)/[K+](i) ratio. We also demonstrated that attenuation of pH from 7.45 to 6.75 did not alter the level of p38 MAPK phosphorylation observed in ouabain-treated cells. Inhibitors of PKA, PKC, and PKG as well as protein phosphatases were unable to abolish p38 MAPK activation triggered by ouabain. Using phosphotyrosine antibodies we did not detect any effect of ouabain on activation of tyrosine kinases. Thus, our results show that activation of p38 MAPK and cytotoxic action of CTS are independent of intracellular Na+, K+, and H+ concentrations. The molecular origin of intermediates of death signaling induced by CTS via conformation changes of Na,K-ATPase with following activation of p38 MAPK should be examined further.

Targeted mutations in the Na,K-ATPase α 2 isoform confer ouabain resistance and result in abnormal behavior in mice

Sodium and potassium-activated adenosine triphosphatases (Na,K-ATPase) are ubiquitous, participate in osmotic balance and membrane potential, and are composed of α, β, and γ subunits. The α subunit is required for the catalytic and transport properties of the enzyme and contains binding sites for cations, ATP, and digitalis-like compounds including ouabain. There are four known α isoforms; three that are expressed in the CNS in a regional and cell-specific manner. The α2 isoform is most commonly found in astrocytes, pyramidal cells of the hippocampus in adults, and developmentally in several other neuronal types. Ouabain-like compounds are thought to be produced endogenously in mammals, bind the Na,K-ATPase, and function as a stress-related hormone, however, the impact of the Na,K-ATPase ouabain binding site on neurobehavioral function is largely unknown. To determine if the ouabain binding site of the α2 isoform plays a physiological role in CNS function, we examined knock-in mice in which the normally ouabain-sensitive α2 isoform was made resistant (α2(R/R) ) while still retaining basal Na,K-ATPase enzymatic function. Egocentric learning (Cincinnati water maze) was impaired in adult α2(R/R) mice compared to wild type (WT) mice. They also exhibited decreased locomotor activity in a novel environment and increased responsiveness to a challenge with an indirect sympathomimetic agonist (methamphetamine) relative to WT mice. The α2(R/R) mice also demonstrated a blunted acoustic startle reflex and a failure to habituate to repeated acoustic stimuli. The α2(R/R) mice showed no evidence of altered anxiety (elevated zero maze) nor were they impaired in spatial learning or memory in the Morris water maze and neither group could learn in a large Morris maze. These results suggest that the ouabain binding site is involved in specific types of learning and the modulation of dopamine-mediated locomotor behavior.

Cardiotonic steroid-resistant alpha1-Na+,K+-ATPase rescues renal epithelial cells from the cytotoxic action of ouabain: evidence for a Nai+,Ki+ -independent mechanism

Mechanisms underlying the tissue-specific impact of cardiotonic steroids (CTS) on cell survival and death remain poorly understood. This study examines the role of Na(+),K(+)-ATPase alpha subunits in death of Madin-Darby canine kidney (MDCK) cells evoked by 24-h exposure to ouabain. MDCK cells expressing a variant of the alpha1 isoform, CTS-sensitive alpha1S, were stably transfected with a cDNA encoding CTS-resistant alpha1R-Na(+),K(+)-ATPase, whose expression was confirmed by RT-PCR. In mock-transfected and alpha1R-cells, maximal inhibition of (86)Rb influx was observed at 10 and 1000 muM ouabain, respectively, thus confirming high abundance of alpha1R-Na(+),K(+)-ATPase in these cells. Six-hour treatment of alpha1R-cells with 1000 muM ouabain led to the same elevation of the [Na(+)](i)/[K(+)](i) ratio that was detected in mock-transfected cells treated with 3 muM ouabain. However, in contrast to the massive death of mock-transfected cells exposed to 3 muM ouabain, alpha1R-cells survived after 24-h incubation with 1000 muM ouabain. Inversion of the [Na(+)](i)/[K(+)](i) ratio evoked by Na(+),K(+)-ATPase inhibition in K(+)-free medium did not affect survival of alpha1R-cells but increased their sensitivity to ouabain. Our results show that the alpha1R subunit rescues MDCK cells from the cytotoxic action of CTS independently of inhibition of Na(+),K(+)-ATPase-mediated Na(+) and K(+) fluxes and inversion of the [Na(+)](i)/[K(+)](i) ratio.

Death of ouabain-treated renal epithelial cells: evidence for p38 MAPK-mediated Na (i) (+) /K (i) (+) -independent signaling

Recent studies demonstrate that cytotoxic actions of ouabain and other cardiotonic steroids (CTS) on renal epithelial cells (REC) are triggered by their interaction with the Na(+),K(+)-ATPase alpha-subunit but not the result of inhibition of Na(+),K(+)-ATPase-mediated ion fluxes and inversion of the [Na(+)](i)/[K(+)](i) ratio. This study examined the role of mitogen-activated protein kinases (MAPK) in the death of ouabain-treated REC. Exposure of C7-MDCK cells that resembled principal cells from canine kidney to 3 microM ouabain led to phosphorylation of p38 without significant impact on phosphorylation of ERK and JNK MAPK. Maximal increment of p38 phosphorylation was observed at 4 h followed by cell death at 12 h of ouabain addition. In contrast to ouabain, neither cell death nor p38 MAPK phosphorylation were affected by elevation of the [Na(+)](i)/[K(+)](i) ratio triggered by Na(+),K(+)-ATPase inhibition in K(+)-free medium. p38 phosphorylation was noted in all other cell types exhibiting death in the presence of ouabain, such as intercalated cells from canine kidney and human colon rectal carcinoma cells. We did not observe any action of ouabain on p38 phosphorylation in ouabain-resistant smooth muscle cells from rat aorta and endothelial cells from human umbilical vein. Both p38 phosphorylation and death of ouabain-treated C7-MDCK cells were suppressed by p38 inhibitor SB 202190 but were resistant to its inactive analogue SB 202474. Our results demonstrate that death of CTS-treated REC is triggered by Na (i) (+) ,K (i) (+) -independent activation of p38 MAPK.

Diverse actions of ouabain and its aglycone ouabagenin in renal cells

The cellular actions of ouabain are complex and involve different pathways, depending on the cell type and experimental conditions. Several studies have reported that Madin-Darby canine kidney (MDCK) cellular sensitivity to ouabain is not related to Na-K-ATPase inhibition, and others showed that some cell types, such as Ma104, are resistant to ouabain toxicity albeit their Na-K-ATPase isoforms possess high affinity for this glycoside. We describe here that the effects of ouabain and ouabagenin also diverge in MDCK and Ma104 cells, being MDCK cells more resistant to ouabagenin, while Ma104 cells are resistant to both molecules. This feature seems to correlate with induction of cell signaling, since ouabain, but not ouabagenin, induced an intense and sustained increase in tyrosine phosphorylation levels in MDCK cells. Moreover, ouabain-induced phosphorylation in Ma104 cells was approximately half than that observed in MDCK cells. The proportion between alpha and beta subunits of Na-K-ATPase was similar in MDCK cells, though Ma104 cells presented more alpha subunits, located mainly at the cytoplasm. Furthermore, a fluorescent ouabain-analog labeled mainly the cytoplasm of Ma104 cells, the opposite of that seen in MDCK cells, corroborating the results using anti-Na-K-ATPase antibodies. Hence, the results suggest that ouabain and ouabagenin differ in terms of Na-K-ATPase inhibition and cell signaling activation in MDCK cells. Additionally, MDCK and Ma104 cell lines respond differently to ouabain, perhaps due to an intrinsic ability of this glycoside to selectively reach the cytoplasm of Ma104 cells.

Diverse biological responses to different cardiotonic steroids

Cardiotonic steroids (CS) such as ouabain, digoxin and bufalin, are steroidal drugs prepared from the seeds and dried leaves of the genus Digitalis, and the skin and parotid gland of amphibians, are used as a cardiac stimulant. Steroids similar or identical to the cardiotonic steroids were identified in human tissues. The available literature unequivocally supports the notion that these endogenous CS function as hormones in mammals. Recent studies show that although similar in structure, the different CS exhibit diverse biological responses. This was shown at the molecular, cellular, tissue and whole animal levels. This review summarizes these diversities, raises a possible explanation for their presence and discusses their implication on the physiological role of the different steroids.

[Na+]i/[K+]i -independent death of ouabain-treated renal epithelial cells is not mediated by Na+,K+ -ATPase internalization and de novo gene expression

The cytotoxic effect of long-term exposure of renal epithelial cells to ouabain and other cardiotonic steroids (CTS) is mediated by the interaction of these compounds with Na(+),K(+)-ATPase but is independent of the inhibition of Na(+),K(+)-ATPase-mediated ion fluxes. Sustained application of CTS also leads to Na(+),K(+)-ATPase endocytosis and its translocation into the nuclei that might trigger the cell death machinery via the regulation of gene expression. This study examines the role of Na(+),K(+)-ATPase internalization and de novo gene expression in the death of ouabain-treated C7-Madin-Darby canine kidney (MDCK) cells derived from distal tubules of the MDCK. In these cells, 6-h exposure to 3 microM ouabain led to the internalization of approximately 50% of plasmalemmal Na(+),K(+)-ATPase. Prolonged incubation in a K(+)-free medium abolished ouabain-induced Na(+),K(+)-ATPase internalization but did not affect the cytotoxic action of ouabain seen after 18-h incubation. Previously, it was shown that CTS-induced Na(+),K(+)-ATPase internalization is mediated by its interaction with Src within caveolae. Neither caveolae damage by cholesterol depletion with methyl-beta-cyclodextrin nor Src inhibition with 4-amino-5(4-chlorophenyl)-7-(t-butyl)pyrazol[3,4-d]pyridine affected the death of ouabain-treated C7-MDCK cells. Actinomycin D at the 0.1-microg/ml concentration almost completely abolished ribonucleic acid synthesis but did not protect C7-MDCK cells from the cytotoxic action of ouabain. Our results show that neither Na(+),K(+)-ATPase endocytosis nor de novo gene expression contributes to Na(+)(i), K(+)(i)-independent cell death signaling evoked by prolonged exposure to CTS.

The digitalis-like steroid hormones: new mechanisms of action and biological significance

Digitalis-like compounds (DLC) are a family of steroid hormones synthesized in and released from the adrenal gland. DLC, the structure of which resembles that of plant cardiac glycosides, bind to and inhibit the activity of the ubiquitous cell surface enzyme Na(+), K(+)-ATPase. However, there is a large body of evidence suggesting that the regulation of ion transport by Na(+), K(+)-ATPase is not the only physiological role of DLC. The binding of DLC to Na(+), K(+)-ATPase induces the activation of various signal transduction cascades that activate changes in intracellular Ca(++) homeostasis, and in specific gene expression. These, in turn, stimulate endocytosis and affect cell growth and proliferation. At the systemic level, DLC were shown to be involved in the regulation of major physiological parameters including water and salt homeostasis, cardiac contractility and rhythm, systemic blood pressure and behavior. Furthermore, the DLC system has been implicated in several pathological conditions, including cardiac arrhythmias, hypertension, cancer and depressive disorders. This review evaluates the evidence for the different aspects of DLC action and delineates open questions in the field.

The death of cardiotonic steroid-treated cells: evidence of Na+i,K+i-independent H+i-sensitive signalling

Na/K-ATPase is the only known target of cardiotonic steroids (CTS) identified in plants, amphibians and later on in several mammalian species, including human. We focus our review on recent data implicating CTS in the tissue-specific regulation of cell survival and death. In vascular smooth muscle cells, CTS inhibited cell death triggered by apoptotic stimuli via a novel Na+i-mediated, Ca2+i-independent mechanism of expression of antiapoptotic genes, including mortalin. In contrast, exposure to CTS in vascular endothelial and renal epithelial cells led to cell death, showing combined markers of apoptosis and necrosis. This mode of cell death, termed oncosis, is caused by CTS interaction with Na/K-ATPase but is independent of the inhibition of Na/K-ATPase-mediated ion fluxes and inversion of the [Na+]i/[K+]i ratio. The intermediates of intracellular signalling involved in Na+i, K+i-independent oncosis of CTS-treated cells remain unknown. Recently, we found that this mode of cell death can be protected by modest intracellular acidification via the expression of H+i-sensitive genes. The molecular origin of intracellular Na+ and H+ sensor involvement in the development of apoptosis and oncosis is currently under investigation.

Modest intracellular acidification suppresses death signaling in ouabain-treated cells

The signaling cascade resulting in the death of several types of cells treated with ouabain or other cardiotonic steroids (CTS) remains poorly understood. Recently, we observed that ouabain kills epithelial and endothelial cells via its interaction with Na(+), K(+) -ATPase, but independently of inhibition of Na(+), K(+) -ATPase-mediated ion fluxes and inversion of the [Na(+)](i)/[K(+)](i) ratio. Here, we report that the death of ouabain-treated epithelial cells from the Madin-Darby canine kidney (C7-MDCK) and endothelial cells from porcine aortae is suppressed by acidification of medium from pH 7.4 to 7.0, i.e. under conditions when pH(i) was decreased from approximately 7.2 to 6.9. The rescue of ouabain-treated C7-MDCK cells was also detected under selective intracellular acidification caused by inhibition of Na(+)/H(+) exchanger. In these cells, neither Na(+), K(+) pump activity nor [(3)H]-ouabain binding was significantly affected by modest acidification. The death of ouabain-treated cells was independent of inhibition of RNA and protein synthesis with actinomycin D and cycloheximide. In contrast, both compounds sharply attenuated the protective action of acidified medium. Thus, our results show that very modest intracellular acidification is sufficient to inhibit the Na(+) (i)/K(+) (i)-independent death signal triggered in epithelial and endothelial cells by CTS. They also suggest that the protective action of acidification is mediated by de novo expression of genes involved in inhibition of the cell death machinery.