Inversion of the intracellular Na(+)/K(+) ratio blocks apoptosis in vascular smooth muscle cells by induction of RNA synthesis

Sodium Ions as Regulators of Transcription in Mammalian Cells

The maintenance of an uneven distribution of Na+ and K+ ions between the cytoplasm and extracellular medium is the basis for the functioning of any animal cell. Changes in the intracellular ratio of these cations occur in response to numerous stimuli and are important for the cell activity regulation. Numerous experimental data have shown that gene transcription in mammalian cells can be regulated by changes in the intracellular [Na+]_i/[K+]_i ratio. Here, we discuss possible mechanisms of such regulation in various cell types, with special attention to the [Ca2+]-independent signaling pathways that suggest the presence of an intracellular sensor of monovalent cations. As such sensor, we propose the secondary structures of nucleic acids called G-quadruplexes. They are widely represented in mammalian genomes and are often found in the promoters of genes encoding transcription factors.

Na⁺i,K⁺i-Dependent and -Independent Signaling Triggered by Cardiotonic Steroids: Facts and Artifacts

Na⁺,K⁺-ATPase is the only known receptor of cardiotonic steroids (CTS) whose interaction with catalytic α-subunits leads to inhibition of this enzyme. As predicted, CTS affect numerous cellular functions related to the maintenance of the transmembrane gradient of monovalent cations, such as electrical membrane potential, cell volume, transepithelial movement of salt and osmotically-obliged water, symport of Na⁺ with inorganic phosphate, glucose, amino acids, nucleotides, etc. During the last two decades, it was shown that side-by-side with these canonical Na⁺_i/K⁺_i-dependent cellular responses, long-term exposure to CTS affects transcription, translation, tight junction, cell adhesion and exhibits tissue-specific impact on cell survival and death. It was also shown that CTS trigger diverse signaling cascades via conformational transitions of the Na⁺,K⁺-ATPase α-subunit that, in turn, results in the activation of membrane-associated non-receptor tyrosine kinase Src, phosphatidylinositol 3-kinase and the inositol 1,4,5-triphosphate receptor. These findings allowed researchers to propose that endogenous CTS might be considered as a novel class of steroid hormones. We focus our review on the analysis of the relative impact Na⁺_i,K⁺_i-mediated and -independent pathways in cellular responses evoked by CTS.

Salt and gene expression: evidence for [Na+]i/[K+]i-mediated signaling pathways

Our review focuses on the recent data showing that gene transcription and translation are under the control of signaling pathways triggered by modulation of the intracellular sodium/potassium ratio ([Na+]i/[K+]i). Side-by-side with sensing of osmolality elevation by tonicity enhancer-binding protein (TonEBP, NFAT5), [Na+]i/[K+]i-mediated excitation-transcription coupling may contribute to the transcriptomic changes evoked by high salt consumption. This novel mechanism includes the sensing of heightened Na+ concentration in the plasma, interstitial, and cerebrospinal fluids via augmented Na+ influx in the endothelium, immune system cells, and the subfornical organ, respectively. In these cells, [Na+]i/[K+]i ratio elevation, triggered by augmented Na+ influx, is further potentiated by increased production of endogenous Na+,K+-ATPase inhibitors documented in salt-sensitive hypertension.

Cell volume and monovalent ion transporters: their role in cell death machinery triggering and progression

Cell death is accompanied by the dissipation of electrochemical gradients of monovalent ions across the plasma membrane that, in turn, affects cell volume via modulation of intracellular osmolyte content. In numerous cell types, apoptotic and necrotic stimuli caused cell shrinkage and swelling, respectively. Thermodynamics predicts a cell type-specific rather than an ubiquitous impact of monovalent ion transporters on volume perturbations in dying cells, suggesting their diverse roles in the cell death machinery. Indeed, recent data showed that apoptotic collapse may occur in the absence of cell volume changes and even follow cell swelling rather than shrinkage. Moreover, side-by-side with cell volume adjustment, monovalent ion transporters contribute to cell death machinery engagement independently of volume regulation via cell type-specific signaling pathways. Thus, inhibition of Na(+)-K(+)-ATPase by cardiotonic steroids (CTS) rescues rat vascular smooth muscle cells from apoptosis via a novel Na(+)i-K(+)i-mediated, Ca(2+)i-independent mechanism of excitation-transcription coupling. In contrast, CTS kill renal epithelial cells independently of Na(+)-K(+)-ATPase inhibition and increased [Na(+)]i/[K(+)]i ratio. The molecular origin of [Na(+)]i/[K(+)]i sensors involved in the inhibition of apoptosis as well as upstream intermediates of Na(+)i/K(+)i-independent death signaling triggered by CTS remain unknown.

Ouabain-induced perturbations in intracellular ionic homeostasis regulate death receptor-mediated apoptosis

Apoptosis is defined by specific morphological and biochemical characteristics including cell shrinkage (termed apoptotic volume decrease), a process that results from the regulation of ion channels and plasma membrane transporter activity. The Na(+)-K(+)-ATPase is the predominant pump that controls cell volume and plasma membrane potential in cells and alterations in its function have been suggested to be associated with apoptosis. We report here that the Na(+)-K(+)-ATPase inhibitor ouabain, potentiates apoptosis in the human lymphoma Jurkat cells exposed to Fas ligand (FasL) or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) but not other apoptotic agents such as H(2)O(2), thapsigargin or UV-C implicating a role for the Na(+)-K(+)-ATPase in death receptor-induced apoptosis. Interestingly, ouabain also potentiated perturbations in cell Ca(2+) homeostasis only in conjunction with the apoptotic inducer FasL but not TRAIL. Ouabain did not affect alterations in the intracellular Ca(2+) levels in response to H(2)O(2), thapsigargin or UV-C. FasL-induced alterations in Ca(2+) were not abolished in Ca(2+)-free medium but incubation of cells with BAPTA-AM inhibited both Ca(2+) perturbations and the ouabain-induced potentiation of FasL-induced apoptosis. Our data suggest that the impairment of the Na(+)-K(+)-ATPase activity during apoptosis is linked to perturbations in cell Ca(2+) homeostasis that modulate apoptosis induced by the activation of Fas by FasL.

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.

The death of ouabain-treated renal epithelial cells: evidence against anoikis occurrence

The mechanisms of cell death signaling triggered by cardiotonic steroids are poorly understood. Based on massive detachment of ouabain-treated Madin-Darby canine kidney (MDCK) cells, it may be proposed that the cytotoxic action of these compounds is mediated by anoikis, i.e. a particular mode of death occurring in cells lacking cell-to-extracellular matrix interactions. We tested this hypothesis. Six hour incubation of MDCK cells with ouabain, marinobufagenin or K+-free medium almost completely blocked Na+,K+-ATPase, increased Na (i) + content by approximately 10-fold and suppressed cell attachment to regular-plastic-plates by up to 5-fold. In contrast, the death of attached cells was observed after 24-h incubation with ouabain but not in the presence of marinobufagenin or K+-free medium. Cells treated with ouabain and undergoing anoikis on ultra-low attachment plates exhibited different cell volume behaviour, i.e. swelling and shrinkage, respectively. The pan-caspase inhibitor z-VAD.fmk and the protein kinase C activator PMA rescued MDCK cells from anoikis but did not influence the survival of ouabain-treated cells, whereas medium acidification from pH 7.2 to 6.7 almost completely abolished the cytotoxic action of ouabain, but did not significantly affect anoikis. Our results show that the Na (i) + ,K (i) + -independent mode of MDCK cell death evoked by ouabain is not mediated by anoikis.

Potassium channels in the regulation of pulmonary artery smooth muscle cell proliferation and apoptosis: pharmacotherapeutic implications

Maintaining the proper balance between cell apoptosis and proliferation is required for normal tissue homeostasis; when this balance is disrupted, disease such as pulmonary arterial hypertension (PAH) can result. Activity of K(+) channels plays a major role in regulating the pulmonary artery smooth muscle cell (PASMC) population in the pulmonary vasculature, as they are involved in cell apoptosis, survival and proliferation. PASMCs from PAH patients demonstrate many cellular abnormalities linked to K(+) channels, including decreased K(+) current, downregulated expression of various K(+) channels, and inhibited apoptosis. K(+) is the major intracellular cation, and the K(+) current is a major determinant of cell volume. Apoptotic volume decrease (AVD), an early hallmark and prerequisite of programmed cell death, is characterized by K(+) and Cl(-) efflux. In addition to its role in AVD, cytosolic K(+) can be inhibitory toward endogenous caspases and nucleases and can suppress mitochondrial cytochrome c release. In PASMC, K(+) channel activation accelerates AVD and enhances apoptosis, while K(+) channel inhibition decelerates AVD and inhibits apoptosis. Finally, inhibition of K(+) channels, by increasing cytosolic [Ca(2+)] as a result of membrane depolarization-mediated opening of voltage-dependent Ca(2+) channels, leads to PASMC contraction and proliferation. The goals of this review are twofold: (1) to elucidate the role of K(+) ions and K(+) channels in the proliferation and apoptosis of PASMC, with an emphasis on abnormal cell growth in human and animal models of PAH, and (2) to elaborate upon the targeting of K(+) flux pathways for pharmacological treatment of pulmonary vascular disease.

[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.

[Na]i -induced c-Fos expression is not mediated by activation of the 5' -promoter containing known transcriptional elements

In vascular smooth muscle cells and several other cell types, inhibition of Na(+)/K(+)-ATPase leads to the expression of early response genes, including c-Fos. We designed this study to examine whether or not a putative Na(+) (i)/K(+) (i)-sensitive element is located within the c-Fos 5'-UTR from - 650 to + 103 containing all known response elements activated by 'classic' stimuli, such as growth factors and Ca(2+) (i)-raising compounds. In HeLa cells, the highest increment of c-Fos mRNA content was noted after 6 h of Na(+)/K(+)-ATPase inhibition with ouabain that was abolished by actinomycin D, an inhibitor of RNA synthesis. c-Fos protein accumulation in ouabain-treated cells correlated with a gain of Na(+) (i) and loss of K(+) (i). Augmented c-Fos expression was also observed under inhibition of Na(+)/K(+)-ATPase in K(+)-free medium and in the presence of the Na(+) ionophore monensin. The effect of ouabain on c-Fos expression was sharply attenuated under dissipation of the transmembrane Na(+) gradient, but was preserved in the presence of Ca(2+) chelators and the extracellular regulated kinase inhibitor PD98059, thus indicating an Na(+) (i)-mediated, Ca(2+) (i)- and extracellular regulated kinase-independent mechanism of gene expression. In contrast to massive c-Fos expression, we failed to detect any effect of ouabain on accumulation of luciferase driven by the c-Fos 5'-UTR. Negative results were also obtained in ouabain-treated vascular smooth muscle cells and C11 Madin-Darby canine kidney cells possessing augmented c-Fos expression. Our results reveal that Na(+) (i)-induced c-Fos expression is not mediated by the 5'-UTR containing transcriptional elements activated by growth factors and other 'classic stimuli'.

Apoptosis vs. oncosis: role of cell volume and intracellular monovalent cations

Several research teams have proposed that shrinkage and swelling in cells undergoing apoptosis and oncosis are not only the earliest morphological markers of the two modes of cell death but are also obligatory steps in the development of the death machinery. We examined this hypothesis as well as the role of monovalent cations as major intracellular osmolytes using vascular smooth muscle cells (VSMC) from the rat aorta and C7-MDCK cells derived from the Madin-Darby canine kidney. 48-hr inhibition of the Na(+)-K+ pump with ouabain did not affect VSMC survival and delayed serum deprivation-induced apoptosis at a step upstream of caspase-3 via elevation of the [Na+]i/[K+]i ratio and the expression of Na+ i-sensitive antiapoptotic genes including mortalin. Transient and modest (15-20%) shrinkage observed in serum-deprived VSMC did not contribute to triggering of the apoptotic machinery. In contrast to VSMC, ouabain led to oncosis of C7-MDCK cells, indicated by swelling and resistance to the pan-caspase inhibitor z-VAD.fmk. In these cells, the death signal was mediated by interaction of ouabain with the Na(+)-K(+)-ATPase alpha-subunit but was independent of the inhibition of Na(+)-K+ pump-mediated ion fluxes and elevation of the [Na+]i/[K+]i ratio.

The MgtC virulence factor of Salmonella enterica serovar Typhimurium activates Na(+),K(+)-ATPase

The mgtC gene of Salmonella enterica serovar Typhimurium encodes a membrane protein of unknown function that is important for full virulence in the mouse. Since mgtC is part of an operon with mgtB which encodes a Mg(2+)-transporting P-type ATPase, MgtC was hypothesized to function in ion transport, possibly in Mg(2+) transport. Consequently, MgtC was expressed in Xenopus laevis oocytes, and its effect on ion transport was evaluated using ion selective electrodes. Oocytes expressing MgtC did not exhibit altered currents or membrane potentials in response to changes in extracellular H(+), Mg(2+), or Ca(2+), thus ruling out a previously postulated function as a Mg(2+)/H(+) antiporter. However, addition of extracellular K(+) markedly hyperpolarized membrane potential instead of the expected depolarization. Addition of ouabain to block the oocyte Na(+),K(+)-ATPase completely prevented hyperpolarization and restored the normal K(+)-induced depolarization response. These results suggested that the Na(+),K(+)-ATPase was constitutively activated in the presence of MgtC resulting in a membrane potential largely dependent on Na(+),K(+)-ATPase. Consistent with the involvement of Na(+),K(+)-ATPase, oocytes expressing MgtC exhibited an increased rate of (86)Rb(+) uptake and had increased intracellular free [K(+)] and decreased free [Na(+)] and ATP. The free concentrations of Mg(2+) and Ca(2+) and cytosolic pH were unchanged, although the total intracellular Ca(2+) content was slightly elevated. These results suggest that the serovar Typhimurium MgtC protein may be involved in regulating membrane potential but does not directly transport Mg(2+) or another ion.

Intracellular monovalent ions as second messengers

It is generally accepted that electrochemical gradients of monovalent ions across the plasma membrane, created by the coupled function of pumps, carriers and channels, are involved in the maintenance of resting and action membrane potential, cell volume adjustment, intracellular Ca(2+ )handling and accumulation of glucose, amino acids, nucleotides and other precursors of macromolecular synthesis. In the present review, we summarize data showing that side-by-side with these classic functions, modulation of the intracellular concentration of monovalent ions in a physiologically reasonable range is sufficient to trigger numerous cellular responses, including changes in enzyme activity, gene expression, protein synthesis, cell proliferation and death. Importantly, the engagement of monovalent ions in regulation of the above-listed cellular responses occurs at steps upstream of Ca(2+) (i) and other key intermediates of intracellular signaling, which allows them to be considered as second messengers. With the exception of HCO (3) (-) -sensitive soluble adenylyl cyclase, the molecular origin of sensors involved in the function of monovalent ions as second messengers remains unknown.

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.

Low-dose ouabain protects against excitotoxic apoptosis and up-regulates nuclear Bcl-2 in vivo

Sodium-potassium ATPase (Na+,K+-ATPase) regulates the electrochemical gradient in cells, thereby providing fluid and ionic homeostasis. Additionally, interaction of the Na+,K+ pump with cardiac glycosides can activate intracellular signaling cascades (resulting in cell growth) and up-regulate transcription factors that promote cell survival. We used an in vivo excitotoxicity model to assess if Na+,K+-ATPase plays a role in neuronal apoptosis. After unilateral, intrastriatal injection of the glutamate receptor agonist kainic acid into postnatal day 7 rats, Na+,K+ pump function was increased at 12 h after excitotoxic challenge, and levels of neuron-specific enzyme subunits were preserved (up to 24 h after injection) in membrane-enriched striatal fractions. In addition, co-injection of kainic acid with a low-dose (0.01 nmol) of the cardiac glycoside ouabain significantly (P<0.05) reduced striatal apoptosis (at 24 h post-injection) without diminishing Na+,K+-ATPase activity. To evaluate the possible mechanisms for this neuroprotection, we examined the levels of nuclear factor kappa B and Bcl-2 after cardiac glycoside exposure. Low-dose ouabain increased nuclear Bcl-2 (but not nuclear factor kappa B) protein levels at 6 h post injection. Our results suggest that Na+,K+-ATPase allows for progression of apoptosis in excitotoxically-injured neurons, and that sublethal concentrations of ouabain provide neuroprotection against excitotoxicity. The mechanism for this ouabain neuroprotection could be intracellular cascades linked to the Na+,K+-ATPase-ouabain interaction that modulate subcellular Bcl-2 levels. Targeted, therapeutic inhibition of apoptosis through cardiac glycosides may represent an effective strategy against excitotoxicity-mediated neuronal injury.

Ouabain induces an increase of retinal ganglion cell survival in vitro: the involvement of protein kinase C

Ouabain is a steroid derivative that can regulate many cellular events such as growth and proliferation. It modulates Na+,K+-ATPase activity leading to the activation of different intracellular pathways through protein-protein interactions that have been characterized during the last few years. The aim of this work was to study the role of ouabain in rat retinal ganglion cell survival after 48 h in culture. Our results demonstrated that ouabain significantly induced an increase in retinal ganglion cell survival. The effect was dose-dependent and was maximal with 3.0 nM. The blockade of protein kinase C activity by 1.25 microM chelerythrine chloride abolished the ouabain effect, indicating an involvement of this intracellular pathway. None of the protein kinase inhibitors usually employed in the study of ouabain-driven intracellular pathways (PD98059, Ly294002, herbimycin, and genistein) was able to influence neuronal survival induced by ouabain. The data presented suggest that ouabain may be the trigger of an intracellular pathway responsible for neuronal survival.

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.

Purinergic-induced signaling in C11-MDCK cells inhibits the secretory Na-K-Cl cotransporter

Purinergic inhibition of Na-K-Cl cotransport has been noted in various renal epithelial cells derived from the collecting tubule, including Madin-Darby canine kidney (MDCK) cells. In recent studies, we have observed purinergic inhibition of Na-K-Cl cotransport in C11-MDCK subclones (alpha-intercalated-like cells). Interestingly, Na-K-Cl cotransport activity was also detected in C7-MDCK subclones (principal-like cells) but was not affected by ATP. In this investigation, we have transfected the human Na-K-Cl cotransporter (huNKCC1) in both C11 and C7 cells to determine whether these differences in NKCC regulation by ATP were due to cell-specific purinoceptor signaling pathways or to cell-specific isoforms/splice variants of the transporter. In both cell lines, we found that endogenous as well as huNKCC1-derived cotransport activity was restricted to the basolateral side. In addition, we were able to show that extracellular application of 100 microM ATP or 100 microM UTP abolished NKCC activity in both mock- and huNKCC1-transfected C11 cells but not in mock- and huNKCC1-transfected C7 cells; in C11 cells, intriguingly, this inhibition was not affected by inhibitors of RNA and protein synthesis and occurred even though expression levels of UTP-sensitive P2Y2-, P2Y4-, and P2Y6-purinoceptors were not different from those observed in C7 cells. These results suggest that C11 cells express an undetermined type of UTP-sensitive P2-purinoceptors or a unique P2Y-purinoceptor-triggered signaling cascade that leads to inhibition of NKCC1.

Na+, K+-ATPase: the new face of an old player in pathogenesis and apoptotic/hybrid cell death

The Na(+), K(+)-ATPase is a ubiquitous membrane transport protein in mammalian cells, responsible for establishing and maintaining high K(+) and low Na(+) in the cytoplasm required for normal resting membrane potentials and various cellular activities. The ionic homeostasis maintained by the Na(+), K(+)-ATPase is also critical for cell growth, differentiation, and cell survival. Although the toxic effects of blocking the Na(+), K(+)-ATPase by ouabain and other selective inhibitors have been known for years, the mechanism of action remained unclear. Recent progress in two areas has significantly advanced our understanding of the role and mechanism of Na(+), K(+)-ATPase in cell death. Along with increased recognition of apoptosis in a wide range of disease states, Na(+), K(+)-ATPase deficiency has been identified as a contributor to apoptosis and pathogenesis. More importantly, accumulating evidence now endorses a close relationship between ionic homeostasis and apoptosis, namely the regulation of apoptosis by K(+) homeostasis. Since Na(+), K(+)-ATPase is the primary system for K(+) uptake, dysfunction of the transport enzyme and resultant disruption of ionic homeostasis have been re-evaluated for their critical roles in apoptosis and apoptosis-related diseases. In this review, instead of giving a detailed description of the structure and regulation of Na(+), K(+)-ATPase, the author will focus on the most recent evidence indicating the unique role of Na(+), K(+)-ATPase in cell death, including apoptosis and the newly recognized "hybrid death" of concurrent apoptosis and necrosis in the same cells. It is also hoped that discussion of some seemingly conflicting reports will inspire further debate and benefit future investigation in this important research field.

Mechanisms of ouabain toxicity

The suggested involvement of ouabain in hypertension raised the need for a better understanding of its cellular action, but the mechanisms of ouabain toxicity are only now being uncovered. In the present study, we show that reduced glutathione (GSH) protected ouabain-sensitive (OS) cells from ouabain-induced toxicity and that the inhibition of GSH synthesis by D, L-buthionine-(S,R)-sulfoximine (BSO) sensitized ouabain-resistant (OR) cells. We could not observe formation of *OH or H2O2, but there was an increase in O2*-only in OS cells. Unexpectedly, an increased number of OR cells depolarized after treatment with ouabain, and BSO blocked this depolarization. Moreover, GSH increased ouabain-induced depolarization in OS cells. A sustained increase in tyrosine phosphorylation (P-Tyr) and Ras expression was observed after treatment of OS cells, and GSH prevented it. Conversely, BSO induced P-Tyr and Ras expression in ouabain-treated OR cells. The results obtained have three major implications: There is no direct correlation between membrane depolarization and ouabain-induced cell death; ouabain toxicity is not directly related to its classical action as a Na+, K+-ATPase inhibitor but seems to be associated to signal transduction, and GSH plays a major role in preventing ouabain-induced cell death.

Effect of ouabain on CFTR gene expression in human Calu-3 cells

We have previously shown that ouabain, which changes the electrochemical properties of cell membranes by inhibiting Na(+),K(+)-ATPase, induces the expression of multidrug resistance (MDR-1) gene in several human cell lines. Because the expressions of the MDR-1 and CFTR (which encodes the cAMP-activated Cl(-) channel associated with cystic fibrosis) genes are physiologically regulated in opposing directions, we wanted to determine whether ouabain also decreases CFTR transcripts and subsequently to analyze its mechanism of action. We found that the submicromolar concentrations of ouabain that increase MDR-1 mRNAs decrease the CFTR transcripts with analogous time-dependency in human pulmonary Calu-3 cells. By altering or reproducing the ouabain-induced changes in intracellular ionic activities (decreasing in external Na(+) or K(+) or using Na(+) ionophore), we show that the ouabain-induced regulations of both CFTR and MDR-1 transcripts depend on the Na(+)/K(+) pump inhibition but that the decrease in CFTR mRNAs also proceeds from cytoplasm reactions simultaneously activated by ouabain. These data, which emphasize the complex mechanism of action of ouabain, suggest that changes in intracellular ionic activities modulate CFTR/MDR-1 gene expressions.

Inhibition of Na+,K+-ATPase by ouabain triggers epithelial cell death independently of inversion of the [Na+]i/[K+]i ratio

Treatment with ouabain led to massive death of principal cells from collecting ducts (C7-MDCK), indicated by cell swelling, loss of mitochondrial function, an irregular pattern of DNA degradation, and insensitivity to pan-caspase inhibitor. Equimolar substitution of extracellular Na(+) by K(+) or choline(+) sharply attenuated the effect of ouabain on intracellular Na(+) and K(+) content but did not protect the cells from death in the presence of ouabain. In contrast to ouabain, inhibition of the Na(+)/K(+) pump in K(+)-free medium increased Na(+)(i) content but did not affect cell survival. In control and K(+)-free medium, ouabain triggered half-maximal cell death at concentrations of approximately 0.5 and 0.05 microM, respectively, which was consistent with elevation of Na(+)/K(+) pump sensitivity to ouabain in K(+)-depleted medium. Our results show for the first time that the death of ouabain-treated renal epithelial cells is independent of the inhibition of Na(+)/K(+) pump-mediated ion fluxes and the [Na(+)](i)]/[K(+)](i) ratio.

Plasma membrane potential interferes with the respiratory burst of peripheral granulocytes

Membrane potential is involved in the regulation of several immune functions developed by granulocytes. The Na(+)/K(+) gradient across the plasma membrane, mainly generated by the Na(+)/K(+) pump, plays a key role in the maintenance of membrane potential. This study is focused on the correlation between plasma membrane potential and the in vitro receptor - triggered respiratory burst of normal human peripheral granulocytes. The respiratory burst was measured as superoxide anion release by the cytochrome c reduction test and plasma membrane potential was modulated by experimental changes of the extracellular potassium concentration. Results show a differentiated cellular response, depending on the in vivo activation state and on the signals received in vitro by granulocytes via CR3 or FcgammaR. Alteration of the membrane potassium gradient modulates the respiratory burst of unstimulated and CR3-activated cells, whilst it does not seem to significantly interfere with the signals delivered by FcgammaR.

c-Fos expression in ouabain-treated vascular smooth muscle cells from rat aorta: evidence for an intracellular-sodium-mediated, calcium-independent mechanism

In this study, we examined the effect of Na(+)-K(+) pump inhibition on the expression of early response genes in vascular smooth muscle cells (VSMC) as possible intermediates of the massive RNA synthesis and protection against apoptosis seen in ouabain-treated VSMC in our previous experiments. Incubation of VSMC with ouabain resulted in rapid induction of c-Fos protein expression with an approximately sixfold elevation after 2 h of incubation. c-Jun expression was increased by approximately fourfold after 12 h, whereas expression of activating transcription factor 2, cAMP/Ca(2+) response element binding protein (CREB)-1 and c-Myc was not altered. Markedly augmented c-Fos expression was also observed under Na(+)-K(+) pump inhibition in potassium-depleted medium. Na(+)-K(+) pump inhibition triggered c-Fos expression via elevation of the [Na(+)](i)/[K(+)](i) ratio. This conclusion follows from experiments showing the lack of effect of ouabain on c-Fos expression in high-potassium-low-sodium medium and from the comparison of dose responses of Na(+)-K(+) pump activity, [Na(+)](i) and [K(+)](i) content and c-Fos expression to ouabain. A fourfold increment of c-Fos mRNA was revealed 30 min following addition of ouabain to the incubation medium. At this time point, treatment with ouabain resulted in an approximately fourfold elevation of [Na(+)](i) but did not affect [K(+)](i). Augmented c-Fos expression was also observed under VSMC depolarization in high-potassium medium. Increments in both c-Fos expression and (45)Ca uptake in depolarized VSMC were abolished under inhibition of L-type Ca(2+) channels with 0.1 microM nicardipine. Ouabain did not affect the free [Ca(2+)](i) or the content of exchangeable [Ca(2+)](i). Ouabain-induced c-Fos expression was also insensitive to the presence of nicardipine and [Ca(2+)](o), as well as chelators of [Ca(2+)](o) (EGTA) and [Ca(2+)](i) (BAPTA). The effect of ouabain and serum on c-Fos expression was additive. In contrast to serum, however, ouabain failed to activate the Elk-1, serum response factor, CREB and activator protein-1 transcription factors identified within the c-Fos promoter. These results suggest that Na(+)-K(+) pump inhibition triggers c-Fos expression via [Na(+)](i)-sensitive [Ca(2+)](i)-independent transcription factor(s) distinct from factors interacting with known response elements of this gene promoter.

Ionic mechanism of ouabain-induced concurrent apoptosis and necrosis in individual cultured cortical neurons

Energy deficiency and dysfunction of the Na+, K+-ATPase are common consequences of many pathological insults. The nature and mechanism of cell injury induced by impaired Na+, K+-ATPase, however, are not well defined. We used cultured cortical neurons to examine the hypothesis that blocking the Na+, K+-ATPase induces apoptosis by depleting cellular K+ and, concurrently, induces necrotic injury in the same cells by increasing intracellular Ca2+ and Na+. The Na+, K+-ATPase inhibitor ouabain induced concentration-dependent neuronal death. Ouabain triggered transient neuronal cell swelling followed by cell shrinkage, accompanied by intracellular Ca2+ and Na+ increase, K+ decrease, cytochrome c release, caspase-3 activation, and DNA laddering. Electron microscopy revealed the coexistence of ultrastructural features of both apoptosis and necrosis in individual cells. The caspase inhibitor Z-Val-Ala-Asp(OMe)-fluoromethyl ketone (Z-VAD-FMK) blocked >50% of ouabain-induced neuronal death. Potassium channel blockers or high K+ medium, but not Ca2+ channel blockade, prevented cytochrome c release, caspase activation, and DNA damage. Blocking of K+, Ca2+, or Na+ channels or high K+ medium each attenuated the ouabain-induced cell death; combined inhibition of K+ channels and Ca2+ or Na+ channels resulted in additional protection. Moreover, coapplication of Z-VAD-FMK and nifedipine produced virtually complete neuroprotection. These results suggest that the neuronal death associated with Na+, K+-pump failure consists of concurrent apoptotic and necrotic components, mediated by intracellular depletion of K+ and accumulation of Ca2+ and Na+, respectively. The ouabain-induced hybrid death may represent a distinct form of cell death related to the brain injury of inadequate energy supply and disrupted ion homeostasis.

Necrotic volume increase and the early physiology of necrosis

Whether a lethally injured mammalian cell undergoes necrosis or apoptosis may be determined by the early activation of specific ion channels at the cell surface. Apoptosis requires K+ and Cl- efflux, which leads to cell shrinking, an active phenomenon termed apoptotic volume decrease (AVD). In contrast, necrosis has been shown to require Na+ influx through membrane carriers and more recently through stress-activated non-selective cation channels (NSCCs). These ubiquitous channels are kept dormant in viable cells but become activated upon exposure to free-radicals. The ensuing Na+ influx leads to cell swelling, an active response that may be termed necrotic volume increase (NVI). This review focuses on how AVD and NVI become conflicting forces at the beginning of cell injury, on the events that determine irreversibility and in particular, on the ion fluxes that decide whether a cell is to die by necrosis or by apoptosis.

Inhibition of Na+,K+ pump affects nucleic acid synthesis and smooth muscle cell proliferation via elevation of the [Na+]i/[K+]i ratio: possible implication in vascular remodelling

OBJECTIVES

Na+,K+ pump inhibition is known to delay the development of apoptosis in vascular smooth muscle cells (VSMC). This study examines Na+,K+ pump involvement in the regulation of VSMC macromolecular synthesis and proliferation.

METHODS

DNA, RNA and protein synthesis in VSMC from the rat aorta was studied by the incorporation of [3H]-labelled thymidine, uridine and leucine. Cell cycle progression was estimated by flow cytometry. Intracellular Na+ and K+ content and Na+,K+ pump activity were quantified as the steady-state distribution of 22Na and 86Rb and the rate of ouabain-sensitive 86Rb uptake in Na+-loaded cells, respectively.

RESULTS

Ouabain inhibited the Na+,K+ pump with a Ki of 0.1 mmol/l. At concentrations less than 0.1 mmol/l, neither [Na+]i nor [K+]i was affected by ouabain; elevation of ouabain concentration sharply increased the [Na+]i/[K+]i ratio with a K0.5 of approximately 0.3 mmol/l. At concentrations higher than 0.1 mmol/l, ouabain time- and dose-dependently activated RNA and DNA syntheses in serum-deprived VSMC and inhibited cell cycle progression triggered by serum. In quiescent VSMC, ouabain did not affect protein synthesis, total cell number, but slightly increased the percentage of cells in the S-phase (4.25 versus 1.46%) and attenuated cell death assessed by staining with trypan blue and lactate dehydrogenase release.

CONCLUSIONS

Elevation of the [Na+]i/[K+]i ratio caused by Na+,K+ pump inhibition markedly enhances nucleic acid synthesis in quiescent VSMC and blocks cell cycle progression in serum-supplied VSMC. The relative contribution of this phenomenon as well as the anti-apoptotic action of increased [Na+]i/[K+]i ratio to vascular remodelling under augmented content of endogenous Na+,K+ pump inhibitors, seen in volume-expanded hypertension, should be investigated by in-vivo studies.

Decreased intracellular potassium levels underlie increased progesterone synthesis during ovarian follicular atresia

More than 99% of ovarian follicles are lost by a degenerative process known as atresia, a phenomenon characterized by apoptosis of granulosa cells. Uniquely, dying granulosa cells also greatly increase their progesterone biosynthesis while reducing estrogen production. Recent studies have documented a dramatic decrease in intracellular K+ concentration during apoptosis that plays an important role in regulating apoptotic enzymes. However, it is unclear whether this ionic change affects related processes such as the change in steroidogenesis in dying granulosa cells. To explore this question, granulosa cells were cultured in hypotonic medium, which initially swells the cells. The cells respond by extruding K+, which we have documented by fluorescence spectrophotometry. The K+ efflux osmotically draws water out the cell, returning it to a near normal volume (as measured by flow cytometry). The result is a cell of normal size with a decreased intracellular K+ concentration. FSH stimulation of these cells caused an increase in progesterone biosynthesis. This response was enhanced at higher doses of FSH, although basal progesterone production was not affected, suggesting that K+ levels may affect the gonadotropin-signaling pathway. No increase in steroidogenic acute regulatory or cholesterol side-chain cleavage cytochrome P450 mRNA was detected, although cAMP production was enhanced. These results suggest that the loss of intracellular K+ by apoptotic granulosa cells greatly facilitates FSH-stimulated progesterone production.