Regulation of a c-Jun amino-terminal kinase/stress-activated protein kinase cascade by a sodium-dependent signal transduction pathway

Ouabain Suppresses IL-6/STAT3 Signaling and Promotes Cytokine Secretion in Cultured Skeletal Muscle Cells

The cardiotonic steroids (CTS), such as ouabain and marinobufagenin, are thought to be adrenocortical hormones secreted during exercise and the stress response. The catalytic α-subunit of Na,K-ATPase (NKA) is a CTS receptor, whose largest pool is located in skeletal muscles, indicating that muscles are a major target for CTS. Skeletal muscles contribute to adaptations to exercise by secreting interleukin-6 (IL-6) and plethora of other cytokines, which exert paracrine and endocrine effects in muscles and non-muscle tissues. Here, we determined whether ouabain, a prototypical CTS, modulates IL-6 signaling and secretion in the cultured human skeletal muscle cells. Ouabain (2.5–50 nM) suppressed the abundance of STAT3, a key transcription factor downstream of the IL-6 receptor, as well as its basal and IL-6-stimulated phosphorylation. Conversely, ouabain (50 nM) increased the phosphorylation of ERK1/2, Akt, p70S6K, and S6 ribosomal protein, indicating activation of the ERK1/2 and the Akt-mTOR pathways. Proteasome inhibitor MG-132 blocked the ouabain-induced suppression of the total STAT3, but did not prevent the dephosphorylation of STAT3. Ouabain (50 nM) suppressed hypoxia-inducible factor-1α (HIF-1α), a modulator of STAT3 signaling, but gene silencing of HIF-1α and/or its partner protein HIF-1β did not mimic effects of ouabain on the phosphorylation of STAT3. Ouabain (50 nM) failed to suppress the phosphorylation of STAT3 and HIF-1α in rat L6 skeletal muscle cells, which express the ouabain-resistant α1-subunit of NKA. We also found that ouabain (100 nM) promoted the secretion of IL-6, IL-8, GM-CSF, and TNF-α from the skeletal muscle cells of healthy subjects, and the secretion of GM-CSF from cells of subjects with the type 2 diabetes. Marinobufagenin (10 nM), another important CTS, did not alter the secretion of these cytokines. In conclusion, our study shows that ouabain suppresses the IL-6 signaling via STAT3, but promotes the secretion of IL-6 and other cytokines, which might represent a negative feedback in the IL-6/STAT3 pathway. Collectively, our results implicate a role for CTS and NKA in regulation of the IL-6 signaling and secretion in skeletal muscle.

Mechanical Strain-Mediated Tenogenic Differentiation of Mesenchymal Stromal Cells Is Regulated through Epithelial Sodium Channels

It has been suggested that mechanical strain may elicit cell differentiation in adult somatic cells through activation of epithelial sodium channels (ENaC). However, such phenomenon has not been previously demonstrated in mesenchymal stromal cells (MSCs). The present study was thus conducted to investigate the role of ENaC in human bone marrow-derived MSCs (hMSCs) tenogenic differentiation during uniaxial tensile loading. Passaged-2 hMSCs were seeded onto silicone chambers coated with collagen I and subjected to stretching at 1 Hz frequency and 8% strain for 6, 24, 48, and 72 hours. Analyses at these time points included cell morphology and alignment observation, immunocytochemistry and immunofluorescence staining (collagen I, collagen III, fibronectin, and N-cadherin), and gene expression (ENaC subunits, and tenogenic markers). Unstrained cells at similar time points served as the control group. To demonstrate the involvement of ENaC in the differentiation process, an ENaC blocker (benzamil) was used and the results were compared to the noninhibited hMSCs. ENaC subunits' (α, β, γ, and δ) expression was observed in hMSCs, although only α subunit was significantly increased during stretching. An increase in tenogenic genes' (collagen1, collagen3, decorin, tenascin-c, scleraxis, and tenomodulin) and proteins' (collagen I, collagen III, fibronectin, and N-cadherin) expression suggests that hMSCs underwent tenogenic differentiation when subjected to uniaxial loading. Inhibition of ENaC function resulted in decreased expression of these markers, thereby suggesting that ENaC plays a vital role in tenogenic differentiation of hMSCs during mechanical loading.

Cardiac glycosides with target at direct and indirect interactions with nuclear receptors

Cardiac glycosides are compounds isolated from plants and animals and have been known since ancient times. These compounds inhibit the activity of the sodium potassium pump in eukaryotic cells. Cardiac glycosides were used as drugs in heart ailments to increase myocardial contraction force and, at the same time, to lower frequency of this contraction. An increasing number of studies have indicated that the biological effects of these compounds are not limited to inhibition of sodium-potassium pump activity. Furthermore, an increasing number of data have shown that they are synthesized in tissues of mammals, where they may act as a new class of steroid hormones or other hormones by mimicry to modulate various signaling pathways and influence whole organisms. Thus, we discuss the interactions of cardiac glycosides with the nuclear receptor superfamily of transcription factors activated by low-weight molecular ligands (including hormones) that regulate many functions of cells and organisms. Cardiac glycosides of endogenous and exogenous origin by interacting with nuclear receptors can affect the processes regulated by these transcription factors, including hormonal management, immune system, body defense, and carcinogenesis. They can also be treated as initial structures for combinatorial chemistry to produce new compounds (including drugs) with the desired properties.

The Na/K-ATPase Signaling: From Specific Ligands to General Reactive Oxygen Species

The signaling function of the Na/K-ATPase has been established for 20 years and is widely accepted in the field, with many excellent reports and reviews not cited here. Even though there is debate about the underlying mechanism, the signaling function is unquestioned. This short review looks back at the evolution of Na/K-ATPase signaling, from stimulation by cardiotonic steroids (also known as digitalis-like substances) as specific ligands to stimulation by reactive oxygen species (ROS) in general. The interplay of cardiotonic steroids and ROS in Na/K-ATPase signaling forms a positive-feedback oxidant amplification loop that has been implicated in some pathophysiological conditions.

Palytoxin-Containing Aquarium Soft Corals as an Emerging Sanitary Problem

Palytoxin (PLTX), one the most potent marine toxins, and/or its analogs, have been identified in different marine organisms, such as Palythoa soft corals, Ostreopsis dinoflagellates, and Trichodesmium cyanobacteria. Although the main concern for human health is PLTXs entrance in the human food chain, there is growing evidence of adverse effects associated with inhalational, cutaneous, and/or ocular exposure to aquarium soft corals contaminated by PLTXs or aquaria waters. Indeed, the number of case reports describing human poisonings after handling these cnidarians is continuously increasing. In general, the signs and symptoms involve mainly the respiratory (rhinorrhea and coughing), skeletomuscular (myalgia, weakness, spasms), cardiovascular (electrocardiogram alterations), gastrointestinal (nausea), and nervous (paresthesia, ataxia, tremors) systems or apparates. The widespread phenomenon, the entity of the signs and symptoms of poisoning and the lack of control in the trade of corals as aquaria decorative elements led to consider these poisonings an emerging sanitary problem. This review summarizes literature data on human poisonings due to, or ascribed to, PLTX-containing soft corals, focusing on the different PLTX congeners identified in these organisms and their toxic potential.

Palytoxin induces dissociation of HSP 27 oligomers through a p38 protein kinase pathway

Palytoxin (PlTX) induces a stress response in MCF-7 cells that involves the phosphorylation of HSP 27 at serines 15, 78, and 82 by an as yet undetermined mechanism. We have studied the involvement of major groups of the mitogen-activated protein kinase (MAPK) family in this molecular response and focused our analyses on the ERK1/2, JNK, p38 protein kinase (p38K), and ERK5 pathways. The results show that PlTX induces the activation of JNK and p38 kinase but not ERK1/2 and 5 in MCF-7 cells. Through the use of protein kinase inhibitors, we established that blocking p38K, but not JNK, prevents the phosphorylation of HSP 27 induced by PlTX and that MAPKAPK2 participates in the response induced by the toxin under our experimental conditions. The cell death response induced by PlTX was inhibited by preventing JNK phosphorylation but not by blocking p38K/MAPKAPK2 and HSP 27 phosphorylation. Sucrose density gradient centrifugation revealed that MCF-7 cell extracts contain a heterodisperse population of HSP 27, including oligomers and smaller forms. Treating MCF-7 cells with PlTX caused the dissociation of HSP 27 oligomers, and using inhibitors of the JNK and p38K pathways showed that the dissociation of HSP 27 oligomers induced by PlTX involves a p38K-dependent process. We conclude that the changes induced by PlTX in the HSP 27 stress response protein system proceed through a molecular mechanism involving the activation of the p38 kinase pathway and its substrate, MAPKAK2, leading to dissociation of HSP 27 oligomers and the stabilization of a cellular pool of monomers phosphorylated at serines 15, 78 and 82, which could play a protective role against the death response induced by PlTX.

Head and neck cancer cells and xenografts are very sensitive to palytoxin: decrease of c-jun n-terminale kinase-3 expression enhances palytoxin toxicity

Objectives

Palytoxin (PTX), a marine toxin isolated from the Cnidaria (zooanthid) Palythoa caribaeorum is one of the most potent non-protein substances known. It is a very complex molecule that presents both lipophilic and hydrophilic areas. The effect of PTX was investigated in a series of experiments conducted in head and neck squamous cell carcinoma (HNSCC) cell lines and xenografts.

Materials and methods

Cell viability, and gene expression of the sodium/potassium-transporting ATPase subumit alpha1 (ATP1AL1) and GAPDH were analyzed in HNSCC cells and normal epithelial cells after treatment with PTX using cytotoxicity-, clonogenic-, and enzyme inhibitor assays as well as RT-PCR and Northern Blotting. For xenograft experiments severe combined immunodeficient (SCID) mice were used to analyze tumor regression. The data were statistically analyzed using One-Way Annova (SPSS vs20).

Results

Significant toxic effects were observed in tumor cells treated with PTX (LD50 of 1.5 to 3.5 ng/ml) in contrast to normal cells. In tumor cells PTX affected both the release of LDH and the expression of the sodium/potassium-transporting ATPase subunit alpha1 gene suggesting loss of cellular integrity, primarily of the plasma membrane. Furthermore, strong repression of the c-Jun N-terminal kinase 3 (JNK3) mRNA expression was found in carcinoma cells which correlated with enhanced toxicity of PTX suggesting an essential role of the mitogen activated protein kinase (MAPK)/JNK signalling cascades pathway in the mechanisms of HNSCC cell resistance to PTX. In mice inoculated with carcinoma cells, injections of PTX into the xenografted tumors resulted within 24 days in extensive tumor destruction in 75% of the treated animals (LD50 of 68 ng/kg to 83 ng/kg) while no tumor regression occurred in control animals.

Conclusions

These results clearly provide evidence that PTX possesses preferential toxicity for head and neck carcinoma cells and therefore it is worth further studying its impact which may extend our knowledge of the biology of head and neck cancer.

Modulation of protein kinase signaling cascades by palytoxin

Although known for its acutely toxic action, palytoxin has also been identified as a type of carcinogenic agent called a tumor promoter. In general tumor promoters do not damage DNA, but instead contribute to carcinogenesis by disrupting the regulation of cellular signaling. The identification of palytoxin as a tumor promoter, together with the recognition that the Na(+), K(+)-ATPase is its receptor, led to research on how palytoxin triggers the modulation of signal transduction pathways. This review focuses on mitogen activated protein (MAP) kinases as mediators of palytoxin-stimulated signaling. MAP kinases are a family of serine/threonine kinases that relay a variety of signals to the cellular machinery that regulates cell fate and function. The studies discussed in this review investigated how palytoxin stimulates MAP kinase activity and, in turn, how MAP kinases mediate the response of cells to palytoxin.

Na+/K+-ATPase inhibitor palytoxin enhances vulnerability of cultured cerebellar neurons to domoic acid via sodium-dependent mechanisms

Dysfunction or deficiency of the Na(+)/K(+)-ATPase appears to be a common event in a variety of pathological conditions in the central nervous system. Studies on neurotoxicity associated to impaired Na(+)/K(+)-ATPase activity have focused on NMDA receptors, while the involvement of non-NMDA receptors has been much less explored. We show that mild, non-toxic, exposures to the Na(+)/K(+)-ATPase inhibitor palytoxin (PTX) synergistically sensitized the vulnerability of neurons to normally non-toxic concentrations of domoic acid, leaving NMDA receptor-mediated excitotoxic response unaltered. Enhancement of excitotoxicity required at least 1 h pre-exposure to PTX, was not observed after longer exposures to PTX, and did not require RNA synthesis. PTX caused a voltage-sensitive Na(+) channel-independent increase in intracellular Na(+). Both intracellular Na(+) increase and potentiation of excitotoxicity depended upon the external concentrations of Na(+) and Cl(-), and were suppressed by the anion exchanger blocker 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid in a dose-dependent manner. Other stilbene derivatives, chloride channel antagonists or Na(+) cotransporter inhibitors proved ineffective. Our results demonstrate a crucial role for Na(+)/K(+)-ATPase activity in determining neuronal vulnerability to domoic acid-mediated excitotoxicity. They also raise reasonable concern about possible risks for human health associated to the ingestion of low amounts of phycotoxins PTX and domoic acid in food.

Extracellular signal regulated kinase 5 mediates signals triggered by the novel tumor promoter palytoxin

Palytoxin is classified as a non-12-O-tetradecanoylphorbol-13-acetate (TPA)-type skin tumor because it does not bind to or activate protein kinase C. Palytoxin is thus a novel tool for investigating alternative signaling pathways that may affect carcinogenesis. We previously showed that palytoxin activates three major members of the mitogen activated protein kinase (MAPK) family, extracellular signal regulated kinase 1 and 2 (ERK1/2), c-Jun N-terminal kinase (JNK), and p38. Here we report that palytoxin also activates another MAPK family member, called ERK5, in HeLa cells and in keratinocytes derived from initiated mouse skin (308 cells). By contrast, TPA does not activate ERK5 in these cell lines. The major cell surface receptor for palytoxin is the Na+,K+-ATPase. Accordingly, ouabain blocked the ability of palytoxin to activate ERK5. Ouabain alone did not activate ERK5. ERK5 thus represents a divergence in the signaling pathways activated by these two agents that bind to the Na+,K+-ATPase. Cycloheximide, okadaic acid, and sodium orthovanadate did not mimic the effect of palytoxin on ERK5. These results indicate that the stimulation of ERK5 by palytoxin is not simply due to inhibition of protein synthesis or inhibition of serine/threonine or tyrosine phosphatases. Therefore, the mechanism by which palytoxin activates ERK5 differs from that by which it activates ERK1/2, JNK, and p38. Finally, studies that used pharmacological inhibitors and shRNA to block ERK5 action indicate that ERK5 contributes to palytoxin-stimulated c-Fos gene expression. These results suggest that ERK5 can act as an alternative mediator for transmitting diverse tumor promoter-stimulated signals.

Carcinogenic aspects of protein phosphatase 1 and 2A inhibitors

Okadaic acid is functionally a potent tumor promoter working through inhibition of protein phosphatases 1 and 2A (PP1 and PP2A), resulting in sustained phosphorylation of proteins in cells. The mechanism of tumor promotion with okadaic acid is thus completely different from that of the classic tumor promoter phorbol ester. Other potent inhibitors of PP1 and PP2A - such as dinophysistoxin-1, calyculins A-H, microcystin-LR and its derivatives, and nodularin - were isolated from marine organisms, and their structural features including the crystal structure of the PP1-inhibitor complex, tumor promoting activities, and biochemical and biological effects, are here reviewed. The compounds induced tumor promoting activity in three different organs, including mouse skin, rat glandular stomach and rat liver, initiated with three different carcinogens. The results indicate that inhibition of PP1 and PP2A is a general mechanism of tumor promotion applicable to various organs. This study supports the concept of endogenous tumor promoters in human cancer development.

A Chemical Screen Identifies Anisomycin as an Anoikis Sensitizer That Functions by Decreasing FLIP Protein Synthesis

Malignant epithelial cells with metastatic potential resist apoptosis that normally occurs upon loss of anchorage from the extracellular matrix, a process termed "anoikis." Resistance to anoikis enables malignant cells to survive in an anchorage-independent manner, which leads to the formation of distant metastases. To understand the regulation of anoikis, we designed, automated, and conducted a high-throughput chemical screen for anoikis sensitizers. PPC-1 anoikis-resistant prostate cancer cells were seeded in hydrogel-coated ultralow binding plates for suspension conditions and standard tissue culture plates to promote adhesion. After seeding, cells were treated with aliquots from a library of previously characterized small molecules, and viability was assessed using the 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt, assay. From this chemical screen, we identified anisomycin that induced apoptosis in suspension conditions, but was not toxic to these cells grown under adherent conditions. Anisomycin sensitized cells to anoikis by decreasing levels of the caspase-8 inhibitor FLIP and subsequently activating the death receptor pathway of caspase activation. Although anisomycin activated c-Jun-NH(2)-kinase and p38, these kinases were not functionally important for the effect of anisomycin on anoikis and FLIP. Rather, anisomycin decreased FLIP and sensitized cells to anoikis by inhibiting its protein synthesis. Finally, we showed that anisomycin decreased distal tumor formation in a mouse model of prostate cancer metastases. Thus, a novel chemical screen identified anisomycin as an anoikis sensitizer that acts by decreasing FLIP protein synthesis. Our results suggest that FLIP is a suppressor of anoikis and inhibiting FLIP protein synthesis may be a useful antimetastatic strategy.

Mitogen-activated protein kinases regulate palytoxin-induced calcium influx and cytotoxicity in cultured neurons

BACKGROUND AND PURPOSE

Palytoxin (PLT) is a potent toxin that binds to the Na,K-ATPase. Palytoxin is highly neurotoxic and increases the cytosolic calcium concentration ([Ca(2+)](c)) while decreasing intracellular pH (pH(i)) in neurons (Vale et al., 2006; Vale-Gonzalez et al., 2007). It is also a tumour promoter that activates several protein kinases.

EXPERIMENTAL APPROACH

The role of different protein kinases in the effects of palytoxin on [Ca(2+)](c), pH(i) and cytoxicity was investigated in cultured neurons.

KEY RESULTS

Palytoxin-induced calcium load was not affected by inhibition of calcium-dependent protein kinase C (PKC) isoforms but it was partially ameliorated by blockade of calcium-independent PKC isozymes. Inhibition of the extracellular signal-regulated kinase (ERK) 2 eliminated the palytoxin-induced rise in calcium and intracellular acidification, whereas inhibition of MEK greatly attenuated the palytoxin effect on calcium without modifying the PLT-evoked intracellular acidification. Blockade of c-Jun N-terminal protein kinases (JNK) somewhat decreased the palytoxin-effect on calcium, whereas inhibition of the p38 mitogen activated protein kinases (MAPKs) delayed the onset of the palytoxin-evoked rise in calcium and acidification. Furthermore, the cytotoxicity of palytoxin was completely blocked by inhibition of ERK 2 and partially prevented by inhibition of MEK. PLT increased phosphorylated ERK immunoreactivity in a concentration-dependent manner.

CONCLUSIONS AND IMPLICATIONS

MAPKs, specifically ERK 2, link palytoxin cytotoxicity with its effects on calcium homeostasis after inhibition of the Na,K-ATPase. Binding of palytoxin to the Na,K-ATPase would alter signal transduction pathways, even in non-dividing cells, and this finding is related to the potent neurotoxicity of this marine toxin.

Regulation of mitogen-activated protein kinase pathways by the plasma membrane Na+/H+ exchanger, NHE1

The mitogen-activated protein kinases (MAPKs), including extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, play a major role in the regulation of pivotal cellular processes such as cell death/survival balance, cell cycle progression, and cell migration. MAPK activity is regulated by a three-tiered phosphorelay system, which is in turn regulated by a complex network of signaling events and scaffolding proteins. The ubiquitous plasma membrane Na(+)/H(+) exchanger NHE1 is activated by, and implicated in, the physiological/pathophysiological responses to many of the same stimuli that modulate MAPK activity. While under some conditions, NHE1 is regulated by MAPKs, a number of studies have, conversely, implicated NHE1 in the regulation of MAPK activity. Here, we discuss the current evidence indicating the involvement of NHE1 in MAPK regulation, the mechanisms by which this may occur, and the possible physiological and pathophysiological relevance of this phenomenon.

Palytoxin: exploiting a novel skin tumor promoter to explore signal transduction and carcinogenesis

Palytoxin is a novel skin tumor promoter, which has been used to help probe the role of different types of signaling mechanisms in carcinogenesis. The multistage mouse skin model indicates that tumor promotion is an early, prolonged, and reversible phase of carcinogenesis. Understanding the molecular mechanisms underlying tumor promotion is therefore important for developing strategies to prevent and treat cancer. Naturally occurring tumor promoters that bind to specific cellular receptors have proven to be useful tools for investigating important biochemical events in multistage carcinogenesis. For example, the identification of protein kinase C as the receptor for the prototypical skin tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) (also called phorbol 12-myristate 13-acetate, PMA) provided key evidence that tumor promotion involves the aberrant modulation of signaling cascades that govern cell fate and function. The subsequent discovery that palytoxin, a marine toxin isolated from zoanthids (genus Palythoa), is a potent skin tumor promoter yet does not activate protein kinase C indicated that investigating palytoxin action could help reveal new aspects of tumor promotion. Interestingly, the putative receptor for palytoxin is the Na(+),K(+)-ATPase. This review focuses on palytoxin-stimulated signaling and how palytoxin has been used to investigate alternate biochemical mechanisms by which important targets in carcinogenesis can be modulated.

Mitogen-activated Protein Kinase Phosphatase-3 Is a Tumor Promoter Target in Initiated Cells That Express Oncogenic Ras

We have capitalized on the unique properties of the skin tumor promoter palytoxin, which does not activate protein kinase C, to investigate alternative mechanisms by which major signaling molecules can be modulated during carcinogenesis. We report here that palytoxin activates extracellular signal-regulated kinase (ERK) through a novel mechanism that involves inactivation of an ERK phosphatase in keratinocytes derived from initiated mouse skin (308 cells). Use of U0126 revealed that palytoxin requires the ERK kinase MEK to stimulate ERK activity, although palytoxin did not activate MEK. We found that 308 keratinocytes highly express mitogen-activated protein kinase phosphatase-3 (MKP-3), which selectively inactivates ERK. Palytoxin induced the loss of MKP-3 in a manner that corresponded to increased ERK phosphorylation. Complementary studies showed that sustained expression of exogenous MKP-3 inhibited palytoxin-stimulated ERK activation. As is characteristic of initiated keratinocytes, 308 cells express activated H-Ras. To investigate whether expression of oncogenic Ras is key to palytoxin-stimulated ERK activation, we determined how palytoxin affected ERK and MKP-3 in MCF10A human breast epithelial cells and in H-ras MCF10A cells, which stably express activated H-Ras. Palytoxin did not affect ERK activity in MCF10A cells, which had no detectable MKP-3. Like 308 cells, H-ras MCF10A cells highly express MKP-3. Strikingly, palytoxin stimulated ERK activity and induced a corresponding loss of MKP-3 in H-ras MCF10A cells. These studies indicate that in initiated cells palytoxin unleashes ERK activity by down-regulating MKP-3, an ERK inhibitor, and further suggest that MKP-3 may be a vulnerable target in cells that express oncogenic Ras.

The toxicity of H2O2 on the ionic homeostasis of airway epithelial cells in vitro

Oxygen species may be formed in the air spaces of the respiratory tract in response to environmental pollution such as particulate matter. The mechanisms and target molecules of these oxidants are still mainly unknown but may involve modifications of the ionic homeostasis in epithelial cells. Cytosolic concentrations of Ca2+ (Fura2) and Na+ (SBFI) and short-circuit current (Isc) were followed in primary cultures of human nasal epithelial cells and in the cell line 16HBE14o- after exposure to H2O2 or *OH (H2O2 + Fe2+). Cells were grown on glass coverslips for ionic imaging or on permeable snapwell inserts for Isc studies. Exposure of the apical as well as the basal side of the cultures to H2O2 or *OH induced a concentration-dependent transient increase in Isc which is due to a transient secretion of Cl-. Cai also increased transiently with approximately the same kinetics. The response was dependent on the release of calcium from intracellular stores. Nai on the contrary increased steadily over more than an hour. When the apical membrane was permeabilized with gramicidin, *OH inhibited the Na+ current (a measure of Na(+)-K(+)-ATPase activity in the baso-lateral membrane). The arrest of the pump was significant after 30 min exposure to oxidant. On the other hand no increase in the apical or baso-lateral sodium conductances could be detected. The progressive arrest of the Na+/K(+)-pump may contribute to the sustained elevation of Nai. This strong modification in the cellular ionic homeostasis may participate in the stress response of the respiratory epithelium through alterations in signal transduction pathways.

Involvement of sodium in early phosphatidylserine exposure and phospholipid scrambling induced by P2X7 purinoceptor activation in thymocytes

Extracellular ATP (ATP(ec)), a possible effector in thymocyte selection, induces thymocyte death via purinoceptor activation. We show that ATP(ec) induced cell death by apoptosis, rather than lysis, and early phosphatidylserine (PS) exposure and phospholipid scrambling in a limited thymocyte population (35-40%). PS externalization resulted from the activation of the cationic channel P2X7 (formerly P2Z) receptor and was triggered in all thymocyte subsets although to different proportions in each one. Phospholipid movement was dependent on ATP(ec)-induced Ca(2+) and/or Na(+) influx. At physiological external Na(+) concentration, without external Ca(2+), PS was exposed in all ATP(ec)-responsive cells. In contrast, without external Na(+), physiological external Ca(2+) concentration promoted a submaximal response. Altogether these data show that Na(+) influx plays a major role in the rapid PS exposure induced by P2X7 receptor activation in thymocytes.

Determination of onset of apoptosis in granulosa cells of the preovulatory follicles in the bonnet monkey (Macaca radiata): correlation with mitogen-activated protein kinase activities

During reproductive life, only a selected few ovarian follicles mature and ovulate, while the vast majority of follicles undergo a degenerative process called atresia. Recent studies have indicated that follicular atresia is mediated through apoptosis of follicular granulosa cells. The objectives of the present study were to determine the time of onset of apoptosis in granulosa cells of preovulatory follicles and to evaluate the consequences of gonadotropin withdrawal on mitogen-activated protein (MAP) kinase activities. Bonnet monkeys (Macaca radiata) undergoing controlled ovarian stimulation cycles were utilized for stimulation of multiple follicles, and granulosa cells were retrieved from preovulatory follicles at 24, 48, 72, and 96 h after stopping gonadotropin treatment. Serum and follicular fluid estradiol concentrations were highest at 24 h but declined precipitously (P < 0.05) to reach the lowest concentrations at 96 h; however, progesterone concentrations during this period did not increase, indicating the absence of luteinization. Quantitative analysis of genomic DNA by 3'-end labeling revealed the presence of low-molecular-weight fragments from 48 h onward, but by agarose gel electrophoresis, DNA laddering could be visualized only after 72 h. Messenger RNA expression for Bax, caspase-2, and caspase-3 increased with the onset of apoptosis. Immunoblot analysis of MAP kinases in lysates of granulosa cells (48-72 h) indicated increased (P < 0.05) levels of phosphorylated extracellular response kinase-1 and -2, Jun N-terminal kinase (JNK)-1 and -2, and p38. However, in vitro kinase assay data indicated that only phospho-JNK and -p38 activities were higher at 72 h compared to 24 h. These results demonstrate that granulosa cells of preovulatory follicles undergo apoptosis and that increased activities of phospho-JNK and -p38 are correlated with apoptosis in the primate.

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.

Na(+)/K(+)-ATPase as a signal transducer

Na(+)/K(+)-ATPase as an energy transducing ion pump has been studied extensively since its discovery in 1957. Although early findings suggested a role for Na(+)/K(+)-ATPase in regulation of cell growth and expression of various genes, only in recent years the mechanisms through which this plasma membrane enzyme communicates with the nucleus have been studied. This research, carried out mostly on cardiac myocytes, shows that in addition to pumping ions, Na(+)/K+-ATPase interacts with neighboring membrane proteins and organized cytosolic cascades of signaling proteins to send messages to the intracellular organelles. The signaling pathways that are rapidly elicited by the interaction of ouabain with Na(+)/K(+)-ATPase, and are independent of changes in intracellular Na(+) and K(+) concentrations, include activation of Src kinase, transactivation of the epidermal growth factor receptor by Src, activation of Ras and p42/44 mitogen-activated protein kinases, and increased generation of reactive oxygen species by mitochondria. In cardiac myocytes, the resulting downstream events include the induction of some early response proto-oncogenes, activation of the transcription factors, activator protein-1 and nuclear factor kappa-B, regulation of a number of cardiac growth-related genes, and stimulation of protein synthesis and myocyte hypertrophy. For these downstream events, the induced reactive oxygen species and rise in intracellular Ca(2+) are essential second messengers. In cells other than cardiac myocytes, the proximal pathways linked to Na(+)/K(+)-ATPase through protein-protein interactions are similar to those reported in myocytes, but the downstream events and consequences may be significantly different. The likely extracellular physiological stimuli for the signal transducing function of Na+/K+-ATPase are the endogenous ouabain-like hormones, and changes in extracellular K+ concentration.

Antitumor activities of a newly synthesized shikonin derivative, 2-hyim-DMNQ-S-33

2- or 6-(1-hydroxyiminoalkyl)-5,8-dimethoxy-1, 4-naphthoquinone(2- or 6-hyim-DMNQ) derived from the roots of Lithospermum erythrorhizon was synthesized for the evaluation of antitumor activities. Among those derivatives, 2-hyim-DMNQ-S33 was found to be a potent anticancer agent. This compound suppressed the proliferation of Radiation Induced Fibrosarcoma (RIF) cells in a dose-dependent manner. 2-hyim-DMNQ-S33 significantly prolonged the survival time by 239% as compared with Sarcoma 180 tumor-bearing control mice in vivo. We found that the compound significantly suppressed phosphorylation of extracellular signal-regulated kinase (pERK) and activated c-jun-N-terminal kinase (JNK) and protein kinase C (PKC)-alpha following 4 h-treatment. These findings indicate that 2-hyim-DMSQ-S33 exerts antitumor activities by regulating pERK, JNK and PKC-alpha.

IFN-gamma downregulates expression of Na(+)/H(+) exchangers NHE2 and NHE3 in rat intestine and human Caco-2/bbe cells

Diarrhea associated with inflammatory bowel diseases has traditionally been attributed to stimulated secretion. The purpose of this study was to determine whether chronic stimulation of intestinal mucosa by interferon-gamma (IFN-gamma) affects expression and function of the apical membrane Na(+)/H(+) exchangers NHE2 and NHE3 in rat intestine and Caco-2/bbe (C2) cells. Confluent C2 cells expressing NHE2 and NHE3 were treated with IFN-gamma for 2, 24, and 48 h. Adult rats were injected with IFN-gamma intraperitoneally for 12 and 48 h. NHE2 and NHE3 activities were measured by unidirectional (22)Na influx across C2 cells and in rat brush-border membrane vesicles. NHE protein and mRNA were assessed by Western and Northern blotting. IFN-gamma treatment of C2 monolayers caused a >50% reduction in NHE2 and NHE3 activities and protein expression. In rats, region-specific, time- and dose-dependent reductions of NHE2 and NHE3 activities, protein expression, and mRNA were observed after exposure to IFN-gamma. Chronic exposure of intestinal epithelial cells to IFN-gamma results in selective downregulation of NHE2 and NHE3 expression and activity, a potential cause of inflammation-associated diarrhea.

Inhibition of Na+,K+-ATPase by interferon gamma down-regulates intestinal epithelial transport and barrier function

BACKGROUND & AIMS

To determine how interferon (IFN)-gamma inhibits epithelial barrier and ion transport functions, intestinal T84 cells were studied.

METHODS

Acute and chronic effects of IFN-gamma on T84 barrier function, Na+,K+-adenosine triphosphatase (ATPase) activity, and certain ion transport and tight junctional proteins were determined. To assess the role of Na+,K+-ATPase and intracellular Na+, similar studies with the Na+,K+-ATPase inhibitor ouabain and Na+ ionophore monensin were performed. To determine the role of nitric oxide (NO), the NO donor SPER-NO was used.

RESULTS

IFN-gamma acutely (<6 hour) decreased cellular Na+,K+-ATPase activity, followed later (>24 hours) by decreases in expression of Na/K/2Cl, the alpha subunit of Na+,K+-ATPase, occludin, and ZO-1. In contrast, cystic fibrosis transmembrane conductance regulator or the Na+ pump beta subunit were unchanged. Ouabain and monensin caused nearly identical changes to IFN-gamma. Incubation in low Na+ media significantly blunted the chronic effects of IFN-gamma. Hypotonic-induced cell swelling, in contrast, had effects similar to IFN-gamma but did not alter the expression of the Na+ pump alpha subunit. The NO donor SPER-NO rapidly inhibited Na+,K+-ATPase and also down-regulated transport and barrier proteins.

CONCLUSIONS

IFN-gamma inhibition of Na+,K+-ATPase activity acutely causes increases in intracellular Na(i) concentration and cell volume, which are distinct signaling events that ultimately result in a leaky and dysfunctional epithelium associated with chronic inflammation.

Sodium-potassium-adenosinetriphosphatase-dependent sodium transport in the kidney: hormonal control

Tubular reabsorption of filtered sodium is quantitatively the main contribution of kidneys to salt and water homeostasis. The transcellular reabsorption of sodium proceeds by a two-step mechanism: Na(+)-K(+)-ATPase-energized basolateral active extrusion of sodium permits passive apical entry through various sodium transport systems. In the past 15 years, most of the renal sodium transport systems (Na(+)-K(+)-ATPase, channels, cotransporters, and exchangers) have been characterized at a molecular level. Coupled to the methods developed during the 1965-1985 decades to circumvent kidney heterogeneity and analyze sodium transport at the level of single nephron segments, cloning of the transporters allowed us to move our understanding of hormone regulation of sodium transport from a cellular to a molecular level. The main purpose of this review is to analyze how molecular events at the transporter level account for the physiological changes in tubular handling of sodium promoted by hormones. In recent years, it also became obvious that intracellular signaling pathways interacted with each other, leading to synergisms or antagonisms. A second aim of this review is therefore to analyze the integrated network of signaling pathways underlying hormone action. Given the central role of Na(+)-K(+)-ATPase in sodium reabsorption, the first part of this review focuses on its structural and functional properties, with a special mention of the specificity of Na(+)-K(+)-ATPase expressed in renal tubule. In a second part, the general mechanisms of hormone signaling are briefly introduced before a more detailed discussion of the nephron segment-specific expression of hormone receptors and signaling pathways. The three following parts integrate the molecular and physiological aspects of the hormonal regulation of sodium transport processes in three nephron segments: the proximal tubule, the thick ascending limb of Henle's loop, and the collecting duct.

Cell-type-specific activation of p38 protein kinase cascades by the novel tumor promoter palytoxin

Palytoxin is a potent non-12-O-tetradecanoylphorbol-13-acetate (TPA)-type skin tumor promoter. We used COS7 and HeLa cells to investigate the protein kinase cascades by which palytoxin activates the mitogen-activated protein kinase (MAPK) p38. Three p38 kinases have been identified: stress-activated protein kinase/extracellular signal-regulated kinase kinase-1 (SEK1), MAPK kinase 3 (MKK3), and MKK6. SEK1 phosphorylates and activates both p38 and c-Jun NH(2)-terminal kinase (JNK), whereas MKK3 and MKK6 selectively phosphorylate and activate p38. Although transiently overexpressed SEK1 activates p38 in cells, the importance of endogenous SEK1 for the activation of p38 by specific types of stimuli is unclear because some agents, such as sorbitol, can activate p38 in cells derived from SEK1 knockout mice. Because we previously showed that palytoxin activates JNK through an SEK1-dependent pathway, we investigated whether SEK1 also mediates the activation of p38 by palytoxin. The results presented here demonstrate that endogenous SEK1 does play an important role in the activation of p38 by palytoxin in specific cell types. In COS7 cells, palytoxin stimulated the phosphorylation of SEK1 and MKK6, and expression of dominant negative mutants of either SEK1 or MKK6 inhibited palytoxin-stimulated p38 activation. In HeLa cells, palytoxin stimulated the phosphorylation of MKK3 in addition to SEK1 and MKK6. In contrast to COS7 cells, in HeLa cells expression of a dominant negative mutant of SEK1 did not inhibit palytoxin-stimulated activation of p38, although expression of dominant negative mutants of either MKK3 or MKK6 did inhibit palytoxin-stimulated p38 activation in this cell type. These studies indicate that the importance of SEK1 in the activation of p38 by palytoxin depends on the ability of palytoxin to activate MKK3 and MKK6.

Epithelial sodium channel in human epidermal keratinocytes: expression of its subunits and relation to sodium transport and differentiation

The amiloride-sensitive epithelial sodium channel (ENaC) is a main determinant of sodium absorption in renal and colonic epithelial cells. Surprisingly, it is also expressed in non-transporting epithelia such as the epidermis. To gain insight into the putative role of ENaC in keratinocytes, we have evaluated its expression in human skin and in cultured human keratinocytes. Our results indicate that (1) ENaC is expressed in the epidermis and in cultured keratinocytes, at the mRNA and at the protein levels, (2) the ratio of expression of the different ENaC subunits is drastically modified at the protein level during cell growth and differentiation, with a selective upregulation of the β subunit, (3) no transepithelial sodium transport function is apparent in cultured keratinocytes, but patch-clamp recordings indicate the existence of functional sodium channels with properties similar to those of the cloned ENaC and (4) ENaC inhibition does not alter keratinocyte proliferation, but it significantly decreases the frequency of dome formation in confluent keratinocyte cultures. These results document for the first time the characteristics of ENaC subunit expression in human keratinocytes, and suggest that ENaC may be important during differentiation.

Shear stress-mediated extracellular signal-regulated kinase activation is regulated by sodium in endothelial cells. Potential role for a voltage-dependent sodium channel

Fluid shear stress is an important regulator of endothelial cell (EC) function. To determine whether mechanosensitive ion channels participate in the EC response to shear stress, we characterized the role of ion transport in shear stress-mediated extracellular signal-regulated kinase (ERK1/2) stimulation. Replacement of all extracellular Na+ with either N-methyl-D-glucamine or choline chloride increased the ERK1/2 stimulation in response to shear stress by 1.89 +/- 0.1-fold. The Na+ effect was concentration-dependent (maximal effect, </=12.5 mM) and was specific for shear stress-mediated ERK1/2 activation as epidermal growth factor-stimulated ERK1/2 activation was unaffected by removal of extracellular Na+. Shear stress-mediated ERK1/2 activation was potentiated by the voltage-gated sodium channel antagonist, tetrodotoxin (100 nM), to a magnitude similar to that achieved with extracellular Na+ withdrawal. Transfection of Chinese hamster ovary cells with a rat brain type IIa voltage-gated sodium channel completely inhibited shear stress-mediated ERK1/2 activation in these cells. Inhibition was reversed by performing the experiment in sodium-free buffer or by including tetrodotoxin in the buffer. Western blotting of bovine and human EC lysates with SP19 antibody detected a 250-kDa protein consistent with the voltage-gated sodium channel. Degenerate polymerase chain reaction of cDNA from primary human EC yielded transcripts whose sequences were identical to the sodium channel SCN4a and SCN8a alpha subunit genes. These results indicate that shear stress-mediated ERK1/2 activation is regulated by extracellular sodium and demonstrate that ion transport via Na+ channels modulates EC responses to shear stress.

Differential activation of mitogen-activated protein kinases by palytoxin and ouabain, two ligands for the Na+,K+-ATPase

We previously demonstrated that the marine toxin and skin tumor promoter palytoxin activates the stress-activated protein kinase/c-Jun N-terminal kinase (JNK), but not the extracellular signal-regulated kinase (ERK), which is typically activated by mitogenic agents. JNK, ERK, and p38, another stress-activated protein kinase, are members of the mitogen-activated protein (MAP) kinase family of serine/threonine kinases, which coordinate the transmission of various signals through the cell. The Na+,K+-ATPase is the putative palytoxin receptor. Therefore, we hypothesized that the Na+,K+-ATPase inhibitor ouabain might also stimulate signaling pathways that activate MAP kinases. Using HeLa and COS7 cells, we found that, although there are similarities between the protein kinase cascades by which palytoxin and ouabain activate JNK, there are also significant differences between the activation of specific MAP kinases by palytoxin and ouabain. Transient expression of dominant negative mutants indicates that ouabain, like palytoxin, activates JNK through a protein kinase cascade that involves the JNK kinase SEK1 but does not require the GTPase Ras. Palytoxin activates JNK and p38 to a greater extent than ouabain. By contrast, ouabain activates ERK to a greater extent than palytoxin. Ouabain blocked palytoxin-stimulated activation of JNK and p38, but not anisomycin-stimulated activation of these kinases, supporting the conclusion that ouabain and palytoxin bind to the same site on the Na+,K+-ATPase. These results suggest that the Na+,K+-ATPase can differentially mediate the activation of MAP kinases by two diverse ligands, palytoxin and ouabain.