Hepatocellular ATP-binding cassette protein expression enhances ATP release and autocrine regulation of cell volume

The Purinergic Nature of Pseudoxanthoma Elasticum

Pseudoxanthoma Elasticum (PXE) is an inherited disease characterized by elastic fiber calcification in the eyes, the skin and the cardiovascular system. PXE results from mutations in ABCC6 that encodes an ABC transporter primarily expressed in the liver and kidneys. It took nearly 15 years after identifying the gene to better understand the etiology of PXE. ABCC6 function facilitates the efflux of ATP, which is sequentially hydrolyzed by the ectonucleotidases ENPP1 and CD73 into pyrophosphate (PPi) and adenosine, both inhibitors of calcification. PXE, together with General Arterial Calcification of Infancy (GACI caused by ENPP1 mutations) as well as Calcification of Joints and Arteries (CALJA caused by NT5E/CD73 mutations), forms a disease continuum with overlapping phenotypes and shares steps of the same molecular pathway. The explanation of these phenotypes place ABCC6 as an upstream regulator of a purinergic pathway (ABCC6 → ENPP1 → CD73 → TNAP) that notably inhibits mineralization by maintaining a physiological Pi/PPi ratio in connective tissues. Based on a review of the literature and our recent experimental data, we suggest that PXE (and GACI/CALJA) be considered as an authentic "purinergic disease". In this article, we recapitulate the pathobiology of PXE and review molecular and physiological data showing that, beyond PPi deficiency and ectopic calcification, PXE is associated with wide and complex alterations of purinergic systems. Finally, we speculate on the future prospects regarding purinergic signaling and other aspects of this disease.

Peroxisomal ATP Uptake Is Provided by Two Adenine Nucleotide Transporters and the ABCD Transporters

Peroxisomes are essential organelles involved in various metabolic processes, including fatty acid β-oxidation. Their metabolic functions require a controlled exchange of metabolites and co-factors, including ATP, across the peroxisomal membrane. We investigated which proteins are involved in the peroxisomal uptake of ATP in the yeast Saccharomyces cerevisiae. Using wild-type and targeted deletion strains, we measured ATP-dependent peroxisomal octanoate β-oxidation, intra-peroxisomal ATP levels employing peroxisome-targeted ATP-sensing reporter proteins, and ATP uptake in proteoliposomes prepared from purified peroxisomes. We show that intra-peroxisomal ATP levels are maintained by different peroxisomal membrane proteins each with different modes of action: 1) the previously reported Ant1p protein, which catalyzes the exchange of ATP for AMP or ADP, 2) the ABC transporter protein complex Pxa1p/Pxa2p, which mediates both uni-directional acyl-CoA and ATP uptake, and 3) the mitochondrial Aac2p protein, which catalyzes ATP/ADP exchange and has a dual localization in both mitochondria and peroxisomes. Our results provide compelling evidence for a complementary system for the uptake of ATP in peroxisomes.

Methods for cell volume measurement

Volume is an essential characteristic of a cell, and this review describes the main methods of its measurement that have been used in the past several decades. The discussed methods include various implementations of light scattering, estimates based on one or two cell dimensions, surface scanning, fluorescence confocal and transmission slice-by-slice imaging, intracellular volume markers, displacement of extracellular solution, quantitative phase imaging, radioactive methods, and some others. Suitability of these methods to some typical samples and applications is discussed. © 2017 International Society for Advancement of Cytometry.

Ectopic adenine nucleotide translocase activity controls extracellular ADP levels and regulates the F1-ATPase-mediated HDL endocytosis pathway on hepatocytes

Ecto-F₁-ATPase is a complex related to mitochondrial ATP synthase which has been identified as a plasma membrane receptor for apolipoprotein A-I (apoA-I), the major protein of high-density lipoprotein (HDL), and has been shown to contribute to HDL endocytosis in several cell types. On hepatocytes, apoA-I binding to ecto-F₁-ATPase stimulates extracellular ATP hydrolysis into ADP, which subsequently activates a P2Y₁₃-mediated HDL endocytosis pathway. Interestingly, other mitochondrial proteins have been found to be expressed at the plasma membrane of several cell types. Among these, adenine nucleotide translocase (ANT) is an ADP/ATP carrier but its role in controlling extracellular ADP levels and F₁-ATPase-mediated HDL endocytosis has never been investigated. Here we confirmed the presence of ANT at the plasma membrane of human hepatocytes. We then showed that ecto-ANT activity increases or reduces extracellular ADP level, depending on the extracellular ADP/ATP ratio. Interestingly, ecto-ANT co-localized with ecto-F₁-ATPase at the hepatocyte plasma membrane and pharmacological inhibition of ecto-ANT activity increased extracellular ADP level when ecto-F₁-ATPase was activated by apoA-I. This increase in the bioavailability of extracellular ADP accordingly translated into an increase of HDL endocytosis on human hepatocytes. This study thus uncovered a new location and function of ANT for which activity at the cell surface of hepatocytes modulates the concentration of extracellular ADP and regulates HDL endocytosis.

Role of Extracellular Adenosine Triphosphate in Human Skin

Background:

The nucleotide adenosine triphosphate (ATP) has long been known to drive and participate in countless intracellular processes. Extracellular ATP and its metabolite adenosine have also been shown to exert a variety of effects on nearly every cell type in human skin. Knowledge of the sources and effects of extracellular ATP in human skin may help shape new therapies for skin injury, inflammation, and numerous other cutaneous disorders.

Objective:

The objective of this review is to introduce the reader to current knowledge regarding the sources and effects of extracellular ATP in human skin and to outline areas in which further research is necessary to clarify the nature and mechanism of these effects.

Conclusion:

Extracellular ATP seems to play a direct role in triggering skin inflammatory, regenerative, and fibrotic responses to mechanical injury, an indirect role in melanocyte proliferation and apoptosis, and a complex role in Langerhans cell-directed adaptive immunity.

Dynamic Regulation of Cell Volume and Extracellular ATP of Human Erythrocytes

Introduction

The peptide mastoparan 7 (MST7) triggered in human erythrocytes (rbcs) the release of ATP and swelling. Since swelling is a well-known inducer of ATP release, and extracellular (ATPe), interacting with P (purinergic) receptors, can affect cell volume (Vr), we explored the dynamic regulation between Vr and ATPe.

Methods and Treatments

We made a quantitative assessment of MST7-dependent kinetics of Vr and of [ATPe], both in the absence and presence of blockers of ATP efflux, swelling and P receptors.

Results

In rbcs 10 μM MST7 promoted acute, strongly correlated changes in [ATPe] and Vr. Whereas MST7 induced increases of 10% in Vr and 190 nM in [ATPe], blocking swelling in a hyperosmotic medium + MST7 reduced [ATPe] by 40%. Pre-incubation of rbcs with 10 μM of either carbenoxolone or probenecid, two inhibitors of the ATP conduit pannexin 1, reduced [ATPe] by 40–50% and swelling by 40–60%, while in the presence of 80 U/mL apyrase, an ATPe scavenger, cell swelling was prevented. While exposure to 10 μM NF110, a blocker of ATP-P2X receptors mediating sodium influx, reduced [ATPe] by 48%, and swelling by 80%, incubation of cells in sodium free medium reduced swelling by 92%.

Analysis and Discussion

Results were analyzed by means of a mathematical model where ATPe kinetics and Vr kinetics were mutually regulated. Model dependent fit to experimental data showed that, upon MST7 exposure, ATP efflux required a fast 1960-fold increase of ATP permeability, mediated by two kinetically different conduits, both of which were activated by swelling and inactivated by time. Both experimental and theoretical results suggest that, following MST7 exposure, ATP is released via two conduits, one of which is mediated by pannexin 1. The accumulated ATPe activates P2X receptors, followed by sodium influx, resulting in cell swelling, which in turn further activates ATP release. Thus swelling and P2X receptors constitute essential components of a positive feedback loop underlying ATP-induced ATP release of rbcs.

Lrp4 in astrocytes modulates glutamatergic transmission

Neurotransmission requires precise control of neurotransmitter release from axon terminals. This process is regulated by glial cells; however, underlying mechanisms are not fully understood. Here we report that glutamate release in the brain is impaired in mice lacking low density lipoprotein receptor-related protein 4 (Lrp4), a protein critical for neuromuscular junction formation. Electrophysiological studies indicate compromised release probability in astrocyte-specific Lrp4 knockout mice. Lrp4 mutant astrocytes suppress glutamate transmission by enhancing the release of ATP, whose levels are elevated in the hippocampus of Lrp4 mutant mice. Consequently, the mutant mice are impaired in locomotor activity and spatial memory and are resistant to seizure induction. These impairments could be ameliorated by adenosine A1 receptor antagonist. The results reveal a critical role of Lrp4, in response to agrin, in modulating astrocytic ATP release and synaptic transmission. Our study provides insight into the interaction between neurons and astrocytes for synaptic homeostasis and/or plasticity.

Cisplatin activates volume-sensitive like chloride channels via purinergic receptor pathways in nasopharyngeal carcinoma cells

Cisplatin-based concomitant chemoradiotherapy is considered as the standard treatment for locally advanced nasopharyngeal carcinoma patients. However, the curative efficacy of cisplatin-based chemotherapy is limited because of the occurrence of cisplatin resistance. Some researches indicate that activating the volume-sensitive Cl(-) channel might be a new strategy for the reduction of cisplatin resistance. However, little is known about the activation pathway of the Cl(-) channels activated by cisplatin. In this study, the cisplatin-activated chloride current was investigated using the whole cell patch-clamp technique in the poorly differentiated nasopharyngeal carcinoma cells (CNE-2Z cells), and the activation pathway of the current was also discussed. The results showed that extracellular application of cisplatin activated a Cl(-) current, showing the properties of significant outward rectification, intracellular ATP dependency, and a selectivity sequence of I(-) > Br(-) > Cl(-) > gluconate, and being inhibited by the Cl(-) channel inhibitors tamoxifen and extracellular ATP. These characteristics are similar to those of the volume-sensitive Cl(-) current in CNE-2Z cells, indicating that cisplatin induces the Cl(-) current by activating the volume-sensitive like chloride channel. The cisplatin-activated current was blocked by suramin (a wide-spectrum purinergic antagonist) and RB2 (a relatively selective P2Y antagonist). In addition, the current was depressed by extracellular application of apyrase. The apoptotic volume decrease induced by cisplatin was also attenuated by RB2. P2Y receptors were expressed in CNE-2Z cells. These results suggest that cisplatin can induce a Cl(-) current by activating volume-sensitive like Cl(-) channels through the P2Y purinoceptor pathway.

Bacterial RTX toxins allow acute ATP release from human erythrocytes directly through the toxin pore

ATP is as an extracellular signaling molecule able to amplify the cell lysis inflicted by certain bacterial toxins including the two RTX toxins α-hemolysin (HlyA) from Escherichia coli and leukotoxin A (LtxA) from Aggregatibacter actinomycetemcomitans. Inhibition of P2X receptors completely blocks the RTX toxin-induced hemolysis over a larger concentration range. It is, however, at present not known how the ATP that provides the amplification is released from the attacked cells. Here we show that both HlyA and LtxA trigger acute release of ATP from human erythrocytes that preceded and were not caused by cell lysis. This early ATP release did not occur via previously described ATP-release pathways in the erythrocyte. Both HlyA and LtxA were capable of triggering ATP release in the presence of the pannexin 1 blockers carbenoxolone and probenecid, and the HlyA-induced ATP release was found to be similar in erythrocytes from pannexin 1 wild type and knock-out mice. Moreover, the voltage-dependent anion channel antagonist TRO19622 had no effect on ATP release by either of the toxins. Finally, we showed that both HlyA and LtxA were able to release ATP from ATP-loaded lipid (1-palmitoyl-2-oleoyl-phosphatidylcholine) vesicles devoid of any erythrocyte channels or transporters. Again we were able to show that this happened in a non-lytic fashion, using calcein-containing vesicles as controls. These data show that both toxins incorporate into lipid vesicles and allow ATP to be released. We suggest that both toxins cause acute ATP release by letting ATP pass the toxin pores in both human erythrocytes and artificial membranes.

ABCC6 prevents ectopic mineralization seen in pseudoxanthoma elasticum by inducing cellular nucleotide release

Pseudoxanthoma elasticum (PXE) is an autosomal recessive disease characterized by progressive ectopic mineralization of the skin, eyes, and arteries, for which no effective treatment exists. PXE is caused by inactivating mutations in the gene encoding ATP-binding cassette sub-family C member 6 (ABCC6), an ATP-dependent efflux transporter present mainly in the liver. Abcc6(-/-) mice have been instrumental in demonstrating that PXE is a metabolic disease caused by the absence of an unknown factor in the circulation, the presence of which depends on ABCC6 in the liver. Why absence of this factor results in PXE has remained a mystery. Here we report that medium from HEK293 cells overexpressing either human or rat ABCC6 potently inhibits mineralization in vitro, whereas medium from HEK293 control cells does not. Untargeted metabolomics revealed that cells expressing ABCC6 excrete large amounts of nucleoside triphosphates, even though ABCC6 itself does not transport nucleoside triphosphates. Extracellularly, ectonucleotidases hydrolyze the excreted nucleoside triphosphates to nucleoside monophosphates and inorganic pyrophosphate (PPi), a strong inhibitor of mineralization that plays a pivotal role in several mineralization disorders similar to PXE. The in vivo relevance of our data are demonstrated in Abcc6(-/-) mice, which had plasma PPi levels <40% of those found in WT mice. This study provides insight into how ABCC6 affects PXE. Our data indicate that the factor that normally prevents PXE is PPi, which is provided to the circulation in the form of nucleoside triphosphates via an as-yet unidentified but ABCC6-dependent mechanism.

Roles of substance P and ATP in the subepithelial fibroblasts of rat intestinal villi

The ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes and also villous movement in the gut. In the intestinal villi, subepithelial fibroblasts under the epithelium form contractile cellular networks and closely contact to the varicosities of substance P and nonsubstance P afferent neurons. Subepithelial fibroblasts of the duodenal villi possess purinergic receptor P2Y1 and tachykinin receptor NK1. ATP and substance P induce increase in intracellular Ca(2+) and cell contraction in subepithelial fibroblasts. They are highly mechanosensitive and release ATP by mechanical stimuli. Released ATP spreads to form an ATP "cloud" with nearly 1μM concentration and activates the surroundings via P2Y1 and afferent neurons via P2X receptors. These findings suggest that villous subepithelial fibroblasts and afferent neurons interact via ATP and substance P. This mutual interaction may play important roles in the signal transduction of mechano reflex pathways including a coordinate villous movement and also in the maturation of the structure and function of the intestinal villi.

Purinergic receptors and regulatory volume decrease in seabream (Sparus aurata) hepatocytes: a videometric study

The response of isolated hepatocytes of Sparus aurata to hypotonic stress was studied by the aid of videometric methods with the aim to investigate the possible involvement of ATP in the regulatory volume decrease (RVD). This study confirms our previous observations showing the ability of these cells to undergo RVD. In addition, it shows that the homeostatic response was inhibited by apyrase, an ATP scavenger, thus suggesting the involvement of extracellular ATP in the RVD response. Experiments performed in the presence of ATPγS or adenosine, agonists of P(2) and P(1) receptors respectively, and in the presence of suramin or 8-PT, antagonists of P(2) and P(1) receptors respectively, suggest that ATP exerts its stimulatory effect on the homeostatic response by interacting with P(2) receptors. On the other hand, the activation of P(1) receptors by ATP metabolites produces opposite effects. In an attempt to clarify the mechanisms involved in ATP release from the cell, we performed some experiments with known inhibitors of the possible mechanisms of regulated ATP release. The results we obtained let us to suppose that the mechanism allowing the exit of ATP from the cell is verapamil sensitive suggesting the involvement of the P-glycoprotein.

Pannexin1 contributes to pathophysiological ATP release in lipoapoptosis induced by saturated free fatty acids in liver cells

Hepatocyte lipoapoptosis induced by saturated free fatty acids (FFA) contributes to hepatic inflammation in lipotoxic liver injury, and the cellular mechanisms involved have not been defined. Recent studies have shown that apoptosis in nonhepatic cells stimulates ATP release via activation of pannexin1 (panx1), and extracellular ATP functions as a proinflammatory signal for recruitment and activation of the inflammatory cells. However, it is not known whether lipoapoptosis stimulates ATP release in liver cells. We found that lipoapoptosis induced by saturated FFA stimulated ATP release in liver cells that increased extracellular ATP concentration by more than fivefold above the values observed in healthy cells. This sustained pathophysiological ATP release was not dependent on caspase-3/7 activation. Inhibition of c-Jun NH(2)-terminal kinase (JNK), a key mediator of lipoapoptosis, with SP600125 blocked pathophysiological ATP release in a dose-dependent manner. RT-PCR analysis indicated that panx1 is expressed in hepatocytes and multiple liver cell lines. Notably, inhibition of panx1 expression with short hairpin (sh)RNA inhibited in part pathophysiological ATP release. Moreover, lipoapoptosis stimulated uptake of a membrane impermeable dye YoPro-1 (indicative of panx1 activation), which was inhibited by panx1 shRNA, probenecid, and mefloquine. These results suggest that panx1 contributes to pathophysiological ATP release in lipoapoptosis induced by saturated FFA. Thus panx1 may play an important role in hepatic inflammation by mediating an increase in extracellular ATP concentration in lipotoxic liver injury.

Millimeter wave-induced modulation of calcium dynamics in an engineered skin co-culture model: role of secreted ATP on calcium spiking

We have previously designed and characterized a 94 GHz exposure system that allows real-time monitoring of subcellular interactions induced by millimeter wave (MMW) stimulation. For example, studies of the calcium dynamics in neuronal cells in response to 94 GHz irradiation suggested that MMW stimulation increased calcium spiking. In this study, we engineered a 3D co-culture model that represents the major constituents of skin. We used this experimental model along with the custom-designed MMW exposure system to investigate the effects of 94 GHz irradiation in the skin-like tissue construct. Unlike typical non-excitable cells, keratinocytes exhibited calcium spikes in their resting state. Exposure to a 94 GHz irradiation induced a statistically significant increase in the calcium spiking. When co-cultured with neuronal cells in the 3D co-culture skin model, changes in the calcium spiking in neuronal cells depended on the MMW input power. Further, the 94 GHz irradiation caused ATP secretion by keratincytes. ATP is a major factor that modulates the calcium spiking in neuronal cells. Surprisingly, while a 5-fold increase in the ATP secretion enhanced the calcium spiking in neuronal cells, a 10-fold increase significantly hindered the calcium dynamics. Computational simulation of ATP-induced calcium dynamics was in general agreement with the experimental findings, suggesting the involvement of the ATP-sensitive purinergic receptors. The engineered co-culture skin model offers a physiologically relevant environment in which the calcium dynamics is regulated both by the cell-MMW and cell-cell interactions.

Mechanisms of ATP release, the enabling step in purinergic dynamics

The only effective intervention to slow onset and progression of glaucomatous blindness is to lower intraocular pressure (IOP). Among other modulators, adenosine receptors (ARs) exert complex regulation of IOP. Agonists of A(3)ARs in the ciliary epithelium activate Cl(-) channels, favoring increased formation of aqueous humor and elevated IOP. In contrast, stimulating A(1)ARs in the trabecular outflow pathway enhances release of matrix metalloproteinases (MMPs) from trabecular meshwork (TM) cells, reducing resistance to outflow of aqueous humor to lower IOP. These opposing actions are thought to be initiated by cellular release of ATP and its ectoenzymatic conversion to adenosine. This view is now supported by our identification of six ectoATPases in trabecular meshwork (TM) cells and by our observation that external ATP enhances TM-cell secretion of MMPs through ectoenzymatic formation of adenosine. ATP release is enhanced by cell swelling and stretch. Also, enhanced ATP release and downstream MMP secretion is one mediator of the action of actin depolymerization to reduce outflow resistance. Inflow and outflow cells share pannexin-1 and connexin hemichannel pathways for ATP release. However, vesicular release and P2X(7) release pathways were functionally limited to inflow and outflow cells, respectively, suggesting that blocking exocytosis might selectively inhibit inflow, lowering IOP.

Prostacyclin receptor-mediated ATP release from erythrocytes requires the voltage-dependent anion channel

Erythrocytes have been implicated as controllers of vascular caliber by virtue of their ability to release the vasodilator ATP in response to local physiological and pharmacological stimuli. The regulated release of ATP from erythrocytes requires activation of a signaling pathway involving G proteins (G(i) or G(s)), adenylyl cyclase, protein kinase A, and the cystic fibrosis transmembrane conductance regulator as well as a final conduit through which this highly charged anion exits the cell. Although pannexin 1 has been shown to be the final conduit for ATP release from human erythrocytes in response to reduced oxygen tension, it does not participate in transport of ATP following stimulation of the prostacyclin (IP) receptor in these cells, which suggests that an additional protein must be involved. Using antibodies directed against voltage-dependent anion channel (VDAC)1, we confirm that this protein is present in human erythrocyte membranes. To address the role of VDAC in ATP release, two structurally dissimilar VDAC inhibitors, Bcl-x(L) BH4(4-23) and TRO19622, were used. In response to the IP receptor agonists, iloprost and UT-15C, ATP release was inhibited by both VDAC inhibitors although neither iloprost-induced cAMP accumulation nor total intracellular ATP concentration were altered. Together, these findings support the hypothesis that VDAC is the ATP conduit in the IP receptor-mediated signaling pathway in human erythrocytes. In addition, neither the pannexin inhibitor carbenoxolone nor Bcl-x(L) BH4(4-23) attenuated ATP release in response to incubation of erythrocytes with the β-adrenergic receptor agonist isoproterenol, suggesting the presence of yet another channel for ATP release from human erythrocytes.

Plasmalemmal VDAC controversies and maxi-anion channel puzzle

The maxi-anion channel has been observed in many cell types from the very beginning of the patch-clamp era. The channel is highly conductive for chloride and thus can modulate the resting membrane potential and play a role in fluid secretion/absorption and cell volume regulation. A wide nanoscopic pore of the maxi-anion channel permits passage of excitatory amino acids and nucleotides. The channel-mediated release of these signaling molecules is associated with kidney tubuloglomerular feedback, cardiac ischemia/hypoxia, as well as brain ischemia/hypoxia and excitotoxic neurodegeneration. Despite the ubiquitous expression and physiological/pathophysiological significance, the molecular identity of the maxi-anion channel is still obscure. VDAC is primarily a mitochondrial protein; however several groups detected it on the cellular surface. VDAC in lipid bilayers reproduced the most important biophysical properties of the maxi-anion channel, such as a wide nano-sized pore, closure in response to moderately high voltages, ATP-block and ATP-permeability. However, these similarities turned out to be superficial, and the hypothesis of plasmalemmal VDAC as the maxi-anion channel did not withstand the test by genetic manipulations of VDAC protein expression. VDAC on the cellular surface could also function as a ferricyanide reductase or a receptor for plasminogen kringle 5 and for neuroactive steroids. These ideas, as well as the very presence of VDAC on plasmalemma, remain to be scrutinized by genetic manipulations of the VDAC protein expression. This article is part of a Special Issue entitled: VDAC structure, function, and regulation of mitochondrial metabolism.

Molecular mechanisms of purine and pyrimidine nucleotide release

Given the widespread importance of purinergic receptor-evoked signaling, understanding how ATP and other nucleotides are released from cells in a regulated manner is an essential physiological question. Nonlytic release of ATP, UTP, UDP-glucose, and other nucleotides occurs in all cell types and tissues via both constitutive mechanisms, that is, in the absence of external stimuli, and to a greater extent in response to biochemical or mechanical/physical stimuli. However, a molecular understanding of the processes regulating nucleotide release has only recently begun to emerge. It is generally accepted that nucleotide release occurs in two different scenarios, exocytotic release from the secretory pathway or via conductive/transport mechanisms, and a critical review of our current understanding of these mechanisms is presented in this chapter.

Evidence for sustained ATP release from liver cells that is not mediated by vesicular exocytosis

Extracellular ATP regulates many important cellular functions in the liver by stimulating purinergic receptors. Recent studies have shown that rapid exocytosis of ATP-enriched vesicles contributes to ATP release from liver cells. However, this rapid ATP release is transient, and ceases in ~30 s after the exposure to hypotonic solution. The purpose of these studies was to assess the role of vesicular exocytosis in sustained ATP release. An exposure to hypotonic solution evoked sustained ATP release that persisted for more than 15 min after the exposure. Using FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide) fluorescence to measure exocytosis, we found that hypotonic solution stimulated a transient increase in FM1-43 fluorescence that lasted ~2 min. Notably, the rate of FM1-43 fluorescence and the magnitude of ATP release were not correlated, indicating that vesicular exocytosis may not mediate sustained ATP release from liver cells. Interestingly, mefloquine potently inhibited sustained ATP release, but did not inhibit an increase in FM1-43 fluorescence evoked by hypotonic solution. Consistent with these findings, when exocytosis of ATP-enriched vesicles was specifically stimulated by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), mefloquine failed to inhibit ATP release evoked by NPPB. Thus, mefloquine can pharmacologically dissociate sustained ATP release and vesicular exocytosis. These results suggest that a distinct mefloquine-sensitive membrane ATP transport may contribute to sustained ATP release from liver cells. This novel mechanism of membrane ATP transport may play an important role in the regulation of purinergic signaling in liver cells.

ATP signalling is crucial for the response of human keratinocytes to mechanical stimulation by hypo-osmotic shock

Touch is detected through receptors located in the skin and the activation of channels in sensory nerve fibres. Epidermal keratinocytes themselves, however, may sense mechanical stimulus and contribute to skin sensation. Here, we showed that the mechanical stimulation of human keratinocytes by hypo-osmotic shock releases adenosine triphosphate (ATP) and increases intracellular calcium. We demonstrated that the release of ATP was found to be calcium independent because emptying the intracellular calcium stores did not cause ATP release; ATP release was still observed in the absence of external calcium and it persisted on chelating cytosolic calcium. On the other hand, the released ATP activated purinergic receptors and mobilized intracellular calcium stores. The resulting depletion of stored calcium led to the activation of capacitative calcium entry. Increase in cytosolic calcium concentration was blocked by the purinergic receptor blocker suramin, phospholipase C inhibitor and apyrase, which hydrolyses ATP. Collectively, our data demonstrate that human keratinocytes are mechanically activated by hypo-osmotic shock, leading first to the release of ATP, which in turn stimulates purinergic receptors, resulting in the mobilization of intracellular calcium and capacitative calcium entry. These results emphasize the crucial role of ATP signalling in the transduction of mechanical stimuli in human keratinocytes.

Involvement of the cystic fibrosis transmembrane conductance regulator in the acidosis-induced efflux of ATP from rat skeletal muscle

The present study was performed to investigate the effect of acidosis on the efflux of ATP from skeletal muscle. Infusion of lactic acid to the perfused hindlimb muscles of anaesthetised rats produced dose-dependent decreases in pH and increases in the interstitial ATP of extensor digitorum longus (EDL) muscle: 10 mM lactic acid reduced the venous pH from 7.22 ± 0.04 to 6.97 ± 0.02 and increased interstitial ATP from 38 ± 8 to 67 ± 11 nM. The increase in interstitial ATP was well-correlated with the decrease in pH (r(2) = 0.93; P < 0.05). Blockade of cellular uptake of lactic acid using α-cyano-hydroxycinnamic acid abolished the lactic acid-induced ATP release, whilst infusion of sodium lactate failed to depress pH or increase interstitial ATP, suggesting that intracellular pH depression, rather than lactate, stimulated the ATP efflux. Incubation of cultured skeletal myoblasts with 10 mM lactic acid significantly increased the accumulation of ATP in the bathing medium from 0.46 ± 0.06 to 0.76 ± 0.08 μM, confirming the skeletal muscle cells as the source of the released ATP. Acidosis-induced ATP efflux from the perfused muscle was abolished by CFTR(inh)-172, a specific inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR), or glibenclamide, an inhibitor of both K(ATP) channels and CFTR, but it was not affected by atractyloside, an inhibitor of the mitochondrial ATP transporter. Silencing of the CFTR gene using an siRNA abolished the acidosis-induced increase in ATP release from cultured myoblasts. CFTR expression on skeletal muscle cells was confirmed using immunostaining in the intact muscle and Western blotting in the cultured cells. These data suggest that depression of the intracellular pH of skeletal muscle cells stimulates ATP efflux, and that CFTR plays an important role in the release mechanism.

Pannexin 1 is the conduit for low oxygen tension-induced ATP release from human erythrocytes

Erythrocytes release ATP in response to exposure to the physiological stimulus of lowered oxygen (O(2)) tension as well as pharmacological activation of the prostacyclin receptor (IPR). ATP release in response to these stimuli requires activation of adenylyl cyclase, accumulation of cAMP, and activation of protein kinase A. The mechanism by which ATP, a highly charged anion, exits the erythrocyte in response to lowered O(2) tension or receptor-mediated IPR activation by iloprost is unknown. It was demonstrated previously that inhibiting pannexin 1 with carbenoxolone inhibits hypotonically induced ATP release from human erythrocytes. Here we demonstrate that three structurally dissimilar compounds known to inhibit pannexin 1 prevent ATP release in response to lowered O(2) tension but not to iloprost-induced ATP release. These results suggest that pannexin 1 is the conduit for ATP release from erythrocytes in response to lowered O(2) tension. However, the identity of the conduit for iloprost-induced ATP release remains unknown.

Negative feedback of extracellular ADP on ATP release in goldfish hepatocytes: a theoretical study

A mathematical model was built to account for the kinetic of extracellular ATP (ATPe) and extracellular ADP (ADPe) concentrations from goldfish hepatocytes exposed to hypotonicity. The model was based on previous experimental results on the time course of ATPe accumulation, ectoATPase activity, and cell viability [Pafundo et al., 2008]. The kinetic of ATPe is controlled by a lytic ATP flux, a non-lytic ATP flux, and ecto-ATPase activity, whereas ADPe kinetic is governed by a lytic ADP flux and both ecto-ATPase and ecto-ADPase activities. Non-lytic ATPe efflux was included as a diffusion equation modulated by ATPe activation (positive feedback) and ADPe inhibition (negative feedback). The model yielded physically meaningful and stable steady-state solutions, was able to fit the experimental time evolution of ATPe and simulated the concomitant kinetic of ADPe. According to the model during the first minute of hypotonicity the concentration of ATPe is mainly governed by both lytic and non-lytic ATP efflux, with almost no contribution from ecto-ATPase activity. Later on, ecto-ATPase activity becomes important in defining the time dependent decay of ATPe levels. ADPe inhibition of the non-lytic ATP efflux was strong, whereas ATPe activation was minimal. Finally, the model was able to predict the consequences of partial inhibition of ecto-ATPase activity on the ATPe kinetic, thus emulating the exposure of goldfish cells to hypotonic medium in the presence of the ATP analog AMP-PCP. The model predicts this analog to both inhibit ectoATPase activity and increase non-lytic ATP release.

Gene expression profiling defines the role of ATP-exposed keratinocytes in skin inflammation

BACKGROUND

Various environmental stimuli, e.g., mechanical stress, osmolarity change, oxidative stress, and microbial products trigger ATP release from cells. It is well known that ATP regulates cell growth, differentiation, terminal differentiation, and cell-to-cell communication in keratinocytes. Moreover, extracellular ATP stimulates the expression and release of IL-6 and modulates the production several chemokines by keratinocytes.

OBJECTIVE

To investigate the role of ATP-stimulated keratinocytes in skin inflammation and immune response.

METHODS

We identified genes whose expression is augmented in ATP-stimulated human keratinocytes by DNA microarray. These microarray data were validated by quantitative real-time RT-PCR. Furthermore, we confirmed the observed mRNA change at protein level by ELISA and Western blotting.

RESULTS

The statistical analysis of the microarray data revealed that, besides IL-6, the expression of several novel genes such as IL-20, CXCL1-3, and ATF3 was significantly augmented in ATP-stimulated keratinocytes. These data was validated by quantitative real-time RT-PCR. We also confirmed the augmented production of IL-6, IL-20, CXCL1 by ELISA and that of ATF3 by Western blotting. Since both IL-6 and IL-20 that can stimulate STAT3 were produced by the ATP-stimulated keratinocytes, we examined their phosphorylation of STAT3. The study demonstrated biphasic activation of STAT3 after ATP stimulation, which was composed of a first peak at 1-2 h and a second peak at 12-24 h. The latter peak was significantly suppressed by anti-IL-6 antibody.

CONCLUSION

These studies characterized (1) STAT3 activation, (2) chemotaxis for neutrophils via CXCL1-3, and (3) ATF3 activation as possible roles of ATP-stimulated keratinocytes in skin inflammation and immune response.

Basal release of ATP: an autocrine-paracrine mechanism for cell regulation

Cells release adenosine triphosphate (ATP), which activates plasma membrane-localized P2X and P2Y receptors and thereby modulates cellular function in an autocrine or paracrine manner. Release of ATP and the subsequent activation of P2 receptors help establish the basal level of activation (sometimes termed "the set point") for signal transduction pathways and regulate a wide array of responses that include tissue blood flow, ion transport, cell volume regulation, neuronal signaling, and host-pathogen interactions. Basal release and autocrine or paracrine responses to ATP are multifunctional, evolutionarily conserved, and provide an economical means for the modulation of cell, tissue, and organismal biology.

The effects of extracellular adenosine 5'-triphosphate on the tobacco proteome

Extracellular adenosine 5'-triphosphate (eATP) is emerging as an important plant signalling compound capable of mobilising intracellular second messengers such as Ca(2+), nitric oxide, and reactive oxygen species. However, the downstream molecular targets and the spectrum of physiological processes that eATP regulates are largely unknown. We used exogenous ATP and a non-hydrolysable analogue as probes to identify the molecular and physiological effects of eATP-mediated signalling in tobacco. 2-DE coupled with MS/MS analysis revealed differential protein expression in response to perturbation of eATP signalling. These proteins are in several functional classes that included photosynthesis, mitochondrial ATP synthesis, and defence against oxidative stress, but the biggest response was in the pathogen defence-related proteins. Consistent with this, impairment of eATP signalling induced resistance against the bacterial pathogen Erwinia carotovora subsp. carotovora. In addition, disease resistance activated by a fungal pathogen elicitor (xylanase from Trichoderma viride) was concomitant with eATP depletion. These results reveal several previously unknown putative molecular targets of eATP signalling, which pinpoint eATP as an important hub at which regulatory signals of some major primary metabolic pathways and defence responses are integrated.

Extracellular ATP is a regulator of pathogen defence in plants

In healthy plants extracellular ATP (eATP) regulates the balance between cell viability and death. Here we show an unexpected critical regulatory role of eATP in disease resistance and defensive signalling. In tobacco, enzymatic depletion of eATP or competition with non-hydrolysable ATP analogues induced pathogenesis-related (PR) gene expression and enhanced resistance to tobacco mosaic virus and Pseudomonas syringae pv. tabaci. Artificially increasing eATP concentrations triggered a drop in levels of the important defensive signal chemical salicylic acid (SA) and compromised basal resistance to viral and bacterial infection. Inoculating tobacco leaf tissues with bacterial pathogens capable of activating PR gene expression triggered a rapid decline in eATP. Conversely, inoculations with mutant bacteria unable to induce defence gene expression failed to deplete eATP. Furthermore, a collapse in eATP concentration immediately preceded PR gene induction by SA. Our study reveals a previously unsuspected role for eATP as a negative regulator of defensive signal transduction and demonstrates its importance as a key signal integrating defence and cell viability in plants.

5-Nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) stimulates cellular ATP release through exocytosis of ATP-enriched vesicles

Cells release ATP in response to physiologic stimuli. Extracellular ATP regulates a broad range of important cellular functions by activation of the purinergic receptors in the plasma membrane. The purpose of these studies was to assess the role of vesicular exocytosis in cellular ATP release. FM1-43 fluorescence was used to measure exocytosis and bioluminescence to measure ATP release in HTC rat hepatoma and Mz-Cha-1 human cholangiocarcinoma cells. Exposure to a Cl(-) channel inhibitor 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) (50-300 microM) stimulated a 5-100-fold increase in extracellular ATP levels within minutes of the exposure. This rapid response was not a result of changes in cell viability or Cl(-) channel activity. NPPB also potently stimulated ATP release in HEK293 cells and HEK293 cells expressing a rat P2X7 receptor indicating that P2X7 receptors are not involved in stimulation of ATP release by NPPB. In all cells studied, NPPB rapidly stimulated vesicular exocytosis that persisted many minutes after the exposure. The kinetics of NPPB-evoked exocytosis and ATP release were similar. Furthermore, the magnitudes of NPPB-evoked exocytosis and ATP release were correlated (correlation coefficient 0.77), indicating that NPPB may stimulate exocytosis of a pool of ATP-enriched vesicles. These findings provide further support for the concept that vesicular exocytosis plays an important role in cellular ATP release and suggest that NPPB can be used as a biochemical tool to specifically stimulate ATP release through exocytic mechanisms.

Imaging localized astrocyte ATP release with firefly luciferase beads attached to the cell surface

Extracellular adenosine triphosphate (ATP) functions as a signaling molecule in many cell regulation processes. The traditional firefly luciferase assays measure the ATP release as a signal increase with time using a luminometer. Recently, advanced cell imaging techniques using charge-coupled device (CCD) cameras have enabled two-dimensional (2D) high-resolution detection providing both spatial and temporal information. Real-time imaging of ATP release from astrocyte cells has been reported. However, the observed chemiluminescence propagation wave reflects both ATP release and diffusion in the extracellular bulk solution. The dynamic ATP efflux at the cell surface could not be accurately measured. Hence, we constructed biotinylated fused firefly luciferase proteins, immobilized the proteins on 1 microm beads, and attached the beads to the cell surface to detect ATP release from mechanically stimulated astrocyte cells. This novel detection method enables us to monitor the actual ATP concentration at the surface of single live cells. The localized ATP release was found to be prominent but lasted only <20 s, which is very different from the results obtained by free firefly luciferase detection.

Autocrine signaling involved in cell volume regulation: the role of released transmitters and plasma membrane receptors

Cell volume regulation is a basic homeostatic mechanism transcendental for the normal physiology and function of cells. It is mediated principally by the activation of osmolyte transport pathways that result in net changes in solute concentration that counteract cell volume challenges in its constancy. This process has been described to be regulated by a complex assortment of intracellular signal transduction cascades. Recently, several studies have demonstrated that alterations in cell volume induce the release of a wide variety of transmitters including hormones, ATP and neurotransmitters, which have been proposed to act as extracellular signals that regulate the activation of cell volume regulatory mechanisms. In addition, changes in cell volume have also been reported to activate plasma membrane receptors (including tyrosine kinase receptors, G-protein coupled receptors and integrins) that have been demonstrated to participate in the regulatory process of cell volume. In this review, we summarize recent studies about the role of changes in cell volume in the regulation of transmitter release as well as in the activation of plasma membrane receptors and their further implications in the regulation of the signaling machinery that regulates the activation of osmolyte flux pathways. We propose that the autocrine regulation of Ca2+-dependent and tyrosine phosphorylation-dependent signaling pathways by the activation of plasma membrane receptors and swelling-induced transmitter release is necessary for the activation/regulation of osmolyte efflux pathways and cell volume recovery. Furthermore, we emphasize the importance of studying these extrinsic signals because of their significance in the understanding of the physiology of cell volume regulation and its role in cell biology in vivo, where the constraint of the extracellular space might enhance the autocrine or even paracrine signaling induced by these released transmitters.

Loss of apical monocilia on collecting duct principal cells impairs ATP secretion across the apical cell surface and ATP-dependent and flow-induced calcium signals

Renal epithelial cells release ATP constitutively under basal conditions and release higher quantities of purine nucleotide in response to stimuli. ATP filtered at the glomerulus, secreted by epithelial cells along the nephron, and released serosally by macula densa cells for feedback signaling to afferent arterioles within the glomerulus has important physiological signaling roles within kidneys. In autosomal recessive polycystic kidney disease (ARPKD) mice and humans, collecting duct epithelial cells lack an apical central cilium or express dysfunctional proteins within that monocilium. Collecting duct principal cells derived from an Oak Ridge polycystic kidney (orpk ( Tg737 ) ) mouse model of ARPKD lack a well-formed apical central cilium, thought to be a sensory organelle. We compared these cells grown as polarized cell monolayers on permeable supports to the same cells where the apical monocilium was genetically rescued with the wild-type Tg737 gene that encodes Polaris, a protein essential to cilia formation. Constitutive ATP release under basal conditions was low and not different in mutant versus rescued monolayers. However, genetically rescued principal cell monolayers released ATP three- to fivefold more robustly in response to ionomycin. Principal cell monolayers with fully formed apical monocilia responded three- to fivefold greater to hypotonicity than mutant monolayers lacking monocilia. In support of the idea that monocilia are sensory organelles, intentionally harsh pipetting of medium directly onto the center of the monolayer induced ATP release in genetically rescued monolayers that possessed apical monocilia. Mechanical stimulation was much less effective, however, on mutant orpk collecting duct principal cell monolayers that lacked apical central monocilia. Our data also show that an increase in cytosolic free Ca(2+) primes the ATP pool that is released in response to mechanical stimuli. It also appears that hypotonic cell swelling and mechanical pipetting stimuli trigger release of a common ATP pool. Cilium-competent monolayers responded to flow with an increase in cell Ca(2+) derived from both extracellular and intracellular stores. This flow-induced Ca(2+) signal was less robust in cilium-deficient monolayers. Flow-induced Ca(2+) signals in both preparations were attenuated by extracellular gadolinium and by extracellular apyrase, an ATPase/ADPase. Taken together, these data suggest that apical monocilia are sensory organelles and that their presence in the apical membrane facilitates the formation of a mature ATP secretion apparatus responsive to chemical, osmotic, and mechanical stimuli. The cilium and autocrine ATP signaling appear to work in concert to control cell Ca(2+). Loss of a cilium-dedicated autocrine purinergic signaling system may be a critical underlying etiology for ARPKD and may lead to disinhibition and/or upregulation of multiple sodium (Na(+)) absorptive mechanisms and a resultant severe hypertensive phenotype in ARPKD and, possibly, other diseases.

Kinetics of ATP release and cell volume regulation of hyposmotically challenged goldfish hepatocytes

In most animal cells, hypotonic swelling is followed by a regulatory volume decrease (RVD) thought to prevent cell death. In contrast, goldfish hepatocytes challenged with hypotonic medium (180 mosM, HYPO) increase their volume 1.7 times but remain swollen and viable for at least 5 h. Incubation with ATPgammaS (an ATP analog) in HYPO triggers a 42% volume decrease. This effect is concentration dependent (K(1/2) = 760 nM) and partially abolished by P2 receptor antagonists (64% inhibition). A similar induction of RVD is observed with ATP, UTP, and UDP, whereas adenosine inhibits RVD. Goldfish hepatocytes release more than 500 nM ATP during the first minutes of HYPO with no induction of RVD. The fact that similar concentrations of ATPgammaS did trigger RVD could be explained by showing that ATPgammaS induced ATP release. Finally, we observed that in a very small extracellular volume, hepatocytes do show a 56% RVD. This response was diminished by P2 receptor antagonists (73%) and increased (73%) when the extracellular ATP hydrolysis was inhibited 72%. Using a mathematical model, we predict that during the first 2 min of HYPO exposure the extracellular [ATP] is mainly governed by ATP diffusion and by both nonlytic and lytic ATP release, with almost no contribution from ecto-ATPase activity. We show that goldfish hepatocytes under standard HYPO (large volume) do not display RVD unless this is triggered by the addition of micromolar concentrations of nucleotides. However, under very low assay volumes, sufficient endogenous extracellular [ATP] can build up to induce RVD.

Enhanced Na+/H+ exchange activity contributes to the pathogenesis of muscular dystrophy via involvement of P2 receptors

A subset of muscular dystrophy is caused by genetic defects in dystrophin-associated glycoprotein complex. Using two animal models (BIO14.6 hamsters and mdx mice), we found that Na(+)/H(+) exchanger (NHE) inhibitors prevented muscle degeneration. NHE activity was constitutively enhanced in BIO myotubes, as evidenced by the elevated intracellular pH and enhanced (22)Na(+) influx, with activation of putative upstream kinases ERK42/44. NHE inhibitor significantly reduced the increases in baseline intracellular Ca(2+) as well as Na(+) concentration and stretch-induced damage, suggesting that Na(+)(i)-dependent Ca(2+)overload via the Na(+)/Ca(2+) exchanger may cause muscle damage. Furthermore, ATP was found to be released continuously from BIO myotubes in a manner further stimulated by stretching and that the P2 receptor antagonists reduce the enhanced NHE activity and dystrophic muscle damage. These observations suggest that autocrine ATP release may be primarily involved in genesis of abnormal ionic homeostasis in dystrophic muscles and that Na(+)-dependent ion exchangers play a critical pathological role in muscular dystrophy.

Dendritic cells: importance in allergy

In this review we discuss the role of dendritic cells (DC) in the pathogenesis of allergic contact hypersensitivity (ACH) and atopic disorders, such as asthma and atopic eczema. In ACH patients, DC recognize the invasion of simple chemicals such as haptens, and trigger antigen-specific T cell responses leading to the characteristic histological and clinical changes such as spongiosis and papulovesicular eruptions. During atopic disorders, it is well known that the Th2-deviated immune response plays a crucial role in their pathogenesis. DC provide T cells with antigen and costimulatory signals (signals 1 and 2, respectively), as well as with a polarizing signal (signal 3). When studying ACH, it is important to understand how simple chemicals induce the activation of DC and their migration to the draining lymph nodes where they supply signals 1 and 2 to naive T cells. The mechanisms by which DC induce the Th2-deviated immune response, namely via the Th2-deviated signal 3, are central topics in the pathogenesis of atopic disorders.

Extracellular phosphorylation of collagen XVII by ecto-casein kinase 2 inhibits ectodomain shedding

Ecto-phosphorylation is emerging as an important mechanism to regulate cellular ligand interactions and signal transduction. Here we show that extracellular phosphorylation of the cell surface receptor collagen XVII regulates shedding of its ectodomain. Collagen XVII, a member of the novel family of collagenous transmembrane proteins and component of the hemidesmosomes, mediates adhesion of the epidermis to the dermis in the skin. The ectodomain is constitutively shed from the cell surface by metalloproteinases of the ADAM (a disintegrin and metalloproteinase) family, mainly by tumor necrosis factor-alpha converting enzyme (TACE). We used biochemical, mutagenesis, and structural modeling approaches to delineate mechanisms controlling ectodomain cleavage. A standard assay for extracellular phosphorylation, incubation of intact keratinocytes with cell-impermeable [gamma-(32)P]ATP, led to collagen XVII labeling. This was significantly diminished by both broad-spectrum extracellular kinase inhibitor K252b and a specific casein kinase 2 (CK2) inhibitor. Collagen XVII peptides containing a putative CK2 recognition site were phosphorylated by CK2 in vitro, disclosing Ser(542) and Ser(544) in the ectodomain as phosphate group acceptors. Phosphorylation of Ser(544) in vivo and in vitro was confirmed by immunoblotting of epidermis and HaCaT keratinocyte extracts with phosphoepitope-specific antibodies. Functionally, inhibition of CK2 kinase activity or mutation of the phosphorylation acceptor Ser(544) to Ala significantly increased ectodomain shedding, whereas overexpression of CK2alpha inhibited cleavage of collagen XVII. Structural modeling suggested that the phosphorylation of serine residues prevents binding of TACE to its substrate. Thus, extracellular phosphorylation of collagen XVII by ecto-CK2 inhibits its shedding by TACE and represents novel mechanism to regulate adhesion and motility of epithelial cells.

Proteomics of ionically bound and soluble extracellular proteins inMedicago truncatula leaves

A large proportion of the apoplast proteome resides in the intercellular fluid (IF) or is ionically bound (IB) to the wall matrix. A combined analysis of IF and IB proteins of the Medicago truncatula leaf apoplast was performed. 2-DE analyses demonstrated the reproducible presence of 220 IF and 84 IB proteins in the apoplast. These two protein populations were largely distinct; 22 proteins could be spatially matched, but MALDI-TOF/TOF analyses suggested a considerably smaller number had common identities. MALDI-TOF/TOF characterisation identified 81 distinct proteins. Analyses of selected IF proteins (45) indicated 17 distinct proteins with mainly defence-related functions, whereas analyses of IB proteins (70) identified 63 distinct proteins of diverse natures, including proteins of non-canonical natures. The presence of non-canonical proteins in IB extracts is discussed in the light of evidence supporting a low level of contamination of purified walls from symplastic proteins. This work indicates that IB and IF proteins are functionally distinct fractions of the apoplast. The data obtained complements earlier studies of the Medicago proteome and therefore will be useful in future studies investigating the role of apoplastic proteins in plant processes.

Subepithelial fibroblasts in intestinal villi: roles in intercellular communication

Ingestion of food and water induces chemical and mechanical signals that trigger peristaltic reflexes in the gut. Intestinal villi are motile, equipped with chemosensors and mechanosensors, and transduce signaling to sensory neurons, but the exact mechanisms have not yet been elucidated. Subepithelial fibroblasts located under the villous epithelium form contractile cellular networks via gap junctions. The networks ensheathe lamina propria and are in close contact with epithelium, neural and capillary networks, smooth muscles, and immune cells. Unique characteristics of subepithelial fibroblasts have been revealed by primary cultures isolated from rat duodenal villi. They include rapid reversal changes in cell shape by cAMP reagents and endothelins, cell shape-dependent mechanosensitivity that induces ATP release as a paracrine mediator, contractile ability, and expression of various receptors for vasoactive and neuroactive substances. Herein, we review these characteristics that play a key role in the villi. They serve as a barrier/sieve, flexible mechanical frame, mechanosensor, and signal transduction machinery in the intestinal villi, which are regulated locally and dynamically by rapid cell shape conversion.

Cell surface adenylate kinase activity regulates the F(1)-ATPase/P2Y (13)-mediated HDL endocytosis pathway on human hepatocytes

We have previously demonstrated on human hepatocytes that apolipoprotein A-I binding to an ecto-F(1)-ATPase stimulates the production of extracellular ADP that activates a P2Y(13)-mediated high-density lipoprotein (HDL) endocytosis pathway. Therefore, we investigated the mechanisms controlling the extracellular ATP/ADP level in hepatic cell lines and primary cultures to determine their impact on HDL endocytosis. Here we show that addition of ADP to the cell culture medium induced extracellular ATP production that was due to adenylate kinase [see text] and nucleoside diphosphokinase [see text] activities, but not to ATP synthase activity. We further observed that in vitro modulation of both ecto-NDPK and AK activities could regulate the ADP-dependent HDL endocytosis. But interestingly, only AK appeared to naturally participate in the pathway by consuming the ADP generated by the ecto-F(1)-ATPase. Thus controlling the extracellular ADP level is a potential target for reverse cholesterol transport regulation.

Anion channels in astrocytes: biophysics, pharmacology, and function

The chloride/anion channels that have been so far identified in cultured astrocytes and those that have been confirmed in situ by a combination of mRNA identification, immunocytochemistry, and biophysical studies are reviewed. It is emphasized that we are just beginning to describe such channels and analyze their functions in astrocytes. The best-studied anion channels studied so far are those known as volume-regulated anion channels (VRACs). These, as for most channels, have been mainly studied in cultured astrocytes, but some correlative studies have been done in situ, because these channels have been emphasized as release routes for transmitters; namely, excitatory amino acids and ATP. They are activated by cell shape changes and cell swelling, and the release of amino acids and ATP and chloride currents, measured by whole cell clamping, by these processes has been well described, as is also their activation by low concentrations of extracellular ATP. However, the identity of these channels in astrocytes, as in all other cells, remains elusive. The potential involvement of VRACs in pathological states such as stroke, metastasis, and spreading depression is also discussed.

Functional down-regulation of volume-regulated anion channels in AQP4 knockdown cultured rat cortical astrocytes

In the brain, the astroglial syncytium is crucially involved in the regulation of water homeostasis. Accumulating evidence indicates that a dysregulation of the astrocytic processes controlling water homeostasis has a pathogenetic role in several brain injuries. Here, we have analysed by RNA interference technology the functional interactions occurring between the most abundant water channel in the brain, aquaporin-4 (AQP4), and the swelling-activated Cl(-) current expressed by cultured rat cortical astrocytes. We show that in primary cultured rat cortical astrocytes transfected with control small interfering RNA (siRNA), hypotonic shock promotes an increase in cellular volume accompanied by augmented membrane conductance mediated by volume-regulated anion channels (VRAC). Conversely, astroglia in which AQP4 was knocked down (AQP4 KD) by transfection with AQP4 siRNA changed their morphology from polygonal to process-bearing, and displayed normal cell swelling but reduced VRAC activity. Pharmacological manipulations of actin cytoskeleton in rat astrocytes, and functional analysis in mouse astroglial cells, which retain their morphology upon knockdown of AQP4, suggest that stellation of AQP4 KD rat cortical astrocytes was not causally linked to reduction of VRAC current. Molecular analysis of possible candidates of swelling-activated Cl(-) current provided evidence that in AQP4 KD astrocytes, there was a down-regulation of chloride channel-2 (CIC-2), which, however, was not involved in VRAC conductance. Inclusion of ATP in the intracellular saline restored VRAC activity upon hypotonicity. Collectively, these results support the view that in cultured astroglial cells, plasma membrane proteins involved in cell volume homeostasis are assembled in a functional platform.

Effects of hypo-osmotic stress on ATP release in isolated turbot (Scophthalmus maximus) hepatocytes

BACKGROUND INFORMATION

ATP is released from many cell types exposed to hypo-osmotic shock and is involved in RVD (regulatory volume decrease). Purinergic signalling events have been extensively investigated in mammals, but not in marine teleosteans.

RESULTS

The effect of hypo-osmotic shock on ATP release was examined in isolated hepatocytes from turbot (Scophthalmus maximus), a marine flatfish. Hypo-osmotic stress (240 mOsm x kg(-1)) induced a significant increase in ATP efflux, and was inhibited by a potential CFTR (cystic fibrosis transmembrane conductance regulator) inhibitor, glibenclamide, but not by the MDR1 (multidrug resistance 1) P-glycoprotein inhibitor, verapamil. ATP efflux could be a cAMP-dependent process, as IBMX (isobutylmethylxanthine) and forskolin triggered the process under iso-osmotic conditions. Protein kinases, including protein kinase C, could also be involved, as staurosporine and chelerythrine inhibited the mechanism. Calcium could contribute to ATP efflux as ionomycin, a calcium ionophore, elicited a rapid release under iso-osmotic conditions, and chelation using EGTA abolished ATP release under hypo-osmotic conditions. RVD was partially abolished by apyrase, an ATP scavenger, and suramin, a purinoceptor antagonist. Moreover, hypo-osmotic shock induced a rise in intracellular calcium which could be involved in RVD. Since extracellular ATP triggered an increase in cellular free-calcium content under iso-osmotic conditions, our results could indicate that hypo-osmotic-induced ATP efflux contributes to RVD in turbot hepatocytes by stimulating purinergic receptors, which may lead to activation of a calcium signalling pathway.

CONCLUSIONS

These data provide the first evidence of volume-sensitive ATP signalling for volume maintenance in a marine teleost fish cell type.

Preface: the concept and consequences of multidrug resistance

The problem of multidrug resistance (MDR) in human cancers led to the discovery 30 years ago of a single protein P-glycoprotein (P-gp), capable of mediating resistance to multiple structurally diverse drugs. P-gp became the archetypal eukaryotic ABC transporter gene, and studies of P-gp and related ABC transporters in both eukaryotes and bacteria have led to a basic mechanistic understanding of the molecular basis of MDR. Particular milestones along the way have been the identification of the homology between P-gp and bacterial transport proteins, the purification and functional reconstitution of P-gp into synthetic lipid systems, and the development of targeted therapies that attempt to overcome MDR by inhibiting P-gp. This preface places into this context some of the less well-explored themes developed in the MDR field, particularly various alternative models of P-gp action, evidence for parallel physiological roles for P-gp, and the unusual relationship between the substrate recognition capabilities of ABC transporters and their evolutionary history.

Purinergic signaling and kinase activation for survival in pulmonary oxidative stress and disease

Stimulus-induced release of endogenous ATP into the extracellular milieu has been shown to occur in a variety of cells, tissues, and organs. Extracellular ATP can propagate signals via P2 receptors that are essential for growth and survival of cells. Abundance of P2 receptors, their multiple isoforms, and their ubiquitous distribution indicate that they transmit vital signals. Pulmonary epithelium and endothelium are rich in both P2X and P2Y receptors. ATP release from lung tissue and cells occurs upon stimulation both in vivo and in vitro. Extracellular ATP can activate signaling cascades composed of protein kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol-3-kinase (PI3K). Here we summarize progress related to release of endogenous ATP and nucleotide signaling in pulmonary tissues upon exposure to oxidant stress. Hypoxic, hyperoxic, and ozone exposures cause a rapid increase of extracellular ATP in primary pulmonary endothelial and epithelial cells. Extracellular ATP is critical for survival of these cells in high oxygen and ozone concentrations. The released ATP, upon binding to its specific receptors, triggers ERK and PI3K signaling and renders cells resistant to these stresses. Impairment of ATP release and transmission of such signals could limit cellular survival under oxidative stress. This may further contribute to disease pathogenesis or exacerbation.

P-glycoprotein expression increases ATP release in respiratory cystic fibrosis cells

P-glycoprotein (Pgp) is a well-defined ATP-binding cassette (ABC) protein and a close relative of cystic fibrosis transmembrane conductance regulator (CFTR), whose dysfunction causes cystic fibrosis (CF). It is postulated that Pgp can complement deficient CFTR functions because of structural and functional homologies. One of the most relevant functions appears to be the regulation of ATP release, which influences mucociliary clearance in respiratory epithelia by nucleotide receptor stimulation. However, mechanisms involved in ATP secretion remain a controversial issue. In the present study, CF epithelial cells (sigmaCFTE29ó) were transduced with the retroviral vector MP1m encoding Pgp, and thus, a stable Pgp-overexpressing CF cell line (sigmaCFTE29óPgp) was established and used for studies of hypothesized CFTR complementation. In addition, overexpression of native Pgp in sigmaCFTE29ó could also be achieved by long-term treatment with colchicine, a drug, which may be of great interest in CF therapy. We confirmed that overexpression of Pgp causes a significant increase in cellular ATP release, which could even be enhanced by stimulation with hypoosmolar medium. A potential clinical benefit is discussed.

ATP release via anion channels

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

The role of anion channels and Ca2+ in addition to K+ channels in the physiological volume regulation of murine spermatozoa

Studies in the human, transgenic mice, and cattle indicate that sperm cell volume regulation plays an important role in male fertility as spermatozoa encounter a hypo-osmotic challenge upon ejaculation into the female tract. Physiological regulatory volume decrease (RVD) was examined using flow cytometry in murine sperm released into incubation medium mimicking uterine osmolality and including putative channel inhibitors. The involvement of K+ channels was indicated by the recovery of volume regulation by the K+ ionophore valinomycin in defective sperm from infertile transgenic mice, and from blockage of RVD by quinine in normal sperm. However, in neither case was the recovery complete. The involvement of volume-sensitive osmolyte and anion channels (VSOAC) were investigated using blockers effective in other cell types. NPPB (5-nitro-2(3-phenylpropylamino) benzoic acid) and tamoxifen inhibited RVD but SITS (4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulphonic acid) at 0.4 and 1 mM had no effect whereas DIDS (di-isothiocyanato-stilbene-2,2'-disulphonic acid) at 1 mM enhanced RVD. Verapamil, but not another P-glycoprotein antagonist cyclosporin, caused sperm swelling which persisted in the presence of valinomycin, in Ca2+-free medium and in the presence of thapsigargin, but swelling was abolished by the Ca2+ ionophore A23187. Nifedipine was slightly effective in blocking RVD. Analysis by Western blotting failed to reveal ClC-2 and ClC-3 members of the chloride channel family in murine or rat sperm proteins despite signal bands in positive tissue controls. These findings implicate the involvement of some unidentified VSOAC in sperm volume regulation, which is probably Ca+-dependent.

Characteristics of subepithelial fibroblasts as a mechano-sensor in the intestine: cell-shape-dependent ATP release and P2Y1 signaling

Subepithelial fibroblasts form a cellular network just under the epithelium of the gastrointestinal tract. Using primary cultured cells isolated from rat duodenal villi, we previously found that subepithelial fibroblasts reversibly changed cell morphology between flat and stellate-shape depending on intracellular cAMP levels. In this paper, we examined cell-cell communication via released ATP and Ca2+ signaling in the cellular network. Subepithelial fibroblasts were sensitive to mechanical stress such as ;touching' a cell with a fine glass rod and ;stretching' cells cultured on elastic silicone chamber. Mechanical stimulations evoked Ca2+-increase in the cells and ATP-release from the cells. The released ATP activated P2Y receptors on the surrounding cells and propagated Ca2+-waves through the network. Concomitant with Ca2+-waves, a transient contraction of the network was observed. Histochemical, RT-PCR, western blotting and Ca2+ response analyses indicated P2Y1 is a dominant functional subtype. ATP-release and Ca2+ signaling were cell-shape dependent, i.e. they were abolished in stellate-shaped cells treated with dBcAMP, and recovered or further enhanced in re-flattened cells treated with endothelin. The response to ATP also decreased in stellate-shaped cells. These findings indicate cAMP-mediated intracellular signaling causes cell-shape change, which accompanies the changes in mechano- and ATP sensitivities. Using a co-culture system of neuronal cells (NG108-15) with subepithelial fibroblasts, we confirmed that mechanically induced Ca2+-waves propagated to neurons. From these findings we propose that subepithelial fibroblasts work as a mechanosensor in the intestine. Uptake of food, water and nutrients may cause mechanical stress on subepithelial fibroblasts in the villi. The ATP released by mechanical stimulation elicits Ca2+-wave propagation through the network via P2Y1 activation and also activates P2X on terminals of mucosal sensory neurons to regulate peristaltic motility.

Human keratinocytes release ATP and utilize three mechanisms for nucleotide interconversion at the cell surface

Nucleotide activation of P2 receptors is important in autocrine and paracrine regulation in many tissues. In the epidermis, nucleotides are involved in proliferation, differentiation, and apoptosis. In this study, we have used a combination of luciferin-luciferase luminometry, pharmacological inhibitors, and confocal microscopy to demonstrate that HaCaT keratinocytes release ATP into the culture medium, and that there are three mechanisms for nucleotide interconversion, resulting in ATP generation at the cell surface. Addition of ADP, GTP, or UTP to culture medium elevated the ATP concentration. ADP to ATP conversion was inhibited by diadenosine pentaphosphate, oligomycin, and UDP, suggesting the involvement of cell surface adenylate kinase, F(1)F(0) ATP synthase, and nucleoside diphosphokinase (NDPK), respectively, which was supported by immunohistochemistry. Simultaneous addition of ADP and GTP elevated ATP above that for each nucleotide alone indicating that GTP acts as a phosphate donor. However, the activity of NDPK, F(1)F(0) ATP synthase or the forward reaction of adenylate kinase could not fully account for the culture medium ATP content. We postulate that this discrepancy is due to the reverse reaction of adenylate kinase utilizing AMP. In normal human skin, F(1)F(0) ATP synthase and NDPK were differentially localized, with mitochondrial expression in the basal layer, and cell surface expression in the differentiated layers. We and others have previously demonstrated that keratinocytes express multiple P2 receptors. In this study we now identify the potential sources of extracellular ATP required to activate these receptors and provide better understanding of the role of nucleotides in normal epidermal homeostasis and wound healing.