Signal transduction pathways coupled to a P2U receptor in neuroblastoma x glioma (NG108-15) cells

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Synthesis of uracil nucleotide analogs with a modified, acyclic ribose moiety as P2Y(2) receptor antagonists

A series of new uracil nucleotide analogs (monophosphates, triphosphates, and phosphonates) was synthesized, in which the ribose moiety was replaced by acyclic chains, including branched or linear alkyl or dialkylether linkers. 1-omega-Bromoalkyluracil derivatives (2) were converted to the corresponding alcohols by treatment with sodium hydroxide and subsequently phosphorylated using phosphorus oxychloride followed by hydrolysis to yield the monophosphates, or by coupling with diphosphate to form the triphosphates. Reaction of 2 with triethyl phosphite followed by deprotection with trimethylsilyl bromide led to the omega-phosphonylalkyluracil derivatives. These products could be further phosphorylated by converting them into their imidazolides and subsequent treatment with diphosphate yielding the corresponding UTP analogs. Nucleoside analogs with an oxygen atom in the 2'-position, which are more similar to the natural ribosides, were synthesized from silylated uracil and trimethylsilyl iodide-treated 1,3-dioxolane, or 1,3-dioxane, respectively, and subsequently phosphorylated by standard procedures. The nucleotide analogs were investigated in a functional assay at NG108-15 cells, a neuroblastomaxglioma hybrid cell line which expresses the UTP- and ATP-activated nucleotide receptor subtype P2Y(2). The acyclic nucleotide analogs were generally weaker ligands than UTP, and-in contrast to UTP-they were antagonistic. The most potent compound was diphosphoric 5-(2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)pentylphosphonic anhydride (5c) with an IC(50) value of 92microM showing that the replacement of the alpha-phosphate by phosphonate, which leads to enhanced stability, was well tolerated.

P2U purinoceptors: cDNA cloning, signal transduction mechanisms and structure-function analysis

The cloning of a P2U purinoceptor cDNA has made it possible to use molecular biological approaches to investigate P2U purinoceptor function. Expression of recombinant P2U purinoceptors in mammalian cells lacking endogenous P2U purinoceptors has enabled us to characterize the receptor protein and its downstream effectors, and has allowed a partial analysis of the role of certain amino acid residues in ligand binding. These approaches have placed the pharmacological classification of the P2U purinoceptor on a firm molecular footing and have generated model systems that can be used to investigate receptor-ligand binding, regulation and signal transduction.

The influence of rutin on the extracellular matrix in streptozotocin-induced diabetic rat kidney

We previously reported that rutin administration to streptozotocin (STZ)-induced diabetic rats decreased plasma glucose and increased plasma insulin levels. In this study, we have examined the role of rutin on matrix remodelling in the kidney of STZ-induced diabetic rats. STZ was administered intraperitoneally (50 mg kg(-1)) to male albino Wistar rats to induce experimental diabetes. Rutin (100 mg kg(-1)) was orally administered to normal and STZ-induced diabetic rats for a period of 45 days and its influence on the content of hydroxyproline and collagen and on the activity of matrix metalloproteinases (MMPs) were studied. We have also studied the levels of tissue inhibitors of metalloproteinases (TIMPs) in the kidney. STZ-induced diabetic control rats showed increased content of hydroxyproline and collagen, decreased activity of MMPs and increased levels of TIMPs in the kidney. These changes were positively modulated by rutin treatment in STZ-induced diabetic rats, thereby protecting the kidney. In normal rats treated with rutin, none of the parameters studied were significantly altered. From the results obtained, we could conclude that rutin influences MMPs and effectively protects kidney against STZ-induced damage in rats. The effects observed are due to the reduction of plasma glucose levels by rutin.

Neuronal and glial cell lines as model systems for studying P2Y receptor pharmacology

Investigation of the role of extracellular nucleotides in nervous system has been one of the main topics of the P2Y receptor research throughout the years. In parallel to numerous studies on primary culture systems, various neuronal and non-neuronal cell lines have been used to model in vitro the processes mediated by extracellular nucleotides. In this review article, a survey of expression profiles of G protein-coupled P2Y receptor subtypes in nervous-system-derived cell lines is presented, by analysing the receptor expression at the mRNA, protein, and functional level. The variability of receptor expression profiles in established cell lines is further discussed, bringing forward some general properties for neuronal and glial malignant cell lines.

Cl- channels are expressed in human normal monocytes: a functional role in migration, adhesion and volume change

Increased adhesion and diapedesis of monocytes appear to be primary initiating factors in the pathophysiology of occlusive vascular diseases, including atherosclerosis and restenosis. However, the underlying mechanisms of transendothelial migration and invasion of monocytes into the blood vessels are not known. Alterations in ion channels on the cell membrane are generally involved in induced changes in shape and volume. In the present study, we investigated the expression and functional role of chloride channels in freshly isolated human blood monocytes. The Cl- currents in whole-cells were measured by the patch-clamp technique. We observed whole cell Cl- currents, which were time-independent and outwardly rectifying. The chloride channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and R(+)-[(6,7-dichloro-2-cyclopentyl-2,3-dihydro-2-methyl-1-oxo-1H-inden-5yl)-oxy]acetic acid 94 (IAA94) attenuated the Cl- currents. NPPB and IAA94 also inhibited chemotaxis of monocytes, as measured in Boyden chemotactic chambers, with the same sensitivity. NPPB but not IAA94, increased the cell volume as measured by shape change, and decreased tumour necrosis factor (TNF)-alpha-induced monocyte adhesion to endothelial cells. These results suggest that monocytes contain Cl- channels which regulate transendothelial migration of monocytes, due presumably to an alteration in cell volume.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Extracellular metabolism of nucleotides in neuroblastoma x glioma NG108-15 cells determined by capillary electrophoresis

1. The metabolism of extracellular nucleotides in NG108-15 cells, a neuroblastoma x glioma hybrid cell line, was studied by means of capillary zone electrophoresis (CZE) and micellar electrokinetic capillary chromatography (MECC). 2. In NG108-15 cells ATP, ADP, AMP, UTP, UDP, and UMP were hydrolyzed to the nucleosides adenosine and uridine indicating the presence of ecto-nucleotidases and ectophosphatases. The hydrolysis of the purine nucleotides ATP and ADP was significantly faster than the hydrolysis of the pyrimidine nucleotides UTP and UDP. 3. ATP and UTP breakdown appeared to be mainly due to an ecto-nucleotide-diphosphohydrolase. ADP, but not UDP, was initially also phosphorylated to some extent to the corresponding triphosphate, indicating the presence of an adenylate kinase on NG108-15 cells. The alkaline phosphatase (ALP) inhibitor levamisole did not only inhibit the hydrolysis of AMP to adenosine and of UMP to uridine, but also the degradation of ADP and to a larger extent that of UDP. ATP and UTP degradation was only slightly inhibited by levamisole. 4. These results underscore the important role of ecto-alkaline phosphatase in the metabolism of adenine as well as uracil nucleotides in NG108-15 cells Dipyridamole, a potent inhibitor of nucleotide breakdown in superior cervical ganglion cells, had no effect on nucleotide degradation in NG108-15 cells. 5. Dipyridamole, which is a therapeutically used nucleoside reuptake inhibitor in humans, reduced the extracellular adenosine accumulation possibly by allosteric enhancement of adenosine reuptake into the cells.

Chick RGS2L demonstrates concentration-dependent selectivity for pertussis toxin-sensitive and -insensitive pathways that inhibit L-type Ca2+ channels

In neuronal cells, the influx of Ca2+ ions through voltage-dependent L-type calcium (L) channels couples excitation to multiple cellular functions. In addition to voltage, several neurotransmitters, hormones and cytokines regulate L channel gating via binding to G-protein-coupled receptors. Intracellular molecules that modify G-protein activity - such as regulator of G-protein-signalling (RGS) proteins - are therefore potential candidates for regulating Ca2+ influx through L channels. Here we show that a novel RGS2 splice variant from chick dorsal root ganglion (DRG) neurons, RGS2L, reduces bradykinin (BK)-mediated inhibition of neuronal L channels and accelerates recovery from inhibition. Chick RGS2 reduces the inhibition mediated by both the pertussis toxin (PTX)-sensitive (Gi/o-coupled) and the PTX-insensitive (presumably Gq/11-coupled) pathways. However, we demonstrate for the first time in a living cell that the extent of coupling to each pathway varies with RGS2L concentration. A low concentration of recombinant chick RGS2L (10 nM) preferentially reduces the inhibition mediated by the PTX-insensitive pathway, whereas a 100-fold higher concentration attenuates both PTX-sensitive- and PTX-insensitive-mediated components equally. Our data suggest that factors promoting RGS2L gene induction may regulate Ca2+ influx through L channels by recruiting low-affinity interactions with Gi/o that are absent at basal RGS2L levels.

Role of PKC and MAPK in cytosolic PLA2 phosphorylation and arachadonic acid release in primary murine astrocytes

Although Group IV cytosolic phospholipase A2 (cPLA2) in astrocytes has been implicated in a number of neurodegenerative diseases, mechanisms leading to its activation and release of arachidonic acid (AA) have not been clearly elucidated. In primary murine astrocytes, phorbol myristate acetate (PMA) and ATP stimulated phosphorylation of ERK1/2 and cPLA2 as well as evoked AA release. However, complete inhibition of phospho-ERK by U0126, an inhibitor of mitogen-activated protein kinase kinase (MEK), did not completely inhibit PMA-stimulated cPLA2 and AA release. Epidermal growth factor (EGF) also stimulated phosphorylation of ERK1/2 and cPLA2[largely through a protein kinase C (PKC)-independent pathway], but EGF did not evoke AA release. These results suggest that phosphorylation of cPLA2 due to phospho-ERK is not sufficient to evoke AA release. However, complete inhibition of ATP-induced cPLA2 phosphorylation and AA release was observed when astrocytes were treated with GF109203x, a general PKC inhibitor, together with U0126, indicating the important role for both PKC and ERK in mediating the ATP-induced AA response. There is evidence that PMA and ATP stimulated AA release through different PKC isoforms in astrocytes. In agreement with the sensitivity of PMA-induced responses to PKC down-regulation, prolonged treatment with PMA resulted in down-regulation of PKCalpha and epsilon in these cells. Furthermore, PMA but not ATP stimulated rapid translocation of PKCalpha from cytosol to membranes. Together, our results provided evidence for an important role of PKC in mediating cPLA2 phosphorylation and AA release in astrocytes through both ERK1/2-dependent and ERK1/2-independent pathways.

The mRNA expression of serotonin 2C subtype receptors uncoupled with inositol hydrolysis in NG108-15 cells

Cell culture systems seem to be useful for clarifying the cellular physiological mechanisms of serotonin 2C subtype receptors (5-HT2CR) and related drug action mechanisms. However, there are still few reports about cells that contain intrinsic 5-HT2CR. This report demonstrates by using RT/PCR that 5-HT2CR mRNA exists in splicing variant forms in NGI08-15 cells. The PCR results using a pair of primers that recognized sequences near the third intracellular loop site showed two neighboring bands at about 500 bp upon electrophoresis in acrylamide gels. The sequence analysis demonstrated that one band was the rat 5-HT2CR sequence and the other one was that of the mouse. Serotonin, however, did not enhance the inositol phosphates formation in NG108-15 cells. It has been reported that post-translational modifications of RNA, splicing and editing, occur at the site of the second intracellular loop domain in 5-HT2CR mRNA. Accordingly, a pair of primers that recognized this site were designed. The molecular size of the PCR product was shorter than that expected based on the sequence of the native 5-HT2CR. The fragment lacked the 95 nucleotides of native 5-HT2CR mRNA. This seems to be the reason why serotonin did not enhance inositol phosphates formation in NG108-15 cells.

An interaction between ATP and high K+: mutual impairment of ATP- and high K(+)-evoked [Ca2+]i increase in NG 108-15 cells

The interaction between ATP- and high K(+)-evoked increase in intracellular free calcium concentration ([Ca2+]i) was investigated to gain an insight into the mechanism of interaction of ATP with voltage-sensitive calcium channels. [Ca2+]i was measured in the neuronal model, neuroblastoma x glioma hybrid cells (NG 108-15), using the fluorescence indicator fura-2. In the presence of 1.8 mM extracellular Ca2+, ATP induced a rapid, concentration-dependent increase in [Ca2+]i. High K+ (50 mM) evoked a [Ca2+]i rise from 109 +/- 11 nM to 387 +/- 81 nM (n = 16). The application of either of these two [Ca2+]i-increase provoking agents in sequence with the other caused impairment of the latter effect. The mutual desensitization of the responses to ATP and high K+ strongly suggests that both agents rely at least in part on the same source of Ca2+ for elevation of [Ca2+]i in NG 108-15 cells.

Pharmacological characterisation of pyrimidinoceptor responses in NG108-15 cells

In the present study, the P2Y receptor(s) mediating the effects of the pyrimidines UTP and UDP on phospholipase C activation in the mouse neuroblastoma x rat glioma hybrid cell line NG108-15 was investigated. Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) analysis detected transcripts for the P2Y(6) and P2Y(2) receptors, but not for P2Y(1) and P2Y(4.) UTP and UDP were equipotent agonists and their effects were partially additive. Suramin, reactive blue 2 and pyridoxal phosphate-6-azophenyl-2',4'disulfonic acid (PPADS) antagonised the phospholipase C response to both UTP and UDP. High micromolar concentrations of adenosine, 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680), 2',3'-O-isopropylideneadenosine (iPAdo) and adenosine 3':5'-cyclic monophosphate (3',5'-cAMP) were able to antagonise the effect of UTP on phospholipase C but not that of UDP. The additivity of the UTP and UDP responses, novel P2 receptor antagonist profile and the distinguishing action of adenosine may indicate the expression of a pyrimidine selective P2Y receptor in addition to the P2Y(6) type in these cells.

Mechanisms of agonist-dependent and -independent desensitization of a recombinant P2Y2 nucleotide receptor

UTP activates P2Y, receptors in both 1321N1 cell transfectants expressing the P2Y2 receptor and human HT-29 epithelial cells expressing endogenous P2Y, receptors with an EC50 of 0.2-1.0 microM. Pretreatment of these cells with UTP diminished the effectiveness of a second dose of UTP (the IC50 for UTP-induced receptor desensitization was 0.3-1.0 microM for both systems). Desensitization and down-regulation of the P2Y2 nucleotide receptor may limit the effectiveness of UTP as a therapeutic agent. The present studies investigated the phenomenon of P2Y2 receptor desensitization in human 1321N1 astrocytoma cells expressing recombinant wild type and C-terminal truncation mutants of the P2Y2 receptor. In these cells, potent P2Y2 receptor desensitization was observed after a 5 min exposure to UTP. Full receptor responsiveness returned 5-10 min after removal of UTP. Thapsigargin, an inhibitor of Ca2+-ATPase in the endoplasmic reticulum, induced an increase in the intracellular free calcium concentration, [Ca2+]i, after addition of desensitizing concentrations of UTP, indicating that P2Y2 receptor desensitization is not due to depletion of calcium from intracellular stores. Single cell measurements of increases in [Ca2+]i induced by UTP in 1321N1 cell transfectants expressing the P2Y2 receptor indicate that time- and UTP concentration-dependent desensitization occurred uniformly across a cell population. Other results suggest that P2Y2 receptor phosphorylation/dephosphorylation regulate receptor desensitization/resensitization. A 5 min preincubation of 1321N1 cell transfectants with the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), reduced the subsequent response to UTP by about 50%, whereas co-incubation of PMA with UTP caused a greater inhibition in the response. The protein phosphatases-1 and -2A inhibitor, okadaic acid, partially blocked resensitization of the receptor. Furthermore, C-terminal truncation mutants of the P2Y2 receptor that eliminated several potential phosphorylation sites including two for PKC were resistant to UTP-, but not phorbol ester-induced desensitization. Down regulation of protein kinase C isoforms prevented phorbol ester-induced desensitization but had no effect on agonist-induced desensitization of wild type or truncation mutant receptors. These results suggest that phosphorylation of the C-terminus of the P2Y2 receptor by protein kinases other than protein kinase C mediates agonist-induced receptor desensitization. A better understanding of the molecular mechanisms of P2Y2 nucleotide receptor desensitization may help optimize a promising cystic fibrosis pharmacotherapy based on the activation of anion secretion in airway epithelial cells by P2Y, receptor agonists.

Effect of ethanol on ATP-induced phospholipases C and D and serine base exchange in glioma C6 cells

The effect of extracellular ATP, a nucleotide receptor agonist in the central nervous system, was investigated in glioma C6 cells on the intracellular Ca2+ level and the formation of phosphatidylethanol and phosphatidic acid in the presence and absence of ethanol (150 mM). In the cells prelabeled with [14C]palmitic acid, 100 microM ATP induced both the hydrolysis and the transphosphatidylation reactions leading to the formation of [14C]phosphatidic acid; addition of ethanol generated [14C]phosphatidylethanol. However, ATP-mediated increase in the level of [14C]phosphatidic acid was not inhibited by ethanol. Furthermore, ethanol augmented ATP-induced transient and sustained increase in the intracellular Ca2+ concentration, whereas ethanol alone did not produce any change in the intracellular Ca2+ level. These results indicate that in glioma C6 cells, ATP induces activation of polyphosphoinositide-specific phospholipase C and phospholipase D and that ethanol enhances this effect. In the present investigation we have also shown that long-term (2 days) ethanol treatment, at concentration relevant to chronic alcoholism (100 mM), decreased the incorporation of [14C]serine into phosphatidylserine. Since the effect of ethanol on ATP-induced activities of phospholipase C and phospholipase D and on serine base-exchange in glioma C6 cells differs significantly from that in cultured neuronal cells, these results may contribute to a better understanding of the mechanisms of ethanol action in cells of glial origin.

Phosphatidic acid as the biosynthetic precursor of the endocannabinoid 2-arachidonoylglycerol in intact mouse neuroblastoma cells stimulated with ionomycin

In mouse neuroblastoma N18TG2 cells prelabeled with [3H]arachidonic acid ([3H]AA) the biosynthesis of 2-arachidonoylglycerol (2-AG) is induced by ionomycin in a fashion sensitive to an inhibitor of diacylglycerol (DAG) lipase, RHC 80267, but not to four different phospholipase C (PLC) blockers. Pulse experiments with [3H]AA showed that ionomycin stimulation leads to the sequential formation of [3H]phosphatidic acid ([3H]PA), [3H]DAG, and [3H]2-AG. [3H]2-AG biosynthesis in N18TG2 cells prelabeled with [3H]AA was counteracted by propranolol and N-ethylmaleimide, two inhibitors of the Mg2+/Ca2(+)-dependent brain PA phosphohydrolase. Pretreatment of cells with exogenous phospholipase D (PLD) led to a strong potentiation of ionomycin-induced [3H]2-AG formation. These data indicate that DAG precursors for 2-AG in intact N18TG2 cells are obtained from the hydrolysis of PA and not through the activation of PLC. The presence of 2% ethanol during ionomycin stimulation failed to elicit the synthesis of [3H]phosphatidylethanol and did not counteract the formation of [3H]PA, thus arguing against the activation of PLD by the Ca2+ ionophore. Selective inhibitors of secretory phospholipase A2 and the acyl-CoA acylase inhibitor thimerosal significantly reduced [3H]2-AG biosynthesis. The implications of these latter findings, and of the PA-dependent pathways of 2-AG formation described here, are discussed.

Voltage-sensitive Ca2+ channels, intracellular Ca2+ stores and Ca2+-release-activated Ca2+ channels contribute to the ATP-induced [Ca2+]i increase in differentiated neuroblastoma x glioma NG 108-15 cells

Activation of ionotropic P2X7 purinoreceptors in NG108-15 cells directly opens non-selective cation channels, leading to an increase in intracellular Ca2+ concentration ([Ca2+]i) and membrane depolarization and, hence, by indirect opening of voltage-stimulated Ca2+ channels (VSCC) to further increases of [Ca2+]i, whereas activation of the metabotropic P2Y receptor causes intracellular Ca2+ release. The quantitative contribution of Ca2+ entry and release to ATP-induced [Ca2+]i increase in differentiated NG108-15 cells is not known. Here we have investigated the Ca2+ influx and Ca2+ release components by studying [Ca2+]i in Fura-2-loaded cells and by using the following tools: nifedipine to block L-type VSCC, omega-conotoxin GVIa (omegaCT) to block N-type VSCC and thapsigargin to deplete intracellular Ca2+ stores. With 1.8 mM Ca2+ in the medium, ATP (600 microM) increased [Ca2+]i by 656 +/- 50 nM (n = 11). This response was reduced to 72% by nifedipine (50 microM), to 63% by omegaCT (1 microM), and to 31% by nifedipine and omegaCT in combination. Since nifedipine and omegaCT completely block VSCC in our model, the remaining 31% of [Ca2+]i increase could be caused by influx via P2X7-activated non-selective channels or by intracellular release mediated by P2Y receptors. When Ca2+-free medium was used to exclude Ca2+ influx, ATP (600 microM) increased [Ca2+]i by only 34 +/- 4 nM (n = 4), indicating that the majority of [Ca2+]i increase depends on Ca2+ influx. A similar rise by 37 +/- 4 nM (n = 4) was observed with the selective P2Y agonist UTP (150 microM). This small response was sensitive to thapsigargin and hence represents Ca2+ release. The remainder (i.e. total [Ca2+]i increase minus nifedipine-, omegaCT- and thapsigargin-sensitive [Ca2+]i increases) should, therefore, represent Ca2+ influx via P2X7 non-selective cation channels.

Structural basis of agonist-induced desensitization and sequestration of the P2Y2 nucleotide receptor. Consequences of truncation of the C terminus

Molecular determinants of P2Y2 receptor desensitization and sequestration have been investigated. Wild-type P2Y2 receptors and a series of five C-terminal truncation mutants of the receptor were epitope-tagged and stably expressed in 1321N1 cells. These constructs were used to assess the importance of the intracellular C terminus on 1) UTP-stimulated increases in intracellular calcium concentration, 2) homologous desensitization of the receptor, and 3) agonist-induced decreases in cell-surface density (receptor sequestration) of epitope-tagged receptors using fluorescence-activated cell sorting. The potency and efficacy of UTP were similar for the wild-type and all mutant P2Y2 receptors. Truncation of 18 or more amino acids from the C terminus increased by approximately 30-fold the concentration of UTP necessary to desensitize the receptor. Both the rate and magnitude of UTP-induced receptor sequestration were decreased with progressively larger truncations of the C terminus. Furthermore, the recovery from sequestration was slower for the most extensively truncated receptor. Complete desensitization was obtained with >50% of the original receptor complement remaining on the cell surface. Protein kinase C activation, which desensitizes the P2Y2 receptor, had no effect on sequestration, consistent with the ideas that desensitization and sequestration are discrete events and that agonist occupancy is required for receptor sequestration.

P2Y2 nucleotide receptors expressed heterologously in sympathetic neurons inhibit both N-type Ca2+ and M-type K+ currents

The P2Y2 receptor is a uridine/adenosine triphosphate (UTP/ATP)-sensitive G-protein-linked nucleotide receptor that previously has been reported to stimulate the phosphoinositide signaling pathway. Messenger RNA for this receptor has been detected in brain tissue. We have investigated the coupling of the molecularly defined rat P2Y2 receptor to neuronal N-type Ca2+ channels and to M-type K+ channels by heterologous expression in rat superior cervical sympathetic (SCG) neurons. After the injection of P2Y2 cRNA, UTP inhibited the currents carried by both types of ion channel. As previously reported [Filippov AK, Webb TE, Barnard EA, Brown DA (1997) Inhibition by heterologously expressed P2Y2 nuerones. Br J Pharmacol 121:849-851], UTP inhibited the Ca2+ current (ICa(N)) by up to 64%, with an IC50 of approximately 0.5 microM. We now find that UTP also inhibited the K+M current (IK(M)) by up to 61%, with an IC50 of approximately 1.5 microM. UTP had no effect on either current in neurons not injected with P2Y2 cRNA. Structure-activity relations for the inhibition of ICa(N) and IK(M) in P2Y2 cRNA-injected neurons were similar, with UTP >/= ATP > ITP >> GTP,UDP. However, coupling to these two channels involved different G-proteins: pretreatment with Pertussis toxin (PTX) did not affect UTP-induced inhibition of IK(M) but reduced inhibition of ICa(N) by approximately 60% and abolished the voltage-dependent component of this inhibition. In unclamped neurons, UTP greatly facilitated depolarization-induced action potential discharges. Thus, the single P2Y2 receptor can couple to at least two G-proteins to inhibit both Ca2+N and K+M channels with near-equal facility. This implies that the P2Y2 receptor may induce a broad range of effector responses in the nervous system.

Diverse signal transduction pathways mediated by endogenous P2 receptors in cultured rat cerebral cortical neurons

The present study was conducted to assess the intracellular signaling pathways mediated by receptors for ATP, uridine triphosphate (UTP), and 2-methylthio ATP (2-MeSATP), by monitoring patch-clamp currents and intracellular calcium mobilization in cultured rat cortical cerebral neurons. All three agonists evoked potassium currents and increased the intracellular free Ca2+ concentration ([Ca2+]i), and these effects were inhibited by the broad G-protein inhibitor guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS) but not by the Gi/o-protein inhibitor pertussis toxin (PTX). UTP-evoked currents were inhibited by either the phospholipase C inhibitor neomycin or the selective protein kinase C (PKC) inhibitor GF109203X, and the rise in cytosolic Ca2+ was inhibited by either neomycin or the inositol 1,4,5-trisphosphate (IP3) receptor antagonist heparin, indicating that the UTP receptor involved phospholipase C-mediated phosphatidylinositol signaling. In contrast, 2-MeSATP-induced currents and rise in cytosolic Ca2+ were not inhibited by either neomycin, or GF109203X, or heparin. 2-MeSATP elicited single-channel currents in the cell-attached patch-clamp configuration and also in excised patches. The G-protein activator GTP gamma S induced single-channel currents in a fashion that mimicked the effect of 2-MeSATP. These data suggest that 2 MeSATP activated potassium channels by a direct action of G-protein beta gamma subunits and increased [Ca2+]i by a mechanism independent of phospholipase C stimulation and IP3 production. ATP-evoked currents were partially inhibited by either neomycin or GF109203X, although the rise in cytosolic Ca2+ was not affected by these inhibitors. ATP produced single-channel currents with two major classes of the slope conductance (86 and 95 pS) in cell-attached patches, each of which is consistent with that achieved by 2-MeSATP (85 pS) or UTP (96 pS); the currents with the lower conductance were observed in the outside-out patch-clamp configuration. These results indicate that P2 receptors for UTP and 2-MeSATP are linked to a PTX-insensitive G-protein involving different signal transduction pathways and that ATP responses are mediated by both of these P2 receptors.

Effects of IL-1 beta on receptor-mediated poly-phosphoinositide signaling pathway in immortalized astrocytes (DITNC)

Astrocytes are known to play multi-functional roles in support of many homeostatic mechanisms in the central nervous system including host defense mechanisms. Despite the ability of cytokines to alter gene expression and cellular activity, their effect on receptor-mediated poly-phosphoinositide (poly-PI) signaling pathway has not been examined in detail. In this study, an immortalized astrocyte cell line (DITNC) was used to test the effect of IL-1 beta exposure on the poly-PI signaling pathway. Similar to primary astrocytes, DITNC cells exhibit P2-purinergic receptor response to ATP and UTP leading to transient increases in inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and intracellular calcium concentration, [Ca2+]i. Upon exposure of DITNC cells to IL-1 beta (100 U/ml) for 24 hrs, an increased response to the poly-PI agonists was observed. The increase in ATP-mediated Ins(1,4,5)P3 release could not be attributed to a shift in the ATP dose or an alteration of the time profile for the release of Ins(1,4,5)P3. Since the increase in response required a lag time of 4 hr after IL-1 beta exposure, it is unlikely that this effect was due to a direct interaction of IL-1 beta with the purinergic receptor. On the other hand, an increase in ATP response could be observed in DITNC cells exposed to conditioned medium obtained after IL-1 beta treatment. It can be concluded that exposure of astrocytes to cytokines may lead to an increase in receptor-mediated poly-PI signaling activity and this may involve compounds secreted into the culture medium, e.g., the secretory phospholipase A2.

HEK293 human embryonic kidney cells endogenously express the P2Y1 and P2Y2 receptors

Adenine and uridine nucleotide-promoted inositol phosphate accumulation was studied in HEK293 cells. Concentration effect curves for ADP, ATP, and 2ClATP were complex and could be resolved by a two-site model into low and high potency components, suggesting the involvement of two receptors. The maximal effect observed for the P2Y1 receptor-selective agonists 2MeSATP and 2MeSADP was 65-70% of that observed with ATP, ADP, or 2ClATP, and the concentration effect curves for these two analogs were consistent with their interaction at a single site. The P2Y1 receptor-selective antagonist PPADS completely blocked 2MeSATP-stimulated inositol phosphate accumulation, but only partially antagonized the response to ATP. UTP also was an agonist, but the maximal effect observed was approximately 25% of that observed with ATP or ADP. In the presence of maximally effective concentrations of UTP, the concentration effect curves to 2C1ATP and ADP followed law of mass action interaction at a single site, and their maximal elevation of inositol phosphate accumulation was equivalent to that observed with 2MeSATP and 2MeSADP. The order of potency of adenine nucleotide agonists in the presence of a maximally effective concentration of UTP was consistent with that for interaction with a P2Y1 receptor. Thus, HEK293 cells apparently express two subtypes of P2Y receptors that respond to ADP or ATP in an additive manner: a P2Y1 receptor, which is selectively activated by 2MeSADP, and a P2Y2 receptor, which is selectively activated by UTP.

Uridine nucleotide receptors and their ligands: structural, physiological, and pathophysiological aspects, with special emphasis on the nervous system

This review presents data on metabotropic uridine nucleotide receptor subtypes (P2UR) activated by UTP, sometimes also by UDP and/or ATP. Some chemical details of receptor subtypes and ligand interactions are described. Ligand-activated P2UR subtypes may couple to different second messengers, yet little is known about the nature of the coupling G-proteins. Data evaluating UTP as a physiological ligand include UTP origin, release and metabolism and illuminate especially roles for P2UR in the nervous system. No evidence shows UTP as a synaptic transmitter; sympathetic neurons may, however, carry P2UR allowing UTP-stimulation of norepinephrine release. UTP and derivatives act as therapeutic agents in several diseases involving mutated genes of transepithelial conductance regulators, including cystic fibrosis. This focuses interest to the synthesis of new compounds. Further, therapeutically used pyrimidine and pyrimidine analogues are suspected to have CNS-pathological effects. The presently scarce information in these areas strongly underlines the need for and importance of intense research on the suspected pyrimidine derivative triggered pathology as well as on the role of P2UR receptors in physiology and pathophysiology.

Intracellular Ca2+ signals induced by ATP and thapsigargin in glioma C6 cells. Calcium pools sensitive to inositol 1,4,5-trisphosphate and thapsigargin

In glioma C6 cells, extracellular ATP generates inositol 1,4,5-trisphosphate (InsP3), indicating the presence of purinergic receptors coupled to phosphoinositide turnover. To identify the effect of ATP (acting via InsP3) and thapsigargin (acting without InsP3 production as a specific inhibitor of the endoplasmic reticulum Ca(2+)-ATPase) on intracellular Ca2+ pools we used video imaging of Fura-2 loaded into single, intact glioma C6 cells. It has been shown that ATP and thapsigargin initiate Ca2+ response consistent with the capacitative model of Ca2+ influx. When the cells were stimulated by increasing concentrations of ATP (1, 10, 50 and 100 microM) the graded, quantal Ca2+ response was observed. In the absence of extracellular Ca2+ thapsigargin and ionomycin-releasable Ca2+ pools are overlapping, demonstrating that Ca2+ stores are located mainly in the endoplasmic reticulum. After maximal Ca2+ mobilization by ATP, thapsigargin causes further increase in cytosolic Ca2+ concentration, whereas emptying of thapsigargin-sensitive intracellular stores prevents any further Ca2+ release by ATP. Thus, the thapsigargin-sensitive intracellular pool of Ca2+ in glioma C6 cells seems to be larger than that sensitive to InsP3. Two hypothesis to explain this result are proposed. One postulates a presence of two different Ca2+ pools, sensitive and insensitive to InsP3 and both discharged by thapsigargin, and the other, the same intracellular pool of Ca2+ completely emptying by thapsigargin and only partially by InsP3. These results may contribute to understanding the mechanism of Ca2+ signalling mediated by ATP, the most potent intracellular Ca2+ mobilizing agonist in all types of glial cells.

Dual role of protein kinase C in the regulation of cPLA2-mediated arachidonic acid release by P2U receptors in MDCK-D1 cells: involvement of MAP kinase-dependent and -independent pathways

Defining the mechanism for regulation of arachidonic acid (AA) release is important for understanding cellular production of AA metabolites, such as prostaglandins and leukotrienes. Here we have investigated the differential roles of protein kinase C (PKC) and mitogen-activated protein (MAP) kinase in the regulation of cytosolic phospholipase A2 (cPLA2)-mediated AA release by P2U-purinergic receptors in MDCK-D1 cells. Treatment of cells with the P2U receptor agonists ATP and UTP increased PLA2 activity in subsequently prepared cell lysates. PLA2 activity was inhibited by the cPLA2 inhibitor AACOCF3, as was AA release in intact cells. Increased PLA2 activity was recovered in anti-cPLA2 immunoprecipitates of lysates derived from nucleotide-treated cells, and was lost from the immunodepleted lysates. Thus, cPLA2 is responsible for AA release by P2U receptors in MDCK-D1 cells. P2U receptors also activated MAP kinase. This activation was PKC-dependent since phorbol 12-myristate 13-acetate (PMA) promoted down-regulation of PKC-eliminated MAP kinase activation by ATP or UTP. Treatment of cells with the MAP kinase cascade inhibitor PD098059, the PKC inhibitor GF109203X, or down-regulation of PKC by PMA treatment, all suppressed AA release promoted by ATP or UTP, suggesting that both MAP kinase and PKC are involved in the regulation of cPLA2 by P2U receptors. Differential effects of GF109203X on cPLA2-mediated AA release and MAP kinase activation, however, were observed: at low concentrations, GF109203X inhibited AA release promoted by ATP, UTP, or PMA without affecting MAP kinase activation. Since GF109203X is more selective for PKCalpha, PKCalpha may act independently of MAP kinase to regulate cPLA2 in MDCK-D1 cells. This conclusion is further supported by data showing that PMA-promoted AA release, but not MAP kinase activation, was suppressed in cells in which PKCalpha expression was decreased by antisense transfection. Based on these data, we propose a model whereby both MAP kinase and PKC are required for cPLA2-mediated AA release by P2U receptors in MDCK-D1 cells. PKC plays a dual role in this process through the utilization of different isoforms: PKCalpha regulates cPLA2-mediated AA release independently of MAP kinase, while other PKC isoforms act through MAP kinase activation. This model contrasts with our recently demonstrated mechanism (J. Clin. Invest. 99:1302-1310.) whereby alpha1-adrenergic receptors in the same cell type regulate cPLA2-mediated AA release only through sequential activation of PKC and MAP kinase.

Desensitization of P2U receptor in neuronal cell line. Different control by the agonists ATP and UTP, as demonstrated by single-cell Ca2+ responses

The neuronal cell line NG108-15 (180CC15) responds to extracellular stimuli of ATP or UTP with a transient increase in the level of cytosolic Ca2+. Desensitization was investigated by recording single-cell Ca2+ responses induced by consecutive, regularly spaced (100 s intervals), brief pulses of the nucleotides. The two natural ligands of the P2U receptor, ATP and UTP, were applied at a concentration that evoked responses of a comparable size. With two pulses of UTP (10 microM), a substantial decrease (of 43%) was observed in the size of the second response. The magnitude of response was determined by measuring either the maximal amplitude or the total response, represented by the area of the Ca2+ transient. The analogous studies with ATP pulses showed a much smaller decrease (of 12%). Comparable experiments performed to investigate the mutual interaction between ATP and UTP revealed that after stimulation with ATP the response to UTP was slightly (12%) diminished, whereas the response to ATP after UTP was greatly (52%) decreased. The different degree of desensitization by either UTP or ATP of P2U receptors could be due to (1) a difference in the mode of activation of the receptor by the two ligands or (2) recruitment of another effector mechanism besides elevated Ca2+. Our results indicate the existence of a novel mechanism of receptor control, involving different modes of the receptor, that are induced by two different, activating ligands. We also investigated the crosstalk between the bradykinin B2 receptor and the nucleotide receptor. ATP and UTP, even when eliciting responses of comparable size in the neuronal cell line, affect the desensitization of the bradykinin receptor differently. This suggests regulatory binding sites for the nucleotides on either the nucleotide receptor or the peptide receptor.

Changes in the levels of glycerophosphoinositols during differentiation of hepatic and neuronal cells

Glycerophosphoinositols are metabolites formed by a phosholipase A2 and a lysolipase specifically acting on membrane phosphoinositol lipids. High levels of these compounds characterize epithelial cells and fibroblasts transformed by ras and other cellular oncogenes. Here we have analyzed the glycerophosphoinositol levels in cells that are considered models of cell differentiation. Using rat hepatocytes at different stages of liver development we have shown that the glycerophosphoinositol basal levels of fetal cells were up to fourfold higher than in adult hepatocytes. No changes in glycerophosphoinositol were observed in regenerating rat liver, a model of differentiated cells proliferating in a synchronous manner, where only glycerophosphoinositol 4-phosphate increased by 80%. Similarly to fetal hepatocytes, a modest but significant increase (30%) in the levels of glycerophosphoinositols was observed in undifferentiated NG-108-15 cells as compared to the same cells induced to differentiate by cAMP. In a different neuronal cell line, PC12 cells, increased glycerophosphoinositol levels characterized the differentiated cells. Based on these observations we suggest that high glycerophosphoinositol levels characterize cellular phenomena associated with the activation of ras/mitogen-activated protein kinase pathways.

Regulation of bradykinin-induced Ins(1,4,5)P3 formation by protein kinase C in human fibroblasts

To better understand the molecular mechanisms that underlie the exaggerated bradykinin (BK)-stimulated release of Ins(1,4,5)P3 in fibroblasts from Alzheimer patients, the role of G-proteins, protein kinase C (PKC) and cyclic AMP in BK-induced Ins(1,4,5)P3 formation was determined. A role for G-proteins in the coupling of the BK receptor to intracellular signals was indicated by guanosine 5'-(3-O-thio)triphosphate (GTP gamma S) enhanced BK-stimulated Ins(1,4,5)P3 release. The coupling of G-proteins to Ins(1,4,5)P3 formation was sensitive to cholera toxin (CTX), but not pertussis toxin (PTX), and was not altered by PKC activation. The inhibition by CTX appeared to be secondary to its ability to increase cyclic AMP, because forskolin also inhibited the BK-mediated Ins (1,4,5)P3 release. Activation of PKC with TPA diminished the number of BK receptors by 33% and proportionally decreased BK-mediated Ins(1,4,5)P3 formation by 28%. The latter response was abolished by PKC inhibitors. Depletion of PKC by prolonged TPA treatment did not further alter the number of BK receptors but further decreased the Ins(1,4,5)P3 response by 65%. Thus, changes in PKC probably do not underlie the enhanced BK-induced Ins(1,4,5)P3 formation in AD fibroblasts, because both activation and depletion of the PKC diminished the Ins(1,4,5)P3 response.

Activation of nucleotide receptors inhibits high-threshold calcium currents in NG108-15 neuronal hybrid cells

A P2U (UTP-sensitive) nucleotide receptor has previously been cloned from NG108-15 neuroblastoma x glioma hybrid cells and it has been shown that activation of this receptor inhibits the M-type K+-current. We now report that UTP also inhibits Ca2+-currents in differentiated NG108-15 cells, but probably through a different nucleotide receptor. UTP (100 microM) inhibited the peak of the high-threshold current by 28.4 +/- 3.1% (n = 28) with no effect on the low-threshold current. Two components of high-threshold current were identified: one inhibited by 100 nM omega-conotoxin (CgTx) and one inhibited by 2 microM nifedipine and enhanced by 1 microM BAY K8644. UTP inhibited the former by 31.0 +/- 3.1%, with an IC50 of 2. 8 +/- 1.1 microM, and the latter 34.2 +/- 6.1% with an IC50 of 1.7 +/- 1.3 microM. Pertussis toxin pretreatment prevented inhibition of the CgTx-sensitive, nifedipine-resistant but not CgTx-resistant current. Inhibition was not prevented by intracellular BAPTA (20 mM) or cAMP (1mM). Effects of UTP on both currents were imitated by UDP, ATP, ADP, AP4A and ATPgammaS but weakly or not at all by 2-MeSATP, GTP, AMP-CPP or ITP. Since the receptors which inhibit Ca2+-currents are activated by ATP, it is suggested that they might mediate auto-inhibition of transmitter release by ATP if present on purinergic nerve terminals.

KEYWORDS

nucleotides, UTP, ATP, calcium currents, neuroblastoma cells

Adenosine activates the K+ channel and enhances cytosolic Ca2+ release via a P2Y purinoceptor in hippocampal neurons

The effects of adenosine on hippocampal neurons were examined by patch-clamp recording and Ca2+ imaging using fura-2 fluorescence. In the whole-cell patch-clamp configuration, adenosine evoked outwardly rectifying K+ currents in a dose-dependent manner. These currents were not inhibited by a nonselective P1 purinoceptor antagonist or selective adenosine A1, A2A receptor antagonists and moreover, selective adenosine A1, A2A receptor agonists evoked no current. In contrast, P2 purinoceptor agonists produced similar outward currents with the order of potency: ADP > or = 2-methylthio ATP > ATP > adenosine >> AMP. No response was obtained to UTP, alpha, beta-methylene ATP or beta, gamma-methylene ATP. The intracellular perfusion of a broad G-protein inactivator, guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), abolished adenosine-evoked currents, whereas a Gi/Go-protein inhibitor, pertussis toxin, had no effect. Furthermore, the currents were blocked by a phospholipase C inhibitor, neomycin, or specific protein kinase C inhibitors, GF109203X (bisindolyl maleimide, C25H24N4O2) and protein kinase C inhibitor peptide. In the cell-attached patch-clamp configuration, adenosine elicited single-channel currents with two major kinds of slope conductances. Likewise, application of adenosine outside the patch electrode again produced single-channel currents with same conductances. A potent protein kinase C activator, 12-O-tetradecanoylphorbol-13-acetate (TPA), induced single-channel currents in a fashion that mimics the effect of adenosine. The evoked currents were blocked by GF109203X. In addition, adenosine enhanced intracellular free Ca2+ concentration ([Ca2+]i). This [Ca2+]i increase was inhibited by GDP beta S or neomycin, but was not affected by pertussis toxin. These results, thus, suggest that adenosine activates the K+ channel and enhances cytosolic Ca2+ release via a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein, which is involved in a phospholipase C-mediated phospholipid-signaling pathway.

ATP-evoked inositol phosphates formation through activation of P2U purinergic receptors in cultured astrocytes: regulation by PKC subtypes alpha, delta, and theta

ATP-induced phosphoinositide (PI) hydrolysis was studied in cultured astrocytes. To characterize the P2 purinergic receptor-mediated effects of ATP, the subtype-specific agonists 2-methylthio ATP (2-MeSATP), UTP, and alpha, beta-methylene ATP were compared. ATP, UTP, or 2-MeSATP induced a dose-dependent increase of inositol phosphates (IP) accumulation; alpha, beta-methylene ATP and adenosine had no effect. The order of potency was ATP > or = UTP >> 2-MeSATP. Cross-desensitization experiments indicated that ATP interacted with both P2U and P2Y receptors. P2U was the predominant P2 receptor in mediating PI hydrolysis in astrocytes. The effect of ATP, UTP, or 2-MeSATP was markedly inhibited by pretreatment of cells with pertussis toxin (PTX), indicating that both P2U and P2Y receptors coupled to phospholipase C through PTX-sensitive G protein. Short-term (10 min) treatment of cells with 1 microM TPA attenuated ATP, UTP, and 2-MeSATP-induced PI breakdown; however, long-term (24 h) pretreatment resulted in marked potentiation of both ATP and UTP, and restoration of 2-MeSATP responses. In a further analysis of the effect of TPA, 10 min and 1.5 h pretreatment attenuated ATP-and UTP-induced PI breakdown, but this inhibitory action was lost after 3 h of treatment. Both 6 and 24 h pretreatments resulted in a potentiation. Western blot analysis showed translocation of protein kinase C (PKC) alpha, -delta, and -theta from the cytosol to the membrane following 10 min and 1.5 h treatments, and restoration to basal levels in the membrane fraction was seen after 3 h of treatment. On the other hand, partial and complete down-regulation of these three isoforms was seen after 6 and 24 h of treatment, respectively. PKC eta was translocated but not down-regulated by TPA. These results suggested that PKC alpha, -delta, and -theta, not -eta may exert tonic inhibition on P2U receptor-mediated PI turnover in unstimulated astrocytes.

ATP-regulated K+ channel and cytosolic Ca2+ mobilization in cultured rat spinal neurons

ATP activated the K+ channel responsible for outwardly rectifying currents via a P2Y purinoceptor linked to a pertussis toxin-insensitive G-protein in cultured rat spinal neurons. The evoked currents were inhibited by a selective protein kinase C inhibitor, GF109203X, whereas a phospholipase C inhibitor, neomycin had no effect. These indicate that the currents are regulated by phospholipase C-independent protein kinase C activation. In addition, ATP enhanced intracellular free Ca2+ concentration. The increase in intracellular free Ca2+ concentration was inhibited by a broad G-protein inhibitor, GDP beta S, but not affected by neomycin or an inositol 1,4,5-triphosphate receptor antagonist, heparin, suggesting that the cytosolic Ca2+ mobilization is regulated by a mechanism independent of a phospholipase C-mediated phosphatidylinositol signaling. These results, thus, demonstrate that ATP has dual actions on the coupled K+ channel and cytosolic Ca2+ release.

Streptozotocin, an inducer of NAD+ decrease, attenuates M-potassium current inhibition by ATP, bradykinin, angiotensin II, endothelin 1 and acetylcholine in NG108-15 cells

The M-potassium current was inhibited by bath application of 100 micron ATP, 10 nM bradykinin, 100 nM angiotensin II and 100 nM endothelin 1 as well as by 10 micron acetylcholine in an m1-muscarinic acetylcholine receptor-transformed NG108-15 cell line. The inhibition of M-current was attenuated in cells pretreated with 5 mM streptozotocin for 5-15 h and restored by simultaneous incubation with 5 mM nicotinamide. The results suggest that signal transduction from these five different receptors to M channels shares a common pathway which is susceptible to a streptozotocin-induced decrease in cellular NAD+ content.

The role of signal transduction systems in mediating cell density dependent changes in tyrosine hydroxylase gene expression

Cell density has been implicated in the regulation of neuronal gene phenotype. This study tested the interaction of signal transduction pathways and the expression of tyrosine hydroxylase (TH) mRNA with varying cell density. Increasing cell density in a parental, wild type PC12 cell line elevated steady state levels of TH mRNA. Three observations suggested that this induction is not related to the cyclic AMP dependent signalling pathway: (1) Forskolin stimulated the level of TH mRNA similarly at multiple densities. (2) PKA deficient mutant PC12 cell lines that have either one third (A123.7, AB11) or 3% (A126-1B2) of normal basal expression of TH mRNA still exhibit the same density induced elevation of TH mRNA levels as the wild type. (3) Different cell densities did not change cyclic AMP concentrations in the basal or in the receptor stimulated state. Increasing cell density did not change basal levels of inositol triphosphate (IP3) levels, which suggests that the phosphatidylinositol cascade (PI) is not responsible for density dependent changes in TH expression. Increasing confluence was highly correlated to [Ca2+]i in control (r = 0.70; P < 0.0001), A123.7 (r = 0.92; P < 0.001), AB11 (r = 0.72; P < 0.0001) and A126 (r = 0.42; P < 0.07). Taken together, the results show that neither cyclic AMP nor the PI cascade is involved in cell density induced changes in TH mRNA and suggest that altered [Ca2+]i may have a role.

Adenosine activates the potassium channel via a P2 purinoceptor but not via an adenosine receptor in cultured rat superior colliculus neurons

The effect of adenosine on superior colliculus neurons was examined by whole-cell patch clamp recording. Adenosine elicited whole-cell potassium currents. A selective A1 or A2a adenosine receptor agonist induced no current and furthermore, adenosine-evoked currents were not inhibited by selective A1 or A2a adenosine receptor antagonists or a non-selective adenosine receptor (P1 purinoceptor) antagonist, indicating that the currents are not mediated by adenosine receptors. In contrast, P2 purinoceptor agonists, such as 2-methylthio ATP (2-MeSATP), ATP, ADP, and AMP, produced similar potassium currents, whereas alpha,beta-methylene ATP, beta,gamma-metylene ATP, or UTP had no response. The order of their potencies for the current amplitudes was 2-MeSATP > ADP > adenosine > ATP >> AMP and this order corresponds to that for the P2Y purinoceptor. These results, thus, suggest that adenosine exerts an inhibitory effect at the postsynaptic site by activating the potassium channel via a P2Y purinoceptor in superior colliculus neurons.

A P2 purinoceptor activated by ADP in rat medullar neurons

ADP evoked outwardly rectifying potassium currents with a latency of 0.6 s in cultured rat medullar neurons. Purinoceptor agonists, such as 2-methylthio ATP (2-MeSATP), ATP, AMP, alpha,beta-methylene ATP (alpha,beta-MeATP), and UTP, produced similar outward currents with the order of their potencies for current amplitudes: 2-MeSATP > ADP > ATP > or = alpha,beta-MeATP > or = AMP > UTP. This order corresponds to that for a subtype of P2Y purinoceptors. ADP-evoked currents were fully blocked by a broad G-protein inhibitor, guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), whereas a G(i)/G(o)-protein inhibitor, pertussis toxin (PTX) had no effect. The currents were not affected by a phospholipase C (PLC) inhibitor, neomycin. Furthermore, a selective protein kinase C inhibitor, GF109203X or a selective cAMP-dependent protein kinase inhibitor, H-89 showed no effect on the currents. These results suggest that ADP activates the potassium channel via a P2Y purinoceptor linked to a PTX-insensitive G-protein and its channel regulation may be due to a direct action of the G-protein beta gamma subunits regardless of second messenger signaling cascades. Additionally, ADP enhanced intracellular free Ca2+ concentration ([Ca2+]i) both in the presence and absence of extracellular calcium, and this [Ca2+]i increase was not inhibited by neomycin. This provides an additional evidence that ADP binds to a subtype of P2Y purinoceptors, which is not involved in PLC stimulation.

Inhibition by Zn2+ of uridine 5'-triphosphate-induced Ca(2+)-influx but not Ca(2+)-mobilization in rat phaeochromocytoma cells

1. Uridine 5'-triphosphate (UTP)-evoked increase in intracellular Ca2+ concentration ([Ca]i) and release of dopamine were investigated in rat phaeochromocytoma PC12 cells. UTP (1-100 microM) evoked an increase in [Ca]i in a concentration-dependent manner. This response was decreased to about 30% by extracellular Ca(2+)-depletion, but not abolished. This [Ca]i rise was mimicked by 100 microM ATP but not by 100 microM 2-methyl-thio-ATP or alpha,beta-methylene-ATP in the absence of external Ca2+, suggesting that the response was mediated by P2U purinoceptors, a subclass of P2-purinoceptors. 2. The UTP-evoked [Ca]i rise consisted of two components; a transient and a sustained one. When external Ca2+ was removed, the sustained component was abolished while the transient component was decreased by about 70% but did not disappear. These results suggest that UTP induces Ca(2+)-mobilization and, subsequently, Ca(2+)-influx. 3. The UTP-evoked increase in [Ca]i was not affected by Cd2+ (100 and 300 microM) or nicardipine (30 microM), inhibitors of voltage-gated calcium channels, but was significantly inhibited by Zn2+ (10-300 microM) in the presence of external Ca2+. Zn2+, however, did not affect the Ca2+ response to UTP in the absence of external Ca2+. 4. UTP (30 microM-1 mM) evoked the release of dopamine from the cells in a concentration-dependent manner. This dopamine release was abolished by Ca(2+)-depletion or Zn2+ but not by Cd2+ or nicardipine. 5. Taken together, the data demonstrate that UTP stimulates P2U-purinoceptors and induces a rise in [Ca]i both by Ca(2+)-mobilization and Ca(2+)-influx in PC12 cells. The dopamine release evoked by UTP requires external Ca2+ which may enter the cells through pathways sensitive to Zn2+ but insensitive to Cd2+ or nicardipine.

Modification of frequency augmentation-potentiation by GTP gamma S in the frog neuromuscular junction

The effect of modifiers of guanine nucleotide-binding proteins (G proteins) on the frequency augmentation-potentiation of transmitter release were studied in the frog neuromuscular junction. Using Genetransfer as a carrier the mean quantal content of the endplate potential increased by penetration of GTP gamma S into the presynaptic nerve terminal. Neither GTP gamma S alone nor carrier alone had any effect. The relationship of log (mean quantal content) versus stimulation frequency changed from a single linear to a dual linear function, suggesting that the immediately releasable pool was modified. GDP beta S + carrier also had similar effects, but was less potent. Aluminium fluoride was without effect. Extracellularly recorded presynaptic nerve action potentials remained unchanged with GTP gamma S + carrier. Also, GTP gamma S + carrier did not affect the action potential nor the cytosolic Ca2+ concentration in differentiated NG108-15 hybrid cells. It is suggested that some smg-type G protein-dependent processes are involved in determining frequency augmentation-potentiation.

ATP-evoked potassium currents in rat striatal neurons are mediated by a P2 purinergic receptor

The effect of ATP on cultured striatal neurons was examined by whole cell voltage clamp recordings. ATP produced outwardly rectifying currents that reversed near the expected equilibrium potential for the potassium ion and the currents were blocked by intracellular Cs+. Purinergic receptor agonists such as ADP, AMP adenosine, and 2-methylthio ATP (2-MeSATP) also evoked similar outward currents. The order of their potencies was ATP >> 2-MeSATP > or = ADP > adenosine > AMP, corresponding to a P2 purinergic receptor. ATP-evoked currents were blocked by a specific protein kinase C (PKC) inhibitor, GF109203X. In addition, the intracellular perfusion of a G-protein inactivator, GDP beta S abolished ATP-induced currents, whereas pertussis toxin (PTX) had no effect on the currents. These results suggest that ATP activates a potassium channel in striatal neurons, which is regulated by protein kinase C (PKC) activation through a P2 purinergic receptor linked to PTX-insensitive G protein.

Biphasic regulation of paracellular permeability in human cervical cells by two distinct nucleotide receptors

The effects of extracellular ATP (ATPe) were characterized in human cervical cells grown on filters. ATPe changed the transepithelial conductance (GT) in a biphasic manner with an initial acute increase of 13 +/- 3% (phase I), lasting approximately 2 min, followed by a sustained decrease of 49 +/- 17% (phase II). ATPe also effected decreases of permeabilities to pyranine and to sucrose, which correspond to the phase II decrease in GT. Both phase I and II effects appear to be mediated by apical nucleotide receptors. However, the phase I effect differed from the phase II effect as follows: 1) a higher 50% effective concentration for ATPe, 22 vs. 3 microM; 2) different nucleotide specificity; 3) lack of influence of pretreatment with pertussis toxin; 4) independence from time in culture after seeding; and 5) involvement of cytosolic calcium, rather than diacylglycerol, as intracellular messenger. These differences suggest the presence of two different types of nucleotide receptors that use different intracellular signaling systems and have opposite effects on the paracellular permeability and electrical conductance. These results suggest that ATPe could regulate cervical mucus production in vivo by regulating the paracellular permeability. Depending on the specific receptors present, ATPe could either increase or decrease mucus production.

Shear stress-induced [Ca2+]i transients and oscillations in mouse fibroblasts are mediated by endogenously released ATP

The effects of ATP, U-73122, apyrase, and saline shear stress on [Ca2+]i homeostasis were studied in fura-2 loaded, mouse fibroblast cells (L929), both in suspension and plated on glass. Release of internal Ca2+ was induced by ATP, via a receptor identified pharmacologically as a P2U type. In single cells, low concentrations of ATP evoked [Ca2+]i oscillations. These events were blocked by the putative phospholipase C inhibitor, U-73122 (but not by the inactive analog U-73343) and by the ATP/ADPase, apyrase. In addition, both these agents reduced the [Ca2+]i of unstimulated cells, especially after stirring, and blocked spontaneously occurring [Ca2+]i oscillations, which suggested an already activated state of the ATP receptor, independent from exogenous stimulations. Moreover, it was found that stirring of the cells was correlated with a steady accumulation of inositol phosphates, also blockable by apyrase, and that [Ca2+]i mobilization could be induced by puffs of saline in single cells. The transition to a Ca(2+)-free environment also provoked [Ca2+]i oscillations, most likely via the increase in ATP4- concentration. This evidence suggests that endogenous ATP is released from L fibroblasts in response to fluid shear stress, and this results in an autocrine, tonic up-regulation of the phosphoinositide signaling system and an ensuing alteration in Ca2+ homeostasis. Up until now, such a response to shear stress was believed to be unique to endothelial cells.

Activation of nucleotide receptors inhibits M-type K current [IK(M)] in neuroblastoma x glioma hybrid cells

A phospholipase-C-linked nucleotide receptor, sensitive to both uridine and adenosine triphosphate (UTP and ATP) has been cloned from NG108-15 neuroblastoma x glioma hybrid cells. We have tested whether activation of this receptor could inhibit the voltage-dependent K+ current [IK(M) or "M-current"] in NG108-15 cells recorded using whole-cell patch-clamp methods. Both UTP and ATP inhibited IK(M) by 44% and 42%, respectively, at 100 microM. Mean IC50 values were: UTP, 0.77 +/- 0.27 microM; ATP, 1.81 +/- 0.82 microM. The order of nucleotide and nucleoside activity at 100 microM was: UTP = ATP > ATP [gamma S] = ITP > 2-MeSATP > ADP = GTP >> AMP-CPP, adenosine, where ATP[gamma S] is adenosine 5'-O-(3-thiotriphosphate), ITP is inosine 5'-triphosphate, 2-MeSATP is 2-methylthio ATP and AMP-CPP is alpha, beta methylene ATP. This rank order accords with their activities at the cloned P2U receptor. Effects were not inhibited by suramin (up to 500 microM) or by pre-incubation for 12 h in 500 ng.ml-1 Pertussis toxin. Inhibition of IK(M) was frequently preceded by a transient outward current, probably a Ca(2+)-activated K+ current, responding to Ca2+ mobilization. No effect on the delayed rectifier K+ current was observed. These observations match those expected from stimulating other phospholipase-C-linked receptors in NG108-15 cells.

Purine- and pyrimidine-stimulated phosphoinositide breakdown and intracellular calcium mobilisation in astrocytes

Phosphoinositide breakdown in cultured cortical astrocytes was assessed by measuring the accumulation of [3H]inositol phosphates (IP's) following incubations with various purines and pyrimidines. Dose-response relationships gave the following order of potency: 2-methylthioadenosine triphosphate (2-MeSATP) > uridine 5'-triphosphate (UTP) > ATP = ADP > inosine 5' triphosphate (ITP). However, 2-MeSATP and UTP were only half as effective as either ATP or ADP in stimulating [3H]IP production. Astrocytes were also challenged with combined additions of maximally effective concentrations of agonists. Responses to ADP plus UTP and 2-MeSATP plus UTP were essentially additive whilst ATP plus UTP evoked a response which was only partially additive. ATP-stimulated [3H]IP accumulation was markedly reduced in the presence of 2-MeSATP suggesting that the latter may be a partial agonist at these receptors. We also examined the ability of ATP and UTP to increase intracellular Ca2+ concentrations in these cells. Greater than 90% of all cells tested responded to ATP with a release from internal Ca2+ stores but less than half of these responded similarly when challenged with UTP. Our results indicate that astrocytes possess both P2Y-purinoceptors and a population of receptors which are also coupled to phosphoinositide metabolism and intracellular Ca2+ mobilisation but recognise ATP and the pyrimidine nucleotide UTP.

Effects of extracellular divalent cations on responses of human blood platelets to adenosine 5'-diphosphate

The effects of extracellular divalent cations on the responses of human platelets to adenosine 5'-diphosphate (ADP) and on its inhibition by the competitive antagonist adenosine 5'-triphosphate (ATP) were investigated. Two responses were studied, shape change and the inhibition of prostaglandin E1 (PGE1)-stimulated adenylate cyclase, and experiments were carried out in the presence of divalent cations (Ca2+ and Mg2+, 1 mM) or in their absence. For each response there was a small leftward shift of the concentration-response curve to ADP in the absence of divalent cations compared to that in their presence, and this leftward shift disappeared when the results were plotted in terms of ADP3- rather than total ADP concentration. The shape change results were, however, complicated by a reduction in the maximal response to ADP in the absence of divalent cations. For each response there was also a marked increase in the pA2 value of ATP in the absence of divalent cations compared to that in their presence, and this difference disappeared if the results were calculated in terms of ATP4- instead of total ATP. These results suggest that the human platelet ADP receptor, in common with other receptors for adenine nucleotides, recognises predominantly the uncomplexed forms of ADP and ATP as ligands.