P2-purinergic receptors activate a guanine nucleotide-dependent phospholipase C in membranes from HL-60 cells

The Signaling Mechanism of Contraction Induced by ATP and UTP in Feline Esophageal Smooth Muscle Cells

P2 receptors are membrane-bound receptors for extracellular nucleotides such as ATP and UTP. P2 receptors have been classified as ligand-gated ion channels or P2X receptors and G protein-coupled P2Y receptors. Recently, purinergic signaling has begun to attract attention as a potential therapeutic target for a variety of diseases especially associated with gastroenterology. This study determined the ATP and UTP-induced receptor signaling mechanism in feline esophageal contraction. Contraction of dispersed feline esophageal smooth muscle cells was measured by scanning micrometry. Phosphorylation of MLC20 was determined by western blot analysis. ATP and UTP elicited maximum esophageal contraction at 30 s and 10 μM concentration. Contraction of dispersed cells treated with 10 μM ATP was inhibited by nifedipine. However, contraction induced by 0.1 μM ATP, 0.1 μM UTP and 10 μM UTP was decreased by U73122, chelerythrine, ML-9, PTX and GDPβS. Contraction induced by 0.1 μM ATP and UTP was inhibited by Gαi3 or Gαq antibodies and by PLCβ1 or PLCβ3 antibodies. Phosphorylated MLC20 was increased by ATP and UTP treatment. In conclusion, esophageal contraction induced by ATP and UTP was preferentially mediated by P2Y receptors coupled to Gαi3 and G q proteins, which activate PLCβ1 and PLCβ3. Subsequently, increased intracellular Ca(2+) and activated PKC triggered stimulation of MLC kinase and inhibition of MLC phosphatase. Finally, increased pMLC20 generated esophageal contraction.

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.

Chemotactic activity of extracellular nucleotideson human immune cells

Purinergic P2 receptors are a class of plasma membrane receptors that are express in many tissues and are ligated by extracellular nucleotides [such as adenosine triphosphate (ATP), adenosine diphosphate (ADP), uridine 5–triphosphate (UTP) and uridine 5–diphosphate (UDP)], which are released as a consequence of cell damage, cell stress, bacterial infection or other noxious stimuli. According to the molecular structure, P2 receptors are divided into two subfamilies: P2X and P2Y receptors. The P2X receptors are ligand-gated channels, whereas P2Y receptors are G-protein-coupled seven-membrane-spanning receptors. Several studies indicate that nucleotides play an important role in immune response modulation through their action on multiple cell types, including monocytes, mast cells, dendritic cells, neutrophils, and eosinophils. Recent work by our group and others identified extracellular nucleotides as chemotaxins for various human immune cells, including eosinophils, neutrophils and dendritic cells. In this review, we summarise recent findings in this field and put forward a hypothesis on the role of P2 receptors in the early recruitment of human immune cells to the site of inflammation.

P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Mechanical or ischemic trauma to the CNS causes the release of nucleotides and other neurotransmitters into the extracellular space. Nucleotides can activate nucleotide receptors that modulate the expression of genes implicated in cellular adaptive responses. In this investigation, we used human 1321N1 astrocytoma cells expressing a recombinant P2Y2 receptor to assess the role of this receptor in the regulation of anti-apoptotic (bcl-2 and bcl-xl) and pro-apoptotic (bax) gene expression. Acute treatment with the P2Y2 receptor agonist UTP up-regulated bcl-2 and bcl-xl, and down-regulated bax, gene expression. Activation of P2Y2 receptors was also coupled to the phosphorylation of cyclic AMP responsive element binding protein that positively regulates bcl-2 and bcl-xl gene expression. Cyclic AMP responsive element decoy oligonucleotides markedly attenuated the UTP-induced increase in bcl-2 and bcl-xl mRNA levels. Activation of P2Y2 receptors induced the phosphorylation of the pro-apoptotic factor Bad and caused a reduction in bax/bcl-2 mRNA expression ratio. All these signaling pathways are known to be involved in cell survival mechanisms. Using cDNA microarray analysis and RT-PCR, P2Y2 receptors were found to up-regulate the expression of genes for neurotrophins, neuropeptides and growth factors including nerve growth factor 2; neurotrophin 3; glia-derived neurite-promoting factor, as well as extracellular matrix proteins CD44 and fibronectin precursor--genes known to regulate neuroprotection. Consistent with this observation, conditioned media from UTP-treated 1321N1 cells expressing P2Y2 receptors stimulated the outgrowth of neurites in PC-12 cells. Taken together, our results suggest an important novel role for the P2Y2 receptor in survival and neuroprotective mechanisms under pathological conditions.

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.

Neutrophil degranulation: coactivation of chemokine receptor(s) is required for extracellular nucleotide-induced neutrophil degranulation

Extracellular nucleotide-induced stimulation of leukocytes and subsequent adhesion to endothelium plays a critical role in inflammatory diseases. The extracellular nucleotides stimulate a P2Y receptor on human PMN with the pharmacological profile of the P2Y2 receptor. Followed by generation of arachidonic acid, subsequently metabolized by 5 lipoxygenase forming the leukotrienes (LT). Of the several LTs generated, LTB(4)is a potent chemokine and upon its release binds to the PMN in an autocrine manner leading to the PMN degranulation. It is known that LTB(4)causes neutrophil degranulation through its receptor specific binding while the molecular mechanism remains not known at present. However, it is not known whether any LTB(4)receptor exists in cytoplasm in any given cell type and also, the existence of any other signaling cascade for the extracellular nucleotide-induced neutrophil degranulation. Based on the few direct experimental and numerous circumstantial evidence, it is conceivable that the extracellular nucleotides require LT generation, as an essential intermediate for mediating neutrophil degranulation.

Nucleotide receptors: an emerging family of regulatory molecules in blood cells

Nucleotides are emerging as an ubiquitous family of extracellular signaling molecules. It has been known for many years that adenosine diphosphate is a potent platelet aggregating factor, but it is now clear that virtually every circulating cell is responsive to nucleotides. Effects as different as proliferation or differentiation, chemotaxis, release of cytokines or lysosomal constituents, and generation of reactive oxygen or nitrogen species are elicited upon stimulation of blood cells with extracellular adenosine triphosphate (ATP). These effects are mediated through a specific class of plasma membrane receptors called purinergic P2 receptors that, according to the molecular structure, are further subdivided into 2 subfamilies: P2Y and P2X. ATP and possibly other nucleotides are released from damaged cells or secreted via nonlytic mechanisms. Thus, during inflammation or vascular damage, nucleotides may provide an important mechanism involved in the activation of leukocytes and platelets. However, the cell physiology of these receptors is still at its dawn, and the precise function of the multiple P2X and P2Y receptor subtypes remains to be understood.

Growth inhibition of HL-60 cells by extracellular ATP: concentration-dependent involvement of a P2 receptor and adenosine generation

A single addition of ATP (20-1000 microM) to cultures of HL-60 cells resulted here in permanent, Ca(2+)-independent inhibition of cellular proliferation, evident 48 h following treatment. Extracellular ATP (ATPo) was maximally effective at 250 microM giving 90 +/- 1.5% growth inhibition. Up to a concentration of 250 microM ATPo, growth inhibition is solely attributable to ATPo, while at higher ATPo concentrations adenosine generated from ATPo hydrolysis contributes to this effect. The order of potency for growth inhibition was ATP = ADP > AMP > adenosine. Suramin, a P2 receptor antagonist, attenuated growth inhibition by ATP and ADP, indicative of P2 receptor involvement. Equipotency of ATP and ADP excludes the involvement of either an ecto-protein kinase or a P2X7 receptor in growth inhibition. Neither UTP (P2Y2 agonist) nor alpha, beta-methyleneATP (P2X1 agonist) inhibited growth, indicating that such inhibition is mediated by a previously undescribed P2 receptor on HL-60 cells.

Coexpression of ligand-gated P2X and G protein-coupled P2Y receptors in smooth muscle. Preferential activation of P2Y receptors coupled to phospholipase C (PLC)-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3

P2 receptor subtypes and their signaling mechanisms were characterized in dispersed smooth muscle cells. UTP and ATP stimulated inositol 1,4,5-triphosphate formation, Ca2+ release, and contraction that were abolished by U-73122 and guanosine 5'-O-(3-thio)diphosphate, and partly inhibited (50-60%) by pertussis toxin (PTX). ATP analogs (adenosine 5'-(alpha, beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methylene)triphosphate, and 2-methylthio-ATP) stimulated Ca2+ influx and contraction that were abolished by nifedipine and in Ca2+-free medium. Micromolar concentrations of ATP stimulated both Ca2+ influx and Ca2+ release. ATP and UTP activated Gq/11 and Gi3 in gastric and aortic smooth muscle and heart membranes, Gq/11 and Gi1 and/or Gi2 in liver membranes, and Go and Gi1-3 in brain membranes. Phosphoinositide hydrolysis stimulated by ATP and UTP was mediated concurrently by Galphaq/11-dependent activation of phospholipase (PL) C-beta1 and Gbetagammai3-dependent activation of PLC-beta3. Phosphoinositide hydrolysis was partially inhibited by PTX or by antibodies to Galphaq/11, Gbeta, PLC-beta1, or PLC-beta3, and completely inhibited by the following combinations (PLC-beta1 and PLC-beta3 antibodies; Galphaq/11 and Gbeta antibodies; PLC-beta1 and Gbeta antibodies; PTX with either PLC-beta1 or Galphaq/11 antibody). The pattern of responses implied that P2Y2 receptors in visceral, and probably vascular, smooth muscle are coupled to PLC-beta1 via Galphaq/11 and to PLC-beta3 via Gbetagammai3. These receptors co-exist with ligand-gated P2X1 receptors activated by ATP analogs and high levels of ATP.

Activation of P2Y2 receptors by UTP and ATP stimulates mitogen-activated kinase activity through a pathway that involves related adhesion focal tyrosine kinase and protein kinase C

We examined downstream signaling events that followed the exposure of PC12 cells to extracellular ATP and UTP, and we compared the effects of these P2 receptor agonists with those of growth factors and other stimuli. Based on early findings, we focused particular attention on the mitogen-activated protein (MAP) kinase pathway. ATP and/or UTP produced increases in tyrosine phosphorylation of multiple proteins, including p42 MAP (ERK2) kinase, related adhesion focal tyrosine kinase (RAFTK) (PYK2, CAKbeta), focal adhesion kinase (FAK), Shc, and protein kinase Cdelta (PKCdelta). MAP (ERK2) kinase activity (quantified by substrate phosphorylation) was increased by UTP, ATP, phorbol 12-myristate 13-acetate, ionomycin, and growth factors. UTP and ATP were equipotent (EC50 approximately 25 microM) in stimulating MAP kinase activity, suggesting that these effects were mediated via the Gi-linked P2Y2 (P2U) receptor. Consistent with this, the UTP- and ATP-promoted activation of MAP kinase was diminished in pertussis toxin-treated cells. Treatment of cells with pertussis toxin also reduced both the UTP-dependent increases in intracellular calcium ion concentration ([Ca2+]i) and the tyrosine phosphorylation of RAFTK. Similarly, when [Ca2+]i elevation was prevented using BAPTA and EGTA, the activation of MAP kinase by UTP and ionomycin was blocked, and the tyrosine phosphorylation of RAFTK was reduced. The UTP-promoted increase in MAP kinase activity was partially reduced in cells in which PKC was down-regulated, suggesting that both PKC-dependent and PKC-independent pathways were involved. PKCdelta, which increases MAP kinase activity in some systems, became tyrosine-phosphorylated within 15 s of exposure of cells to ATP or UTP; but epidermal growth factor, nerve growth factor, and insulin had little effect. UTP also promoted the association of Shc with Grb2. These results suggest that the P2Y2 receptor-initiated activation of MAP kinase was dependent on the elevation of [Ca2+]i, involved the recruitment of Shc and Grb2, and was mediated by RAFTK and PKC.

G-protein-coupled receptors in HL-60 human leukemia cells

1. HL-60 human leukemia cells are a widely employed model system for the analysis of signal transduction processes mediated via regulatory heterotrimeric guanine nucleotide-binding proteins (G-proteins). HL-60 promyelocytes are pluripotent and can be differentiated into neutrophilic or monocytic cells. 2. HL-60 cells express formyl peptide-, complement C5a-, leukotriene B4 (LTB4)- and platelet-activating factor receptors, receptors for purine and pyrimidine nucleotides, histamine H1- and H2-receptors, beta 2-adrenoceptors and prostaglandin receptors. 3. The major G-proteins in HL-60 cells are pertussis toxin (PTX)-sensitive Gi-proteins (Gi2 > Gi3). Gs-proteins and G-proteins of the Gq-family (e.g., G16) are expressed, too. 4. G-protein-regulated effector systems in HL-60 cells are adenylyl cyclase and phospholipase C-beta 2 (PLC-beta 2) and, possibly, phospholipase D (PLD), nonselective cation (NSC) channels and NADPH oxidase. 5. The expression of signal transduction pathways in HL-60 cells strongly depends on the differentiation state of cells. 6. Formyl peptides, via Gi-proteins, mediate activation of PLC, PLD, NSC channels, NADPH oxidase and azurophilic granule release and are referred to as full secretagogues. In dibutyryl cAMP (Bt2cAMP)-differentiated HL-60 cells, C5a and LTB4 are partial and incomplete secretagogues, respectively. There are substantial differences in the Gi-protein activations induced by formyl peptides, C5a and LTB4. 7. In HL-60 promyelocytes, purine and pyrimidine nucleotides mediate activation of PLC and NSC channels largely via PTX-insensitive G-proteins and induce functional differentiation. In Bt2cAMP-differentiated HL-60 cells, they additionally activate PLD, NADPH oxidase and granule release via PTX-sensitive and -insensitive pathways. ATP and UTP are partial secretagogues. Multiple types of receptors (i.e., P2Y- and P2U-receptors and pyrimidinocyeptors) may mediate the effects of nucleotides in HL-60 cells. 8. Bt2cAMP- and 1 alpha,25-dihydroxycholecalciferol-differentiated HL-60 cells express H1-receptors coupled to Gi-proteins and PTX-insensitive G-proteins. In the former cells, histamine mediates activation of PLC and NSC channels, and in the latter, activation of NSC channels. Histamine is an incomplete secretagogue in these cells. 9. HL-60 promyelocytes express H2-receptors coupled to adenylyl cyclase, PLC, and NSC channels. There are substantial differences in the agonist/antagonist profiles of H2-receptor-mediated cAMP formation and rises in cytosolic Ca2+ concentration, indicative of the involvement of different H2-receptor subtypes. H2-receptors mediate functional differentiation of HL-60 cells. 10. Certain cationic-amphiphilic histamine receptor ligands (i.e., 2-substituted histamines, lipophilic guanidines, and a histamine trifluoromethyl-toluidide derivative) show stimulatory effects in HL-60 cells that are attributable to receptor-independent activation of Gi-proteins.

Current state of purinoceptor research

It is hoped that this summary of the history and current status of purinoceptors will convince readers that receptors for purines are now established alongside other well-known extracellular messenger systems. These receptors are primitive, widespread and serve many different systems. Receptors of adenosine (P1-purinoceptors) are clearly different from receptors of ATP (P2-purinoceptors). As for other major transmitters such as acetylcholine, GABA, glutamate and 5-HT, receptors of two major families are activated by ATP, one (the P2X-purinoceptor family) mediates fast responses via ligand-gated ion channels, while the other (the P2Y-purinoceptor family) mediates slower responses via G-proteins (see Table 3). Subclasses of these two families have been suggested on the basis of recent molecular biology studies and the development of new selective agonists and antagonists (Abbracchio and Burnstock, 1994). It would indeed be helpful if the work on purinoceptors could be extended to studies of their chemical structure employing crystallography.

Biphasic and differential modulation of Ca2+ entry by ATP and UTP in promyelocytic leukaemia HL60 cells

ATP and UTP cause mobilization of Ca2+ from the intracellular stores with similar potency in several cell types including both undifferentiated and differentiated HL60 cells. We show here that, in HL60 cells with Ca2+ stores that had been fully and irreversibly emptied using the endomembrane Ca(2+)-ATPase inhibitor thapsigargin, both nucleotides produced a biphasic effect on Ca2+ entry, first rapid inhibition and then delayed (about 15 s) activation. ATP was more effective at producing the initial inhibition of Ca2+ entry, whereas UTP was more effective at activating the delayed Ca2+ entry. Previous incubation with UTP desensitized the Ca2+ mobilization and the delayed activation of Ca2+ entry induced by ATP but not the inhibition of Ca2+ entry. The ATP analogue 2-methylthioATP (2-MeSATP) barely mobilized stored Ca2+ but inhibited Ca2+ entry. These results could be explained by the presence of two receptors: (i) a P2u receptor sensitive to ATP and UTP, responsible for activation of phospholipase C and Ca2+ mobilization, early inhibition of Ca2+ entry and delayed activation of Ca2+ entry and (ii) a P2y-like receptor sensitive to ATP and 2-MeSATP which produces only inhibition of Ca2+ entry. The inhibition of Ca2+ entry by nucleotides increased greatly during differentiation. Given that Ca2+ mobilization by nucleotides is not modified by differentiation, this suggests that a component of the mechanism of inhibition of Ca2+ entry is gradually expressed during differentiation of HL60 cells.

Purinergic and cholinergic components of bladder contractility and flow

The role of ATP as a neurotransmitter/neuromodulator in the urinary tract has been the subject of much study, particularly whether ATP has a functional role in producing urine flow. Recent studies suggested significant species variation, specifically a variation between cat and other species. This study was performed to determine the in vivo response of cat urinary bladder to pelvic nerve stimulation (PNS) and to the exogenous administration of cholinergic and purinergic agents. In anesthetized cats, bladder contractions and fluid expulsion was measured in response to PNS and to the exogenous administration of cholinergic and purinergic agents. Fluid was instilled into the bladder and any fluid expelled by bladder contractions induced by PNS or exogenous agents was collected in a beaker. The volume was measured in a graduated cylinder and recorded. PNS, carbachol and APPCP produced sustained contractions with significant expulsion of fluid. ATP, ACh and hypogastric nerve stimulation did not produce any significant expulsion of fluid. Atropine, a cholinergic antagonist, inhibited PNS contractions and fluid expulsion with no effect on purinergic actions. There was a significant relationship between the magnitude of the contraction, duration of the contractions and volume of fluid expelled. The data and information from other studies, strongly suggests a functional role for ATP as a cotransmitter in the lower urinary tract different from ACh's role. ATP stimulation of a specific purinergic receptor plays a role in initiation of bladder contractions and perhaps in the initiation of urine flow from the bladder. ACh's role is functionally different and appears to be more involved in maintenance of contractile activity and flow.

Activation of specific ATP receptors induces a rapid increase in intracellular calcium ions in rat hypothalamic neurons

We have used real-time dynamic video imaging of Fura-2 fluorescence to study the acute effects of external ATP on [Ca2+]i in cultured rat hypothalamic neurons. The addition of ATP at microM concentrations, but not adenosine, AMP, ADP or GTP, produced a rapid, dose-dependent increase in cytosolic Ca2+. The hydrolysis-resistant ATP analogues 3-thio-ATP and beta,gamma-imido-ATP produced a similar response but alpha,beta-methylene ATP had much lower efficacy. The ATP response was inhibited by 10 microM nifedipine, abolished by 50 microM cadmium and by the absence of extracellular Ca2+, but was unaffected by ryanodine or omega-conotoxin GVIA. The P2-purinoceptor antagonist suramin reversibly and selectively inhibited the ATP response but had no effect on other neurotransmitter-induced Cai2+ responses. Antagonists to muscarinic, nicotinic, NMDA, non-NMDA, GABA, 5-HT and adenosine receptors had no effect on the ATP response. Thus the Ca2+ response of hypothalamic neurons to ATP is mediated by specific suramin-sensitive ATP-receptors, activation of which is independent of ATP hydrolysis and results in an influx of extracellular Ca2+ largely through high voltage-gated Ca2+ channels. These findings support the assertion that ATP acts in the CNS as an excitatory neurotransmitter.

Metabotropic ATP receptor in hippocampal and thalamic neurones: pharmacology and modulation of Ca2+ mobilizing mechanisms

Changes in cytoplasmic free Ca2+ concentration, [Ca]i, elicited by ATP, were studied in neurones cultured from rat hippocampus and thalamus. ATP evoked [Ca]i increases in about 30% of all cells tested and suppressed [Ca]i transients in responsive cells. The number of responses to ATP markedly increased after pretreatment of cells with inhibitors of protein kinase C, H-7 or staurosporine. The potentiation was blocked by a phorbol ester and by dioleylglycerol. In pretreated cells both once peak [Ca]i and the number of successive trials were augmented by an [ATP] increase. The former effect can be described by the Michaelis-Menten equation whereas the latter one has a steeper, leftward-shifted dependence. Both concentration dependences are explained with a model, describing Ca2+ release as a threshold phenomena. ATP analogues had the rank of potency: ATP approximately ADP >> AMP > alpha, beta-MeATP. A single ATP application depleted internal Ca2+ stores which could be replenished by brief membrane depolarization with high-K+. ATP- and caffeine-induced [Ca]i transients were independent, indicating two non-overlapping Ca2+ storage sites. Only caffeine effects were potentiated at an elevated [Ca]i level, showing a Ca(2+)-induced Ca2+ release. Inhibitors of the Ca2+ pump in internal stores, ryanodine and sulphydryl reagents suppressed the ATP-induced [Ca]i transients, acting via different mechanisms.

Increases in intracellular calcium via activation of an endogenous P2-purinoceptor in cultured CHO-K1 cells

1. Increases in intracellular calcium ([Ca2+]i) were measured in chinese hamster cultured ovary cells (clone, CHO-K1), by use of the fluorescent, calcium-sensitive dye, fura-2. 2. Addition of both ATP and UTP elicited rapid increases in [Ca2+]i due to mobilization from intracellular stores and calcium entry across the plasma membrane. 3. Omission of calcium from the extracellular medium and pre-incubation with the inorganic calcium channel blocker, nickel (Ni2+) prevented the calcium entry components of the responses. 4. Investigation of the concentration-response relationships of various analogues of ATP suggests the presence of a purinoceptor which cannot be characterized as P2X or P2Y. In addition, there appears to be a sub-population of P2Y-purinoceptors which do not cross-react with the 'nucleotide' receptor population. 5. Cross-desensitization and additivity experiments suggest that both ATP and UTP activate the same receptor. 6. Pre-incubation with the tumour-promoting agent, beta-phorbol-12,13 dibutyrate (PDBu), caused a reduction in the increases in [Ca2+]i, suggesting a role for protein kinase C in feedback inhibition of purinoceptor responses in this cell line. 7. In summary, we present evidence for the existence of an endogenous P2U-purinoceptor (or 'nucleotide receptor') which is linked to increases in [Ca2+]i in CHO-K1 cells.

Signal transduction via P2-purinergic receptors for extracellular ATP and other nucleotides

Extracellular ATP, at micromolar concentrations, induces significant functional changes in a wide variety of cells and tissues. ATP can be released from the cytosol of damaged cells or from exocytotic vesicles and/or granules contained in many types of secretory cells. There are also efficient extracellular mechanisms for the rapid metabolism of released nucleotides by ecto-ATPases and 5'-nucleotidases. The diverse biological responses to ATP are mediated by a variety of cell surface receptors that are activated when ATP or other nucleotides are bound. The functionally identified nucleotide or P2-purinergic receptors include 1) ATP receptors that stimulate G protein-coupled effector enzymes and signaling cascades, including inositol phospholipid hydrolysis and the mobilization of intracellular Ca2+ stores; 2) ATP receptors that directly activate ligand-gated cation channels in the plasma membranes of many excitable cell types; 3) ATP receptors that, via the rapid induction of surface membrane channels and/or pores permeable to ions and endogenous metabolites, produce cytotoxic or activation responses in macrophages and other immune effector cells; and 4) ADP receptors that trigger rapid ion fluxes and aggregation responses in platelets. Current research in this area is directed toward the identification and structural characterization of these receptors by biochemical and molecular biological approaches.

Cytosolic calcium homeostasis in bovine parathyroid cells and its modulation by protein kinase C

1. The effects of protein kinase C (PKC) activators and inhibitors on the mechanisms regulating cytosolic Ca2+ homeostasis in dissociated bovine parathyroid cells loaded with fura-2 were examined. 2. Stepwise increases in the concentration of extracellular Ca2+ (from 0.5 to 2 or 3 mM) elicited transient followed by sustained increases in the concentration of intracellular free Ca2+ ([Ca2+]i). Cytosolic Ca2+ transients reflected the mobilization of intracellular Ca2+ and influx of extracellular Ca2+ whereas sustained increases in [Ca2+]i resulted from the influx of extracellular Ca2+. Brief (1-2 min) pretreatment with phorbol myristate acetate (PMA) shifted the concentration-response curve for extracellular Ca(2+)-induced cytosolic Ca2+ transients to the right without affecting the maximal response. Cytosolic Ca2+ transients elicited by extracellular Mg2+ were similarly affected by PMA. 3. These effects of PMA were mimicked by various other activators of PKC with the rank order of potency PMA > phorbol dibutyrate > bryostatin , > (-)indolactam V > mezerein. Isomers or analogues of these compounds that do not alter PKC activity (4 alpha-phorbols and (+)indolactam V) did not alter [Ca2+]i. 4. PKC activators depressed evoked increases in [Ca2+]i when influx of extracellular Ca2+ was blocked with Gd3+. Cytosolic Ca2+ transients elicited by extracellular Mg2+ in the absence of extracellular Ca2+ were similarly inhibited by PKC activators. Activation of PKC thus inhibits the mobilization of intracellular Ca2+ elicited by extracellular divalent cations. 5. Increases in the concentration of extracellular Ca2+ caused corresponding increases in the formation of [3H]inositol 1,4,5-trisphosphate ([3H]InsP3). Pretreatment with PMA shifted the concentration-response curve for extracellular Ca(2+)-induced [3H]InsP3 formation to the right without affecting the maximal response. 6. PKC activators also caused some depression of steady-state increases in [Ca2+]i elicited by extracellular Ca2+. In contrast, PMA did not affect increases in [Ca2+]i elicited by ionomycin or thapsigargin. 7. Ba2+ was used to monitor divalent cation influx. PMA decreased the rate of rise of the fluorescent signal elicited by extracellular Ba2+. 8. All these effects of PKC activators on [Ca2+]i were blocked or reversed by staurosporine at concentrations (30-100 nM) that inhibited PKC activity in parathyroid cells. Staurosporine alone potentiated cytosolic Ca2+ responses evoked by submaximal concentrations of extracellular divalent cations. 9. PKC thus depresses both the mobilization of intracellular Ca2+ and the influx of extracellular Ca2+ in parathyroid cells. The effects on [Ca2+]i provide evidence for a Ca2+ receptor on the surface of parathyroid cells that uses transmembrane signalling mechanisms common to some other Ca(2+)-mobilizing receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Enkephalin activates the phospholipase C/Ca2+ system through cross-talk between opioid receptors and P2-purinergic or bradykinin receptors in NG 108-15 cells. A permissive role for pertussis toxin-sensitive G-proteins

In an NG 108-15 neuroblastoma x glioma hybrid cell suspension, extracellular ATP (via P2-purinergic receptors) and bradykinin stimulated Ins(1,4,5)P3 formation, which was accompanied by an increase in the cytosolic Ca2+ concentration ([Ca2+]i). Leucine enkephalin (EK) also slightly increased [Ca2+]i in the absence, but not in the presence, of apyrase, which hydrolyses extracellular ATP and ADP to AMP. When the cells were stimulated by P2-agonists or bradykinin prior to the application of EK, EK induces a remarkable rise in [Ca2+]i. This P2-agonist- or bradykinin-assisted EK action was also observed in single cells on a coverslip. A decrease in the extracellular Ca2+ concentration only slightly lowered the EK-induced rise in [Ca2+]i, but treatment of the cells with thapsigargin, an agent which depletes Ca2+ in the Ins(1,4,5)P3-sensitive pool, almost completely abolished EK action. The observed permissive stimulation by EK of Ins(1,4,5)P3 formation induced by a P2-agonist or bradykinin may be a primary event for the EK-induced [Ca2+]i rise. These actions of EK were antagonized by naloxone and completely reversed by prior treatment of the cells with pertussis toxin, whereas the toxin hardly affected the actions of P2-agonists and bradykinin themselves. Thus EK can induce phospholipase C activation and subsequent Ca2+ mobilization, provided that the cells have been previously or are simultaneously stimulated by endogenous adenine nucleotides or by externally applied P2-agonists or bradykinin. In this cross-talk mechanism between opioid receptors and these Ca(2+)-mobilizing agonist receptors, pertussis toxin-sensitive G-proteins play a permissive role.

Involvement of a signal transduction mechanism in ATP-induced mucin release from cultured airway goblet cells

Release of mucins from cultured airway surface epithelial cells can be stimulated by extracellular ATP via a P2-purinergic receptor-mediated mechanism (K. C. Kim and B. C. Lee. 1991. Br. J. Pharmacol. 103:1053-1056). In this report, we studied the mechanism by which extracellular ATP induces the mucin release. We found that: (1) ATP increased both mucin release and generation of inositol phosphates in a dose-dependent fashion, and their dose-effect relationships were almost superimposed; (2) the increases in both mucin release and the phosphatidylinositol phosphate (PI) turnover by extracellular ATP were partially, but almost equally, blocked by the pretreatment with pertussis toxin (42% for mucin release and 44% for PI turnover). We conclude that in cultured airway goblet cells extracellular ATP stimulates mucin release by a signal transduction mechanism, which seems to involve coupling of ATP-activated P2 purinoceptors with phospholipase C, at least in part, via pertussis toxin-sensitive GTP-binding proteins. This may be an important finding in understanding the regulation of mucin release by airway goblet cells, since a number of agents present in the airway could influence this signal transduction pathway and subsequently modulate the mucin secretion.

Subclasses of purinoceptors in feline bladder

Purines have been shown to inhibit and excite feline detrusor smooth muscle through P1 and P2 receptor activation. Several recent studies have demonstrated differences in agonist potency orders for subclasses of purinoceptors, including P2Y and nucleotide, or P2U receptors. The current studies were performed to determine the presence of such receptor subtypes in feline detrusor smooth muscle. Cats were surgically prepared for monitoring detrusor smooth muscle contractions as increases in intravesical pressure. Contractions were induced by pelvic nerves stimulation (PNS), ATP, and ATP analogs, such as beta, gamma-methylene ATP (APPCP), 5' adenylimido diphosphate (AMP-PNP) and 2-methylthio ATP (2-MeSATP), ATP gamma S, UTP, CTP and GTP. These agents all produced contractions and had an agonist potency order of AMP-PNP = APPCP > ATP gamma S = 2-MeSATP >> ATP > UTP = CTP = GTP. The agonist potency order for inhibition of PNS nerve-evoked bladder contractions was APPCP = AMP-PNP = ATP gamma S > 2-MeSATP = ATP > UTP = CTP = GTP. Reactive Blue 2 and Coomassie's Brilliant Blue G, two putative P2Y receptor antagonists, antagonized purine-induced actions. This antagonism and the agonist potency orders suggest the possible presence of novel receptors in detrusor smooth muscle and/or the presence of multiple receptors in detrusor smooth muscle.

Inositol 1,4,5-trisphosphate generation and calcium mobilisation via activation of an atypical P2 receptor in the neuronal cell line, N1E-115

1. Alterations in the levels of intracellular calcium ([Ca2+]i) and D-myo-inositol-1,4,5-trisphosphate (InsP3) were measured in the murine neuroblastoma cell line clone, N1E-115, by use of the calcium-sensitive dye, fura-2 and a radioreceptor assay, respectively. 2. Exposure of the cells to ATP (100 microM) elicited rapid and transient increases in [Ca2+]i and InsP3, with both responses reaching a maximum between 10-20 s after agonist addition. 3. Investigation of concentration-response data by use of various analogues of ATP suggests the presence of an extracellular receptor which fails to fit into the current classification of purinoceptors. 4. Cross-desensitization experiments suggest that the same receptor can also be activated by the structurally different pyrimidine base, UTP. 5. Application of the tumour-promoting agent, beta-phorbol-12,13 dibutyrate (PDBu) caused a reduction in the increases in both [Ca2+]i and InsP3, suggesting a role for protein kinase C in feedback inhibition of purinoceptor responses in this cell line. 6. In summary, we present the first evidence for the existence of an atypical purinoceptor on a cell line of CNS origin. This receptor is linked to stimulation of phosphoinositide turnover and subsequent mobilisation of intracellular calcium.

Extracellular nucleotides elevate [Ca2+]i in rat osteoblastic cells by interaction with two receptor subtypes

Extracellular nucleotides, through interaction with specific cell-surface receptors, mediate a variety of biological responses, including elevation of cytosolic free Ca2+ concentration ([Ca2+]i) in a number of cell types. The effects of nucleotides on [Ca2+]i in the rat osteoblastic cell line UMR-106 were studied by fluorescence spectrophotometry of indo-1-loaded cells. In response to ATP (100 microM), [Ca2+]i rose to peaks 228 +/- 16 nM (n = 59) above baseline (85 +/- 3 nM) before returning to near basal levels. Half-maximal elevation of [Ca2+]i was observed at an ATP concentration of 3 +/- 1 microM, consistent with a high-affinity interaction. The response arose primarily by release of Ca2+ from internal stores. UTP, ADP, and 2-methylthioadenosine 5'-triphosphate also induced Ca2+ transients, whereas adenosine, AMP, CTP, and TTP did not, demonstrating specificity. Responsiveness to adenosine 5'-O-(3-thiotriphosphate) and inhibition by Mg2+ of the response to ATP indicated that signaling was not dependent on nucleotide hydrolysis. Ca2+ responses to ADP, ATP, and UTP, added sequentially or simultaneously, were consistent with the presence of two distinct P2-purinoceptor subtypes, both linked to Ca2+ mobilization. ADP appeared to interact selectively with one receptor, whereas ATP and UTP interacted selectively with the other. After maximal stimulation with ATP, subsequent responses to ATP were abolished. However, removal of ATP from the extracellular medium rapidly restored responsiveness, suggesting that, with continued receptor occupation, there is time-dependent inactivation of the Ca2+ signaling pathway. Our findings indicate that extracellular nucleotides elevate [Ca2+]i in osteoblastic cells through interaction with two receptor subtypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent developments in the classification and functional significance of receptors for ATP and UTP, evidence for nucleotide receptors

The presence of a nucleotide receptor activated with similar potencies by UTP and ATP is suggested by recent data from a variety of different cell types. This receptor type appears distinct from previously described ATP-sensitive P2-purinoceptor subtypes and, probably, from other UTP-sensitive receptors, however further studies using selective antagonists are necessary to provide a definitive characterisation. Although the functional role of endogenous extracellular ATP has already achieved recognition there are also many diverse examples of cells and tissues which respond to UTP at micromolar or sub-micromolar concentrations. Therefore, the possible physiological importance of UTP is a fertile area for further investigation. The functional significance of ATP/UTP receptors is underlined by recent demonstrations that UTP and ATP modulate chloride ion secretion in human airways epithelium, possibly by activation of a nucleotide receptor, an effect which may have potential clinical utility in the treatment of cystic fibrosis.

Chemotactic peptide stimulation of arachidonic acid release in HL60 cells, an interaction between G protein and phospholipase C mediated signal transduction

The mechanism of phospholipase A2 activation by chemotactic peptide was investigated in human promyelocytic HL60 cells. N-Formyl-methionyl-leucyl-phenylalanine (fMetLeuPhe) and the non-hydrolyzable GTP analogue guanosine 5'-[gamma-thio]triphosphate (GTP[S]) induced arachidonic acid release in permeabilized and metabolically inhibited HL60 cells, a preparation in which calcium was buffered and inositol phospholipid hydrolysis was inhibited. Inositol phosphate generation and arachidonic acid were shown to be temporally dissociated. These results suggest that receptor-dependent phospholipase C activity is not required for fMetLeuPhe to induce arachidonic acid release. However, fMetLeuPhe effects were highly calcium-dependent and inhibition of phospholipase C reduced fMetLeuPhe stimulation of arachidonic acid release even in the permeabilized cell preparation. We conclude that although phospholipase A2 activation is linked to the fMetLeuPhe receptor independent of phospholipase C, actions of phospholipase C to mobilize calcium and release diacylglycerol may be important to phospholipase A2 activation in the intact cell.

Adenosine triphosphate (ATP) and other nucleotides stimulate the hydrolysis of phosphatidylethanolamine in intact fibroblasts

Addition of adenosine triphosphate (ATP) to [14C]ethanolamine-prelabeled NIH 3T3 fibroblasts resulted in rapid formation of [14C]ethanolamine from the prelabeled cellular phosphatidylethanolamine (PtdEtn) pool. After 2-min exposure, 10 microM ATP had near maximal effects on PtdEtn hydrolysis. Several other nucleotides, including UTP, ITP, and the stable ATP analog adenosine 5'-O-(3-thiotriphosphate) (ATP gamma S), also had stimulatory effects on PtdEtn hydrolysis with a potency comparable to that observed with ATP. The same nucleotides which acted on PtdEtn hydrolysis also had similar stimulatory effects on the hydrolysis of phosphatidylcholine (PtdCho) in [14C]choline-labeled cells. In isolated membranes, Mg2+ greatly enhanced the stimulatory effects of ATP and ATP gamma S, but not of other nucleotides, on the hydrolysis of PtdEtn and PtdCho. Results indicate that in isolated membranes, both ATP and ATP gamma S stimulate phospholipid hydrolysis by two different mechanisms, but in intact cells only one of these mechanisms appears to be responsive to externally added nucleotides.

Guanine-nucleotide- and adenine-nucleotide-dependent regulation of phospholipase D in electropermeabilized HL-60 granulocytes

We have characterized the regulation of phospholipase D (PLD) in electropermeabilized HL-60 granulocytes in which endogenous phospholipids were pre-labelled with [3H]oleic acid. Treatment of these permeabilized cells with the non-hydrolysable GTP analogues guanosine 5'-[gamma-thio]triphosphate (GTP[S]) and guanosine 5'-[beta gamma-imido]triphosphate induced a sustained (near-linear for up to 60 min) accumulation of phosphatidic acid (PA). In the presence of ethanol a sustained production of phosphatidylethanol (PEt) was also observed. With increasing concentrations of ethanol, PEt formation increased, whereas PA formation declined; this indicated involvement of a PLD-type effector enzyme. The ability of GTP[S] to stimulate this PLD activity was Mg(2+)-dependent and was inhibited by GDP and its non-hydrolysable beta-thio analogue. Ca2+, at concentrations less than or equal to nM, had no effect on the GTP[S]-dependent PLD activity. However, higher concentrations of Ca2+ produced a significant potentiation of this activity. Inclusion of MgATP (greater than or equal to 0.1 mM), but not other nucleoside triphosphates, also induced a large potentiation of GTP[S]-dependent PLD activation. In the absence of guanine nucleotides, MgATP elicited no significant activation of PLD. Significantly, this effect of ATP was not mimicked by adenosine 5'-[beta gamma-methylene]triphosphate, a non-hydrolysable ATP analogue. Rather, this analogue inhibited both basal and ATP-potentiated GTP[S]-dependent PLD activity. This suggests that the ability of ATP to potentiate GTP[S]-dependent PLD activity involves phosphotransferase action rather than simple allosteric effects induced by adenine nucleotide binding. The absolute magnitude of the GTP[S]-dependent PLD activity which could be potentiated by MgATP was decreased by 90% when the permeabilized cells were preincubated for various times before addition of these stimulatory agents. This time-dependent loss of MgATP-induced potentiation was prevented when the permeabilized cells were preincubated in the presence of GTP[S]. These results demonstrate that electropermeabilized HL-60 granulocytes can be used to discriminate synergistic roles for a GTP-binding protein(s) and an ATP-dependent process (kinase?) in the regulation of phospholipase D activity.

Regulation of phospholipase D and primary granule secretion by P2-purinergic- and chemotactic peptide-receptor agonists is induced during granulocytic differentiation of HL-60 cells

We have compared the abilities of extracellular ATP (acting via P2-purinergic receptors) and formylated peptides (FMLP) to stimulate both phospholipase D (PLD)-based signal transduction and primary granule (azurophilic) secretion in HL-60 cells induced to differentiate along the granulocytic pathway. In undifferentiated HL-60 cells, neither ATP nor FMLP elicited significant PLD activation or increased secretion despite the previously documented ability of ATP to stimulate large increases in polyphosphoinositide hydrolysis and Ca2+ mobilization. Conversely, within 1 d after induction of granulocytic differentiation by dibutyryl cAMP, both ATP and FMLP induced large increases in azurophilic secretion and corresponding increases in PLD activity. ATP-activated PLD activity was near-maximal within 1 d after dibutyryl cAMP treatment, while the FMLP-induced activity increased continuously over 4 d, with a maximal level twice that stimulated by ATP. Additional experiments characterized the activation of PLD by receptor-independent pathways at different stages of differentiation; these included studies of phorbol ester action in intact cells and GTP gamma S action in electropermeabilized cells. An apparent role for guanine nucleotide-binding regulatory proteins in PLD regulation was also indicated by the significant reduction in FMLP- and ATP-stimulated PLD activity observed in cells pretreated with pertussis toxin. At all stages of differentiation, there was good correlation between the relative efficacies of ATP versus FMLP in stimulating both secretion and PLD activity. These data indicate: (a) that the receptor-regulated phospholipase D signaling pathway is induced during differentiation of myeloid progenitor cells; and (b) that differential activation of this signaling system by various Ca(2+)-mobilizing receptor agonists may underlie the differential regulation of secretion and other phagocyte functions by such agents.

Further subclassification of ATP receptors based on agonist studies

Recent studies using agonist analogues of ATP and other nucleotides have generated some surprising observations which may have ramifications for the classification of P2 receptors, particularly for those responses currently attributed to P2Y receptor activation. 2-MethylthioATP (2-MeSATP), the conventional P2Y receptor agonist, does not interact with ATP in the expected fashion in various models of endothelial function, suggesting that it acts by a different mechanism. Furthermore, in certain cell types where responses to ATP are mediated by phospholipase C activation, 2-MeSATP has little or no activity. Interestingly, the pyrimidine uridine triphosphate (UTP) invariably shows similar potency to ATP in systems where 2-MeSATP is inactive. In this article Steve O'Connor and colleagues discuss these data and their significance, and propose that separate receptors may be responsible: one sensitive to 2-MeSATP and the other, a 'nucleotide' receptor, sensitive to UTP.

Dual effects of ATP on phosphatidylinositol breakdown in rat hepatocyte membranes

The mechanisms whereby adenosine-5'-triphosphate (ATP) regulates the inositol phospholipid-signalling system were studied in rat hepatocytes. Intact hepatocytes respond to extracellular ATP, adenosine-5'-O-(3-thiotriphosphate) (ATP gamma S), ADP and weakly to guanosine-5'-triphosphate (GTP), but not to other purine nucleotides (GDP or AMP). This is consistent with the idea that a P2 purinergic receptor is coupled to the phosphatidylinositol metabolism in these cells. Partially purified plasma membranes prepared from myo-[3H]inositol prelabelled hepatocytes exhibit a phosphatidylinositol-4,5-bisphosphate phospholipase C activity sensitive to ATP, ATP gamma S and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S). Moreover the GTP gamma S effect is greatly enhanced by ATP and ATP gamma S. These potentiating effects differ according to the adenylnucleotide considered. ATP produces (1) an increase in the GTP gamma S-PLC sensitivity, (2) a potentiation of the phospholipase C (PLC) response induced by maximal dose of GTP gamma S, and (3) an increase in the inositol lipids pools. At variance, ATP gamma S, a nonhydrolysable analogue of ATP, only increases the PLC-sensitivity towards GTP gamma S. These results may signify that ATP stimulates inositol phosphate accumulation via at least two distinct mechanisms (i) a direct activation of a P2 purinergic receptor coupled to a PLC via a GTP binding protein and (ii) a stimulation of the phosphatidylinositol (PI) and phosphatidylinositol-4-phosphate (PIP) kinases which increased the pool of phospholipase C substrates.