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

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.

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.

Only pyrimidinoceptors are functionally expressed in mouse neuroblastoma cell lines

The ability of UTP, UDP, ATP, and ADP to influence inositol phospholipid hydrolysis in neuroblastoma origin cell lines was assessed. The mouse neuroblastoma lines N1E 115, Neuro 2a, and NB4 1A3 and the rat glioma/mouse neuroblastoma hybrid line NG108-15 gave robust responses to both UTP and UDP, which were essentially equipotent. Thus a range of cell lines of mouse neuroblastoma origin express a pyrimidine-selective P2Y receptor. The NG108-15 cells were the only cell type tested at which ATP and ADP displayed activity with EC50 values of greater than 100 microM, compared with values of 0.58 and 1.25 microM for UTP and UDP, respectively. In contrast to the cell lines derived from mouse neuroblastoma, the human neuroblastoma lines SH-SY5Y and SK-N-SH did not respond to any nucleotides, although both responded well to carbachol.

Cloning and expression of a P2y purinoceptor from the adult bovine corpus callosum

We have isolated an ATP receptor clone by screening a bovine corpus callosum cDNA library. The clone includes one open reading frame encoding for a protein of 373 amino acid residues (42 kDa) which belongs to the G-protein-coupled receptor superfamily. In Xenopus oocytes, this clone expressed an ATP receptor that triggered an oscillatory current in response to ATP (EC50 approximately 20 microM). This current may have resulted from the activation of phospholipase C, the formation of inositol trisphosphate, and the release of Ca2+, which then opens Cl- channels. The order of potency for ATP receptor agonists was 2-MeSATP approximately ATP >> alpha, beta-MeATP > adenosine, and UTP was ineffective, a pharmacological profile consistent with that of a P2y purinoceptor. Northern blot analysis of mRNAs from various bovine brain tissues showed that the gene is expressed in the cerebellum, medulla, corpus callosum, hippocampus, superior colliculus, frontal cortex, and retina. In situ RT-PCR showed transcripts of the gene in many glial cells and endothelial cells of the corpus callosum. The cloned receptor may play an important role in neuron-glial signaling under normal and pathological conditions.

Mechanisms of intracellular calcium regulation in adult astrocytes

Microfluorimetric techniques were used to measure changes in intracellular calcium in astrocytes cultured from the forebrain of the adult rat. Application of ATP consistently raised intracellular calcium. The response persisted in the absence of extracellular calcium, but then quickly declined upon repeated agonist application. Thapsigargin abolished responses to nucleotides following depletion of the endoplasmic reticular calcium stores. Calcium release was inhibited by caffeine, but was dramatically increased through inositol phosphate receptor sensitization by the sulphydryl reagent thimerosal. Responses to repeated nucleotide applications resulted in a gradual decline of peak calcium concentrations, suggesting a (post)receptor-mediated desensitization or gradual depletion of the internal calcium stores. Subsequent application of ionomycin suggested intracellular calcium depletion as the relevant mechanism. Depletion of the internal calcium stores with ATP, ionomycin or thapsigargin failed to reveal a calcium influx pathway. These results suggest that the capacitative mechanism of calcium entry does not operate in response to nucleotide receptor activation in these cells, and that the immediate refilling of the internal calcium stores is primarily determined by re-uptake of cytosolic calcium into the endoplasmic reticulum. A complete refilling of this calcium store by extracellular calcium may be a much slower process. Control of these signal transduction pathways is crucial to the maintenance of the calcium/energy homeostasis of the adult astrocyte in the central nervous system.

Actions of ADP, but not ATP, on cytosolic free Ca2+ in single rat hepatocytes mimicked by 2-methylthioATP

1. Aequorin-injected, single rat hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca2+]i) when stimulated with agonists acting through the phosphoinositide signalling pathway, including ADP and ATP. We have previously described differences in the [Ca2+]i responses of aequorin-injected hepatocytes to ADP and ATP. 2. The effects of the phosphorothioate analogue of ATP, 2-methylthioATP (2-meSATP), have been examined on single rat hepatocytes. This analogue is belived to be the most potent agonist at the P2Y1 subclass of purinoceptor. 3. The [Ca2+]i transients induced by 2-meSATP were indistinguishable from those induced by ADP, and in contrast to those induced by ATP. 4. At hig concentrations, 2-meSATP and ADP both induced transients at high frequency. In contrast, hepatocytes responded to high concentrations of ATP with an initial rapid rise in [Ca2+]i, followed by a slowly decaying fall. 5. The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both 2-meSATP- and ADP-induced [Ca2+]i transients; the peak height and frequency of transients were enhanced. ATP-induced transients, however, underwent either an increase in duration or conversion into a sustained rise in [Ca2+]i. 6. ATP-induced transients were specifically potentiated by the co-addition of alpha, beta-methyleneATP, whereas 2-meSATP- and ADP-induced transients were unaffected by this treatment. 7. We conclude that 2-meSATP acts at the same receptor as ADP on rat hepatocytes, and that this is distinct from teh receptor(s) mediating the effects of ATP.

Synergy between the inositol phosphate responses to transfected human adenosine A1-receptors and constitutive P2-purinoceptors in CHO-K1 cells

1. The effect of adenosine A1-receptor and P2-purinoceptor agonists on [3H]-inositol phosphate accumulation has been investigated in CHO-K1 cells transfected with the human adenosine A1-receptor. 2. Adenosine receptor agonists stimulated [3H]-inositol phosphate accumulation in CHO-K1 cells with a rank potency order of N6-cyclopentyladenosine (CPA) > 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > N6-2-(4-aminophenyl) ethyladenosine (APNEA). The responses to both CPA and APNEA were antagonized by the A1 selective antagonist, 1,3-dipropylcyclopentylxanthine (DPCPX) yielding KD values of 1.2 nM and 4.3 nM respectively. 3. ATP, UTP and ATP gamma S were also able to stimulate [3H]-inositol phosphate accumulation in these cells with EC50 values of 1.9 microM, 1.3 microM and 5.0 microM respectively. 2-Methyl-thio-ATP was a weak agonist of this response (EC50 > 100 microM). 4. The [3H]-inositol phosphate response to CPA was completely attenuated by pertussis toxin treatment (24 h; 100 ng ml-1). In contrast, the responses to ATP, UTP and ATP gamma S were only reduced by circa 30% in pertussis toxin-treated cells. 5. The simultaneous addition of CPA and either ATP, UTP or ATP gamma S produced a large augmentation of [3H]-inositol phospholipid hydrolysis. This was due to an increase in the maximal response and was significantly greater than the predicted additive response for activation of these two receptor systems. The synergy was not observed in pertussis toxin-treated cells. 6. No synergy was observed between the [3H]-inositol phosphate responses to histamine and ATP in CHO-K1 cells transfected with the bovine histamine H1-receptor. In these cells the response to histamine was completely resistant to inhibition by pertussis toxin treatment. 7. This study provides a clear demonstration of a synergy between pertussis toxin-sensitive and insensitive receptor systems in a model cell system which is an ideal host for transfected cDNA sequences. This model system should provide a unique opportunity to unravel the mechanisms underlying this example of receptor cross-talk involving phospholipase C.

Nucleotides as extracellular signalling molecules

There is now wide acceptance that ATP and other nucleotides are ubiquitous extracellular chemical messengers. ATP and diadenosine polyphosphates can be released from synaptosomes. They act on a large and diverse family of P2 purinoceptors, four of which have been cloned. This receptor family can be divided into two distinct classes: ligand-gated ion channels for P2X receptors and G protein-coupled receptors for P2Y, P2U, P2T and P2D receptors. The P2Y, P2U and P2D receptors have a fairly wide tissue distribution, while the P2X receptor is mainly found in neurons and muscles and the P2T and P2Z receptors confined to platelets and immune cells, respectively. Inositol phosphate and calcium signalling appear to be the predominant mechanisms for transducing the G-protein linked P2 receptor signals. Multiple P2 receptors are expressed by neurons and glia in the CNS and also in neuroendocrine cells. ATP and other nucleotides may therefore have important roles not only as a neurotransmitter but also as a neuroendocrine regulatory messenger.

Stimulation by extracellular ATP and UTP of the mitogen-activated protein kinase cascade and proliferation of rat renal mesangial cells

1. Extracellular ATP and UTP have been reported to activate a nucleotide receptor that mediates phosphoinositide and phosphatidylcholine hydrolysis by phospholipases C and D, respectively. Here we report that ATP and UTP potently stimulate mesangial cell proliferation. 2. Both nucleotides stimulate phosphorylation and activation of mitogen-activated protein kinase and a biphasic phosphorylation of the up-stream mitogen-activated protein kinase kinase. 3. When added at 100 microM, ATP gamma S, UTP and ATP were the most potent activators of mitogen-activated protein kinase. beta gamma-imido-ATP was somewhat less active and ADP and 2-methylthio-ATP caused a weak induction of enzyme activity. Activation of mitogen-activated protein kinase by both ATP and UTP is dose-dependently attenuated by the P2-receptor antagonist, suramin. 4. The protein kinase C activator 12-0-tetradecanoylphorbol 13-acetate, but not the biologically inactive 4 alpha-phorbol 12,13-didecanoate, increased mitogen-activated protein kinase activity in mesangial cells, suggesting that protein kinase C may mediate nucleotide-induced stimulation of mitogen-activated protein kinase. 5. Down-regulation of protein kinase C -alpha and -delta isoenzymes by 4 h or 8 h treatment with phorbol ester partially inhibited ATP- and UTP-triggered mitogen-activated protein kinase activation. Moreover, a 24 h treatment of mesangial cells with phorbol ester, a regimen that also causes depletion of protein kinase C-epsilon did not further reduce the level of mitogen-activated protein kinase stimulation. 6. The specific protein kinase C inhibitor, CGP 41251, which displayed a selectivity for the Ca2+-dependent isoenzymes, as compared to the Ca2+-independent isoenzymes did not inhibit nucleotide stimulated mitogen-activated protein kinase phosphorylation, thus implicating the involvement of a Ca2+-independent protein kinase C isoform.7. In summary, these results suggest that ATP and UTP trigger the activation of the mitogen-activated protein kinase signalling cascade in mesangial cells and this may be responsible for the potent mitogenic activity of both nucleotides.

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.

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.

Extracellular ATP and UTP activation of phospholipase D is mediated by protein kinase C-epsilon in rat renal mesangial cells

1. We have studied whether a nucleotide receptor mediates the effects of extracellular ATP and UTP on phosphatidylcholine metabolism in rat cultured glomerular mesangial cells. 2. ATP and UTP stimulated a biphasic 1,2-diacylglycerol (DAG) formation in [3H]-arachidonic acid-labelled mesangial cells. In contrast, in cells labelled with [3H]-myristic acid, a tracer that preferentially marks phosphatidylcholine, both nucleotides induced a delayed monophasic production of DAG with a concomitant increase in phosphatidic acid and choline formation. 3. A phospholipase D-mediated phosphatidylcholine hydrolysis was further suggested by the observation that ATP and UTP stimulate the accumulation of phosphatidylethanol, when ethanol was added to mesangial cells. 4. The rank order of potency of a series of nucleotide analogues for stimulation of phosphatidylethanol formation was UTP = ATP > ITP > ATP gamma S > beta gamma-imido-ATP = ADP > 2-methylthio-ATP = beta gamma-methylene-ATP = ADP beta S, while AMP, adenosine, CTP and GTP were inactive, indicating the presence of a nucleotide receptor. 5. Elevation of cytosolic free Ca2+ by the calcium ionophore A23187 (1 microM) or the Ca(2+)-ATPase inhibitor, thapsigargin (200 nM) slightly increased phosphatidylethanol formation. However, chelation of cytosolic Ca2+ with high concentrations of Quin 2 did not attenuate ATP- and UTP-induced phosphatidylethanol production, thus suggesting that Ca2+ is not crucially involved in agonist-stimulated phospholipase D activation. 6. The protein kinase C (PKC) activator, phorbol 12-myristate 13-acetate (PMA), but not the biologically inactive 4 alpha-phorbol 12,13-didecanoate, increased phospholipase D activity in mesangial cells, suggesting that PKC may mediate nucleotide-induced phosphatidylcholine hydrolysis. 7. Down-regulation of PKC-alpha and -delta isoenzymes by 8 h PMA treatment still resulted in full phospholipase D activation. In contrast, a 24 h treatment of mesangial cells with PMA, a regimen that also causes depletion of PKC-epsilon, markedly attenuated nucleotide-evoked phosphatidylethanol formation. In addition, the selective PKC inhibitor, calphostin C attenuated ATP- and UTP-induced phosphatidylethanol production.8. In summary, these data suggest that extracellular ATP and UTP use a common nucleotide receptor to activate phospholipase D-mediated phosphatidylcholine hydrolysis. Stimulation of phospholipase D appears to involve the PKC-epsilon isoenzyme, activated by DAG derived from phosphoinositide hydrolysis by phospholipase C.

The effects of beta-phorbol-12,13 dibutyrate on agonist-induced increases in [Ca2+]i in N1E-115 cells. Differential modulation of responses to angiotensin II and bradykinin

Addition of angiotensin II (A2; 500 nM) to populations of fura-2-loaded N1E-115 cells resulted in a transient increase in intracellular calcium which was abolished by pre-treatment with the phorbol ester, beta-phorbol-12,13 dibutyrate (PDBu) (1.5 microM). The inhibitory effects were reversed by the protein kinase C inhibitor, staurosporine (150 nM), and down-regulation of protein kinase C was observed over 48 hr. Responses to maximally effective concentrations of histamine (300 microM), ATP (100 microM), UTP (100 microM) and carbachol (100 microM) were similarly inhibited by phorbol pre-treatment but the response to bradykinin (BK) (100 nM) was unaffected. When the concentrations of BK and A2 were adjusted to produce the same-sized calcium signals, PDBu pre-treatment abolished the response to A2 but only partially inhibited the response to BK. From the data presented here we can conclude that the calcium response to BK in N1E-115 cells is less susceptible to the inhibitory effects of protein kinase C activation than the response produced by A2.

Qualitative differences in [Ca2+]i increases and InsP3 generation following stimulation of N1E-115 cells with micromolar and millimolar ATP

Addition of ATP (100 microM) to populations of the neuronal cell line N1E-115 caused a transient increase in intracellular calcium ([Ca2+]i) which rapidly reached a peak (maximum of 243 +/- 7 nM above basal) and returned to basal after approximately 50 sec. The response was concentration-dependent (EC50 21 +/- 4 microM) and was unchanged when calcium was omitted from the extracellular medium. Transient increases in D-myo-inositol-1,4,5-trisphosphate levels (InsP3) were also observed over a similar time period. Addition of millimolar ATP, however, produced characteristically different responses; [Ca2+]i again increased rapidly (reaching a maximum of 639 +/- 23 nM above basal) but returned to a new maintained plateau (274 +/- 34 nM) which was abolished by the inorganic calcium channel blocker, nickel (1 mM), and omission of calcium from the extracellular medium. InsP3 levels were also maintained but were, however, unaffected by nickel or removal of extracellular calcium. The qualitative difference in the mechanism of calcium elevation produced with millimolar ATP, compared with lower concentrations, suggests that the N1E-115 cells might also contain a low affinity P2 receptor coupled with a calcium channel.