Fidelity in functional coupling of the rat P2Y1 receptor to phospholipase C

Cross-Talk in Nucleotide Signaling in Glioma C6 Cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y₁, P2Y₂, P2Y₁₂, P2Y₁₄ and the ionotropic P2X₇ receptor in glioma C6 cells. P2Y₁ and P2Y₁₂ both respond to ADP, but while P2Y₁ links to PLC and elevates cytosolic Ca²⁺ concentration, P2Y₁₂ negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y₁ receptor strongly decreases and P2Y₁₂ becomes a major player responsible for ADP-evoked signal transduction. The P2Y₁₂ receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y₁ has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X₇ receptor, often responsible for apoptotic fate, is not involved in Ca²⁺elevation in C6 cells. The shift in nucleotide receptor expression from P2Y₁ to P2Y₁₂ during serum withdrawal, the cross talk between both receptors and the lack of P2X₇ activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Antagonism of P2Y1-induced vasorelaxation by acyl CoA: a critical role for palmitate and 3'-phosphate

BACKGROUND AND PURPOSE

Acyl derivatives of CoA have been shown to act as antagonists at human platelet and recombinant P2Y1 receptors, but little is known about their effects in the cardiovascular system. This study evaluated the effect of these endogenous nucleotide derivatives at P2Y1 receptors natively expressed in rat and porcine blood vessels.

EXPERIMENTAL APPROACH

Isometric tension recordings were used to evaluate the effects of CoA, acetyl CoA, palmitoyl CoA (PaCoA) and 3'-dephospho-palmitoyl-CoA on concentration relaxation-response curves to ADP and uridine triphosphate (UTP). A FlexStation monitored ADP- and UTP-evoked calcium responses in HEK293 cells.

KEY RESULTS

Acetyl CoA and PaCoA, but not CoA, inhibited endothelium-dependent relaxations to ADP with apparent selectivity for P2Y1 receptors (over P2Y(2/4) receptors) in rat thoracic aorta; PaCoA was more potent than acetyl CoA (331-fold vs. fivefold shift of ADP response curve evoked by 10 μM PaCoA and acetyl CoA, respectively); the apparent pA2 value for PaCoA was 6.44. 3'-dephospho-palmitoyl-CoA (10 μM) was significantly less potent than PaCoA (20-fold shift). In porcine mesenteric arteries, PaCoA and the P2Y1 receptor antagonist MRS2500 blocked ADP-mediated endothelium-dependent relaxations; in contrast, they were ineffective against ADP-mediated endothelium-independent relaxation in porcine coronary arteries (which does not involve P2Y1 receptors). Calcium responses evoked by ADP activation of endogenous P2Y1 receptors in HEK293 cells were inhibited in the presence of PaCoA, which failed to alter responses to UTP (acting at endogenous P2Y(2/4) receptors).

CONCLUSIONS AND IMPLICATIONS

Acyl derivatives of CoA can act as endogenous selective antagonists of P2Y1 receptors in blood vessels, and this inhibitory effect critically depends on the palmitate and 3'-ribose phosphate substituents on CoA.

Cross-talk in nucleotide signaling in glioma C6 cells

The chapter is focused on the mechanism of action of metabotropic P2Y nucleotide receptors: P2Y(1), P2Y(2), P2Y(12), P2Y(14) and the ionotropic P2X(7) receptor in glioma C6 cells. P2Y(1) and P2Y(12) both respond to ADP, but while P2Y(1) links to PLC and elevates cytosolic Ca(2+) concentration, P2Y(12) negatively couples to adenylate cyclase, maintaining cAMP at low level. In glioma C6, these two P2Y receptors modulate activities of ERK1/2 and PI3K/Akt signaling and the effects depend on physiological conditions of the cells. During prolonged serum deprivation, cell growth is arrested, the expression of the P2Y(1) receptor strongly decreases and P2Y(12) becomes a major player responsible for ADP-evoked signal transduction. The P2Y(12) receptor activates ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulates Akt activity, contributing to glioma invasiveness. In contrast, P2Y(1) has an inhibitory effect on Akt pathway signaling. Furthermore, the P2X(7) receptor, often responsible for apoptotic fate, is not involved in Ca(2+)elevation in C6 cells. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) during serum withdrawal, the cross talk between both receptors and the lack of P2X(7) activity shows the precise self-regulating mechanism, enhancing survival and preserving the neoplastic features of C6 cells.

Signalling and pharmacological properties of the P2Y receptor

The P2Y(14) receptor is a relatively broadly expressed G protein-coupled receptor that is prominently associated with immune and inflammatory cells as well as with many epithelia. This receptor historically was thought to be activated selectively by UDP-glucose and other UDP-sugars. However, UDP is also a very potent agonist of this receptor, and may prove to be one of its most important cognate activators.

Gi-dependent cell signaling responses of the human P2Y14 receptor in model cell systems

Eight G protein-coupled receptors comprise the P2Y receptor family of cell signaling proteins. The goal of the current study was to define native cell signaling pathways regulated by the uridine nucleotide sugar-activated P2Y(14) receptor (P2Y(14)-R). The P2Y(14)-R was stably expressed in human embryonic kidney (HEK) 293 and C6 rat glioma cells by retroviral infection. Nucleotide sugar-dependent P2Y(14)-R activation was examined by measuring inhibition of forskolin-stimulated cAMP accumulation. The effect of P2Y(14)-R activation on mitogen-activated protein kinase signaling also was studied in P2Y(14)-HEK293 cells and in differentiated HL-60 human myeloid leukemia cells. UDP-Glc, UDP-galactose, UDP-glucuronic acid, and UDP-N-acetylglucosamine promoted inhibition of forskolin-stimulated cAMP accumulation in P2Y(14)-HEK293 and P2Y(14)-C6 cells, and this signaling effect was abolished by pretreatment of cells with pertussis toxin. Inhibition of cAMP formation by nucleotide sugars also was observed in direct assays of adenylyl cyclase activity in membranes prepared from P2Y(14)-C6 cells. UDP-Glc promoted concentration-dependent and pertussis toxin-sensitive extracellular signal-regulated kinase (ERK) 1/2 phosphorylation in P2Y(14)-HEK293 cells. P2Y(14)-R mRNA was not observed in wild-type HL-60 cells but was readily detected in dimethyl sulfoxide-differentiated cells. Consistent with this observation, no effect of UDP-Glc was observed in wild-type HL-60 cells, but UDP-Glc-promoted pertussis toxin-sensitive activation of ERK1/2 occurred after differentiation. These results illustrate that the human P2Y(14)-R signals through G(i) to inhibit adenylyl cyclase, and P2Y(14)-R activation also leads to ERK1/2 activation. This work also identifies two stable P2Y(14)-R-expressing cell lines and differentiated HL-60 cells as model systems for the study of P2Y(14)-R-dependent signal transduction.

The P2Y14 receptor activity in glioma C6 cells

In this study, we demonstrated the presence and the activity of the P2Y14 receptor in glioma C6 cells. We found that P2Y14 could exist in two forms, highly predominating glycosylated and non-glycosylated. Binding of UDP-glucose evoked two responses: calcium signal and adenylate cyclase inhibition, both pertussis toxin-sensitive. Separate glycosylation pattern and functional profile of these two receptor forms were observed in non-starved and serum-starved cells. During long-term serum deprivation (96 h), the level of glycosylated form strongly decreased, while non-glycosylated increased, what was correlated with the decrease of calcium signaling activity and stronger adenylate cyclase inhibition, suggesting that receptor N-glycosylation may modulate its functional activity.

Expression and functional characterization of P2Y1 and P2Y12 nucleotide receptors in long-term serum-deprived glioma C6 cells

We characterized the expression and functional properties of the ADP-sensitive P2Y(1) and P2Y(12) nucleotide receptors in glioma C6 cells cultured in medium devoid of serum for up to 96 h. During this long-term serum starvation, cell morphology changed from fibroblast-like flat to round, the adhesion pattern changed, cell-cycle arrest was induced, extracellular signal-regulated kinase (ERK1/2) phosphorylation was reduced, Akt phosphorylation was enhanced, and expression of the P2Y(12) receptor relative to P2Y(1) was increased. These processes did not reflect differentiation into astrocytes or oligodendrocytes, as expression of glial fibrillary acidic protein and NG2 proteoglycan (standard markers of glial cell differentiation) was not increased during the serum deprivation. Transfer of the cells into fresh medium containing 10% fetal bovine serum reversed the changes. This demonstrates that serum starvation caused only temporary growth arrest of the glioma C6 cells, which were ready for rapid division as soon as the environment became more favorable. In cells starved for 72 and 96 h, expression of the P2Y(1) receptor was low, and the P2Y(12) receptor was the major player, responsible for ADP-evoked signal transduction. The P2Y(12) receptor activated ERK1/2 kinase phosphorylation (a known cell proliferation regulator) and stimulated Akt activity. These effects were reduced by AR-C69931MX, a specific antagonist of the P2Y(12) receptor. On the other hand, Akt phosphorylation increased in parallel with the low expression of the P2Y(1) receptor, indicating the inhibitory role of P2Y(1) in Akt pathway signaling. The shift in nucleotide receptor expression from P2Y(1) to P2Y(12) would appear to be a new and important self-regulating mechanism that promotes cell growth rather than differentiation and is a defense mechanism against effects of serum deprivation.

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.

Differential coupling of the P2Y1 receptor to Galpha14 and Galphaq/11 proteins during the development of the rat salivary gland

In rat submandibular gland (SMG), the P2Y1 receptor (P2Y1R) mediates increases in the intracellular calcium concentration, [Ca2+]i that diminish as the animal ages from 1 to 4-6 weeks. However, P2Y1R mRNA levels do not change with age, suggesting that the age-dependent decrease in the [Ca2+]i response to P2Y1R agonists may be due to alterations in the activity of a component of the P2Y1R signalling pathway.

OBJECTIVES

To assess whether the decrease in P2Y1R-mediated intracellular calcium signalling in SMG cells as rats age is due to a decrease in P2Y1R coupling to G proteins or to a decrease in the expression of a cognate G protein.

DESIGN

SMG cells were isolated from Sprague-Dawley rats. P2Y1R function was assessed by measuring 2-MeSADP-induced increases in [Ca2+]i and ERK1/2 activation. P2Y(1)R-mediated activation of G proteins was determined by the [35S]GTPgammaS binding assay. Gq protein expression was determined by RT-PCR, Northern, and Western analysis.

RESULTS

In SMG cells from 1-week-old rats, two bands (52 and 42kDa) were detected using anti-Galpha14 antibody, whereas in SMG cells from 4- to 6-week-old rats only the 42 kDa band was detected. Furthermore, 2-MeSADP-induced GTPgamma35S binding to Galpha14 and Galphaq/11 decreases in SMG cells from 4- to 6-week-old rats as compared to 1-week-old rats.

CONCLUSIONS

These findings suggest that the age-dependent decrease in P2Y1R-mediated intracellular calcium signalling in rat SMG cells is due to a loss of 52 kDa Galpha14 and indicate the differential coupling of the P2Y1R to Galpha14 and Galphaq/11 as the gland develops.

Mitochondrial P2Y-Like receptors link cytosolic adenosine nucleotides to mitochondrial calcium uptake

ATP is a known extracellular ligand for cell membrane purinergic receptors. Intracellular ATP can work also as a regulatory ligand via binding sites on functional proteins. We report herein the existence of P2Y(1)-like and P2Y(2)-like receptors in hepatocyte mitochondria (mP2Y(1) and mP2Y(2)), which regulate mCa(2+) uptake though the uniporter. Mitochondrial P2Y(1) activation stimulates mCa(2+) uptake; whereas, mP2Y(2) activation inhibits mCa(2+) uptake. ATP acts preferentially on mP2Y(2) receptors, while ADP and AMP-PNP stimulate both the mP2Y(1) and mP2Y(2). PPADS inhibits ADP stimulated mP2Y(1)-mediated mCa(2+) uptake. In addition, UTP, a selective P2Y(2) agonist, strongly inhibits mCa(2+) uptake. The newly discovered presence and function of these receptors is significant because it explains increased mCa(2+) uptake in the setting of low cytosolic [ATP] and, therefore, establishes a mechanism for direct feedback in which cytosolic [ATP] governs mitochondrial ATP production through regulation of mCa(2+) uptake.

Shift in purine/pyrimidine base recognition upon exchanging extracellular domains in P2Y 1/6 chimeric receptors

P2Y receptors are G protein-coupled receptors stimulated by extracellular nucleotides. Both the P2Y(1) and the P2Y(6) receptors are preferentially activated by nucleoside 5'-diphosphates, but favor different base moieties. In the case of the P2Y(1) receptor the preferred base is adenine, while the P2Y(6) receptor is activated by uracil nucleotides. To identify potential amino acid domains that interact with the base moiety, we used a chimeric receptor approach, employing the human P2Y(1) receptor as core structure to investigate the role in receptor activation of extracellular loops (ELs) and transmembrane domains (TMs) of the rat P2Y(6) receptor. The chimeric receptors were expressed in COS-7 cells and measured for stimulation of phospholipase C (PLC) induced by the potent P2Y(1) receptor agonist 2-MeSADP or the potent P2Y(6) receptor agonist UDP. Replacement of the N-terminus or EL2 resulted in low ( approximately 50 microM) potency of the agonist 2-MeSADP, thus confirming the importance of EL2 in ligand recognition. Upon replacement of several regions, the potency of the P2Y(1) agonist 2-MeSADP was either 1-2 microM (N-terminus and EL1, or EL1 and EL3) or 72 microM (N-terminus and EL3). Concurrent replacement of three regions (N-terminus, EL1, and EL3) completely precluded activation by 2-MeSADP. Our study identified domains of the P2Y(6) receptor that contribute to receptor activation by UDP and hence seem to be involved in uracil recognition. Upon replacement with extracellular domains of the P2Y(6) receptor sequence we observed a trend toward gain of receptor-induced PLC activation by UDP, for example, in the chimera containing replacements of both the N-terminus and EL1. Exchange of three receptor domains led to a construct with an EC(50) value for UDP of 19 microM and a maximal inositol phosphate accumulation similar to the native P2Y(6) receptor. Within receptor constructs of combined domain exchanges the additional substitution of Tyr(110) by the corresponding Asn from the P2Y(6) receptor showed a significant increase for activation by UDP, but only when combined with the N-terminal domain and TM1. The residue Tyr(110) was identified to play an important role in the recognition of the nucleobase in the P2Y(1) and P2Y(6) receptors.

Induction of novel agonist selectivity for the ADP-activated P2Y1 receptor versus the ADP-activated P2Y12 and P2Y13 receptors by conformational constraint of an ADP analog

ADP is the cognate agonist of the P2Y1, P2Y12, and P2Y13 receptors. With the goal of identifying a high potency agonist that selectively activates the P2Y1 receptor, we examined the pharmacological selectivity of the conformationally constrained non-nucleotide analog (N)-methanocarba-2MeSADP [(1'S,2'R, 3'S,4'R,5'S)-4-[(6-amino-2-methylthio-9H-purin-9-yl)-1-diphosphoryloxymethyl]bicyclo[3.1.0]hexane-2,3-diol] among the three ADP-activated receptors. Each P2Y receptor was expressed transiently in COS-7 cells, and inositol lipid hydrolysis was quantified as a measure of receptor activity. In the case of the Gi-linked P2Y12 and P2Y13 receptors, a chimeric G protein, Galphaq/i, was coexpressed to confer a capacity of these Gi-linked receptors to activate phospholipase C. 2MeSADP (2-methylthio-ADP) was a potent agonist at all three receptors exhibiting EC50 values in the sub to low nanomolar range. In contrast, whereas (N)-methanocarba-2MeSADP was an extremely potent (EC50=1.2 +/- 0.2 nM) agonist at the P2Y1 receptor, this non-nucleotide analog exhibited no agonist activity at the P2Y12 receptor and very low activity at the P2Y13 receptor. (N)-Methanocarba-2MeSADP also failed to block the action of 2MeSADP at the P2Y12 and P2Y13 receptors, indicating that the (N)-methanocarba analog is not an antagonist at these receptors. The P2Y1 receptor selectivity of (N)-methanocarba-2MeSADP was confirmed in human platelets where it induced the shape change promoted by P2Y1 receptor activation without inducing the sustained platelet aggregation that requires simultaneous activation of the P2Y12 receptor. These results provide the first demonstration of a high-affinity agonist that discriminates among the three ADP-activated P2Y receptors, and therefore, introduce a potentially important new pharmacological tool for delineation of the relative biological action of these three signaling proteins.

Inhibition of bicarbonate reabsorption in the rat proximal tubule by activation of luminal P2Y1 receptors

The present study used a stationary microperfusion technique to investigate in vivo the effect of P2Y1 receptor activation on bicarbonate reabsorption in the rat proximal tubule. Proximal tubules were perfused with a bicarbonate Ringer solution before flow was stopped by means of an oil block. The recovery of lumen pH from the initial value (pH 8.0) to stationary values (pH approximately 6.7) was recorded by a H+-sensitive microelectrode inserted downstream of the perfusion pipette and oil block. The stationary pH value and the t(1/2) of pH recovery were used to calculate bicarbonate reabsorption (JHCO3). Both EIPA and bafilomycin A1 caused significant reductions in proximal tubule JHCO3, consistent with the established contributions of Na/H exchange and H+-ATPase to proximal tubule HCO3 reabsorption. The nucleotides ADP and, to a lesser extent, ATP reduced JHCO3 but AMP and UTP were without effect. 2MeSADP, a highly selective agonist of the P2Y1 receptor, reduced JHCO3 in a dose-dependent manner. MRS-2179, a P2Y1 receptor-specific antagonist, abolished the effect of 2MeSADP, whereas theophylline, an antagonist of adenosine (P1) receptors, did not. The inhibitory action of 2MeSADP was blocked by inhibition of protein kinase C and reduced by inhibition of protein kinase A. The effects of EIPA and 2MeSADP were not additive. The data provide functional evidence for P2Y1 receptors in the apical membrane of the rat proximal tubule: receptor activation impairs acidification in this nephron segment.

Differential effects of P2Y1 and P2Y12 nucleotide receptors on ERK1/ERK2 and phosphatidylinositol 3-kinase signalling and cell proliferation in serum-deprived and nonstarved glioma C6 cells

We have previously shown that, in glioma C6 cells, two nucleotide ADP-sensitive receptors coexist: P2Y1, coupled to PLC and responsible for Ca2+ release, and P2Y12, negatively coupled to adenylate cyclase. In the present study, we examined the effects of the stimulation of these two receptors on ERK1/2 and PI3-K activation, and cell proliferation in either serum-deprived or nonstarved C6 cells. In response to ADP and its analogues, in serum-starved cells, both p44 ERK1 and p42 ERK2 were activated in a time-dependent manner, as monitored by Western blot analysis using an antiphospho-p42/p44 MAPK antibody. The phosphorylation was reduced both by removal of the extracellular Ca2+ and partially or almost completely by MRS2179 or AR-C69931MX, specific antagonists of the P2Y1 and P2Y12 receptors, respectively. The inhibitory effect of antagonists was additive. These data indicate the involvement of both receptors, P2Y1 and P2Y12, in the ERK1/2 activation, but the P2Y12 receptor contribution predominates. ERK1/2 activity was positively correlated with cell proliferation of cultured glioma C6 cells. In nonstarved cells, ADP markedly decreased the PI3-K activity. In contrast, in serum-starved cells, ADP evoked an increase in the PI3-K activity. Blocking of the P2Y1 receptor by MRS2179 additionally increased this ADP response. These results suggest that the P2Y1 receptor has an inhibitory and the P2Y12 receptor a stimulatory effect on PI3-K signalling pathway. RT-PCR analysis revealed different mRNA expression of both receptors in starved and nonstarved cells. In nonstarved cells, the P2Y1 receptor mRNA predominates, whereas in serum-deprived cells the expression of P2Y12 mRNA becomes more pronounced. British Journal of Pharmacology (2004) 141, 497-507. doi:10.1038/sj.bjp.0705639

Agonist Binding and Gq-Stimulating Activities of the Purified Human P2Y1Receptor

The human P2Y1 receptor (P2Y1-R) was purified after high-level expression from a recombinant baculovirus in Sf9 insect cells. Quantification by protein staining and with a radioligand binding assay using the high-affinity P2Y1-R antagonist [3H]MRS2279 ([3H]2-chloro-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bis-phosphate) indicated a nearly homogenous preparation of receptor protein. Ki values determined in [3H]MRS2279 binding assays for antagonists with the purified P2Y1-R were in good agreement with the Ki and KB values determined for these molecules in membrane binding and activity assays, respectively. Availability of P2Y1-R in purified form allowed direct determination of nucleotide agonist affinities under conditions not compromised by nucleotide metabolism/interconversion, and an order of affinities of 2-methylthio-ADP (2MeSADP) > ADP = 2-methylthioATP = adenosine-5'-O-(3-thio)triphosphate = adenosine-5'-O(2-thiodiphosphate) >> ATP was obtained. The signaling activity of the purified P2Y1-R was quantified after reconstitution in proteoliposomes with heterotrimeric G proteins. Steady-state GTP hydrolysis in vesicles reconstituted with P2Y1-R and Galpha(q)beta(1)gamma(2) was stimulated by the addition of either 2MeADP or RGS4 alone and was increased by up to 50-fold in their combined presence. EC50 values of agonists for activation of the purified P2Y1-R were similar to their respective Ki values determined in radioligand binding experiments with the purified receptor. Moreover, ATP exhibited 20-fold higher EC50 and Ki values than did ADP and was a partial agonist relative to ADP and 2MeSADP under conditions in which no metabolism of the nucleotide occurred. Both RGS4 and PLC-beta1 were potent and efficacious GTPase-activating proteins for Galphaq and Galpha11 in P2Y1-R-containing vesicles. These results illustrate that the binding and signaling properties of the human P2Y1-R can be studied with purified proteins under conditions that circumvent the complications that occur in vivo.

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.

Agonist versus antagonist action of ATP at the P2Y4 receptor is determined by the second extracellular loop

UTP is a potent full agonist at both the human P2Y(4) (hP2Y(4)) and rat P2Y(4) (rP2Y(4)) receptor. In contrast, ATP is a potent full agonist at the rP2Y(4) receptor but is a similarly potent competitive antagonist at the hP2Y(4) receptor. To delineate the structural determinants of agonism versus antagonism in these species homologues, we expressed a series of human/rat P2Y(4) receptor chimeras in 1321N1 human astrocytoma cells and assessed the capacity of ATP and UTP to mobilize intracellular Ca(2+). Replacement of the NH(2) terminus of the hP2Y(4) receptor with the corresponding region of the rP2Y(4) receptor resulted in a receptor that was activated weakly by ATP, whereas replacement of the second extracellular loop (EL2) of the hP2Y(4) receptor with that of the rP2Y(4) receptor yielded a chimeric receptor that was activated fully by UTP and near fully by ATP, albeit with lower potencies than those observed at the rP2Y(4) receptor. These potencies were increased, and ATP was converted to a full agonist by replacing both the NH(2) terminus and EL2 in the hP2Y(4) receptor with the corresponding regions from the rP2Y(4) receptor. Mutational analysis of the five divergent amino acids in EL2 between the two receptors revealed that three amino acids, Asn-177, Ile-183, and Leu-190, contribute to the capacity of EL2 to impart ATP agonism. Taken together, these results suggest that the second extracellular loop and the NH(2) terminus form a functional motif that plays a key role in determining whether ATP functions as an agonist or antagonist at mammalian P2Y(4) receptors.

P2X4, P2Y1 and P2Y2 receptors on rat alveolar macrophages

ATP receptors present on rat alveolar macrophages (NR8383 cells) were identified by recordings of membrane current, measurements of intracellular calcium, RT-PCR and immunocytochemistry. In whole-cell recordings with a sodium-based internal solution, ATP evoked an inward current at -60 mV. This reversed at 0 mV. The EC50 for ATP was 18 microM in normal external solution (calcium 2 mm, magnesium 1 mm). The currents evoked by 2',3-O-(4-benzoyl)benzoyl-ATP were about five-fold smaller than those observed with ATP. ADP, UTP and alphabeta-methylene-ATP (alphabetameATP) (up to 100 microM) had no effect. ATP-evoked currents were potentiated up to ten-fold by ivermectin and were unaffected by suramin (30-100 microM), pyridoxal-phosphate-6-azophenyl-(2,4-sulphonic acid) (30-100 microM), and brilliant blue G (1 microM). In whole-cell recordings with a potassium-based internal solution and low EGTA (0.01 mm), ATP evoked an inward current at -60 mV that was followed by larger outward current. ADP and UTP (1-100 microM) evoked only outward currents; these reversed polarity at the potassium equilibrium potential and were blocked by apamin (10 nm). Outward currents were also blocked by the phospholipase C inhibitor U73122 (1 microM), and they were not seen with higher intracellular EGTA (10 mm). Suramin (30 microM) blocked the outward currents evoked by ATP and UTP, but not that evoked by ADP. PPADS (10 microM) blocked the ADP-evoked outward current without altering the ATP or UTP currents. RT-PCR showed transcripts for P2X subunits 1, 4 and 7 (not 2, 3, 5, 6) and P2Y receptors 1, 2, 4 and 12 (not 6). Immunocytochemistry showed strong P2X4 receptor expression partly associated with the membrane, weak P2X7 staining that was not associated with the cell membrane, and no P2X1 receptor immunoreactivity. We conclude that rat alveolar macrophages express (probably homomeric) P2X4 receptors, but find no evidence for other functional P2X subtypes. The P2Y receptors are most likely P2Y1 and P2Y2 and these couple through phospholipase C to an increase in intracellular calcium and the opening of SK type potassium channels.

ADP stimulation of inositol phosphates in hepatocytes: role of conversion to ATP and stimulation of P2Y2 receptors

1 Accumulation of inositol (poly)phosphates (InsP(x)) has been studied in rat hepatocytes labelled with [(3)H]inositol. Stimulation with ADP resulted in a significant increase in total [(3)H]InsP(x), whereas 2-MeSADP had only a small effect and ADPbetaS was ineffective. UTP and ITP also stimulated substantial increases in [(3)H]InsP(x). 2 The dose-response curve to ADP was largely unaltered by the presence of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P). Similarly, inclusion of MRS 2179, a more selective P2Y(1) antagonist, had no effect on the dose-response curve to ADP. 3 The inclusion of hexokinase in the assay reduced, but did not abolish, the response to ADP. 4 HPLC analysis revealed that ADP in the medium was rapidly converted to AMP and ATP. The inclusion of hexokinase removed ATP, but exacerbated the decline in ADP concentration, leading to increased levels of AMP. 2-MeSADP was stable in the medium and ATP was largely unaffected. 5 The addition of the adenylate kinase inhibitor, diadenosine pentaphosphate (Ap(5)A) significantly reduced the ADP response. HPLC analysis conducted in parallel demonstrated that this treatment inhibited conversion of ADP to ATP and AMP. 6 Inclusion of the P1 antagonist CGS 15943 had no effect on the dose-response curve to ADP. 7 These observations indicate that hepatocytes respond to ADP with an increase in inositol (poly)phosphates following conversion to ATP. P2Y(1) activation in hepatocytes does not appear to be coupled to inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) production.

Evidence for the Recognition of Non-Nucleotide Antagonists Within the Transmembrane Domains of the Human P2Y(1) Receptor

Site-directed mutagenesis was used to search for amino acid residues of the human P2Y(1) receptor involved in the binding of the P2 receptor antagonists pyridoxal-5'-phosphate-6-azophenyl-2,4-disulfonate (PPADS), its analogue 6-(2'-chloro-phenylazo)-pyridoxal-α(5)-phosphate (MRS 2210), the suramin analogue 8-8'-[carbonylbis(imino-3,1-phenylene)]bis(1,3,5-naphthalene-trisulfonate) (NF023), and Reactive blue 2. Receptors containing single amino acid replacements at positions in transmembrane helical domains (TMs) 3, 5, 6, and 7 critical for the activation of the receptor by nucleotide agonists were expressed in COS-7 (African green monkey kidney) cells. Inositol phosphate accumulation was induced by 2-methylthioadenosine 5'-diphosphate (2-MeSADP). In wild type human P2Y(1) receptors, PPADS (10 to 60 µM), MRS 2210 (10 µM), NF023 (100 µM), and Reactive blue 2 (10 µM) shifted the concentration-response curve of 2-MeSADP in a parallel manner to the right. For PPADS, a pA(2) value of 5.2 was estimated. The shifts caused by MRS 2210, NF023, and Reactive blue 2 corresponded to apparent pK(B) values of 5.6, 5.0, and 5.8, respectively. In K280A mutant receptors, the affinities of PPADS, MRS 2210, NF023, and Reactive blue 2 were about 6- to 60-fold lower than those observed at wild type receptors. The K280A mutation also caused an approximately 1,000-fold increase in the EC(50) value of the agonist 2-MeSADP, similar to previous observations. In contrast, no major change in antagonistic potency was observed at receptors with other mutations in TMs 3, 5, 6, and 7. Thus, the residue Lys(280) (6.55), which is located within the upper third of TM 6 of the human P2Y(1) receptor, is not only critical for the activation of the receptor but also plays an important role in the binding of pyridoxal derivatives and a number of other chemically unrelated P2 receptor antagonists. Lys(280) seems to belong to an overlapping region of the respective binding sites.

ATP regulates the differentiation of mammalian skeletal muscle by activation of a P2X5 receptor on satellite cells

ATP is well known for its role as an intracellular energy source. However, there is increasing awareness of its role as an extracellular messenger molecule (Burnstock, 1997). Although evidence for the presence of receptors for extracellular ATP on skeletal myoblasts was first published in 1983 (Kolb and Wakelam), their physiological function has remained unclear. In this paper we used primary cultures of rat skeletal muscle satellite cells to investigate the role of purinergic signaling in muscle formation. Using immunocytochemistry, RT-PCR, and electrophysiology, we demonstrate that the ionotropic P2X5 receptor is present on satellite cells and that activation of a P2X receptor inhibits proliferation, stimulates expression of markers of muscle cell differentiation, including myogenin, p21, and myosin heavy chain, and increases the rate of myotube formation. Furthermore, we demonstrate that ATP application results in a significant and rapid increase in the phosphorylation of MAPKs, particularly p38, and that inhibition of p38 activity can prevent the effect of ATP on cell number. These results not only demonstrate the existence of a novel regulator of skeletal muscle differentiation, namely ATP, but also a new role for ionotropic P2X receptors in the control of cell fate.

ADP-evoked phospholipase C stimulation and adenylyl cyclase inhibition in glioma C6 cells occur through two distinct nucleotide receptors, P2Y(1) and P2Y(12)

In this study we characterized the subtypes of nucleotide P2Y receptors that respond to ADP in glioma C6 cells. Direct visualization of phosphatidylinositol 4,5-bisphosphate at the cell surface revealed that extracellular ADP activates phospholipase C (PLC). Knock-down of P2Y(1) receptor with antisense oligonucleotide, as well as treatment with MRS2179 and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (P2Y(1) antagonists), attenuates receptor-mediated PLC activity. Adenylyl cyclase inhibition by ADP remains unchanged under these conditions. Reverse transcription-PCR analysis showed that P2Y(12) receptor is expressed in C6 cells. We therefore conclude that, in glioma C6 cells, two P2Y receptor subtypes are present: P2Y(1), coupled to PLC, and P2Y(12), negatively coupled to adenylyl cyclase.

Inhibition of adenylyl cyclase by neuronal P2Y receptors

P2Y receptors inhibiting adenylyl cyclase have been found in blood platelets, glioma cells, and endothelial cells. In platelets and glioma cells, these receptors were identified as P2Y(12). Here, we have used PC12 cells to search for adenylyl cyclase inhibiting P2Y receptors in a neuronal cellular environment. ADP and ATP (0.1 - 100 microM) left basal cyclic AMP accumulation unaltered, but reduced cyclic AMP synthesis stimulated by activation of endogenous A(2A) or recombinant beta(2) receptors. Forskolin-dependent cyclic AMP production was reduced by <or=1 microM and enhanced by 10 - 100 microM ADP; this latter effect was turned into an inhibition when A(2A) receptors were blocked. The nucleotide inhibition of cyclic AMP synthesis was not altered when P2X receptors were blocked, but abolished by pertussis toxin. The rank order of agonist potencies for the reduction of cyclic AMP was (IC(50) values): 2-methylthio-ADP (0.12 nM)=2-methylthio-ATP (0.13 nM)>ADPbetaS (71 nM)>ATP (164 nM)=ADP (244 nM). The inhibition by ADP was not antagonized by suramin, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid, or adenosine-3'-phosphate-5'-phosphate, but attenuated by reactive blue 2, ATP(alpha)S, and 2-methylthio-AMP. RT - PCR demonstrated the expression of P2Y(2), P2Y(4), P2Y(6), and P2Y(12), but not P2Y(1), receptors in PC12 cells. In Northern blots, only P2Y(2) and P2Y(12) were detectable. Differentiation with NGF did not alter these hybridization signals and left the nucleotide inhibition of adenylyl cyclase unchanged. We conclude that P2Y(12) receptors are expressed in neuronal cells and inhibit adenylyl cyclase activity.

Agonists of the P2Y(AC)-receptor activate MAP kinase by a ras-independent pathway in rat C6 glioma

We have previously shown that an ecto-NPPase modulates the ATP- and ADP-mediated P2Y(AC)-receptor activation in rat C6 glioma. In the present study, 2MeSADP and Ap(3)A induced no detectable PI turnover and were identified as specific agonists of the P2Y(AC)-receptor with EC(50) values of 250 +/- 37 pM and 1 +/- 0.5 microM, respectively. P2Y(AC)-receptor stimulation increased MAP kinase (ERK1/2) activation that returned to the basal level 4 h after stimulation and was correlated with a gradual desensitization of the P2Y(AC)-purinoceptor. The purinoceptor antagonists DIDS and RB2 blocked MAP kinase activation. An IP(3)-independent Ca(2+)-influx was observed after P2Y(AC)-receptor activation. Inhibition of this influx by Ca(2+)-chelation, did not affect MAP kinase activation. Pertussis toxin, toxin B, selective PKC-inhibitors and a specific MEK-inhibitor inhibited the 2MeSADP- and Ap(3)A-induced MAP kinase activation. In addition, transfection with dominant negative RhoA(Asn19) rendered C6 cells insensitive to P2Y(AC)-receptor-mediated MAP kinase activation whereas dominant negative ras was without effect. Immunoprecipitation experiments indicated a significant increase in the phosphorylation of raf-1 after P2Y(AC)-receptor activation. We may conclude that P2Y(AC)-purinoceptor agonists activate MAP kinase through a G(i)-RhoA-PKC-raf-MEK-dependent, but ras- and Ca(2+)-independent cascade.

P2Y(AC)(-)-receptor agonists enhance the proliferation of rat C6 glioma cells through activation of the p42/44 mitogen-activated protein kinase

1. Extracellularly added P(1),P(3)-di(adenosine-5') triphosphate (Ap(3)A), P(1),P(4)-di(adenosine-5') tetraphosphate (Ap(4)A), ATP, ADP, AMP and adenosine are growth inhibitory for rat C6 glioma cells. Analysis of nucleotide hydrolysis and the use of nucleotidase inhibitors demonstrated that the latter inhibition is due to hydrolysis of the nucleotides to adenosine. 2. Agonists of the P2Y(AC)(-)-receptor enhance the growth of C6 cells if their hydrolysis to adenosine is inhibited by pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). In these conditions, the potency to stimulate cell growth parallels the ranking of the receptor agonists, i.e. 2-methylthioadenosine-5'-diphosphate (2MeSADP)>Ap(3)A>Ap(4)A. ATP and ADP are still hydrolysed in the presence of PPADS and have no proliferative effect on C6 cells. 3. The enhanced growth is due to a P2Y(AC)(-)-receptor-mediated activation of p42/44 mitogen-activated protein kinase (MAPK) as shown by immunoblotting and protein kinase assays for active MAPK and the use of the MAPK/extracellular signal-regulated kinase kinase (MEK) inhibitor PD98059. 4. The UTP-induced enhancement of the growth of C6 cells is due to activation of MAPK by a PPADS sensitive nucleotide receptor. 5. In conclusion, the effect of nucleotides on the growth of C6 cells is determined by ecto-nucleotidases and by activation of nucleotide receptors. Hydrolysis of nucleotides to adenosine induces growth inhibition while inhibition of the hydrolysis of agonists of the P2Y(AC)(-)-receptor enhances cell growth by activation of MAPK.

Extracellular ATP or ADP induce chemotaxis of cultured microglia through Gi/o-coupled P2Y receptors

The initial microglial responses that occur after brain injury and in various neurological diseases are characterized by microglial accumulation in the affected sites of brain that results from the migration and proliferation of these cells. The early-phase signal responsible for this accumulation is likely to be transduced by rapidly diffusible factors. In this study, the possibility of ATP released from injured neurons and nerve terminals affecting cell motility was determined in rat primary cultured microglia. Extracellular ATP and ADP induced membrane ruffling and markedly enhanced chemokinesis in Boyden chamber assay. Further analyses using the Dunn chemotaxis chamber assay, which allows direct observation of cell movement, revealed that both ATP and ADP induced chemotaxis of microglia. The elimination of extracellular calcium or treatment with pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, suramin, or adenosine-3'-phosphate-5'-phosphosulfate did not inhibit ATP- or ADP-induced membrane ruffling, whereas AR-C69931MX or pertussis toxin treatments clearly did so. As an intracellular signaling molecule underlying these phenomena, the small G-protein Rac was activated by ATP and ADP stimulation, and its activation was also inhibited by pretreatment with pertussis toxin. These results strongly suggest that membrane ruffling and chemotaxis of microglia induced by ATP or ADP are mediated by G(i/o)-coupled P2Y receptors.

The C6-2B glioma cell P2Y(AC) receptor is pharmacologically and molecularly identical to the platelet P2Y(12) receptor

P2Y receptor activation in many cell types leads to phospholipase C activation and accumulation of inositol phosphates, while in blood platelets, C6-2B glioma cells, and in B10 microvascular endothelial cells a P2Y receptor subtype, which couples to inhibition of adenylyl cyclase, historically termed P2Y(AC), (P2T(AC) or P(2T) in platelets) has been identified. Recently, this receptor has been cloned and designated P2Y(12) in keeping with current P2 receptor nomenclature. Three selective P(2T) receptor antagonists, with a range of affinities, inhibited ADP-induced aggregation of washed human or rat platelets, in a concentration-dependent manner, with a rank order of antagonist potency (pIC(50), human: rat) of AR-C78511 (8.5 : 9.1)>AR-C69581 (6.2 : 6.0)>AR-C70300 (5.4 : 5.1). However, these compounds had no effect on ADP-induced platelet shape change. All three antagonists had no significant effect on the ADP-induced inositol phosphate formation in 1321N1 astrocytoma cells stably expressing the P2Y(1) receptor, when used at concentrations that inhibit platelet aggregation. These antagonists also blocked ADP-induced inhibition of adenylyl cyclase in rat platelets and C6-2B cells with identical rank orders of potency and overlapping concentration - response curves. RT - PCR and nucleotide sequence analyses revealed that the C6-2B cells express the P2Y(12) mRNA. These data demonstrate that the P2Y(AC) receptor in C6-2B cells is pharmacologically identical to the P2T(AC) receptor in rat platelets.

[(35)S]GTPgammaS autoradiography reveals a wide distribution of G(i/o)-linked ADP receptors in the nervous system: close similarities with the platelet P2Y(ADP) receptor

No G(i)-linked P2Y receptors have been cloned to date but the presence of such receptors is thought to be restricted to platelets and certain clonal cell lines. Using the functional approach of [(35)S]guanosine 5'-[gamma-thio]-triphosphate autoradiography, we uncovered the widespread presence of such receptors in the CNS. Under conditions in which the prominent signal due to tonic adenosine receptor activity is masked, ADP and ATP stimulated G-protein activity in multiple grey and white matter regions. Localization in the grey matter suggests inhibitory auto-/heteroreceptor function. In the white matter, activated G proteins appeared as 'hot spots' (presumed oligodendrocyte progenitors) with scattered distribution along the main fibre tracts. Responses to ATP were diminished under conditions that inhibited degradation, suggesting that prior conversion to ADP explained agonist action. Uracil nucleotides were ineffective but 2-methylthio-ADP activated G proteins approximately 500-fold more potently than ADP, although both were similarly degraded. Throughout the brain, ADP-dependent G-protein activity was reversed by 2-hexylthio-AdoOC(O)Asp(2), a non-phosphate ATP analogue, whereas selective P2Y(1) receptor antagonists proved ineffective. A similar receptor was also disclosed from the adrenal medulla. These data witness a hitherto unrecognized abundance of G(i/o)-linked ADP receptors in the nervous system. Biochemical and pharmacological behaviour suggests striking similarities to the elusive platelet P2Y(ADP) receptor.

Extracellular ATP or ADP induce chemotaxis of cultured microglia through Gi/o-coupled P2Y receptors

The initial microglial responses that occur after brain injury and in various neurological diseases are characterized by microglial accumulation in the affected sites of brain that results from the migration and proliferation of these cells. The early-phase signal responsible for this accumulation is likely to be transduced by rapidly diffusible factors. In this study, the possibility of ATP released from injured neurons and nerve terminals affecting cell motility was determined in rat primary cultured microglia. Extracellular ATP and ADP induced membrane ruffling and markedly enhanced chemokinesis in Boyden chamber assay. Further analyses using the Dunn chemotaxis chamber assay, which allows direct observation of cell movement, revealed that both ATP and ADP induced chemotaxis of microglia. The elimination of extracellular calcium or treatment with pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid, suramin, or adenosine-3'-phosphate-5'-phosphosulfate did not inhibit ATP- or ADP-induced membrane ruffling, whereas AR-C69931MX or pertussis toxin treatments clearly did so. As an intracellular signaling molecule underlying these phenomena, the small G-protein Rac was activated by ATP and ADP stimulation, and its activation was also inhibited by pretreatment with pertussis toxin. These results strongly suggest that membrane ruffling and chemotaxis of microglia induced by ATP or ADP are mediated by G(i/o)-coupled P2Y receptors.

Two subtypes of G protein-coupled nucleotide receptors, P2Y(1) and P2Y(2) are involved in calcium signalling in glioma C6 cells

1. In glioma C6 cells, the stimulation of P2Y receptors by ADP, ATP and UTP initiated an increase in the intracellular Ca2+ concentration, in a process that involved the release of Ca2+ from InsP(3)-sensitive store and the capacitative, extracellular Ca2+ entry. The presence of external Ca2+ was not necessary to elevate Ca(2+). 2. The rank order of potencies of nucleotide analogues in stimulating [Ca2+](i) was: 2MeSADP > ADP > 2MeSATP = 2ClATP > ATP > UTP. alpha,beta-Methylene ATP, adenosine and AMP were ineffective. 3. ADP and UTP effects were additive, while actions of ATP and UTP were not additive on [Ca2+](i) increase. Similarly, cross-desensitization between ATP and UTP but not between ADP and UTP occurred. 4. Suramin, a non-specific nucleotide receptors inhibitor, antagonized ATP-, UTP- and ADP-evoked Ca2+ responses. PPADS, a selective antagonist of the P2Y(1) receptor-generated InsP(3) accumulation, decreased ADP-initiated Ca2+ response with no effect on ATP and UTP. 5. Pertussis toxin (PTX) reduced ADP- and ATP-induced Ca2+ increases. Short-term treatment with TPA, inhibited both ATP and ADP stimulatory effects on [Ca2+](i). 6. ADP inhibited isoproterenol-induced cyclic AMP accumulation. PTX blocked this effect, but PPADS did not. 7. RT - PCR analysis revealed the molecular identity of P2Y receptors expressed by glioma C6 cells to be both P2Y(1) and P2Y(2). 8. It is concluded that both P2Y(1) and P2Y(2) receptors co-exist in glioma C6 cells. ADP acts as agonist of the first, and ATP and UTP of the second one. Both receptors are linked to phospholipase C (PLC).

Pharmacological characterisation of the P2Y11 receptor in stably transfected haematological cell lines

The recently cloned P2Y11 receptor is unique amongst P2Y receptors with its coupling to the adenylyl cyclase pathway. P2Y11 has previously been shown to be expressed in human acute promyelocytic leukemia (APL) HL-60 and NB4 cell lines, and both cell types elevate cyclic AMP (cAMP) levels upon stimulation with extracellular ATP. Acute erythroleukemic K562 cells and acute monocytic leukemia U937 cells did not elevate cAMP levels upon exposure to 1 mM extracellular ATP. However, K562 and U937 cells stably transfected with P2Y11 (K11 and U11 cells, respectively) were responsive to extracellular ATP, with an EC50 of 31 and 21 microM, respectively. The most potent agonists in both K11 and U11 cells were ATPgammaS (adenosine 5'-O-[3-thiotriphosphate]), ATPalphaS (adenosine 5'-O-[1-thiotriphosphate]), dATP and ADPbetaS (adenosine 5'-O-[2-thiobisphosphate]), which were of similar or greater potency compared to ATP itself. ADP and alpha,beta-methylene ATP were less potent compared to ATP. The order of potency for ATP breakdown products was ATP > ADP > AMP > or = Ado. UTP, a known activator of P2Y2 and P2Y4, was largely ineffective. In the transfected cells, ATP-induced cAMP elevation was inhibited by suramin (0.5 mM), but not XAC (20 microM) nor PPADS (100 microM). AMPS inhibited ATP-induced cAMP elevation in both K11 and U11 cells (EC50 approximately 3 mM) and may be a P2Y11-selective inhibitor. These results are similar to those observed for HL-60 cells and NB4 cells implicating P2Y11 as the receptor responsible for the ATP-induced cAMP elevations in these cells.

Advances in signalling by extracellular nucleotides. the role and transduction mechanisms of P2Y receptors

Nucleotides are ubiquitous intercellular messengers whose actions are mediated by specific receptors. Since the first clonings in 1993, it is known that nucleotide receptors belong to two families: the ionotropic P2X receptors and the metabotropic P2Y receptors. Five human P2Y receptor subtypes have been cloned so far and a sixth one must still be isolated. In this review we will show that they differ by their preference for adenine versus uracil nucleotides and triphospho versus diphospho nucleotides, as well as by their transduction mechanisms and cell expression.

Ecto-nucleotide pyrophosphatase modulates the purinoceptor-mediated signal transduction and is inhibited by purinoceptor antagonists

1. The effect of ecto-nucleotide pyrophosphatase (ecto-NPPase; EC 3.6.1. 9) on the ATP- and ADP-mediated receptor activation was studied in rat C6 glioma cells. The P2-purinoceptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) and reactive blue (RB2) are potent inhibitors (IC(50)=12+/-3 microM) of the latter enzyme. 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid (DIDS), 5'-phosphoadenosine 3'-phosphate (PAP) and suramin were less potent inhibitors with an IC(50) of 22+/-4, 36+/-7 and 72+/-11 microM respectively. 2. P1-purinoceptor antagonists CGS 15943, cyclo-pentyl theophylline (CTP) and theophylline did not affect the activity of the ecto-NPPase. 3. ATP- and ADP-mediated P2Y(1)-like receptor activation inhibited the (-)-isoproterenol-induced increase of intracellular cyclic AMP concentration. PPADS, an ineffective P2Y-antagonist in C6, potentiated the ATP and ADP effect approximately 3 fold due to inhibition of nucleotide hydrolysis by the ecto-NPPase. 4. We conclude that ecto-NPPase has a modulator effect on purinoceptor-mediated signalling in C6 glioma cell cultures.

Evidence that rat hepatocytes co-express functional P2Y1 and P2Y2 receptors

Previous studies have indicated the expression of multiple P2Y receptors by rat hepatocytes although they have not been identified. Here we show by reverse transcriptase-polymerase chain reaction (RT - PCR) that rat hepatocytes express mRNA encoding all of the four cloned rat P2Y receptors (P2Y(1), P2Y(2), P2Y(4) and P2Y(6)). The effects of UTP have been examined on single aequorin-injected rat hepatocytes. The [Ca(2+)](i) transients induced by UTP were indistinguishable from those induced by ATP in the same cell. The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both UTP- and ATP-induced [Ca(2+)](i) transients. UDP, an agonist at the P2Y(6) receptor, failed to induce transients in hepatocytes, indicating that functional P2Y(6) receptors coupled to increased [Ca(2+)](i) are not expressed. The transients evoked by ADP were more sensitive to inhibition by suramin than those induced by either ATP or UTP. Within an individual cell, the transients induced by ATP and UTP were inhibited by the same concentration of suramin. This sensitivity of ATP and UTP responses to suramin suggests action through P2Y(2) rather than P2Y(4) receptors. Co-application of 30 microM pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) caused a decrease in frequency and amplitude of transients induced by ADP. ATP- and UTP-induced transients also displayed a decrease in amplitude in response to addition of PPADS, but this was accompanied by an increase in frequency of transients. In conclusion the data presented here are consistent with the co-expression of P2Y(1) and P2Y(2) receptors by rat hepatocytes.

Decreased platelet aggregation, increased bleeding time and resistance to thromboembolism in P2Y1-deficient mice

Platelet activation is characterized by shape change, induction of fibrinogen receptor expression and release of granular contents, leading to aggregation and plug formation. While this response is essential for hemostasis, it is also important in the pathogenesis of a broad spectrum of diseases, including myocardial infarction, stroke and unstable angina. Adenosine 5'-diphosphate (ADP) induces platelet aggregation, but the mechanism for this has not been established, and the relative contribution of ADP in hemostasis and the development of arterial thrombosis is poorly understood. We show here that the purinoceptor P2Y1 is required for platelet shape change in response to ADP and is also a principal receptor mediating ADP-induced platelet aggregation. Activation of P2Y1 resulted in increased intracellular calcium but no alteration in cyclic adenosine monophosphate (cAMP) levels. P2Y1-deficient platelets partially aggregated at higher ADP concentrations, and the lack of P2Y1 did not alter the ability of ADP to inhibit cAMP, indicating that platelets express at least one additional ADP receptor. In vivo, the lack of P2Y1 expression increased bleeding time and protected from collagen- and ADP-induced thromboembolism. These findings support the hypothesis that the ATP receptor P2Y1 is a principal receptor mediating both physiologic and pathological ADP-induced processes in platelets.

Metabotropic receptors for ATP and UTP: exploring the correspondence between native and recombinant nucleotide receptors

In the past five years, an extended series (P2Y1-n) of metabotropic nucleotide (P2) receptors has been cloned from vertebrate tissues; these receptors are activated by either ATP or UTP, or both nucleotides. While certain cloned P2Y receptors appear to correspond functionally to particular native P2 receptor phenotypes, such pharmacological phenotypes could be explained by either a combination of several members of the P2Y1-n series being coexpressed in the same tissue or the existence of novel, uncloned P2Y subtypes. Here, Brian King, Andrea Townsend-Nicholson and Geoffrey Burnstock review recent findings on the matter of pharmacological relationships between native P2 and cloned P2Y receptors.

Analysis of the influence of nucleotidases on the apparent activity of exogenous ATP and ADP at P2Y1 receptors

1. ADP is a potent agonist of rat and human P2Y1 purinoceptors. ATP is a weak competitive antagonist. This study analyses the situation in which P2Y1 receptors are exposed to ATP in the presence of exogenous ecto-nucleotidases (apyrases) that have high or low ATPase/ADPase activity ratio. 2. Rat brain capillary endothelial cells of the B10 clone express P2Y1 receptors that couple to intracellular Ca2+ mobilization. They have low endogenous ecto-ATPase and ecto-ADPase activities. 3. ATP did not raise intracellular Ca2+ in B10 cells. Addition of apyrases III or VII (1 u ml(-1)) to ATP treated cells induced large intracellular Ca2+ transients. Apyrases had no action in the absence of ATP. 4. A 1 u ml(-1) apyrase III solution generated 20 microM ADP from 0.1 mM ATP within 15 s. This concentration of ADP was sufficient to produce maximal activation of P2Y1 receptors. 5. ATP was a full agonist of P2Y1 receptors in the presence of 1 u ml(-1) apyrase III. Dose response curves for the apparent actions of ATP were bell shaped in the presence of 0.1 u ml(-1) apyrase III. Apyrase III did not alter ADP dose response curves when coincubated with ADP for 15 s. 6. Apyrase VII (1 u ml(-1)) shifted dose response curves for the actions of ADP to larger concentrations. It induced a bell shaped ATP dose response curve. 7. Results suggest that ATPDases prevent P2Y1 receptor activation by degrading ADP but may contribute to P2Y1 receptor activation by generating ADP from ATP.

Pharmacological profile of a novel cyclic AMP-linked P2 receptor on undifferentiated HL-60 leukemia cells

1. Extracellular ATP (EC50=146+/-57 microM) and various ATP analogues activated cyclic AMP production in undifferentiated HL-60 cells. 2. The order of agonist potency was: ATPgammaS (adenosine 5'-O-[3-thiotriphosphate]) > or = BzATP (2'&3'O-(4-benzoylbenzoyl)-adenosine-5'-triphosphate) > or = dATP > ATP. The following agonists (in order of effectiveness at 1 mM) were all less effective than ATP at concentrations up to 1 mM: beta,gamma methylene ATP > or = 2-methylthioATP > ADP > or = Ap4A (P1, P4-di(adenosine-5') tetraphosphate) > or = Adenosine > UTP. The poor response to UTP indicates that P2Y2 receptors are not responsible for ATP-dependent activation of adenylyl cyclase. 3. Several thiophosphorylated analogs of ATP were more potent activators of cyclic AMP production than ATP. Of these, ATPgammaS (EC50=30.4+/-6.9 microM) was a full agonist. However, adenosine 5'-O-[1-thiotriphosphate] (ATPalphaS; EC50=45+/-15 microM) and adenosine 5'-O-[2-thiodiphosphate] (ADPbetaS; EC50=33.3+/-5.0 microM) were partial agonists. 4. ADPbetaS (IC50=146+/-32 microM) and adenosine 5'-O-thiomonophosphate (AMPS; IC50=343+/-142 microM) inhibited cyclic AMP production by a submaximal concentration of ATP (100 microM). Consistent with its partial agonist activity, ADPbetaS was estimated to maximally suppress ATP-induced cyclic AMP production by about 65%. AMPS has not been previously reported to inhibit P2 receptors. 5. The broad spectrum P2 receptor antagonist, suramin (500 microM), abolished ATP-stimulated cyclic AMP production by HL-60 cells but the adenosine receptor antagonists xanthine amine congener (XAC; 20 microM) and 8-sulpho-phenyltheophylline (8-SPT; 100 microM) were without effect. 6. Extracellular ATP also activated protein kinase A (PK-A) consistent with previous findings that PK-A activation is involved in ATP-induced differentiation of HL-60 cells (Jiang et al., 1997). 7. Taken together, the data indicate the presence of a novel cyclic AMP-linked P2 receptor on undifferentiated HL-60 cells.

The P2Y1 Receptor Is Necessary for Adenosine 5′-Diphosphate–Induced Platelet Aggregation

The human P2Y1 receptor heterologously expressed in Jurkat cells behaves as a specific adenosine 5′-diphosphate (ADP) receptor at which purified adenosine triphosphate (ATP) is an ineffective agonist, but competitively antagonizes the action of ADP. This receptor is thus a good candidate to be the elusive platelet P2T receptor for ADP. In the present work, we examined the effects on ADP-induced platelet responses of two selective and competitive P2Y1 antagonists, adenosine-2′-phosphate-5′-phosphate (A2P5P) and adenosine-3′-phosphate-5′-phosphate (A3P5P). Results were compared with those for the native P2Y1 receptor expressed on the B10 clone of rat brain capillary endothelial cells (BCEC) and for the cloned human P2Y1 receptor expressed on Jurkat cells. A2P5P and A3P5P inhibited ADP-induced platelet shape change and aggregation (pA2 = 5) and competitively antagonized calcium movements in response to ADP in fura-2–loaded platelets, B10 cells, and P2Y1-Jurkat cells. In contrast, these compounds had no effect on ADP-induced inhibition of adenylyl cyclase in platelets or B10 cells, whereas known antagonists of platelet activation by ADP such as Sp-ATPαS were effective. These identical signaling responses and pharmacologic properties suggest that platelets and BCEC share a common P2Y1 receptor involved in ADP-induced aggregation and vasodilation, respectively. This P2Y1 receptor coupled to the mobilization of intracellular calcium stores was found to be necessary to trigger ADP-induced platelet aggregation. The present results, together with data from the literature, also point to the existence of another as yet unidentified ADP receptor, coupled to adenylyl cyclase and responsible for completion of the aggregation response. Thus, the term, P2T, should no longer be used to designate a specific molecular entity.

P2Y1-receptors in human platelets which are pharmacologically distinct from P2Y(ADP)-receptors

1. In the present study we have classified the receptor(s) mediating increases in intracellular calcium concentration ([Ca2+]i) in human washed platelets and compared the pharmacological profile obtained with that observed in Jurkat cells, stably transfected with a bovine P2Y1-receptor. 2. The P2Y1-receptor antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P), competitively antagonized agonist responses in both Jurkat cells, and in platelets with similar affinities (pK(B) of 5.8 and 6.0, respectively). 3. The selective P2Y(ADP) antagonist, AR-C66096, exhibited partial agonism in the Jurkat cells with an affinity (pK(A)) of 4.9. This value is consistent with its known P2Y1-receptor activity. In platelets, AR-C66096 at a concentration (0.1 microM) approximately 100 fold greater than its known P2Y(ADP) receptor affinity, had no effect on ADP-induced increases in [Ca2+]i. 4. The ability of adenine nucleotide analogues to elevate [Ca2+]i in the Jurkat cells was also determined. The rank order of agonist potency (p[A]50) was: 2-MeSADP (8.3)>2-ClATP (7.8)>ADP (7.5)=2-MeSATP (7.4)>ATPgammaS (6.5)>ATP (6.2), with ATP appearing to be a partial agonist. 5. The same rank order of potency was observed when similar experiments were performed in platelets. However, the absolute potencies of all the agonists and the intrinsic activities of both ATPgammaS and ATP were lower in platelets. 6. The operational model of agonism was used to test whether the agonist concentration-effect profiles obtained in these two cell types could be explained on the basis of differences in receptor reserve. The analysis indicated that the data obtained in platelets closely resembled that predicted for a low density or poorly coupled P2Y1-receptor system. 7. The hypothesis that the observed partial agonist behaviour of ATP was the result of receptor activation by contaminating ADP with concomitant receptor blockade by ATP, was tested in the platelet system. This hypothesis was supported by a theoretical analysis, which yielded an affinity value for ATP similar to that obtained previously at P2Y1-receptors. 8. In summary, the results of this study indicate that human washed platelets contain P2Y1-receptors which mediate increases in [Ca2+]i and that this receptor population is pharmacologically distinct from P2Y(ADP)-receptors.

The regulation of vascular function by P2 receptors: multiple sites and multiple receptors

Although the effects of nucleotides in the cardiovascular system have been known for almost 70 years, it is only in the past few years that some of the P2 receptors at which they act have been cloned and characterized. It is now clear that the control of cardiovascular function by nucleotides is complex, involving multiple receptors and multiple effects in the different cell types of importance. In this review Mike Boarder and Susanna Hourani summarize the P2 receptors that are present in endothelial cells, platelets, smooth muscle and nerves, the signalling pathways that they activate and the responses that are produced. They also discuss the important role of nucleotides in the interactions between the different cell types, and the implications of this in vascular disease.