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

Role and recent progress of P2Y12 receptor in cancer development

P2Y12 receptor (P2Y12R) is an adenosine-activated G protein-coupled receptor (GPCR) that plays a central role in platelet function, hemostasis, and thrombosis. P2Y12R activation can promote platelet aggregation and adhesion to cancer cells, promote tumor angiogenesis, and affect the tumor immune microenvironment (TIME) and tumor drug resistance, which is conducive to the progression of cancers. Meanwhile, P2Y12R inhibitors can inhibit this effect, suggesting that P2Y12R may be a potential therapeutic target for cancer. P2Y12R is involved in cancer development and metastasis, while P2Y12R inhibitors are effective in inhibiting cancer. However, a new study suggests that long-term use of P2Y12R inhibitors may increase the risk of cancer and the mechanism remains to be explored. In this paper, we reviewed the structural and functional characteristics of P2Y12R and its role in cancer. We explored the role of P2Y12R inhibitors in different tumors and the latest advances by summarizing the basic and clinical studies on the effects of P2Y12R inhibitors on tumors.

The Function and Regulation of Platelet P2Y12 Receptor

In addition to the key role in hemostasis and thrombosis, platelets have also been wildly acknowledged as immune regulatory cells and involving in the pathogenesis of inflammation-related diseases. Since purine receptor P2Y12 plays a crucial role in platelet activation, P2Y12 antagonists such as clopidogrel, prasugrel, and ticagrelor have been widely used in cardiovascular diseases worldwide in recent decades due to their potent antiplatelet and antithrombotic effects. Meanwhile, the role of P2Y12 in inflammatory diseases has also been extensively studied. Relatively, there are few studies on the regulation of P2Y12. This review first summarizes the various roles of P2Y12 in the process of platelet activation, as well as downstream effects and signaling pathways; then introduces the effects of P2Y12 in inflammatory diseases such as sepsis, atherosclerosis, cancer, autoimmune diseases, and asthma; and finally reviews the current researches on P2Y12 regulation.

The role and pharmacological properties of P2Y12 receptor in cancer and cancer pain

The G protein-coupled P2Y12 receptor (P2Y12R) was cloned in platelets and found to play a key role in maintaining platelet function in hemostasis and thrombosis, and these effects could be mediated by the P2Y12R. However, it has recently been found that P2Y12R-mediated the progression of tumor through interactions between platelets and tumor and stromal cells, as well as through products secreted by platelets. During tumor progression, tumor cells or other cells in the tumor microenvironment (such as immune cells) can secrete large amounts of ATP into the extracellular matrix, and extracellular ATP can be hydrolyzed into ADP. ADP is a P2Y12R activator and plays an important regulatory role in the proliferation and metastasis of tumor cells. P2Y12R is involved in platelet-cancer cell crosstalk and become a potential target for anticancer therapy. Moreover, tumor progression can induce pain, which seriously affects the quality of life of patients. P2Y12R is expressed in microglia and mediates the activities of microglial and participates in the occurrence of cancer pain. Conversely, inhibiting P2Y12R activation and down-regulating its expression has the effect of inhibiting tumor progression and pain. Therefore, P2Y12R can be a common therapeutic target for both. In this article, we explored the potential link between P2Y12R and cancer, discussed the intrinsic link of P2Y12R in cancer pain and the pharmacological properties of P2Y12R antagonists in the treatment of both.

Intratumoral Platelets: Harmful or Incidental Bystanders of the Tumor Microenvironment?

Simple Summary

The tumor microenvironment (TME) is the complex and heterogenous ecosystem of solid tumors known to influence their growth and their progression. Besides tumor cells, the TME comprises a variety of host-derived cell types, ranging from endothelial cells to fibroblasts and immune cells. Clinical and experimental data are converging to indicate that platelets, originally known for their fundamental hemostatic function, also participate in tumor development and shaping of the TME. Considering the abundance of antiplatelet drugs, understanding if and how platelets contribute to the TME may lead to new therapeutic tools for improved cancer prevention and treatments.

Abstract

The tumor microenvironment (TME) has gained considerable interest because of its decisive impact on cancer progression, response to treatment, and disease recurrence. The TME can favor the proliferation, dissemination, and immune evasion of cancer cells. Likewise, there is accumulating evidence that intratumoral platelets could favor the development and aggressiveness of solid tumors, notably by influencing tumor cell phenotype and shaping the vascular and immune TME components. Yet, in contrast to other tumor-associated cell types like macrophages and fibroblasts, platelets are still often overlooked as components of the TME. This might be due, in part, to a deficit in investigating and reporting the presence of platelets in the TME and its relationships with cancer characteristics. This review summarizes available evidence from clinical and animal studies supporting the notion that tumor-associated platelets are not incidental bystanders but instead integral and active components of the TME. A particular emphasis is given to the description of intratumoral platelets, as well as to the functional consequences and possible mechanisms of intratumoral platelet accumulation.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

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.

P2Y12 Inhibition beyond Thrombosis: Effects on Inflammation

The P2Y₁₂ receptor is a key player in platelet activation and a major target for antithrombotic drugs. The beneficial effects of P2Y₁₂ receptor antagonists might, however, not be restricted to the primary and secondary prevention of arterial thrombosis. Indeed, it has been established that platelet activation also has an essential role in inflammation. Additionally, nonplatelet P2Y₁₂ receptors present in immune cells and vascular smooth muscle cells might be effective players in the inflammatory response. This review will investigate the biological and clinical impact of P2Y₁₂ receptor inhibition beyond its platelet-driven antithrombotic effects, focusing on its anti-inflammatory role. We will discuss the potential molecular and cellular mechanisms of P2Y₁₂-mediated inflammation, including cytokine release, platelet–leukocyte interactions and neutrophil extracellular trap formation. Then we will summarize the current evidence on the beneficial effects of P2Y₁₂ antagonists during various clinical inflammatory diseases, especially during sepsis, acute lung injury, asthma, atherosclerosis, and cancer.

P2Y12 Receptors in Tumorigenesis and Metastasis

Platelets, beyond their role in hemostasis and thrombosis, may sustain tumorigenesis and metastasis. These effects may occur via direct interaction of platelets with cancer and stromal cells and by the release of several platelet products. Platelets and tumor cells release several bioactive molecules among which a great amount of adenosine triphosphate (ATP) and adenosine diphosphate (ADP). ADP is also formed extracellularly from ATP breakdown by the ecto-nucleoside-triphosphate-diphosphohydrolases. Under ATP and ADP stimulation the purinergic P2Y₁ receptor (R) initiates platelet activation followed by the ADP-P2Y₁₂R-mediated amplification. P2Y₁₂R stimulation amplifies also platelet response to several platelet agonists and to flow conditions, acting as a key positive feed-forward signal in intensifying platelet responses. P2Y₁₂R represents a potential target for an anticancer therapy due to its involvement in platelet-cancer cell crosstalk. Thus, P2Y₁₂R antagonists, including clopidogrel, ticagrelor, and prasugrel, might represent potential anti-cancer agents, in addition to their role as effective antithrombotic drugs. However, further studies, in experimental animals and patients, are required before the recommendation of the use of P2Y₁₂R antagonists in cancer prevention and progression can be made.

Dual role of P2 receptors during osteoblast differentiation

P2 receptors activated by ATP are expressed in the skeletal system. However, the role of P2 receptors in osteoblast differentiation remains unclear.

METHODS

Participation of P2 receptors in differentiation was investigated in the preosteoblast MC3T3-M1 cell line. Preosteoblasts were stimulated for 7 or 14 days in the presence of osteogenic medium containing ATP and its analogs, and then alkaline phosphatase (ALP) activity, gene expression analyses, and protein expression were assessed.

RESULTS

We observed that ATP and its analogs promoted increased ALP activity after 7 days of treatment. In contrast, these agonists promoted reductions in ALP activity after 14 days. Some antagonists, such as PPADS (P2 antagonist), MRS2179 (P2Y1 antagonist), MRS2578 (P2Y6 antagonist), and AZ11645373 (P2X7 antagonist) reduced the increases in ALP activity after 7 days. However, only AZ11645373 inhibited the reduction in ALP activity after 14 days. The expression of the P2Y2, P2Y6, P2X4, and P2X7 receptors was observed. Furthermore, treatment with ATP modulated the expression of P2 receptors, increasing P2X4 expression and reducing P2Y6 and P2X7 expression. Similar results were observed after 14 days. In addition, ATP treatment for 7 days increased the expression of transcription factors associated with osteoblast differentiation, such as Runx2, SP7, and Dix5, whereas SP7 and Dix5 expression was reduced at 14 days. These results suggest that P2 receptor activation modulates the differentiation of osteoblasts and is dependent upon the stage of differentiation. These results also suggest that several P2 receptors are involved in this process.

Activation of microglial P2Y12 receptor is required for outward potassium currents in response to neuronal injury

Microglia, the resident immune cells in the central nervous system (CNS), constantly survey the surrounding neural parenchyma and promptly respond to brain injury. Activation of purinergic receptors such as P2Y12 receptors (P2Y12R) in microglia has been implicated in chemotaxis toward ATP that is released by injured neurons and astrocytes. Activation of microglial P2Y12R elicits outward potassium current that is associated with microglial chemotaxis in response to injury. This study aimed at investigating the identity of the potassium channel implicated in microglial P2Y12R-mediated chemotaxis following neuronal injury and understanding the purinergic signaling pathway coupled to the channel. Using a combination of two-photon imaging, electrophysiology and genetic tools, we found the ATP-induced outward current to be largely dependent on P2Y12R activation and mediated by G-proteins. Similarly, P2Y12R-coupled outward current was also evoked in response to laser-induced single neuron injury. This current was abolished in microglia obtained from mice lacking P2Y12R. Dissecting the properties of the P2Y12R-mediated current using a pharmacological approach revealed that both the ATP and neuronal injury-induced outward current in microglia was sensitive to quinine (1mM) and bupivacaine (400μM), but not tetraethylammonium (TEA) (10mM) and 4-aminopyridine (4-AP) (5mM). These results suggest that the quinine/bupivacaine-sensitive potassium channels are the functional effectors of the P2Y12R-mediated signaling in microglia activation following neuronal injury.

Baicalein inhibits agonist- and tumor cell-induced platelet aggregation while suppressing pulmonary tumor metastasis via cAMP-mediated VASP phosphorylation along with impaired MAPKs and PI3K-Akt activation

Recently, the importance of platelet activation in cancer metastasis has become generally accepted. As a result, the development of new platelet inhibitors with minimal adverse effects is now a promising area of targeted cancer therapy. Baicalein is a functional ingredient derived from the root of Scutellaria baicalensis Georgi, a plant used intraditional medicine. The pharmacological effects of this compound including anti-oxidative and anti-inflammatory activities have already been demonstrated. However, its effects on platelet activation are unknown. We therefore investigated the effects of baicalein on ligand-induced platelet aggregation and pulmonary cancer metastasis. In the present study, baicalein inhibited agonist-induced platelet aggregation, granule secretion markers (P-selectin expression and ATP release), [Ca(2+)]i mobilization, and integrin αIIbβ3 expression. Additionally, baicalein attenuated ERK2, p38, and Akt activation, and enhanced VASP phosphorylation. Indeed, baicalein was shown to directly inhibit PI3K kinase activity. Moreover, baicalein attenuated the platelet aggregation induced by C6 rat glioma tumor cells in vitro and suppressed CT26 colon cancer metastasis in mice. These features indicate that baicalein is a potential therapeutic drug for the prevention of cancer metastasis.

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.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Increased platelet activation and thrombosis in transgenic mice expressing constitutively active P2Y12

BACKGROUND

In our previous in vitro study, we reported a constitutively active chimeric P2Y(12) (cP2Y(12)) and found that AR-C78511 is a potent inverse agonist at this receptor. The role of cP2Y(12) in platelet activation and thrombosis is not clear.

OBJECTIVES

To investigate the physiologic implications of cP2Y(12) for platelet activation and thrombus formation, and to evaluate the antiplatelet activity of AR-C78511 as an inverse agonist.

METHODS AND RESULTS

We generated transgenic mice conditionally and platelet-specifically expressing cP2Y(12). High-level expression of cP2Y(12) in platelets increased platelet reactivity, as shown by increased platelet aggregation in response to multiple platelet agonists. Moreover, transgenic mice showed a shortened bleeding time, and more rapid and stable thrombus formation in mesenteric artery injured with FeCl(3). The constitutive activity of cP2Y(12) in platelets was confirmed by decreased platelet cAMP levels and constitutive Akt phosphorylation in the absence of agonists. AR-C78511 reversed the cAMP decrease in transgenic mouse platelets, and exhibited a superior antiplatelet effect to that of AR-C69931MX in transgenic mice.

CONCLUSIONS

These findings further emphasize the importance of P2Y(12) in platelet activation, hemostasis, and thrombosis, as well as the prothrombotic role of the constitutive activity of P2Y(12). Our data also validate the in vivo inverse agonist activity of AR-C78511, and confirm its superior antiplatelet activity over neutral antagonists.

Differential expression of P2Y receptors in the rat cochlea during development

Purinergic signaling has broad physiological significance to the hearing organ, involving signal transduction via ionotropic P2X receptors and metabotropic G-protein-coupled P2Y and P1 (adenosine), alongside conversion of nucleotides and nucleosides by ecto-nucleotidases and ecto-nucleoside diphosphokinase. In addition, ATP release is modulated by acoustic overstimulation or stress and involves feedback regulation. Many of these principal elements of the purinergic signaling complex have been well characterized in the cochlea, while the characterization of P2Y receptor expression is emerging. The present study used immunohistochemistry to evaluate the expression of five P2Y receptors, P2Y(1), P2Y(2), P2Y(4), P2Y(6), and P2Y(12), during development of the rat cochlea. Commencing in the late embryonic period, the P2Y receptors studied were found in the cells lining the cochlear partition, associated with establishment of the electrochemical environment which provides the driving force for sound transduction. In addition, early postnatal P2Y(2) and P2Y(4) protein expression in the greater epithelial ridge, part of the developing hearing organ, supports the view that initiation and regulation of spontaneous activity in the hair cells prior to hearing onset is mediated by purinergic signaling. Sub-cellular compartmentalization of P2Y receptor expression in sensory hair cells, and diversity of receptor expression in the spiral ganglion neurons and their satellite cells, indicates roles for P2Y receptor-mediated Ca(2+)-signaling in sound transduction and auditory neuron excitability. Overall, the dynamics of P2Y receptor expression during development of the cochlea complement the other elements of the purinergic signaling complex and reinforce the significance of extracellular nucleotide and nucleoside signaling to hearing.

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.

P2Y(1) and P2Y(12) receptor cross-talk in calcium signalling: Evidence from nonstarved and long-term serum-deprived glioma C6 cells

The current work presents results of experiments on the calcium response evoked by the stimulation by extracellular nucleotides occurring in control, nonstarved glioma C6 cells and in cells after long-term (96 h) serum starvation. Three nucleotide receptors were studied: P2Y₁, P2Y₂ and P2Y₁₂. Two of them, P2Y₁ and P2Y₂, directly stimulate calcium response. The protein level of the P2Y₂ receptor did not change during the serum starvation, while P2Y₁ protein level fell dramatically. Observed changes in the calcium response generated by P2Y₁ are directly correlated with the receptor protein level as well as with the amount of calcium present in the intracellular calcium stores, partially depleted during starvation process. The third receptor, P2Y₁₂, did not directly evoke calcium response, however it is activated by the same ligand as P2Y₁. The experiments with AR-C69941MX, the P2Y₁₂-specific antagonist, indicated that in control and serum-starved cells, calcium response evoked by P2Y₁ receptor is potentiated by the activity of P2Y₁₂-dependent signaling pathways. This potentiation may be mediated by P2Y₁₂ inhibitory effect on the plasma membrane calcium pump. The calcium influx enhanced by the cooperation of P2Y₁ and P2Y₁₂ receptor activity directly depends on the capacitative calcium entrance mechanism.

Integration of P2Y receptor-activated signal transduction pathways in G protein-dependent signalling networks

The role of nucleotides in intracellular energy provision and nucleic acid synthesis has been known for a long time. In the past decade, evidence has been presented that, in addition to these functions, nucleotides are also autocrine and paracrine messenger molecules that initiate and regulate a large number of biological processes. The actions of extracellular nucleotides are mediated by ionotropic P2X and metabotropic P2Y receptors, while hydrolysis by ecto-enzymes modulates the initial signal. An increasing number of studies have been performed to obtain information on the signal transduction pathways activated by nucleotide receptors. The development of specific and stable purinergic receptor agonists and antagonists with therapeutical potential largely contributed to the identification of receptors responsible for nucleotide-activated pathways. This article reviews the signal transduction pathways activated by P2Y receptors, the involved second messenger systems, GTPases and protein kinases, as well as recent findings concerning P2Y receptor signalling in C6 glioma cells. Besides vertical signal transduction, lateral cross-talks with pathways activated by other G protein-coupled receptors and growth factor receptors are discussed.

Activation of P2Y1 receptor triggers two calcium signaling pathways in bone marrow erythroblasts

In this study, we describe the presence of P2 receptor subtypes and Ca2+ signaling in erythroblasts. ATP and ADP produced a biphasic increase of intracellular Ca2+ concentration ([Ca2+]i), with an initial transient phase followed by a sustained phase. Reverse transcription polymerase chain reaction (RT-PCR) showed the expression of P2Y1, P2Y2 and P2Y12. The selective P2Y1 receptor antagonist 2'-deoxy-N6-methyl-adenosine-3',5'-diphosphate (MRS2179) and the G(i) protein inhibitor pertussis toxin blocked Ca2+ increase. The initial transient [Ca2+]i increase phase was sensitive to the 1,4,5-inositol trisphosphate (IP3) receptor blocker 2-aminoethoxy-diphenylborate (2-APB), while the sustained phase was sensitive to the protein kinase C (PKC) inhibitor 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF109203X) and calcium calmodulin kinase II (CaMKII) inhibitor 1-[N,O-bis(5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl]-4-phenylpiperazine (KN-62). In addition, the PKC activator phorbol-12,13-dibutyrate (PDBu) produced increase of [Ca2+]i. Flow cytometry analysis showed the expression of Ca2+-dependent PKC alpha, betaI, gamma and phospho-CaMKII. These results suggest that the activation of the P2Y1 receptor triggers two different [Ca2+]i increase pathways, one IP3-dependent and the other kinase-dependent.

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.

Identification of a potent inverse agonist at a constitutively active mutant of human P2Y12 receptor

Human platelets express two P2Y receptors: G(q)-coupled P2Y(1), and G(i)-coupled P2Y(12). Both P2Y(1) and P2Y(12) are ADP receptors on human platelets and are essential for ADP-induced platelet aggregation that plays pivotal roles in thrombosis and hemostasis. Numerous constitutively active G protein-coupled receptors have been described in natural or recombinant systems, but in the P2Y receptors, to date, no constitutive activity has been reported. In our effort to identify G protein coupling domains of the human platelet ADP receptor, we constructed a chimeric hemagglutinin-tagged human P2Y(12) receptor with its C terminus replaced by the corresponding part of human P2Y(1) receptor and stably expressed it in Chinese hamster ovary-K1 cells. It is interesting that the chimeric P2Y(12) mutant exhibited a high level of constitutive activity, as evidenced by decreased cAMP levels in the absence of agonists. The constitutive activation of the chimeric P2Y(12) mutant was dramatically inhibited by pertussis toxin, a G(i) inhibitor. The constitutively active P2Y(12) mutant retained normal responses to 2-methylthio-ADP, with an EC(50) of 0.15 +/- 0.04 nM. The constitutively active P2Y(12) mutant caused Akt phosphorylation that was abolished by the addition of pertussis toxin. Pharmacological evaluation of several P2Y(12) antagonists revealed (E)-N-[1-[7-(hexylamino)-5-(propylthio)-3H-1,2,3-triazolo-[4,5-d]-pyrimidin-3-yl]-1,5,6-trideoxy-beta-d-ribo-hept-5-enofuranuronoyl]-l-aspartic acid (AR-C78511) as a potent P2Y(12) inverse agonist and 5'-adenylic acid, N-[2-(methylthio)ethyl]-2-[(3,3,3-trifluoropropyl)thio]-, monoanhydride with (dichloromethylene)bis[phosphonic acid] (AR-C69931MX) as a neutral antagonist. In conclusion, this is the first report of a cell line stably expressing a constitutively active mutant of human platelet P2Y(12) receptor and the identification of potent inverse agonist.

Differential Regulation of Ca2+ Signaling and Membrane Trafficking by Multiple P2 Receptors in Brown Adipocytes

Extracellular ATP triggers changes in intracellular Ca(2+), ion channel function, and membrane trafficking in adipocytes. The aim of the present study was to determine which P2 receptors might mediate the Ca(2+) signaling and membrane trafficking responses to ATP in brown fat cells. RT-PCR was used to determine which P2 receptors are expressed in brown fat cells. Responses to nucleotide agonists and antagonists were characterized using fura-2 fluorescence imaging of Ca(2+) responses, and FM 1-43 fluorescence imaging and membrane capacitance measurements to assess membrane trafficking. The pharmacology of the Ca(2+) responses fits the properties of the P2Y receptors for which mRNA is expressed, but the agonist and antagonist sensitivity of the membrane-trafficking response was not consistent with any P2 receptor described to date. Brown adipocytes expressed mRNA for P2Y(2), P2Y(6), and P2Y(12) metabotropic receptors and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5), and P2X(7) ionotropic receptors. The agonists ATP, ADP, UTP, UDP and 2', 3'-(benzoylbenzoyl) ATP (BzATP) increased intracellular Ca(2+), while 100 microM: suramin, pyridoxal-phosphate-6-azophenyl-2' 4'-disulfonic acid (PPADS), or Reactive Blue 2 partially blocked Ca(2+) responses. ATP, but not ADP, UTP, UDP or BzATP activated membrane trafficking. The membrane response could be blocked completely with 1 microM: PPADS but not by the antagonist MRS2179. We conclude that multiple P2 receptors mediate the ATP responses of brown fat cells, and that membrane trafficking is regulated by a P2 receptor showing unusual properties.

Cerebellar astrocytes co-express several ADP receptors. Presence of functional P2Y(13)-like receptors

Astrocytes exhibit a form of excitability based on variations of intracellular Ca²⁺ concentration in response to various stimuli, including ADP, ATP, UTP and dinucleotides. Here, we investigate the presence of the recently cloned ADP-sensitive receptors, P2Y₁₂ and P2Y₁₃ subtypes, which are negatively coupled to adenylate cyclase, in cerebellar astrocytes. We checked the effect of specific agonists, 2-methylthioadenosine diphosphate (2MeSADP) and ADP, on adenylate cyclase stimulation induced by isoproterenol. Both agonists significantly reduced the cAMP accumulation induced by isoproterenol. The inhibitory effect was concentration-dependent with IC₅₀ values of 46 ± 13 and 23 ± 14 nM for 2MeSADP and ADP, respectively. The experiments were carried out in the presence of MRS-2179, a specific antagonist of P2Y₁ receptor, to avoid any contribution of this receptor. Using fura-2 microfluorimetry we also proved that astrocytes responded to 2MeSADP stimulations with calcium responses in the absence and also in the presence of MRS-2179. Both effects, inhibition of adenylate cyclase and intracellular calcium mobilization, were not modified by 2MeSAMP, an antagonist of P2Y₁₂ receptor, suggesting that were mediated by P2Y₁₃-like receptors.

Functional expression of human and mouse P2Y12 receptors in Saccharomyces cerevisiae

DNA sequences encoding the murine ortholog of the human P2Y12 receptor were cloned. The human and mouse P2Y12 receptors were expressed in a yeast cell-based GPCR expression technology containing chimeric yeast Galpha protein (Gpa1) constructs in which the 5 C-terminal amino acids were identical to corresponding sequences from mammalian Galphai/o proteins. LacZ reporter gene assays of agonist-induced activation of the G protein-coupled mating signal transduction pathway revealed murine P2Y12 functional pharmacological properties that closely resembled those exhibited by the human P2Y12 receptor. In NIH3T3 cells, the mouse P2Y12 stimulated calcium uptake monitored in FLIPR via coupling to a Galphaq/i3 chimeric protein. Murine P2Y12 mRNA was expressed at high levels in the brain and at lower levels in a variety of peripheral tissues. In situ hybridization analysis indicated glia-specific expression within the brain.

Adenine triphosphate nucleotides are antagonists at the P2Y receptor

The aim of the present study was to characterize the pharmacological profile of the P2Y(12) receptor for several adenine triphosphate nucleotides in view of their possible roles as partial agonists or true antagonists. Two distinct cellular systems were used: P2Y(1) receptor deficient mouse platelets ( platelets) previously shown to express a native and functional P2Y(12) receptor and 1321 N1 astrocytoma cells stably expressing the human P2Y(12) receptor (1321 N1 P2Y(12)). ADP and its structural analogues inhibited cAMP accumulation in a dose-dependent manner in both platelets and 1321 N1 P2Y(12) cells with a similar rank order of potency, 2 methylthio-ADP (2MeSADP) >>ADP - Adenosine 5'-(betathio) diphosphate (AlphaDPbetaS). Commercial ATP, 2 chloro; ATP (2ClATP) and 2 methylthio-ATP (2MeSATP) also inhibited cAMP accumulation in both cell systems. In contrast, after creatine phosphate (CP)/creatine phosphokinase (CPK) regeneration, adenine triphosphate nucleotides lost their agonistic effect on platelets and behaved as antagonists of ADP (0.5 microm)-induced adenylyl cyclase inhibition with IC(50) of 13.5 +/- 4.8, 838 +/- 610, 1280 +/- 1246 microm for 2MeSATP, ATP and 2ClATP, respectively. In 1321 N1 P2Y(12) cells, CP/CPK regenerated ATP and 2ClATP lost their agonistic effect only when CP/CPK was maintained during the cAMP assay. The stable ATP analogue ATPgammaS antagonized ADPbetaS-induced inhibition of cAMP accumulation in both platelets and 1321 N1 P2Y(12) cells. Thus, ATP and its triphosphate analogues are not agonists but rather antagonists at the P2Y(12) receptor expressed in platelets or transfected cells, provided care is taken to remove diphosphate contaminants and to prevent the generation of diphosphate nucleotide derivatives by cell ectonucleotidases.

ADP receptor P2Y12 is expressed in vascular smooth muscle cells and stimulates contraction in human blood vessels

OBJECTIVE

ADP plays an important role in platelet aggregation by activating P2Y12 receptors. We assessed the hypothesis that P2Y12 receptors are expressed in vascular smooth muscle cells (VSMC).

METHODS AND RESULTS

P2Y12 receptor mRNA was found to have a high expression among the P2 receptors in human VSMC, significantly higher than the other 2 ADP receptors (P2Y1 and P2Y13, real-time polymerase chain reaction). Western blots gave a band of 50 kD, similar to that in platelets. To unmask a P2Y12 receptor-mediated vasoconstriction by simulating the in vivo situation, vessels were precontracted to a submaximal level. 2-MeSADP stimulated contractions in vessel segments from internal mammary artery (IM), IM branches and small veins (Emax=15+/-6% of 60 mmol/L K+ contraction, pEC50=5.6+/-0.6, Emax=21+/-1%, pEC50=6.8+/-0.1, and Emax=48+/-9%, pEC50=6.6+/-0.4). The selective P2Y12 antagonist AR-C67085 blocked 2-MeSADP contractions. The contraction was not reduced in patients using clopidogrel, a drug inhibiting ADP-induced platelet aggregation by blocking the P2Y12 receptor. This may be explained by the high instability of the active clopidogrel metabolite that never reaches the systemic circulation.

CONCLUSIONS

ADP acting on P2Y12 receptors not only is important for platelet activation but also stimulates vasoconstriction. Stable drugs with antagonistic effects on P2Y12 receptors, affecting both platelets and VSMC, could be of double therapeutic benefit in their prevention of both thrombosis and vasospasm.

P2Y12 receptor stimulation inhibits beta-adrenergic receptor-induced differentiation by reversing the cyclic AMP-dependent inhibition of protein kinase B

Cyclic AMP-dependent induction of differentiation by activation of the beta-adrenergic receptor is correlated with inhibition of protein kinase B activity concomitant with growth arrest and increase in glial fibrillary acidic protein (GFAP) synthesis in rat C6 glioma cells. Costimulation of the beta-adrenergic receptor with purinergic receptors activated by 2-methylthio-adenosine-5'-diphosphate (2MeSADP) increased protein kinase B (PKB) phosphorylation above the level measured in non-stimulated cells and abolished cAMP-dependent differentiation. Transfection of cells with constitutively active PKB confirmed that reactivation of PKB is involved in the 2MeSADP-dependent inhibition of GFAP synthesis. The P2Y(12) and P2Y(13) receptor antagonist AR-C69931MX [N(6)-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloro-methylene ATP] decreased PKB phosphorylation to the level in non-stimulated cells, whereas the P2Y(13) antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) and P(1),P(3)-di(adenosine-5') tetraphosphate (Ap(4)A) did not alter the 2MeSADP-induced phosphorylation of PKB, showing that enhanced PKB activity and subsequent phosphorylation of glycogen synthase kinase-3 is due to stimulation of the P2Y(12) receptor. In addition, experiments in the presence of pertussis toxin and phosphatidylinositol 3-kinase (PI 3-K) activity assays demonstrated that the P2Y(12) receptor-mediated increase in PKB phosphorylation is G(i) protein- and PI 3-K-dependent. The presented data demonstrated that a cAMP-dependent inhibition of PKB induces differentiation of C6 glioma cells and that inhibition of adenylate cyclase and reactivation of the PI 3-K/PKB pathway by the P2Y(12) receptor reverses differentiation into enhanced proliferation.

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

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.

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.

Coexpression of functional P2X and P2Y nucleotide receptors in single cerebellar granule cells

The present study describes the presence and expression of functional nucleotide receptors, both ionotropic and metabotropic, in highly purified cultures of cerebellar granule neurons. Microfluorimetric experiments have been carried out to record specific [Ca(2+)](i) transients in individual granule neurons after challenge with diverse nucleotides. Although great heterogeneity was found in nucleotide responses in single cells, these responses all became modified during the course of granule cell differentiation, not only at the level of the number of responding cells, but also in the magnitude of the response to nucleotides. These in vitro developmental changes were more significant in metabotropic responses to pyrimidine nucleotides, UTP and UDP, which were down- and upregulated, respectively, during the time in culture. At least two types of ADP-specific receptors seem expressed in different granule cell subpopulations responding to 2MeSADP, as the specific P2Y(1) antagonist MRS-2179 inhibited Ca(2+) responses in only one of these populations. The great diversity of metabotropic responses observed was confirmed by the RT-PCR expression of different types of P2Y receptors in granule cell cultures: P2Y(1), P2Y(4), P2Y(6), and P2Y(12). Similarly, ionotropic nucleotide responses were confirmed by the presence of specific messengers for different P2X subunits, and by immunolabeling studies (P2X(1), P2X(2), P2X(3), P2X(4) and P2X(7)). Immunolabeling reflected great variety in the P2X subunit distribution along the granule neuron cytoarchitecture, with P2X(2), P2X(3) and P2X(4) present at somatodendritic locations, and P2X(1), P2X(7), and P2X(3), located at the axodendritic prolongations. The punctuated labeling pattern obtained for P2X(3) and P2X(7) subunits is particularly notable, as it presents a high degree of colocalization with synaptophysin, a specific marker of synaptic vesicles, suggesting specialized localization and function in granule neurons.

Inactivation of the human P2Y12 receptor by thiol reagents requires interaction with both extracellular cysteine residues, Cys17 and Cys270

Human platelets express 2 G protein-coupled nucleotide receptors: the platelet adenosine diphosphate (ADP) receptor coupled to stimulation of phospholipase C (P2Y(1)) via heterotrimeric guanosine 5-triphosphate (GTP)-binding protein G(q), and the platelet ADP receptor coupled to inhibition of adenylyl cyclase (P2Y(12)) via heterotrimeric GTP-binding protein G(i). Although these 2 receptors are encoded on the same chromosome and have similar pharmacologic profiles, they have different reactivities toward thiol reagents. The thiol agent p-chloromercuribenzene sulfonic acid (pCMBS) and the active metabolites from antiplatelet drugs, clopidogrel and CS-747, inactivate the P2Y(12) receptor and are predicted to interact with the extracellular cysteine residues on the P2Y(12) receptor. In this study we identified the reactive cysteine residues on the human P2Y(12) receptor by site-directed mutagenesis using pCMBS as the thiol reagent. Cys97Ser and Cys175Ser mutants of the P2Y(12) receptor did not express when transfected into Chinese hamster ovary (CHO-K1) cells, indicating the essential nature of a disulfide bridge between these residues. The Cys17Ser, Cys270Ser, and Cys17Ser/Cys270Ser double mutants had similar median effective concentration (EC(50)) values for ADP and 2-methylthio-ADP (2-MeSADP) when compared with the wild-type P2Y(12). Similarly, the median inhibitory concentration (IC(50)) values for BzATP (2',3'-O-(4- benzoylbenzoyl) adenosine 5'-triphosphate), an antagonist of the P2Y(12) receptor, also did not differ dramatically among these mutants and the wild-type P2Y(12) receptor. pCMBS inactivated the wild-type P2Y(12) receptor in a concentration-dependent manner, whereas it had no effect on the P2Y(1) receptor. Finally, pCMBS partially affected the G(i) coupling of Cys17Ser or Cys270Ser receptor mutants, but had no effect on Cys17Ser/Cys270Ser P2Y(12) receptor-mediated inhibition of adenylyl cyclase. These results indicate that, unlike the P2Y(1) receptor, which has 2 essential disulfide bridges linking its extracellular domains, the P2Y(12) receptor has 2 free cysteines in its extracellular domains (Cys17 and Cys270), both of which are targets of thiol reagents. We speculate that the active metabolites of clopidogrel and CS-747 form disulfide bridges with both Cys17 and Cys270 in the P2Y(12) receptor, and thereby inactivate the receptor.

Attenuation of the P2Y receptor-mediated control of neuronal Ca2+ channels in PC12 cells by antithrombotic drugs

1. In PC12 cells, adenine nucleotides inhibit voltage-activated Ca(2+) currents and adenylyl cyclase activity, and the latter effect was reported to involve P2Y(12) receptors. To investigate whether these two effects are mediated by one P2Y receptor subtype, we used the antithrombotic agents 2-methylthio-AMP (2-MeSAMP) and N(6)-(2-methyl-thioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (AR-C69931MX). 2. ADP reduced A(2A) receptor-dependent cyclic AMP synthesis with half maximal effects at 0.1-0.17 micro M. In the presence of 30 micro M 2-MeSAMP or 100 nM AR-C69931MX, concentration response curves were shifted to the right by factors of 39 and 30, indicative of pA(2) values of 6.1 and 8.5, respectively. 3. The inhibition of Ca(2+) currents by ADP was attenuated by 10-1000 nM AR-C69931MX and by 3-300 micro M 2-MeSAMP. ADP reinhibited Ca(2+) currents after removal of 2-MeSAMP within less than 15 s, but required 2 min to do so after removal of AR-C69931MX. 4. ADP inhibited Ca(2+) currents with half maximal effects at 5-20 micro M. AR-C69931MX (10-100 nM) displaced concentration response curves to the right, and the resulting Schild plot showed a slope of 1.09 and an estimated pK(B) value of 8.7. Similarly, 10-100 micro M 2-MeSAMP also caused rightward shifts resulting in a Schild plot with a slope of 0.95 and an estimated pK(B) of 5.4. 5. The inhibition of Ca(2+) currents by 2-methylthio-ADP and ADPbetaS was also antagonized by AR-C69931MX, which (at 30 nM) caused a rightward shift of the concentration response curve for ADPbetaS by a factor of 3.8, indicative of a pA(2) value of 8.1. 6. These results show that antithrombotic drugs antagonize the inhibition of neuronal Ca(2+) channels by adenine nucleotides, which suggests that this effect is mediated by P2Y(12) receptors.

Characterization and channel coupling of the P2Y(12) nucleotide receptor of brain capillary endothelial cells

Rat brain capillary endothelial (B10) cells express an unidentified nucleotide receptor linked to adenylyl cyclase inhibition. We show that this receptor in B10 cells is identical in sequence to the P2Y(12) ADP receptor ("P2Y(T)") of platelets. When expressed heterologously, 2-methylthio-ADP (2-MeSADP; EC(50), 2 nm), ADP, and adenosine 5'-O-(2-thio)diphosphate were agonists of cAMP decrease, and 2-propylthio-D-beta,gamma-difluoromethylene-ATP was a competitive antagonist (K(B), 28 nm), as in platelets. However, 2-methylthio-ATP (2-MeSATP) (EC(50), 0.4 nm), ATP (1.9 microm), and 2-chloro-ATP (190 nm), antagonists in the platelet, were also agonists. 2-MeSADP activated (EC(50), 0.1 nm) GIRK1/GIRK2 inward rectifier K(+) channels when co-expressed with P2Y(12) receptors in sympathetic neurons. Surprisingly, P2Y(1) receptors expressed likewise gave that response; however, a full inactivation followed, absent with P2Y(12) receptors. A new P2Y(12)-mediated transduction was found, the closing of native N-type Ca(2+) channels; again both 2-MeSATP and 2-MeSADP are agonists (EC(50), 0.04 and 0.1 nm, respectively). That action, like their cAMP response, was pertussis toxin-sensitive. The Ca(2+) channel inhibition and K(+) channel activation are mediated by beta gamma subunit release from a heterotrimeric G-protein. G alpha subunit types in B10 cells were also identified. The presence in the brain capillary endothelial cell of the P2Y(12) receptor is a significant extension of its functional range.

Adenine nucleotide analogues locked in a Northern methanocarba conformation: enhanced stability and potency as P2Y(1) receptor agonists

Preference for the Northern (N) ring conformation of the ribose moiety of nucleotide 5'-triphosphate agonists at P2Y(1), P2Y(2), P2Y(4), and P2Y(11) receptors, but not P2Y(6) receptors, was established using a ring-constrained methanocarba (a 3.1.0-bicyclohexane) ring as a ribose substitute (Kim et al. J. Med. Chem. 2002, 45, 208-218.). We have now combined the ring-constrained (N)-methanocarba modification of adenine nucleotides with other functionalities known to enhance potency at P2 receptors. The potency of the newly synthesized analogues was determined in the stimulation of phospholipase C through activation of turkey erythrocyte P2Y(1) or human P2Y(1) and P2Y(2) receptors stably expressed in astrocytoma cells. An (N)-methanocarba-2-methylthio-ADP analogue displayed an EC(50) at the hP2Y(1) receptor of 0.40 nM and was 55-fold more potent than the corresponding triphosphate and 16-fold more potent than the riboside 5'-diphosphate. 2-Cl-(N)-methanocarba-ATP and its N(6)-Me analogue were also highly selective, full agonists at P2Y(1) receptors. The (N)-methanocarba-2-methylthio and 2-chloromonophosphate analogues were full agonists exhibiting micromolar potency at P2Y(1) receptors, while the corresponding ribosides were inactive. Although beta,gamma-methylene-ATP was inactive at P2Y receptors, beta,gamma-methylene-(N)-methanocarba-ATP was a potent hP2Y(1) receptor agonist with an EC(50) of 160 nM and was selective versus hP2Y(2) and hP2Y(4) receptors. The rates of hydrolysis of Northern (N) and Southern (S) methanocarba analogues of AMP by rat 5'-ectonucleotidase were negligible. The rates of hydrolysis of the corresponding triphosphates by recombinant rat NTPDase1 and 2 were studied. Both isomers were hydrolyzed by NTPDase 1 at about half the rate of ATP hydrolysis. The (N) isomer was hardly hydrolyzed by NTPDase 2, while the (S) isomer was hydrolyzed at one-third of the rate of ATP hydrolysis. This suggests that new, more stable and selective nucleotide agonists may be designed on the basis of the (N)-conformation, which greatly enhanced potency at P2Y(1) receptors.

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.