P2Y6 nucleotide receptor activates PKC to protect 1321N1 astrocytoma cells against tumor necrosis factor-induced apoptosis

The Involvement of Neuroinflammation in the Onset and Progression of Parkinson's Disease

Parkinson's disease is a neurodegenerative disease exhibiting the fastest growth in incidence in recent years. As with most neurodegenerative diseases, the pathophysiology is incompletely elucidated, but compelling evidence implicates inflammation, both in the central nervous system and in the periphery, in the initiation and progression of the disease, although it is not yet clear what triggers this inflammatory response and where it begins. Gut dysbiosis seems to be a likely candidate for the initiation of the systemic inflammation. The therapies in current use provide only symptomatic relief, but do not interfere with the disease progression. Nonetheless, animal models have shown promising results with therapies that target various vicious neuroinflammatory cascades. Translating these therapeutic strategies into clinical trials is still in its infancy, and a series of issues, such as the exact timing, identifying biomarkers able to identify Parkinson's disease in early and pre-symptomatic stages, or the proper indications of genetic testing in the population at large, will need to be settled in future guidelines.

Titanium dioxide nanotubes increase purinergic receptor P2Y6 expression and activate its downstream PKCα-ERK1/2 pathway in bone marrow mesenchymal stem cells under osteogenic induction

Titanium dioxide (TiO₂) nanotubes can improve the osseointegration of pure titanium implants, but this exact mechanism has not been fully elucidated. The purinergic receptor P2Y6 is expressed in bone marrow mesenchymal stem cells (BMSCs) and participates in the regulation of bone metabolism. However, it is unclear as to whether P2Y6 is involved in the osteogenic differentiation of BMSCs induced by TiO₂ nanotubes. TiO₂ nanotubes were prepared on the surface of titanium specimens using the anodizing method and characterized their features. Quantitative reverse transcriptase polymerase chain reaction and western blotting were used to detect the expression of P2Y6, markers of osteogenic differentiation, and PKCα-ERK1/2. A rat femoral defect model was established to evaluate the osseointegration effect of TiO₂ nanotubes combined with P2Y6 agonists. The results showed that the average inner diameter of the TiO₂ nanotubes increased with an increase in voltage (voltage range of 30-90V), and the expression of P2Y6 in BMSCs could be upregulated by TiO₂ nanotubes in osteogenic culture. Inhibition of P2Y6 expression partially inhibited the osteogenic effect of TiO₂ nanotubes and downregulated the activity of the PKCα-ERK1/2 pathway. When using in vitro and in vivo experiments, the osteogenic effect of TiO₂ nanotubes when combined with P2Y6 agonists was more pronounced. TiO₂ nanotubes promoted the P2Y6 expression of BMSCs during osteogenic differentiation and promoted osteogenesis by activating the PKCα-ERK1/2 pathway. The combined application of TiO₂ nanotubes and P2Y6 agonists may be an effective new strategy to improve the osseointegration of titanium implants. STATEMENT OF SIGNIFICANCE: Titanium dioxide (TiO₂) nanotubes can improve the osseointegration of pure titanium implants, but this exact mechanism has not been fully elucidated. The purinergic receptor P2Y6 is expressed in bone marrow mesenchymal stem cells (BMSCs) and participates in the regulation of bone metabolism. However, it is unclear as to whether P2Y6 is involved in the osteogenic differentiation of BMSCs induced by TiO₂ nanotubes. For the first time, this study revealed the relationship between TiO₂ nanotubes and purine receptor P2Y6, and further explored its mode of action, which may provide clues as to the regulatory role of TiO₂ nanotubes on osteogenic differentiation of BMSCs. These findings will help to develop novel methods for guiding material design and biosafety evaluation of nano implants.

Mechanisms of sympathoexcitation via P2Y6 receptors

Many drugs used in cardiovascular therapy, such as angiotensin receptor antagonists and beta-blockers, may exert at least some of their actions through effects on the sympathetic nervous system, and this also holds true for e.g., P2Y12 antagonists. A new target at the horizon of cardiovascular drugs is the P2Y6 receptor which contributes to the development of arteriosclerosis and hypertension. To learn whether P2Y6 receptors in the sympathetic nervous system might contribute to actions of respective receptor ligands, responses of sympathetic neurons to P2Y6 receptor activation were analyzed in primary cell culture. UDP in a concentration dependent manner caused membrane depolarization and enhanced numbers of action potentials fired in response to current injections. The excitatory action was antagonized by the P2Y6 receptor antagonist MRS2578, but not by the P2Y2 antagonist AR-C118925XX. UDP raised intracellular Ca2+ in the same range of concentrations as it enhanced excitability and elicited inward currents under conditions that favor Cl- conductances, and these were reduced by a blocker of Ca2+-activated Cl- channels, CaCCInh-A01. In addition, UDP inhibited currents through KV7 channels. The increase in numbers of action potentials caused by UDP was not altered by the KV7 channel blocker linopirdine, but was enhanced in low extracellular Cl- and was reduced by CaCCInh-A01 and by an inhibitor of phospholipase C. Moreover, UDP enhanced release of previously incorporated [3H] noradrenaline, and this was augmented in low extracellular Cl- and by linopirdine, but attenuated by CaCCInh-A01. Together, these results reveal sympathoexcitatory actions of P2Y6 receptor activation involving Ca2+-activated Cl- channels.

Changes in P2Y6 receptor-mediated vasoreactivity following focal and global ischemia

Ischemia, both in the form of focal thromboembolic stroke and following subarachnoid hemorrhage (SAH), causes upregulation of vasoconstrictive receptor systems within the cerebral vasculature. Descriptions regarding changes in purinergic signaling following ischemia are lacking, especially when the importance of purinergic signaling in regulating vascular tone is taken into consideration. This prompted us to evaluate changes in P2Y₆ -mediated vasomotor reactivity in two different stroke models in rat. We used wire myography to measure changes in cerebral vasoreactivity to the P2Y₆ agonist UDP-β-S following either experimental SAH or transient middle cerebral artery occlusion. Changes in receptor localization or receptor expression were evaluated using immunohistochemistry and quantitative flow cytometry. Transient middle cerebral artery occlusion caused an increase in Emax when compared to sham (233.6 [206.1-258.5]% vs. 161.1 [147.1-242.6]%, p = 0.0365). No such change was seen following SAH. Both stroke models were associated with increased levels of P2Y₆ receptor expression in the vascular smooth muscle cells (90.94 [86.99-99.15]% and 93.79 [89.96-96.39]% vs. 80.31 [70.80-80.86]%, p = 0.021) and p = 0.039 respectively. There was no change in receptor localization in either of the stroke models. Based on these findings, we conclude that focal ischemic stroke increases vascular sensitivity to UDP-β-S by upregulating P2Y₆ receptors on vascular smooth muscle cells while experimental SAH did not induce changes in vasoreactivity in spite of increased P2Y₆ receptor expression.

Effects of R-flurbiprofen and the oxygenated metabolites of endocannabinoids in inflammatory pain mice models

Pain is one of the cardinal signs accompanying inflammation. The prostaglandins (PGs), synthetized from arachidonic acid by cyclooxygenase (COX)-2, are major bioactive lipids implicated in inflammation and pain. However, COX-2 is also able to metabolize other lipids, including the endocannabinoids 2-arachidonoylglycerol (2-AG) and anandamide (AEA), to give glycerol ester (PG-G) and ethanolamide (PG-EA) derivatives of the PGs. Consequently, COX-2 can be considered as a hub not only controlling PG synthesis, but also PG-G and PG-EA synthesis. As they were more recently characterized, these endocannabinoid metabolites are less studied in nociception compared to PGs. Interestingly R-profens, previously considered as inactive enantiomers of nonsteroidal anti-inflammatory drugs (NSAIDs), are substrate-selective COX inhibitors. Indeed, R-flurbiprofen can selectively block PG-G and PG-EA production, without affecting PG synthesis from COX-2. Therefore, we compared the effect of R-flurbiprofen and S-flurbiprofen in models of inflammatory pain triggered by local administration of lipopolysaccharides (LPS) and carrageenan in mice. Remarkably, the effects of flurbiprofen enantiomers on mechanical hyperalgesia seem to depend on (i) the inflammatory stimuli, (ii) the route of administration, and (iii) the timing of administration. We also assessed the effect of administration of the PG-Gs, PG-EAs, and PGs on LPS-induced mechanical hyperalgesia. Our data support the interest of studying the nonhydrolytic endocannabinoid metabolism in the context of inflammatory pain.

Microglia Purinoceptor P2Y6: An Emerging Therapeutic Target in CNS Diseases

The purinergic receptor P2Y6 is expressed in immune cells, including the microglia that are implicated in neurological disorders. Its ligand, UDP, is a signaling molecule that can serve as an “find-me” signal when released in significant quantities by damaged/dying cells. The binding of UDP by P2Y6R leads to the activation of different biochemical pathways, depending on the disease context and the pathological environment. Generally, P2Y6R stimulates phagocytosis. However, whether or not phagocytosis coincides with cell activation or the secretion of pro-inflammatory cytokines needs further investigation. The current review aims to discuss the various functions of P2Y6R in some CNS disorders. We present evidence that P2Y6R may have a detrimental or beneficial role in the nervous system, in the context of neurological pathologies, such as ischemic stroke, Alzheimer’s disease, Parkinson’s disease, radiation-induced brain injury, and neuropathic pain.

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.

Tools and drugs for uracil nucleotide-activated P2Y receptors

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

UDP/P2Y6 receptor signaling regulates IgE-dependent degranulation in human basophils

BACKGROUND

P2Y purinergic receptors (P2YR) are G protein-coupled receptors that are stimulated by extracellular nucleotides. They mediate cellular effects by regulating cAMP production, protein kinase C activation, inositol trisphosphate generation, and Ca²⁺ release from intracellular stores. The P2Y6 receptor of this family is selectively stimulated by UDP, and selectively inhibited by MRS2578. In the present study, we examined the effect of UDP/P2Y6 receptor signaling on IgE-dependent degranulation in human basophils.

METHODS

Basophils were purified from human peripheral blood. The mRNA expression of genes encoding P2YR and ecto-nucleoside triphosphate diphosphohydrolase (ENTPDase) was measured by RT-PCR. Intracellular Ca²⁺ influx via UDP/P2Y6 receptor signaling in basophils was detected using a calcium probe. The effect of UDP/P2Y6 receptor signaling on IgE-dependent degranulation in basophils was confirmed by measuring CD63 expression by flow cytometry. Autocrine secretion of nucleotides was detected by HPLC analysis.

RESULTS

We showed that purified basophils express P2Y6 mRNA and that UDP increased intracellular Ca²⁺, which was reduced by MRS2578 treatment. UDP promoted IgE-dependent degranulation. Furthermore, MRS2578 inhibited IgE-dependent degranulation in basophils. HPLC analysis indicated that basophils spontaneously secrete UTP. In addition, basophils expressed the extracellular nucleotide hydrolases ENTPDase2, ENTPDase3, and ENTPDase8.

CONCLUSIONS

This study showed that UDP/P2Y6 receptor signaling is involved in the regulation of IgE-dependent degranulation in basophils, which might stimulate the P2Y6 receptor via the autocrine secretion of UTP. Thus, this receptor represents a potential target to regulate IgE-dependent degranulation in basophils during allergic diseases.

Pyrimidine Nucleotides Containing a (S)-Methanocarba Ring as P2Y6 Receptor Agonists

Both agonists and antagonists of the UDP-activated P2Y₆ receptor (P2Y₆R) have been proposed for therapeutic use, in conditions such as cancer, inflammation, neurodegeneration and diabetes. Uracil nucleotides containing a South-bicyclo[3.1.0]hexane ((S)-methanocarba) ring system in place of the ribose ring were synthesized and shown to be potent P2Y₆R agonists in a calcium mobilization assay. The (S)-methanocarba modification was compatible with either a 5-iodo or 4-methoxyimino group on the pyrimidine, but not with a α,β-methylene 5´-diphosphate. (S)-Methanocarba dinucleotide potency was compatible with a N⁴-methoxy modification on the proximal nucleoside that is assumed to bind at the P2Y₆R similarly to UDP; (N)-methanocarba was preferred on the distal nucleoside moiety. This suggests that the distal dinucleotide P2Y₆R binding site prefers a ribose-like group that can attain a (N) conformation, rather than (S). Dinucleotide binding was modeled by homology modeling, docking and molecular dynamics simulations, which suggested the same ribose conformational preferences found empirically.

Microglia P2Y6 receptor is related to Parkinson's disease through neuroinflammatory process

BACKGROUND

Microglia in the central nervous system (CNS) were reported to play crucial role in neurodegeneration. Previous studies showed that P2Y6 receptor (P2Y6R) mainly contributed to microglia activation and phagocytosis in CNS. However, the level of P2Y6R in Parkinson's disease (PD) patients is unclear. Therefore, we measured the level of P2Y6R in PD patients and speculated whether it could be a potential biomarker for PD. Given on the basis that P2Y6R was higher in PD patients, we further explored the mechanisms underlying P2Y6R in the pathogenesis of PD.

METHODS

We tested the expression level of P2Y6R in the peripheral blood mononuclear cells (PBMCs) among 145 PD patients, 170 healthy controls, and 30 multiple system atrophy (MSA) patients. We also used a lipopolysaccharide (LPS)-stimulated microglial cell culture model to investigate (i) the effects of LPS on P2Y6R expression with western blot and RT-PCR, (ii) the effects of LPS on UDP expression using HPLC, (iii) the effects of UDP/P2Y6R signaling on cytokine expression using western blot, RT-PCR, and ELISA, and (iv) the signaling pathways activated by the P2Y6R involved in the neuroinflammation.

RESULTS

Expression levels of P2Y6R in PD patients were higher than healthy controls and MSA patients. P2Y6R could be a good biomarker of PD. P2Y6R was also upregulated in LPS-treated BV-2 cells and involved in proinflammatory cytokine release through an autocrine loop based on LPS-triggered UDP secretion and accelerated neuroinflammatory responses through the ERK1/2 pathway. Importantly, blocking UDP/P2Y6R signaling could reverse these pathological processes.

CONCLUSIONS

P2Y6R may be a potential clinical biomarker of PD. Blocking P2Y6R may be a potential therapeutic approach to the treatment of PD patients through inhibition of microglia-activated neuroinflammation.

TLR-Activated Gap Junction Channels Protect Mice against Bacterial Infection through Extracellular UDP Release

Extracellular UDP (eUDP), released as a danger signal by stressed or apoptotic cells, plays an important role in a series of physiological processes. Although the mechanism of eUDP release in apoptotic cells has been well defined, how the eUDP is released in innate immune responses remains unknown. In this study, we demonstrated that UDP was released in both Escherichia coli-infected mice and LPS- or Pam3CSK4-treated macrophages. Also, LPS-induced UDP release could be significantly blocked by selective TLR4 inhibitor Atractylenolide I and selective gap junction inhibitors carbenoxolone and flufenamic acid (FFA), suggesting the key role of TLR signaling and gap junction channels in this process. Meanwhile, eUDP protected mice from peritonitis by reducing invaded bacteria that could be rescued by MRS2578 (selective P2Y6 receptor inhibitor) and FFA. Then, connexin 43, as one of the gap junction proteins, was found to be clearly increased by LPS in a dose- and time-dependent manner. Furthermore, if we blocked LPS-induced ERK signaling by U0126, the expression of connexin 43 and UDP release was also inhibited dramatically. In addition, UDP-induced MCP-1 secretion was significantly reduced by MRS2578, FFA, and P2Y6 mutation. Accordingly, pretreating mice with U0126 and Gap26 increased invaded bacteria and aggravated mice death. Taken together, our study reveals an internal relationship between danger signals and TLR signaling in innate immune responses, which suggests a potential therapeutic significance of gap junction channel-mediated UDP release in infectious diseases.

Didehydro-cortistatin A inhibits HIV-1 Tat mediated neuroinflammation and prevents potentiation of cocaine reward in Tat transgenic mice

HIV-1 Tat protein has been shown to have a crucial role in HIV-1-associated neurocognitive disorders (HAND), which includes a group of syndromes ranging from undetectable neurocognitive impairment to dementia. The abuse of psychostimulants, such as cocaine, by HIV infected individuals, may accelerate and intensify neurological damage. On the other hand, exposure to Tat potentiates cocaine-mediated reward mechanisms, which further promotes HAND. Here, we show that didehydro-Cortistatin A (dCA), an analog of a natural steroidal alkaloid, crosses the blood-brain barrier, cross-neutralizes Tat activity from several HIV-1 clades and decreases Tat uptake by glial cell lines. In addition, dCA potently inhibits Tat mediated dysregulation of IL-1β, TNF-α and MCP-1, key neuroinflammatory signaling proteins. Importantly, using a mouse model where doxycycline induces Tat expression, we demonstrate that dCA reverses the potentiation of cocaine-mediated reward. Our results suggest that adding a Tat inhibitor, such as dCA, to current antiretroviral therapy may reduce HIV-1-related neuropathogenesis.

Enhancement by Uridine Diphosphate of Macrophage Inflammatory Protein-1 Alpha Production in Microglia Derived from Sandhoff Disease Model Mice

Sandhoff disease (SD) is a lysosomal β-hexosaminidase (Hex) deficiency involving excessive accumulation of undegraded substrates, including GM2 ganglioside, and progressive neurodegeneration. Macrophage inflammatory protein-1α (MIP-1α) is a crucial factor for microglia-mediated neuroinflammation in the onset or progression of SD. However, the transmitter-mediated production of MIP-1α in SD is still poorly understood.Extracellular nucleotides, including uridine diphosphate (UDP), leaked by either injured or damaged neuronal cells activate microglia to trigger chemotaxis, phagocytosis, macropinocytosis, and cytokine production.In this study, we demonstrated that UDP enhanced the production of MIP-1α by microglia derived from SD mice (SD-Mg), but not that from wild-type mice (WT-Mg). The UDP-induced MIP-1α production was mediated by the activation of P2Y6 receptor, ERK, and JNK. We also found the amount of dimeric P2Y6 receptor protein to have increased in SD-Mg in comparison to WT-Mg. In addition, we demonstrated that the disruption of lipid rafts enhanced the effect of UDP on MIP-1α production and the disordered maintenance of the lipid rafts in SD-Mg. Thus, the accumulation of undegraded substrates might cause the enhanced effect of UDP in SD-Mg through the increased expression of the dimeric P2Y6 receptors and the disordered maintenance of the lipid rafts. These findings provide new insights into the pathogenic mechanism and therapeutic strategies for SD.

Supportive or detrimental roles of P2Y receptors in brain pathology?--The two faces of P2Y receptors in stroke and neurodegeneration detected in neural cell and in animal model studies

This review describing the role of P2Y receptors in neuropathological conditions focuses on obvious differences between results demonstrating either a role in neuroprotection or in neurodegeneration, depending on in vitro and in vivo models. Such critical juxtaposition puts special emphasis on discussions of beneficial and detrimental effects of P2Y receptor agonists and antagonists in these models. The mechanisms reported to underlie the protection in vitro include increased expression of oxidoreductase genes, like carbonyl reductase and thioredoxin reductase; increased expression of inhibitor of apoptosis protein-2; extracellular signal-regulated kinase- and Akt-mediated antiapoptotic signaling; increased expression of Bcl-2 proteins, neurotrophins, neuropeptides, and growth factors; decreased Bax expression; non-amyloidogenic APP shedding; and increased neurite outgrowth in neuronal cells. Animal studies investigating the influence of P2Y receptors in middle cerebral artery occlusion (MCAO) models for stroke prove beneficial effects of P2Y receptor antagonists. In MCAO mice and rats, the application of broad-range P2 receptor antagonists decreased the infarct volume and improved neurological outcome. Moreover, antagonists of the P2Y1 receptor, one of the most abundant P2Y receptor subtypes in brain tissue, decreased neuronal loss and improved spatial memory in rats after traumatic brain injury (TBI). Currently available data show a discrepancy between in vitro and in vivo models concerning the benefits of P2Y receptor activation in pathological conditions. In vitro models demonstrate protection by P2Y receptor agonists, but in vivo P2Y receptor activation deteriorates the outcome after MCAO and controlled cortical impact brain injury, a TBI model. To broaden the scope of the review, we additionally discuss publications that demonstrate detrimental effects of P2Y receptor agonists in vitro and publications showing protective effects of agonists in vivo. All these studies help to better understand the significant role of P2Y receptors especially in stroke models and to develop pharmacological strategies for the treatment of stroke.

Enhancement of glucose uptake in mouse skeletal muscle cells and adipocytes by P2Y6 receptor agonists

Glucose uptake by peripheral tissues such as skeletal muscles and adipocytes is important in the maintenance of glucose homeostasis. We previously demonstrated that P2Y₆ receptor (P2Y₆R) agonists protect pancreatic islet cells from apoptosis and stimulate glucose-dependent insulin release. Here, we investigated the effects of P2Y₆R activation on glucose uptake in insulin target tissues. An agonist of the P2Y₆R, P¹-(5′-uridine)-P³-(5′-N⁴-methoxycytidine)-triphosphate (MRS2957), significantly increased the uptake of [³H]2-deoxyglucose in mouse C2C12 myotubes and 3T3-L1 adipocytes, and this stimulation was significantly decreased by a selective P2Y₆R antagonist N,N″-1,4-butanediyl-bis[N′-(3-isothiocyanatophenyl)thiourea] (MRS2578). Pre-incubation with Compound C (an inhibitor of 5′-AMP-activated protein kinase, AMPK), or AMPK siRNA abolished the stimulatory effect of MRS2957 on glucose uptake. Also, MRS2957 (60 min incubation) increased recruitment of the facilitated glucose transporter-4 (GLUT4) to the cell membrane, which was blocked by MRS2578. Treatment of C2C12 myotubes with MRS2957 induced significant phosphorylation of AMPK, which increase GLUT4 expression through histone deacetylase (HDAC)5 signaling. Glucose uptake in primary mouse adipocytes from wild-type mice was stimulated upon P2Y₆R activation by either MRS2957 or native agonist UDP, and the P2Y₆R effect was antagonized by MRS2578. However, in adipocytes from P2Y₆R-knockout mice P2Y₆R agonists had no effect on glucose uptake, and there was no change in the glucose uptake by insulin. Our results indicate that the P2Y₆R promotes glucose metabolism in peripheral tissues, which may be mediated through AMPK signaling.

Extracellular UDP and P2Y6 function as a danger signal to protect mice from vesicular stomatitis virus infection through an increase in IFN-β production

Extracellular nucleotides that constitute a "danger signal" play an important role in the regulation of immune responses. However, the function and mechanism of extracellular UDP and P2Y6 in antiviral immunity remain unknown. In this study, we demonstrated the in vitro and in vivo protection of UDP/P2Y6 signaling in vesicular stomatitis virus (VSV) infection. First, we demonstrated that VSV-infected cells secrete UDP from the cytoplasm as a danger signal to arouse surrounding cells. Meanwhile, expression of the UDP-specific receptor P2Y6 also was enhanced by VSV. Consequently, UDP protects RAW 264.7 cells, murine embryonic fibroblasts, bone marrow-derived macrophages, and L929 cells from VSV and GFP lentivirus infection. This protection can be blocked by the P2Y6 selective antagonist MRS2578 or IFN-α/β receptor-blocking Ab. VSV-induced cell death and virus replication were both enhanced significantly by knocking down and knocking out P2Y6 in different cells. Mechanistically, UDP facilitates IFN-β secretion through the p38/JNK- and ATF-2/c-Jun-signaling pathways, which are crucial in promoting antiviral immunity. Interestingly, UDP was released through a caspase-cleaved pannexin-1 channel in VSV-induced apoptotic cells and protected cells from infection through P2Y6 receptor in an autocrine or paracrine manner. Furthermore, UDP also protected mice from VSV infection through P2Y6 receptors in an acute neurotropic infection mouse model. Taken together, these results demonstrate the important role of extracellular UDP and P2Y6 as a danger signal in antiviral immune responses and suggest a potential therapeutic role for UDP/P2Y6 in preventing and controlling viral diseases.

P2Y6 receptor-mediated proinflammatory signaling in human bronchial epithelia

P2Y receptors are expressed in virtually all epithelia and are responsible for the control of fluid and electrolyte transport. In asthmatic inflammation, the bronchial epithelia are damaged by eosinophil-derived, highly toxic cationic proteins, such as major basic protein (MBP). Consequently, extracellular nucleotides are released into the extracellular space from airway epithelial cells, and act in an autocrine or paracrine fashion to regulate immune functions. Our data show damage to the human bronchial epithelial cell line, 16HBE14o-, by poly-L-arginine-induced UDP release into the extracellular medium. Activation of P2Y6 receptor by its natural ligand, UDP, or its specific agonist, MRS 2693, led to the production of two proinflammatory cytokines, interleukin (IL)-6 and IL-8. This may have resulted from increased IL-6 and IL-8 mRNA expression, and activation of p38 and ERK1/2 MAPK, and NF-κB pathways. Our previous study demonstrated that UDP stimulated transepithelial Cl- secretion via both Ca2+- and cAMP-dependent pathways in 16HBE14o- epithelia. This was further confirmed in this study by simultaneous imaging of Ca2+ and cAMP levels in single cells using the Fura-2 fluorescence technique and a FRET-based approach, respectively. Moreover, the P2Y6 receptor-mediated production of IL-6 and IL-8 was found to be dependent on Ca2+, but not the cAMP/PKA pathway. Together, these studies show that nucleotides released during the airway inflammatory processes will activate P2Y6 receptors, which will lead to further release of inflammatory cytokines. The secretion of cytokines and the formation of such "cytokine networks" play an important role in sustaining the airway inflammatory disease.

Purinergic signalling and cancer

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

Discovery and optimization of 1,3,5-trisubstituted pyrazolines as potent and highly selective allosteric inhibitors of protein kinase C-ζ

There is increasing evidence that the atypical protein kinase C, PKCζ, might be a therapeutic target in pulmonary and hepatic inflammatory diseases. However, targeting the highly conserved ATP-binding pocket in the catalytic domain held little promise to achieve selective inhibition. In the present study, we introduce 1,3,5-trisubstituted pyrazolines as potent and selective allosteric PKCζ inhibitors. The rigid scaffold offered many sites for modification, all acting as hot spots for improving activity, and gave rise to sharp structure-activity relationships. Targeting of PKCζ in cells was confirmed by reporter gene assay, transfection assays, and Western blotting. The strongly reduced cell-free and cellular activities toward a PIF-pocket mutant of PKCζ suggested that the inhibitors most likely bound to the PIF-pocket on the kinase catalytic domain. Thus, using a rigidification strategy and by establishing and optimizing multiple molecular interactions with the binding site, we were able to significantly improve the potency of the previously reported PKCζ inhibitors.

5-OMe-uridine-5'-O-(α-boranodiphosphate), a novel nucleotide derivative highly active at the human P2Y(6) receptor protects against death-receptor mediated glial apoptosis

P2Y receptors are activated by nucleotides and involved in numerous physiological/pathophysiological processes. However, investigations of specific P2Y receptor functions have been hampered by lack of suitable receptor agonists-antagonists. Recently, we identified the nucleotide 5-OMe-UDP as potent and selective agonist for human P2Y6 receptors. We studied a series of derivatives of this analog with a Pα-borano group substituting a non-bridging oxygen and found increased potency and receptor specificity. Rp-5-OMe-UDPαB (Rp-5-OMe-uridine 5'-O-α-boranodiphosphate) was most potent and selective in inducing intracellular calcium signaling in 1321N1 astrocytoma cells expressing the human P2Y6 receptor. Here, we investigated whether Rp-5-OMe-UDPαB evokes cell protection through human P2Y6 receptors. We tested a well-established model, tumor necrosis factor α (TNFα)-induced cell death in 1321N1 astrocytoma cells. Rp-5-OMe-UDPαB inhibited TNFα-induced cell death even stronger than UDP. These first data of a neuro-protective activity of the human P2Y6 receptor emphasize the potential of the stable, selective, and potent Rp-5-OMe-UDPαB analog for exploiting P2Y6 receptor-mediated cellular functions, like cytoprotection in human tissues, with suitability for future neuro-protective drug development.

Permeation of fluorophore-conjugated phalloidin into live hair cells of the inner ear is modulated by P2Y receptors

Phalloidin, a toxin isolated from the death cap mushroom, Amanita phalloides, binds to filamentous actin with high affinity, and this has made fluorophore-conjugated phalloidin a useful tool in cellular imaging. Hepatocytes take up phalloidin via the liver-specific organic anion transporting polypeptide 1b2, but phalloidin does not permeate most living cells. Rapid entry of styryl dyes into live hair cells has been used to evaluate function, but the usefulness of those fluorescence dyes is limited by broad and fixed absorption spectra. Since phalloidin can be conjugated to fluorophores with various spectra, we investigated whether it would permeate living hair cells. When we incubated mouse utricles in 66 nM phalloidin-CF488A and followed that by washes in phalloidin-free medium, we observed that it entered a subset of hair cells and labeled entire hair bundles fluorescently after 20 min. Incubations of 90 min labeled nearly all the hair bundles. When phalloidin-treated utricles were cultured for 24 h after washout, the label disappeared from the hair cells and progressively but heterogeneously labeled filamentous actin in the supporting cells. We investigated how phalloidin may enter hair cells and found that P2 receptor antagonists, pyridoxalphosphate-6-azophenyl-2', 4'-disulfonic acid and suramin, blocked phalloidin entry, while the P2Y receptor ligands, uridine-5'-diphosphate and uridine-5'-triphosphaste, stimulated uptake. Consistent with that, the P2Y6 receptor antagonist, MRS 2578, decreased phalloidin uptake. The results show that phalloidin permeates live hair cells through a pathway that requires metabotropic P2Y receptor signaling and suggest that phalloidin can be transferred from hair cells to supporting cells in culture.

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.

Transcriptional profiling in an MPNST-derived cell line and normal human Schwann cells

cDNA microarrays were utilized to identify abnormally expressed genes in a malignant peripheral nerve sheath tumor (MPNST)-derived cell line, T265, by comparing the mRNA abundance profiles with that of normal human Schwann cells (nhSCs). The findings characterize the molecular phenotype of this important cell-line model of MPNSTs, and elucidate the contribution of Schwann cells in MPNSTs. In total, 4608 cDNA sequences were screened and hybridizations replicated on custom cDNA microarrays. In order to verify the microarray data, a large selection of differentially expressed mRNA transcripts were subjected to semi-quantitative reverse transcription PCR (LightCycler). Western blotting was performed to investigate a selection of genes and signal transduction pathways, as a further validation of the microarray data. The data generated from multiple microarray screens, semi-quantitative RT-PCR and Western blotting are in broad agreement. This study represents a comprehensive gene-expression analysis of an MPNST-derived cell line and the first comprehensive global mRNA profile of nhSCs in culture. This study has identified ~900 genes that are expressed abnormally in the T265 cell line and detected many genes not previously reported to be expressed in nhSCs. The results provide crucial information on the T265 cells that is essential for investigation using this cell line in experimental studies in neurofibromatosis type I (NF1), and important information on normal human Schwann cells that is applicable to a wide range of studies on Schwann cells in cell culture.

UDP made a highly promising stable, potent, and selective P2Y6-receptor agonist upon introduction of a boranophosphate moiety

P2Y(6) nucleotide receptor (P2Y(6)-R) plays important physiological roles, such as insulin secretion and reduction of intraocular pressure. However, this receptor is still lacking potent and selective agonists to be used as potential drugs. Here, we synthesized uracil nucleotides and dinucleotides, substituted at the C5 and/or P(α) position with methoxy and/or borano groups, 18-22. Compound 18A, R(p) isomer of 5-OMe-UDP(α-B), is the most potent and P2Y(6)-R selective agonist currently known (EC(50) 0.008 μM) being 19-fold more potent than UDP and showing no activity at uridine nucleotide receptors, P2Y(2)- and P2Y(4)-R. Analogue 18A was highly chemically stable under conditions mimicking gastric juice acidity (t(1/2) = 16.9 h). It was more stable to hydrolysis by nucleotide pyrophosphatases (NPP1,3) than UDP (15% and 28% hydrolysis by NPP1 and NPP3, respectively, vs 50% and 51% hydrolysis of UDP) and metabolically stable in blood serum (t(1/2) = 17 vs 2.4, 11.9, and 21 h for UDP, 5-OMe-UDP, and UDP(α-B), respectively). This newly discovered highly potent and physiologically stable P2Y(6)-R agonist may be of future therapeutic potential.

P2Y6 receptor contributes to neutrophil recruitment to inflamed intestinal mucosa by increasing CXC chemokine ligand 8 expression in an AP-1-dependent manner in epithelial cells

BACKGROUND

Inflammatory bowel diseases are characterized by the presence of CXCL8 at the site of lesions resulting in neutrophil recruitment and loss of tissue functions. We report that P2Y(6) receptor activation stimulates CXCL8 expression and release by intestinal epithelial cells (IECs). In this context, we investigated if uridine 5'-diphosphate (UDP) enemas stimulate neutrophil recruitment to the mucosa of mice suffering from colitis-like disease and we characterized the signaling events linking P2Y(6) to CXCL8 expression in IEC.

METHODS

Neutrophil recruitment was monitored by immunofluorescence and FACS analysis. Expression of Cxcl1, a mouse functional homolog of CXCL8, was determined by quantitative real-time polymerase chain reaction (qPCR). Pharmacological inhibitors and interfering RNAs were used to characterize the signaling pathway. The outcomes of these treatments on protein phosphorylation and on CXCL8 expression were characterized by western blots, qPCR, luciferase, and chromatin immunoprecipitation (ChIP) assays.

RESULTS

Mutation of the AP-1 site in the CXCL8 core promoter abolished the UDP-stimulating effect. The c-fos/c-jun dimer was identified as the AP-1 complex regulating CXCL8 in response to UDP stimulation. Regulation of CXCL8 expression by P2Y(6) required PKCδ activation upstream of the signaling pathway composed of MEK1/2-ERK1/2 and c-fos. UDP administration to mice suffering from colitis-like disease increased the number of neutrophil infiltrating the mucosa, correlating with Cxcl1 increased expression in IEC and the severity of inflammation.

CONCLUSIONS

This study not only describes the P2Y(6) signaling mechanism regulating CXCL8 expression in IEC, but it also illustrates the potential of targeting P2Y(6) to reduce intestinal inflammation.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Pyrimidine ribonucleotides with enhanced selectivity as P2Y(6) receptor agonists: novel 4-alkyloxyimino, (S)-methanocarba, and 5'-triphosphate gamma-ester modifications

The P2Y(6) receptor is a cytoprotective G-protein-coupled receptor (GPCR) activated by UDP (EC(50) = 0.30 microM). We compared and combined modifications to enhance P2Y(6) receptor agonist selectivity, including ribose ring constraint, 5-iodo and 4-alkyloxyimino modifications, and phosphate modifications such as alpha,beta-methylene and extension of the terminal phosphate group into gamma-esters of UTP analogues. The conformationally constrained (S)-methanocarba-UDP is a full agonist (EC(50) = 0.042 microM). 4-Methoxyimino modification of pyrimidine enhanced P2Y(6), preserved P2Y(2) and P2Y(4), and abolished P2Y(14) receptor potency, in the appropriate nucleotide. N(4)-Benzyloxy-CDP (15, MRS2964) and N(4)-methoxy-Cp(3)U (23, MRS2957) were potent, selective P2Y(6) receptor agonists (EC(50) of 0.026 and 0.012 microM, respectively). A hydrophobic binding region near the nucleobase was explored with receptor modeling and docking. UTP-gamma-aryl and cycloalkyl phosphoesters displayed only intermediate P2Y(6) receptor potency but had enhanced stability in acid and cell membranes. UTP-glucose was inactive, but its (S)-methanocarba analogue and N(4)-methoxycytidine 5'-triphospho-gamma-[1]glucose were active (EC(50) of 2.47 and 0.18 microM, respectively). Thus, the potency, selectivity, and stability of pyrimidine nucleotides as P2Y(6) receptor agonists may be enhanced by modest structural changes.

Activation of distinct P2Y receptor subtypes stimulates insulin secretion in MIN6 mouse pancreatic beta cells

Extracellular nucleotides and their receptor antagonists have therapeutic potential in disorders such as inflammation, brain disorders, and cardiovascular diseases. Pancreatic beta cells express several purinergic receptors, and reported nucleotide effects on insulin secretion are contradictory. We studied the effect of P2Y receptors on insulin secretion and cell death in MIN6, mouse pancreatic beta cells. Expression of P2Y(1) and P2Y(6) receptors was revealed by total mRNA analysis using RT-PCR. MIN6 cells were stimulated in the presence of 16.7 mM glucose with or without P2Y(1) and P2Y(6) agonists, 2-MeSADP and Up(3)U, respectively. Both the agonists increased insulin secretion with EC(50) values of 44.6+/-7.0 nM and 30.7+/-12.7 nM respectively. The insulin secretion by P2Y(1) and P2Y(6) agonists was blocked by their selective antagonists MRS2179 and MRS2578, respectively. Binding of the selective P2Y(1) receptor antagonist radioligand [125I]MRS2500 in MIN6 cell membranes was saturable (K(D) 4.74+/-0.47 nM), and known P2Y(1) ligands competed with high affinities. Inflammation and glucose toxicity lead to pancreatic beta cell death in diabetes. Flow cytometric analysis revealed that Up(3)U but not 2-MeSADP protected MIN6 cells against TNF-alpha induced apoptosis. Overall, the results demonstrate that selective stimulation of P2Y(1) and P2Y(6) receptors increases insulin secretion that accompanies intracellular calcium release, suggesting potential application of P2Y receptor ligands in the treatment of diabetes.

A comparative study of ectonucleotidase and P2 receptor mRNA profiles in C6 cell line cultures and C6 ex vivo glioma model

Glioblastoma multiforme is the most common type of primary brain tumour and has the worst clinical outcome. Nucleotides represent an important class of extracellular molecules involved in cell proliferation, differentiation and apoptosis. Alterations in purinergic signalling have been implicated in pathological processes, such as cancer, and glioma cell lines are widely employed as a model to study the biology of brain tumours. Increasing evidence, however, suggests that glioma cell lines may not present all the phenotypic and genetic characteristics of the primary tumours. We have compared the biological characteristics of C6 rat glioma cells in culture and the same cells after their implantation in the rat brain and growth in culture (denominated as the C6 ex vivo culture model). Parameters evaluated included cell morphology, differentiation, angiogenic markers, purinergic receptors and ecto-nucleotidase mRNA profile/enzymatic activity. Analysis of the C6 glioma cell line and C6 ex vivo glioma cultures revealed distinct cell morphologies, although cell differentiation and angiogenic marker expressions were similar. Both glioma models co-expressed multiple P2X and P2Y receptor subtypes with some differences. In addition, the C6 glioma cell line and C6 ex vivo glioma cultures exhibited similar extracellular ATP metabolism and cell proliferation behaviour when exposed to cytotoxic ATP concentrations. Thus, the disruption of purinergic signalling is a feature shown not only by glioma cell lineages, but also by primary glioma cultures. Our results therefore suggest the participation of the purinergic system in glioma malignancy.

Agonists and antagonists for P2 receptors

Recent work has identified nucleotide agonists selective for P2Y1, P2Y2 and P2Y6 receptors and nucleotide antagonists selective for P2Y1, P2Y12 and P2X1 receptors. Selective non-nucleotide antagonists have been reported for P2Y1, P2Y2, P2Y6, P2Y12, P2Y13, P2X(2/3)/P2X3 and P2X7 receptors. For example, the dinucleotide INS 37217 (Up4dC) potently activates the P2Y2 receptor, and the non-nucleotide antagonist A-317491 is selective for P2X(2/3)/P2X3 receptors. Nucleotide analogues in which the ribose moiety is substituted by a variety of novel ring systems, including conformationally locked moieties, have been synthesized as ligands for P2Y receptors. The focus on conformational factors of the ribose-like moiety allows the inclusion of general modifications that lead to enhanced potency and selectivity. At P2Y1,2,4,11 receptors, there is a preference for the North conformation as indicated with (N)-methanocarba analogues. The P2Y1 antagonist MRS2500 inhibited ADP-induced human platelet aggregation with an IC50 of 0.95 nM. MRS2365, an (N)-methanocarba analogue of 2-MeSADP, displayed potency (EC50) of 0.4nM at the P2Y1 receptor, with >10000-fold selectivity in comparison to P2Y12 and P2Y13 receptors. At P2Y6 receptors there is a dramatic preference for the South conformation. Three-dimensional structures of P2Y receptors have been deduced from structure activity relationships (SAR), mutagenesis and modelling studies. Detailed three-dimensional structures of P2X receptors have not yet been proposed.

Attenuation of apoptosis in vitro and ischemia/reperfusion injury in vivo in mouse skeletal muscle by P2Y6 receptor activation

Activation of the G(q)-coupled P2Y(6) receptor heterologously expressed in astrocytes significantly attenuates apoptosis induced by tumor necrosis factor alpha (TNFalpha). We have extended the analysis of P2Y(6) receptor-induced cytoprotection to mouse skeletal muscle cells endogenously expressing this receptor. The endogenous P2Y(6) receptor agonist UDP and synthetic agonist MRS2693 protected C2C12 skeletal muscle cells against apoptosis in a concentration-dependent manner (0.1-10 nM) as determined by propidium iodide staining, histochemical analysis using hematoxylin and Hoechst 33258, and DNA fragmentation. The insurmountable P2Y(6) receptor antagonist MRS2578 blocked the protection. TNFalpha-induced apoptosis in C2C12 cells correlated with activation of the transcription factor NF-kappaB. The NF-kappaB activation was attenuated by 10nM MRS2693, which activated the antiapoptic ERK1/2 pathway. In an in vivo mouse hindlimb model, MRS2693 protected against skeletal muscle ischemia/reperfusion injury. The P2Y(6) receptor is a novel cytoprotective receptor that deserves further exploration in ameliorating skeletal muscle injury.

P2Y receptor-activating nucleotides modulate cellular reactive oxygen species production in dissociated hippocampal astrocytes and neurons in culture independent of parallel cytosolic Ca(2+) rise and change in mitochondrial potential

With mixed cultures of hippocampal astrocytes and neurons, we investigated the influence of nucleotides on cytosolic Ca(2+) level, generation of reactive oxygen species (ROS), and mitochondrial potential. We employed ATP and four purine/pyrimidine derivates, which are P2Y receptor subtype-preferring agonists. Stimulation with ATP, a P2Y(1/2/4) receptor agonist in rat, caused a large cytosolic Ca(2+) increase in astrocytes and a considerably smaller Ca(2+) response in neighboring neurons. The P2Y(1) receptor antagonist MRS2179 completely blocked the ATP-induced Ca(2+) response in astrocytes and neurons. Application of ATP significantly reduced the mitochondrial potential in neurons, which was not inhibited by MRS2179. Interestingly, MRS2179 mediated a mitochondrial depolarization without affecting the cytosolic Ca(2+) level. Stimulation with UDP, a P2Y(6) receptor agonist; UTP, a P2Y(2/4) receptor agonist; 2MeSATP, a P2Y(1) receptor agonist; or 2MeSADP, a P2Y(1/12/13) receptor agonist, evoked significant Ca(2+) responses in astrocytes but small Ca(2+) responses in neurons. In astrocytes, there was an inverse relationship between the amplitude of the cytosolic Ca(2+) peak and the rate of ROS generation in response to nucleotide application. Activation with UDP resulted in the highest ROS generation that we detected, whereas 2MeSADP and 2MeSATP reduced the ROS generation below the basal level. 2MeSADP and UDP caused mitochondrial depolarization of comparable size. Thus, neither in astrocytes nor in neurons did the degree of mitochondrial depolarization correlate with ROS generation. Nucleotides acting via P2Y receptors can modulate ROS generation of hippocampal neurons without acutely changing the cytosolic Ca(2+) level. Thus, ROS might function as a signaling molecule upon nucleotide-induced P2Y receptor activation in brain.

Uracil nucleotides: from metabolic intermediates to neuroprotection and neuroinflammation

Uracil nucleotides (i.e., UTP and UDP) have been known for years as fundamental intermediates in the de novo synthesis of the other pyrimidine nucleotides, which altogether represent key building blocks for nucleic acid synthesis. In addition, their sugar conjugates (i.e., UDP-glucose and UDP-galactose) enter in several biochemical routes, for example leading to glycogen biosynthesis, and protein and lipid glycosylation, which in turn contribute to the synthesis of essential components of the cellular plasma membrane. More recently, the existence of a "pyrimidinergic transmission" has arisen from the discovery that several purinergic G protein-coupled P2Y receptors can be activated also or exclusively by uracil nucleotides and sugar conjugates. The number of these receptors is continuously growing over years with the discovery that previously "orphan" G protein-coupled receptors are actually responding to this class of molecules. Therefore, new unforeseen effects mediated by uracil derivatives have emerged, in particular in the nervous system, and previously unexplored avenues for the pharmacological manipulation of this system are currently under investigation. In this commentary we shall try to put together our current knowledge on the biochemical and receptor-mediated effects of uracil nucleotide derivatives with a specific focus on the nervous system in order to depict a clearer view of the importance of the pyrimidinergic system in both physiological and pathological conditions.

Purinergic receptor antagonism prevents cold preservation-induced cell death independent of cellular ATP levels

BACKGROUND

Purinergic (P2Y) receptors play an important role in intracellular Ca(2+) regulation in hepatocytes. Prevention of mitochondrial Ca(2+) (mCa(2+)) overload during ischemic conditions prevents cellular cell death during the early reperfusion period. P2Y antagonists are cytoprotective in other settings. We studied the effect of P2Y receptor antagonism on mitochondrial associated cell death during the period of cold storage.

METHODS

HepG2 cells were stored in UW with or without 300 muM reactive blue 2 (RB2) or 10 muM ruthenium red (RR) under either normoxic-hypothermic or hypoxic-hypothermic conditions. Cytoplasmic cytochrome c levels were studied by transfection of cytochrome c-GFP. Immunofluorescence determined the intracellular, spatio-temporal distribution of Bax, and terminal deoxynucleotidyl transferase mediated dUTP nick end labeling staining was used to evaluate cell death. Intracellular compartmental ATP levels were assayed by transfecting with luciferase vectors specific for cytoplasm (PcDNA3-luciferase-LL/V) and mitochondria (PcDNA3-COX8-luciferase).

RESULTS

Bax translocation to the mitochondria occurred immediately following cold storage and was followed by cytochrome c-GFP redistribution to the cytosol during rewarming. RB2 treatment significantly attenuated Bax translocation, cytochrome c-GFP redistribution, and cell death following both storage conditions. Both RR and RB2 provided cytoprotection despite ongoing cytoplasmic ATP consumption during cold ischemia.

CONCLUSION

These data indicate that the cytoprotective effects of mCa(2+) uptake inhibition and P2Y receptor antagonism are independent of cytoplasmic ATP levels during cold ischemia.

Multiple P2X receptors are involved in the modulation of apoptosis in human mesangial cells: evidence for a role of P2X4

Apoptosis, a normal event in renal tissue homeostasis, has been considered as a major mechanism for either resolution of glomerular hypercellularity in glomerulonephritis or loss of cellularity and progression to glomerulosclerosis in chronic renal disease. This study was aimed at investigating the role of extracellular ATP (eATP) in mediating apoptosis in human mesangial cells (HMC) and identifying the subtype(s) of purinergic receptors involved. eATP, but not uridin-5'-triphosphate (UTP), caused dose-dependent modifications of cellular morphology, as assessed by contrast-phase microscopy, and late apoptosis, as measured by Annexin V/propidium iodide-based flow cytometry and caspase-3 activation. Both phenomena were prevented by the P2X antagonist oxidized-ATP. 2', 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate (BzATP) was less effective than ATP, whereas 1[N,O-bis (5-isoquinolinesulfonyl)-N-methyl-L-tyrosyl] -4-phenylpiperazine (KN62), a selective inhibitor of human P2X(7), prevented morphological changes but potentiated apoptosis induced by BzATP. P2X(7) was barely expressed in HMC and showed a relatively scarce functional activity, as assessed by monitoring nucleotide-induced intracellular calcium surge and plasma membrane depolarization by Fura-2/AM and bis[1,3-diethylthiobarbiturate]trimethineoxonal uptake, respectively. These data indicated a negligible role of P2X(7) in eATP-mediated apoptosis and pointed to the involvement of other P2X receptor(s). Molecular and inhibitor studies suggested a main role for P2X(4) receptor in nucleotide-induced apoptosis in HMC, indicating a relevant role for purinergic signaling in regulating death rate in these cells.

A3 adenosine receptors and mitogen-activated protein kinases in lung injury following in vivo reperfusion

INTRODUCTION

Although activation of A3 adenosine receptors attenuates reperfusion lung injury and associated apoptosis, the signaling pathway that mediates this protection remains unclear. Adenosine agonists activate mitogen-activated protein kinases, and these kinases have been implicated in ischemia/reperfusion injury; the purpose of this study was therefore to determine whether A3 adenosine receptor stimulation with reperfusion modulates expression of the different mitogen-activated protein kinases. In addition, we compared the effect of the A3 adenosine agonist IB-MECA with the newly synthesized, highly selective A3 adenosine receptor agonist MRS3558 on injury in reperfused lung.

METHOD

Studies were performed in an in vivo spontaneously breathing cat model, in which the left lower lobe of the lung was isolated and subjected to 2 hours of ischemia and 3 hours of reperfusion. The selective A3 adenosine receptor agonists IB-MECA (0.05 mg/kg, 0.1 mg/kg, or 0.3 mg/kg) and MRS3558 (0.05 mg/kg or 0.1 mg/kg) were administered before reperfusion.

RESULTS

Both A3 adenosine receptor agonists administered before reperfusion markedly (P < 0.01) attenuated indices of injury and apoptosis, including the percentage of injured alveoli, wet/dry weight ratio, myeloperoxidase activity, TUNEL (in situ TdT-mediated dUTP nick end labeling)-positive cells, and caspase 3 activity and expression. The more pronounced effects at low doses were observed with MRS3558. Increases in phosphorylated c-Jun amino-terminal protein kinase (JNK), p38, and extracellular signal-regulated kinase (ERK)1/2 levels were observed by the end of reperfusion compared with controls. Pretreatment with the A3 agonists upregulated phosphorylated ERK1/2 levels but did not modify phosphorylated JNK and p38 levels.

CONCLUSION

The protective effects of A3 adenosine receptor activation are mediated in part through upregulation of phosphorylated ERK. Also, MRS3558 was found to be more potent than IB-MECA in attenuating reperfusion lung injury. The results suggest not only that enhancement of the ERK pathway may shift the balance between cell death and survival toward cell survival, but also that A3 agonists have potential as an effective therapy for ischemia/reperfusion-induced lung injury.

Regulation of death and survival in astrocytes by ADP activating P2Y1 and P2Y12 receptors

ADP is the endogenous agonist for both P2Y(1) and P2Y(12) receptors, which are important therapeutic targets. It was previously demonstrated that ADP and a synthetic agonist, 2-methylthioadenosine 5'-diphosphate (2MeSADP), can induce apoptosis by activating the human P2Y(1) receptor heterologously expressed in astrocytoma cells. However, it was not known whether the P2Y(12) receptor behaved similarly. We demonstrated here that, unlike with the G(q)-coupled P2Y(1) receptor, activation of the G(i)-coupled P2Y(12) receptor does not induce apoptosis. Furthermore, activation of the P2Y(12) receptor by either ADP or 2MeSADP significantly attenuates the tumor necrosis factor alpha (TNFalpha)-induced apoptosis in 1321N1 human astrocytoma cells. This protective effect was blocked by the P2Y(12) receptor antagonist 2-methylthioAMP and by inhibitors of phospholipase C (U73122) and protein kinase C (chelerythrin), but not by the P2Y(1) receptor antagonist MRS2179. Toward a greater mechanistic understanding, we showed that hP2Y(12) receptor activation by 10nM 2MeSADP, activates Erk1/2, Akt, and JNK by phosphorylation. However, at a lower protective concentration of 100pM 2MeSADP, activation of the hP2Y(12) receptor involves only phosphorylated Erk1/2, but not Akt or JNK. This activation is hypothesized as the major mechanism for the protective effect induced by P2Y(12) receptor activation. Apyrase did not affect the ability of TNFalpha to induce apoptosis in hP2Y(12)-1321N1 cells, suggesting that the endogenous nucleotides are not involved. These results may have important implications for understanding the signaling cascades that follow activation of P2Y(1) and P2Y(12) receptors and their opposing effects on cell death pathways.

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.

Human neutrophil peptides induce interleukin-8 production through the P2Y6 signaling pathway

Antimicrobial human neutrophil peptides (HNPs) play a pivotal role in innate host defense against a broad spectrum of prokaryotic pathogens. In addition, HNPs modulate cellular immune responses by producing the chemokine interleukin-8 (IL-8) in myeloid and epithelial cells and by exerting chemotaxis to T cells, immature dendritic cells, and monocytes. However, the mechanisms by which HNPs modulate the immune responses in the eukaryotic cells remain unclear. We demonstrated that, as with adenosine triphosphate (ATP) and uridine diphosphate (UDP), HNP stimulation of human lung epithelial cells selectively induced IL-8 production in 10 pro- and anti-inflammatory cytokines examined. HNP-induced IL-8 release was inhibited by treatment with the nucleotide receptor antagonists suramin and reactive blue. Transfection of lung epithelial cells with antisense oligonucleotides targeting specific purinergic P2Y receptors revealed that the P2Y6 (ligand of UDP) signaling pathway plays a predominant role in mediating HNP-induced IL-8 production.

Influence of CO2 pneumoperitoneum on intracellular pH and signal transduction in cancer cells

OBJECT

The authors studied the influence of CO(2) pneumoperitoneum on intracellular pH and signal transduction arising from cancer cell multiplication in laparoscopic tumor operation.

METHOD

They set up a simulation of pneumoperitoneum under different CO(2) pressure, and then measured the variation of intracellular pH (pHi) at different time and the activity of protein kinase C (PKC) and protein phosphatase 2a (PP2a) at the end of the pneumoperitoneum. After 1 week, the concentration of cancer cells in the culture medium was calculated.

RESULT

When the pressure of CO(2) pneumoperitoneum was 0, 10, 20, 30 mmHg respectively, the average pHi was 7.273, 7.075, 6.783, 6.693 at the end of the pneumoperitoneum; PKC activity was 159.4, 168.5, 178.0, 181.6 nmol/(g.min) and PP2a was 4158.3, 4066.9, 3984.0, 3878.5 nmol/(g.min) respectively. After 1 week, the cancer cells concentration was 2.15 x 10(5), 2.03 x 10(5), 2.20 x 10(5), 2.18 x 10(5) L(-1).

CONCLUSION

CO(2) pneumoperitoneum could promote acidosis in cancer cells, inducing the activation of protein kinase C and deactivation of protein phosphatase 2a, but it could not accelerate the mitosis rate of the cancer cells.

Cytoprotection against oxidative stress-induced damage of astrocytes by extracellular ATP via P2Y1 receptors

Oxidative stress is the main cause of neuronal damage in traumatic brain injury, hypoxia/reperfusion injury, and neurodegenerative disorders. Although extracellular nucleosides, especially adenosine, are well known to protect against neuronal damage in such pathological conditions, the effects of these nucleosides or nucleotides on glial cell damage remain largely unknown. We report that ATP but not adenosine protects against the cell death of cultured astrocytes induced by hydrogen peroxide (H2O2). ATP ameliorated the H2O2-induced decrease in cell viability of astrocytes in an incubation time- and concentration-dependent fashion. Protection by ATP was inhibited by P2 receptor antagonists and was mimicked by P2Y1 receptor agonists but not by adenosine. The expressions of P2Y1 mRNAs and functional P2Y1 receptors in astrocytes were confirmed. Thus, ATP, acting on P2Y1 receptors in astrocytes, showed a protective action against H2O2. The astrocytic protection by the P2Y1 receptor agonist 2-methylthio-ADP was inhibited by an intracellular Ca2+ chelator and a blocker of phospholipase C, indicating the involvement of intracellular signals mediated by Gq/11-coupled P2Y1 receptors. The ATP-induced protection was inhibited by cycloheximide, a protein synthesis inhibitor, and it took more than 12 h for the onset of the protective action. In the DNA microarray analysis, ATP induced a dramatic upregulation of various oxidoreductase genes. Taken together, ATP acts on P2Y1 receptors coupled to Gq/11, resulting in the upregulation of oxidoreductase genes, leading to the protection of astrocytes against H2O2.

P2Y6 nucleotide receptors activate NF-kappaB and increase survival of osteoclasts

Nucleotides, released from cells during inflammation and by mechanical stimulation, act through the P2 family of nucleotide receptors. Previous studies have demonstrated the expression of P2Y1 and P2Y2 receptors in osteoclasts. The aim of this study was to determine whether osteoclast P2Y receptors signal through NF-kappaB, a key transcription factor regulating osteoclastogenesis. Immunofluorescence was used to detect the p65 subunit of NF-kappaB, which upon activation translocates from the cytosol to nuclei. Low levels of NF-kappaB activation were observed in untreated rabbit osteoclasts and in those exposed to 2-methylthio ADP (P2Y1 agonist) or ATP or UTP (P2Y2 agonists). In contrast, UDP or INS48823 (P2Y6 agonists) induced a significant increase in the number of cells exhibiting NF-kappaB activation, a process sensitive to the proteasome inhibitor lactacystin. In osteoclasts purified by micromanipulation, reverse transcription-PCR revealed the presence of P2Y1, P2Y2, and P2Y6 receptor transcripts, and application of agonists for these receptors induced the transient rise of cytosolic calcium. Treatment of rat osteoclasts with UDP or INS48823, but not 2-methylthio ADP or UTP, increased osteoclast survival. Osteoprotegerin (a decoy receptor for RANK ligand) did not significantly alter the effects of UDP on NF-kappaB localization or osteoclast survival, consistent with a direct action. Moreover, SN50 (cell-permeable peptide inhibitor of NF-kappaB) suppressed the enhancement of cell survival induced by UDP and INS48823. Our findings demonstrate the presence of functional P2Y6 receptors in osteoclasts. Thus, nucleotides, following their release at sites of inflammation and mechanical stimulation, can act through P2Y6 receptors to initiate NF-kappaB signaling and enhance osteoclast survival.

Pathophysiological roles of extracellular nucleotides in glial cells: differential expression of purinergic receptors in resting and activated microglia

Microglial cells are the major cellular elements with immune function inside the CNS and play important roles in orchestrating inflammatory brain response to hypoxia and trauma. Although a complete knowledge of the endogenous factors leading to a prompt activation of microglia is not yet available, activation of P2 purinoreceptors by extracellular ATP has been indicated as a primary factor in microglial response. A still unresolved question, however, is which subtype(s) of P2 receptors mediate(s) the response to ATP. By a combination of RT-PCR, Western blotting, and single-cell calcium imaging, we assessed the presence and the activity of P2 receptor subtypes in the mouse microglial cell line N9. All members of the P2 receptor family, including the recently reported receptor for sugar nucleotides (P2Y(14)), were found to be present in these cells at mRNA and/or protein level. The functionality of the receptors was assessed by analysis of the calcium responses evoked by specific agonists both in N9 cells and in primary microglia from rat brain. Interestingly, a different functional profile of P2 receptors was observed in resting or in LPS-activated N9 cells. Overnight exposure to LPS increased P2Y(6) and P2Y(14), decreased P2X(7), and left unchanged P2Y(1) and P2Y(2,4) receptor activity. The change in the P2 receptor profile in activated cells suggests selective roles for specific P2 receptor subtypes in microglial activation triggered by LPS. We speculate that modulation of microglial cell function via subtype-selective P2 receptor ligands may open up new strategies in the therapeutic management of inflammatory neurological diseases characterized by abnormal microglia response.

Diisothiocyanate derivatives as potent, insurmountable antagonists of P2Y6 nucleotide receptors

The physiological role of the P2Y(6) nucleotide receptor may involve cardiovascular, immune and digestive functions based on the receptor tissue distribution, and selective antagonists for this receptor are lacking. We have synthesized a series of symmetric aryl diisothiocyanate derivatives and examined their ability to inhibit phospholipase C (PLC) activity induced by activation of five subtypes of recombinant P2Y receptors. Several derivatives were more potent at inhibiting action of UDP at both human and rat P2Y(6) receptors expressed in 1321N1 human astrocytes than activation of human P2Y(1), P2Y(2), P2Y(4) and P2Y(11) receptors. The inhibition by diisothiocyanate derivatives of 1,2-diphenylethane (MRS2567) and 1,4-di-(phenylthioureido) butane (MRS2578) was concentration-dependent and insurmountable, with IC(50) values of 126+/-15 nM and 37+/-16 nM (human) and 101+/-27 nM and 98+/-11 nM (rat), respectively. A derivative of 1,4-phenylendiisothiocyanate (MRS2575) inhibited only human but not rat P2Y(6) receptor activity. MRS2567 and MRS2578 at 10microM did not affect the UTP (100nM)-induced responses of cells expressing P2Y(2) and P2Y(4) receptors, nor did they affect the 2-methylthio-ADP (30nM)-induced responses at the P2Y(1) receptor or the ATP (10microM)-induced responses at the P2Y(11) receptor. Other antagonists displayed mixed selectivities. The selective antagonists MRS2567, MRS2575 and MRS2578 (1microM) completely blocked the protection by UDP of cells undergoing TNFalpha-induced apoptosis. Thus, we have identified potent, insurmountable antagonists of P2Y(6) receptors that are selective within the family of PLC-coupled P2Y receptors.