An ecto-nucleotide pyrophosphatase is one of the main enzymes involved in the extracellular metabolism of ATP in rat C6 glioma

Enzymology of extracellular NAD metabolism

Extracellular NAD represents a key signaling molecule in different physiological and pathological conditions. It exerts such function both directly, through the activation of specific purinergic receptors, or indirectly, serving as substrate of ectoenzymes, such as CD73, nucleotide pyrophosphatase/phosphodiesterase 1, CD38 and its paralog CD157, and ecto ADP ribosyltransferases. By hydrolyzing NAD, these enzymes dictate extracellular NAD availability, thus regulating its direct signaling role. In addition, they can generate from NAD smaller signaling molecules, like the immunomodulator adenosine, or they can use NAD to ADP-ribosylate various extracellular proteins and membrane receptors, with significant impact on the control of immunity, inflammatory response, tumorigenesis, and other diseases. Besides, they release from NAD several pyridine metabolites that can be taken up by the cell for the intracellular regeneration of NAD itself. The extracellular environment also hosts nicotinamide phosphoribosyltransferase and nicotinic acid phosphoribosyltransferase, which inside the cell catalyze key reactions in NAD salvaging pathways. The extracellular forms of these enzymes behave as cytokines, with pro-inflammatory functions. This review summarizes the current knowledge on the extracellular NAD metabolome and describes the major biochemical properties of the enzymes involved in extracellular NAD metabolism, focusing on the contribution of their catalytic activities to the biological function. By uncovering the controversies and gaps in their characterization, further research directions are suggested, also to better exploit the great potential of these enzymes as therapeutic targets in various human diseases.

Host CD39 Deficiency Affects Radiation-Induced Tumor Growth Delay and Aggravates Radiation-Induced Normal Tissue Toxicity

The ectonucleoside triphosphate diphosphohydrolase (CD39)/5′ ectonuclotidase (CD73)-dependent purinergic pathway emerges as promising cancer target. Yet, except for own previous work revealing a pathogenic role of CD73 and adenosine in radiation-induced lung fibrosis, the role of purinergic signaling for radiotherapy outcome remained elusive. Here we used C57BL/6 wild-type (WT), CD39 knockout (CD39^−/−), and CD73 knockout (CD73^−/−) mice and hind-leg tumors of syngeneic murine Lewis lung carcinoma cells (LLC1) to elucidate how host purinergic signaling shapes the growth of LLC1 tumors to a single high-dose irradiation with 10 Gy in vivo. In complementary in vitro experiments, we examined the radiation response of LLC1 cells in combination with exogenously added ATP or adenosine, the proinflammatory and anti-inflammatory arms of purinergic signaling. Finally, we analyzed the impact of genetic loss of CD39 on pathophysiologic lung changes associated with lung fibrosis induced by a single-dose whole-thorax irradiation (WTI) with 15 Gy. Loss of CD73 in the tumor host did neither significantly affect tumor growth nor the radiation response of the CD39/CD73-negative LLC1 tumors. In contrast, LLC1 tumors exhibited a tendency to grow faster in CD39^−/− mice compared to WT mice. Even more important, tumors grown in the CD39-deficient background displayed a significantly reduced tumor growth delay upon irradiation when compared to irradiated tumors grown on WT mice. CD39 deficiency caused only subtle differences in the immune compartment of irradiated LLC1 tumors compared to WT mice. Instead, we could associate the tumor growth and radioresistance-promoting effects of host CD39 deficiency to alterations in the tumor endothelial compartment. Importantly, genetic deficiency of CD39 also augmented the expression level of fibrosis-associated osteopontin in irradiated normal lungs and exacerbated radiation-induced lung fibrosis at 25 weeks after irradiation. We conclude that genetic loss of host CD39 alters the tumor microenvironment, particularly the tumor microvasculature, and thereby promotes growth and radioresistance of murine LLC1 tumors. In the normal tissue loss of host, CD39 exacerbates radiation-induced adverse late effects. The suggested beneficial roles of host CD39 on the therapeutic ratio of radiotherapy suggest that therapeutic strategies targeting CD39 in combination with radiotherapy have to be considered with caution.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Structure-activity relationship study of NPP1 inhibitors based on uracil-N1-(methoxy)ethyl-β-phosphate scaffold

Overexpression of ecto-nucleotide pyrophosphatase-1 (NPP1) is associated with diseases such as calcium pyrophosphate dihydrate deposition disease, calcific aortic valve disease, and type 2 diabetes. In this context, NPP1 inhibitors are potential drug candidates for the treatment of these diseases. The present study focuses on the analysis of the structure-activity relationship of NPP1 inhibitors based on acyclic uracil-nucleotides. For this purpose, we synthesized acyclic uridine-monophosphate analogs, 10-11, uridine-diphosphate analogs, 12-14, and uridine-P_α,_α-dithio-triphosphate analogs, 15-17. We evaluated their inhibitory activity and selectivity towards NPP1, -3, NTPDase1, -2, -3, and -8, and P2Y_2,4,6 receptors. Analogs 16 and 17 were the most selective and potent NPP1 inhibitors (Ki 0.94 and 0.73 μM, respectively) among the tested molecules. Analogs 10-17 had only minute effect on uracil-nucleotide responding P2Y_2,4,6 receptors. Analog 17 (100 μM) displayed 96% inhibition of NPPase activity in osteoarthritic human chondrocytes. Analogs 14-17 displayed weak inhibitory effect on alkaline phosphatase activity at equimolar concentrations in human chondrocytes. All tested analogs showed no toxicity at human chondrocytes. We concluded that ribose-ring to chain transformation, as well as the type of the nucleobase, are parameters of minor significance to NPP1 inhibition, whereas the major parameter is P_α-dithio-substitution. In addition, the length of the phosphate chain also significantly affects inhibition. Overall, the experimental results were well reproduced by molecular docking. A correlation was observed between the activities of the compounds and the number of H-bonds and salt bridges formed between the inhibitors and NPP1 binding site residues. Uracil-N1-(methoxy)ethyl-β-P_α,α-dithio, P_β,γ-methylene tri-phosphate, 17, was identified as the most potent, selective, and non-toxic NPP1 inhibitor among the tested analogs, and may be used as a lead structure for further drug development.

Coffee, caffeine, chlorogenic acid, and the purinergic system

Coffee is a drink prepared from roasted coffee beans and is lauded for its aroma and flavour. It is the third most popular beverage in the world. This beverage is known by its stimulant effect associated with the presence of methylxanthines. Caffeine, a purine-like molecule (1,3,7 trymetylxantine), is the most important bioactive compound in coffee, among others such as chlorogenic acid (CGA), diterpenes, and trigonelline. CGA is a phenolic acid with biological properties as antioxidant, anti-inflammatory, neuroprotector, hypolipidemic, and hypoglicemic. Purinergic system plays a key role inneuromodulation and homeostasis. Extracellular ATP, other nucleotides and adenosine are signalling molecules that act through their specific receptors, namely purinoceptors, P1 for nucleosides and P2 for nucleotides. They regulate many pathological processes, since adenosine, for instance, can limit the damage caused by ATP in the excitotoxicity from the neuronal cells. The primary purpose of this review is to discuss the effects of coffee, caffeine, and CGA on the purinergic system. This review focuses on the relationship/interplay between coffee, caffeine, CGA, and adenosine, and their effects on ectonucleotidases activities as well as on the modulation of P1 and P2 receptors from central nervous system and also in peripheral tissue.

UDP-sugars activate P2Y14 receptors to mediate vasoconstriction of the porcine coronary artery

Aims

UDP-sugars can act as extracellular signalling molecules, but relatively little is known about their cardiovascular actions. The P2Y₁₄ receptor is a G_i/o-coupled receptor which is activated by UDP-glucose and related sugar nucleotides. In this study we sought to investigate whether P2Y₁₄ receptors are functionally expressed in the porcine coronary artery using a selective P2Y₁₄ receptor agonist, MRS2690, and a novel selective P2Y₁₄ receptor antagonist, PPTN (4,7-disubstituted naphthoic acid derivative).

Methods and results

Isometric tension recordings were used to evaluate the effects of UDP-sugars in porcine isolated coronary artery segments. The effects of the P2 receptor antagonists suramin and PPADS, the P2Y₁₄ receptor antagonist PPTN, and the P2Y₆ receptor antagonist MRS2578, were investigated. Measurement of vasodilator-stimulated phosphoprotein (VASP) phosphorylation using flow cytometry was used to assess changes in cAMP levels. UDP-glucose, UDP-glucuronic acid UDP-N-acetylglucosamine (P2Y₁₄ receptor agonists), elicited concentration-dependent contractions of the porcine coronary artery. MRS2690 was a more potent vasoconstrictor than the UDP-sugars. Concentration dependent contractile responses to MRS2690 and UDP-sugars were enhanced in the presence of forskolin (activator of cAMP), where the level of basal tone was maintained by addition of U46619, a thromboxane A₂ mimetic. Contractile responses to MRS2690 were blocked by PPTN, but not by MRS2578. Contractile responses to UDP-glucose were also attenuated by PPTN and suramin, but not by MRS2578. Forskolin-induced VASP-phosphorylation was reduced in porcine coronary arteries exposed to UDP-glucose and MRS2690, consistent with P2Y₁₄ receptor coupling to G_i/o proteins and inhibition of adenylyl cyclase activity.

Conclusions

Our data support a role of UDP-sugars as extracellular signalling molecules and show for the first time that they mediate contraction of porcine coronary arteries via P2Y₁₄ receptors.

Enzymes involved in metabolism of extracellular nucleotides and nucleosides: functional implications and measurement of activities

Extracellular nucleotides and nucleosides mediate diverse signaling effects in virtually all organs and tissues. Most models of purinergic signaling depend on functional interactions between distinct processes, including (i) the release of endogenous ATP and other nucleotides, (ii) triggering of signaling events via a series of nucleotide-selective ligand-gated P2X and metabotropic P2Y receptors as well as adenosine receptors and (iii) ectoenzymatic interconversion of purinergic agonists. The duration and magnitude of purinergic signaling is governed by a network of ectoenzymes, including the enzymes of the nucleoside triphosphate diphosphohydrolase (NTPDase) family, the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, ecto-5'-nucleotidase/CD73, tissue-nonspecific alkaline phosphatase (TNAP), prostatic acid phosphatase (PAP) and other alkaline and acid phosphatases, adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP). Along with "classical" inactivating ectoenzymes, recent data provide evidence for the co-existence of a counteracting ATP-regenerating pathway comprising the enzymes of the adenylate kinase (AK) and nucleoside diphosphate kinase (NDPK/NME/NM23) families and ATP synthase. This review describes recent advances in this field, with special emphasis on purine-converting ectoenzymes as a complex and integrated network regulating purinergic signaling in such (patho)physiological states as immunomodulation, inflammation, tumorigenesis, arterial calcification and other diseases. The second part of this review provides a comprehensive overview and basic principles of major approaches employed for studying purinergic activities, including spectrophotometric Pi-liberating assays, high-performance liquid chromatographic (HPLC) and thin-layer chromatographic (TLC) analyses of purine substrates and metabolites, capillary electrophoresis, bioluminescent, fluorometric and electrochemical enzyme-coupled assays, histochemical staining, and further emphasizes their advantages, drawbacks and suitability for assaying a particular catalytic reaction.

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.

Highly potent and selective ectonucleotide pyrophosphatase/phosphodiesterase I inhibitors based on an adenosine 5'-(α or γ)-thio-(α,β- or β,γ)-methylenetriphosphate scaffold

Aberrant nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) activity is associated with chondrocalcinosis, osteoarthritis, and type 2 diabetes. The potential of NPP1 inhibitors as therapeutic agents, and the scarceness of their structure-activity relationship, encouraged us to develop new NPP1 inhibitors. Specifically, we synthesized ATP-α-thio-β,γ-CH2 (1), ATP-α-thio-β,γ-CCl2 (2), ATP-α-CH2-γ-thio (3), and 8-SH-ATP (4) and established their resistance to hydrolysis by NPP1,3 and NTPDase1,2,3,8 (<5% hydrolysis) (NTPDase = ectonucleoside triphosphate diphosphohydrolase). Analogues 1-3 at 100 μM inhibited thymidine 5'-monophosphate p-nitrophenyl ester hydrolysis by NPP1 and NPP3 by >90% and 23-43%, respectively, and only slightly affected (0-40%) hydrolysis of ATP by NTPDase1,2,3,8. Analogue 3 is the most potent NPP1 inhibitor currently known, Ki = 20 nM and IC50 = 0.39 μM. Analogue 2a is a selective NPP1 inhibitor with Ki = 685 nM and IC50 = 0.57 μM. Analogues 1-3 were found mostly to be nonagonists of P2Y1/P2Y2/P2Y11 receptors. Docking analogues 1-3 into the NPP1 model suggested that activity correlates with the number of H-bonds with binding site residues. In conclusion, we propose analogues 2a and 3 as highly promising NPP1 inhibitors.

Mechanisms regulating airway nucleotides

In the respiratory system, extracellular nucleotides and nucleosides serve as signaling molecules for a wide spectrum of biological functions regulating airway defenses against infection and toxic material. Their concentrations are controlled by a complex network of cell surface enzymes named ectonucleotidases. This highly integrated metabolic network combines the activities of three dephosphorylating ectonucleotidases, namely nucleoside triphosphate diphosphohydrolases (NTPDases), nucleotide pyrophosphatase/phosphodiesterases (NPPs) and alkaline phosphatases (APs). Extracellular nucleotides are also inter-converted by the transphosphorylating activities of ecto adenylate kinase (ectoAK) and nucleoside diphosphokinase (NDPK). Different cell types use specific combinations of ectonucleotidases to regulate local concentrations of P2 receptor agonists (ATP, UTP, ADP and UDP). In addition, they provide AMP for the activity of ecto 5'-nucleotidase (ecto 5'-NT; CD73), which produces the P1 receptor agonist: adenosine (ADO). Finally, mechanisms are in place to prevent the accumulation of airway ADO, namely adenosine deaminases and nucleoside transporters. This chapter reviews the properties of each enzyme and transporter, and the current knowledge on their distribution and regulation in the airways.

Stem cell characteristics in glioblastoma are maintained by the ecto-nucleotidase E-NPP1

Glioblastomas are highly aggressive brain tumours and are characterised by substantial cellular heterogeneity within a single tumour. A sub-population of glioblastoma stem-like cells (GSCs) that shares properties with neural precursor cells has been described, exhibiting resistance to therapy and therefore being considered responsible for the high recurrence rate in glioblastoma. To elucidate the underlying cellular processes we investigated the role of phosphatases in the GSC phenotype, using an in vitro phosphatome-wide RNA interference screen. We identified a set of genes, the knockdown of which induces a significant decrease in the glioma stem cell marker CD133, indicating a role in the glioblastoma stem-like phenotype. Among these genes, the ecto-nucleotidase ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) was found to be highly expressed in GSCs compared with normal brain and neural stem cells. Knockdown of ENPP1 in cultured GSCs resulted in an overall downregulation of stem cell-associated genes, induction of differentiation into astrocytic cell lineage, impairment of sphere formation, in addition to increased cell death, accumulation of cells in G1/G0 cell cycle phase and sensitisation to chemotherapeutic treatment. Genome-wide gene expression analysis and nucleoside and nucleotide profiling revealed that knockdown of ENPP1 affects purine and pyrimidine metabolism, suggesting a link between ENPP1 expression and a balanced nucleoside-nucleotide pool in GSCs. The phenotypic changes in E-NPP1-deficient GSCs are assumed to be a consequence of decreased transcriptional function of E2F1. Together, these results reveal that E-NPP1, by acting upstream of E2F1, is indispensable for the maintenance of GSCs in vitro and hence required to keep GSCs in an undifferentiated, proliferative state.

Nonhydrolyzable ATP analogues as selective inhibitors of human NPP1: a combined computational/experimental study

Elevated nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) activity is implicated in health disorders including pathological calcification. Specific NPP1 inhibitors would therefore be valuable for studying this enzyme and as potential therapeutic agents. Here we present a combined computational/experimental study characterizing 13 nonhydrolyzable ATP analogues as selective human NPP1 inhibitors. All analogues at 100 μM inhibited (66-99%) the hydrolysis of pnp-TMP by both recombinant NPP1 and cell surface NPP1 activity of osteocarcinoma (HTB-85) cells. These analogues only slightly altered the activity of other ectonucleotidases, NPP3 and NTPDases. The Ki,app values of the seven most potent and selective inhibitors were in the range of 0.5-56 μM, all with mixed type inhibition, predominantly competitive. Those molecules were docked into a newly developed homology model of human NPP1. All adopted ATP-like binding modes, suggesting competitive inhibition with the endogenous ligand. NPP1 selectivity versus NPP3 could be explained in terms of the electrostatic potential of the two proteins that of NPP1 favoring negatively charged ligands. Inhibitor 2 that had the lowest Ki,app (0.5 μM) was also inactive toward P2Y receptors. Overall, analogue 2 is the most potent and selective NPP1 inhibitor described so far.

A CD38/CD203a/CD73 ectoenzymatic pathway independent of CD39 drives a novel adenosinergic loop in human T lymphocytes

The tumor microenvironment is characterized by of high levels of extracellular nucleotides that are metabolized through the dynamic and sequential action of cell surface enzymes (ectoenzymes). These ectoenzymes operate according to their spatial arrangement, as part of (1) continuous (molecules on the same cell) or (2) discontinuous (molecules on different cells) pathways, the latter being facilitated by restricted cellular microenvironment. The outcome of this catabolic activity is an increase in the local concentration of adenosine, a nucleoside involved in the control of inflammation and immune responses. The aim of the work presented here was to demonstrate that a previously unexplored enzymatic pathway may be an alternate route to produce extracellular adenosine. Our data show that this new axis is driven by the nucleotide-metabolizing ectoenzymes CD38 (an NAD⁺ nucleosidase), the ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1, also known as CD203a or PC-1) and the 5′ ectonucleotidase (5′-NT) CD73, while bypassing the canonical catabolic pathway mediated by the nucleoside tri- and diphosphohydrolase (NTPDase) CD39. To determine the relative contributions of these cell surface enzymes to the production of adenosine, we exploited a human T-cell model allowing for the modular expression of the individual components of this alternative pathway upon activation and transfection. The biochemical analysis of the products of these ectoenzymes by high-performance liquid chromatography (HPLC) fully substantiated our working hypothesis. This newly characterized pathway may facilitate the emergence of an adaptive immune response in selected cellular contexts. Considering the role for extracellular adenosine in the regulation of inflammation and immunogenicity, this pathway could constitute a novel strategy of tumor evasion, implying that these enzymes may represent ideal targets for antibody-mediated therapy.

5'-ectonucleotidase mediates multiple-drug resistance in glioblastoma multiforme cells

Glioblastoma multiforme (GBM) cells are characterised by their extreme chemoresistance. The activity of multiple-drug resistance (MDR) transporters that extrude antitumor drugs from cells plays the most important role in this phenomenon. To date, the mechanism controlling the expression and activity of MDR transporters is poorly understood. Activity of the enzyme ecto-5'-nucleotidase (CD73) in tumor cells, which hydrolyses AMP to adenosine, has been linked to immunosuppression and prometastatic effects in breast cancer and to the proliferation of glioma cells. In this study, we identify a high expression of CD73 in surgically resected samples of human GBM. In primary cultures of GBM, inhibition of CD73 activity or knocking down its expression by siRNA reversed the MDR phenotype and cell viability was decreased up to 60% on exposure to the antitumoral drug vincristine. This GBM chemosensitization was caused by a decrease in the expression and activity of the multiple drug associated protein 1 (Mrp1), the most important transporter conferring multiple drug resistance in these cells. Using pharmacological modulators, we have recognized the adenosine A(3) receptor subtype in mediation of the chemoresistant phenotype in these cells. In conclusion, we have determined that the activity of CD73 to trigger adenosine signaling sustains chemoresistant phenotype in GBM cells.

Purinergic signaling in glioma progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X(7), is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Physical training normalizes nucleotide hydrolysis and biochemical parameters in blood serum from streptozotocin-diabetic rats

Ectonucleotidases and the nucleotide metabolism have been implicated as important regulators in diabetes disease. We evaluated the ectonucleotidase activities and biochemical parameters in blood serum of streptozotocin (STZ)-induced diabetic rats submitted a physical training protocol. We observed a raise in ATP, ADP, AMP and p-Nph-5'-TMP hydrolysis rate and in the levels of cholesterol and triglycerides in rat blood serum, after 30 days of diabetes induction. However, in serum of rats submitted a physical training protocol by forced swimming, both the nucleotide hydrolysis rate and the lipids levels returned to the control values. Considering that diabetes leads to multiple pathophysiological alterations, the modulations observed in ectonucleotidase activities may be part of the events involved in these alterations. Then the physical training is a very important way to control the vascular alterations developed in diabetes.

Characterization of ectonucleotidases in human medulloblastoma cell lines: ecto-5'NT/CD73 in metastasis as potential prognostic factor

Medulloblastoma (MB) is the most common malignant brain tumor in children and occurs mainly in the cerebellum. Important intracellular signaling molecules, such those present in the Sonic Hedgehog and Wnt pathways, are involved in its development and can also be employed to determine tumor grade and prognosis. Ectonucleotidases, particularly ecto-5'NT/CD73, are important enzymes in the malignant process of different tumor types regulating extracellular ATP and adenosine levels. Here, we investigated the activity of ectonucleotidases in three malignant human cell lines: Daoy and ONS76, being representative of primary MB, and the D283 cell line, derived from a metastatic MB. All cell lines secreted ATP into the extracellular medium while hydrolyze poorly this nucleotide, which is in agreement with the low expression and activity of pyrophosphate/phosphodiesterase, NTPDases and alkaline phosphatase. The analysis of AMP hydrolysis showed that Daoy and ONS76 completely hydrolyzed AMP, with parallel adenosine production (Daoy) and inosine accumulation (ONS76). On the other hand, D283 cell line did not hydrolyze AMP. Moreover, primary MB tumor cells, Daoy and ONS76 express the ecto-5'NT/CD73 while D283 representative of a metastatic tumor, revealed poor expression of this enzyme, while the ecto-adenosine deaminase showed higher expression in D283 compared to Daoy and ONS76 cells. Nuclear beta-catenin has been suggested as a marker for MB prognosis. Further it can promotes expression of ecto-5'NT/CD73 and suppression of adenosine deaminase. It was observed that Daoy and ONS76 showed greater nuclear beta-catenin immunoreactivity than D283, which presented mainly cytoplasmic immunoreactivity. In summary, the absence of ecto-5'NT/CD73 in the D283 cell line, a metastatic MB phenotype, suggests that high expression levels of this ectonucleotidase could be correlated with a poor prognosis in patients with MB.

Ectonucleotidases in tumor cells and tumor-associated immune cells: an overview

Increasing evidence points out that genetic alteration does not guarantee the development of a tumor and indicates that complex interactions of tumor cells with the microenvironment are fundamental to tumorigenesis. Among the pathological alterations that give tumor cells invasive potential, disruption of inflammatory response and the purinergic signaling are emerging as an important component of cancer progression. Nucleotide/nucleoside receptor-mediated cell communication is orchestrated by ectonucleotidases, which efficiently hydrolyze ATP, ADP, and AMP to adenosine. ATP can act as danger signaling whereas adenosine, acts as a negative feedback mechanism to limit inflammation. Many tumors exhibit alterations in ATP-metabolizing enzymes, which may contribute to the pathological events observed in solid cancer. In this paper, the main changes occurring in the expression and activity of ectonucleotidases in tumor cells as well as in tumor-associated immune cells are discussed. Furthermore, we focus on the understanding of the purinergic signaling primarily as exemplified by research done by the group on gliomas.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

Overexpression of NTPDase2 in gliomas promotes systemic inflammation and pulmonary injury

Gliomas are the most common and devastating type of primary brain tumor. Many non-neoplastic cells, including immune cells, comprise the tumor microenvironment where they create a milieu that appears to dictate cancer development. ATP and the phosphohydrolytic products ADP and adenosine by activating P2 and P1 receptors may participate in these interactions among malignant and immune cells. Purinergic receptor-mediated cell communication is closely regulated by ectonucleotidases, such as by members of the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family, which hydrolyze extracellular nucleotides. We have shown that gliomas, unlike astrocytes, exhibit low NTPDase activity. Furthermore, ATP induces glioma cell proliferation and the co-administration of apyrase decreases progression of injected cells in vivo. We have previously shown that NTPDase2 reconstitution dramatically increases tumor growth in vivo. Here we evaluated whether NTPDase2 reconstitution to gliomas modulates systemic inflammatory responses. We observed that NTPDase2 overexpression modulated pro-inflammatory cytokine production and platelet reactivity. Additionally, pathological alterations in the lungs were observed in rats bearing these tumors. Our results suggest that disruption of purinergic signaling via ADP accumulation creates an inflammatory state that may promote tumor spread and dictate clinical progression.

Impact of ectoenzymes on p2 and p1 receptor signaling

P2 receptors that are activated by extracellular nucleotides (e.g., ATP, ADP, UTP, UDP, Ap(n)A) and P1 receptors activated by adenosine control a diversity of biological processes. The activation of these receptors is tightly regulated by ectoenzymes that metabolize their ligands. This review presents these enzymes as well as their roles in the regulation of P2 and P1 receptor activation. We focus specifically on the role of ectoenzymes in processes of our interest, that is, inflammation, vascular tone, and neurotransmission. An update on the development of ectonucleotidase inhibitors is also presented.

Inactivation of membrane surface ecto-5'-nucleotidase by sodium nitroprusside in C6 glioma cells

Ecto-5'-nucleotidase (NT5E), a predominant enzyme that produces extracellular adenosine from AMP, plays an important role in a variety of physiological and pathophysiological processes. This study was performed to identify agents that affect NT5E activity using C6 glioma cells. When cells were incubated with sodium nitroprusside (SNP), phorbol 12-myristate 13-acetate, forskolin, lipopolysaccharide, or interferon-γ, only SNP inhibited NT5E activity in a time- and concentration-dependent manner (IC(50) = 1.2 µM). The inhibitory effect of SNP was long-lasting even after SNP washout; and its action was not mimicked by nitric oxide generating agents, 8-bromo cyclic GMP, ferricyanide, ferrocyanide, or sodium cyanide. SNP did not change NT5E mRNA level or membrane surface protein expression. Similar to SNP, Fe(2+) inhibited NT5E activity, but to a lesser extent. Although Fe(2+) is known to increase oxidative stress, Fe(2+)-mediated oxidative stress was not involved in SNP inhibition of NT5E because the inhibition of NT5E by SNP was not affected by superoxide dismutase and catalase. In contrast, addition of Zn(2+), an essential metal co-factor of NT5E activity, prevented SNP from inhibiting NT5E. These results suggest that SNP disrupts a critical Zn(2+)-dependent enzyme activity and might be useful as a pharmacological tool for inhibiting NT5E.

ATP hydrolysis pathways and their contributions to pial arteriolar dilation in rats

ATP is thought to be released to the extracellular compartment by neurons and astrocytes during neural activation. We examined whether ATP exerts its effect of promoting pial arteriolar dilation (PAD) directly or upon conversion (via ecto-nucleotidase action) to AMP and adenosine. Blockade of extracellular direct ATP to AMP conversion, with ARL-67156, significantly reduced sciatic nerve stimulation-evoked PADs by 68%. We then monitored PADs during suffusions of ATP, ADP, AMP, and adenosine in the presence and absence of the following: 1) the ecto-5'-nucleotidase inhibitor α,β-methylene adenosine 5'-diphosphate (AOPCP), 2) the A(2) receptor blocker ZM 241385, 3) the ADP P2Y(1) receptor antagonist MRS 2179, and 4) ARL-67156. Vasodilations induced by 1 and 10 μM, but not 100 μM, ATP were markedly attenuated by ZM 241385, AOPCP, and ARL-67156. Substantial loss of reactivity to 100 μM ATP required coapplications of ZM 241385 and MRS 2179. Dilations induced by ADP were blocked by MRS 2179 but were not affected by either ZM 241385 or AOPCP. AMP-elicited dilation was partially inhibited by AOPCP and completely abolished by ZM 241385. Collectively, these and previous results indicate that extracellular ATP-derived adenosine and AMP, via A(2) receptors, play key roles in neural activation-evoked PAD. However, at high extracellular ATP levels, some conversion to ADP may occur and contribute to PAD through P2Y(1) activation.

Pathways and subcellular compartmentation of NAD biosynthesis in human cells: from entry of extracellular precursors to mitochondrial NAD generation

NAD is a vital redox carrier, and its degradation is a key element of important regulatory pathways. NAD-mediated functions are compartmentalized and have to be fueled by specific biosynthetic routes. However, little is known about the different pathways, their subcellular distribution, and regulation in human cells. In particular, the route(s) to generate mitochondrial NAD, the largest subcellular pool, is still unknown. To visualize organellar NAD changes in cells, we targeted poly(ADP-ribose) polymerase activity into the mitochondrial matrix. This activity synthesized immunodetectable poly(ADP-ribose) depending on mitochondrial NAD availability. Based on this novel detector system, detailed subcellular enzyme localizations, and pharmacological inhibitors, we identified extracellular NAD precursors, their cytosolic conversions, and the pathway of mitochondrial NAD generation. Our results demonstrate that, besides nicotinamide and nicotinic acid, only the corresponding nucleosides readily enter the cells. Nucleotides (e.g. NAD and NMN) undergo extracellular degradation resulting in the formation of permeable precursors. These precursors can all be converted to cytosolic and mitochondrial NAD. For mitochondrial NAD synthesis, precursors are converted to NMN in the cytosol. When taken up into the organelles, NMN (together with ATP) serves as substrate of NMNAT3 to form NAD. NMNAT3 was conclusively localized to the mitochondrial matrix and is the only known enzyme of NAD synthesis residing within these organelles. We thus present a comprehensive dissection of mammalian NAD biosynthesis, the groundwork to understand regulation of NAD-mediated processes, and the organismal homeostasis of this fundamental molecule.

NO-induced activation of cyclic GMP-dependent pathway down regulates ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) protein in rat C6 glioma

In rat C6 glioma cells, the ecto-nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1), a modulator of purinergic receptor signaling, is down regulated after an increase in intracellular cAMP by addition of dibutyryl cAMP, a membrane-permeable cAMP-analog, or by activation of the β-adrenoceptor receptor with (-)-isoproterenol (Aerts et al., 2011, Eur. J. Pharmacol. 654, 1-9). In this communication we studied the effect of nitric oxide (NO)/cGMP, a pathway also affecting purinergic receptor signaling, on the level of NPP1 protein. Sodium nitroprusside (SNP), a NO donor, reduces NPP1 protein in a dose-dependent manner. A combination of SNP and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, an inhibitor of soluble guanylate cyclase, demonstrated that NO-dependent down regulation of NPP1 was caused by NO-sensitive guanylyl cyclase. Treatment with Rp-pCPT-cGMPS, an inhibitor of protein kinase G (PKG), showed that PKG is not involved in the down regulation of NPP1. In addition, we have shown that the cAMP- and cGMP-dependent decrease in NPP1 expression is unrelated. These results indicate that NO/cGMP regulates the level of NPP1 protein by a pathway that differs from the cAMP-induced decrease in NPP1.

Cyclic AMP-dependent down regulation of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) in rat C6 glioma

In this communication, we demonstrate that an increase in intracellular cAMP by 1) addition of dibutyrylic cAMP (dbcAMP), a membrane-permeable cAMP-analogue, or 2) activation of the β-adrenoceptor with (-)-isoproterenol, down regulates the levels of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) mRNA, NPP1 protein and ecto-NPPase activity in rat C6 glioma cells. DbcAMP and (-)-isoproterenol inhibit NPP1 expression in a time and dose-dependent manner. After 48h of stimulation, 1mM dbcAMP or 5μM (-)-isoproterenol decreases the amount of NPP1 protein by 75±3% and 81±1% respectively. Contrary to down regulation of NPP1, we observe an up regulation of glial fibrillary acidic protein (GFAP), a differentiation marker for astrocytic cells. Using specific inhibitors and activators, we have shown that Ca(2+), PKA, PI 3-K/PKB/GSK-3, Epac/Rap1/PP2A and MAP kinase modules are not involved in the inhibition of NPP1 gene expression. The transcription factor c-jun is significantly reduced while c-fos becomes up regulated after cAMP elevation. However an electrophoretic mobility shift assay with the activator protein-1 motif present in the promoter of the rat NPP1 gene indicates that this motif is not involved in the cAMP-dependent inhibition of NPP1 expression. In conclusion, these results indicate that intracellular cAMP levels regulate the expression of NPP1 in rat C6 glioma cells by a signalling pathway that is different from the GFAP signal transduction pathway.

Diadenosine 5',5''-(boranated)polyphosphonate analogues as selective nucleotide pyrophosphatase/phosphodiesterase inhibitors

Nucleotide pyrophosphatase/phosphodiesterases (NPPs) hydrolyze extracellular nucleotides and dinucleotides and thus control purinergic signaling. Enhanced NPP activity is implicated in health disorders such as osteoarthritis and cancer. We designed novel diadenosine polyphosphonate derivatives as potential NPP inhibitors. Analogues 1-4 bear a phosphonate and/or boranophosphate group and/or a 2'-H atom instead of a 2'-OH group. In comparison to ATP, analogues 1-4 were barely hydrolyzed by human NTPDase1, -2, -3, and -8 (<5% hydrolysis) and NPP1 and -3 (≤ 13%) and were not hydrolyzed by ecto-5'-nucleotidase, unlike AMP. These derivatives did not affect NTPDase activity, and analogues 1 and 2 did not inhibit ecto-5'-nucleotidase. All analogues blocked ∼80% of the NPP2-dependent hydrolysis of pnp-TMP, a specific NPP substrate, and inhibited the catabolism of pnp-TMP (K(i) and IC₅₀ both found to be between 10 and 60 μM), Ap₅A, and ATP by NPP1. The activity of NPP3 was inhibited to a lesser extent by the new analogues, with compounds 1 and 4 being the most effective in that respect. The analogues dramatically reduced the level of hydrolysis of pnp-TMP at the cell surface of both osteocarcinoma and colon cancer cells. Importantly, analogues 1-4 exhibited significantly reduced agonistic activity toward human P2Y₁,₁₁) receptors (except for analogue 1) and no activity with human P2Y₂ receptor. Our data provide strong evidence that analogue 2 is the first specific NPP inhibitor to be described.

Caffeine and the control of cerebral hemodynamics

While the influence of caffeine on the regulation of brain perfusion has been the subject of multiple publications, the mechanisms involved in that regulation remain unclear. To some extent, that uncertainty is a function of a complex interplay of processes arising from multiple targets of caffeine located on a variety of different cells, many of which have influence, either directly or indirectly, on cerebral vascular smooth muscle tone. Adding to that complexity are the target-specific functional changes that may occur when comparing acute and chronic caffeine exposure. In the present review, we discuss some of the mechanisms behind caffeine influences on cerebrovascular function. The major effects of caffeine on the cerebral circulation can largely be ascribed to its inhibitory effects on adenosine receptors. Herein, we focus mostly on the A1, A2A, and A2B subtypes located in cells comprising the neurovascular unit (neurons, astrocytes, vascular smooth muscle); their roles in the coupling of increased neuronal (synaptic) activity to vasodilation; how caffeine, through blockade of these receptors, may interfere with the "neurovascular coupling" process; and receptor-linked changes that may occur in cerebrovascular regulation when comparing acute to chronic caffeine intake.

ATP-loaded liposomes for treatment of myocardial ischemia

A major obstacle to drug therapy for treatment of potential myocardial infarction is the limited access to the ischemic myocardium by drugs in an active form. Encouraging results have been reported with liposomes loaded with ATP in a variety of in vitro and in vivo models. We describe methods for optimized encapsulation of ATP in liposomes, enhancement of their effectiveness by increasing circulation time, and targeting of injured myocardial cells with surface attached antimyosin. In isolated ischemic rat hearts, ATP-loaded liposomes and ATP-loaded immunoliposomes effectively protected myocardium from ischemia/reperfusion damage as measured by systolic and diastolic functional improvements. In vivo, in rabbits with induced localized myocardial ischemia, liposomal encapsulation of ATP significantly diminished the proportion of ventricular muscle at risk that was irreversibly damaged during reperfusion. Therefore, ATP encapsulated in liposomes can provide an effective exogenous source for in vivo application which can protect ischemically damaged hearts.

Biochemical characterization of soluble nucleotide pyrophosphatase/phosphodiesterase activity in rat serum

Biochemical properties of nucleotide pyrophosphatase/phosphodiesterase (NPP) in rat serum have been described by assessing its nucleotide phosphodiesterase activity, using p-nitrophenyl-5'-thymidine monophosphate (p-Nph-5'-TMP) as a substrate. It was demonstrated that NPP activity shares some typical characteristics described for other soluble NPP, such as divalent cation dependence, strong alkaline pH optimum (pH 10.5), inhibition by glycosaminoglycans, and K (m) for p-Nph-5'-TMP hydrolysis of 61.8 +/- 5.2 microM. In order to characterize the relation between phosphodiesterase and pyrophosphatase activities of NPP, we have analyzed the effects of different natural nucleotides and nucleotide analogs. ATP, ADP, and AMP competitively inhibited p-Nph-5'-TMP hydrolysis with K (i) values ranging 13-43 microM. Nucleotide analogs, alpha,beta-metATP, BzATP, 2-MeSATP, and dialATP behaved as competitive inhibitors, whereas alpha,beta-metADP induced mixed inhibition, with K (i) ranging from 2 to 20 microM. Chromatographic analysis revealed that alpha,beta-metATP, BzATP, and 2-MeSATP were catalytically degraded in the serum, whereas dialATP and alpha,beta-metADP resisted hydrolysis, implying that the former act as substrates and the latter as true competitive inhibitors of serum NPP activity. Since NPP activity is involved in generation, breakdown, and recycling of extracellular adenine nucleotides in the vascular compartment, the results suggest that both hydrolyzable and non-hydrolyzable nucleotide analogs could alter the amplitude and direction of ATP actions and could have potential therapeutic application.

Specific requirements for Vgamma9Vdelta2 T cell stimulation by a natural adenylated phosphoantigen

Human Vgamma9Vdelta2 T lymphocytes recognize phosphorylated alkyl Ags. Isopentenyl pyrophosphate (IPP) was previously proposed as the main Ag responsible for Vgamma9Vdelta2 T cell activation by cancer cells. However, triphosphoric acid 1-adenosin-5'-yl ester 3-(3-methylbut-3-enyl) ester (ApppI), a metabolite in which the isopentenyl moiety is linked to ATP, was reported in cells activated with aminobisphosphonates. The contribution of this compound to tumor-stimulatory activity was thus examined. ApppI induces selective expansion of Vgamma9Vdelta2 T cells from PBMCs. In the absence of APCs, however, ApppI has little stimulatory activity on Vgamma9Vdelta2 T cells, and optimal activation with ApppI requires addition of a nucleotide pyrophosphatase releasing IPP plus AMP. Thus, ApppI has no intrinsic stimulatory activity. Nevertheless, stimulation by ApppI is strengthened by the presence of APCs. Moreover, in contrast to IPP, ApppI can be efficiently pulsed on dendritic cells as well as on nonprofessional APCs. Pulsed APCs display stable and phosphatase-resistant stimulatory activity, indicative of Ag modification. HPLC analysis of tumor cell extracts indicates that latent phosphoantigenic activity is stored intracellularly in the Vgamma9Vdelta2 cell-sensitive tumor Daudi and can be activated by a nucleotide pyrophosphatase activity. The presence of ApppI in Daudi cell extracts was demonstrated by mass spectrometry. Nucleotidic Ags such as ApppI are thus a storage form of phosphoantigen which may represent a major source of phosphoantigenic activity in tumor cells. The unique properties of ApppI may be important for the design of Ags used in anticancer immunotherapeutic protocols using Vgamma9Vdelta2 cells.

Activity and expression of ecto-nucleotide pyrophosphate/phosphodiesterases in a hepatic stellate cell line

Nucleotides and nucleosides represent an important and ubiquitous class of molecules that interact with specific receptors, regulate a variety of activities within the liver, and play a role in the pathogenesis of hepatic fibrosis. Ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPPs) are ecto-enzymes that are located on the cell surface. NPP1, NPP2, and NPP3 (abbreviated as NPP1-3 hereafter) have been implicated in the hydrolysis of nucleotides; together with other ecto-nucleotidases, they control the events induced by extracellular nucleotides. We have identified and compared the expression of E-NPP family members in two different phenotypes of the mouse hepatic stellate cell line (GRX). In quiescent-like hepatic stellate cells (HSCs), E-NPP activity was significantly higher, NPP2 mRNA expression decreased and NPP3 mRNA increased. The differential NPP activity and expression in two phenotypes of GRX cells suggests that they are involved in the regulation of extracellular nucleotide metabolism in HSCs. However, the role of E-NPPs in the liver remains to be clarified.

Relative contribution of ecto-ATPase and ecto-ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

BACKGROUND AND PURPOSE

The relative contribution of distinct ecto-nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

EXPERIMENTAL APPROACH

Extracellular ATP catabolism (assessed by high-performance liquid chromatography) and its influence on [(3)H]acetylcholine ([(3)H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

KEY RESULTS

ATP was primarily metabolized via ecto-ATPDase (adenosine 5'-triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto-5'-nucleotidase. Alternative conversion of ATP into ADP by ecto-ATPase (adenosine 5'-triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [(3)H]ACh outflow in a 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP)-dependent, tetrodotoxin-independent manner. ATP and ATPgammaS (adenosine 5'-[gamma-thio]triphosphate), but not alpha,beta-methyleneATP, decreased [(3)H]ACh release induced by electrical stimulation. ADP and ADPbetaS (adenosine 5'[beta-thio]diphosphate) only decreased evoked [(3)H]ACh release. Inhibition by ADPbetaS was prevented by MRS 2179 (2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt, a selective P2Y(1) antagonist); blockade of ADP inhibition required co-application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentyl xanthine enhanced ADPbetaS inhibition, indicating that P2Y(1) stimulation is cut short by tonic adenosine A(1) receptor activation. MRS 2179 facilitated evoked [(3)H]ACh release, an effect reversed by the ecto-ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

CONCLUSIONS AND IMPLICATIONS

ATP transiently facilitated [(3)H]ACh release from non-stimulated nerve terminals via prejunctional P2X (probably P2X(2)) receptors. Hydrolysis of ATP directly into AMP by ecto-ATPDase and subsequent formation of adenosine by ecto-5'-nucleotidase reduced [(3)H]ACh release via inhibitory adenosine A(1) receptors. Stimulation of inhibitory P2Y(1) receptors by ADP generated alternatively via ecto-ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto-ATPDase activity.

Astrocytic alkalinization by therapeutically relevant lithium concentrations: implications for myo-inositol depletion

RATIONALE

One theory for therapeutic effects of the lithium ion (Li+) in bipolar disorder is that myo-inositol, needed for phospholipase C-mediated signaling, is depleted by Li(+)-induced inhibition of inositolphosphate hydrolysis or of myo-inositol uptake, an effect demonstrated in cultured mouse astrocytes at high myo-inositol concentrations. In contrast, myo-inositol uptake is inhibited at low concentrations, reflecting that it occurs both by the high-affinity Na(+)-dependent myo-inositol transporter (SMIT) and the lower-affinity H(+)-dependent inositol transporter (HMIT). Increased intracellular pH (pHi) stimulates SMIT but inhibits HMIT, suggesting that the effect of Li+ could be caused by intracellular alkalinization. In this study, we therefore investigated Li+ effects on intracellular pH in astrocytes, measured by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence.

RESULTS

Chronic treatment with the therapeutically relevant Li+ concentration of 1 mM for 2 or 3 weeks increased pHi by approximately 0.10, whereas 0.5 mM was ineffective, and 2 mM caused a larger increase. The alkalinization resulted from acute stimulation of the Na+/H+ exchanger (NHE) by extracellular Li+, demonstrated after acid load with NH4Cl. In response to continuous stimulation, NHE1 mRNA was down-regulated, but protein was not.

CONCLUSIONS

Chronic treatment with pharmacologically relevant Li+ concentrations increases pHi in astrocytes, creating conditions for decreased uptake of high myo-inositol concentrations and increased uptake of low concentrations. The pharmacological relevance of this effect is supported by literature data suggesting brain acidosis in bipolar patients and by preliminary observations that carbamazepine and valproate also increase pHi in astrocytes. Stimulation of NHE1-stimulated sodium ion uptake might also trigger uptake of chloride ions and osmotically obliged water.

Biochemical characterization of ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP, E.C. 3.1.4.1) from rat heart left ventricle

In the present study we investigate the biochemical properties of the members of NPP family in synaptosomes prepared from rat heart left ventricles. Using p-nitrophenyl-5'-thymidine monophosphate (p-Nph-5'-TMP) as substrate for E-NPPs in rat cardiac synaptosomes, we observed an alkaline pH dependence, divalent cation dependence and the K ( M ) value corresponded to 91.42 +/- 13.97 microM and the maximal velocity (V ( max )) value calculated was 63.79 +/- 3.59 nmol p-nitrophenol released/min/mg of protein (mean +/- SD, n = 4). Levamisole (1 mM), was ineffective as inhibitor of p-Nph-5'-TMP hydrolysis in pH 8.9 (optimum pH for the enzyme characterized). Suramin (0.25 mM) strongly reduced the hydrolysis of p-Nph-5'-TMP by about 46%. Sodium azide (10 and 20 mM) and gadolinium chloride (0.3 and 0.5 mM), E-NTPases inhibitors, had no effects on p-Nph-5'-TMP hydrolysis. RT-PCR analysis of left ventricle demonstrated the expression of NPP2 and NPP3 enzymes, but excluded the presence of NPP1 member. By quantitative real-time PCR we identified the NPP3 as the enzyme with the highest expression in rat left ventricle. The demonstration of the presence of the E-NPP family in cardiac system, suggest that these enzymes could contribute with the fine-tuning control of the nucleotide levels at the nerve terminal endings of left ventricles that are involved in several cardiac pathologies.

ATP release and hydrolysis contribute to rat pial arteriolar dilatation elicited by neuronal activation

Owing to their intimate anatomical relationship with cerebral arterioles, astrocytes have been postulated as signal transducers, transferring information from activated neurones to the cerebral microcirculation. These forwarded signals may involve the release of vasoactive factors from the end-feet of astrocytes. This mechanism is termed 'neurovascular coupling' and its anatomical components (i.e. neurone, astrocyte and vascular cells) are termed the 'neurovascular unit'. The process of neurovascular coupling often involves upstream dilatation. This is necessary during periods of increased metabolic demand, in order to permit more blood to reach dilated downstream vessels, thereby improving nutrient supply to the activated neurones. Without it, that downstream dilatation might be ineffective, placing neurones at risk, especially during episodes of intense neuronal activity, such as seizure. In the brain, pial arterioles represent important 'upstream' vascular segments. The pial arterioles overlie a thick layer of astrocytic processes, termed the glia limitans. This essentially isolates pial arterioles, anatomically, from the neurones below. Vasodilating signals that originate in the neurones therefore reach the pial arterioles via indirect pathways, primarily involving astrocytes and the glia limitans. Here we discuss a process whereby purinergic mechanisms play a key and neuronal activity-dependent role in astrocyte to astrocyte communication, as well as in glia limitans to pial arteriolar signals leading to vasodilatation.

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

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

Biochemical analysis of ecto-nucleotide pyrophosphatase phosphodiesterase activity in brain membranes indicates involvement of NPP1 isoenzyme in extracellular hydrolysis of diadenosine polyphosphates in central nervous system

Synaptosomes and plasma membranes obtained from rat brain display ectoenzymatic hydrolytic activity responsible for hydrolysis of the neurotransmitter/neuroregulatory nucleotides diadenosine polyphosphates. Intact synaptosomes and plasma and synaptic membranes isolated by sucrose-gradient ultracentrifugation from several brain regions (hypothalamus, hippocampus, temporal cortex, frontal cortex striatum and cerebellum) degraded the fluorogenic substrates diethenoadenosine polyphosphates up to ethenoadenosine as by-product. Purified ectoenzyme cleaved substrates always releasing the mononucleotide moieties ethenoadenosine 5'-monophosphate and the corresponding ethenoadenosine (n-1) 5'-phosphate. Ectoenzymatic hydrolysis reached maximal activity at pH 9.0 (pH range 6.5-9.0) and was activated by Ca(2+) and Mg(2+) ions, with maximal effects around 2.0 mM cation. EDTA drastically reduced activity and Zn(2+) was required for enzyme reactivation. Hydrolysis of substrates followed hyperbolic kinetics with K(m) values in the 3-10 microM range. Diadenosine polyphosphates and heparin behaved as competitive inhibitors in the enzymatic hydrolysis of diethenoadenosine polyphosphates and AMP, ATP, alpha,beta-methyleneADP, ADPbetaS ATPgammaS, beta,gamma-methyleneATP, suramin and diethyl pyrocarbonate were also inhibitors. Ectoenzymatic activity shared the typical characteristics of members of the ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family and inhibition data suggest that NPP1 ectoenzyme is involved in the cleavage of extracellular diadenosine polyphosphates in brain. Synaptic membranes from cerebellum, hypothalamus and hippocampus presented the highest activities and no activity differences were observed between young and aged animals. However, plasma membranes showed a more homogeneous distribution of ectoenzymatic activity but a general increase was detected in aged animals. Enhancement of ectoenzymatic diadenosine polyphosphate cleaving activity found in plasma membranes from old animals could play a deleterious role in aged brain by limiting neuroprotective effects reported for extracellular diadenosine tetraphosphate.

Immunolocalization of ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) in the rat forebrain

Immunohistochemical study was performed to determine distribution of ecto-nucleotide pyrophosphatase/phosphodiesterase1 (NPP1) in adult rat forebrain. The study revealed widespread distribution of NPP1 in rat forebrain, yet with regional differences in the expression pattern and abundance. Strong NPP1 immunoreaction was detected in pyramidal cell layer of cerebral cortex and hippocampus, and in the midline regions of hypothalamus and thalamus. In many immunopositive forebrain areas, NPP1 was mainly localized at neuronal cell bodies. However, prominent immunoreaction was also detected at ependymal cells, tanycytes, endothelial cells of the capillaries and cells of the choroid plexus, suggesting that NPP1 could be involved in some highly specialized transport process.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

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.

Modulation of purinergic signaling by NPP-type ectophosphodiesterases

Extracellular nucleotides can elicit a wide array of cellular responses by binding to specific purinergic receptors. The level of ectonucleotides is dynamically controlled by their release from cells, synthesis by ectonucleoside diphosphokinases and ectoadenylate kinases, and hydrolysis by ectonucleotidases. One of the four structurally unrelated families of ectonucleotidases is represented by the NPP-type ectophosphodiesterases. Three of the seven members of the NPP family, namely NPP1–3, are known to hydrolyze nucleotides. The enzymatic action of NPP1–3 (in)directly results in the termination of nucleotide signaling, the salvage of nucleotides and/or the generation of new messengers like ADP, adenosine or pyrophosphate. NPP2 is unique in that it hydrolyzes both nucleotides and lysophospholipids and, thereby, generates products that could synergistically promote cell motility. We review here the enzymatic properties of NPPs and analyze current evidence that links their nucleotide-hydrolyzing capability to epithelial and neural functions, the immune response and cell motility.

Nucleoside triphosphate diphosphohydrolase-2 (NTPDase2/CD39L1) is the dominant ectonucleotidase expressed by rat astrocytes

Inflammatory and degenerative pathophysiological processes within the CNS are important causes of human disease. Astrocytes appear to modulate these reactions and are a major source of inflammatory mediators, e.g. extracellular adenine nucleotides, in nervous tissues. Actions following extracellular nucleotides binding to type 2 purinergic receptors are regulated by ectonucleotidases, including members of the CD39/ecto-nucleoside triphosphate diphosphohydrolase family. The ectonucleotidases of astrocytes expressed by rat brain rapidly convert extracellular ATP to ADP, ultimately to AMP. RT-PCR, immunocytochemistry as well as Western blotting analysis demonstrated expression of multiple ecto-nucleoside triphosphate diphosphohydrolase family members at both the mRNA and protein level. By quantitative real-time PCR, we identified Entpd2 (CD39L1) as the dominant Entpd gene expressed by rat hippocampal, cortical and cerebellar astrocytes. These data in combination with the elevated ecto-ATPase activity observed in these brain regions, suggest that NTPDase2, an ecto-enzyme that preferentially hydrolyzes ATP, is the major ecto-nucleoside triphosphate diphosphohydrolase expressed by rat astrocytes. NTPDase2 may modulate inflammatory reactions within the CNS and could represent a useful therapeutic target in human disease.

Human keratinocytes release ATP and utilize three mechanisms for nucleotide interconversion at the cell surface

Nucleotide activation of P2 receptors is important in autocrine and paracrine regulation in many tissues. In the epidermis, nucleotides are involved in proliferation, differentiation, and apoptosis. In this study, we have used a combination of luciferin-luciferase luminometry, pharmacological inhibitors, and confocal microscopy to demonstrate that HaCaT keratinocytes release ATP into the culture medium, and that there are three mechanisms for nucleotide interconversion, resulting in ATP generation at the cell surface. Addition of ADP, GTP, or UTP to culture medium elevated the ATP concentration. ADP to ATP conversion was inhibited by diadenosine pentaphosphate, oligomycin, and UDP, suggesting the involvement of cell surface adenylate kinase, F(1)F(0) ATP synthase, and nucleoside diphosphokinase (NDPK), respectively, which was supported by immunohistochemistry. Simultaneous addition of ADP and GTP elevated ATP above that for each nucleotide alone indicating that GTP acts as a phosphate donor. However, the activity of NDPK, F(1)F(0) ATP synthase or the forward reaction of adenylate kinase could not fully account for the culture medium ATP content. We postulate that this discrepancy is due to the reverse reaction of adenylate kinase utilizing AMP. In normal human skin, F(1)F(0) ATP synthase and NDPK were differentially localized, with mitochondrial expression in the basal layer, and cell surface expression in the differentiated layers. We and others have previously demonstrated that keratinocytes express multiple P2 receptors. In this study we now identify the potential sources of extracellular ATP required to activate these receptors and provide better understanding of the role of nucleotides in normal epidermal homeostasis and wound healing.

Degradation of extracellular ATP by the retinal pigment epithelium

Stimulation of ATP or adenosine receptors causes important physiological changes in retinal pigment epithelial (RPE) cells that may influence their relationship to the adjacent photoreceptors. While RPE cells have been shown to release ATP, the regulation of extracellular ATP levels and the production of dephosphorylated purines is not clear. This study examined the degradation of ATP by RPE cells and the physiological effects of the adenosine diphosphate (ADP) that result. ATP was readily broken down by both cultured human ARPE-19 cells and the apical membrane of fresh bovine RPE cells. The compounds ARL67156 and betagamma-mATP inhibited this degradation in both cell types. RT-PCR analysis of ARPE-19 cells found mRNA message for multiple extracellular degradative enzymes; ectonucleotide pyrophosphatase/phosphodiesterase eNPP1, eNPP2, and eNPP3; the ectoATPase ectonucleoside triphosphate diphosphohydrolase NTPDase2, NTPDase3, and some message for NTPDase1. Considerable levels of ADP bathed RPE cells, consistent with a role for NTPDase2. ADP and ATP increased levels of intracellular Ca(2+). Both responses were inhibited by thapsigargin and P2Y(1) receptor inhibitor MRS 2179. Message for both P2Y(1) and P2Y(12) receptors was detected in ARPE-19 cells. These results suggest that extracellular degradation of ATP in subretinal space can result in the production of ADP. This ADP can stimulate P2Y receptors and augment Ca(2+) signaling in the RPE.

Biochemical and molecular characterization of a novel choline-specific glycerophosphodiester phosphodiesterase belonging to the nucleotide pyrophosphatase/phosphodiesterase family

Nucleotide pyrophosphatases/phosphodiesterases (NPPs) are ubiquitous membrane-associated or secreted ectoenzymes that release nucleoside 5'-monophosphate from a variety of nucleotides and nucleotide derivatives. The mammalian NPP family comprises seven members, but only three of these (NPP1-3) have been studied in some detail. Previously we showed that lysophospholipase D, which hydrolyzes lysophosphatidylcholine (LPC) to produce lysophosphatidic acid, is identical to NPP2. More recently an uncharacterized novel NPP member (NPP7) was shown to have alkaline sphingomyelinase activity. These findings raised the possibility that other members of the NPP family act on phospholipids. Here we show that the sixth member of the NPP family, NPP6, is a choline-specific glycerophosphodiester phosphodiesterase. The sequence of NPP6 encodes a transmembrane protein containing an NPP domain with significant homology to NPP4, NPP5, and NPP7/alkaline sphingomyelinase. When expressed in HeLa cells, NPP6 was detected in both the cells and the cell culture medium as judged by Western blotting and by enzymatic activity. Recombinant NPP6 efficiently hydrolyzed the classical substrate for phospholipase C, p-nitrophenyl phosphorylcholine, but not the classical nucleotide phosphodiesterase substrate, p-nitrophenyl thymidine 5'-monophosphate. In addition, NPP6 hydrolyzed LPC to form monoacylglycerol and phosphorylcholine but not lysophosphatidic acid, showing it has a lysophospholipase C activity. NPP6 showed a preference for LPC with short (12:0 and 14:0) or polyunsaturated (18:2 and 20:4) fatty acids. It also hydrolyzed glycerophosphorylcholine and sphingosylphosphorylcholine efficiently. In mice, NPP6 mRNA was predominantly detected in kidney with a lesser expression in brain and heart, and in human it was detected in kidney and brain. The present results suggest that NPP6 has a specific role through the hydrolysis of polyunsaturated LPC, glycerophosphorylcholine, or sphingosylphosphorylcholine in these organs.

Methylene ATP analogs as modulators of extracellular ATP metabolism and accumulation

1 Transient accumulation of extracellular ATP reflects both release of ATP from intracellular stores and altered rates of ATP metabolism by ecto-enzymes. Ecto-nucleoside triphosphate diphosphohydrolases (eNTPDases) and ecto-nucleotide pyrophosphatases (eNPPs) degrade ATP, while ecto-nucleotide diphosphokinases (eNDPKs) synthesize ATP from ambient ADP. 2 Although the methylene ATP analogs betagamma-meATP and alphabeta-meATP are widely used as metabolically stable tools for the analysis of purinergic signaling, their specific effects on eNTPDase, eNPP, and eNDPK activities have not been defined. This study compared the actions of these analogs on extracellular ATP metabolism by human 1321N1 astrocytes, rat PC12 pheochomocytoma cells, and rat C6 glioma cells. 3 Both analogs significantly reduced clearance of extracellular ATP by 1321N1 cells that express both eNTPDases and eNPPs, as well as by C6 cells that exclusively express eNPPs. In contrast, both analogs were much less efficacious in inhibiting ATP clearance by PC12 cells that predominantly express eNTPDases. Betagamma-meATP, but not alphabeta-meATP, was effectively hydrolyzed by the 1321N1 and C6 cells; PC12 cells did not significantly degrade this analog. 4 Alphabeta-meATP, but not betagamma-meATP, acted as a substrate for purified yeast NDPK to generate ATP via trans-phosphorylation of ADP. alphabeta-meATP also acted as substrate for the eNDPK activities expressed by 1321N1, PC12, and C6 cells and thereby induced extracellular ATP accumulation in the presence of ambient or exogenously added ADP. 5 These results indicate that methylene ATP analogs exert complex and cell-specific effects on extracellular ATP metabolism that can significantly modify interpretation of studies that use these reagents as probes of purinergic signal transduction in intact tissues.

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.