Inhibition of rat parotid ecto-ATPase activity

Purinergic ATP triggers moxibustion-induced local anti-nociceptive effect on inflammatory pain model

Purinergic signalling adenosine and its A1 receptors have been demonstrated to get involved in the mechanism of acupuncture (needling therapy) analgesia. However, whether purinergic signalling would be responsible for the local analgesic effect of moxibustion therapy, the predominant member in acupuncture family procedures also could trigger analgesic effect on pain diseases, it still remains unclear. In this study, we applied moxibustion to generate analgesic effect on complete Freund's adjuvant (CFA)-induced inflammatory pain rats and detected the purine released from moxibustioned-acupoint by high-performance liquid chromatography (HPLC) approach. Intramuscular injection of ARL67156 into the acupoint Zusanli (ST36) to inhibit the breakdown of ATP showed the analgesic effect of moxibustion was increased while intramuscular injection of ATPase to speed up ATP hydrolysis caused a reduced moxibustion-induced analgesia. These data implied that purinergic ATP at the location of ST36 acupoint is a potentially beneficial factor for moxibustion-induced analgesia.

MIG-23 is involved in sperm migration by modulating extracellular ATP levels in Ascaris suum

In nematodes, spermiogenesis is a process of sperm activation in which nonmotile spermatids are transformed into crawling spermatozoa. Sperm motility acquisition during this process is essential for successful fertilization, but the underlying mechanisms remain to be clarified. Herein, we have found that extracellular adenosine-5′-triphosphate (ATP) level regulation by MIG-23, which is a homolog of human ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase), was required for major sperm protein (MSP) filament dynamics and sperm motility in the nematode Ascaris suum. During sperm activation, a large amount of ATP was produced in mitochondria and was stored in refringent granules (RGs). Some of the produced ATP was released to the extracellular space through innexin channels. MIG-23 was localized in the sperm plasma membrane and contributed to the ecto-ATPase activity of spermatozoa. Blocking MIG-23 activity resulted in a decrease in the ATP hydrolysis activity of spermatozoa and an increase in the depolymerization rate of MSP filaments in pseudopodia, which eventually affected sperm migration. Overall, our data suggest that MIG-23, which contributes to the ecto-ATPase activity of spermatozoa, regulates sperm migration by modulating extracellular ATP levels.

Inhibition of ATP hydrolysis restores airway surface liquid production in cystic fibrosis airway epithelia

Airway surface dehydration is a pathological feature of cystic fibrosis (CF) lung disease. CF is caused by mutations in the CF transmembrane conductance regulator (CFTR), a cyclic AMP-regulated Cl- channel controlled in part by the adenosine A2B receptor. An alternative CFTR-independent mechanism of fluid secretion is regulated by ATP via the P2Y2 receptor (P2Y2R) that activates Ca2+-regulated Cl- channels (CaCC/TMEM16) and inhibits Na+ absorption. However, due to rapid ATP hydrolysis, steady-state ATP levels in CF airway surface liquid (ASL) are inadequate to maintain P2Y2R-mediated fluid secretion. Therefore, inhibiting airway epithelial ecto-ATPases to increase ASL ATP levels constitutes a strategy to restore airway surface hydration in CF. Using [γ32P]ATP as radiotracer, we assessed the effect of a series of ATPase inhibitory compounds on the stability of physiologically occurring ATP concentrations. We identified the polyoxometalate [Co4(H2O)2(PW9O34)2]10- (POM-5) as the most potent and effective ecto-ATPase inhibitor in CF airway epithelial cells. POM-5 caused long-lasting inhibition of ATP hydrolysis in airway epithelia, which was reversible upon removal of the inhibitor. Importantly, POM-5 markedly enhanced steady-state levels of released ATP, promoting increased ASL volume in CF cell surfaces. These results provide proof of concept for ecto-ATPase inhibitors as therapeutic agents to restore hydration of CF airway surfaces. As a test of this notion, cell-free sputum supernatants from CF subjects were studied and found to have abnormally elevated ATPase activity, which was markedly inhibited by POM-5.

Label-free assay based on immobilized capillary enzyme reactor of Leishmania infantum nucleoside triphosphate diphosphohydrolase (LicNTPDase-2-ICER-LC/UV)

Nucleoside triphosphate diphosphohydrolase (NTPDase) is an enzyme belonging to the apyrase family that participates in the hydrolysis of the nucleosides di- and triphosphate to the corresponding nucleoside monophosphate. This enzyme underlies the virulence of parasites such as Leishmania. Recently, an NTPDase from Leishmania infantum (LicNTPDase-2) was cloned and expressed and has been considered as a new drug target for the treatment of leishmaniasis. With the intent of developing label-free online screening methodologies, LicNTPDase-2 was covalently immobilized onto a fused silica capillary tube in the present study to create an immobilized capillary enzyme reactor (ICER) based on LicNTPDase-2 (LicNTPDase-2-ICER). To perform the activity assays, a multidimensional chromatographic method was developed employing the LicNTPDase-2-ICER in the first dimension, and an analytical Ascentis C8 column was used in the second dimension to provide analytical separation of the substrates and products. The validated LicNTPDase-2-ICER method provided the following kinetic parameters of the immobilized enzyme: KM of 2.2 and 1.8mmolL(-1) for the ADP and ATP substrates, respectively. Suramin (1mmolL(-1)) was also shown to inhibit 32.9% of the enzymatic activity. The developed method is applicable to kinetic studies and enables the recognition of the ligands. Furthermore, a comparison of the values of LicNTPDase-2-ICER with those obtained with an LC method using free enzyme in solution showed that LicNTPDase-2-ICER-LC/UV was an accurate and reproducible method that enabled automated measurements for the rapid screening of ligands.

Inhibition of ecto-ATPase activity by curcumin in hepatocellular carcinoma HepG2 cells

Effects of curcumin, a major constituent of turmeric, on ecto-nucleotidases have not been clarified. Here, we investigated whether curcumin affects ecto-nucleotidase activities in human hepatocellular carcinoma HepG2 cells. In the cells, high levels of Mg(2+)-dependent activity of ecto-nucleotidases were observed in the presence of 1 mM adenosine triphosphate (ATP). The activity was inhibited by ecto-ATPase inhibitors such as suramin, ZnCl(2) and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. On the other hand, the activity was significantly decreased at alkaline pH (pH 9) and was not inhibited by levamisole, an inhibitor of alkaline phosphatase. In the presence of ATP, curcumin inhibited the activity in a concentration-dependent manner (IC(50) = 6.2 μM). In contrast, curcumin had no effects on ecto-nucleotidase activity in the presence of ADP (1 mM) or AMP (1 mM). The K (m) value for ATP hydrolysis of curcumin-sensitive ecto-ATPase was similar to the value of NTPDase2, an isoform of ecto-nucleoside triphosphate diphosphohydrolase. These results suggest that curcumin is a potent inhibitor of ecto-ATPase and may affect extracellular ATP-dependent responses.

Structure-activity relationships of anthraquinone derivatives derived from bromaminic acid as inhibitors of ectonucleoside triphosphate diphosphohydrolases (E-NTPDases)

Reactive blue 2 (RB-2) had been characterized as a relatively potent ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) inhibitor with some selectivity for NTPDase3. In search for the pharmacophore and to analyze structure-activity relationships we synthesized a series of truncated derivatives and analogs of RB-2, including 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinones, 1-amino-2-methyl-4-arylaminoanthraquinones, 1-amino-4-bromoanthraquinone 2-sulfonic acid esters and sulfonamides, and bis-(1-amino-4-bromoanthraquinone) sulfonamides, and investigated them in preparations of rat NTPDase1, 2, and 3 using a capillary electrophoresis assay. Several 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinone derivatives inhibited E-NTPDases in a concentration-dependent manner. The 2-sulfonate group was found to be required for inhibitory activity, since 2-methyl-substituted derivatives were inactive. 1-Amino-2-sulfo-4-p-chloroanilinoanthraquinone (18) was identified as a nonselective competitive blocker of NTPDases1, 2, and 3 (K_i 16–18 μM), while 1-amino-2-sulfo-4-(2-naphthylamino)anthraquinone (21) was a potent inhibitor with preference for NTPDase1 (K_i 0.328 μM) and NTPDase3 (K_i 2.22 μM). Its isomer, 1-amino-2-sulfo-4-(1-naphthylamino)anthraquinone (20), was a potent and selective inhibitor of rat NTPDase3 (K_i 1.5 μM).

Ectonucleotidases in Müller glial cells of the rodent retina: Involvement in inhibition of osmotic cell swelling

Extracellular nucleotides mediate glia-to-neuron signalling in the retina and are implicated in the volume regulation of retinal glial (Müller) cells under osmotic stress conditions. We investigated the expression and functional role of ectonucleotidases in Müller cells of the rodent retina by cell-swelling experiments, calcium imaging, and immuno- and enzyme histochemistry. The swelling of Müller cells under hypoosmotic stress was inhibited by activation of an autocrine purinergic signalling cascade. This cascade is initiated by exogenous glutamate and involves the consecutive activation of P2Y₁ and adenosine A1 receptors, the action of ectoadenosine 5′-triphosphate (ATP)ases, and a nucleoside-transporter-mediated release of adenosine. Inhibition of ectoapyrases increased the ATP-evoked calcium responses in Müller cell endfeet. Müller cells were immunoreactive for nucleoside triphosphate diphosphohydrolases (NTPDase)2 (but not NTPDase1), ecto-5′-nucleotidase, P2Y₁, and A1 receptors. Enzyme histochemistry revealed that ATP but not adenosine 5′-diphosphate (ADP) is extracellularly metabolised in retinal slices of NTPDase1 knockout mice. NTPDase1 activity and protein is restricted to blood vessels, whereas activity of alkaline phosphatase is essentially absent at physiological pH. The data suggest that NTPDase2 is the major ATP-degrading ectonucleotidase of the retinal parenchyma. NTPDase2 expressed by Müller cells can be implicated in the regulation of purinergic calcium responses and cellular volume.

Inhibition of human and mouse plasma membrane bound NTPDases by P2 receptor antagonists

The plasma membrane bound nucleoside triphosphate diphosphohydrolase (NTPDase)-1, 2, 3 and 8 are major ectonucleotidases that modulate P2 receptor signaling by controlling nucleotides' concentrations at the cell surface. In this report, we systematically evaluated the effect of the commonly used P2 receptor antagonists reactive blue 2, suramin, NF279, NF449 and MRS2179, on recombinant human and mouse NTPDase1, 2, 3 and 8. Enzymatic reactions were performed in a Tris/calcium buffer, commonly used to evaluate NTPDase activity, and in a more physiological Ringer modified buffer. Although there were some minor variations, there were no major changes either in the enzymatic activity or in the profile of NTPDase inhibition between the two buffers. Except for MRS2179, all other antagonists significantly inhibited these ecto-ATPases; NTPDase3 being the most sensitive to inhibition and NTPDase8 the most resistant. Estimated IC(50) showed that human NTPDases were generally more sensitive to the P2 receptor antagonists tested than the corresponding mouse isoforms. NF279 and reactive blue 2 were the most potent inhibitors of NTPDases which almost completely abrogated their activity at the concentration of 100 microM. In conclusion, reactive blue 2, suramin, NF279 and NF449, at the concentrations commonly used to antagonize P2 receptors, inhibit the four major ecto-ATPases. This information may reveal useful for the interpretation of some pharmacological studies of P2 receptors. In addition, NF279 is a most potent non-selective NTPDase inhibitor. Although P2 receptor antagonists do not display a strict selectivity toward NTPDases, their IC(50) values may help to discriminate some of these enzymes.

Specificity of the ecto-ATPase inhibitor ARL 67156 on human and mouse ectonucleotidases

BACKGROUND AND PURPOSE

ARL 67156, 6-N,N-Diethyl-D-beta-gamma-dibromomethylene adenosine triphosphate, originally named FPL 67156, is the only commercially available inhibitor of ecto-ATPases. Since the first report on this molecule, various ectonucleotidases responsible for the hydrolysis of ATP at the cell surface have been cloned and characterized. In this work, we identified the ectonucleotidases inhibited by ARL 67156.

EXPERIMENTAL APPROACH

The effect of ARL 67156 on recombinant NTPDase1, 2, 3 & 8 (mouse and human), NPP1, NPP3 and ecto-5'-nucleotidase (human) have been evaluated. The inhibition of the activity of NTPDases (using the following substrates: ATP, ADP, UTP), NPPs (pnp-TMP, Ap(3)A) and ecto-5'-nucleotidase (AMP) was measured by colorimetric or HPLC assays.

KEY RESULTS

ARL 67156 was a weak competitive inhibitor of human NTPDase1, NTPDase3 and NPP1 with K(i) of 11+/-3, 18+/-4 and 12+/-3 microM, respectively. At concentrations used in the literature (50-100 microM), ARL 67156 partially but significantly inhibited the mouse and human forms of these enzymes. NTPDase2, NTPDase8, NPP3 and ecto-5'-nucleotidase activities were less affected. Importantly, ARL 67156 was not hydrolysed by either human NTPDase1, 2, 3, 8, NPP1 or NPP3.

CONCLUSIONS AND IMPLICATIONS

In cell environments where NTPDase1, NTPDase3, NPP1 or mouse NTPDase8 are present, ARL 67156 would prolong the effect of endogenously released ATP on P2 receptors. However, it does not block any ectonucleotidases efficiently when high concentrations of substrates are present, such as in biochemical, pharmacological or P2X(7) assays. In addition, ARL 67156 is not an effective inhibitor of NTPDase2, human NTPDase8, NPP3 and ecto-5'-nucleotidase.

Kinetic and biochemical characterization of an ecto-nucleotide pyrophosphatase/phosphodiesterase (EC 3.1.4.1) in cells cultured from submandibular salivary glands of rats

The participation of ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) activity in the nucleotide hydrolysis by salivary gland cells of rats was evaluated using p-nitrophenyl 5'-thymidine monophosphate (p-Nph-5'-TMP) as a substrate for this enzyme. We investigated the biochemical characteristics of this ectoenzyme in cells cultured from submandibular salivary glands of rats. Primary cell cultures demonstrated ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) activities, which could be observed by extracellular hydrolysis of p-Nph-5'-TMP and other biochemical characteristics such as dependence of metal ions, dependence of pH alkaline and inactivation by a metal ion chelator. The Km value for the hydrolysis of p-Nph-5'-TMP was 280.7+/-34.2 microM (mean+/-S.D., n=4) and Vmax was 721.31+/-225nmol p-nitrophenol/min/mg (mean+/-S.D., n=4). We suggest that E-NPP is co-localized with an ecto-ATP diphosphohydrolase/ecto-NTPDase and an ecto-5'-nucleotidase, since these enzymes probably act under different conditions. It may be postulated that the physiological role for these ecto-enzymes is to terminate the action of the co-transmitter ATP, generating adenosine.

A capillary electrophoresis method for the characterization of ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) and the analysis of inhibitors by in-capillary enzymatic microreaction

A capillary electrophoresis (CE) method for the characterization of recombinant NTPDases 1, 2, and 3, and for assaying NTPDase inhibitors has been developed performing the enzymatic reaction within the capillary. After hydrodynamic injection of plugs of substrate solution with or without inhibitor in reaction buffer, followed by a suspension of an enzyme-containing membrane preparation, and subsequent injection of another plug of substrate solution with or without inhibitor, the reaction took place close to the capillary inlet. After 5 min, the electrophoretic separation of the reaction products was initiated by applying a constant current of -60 muA. The method employing a polyacrylamide-coated capillary and reverse polarity mode provided baseline resolution of substrates and products within a short separation time of less than 7 min. A 50 mM phosphate buffer (pH 6.5) was used for the separations and the products were detected by their UV absorbance at 210 nm. The Michaelis-Menten constants (K (m)) for the recombinant rat NTPDases 1, 2, and 3 obtained with this method were consistent with previously reported data. The inhibition studies revealed pronounced differences in the potency of reactive blue 2, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), suramin, and N (6)-diethyl-beta,gamma-dibromomethylene-ATP (ARL67156) towards the NTPDase isoforms. Notably, ARL67156 does not inhibit all NTPDases, having only a minor inhibitory effect on NTPDase2. Dipyridamole is not an inhibitor of the NTPDase isoforms investigated. The new method is fast and accurate, it requires only tiny amounts of material (nanoliter scale), no sample pretreatment and can be fully automated; thus it is clearly superior to the current standard methods.

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.

Extracellular ATP is an autocrine/paracrine regulator of hypoxia-induced adventitial fibroblast growth. Signaling through extracellular signal-regulated kinase-1/2 and the Egr-1 transcription factor

Important autocrine/paracrine functions for the adenine nucleotides have been proposed in several tissues. We addressed the possibility that extracellular ATP would modulate/mediate hypoxia-induced adventitial fibroblast growth. Acute hypoxia (3% O(2), 10-60 min) increased extracellular ATP concentrations in adventitial fibroblasts and in lung microvascular endothelial cells, and chronic hypoxia (3% O(2), 14-30 days) markedly attenuated the rate of extracellular ATP hydrolysis by ecto-nucleotidase(s). Exogenous ATP stimulated [(3)H]thymidine incorporation in fibroblasts as did UTP, ADPbeta, 2-methylthioadenosine triphosphate, adenosine 5'-(alpha,beta-methylene)triphosphate, and benzoylbenzoyl-ATP (2'-3'-O-(4-benzoylbenzoyl)-ATP), indicating that both P2Y and P2X purinoceptors can mediate mitogenic responses. Suramin (100 microm), Cibacron blue 3GA (100 microm), and pyridoxalphosphate-6-azophenyl-2',-4'-disulfonic acid (100 microm) as well as apyrase (5 units/ml) attenuated hypoxia- and ATP-induced and DNA synthesis, indicating activation and a functional role of purinoceptors under hypoxic conditions. ATP-induced DNA synthesis was augmented by hypoxia in an additive fashion, whereas ATP and hypoxia synergistically increased growth factor-induced DNA synthesis, again suggesting that ATP and hypoxia utilize similar signaling pathways to induce proliferation. Indeed, we found that ATP (100 microm) and hypoxia (3% O(2)) induced expression and activation of Egr-1 transcription factor, and both stimuli acted, in part, through a G(alpha)(i)/ERK1/2-dependent signaling pathway. Suramin, Cibacron blue 3GA, and apyrase attenuated hypoxia-induced ERK1/2 activation and Egr-1 expression. We conclude that hypoxia induces ATP release from endothelial cells and fibroblasts and that the activation of P2 purinoceptors is involved in the regulation of DNA synthesis by fibroblasts under hypoxic conditions.