Nucleotide receptors control IL-8/CXCL8 and MCP-1/CCL2 secretions as well as proliferation in human glioma cells.

Glioma cells release cytokines to stimulate inflammation that facilitates cell proliferation. Here, we show that Lipopolysaccharide (LPS) treatment could induce glioma cells to proliferate and this process was dependent on nucleotide receptor activation as well as interleukin-8 (IL-8/CXCL8) secretion. We observed that extracellular nucleotides controlled IL-8/CXCL8 and monocyte chemoattractant protein 1 (MCP-1/CCL2) release by U251MG and U87MG human glioma cell lines via P2X7 and P2Y6 receptor activation. The LPS-induced release of these cytokines was also modulated by purinergic receptor activation since IL-8 and MCP-1 release was decreased by the nucleotide scavenger apyrase as well as by the pharmacological P2Y6 receptor antagonists suramin and MRS2578. In agreement with these observations, the knockdown of P2Y6 expression decreased LPS-induced IL-8 release as well as the spontaneous release of IL-8 and MCP-1, suggesting an endogenous basal release of nucleotides. Moreover, high millimolar concentrations of ATP increased IL-8 and MCP-1 release by the glioma cells stimulated with suboptimal LPS concentration which were blocked by P2X7 and P2Y6 antagonists. Altogether, these data suggest that extracellular nucleotides control glioma growth via P2 receptor-dependent IL-8 and MCP-1 secretions.

Nucleoside triphosphate diphosphohydrolase-1 ectonucleotidase is required for normal vas deferens contraction and male fertility through maintaining P2X1 receptor function.

In this work, we report that Entpd1(-/-) mice, deficient for the ectonucleotidase nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), produce smaller litters (27% reduction) compared with wild-type C57BL6 animals. This deficit is linked to reduced in vivo oocyte fertilization by Entpd1(-/-) males (61 ± 11% versus 88 ± 7% for Entpd1(+/+)). Normal epididymal sperm count, spermatozoa morphology, capacitation, and motility and reduced ejaculated sperm number (2.4 ± 0.5 versus 3.7 ± 0.4 million for Entpd1(+/+)) pointed to vas deferens dysfunction. NTPDase1 was localized by immunofluorescence in the tunica muscularis of the vas deferens. Its absence resulted in a major ATP hydrolysis deficiency, as observed in situ by histochemistry and in primary smooth muscle cell cultures. In vitro, Entpd1(-/-) vas deferens displayed an exacerbated contraction to ATP, a diminished response to its non-hydrolysable analog αßMeATP, and a reduced contraction to electrical field stimulation, suggesting altered P2X1 receptor function with a propensity to desensitize. This functional alteration was accompanied by a 3-fold decrease in P2X1 protein expression in Entpd1(-/-) vas deferens with no variation in mRNA levels. Accordingly, exogenous nucleotidase activity was required to fully preserve P2X1 receptor activation by ATP in vitro. Our study demonstrates that NTPDase1 is required to maintain normal P2X1 receptor functionality in the vas deferens and that its absence leads to impaired peristalsis, reduced spermatozoa concentration in the semen, and, eventually, reduced fertility. This suggests that alteration of NTPDase1 activity affects ejaculation efficacy and male fertility. This work may contribute to unveil a cause of infertility and open new therapeutic potentials.

Purine-metabolizing ectoenzymes control IL-8 production in human colon HT-29 cells.

Interleukin-8 (IL-8) plays key roles in both chronic inflammatory diseases and tumor modulation. We previously observed that IL-8 secretion and function can be modulated by nucleotide (P2) receptors. Here we investigated whether IL-8 release by intestinal epithelial HT-29 cells, a cancer cell line, is modulated by extracellular nucleotide metabolism. We first identified that HT-29 cells regulated adenosine and adenine nucleotide concentration at their surface by the expression of the ectoenzymes NTPDase2, ecto-5'-nucleotidase, and adenylate kinase. The expression of the ectoenzymes was evaluated by RT-PCR, qPCR, and immunoblotting, and their activity was analyzed by RP-HPLC of the products and by detection of Pi produced from the hydrolysis of ATP, ADP, and AMP. In response to poly (I:C), with or without ATP and/or ADP, HT-29 cells released IL-8 and this secretion was modulated by the presence of NTPDase2 and adenylate kinase. Taken together, these results demonstrate the presence of 3 ectoenzymes at the surface of HT-29 cells that control nucleotide levels and adenosine production (NTPDase2, ecto-5'-nucleotidase and adenylate kinase) and that P2 receptor-mediated signaling controls IL-8 release in HT-29 cells which is modulated by the presence of NTPDase2 and adenylate kinase.

NTPDase1 controls IL-8 production by human neutrophils.

The ectonucleotidase NTPDase1 (CD39) terminates P2 receptor activation by the hydrolysis of extracellular nucleotides (i.e., the P2 receptor ligands). In agreement with that role, exacerbated inflammation has been observed in NTPDase1-deficient mice. In this study, we extend these observations by showing that inhibition of NTPDase1 markedly increases IL-8 production by TLR-stimulated human neutrophils. First, immunolabeling of human blood neutrophils and neutrophil-like HL60 cells displayed the expression of NTPDase1 protein, which correlated with the hydrolysis of ATP at their surface. NTPDase1 inhibitors (e.g., NF279 and ARL 67156) as well as NTPDase1-specific small interfering RNAs markedly increased IL-8 production in neutrophils stimulated with LPS and Pam(3)CSK(4) (agonists of TLR4 and TLR1/2, respectively) but not with flagellin (TLR5) and gardiquimod (TLR7 and 8). This increase in IL-8 release was due to the synergy between TLRs and P2 receptors. Indeed, ATP was released from neutrophils constitutively and accumulated in the medium upon NTPDase1 inhibition by NF279. Likewise, both human blood neutrophils and neutrophil-like HL60 cells produced IL-8 in response to exogenous nucleotides, ATP being the most potent inducer. In agreement, P2Y(2) receptor knockdown in neutrophil-like HL60 cells markedly decreased LPS- and Pam(3)CSK(4)-induced IL-8 production. In line with these in vitro results, injection of LPS in the air pouches of NTPDase1-deficient mice triggered an increased production of the chemokines MIP-2 and keratinocyte-derived chemokine (i.e., the rodent counterparts of human IL-8) compared with that in wild-type mice. In summary, NTPDase1 controls IL-8 production by human neutrophils via the regulation of P2Y(2) activation.

NTPDase1 governs P2X7-dependent functions in murine macrophages.

P2X7 receptor is an adenosine triphosphate (ATP)-gated ion channel within the multiprotein inflammasome complex. Until now, little is known about regulation of P2X7 effector functions in macrophages. In this study, we show that nucleoside triphosphate diphosphohydrolase 1 (NTPDase1)/CD39 is the dominant ectonucleotidase expressed by murine peritoneal macrophages and that it regulates P2X7-dependent responses in these cells. Macrophages isolated from NTPDase1-null mice (Entpd1(-/-)) were devoid of all ADPase and most ATPase activities when compared with WT macrophages (Entpd1(+/+)). Entpd1(-/-) macrophages exposed to millimolar concentrations of ATP were more susceptible to cell death, released more IL-1beta and IL-18 after TLR2 or TLR4 priming, and incorporated the fluorescent dye Yo-Pro-1 more efficiently (suggestive of increased pore formation) than Entpd1(+/+) cells. Consistent with these observations, NTPDase1 regulated P2X7-associated IL-1beta release after synthesis, and this process occurred independently of, and prior to, cytokine maturation by caspase-1. NTPDase1 also inhibited IL-1beta release in vivo in the air pouch inflammatory model. Exudates of LPS-injected Entpd1(-/-) mice had significantly higher IL-1beta levels when compared with Entpd1(+/+) mice. Altogether, our studies suggest that NTPDase1/CD39 plays a key role in the control of P2X7-dependent macrophage responses.

Emerging role of extracellular nucleotides and adenosine in multiple sclerosis.

Extracellular nucleotides and adenosine play important roles in inflammation. These signaling molecules interact with the cell-surface-located P2 and P1 receptors, respectively, that are widely distributed in the central nervous system and generally exert opposite effects on immune responses. Indeed, extracellular ATP, ADP, UTP, and UDP serve as alarmins or damage-associated molecular patterns that activate mainly proinflammatory mechanisms, whereas adenosine has potent anti-inflammatory and immunosuppressive effects. This review discusses the actual and potential role of extracellular nucleotides and adenosine in multiple sclerosis (MS).

Identification of the ectonucleotidases expressed in mouse, rat, and human Langerhans islets: potential role of NTPDase3 in insulin secretion.

Extracellular nucleotides and adenosine regulate endocrine pancreatic functions such as insulin secretion by Langerhans islet ß-cells via the activation of specific P2 and P1 receptors. Membrane-bound ectonucleotidases regulate the local concentration of these ligands and consequently control the activation of their receptors. The objective of this study was to identify and localize the major ectonucleotidases, namely NTPDases and ecto-5'-nucleotidase, present in the endocrine pancreas. In addition, the potential implication of ecto-ATPase activity on insulin secretion was investigated in the rat ß-cell line INS-1 (832/13). The localization of ectonucleotidase activity and protein was carried out in situ by enzyme histochemistry and immunolocalization in mouse, rat, and human pancreas sections. NTPDase1 was localized in all blood vessels and acini, and NTPDase2 was localized in capillaries of Langerhans islets and in peripheral conjunctive tissue, whereas NTPDase3 was detected in all Langerhans islet cell types. Interestingly, among the mammalian species tested, ecto-5'-nucleotidase was present only in rat Langerhans islet cells, where it was coexpressed with NTPDase3. Notably, the inhibition of NTPDase3 activity by BG0136 and NF279 facilitated insulin release from INS-1 (832/13) cells under conditions of low glycemia, probably by affecting P2 receptor activation. NTPDase3 activity also regulated the inhibitory effect of exogenous ATP in the presence of a high glucose concentration most likely by controlling adenosine production. In conclusion, all pancreatic endocrine cells express NTPDase3 that was shown to modulate insulin secretion in rat INS-1 (832/13) ß-cells. Ecto-5'-nucleotidase is expressed in rat Langerhans islet cells but absent in human and mouse endocrine cells.

Concomitant activation of P2Y(2) and P2Y(6) receptors on monocytes is required for TLR1/2-induced neutrophil migration by regulating IL-8 secretion.

Extracellular nucleotides regulate a variety of cellular responses involved in inflammation via the activation of P2 receptors. Here, we show that nucleotides regulate TLR2-induced neutrophil migration both in vivo and in vitro. The nucleotide scavenger apyrase inhibited neutrophil recruitment in murine air pouches injected with the TLR2 agonist Pam(3)CSK(4). In agreement, the supernatants of either human primary monocytes or monocytic cells (THP-1 and U937) treated with Pam(3)CSK(4) recruited significantly fewer neutrophils when the former cells were treated in the presence of apyrase. As demonstrated with inhibitory Ab, these supernatants induced neutrophil migration due to IL-8 secretion. In addition, IL-8 secretion was markedly diminished by the non-selective P2 receptor antagonists reactive blue 2 and suramin, and by a selective P2Y(6) antagonist, MRS2578. Selective antagonists of P2Y(1) (MRS2500) and P2Y(11) (NF157) did not affect IL-8 release. The knockdown of either P2Y(2) or P2Y(6) with specific shRNA diminished IL-8 secretion from Pam(3)CSK(4)-treated THP-1 cells. Altogether, these results show that extracellular nucleotides, via P2Y(2) and P2Y(6) receptors, regulate neutrophil migration by controlling TLR2-induced IL-8 release from human monocytes. In line with our previous work on TLR4, this study further supports the importance of nucleotides in bacterial-induced neutrophil migration.

High expression and activity of ecto-5'-nucleotidase/CD73 in the male murine reproductive tract.

Extracellular ATP and its hydrolysis product adenosine modulate various reproductive functions such as those requiring contraction, steroidogenesis, and maintenance of fluid composition. Interestingly, adenosine might act as a key capacitative effector for mammalian spermatozoa to acquire the capacity for fertilisation. Extracellular nucleotide levels are affected by cell surface ectonucleotidases, amongst which the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family regroups the most abundant and effective enzymes to hydrolyse ATP and ADP to AMP in physiological conditions. In the male reproductive tract three members of this family have been indentified: NTPDase1, NTPDase2 and NTPDase3 (Martín-Satué et al. in Histochem Cell Biol 131:615-628, 2009). The purpose of the present study was to characterize in the male reproductive tract the expression profile of the main enzyme responsible for the generation of adenosine from AMP, namely the ecto-5'-nucleotidase (CD73). The enzyme was identified by immunological techniques and by in situ enzymatic assays, including inhibition experiments with alpha,beta-methylene-ADP, a specific CD73 inhibitor. High levels of ecto-5'-nucleotidase were detected in testes in association with both germinal and somatic cells, in smooth muscle cells throughout the tract, in secretory epithelia from exocrine glands, and remarkably, in principal cells of epididymis, where co-localization with NTPDase3 was found. The relevance of this co-expression on nucleotide hydrolysis in these cells directly involved in the control of sperm fluid composition was addressed biochemically. This study suggests close regulation of extracellular nucleoside and nucleotide levels in the genital tract by ecto-5'-nucleotidase that, in concurrence with NTPDases, may impact male fertility.

Extracellular ATP and P2 receptors are required for IL-8 to induce neutrophil migration.

The chemokine interleukin 8 (IL-8) is a major chemoattractant for human neutrophils. Here, we demonstrate novel evidence that IL-8-induced neutrophil chemotaxis requires a concurrent activation of P2 receptors, most likely the P2Y(2) which is dominantly expressed in these cells. Indeed, the migration of human neutrophils towards IL-8 was significantly inhibited by the P2Y receptor antagonists, suramin and reactive blue 2 (RB-2) and potentiated by a P2Y(2) ligand, ATP, but insensitive to specific antagonists of P2Y(1), P2Y(6) and P2Y(11) receptors. Adenosine had no effect on neutrophil migration towards IL-8 which contrasted with the stimulatory effect of this molecule on neutrophil chemotaxis caused by formyl-Met-Leu-Phe (fMLP or fMLF). Taken together, these data suggest that extracellular ATP is necessary for IL-8 to exert its chemotactic effect on neutrophils.

Extracellular nucleotides mediate LPS-induced neutrophil migration in vitro and in vivo.

Extracellular nucleotides are emerging as important inflammatory mediators. Here, we demonstrate that these molecules mediate LPS-induced neutrophil migration in vitro and in vivo. Apyrase, a nucleotide scavenger, reduced the ability of LPS-stimulated monocytes to recruit neutrophils, as assayed using a modified Boyden chamber. This effect resulted from the inhibition of IL-8 release from monocytes. Furthermore, LPS-induced IL-8 release by monocytes was attenuated significantly by P2Y6 receptor antagonists, RB-2 and MRS2578. Reciprocally, UDP, the selective P2Y6 agonist, induced IL-8 release by monocytes. As for LPS, the media of UDP-stimulated monocytes were chemotactic for neutrophils; IL-8 accounted for approximately 50% of neutrophil migration induced by the media of LPS- or UDP-treated monocytes in transendothelial migration assays. It is important that in the murine air-pouch model, extracellular nucleotides were instrumental in LPS-induced neutrophil migration. Altogether, these data imply that LPS induces the release of nucleotides from monocytes and that by autocrine stimulation, the latter molecules regulate neutrophil migration caused by Gram-negative bacteria, suggesting a proinflammatory role of extracellular nucleotides in innate immunity.

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.

Cloning and characterization of mouse nucleoside triphosphate diphosphohydrolase-3.

We have cloned and characterized the nucleoside triphosphate diphosphohydrolase-3 (NTPDase3) from mouse spleen. Analysis of cDNA shows an open reading frame of 1587 base pairs encoding a protein of 529 amino acids with a predicted molecular mass of 58953Da and an estimated isoelectric point of 5.78. The translated amino acid sequence shows the presence of two transmembrane domains, eight potential N-glycosylation sites and the five apyrase conserved regions. The genomic sequence is located on chromosome 9F4 and is comprised of 11 exons. Intact COS-7 cells transfected with an expression vector containing the coding sequence for mouse NTPDase3 hydrolyzed P2 receptor agonists (ATP, UTP, ADP and UDP) but not AMP. NTPDase3 required divalent cations (Ca2+ > Mg2+) for enzymatic activity. Interestingly, the enzyme had two optimum pHs for ATPase activity (pH 5.0 and 7.4) and one for ADPase activity (pH 8.0). Consequently, the ATP/ADP and UTP/UDP hydrolysis ratios were two to four folds higher at pH 5.0 than at pH 7.4, for both, intact cells and protein extracts. At pH 7.4 mouse NTPDase3 hydrolyzed ATP, UTP, ADP and UDP according to Michaelis-Menten kinetics with apparent K(m)s of 11, 10, 19 and 27 microM, respectively. In agreement with the K(m) values, the pattern of triphosphonucleoside hydrolysis showed a transient accumulation of the corresponding diphosphonucleoside and similar affinity for uracil and adenine nucleotides. NTPDase3 hydrolyzes nucleotides in a distinct manner than other plasma membrane bound NTPDases that may be relevant for the fine tuning of the concentration of P2 receptor agonists.

Comparative hydrolysis of extracellular adenine nucleotides and adenosine in synaptic membranes from porcine brain cortex, hippocampus, cerebellum and medulla oblongata.

We have investigated the metabolism of extracellular adenine nucleotides and adenosine in porcine brain. The cortex synaptic plasma membranes hydrolyzed ATP to ADP, AMP and adenosine. We also observed a slow hydrolysis of adenosine with the concomitant accumulation of inosine. These results indicate that NTPDase1, NTPDase2, ecto-5'-nucleotidase, and adenosine deaminase are present in cortex synaptic membranes from porcine brain. We further showed that all these enzymes are also abundant in synaptic membranes from hippocampus, cerebellum, and medulla oblongata and compared their specific activities. Brain cortex and hippocampus exhibited higher activities of NTPDase1 and NTPDase2 than cerebellum and medulla oblongata. It was consistent with the high level of the expression of NTPDases in the two first structures. Adenosine deaminase activity was found in all brain structures analyzed; however, it was lower than the activity of ecto-nucleotidases. Taken together, our data suggest that investigated enzymes have a ubiquitous abundance in porcine brain, and observed differences in their activities in cortex, hippocampus, cerebellum, and medulla oblongata may correlate with the pattern of P2 receptor expression in these brain areas. In addition, low activity of adenosine deaminase may indicate that nonenzymatic mechanism(s) are responsible for the termination of P1 receptor signaling in porcine brain.

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.

The P2 receptor antagonist PPADS abrogates LPS-induced neutrophil migration in the murine air pouch via inhibition of MIP-2 and KC production.

In this work, we show that P2 nucleotide receptors control lipopolysaccharide (LPS)-induced neutrophil migration in the mouse air pouch model. Neutrophil infiltration in LPS-treated air pouches was reduced by the intravenous (iv) administration of the non-selective P2 receptor antagonist PPADS but not by suramin and RB-2. In addition, the iv administration of a P2 receptor ligand, UTP, enhanced LPS-induced neutrophil migration. In contrast, the iv injection of UDP had no effect on neutrophil migration. These data suggest that LPS-induced neutrophil migration in the air pouch could involve P2Y(4) receptor which is antagonized by PPADS, activated by UTP, but not UDP, and insensitive to suramin. The inhibition of neutrophil migration by PPADS correlated with a diminished secretion of chemokines macrophage inflammatory protein-2 (MIP-2) and keratinocyte-derived chemokine (KC) in the air pouch exudates. As determined in vitro, PPADS did not affect MIP-2 and KC release from air pouch resident cells nor from accumulated neutrophils. MIP-2 and KC production in the LPS-treated air pouches correlated with an early neutrophil migration (1h after LPS injection), and both of these effects were significantly reduced in mice administered with PPADS. Altogether, these data suggest that P2Y(4) receptor expressed in circulating leukocytes and/or endothelium controls LPS-induced acute neutrophil recruitment in mouse air pouch.

Endothelial P2Y2 receptor regulates LPS-induced neutrophil transendothelial migration in vitro.

Previous studies showed that P2 receptors are involved in neutrophil migration via stimulation of chemokine release and by facilitating chemoattractant gradient sensing. Here, we have investigated whether these receptors are involved in LPS-induced neutrophil transendothelial migration (TEM) using a Boyden chamber where neutrophils migrated through a layer of lipopolysaccharide (LPS)-stimulated human umbilical vein endothelial cells (HUVECs). In line with a role of P2 receptors, neutrophil TEM was inhibited by the P2 receptor antagonists suramin and reactive blue 2 (RB-2) acting on the basolateral, but not luminal, HUVECs' P2 receptors. HUVECs express P2Y(1), P2Y(2), P2Y(4), P2Y(6) and P2Y(11). The involvement of P2Y(4) was unlikely as this receptor is insensitive to suramin while P2Y(1), P2Y(6) and P2Y(11) were excluded with available selective antagonists, leaving P2Y(2) as the only candidate. Indeed, the P2Y(2) knockdown in HUVECs inhibited neutrophil TEM compared to control HUVECs transfected with scrambled siRNA. Moreover, UTP, a P2Y(2) ligand, markedly potentiated LPS-induced TEM. Interestingly, IL-8 and ICAM-1 had a modest effect on neutrophil TEM in this 3h assay which was significantly diminished by the inhibition of Rho kinase in HUVECs with Y27632. In summary, endothelial P2Y(2) receptors control the early LPS-induced neutrophil TEM in vitro via Rho kinase activation.

Cloning, purification, and identification of the liver canalicular ecto-ATPase as NTPDase8.

Extracellular nucleotides regulate critical liver functions via the activation of specific transmembrane receptors. The hepatic levels of extracellular nucleotides, and therefore the related downstream signaling cascades, are modulated by cell-surface enzymes called ectonucleotidases, including nucleoside triphosphate diphosphohydrolase-1 (NTPDase1/CD39), NTPDase2/CD39L1, and ecto-5'-nucleotidase/CD73. The goal of this study was to determine the molecular identity of the canalicular ecto-ATPase/ATPDase that we hypothesized to correspond to the recently cloned NTPDase8. Human and rat NTPDase8 cDNAs were cloned, and the genes were located on chromosome loci 9q34 and 3p13, respectively. The recombinant proteins, expressed in COS-7 and HEK293T cells, were biochemically characterized. NTPDase8 was also purified from rat liver by Triton X-100 solubilization, followed by DEAE, Affigel Blue, and concanavalin A chromatographies. Importantly, NTPDase8 was responsible for the major ectonucleotidase activity in liver. The ion requirement, apparent K(m) values, nucleotide hydrolysis profile, and preference as well as the resistance to azide were similar for recombinant NTPDase8s and both purified rat NTPDase8 and porcine canalicular ecto-ATPase/ATPDase. The partial NH(2)-terminal amino acid sequences of all NTPDase8s share high identity with the purified liver canalicular ecto-ATPase/ATPDase. Histochemical analysis showed high ectonucleotidase activities in bile canaliculi and large blood vessels of rat liver, in agreement with the immunolocalization of NTPDase1, 2, and 8 with antibodies developed for this study. No NTPDase3 expression could be detected in liver. In conclusion, NTPDase8 is the canalicular ecto-ATPase/ATPDase and is responsible for the main hepatic NTPDase activity. The canalicular localization of this enzyme suggests its involvement in the regulation of bile secretion and/or nucleoside salvage.

Purification and characterization of NTPDase1 (ecto-apyrase) and NTPDase2 (ecto-ATPase) from porcine brain cortex synaptosomes.

We purified to homogeneity and characterized NTPDase1 and NTPDase2 from porcine brain cortex synaptosomes. SDS/PAGE and immunoblotting with antibodies specific to these enzymes revealed a molecular mass estimated at 72 kDa for NTPDase1 and 66 for NTPDase2. Both enzymes exhibited kinetic properties typical for all members of the NTPDase family, e.g. low substrate specificity for tri- and diphosphonucleosides, divalent cations dependency and insensitivity towards ATPase inhibitors. The calculated Km value for NTPDase1 in respect to ATP as a substrate (97 microm) was three times lower in comparison to analogous values for NTPDase2 (270 microm). Additionally, NTPDase1 had a three times higher Kcat/Km coefficient than NTPDase2 (860 and 833 micromol product.s(-1), respectively). We have also demonstrated that in spite of differences in the affinity of ATP for both hydrolases, these enzymes have similar molecular activity. Taken together, these results indicate that NTPDase1 would terminate P2 receptor-mediated signal transmission whereas activity of NTPDase2 may contribute to decreasing high (toxic) concentrations of ATP and/or to production of another signal molecule, ADP.

Cloning and characterization of mouse nucleoside triphosphate diphosphohydrolase-8.

A novel mammalian plasma membrane bound nucleoside triphosphate diphosphohydrolase (NTPDase), named NTPDase8, has been cloned and characterized. Analysis of cDNA reveals an open reading frame of 1491 base pairs encoding a protein of 497 amino acid residues with an estimated molecular mass of 54650 Da and a predicted isoelectric point of 5.94. In a mouse, the genomic sequence is located on chromosome 2A3 and is comprised of 10 exons. The deduced amino acid sequence reveals eight putative N-glycosylation sites, two transmembrane domains, five apyrase-conserved regions, and 20-50% amino acid identity with other mammalian NTPDases. mRNA expression was detected in liver, jejunum, and kidney. Both intact cells and crude cell lysates from COS-7 cells expressing NTPDase8 hydrolyzed P2 receptor agonists, namely, ATP, ADP, UTP, and UDP, but did not hydrolyze AMP. There was an absolute requirement for divalent cations for the catalytic activity (Ca(2+) > Mg(2+)) with an optimal pH between 5.5 and 8.0 for ATP and 6.4 for ADP hydrolysis. Kinetic parameters derived from analysis of crude cell lysates showed that the enzyme had lower apparent K(m) values for adenine nucleotides and for triphosphonucleosides (K(m,app) of 13 microM for ATP, 41 microM for ADP, 47 microM for UTP, and 171 microM for UDP). Hydrolysis of triphosphonucleosides resulted in a transient accumulation of the corresponding diphosphonucleoside, as expected from the apparent K(m) values. Enzymatic properties of NTPDase8 differ from those of other NTPDases suggesting an alternative way to modulate nucleotide levels and consequently P2 receptor activation.