Extracellular ATP metabolism in B and T lymphocytes

Direct treatment of mouse or human blood with soluble 5'-nucleotidase inhibits platelet aggregation

OBJECTIVE

Adenosine signaling is known to inhibit platelet aggregation. Extracellular adenosine mainly stems from enzymatic phosphohydrolysis of precursor nucleotides via ecto-5'-nucleotidase. Previous studies suggest that soluble 5'-nucleotidase (5'-NT) derived from Crotalus atrox venom may be clinically beneficial in vascular leakage, myocardial, renal, and intestinal ischemia, or acute lung injury. However, the effects of 5'-NT treatment on platelet aggregation remain unknown. We examined the direct effects of 5'-NT treatment on platelet aggregation in vivo and ex vivo using a whole blood aggregation method.

METHODS AND RESULTS

Platelet aggregation in whole human blood was completely inhibited by 5'-NT. When 5'-[alphabeta-methylene] diphosphate (APCP), a specific 5'-ecto-nucleotidase inhibitor, was added together with 5'-NT, APCP fully restored collagen- or ADP-induced aggregation. Adenosine levels in whole blood were significantly increased after 5'-NT treatment compared to controls and inhibition of platelet aggregation by 5'-NT was completely reversed by pretreatment with the nonspecific adenosine receptor antagonist 8-(p-sulfophenyl)theophylline hydrate (8-SPT), suggesting that 5'-NT inhibits aggregation via increased adenosine signaling. Administration of 5'-NT to mice in vivo abolished ADP- and collagen-induced platelet aggregation and increased adenosine concentrations and tail bleeding time.

CONCLUSIONS

5'-NT treatment inhibits platelet aggregation via generation of increased levels of extracellular adenosine and subsequent adenosine receptor signaling.

The nucleoside triphosphate diphosphohydrolase-1/CD39 is incorporated into human immunodeficiency type 1 particles, where it remains biologically active

Human immunodeficiency virus type 1 (HIV-1) carries a variety of host proteins in addition to virus-encoded structural proteins, both in its envelope and inside the viral particle. Previous studies have reported that the HIV-1 life-cycle is affected by such virus-associated host cell surface proteins. The nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), also known as CD39, is a plasma membrane-bound ectoenzyme that hydrolyzes extracellular ATP and ADP to AMP. It has been shown that CD39 inhibits platelet function, and is thus a critical thromboregulatory molecule. We demonstrate here that host-derived CD39 is acquired by both laboratory-adapted and clinical variants of HIV-1 produced in cellular reservoirs of the virus. Moreover, purified CD39-bearing virions, but not isogenic viruses lacking CD39, display strong ATPase and ADPase activities. It is of particular interest that virions bearing this cellular enzyme can inhibit ADP-induced platelet aggregation, an effect blocked by an NTPDase inhibitor. On the basis of published and the present data on the functionality of human cellular proteins embedded within HIV-1, it can be proposed that these proteins might contribute to some of the immunologic deficiencies seen in infected individuals.

Secretion of IL-2 and IFN-gamma, but not IL-4, by antigen-specific T cells requires extracellular ATP

Extracellular ATP and other nucleotides transmit signals to cells via surface-associated molecules whose binding sites face the extracellular milieu. Ecto-nucleoside triphosphate diphosphohydrolase is such an ATP-binding enzyme that is expressed by activated lymphocytes. We have previously shown that nonhydrolyzable ATP analogs block the lytic activity of NK cells and CD8(+) T cells as well as their E-NTPDase activity. These results suggest that the hydrolysis of ATP may play a role in lymphocyte function. Here we report that E-NTPDase activity is up-regulated within 15 min of T cell stimulation and that reversible and irreversible enzyme inhibitors profoundly reduce secretion of IL-2 and IFN-gamma, but not IL-4. TNF-alpha, IL-10, and IL-5 production showed intermediate sensitivity to these ATP analogs. Depletion of extracellular ATP also inhibited secretion of IFN-gamma, but not IL-4, supporting the interpretation that extracellular ATP is required for secretion of some, but not all, cytokines. E-NTPDase antagonists reduced transcription of IL-2 mRNA and inhibited TCR-mediated intracellular calcium flux. These results suggest that extracellular ATP plays an essential role in the TCR-mediated signal transduction cascade for expression of certain cytokine genes.

Tumor-promoting functions of adenosine

Tumor growth is a multifactorial process that, in addition to mutations leading to dysregulated expression of oncogenes and tumor suppressive genes, requires specific conditions that provide a supportive physiological environment at the primary and metastatic sites of the disease. Adenosine is one of the factors potentially contributing to tumor growth that thus far has not received adequate attention, despite evidence for a broad range of cytoprotective, growth-promoting, and immunosuppressive activities. Adenosine accumulates in solid tumors at high concentrations, and has been shown to stimulate tumor growth and angiogenesis and to inhibit cytokine synthesis, adhesion of immune cells to the endothelial wall, and the function of T-cells, macrophages, and natural killer cells. However, the mechanisms whereby adenosine accumulates in cancer and the specific effects that result from this accumulation are not well understood. This article surveys the available evidence that supports an important role of adenosine in cancer.

CFTR-independent ATP release from epithelial cells triggered by mechanical stimuli

Cystic fibrosis transmembrane conductance regulator (CFTR)-mediated ATP efflux has been proposed as an autocrine mechanism for regulating chloride secretion through other types of chloride channels. Although we found in previous studies that wild-type CFTR channels bathed with high-ATP solutions do not conduct ATP at rates that can be measured with the patch-clamp technique, those experiments would not have detected very small or electroneutral ATP fluxes through CFTR or ATP efflux through other pathways that might be regulated by CFTR. To examine these possibilities, we have now used a sensitive luciferase luminometric assay to measure ATP efflux from epithelial and nonepithelial cell lines. Adenosine 3',5'-cyclic monophosphate (cAMP) stimulation did not raise external ATP concentration above the background noise in any of the cell lines tested [T84, Calu-3, 9HTEo- and sigma CFTE29o- (colonic and airway human epithelial cells, respectively), NIH/3T3 fibroblasts, and Chinese hamster ovary cells], and variations in ATP release were not correlated with CFTR expression. The rate of ATP release was unaffected by cAMP but was exquisitely sensitive to mechanical perturbations in both CFTR expressing and nonexpressing cells. Mechanically induced, CFTR-independent ATP release may be a physiologically relevant mechanism of epithelial regulation, which has not previously been fully appreciated.

CD39 is an ecto-(Ca2+,Mg2+)-apyrase

CD39, a 70- to 100-kDa molecule expressed primarily on activated lymphoid cells, was previously identified as a surface marker of Epstein Barr virus (EBV)-transformed B cells. In this report, we show that an ecto-(Ca2+,Mg2+)-apyrase activity is present on EBV-transformed B cells, but not on B or T lymphomas. The coincidence between CD39 expression and ecto-apyrase activity on immune cells suggests that CD39 may be an ecto-apyrase. This supposition is supported by the observation that the amino acid sequence of CD39 is significantly homologous to those of several newly identified nucleotide triphosphatases. Finally, we show that CD39 indeed has ecto-apyrase activity by expression in COS-7 cells.

Identification and partial characterization of an ectoATPase expressed by human natural killer cells

An extracellular membrane-associated ectoATPase has been identified on the human natural killer cell line NK3.3. The enzyme is distinct from other classes of ATPases, kinases, and phosphatases. NK3.3 ectoATPase demonstrated a Km for ATP of 41 microM and a Vmax of 0.2 mumol/min and required both Ca2+ and Mg2+ for maximal activity. Purine and pyrimidine nucleotides were competitive inhibitors of the catalytic reaction. Inhibition increased with the addition of increasing negative charge of the phosphate side chain and was also dependent on contributions from the nucleoside. NK3.3 ectoATPase activity was inhibited by reaction with the affinity label [p-(fluorosulfonyl)benzoyl]-5'-adenosine (5'-FSBA), which is shown to modify the enzyme at or near the ATP-binding domain. Photoaffinity labeling of intact NK3.3 cells with [alpha-32P]-8-azidoATP demonstrated an ATP-binding protein of 68-80 kDa unique to NK3.3 cells. A positive correlation was observed between the ability of the various nucleotides to block photoincorporation into the 68-80-kDa protein and their ability to inhibit ectoATPase activity. NK3.3 cells which were made ectoATPase-deficient by reaction with 5'-FSBA demonstrated that this enzyme does not have a major role in the protection of this cytolytic effector cell from the possible lytic effects of extracellular ATP.