Thromboregulation by endothelial cells: significance for occlusive vascular diseases

Thrombosis and Inflammation—A Dynamic Interplay and the Role of Glycosaminoglycans and Activated Protein C

Hemostasis, thrombosis, and inflammation are tightly interconnected processes which may give rise to thrombo-inflammation, involved in infectious and non-infectious acute and chronic diseases, including cardiovascular diseases (CVD). Traditionally, due to its hemostatic role, blood coagulation is isolated from the inflammation, and its critical contribution in the progressing CVD is underrated, until the full occlusion of a critical vessel occurs. Underlying vascular injury exposes extracellular matrix to deposit platelets and inflammatory cells. Platelets being key effector cells, bridge all the three key processes (hemostasis, thrombosis, and inflammation) associated with thrombo-inflammation. Under physiological conditions, platelets remain in an inert state despite the proximity to the endothelium and other cells which are decorated with glycosaminoglycan (GAG)-rich glycocalyx (GAGs). A pathological insult to the endothelium results in an imbalanced blood coagulation system hallmarked by increased thrombin generation due to losses of anticoagulant and cytoprotective mechanisms, i.e., the endothelial GAGs enhancing antithrombin, tissue factor pathway-inhibitor (TFPI) and thrombomodulin-protein C system. Moreover, the loss of GAGs promotes the release of mediators, such as von Willebrand factor (VWF), platelet factor 4 (PF4), and P-selectin, both locally on vascular surfaces and to circulation, further enhancing the adhesion of platelets to the affected sites. Platelet-neutrophil interaction and formation of neutrophil extracellular traps foster thrombo-inflammatory mechanisms exacerbating the cardiovascular disease course. Therefore, therapies which not only target the clotting mechanisms but simultaneously or independently convey potent cytoprotective effects hemming the inflammatory mechanisms are expected to provide clinical benefits. In this regard, we review the cytoprotective protease activated protein C (aPC) and its strong anti-inflammatory effects thereby preventing the ensuing thrombotic complications in CVD. Furthermore, restoring GAG-like vasculo-protection, such as providing heparin-proteoglycan mimetics to improve regulation of platelet and coagulation activity and to suppress of endothelial perturbance and leukocyte-derived pro-inflammatory cytokines, may provide a path to alleviate thrombo-inflammatory disorders in the future. The vascular tissue-modeled heparin proteoglycan mimic, antiplatelet and anticoagulant compound (APAC), dual antiplatelet and anticoagulant, is an injury-targeting and locally acting arterial antithrombotic which downplays collagen- and thrombin-induced and complement-induced activation and protects from organ injury.

Role of Membrane Lipid Rafts in MRP4 (ABCC4) Dependent Regulation of the cAMP Pathway in Blood Platelets

BACKGROUND

Platelet cytosolic cyclic adenosine monophosphate (cAMP) levels are balanced by synthesis, degradation, and efflux. Efflux can occur via multidrug resistant protein-4 (MRP4; ABCC4) present on dense granule and/or plasma membranes. As lipid rafts have been shown to interfere on cAMP homeostasis, we evaluated the relationships between the distribution and activity of MRP4 in lipid rafts and cAMP efflux.

METHODS

Platelet activation and cAMP homeostasis were analyzed in human and wild-type or MRP4-deleted mouse platelets in the presence of methyl-β-cyclodextrin (MßCD) to disrupt lipid rafts, and of activators of the cAMP signalling pathways. Human platelet MRP4 and effector proteins of the cAMP pathway were analyzed by immunoblots in lipid rafts isolated by differential centrifugation.

RESULTS

MßCD dose dependently inhibited human and mouse platelet aggregation without affecting per se cAMP levels. An additive inhibitory effect existed between the adenylate cyclase (AC) activator forskolin and MßCD that was accompanied by an overincrease of cAMP, and which was significantly enhanced upon MRP4 deletion. Finally, an efflux of cAMP out of resting platelets incubated with prostaglandin E1 (PGE1) was observed that was partly dependent on MRP4. Lipid rafts contained a small fraction (≈15%) of MRP4 and most of the inhibitory G-protein Gi, whereas Gs protein, AC3, and phosphodiesterases PDE2 and PDE3A were all present as only trace amounts.

CONCLUSION

Our results are in favour of part of MRP4 present at the platelet surface, including in lipid rafts. Lipid raft integrity is necessary for cAMP signalling regulation, although MRP4 and most players of cAMP homeostasis are essentially located outside rafts.

Sterile inflammation in thoracic transplantation

The life-saving benefits of organ transplantation can be thwarted by allograft dysfunction due to both infectious and sterile inflammation post-surgery. Sterile inflammation can occur after necrotic cell death due to the release of endogenous ligands [such as damage-associated molecular patterns (DAMPs) and alarmins], which perpetuate inflammation and ongoing cellular injury via various signaling cascades. Ischemia-reperfusion injury (IRI) is a significant contributor to sterile inflammation after organ transplantation and is associated with detrimental short- and long-term outcomes. While the vicious cycle of sterile inflammation and cellular injury is remarkably consistent amongst different organs and even species, we have begun understanding its mechanistic basis only over the last few decades. This understanding has resulted in the developments of novel, yet non-specific therapies for mitigating IRI-induced graft damage, albeit with moderate results. Thus, further understanding of the mechanisms underlying sterile inflammation after transplantation is critical for identifying personalized therapies to prevent or interrupt this vicious cycle and mitigating allograft dysfunction. In this review, we identify common and distinct pathways of post-transplant sterile inflammation across both heart and lung transplantation that can potentially be targeted.

Nucleotide and nucleoside involvement in immunomodulation in experimental Chagas disease

The aim of this study was to evaluate whether Trypanosma cruzi infections cause alterations in the levels of seric purines, which could contribute to host immunomodulation. Twelve mice were divided into two groups identified as control (uninfected) and infected (T. cruzi) groups. The influence of the disease on seric purine levels was verified on day 20 post-infection (PI) by HPLC. Infected mice had circulating trypomastigotes during the experiment, as well as amastigote forms in the heart associated with inflammatory infiltrates. Increases on adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine (ADO), inosine (INO), and uric acid (URIC) levels were observed in the infected animals, while the adenosine monophosphate (AMP) and xanthine (XAN) levels were reduced compared with mice of the control group, indicating a possible impairment on the purinergic system, and consequently, on the immune system during the clinical course of the disease. In summary, the T. cruzi infection alters the seric purine levels, and consequently, modulates the immune system.

Enzymes that hydrolyze adenine nucleotides in a model of hypercholesterolemia induced by Triton WR-1339: protective effects of β-caryophyllene

Purinergic system has been proven to play a critical role in the inflammatory process and to represent an important therapeutic target to improve the immune response during hypercholesterolemia. β-caryophyllene, a phytocannabinoid compound, has a powerful hypolipidemic and anti-inflammatory actions. However, the effects of β-caryophyllene on seric enzymes of purinergic system have not been evaluated. The purpose of this study was to investigate whether β-caryophyllene is able to ameliorate the seric activities of NTPDase and adenosine deaminase (ADA) in a model of hypercholesterolemia induced by Triton WR-1339. The activities of NTPDase and ADA were evaluated enzymatically, and the seric levels of β-caryophyllene were evaluated by high-performance liquid chromatography. We found that treatment with β-caryophyllene ameliorates the enzymatic activities of NTPDase and ADA in serum of hypercholesterolemic rats, in a concentration-dependent manner. These results indicated that β-caryophyllene treatment could improve the immune response during hypercholesterolemia through purinergic pathway.

Role of the CD39/CD73 Purinergic Pathway in Modulating Arterial Thrombosis in Mice

OBJECTIVE

Circulating blood cells and endothelial cells express ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and ecto-5'-nucleotidase (CD73). CD39 hydrolyzes extracellular ATP or ADP to AMP. CD73 hydrolyzes AMP to adenosine. The goal of this study was to examine the interplay between CD39 and CD73 cascade in arterial thrombosis.

APPROACH AND RESULTS

To determine how CD73 activity influences in vivo thrombosis, the time to ferric chloride-induced arterial thrombosis was measured in CD73-null mice. In response to 5% FeCl3, but not to 10% FeCl3, there was a significant decrease in the time to thrombosis in CD73-null mice compared with wild-type mice. In mice overexpressing CD39, ablation of CD73 did not inhibit the prolongation in the time to thrombosis conveyed by CD39 overexpression. However, the CD73 inhibitor α-β-methylene-ADP nullified the prolongation in the time to thrombosis in human CD39 transgenic (hC39-Tg)/CD73-null mice. To determine whether hematopoietic-derived cells or endothelial cell CD39 activity regulates in vivo arterial thrombus, bone marrow transplant studies were conducted. FeCl3-induced arterial thrombosis in chimeric mice revealed a significant prolongation in the time to thrombosis in hCD39-Tg reconstituted wild-type mice, but not on wild-type reconstituted hCD39-Tg mice. Monocyte depletion with clodronate-loaded liposomes normalized the time to thrombosis in hCD39-Tg mice compared with hCD39-Tg mice treated with control liposomes, demonstrating that increased CD39 expression on monocytes protects against thrombosis.

CONCLUSIONS

These data demonstrate that ablation of CD73 minimally effects in vivo thrombosis, but increased CD39 expression on hematopoietic-derived cells, especially monocytes, attenuates in vivo arterial thrombosis.

The effect of reactive oxygen species on whole blood aggregation and the endothelial cell-platelet interaction in patients with coronary heart disease

Background

The effect of reactive oxygen species (ROS) on platelet function in coronary heart disease (CHD) is complex and poorly defined. Platelet aggregation studies in healthy volunteers have demonstrated contrasting results when platelets are exposed to ROS. We investigated the effect of ROS on whole blood aggregation (WBA) and the endothelial cell-platelet interaction in patients with CHD.

Methods and Results

ROS generated by xanthine and xanthine oxidase caused a concentration-dependent inhibition of WBA in blood from healthy donors and patients with CHD. In patients with CHD, 100 μM xanthine and 100 mU/ml xanthine oxidase inhibited WBA in response to 3 μg/ml collagen by 28.9% (95% CI 15.9%-41.8%, p < 0.001) and in response to 5 μM ADP by 36.0% (95% CI 9.6%-62.4%, p = 0.005). Using nitrotyrosine expression, platelets isolated from patients with CHD were found to be susceptible to peroxynitrite damage. The addition of 1 × 10⁵ cultured endothelial cells inhibited WBA in response to 3 μg/ml collagen by 31.2% (95% CI 12.2%-50.2%, p < 0.05) and in response to 5 μM ADP by 31.6% (95% CI 2.5-60.7%, p < 0.05). Addition of xanthine and xanthine oxidase did not alter this effect, however pre-treatment of endothelial cells with a nitric oxide synthase inhibitor (L-NAME) partly reversed the inhibition.

Conclusion

ROS inhibit WBA in blood from patients with CHD. Whilst endothelial cells also inhibit WBA, the effect is attenuated by L-NAME, suggesting that nitric oxide is likely to remain an important protective mechanism against thrombosis in CHD.

Lung cancer alters the hydrolysis of nucleotides and nucleosides in platelets

BACKGROUND

The nucleotides and nucleosides of adenine are signaling molecules related to thromboregulation and modulation of immune responses in patients with malignancies. Thus, this study aims to determine NTPDase, 5'-nucleotidase, ectonucleotide pyrophosphatase/phosphodiesterase (E-NPP) and adenosine deaminase (ADA) activities in the platelets of patients with lung cancer.

METHODS

We collected blood samples from patients (n=33) previously treated for lung cancer with chemotherapy. Patients were classified as stage IIIb and IV according to the Union for International Cancer Control (UICC).

RESULTS

Patients showed a significant decrease in the hydrolysis of adenosine diphosphate (ADP) and adenosine, whereas the adenosine monophosphate (AMP) hydrolysis and platelet aggregation were significantly increased in this group. Adenosine triphosphate (ATP) hydrolysis did not show significant results between the group of patients and the control group.

CONCLUSIONS

We may suggest that ectonucleotidases as well as ADA are enzymes involved in thromboembolic events but especially here we may see that they are also directly involved in the generation of adenosine formation in the cancer patient circulation.

Nucleotide degrading enzymes in platelets from uterine cervical neoplasia patients treated with conization or radiotherapy

BACKGROUND

Uterine cervical neoplasia is an important worldwide malignancy sometimes associated with thrombosis. Ectonucleotidases are membrane-bound enzymes which participate in thromboregulation by hydrolyzing adenine nucleotides in the extracellular medium. In this sense, we aimed to investigate their activity in patients with uterine cervical neoplasia.

METHODS

We evaluated NTPDase and 5'-nucleotidase activities from patients previously treated for uterine cervical neoplasia with either conization or radiotherapy (RTX). These patients were divided into four groups: two conization groups (I and II) and two RTX groups (III and IV), which were further divided based on the amount of time that had passed since the conclusion of their treatment, where groups I and III were extended-remission-period groups (patients with 1 to 5 years elapsed after the conclusion of treatment), and groups II and IV were recently treated patients (treated up to three months before).

RESULTS

For both conization and RTX groups, ATP and ADP hydrolysis decreased in the extended-remission groups when compared to the control and recently treated groups. On the other hand, AMP hydrolysis was decreased in all the treated groups (both conization and RTX) compared to the control. CD39 expression was decreased in extended-remission groups (I and III) when compared to the other groups.

CONCLUSIONS

NTPDase protects against platelet aggregation and 5'-nucleotidase is more involved in the control of adenosine formation.

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.

Pro-healing drug-eluting stents: a role for antioxidants?

Current strategies to lower the incidence of ISR (in-stent restenosis) following PCI (percutaneous coronary intervention) are aimed at modifying arterial healing after stent injury. This can impair endothelial recovery and render the vessel prone to acute thrombosis. As early restoration of endothelial integrity inhibits neointimal growth and thrombosis, alternative approaches which encourage this process may provide a more effective long-term result after PCI. Oxidative stress is enhanced after PCI and participates in the regulation of endothelial regeneration and neointimal growth. Moreover, evidence suggests antioxidants improve re-endothelialization and inhibit ISR. By promoting, rather than blocking, the healing process, antioxidant and other therapies may offer an alternative or additional approach over the antiproliferative approaches common to many current devices.

NTPDase and 5'-nucleotidase activities in platelets of human pregnants with a normal or high risk for thrombosis

The nucleotide degrading enzymes, ectonucleotidases, present on the platelet surface of human pregnant with a normal (without complications) or high risk for thrombosis (hypertension and gestational diabetes) were studied. NTPDase (E.C. 3.6.1.5, CD39) and 5'-nucleotidase (E.C. 3.1.3.5, CD73) activities of four patient groups, non-pregnant (NP, n = 18), pregnant without complications (P, n = 25), pregnant with hypertension (HP, n = 15) and pregnant with gestational diabetes mellitus (GDP, n = 10), were analyzed. Increased NTPDase activities were observed in the groups P (37.0%, S.D. = 2.03 and 34.0%, S.D. = 3.19), HP (40.0%, S.D. = 3.32 and 56.0%, S.D. = 3.25) and GDP (23.0%, S.D. = 2.30 and 42.0%, S.D. = 2.26) in comparison to the control group NP (p < 0.01, S.D. = 1.92 and S.D. = 2.48) when ATP and ADP were used as substrate, respectively. AMP was used as substrate to determine the 5'-nucleotidase activities, which showed to be elevated in the groups P (45.0%, S.D. = 1.73), HP (54.0%, S.D. = 2.64) and GDP (68.0%, S.D. = 1.69) when compared to the control group NP (p < 0.01, S.D. = 1.26). However, no statistically significant differences were observed between the groups P, HP and GDP. As a consequence, the enhanced ATP, ADP and AMP hydrolysis was ascribed to the pregnancy itself, independent of a normal or high risk for thrombosis. The enhanced NTPDase and 5'-nucleotidase activities in platelets suggest that these enzymes are involved in the thromboregulation process in the pregnancy.

Enzymes that hydrolyze adenine nucleotides of patients with hypercholesterolemia and inflammatory processes

The activity of NTPDase (EC 3.6.1.5, apyrase, CD39) was verified in platelets from patients with increasing cholesterol levels. A possible association between cholesterol levels and inflammatory markers, such as oxidized low-density lipoprotein, highly sensitive C-reactive protein and oxidized low-density lipoprotein autoantibodies, was also investigated. Lipid peroxidation was estimated by measurement of thiobarbituric acid reactive substances in serum. The following groups were studied: group I, < 150 mg.dL(-1) cholesterol; group II, 151-200 mg.dL(-1) cholesterol; group III, 201-250 mg.dL(-1) cholesterol; and group IV, > 251 mg.dL(-1) cholesterol. The results demonstrated that both ATP hydrolysis and ADP hydrolysis were enhanced as a function of cholesterol level. Low-density lipoprotein levels increased concomitantly with total cholesterol levels. Triglyceride levels were increased in the groups with total cholesterol above 251 mg.dL(-1). Oxidized low-density lipoprotein levels were elevated in groups II, III, and IV. Highly sensitive C-reactive protein was elevated in the group with cholesterol levels higher than 251 mg.dL(-1). Oxidized low-density lipoprotein autoantibodies were elevated in groups III and IV. Thiobarbituric acid reactive substance content was enhanced as a function of cholesterol level. In summary, hypercholesterolemia is associated with enhancement of inflammatory response, oxidative stress, and ATP and ADP hydrolysis. The increased ATP and ADP hydrolysis in group IV was confirmed by an increase in CD39 expression on its surface. The increase in CD39 activity is possibly related to a compensatory response to the inflammatory and pro-oxidative state associated with hypercholesterolemia.

NTPDase and 5'-nucleotidase activities in physiological and disease conditions: new perspectives for human health

Extracellular nucleotides and nucleosides act as signaling molecules involved in a wide spectrum of biological effects. Their levels are controlled by a complex cell surface-located group of enzymes called ectonucleotidases. There are four major families of ectonucleotidases, nucleoside triphosphate diphosphohydrolases (NTPDases/CD39), ectonucleotide pyrophosphatase/phosphodiesterases (E-NPPs), alkaline phosphatases and ecto-5'-nucleotidase. In the last few years, substantial progress has been made toward the molecular identification of members of the ectonucleotidase families and their enzyme structures and functions. In this review, there is an emphasis on the involvement of NTPDase and 5'-nucleotidase activities in disease processes in several tissues and cell types. Brief background information is given about the general characteristics of these enzymes, followed by a discussion of their roles in thromboregulatory events in diabetes, hypertension, hypercholesterolemia and cancer, as well as in pathological conditions where platelets are less responsive, such as in chronic renal failure. In addition, immunomodulation and cell-cell interactions involving these enzymes are considered, as well as ATP and ADP hydrolysis under different clinical conditions related with alterations in the immune system, such as acute lymphoblastic leukemia (ALL), B-chronic lymphocytic leukemia (B-CLL) and infections associated with human immunodeficiency virus (HIV). Finally, changes in ATP, ADP and AMP hydrolysis induced by inborn errors of metabolism, seizures and epilepsy are discussed in order to highlight the importance of these enzymes in the control of neuronal activity in pathological conditions. Despite advances made toward understanding the molecular structure of ectonucleotidases, much more investigation will be necessary to entirely grasp their role in physiological and pathological conditions.

Relation of CD39 to plaque instability and thrombus formation in directional atherectomy specimens from patients with stable and unstable angina pectoris

To determine whether the expression of CD39 in coronary atherosclerotic lesions is related to plaque instability and thrombus formation, we assessed directional coronary atherectomy (DCA) specimens from patients with stable and unstable angina pectoris. CD39 immunoreactivity was decreased in culprit lesions in patients with unstable angina pectoris compared with those with stable angina pectoris, and was reduced in DCA specimens with thrombus formation. These results suggest that CD39 expressed in atheromatous plaque plays an important role in preventing acute coronary syndromes.

Kinetics of expression of the salivary apyrases in Triatoma infestans

Apyrases are nucleoside triphosphate-diphosphohydrolases that remove Pi from ATP and ADP. The blood feeding reduviid Triatoma infestans, which transmits the Trypanosoma cruzi agent of Chagas disease to animals and man, presents in its salivary glands five apyrases with molecular masses of 88, 82, 79, 68 and 67 kDa. These triatomine apyrases have been associated with the prevention of ADP induced platelet aggregation in the host. Here we provide biochemical data showing that these apyrases are stored in the lumen of the salivary gland D1 pairs, and that about one half of the pool of the enzyme is consumed during feeding. After the feeding recovery of apyrases to maximal activity level takes days, thus suggesting de novo protein synthesis. This hypothesis is supported by quantitative RT-PCR analysis which shows an upregulation of the 79 kDa apyrase mRNA level after feeding.

Heterologous cell-cell interactions: thromboregulation, cerebroprotection and cardioprotection by CD39 (NTPDase-1)

Blood platelets maintain vascular integrity and promote primary and secondary hemostasis following interruption of vessel continuity. Biochemical or physical damage to the coronary, carotid or peripheral arteries is followed by excessive platelet activation and recruitment culminating in vascular occlusion and tissue ischemia. Currently inadequate therapeutic approaches to stroke and coronary artery disease are a public health issue. Following our demonstration of neutrophil leukotriene production from arachidonate released from activated aspirin-treated platelets, we studied interactions between platelets and other blood cells, leading to concepts of transcellular metabolism and thromboregulation. Thrombosis has a proinflammatory component whereby biologically active substances are synthesized by interactions between different cell types that could not individually synthesize the product(s). Endothelial cells control platelet reactivity via three biochemical systems-autacoids leading to production of prostacyclin and nitric oxide, and endothelial ecto-ADPase/CD39/NTPDase-1. The autacoids are fluid-phase reactants, not produced by tissues in the basal state. They are only synthesized intracellularly and released upon interactions of cells with an agonist. When released, autacoids exert fleeting actions in the immediate milieu, and are rapidly inactivated. CD39 is an integral component of the endothelial cell surface and is substrate-activated. It maintains vascular fluidity in the complete absence of prostacyclin and nitric oxide, indicating that they are ancillary components of hemostasis. Therapeutic implications for the autacoids have not been compelling because of their transient, local and fleeting action, and limited potency. Conversely, CD39, acting solely on the platelet releasate, is efficacious in three different animal models. It metabolically neutralizes a prothrombotic platelet releasate via deletion of ADP--the major recruiting agent responsible for formation of an occlusive thrombus. In addition, solCD39 reduced ATP- and ischemia-induced norepinephrine release in the heart. This reduction can prevent fatal arrhythmia. Moreover, solCD39 ameliorated the sequelae of stroke in CD39 null mice. CD39 represents the next generation of cardioprotective and cerebroprotective molecules.

A role of the fast ATP-gated P2X1 cation channel in thrombosis of small arteries in vivo

The P2X1 receptor is a fast ATP-gated cation channel expressed in blood platelets, where its role has been difficult to assess due to its rapid desensitization and the lack of pharmacological tools. In this paper, we have used P2X1-/- and wild-type mouse platelets, treated with apyrase to prevent desensitization, to demonstrate the function of P2X1 in the response to thrombogenic stimuli. In vitro, the collagen-induced aggregation and secretion of P2X1-deficient platelets was decreased, as was adhesion and thrombus growth on a collagen-coated surface, particularly when the wall shear rate was elevated. In vivo, the functional role of P2X1 could be demonstrated using two models of platelet-dependent thrombotic occlusion of small arteries, in which blood flow is characterized by a high shear rate. The mortality of P2X1-/- mice in a model of systemic thromboembolism was reduced and the size of mural thrombi formed after a laser-induced vessel wall injury was decreased as compared with normal mice, whereas the time for complete thrombus removal was shortened. Overall, the P2X1 receptor appears to contribute to the formation of platelet thrombi, particularly in arteries in which shear forces are high.

Adherent leukocytes prevent adenosine formation and impair endothelial barrier function by Ecto-5'-nucleotidase/CD73-dependent mechanism

Extracellular purines are important signaling molecules that mediate both inflammatory (ATP, ADP) and anti-inflammatory (adenosine) effects in the vasculature. The duration and magnitude of purinergic signaling is governed by a network of purine-converting ectoenzymes, and endothelial and lymphoid cells are generally characterized by counteracting ATP-inactivating and ATP-regenerating/adenosine-eliminating, phenotypes, respectively. By using cultured human umbilical vein endothelial cells and normal or leukemic lymphocytes as an in vitro model of leukocyte-endothelial interactions, we have identified a link between the adhesion cascade and extracellular purine turnover. Upon adhesion, lymphocytes suppress endothelial purine metabolism via (i) inhibition of ecto-5'-nucleotidase/CD73-mediated AMP hydrolysis, (ii) rapid deamination of the remaining adenosine, and (iii) maintenance of the sustained pericellular ATP level through continuous nucleotide release and phosphotransfer reactions. Compensation of the loss of adenosine promotes vascular barrier function (measured as a paracellular flux of 70 kDa fluorescein isothiocyanate-dextran) and decreases transendothelial leukocyte migration. Together, these data show that adherent lymphocytes attempt to prevent adenosine formation in the endothelial environment that, as a consequence, may impair the vascular barrier function and facilitate the subsequent step of leukocyte transmigration into the tissue. These leukocyte adhesion-mediated shifts in the local nucleotide and nucleoside concentrations represent a previously unrecognized paracrine mechanism affecting the functional state of the targeted vascular endothelium and coordinately regulating lymphocyte trafficking between the blood and tissues.

Soluble purine-converting enzymes circulate in human blood and regulate extracellular ATP level via counteracting pyrophosphatase and phosphotransfer reactions

Extracellular ATP and other purines play a crucial role in the vasculature, and their turnover is selectively governed by a network of ectoenzymes expressed both on endothelial and hematopoietic cells. By studying the whole pattern of purine metabolism in human serum, we revealed the existence of soluble enzymes capable of both inactivating and transphosphorylating circulating purines. Evidence for this was obtained by using independent assays, including chromatographic analyses with 3H-labeled and unlabeled nucleotides and adenosine, direct transfer of gamma-terminal phosphate from [gamma-32P]ATP to NDP/AMP, and bioluminescent measurement of ATP metabolism. Based on substrate-specificity and competitive studies, we identified three purine-inactivating enzymes in human serum, nucleotide pyrophosphatase (EC 3.6.1.9), 5'-nucleotidase (EC 3.1.3.5), and adenosine deaminase (EC 3.5.4.4), whereas an opposite ATP-generating pathway is represented by adenylate kinase (EC 2.7.4.3) and NDP kinase (EC 2.7.4.6). Comparative kinetic analysis revealed that the Vmax values for soluble nucleotide kinases significantly exceed those of counteracting nucleotidases, whereas the apparent Km values for serum enzymes were fairly comparable and varied within a range of 40-70 micro mol/l. Identification of soluble enzymes contributing, along with membrane-bound ectoenzymes, to the active cycling between circulating ATP and other purines provides a novel insight into the regulatory mechanisms of purine homeostasis in the blood.

Role of extracellular ATP metabolism in regulation of platelet reactivity

Extracellular adenosine triphosphate (ATP) regulates platelet reactivity by way of direct action on platelet purinergic receptors or by hydrolysis to adenosine diphosphate (ADP). Subsequent metabolism of ATP and ADP to adenosine monophosphate (AMP) and adenosine inhibits platelet aggregation. Endothelial cell membrane-bound ecto-ATP/ADPase (CD39, E-NTPDase1) is thought to be the main regulator of platelet responsiveness. However, the findings in studies of CD39-knockout mice imply that nucleotidase(s) in plasma regulates circulating adenine nucleotides levels. Understanding extracellular ATP metabolism by CD39 and plasma nucleotidases is therefore important. In this study, alpha-phosphorus 32- and gamma-phosphorus 32-labeled ATP were rapidly metabolized directly to AMP and pyrophosphate in human plasma at pH 7.4, suggesting the presence of pyrophosphatase/phosphodiesterase-like activity. A specific phosphodiesterase substrate, p-nitrophenol-5'-TMP (p-Nph-5'-TMP), was readily hydrolyzed in human plasma. The antiaggregatory action of beta,gamma-methylene-ATP (AMPPCP) (5 micromol/L) was blocked by DMPX, an adenosine-receptor antagonist, suggesting that in plasma, AMPPCP was metabolized to AMP and adenosine. Recombinant soluble CD39 (solCD39) was used to assess the role of CD39 in ATP metabolism. As little as 0.25 microg/mL of solCD39 inhibited ADP-induced platelet aggregation. However, in the presence of ADP-free ATP (10 micromol/L), solCD39 induced platelet aggregation in a dose-dependent manner. Because AMPPCP could not substitute for ATP in solCD39-stimulated platelet aggregation, it is likely that ADP formation from ATP was required. Endogenous CD39 may thus have a hemostatic function by promoting ADP formation from released ATP, in addition to its antiaggregatory properties. A plasma nucleotidase hydrolyzes ATP directly to AMP. This prevents ADP accumulation and generates adenosine, a potent, locally acting inhibitor of platelet reactivity. The presence of both endothelial CD39 and plasma nucleotidase appears to be important in the maintenance of normal hemostasis and prevention of excessive platelet responsiveness.

Cloning, expression, and characterization of a soluble calcium-activated nucleotidase, a human enzyme belonging to a new family of extracellular nucleotidases

The salivary apyrases of blood-feeding arthropods are nucleotide-hydrolyzing enzymes implicated in the inhibition of host platelet aggregation through the hydrolysis of extracellular adenosine diphosphate. A human cDNA homologous to the apyrase cDNA of the blood-feeding bed bug was identified, revealing an open reading frame encoding a 371-amino acid protein. A cleavable signal peptide generates a secreted protein of 333 residues with a predicted core molecular mass of 37,193 Da. Expression in COS-1 cells produced a secreted apyrase in the cell media. The ADPase and ATPase activities were dependent upon calcium, with a pH optimum between pH 6.2 and 7.2. Interestingly, the preferred substrate was not ADP, as might be expected for an enzyme modulating platelet aggregation, but rather UDP, followed by GDP, UTP, GTP, ADP, and ATP. The nucleotidase did not hydrolyze nucleoside monophosphates. Size-exclusion chromatography and Western blot analysis revealed a molecular mass of approximately 34-37 kDa. Treatment of the enzyme with peptide N-glycosidase F indicated that the protein is glycosylated. Northern analysis identified the transcript in a range of human tissues, including testis, placenta, prostate, and lung. No traditional apyrase-conserved regions or nucleotide-binding domains were identified in this human enzyme, indicating membership in a new family of extracellular nucleotidases.

New mechanism of action for cilostazol: interplay between adenosine and cilostazol in inhibiting platelet activation

Cilostazol, a potent phosphodiesterase 3 inhibitor and anti-thrombotic agent, was recently shown to inhibit adenosine uptake into cardiac myocytes and vascular cells. In the present studies, cilostazol inhibited [ H]-adenosine uptake in both platelets and erythrocytes with a median inhibitory concentration (IC ) of 7 micro M. Next collagen-induced platelet aggregation was studied and it was found that adenosine (1 micro M ), having no effect by itself, shifted the IC of cilostazol from 2.66 micro M to 0.38 micro M (p < 0.01). This shifting was due to an enhanced accumulation of cAMP in platelets and was significantly larger than that by the combination of adenosine and milrinone, which has no effect on adenosine uptake. Similarly, cilostazol, by blocking adenosine uptake, enhanced the adenosine-mediated cAMP increase in Chinese hamster ovary cells that overexpress human A receptor. Furthermore, the inhibitory effect of cilostazol on platelet aggregation in whole blood was significantly reversed by ZM241385 (100 n ), an A adenosine receptor antagonist, and by adenosine deaminase (2 U/ml). These data suggest that the inhibitory effects of cilostazol on adenosine uptake and phosphodiesterase 3 together elevate intracellular cAMP, resulting in greater inhibition of agonist-induced platelet activation.