Purification and characterization of an adenotin-like adenosine binding protein from human platelets

Blood Platelet Adenosine Receptors as Potential Targets for Anti-Platelet Therapy

Adenosine receptors are a subfamily of highly-conserved G-protein coupled receptors. They are found in the membranes of various human cells and play many physiological functions. Blood platelets express two (A_2A and A_2B) of the four known adenosine receptor subtypes (A₁, A_2A, A_2B, and A₃). Agonization of these receptors results in an enhanced intracellular cAMP and the inhibition of platelet activation and aggregation. Therefore, adenosine receptors A_2A and A_2B could be targets for anti-platelet therapy, especially under circumstances when classic therapy based on antagonizing the purinergic receptor P2Y₁₂ is insufficient or problematic. Apart from adenosine, there is a group of synthetic, selective, longer-lasting agonists of A_2A and A_2B receptors reported in the literature. This group includes agonists with good selectivity for A_2A or A_2B receptors, as well as non-selective compounds that activate more than one type of adenosine receptor. Chemically, most A_2A and A_2B adenosine receptor agonists are adenosine analogues, with either adenine or ribose substituted by single or multiple foreign substituents. However, a group of non-adenosine derivative agonists has also been described. This review aims to systematically describe known agonists of A_2A and A_2B receptors and review the available literature data on their effects on platelet function.

IFN-γ Up-Regulates the A2B Adenosine Receptor Expression in Macrophages: A Mechanism of Macrophage Deactivation

Adenosine is a potent endogenous anti-inflammatory agent released by cells in metabolically unfavorable conditions, such as hypoxia or ischemia. Adenosine modulates different functional activities in macrophages. Some of these activities are believed to be induced through the uptake of adenosine into the macrophages, while others are due to the interaction with specific cell surface receptors. In murine bone marrow-derived macrophages, the use of different radioligands for adenosine receptors suggests the presence of A2B and A3 adenosine receptor subtypes. The presence of A2B receptors was confirmed by flow cytometry using specific Abs. The A2B receptor is functional in murine macrophages, as indicated by the fact that agonists of A2B receptors, but not agonists for A1, A2A, or A3, lead to an increase in cAMP levels. IFN-γ up-regulates the surface protein and gene expression of the A2B adenosine receptor by induction of de novo synthesis. The up-regulation of A2B receptors correlates with an increase in cAMP production in macrophages treated with adenosine receptor agonist. The stimulation of A2B receptors by adenosine or its analogues inhibits the IFN-γ-induced expression of MHC class II genes and also the IFN-γ-induced expression of nitric oxide synthase and of proinflammatory cytokines. Therefore, the up-regulation of the A2B adenosine receptor expression induced by IFN-γ could be a feedback mechanism for macrophage deactivation.

Heterogeneous forms of adenotin-1 of different subcellular localization

The localization of the low-affinity adenosine binding protein adenotin-1 with respect to distribution in rat organs and subcellular compartments was investigated. Adenotin-1 was characterized by 5'-N-ethylcarboxamido[2,8-3H]adenosine ([3H]NECA) binding and Western blotting. Cytosolic as well as membrane fractions of all tissues contained adenotin-1. Highest levels of membrane-bound adenotin-1 were found in the liver (liver > kidney approximately spleen approximately lung > forebrain approximately cerebellum > fat heart - striated muscle), whereas highest levels of cytosolic adenotin-1 were detected in spleen, liver, lung and fat. Subcellular fractions from rat liver were prepared by differential and density gradient centrifugation. Like the homologous proteins endoplasmin or gp96, adenotin-1 is enriched in the endoplasmic reticulum. Cytosolic and membrane-bound adenotin-1 species are pharmacologically distinct, because in the liver particulate fraction adenotin-1 showed a more rapid binding kinetics, a twofold lower affinity for [3H]NECA (KD 227 nM vs. 105 nM) and a sevenfold higher affinity for 2-chloroadenosine than the cytosolic protein (Ki 1.48 microM vs. 9.25 microM). In rat liver cytosol, two different binding sites were found, which differed in [3H]NECA binding kinetics and displayed a hundredfold difference in their affinity for 2-chloro-5'-N-methylcarboxamidoadenosine (Ki 45.8 nM vs. 4.76 microM). The presence of adenotin-1 in subcellular fractions, as determined by radioligand binding, was confirmed by Western blotting. Adenotin-1 was detected as a 98-kDa band in all rat liver subcellular fractions, which agrees with the molecular mass determined for the purified protein. In the cytosol, a 65-kDa hand was labeled more intensely than the 98-kDa band. This additional band probably represents the pharmacologically distinct species of adenotin-1 found in the cytosol.

Calcium mobilization in Jurkat cells via A2b adenosine receptors

1. A functional study of cell surface A2b adenosine receptors was performed on the T cell leukaemia line, Jurkat. 2. A2b receptors were coupled both to the adenylate cyclase system and to intracellular calcium channels. In fact, the agonist of A2b receptors, 5'-N-ethylcarboxamidoadenosine (NECA), led to a transient accumulation of intracellular calcium by an inositol phosphate-independent mechanism. 3. The NECA-induced accumulation of cGMP was not responsible for the calcium mobilization via A2b receptors. 4. The calcium response elicited by activation of A2b receptors was independent of that evoked by activation of the T cell receptor. 5. These findings not only delineate a novel transduction mechanism for adenosine but also support a specific role for adenosine in modulating signals elicited via the T cell receptor.

Purinoceptors and platelet aggregation

1. Adenosine 5'-diphosphate (ADP) is a physiologically important mediator, being released from damaged cells and from aggregating platelets. It acts on platelets to cause aggregation via a purinoceptor ('P2T-purinoceptor') at which adenosine 5'-triphosphate (ATP) is a competitive antagonist; however, the way in which it does so is not fully understood. 2. ADP activates a G12 protein, is a weak activator of phospholipase C but causes calcium mobilization from internal stores, and also inhibits adenylate cyclase. It seems likely that these effects are mediated by a single receptor but this is still unclear. 3. ADP also causes a rapid calcium influx which has the characteristics of a receptor-operated channel, and it has been suggested that this is due to a P2X1 receptor. This suggests the presence of at least two types of receptor responding to ADP, one G protein coupled and one a cation channel, and raises questions about the role of ATP in platelet function. 4. Adenosine acts via an A2a receptor to stimulate adenylate cyclase in platelets, and this nonselectively inhibits platelet activation. As ADP released from platelets is broken down to adenosine by ectonucleotidases on endothelial cells, this may provide an important mechanism for limiting inappropriate platelet aggregation in an intact blood vessel.

Characterization of a novel adenosine binding protein sensitive to cyclic AMP in rat brain cytosolic and particulate fractions

A novel binding site for the adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA), which was enriched in rat forebrain, was characterized in cytosolic and particulate preparations. The site showed a pharmacological profile different from other [3H]NECA binding proteins and was named adenotin 2. [3H]NECA was bound in the presence of 100 microM 2-chloroadenosine with a Kd of 45.4 nM and a Bmax of 4711 fmol/mg in the cytosol and a Kd of 72.4 nM and a Bmax of 4844 fmol/mg in the crude membrane fraction. The presence of two different binding sites on adenotin 2 for [3H]NECA was shown in kinetic experiments. This protein showed identical pharmacological profiles in both subcellular preparations. [3H]NECA was displaced by purine analogues with a rank order of potency of NECA > 3'5' cyclic AMP (cAMP) > 5'-deoxy-5'-chloroadenosine > S-adenosylhomocysteine approximately 5'-deoxy-5'-methylthioadenosine (MeSA) > adenosine approximately adenine. cAMP inhibited [3H]NECA binding allosterically, whereas adenine and MeSA acted competitively. Inhibitors and activators of protein kinases such as N-(2-aminoethyl)-5-isoquinolinesulfonamide, Sp-adenosine cyclic monophophothioate and (8R*, 9S*, 11S*)-(-)-9-hydroxy-9-methoxy -carbonyl-8-methyl-2,3,9, 10-tetrahydro-8,11-epoxy-1H, 8H, 11H-2, 7b, 11a-triazadibenzo-(a,g)cycloocta(cde)-trinden-1-one (K 252a) interacted with [3H]NECA binding to adenotin 2 in nanomolar concentrations. Adenosine-5'-O-(3-thiotriphosphate) (100 microM) increased the affinity of [3H]NECA to a Kd of 9 nM and diminished the affinity of cAMP. The pharmacological characteristics of this novel binding site for [3H]NECA resemble those of the inhibition of phosphorylation processes by adenosine and its derivatives in heart and smooth muscle but are distinct from known adenosine receptors, adenosine binding proteins and protein kinases.

The selective adenosine A2A receptor antagonist SCH 58261 discriminates between two different binding sites for [3H]-CGS 21680 in the rat brain

We have used quantitative autoradiography to further examine two previously described binding sites for [3H]-CGS 21680 in cortical regions and in striatum, respectively. The striatal binding sites largely represent classical adenosine A2A receptors whereas the cortical sites show characteristics that differ from those of recognised adenosine receptors. A recently developed non-xanthine A2A receptor antagonist SCH 58261 displaced the binding of [3H]-CGS 21680 from the A2A receptors in striatum with an estimated Ki value of 2.4 nM, but was more than 1000-fold less potent in displacing its binding from cortex. Conversely, the adenosine analogue 2-chloro-NECA was found to be some 10 times more potent in displacing CGS 21680 from the cortical binding sites than from A2A receptors. The results provide additional evidence that CGS 21680 binds not only to classical A2A receptors, but also to sites that differ from defined adenosine receptors. They also suggest that effects of CGS 21680 observed in the presence of SCH 58261 might reveal the functional significance (if any) of these sites.

Pharmacological and biochemical characterization of purified A2a adenosine receptors in human platelet membranes by [3H]-CGS 21680 binding

1. The binding properties of human platelet A2a adenosine receptors, assayed with the A2a-selective agonist, [3H]-2-[p-(2-carboxyethyl)-phenethylamino]-5'-N-ethylcarboxamidoad enosine ([3H]-CGS 21680), are masked by a non-receptorial component, the adenotin site. In order to separate A2a receptors from adenotin sites, human platelet membranes were solubilized with 1% 3-[(3-cholamidopropyl)dimethyl-ammonio]-1-propanesulphonate (CHAPS). The soluble platelet extract was precipitated with polyethylene glycol (PEG) and the fraction enriched in adenosine receptors was isolated from the precipitate by differential centrifugation. 2. The present paper describes the binding characteristics of the selective A2a agonist, [3H]-CGS 21680, to this purified platelet membrane preparation. In addition, receptor affinity and potency of several adenosine agonists and antagonists were determined in binding and adenylyl cyclase studies. 3. Saturation experiments revealed a single class of binding site with Kd and Bmax values of 285 nM and 2.07 pmol mg-1 of protein respectively. Adenosine receptor ligands competed for the binding of 50 nM [3H]-CGS 21680 to purified protein, showing a rank order of potency consistent with that typically found for interactions with the A2a adenosine receptors. In the adenylyl cyclase assay the compounds examined exhibited a rank order of potency very close to that observed in binding experiments. 4. Thermodynamic data indicated that [3H]-CGS 21680 binding to the purified receptor is totally entropy-driven in agreement with results obtained in rat striatal A2a adenosine receptors. 5. It is concluded that in the purified platelet membranes there is a CGS 21680 binding site showing the characteristic properties of the A2a receptor. This makes it possible to use this compound for reliable radioligand binding studies on the A2a adenosine receptor of human platelets.