Identification of a novel high affinity adenosine binding protein from bovine striatum

Pharmacology of a unique adenosine binding site in rat brain using a selective ligand

1. In order to further characterize the adenosine binding sites that we previously purified and termed P3 purinergic receptor-like protein (P3LP), a reliable binding assay method was developed using [3H]-5'-N-ethylcarboxamidoadenosine (NECA) as a radioligand and the newly developed high-affinity selective ligand 9-(6,7-dideoxy-beta-D-allo-hept-5-ynofuranosyl) adenine (HAK2701) as a selective displacer. 2. Using this assay method, it was found that rat brain membranes possess high- and low-affinity [3H]-NECA binding sites. The high-affinity binding site showed KD and Bmax values of 19.7+/-2.5 nmol/L and 0.192+/-0.05 pmol/mg protein, respectively, and the KD value for the low-affinity binding site was 4260+/-330 nmol/L. The KD value for the high-affinity site agreed well with that of the [3H]-NECA binding site determined with the partially purified P3LP preparation described previously. 3. The distribution of P3LP in rat tissues was determined using the [3H]-NECA binding method described above. The highest level of P3LP was in the cerebellum followed by the olfactory bulb and the spinal cord. 4. The order of the affinity for various purinergic or related compounds to P3LP in rat brain preparations was also determined by the [3H]-NECA binding method to be HAK2701 > NECA = adenosine 5'-O-(2-thiodiphosphate) > cAMP = beta,gamma-methyleneadenosine 5'-triphosphate > diadenosine tetraphosphate > alpha,beta-methyleneadenosine 5'-triphosphate > 5'-deoxy-5'-methylthioadenosine > N6-cyclopentyladenosine. 5. These studies reveal that the [3H]-NECA binding assay in combination with HAK2701 is successful in the characterization of P3LP, especially the membrane-bound form.

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.

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.

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.

Binding characteristics of the adenosine A2 receptor ligand [3H]CGS 21680 to human platelet membranes

The binding characteristics of the selective adenosine A2 agonist [3H]CGS 21680 ([3H]2-[p-(2-carboxyethyl)-phenethyl-amino]-5'-N- ethylcarboxamidoadenosine) were determined in human platelet membranes. Specific binding was saturable, reversible and dependent upon protein concentration. Saturation experiments revealed a single class of binding sites with Kd and Bmax values of 1.4 microM and 5.9 pmol/mg of protein, respectively. Adenosine receptor agonists and antagonists competed for the binding of [3H]CGS 21680 (50 or 200 nM) to human platelet membranes showing a rank order of potency consistent with that typically found for interactions at the adenosine A2 receptor. Adenylate cyclase stimulation and platelet aggregation inhibition induced by adenosine agonists exhibited a rank order of potency close to that observed in binding experiments. However, the adenosine A1 receptor agonists, R- and S-N6-(2-phenylisopropyl)adenosine, (R-PIA) and (S-PIA), N6-cyclohexyladenosine (CHA) and 2-chloro-N6-cyclopentyladenosine (CCPA), which stimulate adenylate cyclase and inhibit platelet aggregation in the low microM range, displaced [3H]CGS 21680 only in the high microM range. In conclusion, we have found that [3H]CGS 21680, which is widely used as a specific A2 agonist in binding studies on brain tissues, is not appropriate for the characterization of the human platelet adenosine A2 receptor.

Characterization of the adenosine receptor in microvascular coronary endothelial cells

In the present work we studied the effect of adenosine and various adenosine analogues on cAMP level in guinea pig coronary endothelial cells of microvascular origin. The tested adenosine agonist mediate a concentration-dependent increase in cAMP level. The rank order of potency was 5'-N-ethylcarboxamidoadenosine (NECA) > CGS 21680 > N6-phenylisopropyladenosine (R-PIA) > 2-chloro-N6-cyclopentyladenosine (CCPA) which is typical for an adenosine A2 receptor. Their respective concentrations for half maximal stimulation of cAMP formation were 0.36 microM, 0.82 microM, 4.7 microM and 9.8 microM. The tested agonists showed differences in efficacy, NECA being the most efficacious. R-PIA, CCPA and adenosine were less efficacious, suggesting partial agonism. The efficacy of adenosine was unchanged by the addition of the nucleoside transport inhibitor S(4-nitrobenzyl)-6-thioinosine (NBTI, 10 microM) suggesting that inhibition of adenylyl cyclase through P-site activation is not responsible for the observed low efficacy of adenosine. We could demonstrate CGS 21680 activation of adenylyl cyclase in a peripheral receptor. We therefore suggest that the endothelial adenosine receptor resembles the striatal adenosine A2a receptor.

The binding of the adenosine A2 receptor selective agonist [3H]CGS 21680 to rat cortex differs from its binding to rat striatum

The binding of the reportedly A2A selective agonist CGS 21680 (2-[p-(2-carboxyethyl)phenylethylamino]-5'N-ethylcarboxamidoadenos ine) to cortex and striatum was examined in parallel using quantitative receptor autoradiography. [3H]CGS 21680 bound to a single site in rat striatum with KD 2.3 nM and Bmax 320 fmol/mg grey matter. In addition [3H]CGS 21680 bound to a single site in the cerebral cortex with KD 47 nM and Bmax 100 fmol/mg grey matter. In cat cortex [3H]CGS 21680 (2 nM) binding was strong and particularly evident in the most superficial layers. The potency order for inhibition of 2 nM [3H]CGS 21680 binding to rat striatum was NECA (5'-N-ethylcarboxamidoadenosine; IC50 9.0 nM) > 2-CADO (2-chloroadenosine; 87 nM) > R-PIA (N6-(R)-phenylisopropyladenosine; 110 nM). The potency order for inhibition of 2 nM [3H]CGS 21680 binding to rat cortex was NECA (3.0 nM) > 2-CADO (14 nM) > or = R-PIA (16 nM). Gpp(NH)p (5'-guanylyl imidodiphosphate) inhibited [3H]CGS 21680 binding to both cortex and striatum, but more potently in cortex (IC50 100 nM vs. 470 nM). The present results show that there is a cortical binding site for [3H]CGS 21680 which appears to be different from the the striatal A2A receptor, the A2B receptor and the A1 receptor.

Adenosine receptors and their modulators

The identification and characterization of adenosine receptors and the development of potent, receptor subtype-selective agonists and antagonists has been an active area of research for the past 20 years. Major recent advances in the field have been the cloning of several adenosine receptor subtypes of different species, including the discovery of a new subtype, designated A3, the discovery and development of new agonists and antagonists, particularly those with selectivity for the A2a adenosine receptor, the characterization of signal transduction pathways, and the development of agents which act indirectly on the adenosine receptor system. The present article focusses on aspects of pharmaceutical/medicinal chemistry related to adenosine receptors.

Characterization of membrane-bound and solubilized high-affinity binding sites for 5'-N-ethylcarboxamido[3H]adenosine from bovine cerebral cortex

A high-affinity binding site for 5'-N-ethylcarboxamido[3H]adenosine ([3H]NECA) from bovine cerebral cortex has been characterized in its membrane-bound and solubilized state after gel filtration on Sepharose CL-6B. For detection of this site in membranes, it was necessary to remove metabolites with high affinities for this site enzymatically, e.g., adenosine by addition of adenosine deaminase and inosine by addition of nucleoside phosphorylase. The pore-forming peptide antibiotic alamethicin further enhanced binding of [3H]NECA to this site in membranes. In contrast to adenosine receptors and the adenotin-like low-affinity binding protein, this novel site was extremely sensitive against treatment with the sulfhydryl alkylating agent N-ethylmaleimide. In competition experiments, this site could be differentiated from adenosine receptors by its high affinity for adenine nucleotides and its lack of affinity for adenosine receptor antagonists. Inosine and its derivative S-(4-nitrobenzyl)-6-thioinosine were relatively potent ligands with Ki values in the high nano- and low micromolar range, respectively. We conclude that the high-affinity NECA binding site described previously in bovine striatum is not exclusively located in the striatum, but can also be detected in membrane preparations and soluble extracts of bovine brain cortex.

Adenosine receptors in the central nervous system

Two major subclasses of adenosine receptors have been distinguished in the central nervous system, termed A1 and A2. They are coupled to G-proteins and regulate the activity of adenylyl cyclase, potassium channels and several other effector systems. Autoradiographic studies have shown that A1 receptors are mainly found in the hippocampus and the cerebellum, whereas A2 receptors are almost exclusively located in the striatum and olfactory tubercle. Furthermore, a novel adenosine binding protein was identified in bovine striatum by radioligand binding with [3H]5'-N-ethylcarboxamidoadenosine ([3H]NECA). The pharmacological profile of this NECA binding protein has been determined in competition experiments with adenosine receptor ligands. It can be distinguished from that of A2 adenosine receptors and other adenosine binding proteins such as S-adenosylhomocysteine hydrolase and the adenosine transporter.