Synthetic Strategies for Dinucleotides Synthesis.

Dinucleoside 5',5'-polyphosphates (DNPs) are endogenous substances that play important intra- and extracellular roles in various biological processes, such as cell proliferation, regulation of enzymes, neurotransmission, platelet disaggregation and modulation of vascular tone. Various methodologies have been developed over the past fifty years to access these compounds, involving enzymatic processes or chemical procedures based either on P(III) or P(V) chemistry. Both solution-phase and solid-support strategies have been developed and are reported here. Recently, green chemistry approaches have emerged, offering attracting alternatives. This review outlines the main synthetic pathways for the preparation of dinucleoside 5',5'-polyphosphates, focusing on pharmacologically relevant compounds, and highlighting recent advances.

Synthesis and preclinical validation of novel P2Y1 receptor ligands as a potent anti-prostate cancer agent.

Purinergic receptor is a potential drug target for neuropathic pain, Alzheimer disease, and prostate cancer. Focusing on the structure-based ligand discovery, docking analysis on the crystal structure of P2Y1 receptor (P2Y1R) with 923 derivatives of 1-indolinoalkyl 2-phenolic compound is performed to understand the molecular insights of the receptor. The structural model identified the top novel ligands, 426 (compound 1) and 636 (compound 2) having highest binding affinity with the docking score of -7.38 and -6.92. We have reported the interaction efficacy and the dynamics of P2Y1R protein with the ligands. The best hits synthesized were experimentally optimized as a potent P2Y1 agonists. These ligands exhibits anti-proliferative effect against the PC-3 and DU-145 cells (IC50 = 15 µM - 33 µM) with significant increase in the calcium level in dose- and time-dependent manner. Moreover, the activation of P2Y1R induced the apoptosis via Capase3/7 and ROS signaling pathway. Thus it is evidenced that the newly synthesized ligands, as a P2Y1R agonists could potentially act as a therapeutic drug for treating prostate cancer.

Impact of Test Conditions on ADP-Induced Platelet Function Results With the Multiplate Assay: Is Further Standardization Required?

BACKGROUND: Platelet function testing was suggested to help tailor P2Y12-inhibitor therapy; however, the lack of proper standardization is still a limitation. METHODS: In a prospective study, we enrolled clopidogrel-treated and P2Y12-inhibitor naive patients to investigate the influence of (1) time from blood collection, (2) stability of the stored Adenosine diphosphate (ADP) reagent, and (3) the use of enoxaparin on results of the Multiplate assay. Measurements were performed from samples kept for 0, 30, 60, 120, and 240 minutes at room temperature before processing. To determine the impact of the reagent stability, freshly thawed ADP was compared with ADP kept for 3 to 5 or 8 to 13 days at 2°C to 8°C. Finally, samples containing enoxaparin at therapeutic or prophylactic doses were compared with enoxaparin-free blood. RESULTS: A total of 180 measurements were performed. ADP-stimulated platelet reactivity values decreased significantly over time (67 ± 40 U to 68 ± 37 U to 58 ± 37 U to 45 ± 33 U to 35 ± 33 U; P < .0001). Consequently, a dramatic reduction was observed in the proportion of patients with high platelet reactivity ( P < .0001). A significant drop in platelet reactivity was observed with ADP stored for 8 to 13 days as compared to freshly thawed ADP ( P = .011). Enoxaparin triggered a slight, concentration-dependent increase in platelet reactivity ( P < .05). CONCLUSION: Test conditions may have profound impacts on the obtained results with the Multiplate assay. Our findings highlight the large influence of the time from sample collection until testing, suggesting that measurements should be performed within an hour of blood collection.

Demystifying P2Y1 Receptor Ligand Recognition through Docking and Molecular Dynamics Analyses.

We performed a molecular modeling analysis of 100 nucleotide-like bisphosphates and 46 non-nucleotide arylurea derivatives previously reported as P2Y1R binders using the recently solved hP2Y1R structures. We initially docked the compounds at the X-ray structures and identified the binding modes of representative compounds highlighting key patterns in the structure-activity relationship (SAR). We subsequently subjected receptor complexes with selected key agonists (2MeSADP and MRS2268) and antagonists (MRS2500 and BPTU) to membrane molecular dynamics (MD) simulations (at least 200 ns run in triplicate, simulation time 0.6-1.6 µs per ligand system) while considering alternative protonation states of nucleotides. Comparing the temporal evolution of the ligand-protein interaction patterns with available site-directed mutagenesis (SDM) data and P2Y1R apo state simulation provided further SAR insights and suggested reasonable explanations for loss/gain of binding affinity as well as the most relevant charged species for nucleotide ligands. The MD analysis also predicted local conformational changes required for the receptor inactive state to accommodate nucleotide agonists.

Modeling Membrane Protein-Ligand Binding Interactions: The Human Purinergic Platelet Receptor.

Membrane proteins, due to their roles as cell receptors and signaling mediators, make prime candidates for drug targets. The computational analysis of protein-ligand binding affinities has been widely employed as a tool in rational drug design efforts. Although efficient implicit solvent-based methods for modeling globular protein-ligand binding have been around for many years, the extension of such methods to membrane protein-ligand binding is still in its infancy. In this study, we extended the widely used Amber/MMPBSA method to model membrane protein-ligand systems, and we used it to analyze protein-ligand binding for the human purinergic platelet receptor (P2Y12R), a prominent drug target in the inhibition of platelet aggregation for the prevention of myocardial infarction and stroke. The binding affinities, computed by the Amber/MMPBSA method using standard parameters, correlate well with experiment. A detailed investigation of these parameters was conducted to assess their impact on the accuracy of the method. These analyses show the importance of properly treating the nonpolar solvation interactions and the electrostatic polarization in the binding of nucleotide agonists and non-nucleotide antagonists to P2Y12R. On the basis of the crystal structures and the experimental conditions in the binding assay, we further hypothesized that the nucleotide agonists lose their bound magnesium ion upon binding to P2Y12R, and our computational study supports this hypothesis. Ultimately, this work illustrates the value of computational analysis in the interpretation of experimental binding reactions.

Medicinal chemistry of adenosine, P2Y and P2X receptors.

Pharmacological tool compounds are now available to define action at the adenosine (ARs), P2Y and P2X receptors. We present a selection of the most commonly used agents to study purines in the nervous system. Some of these compounds, including A1 and A3 AR agonists, P2Y1R and P2Y12R antagonists, and P2X3, P2X4 and P2X7 antagonists, are potentially of clinical use in treatment of disorders of the nervous system, such as chronic pain, neurodegeneration and brain injury. Agonists of the A2AAR and P2Y2R are already used clinically, P2Y12R antagonists are widely used antithrombotics and an antagonist of the A2AAR is approved in Japan for treating Parkinson's disease. The selectivity defined for some of the previously introduced compounds has been revised with updated pharmacological characterization, for example, various AR agonists and antagonists were deemed A1AR or A3AR selective based on human data, but species differences indicated a reduction in selectivity ratios in other species. Also, many of the P2R ligands still lack bioavailability due to charged groups or hydrolytic (either enzymatic or chemical) instability. X-ray crystallographic structures of AR and P2YRs have shifted the mode of ligand discovery to structure-based approaches rather than previous empirical approaches. The X-ray structures can be utilized either for in silico screening of chemically diverse libraries for the discovery of novel ligands or for enhancement of the properties of known ligands by chemical modification. Although X-ray structures of the zebrafish P2X4R have been reported, there is scant structural information about ligand recognition in these trimeric ion channels. In summary, there are definitive, selective agonists and antagonists for all of the ARs and some of the P2YRs; while the pharmacochemistry of P2XRs is still in nascent stages. The therapeutic potential of selectively modulating these receptors is continuing to gain interest in such fields as cancer, inflammation, pain, diabetes, ischemic protection and many other conditions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Identification of Highly Promising Antioxidants/Neuroprotectants Based on Nucleoside 5'-Phosphorothioate Scaffold. Synthesis, Activity, and Mechanisms of Action.

With a view to identify novel and biocompatible neuroprotectants, we designed nucleoside 5'-thiophosphate analogues, 6-11. We identified 2-SMe-ADP(α-S), 7A, as a most promising neuroprotectant. 7A reduced ROS production in PC12 cells under oxidizing conditions, IC50 of 0.08 vs 21 µM for ADP. Furthermore, 7A rescued primary neurons subjected to oxidation, EC50 of 0.04 vs 19 µM for ADP. 7A is a most potent P2Y1-R agonist, EC50 of 0.0026 µM. Activity of 7A in cells involved P2Y1/12-R as indicated by blocking P2Y12-R or P2Y1-R. Compound 7A inhibited Fenton reaction better than EDTA, IC50 of 37 vs 54 µM, due to radical scavenging, IC50 of 12.5 vs 30 µM for ADP, and Fe(II)-chelation, IC50 of 80 vs >200 µM for ADP (ferrozine assay). In addition, 7A was stable in human blood serum, t1/2 of 15 vs 1.5 h for ADP, and resisted hydrolysis by NPP1/3, 2-fold vs ADP. Hence, we propose 7A as a highly promising neuroprotectant.

Modeling ligand recognition at the P2Y12 receptor in light of X-ray structural information.

The G protein-coupled P2Y12 receptor (P2Y12R) is an important antithrombotic target and of great interest for pharmaceutical discovery. Its recently solved, highly divergent crystallographic structures in complex either with nucleotides (full or partial agonist) or with a nonnucleotide antagonist raise the question of which structure is more useful to understand ligand recognition. Therefore, we performed extensive molecular modeling studies based on these structures and mutagenesis, to predict the binding modes of major classes of P2Y12R ligands previously reported. Various nucleotide derivatives docked readily to the agonist-bound P2Y12R, but uncharged nucleotide-like antagonist ticagrelor required a hybrid receptor resembling the agonist-bound P2Y12R except for the top portion of TM6. Supervised molecular dynamics (SuMD) of ticagrelor binding indicated interactions with the extracellular regions of P2Y12R, defining possible meta-binding sites. Ureas, sulfonylureas, sulfonamides, anthraquinones and glutamic acid piperazines docked readily to the antagonist-bound P2Y12R. Docking dinucleotides at both agonist- and antagonist-bound structures suggested interactions with two P2Y12R pockets. Thus, our structure-based approach consistently rationalized the main structure-activity relationships within each ligand class, giving useful information for designing improved ligands.

Nucleotides Acting at P2Y Receptors: Connecting Structure and Function.

Eight G protein-coupled P2Y receptor (P2YR) subtypes are important physiologic mediators. The human P2YRs are fully activated by ATP (P2Y2 and P2Y11), ADP (P2Y1, P2Y12, and P2Y13), UTP (P2Y2 and P2Y4), UDP (P2Y6 and P2Y14), and UDP glucose (P2Y14). Their structural elucidation is progressing rapidly. The X-ray structures of three ligand complexes of the Gi-coupled P2Y12R and two of the Gq-coupled P2Y1Rs were recently determined and will be especially useful in structure-based ligand design at two P2YR subfamilies. These high-resolution structures, which display unusual binding site features, complement mutagenesis studies for probing ligand recognition and activation. The structural requirements for nucleotide agonist recognition at P2YRs are relatively permissive with respect to the length of the phosphate moiety, but less so with respect to base recognition. Nucleotide-like antagonists and partial agonists are also known for P2Y1, P2Y2, P2Y4, and P2Y12Rs. Each P2YR subtype has the ability to be activated by structurally bifunctional agonists, such as dinucleotides, typically, dinucleoside triphosphates or tetraphosphates, and nucleoside polyphosphate sugars (e.g., UDP glucose) as well as the more conventional mononucleotide agonists. A range of dinucleoside polyphosphates, from triphosphates to higher homologs, occurs naturally. Earlier modeling predictions of the P2YRs were not very accurate, but recent findings have provided much detailed structural insight into this receptor family to aid in the rational design of new drugs.

Agonist-bound structure of the human P2Y12 receptor.

The P2Y12 receptor (P2Y12R), one of eight members of the P2YR family expressed in humans, is one of the most prominent clinical drug targets for inhibition of platelet aggregation. Although mutagenesis and modelling studies of the P2Y12R provided useful insights into ligand binding, the agonist and antagonist recognition and function at the P2Y12R remain poorly understood at the molecular level. Here we report the structures of the human P2Y12R in complex with the full agonist 2-methylthio-adenosine-5'-diphosphate (2MeSADP, a close analogue of endogenous agonist ADP) at 2.5 Šresolution, and the corresponding ATP derivative 2-methylthio-adenosine-5'-triphosphate (2MeSATP) at 3.1 Šresolution. These structures, together with the structure of the P2Y12R with antagonist ethyl 6-(4-((benzylsulfonyl)carbamoyl)piperidin-1-yl)-5-cyano-2-methylnicotinate (AZD1283), reveal striking conformational changes between nucleotide and non-nucleotide ligand complexes in the extracellular regions. Further analysis of these changes provides insight into a distinct ligand binding landscape in the δ-group of class A G-protein-coupled receptors (GPCRs). Agonist and non-nucleotide antagonist adopt different orientations in the P2Y12R, with only partially overlapped binding pockets. The agonist-bound P2Y12R structure answers long-standing questions surrounding P2Y12R-agonist recognition, and reveals interactions with several residues that had not been reported to be involved in agonist binding. As a first example, to our knowledge, of a GPCR in which agonist access to the binding pocket requires large-scale rearrangements in the highly malleable extracellular region, the structural and docking studies will therefore provide invaluable insight into the pharmacology and mechanisms of action of agonists and different classes of antagonists for the P2Y12R and potentially for other closely related P2YRs.

Novel vasocontractile role of the P2Y14 receptor: characterization of its signalling in porcine isolated pancreatic arteries.

BACKGROUND AND PURPOSE: The P2Y14 receptor is the newest member of the P2Y receptor family; it is G(i/o) protein-coupled and is activated by UDP and selectively by UDP-glucose and MRS2690 (2-thiouridine-5'-diphosphoglucose) (7-10-fold more potent than UDP-glucose). This study investigated whether P2Y14 receptors were functionally expressed in porcine isolated pancreatic arteries. EXPERIMENTAL APPROACH: Pancreatic arteries were prepared for isometric tension recording and UDP-glucose, UDP and MRS2690 were applied cumulatively after preconstriction with U46619, a TxA2 mimetic. Levels of phosphorylated myosin light chain 2 (MLC2) were assessed with Western blotting. cAMP concentrations were assessed using a competitive enzyme immunoassay kit. KEY RESULTS: Concentration-dependent contractions with a rank order of potency of MRS2690 (10-fold) > UDP-glucose ≥ UDP were recorded. These contractions were reduced by PPTN {4-[4-(piperidin-4-yl)phenyl]-7-[4-(trifluoromethyl)phenyl]-2-naphthoic acid}, a selective antagonist of P2Y14 receptors, which did not affect responses to UTP. Contraction to UDP-glucose was not affected by MRS2578, a P2Y6 receptor selective antagonist. Raising cAMP levels and forskolin, in the presence of U46619, enhanced contractions to UDP-glucose. In addition, UDP-glucose and MRS2690 inhibited forskolin-stimulated cAMP levels. Removal of the endothelium and inhibition of endothelium-derived contractile agents (TxA2, PGF(2α) and endothelin-1) inhibited contractions to UDP glucose. Y-27632, nifedipine and thapsigargin also reduced contractions to the agonists. UDP-glucose and MRS2690 increased MLC2 phosphorylation, which was blocked by PPTN. CONCLUSIONS AND IMPLICATIONS: P2Y14 receptors play a novel vasocontractile role in porcine pancreatic arteries, mediating contraction via cAMP-dependent mechanisms, elevation of intracellular Ca²âº levels, activation of RhoA/ROCK signalling and MLC2, along with release of TxA2, PGF(2α) and endothelin-1.

Potent P2Y1 urea antagonists bearing various cyclic amine scaffolds.

A number of new amine scaffolds with good inhibitory activity in the ADP-induced platelet aggregation assay have been found to be potent antagonists of the P2Y1 receptor. SAR optimization led to the identification of isoindoline 3c and piperidine 4a which showed good in vitro binding and functional activities, as well as improved aqueous solubility. Among them, the piperidine 4a showed the best overall profile with favorable PK parameters.

Highly efficient biocompatible neuroprotectants with dual activity as antioxidants and P2Y receptor agonists.

Currently, there is a need for novel, biocompatible, and effective neuroprotectants for the treatment of neurodegenerative diseases and brain injury associated with oxidative damage. Here, we developed nucleotide-based neuroprotectants acting dually as antioxidants and P2Y-R agonists. To improve the potency, selectivity, and metabolic stability of ATP/ADP, we substituted adenine C2-position by Cl and Pα/Pß position by borano group, 6-9. Nucleotides 6-9 inhibited oxidation in cell-free systems (Fe(II)-H2O2), as detected by ESR (IC50 up to 175 µM), and ABTS assay (IC50 up to 40 µM). They also inhibited FeSO4-induced oxidative stress in PC12 cells (IC50 of 80-200 nM). 2-Cl-ADP(α-BH3), 7a, was found to be the most potent P2Y1-R agonist currently known (EC50 7 nM) and protected primary cortical neurons from FeSO4 insult (EC50 170 nM). In addition, it proved to be metabolically stable in human blood serum (t(1/2) 7 vs 1.5 h for ADP). Hence, we propose 7a as a highly promising neuroprotectant.

Identification of determinants required for agonistic and inverse agonistic ligand properties at the ADP receptor P2Y12.

The ADP receptor P2Y(12) belongs to the superfamily of G protein-coupled receptors (GPCRs), and its activation triggers platelet aggregation. Therefore, potent antagonists, such as clopidogrel, are of high clinical relevance in prophylaxis and treatment of thromboembolic events. P2Y(12) displays an elevated basal activity in vitro, and as such, inverse agonists may be therapeutically beneficial compared with antagonists. Only a few inverse agonists of P2Y(12) have been described. To expand this limited chemical space and improve understanding of structural determinants of inverse agonist-receptor interaction, this study screened a purine compound library for lead structures using wild-type (WT) human P2Y(12) and 28 constitutively active mutants. Results showed that ATP and ATP derivatives are agonists at P2Y(12). The potency at P2Y(12) was 2-(methylthio)-ADP > 2-(methylthio)-ATP > ADP > ATP. Determinants required for agonistic ligand activity were identified. Molecular docking studies revealed a binding pocket for the ATP derivatives that is bordered by transmembrane helices 3, 5, 6, and 7 in human P2Y(12,) with Y(105), E(188), R(256), Y(259), and K(280) playing a particularly important role in ligand interaction. N-Methyl-anthraniloyl modification at the 3'-OH of the 2'-deoxyribose leads to ligands (mant-deoxy-ATP [dATP], mant-deoxy-ADP) with inverse agonist activity. Inverse agonist activity of mant-dATP was found at the WT human P2Y(12) and half of the constitutive active P2Y(12) mutants. This study showed that, in addition to ADP and ATP, other ATP derivatives are not only ligands of P2Y(12) but also agonists. Modification of the ribose within ATP can result in inverse activity of ATP-derived ligands.

Identification of a promising drug candidate for the treatment of type 2 diabetes based on a P2Y(1) receptor agonist.

The activation by extracellular nucleotides of pancreatic P2Y receptors, particularly, the P2Y(1)R subtype, increases insulin secretion. Therefore, we developed analogues of the P2Y(1)R receptor agonist 2-MeS-ADP, as potential antidiabetic drugs. Analogue 3A was found to be a potent P2Y(1)R agonist (EC(50) = 0.038 µM vs 0.0025 µM for 2-MeS-ADP) showing no activity at P2Y(2/4/6)Rs. Analogue 3A was stable at pH 1.4 (t(1/2) = 7.3 h) and resistant to hydrolysis vs 2-MeS-ADP by alkaline phosphatase (t(1/2) = 6 vs 4.5 h), human e-NPP1 (4% vs 16% hydrolysis after 20 min), and human blood serum (30% vs 50% hydrolysis after 24 h). Intravenous administration of 3A in naive rats decreased blood glucose from 155 mg/dL to normal values, ca. 87 mg/dL, unlike glibenclamide, leading to subnormal values (i.e., 63 mg/dL). Similar observations were made for streptozotocin (STZ)-treated and db(+)/db(-) mouse models. Furthermore, 3A inhibits platelet aggregation in vitro and elongates bleeding time in mice (iv administration of 30 mg of 3A/kg), increasing bleeding time to 16 vs 9 min for Prasugrel. Oral administration of 30 mg/kg 3A to rats increased tail bleeding volume, similar to aspirin. These findings suggest that 3A may be an effective treatment for type 2 diabetes by reducing both blood glucose levels and platelet aggregation.

Synthesis and P2Y2 receptor agonist activities of uridine 5'-phosphonate analogues.

We explored the influence of modifications of uridine 5'-methylenephosphonate on biological activity at the human P2Y(2) receptor. Key steps in the synthesis of a series of 5-substituted uridine 5'-methylenephosphonates were the reaction of a suitably protected uridine 5'-aldehyde with [(diethoxyphosphinyl)methylidene]triphenylphosphorane, C-5 bromination and a Suzuki-Miyaura coupling. These analogues behaved as selective agonists at the P2Y(2) receptor, with three analogues exhibiting potencies in the submicromolar range. Although maximal activities observed with the phosphonate analogues were much less than observed with UTP, high concentrations of the phosphonates had no effect on the stimulatory effect of UTP. These results suggest that these phosphonates bind to an allosteric site of the P2Y(2) receptor.

G protein-coupled adenosine (P1) and P2Y receptors: ligand design and receptor interactions.

The medicinal chemistry and pharmacology of the four subtypes of adenosine receptors (ARs) and the eight subtypes of P2Y receptors (P2YRs, activated by a range of purine and pyrimidine mono- and dinucleotides) has recently advanced significantly leading to selective ligands. X-ray crystallographic structures of both agonist- and antagonist-bound forms of the A(2A)AR have provided unprecedented three-dimensional detail concerning molecular recognition in the binding site and the conformational changes in receptor activation. It is apparent that this ubiquitous cell signaling system has implications for understanding and treating many diseases. ATP and other nucleotides are readily released from intracellular sources under conditions of injury and organ stress, such as hypoxia, ischemia, or mechanical stress, and through channels and vesicular release. Adenosine may be generated extracellularly or by cellular release. Therefore, depending on pathophysiological factors, in a given tissue, there is often a tonic activation of one or more of the ARs or P2YRs that can be modulated by exogenous agents for a beneficial effect. Thus, this field has provided fertile ground for pharmaceutical development, leading to clinical trials of selective receptor ligands as imaging agents or for conditions including cardiac arrhythmias, ischemia/reperfusion injury, diabetes, pain, thrombosis, Parkinson's disease, rheumatoid arthritis, psoriasis, dry eye disease, pulmonary diseases such as cystic fibrosis, glaucoma, cancer, chronic hepatitis C, and other diseases.

Boranophosphate isoster controls P2Y-receptor subtype selectivity and metabolic stability of dinucleoside polyphosphate analogues.

Dinucleoside polyphosphates, Np(n)N', exert their physiological effects via P2 receptors (P2Rs). Np(n)N' are attractive drug candidates as they offer better stability and specificity compared to nucleotides, the most common P2R ligands. To further improve the agonist properties of Np(n)N', we synthesized novel isosters of dinucleoside polyphosphates where N and N' are A or U and where the Pα or Pß phosphate groups are replaced by boranophosphate, denoted as Np(n)(α-B)N' or Np(n)(ß-B)N' (n = 3, 4), respectively. The potency of Np(n)(α/ß-B)N' analogues was evaluated at tP2Y(1), hP2Y(2), hP2Y(4), and rP2Y(6) receptors. The most potent P2Y(1)R and P2Y(6)R agonists were the Up(4)(ß-B)A (A isomer, EC(50) of 0.5 µM vs 0.004 µM for 2-SMe-ADP) and Up(3)(α-B)U (B isomer, EC(50) of 0.3 µM vs 0.2 µM for UDP), respectively. The receptor subtype selectivity is controlled by the position of the borano moiety on the Np(n)N' polyphosphate chain and the type of the nucleobase. In addition, Np(n)(α/ß-B)N' proved ∼22-fold more resistant to hydrolysis by e-NPP1, as compared to the corresponding Np(n)N' analogues. In summary, Up(4)(ß-B)A and Up(3)(α-B)U are potent, stable, and highly selective P2Y(1) and P2Y(6) receptor agonists, respectively.

Purinergic P2Y2 receptors mediate rapid Ca(2+) mobilization, membrane hyperpolarization and nitric oxide production in human vascular endothelial cells.

In blood vessels, stimulation of the vascular endothelium by the Ca(2+)-mobilizing agonist ATP initiates a number of cellular events that cause relaxation of the adjacent smooth muscle layer. Although vascular endothelial cells are reported to express several subtypes of purinergic P2Y and P2X receptors, the major isoform(s) responsible for the ATP-induced generation of vasorelaxant signals in human endothelium has not been well characterized. To address this issue, ATP-evoked changes in cytosolic Ca(2+), membrane potential and acute nitric oxide production were measured in isolated human umbilical vein endothelial cells (HUVECs) and profiled using established P2X and P2Y receptor probes. Whereas selective P2X agonist (i.e. α,ß-methyl ATP) and antagonists (i.e. TNP-ATP and PPADS) could neither mimic nor block the observed ATP-evoked cellular responses, the specific P2Y receptor agonist UTP functionally reproduced all the ATP-stimulated effects. Furthermore, both ATP and UTP induced intracellular Ca(2+) mobilization with comparable EC(50) values (i.e. 1-3µM). Collectively, these functional and pharmacological profiles strongly suggest that ATP acts primarily via a P2Y2 receptor sub-type in human endothelial cells. In support, P2Y2 receptor mRNA and protein were readily detected in isolated HUVECs, and siRNA-mediated knockdown of endogenous P2Y2 receptor protein significantly blunted the cytosolic Ca(2+) elevations in response to ATP and UTP, but did not affect the histamine-evoked response. In summary, these results identify the P2Y2 isoform as the major purinergic receptor in human vascular endothelial cells that mediates the cellular actions of ATP linked to vasorelaxation.

Molecular mechanism of action for reversible P2Y12 antagonists.

Recently, reversible antagonists of the P2Y(12) receptor have been reported. However, the mechanisms of binding have not been elucidated. To this end, a number of homology models were built by means of three programs from four templates. A consensus model was derived from those initial models. The final model was created by refining the consensus model with molecular dynamics simulations. The agonist and antagonists of P2Y(12) have been docked in the final model. For the agonist, the Arg256, Lys280, and Phe252 are "hot" residues. For the antagonists, the Lys280 and Phe252 are "hot" residues that have hydrogen bonding contacts and π-π interactions, respectively. These results can explain the observations of mutation experiments and can guide the design of new inhibitors.