Pharmacological characterization of novel A3 adenosine receptor-selective antagonists

Medicinal chemistry of the A3 adenosine receptor: agonists, antagonists, and receptor engineering

A(3) adenosine receptor (A(3)AR) ligands have been modified to optimize their interaction with the A(3)AR. Most of these modifications have been made to the N(6) and C2 positions of adenine as well as the ribose moiety, and using a combination of these substitutions leads to the most efficacious, selective, and potent ligands. A(3)AR agonists such as IB-MECA and Cl-IB-MECA are now advancing into Phase II clinical trials for treatments targeting diseases such as cancer, arthritis, and psoriasis. Also, a wide number of compounds exerting high potency and selectivity in antagonizing the human (h)A(3)AR have been discovered. These molecules are generally characterized by a notable structural diversity, taking into account that aromatic nitrogen-containing monocyclic (thiazoles and thiadiazoles), bicyclic (isoquinoline, quinozalines, (aza)adenines), tricyclic systems (pyrazoloquinolines, triazoloquinoxalines, pyrazolotriazolopyrimidines, triazolopurines, tricyclic xanthines) and nucleoside derivatives have been identified as potent and selective A(3)AR antagonists. Probably due to the "enigmatic" physiological role of A(3)AR, whose activation may produce opposite effects (for example, concerning tissue protection in inflammatory and cancer cells) and may produce effects that are species dependent, only a few molecules have reached preclinical investigation. Indeed, the most advanced A(3)AR antagonists remain in preclinical testing. Among the antagonists described above, compound OT-7999 is expected to enter clinical trials for the treatment of glaucoma, while several thiazole derivatives are in development as antiallergic, antiasthmatic and/or antiinflammatory drugs.

The P2X(7) receptor in retinal ganglion cells: A neuronal model of pressure-induced damage and protection by a shifting purinergic balance

Retinal ganglion cells process the visual signal and transmit it along their axons in the optic nerve to the brain. Molecular, immunohistochemical, and functional analyses indicate that the majority of retinal ganglion cells express the ionotropic P2X(7) receptor. Stimulation of the receptor can lead to a rise in intracellular calcium and cell death, although death does not involve the opening of a large diameter pore. Adenosine acting at A(3) receptors can attenuate the rise in calcium and death accompanying P2X(7) receptor activation, suggesting that dephosphorylation of ATP into adenosine is neuroprotective and that the balance of extracellular purines can influence neuronal survival. Increased intraocular pressure can lead to release of excessive extracellular ATP in the retina and damage ganglion cells by acting on P2X(7) receptors, implicating a role for the receptor in the loss of ganglion cell activity in glaucoma. In summary, the activation of P2X(7) receptors has both physiologic and pathophysiologic implications for ganglion cell function. These characteristics may also provide an insight into the contributions the P2X(7) receptor makes to neurons elsewhere.

A2A receptors in inflammation and injury: lessons learned from transgenic animals

Adenosine regulates the function of the innate and adaptive immune systems through targeting virtually every cell type that is involved in orchestrating an immune/inflammatory response. Of the four adenosine receptors (A(1), A(2A), A(2B), A(3)), A(2A) receptors have taken center stage as the primary anti-inflammatory effectors of extracellular adenosine. This broad, anti-inflammatory effect of A(2A) receptor activation is a result of the predominant expression of A(2A) receptors on monocytes/macrophages, dendritic cells, mast cells, neutrophils, endothelial cells, eosinophils, epithelial cells, as well as lymphocytes, NK cells, and NKT cells. A(2A) receptor activation inhibits early and late events occurring during an immune response, which include antigen presentation, costimulation, immune cell trafficking, immune cell proliferation, proinflammatory cytokine production, and cytotoxicity. In addition to limiting inflammation, A(2A) receptors participate in tissue remodeling and reparation. Consistent with their multifaceted, immunoregulatory action on immune cells, A(2A) receptors have been shown to impact the course of a wide spectrum of ischemic, autoimmune, infectious, and allergic diseases. Here, we review the regulatory roles of A(2A) receptors in immune/inflammatory diseases of various organs, including heart, lung, gut, liver, kidney, joints, and brain, as well as the role of A(2A) receptors in regulating multiple organ failure and sepsis.

Design, synthesis, and biological evaluation of LNA nucleosides as adenosine A3 receptor ligands

We have prepared a series of adenosine analogs based on the bicyclo[2.2.1]heptane scaffold of locked nucleic acid (LNA) and tested them for both agonist and antagonist activity at the adenosine A(3) receptor. The design of these derivatives was based on the known A(3) agonist IB-MECA and related compounds. Modifications thus include the 5'-uronamides and N(6)-(3-iodobenzyl) derivatives. In this way we have prepared analogs of known A(3) agonists with the sugar ring restricted in an N-conformation. For comparison we have also prepared 2'-O-methyl derivatives of IB-MECA. The LNA nucleosides showed no agonist activity but some of them are potent antagonists. The 2'-O-methyl derivative of IB-MECA is an agonist with similar potency as the parent compound.

Barrier qualities of the mouse eye to topically applied drugs

The mouse eye displays unusually rapid intraocular pressure (IOP) responses to topically applied drugs as measured by the invasive servo-null micropipette system (SNMS). To learn if the time course reflected rapid drug transfer across the thin mouse cornea and sclera, we monitored a different parameter, pupillary size, following topical application of droplets containing 40 microM (0.073 microg) carbachol. No miosis developed from this low carbachol concentration unless the cornea was impaled with an exploring micropipette as used in the SNMS. We also compared the mouse IOP response to several purinergic drugs, measured by the invasive SNMS and non-invasive pneumotonometry. Responses to the previously studied non-selective adenosine-receptor (AR) agonist adenosine, the A(3)-selective agonist Cl-IB-MECA and the A(3)-selective antagonist MRS 1191 were all enhanced to varying degrees, in time and magnitude, by corneal impalement. We conclude that the thin ocular coats of the mouse eye actually present a substantial barrier to drug penetration. Corneal impalement with even fine-tipped micropipettes can significantly enhance entry of topically-applied drugs into the mouse aqueous humor, reflecting either direct diffusion around the tip or a more complex impalement-triggered change in ocular barrier properties. Comparison of invasive and non-invasive measurement methods can document drug efficacy at intraocular target sites even if topical drug penetration is too slow to manifest convincing physiologic effects in intact eyes.

Radioligand binding and functional responses of ligands for human recombinant adenosine A(3) receptors

The binding and functional properties of adenosine receptor ligands were compared in Chinese hamster ovary cells transfected with human adenosine A(3) receptors. Inhibition of [(125)I]-aminobenzyl-5'-N-methylcarboamidoadenosine ([(125)I]-AB-MECA) binding by adenosine receptor ligands was examined in membrane preparations. Inhibition of forskolin-induced cAMP accumulation by agonists was measured using a cAMP enzyme immunoassay. The rank order of agonist potency for both assays was N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA) > 5'-N-ethylcarboxamidoadenosine (NECA) > (-)-N(6)-[(R)-phenylisopropyl] adenosine (R-PIA) > 4-aminobenzyl-5'-N-methylcarboxamidoadenosine (AB-MECA) > N(6)-cyclopentyl adenosine (CPA) > adenosine. The radioligand binding rank order of antagonist potency was N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide (MRS1220) > 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) > 8-phenyltheophylline (8-PT) > 8-(p-sulfophenyl)-theophylline (8-SPT). MRS1220 competitively inhibited the effect of IB-MECA on cAMP production, with a K(B) value of 0.35 nm. These data are characteristic of adenosine A(3) receptors. The absence of Mg(2+) and presence of guanosine 5'-(gamma-thio)triphosphate (GTPgammaS) significantly reduced agonist binding inhibition potency, indicating binding to high- and low-affinity states. The IB-MECA, NECA and R-PIA IC(50) values were greater for the cAMP assay than for radioligand binding, suggesting an efficient stimulus-response transduction pathway.

Balance of purines may determine life or death of retinal ganglion cells as A3 adenosine receptors prevent loss following P2X7 receptor stimulation

The purines ATP and adenosine can act as a coordinated team of transmitters. As extracellular adenosine is frequently derived from the enzymatic dephosphorylation of released ATP, the distinct actions of the two purines can be synchronized. In retinal ganglion cells (RGCs), stimulation of the P2X7 receptor for ATP leads to increased intracellular Ca2+ and death. Here we define the contrasting effects of adenosine and identify protective actions mediated by the A3 receptor. Adenosine attenuated the rise in Ca2+ produced by the P2X7 agonist 3'-O-(4-benzoylbenzoyl)ATP (BzATP). Adenosine was also neuroprotective, increasing the survival of ganglion cells exposed to BzATP. The A3 adenosine receptor agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronimide (Cl-IB-MECA) mimicked the inhibition of the Ca2+ rise, whereas the A3 antagonist 3-Ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191) reduced the protective effects of adenosine. Both Cl-IB-MECA and a second A3 receptor agonist IB-MECA reduced the cell loss triggered by BzATP. The actions of BzATP were mimicked by ATPgammaS, but not by ATP. In summary, adenosine can stop the rise in Ca2+ and cell death resulting from stimulation of the P2X7 receptor on RGCs, with the A3 adenosine receptor contributing to this protection. Hydrolysis of ATP into adenosine and perhaps inosine shifts the balance of purinergic action from that of death to the preservation of life.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

A3 adenosine receptor antagonists delay irreversible synaptic failure caused by oxygen and glucose deprivation in the rat CA1 hippocampus in vitro

The role of adenosine A3 receptor activation during ischaemia-like conditions produced by oxygen and glucose deprivation (OGD) was evaluated with extracellular recordings from the CA1 region of rat hippocampal slices. In all, 7 min of OGD evoked tissue anoxic depolarisation (AD, peak at approximately 7 min from OGD start, n=20) and were invariably followed by irreversible loss of electrically evoked field epsps (fepsps, n=42).The selective adenosine A3 antagonists 3-propyl-6-ethyl-5[(ethylthio)carbonyl]-2-phenyl-4-propyl-3-pyridinecarboxylate (MRS 1523, 1-100 nM, n=31), N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-yl]benzeneacetamide (MRS 1220, 100 nM, n=7), N-(2-methoxyphenyl)-N'-[2-(3-pyrindinyl)-4-quinazolinyl]-urea, (VUF 5574, 100 nM, n=3) and 5-[[(4-pyridyl)amino]carbonyl]amino-8-methyl-2-(2-furyl)-pyrazolo[4,3-e]1,2,4-triazolo[1,5-c]pyrimidine hydrochloride (1 nM, n=4), prevented the irreversible failure of neurotransmission induced by 7 min OGD (n=45) and the development of AD in 20 out of 22 monitored slices. When tested on OGD episodes of longer duration (8-10 min, n=18), 100 nM MRS 1523 prevented or delayed the appearance of AD and exerted a protective effect on neurotransmission for episodes of up to 9 min duration. In the absence of AD, the fepsp recovery was almost total, regardless of OGD episode duration. These findings support the notion that A3 receptor stimulation is deleterious during ischaemia and suggest that selective A3 receptor block may substantially increase the resistance of the CA1 hippocampal region to ischaemic damage.

Swelling-activated chloride channels in aqueous humour formation: on the one side and the other

Aqueous humour is secreted by the ciliary epithelium comprising pigmented and non-pigmented cell layers facing the stroma and aqueous humour respectively. Net chloride secretion likely limits the rate of aqueous humour formation and proceeds in three steps: stromal chloride entry into pigmented cells, diffusion through gap junctions and final non-pigmented cell secretion. Swelling-activated chloride channels function on both epithelial surfaces. At the stromal surface, swelling- and cyclic adenosine monophosphate-activated maxi-chloride channels can recycle chloride, reducing net chloride secretion. At the aqueous-humour surface, swelling- and A3 adenosine receptor-activated chloride channels subserve chloride release into the aqueous humour. The similar macroscopic properties of the two non-pigmented cell chloride currents suggest that both flow through a common conduit. In addition, measurements of intraocular pressure (IOP) in living wild-type and mutant mice have confirmed that A3 adenosine receptor-activated agonists and antagonists increase and lower IOP respectively. Isolated ciliary epithelial cells are commonly perfused with hypotonic solution to probe and characterize chloride channels, but the physiological role of swelling-activated channels has been unclear without knowing their epithelial distribution. Recently, hypotonic challenge has been found to stimulate the chloride-sensitive short-circuit current across the intact bovine ciliary epithelium, suggesting that the net effect of the swelling-activated chloride currents is oriented to enhance aqueous humour formation. Taken together, the results suggest that swelling-activated chloride channels are predominantly oriented to enhance aqueous humour secretion, and these chloride channels at the aqueous surface may be identical with adenosine receptor-activated chloride channels which likely modulate aqueous inflow and IOP in the living mouse.

Progress in the pursuit of therapeutic adenosine receptor antagonists

Ever since the discovery of the hypotensive and bradycardiac effects of adenosine, adenosine receptors continue to represent promising drug targets. First, this is due to the fact that the receptors are expressed in a large variety of tissues. In particular, the actions of adenosine (or methylxanthine antagonists) in the central nervous system, in the circulation, on immune cells, and on other tissues can be beneficial in certain disorders. Second, there exists a large number of ligands, which have been generated by introducing several modifications in the structure of the lead compounds (adenosine and methylxanthine), some of them highly specific. Four adenosine receptor subtypes (A1, A2A, A2B, and A3) have been cloned and pharmacologically characterized, all of which are G protein-coupled receptors. Adenosine receptors can be distinguished according to their preferred mechanism of signal transduction: A1 and A3 receptors interact with pertussis toxin-sensitive G proteins of the Gi and Go family; the canonical signaling mechanism of the A2A and of the A2B receptors is stimulation of adenylyl cyclase via Gs proteins. In addition to the coupling to adenylyl cyclase, all four subtypes may positively couple to phospholipase C via different G protein subunits. The development of new ligands, in particular, potent and selective antagonists, for all subtypes of adenosine receptors has so far been directed by traditional medicinal chemistry. The availability of genetic information promises to facilitate understanding of the drug-receptor interaction leading to the rational design of a potentially therapeutically important class of drugs. Moreover, molecular modeling may further rationalize observed interactions between the receptors and their ligands. In this review, we will summarize the most relevant progress in developing new therapeutic adenosine receptor antagonists.

Mechanisms of adenosine-induced cytotoxicity and their clinical and physiological implications

Extracellular ATP (ATPo) and adenosine are cytotoxic to several cancer cell lines, suggesting their potential use for anticancer therapy. Adenosine causes cytotoxicity, either when added exogenously or when generated from ATPo hydrolysis, via mechanisms which are not mutually exclusive and which involve, adenosine receptor activation, pyrimidine starvation and/or increases in intracellular S-adenosylhomocysteine: S-adenosylmethionine ratio. Given that adenosine also appears to protect against cytotoxicity via mechanisms including immunity against damage by oxygen free radicals, an understanding of the contribution of adenosine to ATPo-induced cytotoxicity is thus crucial, when considering any potential therapeutic use for these compounds. However, such an understanding has been largely hindered by the fact that many studies have not focused enough on the possibility that both ATPo and adenosine may mediate cytotoxicity in the same system. Such studies can benefit from use a range of ATPo concentrations when assessing the contribution of adenosine to ATPo-induced cytotoxicity. Whilst future molecular and pharmacological studies are needed to establish the nature of the cytotoxic adenosine receptor, it is possible that more than just one adenosine receptor type is involved and that the cytotoxic receptor(s) type is more likely to have a low affinity for adenosine. Activation of the adenosine receptor(s) would thus lead to cytotoxicity only at relatively high adenosine concentrations, while lower adenosine concentrations mediate non-cytotoxic physiological effects.

2,9-disubstituted-N6-(arylcarbamoyl)-8-azaadenines as new selective A3 adenosine receptor antagonists: synthesis, biochemical and molecular modelling studies

A number of N6-(N-arylcarbamoyl)-2-substituted-9-benzyl-8-azaadenines, obtained by a modification of the synthetic scheme used to prepare selective A1 ligands, by only three or two steps, are described. At first we prepared a series of 2-phenyl-9-benzyl-8-azaadenines having as N6 substituent a variously substituted N-phenylcarbamoyl group. Some of these derivatives demonstrated good affinity towards the A3 subtype but low selectivity. Compounds having p-CF3, p-F and p-OCH3, as substituents on the phenylcarbamoyl group were selected as lead compounds for the second part of this study. Without modifying the N6 substituent, which would assure A3 affinity, we varied the 9 and 2 positions on these molecules to enhance selectivity. Some compounds having a p-methyl group on the 2-phenyl substituent showed a very good affinity and selectivity for the A3 subtype, revealing the first class of A3 adenosine receptor selective antagonists with a bicyclic structure strictly correlated to the adenine nucleus. The molecular modelling work, carried out using the DOCK program, supplied two models which may be useful for a better understanding of the binding modes. Both models highlighted the preferred interacting tautomeric forms of the antagonists for human A1 and A3 receptors.

The cross-species A3 adenosine-receptor antagonist MRS 1292 inhibits adenosine-triggered human nonpigmented ciliary epithelial cell fluid release and reduces mouse intraocular pressure

PURPOSE

Antagonists to A3 adenosine receptors (ARs) lower mouse intraocular pressure (IOP), but extension to humans is limited by species variability. We tested whether the specific A3AR antagonist MRS 1292, designed to cross species, mimicks the effects of other A3AR antagonists on cultured human nonpigmented ciliary epithelial (NPE) cells and mouse IOP.

METHODS

NPE cell volume was monitored by electronic cell sorting. Mouse IOP was measured with the Servo-Null Micropipette System.

RESULTS

Adenosine triggered A3AR-mediated shrinkage of human NPE cells. Shrinkage was blocked by MRS 1292 (IC50 = 42 +/- 11 nM, p < 0.01) and by another A3AR antagonist effective in this system, MRS 1191. Topical application of the A3AR agonist IB-MECA increased mouse IOP. MRS 1292 reduced IOP by 4.0 +/- 0.8 mmHg at 25-microM droplet concentration (n = 10, p < 0.005).

CONCLUSIONS

MRS 1292 inhibits A3AR-mediated shrinkage of human NPE cells and reduces mouse IOP, consistent with its putative action as a cross-species A3 antagonist.

Modulation of Seizures and Synaptic Plasticity by Adenosinergic Receptors in an Experimental Model of Temporal Lobe Epilepsy Induced by Pilocarpine in Rats

PURPOSE

Adenosine is a major negative neuromodulator of synaptic activity in the central nervous system and can exert anticonvulsant and neuroprotective effects in many experimental models of epilepsy. Extracellular adenosine can be formed by a membrane-anchored enzyme ecto-5'-nucleotidase. The purposes of this study were to characterize the role of adenosine receptors in modulating status epilepticus (SE) induced by pilocarpine and evaluate its neuroprotective action. Ecto-5'-nucleotidase activity was studied during the different phases of pilocarpine-induced epilepsy in rats.

METHODS

Adult rats were pretreated with different adenosinergic agents to evaluate the latency and incidence of SE induced by pilocarpine in rats. The neuroprotective effect also was evaluated.

RESULTS

A proconvulsant effect was observed with DPCPX and DMPX that reduced the latency of SE in almost all rats. Pretreatment with the MRS 1220 did not alter the incidence of SE but reduced the latency to develop SE. An anticonvulsant and neuroprotective effect was detected with R-PIA. Rats pretreated with R-PIA had a decreased number of apoptotic cells in the hippocampus, whereas pretreatment with DPCPX did not modify the hippocampal damage. An intensification of neuronal death was observed in the dentate gyrus and CA3 when rats were pretreated with DMPX. MRS-1220 did not modify the number of apoptotic cells in the hippocampus. An increase in the ecto-5 -nucleotidase staining was detected in the hippocampus during silent and chronic phases.

CONCLUSIONS

The present data show that adenosine released during pilocarpine-induced SE via A1-receptor stimulation can exhibit neuroprotective and anticonvulsant roles. Similar effects could also be inferred with A2a and A3 adenosinergic agents, but further experiments are necessary to confirm their roles. Ecto-5 -nucleotidase activity during silent and chronic phases might have a role in blocking spontaneous seizures by production of inhibitory neuromodulator adenosine, besides taking part in the mechanism that controls sprouting.

Basis of chloride transport in ciliary epithelium

The aqueous humor is formed by the bilayered ciliary epithelium. The pigmented ciliary epithelium (PE) faces the stroma and the nonpigmented ciliary epithelium (NPE) contacts the aqueous humor. Cl(-) secretion likely limits the rate of aqueous humor formation. Many transport components underlying Cl(-) secretion are known. Cl(-) is taken up from the stroma into PE cells by electroneutral transporters, diffuses to the NPE cells through gap junctions and is released largely through Cl(-) channels. Recent work suggests that significant Cl(-) recycling occurs at both surfaces of the ciliary epithelium, providing the basis for modulation of net secretion. The PE-NPE cell couplet likely forms the fundamental unit of secretion; gap junctions within the PE and NPE cell layers are inadequate to maintain constancy of ionic composition throughout the epithelium under certain conditions. Although many hormones, drugs and signaling cascades are known to have effects, a persuasive model of the regulation of aqueous humor formation has not yet been developed. cAMP likely plays a central role, potentially both enhancing and reducing secretion by actions at both surfaces of the ciliary epithelium. Among other hormone receptors, A(3) adenosine receptors likely alter intraocular pressure by regulating NPE-cell Cl(-) channel activity. Recently, functional evidence for the regional variation in ciliary epithelial secretion has been demonstrated; the physiologic and pathophysiologic implications of this regional variation remain to be addressed.

Differential P1-purinergic modulation of human Schlemm's canal inner-wall cells

Intraocular pressure is directly dependent on aqueous humor flow into, and resistance to flow out of, the eye. Adenosine has complex effects on intraocular pressure. Stimulation of A1and A2Aadenosine receptors changes intraocular pressure oppositely, likely through opposing actions on the outflow of aqueous humor. While the cellular sites regulating outflow resistance are unknown, the cells lining the inner wall of Schlemm's canal (SC) are a likely regulatory site. We applied selective adenosine receptor agonists to SC cells in vitro to compare the responses to A1and A2Astimulation. Parallel studies were conducted with human inner-wall SC cells isolated by a novel enzyme-assisted technique and with cannula-derived mixed inner- and outer-wall SC cells. A1agonists increased whole cell currents of both inner-wall and cannula-derived SC cells. An A2Aagonist reduced currents most consistently in specifically inner-wall SC cells. Those currents were also increased by A2B, but not consistently affected by A3, stimulation. A1, A2A, and A3agonists all increased SC-cell intracellular Ca2+. The electrophysiological results are consistent with the possibility that inner-wall SC cells may mediate the previously reported modulatory effects of adenosine on outflow resistance. The results are also consistent with the presence of functional A2B, as well as A1, A2A, and A3adenosine receptors in SC cells.

Inosine reduces ischemic brain injury in rats

BACKGROUND AND PURPOSE

Purinergic nucleoside inosine elicits protection and regeneration during various injuries. The purpose of this study was to examine the protective effects of inosine against cerebral ischemia.

METHODS

Adult Sprague-Dawley rats were anesthetized. Inosine, hypoxathine, or vehicle was administered intracerebroventricularly before transient right middle cerebral artery occlusion (MCAo). Animals were placed in behavioral chambers 2 days to 2 weeks after MCAo and then euthanized for tri-phenyl-tetrazolium chloride staining. Glutamate release was measured by microdialysis/high-performance liquid chromatography, and single-unit action potentials were recorded from neurons in the parietal cortex.

RESULTS

Stroke animals receiving inosine pretreatment demonstrated a higher level of locomotor activity and less cerebral infarction. Intracerebroventricular administration of the same dose of hypoxanthine did not confer protection. Coadministration of selective A3 receptor antagonist 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1, 4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS1191) significantly reduced inosine-mediated protection. Inosine did not alter basal glutamate release, nor did it reduce ischemia-evoked glutamate overflow from cerebral cortex. However, inosine antagonized glutamate-induced electrophysiological excitation in cerebral cortical neurons.

CONCLUSIONS

Inosine inhibits glutamate postsynaptic responses and reduces cerebral infarction. Its protective effect against ischemia/reperfusion-related insults may involve activation of adenosine A3 receptors.

Proteomic analysis of hyperdynamic mouse hearts with enhanced sarcoplasmic reticulum calcium cycling

Depressed sarcoplasmic reticulum (SR) Ca-cycling is a hallmark of human and experimental heart failure. Strategies to improve this impairment by either increasing SERCA2a levels or decreasing phospholamban (PLN) activity have been suggested as promising therapeutic targets. Indeed, ablation of PLN gene in mice was associated with greatly enhanced cardiac Ca-cycling and performance. Intriguingly, this hyperdynamic cardiac function was maintained throughout the lifetime of the mouse without observable pathological consequences. To determine the cellular alterations in the expression or modification of myocardial proteins, which are associated with the enhanced cardiac contractility, we performed a proteomics-based analysis of PLN knockout (PLN-KO) hearts in comparison to isogenic wild-types. By use of 2-dimensional gel electrophoresis (2-DE), approximately 3300 distinct protein spots were detected in either wild-type or PLN-KO ventricles. Protein spots observed to be altered between PLN-KO and wild-type hearts were subjected to tryptic peptide mass fingerprinting for identification by MALDI-TOF mass spectrometry in combination with LC/MS/MS analysis. In addition, two-dimensional 32P-autoradiography was performed to analyze the phosphorylation profiles of PLN-KO cardiomyocytes. We identified alterations in the expression level of more than 100 ventricular proteins, along with changes in phosphorylation status of important regulatory proteins in the PLN-KO. These protein changes were observed mainly in two subcellular compartments: the cardiac contractile apparatus, and metabolism/energetics. Our findings suggest that numerous alterations in protein expression and phosphorylation state occurred upon ablation of PLN and that a complex functional relationship among proteins involved in calcium handling, myofibrils, and energy production may exist to coordinately maintain the hyperdynamic cardiac contractile performance of the PLN-KO mouse in the long term.

Adenosine up-regulation of the mucin gene, MUC2, in asthma

Mucus hypersecretion is a hallmark of asthma that contributes to airway obstruction. While the etiology is not well understood, hypersecretion has been linked to the presence of cytokines such as IL-4, IL-5, IL-9, and IL-13 in the inflamed airway. The presence of adenosine has also been noted in asthmatic airways, and adenosine-mediated signaling in mast cells has been implicated in the severe bronchoconstriction and inflammation prevalent in these patients (1, 2). Here we examine the possibility that adenosine also contributes to mucus hypersecretion by airway epithelial cells. Results in cultured airway epithelial cells showed that MUC2 mucin expression increased in response to adenosine. This appeared to be mediated by a pathway initiated at the adenosine A1 receptor that transduced signals through a Ca2+-activated Cl- channel and EGFR. That this signaling cascade is relevant to asthmatic hypersecretion was indicated by results showing that mucin induction by asthmatic tracheal aspirates was reduced by A1, CLCA1, and EGFR inhibitors. These results suggest that adenosine cooperates with inflammatory cytokines to stimulate mucin production in the asthmatic airway and supports the use of A1, CLCA1, and EGFR inhibitors in the treatment of asthma.

Effects of Endogenous Adenosine and Adenosine Receptor Agonists on Hypoxia-Induced Myocardial Stunning in Guinea-Pig Atria and Papillary Muscles

The effects of endogenous adenosine and adenosine receptor agonists were examined on hypoxia-induced myocardial stunning of guinea-pig isolated paced left atria and papillary muscles. Hypoxia (30 minutes) reduced developed tension and increased diastolic tension (contracture) of left atria (41.8 +/- 11.5%) and papillary muscles (17.7 +/- 6.2%). Developed tension recovered to 80.8 +/- 3.15 and 77.2 +/- 5.3% 15 minutes after reoxygenation (stunning). Recovery of left atria was unaffected by adenosine deaminase (1 IU mL) but was depressed in papillary muscles (15 minutes, 48.6 +/- 4.3%) and contracture (46.1 +/- 7.5%) increased. Endogenous adenosine therefore protects from ventricular but not atrial stunning. Adenosine receptor agonists were introduced at 10 minutes into hypoxia. CPA (A1 selective, 3 x 10 M) impaired left atrial recovery (5 minutes, 38.1 +/- 5.0%), through direct negative inotropy, but did not affect papillary muscles. CGS21680 (A2A selective, 3 x 10 M) did not affect recovery. APNEA (A1/A3 receptor agonist, 10 M), increased recovery rate of left atria. Improved rate and extent of recovery of papillary muscles by APNEA (15 minutes, 94.8 +/- 3.1%) was prevented by the A3 receptor antagonist, MRS-1220 (10 M). IB-MECA (A3 selective, 3 x 10 M) increased atrial recovery rate but not the maximum developed tension reached in either tissue. However, when added at reoxygenation, IB-MECA caused complete recovery of both tissues, in the absence or presence of adenosine deaminase. Thus, A3 receptor stimulation reverses myocardial stunning of isolated atria and papillary muscles.

Animal models for the study of adenosine receptor function

Adenosine receptors represent a family of G-protein coupled receptors that are ubiquitously expressed in a wide variety of tissues. This family contains four receptor subtypes: A1 and A3, which mediate inhibition of adenylyl cyclase; and A2a and A2b, which mediate stimulation of this enzyme. Currently, all receptor subtypes have been genetically deleted in mouse models except for the A2b adenosine receptor, and some have been overexpressed in selective tissues of transgenic mice. Studies involving these transgenic mice indicated that receptor levels are rate limiting, as effects were amplified upon increases in receptor level. The knockout models pointed to clusters of activities related to the physiologies of the cardiovascular and the nervous systems, which are either reduced or enhanced upon specific receptor deletion. Interestingly, the trend of effects on these systems is similar in the A1 and A3 adenosine receptor knockout mice and opposite to the effects observed in the A2a adenosine receptor knockout model. This review summarizes in vitro studies on pathways affected by each adenosine receptor, and primarily focuses on the above in vivo models generated to investigate the physiologic role of adenosine receptors. Furthermore, it illustrates the need for multiple adenosine receptor subtype deficiency studies in mice and the deletion of the A2b subtype.

Behavioral characterization of mice lacking the A3 adenosine receptor: sensitivity to hypoxic neurodegeneration

1. The potential neuroprotective actions of the A3 adenosine receptor (A3AR) were investigated using mice with functional deletions of the A3AR (A3AR-/-) in behavioral assessments of analgesia, locomotion, tests predictive of depression and anxiety, and the effects of mild hypoxia on cognition and neuronal survival. 2. Untreated A3AR-/- mice were tested in standard behavioral paradigms, including activity in the open field, performance in the hot-plate, tail-flick, tail-suspension, and swim tests, and in the elevated plus maze. In addition, mice were exposed repeatedly to a hypoxic environment containing carbon monoxide (CO). The cognitive effects of this treatment were assessed using the contextual fear conditioning test. After testing, the density of pyramidal neurons in the CA1, 2, and 3 subfields of the hippocampus was determined using standard histological and morphometric techniques. 3. A3AR-/- mice showed increased locomotion in the open field test, elevated plus maze (number of arm entries) and light/dark box (number of transitions). However, they spent more time immobile in two different tests of antidepressant activity (Swim and tail suspension tests). A3AR-/- mice also showed evidence of decreased nociception in the hotplate, but not tail-flick tests. Further, A3AR-/- mice were more vulnerable to hippocampal pyramidal neuron damage following episodes of carbon monoxide (CO)-induced hypoxia. One week after exposure to CO a moderate loss of pyramidal neurons was observed in all hippocampal subfields of both wild-type (A3AR+/+) and A3AR-/- mice. However, the extent of neuronal death in the CA2-3 subfields was less pronounced in A3AR+/+ than A3AR-/- mice. This neuronal loss was accompanied by a decline in cognitive function as determined using contextual fear conditioning. These histological and cognitive changes were reproduced in wild-type mice by repeatedly administering the A3AR-selective antagonist MRS 1523 (5-propyl-2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate 1 mg/kg i.p.). 4. These results indicate that pharmacologic or genetic suppression of A3AR function enhances some aspects of motor function and suppresses pain processing at supraspinal levels, while acting as a depressant in tests predictive of antidepressant action. Consistent with previous reports of the neuroprotective actions of A3AR agonists, A3AR-/- mice show an increase in neurodegeneration in response to repeated episodes of hypoxia.

N6-Substituted adenosine derivatives: selectivity, efficacy, and species differences at A3 adenosine receptors

The activation of the human A(3) adenosine receptor (AR) by a wide range of N(6)-substituted adenosine derivatives was studied in intact CHO cells stably expressing this receptor. Selectivity of binding at rat and human ARs was also determined. Among N(6)-alkyl substitutions, small N(6)-alkyl groups were associated with selectivity for human A(3)ARs vs. rat A(3)ARs, and multiple points of branching were associated with decreased hA(3)AR efficacy. N(6)-Cycloalkyl-substituted adenosines were full (</=5 carbons) or partial (>/=6 carbons) hA(3)AR agonists. N(6)-(endo-Norbornyl)adenosine 13 was the most selective for both rat and human A(1)ARs. Numerous N(6)-arylmethyl analogues, including substituted benzyl, tended to be more potent in binding to A(1) and A(3) vs. A(2A)ARs (with variable degrees of partial to full A(3)AR agonisms). A chloro substituent decreased the efficacy depending on its position on the benzyl ring. The A(3)AR affinity and efficacy of N(6)-arylethyl adenosines depended highly on stereochemistry, steric bulk, and ring constraints. Stereoselectivity of binding was demonstrated for N(6)-(R-1-phenylethyl)adenosine vs. N(6)-(S-1-phenylethyl)adenosine, as well as for the N(6)-(1-phenyl-2-pentyl)adenosine, at the rat, but not human A(3)AR. Interestingly, DPMA, a potent agonist for the A(2A)AR (K(i)=4nM), was demonstrated to be a moderately potent antagonist for the human A(3)AR (K(i)=106nM). N(6)-[(1S,2R)-2-Phenyl-1-cyclopropyl]adenosine 48 was 1100-fold more potent in binding to human (K(i)=0.63nM) than rat A(3)ARs. Dual acting A(1)/A(3) agonists (N(6)-3-chlorobenzyl- 29, N(6)-(S-1-phenylethyl)- 39, and 2-chloro-N(6)-(R-phenylisopropyl)adenosine 53) might be useful for cardioprotection.

Enhanced adenosine A(2B) mediated coronary response in reserpinised rat heart

In this study, we investigated the effect of noradrenaline depletion on contractile recovery in rat isolated heart following myocardial ischaemia. Groups tested included control tissues and hearts from reserpinised rats. Reserpine 1 mg/kg s.c. was injected into rats 18 to 24 h prior to experiments. Hearts underwent 15 min global normothermic ischaemia followed by 30 min reperfusion. Functional data (end diastolic pressure (EDP), heart rate (HR), left ventricular developed pressure (LVDP), dP/dt(max), dP/dt(min)) showed that contractile function following ischaemia-reperfusion is unaffected by reserpinisation. However, pre- and post-ischaemic coronary flow rates (CFR) were increased by 16 to 38% in hearts from reserpinised rats versus control hearts. Pre-ischaemic CFRs in control hearts (11.17+/-0.67 ml/in(-1) x g tissue(-1), n=9) were significantly lower then CFRs derived from reserpinised rat hearts (14.57+/-0.72 ml/min(-1)/g tissue(-1), n=10). Post-ischaemic reactive hyperaemia was evident in all groups. CFRs in reserpinised hearts remained elevated when compared to pre-ischaemic values through reperfusion (P<0.05). Reserpine treatment did not significantly alter pre- or post-ischaemic adenosine efflux. The A(2B) adenosine receptor antagonist alloxazine (10 microM) attenuated pre- and post-ischaemic CFRs in both control and reserpinised hearts (P<0.05) without altering the hyperaemic response while the A(2A) adenosine receptor antagonist 8-(3-chlorostyryl) caffeine (1 microM) did not alter CFRs in both groups. The A(3) adenosine receptor antagonist MRS1191 (0.1 microM) increased CFR in control and reserpinised hearts (P<0.05). Catecholamine depletion with reserpinisation enhances the responsiveness of the coronary resistance vessels to endogenous adenosine through activation of the A(2B) adenosine receptor.

Differential allosteric modulation by amiloride analogues of agonist and antagonist binding at A(1) and A(3) adenosine receptors

The diuretic drug amiloride and its analogues were found previously to be allosteric modulators of antagonist binding to A(2A) adenosine receptors. In this study, the possibility of the allosteric modulation by amiloride analogues of antagonist binding at A(1) and A(3) receptors, as well as agonist binding at A(1), A(2A), and A(3) receptors, was explored. Amiloride analogues increased the dissociation rates of two antagonist radioligands, [3H]8-cyclopentyl-1,3-dipropylxanthine ([3H]DPCPX) and [3H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2,1-i]purin-5-one ([3H]PSB-11), from A(1) and A(3) receptors, respectively. Amiloride and 5-(N,N-dimethyl)amiloride (DMA) were more potent at A(1) receptors than at A(3) receptors, while 5-(N,N-hexamethylene)amiloride (HMA) was more potent at A(3) receptors. Thus, amiloride analogues are allosteric inhibitors of antagonist binding at A(1), A(2A), and A(3) adenosine receptor subtypes. In contrast to their effects on antagonist-occupied receptors, amiloride analogues did not affect the dissociation rates of the A(1) agonist [3H]N(6)-[(R)-phenylisopropyl]adenosine ([3H]R-PIA) from A(1) receptors or the A(2A) agonist [3H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine ([3H]CGS21680) from A(2A) receptors. The dissociation rate of the A(3) agonist radioligand [125I]N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]I-AB-MECA) from A(3) receptors was decreased significantly by amiloride analogues. The binding modes of amiloride analogues at agonist-occupied and antagonist-occupied receptors differed markedly, which was demonstrated in all three subtypes of adenosine receptors tested in this study. The effects of the amiloride analogues on the action of the A(3) receptor agonist were explored further using a cyclic AMP functional assay in intact CHO cells expressing the human A(3) receptor. Both binding and functional assays support the allosteric interactions of amiloride analogues with A(3) receptors.

A3 adenosine receptor knockout mice are protected against ischemia- and myoglobinuria-induced renal failure

A(3) adenosine receptor (AR) activation and inhibition worsen and improve, respectively, renal function after ischemia-reperfusion (I/R) injury in rats. We sought to further characterize the role of A(3) ARs in modulating renal function after either I/R or myoglobinuric renal injury. A(3) knockout mice had significantly lower plasma creatinines compared with C57 controls 24 h after I/R or myoglobinuric renal injury. C57 control mice pretreated with the A(3) AR antagonist [3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(+/-)-dihydropyridine-3,5 dicarboxylate] or agonist [0.125 mg/kg N(6)-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (IB-MECA)] demonstrated improved or worsened renal function, respectively, after I/R or myoglobinuric renal injury. Higher doses of IB-MECA were lethal in C57 mice subjected to renal ischemia. H(1) but not H(2) histamine receptor antagonist prevented death in mice pretreated with IB-MECA before renal ischemia. Improvement in renal function was associated with significantly improved renal histology. In conclusion, preischemic A(3) AR activation (0.125 mg/kg IB-MECA) exacerbated renal I/R injury in mice. Mice lacking A(3) ARs or blocking A(3) ARs in wild-type mice resulted in significant renal protection from ischemic or myoglobinuric renal failure.

Positive coupling of atypical adenosine A3 receptors on human eosinophils to adenylyl cyclase

Adenosine A(3) receptors are reported to couple negatively to adenylyl cyclase (AC) but their mediation of anti-inflammatory effects in human eosinophils prompted us to investigate their coupling to AC. The A(3)-selective agonists IB-MECA and Cl-IB-MECA evoked a concentration-dependent generation of cAMP (EC(50), 3.2 and 1.8 microM, respectively) and were more potent than the A(2A) agonist CGS 21680 (EC(50)=15.4 microM) and adenosine (EC(50)=19.2 microM). The cAMP response was additive to that produced by forskolin (10 microM). The effect of IB-MECA was insensitive to A(1) and A(2A) receptor antagonists, but was antagonized by the A(3)-selective antagonist MRS 1220 (0.1-2.5 microM) in a competitive manner. The estimated K(B) of 190 nM was, however, atypical. The cyclo-oxygenase inhibitor, indomethacin, had no effect on the cAMP response. A general inverse relationship between cAMP generation and inhibition of degranulation was seen. We conclude that in human eosinophils, an atypical form of A(3) receptors positively coupled to AC may exist. The resulting cAMP generation may underlie the anti-inflammatory actions of A(3) agonists in eosinophils.

A3 adenosine receptors in human astrocytoma cells: agonist-mediated desensitization, internalization, and down-regulation

A(3) adenosine receptor activation has been previously demonstrated to result in both neuroprotective and neurodegenerative effects, depending upon specific pathophysiological conditions. This dual effect may depend on receptor regulation mechanisms that are able to change receptor availability and/or function. In the present study, we investigated desensitization, internalization, and down-regulation of native A(3) adenosine receptors in human astrocytoma cells after exposure to the agonist 2-chloro-N6-(3-iodobenzyl)-N-methyl-5'-carbamoyladenosine (Cl-IBMECA). Cl-IBMECA induced a concentration-dependent inhibition of adenylyl cyclase activity with an EC(50) value of 2.9 +/- 0.1 nM. The effect was suggested to be mediated by A(3) adenosine receptor subtype by the use of selective adenosine receptor antagonists. Cell treatment with pertussis toxin abolished Cl-IBMECA-mediated inhibition of adenylyl cyclase activity, evidencing an A(3) receptor coupling to inhibitory G protein. Short-term exposure to the agonist Cl-IBMECA (100 nM) caused rapid receptor desensitization, within 15 min. Agonist-induced desensitization was accompanied by receptor internalization: A(3) adenosine receptor internalized with rapid kinetics, within 30 min, after cell exposure to 100 nM Cl-IBMECA. The localization of A(3) adenosine receptors on the plasma membrane and in intracellular compartments was directly revealed by immunogold electron microscopy. After desensitization, the removal of agonist led to the restoration of A(3) adenosine receptor functioning through receptor recycling to the cell surface within 120 min. Prolonged agonist exposure (1-24 h) resulted in a marked down-regulation of A(3) adenosine receptors that reached 21.9 +/- 2.88% of control value after 24 h. After down-regulation, the recovery of receptor functioning was slow (24 h) and associated with the restoration of receptor levels close to control values. In conclusion, our results demonstrated that A(3) receptors, in astrocytoma cells, are regulated after short- and long-term agonist exposure.

Structural determinants of A(3) adenosine receptor activation: nucleoside ligands at the agonist/antagonist boundary

Mutagenesis of the human A(3) adenosine receptor (AR) suggested that certain amino acid residues contributed differently to ligand binding and activation processes. Here we demonstrated that various adenosine modifications, including adenine substitution and ribose ring constraints, also contributed differentially to these processes. The ligand effects on cyclic AMP production in intact CHO cells expressing the A(3)AR and in receptor binding were compared. Notably, the simple 2-fluoro group alone or 2-chloro in combination with N(6)-substitution dramatically diminished the efficacy of adenosine derivatives, even converting agonist into antagonist. Other affinity-increasing substitutions, including N(6)-(3-iodobenzyl) 4 and the (Northern)-methanocarba 15, also reduced efficacy, except in combination with a flexible 5'-uronamide. 2-Cl-N(6)-(3-iodobenzyl) derivatives, both in the (N)-methanocarba (i.e., of the Northern conformation) and riboside series 18 and 5, respectively, were potent antagonists with little residual agonism. Ring-constrained 2',3'-epoxide derivatives in both riboside and (N)-methanocarba series 13 and 21, respectively, and a cyclized (spiral) 4',5'-uronamide derivative 14 were synthesized and found to be human A(3)AR antagonists. 14 bound potently at both human (26 nM) and rat (49 nM) A(3)ARs. A rhodopsin-based A(3)AR model, containing all domains except the C-terminal region, indicated separate structural requirements for receptor binding and activation for these adenosine analogues. Ligand docking, taking into account binding of selected derivatives at mutant A(3)ARs, featured interactions of TM3 (His95) with the adenine moiety and TMs 6 and 7 with the ribose 5'-region. The 5'-OH group of antagonist N(6)-(3-iodobenzyl)-2-chloroadenosine 5 formed a H-bond with N274 but not with S271. The 5'-substituent of nucleoside antagonists moved toward TM7 and away from TM6. The conserved Trp243 (6.48) side chain, involved in recognition of the classical (nonnucleoside) A(3)AR antagonists but not adenosine-derived ligands, displayed a characteristic movement exclusively upon docking of agonists. Thus, A(3)AR activation appeared to require flexibility at the 5'- and 3'-positions, which was diminished in (N)-methanocarba, spiro, and epoxide analogues, and was characteristic of ribose interactions at TM6 and TM7.

Selective allosteric enhancement of agonist binding and function at human A3 adenosine receptors by a series of imidazoquinoline derivatives

We have identified a series of 1H-imidazo-[4,5-c]quinolines as selective allosteric enhancers of human A3 adenosine receptors. Several of these compounds potentiated both the potency and maximal efficacy of agonist-induced responses and selectively decreased the dissociation of the agonist N6-(4-amino-3-[125I]iodobenzyl)-5'-N-methylcarboxamidoadenosine from human A3 adenosine receptors. There was no effect on the dissociation of the antagonist [3H]8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i]purin-5-one (PSB-11) from the A3 receptors, as well as [3H]N6-[(R)-phenylisopropyl]adenosine from rat brain A1 receptors and [3H]2-[p-(2-carboxyethyl)phenyl-ethylamino]-5'-N-ethylcarboxamidoadenosine from rat striatal A2A receptors, suggesting the selective enhancement of agonist binding at A3 receptors. The analogs were tested as antagonists of competitive binding at human A3 receptors, and K(i) values ranging from 120 nM to 101 microM were observed; as for many allosteric modulators of G protein-coupled receptors, an orthosteric effect was also present. The most promising leads from the present set of analogs seem to be the 2-cyclopentyl-1H-imidazo[4,5-c]quinoline derivatives, of which the 4-phenylamino analog DU124183 had the most favorable degree of allosteric modulation versus receptor antagonism. The inhibition of forskolin-stimulated cyclic AMP accumulation in intact cells that express human A3 receptors was employed as a functional index of A3 receptor activation. The enhancer DU124183 caused a marked leftward shift of the concentration-response curve of the A3 receptor agonists in the presence of antagonist and, surprisingly, a potentiation of the maximum agonist efficacy by approximately 30%. Thus, we have identified a novel structural lead for developing allosteric enhancers of A3 adenosine receptors; such enhancers may be useful for treating brain ischemia and other hypoxic conditions.

Targeted deletion of adenosine A(3) receptors augments adenosine-induced coronary flow in isolated mouse heart

To determine whether adenosine A(3) receptors participate in adenosine-induced changes in coronary flow, isolated hearts from wild-type (WT) and A(3) receptor knockout (A(3)KO) mice were perfused under constant pressure and effects of nonselective and selective agonists were examined. Adenosine and the selective A(2A) agonist 2-[p-(2-carboxyethyl)]phenylethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680) produced augmented maximal coronary vasodilation in A(3)KO hearts compared with WT hearts. Selective activation of A(3) receptors with 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) at nanomolar concentrations did not effect coronary flow, but at higher concentrations it produced coronary vasodilation both in WT and A(3)KO hearts. Cl-IB-MECA-induced increases in coronary flow were susceptible to both pharmacological blockade and genetic deletion of A(2A) receptors. Because deletion or blockade of adenosine A(3) receptors augmented coronary flow induced by nonselective adenosine and the selective A(2A) receptor agonist CGS-21680, we speculate that this is due to removal of an inhibitory influence associated with the A(3) receptor subtype. These data indicate that the presence of adenosine A(3) receptors may either inhibit or negatively modulate coronary flow mediated by other adenosine receptor subtypes.

KF26777 (2-(4-bromophenyl)-7,8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one dihydrochloride), a new potent and selective adenosine A3 receptor antagonist

We investigated the biochemical and pharmacological properties of a new adenosine A(3) receptor antagonist, KF26777 (2-(4-bromophenyl)-7,8-dihydro-4-propyl-1H-imidazo[2,1-i]purin-5(4H)-one dihydrochloride). This compound was characterized using N(6)-(4-amino-3-iodobenzyl)adenosine-5'-N-methyluronamide ([125I]AB-MECA) or [35S]guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) binding to membranes from human embryonic kidney 293 (HEK293) cells expressing human adenosine A(3) receptors. KF26777 showed a K(i) value of 0.20+/-0.038 nM for human adenosine A(3) receptors labeled with [125I]AB-MECA and possessed 9000-, 2350- and 3100-fold selectivity vs. human adenosine A(1), A(2A) and A(2B) receptors, respectively. The inhibitory mode of binding was competitive. KF26777 inhibited the binding of [35S]GTPgammaS stimulated by 1 microM 2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IB-MECA). The IC(50) value was 270+/-85 nM; the compound had no effect on basal activity. Dexamethasone treatment for HL-60 cells, human promyelocytic leukemia, up-regulated functional adenosine A(3) receptors expression, and resulted in the enhanced elevation of intracellular Ca(2+) concentration ([Ca(2+)](i)) via the adenosine A(3) receptor. KF26777 antagonized this [Ca(2+)](i) mobilization induced by Cl-IB-MECA, with a K(B) value of 0.42+/-0.14 nM. These results indicate that KF26777 is a highly potent and selective antagonist of the human adenosine A(3) receptor.

Identification by site-directed mutagenesis of residues involved in ligand recognition and activation of the human A3 adenosine receptor

Ligand recognition has been extensively explored in G protein-coupled A(1), A(2A), and A(2B) adenosine receptors but not in the A(3) receptor, which is cerebroprotective and cardioprotective. We mutated several residues of the human A(3) adenosine receptor within transmembrane domains 3 and 6 and the second extracellular loop, which have been predicted by previous molecular modeling to be involved in the ligand recognition, including His(95), Trp(243), Leu(244), Ser(247), Asn(250), and Lys(152). The N250A mutant receptor lost the ability to bind both radiolabeled agonist and antagonist. The H95A mutation significantly reduced affinity of both agonists and antagonists. In contrast, the K152A (EL2), W243A (6.48), and W243F (6.48) mutations did not significantly affect the agonist binding but decreased antagonist affinity by approximately 3-38-fold, suggesting that these residues were critical for the high affinity of A(3) adenosine receptor antagonists. Activation of phospholipase C by wild type (WT) and mutant receptors was measured. The A(3) agonist 2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine stimulated phosphoinositide turnover in the WT but failed to evoke a response in cells expressing W243A and W243F mutant receptors, in which agonist binding was less sensitive to guanosine 5'-gamma-thiotriphosphate than in WT. Thus, although not important for agonist binding, Trp(243) was critical for receptor activation. The results were interpreted using a rhodopsin-based model of ligand-A(3) receptor interactions.

2-Chloro-N(6)-cyclopentyladenosine, adenosine A(1) receptor agonist, antagonizes the adenosine A(3) receptor

The potent adenosine A(1) receptor agonists, N(6)-cyclopentyladenosine (CPA) and 2-chloro-N(6)-cyclopentyladenosine (CCPA), were studied in Chinese hamster ovary (CHO) cells expressing the human adenosine A(3) receptor. CPA, but not CCPA, induced phosphoinositide turnover. CPA inhibited forskolin-stimulated cyclic AMP production (EC(50) value of 242+/-47 nM). CCPA competitively antagonized the effects of agonist Cl-IB-MECA (2-chloro-N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine) with K(B) value of 5.0 nM. CPA competition curves versus the A(3) antagonist radioligand [3H]PSB-11 (8-ethyl-4-methyl-2-phenyl-(8R)-4,5,7,8-tetrahydro-1H-imidazo[2.1-i]purin-5-one) were right-shifted four-fold by 100 microM GTP, which had no effect on binding of CCPA or the antagonist MRS 1220 (N-[9-chloro-2-(2-furanyl)[1,2,4]triazolo[1,5-c]quinazolin-5-yl]benzene-acetamide). Thus, CCPA is a moderately potent antagonist (K(i)=38 nM) of the human A(3) adenosine receptor.

Adenosine A(3) receptor-mediated potentiation of mucociliary transport and epithelial ciliary motility

To examine the effect of adenosine A(3) receptor stimulation on airway mucociliary clearance, we measured transport of Evans blue dye in rabbit trachea in vivo and ciliary motility of epithelium by the photoelectric method in vitro. Mucociliary transport was enhanced dose dependently by the selective A(3) agonist N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) and to a lesser extent by the less-selective N(6)-2-(4-amino-3-iodophenyl)ethyladenosine, whereas the A(1) agonist N-cyclopentyladenosine (CPA) and the A(2) agonist CGS-21680 had no effect. The effect of IB-MECA was abolished by pretreatment with the selective A(3) antagonist MRS-1220 but not by the A(1) antagonist 1,3-dipropyly-8-cyclopentylxanthine or the A(2) antagonist 3,7-dimethyl-L-propargylxanthine. Epithelial ciliary beat frequency was increased by IB-MECA in a concentration-dependent manner, the maximal increase being 33%, and this effect was inhibited by MRS-1220. The IB-MECA-induced ciliary stimulation was not altered by the Rp diastereomer of cAMP but was greatly inhibited by Ca(2+)-free medium containing BAPTA-AM. Incubation with IB-MECA increased intracellular Ca(2+) contents. Therefore, A(3) agonist enhances airway mucociliary clearance probably through Ca(2+)-mediated stimulation of ciliary motility of airway epithelium.

Interactions of flavones and other phytochemicals with adenosine receptors

Dietary flavonoids have varied effects on animal cells, such as inhibition of platelet binding and aggregation, inhibition of inflammation, and anticancer properties, but the mechanisms of these effects remain largely unexplained. Adenosine receptors are involved in the homeostasis of the immune, cardiovascular, and central nervous systems, and adenosine agonists/antagonists exert many similar effects. The affinity of flavonoids and other phytochemicals to adenosine receptors suggests that a wide range of natural substances in the diet may potentially block the effects of endogenous adenosine. We used competitive radioligand binding assays to screen flavonoid libraries for affinity and a computational CoMFA analysis of flavonoids to compare steric and electrostatic requirements for ligand recognition at three subtypes of adenosine receptors. Flavone derivatives, such as galangin, were found to bind to three subtypes of adenosine receptors in the microM range. Pentamethylmorin (Ki 2.65 microM) was 14- to 17-fold selective for human A3 receptors than for A1 and A2A receptors. An isoflavone, genistein, was found to bind to A1 receptors. Aurones, such as hispidol (Ki 350 nM) are selective A1 receptor antagonists, and, like genistein, are present in soy. The flavones, chemically optimized for receptor binding, have led to the antagonist, MRS 1067 (3,6-dichloro-2'-(isopropoxy)4'-methylflavone), which is 200-fold more selective for human A3 than A1 receptors. Adenosine receptor antagonism, therefore, may be important in the spectrum of biological activities reported for the flavonoids.

Roles of BCL-2 and caspase 3 in the adenosine A3 receptor-induced apoptosis

Selective A3 adenosine receptor agonists have been shown to induce apoptosis in a variety of cell types. In this study we examined the effects of adenosine receptor agonists selective for A1, A2A, or A3 receptors on the induction of apoptosis in primary cultures of rat astrocytes and in C6 glial cells. Treatment of the cells with the A3 receptor agonist Cl-IB-MECA (10 microM) induced apoptosis in both cell types. The effects of Cl-IB-MECA were partially antagonized by the A3 receptor-selective antagonist MRS 1191. In contrast, the A1 and A2A receptor agonists, CPA and CGS 21680, respectively, did not have significant effects on apoptosis in these cells. Cl-IB-MECA reduced the expression of endogenous Bcl-2, whereas it did not affect the expression of Bax. Overexpression of Bcl-2 in C6 cells abrogated the induction of apoptosis induced by the A3 agonist. Cl-IB-MECA also induced an increase in caspase 3 activity and caspase inhibitors decreased the apoptosis induced by the A3 agonist. These findings suggest that intense activation of the A3 receptor is pro-apoptotic in glial cells via bcl2 and caspase-3 dependent pathways.

Pharmacological characterization of adenosine receptors in PGT-beta mouse pineal gland tumour cells

1. The adenosine receptor in mouse pinealocytes was identified and characterized using pharmacological and physiological approaches. 2. Expression of the two adenosine receptor subtypes A2B and A3 was detected in mouse pineal glands and PGT-beta cells by polymerase chain reaction and nucleotide sequencing. 3. Adenosine and 5'-N-ethylcarboxamidoadenosine (NECA) evoked cyclic AMP generation but the A2)-selective agonist 2-(4-(2-carboxyethyl)phenylethylamino)adenosine-5'-N-ethylcarboxamideadenosine (CGS 21680) and the A1-specific agonists R-N(6)-(2-phenylisopropyl)adenosine (R-PIA) and N(6)-cyclopentyladenosine (CPA) had little effect on intracellular cyclic AMP levels. The A2B receptor selective antagonists alloxazine and enprofylline completely blocked NECA-mediated cyclic AMP accumulation. 4. Treatment of cells with the A3-selective agonist N(6)-(3-iodobenzyl)-5'-(N-methylcarbamoyl)adenosine (IB-MECA) inhibited the elevation of the cyclic AMP level induced by NECA or isoproterenol in a concentration-dependent manner with maximal inhibition of 40 - 50%. These responses were blocked by the specific A3 adenosine receptor antagonist MRS 1191. Pretreatment of the cells with pertussis toxin attenuated the IB-MECA-induced responses, suggesting that this effect occurred via the pertussis toxin-sensitive inhibitory G proteins. 5. IB-MECA also caused a concentration-dependent elevation in [Ca(2+)]i and IP3 content. Both the responses induced by IB-MECA were attenuated by treatment with U73122 or phorbol 12-myristate 13-acetate. 6. These data suggest the presence of both A2B and A3 adenosine receptors in mouse pineal tumour cells and that the A2B receptor is positively coupled to adenylyl cyclase whereas the A3 receptor is negatively coupled to adenylyl cyclase and also coupled to phospholipase C.

A(1)-, A(2A)- and A(3)-subtype adenosine receptors modulate intraocular pressure in the mouse

Despite the potential importance of the mouse in studying the pharmacology of aqueous dynamics, measurement of intraocular pressure (IOP) in its very small eye has been problematic. Utilizing a novel servo-null electrophysiologic approach recently applied to the mouse, we have identified a diversity of adenosine-receptor mechanisms in modulating IOP in this species. We report the first evidence that A(3) receptors increase IOP in any species, and verify in the mouse reports with larger mammals that A(1) receptors lower and A(2A) receptors increase IOP.

Involvement of A(3) receptors in the potentiation by adenosine of the inhibitory effect of theophylline on human eosinophil degranulation: possible novel mechanism of the anti-inflammatory action of theophylline

The current use of theophylline in asthma is based on both the bronchodilatory and the anti-inflammatory effects. The exact mechanism of these actions is still controversial and may include the inhibition of adenosine 3',5'-monophosphate phosphodiesterase enzyme (PDE) and antagonism of adenosine receptors. In this study, the mechanism of the anti-inflammatory action was investigated by studying the inhibition by theophylline of complement C5a (C5a)-induced degranulation of human eosinophils and its interaction with adenosine. Theophylline (10-1000 microM) inhibited C5a-induced release of eosinophil peroxidase (EPO) in a concentration-dependent manner with an IC(50) of 233.5 microM and a maximal inhibition of 90.3 +/- 3.0%. In contrast, the PDE4 inhibitor rolipram (up to 50 microM) had no effect. The adenosine A(3) receptor agonist N(6)-(3-iodobenzyl)-5'-N-methylcarbamoyladenosine (IB-MECA) also inhibited release (IC(50) = 7.5 microM), but neither adenosine itself nor the selective A(1) and A(2) agonists and antagonists had any significant effect, even at 100 microM. The inhibition produced by clinically relevant concentration of theophylline (50 microM) was potentiated by ineffective concentrations of exogenous adenosine and additive to that produced by IB-MECA. The potent and selective A(3) antagonist MRS 1220, but not the A(1) or A(2) antagonists, significantly reversed the inhibitory effect of theophylline. These results suggest that therapeutic concentrations of theophylline inhibit human eosinophil partly by acting as an A(3) agonist. Together with the potentiation of theophylline action by adenosine, perhaps via the A(3) receptors, these novel actions may, at least in part, contribute to the mechanism of the anti-inflammatory action of this drug in vivo.

A3 receptors mediate rapid inflammatory cell influx into the lungs of sensitized guinea-pigs

BACKGROUND

Inhaled adenosine causes bronchoconstriction in asthmatics and may modulate inflammatory cell activity. Elevated adenosine levels occur in the lungs after antigen challenge of asthmatics.

OBJECTIVES

The aim of this study was to investigate whether the bronchoconstrictor effects of the adenosine derivative, 5'-AMP, were associated with altered migration of inflammatory cells into the airways using a sensitized atopic guinea-pig model previously shown to display a bronchoconstrictor response. Comparisons were made with the effects of inhaled antigen.

METHODS

Airway responses of conscious sensitized guinea-pigs to inhalation exposures of 5'-AMP were determined by whole body plethysmography as the change in specific airway conductance (sGaw). Influx of leucocytes into the airways was determined by bronchoalveolar lavage (BAL).

RESULTS

5'-AMP caused bronchoconstrictor airway responses in sensitized animals. Dose-dependent infiltration of inflammatory cells into the lungs occurred 1 h after 5'-AMP exposure. No bronchoconstriction or cell influx was seen in unsensitized guinea-pigs. Exposure to ovalbumin (OA) also caused influx of inflammatory cells. Twenty-four hours after an OA exposure, 5'-AMP produced no bronchoconstriction. The P1-receptor antagonists, 8-PT and 8-SPT, inhibited the 5'-AMP-induced bronchoconstriction, indicating that the bronchoconstriction seen in sensitized animals is mediated by A1 or A2 receptors. They had no effect on the cell influx, whereas the A3 antagonist, MRS-1220, significantly inhibited cellular infiltration, suggesting mediation through A3 receptors. At 24 h after an OA challenge and accompanying the cellular influx, there was airway hyper-responsiveness to the bronchoconstriction by histamine. In contrast, no hyper-responsiveness to histamine was seen 1 h after 3 mM or 24 h after 300 mM 5'-AMP.

CONCLUSIONS

5'-AMP caused a rapid migration of eosinophils and macrophages into the airways only in sensitized guinea-pigs, and this was blocked by the A3 antagonist MRS-1220. This was not associated with bronchial hyper-reactivity to histamine.

The A3 adenosine receptor induces cytoskeleton rearrangement in human astrocytoma cells via a specific action on Rho proteins

In previous studies, we have demonstrated that exposure of astroglial cells to A3 adenosine receptor agonists results in dual actions on cell survival, with "trophic" and antiapoptotic effects at nanomolar concentrations and induction of cell death at micromolar agonist concentrations. The protective actions of A3 agonists have been associated with a reinforcement of the actin cytoskeleton, which likely results in increased resistance of cells to cytotoxic stimuli. The molecular mechanisms at the basis of this effect and the signalling pathway(s) linking the A3 receptor to the actin cytoskeleton have never been elucidated. Based on previous literature data suggesting that the actin cytoskeleton is controlled by small GTP-binding proteins of the Rho family, in the study reported here we investigated the involvement of these proteins in the effects induced by A3 agonists on human astrocytoma ADF cells. The presence of the A3 adenosine receptor in these cells has been confirmed by immunoblotting analysis. As expected, exposure of human astrocytoma ADF cells to nanomolar concentrations of the selective A3 agonist 2-chloro-N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (CI-IB-MECA) resulted in formation of thick actin positive stress fibers. Preexposure of cells to the C3B toxin that inactivates Rho-proteins completely prevented the actin changes induced by CI-IB-MECA. Exposure to the A3 agonist also resulted in significant reduction of Rho-GDI, an inhibitory protein known to maintain Rho proteins in their inactive state, suggesting a potentiation of Rho-mediated effects. This effect was fully counteracted by the concomitant exposure to the selective A3 receptor antagonist MRS1191. These results suggest that the reinforcement of the actin cytoskeleton induced by A3 receptor agonists is mediated by an interference with the activation/inactivation cycle of Rho proteins, which may, therefore, represent a biological target for the identification of novel neuroprotective strategies.

[3H]MRS 1754, a selective antagonist radioligand for A(2B) adenosine receptors

MRS 1754 [N-(4-cyanophenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)-phenoxy]acetamide] is a selective antagonist ligand of A(2B) adenosine receptors. This is the least well-defined adenosine receptor subtype, and A(2B) antagonists have potential as antiasthmatic drugs. For use as a radioligand, MRS 1754, a p-cyanoanilide xanthine derivative, was tritiated on the propyl groups in a two-step reaction using a p-carboxamido precursor, which was dehydrated to the cyano species using trifluoroacetic anhydride. [3H]MRS 1754 (150 Ci/mmol) bound to recombinant human A(2B) adenosine receptors in membranes of stably transfected HEK-293 cells. Specific binding was saturable, competitive, and followed a one-site model, with a K(D) value of 1.13 +/- 0.12 nM and a B(max) value of 10.9 +/- 0.6 pmol/mg protein. Specific binding utilizing 0.7 nM [3H]MRS 1754 was > 70% of total binding. The affinity calculated from association and dissociation binding constants was 1.22 nM (N = 4). Binding to membranes expressing rat and human A(1) and A(3) adenosine receptors was not significant, and binding in membranes of HEK-293 cells expressing human A(2A) receptors was of low affinity (K(D) > 50 nM). The effects of cations and chelators were explored. Specific binding was constant over a pH range of 4.5 to 6.5, with reduced binding at higher pH. The pharmacological profile in competition experiments with [3H]MRS 1754 was consistent with the structure-activity relationship for agonists and antagonists at A(2B) receptors. The K(i) values of XAC (xanthine amine congener) and CPX (8-cyclopentyl-1,3-dipropylxanthine) were 16 and 55 nM, respectively. NECA (5'-N-ethylcarboxamidoadenosine) competed for [3H]MRS 1754 binding with a K(i) of 570 nM, similar to its potency in functional assays. Thus, [3H]MRS 1754 is suitable as a selective, high-affinity radioligand for A(2B) receptors.

Adenosine A(1) and A(3) receptors mediate inhibition of synaptic transmission in rat cortical neurons

Intracellular recordings were made in rat brain slice preparations containing pyramidal cells of the associative frontal cortex in order to characterize the action of selective adenosine A(1) and A(3) receptor ligands on synaptic neurotransmission. The selective A(1) receptor agonist N(6)-cyclopentyladenosine (CPA) inhibited concentration-dependently the excitatory postsynaptic potentials (PSPs) which were evoked by focal electrical stimulation. The CPA-mediated inhibition was blocked by 1, 3-dipropyl-8-cyclopentylxanthine (DPCPX), a highly A(1) receptor-selective antagonist. The A(3) receptor agonist N(6)-(3-iodobenzyl)-adenosine-5'-N-methylcarboxamide (IB-MECA) inhibited concentration-dependently the evoked PSPs while the A(1) receptors were blocked continuously by DPCPX. Under these conditions, the A(3) receptor antagonist 9-chloro-2-(2-furanyl)-5-[(phenylacetyl)amino]-1,2,4-triazolo[1, 5-c]quinazoline (MRS 1220) did not influence the PSPs but inhibited completely the effect of IB-MECA. The inhibitory effect of IB-MECA was unaffected by DPCPX. CPA additionally inhibited the PSPs when applied after IB-MECA. Pharmacological dissociation of the N-methyl-D-aspartate (NMDA) and non-NMDA receptor components of the PSPs showed that CPA as well as IB-MECA reduced both. We conclude that adenosine A(1) and A(3) receptors are present on cortical pyramidal cells and involved in the inhibition of excitatory neurotransmission. Our results indicate no interplay between the two receptor subtypes. The separate inhibition may become particularly evident in situations where there are high levels of endogenously released adenosine, as seen in hypoxia.

Adenosine-enhanced ischemic preconditioning: adenosine receptor involvement during ischemia and reperfusion

Adenosine-enhanced ischemic preconditioning (APC) extends the cardioprotection of ischemic preconditioning (IPC) by both significantly decreasing myocardial infarct size and significantly enhancing postischemic functional recovery. In this study, the role of adenosine receptors during ischemia-reperfusion was determined. Rabbit hearts (n = 92) were used for Langendorff perfusion. Control hearts were perfused for 180 min, global ischemia hearts received 30-min ischemia and 120-min reperfusion, and IPC hearts received 5-min ischemia and 5-min reperfusion before ischemia. APC hearts received a bolus injection of adenosine coincident with IPC. Adenosine receptor (A(1), A(2), and A(3)) antagonists were used with APC before ischemia and/or during reperfusion. GR-69019X (A(1)/A(3)) and MRS-1191/MRS-1220 (A(3)) significantly increased infarct size in APC hearts when administered before ischemia and significantly decreased functional recovery when administered during both ischemia and reperfusion (P < 0.05 vs. APC). DPCPX (A(1)) administered either before ischemia and/or during reperfusion had no effect on APC cardioprotection. APC-enhanced infarct size reduction is modulated by adenosine receptors primarily during ischemia, whereas APC-enhanced postischemic functional recovery is modulated by adenosine receptors during both ischemia and reperfusion.