Distinct cardioprotective effects of adenosine mediated by differential coupling of receptor subtypes to phospholipases C and D

Remifentanil-induced preconditioning has cross-talk with A1 and A2B adenosine receptors in ischemic-reperfused rat heart

The purpose of this study was to determine whether there is a cross-talk between opioid receptors (OPRs) and adenosine receptors (ADRs) in remifentanil preconditioning (R-Pre) and, if so, to investigate the types of ADRs involved in the cross-talk. Isolated rat hearts received 30 min of regional ischemia followed by 2 hr of reperfusion. OPR and ADR antagonists were perfused from 10 min before R-Pre until the end of R-Pre. The heart rate, left ventricular developed pressure (LVDP),velocity of contraction (+dP/dtmax), and coronary flow (CF) were recorded. The area at risk and area of necrosis were measured. After reperfusion, the LVDP, +dP/dtmax,and CF showed a significant increase in the R-Pre group compared with the control group (no intervention before or after regional ischemia). These increases in the R-Pre group were blocked by naloxone, a nonspecific ADR antagonist, an A1 ADR antagonist, and an A2B ADR antagonist. The infarct size was reduced significantly in the R-Pre group compared with the control group. The infarct-reducing effect in the R-Pre group was blocked by naloxone, the nonspecific ADR antagonist, the A1 ADR antagonist, and the A2B ADR antagonist. The results of this study demonstrate that there is cross-talk between ADRs and OPRs in R-Pre and that A1 ADR and A2B ADR appear to be involved in the cross-talk.

The ral exchange factor rgl2 promotes cardiomyocyte survival and inhibits cardiac fibrosis

Cardiomyocytes compensate to acute cardiac stress by increasing in size and contractile function. However, prolonged stress leads to a decompensated response characterized by cardiomyocyte death, tissue fibrosis and loss of cardiac function. Identifying approaches to inhibit this transition to a decompensated response may reveal important targets for treating heart failure. The Ral guanine nucleotide disassociation (RalGDS) proteins are Ras-interacting proteins that are upregulated by hypertrophic stimuli. The Ral guanine nucleotide dissociation stimulator-like 2 (Rgl2) is a member of the RalGDS family that modulates expression of hypertrophic genes in cardiomyocytes. However, the pathophysiologic consequence of increased Rgl2 expression in cardiomyoctyes remains unclear. To evaluate the effect of increasing Rgl2 activity in the heart, transgenic mice with cardiac-targeted over-expression of Rgl2 were generated. Although Ral activation was increased, there were no apparent morphologic or histological differences between the hearts of Rgl2 transgenic and nontransgenic mice indicating that increased Rgl2 expression had no effect on basal cardiac phenotype. To determine if Rgl2 modulates the cardiac response to stress, mice were infused with the ß-adrenergic receptor agonist, isoproterenol. Isoproterenol infusion increased heart mass in both Rgl2 transgenic and nontransgenic mice. However, unlike nontransgenic mice, Rgl2 transgenic mice showed no morphologic evidence of cardiomyocyte damage or increased cardiac fibrosis following isoproterenol infusion. Increased Rgl2 expression in cultured cardiomyocytes stimulated Ral activation and inhibited staurosporine-induced apoptosis via increased activation of PI3-kinase. Activation of the PI3-kinase signaling pathway was confirmed in hearts isolated from Rgl2 transgenic mice. Increased expression and function of Rgl2 in cardiomyocytes promotes activation of the PI3-kinase signaling cascade and protects from carciomyocyte death and pathologic cardiac fibrosis. Taken further, these results suggest that Rgl2 upregulation in hypertrophic hearts may be a protetive mechanism, and that Rgl2 may be a novel therapeutic target in treating heart disease.

Activation of transient receptor potential canonical 3 (TRPC3)-mediated Ca2+ entry by A1 adenosine receptor in cardiomyocytes disturbs atrioventricular conduction

Although the activation of the A(1)-subtype of the adenosine receptors (A(1)AR) is arrhythmogenic in the developing heart, little is known about the underlying downstream mechanisms. The aim of this study was to determine to what extent the transient receptor potential canonical (TRPC) channel 3, functioning as receptor-operated channel (ROC), contributes to the A(1)AR-induced conduction disturbances. Using embryonic atrial and ventricular myocytes obtained from 4-day-old chick embryos, we found that the specific activation of A(1)AR by CCPA induced sarcolemmal Ca(2+) entry. However, A(1)AR stimulation did not induce Ca(2+) release from the sarcoplasmic reticulum. Specific blockade of TRPC3 activity by Pyr3, by a dominant negative of TRPC3 construct, or inhibition of phospholipase Cs and PKCs strongly inhibited the A(1)AR-enhanced Ca(2+) entry. Ca(2+) entry through TRPC3 was activated by the 1,2-diacylglycerol (DAG) analog OAG via PKC-independent and -dependent mechanisms in atrial and ventricular myocytes, respectively. In parallel, inhibition of the atypical PKCζ by myristoylated PKCζ pseudosubstrate inhibitor significantly decreased the A(1)AR-enhanced Ca(2+) entry in both types of myocytes. Additionally, electrocardiography showed that inhibition of TRPC3 channel suppressed transient A(1)AR-induced conduction disturbances in the embryonic heart. Our data showing that A(1)AR activation subtly mediates a proarrhythmic Ca(2+) entry through TRPC3-encoded ROC by stimulating the phospholipase C/DAG/PKC cascade provide evidence for a novel pathway whereby Ca(2+) entry and cardiac function are altered. Thus, the A(1)AR-TRPC3 axis may represent a potential therapeutic target.

Expression of mRNA for Adenosine A1, A2a, A2b, and A3 Receptors in HL-60 Cells: Dependence on Cell Cycle Phases

The present studies investigated changes in expression of mRNA for adenosine A1, A2a, A2b, and A3 receptors in samples of HL-60 promyelocytic cells differing in the actual presence of cells in various phases of the cell cycle induced by the double thymidine block method. Real-time PCR technique was used for obtaining data on mRNA expression. Statistical analysis of the data revealed that the mRNA expression of adenosine A1, A2a, and A3 receptors is dependent on the cell cycle phase. G0/G1 and G2/M phases were characterized by a higher mRNA expression of adenosine A1 receptors and a lower one of adenosine A2a and A3 receptors whereas the opposite was true for the S phase. Interestingly, expression of mRNA of the adenosine A2b receptors was independent on the cell cycle phase. The results indicate the plasticity of mRNA expression of adenosine receptors in the investigated promyelocytic cells and its interaction with physiological mechanisms of the cell cycle.

Adenosine A3 receptor stimulation reduces muscle injury following physical trauma and is associated with alterations in the MMP/TIMP response

We have previously demonstrated that in response to traumatic injury in skeletal muscle, there is a dysregulation of the matrix metalloproteases (MMPs) and their inhibitors (TIMPs), a response hypothesized to interfere with proper skeletal muscle regeneration. Moreover, we have shown that pharmacological activation of the adenosine A(3) receptor by Cl-IBMECA in skeletal muscle can protect against ischemia-reperfusion and eccentric exercise injury. However, the mechanism by which Cl-IBMECA protects muscle tissue is poorly defined. This study evaluated the effects of Cl-IBMECA on MMP/TIMP expression in skeletal muscle and tested the hypothesis that adenosine A(3) receptor-stimulated protection of skeletal muscle following traumatic injury is associated with a blunting of MMPs involved in inflammatory processes and collagen degradation, and an increase in MMPs associated with extracellular matrix remodeling. Sixty C57BL/6J male mice were injected with Cl-IBMECA (n = 30) or a vehicle (n = 30), and Evans blue dye. Injury was induced by applying a cold steel probe (-79°C) to the tibialis anterior (TA) muscle for 10 s. TA muscles from uninjured and injured legs were collected 3, 10, and 24 h postinjury for analysis of muscle injury and MMP/TIMP mRNA and protein levels. Twenty-four hours postinjury, 56.8% of the fibers were damaged in vehicle-treated mice vs. 35.4% in Cl-IBMECA-treated mice (P = 0.02). Cl-IBMECA treatment reduced membrane type 1 (MT1)-MMP, MMP-3, MMP-9, and TIMP-1 mRNA expression 2- to 20-fold compared with vehicle-treated mice (P < 0.05). Cl-IBMECA decreased protein levels of latent/shed MT1-MMP 23-2,000%, respectively, 3-10 h postinjury. In Cl-IBMECA-treated mice, latent MMP-2 was decreased 20% 3 h postinjury, active MMP-3 was decreased 64% 3 h postinjury, and latent/active MMP-9 was decreased 417,631% 3 h postinjury and 20% 10 h postinjury. Protein levels of active MMP-2 and latent MMP-3 were increased 25% and 74% 3 h postinjury, respectively. The present study elucidates a new protective role of adenosine A(3) receptor stimulation in posttraumatic skeletal muscle injury.

Adenosine A1 receptor activation is arrhythmogenic in the developing heart through NADPH oxidase/ERK- and PLC/PKC-dependent mechanisms

Whether adenosine, a crucial regulator of the developing cardiovascular system, can provoke arrhythmias in the embryonic/fetal heart remains controversial. Here, we aimed to establish a mechanistic basis of how an adenosinergic stimulation alters function of the developing heart. Spontaneously beating hearts or dissected atria and ventricle obtained from 4-day-old chick embryos were exposed to adenosine or specific agonists of the receptors A(1)AR (CCPA), A(2A)AR (CGS-21680) and A(3)AR (IB-MECA). Expression of the receptors was determined by quantitative PCR. The functional consequences of blockade of NADPH oxidase, extracellular signal-regulated kinase (ERK), phospholipase C (PLC), protein kinase C (PKC) and L-type calcium channel (LCC) in combination with adenosine or CCPA, were investigated in vitro by electrocardiography. Furthermore, the time-course of ERK phosphorylation was determined by western blotting. Expression of A(1)AR, A(2A)AR and A(2B)AR was higher in atria than in ventricle while A(3)AR was equally expressed. Adenosine (100μM) triggered transient atrial ectopy and second degree atrio-ventricular blocks (AVB) whereas CCPA induced mainly Mobitz type I AVB. Atrial rhythm and atrio-ventricular propagation fully recovered after 60min. These arrhythmias were prevented by the specific A(1)AR antagonist DPCPX. Adenosine and CCPA transiently increased ERK phosphorylation and induced arrhythmias in isolated atria but not in ventricle. By contrast, A(2A)AR and A(3)AR agonists had no effect. Interestingly, the proarrhythmic effect of A(1)AR stimulation was markedly reduced by inhibition of NADPH oxidase, ERK, PLC, PKC or LCC. Moreover, NADPH oxidase inhibition or antioxidant MPG prevented both A(1)AR-mediated arrhythmias and ERK phosphorylation. These results suggest that pacemaking and conduction disturbances are induced via A(1)AR through concomitant stimulation of NADPH oxidase and PLC, followed by downstream activation of ERK and PKC with LCC as possible target.

The in vivo and in vitro stimulatory effects of cordycepin on mouse leydig cell steroidogenesis

Cordycepin, a pure compound of Cordyceps sinensis (CS), is known as an adenosine analog. We have found that CS stimulated Leydig cell steroidogenesis. Here we investigated the in vivo and in vitro effects of cordycepin in primary mouse Leydig cell steroidogenesis. The results indicate that cordycepin increased the plasma testosterone concentration. Cordycepin also stimulated in vitro mouse Leydig cell testosterone production in dose- and time-dependent manners. We further observed that cordycepin regulated the mRNA expression of the A1, A2a, A2b, and A3 adenosine receptors in the mouse Leydig cells, and that antagonists of A1, A2a, and A3 suppressed testosterone production 20-50% testosterone production. Furthermore, Rp-cAMPS (cAMP antagonist) and Protein Kinase A (PKA) inhibitors (H89 and PKI) significantly decreased cordycepin-induced testosterone production, indicating that the PKA-cAMP signal pathway was activated by cordycepin through adenosine receptors. Moreover, cordycepin induced StAR protein expression, and H89 suppressed cordycepin-induced steroidogenic acute regulatory (StAR) protein expression. Conclusively, cordycepin associated with adenosine receptors to activate cAMP-PKA-StAR pathway and steroidogenesis in the mouse Leydig cells.

The role of adenosine receptor agonists in regulation of hematopoiesis

The review summarizes data evaluating the role of adenosine receptor signaling in murine hematopoietic functions. The studies carried out utilized either non-selective activation of adenosine receptors induced by elevation of extracellular adenosine or by administration of synthetic adenosine analogs having various proportions of selectivity for a particular receptor. Numerous studies have described stimulatory effects of non-selective activation of adenosine receptors, manifested as enhancement of proliferation of cells at various levels of the hematopoietic hierarchy. Subsequent experimental approaches, considering the hematopoiesis-modulating action of adenosine receptor agonists with a high level of selectivity to individual adenosine receptor subtypes, have revealed differential effects of various adenosine analogs. Whereas selective activation of A₁ receptors has resulted in suppression of proliferation of hematopoietic progenitor and precursor cells, that of A₃ receptors has led to stimulated cell proliferation in these cell compartments. Thus, A₁ and A₃ receptors have been found to play a homeostatic role in suppressed and regenerating hematopoiesis. Selective activation of adenosine A₃ receptors has been found to act curatively under conditions of drug- and radiation-induced myelosuppression. The findings in these and further research areas will be summarized and mechanisms of hematopoiesis-modulating action of adenosine receptor agonists will be discussed.

Multivalent dendrimeric and monomeric adenosine agonists attenuate cell death in HL-1 mouse cardiomyocytes expressing the A(3) receptor

Multivalent dendrimeric conjugates of GPCR ligands may have increased potency or selectivity in comparison to monomeric ligands, a phenomenon that was tested in a model of cytoprotection in mouse HL-1 cardiomyocytes. Quantitative RT-PCR indicated high expression levels of endogenous A(1) and A(2A) adenosine receptors (ARs), but not of A(2B) and A(3)ARs. Activation of the heterologously expressed human A(3)AR in HL-1 cells by AR agonists significantly attenuated cell damage following 4h exposure to H(2)O(2) (750 microM) but not in untransfected cells. The A(3) agonist IB-MECA (EC(50) 3.8 microM) and the non-selective agonist NECA (EC(50) 3.9 microM) protected A(3) AR-transfected cells against H(2)O(2) in a concentration-dependent manner, as determined by lactate dehydrogenase release. A generation 5.5 PAMAM (polyamidoamine) dendrimeric conjugate of a N(6)-chain-functionalized adenosine agonist was synthesized and its mass indicated an average of 60 amide-linked nucleoside moieties out of 256 theoretical attachment sites. It non-selectively activated the A(3)AR to inhibit forskolin-stimulated cAMP formation (IC(50) 66nM) and, similarly, protected A(3)-transfected HL-1 cells from apoptosis-inducing H(2)O(2) with greater potency (IC(50) 35nM) than monomeric nucleosides. Thus, a PAMAM conjugate retained AR binding affinity and displayed greatly enhanced cardioprotective potency.

Synthesis and evaluation of new N6-substituted adenosine-5'-N-methylcarboxamides as A3 adenosine receptor agonists

A number of N(6)-substituted adenosine-5'-N-methylcarboxamides were synthesised and their pharmacology, in terms of their receptor affinity, selectivity and cardioprotective effects, were explored. The first series of compounds, 4a-4f and 5a-5f, showed modest receptor affinity for the A(3)AR with K(i) values in the low to mid muM range. However, the incorporation of a 4-(2-aminoethyl)-2,6-di-tert-butylphenol group in the N(6)-position (in compounds 4g and 5g) significantly improved the affinity with K(i) values of 30 and 9 nM, respectively. Improvements in affinity, as well as selectivity were seen when a functionalized linker was introduced. The N(6)-phenyl series, compounds 7a-7d, demonstrated low to mid nanomolar receptor affinities (K(i)=2.3-45.0 nM), with 7b displaying 109-fold selectivity for the A(3)AR (vs A(1)). The N(6)-benzyl series 9a-9c also proved to be potent and selective A(3)AR agonists and the longer chain length linker 13 was tolerated at the A(3)AR without abrogation of affinity or selectivity. Cardioprotection was demonstrated by a simulated ischaemia cell culture assay, whereby 7b, 7c, 9a, 9b and 9c all showed cardioprotective effects at 100 nM comparable or better than the benchmark A(3)AR agonist IB-MECA, but which were indistinguishable by statistical analysis. For example, compound 9c reduced cell death by 68.0+/-3.6%.

A3 adenosine receptor: pharmacology and role in disease

The study of the A(3) adenosine receptor (A(3)AR) represents a rapidly growing and intense area of research in the adenosine field. The present chapter will provide an overview of the expression patterns, molecular pharmacology and functional role of this A(3)AR subtype under pathophysiological conditions. Through studies utilizing selective A(3)AR agonists and antagonists, or A(3)AR knockout mice, it is now clear that this receptor plays a critical role in the modulation of ischemic diseases as well as in inflammatory and autoimmune pathologies. Therefore, the potential therapeutic use of agonists and antagonists will also be described. The discussion will principally address the use of such compounds in the treatment of brain and heart ischemia, asthma, sepsis and glaucoma. The final part concentrates on the molecular basis of A(3)ARs in autoimmune diseases such as rheumatoid arthritis, and includes a description of clinical trials with the selective agonist CF101. Based on this chapter, it is evident that continued research to discover agonists and antagonists for the A(3)AR subtype is warranted.

Adenoprotection of the heart involves phospholipase C-induced activation and translocation of PKC-epsilon to RACK2 in adult rat and mouse

Adenosine protects the heart from adrenergic overstimulation. This adenoprotection includes the direct anti-adrenergic action via adenosine A(1) receptors (A(1)R) on the adrenergic signaling pathway. An indirect A(1)R-induced attenuation of adrenergic responsiveness involves the translocation of PKC-epsilon to t-tubules and Z-line of cardiomyocytes. We investigated with sarcomere imaging, immunocytochemistry imaging, and coimmunoprecipitation (co-IP) whether A(1)R activation of PKC-epsilon induces the kinase translocation to receptor for activated C kinase 2 (RACK2) in isolated rat and mouse hearts and whether phospholipase C (PLC) is involved. Rat cardiomyocytes were treated with the A(1)R agonist chlorocyclopentyladenosine (CCPA) and exposed to primary PKC-epsilon and RACK2 antibodies with secondaries conjugated to Cy3 and Cy5 (indodicarbocyanine), respectively. Scanning confocal microscopy showed that CCPA caused PKC-epsilon to reversibly colocalize with RACK2 within 3 min. Additionally, rat and mouse hearts were perfused and stimulated with CCPA or phenylisopropyladenosine to activate A(1)R, or with phorbol 12-myristate 13-acetate to activate PKC. RACK2 was immunoprecipitated from heart extracts and resolved with SDS-PAGE. Western blotting showed that CCPA, phenylisopropyladenosine, and phorbol 12-myristate 13-acetate in the rat heart increased the PKC-epsilon co-IP with RACK2 by 186, 49, and >1,000%, respectively. The A(1)R antagonist 8-cyclopentyl-1,3-dipropylxanthine prevented the CCPA-induced co-IP with RACK2. In mouse hearts, CCPA increased the co-IP of PKC-epsilon with RACK2 by 61%. With rat cardiomyocytes, the beta-adrenergic agonist isoproterenol increased sarcomere shortening by 177%. CCPA reduced this response by 47%, an action inhibited by the PLC inhibitor U-73122 and 8-cyclopentyl-1,3-dipropylxanthine. In conclusion, A(1)R stimulation of the heart is associated with PLC-initiated PKC-epsilon translocation and association with RACK2.

Isoform-specific regulation of the Na+ -K+ pump by adenosine in guinea pig ventricular myocytes

AIM

The present study investigated the effect of adenosine on Na(+)-K(+) pumps in acutely isolated guinea pig (Cavia sp.) ventricular myocytes.

METHODS

The whole-cell, patch-clamp technique was used to record the Na(+)-K(+) pump current (I(p)) in acutely isolated guinea pig ventricular myocytes.

RESULTS

Adenosine inhibited the high DHO-affinity pump current (I(h)) in a concentration-dependent manner, which was blocked by the selective adenosine A(1) receptor antagonist DPCPX and the general protein kinase C (PKC) antagonists staurosporine, GF 109203X or the specific delta isoform antagonist rottlerin. In addition, the inhibitory action of adenosine was mimicked by a selective A(1) receptor agonist CCPA and a specific activator peptide of PKC-delta, PP114. In contrast, the selective A(2A) receptor agonist CGS21680 and A(3) receptor agonist Cl-IB-MECA did not affect I(h). Application of the selective A(2A) receptor antagonist SCH58261 and A(3) receptor antagonist MRS1191 also failed to block the effect of adenosine. Furthermore, H89, a selective protein kinase A (PKA) antagonist, did not exert any effect on adenosine-induced I(h) inhibition.

CONCLUSION

The present study provides the electrophysiological evidence that adenosine can induce significant inhibition of I(h) via adenosine A(1) receptors and the PKC-delta isoform.

Protective roles of adenosine A1, A2A, and A3 receptors in skeletal muscle ischemia and reperfusion injury

Although adenosine exerts cardio-and vasculoprotective effects, the roles and signaling mechanisms of different adenosine receptors in mediating skeletal muscle protection are not well understood. We used a mouse hindlimb ischemia-reperfusion model to delineate the function of three adenosine receptor subtypes. Adenosine A(3) receptor-selective agonist 2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide (Cl-IBMECA; 0.07 mg/kg ip) reduced skeletal muscle injury with a significant decrease in both Evans blue dye staining (5.4 +/- 2.6%, n = 8 mice vs. vehicle-treated 28 +/- 6%, n = 7 mice, P < 0.05) and serum creatine kinase level (1,840 +/- 910 U/l, n = 13 vs. vehicle-treated 12,600 +/- 3,300 U/l, n = 14, P < 0.05), an effect that was selectively blocked by an A(3) receptor antagonist 3-ethyl-5-benzyl-2-methyl-6-phenyl-4-phenylethynyl-1,4-(+/-)-dihydropyridine-3,5-dicarboxylate (MRS-1191; 0.05 mg/kg). The adenosine A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine (CCPA; 0.05 mg/kg) also exerted a cytoprotective effect, which was selectively blocked by the A(1) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 0.2 mg/kg). The adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS-21680; 0.07 mg/kg)-induced decrease in skeletal muscle injury was selectively blocked by the A(2A) antagonist 2-(2-furanyl)-7-[3-(4-methoxyphenyl)propyl]-7H-pyrazolo[4,3-e] [1,2,4]triazolo[1,5-C]pyrimidin-5-amine (SCH-442416; 0.017 mg/kg). The protection induced by the A(3) receptor was abrogated in phospholipase C-beta2/beta3 null mice, but the protection mediated by the A(1) or A(2A) receptor remained unaffected in these animals. The adenosine A(3) receptor is a novel cytoprotective receptor that signals selectively via phospholipase C-beta and represents a new target for ameliorating skeletal muscle injury.

The A3 adenosine receptor: an enigmatic player in cell biology

Adenosine is a primordial signaling molecule present in every cell of the human body that mediates its physiological functions by interacting with 4 subtypes of G-protein-coupled receptors, termed A1, A2A, A2B and A3. The A3 subtype is perhaps the most enigmatic among adenosine receptors since, although several studies have been performed in the years to elucidate its physiological function, it still presents in several cases a double nature in different pathophysiological conditions. The 2 personalities of A3 often come into direct conflict, e.g., in ischemia, inflammation and cancer, rendering this receptor as a single entity behaving in 2 different ways. This review focuses on the most relevant aspects of A3 adenosine subtype activation and summarizes the pharmacological evidence as the basis of the dichotomy of this receptor in different therapeutic fields. Although much is still to be learned about the function of the A3 receptor and in spite of its duality, at the present time it can be speculated that A3 receptor selective ligands might show utility in the treatment of ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The biggest and most intriguing challenge for the future is therefore to understand whether and where selective A3 agonists or antagonists are the best choice.

Adenosinergic cardioprotection: Multiple receptors, multiple pathways

Adenosine, formed primarily via hydrolysis of 5'-AMP, has been historically dubbed a "retaliatory" metabolite due to enhanced local release and beneficial actions during cellular/metabolic stress. From a cardiovascular perspective, evidence indicates the adenosinergic system is essential in mediation of intrinsic protection (e.g., pre- and postconditioning) and determining myocardial resistance to insult. Modulation of adenosine and its receptors thus remains a promising, though as yet not well-realized, approach to amelioration of injury in ischemic-reperfused myocardium. Adenosine exerts effects through A(1), A(2A), A(2B), and A(3) adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR), which are all expressed in myocardial and vascular cells, and couple to G proteins to trigger a range of responses (generally, but not always, beneficial). Adenosine can also enhance tolerance to injurious stimuli via receptor-independent metabolic effects. Given adenosines contribution to preconditioning, it is no surprise that postreceptor signaling typically mimics that associated with preconditioning. This involves activation/translocation of PKC, PI3 kinase, and MAPKs, with ultimate effects at the level of mitochondrial targets-the mitochondrial K(ATP) channel and/or the mitochondrial permeability transition pore (mPTP). Nonetheless, differences in cytoprotective signaling and actions of the different adenosine receptor subtypes have been recently revealed. Our understanding of adenosinergic cytoprotection continues to evolve, with roles for the A(2) subtypes emerging, together with evidence of essential receptor "cross-talk" in mediation of protection. This review focuses on current research into adenosine-mediated cardioprotection, highlighting recent findings which, together with a wealth of prior knowledge, may ultimately facilitate adenosinergic approaches to clinical cardiac protection.

Rapid stimulation of presynaptic serotonin transport by A(3) adenosine receptors

The inactivation of synaptic serotonin (5-hydroxytryptamine, 5-HT) is largely established through the actions of the presynaptic, antidepressant-sensitive 5-HT transporter (SERT, SLC6A4). Recent studies have demonstrated post-translational regulation of SERT mediated by multiple Ser/Thr kinases, including protein kinases C and G (PKC and PKG) and p38 mitogen-activated protein kinase (MAPK), as well as the Ser/Thr phosphatase PP2A. Less well studied are specific surface receptors that target these signaling pathways to control SERT surface expression and/or catalytic rates. Using rat basophilic leukemia 2H3 cell line (RBL-2H3), we previously established that activation of A(3) adenosine receptors (A(3)AR) stimulates SERT activity via both PKG and p38 MAPK (Zhu et al., 2004a). Whether A(3)ARs regulate SERT in the central nervous system (CNS) is unknown. Here we report that the A(3)AR agonist N(6)-(3-iodobenzyl)-N-methyl-5'carbamoyladenosine (IB-MECA) rapidly (10 min) and selectively stimulates 5-HT transport in mouse midbrain, hippocampal, and cortical synaptosomes. IB-MECA-induced stimulation of 5-HT uptake is blocked by the selective A(3)AR antagonist 3-ethyl-5-benzyl-2-methyl-phenylethynyl-6-phenyl-1,4(+/-)dihydropyridine-3,5-dicarboxylate (MRS1191) and is absent from synaptosomes prepared from A(3)AR knockout mice. Kinetic analyses demonstrate that IB-MECA induces an increase of 5-HT transport V(max) with no significant change in K(m). As in RBL-2H3 cells, IB-MECA stimulation of synaptosomal 5-HT uptake can be blocked by preincubation with PKG antagonists N-[2-(methylamino)ethy]-5-isoquinoline-sulfonamide (H8) and DT-2 (YGRKKRRQRRRPPLRK(5)H), as well as by the p38 MAPK inhibitor SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)-1H-imidazole]. Chronoamperometry studies in the anesthetized rat hippocampus support a role for A(3)ARs in SERT regulation in vivo. Together, these results identify a novel, region-specific action of CNS A(3)ARs in the modulation of SERT-mediated 5-HT transport that may be relevant for the etiology and/or therapy of 5-HT-linked brain disorders.

The neuroprotective adenosine-activated signal transduction pathway involves activation of phospholipase C

We have demonstrated before that exposure of neuronal cultures to poisoning by iodoacetic acid (IAA) followed by "reperfusion" (IAA-R insult), results in severe cytotoxicity, which could be markedly attenuated by prior activation of the adenosine A1 receptors. We also have demonstrated that adenosine activates a signal transduction pathway (STP), which involves activation of PKC epsilon and opening of KATP channels. Here, we provide proof for the involvement also of phospholipase C (PLC) in the neuronal protective adenosine-activated STP. R-PIA, a specific A1 adenosine receptor agonist, was found to enhance neuronal PLC activity and protect against the IAA-R insult. The PLC inhibitor U73122, abrogated both R-PIA-induced effects. These results demonstrate that activation of PLC is a vital step in the neuronal protective adenosine-induced STP.

Modulation of cardiac sarcoplasmic reticulum calcium release by aenosine: a protein kinase C- dependent pathway

We have already reported that A(3) adenosine receptor stimulation reduces [(3)H]-ryanodine binding and sarcoplasmic reticulum Ca(2+) release in rat heart. In the present work we have investigated the transduction pathway responsible for this effect. Isolated rat hearts were perfused for 20 min in the presence of the following substances: 100 nM N(6)-(iodobenzyl)-adenosine-5'-N-methyluronamide (IB-MECA), an A(3) adenosine agonist; 10 muM U-73122, a phospholipase C inhibitor; 2 muM chelerythrine, a protein kinase C inhibitor. At the end of perfusion, the hearts were homogenized and [(3)H]-ryanodine binding was assayed. IB-MECA produced a significant decrease in ryanodine binding, which was abolished in the presence of chelerythrine but not in the presence of U-73122. RT-PCR experiments showed that ryanodine receptor gene expression was not affected by IB-MECA. In Western blot experiments, ryanodine receptor phosphorylation on serine 2809 was not modified after perfusion with IB-MECA. We conclude that modulation of SR Ca(2+) release channel by IB-MECA is dependent on protein kinase C activation. However, in this model protein kinase C activation is not due to phospholipase C activation. In addition, changes in ryanodine receptor gene expression or direct phosphorylation of the ryanodine receptor on serine 2809 residue do not appear to occur.

Semi-rational design of (north)-methanocarba nucleosides as dual acting A(1) and A(3) adenosine receptor agonists: novel prototypes for cardioprotection

Ring-constrained adenosine analogues have been designed to act as dual agonists at tissue-protective A(1) and A(3) adenosine receptors (ARs). 9-Ribosides transformed into the ring-constrained (N)-methanocarba-2-chloro-5'-uronamides consistently lost affinity at A(1)/A(2A)ARs and gained at A(3)AR. Among 9-riboside derivatives, only N(6)-cyclopentyl and 7-norbornyl moieties were extrapolated for mixed A(1)/A(3) selectivity and rat/human A(3)AR equipotency. Consequently, 2 was balanced in affinity and potency at A(1)/A(3)ARs as envisioned and dramatically protected in an intact heart model of global ischemia and reperfusion.

A3 adenosine receptor-mediated protection of the ischemic heart

The A3 adenosine receptor (A3AR) is attributed with multiple beneficial actions in ischemic-reperfused myocardium, including modulation of oncotic and apoptotic cell death and enhancement of contractile function. Additionally, the A3AR may attenuate vascular dysfunction and improve long-term outcome from myocardial insult (modulating hypertrophy and angiogenesis). Available evidence indicates that this receptor sub-type is minimally activated by endogenous adenosine during ischemia (A3AR antagonists exerting no effects on ischemic outcome), and is thus amenable to activation with exogenous agonists. Protected phenotypes arise with both pre- and post-ischemic treatment with A3AR agonists, and transient A3AR agonism also triggers early and delayed preconditioned states. The molecular basis for the varied protective actions of the A3AR remains poorly defined, and may well vary between species (e.g. rodent vs. human) and protective responses (e.g. acute vs. delayed protection). Nonetheless, A3ARs may be more promising as therapeutic "anti-ischemic" targets compared with other adenosine receptor subtypes, since A3AR agonists elicit fewer and less significant side-effects. This review addresses current knowledge and controversy regarding the protective actions (and associated signaling) of A3ARs in ischemic-reperfused heart.

A neoceptor approach to unraveling microscopic interactions between the human A2A adenosine receptor and its agonists

Strategically mutated neoceptors, e.g., with anionic residues in TMs 3 and 7 intended for pairing with positively charged amine-modified nucleosides, were derived from the antiinflammatory A(2A) adenosine receptor (AR). Adenosine derivatives functionalized at the 5', 2, and N(6) positions were synthesized. The T88D mutation selectively enhanced the binding of the chain-length-optimized 5'-(2-aminoethyl)uronamide but not 5'-(2-hydroxyethyl)uronamide, suggesting a critical role of the positively charged amine. Combination of this modification with the N(6)-(2-methylbenzyl) group enhanced affinity at the Q89D- and N181D- but not the T88D-A(2A)AR. Amino groups placed near the 2- or N(6)-position only slightly affected the binding to mutant receptors. The 5'-hydrazide MRS3412 was 670- and 161-fold enhanced, in binding and functionally, respectively, at the Q89D-A(2A)AR compared to the wild-type. Thus, we identified and modeled pairs of A(2A)AR-derived neoceptor-neoligand, which are pharmacologically orthogonal with respect to the native species.

Techniques: Recent developments in computer-aided engineering of GPCR ligands using the human adenosine A3 receptor as an example

G-protein-coupled receptors (GPCRs) represent the largest known family of signal-transducing molecules, and convey signals for light and many extracellular regulatory molecules. GPCRs are dysfunctional or dysregulated in several human diseases and are estimated to be the targets of >40% of the drugs used in clinical medicine today. The crystal structure of rhodopsin provides the first information on the three-dimensional structure of GPCRs, which now supports homology modeling studies and structure-based drug-design approaches. In this article, we review recent work on adenosine receptors, a family of GPCRs, and, in particular, on adenosine A(3) receptor antagonists. We focus on an iterative, bi-directional approach in which models are used to generate hypotheses that are tested by experimentation; the experimental findings are, in turn, used to refine the model. The success of this approach is due to the synergistic interaction between theory and experimentation.

Phospholipase C is involved in the adenosine-activated signal transduction pathway conferring protection against iodoacetic acid-induced injury in primary rat neuronal cultures

We have demonstrated before that exposure of neuronal cultures to poisoning by iodoacetic acid, followed by "reperfusion" (iodoacetate-"reperfusion" insult; IAA-R insult), results in severe cytotoxicity. This insult was found to be associated with ATP depletion and generation of reactive oxygen species. The cultured neurons could be protected against the insult by activation of the adenosine A1 receptors and by presence of antioxidants. Previous studies in our laboratory demonstrated that the adenosine-activated signal transduction pathway (Ado-STP) conferring protection against the IAA-R insult, involves activation of protein kinase C-epsilon (PKCepsilon) and opening of ATP sensitive potassium (K(ATP)) channels. In this respect, the adenosine-activated protective mechanism against the IAA-R insult is similar to the Ado-STP in the neurons and in cardiomyocytes against ischemia-reperfusion injury. Phospholipase C (PLC) is an additional component demonstrated recently to participate in the myocardial Ado-STP protecting against ischemia-reperfusion. Here we provide proof for the involvement of PLC also in the neuronal Ado-STP protecting against the IAA-R insult. Primary rat neuronal cultures were exposed to the IAA-R insult. The neurons could be protected against this insult by activation of the adenosine A1 receptors by N6-(R)-phenylisopropyladenosine (R-PIA), a specific A1 adenosine receptor agonist. Exposure of the cultures to the PLC inhibitor U73122, abrogated the protection. The exposure of the cultures to R-PIA was found to enhance PLC activity, an effect that could be abrogated by presence of U73122. The R-PIA-induced increase in PLC activity was short-lived, in the range of minutes. These results demonstrate that activation of PLC is a vital step in the neuronal protective Ado-STP, but that it does not contribute directly to the relatively long time window of the protection signal shown previously to characterize the neuronal mechanism. The results also support the suggestion that the Ado-STP protecting against the IAA-R insult and that protecting against ischemia-reperfusion may represent the same mechanism.

Protein kinase Cepsilon and the antiadrenergic action of adenosine in rat ventricular myocytes

Adenosine-induced antiadrenergic effects in the heart are mediated by adenosine A(1) receptors (A(1)R). The role of PKCepsilon in the antiadrenergic action of adenosine was explored with adult rat ventricular myocytes in which PKCepsilon was overexpressed. Myocytes were transfected with a pEGFP-N1 vector in the presence or absence of a PKCepsilon construct and compared with normal myocytes. The extent of myocyte shortening elicited by electrical stimulation of quiescent normal and transfected myocytes was recorded with video imaging. PKCepsilon was found localized primarily in transverse tubules. The A(1)R agonist chlorocyclopentyladenosine (CCPA) at 1 microM rendered an enhanced localization of PKCepsilon in the t-tubular system. The beta-adrenergic agonist isoproterenol (Iso; 0.4 microM) elicited a 29-36% increase in myocyte shortening in all three groups. Although CCPA significantly reduced the Iso-produced increase in shortening in all three groups, the reduction caused by CCPA was greatest with PKCepsilon overexpression. The CCPA reduction of the Iso-elicited shortening was eliminated in the presence of a PKCepsilon inhibitory peptide. These results suggest that the translocation of PKCepsilon to the t-tubular system plays an important role in A(1)R-mediated antiadrenergic actions in the heart.

Epsilon protein kinase C mediated ischemic tolerance requires activation of the extracellular regulated kinase pathway in the organotypic hippocampal slice

Ischemic preconditioning (IPC) promotes brain tolerance against subsequent ischemic insults. Using the organotypic hippocampal slice culture, we conducted the present study to investigate (1) the role of adenosine A1 receptor (A1AR) activation in IPC induction, (2) whether epsilon protein kinase C (epsilonPKC) activation after IPC is mediated by the phosphoinositol pathway, and (3) whether epsilonPKC protection is mediated by the extracellular signal-regulated kinase (ERK) pathway. Our results demonstrate that activation of A1AR emulated IPC, whereas blockade of the A1AR during IPC diminished neuroprotection. The neuroprotection promoted by the A1AR was also reduced by the epsilonPKC antagonist. To determine whether epsilonPKC activation in IPC and A1AR preconditioning is mediated by activation of the phosphoinositol pathway, we incubated slices undergoing IPC or adenosine treatment with a phosphoinositol phospholipase C inhibitor. In both cases, preconditioning neuroprotection was significantly attenuated. To further characterize the subsequent signal transduction pathway that ensues after epsilonPKC activation, mitogen-activated protein kinase kinase was blocked during IPC and pharmacologic preconditioning (PPC) (with epsilonPKC, NMDA, or A1AR agonists). This treatment significantly attenuated IPC- and PPC-induced neuroprotection. In conclusion, we demonstrate that epsilonPKC activation after IPC/PPC is essential for neuroprotection against oxygen/glucose deprivation in organotypic slice cultures and that the ERK pathway is downstream to epsilonPKC.

Structural determinants of efficacy at A3 adenosine receptors: modification of the ribose moiety

We have found previously that structural features of adenosine derivatives, particularly at the N6- and 2-positions of adenine, determine the intrinsic efficacy as A3 adenosine receptor (AR) agonists. Here, we have probed this phenomenon with respect to the ribose moiety using a series of ribose-modified adenosine derivatives, examining binding affinity and activation of the human A3 AR expressed in CHO cells. Both 2'- and 3'-hydroxyl groups in the ribose moiety contribute to A3 AR binding and activation, with 2'-OH being more essential. Thus, the 2'-fluoro substitution eliminated both binding and activation, while a 3'-fluoro substitution led to only a partial reduction of potency and efficacy at the A3 AR. A 5'-uronamide group, known to restore full efficacy in other derivatives, failed to fully overcome the diminished efficacy of 3'-fluoro derivatives. The 4'-thio substitution, which generally enhanced A3 AR potency and selectivity, resulted in 5'-CH2OH analogues (10 and 12) which were partial agonists of the A3 AR. Interestingly, the shifting of the N6-(3-iodobenzyl)adenine moiety from the 1'- to 4'-position had a minor influence on A3 AR selectivity, but transformed 15 into a potent antagonist (16) (Ki = 4.3 nM). Compound 16 antagonized human A3 AR agonist-induced inhibition of cyclic AMP with a K(B) value of 3.0 nM. A novel apio analogue (20) of neplanocin A, was a full A3 AR agonist. The affinities of selected, novel analogues at rat ARs were examined, revealing species differences. In summary, critical structural determinants for human A3 AR activation have been identified, which should prove useful for further understanding the mechanism of receptor activation and development of more potent and selective full agonists, partial agonists and antagonists for A3 ARs.

Exploring distal regions of the A3 adenosine receptor binding site: sterically constrained N6-(2-phenylethyl)adenosine derivatives as potent ligands

We synthesized phenyl ring-substituted analogues of N(6)-(1S,2R)-(2-phenyl-1-cyclopropyl)adenosine, which is highly potent in binding to the human A(3)AR with a Ki value of 0.63 nM. The effects of these structural changes on affinity at human and rat adenosine receptors and on intrinsic efficacy at the hA(3)AR were measured. A 3-nitrophenyl analogue was resolved chromatographically into pure diastereomers, which displayed 10-fold stereoselectivity in A(3)AR binding in favor of the 1S,2R isomer. A molecular model defined a hydrophobic region (Phe168) in the putative A(3)AR binding site around the phenyl moiety. A heteroaromatic group (3-thienyl) could substitute for the phenyl moiety with retention of high affinity of A(3)AR binding. Other related N(6)-substituted adenosine derivatives were included for comparison. Although the N(6)-(2-phenyl-1-cyclopropyl) derivatives were full A(3)AR agonists, several other derivatives had greatly reduced efficacy. N(6)-Cyclopropyladenosine was an A(3)AR antagonist, and adding either one or two phenyl rings at the 2-position of the cyclopropyl moiety restored efficacy. N(6)-(2,2-Diphenylethyl)adenosine was an A(3)AR antagonist, and either adding a bond between the two phenyl rings (N(6)-9-fluorenylmethyl) or shortening the ethyl moiety (N(6)-diphenylmethyl) restored efficacy. A QSAR study of the N(6) region provided a model that was complementary to the putative A(3)AR binding site in a rhodopsin-based homology model. Thus, a new series of high-affinity A(3)AR agonists and related nucleoside antagonists was explored through both empirical and theoretical approaches.

Characterization of ERK1/2 signalling pathways induced by adenosine receptor subtypes in newborn rat cardiomyocytes

1. Adenosine A(1), A(2A), and A(3) receptors (ARs) and extracellular signal-regulated kinase 1/2 (ERK1/2) play a major role in myocardium protection from ischaemic injury. In this study, we have characterized the adenosine receptor subtypes involved in ERK1/2 activation in newborn rat cardiomyocytes. 2. Adenosine (nonselective agonist), CPA (A(1)), CGS 21680 (A(2A)) or Cl-IB-MECA (A(3)), all increased ERK1/2 phosphorylation in a time- and dose-dependent manner. The combined maximal response of the selective agonists was similar to adenosine alone. Theophylline (nonselective antagonist) inhibited completely adenosine-mediated ERK1/2 activation, whereas a partial inhibition was obtained with DPCPX (A(1)), ZM 241385 (A(2A)), and MRS 1220 (A(3)). 3. PD 98059 (MEK1; ERK kinase inhibitor) abolished all agonist-mediated ERK1/2 phosphorylation. Pertussis toxin (PTX, G(i/o) blocker) inhibited completely CPA- and partially adenosine- and Cl-IB-MECA-induced ERK1/2 activation. Genistein (tyrosine kinase inhibitor) and Ro 318220 (protein kinase C, PKC inhibitor) partially reduced adenosine, CPA and Cl-IB-MECA responses, without any effect on CGS 21680-induced ERK1/2 phosphorylation. H89 (protein kinase A, PKA inhibitor) abolished completely CGS 21680 and partially adenosine and Cl-IB-MECA responses, without any effect on CPA response. 4. Cl-IB-MECA-mediated increases in cAMP accumulation suggest that A(3)AR-induced ERK1/2 phosphorylation involves adenylyl cyclase activation via phospholipase C (PLC) and PKC stimulation. 5. In summary, we have shown that ERK1/2 activation by adenosine in cardiomyocytes results from an additive stimulation of A(1), A(2A), and A(3)ARs, which involves G(i/o) proteins, PKC, and tyrosine kinase for A(1) and A(3)ARs, and Gs and PKA for A(2A)ARs. Moreover, the A(3)AR response also involves a cAMP/PKA pathway via PKC activation.

Role of direct RhoA-phospholipase D1 interaction in mediating adenosine-induced protection from cardiac ischemia

Activation of adenosine A1 or A3 receptors protects heart cells from ischemia-induced injury. The A3 receptor signals via RhoA and phospholipase D (PLD) to induce cardioprotection. The objective of the study was to investigate how RhoA activates PLD to achieve the anti-ischemic effect of adenosine A3 receptors. In an established cardiac myocyte model of preconditioning using the cultured chick embryo heart cells, overexpression of the RhoA-noninteracting PLD1 mutant I870R selectively blocked the A3 agonist (Cl-IBMECA, 10 nM)-induced cardioprotection. I870R caused a significantly higher percentage of cardiac cells killed in A3 agonist-treated than in A1 agonist (CCPA, 10 nM)-treated myocytes (ANOVA and posttest comparison, P<0.01). Consistent with its inhibitory effect on the PLD activity, I870R attenuated the Cl-IBMECA-mediated PLD activation. Cl-IBMECA caused a 41 +/- 15% increase in PLD activity in mock-transfected myocytes (P<0.01, paired t test) while having only a slight stimulatory effect on the PLD activity in I870R-transfected cells. To further test the anti-ischemic role of a direct RhoA-PLD1 interaction, atrial cardiac myocytes were rendered null for native adenosine receptors by treatment with irreversible A1 antagonist m-DITC-XAC and were selectively transfected with the human adenosine A1 or A3 receptor cDNA individually or they were cotransfected with cDNAs encoding either receptor plus I870R. I870R preferentially inhibited the human A3 receptor-mediated protection from ischemia. The RhoA-noninteracting PLD1 mutant caused a significantly higher percentage of cardiac cells killed in myocytes cotransfected with the human A3 receptor than in those cells expressing the human A1 receptor (ANOVA and posttest comparison, P<0.01). The present data provided the first demonstration of a novel physiological role for the direct RhoA-PLD1 interaction, that of potent protection from cardiac ischemia. The study further supported the concept that a divergent signaling mechanism mediates the anti-ischemic effect of adenosine A1 and A3 receptors.

Novel N6-substituted adenosine 5'-N-methyluronamides with high selectivity for human adenosine A3 receptors reduce ischemic myocardial injury

We recently reported the identification of a novel human adenosine A3 receptor-selective agonist, (2S,3S,4R,5R)-3-amino-5-[6-[5-chloro-2-(3-methylisoxazol-5-ylmethoxy)benzylamino]purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-608,039), with 1,260-fold selectivity for the human A3 versus human A1 receptor (DeNinno et al., J Med Chem 46: 353-355, 2003). However, because the modest (20-fold) rabbit A3 receptor selectivity of CP-608,039 precludes demonstration of A3-mediated cardioprotection in rabbit models, we identified another member of this class, (2S,3S,4R,5R)-3-amino-5-[6-(2,5-dichlorobenzylamino)purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-532,903), which both retained human A3 receptor selectivity (210-fold; human A3/human A1 Ki: 23/4,800 nM) and had improved rabbit A3 receptor selectivity (90-fold; rabbit A3/rabbit A1 Ki: 23/2,000 nM). Infarct size was measured in Langendorff hearts or in vivo after 30 min of regional ischemia and 120 min of reperfusion. Five-minute perfusion with CP-532,903 before ischemia-reperfusion elicited a concentration-dependent reduction in infarct size in isolated hearts (EC50: 0.97 nM; maximum reduction in infarct size: 77%, P < 0.05 vs. control). Furthermore, administration of CP-532,903 (150 nM) at reperfusion also significantly reduced infarct size by 64% (P < 0.05 vs. control), which was not different (P > or = 0.05) from the cardioprotection provided by the same concentration of drug given before ischemia. The selective rabbit A1 receptor antagonist BWA1433 did not affect CP-532,903-dependent cardioprotection. In vivo, CP-532,903 (1 mg/kg) reduced infarct size by 50% in the absence of significant hemodynamic effects (mean arterial pressure, heart rate, rate-pressure product). CP-532,903 and CP-608,039 represent a novel class of human A3 receptor-selective agonists that may prove suitable for investigation of the clinical cardioprotective efficacy of A3 receptor activation.

Acute adenosinergic cardioprotection in ischemic-reperfused hearts

Cells of the cardiovascular system generate and release purine nucleoside adenosine in increasing quantities when constituent cells are "stressed" or subjected to injurious stimuli. This increased adenosine can interact with surface receptors in myocardial, vascular, fibroblast, and inflammatory cells to modulate cellular function and phenotype. Additionally, adenosine is rapidly reincorporated back into 5'-AMP to maintain the adenine nucleotide pool. Via these receptor-dependent and independent (metabolic) paths, adenosine can substantially modify the acute response to ischemic insult, in addition to generating a more sustained ischemia-tolerant phenotype (preconditioning). However, the molecular basis for acute adenosinergic cardioprotection remains incompletely understood and may well differ from more widely studied preconditioning. Here we review current knowledge and some controversies regarding acute cardioprotection via adenosine and adenosine receptor activation.

Molecular mechanisms of reduced beta-adrenergic signaling in the aged heart as revealed by genomic profiling

Myocardial aging leads to a reduction of beta-adrenergic receptor-induced metabolic and contractile responsiveness. We hypothesize that a change in the patterns of gene expression is important in these age-related events. To test this, hearts were harvested from young and aged male rats (3-4 and 20-22 mo, respectively). Total mRNA was extracted and prepared for hybridization to Affymetrix U34A GeneChips. Filtering criteria, involving fold change and a statistical significance cutoff were employed, yielding 263 probe pairs exhibiting differential signals. Of the 163 annotated genes, at least 56 (34%) were classified as signaling/cell communication. Of these 56, approximately half were directly involved in G protein-coupled receptor signaling pathways. We next determined which of these changes might be involved in anti-adrenergic activity and identified 19 potentially important gene products. Importantly, we observed a decrease in beta1-adrenergic receptor and adenylyl cyclase mRNAs, whereas the mRNA encoding beta-arrestin increased. Furthermore, the results demonstrate an increase in mRNAs encoding the adenosine A1 receptor and phospholipase D, which could increase anti-adrenergic effects. Moreover, the mRNAs encoding the muscarinic M3 receptor, nicotinic acetylcholine receptor beta3, and nicotinic acetylcholine receptor-related protein were increased as was the mRNA encoding guanylate kinase-associated protein. Interestingly, we also observed eight mRNAs whose abundance changed three- to sixfold with aging that could be considered as being compensatory. Although these results do not prove causality, they demonstrate that cardiac aging is associated with changes in the profiles of gene expression and that many of these changes may contribute to reduced adrenergic signaling.

Combined pharmacological preconditioning with a G-protein-coupled receptor agonist, a mitochondrial KATP channel opener and a nitric oxide donor mimics ischaemic preconditioning

1. Although pharmacological preconditioning (PPC) has emerged as an alternative to ischaemic preconditioning (IPC) in cardioprotection, the efficacy of PPC compared with IPC has not been investigated. Because IPC is mediated by complex signalling cascades arising from multiple triggers, we have hypothesized that combined PPC is necessary to mimic IPC. 2. Isolated and perfused rat hearts underwent IPC by three cycles of 5 min ischaemia and 5 min reperfusion before 30 min global ischaemia followed by 120 min reperfusion. Adenosine (30 micromol/L), diazoxide (50 micromol/L) and s-nitroso-N-acetylpenicillamine (SNAP; 50 micromol/L) were added for 25 min just before (pretreatment modality) or 45 min before (PPC modality) the index ischaemia. 3. Ischaemic preconditioning significantly improved isovolumic left ventricular (LV) function and reduced infarct size. Although pretreatment with adenosine, diazoxide or SNAP alone was capable of reducing infarct size, PPC with each drug alone or in a combination of two drugs except for diazoxide plus SNAP failed to reduce infarct size. In contrast, PPC in combination with adenosine, diazoxide and SNAP (triple combination PPC) conferred significant improvement of LV function and reduction of infarct size that was as effective as IPC. 4. Cardioprotection afforded by triple combination PPC was abolished by the Gi/o-protein inhibitor pertussis toxin, the mitochondiral KATP channel inhibitor 5-hydroxydecanoate or the nitric oxide (NO) scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl imidazoline-1-oxyl 3-oxide (carboxy-PTIO). 5. Protein kinase C (PKC)-epsilon in the particulate fraction was activated throughout preconditioning ischaemia and reperfusion. Although PKC-epsilon was activated during treatment with adenosine, diazoxide or SNAP alone, it was inactivated after washout. In contrast, PKC-epsilon remained activated after triple combination PPC. The PKC inhibitor chelerythrine abolished activation of PKC-epsilon and cardioprotection afforded by IPC and triple combination PPC. 6. These results demonstrate that combined PPC with a G-protein-coupled receptor agonist, a mitochondrial KATP channel opener and an NO donor is necessary to mimic IPC and such synergistic cardioprotection is associated with enhanced and sustained activation of PKC-epsilon.

Endothelial and Microvascular Function

The endothelium is more than just a passive vessel lining. New advances have revealed and expanded the multifactorial role of the endothelium in the homeostatic regulation of the microvasculature, including control of primary hemostasis, blood coagulation and fibrinolysis, platelet and leukocyte interactions with the vessel wall, lipoprotein metabolism, presentation of histocompatibility antigens, regulation of vascular tone and growth, and regulation of blood pressure. It possesses numerous receptors and releases compounds that affect the regulation of vascular tone and contribute to vascular permeability. Many crucial vasoactive endogenous compounds are formed in the endothelial cells to control the functions of vascular smooth muscle cells and circulating blood cells. Gap junctions facilitate the exchange of metabolites, ions, and other messenger molecules among endothelial cells and smooth muscle cells, and regulate cell growth. Among the numerous regulatory systems affecting microvascular function are the cholinergic and adrenergic (α1, α2, and β) systems. Flow-metabolism coupling is affected by a variety of signaling systems, including adenosine, oxygen, carbon dioxide, lactate, nitric oxide, and others. Agents such as the angiotensin system and endothelin, as well as others, play a role in autoregulation (maintenance of constant flow in the face of changing pressure). All of these are discussed in detail.

Signalling from adenosine receptors to mitogen-activated protein kinases

The purine nucleoside adenosine acts via four distinct adenosine receptor subtypes: the adenosine A(1), A(2A), A(2B), and A(3) receptor. They are all G protein-coupled receptors (GPCR) coupling to classical second messenger pathways such as modulation of cAMP production or the phospholipase C (PLC) pathway. In addition, they couple to mitogen-activated protein kinases (MAPK), which could give them a role in cell growth, survival, death and differentiation. Although each of the adenosine receptors can activate one or more of the MAPKs, the mechanisms appear to differ substantially, both between receptor subtypes in the same cell type and between the same receptor in different cell types.

Adenosine and opioid receptor-mediated cardioprotection in the rat: evidence for cross-talk between receptors

The relative roles of free-radical production, mitochondrial ATP-sensitive K+ (mitoKATP) channels and possible receptor cross-talk in both opioid and adenosine A1 receptor (A1AR) mediated protection were assessed in a rat model of myocardial infarction. Sprague-Dawley rats were subjected to 30 min of occlusion and 90 min of reperfusion. The untreated rats exhibited an infarct of 58.8 +/- 2.9% [infarct size (IS)/area at risk (AAR), %] at the end of reperfusion. Pretreatment with either the nonselective opioid receptor agonist morphine or the selective A1AR agonist 2-chloro-cyclopentyladenosine (CCPA) dramatically reduced IS/AAR to 41.1 +/- 2.2% and 37.9 +/- 5.5%, respectively (P < 0.05). Protection afforded by either morphine or CCPA was abolished by the reactive oxygen species scavenger N-(2-mercaptopropionyl)glycine or the mitoKATP channel blocker 5-hydroxydecanoate. Both morphine- and CCPA-mediated protection were attenuated by the selective A1AR antagonist 1,3-dipropyl-8-cyclopentylxanthine and the selective delta1-opioid receptor (DOR) antagonist 7-benzylidenealtrexone. Simultaneous administration of morphine and CCPA failed to enhance the infarct-sparing effect of either agonist alone. These data suggest that both DOR and A1AR-mediated cardioprotection are mitoKATP and reactive oxygen species dependent. Furthermore, these data suggest that there are converging pathways and/or receptor cross-talk between A1AR- and DOR-mediated cardioprotection.

Effect of protein kinase C during hepatocyte hypoxic precondition

AIM

To investigate the effects of protein kinase C (PKC) on hypoxic preconditioning (HP) for hepatocyte.

METHODS

Through a normal liver cell HP model, PKC inhibitor and activator were utilized to analyze the phosphorylation of PKC. The cellular structure and viability were also observed. All the data were statistically analyzed.

RESULTS

Compared with the phosphorylation of PKC in the control without HP [(710.5±78.8) fkat/g], the phosphorylation of PKC was obviously increased in HP treated model [(1823.7±268.2) fkat/g] and PMA treated model [(2 541.2±326.5) fkat/g] (P<0.01). Cellular changes were less. In addition, opposite changes were found in PKC inhibited groups, and the phosphorylation of PKC was [(1 088.0±89.3) fkat/g] (P<0.01).

CONCLUSION

The activation of PKC is the important chain of HP in the preservation of liver cell, and its mechanism may be involved in protein phosphorylation.

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.

Receptor and non-receptor-dependent mechanisms of cardioprotection with adenosine

The relative roles of mitochondrial (mito) ATP-sensitive K(+) (mitoK(ATP)) channels, protein kinase C (PKC), and adenosine kinase (AK) in adenosine-mediated protection were assessed in Langendorff-perfused mouse hearts subjected to 20-min ischemia and 45-min reperfusion. Control hearts recovered 72 +/- 3 mmHg of ventricular pressure (50% preischemia) and released 23 +/- 2 IU/g lactate dehydrogenase (LDH). Adenosine (50 microM) during ischemia-reperfusion improved recovery (149 +/- 8 mmHg) and reduced LDH efflux (5 +/- 1 IU/g). Treatment during ischemia alone was less effective. Treatment with 50 microM diazoxide (mitoK(ATP) opener) during ischemia and reperfusion enhanced recovery and was equally effective during ischemia alone. A(3) agonism [100 nM 2-chloro-N(6)-(3-iodobenzyl)-adenosine-5'-N-methyluronamide], A(1) agonism (N(6)-cyclohexyladenosine), and AK inhibition (10 microM iodotubercidin) all reduced necrosis to the same extent as adenosine, but less effectively reduced contractile dysfunction. These responses were abolished by 100 microM 5-hydroxydecanoate (5-HD, mitoK(ATP) channel blocker) or 3 microM chelerythrine (PKC inhibitor). However, the protective effects of adenosine during ischemia-reperfusion were resistant to 5-HD and chelerythrine and only abolished when inhibitors were coinfused with iodotubercidin. Data indicate adenosine-mediated protection via A(1)/A(3) adenosine receptors is mitoK(ATP) channel and PKC dependent, with evidence for a downstream location of PKC. Adenosine provides additional and substantial protection via phosphorylation to 5'-AMP, primarily during reperfusion.

Acetylcholine but not adenosine triggers preconditioning through PI3-kinase and a tyrosine kinase

Adenosine and acetylcholine (ACh) trigger preconditioning by different signaling pathways. The involvement of phosphatidylinositol 3-kinase (PI3-kinase), a protein tyrosine kinase, and Src family tyrosine kinase in preconditioning was evaluated in isolated rabbit hearts. Either wortmannin (PI3-kinase blocker), genistein (tyrosine kinase blocker), lavendustin A (tyrosine kinase blocker), or 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolol[3,4-d]pyrimidine (PP2; Src family tyrosine kinase blocker) was given for 15 min to bracket a 5-min infusion of either adenosine or ACh (trigger phase). The hearts then underwent 30 min of regional ischemia. Infarct size for ACh alone was 9.3 +/- 3.5% of the risk zone versus 34.3 +/- 4.1% in controls. All four inhibitors blocked ACh-induced protection. When wortmannin or PP2 was infused only during the 30-min ischemic period (mediator phase), ACh-induced protection was not affected (7.4 +/- 2.1% and 9.7 +/- 1.7% infarction, respectively). Adenosine-triggered protection was not blocked by any of the inhibitors. Therefore, PI3-kinase and at least one protein tyrosine kinase, probably Src kinase, are involved in the trigger phase of ACh-induced, but not adenosine-induced, preconditioning. Neither PI3-kinase nor Src kinase is a mediator of the protection of ACh.

Differential aspects in ratio measurements of [Ca(2+)](i) relaxation in cardiomyocyte contraction following various drug treatments

This study is concerned with the analysis of the time dependency of [Ca(2+)](i), monitored by indo-1-AM, via the ratiometric time response curve R(t) as measured during contractions of spontaneous or electrical stimulated cardiomyocytes (in culture). A mathematical formulation which describes the relaxation phase of R(t) was developed. By fitting formulation to the measured data of R(t), the extraction of characteristic parameters is feasible, which may reflect the factors regulating intracellular Ca concentration. The usefulness of the suggested formulation was examined by monitoring changes induced in those parameters following the exposure of the myocytes to different drugs, among which are: caffeine, ryanodine, thapsigargin db, cyclic AMP, isoprenaline, doxorubicin, and Cl-IB-MECA.

Cardiomyocyte resistance to doxorubicin mediated by A(3) adenosine receptor

Recently, we reported that the activation of A(3) adenosine receptor (A(3)R) in newborn cultured cardiomyocytes by highly selective agonist Cl-IB-MECA (2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methyluronamide) induces protection against the anthracycline antibiotic doxorubicin (DOX) cardiotoxicity. The present study was undertaken to further characterize the cardioprotective action of A(3)R activation by revealing the structural changes in cardiomyocytes elicited upon exposure to DOX. Morphological observations (ultrastructural and immunocytochemical) indicate that after DOX treatment, the cardiomyocytes undergo destructive alterations, and protective action of A(3)R is not connected with its anti-apoptotic activity. A(3)R activation appeared to prevent destructive alterations of cardiomyocyte mitochondria and dissipation of mitochondrial membrane potential. In DOX-treated cardiomyocytes, appearance of disorganized desmin and contractile filaments was related to detrimental alterations in the mitochondrial structure, in particular their position and transmembrane potential. In intact cardiomyocytes, diazoxide, a selective mitochondrial K(ATP) channel opener, induced an increase in ATP synthesis within 15 min of application. Similar effect was obtained by activation of adenosine A(1)R. However, A(3)R agonist Cl-IB-MECA did not affect ATP synthesis. Neither A(1)R agonist CCPA (2-chloro-N(6)-cyclopentyladenosine) nor diazoxide protected cardiomyocytes from the detrimental effects of DOX. Thus, the opening of mitochondrial K(ATP) channels does not seem to be effective during the slow development of anthracycline cytotoxicity. Our results indicate that DOX increases the activity of lysosomes, which may contribute to cell injury in an "oncotic" manner and also demonstrate the proinflammatory potency of the drug. Furthermore, the decreased acidification of cytoplasm upon activation of A(3)R may attenuate the ongoing inflammatory response. The present study identifies a novel role for A(3)R selective agonist Cl-IB-MECA and suggests its importance in regulating cardiac cellular function.

Late preconditioning elicited by activation of adenosine A(3) receptor in heart: role of NF- kappa B, iNOS and mitochondrial K(ATP) channel

Activation of adenosine A(3) receptor (A(3)AR) protects against ischemia/reperfusion injury in the heart. However, the downstream signaling mechanisms leading to its delayed anti-ischemic effects remain unclear. We hypothesized that A(3)AR stimulation protects the heart via activation of nuclear transcription factor kappa B (NF-kappa B) and synthesis of inducible nitric oxide synthase (iNOS). Mice were treated with selective A(3)AR agonist, N(6)-(3-iodobenzyl) adenosine-5;-N-methyluronamide (IB-MECA). Twenty-four h later, hearts were perfused in Langendorff mode and subjected to 30 min of global ischemia and 30 min of reperfusion. IB-MECA caused post-ischemic reduction in necrosis and improvement in myocardial performance which was abolished by A(3)AR antagonist, MRS1191. Electrophoretic mobility shift assay demonstrated increased NF-kappa B binding in nuclear extracts following A(3)AR stimulation, which was diminished by MRS1191 and NF-kappa B inhibitor, pyrrolidinediethyldithiocarbamate (PDTC). The cardioprotection was abrogated by PDTC and targeted ablation of p50 subunit of NF-kappa B in mice. The inhibition of iNOS with S-methylisothiourea and targeted disruption of the iNOS gene also abolished the protective effect of A(3)AR stimulation. Expression of iNOS mRNA and NO production were enhanced after 6 and 24 h respectively of IB-MECA treatment. MRS1191 and PDTC blocked IB-MECA induced NO production after A(3)AR stimulation. MitoK(ATP) channel blocker, 5-hydroxydecanoate abolished the protective effect of A(3)AR. For the first time, we have provided direct evidence of an essential role of NF- kappa B activation and iNOS in A(3)AR-induced late preconditioning. Selective activation of A(3)AR with IB-MECA can be used to trigger long-lasting ischemic protection in the heart.

Adenosine-mediated cardioprotection in ischemic-reperfused mouse heart

We investigated the roles of A1, A2A, or A3 receptors and purine salvage in cardioprotection with exogenous adenosine, and tested whether A2A -mediated reductions in perfusion pressure modify post-ischemic recovery. Treatment with 10(-5) or 5 x 10(-5) M adenosine improved contractile recovery from 20 min ischemia 45 min reperfusion in isolated mouse hearts. Protection was attenuated by adenosine kinase inhibition (10(-5) M iodotubercidin) and receptor antagonism (5 x 10(-5) M 8-rho-sulfophenyltheophylline, 8-SPT). Enzyme efflux mirrored contractile recoveries. A 3 agonism with 10(-7) M 2-chloro- N6-(3-iodobenzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) improved ischemic tolerance whereas A1 agonism with 5 x 10(-8) M N6-cyclopentyladenosine (CPA) and A2A agonism with 10(-9) M 2-[p-(2-carboxyethyl) phenethylamino]-5'-N-ethylcarboxamidoadenosine (CGS21680) or 2 x 10(-8) M methyl-4-(3-[9-[4S,5S,2R,3R)-5-(N-ethylcarbamoyl)-3,4-dihydroxyoxolan-2-yl]-6-aminopurin-2-yl)]prop-2-ynyl) cyclohexane-carboxylate (ATL-146e) were ineffective. Protection via A1 receptor overexpression was enhanced by adenosine, but unaltered by A1 or A2A agonists. Finally, post-ischemic dysfunction in hearts perfused at constant flow was dependent on coronary pressure, with A2A AR-mediated reductions in pressure reducing diastolic contracture, and elevated perfusion pressure worsening contracture. Data indicate that cardioprotection with exogenous adenosine in asanguinous hearts involves purine salvage and activation of A3 but not A1 or A2A receptors.