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

Purinergic Signaling and its Role in the Stem Cell Differentiation

Purinergic signaling is a mechanism in which extracellular purines and pyrimidines interact with specialized cell surface receptors known as purinergic receptors. These receptors are divided into two families of P1 and P2 receptors, each responding to different nucleosides and nucleotides. P1 receptors are activated by adenosine, while P2 receptors are activated by pyrimidine and purines. P2X receptors are ligand-gated ion channels, including seven subunits (P2X1-7). However, P2Y receptors are the G-protein coupled receptors comprising eight subtypes (P2Y1/2/4/6/11/12/13/14). The disorder in purinergic signaling leads to various health-related issues and diseases. In various aspects, it influences the activity of non-neuronal cells and neurons. The molecular mechanism of purinergic signaling provides insight into treating various human diseases. On the contrary, stem cells have been investigated for therapeutic applications. Purinergic signaling has shown promising effect in stem cell engraftment. The immune system promotes the autocrine and paracrine mechanisms and releases the significant factors essential for successful stem cell therapy. Each subtype of purinergic receptor exerts a beneficial effect on the damaged tissue. The most common effect caused by purinergic signaling is the proliferation and differentiation that treat different health-related conditions.

Loss of adenosine A3 receptors accelerates skeletal muscle regeneration in mice following cardiotoxin-induced injury

Skeletal muscle regeneration is a complex process orchestrated by multiple interacting steps. An increasing number of reports indicate that inflammatory responses play a central role in linking initial muscle injury responses to timely muscle regeneration following injury. The nucleoside adenosine has been known for a long time as an endogenously produced anti-inflammatory molecule that is generated in high amounts during tissue injury. It mediates its physiological effects via four types of adenosine receptors. From these, adenosine A3 receptors (A3Rs) are not expressed by the skeletal muscle but are present on the surface of various inflammatory cells. In the present paper, the effect of the loss of A3Rs was investigated on the regeneration of the tibialis anterior (TA) muscle in mice following cardiotoxin-induced injury. Here we report that regeneration of the skeletal muscle from A3R-/- mice is characterized by a stronger initial inflammatory response resulting in a larger number of transmigrating inflammatory cells to the injury site, faster clearance of cell debris, enhanced proliferation and faster differentiation of the satellite cells (the muscle stem cells), and increased fusion of the generated myoblasts. This leads to accelerated skeletal muscle tissue repair and the formation of larger myofibers. Though the infiltrating immune cells expressed A3Rs and showed an increased inflammatory profile in the injured A3R-/- muscles, bone marrow transplantation experiments revealed that the increased response of the tissue-resident cells to tissue injury is responsible for the observed phenomenon. Altogether our data indicate that A3Rs are negative regulators of injury-related regenerative inflammation and consequently also that of the muscle fiber growth in the TA muscle. Thus, inhibiting A3Rs might have a therapeutic value during skeletal muscle regeneration following injury.

Adenosine receptors as emerging therapeutic targets for diabetic kidney disease

Diabetic kidney disease (DKD) is now a pandemic worldwide, and novel therapeutic options are urgently required. Adenosine, an adenosine triphosphate metabolite, plays a role in kidney homeostasis through interacting with four types of adenosine receptors (ARs): A1AR, A2AAR, A2BAR, and A3AR. Increasing evidence highlights the role of adenosine and ARs in the development and progression of DKD: 1) increased adenosine in the plasma and urine of diabetics with kidney injury, 2) increased expression of each of the ARs in diabetic kidneys, 3) the protective effect of coffee, a commonly ingested nonselective AR antagonist, on DKD, and 4) the protective effect of AR modulators in experimental DKD models. We propose AR modulators as a new therapeutic option to treat DKD. Detailed mechanistic studies on the pharmacology of AR modulators will help us to develop effective first-in-class AR modulators against DKD.

Exploration of chalcones and related heterocycle compounds as ligands of adenosine receptors: therapeutics development

Adenosine receptors (ARs) are ubiquitously distributed throughout the mammalian body where they are involved in an extensive list of physiological and pathological processes that scientists have only begun to decipher. Resultantly, AR agonists and antagonists have been the focus of multiple drug design and development programmes within the past few decades. Considered to be a privileged scaffold in medicinal chemistry, the chalcone framework has attracted a substantial amount of interest in this regard. Due to the potential liabilities associated with its structure, however, it has become necessary to explore other potentially promising compounds, such as heterocycles, which have successfully been obtained from chalcone precursors in the past. This review aims to summarise the emerging therapeutic importance of adenosine receptors and their ligands, especially in the central nervous system (CNS), while highlighting chalcone and heterocyclic derivatives as promising AR ligand lead compounds.

Dual Actions of A2A and A3 Adenosine Receptor Ligand Prevents Obstruction-Induced Kidney Fibrosis in Mice

Kidney fibrosis is the final outcome of chronic kidney disease (CKD). Adenosine plays a significant role in protection against cellular damage by activating four subtypes of adenosine receptors (ARs), A₁AR, A_2AAR, A_2BAR, and A₃AR. A_2AAR agonists protect against inflammation, and A₃AR antagonists effectively inhibit the formation of fibrosis. Here, we showed for the first time that LJ-4459, a newly synthesized dual-acting ligand that is an A_2AAR agonist and an A₃AR antagonist, prevents the progression of tubulointerstitial fibrosis. Unilateral ureteral obstruction (UUO) surgery was performed on 6-week-old male C57BL/6 mice. LJ-4459 (1 and 10 mg/kg) was orally administered for 7 days, started at 1 day before UUO surgery. Pretreatment with LJ-4459 improved kidney morphology and prevented the progression of tubular injury as shown by decreases in urinary kidney injury molecular-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) excretion. Obstruction-induced tubulointerstitial fibrosis was attenuated by LJ-4459, as shown by a decrease in fibrotic protein expression in the kidney. LJ-4459 also inhibited inflammation and oxidative stress in the obstructed kidney, with reduced macrophage infiltration, reduced levels of pro-inflammatory cytokines, as well as reduced levels of reactive oxygen species (ROS). These data demonstrate that LJ-4459 has potential as a therapeutic agent against the progression of tubulointerstitial fibrosis.

Heme Oxygenase-1 Contributes to Both the Engulfment and the Anti-Inflammatory Program of Macrophages during Efferocytosis

Heme oxygenase-1 (HO-1) plays a vital role in the catabolism of heme and yields equimolar amounts of biliverdin, carbon monoxide, and free iron. We report that macrophages engulfing either the low amount of heme-containing apoptotic thymocytes or the high amount of heme-containing eryptotic red blood cells (eRBCs) strongly upregulate HO-1. The induction by apoptotic thymocytes is dependent on soluble signals, which do not include adenylate cyclase activators but induce the p38 mitogen-activated protein (MAP) kinase pathway, while in the case of eRBCs, it is cell uptake-dependent. Both pathways might involve the regulation of BTB and CNC homology 1 (BACH1), which is the repressor transcription regulator factor of the HO-1 gene. Long-term continuous efferocytosis of apoptotic thymocytes is not affected by the loss of HO-1, but that of eRBCs is inhibited. This latter is related to an internal signaling pathway that prevents the efferocytosis-induced increase in Rac1 activity. While the uptake of apoptotic cells suppressed the basal pro-inflammatory cytokine production in wild-type macrophages, in the absence of HO-1, engulfing macrophages produced enhanced amounts of pro-inflammatory cytokines. Our data demonstrate that HO-1 is required for both the engulfment and the anti-inflammatory response parts of the efferocytosis program.

"Adenosine an old player with new possibilities in kidney diseases": Preclinical evidences and clinical perspectives

Renal injury might originate from multiple factors like ischemia reperfusion (I/R), drug toxicity, cystic fibrosis, radio contrast agent etc. The four adenosine receptor subtypes have been identified and found to show diverse physiological and pathological roles in kidney diseases. The activation of A₁ adenosine receptor (A₁) protects against acute kidney injury by improving renal hemodynamic alterations, decreasing tubular necrosis and its inhibition might facilitate removal of toxin or drug metabolite in chronic kidney disease models. Furthermore, recent findings revealed that A_2A receptor subtype activation regulates macrophage phenotype in experimental models of nephritis. Interestingly the emerging role of adenosine kinase inhibitors in kidney diseases has been discussed which act by increasing adenosine availability at target sites and thereby promote A_2A receptor stimulation. In addition, the least explored adenosine receptor subtype A₃ inhibition was observed to exert anti- oxidant, immunosuppressive and anti-fibrotic effects, but more studies are required to confirm its benefits in other renal injury models. The clinical studies targeting A₁ receptor in patients with pre-existing kidney disease have yielded disappointing results, perhaps owing to the origin of unexpected neurological complications during the course of trial. Importantly, conducting well designed clinical trials and testing adenosine modulators with lesser brain penetrability could clear the way for clinical approval of these agents for patients with renal functional impairments.

Adenosine Receptor Ligands: Coumarin-Chalcone Hybrids as Modulating Agents on the Activity of hARs

Adenosine receptors (ARs) play an important role in neurological and psychiatric disorders such as Alzheimer’s disease, Parkinson’s disease, epilepsy and schizophrenia. The different subtypes of ARs and the knowledge on their densities and status are important for understanding the mechanisms underlying the pathogenesis of diseases and for developing new therapeutics. Looking for new scaffolds for selective AR ligands, coumarin–chalcone hybrids were synthesized (compounds 1–8) and screened in radioligand binding (hA₁, hA_2A and hA₃) and adenylyl cyclase (hA_2B) assays in order to evaluate their affinity for the four human AR subtypes (hARs). Coumarin–chalcone hybrid has been established as a new scaffold suitable for the development of potent and selective ligands for hA₁ or hA₃ subtypes. In general, hydroxy-substituted hybrids showed some affinity for the hA₁, while the methoxy counterparts were selective for the hA₃. The most potent hA₁ ligand was compound 7 (K_i = 17.7 µM), whereas compound 4 was the most potent ligand for hA₃ (K_i = 2.49 µM). In addition, docking studies with hA₁ and hA₃ homology models were established to analyze the structure–function relationships. Results showed that the different residues located on the protein binding pocket could play an important role in ligand selectivity.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Knockout and Knock-in Mouse Models to Study Purinergic Signaling

Purinergic signaling involves extracellular purines and pyrimidines acting upon specific cell surface purinoceptors classified into the P1, P2X, and P2Y families for nucleosides and nucleotides. This widespread signaling mechanism is active in all major tissues and influences a range of functions in health and disease. Orthologs to all but one of the human purinoceptors have been found in mouse, making this laboratory animal a useful model to study their function. Indeed, analyses of purinoceptors via knock-in or knockout approaches to produce gain or loss of function phenotypes have revealed several important therapeutic targets. None of the homozygous purinoceptor knockouts proved to be developmentally lethal, which suggest that either these receptors are not involved in key developmental processes or that the large number of receptors in each family allowed for functional compensation. Different models for the same purinoceptor often show compatible phenotypes but there have been examples of significant discrepancies. These revealed unexpected differences in the structure of human and mouse genes and emphasized the importance of the genetic background of different mouse strains. In this chapter, we provide an overview of the current knowledge and new trends in the modifications of purinoceptor genes in vivo. We discuss the resulting phenotypes, their applications and relative merits and limitations of mouse models available to study purinoceptor subtypes.

Mechanisms and therapeutic targets of ischemic acute kidney injury

Acute kidney injury (AKI) due to renal ischemia reperfusion (IR) is a major clinical problem without effective therapy and is a significant and frequent cause of morbidity and mortality during the perioperative period. Although the pathophysiology of ischemic AKI is not completely understood, several important mechanisms of renal IR-induced AKI have been studied. Renal ischemia and subsequent reperfusion injury initiates signaling cascades mediating renal cell necrosis, apoptosis, and inflammation, leading to AKI. Better understanding of the molecular and cellular pathophysiological mechanisms underlying ischemic AKI will provide more targeted approach to prevent and treat renal IR injury. In this review, we summarize important mechanisms of ischemic AKI, including renal cell death pathways and the contribution of endothelial cells, epithelial cells, and leukocytes to the inflammatory response during ischemic AKI. Additionally, we provide some updated potential therapeutic targets for the prevention or treatment of ischemic AKI, including Toll-like receptors, adenosine receptors, and peptidylarginine deiminase 4. Finally, we propose mechanisms of ischemic AKI-induced liver, intestine, and kidney dysfunction and systemic inflammation mainly mediated by Paneth cell degranulation as a potential explanation for the high mortality observed with AKI.

Free-Energy Calculations for Bioisosteric Modifications of A3 Adenosine Receptor Antagonists

Adenosine receptors are a family of G protein-coupled receptors with increased attention as drug targets on different indications. We investigate the thermodynamics of ligand binding to the A₃ adenosine receptor subtype, focusing on a recently reported series of diarylacetamidopyridine inhibitors via molecular dynamics simulations. With a combined approach of thermodynamic integration and one-step perturbation, we characterize the impact of the charge distribution in a central heteroaromatic ring on the binding affinity prediction. Standard charge distributions according to the GROMOS force field yield values in good agreement with the experimental data and previous free energy calculations. Subsequently, we examine the thermodynamics of inhibitor binding in terms of the energetic and entropic contributions. The highest entropy penalties are found for inhibitors with methoxy substituents in meta position of the aryl groups. This bulky group restricts rotation of aromatic rings attached to the pyrimidine core which leads to two distinct poses of the ligand. Our predictions support the previously proposed binding pose for the o-methoxy ligand, yielding in this case a very good correlation with the experimentally measured affinities with deviations below 4 kJ/mol.

Amitriptyline inhibits the MAPK/ERK and CREB pathways and proinflammatory cytokines through A3AR activation in rat neuropathic pain models

Background

The pain-relief properties of tricyclic antidepressants can be attributed to several actions. Recent observations suggest that adenosine is involved in the antinociceptive effect of amitriptyline. The A3 adenosine receptor (A3AR) is the only adenosine subtype overexpressed in inflammatory and cancer cells. This study was performed to investigate the role of A3AR in the anti-nociceptive effect of amitriptyline.

Methods

Spinal nerve-ligated neuropathic pain was induced by ligating the L5 and L6 spinal nerves of male Sprague-Dawley rats. The neuropathic rats were randomly assigned to one of the following three groups (8 per group): a neuropathic pain with normal saline group, a neuropathic pain with amitriptyline group, and a neuropathic pain with amitriptyline and 3-ethyl-5-benzyl- 2-methyl-4-phenylethynyl-6-phenyl-1,4-(±)-dihydropyridine-3,5-dicarboxylate (MRS) group. Amitriptyline or saline was administered intraperitoneally and 3-ethyl-5-benzyl-2-methyl-4-phenylethynyl-6-phenyl-1,4-(±)-dihydropyridine-3,5-dicarboxylate (MRS-1191), an A3AR antagonist, was injected subcutaneously immediately before amitriptyline administration. The level of extracellular signal-regulated kinase P44/42 (ERK1/2), cyclic AMP response element-binding protein (CREB), and proinflammatory cytokines were assessed using immunoblotting or reverse-transciption polymerase chain reaction.

Results

Amitriptyline increased the mechanical withdrawal threshold of the neuropathic rats. The level of phospho-ERK1/2 and phospho-CREB proteins, and proinflammatory cytokines produced by spinal nerve ligation were significantly reduced by amitriptyline administration. However, the use of MRS-1191 before amitriptyline administration not only reduced the threshold of mechanical allodynia, but also increased the signaling protein and proinflammatory cytokine levels, which were reduced by amitriptyline.

Conclusions

The results of this study suggest that the anti-nociceptive effect of amitriptyline involves the suppression of ERK1/2 and CREB signaling proteins, and A3AR activation also affects the alleviation of the inflammatory response.

Orally active, species-independent novel A3 adenosine receptor antagonist protects against kidney injury in db/db mice

Diabetic kidney disease (DKD) is the leading cause of end-stage kidney disease, and the current pharmacological treatment for DKD is limited to renin-angiotensin system (RAS) inhibitors. Adenosine is detectable in the kidney and is significantly elevated in response to cellular damage. While all 4 known subtypes of adenosine receptors, namely, A₁AR, A_2aAR, A_2bAR, and A₃AR, are expressed in the kidney, our previous study has demonstrated that a novel, orally active, species-independent, and selective A₃AR antagonist, LJ-1888, ameliorates unilateral ureteral obstruction-induced tubulointerstitial fibrosis. The present study examined the protective effects of LJ-2698, which has higher affinity and selectivity for A₃AR than LJ-1888, on DKD. In experiment I, dose-dependent effects of LJ-2698 were examined by orally administering 1.5, 5, or 10 mg/kg for 12 weeks to 8-week-old db/db mice. In experiment II, the effects of LJ-2698 (10 mg/kg) were compared to those of losartan (1.5 mg/kg), which is a standard treatment for patients with DKD. LJ-2698 effectively prevented kidney injuries such as albuminuria, glomerular hypertrophy, tubular injury, podocyte injury, fibrosis, inflammation, and oxidative stress in diabetic mice as much as losartan. In addition, inhibition of lipid accumulation along with increases in PGC1α, a master regulator of mitochondrial biogenesis, were demonstrated in diabetic mice treated with either LJ-2698 or losartan. These results suggest that LJ-2698, a selective A₃AR antagonist, may become a novel therapeutic agent against DKD.

A therapeutic treatment targeting a protein involved in the progression of diabetic kidney disease (DKD) shows promise in mouse trials. Between 30 and 40 per cent of diabetic patients suffer from DKD, a common cause to fatal end-stage kidney disease. Protein receptors, commonly expressed on cell surfaces throughout the body, play both positive and negative roles in diseases. The A₃ adenosine receptor (A₃AR) is highly expressed in diabetic kidney tissue, and is linked to disease progression. Hunjoo Ha at Ewha Womans University in Seoul, Republic of Korea, and co-workers demonstrated the positive effect of a novel drug in targeting A₃AR in mice with DKD. A 12-week treatment of the drug prevented kidney injury, lowered oxidative stress and inflammation, and improved kidney function. It may prove an invaluable drug, particularly in combination with an existing DKD drug.

Involvement of adenosine A3 receptors in the chemotactic navigation of macrophages towards apoptotic cells

The first step in the clearance of apoptotic cells is chemotactic migration of macrophages towards the apoptotic cells guided by find-me signals provided by the dying cells. Upon sensing the chemotactic signals, macrophages release ATP. ATP is then degraded to ADP, AMP and adenosine to trigger purinergic receptors concentrated at the leading edge of the cell. Previous studies have shown that in addition to the chemotactic signals, this purinergic autocrine signaling is required to amplify and translate chemotactic signals into directional motility. In the present study the involvement of adenosine A₃ receptors (A3R) was studied in the chemotactic migration of macrophages directed by apoptotic thymocyte-derived find-me signals. By taking video images in vitro, we demonstrate 1, by administering apyrase, which degrades ATP and ADP, that the purinergic autocrine signaling is required for maintaining both the velocity and the directionality of macrophage migration towards the apoptotic thymocytes; 2, by readding 5'-N-ethylcarboxamidoadenosine, an adenosine analogue, to apyrase treated cells that the adenosine receptor signaling alone is sufficient to act so; and 3, by studying migration of various adenosine receptor null or adenosine receptor antagonist-treated macrophages, that the individual loss of the A3R signaling leads to the loss of chemotactic navigation. Though loss of A3Rs does not affect the phagocytotic capacity of macrophages, intraperitoneally-injected apoptotic thymocytes were cleared with a delayed kinetics by A3R null macrophages in vivo due to the impaired chemotactic navigation. All together these data demonstrate the involvement of macrophage A3Rs in the proper chemotactic navigation and consequent in vivo clearance of apoptotic cells. Interestingly, loss of A3Rs did not affect the in vivo clearance of apoptotic thymocytes in the dexamethasone-treated thymus.

Adenosine contribution to normal renal physiology and chronic kidney disease

Adenosine is a nucleoside that is particularly interesting to many scientific and clinical communities as it has important physiological and pathophysiological roles in the kidney. The distribution of adenosine receptors has only recently been elucidated; therefore it is likely that more biological roles of this nucleoside will be unveiled in the near future. Since the discovery of the involvement of adenosine in renal vasoconstriction and regulation of local renin production, further evidence has shown that adenosine signaling is also involved in the tubuloglomerular feedback mechanism, sodium reabsorption and the adaptive response to acute insults, such as ischemia. However, the most interesting finding was the increased adenosine levels in chronic kidney diseases such as diabetic nephropathy and also in non-diabetic animal models of renal fibrosis. When adenosine is chronically increased its signaling via the adenosine receptors may change, switching to a state that induces renal damage and produces phenotypic changes in resident cells. This review discusses the physiological and pathophysiological roles of adenosine and pays special attention to the mechanisms associated with switching homeostatic nucleoside levels to increased adenosine production in kidneys affected by CKD.

Renoprotective Effects of a Highly Selective A3 Adenosine Receptor Antagonist in a Mouse Model of Adriamycin-induced Nephropathy

The concentration of adenosine in the normal kidney increases markedly during renal hypoxia, ischemia, and inflammation. A recent study reported that an A3 adenosine receptor (A3AR) antagonist attenuated the progression of renal fibrosis. The adriamycin (ADX)-induced nephropathy model induces podocyte injury, which results in severe proteinuria and progressive glomerulosclerosis. In this study, we investigated the preventive effect of a highly selective A3AR antagonist (LJ1888) in ADX-induced nephropathy. Three groups of six-week-old Balb/c mice were treated with ADX (11 mg/kg) for four weeks and LJ1888 (10 mg/kg) for two weeks as following: 1) control; 2) ADX; and 3) ADX + LJ1888. ADX treatment decreased body weight without a change in water and food intake, but this was ameliorated by LJ1888 treatment. Interestingly, LJ1888 lowered plasma creatinine level, proteinuria, and albuminuria, which had increased during ADX treatment. Furthermore, LJ1888 inhibited urinary nephrin excretion as a podocyte injury marker, and urine 8-isoprostane and kidney lipid peroxide concentration, which are markers of oxidative stress, increased after injection of ADX. ADX also induced the activation of proinflammatory and profibrotic molecules such as TGF-β1, MCP-1, PAI-1, type IV collagen, NF-κB, NOX4, TLR4, TNFα, IL-1β, and IFN-γ, but they were remarkably suppressed after LJ1888 treatment. In conclusion, our results suggest that LJ1888 has a renoprotective effect in ADX-induced nephropathy, which might be associated with podocyte injury through oxidative stress. Therefore, LJ1888, a selective A3AR antagonist, could be considered as a potential therapeutic agent in renal glomerular diseases which include podocyte injury and proteinuria.

Genetic Abrogation of Adenosine A3 Receptor Prevents Uninephrectomy and High Salt-Induced Hypertension

Background

Early‐life reduction in nephron number (uninephrectomy [UNX]) and chronic high salt (HS) intake increase the risk of hypertension and chronic kidney disease. Adenosine signaling via its different receptors has been implicated in modulating renal, cardiovascular, and metabolic functions as well as inflammatory processes; however, the specific role of the A₃ receptor in cardiovascular diseases is not clear. In this study, gene‐modified mice were used to investigate the hypothesis that lack of A₃ signaling prevents the development of hypertension and attenuates renal and cardiovascular injuries following UNX in combination with HS (UNX‐HS) in mice.

Methods and Results

Wild‐type (A₃^+/+) mice subjected to UNX‐HS developed hypertension compared with controls (mean arterial pressure 106±3 versus 82±3 mm Hg; P<0.05) and displayed an impaired metabolic phenotype (eg, increased adiposity, reduced glucose tolerance, hyperinsulinemia). These changes were associated with both cardiac hypertrophy and fibrosis together with renal injuries and proteinuria. All of these pathological hallmarks were significantly attenuated in the A₃^−/− mice. Mechanistically, absence of A₃ receptors protected from UNX‐HS–associated increase in renal NADPH oxidase activity and Nox2 expression. In addition, circulating cytokines including interleukins 1β, 6, 12, and 10 were increased in A₃^+/+ following UNX‐HS, but these cytokines were already elevated in naïve A₃^−/− mice and did not change following UNX‐HS.

Conclusions

Reduction in nephron number combined with chronic HS intake is associated with oxidative stress, chronic inflammation, and development of hypertension in mice. Absence of adenosine A₃ receptor signaling was strongly protective in this novel mouse model of renal and cardiovascular disease.

Adenosine produced from adenine nucleotides through an interaction between apoptotic cells and engulfing macrophages contributes to the appearance of transglutaminase 2 in dying thymocytes

Transglutaminase 2 (TG2) has been known for a long time to be associated with the in vivo apoptosis program of various cell types, including T cells. Though the expression of the enzyme is strongly induced in mouse thymocytes following apoptosis induction in vivo, no significant induction of TG2 can be detected, when thymocytes are induced to die by the same stimuli in vitro indicating that signals arriving from the tissue environment are required for the proper in vivo induction of the enzyme. Previous studies from our laboratory have demonstrated that two of these signals, transforming growth factor-β (TGF-β) and retinoids, are produced by macrophages engulfing apoptotic cells. However, in addition to TGF-β and retinoids, engulfing macrophages produce adenosine as well. Here, we show that in vitro adenosine, adenosine, and retinoic acid or adenosine, TGF-β and retinoic acids together can significantly enhance the TG2 mRNA expression in dying thymocytes. The effect of adenosine is mediated via adenosine A2A receptors (A2ARs) and the A2AR-triggered adenylate cyclase signaling pathway. In accordance, loss of A2ARs in A2AR null mice significantly attenuates the in vivo induction of TG2 following apoptosis induction in the thymus indicating that adenosine indeed contributes in vivo to the apoptosis-related appearance of the enzyme. We also demonstrate that adenosine is produced extracellularly during engulfment of apoptotic thymocytes, partly from adenine nucleotides released via thymocyte pannexin-1 channels. Our data reveal a novel crosstalk between macrophages and apoptotic cells, in which apoptotic cell uptake-related adenosine production contributes to the appearance of TG2 in the dying thymocytes.

Adenosine receptors and renal ischaemia reperfusion injury

One of the frequent clinical complications that results in billions of dollars in healthcare costs annually in the United States is acute kidney injury (AKI). Ischaemia reperfusion (IR) injury is a major cause AKI. Unfortunately, no effective treatment or preventive measure for AKI exists. With increased surgical complexity coupled with increasing number of elderly, the incidence of AKI is becoming more frequent. Adenosine is a metabolic breakdown product of adenosine triphosphate (ATP) and contributes to the regulation of multiple physiological events. Extracellular adenosine activates four subtypes of adenosine receptors (AR) including A1 AR, A2 A AR, A2 B AR and A3 AR. In the kidney, adenosine regulates glomerular filtration rate, vascular tone, renin release and is an integrative part of tubular glomerular feedback signal to the afferent arterioles. In addition, each AR subtype powerfully modulates renal IR injury. The A1 AR activation protects against ischaemic insult by reducing apoptosis, necrosis and inflammation. Activation of A2 A AR protects against renal injury by modulating leucocyte-mediated inflammation as well as directly reducing renal tubular inflammation. Activation of A2 B AR acts via direct activation of renal parenchymal as well as renovascular receptors and is important in kidney preconditioning. Finally, activation of A3 AR exacerbates renal damage following renal IR injury while A3 AR antagonism attenuates renal damage following ischaemic insult. Latest body of research suggests that kidney AR modulation may be a promising approach to treat ischaemic AKI. This brief review focuses on the signalling pathways of adenosine in the kidney followed by the role for various AR modulations in protecting against ischaemic AKI.

Adenosine, lidocaine and Mg2+ improves cardiac and pulmonary function, induces reversible hypotension and exerts anti-inflammatory effects in an endotoxemic porcine model

Introduction

The combination of Adenosine (A), lidocaine (L) and Mg²⁺ (M) (ALM) has demonstrated cardioprotective and resuscitative properties in models of cardiac arrest and hemorrhagic shock. This study evaluates whether ALM also demonstrates organ protective properties in an endotoxemic porcine model.

Methods

Pigs (37 to 42 kg) were randomized into: 1) Control (n = 8) or 2) ALM (n = 8) followed by lipopolysaccharide infusion (1 μg∙kg^-1∙h^-1) for five hours. ALM treatment consisted of 1) a high dose bolus (A (0.82 mg/kg), L (1.76 mg/kg), M (0.92 mg/kg)), 2) one hour continuous infusion (A (300 μg∙kg^-1 ∙min^-1), L (600 μg∙kg^-1 ∙min^-1), M (336 μg∙kg^-1 ∙min^-1)) and three hours at a lower dose (A (240∙kg^-1∙min^-1), L (480 μg∙kg^-1∙min^-1), M (268 μg∙kg^-1 ∙min^-1)); controls received normal saline. Hemodynamic, cardiac, pulmonary, metabolic and renal functions were evaluated.

Results

ALM lowered mean arterial pressure (Mean value during infusion period: ALM: 47 (95% confidence interval (CI): 44 to 50) mmHg versus control: 79 (95% CI: 75 to 85) mmHg, P <0.0001). After cessation of ALM, mean arterial pressure immediately increased (end of study: ALM: 88 (95% CI: 81 to 96) mmHg versus control: 86 (95% CI: 79 to 94) mmHg, P = 0.72). Whole body oxygen consumption was significantly reduced during ALM infusion (ALM: 205 (95% CI: 192 to 217) ml oxygen/min versus control: 231 (95% CI: 219 to 243) ml oxygen/min, P = 0.016). ALM treatment reduced pulmonary injury evaluated by PaO₂/FiO₂ ratio (ALM: 388 (95% CI: 349 to 427) versus control: 260 (95% CI: 221 to 299), P = 0.0005). ALM infusion led to an increase in heart rate while preserving preload recruitable stroke work. Creatinine clearance was significantly lower during ALM infusion but reversed after cessation of infusion. ALM reduced tumor necrosis factor-α peak levels (ALM 7121 (95% CI: 5069 to 10004) pg/ml versus control 11596 (95% CI: 9083 to 14805) pg/ml, P = 0.02).

Conclusion

ALM infusion induces a reversible hypotensive and hypometabolic state, attenuates tumor necrosis factor-α levels and improves cardiac and pulmonary function, and led to a transient drop in renal function that was reversed after the treatment was stopped.

Adenosine A3 receptors negatively regulate the engulfment-dependent apoptotic cell suppression of inflammation

Timed initiation of apoptotic cell death followed by efficient removal mediated by professional macrophages is a key mechanism in maintaining tissue homeostasis. Besides phagocytosis, clearance of apoptotic cells also involves suppression of inflammatory responses by apoptotic cells mediated by both direct inhibition of pro-inflammatory cytokine production and release of soluble anti-inflammatory factors, which act in a paracrine or autocrine fashion to amplify or sustain the anti-inflammatory response. Previous work has demonstrated that during engulfment of apoptotic cells adenosine is produced in sufficient amounts to trigger both adenosine A2A receptors (A2ARs) and A3 receptors (A3Rs). Adenosine bound to A2ARs of macrophages activated the adenylate cyclase pathway to suppress the apoptotic-cell induced, NO-dependent formation of neutrophil migration factors. Here we show by using A3R null engulfing macrophages that the adenosine produced triggers the A3Rs as well, which attenuate the A2AR signaling by inhibiting adenylate cyclase. As a result, the balance in the activation of A2ARs and A3Rs determines the amounts of NO and consequently the levels of neutrophil chemoattractants formed. Since during phagocytosis of apoptotic cells the expression of A2ARs increases, while that of A3Rs decreases, on long term adenosine suppresses the proinflammatory responses in engulfing macrophages.

The conundrum of protection from AKI by adenosine in rodent clamp ischemia models

Kim et al. show that isoflurane uses a tubule-based transforming growth factor-β/CD73-dependent process that generates adenosine to protect mice from ischemic acute kidney injury (AKI) with effects to prevent the 'no-reflow phenomenon' and decrease inflammation. While direct cytoprotection occurred in culture, extensive research suggests that in vivo adenosine protection from rodent ischemic AKI is mediated by a mutually cooperative mechanism involving blood flow, inflammation, and innate immunity through multiple adenosine receptors with promiscuous actions on diverse cell types.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

Synthesis and anti-renal fibrosis activity of conformationally locked truncated 2-hexynyl-N(6)-substituted-(N)-methanocarba-nucleosides as A3 adenosine receptor antagonists and partial agonists

Truncated N⁶-substituted-(N)-methanocarba-adenosine derivatives with 2-hexynyl substitution were synthesized to examine parallels with corresponding 4′-thioadenosines. Hydrophobic N⁶ and/or C2 substituents were tolerated in A₃AR binding, but only an unsubstituted 6-amino group with a C2-hexynyl group promoted high hA_2AAR affinity. A small hydrophobic alkyl (4b and 4c) or N⁶-cycloalkyl group (4d) showed excellent binding affinity at the hA₃AR and was better than an unsubstituted free amino group (4a). A₃AR affinities of 3-halobenzylamine derivatives 4f–4i did not differ significantly, with K_i values of 7.8–16.0 nM. N⁶-Methyl derivative 4b (K_i = 4.9 nM) was a highly selective, low efficacy partial A₃AR agonist. All compounds were screened for renoprotective effects in human TGF-β1-stimulated mProx tubular cells, a kidney fibrosis model. Most compounds strongly inhibited TGF-β1-induced collagen I upregulation, and their A₃AR binding affinities were proportional to antifibrotic effects; 4b was most potent (IC₅₀ = 0.83 μM), indicating its potential as a good therapeutic candidate for treating renal fibrosis.

The selective A3AR antagonist LJ-1888 ameliorates UUO-induced tubulointerstitial fibrosis

Adenosine in the normal kidney significantly elevates in response to cellular damage. The renal A3 adenosine receptor (A3AR) is up-regulated under stress, but the therapeutic effects of A3AR antagonists on chronic kidney disease are not fully understood. The present study examined the effect of LJ-1888 [(2R,3R,4S)-2-[2-chloro-6-(3-iodobenzylamino)-9H-purine-9-yl]-tetrahydrothiophene-3,4-diol], a newly developed potent, selective, species-independent, and orally active A3AR antagonist, on unilateral ureteral obstruction (UUO)-induced renal fibrosis. Pretreatment with LJ-1888 inhibited UUO-induced fibronectin and collagen I up-regulation in a dose-dependent manner. Masson's trichrome staining confirmed that LJ-1888 treatment effectively reduced UUO-induced interstitial collagen accumulation. Furthermore, delayed administration of LJ-1888 showed an equivalent therapeutic effect on tubulointerstitial fibrosis to that of losartan. Small-interfering A3AR transfection effectively inhibited transforming growth factor-β1 (TGF-β1)-induced fibronectin and collagen I up-regulation in proximal tubular cells similar to LJ-1888, confirming that the renoprotective effect of LJ-1888 resulted from A3AR blockade. UUO- or TGF-β1-induced c-Jun N-terminal kinase and extracellular signal-regulated kinase phosphorylation decreased significantly after LJ-1888 administration. A3AR blockade reduced UUO- or TGF-β1-induced up-regulation of lysyl oxidase, which induces cross-linking of extracellular matrix, suggesting that LJ-1888 may also regulate extracellular matrix accumulation via post-translational regulation. In conclusion, the present data demonstrate that the A3AR antagonist, LJ-1888, blocked the development and attenuated the progression of renal fibrosis, and they suggest that LJ-1888 may become a new therapeutic modality for renal interstitial fibrosis.

Adenosine A3 receptor activation attenuates lung ischemia-reperfusion injury

BACKGROUND

Severe ischemia-reperfusion (IR) injury leads to primary graft dysfunction after lung transplantation. Adenosine receptors modulate inflammation after IR, and the adenosine A3 receptor (A3R) is expressed in lung tissue and inflammatory cells. This study tests the hypothesis that A3R agonism attenuates lung IR injury by a neutrophil-dependent mechanism.

METHODS

Wild-type and A3R knockout (A3R-/-) mice underwent 1-hour left lung ischemia followed by 2-hours reperfusion (IR). A selective A3R agonist, Cl-IB-MECA, was administered (100 μg/kg intravenously) 5 minutes prior to ischemia. Study groups included sham, IR, and IR+Cl-IB-MECA (n = 6/group). Lung injury was assessed by measuring lung function, pulmonary edema, histopathology, and proinflammatory cytokines, and myeloperoxidase levels in bronchoalveolar lavage fluid. Parallel in vitro experiments were performed to evaluate neutrophil chemotaxis, and neutrophil activation was measured after exposure to acute hypoxia and reoxygenation.

RESULTS

Treatment of wild-type mice with Cl-IB-MECA significantly improved lung function and decreased edema, cytokine expression, and neutrophil infiltration after IR. The Cl-IB-MECA had no effects in A3R-/- mice; Cl-IB-MECA significantly decreased activation of wild-type, but not A3R-/-, neutrophils after acute hypoxia and reoxygenation and inhibited chemotaxis of wild-type neutrophils.

CONCLUSIONS

Exogenous activation of A3R by Cl-IB-MECA attenuates lung dysfunction, inflammation, and neutrophil infiltration after IR in wild-type but not A3R-/- mice. Results with isolated neutrophils suggest that the protective effects of Cl-IB-MECA are due, in part, to the prevention of neutrophil activation and chemotaxis. The use of A3R agonists may be a novel therapeutic strategy to prevent lung IR injury and primary graft dysfunction after transplantation.

The CD39-adenosinergic axis in the pathogenesis of renal ischemia-reperfusion injury

Hypoxic injury occurs when the blood supply to an organ is interrupted; subsequent reperfusion halts ongoing ischemic damage but paradoxically leads to further inflammation. Together this is termed ischemia-reperfusion injury (IRI). IRI is inherent to organ transplantation and impacts both the short- and long-term outcomes of the transplanted organ. Activation of the purinergic signalling pathway is intrinsic to the pathogenesis of, and endogenous response to IRI. Therapies targeting the purinergic pathway in IRI are an attractive avenue for the improvement of transplant outcomes and the basis of ongoing research. This review aims to examine the role of adenosine receptor signalling and the ecto-nucleotidases, CD39 and CD73, in IRI, with a particular focus on renal IRI.

Adora2b adenosine receptor signaling protects during acute kidney injury via inhibition of neutrophil-dependent TNF-α release

Renal ischemia is among the leading causes of acute kidney injury (AKI). Previous studies have shown that extracellular adenosine is a prominent tissue-protective cue elicited during ischemia, including signaling events through the adenosine receptor 2b (Adora2b). To investigate the functional role of Adora2b signaling in cytokine-mediated inflammatory pathways, we screened wild-type and Adora2b-deficient mice undergoing renal ischemia for expression of a range of inflammatory cytokines. These studies demonstrated a selective and robust increase of TNF-α levels in Adora2b-deficient mice following renal ischemia and reperfusion. Based on these findings, we next sought to understand the contribution of TNF-α on ischemic AKI through a combination of loss- and gain-of-function studies. Loss of TNF-α, through either Ab blockade or study of Tnf-α-deficient animals, resulted in significantly attenuated tissue injury and improved kidney function following renal ischemia. Conversely, transgenic mice with overexpression of TNF-α had significantly pronounced susceptibility to AKI. Furthermore, neutrophil depletion or reconstitution of Adora2b(-/-) mice with Tnf-α-deficient neutrophils rescued their phenotype. In total, these data demonstrate a critical role of adenosine signaling in constraining neutrophil-dependent production of TNF-α and implicate therapies targeting TNF-α in the treatment of ischemic AKI.

A1 adenosine receptor allosteric enhancer PD-81723 protects against renal ischemia-reperfusion injury

Activation of A(1) adenosine receptors (ARs) protects against renal ischemia-reperfusion (I/R) injury by reducing necrosis, apoptosis, and inflammation. However, extrarenal side effects (bradycardia, hypotension, and sedation) may limit A(1)AR agonist therapy for ischemic acute kidney injury. Here, we hypothesized that an allosteric enhancer for A(1)AR (PD-81723) protects against renal I/R injury without the undesirable side effects of systemic A(1)AR activation by potentiating the cytoprotective effects of renal adenosine generated locally by ischemia. Pretreatment with PD-81723 produced dose-dependent protection against renal I/R injury in A(1)AR wild-type mice but not in A(1)AR-deficient mice. Significant reductions in renal tubular necrosis, neutrophil infiltration, and inflammation as well as tubular apoptosis were observed in A(1)AR wild-type mice treated with PD-81723. Furthermore, PD-81723 decreased apoptotic cell death in human proximal tubule (HK-2) cells in culture, which was attenuated by a specific A(1)AR antagonist (8-cyclopentyl-1,3-dipropylxanthine). Mechanistically, PD-81723 induced sphingosine kinase (SK)1 mRNA and protein expression in HK-2 cells and in the mouse kidney. Supporting a critical role of SK1 in A(1)AR allosteric enhancer-mediated renal protection against renal I/R injury, PD-81723 failed to protect SK1-deficient mice against renal I/R injury. Finally, proximal tubule sphingosine-1-phosphate type 1 receptors (S1P(1)Rs) are critical for PD-81723-induced renal protection, as mice selectively deficient in renal proximal tubule S1P(1)Rs (S1P(1)R(flox/flox) PEPCK(Cre/-) mice) were not protected against renal I/R injury with PD-81723 treatment. Taken together, our experiments demonstrate potent renal protection with PD-81723 against I/R injury by reducing necrosis, inflammation, and apoptosis through the induction of renal tubular SK1 and activation of proximal tubule S1P(1)Rs. Our findings imply that selectively enhancing A(1)AR activation by locally produced renal adenosine may be a clinically useful therapeutic option to attenuate ischemic acute kidney injury without systemic side effects.

Adenosine and protection from acute kidney injury

PURPOSE OF REVIEW

Acute kidney injury (AKI) is a major clinical problem without effective therapy. Development of AKI among hospitalized patients drastically increases mortality and morbidity. With increases in complex surgical procedures together with a growing elderly population, the incidence of AKI is rising. Renal adenosine receptor manipulation may have great therapeutic potential in mitigating AKI. In this review, we discuss renal adenosine receptor biology and potential clinical therapies for AKI.

RECENT FINDINGS

The four adenosine receptor subtypes (A(1)AR, A(2A)AR, A(2B)AR, and A(3)AR) have diverse effects on the kidney. The pathophysiology of AKI may dictate the specific adenosine receptor subtype activation needed to produce renal protection. The A(1)AR activation in renal tubules and endothelial cells produces beneficial effects against ischemia and reperfusion injury by modulating metabolic demand, decreasing necrosis, apoptosis, and inflammation. The A(2A)AR protects against AKI by modulating leukocyte-mediated renal and systemic inflammation, whereas the A(2B)AR activation protects by direct activation of renal parenchymal adenosine receptors. In contrast, the A(1)AR antagonism may play a protective role in nephrotoxic AKI and radiocontrast induced nephropathy by reversing vascular constriction and inducing naturesis and diuresis. Furthermore, as the A(3)AR activation exacerbates apoptosis and tissue damage due to renal ischemia and reperfusion, selective A(3)AR antagonism may hold promise to attenuate renal ischemia and reperfusion injury. Finally, renal A(1)AR activation also protects against renal endothelial dysfunction caused by hepatic ischemia and reperfusion injury.

SUMMARY

Despite the current lack of therapies for the treatment and prevention of AKI, recent research suggests that modulation of renal adenosine receptors holds promise in treating AKI and extrarenal injury.

Role of the adenosine(2A) receptor-epoxyeicosatrienoic acid pathway in the development of salt-sensitive hypertension

Activation of rat adenosine(2A) receptors (A(2A) R) dilates preglomerular microvessels, an effect mediated by epoxyeicosatrienoic acids (EETs). High salt (HS) intake increases epoxygenase activity and adenosine levels. A greater vasodilator response to a stable adenosine analog, 2-chloroadenosine (2-CA), was seen in kidneys obtained from HS-fed rats which was mediated by increased EET release. Because this pathway is antipressor, we examined the role of the A(2A) R-EET pathway in a genetic model of salt-sensitive hypertension, the Dahl salt-sensitive (SS) rats. Dahl salt resistant (SR) rats fed a HS diet demonstrated a greater renal vasodilator response to 2-CA. In contrast, Dahl SS rats did not exhibit a difference in the vasodilator response to 2-CA whether fed normal salt (NS) or HS diet. In Dahl SR but not Dahl SS rats, HS intake significantly increased purine flux, augmented the protein expression of A(2A) R and cytochrome P450 2C23 and 2C11 epoxygenases, and elevated the renal efflux of EETs. Thus the Dahl SR rat is able to respond to HS intake by recruiting EET formation, whereas the Dahl SS rat appears to have exhausted its ability to increase EET synthesis above the levels observed on NS intake. In vivo inhibition of the A(2A) R-EET pathway in Dahl SR rats fed a HS diet results in reduced renal EETs levels, diminished natriuretic capacity and hypertension, thus supporting a role for the A(2A) R-EET pathway in the adaptive natriuretic response to modulate blood pressure during salt loading. An inability of Dahl SS rats to upregulate the A(2A) R-EET pathway in response to salt loading may contribute to the development of salt-sensitive hypertension.

The A3 adenosine receptor as multifaceted therapeutic target: pharmacology, medicinal chemistry, and in silico approaches

Adenosine is an ubiquitous local modulator that regulates various physiological and pathological functions by stimulating four membrane receptors, namely A(1), A(2A), A(2B), and A(3). Among these G protein-coupled receptors, the A(3) subtype is found mainly in the lung, liver, heart, eyes, and brain in our body. It has been associated with cerebroprotection and cardioprotection, as well as modulation of cellular growth upon its selective activation. On the other hand, its inhibition by selective antagonists has been reported to be potentially useful in the treatment of pathological conditions including glaucoma, inflammatory diseases, and cancer. In this review, we focused on the pharmacology and the therapeutic implications of the human (h)A(3) adenosine receptor (AR), together with an overview on the progress of hA(3) AR agonists, antagonists, allosteric modulators, and radioligands, as well as on the recent advances pertaining to the computational approaches (e.g., quantitative structure-activity relationships, homology modeling, molecular docking, and molecular dynamics simulations) applied to the modeling of hA(3) AR and drug design. 

Enhanced neurodegeneration after a high dose of methamphetamine in adenosine A3 receptor null mutant mice

Previous reports have indicated that adenosine A3 receptor (A3R) knockout mice are more sensitive to ischemic or hypoxic brain injury. The purpose of this study was to examine if suppression of A3R expression is associated with increase in sensitivity to injury induced by a high dose of methamphetamine (Meth). Adult male A3R null mutant (-/-) mice and their controls (+/+) were injected with four doses (2 h apart) of Meth (10 mg/kg) or saline. Animals were placed in a behavioral activity chamber, equipped with food and water, for 52 h starting from one day after injections. The first 4 h were used for studying exploratory behaviors, and the next 48 h were used to measure locomotor activity. High doses of Meth equally reduced the 4-h exploratory behavior in -/- and +/+ mice. Meth suppressed locomotor activity between 4 and 52 h in both groups, with a greater reduction being found in the -/- mice. Brain tissues were collected at 3 days after the Meth or saline injections. Meth treatment reduced striatal dopamine (DA) levels in both +/+ and -/- mice with an increase in 3,4-dihydroxyphenylacetic acid (DOPAC)/DA ratio being found only in -/- animals. Meth also significantly increased ionized calcium-binding adaptor molecule 1 (Iba-1) and cleaved caspase-3 level in striatum, as well as Iba-1 and TNFα mRNA expression in nigra in -/-, compared to +/+, mice. Previous studies have shown that pharmacological suppression of vesicular monoamine transport 2 (VMAT2) by reserpine enhanced Meth toxicity by increasing cytosolic DA and inflammation. A significant reduction in striatal VMAT2 expression was found in -/- mice compared to +/+ mice, suggesting that increase in sensitivity to Meth injury in -/- mice may be related to a reduction in VMAT2 expression in these mice. In conclusion, our data suggest that A3R -/- mice are more sensitive to high doses of Meth.

Investigational A₃ adenosine receptor targeting agents

INTRODUCTION

Adenosine is an endogenous nucleoside that accumulates in the extracellular space in response to metabolic stress and cell damage. Extracellular adenosine is a signaling molecule that signals by activating four GPCRs: the A(1), A(2A), A(2B) and A(3) receptors. Since the discovery of A(3) adenosine receptors, accumulating evidence has identified these receptors as potential targets for therapeutic intervention.

AREAS COVERED

A(3) adenosine receptors are expressed on the surface of most immune cell types, including neutrophils, macrophages, dendritic cells, lymphocytes and mast cells. A(3) adenosine receptor activation on immune cells governs a broad array of immune cell functions, which include cytokine production, degranulation, chemotaxis, cytotoxicity, apoptosis and proliferation. In accordance with their multitudinous immunoregulatory actions, targeting A(3) adenosine receptors has been shown to impact the course of a wide spectrum of immune-related diseases, such as asthma, rheumatoid arthritis, cancer, ischemia and inflammatory disorders.

EXPERT OPINION

Given the existence of both preclinical and early clinical data supporting the utility of A(3) adenosine receptor ligands in treating immune-related diseases, further development of A(3) adenosine receptor ligands is anticipated.

The identification of the 2-phenylphthalazin-1(2H)-one scaffold as a new decorable core skeleton for the design of potent and selective human A3 adenosine receptor antagonists

Following a molecular simplification approach, we have identified the 2-phenylphthalazin-1(2H)-one (PHTZ) ring system as a new decorable core skeleton for the design of novel hA(3) adenosine receptor (AR) antagonists. Interest for this new series was driven by the structural similarity between the PHTZ skeleton and both the 2-aryl-1,2,4-triazolo[4,3-a]quinoxalin-1-one (TQX) and the 4-carboxamido-quinazoline (QZ) scaffolds extensively investigated in our previously reported studies. Our attention was focused at position 4 of the phthalazine nucleus where different amido and ureido moieties were introduced (compounds 2-20). Some of the new PHTZ compounds showed high hA(3) AR affinity and selectivity, the 2,5-dimethoxyphenylphthalazin-1(2H)-one 18 being the most potent and selective hA(3) AR antagonist among this series (K(i) = 0.776 nM; hA(1)/hA(3) and hA(2A)/hA(3) > 12000). Molecular docking studies on the PHTZ derivatives revealed for these compounds a binding mode similar to that of the previously reported TQX and QZ series, as was expected from the simplification approach.

Targeting of adenosine receptors in ischemia-reperfusion injury

IMPORTANCE OF THE FIELD

Ischemia-reperfusion (IR) injury is a common problem after transplantation as well as myocardial infarction and stroke. IR initiates an inflammatory response leading to rapid tissue damage. Adenosine, produced in response to IR, is generally considered a protective signaling molecule and elicits its physiological responses through four distinct adenosine receptors. The short half-life, lack of specificity and rapid metabolism limits the use of adenosine as a therapeutic agent. Thus, intense research efforts have focused on the synthesis and implementation of specific adenosine receptor agonists and antagonists as potential therapeutic agents for a variety of inflammatory conditions including IR injury.

AREAS COVERED IN THIS REVIEW

Current knowledge on IR injury with a focus on lung, heart and kidney and studies that have advanced our understanding of the role of adenosine receptors and the therapeutic potential of adenosine receptor agonists and antagonists for the prevention of IR injury.

WHAT THE READER WILL GAIN

Insight into the role of adenosine receptor signaling in IR injury.

TAKE HOME MESSAGE

No therapies are currently available that specifically target IR injury; however, targeting of specific adenosine receptors may offer therapeutic strategies in this regard.

Novel aspects of pharmacological therapies for acute renal failure

Acute renal failure (ARF) comprises several syndromes that are associated with a sudden decrease in renal function. ARF is common among critically ill patients, is typically multifactorial and is of great prognostic significance. Indeed, even moderate changes in renal function significantly add to the morbidity and worsen mortality associated with ARF. Recent definitions, namely the renal Risk, Injury, Failure, Loss of renal function and End-stage kidney disease (RIFLE) classification or Acute Kidney Injury Network (AKIN) criteria, which incorporate the levels of oliguria in addition to fractional serum creatinine elevation, are important because the magnitude of kidney injury according to those definitions correlates very well with both short- and long-term patient survival. However, preventive strategies are most effective when started before oliguria or elevated serum creatinine is detectable, as those criteria already reflect established renal tubular cell injury. New biomarkers, including neutrophil gelatinase-associated lipocalin (NGAL), liver-type fatty acid binding protein (L-FABP) or kidney injury molecule-1 (KIM-1) that increase prior to the serum creatinine elevation are promising and have been proven to be useful in this regard in a few clinical trials. In addition, genetic profiling may define patients at risk earlier and help to individualize preventive strategies. Well established strategies include limiting dehydration and hypotension by the use of intravenous isotonic fluids at an optimal and individualized rate, as well as avoiding exposure to nephrotoxins, which include aminoglycosides, amphotericin or non-ionic contrast. Generally accepted and evidence-based pharmacological preventive or therapeutic options have not yet been established, although many drugs (e.g. renal vasodilators, diuretics and HMG-CoA reductase inhibitors [statins]) have been tested. New promising agents interfere with the apoptotic signalling that can occur in the setting of toxin exposure or ischaemia-reperfusion injury, limit inflammatory responses or modulate endothelial cell activation. In the future, these new approaches will enable us to extend our therapeutic repertoire.

Adenosine receptor A3 is a critical mediator in LPS-induced pulmonary inflammation

Adenosine receptor A(3) (A(3)) regulates directed movement of polymorphonuclear cells (PMNs) to sites of inflammation and has been implicated as a relevant mediator in models of inflammatory diseases. Here, we sought to characterize the role of A(3) in a murine model of lung inflammation. Initial studies revealed that pulmonary A(3) transcript levels were elevated following LPS exposure in vivo. In addition, inhalation of LPS increased the accumulation of PMNs in wild-type and A(3)(-/-) mice in all lung compartments. Pretreatment with the specific A(3)-agonist Cl-IB-MECA significantly decreased migration of PMNs into lung interstitium and alveolar air space of wild-type mice but not of A(3)(-/-) mice. Lower PMN counts were associated with reduced levels of TNF-α and IL-6 in the alveolar space of wild-type mice that received Cl-IB-MECA. In addition, Cl-IB-MECA attenuated LPS-induced microvascular permeability in wild-type mice as assessed by the extravasation of Evans blue. In pulmonary microvascular endothelial cells, Cl-IB-MECA reduced LPS-induced cytoskeletal remodeling and cell retraction, consistent with a specific role of A(3) for maintaining endothelial integrity. Migratory activity of human PMNs across an endothelial or epithelial monolayer was reduced when A(3) was activated on PMNs. Studies in chimeric mice, however, revealed that Cl-IB-MECA required A(3) on both hematopoietic and nonhematopoietic cells to reduce transmigration in vivo. Together, our results shed new light on the role of A(3) in LPS-induced PMN trafficking in the lung and suggest pharmacological modulation of A(3)-dependent pathways as a promising approach in lung inflammation.

Adenosine A(3) receptors regulate heart rate, motor activity and body temperature

AIM

To examine the phenotype of mice that lack the adenosine A(3) receptor (A(3)R).

METHODS

We examined the heart rate, body temperature and locomotion continuously by telemetry over several days. In addition, the effect of the adenosine analogue R-N(6)-phenylisopropyl-adenosine (R-PIA) was examined. We also examined heat production and food intake.

RESULTS

We found that the marked diurnal variation in activity, heart rate and body temperature, with markedly higher values at night than during day time, was reduced in the A(3)R knock-out mice. Surprisingly, the reduction in heart rate, activity and body temperature seen after injection of R-PIA in wild type mice was virtually eliminated in the A(3)R knock-out mice. The marked reduction in activity was associated with a decreased heat production, as expected. However, the A(3)R knock-out mice, surprisingly, had a higher food intake but no difference in body weight compared to wild type mice.

CONCLUSIONS

The mice lacking adenosine A(3) receptors exhibit a surprisingly clear phenotype with changes in diurnal rhythm and temperature regulation. Whether these effects are due to a physiological role of A(3) receptors in these processes or whether they represent a role in development remains to be elucidated.

Cytoprotective effects of adenosine and inosine in an in vitro model of acute tubular necrosis

BACKGROUND AND PURPOSE

We have established an in vitro model of acute tubular necrosis in rat kidney tubular cells, using combined oxygen-glucose deprivation (COGD) and screened a library of 1280 pharmacologically active compounds for cytoprotective effects.

EXPERIMENTAL APPROACH

We used in vitro cell-based, high throughput, screening, with cells subjected to COGD using hypoxia chambers, followed by re-oxygenation. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and the Alamar Blue assay measured mitochondrial respiration and the lactate dehydrogenase assay was used to indicate cell death. ATP levels were measured using a luminometric assay.

KEY RESULTS

Adenosine markedly reduced cellular injury, with maximal cytoprotective effect at 100 microM and an EC(50) value of 14 microM. Inosine was also found to be cytoprotective. The selective A(3) adenosine receptor antagonist MRS 1523 attenuated the protective effects of adenosine and inosine, while an A(3) adenosine receptor agonist provided a partial protective effect. Adenosine deaminase inhibition attenuated the cytoprotective effect of adenosine but not of inosine during COGD. Inhibition of adenosine kinase reduced the protective effects of both adenosine and inosine during COGD. Pretreatment of the cells with adenosine or inosine markedly protected against the fall in cellular ATP content in the cells subjected to COGD.

CONCLUSIONS AND IMPLICATIONS

The cytoprotection elicited by adenosine and inosine in a model of renal ischaemia involved both interactions with cell surface adenosine receptors on renal tubular epithelial cells and intracellular metabolism and conversion of adenosine to ATP.

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.

Pyrido[2,3-e]-1,2,4-triazolo[4,3-a]pyrazin-1-one as a new scaffold to develop potent and selective human A3 adenosine receptor antagonists. Synthesis, pharmacological evaluation, and ligand-receptor modeling studies

The paper describes a new class of human (h) A(3) adenosine receptor antagonists, the 2-arylpyrido[2,3-e]-1,2,4-triazolo[4,3-a]pyrazin-1-one derivatives (PTP), either 4-oxo (1-6, series A) or 4-amino-substituted (7-20, series B). In both series A and B, substituents able to act as hydrogen bond acceptors (OMe, OH, F, COOEt) were inserted on the 2-phenyl ring. In series B, cycloalkyl and acyl residues were introduced on the 4-amino group. Some of the new derivatives showed high hA(3) AR affinities (K(i) < 50 nM) and selectivities vs both hA(1) and hA(2A) receptors. The selected 4-benzoylamino-2-(4-methoxyphenyl)pyrido[2,3-e]-1,2,4-triazolo[4,3-a]pyrazin-1-one (18), tested in an in vitro rat model of cerebral ischemia, proved to be effective in preventing the failure of synaptic activity induced by oxygen and glucose deprivation in the hippocampus. Molecular docking of this new class of hA(3) AR antagonists was carried out to depict their hypothetical binding mode to our refined model of hA(3) receptor.

Adenosine receptors and the kidney

The autacoid, adenosine, is present in the normoxic kidney and generated in the cytosol as well as at extracellular sites. The rate of adenosine formation is enhanced when the rate of ATP hydrolysis prevails over the rate of ATP synthesis during increased tubular transport work or during oxygen deficiency. Extracellular adenosine acts on adenosine receptor subtypes (A(1), A(2A), A(2B), and A(3)) in the cell membranes to affect vascular and tubular functions. Adenosine lowers glomerular filtration rate by constricting afferent arterioles, especially in superficial nephrons, and thus lowers the salt load and transport work of the kidney consistent with the concept of metabolic control of organ function. In contrast, it leads to vasodilation in the deep cortex and the semihypoxic medulla, and exerts differential effects on NaCl transport along the tubular and collecting duct system. These vascular and tubular effects point to a prominent role of adenosine and its receptors in the intrarenal metabolic regulation of kidney function, and, together with its role in inflammatory processes, form the basis for potential therapeutic approaches in radiocontrast media-induced acute renal failure, ischemia reperfusion injury, and in patients with cardiorenal failure.

Adenosine and renal tubular function

PURPOSE OF REVIEW

Intrarenal adenosine is present in the cytoplasm of renal epithelial cells and in the extracellular space. Adenosine is generated at high levels in response to imbalance between energy demand and supply (e.g. increased tubular sodium chloride transport or hypoxia) and activates cell membrane adenosine receptors to affect renal vascular and tubular functions. Adenosine regulates renal sodium and water excretion via a myriad of effects on renal hemodynamic, glomerular filtration rate, renin secretion and direct effects on the renal tubule epithelium. This review examines the direct effects of adenosine on renal tubular epithelial transport in light of the most recent evidence and discusses some physiologic and pathophysiologic implications.

RECENT FINDINGS

Intrarenal adenosine affects proximal fluid and solute transport in a biphasic fashion. Under physiological conditions adenosine stimulates proximal tubular re-absorption, thus reducing the load delivered to the distal nephron. A supra-physiologic increase in adenosine such as in ischemia reduces reabsorption in the proximal tubule, thus reducing renal oxygen consumption.

SUMMARY

Intrarenal adenosine and its receptors have important regulatory functions in the renal epithelium. A complete understanding of this autocrine/paracrine system holds great potential for novel therapeutic strategies, such as the use of nucleoside analogues for reno-protection in renal ischemia.

A3 and P2Y2 receptors control the recruitment of neutrophils to the lungs in a mouse model of sepsis

We have recently shown that A3 adenosine receptors and P2Y2 purinergic receptors play an important role in neutrophil chemotaxis. Chemotaxis of neutrophils to sites of infections is critical for immune defense. However, excessive accumulation of neutrophils in the lungs can cause acute lung tissue damage. Here we assessed the role of A3 and P2Y2 receptors in neutrophil sequestration to the lungs in a mouse model of sepsis. Sepsis was induced by cecal ligation and puncture (CLP) using adult male C57BL/6J mice (wild type [WT]), homozygous A3 receptor knockout (A3KO) mice, and P2Y2 receptor knockout (P2Y2KO) mice. Animals were killed 2, 4, 6, or 8 h after CLP, and peritoneal lavage fluid and blood were collected. Lungs were removed, and neutrophil infiltration was evaluated using elastase as a marker. Leukocyte and bacterial counts in peritoneal lavage fluid and blood samples were determined. Survival after sepsis was determined in a separate group. Leukocyte counts in the peritoneum were lower in A3KO and P2Y2KO mice than in WT mice. Conversely, initial leukocyte counts in the peripheral blood were higher in KO mice than in WT mice. Neutrophil sequestration to the lungs reached a maximum 2 h after CLP and remained significantly higher in WT mice compared with A3KO and P2Y2KO mice (P < 0.001). Survival after 24 h was significantly lower in WT mice (37.5%) than in A3KO or P2Y2KO mice (82.5%; P < 0.05). These data suggest that A3 and P2Y2 receptors are involved in the influx of neutrophils into the lungs after sepsis. Thus, pharmaceutical approaches that target these receptors might be useful to control acute lung tissue injury in sepsis.

Short- and long-term A3 adenosine receptor activation inhibits the Na+/H+ exchanger NHE3 activity and expression in opossum kidney cells

The renal function of the A(3) adenosine receptor (A3AR) is poorly characterized. In this study, we report that the A3AR-selective agonist, 1-[2-chloro-6-[[(3-iodophenyl)methyl]amino]-9H-purine-9-yl]-1-deoxy-N-methyl-b-D-ribofuranuronamide (2-Cl-IBMECA) regulates the Na+/H+ exchanger-3 (NHE3) in a dose- and time-dependent fashion. In opossum kidney (OK) cells, 2-Cl-IBMECA at high (10(-6) M) and low (10(-8) M) dose inhibits NHE3 by a multiphasic time course with an acute phase of NHE3 inhibition from 15 min to 1 h, followed by a chronic phase of NHE3 inhibition from 24 to 48 h. Pre-incubation with either the selective A3AR-antagonist MRS1523 (10(-7) M) or the protein kinase C inhibitor, Calphostin C (10(-8) M) completely blocked 10(-6) M 2-Cl-IBMECA-induced acute (15 min) and chronic (24 h) phases of NHE3 inhibition. In contrast, the acute inhibitory phase (15 min) of 10(-8) M 2-Cl-IBMECA was completely prevented only when Calphostin C (10(-8) M) was added in conjunction with the protein kinase A inhibitor, H89 (10(-7) M). Acute (15 or 30 min depending on the A3AR-agonist concentration) A3AR-dependent inhibition of NHE3 activity was accompanied by decrease in cell surface NHE3 protein with no change in total NHE3 antigen. Chronic (24 h) A3AR-mediated down-regulation of NHE3 was associated with reduction of surface NHE3, decreased total NHE3 protein (70%) and a paradoxical rise of NHE3 RNA (40%). In summary, these results indicate that A3AR directly regulates NHE3 at multiple levels in a complex pattern. A3AR-dependent short- and long-term inhibition of NHE3 may be a fundamental mechanism of net sodium and fluid balance.