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

A2B adenosine receptor signaling and regulation

The A2B adenosine receptor (A2BR) is one of the four adenosine-activated G protein-coupled receptors. In addition to adenosine, protein kinase C (PKC) was recently found to activate the A2BR. The A2BR is coupled to both Gs and Gi, as well as Gq proteins in some cell types. Many primary cells and cell lines, such as bladder and breast cancer, bronchial smooth muscle, skeletal muscle, and fat cells, express the A2BR endogenously at high levels, suggesting its potentially important role in asthma, cancer, diabetes, and other conditions. The A2BR has been characterized as both pro- and anti-inflammatory, inducing cell type-dependent secretion of IL-6, IL-8, and IL-10. Theophylline and enprofylline have long been used for asthma treatment, although it is still not entirely clear if their A2BR antagonism contributes to their therapeutic effects or side effects. The A2BR is required in ischemic cardiac preconditioning by adenosine. Both A2BR and protein kinase C (PKC) contribute to cardioprotection, and both modes of A2BR signaling can be blocked by A2BR antagonists. Inhibitors of PKC and A2BR are in clinical cancer trials. Sulforaphane and other isothiocyanates from cruciferous vegetables such as broccoli and cauliflower have been reported to inhibit A2BR signaling via reaction with an intracellular A2BR cysteine residue (C210). A full, A2BR-selective agonist, critical to elucidate many controversial roles of the A2BR, is still not available, although agonist-bound A2BR structures have recently been reported.

Neutrophil depletion attenuates acute renal injury after exhaustive exercise in mice

Prolonged intense exercise induces acute renal injury; however, the precise mechanism remains unclear. We investigated the effects of neutrophil depletion in male C57BL/6J mice. Male C57BL/6J mice were divided into four groups: sedentary with control antibody; sedentary with antineutrophil antibody; exhaustive exercise with control antibody; and exhaustive exercise with antineutrophil antibody. Antineutrophil (1A8) or control antibody was administered i.p. to the mice before they ran on a treadmill. Plasma levels of creatinine and blood urea nitrogen (BUN) were measured. Renal histology was assessed 24 h after exhaustive exercise, and the concentration of kidney injury molecule (KIM)-1 was measured using an enzyme-linked immunosorbent assay. The expression levels of inflammatory cytokines were measured using qRT-PCR. Furthermore, NADPH oxidase activity and the hydrogen peroxide concentration in the kidney were measured. Immediately after exhaustive exercise, plasma BUN was significantly increased, but creatinine was not. The increase in BUN after exercise was suppressed by 1A8 treatment. The pathological changes manifested as congested and swollen glomeruli and nuclear infiltration after exhaustive exercise. These changes were suppressed by treatment with the 1A8 antibodies. The KIM-1 concentration increased after exhaustive exercise but was reduced by the 1A8 antibodies. Treatment with the 1A8 antibody also decreased exhaustive exercise-induced inflammation and reactive oxygen species levels in the kidney. These results suggest that neutrophils contribute to exercise-induced acute renal injury by regulating inflammation and oxidative stress.

Parthenolide Attenuates Sepsis-Induced Acute Kidney Injury in Rats by Reducing Inflammation

Background

Sepsis is a common complication of severe trauma, burns, infection, or major surgery. This disease-related end-organ dysfunction results from systemic inflammatory response syndrome (SIRS). Acute kidney damage (AKI), also known as acute renal failure, is one of the most frequent and serious sequelae of sepsis. Nuclear transcription factor-κB (NF-κB) regulates the transcription of inflammation-related genes and operates as a mediator in the immune system. While parthenolide (PTL) has been reported to prevent harmful inflammatory reactions, its effects on sepsis-associated AKI are unknown. The current study investigates the effects of PTL in sepsis-associated AKI using cell and cecal ligation and puncture (CLP) models.

Methods

Lipopolysaccharide (LPS)-stimulated rat glomerular mesangial cells were treated with 10 μM PTL. Inflammatory mediators, including TNF-α, IL-6, and IL-1β, in the culture supernatants were measured by ELISA, and NF-κB levels were assessed by qPCR. After the generation of the septic CLP model, rats were intraperitoneally injected with 500 g/kg PTL and were euthanized after 72 h. Serum and kidney samples were analyzed.

Results

TNF-α, IL-1β, and IL-6 levels were elevated after LPS treatment of rat glomerular mesangial cells (p=0.004, p=0.002, and p=0.004, respectively) but were significantly reduced in the PTL treatment group (p ≤ 0.001, p=0.01, and p ≤ 0.001). NF-κB p65 levels were also increased after LPS treatment in this group and were reduced in the PTL treatment group. PTL treatment also reduced kidney damage after CLP induction, as shown by histological analysis and reductions in the levels of BUN, Cre, KIM-1, and NAGL. CLP-induced kidney inflammation together with increased levels of proinflammatory cytokines and inflammatory-related proteins. The elevated levels of renal TNF-α, IL-6, and IL-1β were downregulated after PTL treatment. The PTL treatment also reduced the CLP-induced activation of NF-κB p65 in the damaged kidneys.

Conclusion

PTL reduced inflammation induced by CLP-induced AKI in rat models and LPS-induced damage to glomerular mesangial cells by suppressing NF-κB signaling.

Endless Journey of Adenosine Signaling in Cardioprotective Mechanism of Conditioning Techniques: Clinical Evidence

Myocardial ischemic injury is a primary cause of death among various cardiovascular disorders. The condition occurs due to an interrupted supply of blood and vital nutrients (necessary for normal cellular activities and viability) to the myocardium, eventually leading to damage. Restoration of blood supply to ischemic tissue is noted to cause even more lethal reperfusion injury. Various strategies, including some conditioning techniques, like preconditioning and postconditioning, have been developed to check the detrimental effects of reperfusion injury. Many endogenous substances have been proposed to act as initiators, mediators, and end effectors of these conditioning techniques. Substances, like adenosine, bradykinin, acetylcholine, angiotensin, norepinephrine, opioids, etc., have been reported to mediate cardioprotective activity. Among these agents, adenosine has been widely studied and suggested to have the most pronounced cardioprotective effects. The current review article highlights the role of adenosine signaling in the cardioprotective mechanism of conditioning techniques. The article also provides an insight into various clinical studies that substantiate the applicability of adenosine as a cardioprotective agent in myocardial reperfusion injury.

Macrophage depletion attenuates acute renal damage after exhaustive exercise in mice

Exhaustive exercise is known to induce acute renal damage. However, the precise mechanisms remain unclear. We investigated the effects of macrophage depletion on exhaustive exercise-induced acute renal damage. Male C57BL/6 J mice were divided into four groups: sedentary with control liposome (n=8), sedentary with clodronate liposome (n=8), exhaustive exercise with control liposome (n=8), and exhaustive exercise with clodronate liposome (n=8). Mice were treated with clodronate liposomes or control liposomes intraperitoneally for 48 h before undergoing exhaustive exercise. Renal function and renal histology were tested at 24 h. The expression levels of kidney injury molecule (KIM)-1 and inflammatory cytokines in kidney tissues were measured by quantitative RT-PCR, and KIM-1 concentration was semi-quantified by immunostaining. As a result, exhaustive exercise increased macrophage infiltration into the kidney. However, clodronate reduced it. Although exhaustive exercise resulted in an increase in KIM-1 mRNA expression levels and concentration, injection of clodronate liposome reduced it. In addition, TUNEL positive apoptotic cells were increased after exercise, but significantly reduced by clodronate. Clodronate liposome treatment also decreased the mRNA expression levels of inflammatory cytokines (TNF-α, IL-1β, and IL-6) in the kidney after exhaustive exercise. These results suggest that macrophages play a critical role in increasing renal damage by regulating inflammation.

Enhancing Extracellular Adenosine Levels Restores Barrier Function in Acute Lung Injury Through Expression of Focal Adhesion Proteins

Background: Acute respiratory distress syndrome (ARDS) is a clinical presentation of acute lung injury (ALI) with often fatal lung complication. Adenosine, a nucleoside generated following cellular stress provides protective effects in acute injury. The levels of extracellular adenosine can be depleted by equilibrative nucleoside transporters (ENTs). ENT inhibition by pharmaceutical agent dipyridamole promotes extracellular adenosine accumulation and is protective in ARDS. However, the therapeutic potential of dipyridamole in acute lung injury has not yet been evaluated. Methods: Adenosine acts on three adenosine receptors, the adenosine A1 (Adora1), A2a (Adora2a), the A2b (Adora2b) or the adenosine A3 (Adora 3) receptor. Accumulation of adenosine is usually required to stimulate the low-affinity Adora2b receptor. In order to investigate the effect of adenosine accumulation and the contribution of epithelial-specific ENT2 or adora2b expression in experimental ALI, dipyridamole, and epithelial specific ENT2 or Adora2b deficient mice were utilized. MLE12 cells were used to probe downstream Adora2b signaling. Adenosine receptors, transporters, and targets were determined in ARDS lungs. Results: ENT2 is mainly expressed in alveolar epithelial cells and is negatively regulated by hypoxia following tissue injury. Enhancing adenosine levels with ENT1/ENT2 inhibitor dipyridamole at a time when bleomycin-induced ALI was present, reduced further injury. Mice pretreated with the ADORA2B agonist BAY 60-6583 were protected from bleomycin-induced ALI by reducing vascular leakage (558.6 ± 50.4 vs. 379.9 ± 70.4, p < 0.05), total bronchoalveolar lavage fluid cell numbers (17.9 ± 1.8 to 13.4 ± 1.4 e4, p < 0.05), and neutrophil infiltration (6.42 ± 0.25 vs. 3.94 ± 0.29, p < 0.05). While mice lacking Adora2b in AECs were no longer protected by dipyridamole. We also identified occludin and focal adhesion kinase as downstream targets of ADORA2B, thus providing a novel mechanism for adenosine-mediated barrier protection. Similarly, we also observed similar enhanced ADORA2B (3.33 ± 0.67 to 16.12 ± 5.89, p < 0.05) and decreased occludin (81.2 ± 0.3 to 13.3 ± 0.4, p < 0.05) levels in human Acute respiratory distress syndrome lungs. Conclusion: We have highlighted a role of dipyridamole and adenosine signaling in preventing or treating ALI and identified Ent2 and Adora2b as key mediators in important for the resolution of ALI.

Oral delivery of folate-targeted resveratrol-loaded nanoparticles for inflammatory bowel disease therapy in rats

AIMS

In the current study, resveratrol-loaded PLGA nanoparticles targeted with folate were developed in order to protect resveratrol from fast degradation, modify its pharmacokinetics and increase its intestinal permeation. Then, the therapeutic efficacy of the prepared system was evaluated in suppression of colon inflammation on TNBS-induced colitis model.

MAIN METHODS

In this regard, resveratrol was encapsulated in PLGA and FA-conjugated PLGA in order to prepare non-targeted (PLGA-RSV) and targeted (PLGA-FA-RSV) platforms, respectively.

KEY FINDINGS

Obtained results demonstrated that the prepared formulations encapsulated the resveratrol with high encapsulation efficiency of 90.7% ± 5.1% for PLGA-RSV and 59.1% ± 3.3% for PLGA-FA-RSV. In vitro release experiment showed that the prepared formulations were capable of retaining good amount of resveratrol under the simulated gastric condition (HCl 0.1 N, pH 1.2), while significant amount of resveratrol was released under simulated intestinal condition (PBS, pH 7.4). The trans-well permeability rates through Caco-2 monolayer during 180 min, was determined to be 4.5%, 61% and 99% for resveratrol, PLGA-RSV and PLGA-FA-RSV respectively. The pathological analysis of the rat intestinal sections (hematoxylin & eosin staining) at 7th day post-TNBS colonic inflammation induction illustrated that the oral administrations of FA-PLGA-RSV and PLGA-RSV were able to significantly inhibit the inflammation and reduce neutrophil and lymphocytes accumulation. It is worth noting that the folate-targeted system demonstrated highest efficacy in suppressing colon inflammation.

SIGNIFICANCE

It could be concluded that the encapsulation of resveratrol into biodegradable folate-targeted PLGA nanoparticles could introduce a potent platform in suppressing colonic inflammation thus offering a great capability for clinical translation.

Does ischemic preconditioning increase flap survival by ADORA2B receptor activation?

BACKGROUND

Ischemic preconditioning (IPC) is defined as raising tolerance to subsequent ischemic stress by exposing tissues to sub-lethal ischemia. Although many candidates have been suggested, recent studies have clearly demonstrated that adenosine-mediated ADORA2B receptor (ADORA2BR) activation is the main mechanism involved in IPC. While the tissue-protective role of this mechanism has been demonstrated in different ischemia/reperfusion (I/R) models, its role in flap surgery-derived I/R damage has not to date been investigated.

OBJECTIVE

To investigate the role of adenosine and ADORA2BR activation in IPC-mediated tissue protection in an epigastric flap model.

METHODS

Animals were divided into five main groups, all of which were then divided into two subgroups depending on whether or not they were exposed to IPC before the I/R procedure, which consisted of 6 hours of ischemia and 6 days of reperfusion. No drugs were administered in Group 1 (the control group). Animals in Group 2 were pretreated with CD73-inhibitor before IPC application or the ischemic period. Animals in Group 3 were pretreated with adenosine. Animals in Group 4 were pretreated with an ADORA2BR antagonist, and those in Group 5 with an ADORA2BR agonist. After 6 days of reperfusion, tissue survival was evaluated via histological and macroscopic analysis.

RESULTS

IPC application significantly enhanced CD73 expressions and adenosine concentrations (p < 0.01). Flap survivals were increased by IPC in Group 1 (p < 0.05). However, CD73 inhibition blocked this increase (Group 2). In Group 3, adenosine improved flap survival even in the absence of IPC (p < 0.01). While an ADORA2BR antagonist attenuated the tissue-protective effect of IPC (p < 0.01), the ADORA2BR agonist improved flap survival by mimicking IPC in groups 4 and 5.

CONCLUSION

These results provide pharmacological evidence for a contribution of CD73 enzyme-dependent adenosine generation and signaling through ADORA2BR to IPC-mediated tissue protection. They also suggest for the first time that ADORA2BR agonists may be used as a potential preventive therapy against I/R injury in flap surgeries.

Therapeutic Potentials of A2B Adenosine Receptor Ligands: Current Status and Perspectives

BACKGROUND

Adenosine receptors (ARs) are classified as A1, A2A, A2B, and A3 subtypes belong to the superfamily of G-protein coupled receptors (GPCRs). More than 40% of modern medicines act through either activation or inhibition of signaling processes associated with GPCRs. In particular, A2B AR signaling pathways are implicated in asthma, inflammation, cancer, ischemic hyperfusion, diabetes mellitus, cardiovascular diseases, gastrointestinal disorders, and kidney disease.

METHODS

This article reviews different disease segments wherein A2B AR is implicated and discusses the potential role of subtype-selective A2B AR ligands in the management of such diseases or disorders. All the relevant publications on this topic are reviewed and presented scientifically.

RESULTS

This review provides an up-to-date highlight of the recent advances in the development of novel and selective A2B AR ligands and their therapeutic role in treating various disease conditions. A special focus has been given to the therapeutic potentials of selective A2B AR ligands in the management of airway inflammatory conditions and cancer.

CONCLUSIONS

This systematic review demonstrates the current status and perspectives of A2B AR ligands as therapeutically useful agents that would assist medicinal chemists and pharmacologists in discovering novel and subtype-selective A2B AR ligands as potential drug candidates.

Short-Term Hypoxia Dampens Inflammation in vivo via Enhanced Adenosine Release and Adenosine 2B Receptor Stimulation

Hypoxia and inflammation are closely intertwined phenomena. Critically ill patients often suffer from systemic inflammatory conditions and concurrently experience short-lived hypoxia. We evaluated the effects of short-term hypoxia on systemic inflammation, and show that it potently attenuates pro-inflammatory cytokine responses during murine endotoxemia. These effects are independent of hypoxia-inducible factors (HIFs), but involve augmented adenosine levels, in turn resulting in an adenosine 2B receptor-mediated post-transcriptional increase of interleukin (IL)-10 production. We translated our findings to humans using the experimental endotoxemia model, where short-term hypoxia resulted in enhanced plasma concentrations of adenosine, augmentation of endotoxin-induced circulating IL-10 levels, and concurrent attenuation of the pro-inflammatory cytokine response. Again, HIFs were shown not to be involved. Taken together, we demonstrate that short-term hypoxia dampens the systemic pro-inflammatory cytokine response through enhanced purinergic signaling in mice and men. These effects may contribute to outcome and provide leads for immunomodulatory treatment strategies for critically ill patients.

mROS-TXNIP axis activates NLRP3 inflammasome to mediate renal injury during ischemic AKI

Ischemia/reperfusion (I/R) is a critical risk factor for acute kidney injury (AKI). Recent studies provided evidence that tubular epithelial cells (TEC)-associated inflammation aggravates kidney injury and impairs tissue repair after I/R injury. Here we demonstrated that the Nod-like receptor protein 3 (NLRP3) inflammasome is activated by mitochondrial reactive oxygen species (mROS) during I/R injury via direct interactions between the inflammasome and thioredoxin-interacting protein (TXNIP). Firstly, we found that NLRP3 inflammasome activation was induced by I/R injury, peaking at day 3 after reperfusion. Consistent with this observation, NLRP3 deletion significantly attenuated I/R-induced kidney damage and markers of inflammasome activation. Then, we observed mitochondrial dysfunction, characterized by ultrastructural changes and cytochrome C (Cyt c) redistribution. Mitochondria-targeted antioxidant MitoTEMPO prevented mROS overproduction and the decline in mitochondrial membrane potential (MMP) in vitro. MitoTEMPO treatment also inhibited NLRP3 inflammasome activation and co-localization of NLRP3 and TXNIP after simulated ischemia/reperfusion (SI/R) injury. Finally, we transfected HK-2 cells with TXNIP siRNA to explore the role of TXNIP in mROS-induced NLRP3 inflammasome activation. We found that TXNIP siRNA significantly inhibited NLRP3 inflammasome activation. These results demonstrate that NLRP3 inflammasome is activated through the mROS-TXNIP-NLRP3 pathway and provide a potential therapeutic target in ischemic AKI.

AMP and adenosine are both ligands for adenosine 2B receptor signaling

Adenosine is considered the canonical ligand for the adenosine 2B receptor (A_2BR). A_2BR is upregulated following kidney ischemia augmenting post ischemic blood flow and limiting tubular injury. In this context the beneficial effect of A_2BR signaling has been attributed to an increase in the pericellular concentration of adenosine. However, following renal ischemia both kidney adenosine monophosphate (AMP) and adenosine levels are substantially increased. Using computational modeling and calcium mobilization assays, we investigated whether AMP could also be a ligand for A_2BR. The computational modeling suggested that AMP interacts with more favorable energy to A_2BR compared with adenosine. Furthermore, AMPαS, a non-hydrolyzable form of AMP, increased calcium uptake by Chinese hamster ovary (CHO) cells expressing the human A_2BR, indicating preferential signaling via the G_q pathway. Therefore, a putative AMP-A_2BR interaction is supported by the computational modeling data and the biological results suggest this interaction involves preferential G_q activation. These data provide further insights into the role of purinergic signaling in the pathophysiology of renal IRI.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

Targeting Adenosine Receptors for the Treatment of Cardiac Fibrosis

Adenosine is a ubiquitous molecule with key regulatory and cytoprotective mechanisms at times of metabolic imbalance in the body. Among a plethora of physiological actions, adenosine has an important role in attenuating ischaemia-reperfusion injury and modulating the ensuing fibrosis and tissue remodeling following myocardial damage. Adenosine exerts these actions through interaction with four adenosine G protein-coupled receptors expressed in the heart. The adenosine A_2B receptor (A_2BAR) is the most abundant adenosine receptor (AR) in cardiac fibroblasts and is largely responsible for the influence of adenosine on cardiac fibrosis. In vitro and in vivo studies demonstrate that acute A_2BAR stimulation can decrease fibrosis through the inhibition of fibroblast proliferation and reduction in collagen synthesis. However, in contrast, there is also evidence that chronic A_2BAR antagonism reduces tissue fibrosis. This review explores the opposing pro- and anti-fibrotic activity attributed to the activation of cardiac ARs and investigates the therapeutic potential of targeting ARs for the treatment of cardiac fibrosis.

Protective role of hypoxia-inducible factor-1α-dependent CD39 and CD73 in fulminant acute liver failure

Acute liver failure (ALF) is a severe life-threatening disease which usually arises in patients with-irreversible liver illnesses. Although human ectonucleotide triphosphate diphosphohydrolase-1, E-NTPDase1 (CD39) and ecto-5'-nucleotidase, Ecto5'NTase (CD73) are known to protect tissues from ALF, the expression and function of CD39 and CD73 during ALF are currently not fully investigated. We tested whether CD39 and CD73 are upregulated by hypoxia inducible factor (HIF)-1α, and improve ischemic tolerance to ALF. To test our hypothesis, liver biopsies were obtained and we found that CD39 and CD73 mRNA and proteins from human specimens were dramatically elevated in ALF. We investigated that induction of CD39 and CD73 in ALF-related with wild type mice. In contrast, deletion of cd39 and cd73 mice has severe ALF. In this study, we concluded that CD39 and CD73 are molecular targets for the development of drugs for ALF patients care.

Unlocking the Potential of Purinergic Signaling in Transplantation

Purinergic signaling has been recognized as playing an important role in inflammation, angiogenesis, malignancy, diabetes and neural transmission. Activation of signaling pathways downstream from purinergic receptors may also be implicated in transplantation and related vascular injury. Following transplantation, the proinflammatory "danger signal" adenosine triphosphate (ATP) is released from damaged cells and promotes proliferation and activation of a variety of immune cells. Targeting purinergic signaling pathways may promote immunosuppression and ameliorate inflammation. Under pathophysiological conditions, nucleotide-scavenging ectonucleotidases CD39 and CD73 hydrolyze ATP, ultimately, to the anti-inflammatory mediator adenosine. Adenosine suppresses proinflammatory cytokine production and is associated with improved graft survival and decreased severity of graft-versus-host disease. Furthermore, purinergic signaling is involved both directly and indirectly in the mechanism of action of several existing immunosuppressive drugs, such as calcineurin inhibitors and mammalian target of rapamycin inhibitors. Targeting of purinergic receptor pathways, particularly in the setting of combination therapies, could become a valuable immunosuppressive strategy in transplantation. This review focuses on the role of the purinergic signaling pathway in transplantation and immunosuppression and explores possible future applications in clinical practice.

Adenosine A2B Receptor: From Cell Biology to Human Diseases

Extracellular adenosine is a ubiquitous signaling molecule that modulates a wide array of biological processes. Recently, significant advances have been made in our understanding of A2B adenosine receptor (A2BAR). In this review, we first summarize some of the general characteristics of A2BAR, and then we describe the multiple binding partners of the receptor, such as newly identified α-actinin-1 and p105, and discuss how these associated proteins could modulate A2BAR's functions, including certain seemingly paradoxical functions of the receptor. Growing evidence indicates a critical role of A2BAR in cancer, renal disease, and diabetes, in addition to its importance in the regulation of vascular diseases, and lung disease. Here, we also discuss the role of A2BAR in cancer, renal disease, and diabetes and the potential of the receptor as a target for treating these three diseases.

Purinergic regulation of the immune system

Cellular stress or apoptosis triggers the release of ATP, ADP and other nucleotides into the extracellular space. Extracellular nucleotides function as autocrine and paracrine signalling molecules by activating cell-surface P2 purinergic receptors that elicit pro-inflammatory immune responses. Over time, extracellular nucleotides are metabolized to adenosine, leading to reduced P2 signalling and increased signalling through anti-inflammatory adenosine (P1 purinergic) receptors. Here, we review how local purinergic signalling changes over time during tissue responses to injury or disease, and we discuss the potential of targeting purinergic signalling pathways for the immunotherapeutic treatment of ischaemia, organ transplantation, autoimmunity or cancer.

Neutrophil MiRNA-128-3p is Decreased During Active Phase of Granulo-matosis with Polyangiitis

Granulomatosis with polyangiitis is a rare chronic inflammatory disease. In this multisystem autoimmune disorder neutrophils cause small vessels necrosis and infiltrate perivascular tissue to form granulomas. Progression of the disease is evaluated by the symptoms score and by a titer of anti-neutrophil cytoplasm antibodies. Despite glucocorticoid and immunosuppressive therapy, prognosis is complicated by chronic renal insufficiency, hearing loss and skin ulceration. In this preliminary study we tested the hypothesis that altered neutrophil expression of miRNAs can contribute to the cell activation, extracellular traps formation and decreased apoptosis. First we compared a profile of 728 miRNAs expressed in circulating neutrophils of patients with active disease and matched healthy donors. Subsequently, candidate miRNAs were quantified in neutrophils from 16 subjects with active disease, 16 asymptomatic patients at the remission and in 16 healthy controls. Out of 11 candidate miRNAs, only miR-128-3p was both biologically (relative quantity < 30% control or remission patients) and statistically (p<0.01) decreased in the cells during active stage of the disease. This miRNA correlated with a clinical score of the disease well. A set of 10 transcripts involved in the mechanism of the disease was quantified from the same neutrophils RNA. Relative expression of MMP9 was higher in neutrophils from the patients with active disease and correlated negatively with miR-128-3p. The opposite finding was present for MTA1 transcripts. Despite surprisingly scarce changes in the expression of neutrophil miRNAs, miR-128-3p is the best candidate for deciphering etiology of granulomatosis with polyangiitis.

Glycyrrhizic Acid Attenuates Sepsis-Induced Acute Kidney Injury by Inhibiting NF-κB Signaling Pathway

Glycyrrhizic acid (GA) is a major active ingredient in licorice. In our study, the effects of GA on acute kidney injury (AKI) in rats and its underlying molecular mechanisms were investigated. The sepsis model was produced by caecal ligation and puncture (CLP) in rats. The molecular and histological experiments were performed in the kidney tissues and serum samples of rats. According to the results obtained, GA alleviated sepsis-induced AKI by improving the pathological changes, decreasing the levels of blood urea nitrogen (BUN), creatinine (Cre), and increasing the survival rate of rats with AKI significantly. The production of inflammatory cytokines, such as TNF-α, IL-1β, and IL-6, was markedly inhibited by GA. Moreover, treatment with GA inhibited the production of nitric oxide (NO) and prostaglandin E2 (PGE2) and expression levels of induced nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in kidney tissues. Furtherly, the apoptosis in kidney tissue induced by AKI was suppressed by GA. Finally, GA could inhibit the activation of NF-κB signaling pathway. Our study suggests that GA alleviates sepsis-induced AKI by inhibiting the NF-κB signaling pathway, which provides a strong evidence for a new approach for treating sepsis-induced AKI.

Nephroprotective Effects of Polydatin against Ischemia/Reperfusion Injury: A Role for the PI3K/Akt Signal Pathway

Oxidative stress and inflammation are involved in the pathogenesis in renal ischemia/reperfusion (I/R) injury. It has been demonstrated that polydatin processed the antioxidative, anti-inflammatory, and nephroprotective properties. However, whether it has beneficial effects and the possible mechanisms on renal I/R injury remain unclear. In our present study I/R models were simulated both in vitro and in vivo. Compared with vehicle control, the administration of polydatin significantly improved the renal function, accelerated the mitogenic response and reduced cell apoptosis in renal I/R injury models, strongly suppressed the I/R-induced upregulation of the expression of tumor necrosis factor-α, interleukin-1β, cyclooxygenase-2, inducible nitric oxide synthase, prostaglandin E-2, and nitric oxide levels, and dramatically decreased contents of malondialdehyde, but it increased the activity of superoxide dismutase, glutathione transferase, glutathione peroxidase and catalase, and the level of glutathione. Further investigation showed that polydatin upregulated the phosphorylation of Akt in kidneys of I/R injury dose-dependently. However, all beneficial effects of polydatin mentioned above were counteracted when we inhibited PI3K/Akt pathway with its specific inhibitor, wortmannin. Taken together, the present findings provide the first evidence demonstrating that PD exhibited prominent nephroprotective effects against renal I/R injury by antioxidative stress and inflammation through PI3-K/Akt-dependent molecular mechanisms.

Trauma/hemorrhagic shock instigates aberrant metabolic flux through glycolytic pathways, as revealed by preliminary (13)C-glucose labeling metabolomics

BACKGROUND

Metabolic derangement is a key hallmark of major traumatic injury. The recent introduction of mass spectrometry-based metabolomics technologies in the field of trauma shed new light on metabolic aberrations in plasma that are triggered by trauma and hemorrhagic shock. Alteration in metabolites associated with catabolism, acidosis and hyperglycemia have been identified. However, the mechanisms underlying fluxes driving such metabolic adaptations remain elusive.

METHODS

A bolus of U-(13)C-glucose was injected in Sprague-Dawley rats at different time points. Plasma extracts were analyzed via ultra-high performance liquid chromatography-mass spectrometry to detect quantitative fluctuations in metabolite levels as well as to trace the distribution of heavy labeled carbon isotopologues.

RESULTS

Rats experiencing trauma did not show major plasma metabolic aberrations. However, trauma/hemorrhagic shock triggered severe metabolic derangement, resulting in increased glucose levels, lactate and carboxylic acid accumulation. Isotopologue distributions in late Krebs cycle metabolites (especially succinate) suggested a blockade at complex I and II of the electron transport chain, likely due to mitochondrial uncoupling. Urate increased after trauma and hemorrhage. Increased levels of unlabeled mannitol and citramalate, metabolites of potential bacterial origin, were also observed in trauma/hemorrhagic shock rats, but not trauma alone or controls.

CONCLUSIONS

These preliminary results are consistent with observations we have recently obtained in humans, and expand upon our early results on rodent models of trauma and hemorrhagic shock by providing the kinetics of glucose fluxes after trauma and hemorrhage. Despite the preliminary nature of this study, owing to the limited number of biological replicates, results highlight a role for shock, rather than trauma alone, in eliciting systemic metabolic aberrations. This study provides the foundation for tracing experiments in rat models of trauma. The goal is to improve our understanding of substrate specific metabolic derangements in trauma/hemorrhagic shock, so as to design resuscitative strategies tailored toward metabolic alterations and the severity of trauma.

Blocking neutrophil integrin activation prevents ischemia-reperfusion injury

Neutrophil recruitment, mediated by β2 integrins, combats pyogenic infections but also plays a key role in ischemia-reperfusion injury and other inflammatory disorders. Talin induces allosteric rearrangements in integrins that increase affinity for ligands (activation). Talin also links integrins to actin and other proteins that enable formation of adhesions. Structural studies have identified a talin1 mutant (L325R) that perturbs activation without impairing talin's capacity to link integrins to actin and other proteins. Here, we found that mice engineered to express only talin1(L325R) in myeloid cells were protected from renal ischemia-reperfusion injury. Dissection of neutrophil function in vitro and in vivo revealed that talin1(L325R) neutrophils had markedly impaired chemokine-induced, β2 integrin-mediated arrest, spreading, and migration. Surprisingly, talin1(L325R) neutrophils exhibited normal selectin-induced, β2 integrin-mediated slow rolling, in sharp contrast to the defective slow rolling of neutrophils lacking talin1 or expressing a talin1 mutant (W359A) that blocks talin interaction with integrins. These studies reveal the importance of talin-mediated activation of integrins for renal ischemia-reperfusion injury. They further show that neutrophil arrest requires talin recruitment to and activation of integrins. However, although neutrophil slow rolling requires talin recruitment to integrins, talin-mediated integrin activation is dispensable.

Alveolar Epithelial A2B Adenosine Receptors in Pulmonary Protection during Acute Lung Injury

Acute lung injury (ALI) is an acute inflammatory lung disease that causes morbidity and mortality in critically ill patients. However, there are many instances where ALI resolves spontaneously through endogenous pathways that help to control excessive lung inflammation. Previous studies have implicated the extracellular signaling molecule adenosine and signaling events through the A2B adenosine receptor in lung protection. In this context, we hypothesized that tissue-specific expression of the A2B adenosine receptor is responsible for the previously described attenuation of ALI. To address this hypothesis, we exposed mice with tissue-specific deletion of Adora2b to ALI, utilizing a two-hit model where intratracheal LPS treatment is followed by injurious mechanical ventilation. Interestingly, a head-to-head comparison of mice with deletion of Adora2b in the myeloid lineage (Adora2b(loxP/loxP) LysM Cre(+)), endothelial cells (Adora2b(loxP/loxP) VE-cadherin Cre(+)), or alveolar epithelial cells (Adora2b(loxP/loxP) SPC Cre(+)) revealed a selective increase in disease susceptibility in Adora2b(loxP/loxP) SPC Cre(+) mice. More detailed analysis of Adora2b(loxP/loxP) SPC Cre(+) mice confirmed elevated lung inflammation and attenuated alveolar fluid clearance. To directly deliver an A2B adenosine receptor-specific agonist to alveolar epithelial cells, we subsequently performed studies with inhaled BAY 60-6583. Indeed, aerosolized BAY 60-6583 treatment was associated with attenuated pulmonary edema, improved histologic lung injury, and dampened lung inflammation. Collectively, these findings suggest that alveolar epithelial A2B adenosine receptor signaling contributes to lung protection, and they implicate inhaled A2B adenosine receptor agonists in ALI treatment.

TNF-α-mediated cardiorenal injury after rhabdomyolysis in rats

The TNF-α serum level increases after rhabdomyolysis and is involved in the subsequent cardiorenal injury. In the present study, we investigated the TNF-α-dependent cell signaling pathways implicated in cellular injury in these organs. Rhabdomyolysis was induced by intramuscular glycerol injection in rats. Renal function, cardiac and renal pathology, and activation of caspases were evaluated during the first 24 h after glycerol injection. TNF-α blockade with infliximab reduced tubular necrosis and cardiorenal apoptosis. Cellular Fas-associated protein with death domain-like IL-1β-converting enzyme inhibitory protein (cFLIP), an inhibitor of caspase-8, was overexpressed in the kidney but not in the heart. The inhibitory effect of cFLIP blunted caspase-8 activation in the kidney. In this condition, the cellular response to the TNF-α stimulus was driven to receptor-interacting protein-1 (RIP1)-mediated necroptosis. Treatment with RIP1 inhibitor (necrostatin-1) isolated or in combination with infliximab showed a similar reduction in tubular necrosis, underscoring the importance of TNF-α-mediated tubular necroptosis in this model. TNF-α played a positive regulatory role in the transcription of proapoptotic Bax and p53-upregulated modulator of apoptosis (PUMA) proteins. Infliximab treatment reduced caspase-9-mediated apoptosis in both organs. Treatment with a caspase-8 inhibitor showed that caspase-8 participated in the process of apoptosis only in the heart, upstream of caspase-9 activation. TNF-α-mediated necroptosis is the predominant form of tubular injury observed in the glycerol model. TNF-α up regulates Bax and PUMA proapoptotic proteins, resulting in activation of the intrinsic pathway of apoptosis in the kidney and heart.

Early hemorrhage triggers metabolic responses that build up during prolonged shock

Metabolic staging after trauma/hemorrhagic shock is a key driver of acidosis and directly relates to hypothermia and coagulopathy. Metabolic responses to trauma/hemorrhagic shock have been assayed through classic biochemical approaches or NMR, thereby lacking a comprehensive overview of the dynamic metabolic changes occurring after shock. Sprague-Dawley rats underwent progressive hemorrhage and shock. Baseline and postshock blood was collected, and late hyperfibrinolysis was assessed (LY30 >3%) in all of the tested rats. Extreme and intermediate time points were collected to assay the dynamic changes of the plasma metabolome via ultra-high performance liquid chromatography-mass spectrometry. Sham controls were used to determine whether metabolic changes could be primarily attributable to anesthesia and supine positioning. Early hemorrhage-triggered metabolic changes that built up progressively and became significant during sustained hemorrhagic shock. Metabolic phenotypes either resulted in immediate hypercatabolism, or late hypercatabolism, preceded by metabolic deregulation during early hemorrhage in a subset of rats. Hemorrhagic shock consistently promoted hyperglycemia, glycolysis, Krebs cycle, fatty acid, amino acid, and nitrogen metabolism (urate and polyamines), and impaired redox homeostasis. Early dynamic changes of the plasma metabolome are triggered by hemorrhage in rats. Future studies will determine whether metabolic subphenotypes observed in rats might be consistently observed in humans and pave the way for tailored resuscitative strategies.

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.

Proximal tubule epithelial cell specific ablation of the spermidine/spermine N1-acetyltransferase gene reduces the severity of renal ischemia/reperfusion injury

BACKGROUND

Expression and activity of spermidine/spermine N1-acetyltransferase (SSAT) increases in kidneys subjected to ischemia/reperfusion (I/R) injury, while its ablation reduces the severity of such injuries. These results suggest that increased SSAT levels contribute to organ injury; however, the role of SSAT specifically expressed in proximal tubule epithelial cells, which are the primary targets of I/R injury, in the mediation of renal damage remains unresolved.

METHODS

Severity of I/R injury in wt and renal proximal tubule specific SSAT-ko mice (PT-SSAT-Cko) subjected to bilateral renal I/R injury was assessed using cellular and molecular biological approaches.

RESULTS

Severity of the loss of kidney function and tubular damage are reduced in PT-SSAT-Cko- compared to wt-mice after I/R injury. In addition, animals treated with MDL72527, an inhibitor of polyamine oxidases, had less severe renal damage than their vehicle treated counter-parts. The renal expression of HMGB 1 and Toll like receptors (TLR) 2 and 4 were also reduced in PT-SSAT-Cko- compared to wt mice after I/R injury. Furthermore, infiltration of neutrophils, as well as expression of tumor necrosis factor-α (TNF-α), monocyte chemoattractant protein-1 (MCP-1) and interleukin-6 (IL-6) transcripts were lower in the kidneys of PT-SSAT-Cko compared to wt mice after I/R injury. Finally, the activation of caspase3 was more pronounced in the wt compared to PT-SSAT-Cko animals.

CONCLUSIONS

Enhanced SSAT expression by proximal tubule epithelial cells leads to tubular damage, and its deficiency reduces the severity of renal I/R injury through reduction of cellular damage and modulation of the innate immune response.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

A tissue-specific role for Nlrp3 in tubular epithelial repair after renal ischemia/reperfusion

Ischemia/reperfusion injury is a major cause of acute kidney injury. Improving renal repair would represent a therapeutic strategy to prevent renal dysfunction. The innate immune receptor Nlrp3 is involved in tissue injury, inflammation, and fibrosis; however, its role in repair after ischemia/reperfusion is unknown. We address the role of Nlrp3 in the repair phase of renal ischemia/reperfusion and investigate the relative contribution of leukocyte- versus renal-associated Nlrp3 by studying bone marrow chimeric mice. We found that Nlrp3 expression was most profound during the repair phase. Although Nlrp3 expression was primarily expressed by leukocytes, both leukocyte- and renal-associated Nlrp3 was detrimental to renal function after ischemia/reperfusion. The Nlrp3-dependent cytokine IL-1β remained unchanged in kidneys of all mice. Leukocyte-associated Nlrp3 negatively affected tubular apoptosis in mice that lacked Nlrp3 expression on leukocytes, which correlated with reduced macrophage influx. Nlrp3-deficient (Nlrp3KO) mice with wild-type bone marrow showed an improved repair response, as seen by a profound increase in proliferating tubular epithelium, which coincided with increased hepatocyte growth factor expression. In addition, Nlrp3KO tubular epithelial cells had an increased repair response in vitro, as seen by an increased ability of an epithelial monolayer to restore its structural integrity. In conclusion, Nlrp3 shows a tissue-specific role in which leukocyte-associated Nlrp3 is associated with tubular apoptosis, whereas renal-associated Nlrp3 impaired wound healing.

Netrin-1 guides inflammatory cell migration to control mucosal immune responses during intestinal inflammation

The intestinal epithelium is a dynamic barrier playing an active role in intestinal homeostasis and inflammation. Intestinal barrier function is dysregulated during inflammatory bowel disease (IBD), with epithelial cells playing a significant part in generating an inflammatory milieu through the release of signals that attract leukocytes to the intestinal lamina propria. However, it is increasingly appreciated that the intestinal epithelium mediates a counterbalancing response to drive resolution. Drawing analogies with neuronal development, where the balance of chemoattractive and chemorepellent signals is key to directed neuronal movement it has been postulated that such secreted cues play a role in leukocyte migration. Netrin-1 is one of the best-described neuronal guidance molecules, which has been shown to play a significant role in directed migration of leukocytes. Prior to our study the potential role of netrin-1 in IBD was poorly characterized. We defined netrin-1 as an intestinal epithelial-derived protein capable of limiting neutrophil recruitment to attenuate acute colitis. Our study highlights that the intestinal epithelium releases factors during acute inflammation that are responsible for fine-tuning the immune response. Exploration of these epithelial-mediated protective mechanisms will shed light on the complexity of the intestinal epithelial barrier in health and disease.

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.

CD73+ regulatory T cells contribute to adenosine-mediated resolution of acute lung injury

Acute lung injury (ALI) is characterized by alveolar injury and uncontrolled inflammation. Since most cases of ALI resolve spontaneously, understanding the endogenous mechanisms that promote ALI resolution is important to developing effective therapies. Previous studies have implicated extracellular adenosine signaling in tissue adaptation and wound healing. Therefore, we hypothesized a functional contribution for the endogenous production of adenosine during ALI resolution. As a model, we administered intratracheal LPS and observed peak lung injury at 3 d, with resolution by d 14. Treatment with pegylated adenosine-deaminase to enhance extracellular adenosine breakdown revealed impaired ALI resolution. Similarly, genetic deletion of cd73, the pacemaker for extracellular adenosine generation, was associated with increased mortality (0% wild-type and 40% in cd73(-/-) mice; P<0.05) and failure to resolve ALI adequately. Studies of inflammatory cell trafficking into the lungs during ALI resolution revealed that regulatory T cells (Tregs) express the highest levels of CD73. While Treg numbers in cd73(-/-) mice were similar to controls, cd73-deficient Tregs had attenuated immunosuppressive functions. Moreover, adoptive transfer of cd73-deficient Tregs into Rag(-/-) mice emulated the observed phenotype in cd73(-/-) mice, while transfer of wild-type Tregs was associated with normal ALI resolution. Together, these studies implicate CD73-dependent adenosine generation in Tregs in promoting ALI resolution.

Innate immunity in donor procurement

PURPOSE OF REVIEW

Ischaemic kidney injury occurs during organ procurement and can lead to delayed graft function or nonviable grafts. The innate immune system is a key trigger of inflammation in renal ischaemia. This review discusses the components of innate immunity known to be involved in renal ischaemic reperfusion injury (IRI). Understanding how inflammatory damage is initiated in renal IRI is important for the development of targeted therapies aimed at preserving the donor organ.

RECENT FINDINGS

Much remains to be determined about the role of innate immune signalling in renal ischaemia/reperfusion injury. Recently, discoveries about complement receptors, Toll-like receptors (TLRs), NOD-like receptors (NLRs) and inflammasomes have opened new avenues of exploration. We are also now learning that macrophages, complement and TLR activation may have additional roles in renal repair following IRI.

SUMMARY

A greater understanding of the mechanisms that contribute to innate immune-mediated renal ischaemic damage will allow for the development of therapeutics targeted to the donor organ. New data suggest that treatment limited to specific receptors on specific cells, or localized to specific regions within the kidney, may provide novel approaches to maximize our use of donor organs, particularly those that may have been discarded due to prolonged preimplantation ischaemia.

Attenuating myocardial ischemia by targeting A2B adenosine receptors

Myocardial ischemia is associated with profound tissue hypoxia due to an imbalance in oxygen supply and demand, and studies of hypoxia-elicited adaptive responses during myocardial ischemia revealed a cardioprotective role for the signaling molecule adenosine. In ischemic human hearts, the A2B adenosine receptor (ADORA2B) is selectively induced. Functional studies in genetic models show that ADORA2B signaling attenuates myocardial infarction by adapting metabolism towards more oxygen efficient utilization of carbohydrates. This adenosine-mediated cardio-adaptive response involves the transcription factor hypoxia-inducible factor HIF1α and the circadian rhythm protein PER2. In this article, we discuss advances in the understanding of adenosine-elicited cardioprotection with particular emphasis on ADORA2B, its downstream targets, and the implications for novel strategies to prevent or treat myocardial ischemia.

Transcriptional control of adenosine signaling by hypoxia-inducible transcription factors during ischemic or inflammatory disease

Inflammatory lesions, ischemic tissues, or solid tumors are characterized by the occurrence of severe tissue hypoxia within the diseased tissue. Subsequent stabilization of hypoxia-inducible transcription factors-particularly of hypoxia-inducible factor 1α (HIF1A)--results in significant alterations of gene expression of resident cells or inflammatory cells that have been recruited into such lesions. Interestingly, studies of hypoxia-induced changes of gene expression identified a transcriptional program that promotes extracellular adenosine signaling. Adenosine is a signaling molecule that functions through the activation of four distinct adenosine receptors--the ADORA1, ADORA2A, ADORA2B, and ADORA3 receptors. Extracellular adenosine is predominantly derived from the phosphohydrolysis of precursor nucleotides, such as adenosine triphosphate or adenosine monophosphate. HIF1A-elicited alterations in gene expression enhance the enzymatic capacity within inflamed tissues to produce extracellular adenosine. Moreover, hypoxia-elicited induction of adenosine receptors--particularly of ADORA2B--results in increased signal transduction. Functional studies in genetic models for HIF1A or adenosine receptors implicate this pathway in an endogenous feedback loop that dampens excessive inflammation and promotes injury resolution, while at the same time enhancing ischemia tolerance. Therefore, pharmacological strategies to enhance HIF-elicited adenosine production or to promote adenosine signaling through adenosine receptors are being investigated for the treatment of acute inflammatory or ischemic diseases characterized by tissue hypoxia.