Targeting the A2B adenosine receptor during gastrointestinal ischemia and inflammation

Alternative adenosine Receptor activation: The netrin-Adora2b link

During hypoxia or inflammation, extracellular adenosine levels are elevated. Studies using pharmacologic approaches or genetic animal models pertinent to extracellular adenosine signaling implicate this pathway in attenuating hypoxia-associated inflammation. There are four distinct adenosine receptors. Of these, it is not surprising that the Adora2b adenosine receptor functions as an endogenous feedback loop to control hypoxia-associated inflammation. First, Adora2b activation requires higher adenosine concentrations compared to other adenosine receptors, similar to those achieved during hypoxic inflammation. Second, Adora2b is transcriptionally induced during hypoxia or inflammation by hypoxia-inducible transcription factor HIF1A. Studies seeking an alternative adenosine receptor activation mechanism have linked netrin-1 with Adora2b. Netrin-1 was originally discovered as a neuronal guidance molecule but also functions as an immune-modulatory signaling molecule. Similar to Adora2b, netrin-1 is induced by HIF1A, and has been shown to enhance Adora2b signaling. Studies of acute respiratory distress syndrome (ARDS), intestinal inflammation, myocardial or hepatic ischemia and reperfusion implicate the netrin-Adora2b link in tissue protection. In this review, we will discuss the potential molecular linkage between netrin-1 and Adora2b, and explore studies demonstrating interactions between netrin-1 and Adora2b in attenuating tissue inflammation.

Enteric glial adenosine 2B receptor signaling mediates persistent epithelial barrier dysfunction following acute DSS colitis

Intestinal epithelial barrier function is compromised in inflammatory bowel disease and barrier dysfunction contributes to disease progression. Extracellular nucleotides/nucleosides generated in gut inflammation may regulate barrier function through actions on diverse cell types. Enteric glia modulate extracellular purinergic signaling and exert pathophysiological effects on mucosal permeability. These glia may regulate inflammation with paracrine responses, theoretically mediated via adenosine 2B receptor (A2BR) signaling. As the cell-specific roles of A2BRs in models of colitis and barrier dysfunction are unclear, we studied glial A2BRs in acute dextran sodium sulfate (DSS) colitis. We performed and validated conditional ablation of glial A2BRs in Sox10CreERT2+/-;Adora2bf/f mice. Overt intestinal disease activity indices in DSS-colitis were comparable between Sox10CreERT2+/-;Adora2bf/f mice and littermate controls. However, ablating glial A2BRs protected against barrier dysfunction following acute DSS-colitis. These benefits were associated with the normalization of tight junction protein expression and localization including claudin-1, claudin-8, and occludin. Glial A2BR signaling increased levels of proinflammatory mediators in the colon and cell-intrinsic regulation of genes including Csf3, Cxcl1, Cxcl10, and Il6. Our studies show that glial A2BR signaling exacerbates immune responses during DSS-colitis and that this adenosinergic cell-specific mechanism contributes to persistent gut epithelial barrier dysfunction.

Characterization of a Murine Model System to Study MicroRNA-147 During Inflammatory Organ Injury

Inflammatory organ injury and sepsis have profound impacts on the morbidity and mortality of surgical and critical care patients. MicroRNAs are small RNAs composed of 20-25 nucleotides that have a significant contribution to gene regulation. MicroRNA-147 (miR-147), in particular, has been shown to have an emerging role in different physiological functions such as cell cycle regulation and inflammatory responses. However, animal model systems to study tissue-specific functions of miR-147 during inflammatory conditions in vivo are lacking. In the present study, we characterize miR-147 expression in different organs and cell types. Next, we generated a transgenic mouse line with a floxed miR-147 gene. Subsequently, we used this mouse line to generate mice with whole-body deletion of miR-147 (miR-147 -/-) by crossing "floxed" miR-147 mice with transgenic mice expressing Cre recombinase in all tissues (CMVcre mice). Systematic analysis of miR-147 -/- mice demonstrates normal growth, development, and off-spring. In addition, deletion of the target gene in different organs was successful at baseline or during inflammation, including the heart, intestine, stomach, liver, spleen, bone marrow, lungs, kidneys, or stomach. Moreover, miR-147 -/- mice have identical baseline inflammatory gene expression compared to C57BL/6 mice, except elevated IL-6 expression in the spleen (7.5 fold, p < 0.05). Taken together, our data show the successful development of a transgenic animal model for tissue and cell-specific deletion of miR-147 that can be used to study the functional roles of miR-147 during inflammatory organ injury.

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.

Adenosine at the Interphase of Hypoxia and Inflammation in Lung Injury

Hypoxia and inflammation often coincide in pathogenic conditions such as acute respiratory distress syndrome (ARDS) and chronic lung diseases, which are significant contributors to morbidity and mortality for the general population. For example, the recent global outbreak of Coronavirus disease 2019 (COVID-19) has placed viral infection-induced ARDS under the spotlight. Moreover, chronic lung disease ranks the third leading cause of death in the United States. Hypoxia signaling plays a diverse role in both acute and chronic lung inflammation, which could partially be explained by the divergent function of downstream target pathways such as adenosine signaling. Particularly, hypoxia signaling activates adenosine signaling to inhibit the inflammatory response in ARDS, while in chronic lung diseases, it promotes inflammation and tissue injury. In this review, we discuss the role of adenosine at the interphase of hypoxia and inflammation in ARDS and chronic lung diseases, as well as the current strategy for therapeutic targeting of the adenosine signaling pathway.

Approaches for designing and discovering purinergic drugs for gastrointestinal diseases

INTRODUCTION

Purines finely modulate physiological motor, secretory, and sensory functions in the gastrointestinal tract. Their activity is mediated by the purinergic signaling machinery, including receptors and enzymes regulating their synthesis, release, and degradation. Several gastrointestinal dysfunctions are characterized by alterations affecting the purinergic system.

AREAS COVERED

The authors provide an overview on the purinergic receptor signaling machinery, the molecules and proteins involved, and a summary of medicinal chemistry efforts aimed at developing novel compounds able to modulate the activity of each player involved in this machinery. The involvement of purinergic signaling in gastrointestinal motor, secretory, and sensory functions and dysfunctions, and the potential therapeutic applications of purinergic signaling modulators, are then described.

EXPERT OPINION

A number of preclinical and clinical studies demonstrate that the pharmacological manipulation of purinergic signaling represents a viable way to counteract several gastrointestinal diseases. At present, the paucity of purinergic therapies is related to the lack of receptor-subtype-specific agonists and antagonists that are effective in vivo. In this regard, the development of novel therapeutic strategies should be focused to include tools able to control the P1 and P2 receptor expression as well as modulators of the breakdown or transport of purines.

Contribution of Wound-Associated Cells and Mediators in Orchestrating Gastrointestinal Mucosal Wound Repair

The gastrointestinal mucosa, structurally formed by the epithelium and lamina propria, serves as a selective barrier that separates luminal contents from the underlying tissues. Gastrointestinal mucosal wound repair is orchestrated by a series of spatial and temporal events that involve the epithelium, recruited immune cells, resident stromal cells, and the microbiota present in the wound bed. Upon injury, repair of the gastrointestinal barrier is mediated by collective migration, proliferation, and subsequent differentiation of epithelial cells. Epithelial repair is intimately regulated by a number of wound-associated cells that include immune cells and stromal cells in addition to mediators released by luminal microbiota. The highly regulated interaction of these cell types is perturbed in chronic inflammatory diseases that are associated with impaired wound healing. An improved understanding of prorepair mechanisms in the gastrointestinal mucosa will aid in the development of novel therapeutics that promote mucosal healing and reestablish the critical epithelial barrier function.

The adenosine A2B G protein-coupled receptor: Recent advances and therapeutic implications

The adenosine A_2B receptor (A_2BAR) is one of four adenosine receptor subtypes belonging to the Class A family of G protein-coupled receptors (GPCRs). Until recently, the A_2BAR remained poorly characterised, in part due to its relatively low affinity for the endogenous agonist adenosine and therefore presumed minor physiological significance. However, the substantial increase in extracellular adenosine concentration, the sensitisation of the receptor and the upregulation of A_2BAR expression under conditions of hypoxia and inflammation, suggest the A_2BAR as an exciting therapeutic target in a variety of pathological disease states. Here we discuss the pharmacology of the A_2BAR and outline its role in pathophysiology including ischaemia-reperfusion injury, fibrosis, inflammation and cancer.

A2A adenosine receptors control pancreatic dysfunction in high-fat-diet-induced obesity

Adenosine, a key extracellular signaling mediator, regulates several aspects of metabolism by activating 4 G-protein-coupled receptors, the A₁, A_2A, A_2B, and A₃ adenosine receptors (ARs). The role of A_2AARs in regulating high-fat-diet (HFD)-induced metabolic derangements is unknown. To evaluate the role of A_2AARs in regulating glucose and insulin homeostasis in obesity, we fed A_2AAR-knockout (KO) and control mice an HFD for 16 wk to initiate HFD-induced metabolic disorder. We found that genetic deletion of A_2AARs caused impaired glucose tolerance in mice fed an HFD. This impaired glucose tolerance was caused by a decrease in insulin secretion but not in insulin sensitivity. Islet size and insulin content in pancreata of A_2AAR-deficient mice were decreased compared with control mice after consuming an HFD. A_2AAR-KO mice had decreased expression of the β-cell-specific markers pdx1, glut2, mafA, and nkx6.1 and increased expression of the dedifferentiation markers sox2 and hes1. Ex vivo islet experiments confirmed the role of A_2AARs in protecting against decreased insulin content and release caused by HFD. Other experiments with bone marrow chimeras revealed that inflammation was not the primary cause of decreased insulin secretion in A_2AAR-KO mice. Altogether, our data showed that A_2AARs control pancreatic dysfunction in HFD-induced obesity.-Csóka, B., Törő, G., Vindeirinho, J., Varga, Z. V., Koscsó, B., Németh, Z. H., Kókai, E., Antonioli, L., Suleiman, M., Marchetti, P., Cseri, K., Deák, Á., Virág, L., Pacher, P., Bai, P., Haskó, G. A_2A adenosine receptors control pancreatic dysfunction in high-fat-diet-induced obesity.

Expression of Adenosine A2B Receptor and Adenosine Deaminase in Rabbit Gastric Mucosa ECL Cells

Adenosine is readily available to the glandular epithelium of the stomach. Formed continuously in intracellular and extracellular locations, it is notably produced from ATP released in enteric cotransmission. Adenosine analogs modulate chloride secretion in gastric glands and activate acid secretion in isolated parietal cells through A_2B adenosine receptor (A2BR) binding. A functional link between surface A2BR and adenosine deaminase (ADA) was found in parietal cells, but whether this connection is a general feature of gastric mucosa cells is unknown. Here we examine whether A2BR is expressed at the membrane of histamine-producing enterochromaffin-like (ECL) cells, the major endocrine cell type in the oxyntic mucosa, and if so, whether it has a vicinity relationship with ADA. We used a highly homogeneous population of rabbit ECL cells (size 7.5–10 µm) after purification by elutriation centrifugation. The surface expression of A2BR and ADA proteins was assessed by flow cytometry and confocal microscopy. Our findings demonstrate that A2BR and ADA are partially coexpressed at the gastric ECL cell surface and that A2BR is functional, with regard to binding of adenosine analogs and adenylate cyclase activation. The physiological relevance of A2BR and ADA association in regulating histamine release is yet to be explained.

Erythrocytes retain hypoxic adenosine response for faster acclimatization upon re-ascent

Faster acclimatization to high altitude upon re-ascent is seen in humans; however, the molecular basis for this enhanced adaptive response is unknown. We report that in healthy lowlanders, plasma adenosine levels are rapidly induced by initial ascent to high altitude and achieved even higher levels upon re-ascent, a feature that is positively associated with quicker acclimatization. Erythrocyte equilibrative nucleoside transporter 1 (eENT1) levels are reduced in humans at high altitude and in mice under hypoxia. eENT1 deletion allows rapid accumulation of plasma adenosine to counteract hypoxic tissue damage in mice. Adenosine signalling via erythrocyte ADORA2B induces PKA phosphorylation, ubiquitination and proteasomal degradation of eENT1. Reduced eENT1 resulting from initial hypoxia is maintained upon re-ascent in humans or re-exposure to hypoxia in mice and accounts for erythrocyte hypoxic memory and faster acclimatization. Our findings suggest that targeting identified purinergic-signalling network would enhance the hypoxia adenosine response to counteract hypoxia-induced maladaptation.

Humans that reach high altitude soon after the first ascent show faster adaptation to hypoxia. Song et al. show that this adaptive response relies on decreased red blood cell uptake of plasma adenosine due to reduced levels of nucleoside transporter ENT1 resulting from coordinated adenosine generation by ectonucleotidase CD73 and activation of A2B receptors.

Adenosine Receptor Adora2b Plays a Mechanistic Role in the Protective Effect of the Volatile Anesthetic Sevoflurane during Liver Ischemia/Reperfusion

Background

Liver ischemia/reperfusion (IR) injury is characterized by hepatic tissue damage and an inflammatory response. This is accompanied by the formation and vascular sequestration of platelet–neutrophil conjugates (PNCs). Signaling through Adora2b adenosine receptors can provide liver protection. Volatile anesthetics may interact with adenosine receptors. This study investigates potential antiinflammatory effects of the volatile anesthetic sevoflurane during liver IR.

Methods

Experiments were performed ex vivo with human blood and in a liver IR model with wild-type, Adora2a−/−, and Adora2b−/− mice. The effect of sevoflurane on platelet activation, PNC formation and sequestration, cytokine release, and liver damage (alanine aminotransferase release) was analyzed using flow cytometry, luminometry, and immunofluorescence. Adenosine receptor expression in liver tissue was analyzed using immunohistochemistry and real-time polymerase chain reaction.

Results

Ex vivo experiments indicate that sevoflurane inhibits platelet and leukocyte activation (n = 5). During liver IR, sevoflurane (2 Vol%) decreased PNC formation 2.4-fold in wild-type (P &lt; 0.05) but not in Adora2b−/− mice (n ≥ 5). Sevoflurane reduced PNC sequestration 1.9-fold (P &lt; 0.05) and alanine aminotransferase release 3.5-fold (P &lt; 0.05) in wild-type but not in Adora2b−/− mice (n = 5). In Adora2a−/− mice, sevoflurane also inhibited PNC formation and cytokine release. Sevoflurane diminished cytokine release (n ≥ 3) and increased Adora2b transcription and expression in liver tissue of wild-types (n = 4).

Conclusions

Our experiments highlight antiinflammatory and tissue-protective properties of sevoflurane during liver IR and reveal a mechanistic role of Adora2b in sevoflurane-associated effects. The targeted use of sevoflurane not only as an anesthetic but also to prevent IR damage is a promising approach in the treatment of critically ill patients.

Targeting hypoxia in inflammatory bowel disease

In this review, I summarize some of the recent insight into pharmacological targeting of hypoxia in disease models. Studies from cultured cell systems, animal models, and translation to human patients have revealed that posttranslational modifications of individual proteins within NF-κB and hypoxia-inducible factor pathways serve as ideal targets for analysis in disease models. Studies defining differences and similarities between these responses have taught us a number of important lessons about the complexity of the inflammatory response. A clearer definition of these pathways has provided new insight into disease pathogenesis and, importantly, the potential for new therapeutic targets.

Stimulation of gastric acid secretion by rabbit parietal cell A2B adenosine receptor activation

Adenosine modulates different functional activities in many cells of the gastrointestinal tract; some of them are believed to be mediated by interaction with its four G protein-coupled receptors. The renewed interest in the adenosine A2B receptor (A2BR) subtype can be traced by studies in which the introduction of new genetic and chemical tools has widened the pharmacological and structural knowledge of this receptor as well as its potential therapeutic use in cancer and inflammation- or hypoxia-related pathologies. In the acid-secreting parietal cells of the gastric mucosa, the use of various radioligands for adenosine receptors suggested the presence of the A2 adenosine receptor subtype(s) on the cell surface. Recently, we confirmed A2BR expression in native, nontransformed parietal cells at rest by using flow cytometry and confocal microscopy. In this study, we show that A2BR is functional in primary rabbit gastric parietal cells, as indicated by the fact that agonist binding to A2BR increased adenylate cyclase activity and acid production. In addition, both acid production and radioligand binding of adenosine analogs to isolated cell membranes were potently blocked by selective A2BR antagonists, whereas ligands for A1, A2A, and A3 adenosine receptors failed to abolish activation. We conclude that rabbit gastric parietal cells possess functional A2BR proteins that are coupled to Gs and stimulate HCl production upon activation. Whether adenosine- and A2BR-mediated functional responses play a role in human gastric pathophysiology is yet to be elucidated.

The regulation of pulmonary inflammation by the hypoxia-inducible factor-hydroxylase oxygen-sensing pathway

Although the hypoxia-inducible factor (HIF)-hydroxylase oxygen-sensing pathway has been extensively reviewed in the context of cellular responses to hypoxia and cancer biology, its importance in regulating innate immune biology is less well described. In this review, we focus on the role of the HIF-hydroxylase pathway in regulating myeloid cell responses and its relevance to inflammatory lung disease. The more specific roles of individual HIF/ prolyl hydroxylase pathway members in vivo are discussed in the context of lineage-specific rodent models of inflammation, with final reference made to the therapeutic challenges of targeting the HIF/hydroxylase pathway in immune cells.

Beneficial and detrimental role of adenosine signaling in diseases and therapy

Adenosine is a major signaling nucleoside that orchestrates cellular and tissue adaptation under energy depletion and ischemic/hypoxic conditions by activation of four G protein-coupled receptors (GPCR). The regulation and generation of extracellular adenosine in response to stress are critical in tissue protection. Both mouse and human studies reported that extracellular adenosine signaling plays a beneficial role during acute states. However, prolonged excess extracellular adenosine is detrimental and contributes to the development and progression of various chronic diseases. In recent years, substantial progress has been made to understand the role of adenosine signaling in different conditions and to clarify its significance during the course of disease progression in various organs. These efforts have and will identify potential therapeutic possibilities for protection of tissue injury at acute stage by upregulation of adenosine signaling or attenuation of chronic disease progression by downregulation of adenosine signaling. This review is to summarize current progress and the importance of adenosine signaling in different disease stages and its potential therapeutic effects.

Metabolic regulation of intestinal epithelial barrier during inflammation

The gastrointestinal mucosa has proven to be an interesting tissue for which to investigate disease-related metabolism. In this review, we outline some evidence that implicates metabolic signaling as important features of barrier in the healthy and disease. Studies from cultured cell systems, animal models and human patients have revealed that metabolites generated within the inflammatory microenvironment are central to barrier regulation. These studies have revealed a prominent role for hypoxia and hypoxia-inducible factor (HIF) at key steps in adenine nucleotide metabolism and within the creatine kinase pathway. Results from animal models of intestinal inflammation have demonstrated an almost uniformly beneficial influence of HIF stabilization on disease outcomes and barrier function. Studies underway to elucidate the contribution of immune responses will provide additional insight into how metabolic changes contribute to the complexity of the gastrointestinal tract and how such information might be harnessed for therapeutic benefit.

Targeting hypoxia signalling for the treatment of ischaemic and inflammatory diseases

Hypoxia-inducible factors (HIFs) are stabilized during adverse inflammatory processes associated with disorders such as inflammatory bowel disease, pathogen infection and acute lung injury, as well as during ischaemia-reperfusion injury. HIF stabilization and hypoxia-induced changes in gene expression have a profound impact on the inflamed tissue microenvironment and on disease outcomes. Although the mechanism that initiates HIF stabilization may vary, the final molecular steps that control HIF stabilization converge on a set of oxygen-sensing prolyl hydroxylases (PHDs) that mark HIFs for proteasomal degradation. PHDs are therefore promising therapeutic targets. In this Review, we discuss the emerging potential and associated challenges of targeting the PHD-HIF pathway for the treatment of inflammatory and ischaemic diseases.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

A novel adenosine precursor 2',3'-cyclic adenosine monophosphate inhibits formation of post-surgical adhesions

BACKGROUND

Intraperitoneal adenosine reduces abdominal adhesions. However, because of the ultra-short half-life and low solubility of adenosine, optimal efficacy requires multiple dosing.

AIM

Here, we compared the ability of potential adenosine prodrugs to inhibit post-surgical abdominal adhesions after a single intraperitoneal dose.

METHODS

Abdominal adhesions were induced in mice using an electric toothbrush to damage the cecum. Also, 20 μL of 95 % ethanol was applied to the cecum to cause chemically induced injury. After injury, mice received intraperitoneally either saline (n = 18) or near-solubility limit of adenosine (23 mmol/L; n = 12); 5'-adenosine monophosphate (75 mmol/L; n = 11); 3'-adenosine monophosphate (75 mmol/L; n = 12); 2'-adenosine monophosphate (75 mmol/L; n = 12); 3',5'-cyclic adenosine monophosphate (75 mmol/L; n = 19); or 2',3'-cyclic adenosine monophosphate (75 mmol/L; n = 20). After 2 weeks, adhesion formation was scored by an observer blinded to the treatments. In a second study, intraperitoneal adenosine levels were measured using tandem mass spectrometry for 3 h after instillation of 2',3'-cyclic adenosine monophosphate (75 mmol/L) into the abdomen.

RESULTS

The order of efficacy for attenuating adhesion formation was: 2',3'-cyclic adenosine monophosphate > 3',5'-cyclic adenosine monophosphate ≈ adenosine > 5'-adenosine monophosphate ≈ 3'-adenosine monophosphate ≈ 2'-adenosine monophosphate. The groups were compared using a one-factor analysis of variance, and the overall p value for differences between groups was p < 0.000001. Intraperitoneal administration of 2',3'-cAMP yielded pharmacologically relevant levels of adenosine in the abdominal cavity for >3 h.

CONCLUSION

Administration of 2',3'-cyclic adenosine monophosphate into the surgical field is a unique, convenient and effective method of preventing post-surgical adhesions by acting as an adenosine prodrug.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Equilibrative nucleoside transporter (ENT)-1-dependent elevation of extracellular adenosine protects the liver during ischemia and reperfusion

Ischemia and reperfusion-elicited tissue injury contributes to morbidity and mortality of hepatic surgery and during liver transplantation. Previous studies implicated extracellular adenosine signaling in liver protection. Based on the notion that extracellular adenosine signaling is terminated by uptake from the extracellular towards the intracellular compartment by way of equilibrative nucleoside transporters (ENTs), we hypothesized a functional role of ENTs in liver protection from ischemia. During orthotopic liver transplantation in humans, we observed higher expressional levels of ENT1 than ENT2, in conjunction with repression of ENT1 and ENT2 transcript and protein levels following warm ischemia and reperfusion. Treatment with the pharmacologic ENT inhibitor dipyridamole revealed elevations of hepatic adenosine levels and robust liver protection in a murine model of liver ischemia and reperfusion. Studies in gene-targeted mice for Ent1 or Ent2 demonstrated selective protection from liver injury in Ent1(-/-) mice. Treatment with selective adenosine receptor antagonists indicated a contribution of Adora2b receptor signaling in ENT-dependent liver protection.

CONCLUSION

These findings implicate ENT1 in liver protection from ischemia and reperfusion injury and suggest ENT inhibitors may be of benefit in the prevention or treatment of ischemic liver injury.

Role of extracellular ATP and P2 receptor signaling in regulating renal cyst growth and interstitial inflammation in polycystic kidney disease

Polycystic kidney diseases (PKD) are a group of inherited ciliopathies in which the formation and growth of multiple cysts derived from the distal nephron and collecting duct leads to the disruption of normal kidney architecture, chronic interstitial inflammation/fibrosis and hypertension. Kidney failure is the most life-threatening complication of PKD, and is the consequence of cyst expansion, renal interstitial disease and loss of normal kidney tissue. Over the last decade, accumulating evidence suggests that the autocrine and paracrine effects of ATP (through its receptor family P2X and P2Y), could be detrimental for the progression of PKD. (2009). In vitro, ATP-P2 signaling promotes cystic epithelial cell proliferation, chloride-driven fluid secretion and apoptosis. Furthermore, dysfunction of the polycystin signal transduction pathways promotes the secretagogue activity of extracellular ATP by activating a calcium-activated chloride channel via purinergic receptors. Finally, ATP is a danger signal and could potentially contribute to interstitial inflammation associated with PKD. These data suggest that ATP-P2 signaling worsens the progression of cyst enlargement and interstitial inflammation in PKD.

Trefoil factor 2 expression and its significance as a predictor of severity of sepsis in children

Intestinal injury significantly contributes to critical illness, sepsis and multiorgan failure. TFF2 (Trefoil Factor 2) is expressed and secreted preferentially by gastric mucous neck cells. TFF2 gene expression is promptly increased after gut injury, and its expression profile broadens to include the regenerative epithelia of virtually the entire gastrointestinal tract. The first objective of our study was an analysis of TFF2 levels dynamics in patients with Systemic Inflammatory Response Syndrome (SIRS) or septic condition during a 5-day period after admission. The second objective was to determine optimal cut-off value and quantify diagnostic characteristics of TFF2 between controls and patients with various septic states. The study included 57 children aged 0-19 years, with expected or proven SIRS and septic condition. The degree of severity was evaluated according to PELOD Score (Pediatric Logistic Organ Dysfunction). Blood samples to determine levels of TFF2 factor were taken during the time patient met the criteria of SIRS or sepsis. Control group samples to determine the serum levels of TFF2 were taken from patients undergoing elective surgery. Analysis of TFF2 levels dynamics revealed that TFF2 levels kept steady state during the 5-day period. Significantly higher levels of TFF2 were in patients with Multiple Organ Dysfunction Syndrome (MODS). The difference was noticed also in ROC analysis.

Adenosine kinase: exploitation for therapeutic gain

Adenosine kinase (ADK; EC 2.7.1.20) is an evolutionarily conserved phosphotransferase that converts the purine ribonucleoside adenosine into 5'-adenosine-monophosphate. This enzymatic reaction plays a fundamental role in determining the tone of adenosine, which fulfills essential functions as a homeostatic and metabolic regulator in all living systems. Adenosine not only activates specific signaling pathways by activation of four types of adenosine receptors but it is also a primordial metabolite and regulator of biochemical enzyme reactions that couple to bioenergetic and epigenetic functions. By regulating adenosine, ADK can thus be identified as an upstream regulator of complex homeostatic and metabolic networks. Not surprisingly, ADK dysfunction is involved in several pathologies, including diabetes, epilepsy, and cancer. Consequently, ADK emerges as a rational therapeutic target, and adenosine-regulating drugs have been tested extensively. In recent attempts to improve specificity of treatment, localized therapies have been developed to augment adenosine signaling at sites of injury or pathology; those approaches include transplantation of stem cells with deletions of ADK or the use of gene therapy vectors to downregulate ADK expression. More recently, the first human mutations in ADK have been described, and novel findings suggest an unexpected role of ADK in a wider range of pathologies. ADK-regulating strategies thus represent innovative therapeutic opportunities to reconstruct network homeostasis in a multitude of conditions. This review will provide a comprehensive overview of the genetics, biochemistry, and pharmacology of ADK and will then focus on pathologies and therapeutic interventions. Challenges to translate ADK-based therapies into clinical use will be discussed critically.

The role of purinergic pathways in the pathophysiology of gut diseases: pharmacological modulation and potential therapeutic applications

Gut homeostasis results from complex neuro-immune interactions aimed at triggering stereotypical and specific programs of coordinated mucosal secretion and powerful motor propulsion. A prominent role in the regulation of this highly integrated network, comprising a variety of immune/inflammatory cells and the enteric nervous system, is played by purinergic mediators. The cells of the digestive tract are literally plunged into a "biological sea" of functionally active nucleotides and nucleosides, which carry out the critical task of driving regulatory interventions on cellular functions through the activation of P1 and P2 receptors. Intensive research efforts are being made to achieve an integrated view of the purinergic system, since it is emerging that the various components of purinergic pathways (i.e., enzymes, transporters, mediators and receptors) are mutually linked entities, deputed to finely modulating the magnitude and the duration of purinergic signaling, and that alterations occurring in this balanced network could be intimately involved in the pathophysiology of several gut disorders. This review article intends to provide a critical appraisal of current knowledge on the purinergic system role in the regulation of gastrointestinal functions, considering these pathways as a whole integrated network, which is capable of finely controlling the levels of bioactive nucleotides and nucleosides in the biophase of their respective receptors. Special attention is paid to the mechanisms through which alterations in the various compartments of the purinergic system could contribute to the pathophysiology of gut disorders, and to the possibility of counteracting such dysfunctions by means of pharmacological interventions on purinergic molecular targets.

Adenosine receptors as drug targets--what are the challenges?

Adenosine signalling has long been a target for drug development, with adenosine itself or its derivatives being used clinically since the 1940s. In addition, methylxanthines such as caffeine have profound biological effects as antagonists at adenosine receptors. Moreover, drugs such as dipyridamole and methotrexate act by enhancing the activation of adenosine receptors. There is strong evidence that adenosine has a functional role in many diseases, and several pharmacological compounds specifically targeting individual adenosine receptors--either directly or indirectly--have now entered the clinic. However, only one adenosine receptor-specific agent--the adenosine A2A receptor agonist regadenoson (Lexiscan; Astellas Pharma)--has so far gained approval from the US Food and Drug Administration (FDA). Here, we focus on the biology of adenosine signalling to identify hurdles in the development of additional pharmacological compounds targeting adenosine receptors and discuss strategies to overcome these challenges.

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.

Adenosine signaling during acute and chronic disease states

Adenosine is a signaling nucleoside that is produced following tissue injury, particularly injury involving ischemia and hypoxia. The production of extracellular adenosine and its subsequent signaling through adenosine receptors plays an important role in orchestrating injury responses in multiple organs. There are four adenosine receptors that are widely distributed on immune, epithelial, endothelial, neuronal,and stromal cells throughout the body. Interestingly, these receptors are subject to altered regulation following injury. Studies in mouse models and human cells and tissues have identified that the production of adenosine and its subsequent signaling through its receptors plays largely beneficial roles in acute disease states, with the exception of brain injury. In contrast, if elevated adenosine levels are sustained beyond the acute injury phase, adenosine responses can become detrimental by activating pathways that promote tissue injury and fibrosis. Understanding when during the course of disease adenosine signaling is beneficial as opposed to detrimental and defining the mechanisms involved will be critical for the advancement of adenosine-based therapies for acute and chronic diseases. The purpose of this review is to discuss key observations that define the beneficial and detrimental aspects of adenosine signaling during acute and chronic disease states with an emphasis on cellular processes, such as inflammatory cell regulation, vascular barrier function, and tissue fibrosis.

Adenosine and gastrointestinal inflammation

Nucleosides such as adenosine (Ado) influence nearly every aspect of physiology and pathophysiology. Extracellular nucleotides liberated at local sites of inflammation are metabolized through regulated phosphohydrolysis by a series of ecto-nucleotidases including ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and ecto-5'-nucleotidase (CD73), found on the surface of a variety of cell types. Once generated, Ado is made available to bind and activate one of four G protein-coupled Ado receptors. Recent in vitro and in vivo studies implicate Ado in a broad array of tissue-protective mechanisms that provide new insight into adenosine actions. Studies in cultured cells and murine tissues have indicated that Ado receptors couple to novel posttranslational protein modifications, including Cullin deneddylation, as a new anti-inflammatory mechanism. Studies in Ado receptor-null mice have been revealing and indicate a particularly important role for the Ado A2B receptor in animal models of intestinal inflammation. Here, we review contributions of Ado to cell and tissue stress responses, with a particular emphasis on the gastrointestinal mucosa.

Blocking A2B adenosine receptor alleviates pathogenesis of experimental autoimmune encephalomyelitis via inhibition of IL-6 production and Th17 differentiation

Adenosine is a key endogenous signaling molecule that regulates immune responses. A(2B) adenosine receptor (AR) is a relatively low-affinity receptor for adenosine, and the activation of A(2B)AR is believed to require pathological level of adenosine that is associated with ischemia, inflammation, trauma, or other types of stress. The role of A(2B)AR in the pathogenesis of multiple sclerosis (MS) is still unclear. In this study, we discovered that A(2B)AR was upregulated both in the peripheral blood leukocytes of MS patients and the peripheral lymphoid tissues of experimental autoimmune encephalomyelitis (EAE) mice. A(2B)AR-specific antagonists, CVT-6883 and MRS-1754, alleviated the clinical symptoms of EAE and protected the CNS from immune damage. A(2B)AR-knockout mice also developed less severe EAE. Further study indicated that blocking or deleting A(2B)AR inhibited Th17 cell differentiation by blocking IL-6 production from APCs such as dendritic cells. In dendritic cells, A(2B)AR was also upregulated during the development of EAE. CVT-6883 and genetic deletion of A(2B)AR significantly reduced adenosine-mediated IL-6 production. The phospholipase Cβ-protein kinase C and p38 MAPK pathways were found to be involved in the A(2B)AR-mediated IL-6 production. Our findings not only revealed the pathological role of A(2B)AR in EAE, but also suggested that this receptor might be a new therapeutic target for the development of anti-MS drugs.

Implications of protein post-translational modifications in IBD

In recent years our understanding of the pathogenesis of inflammatory bowel disease (IBD) has greatly increased. Hallmarks of IBD include loss of intestinal barrier function, increased cytokine production, and failed resolution of tissue damage. Lasting treatments are still lacking and, therefore, a better understanding of the underlying molecular mechanisms is necessary to design novel therapeutic approaches. Apart from transcriptional and posttranscriptional regulation of relevant genes, mammals have evolved a complex and efficient series of mechanisms to rapidly modify newly made proteins for the purposes of signaling and adaptation. These posttranslational protein modifications include, among others, phosphorylation, hydroxylation, neddylation, and cytokine cleavage by the inflammasome. This review focuses on our current understanding of posttranslational protein modifications with a particular focus on their relevance to IBD pathogenesis.

Hypoxia signaling during intestinal ischemia and inflammation

PURPOSE OF REVIEW

During critical illness, alterations of intestinal blood supply and inflammatory activation can result in severe intestinal hypoxia (limited oxygen availability). Conditions of hypoxia lead to the activation of a transcriptional program that is under the control of the transcription factor hypoxia-inducible factor (HIF). In many instances, HIF-dependent alterations of gene expression represent endogenous adaptive responses that dampen pathologic inflammation and could be targeted to treat intestinal injury.

RECENT FINDINGS

Post-translational stabilization of the HIF transcription factor and corresponding changes in gene expression are central to the resolution of intestinal injury. Examples for such responses that we discuss in this review include hypoxia-elicited increases in extracellular adenosine production and signaling, particularly through the A2B adenosine receptor, and intestinal protection provided by hypoxia-inducible netrin-1.

SUMMARY

The present review focuses on HIF-elicited anti-inflammatory pathways that result in intestinal protection during critical illness. Many of these pathways represent novel therapeutic targets for attenuating multiorgan failure and critical illness. Whereas these therapeutic approaches are currently being investigated in cell culture models or in genetic mouse models, we are optimistic that at least some of these novel targets can be translated from bench to bedside in the near future.

Targeting Hypoxia to Augment Mucosal Barrier Function

Sites of inflammation are associated with profound changes in tissue metabolism. Studies in vitro and in vivo have shown that the activation of the hypoxia-inducible factor (HIF) serves as an adaptive pathway for the resolution of inflammation associated with various murine disease models. The resolution of disease occurs, at least in part, through transcriptional regulation of non-classical epithelial barrier genes. There is significant recent interest in harnessing hypoxia-inducible pathways, including targeting the HIF and the proyl-hydroxylase (PHD) enzymes that stabilize HIF, to promote mucosal healing. Here, we review the signaling pathways involved and define how hypoxia-associated signaling provides mechanistic insight into augmenting barrier function in mucosal inflammatory disease.

The hypoxia-inflammation link and potential drug targets

PURPOSE OF REVIEW

Hypoxia represents one of the strongest transcriptional stimuli known to us. In most cases, hypoxia-induced changes in gene expression are directed towards adapting tissues to conditions of limited oxygen availability.

RECENT FINDINGS

As a well known example, physical exercise at high altitude results in the transcriptional induction of erythropoietin that functions to increase oxygen carrying capacity and red cell volume. Studies of the transcriptional pathway responsible for the induction of erythropoietin during conditions of hypoxia led to the discovery of the transcription factor hypoxia-inducible factor (HIF) that is known today as the key transcription factor for hypoxia adaptation. Surgical patients are frequently at risk for experiencing detrimental effects of hypoxia or ischemia, for example, in the context of acute kidney injury, myocardial, intestinal or hepatic ischemia, acute lung injury, or during organ transplantation.

SUMMARY

In the present review, we discuss the mechanisms of transcriptional adaptation to hypoxia and provide evidence supporting the hypothesis that targeting hypoxia-induced inflammation can represent novel pharmacologic strategies to improve perioperative outcomes. Currently, such strategies are being explored at an experimental level, but we hope that some of these targets can be translated into perioperative patient care within the next decade.

Adenosine receptor targeting in health and disease

INTRODUCTION

The adenosine receptors A(1), A(2A), A(2B) and A(3) are important and ubiquitous mediators of cellular signaling that play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including increasing the oxygen supply:demand ratio, pre-conditioning, anti-inflammatory effects and the stimulation of angiogenesis.

AREAS COVERED

The state of the art of the role of adenosine receptors which have been proposed as targets for drug design and discovery, in health and disease, and an overview of the ligands for these receptors in clinical development.

EXPERT OPINION

Selective ligands of A(1), A(2A), A(2B) and A(3) adenosine receptors are likely to find applications in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The aim of this review is to provide an overview of the present knowledge regarding the role of these adenosine receptors in health and disease.

Extracellular adenosine: a safety signal that dampens hypoxia-induced inflammation during ischemia

Traditionally, the single most unique feature of the immune system has been attributed to its capability to discriminate between self (e.g., host proteins) and nonself (e.g., pathogens). More recently, an emerging immunologic concept involves the notion that the immune system responds via a complex system for sensing signals of danger, such as pathogens or host-derived signals of cellular distress (e.g., ischemia), while remaining unresponsive to nondangerous motifs. Experimental studies have provided strong evidence that the production and signaling effects of extracellular adenosine are dramatically enhanced during conditions of limited oxygen availability as occurs during ischemia. As such, adenosine would fit the bill of signaling molecules that are enhanced during situations of cellular distress. In contrast to a danger signal, we propose here that extracellular adenosine operates as a countermeasure, in fact as a safety signal, to both restrain potentially harmful immune responses and to maintain and promote general tissue integrity during conditions of limited oxygen availability.

Interplay of hypoxia and A2B adenosine receptors in tissue protection

That adenosine signaling can elicit adaptive tissue responses during conditions of limited oxygen availability (hypoxia) is a long-suspected notion that recently gained general acceptance from genetic and pharmacologic studies of the adenosine signaling pathway. As hypoxia and inflammation share an interdependent relationship, these studies have demonstrated that adenosine signaling events can be targeted to dampen hypoxia-induced inflammation. Here, we build on the hypothesis that particularly the A(2B) adenosine receptor (ADORA(2B)) plays a central role in tissue adaptation to hypoxia. In fact, the ADORA(2B) requires higher adenosine concentrations than any of the other adenosine receptors. However, during conditions of hypoxia or ischemia, the hypoxia-elicited rise in extracellular adenosine is sufficient to activate the ADORA(2B). Moreover, several studies have demonstrated very robust induction of the ADORA(2B) elicited by transcriptional mechanisms involving hypoxia-dependent signaling pathways and the transcription factor "hypoxia-induced factor" 1. In the present chapter, genetic and pharmacologic evidence is presented to support our hypothesis of a tissue protective role of ADORA(2B) signaling during hypoxic conditions, including hypoxia-elicited vascular leakage, organ ischemia, or acute lung injury. All these disease models are characterized by hypoxia-elicited tissue inflammation. As such, the ADORA(2B) has emerged as a therapeutic target for dampening hypoxia-induced inflammation and tissue adaptation to limited oxygen availability.

Adenosine receptors in health and disease

The adenosine receptors A(1), A(2A), A(2B), and A(3) are important and ubiquitous mediators of cellular signaling, which play vital roles in protecting tissues and organs from damage. In particular, adenosine triggers tissue protection and repair by different receptor-mediated mechanisms, including an increase of oxygen supply/demand ratio, preconditioning, anti-inflammatory effects, and stimulation of angiogenesis. Considerable advances have been recently achieved in the pharmacological and molecular characterization of adenosine receptors, which have been proposed as targets for drug design and discovery. At the present time, it can be speculated that adenosine A(1), A(2A), A(2B), and A(3) receptor-selective ligands may show utility in the treatment of pain, ischemic conditions, glaucoma, asthma, arthritis, cancer, and other disorders in which inflammation is a feature. This chapter documents the present state of knowledge of adenosine receptors' role in health and disease.

A role for the low-affinity A2B adenosine receptor in regulating superoxide generation by murine neutrophils

The formation of adenosine dampens inflammation by inhibiting most cells of the immune system. Among its actions on neutrophils, adenosine suppresses superoxide generation and regulates chemotactic activity. To date, most evidence implicates the G(s) protein-coupled A(2A) adenosine receptor (AR) as the primary AR subtype responsible for mediating the actions of adenosine on neutrophils by stimulating cAMP production. Given that the A(2B)AR is now known to be expressed in neutrophils and that it is a G(s) protein-coupled receptor, we examined in this study whether it signals to suppress neutrophil activities by using 2-[6-amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]pyridin-2-ylsulfanyl]acetamide (BAY 60-6583), a new agonist for the human A(2B)AR that was confirmed in preliminary studies to be a potent and highly selective agonist for the murine A(2B)AR. We found that treating mouse neutrophils with low concentrations (10(-9) and 10(-8) M) of BAY 60-6583 inhibited formylated-methionine-leucine-phenylalanine (fMLP)-stimulated superoxide production by either naive neutrophils, tumor necrosis factor-α-primed neutrophils, or neutrophils isolated from mice treated systemically with lipopolysaccharide. This inhibitory action of BAY 60-6583 was confirmed to involve the A(2B)AR in experiments using neutrophils obtained from A(2B)AR gene knockout mice. It is noteworthy that BAY 60-6583 increased fMLP-stimulated superoxide production at higher concentrations (>1 μM), which was attributed to an AR-independent effect. In a standard Boyden chamber migration assay, BAY 60-6583 alone did not stimulate neutrophil chemotaxis or influence chemotaxis in response to fMLP. These results indicate that the A(2B)AR signals to suppress oxidase activity by murine neutrophils, supporting the idea that this low-affinity receptor for adenosine participates along with the A(2A)AR in regulating the proinflammatory actions of neutrophils.

Partial netrin-1 deficiency aggravates acute kidney injury

The netrin family of secreted proteins provides migrational cues in the developing central nervous system. Recently, netrins have also been shown to regulate diverse processes beyond their functions in the brain, incluing the ochrestration of inflammatory events. Particularly netrin-1 has been implicated in dampening hypoxia-induced inflammation. Here, we hypothesized an anti-inflammatory role of endogenous netrin-1 in acute kidney injury (AKI). As homozygous deletion of netrin-1 is lethal, we studied mice with partial netrin-1 deletion (Ntn-1^+/− mice) as a genetic model. In fact, Ntn-1^+/− mice showed attenuated Ntn-1 levels at baseline and following ischemic AKI. Functional studies of AKI induced by 30 min of renal ischemia and reperfusion revealed enhanced kidney dysfunction in Ntn-1^+/− mice as assessed by measurements of glomerular filtration, urine flow rate, urine electrolytes, serum creatinine and creatinine clearance. Consistent with these findings, histological studies indicated a more severe degree kidney injury. Similarly, elevations of renal and systemic inflammatory markers were enhanced in mice with partial netrin-1 deficiency. Finally, treatment of Ntn-1^+/− mice with exogenous netrin-1 restored a normal phenotype during AKI. Taking together, these studies implicate endogenous netrin-1 in attenuating renal inflammation during AKI.

The A2B adenosine receptor promotes Th17 differentiation via stimulation of dendritic cell IL-6

Adenosine is an endogenous metabolite produced during hypoxia or inflammation. Previously implicated as an anti-inflammatory mediator in CD4(+) T cell regulation, we report that adenosine acts via dendritic cell (DC) A(2B) adenosine receptor (A(2B)AR) to promote the development of Th17 cells. Mouse naive CD4(+) T cells cocultured with DCs in the presence of adenosine or the stable adenosine mimetic 5'-(N-ethylcarboximado) adenosine resulted in the differentiation of IL-17- and IL-22-secreting cells and elevation of mRNA that encode signature Th17-associated molecules, such as IL-23R and RORγt. The observed response was similar when DCs were generated from bone marrow or isolated from small intestine lamina propria. Experiments using adenosine receptor antagonists and cells from A(2B)AR(-/-) or A(2A)AR(-/-)/A(2B)AR(-/-) mice indicated that the DC A(2B)AR promoted the effect. IL-6, stimulated in a cAMP-independent manner, is an important mediator in this pathway. Hence, in addition to previously noted direct effects of adenosine receptors on regulatory T cell development and function, these data indicated that adenosine also acts indirectly to modulate CD4(+) T cell differentiation and suggested a mechanism for putative proinflammatory effects of A(2B)AR.

Use of a hanging weight system for coronary artery occlusion in mice

Murine studies of acute injury are an area of intense investigation, as knockout mice for different genes are becoming increasingly available. Cardioprotection by ischemic preconditioning (IP) remains an area of intense investigation. To further elucidate its molecular basis, the use of knockout mouse studies is particularly important. Despite the fact that previous studies have already successfully performed cardiac ischemia and reperfusion in mice, this model is technically very challenging. Particularly, visual identification of the coronary artery, placement of the suture around the vessel and coronary occlusion by tying off the vessel with a supported knot is technically difficult. In addition, re-opening the knot for intermittent reperfusion of the coronary artery during IP without causing surgical trauma adds additional challenge. Moreover, if the knot is not tied down strong enough, inadvertent reperfusion due to imperfect occlusion of the coronary may affect the results. In fact, this can easily occur due to the movement of the beating heart. Based on potential problems associated with using a knotted coronary occlusion system, we adopted a previously published model of chronic cardiomyopathy based on a hanging weight system for intermittent coronary artery occlusion during IP. In fact, coronary artery occlusion can thus be achieved without having to occlude the coronary by a knot. Moreover, reperfusion of the vessel can be easily achieved by supporting the hanging weights which are in a remote localization from cardiac tissues. We tested this system systematically, including variation of ischemia and reperfusion times, preconditioning regiments, body temperature and genetic backgrounds. In addition to infarct staining, we tested cardiac troponin I (cTnI) as a marker of myocardial infarction in this model. In fact, plasma levels of cTnI correlated with infarct sizes (R2=0.8). Finally, we could show in several studies that this technique yields highly reproducible infarct sizes during murine IP and myocardial infarction. Therefore, this technique may be helpful for researchers who pursue molecular mechanisms involved in cardioprotection by IP using a genetic approach in mice with targeted gene deletion. Further studies on cardiac IP using transgenic mice may consider this technique.

Anti-inflammatory actions of adrenomedullin through fine tuning of HIF stabilization

In intact mucosal tissues, epithelial cells are anatomically positioned in proximity to a number of subepithelial cell types, including endothelia. A number of recent studies have suggested that imbalances between energy supply and demand can result in "inflammatory hypoxia." Given these associations, we hypothesized that endothelial-derived, hypoxia-inducible mediators might influence epithelial function. Guided by cDNA microarray analysis of human microvascular endothelial cells (HMEC-1 line) subjected to hypoxia (pO(2) 20 torr, 8 h), we identified adrenomedullin (ADM) as a prominent hypoxia-inducible factor (HIF) that acts on epithelial cells through cell surface receptors. We assessed the functional ability for exogenous ADM to signal in human intestinal Caco2 cells in vitro by demonstrating a dose-dependent induction of Erk1/2phosphorylation. Further analysis revealed that ADM deneddylates cullin-2 (Cul2), whose action has been demonstrated to control the activity of HIF. Caco2 cells stably expressing a hypoxic response element (HRE)-driven luciferase promoter confirmed that ADM activates the HIF signaling pathway. Extensions of these studies revealed an increase in canonical HIF-1-dependent genes following stimulation with ADM. To define physiological relevance, we investigated the effect of ADM in a DSS model of murine colitis. Administration of ADM resulted in reduced inflammatory indices and less severe histological inflammation compared to vehicle controls. Analysis of tissue and serum cytokines showed a marked and significant inhibition of colitis-associated TNF-α, IL-1β, and KC. Analysis of circulating ADM demonstrated an increase in serum ADM in murine models of colitis. Taken together, these results identify ADM as an endogenously generated vascular mediator that functions as a mucosal protective factor through fine tuning of HIF activity.

The resurgence of A2B adenosine receptor signaling

Since its discovery as a low-affinity adenosine receptor (AR), the A2B receptor (A2BAR), has proven enigmatic in its function. The previous discovery of the A2AAR, which shares many similarities with the A2BAR but demonstrates significantly greater affinity for its endogenous ligand, led to the original perception that the A2BAR was not of substantial physiologic relevance. In addition, lack of specific pharmacological agents targeting the A2BAR made its initial characterization challenging. However, the importance of this receptor was reconsidered when it was observed that the A2BAR is highly transcriptionally regulated by factors implicated in inflammatory hypoxia. Moreover, the notion that during ischemia or inflammation extracellular adenosine is dramatically elevated to levels sufficient for A2BAR activation, indicated that A2BAR signaling may be important to dampen inflammation particularly during tissue hypoxia. In addition, the recent advent of techniques for murine genetic manipulation along with development of pharmacological agents with enhanced A2BAR specificity has provided invaluable tools for focused studies on the explicit role of A2BAR signaling in different disease models. Currently, studies performed with combined genetic and pharmacological approaches have demonstrated that A2BAR signaling plays a tissue protective role in many models of acute diseases e.g. myocardial ischemia, or acute lung injury. These studies indicate that the A2BAR is expressed on a wide variety of cell types and exerts tissue/cell specific effects. This is an important consideration for future studies where tissue or cell type specific targeting of the A2BAR may be used as therapeutic approach.

Metabolic shifts in immunity and inflammation

Sites of ongoing inflammation and triggered immune responses are characterized by significant changes in metabolic activity. Recent studies have indicated that such shifts in tissue metabolism result from a combination of profound recruitment of inflammatory cells (neutrophils and monocytes) and high proliferation rates among lymphocyte populations. The resultant shifts in energy supply and demand can result in metabolic acidosis and diminished delivery and/or availability of oxygen, leading to hypoxia extensive enough to trigger transcriptional and translation changes in tissue phenotype. Such phenotypic shifts can imprint fundamental changes to tissue metabolism. In this study, we review recent work addressing metabolic changes and metabolic control of inflammation and immunity.