HIF-dependent induction of adenosine A2B receptor in hypoxia

Interactions of PPAR-alpha and adenosine receptors in hypoxia-induced angiogenesis

Hypoxia and adenosine are known to upregulate angiogenesis; however, the role of peroxisome proliferator-activated receptor alpha (PPARα) in angiogenesis is controversial. Using transgenic Tg(fli-1:EGFP) zebrafish embryos, interactions of PPARα and adenosine receptors in angiogenesis were evaluated under hypoxic conditions. Epifluorescent microscopy was used to assess angiogenesis by counting the number of intersegmental (ISV) and dorsal longitudinal anastomotic vessel (DLAV) at 28 h post-fertilization (hpf). Hypoxia (6h) stimulated angiogenesis as the number of ISV and DLAV increased by 18-fold (p<0.01) and 100 ± 8% (p<0.001), respectively, at 28 hpf. Under normoxic and hypoxic conditions, WY-14643 (10 μM), a PPARα activator, stimulated angiogenesis at 28 hpf, while MK-886 (0.5 μM), an antagonist of PPARα, attenuated these effects. Compared to normoxic condition, adenosine receptor activation with NECA (10 μM) promoted angiogenesis more effectively under hypoxic conditions. Involvement of A2B receptor was implied in hypoxia-induced angiogenesis as MRS-1706 (10nM), a selective A2B antagonist attenuated NECA (10 μM)-induced angiogenesis. NECA- or WY-14643-induced angiogenesis was also inhibited by miconazole (0.1 μM), an inhibitor of epoxygenase dependent production of eicosatrienoic acid (EET) epoxide. Thus, we conclude that: activation of PPARα promoted angiogenesis just as activation of A2B receptors through an epoxide dependent mechanism.

New insights into sickle cell disease: a disease of hypoxia

PURPOSE OF REVIEW

Sickle cell disease (SCD) is a devastating genetic disorder caused by a single amino acid substitution in β-globin. Although the condition was first described more than a 100 years ago, treatment options remain scarce and unsatisfactory. This review summarizes recent findings that may provide novel insight into therapeutic approaches to SCD treatment.

RECENT FINDINGS

Because of insufficient numbers of erythrocytes for oxygen delivery, SCD patients constantly face hypoxia. Adenosine is well known as a key signaling nucleoside that orchestrates a multifaceted physiological response to hypoxia. Recent studies have revealed that adenosine concentrations are significantly elevated in SCD and contribute to disease pathology by activating adenosine receptors on red blood cells. Apart from adenosine, hypoxia also causes hemoglobin release via hemolysis. Studies on free hemoglobin in circulation have uncovered another two important molecules: nitric oxide and heme oxygenase-1.

SUMMARY

The core of SCD pathology is erythrocyte sickling under hypoxic conditions, leading to vaso-occlusion and hemolysis. Deeper and more comprehensive understanding of SCD as a disease of hypoxia will provide us new therapeutic targets for SCD treatment.

HIF-dependent regulation of AKAP12 (gravin) in the control of human vascular endothelial function

Hypoxia has been widely implicated in many pathological conditions, including those associated with inflammation and tumorigenesis. A number of recent studies have implicated hypoxia in the control of vasculogenesis and permeability, the basis for which is not fully understood. Here we examine the transcriptional regulation of angiogenesis and permeability by hypoxia in endothelial cells. Guided by a global profiling approach in cultured endothelial cells, these studies revealed the selective induction of human gravin (protein kinase A anchoring protein 12) by hypoxia. Analysis of the cloned gravin promoter identified a functional hypoxia-responsive region including 2 binding sites for hypoxia-inducible factor (HIF). Site-directed mutagenesis identified the most distal HIF-binding site as essential for the induction of gravin by hypoxia. Further studies examining gravin gain and loss of function confirmed strong dependence of gravin in control of microvascular endothelial tube formation, wherein gravin functions as a "braking" system for angiogenesis. Additional studies in confluent endothelia revealed that gravin functionally couples to control endothelial barrier function in response to protein kinase A (PKA) agonists. Taken together, these results demonstrate transcriptional coordination of gravin by HIF-1α and amplified PKA-dependent endothelial responses. These findings provide an important link between hypoxia and metabolic conditions associated with inflammation and angiogenesis.

Hypoxia-inducible factor 1 and cardiovascular disease

Cardiac function is required for blood circulation and systemic oxygen delivery. However, the heart has intrinsic oxygen demands that must be met to maintain effective contractility. Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that functions as a master regulator of oxygen homeostasis in all metazoan species. HIF-1 controls oxygen delivery, by regulating angiogenesis and vascular remodeling, and oxygen utilization, by regulating glucose metabolism and redox homeostasis. Analysis of animal models suggests that by activation of these homeostatic mechanisms, HIF-1 plays a critical protective role in the pathophysiology of ischemic heart disease and pressure-overload heart failure.

Signaling through hepatocellular A2B adenosine receptors dampens ischemia and reperfusion injury of the liver

Ischemia and reperfusion significantly contributes to the morbidity and mortality of liver surgery and transplantation. Based on studies showing a critical role for adenosine signaling in mediating tissue adaptation during hypoxia, we hypothesized that signaling events through adenosine receptors (ADORA1, ADORA2A, ADORA2B, or ADORA3) attenuates hepatic ischemia and reperfusion injury. Initial screening studies of human liver biopsies obtained during hepatic transplantation demonstrated a selective and robust induction of ADORA2B transcript and protein following ischemia and reperfusion. Subsequent exposure of gene-targeted mice for each individual adenosine receptor to liver ischemia and reperfusion revealed a selective role for the Adora2b in liver protection. Moreover, treatment of wild-type mice with an Adora2b-selective antagonist resulted in enhanced liver injury, whereas Adora2b-agonist treatment was associated with attenuated hepatic injury in wild-type, but not in Adora2b(-/-) mice. Subsequent studies in mice with Adora2b deletion in different tissues--including vascular endothelia, myeloid cells, and hepatocytes--revealed a surprising role for hepatocellular-specific Adora2b signaling in attenuating nuclear factor NF-κB activation and thereby mediating liver protection from ischemia and reperfusion injury. These studies provide a unique role for hepatocellular-specific Adora2b signaling in liver protection during ischemia and reperfusion injury.

Adenosine A2A receptor (A2AR) is a fine-tune regulator of the collagen1:collagen3 balance

Adenosine is a potent endogenous anti-inflammatory and immunosuppressive metabolite that is a potent modulator of tissue repair. However, the adenosine A2A receptor (A2AR)-mediated promotion of collagen synthesis is detrimental in settings such as scarring and scleroderma. The signaling cascade from A2AR stimulation to increased collagen production is complex and obscure, not least because cAMP and its downstream molecules PKA and Epac1 have been reported to inhibit collagen production. We therefore examined A2AR-stimulated signaling for collagen production by normal human dermal fibroblasts (NHDF). Collagen1 (Col1) and collagen3 (Col3) content after A2AR activation by CGS21680 was studied by western blotting. Contribution of PKA and Epac was analyzed by the PKA inhibitor PKI and by knockdowns of the PKA-Cα, -Cβ, -Cγ, Epac1, and Epac2. CGS21680 stimulates Col1 expression at significantly lower concentrations than those required to stimulate Col3 expression. A2AR stimulates Col1 expression by a PKA-dependent mechanism since PKA inhibition or PKA-Cα and -Cβ knockdown prevents A2AR-mediated Col1 increase. In contrast, A2AR represses Col3 via PKA but stimulates both Col1 and Col3 via an Epac2-dependent mechanism. A2AR stimulation with CGS21680 at 0.1 μM increased Col3 expression only upon PKA blockade. A2AR activation downstream signaling for Col1 and Col3 expression proceeds via two distinct pathways with varying sensitivity to cAMP activation; more highly cAMP-sensitive PKA activation stimulates Col1 expression, and less cAMP-sensitive Epac activation promotes both Col1 and Col3 expression. These observations may explain the dramatic change in Col1:Col3 ratio in hypertrophic and immature scars, where adenosine is present in higher concentrations than in normal skin.

Targeting the HIF pathway in inflammation and immunity

Oxygen deprivation (hypoxia) is a frequently encountered condition in both health and disease. Metazoans have evolved an elegant and direct cellular mechanism by which to sense local oxygen levels and mount an adaptive transcriptional response to hypoxia which is mediated by a transcription factor termed the hypoxia-inducible factor (HIF). In normoxia, HIF is repressed primarily through the action of a family of hydroxylases, which target HIFα subunits for degradation in an oxygen-dependent manner. In hypoxia, HIF is rapidly stabilized in cells thus allowing it to regulate the expression of hundreds of genes which promote an adaptive response including genes expressing regulators of angiogenesis, metabolism, growth and survival. Initial studies into the HIF pathway focused mainly on its role in supporting tumor adaptation through enhancing processes such as angiogenesis, glycolytic metabolism and cell survival. More recently however, it has become clear that the HIF pathway also plays a key role in the regulation of immunity and inflammation. In fact, conditional knockout of the HIF-1α subunit has identified key immune roles in T-cells, dendritic cells, macrophages, neutrophils and epithelial cells. In this review, we will consider the role for HIF in the regulation of the immune response and its possible contribution to inflammation. Furthermore, we will consider potential therapeutic strategies, which target the HIF pathway in chronic inflammatory and infectious disease.

Protective role for netrin-1 during diabetic nephropathy

Recent studies implicate neuronal guidance molecules in the orchestration of inflammatory events. For example, previous studies demonstrate a functional role for netrin-1 in attenuating acute kidney injury. Here, we hypothesized a kidney-protective role for netrin-1 during chronic kidney disease, such as occurs during diabetic nephropathy. To study the role of netrin-1 during diabetic nephropathy, we induced diabetes in mice at the age of 8 weeks by streptocotozin (STZ) treatment. Sixteen weeks after STZ treatment, we examined the kidneys. Initial studies in wild-type mice demonstrated robust induction of renal, urinary, and plasma netrin-1 protein levels during diabetic nephropathy. Subsequent genetic studies in mice with partial netrin-1 deficiency (Ntrn1(+/-) mice) revealed a more severe degree of diabetic nephropathy, including more severe loss of kidney function (albuminuria, glomerular filtration rate, histology). We subsequently performed pharmacologic studies with recombinant netrin-1 treatment given continuously via osmotic pump. Indeed, netrin-1 treatment was associated with attenuated albuminuria and improved histologic scores for diabetic nephropathy compared to controls. Consistent with previous studies implicating purinergic signaling in netrin-1-elicited tissue protection, mice deficient in the Adora2b adenosine receptor were not protected. Taken together, these studies demonstrate a functional role for endogenous netrin-1 in attenuating diabetic kidney disease.

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.

Hydroxylases as therapeutic targets in inflammatory bowel disease

Inflammatory bowel disease (IBD) is a common and debilitating clinical disorder comprising ulcerative colitis and Crohn's disease. IBD occurs when inappropriate immunological activity in the intestinal mucosa results in epithelial barrier dysfunction leading to exposure of the mucosal immune system to luminal antigenic material. This in turn results in the cycles of inflammation and further barrier dysfunction which underlie disease progression. Although significant therapeutic advances have been made over the last decade, current immunosuppressive and anti-inflammatory treatments for IBD have significant limitations due to lack of treatment response in some patients and adverse effects, including increased risk of infection and malignancy. Recent studies using experimental models of IBD have identified that intracellular hydroxylases, a group of enzymes responsible for oxygen sensing and activation of adaptive transcriptional responses to hypoxia may represent a new class of therapeutic targets in IBD. Hydroxylase inhibitors are effective in ameliorating symptoms of colitis at least in part through the promotion of intestinal epithelial barrier function. The mechanism of this protection is due to activation of hypoxia-sensitive transcription factors, including the hypoxia-inducible factor (HIF) and nuclear factor kappa-B (NF-κB), which activate specific epithelial barrier-protective transcriptional programs. In this review, the mechanism(s) of action and the therapeutic potential of small molecule hydroxylase inhibitors for the treatment of IBD will be discussed.

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.

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.

Hypoxia-induced deoxycytidine kinase expression contributes to apoptosis in chronic lung disease

Chronic obstructive pulmonary disease (COPD) is characterized by persistent inflammation and tissue remodeling and is a leading cause of death in the United States. Increased apoptosis of pulmonary epithelial cells is thought to play a role in COPD development and progression. Identification of signaling pathways resulting in increased apoptosis in COPD can be used in the development of novel therapeutic interventions. Deoxyadenosine (dAdo) is a DNA breakdown product that amplifies lymphocyte apoptosis by being phosphorylated to deoxyadenosine triphosphate (dATP). dAdo is maintained at low levels by adenosine deaminase (ADA). This study demonstrated that mice lacking ADA developed COPD manifestations in association with elevated dAdo and dATP levels and increased apoptosis in the lung. Deoxycitidine kinase (DCK), a major enzyme for dAdo phosphorylation, was up-regulated in mouse and human airway epithelial cells in association with air-space enlargement. Hypoxia was identified as a novel regulator of DCK, and inhibition of DCK resulted in diminished dAdo-mediated apoptosis in the lungs. Our results suggest that activating the dAdo-DCK-dATP pathway directly results in increased apoptosis in the lungs of mice with air-space enlargement and suggests a novel therapeutic target for the treatment of COPD.

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.

Targeting the hypoxia-adenosinergic signaling pathway to improve the adoptive immunotherapy of cancer

The recent approval by the FDA of cancer vaccines and drugs that blockade immunological negative regulators has further enhanced interest in promising approaches of the immunotherapy of cancer. However, the disappointingly short life extension has also underscored the need to better understand the mechanisms that prevent tumor rejection and survival even after the blockade of immunological negative regulators. Here, we describe the implications of the "metabolism-based" immunosuppressive mechanism, where the local tissue hypoxia-driven accumulation of extracellular adenosine triggers suppression via A2 adenosine receptors on the surface of activated immune cells. This molecular pathway is of critical importance in mechanisms of immunosuppression in inflamed and cancerous tissue microenvironments. The protection of tumors by tumor-generated extracellular adenosine and A2 adenosine receptors could be the misguided application of the normal tissue-protecting mechanism that limits excessive collateral damage to vital organs during the anti-pathogen immune response. The overview of the current state of the art regarding the immunosuppressive effects of extracellular adenosine is followed by a historical perspective of studies focused on the elucidation of the physiological negative regulators that protect tissues of vital organs from excessive collateral damage, but, as a trade-off, may also weaken the anti-pathogen effector functions and negate the attempts of anti-tumor immune cells to destroy cancerous cells.

Ligand-specific binding and activation of the human adenosine A(2B) receptor

Adenosine A(2B) receptors, which play a role in inflammation and cancer, are of considerable interest as novel drug targets. To gain deeper insights into ligand binding and receptor activation, we exchanged amino acids predicted to be close to the binding pocket. The alanine mutants were stably expressed in CHO cells and characterized by radioligand binding and cAMP assays using three structural classes of ligands: xanthine (antagonist), adenosine, and aminopyridine derivatives (agonists). Asn282(7.45) and His280(7.43) were found to stabilize the binding site by intramolecular hydrogen bond formation as in the related A(2A) receptor subtype. Trp247(6.48), Val250(6.51), and particularly Ser279(7.42) were shown to be important for binding of nucleosidic agonists. Leu81(3.28), Asn186(5.42), and Val250(6.51) were discovered to be crucial for binding of the xanthine-derived antagonist PSB-603. Leu81(3.28), which is not conserved among adenosine receptor subtypes, may be important for the high selectivity of PSB-603. The N186(5.42)A mutant resulted in an increased potency for agonists. The interactions of the non-nucleosidic agonist BAY60-6583 were different from those of the nucleosides: while BAY60-6583 appeared not to interact with Ser279(7.42), its interactions with Trp247(6.48) and Val250(6.51) were significantly weaker compared to those of NECA. Moreover, our results discount the hypothesis of Trp247(6.48) serving as a "toogle switch" because BAY60-6583 was able to activate the corresponding mutant. This study reveals distinct interactions of structurally diverse ligands with the human A(2B) receptor and differences between closely related receptor subtypes (A(2B) and A(2A)). It will contribute to the understanding of G protein-coupled receptor function and advance A(2B) receptor ligand design.

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

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

CD73-generated adenosine: orchestrating the tumor-stroma interplay to promote cancer growth

Despite the coming of age of cancer immunotherapy, clinical benefits are still modest. An important barrier to successful cancer immunotherapy is that tumors employ a number of mechanisms to facilitate immune escape, including the production of anti-inflammatory cytokines, the recruitment of regulatory immune subsets, and the production of immunosuppressive metabolites. Significant therapeutic opportunity exists in targeting these immunosuppressive pathways. One such immunosuppressive pathway is the production of extracellular adenosine by CD73, an ectonucleotidase overexpressed in various types of cancer. We hereafter review the biology of CD73 and its role in cancer progression and metastasis. We describe the role of extracellular adenosine in promoting tumor growth through paracrine and autocrine action on tumor cells, endothelial cells, and immune cells.

Production of adenosine by ectonucleotidases: a key factor in tumor immunoescape

It is now well known that tumor immunosurveillance contributes to the control of cancer growth. Many mechanisms can be used by cancer cells to avoid the antitumor immune response. One such mechanism relies on the capacity of cancer cells or more generally of the tumor microenvironment to generate adenosine, a major molecule involved in antitumor T cell response suppression. Adenosine is generated by the dephosphorylation of extracellular ATP released by dying tumor cells. The conversion of ATP into adenosine is mediated by ectonucleotidase molecules, namely, CD73 and CD39. These molecules are frequently expressed in the tumor bed by a wide range of cells including tumor cells, regulatory T cells, Th17 cells, myeloid cells, and stromal cells. Recent evidence suggests that targeting adenosine by inhibiting ectonucleotidases may restore the resident antitumor immune response or enhance the efficacy of antitumor therapies. This paper will underline the impact of adenosine and ectonucleotidases on the antitumor response.

Adenosine receptor A2b on hematopoietic cells mediates LPS-induced migration of PMNs into the lung interstitium

Uncontrolled transmigration of polymorphonuclear leukocytes (PMNs) into the different compartments of the lungs (intravascular, interstitial, alveolar) is a critical event in the early stage of acute lung injury and acute respiratory distress syndrome. Adenosine receptor A2b is highly expressed in the inflamed lungs and has been suggested to mediate cell trafficking. In a murine model of LPS-induced lung inflammation, we investigated the role of A2b on migration of PMNs into the different compartments of the lung. In A2b−/− mice, LPS-induced accumulation of PMNs was significantly higher in the interstitium, but not in the alveolar space. In addition, pulmonary clearance of PMNs was delayed in A2b−/− mice. Using chimeric mice, we identified A2b on hematopoietic cells as crucial for PMN migration. A2b did not affect the release of relevant chemokines into the alveolar space. LPS-induced microvascular permeability was under the control of A2b on both hematopoietic and nonhematopoietic cells. Activation of A2b on endothelial cells also reduced formation of LPS-induced stress fibers, highlighting its role for endothelial integrity. A specific A2b agonist (BAY 60–6583) was effective in decreasing PMN migration into the lung interstitium and microvascular permeability. In addition, in vitro transmigration of human PMNs through a layer of human endothelial or epithelial cells was A2b dependent. Activation of A2b on human PMNs reduced oxidative burst activity. Together, our results demonstrate anti-inflammatory effects of A2b on two major characteristics of acute lung injury, with a distinct role of hematopoietic A2b for cell trafficking and endothelial A2b for microvascular permeability.

Hypoxia-inducible factor as a therapeutic target for cardioprotection

Hypoxia inducible factor (HIF) is an oxygen-sensitive transcription factor that enables aerobic organisms to adapt to hypoxia. This is achieved through the transcriptional activation of up to 200 genes, many of which are critical to cell survival. Under conditions of normoxia, the hydroxylation of HIF by prolyl hydroxylase domain-containing (PHD) enzymes targets it for polyubiquitination and proteosomal degradation by the von Hippel-Lindau protein (VHL). However, under hypoxic conditions, PHD activity is inhibited, thereby allowing HIF to accumulate and translocate to the nucleus, where it binds to the hypoxia-responsive element sequences of target gene promoters. Experimental studies suggest that HIF may act as a mediator of ischemic preconditioning, and that the genetic or pharmacological stabilization of HIF under normoxic conditions, may protect the heart against the detrimental effects of acute ischemia-reperfusion injury. The mechanisms underlying the cardioprotective effect of HIF are unclear, but it may be attributed to the transcriptional activation of genes associated with cardioprotection such as erythropoietin, heme oxygenase-1, and inducible nitric oxide synthase or it may be due to reprogramming of cell metabolism. In this review article, we highlight the role of HIF in mediating both adaptive and pathological processes in the heart, as well as focusing on the therapeutic potential of the HIF-signaling pathway as a target for cardioprotection.

A novel mechanism of control of NFκB activation and inflammation involving A2B adenosine receptors

The nuclear factor kappa B (NFκB) pathway controls a variety of processes, including inflammation, and thus, the regulation of NFκB has been a continued focus of study. Here, we report a newly identified regulation of this pathway, involving direct binding of the transcription factor NFκB1 (the p105 subunit of NFκB) to the C-terminus of the A(2B) adenosine receptor (A(2B)AR), independent of ligand activation. Intriguingly, binding of A(2B)AR to specific sites on p105 prevents polyubiquitylation and degradation of p105 protein. Ectopic expression of the A(2B)AR increases p105 levels and inhibits NFκB activation, whereas p105 protein levels are reduced in cells from A(2B)AR-knockout mice. In accordance with the known regulation of expression of anti- and pro-inflammatory cytokines by p105, A(2B)AR-null mice generate less interleukin (IL)-10, and more IL-12 and tumor necrosis factor (TNF-α). Taken together, our results show that the A(2B)AR inhibits NFκB activation by physically interacting with p105, thereby blocking its polyubiquitylation and degradation. Our findings unveil a surprising function for the A(2B)AR, and provide a novel mechanistic insight into the control of the NFκB pathway and inflammation.

Sustained adenosine exposure causes lung endothelial barrier dysfunction via nucleoside transporter-mediated signaling

Previous studies by our group as well as others have shown that acute adenosine exposure enhances lung vascular endothelial barrier integrity and protects against increased permeability lung edema. In contrast, there is growing evidence that sustained adenosine exposure has detrimental effects on the lungs, including lung edema. It is well established that adenosine modulates lung inflammation. However, little is known concerning the effect of sustained adenosine exposure on lung endothelial cells (ECs), which are critical to the maintenance of the alveolar-capillary barrier. We show that exogenous adenosine plus adenosine deaminase inhibitor caused sustained elevation of adenosine in lung ECs. This sustained adenosine exposure decreased EC barrier function, elevated cellular reactive oxygen species levels, and activated p38, JNK, and RhoA. Inhibition of equilibrative nucleoside transporters (ENTs) prevented sustained adenosine-induced p38 and JNK activation and EC barrier dysfunction. Inhibition of p38, JNK, or RhoA also partially attenuated sustained adenosine-induced EC barrier dysfunction. These data indicate that sustained adenosine exposure causes lung EC barrier dysfunction via ENT-dependent intracellular adenosine uptake and subsequent activation of p38, JNK, and RhoA. The antioxidant N-acetylcysteine and the NADPH inhibitor partially blunted sustained adenosine-induced JNK activation but were ineffective in attenuation of p38 activation or barrier dysfunction. p38 was activated exclusively in mitochondria, whereas JNK was activated in mitochondria and cytoplasm by sustained adenosine exposure. Our data further suggest that sustained adenosine exposure may cause mitochondrial oxidative stress, leading to activation of p38, JNK, and RhoA in mitochondria and resulting in EC barrier dysfunction.

Hypoxia-inducible factor 1 transcriptional activity in endothelial cells is required for acute phase cardioprotection induced by ischemic preconditioning

Infarction occurs when myocardial perfusion is interrupted for prolonged periods of time. Short episodes of ischemia and reperfusion protect against tissue injury when the heart is subjected to a subsequent prolonged ischemic episode, a phenomenon known as ischemic preconditioning (IPC). Hypoxia-inducible factor 1 (HIF-1) is a transcription factor that mediates adaptive responses to hypoxia/ischemia and is required for IPC. In this study, we performed a cellular and molecular characterization of the role of HIF-1 in IPC. We analyzed mice with knockout of HIF-1α or HIF-1β in Tie2(+) lineage cells, which include bone marrow (BM) and vascular endothelial cells, compared with control littermates. Hearts were subjected to 30 min of ischemia and 120 min of reperfusion, either as ex vivo Langendorff preparations or by in situ occlusion of the left anterior descending artery. The IPC stimulus consisted of two cycles of 5-min ischemia and 5-min reperfusion. Mice lacking HIF-1α or HIF-1β in Tie2(+) lineage cells showed complete absence of protection induced by IPC, whereas significant protection was induced by adenosine infusion. Treatment of mice with a HIF-1 inhibitor (digoxin or acriflavine) 4 h before Langendorff perfusion resulted in loss of IPC, as did administration of acriflavine directly into the perfusate immediately before IPC. We conclude that HIF-1 activity in endothelial cells is required for acute IPC. Expression and dimerization of the HIF-1α and HIF-1β subunits is required, suggesting that the heterodimer is functioning as a transcriptional activator, despite the acute nature of the response.

Physiological and pharmacological roles of vascular nucleoside transporters

Adenosine modulates various vascular functions such as vasodilatation and anti-inflammation. The local concentration of adenosine in the vicinity of adenosine receptors is fine tuned by 2 classes of nucleoside transporters: equilibrative nucleoside transporters (ENTs) and concentrative nucleoside transporters (CNTs). In vascular smooth muscle cells, 95% of adenosine transport is mediated by ENT-1 and the rest by ENT-2. In endothelial cells, 60%, 10%, and 30% of adenosine transport are mediated by ENT-1, ENT-2, and CNT-2, respectively. In vitro studies show that glucose per se increases the expression level of ENT-1 via mitogen-activating protein kinase-dependent pathways. Similar results have been demonstrated in diabetic animal models. Hypertension is associated with the increased expression of CNT-2. It has been speculated that the increase in the activities of ENT-1 and CNT-2 may reduce the availability of adenosine to adenosine receptors, thereby weakening the vascular functions of adenosine. This may explain why patients with diabetes and hypertension suffer greater morbidity from ischemia and atherosclerosis. No oral hypoglycemic agents can inhibit ENTs, but an exception is troglitazone (a thiazolidinedione that has been withdrawn from the market). ENTs are also sensitive to dihydropyridine-type calcium-channel blockers, particularly nimodipine, which can inhibit ENT-1 in the nanomolar range. Those calcium-channel blockers are noncompetitive inhibitors of ENTs, probably working through the reversible interactions with allosteric sites. The nonsteroidal anti-inflammatory drug sulindac sulfide is a competitive inhibitor of ENT-1. In addition to their original pharmacological actions, it is believed that the drugs mentioned above may regulate vascular functions through potentiation of the effects of adenosine.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Adora2b-elicited Per2 stabilization promotes a HIF-dependent metabolic switch crucial for myocardial adaptation to ischemia

Adenosine signaling has been implicated in cardiac adaptation to limited oxygen availability. In a wide search for adenosine receptor A2b (Adora2b)-elicited cardioadaptive responses, we identified the circadian rhythm protein period 2 (Per2) as an Adora2b target. Adora2b signaling led to Per2 stabilization during myocardial ischemia, and in this setting, Per2(-/-) mice had larger infarct sizes compared to wild-type mice and loss of the cardioprotection conferred by ischemic preconditioning. Metabolic studies uncovered a limited ability of ischemic hearts in Per2(-/-) mice to use carbohydrates for oxygen-efficient glycolysis. This impairment was caused by a failure to stabilize hypoxia-inducible factor-1α (Hif-1α). Moreover, stabilization of Per2 in the heart by exposing mice to intense light resulted in the transcriptional induction of glycolytic enzymes and Per2-dependent cardioprotection from ischemia. Together, these studies identify adenosine-elicited stabilization of Per2 in the control of HIF-dependent cardiac metabolism and ischemia tolerance and implicate Per2 stabilization as a potential new strategy for treating myocardial ischemia.

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.

A2 adenosine receptors and vascular pathologies

Cardiovascular disease, a leading cause of death and morbidity, is regulated, among various factors, by inflammation. The level of the metabolite adenosine is augmented under stress, including inflammatory, hypoxic, or injurious events. Adenosine has been shown to affect various physiological and pathological processes, largely through 1 or more of its 4 types of receptors: the A1 and A3 adenylyl cyclase inhibitory receptors and the A2A and A2B adenylyl cyclase stimulatory receptors. This article focuses on reviewing common and distinct effects of the 2 A2-type adenosine receptors on vascular disease and the mechanisms involved. Understanding the pathogenesis of vascular disease mediated by these receptors is important to the development of therapeutics and to the prevention and management of disease.

Expression of prolyl hydroxylases (PHDs) is selectively controlled by HIF-1 and HIF-2 proteins in nucleus pulposus cells of the intervertebral disc: distinct roles of PHD2 and PHD3 proteins in controlling HIF-1α activity in hypoxia

Adaptive response to hypoxia in nucleus pulposus cells of the intervertebral disc is regulated by the hypoxia-inducible factors, HIF-1α and HIF-2α. Moreover, oxygen-dependent turnover of HIF-1α in these cells is controlled by the prolyl-4-hydroxylase domain (PHD) family of proteins. Whether HIF homologues control expression of PHDs and whether PHDs control hypoxia-inducible factor (HIF) turnover and/or activity under hypoxia is not known. Here, we show that in nucleus pulposus cells, hypoxia robustly induces PHD3 expression and, to a lesser extent, of PHD2 and PHD1. Reporter analysis shows that the hypoxic induction of the PHD2 promoter is HIF-1α dependent, whereas PHD3 promoter/enhancer activity is dependent on both HIF-1α and HIF-2α. Lentiviral delivery of HIF-1α, ShHIF-1α, and ShHIF-1β confirmed these observations. Noteworthy, HIF-1α maintains basal expression of PHD1 in hypoxia at the posttranscriptional level. Finally, loss of function studies using lentiviral transduction of ShPHDs clearly shows that even at 1% O(2), PHD2 selectively degrades HIF-1α. In contrast, in hypoxia, PHD3 enhances HIF-1α transcriptional activity without affecting protein levels. To correlate these observations with disc disease, a condition characterized by tissue vascularization, we analyzed human tissues. Increased PHD1 mRNA expression but decreased PHD2 and PHD3 expression is observed in degenerate tissues. Interestingly, the hypoxic responsiveness of all the PHDs is maintained in isolated nucleus pulposus cells regardless of the disease state. We propose that PHD2 and PHD3 can be used as a biomarker of tissue oxygenation in the disc and that, as such, it may have important clinical implications.

Adora2b adenosine receptor engagement enhances regulatory T cell abundance during endotoxin-induced pulmonary inflammation

Anti-inflammatory signals play an essential role in constraining the magnitude of an inflammatory response. Extracellular adenosine is a critical tissue-protective factor, limiting the extent of inflammation. Given the potent anti-inflammatory effects of extracellular adenosine, we sought to investigate how extracellular adenosine regulates T cell activation and differentiation. Adenosine receptor activation by a pan adenosine-receptor agonist enhanced the abundance of murine regulatory T cells (Tregs), a cell type critical in constraining inflammation. Gene expression studies in both naïve CD4 T cells and Tregs revealed that these cells expressed multiple adenosine receptors. Based on recent studies implicating the Adora2b in endogenous anti-inflammatory responses during acute inflammation, we used a pharmacologic approach to specifically activate Adora2b. Indeed, these studies revealed robust enhancement of Treg differentiation in wild-type mice, but not in Adora2b^−/− T cells. Finally, when we subjected Adora2b-deficient mice to endotoxin-induced pulmonary inflammation, we found that these mice experienced more severe inflammation, characterized by increased cell recruitment and increased fluid leakage into the airways. Notably, Adora2b-deficient mice failed to induce Tregs after endotoxin-induced inflammation and instead had an enhanced recruitment of pro-inflammatory effector T cells. In total, these data indicate that the Adora2b adenosine receptor serves a potent anti-inflammatory role, functioning at least in part through the enhancement of Tregs, to limit inflammation.

Adenosine A2B receptor blockade slows growth of bladder and breast tumors

The accumulation of high levels of adenosine in tumors activates A(2A) and A(2B) receptors on immune cells and inhibits their ability to suppress tumor growth. Deletion of adenosine A(2A) receptors (A(2A)ARs) has been reported to activate antitumor T cells, stimulate dendritic cell (DC) function, and inhibit angiogenesis. In this study, we evaluated the effects of intermittent intratumor injection of a nonselective adenosine receptor antagonist, aminophylline (AMO; theophylline ethylenediamine) and, for the first time to our knowledge, a selective A(2B)AR antagonist, ATL801. AMO and ATL801 slowed the growth of MB49 bladder and 4T1 breast tumors in syngeneic mice and reduced by 85% metastasizes of breast cancer cells from mammary fat to lung. Based on experiments with A(2A)AR(-/-) or adenosine A(2B) receptor(-/-) mice, the effect of AMO injection was unexpectedly attributed to A(2B)AR and not to A(2A)AR blockade. AMO and ATL801 significantly increased tumor levels of IFN-γ and the IFN-inducible chemokine CXCL10, which is a ligand for CXCR3. This was associated with an increase in activated tumor-infiltrating CXCR3(+) T cells and a decrease in endothelial cell precursors within tumors. Tumor growth inhibition by AMO or ATL801 was eliminated in CXCR3(-/-) mice and RAG1(-/-) mice that lack mature T cells. In RAG1(-/-) mice, A(2B)AR deletion enhanced CD86 expression on CD11b(-) DCs. Bone marrow chimera experiments demonstrated that CXCR3 and A(2B)AR expression on bone marrow cells is required for the antitumor effects of AMO. The data suggest that blockade of A(2B)ARs enhances DC activation and CXCR3-dependent antitumor responses.

Adenosine A2A receptor contributes to the anti-inflammatory effect of the fixed herbal combination STW 5 (Iberogast®) in rat small intestinal preparations

STW 5 (Iberogast®), an established herbal combination, was effective in randomized, double blind clinical studies in functional dyspepsia and irritable bowel syndrome. Since STW 5 was found to influence intestinal motility and has anti-inflammatory properties, this study investigated the expression of adenosine receptors and characterized their role in the control of the anti-inflammatory action of STW 5 and its fresh plant component STW 6 in inflammation-disturbed rat small intestinal preparations. The inflammation was induced by intraluminal instillation of 2,4,6-trinitrobenzene sulfonic acid (TNBS, 0.01 M). The effects of coincubation with selective receptor agonists and antagonists, STW 5, STW 6, or combinations of these compounds on acetylcholine (ACh)-evoked contraction of ileum/jejunum preparations were tested. Adenosine receptor mRNA expression was examined by reverse transcription-polymerase chain reaction (RT-PCR). In untreated preparations, RT-PCR revealed the presence of all adenosine receptor subtypes. Suppressed expression was detected for all subtypes in inflamed tissues, except for A(2B)R mRNA, which was unaffected. STW 5 reversed these effects and enhanced A(2A)R expression above control levels. Radioligand binding assays confirm the affinity of STW 5 to the A(2A)R, and the A(2A)R antagonist was able to prevent the effect of STW 5 on TNBS-induced attenuation of the ACh contraction. Our findings provide evidence that STW 5, but not STW 6 interacts with A(2A)R, which is involved in the anti-inflammatory action of STW 5. STW 6 did not contribute to adenosine A(2A)R-mediated anti-inflammatory effect of STW 5. Other signaling pathways could be involved in the mechanism of action of STW 6.

Adenosine and hypoxia-inducible factor signaling in intestinal injury and recovery

The gastrointestinal mucosa has proven to be an interesting tissue in which to investigate disease-related metabolism. In this review, we outline some of the evidence that implicates hypoxia-mediated adenosine signaling as an important signature within both healthy and diseased mucosa. Studies derived from cultured cell systems, animal models, and human patients have revealed that hypoxia is a significant component of the inflammatory microenvironment. These studies have revealed a prominent role for hypoxia-induced factor (HIF) and hypoxia signaling at several steps along the adenine nucleotide metabolism and adenosine receptor signaling pathways. Likewise, studies to date in animal models of intestinal inflammation have demonstrated an almost uniformly beneficial influence of HIF stabilization on disease outcomes. Ongoing studies to define potential similarities with and differences between innate and adaptive immune responses will continue to teach us important lessons about the complexity of the gastrointestinal tract. Such information has provided new insights into disease pathogenesis and, importantly, will provide insights into new therapeutic targets.

CD73 protects kidney from ischemia-reperfusion injury through reduction of free radicals

Renal ischemia-reperfusion injury (IRI) may cause severe systemic diseases. Extracellular adenosine is anti-inflammatory especially during hypoxemia. As ecto-5'-nucleotidase (CD73) is the rate-limiting enzyme for extracellular adenosine generation, it may protect renal IRI through adenosine production. In the current studies, we investigated the effects of CD73 in genetically modified mice. We found that renal IRI caused more serious histological injury, vascular permeability, and lipid peroxidation in CD73(-/-) than that in CD73(+/+) mice. In addition, AMP and free radical concentrations were much higher in CD73(-/-) than that in CD73(+/+) mice. Our data support the fact that CD73 may protect the kidney from IRI through adenosine production and a reduction of free radicals.

The protective effects of inosine against chemical hypoxia on cultured rat oligodendrocytes

Inosine is a purine nucleoside and is considered protective to neural cells including neurons and astrocytes against hypoxic injury. However, whether oligodendrocytes (OLs) could also be protected from hypoxia by inosine is not known. Here we investigated the effects of inosine on primarily cultured rat OLs injured by rotenone-mediated chemical hypoxia, and the mechanisms of the effects using ATP assay, MTT assay, PI-Hoechst staining, TUNEL, and immunocytochemistry. Results showed that rotenone exposure for 24 h caused cell death and impaired viability in both immature and mature OLs, while pretreatment of 10 mM inosine 30 min before rotenone administration significantly reduced cell death and improved the viability of OLs. The same concentration of inosine given 120 min after rotenone exposure also improved viability of injured mature OLs. Immunocytochemistry for nitrotyrosine and cellular ATP content examination indicated that inosine may protect OLs by providing ATP and scavenging peroxynitrite for cells. In addition, immature OLs were more susceptible to hypoxia than mature OLs; and at the similar degree of injury, inosine protected immature and mature OLs differently. Quantitative real-time PCR revealed that expression of adenosine receptors was different between these two stages of OLs. These data suggest that inosine protect OLs from hypoxic injury as an antioxidant and ATP provider, and the protective effects of inosine on OLs vary with cell differentiation, possibly due to the adenosine receptors expression profile. As OLs form myelin in the central nervous system, inosine could be used as a promising drug to treat demyelination-involved disorders.

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.

Activation of hypoxia-inducible factor-1 protects airway epithelium against oxidant-induced barrier dysfunction

The respiratory epithelium forms an important barrier against inhaled pollutants and microorganisms, and its barrier function is often compromised during inflammatory airway diseases. Epithelial activation of hypoxia-inducible factor-1 (HIF-1) represents one feature of airway inflammation, but the functional importance of HIF-1 within the respiratory epithelium is largely unknown. Using primary mouse tracheal epithelial (MTE) cells or immortalized human bronchial epithelial cells (16HBE14o-), we evaluated the impact of HIF-1 activation on loss of epithelial barrier function during oxidative stress. Exposure of either 16HBE14o- or MTE cells to H(2)O(2) resulted in significant loss of transepithelial electrical resistance and increased permeability to fluorescein isothiocyanate-dextran (4 kDa), and this was attenuated significantly after prior activation of HIF-1 by preexposure to hypoxia (2% O(2); 6 h) or the hypoxia mimics CoCl(2) or dimethyloxalylglycine (DMOG). Oxidative barrier loss was associated with reduced levels of the tight junction protein occludin and with hyperoxidation of the antioxidant enzyme peroxiredoxin (Prx-SO(2)H), events that were also attenuated by prior activation of HIF-1. Involvement of HIF-1 in these protective effects was confirmed using the pharmacological inhibitor YC-1 and by short-hairpin RNA knockdown of HIF-1α. The protective effects of HIF-1 were associated with induction of sestrin-2, a hypoxia-inducible enzyme known to reduce oxidative stress and minimize Prx hyperoxidation. Together, our results suggest that loss of epithelial barrier integrity by oxidative stress is minimized by activation of HIF-1, in part by induction of sestrin-2.

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.

Immunological alterations mediated by adenosine during host-microbial interactions

Adenosine accumulates in inflammation and ischemia but it is more than an end-product of ATP catabolism. Signaling through different receptors with distinct, cell-specific cytoplasmic pathways, adenosine is now recognized as an inducible switch that regulates the immune system. By acting through the A(2A)AR, adenosine shapes T cell function, largely by conferring an anti-inflammatory tone on effector Th cells (Teff) and natural killer (NK)T cells. In contrast, both the A(2A)AR and A(2B)AR are expressed by antigen-presenting cells (APC) which have been shown to regulate innate responses and the transition to adaptive immunity. There is also emerging evidence that adenosine production is one mechanism that allows some pathogens as well as neoplasms to evade host defenses. This review discusses the immunoregulatory functions of adenosine and some of the interactions it may have in regulating host-microbial interactions.

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.

Interactions between nitric oxide and hypoxia-inducible factor signaling pathways in inflammatory disease

Induction and activation of nitric oxide (NO) synthases (NOS) and excessive production of NO are common features of almost all diseases associated with infection and acute or chronic inflammation, although the contribution of NO to the pathophysiology of these diseases is highly multifactorial and often still a matter of controversy. Because of its direct impact on tissue oxygenation and cellular oxygen (O(2)) consumption and re-distribution, the ability of NO to regulate various aspects of hypoxia-induced signaling has received widespread attention. Conditions of tissue hypoxia and the activation of hypoxia-inducible factors (HIF) have been implicated in hypoxia or in cancer biology, but are also being increasingly recognized as important features of acute and chronic inflammation. Thus, the activation of HIF transcription factors has been increasingly implicated in inflammatory diseases, and recent studies have indicated its critical importance in regulating phagocyte function, inflammatory mediator production, and regulation of epithelial integrity and repair processes. Finally, HIF also appears to contribute to important features of tissue fibrosis and epithelial-to-mesenchymal transition, processes that are associated with tissue remodeling in various non-malignant chronic inflammatory disorders. In this review, we briefly summarize the current state of knowledge with respect to the general mechanisms involved in HIF regulation and the impact of NO on HIF activation. Secondly, we will summarize the major recent findings demonstrating a role for HIF signaling in infection, inflammation, and tissue repair and remodeling, and will address the involvement of NO. The growing interest in hypoxia-induced signaling and its relation with NO biology is expected to lead to further insights into the complex roles of NO in acute or chronic inflammatory diseases and may point to the importance of HIF signaling as key feature of NO-mediated events during these disorders.

Nucleoside/nucleobase transporters: drug targets of the future?

BACKGROUND

Nucleoside/nucleobase transporters have been investigated since the 1960s. In particular, equilibrative nucleoside transporters were thought to be valuable drug targets, since they are involved in various kinds of viral and parasitic diseases as well as cancers.

DISCUSSION

In the postgenomic era multiple transporters, including different subtypes, have been cloned and characterized on the molecular level. In this article we summarize recent advances regarding structure, function and localization of nucleoside/nucleobase transporters as well as the pharmacological profile of selected drugs.

CONCLUSION

Knowledge of the different kinetic properties and structural features of nucleoside transporters can either be used for the rational design of therapeutics directly targeting the transporter itself or for the delivery of drugs using the transporter as a port of entry into the target cell. Equilibrative nucleoside transporters are of considerable pharmacological interest as drug targets for the development of drugs tailored to each patient's need for the treatment of cardiac disease, cancer and viral infections.

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 four cysteine residues in the second extracellular loop of the human adenosine A2B receptor: role in ligand binding and receptor function

The adenosine A(2B) receptor is of considerable interest as a new drug target for the treatment of asthma, inflammatory diseases, pain, and cancer. In the present study we investigated the role of the cysteine residues in the extracellular loop 2 (ECL2) of the receptor, which is particularly cysteine-rich, by a combination of mutagenesis, molecular modeling, chemical and pharmacological experiments. Pretreatment of CHO cells recombinantly expressing the human A(2B) receptor with dithiothreitol led to a 74-fold increase in the EC(50) value of the agonist NECA in cyclic AMP accumulation. In the C78(3.25)S and the C171(45.50)S mutant high-affinity binding of the A(2B) antagonist radioligand [(3)H]PSB-603 was abolished and agonists were virtually inactive in cAMP assays. This indicates that the C3.25-C45.50 disulfide bond, which is highly conserved in GPCRs, is also important for binding and function of A(2B) receptors. In contrast, the C166(45.45)S and the C167(45.46)S mutant as well as the C166(45.45)S-C167(45.46)S double mutant behaved like the wild-type receptor, while in the C154(45.33)S mutant significant, although more subtle effects on cAMP accumulation were observed - decrease (BAY60-6583) or increase (NECA) - depending on the structure of the investigated agonist. In contrast to the X-ray structure of the closely related A(2A) receptor, which showed four disulfide bonds, the present data indicate that in the A(2B) receptor only the C3.25-C45.50 disulfide bond is essential for ligand binding and receptor activation. Thus, the cysteine residues in the ECL2 of the A(2B) receptor not involved in stabilization of the receptor structure may have other functions.

Hypoxia and metabolic factors that influence inflammatory bowel disease pathogenesis

The gastrointestinal epithelium is anatomically positioned to provide a selective barrier between the anaerobic lumen and lamina propria, which has a high rate of metabolism. Supported by a complex vasculature, this important barrier is affected by reduced blood flow and resultant tissue hypoxia, particularly during the severe metabolic shifts associated with active inflammation in individuals with inflammatory bowel disease. Activation of hypoxia-inducible factor (HIF) under these conditions promotes resolution of inflammation in mouse models of disease. Protective influences of HIF are attributed, in part, to the complex regulation of barrier protection with the intestinal mucosa. Reagents that activate HIF, via inhibition of the prolyl hydroxylase enzymes, might be developed to induce hypoxia-mediated resolution in patients with intestinal mucosal inflammatory disease.

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.