Oxygenation inhibits the physiological tissue-protecting mechanism and thereby exacerbates acute inflammatory lung injury

Therapeutic anti-inflammatory effects of myeloid cell adenosine receptor A2a stimulation in lipopolysaccharide-induced lung injury

To determine the role of the adenosine receptor A2a in a murine model of LPS-induced lung injury, migration of polymorphonuclear leukocytes (PMNs) into the different compartments of the lung was determined by flow cytometry, microvascular permeability was assessed by the extravasation of Evans blue, and the release of chemotactic cytokines into the alveolar airspace was determined by ELISA. Measurements were performed in wild-type and A2a gene-deficient mice (A2a(-/-)). To differentiate the role of A2a on hemopoietic and nonhemopoietic cells, we created chimeric mice by transfer of bone marrow (BM) between wild-type and A2a(-/-) mice and used mice that lacked A2a expression selectively on myeloid cells (A2a(flox/flox) x LysM-cre). A specific A2a receptor agonist (ATL202) was used to evaluate its potential to reduce lung injury in vivo. In wild-type mice, therapeutic treatment with ATL202 reduced LPS-induced PMN recruitment, and release of cytokines. Pretreatment, but not posttreatment, also reduced Evans blue extravasation. In the BM chimeric mice lacking A2a on BM-derived cells, PMN migration into the alveolar space was increased by approximately 50%. These findings were confirmed in A2a(flox/flox) x LysM-cre mice. ATL202 was only effective when A2a was present on BM-derived cells. A2a agonists may be effective at curbing inflammatory lung tissue damage.

Hypoxia-dependent anti-inflammatory pathways in protection of cancerous tissues

The evolutionarily selected tissue-protecting mechanisms are likely to be triggered by an event of universal significance for all surrounding cells. Such an event could be damage to blood vessels, which would result in local tissue hypoxia. It is now recognized that tissue hypoxia can initiate the tissue-protecting mechanism mediated by at least two different biochemical pathways. The central message of this review is that tumor cells are protected from immune damage in hypoxic and immunosuppressive tumor microenvironments due to the inactivation of anti-tumor T cells by the combined action of these two hypoxia-driven mechanisms. Firstly, tumor hypoxia-produced extracellular adenosine inhibits anti-tumor T cells via their G(s)-protein-coupled and cAMP-elevating A2A and A2B adenosine receptors (A2AR/A2BR). Levels of extracellular adenosine are increased in tumor microenvironments due to the changes in activities of enzymes involved in adenosine metabolism. Secondly, TCR-activated and/or tumor hypoxia-exposed anti-tumor T cells may be inhibited in tumor microenvironments by Hypoxia-inducible Factor 1alpha (HIF-1alpha) Hence, HIF-1alpha activity in T cells may contribute to the tumor-protecting immunosuppressive effects of tumor hypoxia. Here, we summarize the data that support the view that protection of hypoxic cancerous tissues from anti-tumor T cells is mediated by the same mechanism that protects normal tissues from the excessive collateral damage by overactive immune cells during acute inflammation.

Targeted deletion of HIF-1alpha gene in T cells prevents their inhibition in hypoxic inflamed tissues and improves septic mice survival

BACKGROUND

Sepsis patients may die either from an overwhelming systemic immune response and/or from an immunoparalysis-associated lack of anti-bacterial immune defence. We hypothesized that bacterial superantigen-activated T cells may be prevented from contribution into anti-bacterial response due to the inhibition of their effector functions by the hypoxia inducible transcription factor (HIF-1alpha) in inflamed and hypoxic areas.

METHODOLOGY/PRINCIPAL FINDINGS

Using the Cre-lox-P-system we generated mice with a T-cell targeted deletion of the HIF-1alpha gene and analysed them in an in vivo model of bacterial sepsis. We show that deletion of the HIF-1alpha gene leads to higher levels of pro-inflammatory cytokines, stronger anti-bacterial effects and much better survival of mice. These effects can be at least partially explained by significantly increased NF-kappaB activation in TCR activated HIF-1 alpha deficient T cells.

CONCLUSIONS/SIGNIFICANCE

T cells can be recruited to powerfully contribute to anti-bacterial response if they are relieved from inhibition by HIF-1alpha in inflamed and hypoxic areas. Our experiments uncovered the before unappreciated reserve of anti-bacterial capacity of T cells and suggest novel therapeutic anti-pathogen strategies based on targeted deletion or inhibition of HIF-1 alpha in T cells.

Contribution of E-NTPDase1 (CD39) to renal protection from ischemia-reperfusion injury

Previous studies showed increased extracellular nucleotides during renal ischemia-reperfusion. While nucleotides represent the main source for extracellular adenosine and adenosine signaling contributes to renal protection from ischemia, we hypothesized a role for ecto-nucleoside-triphosphate-diphosphohydrolases (E-NTPDases) in renal protection. We used a model of murine ischemia-reperfusion and in situ ischemic preconditioning (IP) via a hanging weight system for atraumatic renal artery occlusion. Initial studies with a nonspecific inhibitor of E-NTPDases (POM-1) revealed inhibition of renal protection by IP. We next pursued transcriptional responses of E-NTPDases (E-NTPDase1-3, and 8) to renal IP, and found a robust and selective induction of E-NTPDase1/CD39 transcript and protein. Moreover, based on clearance studies, plasma electrolytes, and renal tubular histology, IP protection was abolished in gene-targeted mice for cd39 whereas increased renal adenosine content with IP was attenuated. Furthermore, administration of apyrase reconstituted renal protection by IP in cd39-/- mice. Finally, apyrase treatment of wild-type mice resulted in increased renal adenosine concentrations and a similar degree of renal protection from ischemia as IP treatment. Taken together, these data identify CD39-dependent nucleotide phosphohydrolysis in renal protection. Moreover, the present studies suggest apyrase treatment as a novel pharmacological approach to renal diseases precipitated by limited oxygen availability.

Aspects of the general biology of adenosine A2A signaling

Many of our current hopes of finding better ways to treat Parkinson's disease or to stop its progression rely on studies of adenosine A2A receptors in the brain. Yet any drug targeting central receptors will also potentially affect receptors in other sites. Furthermore, several fundamental aspects of adenosine receptor biology must be taken into account. For these reasons the "Targeting adenosine A2A receptors in Parkinson's disease and other CNS disorders" meeting in Boston included selected aspects of the general biology of adenosine A2A receptor signaling. Some of the presentations from this part of the meeting are summarized in this first chapter. As will be apparent to the reader, these different parts do not form an integrated whole, but they do indicate areas the organizers felt might illuminate remaining questions regarding the roles of adenosine A2A receptors. The contributors to this part of the meeting have summarized some of the key questions below.

Requirements for T lymphocyte migration in explanted lymph nodes

Although the requirements for T lymphocyte homing to lymph nodes (LNs) are well studied, much less is known about the requirements for T lymphocyte locomotion within LNs. Imaging of murine T lymphocyte migration in explanted LNs using two-photon laser-scanning fluorescence microscopy provides an opportunity to systematically study these requirements. We have developed a closed system for imaging an intact LN with controlled temperature, oxygenation, and perfusion rate. Naive T lymphocyte locomotion in the deep paracortex of the LN required a perfusion rate of >13 microm/s and a partial pressure of O(2) (pO(2)) of >7.4%. Naive T lymphocyte locomotion in the subcapsular region was 38% slower and had higher turning angles and arrest coefficients than naive T lymphocytes in the deep paracortex. T lymphocyte activation decreased the requirement for pO(2), but also decreased the speed of locomotion in the deep paracortex. Although CCR7(-/-) naive T cells displayed a small reduction in locomotion, systemic treatment with pertussis toxin reduced naive T lymphocyte speed by 59%, indicating a contribution of Galpha(i)-mediated signaling, but involvement of other G protein-coupled receptors besides CCR7. Receptor knockouts or pharmacological inhibition in the adenosine, PG/lipoxygenase, lysophosphatidylcholine, and sphingosine-1-phosphate pathways did not individually alter naive T cell migration. These data implicate pO(2), tissue architecture, and G-protein coupled receptor signaling in regulation of naive T lymphocyte migration in explanted LNs.

Identification of ectonucleotidases CD39 and CD73 in innate protection during acute lung injury

Acute lung injury (ALI), such as that which occurs with mechanical ventilation, contributes to morbidity and mortality of critical illness. Nonetheless, in many instances, ALI resolves spontaneously through unknown mechanisms. Therefore, we hypothesized the presence of innate adaptive pathways to protect the lungs during mechanical ventilation. In this study, we used ventilator-induced lung injury as a model to identify endogenous mechanisms of lung protection. Initial in vitro studies revealed that supernatants from stretch-induced injury contained a stable factor which diminished endothelial leakage. This factor was subsequently identified as adenosine. Additional studies in vivo revealed prominent increases in pulmonary adenosine levels with mechanical ventilation. Because ectoapyrase (CD39) and ecto-5'-nucleotidase (CD73) are rate limiting for extracellular adenosine generation, we examined their contribution to ALI. In fact, both pulmonary CD39 and CD73 are induced by mechanical ventilation. Moreover, we observed pressure- and time-dependent increases in pulmonary edema and inflammation in ventilated cd39(-/-) mice. Similarly, pharmacological inhibition or targeted gene deletion of cd73 was associated with increased symptom severity of ventilator-induced ALI. Reconstitution of cd39(-/-) or cd73(-/-) mice with soluble apyrase or 5'-nucleotidase, respectively, reversed such increases. In addition, ALI was significantly attenuated and survival improved after i.p. treatment of wild-type mice with soluble apyrase or 5'-nucleotidase. Taken together, these data reveal a previously unrecognized role for CD39 and CD73 in lung protection and suggest treatment with their soluble compounds as a therapeutic strategy for noninfectious ALI.

A1 adenosine receptors play an essential role in protecting the embryo against hypoxia

Embryos can be exposed to environmental factors that induce hypoxia. Currently, our understanding of the effects of hypoxia on early mammalian development is modest. Potential mediators of hypoxia action include the nucleoside adenosine, which acts through A(1) adenosine receptors (A(1)ARs) and mediates adverse effects of hypoxia on the neonatal brain. We hypothesized that A(1)ARs may also play a role in mediating effects of hypoxia on the embryo. When pregnant dams were exposed to hypoxia (10% O(2)) beginning at embryonic day (E) 7.5 or 8.5 and continued for 24-96 h, A(1)AR+/+ embryos manifested growth inhibition and a disproportionate reduction in heart size, including thinner ventricular walls. Yet, when dams were exposed to hypoxia, embryos lacking A(1)ARs (A(1)AR-/-) had much more severe growth retardation than A(1)AR+/+ or +/- embryos. When levels of hypoxia-inducible factor 1alpha (HIF1alpha) were examined, A(1)AR-/- embryos had less stabilized HIF1alpha protein than A(1)AR+/- littermates. Normal patterns of cardiac gene expression were also disturbed in A(1)AR-/- embryos exposed to hypoxia. These results show that short periods of hypoxia during early embryogenesis can result in intrauterine growth retardation. We identify adenosine and A(1)ARs as playing an essential role in protecting the embryo from hypoxia.

Increased hyperoxia-induced mortality and acute lung injury in IL-13 null mice

IL-13 is a critical effector at sites of Th2 inflammation and remodeling. As a result, anti-IL-13-based therapies are being actively developed to treat a variety of diseases and disorders. However, the beneficial effects of endogenous IL-13 in the normal and diseased lung have not been adequately defined. We hypothesized that endogenous IL-13 is an important regulator of oxidant-induced lung injury and inflammation. To test this hypothesis, we compared the effects of 100% O(2) in mice with wild-type and null IL-13 loci. In this study, we demonstrate that hyperoxia significantly augments the expression of the components of the IL-13R, IL-13Ralpha1, and IL-4Ralpha. We also demonstrate that, in the absence of IL-13, hyperoxia-induced tissue inflammation is decreased. In contrast, in the IL-13 null mice, DNA injury, cell death, caspase expression, and activation and mortality are augmented. Interestingly, the levels of the cytoprotective cytokines vascular endothelial cell growth factor, IL-6, and IL-11 were decreased in the bronchoalveolar lavage fluid. These studies demonstrate that the expression of the IL-13R is augmented and that the endogenous IL-13-IL-13R pathway contributes to the induction of inflammation and the inhibition of injury in hyperoxic acute lung injury.

Adenosine A2areceptor-mediated, normoxic induction of HIF-1 through PKC and PI-3K-dependent pathways in macrophages

Adenosine released by cells in injurious or hypoxic environments has tissue-protecting and anti-inflammatory effects, which are also a result of modulation of macrophage functions, such as vascular endothelial growth factor (VEGF) production. As VEGF is a well-known target of hypoxia-inducible factor 1 (HIF-1), we hypothesized that adenosine may activate HIF-1 directly. Our studies using subtype-specific adenosine receptor agonists and antagonists showed that by activating the A(2A) receptor, adenosine treatment induced HIF-1 DNA-binding activity, nuclear accumulation, and transactivation capacity in J774A.1 mouse macrophages. Increased HIF-1 levels were also found in adenosine-treated mouse peritoneal macrophages. The HIF-1 activation induced by the A(2A) receptor-specific agonist CGS21680 required the PI-3K and protein kinase C pathways but was not mediated by changes in iron levels. Investigation of the molecular basis of HIF-1 activation revealed the involvement of transcriptional and to a larger extent, translational mechanisms. HIF-1 induction triggered the expression of HIF-1 target genes involved in cell survival (aldolase, phosphoglycerate kinase) and VEGF but did not induce inflammation-related genes regulated by HIF-1, such as TNF-alpha or CXCR4. Our results show that the formation of adenosine and induction of HIF-1, two events which occur in response to hypoxia, are linked directly and suggest that HIF-1 activation through A(2A) receptors may contribute to the anti-inflammatory and tissue-protecting activity of adenosine.

VEGF-induced heme oxygenase-1 confers cytoprotection from lethal hyperoxia in vivo

Prolonged exposure to hyperoxia results in hyperoxic acute lung injury (HALI). Vascular endothelial growth factor (VEGF) has been shown to have cytoprotective effects and prolong survival in an in vivo model of HALI. Heme oxygenase-1 (HO-1) has protective effects in hyperoxia; therefore, we hypothesized that induction of HO-1 would be an important contributor to VEGF-induced cytoprotection. Using inducible, lung-specific VEGF overexpressing transgenic mice, we demonstrated that VEGF is a potent inducer of HO-1 mRNA and protein in mouse lung. To evaluate the effect of inhibition of HO-1 on injury, VEGF transgenic mice were treated with HO-1 short interfering RNA (HO-1 siRNA) and exposed to hyperoxia. Total lung lavage protein concentration, TUNEL staining, lipid peroxidation, and wet-to-dry ratio were all increased, consistent with increased injury. These findings were corroborated by survival studies in which inhibition of HO-1 function using tin-protoporphryin or silencing of HO-1 with lentiviral HO-1 short hairpin RNA (ShRNA) significantly decreased survival in hyperoxia. We conclude from these data that VEGF-induced HO-1 is an important contributor to cytoprotection in this in vivo model of acute lung injury and that alterations in VEGF function in the lung are likely to be important determinants of the outcome of acute lung injury.

Adenosine, an endogenous distress signal, modulates tissue damage and repair

Adenosine is formed inside cells or on their surface, mostly by breakdown of adenine nucleotides. The formation of adenosine increases in different conditions of stress and distress. Adenosine acts on four G-protein coupled receptors: two of them, A(1) and A(3), are primarily coupled to G(i) family G proteins; and two of them, A(2A) and A(2B), are mostly coupled to G(s) like G proteins. These receptors are antagonized by xanthines including caffeine. Via these receptors it affects many cells and organs, usually having a cytoprotective function. Joel Linden recently grouped these protective effects into four general modes of action: increased oxygen supply/demand ratio, preconditioning, anti-inflammatory effects and stimulation of angiogenesis. This review will briefly summarize what is known and what is not in this regard. It is argued that drugs targeting adenosine receptors might be useful adjuncts in many therapeutic approaches.

Physiological roles of vascular nucleoside transporters

Nucleoside transporters (NTs) comprise 2 widely expressed families, the equilibrative nucleoside transporters (diffusion-limited channels) and concentrative nucleoside transporters (sodium-dependent transporters). Because of their anatomic position at the blood-tissue interface, vascular NTs are in an ideal position to influence vascular nucleoside levels, particularly adenosine, which among others plays an important role in tissue protection during acute injury. For example, endothelial NTs contribute to preserving the vascular integrity during conditions of limited oxygen availability (hypoxia). Indeed, hypoxia-inducible factor-1-dependent repression of NTs results in enhanced extracellular adenosine signaling and thus attenuates hypoxia-associated increases in vascular leakage. In addition, vascular NTs also contribute to cardiac ischemic preconditioning, coronary vasodilation, and inhibition of platelet aggregation. Moreover, vascular nucleoside uptake via NTs is important for nucleoside recovery, particularly in cells lacking de novo nucleotide synthesis pathways (erythrocytes, leukocytes). Taken together, vascular NTs are critical in modulating adenosine-mediated responses during conditions such as inflammation or hypoxia.

Effects of adenosine on adhesion molecule expression and cytokine production in human PBMC depend on the receptor subtype activated

BACKGROUND AND PURPOSE

Adenosine suppresses immune responses through adenosine(2A) (A(2A)) receptors, by raising intracellular cAMP. Interleukin (IL)-18 up-regulates the expression of intercellular adhesion molecule (ICAM)-1 on monocytes, leading to production of pro-inflammatory cytokines such as IL-12, interferon (IFN)-gamma and tumor necrosis factor (TNF)-alpha by human peripheral blood mononuclear cells (PBMC). We have previously demonstrated that elevation of cAMP inhibits this IL-18-induced expression of adhesion molecules. In the present study, we examined the effect of adenosine on the IL-18-induced up-regulation of ICAM-1 on human monocytes and production of IL-12, IFN-gamma and TNF-alpha by PBMC.

EXPERIMENTAL APPROACH

The expression of ICAM-1 was examined by flow cytometry. IL-12, IFN-gamma and TNF-alpha were determined by ELISA assay.

KEY RESULTS

Adenosine inhibited the IL-18-induced up-regulation of ICAM-1 on human monocytes and it abolished the IL-18-enhanced production of IL-12, IFN-gamma and TNF-alpha. While an A(2A) receptor antagonist reversed the action of adenosine, an A(1) or A(3) receptor antagonist enhanced them. An A(2A) receptor agonist, CGS21680, mimicked the effects of adenosine and its effects were abolished not only by the A(2A) receptor antagonist but also by A(1) or A(3) receptor agonists. Activation via A(2A) receptors resulted in elevation of cAMP in monocytes, whereas the stimulation of A(1) or A(3) receptors inhibited it, suggesting that intracellular signal transduction following ligation of A(2A) receptors might be blocked by activation of A(1) or A(3) receptors.

CONCLUSIONS AND IMPLICATIONS

Adenosine differentially regulates IL-18-induced adhesion molecule expression and cytokine production through several subtypes of its receptors.

Cutting edge: hypoxia-inducible factor 1alpha and its activation-inducible short isoform I.1 negatively regulate functions of CD4+ and CD8+ T lymphocytes

To evaluate the role of hypoxia-inducible factor 1alpha (HIF-1alpha) and its TCR activation-inducible short isoform I.1 in T cell functions, we genetically engineered unique mice with: 1) knockout of I.1 isoform of HIF-1alpha; 2) T cell-targeted HIF-1alpha knockdown; and 3) chimeric mice with HIF-1alpha gene deletion in T and B lymphocytes. In all three types of mice, the HIF-1alpha-deficient T lymphocytes, which were TCR-activated in vitro, produced more proinflammatory cytokines compared with HIF-1alpha-expressing control T cells. Surprisingly, deletion of the I.1 isoform, which represents < 30% of total HIF-1alpha mRNA in activated T cells, was sufficient to markedly enhance TCR-triggered cytokine secretion. These data suggest that HIF-1alpha not only plays a critical role in oxygen homeostasis but also may serve as a negative regulator of T cells.

Protective role of ecto-5'-nucleotidase (CD73) in renal ischemia

Acute renal failure from ischemia significantly contributes to cardiovascular morbidity and mortality. Extracellular adenosine has been implicated as an anti-inflammatory metabolite particularly during conditions of limited oxygen availability (e.g., ischemia). Because ecto-5'-nucleotidase (CD73) is rate limiting for extracellular adenosine generation, this study examined the contribution of CD73-dependent adenosine production to ischemic preconditioning (IP) of the kidneys. After the initial observation that murine CD73 transcript, protein, and function are induced by renal IP, its role in IP-mediated kidney protection was studied. In fact, increases in renal adenosine concentration with IP are attenuated in cd73(-/-) mice. Moreover, pharmacologic inhibition of CD73 or its targeted gene deletion abolished renal protection by IP as measured by clearance studies, plasma electrolytes, and renal tubular destruction, and reconstitution of cd73(-/-) mice with soluble 5'-nucleotidase resulted in complete restoration of renal protection by IP. Finally, renal injury after ischemia was attenuated by intraperitoneal treatment of wild-type mice with soluble 5'-nucleotidase to a similar degree as by IP. Taken together, these data reveal what is believed to be a previously unrecognized role of CD73 in renal protection from ischemia and suggest treatment with soluble 5'-nucleotidase as a novel therapeutic approach in the treatment of renal diseases that are precipitated by limited oxygen availability.

From "Hellstrom Paradox" to anti-adenosinergic cancer immunotherapy

Cancer therapy by endogenous or adoptively transferred anti-tumor T cells is considered complementary to conventional cancer treatment by surgery, radiotherapy or chemotherapy. However, the scope of promising immunotherapeutic protocols is currently limited because tumors can create a ‘hostile–immunosuppressive microenvironment that prevents their destruction by anti-tumor T cells. There is a possibility to develop better and more effective immunotherapies by inactivating mechanisms that inhibit anti-tumor T cells in the tumor microenvironment and thereby protect cancerous tissues from immune damage. This may be now possible because of the recent demonstration that genetic deletion of immunosuppressive A2A and A2B adenosine receptors (A2AR and A2BR) or their pharmacological inactivation can prevent the inhibition of anti-tumor T cells by the hypoxic tumor microenvironment and as a result facilitate full tumor rejection [Ohta A, Gorelik E, Prasad SJ et al (2006) Proc Natl Acad Sci USA 103(35):13132–3137]. This approach is based on in vivo genetic evidence that A2AR play a critical role in the protection of normal tissues from overactive immune cells in acutely inflamed and hypoxic areas. The observations of much improved T-cell-mediated rejection of tumors in mice with inactivated A2AR strongly suggest that A2AR also protects hypoxic cancerous tissues and that A2AR should be inactivated in order to improve tumor rejection by anti-tumor T cells.

EFFECTS OF ADENOSINE ON FUNCTIONS OF POLYMORPHONUCLEAR LEUKOCYTES FROM PATIENTS WITH SEPTIC SHOCK

Inasmuch as polymorphonuclear leukocytes (PMNs) play a major role in antibacterial defense but can also cause substantial tissue injury, drugs are needed which are able to attenuate tissue-toxic PMN reactions without inhibiting bactericidal mechanisms. Adenosine as a retaliatory metabolite is produced in response to metabolically unfavorable conditions like inflammation. However, it is not known whether adenosine can selectively downregulate adverse PMN reactions in sepsis. In this prospective clinical study, we characterized the effects of adenosine ex vivo on PMN functions in patients with septic shock ([SS] n = 33) and healthy volunteers ([HV] n = 33). The PMNs were primed by tumor necrosis factor-alpha (TNF-alpha) and subsequently stimulated with N-formyl methionyl-leucyl-phenylalanine (fMLP) to test for the formation of hydrogen peroxide (H2O2) in response to soluble inflammatory stimuli. The PMNs were also challenged by opsonized zymosan particles to assess adhesion, phagocytosis, and the associated H2O2 production. As compared with HV, PMNs from SS patients showed strongly enhanced tissue-toxic H2O2 production elicited by TNF-alpha/fMLP. Increasing concentrations of adenosine dose-dependently reduced this tissue-toxic H2O2 production in both groups with a half-maximal inhibitory concentration of 25 nmol/L and 114 nmol/L in HV and SS patients, respectively. This 4.6-fold decrease in the adenosine-mediated inhibition of PMNs from patients with septic shock was compensated by a 3-fold increase in the plasma concentrations of the nucleoside (HV, 42.5 +/- 2.9 nmol/L vs. SS, 125.6 +/- 18.2 nmol/L; mean +/- SEM). When the effects of adenosine were tested at a very high A2A receptor saturating concentration of 10 mol/L, neither adhesion, phagocytosis, nor the associated H2O2 production induced by opsonized zymosan was affected in both groups. These results were confirmed by the highly selective A2A agonist, CGS21680.Thus, adenosine or A2A agonists may be useful to selectively inhibit the potentially tissue-toxic H2O2 production elicited by soluble inflammatory mediators in patients with septic shock.

Endothelial STAT3 is essential for the protective effects of HO-1 in oxidant-induced lung injury

Administering high levels of inspired oxygen, or hyperoxia, is commonly used as a life-sustaining measure in critically ill patients. Unfortunately, the oxidant stress generated by prolonged hyperoxia can lead to respiratory failure, multiorgan failure, and death. Although the endothelial cell is known to be a target for hyperoxia-induced injury, its precise role is unclear. Heme oxygenase-1 (HO-1) and "signal transducer and activator of transcription 3" (STAT3) have been found to confer protection against endothelial cell injury. We sought to elucidate the specific roles of HO-1 and STAT3 in hyperoxic lung and endothelial cell injury. Mice or murine lung endothelial cells (MLEC) administered HO-1 siRNA exhibited marked injury and death compared with nonspecific siRNA. Overexpression of either HO-1 or STAT3 confers protection. However, HO-1 and its reaction product carbon monoxide (CO) lose their protective effects in the presence of STAT3 siRNA in MLEC or in endothelial-specific, STAT3-deficient mice. STAT3 overexpression is able to partially rescue HO-1-deficient MLEC from hyperoxia-induced cell death. Our results demonstrate 1) the importance of the endothelium in lethal hyperoxic injury, 2) HO-1 and CO require endothelial STAT3 for their protective effects, and 3) STAT3 confers endothelial cell protection via both HO-1-dependent and independent mechanisms.

Protecting motor neurons from toxic insult by antagonism of adenosine A2a and Trk receptors

The death of motor neurons in amyotrophic lateral sclerosis (ALS) is thought to result from the interaction of a variety of factors including excitotoxicity, accumulation of toxic proteins, and abnormal axonal transport. Previously, we found that the susceptibility of motor neurons to excitotoxic insults can be limited by inhibiting signals evoked by brain-derived neurotrophic factor (BDNF) activation of the receptor tyrosine kinase B (TrkB). Here we show that this can be achieved by direct kinase inhibition or by blockade of a transactivation pathway that uses adenosine A2a receptors and src-family kinases (SFKs). Downstream signaling cascades (such as mitogen-activated protein kinase and phosphatidylinositol-3 kinase) are inhibited by these blockers. In addition to protecting motor neurons from excitotoxic insult, these agents also prevent toxicity that follows from the expression of mutant proteins (G85R superoxide dismutase 1; G59S p150(glued)) that cause familial motor neuron disease. TrkB, adenosine A2a receptors, and SFKs associate into complexes in lipid raft and nonlipid raft membranes and the signaling from lipids rafts may be particularly important because their disruption by cholesterol depletion blocks the ability of BDNF to render motor neurons vulnerable to insult. The neuroprotective versatility of Trk antagonism suggests that it may have broad utility in the treatment of ALS patients.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Adenosine in the airways: implications and applications

Adenosine in a signaling nucleoside eliciting many physiological responses. Elevated levels of adenosine have been found in bronchoalveolar lavage, blood and exhaled breath condensate of patients with asthma a condition characterized by chronic airway inflammation. In addition, inhaled adenosine-5'-monophosphate induces bronchoconstriction in asthmatics but not in normal subjects. Studies on animals and humans have shown that bronchoconstriction is most likely due to the release of inflammatory mediators from mast cells. However a number of evidences suggest that adenosine modulates the function of many other cells involved in airway inflammation such as neutrophils, eosinophils, lymphocytes and macrophages. Although this clear pro-inflammatory role in the airways, adenosine may activate also protective mechanisms particularly against lung injury. For many years this dual role of adenosine in the respiratory system has represented an enigma, and only recently it has become clear that biological functions of adenosine are mediated by four distinct subtypes of receptors (A1, A2A, A2B, and A3) and that biological responses are determined by the different pattern of receptors distribution in specific cells. Therefore, pharmacological modulation of adenosine receptors, particularly A2B, may represent a novel therapeutic approach for inflammatory diseases. Moreover, as bronchial response to adenosine strictly reflects airway inflammation in asthma, bronchial challenge with adenosine is considered a valuable clinical tool to monitor airway inflammation, to follow the response to anti-inflammatory treatments and to help in the diagnostic discrimination between asthma and chronic obstructive lung disease.

Regulation of immune cells by local-tissue oxygen tension: HIF1 alpha and adenosine receptors

Immune cells are often exposed to low oxygen tensions, which markedly affect cellular metabolism. We describe how activated T cells adapt to the changing energy supplies in hypoxic areas of inflamed tissues by using hypoxia-inducible factor 1 (HIF1) to switch to glycolysis as the main source of energy and by signalling through extracellular-adenosine receptors. This hypoxic regulation might alter the balance between T helper 1 cells and T helper 2 cells and might alter the activities of cells of the innate immune system, thereby qualitatively and quantitatively affecting immune responses. This regulatory mechanism should be taken into account in the design and interpretation of in vitro and in vivo studies of immune-cell effector functions.

Hooray for hypoxia?

The mortality of critically ill patients rises steadily as the partial pressure of arterial oxygen falls below about 11 kPa (80 mm Hg). A new animal study in the May 2005 issue of PLoS Biology showing a potential benefit for hypoxia is thus a challenge to current thinking.