Protective effect of purinergic agonist ATPgammaS against acute lung injury

Erythronecroptosis: an overview of necroptosis or programmed necrosis in red blood cells

Necroptosis is considered a programmed necrosis that requires receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3), and pore-forming mixed lineage kinase domain-like protein (MLKL) to trigger a regulated cell membrane lysis. Membrane rupture in necroptosis has been shown to fuel innate immune response due to release of damage-associated molecular patterns (DAMPs). Recently published studies indicate that mature erythrocytes can undergo necroptosis as well. In this review, we provide an outline of multiple cell death modes occurring in erythrocytes, discuss possible immunological aspects of diverse erythrocyte cell deaths, summarize available evidence related to the ability of erythrocytes to undergo necroptosis, outline key involved molecular mechanisms, and discuss the potential implication of erythrocyte necroptosis in the physiology and pathophysiology. Furthermore, we aim to highlight the interplay between necroptosis and eryptosis signaling in erythrocytes, emphasizing specific characteristics of these pathways distinct from their counterparts in nucleated cells. Thus, our review provides a comprehensive summary of the current knowledge of necroptosis in erythrocytes. To reflect critical differences between necroptosis of nucleated cells and necroptosis of erythrocytes, we suggest a term erythronecroptosis for necroptosis of enucleated cells.

Generation and Export of Red Blood Cell ATP in Health and Disease

Metabolic homeostasis in animals depends critically on evolved mechanisms by which red blood cell (RBC) hemoglobin (Hb) senses oxygen (O₂) need and responds accordingly. The entwined regulation of ATP production and antioxidant systems within the RBC also exploits Hb-based O₂-sensitivity to respond to various physiologic and pathophysiologic stresses. O₂ offloading, for example, promotes glycolysis in order to generate both 2,3-DPG (a negative allosteric effector of Hb O₂ binding) and ATP. Alternatively, generation of the nicotinamide adenine dinucleotide phosphate (NADPH) critical for reducing systems is favored under the oxidizing conditions of O₂ abundance. Dynamic control of ATP not only ensures the functional activity of ion pumps and cellular flexibility, but also contributes to the availability of vasoregulatory ATP that can be exported when necessary, for example in hypoxia or upon RBC deformation in microvessels. RBC ATP export in response to hypoxia or deformation dilates blood vessels in order to promote efficient O₂ delivery. The ability of RBCs to adapt to the metabolic environment via differential control of these metabolites is impaired in the face of enzymopathies [pyruvate kinase deficiency; glucose-6-phosphate dehydrogenase (G6PD) deficiency], blood banking, diabetes mellitus, COVID-19 or sepsis, and sickle cell disease. The emerging availability of therapies capable of augmenting RBC ATP, including newly established uses of allosteric effectors and metabolite-specific additive solutions for RBC transfusates, raises the prospect of clinical interventions to optimize or correct RBC function via these metabolite delivery mechanisms.

Adenosine and ATPγS protect against bacterial pneumonia-induced acute lung injury

Lipopolysaccharide (LPS), a component of the outer membrane of gram-negative bacteria, disrupts the alveolar-capillary barrier, triggering pulmonary vascular leak thus inducing acute lung injury (ALI). Extracellular purines, adenosine and ATP, protected against ALI induced by purified LPS. In this study, we investigated whether these purines can impact vascular injury in more clinically-relevant E.coli (non-sterile LPS) murine ALI model. Mice were inoculated with live E. coli intratracheally (i.t.) with or without adenosine or a non-hydrolyzable ATP analog, adenosine 5'-(γ-thio)-triphosphate (ATPγS) added intravenously (i.v.). After 24 h of E. coli treatment, we found that injections of either adenosine or ATPγS 15 min prior or adenosine 3 h after E.coli insult significantly attenuated the E.coli-mediated increase in inflammatory responses. Furthermore, adenosine prevented weight loss, tachycardia, and compromised lung function in E. coli-exposed mice. Accordingly, treatment with adenosine or ATPγS increased oxygen saturation and reduced histopathological signs of lung injury in mice exposed to E. coli. Lastly, lung-targeting gene delivery of adenosine or ATPγS downstream effector, myosin phosphatase, significantly attenuated the E. coli-induced compromise of lung function. Collectively, our study has demonstrated that adenosine or ATPγS mitigates E. coli-induced ALI in mice and may be useful as an adjuvant therapy in future pre-clinical studies.

P2Y Purinergic Receptors, Endothelial Dysfunction, and Cardiovascular Diseases

Purinergic G-protein-coupled receptors are ancient and the most abundant group of G-protein-coupled receptors (GPCRs). The wide distribution of purinergic receptors in the cardiovascular system, together with the expression of multiple receptor subtypes in endothelial cells (ECs) and other vascular cells demonstrates the physiological importance of the purinergic signaling system in the regulation of the cardiovascular system. This review discusses the contribution of purinergic P2Y receptors to endothelial dysfunction (ED) in numerous cardiovascular diseases (CVDs). Endothelial dysfunction can be defined as a shift from a “calm” or non-activated state, characterized by low permeability, anti-thrombotic, and anti-inflammatory properties, to a “activated” state, characterized by vasoconstriction and increased permeability, pro-thrombotic, and pro-inflammatory properties. This state of ED is observed in many diseases, including atherosclerosis, diabetes, hypertension, metabolic syndrome, sepsis, and pulmonary hypertension. Herein, we review the recent advances in P2Y receptor physiology and emphasize some of their unique signaling features in pulmonary endothelial cells.

P2 Purinergic Signaling in the Distal Lung in Health and Disease

The distal lung provides an intricate structure for gas exchange in mammalian lungs. Efficient gas exchange depends on the functional integrity of lung alveoli. The cells in the alveolar tissue serve various functions to maintain alveolar structure, integrity and homeostasis. Alveolar epithelial cells secrete pulmonary surfactant, regulate the alveolar surface liquid (ASL) volume and, together with resident and infiltrating immune cells, provide a powerful host-defense system against a multitude of particles, microbes and toxicants. It is well established that all of these cells express purinergic P2 receptors and that purinergic signaling plays important roles in maintaining alveolar homeostasis. Therefore, it is not surprising that purinergic signaling also contributes to development and progression of severe pathological conditions like pulmonary inflammation, acute lung injury/acute respiratory distress syndrome (ALI/ARDS) and pulmonary fibrosis. Within this review we focus on the role of P2 purinergic signaling in the distal lung in health and disease. We recapitulate the expression of P2 receptors within the cells in the alveoli, the possible sources of ATP (adenosine triphosphate) within alveoli and the contribution of purinergic signaling to regulation of surfactant secretion, ASL volume and composition, as well as immune homeostasis. Finally, we summarize current knowledge of the role for P2 signaling in infectious pneumonia, ALI/ARDS and idiopathic pulmonary fibrosis (IPF).

Purinergic Signaling in Pulmonary Inflammation

Purine nucleotides and nucleosides are at the center of biologic reactions. In particular, adenosine triphosphate (ATP) is the fundamental energy currency of cellular activity and adenosine has been demonstrated to play essential roles in human physiology and pathophysiology. In this review, we examine the role of purinergic signaling in acute and chronic pulmonary inflammation, with emphasis on ATP and adenosine. ATP is released into extracellular space in response to cellular injury and necrosis. It is then metabolized to adenosine monophosphate (AMP) via ectonucleoside triphosphate diphosphohydrolase-1 (CD39) and further hydrolyzed to adenosine via ecto-5'-nucleotidase (CD73). Adenosine signals via one of four adenosine receptors to exert pro- or anti-inflammatory effects. Adenosine signaling is terminated by intracellular transport by concentrative or equilibrative nucleoside transporters (CNTs and ENTs), deamination to inosine by adenosine deaminase (ADA), or phosphorylation back into AMP via adenosine kinase (AK). Pulmonary inflammatory and hypoxic conditions lead to increased extracellular ATP, adenosine diphosphate (ADP) and adenosine levels, which translates to increased adenosine signaling. Adenosine signaling is central to the pulmonary injury response, leading to various effects on inflammation, repair and remodeling processes that are either tissue-protective or tissue destructive. In the acute setting, particularly through activation of adenosine 2A and 2B receptors, adenosine signaling serves an anti-inflammatory, tissue-protective role. However, excessive adenosine signaling in the chronic setting promotes pro-inflammatory, tissue destructive effects in chronic pulmonary inflammation.

Rosiglitazone promotes ENaC-mediated alveolar fluid clearance in acute lung injury through the PPARγ/SGK1 signaling pathway

Background

Pulmonary edema is one of the pathological characteristics of acute respiratory distress syndrome (ARDS). The epithelial sodium channel (ENaC) is thought to be the rate-limiting factor for alveolar fluid clearance (AFC) during pulmonary edema. The peroxisome proliferator-activated receptor γ (PPARγ) agonist rosiglitazone was shown to stimulate ENaC-mediated salt absorption in the kidney. However, its role in the lung remains unclear. Here, we investigated the role of the PPARγ agonist in the lung to find out whether it can regulate AFC during acute lung injury (ALI). We also attempted to elucidate the mechanism for this.

Methods

Our ALI model was established through intratracheal instillation of lipopolysaccharide (LPS) in C57BL/6 J mice. The mice were randomly divided into 4 groups of 10. The control group underwent a sham operation and received an equal quantity of saline. The three experimental groups underwent intratracheal instillation of 5 mg/kg LPS, followed by intraperitoneal injection of 4 mg/kg rosiglitazone, 4 mg/kg rosiglitazone plus 1 mg/kg GW9662, or only equal quantity of saline. The histological morphology of the lung, the levels of TNF-α and IL-1β in the bronchoalveolar lavage fluid (BALF), the level of AFC, and the expressions of αENaC and serum and glucocorticoid-induced kinase-1 (SGK1) were determined. Type 2 alveolar (AT II) cells were incubated with rosiglitazone (15 μM) with or without GW9662 (10 μM). The expressions of αENaC and SGK1 were determined 24 h later.

Results

A mouse model of ALI was successfully established. Rosiglitazone significantly ameliorated the lung injury, decreasing the TNF-α and IL-1β levels in the BALF, enhancing AFC, and promoting the expressions of αENaC and SGK1 in ALI mice, which were abolished by the specific PPARγ blocker GW9662. In vitro, rosiglitazone increased the expressions of αENaC and SGK1. This increase was prevented by GW9662.

Conclusions

Rosiglitazone ameliorated the lung injury and promoted ENaC-mediated AFC via a PPARγ/SGK1-dependent signaling pathway, alleviating pulmonary edema in a mouse model of ALI.

Differential mechanisms of adenosine- and ATPγS-induced microvascular endothelial barrier strengthening

Maintenance of the endothelial cell (EC) barrier is critical to vascular homeostasis and a loss of barrier integrity results in increased vascular permeability. While the mechanisms that govern increased EC permeability have been under intense investigation over the past several decades, the processes regulating the preservation/restoration of the EC barrier remain poorly understood. Herein we show that the extracellular purines, adenosine (Ado) and adenosine 5'-[γ-thio]-triphosphate (ATPγS) can strengthen the barrier function of human lung microvascular EC (HLMVEC). This ability involves protein kinase A (PKA) activation and decreases in myosin light chain 20 (MLC20) phosphorylation secondary to the involvement of MLC phosphatase (MLCP). In contrast to Ado, ATPγS-induced PKA activation is accompanied by a modest, but significant decrease in cyclic adenosine monophosphate (cAMP) levels supporting the existence of an unconventional cAMP-independent pathway of PKA activation. Furthermore, ATPγS-induced EC barrier strengthening does not involve the Rap guanine nucleotide exchange factor 3 (EPAC1) which is directly activated by cAMP but is instead dependent upon PKA-anchor protein 2 (AKAP2) expression. We also found that AKAP2 can directly interact with the myosin phosphatase-targeting protein MYPT1 and that depletion of AKAP2 abolished ATPγS-induced increases in transendothelial electrical resistance. Ado-induced strengthening of the HLMVEC barrier required the coordinated activation of PKA and EPAC1 in a cAMP-dependent manner. In summary, ATPγS-induced enhancement of the EC barrier is EPAC1-independent and is instead mediated by activation of PKA which is then guided by AKAP2, in a cAMP-independent mechanism, to activate MLCP which dephosphorylates MLC20 resulting in reduced EC contraction and preservation.

Cytokine-Ion Channel Interactions in Pulmonary Inflammation

The lungs conceptually represent a sponge that is interposed in series in the bodies’ systemic circulation to take up oxygen and eliminate carbon dioxide. As such, it matches the huge surface areas of the alveolar epithelium to the pulmonary blood capillaries. The lung’s constant exposure to the exterior necessitates a competent immune system, as evidenced by the association of clinical immunodeficiencies with pulmonary infections. From the in utero to the postnatal and adult situation, there is an inherent vital need to manage alveolar fluid reabsorption, be it postnatally, or in case of hydrostatic or permeability edema. Whereas a wealth of literature exists on the physiological basis of fluid and solute reabsorption by ion channels and water pores, only sparse knowledge is available so far on pathological situations, such as in microbial infection, acute lung injury or acute respiratory distress syndrome, and in the pulmonary reimplantation response in transplanted lungs. The aim of this review is to discuss alveolar liquid clearance in a selection of lung injury models, thereby especially focusing on cytokines and mediators that modulate ion channels. Inflammation is characterized by complex and probably time-dependent co-signaling, interactions between the involved cell types, as well as by cell demise and barrier dysfunction, which may not uniquely determine a clinical picture. This review, therefore, aims to give integrative thoughts and wants to foster the unraveling of unmet needs in future research.

Elevated uric acid and adenosine triphosphate concentrations in bronchoalveolar lavage fluid of eosinophilic pneumonia

BACKGROUND

Recent evidence has suggested that the innate immune response may play a role in the development of eosinophilic airway inflammation. We previously reported that uric acid (UA) and adenosine triphosphate (ATP), two important damage-associated molecular pattern molecules (DAMPs), activate eosinophil functions, suggesting that these molecules may be involved in the development of eosinophilic airway inflammation. The objective of this study was to measure the concentrations of DAMPs including UA and ATP in the bronchoalveolar lavage fluid (BALF) of patients with eosinophilic pneumonia (EP).

METHODS

BAL was performed in patients with EP including acute and chronic eosinophilic pneumonia, and in patients with hypersensitivity pneumonia, and sarcoidosis. UA, ATP, and cytokine concentrations in the BALF were then measured.

RESULTS

The UA concentration was increased in the BALF of EP patients. UA concentrations correlated with eosinophil numbers, and with eosinophil-derived neurotoxin and interleukin (IL)-5 concentrations. Furthermore, the ATP concentration was increased in the BALF of EP patients and ATP concentrations correlated with UA concentrations. Moreover, IL-33 was increased in EP patients and IL-33 concentrations correlated with UA and ATP concentrations.

CONCLUSIONS

The UA and ATP concentration was increased in the BALF of EP patients. UA concentrations correlated with eosinophil numbers, and with ATP and IL-33 concentrations. Our findings suggest that DAMPs such as UA and ATP play a role in the pathogenesis of EP.

Actin dynamics in the regulation of endothelial barrier functions and neutrophil recruitment during endotoxemia and sepsis

Sepsis is a leading cause of death worldwide. Increased vascular permeability is a major hallmark of sepsis. Dynamic alterations in actin fiber formation play an important role in the regulation of endothelial barrier functions and thus vascular permeability. Endothelial integrity requires a delicate balance between the formation of cortical actin filaments that maintain endothelial cell contact stability and the formation of actin stress fibers that generate pulling forces, and thus compromise endothelial cell contact stability. Current research has revealed multiple molecular pathways that regulate actin dynamics and endothelial barrier dysfunction during sepsis. These include intracellular signaling proteins of the small GTPases family (e.g., Rap1, RhoA and Rac1) as well as the molecules that are directly acting on the actomyosin cytoskeleton such as myosin light chain kinase and Rho kinases. Another hallmark of sepsis is an excessive recruitment of neutrophils that also involves changes in the actin cytoskeleton in both endothelial cells and neutrophils. This review focuses on the available evidence about molecules that control actin dynamics and regulate endothelial barrier functions and neutrophil recruitment. We also discuss treatment strategies using pharmaceutical enzyme inhibitors to target excessive vascular permeability and leukocyte recruitment in septic patients.

The protective role of MLCP-mediated ERM dephosphorylation in endotoxin-induced lung injury in vitro and in vivo

The goal of this study was to investigate the role of MLC phosphatase (MLCP) in a LPS model of acute lung injury (ALI). We demonstrate that ectopic expression of a constitutively-active (C/A) MLCP regulatory subunit (MYPT1) attenuates the ability of LPS to increase endothelial (EC) permeability. Down-regulation of MYPT1 exacerbates LPS-induced expression of ICAM1 suggesting an anti-inflammatory role of MLCP. To determine whether MLCP contributes to LPS-induced ALI in vivo, we utilized a nanoparticle DNA delivery method to specifically target lung EC. Expression of a C/A MYPT1 reduced LPS-induced lung inflammation and vascular permeability. Further, increased expression of the CS1β (MLCP catalytic subunit) also reduced LPS-induced lung inflammation, whereas the inactive CS1β mutant increased vascular leak. We next examined the role of the cytoskeletal targets of MLCP, the ERM proteins (Ezrin/Radixin/Moesin), in mediating barrier dysfunction. LPS-induced increase in EC permeability was accompanied by PKC-mediated increase in ERM phosphorylation, which was more prominent in CS1β-depleted cells. Depletion of Moesin and Ezrin, but not Radixin attenuated LPS-induced increases in permeability. Further, delivery of a Moesin phospho-null mutant into murine lung endothelium attenuated LPS-induced lung inflammation and vascular leak suggesting that MLCP opposes LPS-induced ALI by mediating the dephosphorylation of Moesin and Ezrin.

MicroRNA-7-5p regulates human alveolar epithelial sodium channels by targeting the mTORC2/SGK-1 signaling pathway

PURPOSE

The aim of this study was to investigate the effect of miRNA-7-5p on human alveolar epithelial sodium channels and clarify the pathway in which miRNA-7-5p regulates the expression of ENaC in ARDS.

MATERIALS AND METHODS

Dual luciferase target gene validation experiments were used to confirm whether mTOR and SGK-1 are the target genes of miRNA-7-5p. Then, we overexpressed and inhibited miRNA-7-5p in the human alveolar epithelial cell line A549, respectively. LPS at a concentration of 100 ng/mL was used to stimulate the cells. The expressions ofmiRNA-7-5p, mTOR, SGK-1, p-Akt-Ser473, α-, β-, and γ-ENaC were detected by quantitative RT polymerase chain reaction (qRT-PCR) and western blotting.

RESULTS

In this study, we first confirmed that mTOR and SGK-1 are the target genes of miRNA-7-5p. Then, we found that mRNA expression levels of both mTOR and SGK-1 were downregulated to 0.54- and 0.3-fold, respectively, in the miRNA-7-5p mimic groups than the blank controls (P < 0.01). MiRNA-7-5p overexpression inhibited mTORC2/SGK-1 signaling pathway activity and reduced ENaC expression. The expression of miRNA-7-5p was significantly upregulated in A549 cells stimulated with lipopolysaccharide (LPS) and downregulated mRNA expression levels of both mTOR and SGK-1. After transfection with miRNA-7-5p inhibitors, we found that the mTORC2/SGK-1 pathway activity was restored compared to the group with LPS stimulation only, and the ENaC expression was also obviously increased.

CONCLUSION

Our results demonstrate that miRNA-7-5p can regulate the expression of human alveolar ENaC by targeting the mTORC2/SGK-1 signaling pathway. The inhibition of miRNA-7-5p can enhance the expression of ENaC, which may provide a new target for the treatment of ARDS.

Suppression of endothelial CD39/ENTPD1 is associated with pulmonary vascular remodeling in pulmonary arterial hypertension

Endothelial cell (EC) dysfunction plays a role in the pathobiology of occlusive vasculopathy in pulmonary arterial hypertension (PAH). Purinergic signaling pathways, which consist of extracellular nucleotide and nucleoside-mediated cell signaling through specific receptors, are known to be important regulators of vascular tone and remodeling. Therefore, we hypothesized that abnormalities in the vascular purinergic microenvironment are associated with PAH. Enzymatic clearance is crucial to terminate unnecessary cell activation; one of the most abundantly expressed enzymes on the EC surface is E-NTPDase1/CD39, which hydrolyzes ATP and ADP to AMP. we used histological samples from patients and healthy donors, radioisotope-labeled substrates to measure ectoenzyme activity, and a variety of in vitro approaches to study the role of CD39 in PAH. Immunohistochemistry on human idiopathic PAH (IPAH) patients' lungs demonstrated that CD39 was significantly downregulated in the endothelium of diseased small arteries. Similarly, CD39 expression and activity were decreased in cultured pulmonary ECs from IPAH patients. Suppression of CD39 in vitro resulted in EC phenotypic switch that gave rise to apoptosis-resistant pulmonary arterial endothelial cells and promoted a microenvironment that induced vascular smooth muscle cell migration. we also identified that the ATP receptor P2Y11 is essential for ATP-mediated EC survival. Furthermore, we report that apelin, a known regulator of pulmonary vascular homeostasis, can potentiate the activity of CD39 both in vitro and in vivo. we conclude that sustained attenuation of CD39 activity through ATP accumulation is tightly linked to vascular dysfunction and remodeling in PAH and could represent a novel target for therapy.

The Effects of Portulaca oleracea on Hypoxia-Induced Pulmonary Edema in Mice

Portulaca oleracea L. (PO) is known as "a vegetable for long life" due to its antioxidant, anti-inflammatory, and other pharmacological activities. However, the protective activity of the ethanol extract of PO (EEPO) against hypoxia-induced pulmonary edema has not been fully investigated. In this study, we exposed mice to a simulated altitude of 7000 meters for 0, 3, 6, 9, and 12 h to observe changes in the water content and transvascular leakage of the mouse lung. It was found that transvascular leakage increased to the maximum in the mouse lung after 6 h exposure to hypobaric hypoxia. Prophylactic administration of EEPO before hypoxic exposure markedly reduced the transvascular leakage and oxidative stress, and inhibited the upregulation of NF-kB in the mouse lung, as compared with the control group. In addition, EEPO significantly reduced the levels of proinflammatory cytokines and cell adhesion molecules in the lungs of mice, as compared with the hypoxia group. Our results show that EEPO can reduce initial transvascular leakage and pulmonary edema under hypobaric hypoxia conditions.

Extracellular ATP mediates the late phase of neutrophil recruitment to the lung in murine models of acute lung injury

Acute lung injury (ALI) is a severe inflammatory condition whose pathogenesis is irrevocably linked to neutrophil emigration to the lung. Activation and recruitment of neutrophils to the lung is mostly attributable to local production of the chemokines. However, much of our understanding of neutrophil recruitment to the lung is based on studies focusing on early time points after initiation of injury. In this study, we sought to evaluate the extended temporal relationship between neutrophil chemotactic factor expression and influx of neutrophils into the lung after intratracheal administration of either LPS or bleomycin. In both models, results demonstrated two phases of neutrophil chemotactic factor expression; first, an early phase characterized by high levels of CXCL1/keratinocyte-derived chemokine, CXCL2/monocyte-inhibitory protein-2, and CXCL5/LPS-induced chemokine expression, and second, a late phase distinguished by increases in extracellular ATP. Furthermore, we show that strategies aimed at either enhancing ATP catabolism (ip ecto-5'-nucleotidase administration) or inhibiting glycolytic ATP production (ip 2-deoxy-d-glucose treatment) reduce extracellular ATP accumulation, limit vascular leakage, and effectively block the late, but not the early, stages of neutrophil recruitment to the lung after LPS instillation. In conclusion, this study illustrates that neutrophil recruitment to the lung is mediated by the time-dependent expression of chemotactic factors and suggests that novel strategies, which reduce extracellular ATP accumulation, may attenuate late neutrophil recruitment and limit lung injury during ALI.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Effective treatment of mouse sepsis with an inhibitory antibody targeting integrin αvβ5

OBJECTIVE

Integrin αvβ5 has been identified as a regulator of vascular leak and endothelial permeability. We hypothesized that targeting αvβ5 could represent a viable treatment strategy for sepsis.

DESIGN

Integrin β5 subunit knockout and wild-type 129/svJae mice and wild-type mice treated with αvβ5 blocking or control antibodies were tested in models of intraperitoneal lipopolysaccharide and cecal ligation and puncture. Human umbilical vein endothelial cell and human lung microvascular endothelial cell monolayers were treated with αvβ5 antibodies to assess for effects on lipopolysaccharide-induced changes in transendothelial resistance and on patterns of cytoskeletal reorganization.

SETTING

Laboratory-based research.

SUBJECTS

Mice and endothelial cell monolayers.

INTERVENTIONS, MEASUREMENTS, AND MAIN RESULTS

Measurements taken after intraperitoneal lipopolysaccharide and/or cecal ligation and puncture included mortality, vascular leak, hematocrit, quantification of a panel of serum cytokines/chemokines, and assessment of thioglyccolate-induced leukocyte migration. β5 knockout mice had decreased mortality after intraperitoneal lipopolysaccharide and cecal ligation and puncture and decreased vascular leak, as measured by extravasation of an I-labeled intravascular tracer. Treating clinically ill mice with αvβ5 antibodies, up to 20 hrs after intraperitoneal lipopolysaccharide and cecal ligation and puncture, also resulted in decreased mortality. αvβ5 antibodies attenuated lipopolysaccharide-induced transendothelial resistance changes and cytoskeletal stress fiber formation in both human umbilical vein endothelial cell and human lung microvascular endothelial cell monolayers. αvβ5 antibodies had no effect on cytokine/chemokine serum levels after cecal ligation and puncture. β5 knockout mice and wild-type controls did not exhibit differences in thioglyccolate-induced leukocyte migration.

CONCLUSIONS

Our studies suggest that αvβ5 is an important regulator of the vascular endothelial leak response in sepsis and that αvβ5 blockade may provide a novel approach to treating this devastating disease syndrome.

Critical role of S1PR1 and integrin β4 in HGF/c-Met-mediated increases in vascular integrity

Vascular endothelial cell (EC) barrier integrity is critical to vessel homeostasis whereas barrier dysfunction is a key feature of inflammatory disorders and tumor angiogenesis. We previously reported that hepatocyte growth factor (HGF)-mediated increases in EC barrier integrity are signaled through a dynamic complex present in lipid rafts involving its receptor, c-Met. We extended these observations to confirm that S1PR1 (sphingosine 1-phosphate receptor 1) and integrin β4 (ITGB4) are essential participants in HGF-induced EC barrier enhancement. Immunoprecipitation experiments demonstrated HGF-mediated recruitment of c-Met, ITGB4 and S1PR1 to caveolin-enriched lipid rafts in human lung EC with direct interactions of c-Met with both S1PR1 and ITGB4 accompanied by c-Met-dependent S1PR1 and ITGB4 transactivation. Reduced S1PR1 expression (siRNA) attenuated both ITGB4 and Rac1 activation as well as c-Met/ITGB4 interaction and resulted in decreased transendothelial electrical resistance. Furthermore, reduced ITGB4 expression attenuated HGF-induced c-Met activation, c-Met/S1PR1 interaction, and effected decreases in S1P- and HGF-induced EC barrier enhancement. Finally, the c-Met inhibitor, XL880, suppressed HGF-induced c-Met activation as well as S1PR1 and ITGB4 transactivation. These results support a critical role for S1PR1 and ITGB4 transactivation as rate-limiting events in the transduction of HGF signals via a dynamic c-Met complex resulting in enhanced EC barrier integrity.

Transgenic expression of FoxM1 promotes endothelial repair following lung injury induced by polymicrobial sepsis in mice

Enhancing endothelial barrier integrity for the treatment of acute lung injury (ALI) is an emerging novel therapeutic strategy. Our previous studies have demonstrated the essential role of FoxM1 in mediating endothelial regeneration and barrier repair following lipopolysaccharide-induced lung injury. However, it remains unclear whether FoxM1 expression is sufficient to promote endothelial repair in experimental models of sepsis. Here, employing the FoxM1 transgenic (FoxM1 Tg) mice, we showed that transgenic expression of FoxM1 promoted rapid recovery of endothelial barrier function and survival in a clinically relevant model of sepsis induced by cecal ligation and puncture (CLP). We observed lung vascular permeability was rapidly recovered and returned to levels similar to baseline at 48 h post-CLP challenge in FoxM1 Tg mice whereas it remained markedly elevated in WT mice. Lung edema and inflammation were resolved only in FoxM1 Tg mice at 24 h post-CLP. 5-bromo-2-deoxyuridine incorporation assay revealed a drastic induction of endothelial proliferation in FoxM1 Tg lungs at 24h post-CLP, correlating with early induction of expression of FoxM1 target genes essential for cell cycle progression. Additionally, deletion of FoxM1 in endothelial cells, employing the mouse model with endothelial cell-restricted disruption of FoxM1 (FoxM1 CKO) resulted in impaired endothelial repair following CLP challenge. Together, these data suggest FoxM1 expression in endothelial cells is necessary and sufficient to mediate endothelial repair and thereby promote survival following sepsis challenge.

β-Nicotinamide adenine dinucleotide attenuates lipopolysaccharide-induced inflammatory effects in a murine model of acute lung injury

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) occur in approximately 200,000 patients per year. Studies indicate that lung endothelium plays a significant role in ALI. The authors' recent in vitro studies demonstrate a novel mechanism of β-nicotinamide adenine dinucleotide (β-NAD)-induced protection against gram-positive (pneumolysin, PLY) and gram-negative (lipopolysaccharide, LPS) toxin-induced lung endothelial cell (EC) barrier dysfunction. The objective of the current study was to evaluate the protective effect of β-NAD against LPS-induced ALI in mice. C57BL/6J mice were randomly divided into 4 groups: vehicle, β-NAD, LPS, and LPS/β-NAD. After surgery, mice were allowed to recover for 24 hours. Evans blue dye-albumin (EBA) was given through the internal jugular vein 2 hours prior to the termination of the experiments. Upon sacrificing the animals, bronchoalveolar lavage fluid (BALF) was collected and the lungs were harvested. β-NAD treatment significantly attenuated the inflammatory response by means of reducing the accumulation of cells and protein in BALF, blunting the parenchymal neutrophil infiltration, and preventing capillary leak. In addition, the histological examination demonstrated decreased interstitial edema in the LPS/β-NAD specimens, as compared to the LPS-only specimens. The mRNA levels of the anti-inflammatory cytokines were up-regulated in the LPS group treated with β-NAD compared to the LPS-only-treated group. β-NAD treatment down-regulated the mRNA levels of the proinflammatory cytokines. These findings suggest that β-NAD could be investigated as a therapeutic option against bacterial toxin-induced lung inflammation and ALI in mice.

Purinergic signaling in the airways

Evidence for a significant role and impact of purinergic signaling in normal and diseased airways is now beyond dispute. The present review intends to provide the current state of knowledge of the involvement of purinergic pathways in the upper and lower airways and lungs, thereby differentiating the involvement of different tissues, such as the epithelial lining, immune cells, airway smooth muscle, vasculature, peripheral and central innervation, and neuroendocrine system. In addition to the vast number of well illustrated functions for purinergic signaling in the healthy respiratory tract, increasing data pointing to enhanced levels of ATP and/or adenosine in airway secretions of patients with airway damage and respiratory diseases corroborates the emerging view that purines act as clinically important mediators resulting in either proinflammatory or protective responses. Purinergic signaling has been implicated in lung injury and in the pathogenesis of a wide range of respiratory disorders and diseases, including asthma, chronic obstructive pulmonary disease, inflammation, cystic fibrosis, lung cancer, and pulmonary hypertension. These ostensibly enigmatic actions are based on widely different mechanisms, which are influenced by the cellular microenvironment, but especially the subtypes of purine receptors involved and the activity of distinct members of the ectonucleotidase family, the latter being potential protein targets for therapeutic implementation.

Opposing effects of ATP and adenosine on barrier function of rat coronary microvasculature

ATP can differentially affect the micro- and macrovascular endothelial barrier. It has been shown that it can both increase and/or decrease macromolecule permeability of microvascular endothelial cells and microvessels, in vivo. We hypothesised that the barrier stabilising effect is mediated by ATP itself via P2 receptors, while barrier-disrupting effect is mediated by its metabolite adenosine via adenosine receptors. The effects of ATP, ADP, AMP and adenosine on barrier function were studied in cultured rat coronary microvascular endothelial monolayers (RCEC) in vitro, as well as in rat mesentery vessels, and in rat hearts in vivo. ATP and ADP showed a biphasic effect on permeability of RCEC monolayers with a reduction followed by a later increase in albumin permeability. The permeability decreasing effect of ATP was enhanced by ecto-nucleotidase inhibitor ARL67156 while permeability increasing effect was enhanced by apyrase, an extracellular ecto-nucleotidase. Moreover, the permeability increasing effect was abrogated by adenosine receptor antagonists, 8-phenyltheophylline (8-PT) and DMPX. Adenosine and adenosine receptor agonists 5'-(N-ethylcarboxamido)-adenosine (NECA), CGS21680, and R-PIA enhanced albumin permeability which was antagonised by 8-PT, A(1), and A(2) but not by A(3) receptor antagonists. Likewise, immunofluorescence microscopy of VE-cadherin and actin showed that NECA induces a disturbance of intercellular junctions. Pre-incubation of ATP antagonised the effects of NECA on permeability, actin cytoskeleton and intercellular junctions. Similar effects of the applied substances were observed in rat mesentery artery by determining the vascular leakage using intravital microscopy as well as in rat hearts by assessing myocardial water contents in vivo. In conclusion, the study demonstrates that in RCEC, ATP, ADP, and its metabolite adenosine play opposing roles on endothelial barrier function.

The suppression of myosin light chain (MLC) phosphorylation during the response to lipopolysaccharide (LPS): beneficial or detrimental to endothelial barrier?

Sepsis-induced vascular leakage is a major underlying cause of the respiratory dysfunction seen in severe sepsis. Here, we studied the role of MLC phosphorylation in LPS-induced endothelial hyperpermeability and assessed how the changes in phospho-MLC distribution affect LPS-induced barrier dysfunction. We demonstrated that the changes in human lung microvascular endothelial permeability are preceded by the increase in intracellular calcium level, and increase in MYPT and MLC phosphorylation. Using the siRNA approach, we showed that both LPS-induced barrier dysfunction and MLC phosphorylation are attenuated by the depletion of the smooth muscle isoform of MLC kinase (MLCK) and Rho kinase 2 (ROCK2). Surprisingly, pharmacological inhibition of both ROCK1 and 2 with Y-27632 exacerbated LPS-induced drop in transendothelial resistance, although significantly decreasing MLC phosphorylation level. We next studied the involvement of protein kinase A (PKA)-dependent pathways in LPS-induced barrier dysfunction. We showed that LPS decreased the level of PKA-dependent phosphorylation in endothelial cells; and the pretreatment with forskolin or PKA activator bnz-cAMP counteracted this effect. Forskolin and bnz-cAMP also attenuated LPS-induced increase in MLC phosphorylation level. As we have shown earlier (Bogatcheva et al., 2009), forskolin and bnz-cAMP provide protection from LPS-induced barrier dysfunction. We compared the effects of bnz-cAMP and Y-27632 on phospho-MLC distribution and observed that while bnz-cAMP increased the association of the phospho-MLC signal with the cortical structures, Y-27632 decreased this association. These data indicate that an overall decrease in MLC phosphorylation could be either beneficial or detrimental to endothelial barrier, depending on the intracellular locale of major phospho-MLC changes.

The role of purinergic signaling on deformation induced injury and repair responses of alveolar epithelial cells

Cell wounding is an important driver of the innate immune response of ventilator-injured lungs. We had previously shown that the majority of wounded alveolus resident cells repair and survive deformation induced insults. This is important insofar as wounded and repaired cells may contribute to injurious deformation responses commonly referred to as biotrauma. The central hypothesis of this communication states that extracellular adenosine-5′ triphosphate (ATP) promotes the repair of wounded alveolus resident cells by a P2Y2-Receptor dependent mechanism. Using primary type 1 alveolar epithelial rat cell models subjected to micropuncture injury and/or deforming stress we show that 1) stretch causes a dose dependent increase in cell injury and ATP media concentrations; 2) enzymatic depletion of extracellular ATP reduces the probability of stretch induced wound repair; 3) enriching extracellular ATP concentrations facilitates wound repair; 4) purinergic effects on cell repair are mediated by ATP and not by one of its metabolites; and 5) ATP mediated cell salvage depends at least in part on P2Y2-R activation. While rescuing cells from wounding induced death may seem appealing, it is possible that survivors of membrane wounding become governors of a sustained pro-inflammatory state and thereby perpetuate and worsen organ function in the early stages of lung injury syndromes. Means to uncouple P2Y2-R mediated cytoprotection from P2Y2-R mediated inflammation and to test the preclinical efficacy of such an undertaking deserve to be explored.

Adenosine triphosphate-dependent calcium signaling during ventilator-induced lung injury is amplified by hypercapnia

Adenosine triphosphate (ATP) is released by alveolar epithelial cells during ventilator-induced lung injury (VILI) and regulates fluid transport across epithelia. High CO(2) levels are observed in patients with "permissive hypercapnia," which inhibits alveolar fluid reabsorption (AFR) in alveolar epithelial cells. The authors set out to determine whether VILI affects AFR and whether the purinergic pathway is modulated in cells exposed to hypercapnia. Control group was compared against VILI (tidal volume [Vt] = 35 mL/kg, zero positive end-expiratory pressure [PEEP]) and protective ventilation (Vt = 6 mL/kg, PEEP = 10 cm H(2)O) groups. Lung mechanics, histology, and AFR were evaluated. Alveolar epithelial cells (AECs) were loaded with Fura 2-AM to measure intracellular calcium in the presence ATP (10 μM) at 5% or 10% CO(2) as compared with baseline. High tidal volume ventilation impairs lung mechanics and AFR. Hypercapnia (HC) increases intracellular calcium levels in response to ATP stimulation. HC + ATP is the most detrimental combination decreasing AFR. Purinergic signaling in AECs is modulated by high CO(2) levels via increased cytosolic calcium. The authors reason that this modulation may play a role in the impairment of alveolar epithelial functions induced by hypercapnia.

R-Ras interacts with filamin a to maintain endothelial barrier function

The molecular mechanisms regulating vascular barrier integrity remain incompletely elucidated. We have previously reported an association between the GTPase R-Ras and repeat 3 of Filamin A (FLNa). Loss of FLNa has been linked to increased vascular permeability. We sought to determine whether FLNa's association with R-Ras affects endothelial barrier function. We report that in endothelial cells endogenous R-Ras interacts with endogenous FLNa as determined by co-immunoprecipitations and pulldowns with the FLNa-GST fusion protein repeats 1-10. Deletion of FLNa repeat 3 (FLNaΔ3) abrogated this interaction. In these cells FLNa and R-Ras co-localize at the plasma membrane. Knockdown of R-Ras and/or FLNa by siRNA promotes vascular permeability, as determined by TransEndothelial Electrical Resistance and FITC-dextran transwell assays. Re-expression of FLNa restored endothelial barrier function in cells lacking FLNa whereas re-expression of FLNaΔ3 did not. Immunostaining for VE-Cadherin in cells with knocked down R-Ras and FLNa demonstrated a disorganization of VE-Cadherin at adherens junctions. Loss of R-Ras and FLNa or blocking R-Ras function via GGTI-2133, a selective R-Ras inhibitor, induced vascular permeability and increased phosphorylation of VE-Cadherin (Y731) and Src (Y416). Expression of dominant negative R-Ras promoted vascular permeability that was blocked by the Src inhibitor PP2. These findings demonstrate that maintaining endothelial barrier function is dependent upon active R-Ras and association between R-Ras and FLNa and that loss of this interaction promotes VE-Cadherin phosphorylation and changes in downstream effectors that lead to endothelial leakiness.

P2Y receptors as regulators of lung endothelial barrier integrity

Endothelial cells (ECs), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, control such diverse processes as vascular tone, homeostasis, adhesion of platelets, and leukocytes to the vascular wall and permeability of vascular wall for cells and fluids. Mechanisms which govern the highly clinically relevant process of increased EC permeability are under intense investigation. It is well known that loss of this barrier (permeability increase) results in tissue inflammation, the hall mark of inflammatory diseases such as acute lung injury and its severe form, acute respiratory distress syndrome. Little is known about processes which determine the endothelial barrier enhancement or protection against permeability increase. It is now well accepted that extracellular purines and pyrimidines are promising and physiologically relevant barrier-protective agents and their effects are mediated by interaction with cell surface P2Y receptors which belong to the superfamily of G-protein-coupled receptors. The therapeutic potential of P2Y receptors is rapidly expanding field in pharmacology and some selective agonists became recently available. Here, we present an overview of recently identified P2Y receptor agonists that enhance the pulmonary endothelial barrier and inhibit and/or reverse endothelial barrier disruption.

An official American Thoracic Society workshop report: features and measurements of experimental acute lung injury in animals

Acute lung injury (ALI) is well defined in humans, but there is no agreement as to the main features of acute lung injury in animal models. A Committee was organized to determine the main features that characterize ALI in animal models and to identify the most relevant methods to assess these features. We used a Delphi approach in which a series of questionnaires were distributed to a panel of experts in experimental lung injury. The Committee concluded that the main features of experimental ALI include histological evidence of tissue injury, alteration of the alveolar capillary barrier, presence of an inflammatory response, and evidence of physiological dysfunction; they recommended that, to determine if ALI has occurred, at least three of these four main features of ALI should be present. The Committee also identified key "very relevant" and "somewhat relevant" measurements for each of the main features of ALI and recommended the use of least one "very relevant" measurement and preferably one or two additional separate measurements to determine if a main feature of ALI is present. Finally, the Committee emphasized that not all of the measurements listed can or should be performed in every study, and that measurements not included in the list are by no means "irrelevant." Our list of features and measurements of ALI is intended as a guide for investigators, and ultimately investigators should choose the particular measurements that best suit the experimental questions being addressed as well as take into consideration any unique aspects of the experimental design.

Role of sphingolipids in murine radiation-induced lung injury: protection by sphingosine 1-phosphate analogs

Clinically significant radiation-induced lung injury (RILI) is a common toxicity in patients administered thoracic radiotherapy. Although the molecular etiology is poorly understood, we previously characterized a murine model of RILI in which alterations in lung barrier integrity surfaced as a potentially important pathobiological event and genome-wide lung gene mRNA levels identified dysregulation of sphingolipid metabolic pathway genes. We hypothesized that sphingolipid signaling components serve as modulators and novel therapeutic targets of RILI. Sphingolipid involvement in murine RILI was confirmed by radiation-induced increases in lung expression of sphingosine kinase (SphK) isoforms 1 and 2 and increases in the ratio of ceramide to sphingosine 1-phosphate (S1P) and dihydro-S1P (DHS1P) levels in plasma, bronchoalveolar lavage fluid, and lung tissue. Mice with a targeted deletion of SphK1 (SphK1(-/-)) or with reduced expression of S1P receptors (S1PR1(+/-), S1PR2(-/-), and S1PR3(-/-)) exhibited marked RILI susceptibility. Finally, studies of 3 potent vascular barrier-protective S1P analogs, FTY720, (S)-FTY720-phosphonate (fTyS), and SEW-2871, identified significant RILI attenuation and radiation-induced gene dysregulation by the phosphonate analog, fTyS (0.1 and 1 mg/kg i.p., 2×/wk) and to a lesser degree by SEW-2871 (1 mg/kg i.p., 2×/wk), compared with those in controls. These results support the targeting of S1P signaling as a novel therapeutic strategy in RILI.

Animal models of airway diseases

Over the past 20 years, the growing awareness that purinergic signaling events literally shape the immune and inflammatory responses to infection and allergic reactions warranted the development of animal models to assess their importance in vivo in acute lung injury and chronic airway diseases. The pioneer work conducted with the adenosine deaminase (ADA)-deficient mouse provided irrefutable evidence that excess adenosine (ADO) accumulating in the lungs of asthmatic patients, constitutes a powerful mediator of disease severity. These original studies launched the development of murine strains for the two major ectonucleotidases responsible for the generation of airway ADO from ATP release: CD39 and CD73. The dramatic acute lung injury and chronic lung complications, manifested by these knockout mice in response to allergens and endotoxin, demonstrated the critical importance of regulating the availability of ATP and ADO for their receptors. Therapeutic targets are currently evaluated using knockout mice and agonists/antagonists for each ADO receptor (A(1)R, A(2A)R, A(2B)R, and A(3)R) and the predominant ATP receptors (P2Y(2)R and P2X(7)R). This chapter provides an in-depth description of each in vivo study, and a critical view of the therapeutic potentials for the treatment of airway diseases.

Role of Apoptosis in Amplifying Inflammatory Responses in Lung Diseases

Apoptosis is an important contributor to the pathophysiology of lung diseases such as acute lung injury (ALI) and chronic obstructive pulmonary disease (COPD). Furthermore, the cellular environment of these acute and chronic lung diseases favors the delayed clearance of apoptotic cells. This dysfunctional efferocytosis predisposes to the release of endogenous ligands from dying cells. These so-called damage-associated molecular patterns (DAMPs) play an important role in the stimulation of innate immunity as well as in the induction of adaptive immunity, potentially against autoantigens. In this review, we explore the role of apoptosis in ALI and COPD, with particular attention to the contribution of DAMP release in augmenting the inflammatory response in these disease states.

Extracellular beta-nicotinamide adenine dinucleotide (beta-NAD) promotes the endothelial cell barrier integrity via PKA- and EPAC1/Rac1-dependent actin cytoskeleton rearrangement

Extracellular beta-NAD is known to elevate intracellular levels of calcium ions, inositol 1,4,5-trisphate and cAMP. Recently, beta-NAD was identified as an agonist for P2Y1 and P2Y11 purinergic receptors. Since beta-NAD can be released extracellularly from endothelial cells (EC), we have proposed its involvement in the regulation of EC permeability. Here we show, for the first time, that endothelial integrity can be enhanced in EC endogenously expressing beta-NAD-activated purinergic receptors upon beta-NAD stimulation. Our data demonstrate that extracellular beta-NAD increases the transendothelial electrical resistance (TER) of human pulmonary artery EC (HPAEC) monolayers in a concentration-dependent manner indicating endothelial barrier enhancement. Importantly, beta-NAD significantly attenuated thrombin-induced EC permeability as well as the barrier-compromising effects of Gram-negative and Gram-positive bacterial toxins representing the barrier-protective function of beta-NAD. Immunofluorescence microscopy reveals more pronounced staining of cell-cell junctional protein VE-cadherin at the cellular periphery signifying increased tightness of the cell-cell contacts after beta-NAD stimulation. Interestingly, inhibitory analysis (pharmacological antagonists and receptor sequence specific siRNAs) indicates the participation of both P2Y1 and P2Y11 receptors in beta-NAD-induced TER increase. beta-NAD-treatment attenuates the lipopolysaccharide (LPS)-induced phosphorylation of myosin light chain (MLC) indicating its involvement in barrier protection. Our studies also show the involvement of cAMP-dependent protein kinase A and EPAC1 pathways as well as small GTPase Rac1 in beta-NAD-induced EC barrier enhancement. With these results, we conclude that beta-NAD regulates the pulmonary EC barrier integrity via small GTPase Rac1- and MLCP- dependent signaling pathways.

ATP activates regulatory T Cells in vivo during contact hypersensitivity reactions

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) require activation to develop their full suppressive capacity. Similar to conventional T cells, Tregs can be activated via their TCRs; however, other means may be in place. We injected naive and nonactivated Tregs, being CD69(-)CD44(low)CD62L(+) into mice, and analyzed their phenotype after sensitization or challenge with the contact sensitizer 2,4,6-trinitro-1-chlorobenzene. We found that Tregs acquired an activated phenotype (CD69(+)CD44(high)CD62L(-)) in the draining lymph node after sensitization. In contrast, Ag challenge activated Tregs in the blood. This tissue-specific activation was induced by ATP, which was released at the respective tissue sites after sensitization or challenge, respectively. To demonstrate that activation was also essential for the induction of the suppressive function of Tregs, Tregs were treated with ATP receptor antagonists. In this study, we show that ATP receptor antagonists abrogated the suppressive effects of injected naive Tregs in contact hypersensitivity reactions. Thus, these data indicate that activation of Tregs via ATP in vivo provides a novel pathway of stimulating the suppressive function of Tregs.

Role of protein tyrosine phosphatase SHP2 in barrier function of pulmonary endothelium

Pulmonary edema is mediated in part by disruption of interendothelial cell contacts. Protein tyrosine phosphatases (PTP) have been shown to affect both cell-extracellular matrix and cell-cell junctions. The SH2 domain-containing nonreceptor PTP, SHP2, is involved in intercellular signaling through direct interaction with adherens junction proteins. In this study, we examined the role of SHP2 in pulmonary endothelial barrier function. Inhibition of SHP2 promoted edema formation in rat lungs and increased monolayer permeability in cultured lung endothelial cells. In addition, pulmonary endothelial cells demonstrated a decreased level of p190RhoGAP activity following inhibition of SHP2, events that were accompanied by a concomitant increase in RhoA activity. Furthermore, immunofluorescence microscopy confirmed enhanced actin stress fiber formation and diminished interendothelial staining of adherens junction complex-associated proteins upon SHP2 inhibition. Finally, immunoprecipitation and immunoblot analyses demonstrated increased tyrosine phosphorylation of VE-cadherin, beta-catenin, and p190RhoGAP proteins, as well as decreased association between p120-catenin and VE-cadherin proteins. Our findings suggest that SHP2 supports basal pulmonary endothelial barrier function by coordinating the tyrosine phosphorylation profile of VE-cadherin, beta-catenin, and p190RhoGAP and the activity of RhoA, signaling molecules important in adherens junction complex integrity.

Molecular mechanisms mediating protective effect of cAMP on lipopolysaccharide (LPS)-induced human lung microvascular endothelial cells (HLMVEC) hyperpermeability

Up to date, the nature of the sepsis-induced vascular leakage is understood only partially, which limits pharmacological approaches for its management. Here we studied the protective effect of cAMP using endotoxin-induced hyperpermeability as a model for barrier dysfunction observed in gram-negative sepsis. We demonstrated that the alleviation of lipopolysaccharide (LPS)-induced barrier compromise could be achieved by the specific activation of either protein kinase A (PKA) or Epac with cAMP analogs Bnz-cAMP or O-Me-cAMP, respectively. We next studied the involvement of PKA substrates VASP and filamin1 in barrier maintenance and LPS-induced barrier compromise. Depletion of both VASP and filamin1 with the specific siRNAs significantly exacerbated both the quiescent cells barrier and LPS-induced barrier dysfunction, suggesting barrier-protective role of these proteins. VASP depletion was associated with the more severe loss of ZO-1 peripheral staining in response to LPS, whereas filamin1-depleted cells reacted to LPS with more robust stress fiber induction and more profound changes in ZO-1 and VE-cadherin peripheral organization. Both VASP and filamin1 phosphorylation was significantly increased as a result of PKA activation. We next analyzed the effect of VASP and filamin1 depletion on the PKA-dependent alleviation of LPS-induced barrier compromise. We observed that Bnz-cAMP ability to counteract LPS-induced hyperpermeability was attenuated only by VASP, but not filamin1 depletion. Our data indicate that while PKA-dependent VASP phosphorylation contributes to the protective effect of cAMP elicited on LPS-compromised monolayers, filamin1 phosphorylation is unlikely to play a significant role in this process.

DARC on RBC limits lung injury by balancing compartmental distribution of CXC chemokines

The Duffy antigen receptor for chemokines (DARC) has a high affinity for CC and CXC chemokines. However, it lacks the ability to induce cell responses that are typical for classical chemokine receptors. The role of DARC in inflammatory conditions remains to be elucidated. We studied the role of DARC in a murine model of acute lung injury. We found that in Darc-gene-deficient (Darc(-/-)) mice, LPS-induced PMN migration into the alveolar space was elevated more than twofold. In contrast, PMN adhesion to endothelial cells and within the interstitial space was reduced in Darc(-/-) mice. Darc(-/-) mice also exhibited increased microvascular permeability. Elevated PMN migration in Darc(-/-) mice was associated with increased concentrations of two essential CXCR2 ligands, CXCL1 and CXCL2/3 in the alveolar space. In the blood, CXCL1 was mostly associated with RBC in WT mice and with plasma in Darc(-/-) mice. We found that DARC on RBC prevented excessive PMN migration into the alveolar space. In contrast, DARC on non-hematopoietic cells appeared to have only minor effects on leukocyte trafficking in this model. These findings show how DARC regulates lung inflammation by controlling the distribution and presentation of chemokines that bind CXCR2.

Regulators of endothelial and epithelial barrier integrity and function in acute lung injury

Permeability edema is a life-threatening complication accompanying acute lung injury (ALI), severe pneumonia and the acute respiratory distress syndrome (ARDS), which can be associated with a reduced alveolar liquid clearance (ALC) capacity, a disruption of the alveolar epithelial barrier, and an increased capillary endothelial permeability. Bacterial and viral infections can directly promote pulmonary endothelial hyperpermeability and indirectly decrease the function and/or expression of ion transporters regulating ALC in type II alveolar epithelial cells, by means of inducing a strong inflammatory and oxidative stress response in the infected lungs. Apart from ventilation strategies, no standard treatment exists for permeability edema, making the search for novel regulators of endothelial and epithelial hyperpermeability and dysfunction important. Here, we present an overview of recently identified substances that inhibit and/or reverse endothelial barrier disruption and permeability or alveolar epithelial dysfunction: (1) zinc chelators, which were shown to attenuate the effects of oxidative stress on the pulmonary endothelium; (2) peroxisome proliferator activated receptor (PPAR) ligands, which have been shown to exert anti-inflammatory effects, by decreasing the expression of pro-inflammatory genes; (3) extracellular ATP, produced during inflammation, which induces a rapid and dose-dependent increase in transendothelial electrical resistance (TER) across pulmonary endothelial cells; (4) the lectin-like domain of TNF, which is spatially distinct from the receptor binding sites and which protects from hydrostatic and permeability edema and (5) Hsp90 inhibitors, which prevent and repair toxin-induced hyperpermeability. Unraveling the mechanism of action of these agents could contribute to the development of novel therapeutic strategies to combat permeability edema.

Acute lung inflammation and ventilator-induced lung injury caused by ATP via the P2Y receptors: an experimental study

BACKGROUND

Extracellular adenosine 5'-triphosphate (ATP) is an endogenous signaling molecule involved in multiple biological phenomena, including inflammation. The effects of extracellular ATP in the lung have not been fully clarified. This study examined 1) the biological roles of extracellular ATP in the pathogenesis of lung inflammation and 2) the possibility of involvement of extracellular ATP in mechanical ventilation-induced lung injury.

METHODS

The effects of intratracheal ATP on lung permeability, edema or lung inflammation were assessed by measurements of the lung wet-to-dry weight ratio and lung permeability index, immunohistochemistry and expression of key cytokines by real-time polymerase chain reaction. The ATP concentration in broncho-alveolar lavage (BAL) fluid from mice mechanically ventilated was measured by luciferin-luciferase assay. The suppressive effects of a P2 receptor antagonist on ventilator-induced lung inflammation were also examined.

RESULTS

ATP induced inflammatory reactions in the lung mainly via the ATP-P2Y receptor system. These reactions were alleviated by the co-administration of a specific P2 receptor antagonist. Mechanical ventilation with a large tidal volume caused lung inflammation and increased the ATP concentration in BAL fluid. P2 receptor antagonism partially mitigated the inflammatory effects of large tidal volume ventilation.

CONCLUSION

Our observations suggest that the ATP-P2Y receptor system is partially involved in the pathogenesis of ventilator-induced lung injury.

Effects of extracellular ATP on bovine lung endothelial and epithelial cell monolayer morphologies, apoptoses, and permeabilities

Pneumonia in cattle is an important disease both economically and in terms of animal welfare. Recent evidence in other species has shown ATP to be an important modulator of inflammation in the lung, where it is released by activated alveolar macrophages and damaged lung cells. Whether ATP serves a similar process during infection in the bovine lung is unknown. In the present study, we examined the effects of ATP treatment on the morphology, apoptosis, and permeability of bovine pulmonary epithelial (BPE) cells and bovine pulmonary microvascular endothelial cells (BPMEC). Monolayers of BPE cells underwent striking morphological changes when exposed to ATP that included separation of the cells. Neither BPE cells nor BPMEC exhibited increased apoptosis in response to ATP. BPE cell and BPMEC monolayers displayed virtually identical increases in permeability when exposed to ATP, with a 50% change occurring within the first hour of exposure. Both cell types contained mRNA for the P2X(7) receptor, a known receptor for ATP. In BPE cells, but not BPMEC, the change in permeability in response to ATP was reversed by the addition of a P2X(7) receptor antagonist. If similar permeability changes occur in vivo, they could be a factor in vascular leakage into lung airspaces during pneumonia.