Protective role of the leukotriene B4 receptor BLT2 in murine inflammatory colitis

Neutrophils secrete exosome-associated DNA to resolve sterile acute inflammation

Acute inflammation, characterized by a rapid influx of neutrophils, is a protective response that can lead to chronic inflammatory diseases when left unresolved. Secretion of LTB 4 -containing exosomes is required for effective neutrophil infiltration during inflammation. In this study, we show that neutrophils release nuclear DNA in a non-lytic, rapid, and repetitive manner, via a mechanism distinct from suicidal NET release and cell death. The packaging of nuclear DNA occurs in the lumen of nuclear envelope (NE)-derived multivesicular bodies (MVBs) that harbor the LTB 4 synthesizing machinery and is mediated by the lamin B receptor (LBR) and chromatin decondensation. Disruption of secreted exosome-associated DNA (SEAD) in a model of sterile inflammation in mouse skin amplifies and prolongs the presence of neutrophils, impeding the onset of resolution. Together, these findings advance our understanding of neutrophil functions during inflammation and the physiological significance of NETs, with implications for novel treatments for inflammatory disorders.

Two distinct forms of human BLT2: long-form and short-form BLT2

BLT2 is a low-affinity leukotriene B4 receptor that plays an essential role in the pathogenesis of various inflammatory diseases, including asthma and cancer. BLT2 is minimally expressed in a normal internal environment but is overexpressed in a stress-induced inflammatory environment. Recent research indicated that human BLT2 has two distinct forms. Although their functions are likely to be different, very few studies investigated these differences. Therefore, this paper will discuss about the two distinct forms of human BLT2; the short-form of BLT2 and the long-form of BLT2.

Associations of serum long-chain fatty acids with multiple organ involvement in patients with sarcoidosis

Background

Fatty acids have diverse immunomodulatory functions and the potential to be associated with inflammatory responses in sarcoidosis.

Methods

The serum levels of multiple long-chain fatty acids (LCFAs) were compared between 63 patients with sarcoidosis and 38 healthy controls. The associations of LCFAs with clinical outcomes of sarcoidosis were also evaluated.

Results

The patients with sarcoidosis had significantly lower levels of n-3 poly-unsaturated fatty acids (PUFAs) (p < 0.001) and n-6 PUFAs (p < 0.001) than the healthy controls. However, there were no significant differences in the levels of saturated fatty acids (SFAs) and mono-unsaturated fatty acids (MUFAs) between the two groups. On multivariate logistic analysis, lower levels of n-3 PUFAs, n-6 PUFAs, and n-3/n-6 ratio were predictive of sarcoidosis. Among the patients with sarcoidosis, those with multiple organ involvement had significantly lower levels of n-3 PUFAs and n-3/n-6 ratio than those with single organ involvement. There were no significant differences in the levels of n-6 PUFAs, SFAs, and MUFAs between the patients with multiple and single organ involvement. On multivariate logistic analysis, lower levels of SFAs and n-3/n-6 ratio were predictive of multiple organ involvement. The levels of LCFAs had no significant association with radiographic stage or spontaneous remission.

Conclusions

Assessment of LCFA profiles may be useful for the diagnosis of sarcoidosis and evaluation of the disease activity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12890-022-02084-x.

Lipids in Liver Failure Syndromes: A Focus on Eicosanoids, Specialized Pro-Resolving Lipid Mediators and Lysophospholipids

Lipids are organic compounds insoluble in water with a variety of metabolic and non-metabolic functions. They not only represent an efficient energy substrate but can also act as key inflammatory and anti-inflammatory molecules as part of a network of soluble mediators at the interface of metabolism and the immune system. The role of endogenous bioactive lipid mediators has been demonstrated in several inflammatory diseases (rheumatoid arthritis, inflammatory bowel disease, atherosclerosis, cancer). The liver is unique in providing balanced immunotolerance to the exposure of bacterial components from the gut transiting through the portal vein and the lymphatic system. This balance is abruptly deranged in liver failure syndromes such as acute liver failure and acute-on-chronic liver failure. In these syndromes, researchers have recently focused on bioactive lipid mediators by global metabonomic profiling and uncovered the pivotal role of these mediators in the immune dysfunction observed in liver failure syndromes explaining the high occurrence of sepsis and subsequent organ failure. Among endogenous bioactive lipids, the mechanistic actions of three classes (eicosanoids, pro-resolving lipid mediators and lysophospholipids) in the pathophysiological modulation of liver failure syndromes will be the topic of this narrative review. Furthermore, the therapeutic potential of lipid-immune pathways will be described.

2-Deoxy-d-glucose induces deglycosylation of proinflammatory cytokine receptors and strongly reduces immunological responses in mouse models of inflammation

Anti‐proinflammatory cytokine therapies against interleukin (IL)‐6, tumor necrosis factor (TNF)‐α, and IL‐1 are major advancements in treating inflammatory diseases, especially rheumatoid arthritis. Such therapies are mainly performed by injection of antibodies against cytokines or cytokine receptors. We initially found that the glycolytic inhibitor 2‐deoxy‐d‐glucose (2‐DG), a simple monosaccharide, attenuated cellular responses to IL‐6 by inhibiting N‐linked glycosylation of the IL‐6 receptor gp130. Aglycoforms of gp130 did not bind to IL‐6 or activate downstream intracellular signals that included Janus kinases. 2‐DG completely inhibited dextran sodium sulfate‐induced colitis, a mouse model for inflammatory bowel disease, and alleviated laminarin‐induced arthritis in the SKG mouse, an experimental model for human rheumatoid arthritis. These diseases have been shown to be partially dependent on IL‐6. We also found that 2‐DG inhibited signals for other proinflammatory cytokines such as TNF‐α, IL‐1β, and interferon ‐γ, and accordingly, prevented death by another inflammatory disease, lipopolysaccharide (LPS) shock. Furthermore, 2‐DG prevented LPS shock, a model for a cytokine storm, and LPS‐induced pulmonary inflammation, a model for acute respiratory distress syndrome of coronavirus disease 2019 (COVID‐19). These results suggest that targeted therapies that inhibit cytokine receptor glycosylation are effective for treatment of various inflammatory diseases.

Lysophosphatidic acid receptor1/3 antagonist inhibits the activation of satellite glial cells and reduces acute nociceptive responses

Lysophosphatidic acid (LPA) exerts various biological activities through six characterized G protein-coupled receptors (LPA_1-6 ). While LPA-LPA₁  signaling contributes toward the demyelination and retraction of C-fiber and induces neuropathic pain, the effects of LPA-LPA₁  signaling on acute nociceptive pain is uncertain. This study investigated the role of LPA-LPA₁  signaling in acute nociceptive pain using the formalin test. The pharmacological inhibition of the LPA-LPA₁ axis significantly attenuated formalin-induced nociceptive behavior. The LPA₁  mRNA was expressed in satellite glial cells (SGCs) in dorsal root ganglion (DRG) and was particularly abundant in SGCs surrounding large DRG neurons, which express neurofilament 200. Treatment with LPA_1/3 receptor (LPA_1/3 ) antagonist inhibited the upregulation of glial markers and inflammatory cytokines in DRG following formalin injection. The LPA_1/3 antagonist also attenuated phosphorylation of extracellular signal-regulated kinase, especially in SGCs and cyclic AMP response element-binding protein in the dorsal horn following formalin injection. LPA amounts after formalin injection to the footpad were quantified by liquid chromatography/tandem mass spectrometry, and LPA levels were found to be increased in the innervated DRGs. Our results indicate that LPA produced in the innervated DRGs promotes the activation of SGCs through LPA₁ , increases the sensitivity of primary neurons, and modulates pain behavior. These results facilitate our understanding of the pathology of acute nociceptive pain and demonstrate the possibility of the LPA₁ on SGCs as a novel target for acute pain control.

Acceleration of Trichinella spiralis worm expulsion by leukotriene B4 receptor binding inhibition

AIMS

Helminth infection typically induces a Th2 inflammatory response that is characterized by eosinophilia, high levels of IgE and mast cells. LTB4 is generated from innate immune cells, such as neutrophils, macrophages and mast cells, in response to a range of stimuli. It mainly acts on myeloid leukocytes, inducing the activation of integrins, adhesion to endothelium walls, and chemotaxis.

METHODS AND RESULTS

The objective of the present study was to determine the role of the LTB4 receptor in Trichinella spiralis expulsion. We treated mice with the LTB4 receptor antagonist before infection with T. spiralis. We observed that the number of mast cells and worm infection decreased following treatment with the BLT antagonist during the intestinal phase. We also demonstrated that blocking the LTB4 receptor inhibited neutrophil and eosinophil infiltration.

CONCLUSIONS

Further studies are required to investigate the specific mechanism of mast cell number decrease and worm infection and the in vitro interactions between LTB4 and worm expulsion.

Elevated cerebrospinal fluid cytokine levels in tuberculous meningitis predict survival in response to dexamethasone

Adjunctive treatment with antiinflammatory corticosteroids like dexamethasone increases survival in tuberculosis meningitis. Dexamethasone responsiveness associates with a C/T variant in Leukotriene A4 Hydrolase (LTA4H), which regulates expression of the proinflammatory mediator leukotriene B4 (LTB4). TT homozygotes, with increased expression of LTA4H, have the highest survival when treated with dexamethasone and the lowest survival without. While the T allele is present in only a minority of the world's population, corticosteroids confer modest survival benefit worldwide. Using Bayesian methods, we examined how pretreatment levels of cerebrospinal fluid proinflammatory cytokines affect survival in dexamethasone-treated tuberculous meningitis. LTA4H TT homozygosity was associated with global cytokine increases, including tumor necrosis factor. Association between higher cytokine levels and survival extended to non-TT patients, suggesting that other genetic variants may also induce dexamethasone-responsive pathological inflammation. These findings warrant studies that tailor dexamethasone therapy to pretreatment cerebrospinal fluid cytokine concentrations, while searching for additional genetic loci shaping the inflammatory milieu.

Emerging roles of metabolites of ω3 and ω6 essential fatty acids in the control of intestinal inflammation

The gastrointestinal tract is continuously exposed to the external environment, which contains numerous non-self antigens, including food materials and commensal micro-organisms. For the maintenance of mucosal homeostasis, the intestinal epithelial layer and mucosal immune system simultaneously provide the first line of defense against pathogens and are tightly regulated to prevent their induction of inflammatory responses to non-pathogenic antigens. Defects in mucosal homeostasis lead to the development of inflammatory and associated intestinal diseases, such as Crohn’s disease, ulcerative colitis, food allergy and colorectal cancer. The recent discovery of novel dietary ω3 and ω6 lipid-derived metabolites—such as resolvin, protectin, maresin, 17,18-epoxy-eicosatetraenoic acid and microbe-dependent 10-hydroxy-cis-12-octadecenoic acid—and their potent biologic effects on the regulation of inflammation have initiated a new era of nutritional immunology. In this review, we update our understanding of the role of lipid metabolites in intestinal inflammation.

Leukotriene B4 Receptor Type 2 Accelerates the Healing of Intestinal Lesions by Promoting Epithelial Cell Proliferation

Leukotriene B₄ receptor type 2 (BLT2) is a low-affinity leukotriene B₄ receptor that is highly expressed in intestinal epithelial cells. Previous studies demonstrated the protective role of BLT2 in experimentally induced colitis. However, its role in intestinal lesion repair is not fully understood. We investigated the role of BLT2 in the healing of indomethacin-induced intestinal lesions in mice. There was no significant different between wild-type (WT) and BLT2-deficient (BLT2KO) mice in terms of the development of indomethacin-induced intestinal lesions. However, healing of these lesions was significantly impaired in BLT2KO mice compared with WT mice. In contrast, transgenic mice with intestinal epithelium-specific BLT2 overexpression presented with superior ileal lesion healing relative to WT mice. An immunohistochemical study showed that the number of Ki-67-proliferative cells was markedly increased during the healing of intestinal lesions in WT mice but significantly attenuated in BLT2KO mice. Exposure of cultured mouse intestinal epithelial cells to CAY10583, a BLT2 agonist, promoted wound healing and cell proliferation in a concentration-dependent manner. Nevertheless, these responses were abolished under serum-free conditions. The CAY10583-induced proliferative effect was also negated by Go6983, a protein kinase C (PKC) inhibitor, U-73122, a phospholipase C (PLC) inhibitor, LY255283, a BLT2 antagonist, and pertussis toxin that inhibits G protein-coupled receptor signaling via G_i/o proteins. Thus, BLT2 plays an important role in intestinal wound repair. Moreover, this effect is mediated by the promotion of epithelial cell proliferation via the G_i/o protein-dependent and PLC/PKC signaling pathways. The BLT2 agonists are potential therapeutic agents for the treatment of intestinal lesions. SIGNIFICANCE STATEMENT: The healing of indomethacin-induced Crohn's disease-like intestinal lesions was impaired in mice deficient in low-affinity leukotriene B₄ receptor type 2 (BLT2). They presented with reduced epithelial cell proliferation during the healing. In contrast, healing was promoted in mice overexpressing intestinal epithelial BLT2. In cultured intestinal epithelial cells, the BLT2 agonist CAY10583 substantially accelerated wound repair by enhancing cell proliferation rather than migration. Thus, BLT2 plays an important role in the intestinal lesions via acceleration of epithelial cell proliferation.

Structure of the agonist 12-HHT in its BLT2 receptor-bound state

G Protein-Coupled receptors represent the main communicating pathway for signals from the outside to the inside of most of eukaryotic cells. They define the largest family of integral membrane receptors at the surface of the cells and constitute the main target of the current drugs on the market. The low affinity leukotriene receptor BLT2 is a receptor involved in pro- and anti-inflammatory pathways and can be activated by various unsaturated fatty acid compounds. We present here the NMR structure of the agonist 12-HHT in its BLT2-bound state and a model of interaction of the ligand with the receptor based on a conformational homology modeling associated with docking simulations. Put into perspective with the data obtained with leukotriene B4, our results illuminate the ligand selectivity of BLT2 and may help define new molecules to modulate the activity of this receptor.

Intestinal Barrier Function in Gluten-Related Disorders

Gluten-related disorders include distinct disease entities, namely celiac disease, wheat-associated allergy and non-celiac gluten/wheat sensitivity. Despite having in common the contact of the gastrointestinal mucosa with components of wheat and other cereals as a causative factor, these clinical entities have distinct pathophysiological pathways. In celiac disease, a T-cell mediate immune reaction triggered by gluten ingestion is central in the pathogenesis of the enteropathy, while wheat allergy develops as a rapid immunoglobulin E- or non-immunoglobulin E-mediated immune response. In non-celiac wheat sensitivity, classical adaptive immune responses are not involved. Instead, recent research has revealed that an innate immune response to a yet-to-be-defined antigen, as well as the gut microbiota, are pivotal in the development in this disorder. Although impairment of the epithelial barrier has been described in all three clinical conditions, its role as a potential pathogenetic co-factor, specifically in celiac disease and non-celiac wheat sensitivity, is still a matter of investigation. This article gives a short overview of the mucosal barrier of the small intestine, summarizes the aspects of barrier dysfunction observed in all three gluten-related disorders and reviews literature data in favor of a primary involvement of the epithelial barrier in the development of celiac disease and non-celiac wheat sensitivity.

The Role of Leukotrienes as Potential Therapeutic Targets in Allergic Disorders

Leukotrienes (LTs) are lipid mediators that play pivotal roles in acute and chronic inflammation and allergic diseases. They exert their biological effects by binding to specific G-protein-coupled receptors. Each LT receptor subtype exhibits unique functions and expression patterns. LTs play roles in various allergic diseases, including asthma (neutrophilic asthma and aspirin-sensitive asthma), allergic rhinitis, atopic dermatitis, allergic conjunctivitis, and anaphylaxis. This review summarizes the biology of LTs and their receptors, recent developments in the area of anti-LT strategies (in settings such as ongoing clinical studies), and prospects for future therapeutic applications.

BLT1 in dendritic cells promotes Th1/Th17 differentiation and its deficiency ameliorates TNBS-induced colitis

Leukotriene B4 (LTB4) synthesis is enhanced in the colonic mucosa in patients with inflammatory bowel disease (IBD). BLT1, a high-affinity receptor for LTB4, exhibits no effect on the progression of dextran sodium sulfate (DSS)-induced colitis, which mostly relies on innate immunity. Here, we reported that BLT1 regulates trinitrobenzene sulfonic acid (TNBS)-induced colitis, which reflects CD4+ T-cell-dependent adaptive immune mechanisms of IBD. We found that BLT1 signaling enhanced the progression of colitis through controlling the production of proinflammatory cytokines by dendritic cells (DCs) and modulating the differentiation of Th1 and Th17. BLT1-/- mice displayed an alleviated severity of TNBS-induced colitis with reduced body weight loss and infiltrating cells in the lamina propria. BLT1 deficiency in DCs led to reduced production of proinflammatory cytokines, including IL-6, TNF-α, and IL-12, and these results were further confirmed via treatment with a BLT1 antagonist. The impaired cytokine production by BLT1-/- DCs subsequently led to reduced Th1 and Th17 differentiation both in vitro and in vivo. We further performed a conditional DC reconstitution experiment to assess whether BLT1 in DCs plays a major role in regulating the pathogenesis of TNBS-induced colitis, and the results indicate that BLT1 deficiency in DCs also significantly reduces disease severity. The mechanistic study demonstrated that BLT1-regulated proinflammatory cytokine production through the Gαi βγ subunit-phospholipase Cβ (PLCβ)-PKC pathway. Notably, we found that treatment with the BLT1 antagonist also reduced the production of proinflammatory cytokines by human peripheral blood DCs. Our findings reveal the critical role of BLT1 in regulating adaptive immunity and TNBS-induced colitis, which further supports BLT1 as a potential drug target for adaptive immunity-mediated IBD.

Biological functions of 12(S)-hydroxyheptadecatrienoic acid as a ligand of leukotriene B4 receptor 2

Although 12(S)-hydroxyheptadecatrienoic acid (12-HHT) is an abundant fatty acid, it is long considered a byproduct of thromboxane A₂ production. We identified a leukotriene B₄ receptor 2 (BLT2)-specific agonistic activity in lipid extracts from rat small intestine, and mass spectrometric analysis of partially purified lipids containing BLT2 agonistic activity revealed that 12-HHT is an endogenous ligand of BLT2. In a dextran sulfate sodium (DSS)-induced inflammatory colitis model, BLT2-deficient mice exhibited enhanced intestinal inflammation, possibly due to impaired epithelial barrier function. In a skin wound healing model, BLT2-deficient mice exhibited delayed wound healing via dampened keratinocyte migration. BLT2 also accelerates corneal wound healing, and eye drops containing a non-steroidal anti-inflammatory drug (NSAID) inhibit the production of 12-HHT, resulting in delayed corneal wound healing. Furthermore, BLT2 is expressed in pulmonary epithelial type II cells and vascular endothelial cells in the mouse lung, and BLT2-deficient mice are more susceptible to lung damage by pneumolysin. In this review, we summarize the identification and characterization of 12-HHT as a ligand for BLT2 and discuss recent research on the physiological and pathophysiological roles of the 12-HHT-BLT2 axis. Some side effects of NSAIDs such as delayed wound healing may be caused by reduced 12-HHT production rather than diminished production of prostaglandins.

Arachidonic Acid Metabolites in Cardiovascular and Metabolic Diseases

Lipid and immune pathways are crucial in the pathophysiology of metabolic and cardiovascular disease. Arachidonic acid (AA) and its derivatives link nutrient metabolism to immunity and inflammation, thus holding a key role in the emergence and progression of frequent diseases such as obesity, diabetes, non-alcoholic fatty liver disease, and cardiovascular disease. We herein present a synopsis of AA metabolism in human health, tissue homeostasis, and immunity, and explore the role of the AA metabolome in diverse pathophysiological conditions and diseases.

Leukotriene B4 promotes neovascularization and macrophage recruitment in murine wet-type AMD models

Age-related macular degeneration (AMD), a progressive chronic disease of the central retina, is associated with aging and is a leading cause of blindness worldwide. Here, we demonstrate that leukotriene B4 (LTB4) receptor 1 (BLT1) promotes laser-induced choroidal neovascularization (CNV) in a mouse model for wet-type AMD. CNV was significantly less in BLT1-deficient (BLT1-KO) mice compared with BLT1-WT controls. Expression of several proangiogenic and profibrotic factors was lower in BLT1-KO eyes than in BLT1-WT eyes. LTB4 production in the eyes was substantially increased in the early phase after laser injury. BLT1 was highly expressed in M2 macrophages in vitro and in vivo, and ocular BLT1+ M2 macrophages were increased in the aged eyes after laser injury. Furthermore, M2 macrophages were rapidly attracted by LTB4 and subsequently produced VEGF-A- through BLT1-mediated signaling. Consequently, intravitreal injection of M2 macrophages augmented CNV formation, which was attenuated by BLT1 deficiency. Thus, laser-induced injury to the retina triggered LTB4 production and attracted M2 macrophages via BLT1, leading to development of CNV. A selective BLT1 antagonist (CP105696) and 3 LTB4 inhibitors (zileuton, MK-886, and bestatin) reduced CNV in a dose-dependent manner. CP105696 also inhibited the accumulation of BLT1+ M2 macrophages in the laser-injured eyes of aged mice. Together, these results indicate that the LTB4-BLT1 axis is a potentially novel therapeutic target for CNV of wet-type AMD.

Importance of the leukotriene B4-BLT1 and LTB4-BLT2 pathways in asthma

For several decades, the leukotriene pathways have been implicated as playing a central role in the pathophysiology of asthma. The presence and elevation of numerous metabolites in the blood, sputum, and bronchoalveolar lavage fluid from asthmatics or experimental animals adds support to this notion. However, targeting of the leukotriene pathways has had, in general, limited success. The single exception in asthma therapy has been targeting of the cysteinyl leukotriene receptor 1, which clinically has proven effective but only in certain clinical situations. Interference with 5-lipoxygenase has had limited success, in part due to adverse drug effects. The importance of the LTB4-BLT1 pathway in asthma pathogenesis has extensive experimental support and findings, albeit limited, from clinical samples. The LTB4-BLT1 pathway was shown to be important as a neutrophil chemoattractant. Despite observations made more than two decades ago, the LTB4-BLT1 pathway has only recently been shown to exhibit important activities on subsets of T lymphocytes, both as a chemoattractant and on lymphocyte activation, as well as on dendritic cells, the major antigen presenting cell in the lung. The role of BLT2 in asthma remains unclear. Targeting of components of the LTB4-BLT1 pathway offers innovative therapeutic opportunities especially in patients with asthma that remain uncontrolled despite intensive corticosteroid treatment.

Identification, signaling, and functions of LTB4 receptors

Leukotriene B₄ (LTB₄), a lipid mediator produced from arachidonic acid, is a chemoattractant for inflammatory leukocytes. We identified two receptors for LTB₄, the high-affinity receptor BLT1 and the low-affinity receptor BLT2. BLT1 is expressed in various subsets of leukocytes, and analyses of BLT1-deficient mice revealed that the LTB₄/BLT1 axis enhances leukocyte recruitment to infected sites, and is involved in the elimination of pathogens. Hyperactivation of the LTB₄/BLT1 axis induces acute and chronic inflammation, resulting in various inflammatory diseases. BLT2 was originally identified as a low-affinity receptor for LTB₄, and we later identified 12(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT) as a high-affinity ligand for BLT2. BLT2 is highly expressed in epithelial cells in various tissues including intestine and skin. Large quantities of 12-HHT are produced by activated platelets during skin injury, and activation of BLT2 on epidermal keratinocytes accelerates skin wound healing by enhancing cell migration. BLT2 signaling also enhances cell-cell junctions, protectes against transepidermal water loss, and preventes entry of environmental substances into the body.

The role of the LTB4-BLT1 axis in chemotactic gradient sensing and directed leukocyte migration

Directed leukocyte migration is a hallmark of inflammatory immune responses. Leukotrienes are derived from arachidonic acid and represent a class of potent lipid mediators of leukocyte migration. In this review, we summarize the essential steps leading to the production of LTB₄ in leukocytes. We discuss the recent findings on the exosomal packaging and transport of LTB₄ in the context of chemotactic gradients formation and regulation of leukocyte recruitment. We also discuss the dynamic roles of the LTB₄ receptors, BLT1 and BLT2, in mediating chemotactic signaling in leukocytes and contrast them to other structurally related leukotrienes that bind to distinct GPCRs. Finally, we highlight the specific roles of the LTB₄-BLT1 axis in mediating signal-relay between chemotaxing neutrophils and its potential contribution to a wide variety of inflammatory conditions including tumor progression and metastasis, where LTB₄ is emerging as a key signaling component.

Leukotriene receptors as potential therapeutic targets

Leukotrienes, a class of arachidonic acid-derived bioactive molecules, are known as mediators of allergic and inflammatory reactions and considered to be important drug targets. Although an inhibitor of leukotriene biosynthesis and antagonists of the cysteinyl leukotriene receptor are clinically used for bronchial asthma and allergic rhinitis, these medications were developed before the molecular identification of leukotriene receptors. Numerous studies using cloned leukotriene receptors and genetically engineered mice have unveiled new pathophysiological roles for leukotrienes. This Review covers the recent findings on leukotriene receptors to revisit them as new drug targets.

Illuminating the Energy Landscape of GPCRs: The Key Contribution of Solution-State NMR Associated with Escherichia coli as an Expression Host

Conformational dynamics of GPCRs are central to their function but are difficult to explore at the atomic scale. Solution-state NMR has provided the major contribution in that area of study during the past decade, despite nonoptimized labeling schemes due to the use of insect cells and, to a lesser extent, yeast as the main expression hosts. Indeed, the most efficient isotope-labeling scheme ever to address energy landscape issues for large proteins or protein complexes relies on the use of ¹³CH₃ probes immersed in a perdeuterated dipolar environment, which is essentially out of reach of eukaryotic expression systems. In contrast, although its contribution has been underestimated because of technical issues, Escherichia coli is by far the best-adapted host for such labeling. As it is now tightly controlled, we show in this review that bacterial expression can provide an NMR spectral resolution never achieved in the GPCR field.

Leukotriene B4 receptor 2 regulates the proliferation, migration, and barrier integrity of bronchial epithelial cells

The airway epithelium plays a crucial role in the pathogenesis of asthma. The functions of leukotriene B4 receptor 2 (BLT2) on the airway epithelial cells remains unknown. In our study, BLT2 expression in 16HBE bronchial epithelial cells were manipulated by transfection with BLT2 overexpression plasmid or BLT2 small interference RNA. 16HBE cells were then exposed to BLT2 antagonist (LY255283) or BLT2 agonist (12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid [12-HHT] or CAY10583). The results showed that BLT2 overexpression, 12-HHT stimulation, or CAY10583 treatment resulted in the enhanced proliferation and migration of 16HBE cells. In addition, BLT2 showed an inhibitory effect on epithelial permeability as illustrated by the measurement of transepithelial electrical resistance (TER) and epithelial permeability, and a promoting effect on the levels of tight junction proteins (occludin and claudin-4) and phosphorylated p38 as demonstrated by real-time PCR and Western blotting analyses. These results suggest BLT2 as a key determinant of airway epithelial barrier integrity. On the contrary, RNAi-mediated knockdown or LY255283 treatment had reversed effects on the proliferation, migration, and epithelial barrier integrity. Together, our findings suggest the critical roles of BLT2 on the functions of bronchial epithelial cells and that BLT2 agonists are potential therapeutic agents for asthma treatment.

Relationship between G proteins coupled receptors and tight junctions

Tight junctions (TJs) are sites of cell-cell adhesion, constituted by a cytoplasmic plaque of molecules linked to integral proteins that form a network of strands around epithelial and endothelial cells at the uppermost portion of the lateral membrane. TJs maintain plasma membrane polarity and form channels and barriers that regulate the transit of ions and molecules through the paracellular pathway. This structure that regulates traffic between the external milieu and the organism is affected in numerous pathological conditions and constitutes an important target for therapeutic intervention. Here, we describe how a wide array of G protein-coupled receptors that are activated by diverse stimuli including light, ions, hormones, peptides, lipids, nucleotides and proteases, signal through heterotrimeric G proteins, arrestins and kinases to regulate TJs present in the blood-brain barrier, the blood-retinal barrier, renal tubular cells, keratinocytes, lung and colon, and the slit diaphragm of the glomerulus.

Non-steroidal anti-inflammatory drug delays corneal wound healing by reducing production of 12-hydroxyheptadecatrienoic acid, a ligand for leukotriene B4 receptor 2

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used to reduce inflammation by suppressing cyclooxygenases (COXs). NSAID eye drops are frequently prescribed after ocular surgery to reduce inflammation and pain, but this treatment has clinically significant side effects, including corneal ulcer and perforation. The molecular mechanisms underlying these side effects remain unknown. Recently, the COX product 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) was identified as an endogenous ligand for leukotriene B₄ receptor 2 (BLT2), which is important in maintenance of epithelial homeostasis. We hypothesized that NSAID-dependent corneal damage is caused by reduced production of 12-HHT. Diclofenac eye drops decreased the abundance of downstream products of COX and delayed corneal wound healing in BALB/c mice. Expression of BLT2 was observed in murine ocular tissues including cornea, and in human corneal epithelial cell line and human primary corneal epithelial cells. In BLT2-knockout mice, corneal wound healing was delayed, but the diclofenac-dependent delay in corneal wound healing disappeared. 12-HHT accelerated wound closure both in BLT2-transfected corneal cell line and human primary corneal epithelial cells. Thus, our results reveal that NSAIDs delay corneal wound healing by inhibiting 12-HHT production, and suggest that stimulation of the 12-HHT/BLT2 axis represents a novel therapeutic approach to corneal wound healing.

The yin and yang of leukotriene B4 mediated inflammation in cancer

The high affinity leukotriene B₄ receptor, BLT1 mediates chemotaxis of diverse leukocyte subsets to the sites of infection or inflammation. Whereas the pathological functions of LTB₄/BLT1 axis in allergy, autoimmunity and cardiovascular disorders are well established; its role in cancer is only beginning to emerge. In this review, we summarize recent findings on LTB₄/BLT1 axis enabling distinct outcomes toward tumor progression. In a mouse lung tumor model promoted by silicosis-induced inflammation, genetic deletion of BLT1 attenuated neutrophilic inflammation and tumor promotion. In contrast, in a spontaneous model of intestinal tumorigenesis, absence of BLT1 led to defective mucosal host response, altered microbiota and bacteria dependent colon tumor progression. Furthermore, BLT1 mediated CD8⁺ T cell recruitment was shown to be essential for initiating anti-tumor immunity in number of xenograft models and is critical for effective PD1 based immunotherapy. BLT2 mediated chemotherapy resistance, tumor promotion and metastasis are also discussed. This new information points to a paradigm shift in our understanding of the LTB₄ pathways in cancer.

Group III phospholipase A2 promotes colitis and colorectal cancer

Lipid mediators play pivotal roles in colorectal cancer and colitis, but only a limited member of the phospholipase A₂ (PLA₂) subtypes, which lie upstream of various lipid mediators, have been implicated in the positive or negative regulation of these diseases. Clinical and biochemical evidence suggests that secreted PLA₂ group III (sPLA₂-III) is associated with colorectal cancer, although its precise role remains obscure. Here we have found that sPLA₂-III-null (Pla2g3 ^-/-) mice are highly resistant to colon carcinogenesis. Furthermore, Pla2g3 ^-/- mice are less susceptible to dextran sulfate-induced colitis, implying that the amelioration of colonic inflammation by sPLA₂-III ablation may underlie the protective effect against colon cancer. Lipidomics analysis of the colon revealed significant reduction of pro-inflammatory/pro-tumorigenic lysophosholipids as well as unusual steady-state elevation of colon-protective fatty acids and their oxygenated metabolites in Pla2g3 ^-/- mice. Overall, our results establish a role of sPLA₂-III in the promotion of colorectal inflammation and cancer, expand our understanding of the divergent roles of multiple PLA₂ enzymes in the gastrointestinal tract, and point to sPLA₂-III as a novel druggable target for colorectal diseases.

Biochemical and immunological characterization of a novel monoclonal antibody against mouse leukotriene B4 receptor 1

Leukotriene B₄ (LTB₄) receptor 1 (BLT1) is a G protein-coupled receptor expressed in various leukocyte subsets; however, the precise expression of mouse BLT1 (mBLT1) has not been reported because a mBLT1 monoclonal antibody (mAb) has not been available. In this study, we present the successful establishment of a hybridoma cell line (clone 7A8) that produces a high-affinity mAb for mBLT1 by direct immunization of BLT1-deficient mice with mBLT1-overexpressing cells. The specificity of clone 7A8 was confirmed using mBLT1-overexpressing cells and mouse peripheral blood leukocytes that endogenously express BLT1. Clone 7A8 did not cross-react with human BLT1 or other G protein-coupled receptors, including human chemokine (C-X-C motif) receptor 4. The 7A8 mAb binds to the second extracellular loop of mBLT1 and did not affect LTB₄ binding or intracellular calcium mobilization by LTB₄. The 7A8 mAb positively stained Gr-1-positive granulocytes, CD11b-positive granulocytes/monocytes, F4/80-positive monocytes, CCR2-high and CCR2-low monocyte subsets in the peripheral blood and a CD4-positive T cell subset, Th1 cells differentiated in vitro from naïve CD4-positive T cells. This mAb was able to detect Gr-1-positive granulocytes and monocytes in the spleens of naïve mice by immunohistochemistry. Finally, intraperitoneal administration of 7A8 mAb depleted granulocytes and monocytes in the peripheral blood. We have therefore succeeded in generating a high-affinity anti-mBLT1 mAb that is useful for analyzing mBLT1 expression in vitro and in vivo.

The leukotriene B4 receptors BLT1 and BLT2 form an antagonistic sensitizing system in peripheral sensory neurons

Sensitization of the heat-activated ion channel transient receptor potential vanilloid 1 (TRPV1) through lipids is a fundamental mechanism during inflammation-induced peripheral sensitization. Leukotriene B4 is a proinflammatory lipid mediator whose role in peripheral nociceptive sensitization is not well understood to date. Two major G-protein-coupled receptors for leukotriene B4 have been identified: the high-affinity receptor BLT1 and the low-affinity receptor BLT2. Transcriptional screening for the expression G-protein-coupled receptors in murine dorsal root ganglia showed that both receptors were among the highest expressed in dorsal root ganglia. Calcium imaging revealed a sensitization of TRPV1-mediated calcium increases in a relative narrow concentration range for leukotriene B4 (100-200 nm). Selective antagonists and neurons from knock-out mice demonstrated a BLT1-dependent sensitization of TRPV1-mediated calcium increases. Accordingly, leukotriene B4-induced thermal hyperalgesia was mediated through BLT1 and TRPV1 as shown using the respective knock-out mice. Importantly, higher leukotriene B4 concentrations (>0.5 μm) and BLT2 agonists abolished sensitization of the TRPV1-mediated calcium increases. Also, BLT2 activation inhibited protein kinase C- and protein kinase A-mediated sensitization processes through the phosphatase calcineurin. Consequently, a selective BLT2-receptor agonist increased thermal and mechanical withdrawal thresholds during zymosan-induced inflammation. In accordance with these data, immunohistochemical analysis showed that both leukotriene B4 receptors were expressed in peripheral sensory neurons. Thus, the data show that the two leukotriene B4 receptors have opposing roles in the sensitization of peripheral sensory neurons forming a self-restricting system.

Macrophage LTB4 drives efficient phagocytosis of Borrelia burgdorferi via BLT1 or BLT2

Unresolved experimental Lyme arthritis in C3H 5-lipoxygenase (5-LOX)^-/- mice is associated with impaired macrophage phagocytosis of Borrelia burgdorferi In the present study, we further investigated the effects of the 5-LOX metabolite, leukotriene (LT)B₄ on phagocytosis of B. burgdorferi Bone marrow-derived macrophages (BMDMs) from 5-LOX^-/- mice were defective in the uptake and killing of B. burgdorferi from the earliest stages of spirochete internalization. BMDMs from mice deficient for the LTB₄ high-affinity receptor (BLT1^-/-) were also unable to efficiently phagocytose B. burgdorferi Addition of exogenous LTB₄ augmented the phagocytic capability of BMDMs from both 5-LOX^-/- and BLT1^-/- mice, suggesting that the low-affinity LTB₄ receptor, BLT2, might be involved. Blocking BLT2 activity with the specific antagonist, LY255283, inhibited phagocytosis in LTB₄-stimulated BLT1^-/- BMDMs, demonstrating a role for BLT2. However, the lack of a phagocytic defect in BLT2^-/- BMDMs suggested that this was a compensatory effect. In contrast, 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid, a natural BLT2-specific high-affinity ligand, and resolvin E1, a BLT1 agonist, were both unable to boost phagocytosis in BMDMs from either 5-LOX^-/- or BLT1^-/- mice, suggesting a specific role for LTB₄ in mediating phagocytosis in murine macrophages. This study demonstrates that LTB₄ promotes macrophage phagocytosis of bacteria via BLT1, and that BLT2 can fulfill this role in the absence of BLT1.

A synthetic leukotriene B4 receptor type 2 agonist accelerates the cutaneous wound healing process in diabetic rats by indirect stimulation of fibroblasts and direct stimulation of keratinocytes

AIMS

The synthetic leukotriene B₄ receptor type 2 (BLT2) agonist CAY10583 (CAY) accelerates wound healing in diabetic mice by promoting keratinocyte migration. However, its effects on fibroblast activity and granulation are unknown. We investigated the mechanisms by which CAY promotes wound healing.

METHODS

CAY was applied to wounds on streptozotocin-induced diabetic rats, and wound closure, granulation thickness, and epithelialization gaps were analyzed. BLT2 expression was examined by RT-PCR. Migration and proliferation were studied by scratch assays and MTS assays. Keratinocyte supernatants with CAY were applied to fibroblasts, and cytokines were measured by enzyme-linked immunosorbent assays.

RESULTS

CAY significantly accelerated wound healing in diabetic rats (CAY, 78.05±12.22% vs. control, 59.84±11.09%; p=0.0222), with increased re-epithelialization and granulation compared to controls. BLT2 was expressed in keratinocytes, but not in fibroblasts. Keratinocyte treatment with the CAY supernatant enhanced fibroblast proliferation and migration (fibroblast scratch closure: CAY, 75.95±4.09% vs. control, 49.69±4.49%; p<0.0001). CAY-treated keratinocytes exhibited increased TGF-β1 and bFGF expression.

CONCLUSIONS

CAY directly promotes keratinocyte migration and indirectly enhances fibroblast proliferation by increasing keratinocyte production of TGF-β1 and bFGF, accelerating wound closure. CAY is a promising pharmaceutical agent for diabetic wounds.

BLT2 expression improves skin integrity and protects from alterations caused by hyperglycemia in type 2 diabetes

Type 2 diabetes (T2D) can go undiagnosed for years, leading to a stage where chronic high blood sugar produces complications such as delayed wound healing. Reports have shown that BLT2 activation improves keratinocyte migration and wound healing, as well as protecting the epidermal barrier through the promotion of actin polymerization. The goal of this study was to elucidate the role of BLT2 expression in skin epithelial integrity in T2D. For this purpose, we used both wild type (WT) and BLT2 knockout mice in a model, in which a T2D-like phenotype was induced by keeping the animals on a high fat (HF) diet over 5 weeks. In a parallel in vitro approach, we cultured BLT2-transfected HaCaT cells at both low and high glucose concentrations for 48 h. Structure, transepithelial resistance (TEER), IL-1ß, IL-8 or CXCL2, MMP9, Filaggrin, Loricrin and Keratin 10 (K10) were evaluated ex vivo and in vitro. Additionally, wound healing (WH) was studied in vitro. The skin from T2D and BLT2 knockout mice showed a reduction in TEER and the expression of IL-1ß, and in increase in CXCL2, MMP9, Filaggrin, Loricrin and K10 expression. The structure suggested an atrophic epidermis; however, the skin was dramatically affected in the BLT2 knockout mice kept on a HF diet. HaCaT-BLT2 cells presented as an organized monolayer and showed higher TEER and wound healing compared with vector only-transfected HaCaT-Mock cells. Likewise, alterations in the expression of skin inflammatory, matrix degradation and differentiation markers under low and high glucose conditions were less severe than in HaCaT-Mock cells. Our results suggest that BLT2 improves epithelial integrity and function by regulating differentiation markers, cytokines and MMP9. Furthermore, BLT2 attenuates the damaging effects of high glucose levels, thereby accelerating wound healing.

Eicosanoid receptors: Targets for the treatment of disrupted intestinal epithelial homeostasis

The importance of cyclooxygenase and lipoxygenase pathways and the consequent eicosanoid synthesis in the physiology and pathophysiology of the intestinal epithelium is currently being established. Each eicosanoid (prostanoid, leukotriene, hydroxyeicosatetraenoic acid) preferentially recognizes one or more receptors coupled to one or more signal-transduction processes. This overview focuses on the role of eicosanoid receptors in the maintenance of intestinal epithelium physiology through the control of proliferation/differentiation/apoptosis processes. Furthermore, it is reported that the role of these receptors on the regulation of the barrier function of the intestinal epithelium have arisen through the regulation of absorption/secretion processes, tight-junction state and the control of the intestinal immune response. Also, this review considers the implication of AA cascade in the disruption of epithelial homeostasis during inflammatory bowel diseases and colorectal cancer as well as the therapeutic values and potential of the eicosanoid receptors as novel targets for the treatments of the pathologies above mentioned.

Receptor for Advanced Glycation End Products Regulates Leukotriene B4 Receptor 1 Signaling

Leukotriene B₄ receptor 1 (BLT1), a high-affinity G protein-coupled receptor (GPCR) for leukotriene B₄ (LTB₄), plays important roles in inflammatory and immune reactions. Although the LTB₄-BLT1 axis is known to promote inflammation, the binding proteins that modulate LTB₄-BLT1 signaling have not been identified. Recently, we discovered that receptor for advanced glycation end products (RAGE) interacts with BLT1 and modulates LTB₄-BLT1 signaling. We propose RAGE as a new class of GPCR modulator and a new target of future GPCR studies.

Asymmetric synthesis of 12-hydroxyheptadecatrienoic acid and its 5,6-dihydro- and 14,15-dehydro-derivatives

Natural 12-hydroxyheptadecatrienoic acid (12-HHT) with an S configuration was synthesised by a Suzuki-Miyaura coupling of C10-C17 iodo alcohol with C1-C9 vinylborane. The iodo alcohol was synthesised by utilising Sharpless asymmetric epoxidation of the corresponding trimethylsilyl alcohol. The method yielded more than 100 mg of 12-HHT. Similarly, syntheses of 5,6-dihydro- and 14,15-dehydro derivatives of 12-HHT, known as HHD and HHTE, respectively, were completed in a stereoselective manner.

Leukotriene B4 receptor type 2 protects against pneumolysin-dependent acute lung injury

Although pneumococcal infection is a serious problem worldwide and has a high mortality rate, the molecular mechanisms underlying the lethality caused by pneumococcus remain elusive. Here, we show that BLT2, a G protein-coupled receptor for leukotriene B₄ and 12(S)-hydroxyheptadecatrienoic acid (12-HHT), protects mice from lung injury caused by a pneumococcal toxin, pneumolysin (PLY). Intratracheal injection of PLY caused lethal acute lung injury (ALI) in BLT2-deficient mice, with evident vascular leakage and bronchoconstriction. Large amounts of cysteinyl leukotrienes (cysLTs), classically known as a slow reactive substance of anaphylaxis, were detected in PLY-treated lungs. PLY-dependent vascular leakage, bronchoconstriction, and death were markedly ameliorated by treatment with a CysLT1 receptor antagonist. Upon stimulation by PLY, mast cells produced cysLTs that activated CysLT1 expressed in vascular endothelial cells and bronchial smooth muscle cells, leading to lethal vascular leakage and bronchoconstriction. Treatment of mice with aspirin or loxoprofen inhibited the production of 12-HHT and increased the sensitivity toward PLY, which was also ameliorated by the CysLT1 antagonist. Thus, the present study identifies the molecular mechanism underlying PLY-dependent ALI and suggests the possible use of CysLT1 antagonists as a therapeutic tool to protect against ALI caused by pneumococcal infection.

Leukotriene receptor expression in esophageal squamous cell cancer and non-transformed esophageal epithelium: a matched case control study

BACKGROUND

Leukotriene B4 (LTB4R and LTB4R2) and cysteinyl leukotriene receptors (CYSLTR1 and CYSLTR2) contribute to malignant cell transformation. We aimed to investigate the expression of LTB4R, LTB4R2, CYSLTR1 and CYSLTR2 in esophageal squamous cell carcinoma and adjacent non-transformed squamous epithelium of the esophagus, as well as in control biopsy samples from esophageal squamous epithelium of patients with functional dyspepsia.

METHODS

Expression of LTB4R, LTB4R2, CYSLTR1 and CYSLTR2 was analyzed by immunohistochemistry (IHC) and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) in biopsy samples of 19 patients with esophageal squamous cell cancer and 9 sex- and age-matched patients with functional dyspepsia.

RESULTS

LTB4R, LTB4R2, CYSLTR1 and CYSLTR2 were expressed in all biopsy samples. Major findings were: 1) protein levels of all leukotriene receptors were significantly increased in esophageal squamous cell cancer compared to control mucosa (p < 0.05); 2) CYSLTR1 and CYSLTR2 gene expression was decreased in cancer tissue compared to control at 0.26-fold and 0.23-fold respectively; 3) an up-regulation of LTB4R (mRNA and protein expression) and a down-regulation of CYSLTR2 (mRNA expression) in non-transformed epithelium of cancer patients compared to control (p < 0.05) was observed.

CONCLUSIONS

The expression of leukotriene receptors was deregulated in esophageal squamous cell cancer. Up-regulation of LTB4R and down-regulation of CYSLTR2 gene expression may occur already in normal squamous esophageal epithelium of patients with esophageal cancer suggesting a potential role of these receptors in early steps of esophageal carcinogenesis. Larger studies are warranted to confirm these observations.

Group X Secreted Phospholipase A2 Releases ω3 Polyunsaturated Fatty Acids, Suppresses Colitis, and Promotes Sperm Fertility

Within the secreted phospholipase A2(sPLA2) family, group X sPLA2(sPLA2-X) has the highest capacity to hydrolyze cellular membranes and has long been thought to promote inflammation by releasing arachidonic acid, a precursor of pro-inflammatory eicosanoids. Unexpectedly, we found that transgenic mice globally overexpressing human sPLA2-X (PLA2G10-Tg) displayed striking immunosuppressive and lean phenotypes with lymphopenia and increased M2-like macrophages, accompanied by marked elevation of free ω3 polyunsaturated fatty acids (PUFAs) and their metabolites. Studies usingPla2g10-deficient mice revealed that endogenous sPLA2-X, which is highly expressed in the colon epithelium and spermatozoa, mobilized ω3 PUFAs or their metabolites to protect against dextran sulfate-induced colitis and to promote fertilization, respectively. In colitis, sPLA2-X deficiency increased colorectal expression of Th17 cytokines, and ω3 PUFAs attenuated their production by lamina propria cells partly through the fatty acid receptor GPR120. In comparison, cytosolic phospholipase A2(cPLA2α) protects from colitis by mobilizing ω6 arachidonic acid metabolites, including prostaglandin E2 Thus, our results underscore a previously unrecognized role of sPLA2-X as an ω3 PUFA mobilizerin vivo, segregated mobilization of ω3 and ω6 PUFA metabolites by sPLA2-X and cPLA2α, respectively, in protection against colitis, and the novel role of a particular sPLA2-X-driven PUFA in fertilization.

Differential Contribution of BLT1 and BLT2 to Leukotriene B4-Induced Human NK Cell Cytotoxicity and Migration

Accumulating evidence indicates that leukotriene B₄ (LTB₄) via its receptors BLT₁ and/or BLT₂ (BLTRs) could have an important role in regulating infection, tumour progression, inflammation, and autoimmune diseases. In the present study, we showed that LTB₄ not only augments cytotoxicity by NK cells but also induces their migration. We found that approximately 30% of fresh NK cells express BLT₁, 36% express BLT₂, and 15% coexpress both receptors. The use of selective BLTR antagonists indicated that BLT₁ was involved in both LTB₄-induced migration and cytotoxicity, whereas BLT₂ was involved exclusively in NK cell migration, but only in response to higher concentrations of LTB₄. BLT₁ and BLT₂ expression increased after activation of NK cells with IL-2 and IL-15. These changes of BLTR expression by cytokines were reflected in enhanced NK cell responses to LTB₄. Our findings suggest that BLT₁ and BLT₂ play differential roles in LTB₄-induced modulation of NK cell activity.

Eicosanoid storm in infection and inflammation

Controlled immune responses to infection and injury involve complex molecular signalling networks with coordinated and often opposing actions. Eicosanoids and related bioactive lipid mediators derived from polyunsaturated fatty acids constitute a major bioactive lipid network that is among the most complex and challenging pathways to map in a physiological context. Eicosanoid signalling, similar to cytokine signalling and inflammasome formation, has primarily been viewed as a pro-inflammatory component of the innate immune response; however, recent advances in lipidomics have helped to elucidate unique eicosanoids and related docosanoids with anti-inflammatory and pro-resolution functions. This has advanced our overall understanding of the inflammatory response and its therapeutic implications. The induction of a pro-inflammatory and anti-inflammatory eicosanoid storm through the activation of inflammatory receptors by infectious agents is reviewed here.

Quality of methods reporting in animal models of colitis

BACKGROUND

Current understanding of the onset of inflammatory bowel diseases relies heavily on data derived from animal models of colitis. However, the omission of information concerning the method used makes the interpretation of studies difficult or impossible. We assessed the current quality of methods reporting in 4 animal models of colitis that are used to inform clinical research into inflammatory bowel disease: dextran sulfate sodium, interleukin-10, CD45RB T cell transfer, and 2,4,6-trinitrobenzene sulfonic acid (TNBS).

METHODS

We performed a systematic review based on PRISMA guidelines, using a PubMed search (2000-2014) to obtain publications that used a microarray to describe gene expression in colitic tissue. Methods reporting quality was scored against a checklist of essential and desirable criteria.

RESULTS

Fifty-eight articles were identified and included in this review (29 dextran sulfate sodium, 15 interleukin-10, 5 T cell transfer, and 16 TNBS; some articles use more than 1 colitis model). A mean of 81.7% (SD = ±7.038) of criteria were reported across all models. Only 1 of the 58 articles reported all essential criteria on our checklist. Animal age, gender, housing conditions, and mortality/morbidity were all poorly reported.

CONCLUSIONS

Failure to include all essential criteria is a cause for concern; this failure can have large impact on the quality and replicability of published colitis experiments. We recommend adoption of our checklist as a requirement for publication to improve the quality, comparability, and standardization of colitis studies and will make interpretation and translation of data to human disease more reliable.

The absence of the leukotriene B4 receptor BLT1 attenuates peripheral inflammation and spinal nociceptive processing following intraplantar formalin injury

BACKGROUND

Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation, and its biological effects are mediated primarily through the high affinity LTB4 receptor BLT1. Although numerous studies have reported that LTB4-BLT1 signaling is involved in inflammatory diseases, the role of BLT1 signaling in pain remains undefined. To clarify the role of LTB4-BLT1 signaling in acute inflammatory pain induced by tissue injury, we performed pain behavioral analysis and assessment of local inflammation induced by peripheral formalin injections in BLT1 knockout mice. We examined the phosphorylation of cAMP response element-binding protein (CREB) in the spinal cord both in wild-type and BLT1 knockout mice because phosphorylation of CREB in spinal cord neurons is important for nociceptive sensitization following peripheral injury. We also examined the effect of a BLT1 antagonist on formalin-induced pain responses in mice.

RESULTS

BLT1 knockout mice exhibited markedly attenuated nociceptive responses induced by intraplantar formalin injections. Edema formation and neutrophil infiltration in the paw were significantly decreased in BLT1 knockout mice compared with wild-type mice. Phosphorylation of CREB in the spinal cord after the intraplantar formalin injection was decreased in BLT1 knockout mice. In addition, mice pretreated with a BLT1 antagonist showed reduced nociception and attenuated CREB phosphorylation in the spinal cord after the formalin injection.

CONCLUSIONS

Our data suggest that LTB4-BLT1 axis contributes not only to the peripheral inflammation but also to the neuronal activation in the spinal cord induced by intraplantar formalin injections. Thus, LTB4-BLT1 signaling is a potential target for therapeutic intervention of acute and persistent pain induced by tissue injury.

Biochemical characterization of three BLT receptors in zebrafish

The leukotriene B₄ (LTB₄) receptor 1 (BLT1) is a high affinity receptor for LTB₄, a chemotactic and inflammatory eicosanoid. The LTB₄ receptor 2 (BLT2) was originally identified as a low affinity receptor for LTB₄, and, more recently, as a high affinity receptor for 12-hydroxyheptadecatrienoic acid (12-HHT). The zebrafish BLT receptors have not been previously identified and the in vivo functions of these receptors have been unknown. In this paper, we describe one zebrafish BLT1-like receptor, Blt1, and two zebrafish BLT2-like receptors, Blt2a and Blt2b. Cells expressing Blt1 exhibited LTB₄-induced intracellular [Ca²⁺] increases, inhibition of cAMP production, ligand-dependent [³⁵S]GTPγS binding, and transforming growth factor-α (TGFα) shedding activity in a dose-dependent manner, similar to human BLT1. Cells expressing Blt2a and Blt2b exhibited 12-HHT- and LTB₄-induced intracellular [Ca²⁺] increases, inhibition of cAMP production, [³⁵S]GTPγS binding, and TGFα shedding activity, with a dose-dependency similar to human BLT2. Reverse transcription (RT)-PCR analysis and whole-mount in situ hybridization revealed that blt1, blt2a, blt2b, zebrafish LTA₄ hydrolase (lta4h), and zebrafish 5-lipoxiganase (5lo) are expressed in zebrafish embryos. Knockdown of blt1 by morpholino antisense oligonucleotides resulted in delayed epiboly at gastrulation. Consistently, knockdown of lta4h, an enzyme mediating LTB₄ production, induced a phenotype similar to knockdown of blt1. These results suggest that the LTB₄-BLT1 axis is involved in epiboly in zebrafish development.

Two distinct leukotriene B4 receptors, BLT1 and BLT2

Leukotriene B4 (LTB4) is a potent inflammatory mediator derived from arachidonic acid. Two G protein-coupled receptors for LTB4 have been identified: a high-affinity receptor, BLT1, and a low-affinity receptor, BLT2. Both receptors mainly couple to pertussis toxin-sensitive Gi-like G proteins and induce cell migration. 12(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT) was identified to bind BLT2 with higher affinity than LTB4. Expression of BLT1 was confirmed in type 1 helper T cells, type 2 helper T cells, type 17 helper T cells, effector CD8(+) T cells, dendritic cells and osteoclasts in addition to granulocytes, eosinophils and macrophages, and BLT1-deficient mice showed greatly reduced phenotypes in models of various inflammatory diseases, such as peritonitis, bronchial asthma, rheumatoid arthritis, atherosclerosis and osteoporosis. In mice, BLT2 expression is restricted to intestinal epithelial cells and epidermal keratinocytes. BLT2-deficient mice showed enhanced colitis after administration of dextran sulfate, possibly due to reduced intestinal barrier function. An aspirin-dependent reduction in 12-HHT production was responsible for delayed skin wound healing, showing that the 12-HHT/BLT2 axis also plays an important role in skin biology. BLT1 and BLT2 are therefore potential targets for the development of novel drugs.

Differential cell growth/apoptosis behavior of 13-hydroxyoctadecadienoic acid enantiomers in a colorectal cancer cell line

Cyclooxygenases (COXs) and lipoxygenases (LOXs) are important enzymes that metabolize arachidonic and linoleic acids. Various metabolites generated by the arachidonic acid cascade regulate cell proliferation, apoptosis, differentiation, and senescence. Hydroxyoctadecadienoic acids (HODEs) are synthesized from linoleic acid, giving two enantiomeric forms for each metabolite. The aim was to investigate the effect of 13-HODE enantiomers on nondifferentiated Caco-2 cell growth/apoptosis. Our results indicate that 13(S)-HODE decreases cell growth and DNA synthesis of nondifferentiated Caco-2 cells cultured with 10% fetal bovine serum (FBS). Moreover, 13(S)-HODE showed an apoptotic effect that was reduced in the presence of a specific peroxisome proliferator-activated receptor-γ (PPARγ) antagonist. In addition, we observed that 13(S)-HODE but not 13(R)-HODE is a ligand to PPARγ, confirming the implication of this nuclear receptor in 13(S)-HODE actions. In contrast, 13(R)-HODE increases cell growth and DNA synthesis in the absence of FBS. 13(R)-HODE interaction with BLT receptors activates ERK and CREB signaling pathways, as well as PGE2 synthesis. These results suggest that the proliferative effect of 13(R)-HODE could be due, at least in part, to COX pathway activation. Thus both enantiomers use different receptors and have contrary effects. We also found these differential effects of 9-HODE enantiomers on cell growth/apoptosis. Therefore, the balance between (R)-HODEs and (S)-HODEs in the intestinal epithelium could be important to its cell growth/apoptosis homeostasis.