Characterization of a Mouse Second Leukotriene B4 Receptor, mBLT2

A LTB4/CD11b self-amplifying loop drives pyogranuloma formation in chronic granulomatous disease

Sterile pyogranulomas and heightened cytokine production are hyperinflammatory hallmarks of Chronic Granulomatous Disease (CGD). Using peritoneal cells of zymosan-treated CGD (gp91phox-/-) versus wild-type (WT) mice, an ex vivo system of pyogranuloma formation was developed to determine factors involved in and consequences of recruitment of neutrophils and monocyte-derived macrophages (MoMacs). Whereas WT cells failed to aggregate, CGD cells formed aggregates containing neutrophils initially, and MoMacs recruited secondarily. LTB4 was key, as antagonizing BLT1 blocked neutrophil aggregation, but acted only indirectly on MoMac recruitment. LTB4 upregulated CD11b expression on CGD neutrophils, and the absence/blockade of CD11b inhibited LTB4 production and cell aggregation. Neutrophil-dependent MoMac recruitment was independent of MoMac Nox2 status, BLT1, CCR1, CCR2, CCR5, CXCR2, and CXCR6. As proof of concept, CD11b-deficient CGD mice developed disrupted pyogranulomas with poorly organized neutrophils and diminished recruitment of MoMacs. Importantly, the disruption of cell aggregation and pyogranuloma formation markedly reduced proinflammatory cytokine production.

Leukotriene B4 receptor 2 governs macrophage migration during tissue inflammation

Chronic inflammation is the underlying cause of many diseases, including type 1 diabetes, obesity, and non-alcoholic fatty liver disease. Macrophages are continuously recruited to tissues during chronic inflammation where they exacerbate or resolve the pro-inflammatory environment. Although leukotriene B4 receptor 2 (BLT2) has been characterized as a low affinity receptor to several key eicosanoids and chemoattractants, its precise roles in the setting of inflammation and macrophage function remain incompletely understood. Here we used zebrafish and mouse models to probe the role of BLT2 in macrophage function during inflammation. We detected BLT2 expression in bone marrow derived and peritoneal macrophages of mouse models. Transcriptomic analysis of Ltb4r2-/- and WT macrophages suggested a role for BLT2 in macrophage migration, and studies in vitro confirmed that whereas BLT2 does not mediate macrophage polarization, it is required for chemotactic function, possibly mediated by downstream genes Ccl5 and Lgals3. Using a zebrafish model of tailfin injury, we demonstrated that antisense morpholino-mediated knockdown of blt2a or chemical inhibition of BLT2 signaling impairs macrophage migration. We further replicated these findings in zebrafish models of islet injury and liver inflammation. Moreover, we established the applicability of our zebrafish findings to mammals by showing that macrophages of Ltb4r2-/- mice have defective migration during lipopolysaccharide stimulation in vivo. Collectively, our results demonstrate that BLT2 mediates macrophage migration during inflammation, which implicates it as a potential therapeutic target for inflammatory pathologies.

Discovery of a novel BLT2 antagonist for the treatment of inflammatory airway diseases

Leukotriene B4 (LTB4) is a potent chemoattractant that can recruit and activate immune cells such as neutrophils, eosinophils, and monocytes to sites of inflammation. Excessive production of LTB4 has been linked to acute and chronic inflammatory diseases, including asthma, rheumatoid arthritis, and psoriasis. Inhibiting the binding of LTB4 to its receptors, BLT1 and BLT2, is a potential strategy for treating these conditions. While several BLT1 antagonists have been developed for clinical trials, most have failed due to efficacy and safety issues. Therefore, discovering selective BLT2 antagonists could improve our understanding of the distinct functions of BLT1 and BLT2 receptors and their pharmacological implications. In this study, we aimed to discover novel BLT2 antagonists by synthesizing a series of biphenyl analogues based on a BLT2 selective agonist, CAY10583. Among the synthesized compounds, 15b was found to selectively inhibit the chemotaxis of CHO-BLT2 cells with an IC50 value of 224 nM without inhibiting the chemotaxis of CHO-BLT1 cells. 15b also inhibited the binding of LTB4 and BLT2 with a Ki value of 132 nM. Furthermore, 15b had good metabolic stability in liver microsomes and moderate bioavailability (F = 34%) in in vivo PK studies. 15b also showed in vivo efficacy in a mouse model of asthma, reducing airway hyperresponsiveness by 59% and decreasing Th2 cytokines by up to 46%. Our study provides a promising lead for the development of selective BLT2 antagonists as potential therapeutics for inflammatory airway diseases such as asthma and chronic obstructive pulmonary disease.

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.

Structural diversity of leukotriene G-protein coupled receptors

Dihydroxy acid leukotriene (LTB4) and cysteinyl leukotrienes (LTC4, LTD4, and LTE4) are inflammatory mediators derived from arachidonic acid via the 5-lipoxygenase pathway. While structurally similar, these two types of leukotrienes (LTs) exert their functions through interactions with two distinct G protein-coupled receptor (GPCR) families, BLT and CysLT receptors, which share low sequence similarity and belong to phylogenetically divergent GPCR groups. Selective antagonism of LT receptors has been proposed as a promising strategy for the treatment of many inflammation-related diseases including asthma and chronic obstructive pulmonary disease, rheumatoid arthritis, cystic fibrosis, diabetes, and several types of cancer. Selective CysLT1R antagonists are currently used as antiasthmatic drugs, however, there are no approved drugs targeting CysLT2 and BLT receptors. In this review, we highlight recently published structures of BLT1R and CysLTRs revealing unique structural features of the two receptor families. X-ray and cryo-EM data shed light on their overall conformations, differences in functional motifs involved in receptor activation, and details of the ligand-binding pockets. An unexpected binding mode of the selective antagonist BIIL260 in the BLT1R structure makes it the first example of a compound targeting the sodium-binding site of GPCRs and suggests a novel strategy for the receptor activity modulation. Taken together, these recent structural data reveal dramatic differences in the molecular architecture of the two LT receptor families and pave the way to new therapeutic strategies of selective targeting individual receptors with novel tool compounds obtained by the structure-based drug design approach.

The leukotriene B4 receptors BLT1 and BLT2 as potential therapeutic targets

Leukotriene B4 (LTB4 ) was recognized as an arachidonate-derived chemotactic factor for inflammatory cells and an important drug target even before the molecular identification of its receptors. We cloned the high- and low-affinity LTB4 receptors, BLT1 and BLT2, respectively, and examined their functions by generating and studying gene-targeted mice. BLT1 is involved in the pathogenesis of various inflammatory and immune diseases, including asthma, psoriasis, contact dermatitis, allergic conjunctivitis, age-related macular degeneration, and immune complex-mediated glomerulonephritis. Meanwhile, BLT2 is a high-affinity receptor for 12-hydroxyheptadecatrienoic acid, which is involved in the maintenance of dermal and intestinal barrier function, and the acceleration of skin and corneal wound healing. Thus, BLT1 antagonists and BLT2 agonists are promising candidates in the treatment of inflammatory diseases.

Recent advances in function and structure of two leukotriene B4 receptors: BLT1 and BLT2

Leukotriene B₄ (LTB₄) is generated by the enzymatic oxidation of arachidonic acid, which is then released from the cell membrane and acts as a potent activator of leukocytes and other inflammatory cells. Numerous studies have demonstrated the physiological and pathophysiological significance of this lipid in various diseases. LTB₄ exerts its activities by binding to its specific G protein-coupled receptors (GPCRs): BLT1 and BLT2. In mouse disease models, treatment with BLT1 antagonists or BLT1 gene ablation attenuated various diseases, including bronchial asthma, arthritis, and psoriasis, whereas BLT2 deficiency exacerbated several diseases in the skin, cornea, and small intestine. Therefore, BLT1 inhibitors and BLT2 activators could be beneficial for the treatment of several inflammatory and immune disorders. As a result, attractive compounds targeting LTB₄ receptors have been developed by several pharmaceutical companies. This review aims to understand the potential of BLT1 and BLT2 as therapeutic targets for the treatment of various inflammatory diseases. In addition, recent topics are discussed with major focuses on the structure and post-translational modifications of BLT1 and BLT2. Collectively, current evidence on modulating LTB₄ receptor functions provides new strategies for the treatment of various diseases.

Development and Characterization of a Fluorescent Ligand for Leukotriene B4 Receptor 2 in Cells and Tissues

The leukotriene B4 receptor 2 (BLT2) is a G-protein coupled receptor activated by 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT), which has been proposed as a promising therapeutic target for diabetic wound healing and gastrointestinal lesions. In this study, the rational design of a fluorescent probe based on the synthetic BLT2 agonist CAY10583 is described. The synthesis of several derivatives of CAY10583 coupled to fluorescein resulted in a traceable ligand suitable for different fluorescence-based techniques. An HTRF-based displacement assay (Tag-lite) on stably transfected CHO-K1 cells was developed to characterize binding properties of diverse BLT2 ligands. Highly specific binding to the BLT2 receptor was demonstrated in staining experiments on mouse skin tissue, and specific modulation of BLT2-induced cAMP signaling provided further evidence for receptor binding and ligand functionality. In conclusion, the fluorescent ligands developed in this study are suitable to investigate the pharmacology of BLT2 receptor ligands in a variety of assay systems.

12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) induces cell growth and improves barrier function through BLT2 interaction in intestinal epithelial Caco-2 cell cultures

12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) is an unusual product of the cyclooxygenase pathway that is an endogenous ligand of the low-affinity receptor for leukotriene 4 (LTB₄), BLT2. Recent findings suggested that BLT2 possibly plays an important role in the healing of intestinal lesions and the regulation of barrier function. Here, we studied the role of 12-HHT on intestinal epithelial cell growth and the paracellular permeability of intestinal epithelium using Caco-2 cell cultures as experimental model. Our results demonstrated that 12-HHT stimulates intestinal epithelial Caco-2 cell growth through 12-HHT-BLT2-p38-PKC axis and improves paracellular permeability in differentiated Caco-2 cell cultures through the regulation of tight junction elements such as myosin light chain phosphorylation through 12-HHT-BLT2-p38-PKC-MYPT1 axis. Thus, 12-HHT-BLT2 interaction can be involved in intestinal epithelial cell growth and consequently in the epithelium regeneration/repair processes, together with an interesting improvement on the paracellular permeability. These effects appoint that 12-HHT/BLT2 axis may be a suitable strategy for treating wound healing epithelium and barrier-disrupted intestinal processes.

Metabolism and biological functions of 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid

12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) is a 17-carbon hydroxy fatty acid that is biosynthesized either by enzymatic pathways, like thromboxane synthase (TXAS) and cytochrome P450 or a non-enzymatic pathway. TXAS catalyzes the isomerization reaction from PGH2 to 12-HHT, malondialdehyde, and TXA2 at a ratio of 1:1:1. Furthermore, 12-HHT has been considered as a mere byproduct of TXA2 biosynthesis, and its biological function has long been uncertain. BLT2 was initially identified as a low-affinity leukotriene B4 (LTB4) receptor, which is also activated by various hydroxy-eicosatetraenoic acids (HETEs), suggesting that BLT2 may be activated by other endogenous ligands apart from LTB4 and HETEs. By unbiased ligand screening using crude lipids from rat organs, 12-HHT has been identified as an endogenous agonist for BLT2. The 12-HHT-BLT2 axis induces mast cell migration and contributes to allergic inflammation. BLT2 is also expressed in epithelial cells of the small intestine and skin in mice and contributes to in vivo epithelial barrier functions.

First Structure-Activity Relationship Study of Potent BLT2 Agonists as Potential Wound-Healing Promoters

The first potent leukotriene B4 (LTB4) receptor type 2 (BLT2) agonists, endogenous 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT), and synthetic CAY10583 (CAY) have been recently described to accelerate wound healing by enhanced keratinocyte migration and indirect stimulation of fibroblast activity in diabetic rats. CAY represents a very valuable starting point for the development of novel wound-healing promoters. In this work, the first structure-activity relationship study for CAY scaffold-based BLT2 agonists is presented. The newly prepared derivatives showed promising in vitro wound-healing activity.

Elucidation of Mechanism for Ligand Efficacy at Leukotriene B4 Receptor 2 (BLT2)

G protein-coupled receptors (GPCRs) have always been important drug targets in the pharmaceutical industry. One major question for the current GPCR drug discovery is how drugs have distinct efficacies at the same GPCR target. Related to this question, we studied how different ligands can have disparate efficacies at Leukotriene B₄ receptor (BLT2). By using molecular modeling studies, we predicted that Tyr271^6.51 located at TM6 of BLT2 performs as a key trigger for its activation and verified the prediction by site-directed mutagenesis, chemotactic motility studies, which included a chemical derivative of agonist CAY10583. We further identified Asn275^6.55 located at TM6 as a weak activation trigger in BLT2 and performed double mutation studies to confirm our computational results. Our results provide strong evidence for the exact mechanism of ligand efficacy at BLT2.

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.

The 12-HHT/BLT2/NO Axis Is Associated to the Wound Healing and Skin Condition in Different Glycaemic States

Type 2 diabetes affects over 340 million people worldwide. This condition can go unnoticed and undiagnosed for years, leading to a late stage where high glycaemia produces complications such as delayed wound healing. Studies have shown that 12-HHT through BLT2, accelerates keratinocyte migration and wound healing. Additionally, evidence has shown the role of nitric oxide as a pro-regenerative mediator, which is decreased in diabetes. Our main goal was to study the association between the 12-HHT/BLT2 axis and the nitric oxide production in wound healing under different glycaemia conditions. For that purpose, we used in vivo and in vitro models. Our results show that the skin from diabetic mice showed reduced BLT2 and iNOS mRNA, TEER, 12-HHT, nitrites, and tight junction levels, accompanied by higher MMP9 mRNA levels. Furthermore, a positive correlation between BLT2 mRNA and nitrites was observed. In vitro, HaCaT-BLT2 cells showed higher nitric oxide and tight junction levels, and reduced MMP9 mRNA levels, compared to mock-keratinocytes under low and high glucose condition. The wound healing capacity was associated with higher nitric oxide production and was affected by the NOS inhibition. We suggest that the BLT2 expression improves the keratinocyte response to hyperglycaemia, associated with the production of nitric oxide.

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.

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.

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.

Leukotriene B4 receptor 2 gene polymorphism (rs1950504, Asp196Gly) leads to enhanced cell motility under low-dose ligand stimulation

Recently, single-nucleotide polymorphisms (SNPs) in G-protein-coupled receptors (GPCRs) have been suggested to contribute to physiopathology and therapeutic effects. Leukotriene B₄ receptor 2 (BLT2), a member of the GPCR family, plays a critical role in the pathogenesis of several inflammatory diseases, including cancer and asthma. However, no studies on BLT2 SNP effects have been reported to date. In this study, we demonstrate that the BLT2 SNP (rs1950504, Asp196Gly), a Gly-196 variant of BLT2 (BLT2 D196G), causes enhanced cell motility under low-dose stimulation of its ligands. In addition, we demonstrated that Akt activation and subsequent production of reactive oxygen species (ROS), both of which act downstream of BLT2, are also increased by BLT2 D196G in response to low-dose ligand stimulation. Furthermore, we observed that the ligand binding affinity of BLT2 D196G was enhanced compared with that of BLT2. Through homology modeling analysis, it was predicted that BLT2 D196G loses ionic interaction with R197, potentially resulting in increased agonist-receptor interaction. To the best of our knowledge, this report is the first to describe a SNP study on BLT2 and shows that BLT2 D196G enhances ligand sensitivity, thereby increasing cell motility in response to low-dose ligand stimulation.

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 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.

Leukotriene B4 induces EMT and vimentin expression in PANC-1 pancreatic cancer cells: Involvement of BLT2 via ERK2 activation

Leukotriene B₄ (LTB₄) is a leukocyte chemoattractant and plays a major role controlling inflammatory responses including pancreatitis. LTB₄ is known to be correlated with cancer progression. LTB₄ induces keratin phosphorylation and reorganization by activating extracellular regulated kinase (ERK) in PANC-1 pancreatic cancer cell lines. However, the role of LTB₄ in epithelial mesenchymal transition (EMT) and vimentin expression in pancreatic cancer cells is unknown. We examined whether LTB₄ induces EMT and vimentin expression by Western blot, si-RNA, and RT-PCR. LTB₄ induced morphological change, decreased E-cadherin expression and increased N-cadherin and vimentin expression. LTB4 increased migration and invasion of PANC-1 cancer cells. LTB₄ dose-dependently upregulated expression of vimentin in PANC-1 cancer cells. LTB₄-induced vimentin expression was suppressed by LY255283 (BLT2 antagonist). Comp A, a BLT2 agonist, further increased vimentin expression. Gene silencing of BLT2 suppressed LTB₄-or Comp A-induced vimentin expression in PANC-1 cells. The MEK inhibitor, PD98059 suppressed Comp A-induced vimentin expression. Comp A or transfection of plasmid containing BLT2 cDNA (pC_BLT2) activated ERK, and BLT2 gene silencing suppressed Comp A-induced ERK activation. ERK2 siRNA abrogated Comp A-induced vimentin expression and ERK2 overexpression enhanced vimentin expression. One of well-known cause of ras mutation, cigarette smoke extracts increased BLT2 expression in PANC-1 cancer cells. Taken together, these results suggest that BLT2 is involved in LTB₄-induced vimentin expression through ERK2 in PANC-1 cells.

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.

The efficacy and safety of H1-antihistamine versus Montelukast for allergic rhinitis: A systematic review and meta-analysis

PURPOSE

In order to verify the differences of effectiveness and safety between SAHs and Montelukast, and to find out potential uncared-for problems, we performed a systematic review and Meta-analysis to proceed a qualitative describe and quantitative assessment.

METHODS

We searched the databases of Pubmed, the Cochrane Library, Nature and Science as well as Wanfang data and CNKI from 2000 to March 2016, using key words "Montelukast SAH" or "H1-antihistamine Montelukast", or "Loratadine Montelukast", or "Desloratadine Montelukast", or "Levocetirizine Montelukast", or "Cetirizen Montelukast", or "Fexofenadine Montelukast". And also we included studies through relevant citations in related literature. Meta-analysis and bias of risk were performed. We analyzed Heterogeneity and publish bias as well.

RESULT

Montelukast seems more effective in nighttime symptoms compare with SAHs (P=0.008, MD=-0.04, 95%CI: -0.08, -0.01). No significant difference was found between Montelukast and SAHs in CSS (P=0.10, MD=0.03, 95%CI: -0.01, 0.07). Montelukast and SAHs combined therapy was more effective than Montelukast DNSS (P=0.0006, MD=0.15, 95%CI: 0.07, 0.24) but not in CSS (P=0.04, MD=0.08, 95%CI: 0.00, 0.15; Bonferroni correction α=0.017).

CONCLUSION

Montelukast has a significant influence in improving patients' nasal symptoms quality of live but is not as effective as SAHs, and may have a slight advantage over SAHs in relieving nighttime symptoms significantly. Combined therapy is more effective in improving patients' day time symptom than Montelukast. Probably, patients might have a lower asthenia incidence rate when using Montelukas.

The role of leukotrienes in allergic diseases

Leukotrienes (LTs), both LTB4 and the cysteinyl LTs (CysLTs) LTC4, LTD4 and LTE4, are implicated in a wide variety of inflammatory disorders. These lipid mediators are generated from arachidonic acid via multistep enzymatic reactions through which arachidonic acid is liberated from membrane phospholipids through the action of phospholipase A2. LTB4 and CysLTs exert their biological effects by binding to cognate receptors, which belong to the G protein-coupled receptor superfamily. LTB4 is widely considered to be a potent chemoattractant for most subsets of leukocytes, whereas CysLTs are potent bronchoconstrictors that have effects on airway remodeling. LTs play a central role in the pathogenesis of asthma and many other inflammatory diseases. This review will provide an update on the synthesis, biological function, and relevance of LTs to the pathobiology of allergic diseases, and examine the current and future therapeutic prospects of LT modifiers.

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.

Eicosanoids and Keratinocytes in Wound Healing

Significance: Eicosanoids are biologically active lipid mediators derived from arachidonic acid that are important in injury and inflammatory responses. Cyclooxygenase-1 and cyclooxygenase-2 mediate the production of prostanoids, whereas 5-lipoxygenase mediates the production of leukotrienes and hydroxyeicosatetraenoic acids. These lipid mediators have traditionally been known to recruit cells of the immune system to a site of injury and inflammation. However, they also interact with various cells that are resident to the wound bed, including modulation of keratinocyte activity. Recent Advances: Recent work has identified multiple prostanoid and leukotriene receptors on keratinocytes, indicating that eicosanoids directly interact with them. Recent work also shows that keratinocytes are capable of producing prostanoids and leukotrienes. Critical Issues: Much of the critical work has been performed in cell culture and mouse in vivo models. This has greatly expanded our understanding of the eicosanoid interactions with keratinocytes and wound healing in general. However, few of these in vivo models have been able to critically evaluate keratinocyte migration and re-epithelialization. Future Directions: As research continues in this exciting field, the cellular pathways stimulated by the eicosanoids will become better defined. Future research with excisional wound models in mice and pigs and ex vivo human skin models will better isolate the contribution of eicosanoid-mediated effects on keratinocyte migration and re-epithelialization.

12-Hydroxyheptadecatrienoic acid promotes epidermal wound healing by accelerating keratinocyte migration via the BLT2 receptor

Endogenous 12-HHT, or a synthetic BLT2 agonist promotes epidermal wound closure by stimulating BLT2 on keratinocytes, inducing TNF and MMP production.

Leukotriene B₄ (LTB₄) receptor type 2 (BLT2) is a G protein–coupled receptor (GPCR) for 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) and LTB₄. Despite the well-defined proinflammatory roles of BLT1, the in vivo functions of BLT2 remain elusive. As mouse BLT2 is highly expressed in epidermal keratinocytes, we investigated the role of the 12-HHT/BLT2 axis in skin wound healing processes. 12-HHT accumulated in the wound fluid in mice, and BLT2-deficient mice exhibited impaired re-epithelialization and delayed wound closure after skin punching. Aspirin administration reduced 12-HHT production and resulted in delayed wound closure in wild-type mice, which was abrogated in BLT2-deficient mice. In vitro scratch assay using primary keratinocytes and a keratinocyte cell line also showed that the 12-HHT/BLT2 axis accelerated wound closure through the production of tumor necrosis factor α (TNF) and matrix metalloproteinases (MMPs). A synthetic BLT2 agonist accelerated wound closure in cultured cells as well as in C57BL/6J and diabetic mice. These results identify a novel mechanism underlying the action of the 12-HHT/BLT2 axis in epidermal keratinocytes and accordingly suggest the use of BLT2 agonists as therapeutic agents to accelerate wound healing, particularly for intractable wounds, such as diabetic ulcers.

RanBPM protein acts as a negative regulator of BLT2 receptor to attenuate BLT2-mediated cell motility

BLT2, a low affinity receptor for leukotriene B4 (LTB4), is a member of the G protein-coupled receptor family and is involved in many signal transduction pathways associated with various cellular phenotypes, including chemotactic motility. However, the regulatory mechanism for BLT2 has not yet been demonstrated. To understand the regulatory mechanism of BLT2, we screened and identified the proteins that bind to BLT2. Using a yeast two-hybrid assay with the BLT2 C-terminal domain as bait, we found that RanBPM, a previously proposed scaffold protein, interacts with BLT2. We demonstrated the specific interaction between BLT2 and RanBPM by GST pulldown assay and co-immunoprecipitation assay. To elucidate the biological function of the RanBPM-BLT2 interaction, we evaluated the effects of RanBPM overexpression or knockdown. We found that BLT2-mediated motility was severely attenuated by RanBPM overexpression and that knockdown of endogenous RanBPM by shRNA strongly promoted BLT2-mediated motility, suggesting a negative regulatory function of RanBPM toward BLT2. Furthermore, we observed that the addition of BLT2 ligands caused the dissociation of BLT2 and RanBPM, thus releasing the negative regulatory effect of RanBPM. Finally, we propose that Akt-induced BLT2 phosphorylation at residue Thr(355), which occurs after the addition of BLT2 ligands, is a potential mechanism by which BLT2 dissociates from RanBPM, resulting in stimulation of BLT2 signaling. Taken together, our results suggest that RanBPM acts as a negative regulator of BLT2 signaling to attenuate BLT2-mediated cell motility.

Leukotriene B4 receptor BLT2 negatively regulates allergic airway eosinophilia

Leukotriene B4 (LTB4) has been implicated in the pathogenesis of allergic diseases. BLT2, a low-affinity LTB4 receptor, is activated by LTB4 and 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT). Although the high-affinity LTB4 receptor BLT1 has been shown to exert proinflammatory roles, the role of BLT2 in allergic inflammation has not been clarified. To study the function of BLT2 in development of asthma, we used mice model of ovalbumin (OVA)-induced allergic airway disease. The 12-HHT levels were elevated in bronchoalveolar lavage (BAL) fluids of OVA-sensitized/challenged wild-type mice. BLT2-deficient mice exhibited enhanced eosinophilia in BAL fluids after OVA exposure. Interleukin (IL)-13 levels in BAL fluids and IL-13-producing CD4(+) T cells in the lungs were elevated in BLT2-deficient mice compared to wild-type mice, whereas the levels of IL-4, IL-5, and interferon (IFN)-γ in BAL fluids and serum OVA-specific IgE were comparable. Transfection of BLT2-specific small interfering RNA enhanced IL-13 production in CD4(+) T cells in vitro. Expression of BLT2 mRNA in CD4(+) T cells was significantly reduced in patients with asthma compared to healthy control subjects. These findings indicate that BLT2 has a protective role in allergic airway inflammation and that diminished BLT2 expression in CD4(+) T cells may contribute to the pathophysiology of asthma.

Leukotriene B4 enhances innate immune defense against the puerperal sepsis agent Streptococcus pyogenes

Puerperal sepsis is a leading cause of maternal mortality worldwide. Streptococcus pyogenes [group A Streptococcus; (GAS)] is a major etiologic agent of severe postpartum sepsis, yet little is known regarding the pathogenesis of these infections. Tissue macrophages provide innate defense against GAS, and their actions are highly regulated. The intracellular second messenger cAMP can negatively regulate macrophage actions against GAS. Because leukotriene (LT) B(4) has been shown to suppress intracellular cAMP in macrophages, we hypothesized that it could enhance innate defenses against GAS. We assessed the capacity of LTB(4) to modulate antistreptococcal actions of human macrophages, including placental and decidual macrophages and used a novel intrauterine infection model of GAS in mice lacking the 5-lipoxygenase enzyme to determine the role of endogenous LTs in host defense against this pathogen. Animals lacking 5-lipoxygenase were significantly more vulnerable to intrauterine GAS infection than were wild-type mice and showed enhanced dissemination of bacteria out of the uterus and a more robust inflammatory response than did wild-type mice. In addition, LTB(4) reduced intracellular cAMP levels via the BLT1 receptor and was a potent stimulant of macrophage phagocytosis and NADPH oxidase-dependent intracellular killing of GAS. Importantly, interference was observed between the macrophage immunomodulatory actions of LTB(4) and the cAMP-inducing lipid PGE(2), suggesting that interplay between pro- and anti-inflammatory compounds may be important in vivo. This work underscores the potential for pharmacological targeting of lipid mediator signaling cascades in the treatment of invasive GAS infections.

Novel involvement of leukotriene B₄ receptor 2 through ERK activation by PP2A down-regulation in leukotriene B₄-induced keratin phosphorylation and reorganization of pancreatic cancer cells

Perinuclear reorganization via phosphorylation of specific serine residues in keratin is involved in the deformability of metastatic cancer cells. The level of leukotriene B₄ is high in pancreatic cancers. However, the roles of LTB₄ and its cognate receptors in keratin reorganization of pancreatic cancers are not known. LTB₄ dose-dependently induced phosphorylation and reorganization of Keratin 8 (K8) and these processes were reversed by LY255283 (BLT2 antagonist). BLT2 agonists such as Comp A and 15(S)-HETE also induced phosphorylation of serine 431 in K8. Moreover, Comp A-induced K8 phosphorylation and reorganization were blocked by LY255283. Gene silencing of BLT2 suppressed Comp A-induced K8 phosphorylation and reorganization in PANC-1 cells. Over-expression of BLT2 promoted K8 phosphorylation. Comp A promoted the migration of PANC-1 cells in a dose-dependent manner, and LY255283 blocked Comp A-induced migration, respectively. PD98059 (ERK inhibitor) suppressed Comp A-induced phosphorylation of serine 431 and reorganization of K8. Gene silencing of BLT2 suppressed the expression of pERK, and over-expression of BLT2 increased the expression of pERK even without Comp A. Comp A induced the expression of active ERK (pERK) and BLT2. These inductions were blocked by PD98059. Comp A decreased PP2A expression and hindered the binding of PP2A to the K8, leading to the activation of ERK. PD98059 suppressed the Comp A-induced migration of PANC-1 cells and BLT2 over-expression-induced migration of PANC-1 cells. Overall, these results suggest that BLT2 is involved in LTB(4)-induced phosphorylation and reorganization through ERK activation by PP2A downregulation, leading to increased migration of PANC-1 cells.

Leukotriene A4 Hydrolase: Biology, Inhibitors and Clinical Applications

Leukotriene A4 hydrolase is a zinc-containing cytosolic enzyme with both hydrolase and aminopeptidase activity. LTA4H stereospecifically catalyzes the transformation of the unstable epoxide LTA4 to the potent pro-inflammatory mediator LTB4. Variations in the lta4h gene have been linked to susceptibility to multiple diseases including myocardial infarction, stroke and asthma. Pre-clinical animal models and human biomarker data have implicated LTB4 in inflammatory diseases. Several groups have now identified selective inhibitors of LTA4H, many of which were influenced by the disclosure of a protein crystal structure a decade ago. Clinical validation of LTA4H remains elusive despite the progression of inhibitors into pre-clinical and clinical development.

Androgen receptor is up-regulated by a BLT2-linked pathway to contribute to prostate cancer progression

The androgen receptor (AR) plays a central role in the development and progression of prostate cancer. AR expression is maintained throughout the progression of prostate cancer and is also associated with an aggressive, castration-resistant (CR) phenotype. Despite the critical roles of AR expression in prostate cancer progression, the exact signaling mechanism regulating AR expression remains unclear. In this study, we demonstrated that AR expression was increased by a low-affinity leukotriene B(4) receptor (BLT2)-linked pathway. We found that BLT2 was overexpressed in AR-positive prostate cancer cells, such as LNCaP cells, and BLT2 inhibition, using an inhibitor or siRNA knockdown, clearly attenuated AR expression and triggered apoptosis in these cells. These results suggest a role for BLT2 in AR expression and the survival of AR-positive prostate cancer cells. Moreover, we found that the NADPH oxidase family protein, Nox4, lay downstream of BLT2 and mediated the production of reactive oxygen species (ROS) and subsequent NF-κB stimulation, thereby inducing AR expression. Taken together, our results demonstrate that BLT2 plays a critical role in AR expression via a Nox4-ROS-NF-κB-linked pathway, thereby mediating the survival of AR-positive prostate cancer cells. Our findings point to BLT2 as a key regulator of AR expression and will contribute to the development of novel therapies for AR-positive prostate cancers, including androgen-responsive and CR prostate cancers.

Low-dose UVB irradiation stimulates matrix metalloproteinase-1 expression via a BLT2-linked pathway in HaCaT cells

Skin exposure to low-dose ultraviolet B (UVB) light up-regulates the expression of matrix metalloproteinase-1 (MMP-1), thus contributing to premature skin aging (photo-aging). Although cyclooxygenase-2 (COX- 2) and its product, prostaglandin E(2) (PGE((2))), have been associated with UVB-induced signaling to MMP expression, very little are known about the roles of lipoxygenases and their products, especially leukotriene B((4)) (LTB((4))) and 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), in MMP-1 expression in skin keratinocytes. In the present study, we demonstrate that BLT2, a cell surface receptor for LTB((4)) and 12(S)-HETE, plays a critical role in UVB-mediated MMP-1 upregulation in human HaCaT keratinocytes. Moreover, our results demonstrated that BLT2-mediated MMP-1 upregulation occurs through a signaling pathway dependent on reactive oxygen species (ROS) production and the subsequent stimulation of ERK. Blockage of BLT2 via siRNA knockdown or with the BLT2-antagonist LY255283 completely abolished the up-regulated expression of MMP-1 induced by low-dose UVB irradiation. Finally, when HaCaT cells were transiently transfected with a BLT2 expression plasmid, MMP-1 expression was significantly enhanced, along with ERK phosphorylation, suggesting that BLT2 overexpression alone is sufficient for MMP-1 up-regulation. Together, our results suggest that the BLT2-ROS- ERK-linked cascade is a novel signaling mechanism for MMP-1 upregulation in low-dose UVB- irradiated keratinocytes and thus potentially contributes to photo-aging.

Leukotriene B4 receptors: novel roles in immunological regulations

Mammals have at least two receptors for LTB4; high-affinity BLT1 and low-affinity BLT2, both of which are GPCRs. 12-HHT serves as a more potent and abundant ligand for BLT2 than LTB4. BLT1 is expressed in a variety of inflammatory and immune cells including granulocytes, eosinophils, macrophages, differentiated Th1, Th2 and Th17 cells, effecter CD8+ T cells, dendritic cells and osteoclasts. BLT1 antagonists will be beneficial for the treatment of various diseases such as bronchial asthma, multiple sclerosis, contact dermatitis, and postmenopausal osteoporosis. BLT2 plays different roles from BLT1, and one important role of BLT2 is the maintenance of mucosal integrity in the colon.

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

BLT2 is a low-affinity leukotriene B(4) (LTB(4)) receptor that is activated by 12(S)-hydroxyheptadeca-5Z,8E,10E-trienoic acid (12-HHT) and LTB(4). Despite the well-defined proinflammatory roles of BLT1, the in vivo functions of BLT2 remain elusive. To clarify the role of BLT receptors in intestinal inflammation, we assessed susceptibility to dextran sodium sulfate (DSS)-induced colitis in mice lacking either BLT1 or BLT2. BLT2(-/-) mice exhibited increased sensitivity to DSS as compared to wild-type and BLT1(-/-) mice, with more severe body weight loss and inflammation. Expression of inflammatory cytokines such as interferon (IFN)-γ, interleukin (IL)-1β, and IL-6, chemokines such as CXC chemokine ligand 9 (CXCL9) and C-C motif chemokine 19 (CCL19), and metalloproteinases was highly up-regulated in the colons of DSS-treated BLT2(-/-) mice, and there was an enhanced accumulation of activated macrophages. Phosphorylation of the signal transducer and activator of transcription 3 (STAT3) was also markedly accelerated in the crypts of DSS-treated BLT2(-/-) mice. Madin-Darby canine kidney II (MDCKII) cells transfected with BLT2 exhibited enhanced barrier function as measured by transepithelial electrical resistance (TER) and FITC-dextran leakage through MDCK monolayers. Thus, BLT2 is expressed in colon cryptic cells and appears to protect against DSS-induced colitis, possibly by enhancing barrier function in epithelial cells of the colon. These novel results suggest a direct anti-inflammatory role of BLT2 that is distinct from the proinflammatory roles of BLT1.

Blockade of LTB4-induced chemotaxis by bioactive molecules interfering with the BLT2-Galphai interaction

BLT2, a low-affinity leukotriene B4 (LTB4) receptor, is a member of the G-protein coupled receptor (GPCR) family and is involved in the pathogenesis of inflammatory diseases such as asthma. Despite its clinical implications, however, no pharmacological inhibitors are available. In the present study, we screened for small molecules that interfere with the interaction between the third intracellular loop region of BLT2 (BLT2iL3) and the Galphai3 protein subunit (Galphai3), using a high-throughput screening (HTS) assay with a library of 1040 FDA-approved drugs and bioactive compounds. We identified two small molecules-purpurin [1,2,4-trihydroxy-9,10-anthraquinone; IC50 = 1.6 microM for BLT2] and chloranil [tetrachloro-1,4-benzoquinone; IC50 = 0.42 microM for BLT2]-as specific BLT2-blocking agents. We found that blockade of the BLT2iL3-Galphai3 interaction by these small molecules inhibited the BLT2-downstream signaling cascade. For example, BLT2-signaling to phosphoinositide-3 kinase (PI3K)/Akt phosphorylation was completely abolished by these molecules. Furthermore, we observed that these small molecules blocked LTB4-induced chemotaxis by inhibiting the BLT2-PI3K/Akt-downstream, Rac1-reactive oxygen species-dependent pathway. Taken together, our results show that purpurin and chloranil interfere with the interaction between BLT2iL3 and Galphai3 and thus block the biological functions of BLT2 (e.g., chemotaxis). The present findings suggest a potential application of purpurin and chloranil as pharmacological therapeutic agents against BLT2-associated inflammatory human diseases.

The LTB4-BLT1 axis mediates neutrophil infiltration and secondary injury in experimental spinal cord injury

Traumatic injury in the central nervous system induces inflammation; however, the role of this inflammation is controversial. Precise analysis of the inflammatory cells is important to gain a better understanding of the inflammatory machinery in response to neural injury. Here, we demonstrated that leukotriene B4 plays a significant role in mediating leukocyte infiltration after spinal cord injury. Using flow cytometry, we revealed that neutrophil and monocyte/macrophage infiltration peaked 12 hours after injury and was significantly suppressed in leukotriene B4 receptor 1 knockout mice. Similar findings were observed in mice treated with a leukotriene B4 receptor antagonist. Further, by isolating each inflammatory cell subset with a cell sorter, and performing quantitative reverse transcription-PCR, we demonstrated the individual contributions of more highly expressed subsets, ie, interleukins 6 and 1beta, tumor necrosis factor-alpha, and FasL, to the inflammatory reaction and neural apoptosis. Inhibition of leukotriene B4 suppressed leukocyte infiltration after injury, thereby attenuating the inflammatory reaction, sparing the white matter, and reducing neural apoptosis, as well as inducing better functional recovery. These findings are the first to demonstrate that leukotriene B4 is involved in the pathogenesis of spinal cord injury through the amplification of leukocyte infiltration, and provide a potential therapeutic strategy for traumatic spinal cord injury.

The leukotriene B4 receptor, BLT1, is required for the induction of experimental autoimmune encephalomyelitis

Leukotriene B(4) (LTB(4)) is a potent chemoattractant and activator of neutrophils, macrophages and T cells. These cells are a key component of inflammation and all express BLT1, a high affinity G-protein-coupled receptor for LTB(4). However, little is known about the neuroimmune functions of BLT1. In this study, we describe a distinct role for BLT1 in the pathology of experimental autoimmune encephalomyelitis (EAE) and T(H)1/T(H)17 immune responses. BLT1 mRNA was highly upregulated in the spinal cord of EAE mice, especially during the induction phase. BLT1(-/-) mice had delayed onset and less severe symptoms of EAE than BLT1(+/+) mice. Additionally, inflammatory cells were recruited to the spinal cord of asymptomatic BLT1(+/+), but not BLT1(-/-) mice before the onset of disease. Ex vivo studies showed that both the proliferation and the production of IFN-gamma, TNF-alpha, IL-17 and IL-6 were impaired in BLT1(-/-) cells, as compared with BLT1(+/+) cells. Thus, we suggest that BLT1 exacerbates EAE by regulating the migration of inflammatory cells and T(H)1/T(H)17 immune responses. Our findings provide a novel therapeutic option for the treatment of multiple sclerosis and other T(H)17-mediated diseases.