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

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.

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.

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.

Hepatic stellate cell stearoyl co-A desaturase activates leukotriene B4 receptor 2 - β-catenin cascade to promote liver tumorigenesis

Hepatocellular carcinoma (HCC) is the 3rd most deadly malignancy. Activated hepatic stellate cells (aHSC) give rise to cancer-associated fibroblasts in HCC and are considered a potential therapeutic target. Here we report that selective ablation of stearoyl CoA desaturase-2 (Scd2) in aHSC globally suppresses nuclear CTNNB1 and YAP1 in tumors and tumor microenvironment and prevents liver tumorigenesis in male mice. Tumor suppression is associated with reduced leukotriene B4 receptor 2 (LTB4R2) and its high affinity oxylipin ligand, 12-hydroxyheptadecatrienoic acid (12-HHTrE). Genetic or pharmacological inhibition of LTB4R2 recapitulates CTNNB1 and YAP1 inactivation and tumor suppression in culture and in vivo. Single cell RNA sequencing identifies a subset of tumor-associated aHSC expressing Cyp1b1 but no other 12-HHTrE biosynthetic genes. aHSC release 12-HHTrE in a manner dependent on SCD and CYP1B1 and their conditioned medium reproduces the LTB4R2-mediated tumor-promoting effects of 12-HHTrE in HCC cells. CYP1B1-expressing aHSC are detected in proximity of LTB4R2-positive HCC cells and the growth of patient HCC organoids is blunted by LTB4R2 antagonism or knockdown. Collectively, our findings suggest aHSC-initiated 12-HHTrE-LTB4R2-CTNNB1-YAP1 pathway as a potential HCC therapeutic target.

Synthesis and Significance of Arachidonic Acid, a Substrate for Cyclooxygenases, Lipoxygenases, and Cytochrome P450 Pathways in the Tumorigenesis of Glioblastoma Multiforme, Including a Pan-Cancer Comparative Analysis

Glioblastoma multiforme (GBM) is one of the most aggressive gliomas. New and more effective therapeutic approaches are being sought based on studies of the various mechanisms of GBM tumorigenesis, including the synthesis and metabolism of arachidonic acid (ARA), an omega-6 polyunsaturated fatty acid (PUFA). PubMed, GEPIA, and the transcriptomics analysis carried out by Seifert et al. were used in writing this paper. In this paper, we discuss in detail the biosynthesis of this acid in GBM tumors, with a special focus on certain enzymes: fatty acid desaturase (FADS)1, FADS2, and elongation of long-chain fatty acids family member 5 (ELOVL5). We also discuss ARA metabolism, particularly its release from cell membrane phospholipids by phospholipase A₂ (cPLA₂, iPLA₂, and sPLA₂) and its processing by cyclooxygenases (COX-1 and COX-2), lipoxygenases (5-LOX, 12-LOX, 15-LOX-1, and 15-LOX-2), and cytochrome P450. Next, we discuss the significance of lipid mediators synthesized from ARA in GBM cancer processes, including prostaglandins (PGE₂, PGD₂, and 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂)), thromboxane A₂ (TxA₂), oxo-eicosatetraenoic acids, leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄), lipoxins, and many others. These lipid mediators can increase the proliferation of GBM cancer cells, cause angiogenesis, inhibit the anti-tumor response of the immune system, and be responsible for resistance to treatment.

Topical Administration of a Marine Oil Rich in Pro-Resolving Lipid Mediators Accelerates Wound Healing in Diabetic db/db Mice through Angiogenesis and Macrophage Polarization

Impaired wound healing in patients with type 2 diabetes (DM2) is characterized by chronic inflammation, which delays wound closure. Specialized pro-resolving lipid mediators (SPMs) are bioactive molecules produced from essential polyunsaturated fatty acids (PUFAs), principally omega-3 docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). SPMs are potent regulators of inflammation and have been used to suppress chronic inflammation in peripheral artery disease, non-alcoholic fatty liver disease, and central nervous system syndromes. LIPINOVA^® is a commercially available safe-grade nutritional supplement made from a fractionated marine lipid concentrate derived from anchovy and sardine oil that is rich in SPMs and EPA, as well as DHA precursors. Here, we assessed the effect of LIPINOVA^® in wound dressing applications. LIPINOVA^® showed biocompatibility with keratinocytes and fibroblasts, reduced the abundance of pro-inflammatory macrophages (Mφ1), and promoted in vitro wound closure. Daily application of the marine oil to open wounds made by punch biopsy in db/db mice promoted wound closure by accelerating the resolution of inflammation, inducing neoangiogenesis and Mφ1/Mφ2 macrophage polarization. In conclusion, LIPINOVA^® displays pro-resolutive properties and could be exploited as a therapeutic agent for the treatment of diabetic ulcers.

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.

Eicosanoids in the pancreatic tumor microenvironment - a multicellular, multifaceted progression

BACKGROUND AND AIMS

Eicosanoids, oxidized fatty acids that serve as cell-signaling molecules, have been broadly implicated in tumorigenesis. Here, we aimed to identify eicosanoids associated with pancreatic tumorigenesis and the cell types responsible for their synthesis.

METHODS

We profiled normal pancreas and pancreatic ductal adenocarcinoma (PDAC) in mouse models and patient samples using mass spectrometry. We interrogated RNA sequencing datasets for eicosanoid synthase or receptor expression. Findings were confirmed by immunostaining.

RESULTS

In murine models, we identified elevated levels of PGD₂, prostacyclin, and thromboxanes in neoplasia while PGE₂, 12-HHTre, HETEs, and HDoHEs are elevated specifically in tumors. Analysis of scRNA-seq datasets suggests that PGE₂ and prostacyclins are derived from fibroblasts, PGD₂ and thromboxanes from myeloid cells, and PGD₂ and 5-HETE from tuft cells. In patient samples, we identified a transition from PGD₂ to PGE₂-producing enzymes in the epithelium during the transition to PDAC, fibroblast/tumor expression of PTGIS, and myeloid/tumor cell expression of TBXAS1.

CONCLUSIONS

Our analyses identify key changes in eicosanoid species during pancreatic tumorigenesis and the cell types that contribute to their synthesis. Thromboxane and prostacyclin expression is conserved between animal models and human disease and may represent new druggable targets.

Bioengineered Wound Healing Skin Models: The Role of Immune Response and Endogenous ECM to Fully Replicate the Dynamic of Scar Tissue Formation In Vitro

The healing of deep skin wounds is a complex phenomenon evolving according with a fine spatiotemporal regulation of different biological events (hemostasis, inflammation, proliferation, remodeling). Due to the spontaneous evolution of damaged human dermis toward a fibrotic scar, the treatment of deep wounds still represents a clinical concern. Bioengineered full-thickness skin models may play a crucial role in this direction by providing a deep understanding of the process that leads to the formation of fibrotic scars. This will allow (i) to identify new drugs and targets/biomarkers, (ii) to test new therapeutic approaches, and (iii) to develop more accurate in silico models, with the final aim to guide the closure process toward a scar-free closure and, in a more general sense, (iv) to understand the mechanisms involved in the intrinsic and extrinsic aging of the skin. In this work, the complex dynamic of events underlaying the closure of deep skin wound is presented and the engineered models that aim at replicating such complex phenomenon are reviewed. Despite the complexity of the cellular and extracellular events occurring during the skin wound healing the gold standard assay used to replicate such a process is still represented by planar in vitro models that have been largely used to identify the key factors regulating the involved cellular processes. However, the lack of the main constituents of the extracellular matrix (ECM) makes these over-simplistic 2D models unable to predict the complexity of the closure process. Three-dimensional bioengineered models, which aim at recreating the closure dynamics of the human dermis by using exogenous biomaterials, have been developed to fill such a gap. Although interesting mechanistic effects have been figured out, the effect of the inflammatory response on the ECM remodelling is not replicated yet. We discuss how more faithful wound healing models can be obtained by creating immunocompetent 3D dermis models featuring an endogenous ECM.

Inflammation, It's Regulation and Antiphlogistic Effect of the Cyanogenic Glycoside Amygdalin

The inflammatory reaction accompanies in part or in full any disease process in the vascularized metazoan. This complicated reaction is controlled by regulatory mechanisms, some of which produce unpleasant symptomatic manifestations of inflammation. Therefore, there has been an effort to develop selective drugs aimed at removing pain, fever, or swelling. Gradually, however, serious adverse side effects of such inhibitors became apparent. Scientific research has therefore continued to explore new possibilities, including naturally available substances. Amygdalin is a cyanogenic glycoside present, e.g., in bitter almonds. This glycoside has already sparked many discussions among scientists, especially about its anticancer potential and related toxic cyanides. However, toxicity at different doses made it generally unacceptable. Although amygdalin given at the correct oral dose may not lead to poisoning, it has not yet been accurately quantified, as its action is often affected by different intestinal microbial consortia. Its pharmacological activities have been studied, but its effects on the body's inflammatory response are lacking. This review discusses the chemical structure, toxicity, and current knowledge of the molecular mechanism of amygdalin activity on immune functions, including the anti-inflammatory effect, but also discusses inflammation as such, its mediators with diverse functions, which are usually targeted by drugs.

A review of diabetic wound models-Novel insights into diabetic foot ulcer

Diabetic foot ulcer (DFU) is a major debilitating complication of diabetes. Many research investigations have been conducted with the aims to uncover the diabetic wound healing mechanisms, develop novel therapeutics, and screen bioactive wound dressings in order to improve the current management of DFU. These would have not been possible without the utilization of an appropriate wound model, especially in a diabetic wound context. This review focuses on the different in vitro research models used in DFU investigations such as the 2D scratch wound assay, 3D skin model, and 3D angiogenesis model as well as their limitations. The current efforts and challenges to apply the 2D and 3D in vitro models in a hyperglycemic context to provide insights into DFU modeling will be reviewed. Perspectives of utilizing 3D bioprinting and skin-on-the-chip model as a diabetic wound model in the future will also be highlighted. By leveraging knowledge from past experiences and current research, an improved experimental model for DFU is anticipated to be established in near future.

A review of non-prostanoid, eicosanoid receptors: expression, characterization, regulation, and mechanism of action

Eicosanoid signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain and to cell survival itself. Disruption of normal eicosanoid signaling is implicated in numerous disease states. Eicosanoid signaling is facilitated by G-protein-coupled, eicosanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of non-prostanoid, eicosanoid receptors.

The c-terminal region of BLT2 restricts its localization to the lateral membrane in a LIN7C-dependent manner

Leukotriene B₄ receptor type 2 (BLT2) is a G protein-coupled receptor (GPCR) mainly expressed in epithelial cells, where it enhances barrier function. A unique characteristic of BLT2 is its restricted localization to the lateral membrane. However, the molecular mechanism underlying the localization of BLT2 to the lateral membrane and the physiological roles of laterally localized BLT2 are unknown. BLT1 is the most homologous GPCR to BLT2 and localizes to both the apical and lateral membranes. In this study, we generated chimeric receptors of BLT2 and BLT1 as well as deletion mutants of BLT2 to determine the region(s) of BLT2 responsible for its localization. Chimeric receptors containing the C-terminal domain of BLT2 localized only to the lateral membrane, and the C-terminal deletion mutant of BLT2 accumulated at the Golgi apparatus. Furthermore, the middle and C-terminal regions of BLT2 were important for maintaining epithelial barrier function. Proteomics analysis using the chimeric BLT-ascorbate peroxidase 2 biotinylation method showed that some proteins involved in intracellular protein transport, cell-cell junctions, and actin filament binding were located very close to the C-terminal domain of BLT2. Knockdown of lin-7 homolog C (LIN7C), a membrane trafficking protein, led to accumulation of BLT2 in the Golgi apparatus, resulting in diminished epithelial barrier function. These results suggest that the C-terminal region of BLT2 plays an important role in the transport of BLT2 from the Golgi apparatus to the plasma membrane in a LIN7C-dependent manner.

The role of lipid-based signalling in wound healing and senescence

Lipid-based signalling modulates several cellular processes and intercellular communication during wound healing and tissue regeneration. Bioactive lipids include but are not limited to the diverse group of eicosanoids, phospholipids, and extracellular vesicles and mediate the attraction of immune cells, initiation of inflammatory responses, and their resolution. In aged individuals, wound healing and tissue regeneration are greatly impaired, resulting in a delayed healing process and non-healing wounds. Senescent cells accumulate with age in vivo, preferably at sites implicated in age-associated pathologies and their elimination was shown to alleviate many age-associated diseases and disorders. In contrast to these findings, the transient presence of senescent cells in the process of wound healing exerts beneficial effects and limits fibrosis. Hence, clearance of senescent cells during wound healing was repeatedly shown to delay wound closure in vivo. Recent findings established a dysregulated synthesis of eicosanoids, phospholipids and extracellular vesicles as part of the senescent phenotype. This intriguing connection between cellular senescence, lipid-based signalling, and the process of wound healing and tissue regeneration prompts us to compile the current knowledge in this review and propose future directions for investigation.

Effect of bovine bone collagen oligopeptides on wound healing in mice

Impaired wound healing often brings a set of problems in clinical practice. This study aimed to observe the wound healing potential of bovine bone collagen oligopeptides (BCOP) in mice. After an operation, mice in BCOP-treated groups were given intragastric administration of BCOP, while others were administered vehicle. Mice were sacrificed at different points. The wound healing condition and the tensile strength were observed, serum biochemical indexes and mRNA expression of level of related genes were measured. Compared with the normal control group, albumin (ALB), prealbumin (PA), transferrin (TRF), hydroxyproline (Hyp) levels and tension strength in the BCOP-treated groups increased significantly (p < 0.05). A pathological report showed that neutrophil granulocyte in the BCOP-treated groups decreased, while blood capillary and fibroblasts increased. The levels of serum inflammation indexes like interleukin (IL)-8, tumor necrosis factor (TNF)-α, chemokine (C-C motif) ligand 2 (CCL2) and C-reactive protein (CRP) significantly decreased in full-thickness incision model, whereas increased in full-thickness excision model (p < 0.05). Furthermore, IL-10, stromal cell-derived factor-1 alpha (SDF-1α) levels and the mRNA expression of vascular endothelial growth factor (VEGF) significantly increased in both models (p < 0.05). These results suggested that oral administration of BCOP could promote wound healing in mice.

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.

Eicosanoids in Skin Wound Healing

Wound healing is an important process in the human body to protect against external threats. A dysregulation at any stage of the wound healing process may result in the development of various intractable ulcers or excessive scar formation. Numerous factors such as growth factors, cytokines, and chemokines are involved in this process and play vital roles in tissue repair. Moreover, recent studies have demonstrated that lipid mediators derived from membrane fatty acids are also involved in the process of wound healing. Among these lipid mediators, we focus on eicosanoids such as prostaglandins, thromboxane, leukotrienes, and specialized pro-resolving mediators, which are produced during wound healing processes and play versatile roles in the process. This review article highlights the roles of eicosanoids on skin wound healing, especially focusing on the biosynthetic pathways and biological functions, i.e., inflammation, proliferation, migration, angiogenesis, remodeling, and scarring.

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.

Targeting Leukotrienes as a Therapeutic Strategy to Prevent Comorbidities Associated with Metabolic Stress

Leukotrienes (LTs) are potent lipid mediators that exert a variety of functions, ranging from maintaining the tone of the homeostatic immune response to exerting potent proinflammatory effects. Therefore, LTs are essential elements in the development and maintenance of different chronic diseases, such as asthma, arthritis, and atherosclerosis. Due to the pleiotropic effects of LTs in the pathogenesis of inflammatory diseases, studies are needed to discover potent and specific LT synthesis inhibitors and LT receptor antagonists. Even though most clinical trials using LT inhibitors or antagonists have failed due to low efficacy and/or toxicity, new drug development strategies are driving the discovery for LT inhibitors to prevent inflammatory diseases. A newly important detrimental role for LTs in comorbidities associated with metabolic stress has emerged in the last few years and managing LT production and/or actions could represent an exciting new strategy to prevent or treat inflammatory diseases associated with metabolic disorders. This review is intended to shed light on the synthesis and actions of leukotrienes, the most common drugs used in clinical trials, and discuss the therapeutic potential of preventing LT function in obesity, diabetes, and hyperlipidemia.

The landscape of GPCR signaling in the regulation of epidermal stem cell fate and skin homeostasis

Continuous integration of signals from the micro and macro-environment is necessary for somatic stem cells to adapt to changing conditions, maintain tissue homeostasis and activate repair mechanisms. G-protein coupled receptors (GPCRs) facilitate this integration by binding to numerous hormones, metabolites and inflammatory mediators, influencing a diverse network of pathways that regulate stem cell fate. This adaptive mechanism is particularly relevant for tissues that are exposed to environmental assault, like skin. The skin is maintained by a set of basal keratinocyte stem and progenitor cells located in the hair follicle and interfollicular epidermis, and several GPCRs and their signaling partners serve as makers and regulators of epidermal stem cell activity. GPCRs utilize heterotrimeric G protein dependent and independent pathways to translate extracellular signals into intracellular molecular cascades that dictate the activation of keratinocyte proliferative and differentiation networks, including Hedgehog GLI, Hippo YAP1 and WNT/β-catenin, ultimately regulating stem cell identity. Dysregulation of GPCR signaling underlines numerous skin inflammatory diseases and cancer, with smoothened-driven basal cell carcinoma being a main example of a GPCR associated cancer. In this review, we discuss the impact of GPCRs and their signaling partners in skin keratinocyte biology, particularly in the regulation of the epidermal stem cell compartment.

Bioactive Fatty Acids in the Resolution of Chronic Inflammation in Skin Wounds

Significance: Optimal skin wound healing is crucial for maintaining tissue homeostasis, particularly in response to an injury. The skin immune system is under regulation of mediators such as bioactive lipids and cytokines that can initiate an immune response with controlled inflammation, followed by efficient resolution. However, nutritional deficiency impacts wound healing by hindering fibroblast proliferation, collagen synthesis, and epithelialization, among other crucial functions. In this way, the correct nutritional support of bioactive lipids and of other essential nutrients plays an important role in the outcome of the wound healing process. Recent Advances and Critical Issues: Several studies have revealed the potential role of lipids as a treatment for the healing of skin wounds. Unsaturated fatty acids such as linoleic acid, α-linolenic acid, oleic acid, and most of their bioactive products have shown an effective role as a topical treatment of chronic skin wounds. Their effect, when the treatment starts at day 0, has been observed mainly in the inflammatory phase of the wound healing process. Moreover, some of them were associated with different dressings and were tested for clinical purposes, including pluronic gel, nanocapsules, collagen films and matrices, and polymeric bandages. Therefore, future research is still needed to evaluate these dressing technologies in association with different bioactive fatty acids in a wound healing context. Future Directions: This review summarizes the main results of the available clinical trials and basic research studies and provides evidence-based conclusions. Together, current data encourage the use of bioactive fatty acids for an optimal wound healing resolution.

The skeletal muscles of mice infected with Plasmodium berghei and Plasmodium chabaudi reveal a crosstalk between lipid mediators and gene expression

Background

Malaria is one of the most prevalent infectious disease in the world with 3.2 billion humans at risk. Malaria causes splenomegaly and damage in other organs including skeletal muscles. Skeletal muscles comprise nearly 50% of the human body and are largely responsible for the regulation and modulation of overall metabolism. It is essential to understand how malaria damages muscles in order to develop effective preventive measures and/or treatments. Using a pre-clinical animal model, the potential molecular mechanisms of Plasmodium infection affecting skeletal muscles of mice were investigated.

Methods

Mouse Signal Transduction Pathway Finder PCR Array was used to monitor gene expression changes of 10 essential signalling pathways in skeletal muscles from mice infected with Plasmodium berghei and Plasmodium chabaudi. Then, a new targeted-lipidomic approach using liquid chromatography with tandem mass spectrometry (LC–MS/MS) to profile 158 lipid signalling mediators (LMs), mostly eicosanoids derived from arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), was applied. Finally, 16 key LMs directly associated with inflammation, oxidative stress, and tissue healing in skeletal muscles, were quantified.

Results

The results showed that the expression of key genes altered by Plasmodium infection is associated with inflammation, oxidative stress, and atrophy. In support to gene profiling results, lipidomics revealed higher concentrations of LMs in skeletal muscles directly related to inflammatory responses, while on the levels of LMs crucial in resolving inflammation and tissue repair reduced significantly.

Conclusion

The results provide new insights into the molecular mechanisms of malaria-induced muscle damage and revealed a potential mechanism modulating inflammation in malarial muscles. These pre-clinical studies should help with future clinical studies in humans aimed at monitoring of disease progression and development of specific interventions for the prevention and mitigation of long-term chronic effects on skeletal muscle function.

Leukotriene B4 receptors as therapeutic targets for ophthalmic diseases

Leukotriene B₄ (LTB₄) is an inflammatory lipid mediator produced from arachidonic acid by multiple reactions catalyzed by two enzymes 5-lipoxygenase (5-LOX) and LTA₄ hydrolase (LTA₄H). The two receptors for LTB₄ have been identified: a high-affinity receptor, BLT1, and a low-affinity receptor, BLT2. Our group identified 12(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT) as a high-affinity BLT2 ligand. Numerous studies have revealed critical roles for LTB₄ and its receptors in various systemic diseases. Recently, we also reported the roles of LTB₄, BLT1 and BLT2 in the murine ophthalmic disease models of mice including cornea wound, allergic conjunctivitis, and age-related macular degeneration. Moreover, other groups revealed the evidence of the ocular function of LTB₄. In the present review, we introduce the roles of LTB₄ and its receptors both in ophthalmic diseases and systemic inflammatory diseases. LTB₄ and its receptors are putative novel therapeutic targets for systemic and ophthalmic diseases.

The role of the LTB4-BLT1 axis in health and disease

Leukotriene B4 (LTB4) is a major type of lipid mediator that is rapidly generated from arachidonic acid through sequential action of 5-lipoxygenase (5-LO), 5-lipoxygenase-activating protein (FLAP) and LTA4 hydrolase (LTA4H) in response to various stimuli. LTB4 is well known to be a chemoattractant for leukocytes, particularly neutrophils, via interaction with its high-affinity receptor BLT1. Extensive attention has been paid to the role of the LTB4-BLT1 axis in acute and chronic inflammatory diseases, such as infectious diseases, allergy, autoimmune diseases, and metabolic disease via mediating recruitment and/or activation of different types of inflammatory cells depending on different stages or the nature of inflammatory response. Recent studies also demonstrated that LTB4 acts on non-immune cells via BLT1 to initiate and/or amplify pathological inflammation in various tissues. In addition, emerging evidence reveals a complex role of the LTB4-BLT1 axis in cancer, either tumor-inhibitory or tumor-promoting, depending on the different target cells. In this review, we summarize both established understanding and the most recent progress in our knowledge about the LTB4-BLT1 axis in host defense, inflammatory diseases and cancer.

Promotion of Myofibroblast Differentiation and Tissue Fibrosis by the Leukotriene B4 -Leukotriene B4 Receptor Axis in Systemic Sclerosis

OBJECTIVE

To investigate the role of the inflammatory lipid mediator leukotriene B₄ (LTB₄ ) and its receptor, BLT1, in the development and progression of systemic sclerosis (SSc).

METHODS

Serum levels of LTB₄ were compared in 64 patients with SSc and 80 healthy controls. Skin and lung tissue sections from patients with SSc and healthy donors were immunostained for leukotriene A₄ hydrolase (LTA₄ H), the critical enzyme for LTB₄ synthesis, and BLT1, in combination with different cell markers. In mouse models of SSc using bleomycin or angiotensin II challenge or immunization with the DNA topoisomerase I, genetic or pharmacologic interruption of the LTB₄ -BLT1 axis in mice was carried out to assess its effects on systemic disease features and myofibroblast markers. Immunoblotting was performed to examine the signaling pathway in fibroblasts and endothelial cells following stimulation with LTB₄ or with serum from SSc patients.

RESULTS

Serum LTB₄ levels were 44.93% higher in patients with SSc than in matched healthy controls (mean ± SD 220.3 ± 74.75 pg/ml versus 152.0 ± 68.05 pg/ml; P < 0.0001), and this was associated with the patient subsets of SSc-associated interstitial lung disease and diffuse cutaneous SSc. Levels of LTA₄ H and BLT1 were increased in lesional areas of the skin and lungs of SSc patients, and both were abundant in myofibroblasts and endothelial cells. Interruption of the LTB₄ -BLT1 axis in mouse models of SSc significantly mitigated dermal and pulmonary fibrosis, with 54.00% and 52.65% fewer α-smooth muscle actin-positive myofibroblasts accumulating in the skin and lungs of mice, respectively, after bleomycin challenge. Immunoblotting of cultures with recombinant LTB₄ -stimulated fibroblasts and endothelial cells or with serum from SSc patients showed that fibroblast-myofibroblast and endothelial-mesenchymal transitions were promoted via BLT1, and that this was dependent on activation of the phosphatidylinositol 3-kinase (PI3K)/Akt/mechanistic target of rapamycin (mTOR) pathway but independent of the release of transforming growth factor β (TGFβ) by fibroblasts or endothelial cells.

CONCLUSION

The LTB₄ -BLT1 axis may contribute to fibrosis in SSc by directly promoting myofibroblast differentiation via the PI3K/Akt/mTOR pathway, and this appears to operate independently of autocrine secretion of TGFβ.

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.

The Immunosuppressant Fingolimod (FTY720) for the Treatment of Mechanical Force-Induced Abnormal Scars

Aim

Abnormal scars such as hypertrophic scars (HSs) and keloids are excessively growing scars that exhibit chronic inflammation and capillary vasculogenesis. The lipid mediator sphingosine-1-phosphate (S1P) is important in inflammatory cell recruitment and angiogenesis. Fingolimod (FTY720) is an analog of S1P and thus functionally antagonizes S1P receptors and inhibits the enzyme that produces S1P. We examined the effects of topical FTY720 injections on mechanical force-induced HS progression.

Methods

Mechanical force-induced HSs were generated in C57BL6/J mice by suturing a dorsal incision and applying a stretching device on Days 6, 8, 10, and 12. On Days 8, 10, and 12, intracutaneous FTY720 (10 μM) or control vehicle injections were performed. On Day 14, scar tissues and blood were procured and subjected to histology and flow cytometry.

Results

Flow cytometry showed that FTY720 decreased the frequencies of macrophages with M2 predominance in the scars but had no effect on total, CD4⁺, or CD8a⁺ T cell frequencies. FTY720 also decreased the vascular endothelial cell frequencies in the scar along with the microvessels, as determined by immunohistochemistry. Compared to the vehicles, FTY720 treatment significantly reduced the gross scar area and the cross-sectional scar area on histology. On the other hand, FTY720 tended to reduce white blood cells and significantly reduced the lymphocyte frequencies in the blood.

Conclusion

Topical FTY720 induces M2 predominance and impairs angiogenesis. Therefore, its local immunosuppressive mechanisms differ from those of conventional immunosuppressive agents. Topical FTY720 can be a novel therapeutic option for abnormal scars that are difficult to control with corticosteroids. Its lymphocytopenic effects may be limited by careful optimization of the treatment regimen.

Untargeted metabolomics reveals alterations in metabolites of lipid metabolism and immune pathways in the serum of rats after long-term oral administration of Amalaki rasayana

Amalaki rasayana, a traditional preparation, is widely used by Ayurvedic physicians for the treatment of inflammatory conditions, cardiovascular diseases, and cancer. Metabolic alterations induced by Amalaki rasayana intervention are unknown. We investigated the modulations in serum metabolomic profiles in Wistar rats following long-term oral administration of Amalaki rasayana. Global metabolic profiling was performed of the serum of rats administered with either Amalaki rasayana (AR) or ghee + honey (GH) for 18 months and control animals which were left untreated. Amalaki rasayana components were confirmed from AR extract using HR-LCMS analysis. Significant reductions in prostaglandin J2, 11-dehydrothromboxane B2, and higher levels of reduced glutathione and glycitein metabolites were observed in the serum of AR administered rats compared to the control groups. Eleven different metabolites classified as phospholipids, glycerophospholipids, glucoside derivatives, organic acids, and glycosphingolipid were exclusively observed in the AR administered rats. Pathway analysis suggests that altered metabolites in AR administered rats are those associated with different biochemical pathways of arachidonic acid metabolism, fatty acid metabolism, leukotriene metabolism, G-protein mediated events, phospholipid metabolism, and the immune system. Targeted metabolomics confirmed the presence of gallic acid, ellagic acid, and arachidonic acid components in the AR extract. The known activities of these components can be correlated with the altered metabolic profile following long-term AR administration. AR also activates IGF1R-Akt-Foxo3 signaling axis in heart tissues of rats administered with AR. Our study identifies AR components that induce alterations in lipid metabolism and immune pathways in animals which consume AR for an extended period.

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.

Sphingosine-1-Phosphate Facilitates Skin Wound Healing by Increasing Angiogenesis and Inflammatory Cell Recruitment with Less Scar Formation

Wound healing starts with the recruitment of inflammatory cells that secrete wound-related factors. This step is followed by fibroblast activation and tissue construction. Sphingosine-1-phosphate (S1P) is a lipid mediator that promotes angiogenesis, cell proliferation, and attracts immune cells. We investigated the roles of S1P in skin wound healing by altering the expression of its biogenic enzyme, sphingosine kinase-1 (SphK1). The murine excisional wound splinting model was used. Sphingosine kinase-1 (SphK1) was highly expressed in murine wounds and that SphK1^−/− mice exhibit delayed wound closure along with less angiogenesis and inflammatory cell recruitment. Nanoparticle-mediated topical SphK1 overexpression accelerated wound closure, which associated with increased angiogenesis, inflammatory cell recruitment, and various wound-related factors. The SphK1 overexpression also led to less scarring, and the interaction between transforming growth factor (TGF)-β1 and S1P receptor-2 (S1PR2) signaling is likely to play a key role. In summary, SphK1 play important roles to strengthen immunity, and contributes early wound healing with suppressed scarring. S1P can be a novel therapeutic molecule with anti-scarring effect in surgical, trauma, and chronic wound management.

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.

Eicosanoids in tissue repair

Trauma or infection can result in tissue damage, which needs to be repaired in a well-orchestrated manner to restore tissue function and homeostasis. Lipid mediators derived from arachidonic acid (termed eicosanoids) play central and versatile roles in the regulation of tissue repair. Here, I summarize the current state-of the-art regarding the functional activities of eicosanoids in tissue repair responses during homeostasis and disease. I also describe how eicosanoids are produced during tissue damage and repair in a time-, cell- and tissue-dependent fashion. In particular, recent insights into the roles of eicosanoids in epithelial barrier repair are reviewed. Furthermore, the distinct roles of different eicosanoids in settings of pathological tissue repair such as chronic wounds, scarring or fibrosis are discussed. Finally, an outlook is provided on how eicosanoids may be targeted by future therapeutic strategies to achieve physiological tissue repair and prevent scarring and loss of tissue function in various disease contexts.

Psoriasis and the TNF/IL23/IL17 axis

The excellent response of psoriasis to anti-TNF-α(TNF)/IL23/IL17A biologics implies a crucial role for the TNF/IL23/IL17 axis in developing psoriasis. In addition to the TNF/IL23/IL17 axis provided by immune cells, current evidence points to an important contribution of TNF, IL23 and IL17C produced from non-hematopoietic keratinocytes. Therefore, crosstalk between immune cells and keratinocytes forms a multilayered feed-forward loop to accelerate the TNF/IL23/IL17A axis. Many biologics have already been licensed or are under clinical trials. Given that the IL-17 signature is more upregulated in the skin than in synovium in psoriatic arthritis, anti-IL-23/IL-17 agents seem to be superior to anti-TNF-α remedies in the treatment of skin lesions. In this review, we summarize recent topics in psoriasis and the TNF/IL23/IL17 axis.

Phospholipase A2 in skin biology: new insights from gene-manipulated mice and lipidomics

The skin represents one of the tissues that are most profoundly influenced by alterations in the quality of lipids (lipoquality). Lipids not only constitute cellular membranes, but also serve as bioactive lipid mediators and essential components of the skin barrier. Phospholipase A₂ (PLA₂) enzymes supply fatty acids and lysophospholipids from membrane phospholipids, thereby variably affecting cutaneous homeostasis. Accordingly, perturbation of particular PLA₂-driven lipid pathways can be linked to various forms of skin disease. In this review article, we highlight the roles of several PLA₂ subtypes in cutaneous pathophysiology, as revealed by transgenic/knockout studies in combination with comprehensive lipidomics. We focus mainly on secreted PLA₂ group IIF (sPLA₂-IIF), which is associated with epidermal hyperplasia through mobilization of a unique lipid metabolite. We also address the distinct roles of sPLA₂-IIE in hair follicles and sPLA₂-IID in lymphoid immune cells that secondarily affect cutaneous inflammation, and provide some insights into species differences in sPLA₂s. Additionally, we briefly overview the patatin-like phospholipase PNPLA1, which belongs to the Ca²⁺-independent PLA₂ (iPLA₂) family, as a key regulator of skin barrier function through catalysis of a unique non-PLA₂ reaction. These knowledges on lipid metabolism driven by various PLA₂ subtypes will open novel opportunities for translated studies toward diagnosis and therapy of human skin diseases.

Elovl6 regulates mechanical damage-induced keratinocyte death and skin inflammation

Mechanical damage on the skin not only affects barrier function but also induces various immune responses, which trigger or exacerbate skin inflammation. However, how mechanical damage-induced skin inflammation is regulated remains incompletely understood. Here, we show that keratinocytes express the long-chain fatty-acid elongase Elovl6. Mice deficient in Elovl6 showed higher levels of cis-vaccenic acid (CVA) in the epidermis and severe skin inflammation induced by mechanical damage due to tape stripping than did wild-type mice. CVA accelerated tape stripping-triggered keratinocyte death and release of danger-associated molecular patterns (DAMPs) such as high-mobility group box 1 protein (HMGB-1) and IL-1α, which induced production of proinflammatory cytokines and chemokines IL-1β and CXCL-1 by keratinocytes. Our results demonstrate that Elovl6 regulates mechanical damage—triggered keratinocyte death and the subsequent dermatitis.

The streptozotocin-high fat diet induced diabetic mouse model exhibits severe skin damage and alterations in local lipid mediators

BACKGROUND

Type 2 diabetes (T2D) can go undiagnosed for years, leading to a stage where produces complications such as delayed skin wound healing. Animal models have been developed in the last decades to study the pathological progression in this disease. Streptozotocin (STZ), that has a selective pharmacological toxicity toward pancreatic β cells, in addition to high fat diet has been widely used to induce diabetes however no evidence has shown its effects on the skin integrity.

METHODS

Eighteen C57BL/6J male mice, were divided in 3 groups; the first was fed with chow diet and the second was kept on a high fat diet and the third injected with STZ intraperitoneal for 5 days consecutively before starting the diet protocol with high fat. Mice were maintained 5 weeks in total.

RESULTS

We show that animals treated with STZ-high fat diet exhibit skin injuries without significant alterations on basal insulin and triglycerides, compared to the control. The skin from these animals presents higher levels of oxidative stress, lower levels of adhesion proteins and alterations in lipid mediators, effects that are not produced by the high fat diet itself.

CONCLUSION

Our results suggest that this in vivo model represents a relevant approach for studying skin damage induced by diabetes.

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.

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.

Excessive localized leukotriene B4 levels dictate poor skin host defense in diabetic mice

Poorly controlled diabetes leads to comorbidities and enhanced susceptibility to infections. While the immune components involved in wound healing in diabetes have been studied, the components involved in susceptibility to skin infections remain unclear. Here, we examined the effects of the inflammatory lipid mediator leukotriene B4 (LTB4) signaling through its receptor B leukotriene receptor 1 (BLT1) in the progression of methicillin-resistant Staphylococcus aureus (MRSA) skin infection in 2 models of diabetes. Diabetic mice produced higher levels of LTB4 in the skin, which correlated with larger nonhealing lesion areas and increased bacterial loads compared with nondiabetic mice. High LTB4 levels were also associated with dysregulated cytokine and chemokine production, excessive neutrophil migration but impaired abscess formation, and uncontrolled collagen deposition. Both genetic deletion and topical pharmacological BLT1 antagonism restored inflammatory response and abscess formation, followed by a reduction in the bacterial load and lesion area in the diabetic mice. Macrophage depletion in diabetic mice limited LTB4 production and improved abscess architecture and skin host defense. These data demonstrate that exaggerated LTB4/BLT1 responses mediate a derailed inflammatory milieu that underlies poor host defense in diabetes. Prevention of LTB4 production/actions could provide a new therapeutic strategy to restore host defense in diabetes.

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.

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.