Sepsis, a systemic inflammatory response syndrome caused by infection, is the most common disease in patients treated in intensive care units. Endotoxic shock, the most critical form of sepsis, is caused by gram-negative bacterial infection. However, the detailed mechanism of endotoxic shock remains unclear. In the present study, we observed that the production of leukotriene B4 (LTB4) and 12(S)-hydroxyeicosatetraenoic acid (HETE), inflammatory lipid mediators acting on LTB4 receptors (BLT1 and BLT2), was significantly upregulated in peritoneal lavage fluid (PF) and serum from an LPS-induced endotoxic shock mouse model. Furthermore, BLT1/2-dependent signaling pathways mediated the expression of IL-17, IL-6, and IL-1β, key cytokines for the development of endotoxic shock, via NF-κB activation in the LPS-induced endotoxic shock mouse model. Additionally, inhibition of BLT1/2 significantly attenuated inflammation and tissue damage associated with endotoxic shock and enhanced the survival rate of mice with this inflammatory complication. Together, these results suggest that LTB4 receptors play critical mediatory roles in the development of endotoxic shock. Our findings point to LTB4 receptors as potential therapeutic targets for the treatment of endotoxic shock.

Leukotriene B4 (LTB4) is a lipid mediator of inflammation that is generated from arachidonic acid via the 5-lipoxygenase pathway. Previous studies have reported that the receptors of LTB4, BLT1, and BLT2 play mediatory roles in the allergic airway inflammation induced by ovalbumin (OVA). However, considering that house dust mites (HDMs) are the most prevalent allergen and well-known risk factor for asthmatic allergies, we are interested in elucidating the contributory roles of BLT1/2 in HDMinduced allergic airway inflammation. Our aim in this study was to investigate whether BLT1/2 play any roles in HDM-induced allergic airway inflammation. In this study, we observed that the levels of ligands for BLT1/2 [LTB4 and 12(S)-HETE (12(S)- hydroxyeicosatetraenoic acid)] were significantly increased in bronchoalveolar lavage fluid (BALF) after HDM challenge. Blockade of BLT1 or BLT2 as well as of 5-lipoxygenase (5-LO) or 12-lipoxygenase (12-LO) markedly suppressed the production of TH2 cytokines (IL-4, IL-5, and IL-13) and alleviated lung inflammation and mucus secretion in an HDM-induced eosinophilic airway-inflammation mouse model. Together, these results indicate that the 5-/12-LO-BLT1/2 cascade plays a role in HDMinduced airway inflammation by mediating the production of TH2 cytokines. Our findings suggest that BLT1/2 may be a potential therapeutic target for patients with HDM-induced allergic asthma. [BMB Reports 2021; 54(3): 182-187].

Increased levels of neutrophils in bronchoalveolar lavage fluid (BALF) were associated with asthma severity. As leukotriene B4 (LTB4) is a principal chemoattractant molecule for neutrophils, its receptors, BLT1 and BLT2, may contribute to neutrophil-dominant airway inflammation. In the present study, we established a mouse model of steroid-resistant, neutrophil-dominant airway inflammation by house dust mite (HDM)/lipopolysaccharide (LPS) sensitization and HDM challenge, and we investigated whether BLT1/BLT2 signaling was associated with the development of neutrophilic airway inflammation. Blockade of BLT1 or BLT2 significantly suppressed airway inflammation and IL-17 production in this mouse model. The 5-LO and 12-LO enzymes, which catalyze the synthesis of BLT1/BLT2 ligands, were also critically associated with neutrophil-dominant airway inflammation and the synthesis of IL-17. Collectively, our results suggest that the 5-/12-LO-BLT1/BLT2-linked cascade significantly contributes to neutrophil-dominant severe airway inflammation via IL-17 synthesis in HDM-induced neutrophilic asthma.

The stimulation of the NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome and IL-1β synthesis are associated with chronic respiratory diseases such as neutrophil-dominant severe asthma. Leukotriene B4 (LTB4) is a principal chemoattractant molecule for neutrophil recruitment, and its receptors BLT1 and BLT2 have been suggested to contribute to neutrophil-dominant asthmatic airway inflammation. However, the relationship between BLT1/2 and NLRP3 in neutrophil-dominant asthmatic airway inflammation has not been previously studied. In the present study, we investigated whether BLT1/2 play any roles in stimulating the NLRP3 inflammasome and IL-1βsynthesis. The blockade of BLT1 or BLT2 clearly suppressed the stimulation of the NLRP3 inflammasome and IL-1β synthesis in house dust mite (HDM)/lipopolysaccharide (LPS)-induced neutrophilic airway inflammation. The enzymes 5-lipoxygenase and 12-lipoxygenase, which catalyze the synthesis of BLT1/2 ligands [LTB4, 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE), and 12-hydroxyheptadecatreinoic acid (12-HHT)], were also critically associated with the stimulation of NLRP3 and IL-1β synthesis. Together, our results suggest that the 5-/12-LOX-BLT1/2-linked cascade are necessary for the simulation of the NLRP3 inflammasome and IL-1β synthesis, thus contributing to HDM/LPS-induced neutrophil-dominant airway inflammation.

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.