Relationship between leukotriene B4 and immunological parameters in rheumatoid synovial fluids

The Multifarious Functions of Leukotrienes in Bone Metabolism

Leukotrienes (LTs) are derived from arachidonic acid metabolism by the 5-lipoxygenase (5-LO) enzyme. The production of LTs is stimulated in the pathogenesis of rheumatoid arthritis (RA), osteoarthritis, and periodontitis, with a relevant contribution to bone resorption. However, its role in bone turnover, particularly the suppression of bone formation by modulating the function of osteoclasts and osteoblasts, remains unclear. We investigated the effects of LTs on bone metabolism and their impact on osteogenic differentiation and osteoclastogenesis using a 5-LO knockout (KO) mouse model. Results from micro-computed tomography (μCT) analysis of femur from 8-week-old 5-LO-deficient mice showed increased cortical bone and medullary region in females and males and decreased trabecular bone in females. In the vertebra, we observed increased marrow area in both females and males 5-LO KO and decreased trabecular bone only in females 5-LO KO. Immunohistochemistry (IHC) analysis showed higher levels of osteogenic markers tissue-nonspecific alkaline phosphatase (TNAP) and osteopontin (OPN) and lower expression of osteoclastogenic marker tartrate-resistant acid phosphatase (TRAP) in the femurs of 5-LO KO mice versus wild-type (WT). Alkaline phosphatase activity and mineralization assay results showed that the 5-LO absence enhances osteoblasts differentiation and mineralization but decreases the proliferation. Alkaline phosphatase (ALP), Bglap, and Sp7 gene expression were higher in 5-LO KO osteoblasts compared to WT cells. Eicosanoids production was higher in 5-LO KO osteoblasts except for thromboxane 2, which was lower in 5-LO-deficient mice. Proteomic analysis identified the downregulation of proteins related to adenosine triphosphate (ATP) metabolism in 5-LO KO osteoblasts, and the upregulation of transcription factors such as the adaptor-related protein complex 1 (AP-1 complex) in long bones from 5-LO KO mice leading to an increased bone formation pattern in 5-LO-deficient mice. We observed enormous differences in the morphology and function of osteoclasts with reduced bone resorption markers and impaired osteoclasts in 5-LO KO compared to WT osteoclasts. Altogether, these results demonstrate that the absence of 5-LO is related to the greater osteogenic profile. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

New and promising type of leukotriene B4 (LTB4) antagonists based on the 1,4-benzodioxine structure

New leukotriene B4 (LTB4) antagonists have been synthesized that can be considered as potential anti-inflammatory drugs. Structures containing the dioxygenated nucleus of 1,4-benzodioxine constitute a potential group of leukotriene B4 (LTB4) antagonists. The objective of this study was to access efficient and selective LTB4 antagonists as a way to elucidate the role of LTB4 in inflammatory processes and therefore allow the development of new types of structures based on 1,4-benzodioxine. Forty-one new 1,4-benzodioxine molecules substituted at different positions of the heterocyclic nucleus were synthesized to determine the minimum structural requirements by studying structure-activity relationships. Eighteen of them were tested in vitro and in vivo for their anti-inflammatory activity related to the antagonist character of LTB4. Pharmacological tests have shown satisfactory in vitro activity for compounds 24b, 24c and 24e with IC50's of 288, 439, 477 nM respectively. The results of the in vivo tests, carried out with the compound that presented greater activity in the in vitro tests 24b, have shown significant anti-inflammatory properties.

Interrelationship between the 5-lipoxygenase pathway and microbial dysbiosis in the progression of Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disorder involving neurofibrillary tangles and amyloid plaques. The tau phosphorylation responsible for neurofibrillary tangles and amyloid deposition which causes plaques are both accelerated through the activity of 5-lipoxygenase (5-LO). In addition to these pathological pathways, 5-LO has also been linked to the neuro-inflammation associated with disease progression as well as to dysbiosis in the gut. Interestingly, gut dysbiosis itself has been correlated to AD development. Not only do gut metabolites have direct effects on the brain, but pro-inflammatory mediators such as LPS, BMAA and bacterial amyloids produced in the gut due to dysbiosis reach the brain causing increased neuro-inflammation. While microbial dysbiosis and 5-LO exert detrimental effects in the brain, the cause/effect relationship between these factors remain unknown. These issues may be addressed using mouse models of AD in the context of different knockout mice in the 5-LO pathway in specific pathogen-free, germ-free as well as gnotobiotic conditions.

Patients with asthma have a higher risk of rheumatoid arthritis: A systematic review and meta-analysis

OBJECTIVE

This study aimed to investigate whether patients with asthma have a higher risk of incident rheumatoid arthritis (RA) using systematic review and meta-analysis technique.

METHODS

Potentially eligible studies were identified from Medline and EMBASE databases from inception to May 2020 using search strategy that comprised of terms for "Asthma" and "Rheumatoid arthritis". Eligible cohort study must consist of one cohort of patients with asthma and another cohort of individuals without asthma. Then, the study must effect estimates with 95% confidence intervals (95% CIs) comparing incident RA between the groups. Eligible case-control studies must include cases with RA and controls without RA. Then, the study must explore their history of asthma. Odds ratio (OR) with 95% CIs of the association between asthma status and RA must be reported. Point estimates with standard errors were retrieved from each study and were combined together using the generic inverse variance method.

RESULTS

A total of 22,442 articles were identified. After two rounds of independent review by three investigators, six cohort studies and fourteen case-control studies met the eligibility criteria and were included into the meta-analysis. Meta-analysis of cohort studies found that patients with asthma had a significantly higher risk of RA compared with individuals without asthma with the pooled hazard ratio of 1.42 (95% CI, 1.18 - 1.70). Meta-analysis of case-control studies revealed that patients with asthma had a higher risk of RA compared with individuals without asthma with the pooled odds ratio of 1.33 (95%CI, 0.97 - 1.83). Funnel plot for the meta-analysis of case-control studies was asymmetric, suggesting the presence of publication bias.

CONCLUSIONS

This systematic review and meta-analysis found a significant association between asthma and higher risk of incident RA.

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.

Leukotriene B4 receptor BLT1 signaling is critical for neutrophil apoptosis and resolution of experimental Lyme arthritis

Eicosanoids are powerful mediators of inflammation and are known to drive both the progression and regression of arthritis. We previously reported the infection of C3H 5-lipoxygenase (LO)-deficient mice with Borrelia burgdorferi results in prolonged nonresolving Lyme arthritis. Here we define the role of the 5-LO metabolite leukotriene (LT)B₄ and its high-affinity receptor, BLT1, in this response. C3H and C3H BLT1^-/- mice were infected with B. burgdorferi and arthritis progression was monitored by ankle swelling over time. Similar to 5-LO^-/- mice, BLT1^-/- mice developed nonresolving Lyme arthritis characterized by increased neutrophils in the joint at later time points than WT mice, but with fewer apoptotic (caspase-3⁺ ) neutrophils. In vitro, BLT1^-/- neutrophils were defective in their ability to undergo apoptosis due to the lack of LTB₄ -mediated down-regulation of cAMP, subsequent failure to induce Death-Inducing Signaling Complex (DISC) components, and decreased FasL and CD36 expression. Inhibition of adenylyl cyclase with SQ 22,536 restored BLT1^-/- BMN apoptosis, FasL and CD36 expression, and clearance by macrophages. We conclude that LTB4/BLT1 signaling has an unexpected critical role in mediating neutrophil apoptosis via the down-regulation of cAMP. Loss of BLT1 signaling led to defective clearance of neutrophils from the inflamed joint and failed arthritis resolution.

Targeting nitric oxide as a key modulator of sepsis, arthritis and pain

Nitric oxide (NO) is produced by enzymatic activity of neuronal (nNOS), endothelial (eNOS), and inducible nitric oxide synthase (iNOS) and modulates a broad spectrum of physiological and pathophysiological conditions. The iNOS isoform is positively regulated at transcriptional level and produces high levels of NO in response to inflammatory mediators and/or to pattern recognition receptor signaling, such as Toll-like receptors. In this review, we compiled the main contributions of our group for understanding of the role of NO in sepsis and arthritis outcome and the peripheral contributions of NO to inflammatory pain development. Although neutrophil iNOS-derived NO is necessary for bacterial killing, systemic production of high levels of NO impairs neutrophil migration to infections through inhibiting neutrophil adhesion on microcirculation and their locomotion. Moreover, neutrophil-derived NO contributes to multiple organ dysfunction in sepsis. In arthritis, NO is chief for bacterial clearance in staphylococcal-induced arthritis; however, it contributes to articular damage and bone mass degradation. NO produced in inflammatory sites also downmodulates pain. The mechanism involved in analgesic effect and inhibition of neutrophil migration is dependent on the activation of the classical sGC/cGMP/PKG pathway. Despite the increasing number of studies performed after the identification of NO as an endothelium-derived relaxing factor, the underlying mechanisms of NO in inflammatory diseases remain unclear.

Neutrophils: Novel key players in Rheumatoid Arthritis. Current and future therapeutic targets

Rheumatoid Arthritis (RA) is a complex systemic autoimmune disease in which various cell types are involved. Among them, neutrophils have been recognized as important players in the onset and the progression of RA. The pathogenic role of neutrophils in RA lies in the alteration of several processes, including increased cell survival and migratory capacity, abnormal inflammatory activity, elevated oxidative stress and an exacerbated release of neutrophil extracellular traps. Through these mechanisms, neutrophils can activate other immune cells, thus perpetuating inflammation and leading to the destruction of the cartilage and bone of the affected joint. Given the considerable contribution of neutrophils to the pathophysiology of RA, several studies have attempted to clarify the effects of various therapeutic agents on this subtype of leukocyte. To date, recent studies have envisaged the role of new molecules on the pathogenic profile of neutrophils in RA, which could represent novel targets in future therapies. In this review, we aim to review the pathogenic role of neutrophils in RA, the effect of conventional treatments and biologic therapies, and the new, potential targets of neutrophil-derived molecules for the treatment of RA.

Too much of a good thing: How modulating LTB4 actions restore host defense in homeostasis or disease

The ability to regulate inflammatory pathways and host defense mechanisms is critical for maintaining homeostasis and responding to infections and tissue injury. While unbalanced inflammation is detrimental to the host; inadequate inflammation might not provide effective signals required to eliminate pathogens. On the other hand, aberrant inflammation could result in organ damage and impair host defense. The lipid mediator leukotriene B₄ (LTB₄) is a potent neutrophil chemoattractant and recently, its role as a dominant molecule that amplifies many arms of phagocyte antimicrobial effector function has been unveiled. However, excessive LTB₄ production contributes to disease severity in chronic inflammatory diseases such as diabetes and arthritis, which could potentially be involved in poor host defense in these groups of patients. In this review we discuss the cellular and molecular programs elicited during LTB₄ production and actions on innate immunity host defense mechanisms as well as potential therapeutic strategies to improve host defense.

Label-Free, LC-MS-Based Assays to Quantitate Small-Molecule Antagonist Binding to the Mammalian BLT1 Receptor

We have developed and validated label-free, liquid chromatography-mass spectrometry (LC-MS)-based equilibrium direct and competition binding assays to quantitate small-molecule antagonist binding to recombinant human and mouse BLT1 receptors expressed in HEK 293 cell membranes. Procedurally, these binding assays involve (1) equilibration of the BLT1 receptor and probe ligand, with or without a competitor; (2) vacuum filtration through cationic glass fiber filters to separate receptor-bound from free probe ligand; and (3) LC-MS analysis in selected reaction monitoring mode for bound probe ligand quantitation. Two novel, optimized probe ligands, compounds 1 and 2, were identified by screening 20 unlabeled BLT1 antagonists for direct binding. Saturation direct binding studies confirmed the high affinity, and dissociation studies established the rapid binding kinetics of probe ligands 1 and 2. Competition binding assays were established using both probe ligands, and the affinities of structurally diverse BLT1 antagonists were measured. Both binding assay formats can be executed with high specificity and sensitivity and moderate throughput (96-well plate format) using these approaches. This highly versatile, label-free method for studying ligand binding to membrane-associated receptors should find broad application as an alternative to traditional methods using labeled ligands.

Asthma and the Risk of Rheumatoid Arthritis: An Insight into the Heterogeneity and Phenotypes of Asthma

Asthma is traditionally regarded as a chronic airway disease, and recent literature proves its heterogeneity, based on distinctive clusters or phenotypes of asthma. In defining such asthma clusters, the nature of comorbidity among patients with asthma is poorly understood, by assuming no causal relationship between asthma and other comorbid conditions, including both communicable and noncommunicable diseases. However, emerging evidence suggests that the status of asthma significantly affects the increased susceptibility of the patient to both communicable and noncommunicable diseases. Specifically, the impact of asthma on susceptibility to noncommunicable diseases such as chronic systemic inflammatory diseases (e.g., rheumatoid arthritis), may provide an important insight into asthma as a disease with systemic inflammatory features, a conceptual understanding between asthma and asthma-related comorbidity, and the potential implications on the therapeutic and preventive interventions for patients with asthma. This review discusses the currently under-recognized clinical and immunological phenotypes of asthma; specifically, a higher risk of developing a systemic inflammatory disease such as rheumatoid arthritis and their implications, on the conceptual understanding and management of asthma. Our discussion is divided into three parts: literature summary on the relationship between asthma and the risk of rheumatoid arthritis; potential mechanisms underlying the association; and implications on asthma management and research.

Esculetin reduces leukotriene B4 level in plasma of rats with adjuvant-induced arthritis

Objectives

Esculetin (6,7-dihydroxycoumarin) is a natural coumarin with anti-oxidant, anti-inflammatory and anti-nociceptive activity. It acts as a potent inhibitor of lipoxygenases (5-LOX and 12-LOX) and decreases the production of matrix metalloproteinases (MMP-1, MMP-3 and MMP-9). Because both inhibition of lipoxygenases and inhibition of matrix metalloproteinases are effective strategies in the treatment of rheumatoid arthritis, we investigated whether esculetin may be effective in adjuvant-induced arthritis in rats.

Material and methods

The study was performed on male Lewis rats, in the adjuvant-induced arthritis model. Rats were divided into two groups: control (treated with 1% methylcellulose) and experimental (treated with esculetin – 10 mg/kg ip.). The tested compound was administered for 5 consecutive days starting on the 21^st day after induction of arthritis. Each group consisted of 7 animals. After 5 days of treatment, rats were anesthetized. The concentration of leukotriene B4 (LTB4) in plasma was determined by a competitive enzyme immunoassay.

Results

The LTB4 level in plasma of rats with adjuvant-induced arthritis is increased in comparison to rats without inflammation (362 ±34 vs. 274 ±15 pg/ml, p < 0.01, respectively). Five-day treatment with esculetin in adjuvant-induced arthritis rats decreases the LTB4 level to a level comparable with rats without inflammation (284 ±23 pg/ml, p < 0.01).

Conclusions

LTB4 is the most potent chemotactic agent influencing neutrophil migration into the joint. It is known that its level in serum of patients with active rheumatoid arthritis is increased and correlates with disease severity. Some other lipoxygenase inhibitors have already been tested as potential drug candidates in clinical and preclinical trials for rheumatoid arthritis (Zileuton, PF-4191834). Because esculetin decreases the LTB4 level in plasma of rats in adjuvant-induced arthritis, it may also be considered as an attractive drug candidate for patients with rheumatoid arthritis.

Human articular chondrocytes express functional leukotriene B4 receptors

Leukotriene B4 (LTB4) is a potent chemoattractant associated with the development of osteoarthritis (OA), while its receptors BLT1 and BLT2 have been found in synovium and subchondral bone. In this study, we have investigated whether these receptors are also expressed by human cartilage cells and their potential effects on cartilage cells. The expression of LTB4 receptors in native tissue and cultured cells was assessed by immunohistochemistry, immunocytochemistry, polymerase chain reaction (PCR) and electron microscopy. The functional significance of the LTB4 receptor expression was studied by Western blotting, using phospho-specific antibodies in the presence or absence of receptor antagonists. In further studies, the secretion of pro-inflammatory cytokines, growth factors and metalloproteinases by LTB4-stimulated chondrocytes was measured by multiplex protein assays. The effects of LTB4 in cartilage signature gene expression in cultured cells were assessed by quantitative PCR, whereas the LTB4-promoted matrix synthesis was determined using 3D pellet cultures. Both receptors were present in cultured chondrocytes, as was confirmed by immunolabelling and PCR. The relative quantification by PCR demonstrated a higher expression of the receptors in cells from healthy joints compared with OA cases. The stimulation of cultured chondrocytes with LTB4 resulted in a phosphorylation of downstream transcription factor Erk 1/2, which was reduced after blocking BLT1 signalling. No alteration in the secretion of cytokine and metalloproteinases was recorded after challenging cultured cells with LTB4; likewise, cartilage matrix gene expression and 3D tissue synthesis were unaffected. Chondrocytes express BLT1 and BLT2 receptors, and LTB4 activates the downstream Erk 1/2 pathway by engaging the high-affinity receptor BLT1. However, any putative role in cartilage biology could not be revealed, and remains to be clarified.

5-Lipoxygenase inhibitors attenuate TNF-α-induced inflammation in human synovial fibroblasts

The lipoxygenase isoform of 5-lipoxygenase (5-LOX) is reported to be overexpressed in human rheumatoid arthritis synovial tissue and involved in the progress of inflammatory arthritis. However, the detailed mechanism of how 5-lipoxygenase regulates the inflammatory response in arthritis synovial tissue is still unclear. The aim of this study was to investigate the involvement of lipoxygenase pathways in TNF-α-induced production of cytokines and chemokines. Human synovial fibroblasts from rheumatoid patients were used in this study. 5-LOX inhibitors and shRNA were used to examine the involvement of 5-LOX in TNF-α-induced cytokines and chemokines expression. The signaling pathways were examined by Western Blotting or immunofluorescence staining. The effect of 5-LOX inhibitor on TNF-α-induced chemokine expression and paw edema was also explored invivo in C57BL/6 mice. Treatment with 5-LOX inhibitors significantly decreased TNF-α-induced pro-inflammatory mediators including interleukin-6 (IL-6) and monocyte chemo-attractant protein-1 (MCP-1) in human synovial fibroblasts. Knockdown of 5-LOX using shRNA exerted similar inhibitory effects. The abrogation of NF-κB activation was involved in the antagonizing effects of these inhibitors. Furthermore, 5-LOX inhibitor decreased TNF-α-induced up-regulation of serum MCP-1 level and paw edema in mouse model. Our results provide the evidence that the administration of 5-LOX inhibitors is able to ameliorate TNF-α-induced cytokine/chemokine release and paw edema, indicating that 5-LOX inhibitors may be developed for therapeutic treatment of inflammatory arthritis.

A randomized double blind, placebo controlled phase 2 trial of BIIL 284 BS (an LTB4 receptor antagonist) for the treatment of lung disease in children and adults with cystic fibrosis

BACKGROUND

Airway inflammation, mediated in part by LTB4, contributes to lung destruction in patients with cystic fibrosis (CF). LTB(4)-receptor inhibition may reduce airway inflammation. We report the results of a randomized, double-blind, placebo-controlled study of the efficacy and safety of the leukotriene B(4) (LTB(4))-receptor antagonist BIIL 284 BS in CF patients.

METHODS

CF patients aged ≥6 years with mild to moderate lung disease were randomized to oral BIIL 284 BS or placebo once daily for 24 weeks. Co-primary endpoints were change in FEV(1) and incidence of pulmonary exacerbation.

RESULTS

After 420 (155 children, 265 adults) of the planned 600 patients were randomized, the trial was terminated after a planned interim analysis revealed a significant increase in pulmonary related serious adverse events (SAEs) in adults receiving BIIL 284 BS. Final analysis revealed SAEs in 36.1% of adults receiving BIIL 284 BS vs. 21.2% receiving placebo (p = 0.007), and in 29.6% of children receiving BIIL 284 BS vs. 22.9% receiving placebo (p = 0.348). In adults, the incidence of protocol-defined pulmonary exacerbation was greater in those receiving BIIL 284 BS than in those receiving placebo (33.1% vs. 18.2% respectively; p = 0.005). In children, the incidence of protocol-defined pulmonary exacerbation was 19.8% in the BIIL 284 BS arm, and 25.7% in the placebo arm (p = 0.38).

CONCLUSIONS

While the cause of increased SAEs and exacerbations due to BIIL 284 BS is unknown, the outcome of this trial provides a cautionary tale for the administration of potent anti-inflammatory compounds to individuals with chronic infections, as the potential to significantly suppress the inflammatory response may increase the risk of infection-related adverse events.

Targeting leukotriene B4 in inflammation

INTRODUCTION

Leukotriene (LT) B(4) is a powerful proinflammatory lipid mediator and triggers adherence to the endothelium, activates and recruits leukocytes to the site of injury. When formed in excess, LTB(4) plays a pathogenic role and may sustain chronic inflammation in diseases such as asthma, rheumatoid arthritis, and inflammatory bowel disease. Recent investigations have also indicated that LTB(4) is involved in cardiovascular diseases.

AREAS COVERED

As the 5-lipoxygenase pathway involves several discrete, tightly coupled, enzymes, which convert the substrate, 'step by step', into bioactive products, several different strategies have been used to target LTB(4) as a means to treat inflammation. Here, we discuss recent findings regarding the development of selective enzyme inhibitors and antagonists for LTB(4) receptors, as well as their application in preclinical and clinical studies.

EXPERT OPINION

Components of the 5-lipoxygenase pathway have received considerable attention as candidate drug targets resulting in one new class of medications against asthma, that is, the antileukotrienes. However, efforts to specifically target LTB(4) have not yet been fruitful in the clinical setting, in spite of very promising preclinical data. Recently, crystal structures along with hitherto unknown functions of key enzymes in the leukotriene cascade have emerged, offering new opportunities for drug development and, with time, pharmacological intervention in LTB(4)-mediated pathologies.

The role of leukotrienes in immunopathogenesis of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disorder of joints for which there is no strict cure. However, conventional medications can reduce inflammation, relieve pain, and slow joint damage. Leukotrienes are a family of paracrine agents derived from oxidative metabolism of arachidonic acid. Synthesis of lipid mediators and subsequent induction of receptor activity are tightly regulated under normal physiological conditions, so that enzyme and/or receptor dysfunction can lead to a variety of clinical signs and symptoms of disease, such as local pain and tissue edema. In these tissues, immunocompetent cells accumulate at the site of injury, contributing to tissue damage and perpetuation of the disease process. Leukotrienes (often leukotriene B4) as potent chemotactic agents can provoke most signs and symptoms in rheumatoid arthritis by initiating, coordinating, sustaining, and amplifying the inflammatory response, through recruitment of leukocytes. A number of studies have reported that pharmacological modulation in this field can significantly attenuate clinical manifestations associated with different inflammatory pathologies.

Relationship between salivary leukotriene B4 levels and salivary mucin or alveolar bone resorption, in subjects with periodontal health and disease

OBJECTIVE

Here we determine the salivary levels of leukotriene B4 (LTB4 ) and its relation with salivary mucin and alveolar bone level.

BACKGROUND

LTB4 is a membrane-derived lipid mediator formed from arachidonic acid. It is among the most potent stimulants of polymorphonuclear leukocytes providing the first host defense against infections. Leukotrienes also induce bone resorption. Because LTB4 is present in the oral cavity the aim of the present study was to explore the role of LTB4 in patients with periodontal disease.

METHODS

Eighty-one subjects were clinically examined and distributed into four groups, namely, clinically healthy, mild, moderate and severe periodontitis, according to periodontal status, classified into values of clinical attachment level and probing pocket depth. Unstimulated saliva was collected for 5 min. Salivary LTB4 was determined by an immune assay method, mucin was determined by a colorimetric method and radiographic assessment used to determine alveolar bone level.

RESULTS

Patients with mild periodontitis showed a decrease in salivary LTB4 levels while patients with severe periodontitis showed increased LTB4 levels. A significant positive correlation was observed between salivary LTB4 and clinical attachment level, salivary mucin concentration or alveolar bone level.

CONCLUSION

The close relation between salivary LTB4 and mucin levels suggested that LTB4 might be involved in the defense mechanism of the oral cavity. The correlation of LTB4 with the alveolar bone level indicates that they are one of the mediators responsible for bone resorption.

Leukotrienes are upregulated and associated with human T-lymphotropic virus type 1 (HTLV-1)-associated neuroinflammatory disease

Leukotrienes (LTs) are lipid mediators involved in several inflammatory disorders. We investigated the LT pathway in human T-lymphotropic virus type 1 (HTLV-1) infection by evaluating LT levels in HTLV-1-infected patients classified according to the clinical status as asymptomatic carriers (HACs) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) patients. Bioactive LTB₄ and CysLTs were both increased in the plasma and in the supernatant of peripheral blood mononuclear cell cultures of HTLV-1-infected when compared to non-infected. Interestingly, CysLT concentrations were increased in HAM/TSP patients. Also, the concentration of plasma LTB₄ and LTC₄ positively correlated with the HTLV-1 proviral load in HTLV-1-infected individuals. The gene expression levels of LT receptors were differentially modulated in CD4⁺ and CD8⁺ T cells of HTLV-1-infected patients. Analysis of the overall plasma signature of immune mediators demonstrated that LT and chemokine amounts were elevated during HTLV-1 infection. Importantly, in addition to CysLTs, IP-10 was also identified as a biomarker for HAM/TSP activity. These data suggest that LTs are likely to be associated with HTLV-1 infection and HAM/TSP development, suggesting their putative use for clinical monitoring.

Neutrophils orchestrate their own recruitment in murine arthritis through C5aR and FcγR signaling

Neutrophil recruitment into the joint is a hallmark of inflammatory arthritides, including rheumatoid arthritis (RA). In a mouse model of autoantibody-induced inflammatory arthritis, neutrophils infiltrate the joint via multiple chemoattractant receptors, including the leukotriene B(4) (LTB(4)) receptor BLT1 and the chemokine receptors CCR1 and CXCR2. Once in the joint, neutrophils perpetuate their own recruitment by releasing LTB(4) and IL-1β, presumably after activation by immune complexes deposited on joint structures. Two pathways by which immune complexes may activate neutrophils include complement fixation, resulting in the generation of C5a, and direct engagement of Fcγ receptors (FcγRs). Previous investigations showed that this model of autoantibody-induced arthritis requires the C5a receptor C5aR and FcγRs, but the simultaneous necessity for both pathways was not understood. Here we show that C5aR and FcγRs work in sequence to initiate and sustain neutrophil recruitment in vivo. Specifically, C5aR activation of neutrophils is required for LTB(4) release and early neutrophil recruitment into the joint, whereas FcγR engagement upon neutrophils induces IL-1β release and subsequent neutrophil-active chemokine production, ensuring continued inflammation. These findings support the concept that immune complex-mediated leukocyte activation is not composed of overlapping and redundant pathways, but that each element serves a distinct and critical function in vivo, culminating in tissue inflammation.

Leukotriene B4 augments and restores Fc gammaRs-dependent phagocytosis in macrophages

Phagocytosis by macrophages is essential for host defense, i.e. preventing invasion of pathogens and foreign materials. Macrophages engulf immunoglobulin G (IgG)-opsonized particles through the action of the receptors for the Fc of IgG (FcγRs). Leukotriene B(4) (LTB(4)) is a classical lipid chemoattractant derived from arachidonic acid. Leukotriene B(4) receptor 1 (BLT1), a high affinity LTB(4) receptor, is expressed in a variety of immune cells such as neutrophils, macrophages, and dendritic cells. Although LTB(4) has been shown to enhance macrophage phagocytosis, few studies have investigated the intracellular mechanisms involved in this in detail. Furthermore, there have been no reports of the direct cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling. Here, we show that FcγRs-dependent phagocytosis was attenuated in BLT1-deficient macrophages as compared with wild-type (WT) cells. Moreover, cross-talk between LTB(4)-BLT1 and IgG-FcγRs signaling was identified at the level of phosphatidylinositol 3-OH kinase (PI3K) and Rac, downstream of Syk. In addition, the trimeric G(i) protein (G(i)) was found to be essential for BLT1-dependent phagocytosis. Surprisingly, we found that LTB(4)-BLT1 signaling restores phagocytosis in the absence of FcγRs signaling. These data indicate that LTB(4)-BLT1 signaling plays a pivotal role in macrophage phagocytosis and innate immunity.

LTB4 can stimulate human osteoclast differentiation dependent of RANKL

Our previous study showed that Leukotriene B4 can directly stimulate osteoclast differentiation independent of RANKL. In order to determine whether Leukotriene B4 could indirectly stimulate human osteoclast differentiation through increasing RANKL expression of rheumatoid arthritis fibroblast-like synoviocytes, we utilize the coculture model of rheumatoid arthritis fibroblast-like synoviocytes and monocyte, which were stimulated in the presence of 2.5 ng/ml M-CSF in the control group, 2.5 ng/ml M-CSF+10(-8)M LTB4 in the experimental group a, and 2.5 ng/ml M-CSF+10(-8)M LTB4+100 ng/ml OPG in the experimental group b. After culture for 3 weeks, the number of multinucleated TRAP staining positive osteoclast-like cells stained with TRAP was counted to evaluate the differentiation effect in each group. There was almost no osteoclast-like cell in the control group and the experimental group b. There were many osteoclast-like cells in the experimental group a. These results indicated that Leukotriene B4 is capable of inducing osteoclast differentiation by a RANKL-dependent mechanism.

5-lipoxygenase is a key determinant of acute myocardial inflammation and mortality during Trypanosoma cruzi infection

This study provides evidence supporting the idea that although inflammatory cells migration to the cardiac tissue is necessary to control the growth of Trypanosoma cruzi, the excessive influx of such cells during acute myocarditis may be deleterious to the host. Production of lipid mediators of inflammation like leukotrienes (LTs) along with cytokines and chemokines largely influences the severity of inflammatory injury in response to tissue parasitism. T. cruzi infection in mice deficient in 5-lipoxygenase (5-LO), the enzyme responsible for the synthesis of LTs and other lipid inflammatory mediators, resulted in transiently increased parasitemia, and improved survival rate compared with WT mice. Myocardia from 5-LO(-/-) mice exhibited reduced inflammation, collagen deposition, and migration of CD4(+), CD8(+), and IFN-gamma-producer cells compared with WT littermates. Moreover, decreased amounts of TNF-alpha, IFN-gamma, and nitric oxide synthase were found in the hearts of 5-LO(-/-) mice. Interestingly, despite of early higher parasitic load, 5-LO(-/-) mice survived, and controlled T. cruzi infection. These results show that efficient parasite clearance is possible in a context of moderate inflammatory response, as occurred in 5-LO(-/-) mice, in which reduced myocarditis protects the animals during T. cruzi infection.

A distinctive role of the leukotriene B4 receptor BLT1 in osteoclastic activity during bone loss

Although leukotriene B(4) (LTB(4)) is produced in various inflammatory diseases, its functions in bone metabolism remain unknown. Using mice deficient in the high-affinity LTB(4) receptor BLT1, we evaluated the roles of BLT1 in the development of two bone resorption models, namely bone loss induced by ovariectomy and lipopolysaccharide. Through observations of bone mineral contents and bone morphometric parameters, we found that bone resorption in both models was significantly attenuated in BLT1-deficient mice. Furthermore, osteoclasts from BLT1-deficient mice showed reduced calcium resorption activities compared with wild-type osteoclasts. Osteoclasts expressed BLT1, but not the low-affinity LTB(4) receptor BLT2, and produced LTB(4). LTB(4) changed the cell morphology of osteoclasts through the BLT1-Gi protein-Rac1 signaling pathway. Given the causal relationship between osteoclast morphology and osteoclastic activity, these findings suggest that autocrine/paracrine LTB(4) increases the osteoclastic activity through the BLT1-Gi protein-Rac1 signaling pathway. Inhibition of BLT1 functions may represent a strategy for preventing bone resorption diseases.

LTB4 Can Directly Stimulate Human Osteoclast Formation from PBMC Independent of RANKL

Leukotriene B4, as a kind of 5-lipoxygenase metabolite of arachidonic acid, is known to influence osteoclast formation and bone resorption. In order to determine whether Leukotriene B4 could directly stimulate human osteoclast differentiation and activation independent of RANKL (ODF), three different concentrations of Leukotriene B4 (10(-9)M, 10(-8)M, 10(-7)M) were added to the culture of CD14+ monocyte fraction of peripheral blood mononuclear cell (PBMC) in the presence of macrophage colony-stimulating factor (M-CSF). Under these conditions, Leukotriene B4 could induce multinucleated cells, which were positive for Tartrate-resistant acidic phosphatase (TRAP) staining and capable of bone resorption. Addition of osteoprotegerin (OPG) to PBMC cultures does not abrogate osteoclast formation induced by LTB4. Osteoclastogenesis induced by Leukotriene B4 were dose-dependently increased and weaker than that of RANKL. These results indicated that Leukotriene B4, elevated in many inflammatory diseases, is directly capable of inducing osteoclast formation by a RANKL-independent mechanism.

Expression of 5-lipoxygenase and 15-lipoxygenase in rheumatoid arthritis synovium and effects of intraarticular glucocorticoids

Introduction

It was previously shown that lipoxygenase (LO) pathways are important in the rheumatoid arthritis (RA) inflammatory process and that synovial fluid from RA patients contains high amounts of leukotrienes. We therefore aimed to investigate the 5-LO and 15-LO-1 expression pattern in RA and ostheoarthritis (OA) synovial tissue and to study the effect of intraarticular glucocorticoid (GC) therapy on enzyme expression.

Methods

Expression of LOs was evaluated by immunohistochemistry in RA and OA synovial biopsies. Cellular localization of these enzymes was analyzed by double immunofluorescence. In synovial biopsies from 11 RA patients, 5-LO and 15-LO-1 expression was evaluated before and after triamcinolone hexacetonide knee injection and assessed by image analysis to quantify their expression. We also investigated the presence of 15-LO-1 by immunohistochemistry in synovial fluid (SF) cells as well as their ability to form 15-hydroxyeicosatetraenoic acid (15-HETE) following treatment with arachidonic acid (AA).

Results

5-LO and 15-LO-1 are present in RA and OA synovium, with 5-LO being mostly expressed in lining and sublining macrophages, neutrophils and mast cells and 15-LO-1 mainly in lining macrophages, fibroblasts and sublining endothelial cells. Intraarticular GC treatment resulted in a significant suppression of 5-LO expression, but did not influence the 15-LO-1 enzyme significantly. Also, SF cells express a functional 15-LO-1 and produce 15-HETE when challenged with AA.

Conclusions

These data demonstrate that local therapy with GC decreases 5-LO expression in RA synovium and offer an additional possible mechanism for the efficiency of intraarticular adjuvant therapy in RA.

Involvement of LTB4 in zymosan-induced joint nociception in mice: participation of neutrophils and PGE2

Leukotriene B4 (LTB4) mediates different inflammatory events such as neutrophil migration and pain. The present study addressed the mechanisms of LTB4-mediated joint inflammation-induced hypernociception. It was observed that zymosan-induced articular hypernociception and neutrophil migration were reduced dose-dependently by the pretreatment with MK886 (1-9 mg/kg; LT synthesis inhibitor) as well as in 5-lypoxygenase-deficient mice (5LO(-/-)) or by the selective antagonist of the LTB(4) receptor (CP105696; 3 mg/kg). Histological analysis showed reduced zymosan-induced articular inflammatory damage in 5LO(-/-) mice. The hypernociceptive role of LTB4 was confirmed further by the demonstration that joint injection of LTB4 induces a dose (8.3, 25, and 75 ng)-dependent articular hypernociception. Furthermore, zymosan induced an increase in joint LTB4 production. Investigating the mechanism underlying LTB4 mediation of zymosan-induced hypernociception, LTB4-induced hypernociception was reduced by indomethacin (5 mg/kg), MK886 (3 mg/kg), celecoxib (10 mg/kg), antineutrophil antibody (100 mug, two doses), and fucoidan (20 mg/kg) treatments as well as in 5LO(-/-) mice. The production of LTB4 induced by zymosan in the joint was reduced by the pretreatment with fucoidan or antineutrophil antibody as well as the production of PGE2 induced by LTB4. Therefore, besides reinforcing the role of endogenous LTB4 as an important mediator of inflamed joint hypernociception, these results also suggested that the mechanism of LTB4-induced articular hypernociception depends on prostanoid and neutrophil recruitment. Furthermore, the results also demonstrated clearly that LTB4-induced hypernociception depends on the additional release of endogenous LTs. Concluding, targeting LTB4 synthesis/action might constitute useful therapeutic approaches to inhibit articular inflammatory hypernociception.

Preparation of 3-(4-chlorophenyl)-2-(2-aminothiazol-4-yl)-5-methoxybenzo[b]furan derivatives and their leukotriene B4 inhibitory activity

A series of 3-(4-chlorophenyl)-2-(2-aminothiazol-4-yl)benzo[b]furan derivatives 6-10 were prepared and their leukotriene B(4) inhibitory activity was evaluated. We found that several compounds showed strong inhibition of calcium mobilization in CHO cells overexpressing human BLT(1) and BLT(2) receptors. Among them, 3-(4-chlorophenyl)-2-[5-formyl-2-[(dimethylamino)methyleneamino]thiazol-4-yl]-5-methoxybenzo[b]furan 9b showed the most potent and selective inhibition for the human BLT(2) receptor, and its IC(50) value was smaller than that of the selected positive control compound, ZK-158252.

Targeted Disruption of Leukotriene B4 Receptors BLT1 and BLT2: A Critical Role for BLT1 in Collagen-Induced Arthritis in Mice

Leukotriene B(4) mediates diverse inflammatory diseases through the G protein-coupled receptors BLT1 and BLT2. In this study, we developed mice deficient in BLT1 and BLT2 by simultaneous targeted disruption of these genes. The BLT1/BLT2 double-deficient mice developed normally and peritoneal exudate cells showed no detectable responses to leukotriene B(4) confirming the deletion of the BLT1/BLT2 locus. In a model of collagen-induced arthritis on the C57BL/6 background, the BLT1/BLT2(-/-) as well as the previously described BLT1(-/-) animals showed complete protection from disease development. The disease severity correlated well with histopathology, including loss of joint architecture, inflammatory cell infiltration, fibrosis, pannus formation, and bone erosion in joints of BLT1/BLT2(+/+) animals and a total absence of disease pathology in leukotriene receptor-deficient mice. Despite these differences, all immunized BLT1(-/-) and BLT1/BLT2(-/-) animals had similar serum levels of anti-collagen Abs relative to BLT1/BLT2(+/+) animals. Thus, BLT1 may be a useful target for therapies directed at treating inflammation associated with arthritis.

A unique requirement for the leukotriene B4 receptor BLT1 for neutrophil recruitment in inflammatory arthritis

Neutrophil recruitment into tissue plays an important role in host defense and disease pathogenesis, including the inflammatory arthritides. A multitude of diverse chemoattractants have been implicated in neutrophil recruitment, suggesting that they have overlapping functions in mediating this critical biological response. However, here we demonstrate a unique, non-redundant role for the leukotriene B₄ receptor BLT1 in mediating neutrophil recruitment into the joint in the K/BxN mouse model of inflammatory arthritis. We demonstrate that neutrophil expression of BLT1 was absolutely required for arthritis generation and chemokine production in this model, and that specific BLT1 inhibition reversed established disease. Adoptive transfer of wild-type (WT) neutrophils restored arthritis and chemokine production in BLT1^−/− mice. Surprisingly, the primary effect of the transferred WT neutrophils into BLT1^−/− mice was to promote the entry of endogenous BLT1^−/− neutrophils into the joints of these mice. However, continued joint inflammation was dependent on the presence of WT neutrophils, indicating an ongoing specific requirement for BLT1-activated neutrophils in mediating BLT1^−/− neutrophil recruitment by other chemoattractants. These experiments demonstrate that neutrophil BLT1 functions in a novel and essential non–cell-autonomous manner to enable the recruitment of additional neutrophils not expressing this receptor, thereby amplifying the inflammatory response in autoantibody-induced arthritis.

Pro- and anti-inflammatory substances modulate expression of the leukotriene B4receptor, BLT1, in human monocytes

The high-affinity leukotriene B(4) (LTB(4)) receptor, BLT1, is a chemotactic receptor involved in inflammatory responses. In this study, we have explored the regulation of BLT1 expression in human monocytes by pro- and anti-inflammatory cytokines, lipopolysaccharide (LPS), and dexamethasone. We found that proinflammatory mediators, such as interferon-gamma (IFN-gamma), tumor necrosis factor-alpha, and LPS, down-regulated expression, whereas the anti-inflammatory cytokine, interleukin-10, and dexamethasone up-regulated BLT1 mRNA expression. The effect of IFN-gamma on BLT1 mRNA expression was rapidly detectable (<4 h) and concentration-dependent (1-50 ng/ml) and seems to be exerted through a block in transcriptional activity. Alterations in mRNA expression were accompanied by changes in BLT1 surface expression, and receptor down-modulation following IFN-gamma stimulation resulted in a diminished chemotactic response to LTB(4). The regulation of BLT1 mRNA and receptor protein expression was similar to the regulation of the monocyte chemoattractant protein-1 chemokine receptor, CC chemokine recptor 2 (CCR2). Flow cytometric analysis of fresh peripheral blood cells revealed that classical (CD14(++)CD16(-)) monocytes express high levels of BLT1 and CCR2 and that both receptors are down-regulated on CD14(+)CD16(+) monocytes. Apart from providing insight into the regulation of BLT1 in human monocytes, our results reveal a parallel expression and regulation of BLT1 and CCR2, which may help to understand monocyte trafficking during pathophysiological conditions.

BLT1 and BLT2: the leukotriene B(4) receptors

Two receptors for leukotriene B(4) (LTB(4)) have been molecularly identified: BLT1 and BLT2. Both receptors are G protein-coupled seven transmembrane domain receptors, whose genes are located in very close proximity to each other in the human and mouse genomes. The two receptors differ in their affinity and specificity for LTB(4): BLT1 is a high-affinity receptor specific for LTB(4), whereas BLT2 is a low-affinity receptor that also binds other eicosanoids. The two receptors also differ in their pattern of expression with BLT1 being expressed primarily in leukocytes, whereas BLT2 is expressed more ubiquitously. By mediating the activities of LTB(4), these receptors participate both in host immune responses and in the pathogenesis of inflammatory diseases. Reduced disease severity in animal inflammatory models seen with LTB(4) receptor antagonists and in mice with targeted deletion of BLT1 have revealed important roles for LTB(4) and its receptors in regulating pathologic inflammation.

The critical role of leukotriene B4 in antigen-induced mechanical hyperalgesia in immunised rats

1. We investigated the mediators responsible for mechanical hypersensitivity induced by antigen challenge in rats immunised with ovalbumin (OVA). 2. Challenge with OVA (12.5-100 micro g, intraplantar) caused a dose- and time-dependent mechanical hypersensitivity, which peaked 3 h after, decreased thereafter and reached control levels 24 h later. 3. Levels of TNFalpha, IL-1beta and cytokine-induced neutrophil chemoattractant 1 (CINC-1) were increased in paw skin after antigen challenge. 4. OVA-evoked hypersensitivity was partially inhibited (about 51%) by pretreatment with anti-TNFalpha, IL-1beta and IL-8 sera or with IL-1 receptor antagonist (IL-1ra), but not anti-NGF serum. Pretreatment with thalidomide (45 mg kg(-1)) or pentoxifylline (100 mg kg(-1)) also partially inhibited the hypersensitivity at 1-3 h after challenge. 5. Pretreatment with indomethacin (5 mg kg(-1)) or atenolol (1 mg kg(-1)) reduced the OVA-induced hypersensitivity at 1 and 3 h, but not at 5 h after challenge, while the combination of B(1) and B(2) bradykinin receptor antagonists was ineffective over the same times. 6. Pretreatment with MK886 (5-lipoxygenase-activating protein inhibitor, 3 mg kg(-1)), CP 105696 (LTB(4) receptor antagonist; 3 mg kg(-1)) or dexamethasone (0.5 mg kg(-1)) inhibited the hypersensitivity from 1 to 5 h. Furthermore, LTB(4) levels were increased in the paw skin of challenged rats. 7. In conclusion, our results suggest that the TNFalpha-, IL-1beta- and CINC-1-driven release of prostaglandins, sympathetic amines and LTB(4) mediates the first 3 h of mechanical hypersensitivity induced by antigen challenge in rats. At 5 h after OVA administration, although TNFalpha has some role, LTB(4) is the critical nociceptive mediator.

Effect on prostaglandin E2 and leukotriene B4 levels by local administration of glucocorticoid in human masseter muscle myalgia

Our aim was to determine whether masseter muscle (M) and plasma (P) levels of prostaglandin E2 (PGE2) or leukotriene B4 (LTB4) are influenced by local glucocorticoid administration and whether such changes would be associated with corresponding changes in local pain or hyperalgesia. Eighteen patients with fibromyalgia and 15 with local masseter myalgia were examined immediately before and 2 weeks after intramuscular administration of glucocorticoid with regard to masseter muscle resting pain and tenderness to palpation, pressure pain threshold, maximum voluntary mouth opening (MVM), and pain on maximum voluntary mouth opening. The primary criteria for inclusion were presence of pain for a period of at least 3 months and tenderness to digital palpation in the masseter muscle region. At both visits microdialysis samples were obtained from the masseter muscle, and venous blood was collected for analysis of PGE2 and LTB4. Dialysate levels of M-PGE2 did not change significantly after glucocorticoid administration, but reduction of masseter resting pain and increase of MVM were associated with decrease of M-PGE2 in the patients with fibromyalgia. Dialysate levels of M-LTB4 increased in both groups. In the patients with local myalgia the plasma level of LTB4 also increased, and this increase was associated with a decrease of pain and masseter tenderness. In conclusion, this study shows that reduction of masseter level of PGE2 after intramuscular glucocorticoid administration is associated with a decrease of resting pain in patients with fibromyalgia. In addition, the masseter muscle level of LTB4 increases in patients with fibromyalgia and local myalgia.