Leukotriene B 4 Strongly Increases Monocyte Chemoattractant Protein-1 in Human Monocytes

Inhibition of host 5-lipoxygenase reduces overexuberant inflammatory responses and mortality associated with Cryptococcus meningoencephalitis

Cryptococcosis, caused by fungi of the genus Cryptococcus, manifests in a broad range of clinical presentations, including severe pneumonia and disease of the central nervous system (CNS) and other tissues (bone and skin). Immune deficiency or development of overexuberant inflammatory responses can result in increased susceptibility or host damage, respectively, during fungal encounters. Leukotrienes help regulate inflammatory responses against fungal infections. Nevertheless, studies showed that Cryptococcus exploits host 5-lipoxygenase (5-LO), an enzyme central to the metabolism of arachidonic acid into leukotrienes, to facilitate transmigration across the brain-blood barrier. To investigate the impact of host 5-LO on the development of protective host immune responses and mortality during cryptococcosis, wild-type (C57BL/6) and 5-lipoxygenase-deficient (5-LO-/-) mice were given experimental pulmonary and systemic Cryptococcus sp., infections. Our results showed that 5-LO-/- mice exhibited reduced pathology and better disease outcomes (i.e., no mortality or signs associated with cryptococcal meningoencephalitis) following pulmonary infection with C. deneoformans, despite having detectable yeast in the brain tissues. In contrast, C57BL/6 mice exhibited classical signs associated with cryptococcal meningoencephalitis. Additionally, brain tissues of 5-LO-/- mice exhibited lower levels of cytokines (CCL2 and CCL3) clinically associated with Cryptococcus-related immune reconstitution inflammatory syndrome (C-IRIS). In a systemic mouse model of cryptococcosis, 5-LO-/- mice and those treated with a Federal Drug Administration (FDA)-approved 5-LO synthesis inhibitor, zileuton, displayed significantly reduced mortality compared to C57BL/6 infected mice. These results suggest that therapeutics designed to inhibit host 5-LO signaling could reduce disease pathology and mortality associated with cryptococcal meningoencephalitis.

IMPORTANCE

Cryptococcosis is a mycosis with worldwide distribution and has a broad range of clinical manifestations, including diseases of the CNS. Globally, there is an estimated 179,000 cases of cryptococcal meningitis, resulting in approximately 112,000 fatalities per annum and 19% of AIDS-related deaths. Understanding how host immune responses are modulated during cryptococcosis is central to mitigating the morbidity and mortality associated with cryptococcosis. Leukotrienes (LTs) have been shown to modulate inflammatory responses during infection. In this study, we show that mice deficient in 5-lipoxygenase (5-LO), an enzyme central to the metabolism of arachidonic acid into leukotrienes, exhibit reduced pathology, disease, and neurological signs associated with cryptococcal meningitis. Additionally, mice given an experimental cryptococcal infection and subsequently treated with an FDA-approved 5-LO synthesis inhibitor exhibited significantly reduced mortality rates. These results suggest that therapeutics designed to inhibit host 5-LO activity could significantly reduce pathology and mortality rates associated with cryptococcal meningitis.

Fine particulate matter exposure and systemic inflammation: A potential mediating role of bioactive lipids

Inflammation is a key mechanism underlying the adverse health effects of exposure to fine particulate matter (PM2.5). Bioactive lipids in the arachidonic acid (ARA) pathway are important in the regulation of inflammation and are reportedly altered by PM2.5 exposure. Ceramide-1-phosphate (C1P), a class of sphingolipids, is required to initiate ARA metabolism. We examined the role of C1P in the alteration of ARA metabolism after PM2.5 exposure and explored whether changes in the ARA pathway promoted systemic inflammation based on a panel study involving 112 older adults in Beijing, China. Ambient PM2.5 levels were continuously monitored at a fixed station from 2013 to 2015. Serum cytokine levels were measured to assess systemic inflammation. Multiple bioactive lipids in the ARA pathway and three subtypes of C1P were quantified in blood samples. Mediation analyses were performed to test the hypotheses. We observed that PM2.5 exposure was positively associated with inflammatory cytokines and the three subtypes of C1P. Mediation analyses showed that C1P significantly mediated the associations of ARA and 5, 6-dihydroxyeicosatrienoic acid (5, 6-DHET), an ARA metabolite, with PM2.5 exposure. ARA, 5, 6-DHET, and leukotriene B4 mediated systemic inflammatory response to PM2.5 exposure. For example, C1P C16:0 (a subtype of C1P) mediated a 12.9 % (95 % confidence interval: 3.7 %, 32.5 %) increase in ARA associated with 3-day moving average PM2.5 exposure, and ARA mediated a 27.1 % (7.8 %, 61.2 %) change in interleukin-8 associated with 7-day moving average PM2.5 exposure. Our study indicates that bioactive lipids in the ARA and sphingolipid metabolic pathways may mediate systemic inflammation after PM2.5 exposure.

Lipid Metabolism Modulation during SARS-CoV-2 Infection: A Spotlight on Extracellular Vesicles and Therapeutic Prospects

Extracellular vesicles (EVs) have a significant impact on the pathophysiological processes associated with various diseases such as tumors, inflammation, and infection. They exhibit molecular, biochemical, and entry control characteristics similar to viral infections. Viruses, on the other hand, depend on host metabolic machineries to fulfill their biosynthetic requirements. Due to potential advantages such as biocompatibility, biodegradation, and efficient immune activation, EVs have emerged as potential therapeutic targets against the SARS-CoV-2 infection. Studies on COVID-19 patients have shown that they frequently have dysregulated lipid profiles, which are associated with an increased risk of severe repercussions. Lipid droplets (LDs) serve as organelles with significant roles in lipid metabolism and energy homeostasis as well as having a wide range of functions in infections. The down-modulation of lipids, such as sphingolipid ceramide and eicosanoids, or of the transcriptional factors involved in lipogenesis seem to inhibit the viral multiplication, suggesting their involvement in the virus replication and pathogenesis as well as highlighting their potential as targets for drug development. Hence, this review focuses on the role of modulation of lipid metabolism and EVs in the mechanism of immune system evasion during SARS-CoV-2 infection and explores the therapeutic potential of EVs as well as application for delivering therapeutic substances to mitigate viral infections.

Metabolic alterations upon SARS-CoV-2 infection and potential therapeutic targets against coronavirus infection

The coronavirus disease 2019 (COVID-19) caused by coronavirus SARS-CoV-2 infection has become a global pandemic due to the high viral transmissibility and pathogenesis, bringing enormous burden to our society. Most patients infected by SARS-CoV-2 are asymptomatic or have mild symptoms. Although only a small proportion of patients progressed to severe COVID-19 with symptoms including acute respiratory distress syndrome (ARDS), disseminated coagulopathy, and cardiovascular disorders, severe COVID-19 is accompanied by high mortality rates with near 7 million deaths. Nowadays, effective therapeutic patterns for severe COVID-19 are still lacking. It has been extensively reported that host metabolism plays essential roles in various physiological processes during virus infection. Many viruses manipulate host metabolism to avoid immunity, facilitate their own replication, or to initiate pathological response. Targeting the interaction between SARS-CoV-2 and host metabolism holds promise for developing therapeutic strategies. In this review, we summarize and discuss recent studies dedicated to uncovering the role of host metabolism during the life cycle of SARS-CoV-2 in aspects of entry, replication, assembly, and pathogenesis with an emphasis on glucose metabolism and lipid metabolism. Microbiota and long COVID-19 are also discussed. Ultimately, we recapitulate metabolism-modulating drugs repurposed for COVID-19 including statins, ASM inhibitors, NSAIDs, Montelukast, omega-3 fatty acids, 2-DG, and metformin.

Iron Dyshomeostasis in COVID-19: Biomarkers Reveal a Functional Link to 5-Lipoxygenase Activation

Coronavirus disease 2019 (COVID-19) is characterized by a broad spectrum of clinical symptoms. After acute infection, some subjects develop a post-COVID-19 syndrome known as long-COVID. This study aims to recognize the molecular and functional mechanisms that occur in COVID-19 and long-COVID patients and identify useful biomarkers for the management of patients with COVID-19 and long-COVID. Here, we profiled the response to COVID-19 by performing a proteomic analysis of lymphocytes isolated from patients. We identified significant changes in proteins involved in iron metabolism using different biochemical analyses, considering ceruloplasmin (Cp), transferrin (Tf), hemopexin (HPX), lipocalin 2 (LCN2), and superoxide dismutase 1 (SOD1). Moreover, our results show an activation of 5-lipoxygenase (5-LOX) in COVID-19 and in long-COVID possibly through an iron-dependent post-translational mechanism. Furthermore, this work defines leukotriene B4 (LTB4) and lipocalin 2 (LCN2) as possible markers of COVID-19 and long-COVID and suggests novel opportunities for prevention and treatment.

Blockade of the BLT1-LTB4 axis does not affect mast cell migration towards advanced atherosclerotic lesions in LDLr-/- mice

Mast cells have been associated with the progression and destabilization of advanced atherosclerotic plaques. Reducing intraplaque mast cell accumulation upon atherosclerosis progression could be a potent therapeutic strategy to limit plaque destabilization. Leukotriene B₄ (LTB₄) has been reported to induce mast cell chemotaxis in vitro. Here, we examined whether antagonism of the LTB₄-receptor BLT1 could inhibit mast cell accumulation in advanced atherosclerosis. Expression of genes involved in LTB₄ biosynthesis was determined by single-cell RNA sequencing of human atherosclerotic plaques. Subsequently, Western-type diet fed LDLr^-/- mice with pre-existing atherosclerosis were treated with the BLT1-antagonist CP105,696 or vehicle control three times per week by oral gavage. In the spleen, a significant reduction in CD11b⁺ myeloid cells was observed, including Ly6C^lo and Ly6C^hi monocytes as well as dendritic cells. However, atherosclerotic plaque size, collagen and macrophage content in the aortic root remained unaltered upon treatment. Finally, BLT1 antagonism did not affect mast cell numbers in the aortic root. Here, we show that human intraplaque leukocytes may be a source of locally produced LTB₄. However, BLT1-antagonism during atherosclerosis progression does not affect either local mast cell accumulation or plaque size, suggesting that other mechanisms participate in mast cell accumulation during atherosclerosis progression.

Role of leukotriene B4 (LTB4)-LTB4 receptor 1 signaling in post-incisional nociceptive sensitization and local inflammation in mice

Leukotriene B4 (LTB4) is a potent lipid mediator involved in the recruitment and activation of neutrophils, which is an important feature of tissue injury and inflammation. The biological effects of LTB4 are primarily mediated through the high-affinity LTB4 receptor, BLT1. Postoperative incisional pain is characterized by persistent acute pain at the site of tissue injury and is associated with local inflammation. Here, we compared the role of LTB4-BLT1 signaling in postoperative incisional pain between BLT1-knockout (BLT1KO) and wild-type (BLT1WT) mice. A planter incision model was developed, and mechanical pain hypersensitivity was determined using the von Frey test before and after incision. Local infiltration of neutrophils and inflammatory monocytes was quantified by flow cytometry. Inflammatory cytokine levels in the incised tissue were also determined. Mechanical pain hypersensitivity was significantly reduced in BLT1KO mice compared to BLT1WT mice at 2, 3, and 4 days after incision. LTB4 levels in the tissue at the incision site peaked 3 hours after the incision. Infiltrated neutrophils peaked 1 day after the incision in both BLT1KO and BLT1WT mice. The accumulation of inflammatory monocytes increased 1-3 days after the incision and was significantly more reduced in BLT1KO mice than in BLT1WT mice. In BLT1KO mice, Interleukin-1β and Tumor Necrosis Factor-α levels 1 day after the incision were significantly lower than those of BLT1WT mice. Our data suggest that LTB4 is produced and activates its receptor BLT1 in the very early phase of tissue injury, and that LTB4-BLT1 signaling exacerbates pain responses by promoting local infiltration of inflammatory monocytes and cytokine production. Thus, LTB4-BLT1 signaling is a potential target for therapeutic intervention of acute and persistent pain induced by tissue injury.

Eicosanoids in inflammation in the blood and the vessel

Polyunsaturated fatty acids (PUFAs) are structural components of membrane phospholipids in cells. PUFAs regulate cellular function through the formation of derived lipid mediators termed eicosanoids. The oxygenation of 20-carbon PUFAs via the oxygenases cyclooxygenases, lipoxygenases, or cytochrome P450, generates a class of classical eicosanoids including prostaglandins, thromboxanes and leukotrienes, and also the more recently identified hydroxy-, hydroperoxy-, epoxy- and oxo-eicosanoids, and the specialized pro-resolving (lipid) mediators. These eicosanoids play a critical role in the regulation of inflammation in the blood and the vessel. While arachidonic acid-derived eicosanoids are extensively studied due to their pro-inflammatory effects and therefore involvement in the pathogenesis of inflammatory diseases such as atherosclerosis, diabetes mellitus, hypertension, and the coronavirus disease 2019; in recent years, several eicosanoids have been reported to attenuate exacerbated inflammatory responses and participate in the resolution of inflammation. This review focused on elucidating the biosynthesis and the mechanistic signaling of eicosanoids in inflammation, as well as the pro-inflammatory and anti-inflammatory effects of these eicosanoids in the blood and the vascular wall.

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.

Toxoplasma gondii Infection Decreases Intestinal 5-Lipoxygenase Expression, while Exogenous LTB4 Controls Parasite Growth

5-Lipoxygenase (5-LO) is an enzyme required for the production of leukotrienes and lipoxins and interferes with parasitic infections. In vitro, Toxoplasma gondii inhibits leukotriene B₄ (LTB₄) production, and mice deficient in 5-LO are highly susceptible to infection. The aim of this study was to investigate the effects of the pharmacological inhibition of the 5-LO pathway and exogenous LTB₄ supplementation during experimental toxoplasmosis. For this purpose, susceptible C57BL/6 mice were orally infected with T. gondii and treated with LTB₄ or MK886 (a selective leukotriene inhibitor through inhibition of 5-LO-activating protein [FLAP]). The parasitism, histology, and immunological parameters were analyzed. The infection decreased 5-LO expression in the small intestine, and treatment with MK886 reinforced this reduction during infection; in addition, MK886-treated infected mice presented higher intestinal parasitism, which was associated with lower local interleukin-6 (IL-6), interferon gamma (IFN-γ), and tumor necrosis factor (TNF) production. In contrast, treatment with LTB₄ controlled parasite replication in the small intestine, liver, and lung and decreased pulmonary pathology. Interestingly, treatment with LTB₄ also preserved the number of Paneth cells and increased α-defensins expression and IgA levels in the small intestine of infected mice. Altogether, these data demonstrated that T. gondii infection is associated with a decrease in 5-LO expression, and on the other hand, treatment with the 5-LO pathway product LTB₄ resulted in better control of parasite growth in the organs, adding to the knowledge about the pathogenesis of T. gondii infection.

Mild COVID-19 imprints a long-term inflammatory eicosanoid- and chemokine memory in monocyte-derived macrophages

Monocyte-derived macrophages (MDM) drive the inflammatory response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and they are a major source of eicosanoids in airway inflammation. Here we report that MDM from SARS-CoV-2-infected individuals with mild disease show an inflammatory transcriptional and metabolic imprint that lasts for at least 5 months after SARS-CoV-2 infection. MDM from convalescent SARS-CoV-2-infected individuals showed a downregulation of pro-resolving factors and an increased production of pro-inflammatory eicosanoids, particularly 5-lipoxygenase-derived leukotrienes. Leukotriene synthesis was further enhanced by glucocorticoids and remained elevated at 3–5 months, but had returned to baseline at 12 months post SARS-CoV-2 infection. Stimulation with SARS-CoV-2 spike protein or LPS triggered exaggerated prostanoid-, type I IFN-, and chemokine responses in post COVID-19 MDM. Thus, SARS-CoV-2 infection leaves an inflammatory imprint in the monocyte/ macrophage compartment that drives aberrant macrophage effector functions and eicosanoid metabolism, resulting in long-term immune aberrations in patients recovering from mild COVID-19.

Lipid nutrition: "In silico" studies and undeveloped experiments

This review examines lipids and lipid-binding sites on proteins in relation to cardiovascular disease. Lipid nutrition involves food energy from ingested fatty acids plus fatty acids formed from excess ingested carbohydrate and protein. Non-esterified fatty acids (NEFA) and lipoproteins have many detailed attributes not evident in their names. Recognizing attributes of lipid-protein interactions decreases unexpected outcomes. Details of double bond position and configuration interacting with protein binding sites have unexpected consequences in acyltransferase and cell replication events. Highly unsaturated fatty acids (HUFA) have n-3 and n-6 motifs with documented differences in intensity of destabilizing positive feedback loops amplifying pathophysiology. However, actions of NEFA have been neglected relative to cholesterol, which is co-produced from excess food. Native low-density lipoproteins (LDL) bind to a high-affinity cell surface receptor which poorly recognizes biologically modified LDLs. NEFA increase negative charge of LDL and decrease its processing by "normal" receptors while increasing processing by "scavenger" receptors. A positive feedback loop in the recruitment of monocytes and macrophages amplifies chronic inflammatory pathophysiology. Computer tools combine multiple components in lipid nutrition and predict balance of energy and n-3:n-6 HUFA. The tools help design and execute precise clinical nutrition monitoring that either supports or disproves expectations.

5-LO-derived LTB4 plays a key role in MCP-1 expression in HMGB1-exposed VSMCs via a BLTR1 signaling axis

Monocyte chemoattractant protein-1 (MCP-1) plays an important role in initiating vascular inflammation; however, its cellular source in the injured vasculatures is unclear. Given the importance of high mobility group box 1 (HMGB1) in tissue injury, we investigated the role of vascular smooth muscle cells (VSMCs) in MCP-1 production in response to HMGB1. In primary cultured rat aortic VSMCs stimulated with HMGB1, the expression of MCP-1 and 5-lipoxygenase (LO) was increased. The increased MCP-1 expression in HMGB1 (30 ng/ml)-stimulated cells was significantly attenuated in 5-LO-deficient cells as well as in cells treated with zileuton, a 5-LO inhibitor. Likewise, MCP-1 expression and production were also increased in cells stimulated with exogenous leukotriene B4 (LTB4), but not exogenous LTC4. LTB4-induced MCP-1 expression was attenuated in cells treated with U75302, a LTB4 receptor 1 (BLTR1) inhibitor as well as in BLTR1-deficient cells, but not in 5-LO-deficient cells. Moreover, HMGB1-induced MCP-1 expression was attenuated in BLTR1-deficient cells or by treatment with a BLTR1 inhibitor, but not other leukotriene receptor inhibitors. In contrast to MCP-1 expression in response to LTB4, the increased MCP-1 production in HMGB1-stimulated VSMC was markedly attenuated in 5-LO-deficient cells, indicating a pivotal role of LTB4-BLTR1 signaling in MCP-1 expression in VSMCs. Taken together, 5-LO-derived LTB4 plays a key role in MCP-1 expression in HMGB1-exposed VSMCs via BLTR1 signaling, suggesting the LTB4-BLTR1 signaling axis as a potential therapeutic target for vascular inflammation in the injured vasculatures.

Leukotriene B4 licenses inflammasome activation to enhance skin host defense

The initial production of inflammatory mediators dictates host defense as well as tissue injury. Inflammasome activation is a constituent of the inflammatory response by recognizing pathogen and host-derived products and eliciting the production of IL-1β and IL-18 in addition to inducing a type of inflammatory cell death termed "pyroptosis." Leukotriene B4 (LTB4) is a lipid mediator produced quickly (seconds to minutes) by phagocytes and induces chemotaxis, increases cytokine/chemokine production, and enhances antimicrobial effector functions. Whether LTB4 directly activates the inflammasome remains to be determined. Our data show that endogenously produced LTB4 is required for the expression of pro-IL-1β and enhances inflammasome assembly in vivo and in vitro. Furthermore, LTB4-mediated Bruton's tyrosine kinase (BTK) activation is required for inflammasome assembly in vivo as well for IL-1β-enhanced skin host defense. Together, these data unveil a new role for LTB4 in enhancing the expression and assembly of inflammasome components and suggest that while blocking LTB4 actions could be a promising therapeutic strategy to prevent inflammasome-mediated diseases, exogenous LTB4 can be used as an adjuvant to boost inflammasome-dependent host defense.

Leukotriene B4 and Its Receptor in Experimental Autoimmune Uveitis and in Human Retinal Tissues: Clinical Severity and LTB4 Dependence of Retinal Th17 Cells

Nomacopan, a drug originally derived from tick saliva, has dual functions of sequestering leukotriene B₄ (LTB₄) and inhibiting complement component 5 (C5) activation. Nomacopan has been shown to provide therapeutic benefit in experimental autoimmune uveitis (EAU). Longer acting forms of nomacopan were more efficacious in mouse EAU models, and the long-acting variant that inhibited only LTB₄ was at least as effective as the long-acting variant that inhibited both C5 and LTB₄, preventing structural damage to the retina and a significantly reducing effector T helper 17 cells and inflammatory macrophages. Increased levels of LTB₄ and C5a (produced upon C5 activation) were detected during disease progression. Activated retinal lymphocytes were shown to express LTB₄ receptors (R) in vitro and in inflamed draining lymph nodes. Levels of LTB₄R-expressing active/inflammatory retinal macrophages were also increased. Within the draining lymph node CD4⁺ T-cell population, 30% expressed LTB₄R⁺ following activation in vitro, whereas retinal infiltrating cells expressed LTB₄R and C5aR. Validation of expression of those receptors in human uveitis and healthy tissues suggests that infiltrating cells could be targeted by inhibitors of the LTB₄-LTB4 receptor 1 (BLT1) pathway as a novel therapeutic approach. This study provides novel data on intraocular LTB₄ and C5a in EAU, their associated receptor expression by retinal infiltrating cells in mouse and human tissues, and in attenuating EAU via the dual inhibitor nomacopan.

Control of myeloid cell density in barrier tissues

The interface between the mammalian host and its environment is formed by barrier tissues, for example, of the skin, and the respiratory and the intestinal tracts. On the one hand, barrier tissues are colonized by site-adapted microbial communities, and on the other hand, they contain specific myeloid cell networks comprising macrophages, dendritic cells, and granulocytes. These immune cells are tightly regulated in function and cell number, indicating important roles in maintaining tissue homeostasis and immune balance in the presence of commensal microorganisms. The regulation of myeloid cell density and activation involves cell-autonomous 'single-loop circuits' including autocrine mechanisms. However, an array of microenvironmental factors originating from nonimmune cells and the microbiota, as well as the microanatomical structure, impose additional layers of regulation onto resident myeloid cells. This review discusses models integrating these factors into cell-specific programs to instruct differentiation and proliferation best suited for the maintenance and renewal of immune homeostasis in the tissue-specific environment.

The anti-inflammatory agent bindarit acts as a modulator of fatty acid-binding protein 4 in human monocytic cells

We investigated the cellular and molecular mechanisms by which bindarit, a small indazolic derivative with prominent anti-inflammatory effects, exerts its immunoregulatory activity in lipopolysaccharide (LPS) stimulated human monocytic cells. We found that bindarit differentially regulates the release of interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1), enhancing the release of IL-8 and reducing that of MCP-1. These effects specifically required a functional interaction between bindarit and fatty acid binding protein 4 (FABP4), a lipid chaperone that couples intracellular lipid mediators to their biological targets and signaling pathways. We further demonstrated that bindarit can directly interact with FABP4 by increasing its expression and nuclear localization, thus impacting on peroxisome proliferator-activated receptor γ (PPARγ) and LPS-dependent kinase signaling. Taken together, these findings suggest a potential key-role of FABP4 in the immunomodulatory activity of bindarit, and extend the spectrum of its possible therapeutic applications to FABP4 modulation.

The Cooperative Induction of CCL4 in Human Monocytic Cells by TNF-α and Palmitate Requires MyD88 and Involves MAPK/NF-κB Signaling Pathways

Chronic low-grade inflammation, also known as metabolic inflammation, is a hallmark of obesity and parallels with the presence of elevated circulatory levels of free fatty acids and inflammatory cytokines/chemokines. CCL4/MIP-1β chemokine plays a key role in the adipose tissue monocyte recruitment. Increased circulatory levels of TNF-α, palmitate and CCL4 are co-expressed in obesity. We asked if the TNF-α/palmitate could interact cooperatively to augment the CCL4 production in human monocytic cells and macrophages. THP-1 cells/primary macrophages were co-treated with TNF-α/palmitate and CCL4 mRNA/protein expression was assessed using qRT-PCR/ELISA. TLR4 siRNA, a TLR4 receptor-blocking antibody, XBlue™-defMyD cells and pathway inhibitors were used to decipher the signaling mechanisms. We found that TNF-α/palmitate co-stimulation augmented the CCL4 expression in monocytic cells and macrophages compared to controls (p < 0.05). TLR4 suppression or neutralization abrogated the CCL4 expression in monocytic cells. Notably, CCL4 cooperative induction in monocytic cells was: (1) Markedly less in MyD88-deficient cells, (2) IRF3 independent, (3) clathrin dependent and (4) associated with the signaling mechanism involving ERK1/2, c-Jun, JNK and NF-κB. In conclusion, TNF-α/palmitate co-stimulation promotes the CCL4 expression in human monocytic cells through the mechanism involving a TLR4-MyD88 axis and MAPK/NF-κB pathways. These findings unravel a novel mechanism of the cooperative induction of CCL4 by TNF-α and palmitate which could be relevant to metabolic inflammation.

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.

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.

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.

Arachidonic acid supplementation transiently augments the acute inflammatory response to resistance exercise in trained men

Strenuous exercise can result in skeletal muscle damage, leading to the systemic mobilization, activation, and intramuscular accumulation of blood leukocytes. Eicosanoid metabolites of arachidonic acid (ARA) are potent inflammatory mediators, but whether changes in dietary ARA intake influence exercise-induced inflammation is not known. This study investigated the effect of 4 wk of dietary supplementation with 1.5 g/day ARA ( n = 9, 24 ± 1.5 yr) or corn-soy oil placebo ( n = 10, 26 ± 1.3 yr) on systemic and intramuscular inflammatory responses to an acute bout of resistance exercise (8 sets each of leg press and extension at 80% one-repetition maximum) in previously trained men. Whole EDTA blood, serum, peripheral blood mononuclear cells (PMBCs), and skeletal muscle biopsies were collected before exercise, immediately postexercise, and at 2, 4, and 48 h of recovery. ARA supplementation resulted in higher exercise-stimulated serum creatine kinase activity [incremental area under the curve (iAUC) P = 0.046] and blood leukocyte counts (iAUC for total white cells, P < 0.001; neutrophils: P = 0.007; monocytes: P = 0.015). The exercise-induced fold change in peripheral blood mononuclear cell mRNA expression of interleukin-1β ( IL1B), CD11b ( ITGAM), and neutrophil elastase ( ELANE), as well as muscle mRNA expression of the chemokines interleukin-8 ( CXCL8) and monocyte chemoattractant protein 1 ( CCL2) was also greater in the ARA group than placebo. Despite this, ARA supplementation did not influence the histological presence of leukocytes within muscle, perceived muscle soreness, or the extent and duration of muscle force loss. These data show that ARA supplementation transiently increased the inflammatory response to acute resistance exercise but did not impair recovery. NEW & NOTEWORTHY Daily arachidonic acid supplementation for 4 wk in trained men augmented the acute systemic and intramuscular inflammatory response to a subsequent bout of resistance exercise. Greater exercise-induced inflammatory responses in men receiving arachidonic acid supplementation were not accompanied by increased symptoms of exercise-induced muscle damage. Although increased dietary arachidonic acid intake does not appear to influence basal inflammation in humans, the acute inflammatory response to exercise stress is transiently increased following arachidonic acid supplementation.

Adipose tissue B2 cells promote insulin resistance through leukotriene LTB4/LTB4R1 signaling

Tissue inflammation is a key component of obesity-induced insulin resistance, with a variety of immune cell types accumulating in adipose tissue. Here, we have demonstrated increased numbers of B2 lymphocytes in obese adipose tissue and have shown that high-fat diet-induced (HFD-induced) insulin resistance is mitigated in B cell-deficient (Bnull) mice. Adoptive transfer of adipose tissue B2 cells (ATB2) from wild-type HFD donor mice into HFD Bnull recipients completely restored the effect of HFD to induce insulin resistance. Recruitment and activation of ATB2 cells was mediated by signaling through the chemokine leukotriene B4 (LTB4) and its receptor LTB4R1. Furthermore, the adverse effects of ATB2 cells on glucose homeostasis were partially dependent upon T cells and macrophages. These results demonstrate the importance of ATB2 cells in obesity-induced insulin resistance and suggest that inhibition of the LTB4/LTB4R1 axis might be a useful approach for developing insulin-sensitizing therapeutics.

Targeting STOX1 in the therapy of preeclampsia

INTRODUCTION

Preeclampsia is a major pregnancy disease, explained partly by genetic predispositions. STOX1, a transcription factor discovered in 2005, was the first gene directly associated with genetic forms of the disease. Alterations of STOX1 expression as well as STOX1 variants have also been associated to Alzheimer's disease. These observations make of this gene a putative therapeutic target. Area covered: Two major isoforms (STOX1A and STOX1B) are encoded by the gene and are theoretically able to compete for the same binding site, while only the most complete (STOX1A) is supposed to be able to activate gene expression. This makes the ratio between STOX1A and STOX1B as well as their position inside the cell (nucleus or cytoplasm) crucial to understand how STOX1 functions. STOX1 appears to have multiple gene targets, especially in pathways connected to inflammation, oxidative stress, and cell cycle. Expert opinion: STOX1-directed therapies, could be directed either towards its targets (genes or pathways), or directly at STOX1. For this the addressing of STOX1 to various cell compartments could theoretically be modified; also it could be possible of altering the balance between the two isoforms, through selectively inhibiting one of them, possibly improving the outcomes in severe preeclampsia.

Systemic leukotriene B4 receptor antagonism lowers arterial blood pressure and improves autonomic function in the spontaneously hypertensive rat

KEY POINTS

Evidence indicates an association between hypertension and chronic systemic inflammation in both human hypertension and experimental animal models. Previous studies in the spontaneously hypertensive rat (SHR) support a role for leukotriene B₄ (LTB₄ ), a potent chemoattractant involved in the inflammatory response, but its mode of action is poorly understood. In the SHR, we observed an increase in T cells and macrophages in the brainstem; in addition, gene expression profiling data showed that LTB₄ production, degradation and downstream signalling in the brainstem of the SHR are dynamically regulated during hypertension. When LTB₄ receptor 1 (BLT1) receptors were blocked with CP-105,696, arterial pressure was reduced in the SHR compared to the normotensive control and this reduction was associated with a significant decrease in systolic blood pressure (BP) indicators. These data provide new evidence for the role of LTB₄ as an important neuro-immune pathway in the development of hypertension and therefore may serve as a novel therapeutic target for the treatment of neurogenic hypertension.

ABSTRACT

Accumulating evidence indicates an association between hypertension and chronic systemic inflammation in both human hypertension and experimental animal models. Previous studies in the spontaneously hypertensive rat (SHR) support a role for leukotriene B₄ (LTB₄ ), a potent chemoattractant involved in the inflammatory response. However, the mechanism for LTB₄ -mediated inflammation in hypertension is poorly understood. Here we report in the SHR, increased brainstem infiltration of T cells and macrophages plus gene expression profiling data showing that LTB₄ production, degradation and downstream signalling in the brainstem of the SHR are dynamically regulated during hypertension. Chronic blockade of the LTB₄ receptor 1 (BLT1) receptor with CP-105,696, reduced arterial pressure in the SHR compared to the normotensive control and this reduction was associated with a significant decrease in low and high frequency spectra of systolic blood pressure, and an increase in spontaneous baroreceptor reflex gain (sBRG). These data provide new evidence for the role of LTB₄ as an important neuro-immune pathway in the development of hypertension and therefore may serve as a novel therapeutic target for the treatment of neurogenic hypertension.

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

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

ω-3 Fatty Acids and Cardiovascular Diseases: Effects, Mechanisms and Dietary Relevance

ω-3 fatty acids (n-3 FA) have, since the 1970s, been associated with beneficial health effects. They are, however, prone to lipid peroxidation due to their many double bonds. Lipid peroxidation is a process that may lead to increased oxidative stress, a condition associated with adverse health effects. Recently, conflicting evidence regarding the health benefits of intake of n-3 from seafood or n-3 supplements has emerged. The aim of this review was thus to examine recent literature regarding health aspects of n-3 FA intake from fish or n-3 supplements, and to discuss possible reasons for the conflicting findings. There is a broad consensus that fish and seafood are the optimal sources of n-3 FA and consumption of approximately 2-3 servings per week is recommended. The scientific evidence of benefits from n-3 supplementation has diminished over time, probably due to a general increase in seafood consumption and better pharmacological intervention and acute treatment of patients with cardiovascular diseases (CVD).

Atherosclerosis - A matter of unresolved inflammation

Atherosclerosis is commonly looked upon as a chronic inflammatory disease of the arterial wall arising from an unbalanced lipid metabolism and a maladaptive inflammatory response. However, atherosclerosis is not merely an inflammation of the vessel wall. In fact, the cardinal signs of unstable atherosclerotic lesions are primarily characteristics of failed resolution of a chronic inflammation. In contrast to acute inflammatory events which are typically self-limiting, atherosclerosis is an unresolved inflammatory condition, lacking the switch from the pro-inflammatory to the pro-resolving phase, the latter characterized by termination of inflammatory cell recruitment, removal of inflammatory cells from the site of inflammation by apoptosis and dead cell clearance, reprogramming of macrophages toward an anti-inflammatory, regenerative phenotype, and finally egress of effector cells and tissue regeneration. Here we present an overview on mechanisms of failed resolution contributing to atheroprogression and deliver a summary of novel therapeutic strategies to restore resolution in inflamed arteries.

Cytosolic phospholipase A₂: physiological function and role in disease

The group IV phospholipase A2 (PLA2) family is comprised of six intracellular enzymes (GIVA, -B, -C, -D, -E, and -F) commonly referred to as cytosolic PLA2 (cPLA2)α, -β, -γ, -δ, -ε, and -ζ. They contain a Ser-Asp catalytic dyad and all except cPLA2γ have a C2 domain, but differences in their catalytic activities and subcellular localization suggest unique regulation and function. With the exception of cPLA2α, the focus of this review, little is known about the in vivo function of group IV enzymes. cPLA2α catalyzes the hydrolysis of phospholipids to arachidonic acid and lysophospholipids that are precursors of numerous bioactive lipids. The regulation of cPLA2α is complex, involving transcriptional and posttranslational processes, particularly increases in calcium and phosphorylation. cPLA2α is a highly conserved widely expressed enzyme that promotes lipid mediator production in human and rodent cells from a variety of tissues. The diverse bioactive lipids produced as a result of cPLA2α activation regulate normal physiological processes and disease pathogenesis in many organ systems, as shown using cPLA2α KO mice. However, humans recently identified with cPLA2α deficiency exhibit more pronounced effects on health than observed in mice lacking cPLA2α, indicating that much remains to be learned about this interesting enzyme.

Variants in ALOX5, ALOX5AP and LTA4H are not associated with atherosclerotic plaque phenotypes: the Athero-Express Genomics Study

BACKGROUND

The eicosanoid genes ALOX5, ALOX5AP and LTA4H have been implicated in atherosclerosis. We assessed the impact of common variants in these genes on gene expression, circulating protein levels, and atherosclerotic plaque phenotypes.

METHODS

We included patients from the Stockholm Atherosclerosis Gene Expression study (STAGE, N = 109), and the Athero-Express Biobank Study (AE, N = 1443). We tested 1453 single-nucleotide variants (SNVs) in ALOX5, ALOX5AP and LTA4H for association with gene expression in STAGE. We also tested these SNVs for association with seven histologically defined plaque phenotypes in the AE (which included calcification, collagen, cellular content, atheroma size, and intraplaque vessel density and hemorrhage).

RESULTS

We replicate a known cis-eQTL (rs6538697, p = 1.96 × 10(-6)) for LTA4H expression in whole blood of patients from STAGE. We found no significant association for any of the SNVs tested with serum levels of ALOX5 or ALOX5AP (p > 5.79 × 10(-4)). For atherosclerotic plaque phenotypes the strongest associations were found for intraplaque vessel density and smooth muscle cells in the ALOX5AP locus (p > 1.67 × 10(-4)).

CONCLUSIONS

We replicate a known eQTL for LTA4H expression in whole blood using STAGE data. We found no associations of variants in and around ALOX5, ALOX5AP and LTA4H with serum ALOX5 or ALOX5AP levels, or plaque phenotypes. On the supposition that these genes play a causal role in atherosclerosis, these results suggest that common variants in these loci play a limited role (if any) in influencing advanced atherosclerotic plaque morphology to the extent that it impacts atherosclerotic disease.

Microvesicles: ubiquitous contributors to infection and immunity

MVs, which can be subgrouped into exosomes, SVs, and OMVs, are secreted by eukaryotic and prokaryotic cells. Many previously inexplicable phenomena can be explained by the existence of these vesicles, as they appear to be important in a wide range of biologic processes, such as intercellular communication and transfer of functional genetic information. In this review, we discuss the immunologic roles of MVs during sterile insult and infectious disease. MVs contribute to clotting initiation, cell recruitment, and neovascularization during wound healing. In the context of pathogen infection, both the host and the pathogen use MVs for communication and defense. MVs are exploited by various viruses to evade the host immune response and contribute to viral spread. Bacteria produce MVs that contain virulence factors that contribute to disease pathology and antibiotic resistance. This review summarizes the role of MVs in the pathology and resolution of disease.

NOX2-derived ROS-mediated surface translocation of BLT1 is essential for exocytosis in human eosinophils induced by LTB4

BACKGROUND

Leukotriene B4 (LTB4) is a proinflammatory lipid mediator that elicits eosinophil exocytosis, leading to allergic inflammation. However, the detailed intracellular signaling mechanisms of eosinophil exocytosis induced by LTB4 are poorly understood. Herein, we report that NADPH oxidase (NOX)2-derived reactive oxygen species (ROS)-mediated BLT1 migration to the cell surface is required for exocytosis in human eosinophils induced by LTB4.

METHODS

Peripheral blood eosinophils were purified and stimulated for up to 60 min with LTB4. The signaling role of NOX2-derived ROS in BLT1-dependent exocytosis in LTB4-stimulated eosinophils was investigated.

RESULTS

Stimulating eosinophils with LTB4 induced intracellular ROS production and surface upregulation of the exocytosis marker protein CD63 via BLT1-mediated signaling. LTB4 induced p47(phox) phosphorylation and 91(phox) expression required for NOX2 activation in a BLT1-dependent manner. Pretreatment with NOX2 inhibitors, but not mitochondria inhibitor, prevented LTB4-induced ROS generation and exocytosis. At 30 min after stimulation with LTB4, BLT1 expression at the cell surface was upregulated. LTB4-triggered surface upregulation of BLT1 was also blocked by inhibition of ROS generation with NOX2 inhibitors. Moreover, stimulation for 30 min with LTB4 resulted in the interaction of BLT1 with NOX2 by immunoprecipitation. LTB4-induced ROS generation, surface upregulation of BLT1 and exocytosis was also inhibited by pretreatment with a lipid raft disruptor, protein kinase C inhibitor, or Src kinase inhibitor.

CONCLUSION

These results suggest that NOX2-derived ROS-mediated BLT1 trafficking to the cell surface plays a key role in the exocytosis of human eosinophils induced by LTB4.

Leukotriene B4 levels in human atherosclerotic plaques and abdominal aortic aneurysms

BACKGROUND

Leukotriene B4 (LTB4) has been associated with the initiation and progression of atherosclerosis and abdominal aortic aneurysm (AAA) formation. However, associations of LTB4 levels with tissue characteristics and adverse clinical outcome of advanced atherosclerosis and AAA are scarcely studied. We hypothesized that LTB4 levels are associated with a vulnerable plaque phenotype and adverse clinical outcome. Furthermore, that LTB4 levels are associated with inflammatory AAA and adverse clinical outcome.

METHODS

Atherosclerotic plaques and AAA specimens were selected from two independent databases for LTB4 measurements. Plaques were isolated during carotid endarterectomy from asymptomatic (n = 58) or symptomatic (n = 317) patients, classified prior to surgery. LTB4 levels were measured without prior lipid extraction and levels were corrected for protein content. LTB4 levels were related to plaque phenotype, baseline patient characteristics and clinical outcome within three years following surgery. Seven non-diseased mammary artery specimens served as controls. AAA specimens were isolated during open repair, classified as elective (n = 189), symptomatic (n = 29) or ruptured (n = 23). LTB4 levels were measured similar to the plaque measurements and were related to tissue characteristics, baseline patient characteristics and clinical outcome. Twenty-six non-diseased aortic specimens served as controls.

RESULTS

LTB4 levels corrected for protein content were not significantly associated with histological characteristics specific for vulnerable plaques or inflammatory AAA as well as clinical presentation. Moreover, it could not predict secondary manifestations independently investigated in both databases. However, LTB4 levels were significantly lower in controls compared to plaque (p = 0.025) or AAA (p = 0.017).

CONCLUSIONS

LTB4 levels were not associated with a vulnerable plaque phenotype or inflammatory AAA or clinical presentation. This study does not provide supportive evidence for a role of LTB4 in atherosclerotic plaque destabilization or AAA expansion. However, these data should be interpreted with care, since LTB4 measurements were performed without prior lipid extractions.

The Kaposi's sarcoma-associated herpesvirus (KSHV)-induced 5-lipoxygenase-leukotriene B4 cascade plays key roles in KSHV latency, monocyte recruitment, and lipogenesis

Kaposi's sarcoma-associated herpesvirus (KSHV) is etiologically associated with Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL). KS lesions are characterized by endothelial cells with multiple copies of the latent KSHV episomal genome, lytic replication in a low percentage of infiltrating monocytes, and inflammatory cytokines plus growth factors. We demonstrated that KSHV utilizes inflammatory cyclooxygenase 2/prostaglandin E2 to establish and maintain latency (Sharma-Walia, N., A. G. Paul, V. Bottero, S. Sadagopan, M. V. Veettil, N. Kerur, and B. Chandran, PLoS Pathog 6:e1000777, 2010 [doi:10.1371/journal.ppat.1000777]). Here, we evaluated the role of 5-lipoxygenase (5LO) and its chemotactic metabolite leukotriene B4 (LTB4) in KSHV biology. Abundant staining of 5LO was detected in human KS tissue sections. We observed elevated levels of 5LO and high levels of secretion of LTB4 during primary KSHV infection of endothelial cells and in PEL B cells (BCBL-1 and BC-3 cells). Blocking the 5LO/LTB4 cascade inhibited viral latent ORF73, immunomodulatory K5, viral macrophage inflammatory protein 1 (MIP-1), and viral MIP-2 gene expression, without much effect on lytic switch ORF50, immediate early lytic K8, and viral interferon-regulatory factor 2 gene expression. 5LO inhibition significantly downregulated latent viral Cyclin and latency-associated nuclear antigen 2 levels in PEL cells. 5LO/LTB4 inhibition downregulated TH2-related cytokine secretion, elevated TH1-related cytokine secretion, and reduced human monocyte recruitment, adhesion, and transendothelial migration. 5LO/LTB4 inhibition reduced fatty acid synthase (FASN) promoter activity and its expression. Since FASN, a key enzyme required in lipogenesis, is important in KSHV latency, these findings collectively suggest that 5LO/LTB4 play important roles in KSHV biology and that effective inhibition of the 5LO/LTB4 pathway could potentially be used in treatment to control KS/PEL.

Augmentation of 5-lipoxygenase activity and expression during dengue serotype-2 infection

BACKGROUND

Leukotriene B4, a 5-lipoxygenase product of arachidonic acid with potent chemotactic effects on neutrophils, has not been assessed in dengue patients. In this study, plasma leukotriene B4 and serum high-sensitivity C-reactive protein levels were determined in adult patients during the febrile, convalescent and defervescent stages of dengue serotype-2 (DENV-2) infection, and compared with those of age-matched healthy and non-dengue febrile subjects. In vitro studies were performed to examine the effects of live and heat-inactivated DENV-2 on the activities and expression of 5-lipoxygenase in human neutrophils.

RESULTS

Plasma leukotriene B4 was elevated during the febrile stages of dengue infection compared to levels during convalescence and in study controls. Plasma leukotriene B4 also correlated with serum high-sensitivity C-reactive protein in dengue patients (febrile, r = 0.91, p < 0.001; defervescence, r = 0.87, p < 0.001; convalescence, r = 0.87, p < 0.001). Exposure of human neutrophils to DENV-2 resulted in a significant rise in leukotriene B4; the extent of increase, however, did not differ between exposure to live and heat-inactivated DENV-2. Pre-incubation of either live or heat-inactivated DENV-2 resulted in reduced leukotriene B4 release by neutrophils, indicating that contact with dengue antigens (and not replication) triggers the neutrophil response. Production of leukotriene B4 was associated with an increase in 5-lipoxygenase expression in human neutrophils; addition of MK886 (a 5-lipoxygenase activating protein inhibitor) attenuated further increase in leukotriene B4 production.

CONCLUSION

These findings provide important clinical and mechanistic data on the involvement of 5-lipoxygenase and its metabolites in dengue infection. Further studies are needed to elucidate the therapeutic implications of these findings.

Leukotriene B4, an endogenous stimulator of the innate immune response against pathogens

Leukotriene B4 (LTB4) is an endogenous lipid mediator of inflammation derived from arachidonic acid by the sequential action of cytosolic phospholipase A2 and 5-lipoxygenase. This mediator was initially recognized for its involvement in the recruitment of neutrophils. However, in the last decade, LTB4 has been clearly demonstrated to play a significant role in the control of microbial infections through its ability to activate host innate defenses. In this review, we will focus on the modulator effects of LTB4 on the innate defenses and discuss its therapeutic potential against viral pathogens.

The pivotal role of 5-lipoxygenase-derived LTB4 in controlling pulmonary paracoccidioidomycosis

Leukotrienes (LTs) produced from arachidonic acid by the action of 5-lipoxygenase (5-LO) are classical mediators of inflammatory responses. However, studies published in the literature regarding these mediators are contradictory and it remains uncertain whether these lipid mediators play a role in host defense against the fungal pathogen Paracoccidioides brasiliensis. To determine the involvement of LTs in the host response to pulmonary infection, wild-type and LT-deficient mice by targeted disruption of the 5-lipoxygenase gene (knockout mice) were studied following intratracheal challenge with P. brasiliensis yeasts. The results showed that infection is uniformly fatal in 5-LO-deficient mice and the mechanisms that account for this phenotype are an exacerbated lung injury and higher fungal pulmonary burden. Genetic ablation or pharmacological inhibition of LTs resulted in lower phagocytosis and fungicidal activity of macrophages in vitro, suggesting that deficiency in fungal clearance seems to be secondary to the absence of activation in 5-LO(-/-) macrophages. Exogenous LTB4 restored phagocytosis and fungicidal activity of 5-LO(-/-) macrophages. Moreover, P. brasiliensis killing promoted by LTB4 was dependent on nitric oxide (NO) production by macrophages. Taken together, these results reveal a fundamental role for 5-LO-derived LTB4 in the protective response to P. brasiliensis infection and identify relevant mechanisms for the control of fungal infection during the early stages of the host immune response.

Lipoxygenase and Cyclooxygenase Pathways and Colorectal Cancer Prevention

Colorectal cancer is one of the commonest malignancies in both men and women. In spite of significant progress in screening and in surgical and therapeutic interventions, colorectal cancer (CRC) is still a major public health problem. Accumulating evidence suggests that targeting inflammatory pathways may provide protection against the development of CRC. Eicosanoids derived from the enzymes cyclooxygenase (COX) and lipoxygenase (LOX) may contribute to CRC carcinogenesis. Approaches for targeting COX-1 and COX-2 with traditional nonsteroidal anti-inflammatory agents or targeting COX-2 with specific inhibitors are highly successful at the preclinical and clinical levels; however, large-scale clinical applicability of these agents is limited owing to unwanted side effects. Emerging studies suggests that 5-LOX-derived leukotrienes may contribute to colon tumor development and risk of thrombotic events. Thus, developing drugs that target both 5-LOX and COX-2 may provide a safer strategy. In this review, we discuss evidence for the involvement of 5-LOX in colon tumor development and targeting 5-LOX and COX-2 with synthetic and naturally occurring agents for CRC prevention.

Excessive leukotriene B4 in nucleus tractus solitarii is prohypertensive in spontaneously hypertensive rats

Inflammation within the brain stem microvasculature has been associated with chronic cardiovascular diseases. We found that the expression of several enzymes involved in arachidonic acid-leukotriene B4 (LTB4) production was altered in nucleus tractus solitarii (NTS) of spontaneously hypertensive rat (SHR). LTB4 produced from arachidonic acid by 5-lipoxygenase is a potent chemoattractant of leukocytes. Leukotriene B4-12-hydroxydehydrogenase (LTB4-12-HD), which degrades LTB4, was downregulated in SHR rats compared with that in Wistar-Kyoto rats. Quantitative real-time PCR revealed that LTB4-12-HD was reduced by 63% and 58% in the NTS of adult SHR and prehypertensive SHR, respectively, compared with that in age-matched Wistar-Kyoto rats (n=6). 5-lipoxygenase gene expression was upregulated in the NTS of SHR (≈50%; n=6). LTB4 levels were increased in the NTS of the SHR, (17%; n=10, P<0.05). LTB4 receptors BLT1 (but not BLT2) were expressed on astroglia in the NTS but not neurons or vessels. Microinjection of LTB4 into the NTS of Wistar-Kyoto rats increased both leukocyte adherence and arterial pressure for over 4 days (peak: +15 mm Hg; P<0.01). In contrast, blockade of NTS BLT1 receptors lowered blood pressure in the SHR (peak: -13 mm Hg; P<0.05) but not in Wistar-Kyoto rats. Thus, excessive amounts of LTB4 in NTS of SHR, possibly as a result of upregulation of 5-lipoxygenase and downregulation of LTB4-12-HD, can induce inflammation. Because blockade of NTS BLT1 receptors lowered arterial pressure in the SHR, their endogenous activity may contribute to the hypertensive state of this rodent model. Thus, inflammatory reactions in the brain stem are causally associated with neurogenic hypertension.

Adipocytes secrete leukotrienes: contribution to obesity-associated inflammation and insulin resistance in mice

Leukotrienes (LTs) are potent proinflammatory mediators, and many important aspects of innate and adaptive immune responses are regulated by LTs. Key members of the LT synthesis pathway are overexpressed in adipose tissue (AT) during obesity, resulting in increased LT levels in this tissue. We observed that several mouse adipocyte cell lines and primary adipocytes from mice and humans both can secrete large amounts of LTs. Furthermore, this production increases with a high-fat diet (HFD) and positively correlates with adipocyte size. LTs produced by adipocytes play an important role in attracting macrophages and T cells in in vitro chemotaxis assays. Mice that are deficient for the enzyme 5-lipoxygenase (5-LO), and therefore lack LTs, exhibit a decrease in HFD-induced AT macrophage and T-cell infiltration and are partially protected from HFD-induced insulin resistance. Similarly, treatment of HFD-fed wild-type mice with the 5-LO inhibitor Zileuton also results in a reduction of AT macrophages and T cells, accompanied by a decrease in insulin resistance. Together, these findings suggest that LTs represent a novel target in the prevention or treatment of obesity-associated inflammation and insulin resistance.

TAK1 contributes to the enhanced responsiveness of LTB(4)-treated neutrophils to Toll-like receptor ligands

Pattern-recognition receptors such as Toll-like receptors (TLRs) are essential sensors implicated in the early and efficient innate immune response against pathogens. We have previously demonstrated that leukotriene B(4)(LTB(4)) has the capacity to enhance leukocyte responses to TLR9 ligands and to control viral infection. In this report, we provide evidence that LTB(4) treatment of human neutrophils leads to a potentiation in proinflammatory cytokine secretion induced by various myeloid differentiation factor 88-dependent TLR agonists. LTB(4) failed to enhance TLR mRNA levels as well as expression of TLR2 and TLR4 receptors, suggesting that LTB(4) acts through intracellular mechanism(s) to potentiate neutrophil responses to TLR ligands. We found that while IRAK can be activated by LTB(4), this process is dispensable to LTB(4) to potentiate neutrophil responses to TLR ligands since pretreatment of neutrophils with IRAK1/4 inhibitor did not affect its potentiating effects. However, our data clearly show that LTB(4) treatment of neutrophils led to the phosphorylation of downstream signaling molecules, TAK1 and p38, a process found essential to observe an increased secretion of cytokines by neutrophils activated with TLR ligands. Pretreatment of neutrophils with TAK1 or p38 kinase inhibitors strongly repressed the effect of LTB(4) on cytokine synthesis by neutrophils stimulated with LTA, LPS or CpG. The same pattern was observed in agonist-treated human embryonic kidney 293 cells transfected with TAK1-targeting siRNA where secretion of IL-8 was significantly reduced to basal levels. These results indicate that TAK1 and p38 kinases appear to be central in the 'priming effect' of LTB(4) on neutrophils to enhance response to TLR ligands.

Leukotriene A4 Hydrolase: Biology, Inhibitors and Clinical Applications

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

Analyses of single nucleotide polymorphisms in selected nutrient-sensitive genes in weight-regain prevention: the DIOGENES study

BACKGROUND

Differences in the interindividual response to dietary intervention could be modified by genetic variation in nutrient-sensitive genes.

OBJECTIVE

This study examined single nucleotide polymorphisms (SNPs) in presumed nutrient-sensitive candidate genes for obesity and obesity-related diseases for main and dietary interaction effects on weight, waist circumference, and fat mass regain over 6 mo.

DESIGN

In total, 742 participants who had lost ≥ 8% of their initial body weight were randomly assigned to follow 1 of 5 different ad libitum diets with different glycemic indexes and contents of dietary protein. The SNP main and SNP-diet interaction effects were analyzed by using linear regression models, corrected for multiple testing by using Bonferroni correction and evaluated by using quantile-quantile (Q-Q) plots.

RESULTS

After correction for multiple testing, none of the SNPs were significantly associated with weight, waist circumference, or fat mass regain. Q-Q plots showed that ALOX5AP rs4769873 showed a higher observed than predicted P value for the association with less waist circumference regain over 6 mo (-3.1 cm/allele; 95% CI: -4.6, -1.6; P/Bonferroni-corrected P = 0.000039/0.076), independently of diet. Additional associations were identified by using Q-Q plots for SNPs in ALOX5AP, TNF, and KCNJ11 for main effects; in LPL and TUB for glycemic index interaction effects on waist circumference regain; in GHRL, CCK, MLXIPL, and LEPR on weight; in PPARC1A, PCK2, ALOX5AP, PYY, and ADRB3 on waist circumference; and in PPARD, FABP1, PLAUR, and LPIN1 on fat mass regain for dietary protein interaction.

CONCLUSION

The observed effects of SNP-diet interactions on weight, waist, and fat mass regain suggest that genetic variation in nutrient-sensitive genes can modify the response to diet. This trial was registered at clinicaltrials.gov as NCT00390637.

5-(4-Hydroxy-2,3,5-trimethylbenzylidene) thiazolidine-2,4-dione attenuates atherosclerosis possibly by reducing monocyte recruitment to the lesion

A variety of benzylidenethiazole analogs have been demonstrated to inhibit 5-lipoxygenase (5-LOX). Here we report the anti-atherogenic potential of 5-(4-hydroxy- 2,3,5-trimethylbenzylidene) thiazolidin-2,4-dione (HMB-TZD), a benzylidenethiazole analog, and its potential mechanism of action in LDL receptor-deficient (Ldlr-/-) mice. HMB-TZD Treatment reduced leukotriene B4 (LTB4) production significantly in RAW264.7 macrophages and SVEC4-10 endothelial cells. Macrophages or endothelial cells pre-incubated with HMB-TZD for 2 h and then stimulated with lipopolysaccharide or tumor necrosis factor-alpha (TNF-α) displayed reduced cytokine production. Also, HMB-TZD reduced cell migration and adhesion in accordance with decreased proinflammatory molecule production in vitro and ex vivo. HMB-TZD treatment of 8-week-old male Ldlr-/- mice resulted in significantly reduced atherosclerotic lesions without a change to plasma lipid profiles. Moreover, aortic expression of pro-atherogenic molecules involved in the recruitment of monocytes to the aortic wall, including TNF-α , MCP-1, and VCAM-1, was downregulated. HMB-TZD also reduced macrophage infiltration into atherosclerotic lesions. In conclusion, HMB-TZD ameliorates atherosclerotic lesion formation possibly by reducing the expression of proinflammatory molecules and monocyte/macrophage recruitment to the lesion. These results suggest that HMB-TZD, and benzylidenethiazole analogs in general, may have therapeutic potential as treatments for atherosclerosis.