A novel hepatointestinal leukotriene B4 receptor. Cloning and functional characterization

Two distinct forms of human BLT2: long-form and short-form BLT2

BLT2 is a low-affinity leukotriene B4 receptor that plays an essential role in the pathogenesis of various inflammatory diseases, including asthma and cancer. BLT2 is minimally expressed in a normal internal environment but is overexpressed in a stress-induced inflammatory environment. Recent research indicated that human BLT2 has two distinct forms. Although their functions are likely to be different, very few studies investigated these differences. Therefore, this paper will discuss about the two distinct forms of human BLT2; the short-form of BLT2 and the long-form of BLT2.

The leukotriene B4 receptors BLT1 and BLT2 as potential therapeutic targets

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

Involvement of Proinflammatory Arachidonic Acid (ARA) Derivatives in Crohn’s Disease (CD) and Ulcerative Colitis (UC)

Recently, an increase in the incidence of inflammatory bowel disease (IBD) has been observed, especially among children and adolescents. Currently, few studies focus on the differentiation of inflammation in IBD subunits, i.e., Crohn’s Disease (CD) and Ulcerative Colitis (UC). The aim of this study was to compare the concentrations of proinflammatory mediators of arachidonic acid (ARA) and linoleic acid (LA) in patients with CD (n = 34) and UC (n = 30), in order to identify differences in inflammation in both diseases and within the same entity, according to disease activity. Sixty-four adolescents with a mean age of 13.76 ± 2.69 and 14.15 ± 3.31, for CD and UC, respectively, were enrolled in the study. Biochemical analysis of ARA and LA derivatives was performed using a liquid chromatography. A trend was observed in the concentration of 15S-HETE (hydroxyeicosatetraenoic acids) in CD relative to UC. The active phase of both diseases showed a higher 15S-HETE concentration in active CD relative to active UC. Comparing patients with CD with active and inactive disease showed a trend of increased levels of thromboxane B2, leukotriene B4 and 9S-HODE (hydroxyoctadecadienoic acid) in the active versus the inactive disease. We also observed statistically significantly higher levels of 12S-HETE in inactive CD relative to active CD. In the UC group, on the other hand, statistically significantly higher levels of prostaglandin E2 and 16RS-HETE were observed in active UC relative to inactive UC. Moreover, significantly higher concentrations of LTX A4 5S, 6R were observed in inactive UC relative to the active phase. In conclusion, the present study indicated the activity of the 15-LOX pathway in CD. Further studies involving lipid mediators in patients with IBD may contribute to the development of new therapies for the treatment of IBD. The identification of differences in the course of inflammation may help to target therapy in CD and UC, and perhaps allow the introduction of an additional diagnostic marker between the two main IBD subtypes.

Structural insights on ligand recognition at the human leukotriene B4 receptor 1

The leukotriene B4 receptor 1 (BLT1) regulates the recruitment and chemotaxis of different cell types and plays a role in the pathophysiology of infectious, allergic, metabolic, and tumorigenic human diseases. Here we present a crystal structure of human BLT1 (hBLT1) in complex with a selective antagonist MK-D-046, developed for the treatment of type 2 diabetes and other inflammatory conditions. Comprehensive analysis of the structure and structure-activity relationship data, reinforced by site-directed mutagenesis and docking studies, reveals molecular determinants of ligand binding and selectivity toward different BLT receptor subtypes and across species. The structure helps to identify a putative membrane-buried ligand access channel as well as potential receptor binding modes of endogenous agonists. These structural insights of hBLT1 enrich our understanding of its ligand recognition and open up future avenues in structure-based drug design.

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

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

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.

Structure of the agonist 12-HHT in its BLT2 receptor-bound state

G Protein-Coupled receptors represent the main communicating pathway for signals from the outside to the inside of most of eukaryotic cells. They define the largest family of integral membrane receptors at the surface of the cells and constitute the main target of the current drugs on the market. The low affinity leukotriene receptor BLT2 is a receptor involved in pro- and anti-inflammatory pathways and can be activated by various unsaturated fatty acid compounds. We present here the NMR structure of the agonist 12-HHT in its BLT2-bound state and a model of interaction of the ligand with the receptor based on a conformational homology modeling associated with docking simulations. Put into perspective with the data obtained with leukotriene B4, our results illuminate the ligand selectivity of BLT2 and may help define new molecules to modulate the activity of this receptor.

Combining Machine Learning and Metabolomics to Identify Weight Gain Biomarkers

Weight gain is a metabolic disorder that often culminates in the development of obesity and other comorbidities such as diabetes. Obesity is characterized by the development of a chronic, subclinical systemic inflammation, and is regarded as a remarkably important factor that contributes to the development of such comorbidities. Therefore, laboratory methods that allow the identification of subjects at higher risk for severe weight-associated morbidity are of utter importance, considering the health, and safety of populations. This contribution analyzed the plasma of 180 Brazilian individuals, equally divided into a eutrophic control group and case group, to assess the presence of biomarkers related to weight gain, aiming at characterizing the phenotype of this population. Samples were analyzed by mass spectrometry and most discriminant features were determined by a machine learning approach using Random Forest algorithm. Five biomarkers related to the pathogenesis and chronicity of inflammation in weight gain were identified. Two metabolites of arachidonic acid were upregulated in the case group, indicating the presence of inflammation, as well as two other molecules related to dysfunctions in the cycle of nitric oxide (NO) and increase in superoxide production. Finally, a fifth case group marker observed in this study may indicate the trigger for diabetes in overweight and obesity individuals. The use of mass spectrometry combined with machine learning analyses to prospect and characterize biomarkers associated with weight gain will pave the way for elucidating potential therapeutic and prognostic targets.

Identification, signaling, and functions of LTB4 receptors

Leukotriene B₄ (LTB₄), a lipid mediator produced from arachidonic acid, is a chemoattractant for inflammatory leukocytes. We identified two receptors for LTB₄, the high-affinity receptor BLT1 and the low-affinity receptor BLT2. BLT1 is expressed in various subsets of leukocytes, and analyses of BLT1-deficient mice revealed that the LTB₄/BLT1 axis enhances leukocyte recruitment to infected sites, and is involved in the elimination of pathogens. Hyperactivation of the LTB₄/BLT1 axis induces acute and chronic inflammation, resulting in various inflammatory diseases. BLT2 was originally identified as a low-affinity receptor for LTB₄, and we later identified 12(S)-hydroxy-5Z,8E,10E-heptadecatrienoic acid (12-HHT) as a high-affinity ligand for BLT2. BLT2 is highly expressed in epithelial cells in various tissues including intestine and skin. Large quantities of 12-HHT are produced by activated platelets during skin injury, and activation of BLT2 on epidermal keratinocytes accelerates skin wound healing by enhancing cell migration. BLT2 signaling also enhances cell-cell junctions, protectes against transepidermal water loss, and preventes entry of environmental substances into the body.

Leukotriene receptors as potential therapeutic targets

Leukotrienes, a class of arachidonic acid-derived bioactive molecules, are known as mediators of allergic and inflammatory reactions and considered to be important drug targets. Although an inhibitor of leukotriene biosynthesis and antagonists of the cysteinyl leukotriene receptor are clinically used for bronchial asthma and allergic rhinitis, these medications were developed before the molecular identification of leukotriene receptors. Numerous studies using cloned leukotriene receptors and genetically engineered mice have unveiled new pathophysiological roles for leukotrienes. This Review covers the recent findings on leukotriene receptors to revisit them as new drug targets.

A novel monoclonal antibody suitable for the detection of leukotriene B4

Leukotriene B4 as an inflammatory mediator is an important biomarker for different respiratory diseases like asthma, chronic obstructive pulmonary disease or cystic lung fibrosis. Therefore the detection of LTB4 is helpful in the diagnosis of these pulmonary diseases. However, until now its determination in exhaled breath condensates suffers from problems of accuracy. Reasons for that could be improper sample collection and preparation methods of condensates and the lack of consistently assay specificity and reproducibility of the used immunoassay detection system. In this study we describe the development and the characterization of a specific monoclonal antibody (S27BC6) against LTB4, its use as molecular recognition element for the development of an enzyme-linked immunoassay to detect LTB4 and discuss possible future diagnostic applications.

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.

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

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

Endothelial 12(S)-HETE vasorelaxation is mediated by thromboxane receptor inhibition in mouse mesenteric arteries

Arachidonic acid (AA) metabolites mediate endothelium-dependent relaxation in many vascular beds. Previously, we identified the major AA 12/15-lipoxygenase (12/15-LO) metabolite of mouse arteries as 12-hydroxyeicosatetraenoic acid (12-HETE). The goal was to determine the stereospecific configuration of mouse vascular 12-HETE and characterize the role of 12-HETE stereoisomers in the regulation of vascular tone. Using normal, reverse phase, and chiral HPLC, the stereospecific configuration was identified as 12(S)-HETE. 12(S)-HETE relaxed U46619-, carbocyclic thromboxane A(2)-, PGF(2α)-, and 8-iso PGF(2α)-preconstricted mesenteric arteries, but not phenylephrine-preconstricted arteries. 12(R)-HETE was more potent than 12(S)-HETE in relaxing U46619-preconstricted mouse arteries (maximum relaxations = 91.4 ± 2.7% and 71.8 ± 5.9%, respectively). Neither 12-HETE isomer caused constriction. Pretreatment with 12(S)- or 12(R)-HETE (1 μM) inhibited constrictions to U46619 but not phenylephrine. To investigate the role of thromboxane A(2) (TP) receptors in 12-HETE vascular actions, [(3)H]SQ29548 radioligand binding studies were performed in mouse platelets. U46619, 12(R)-HETE, and 12(S)-HETE displaced [(3)H]SQ29548 binding with IC(50)s of 0.07, 0.32, and 1.73 μM, respectively. Both 12(S)- and 12(R)-HETE inhibited intracellular calcium increases induced by U46619 (10 nM) in HEK293 cells overexpressing TP(α) receptor (65.5% and 45.1%, respectively) and coexpressing prostacyclin (IP) and TP(α) receptors (58.0% and 27.1%, respectively). The LO inhibitor NDGA (10 μM) reduced AA relaxations in arteries preconstricted with U46619 but not phenylephrine. These results indicate that exogenous and endogenous 12(S)-HETE relax mouse mesenteric arteries that are preconstricted with thromboxane agonists. These 12(S)-HETE relaxations are mediated by TP receptor competitive inhibition and inhibition of TP agonist-induced increases in intracellular calcium.

BLT1-dependent alveolar recruitment of CD4(+)CD25(+) Foxp3(+) regulatory T cells is important for resolution of acute lung injury

RATIONALE

Recent study has demonstrated that CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs) present in bronchoalveolar lavage fluid (BALF) contribute to the resolution of an experimental acute lung injury (ALI). However, the molecular mechanism underlying the alveolar recruitment of Treg remains unclear.

OBJECTIVES

To determine the role of BLT1, a chemotactic receptor for leukotriene B4 (LTB4), in Treg recruitment to BALF of LPS-induced ALI.

METHODS

We examined BLT1 expression in mouse and human Tregs and evaluated its role in mediating Treg migration in vitro and in vivo.

MEASUREMENTS AND MAIN RESULTS

We found that BLT1 expression was strongly up-regulated in Tregs on activation, and that BLT1 mediated the migration of activated, but not resting, Tregs toward LTB4 in vitro. LTB4 levels were persistently elevated in BALF of LPS-induced ALI. Blockade of LTB4-BLT1 pathway by administrating antagonists 1 day after LPS exposure significantly decreased BALF Treg numbers and impaired resolution of ALI characterized by persistent BALF protein, neutrophilic infiltrates, and elevated proinflammatory cytokines. Furthermore, there were significantly less BLT1(-/-) Tregs than wild-type Tregs migrating to BALF of LPS-exposed recipient Rag-1(-/-) mice after adoptive transfer (point estimate 299.73; 95% confidence interval, 255.77-343.69; P < 0.00001), and the impaired alveolar recruitment of BLT1(-/-) Tregs caused the inability to restore the resolution of ALI.

CONCLUSIONS

Our findings reveal a novel antiinflammatory role of BLT1 in the resolution of ALI by mediating the alveolar recruitment of Tregs, and indicate that therapies aimed at interrupting the LTB4-BLT1 pathway after ALI onset could be harmful to the resolution of ALI.

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.

Leukotrienes antagonize the antiproliferative effect of Larrea divaricata Cav. on a lymphoma cell line interfering with cAMP intracellular level and PKC activity

Larrea divaricata exerts a dual effect on the proliferation of BW5147 cells. The aim of this work was to analyze the effect of leukotrienes on the antiproliferative action of the extract in relation to cyclic adenosine monophosphate production and protein kinase C activation. The extracts induced cyclic adenosine monophosphate and inhibited protein kinase C but induced leukotriene B4 production, which exerted contrary effects. These results suggest the finding of an extract free of compounds, which can increase leukotriene B4 level, in order to obtain a more active extract to be used in the treatment of lymphomas, positively modulated by leukotrienes.

Leukotriene B4 receptors: novel roles in immunological regulations

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

Structure of a GPCR ligand in its receptor-bound state: leukotriene B4 adopts a highly constrained conformation when associated to human BLT2

G protein-coupled receptors (GPCRs) are key players in signal recognition and cell communication and are among the most important targets for drug development. Direct structural information on the conformation of GPCR ligands bound to their receptors is scarce. Using a leukotriene receptor, BLT2, expressed under a perdeuterated form in Escherichia coli , purified in milligram amounts, and folded to its native state using amphipols, we have solved, by (1)H NMR, the structure of receptor-bound leukotriene B4 (LTB4). Upon binding, LTB4 adopts a highly constrained seahorse conformation, at variance with the free state, where it explores a wide range of conformations. This structure provides an experimentally determined template of a pro-inflammatory compound for further pharmacological studies. The novel approach used for its determination could prove powerful to investigate ligand binding to GPCRs and membrane proteins in general.

A novel chemogenomics analysis of G protein-coupled receptors (GPCRs) and their ligands: a potential strategy for receptor de-orphanization

Background

G protein-coupled receptors (GPCRs) represent a family of well-characterized drug targets with significant therapeutic value. Phylogenetic classifications may help to understand the characteristics of individual GPCRs and their subtypes. Previous phylogenetic classifications were all based on the sequences of receptors, adding only minor information about the ligand binding properties of the receptors. In this work, we compare a sequence-based classification of receptors to a ligand-based classification of the same group of receptors, and evaluate the potential to use sequence relatedness as a predictor for ligand interactions thus aiding the quest for ligands of orphan receptors.

Results

We present a classification of GPCRs that is purely based on their ligands, complementing sequence-based phylogenetic classifications of these receptors. Targets were hierarchically classified into phylogenetic trees, for both sequence space and ligand (substructure) space. The overall organization of the sequence-based tree and substructure-based tree was similar; in particular, the adenosine receptors cluster together as well as most peptide receptor subtypes (e.g. opioid, somatostatin) and adrenoceptor subtypes. In ligand space, the prostanoid and cannabinoid receptors are more distant from the other targets, whereas the tachykinin receptors, the oxytocin receptor, and serotonin receptors are closer to the other targets, which is indicative for ligand promiscuity. In 93% of the receptors studied, de-orphanization of a simulated orphan receptor using the ligands of related receptors performed better than random (AUC > 0.5) and for 35% of receptors de-orphanization performance was good (AUC > 0.7).

Conclusions

We constructed a phylogenetic classification of GPCRs that is solely based on the ligands of these receptors. The similarities and differences with traditional sequence-based classifications were investigated: our ligand-based classification uncovers relationships among GPCRs that are not apparent from the sequence-based classification. This will shed light on potential cross-reactivity of GPCR ligands and will aid the design of new ligands with the desired activity profiles. In addition, we linked the ligand-based classification with a ligand-focused sequence-based classification described in literature and proved the potential of this method for de-orphanization of GPCRs.

Leukotriene inhibition in small animal medicine

Leukotrienes are important mediators of inflammatory and allergic conditions in people and are suspected to play an important role in tumorigenesis and tumor growth of several different tumor types. Based on this, researchers are making great progress in identifying novel pharmacologic targets for several human diseases. Leukotriene inhibition has resulted in therapeutic benefit in clinical trials involving people with osteoarthritis, allergic asthma, and atopic dermatitis. Despite this progress and the possibility that leukotriene inhibition may also play an important therapeutic role in veterinary patients, parallel advances have not yet been made in veterinary medicine. This article summarizes leukotriene function and synthesis. It also reviews the published literature regarding potential therapeutic applications of leukotriene inhibition in both human and veterinary medicine, focusing primarily on osteoarthritis, NSAID induced gastrointestinal mucosal damage, allergic asthma, atopic dermatitis, and cancer.

Catalytic mechanism and substrate specificity of the beta-subunit of the voltage-gated potassium channel

The beta-subunits of voltage-gated potassium (Kv) channels are members of the aldo-keto reductase (AKR) superfamily. These proteins regulate inactivation and membrane localization of Kv1 and Kv4 channels. The Kvbeta proteins bind to pyridine nucleotides with high affinity; however, their catalytic properties remain unclear. Here we report that recombinant rat Kvbeta2 catalyzes the reduction of a wide range of aldehydes and ketones. The rate of catalysis was slower (0.06-0.2 min(-1)) than those of most other AKRs but displayed the expected hyperbolic dependence on substrate concentration, with no evidence of allosteric cooperativity. Catalysis was prevented by site-directed substitution of Tyr-90 with phenylalanine, indicating that the acid-base catalytic residue, identified in other AKRs, has a conserved function in Kvbeta2. The protein catalyzed the reduction of a broad range of carbonyls, including aromatic carbonyls, electrophilic aldehydes and prostaglandins, phospholipids, and sugar aldehydes. Little or no activity was detected with carbonyl steroids. Initial velocity profiles were consistent with an ordered bi-bi rapid equilibrium mechanism in which NADPH binding precedes carbonyl binding. Significant primary kinetic isotope effects (2.0-3.1) were observed under single- and multiple-turnover conditions, indicating that the bond-breaking chemical step is rate-limiting. Structure-activity relationships with a series of para-substituted benzaldehydes indicated that the electronic interactions predominate during substrate binding and that no significant charge develops during the transition state. These data strengthen the view that Kvbeta proteins are catalytically active AKRs that impart redox sensitivity to Kv channels.

Identification of a potent, selective, and orally active leukotriene a4 hydrolase inhibitor with anti-inflammatory activity

LTA 4H is a ubiquitously distributed 69 kDa zinc-containing cytosolic enzyme with both hydrolase and aminopeptidase activity. As a hydrolase, LTA 4H stereospecifically catalyzes the transformation of the unstable epoxide LTA 4 to the diol LTB 4, a potent chemoattractant and activator of neutrophils and a chemoattractant of eosinophils, macrophages, mast cells, and T cells. Inhibiting the formation of LTB 4 is expected to be beneficial in the treatment of inflammatory diseases such as inflammatory bowel disease (IBD), asthma, and atherosclerosis. We developed a pharmacophore model using a known inhibitor manually docked into the active site of LTA 4H to identify a subset of compounds for screening. From this work we identified a series of benzoxazole, benzthiazole, and benzimidazole inhibitors. SAR studies resulted in the identification of several potent inhibitors with an appropriate cross-reactivity profile and excellent PK/PD properties. Our efforts focused on further profiling JNJ 27265732, which showed encouraging efficacy in a disease model relevant to IBD.

Constitutional polymorphisms of prostate cancer: prognostic and diagnostic implications

Prostate cancer is the most common cancer diagnosis in men. While often perceived as a slow, indolent malignancy, prostate cancer trails only lung cancer among cancer-related mortality in men. Current diagnosis and treatment algorithms are plagued by overdiagnosis of non-lethal indolent prostate cancer with no proven means to predict, detect, and prevent aggressive lethal prostate cancer in men most at risk. These challenges are particularly concerning for African-American men who demonstrate increased rates of prostate cancer incidence and mortality when compared to other ethnic groups. With the completion of the human genome project, technology and techniques now exist to differentiate cancer from normal tissues based on the expression patterns of thousands of genes assessed simultaneously on a single microarray gene 'chip'. This platform has greatly improved our understanding of genes that regulate tumor behavior once cancer is established. Microarrays can also be utilized in patients without cancer to determine which patients are at high risk for tumor development and in need of rational prevention strategies. Constitutional single nucleotide polymorphisms (SNPs) are one source of genetic variation and may serve as a means to identify these high-risk individuals. SNPs are single nucleotide base pair changes within a gene which occur in one percent or more of the population. SNPs can contribute to a disease state by altering the function of a protein encoded by a gene without affecting gene expression. This review will examine the current understanding of constitutional SNPs associated with prostate cancer carcinogenesis, highlight two current diagnostic array platforms and discuss implications for future prevention and screening programs.

Impaired ex vivo leukotriene B4 production characterizes the metabolic syndrome and is improved after weight reduction

CONTEXT

Neutrophil (polymorphonuclear neutrophil) production of leukotriene B4 (LTB4) may be associated with alterations in immune and inflammatory function that characterize the metabolic syndrome (MetS).

OBJECTIVE

We investigated whether polymorphonuclear neutrophil production of LTB(4) and its metabolites 20-hydroxy-LTB4 (20-OH-LTB4) and 20-carboxyl-LTB4 were altered in subjects with features of the MetS before and after weight reduction.

DESIGN, SETTING, PATIENTS, AND INTERVENTION

In a case-controlled comparison, men and postmenopausal women with features of the MetS were matched with controls. Subjects with MetS were then matched and randomly assigned to either a 12-wk weight reduction study followed by 4-wk weight stabilization or 16-wk weight maintenance.

MAIN OUTCOME MEASURES

Measurements were performed at baseline and at the end of the 16-wk period. Stimulated neutrophil LTB4 and its metabolites were measured by HPLC.

RESULTS

In the case-controlled study, body mass index, waist circumference, blood pressure, fasting triglycerides, and glucose were all significantly increased in subjects with features of the MetS (P < 0.05). Production of LTB4 and 20-OH-LTB4 was significantly lower compared with controls (P < 0.005). The weight loss intervention resulted in a 4.6-kg reduction in body weight and 6.6-cm decrease in waist circumference relative to controls and a significant increase in LTB4 and 20-OH-LTB4.

CONCLUSIONS

Subjects with features of the MetS have lower stimulated LTB4, which is not due to increased metabolism of LTB4. Weight reduction restored the production of neutrophil LTB4, suggesting that in addition to modifying cardiovascular risk, weight loss may also help with the management of perturbed inflammatory responses in overweight subjects.

Inflammation resolved by retinoid X receptor-mediated inactivation of leukotriene signaling pathways

Leukotrienes are implicated in the pathogenesis of diverse, inflammation-driven diseases. Metabolic inactivation of leukotriene signaling is an innate response to resolve inflammation, yet little is known of mechanisms regulating disposition of leukotrienes in peripheral tissues afflicted in common inflammatory diseases. We studied leukotriene hydroxylases (CYP4F gene products) in human skin, a common target of inflammation and adverse drug reactions. Epidermal keratinocytes express at least six CYP4F enzymes; the most highly expressed and highly regulated is CYP4F3A-the main neutrophil leukotriene hydroxylase. Differentiation-specific factors and retinoids are positive CYP4F regulators in vitro, effecting increased leukotriene B4 hydroxylation (inactivation). CYP4F expression is up-regulated in situ in hyperproliferative dermatoses-an innate mechanism to repair and restore epidermal barrier competency-and after retinoid therapy. Enhanced CYP4F-mediated inactivation of leukotriene signaling is a previously unrecognized antiinflammatory property of therapeutic retinoids mediated by preferential interactions between retinoid X receptors and CYP4F promoter elements in epidermal cells.

Leukotriene B4 triggers the in vitro and in vivo release of potent antimicrobial agents

Leukotriene B(4) (LTB(4)) is a bioactive lipid derived from the metabolism of arachidonic acid. Mainly produced by polymorphonuclear leukocytes (PMN) and macrophages, LTB(4) triggers several functional responses important in host defense, including the secretion of lysosomal enzymes, the activation of NADPH oxidase activity, NO formation, and phagocytosis. We report that LTB(4), but not structural analogs thereof, stimulates primed human PMN to release molecules having potent antimicrobial activities. Exposure of bacteria (Escherichia coli and Staphylococcus aureus) or viruses (herpes simplex virus type 1 and HIV type 1) to supernatants of LTB(4)-activated PMN led to > or =90% reduction in infectivity. ELISA and mass spectroscopy analysis of proteins released from LTB(4)-activated PMN have identified several antimicrobial proteins, including alpha-defensins, cathepsin G, elastase, lysozyme C, and LL-37, that are likely to participate in the killing of microorganisms. In addition to these in vitro observations, i.v. injections of LTB(4) (50 microg/kg) to monkeys led to an increase in alpha-defensin plasmatic levels and enhanced ex vivo antimicrobial activities of plasma. These results demonstrate the ability of LTB(4) to cause the release of potent antimicrobial agents from PMN in vitro as well as in vivo and add further support to the important role of LTB(4) in host defense.

Biosynthesis and metabolism of leukotrienes

Leukotrienes are metabolites of arachidonic acid derived from the action of 5-LO (5-lipoxygenase). The immediate product of 5-LO is LTA4 (leukotriene A4), which is enzymatically converted into either LTB4 (leukotriene B4) by LTA4 hydrolase or LTC4 (leukotriene C4) by LTC4 synthase. The regulation of leukotriene production occurs at various levels, including expression of 5-LO, translocation of 5-LO to the perinuclear region and phosphorylation to either enhance or inhibit the activity of 5-LO. Several other proteins, including cPLA2α (cytosolic phospholipase A2α) and FLAP (5-LO-activating protein) also assemble at the perinuclear region before production of LTA4. LTC4 synthase is an integral membrane protein that is present at the nuclear envelope; however, LTA4 hydrolase remains cytosolic. Biologically active LTB4 is metabolized by ω-oxidation carried out by specific cytochrome P450s (CYP4F) followed by β-oxidation from the ω-carboxy position and after CoA ester formation. Other specific pathways of leukotriene metabolism include the 12-hydroxydehydrogenase/15-oxo-prostaglandin-13-reductase that forms a series of conjugated diene metabolites that have been observed to be excreted into human urine. Metabolism of LTC4 occurs by sequential peptide cleavage reactions involving a γ-glutamyl transpeptidase that forms LTD4 (leukotriene D4) and a membrane-bound dipeptidase that converts LTD4 into LTE4 (leukotriene E4) before ω-oxidation. These metabolic transformations of the primary leukotrienes are critical for termination of their biological activity, and defects in expression of participating enzymes may be involved in specific genetic disease.

Identification of the Intracellular Region of the Leukotriene B4 Receptor Type 1 That Is Specifically Involved in Gi Activation

Many G-protein-coupled receptors can activate more than one G-protein subfamily member. Leukotriene B4 receptor type 1 (BLT1) is a high affinity G-protein-coupled receptors for leukotriene B4 functioning in host defense, inflammation, and immunity. Previous studies have shown that BLT1 utilizes different G-proteins (the Gi family and G16 G-proteins) in mediating diverse cellular events and that truncation of the cytoplasmic tail of BLT1 does not impair activation of Gi and G16 proteins. To determine responsive regions of BLT1 for G-protein coupling, we performed an extensive mutagenesis study of its intracellular loops. Three intracellular loops (i1, i2, and i3) of BLT1 were found to be important for both Gi and G16 coupling, as judged by Gi-dependent guanosine 5'-(gamma-thio) triphosphate (GTPgammaS) binding and G16-dependent inositol phosphate accumulation assays. The i3-1 mutant, with a mutation at the i3 amino terminus, exhibited greatly reduced GTPgammaS binding but intact inositol phosphate accumulation triggered by leukotriene B4 stimulation. These results suggest that the i3-1 region is required only for Gi activation. Moreover, in the i3-1 mutant, the deficiency in Gi activation was accompanied by a loss of the high affinity leukotriene B4 binding state seen with the wild type receptor. A three-dimensional model of BLT1 constructed based on the structure of bovine rhodopsin suggests that the i3-1 region may consist of the cytoplasmic end of the transmembrane helix V, which protrudes the helix into the cytoplasm. From mutational studies and three-dimensional modeling, we propose that the extended cytoplasmic helix connected to the transmembrane helix V of BLT1 might be a key region for selective activation of Gi proteins.

Time-dependent expression of leukotriene B4 receptors in rat collagen-induced arthritis

Leukotriene B4 acts through its receptors, BLT(1) and BLT(2), however, their expression in rheumatoid arthritis is unknown. In this experiment, BLT(1) and BLT(2) mRNA expressions in the synovium of rats with collagen-induced arthritis (CIA) at days 1, 3, 7 and 14 after CIA onset were analyzed by RT-PCR. The expression of two immunological and inflammatory factors, S100A8 and S100A9, in the synovium of the arthritic rats was also determined at the indicated time. At d14, the differential expressions of BLT(1) and BLT(2) in the synovium, spleen, peripheral blood mononuclear cells (PBMC) and thymus of CIA rats were analyzed. The results showed that, in the synovium of the arthritic rats, the BLT(1) mRNA expression increased after CIA onset, reached the highest value between d1 and d3, and declined afterwards while the BLT(2) expression increased with time and reached its peak at d14. Both S100A8 and S100A9 expression reached the peak levels between d1 and d3, and decreased to lower levels between d7 and d14. For the analyzed tissues from CIA rats at d14, BLT(1) mRNA was expressed in the thymus with the highest level, followed by the spleen, PBMC and synovium. BLT(2) mRNA was expressed in the thymus the highest as well, but followed by the synovium, spleen and PBMC. Since BLT(1) and BLT(2) play distinct roles during CIA, this study may provide basis for new therapies targeting BLT(1) and BLT(2), respectively, for the treatment of arthritic inflammation at different stages.

Leukotriene B4Receptors BLT1 and BLT2: Expression and Function in Human and Murine Mast Cells

Leukotriene B4 (LTB4) is a potent activator and chemoattractant for leukocytes and is implicated in several inflammatory diseases. The actions of LTB4 are mediated by two cell surface receptors, BLT1, which is predominantly expressed in peripheral blood leukocytes, and BLT2, which is expressed more ubiquitously. Recently, BLT1 expression and LTB4-dependent chemotaxis have been reported in immature mast cells (MCs). We now show the first evidence for BLT2 mRNA expression, in addition to BLT1, in murine bone marrow-derived MCs (mBMMCs) and in a human MC line (HMC-1). Protein expression of BLT1 was confirmed by mAb staining in HMC-1 cells and shown to be predominantly intracellular. Both HMC-1 cells and mBMMCs migrated to LTB4 in a dose-dependent manner in chemotaxis assays. Migration to LTB4 could be inhibited by either a BLT1- or BLT2-selective antagonist. Significant dose-dependent migration of mBMMCs also was observed to 12-(S)-hydroxyeicosotetraenoic acid, a BLT2-selective agonist, demonstrating functional BLT2 activity in these cells. Stimulation of mBMMCs with LTB4 induced transient, dose-dependent, ERK phosphorylation and changes in Akt phosphorylation. Dose-dependent ERK phosphorylation also was observed in response to 12-(S)-hydroxyeicosotetraenoic acid, indicating signaling downstream of BLT2. Pretreatment of mBMMCs with stem cell factor significantly down-regulated expression of BLT1 and BLT2 mRNA and inhibited their migration to LTB4. This study demonstrates expression of functional LTB4 receptors, both BLT1 and BLT2, in murine and human MCs and a regulatory role for stem cell factor in their expression. These receptors may mediate recruitment and accumulation of MCs in response to LTB4 production in areas of inflammation.

Leukotriene receptors in atherosclerosis

Leukotriene-forming enzymes are expressed within atherosclerotic lesions and locally produced leukotrienes exert pro-inflammatory actions within the vascular wall by means of cell surface receptors of the BLT and CysLT receptor subtypes. The migration and accumulation of inflammatory cells that follow leukotriene receptor activation have been implicated in atherosclerosis initiation and progression. Leukotriene receptors are in addition expressed on endothelial and vascular smooth muscle cells, associated with intimal hyperplasia in early atherosclerosis and restenotic lesions after angioplasty. Taken together, recent evidence suggests that leukotriene receptors may be a potential target in the treatment of atherosclerosis and in the prevention of restenosis after coronary interventions.

Leukotriene B4 receptor and the function of its helix 8

More than 30 lipid ligands, which express their biological activities through cognate G-protein-coupled receptors (GPCRs), have been reported. Among them, leukotriene B(4) (LTB(4)) is a potent lipid mediator involved in host defense, inflammation, and the immune responses. Two GPCRs for LTB(4) (BLT1 and BLT2) have been cloned and analyzed. Recent studies using genetically engineered mice suggest that BLT1 plays an important role in several inflammatory diseases including ischemic reperfusion tissue injury, atherosclerosis, and bronchial asthma. BLT1 is also a good tool to study the molecular mechanism of GPCR activation and inactivation in vitro. In this brief review, we focus on the biological and biochemical properties of BLT1 with special attention to the putative helix 8 of the receptor.

Characterization of a Mouse Second Leukotriene B4 Receptor, mBLT2

Leukotriene B4 (LTB4) is a potent chemoattractant and activator for granulocytes and macrophages and is considered to be an inflammatory mediator. Two G-protein-coupled receptors for LTB4, BLT1 and BLT2, have been cloned from human and shown to be high and low affinity LTB4 receptors, respectively. To reveal the biological roles of BLT2 using mouse disease models, we cloned and characterized mouse BLT2. Chinese hamster ovary cells stably expressing mouse BLT2 exhibited specific binding to LTB4, LTB4-induced calcium mobilization, inhibition of adenylyl cyclase, and phosphorylation of extracellular signal-regulated kinase. We found that Compound A (4'-{[pentanoyl (phenyl) amino] methyl}-1, 1'-biphenyl-2-carboxylic acid) was a BLT2-selective agonist and induced Ca(2+) mobilization and phosphorylation of extracellular signal-regulated kinase through BLT2, whereas it had no effect on BLT1. 12-epi LTB4 exhibited a partial agonistic activity against mBLT1 and mBLT2, whereas 6-trans-12-epi LTB4 did not. Northern blot analysis showed that mouse BLT2 is expressed highly in small intestine and skin in contrast to the ubiquitous expression of human BLT2. By in situ hybridization and the reverse transcriptase polymerase chain reaction, we demonstrated that mouse BLT2 is expressed in follicular and interfollicular keratinocytes. Compound A, LTB4, and 12-epi LTB4 all induced phosphorylation of extracellular signal-regulated kinase in primary mouse keratinocytes. Furthermore, Compound A and LTB4 induced chemotaxis in primary mouse keratinocytes. These data suggest the presence of functional BLT2 in primary keratinocytes.

Fluorescent leukotriene B4: potential applications

Leukotriene B4 (LTB4) is a potent lipid mediator of inflammation that acts primarily via a seven-transmembrane-spanning, G-protein-coupled receptor denoted BLT1. Here, we describe the synthesis and characterization of fluorescent analogs of LTB4 that are easy to produce, inexpensive, and without the disadvantages of a radioligand. Fluorescent LTB4 is useful for labeling LTB4 receptors for which no antibodies are available and for performing one-step fluorescence polarization assays conducive to high-throughput screening. We found that orange and green fluorescent LTB4 were full agonists that activated the LTB4 receptor BLT1 with EC50 values of 68 and 40 nM, respectively (4.5 nM for unmodified LTB4). Flow cytometric measurements and confocal imaging showed that fluorescent LTB4 colocalized with BLT1. Fluorescence polarization measurements showed that orange fluorescent LTB4 bound to BLT1 with a Kd of 66 nM and that this binding could be displaced by unlabeled LTB4 and other BLT1-specific ligands. Fluorescent LTB4 analogs were also able to displace tritiated LTB4. Orange fluorescent LTB4 binding to enhanced green fluorescent protein-tagged BLT1 could be observed using fluorescence resonance energy transfer. In addition to being a useful alternative to radiolabeled LTB4, the unique properties of fluorescently labeled LTB4 allow a variety of detection technologies to be used.

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.

Leukotrienes and atherosclerosis: new roles for old mediators

Lipid mediators generated from arachidonic acid through the action of 5-lipoxygenase have been known for over two decades and are implicated in a wide variety of inflammatory disorders. G-protein-coupled receptors mediate the effects of different leukotrienes in distinct cell types. Novel cellular and molecular targets were recently discovered for these mediators, with important consequences for the function of both adaptive and innate immune systems. These studies have outlined crucial new roles for leukotrienes in the recruitment of T lymphocytes and in the development of atherosclerotic lesions, suggesting novel mechanisms for their actions. Through the development of appropriate animal models, leukotrienes are becoming renewed targets for treatment of many inflammatory diseases including atherosclerosis.

Exploring the pharmacology of the leukotriene B4 receptor BLT1, without the confounding effects of BLT2

Most previous studies of leukotriene B4 (LTB4) pharmacology using primary leukocyte cultures and myeloid cell lines do not differentiate between leukotriene BLT1 and BLT2 receptor activation because both receptors are often expressed by these cells. Here we show that in HeLa cells expressing BLT1 but not BLT2 receptors, BLT1 receptor activation resulted in IP3 mediated calcium release from intracellular stores initially, followed by calcium influx through cell membrane channels. BLT1 calcium signalling was sensitive to the activity of protein kinase C (PKC), protein kinase A (PKA) and protein-tyrosine kinases (PTKs), as well as changes in membrane cholesterol levels and treatments that are known to disrupt normal membrane physiology and/or lipid rafts. Inhibition of MAP kinases, Rho-associated kinases, or phosphoinositol-3-kinases (PI3K) had no effect on BLT1 receptor induced calcium signalling, and the receptor was insensitive to the redox state of the extracellular compartment.

The contractile action of leukotriene B4 in the guinea-pig lung involves a vascular component

Leukotriene B4 (LTB4) is a potent leukocyte chemoattractant, acting on specific receptors, BLT receptors. The aim of this study was to examine the mechanism of action of LTB4 in the guinea-pig lung, using strips of lung parenchyma (GPLP), spirals of trachea (GPT) and bronchus (GPB) and rings of pulmonary artery (GPPA). Mechanical responses were studied in organ baths, and mediator release was assessed using enzyme immuno assay. LTB4 induced similar contractions of GPLP and GPPA, whereas LTB4 had only small contractile effects in GPT and GPB. In addition, the contractile response to LTB4 was reproduced in the human pulmonary artery. In the GPLP, the unselective BLT receptor antagonist ONO-4057 abolished the contractions induced by LTB4, whereas the selective BLT1 receptor antagonist U-75302 only partly inhibited the LTB4-induced contractions. In the GPPA, both antagonists abolished the response to LTB4. The effect of LTB4 in GPPA and GPLP was indirect and mediated by the release of thromboxane A2 and histamine, as supported by selective pharmacologic interventions and measurements of thromboxane B2 and histamine in the organ baths. In conclusion, the results indicate a new biological function of LTB4, namely to constrict isolated pulmonary arteries. Moreover, the findings suggest that the LTB4-induced contractions of GPPA were mediated by a BLT1 receptor, whereas BLT2 receptor activation accounted for a major part of the contraction of GPLP, making the latter preparation a suitable assay for BLT2 receptors. British Journal of Pharmacology (2004) 141, 449-456. doi:10.1038/sj.bjp.0705641

Flow cytometric mapping of the leukotriene B4 receptor, BLT1, in human bone marrow and peripheral blood using specific monoclonal antibodies

We have previously raised two monoclonal antibodies (7B1, 14F11) recognizing the high-affinity leukotriene B4 receptor, BLT1. They were presently used to determine receptor surface expression in the hematopoietic system. In peripheral blood, BLT1 was primarily recognized in granulocytes, monocytes and, to a lower extent, in certain lymphocytes except the CD4 subpopulation. The expression pattern was similar in bone marrow cells. In vitro differentiation of CD34+ progenitor cells induced BLT1 expression within 7 days, which remained constant up to day 17 when a further increase was measured and maintained up to day 20. BLT1 expression was modified by inflammatory mediators: LPS, TNFalpha, fMLP, as well as LTB4 itself, caused a slight down-regulation at 30 min, an effect that was particularly marked with PMA, whereas the effect was least pronounced with IL-8. The antibodies have proved to be useful in an extensive mapping of BLT1 in both peripheral blood and bone marrow and as a tool to elucidate changes in the receptor expression.

Structure-based analysis of GPCR function: conformational adaptation of both agonist and receptor upon leukotriene B4 binding to recombinant BLT1

We produced the human leukotriene B(4) (LTB(4)) receptor BLT1, a G-protein-coupled receptor, in Escherichia coli with yields that are sufficient for the first structural characterization of this receptor in solution. Overexpression was achieved through codon optimization and the search for optimal refolding conditions of BLT1 recovered from inclusion bodies. The detergent-solubilized receptor displays a 3D-fold compatible with a seven transmembrane (TM) domain with ca 50% alpha-helix and an essential disulfide bridge (circular dichroism evidence); it binds LTB(4) with K(a)=7.8(+/-0.2)x10(8)M(-1) and a stoichiometric ratio of 0.98(+/-0.02). Antagonistic effects were investigated using a synthetic molecule that shares common structural features with LTB(4). We report evidence that both partners, LTB(4) and BLT1, undergo a rearrangement of their respective conformations upon complex formation: (i) a departure from planarity of the LTB(4) conjugated triene moiety; (ii) a change in the environment of Trp234 (TM-VI helix) and in the exposure of the cytoplasmic region of this transmembrane helix.

Leukotriene B4 omega-side chain hydroxylation by CYP4F5 and CYP4F6

Leukotriene B(4) (LTB(4)) is a lipid mediator that plays an important role in inflammation. Metabolism of LTB(4) by cytochrome P450 (CYP) enzymes belonging to the CYP4F subfamily is considered to be of importance for the regulation of inflammation. This study investigates LTB(4) metabolism by recombinant rat CYP4F5 and CYP4F6 expressed in a yeast system and by microsomes isolated from rat organs expressing CYP4F mRNA. CYP4F6 was found to convert LTB(4) into 19-hydoxy- and 18-hydroxy-LTB(4) with an apparent K(m) of 26 microM, and CYP4F5 was found to convert LTB(4) primarily into 18-hydroxy-LTB(4) with an apparent K(m) of 9.7 microM. The rate of formation of 18-hydroxy-LTB(4) by CYP4F5 was surprisingly high. At a substrate concentration of 30 microM, the rate of formation was about 15 nmol/min/mg microsomal protein, approximately 30 times faster than the reaction catalyzed by CYP4F6. Analysis of LTB(4) metabolism by microsomes isolated from various tissues from the rat suggests that CYP4F5 and CYP4F6 are active in the lung and to some extent in the brain, kidney, and testis. CYP4F5 and CYP4F6, due to their capacities to metabolize LTB(4), may play important roles in modulating inflammatory response in these organs.

International Union of Pharmacology XXXVII. Nomenclature for leukotriene and lipoxin receptors

The leukotrienes and lipoxins are biologically active metabolites derived from arachidonic acid. Their diverse and potent actions are associated with specific receptors. Recent molecular techniques have established the nucleotide and amino acid sequences and confirmed the evidence that suggested the existence of different G-protein-coupled receptors for these lipid mediators. The nomenclature for these receptors has now been established for the leukotrienes. BLT receptors are activated by leukotriene B(4) and related hydroxyacids and this class of receptors can be subdivided into BLT(1) and BLT(2). The cysteinyl-leukotrienes (LT) activate another group called CysLT receptors, which are referred to as CysLT(1) and CysLT(2). A provisional nomenclature for the lipoxin receptor has also been proposed. LXA(4) and LXB(4) activate the ALX receptor and LXB(4) may also activate another putative receptor. However this latter receptor has not been cloned. The aim of this review is to provide the molecular evidence as well as the properties and significance of the leukotriene and lipoxin receptors, which has lead to the present nomenclature.

Leukotriene B4 receptors

Leukotriene B4 (LTB4) is a potent chemotactic agent and activating factor for granulocytes. Two cell surface receptors for LTB4 (BLT1 and BLT2) have been isolated in the last few years. These receptors are G-protein-coupled receptors (GPCR), and they have 45% amino acid identity. BLT1 and BLT2 are high- and low- affinity receptors, respectively. Cells transfected with BLT1 and BLT2 show LTB4-dependent intracellular signal transduction and chemotaxis in vitro. The distribution and pharmacological characteristics of BLT1 and BLT2 are different, suggesting distinct roles for these receptors in vivo. The open reading frame (ORF) of BLT2 overlaps the promoter of BLT1, a so called 'promoter in ORF'. Based on recent publications on BLT1 transgenic and knock out mice phenotypes, it appears that LTB4 plays important roles in inflammation in addition to host defense in vivo.