Cloning and characterization of rat leukotriene B(4) receptor

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.

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.

Chemical Tools for Studying Lipid-Binding Class A G Protein-Coupled Receptors

Cannabinoid, free fatty acid, lysophosphatidic acid, sphingosine 1-phosphate, prostanoid, leukotriene, bile acid, and platelet-activating factor receptor families are class A G protein-coupled receptors with endogenous lipid ligands. Pharmacological tools are crucial for studying these receptors and addressing the many unanswered questions surrounding expression of these receptors in normal and diseased tissues. An inherent challenge for developing tools for these lipid receptors is balancing the often lipophilic requirements of the receptor-binding pharmacophore with favorable physicochemical properties to optimize highly specific binding. In this study, we review the radioligands, fluorescent ligands, covalent ligands, and antibodies that have been used to study these lipid-binding receptors. For each tool type, the characteristics and design rationale along with in vitro and in vivo applications are detailed.

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.

The role of leukotrienes in allergic diseases

Leukotrienes (LTs), both LTB4 and the cysteinyl LTs (CysLTs) LTC4, LTD4 and LTE4, are implicated in a wide variety of inflammatory disorders. These lipid mediators are generated from arachidonic acid via multistep enzymatic reactions through which arachidonic acid is liberated from membrane phospholipids through the action of phospholipase A2. LTB4 and CysLTs exert their biological effects by binding to cognate receptors, which belong to the G protein-coupled receptor superfamily. LTB4 is widely considered to be a potent chemoattractant for most subsets of leukocytes, whereas CysLTs are potent bronchoconstrictors that have effects on airway remodeling. LTs play a central role in the pathogenesis of asthma and many other inflammatory diseases. This review will provide an update on the synthesis, biological function, and relevance of LTs to the pathobiology of allergic diseases, and examine the current and future therapeutic prospects of LT modifiers.

Commonly used leukotriene B4 receptor antagonists possess intrinsic activity as agonists in human endothelial cells: Effects on calcium transients, adhesive events and mediator release

Leukotriene B4 (LTB4), a potent chemotactic and immune-modulating lipid mediator, signals via two receptors, BLT1 and BLT2, leading to pro-inflammatory responses in phagocytes. Recently, we reported that BLT1 is the predominating BLT on human umbilical vein endothelial cells (HUVEC) and transmits a variety of functional responses. Here, we demonstrate that, in HUVEC, two BLT1 antagonists (U75302, CP105696) and one BLT2 antagonist (LY255283) possess intrinsic but varying agonist activity for adhesion of neutrophils, up-regulation of E-selectin, ICAM-1 and VCAM-1, and release of MCP-1. These effects were observed after exposure of HUVEC for the drugs for 0.25-6h, persisted for several hours, and were less potent in magnitude as those elicited by LPS. Our findings may have consequences for interpretation of in vitro BLT blockade experiments.

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.

Study on correlation of signal molecule genes and their receptor-associated genes with rat liver regeneration

To investigate the effect of signal molecules and their receptor-associated genes on rat liver regeneration (LR) at the transcriptional level, the associated genes were originally obtained by retrieving the databases and related scientific publications; their expression profiles in rat LR were then checked using the Rat Genome 230 2.0 microarray. The LR-associated genes were identified by comparing gene expression difference between partial hepatectomy groups and operation-control groups. A total of 454 genes were proved to be LR related. The genes associated with the seven kinds of signal molecules (steroid hormones, fatty acid derivatives, protein and polypeptide hormones, amino acids and their derivatives, choline, cytokines, and gas signal molecules) were detected to be enriched in a cluster characterized by upregulated expression in LR. The number of genes related to the seven kinds of signal molecules was, in sequence, 63, 27, 100, 102, 16, 166, and 18. The 1027 frequencies of upregulation and 823 frequencies of downregulation in total as well as 42 types of different expression patterns suggest the complex and diverse gene expression changes in LR. It is presumed that signal molecules played an important role in metabolism, inflammation, cell proliferation, growth and differentiation, etc., during rat LR.

Expression patterns and action analysis of genes associated with inflammatory responses during rat liver regeneration

AIM: To study the relationship between inflammatory response and liver regeneration (LR) at transcriptional level.

METHODS: After partial hepatectomy (PH) of rats, the genes associated with inflammatory response were obtained according to the databases, and the gene expression changes during LR were checked by the Rat Genome 230 2.0 array.

RESULTS: Two hundred and thirty-nine genes were associated with liver regeneration. The initial and total expressing gene numbers found in initiation phase (0.5-4 h after PH), G₀/G₁ transition (4-6 h after PH), cell proliferation (6-66 h after PH), cell differentiation and structure-function reconstruction (66-168 h after PH) of liver regeneration were 107, 34, 126, 6 and 107, 92, 233, 145 respectively, showing that the associated genes were mainly triggered at the beginning of liver regeneration, and worked at different phases. According to their expression similarity, these genes were classified into 5 groups: only up-regulated, predominantly up-, only down-, predominantly down-, up- and down-, involving 92, 25, 77, 14 and 31 genes, respectively. The total times of their up- and down-regulated expression were 975 and 494, respectively, demonstrating that the expressions of the majority of genes were increased, and that of a few genes were decreased. Their time relevance was classified into 13 groups, showing that the cellular physiological and biochemical activities were staggered during liver regeneration. According to gene expression patterns, they were classified into 33 types, suggesting that the activities were diverse and complex during liver regeneration.

CONCLUSION: Inflammatory response is closely associated with liver regeneration, in which 239 LR-associated genes play an important role.

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.

Differential induction of BLT receptor expression on human endothelial cells by lipopolysaccharide, cytokines, and leukotriene B4

Leukotriene (LT) B4 is a powerful chemotactic and immune modulating agent that signals via two receptors denoted BLT1 and BLT2. Here we report that BLT1 and BLT2 are expressed at low levels in an apparently silent state in human umbilical vein endothelial cells (HUVEC). However, treatment with LPS leads to a >10 fold increase in the levels of BLT1 mRNA without any significant effects on BLT2 mRNA. In parallel, LPS also increases the amounts of BLT1 protein. Tumor necrosis factor-alpha (TNF-alpha) increases the expression of BLT2 mRNA approximately 6 times above basal levels with only a modest increase in BLT1 mRNA. Interleukin-1beta causes variable and parallel increases of both BLT1 and BLT2 mRNA. The natural ligand LTB4 also increases BLT1, but not BLT2, mRNA and protein expression. Along with the induction of BLT1 and/or BLT2, HUVEC acquire the capacity to respond to LTB4 with increased levels of intracellular calcium and these signals can be blocked by isotype selective BLT antagonists, CP-105696 and LY-255283. In addition, treatment of HUVEC with LTB4 causes increased release of both nitrite, presumably reflecting nitric oxide (NO), and monocyte chemoattractant protein-1. Our data indicate that expression of functional BLT receptors may occur at the surface of endothelial cells in response to LPS, cytokines, and ligand, which in turn may have functional consequences during the early vascular responses to inflammation. Moreover, the results point to BLT receptors as potential targets for pharmacological intervention in LT-dependent inflammatory diseases such as asthma, rheumatoid arthritis, and arteriosclerosis.

Involvement of BLT1 Endocytosis and Yes Kinase Activation in Leukotriene B4-Induced Neutrophil Degranulation

One of the important biological activities of human neutrophils is degranulation, which can be induced by leukotriene B4 (LTB4). Here we investigated the intracellular signaling events involved in neutrophil degranulation mediated by the high affinity LTB4 receptor, BLT1. Peripheral blood neutrophils as well as the promyeloid PLB-985 cell line, stably transfected with BLT1 cDNA and differentiated into a neutrophil-like cell phenotype, were used throughout this study. LTB4-induced enzyme release was inhibited by 50-80% when cells were pretreated with the pharmacological inhibitors of endocytosis sucrose, Con A and NH4Cl. In addition, transient transfection with a dominant negative form of dynamin (K44A) resulted in approximately 70% inhibition of ligand-induced degranulation. Pretreating neutrophils or BLT1-expressing PLB-985 cells with the Src family kinase inhibitor PP1 resulted in a 30-60% inhibition in BLT1-mediated degranulation. Yes kinase, but not c-Src, Fgr, Hck, or Lyn, was found to exhibit up-regulated kinase activity after LTB4 stimulation. Moreover, BLT1 endocytosis was found to be necessary for Yes kinase activation in neutrophils. LTB4-induced degranulation was also sensitive to inhibition of PI3K. In contrast, it was not affected by inhibition of the mitogen-activated protein kinase MEK kinase, the Janus kinases, or the receptor tyrosine kinase epidermal growth factor receptor or platelet-derived growth factor receptor. Taken together, our results suggest an essential role for BLT1 endocytosis and Yes kinase activation in LTB4-mediated degranulation of human neutrophils.

Role of Leukotriene B 4 Receptors in the Development of Atherosclerosis: Potential Mechanisms

Objective— Leukotriene B 4 (LTB 4 ), a potent leukocyte chemoattractant, is known to promote several inflammatory diseases, including atherosclerosis. We sought to determine mechanisms through which LTB 4 modulates atherosclerosis in cell lines expressing LTB 4 receptors, BLT-1, and in mice deficient in BLT-1 as well as macrophage cell lines derived from BLT-1 +/+ and BLT-1 −/− mice.

Methods and Results— Analysis of global changes in gene expression induced by LTB 4 in rat basophilic leukemia cells (RBL-2H3) expressing the human BLT-1 showed highest-fold increase in expression of fatty acid translocase/CD36 and the chemokine MCP1/JE/CCL2 , which are critical in atherogenesis. To determine the importance of BLT-1 in atherogenesis, we crossed BLT-1-null mice with apolipoprotein (apo)-E-deficient mice, which develop severe atherosclerosis. Deletion of BLT-1 significantly reduced the lesion formation in apo-E −/− mice only during initiating stages (4 and 8 weeks) but had no effect on the lesion size in mice fed atherogenic diet for 19 weeks. Macrophage cell lines from BLT-1-deficient mice expressed the low-affinity LTB 4 receptor, BLT-2, and exhibited chemotaxis to LTB 4 .

Conclusions— The effects of LTB 4 in atherosclerosis are likely mediated through the high-affinity BLT-1 and the low-affinity BLT-2 receptors. LTB 4 promotes atherosclerosis by chemo-attracting monocytes, by providing an amplification loop of monocyte chemotaxis via CCL2 production, and by converting monocytes to foam cells by enhanced expression of CD36 and fatty acid accumulation.

Helix 8 of the leukotriene B4 receptor is required for the conformational change to the low affinity state after G-protein activation

Recent studies have revealed that G-protein-coupled receptors contain a putative cytoplasmic helical domain, helix 8. Leukotriene B4 (LTB4) receptor 1 derivatives with truncated or mutated helix 8 showed much higher LTB4 binding than wild-type (WT) receptors. Similar to the WT receptor, LTB4 promoted guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding in these mutants. Unlike the WT receptor, however, the addition of GTPgammaS did not inhibit LTB4 binding to the mutant receptors. Scatchard analyses revealed that mutants maintained high affinity for LTB4, even in the presence of excess GTPgammaS. Consistently, mutant receptors showed a more prolonged Ca2+ mobilization and cellular metabolic activation than the WT receptor. From mutational studies and three-dimensional modeling based on the structure of bovine rhodopsin, we conclude that the helix 8 of LTB4 receptor 1 plays an important role in the conformational change of the receptor to the low affinity state after G-protein activation, possibly by sensing the status of coupling Galpha subunits as GTP-bound.

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.

Implication of the small GTPase Rac1 in the generation of reactive oxygen species in response to beta-amyloid in C6 astroglioma cells

Exogenous application of beta-amyloid (Abeta(25-35), a fragment of Abeta(1-42)) significantly elevated levels of reactive oxygen species (ROS) in C6 astroglioma cells, as measured by confocal microscopic analysis of H(2)O(2)-sensitive 2',7'-dichlorofluorescin fluorescence. Subsequent characterization of the signalling pathway revealed that expression of RacN17, a dominant-negative Rac1 mutant, completely blocked Abeta(25-35)-induced generation of ROS, which is indicative of the crucial role played by Rac GTPase in this process. To better understand the downstream mediators affected by Rac, we assessed the degree to which inhibition of cytosolic phospholipase A(2) (cPLA(2)) and 5-lipoxygenase (5-LO) contributed to the response and found that inhibition of either enzyme completely blocked Abeta(25-35)-induced ROS generation, indicating its dependence on arachidonic acid synthesis and metabolism to leukotrienes (e.g. leukotriene B(4)). Consistent with those findings, Abeta(25-35) Rac-dependently stimulated translocation of 5-LO to the nuclear envelope and increased intracellular levels of leukotriene B(4), while exogenous application of leukotriene B(4) increased intracellular H(2)O(2) via BLT, its cell-surface receptor. In addition to the aforementioned downstream mediators, inhibition of phosphoinositide 3-kinase (PI 3-kinase), an enzyme situated upstream of Rac, also completely blocked Abeta(25-35)-induced H(2)O(2) generation. Our findings thus demonstrate that PI 3-kinase, Rac, cPLA(2) and 5-LO are all essential components of the beta-amyloid signaling cascade leading to generation of ROS.

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.

Dietary fish oil protects against lung and liver inflammation and fibrosis in monocrotaline treated rats

The purpose of the present study was to evaluate the effectiveness of fish oil in preventing tissue pathologies associated with monocrotaline (MCT) toxicity. Twenty-four weanling rats were randomly assigned to one of two groups: (1) 12 to a group fed a diet containing 15% (w/w) corn oil (control) and (2) 12 to a group fed a diet containing fish oil (13%) and corn oil (2%) as the source of fat. Rats were fed for 4 weeks prior to MCT treatment. Six rats in each group were subcutaneously injected with MCT and six injected with its vehicle (water) and all were continued on their respective diets. All rats were sacrificed 3 weeks after injection. In rats receiving MCT, we observed severe interstitial pneumonia, septal fibrosis, vasculitis with virtual obliteration of the lumen of the small arteries and arterioles, right ventricular hypertrophy (RVH), and hepatomegaly and hepatocyte vacuole formation. Dietary fish oil significantly reduced septal fibrosis and development of pneumonia. There was a slight, but statistically insignificant decrease in vasculitis and fish oil did not prevent RVH (pulmonary hypertension). In addition, fish oil effectively protected the MCT-treated rats from development of hepatocyte vacuoles (steatosis), hepatic inflammation and vasculitis, increased presence of fibroblasts and collagen deposition in the centrilobular and, to a lesser extent, in the periportal spaces. These results suggest that lung parenchymal inflammation can be attenuated without altering the course of development of pulmonary hypertension in the MCT model. These results also indicate that fish oil protects against inflammation and fibrosis in the lung and liver, and against hepatocyte vacuole formation in MCT-treated rats.

Leukotriene B(4) stimulates Rac-ERK cascade to generate reactive oxygen species that mediates chemotaxis

Leukotriene B(4) is a potent chemoattractant known to be involved mainly in inflammation, immune responses, and host defense against infection, although the exact signaling mechanisms by which it exerts its effects are not well understood. Here we show that exogenous leukotriene B(4) induces reactive oxygen species (ROS) generation via a Rac-dependent pathway, and that stable expression of Rac(N17), a dominant negative Rac1 mutant, completely blocks leukotriene B(4)-induced ROS generation. In addition, leukotriene B(4)-induced ROS generation is selectively blocked by inhibition of ERK or cytosolic phospholipase A(2), but not p38 kinase, which is indicative of its dependence on ERK activation and synthesis of arachidonic acid. Consistent with those findings, leukotriene B(4) Rac-dependently stimulates ERK and cytosolic phospholipase A(2) activity, and transient transfection with plasmid expressing Rac(V12), a constitutively activated Rac1 mutant, also dose-dependently stimulates ERK activity. Our findings suggest that ERK and cytosolic phospholipase A(2) are situated downstream of Rac, and we conclude that Rac, ERK, and cytosolic phospholipase A(2) all play pivotal roles in mediating the ROS generation that appears to be a prerequisite for leukotriene B(4)-induced chemotaxis and cell proliferation.

Leukotriene B4: metabolism and signal transduction

Leukotriene B4 (LTB4) is known as one of the most potent chemoattractants and activators of leukocytes and is involved in inflammatory diseases. Enzymes involved in the biosynthesis and metabolism of LTB4 have been cloned, and their properties are well understood. Two G-protein-coupled receptors (BLT1 and BLT2) have been cloned and characterized. BLT1 and BLT2 are high- and low-affinity LTB4 receptors, respectively, and form a gene cluster in human and mouse. In this article recent findings on the metabolism of and the receptors for LTB4 are reviewed. We also discuss briefly a coreceptor role of BLT in HIV infection, and ion channel modification by LTB4.

Hydroxyeicosanoids bind to and activate the low affinity leukotriene B4 receptor, BLT2

Leukotriene B(4), an arachidonate metabolite, is a potent chemoattractant of leukocytes involved in various inflammatory diseases. Two G-protein-coupled receptors for leukotriene B(4) have been cloned and characterized. BLT1 (Yokomizo, T., Izumi, T., Chang, K., Takuwa, Y., and Shimizu, T. (1997) Nature 387, 620-624) is a high affinity receptor exclusively expressed in leukocytes, and BLT2 (Yokomizo, T., Kato, K., Terawaki, K., Izumi, T., and Shimizu, T. (2000) J. Exp. Med. 192, 421-432) is a low affinity receptor expressed more ubiquitously. Here we report the binding profiles of various BLT antagonists and eicosanoids to either BLT1 or BLT2 using the membrane fractions of Chinese hamster ovary cells stably expressing the receptor. BLT antagonists are grouped into three classes: BLT1-specific U-75302, BLT2-specific LY255283, and BLT1/BLT2 dual-specific ZK 158252 and CP 195543. We also show that 12(S)-hydroxyeicosatetraenoic acid, 12(S)-hydroperxyeicosatetraenoic acid, and 15(S)-hydroxyeicosatetraenoic acid competed with [(3)H]LTB(4) binding to BLT2, but not BLT1, dose dependently. These eicosanoids also cause calcium mobilization and chemotaxis through BLT2, again in contrast to BLT1. These findings suggest that BLT2 functions as a low affinity receptor, with broader ligand specificity for various eicosanoids, and mediates distinct biological and pathophysiological roles from BLT1.

Leukotrienes as mediators of asthma

This review describes the aspects of leukotriene (LT) pharmacology and biology that are relevant to their important role in asthma. The biosynthesis and metabolism, including transcellular metabolism, of LTB4 and the cysteinyl-LTs (i.e. LTC4, LTD4 and LTE4) are described, and their transport is briefly outlined. The existence, distribution and pharmacological characterization of the receptors (BLT, CysLT1, CysLT2), as well as the transduction mechanisms triggered, are discussed in detail. We also describe their effects on airway smooth muscle tone, hyperresponsiveness and proliferation, on vascular tone and permeability, on mucus secretion, on neural fibers and inflammatory cell functions. Finally, the evidence supporting their role as asthma mediators is reviewed, including the effects of anti LT drugs (both biosynthesis inhibitors and receptor antagonists) in experimental and clinical asthma.

Eicosanoid regulation of the renal vasculature

Even though it has been recognized that arachidonic acid metabolites, eicosanoids, play an important role in the control of renal blood flow and glomerular filtration, several key observations have been made in the past decade. One major finding was that two distinct cyclooxygenase (COX-1 and COX-2) enzymes exist in the kidney. A renewed interest in the contribution of cyclooxygenase metabolites in tubuloglomerular feedback responses has been sparked by the observation that COX-2 is constitutively expressed in the macula densa area. Arachidonic acid metabolites of the lipoxygenase pathway appear to be significant factors in renal hemodynamic changes that occur during disease states. In particular, 12(S)- hydroxyeicosatetraenoic acid may be important for the full expression of the renal hemodynamic actions in response to angiotensin II. Cytochrome P-450 metabolites have been demonstrated to possess vasoactive properties, act as paracrine modulators, and be a critical component in renal blood flow autoregulatory responses. Last, peroxidation of arachidonic acid metabolites to isoprostanes appears to be involved in renal oxidative stress responses. The recent developments of specific enzymatic inhibitors, stable analogs, and gene-disrupted mice and in antisense technology are enabling investigators to understand the complex interplay by which eicosanoids control renal blood flow.

Tumor necrosis factor-alpha generates reactive oxygen species via a cytosolic phospholipase A2-linked cascade

Reactive oxygen species (ROS) are important regulatory molecules implicated in the signaling cascade triggered by tumor necrosis factor (TNF)-alpha, although the events through which TNF-alpha induces ROS generation are not yet well characterized. We therefore investigated selected candidates likely to mediate TNF-alpha-induced ROS generation. Consistent with the role of Rac in that process, stable expression of Rac(Asn-17), a dominant negative Rac1 mutant, completely blocked TNF-alpha-induced ROS generation. To understand better the mediators downstream of Rac, we investigated the involvement of cytosolic phospholipase A(2) (cPLA(2)) activation and metabolism of the resultant arachidonic acid (AA) by 5-lipoxygenase (5-LO). TNF-alpha-induced ROS generation was blocked by inhibition of cPLA(2) or 5-LO, but not cyclooxygenase, suggesting that TNF-alpha-induced ROS generation is dependent on synthesis of AA and its subsequent metabolism to leukotrienes. Consistent with that hypothesis, TNF-alpha Rac-dependently stimulated endogenous production of leukotriene B(4) (LTB(4)), while exogenous application of LTB(4) increased levels of ROS. In contrast, application of leukotrienes C(4), D(4), and E(4) or prostaglandin E(2) had little effect. Our findings suggest that LTB(4) production by 5-LO is situated downstream of the Rac-cPLA(2) cascade, and we conclude that Rac, cPLA(2), and LTB(4) play pivotal roles in the ROS-generating cascade triggered by TNF-alpha.

A second leukotriene B(4) receptor, BLT2. A new therapeutic target in inflammation and immunological disorders

Leukotriene B(4) (LTB(4)) is a potent chemoattractant and activator of both granulocytes and macrophages. The actions of LTB(4) appear to be mediated by a specific G protein-coupled receptor (GPCR) BLT1, originally termed BLT (Yokomizo, T., T. Izumi, K. Chang, Y. Takuwa, and T. Shimizu. 1997. Nature. 387:620-624). Here, we report the molecular cloning of a novel GPCR for LTB(4), designated BLT2, which binds LTB(4) with a Kd value of 23 nM compared with 1.1 nM for BLT1, but still efficiently transduces intracellular signaling. BLT2 is highly homologous to BLT1, with an amino acid identity of 45.2%, and its open reading frame is located in the promoter region of the BLT1 gene. BLT2 is expressed ubiquitously, in contrast to BLT1, which is expressed predominantly in leukocytes. Chinese hamster ovary cells expressing BLT2 exhibit LTB(4)-induced chemotaxis, calcium mobilization, and pertussis toxin-insensitive inhibition of adenylyl cyclase. Several BLT1 antagonists, including U 75302, failed to inhibit LTB(4) binding to BLT2. Thus, BLT2 is a pharmacologically distinct receptor for LTB(4), and may mediate cellular functions in tissues other than leukocytes. BLT2 provides a novel target for antiinflammatory therapy and promises to expand our knowledge of LTB(4) function. The location of the gene suggests shared transcriptional regulation of these two receptors.

Cell-specific transcriptional regulation of human leukotriene B(4) receptor gene

Leukotriene B(4) (LTB(4)) is a lipid mediator that activates leukocytes and is involved in host defense and inflammation. BLT1, a high-affinity receptor for LTB(4) (originally termed BLT), is expressed exclusively in inflammatory cells and is inducible in macrophages upon activation. The mechanisms of tissue-specific expression and induction of BLT1 are important for the understanding of mechanism of onset and the potential treatment of inflammatory disorders. Here, we report the genomic structure and a promoter analysis of the human BLT1 gene, with an emphasis on the mechanism of cell-specific transcription. No TATA or CAAT elements exist around the transcription initiation sites, but a GC-rich sequence is observed in this region. A reporter gene assay revealed that a region approximately 80 basepair upstream from the initiator sequence is required for the basal transcription of the BLT1 gene. Sp1 was found to be a major activator of basal transcription by electrophoretic mobility shift assays and site-directed mutagenesis. The CpG sites of the BLT1 promoter region were highly methylated in BLT1-nonexpressing cells, but not methylated in BLT1-expressing cells. Further, methylation of this region in vitro inhibited the promoter activity to approximately 15% of the control. Thus, methylation at CpG sites in the promoter region is important for cell-specific transcription of the BLT1 gene. The promoter region of the BLT1 gene is localized within the open reading frame (ORF) of the BLT2 gene, which encodes a low-affinity receptor for LTB(4) (Yokomizo, T., K. Kato, K. Terawaki, T. Izumi, and T. Shimizu. 2000. J. Exp. Med. 192:421-431). To our knowledge, this is the first example of "promoter in ORF" in higher eukaryotes.

An in vivo approach showing the chemotactic activity of leukotriene B(4) in acute renal ischemic-reperfusion injury

Neutrophil migration protects the body against foreign invasion. Sequestration and activation of neutrophils, however, require stringent regulation because they may also cause tissue damage by the release of lysosomal enzymes and reactive oxygen species. The activity of various chemoattractants [e.g., leukotriene B(4) (LTB(4)), interleukin-8, and complements] has been documented by in vitro assays, whereas in vivo data have been limited mostly to histology. To examine in an in vivo model the chemotactic activity and subsequent tissue infiltration and the role of a specific chemoattractant, LTB(4), we used a rat renal ischemia-reperfusion injury model. Fluorescence-labeled Chinese hamster ovary (CHO) cells stably expressing the LTB(4) receptor (CHO-BLT) were able to accumulate along with neutrophils in the postischemic kidney, in contrast to vector control CHO cells. Furthermore, LTB(4) antagonists that protect against the decrease in renal function and diminish the tissue myeloperoxidase activity also led to the marked decrease in the number of CHO-BLT cells and neutrophils. Thus, LTB(4) alone appears sufficient to cause cells to migrate into postischemic tissues, and its dominant role in reperfusion injury has been demonstrated. The utilization of transfectants to pinpoint the role of LTB(4) in these in vivo experiments suggests their potential use with other ligands and/or in other pathological conditions.