Levels of phospholipids and triacylglycerol-containing omega 3 fatty acids in myocardial tissue of patients with myocardial infarction: analyzed by a lipidomics profiling method

Objective

According to population-based studies, low omega 3 fatty acid (omega3FA) intake and high levels of serum triacylglycerol (TAG) are associated with cardiovascular diseases. Recent advances in mass spectrometry allow molecular lipid (lipidomics) profiling, which may enhance cardiovascular risk prediction. In this study, we assessed the levels of omega3FA-containing phospholipids (PL) and TAG in myocardial tissues of patients with and without myocardial infarction (MI) using a lipidomics profiling method.

Methods

We performed lipidomics profiling of human left atrial appendage (LAA) tissue of 29 consecutive patients receiving off-pump coronary bypass surgery with standard LAA resection. The patients were divided into the MI group (n=7) and an age- and gender-matched non-MI group (n=7).

Results

Lipidomics profiling revealed that the MI group tended to have low levels of phosphatidylcholines (PC), phosphatidylethanolamine (PE), lysophosphatidylethanolamine (LPE), and plasmalogen, and high levels of TAG species. Individual molecular species containing omega3FA, such as PC (18:0/20:5; 3,200±1,200 vs. 4,500±910 pmol/g tissue, p=0.04) and plasmalogen (18:1/20:5; 57,000±21,000 vs. 91,000±28,000 pmol/g tissue, p=0.02), were significantly lower in the MI group than in the non-MI group.

Conclusions

To our knowledge, this is the first study to determine the levels of omega3FA-containing PL and TAG in myocardial tissue using lipidomics profiling. We discovered that lower levels of omega3FA-containing PL and higher levels of TAG existed in myocardial tissues of patients with MI than in those of patients without MI. Accordingly, the lipidomics profiling method for human myocardial tissue may be useful for developing therapy targets for cardiovascular diseases.

Funding Acknowledgement

Type of funding source: Public grant(s) – National budget only. Main funding source(s): MEXT/JSPS KAKENHI Grant

Epithelial-mesenchymal status renders differential responses to cisplatin in ovarian cancer

Chemoresistance to platinums, such as cisplatin, is of critical concern in the treatment of ovarian cancer. Recent evidence has linked epithelial-mesenchymal transition (EMT) as a contributing mechanism. The current study explored the connection between cellular responses to cisplatin and EMT in ovarian cancer. Expression microarrays were utilized to estimate the EMT status as a binary phenotype, and the transcriptional responses of 46 ovarian cancer cell lines to cisplatin were measured at dosages equivalent to 50% growth inhibition. Phenotypic responses to cisplatin were quantified with respect to cell number, proliferation rate and apoptosis, and then compared with the epithelial or mesenchymal status. Ovarian cancer cell lines with an epithelial status exhibited higher resistance to cisplatin treatment in the MTS assay than those with a mesenchymal status. Pathway analyses revealed the induction of G1/S- and S-phase genes (P=0.001) and the activation of multiple NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) downstream genes (P=0.0016) by cisplatin selectively in epithelial-like cell lines. BrdU incorporation and Caspase-3/7 release assays confirmed impaired apoptosis in epithelial-like ovarian cancer cells. In clinical samples, we observed resistance to single platinum treatment and the selective activation of the NF-κB pathway by platinum in ovarian cancers with an epithelial status. Overall, our results suggest that, in epithelial-like ovarian cancer cells, NF-κB activation by cisplatin may lead to defective apoptosis, preferential proliferation arrest and a consequential decreased sensitivity to cisplatin.

Immunohistochemical localization of guinea-pig leukotriene B4 12-hydroxydehydrogenase/15-ketoprostaglandin 13-reductase

We have cloned cDNA for leukotriene B4 12-hydroxydehydrogenase (LTB4 12-HD)/15-ketoprostaglandin 13-reductase (PGR) from guinea-pig liver. LTB4 12-HD catalyzes the conversion of LTB4 into 12-keto-LTB4 in the presence of NADP+, and plays an important role in inactivating LTB4. The cDNA contained an ORF of 987 bp that encodes a protein of 329 amino-acid residues with a 78% identity with porcine LTB4 12-HD. The amino acids in the putative NAD+/NADP+ binding domain are well conserved among the pig, guinea-pig, human, rat, and rabbit enzymes. The guinea-pig LTB4 12-HD (gpLTB4 12-HD) was expressed as a glutathione S-transferase (GST) fusion protein in Escherichia coli, which exhibited similar enzyme activities to porcine LTB4 12-HD. We examined the 15-ketoprostaglandin 13-reductase (PGR) activity of recombinant gpLTB4 12-HD, and confirmed that the Kcat of the PGR activity is higher than that of LTB4 12-HD activity by 200-fold. Northern and Western blot analyses revealed that gpLTB4 12-HD/PGR is widely expressed in guinea-pig tissues such as liver, kidney, small intestine, spleen, and stomach. We carried out immunohistochemical analyses of this enzyme in various guinea-pig tissues. Epithelial cells of calyx and collecting tubules in kidney, epithelial cells of airway, alveoli, epithelial cells in small intestine and stomach, and hepatocytes were found to express the enzyme. These findings will lead to the identification of the unrevealed roles of PGs and LTs in these tissues.

Step-wise divergence of primitive and definitive haematopoietic and endothelial cell lineages during embryonic stem cell differentiation

BACKGROUND

The developmental processes leading from the mesoderm to primitive and definitive haematopoietic and endothelial lineages, although of great importance, are still poorly defined. Recent studies have suggested a model in which common precursors give rise to endothelial progenitors and haematopoietic progenitors, the latter subsequently generating both primitive and definitive haematopoietic lineages. However, this model is contradicted by findings that suggest the emergence of haematopoietic cells from the endothelial lineage.

RESULTS

We found sequential steps in the differentiation of FLK1+ mesoderm into haematopoietic and endothelial lineages in an in vitro differentiation system of embryonic stem (ES) cells: (i) the GATA-1+ subset of FLK1+ mesodermal cells loses the capacity to give rise to endothelial cells and is restricted to primitive erythroid, macrophage and definitive erythroid progenitors; (ii) the remaining GATA-1- cells give rise to VE-cadherin+ endothelial cells; and subsequently (iii) multiple definitive haematopoietic progenitors and endothelial cells branch off from a subset of VE-cadherin+ cells.

CONCLUSIONS

These observations strongly suggest that the divergence of primitive and multilineage definitive haematopoietic/endothelial lineages occurs first, and then multilineage definitive haematopoietic progenitors arise from VE-cadherin+ endothelial cells in the development of haematopoietic and endothelial cells.

Single nucleotide polymorphism of human platelet-activating factor receptor impairs G-protein activation

Various proinflammatory and vasoactive actions of platelet-activating factor (PAF) are mediated through a specific G-protein-coupled PAF receptor (PAFR). We identified a novel DNA variant in the human PAFR gene, which substitutes an aspartic acid for an alanine residue at position 224 (A224D) in the putative third cytoplasmic loop. This mutation was observed in a Japanese population at an allele frequency of 7.8%. To delineate the functional consequences of this structural alteration, Chinese hamster ovary cells were stably transfected with constructs encoding either wild-type or A224D mutated PAFR. No significant difference was observed in the expression level of the receptor or the affinity to PAF or to an antagonist, WEB2086, between the cells transfected with wild-type and mutant PAFR. Chinese hamster ovary cells expressing A224D mutant PAFR displayed partial but significant reduction of PAF-induced intracellular signals such as calcium mobilization, inositol phosphate production, inhibition of adenylyl cyclase, and chemotaxis. These findings suggest that this variant receptor produced by a naturally occurring mutation exhibits impaired coupling to G-proteins and may be a basis for interindividual variation in PAF-related physiological responses, disease predisposition or phenotypes, and drug responsiveness.

Co-expression of two LTB4 receptors in human mononuclear cells

Leukotriene B4 (LTB4) is one of the most potent chemoattractants and activators of leukocytes, and is involved in inflammatory diseases. Two G-protein-coupled-receptors for LTB4, BLT1 and BLT2, have been isolated, and shown to be a high- and low-affinity receptor, respectively. The tissue distributions of these receptors are different, and distinct roles of each receptor remain elusive. We compared the expression of these two receptors using semi-quantitative PCR analyses, and show that these two receptors are expressed in various subsets of human lymphocytes in different quantities. BLT1 expression is highest in CD14+ monocytes, while BLT2 expression is high in CD8+ cytotoxic T-, CD4+ helper T-, and CD19+ B-cells. Moreover, BLT2 expression in these lymphocytes decreased upon activation of the cells. We also established CHO cells stably expressing both receptors, and found that these cells could migrate toward LTB4 with a broad range of LTB4. These findings suggest novel roles of LTB4 in immune system, and the biological significance of high- and low- affinity LTB4 receptors in chemotaxis.

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.

Requirement of Runx1/AML1/PEBP2alphaB for the generation of haematopoietic cells from endothelial cells

Recent studies revealing that endothelial cells derived from E8.5-E10.5 mouse embryos give rise to haematopoietic cells appear to correspond to previous histological observations that haematopoietic cell clusters are attached to the ventral aspect of dorsal aorta in such a way as if they were budding from the endothelial cell layer. Gene disruption studies have revealed that Runx1/AML1 is required for definitive haematopoiesis but not for primitive haematopoiesis, but the precise stage of gene function is not yet known. We found that mice deficient in Runx1/AML1 (an alpha subunit of the transcription factor PEBP2/CBF) lack c-Kit+ haematopoietic cell clusters in the dorsal aorta, omphalomesenteric and umbilical arteries, as well as yolk sac vessels. Moreover, endothelial cells sorted from the embryo proper and the yolk sac of AML1-/- embryos are unable to differentiate into haematopoietic cells on OP9 stromal cells, whereas colonies of AML1-/- endothelial cells can be formed in culture. These results strongly suggest that the emergence of haematopoietic cells from endothelial cells represents a major pathway of definitive haematopoiesis and is an event that also occurs in the yolk sac in vivo, as suggested by earlier in vitro experiments.

Inhibitory regulation of Rac activation, membrane ruffling, and cell migration by the G protein-coupled sphingosine-1-phosphate receptor EDG5 but not EDG1 or EDG3

Sphingosine-1-phosphate (S1P) is a bioactive lysophospholipid that induces a variety of biological responses in diverse cell types. Many, if not all, of these responses are mediated by members of the EDG (endothelial differentiation gene) family G protein-coupled receptors EDG1, EDG3, and EDG5 (AGR16). Among prominent activities of S1P is the regulation of cell motility; S1P stimulates or inhibits cell motility depending on cell types. In the present study, we provide evidence for EDG subtype-specific, contrasting regulation of cell motility and cellular Rac activity. In CHO cells expressing EDG1 or EDG3 (EDG1 cells or EDG3 cells, respectively) S1P as well as insulin-like growth factor I (IGF I) induced chemotaxis and membrane ruffling in phosphoinositide (PI) 3-kinase- and Rac-dependent manners. Both S1P and IGF I induced a biphasic increase in the amount of the GTP-bound active form of Rac. In CHO cells expressing EDG5 (EDG5 cells), IGF I similarly stimulated cell migration; however, in contrast to what was found for EDG1 and EDG3 cells, S1P did not stimulate migration but totally abolished IGF I-directed chemotaxis and membrane ruffling, in a manner dependent on a concentration gradient of S1P. In EDG5 cells, S1P stimulated PI 3-kinase activity as it did in EDG1 cells but inhibited the basal Rac activity and totally abolished IGF I-induced Rac activation, which involved stimulation of Rac-GTPase-activating protein activity rather than inhibition of Rac-guanine nucleotide exchange activity. S1P induced comparable increases in the amounts of GTP-RhoA in EDG3 and EDG5 cells. Neither S1P nor IGF I increased the amount of GTP-bound Cdc42. However, expression of N(17)-Cdc42, but not N(19)-RhoA, suppressed S1P- and IGF I-directed chemotaxis, suggesting a requirement for basal Cdc42 activity for chemotaxis. Taken together, the present results demonstrate that EDG5 is the first example of a hitherto-unrecognized type of receptors that negatively regulate Rac activity, thereby inhibiting cell migration and membrane ruffling.

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.

Characterization of the cloned guinea pig leukotriene B4 receptor: comparison to its human orthologue

A cDNA clone coding for the guinea pig leukotriene B4 (BLT) receptor has been isolated from a lung cDNA library. The guinea pig BLT receptor has an open reading frame corresponding to 348 amino acids and shares 73% and 70% identity with human and mouse BLT receptors, respectively. Scatchard analysis of membranes prepared from guinea pig and human BLT receptor-transfected human embryonic kidney (HEK) 293 EBNA (Epstein-Bar Virus Nuclear Antigen) cells showed that both receptors displayed high affinity for leukotriene B4 (Kd value of approximately 0.4 nM) and were expressed at high levels (Bmax values ranging from 9 to 12 pmol/mg protein). The rank order of potency for leukotrienes and related analogs in competition for [3H]leukotriene B4 specific binding at the recombinant guinea pig BLT receptor is leukotriene B4 > 20-OH-leukotriene B4 > 12(R)-HETE ((5Z,8Z,10E,12(R)14Z)-12-hydroxyeicosatetraen -1-oic acid) > 12(S)-HETE ((5Z,8Z,10E,12(S)14Z)-12-Hydroxyeicosatetraen -1-oic acid) > 20-COOH-leukotriene B4 > U75302 (6-(6-(3-hydroxy-1E,5Z-undecadienyl)-2-pyridinyl)-1,5-hexane diol) >> leukotriene C4 = leukotriene D4 = leukotriene E4. For the human receptor the rank order of 12(S)-HETE, 20-COOH-leukotriene B4 and U75302 was reversed. Xenopus melanophore and HEK aequorin-based reporter gene assays were used to demonstrate that the guinea pig and human BLT receptors can couple to both the cAMP inhibitory and intracellular Ca2+ mobilization signaling pathways. However, in the case of the aequorin-expressing HEK cells (designated AEQ17-293) transfected with either the guinea pig or human BLT receptor, expression of Galpha16 was required to achieve a robust Ca2+ driven response. Leukotriene B4 was a potent agonist in functional assays of both the guinea pig and human BLT receptors. U-75302 a leukotriene B4 analogue which possesses both agonistic and antagonistic properties behaved as a full agonist of the guinea pig and human BLT receptors in AEQ17-293 cells and not as an antagonist. The recombinant guinea pig BLT receptor will permit the comparison of the intrinsic potencies of leukotriene B4 receptor antagonists used in guinea pig in vivo models of allergic and inflammatory disorders.

Cloning and characterization of rat leukotriene B(4) receptor

Leukotriene B(4) (LTB(4)) is a potent chemoattractant for neutrophils and eosinophils. cDNAs for LTB(4) receptor (BLT) have been cloned from human, mouse, and guinea pig. Here we report the isolation of BLT from rat genomic library. Rat BLT consists of 351 amino acids with homologies of 80.2, 93.2, and 71.6%, to human, mouse, and guinea pig BLT, respectively. When expressed in human embryonic kidney (HEK)-293 cells, rat BLT showed a specific and high-affinity binding to LTB(4) with a Kd value of 0.68 nM (mean, n = 3). Northern blot analysis showed that BLT is exclusively expressed in polymorphonuclear leukocytes. Furthermore, the expression of BLT was high in proteosepeptone-activated peritoneal macrophages, while the resident macrophages did not show significant expression. The present results suggest important roles of LTB(4) in macrophage recruitment and activation.

cDNA cloning and characterization of guinea-pig leukotriene B4 receptor

The cDNA for leukotriene B(4) (LTB(4)) receptor (BLT) was cloned from a guinea-pig leucocyte cDNA library. The cloned receptor cDNA encodes 348 amino acid residues and shares 73% identity with the amino acid sequence of human BLT. Northern blot analysis showed the highest expression of the receptor mRNA in leucocytes, followed by lung and spleen. The membrane fractions of HEK-293 and Cos-7 cells transfected with the cDNA showed specific LTB(4)-binding activities, with K(d) values of 0.27 and 0.17 nM respectively. Xenopus laevis oocytes injected with the cRNA of guinea-pig BLT showed LTB(4)-induced Cl(-) currents, indicating that the cloned receptor is functional. LTB(4) is metabolized to 20-hydroxy-LTB(4) and then to 20-carboxy-LTB(4), a transformation considered as a major inactivation pathway of the compound. Using the cloned receptor, we analysed the agonistic effects of LTB(4) and these two metabolites. 20-Carboxy-LTB(4) is a much weaker agonist, with a K(d) value higher than that of LTB(4) by three orders of magnitude, corresponding to a much weaker chemotactic activity. Although 20-hydroxy-LTB(4) is as potent as LTB(4) in inhibiting [(3)H]LTB(4) binding and cAMP formation, it is less potent than LTB(4) in the mobilization of intracellular Ca(2+) and the chemotaxis of Chinese hamster ovary cells expressing the guinea-pig BLT. The present study demonstrated that although LTB(4) and 20-hydroxy-LTB(4) bind to the receptor with similar affinities, they do differ in activating intracellular signalling.

Critical duration of intracellular Ca2+ response required for continuous translocation and activation of cytosolic phospholipase A2

When cells are exposed to certain external stimuli, arachidonic acid (AA) is released from the membrane and serves as a precursor of various types of eicosanoids. A Ca2+-regulated cytosolic phospholipase A2 (cPLA2) plays a dominant role in the release of AA. To closely examine the relation between Ca2+ response and AA release by stimulation of G protein-coupled receptors, we established several lines of Chinese hamster ovary cells expressing platelet-activating factor receptor or leukotriene B4 receptor. Measurement of intracellular Ca2+ concentration ([Ca2+]i) demonstrated that cell lines capable of releasing AA elicited a sustained [Ca2+]i increase when stimulated by agonists. The prolonged [Ca2+]i elevation is the result of Ca2+ entry, because this elevation was blocked by EGTA treatment or in the presence of Ca2+ channel blockers (SKF 96365 and methoxyverapamil). cPLA2 fused with a green fluorescent protein (cPLA2-GFP) translocated from the cytosol to the perinuclear region in response to increases in [Ca2+]i. When EGTA was added shortly after [Ca2+]i increase, the cPLA2-GFP returned to the cytosol, without liberating AA. After a prolonged [Ca2+]i increase, even by EGTA treatment, the enzyme was not readily redistributed to the cytosol. Thus, we propose that a critical time length of [Ca2+]i elevation is required for continuous membrane localization and full activation of cPLA2.

Characterization of the leukotriene B4 receptor in porcine leukocytes. Separation and reconstitution with heterotrimeric GTP-binding proteins

Leukotriene B4 (LTB4) is a potent chemoattractant derived from arachidonic acid. When cDNAs for LTB4 receptor (BLT) were cloned it was found that they belong to a guanine nucleotide-binding regulatory protein (G-protein)-coupled receptor superfamily. However, purification of BLT from inflammatory cells and reconstitution with various types of G-proteins have not been successful. In the present study, BLT from porcine leukocytes was solubilized, separated from associated G-proteins by Ricinus communis agglutinin (RCA) 120 chromatography, and reconstituted with several endogenous and exogenous G-proteins, in combination with the fraction which contained endogenous phospholipids and Gbeta gamma. Kinetic studies of LTB4 were performed to determine the association with G-proteins. A partially purified BLT fraction (retained on an RCA120 column) free of G-proteins showed a lower affinity for LTB4 (Kd = 500 nm), but reconstitution of the BLT fraction with a G-protein-rich fraction (flow-through of an RCA column) increased the affinity for LTB4 10-fold (Kd = 50 nm). The partially purified BLT fraction was also reconstituted with exogenous G-proteins such as a heterotrimeric Gi2 purified from bovine brain or recombinant alpha subunits of Gi1, Gi2, Gi3, and Go expressed in Spodoptera frugiperda-9 cells. These increases in LTB4 bindings demonstrate that the BLT of porcine leukocytes can interact with pertussis toxin-sensitive G-proteins in vitro. The method is useful for the purification and reconstitution of other, as yet unisolated, G-protein-coupled receptors.

A G-protein-coupled receptor for leukotriene B4 that mediates chemotaxis

Leukotriene B4 (LTB4) is a potent chemoattractant that is primarily involved in inflammation, immune responses and host defence against infection. LTB4 activates inflammatory cells by binding to its cell-surface receptor (BLTR). LTB4 can also bind and activate the intranudear transcription factor PPAR alpha, resulting in the activation of genes that terminate inflammatory processes. Here we report the cloning of the complementary DNA encoding a cell-surface LTB4 receptor that is highly expressed in human leukocytes. Using a subtraction strategy, we isolated two cDNA clones (HL-1 and HL-5) from retinoic acid-differentiated HL-60 cells. These two clones contain identical open reading frames encoding a protein of 352 amino acids and predicted to contain seven membrane-spanning domains, but different 5'-untranslated regions. Membrane fractions of Cos-7 cells transfected with an expression construct containing the open reading frame of HL-5 showed specific LTB4 binding, with a K(d) (0.154nM) comparable to that observed in retinoic acid-differentiated HL-60 cells. In CHO cells stably expressing this receptor, LTB4 induced increases in intracellular calcium, D-myo-inositol-1,4,5-triphosphate (InsP3) accumulation, and inhibition of adenylyl cyclase. Furthermore, CHO cells expressing exogenous BLTR showed marked chemotactic responses towards low concentrations of LTB4 in a pertussis-toxin-sensitive manner. Our findings, together with previous reports, show that LTB4 is a unique lipid mediator that interacts with both cell-surface and nuclear receptors.

cDNA cloning, expression, and mutagenesis study of leukotriene B4 12-hydroxydehydrogenase

Leukotriene B4 12-hydroxydehydrogenase catalyzes the conversion of leukotriene B4 into its biologically less active metabolite, 12-oxo-leukotriene B4. This is an initial and key step of metabolic inactivation of leukotriene B4 in various tissues other than leukocytes. Here we report the cDNA cloning for porcine and human enzymes from kidney cDNA libraries. A full-length cDNA of the porcine enzyme contains an open reading frame consisting of 987 base pairs, corresponding to 329 amino acids. The human enzyme showed a 97.1% homology with the porcine enzyme. Northern blotting of human tissues revealed its high expression in the kidney, liver, and intestine but not in leukocytes. The porcine enzyme was expressed as a glutathione S-transferase fusion protein in Escherichia coli, which exhibited similar characteristics with the native enzyme. Because the enzymes have a homology, in part, with NAD(P)(+)-dependent alcohol dehydrogenases, a site-directed mutagenesis study was carried out. We found that three glycines at 152, 155, and 166 have crucial roles in the enzyme activity, possibly by producing an NADP+ binding pocket.

Enzymatic inactivation of leukotriene B4 by a novel enzyme found in the porcine kidney. Purification and properties of leukotriene B4 12-hydroxydehydrogenase

Leukotriene B4 (LTB4) 12-hydroxydehydrogenase was purified to apparent homogeneity from the cytosol fraction of the porcine kidney. The N-terminal amino acid sequence analysis revealed that this enzyme is a novel protein with a molecular weight of 35,000. Although the enzyme is ubiquitously distributed in various tissues and leukocytes of porcine, the kidney and liver had the highest enzyme activities. In the presence of NADP+ as a cofactor, the enzyme catalyzes the conversion of LTB4 to 12-oxo-LTB4, the structure identified by gas chromatography/mass spectrometry. 12-Oxo-LTB4 was further converted by other enzymes to 10,11,14,15-tetrahydro-12-oxo-LTB4, which was determined by proton NMR and gas chromatography/mass spectrometry. 12-Oxo-LTB4 was 100-fold less potent than LTB4 in increasing intracellular calcium concentrations of human leukocytes. 6-trans-LTB4 and LTB4 proved to be the best substrates of the enzyme, whereas various types of monohydroxyeicosatetraenoic acids, 5(S),12(S)-dihydroxyeicosatetraenoic acid, prostaglandins, cortisol, or pregnenolone could not serve as a substrate. These results suggest that the enzyme acts specifically on the 12(R)-hydroxy group of leukotriene B4 and is involved in the metabolic inactivation of LTB4 in the porcine kidney.