Purification to homogeneity and the N-terminal sequence of human leukotriene C4 synthase: a homodimeric glutathione S-transferase composed of 18-kDa subunits

Identification of glutathione-S-transferase m19 and m34 among responsive GST genes against 1-chloro-2,4-dinitrobenzene treatment of Tetrahymena thermophila

Industrial xenobiotic pollutants have toxic effects on diverse organisms in their natural environments. This study aims to identify the Glutathione-S-transferases (GST) from Tetrahymena thermophila that are highly responsive to the treatment of synthetic substrate 1-chloro-2,4-dinitrobenzene (CDNB). The LD50 value of CDNB was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) test as 0.079 mM at 9 h exposure. The glutathione affinity-purified 22 kDa and 23 kDa GSTs from CDNB-treated cells were identified as GSTm19 and GSTm34 with 2D-gel electrophoresis coupled MALDI-Tof MS/MS analysis. The specific activitiy of the affinity-purified GSTs was upregulated upon the treatment of 0.072 mM CDNB with the decreased cell survival. GSTm19 and GSTm34 had also upregulated the mRNA expression under the highest dose treatment. The high cell survival and elevated total GST enzyme activity at 9 h under CDNB doses could be the result of both transcriptional upregulations as well as post-translational modifications. As a result, the cell survival of Tetrahymena thermophila was significantly affected by CDNB exposure in a concentration-dependent manner with the effect of low-dose stimulation and high-dose inhibition.

Integral Membrane Enzymes in Eicosanoid Metabolism: Structures, Mechanisms and Inhibitor Design

Eicosanoids are potent lipid mediators involved in central physiological processes such as hemostasis, renal function and parturition. When formed in excess, eicosanoids become critical players in a range of pathological conditions, in particular pain, fever, arthritis, asthma, cardiovascular disease and cancer. Eicosanoids are generated via oxidative metabolism of arachidonic acid along the cyclooxygenase (COX) and lipoxygenase (LOX) pathways. Specific lipid species are formed downstream of COX and LOX by specialized synthases, some of which reside on the nuclear and endoplasmic reticulum, including mPGES-1, FLAP, LTC4 synthase, and MGST2. These integral membrane proteins are members of the family "membrane-associated proteins in eicosanoid and glutathione metabolism" (MAPEG). Here we focus on this enzyme family, which encompasses six human members typically catalyzing glutathione dependent transformations of lipophilic substrates. Enzymes of this family have evolved to combat the topographical challenge and unfavorable energetics of bringing together two chemically different substrates, from cytosol and lipid bilayer, for catalysis within a membrane environment. Thus, structural understanding of these enzymes are of utmost importance to unravel their molecular mechanisms, mode of substrate entry and product release, in order to facilitate novel drug design against severe human diseases.

Discovery of the Oral Leukotriene C4 Synthase Inhibitor (1S,2S)-2-({5-[(5-Chloro-2,4-difluorophenyl)(2-fluoro-2-methylpropyl)amino]-3-methoxypyrazin-2-yl}carbonyl)cyclopropanecarboxylic Acid (AZD9898) as a New Treatment for Asthma

While bronchodilators and inhaled corticosteroids are the mainstay of asthma treatment, up to 50% of asthmatics remain uncontrolled. Many studies show that the cysteinyl leukotriene cascade remains highly activated in some asthmatics, even those on high-dose inhaled or oral corticosteroids. Hence, inhibition of the leukotriene C4 synthase (LTC4S) enzyme could provide a new and differentiated core treatment for patients with a highly activated cysteinyl leukotriene cascade. Starting from a screening hit (3), a program to discover oral inhibitors of LTC4S led to (1S,2S)-2-({5-[(5-chloro-2,4-difluorophenyl)(2-fluoro-2-methylpropyl)amino]-3-methoxypyrazin-2-yl}carbonyl)cyclopropanecarboxylic acid (AZD9898) (36), a picomolar LTC4S inhibitor (IC₅₀ = 0.28 nM) with high lipophilic ligand efficiency (LLE = 8.5), which displays nanomolar potency in cells (peripheral blood mononuclear cell, IC_50,free = 6.2 nM) and good in vivo pharmacodynamics in a calcium ionophore-stimulated rat model after oral dosing (in vivo, IC_50,free = 34 nM). Compound 36 mitigates the GABA binding, hepatic toxicity signal, and in vivo toxicology findings of an early lead compound 7 with a human dose predicted to be 30 mg once daily.

Stage-specific Proteomes from Onchocerca ochengi, Sister Species of the Human River Blindness Parasite, Uncover Adaptations to a Nodular Lifestyle

Despite 40 years of control efforts, onchocerciasis (river blindness) remains one of the most important neglected tropical diseases, with 17 million people affected. The etiological agent, Onchocerca volvulus, is a filarial nematode with a complex lifecycle involving several distinct stages in the definitive host and blackfly vector. The challenges of obtaining sufficient material have prevented high-throughput studies and the development of novel strategies for disease control and diagnosis. Here, we utilize the closest relative of O. volvulus, the bovine parasite Onchocerca ochengi, to compare stage-specific proteomes and host-parasite interactions within the secretome. We identified a total of 4260 unique O. ochengi proteins from adult males and females, infective larvae, intrauterine microfilariae, and fluid from intradermal nodules. In addition, 135 proteins were detected from the obligate Wolbachia symbiont. Observed protein families that were enriched in all whole body extracts relative to the complete search database included immunoglobulin-domain proteins, whereas redox and detoxification enzymes and proteins involved in intracellular transport displayed stage-specific overrepresentation. Unexpectedly, the larval stages exhibited enrichment for several mitochondrial-related protein families, including members of peptidase family M16 and proteins which mediate mitochondrial fission and fusion. Quantification of proteins across the lifecycle using the Hi-3 approach supported these qualitative analyses. In nodule fluid, we identified 94 O. ochengi secreted proteins, including homologs of transforming growth factor-β and a second member of a novel 6-ShK toxin domain family, which was originally described from a model filarial nematode (Litomosoides sigmodontis). Strikingly, the 498 bovine proteins identified in nodule fluid were strongly dominated by antimicrobial proteins, especially cathelicidins. This first high-throughput analysis of an Onchocerca spp. proteome across the lifecycle highlights its profound complexity and emphasizes the extremely close relationship between O. ochengi and O. volvulus The insights presented here provide new candidates for vaccine development, drug targeting and diagnostic biomarkers.

Sickle erythrocytes and platelets augment lung leukotriene synthesis with downregulation of anti-inflammatory proteins: relevance in the pathology of the acute chest syndrome

Initiation, progression, and resolution of vaso-occlusive pain episodes in sickle cell disease (SCD) have been recognized as reperfusion injury, which provokes an inflammatory response in the pulmonary circulation. Some 5-lipoxygenase (5-lox) metabolites are potent vasoconstrictors in the pulmonary circulation. We studied stimulation of production of the inflammatory eicosanoids leukotrienes (LTs) and prostaglandin E2 (PGE2) by isolated rat lungs perfused with sickle (HbSS) erythrocytes. Our hypothesis is that HbSS erythrocytes produce more LTs than normal (HbAA) erythrocytes, which can induce vaso-occlusive episodes in SCD patients. Lung perfusates were collected at specific time points and purified by high-pressure liquid chromatography, and LTC4 and PGE2 contents were measured by enzyme-linked immunosorbent assay (ELISA). Rat lung explants were also cultured with purified HbAA and HbSS peptides, and 5-lox, cyclooxygenase 1/2, and platelet-activating factor receptor (PAFR) proteins were measured by Western blotting, while prostacyclin and LTs produced by cultured lung explants were measured by ELISA. Lung weight gain and blood gas data were not different among the groups. HbSS-perfused lungs produced more LTC4 and PGE2 than HbAA-perfused lungs: 10.40 ± 0.62 versus 0.92 ± 0.2 ng/g dry lung weight (mean ± SEM; P = 0.0001) for LTC4. Inclusion of autologous platelets (platelet-rich plasma) elevated LTC4 production to 12.6 ± 0.96 and 7 ± 0.60 ng/g dry lung weight in HbSS and HbAA perfusates, respectively. HbSS lungs also expressed more 5-lox and PAFR. The data suggest that HbSS erythrocytes and activated platelets in patient's pulmonary microcirculation will enhance the synthesis and release of the proinflammatory mediators LTC4 and PGE2, both of which may contribute to onset of the acute chest syndrome in SCD.

Prostaglandin D₂: a dominant mediator of aspirin-exacerbated respiratory disease

BACKGROUND

Aspirin desensitization followed by high-dose aspirin therapy is routinely performed for patients with aspirin-exacerbated respiratory disease (AERD). Little is known about the contributions of mediators other than cysteinyl leukotrienes to aspirin reactions and to the therapeutic benefit of high-dose aspirin therapy.

OBJECTIVE

We investigated differences in urinary eicosanoid metabolite levels and blood eosinophil counts in patients with AERD who tolerate and those who fail aspirin desensitization and also in patients with AERD who were successfully treated with high-dose aspirin therapy.

METHODS

Twenty-nine patients with AERD were stratified into those who tolerated aspirin desensitization (group I) and those who did not (group II). Urine was analyzed for eicosanoid metabolites at baseline, during aspirin reactions, and during high-dose aspirin therapy. Blood was analyzed for cell differentials at baseline and during aspirin therapy.

RESULTS

Basal prostaglandin D2 metabolite (PGD-M; 13.6 ± 2.7 vs 7.0 ± 0.8 pmol/mg creatinine [Cr], P < .05) and thromboxane metabolite (TX-M; 1.4 ± 0.3 vs 0.9 ± 0.1 pmol/mg Cr, P < .01) levels were higher in group II than in group I. During aspirin reactions, PGD-M levels remained unchanged, whereas TX-M levels (0.7 ± 0.1 pmol/mg Cr, P = .07) tended to decrease in group I. In contrast, PGD-M levels increased dramatically in group II (61.3 ± 19.9 pmol/mg Cr, P < .05), whereas TX-M levels did not change. The decrease in FEV1 inversely correlated with basal urinary levels of both leukotriene E4 and PGD-M. Blood eosinophil and basophil levels increased and urinary PGD-M levels (2.2 ± 0.8 pmol/mg Cr, P < .001) decreased on 2 months of high-dose aspirin therapy in group I.

CONCLUSION

Failure to tolerate aspirin desensitization in a subset of patients with AERD is associated with prostaglandin D2 overproduction. The increase in blood eosinophil and basophil counts during high-dose aspirin therapy might reflect the functional consequences of decreased prostaglandin D2 release and the therapeutic benefit of aspirin.

Cysteinyl leukotrienes impair hypoxic pulmonary vasoconstriction in endotoxemic mice

BACKGROUND

Sepsis impairs hypoxic pulmonary vasoconstriction (HPV) in patients and animal models, contributing to systemic hypoxemia. Concentrations of cysteinyl leukotrienes are increased in the bronchoalveolar lavage fluid of patients with sepsis, but the contribution of cysteinyl leukotrienes to the impairment of HPV is unknown.

METHODS

Wild-type mice, mice deficient in leukotriene C(4) synthase, the enzyme responsible for cysteinyl leukotriene synthesis, and mice deficient in cysteinyl leukotriene receptor 1 were studied 18 h after challenge with either saline or endotoxin. HPV was measured by the increase in left pulmonary vascular resistance induced by left mainstem bronchus occlusion. Concentrations of cysteinyl leukotrienes were determined in the bronchoalveolar lavage fluid.

RESULTS

In the bronchoalveolar lavage fluid of all three strains, cysteinyl leukotrienes were not detectable after saline challenge; whereas endotoxin challenge increased cysteinyl leukotriene concentrations in wild-type mice and mice deficient in cysteinyl leukotriene receptor 1, but not in mice deficient in leukotriene C(4) synthase. HPV did not differ among the three mouse strains after saline challenge (120 ± 26, 114 ± 16, and 115 ± 24%, respectively; mean ± SD). Endotoxin challenge markedly impaired HPV in wild-type mice (41 ± 20%) but only marginally in mice deficient in leukotriene C(4) synthase (96 ± 16%, P < 0.05 vs. wild-type mice), thereby preserving systemic oxygenation. Although endotoxin modestly decreased HPV in mice deficient in cysteinyl leukotriene receptor 1 (80 ± 29%, P < 0.05 vs. saline challenge), the magnitude of impairment was markedly less than in endotoxin-challenged wild-type mice.

CONCLUSION

Cysteinyl leukotrienes importantly contribute to endotoxin-induced impairment of HPV in part via a cysteinyl leukotriene receptor 1-dependent mechanism.

Mechanisms of induction of cytosolic and microsomal glutathione transferase (GST) genes by xenobiotics and pro-inflammatory agents

Glutathione transferase (GST) isoezymes are encoded by three separate families of genes (designated cytosolic, microsomal and mitochondrial transferases), with distinct evolutionary origins, that provide mammalian species with protection against electrophiles and oxidative stressors in the environment. Members of the cytosolic class Alpha, Mu, Pi and Theta GST, and also certain microsomal transferases (MGST2 and MGST3), are up-regulated by a diverse spectrum of foreign compounds typified by phenobarbital, 1,4-bis[2-(3,5-dichloropyridyloxy)]benzene, pregnenolone-16α-carbonitrile, 3-methylcholanthrene, 2,3,7,8-tetrachloro-dibenzo-p-dioxin, β-naphthoflavone, butylated hydroxyanisole, ethoxyquin, oltipraz, fumaric acid, sulforaphane, coumarin, 1-[2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oyl]imidazole, 12-O-tetradecanoylphorbol-13-acetate, dexamethasone and thiazolidinediones. Collectively, these compounds induce gene expression through the constitutive androstane receptor (CAR), the pregnane X receptor (PXR), the aryl hydrocarbon receptor (AhR), NF-E2-related factor 2 (Nrf2), peroxisome proliferator-activated receptor-γ (PPARγ) and CAATT/enhancer binding protein (C/EBP) β. The microsomal T family includes 5-lipoxygenase activating protein (FLAP), leukotriene C(4) synthase (LTC4S) and prostaglandin E(2) synthase (PGES-1), and these are up-regulated by tumour necrosis factor-α, lipopolysaccharide and transforming growth factor-β. Induction of genes encoding FLAP, LTC4S and PGES-1 is mediated by the transcription factors C/EBPα, C/EBPδ, C/EBPϵ, nuclear factor-κB and early growth response-1. In this article we have reviewed the literature describing the mechanisms by which cytosolic and microsomal GST are up-regulated by xenobiotics, drugs, cytokines and endotoxin. We discuss cross-talk between the different induction mechanisms, and have employed bioinformatics to identify cis-elements in the upstream regions of GST genes to which the various transcription factors mentioned above may be recruited.

Oxidative stress suppresses cysteinyl leukotriene generation by mouse bone marrow-derived mast cells

Cysteinyl leukotrienes and oxidative stress have both been implicated in bronchial asthma; however, there is no previous study that focused on the ability of oxidative stress to alter cysteinyl leukotriene generation. In this study, treatment of bone marrow-derived mast cells with prostaglandin D(2) reduced their ability to generate leukotriene (LT) C(4) upon calcium ionophore stimulation but had little effect on LTB(4) generation. This effect could be reproduced by a selective agonist of the DP(2) receptor, 15R-methyl prostaglandin D(2) (15R-D(2)). 15R-D(2) dose-dependently inhibited LTC(4) generation with an IC(50) of 2 μM, and the effect was not altered by a DP(2)/thromboxane antagonist or by a peroxisome proliferator-activated receptor-γ antagonist. 15R-D(2) exerted its suppressive effect via a reduction in intracellular GSH, a mechanism that involved the conjugation of its non-enzymatic breakdown product to GSH. At 10 μM, 15R-D(2) reduced LTC(4) generation to 10%, intracellular GSH to 50%, and LTC(4) synthase (LTC(4)S) activity to 33.5% of untreated cells without altering immunoreactive LTC(4)S protein expression or 5-lipoxygenase activity. The effects of 15R-D(2) on LTC(4)S activity could be partially reversed by reducing reagent. The sulfhydryl-reactive oxidative agent diamide suppressed LTC(4)S activity and induced a reversible formation of covalent dimer LTC(4)S. LTC(4)S bearing a C56S mutation was resistant to the effect of diamide. Covalent dimer LTC(4)S was observed in nasal polyp biopsies, indicating that dimerization and inactivation of LTC(4)S can occur at the site of inflammation. These results suggest a cellular redox regulation of LTC(4)S function through a post-translational mechanism.

Dissection of glutathione conjugate turnover in yeast

Xenobiotics are widely used as pesticides. The detoxification of xenobiotics frequently involves conjugation to glutathione prior to compartmentalization and catabolism. In plants, degradation of glutathione-S-conjugates is initiated either by aminoterminal or carboxyterminal amino acid cleavage catalyzed by a gamma-glutamyl transpeptidase and phytochelatin synthase, respectively. In order to establish yeast as a model system for the analysis of the plant pathway, we used monochlorobimane as a model xenobiotic in Saccharomyces cerevisiae and mutants thereof. The catabolism of monochlorobimane is initiated by conjugation to form glutathione-S-bimane, which is then turned over into a gamma-GluCys-bimane conjugate by the vacuolar serine carboxypeptidases CPC and CPY. Alternatively, the glutathione-S-bimane conjugate is catabolized by the action of the gamma-glutamyl transpeptidase Cis2p to a CysGly-conjugate. The turnover of glutathione-S-bimane was impaired in yeast cells deficient in Cis2p and completely abolished by the additional inactivation of CPC and CPY in the corresponding triple knockout. Inducible expression of the Arabidopsis phytochelatin synthase AtPCS1 in the triple knockout resulted in the turnover of glutathione-S-bimane to the gamma-GluCys-bimane conjugate as observed in plants. Challenge of AtPCS1-expressing yeast cells with zinc, cadmium, and copper ions, which are known to activate AtPCS1, enhanced gamma-GluCys-bimane accumulation. Thus, initial catabolism of glutathione-S-conjugates is similar in plants and yeast, and yeast is a suitable system for a study of enzymes of the plant pathway.

Glutathione transferase (GST) as a candidate molecular-based biomarker for soil toxin exposure in the earthworm Lumbricus rubellus

The earthworm Lumbricus rubellus (Hoffmeister, 1843) is a terrestrial pollution sentinel. Enzyme activity and transcription of phase II detoxification superfamily glutathione transferases (GST) is known to respond in earthworms after soil toxin exposure, suggesting GST as a candidate molecular-based pollution biomarker. This study combined sub-proteomics, bioinformatics and biochemical assay to characterise the L. rubellus GST complement as pre-requisite to initialise assessment of the applicability of GST as a biomarker. L. rubellus possesses a range of GSTs related to known classes, with evidence of tissue-specific synthesis. Two affinity-purified GSTs dominating GST protein synthesis (Sigma and Pi class) were cloned, expressed and characterised for enzyme activity with various substrates. Electrospray ionisation mass spectrometry (ESI-MS) and tandem mass spectrometry (MS/MS) following SDS-PAGE were superior in retaining subunit stability relative to two-dimensional gel electrophoresis (2-DE). This study provides greater understanding of Phase II detoxification GST superfamily status of an important environmental pollution sentinel organism.

Regulation of the eicosanoid pathway by tumour necrosis factor alpha and leukotriene D4 in intestinal epithelial cells

In this study the mRNA and protein levels of the key enzymes involved in eicosanoid biosynthesis and the cysteinyl leukotriene receptors (CysLT1R and CysLT2R) have been analysed in non-transformed intestinal epithelial and colon cancer cell lines. Our results revealed that tumour necrosis factor alpha (TNF-alpha), and leukotriene D4 (LTD4), which are inflammatory mediators implicated in carcinogenesis, stimulated an increase of cyclooxygenase-2 (COX-2), in non-transformed epithelial cells, and 5-lipoxygenase (5-LO) in both non-transformed and cancer cell lines. Furthermore, these mediators also stimulated an up-regulation of LTC4 synthase in cancer cells as well as non-transformed cells. We also observed an endogenous production of CysLTs in these cells. TNF-alpha and LTD4, to a lesser extent, up-regulate the CysLT1R levels. Interestingly, TNF-alpha also reduced CysLT2R expression in cancer cells. Our results demonstrate that inflammatory mediators can cause intestinal epithelial cells to up-regulate the expression of enzymes needed for the biosynthesis of eicosanoids, including the cysteinyl leukotrienes, as well as the signal transducing proteins, the CysLT receptors, thus providing important mechanisms for both maintaining inflammation and for tumour progression.

Leukotriene E4 activates peroxisome proliferator-activated receptor gamma and induces prostaglandin D2 generation by human mast cells

Cysteinyl leukotrienes (cys-LTs) are potent inflammatory lipid mediators, of which leukotriene (LT) E(4) is the most stable and abundant in vivo. Although only a weak agonist of established G protein-coupled receptors (GPCRs) for cys-LTs, LTE(4) potentiates airway hyper-responsiveness (AHR) by a cyclooxygenase (COX)-dependent mechanism and induces bronchial eosinophilia. We now report that LTE(4) activates human mast cells (MCs) by a pathway involving cooperation between an MK571-sensitive GPCR and peroxisome proliferator-activated receptor (PPAR)gamma, a nuclear receptor for dietary lipids. Although LTD(4) is more potent than LTE(4) for inducing calcium flux by the human MC sarcoma line LAD2, LTE(4) is more potent for inducing proliferation and chemokine generation, and is at least as potent for upregulating COX-2 expression and causing prostaglandin D(2) (PGD(2)) generation. LTE(4) caused phosphorylation of extracellular signal-regulated kinase (ERK), p90RSK, and cyclic AMP-regulated-binding protein (CREB). ERK activation in response to LTE(4), but not to LTD(4), was resistant to inhibitors of phosphoinositol 3-kinase. LTE(4)-mediated COX-2 induction, PGD(2) generation, and ERK phosphorylation were all sensitive to interference by the PPARgamma antagonist GW9662 and to targeted knockdown of PPARgamma. Although LTE(4)-mediated PGD(2) production was also sensitive to MK571, an antagonist for the type 1 receptor for cys-LTs (CysLT(1)R), it was resistant to knockdown of this receptor. This LTE(4)-selective receptor-mediated pathway may explain the unique physiologic responses of human airways to LTE(4) in vivo.

Identification of regions of leukotriene C4synthase which direct the enzyme to its nuclear envelope localization

Leukotrienes (LTs) are fatty acid derivatives formed by oxygenation of arachidonic acid via the 5-lipoxygenase (5-LO) pathway. Upon activation of inflammatory cells 5-LO is translocated to the nuclear envelope (NE) where it converts arachidonic acid to the unstable epoxide LTA4. LTA4 is further converted to LTC4 by conjugation with glutathione, a reaction catalyzed by the integral membrane protein LTC4 synthase (LTC4S), which is localized on the NE and endoplasmic reticulum (ER). We now report the mapping of regions of LTC4S that are important for its subcellular localization. Multiple constructs encoding fusion proteins of green fluorescent protein (GFP) as the N-terminal part and various truncated variants of human LTC4S as C-terminal part were prepared and transfected into HEK 293/T or COS-7 cells. Constructs encoding hydrophobic region 1 of LTC4S (amino acids 6-27) did not give distinct membrane localized fluorescence. In contrast hydrophobic region 2 (amino acids 60-89) gave a localization pattern similar to that of full length LTC4S. Hydrophobic region 3 (amino acids 114-135) directed GFP to a localization indistinguishable from that of full length LTC4S. A minimal directing sequence, amino acids 117-132, was identified by further truncation. The involvement of the hydrophobic regions in the homo-oligomerization of LTC4S was investigated using bioluminescence resonance energy transfer (BRET) analysis in living cells. BRET data showed that hydrophobic regions 1 and 3 each allowed oligomerization to occur. These regions most likely form transmembrane helices, suggesting that homo-oligomerization of LTC4S is due to helix-helix interactions in the membrane.

Modulation of gentamicin-induced renal dysfunction and injury by the phenolic extract of soybean (Glycine max)

Gentamicin (GM) is one of the most important of the aminoglycoside antibiotics used widely for the treatment of serious and life-threatening infections and whose clinical use is limited by its nephrotoxicity. As the pathogenesis of GM-induced renal dysfunction and injury involves reactive oxygen species, the polyphenolic constituents of soybean with antioxidant property may protect against GM-induced renal toxicity. We therefore tested this hypothesis using phenolic extract of soybean (PESB) on GM-induced nephrotoxicity rat model. Administration of GM (80 mg/kg, s.c.) for 12 days to rats induced marked renal failure, characterized by a significantly increased plasma creatinine, urea and Na(+) ions levels, with K(+) depletion. This was also associated with decreases in the activity of the renal antioxidant enzymes [superoxide dismutase (SOD), catalase (CAT), glutathione-S-transferase (GST)] measured and depletion of both blood and renal reduced glutathione (GSH) levels. The activities of membrane-bound glucose-6-phosphatase (G6Pase) and 5(1)-nucleotidase (5(1)-NTD) enzymes as well as gamma-glutamyltransferase (gamma-GT) and aspartate aminotransferase (AST) (enzymes that are located in the proximal tubule) were decreased. Renal histology examination further confirmed the damage to the kidney as it reveals severe necrosis of the proximal renal tubules with deposition of colloid casts. These alterations were ameliorated in rats pretreated with PESB. The decrease in the activities of SOD, CAT, GST as well as GSH depletion observed in GM-treated rats was prevented in the rats pretreated with PESB. The activities of gamma-GT, AST and G6Pase were also increased in the kidney. These protective effects were dose dependent except for G6Pase activity and GSH levels that were preserved only at 500 mg/kg dose of PESB, and 5'-NTD activity that was dose dependently decreased. Furthermore, the extent of tubular damage induced by GM was reduced in rats that also received PESB. The lower dose (500 mg/kg) of the extract, however, appeared to provide better histological protection. These results suggest that the PESB has protective effects on GM-mediated nephropathy and this may be related to the action of the antioxidant polyphenolic content of the soybean.

5-Lipoxygenase-activating protein homodimer in human neutrophils: evidence for a role in leukotriene biosynthesis

FLAP (5-lipoxygenase-activating protein) is a nuclear transmembrane protein involved in the biosynthesis of LTs (leukotrienes) and other 5-LO (5-lipoxygenase) products. However, little is known about its mechanism of action. In the present study, using cross-linkers, we demonstrate that FLAP is present as a monomer and a homodimer in human PMN (polymorphonuclear cells). The functional relevance of the FLAP dimer in LT biosynthesis was assessed in different experimental settings. First, the 5-LO substrate AA (arachidonic acid) concomitantly disrupted the FLAP dimer (at > or =10 microM) and inhibited LT biosynthesis. Secondly, using Sf9 cells expressing active and inactive FLAP mutants and 5-LO, we observed that the FLAP mutants capable of supporting 5-LO product biosynthesis also form the FLAP dimer, whereas inactive FLAP mutants do not. Finally, we showed that FLAP inhibitors such as MK-0591 which block LT biosynthesis in human PMN, disrupt the FLAP dimer in PMN membranes with a similar IC50. The present study demonstrates that LT biosynthesis in intact cells not only requires the presence of FLAP but its further organization into a FLAP homodimer.

5-Lipoxygenase-activating protein homodimer in human neutrophils: evidence for a role in leukotriene biosynthesis

FLAP (5-lipoxygenase-activating protein) is a nuclear transmembrane protein involved in the biosynthesis of LTs (leukotrienes) and other 5-LO (5-lipoxygenase) products. However, little is known about its mechanism of action. In the present study, using cross-linkers, we demonstrate that FLAP is present as a monomer and a homodimer in human PMN (polymorphonuclear cells). The functional relevance of the FLAP dimer in LT biosynthesis was assessed in different experimental settings. First, the 5-LO substrate AA (arachidonic acid) concomitantly disrupted the FLAP dimer (at > or =10 microM) and inhibited LT biosynthesis. Secondly, using Sf9 cells expressing active and inactive FLAP mutants and 5-LO, we observed that the FLAP mutants capable of supporting 5-LO product biosynthesis also form the FLAP dimer, whereas inactive FLAP mutants do not. Finally, we showed that FLAP inhibitors such as MK-0591 which block LT biosynthesis in human PMN, disrupt the FLAP dimer in PMN membranes with a similar IC50. The present study demonstrates that LT biosynthesis in intact cells not only requires the presence of FLAP but its further organization into a FLAP homodimer.

Analysis of the glutathione S-transferase (GST) gene family

The glutathione S-transferase (GST) gene family encodes genes that are critical for certain life processes, as well as for detoxication and toxification mechanisms, via conjugation of reduced glutathione (GSH) with numerous substrates such as pharmaceuticals and environmental pollutants. The GST genes are upregulated in response to oxidative stress and are inexplicably overexpressed in many tumours, leading to problems during cancer chemotherapy. An analysis of the GST gene family in the Human Genome Organization-sponsored Human Gene Nomenclature Committee database showed 21 putatively functional genes. Upon closer examination, however, GST-kappa 1 (GSTK1), prostaglandin E synthase (PTGES) and three microsomal GSTs (MGST1, MGST2, MGST3) were determined as encoding membrane-bound enzymes having GST-like activity, but these genes are not evolutionarily related to the GST gene family. It is concluded that the complete GST gene family comprises 16 genes in six subfamilies--alpha (GSTA), mu (GSTM), omega (GSTO), pi (GSTP), theta (GSTT) and zeta (GSTZ).

Human leukotriene C(4) synthase at 4.5 A resolution in projection

Leukotriene (LT) C(4) synthase, an 18 kDa integral membrane enzyme, conjugates LTA(4) with reduced glutathione to form LTC(4), the parent compound of all cysteinyl leukotrienes that play a crucial role in the pathobiology of bronchial asthma. We have calculated a projection map of recombinant human LTC(4) synthase at a resolution of 4.5 A by electron crystallography, which shows that the enzyme is a trimer. A map truncated at 7.5 A visualizes four transmembrane alpha helices per protein monomer. The densities in projection indicate that most of the alpha helices run nearly perpendicular to the plane of the membrane. At this resolution, LTC(4) synthase is strikingly similar to microsomal glutathione S-transferase 1, which belongs to the same gene family but bears little sequence identity and no resemblance in substrate specificity to the LTC(4) synthase. These results provide new insight into the structure and function of membrane proteins involved in eicosanoid and glutathione metabolism.

Human microsomal prostaglandin E synthase 1: a member of the MAPEG protein superfamily

In this chapter, we briefly review the MAPEG superfamily (membrane associated proteins in eicosanoid and glutathione metabolism), a family of proteins in which all human members except one possess glutathione conjugating capacity. Recent findings regarding the biological functions of MAPEG proteins are highlighted. More extensively, the characterization of human microsomal prostaglandin E synthase 1 is presented, including results and applied methodology.

Cysteinyl leukotrienes and their receptors: cellular distribution and function in immune and inflammatory responses

The cysteinyl leukotrienes (cys-LTs) are a family of potent bioactive lipids that act through two structurally divergent G protein-coupled receptors, termed the CysLT(1) and CysLT(2) receptors. The cloning and characterization of these two receptors has not only reconciled findings of previous pharmacologic profiling studies of contractile tissues, but also has uncovered their expression on a wide array of circulating and tissue-dwelling leukocytes. With the development of receptor-selective reagents, as well as mice lacking critical biosynthetic enzymes, transporter proteins, and the CysLT(1) receptor, diverse functions of cys-LTs and their receptors in immune and inflammatory responses have been identified. We review cys-LT biosynthesis; the molecular biology and distribution of the CysLT(1) and CysLT(2) receptors; the functions of cys-LTs and their receptors in the recruitment and activation of effector leukocytes and induction of adaptive immunity; and the development of fibrosis and airway remodeling in animal models of lung injury and allergic inflammation.

Does glutathione S-transferase Pi (GST-Pi) a marker protein for cancer?

Glutathione S-transferases (GSTs, EC 2.5.1.18) are multifunctional and multigene products. They are versatile enzymes and participate in the nucleophilic attack of the sulphur atom of glutathione on the electrophilic centers of various endogenous and xenobiotic compounds. Out of the five, alpha, micro, pi, sigma and theta, major classes of GSTs, GST-pi has significance in the diagnosis of cancers as it is expressed abundantly in tumor cells. This protein is a single gene product, coded by seven exons, that is having 24 kDa mass and pI value of 7.0. Four upstream elements such as two enhancers, and one of each of AP-1 site and GC box regulate pi gene. During chemical carcinogenesis because of jun/fos oncogenes (AP-1) regulatory elements, specifically GST-pi is expressed in liver. Therefore this gene product could be used as marker protein for the detection of chemical toxicity and carcinogenesis.

Molecular and catalytic properties of three rat leukotriene C4 synthase homologs

The committed step in the biosynthesis of cysteinyl-leukotrienes is catalyzed by leukotriene C(4) synthase as well as microsomal glutathione S-transferase (MGST) type 2 and type 3, which belong to a family of membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG). We cloned and characterized these three enzymes from the rat to allow a side-by-side comparison of structural and catalytic properties. The proteins are 79.6-86.7% identical to the human orthologs. Rat MGST3 fails to convert leukotriene A(4) into leukotriene C(4), which in turn challenges the proposed catalytic role of a conserved Arg and Tyr residue for the leukotriene C(4) synthase reaction. Comparative inhibitor studies of all three enzymes, using MK-886 and cysteinyl-leukotrienes, indicate that their catalytic centers originate from structurally related and overlapping active sites. Hence, it seems feasible to design enzyme inhibitors, which simultaneously target several members of this protein family to yield compounds with increased anti-inflammatory action.

5-lipoxygenase and FLAP

The initial steps in the biosynthesis of leukotrienes from arachidonic acid are carried out by the enzyme 5-lipoxygenase (5-LO). In intact cells, the helper protein 5-LO activating protein (FLAP) is necessary for efficient enzyme utilization of endogenous substrate. The last decade has witnessed remarkable progress in our understanding of these two proteins. Here we review the molecular and cellular aspects of the expression, function, and regulation of 5-LO and FLAP.

Leukotriene C(4) synthase

LTC(4) synthase conjugates LTA(4) with glutathione (GSH) to form LTC(4), the parent compound of the cysteinyl leukotrienes. LTC(4) synthase is a membrane protein that functions as a non-covalent homodimer of two 18-kDa polypeptides. The enzymatic activity of LTC(4) synthase is augmented by Mg(2+) and inhibited by Co(2+) and the FLAP inhibitor MK-886. The K(m) and V(max) values of human LTC(4) synthase are 3.6 microM and 1.3 micromol/mg/min for LTA(4) and 1.6 mM and 2.7 micromol/mg/min for GSH, respectively. The deduced amino acid sequence and the predicted secondary structure of LTC(4) synthase share significant homology to FLAP, mGST-2, and mGST-3. Site-directed mutagenesis of LTC(4) synthase suggests that Arg-51 is involved in opening the epoxide ring of LTA(4) and Tyr-93 in GSH thiolate anion formation during catalytic conjugation. LTC(4) synthase is a TATA-less gene whose transcription involved both cell- and non-specific regulatory elements. LTC(4) synthase gene disrupted mice grow normally, and are attenuated for innate and adaptive immune inflammatory permeability responses.

Induction of leukotriene C4 synthase after the differentiation of rat basophilic leukemia cells with retinoic acid and a low dose of actinomycin D and its suppression with methylprednisolone

Leukotriene C(4) synthase (LTC(4) S) is a pivotal enzyme for generation of cysteinyl-leukotrienes (cysLTs). LTC(4) S activity in rat basophilic leukemia-1 (RBL-1) cells increased after culture in the presence of retinoic acid (RA) analogues, which was inhibited by cycloheximide or actinomycin D (ACD). Unexpectedly, the co-addition of a low dose of ACD with RA further potentiated the upregulation of the LTC(4) S activity. Daunorubicin and mitomycin C also had a similar effect. When stimulated with calcium ionophore A23187, control cells did not produce cysLTs, but RA-treated cells generated cysLTs and the co-addition of ACD further increased. While LTC(4) S mRNA and protein increased in the cells treated with RA, the co-addition of ACD further potentiated both in proportion to the LTC(4) S activity. The effect of ACD was considered to enhance the transcription rate of LTC(4) S gene, but not the mRNA-stability. The addition of methylprednisolone (MP) inhibited generation of cysLTs from the cells with A23187-stimulation and also did LTC(4) S activity, but did not inhibit 5-lipoxygenase (5-LOX). The suppression of LTC(4) S with MP showed a dependent manner on the time-point and duration of MP-treatment after RA-addition which was correlated with reduction in LTC(4) S mRNA and protein. The cells cultured with RA plus ACD contained more histamine, chymase activity, and granules in the cytoplasm than the control cells, suggesting differentiation to mature mast cells. In consideration of RA-differentiation therapy, it may be of pathophysiological relevance that the antineoplastic agents potentiate RA-induced, steroid-sensitive, induction of LTC(4) S in RBL-1 cells.

Cyclic AMP mediated GSTP1 gene activation in tumor cells involves the interaction of activated CREB-1 with the GSTP1 CRE: a novel mechanism of cellular GSTP1 gene regulation

The human GSTP1 gene is frequently over-expressed in many human cancers and the expression increases with tumor progression and is associated with a more aggressive biology, poor patient survival, and resistance to therapy. The molecular regulation of the human GSTP1 gene during malignancy is, however, still not well understood. Recently, we reported the presence of a cAMP response element (CRE) in the 5'-region of the human GSTP1 gene, raising the possibility that the cAMP signaling pathway, frequently aberrant in human cancers, may play an important role in the transcriptional activation of the GSTP1 gene in human tumors. In this study, we report that the GSTP1 gene is an early cAMP response gene. Treatment of cells of the human lung carcinoma cell line, Calu-6, with 25 microM forskolin to activate the cAMP pathway resulted in a rapid and significant (sevenfold after 30 min) increase in GSTP1 gene transcripts, which peaked at 12-fold after 4 h. The forskolin-activated GSTP1 transcription in Calu-6 cells was suppressed dose-dependently by a 2-h pre-treatment with 0.1, 1.0, and 10 microM of the adenylate cyclase inhibitor, 2', 5'-dideoxyadenosine. Western blot analysis showed a rapid, fivefold increase, in GSTP1 protein levels after treatment with 25 microM forskolin, with a peak at 2 h post-treatment. The levels of phosphorylated CRE (Ser133) binding protein-1 (CREB-1) increased rapidly, sevenfold at 30 min, and reached 10-fold at 4 h following forskolin treatment. Intracellular cAMP levels also increased rapidly reaching 12-fold at 30 min. Gel mobility shift and supershift assays and DNase/footprinting analyses demonstrated that CREB-1 bZIP and CREB-containing nuclear extracts recognized the GSTP1 CRE with high affinity and specificity. Binding of CREB-1 bZIP to the GSTP1 CRE was abolished when the GSTP1 CRE sequence 5'-CGTCA-3', was mutated at the core nucleotides. Finally, transfection studies using luciferase plasmid constructs showed the GSTP1 CRE to be required for the cAMP-activated gene expression. Together, these findings describe a novel cAMP- and CREB-1-mediated mechanism of transcriptional regulation of the GSTP1 gene and suggest that this may be an important mechanism underlying the increased GSTP1 expression observed in tumors with an aberrant cAMP signaling pathway and in normal cells under conditions of stress, associated with increased intracellular cAMP.

cDNA cloning and expression of rat leukotriene C(4) synthase: elevated expression in rat basophilic leukemia-1 cells after treatment with retinoic acid

Leukotriene C(4) synthase (LTC(4) S) is considered a pivotal enzyme for generation of potent proinflammatory mediators, cysteinyl-leukotrienes (cysLTs). LTC(4) S cDNA was cloned in rat basophilic leukemia-1 (RBL-1) cells, and exhibited 84.8% and 94.5% identity with the reported human and mouse LTC(4) S cDNA sequences, respectively. Homology between the rat LTC(4) S amino acid sequence and the corresponding sequences from the other species was 86.5% and 95.3% with human and mouse sequences, respectively. Rat LTC(4) S thus showed extensive homology with both mouse and human cDNA sequences. The active enzyme as assessed by LTC(4) S activity was expressed in COS-7 cells. While RBL-1 cells after the culture for 48 h in the presence of 0.1 microg/ml all trans -retinoic acid (RA) exhibited 27 times higher LTC(4) S activity than control cells, Northern-blot analysis of RA-treated cells showed upregulation of LTC(4) S mRNA. Polyclonal antibody was raised against the synthesized peptide deduced from the nucleotide sequence. Thus, Western-blot analysis of RBL-1 cells treated with RA and COS-7 cells transfected with pcDNA-LTC(4) S commonly showed a band at approximately 18 kDa in each solubilized enzyme solution, but either control cells did not. This cDNA probe and antibody may be useful for investigating the roles of cysLTs in various experimental models of rats.

Cloning and expression of a novel Mu class murine glutathione transferase isoenzyme

The present study describes the cDNA cloning, expression and characterization of a novel Mu class murine glutathione transferase (GST) isoenzyme. Screening of a cDNA library from the small intestine of a female A/J mouse using consensus probes derived from Mu class murine GST genes (mGSTM1-mGSTM5) resulted in the isolation of a full-length cDNA clone of a previously unknown Mu class GST gene (designated as mGSTM7). The choice of tissue was based on our previous identification in female A/J mouse small intestine of a potentially novel Mu class GST isoenzyme. The deduced amino acid sequence of mGSTM7, which comprises of 218 amino acid residues, exhibited about 67-78% identity with other Mu class murine GSTs. Recombinant mGSTM7-7 cross-reacted with anti-(GST Mu) antibodies, but not with anti-(GST Alpha) or anti-(GST Pi) antibodies. The pI and the reverse-phase-HPLC elution profile of recombinant mGSTM7-7 were different from those of other Mu class murine GSTs. The substrate specificity of mGSTM7-7 was also different compared with other Mu class murine GSTs. Interestingly, mGSTM7 had a higher identity with the human Mu class isoenzyme hGSTM4 (87% identity and 94% similarity in the amino acid sequence) than with any of the known mouse Mu class GSTs. Specific activities of recombinant mGSTM7-7 and human GSTM4-4 were comparable towards several substrates. For example, similar to hGSTM4-4, recombinant mGSTM7-7 was poorly active in catalysing the GSH conjugation of 1-chloro-2,4-dinitrobenzene and ethacrynic acid, and lacked activity towards 1,2-dichloro-4-nitrobenzene and 1,2-epoxy-3-(p-nitrophenoxy)propane. These results suggested that hGSTM4-4 might be the human counterpart of mouse GSTM7-7. Reverse transcription-PCR analysis using mGSTM7-specific primers revealed that mGSTM7 is widely expressed in tissues of female A/J mice, including liver, forestomach, lung, kidney, colon and spleen.

Leukotriene C4 synthase: a pivotal enzyme in cellular biosynthesis of the cysteinyl leukotrienes

Leukotriene C4 synthase (LTC4S) conjugates LTA4 with glutathione (GSH) to form LTC4, the parent compound of the cysteinyl LTs. LTC4S is an 18 kDa membrane protein and functions as a noncovalent homodimer. The enzyme activity of LTC4S is augmented by Mg2+ and inhibited by Co2+ and the function of 5-lipoxygenase (LO) activating protein (FLAP) inhibitor MK-886. The Km and Vmax values are 3.6 microM and 1.3 micromol/mg/min for LTA4 and 1.6 mM and 2.7 micromol/mg/min for GSH, respectively. The deduced amino acid sequence and the predicted secondary of LTC4S shares significant homology to FLAP, mGST-2 and mGST-3 which are all members of MAPEG protein superfamily. LTC4S and FLAP exhibited identical genomic organization of five exons and four introns. Site-directed mutagenesis suggests that Arg-51 is involved in opening the epoxide ring of LTA4 and Tyr-93 in GSH thiolate anion formation during catalytic conjugation. LTC4S is a TATA-less gene whose transcription assessed in a reporter construct involved both cell-specific and nonspecific regulatory elements. LTC4S-/- mice grow normally, and are attenuated for innate and adaptive immune inflammatory permeability responses.

Cysteinyl leukotrienes and uridine diphosphate induce cytokine generation by human mast cells through an interleukin 4-regulated pathway that is inhibited by leukotriene receptor antagonists

We previously reported that interleukin (IL)-4 upregulates the expression of leukotriene C(4) synthase (LTC(4)S) by human cord blood--derived mast cells (hMCs), augments their high-affinity Fc receptor for IgE (Fc(epsilon)RI)-dependent generation of eicosanoids and cytokines, and induces a calcium flux in response to cysteinyl leukotrienes (cys-LTs) and uridine diphosphate (UDP) that is blocked by cys-LT receptor antagonists. We speculated that this IL-4-dependent, receptor-mediated response to the cys-LTs and UDP might induce cytokine generation by hMCs without concomitant exocytosis. Unlike hMCs maintained in cytoprotective stem cell factor (SCF) alone, hMCs primed for 5 d with IL-4 responded to UDP (1microM), LTC(4) (100 nM), and LTD(4) (100 nM) by producing IL-5, tumor necrosis factor (TNF)-alpha, and especially large quantities of macrophage inflammatory protein (MIP)-1beta de novo at 6 h, preceded by the induced expression of the corresponding mRNAs. Cys-LT- and UDP-mediated cytokine production by the primed hMCs occurred without histamine release or PGD(2) generation and was inhibited by the CysLT1 receptor antagonist MK571. Additionally, pretreatment of hMCs with MK571 or with the cys-LT biosynthetic inhibitor MK886 decreased IL-5 and TNF-alpha production in response to IgE receptor cross-linkage, implying a positive feedback by endogenously produced cys-LTs. Cys-LTs and UDP thus orchestrate a novel, IL-4-regulated, non-IgE-dependent hMC activation for cytokine gene induction that could be initiated by microbes, cellular injury, or neurogenic or inflammatory signals; and this pathobiologic event would not be recognized in tissue studies where hMC activation is classically defined by exocytosis.

The 3-D structure of microsomal glutathione transferase 1 at 6 A resolution as determined by electron crystallography of p22(1)2(1) crystals

Pure solubilised microsomal glutathione transferase 1 (MGST1) forms well-ordered two-dimensional (2-D) crystals of two different symmetries, one orthorhombic (p22(1)2(1)) and one hexagonal (p6), both diffracting electrons to a resolution beyond 3 A. A three-dimensional (3-D) map has previously been calculated to 6 A resolution from the hexagonal crystal form. From orthorhombic crystals we have now calculated a 6 A 3-D reconstruction displaying three repeats of four rod-like densities. These are inclined relative to the normal of the membrane plane and consistent with arising from a left-handed four-helix bundle fold. The rendered volume clearly displays the same structural features as the map previously calculated from the p6 crystal type including similar lengths and substructure of the helices, but several distinguishing features do exist. The helices are more tilted in the map calculated from the orthorhombic crystals indicating conformational flexibility. Density present on the cytosolic side is consistent with the location of the active site. In addition, the current map displays the noted similarity to subunit I of cytochrome c oxidase.

Mice lacking the multidrug resistance protein 1 are resistant to Streptococcus pneumoniae-induced pneumonia

Leukotrienes (LTs) are considered important for antibacterial defense in the lung. Multidrug resistance protein 1 (mrp1) is a transmembrane protein responsible for the cellular extrusion of LTC(4). To determine the role of mrp1 in host defense against pneumonia, mrp1(-/-) and wild-type mice were intranasally inoculated with Streptococcus pneumoniae. mrp1(-/-) mice displayed a diminished outgrowth of pneumococci in lungs and a strongly reduced mortality. These findings were related to an effect of mrp1 on LT metabolism, because survival was similar in mrp1(-/-) and wild-type mice treated with the 5-lipoxygenase-activating protein inhibitor MK-886. Although LTC(4) levels remained low in the bronchoalveolar lavage fluid of mrp1(-/-) mice, LTB(4) concentrations were higher than in wild-type mice. These elevated LTB(4) concentrations were important for the relative protection of mrp1(-/-) mice, because the LTB(4) antagonist LTB(4)-dimethyl amide abolished their survival advantage. In vitro experiments suggested that the intracellullar accumulation of LTC(4) in mrp1(-/-) mice results in product inhibition of LTC(4)-synthase, diminishing substrate competition between LTA(4)-hydrolase (which yields LTB(4)) and LTC(4)-synthase for the available LTA(4). We conclude that mrp1(-/-) mice are resistant against pneumococcal pneumonia by a mechanism that involves increased release of LTB(4). These results identify mrp1 as a novel target for adjunctive therapy in pneumonia.

T helper cell type 2 cytokines coordinately regulate immunoglobulin E-dependent cysteinyl leukotriene production by human cord blood-derived mast cells: profound induction of leukotriene C(4) synthase expression by interleukin 4

Human mast cells (hMCs) derived in vitro from cord blood mononuclear cells exhibit stem cell factor (SCF)-dependent comitogenic responses to T helper cell type 2 (Th2) cytokines. As cysteinyl leukotriene (cys-LT) biosynthesis is a characteristic of immunoglobulin (Ig)E-activated mucosal hMCs, we speculated that Th2 cytokines might regulate eicosanoid generation by hMCs. After passive sensitization for 5 d with IgE in the presence of SCF, anti-IgE-stimulated hMCs elaborated minimal cys-LT (0.1 +/- 0.1 ng/10(6) hMCs) and abundant prostaglandin (PG)D(2) (16.2 +/- 10.3 ng/10(6) hMCs). Priming of hMCs by interleukin (IL)-4 with SCF during passive sensitization enhanced their anti-IgE-dependent histamine exocytosis and increased their generation of both cys-LT (by 27-fold) and PGD(2) (by 2. 5-fold). Although priming with IL-3 or IL-5 alone for 5 d with SCF minimally enhanced anti-IgE-mediated cys-LT generation, these cytokines induced further six- and fourfold increases, respectively, in IgE-dependent cys-LT generation when provided with IL-4 and SCF; this occurred without changes in PGD(2) generation or histamine exocytosis relative to hMCs primed with IL-4 alone. None of these cytokines, either alone or in combination, substantially altered the levels of cytosolic phospholipase A(2) (cPLA(2)), 5-lipoxygenase (5-LO), or 5-LO activating protein (FLAP) protein expression by hMCs. In contrast, IL-4 priming dramatically induced the steady-state expression of leukotriene C(4) synthase (LTC(4)S) mRNA within 6 h, and increased the expression of LTC(4)S protein and functional activity in a dose- and time-dependent manner, with plateaus at 10 ng/ml and 5 d, respectively. Priming by either IL-3 or IL-5, with or without IL-4, supported the localization of 5-LO to the nucleus of hMCs. Thus, different Th2-derived cytokines target distinct steps in the 5-LO/LTC(4)S biosynthetic pathway (induction of LTC(4)S expression and nuclear import of 5-LO, respectively), each of which is necessary for a full integrated functional response to IgE-dependent activation, thus modulating the effector phenotype of mature hMCs.

The three-dimensional map of microsomal glutathione transferase 1 at 6 A resolution

Microsomal glutathione transferase 1 (MGST1) is representative of a superfamily of membrane proteins where different members display distinct or overlapping physiological functions, including detoxication of reactive electrophiles (glutathione transferase), reduction of lipid hydroperoxides (glutathione peroxidase), and production of leukotrienes and prostaglandin E. It follows that members of this superfamily constitute important drug targets regarding asthma, inflammation and the febrile response. Here we propose that this superfamily consists of a new class of membrane proteins built on a common left-handed four-helix bundle motif within the membrane, as determined by electron crystallography of MGST1 at 6 A resolution. Based on the 3D map and biochemical data we discuss a model for the membrane topology. The 3D structure differs significantly from that of soluble glutathione transferases, which display overlapping substrate specificity with MGST1.

Cell-specific transcription of leukotriene C(4) synthase involves a Kruppel-like transcription factor and Sp1

Leukotriene C(4) synthase (LTC(4)S) is responsible for the biosynthesis of cysteinyl leukotrienes that participate in allergic and asthmatic inflammation. We analyzed 2.1 kilobases of the 5'-flanking region of the human LTC(4)S gene, which contains three DNase I hypersensitivity sites, for its transcriptional activity when fused to a promoterless and enhancerless luciferase gene. Deletion analysis revealed a nonspecific basal promoter region between nucleotides -122 and -56 upstream of the translation start site which contains a consensus Sp1 binding site and a putative initiator element (Inr) and cell-specific enhancer regions further upstream. A single mutation of either the Sp1 binding site between nucleotides -120 and -115 or the Inr (CAGAC) between nucleotides -66 and -62 reduced the expression of the reporter gene by approximately 60%, whereas double mutations decreased the expression by approximately 80%. The incubation of nuclear extracts from THP-1 and K562 cells with a (32)P-labeled oligonucleotide containing the Sp1 site or the Inr sequence gave gel-shifted complexes that were blocked by their respective cold oligonucleotides, and antisera specific for Sp1 and Sp3 provided supershifts for the former. Linker-scanning mutations of a cell-specific regulatory region revealed that mutations from nucleotides -165 to -125 reduced reporter activity. This region contains a tandem CACCC repeat (at nucleotides -149 to -145 and -139 to -135). An oligonucleotide containing the distal CACCC motif was gel shifted by THP-1 cell nuclear extract and was supershifted by antisera to Sp1 and Sp3. Cotransfection of an Sp1 expression plasmid into Drosophila SL2 cells with a -228 to -3 LTC(4)S reporter construct transactivated the reporter gene, whereas mutations at the CACCC repeat region reduced Sp1 transactivation by approximately 66%. Similarly, the Kruppel-like factor Zf9/CPBP (core promoter-binding protein) transactivated the -228 construct in COS cells but not its CACCC mutant construct. These findings indicate the involvement of Sp1 and an Inr in non-cell-specific regulation and a Kruppel-like transcription factor and Sp1 in the cell-specific regulation of the LTC(4)S gene. These are the first such analyses of a member of a newly recognized superfamily of membrane-associated proteins involved in eicosanoid and glutathione metabolism, which contains key proteins involved in the generation of both prostanoids and cysteinyl leukotrienes.

Membrane-associated proteins in eicosanoid and glutathione metabolism (MAPEG). A widespread protein superfamily

The members of the MAPEG superfamily have been aligned and found to be distantly related, with a common pattern of hydropathy. Figure 2A shows the multiple sequence alignments of the human members and Figure 2B the corresponding superimposed hydropathy profiles. The alignment in Figure 2A demonstrates a total of six strictly conserved residues. The Arg-51 in LTC4 synthase has been suggested to function as proton donor for the opening of the LTA4 epoxide. This arginine is found in all but the FLAP sequences in accordance with the observation that FLAP has no known enzyme activity. Also the Tyr-93 in LTC4 synthase has been suggested to function as a base for the formation of the thiolate anion of glutathione. This tyrosine is not conserved in MGST1 or MGST1-L1. Table 1 summarizes some other properties of the individual human proteins. They are all of the same size, ranging from 147 to 161 amino acids. Only FLAP differs in that its isoelectric point is more neutral than that of the other, more basic proteins. The genes encoding these proteins all reside on different chromosomes (when known) (Table 1). In addition to the human proteins, MAPEG members have been identified in plants, fungi, and bacteria. It is clearly a challenge to elucidate their role in these different phyla in relation to their defined physiological functions in humans.

The biochemical, molecular, and genomic aspects of leukotriene C4 synthase

Leukotriene C4 (LTC4) synthase is an 18 kD integral membrane enzyme of the 5-lipoxygenase/LTC4 synthase pathway and is positioned as the pivotal and only committed enzyme for the formation of the cysteinyl leukotrienes. Although its function is to conjugate catalytically LTA4 to reduced glutathione, LTC4 synthase is differentiated from other glutathione S-transferase family members by its lack of amino acid homology, substrate specificity, and kinetics. LTC4 synthase (LTC4S) protein is present in the perinuclear membranes of a limited number of hematopoietic cells involved in allergic inflammation, including mast cells, eosinophils, basophils, and macrophages. The cDNA encodes a monomeric protein of 150 amino acids with three hydrophobic domains interspersed with two hydrophilic loops. Site-directed mutagenic studies reveal that the enzyme functions as a homodimer and that arginine-51 in the first hydrophilic loop, and tyrosine-93 in the second hydrophilic loop, are involved in the acid and base catalysis of LTA4 and glutathione, respectively. Homology and secondary structural predictions indicate that LTC4S is a novel member of a new gene superfamily of integral membrane proteins, each with the capacity to participate in leukotriene biosynthesis. The gene for LTC4S is 2.5 kb in length and is localized on chromosome 5q35, distal to that of the genes for cytokines and receptors important in the development and perpetuation of allergic inflammation. Immunohistochemical studies of mucosal biopsies from the bronchi of aspirin-intolerant asthmatics show that LTC4S is overrepresented in individuals with this phenotype, and this finding correlates with overproduction of cysteinyl leukotrienes and lysine-aspirin bronchial hyperreactivity.

Glutathione and glutathione-dependent enzymes represent a co-ordinately regulated defence against oxidative stress

Increases in the intracellular levels of reactive oxygen species (ROS), frequently referred to as oxidative stress, represents a potentially toxic insult which if not counteracted will lead to membrane dysfunction, DNA damage and inactivation of proteins. Chronic oxidative stress has numerous pathological consequences including cancer, arthritis and neurodegenerative disease. Glutathione-associated metabolism is a major mechanism for cellular protection against agents which generate oxidative stress. It is becoming increasingly apparent that the glutathione tripeptide is central to a complex multifaceted detoxification system, where there is substantial inter-dependence between separate component members. Glutathione participates in detoxification at several different levels, and may scavenge free radicals, reduce peroxides or be conjugated with electrophilic compounds. Thus, glutathione provides the cell with multiple defences not only against ROS but also against their toxic products. This article discusses how glutathione biosynthesis, glutathione peroxidases, glutathione S-transferases and glutathione S-conjugate efflux pumps function in an integrated fashion to allow cellular adaption to oxidative stress. Co-ordination of this response is achieved, at least in part, through the antioxidant responsive element (ARE) which is found in the promoters of many of the genes that are inducible by oxidative and chemical stress. Transcriptional activation through this enhancer appears to be mediated by basic leucine zipper transcription factors such as Nrf and small Maf proteins. The nature of the intracellular sensor(s) for ROS and thiol-active chemicals which induce genes through the ARE is described. Gene activation through the ARE appears to account for the enhanced antioxidant and detoxification capacity of normal cells effected by many cancer chemopreventive agents. In certain instances it may also account for acquired resistance of tumours to cancer chemotherapeutic drugs. It is therefore clear that determining the mechanisms involved in regulation of ARE-driven gene expression has enormous medical implications.

LTC4 synthase. Enzymology, biochemistry, and molecular characterization

LTC4S conjugates reduce glutathione to LTA4 and is positioned as the pivotal and only committed enzyme involved in the formation of cysteinyl LTs. Despite its function as an enzyme that conjugates glutathione to LTA4, it is abundantly clear that LTC4S differs from the classic glutathione S-transferase (GST) families. This distinction is based on narrow substrate specificity, inability to conjugate GSH to xenobiotics, differential susceptibility to inhibitors, lack of homology, and failure to be immunorecognized by specific microsomal GST antibodies. The presence of LTC4S protein is restricted to a limited number of hematopoietic cells to include mast cells, eosinophils, basophils, monocytes/macrophages, and platelets, with the platelet being unique in its lack of the complete biosynthetic pathway for cysteinyl LTs. The purification of the protein and the cloning of the cDNA have demonstrated that the kinetic parameters of LTC4S are similar for the isolated natural or recombinant proteins. The protein is an 18-kDa integral perinuclear membrane enzyme, which is functional as a homodimer. The cDNA encodes a 150 amino-acid polypeptide monomer with three hydrophobic domains interspersed by two hydrophilic loops. Homology and secondary structural predictions have revealed that LTC4S is a member of a novel gene family that includes FLAP, mGST II, and mGST III. Each of these molecules is an integral membrane protein with the capacity to participate in LT biosynthesis: LTC4S as the terminal and only committed enzyme in cysteinyl LT formation, FLAP as an arachidonic acid presentation protein, and mGST II and mGST III as unique dual-function enzymes with primary detoxification functions. Site directed mutagenic studies of LTC4S have revealed that two residues, R51 and Y93, are involved in the acid and base catalysis, respectively, of LTA4 and GSH. Alignment of molecules with LTA4 conjugating ability demonstrates conservation of amino acid residues R51 and Y93, which appear necessary for this specific enzymatic function. The 2.5-Kb gene for human LTC4S contains five small exons and four introns, and the 5' UTR contains consensus sequences for AP-1 and AP-2 sites as well as an SP-1 site. The chromosomal localization of this gene is 5q35, distal to that of cytokine, growth factor, and receptor genes that have relevance to the development of allergic inflammation. Furthermore, there is genetic linkage of this region of human chromosome 5 to atopy and asthma, whereas no linkage exists for the chromosomal localization of the other family members, FLAP and mGST II, distinguishing LTC4S as a unique member of the novel gene family. LTC4S is profoundly overexpressed in the aspirin-induced asthmatic phenotype and correlates with overproduction of cysteinyl LTs and bronchial hyperreactivity to lysine aspirin. Ongoing studies are directed to the genomic regulation and additional polymorphisms within the gene of this pivotal enzyme, as well as to further identification of the amino acid residues central to its catalytic function.

Demonstration of cell-specific phosphorylation of LTC4 synthase

PMA-induced leukotriene C4 synthase (LTC4S) phosphorylation was investigated over a period of 8 h in a monocytic cell line (THP-1). The level of LTC4S phosphorylation was increased 3-5 fold over a 4 h period decreasing to basal levels after 8 h. This phosphorylation event was found to be specific to THP-1 cells as there was a lack of LTC4S phosphorylation in both COS-7 and K-562 cells, and was also found to be dependent on the cellular confluency. In the presence of specific protein kinase C (PKC) inhibitors, a dose-dependent inhibition of the phosphorylation of LTC4S became evident, an effect not seen with PKA and tyrosine kinase inhibitors. This represents the first direct demonstration of LTC4S phosphorylation in whole cells.

Asthma and leukotrienes: antileukotrienes as novel anti-asthmatic drugs

Antileukotriene drugs inhibit the formation or action of leukotrienes, which are potent lipid mediators generated from arachidonic acid in lung tissue and inflammatory cells. The leukotrienes were discovered in basic studies of arachidonic acid metabolism in leucocytes 20 years ago and were found to display a number of biological activities which may contribute to airway obstruction. Clinical studies with antileukotriene drugs have indeed demonstrated that leukotrienes are significant mediators of airway obstruction evoked by many common trigger factors in asthma. Moreover, treatment trials have established that this new class of drugs has beneficial anti-asthmatic properties, and several antileukotrienes have recently been introduced as new therapy of asthma. This communication presents an overview of the biosynthesis of leukotrienes, their biological effects and clinical effects of antileukotrienes in the treatment of asthama.