Tyrosine kinase activity modulates catalysis and translocation of cellular 5-lipoxygenase

Insights into topology and membrane interaction characteristics of plastoglobule-localized AtFBN1a and AtLOX2

Plant chloroplasts harbor ubiquitous lipid droplets called plastoglobules. While physically connected to the thylakoid membrane, they are characterized by a unique set of about 30 proteins specifically associated with the plastoglobule. How these proteins selectively target the plastoglobule remains unknown. Protease shaving assays with isolated Arabidopsis thaliana thylakoid and plastoglobule show that a ca. 25 kD portion of the abundant structural protein of plastoglobules, Fibrillin 1a, is protected from protease digestion. Mapping of protease cleavage sites and experimentally identified phosphorylation sites onto a homology model of Fibrillin 1a indicates that this protected sequence corresponds to the C-terminal lipocalin-like domain, implicated in specific lipid binding. In contrast, protease shaving and membrane washing assays with another plastoglobule-associated protein harboring a C-terminal PLAT domain, Lipoxygenase 2, is consistent with an exposed PLAT domain positioned parallel with, and upon, the surface of the plastoglobule. We propose a model where conserved lipid-binding domains associate with either the surface or neutral core of the lipid droplet. Our study provides insight into the topology and membrane interactions of two plastoglobule-localized proteins.

Emerging Roles of 5-Lipoxygenase Phosphorylation in Inflammation and Cell Death

5-Lipoxygenase (ALOX5) is an iron-containing and nonheme dioxygenase that catalyzes the peroxidation of polyunsaturated fatty acids such as arachidonic acid. ALOX5 is the rate-limiting enzyme for the biosynthesis of leukotrienes, a family of proinflammatory lipid mediators derived from arachidonic acid. ALOX5 also make great contributions to mediating lipid peroxidation. In recent years, it has been discovered that ALOX5 plays a central role in cell death including apoptosis, pyroptosis, and ferroptosis, a newly discovered type of cell death. According to the previous studies, ALOX5 can regulate cell death in two ways: one is inflammation and the other is lipid peroxidation. Meanwhile, it has been shown that ALOX5 activity is regulated by several factors including protein phosphorylation, ALOX5-interactng protein, redox state, and metal ions such as iron and calcium. In this review, we aim to summarize the knowledge on the emerging roles of ALOX5 protein phosphorylation in the regulation of cell death and inflammation in order to explore a potential target for human diseases.

Impact of food polyphenols on oxylipin biosynthesis in human neutrophils

The intake of food polyphenols is associated with beneficial impacts on health. Besides anti-oxidative effects, anti-inflammatory properties have been suggested as molecular modes of action, which may result from modulations of the arachidonic acid (AA) cascade. Here, we investigated the effects of a library of food polyphenols on 5-lipoxygenase (5-LOX) activity in a cell-free assay, and in human neutrophils. Resveratrol, its dimer (ε-viniferin), and its imine analogue (IRA) potently blocked the 5-LOX-mediated LT formation in neutrophils with IC₅₀ values in low μM-range. Among the tested flavonoids only the isoflavone genistein showed potent 5-LOX inhibition in neutrophils (IC₅₀ = 0.4 ± 0.1 μM), however was ineffective on isolated 5-LOX. We exclude an interference with the 5-LOX-activating protein (FLAP) in HEK_5-LOX/±FLAP cells and suggest global effects on intact immune cells. Using LC-MS based targeted oxylipin metabolomics, we analyzed the effects of 5-LOX-inhibiting polyphenols on all branches of the AA cascade in Ca²⁺-ionophore-challenged neutrophils. While ε-viniferin causes a clear substrate shunt towards the remaining AA cascade enzymes (15-LOX, cyclooxygenase - COX-1/2, cytochrome P450), resveratrol inhibited the COX-1/2 pathway and showed a weak attenuation of 12/15-LOX activity. IRA had no impact on 15-LOX activity, but elevated the formation of COX-derived prostaglandins, having no inhibitory effects on COX-1/2. Overall, we show that food polyphenols have the ability to block 5-LOX activity and the oxylipin pattern is modulated with a remarkable compound/structural specificity. Taken the importance of polyphenols for a healthy diet and their concentration in food supplements into account, this finding justifies further investigation.

Different roles of Nrf2 and NFKB in the antioxidant imbalance produced by esculetin or quercetin on NB4 leukemia cells

Esculetin (6,7-dihydrocoumarin) and the flavonoid quercetin (3,5,7,3',4' pentahydroxyflavone) are compounds that could change the balance of redox homeostasis. NB4 leukemia cells were treated with 25 μM quercetin for 24 h and with esculetin at either 100 or 500 μM for different times. Quercetin increased the levels of pro-inflammatory NFkB p65 in the nucleus correspondingly reducing them in the cytosol. The levels of NFkB p65 decreased in the nucleus at high esculetin concentration treatments for long times (19 h), concomitantly increasing the levels of anti-inflammatory NFkB p50 in the nucleus. This could suggest formation of inhibitory p50 homodimers possibly related with anti-inflammatory response. Lipoxygenase expression was reduced either by esculetin or quercetin. A significant increase of Nrf2 in the nucleus of NB4 cells treated with 100 μM esculetin for 19 h was observed. Quercetin increased the levels of Nrf2 in the cytosol reducing them in the nucleus. Superoxide dismutase expression increased in NB4 cells treated with esculetin in contrast with quercetin. All these data support a relevant differential role for NFkB and Nrf2 in anti-inflammatory and redox response when apoptosis was induced by esculetin or quercetin in human leukemia NB4 cells.

Eicosanoids and cancer

Eicosanoids are 20-carbon bioactive lipids derived from the metabolism of polyunsaturated fatty acids, which can modulate various biological processes including cell proliferation, adhesion and migration, angiogenesis, vascular permeability and inflammatory responses. In recent years, studies have shown the importance of eicosanoids in the control of physiological and pathological processes associated with several diseases, including cancer. The polyunsaturated fatty acid predominantly metabolized to generate 2-series eicosanoids is arachidonic acid, which is the major n-6 polyunsaturated fatty acid found in animal fat and in the occidental diet. The three main pathways responsible for metabolizing arachidonic acid and other polyunsaturated fatty acids to generate eicosanoids are the cyclooxygenase, lipoxygenase and P450 epoxygenase pathways. Inflammation plays a decisive role in various stages of tumor development including initiation, promotion, invasion and metastasis. This review will focus on studies that have investigated the role of prostanoids and lipoxygenase-derived eicosanoids in the development and progression of different tumors, highlighting the findings that may provide insights into how these eicosanoids can influence cell proliferation, cell migration and the inflammatory process. A better understanding of the complex role played by eicosanoids in both tumor cells and the tumor microenvironment may provide new markers for diagnostic and prognostic purposes and identify new therapeutic strategies in cancer treatment.

Phosphorylation of Leukotriene C4 Synthase at Serine 36 Impairs Catalytic Activity

Leukotriene C4 synthase (LTC4S) catalyzes the formation of the proinflammatory lipid mediator leukotriene C4 (LTC4). LTC4 is the parent molecule of the cysteinyl leukotrienes, which are recognized for their pathogenic role in asthma and allergic diseases. Cellular LTC4S activity is suppressed by PKC-mediated phosphorylation, and recently a downstream p70S6k was shown to play an important role in this process. Here, we identified Ser(36) as the major p70S6k phosphorylation site, along with a low frequency site at Thr(40), using an in vitro phosphorylation assay combined with mass spectrometry. The functional consequences of p70S6k phosphorylation were tested with the phosphomimetic mutant S36E, which displayed only about 20% (20 μmol/min/mg) of the activity of WT enzyme (95 μmol/min/mg), whereas the enzyme activity of T40E was not significantly affected. The enzyme activity of S36E increased linearly with increasing LTA4 concentrations during the steady-state kinetics analysis, indicating poor lipid substrate binding. The Ser(36) is located in a loop region close to the entrance of the proposed substrate binding pocket. Comparative molecular dynamics indicated that Ser(36) upon phosphorylation will pull the first luminal loop of LTC4S toward the neighboring subunit of the functional homotrimer, thereby forming hydrogen bonds with Arg(104) in the adjacent subunit. Because Arg(104) is a key catalytic residue responsible for stabilization of the glutathione thiolate anion, this phosphorylation-induced interaction leads to a reduction of the catalytic activity. In addition, the positional shift of the loop and its interaction with the neighboring subunit affect active site access. Thus, our mutational and kinetic data, together with molecular simulations, suggest that phosphorylation of Ser(36) inhibits the catalytic function of LTC4S by interference with the catalytic machinery.

Pharmacological inhibition of eicosanoids and platelet-activating factor signaling impairs zymosan-induced release of IL-23 by dendritic cells

The engagement of the receptors for fungal patterns induces the expression of cytokines, the release of arachidonic acid, and the production of PGE2 in human dendritic cells (DC), but few data are available about other lipid mediators that may modulate DC function. The combined antagonism of leukotriene (LT) B4, cysteinyl-LT, and platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) inhibited IL23A mRNA expression in response to the fungal surrogate zymosan and to a lower extent TNFA (tumor necrosis factor-α) and CSF2 (granulocyte macrophage colony-stimulating factor) mRNA. The combination of lipid mediators and the lipid extract of zymosan-conditioned medium increased the induction of IL23A by LPS (bacterial lipopolysaccharide), thus suggesting that unlike LPS, zymosan elicits the production of mediators at a concentration enough for optimal response. Zymosan induced the release of LTB4, LTE4, 12-hydroxyeicosatetraenoic acid (12-HETE), and PAF C16:0. DC showed a high expression and detectable Ser663 phosphorylation of 5-lipoxygenase in response to zymosan, and a high expression and activity of LPCAT1/2 (lysophosphatidylcholine acyltransferase 1 and 2), the enzymes that incorporate acetate from acetyl-CoA into choline-containing lysophospholipids to produce PAF. Pharmacological modulation of the arachidonic acid cascade and the PAF receptor inhibited the binding of P-71Thr-ATF2 (activating transcription factor 2) to the IL23A promoter, thus mirroring their effects on the expression of IL23A mRNA and IL-23 protein. These results indicate that LTB4, cysteinyl-LT, and PAF, acting through their cognate G protein-coupled receptors, contribute to the phosphorylation of ATF2 and play a central role in IL23A promoter trans-activation and the cytokine signature induced by fungal patterns.

ATP allosterically activates the human 5-lipoxygenase molecular mechanism of arachidonic acid and 5(S)-hydroperoxy-6(E),8(Z),11(Z),14(Z)-eicosatetraenoic acid

5-Lipoxygenase (5-LOX) reacts with arachidonic acid (AA) to first generate 5(S)-hydroperoxy-6(E),8(Z),11(Z),14(Z)-eicosatetraenoic acid [5(S)-HpETE] and then an epoxide from 5(S)-HpETE to form leukotriene A₄, from a single polyunsaturated fatty acid. This work investigates the kinetic mechanism of these two processes and the role of ATP in their activation. Specifically, it was determined that epoxidation of 5(S)-HpETE (dehydration of the hydroperoxide) has a rate of substrate capture (V_max/K_m) significantly lower than that of AA hydroperoxidation (oxidation of AA to form the hydroperoxide); however, hyperbolic kinetic parameters for ATP activation indicate a similar activation for AA and 5(S)-HpETE. Solvent isotope effect results for both hydroperoxidation and epoxidation indicate that a specific step in its molecular mechanism is changed, possibly because of a lowering of the dependence of the rate-limiting step on hydrogen atom abstraction and an increase in the dependency on hydrogen bond rearrangement. Therefore, changes in ATP concentration in the cell could affect the production of 5-LOX products, such as leukotrienes and lipoxins, and thus have wide implications for the regulation of cellular inflammation.

Effects of menadione, a reactive oxygen generator, on leukotriene secretion from RBL-2H3 cells

Reactive oxygen species (ROS) are produced in various cells and affect many biological processes. We previously reported that 2-methyl-1,4-naphtoquinone (menadione) inhibited Ca(2+) influx from the extracellular medium and exocytosis evoked by antigen stimulation in the mast cell line, RBL-2H3. Mast cells release various inflammatory mediators such as leukotrienes (LTs) and cytokines in addition to the exocytotic secretion of histamine. In this study, we investigated the effects of menadione on LT release in RBL-2H3. Treatment of RBL cells with menadione inhibited LTC(4) secretion induced by antigen stimulation. To elucidate the mechanism of this inhibition, we examined the effects of menadione on the activation process of 5-lipoxygenase that is responsible for the synthesis of LTs from arachidonic acid. Menadione did not affect the phosophorylation of mitogen activated protein (MAP) kinases, extracellular signal-regulated kinase (ERK) and p38, which regulates phosphorylation of 5-lipoxygenase. However, menadione inhibited the translocation of 5-lipoxygenase from the cytoplasm to the nuclear membrane. Together with the result that LT secretion was severely impaired in the absence of extracellular Ca2(2+), it is suggested that ROS produced by menadione inhibited LT secretion through impaired Ca2(2+) influx and 5-lipoxygenase translocation to the nuclear membrane.

HNE-induced 5-LO expression is regulated by NF-{kappa}B/ERK and Sp1/p38 MAPK pathways via EGF receptor in murine macrophages

AIMS

5-Lipoxygenase (5-LO) has been suggested to be a modulator of atherosclerotic plaque instability and co-exists with 4-hydroxynonenal (HNE) in macrophages in atherosclerotic lesions. To determine the potential role for HNE in 5-LO expression, the molecular mechanisms of 5-LO expression were evaluated in HNE-stimulated macrophages.

METHODS AND RESULTS

A genomic sequence of the promoter 2.0 kb upstream of the transcription initiation site was amplified, and a series of sequentially deleted fragments were then fused to a luciferase reporter gene. The promoter region 213 bp upstream of the transcription start site was responsible for the HNE-enhanced transcriptional activity of 5-LO. Site-directed mutagenesis of this region showed that the transcription factors, including stimulating protein 1 (Sp1) and nuclear factor-κB (NF-κB), were associated with up-regulation of HNE-induced 5-LO transcription. Moreover, the role of Sp1 and NF-κB in HNE-induced 5-LO expression was confirmed by siRNA knockdown of Sp1 and NF-κB. The HNE-enhanced Sp1 and NF-κB activities were attenuated by SB203580, a p38 mitogen-activated protein kinase (MAPK) inhibitor, and PD98059, an extracellular signal-regulated kinase (ERK) inhibitor, respectively. In addition, the HNE-enhanced phosphorylation of p38 MAPK and ERK was inhibited by AG1478, an epidermal growth factor receptor (EGFR) antagonist, but not by AG1295, a platelet-derived growth factor receptor (PDGFR) antagonist.

CONCLUSION

5-LO expression by HNE was regulated at the transcriptional level by the EGFR-mediated activation of Sp1/p38 MAPK and NF-κB/ERK pathways in macrophages, which may lead to the development of therapeutic interventions for regulating 5-LO expression in atherosclerosis.

4-Hydroxynonenal enhances CD36 expression on murine macrophages via p38 MAPK-mediated activation of 5-lipoxygenase

Increased levels of 4-hydroxynonenal (HNE) and 5-lipoxygenase (5-LO) coexist in atherosclerotic lesions but their relationship in atherogenesis is unclear. This study investigated the role of 5-LO in HNE-induced CD36 expression and macrophage foam cell formation, and the link between HNE and 5-LO. In J774A.1 murine macrophages, HNE (10 microM) enhanced CD36 expression in association with an increased uptake of oxLDL, which was blunted by inhibition of 5-LO with MK886, a 5-LO inhibitor, or with 5-LO siRNA. In peritoneal macrophages from 5-LO-deficient mice, HNE-induced CD36 expression was markedly attenuated, confirming a pivotal role of 5-LO in HNE-induced CD36 expression. In an assay for 5-LO activity, stimulation of macrophages with HNE led to increased leukotriene B(4) production in the presence of exogenous arachidonic acid in association with an increased association of 5-LO to the nuclear membrane. Among the mitogen-activated protein kinase (MAPK) pathways involved in 5-LO phosphorylation, HNE predominantly activated p38 MAPK in macrophages, and the p38 MAPK inhibitor SB203580, but not an extracellular signal-regulated kinase inhibitor, suppressed HNE-induced LTB(4) production. Collectively, these data suggest that p38 MAPK-mediated activation of 5-LO by HNE might enhance CD36 expression, consequently leading to the formation of macrophage foam cells.

Physiology of cell volume regulation in vertebrates

The ability to control cell volume is pivotal for cell function. Cell volume perturbation elicits a wide array of signaling events, leading to protective (e.g., cytoskeletal rearrangement) and adaptive (e.g., altered expression of osmolyte transporters and heat shock proteins) measures and, in most cases, activation of volume regulatory osmolyte transport. After acute swelling, cell volume is regulated by the process of regulatory volume decrease (RVD), which involves the activation of KCl cotransport and of channels mediating K(+), Cl(-), and taurine efflux. Conversely, after acute shrinkage, cell volume is regulated by the process of regulatory volume increase (RVI), which is mediated primarily by Na(+)/H(+) exchange, Na(+)-K(+)-2Cl(-) cotransport, and Na(+) channels. Here, we review in detail the current knowledge regarding the molecular identity of these transport pathways and their regulation by, e.g., membrane deformation, ionic strength, Ca(2+), protein kinases and phosphatases, cytoskeletal elements, GTP binding proteins, lipid mediators, and reactive oxygen species, upon changes in cell volume. We also discuss the nature of the upstream elements in volume sensing in vertebrate organisms. Importantly, cell volume impacts on a wide array of physiological processes, including transepithelial transport; cell migration, proliferation, and death; and changes in cell volume function as specific signals regulating these processes. A discussion of this issue concludes the review.

A mechanism of benefit of soy genistein in asthma: inhibition of eosinophil p38-dependent leukotriene synthesis

BACKGROUND

Dietary intake of the soy isoflavone genistein is associated with reduced severity of asthma, but the mechanisms responsible for this effect are unknown.

OBJECTIVE

To determine whether genistein blocks eosinophil leukotriene C(4) (LTC(4)) synthesis and to evaluate the mechanism of this effect, and to assess the impact of a 4-week period of soy isoflavone dietary supplementation on indices of eosinophilic inflammation in asthma patients.

METHODS

Human peripheral blood eosinophils were stimulated in the absence and presence of genistein, and LTC(4) synthesis was measured. 5-lipoxygenase (5-LO) nuclear membrane translocation was assessed by confocal immunofluorescence microscopy. Mitogen-activated protein (MAP) kinase activation was determined by immunoblot. Human subjects with mild-to-moderate persistent asthma and minimal or no soy intake were given a soy isoflavone supplement (100 mg/day) for 4 weeks. The fraction of exhaled nitric oxide (FE(NO)) and ex vivo eosinophil LTC(4) production were assessed before and after the soy isoflavone treatment period.

RESULTS

Genistein inhibited eosinophil LTC(4) synthesis (IC(50) 80 nm), blocked phosphorylation of p38 MAP kinase and its downstream target MAPKAP-2, and reduced translocation of 5-LO to the nuclear membrane. In patients with asthma, following 4 weeks of dietary soy isoflavone supplementation, ex vivo eosinophil LTC(4) synthesis decreased by 33% (N=11, P=0.02) and FE(NO) decreased by 18% (N=13, P=0.03).

CONCLUSION

At physiologically relevant concentrations, genistein inhibits eosinophil LTC(4) synthesis in vitro, probably by blocking p38- and MAPKAP-2-dependent activation of 5-LO. In asthma patients, dietary soy isoflavone supplementation reduces eosinophil LTC(4) synthesis and eosinophilic airway inflammation. These results support a potential role for soy isoflavones in the treatment of asthma.

Oxidation of 2-Cys-peroxiredoxins by arachidonic acid peroxide metabolites of lipoxygenases and cyclooxygenase-2

Human peroxiredoxins serve dual roles as anti-oxidants and regulators of H(2)O(2)-mediated cell signaling. The functional versatility of peroxiredoxins depends on progressive oxidation of key cysteine residues. The sulfinic or sulfonic forms of peroxiredoxin lose their peroxidase activity, which allows cells to accumulate H(2)O(2) for signaling or pathogenesis in inflammation, cancer, and other disorders. We report that arachidonic acid lipid hydroperoxide metabolites of 5-, 12-, 15-lipoxygenase-1, and cyclooxygenase-2 oxidize the 2-Cys-peroxiredoxins 1, 2, and 3 to their sulfinic and sulfonic forms. When added exogenously to cells, 5-, 12- and 15-hydroperoxy-eicosatetraenoic acids also over-oxidized peroxiredoxins. Our results suggest that lipoxygenases and cyclooxygenases may affect 2-Cys peroxiredoxin signaling, analogous to NADPH oxidases in the "floodgate" model (Wood, Z. A., Poole, L. B, and Karplus P. A. (2003) Science 300, 600-653). Peroxiredoxin-dependent mechanisms may modulate the receptor-dependent actions of autocoids derived from cellular lipoxygenase and cyclooxygenase catalysis.

5-Lipoxygenase: regulation of expression and enzyme activity

5-Lipoxygenase (5-LO) catalyzes the first two steps in the biosynthesis of leukotrienes, a group of pro-inflammatory lipid mediators derived from arachidonic acid. Leukotriene antagonists are used in the treatment of asthma, and the potential role of leukotrienes in atherosclerosis, another chronic inflammatory disease, has recently received considerable attention. In addition, some possible effects of 5-LO metabolites in tumorigenesis have emerged. Thus, knowledge of the biochemistry of this enzyme has potential implications for the treatment of various diseases. Recent advances have expanded our understanding of the regulatory mechanisms underlying the expression and control of 5-LO activity. With regard to the control of enzyme activity, many of these findings focus on the N-terminal domain of 5-LO.

5-Lipoxygenase: Regulation and possible involvement in atherosclerosis

This review article focuses on two aspects regarding 5-lipoxygenase. First, mechanisms for activation of the enzyme. Second, the involvement of 5-lipoxygenase and leukotrienes in atherosclerosis.

Tocotrienols in health and disease: the other half of the natural vitamin E family

Tocochromanols encompass a group of compounds with vitamin E activity essential for human nutrition. Structurally, natural vitamin E includes eight chemically distinct molecules: alpha-, beta-, gamma- and delta-tocopherol; and alpha-, beta-, gamma- and delta-tocotrienol. Symptoms caused by alpha-tocopherol deficiency can be alleviated by tocotrienols. Thus, tocotrienols may be viewed as being members of the natural vitamin E family not only structurally but also functionally. Palm oil and rice bran oil represent two major nutritional sources of natural tocotrienol. Taken orally, tocotrienols are bioavailable to all vital organs. The tocotrienol forms of natural vitamin E possesses powerful hypocholesterolemic, anti-cancer and neuroprotective properties that are often not exhibited by tocopherols. Oral tocotrienol protects against stroke-associated brain damage in vivo. Disappointments with outcomes-based clinical studies testing the efficacy of alpha-tocopherol need to be handled with caution and prudence recognizing the untapped opportunities offered by the other forms of natural vitamin E. Although tocotrienols represent half of the natural vitamin E family, work on tocotrienols account for roughly 1% of the total literature on vitamin E. The current state of knowledge warrants strategic investment into investigating the lesser known forms of vitamin E.

Modulation of signal transduction by vitamin E

The ability of vitamin E to modulate signal transduction and gene expression has been observed in numerous studies; however, the detailed molecular mechanisms involved are often not clear. The eight natural vitamin E analogues and synthetic derivatives affect signal transduction with different potency, possibly reflecting their different ability to interact with specific proteins. Vitamin E modulates the activity of several enzymes involved in signal transduction, such as protein kinase C, protein kinase B, protein tyrosine kinases, 5-, 12-, and 15-lipoxygenases, cyclooxygenase-2, phospholipase A2, protein phosphatase 2A, protein tyrosine phosphatase, and diacylglycerol kinase. Activation of some these enzymes after stimulation of cell surface receptors with growth factors or cytokines can be normalized by vitamin E. At the molecular level, the translocation of several of these enzymes to the plasma membrane is affected by vitamin E, suggesting that the modulation of protein-membrane interactions may be a common theme for vitamin E action. In this review the main effects of vitamin E on enzymes involved in signal transduction are summarized and the possible mechanisms leading to enzyme modulation evaluated. The elucidation of the molecular and cellular events affected by vitamin E could reveal novel strategies and molecular targets for developing similarly acting compounds.

Tocotrienols: the emerging face of natural vitamin E

Natural vitamin E includes eight chemically distinct molecules: alpha-, beta-, gamma-, and delta-tocopherols and alpha-, beta-, gamma-, and delta-tocotrienols. More than 95% of all studies on vitamin E are directed toward the specific study of alpha-tocopherol. The other forms of natural vitamin E remain poorly understood. The abundance of alpha-tocopherol in the human body and the comparable efficiency of all vitamin E molecules as antioxidants led biologists to neglect the non-tocopherol vitamin E molecules as topics for basic and clinical research. Recent developments warrant a serious reconsideration of this conventional wisdom. The tocotrienol subfamily of natural vitamin E possesses powerful neuroprotective, anticancer, and cholesterol-lowering properties that are often not exhibited by tocopherols. Current developments in vitamin E research clearly indicate that members of the vitamin E family are not redundant with respect to their biological functions. alpha-Tocotrienol, gamma-tocopherol, and delta-tocotrienol have emerged as vitamin E molecules with functions in health and disease that are clearly distinct from that of alpha-tocopherol. At nanomolar concentration, alpha-tocotrienol, not alpha-tocopherol, prevents neurodegeneration. On a concentration basis, this finding represents the most potent of all biological functions exhibited by any natural vitamin E molecule. Recently, it has been suggested that the safe dose of various tocotrienols for human consumption is 200-1000/day. A rapidly expanding body of evidence supports that members of the vitamin E family are functionally unique. In recognition of this fact, title claims in publications should be limited to the specific form of vitamin E studied. For example, evidence for toxicity of a specific form of tocopherol in excess may not be used to conclude that high-dosage "vitamin E" supplementation may increase all-cause mortality. Such conclusion incorrectly implies that tocotrienols are toxic as well under conditions where tocotrienols were not even considered. The current state of knowledge warrants strategic investment into the lesser known forms of vitamin E. This will enable prudent selection of the appropriate vitamin E molecule for studies addressing a specific health need.

Cyclooxygenases, lipoxygenases, and epoxygenases in CNS: Their role and involvement in neurological disorders

Three enzyme systems, cyclooxygenases that generate prostaglandins, lipoxygenases that form hydroxy derivatives and leukotrienes, and epoxygenases that give rise to epoxyeicosatrienoic products, metabolize arachidonic acid after its release from neural membrane phospholipids by the action of phospholipase A(2). Lysophospholipids, the other products of phospholipase A(2) reactions, are either reacylated or metabolized to platelet-activating factor. Under normal conditions, these metabolites play important roles in synaptic function, cerebral blood flow regulation, apoptosis, angiogenesis, and gene expression. Increased activities of cyclooxygenases, lipoxygenases, and epoxygenases under pathological situations such as ischemia, epilepsy, Alzheimer's disease, Parkinson disease, amyotrophic lateral sclerosis, and Creutzfeldt-Jakob disease produce neuroinflammation involving vasodilation and vasoconstriction, platelet aggregation, leukocyte chemotaxis and release of cytokines, and oxidative stress. These are closely associated with the neural cell injury which occurs in these neurological conditions. The metabolic products of docosahexaenoic acid, through these enzymes, generate a new class of lipid mediators, namely docosatrienes and resolvins. These metabolites antagonize the effect of metabolites derived from arachidonic acid. Recent studies provide insight into how these arachidonic acid metabolites interact with each other and other bioactive mediators such as platelet-activating factor, endocannabinoids, and docosatrienes under normal and pathological conditions. Here, we review present knowledge of the functions of cyclooxygenases, lipoxygenases, and epoxygenases in brain and their association with neurodegenerative diseases.

Metabolism of arachidonic acid to eicosanoids within the nucleus

The eicosanoids are a diverse family of molecules that have powerful effects on cell function. They are best known as intercellular messengers, having autocrine and paracrine effects following their secretion from the cells that synthesize them. Many of the eicosanoids are produced from one polyunsaturated fatty acid, arachidonic acid. The diversity of possible products that can be synthesized from arachidonic acid is due, in part to the variety of enzymes that can act on it. Over the past 15 years, studies have placed many, but not all, of these enzymes at or inside the nucleus. In some cases, the nuclear import or export of arachidonic acid-processing enzymes is highly regulated. Furthermore, nuclear receptors that are activated by specific eicosanoids are known to exist. Taken together, these findings indicate that the enzymatic conversion of arachidonic acid to specific signaling molecules can occur in the nucleus, that it is regulated, and that the synthesized products may act within the nucleus. The objectives of this commentary are to review what is known about the metabolism of arachidonic acid to eicosanoids within the nucleus and to point to important areas for future discovery.

Activation of PLA2 isoforms by cell swelling and ischaemia/hypoxia

Phospholipase A2 (PLA2) activity is increased in mammalian cells in response to numerous stimuli such as osmotic challenge, oxidative stress and exposure to allergens. The increased PLA2 activity is seen as an increased release of free, polyunsaturated fatty acids, e.g. arachidonic acid and membrane-bound lysophospholipids. Even though arachidonic acid acts as a second messenger in its own most mammalian cells seem to rely on oxidation of the fatty acid into highly potent second messengers via, e.g. cytochrome P450, the cyclo-oxygenase, or the lipoxygenase systems for downstream signalling. Here, we review data that illustrates that stress-induced PLA2 activity involves various PLA2 subtypes and that the PLA2 in question is determined by the cell type and the physiological stress condition.

Activation of 5-lipoxygenase after oxygen-glucose deprivation is partly mediated via NMDA receptor in rat cortical neurons

5-Lipoxygenase (5-LOX) is the enzyme metabolizing arachidonic acid to produce pro-inflammatory leukotrienes. We have reported that 5-LOX is translocated to the nuclear envelope after ischemic-like injury in PC12 cells. In the present study, we determined whether 5-LOX is activated (translocation and production of leukotrienes) after oxygen-glucose deprivation (OGD) in primary rat cortical neurons; if so, whether this activation is mediated by NMDA receptor. After OGD, 5-LOX was translocated to the nuclear envelope as detected by immunoblotting, immunostaining and green fluorescent protein-5-LOX transfection. 5-LOX metabolites, cysteinyl-leukotrienes (CysLTs) but not leukotriene B4, in the culture media were increased 0.5-1.5 h after recovery. Similarly, NMDA (100 microm) also induced 5-LOX translocation, and increased the production of CysLTs during 0.5-1 h NMDA exposure. Both OGD and NMDA reduced neuron viability. NMDA receptor antagonist MK-801 inhibited almost all the responses to OGD and NMDA; whereas 5-LOX activating protein inhibitor MK-886 and 5-LOX inhibitor caffeic acid inhibited the reduction of neuron viability and the production of CysLTs, but did not affect 5-LOX translocation. From these results, we conclude that OGD can activate 5-LOX in primary rat cortical neurons, and that this activation may be partly mediated via activating NMDA receptor.

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.

Increased Expression of 5-Lipoxygenase in High-Grade Astrocytomas

OBJECTIVE

5-Lipoxygenase (5-LO) oxidizes arachidonic acid into proinflammatory eicosanoids that may promote tumorigenesis. In this study, we investigated whether 5-LO is expressed in human astrocytomas and what effect its expression may have on patient outcome.

METHODS

Increased 5-LO messenger ribonucleic acid and protein expression was detected by the polymerase chain reaction and antibody-based approaches, respectively, in surgical astrocytoma specimens and established glioblastoma multiforme cell lines compared with primary cell culture from the human white matter.

RESULTS

Immunohistochemical analysis revealed predominantly nuclear 5-LO staining in 44 of 49 glioblastoma multiforme samples (90%), 8 of 10 (80%) anaplastic astrocytomas samples, and 3 of 13 (23%) low-grade astrocytoma samples analyzed. Double-staining experiments with anti-CD-68 (macrophage/microglial marker) and anti-5-LO antibodies suggest that both CD-68-positive and CD-68-negative tumor cells express 5-LO protein. Staining of 5-LO was significantly more frequent in high-grade than in low-grade tumors (P = 0.001). Patients whose tumors expressed 5-LO were significantly older, had lower preoperative Karnofsky performance scores and shorter survival than patients whose tumors did not express 5-LO. After adjusting for pathological diagnosis and age, respectively, neither Karnofsky performance score nor survival were significantly associated with 5-LO staining.

CONCLUSION

These data indicate that 5-LO is overexpressed in high-grade astrocytomas and supports the idea that eicosanoids may play a role in tumorigenesis of these brain tumors.

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.

Regulation of 5-lipoxygenase enzyme activity

In this article, regulation of human 5-lipoxygenase enzyme activity is reviewed. First, structural properties and enzyme activities are described. This is followed by the activating factors: Ca2+, membranes, ATP, and lipid hydroperoxide. Also, studies on phosphorylation of 5-lipoxygenase and nuclear localization sequences are reviewed.

Arachidonic acid regulates the translocation of 5-lipoxygenase to the nuclear membranes in human neutrophils

Elevation of the intracellular cAMP concentration in agonist-activated human neutrophils (PMN) leads to the concomitant inhibitions of arachidonic acid (AA) release, 5-lipoxygenase (5-LO) translocation, and leukotriene (LT) biosynthesis. We report herein that exogenous AA completely prevents cAMP-dependent inhibition of 5-LO translocation and LT biosynthesis in agonist-activated PMN. Moreover, the group IVA phospholipase A2 inhibitor pyrrophenone and the MEK inhibitor U-0126 inhibited AA release and 5-LO translocation in activated PMN, and these effects were also prevented by exogenous AA, demonstrating a functional link between AA release and 5-LO translocation. Polyunsaturated fatty acids of the C18 and C20 series containing at least three double bonds located from carbon 9 (or closer to the carboxyl group) were equally effective as AA in restoring 5-LO translocation in pyrrophenone-treated agonist-activated PMN. Importantly, experiments with the 5-LO-activating protein inhibitor MK-0591 and the intracellular Ca2+ chelator BAPTA-AM demonstrated that the AA-regulated 5-LO translocation is FLAP- and Ca2+-dependent. Finally, the redox and competitive 5-LO inhibitors L-685,015, L-739,010, and L-702,539 (but not cyclooxygenase inhibitors) efficiently substituted for AA to reverse the pyrrophenone inhibition of 5-LO translocation, indicating that the site of regulation of 5-LO translocation by AA is at or in the vicinity of the catalytic site. This report demonstrates that AA regulates the translocation of 5-LO in human PMN and unravels a novel mechanism of the cAMP-mediated inhibition of LT biosynthesis.

Muscarinic receptors, leukotriene B4 production and neutrophilic inflammation in COPD patients

BACKGROUND

Acetylcholine (ACh) plays an important role in smooth muscle contraction and in the development of airway narrowing; preliminary evidences led us to hypothesize that ACh might also play a role in the development of airways inflammation in chronic obstructive pulmonary disease (COPD).

METHODS

We evaluated the concentrations of leukotriene B4 (LTB4) in induced sputum, and the expression of Ach M1, M2, and M3 receptors in sputum cells (SC) obtained from 16 patients with COPD, 11 smokers, and 14 control subjects. The SC were also treated with ACh and the production of LTB4 assessed in the presence or absence of a muscarinic antagonist (oxitropium). In blood monocytes, we evaluated LTB4 release and activation of the extracellular signal-regulated kinases (ERK) pathway after treatment with Ach.

RESULTS

The LTB4 concentrations were higher in COPD than in controls (P < 0.01) and correlated with the number of neutrophil (P < 0.01). The M3 receptors expression was increased in COPD subjects when compared to smokers and control (P < 0.05 and 0.0001, respectively), while M2 expression resulted decreased (P < 0.05 and 0.01). The ACh-induced LTB(4) production was observed in peripheral blood monocytes, and was sensitive to ERK inhibition. Similarly, ACh significantly increased neutrophil chemotactic activity and LTB4 released from SC of COPD patients only, and these effects were blocked by pretreatment with the inhibitor of ERK pathway PD98059.

CONCLUSIONS

The results obtained show that muscarinic receptors may be involved in airway inflammation in COPD subjects through ACh-induced, ERK1/2-dependent LTB4 release. Muscarinic antagonism may contribute to reduce neutrophil infiltration and activation in COPD.

1-Oleoyl-2-acetylglycerol stimulates 5-lipoxygenase activity via a putative (phospho)lipid binding site within the N-terminal C2-like domain

5-Lipoxygenase (5-LO) catalysis is positively regulated by Ca2+ ions and phospholipids that both act via the N-terminal C2-like domain of 5-LO. Previously, we have shown that 1-oleoyl-2-acetylglycerol (OAG) functions as an agonist for human polymorphonuclear leukocytes (PMNL) in stimulating 5-LO product formation. Here we have demonstrated that OAG directly stimulates 5-LO catalysis in vitro. In the absence of Ca2+ (chelated using EDTA), OAG strongly and concentration-dependently stimulated crude 5-LO in 100,000 x g supernatants as well as purified 5-LO enzyme from PMNL. Also, the monoglyceride 1-O-oleyl-rac-glycerol and 1,2-dioctanoyl-sn-glycerol were effective, whereas various phospholipids did not stimulate 5-LO. However, in the presence of Ca2+, OAG caused no stimulation of 5-LO. Also, phospholipids or cellular membranes abolished the effects of OAG. As found previously for Ca2+, OAG renders 5-LO activity resistant against inhibition by glutathione peroxidase activity, and this effect of OAG is reversed by phospholipids. Intriguingly, a 5-LO mutant lacking tryptophan residues (Trp-13, -75, and -102) important for the binding of the 5-LO C2-like domain to phospholipids was not stimulated by OAG. We conclude that OAG directly stimulates 5-LO by acting at a phospholipid binding site located within the C2-like domain.

Tocotrienol: The Natural Vitamin E to Defend the Nervous System?

Vitamin E is essential for normal neurological function. It is the major lipid-soluble, chain-breaking antioxidant in the body, protecting the integrity of membranes by inhibiting lipid peroxidation. Mostly on the basis of symptoms of primary vitamin E deficiency, it has been demonstrated that vitamin E has a central role in maintaining neurological structure and function. Orally supplemented vitamin E reaches the cerebrospinal fluid and brain. Vitamin E is a generic term for all tocopherols and their derivatives having the biological activity of RRR-alpha-tocopherol, the naturally occurring stereoisomer compounds with vitamin E activity. In nature, eight substances have been found to have vitamin E activity: alpha-, beta-, gamma- and delta-tocopherol; and alpha-, beta-, gamma- and delta-tocotrienol. Often, the term vitamin E is synonymously used with alpha-tocopherol. Tocotrienols, formerly known as zeta, , or eta-tocopherols, are similar to tocopherols except that they have an isoprenoid tail with three unsaturation points instead of a saturated phytyl tail. Although tocopherols are predominantly found in corn, soybean, and olive oils, tocotrienols are particularly rich in palm, rice bran, and barley oils. Tocotrienols possess powerful antioxidant, anticancer, and cholesterol-lowering properties. Recently, we have observed that alpha-tocotrienol is multi-fold more potent than alpha-tocopherol in protecting HT4 and primary neuronal cells against toxicity induced by glutamate as well as by a number of other toxins. At nanomolar concentration, tocotrienol, but not tocopherol, completely protected neurons by an antioxidant-independent mechanism. Our current work identifies two major targets of tocotrienol in the neuron: c-Src kinase and 12-lipoxygenase. Dietary supplementation studies have established that tocotrienol, fed orally, does reach the brain. The current findings point towards tocotrienol as a potent neuroprotective form of natural vitamin E.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Characterization of calcium and magnesium binding domains of human 5-lipoxygenase

Two calcium binding sites, separated by about 9.3A, present in the loops that connect the beta-sheets of N-terminal domain contain the ligating residues F14, A15, G16, D79, and D18, D19, L76, respectively. Magnesium is found to bind in regions, which are marginally different owing to the disparity in the ionic radii of Ca2+ and Mg2+. The entropy analysis on the loops of 5-lipoxygenase, implementing the wormlike chain model, explains that the N-terminal beta-barrel is well suited to accommodate calcium binding sites. The large buried side chain area of W102 (compared to W13 and W75) and comparatively smaller fraction of side chain exposed to polar atoms corroborate the calcium induced higher affinity to phosphatidylcholine (PC). However, W80 lying in close proximity of the calcium binding sites is expected to have considerable PC affinity but negligible calcium induced effect on PC binding.

Platelet 12-lipoxygenase activation via glycoprotein VI: involvement of multiple signaling pathways in agonist control of H(P)ETE synthesis

Lipoxygenases (LOX) contribute to vascular disease and inflammation through generation of bioactive lipids, including 12-hydro(pero)xyeicosatetraenoic acid (12-H(P)ETE). The physiological mechanisms that acutely control LOX product generation in mammalian cells are uncharacterized. Human platelets that contain a 12-LOX isoform (p12-LOX) were used to define pathways that activate H(P)ETE synthesis in the vasculature. Collagen and collagen-related peptide (CRP) (1 to 10 microg/mL) acutely induced platelet 12-H(P)ETE synthesis. This implicated the collagen receptor glycoprotein VI (GPVI), which signals via the immunoreceptor-based activatory motif (ITAM)-containing FcRgamma chain. Conversely, thrombin only activated at high concentrations (> 0.2 U/mL), whereas U46619 and ADP alone were ineffective. Collagen or CRP-stimulated 12-H(P)ETE generation was inhibited by staurosporine, PP2, wortmannin, BAPTA/AM, EGTA, and L-655238, implicating src-tyrosine kinases, PI3-kinase, Ca2+ mobilization, and p12-LOX translocation. In contrast, protein kinase C (PKC) inhibition potentiated 12-H(P)ETE generation. Finally, activation of the immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing platelet endothelial cell adhesion molecule (PECAM-1) inhibited p12-LOX product generation. This study characterizes a receptor-dependent pathway for 12-H(P)ETE synthesis via the collagen receptor GPVI, which is negatively regulated by PECAM-1 and PKC, and demonstrates a novel link between immune receptor signaling and lipid mediator generation in the vasculature.

Structural basis of leukotriene B4 12-hydroxydehydrogenase/15-Oxo-prostaglandin 13-reductase catalytic mechanism and a possible Src homology 3 domain binding loop

The bifunctional leukotriene B(4) 12-hydroxydehydrogenase/15-oxo-prostaglandin 13-reductase (LTB(4) 12-HD/PGR) is an essential enzyme for eicosanoid inactivation. It is involved in the metabolism of the E and F series of 15-oxo-prostaglandins (15-oxo-PGs), leukotriene B(4) (LTB(4)), and 15-oxo-lipoxin A(4) (15-oxo-LXA(4)). Some nonsteroidal anti-inflammatory drugs (NSAIDs), which primarily act as cyclooxygenase inhibitors also inhibit LTB(4) 12-HD/PGR activity. Here we report the crystal structure of the LTB(4) 12-HD/PGR, the binary complex structure with NADP(+), and the ternary complex structure with NADP(+) and 15-oxo-PGE(2). In the ternary complex, both in the crystalline form and in solution, the enolate anion intermediate accumulates as a brown chromophore. PGE(2) contains two chains, but only the omega-chain of 15-oxo-PGE(2) was defined in the electron density map in the ternary complex structure. The omega-chain was identified at the hydrophobic pore on the dimer interface. The structure showed that the 15-oxo group forms hydrogen bonds with the 2'-hydroxyl group of nicotine amide ribose of NADP(+) and a bound water molecule to stabilize the enolate intermediate during the reductase reaction. The electron-deficient C13 atom of the conjugated enolate may be directly attacked by a hydride from the NADPH nicotine amide in a stereospecific manner. The moderate recognition of 15-oxo-PGE(2) is consistent with a broad substrate specificity of LTB(4) 12-HD/PGR. The structure also implies that a Src homology domain 3 may interact with the left-handed proline-rich helix at the dimer interface and regulate LTB(4) 12-HD/PGR activity by disruption of the substrate binding pore to accommodate the omega-chain.

Molecular basis of vitamin E action: tocotrienol modulates 12-lipoxygenase, a key mediator of glutamate-induced neurodegeneration

Vitamin E is a generic term for tocopherols and tocotrienols. This work is based on our striking evidence that, in neuronal cells, nanomolar concentrations of alpha-tocotrienol, but not alpha-tocopherol, block glutamate-induced death by suppressing early activation of c-Src kinase (Sen, C. K., Khanna, S., Roy, S., and Packer, L. (2000) J. Biol. Chem. 275, 13049-13055). This study on HT4 and immature primary cortical neurons suggests a central role of 12-lipoxygenase (12-LOX) in executing glutamate-induced neurodegeneration. BL15, an inhibitor of 12-LOX, prevented glutamate-induced neurotoxicity. Moreover, neurons isolated from 12-LOX-deficient mice were observed to be resistant to glutamate-induced death. In the presence of nanomolar alpha-tocotrienol, neurons were resistant to glutamate-, homocysteine-, and l-buthionine sulfoximine-induced toxicity. Long-term time-lapse imaging studies revealed that neurons and their axo-dendritic network are fairly motile under standard culture conditions. Such motility was arrested in response to glutamate challenge. Tocotrienol-treated primary neurons maintained healthy growth and motility even in the presence of excess glutamate. The study of 12-LOX activity and metabolism revealed that this key mediator of glutamate-induced neurodegeneration is subject to control by the nutrient alpha-tocotrienol. In silico docking studies indicated that alpha-tocotrienol may hinder the access of arachidonic acid to the catalytic site of 12-LOX by binding to the opening of a solvent cavity close to the active site. These findings lend further support to alpha-tocotrienol as a potent neuroprotective form of vitamin E.

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.

Eosinophils and cysteinyl leukotrienes

Eosinophils are the main source of the cysteinyl leukotrienes, LTC(4)/D(4)/E(4), which are lipid mediators that play major roles in the pathogenesis of asthma and other forms of allergic inflammation. Here, we review the mechanisms governing eosinophil LTC(4) synthesis, focusing on the distinct intracellular domains that regulate eicosanoid formation and function within eosinophils. Cysteinyl leukotrienes exert their actions by engaging specific receptors. As recently shown, eosinophils express CysLT1 and CysLT2, the only cloned receptors for cysteinyl leukotrienes. Therefore, here we also present some of the new findings regarding the paracrine/autocrine activation of these CysLT receptors on eosinophils, and discuss some data on novel intracrine effects of LTC(4) triggered by a putative third CysLT receptor expressed intracellularly within eosinophils.

Identification of two novel nuclear import sequences on the 5-lipoxygenase protein

The nuclear import of 5-lipoxygenase modulates its capacity to produce leukotrienes from arachidonic acid. However, the molecular determinants of its nuclear import are unknown. Recently, we used structural and functional criteria to identify a novel import sequence at Arg(518) on human 5-lipoxygenase (Jones, S. M., Luo, M., Healy, A. M., Peters-Golden, M., and Brock, T. G. (2002) J. Biol. Chem. 277, 38550-38556). However, this analysis also indicated that other import sequences must exist. Here, we identify two additional sites, at Arg(112) and Lys(158), as nuclear import sequences. Both sites were found to be common to 5-lipoxygenases from different species but not found on other lipoxygenases. Both sites also appeared to be a part of structures that were predominantly random loops. Peptide sequences at these sites were sufficient to direct nuclear import of green fluorescent protein. Mutation of basic residues in these sites impaired nuclear import and combinations of mutations at different sites were additive in effect. Mutations in all three sites were required to disable nuclear accumulation of 5-lipoxygenase in all cells. Significantly, mutation in these sites did not inhibit catalytic function. Taken together, these results indicate that nuclear import of 5-lipoxygenase may reflect the combined functional effects of three discrete import sequences. Mutation of individual sites can, by itself, impair nuclear import, which in turn could impact arachidonic acid metabolism.