Arachidonate 12-lipoxygenases

Cell signaling and gene regulation of human 12(S)-lipoxygenase expression

Human 12(S)-lipoxygenase is a platelet-type 12(S)-lipoxyenase. Its expression is detected in human erythroleukemia cells, human skin epidermal cells and human epidermoid carcinoma A431 cells. Treatment of A431 cells with EGF or PMA induces the gene expression of human 12(S)-lipoxygenase. The induction of gene expression is mediated through the cell signaling of MAPK activation, followed by the induction of c-Jun expression. The transcription factor Sp1 binding to the two Sp1 recognition motifs residing at -158 to 150 bp and -123 to 114 bp in the gene promoter is found to be essential for both EGF- and PMA-induced gene expression of human 12(S)-lipoxygenase. However, no change of Sp1 binding to GC-rich sequence was observed while no AP-1-binding site can be found in the responsive region of the promoter in EGF- and PMA-induced promoter activation of the human 12(S)-lipoxygenase gene. Since both of the transcription factors c-Jun and Sp1 are prerequisite for EGF and PMA response, interaction between c-Jun and Sp1 may account for the functional regulation of human 12(S)-lipoxygenase gene regulation. The direct and cooperative interaction between c-Jun and Sp1 induced by EGF or PMA activates the expression of the human 12(S)-lipoxygenase gene. Therefore, Sp1 may serve at least in part as a carrier to bring c-Jun to the promoter, thu's transactivating the transcriptional activity of the human 12(S)-lipoxygenase gene.

Nitric oxide triggers the toxicity due to glutathione depletion in midbrain cultures through 12-lipoxygenase

Glutathione (GSH) depletion is the earliest biochemical alteration shown to date in brains of Parkinson's disease patients. However, data from animal models show that GSH depletion by itself is not sufficient to induce nigral degeneration. We have previously shown that non-toxic inhibition of GSH synthesis with l-buthionine-(S,R)-sulfoximine in primary midbrain cultures transforms a nitric oxide (NO) neurotrophic effect, selective for dopamine neurons, into a toxic effect with participation of guanylate cyclase (GC) and cGMP-dependent protein kinase (PKG) (Canals, S., Casarejos, M. J., de Bernardo, S., Rodríguez-Martín, E., and Mena, M. A. (2001) J. Neurochem. 79, 1183-1195). Here we demonstrate that arachidonic acid (AA) metabolism through the 12-lipoxygenase (12-LOX) pathway is also central for this GSH-NO interaction. LOX inhibitors (nordihydroguaiaretic acid and baicalein), but not cyclooxygenase (indomethacin) or epoxygenase (clotrimazole) ones, prevent cell death in the culture, even when added 10 h after NO treatment. Furthermore, the addition of AA to GSH-depleted cultures precipitates a cell death process that is indistinguishable from that initiated by NO in its morphology, time course, and 12-LOX, GC, and PKG dependence. The first AA metabolite through the 12-LOX enzyme, 12-hydroperoxyeicosatetraenoic acid, induces cell death in the culture, and its toxicity is greatly enhanced by GSH depletion. In addition we show that if GSH synthesis inhibition persists for up to 4 days without any additional treatment, it will induce a cell death process that also depends on 12-LOX, GC, and PKG activation. In this study, therefore, we show that the signaling pathway AA/12-LOX/12-HPETE/GC/PKG may be important in several pathologies in which GSH decrease has been documented, such as Parkinson's disease. The potentiating effect of NO over such a signaling pathway may be of relevance as part of the cascade of events leading to and sustaining nerve cell death.

Suicidal inactivation of the rabbit 15-lipoxygenase by 15S-HpETE is paralleled by covalent modification of active site peptides

Lipoxygenases (LOXs) are multifunctional enzymes that catalyze the oxygenation of polyunsaturated fatty acids to hydroperoxy derivatives; they also convert hydroperoxy fatty acids to epoxy leukotrienes and other secondary products. LOXs undergo suicidal inactivation but the mechanism of this process is still unclear. We investigated the mechanism of suicidal inactivation of the rabbit 15-lipoxygenase by [1-(14)C]-(15S,5Z,8Z,11Z,13E)-15-hydroperoxyeicosa-5,8,11,13-tetraenoic acid (15-HpETE) and observed covalent modification of the enzyme protein. In contrast, nonlipoxygenase proteins (bovine serum albumin and human gamma-globulin) were not significantly modified. Under the conditions of complete enzyme inactivation we found that 1.3 +/- 0.2 moles (n = 10) of inactivator were bound per mole lipoxygenase, and this value did depend neither on the enzyme/inactivator ratio nor on the duration of the inactivation period. Covalent modification required active enzyme protein and proceeded to a similar extent under aerobic and anaerobic conditions. In contrast, [1-(14)C]-(15S,5Z,8Z,11Z,13E)-15-hydroxyeicosa-5,8,11,13-tetraenoic acid (15-HETE), which is no substrate for epoxy-leukotriene formation, did not inactivate the enzyme and protein labeling was minimal. Separation of proteolytic cleavage peptides (Lys-C endoproteinase digestion) by tricine SDS-PAGE and isoelectric focusing in connection with N-terminal amino acid sequencing revealed covalent modification of several active site peptides. These data suggest that 15-lipoxygenase-catalyzed conversion of (15S,5Z,8Z,11Z,13E)-15-hydroperoxyeicosa-5,8,11,13-tetraenoic acid to 14,15-epoxy-leukotriene leads to the formation of reactive intermediate(s), which are covalently linked to the active site. Therefore, this protein modification contributes to suicidal inactivation.

Kinetics of inhibition of leukocyte 12-lipoxygenase by the isoform-specific inhibitor 4-(2-oxapentadeca-4-yne)phenylpropanoic acid

Lipoxygenases (LOXs) are a ubiquitous family of enzymes that catalyze the dioxygenation of polyunsaturated fatty acids. Their role in a diverse range of biological processes has prompted the development of a large number of lipoxygenase inhibitors of possible therapeutic and probative value. The isoform-selective inhibitor 4-(2-oxapentadeca-4-yne)phenylpropanoic acid (OPP) was previously shown to inhibit leukocyte-type 12-LOX by a novel mechanism in which it binds to both the ferrous and ferric forms of the enzyme. The current study provides a detailed kinetic model of this inhibition. Nonlinear regression analysis of OPP's inhibition of arachidonic acid dioxygenation indicated mixed inhibition toward the ferric form of 12-LOX with apparent K(I) values in the low micromolar range: 2.0 +/- 0.2 microM for the free enzyme and 4.5 +/- 0.7 microM for the substrate-bound form of the enzyme. Rapid kinetic techniques allowed OPP's inhibition of the activation of the enzyme from the ferrous to the ferric form to be investigated. Titration of ferrous 12-LOX with OPP indicated that it bound to the ferrous form with an apparent K(I) value of 70 +/- 20 nM, suggesting a significantly higher affinity for the ferrous form than for the ferric form of the enzyme. Investigation of the LOX inhibitors nordihydroguaiaretic acid, N-(4-chlorophenyl)-N-hydroxy-N'-(3-chlorophenyl)urea, BWA137C, and eicosatetraynoic acid revealed that eicosatetraynoic acid also inhibited the activation of 12-LOX. These results demonstrate that LOX inhibitors are capable of binding to multiple forms of LOXs with high affinity and suggest that inhibition of enzyme activation may be an unrecognized mechanism of inhibition of additional LOX inhibitors.

Arachidonate 12-lipoxygenases

Arachidonate 12-lipoxygenase introduces a molecular oxygen at carbon 12 of arachidonic acid to generate a 12-hydroperoxy derivative. The enzymes generate 12-hydroperoxy derivatives with either S- or R-configurations. There are three isoforms of 12S-lipoxygenases named after the cells where they were first identified; platelet, leukocyte and epidermis. The leukocyte-type enzyme is widely distributed among cells, but the tissue distribution varies substantially from species to species. The platelet and epidermal enzymes are present in only a relatively limited number of cell types. Although the structures and enzymatic properties of the three isoforms of 12S-lipoxygenases have been elucidated, the physiological roles of the 12S-lipoxygenases are not yet fully understood. There are important roles for the enzymes and their products in several biological systems including those involved in atherosclerosis and neurotransmission.

15-lipoxygenase-1: a prooxidant enzyme

Human and rabbit reticulocyte 15-lipoxygenase (15-lipoxygenase-1) and the leukocyte-type 12-lipoxygenases (12/15-lipoxygenases) of pig, beef, mouse and rat constitute a particular subfamily of mammalian lipoxygenases (reticulocyte-type lipoxygenases) with unique properties and functions. They catalyze enzymatic lipid peroxidation in complex biological structures via direct dioxygenation of phospholipids and cholesterol esters of biomembranes and plasma lipoproteins. Moreover, they are a source of free radicals initiating non-enzymatic lipid peroxidation and other oxidative processes. Expression and activity of reticulocyte-type lipoxygenases are highly regulated. Moreover, the susceptibility of intracellular membranes toward these lipoxygenases is controlled and may be increased together with lipoxygenase activity under conditions of oxidative stress. Thus, oxidative stress may favor a concerted package of lipoxygenase-mediated enzymatic and non-enzymatic lipid peroxidation and co-oxidative processes. Reaction of reticulocyte-type lipoxygenases with low-density lipoprotein renders the latter atherogenic and appears to be involved in the formation of atherosclerotic lesions.

Eicosanoids participate in the regulation of cardiac glucose transport by contribution to a rearrangement of actin cytoskeletal elements

Intact actin microfilaments are required for insulin-regulated glucose transporter isoform 4 (GLUT4) translocation to the plasma membrane. Lipoxygenase (LO) metabolites have recently been shown to contribute to the regulation of actin cytoskeleton rearrangement. In the present investigation, ventricular cardiomyocytes were used to study the effects of two structurally different LO inhibitors (esculetin and nordihydroguaiaretic acid) on insulin signalling events, glucose uptake, GLUT4 translocation and the actin network organization. Insulin stimulation increased glucose uptake 3-fold in control cells, whereas LO inhibition completely blocked this effect. This was paralleled by a slight reduction in the insulin-induced tyrosine phosphorylation of insulin receptor substrate (IRS)-1 and IRS-2. However, inhibition of 12-LO did not affect the association of phosphatidylinositol 3-kinase with IRS-1 and the phosphorylation of Akt/protein kinase B in response to insulin. Addition of 12(S)-hydroxyeicosatetraenoic acid almost completely restored the insulin action in cells exposed to nordihydroguaiaretic acid. Insulin stimulation increased cell surface GLUT4 2-fold in control cells, whereas LO inhibition abrogated the insulin-stimulated GLUT4 translocation. LO inhibition induced a prominent disassembly of actin fibres compared with control cells. In conclusion, we show here that 12(S)-hydroxyeicosatetraenoic acid plays a role in the organization of the actin network in cardiomyocytes. LO inhibition blocks GLUT4 translocation without affecting downstream insulin signalling. These data suggest that LO metabolites participate in the regulation of glucose transport by contributing to a rearrangement of actin cytoskeletal elements.

A Gene Cluster Encoding Human Epidermis-type Lipoxygenases at Chromosome 17p13.1: Cloning, Physical Mapping, and Expression

Epidermis-type lipoxygenases, a distinct subclass within the multigene family of mammalian lipoxygenases (LOX), comprise recently discovered novel isoenzymes isolated from human and mouse skin including human 15-LOX-2, human and mouse 12R-LOX, mouse 8S-LOX, and mouse e-LOX-3. We have isolated the human homologue of mouse e-LOX-3. The cDNA of 3362 bp encodes a 711-amino-acid protein displaying 89% sequence identity with the mouse protein and exhibiting the same unusual structural feature, i.e., an extra segment of 41 amino acids, which can be located beyond the N-terminal beta-barrel domain at the surface of the C-terminal catalytic domain. The gene encoding e-LOX-3, ALOXE3, was found to be part of a gene cluster of approximately 100 kb on human chromosome 17p13.1 containing in addition the 12R-LOX gene, ALOX12B, the 15-LOX-2 gene, ALOX15B, and a novel 15-LOX pseudogene, ALOX15P. ALOXE3 and ALOX12B are arranged in a head-to-tail fashion separated by 8.5 kb. The genes are split into 15 exons and 14 introns spanning 22 and 15 kb, respectively. ALOX15P was found on the opposite DNA strand directly adjacent to the 3'-untranslated region of ALOX12B. ALOX15B is located in the same orientation 25 kb downstream of ALOX12B, and is composed of 14 exons and 13 introns spanning a total of 9.7 kb of genomic sequence. RT-PCR analysis demonstrated a predominant expression of ALOXE3, ALOX12B, and ALOX15B in skin.

Programmed cell death induced by glutathione depletion in PC 12 cells is blocked by inhibitors of 12 lipoxygenase, but does not appear to be mediated through the formation of 12 HETE derivatives

Lipoxygenase metabolites have been postulated to be involved in the degenerative events provoked by oxidative stress in neuronal and nonneuronal targets, but their roles remain controversial. In the present work, we investigated the putative role of 12 lipoxygenase metabolites in the programmed cell death induced by glutathione depletion in PC 12 cells. Determinations of 12 lipoxygenase expression and activity reveal the presence of the enzyme in PC 12 cells, but the formation of arachidonate metabolites appears rather low and is not influenced by glutathione depletion. In addition, although the death induced by buthionine sulfoximine (BSO) treatment is abolished by known inhibitors of lipoxygenase enzymes, dexamethasone, a potent steroidal inhibitor of both cyclooxygenase and lipoxygenase pathways, fails to protect the cells from BSO-induced degeneration. Finally, incubation of the cells for 24 h in the presence of exogenous 12 HETE did not induce any significant decrease in cell viability. Our results indicate that 12 lipoxygenase is unlikely to play a major role in the process of cell degeneration provoked by glutathione depletion.

Partial purification and characterization of 12-lipoxygenase in bullfrog erythrocytes

12-Lipoxygenase (12-LO) in bullfrog (Rana catesbeiana) erythrocytes was purified partially by ion exchange chromatography and affinity chromatography. Bullfrog 12-LO was a single chain protein with a pI of 7.1-7.8 and MW of 7.77 kDa. This enzyme did not show typical Michaelis Menten type kinetics. At low substrate concentrations, it had a lag phase and at higher substrate concentrations, the activity was inhibited. The product of linoleic acid (LA), 13-hydroperoxy-9, 11-octadecadienoic acid (13-HpODE), was an activator for the enzyme. When arachidonic acid (AA) was used as substrate, 13-HpODE also affected the Km of bullfrog 12-LO towards AA. The affinity of LA towards bullfrog 12-LO was higher than the affinity of AA. Suicide inactivation was much more rapid than that of any mammalian 12-LO reported. Hemoglobin (Hb) inhibited the activity of 12-LO partially and removing Hb eliminated this inhibition. Both Hb and Met-Hb inhibited the 12-LO activity but did not denatured completely the Hb, suggesting that the inhibition was a direct interaction between 12-LO and Hb protein chain and was not due to competition between 12-LO and Hb for oxygen. This study characterizes bullfrog 12-LO with respect to stability, optimal pH, suicide inactivation and interaction with Hb and provides important evolutionary information about this enzyme.

Hinokitiol, a selective inhibitor of the platelet-type isozyme of arachidonate 12-lipoxygenase

Hinokitiol (4-isopropyltropolone), a constituent of Japanese cypress, reversibly inhibited platelet-type 12-lipoxygenase with an IC(50) of 0.1 microM, and the enzyme activity was almost lost at 1 microM. The compound was much less active with other lipoxygenase enzymes with higher IC(50) values (leukocyte-type 12-lipoxygenase, 50 microM; soybean lipoxygenase, 17 microM; 15-lipoxygenase-1, >100 microM; 5-lipoxygenase, 17 microM). Hinokitiol up to 100 microM had almost no effect on cyclooxygenases-1 and -2. Their structure-activity relationship examined with various tropolone derivatives indicated the requirements of the 2-hydroxyl group and 4-alkyl group for the potent and selective inhibition of platelet-type 12-lipoxygenase.

Potential use of lipoxygenase inhibitors for cancer chemoprevention

Increasing evidence suggests that lipoxygenase (LO)-catalysed metabolites have a profound influence on the development and progression of human cancers. Compared with normal tissues, significantly elevated levels of LO products have been found in breast tumours, colon cancers, lung, skin and prostate cancers, as well as in cells from patients with both acute and chronic leukaemias. LO-mediated products elicit diverse biological activities needed for neoplastic cell growth, influencing growth factor and transcription factor activation, oncogene induction, stimulation of tumour cell adhesion and regulation of apoptotic cell death. Agents that block LO catalytic activity may be effective in preventing cancer by interfering with signalling events needed for tumour growth. In the past ten years, pharmaceuticals agents that specifically inhibit the 5-LO metabolic pathway have been developed to treat inflammatory diseases such as asthma, arthritis and psoriasis. Some of these compounds possess anti-oxidant properties and may be effective in preventing cancer by blocking free radical-induced genetic damage or by preventing the metabolic activation of carcinogens. Other compounds may work by negatively modulating DNA synthesis. Pharmacological profiles of potential chemopreventive agents are compiled from enzyme assays, in vitro testing (e.g., cell proliferation inhibition in human cancer cells) and in vivo animal carcinogenesis models (e.g., N-methyl-N-nitrosourea-induced rat mammary cancer, benzo(a)pyrene-induced lung tumours in strain A/J mice and hormone-induced prostate tumours in rats). In this way, compounds are identified for chemoprevention trials in human subjects. Based on currently available data, it is expected that the prevention of lung and prostate cancer will be initially studied in human trials of LO inhibitors.

A short catalytic enantioselective synthesis of the proinflammatory eicosanoid 12(R)-hydroxy- 5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid (12(R)-HETE)

A new and effective pathway is described for the synthesis of 12(R)-HETE and the 12(S)-enantiomer from the common intermediates 4 and 8.

A GATA binding site is involved in the regulation of 15-lipoxygenase-1 expression in human colorectal carcinoma cell line, caco-2

The data presented implicate a GATA binding site in the transcriptional regulation of 15-lipoxygenase-1 (15-LO-1) gene expression in human colorectal carcinoma Caco-2 cells. High expression of GATA-6 mRNA and protein was observed, while GATA-4 mRNA was expressed at a very low level in Caco-2 cells. The expression of GATA-6 was down-regulated, while 15-LO-1 expression was dramatically up-regulated after treatment with sodium butyrate (NaBT). A study using an electrophoretic mobility shift assay indicated that a GATA binding site of the 15-LO-1 promoter region binds to GATA proteins present in both undifferentiated and, to a lesser extent, NaBT-treated (differentiated) Caco-2 cells. Moreover, that DNA binding shift band was disrupted after the addition of GATA-6 antibody in a supershift assay in the absence of NaBT, suggesting that GATA-6 is bound to the GATA binding site of the 15-LO-1 promoter in undifferentiated cells. In contrast, the addition of GATA-6 antibody did not affect the DNA binding ability in NaBT-induced differentiated cells. On the other hand, mutation of the GATA site of the 15-LO-1 promoter decreased the transactivation of the 15-LO-1 promoter as measured by luciferase activity in both FBS and NaBT cultured cells, indicating an unknown GATA binding protein to up-regulate 15-LO-1 expression. These implicate the GATA site at -240 of the proximal region of the 15-LO-1 promoter in the basic transcription of 15-LO-1 gene expression in Caco-2 cells, with GATA-6 acting to repress 15-LO-1 expression.

Thrombin-induced growth cone collapse: involvement of phospholipase A(2) and eicosanoid generation

The studies presented here explore intracellular signals resulting from the action of repellents on growth cones. Growth cone challenge with thrombin or thrombin receptor-activating peptide (TRAP) triggers collapse via a receptor-mediated process. The results indicate that this involves activation of cytosolic phospholipase A(2) (PLA(2)) and eicosanoid synthesis. The collapse response to repellents targets at least two functional units of the growth cone, the actin cytoskeleton and substratum adhesion sites. We show in a cell-free assay that thrombin and TRAP cause the detachment of isolated growth cones from laminin. Biochemical analyses of isolated growth cones reveal that thrombin and TRAP stimulate cytosolic PLA(2) but not phospholipase C. In addition, thrombin stimulates synthesis of 12- and 15-hydroxyeicosatetraenoic acid (HETE) from the released arachidonic acid via a lipoxygenase (LO) pathway. A selective LO inhibitor blocks 12/15-HETE synthesis in growth cones and inhibits thrombin-induced growth cone collapse. Exogenously applied 12(S)-HETE mimics the thrombin effect and induces growth cone collapse in culture. These observations indicate that thrombin-induced growth cone collapse occurs by a mechanism that involves the activation of cytosolic PLA(2) and the generation of 12/15-HETE.

Mechanism of inhibition of porcine leukocyte 12-lipoxygenase by the isoform-specific inhibitor 4-(2-oxapentadeca-4-yne)phenylpropanoic acid

The mechanism of inhibition of porcine leukocyte 12-lipoxygenase by 4-(2-oxapentadeca-4-yne)phenylpropanoic acid (OPP) was investigated. This compound is selective for the leukocyte form of the 12-lipoxygenase and inhibits the purified recombinant enzyme with an IC(50) value of approximately 2 microM. OPP induced a concentration-dependent lag phase in the oxygenation of arachidonic acid and decreased the maximal rate of reaction. Addition of the fatty acid hydroperoxide 13(S)-hydroperoxyoctadecadienoic acid (13-HPODE) to the reaction greatly reduced the OPP-induced lag. Lineweaver-Burk analysis of the effect of OPP on 12-lipoxygenase kinetics with arachidonic acid indicated that it was a mixed-type inhibitor. OPP was not metabolized by 12-lipoxygenase as evidenced by its quantitative recovery from incubations with stoichiometric amounts of enzyme and 13-HPODE or arachidonic acid. OPP inhibited the pseudoperoxidase activity of the enzyme with 13-HPODE and the reducing agent, BWA137C. Lineweaver-Burk analysis of the effect of OPP on pseudoperoxidase kinetics suggested that OPP was competitive with 13-HPODE. Single-turnover experiments indicated that OPP inhibited the reduction of 13-HPODE by a stoichiometric amount of ferrous 12-lipoxygenase. Addition of 13-HPODE shortened the OPP-induced lag phase but did not affect the maximal rate of enzyme activity. In addition, OPP had no effect on total product formation in either the presence or the absence of 5 microM 13-HPODE when the reaction was allowed to go to completion. All of these observations are consistent with a model for inhibition of 12-lipoxygenase activity in which OPP slows the oxidation of the inactive ferrous enzyme to the active ferric enzyme and competes with arachidonic acid for the ferric enzyme.

Psoriasis and the arachidonic acid cascade

Arachidonic acid (5.8,11,14-eicosatetraenoic acid C20:4, n-6) is released from the cell membrane by the action of phospholipases on membrane phospholipids. Metabolites of arachidonic acid, which are generically termed eicosanoids, including prostaglandins, thromboxane, leukotrienes and hydroxyeicosatetraenoic acids, have been implicated as mediators or modulators of a number of physiological functions and pathological conditions in both normal and diseased human skin. Particularly, eicosanoids have been suspected to play an important role in the pathogenesis of psoriasis, because a number of phenomena observed in psoriasis can be explained, at least in part, by the action of eicosanoids. This review will focus on recent progress regarding the significance of eicosanoids in the pathogenesis of psoriasis. Recent developments in the molecular biology in the eicosanoids have renewed interest in the role of eicosanoids in psoriasis. New understanding of the etiology of psoriasis and advances in its treatment due to recent progress in eicosanoid biology will also be presented.

Change in the positional specificity of lipoxygenase 1 due to insertion of fatty acids into phosphatidylcholine deoxycholate mixed micelles

Linoleic and arachidonic acids were inserted into phosphatidylcholine deoxycholate mixed micelles (PDM-micelles) with their tail groups buried inside and carboxylic groups exposed outside. The fatty acid hydrophobic tail had a high affinity for the hydrophobic region of phosphatidylcholine micelles. The fatty acids inserted into phosphatidylcholine micelles were better substrates for soybean lipoxygenase 1 (LOX1) with two distinct pH optima at 7.0 and 10.0. With Tween 20-solubilized linoleic acid, the enzyme had a pH optimum at 9.0, exclusively forming 13-hydroperoxides. However, with linoleic and arachidonic acids inserted into PDM-micelles, LOX1 synthesized exclusively 9- and 5-hydroperoxides, respectively. The enzyme brought about the transformation of the substrate either at pH 7.4 or at 10.0, less efficiently at pH 10.0. However, the regioselectivity of the enzyme was not altered by increasing the pH from 7.4 to 10.0. Thus, LOX1 could utilize fatty acids bound to membranes as physiological substrates. The enzyme utilized the carboxylic group of linoleic and arachidonic acids inserted into the PDM-micelles as a recognition site to convert the compounds into 9- and 5-hydroperoxides, respectively. This was confirmed by activity measurements using methyl linoleate as the substrate. Circular dichroism measurement of LOX1 with PDM-micelles suggested that while there was a small change in the tertiary structure of LOX1, the secondary structure was unaffected. Soybean LOX1, which is arachidonate 15-LOX, acted as "5-LOX", thus making it possible to change the regiospecificity of the LOX1-catalyzed reaction by altering the physical state of the substrate.

Metabolic suppression of platelet-type 12-lipoxygenase in human uterine cervix with invasive carcinoma

Several types of lipoxygenases with various functions occur in mammalian cells. Although the presence of 12-lipoxygenase activity has been reported in uterine tissues, neither its type nor its biological functions have yet been established. Moreover, the putative role of uterine 12-lipoxygenase in cervical cancer has not been addressed before. Homogenates of uterine tissues from women without cancer and from patients with invasive cervical carcinoma were incubated with (1-(14)C)-arachidonic acid under various conditions and the labelled reaction products were analyzed both by thin-layer chromatography and by high-pressure liquid chromatography. 12-Lipoxygenase protein was estimated by Western blot using anti-serum against recombinant human platelet-type 12-lipoxygenase. Highest concentrations and activities of 12-lipoxygenase were found in the exocervix. The formation of 12S-hydroxy-5Z,8Z,10E, 14Z-eicosatetraenoic acid (12-HETE) was stimulated by micromolar concentrations of 13S-hydroperoxy-9Z,11E-octadecadienoic acid, suggesting metabolic control of the 12-lipoxygenase activity via the hydroperoxide tone. Immunohistochemical investigation revealed that the enzyme is mainly located in the squamous epithelium, and is of platelet-type. Significantly lower values for the 12-HETE formation were found in samples from patients with invasive cervical carcinoma, whereas the amount of immunochemically detectable 12-lipoxygenase protein was unaltered. At the same time the expression levels of the bcl-2 gene were enhanced. Thus, it is concluded that during carcinogenesis the hydroperoxide-reducing capacity of the uterine cervix tissue is enhanced, possibly mediated by bcl-2 protein, and in turn metabolically suppresses the 12-lipoxygenase activity. Furthermore, the data suggest an anti-carcinogenic action of 12-lipoxygenase in human cervix, in contrast to its reported pro-carcinogenic action in breast cancer.

Regulation of 12-lipoxygenase in rat intestinal epithelial cells during differentiation and apoptosis induced by sodium butyrate

We evaluated the expression and activity of rat 12-lipoxygenase (LO) in rat intestinal epithelial (RIE) cells during apoptosis and cell differentiation. Sodium butyrate (NaBT) treatment induced wild-type RIE (W-RIE) cells to undergo differentiation and apoptosis. Alkaline phosphatase (ALP) activity, a marker of cell differentiation, and DNA fragmentation, an index of apoptosis, were increased by NaBT treatment. Arachidonic acid was metabolized primarily to 12-hydroxyeicosatetraenoic acid (HETE) suggesting induction of 12-LO activity. In contrast, sense-RIE (S-RIE) cells engineered to overexpress COX-2 were resistant to apoptosis by treatment with 5 mM NaBT and NaBT did not induce 12-LO activity. The upregulation of 12-LO expression by NaBT in W-RIE cells was confirmed at both the transcriptional and translational level but 12-LO was undetectable in S-RIE cells following NaBT treatment. The expression of 12-LO mRNA in W-RIE cells occurs as early as 6 h after treatment and reaches maximum expression at 24 h following treatment. This inducible 12-LO was isolated by RT-PCR and identified as rat "leukocyte-type" 12-LO. The level of 12-LO expression in W-RIE cells was dependent on the concentration of NaBT and appears to reflect the extent of cell differentiation. NDGA, a lipoxygenase inhibitor, attenuated induction of ALP activity by NaBT treatment of W-RIE cells. These observations suggested that 12-LO is regulated by treatment with NaBT and is associated with cell differentiation in rat intestinal epithelial cells.

Role of group II secretory phospholipase A2 in atherosclerosis: 2. Potential involvement of biologically active oxidized phospholipids

Secretory nonpancreatic phospholipase A2 (group II sPLA2) is induced in inflammation and present in atherosclerotic lesions. In an accompanying publication we demonstrate that transgenic mice expressing group II sPLA2 developed severe atherosclerosis. The current study was undertaken to determine whether 1 mechanism by which group II sPLA2 might contribute to the progression of inflammation and atherosclerosis is by increasing the formation of biologically active oxidized phospholipids. In vivo measurements of bioactive lipids were performed, and in vitro studies tested the hypothesis that sPLA2 can increase the accumulation of bioactive phospholipids. We have shown previously that 3 oxidized phospholipids derived from the oxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (PAPC) stimulated endothelial cells to bind monocytes, a process that is known to be an important step in atherogenesis. We now show that these 3 biologically active phospholipids are significantly increased in livers of sPLA2 transgenic mice fed a high-fat diet as compared with nontransgenic littermates. We present in vitro evidence for several mechanisms by which these phospholipids may be increased in sPLA2 transgenics. These studies demonstrated that polyunsaturated free fatty acids, which are liberated by sPLA2, increased the formation of bioactive phospholipids in LDL, resulting in increased ability to stimulate monocyte-endothelial interactions. Moreover, sPLA2-treated LDL was oxidized by cocultures of human aortic endothelial cells and smooth muscle cells more efficiently than untreated LDL. Analysis by electrospray ionization-mass spectrometry revealed that the bioactive phospholipids, compared with unoxidized PAPC, were less susceptible to hydrolysis by human recombinant group II sPLA2. In addition, HDL from the transgenic mice and human HDL treated with recombinant sPLA2 in vitro failed, in the coculture system, to protect against the formation of biologically active phospholipids in LDL. This lack of protection may in part relate to the decreased levels of paraoxonase seen in the HDL isolated from the transgenic animals. Taken together, these studies show that levels of biologically active oxidized phospholipids are increased in sPLA2 transgenic mice; they also suggest that this increase may be mediated by effects of sPLA2 on both LDL and HDL.

Leukotriene A synthase activity of purified mouse skin arachidonate 8-lipoxygenase expressed in Escherichia coli

Mouse skin 8-lipoxygenase was expressed in COS-7 cells by transient transfection of its cDNA in pEF-BOS carrying an elongation factor-1alpha promoter. When crude extract of the transfected COS-7 cells was incubated with arachidonic acid, 8-hydroxy-5,9,11, 14-eicosatetraenoic acid was produced as assessed by reverse- and straight-phase high performance liquid chromatographies. The recombinant enzyme also reacted on alpha-linolenic and docosahexaenoic acids at almost the same rate as that with arachidonic acid. Eicosapentaenoic and gamma-linolenic acids were also oxygenated at 43% and 56% reaction rates of arachidonic acid, respectively. In contrast, linoleic acid was a poor substrate for this enzyme. The 8-lipoxygenase reaction with these fatty acids proceeded almost linearly for 40 min. The 8-lipoxygenase was also expressed in an Escherichia coli system using pQE-32 carrying six histidine residues at N-terminal of the enzyme. The expressed enzyme was purified over 380-fold giving a specific activity of approximately 0.2 micromol/45 min per mg protein by nickel-nitrilotriacetate affinity chromatography. The enzymatic properties of the purified 8-lipoxygenase were essentially the same as those of the enzyme expressed in COS-7 cells. When the purified 8-lipoxygenase was incubated with 5-hydroperoxy-6,8,11, 14-eicosatetraenoic acid, two epimers of 6-trans-leukotriene B4, degradation products of unstable leukotriene A4, were observed upon high performance liquid chromatography. Thus, the 8-lipoxygenase catalyzed synthesis of leukotriene A4 from 5-hydroperoxy fatty acid. Reaction rate of the leukotriene A synthase was approximately 7% of arachidonate 8-lipoxygenation. In contrast to the linear time course of 8-lipoxygenase reaction with arachidonic acid, leukotriene A synthase activity leveled off within 10 min, indicating suicide inactivation.

The diversity of the lipoxygenase family. Many sequence data but little information on biological significance

Lipoxygenases form a family of lipid peroxidising enzymes, which oxygenate free and esterified polyenoic fatty acids to the corresponding hydroperoxy derivatives. They are widely distributed in both the plant and animal kingdoms. During the last couple of years more and more lipoxygenase isoforms have been discovered but for most of them the biological significance remains unclear. This review attempts to classify the currently known mammalian lipoxygenase isoforms and critically reviews the concepts for their biological importance.

Murine 12(R)-lipoxygenase: functional expression, genomic structure and chromosomal localization

A cDNA, recently cloned (by Krieg et al. (1998)) from mouse skin, was shown to encode a 12(R)-lipoxygenase. When expressed in HEK cells, the recombinant protein converted methyl arachidonate into the corresponding 12-HETE ester which was shown to be the R-enantiomer by chiral phase chromatography. Neither arachidonic acid nor linoleic acid were substrates for the recombinant protein. The structure of the 12(R)-lipoxygenase gene is unique among all animal lipoxygenases in that it is divided into 15 exons and 14 introns spanning approximately 12.5 kb. By interspecific backcross analysis, the 12(R)-lipoxygenase gene was localized to the central region of mouse chromosome 11.

cDNA cloning of 15-lipoxygenase type 2 and 12-lipoxygenases of bovine corneal epithelium

Bovine corneal epithelium contains arachidonate 12- and 15-lipoxygenase activity, while human corneal epithelium contains only 15-lipoxygenase activity. Our purpose was to identify the corneal 12- and 15-lipoxygenase isozymes. We used cDNA cloning to isolate the amino acid coding nucleotide sequences of two bovine lipoxygenases. The translated sequence of one lipoxygenase was 82% identical with human 15-lipoxygenase type 2 and 75% identical with mouse 8-lipoxygenase, whereas the other translated nucleotide sequence was 87% identical with human 12-lipoxygenase of the platelet type. Expression of 15-lipoxygenase type 2 and platelet type 12-lipoxygenase mRNAs were detected by Northern analysis. In addition to these two lipoxygenases, 12-lipoxygenase of leukocyte (tracheal) type was detected by polymerase chain reaction (PCR), sequencing, and Northern analysis. Finally, PCR and sequencing suggested that human corneal epithelium contains 15-lipoxygenase types 1 and 2.

Inverse regulation of lipid-peroxidizing and hydroperoxyl lipid-reducing enzymes by interleukins 4 and 13

12/15-lipoxygenases and phospholipid hydroperoxide glutathione peroxidases are opposite enzymes balancing the intracellular concentration of hydroperoxy lipids. We studied the regulation of both enzymes by interleukins 4 and 13 and found an inverse response. When human lung carcinoma cells A549 were cultured in vitro in the presence of these cytokines, an up-regulation of the 12/15-lipoxygenase and a down-regulation of the phospholipid hydroperoxide glutathione peroxidase were observed. A similar inverse regulation was found in human peripheral monocytes. Interleukin 4-treated A549 cells exhibited an impaired capability of reducing exogenous hydroperoxyl lipids and their levels of endogenous lipid hydroperoxides were markedly increased. To find out whether these regulatory processes also occur in vivo, arachidonic acid oxygenase and phospholipid hydroperoxide glutathione peroxidase activity was assayed in various tissues of transgenic mice that systemically overexpress interleukin 4. In lung, spleen, kidney, and heart, an increased arachidonic acid oxygenase activity was detected when transgenic mice were compared with inbred controls. The phospholipid hydroperoxide glutathione peroxidase activity was impaired in lung, liver, and spleen of the transgenic animals. These data indicate that lipid-peroxidizing and lipid peroxide-reducing enzymes are inversely regulated in various mammalian cells. Up-regulation of the 12/15-lipoxygenase and simultaneous down-regulation of the phospholipid hydroperoxide glutathione peroxidase may lead to an increased oxidizing potential, which is reflected by an augmented intracellular peroxide tone.

Lipoxin B4 regulates human monocyte/neutrophil adherence and motility: design of stable lipoxin B4 analogs with increased biologic activity

Lipoxins are biologically active products of arachidonic acid that are formed via cell-cell interactions, particularly those involving leukocytes. Lipoxin A4 and lipoxin B4 (LXB4), within similar concentration ranges, each inhibit human neutrophil, activate monocyte adherence and motility, and are rapidly converted by initial dehydrogenation to other inactive metabolites by human monocytes. Here, we exposed LXB4 to isolated recombinant 15-hydroxy-prostaglandin dehydrogenase (15-PGDH) and found that it was a good substrate for the enzyme (Km=6.9 microM); we identified the major product as 5-oxo-LXB4 via physical methods including liquid chromatography/tandem mass spectrometry. This is the first evidence of 15-PGDH converting a substrate hydroxyl group at a position other than the omega-6 carbon. Based on these observations, several LXB4 analogs were designed and prepared by total organic synthesis to test as stable mimetics: 5(S)-methyl-LXB4-me, 5(R)-methyl-LXB4-me, and 15-epi-LXB4-me (the aspirin-triggered form of LXB4). Both 5(S)-methyl-LXB4-me and 5(R)-methyl-LXB4-me were resistant to rapid conversion. In addition, actions of the stable analogs were evaluated separately with human mono-cytic cells and neutrophils, and 5(S)-methyl-LXB4-me was more potent (nM range) than LXB4 for both cell types. In contrast, 5(R)-methyl-LXB4-me was potent in inhibiting neutrophil transmigration across endothelial monolayers, but did not stimulate monocyte adherence. These results indicate that LXB4 analogs can be designed to resist rapid transformation and retain bioactivity with both monocytes and neutrophils. Moreover, they suggest that LXB4 stable analogs are useful tools to selectively evaluate the modes of actions of LXB4 with different tissues.