Growth State-Dependent Expression of Arachidonate Lipoxygenases in the Human Endothelial Cell Line EA.hy926

Endothelial cells regulate vascular homeostasis through the secretion of various paracrine molecules, including bioactive lipids, but little is known regarding the enzymes responsible for generating these lipids under either physiological or pathophysiological conditions. Arachidonate lipoxygenase (ALOX) expression was therefore investigated in confluent and nonconfluent EA.h926 endothelial cells, which represent the normal quiescent and proliferative states, respectively. mRNAs for ALOX15, ALOX15B, and ALOXE3 were detected in EA.hy926 cells, with the highest levels present in confluent cells compared to nonconfluent cells. In contrast, ALOX5, ALOX12, and ALOX12B mRNAs were not detected. At the protein level, only ALOX15B and ALOXE3 were detected but only in confluent cells. ALOXE3 was also observed in confluent human umbilical artery endothelial cells (HUAEC), indicating that its expression, although previously unreported, may be a general feature of endothelial cells. Exposure to laminar flow further increased ALOXE3 levels in EA.hy926 cells and HUAECs. The evidence obtained in this study indicates that proliferative status and shear stress are both important factors that mediate endothelial ALOX gene expression. The presence of ALOX15B and ALOXE3 exclusively in quiescent human endothelial cells suggests their activity likely contributes to the maintenance of a healthy endothelium.

Free docosahexaenoic acid promotes ferroptotic cell death via lipoxygenase dependent and independent pathways in cancer cells

PURPOSE

Ferroptosis is a form of regulated cell death that has the potential to be targeted as a cancer therapeutic strategy. But cancer cells have a wide range of sensitivities to ferroptosis, which limits its therapeutic potential. Accumulation of lipid peroxides determines the occurrence of ferroptosis. However, the type of lipid involved in peroxidation and the mechanism of lipid peroxide accumulation are less studied.

METHODS

The effects of fatty acids (10 μM) with different carbon chain length and unsaturation on ferroptosis were evaluated by MTT and LDH release assay in cell lines derived from prostate cancer (PC3, 22RV1, DU145 and LNCaP), colorectal cancer (HT-29), cervical cancer (HeLa) and liver cancer (HepG2). Inhibitors of apoptosis, necroptosis, autophagy and ferroptosis were used to determine the type of cell death. Then the regulation of reactive oxygen species (ROS) and lipid peroxidation by docosahexaenoic acid (DHA) was measured by HPLC-MS and flow cytometry. The avtive form of DHA was determined by siRNA mediated gene silencing. The role of lipoxygenases was checked by inhibitors and gene silencing. Finally, the effect of DHA on ferroptosis-mediated tumor killing was verified in xenografts.

RESULTS

The sensitivity of ferroptosis was positively correlated with the unsaturation of exogenously added fatty acid. DHA (22:6 n-3) sensitized cancer cells to ferroptosis-inducing reagents (FINs) at the highest level in vitro and in vivo. In this process, DHA increased ROS accumulation, lipid peroxidation and protein oxidation independent of its membrane receptor, GPR120. Inhibition of long chain fatty acid-CoA ligases and lysophosphatidylcholine acyltransferases didn't affect the role of DHA. DHA-involved ferroptosis can be induced in both arachidonate lipoxygenase 5 (ALOX5) negative and positive cells. Down regulation of ALOX5 inhibited ferroptosis, while overexpression of ALOX5 promoted ferroptosis.

CONCLUSION

DHA can effectively promote ferroptosis-mediated tumor killing by increasing intracellular lipid peroxidation. Both ALOX5 dependent and independent pathways are involved in DHA-FIN induced ferroptosis. And during this process, free DHA plays an important role.

Production of 8S- and 10S-hydroxy polyunsaturated fatty acids by recombinant Escherichia coli cells expressing mouse arachidonate 8S-lipoxygenase

OBJECTIVE

To quantitatively hydroxylate 8S- and 10S-positions on polyunsaturated fatty acids by recombinant Escherichia coli cells expressing mouse arachidonate 8S-lipoxygenase (8S-LOX).

RESULTS

Hydroxylated products gained from the conversion of arachidonic acid (20:4^{Δ5Z,8Z,11Z,14Z}, AA), eicosapentanoic acid (20:5^{Δ5Z,8Z,11Z,14Z,17Z}, EPA), and (22:6^{Δ4Z,7Z,10Z,13Z,16Z,19Z}, DHA) by recombinant E. coli cells containing 8S-LOX from mouse were identified as 8S-hydroxy-5,9,11,14(Z,E,Z,Z)-eicosatetranoic acid (8S-HETE), 8S-hydroxy-5,9,11,14,17(Z,E,Z,Z,Z)-eicosapentanoic acid (8S-HEPE), and 10S-hydroxy-4,8,12,14,16,19(Z,E,Z,Z,Z,Z)-docosahexaenoic acid (10S-HDoHE), respectively. Under the optimal hydroxylation conditions of pH 7.5, 30 °C, 5% (v/v) ethanol, 15 g cells l^-1, and 5 mM substrate, AA, EPA, and DHA were hydroxylated into 4.37 mM 8S-HETE, 3.77 mM 8S-HEPE, and 3.13 mM 10S-HDoHE for 60, 90, and 60 min, with 87, 75, and 63% molar conversions, respectively.

CONCLUSION

To the best of our knowledge, this is the first quantitatively biotechnological production of 8S-HETE, 8S-HEPE, and 10S-HDoHE.

Gaining insight into the chemistry of lipoxygenases: a computational investigation into the catalytic mechanism of (8R)-lipoxygenase

Lipoxygenases (LOXs) are ubiquitous in nature and catalyze a range of life-essential reactions within organisms. In particular they are critical to the formation of eicosanoids, which are critical for normal cell function. However, a number of important questions about the reactivity and mechanism of these enzymes still remain. Specifically, although the initial step in the mechanism of LOXs has been well studied, little is known of subsequent steps. Thus, with use of a quantum mechanical/molecular mechanical approach, the complete catalytic mechanism of (8R)-LOX was investigated. The results have provided a better understanding of the general chemistry of LOXs as a whole. In particular, from comparisons with soybean LOX-1, it appears that the initial proton-coupled electron transfer may be very similar among all LOXs. Furthermore, LOXs appear to undergo multistate reactivity where potential spin inversion of an electron may occur either in the attack of O(2) or in the regeneration of the active site. Lastly, it is shown that with the explicit modeling of the environment, the regeneration of the active center likely occurs via the rotation of the intermediate followed by an outer-sphere [Formula: see text] transfer as opposed to the formation of a "purple intermediate" complex.

Study of arachidonoyl specificity in two enzymes of the PI cycle

We identified a conserved pattern of residues L-X((3-4))-R-X((2))-L-X((4))-G, in which -X((n))- is n residues of any amino acid, in two enzymes acting on the polyunsaturated fatty acids, diacylglycerol kinase epsilon (DGKɛ) and phosphatidylinositol-4-phosphate-5-kinase Iα (PIP5K Iα). DGKɛ is the only one of the 10 mammalian isoforms of DGK that exhibits arachidonoyl specificity and is the only isoform with the motif mentioned above. Mutations of the essential residues in this motif result in the loss of arachidonoyl specificity. Furthermore, DGKα can be converted to an enzyme having this motif by substituting only one residue. When DGKα was mutated so that it gained the motif, the enzyme also gained some specificity for arachidonoyl-containing diacylglycerol. This motif is present also in an isoform of phosphatidylinositol-4-phosphate-5-kinase that we demonstrated had arachidonoyl specificity for its substrate. Single residue mutations within the identified motif of this isoform result in the loss of activity against an arachidonoyl substrate. The importance of acyl chain specificity for the phosphatidic acid activation of phosphatidylinositol-4-phosphate-5-kinase is also shown. We demonstrate that the acyl chain dependence of this phosphatidic acid activation is dependent on the substrate. This is the first demonstration of a motif that endows specificity for an acyl chain in enzymes DGKε and PIP5K Iα.

[Resistance to antiplatelet drugs in patients with cerebrovascular disorders]

This review concerns clinical and laboratory resistance to antiplatelet drugs (aspirin and clopidogrel) in patients with cerebrovascular disorders. Results of certain clinical trials showed that laboratory resistance to antiaggregants is associated with recurrent thromboembolic vascular events. The commonest causes of aspirin resistance are production of arachidonic acid metabolites via the lipoxygenase pathway, poor compliance with the treatment, polymorphism of the genes encoding for cyclooxygenase and glycoprotein (GP) IIb/IIIa, endothelial dysfunction. The causes of clopidogrel resistance include inadequate doses of the drug, its low absorption, poor compliance with the treatment, polymorphism of ADP receptors, GP IIb/IIIa and cytochrome P450 genes, acute coronary syndrome and stroke, metabolic syndrome. Therapeutic efficacy of antiaggregants can be improved by increasing their doses, using membranotropic agents, correcting endothelial dysfunction, etc. Because the apparent variability of antiplatelet drug resistance is currently due to the use of different test-systems by different authors, the evaluation of individual sensitivity to a given drug showing laboratory resistance and the choice of alternative therapy are thus far possible only in the framework of clinical studies. Large-scale prospective multicenter trials of antiplatelet drug resistance are needed along with research for better understanding mechanisms of individual platelet sensitivity and resistance to antiaggregants and developing efficacious methods for their correction.

Oxygenation of arachidonoyl lysophospholipids by lipoxygenases from soybean, porcine leukocyte, or rabbit reticulocyte

Oxygenation of arachidonoyl lysophosphatidylcholine (lysoPC) or arachidonoyl lysophosphatidic acid (lysoPA) by lipoxygenase (LOX) was examined. The oxidized products were identified by HPLC/UV spectrophotometry/mass spectrometry analyses. Straight-phase and chiral-phase HPLC analyses indicated that soybean LOX-1 and rabbit reticulocyte LOX oxygenated arachidonoyl lysophospholipids mainly at C-15 with the S form as major enantiomer, whereas porcine leukocyte LOX oxygenated at C-12 with the S form. Next, the sequential exposure of arachidonoyl-lysoPC to soybean LOX-1 and porcine leukocyte LOX afforded two major isomers of dihydroxy derivatives with conjugated triene structure, suggesting that 15(S)-hydroperoxyeicosatetraenoyl derivatives were converted to 8,15(S)-dihydroxyeicosatetraenoyl derivatives. Separately, arachidonoyl-lysoPA, but not arachidonoyl-lysoPC, was found to be susceptible to double oxygenation by soybean LOX-1 to generate a dihydroperoxyeicosatetraenoyl derivative. Overall, arachidonoyl lysophospholipids were more efficient than arachidonic acid as LOX substrate. Moreover, the catalytic efficiency of arachidonoyl-lysoPC as substrate of three lipoxygenases was much greater than that of arachidonoyl-lysoPA or arachidonic acid. Taken together, it is proposed that arachidonoyl-lysoPC or arachidonoyl-lysoPA is efficiently oxygenated by plant or animal lipoxygenases, C12- or C15-specific, to generate oxidized products with conjugated diene or triene structure.

Epidermal lipoxygenase products of the hepoxilin pathway selectively activate the nuclear receptor PPARalpha

Arachidonic acid can be transformed into a specific epoxyalcohol product via the sequential action of two epidermal lipoxygenases, 12R-LOX and eLOX3. Functional impairment of either lipoxygenase gene (ALOX12B or ALOXE3) results in ichthyosis, suggesting a role for the common epoxyalcohol product or its metabolites in the differentiation of normal human skin. Here we tested the ability of products derived from the epidermal LOX pathway to activate the peroxisome proliferator-activated receptors PPARalpha, gamma, and delta, which have been implicated in epidermal differentiation. Using a dual luciferase reporter assay in PC3 cells, the 12R-LOX/eLOX3-derived epoxyalcohol, 8R-hydroxy-11R,12R-epoxyeicosa-5Z,9E,14Z-trienoic acid, activated PPARalpha with similar in potency to the known natural ligand, 8S-hydroxyeicosatetraenoic acid (8S-HETE) (both at 10 microM concentration). In contrast, the PPARgamma and PPARdelta receptor isoforms were not activated by the epoxyalcohol. Activation of PPARalpha was also observed using the trihydroxy hydrolysis products (trioxilins) of the unstable epoxyalcohol. Of the four trioxilins isolated and characterized, the highest activation was observed with the isomer that is also formed by enzymatic hydrolysis of the epoxyalcohol. Formation of a ligand for the nuclear receptor PPARalpha may be one possibility by which 12R-LOX and eLOX3 contribute to epidermal differentiation.

Inducible expression of 15-lipoxygenase-2 and 8-lipoxygenase inhibits cell growth via common signaling pathways

Human 15-lipoxygenase (LOX)-2 and mouse 8-LOX represent orthologous members of the LOX family but display different positional specificities and tissue distribution. To study the functional role of 15-LOX-2 and 8-LOX in keratinocytes, an inducible Tet-On gene expression system was established in the premalignant mouse keratinocyte cell line 308. Doxycycline (dox)-induced expression of enzymatically active 15-LOX-2 and 8-LOX led to an inhibition of cell growth that was associated with an inhibition of DNA synthesis, as shown by a 15-46% reduction of 5-bromo-2-deoxy-uridine (BrdU) incorporation. The inhibitory effects were increased in the presence of exogenous arachidonic acid. In contrast, addition of linoleic acid or the LOX inhibitor baicalein reversed the growth-inhibitory effects. Treatment of the cells with 15-hydroxyeicosatetraenoic acid (HETE) or 8-HETE resulted in a similar inhibition of BrdU incorporation, whereas 13-hydroxyoctadecadienoic acid (HODE) and 9-HODE, in contrast, had no effects. Dox-induced keratinocytes showed increased levels of reactive oxygen species (ROS). The antioxidant N-acetyl-L-cysteine and a specific inhibitor of p38 mitogen-activated protein kinase, but not of extracellular signal-regulated kinase 1/2 or c-Jun N-terminal kinase/stress-activated kinases, completely abolished the LOX-induced growth inhibition, indicating a critical role of ROS and p38. Our data suggest that 15-LOX-2 and 8-LOX, although displaying different positional specificity, may use common signaling pathways to induce growth inhibition in premalignant epithelial cells.

A lipoxygenase inhibitor in breast cancer brain metastases

The complication of multiple brain metastases in breast cancer patients is a life threatening condition with limited success following standard therapies. The arachidonate lipoxygenase pathway appears to play a role in brain tumor growth as well as inhibition of apoptosis in in-vitro studies. The down regulation of these arachidonate lipoxygenase growth stimulating products therefore appeared to be a worthwhile consideration for testing in brain metastases not responding to standard therapy. Boswellia serrata, a lipoxygenase inhibitor was applied for this inhibition. Multiple brain metastases were successfully reversed using this method in a breast cancer patient who had not shown improvement after standard therapy. The results suggest a potential new area of therapy for breast cancer patients with brain metastases that may be useful as an adjuvant to our standard therapy.

What are cyclooxygenases and lipoxygenases doing in the driver's seat of carcinogenesis?

Substantial evidence supports a functional role for cyclooxygenase- and lipoxygenase-catalyzed arachidonic and linoleic acid metabolism in cancer development. Genetic intervention studies firmly established cause-effect relations for cyclooxygenase-2, but cyclooxygenase-1 may also be involved. In addition, pharmacologic cyclooxygenase inhibition was found to suppress carcinogenesis in both experimental mouse models and several cancers in humans. Arachidonic acid-derived eicosanoid or linoleic acid-derived hydro[peroxy]fatty acid signaling are likely to be involved impacting fundamental biologic phenomena as diverse as cell growth, cell survival, angiogenesis, cell invasion, metastatic potential and immunomodulation. However, long chain unsaturated fatty acid oxidation reactions indicate antipodal functions of distinct lipoxygenase isoforms in carcinogenesis, i.e., the 5- and platelet-type 12-lipoxygenase exhibit procarcinogenic activities, while 15-lipoxygenase-1 and 15-lipoxygenase-2 may suppress carcinogenesis.

Regulation of neurotensin receptor function by the arachidonic acid-lipoxygenase pathway in prostate cancer PC3 cells

Neurotensin (NT) elevates leukotriene levels in animals and stimulates 5-HETE formation in prostate cancer PC3 cells. PC3 cell growth is stimulated by NT and inhibited by lipoxygenase (LOX) blockers. This led us to test LOX blockers (NDGA, MK886, ETYA, Rev5901, AA861 and others) for effects on NT binding and signaling. LOX blockers dramatically enhanced 125I-neurotensin binding to NT receptor NTR1 in PC3 cells, whereas they inhibited NT-induced inositol phosphate formation. These effects were indirect (binding to isolated membranes was unaffected), receptor-specific (binding to beta2-adrenergic, V1a-vasopressin, EGF and bombesin receptor was unaffected) and pathway-specific (cyclooxygenase inhibitors were inactive). NT receptor affinity was increased but receptor number and % internalization were unchanged. Also supporting the involvement of arachidonic acid metabolism in NTR1 regulation was the finding that inhibitors of PLA2 and DAG lipase enhanced NT binding. These findings suggest that NTR1 is regulated by specific feedback mechanism(s) involving lipid peroxidation and/or LOX-dependent processes.

Identification and characterization of an arachidonate 11R-lipoxygenase

11R-Lipoxygenase (11R-LOX) activity has been detected in several marine invertebrates, and here we report the first cloning and expression of the enzyme. The cDNA encoding a protein of 77kDa was isolated by RT-PCR from the soft coral Gersemia fruticosa and expressed in Escherichia coli. Incubations of recombinant enzyme with arachidonic acid yielded a single product, identified by RP-HPLC, GC-MS, and chiral phase-HPLC as 11R-hydroperoxyeicosatetraenoic acid. Other C18, C20, and C22 substrates are also oxygenated, preferentially at the omega10 position. Significantly, both Ca(2+)-ions and a membrane fraction are required for catalytic activity. Calcium effects translocation of the soluble 11R-LOX to the membrane and this association is reversible by Ca(2+) chelation. The enzyme sequence contains some conserved amino acids implicated in calcium activation of mammalian 5-LOX, and with its obligate requirement for membrane interaction the 11R-LOX may thus provide a new model for further analysis of this aspect of lipoxygenase activation.

Lipoxygenase Pathway of Arachidonic Acid Metabolism in Growth Control of Tumor Cells of Different Type

The influence of inhibitors of different lipoxygenases (LOX) on the growth of human tumor cells with different profiles of synthesized eicosanoids was studied. The studied LOX inhibitors had virtually no influence on the growth of A549 cells actively synthesizing cyclooxygenase and lipoxygenase metabolites of arachidonic acid (AA). The inhibitor of 12-LOX, baicalein, significantly inhibited proliferation in cultures of A431 epidermoid carcinoma cells with a characteristic domination of the major lipoxygenase metabolite of AA, 12-hydroxyeicosatetraenoic acid (12-HETE), in the profile of synthesized eicosanoids and reduced to 70% the incorporation of [3H]thymidine into DNA. Treatment of these cultures with 12-HETE virtually restored the growth potential of the tumor cells. The findings suggest that the lipoxygenase metabolite of AA, 12-HETE, is a growth-limiting factor for tumor cells of definite type.

Insights from the X-ray crystal structure of coral 8R-lipoxygenase: calcium activation via a C2-like domain and a structural basis of product chirality

Lipoxygenases (LOXs) catalyze the regio- and stereospecific dioxygenation of polyunsaturated membrane-embedded fatty acids. We report here the 3.2 A resolution structure of 8R-LOX from the Caribbean sea whip coral Plexaura homomalla, a LOX isozyme with calcium dependence and the uncommon R chiral stereospecificity. Structural and spectroscopic analyses demonstrated calcium binding in a C2-like membrane-binding domain, illuminating the function of similar amino acids in calcium-activated mammalian 5-LOX, the key enzyme in the pathway to the pro-inflammatory leukotrienes. Mutation of Ca(2+)-ligating amino acids in 8R-LOX resulted not only in a diminished capacity to bind membranes, as monitored by fluorescence resonance energy transfer, but also in an associated loss of Ca(2+)-regulated enzyme activity. Moreover, a structural basis for R chiral specificity is also revealed; creation of a small oxygen pocket next to Gly(428) (Ala in all S-LOX isozymes) promoted C-8 oxygenation with R chirality on the activated fatty acid substrate.

Identification of an endogenous inhibitor of arachidonate metabolism in human epidermoid carcinoma A431 cells

Eicosanoids, which include prostaglandins, thromboxanes, and leukotrienes, are produced from arachidonic acid by three main pathways in cells, including cyclooxygenases and lipoxygenases, and cytochrome P450 enzymes. Accumulated evidence indicates that a certain peroxide tone is required for the initiation of reaction by lipoxygenases and cyclooxygenases. An endogenous inhibitor of arachidonate oxygenation was suspected in the cytosolic fraction of human epidermoid carcinoma A431 cells. After a series of studies, the existence of this inhibitor was confirmed, while it was purified and characterized. By amino acid sequence analysis, the inhibitor in A431 cells was subsequently identified as a phospholipid hydroperoxide glutathione peroxidase (PHGPx). Depletion of cellular glutathione in cells by diethyl maleate or by dibuthionine-sulfoximine results in an increase in enzyme activities of 12(S)-lipoxygenase and cyclooxygenase, suggesting that glutathione-depleting agents abolish the enzyme activity of PHGPx in cells. Stable transfectants of A431 cells with overexpression and depletion of PHGPx have been constructed, respectively. Reduction of arachidonate metabolism through 12(S)-lipoxygenase and cyclooxygenase 1 and that of the arsenite-induced generation of reactive oxygen species are observed in cells overexpressing PHGPx. On the other hand, enhancement of arachidonate metabolism and the arsenite-induced generation of reactive oxygen species is detected in PHGPx-depleted cells. In conclusion, the endogenous inhibitor of arachidonate metabolism present in A431 cells is a PHGPx, which plays a functional role in the down-regulation of arachidonate oxygenation catalyzed by 12(S)-lipoxygenase and cyclooxygenase 1 through the reduction of the level of intracellular lipid hydroperoxides. The latter acts as the peroxide tone for arachidonate metabolism in A431 cells.

Arachidonate 8(S)-lipoxygenase

The recently identified mouse 8(S)-lipoxygenase almost exclusively directs oxygen insertion into the 8(S) position of arachidonic acid and, with lower efficiency, into the 9(S) position of linoleic acid. The protein of 677 amino acids displays 78% sequence identity to human 15(S)-lipoxygenase-2 which is considered to be its human orthologue. The 8(S)-lipoxygenase gene, Alox15b, consisting of 14 exons and spanning 14.5 kb is located within a gene cluster of related epidermis-type lipoxygenases at the central region of mouse chromosome 11. 8(S)-Lipoxygenase is predominantly expressed in stratifying epithelia of mice, constitutively in the hair follicle, forestomach, and foot-sole and inducible in the back skin with strain-dependent variations. The expression is restricted to terminally differentiating keratinocytes, in particular the stratum granulosum and 8(S)-lipoxygenase activity seems to be involved in terminal differentiation of mouse epidermis. Tumor-specific up-regulation of 8(S)-lipoxygenase expression and activity indicate a critical role of this enzyme in malignant progression during tumor development in mouse skin.

A Pertussis Toxin-Sensitive 8-Lipoxygenase Pathway Is Activated by a Nicotinic Acetylcholine Receptor inAplysia Neurons

Acetylcholine (ACh) activates two types of chloride conductances in Aplysia neurons that can be distinguished by their kinetics and pharmacology. One is a rapidly desensitizing current that is blocked by α-conotoxin-ImI and the other is a sustained current that is insensitive to the toxin. These currents are differentially expressed in Aplysia neurons. We report here that neurons that respond to ACh with a sustained chloride conductance also generate 8-lipoxygenase metabolites. The sustained chloride conductance and the activation of 8-lipoxygenase have similar pharmacological profiles. Both are stimulated by suberyldicholine and nicotine, and both are inhibited by α-bungarotoxin. Like the sustained chloride conductance, the activation of 8-lipoxygenase is not blocked by α-conotoxin-ImI. In spite of the similarities between the metabolic and electrophysiological responses, the generation of 8-lipoxygenase metabolites does not appear to depend on the ion current since an influx of chloride ions is neither necessary nor sufficient for the formation of the lipid metabolites. In addition, the application of pertussis toxin blocked the ACh-activated release of arachidonic acid and the subsequent production of 8-lipoxygenase metabolites, yet the ACh-induced activation of the chloride conductance is not dependent on a G protein. Our results are consistent with the idea that the nicotinic ACh receptor that activates the sustained chloride conductance can, independent of the chloride ion influx, initiate lipid messenger synthesis.

AMPA receptor modulation in previously frozen mouse brain sections: opposite effects of calcium in the cortex and hippocampus

Various forms of synaptic plasticity in the brain have been proposed to result from modifications in the properties of glutamate receptors by calcium-dependent mechanisms. In the present study, changes in glutamate receptors elicited by calcium treatment of previously frozen mouse brain sections were evaluated by qualitative as well as quantitative analysis of tritiated ligand binding to both alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) and N-methyl-D-aspartate (NMDA) glutamate receptor subtypes. Quantitative analysis revealed that 3H-AMPA binding was reduced in a dose-dependent manner by calcium in the cerebral cortex and striatum formations. However, an opposite change in AMPA receptor properties was observed in the hippocampus, as calcium generated an increase of AMPA binding in all hippocampal fields. Analysis of the saturation kinetics of 3H-AMPA binding showed that the calcium-induced augmentation of AMPA binding in the stratum radiatum of the CA1 region was due to an alteration in the maximal number of sites, while the reduction of binding elicited by calcium in the cortex appeared to be due to modified AMPA receptor affinity. Calcium-induced downregulation of AMPA receptor affinity in the cortex and striatum was affected by baicalein, a selective inhibitor of the lipoxygenase pathways of arachidonic acid metabolism, whereas the same inhibitor did not modify calcium-mediated upregulation of receptor number in the CA1 region of the hippocampus. On the other hand, the effect of calcium appeared to be specific for the AMPA receptor, as the same treatment did not affect glutamate binding to the NMDA glutamate receptor subtype. Our results suggest the possibility that, depending on the brain regions, calcium ions may generate opposite modulation of AMPA receptor properties. Because the regulation of AMPA receptors by calcium-dependent enzymes has been implicated in synaptic plasticity, our results suggest that regional variations in the effect of calcium on AMPA binding account for differential plasticity at glutamatergic synapses.

Site-Directed Mutagenesis Studies on a Putative Fifth Iron Ligand of Mouse 8S-Lipoxygenase: Retention of Catalytic Activity on Mutation of Serine-558 to Asparagine, Histidine, or Alanine

The reported crystal structures of plant and animal lipoxygenases (LOX) show that the nonheme iron in the catalytic domain is ligated by three histidines, the C-terminal isoleucine, and in certain structures also by a fifth iron ligand, an asparagine or histidine residue. Mouse 8-LOX and its homologues (e.g., human 15-LOX-2) are unique in having a serine in place of the usual Asn or His in this fifth position. To investigate the importance of the residue in mouse 8-LOX structure-function, the serine-558 was replaced by asparagine, histidine, or alanine using oligonucleotide-directed mutagenesis. Wild-type mouse 8-LOX and the mutant cDNAs were expressed in HeLa cells infected with vaccinia virus encoding T7 RNA polymerase and their relative lipoxygenase activities assessed by incubation with [14C]arachidonic acid or [14C]linoleic acid followed by HPLC analysis of the products. The Ser558Asn and Ser558His mutants had equivalent or greater activity than wild-type 8-LOX. They also exhibited some 15-LOX activity, indicating that small structural perturbations (in this case to a residue identical in mouse 8-LOX and its 15-LOX-2 homologues) can interchange the positional specificity of these closely related enzymes. Remarkably, the Ser558Ala mutant exhibited significant 8-LOX activity, indicating that this position is not an essential iron ligand in the enzyme. We conclude that mouse 8-LOX is catalytically competent with only four amino acid iron ligands, and that Ser-558 of the wild-type enzyme does not play an essential role in catalysis.

Discovery of an 11 (R)- and 12(S)-lipoxygenase activity in ovaries of the mussel Mytilus edulis

Eicosanoid biosynthesis was investigated in mussel gonads by incubation of tissue homogenates with radiolabeled arachidonic acid and analysis of the products by radio-high-performance liquid chromatography. No radiolabeled metabolites were formed in homogenates of testes, but two major metabolites were synthesized by ovarian preparations. The radiolabeled metabolites were analyzed by mass spectrometry and chiral chromatography and identified as 11 (R)-hydroxy-5,8,1 2,14-eicosatetraenoic acid and 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid. In addition, four other nonlabeled metabolites, formed from endogenous substrates, were detected in ovarian extracts. Their structures, determined by mass spectrometric analysis, were the corresponding 11- and 12-hydroxy analogs formed from eicosapentaenoic acid (11-HEPE and 12-HEPE) and 9-hydroxy-6,10,12,15-octadecatetraenoic acid (9-HOTE) and 13-hydroxy-6,9,11,15-octadecatetraenoic acid formecl from stearidonic acid. The biosynthesis of the 11 - and 12-hydroxy products was calcium dependent, localized to the 100,000 x g supernatant cell fraction, and was inhibited by nordihydroguaiaretic acid, but not inhibited by the prostaglandin synthase inhibitors aspirin and indomethacin, or the monoxygenase inhibitor proadifen. Together these data suggested that both the 11 (R)- and 12(S)-hydroxy products were formed from lipoxygenase-type enzymes. Incubation of homogenates of immature ovaries with eicosapentaenoic acid revealed the major product to be I2-HEPE, whereas in mature ovaries mainly 11-HEPE was formed. Extraction of spawned eggs with methanol revealed that predominantly 11-HEPE and 9-HOTE were formed from endogenous substrates. This study shows that female gonads of the mussel express an 11(R)- and 12(S)-lipoxygenase activity whose expression is dependent on differentiation of the ovary.

8S-lipoxygenase products activate peroxisome proliferator-activated receptor alpha and induce differentiation in murine keratinocytes

To determine the function and mechanism of action of the 8S-lipoxygenase (8-LOX) product of arachidonic acid, 8S-hydroxyeicosatetraenoic acid (8S-HETE), which is normally synthesized only after irritation of the epidermis, transgenic mice with 8-LOX targeted to keratinocytes through the use of a loricrin promoter were generated. Histological analyses showed that the skin, tongue, and stomach of transgenic mice are highly differentiated, and immunoblotting and immunohistochemistries of skin showed higher levels of keratin-1 expression compared with wild-type mice. The labeling index, however, of the transgenic epidermis was twice that of the wild-type epidermis. Furthermore, 8S-HETE treatment of wild-type primary keratinocytes induced keratin-1 expression. Peroxisome proliferator activated receptor alpha (PPARalpha) was identified as a crucial component of keratin-1 induction through transient transfection with expression vectors for PPARalpha, PPARgamma, and a dominant-negative PPAR, as well as through the use of known PPAR agonists. From these studies, it is concluded that 8S-HETE plays an important role in keratinocyte differentiation and that at least some of its effects are mediated by PPARalpha.

Structural basis for the positional specificity of lipoxygenases

The positional specificity of arachidonic acid oxygenation is currently the decisive parameter for classification of mammalian lipoxygenases but, unfortunately, the structural reasons for lipoxygenase specificity are not well understood. Although there are no direct structural data on lipoxygenase/substrate interaction, experiments with modified fatty acid substrates and mutagenesis studies suggest that for 12- and 15-lipoxygenases, arachidonic acid slides into the substrate-binding pocket with its methyl end ahead. For arachidonate 5- and/or 8-lipoxygenation two alternative models for the enzyme/substrate interaction have been developed: 1) The orientation-determined model and 2) the space-determined model. This review explores the experimental data available on the mechanistic reasons for lipoxygenase specificity and concludes that each of the above-mentioned hypotheses may be valid for arachidonate 5-lipoxygenation under certain circumstances.

Identification of amino acid determinants of the positional specificity of mouse 8S-lipoxygenase and human 15S-lipoxygenase-2

Phorbol ester-inducible mouse 8S-lipoxygenase (8-LOX) and its human homologue, 15S-lipoxygenase-2 (15-LOX-2), share 78% identity in amino acid sequences, yet there is no overlap in their positional specificities. In this study, we investigated the determinants of positional specificity using a random chimeragenesis approach in combination with site-directed mutagenesis. Exchange of the C-terminal one-third of the 8-LOX with the corresponding portion of 15-LOX-2 yielded a chimeric enzyme with exclusively 15S-lipoxygenase activity. The critical region was narrowed down to a cluster of five amino acids by expression of multiple cDNAs obtained by in situ chimeragenesis in Escherichia coli. Finally, a pair of amino acids, Tyr(603) and His(604), was identified as the positional determinant by site-directed mutagenesis. Mutation of both of these amino acids to the corresponding amino acids in 15-LOX-2 (Asp and Val, respectively) converted the positional specificity from 8S to 90% 15S without yielding any other by-products. Mutation of the corresponding residues in 15-LOX-2 to the 8-LOX sequence changed specificity to 50% oxygenation at C-8 for one amino acid substitution and 70% at C-8 for the double mutant. Based on the crystal structure of the reticulocyte 15-LOX, these two amino acids lie opposite the open coordination position of the catalytic iron in a likely site for substrate binding. The change from 8 to 15 specificity entails a switch in the head to tail binding of substrate. Enzymes that react with substrate "head first" (5-LOX and 8-LOX) have a bulky aromatic amino acid and a histidine in these positions, whereas lipoxygenases that accept substrates "tail first" (12-LOX and 15-LOX) have an aliphatic residue with a glutamine or aspartate. Thus, this positional determinant of the 8-LOX and 15-LOX-2 may have significance for other lipoxygenases.

Affinities of various mammalian arachidonate lipoxygenases and cyclooxygenases for molecular oxygen as substrate

In an attempt to study affinities for molecular oxygen of mammalian arachidonate oxygenases, which remain unclarified at present, we determined activities of platelet-type 12-lipoxygenase, leukocyte-type 12-lipoxygenase, 5-lipoxygenase, 15-lipoxygenase, cyclooxygenase-1 and cyclooxygenase-2 at various oxygen concentrations. Activities of all the tested enzymes were assessed by oxygenation of radioactive arachidonic acid under hypoxic conditions, and part of the enzymes were also assayed by monitoring oxygen consumption. Their Km values for oxygen ranged between 10 and 26 microM. These results should be considered in investigations of arachidonic acid metabolism in pathophysiological processes associated with hypoxia.

With anandamide as substrate plant 5-lipoxygenases behave like 11-lipoxygenases

Anandamide, an endogenous ligand for cannabinoid receptors CB1 and CB2, was incubated with purified 5-lipoxygenases from barley and tomato. This yielded 11S-hydroperoxy-5,8,12,14-eicosatetraenoylethanolamide (11S-HPANA) as major product (about 70%). This is in contrast with the dioxygenation of arachidonic acid, where 5S-HPETE is the major product. This observation implies that the regiospecificity of the dioxygenation, catalyzed by nonmammalian 5-lipoxygenases, is altered by a modification at the carboxylic end of the substrate. Soybean 15-lipoxygenase forms 15S-HPANA (95%) and 11S-HPANA (5%), and in the second dioxygenation 5,15-diHPANA (45%) and 8,15-diHPANA (55%) are formed. Apparently, the regiospecificity of the soybean 15-lipoxygenase reaction is only slightly affected using anandamide as substrate.

Molecular cloning and functional expression of a phorbol ester-inducible 8S-lipoxygenase from mouse skin

One of the effects of topical application of phorbol ester to mouse skin is the induction of an 8S-lipoxygenase in association with the inflammatory response. Here we report the molecular cloning and characterization of this enzyme. The cDNA was isolated by polymerase chain reaction from mouse epidermis and subsequently from a mouse epidermal cDNA library. The cDNA encodes a protein of 677 amino acids with a calculated molecular mass of 76 kDa. The amino acid sequence has 78% identity to a 15S-lipoxygenase cloned recently from human skin and approximately 40% identity to other mammalian lipoxygenases. When expressed in vaccinia virus-infected Hela cells, the mouse enzyme converts arachidonic acid exclusively to 8S-hydroperoxyeicosatetraenoic acid while linoleic acid is converted to 9S-hydroperoxy-linoleic acid in lower efficiency. Phorbol ester treatment of mouse skin is associated with strong induction of 8S-lipoxygenase mRNA and protein. By Northern analysis, expression of 8S-lipoxygenase mRNA was also detected in brain. Immunohistochemical analysis of phorbol ester-treated mouse skin showed the strongest reaction to 8S-lipoxygenase in the differentiated epidermal layer, the stratum granulosum. The inducibility may be a characteristic feature of the mouse 8S-lipoxygenase and its human 15S-lipoxygenase homologue.

Identification of an 8-lipoxygenase pathway in nervous tissue of Aplysia californica

Arachidonic acid is converted to (8R)-hydroperoxyeicosa-5,9,11, 14-tetraenoic acid (8-HPETE) during incubations with homogenates of the central nervous system of the marine mollusc, Aplysia californica. 8-HPETE can be reduced to the corresponding hydroxy acid or be enzymatically converted to a newly identified metabolite, 8-ketoeicosa-5,9,11,14-tetraenoic acid (8-KETE). These metabolites were identified by high performance liquid chromatography, UV absorbance, and gas chromatography/mass spectrometry. Stereochemical analysis of the products demonstrate that the neuronal enzyme is an (8R)-lipoxygenase. Previously we have shown that the neurotransmitters, histamine and Phe-Met-Arg-Phe-amide, activate 12-lipoxygenase metabolism in isolated identified Aplysia neurons. We now show that acetylcholine activates the (8R)-lipoxygenase pathway within intact nerve cells. Thus, both (12S)- and (8R)-lipoxygenase co-exist in intact Aplysia nervous tissue but are differentially activated by several neurotransmitters. The precise physiological role of the 8-lipoxygenase products is currently under investigation, but by analogy to the well-described 12-lipoxygenase pathway, we suggest that (8R)-HPETE and 8-KETE may serve as second messengers in Aplysia cholinoceptive neurons.

5-HPETE is a potent inhibitor of neuronal Na+, K(+)-ATPase activity

The effects of 1 microM concentrations of arachidonic acid hydroperoxide (HPETES) products of 5-, 12- and 15-lipoxygenase on Na+, K(+)-ATPase activity were investigated in synaptosomal membrane preparations from rat cerebral cortex. 5-HPETE inhibited Na+, K(+)-ATPase activity by up to 67 %. In contrast, 12-HPETE and 15-HPETE did not inhibit Na+, K(+)-ATPase activity. In addition, neither 5-HETE or LTA4 inhibited Na+, K(+)-ATPase activity. Dose-response studies indicated that 5-HPETE was a potent (IC25 = 10(-8) M) inhibitor of Na+, K(+)-ATPase activity. These findings indicate that 5-HPETE inhibits Na+, K(+)-ATPase activity by a mechanism that is dependent on the hydroperoxide position and independent of further metabolism by 5-lipoxygenase. It is proposed that 5-HPETE production by 5-lipoxygenase and subsequent inhibition of neuronal Na+, K(+)-ATPase activity may be a mechansim for modulating synaptic transmission.

Novel oxylipins from the temperate red alga Polyneura latissima: evidence for an arachidonate 9(S)-lipoxygenase

The oxylipin chemistry of the temperate red alga Polyneura latissima has been investigated. The structures of three novel oxylipins, 8-[1'(Z),3'(Z),6'(Z)-dodecatriene-1'-oxy]- 5(Z),7(E)-octadienoic acid, 7(S*)-hydroxy-8(S*),9(S*)-epoxy-5(Z),11(Z),14(Z)-eicosatrienoic acid, 7(R*)-hydroxy-8(S*),9(S*)-epoxy-5(Z),11(Z),14(Z)-eicosatrienoic acid, together with two known eicosanoids, 9(S)-hydroxy-5(Z),7(E),11(Z),14(Z)-eicosatetraenoic acid, and 9,15-dihydroxy-5(Z),7(E),11(Z),13(E)-eicosatetraenoic acid, were elucidated by spectroscopic methods and chemical degradation. The oxygenation pattern of these oxylipins suggests that P. latissima metabolizes polyunsaturated fatty acids via a 9(S)-lipoxygenase.

Epidermal fatty acid oxygenases are activated in non-psoriatic dermatoses

The extent of epidermal fatty acid oxygenase activation in non-psoriatic dermatoses and the nature of these oxygenases are not known. The monohydroxylated fatty acid derivatives produced in vivo and trapped in skin scales or produced in vitro by oxygenases preserved in scales were analyzed by high performance liquid chromatography in 10 patients with non-psoriatic dermatoses. Evidence for 15-lipoxygenase activation included the finding of 15(S)-hydroxyeicosatetraenoic acid (HETE) in scales from seven patients and the production of 15(S)-[14C]HETE and 13(S)-[14C]hydroxyoctadecadienoic acid (HODE) during scale incubations, respectively, with [14C]arachidonic and [14C]linoleic acid. Evidence for the activation of an arachidonic acid 12(R)-oxygenase included the finding of 12(R)-HETE in scales from eight patients and the production of 12(R)-[14C]HETE during scale incubations with [14C]arachidonic acid. 13-HODE was the predominant fatty acid derivative present in scale extracts; its lack of enantiopurity (mean S/R = 3.1) and the substantial formation of 9-HODE (mean S/R = 0.6; 9/13-HODE = 0.43) suggest its derivation from 15-lipoxygenase and a second oxygenase. The levels of 15(S)-HETE and 12(R)-HETE had a 125- to 144-fold range and were highest in scales from a patient with erythroderma and in three psoriatic scale samples similarly analyzed. These findings indicate that 15-lipoxygenase, most likely of keratinocyte origin, and an arachidonic acid 12(R)oxygenase of unknown type and cell origin are activated in diverse dermatoses.

[Biosynthesis and functions of eicosanoids. Recent data]

Eicosanoids are oxygenated derivatives from 20-carbon polyunsaturated fatty acids. Among these, arachidonic acid is the reference precursor from which most metabolic pathways have been described. These pathways correspond to the biosynthesis of prostanoids with a recent revival interest after the discovery of an inducible form of prostaglandin H synthase, the formation of lipoxygenase products catalyzed by three different enzymes according to the initial carbon position of the oxygenation, and the oxygenation into epoxides and into hydroxy derivatives by the cytochrome P450 family. In addition to these enzyme pathways, the formation of prostaglandin isomers by radical-induced oxygenation and cyclization has been recently described and named isoprostanes. Finally, it is noteworthy to state the interest for two fatty acids of marine origin, namely eicosapentaenoic acid, an arachidonic acid analogue competing with it in different metabolic pathways, and docosahexaenoic acid, precursor of few docosanoids and transcriptional regulator of several activities.

Modulatory effects of eicosanoids on mesangial cell growth in response to immune injury

In a rat model of glomerular mesangial cell immune injury induced by a monoclonal antibody (ER4) against the mesangial cell membrane antigen Thy 1.1 and in which mesangial cell proliferation is a prominent feature, we examined the role of arachidonate 5- and 12-lipoxygenation (LO) eicosanoids and of thromboxane (Tx) in modulating the proliferative response. Significant increments in glomerular cell proliferation, assessed by counting glomerular cells positive for the Proliferating Cell Nuclear Antigen (PCNA) and by the incorporation of [3H]thymidine ([3H]TdR) in mesangial cell outgrowths from explanted glomeruli, occurred during the mesangioproliferative phase of injury. This event was abrogated in animals depleted of leukocytes or platelets prior to administration of ER4 and in animals pretreated with the arachidonate 5-LO inhibitor MK886. Pretreatment with the Tx synthase inhibitor, Furegrelate, or the arachidonate 12-LO inhibitor, Baicalein, had no effect, indicating that eicosanoids of arachidonate 5-LO but not those of 12-LO or Tx modulate mesangial cell proliferation following immune injury. We further identified those 5-lipoxygenation eicosanoids with growth modulatory effects on cultured mesangial cells. Leukotriene (LT)C4 and D4 but not LTB4 or 5-hydroxyeicosatetraenoic (HETE) acid enhanced [3H]TdR incorporation in growth-arrested mesangial cells. This effect of LTC4 and LTD4 was abrogated by the specific protein kinase C (PKC) inhibitor calphostin C, indicating a PKC-dependent mechanism. LTC4 and LTD4 but not 5-HETE or LTB4 also increased mesangial cell mass levels of the endogenous PKC activator diacylglycerol. The observations indicate that leukocyte-derived arachidonate 5-LO eicosanoids modulate mesangial cell proliferation following immune injury. Of these LTC4 and LTD4 are the likely candidates as they promote mesangial cell growth via a PKC-dependent mechanisms.

Co-regulated expression of glomerular 12/15-lipoxygenase and interleukin-4 mRNAs in rat nephrotoxic nephritis

Arachidonate 12- and 15-lipoxygenase (LO) products are generated in experimental glomerulonephritis. 15-S-HETE (a 15-LO product) and lipoxins (interaction products between 5-LO and either 12-LO or 15-LO) counteract the proinflammatory actions of leukotrienes. IL-4 has been shown to up-regulate 15-LO gene expression in human leukocytes. Based on homology with human 15-LO, we cloned a 0.76 kbp fragment of a rat LO cDNA from leukocytes stimulated by interleukin-4 (IL-4). The deduced amino acid sequence shows 71.0% and 60.1% homology to human 15-LO and 12-LO, respectively, and 100% homology to a recently cloned "leukocyte type" rat 12-lipoxygenase enzyme, which possesses significant 15-lipoxygenase activity (heretofore referred to as "12/15-LO"). A deletion mutant was utilized to generate internal standard cRNA in quantitative PCR assays. Glomerular 12/15-LO mRNA increased significantly over controls 24 and 48 hours after NTS injection, then decreased at 72 hours. RNA from NTS glomeruli contained higher levels of 12/15-LO mRNA than that from unstimulated peripheral leukocytes, suggesting that 12/15-LO transcription is up-regulated locally in native and/or infiltrating glomerular cells. Glomerular IL-4 mRNA increased markedly 16 hours post-NTS, and was then reduced, suggesting a potential role for T cell-derived IL-4 in directing the expression of 12/15-LO during glomerulonephritis. This represents the first demonstration of tandem regulated in vivo gene expression for a lymphokine (IL-4) and a lipoxygenase, both of which promote counter-inflammatory influences in immune complex-mediated injury.

Arachidonate lipoxygenase metabolite formation in gestational tissues

Products of arachidonic acid metabolism via the lipoxygenase pathways have a potential role in the onset of human labor. We have determined whether activation of protein kinase C can modulate the rate of biosynthesis within the uterus of five important arachidonate lipoxygenase metabolites, i.e., leukotriene B4 (LTB4), LTC4, 5-hydroxyeicosatetraenoic acid (5-HETE), 12-HETE and 15-HETE. Amnion, chorion laeve and decidual cells were isolated, grown to confluence and incubated with phorbol 12-myristate 13-acetate (PMA). PMA caused concentration-related stimulation of 5-HETE, 12-HETE and 15-HETE (but not LTB4) production; there were some stimulatory actions on decidual and amnion (but not chorion) LTC4 production. We postulate that activation of protein kinase C can result in enhanced production of arachidonate lipoxygenase metabolites that may have actions on the parturient process.

Regio- and stereochemistry of the dioxygenation reaction catalyzed by (S)-type lipoxygenases or by the cyclooxygenase activity of prostaglandin H synthases

Investigations on the regio- and stereochemistry of the reactions of mammalian lipoxygenases and of prostaglandin H synthases are reviewed. The results and concepts are summarized as two reaction box models. The structures of all known (S)-type lipoxygenase products of long-chain fatty acids carrying an all-cis-1,4-diene structural element including mono-, di-, and tri-hydroxyl products can be accommodated by this model. The model also provides an explanation for leukotriene formation by mammalian lipoxygenases and for the substrate specificity of lipoxygenases towards esterified fatty acids. The reaction box model for the first dioxygenation step of the cyclooxygenase activity of prostaglandin H synthase is stereochemically different from the (S)-type lipoxygenase box model.

Free radical generation coupled with arachidonate lipoxygenase reaction relates to reoxygenation induced myocardial cell injury

OBJECTIVE

The role of arachidonate lipoxygenase activity in reoxygenation induced cell injury in adult canine cardiac myocytes was investigated.

METHODS

The production of hydroxyeicosatetraenoic acids (HETEs), which are lipoxygenase metabolites, was measured with high pressure liquid chromatography in canine cardiac myocytes cultured under hypoxic conditions and then reoxygenated. Free radical generation was evaluated by electron paramagnetic resonance spectroscopy with a spin trapper, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and luminol enhanced chemiluminescence emission. Cell injury was estimated in terms of morphological changes and release of intracellular enzymes. Morphological damage to myocytes was quantified in terms of the percentage of hypercontracted "round" cells. The effects of nordihydroguaiaretic acid, AA-861, mepacrine, indomethacin, aspirin, alpha tocopherol, and 2-0-octadecylascorbic acid (CV-3611) on lipoxygenase metabolism, free radical generation and cell injury were also assessed.

RESULTS

Cardiac myocytes produced 5-HETE and 12-HETE at less than 0.1 ng.mg-1 protein under normoxic conditions. Production of HETE was greatly increased at five hours of reoxygenation after 45 minutes of hypoxia [5-HETE = 12.0(SEM 0.5), 12-HETE = 23.6(1.1) ng.mg-1 protein]. Both DMPO-OH adduct generation and chemiluminescence emission were considerably increased after one to three hours of reoxygenation, although they increased only slightly after 45 minutes of hypoxia. After five hours of reoxygenation, long rod cells gradually became deformed; 92.0% of the cells were converted to hypercontracted "round" cells. Cell injury and HETE production were significantly suppressed by nordihydroguaiaretic acid (10 microM), AA-861 (2 microM), and mepacrine (10 microM). Indomethacin (10 microM) and aspirin (50 microM) enhanced cell injury and HETE production. alpha Tocopherol and CV-3611 greatly suppressed cell injury and free radical generation, but not HETE production.

CONCLUSION

The arachidonate lipoxygenase metabolic pathway may have an important role in reoxygenation induced myocardial cell injury in adult cardiac myocytes, possibly because of the generation of free radicals.

Energy-dependent export of leukotriene B4 in Xenopus oocytes

The export of leukotriene (LT) B4 was studied using the Xenopus oocyte system. The oocytes were microinjected intracellularly with [3H]LTB4, and the export/injection ratios were determined. The ratios decreased with increasing doses of LTB4, converging upon 4%. The export was temperature-dependent and decreased with ATP depletion, thereby suggesting that the export is energy-dependent and carrier-mediated. The LTB4 export was inhibited by 6-trans-LTB4 and its 12-epi isomer, but much less by LTD4. Neither verapamil nor quinidine significantly inhibited LTB4 export. These results suggest that oocytes have an export system specific for LTB4 and its isomers, and that the multidrug-resistant transporter is not involved in this system. By contrast, much of platelet-activating factor, another lipid mediator, was retained in oocytes at different temperatures after injection. It was thus shown that the two potent lipid mediators are exported from cells in a different manner and that oocytes are a good model for analyzing export systems for bioactive mediators.

Mechanism for endothelial cell injury induced by 15-hydroperoxyeicosatetraenoic acid, an arachidonate lipoxygenase product

The mechanisms for endothelial cell injury induced by the lipid hydroperoxide 15-hydroperoxyeicosatetraenoic acid (15-HPETE), an arachidonate lipoxygenase product, were explored in cultured bovine endothelial cells. In serum-free medium, there was significant incorporation of [3H]-15-HPETE into the phospholipids of endothelial monolayers, and 15-HPETE induced severe endothelial cell injury, which was determined by the 51Cr-release assay. In contrast, in serum containing medium, there was little incorporation of [3H]-15-HPETE into the cells, and no cellular injury occurred. In the serum free condition, [3H]-15-HPETE was mainly incorporated into the phospholipids. The incorporated 15-HPETE produced lipid peroxidation, which was determined by the accumulation of malondialdehyde in the cells. The 15-HPETE-induced lipid peroxidation was suppressed by radical scavengers (MK-447, MCI-186), anti-oxidants (alpha-tocopherol, butylated hydroxytoluene) and iron chelators (desferrioxamine,2,2'-bipyridine). Furthermore, these agents also suppressed the 15-HPETE-induced cytotoxicity. These results indicate that 15-HPETE-induced endothelial cell injury depends on iron-mediated lipid peroxidation.

Mutagenesis of some conserved residues in human 5-lipoxygenase: effects on enzyme activity

Recombinant human 5-lipoxygenase (arachidonate:oxygen 5-oxidoreductase, EC 1.13.11.34) was expressed in Escherichia coli. In incubations of E. coli supernatants with arachidonic acid, 5-hydroxy-7,9,11,14-eicosatetraenoic acid and leukotriene A4 were formed, while incubation with 8,11,14-eicosatrienoic acid gave 8-hydroxy-9,11,14-eicosatrienoic acid. Six conserved histidine residues in 5-lipoxygenase were subjected to site-directed mutagenesis. Exchanges of His-367, -372, or -551 gave mutants for which no enzyme activities were detectable. On the other hand, exchanges of His-362, -390, or -399 gave mutants that were enzymatically active, but less so than the nonmutated control. For two of these (exchanges of His-390 or -399), the activities of the mutants were dependent on the expression temperature. Thus, the histidines in the first group (His-367, -372, -551) were crucial for 5-lipoxygenase activity, possibly because of a function of these residues as metal ligands. Mutagenesis aimed at two other conserved elements in 5-lipoxygenase, Gln-558 and the C terminus, gave mutated proteins with only a small residual activity (substitution of Gln-558), or with no detectable activity (deletion of six C-terminal amino acids), indicating that these regions are important for the function of 5-lipoxygenase.

Type II alveolar epithelial eicosanoid metabolism: predominance of cyclooxygenase pathways and transcellular lipoxygenase metabolism in co-culture with neutrophils

Arachidonic acid (AA) metabolism was studied in freshly isolated type II alveolar epithelial cells of rabbits. Substantial basal secretion of prostanoids with predominance of prostaglandin (PG) I2 was noted. Challenge with the calcium ionophore A23187 resulted in a time- and dose-dependent increase in the generation of all AA cyclooxygenase products to severalfold values following the rank order of 12-heptadecatrienoic acid (12-HHT) greater than PGI2 greater than PGE2 greater than or equal to thromboxane A2 greater than PGF2 alpha approximately PGD2. Even larger augmentation of prostanoid generation was evoked by challenge with free exogenous AA. Generation of the different AA cyclooxygenase products was inhibited by acetylsalicylic acid with IC50 in the range between 250 and 500 microM. In addition to the prostanoid release, ionophore-challenged type II pneumocytes liberated substantial amounts of AA lipoxygenase products with leukotriene (LT) B4 greater than 15-hydroxyeicosatetraenoic acid (HETE) greater than 12-HETE greater than 5-HETE. Generation of LTs and HETEs was markedly increased upon simultaneous disposal of free exogenous AA. No omega-oxidation of LTB4 was noted, and no evidence for secretion of intact LTA4 was obtained. The epithelial cells displayed avid uptake of exogenously offered LTA4 with subsequent enzymatic conversion to LTB4. Co-stimulation of pneumocytes with neutrophils (PMN) resulted in an amplification of LTB4 generation, paralleled by a decrease in nonenzymatic decay products of PMN-derived LTA4; both phenomena were dose dependent on the pneumocyte-PMN ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigation of the allene oxide pathway in the coral Plexaura homomalla: formation of novel ketols and isomers of prostaglandin A2 from 15-hydroxyeicosatetraenoic acid

Prostaglandin A2 is a major constituent of the gorgonian Plexaura homomalla, and there is evidence that its biosynthesis involves a noncyclooxygenase pathway. The coral contains an 8(R)-lipoxygenase and an allene oxide synthase; from arachidonic acid, the sequential action of these enzymes gives an allene epoxide, the cyclization of which forms an analogue of prostaglandin A2 (PGA2) with no 15-hydroxyl group. In this study we examined the metabolic fate of 15-hydroxyeicosatetraenoic acid (15-HETE), which via analogous reactions could lead to PGA2. The 8(R)-lipoxygenase metabolized preferentially the 15(R) enantiomer of 15-HETE, and this reaction was stimulated fivefold by including 1 M NaCl in the incubation. Further enzymic steps were detected by comparing the metabolic profiles of the 8(R)-hydroperoxy-15(R)-hydroxy intermediate with that of its 8(S),15(S) enantiomer. Two main products were formed exclusively from the 8(R),15(R) enantiomer: an allene epoxide and the comparatively stable epoxide, 8,9-epoxy-10,15-dihydroxyeicosa-5,11,14-trienoic acid. Formation of the allene oxide was inferred from detection of its hydrolysis and cyclization products. It cyclized to give two isomers of PGA2 which have a "cis" arrangement of the side chains. The main hydrolysis product (8,15-dihydroxy-9-ketoeicosa-5,11,13-trienoic acid) was unstable and prone to oxygenation, giving 8,14,15-trihydroxy-9-ketoeicosa-5,10,12-trienoic acids after reduction of the 14-hydroperoxide. We conclude that metabolism of a 15-hydroxy eicosanoid is a potential route to the A series prostaglandins, although the low yield and lack of stereochemical control suggest that this is not the natural pathway of biosynthesis in P. homomalla. Unexpectedly, the major end products of the pathway are trihydroxy ketols and the single diastereomer of a stable epoxyalcohol.

Characterization of an 8-lipoxygenase activity induced by the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate in mouse skin in vivo

An enzymatic activity has been found in cytosolic preparations from mouse epidermis which catalyzes the formation of 8-hydroperoxyeicosatetraenoic acid/8-hydroxyeicosatetraenoic acid (8-HPETE/8-HETE) from arachidonate. In contrast to 12-lipoxygenase this enzyme activity was not detectable in normal (untreated) mouse skin but only after in vivo treatment with the phorbol ester tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate). The induction showed a maximum at 24 h after TPA treatment strictly depended on the age of the mice and the TPA dose and was prevented by cycloheximide. The primary product formed from arachidonic acid was 8-HPETE, and the enzyme seems not to possess a significant peroxidase activity. This result as well as studies with specific inhibitors and its cytosolic localization indicates this enzyme to be a member of the lipoxygenase family. Most of the 8-lipoxygenase activity is located in cells of the suprabasal compartment of the epidermis. In spite of being a cytosolic enzyme 8-lipoxygenase appeared to be lipophilic to some extent and was activated by lecithin. The enzyme did not require calcium ions or ATP and showed a pH optimum at 7.5-8.0. 8-HPETE/8-HETE levels in mouse epidermis in vivo were determined by gas chromatography-mass spectrometry and found to be strongly increased after phorbol ester treatment, in agreement with the induction of 8-lipoxygenase observed.

Prostacyclin does not play any cytoprotective role in endothelial cell injury induced by 15-hydroperoxy-eicosatetraenoic acid, an arachidonate lipoxygenase product

Among various arachidonic acid metabolites examined, only 15(S)-hydroxperoxy-5,8,11,13-eicosatetraenoic acid (15-HPETE), a lipoxygenase product, caused a time- and dose-dependent injury to bovine endothelial cells in culture. There also occurred a significant inhibition of endothelial prostacyclin (PGI2) production due to 15-HPETE. But there were obvious dissociations in time course and dose dependence between 15-HPETE-induced cellular injury and 15-HPETE-induced inhibition of PGI2 synthesis. In addition, the cytotoxicity of 15-HPETE was not aggravated even when the endothelial monolayers were pretreated with several inhibitors of PGI2 synthesis. Also, some stable analogues of PGI2 had no protective effect on the injury. These results suggest that the reduced production of PGI2 caused by 15-HPETE is not directly associated with the onset of cellular injury, and that PGI2 does not play any cytoprotective role in endothelial cell injury induced by at least such lipid peroxides as 15-HPETE.