12-HETE Formation Research Articles

15-lipoxygenase-2 expression in benign and neoplastic lung: an immunohistochemical study and correlation with tumor grade and proliferation

15-Lipoxygenase-2 (15-LOX-2) is an arachidonic acid-metabolizing enzyme expressed in prostate, lung, skin, esophagus, and cornea. In the benign prostate, it is expressed in differentiated secretory epithelial cells, where its enzymatic product 15-HETE may regulate transcription by activating the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARγ). 15-LOX-2 and 15-HETE formation are reduced in prostate carcinoma. The distribution of 15-LOX-2 in the normal lung and its expression in lung carcinomas has not been reported and was investigated in the current study by using immunohistochemistry and tissue microarrays (TMAs). In benign lung, 15-LOX-2 immunostaining was noted exclusively in type II pneumocytes, which are known to express PPARγ. Of 160 lung carcinomas, 15-LOX-2 was expressed in non-small cell carcinomas (NSCLC), including 33 of 69 (48%) adenocarcinomas, with 10 of 16 (63%) bronchioloalveolar carcinomas immunopositive. Fourteen of 55 (25%) squamous cell carcinomas and 2 of 14 (14%) large cell carcinomas showed weak immunostaining. All 19 neuroendocrine tumors were negative. Better differentiated NSCLCs showed greater 15-LOX-2 expression, with a significant inverse correlation between 15-LOX-2 immunostaining and tumor grade ( P < 0.03). A significant inverse correlation was also noted between 15-LOX-2 immunostaining and tumor cell proliferation (Ki-67 immunostaining; P < 0.0001). These findings suggest a possible role of 15-LOX-2 in regulating secretory differentiation and proliferation in benign lung and NSCLCs, particularly adenocarcinomas.

Excess nitric oxide decreases cytochromeP-450 2J4 content andP-450-dependent arachidonic acid metabolism in lungs of rats with acute pneumonia

Recently, we demonstrated that pulmonary CYP2J4 content, a prominent source of EETs and HETEs formation in rat lungs, is reduced in pneumonia. Therefore, the purpose of this study was to determine the role of iNOS-derived NO in reduced pulmonary CYP2J4 protein content and decreased CYP metabolites in pneumonia. Rats were randomized to control, control plus 1400W (iNOS inhibitor), pneumonia, and pneumonia plus 1400W groups. Pseudomonas organisms were injected into lungs of pneumonia rats. At 40 h after surgery, rats were treated with either saline or 1400W for 4 h before death. Venous plasma samples were obtained for measuring nitrites/nitrates (NOx). There was no significant effect of 1400W on blood pressure measured in control or pneumonia rats, whereas 1400W reduced the elevated plasma NOx levels in pneumonia rats by half. CYP primary metabolites of AA formed at significantly lower rates in pulmonary microsomes from pneumonia rats compared with control rats. Treatment of pneumonia rats with 1400W resulted in a significant increase in the rate of formation of pulmonary EETs and omega-terminal HETEs compared with untreated pneumonia rats. The reduction in CYP2J4 protein content in pneumonia lung microsomes was also partially prevented by 1400W. Therefore, excess NO from iNOS decreases the pulmonary production of EETs and omega-HETEs in acute pneumonia. Inhibition of iNOS restores CYP2J4 protein content and CYP activity in acute pneumonia, indicating an important NO-CYP interaction in pulmonary responses to infection. We speculate CYP2J4 and its AA metabolites are involved in the modulation of pulmonary function in health and disease.

Effects of endocrine disruptors on arachidonic acid metabolism in rabbit platelets

To explore the possible actions of endocrine disruptors on the autacoid synthesis in the body, we investigated the effects of nonylphenol (NP), bisphenol A (BPA), di- n-butyl phthalate (DBP), benzyl- n-butyl phthalate (BBP), and di-2-ethylhexyl phthalate (DEHP) on the formation of 12-lipoxygenase metabolite, 12-HETE, and cyclooxygenase metabolites, TXB 2 and 12-HHT, from exogenous arachidonic acid (AA) in rabbit platelets. NP (10–50 μM) showed strong inhibition on the formation of cyclooxygenase metabolites (TXB 2, 34–95% inhibition; 12-HHT, 13–78% inhibition) and weaker inhibition on the formation of 12-HETE (0–49% inhibition). BPA, DBP, BBP, DEHP, and 17β-estradiol (endogenous estrogen) failed to show any effect on the formation of cyclooxygenase and 12-lipoxygenase metabolites at concentrations up to 100 μM. These results suggest that NP inhibits AA metabolism in platelets and that its effects on the cyclooxygenase pathway predominate over those exerted via the 12-lipoxygenase pathway.

Inhibition of proliferation of PC3 cells by the branched-chain fatty acid, 12-methyltetradecanoic acid, is associated with inhibition of 5-lipoxygenase.

Branched-chain fatty acids or fatty alcohols have been reported to possess anti-tumor activity in various tumor models. Here we study 12-methyltetradecanoic acid (12-MTA), a branched-chain fatty acid, isolated from a sea cucumber extract, on the growth of prostate cancer cells and investigate the underlying mechanisms of its effect. 12-MTA was evaluated by MTT assay for its ability to inhibit cell proliferation in various cancer types. The ability of 12-MTA to induce apoptosis of PC3 cells was examined by morphologic changes, propidium iodide (PI) staining, and caspase-3 activation. Furthermore, alteration of eicosanoid metabolism by 12-MTA was examined in PC3 and RBL-1 cells and in purified lipoxygenase (LOX) and cyclooxygenase (COX) enzymes. 12-MTA inhibited proliferation of various cell lines, with IC50s ranging from 17.99 to 35.44 microg/ml. PI staining clearly showed that 12-MTA caused PC3 cell death through induction of apoptosis. At 50 microg/ml, 12-MTA increased caspase-3 activity four to seven-fold compared with that in control cells. Examination of cellular arachidonate metabolism showed that at 25 microg/ml, 12-MTA reduced the level of 5-hydroxyeicosatetraenoic acid (5-HETE) by 45%. Furthermore, exogenous 5-HETE protects PC3 cells from 12-MTA induced cell death. 12-MTA inhibited proliferation of cancer cells via apoptosis, in which caspase-3 may play a role. At relevant concentrations, 12-MTA can selectively inhibit the formation of 5-HETE, a metabolite of 5-lipoxygenase. This agent may be a novel adjunctive therapy for selected malignancies including prostate cancer.

Chronic hypoxia activates lung 15-lipoxygenase, which catalyzes production of 15-HETE and enhances constriction in neonatal rabbit pulmonary arteries.

Hypoxia causes localized pulmonary arterial (PA) constriction to divert blood flow to optimally ventilated regions of the lung. The biochemical mechanisms for this have remained elusive, especially during prolonged exposures to reduced PO2. We have evidence that subacute hypoxia activates 15-lipoxygenase (15-LO) in small PAs of neonatal rabbits maintained for 9 days in hypoxic environments (FiO2=0.12) compared with siblings raised under normoxia. PA microsomal products of 15-LO, 15-hydroxyeicosatetraenoic acid (HETE), 11,14,15-trihydroxyeicosatrienoic acid (THETA), and 11,12,15-THETA were identified by gas chromatography/mass spectrometry. Increased amounts of these products are synthesized in vivo and in vitro by the lungs of animal raised in hypoxic versus normoxic environments. 15-HETE formation is attenuated by lipoxygenase, but not cytochrome P450 or cyclooxygenase inhibitors. Activation of 15-LO is associated with translocation of the enzyme from the cytosol to membrane as seen by Western immunoblotting. Immunohistochemical analysis demonstrates that 15-LO expression is clearly localized in vascular cells in lungs from normoxic and hypoxic kits. 15-HETE causes concentration-dependent constriction of PA rings from animals exposed to hypoxic but not normoxic environments. In addition, lipoxygenase inhibitors reduce phenylephrine-induced constriction of PA rings. Therefore, subacute hypoxia increases expression of and activates 15-LO, and enhances sensitivity of pulmonary arteries to its product, 15-HETE. Because 15-HETE is a constrictor in this vascular bed, it may play an important role in hypoxia-induced pulmonary vasoconstriction in rabbit kits. Although a clear causal relationship remains to be demonstrated, these data suggest a previously unrecognized role for 15-LO in hypoxic vasoconstriction in neonatal mammals.

Platelets Abrogate Leukotriene B4 Generation by Human Blood Neutrophils Stimulated with Monosodium Urate Monohydrate or f-Met-Leu-Phe In Vitro

Neutrophils are physiologically associated with platelets in whole blood. Inflammatory reactions can be modulated by the presence of platelets. To investigate the influence of platelets on neutrophil activity, we studied the 5-lipoxygenase (5-LOX) metabolic pathway in normal human blood neutrophils stimulated with f-Met-Leu-Phe (fMLP) or monosodium urate monohydrate (MSUM) in the presence of autologous platelets. Platelets inhibited by more than 90% the synthesis of leukotriene B4 and 5-HETE in neutrophils activated with fMLP or MSUM. The addition of exogenous arachidonic acid did not reverse the inhibitory effect of platelets on 5-LOX–generated metabolites in fMLP- or MSUM-activated neutrophils. Preincubation of neutrophils with adenosine deaminase reversed the inhibitory effect of platelets in fMLP-treated neutrophils, indicating that adenosine was responsible for the platelet inhibition of leukotriene B4 and 5-HETE formation. In contrast, adenosine deaminase had no influence on the inhibitory effects of platelets in MSUM-stimulated cells. These results suggest that platelets can inhibit the synthesis of 5-LOX products (a) by acting mainly downstream to phospholipase A2 in cells stimulated by fMLP or MSUM, (b) through adenosine when neutrophils are activated with fMLP, and (c) by an adenosine-independent mechanism in MSUM-activated neutrophils by an as-yet-unidentified mediator.

Platelets may inhibit leucotriene biosynthesis by human neutrophils at the integrin level.

Polymorphonuclear leucocytes and blood platelets co-operate in several pathophysiological processes, and arachidonic acid (AA) metabolites produced in response to the activation of these cells are potent mediators of their functions. We studied the role of platelets in the formation of 5-lipoxygenase products from AA by autologous neutrophils, especially the chemotactic agent leucotriene (LT) B4. The formation of all products, namely 5-hydroxy-eicosatetraenoic acid (5-HETE), LTB4 and the other LTA4-derived metabolites, in response to the calcium ionophore A23187 was evaluated by high-performance liquid chromatography. All the 5-lipoxygenase products were significantly diminished by physiological concentrations of platelets. This inhibitory effect was lost when platelets were previously degranulated by thrombin in non-aggregating conditions. Peptides containing the Arg-Gly-Asp-Ser or His-His-Leu-Gly-Gly-Ala-Lys-Gln-Ala-Gly-Asp-Val sequence, which prevent the adhesion of platelets to neutrophils via the fibrinogen released from platelet granules and the integrin glycoprotein IIb/IIIa, markedly decreased the inhibitory effect of non-degranulated platelets. The production of transcellular metabolites of AA such as LTC4, the dual 5- and 12-lipoxygenase product 5,12-diHETE and lipoxins could not account for the decreased formation of 5-HETE and LTA4-derived metabolites. It is concluded that platelets may inhibit the neutrophil 5-lipoxygenase activity at the integrin level and in turn may play a role in slowing down the production of LTB4 in the course of inflammation.

Increased neutrophil mediator release in patients with pulmonary hypertension--suppression by inhaled iloprost.

Polymorphonuclear neutrophils (PMN) have been implicated in various vascular inflammatory processes. We isolated PMN from venous blood samples of 10 patients with severe primary pulmonary arterial hypertension (PPH), 7 patients with pulmonary hypertension secondary to chronic thromboembolism (CTEPH), and 12 healthy controls. When stimulated with the calcium-ionophore A23187, platelet activating factor (PAF) or the microbial agent n-formyl-Methionyl-Leucyl-Phenylalanine (fMLP), significantly increased release of elastase and superoxide anion was noted in both groups with pulmonary hypertension. Moreover, the neutrophils of CTEPH patients responded with an enhanced liberation of leukotriene (LT) B(4) and 5-hydroxyeicosatetraenoic acid (5-HETE). Inhalation of aerosolized iloprost (5 microg) caused a rapid decline in pulmonary vascular resistance, in both PPH and CTEPH. This hemodynamic response was paralleled by a significant suppression of ionophore- and ligand-induced elastase and superoxide release, as well as LTB(4) and 5-HETE formation. The neutrophil inhibitory effect of the inhalation maneuver was fully reproduced by in vitro incubation of neutrophils with 1-10 pg/ml iloprost for 2 hours. This is the first study to demonstrate that circulating neutrophils from patients with PPH and CTEPH possess an enhanced readiness to respond with inflammatory mediator generation to different stimulatory agents ex-vivo, and that PMN respiratory burst, elastase secretion and leukotriene generation are promptly reduced by an iloprost inhalation maneuver. Neutrophils might participate in the inflammatory processes in pulmonary arterial hypertension.

Analysis of Monohydroxyeicosatetraenoic Acids and F2 -isoprostanes as Markers of Lipid Peroxidation in Rat Brain Mitochondria

We have introduced two specific techniques for the quantitative measurement of monohydroxyeicosatetraenoic acids (HETEs) and F2-isoprostanes by gas chromatography-mass spectrometry/negative ion chemical ionization (GC-MS/NICI) to study lipid peroxidation in isolated rat brain mitochondria by iron/ascorbate. The analysis of HETEs involved hydrogenation, solid phase extraction on a C18-cartridge, formation of pentafluorobenzyl bromide and trimethylsilyl ether derivatives. In the case of F2-isoprostanes, the analytical procedure was similar to that of HETEs except that the hydrogenation step was omitted. We found that HETE content (sum of 5-, 8-12-, and 15-isomers) in freshly prepared rat brain mitochondria was 220 - 40pmol/mg protein. The corresponding content for the F2-isoprostane, 8-iso-PGF 2α, was 0.21 - 0.10pmol/mg protein. HETEs and 8-iso-PGF 2α were predominantly present in the esterified form. The content of both HETEs and 8-iso-PGF 2α were increased in presence of iron/ascorbate as oxidation system. After 30min incubation with Fe2+ + ascorbate, the content of HETE isomers was increased about 6-fold compared with base-line levels whereas that for 8-iso-PGF2α was elevated 100-fold. Formation of HETEs and F2-isoprostanes corresponded to the consumption of arachidonic acid (AA) and α-tocopherol, respectively. There were almost no changes in the content of free (non-esterified) HETEs and 8-iso-PGF2α during the course of iron/ascorbate induced oxidation of the brain mitochondria. Our data provide the first direct evidence for the presence of HETEs and F2-isoprostanes in freshly isolated rat brain mitochondria and that esterified HETEs and 8-iso-PGF2α are predominantly generated during iron/ascorbate induced lipid peroxidation. Sensitive quantification of these products of non-enzymatic lipid peroxidation as indicators of oxidant injury opens new areas of investigation regarding the role of free radicals in the pathogenesis of human diseases. In addition, HETEs and F2-isoprostanes may be important mediators for mitochondrial functions.

The lipoxygenase pathways are involved in LH-stimulated progesterone production in bovine corpus luteum

We have examined the effects of endogenous lipoxygenase products on basal progesterone (P4) production by cultured bovine mid-luteal cells. The involvement of lipoxygenase products in the stimulatory effect of LH on luteal cAMP accumulation and P4 production was also examined. Bovine luteal cells from mid-cycle corpora lutea (CL) were exposed for 16 h to a lipoxygenase inhibitor (nordihydroguaiaretic acid: NDGA; 0.33–33 μM). For the last 4 h of incubation, the cells were exposed to LH and/or three different lipoxygenase products, 5-, 12- and 15-hydroxyeicosatetraenoic acid (HETE). NDGA inhibited P4 production by the cells in a dose-dependent manner ( P < 0.05). NDGA-reduced P4 production was reversed by the addition of 12-HETE, but not 5- or 15-HETE, whereas 5-, 12- and 15-HETE alone showed no significant effect on P4 production in the intact cells. Furthermore, NDGA (33 μM) blocked the stimulatory action of LH on P4 production ( P < 0.05), without changing cAMP accumulation ( P > 0.1). When the cells were exposed to 5-, 12- or 15-HETE with LH and NDGA, only 15-HETE maintained the stimulatory effect of LH on P4 production in the cells ( P < 0.05). These results suggest that endogenous lipoxygenase products play important roles in P4 production by bovine CL, i.e. basal P4 production is supported by 12-HETE, and LH-stimulated P4 production is partially mediated via the activation of lipoxygenase and subsequent 15-HETE formation downstream of the LH-activated cAMP-PKA-phosphorylation pathway.

Downregulation of Ubiquitin-Dependent Proteolysis by Eicosapentaenoic Acid in Acute Starvation

A number of acute wasting conditions are associated with an upregulation of the ubiquitin-proteasome system in skeletal muscle. Eicosapentaenoic acid (EPA) is effective in attenuating the increased protein catabolism in muscle in cancer cachexia, possibly due to inhibition of 15-hydroxyeicosatetraenoic acid (15-HETE) formation. To determine if a similar pathway is involved in other catabolic conditions, the effect of EPA on muscle protein degradation and activation of the ubiquitin-proteasome pathway has been determined during acute fasting in mice. When compared with a vehicle control group (olive oil) there was a significant decrease in proteolysis of the soleus muscles of mice treated with EPA after starvation for 24 h, together with an attenuation of the proteasome ‘chymotryptic-like’ enzyme activity and the induction of the expression of the 20S proteasome α-subunits, the 19S regulator and p42, an ATPase subunit of the 19S regulator in gastrocnemius muscle, and the ubiquitin-conjugating enzyme E214k. The effect was not shown with the related (n-3) fatty acid docosahexaenoic acid (DHA) or with linoleic acid. However, 2,3,5-trimethyl-6-(3-pyridylmethyl)1,4-benzoquinone (CV-6504), an inhibitor of 5-, 12- and 15-lipoxygenases also attenuated muscle protein catabolism, proteasome ‘chymotryptic-like’ enzyme activity and expression of proteasome 20S α-subunits in soleus muscles from acute fasted mice. These results suggest that protein catabolism in starvation and cancer cachexia is mediated through a common pathway, which is inhibited by EPA and is likely to involve a lipoxygenase metabolite as a signal transducer.

Novel Platelet Antiaggregating Substances

We describe herein a novel action of four stable analogs of the hepoxilins. These analogs inhibit to different degrees, the aggregation of washed human platelets evoked by collagen. One of the analogs, PBT-3, is particularly effective with an IC50 = 8 × 10−7 M. The other analogs are about 5-fold less active, but all analogs are about 500-fold more active than the native hepoxilins. The PBT analogs inhibit the collagen-enhanced formation of thromboxane A2 and HHT but do not affect the formation of 12-HETE or the release of arachidonic acid except at doses higher than those needed to block platelet aggregation. These results demonstrate that these novel compounds may have potential for development into drugs in the treatment of thromboxane-mediated cardiovascular disease.

Alterations in Lipoxygenase and Cyclooxygenase-2 Catalytic Activity and mRNA Expression in Prostate Carcinoma

Recent studies in prostate tissues and especially cell lines have suggested roles for arachidonic acid (AA) metabolizing enzymes in prostate adenocarcinoma (Pca) development or progression. The goal of this study was to more fully characterize lipoxygenase (LOX) and cyclooxygenase-2 (COX-2) gene expression and AA metabolism in benign and malignant prostate using snap-frozen tissues obtained intraoperatively and mRNA analyses and enzyme assays. Formation of 15-hydroxyeicosatetraenoic acid (15-HETE) was detected in 23/29 benign samples and 15-LOX-2 mRNA was detected in 21/25 benign samples. In pairs of pure benign and Pca from the same patients, 15-HETE production and 15-LOX-2 mRNA were reduced in Pca versus benign in 9/14 (P=.04) and 14/17 (P=.002), respectively. Under the same conditions, neither 5HETE nor 12-HETE formation was detectable in 29 benign and 24 tumor samples; with a more sensitive assay, traces were detected in some samples, but there was no clear association with tumor tissue. COX-2 mRNA was detected by nuclease protection assay in 7/16 benign samples and 5/16 tumors. In benign and tumor pairs from 10 patients, COX-2 was higher in tumor versus benign in only 2, with similar results by in situ hybridization. Paraffin immunoperoxidase for COX2 was performed in whole mount sections from 87 additional radical prostatectomy specimens, with strong expression in ejaculatory duct as a positive control and corroboration with in situ hybridization. No immunostaining was detected in benign prostate or tumor in 45% of cases. Greater immunostaining in tumor versus benign was present in only 17% of cases, and correlated with high tumor grade (Gleason score 8 and 9 vs. 5 to 7). In conclusion, reduced 15-LOX-2 expression and 15-HETE formation is the most characteristic alteration of AA metabolism in Pca. Increased 12-HETE and 5-HETE formation in Pca were not discernible. Increased COX-2 expression is not a typical abnormality in Pca in general, but occurs in high-grade tumors.

Evidence for the presence of phospholipid hydroperoxide glutathione peroxidase in human platelets: implications for its involvement in the regulatory network of the 12-lipoxygenase pathway of arachidonic acid metabolism

The 12-lipoxygenase pathway of arachidonic acid metabolism in platelets and other cells is bifurcated into a reduction route yielding 12-hydroxyeicosatetraenoic acid (12-HETE) and an isomerization route forming hepoxilins. Here we show for the first time the presence of phospholipid hydroperoxide glutathione peroxidase (PHGPx) protein and its activity in platelets. The ratio of the activity of PHGPx to that of cytosolic glutathione peroxidase (GPx-1) was consistently found to be approx. 1:60 in platelets and UT7 megakaryoblasts. Moreover, short-lived PHGPx mRNA was detected in megakaryocytes but not in platelets. Carboxymethylation of selenium-containing glutathione peroxidases by iodoacetate, which results in the inactivation of PHGPx and GPx-1 without inhibition of 12-lipoxygenase, markedly altered the pattern of arachidonic acid metabolism in human platelets. Whereas the formation of 12-HETE was inhibited by 80%, a concomitant accumulation of 12-hydroperoxyeicosatetraenoic acid (12-HpETE) by two orders of magnitude as well as the formation of hepoxilins A(3) and B(3) were observed. The formation of hepoxilins also occurred when 12-HpETE was added to untreated platelets. In selenium-deficient UT7 cells, which were devoid of GPx-1 but not of PHGPx, the reduction of 12-HPETE was retained, albeit with a lower rate than in control cells containing GPx-1. We therefore believe that both GPx-1 and PHGPx are involved in the regulatory network of the 12-lipoxygenase pathway in platelets and other mammalian cells. Moreover, the diminution of hydroperoxide tone in platelets incubated with arachidonic acid leads primarily to the formation of 12-HETE, whereas the increase in hydroperoxide tone (a situation found under oxidative stress or selenium deficiency or on incubation with 12-HPETE) partly diverts the 12-lipoxygenase pathway from the reduction route to the isomerization route, thus resulting in the formation of hepoxilins.

15-Lipoxygenation of leukotriene A 4: Studies of 12- and 15-lipoxygenase efficiency to catalyze lipoxin formation

The unstable epoxide leukotriene (LT) A 4 is a key intermediate in leukotriene biosynthesis, but may also be transformed to lipoxins via a second lipoxygenation at C-15. The capacity of various 12- and 15-lipoxygenases, including porcine leukocyte 12-lipoxygenase, a human recombinant platelet 12-lipoxygenase preparation, human platelet cytosolic fraction, rabbit reticulocyte 15-lipoxygenase, soybean 15-lipoxygenase and human eosinophil cytosolic fraction, to catalyze conversion of LTA 4 to lipoxins was investigated and standardized against the ability of the enzymes to transform arachidonic acid to 12- or 15-hydroxyeicosatetraenoic acids (HETE), respectively. The highest ratio between the capacity to produce lipoxins and HETE (LX/HETE ratio) was obtained for porcine leukocyte 12-lipoxygenase with an LX/HETE ratio of 0.3. In addition, the human platelet 100 000× g supernatant 12-lipoxygenase preparation and the human platelet recombinant 12-lipoxygenase and human eosinophil 100 000× g supernatant 15-lipoxygenase preparation possessed considerable capacity to produce lipoxins (ratio 0.07, 0.01 and 0.02 respectively). In contrast, lipoxin formation by the rabbit reticulocyte and soybean 15-lipoxygenases was much less pronounced (LX/HETE ratios <0.002). Kinetic studies of the human lipoxygenases revealed lower apparent K m for LTA 4 (9–27 μM), as compared to the other lipoxygenases tested (58–83 μM). The recombinant human 12-lipoxygenase demonstrated the lowest K m value for LTA 4 (9 μM) whereas the porcine leukocyte 12-lipoxygenase had the highest V max. The profile of products was identical, irrespective of the lipoxygenase used. Thus, LXA 4 and 6 S-LXA 4 together with the all- trans LXA 4 and LXB 4 isomers were isolated. Production of LXB 4 was not observed with any of the lipoxygenases. The lipoxygenase inhibitor cinnamyl-3,4-dihydroxy-α-cyanocinnamate was considerably more efficient to inhibit conversion of LTA 4 to lipoxins, as compared to the inhibitory effect on 12-HETE formation from arachidonic acid (IC 50 1 and 50 μM, respectively) in the human platelet cytosolic fraction.

Different Catalytically Competent Arrangements of Arachidonic Acid within the Cyclooxygenase Active Site of Prostaglandin Endoperoxide H Synthase-1 Lead to the Formation of Different Oxygenated Products

Arachidonic acid is converted to prostaglandin G(2) (PGG(2)) by the cyclooxygenase activities of prostaglandin endoperoxide H synthases (PGHSs) 1 and 2. The initial, rate-limiting step is abstraction of the 13-proS hydrogen from arachidonate which, for PGG(2) formation, is followed by insertion of O(2) at C-11, cyclization, and a second O( 2) insertion at C-15. As an accompaniment to ongoing structural studies designed to determine the orientation of arachidonate in the cyclooxygenase site, we analyzed the products formed from arachidonate by (a) solubilized, partially purified ovine (o) PGHS-1; (b) membrane-associated, recombinant oPGHS-1; and (c) a membrane-associated, recombinant active site mutant (V349L oPGHS-1) and determined kinetic values for formation of each product. Native forms of oPGHS-1 produced primarily PGG(2) but also several monohydroxy acids, which, in order of abundance, were 11R-hydroxy-5Z, 8Z,12E,14Z-eicosatetraenoic acid (11R-HETE), 15S-hydroxy-5Z,8Z,11Z, 13E-eicosatetraenoic acid (15S-HETE), and 15R-HETE. V349L oPGHS-1 formed primarily PGG(2), 15S-HETE, and 15R-HETE but only trace amounts of 11R-HETE. With native enzyme, the K(m) values for PGG(2), 11-HETE, and 15-HETE formation were each different (5.5, 12.1, and 19.4 microM, respectively); similarly, the K(m) values for PGG(2) and 15-HETE formation by V349L oPGHS-1 were different (11 and 5 microM, respectively). These results establish that arachidonate can assume at least three catalytically productive arrangements within the cyclooxygenase site of oPGHS-1 leading to PGG(2), 11R-HETE, and 15S-HETE and/or 15R-HETE, respectively. IC(50) values for inhibition of formation of the individual products by the competitive inhibitor, ibuprofen, were determined and found to be the same for a given enzyme form (i.e. 175 microM for oPGHS-1 and 15 microM for V349L oPGHS-1). These latter results are most simply rationalized by a kinetic model in which arachidonate forms various catalytically competent arrangements only after entering the cyclooxygenase active site.

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.

Sphingomyelin inhibits platelet 12-lipoxygenase activity

The effect of sphingomyelin on the formation of 12-hydroxy-5,8,10,14-eicosatetraenoic acid (12-HETE), thromboxane B 2 and 12-hydroxy-5,8,10-heptadecatrienoic acid (HHT) in washed rabbit platelets was examined. Sphingomyelin had a powerful inhibitory effect on 12-HETE formation, while it produced only a small increase in thromboxane B 2 and HHT formation. The sphingomyelin metabolite ceramide did not affect the formation of 12-HETE, thromboxane B 2 and HHT. These results suggest that sphingomyelin is a selective inhibitor of platelet 12-lipoxygenase and may have functional effects in platelets.

15-Lipoxygenase-2 (15-LOX-2) Is Expressed in Benign Prostatic Epithelium and Reduced in Prostate Adenocarcinoma

Human 15S-lipoxygenase-2 (15-LOX-2) is a recently identified lipoxygenase that has approximately 40% sequence identity to the known human 5S-, 12S-, and 15S-lipoxygenases. 15-LOX-2 has a limited tissue distribution, with mRNA detected in prostate, lung, skin, and cornea, but not in numerous other tissues, including peripheral blood leukocytes. In the current study, we have characterized the distribution of 15-LOX-2 in the human prostate by immunohistochemistry, demonstrated the ability of benign prostate tissue to form 15S-hydroxyeicosatetraenoic acid (15S-HETE) from exogenous arachidonic acid (AA), and begun characterizing possible alterations in 15-LOX-2 in prostate adenocarcinoma. Incubation of benign prostate tissue with [14C]AA resulted in formation of [14C]15-HETE, as determined by reverse- and straight-phase high-performance liquid chromatography. 15-HETE was the major AA metabolite formed. By immunohistochemistry, 15-LOX-2 is located in secretory cells of peripheral zone glands and large prostatic ducts and somewhat less uniformly in apical cells of transition and central zone glands. 15-LOX-2 was not detected in the basal cell layer, stroma, ejaculatory ducts, seminal vesicles, or transitional epithelium. Immunostaining of 18 radical prostatectomy specimens showed a loss of 15-LOX-2 in the majority of prostate adenocarcinomas; 14 of 18 cases showed loss of 15-LOX-2 in >25% of the tumor (mean, 74.9% negative for 15-LOX-2; range, 38.9% to 100%). Incubation of paired pure benign and pure malignant prostate tissue from the same radical prostatectomies showed that 15-HETE formation was markedly reduced (>90%) or undetectable in incubations of prostate adenocarcinoma. 15-LOX-2 is a novel human lipoxygenase with a limited tissue distribution that is strongly expressed in benign prostate glandular epithelium and lost to a variable degree in the majority of prostate adenocarcinomas.

Regulation of 15-lipoxygenase expression and mucus secretion by IL-4 in human bronchial epithelial cells.

Our laboratory has recently shown that mucus differentiation of cultured normal human tracheobronchial epithelial (NHTBE) cells is accompanied by the increased expression of 15-lipoxygenase (15-LO). We used differentiated NHTBE cells to investigate the regulation of 15-LO expression and mucus secretion by inflammatory cytokines. Interleukin (IL)-4 and IL-13 dramatically enhanced the expression of 15-LO, whereas tumor necrosis factor-alpha, IL-1beta, and interferon (IFN)-gamma had no effect. These cytokines did not increase the expression of cyclooxygenase-2, with the exception of a modest induction by IL-1beta. The IL-4-induced 15-LO expression was concentration dependent, and mRNA and protein expression increased within 3 and 6 h, respectively, after IL-4 treatment. In metabolism studies with intact cells, 15-hydroxyeicosatetraenoic acid (15-HETE) and 13-hydroxyoctadecadienoic acid (13-HODE) were the major metabolites formed from exogenous arachidonic acid and linoleic acid. No prostaglandins were detected. IL-4 treatment dramatically increased the formation of 13-HODE and 15-HETE compared with that in untreated NHTBE cells, and several additional 15-LO metabolites were observed. Pretreatment of NHTBE cells with IFN-gamma or dexamethasone did not inhibit the IL-4-induced expression of 15-LO except at high concentrations (100 ng/ml of IFN-gamma and 10 microM dexamethasone). IL-4 treatment inhibited mucus secretion and attenuated the expression of the mucin genes MUC5AC and MUC5B at 12-24 h after treatment. Addition of 15-HETE precursor and 13-HODE precursor to the cultures did not alter mucin secretion or mucin gene expression. On the basis of the data presented, we conclude that the increase in 15-LO expression by IL-4 and attenuation of mucus secretion may be independent biological events.