Hepoxilin A3 formation in the rat pineal gland selectively utilizes (12S)-hydroperoxyeicosatetraenoic acid (HPETE), but not (12R)-HPETE

Synthesis and Significance of Arachidonic Acid, a Substrate for Cyclooxygenases, Lipoxygenases, and Cytochrome P450 Pathways in the Tumorigenesis of Glioblastoma Multiforme, Including a Pan-Cancer Comparative Analysis

Glioblastoma multiforme (GBM) is one of the most aggressive gliomas. New and more effective therapeutic approaches are being sought based on studies of the various mechanisms of GBM tumorigenesis, including the synthesis and metabolism of arachidonic acid (ARA), an omega-6 polyunsaturated fatty acid (PUFA). PubMed, GEPIA, and the transcriptomics analysis carried out by Seifert et al. were used in writing this paper. In this paper, we discuss in detail the biosynthesis of this acid in GBM tumors, with a special focus on certain enzymes: fatty acid desaturase (FADS)1, FADS2, and elongation of long-chain fatty acids family member 5 (ELOVL5). We also discuss ARA metabolism, particularly its release from cell membrane phospholipids by phospholipase A₂ (cPLA₂, iPLA₂, and sPLA₂) and its processing by cyclooxygenases (COX-1 and COX-2), lipoxygenases (5-LOX, 12-LOX, 15-LOX-1, and 15-LOX-2), and cytochrome P450. Next, we discuss the significance of lipid mediators synthesized from ARA in GBM cancer processes, including prostaglandins (PGE₂, PGD₂, and 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂)), thromboxane A₂ (TxA₂), oxo-eicosatetraenoic acids, leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄), lipoxins, and many others. These lipid mediators can increase the proliferation of GBM cancer cells, cause angiogenesis, inhibit the anti-tumor response of the immune system, and be responsible for resistance to treatment.

Single-cell RNA sequencing of the mammalian pineal gland identifies two pinealocyte subtypes and cell type-specific daily patterns of gene expression

The vertebrate pineal gland is dedicated to the production of the hormone melatonin, which increases at night to influence circadian and seasonal rhythms. This increase is associated with dramatic changes in the pineal transcriptome. Here, single-cell analysis of the rat pineal transcriptome was approached by sequencing mRNA from ~17,000 individual pineal cells, with the goals of profiling the cells that comprise the pineal gland and examining the proposal that there are two distinct populations of pinealocytes differentiated by the expression of Asmt, which encodes the enzyme that converts N-acetylserotonin to melatonin. In addition, this analysis provides evidence of cell-specific time-of-day dependent changes in gene expression. Nine transcriptomically distinct cell types were identified: ~90% were classified as melatonin-producing α- and β-pinealocytes (1:19 ratio). Non-pinealocytes included three astrocyte subtypes, two microglia subtypes, vascular and leptomeningeal cells, and endothelial cells. α-Pinealocytes were distinguished from β-pinealocytes by ~3-fold higher levels of Asmt transcripts. In addition, α-pinealocytes have transcriptomic differences that likely enhance melatonin formation by increasing the availability of the Asmt cofactor S-adenosylmethionine, resulting from increased production of a precursor of S-adenosylmethionine, ATP. These transcriptomic differences include ~2-fold higher levels of the ATP-generating oxidative phosphorylation transcriptome and ~8-fold lower levels of the ribosome transcriptome, which is expected to reduce the consumption of ATP by protein synthesis. These findings suggest that α-pinealocytes have a specialized role in the pineal gland: efficiently O-methylating the N-acetylserotonin produced and released by β-pinealocytes, thereby improving the overall efficiency of melatonin synthesis. We have also identified transcriptomic changes that occur between night and day in seven cell types, the majority of which occur in β-pinealocytes and to a lesser degree in α-pinealocytes; many of these changes were mimicked by adrenergic stimulation with isoproterenol. The cellular heterogeneity of the pineal gland as revealed by this study provides a new framework for understanding pineal cell biology at single-cell resolution.

The hepoxilins and some analogues: a review of their biology

The hepoxilin pathway was discovered over two decades ago. Products in this pathway are derived through the 12S-lipoxygenase/hepoxilin synthase enzyme system and contain intrinsic biological activity. This activity relates to the reorganization of calcium and potassium ions within the cell, and in inflammation and insulin secretion. Although the natural hepoxilins are chemically unstable, chemical analogues (PBTs) have been synthesized with chemical and biological stability. The PBTs antagonize the natural hepoxilins. The PBTs showed bioavailability, excellent tolerance and stability in vivo. In proof of principle studies in vivo in animal models, the PBTs have shown actions as anti-inflammatory agents, anti-thrombotic agents, anti-cancer agents and anti-diabetic agents. These studies demonstrate the effectiveness of the base structure of the hepoxilin (and PBT) molecule and serve as an excellent framework for the design and preparation of second-generation compounds with improved pharmaceutical properties as therapeutics for the above-mentioned diseases.

Night/day changes in pineal expression of >600 genes: central role of adrenergic/cAMP signaling

The pineal gland plays an essential role in vertebrate chronobiology by converting time into a hormonal signal, melatonin, which is always elevated at night. Here we have analyzed the rodent pineal transcriptome using Affymetrix GeneChip(R) technology to obtain a more complete description of pineal cell biology. The effort revealed that 604 genes (1,268 probe sets) with Entrez Gene identifiers are differentially expressed greater than 2-fold between midnight and mid-day (false discovery rate <0.20). Expression is greater at night in approximately 70%. These findings were supported by the results of radiochemical in situ hybridization histology and quantitative real time-PCR studies. We also found that the regulatory mechanism controlling the night/day changes in the expression of most genes involves norepinephrine-cyclic AMP signaling. Comparison of the pineal gene expression profile with that in other tissues identified 334 genes (496 probe sets) that are expressed greater than 8-fold higher in the pineal gland relative to other tissues. Of these genes, 17% are expressed at similar levels in the retina, consistent with a common evolutionary origin of these tissues. Functional categorization of the highly expressed and/or night/day differentially expressed genes identified clusters that are markers of specialized functions, including the immune/inflammation response, melatonin synthesis, photodetection, thyroid hormone signaling, and diverse aspects of cellular signaling and cell biology. These studies produce a paradigm shift in our understanding of the 24-h dynamics of the pineal gland from one focused on melatonin synthesis to one including many cellular processes.

Structure, biochemistry and biology of hepoxilins

Hepoxilins are biologically relevant epoxy-hydroxy eicosanoids synthesized through the 12S-lipoxygenase (12S-LOX) pathway of the arachidonic acid (AA) metabolism. The pathway is bifurcated at the level of 12S-hydroperoxy-eicosatetraenoic acid (12S-HpETE), which can either be reduced to 12S-hydro-eicosatetraenoic acid (12S-HETE) or converted to hepoxilins. The present review gives an update on the biochemistry, biology and clinical aspects of hepoxilin-based drug development. The isolation, cloning and characterization of a rat leukocyte-type 12S-LOX from rat insulinoma RINm5F cells revealed a 12S-LOX possessing an intrinsic 8S/R-hydroxy-11,12-epoxyeicosa-5Z,9E,14Z-trienoic acid (HXA(3)) synthase activity. Site-directed mutagenesis studies on rat 12S-LOX showed that the HXA(3) synthase activity was impaired when the positional specificity of AA was altered. Interestingly, amino acid Leu353, and not conventional sequence determinants Met419 and Ile418, was found to be a crucial sequence determinant for AA oxygenation. The regulation of HXA(3) formation is dependent on the cellular overall peroxide tone. Cellular glutathione peroxidases (cGPxs) compete with HXA(3) synthase for 12S-HpETE as substrate either to reduce to 12S-HETE or to convert to HXA(3), respectively. Therefore, RINm5F cells, which are devoid of GPxs, are capable of converting AA or 12S-HpETE to HXA(3) under basal conditions, whereas cells overexpressing cGPx are unable to do so. HXA(3) exhibits a myriad of biological effects, most of which are associated with the stimulation of intracellular calcium or the transport of calcium across the membrane. The activation of HXA(3)-G-protein-coupled receptors explains many of the extracellular effects of HXA(3), including AA- and diacylglycerol (DAG) release in human neutrophils, insulin secretion in rat pancreatic beta-cells or islets, and synaptic actions in the brain. The availability of stable analogs of HXA(3), termed 10-hydroxy-11,12-cyclopropyl-eicosa-5Z,8Z,14Z-trienoic acid derivatives (PBTs), recently made several animal studies possible and explored the role of HXA(3) as a therapeutic in treatment of diseases. Thus, PBT-3 induced apoptosis in K562 tumour cells and inhibited growth of K562 CML solid tumours in nude mice. HXA(3) inhibited bleomycin-evoked lung fibrosis and inflammation in mice and the raised insulin level in the circulation of rats. At low glucose concentrations (0-3 mm), HXA(3) also stimulated insulin secretion in RINm5F cells through the activation of IRE1alpha, an endoplasmic reticulum-resident kinase. The latter regulates the protein folding for insulin biosynthesis. In conclusion, HXA(3)-mediated signaling may be involved in normal physiological functions, and hepoxilin-based drugs may serve as therapeutics in diseases such as type II diabetes and idiopathic lung fibrosis.

Hepoxilin A3 synthase

Hepoxilins constitute a group of 12S-hydroperoxyeicosatetraenoic acid (12S-HpETE)-derived epoxy-hydroxy fatty acids that have been detected in various cell types and tissues. Although hepoxilin A3 (HXA3) exhibits a myriad of biological activities, its biosynthetic mechanism was not investigated in detail. Here we review the isolation, cloning, and characterization of a leukocyte-type 12S-lipoxygenase (12S-LOX) from rat insulinoma cells RINm5F, which exhibits an intrinsic hepoxilin A3 synthase activity. Confirmation for this observation was achieved by coimmunoprecipitation of HXA3 synthase activity with an anti-leukocyte 12S-LOX antibody, preparation of recombinant rat 12S-LOX enzyme from RINm5F cells, and assay of HXA3 synthase activity therein. Site-directed mutagenesis studies performed on rat 12S-LOX showed that 12-lipoxygenating enzyme species exhibit a strong HXA3 synthase activity that is impaired when the positional specificity of arachidonic acid is altered in favor of 15-lipoxygenation. Inasmuch as cellular glutathione peroxidases (cGPx and PHGPx) and HXA3 synthase compete for the same substrate 12S-HpETE, it can be proposed that the overall activity of glutathione peroxidases, representing the overall peroxide tone, finely tunes the rate of HXA3 formation.

Biosynthesis of hepoxilins: evidence for the presence of a hepoxilin synthase activity in rat insulinoma cells

The 12(S)-lipoxygenase (12-LOX) pathway of arachidonic acid (AA) metabolism after dioxygenation to 12(S)-hydroperoxy-eicosatetraenoic acid is bifurcated in a reduction route to formation of 12(S)-hydroxy-eicosatetraenoic acid (12-HpETE) and an isomerization route to formation of hepoxilins. Interestingly, we found that the rat insulinoma RINm5F cells, which are devoid of cytoplasmic glutathione peroxidase (cGPx)/phospholipid hydroperoxide glutathione peroxidase (PHGPx), produce solely hepoxilin A(3) (HXA(3)). Since HXA(3) synthesis was abolished in heat-denatured or cGPx- or PHGPx-transfected cells, it was tempting to speculate that a HXA(3) synthase activity regulated by cGPx/PHGPx is present. To confirm this assumption we incubated AA with HeLa cells overexpressing the rat leukocyte-type 12-LOX. Neither HXA(3) nor 12(S)-HETE were detected due to abundance of cGPx/PHGPx. But, pretreatment of transfected cells with diethyl maleate, an inhibitor of glutathione and PHGPx, restored HXA(3) synthase and 12-LOX activities. Thus, we conclude, that cells containing rat leukocyte-type 12-LOX also possess an intrinsic HXA(3) synthase activity, which is activated by inhibition of cGPx/PHGPx. In normal cells HXA(3) is down-regulated by cGPx/PHGPx, but, it is persistently activated in oxidatively stressed cells deficient in cGPx/PHGPx, such as RINm5F.

Comparisons of hydroperoxide isomerase and monooxygenase activities of cytochrome P450 for conversions of allylic hydroperoxides and alcohols to epoxyalcohols and diols: probing substrate reorientation in the active site

Ferric cytochrome P450 (P450) catalyzes intramolecular rearrangements of hydroperoxides to diols by heterolytic scission of the O-O bond and insertion of the terminal oxygen atom into the resulting alcohol. The goals of this work were to further characterize the regio- and stereochemistry of P450 isomerase activity using allylic hydroperoxides and to compare these reactions with NADPH-supported monooxygenations of the corresponding alcohols. Microsomes from phenobarbital-treated rats or purified P450 2B1 catalyzed the conversions of several peroxyquinols, including 2-tert-butyl-4-hydroperoxy-4-methylcyclohexa-2,5-dien-1-one (BMPOOH) and its 2,6-dimethyl derivative (TMPOOH), to diols and to alpha,beta-epoxyquinols by predominant (>93%) cis addition of oxygen to the least hindered double bond. Monooxygenation of the 4-hydroxy analogues (quinols) yielded identical cis-epoxyquinols, and both isomerization and monooxygenation reactions exhibited similar enantioselectivities (32-74% enantiomer excess). Regioselectivities were similar for BMPOOH and BMPOH (epoxyquinol:diol ratios of 5.8-7.6), but quite different for TMPOOH and TMPOH (ratios of 0.6 and 6.4, respectively). Bulky peroxyquinols and quinols derived from the A-rings of 17beta-estradiol and estrone were utilized to further compare these reactions. Both estradiol derivatives underwent approximately equal amounts of 6beta-hydroxylation and 1,2-epoxidation. The estrone derivatives also underwent 6beta-hydroxylation, but only estrone quinol yielded a second product consistent with hydroxylation at position 16. The results support several conclusions. (i) Allylic hydroperoxides may be isomerized to alpha,beta-epoxyalcohols by a heterolytic O-O cleavage mechanism with high stereoselectivity. (ii) Hydroperoxide isomerization is an efficient process relative to monooxygenation. (iii) Isomerase substrates remain in proximity to the P450 oxoferryl intermediate and are rapidly captured by the oxidant. Monooxygenase substrates, on the other hand, may bind to ferric P450 in multiple orientations and undergo more extensive substrate reorientation prior to oxidative attack.

Epoxide hydrolases: biochemistry and molecular biology

Epoxides are organic three-membered oxygen compounds that arise from oxidative metabolism of endogenous, as well as xenobiotic compounds via chemical and enzymatic oxidation processes, including the cytochrome P450 monooxygenase system. The resultant epoxides are typically unstable in aqueous environments and chemically reactive. In the case of xenobiotics and certain endogenous substances, epoxide intermediates have been implicated as ultimate mutagenic and carcinogenic initiators Adams et al. (Chem. Biol. Interact. 95 (1995) 57-77) Guengrich (Properties and Metabolic roles 4 (1982) 5-30) Sayer et al. (J. Biol. Chem. 260 (1985) 1630-1640). Therefore, it is of vital importance for the biological organism to regulate levels of these reactive species. The epoxide hydrolases (E.C. 3.3.2. 3) belong to a sub-category of a broad group of hydrolytic enzymes that include esterases, proteases, dehalogenases, and lipases Beetham et al. (DNA Cell Biol. 14 (1995) 61-71). In particular, the epoxide hydrolases are a class of proteins that catalyze the hydration of chemically reactive epoxides to their corresponding dihydrodiol products. Simple epoxides are hydrated to their corresponding vicinal dihydrodiols, and arene oxides to trans-dihydrodiols. In general, this hydration leads to more stable and less reactive intermediates, however exceptions do exist. In mammalian species, there are at least five epoxide hydrolase forms, microsomal cholesterol 5,6-oxide hydrolase, hepoxilin A(3) hydrolase, leukotriene A(4) hydrolase, soluble, and microsomal epoxide hydrolase. Each of these enzymes is distinct chemically and immunologically. Table 1 illustrates some general properties for each of these classes of hydrolases. Fig. 1 provides an overview of selected model substrates for each class of epoxide hydrolase.

The effect of the membrane fluidity on pharmacokinetics for lipid A analog E5531

The effect of the dispersing procedure on the aggregate size, membrane fluidity and the pharmacokinetics were evaluated for the lipid A analog E5531. The size of the aggregates prepared by the pH-jump method (pH 11.0-->7.3) was decreased, reaching 20 nm with increasing dispersing time in 0.003 N NaOH (pH 11.0). The membrane fluidity of the aggregates increased with increasing dispersing time. When prepared by the normal dilution method (pH 7.3-->7.3), the size of the aggregates remained constant at 150 nm and the membrane fluidity was smaller compared to samples prepared by the pH-jump method. Using samples with different degrees of hydration and different membrane fluidities prepared by the pH-jump method, the pharmacokinetics after intravenous administration into rats were evaluated, and the data obtained confirmed that the membrane fluidity was correlated with the pharmacokinetics in rat. In addition, E5531 vials were stable for 24 months at room temperature when used within 24 hr after reconstitution.

Stereoselective biosynthesis of hepoxilin B3 in human epidermis

We previously reported that normal human epidermis forms 12-oxo-eicosatetraenoic acid and hepoxilin B3 as major eicosanoids and that hepoxilins and trioxilins are dramatically elevated in psoriatic lesions. We also observed that normal epidermis only synthesized one of the two possible 10-hydroxy- epimers of hepoxilin B3, suggesting its enzymatic origin. This study investigated the enzymatic pathways involved in the formation of hepoxilin B3 in human epidermis. Human epidermal fragments or cell fractions were incubated with [14C]-arachidonic acid or authentic 12(S)-hydroperoxyeicosatetraenoic acid. Products were analyzed by high-performance liquid chromatography, gas chromatography-mass spectrometry or a combination of both techniques. Esculetin and nordihydroguaiaretic acid inhibited formation of hepoxilin B3, 12-oxo-eicosatetraenoic acid, trioxilins, and 12-hydroxyeicosatetraenoic acid in a concentration-dependent manner. 12-Lipoxygenase activity was mainly located in the microsomal fraction (100,000 x g pellet) and 12-hydroxyeicosatetraenoic acid, hepoxilin B3, and 12-oxo-eicosatetraenoic acid were formed. The hepoxilin B3-synthesizing activity was not observed in subcellular fractions incubated with authentic 12(S)-hydroperoxyeicosatetraenoic acid, although it was located at least in the microsomal fraction when incubated with arachidonic acid. Similar results were obtained using preparations of recombinant platelet-type 12-lipoxygenase that yielded 12-oxo-eicosatetraenoic acid and hepoxilin B3 in addition to 12-hydroxyeicosatetraenoic acid, when incubated with arachidonic acid but not when incubated with 12-hydroperoxyeicosatetraenoic acid. Nevertheless, recombinant 12-lipoxygenase produced a lower ratio of 12-oxo-eicosatetraenoic acid and hepoxilin B3-12-hydroxyeicosatetraenoic acid than epidermis. Our results support the concept that 12-lipoxygenase catalyzes the formation of hepoxilin B3 and 12-oxo-eicosatetraenoic acid.

Biological actions of the free acid of hepoxilin A3 on human neutrophils

In earlier reports and reviews, it was suggested that unlike its methyl ester, the free acid form of the 12-lipoxygenase-derived eicosanoid hepoxilin A3 (HXA3) does not enter neutrophils and other cells. Therefore, in the past, most studies on the biological activities of HXA3 on human neutrophils were conducted with its methyl ester. Here, we present evidence that free HXA3 is biologically active towards human neutrophils at submicromolar concentrations, which may occur under certain circumstances in vivo. Thus, HXA3 caused chemotaxis at concentrations as low as 30-40 nM, an effect which was attenuated at higher concentrations of this eicosanoid. Its chemotactic potency proved to be comparable to that of leukotriene B4, but higher than that of the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP), and greatly exceeded that of the other 12-lipoxygenase metabolite, 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid, which was inactive at comparable concentrations. The chemotactic activity of HXA3 was not abolished by serum albumin, but it was suppressed by pertussis toxin. Unlike fMLP, at this concentration range HXA3 did not cause respiratory burst or aggregation of the neutrophils or activation of protein kinase C. These observations suggest a remarkably selective and specific receptor-mediated process. At concentrations higher than 1 microM, HXA3 gives rise to an instantaneous release of calcium from intracellular stores which causes, however, only a slight, if any, liberation of arachidonic acid. On the other hand, pretreatment of the neutrophils with submicromolar concentrations of HXA3 significantly blunts the liberation of arachidonic acid caused by fMLP.

The effect of divalent cations on the membrane properties and pharmacokinetics in rat of the lipid A analogue E5531

To obtain information on the effects of Ca2+ on the membrane properties of the lipid A analogue E5531, we have determined the aggregate size, zeta potential, membrane fluidity, micropolarity and permeability of the E5531 membrane as a function of Ca2+ levels. Within the molar ratios of [Ca2+]/[E5531] = 1 and 3, Ca2+ increased the zeta potential of the E5531 membrane but had no effect on aggregate size (approximately 20 nm). Within the above ratios, Ca2+ decreased the membrane fluidity, as measured by micropolarity of E5531 and increased the phase transition temperature. The pharmacokinetics in rats for these samples with different membrane fluidity, prepared by changing the pre-dose formulation concentration of Ca2+, was determined and a correlation between membrane fluidity and pharmacokinetics was clearly observed. It thus appears that Ca2+ effects the membrane fluidity of E5531 as well as its pharmacokinetics in rats.

Hepoxilins raise circulating insulin levels in vivo

We have demonstrated over a decade ago that hepoxilins cause the release of insulin from isolated pancreatic islets of Langerhans in vitro. However, no studies are available so far to indicate whether these compounds are active in vivo. The present study is the first to our knowledge which demonstrates that hepoxilins administered intra-arterially in the anaesthetized rat cause the release of insulin in the circulation. This release is dependent on the glucose status of the rat. Hence, animals fasted overnight do not respond to hepoxilin administration, while animals that have had free access to food respond to hepoxilins with a rise in insulin concentrations in blood. The hepoxilin effect is rapid and varies with different hepoxilins, the most potent of which is hepoxilin A(3) (HxA(3)) (both the 8S and the 8R enantiomers). Administration of 100 microg HxA(3) produces a rise in blood insulin equivalent to that caused by the administration of 5 mg glucose. In view of earlier evidence showing that these compounds cause a rise in intracellular calcium levels in vitro at a <1 microg/ml concentration through a receptor-mediated mechanism, we speculate that the actions of hepoxilins in causing the release of insulin from the pancreas may be due to alterations in calcium levels within the beta-cell. We believe that hepoxilins may represent new lead compounds as therapeutics in type II diabetes mellitus.

Stereoselective actions of hepoxilins A3 and B3 and their cyclopropane analogs (HxdeltaA3 and HxdeltaB3) on bradykinin and PAF-evoked potentiation of vascular leakage in rat skin

Native hepoxilins (Hx) A3 and B3 as well as their synthetic cyclopropane analogs, HxdeltaA3 and HxdeltaB3 are inactive on their own in causing changes in vascular permeability in rat skin measured by leakage of plasma-bound Evans Blue dye. Several of these compounds, however, were observed to potentiate the leakage of dye evoked by bradykinin (BK) and platelet-activating factor (PAF). The syn epimer of HxA3 was effective in potentiating dye leakage evoked by BK but not by PAF. The syn epimer of HxB3, on the other hand, was capable of potentiating both BK- and PAF-evoked plasma protein leakage. The anti epimer of both hepoxilins was inactive. In contrast, the anti epimer of the cyclopropane analog HxdeltaA3 potentiated only the BK-evoked changes, whereas the anti epimer of HxdeltaB3 potentiated only the PAF-evoked changes in dye leakage. The corresponding other epimer of each compound was inactive. Our findings indicate that the hepoxilin cyclopropane analogs appear to mimic the actions of the native compounds.

Formation of 14,15-Hepoxilins of the A3 and B3 Series through a 15-Lipoxygenase and Hydroperoxide Isomerase Present in Garlic Roots

We report herein for the first time the formation by freshly grown garlic roots and the structural characterization of 14,15-epoxide positional analogs of the hepoxilins formed via the 15-lipoxygenase-induced oxygenation of arachidonic acid. These compounds are formed through the combined actions of a 15(S)-lipoxygenase and a hydroperoxyeicosatetraenoic acid (HPETE) isomerase. The compounds were formed when either arachidonic acid or 15-HPETE were used as substrates. Both the "A"-type and the "B"-type products are formed although the B-type compounds are formed in greater relative quantities. Chiral phase high performance liquid chromatography analysis confirmed the formation of hepoxilins from 15(S)- but not 15(R)-HPETE, indicating high stereoselectivity of the isomerase. Additionally, the lipoxygenase was of the 15(S)-type as only 15(S)-hydroxyeicosatetraenoic acid was formed when arachidonic acid was used as substrate. The structures of the products were confirmed by gas chromatography-mass spectrometry of the methyl ester trimethylsilyl ether derivatives as well as after characteristic epoxide ring opening catalytically with hydrogen leading to dihydroxy products. That 15(S)-lipoxygenase activity is of functional importance in garlic was shown by the inhibition of root growth by BW 755C, a dual cyclooxygenase/lipoxygenase inhibitor and nordihydroguaiaretic acid, a lipoxygenase inhibitor. Additional biological studies were carried out with the purified intact 14(S), 15(S)-hepoxilins, which were investigated for hepoxilin-like actions in causing the release of intracellular calcium in human neutrophils. The 14,15-hepoxilins dose-dependently caused a rise in cytosolic calcium, but their actions were 5-10-fold less active than 11(S), 12(S)-hepoxilins derived from 12(S)-HPETE. These studies provide evidence that 15(S)-lipoxygenase is functionally important to normal root growth and that HPETE isomerization into the hepoxilin-like structure may be ubiquitous; the hepoxilin-evoked release of calcium in human neutrophils, which is receptor-mediated, is sensitive to the location within the molecule of the hydroxyepoxide functionality.

In vivo stimulation of 12(S)-lipoxygenase in the rat skin by bradykinin and platelet activating factor: formation of 12(S)-HETE and hepoxilins, and actions on vascular permeability

In this study we set out to investigate whether the inflammatory agents, bradykinin (BK) and platelet activating factor (PAF), affect the lipoxygenase pathway in rat skin in vivo and whether the main products so formed may be involved in the inflammatory actions of these agents. In vitro preparations of epidermis were also investigated to determine whether lipoxygenases are stimulated by these agents. We also investigated the actions of arachidonic acid and 12(S)-HPETE as substrates for the lipoxygenases. Our results indicated that 12-lipoxygenase is actively and selectively stimulated in a dose-dependent way in both preparations by the administration of BK and PAF; the main product, 12-HETE, was shown by chiral analysis to be exclusively of the S-configuration, indicating that 12(S)-lipoxygenase was present in the rat skin and was stimulated by these inflammatory agents. Hepoxilins were also formed but to a lesser extent in both in vivo and in vitro preparations. In separate experiments, 12(S)-HETE administered intradermally on its own (40 ng/site), increased vascular permeability as also seen with bradykinin (100 ng/site) and PAF (10 ng/site). However, unlike previously observed with hepoxilin A3 administration, 12(S)-HETE did not stimulate the action of BK on vascular permeability, suggesting that the two compounds may have different mechanisms of action to enhance inflammation. These observations suggest that the vascular permeability and plasma extravasation observed with both inflammatory agents (BK and PAF) may be mediated at least in part through the activation of 12(S)-lipoxygenase, resulting in enhanced formation of 12(S)-HETE which causes acute inflammation.

Occurrence of hepoxilins and trioxilins in psoriatic lesions

We recently found that normal human epidermis produces relatively high amounts of hepoxilins and trioxilins in vitro. Therefore, the aim of this study was to demonstrate the presence of these compounds in psoriatic lesions. Extracts from scales of patients with chronic stable plaque psoriasis were analyzed by a combination of high performance liquid chromatography and gas chromatography-mass spectrometry techniques. We found that the levels of hepoxilin B3 were more than 16-fold higher in psoriatic scales than in normal epidermis (3.2+/-2.3 and < 0.2 ng per mg, respectively), whereas hepoxilin A3 was not detected in any sample. Trioxilins were semiquantitated and referred to 12-hydroxyeicosatetraenoic acid, ratios of trioxilins A3 and B3 12-hydroxyeicosatetraenoic acid in psoriatic lesions were 0.65+/-0.23 and 0.32+/-0.28, respectively, and they were not detected in normal epidermis. The presence of a great amount of trioxilin A3 strongly suggests that hepoxilin A3 was present in psoriatic lesions and it was totally degraded to trioxilin A3 during the analysis procedure. Our results demonstrate that hepoxilins and trioxilins are produced by human skin in vivo and that the levels of these compounds are increased in psoriasis. The reported biologic activities of hepoxilins indicate that they could amplify and maintain the inflammatory response. Our results reinforce the idea that these compounds could play a role as mediators in the inflammatory response in skin, particularly in psoriasis.

Hepoxilin A3 is oxidized by human neutrophils into its omega-hydroxy metabolite by an activity independent of LTB4 omega-hydroxylase

Hepoxilin A3-methyl ester is taken up by intact human neutrophils where it is first hydrolyzed into the free acid which is subsequently converted into a single major metabolite. The structure of this metabolite was determined through mass spectral analysis of several derivatives, and through identity with an authentic compound prepared by chemical synthesis. The metabolite was identified as omega-hydroxy-hepoxilin A3 showing that the epoxide functionality of the parent hepoxilin is not opened during incubation with human neutrophils. All attempts to investigate hepoxilin metabolism in broken cells, despite the presence of protease inhibitors (Aproteinin, PMSF, DFP) and supplementation with NADPH were unsuccessful. Metabolism of hepoxilin A3 required the intact cell, while parallel experiments with LTB4 as substrate demonstrated that this eicosanoid was metabolized into its omega-hydroxy metabolite regardless of whether intact or broken cell preparations were used provided that NADPH was present in the latter. Hepoxilin metabolism in intact cells was inhibited dose-dependently by CCCP (0.01-100 microM), a mitochondrial uncoupler, whereas LTB4 metabolism was unaffected by CCCP. This data suggests that metabolism of hepoxilin A3 occurs in intact human neutrophils through omega-oxidation, is likely located in the mitochondrial compartment of the cell (inhibition by CCCP) and is carried out by an activity that is independent of the well characterized, relatively stable microsomal LTB4 omega-hydroxylase.

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.

Hepoxilin-evoked intracellular reorganization of calcium in human neutrophils: a confocal microscopy study

Hepoxilin A3 has previously been shown to cause a rapid dose-dependent rise in intracellular calcium in intact human neutrophils in suspension. Two components have been observed, an initial rapid phase of intracellular calcium rise, followed by a slow decline to plateau levels that remain above the original baseline calcium levels. These changes have been suggested to involve the release of calcium from intracellular stores in the ER (initial rapid phase), while the slower rate of decline (plateau phase) was presumed to be due to calcium influx as it was abolished in zero calcium extracellular medium. The present study used confocal microscopy to examine the response to hepoxilin A3 at the subcellular level. Our results show that calcium dynamics in response to hepoxilin A3 varies in different subcellular compartments within the cell and that hepoxilin A3 evoked a persistent accumulation of calcium in organelles. The hepoxilin-evoked calcium sequestration was eliminated by prior exposure to CCCP, a mitochondrial uncoupler. CCCP also eliminated the plateau phase of the calcium response in cell suspension, suggesting that this phase was due to mitochondrial accumulation of calcium rather than calcium influx. Experiments with DiI-loaded cells, a membrane marker, showed that the nuclear calcium was not elevated by hepoxilin addition to the cells. These results demonstrate that hepoxilins evoke the release of calcium from the ER which is taken up by the mitochondria where it is tightly sequestered. These results offer an explanation of observations previously made with cell suspensions in which hepoxilin A3 was shown to inhibit the calcium mobilizing effects of chemotactic agents.

12-HETE and 12-HPETE potently stimulate intracellular release of calcium in intact human neutrophils

We describe here the effects of two 12-lipoxygenase products, 12-HETE (12-hydroxyeicosa (5Z,8Z,10E,14Z) tetraenoic acid) and 12-HPETE (12-hydroperoxyeicosa (5Z,8Z,10E,14Z) tetraenoic acid), on the release of intracellular calcium in intact human neutrophils using the INDO-1 AM fluorescent dye technique. Both products dose dependently stimulate intracellular release, with 12-HETE being more powerful than 12-HPETE. The threshold concentration for 12-HETE was 5 ng/ml (1.5 x 10-8 M), while that for 12-HPETE was 10 ng/ml. The (12S) regioisomer was slightly more active than the (12R) isomer. The laser potency of 12-HPETE may be due to its conversion into the less active hepoxilins as incubation of neutrophils with (12S/R)-HPETE in a nonradioactive assay, using fluorescent ADAM esters of the products, generated mostly hepoxilin A3 (8-hydroxy-(11S,12S) epoxyeicosa (5Z,9E,14Z)trienoic acid), indicative of an enzymatic process. In contrast, boiled neutrophil preparations converted 12-HPETE primarily into hepoxilin B3 which previously showed to be derived nonenzymatically. This data demonstrates that 12-HETE, known to be generated in significant amounts by platelets, can act transcellularly to modify intracellular concentrations of calcium in neutrophils. This may in turn affect the responsiveness of these cells to other chemotactic factors.

Cytochrome P450-dependent transformations of 15R- and 15S-hydroperoxyeicosatetraenoic acids: stereoselective formation of epoxy alcohol products

Although there are many reports of epoxy alcohol synthesis from lipoxygenase products (fatty acid hydroperoxides) in mammalian tissues, there are no well-defined examples of the stereoselective synthesis of individual epoxy alcohol diastereomers. An earlier report on the metabolism of 15S-hydroperoxyeicosatetraenoic acid (15S-HPETE) in rat liver microsomes suggested such a specific reaction [Weiss, R. H., et al. (1987) Arch. Biochem. Biophys. 252, 334-338]. To characterize this reaction further, we set out to determine the precise structures and mechanism of biosynthesis of the epoxy alcohol products. We compared the products formed from 15R- and 15S-HPETE by hematin (a nonenzymatic reaction), by liver microsomes isolated from control and phenobarbital-treated rats, and by purified cytochrome P450 2B1. Eight epoxy alcohol isomers were identified by mass spectrometry and 1H NMR. In the hematin reaction, the major products are four epoxy alcohols with the epoxide in the trans configuration, diastereomers are formed in similar amounts, and the 15-HPETE enantiomers give indistinguishable patterns of products. By contrast, the liver microsomes and P450 2B1 enzyme form predominantly single diastereomers, and the configuration of the epoxide is dependent on the stereochemistry of the substrate. The main product formed from 15S-HPETE is 11S-hydroxy-14S,15S-trans-epoxyeicosa-5Z,8Z,12E- trienoic acid, and the amounts increase upon phenobarbital induction. The main products from 15R-HPETE are 11-hydroxy-14S,15R-epoxyeicosa-5Z,8Z,12E-t rienoic acid from microsomes from control rats and 13-hydroxy-14S,15R-cis-epoxyeicosa-5,8,11-trienoic acid in microsomes from phenobarbital-induced rats. The P450 2B1 enzyme gave products similar to those from the phenobarbital-induced microsomes. Analysis of an incubation using the 18O-labeled 15S-HPETE substrate demonstrated 97.6% retention of both hydroperoxy oxygens in the major product with progressively lower 18O retentions in the minor products (74-32%), possibly reflecting degrees of enzymatic control of these reactions. These results establish a precedent for the stereoselective synthesis of epoxy alcohols by mammalian cytochrome P450s.

Chemical synthesis and actions of 11,12-thiirano-hepoxilin A3

A novel analog of hepoxilin A3 has been chemically synthesized in which the 11,12-epoxide group has been altered to a thiirano group. This has been accomplished through allylic rearrangement of unnatural (11 R, 12 R)-hepoxilin B3 under Mitsunobu conditions, first into unnatural (11 R, 12 R)-hepoxilin A3, followed by conversion of this compound with inversion of the epoxide centers into the thiirano-hepoxilin A3 having the natural 11 S, 12 S configuration. We also report herein evidence showing that thiirano-hepoxilin A3 raises intracellular calcium concentrations in intact human neutrophils.

High-performance liquid chromatographic separation of fluorescent esters of hepoxilin enantiomers on a chiral stationary phase

Fluorescent anthryl (ADAM) derivatives of hepoxilins have been shown to possess good chromatographic properties affording good sensitivity for the high-performance liquid chromatographic analysis and detection of these compounds and related eicosanoids (12-hydroxyeicosatetraenoic acid) in biological samples. We report herein the separation of all possible stereoisomers of hepoxilins A3 and B3 as their methyl esters as well as their ADAM ester and acetate derivatives on a cellulose trisdimethyphenylcarbamate chiral stationary phase (Chiracel OD) in the normal-phase mode. This methodology is important to address the mechanistic route of biosynthesis of these products.

Hepoxilins: a review on their enzymatic formation, metabolism and chemical synthesis

This article reviews published evidence describing the enzymatic and nonenzymatic formation and the routes of metabolism of the hepoxilins. Also treated are the major approaches used for the chemical synthesis of these compounds and for some of their analogs.