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

Stable analogs of 13‑hydroxy-9,10-trans-epoxy-(11E)-octadecenoate (13,9-HEL), an oxidized derivative of linoleic acid implicated in the epidermal skin barrier

Hydroxy-epoxy- and trihydroxy derivatives of linoleic acid are proposed to play an essential function in formation of the mammalian skin permeability barrier, which could account for the essential nature of its precursor, linoleic acid. Recent literature suggests that a specific oxidized enone derivative of LA esterified in ceramides facilitates binding to proteins, potentially serving a structural role in formation of the epidermal skin barrier. However, it is still to be established if other linoleic acid derivatives are also required for skin barrier formation, and whether the essential role is performed exclusively by an esterified, structural lipid or as an unesterified, labile signaling lipid, or by some combination of these derivatives. Progress in this domain is limited by lack of availability of hydroxy‑epoxy-and trihydroxy- and octadecenoate derivatives of linoleic acid and related compounds, and challenges in maintaining them in the unesterified lipid pool. Here we describe methods for the total synthesis of hydroxy‑epoxy-octadecenoate derivatives of linoleic acid (HEL¹), and stable analogs that are designed to be resistant to inactivation by: (a) acylation/esterification (thus trapping these lipids in the free acid pool), (b) dehydrogenation, and (c) analogs combining both modifications. We further provide a total synthesis of corresponding hydroxy‑epoxy- derivatives of sebaleic acid (a regioisomer of linoleic acid present in skin), and of small molecule scaffolds containing the allylic and non-allylic epoxide 7-carbon substructures shared by both families of hydroxy‑epoxy-and trihydroxy- octadecenoates. Finally, we demonstrate that 2,2-dimethyl analogs of hydroxy‑epoxy-and trihydroxy- octadecenoates are resistant to esterification with an in vitro assay and thus provide a novel template for stabilizing labile, bioactive lipids as free acids by preventing acylation/esterification.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

Eicosanoids are short-lived derivatives of polyunsaturated fatty acids that serve as autocrine and paracrine signaling molecules. They are involved numerous biological processes of both the well state and disease states. A thorough understanding of the progression the disease state and homeostasis of the well state requires a complete evaluation of the systems involved. This review examines the enzymology for the enzymes involved in the production of eicosanoids along the lipoxygenase branches of the eicosanoid pathways with particular emphasis on those derived from arachidonic acid. The enzymatic parameters, protocols to measure them, and proposed catalytic mechanisms are presented in detail.

Lipidomic profiling of targeted oxylipins with ultra-performance liquid chromatography-tandem mass spectrometry

Oxylipins are bioactive mediators that play diverse roles in (patho)physiology. We developed a sensitive and selective ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method for the simultaneous profiling of 57 targeted oxylipins derived from five major n-6 and n-3 polyunsaturated fatty acids (PUFAs) that serve as oxylipin precursors, including linoleic (LA), arachidonic (AA), alpha-linolenic (ALA), eicosapentaenoic (EPA), and docosahexaenoic (DHA) acids. The targeted oxylipin panel provides broad coverage of lipid mediators and pathway markers generated from cyclooxygenases, lipoxygenases, cytochrome P450 epoxygenases/hydroxylases, and non-enzymatic oxidation pathways. The method is based on combination of protein precipitation and solid-phase extraction (SPE) for sample preparation, followed by UPLC-MS/MS. This is the first methodology to incorporate four hydroxy-epoxy-octadecenoic acids and four keto-epoxy-octadecenoic acids into an oxylipin profiling network. The novel method achieves excellent resolution and allows in-depth analysis of isomeric and isobaric species of oxylipin extracts in biological samples. The method was quantitatively characterized in human plasma with good linearity (R = 0.990-0.999), acceptable reproducibility (relative standard deviation (RSD) < 20% for the majority of analytes), accuracy (67.8 to 129.3%) for all analytes, and recovery (66.8-121.2%) for all analytes except 5,6-EET. Ion enhancement effects for 28% of the analytes in tested concentrations were observed in plasma, but were reproducible with RSD < 17.2%. Basal levels of targeted oxylipins determined in plasma and serum are in agreement with those previously reported in literature. The method has been successfully applied in clinical and preclinical studies.

Distinct cellular sources of hepoxilin A3 and leukotriene B4 are used to coordinate bacterial-induced neutrophil transepithelial migration

Neutrophilic infiltration is a leading contributor to pathology in a number of pulmonary disease states, including cystic fibrosis. Hepoxilin A3 (HXA3) is a chemotactic eicosanoid shown to mediate the transepithelial passage of neutrophils in response to infection in several model systems and at multiple mucosal surfaces. Another well-known eicosanoid mediating general neutrophil chemotaxis is leukotriene B4 (LTB4). We sought to distinguish the roles of each eicosanoid in the context of infection of lung epithelial monolayers by Pseudomonas aeruginosa. Using human and mouse in vitro transwell model systems, we used a combination of biosynthetic inhibitors, receptor antagonists, as well as mutant sources of neutrophils to assess the contribution of each chemoattractant in driving neutrophil transepithelial migration. We found that following chemotaxis to epithelial-derived HXA3 signals, neutrophil-derived LTB4 is required to amplify the magnitude of neutrophil migration. LTB4 signaling is not required for migration to HXA3 signals, but LTB4 generation by migrated neutrophils plays a significant role in augmenting the initial HXA3-mediated migration. We conclude that HXA3 and LTB4 serve independent roles to collectively coordinate an effective neutrophilic transepithelial migratory response.

Steric analysis of epoxyalcohol and trihydroxy derivatives of 9-hydroperoxy-linoleic acid from hematin and enzymatic synthesis

We characterize the allylic epoxyalcohols and their trihydroxy hydrolysis products generated from 9R- and 9S-hydroperoxy-octadecenoic acid (HPODE) under non-enzymatic conditions, reaction with hematin and subsequent acid hydrolysis, and enzymatic conditions, incubation with Beta vulgaris containing a hydroperoxide isomerase and epoxide hydrolase. The products were resolved by HPLC and the regio and stereo-chemistry of the transformations were determined through a combination of (1)H NMR and GC-MS analysis of dimethoxypropane derivatives. Four trihydroxy isomers were identified upon mild acid hydrolysis of 9S,10S-trans-epoxy-11E-13S-hydroxyoctadecenoate: 9S,10R,13S, 9S,12R,13S, 9S,10S,13S and 9S,12S,13S-trihydroxy-octadecenoic acids, in the ratio 40:26:22:12. We also identified a prominent δ-ketol rearrangement product from the hydrolysis as mainly the 9-hydroxy-10E-13-oxo isomer. Short incubation (5 min) of 9R- and 9S-HPODE with B. vulgaris extract yielded the 9R- and 9S-hydroxy-10E-12R,13S-cis-epoxy products respectively. Longer incubation (60 min) gave one specific hydrolysis product via epoxide hydrolase, the 9R/S,12S,13S-trihydroxyoctadecenoate. These studies provide a practical approach for the isolation and characterization of allylic epoxy alcohol and trihydroxy products using a combination of HPLC, GC-MS and (1)H NMR.

8R-Lipoxygenase-catalyzed synthesis of a prominent cis-epoxyalcohol from dihomo-γ-linolenic acid: a distinctive transformation compared with S-lipoxygenases

Conversion of fatty acid hydroperoxides to epoxyalcohols is a well known secondary reaction of lipoxygenases, described for S-specific lipoxygenases forming epoxyalcohols with a trans-epoxide configuration. Here we report on R-specific lipoxygenase synthesis of a cis-epoxyalcohol. Although arachidonic and dihomo-γ-linolenic acids are metabolized by extracts of the Caribbean coral Plexaura homomalla via 8R-lipoxygenase and allene oxide synthase activities, 20:3ω6 forms an additional prominent product, identified using UV, GC-MS, and NMR in comparison to synthetic standards as 8R,9S-cis-epoxy-10S-erythro-hydroxy-eicosa-11Z,14Z-dienoic acid. Both oxygens of (18)O-labeled 8R-hydroperoxide are retained in the product, indicating a hydroperoxide isomerase activity. Recombinant allene oxide synthase formed only allene epoxide from 8R-hydroperoxy-20:3ω6, whereas two different 8R-lipoxygenases selectively produced the epoxyalcohol.A biosynthetic scheme is proposed in which a partial rotation of the reacting intermediate is required to give the observed erythro epoxyalcohol product. This characteristic and the synthesis of cis-epoxy epoxyalcohol may be a feature of R-specific lipoxygenases.

LIPOXYGENASE PRODUCTS IN MARINE DIATOMS: A CONCISE ANALYTICAL METHOD TO EXPLORE THE FUNCTIONAL POTENTIAL OF OXYLIPINS(1)

Oxylipins are oxygenated derivatives of polyunsaturated fatty acids (PUFAs) that act as chemical mediators in many ecological and physiological processes in marine and freshwater diatoms. The occurrence and distribution of these molecules are relatively widespread within the lineage with considerable species-specific differences due to the variability of both the fatty acids recognized as substrates and the enzymatic transformations. The present review provides a general introduction to recent studies on diatom oxylipins and describes an analytical method for the detection and assessment of these elusive molecules in laboratory and field samples. This methodology is based on selective enrichment of the oxylipin fraction by solvent extraction, followed by parallel acquisition of full-scan UV and tandem mass spectra on reverse phase liquid chromatography (LC) peaks. The analytical procedure enables identification of potential genetic differences, enzymatic regulation, and ecophysiological conditions that result in different oxylipin signatures, thus providing an effective tool for probing the functional relevance of this class of lipids in plankton communities. Examples of oxylipin measurements in field samples are also provided as a demonstration of the analytical potential of the methodology.

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.

Mammalian epoxide hydrolases in xenobiotic metabolism and signalling

Epoxide hydrolases catalyse the hydrolysis of electrophilic--and therefore potentially genotoxic--epoxides to the corresponding less reactive vicinal diols, which explains the classification of epoxide hydrolases as typical detoxifying enzymes. The best example is mammalian microsomal epoxide hydrolase (mEH)-an enzyme prone to detoxification-due to a high expression level in the liver, a broad substrate selectivity, as well as inducibility by foreign compounds. The mEH is capable of inactivating a large number of structurally different, highly reactive epoxides and hence is an important part of the enzymatic defence of our organism against adverse effects of foreign compounds. Furthermore, evidence is accumulating that mammalian epoxide hydrolases play physiological roles other than detoxification, particularly through involvement in signalling processes. This certainly holds true for soluble epoxide hydrolase (sEH) whose main function seems to be the turnover of lipid derived epoxides, which are signalling lipids with diverse functions in regulatory processes, such as control of blood pressure, inflammatory processes, cell proliferation and nociception. In recent years, the sEH has attracted attention as a promising target for pharmacological inhibition to treat hypertension and possibly other diseases. Recently, new hitherto uncharacterised epoxide hydrolases could be identified in mammals by genome analysis. The expression pattern and substrate selectivity of these new epoxide hydrolases suggests their participation in signalling processes rather than a role in detoxification. Taken together, epoxide hydrolases (1) play a central role in the detoxification of genotoxic epoxides and (2) have an important function in the regulation of physiological processes by the control of signalling molecules with an epoxide structure.

LOX-Induced Lipid Peroxidation Mechanism Responsible for the Detrimental Effect of Marine Diatoms on Zooplankton Grazers

Some marine diatoms negatively affect the reproduction of dominant zooplankton grazers such as copepods, thus compromising the transfer of energy through the marine food chains. In this paper, the metabolic mechanism that leads to diatom-induced toxicity is investigated in three bloom-forming microalgae. We show that copepod dysfunctions can be induced by highly reactive oxygen species (hROS) and a blended mixture of diatom products, including fatty acid hydroperoxides (FAHs); these compounds display teratogenic and proapoptotic properties. The process is triggered by the early onset of lipoxygenase activities that elicit the synthesis of species-specific products, the basic structures of which were established (1-20); these compounds boost oxidative stress by massive lipid peroxidation. Our study might explain past laboratory and field results showing how diatoms damage zooplankton grazers even in the absence of polyunsaturated aldehydes, a class of molecules that has been formerly implicated in mediating the toxic activity of diatoms on copepods.

Biological role of hepoxilins: upregulation of phospholipid hydroperoxide glutathione peroxidase as a cellular response to oxidative stress?

The 12S-lipoxygenase (12S-LOX) pathway of arachidonic acid (AA) metabolism is bifurcated at 12(S)-hydroperoxy-5Z,8Z,10E (12S-HpETE) in the reduction route to form 12S-hydroxy-eicosatetraenoic acid (12S-HETE) and in 8(S/R)-hydroxy-11(S),12S-trans-epoxyeicosa-5Z,9E,14Z-trienoic acid (HXA3) synthase pathway, previously known as isomerization route, to form hepoxilins. Earlier we showed that the HXA3 formation is restricted to cellular systems devoid of hydroperoxide reducing enzymes, e.g. GPxs, thus causing a persistent oxidative stress situation. Here, we show that HXA3 at as low as 100 nM concentration upregulates phospholipid hydroperoxide glutathione peroxidase (PHGPx) mRNA and protein expressions, whereas other metabolites of AA metabolism 12S-HpETE and 12S-HETE failed to stimulate the PHGPx. Moreover, the decrease in 12S-HpETE below a threshold value of the hydroperoxide tone causes both suppression of the overall 12S-LOX activity and a shift from HXA3 formation towards 12S-HETE formation. We therefore propose that under persistent oxidative stress the formation of HXA3 and the HXA3-induced upregulation of PHGPx constitute a compensatory defense response to protect the vitality and functionality of the cell.

The hepoxilin connection in the epidermis

The recent convergence of genetic and biochemical evidence on the activities of lipoxygenase (LOX) enzymes has implicated the production of hepoxilin derivatives (fatty acid epoxyalcohols) in the pathways leading to formation of the water-impermeable barrier of the outer epidermis. The enzymes 12R-LOX and eLOX3 are mutated in a rare form of congenital ichthyosis, and, in vitro, the two enzymes function together to convert arachidonic acid to a specific hepoxilin. Taken together, these lines of evidence suggest an involvement of these enzymes and their products in skin barrier function in all normal subjects. The natural occurrence of the specific hepoxilin products, and their biological role, whether structural or signaling, remain to be defined.

Human and mouse eLOX3 have distinct substrate specificities: implications for their linkage with lipoxygenases in skin

Genetic and biochemical evidence suggests a functional link between human 12R-lipoxygenase (12R-LOX) and epidermal lipoxygenase-3 (eLOX3) in normal differentiation of the epidermis; LOX-derived fatty acid hydroperoxide is isomerized by the atypical eLOX3 into a specific epoxyalcohol that is a potential mediator in the pathway. Mouse epidermis expresses a different complement of LOX enzymes, and therefore this metabolic linkage could differ. To test this concept, we compared the substrate specificities of recombinant mouse and human eLOX3 toward sixteen hydroperoxy stereoisomers of arachidonic and linoleic acids. Both enzymes metabolized R-hydroperoxides 2-3 times faster than the corresponding S enantiomers. Whereas 12R-hydroperoxyeicosatetraenoic acid (12R-HPETE) is the best substrate for human eLOX3 (2.4 s(-1); at 30 microM substrate), mouse eLOX3 shows the highest turnover with 8R-HPETE (2.9 s(-1)) followed by 8S-HPETE (1.3 s(-1)). Novel product structures were characterized from reactions of mouse eLOX3 with 5S-, 8R-, and 8S-HPETEs. 8S-HPETE is converted specifically to a single epoxyalcohol, identified as 10R-hydroxy-8S,9S-epoxyeicosa-5Z,11Z,14Z-trienoic acid. The substrate preference of mouse eLOX3 and the unique occurrence of an 8S-LOX enzyme in mouse skin point to a potential LOX pathway for the production of epoxyalcohol in murine epidermal differentiation.

Arachidonate 12-lipoxygenases with reference to their selective inhibitors

Lipoxygenase is a dioxygenase recognizing a 1-cis,4-cis-pentadiene of polyunsaturated fatty acids. The enzyme oxygenates various carbon atoms of arachidonic acid as a substrate and produces 5-, 8-, 12- or 15-hydroperoxyeicosatetraenoic acid with a conjugated diene chromophore. The enzyme is referred to as 5-, 8-, 12- or 15-lipoxygenase, respectively. Earlier we found two isoforms of 12-lipoxygenase, leukocyte- and platelet-type enzymes, which were distinguished by substrate specificity, catalytic activity, primary structure, gene intron size, and antigenicity. Recently, the epidermis-type enzyme was found as the third isoform. Attempts have been made to find isozyme-specific inhibitors of 12-lipoxygenase, and earlier we found hinokitiol, a tropolone, as a potent inhibitor selective for the platelet-type 12-lipoxygenase. More recently, we tested various catechins of tea leaves and found that (-)-gallocatechin gallate was a potent and selective inhibitor of human platelet 12-lipoxygenase with an IC50 of 0.14 microM. The compound was much less active with 12-lipoxygenase of leukocyte-type, 15-, 8-, and 5-lipoxygenases, and cyclooxygenases-1 and -2.

The rat leukocyte-type 12-lipoxygenase exhibits an intrinsic hepoxilin A3 synthase activity

Hepoxilins are biologically relevant eicosanoids formed via the 12-lipoxygenase pathway of the arachidonic acid cascade. Although these eicosanoids exhibit a myriad of biological activities, their biosynthetic mechanism has not been investigated in detail. We examined the arachidonic acid metabolism of RINm5F rat insulinoma cells and found that they constitutively express a leukocyte-type 12S-lipoxygenase. Moreover, we observed that RINm5F cells exhibit an active hepoxilin A(3) synthase that converts exogenous 12S-HpETE (12S-5Z,8-Z,10E,14Z-12-hydro(pero)xy-eicosa-5,8,10,14-tetraenoic acid) or arachidonic acid predominantly to hepoxilin A(3). 12S-lipoxygenase and hepoxilin A(3) synthase activities were co-localized in the cytosol; immunoprecipitation with an anti-12S-lipoxygenase antibody co-precipitated the two catalytic activities. These data suggested that hepoxilin A(3) synthase activity may be considered an intrinsic catalytic property of the leukocyte-type 12S-lipoxygenase. To test this hypothesis we cloned the leukocyte-type 12S-LOX from RINm5F cells, expressed it in Pichia pastoris, and found that the recombinant enzyme exhibited both 12S-lipoxygenase and hepoxilin A(3) synthase activities. The recombinant human platelet-type 12S-lipoxygenase and the porcine leukocyte-type 12S-lipoxygenase also exhibited hepoxilin A(3) synthase activity. In contrast, the native rabbit reticulocyte-type 15S-lipoxygenase did not convert 12S-HpETE to hepoxilin isomers. These data suggest that the positional specificity of lipoxygenases may be crucial for this catalytic function. This hypothesis was confirmed by site-directed mutagenesis studies that altered the positional specificity of the rat leukocyte-type 12S- and the rabbit reticulocyte-type 15-lipoxygenase. In summary, it may be concluded that naturally occurring 12S-lipoxygenases exhibit an intrinsic hepoxilin A(3) synthase activity that is minimal in lipoxygenase isoforms with different positional specificity.

The lipoxygenase gene ALOXE3 implicated in skin differentiation encodes a hydroperoxide isomerase

Lipoxygenase (LOX) enzymes form fatty acid hydroperoxides used in membrane remodeling and cell signaling. Mammalian epidermal LOX type 3 (eLOX3) is distinctive in totally lacking this typical oxygenase activity. Surprisingly, genetic evidence has linked mutations in eLOX3 or a colocalizing enzyme, 12R-LOX, to disruption of the normal permeability barrier of the skin [Jobard, F., Lefèvre, C., Karaduman, A., Blanchet-Bardon, C., Emre, S., Weissenbach, J., Ozgüc, M., Lathrop, M., Prud'homme, J. F. & Fischer, J. (2002) Hum. Mol. Genet. 11, 107-113]. Herein we identify a logical link of the biochemistry to the genetics. eLOX3 functions as a hydroperoxide isomerase (epoxyalcohol synthase) by using the product of 12R-LOX as the preferred substrate. 12R-Hydroperoxyeicosatetraenoic acid (12R-HPETE) is converted to 8R-hydroxy-11R,12R-epoxyeicosa-5Z,9E,14Z-trienoic acid, one of the isomers of hepoxilin A3, and to 12-ketoeicosatetraenoic acid in a 2:1 ratio. Other hydroperoxides, including 8R-HPETE, 12S-HPETE, and 15S-HPETE, as well as the 13S- and 13R-hydroperoxides of linoleic acid are converted less efficiently. Mass spectrometric analysis of the epoxyalcohol formed from [18O]15S-HPETE showed that both hydroperoxy oxygens are retained in the product. We propose that the ferrous form of eLOX3 initiates a redox cycle, unprecedented among LOX in being autocatalytic, in which the hydroperoxy substrate is isomerized to the epoxyalcohol or keto product. Our results provide strong biochemical evidence for a functional linkage of 12R-LOX and eLOX3 and clues into skin biochemistry and the etiology of ichthyosiform diseases in humans.

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.

Hepoxilin analogs inhibit bleomycin-induced pulmonary fibrosis in the mouse

Bleomycin has been suggested to incite plasma extravasation and influx of inflammatory cells leading to pulmonary fibrosis. We hypothesized that stable analogs of the 12-lipoxygenase product, hepoxilin, may attenuate these effects. We initially investigated the effects of the four hepoxilin analogs (PBT-1 to -4) coadministered intradermally with bleomycin and found that PBT-1 and -2 significantly opposed the vascular permeability effects of bleomycin in rat skin. We subsequently tested the hepoxilin analogs for their actions in opposing the intratracheal bleomycin-evoked acute inflammatory phase of lung fibrosis in the mouse, characterized by a marked accumulation of macrophages and an increase in the rate of collagen synthesis and deposition. We found that the bleomycin-evoked effects on macrophage influx were inhibited by all the hepoxilin analogs (PBT-1, -3, and -4 > PBT-2) administered i.p. for 8 days. Increased total lung collagen was completely abrogated by PBT-1 and -2, whereas PBT-3 and -4 had little effect. A dose-response study with PBT-1 indicated that the effective dose for inhibition of bleomycin-induced inflammatory and histological changes was below 10 microg/day. These studies demonstrate an in vivo action of stable analogs of hepoxilin and support an effect on inflammation and vascular permeability from these novel compounds, especially for PBT-1.

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 IC(50) = 8 x 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 A(2) 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.

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.

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.

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.

Docosahexaenoic acid causes accumulation of free arachidonic acid in rat pineal gland and hippocampus to form hepoxilins from both substrates

Hepoxilins (Hx) are biologically active metabolites of arachidonic acid (AA) formed regioselectively from 12(S)-HPETE by 'hepoxilin synthase'. Hx modulate synaptic neurotransmission in hippocampal CA1 neurons, and inhibit norepinephrine release in hippocampal slices. During the course of our studies we investigated whether docosahexaenoic acid (DHA) was a substrate for hepoxilin formation. We used two tissues, the pineal gland and hippocampal slices. Tissues were incubated alone or with AA (20 microg/ml) or DHA (20 microg/ml). After 60 min at 37 degrees C, samples were acid-extracted to convert Hx into their stable trioxilin (TrX) form and analyzed as the Me-TMSi derivatives by EI-GC/MS to determine the structures of the DHA metabolites, and as PFB-TMSi derivatives by GC/MS in the NICI mode using SIM to simultaneously quantify TrX products of the 3-series (derived from AA) monitored at m/z 569, while those of the 5-series (derived from DHA) were monitored at m/z 593. Results show good conversion of both substrate fatty acids by the rat pineal gland and hippocampal slices, into the 3-series (21.3 +/- 5.8 and 12.5 +/- 2.2 ng/microg protein, respectively) and 5-series TrX (12.3 +/- 2.7 and 2.9 +/- 0.4 ng/microg protein, respectively). Surprisingly though, experiments with DHA, in both tissues, also showed formation of TrX derived from endogenous AA (3-series) (10.4 +/- 8.3 and 3.1 +/- 2.1 ng/microg protein, respectively). These experiments demonstrate previously unreported actions of DHA causing the accumulation of AA, which is converted into hepoxilins. In order to prove that AA is accumulated during DHA stimulation of the tissue, we carried out separate experiments with hippocampal slices in which the neutral lipids and phospholipids were labeled with [14C]AA. DHA caused a time-dependent appearance of free [14C]AA which was released mostly from the TG pool. Measurement of the AA/DHA ratio in the TG pool by GC/MS further indicated that DHA is incorporated into the TG at the expense of AA. These results demonstrate that DHA competes with AA for acylation into the metabolically active TG fraction, and both fatty acids are converted into hepoxilins of the corresponding series.

Epoxy allylic carbocations as conceptual intermediates in the biogenesis of diverse marine oxylipins

Marine organisms, especially marine algae, are extremely rich in a diversity of novel oxylipin structures. Many of these oxylipins contain functionalities and rings of a type and location unknown in mammalian systems. In this perspective reviewing marine oxylipins, a proposal is formulated for the central intermediacy of an epoxy allylic carbocation in the biogenesis of these diverse structures. This proposal is strengthened by the relatively large number of examples which are consistent with this type of mechanistic transformation.