A 12R-lipoxygenase in human skin: mechanistic evidence, molecular cloning, and expression

Non-enantioselective, enantioselective, and two-dimensional liquid chromatography coupled with tandem mass spectrometry for the study of stereochemical disposition of oxylipins in cGMP-regulated hemin-treated platelets

Oxylipins are important low abundant signaling molecules in living organisms. In platelets they play a primary role in platelet activation and aggregation in the course of thrombotic events. In vivo, they are enzymatically synthesized by cyclooxygenases, lipoxygenases, or cytochrome P450 isoenzmes, resulting in diverse polyunsaturated fatty acid (FA) metabolites including hydroxy-, epoxy-, oxo-FAs, and endoperoxides with pro-thrombotic or anti-thrombotic effects. In a recent study, it was reported that hemin induces platelet death which was accompanied by enhanced reactive oxygen species (ROS) production (measured by flow cytometry) and lipid peroxidation (as determined by proxy using flow cytometry with BODIPY-C11 as sensor). Lipidomic studies further indicated significant changes of the platelet lipidome upon ex vivo hemin treatment, amongst others oxylipins were increased. The effect could be (at least partly) reversed by riociguat/diethylamine NONOate diethylammonium salt (DEA/NO) which modulates the soluble guanylate cyclase(sGC)-cGMP-cGMP-dependent protein kinase I(cGKI) signaling axis. In the original work, oxylipins were measured by a non-enantioselective UHPLC-tandem-MS assay which may not give the full picture whether oxylipin elevation is due to ROS or by enzymatic processes. We present here the study of the stereochemical disposition of hemin-induced platelet lipidome alterations using Chiralpak IA-U column with amylose tris(3,5-dimethylphenylcarbamate) chiral selector immobilized on 1.6 µm silica particles. It was found that the major platelet oxylipins 12-HETE, 12-HEPE and 14-HDoHE (from 12-LOX) and 12-HHT (from COX-1) were present in S-configuration indicating their enzymatic formation. On the other hand, both R and S enantiomers of 9- and 13-HODE, 11- and 15-HETE were detected, possibly due to enzyme promiscuity rather than non-specific oxidation (by ROS or autoxidation), as confirmed by multi-loop based two-dimensional LC-MS using selective comprehensive mode with achiral RPLC in the 1st dimension and chiral LC in the 2nd using a multiple heart-cutting interface. For 12-HETrE, a peak at the retention time of the R-enantiomer was ruled out as isobaric interference by 2D-LC-MS. In particular, arachidonic acid derivates 12(S)-HHT, 11(R)-HETE and 15(S)-HETE were found to be sensitive to hemin and cGMP modulation.

Lipoxygenases at the Intersection of Infection and Carcinogenesis

The persisting presence of opportunistic pathogens like Pseudomonas aeruginosa poses a significant threat to many immunocompromised cancer patients with pulmonary infections. This review highlights the complexity of interactions in the host's defensive eicosanoid signaling network and its hijacking by pathogenic bacteria to their own advantage. Human lipoxygenases (ALOXs) and their mouse counterparts are integral elements of the innate immune system, mostly operating in the pro-inflammatory mode. Taking into account the indispensable role of inflammation in carcinogenesis, lipoxygenases have counteracting roles in this process. In addition to describing the structure-function of lipoxygenases in this review, we discuss their roles in such critical processes as cancer cell signaling, metastases, death of cancer and immune cells through ferroptosis, as well as the roles of ALOXs in carcinogenesis promoted by pathogenic infections. Finally, we discuss perspectives of novel oncotherapeutic approaches to harness lipoxygenase signaling in tumors.

Enantioselectivity in some physiological and pathophysiological roles of hydroxyeicosatetraenoic acids

The phenomenon of chirality has been shown to greatly impact drug activities and effects. Different enantiomers may exhibit different effects in a certain biological condition or disease state. Cytochrome P450 (CYP) enzymes metabolize arachidonic acid (AA) into a large variety of metabolites with a wide range of activities. Hydroxylation of AA by CYP hydroxylases produces hydroxyeicosatetraenoic acids (HETEs), which are classified into mid-chain (5, 8, 9, 11, 12, and 15-HETE), subterminal (16-, 17-, 18- and 19-HETE) and terminal (20-HETE) HETEs. Except for 20-HETE, these metabolites exist as a racemic mixture of R and S enantiomers in the physiological system. The two enantiomers could have different degrees of activity or sometimes opposing effects. In this review article, we aimed to discuss the role of mid-chain and subterminal HETEs in different organs, importantly the heart and the kidneys. Moreover, we summarized their effects in some conditions such as neutrophil migration, inflammation, angiogenesis, and tumorigenesis, with a focus on the reported enantiospecific effects. We also reported some studies using genetically modified models to investigate the roles of HETEs in different conditions.

Design, synthesis and evaluation of 2-aryl quinoline derivatives against 12R-lipoxygenase (12R-LOX): Discovery of first inhibitor of 12R-LOX

The 12R-lipoxygenase (12R-LOX), a (non-heme) iron-containing metalloenzyme belonging to the lipoxygenase (LOX) family catalyzes the conversion of arachidonic acid (AA) to its key metabolites. Studies suggested that 12R-LOX plays a critical role in immune modulation for the maintenance of skin homeostasis and therefore can be considered as a potential drug target for psoriasis and other skin related inflammatory diseases. However, unlike 12-LOX (or 12S-LOX) the enzyme 12R-LOX did not receive much attention till date. In our effort, the 2-aryl quinoline derivatives were designed, synthesized and evaluated for the identification of potential inhibitors of 12R-hLOX. The merit of selection of 2-aryl quinolines was assessed by in silico docking studies of a representative compound (4a) using the homology model of 12R-LOX. Indeed, in addition to participating in H-bonding with THR628 and LEU635 the molecule formed a hydrophobic interaction with VAL631. The desired 2-aryl quinolines were synthesized either via the Claisen-Schmidt condensation followed by one-pot reduction-cyclization or via the AlCl3 induced heteroarylation or via the O-alkylation approach in good to high (82-95%) yield. When screened against human 12R-LOX (12R-hLOX) in vitro four compounds (e.g. 4a, 4d, 4e and 7b) showed encouraging (>45%) inhibition at 100 μM among which 7b and 4a emerged as the initial hits. Both the compounds showed selectivity towards 12R-hLOX over 12S-hLOX, 15-hLOX and 15-hLOXB and concentration dependent inhibition of 12R-hLOX with IC50 = 12.48 ± 2.06 and 28.25 ± 1.63 μM, respectively. The selectivity of 4a and 7b towards 12R-LOX over 12S-LOX was rationalized with the help of molecular dynamics simulations. The SAR (Structure-Activity Relationship) within the present series of compounds suggested the need of a o-hydroxyl group on the C-2 phenyl ring for the activity. The compound 4a and 7b (at 10 and 20 µM) reduced the hyper-proliferative state and colony forming potential of IMQ-induced psoriatic keratinocytes in a concentration dependent manner. Further, both compounds decreased the protein levels of Ki67 and the mRNA expression of IL-17A in the IMQ-induced psoriatic-like keratinocytes. Notably, 4a but not 7b inhibited the production of IL-6 and TNF-α in the keratinocyte cells. In the preliminary toxicity studies (i.e. teratogenicity, hepatotoxicity and heart rate assays) in zebrafish both the compounds showed low safety (<30 µM) margin. Overall, being the first identified inhibitors of 12R-LOX both 4a and 7b deserve further investigations.

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.

Moisturizing at a molecular level - The basis of Corneocare

BACKGROUND

This review covers the last 20 years of research we and our collaborators have conducted on ethnic differences in facial skin moisturization placed in historical context with previous research.

METHODS

We have focussed particularly on the biochemical and cellular gradients of the stratum corneum (SC) with the aim of discovering new skin moisturization and SC maturation mechanisms, identifying new technologies and/or providing conceptual innovations for ingredients that will improve our understanding and treatment of dry skin. Specifically, we discuss gradients for corneodesmosomes and proteases, corneocyte phenotype-inducing enzymes, filaggrin and natural moisturizing factor (NMF), and barrier lipids. These gradients are interdependent and influence greatly corneocyte maturation.

RESULTS

The interrelationship between corneodesmolysis and the covalent attachment of ω-hydroxy ceramides and ω-hydroxy fatty acids to the corneocyte protein envelope forming the corneocyte lipid envelope is especially relevant in our new understanding of mechanisms leading to dry skin. This process is initiated by a linoleoyl-ω-acyl ceramide transforming enzyme cascade including 12R lipoxygenase (12R-LOX), epidermal lipoxygenase-3 (eLOX3), epoxide hydrolase 3 (EPHX3), short-chain dehydrogenase/reductase family 9C member 7 (SDR9C7), ceramidase and transglutaminase 1.

CONCLUSION

Our research has opened the opportunity of using novel treatment systems for dry skin based on lipids, humectants, niacinamide and inhibitors of the plasminogen system. It is clear that skin moisturization is a more complex mechanism than simple skin hydration.

Bioconversion of C20- and C22-polyunsaturated fatty acids into 9S,15S- and 11S,17S-dihydroxy fatty acids by Escherichia coli expressing double-oxygenating 9S-lipoxygenase from Sphingopyxis macrogoltabida

Double-oxygenating lipoxygenase (LOX) converted C20- and C22-polyunsaturated fatty acids (PUFAs) into C20 dihydroxy fatty acids (DiHFAs) as inflammatory mediators and C22 DiHFAs as specialized pro-resolving mediators, which are involved in the resolution of inflammation and infection in humans, and their isomers, respectively. However, the quantitative bioconversion of C20- and C22-PUFAs into 9S,15S- and 11S,17S-DiHFAs has been not attempted to date, respectively. Here, we performed the efficient quantitative production of 9S,15S- and 11S,17S-DiHFAs by Escherichia coli expressing 9S-LOX from Sphingopyxis macrogoltabida. The optimal bioconversion conditions of the C20 PUFA arachidonic acid or the C22-PUFA docosahexaenoic acid were pH 8.5, 35 °C, 6 mM substrate, and 5 g dry cells/L for C20 PUFAs or 7 g dry cells/L for C22 PUFAs, respectively. Under these conditions, E. coli expressing double-oxygenating 9S-LOX from S. macrogoltabida converted arachidonic acid, eicosapentaenoic acid, docosapentaenoic acid_n-3, and docosahexaenoic acid into 5.85 mM 9S,15S-dihydroxyeicosatetraenoic acid, 5.79 mM 9S,15S-dihydroxyeicosapentaenoic acid, 5.89 mM 11S,17S-hydroxydocosapentaenoic acid_n-3, and 5.24 mM 11S,17S-dihydroxydocosahexaenoic acid in 40, 30, 50, and 60 min, with conversion yields of 97.5%, 96.5%, 98.1%, and 87.3%; and volumetric productivities of 8.78, 11.6, 7.07, and 5.24 mM/h, respectively. To date, these are the highest concentrations, conversion yields, and volumetric productivities reported in the bioconversion of C20- and C22-PUFAs into DiHFAs.

The Reaction Specificity of Mammalian ALOX15 Orthologs is Changed During Late Primate Evolution and These Alterations Might Offer Evolutionary Advantages for Hominidae

Arachidonic acid lipoxygenases (ALOXs) have been implicated in the immune response of mammals. The reaction specificity of these enzymes is decisive for their biological functions and ALOX classification is based on this enzyme property. Comparing the amino acid sequences and the functional properties of selected mammalian ALOX15 orthologs we previously hypothesized that the reaction specificity of these enzymes can be predicted based on their amino acid sequences (Triad Concept) and that mammals, which are ranked in evolution below gibbons, express arachidonic acid 12-lipoxygenating ALOX15 orthologs. In contrast, Hominidae involving the great apes and humans possess 15-lipoxygenating enzymes (Evolutionary Hypothesis). These two hypotheses were based on sequence data of some 60 mammalian ALOX15 orthologs and about half of them were functionally characterized. Here, we compared the ALOX15 sequences of 152 mammals representing all major mammalian subclades expressed 44 novel ALOX15 orthologs and performed extensive mutagenesis studies of their triad determinants. We found that ALOX15 genes are absent in extant Prototheria but that corresponding enzymes frequently occur in Metatheria and Eutheria. More than 90% of them catalyze arachidonic acid 12-lipoxygenation and the Triad Concept is applicable to all of them. Mammals ranked in evolution above gibbons express arachidonic acid 15-lipoxygenating ALOX15 orthologs but enzymes with similar specificity are only present in less than 5% of mammals ranked below gibbons. This data suggests that ALOX15 orthologs have been introduced during Prototheria-Metatheria transition and put the Triad Concept and the Evolutionary Hypothesis on a much broader and more reliable experimental basis.

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.

Lipidomic and transcriptional analysis of the linoleoyl-omega-hydroxyceramide biosynthetic pathway in human psoriatic lesions

A complex assembly of lipids including fatty acids, cholesterol, and ceramides is vital to the integrity of the mammalian epidermal barrier. The formation of this barrier requires oxidation of the substrate fatty acid, linoleic acid (LA), which is initiated by the enzyme 12R-lipoxygenase (LOX). In the epidermis, unoxidized LA is primarily found in long-chain acylceramides termed esterified omega-hydroxy sphingosine (EOS)/phytosphingosine/hydroxysphingosine (collectively EOx). The precise structure and localization of LOX-oxidized EOx in the human epidermis is unknown, as is their regulation in diseases such as psoriasis, one of the most common inflammatory diseases affecting the skin. Here, using precursor LC/MS/MS, we characterized multiple intermediates of EOx, including 9-HODE, 9,10-epoxy-13-HOME, and 9,10,13-TriHOME, in healthy human epidermis likely to be formed via the epidermal LOX pathways. The top layers of the skin contained more LA, 9-HODE, and 9,10,13-TriHOME EOSs, whereas 9,10-epoxy-13-HOME EOS was more prevalent deeper in the stratum corneum. In psoriatic lesions, levels of native EOx and free HODEs and HOMEs were significantly elevated, whereas oxidized species were generally reduced. A transcriptional network analysis of human psoriatic lesions identified significantly elevated expression of the entire biosynthetic/metabolic pathway for oxygenated ceramides, suggesting a regulatory function for EOx lipids in reconstituting epidermal integrity. The role of these new lipids in progression or resolution of psoriasis is currently unknown. We also discovered the central coordinated role of the zinc finger protein transcription factor, ZIC1, in driving the phenotype of this disease. In summary, long-chain oxygenated ceramide metabolism is dysregulated at the lipidomic level in psoriasis, likely driven by the transcriptional differences also observed, and we identified ZIC1 as a potential regulatory target for future therapeutic interventions.

Molecular Pathways Linking Oxylipins to Nociception in Rats

Oxylipins are lipid peroxidation products that participate in nociceptive, inflammatory, and vascular responses to injury. Effects of oxylipins depend on tissue-specific differences in accumulation of precursor polyunsaturated fatty acids and the expression of specific enzymes to transform the precursors. The study of oxylipins in nociception has presented technical challenges leading to critical knowledge gaps in the way these molecules operate in nociception. We applied a systems-based approach to characterize oxylipin precursor fatty acids, and expression of genes coding for proteins involved in biosynthesis, transport, signaling and inactivation of pro- and antinociceptive oxylipins in pain circuit tissues. We further linked these pathways to nociception by demonstrating intraplantar carrageenan injection induced gene expression changes in oxylipin biosynthetic pathways. We determined functional-biochemical relevance of the proposed pathways in rat hind paw and dorsal spinal cord by measuring basal and stimulated levels of oxylipins throughout the time-course of carrageenan-induced inflammation. Finally, when oxylipins were administered by intradermal injection we observed modulation of nociceptive thermal hypersensitivity, providing a functional-behavioral link between oxylipins, their molecular biosynthetic pathways, and involvement in pain and nociception. Together, these findings advance our understanding of molecular lipidomic systems linking oxylipins and their precursors to nociceptive and inflammatory signaling pathways in rats. PERSPECTIVE: We applied a systems approach to characterize molecular pathways linking precursor lipids and oxylipins to nociceptive signaling. This systematic, quantitative evaluation of the molecular pathways linking oxylipins to nociception provides a framework for future basic and clinical research investigating the role of oxylipins in pain.

Structural considerations on lipoxygenase function, inhibition and crosstalk with nitric oxide pathways

Lipoxygenases (LOX) are non-heme iron-containing enzymes that catalyze regio- and stereo-selective dioxygenation of polyunsaturated fatty acids (PUFA). Mammalian LOXs participate in the eicosanoid cascade during the inflammatory response, using preferentially arachidonic acid (AA) as substrate, for the synthesis of leukotrienes (LT) and other oxidized-lipid intermediaries. This review focus on lipoxygenases (LOX) structural and kinetic implications on both catalysis selectivity, as well as the basic and clinical implications of inhibition and interactions with nitric oxide (^•NO) and nitroalkenes pathways. During inflammation ^•NO levels are increasingly favoring the formation of reactive nitrogen species (RNS). ^•NO may act itself as an inhibitor of LOX-mediated lipid oxidation by reacting with lipid peroxyl radicals. Besides, ^•NO may act as an O₂ competitor in the LOX active site, thus displaying a protective role on lipid-peroxidation. Moreover, RNS such as nitrogen dioxide (^•NO₂) may react with lipid-derived species formed during LOX reaction, yielding nitroalkenes (NO₂FA). NO₂FA represents electrophilic compounds that could exert anti-inflammatory actions through the interaction with critical LOX nucleophilic amino acids. We will discuss how nitro-oxidative conditions may limit the availability of common LOX substrates, favoring alternative routes of PUFA metabolization to anti-inflammatory or pro-resolutive pathways.

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.

The Biosynthesis of Enzymatically Oxidized Lipids

Enzymatically oxidized lipids are a specific group of biomolecules that function as key signaling mediators and hormones, regulating various cellular and physiological processes from metabolism and cell death to inflammation and the immune response. They are broadly categorized as either polyunsaturated fatty acid (PUFA) containing (free acid oxygenated PUFA "oxylipins", endocannabinoids, oxidized phospholipids) or cholesterol derivatives (oxysterols, steroid hormones, and bile acids). Their biosynthesis is accomplished by families of enzymes that include lipoxygenases (LOX), cyclooxygenases (COX), cytochrome P450s (CYP), and aldo-keto reductases (AKR). In contrast, non-enzymatically oxidized lipids are produced by uncontrolled oxidation and are broadly considered to be harmful. Here, we provide an overview of the biochemistry and enzymology of LOXs, COXs, CYPs, and AKRs in humans. Next, we present biosynthetic pathways for oxylipins, oxidized phospholipids, oxysterols, bile acids and steroid hormones. Last, we address gaps in knowledge and suggest directions for future work.

MS-based targeted metabolomics of eicosanoids and other oxylipins: Analytical and inter-individual variabilities

Oxylipins, including the well-known eicosanoids, are potent lipid mediators involved in numerous physiological and pathological processes. Therefore, their quantitative profiling has gained a lot of attention during the last years notably in the active field of health biomarker discovery. Oxylipins include hundreds of structurally and stereochemically distinct lipid species which today are most commonly analyzed by (ultra) high performance liquid chromatography-mass spectrometry based ((U)HPLC-MS) methods. To maximize the utility of oxylipin profiling in clinical research, it is crucial to understand and assess the factors contributing to the analytical and biological variability of oxylipin profiles in humans. In this review, these factors and their impacts are summarized and discussed, providing a framework for recommendations expected to enhance the interlaboratory comparability and biological interpretation of oxylipin profiling in clinical research.

Endocervical and Neutrophil Lipoxygenases Coordinate Neutrophil Transepithelial Migration to Neisseria gonorrhoeae

Background

Infection with Neisseria gonorrhoeae (GC) is characterized by robust neutrophil influx that is insufficient to clear the bacteria. Sustained neutrophilic inflammation contributes to serious clinical sequelae that particularly affect women, including pelvic inflammatory disease and infertility.

Methods

We established a 3-component system using GC, End1 polarized human endocervical cells, and primary human neutrophils to investigate neutrophil transepithelial migration following infection.

Results

Neutrophil migration across endocervical monolayers increased with the infectious dose and required GC-epithelial cell contact. Epithelial protein kinase C, cytosolic phospholipase A2, 12R-lipoxygenase (LOX), and eLOX3 hepoxilin synthase were required for neutrophil transmigration to GC, and migration was abrogated by blocking the MRP2 efflux pump and by adding recombinant soluble epoxide hydrolase. These results are all consistent with epithelial cell production of the neutrophil chemoattractant hepoxilin A3 (HXA3). Neutrophil transmigration was also accompanied by increasing apical concentrations of leukotriene B4 (LTB4). Neutrophil 5-lipoxygenase and active BLT1 receptor were required for apical LTB4 and neutrophil migration.

Conclusions

Our data support a model in which GC-endocervical cell contact infection stimulates HXA3 production, driving neutrophil migration that is amplified by neutrophil-derived LTB4. Therapeutic targeting of these pathways could limit inflammation and deleterious clinical sequelae in women with gonorrhea.

12R-lipoxygenase activity is reduced in photodamaged facial stratum corneum. A novel activity assay indicates a key function in corneocyte maturation

Background

During the late stage of keratinocyte differentiation, corneocytes gain a strong protein–lipid structure: the corneocyte envelopes (CE), composed of the inner corneocyte protein envelope (CPE) and the outer corneocyte lipid envelope (CLE). The hydrophobicity of CEs depends on the covalent attachment of linoleoyl‐acyl‐ceramides by transglutaminases (TG). These ceramides are processed by a range of other enzymes, including 12R‐lipoxygenase (12R‐LOX), before the covalent attachment of the free ω‐hydroxyceramides to the CPE surface to form the CLE. The mechanical strength of CE is obtained with the formation of isodipeptide bonds by TG. The increase in hydrophobicity and rigidity leads to CE maturation which supports the integrity and mechanical resistance of the stratum corneum (SC).

Objectives

The aim of this work was to develop and validate a novel enzyme activity assay for 12R‐LOX in tape strippings of photo‐exposed (PE) cheek and photo‐protected (PP) post‐auricular SC of healthy Chinese volunteers (n = 12; age 25 ± 3 years).

Results

A fluorescence‐based assay was developed with ethyl linoleic acid as the substrate and a polyclonal antibody against 12R‐LOX as an inhibitor. The specificity was shown by the lack of effect by a LOX inhibitor (ML351) and an epidermal‐type lipoxygenase 3 (eLOX3) antibody on the acquired 12R‐LOX activity. Reduced 12R‐LOX activity was observed in the outer compared to the inner SC layers. Moreover, dramatically lower activity was shown in the PE vs. PP samples. Furthermore, the enzyme activity has a positive correlation (r = 0.94 ± 0.03) with CE maturity, in particular hydrophobicity, and a negative correlation (r = −0.96 ± 0.01) with transepidermal water loss (TEWL).

Conclusion

This novel enzyme assay revealed a lower 12R‐LOX activity in tape strippings from PE cheek for the first time. This finding is in line with less mature CEs and higher TEWL compared to PP post‐auricular samples. This study indicates a strong link between 12R‐LOX activity and CE maturation and SC integrity.

Probing the Electrostatic and Steric Requirements for Substrate Binding in Human Platelet-Type 12-Lipoxygenase

Human platelet ALOX12 (hALOX12 or h12-LOX) has been implicated in a variety of human diseases. The present study investigates the active site of hALOX12 to more thoroughly understand how it positions the substrate and achieves nearly perfect regio- and stereospecificities (i.e., 100 ± 5% of the 12(S)-hydroperoxide product), utilizing site-directed mutagenesis. Specifically, we have determined that Arg402 is not as important in substrate binding as previously seen for hALOX15 but that His596 may play a role in anchoring the carboxy terminal of the arachidonic acid during catalysis. In addition, Phe414 creates a π-stacking interaction with a double bond of arachidonic acid (Δ¹¹), and Ala417/Val418 define the bottom of the cavity. However, the influence of Ala417/Val418 on the profile is markedly less for hALOX12 than that seen in hALOX15. Mutating these two residues to larger amino acids (Ala417Ile/Val418Met) only increased the generation of 15-HpETE by 24 ± 2%, but conversely, smaller residues at these positions converted hALOX15 to almost 100% hALOX12 reactivity [Gan et al. (1996) J. Biol. Chem. 271, 25412-25418]. However, we were able to increase 15-HpETE to 46 ± 3% by restricting the width of the active site with the Ala417Ile/Val418Met/Ser594Thr mutation, indicating both depth and width of the active site are important. Finally, residue Leu407 is shown to play a critical role in positioning the substrate correctly, as seen by the increase of 15-HpETE to 21 ± 1% for the single Leu407Gly mutant. These results outline critical differences between the active site requirements of hALOX12 relative to hALOX15 and explain both their product specificity and inhibitory differences.

Emerging role of 12/15-Lipoxygenase (ALOX15) in human pathologies

12/15-lipoxygenase (12/15-LOX) is an enzyme, which oxidizes polyunsaturated fatty acids, particularly omega-6 and -3 fatty acids, to generate a number of bioactive lipid metabolites. A large number of studies have revealed the importance of 12/15-LOX role in oxidative and inflammatory responses. The in vitro studies have demonstrated the ability of 12/15-LOX metabolites in the expression of various genes and production of cytokine related to inflammation and resolution of inflammation. The studies with the use of knockout and transgenic animals for 12/15-LOX have further shown its involvement in the pathogenesis of a variety of human diseases, including cardiovascular, renal, neurological and metabolic disorders. This review summarizes our current knowledge on the role of 12/15-LOX in inflammation and various human diseases.

Hepatocyte ALOXE3 is induced during adaptive fasting and enhances insulin sensitivity by activating hepatic PPARγ

The hepatic glucose fasting response is gaining traction as a therapeutic pathway to enhance hepatic and whole-host metabolism. However, the mechanisms underlying these metabolic effects remain unclear. Here, we demonstrate the epidermal-type lipoxygenase, eLOX3 (encoded by its gene, Aloxe3), is a potentially novel effector of the therapeutic fasting response. We show that Aloxe3 is activated during fasting, glucose withdrawal, or trehalose/trehalose analogue treatment. Hepatocyte-specific Aloxe3 expression reduced weight gain and hepatic steatosis in diet-induced and genetically obese (db/db) mouse models. Aloxe3 expression, moreover, enhanced basal thermogenesis and abrogated insulin resistance in db/db diabetic mice. Targeted metabolomics demonstrated accumulation of the PPARγ ligand 12-KETE in hepatocytes overexpressing Aloxe3. Strikingly, PPARγ inhibition reversed hepatic Aloxe3–mediated insulin sensitization, suppression of hepatocellular ATP production and oxygen consumption, and gene induction of PPARγ coactivator-1α (PGC1α) expression. Moreover, hepatocyte-specific PPARγ deletion reversed the therapeutic effect of hepatic Aloxe3 expression on diet-induced insulin intolerance. Aloxe3 is, therefore, a potentially novel effector of the hepatocellular fasting response that leverages both PPARγ-mediated and pleiotropic effects to augment hepatic and whole-host metabolism, and it is, thus, a promising target to ameliorate metabolic disease.

The lipoxygenase ALOXE3 is an effector of the hepatic fasting response that improves insulin sensitivity by activating hepatic PPARγ.

Characterization of Epidermal Lipoxygenase Expression in Normal Human Skin and Tissue-Engineered Skin Substitutes

Lipoxygenases (LOXs) are enzymes likely to be involved in corneocyte lipid envelope formation and skin barrier function. In humans, mutations in epidermis-type lipoxygenase 3 ( eLOX-3) and 12R-lipoxygenase ( 12R-LOX) genes are associated with autosomal recessive congenital ichthyosis (ARCI), whereas deletion of these genes in mice causes epidermal defects. LOXs also represent a matter of interest in psoriasis as well as in cancer research. However, their expression as well as the exact role of these enzymes in normal human skin have not been fully described. Our goal was to characterize the expression of epidermal LOXs in both normal human skin and Tissue-Engineered Skin Substitutes (TESS) and to consider TESS as a potential model for LOX functional studies. Staining for epidermal differentiation markers and LOXs was performed, in parallel, on normal human skin and TESS. Our results showed similar expression profiles in TESS when compared with native skin for e-LOX3, 12R-LOX, 12S-lipoxygenase (12S-LOX), and 15-lipoxygenase 2 (15-LOX-2) but not for 15-lipoxygenase 1 (15-LOX-1). Because of their appropriate epidermal differentiation and LOX expression, TESS represent an alternative model for future studies on LOX function.

Fatty acids and related lipid mediators in the regulation of cutaneous inflammation

Human skin has a distinct profile of fatty acids and related bioactive lipid mediators that regulate many aspects of epidermal and dermal homeostasis, including immune and inflammatory reactions. Sebum lipids act as effective antimicrobial agents, shape immune cell communications and contribute to the epidermal lipidome. The essential fatty acid linoleic acid is crucial for the structure of the epidermal barrier, while polyunsaturated fatty acids act as precursors to eicosanoids, octadecanoids and docosanoids through cyclooxygenase, lipoxygenase and cytochrome P450 monooxygenase-mediated reactions, and endocannabinoids and N-acyl ethanolamines. Cross-communication between these families of bioactive lipids suggests that their cutaneous activities should be considered as part of a wider metabolic network that can be targeted to maintain skin health, control inflammation and improve skin pathologies.

A systems approach for discovering linoleic acid derivatives that potentially mediate pain and itch

Chronic pain and itch are common hypersensitivity syndromes that are affected by endogenous mediators. We applied a systems-based, translational approach to predict, discover, and characterize mediators of pain and itch that are regulated by diet and inflammation. Profiling of tissue-specific precursor abundance and biosynthetic gene expression predicted that inflamed skin would be abundant in four previously unknown 11-hydroxy-epoxy- or 11-keto-epoxy-octadecenoate linoleic acid derivatives and four previously identified 9- or 13-hydroxy-epoxy- or 9- or 13-keto-epoxy-octadecenoate linoleic acid derivatives. All of these mediators were confirmed to be abundant in rat and human skin by mass spectrometry. However, only the two 11-hydroxy-epoxy-octadecenoates sensitized rat dorsal root ganglion neurons to release more calcitonin gene-related peptide (CGRP), which is involved in pain transmission, in response to low pH (which mimics an inflammatory state) or capsaicin (which activates ion channels involved in nociception). The two 11-hydroxy-epoxy-octadecenoates share a 3-hydroxy-Z-pentenyl-E-epoxide moiety, thus suggesting that this substructure could mediate nociceptor sensitization. In rats, intradermal hind paw injection of 11-hydroxy-12,13-trans-epoxy-(9Z)-octadecenoate elicited C-fiber-mediated sensitivity to thermal pain. In a randomized trial testing adjunctive strategies to manage refractory chronic headaches, reducing the dietary intake of linoleic acid was associated with decreases in plasma 11-hydroxy-12,13-trans-epoxy-(9Z)-octadecenoate, which correlated with clinical pain reduction. Human psoriatic skin had 30-fold higher 9-keto-12,13-trans-epoxy-(10E)-octadecenoate compared to control skin, and intradermal injection of this compound induced itch-related scratching behavior in mice. Collectively, these findings define a family of endogenous mediators with potential roles in pain and itch.

Who is the real 12-HETrE?

Oxygenases, including lipoxygenases and cytochrome P450s, generate an array of structurally diverse oxylipins that modulate distinct biological responses in mammals. Depending on the source of tissues and enzymes, distinct oxylipins are generated with inherent cellular function. Here, we report structurally different forms of 12-HETrE, with distinct biological function in tissues as well as their derived enzymatic source.

The crystal structure of Pseudomonas aeruginosa lipoxygenase Ala420Gly mutant explains the improved oxygen affinity and the altered reaction specificity

Secreted LOX from Pseudomonas aeruginosa (PA-LOX) has previously been identified as arachidonic acid 15S-lipoxygenating enzyme. Here we report that the substitution of Ala420Gly in PA-LOX leads to an enzyme variant with pronounced dual specificity favoring arachidonic acid 11R-oxygenation. When compared with other LOX-isoforms the molecular oxygen affinity of wild-type PA-LOX is 1-2 orders of magnitude lower (Km O₂ of 0.4mM) but Ala420Gly exchange improved the molecular oxygen affinity (Km O₂ of 0.2mM). Experiments with stereo-specifically deuterated linoleic acid indicated that the formation of both 13S- and 9R-HpODE involves abstraction of the proS-hydrogen from C11 of the fatty acid backbone. To explore the structural basis for the observed functional changes (altered specificity, improved molecular oxygen affinity) we solved the crystal structure of the Ala420Gly mutant of PA-LOX at 1.8Å resolution and compared it with the wild-type enzyme. Modeling of fatty acid alignment at the catalytic center suggested that in the wild-type enzyme dioxygen is directed to C15 of arachidonic acid by a protein tunnel, which interconnects the catalytic center with the protein surface. Ala420Gly exchange redirects intra-enzyme O₂ diffusion by bifurcating this tunnel so that C11 of arachidonic acid also becomes accessible for O₂ insertion.

The role of lipoxygenases in pathophysiology; new insights and future perspectives

Lipoxygenases (LOXs) are dioxygenases that catalyze the formation of corresponding hydroperoxides from polyunsaturated fatty acids such as linoleic acid and arachidonic acid. LOX enzymes are expressed in immune, epithelial, and tumor cells that display a variety of physiological functions, including inflammation, skin disorder, and tumorigenesis. In the humans and mice, six LOX isoforms have been known. 15-LOX, a prototypical enzyme originally found in reticulocytes shares the similarity of amino acid sequence as well as the biochemical property to plant LOX enzymes. 15-LOX-2, which is expressed in epithelial cells and leukocytes, has different substrate specificity in the humans and mice, therefore, the role of them in mammals has not been established. 12-LOX is an isoform expressed in epithelial cells and myeloid cells including platelets. Many mutations in this isoform are found in epithelial cancers, suggesting a potential link between 12-LOX and tumorigenesis. 12R-LOX can be found in the epithelial cells of the skin. Defects in this gene result in ichthyosis, a cutaneous disorder characterized by pathophysiologically dried skin due to abnormal loss of water from its epithelial cell layer. Similarly, eLOX-3, which is also expressed in the skin epithelial cells acting downstream 12R-LOX, is another causative factor for ichthyosis. 5-LOX is a distinct isoform playing an important role in asthma and inflammation. This isoform causes the constriction of bronchioles in response to cysteinyl leukotrienes such as LTC4, thus leading to asthma. It also induces neutrophilic inflammation by its recruitment in response to LTB4. Importantly, 5-LOX activity is strictly regulated by 5-LOX activating protein (FLAP) though the distribution of 5-LOX in the nucleus. Currently, pharmacological drugs targeting FLAP are actively developing. This review summarized these functions of LOX enzymes under pathophysiological conditions in mammals.

Distribution of bioactive lipid mediators in human skin

The skin produces bioactive lipids that participate in physiological and pathological states, including homeostasis, induction, propagation, and resolution of inflammation. However, comprehension of the cutaneous lipid complement, and contribution to differing roles of the epidermal and dermal compartments, remains incomplete. We assessed the profiles of eicosanoids, endocannabinoids, N-acyl ethanolamides, and sphingolipids, in human dermis, epidermis, and suction blister fluid. We identified 18 prostanoids, 12 hydroxy-fatty acids, 9 endocannabinoids and N-acyl ethanolamides, and 21 non-hydroxylated ceramides and sphingoid bases, several demonstrating significantly different expression in the tissues assayed. The array of dermal and epidermal fatty acids was reflected in the lipid mediators produced, whereas similarities between lipid profiles in blister fluid and epidermis indicated a primarily epidermal origin of suction blister fluid. Supplementation with omega-3 fatty acids ex vivo showed that their action is mediated through perturbation of existing species and formation of other anti-inflammatory lipids. These findings demonstrate the diversity of lipid mediators involved in maintaining tissue homeostasis in resting skin and hint at their contribution to signaling, cross-support, and functions of different skin compartments. Profiling lipid mediators in biopsies and suction blister fluid can support studies investigating cutaneous inflammatory responses, dietary manipulation, and skin diseases lacking biomarkers and therapeutic targets.

Evolutionary aspects of lipoxygenases and genetic diversity of human leukotriene signaling

Leukotrienes are pro-inflammatory lipid mediators, which are biosynthesized via the lipoxygenase pathway of the arachidonic acid cascade. Lipoxygenases form a family of lipid peroxidizing enzymes and human lipoxygenase isoforms have been implicated in the pathogenesis of inflammatory, hyperproliferative (cancer) and neurodegenerative diseases. Lipoxygenases are not restricted to humans but also occur in a large number of pro- and eucaryotic organisms. Lipoxygenase-like sequences have been identified in the three domains of life (bacteria, archaea, eucarya) but because of lacking functional data the occurrence of catalytically active lipoxygenases in archaea still remains an open question. Although the physiological and/or pathophysiological functions of various lipoxygenase isoforms have been studied throughout the last three decades there is no unifying concept for the biological importance of these enzymes. In this review we are summarizing the current knowledge on the distribution of lipoxygenases in living single and multicellular organisms with particular emphasis to higher vertebrates and will also focus on the genetic diversity of enzymes and receptors involved in human leukotriene signaling.

Leukotriene signaling in the extinct human subspecies Homo denisovan and Homo neanderthalensis. Structural and functional comparison with Homo sapiens

Mammalian lipoxygenases (LOXs) have been implicated in cell differentiation and in the biosynthesis of pro- and anti-inflammatory lipid mediators. The initial draft sequence of the Homo neanderthalensis genome (coverage of 1.3-fold) suggested defective leukotriene signaling in this archaic human subspecies since expression of essential proteins appeared to be corrupted. Meanwhile high quality genomic sequence data became available for two extinct human subspecies (H. neanderthalensis, Homo denisovan) and completion of the human 1000 genome project provided a comprehensive database characterizing the genetic variability of the human genome. For this study we extracted the nucleotide sequences of selected eicosanoid relevant genes (ALOX5, ALOX15, ALOX12, ALOX15B, ALOX12B, ALOXE3, COX1, COX2, LTA4H, LTC4S, ALOX5AP, CYSLTR1, CYSLTR2, BLTR1, BLTR2) from the corresponding databases. Comparison of the deduced amino acid sequences in connection with site-directed mutagenesis studies and structural modeling suggested that the major enzymes and receptors of leukotriene signaling as well as the two cyclooxygenase isoforms were fully functional in these two extinct human subspecies.

Biosynthesis, biological effects, and receptors of hydroxyeicosatetraenoic acids (HETEs) and oxoeicosatetraenoic acids (oxo-ETEs) derived from arachidonic acid

Arachidonic acid can be oxygenated by a variety of different enzymes, including lipoxygenases, cyclooxygenases, and cytochrome P450s, and can be converted to a complex mixture of oxygenated products as a result of lipid peroxidation. The initial products in these reactions are hydroperoxyeicosatetraenoic acids (HpETEs) and hydroxyeicosatetraenoic acids (HETEs). Oxoeicosatetraenoic acids (oxo-ETEs) can be formed by the actions of various dehydrogenases on HETEs or by dehydration of HpETEs. Although a large number of different HETEs and oxo-ETEs have been identified, this review will focus principally on 5-oxo-ETE, 5S-HETE, 12S-HETE, and 15S-HETE. Other related arachidonic acid metabolites will also be discussed in less detail. 5-Oxo-ETE is synthesized by oxidation of the 5-lipoxygenase product 5S-HETE by the selective enzyme, 5-hydroxyeicosanoid dehydrogenase. It actions are mediated by the selective OXE receptor, which is highly expressed on eosinophils, suggesting that it may be important in eosinophilic diseases such as asthma. 5-Oxo-ETE also appears to stimulate tumor cell proliferation and may also be involved in cancer. Highly selective and potent OXE receptor antagonists have recently become available and could help to clarify its pathophysiological role. The 12-lipoxygenase product 12S-HETE acts by the GPR31 receptor and promotes tumor cell proliferation and metastasis and could therefore be a promising target in cancer therapy. It may also be involved as a proinflammatory mediator in diabetes. In contrast, 15S-HETE may have a protective effect in cancer. In addition to GPCRs, higher concentration of HETEs and oxo-ETEs can activate peroxisome proliferator-activated receptors (PPARs) and could potentially regulate a variety of processes by this mechanism. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Mammalian lipoxygenases and their biological relevance

Lipoxygenases (LOXs) form a heterogeneous class of lipid peroxidizing enzymes, which have been implicated not only in cell proliferation and differentiation but also in the pathogenesis of various diseases with major public health relevance. As other fatty acid dioxygenases LOXs oxidize polyunsaturated fatty acids to their corresponding hydroperoxy derivatives, which are further transformed to bioactive lipid mediators (eicosanoids and related substances). On the other hand, lipoxygenases are key players in the regulation of the cellular redox homeostasis, which is an important element in gene expression regulation. Although the first mammalian lipoxygenases were discovered 40 years ago and although the enzymes have been well characterized with respect to their structural and functional properties the biological roles of the different lipoxygenase isoforms are not completely understood. This review is aimed at summarizing the current knowledge on the physiological roles of different mammalian LOX-isoforms and their patho-physiological function in inflammatory, metabolic, hyperproliferative, neurodegenerative and infectious disorders. This article is part of a Special Issue entitled "Oxygenated metabolism of PUFA: analysis and biological relevance".

Lysophospholipid acyltransferases and eicosanoid biosynthesis in zebrafish myeloid cells

Eicosanoids derived from the enzymatic oxidation of arachidonic acid play important roles in a large number of physiological and pathological processes in humans. Many animal and cellular models have been used to investigate the intricate mechanisms regulating their biosynthesis and actions. Zebrafish is a widely used model to study the embryonic development of vertebrates. It expresses homologs of the key enzymes involved in eicosanoid production, and eicosanoids have been detected in extracts from adult or embryonic fish. In this study we prepared cell suspensions from kidney marrow, the main hematopoietic organ in fish. Upon stimulation with calcium ionophore, these cells produced eicosanoids including PGE2, LTB4, 5-HETE and, most abundantly, 12-HETE. They also produced small amounts of LTB5 derived from eicosapentaenoic acid. These eicosanoids were also produced in kidney marrow cells stimulated with ATP, and this production was greatly enhanced by preincubation with thimerosal, an inhibitor of arachidonate reacylation into phospholipids. Microsomes from these cells exhibited acyltransferase activities consistent with expression of MBOAT5/LPCAT3 and MBOAT7/LPIAT1, the main arachidonoyl-CoA:lysophospholipid acyltransferases. In summary, this work introduces a new cellular model to study the regulation of eicosanoid production through a phospholipid deacylation-reacylation cycle from a well-established, versatile vertebrate model species.

12/15-Lipoxygenase deficiency reduces densities of mesenchymal stem cells in the dermis of wounded and unwounded skin

BACKGROUND

Mesenchymal stem cells (MSCs) promote skin healing. 12/15-Lipoxgenase (LOX) is crucial in producing specific lipid mediators in wounded skin. The consequences of 12/15-LOX deficiency in MSC densities in skin are unknown.

OBJECTIVES

To determine the effect of 12/15-LOX deficiency in MSC densities in wounded and unwounded dermis.

METHODS

Full-thickness skin incisional wounds were made to 12/15-LOX-deficient (12/15-LOX(-/-) ) and wild-type (WT) C57BL/6 mice. Wounded skin was collected at 3, 8, or 14 days postwounding (dpw). MSCs were analysed in skin sections using histology. 12S- or 15S-hydroxy-eicosatetraenoic acid (HETE) was analysed using a reversed-phase Chiral liquid chromatography-ultraviolet-tandem mass spectrometer.

RESULTS

There were more stem cell antigen (Sca)1(+) CD29(+) MSCs (cells/field) at 3, 8, and 14 dpw, more Sca1(+) CD106(+) MSCs at 3 and 14 dpw in the wounded dermis, more MSCs in unwounded dermis of WT mice compared with 12/15-LOX(-/-) mice, and more MSCs in the wounded dermis than in the unwounded dermis. For 12/15-LOX(-/-) dermis, Sca1(+) CD106(+) MSCs peaked and Sca1(+) CD29(+) MSCs reached a flat level at 8 dpw. However, for the WT dermis, MSCs increased from 8 to 14 dpw. There were more Sca1(+) CD106(+) MSCs than Sca1(+) CD29(+) MSCs in the 12/15-LOX(-/-) wounded dermis at 8 dpw. However, there were more Sca1(+) CD29(+) MSCs in the 12/15-LOX(-/-) than Sca1(+) CD106(+) MSCs in the WT wounded dermis at 3 dpw, and Sca1(+) CD106(+) MSCs and Sca1(+) CD29(+) MSCs were at comparable levels in other conditions. 12/15-LOX deficiency suppressed levels of 12/15-LOX protein and their products, 12S-HETE and 15S-HETE, in wounds.

CONCLUSIONS

12/15-LOX deficiency reduces MSC densities in the dermis, which correlates with the suppressed 12/15-LOX pathways in wounded and unwounded skin.

The importance of the lipoxygenase-hepoxilin pathway in the mammalian epidermal barrier

This review covers the background to discovery of the two key lipoxygenases (LOX) involved in epidermal barrier function, 12R-LOX and eLOX3, and our current views on their functioning. In the outer epidermis, their consecutive actions oxidize linoleic acid esterified in ω-hydroxy-ceramide to a hepoxilin-related derivative. The relevant background to hepoxilin and trioxilin biochemistry is briefly reviewed. We outline the evidence that linoleate in the ceramide is the natural substrate of the two LOX enzymes and our proposal for its importance in construction of the epidermal water barrier. Our hypothesis is that the oxidation promotes hydrolysis of the oxidized linoleate moiety from the ceramide. The resulting free ω-hydroxyl of the ω-hydroxyceramide is covalently bound to proteins on the surface of the corneocytes to form the corneocyte lipid envelope, a key barrier component. Understanding the role of the LOX enzymes and their hepoxilin products should provide rational approaches to ameliorative therapy for a number of the congenital ichthyoses involving compromised barrier function. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Functional characterization of genetic enzyme variations in human lipoxygenases

Mammalian lipoxygenases play a role in normal cell development and differentiation but they have also been implicated in the pathogenesis of cardiovascular, hyperproliferative and neurodegenerative diseases. As lipid peroxidizing enzymes they are involved in the regulation of cellular redox homeostasis since they produce lipid hydroperoxides, which serve as an efficient source for free radicals. There are various epidemiological correlation studies relating naturally occurring variations in the six human lipoxygenase genes (SNPs or rare mutations) to the frequency for various diseases in these individuals, but for most of the described variations no functional data are available. Employing a combined bioinformatical and enzymological strategy, which included structural modeling and experimental site-directed mutagenesis, we systematically explored the structural and functional consequences of non-synonymous genetic variations in four different human lipoxygenase genes (ALOX5, ALOX12, ALOX15, and ALOX15B) that have been identified in the human 1000 genome project. Due to a lack of a functional expression system we resigned to analyze the functionality of genetic variations in the hALOX12B and hALOXE3 gene. We found that most of the frequent non-synonymous coding SNPs are located at the enzyme surface and hardly alter the enzyme functionality. In contrast, genetic variations which affect functional important amino acid residues or lead to truncated enzyme variations (nonsense mutations) are usually rare with a global allele frequency<0.1%. This data suggest that there appears to be an evolutionary pressure on the coding regions of the lipoxygenase genes preventing the accumulation of loss-of-function variations in the human population.

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Non-synonymous coding variations in human lipoxygenases are mostly rare with a global allele frequency <1%.

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Common ALOX SNPs are mainly localized on the enzyme surface and hardly effect the enzyme functionality.

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hALOX15B Ala416Asp is a newly discovered loss-of-function mutation in the hALOX gene family while inactivity seems to be caused by severe structural alterations.

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Our data indicate that there is evolutionary pressure on these redox enzymes preventing the accumulation of loss-of-function variations in the human population.

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1000 Genome database is a useful tool to analyze the distribution and functionality of variations in genes of interest.

The role of lipoxygenases in epidermis

Lipoxygenases (LOX) are key enzymes in the biosynthesis of a variety of highly active oxylipins which act as signaling molecules involved in the regulation of many biological processes. LOX are also able to oxidize complex lipids and modify membrane structures leading to structural changes that play a role in the maturation and terminal differentiation of various cell types. The mammalian skin represents a tissue with highly abundant and diverse LOX metabolism. Individual LOX isozymes are thought to play a role in the modulation of epithelial proliferation and/or differentiation as well as in inflammation, wound healing, inflammatory skin diseases and cancer. Emerging evidence indicates a structural function of a particular LOX pathway in the maintenance of skin permeability barrier. Loss-of-function mutations in the LOX genes ALOX12B and ALOXE3 have been found to represent the second most common cause of autosomal recessive congenital ichthyosis and targeted disruption of the corresponding LOX genes in mice resulted in neonatal death due to a severely impaired permeability barrier function. Recent data indicate that LOX action in barrier function can be traced back to the oxygenation of linoleate-containing ceramides which constitutes an important step in the formation of the corneocyte lipid envelope. This article is part of a Special Issue entitled The Important Role of Lipids in the Epidermis and their Role in the Formation and Maintenance of the Cutaneous Barrier. Guest Editors: Kenneth R. Feingold and Peter Elias.

Ratio of pro-resolving and pro-inflammatory lipid mediator precursors as potential markers for aggressive periodontitis

Aggressive periodontitis (AgP) is a rapidly progressing type of periodontal disease in otherwise healthy individuals which causes destruction of the supporting tissues of the teeth. The disease is initiated by pathogenic bacteria in the dental biofilm, and the severity of inflammation and attachment loss varies with the host response. Recently, there has been an increased interest in determining the role of lipid mediators in inflammatory events and the concept of pro-inflammatory and pro-resolution lipid mediators has been brought into focus also in periodontal disease. The present study aimed to determine the profile of omega-3 or n3- as well as omega-6 or n6- polyunsaturated fatty acids (PUFAs) and PUFA-metabolites of linoleic acid, arachidonic acid (AA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in gingival crevicular fluid (GCF), saliva and serum in AgP patients and healthy controls. In total, 60 selected n3- and n6-PUFAs and various PUFA metabolites were measured using high performance liquid chromatography-tandem electrospray ionisation mass spectrometry (HPLC-ESI-MS-MS). Of these, 51 could be quantified in this study. The concentrations of the majority were low in saliva samples compared with serum and GCF, but were mainly higher in AgP patients compared with healthy controls in all three kinds of sample. Ratios of n3- to n6-PUFAs (DHA + EPA)/AA were significantly lower in the GCF of AgP patients than in the healthy controls. Furthermore, various ratios of the direct precursors of the pro-resolution lipid mediators (precursors of resolvins and protectins) were calculated against the precursors of mainly pro-inflammatory lipid mediators. These ratios were mainly lower in GCF and saliva of AgP patients, compared with healthy controls, but only reached significance in GCF (P<0.05). To conclude, the ratios of precursors of pro-resolution/pro-inflammatory lipid mediators seem to be more relevant for describing the disease status of AgP than the concentration of specific lipid mediators.

Quantitative metabolic profiling of lipid mediators

Lipids are heterogeneous biological molecules that possess multiple physiological roles including cell structure, homeostasis, and restoration of tissue functionality during and after inflammation. Lipid metabolism constitutes a network of pathways that are related at multiple biosynthetic hubs. Disregulation of lipid metabolism can lead to pathophysiological effects and multiple lipid mediators have been described to be involved in physiological processes, (e.g. inflammation). Accordingly, a thorough description of these pathways may shed light on putative relations in multiple complex diseases, including chronic obstructive pulmonary disease, asthma, Alzheimer's disease, multiple sclerosis, obesity, and cancer. Due to the structural complexity of lipids and the low abundance of many lipid mediators, mass spectrometry is the most commonly employed method for analysis. However, multiple challenges remain in the efforts to analyze every lipid subfamily. In this review, the biological role of sphingolipids, glycerolipids, oxylipins (e.g. eicosanoids), endocannabinoids, and N-acylethanolamines in relation to health and disease and the state-of-the-art analyses are summarized. The characterization and understanding of these pathways will increase our ability to examine for interrelations among lipid pathways and improve the knowledge of biological mechanisms in health and disease.

Lipoxygenase pathways in Homo neanderthalensis: functional comparison with Homo sapiens isoforms

Lipoxygenases (LOX) have been implicated in biosynthesis of pro- and anti-inflammatory mediators, and a previous report suggested compromised leukotriene signaling in H. neanderthalensis. To search for corresponding differences in leukotriene biosynthesis, we screened the Neandertal genome for LOX genes and found that, as modern humans, this archaic hominid contains six LOX genes (nALOX15, nALOX12, nALOX5, nALOX15B, nALOX12B, and nALOXE3) and one pseudogene. In the Neandertal genome, 60-75% of the amino acids of the different human LOX isoforms have been identified, and the degree of identity varies between 96 and 99%. Most functional amino acids (iron ligands, specificity determinants, calcium and ATP-binding sites, membrane-binding determinants, and phosphorylation sites) are well conserved in the Neandertal LOX isoforms, and expression of selected neandertalized human LOX mutants revealed no major functional defects. However, in nALOX12 and nALOXE3, two premature stop codons were found, leading to inactive enzyme species. These data suggest that ALOX15, ALOX5, ALOX15B, and ALOX12B should have been present as functional enzymes in H. neanderthalensis and that in contrast to lower nonhuman primates (M. mulatta) and other mammals (mice, rats), this ancient hominid expressed a 15-lipoxygenating ALOX15. Expression of ALOXE3 and ALOX12 was compromised, which might have caused problems in epidermal differentiation.

Eicosanoids in skin inflammation

Eicosanoids play an integral part in homeostatic mechanisms related to skin health and structural integrity. They also mediate inflammatory events developed in response to environmental factors, such as exposure to ultraviolet radiation, and inflammatory and allergic disorders, including psoriasis and atopic dermatitis. This review article discusses biochemical aspects related to cutaneous eicosanoid metabolism, the contribution of these potent autacoids to skin inflammation and related conditions, and considers the importance of nutritional supplementation with bioactives such as omega-3 and omega-6 polyunsaturated fatty acids and plant-derived antioxidants as means of addressing skin health issues.

Bioactive lipid mediators in skin inflammation and immunity

The skin is the primary barrier from the outside environment, protecting the host from injury, infectious pathogens, water loss and solar ultraviolet radiation. In this role, it is supported by a highly organized system comprising elements of innate and adaptive immunity, responsive to inflammatory stimuli. The cutaneous immune system is regulated by mediators such as cytokines and bioactive lipids that can initiate rapid immune responses with controlled inflammation, followed by efficient resolution. However, when immune responses are inadequate or mounted against non-infectious agents, these mediators contribute to skin pathologies involving unresolved or chronic inflammation. Skin is characterized by active lipid metabolism and fatty acids play crucial roles both in terms of structural integrity and functionality, in particular when transformed to bioactive mediators. Eicosanoids, endocannabinoids and sphingolipids are such key bioactive lipids, intimately involved in skin biology, inflammation and immunity. We discuss their origins, role and influence over various cells of the epidermis, dermis and cutaneous immune system and examine their function in examples of inflammatory skin conditions. We focus on psoriasis, atopic and contact dermatitis, acne vulgaris, wound healing and photodermatology that demonstrate dysregulation of bioactive lipid metabolism and examine ways of using this insight to inform novel therapeutics.

Investigations of human platelet-type 12-lipoxygenase: role of lipoxygenase products in platelet activation

Human platelet-type 12-lipoxygenase (12-LOX) has recently been shown to play an important role in regulation of human platelet function by reacting with arachidonic acid (AA). However, a number of other fatty acids are present on the platelet surface that, when cleaved from the phospholipid, can be oxidized by 12-LOX. We sought to characterize the substrate specificity of 12-LOX against six essential fatty acids: AA, dihomo-γ-linolenic acid (DGLA), eicosapentaenoic acid (EPA), α-linolenic acid (ALA), eicosadienoic acid (EDA), and linoleic acid (LA). Three fatty acids were comparable substrates (AA, DGLA, and EPA), one was 5-fold slower (ALA), and two showed no reactivity with 12-LOX (EDA and LA). The bioactive lipid products resulting from 12-LOX oxidation of DGLA, 12-(S)-hydroperoxy-8Z,10E,14Z-eicosatrienoic acid [12(S)-HPETrE], and its reduced product, 12(S)-HETrE, resulted in significant attenuation of agonist-mediated platelet aggregation, granule secretion, αIIbβ3 activation, Rap1 activation, and clot retraction. Treatment with DGLA similarly inhibited PAR1-mediated platelet activation as well as platelet clot retraction. These observations are in surprising contrast to our recent work showing 12(S)-HETE is a prothrombotic bioactive lipid and support our hypothesis that the overall effect of 12-LOX oxidation of fatty acids in the platelet is dependent on the fatty acid substrates available at the platelet membrane.

Targeted chiral analysis of bioactive arachidonic Acid metabolites using liquid-chromatography-mass spectrometry

A complex structurally diverse series of eicosanoids arises from the metabolism of arachidonic acid. The metabolic profile is further complicated by the enantioselectivity of eicosanoid formation and the variety of regioisomers that arise. In order to investigate the metabolism of arachidonic acid in vitro or in vivo, targeted methods are advantageous in order to distinguish between the complex isomeric mixtures that can arise by different metabolic pathways. Over the last several years this targeted approach has become more popular, although there are still relatively few examples where chiral targeted approaches have been employed to directly analyze complex enantiomeric mixtures. To efficiently conduct targeted eicosanoid analyses, LC separations are coupled with collision induced dissociation (CID) and tandem mass spectrometry (MS/MS). Product ion profiles are often diagnostic for particular regioisomers. The highest sensitivity that can be achieved involves the use of selected reaction monitoring/mass spectrometry (SRM/MS); whereas the highest specificity is obtained with an SRM transitions between an intense parent ion, which contains the intact molecule (M) and a structurally significant product ion. This review article provides an overview of arachidonic acid metabolism and targeted chiral methods that have been utilized for the analysis of the structurally diverse eicosanoids that arise.

Lipidomics of polyunsaturated-fatty-acid-derived oxygenated metabolites

Nutritionally important PUFAs (polyunsaturated fatty acids) mediate some of their bioactivities through formation of oxygenated metabolites. These bioactive lipids are formed by COX (cyclo-oxygenase), LOX (lipoxygenase) and cytochrome-P450-catalysed reactions, as well as non-enzymatic lipid peroxidation. These reactions produce numerous species, some of which can be formed through more than one pathway. MS-based lipidomics offers the selectivity and sensitivity required for qualitative and quantitative analysis of multiple lipid species, in a variety of biological systems, and can facilitate the study of these mediators.

Targeted knock-down of a structurally atypical zebrafish 12S-lipoxygenase leads to severe impairment of embryonic development

Lipoxygenases (LO) are a class of dioxygenases, which form hydroperoxy, hydroxy, and epoxy derivatives of arachidonic acid with distinct positional and stereochemical configurations. In man, there are two known types of 12-LO that are distinguished by their expression patterns and catalytic properties. The platelet 12S-LO plays a role in platelet aggregation and 12R-LO seems to be important for normal skin function. Using BLAST searches of the zebrafish (zf) genome we identified one candidate zf12-LO gene with 43% identity with human 12R-LO at the mRNA level and the deduced primary sequence carried the so called "Coffa" structural determinant (Gly residue) for R stereoselectivity of LOs. However, incubations of recombinant, purified, zf12-LO with arachidonic acid revealed exclusive formation of 12(S)-hydroperoxy-eicosatetraenoic acid. Further studies with immunohistochemistry showed prominent expression of zf12-LO in the cell nuclei of skin epithelium, the epithelial lining of the stomodeum, and the pharyngeal pouches in zf embryos. To probe its function, zf12-LO was subjected to targeted knock-down in zf embryos, resulting in the development of a severe phenotype, characterized by abnormal development of the brain, the eyes, and the tail as well as pericardial and yolk sac edema. Hence, we have identified a unique vertebrate 12S-LO that breaks the current structure-function paradigms for S and R stereo-specificity and with critical roles in normal embryonic development.

Smoking and pathogenesis of psoriasis: a review of oxidative, inflammatory and genetic mechanisms

Recent studies suggest that cigarette smoking may trigger the development of psoriasis through oxidative, inflammatory and genetic mechanisms. Smoking initiates formation of free radicals that stimulate cell signalling pathways active in psoriasis including mitogen-activated protein kinase (MAPK), nuclear factor-κB (NF-κB) and Janus kinase/signal transducers and activators of transcription (JAK-STAT). Smoking damages the skin by increasing formation of reactive oxygen species and decreasing the gene expression of antioxidants. Nicotine also stimulates innate immune cells integral to the pathogenesis of psoriasis including dendritic cells, macrophages and keratinocytes. These cells release cytokines that activate T lymphocytes and perpetuate a cycle of chronic inflammation. Smoking also enhances expression of genes known to confer an increased risk of psoriasis, including HLA-Cw6, HLA-DQA1*0201 and CYP1A1. Improved understanding of the possible link between smoking and psoriasis pathogenesis may provide further insight into mechanisms underlying smoking, psoriasis and risk of subsequent cardiovascular disease.

Applications of stereospecifically-labeled Fatty acids in oxygenase and desaturase biochemistry

Oxygenation and desaturation reactions are inherently associated with the abstraction of a hydrogen from the fatty acid substrate. Since the first published application in 1965, stereospecific placement of a labeled hydrogen isotope (deuterium or tritium) at the reacting carbons has proven a highly effective strategy for investigating the chemical mechanisms catalyzed by lipoxygenases, hemoprotein fatty acid dioxygenases including cyclooxygenases, cytochromes P450, and also the desaturases and isomerases. This review presents a synopsis of all published studies through 2010 on the synthesis and use of stereospecifically labeled fatty acids (71 references), and highlights some of the mechanistic insights gained by application of stereospecifically labeled fatty acids.