12R-lipoxygenase deficiency disrupts epidermal barrier function

Evolutionary alteration of ALOX15 specificity optimizes the biosynthesis of antiinflammatory and proresolving lipoxins

ALOX15 (12/15-lipoxygenase) orthologs have been implicated in maturational degradation of intracellular organelles and in the biosynthesis of antiinflammatory and proresolving eicosanoids. Here we hypothesized that lower mammals (mice, rats, pigs) express 12-lipoxygenating ALOX15 orthologs. In contrast, 15-lipoxygenating isoforms are found in higher primates (orangutans, men), and these results suggest an evolution of ALOX15 specificity. To test this hypothesis we first cloned and characterized ALOX15 orthologs of selected Catarrhini representing different stages of late primate evolution and found that higher primates (men, chimpanzees) express 15-lipoxygenating orthologs. In contrast, lower primates (baboons, rhesus monkeys) express 12-lipoxygenating enzymes. Gibbons, which are flanked in evolution by rhesus monkeys (12-lipoxygenating ALOX15) and orangutans (15-lipoxygenating ALOX15), express an ALOX15 ortholog with pronounced dual specificity. To explore the driving force for this evolutionary alterations, we quantified the lipoxin synthase activity of 12-lipoxygenating (rhesus monkey, mouse, rat, pig, humIle418Ala) and 15-lipoxygenating (man, chimpanzee, orangutan, rabbit, ratLeu353Phe) ALOX15 variants and found that, when normalized to their arachidonic acid oxygenase activities, the lipoxin synthase activities of 15-lipoxygenating ALOX15 variants were more than fivefold higher (P < 0.01) [corrected]. Comparative molecular dynamics simulations and quantum mechanics/molecular mechanics calculations indicated that, for the 15-lipoxygenating rabbit ALOX15, the energy barrier for C13-hydrogen abstraction (15-lipoxygenation) was 17 kJ/mol lower than for arachidonic acid 12-lipoxygenation. In contrast, for the 12-lipoxygenating Ile418Ala mutant, the energy barrier for 15-lipoxygenation was 10 kJ/mol higher than for 12-lipoxygenation. Taken together, our data suggest an evolution of ALOX15 specificity, which is aimed at optimizing the biosynthetic capacity for antiinflammatory and proresolving lipoxins.

The Precise Structures and Stereochemistry of Trihydroxy-linoleates Esterified in Human and Porcine Epidermis and Their Significance in Skin Barrier Function: IMPLICATION OF AN EPOXIDE HYDROLASE IN THE TRANSFORMATIONS OF LINOLEATE

Creation of an intact skin water barrier, a prerequisite for life on dry land, requires the lipoxygenase-catalyzed oxidation of the essential fatty acid linoleate, which is esterified to the ω-hydroxyl of an epidermis-specific ceramide. Oxidation of the linoleate moiety by lipoxygenases is proposed to facilitate enzymatic cleavage of the ester bond, releasing free ω-hydroxyceramide for covalent binding to protein, thus forming the corneocyte lipid envelope, a key component of the epidermal barrier. Herein, we report the transformations of esterified linoleate proceed beyond the initial steps of oxidation and epoxyalcohol synthesis catalyzed by the consecutive actions of 12R-LOX and epidermal LOX3. The major end product in human and porcine epidermis is a trihydroxy derivative, formed with a specificity that implicates participation of an epoxide hydrolase in converting epoxyalcohol to triol. Of the 16 possible triols arising from hydrolysis of 9,10-epoxy-13-hydroxy-octadecenoates, using LC-MS and chiral analyses, we identify and quantify specifically 9R,10S,13R-trihydroxy-11E-octadecenoate as the single major triol esterified in porcine epidermis and the same isomer with lesser amounts of its 10R diastereomer in human epidermis. The 9R,10S,13R-triol is formed by S_N2 hydrolysis of the 9R,10R-epoxy-13R-hydroxy-octadecenoate product of the LOX enzymes, a reaction specificity characteristic of epoxide hydrolase. The high polarity of triol over the primary linoleate products enhances the concept that the oxidations disrupt corneocyte membrane lipids, promoting release of free ω-hydroxyceramide for covalent binding to protein and sealing of the waterproof barrier.

Synthesis and degradation pathways, functions, and pathology of ceramides and epidermal acylceramides

Ceramide (Cer) is a structural backbone of sphingolipids and is composed of a long-chain base and a fatty acid. Existence of a variety of Cer species, which differ in chain-length, hydroxylation status, and/or double bond number of either of their hydrophobic chains, has been reported. Ceramide is produced by Cer synthases. Mammals have six Cer synthases (CERS1-6), each of which exhibits characteristic substrate specificity toward acyl-CoAs with different chain-lengths. Knockout mice for each Cer synthase show corresponding, isozyme-specific phenotypes, revealing the functional differences of Cers with different chain-lengths. Cer diversity is especially prominent in epidermis. Changes in Cer levels, composition, and chain-lengths are associated with atopic dermatitis. Acylceramide (acyl-Cer) specifically exists in epidermis and plays an essential role in skin permeability barrier formation. Accordingly, defects in acyl-Cer synthesis cause the cutaneous disorder ichthyosis with accompanying severe skin barrier defects. Although the molecular mechanism by which acyl-Cer is generated was long unclear, most genes involved in its synthesis have been identified recently. In Cer degradation pathways, the long-chain base moiety of Cer is converted to acyl-CoA, which is then incorporated mainly into glycerophospholipids. This pathway generates the lipid mediator sphingosine 1-phosphate. This review will focus on recent advances in our understanding of the synthesis and degradation pathways, physiological functions, and pathology of Cers/acyl-Cers.

The role of epidermal sphingolipids in dermatologic diseases

Sphingolipids, a group of lipids containing the sphingoid base, have both structural and biological functions in human epidermis. Ceramides, as a part of extracellular lipids in the stratum corneum, are important elements of the skin barrier and are involved in the prevention of transepidermal water loss. In addition, ceramides regulate such processes as proliferation, differentiation and apoptosis of keratinocytes. Another important sphingolipid, sphingosine-1-phosphate (S1P), inhibits proliferation and induces differentiation of keratinocytes. A recent clinical study of the efficacy and safety of ponesimod (a selective modulator of the S1P receptor 1) suggested that sphingolipid metabolism may become a new target for the pharmacological treatment of psoriasis. The role of sphingolipids in some dermatologic diseases, including psoriasis, atopic dermatitis and ichthyoses was summarized in this article.

Establishment of Two Mouse Models for CEDNIK Syndrome Reveals the Pivotal Role of SNAP29 in Epidermal Differentiation

Loss-of-function mutations in the synaptosomal-associated protein 29 (SNAP29) gene cause the cerebral dysgenesis, neuropathy, ichthyosis, and keratoderma syndrome. In this study, we created total (Snap29(-/-)) as well as keratinocyte-specific (Snap29(fl/fl)/K14-Cre) Snap29 knockout mice. Both mutant mice exhibited a congenital distinct ichthyotic phenotype resulting in neonatal lethality. Mutant mice revealed acanthosis and hyperkeratosis as well as abnormal keratinocyte differentiation and increased proliferation. In addition, the epidermal barrier was severely impaired. These results indicate an essential role of SNAP29 in epidermal differentiation and barrier formation. Markedly decreased deposition of lamellar body contents in mutant mice epidermis and the observation of malformed lamellar bodies indicate severe impairments in lamellar body function due to the Snap29 knockout. We also found increased microtubule associated protein-1 light chain 3, isoform B-II levels, unchanged p62/SQSTM1 protein amounts, and strong induction of the endoplasmic reticulum stress marker C/EBP homologous protein in mutant mice. This emphasizes a role of SNAP29 in autophagy and endoplasmic reticulum stress. Our murine models serve as powerful tools for investigating keratinocyte differentiation processes and provide insights into the essential contribution of SNAP29 to epidermal differentiation.

Structural and functional biology of arachidonic acid 15-lipoxygenase-1 (ALOX15)

Lipoxygenases (LOX) form a family of lipid peroxidizing enzymes, which have been implicated in a number of physiological processes and in the pathogenesis of inflammatory, hyperproliferative and neurodegenerative diseases. They occur in two of the three domains of terrestrial life (bacteria, eucarya) and the human genome involves six functional LOX genes, which encode for six different LOX isoforms. One of these isoforms is ALOX15, which has first been described in rabbits in 1974 as enzyme capable of oxidizing membrane phospholipids during the maturational breakdown of mitochondria in immature red blood cells. During the following decades ALOX15 has extensively been characterized and its biological functions have been studied in a number of cellular in vitro systems as well as in various whole animal disease models. This review is aimed at summarizing the current knowledge on the protein-chemical, molecular biological and enzymatic properties of ALOX15 in various species (human, mouse, rabbit, rat) as well as its implication in cellular physiology and in the pathogenesis of various diseases.

Transcriptome immunomodulation of in-feed additives in Atlantic salmon Salmo salar infested with sea lice Caligus rogercresseyi

One of the most significant threats to the Chilean salmon aquaculture industry is the ectoparasitic sea louse Caligus rogercresseyi. To cope with sea lice infestations, functional diets have become an important component in strengthening the host immune response. The aim of this study was to evaluate molecular mechanisms activated through immunostimulation by in-feed plant-derived additives in Atlantic salmon infected with sea lice. Herein, a transcriptome-wide sequencing analysis was performed from skin and head kidney tissues, evidencing that the immune response genes were the most variable after the challenge, especially in the head kidney, while other genes involved in metabolism were highly expressed individuals fed with the immunostimulants. Interestingly, defensive enzymes such as Cytochrome p450 and serpins were down-regulated in infested individuals, especially in skin tissue. Additionally, MHC-I and MHC-II genes were differentially expressed after the incorporation of the in-feed additives, giving some cues about the protection mechanisms of plant-derived compound as immunostimulants for infested salmons. This is the first published study that evaluates the transcriptomic response of sea lice-infested Atlantic salmon fed with in-feed additives.

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.

Dissecting the formation, structure and barrier function of the stratum corneum

The skin is the largest organ of the mammalian body. The outermost layer of mammalian skin, the stratum corneum (SC) of the epidermis, consists of piles of dead corneocytes that are the end-products of terminal differentiation of epidermal keratinocytes. The SC performs a crucial barrier function of epidermis. Langerhans cells, when activated, extend their dendrites through tight junctions just beneath the SC to capture external antigens. Recently, knowledge of the biology of corneocytes ('corneobiology') has progressed rapidly and many key factors that modulate its barrier function have been identified and characterized. In this review article on the SC, we summarize its evolution, formation, structure and function. Cornification is an important step of SC formation at the conversion of living epithelial cells to dead corneocytes, and consists of three major steps: formation of the intracellular keratin network, cornified envelopes and intercellular lipids. After cornification, the SC undergoes chemical reactions to form the mature SC with different functional layers. Finally, the SC is shed off at the surface ('desquamation'), mediated by a cascade of several proteases. This review will be helpful to understand our expanding knowledge of the biology of the SC, where immunity meets external antigens.

Quantitative determination of 12-hydroxyeicosatetraenoic acids by chiral liquid chromatography tandem mass spectrometry in a murine atopic dermatitis model

Atopic dermatitis, one of the most important skin diseases, is characterized by both skin barrier impairment and immunological abnormalities. Although several studies have demonstrated the significant relationship between atopic dermatitis and immunological abnormalities, the role of hydroxyeicosatetraenoic acids (HETE) in atopic dermatitis remains unknown. To develop chiral methods for characterization of 12-HETE enantiomers in a 1-chloro-2,4-dinitrochlorobenzene (DNCB)-induced atopic dermatitis mouse model and evaluate the effects of 12-HETE on atopic dermatitis, BALB/c mice were treated with either DNCB or acetone/olive oil (AOO) to induce atopic dermatitis, after which 12(R)- and 12(S)-HETEs in the plasma, skin, spleen, and lymph nodes were quantified by chiral liquid chromatography-tandem mass spectrometry. 12(R)- and 12(S)-HETEs in biological samples of DNCB-induced atopic dermatitis mice increased significantly compared with the AOO group, reflecting the involvement of 12(R)- and 12(S)-HETEs in atopic dermatitis. These findings indicate that 12(R)- and 12(S)-HETEs could be a useful guide for understanding the pathogenesis of atopic dermatitis.

Oxidative stress, unfolded protein response, and apoptosis in developmental toxicity

Physiological development requires precise spatiotemporal regulation of cellular and molecular processes. Disruption of these key events can generate developmental toxicity in the form of teratogenesis or mortality. The mechanism behind many developmental toxicants remains unknown. While recent work has focused on the unfolded protein response (UPR), oxidative stress, and apoptosis in the pathogenesis of disease, few studies have addressed their relationship in developmental toxicity. Redox regulation, UPR, and apoptosis are essential for physiological development and can be disturbed by a variety of endogenous and exogenous toxicants to generate lethality and diverse malformations. This review examines the current knowledge of the role of oxidative stress, UPR, and apoptosis in physiological development as well as in developmental toxicity, focusing on studies and advances in vertebrates model systems.

Lack of association of ALOX12 and ALOX15B polymorphisms with psoriasis despite altered urinary excretion of 12(S)-hydroxyeicosatetraenoic acid

BACKGROUND

Pro- and anti-inflammatory metabolites of arachidonic acid - eicosanoids - participate in skin homeostasis, affecting the growth and differentiation of keratinocytes. Alterations of 12-lipoxygenase (LOX) and 15-LOX and their metabolites have been described in the epidermis of patients with psoriasis, but systemic production of 12-LOX and 15-LOX eicosanoids has not been studied in the disease.

OBJECTIVES

To ascertain the frequencies of the genetic variants ALOX12 rs1126667 and ALOX15 rs11568070 in cases and controls, and to compare urinary metabolites of 12(S)-hydroxyeicosatetraenoic acid (HETE) between patients with psoriasis and healthy controls.

METHODS

Patients with psoriasis (n = 200) were stratified depending on the severity of their dermal lesions. Genotyping was performed using a 5'-nuclease real-time assay. The concentrations of 12(S)-HETE, its metabolites and 15(S)-HETE were determined in urine samples using high-performance liquid chromatography-tandem mass spectrometry.

RESULTS

Tetranor-12(S)-HETE metabolite excretion was significantly higher in urine of patients with psoriasis, while excretion of 12(S)-HETE was decreased. Neither 12(S)-HETE nor tetranor-12(S)-HETE correlated with the type of disease or severity score. No difference in urinary 15(S)-HETE was found between the study groups. Genotype distribution of the ALOX12 rs1126667 or ALOX15 rs11568070 polymorphisms did not discriminate for the disease or its severity.

CONCLUSIONS

Systemic metabolism of 12(S)-HETE is accelerated in psoriasis because excretion of the tetranor-12(S)-HETE inactivation product is elevated. No correlation with the severity or extent of psoriasis is detectable. We propose that in patients with psoriasis, 12(S)-HETE to tetranor-12(S)-HETE conversion could be at least a marker for this disease, in which inflammation of the skin can induce microsomal beta-oxidation of this eicosanoid.

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.

A mouse organotypic tissue culture model for autosomal recessive congenital ichthyosis

BACKGROUND

Autosomal recessive congenital ichthyoses (ARCIs) are keratinization disorders caused by impaired skin barrier function. Mutations in the genes encoding the lipoxygenases 12R-LOX and eLOX-3 are the second most common cause of ARCIs. In recent years, human skin equivalents recapitulating the ARCI phenotype have been established.

OBJECTIVES

To develop a murine organotypic tissue culture model for ARCI.

METHODS

Epidermal keratinocytes were isolated from newborn 12R-LOX-deficient mice and cocultivated with mouse dermal fibroblasts embedded in a scaffold of native collagen type I.

RESULTS

With this experimental set-up the keratinocytes formed a well-organized multilayered stratified epithelium resembling skin architecture in vivo. All epidermal layers were present and the keratinocytes within showed the characteristic morphological features. Markers for differentiation and maturation indicated regular epidermal morphogenesis. The major components of epidermal structures were expressed, and were obviously processed and assembled properly. In contrast to their wild-type counterparts, 12R-LOX-deficient skin equivalents showed abnormal vesicular structures in the upper epidermal layers correlating with altered lipid composition and increased transepidermal water loss, comparable with 12R-LOX-deficient mice.

CONCLUSIONS

The mouse skin equivalents faithfully recapitulate the 12R-LOX-deficient phenotype observed in vivo, classifying them as appropriate in vitro models to study molecular mechanisms involved in the development of ARCI and to evaluate novel therapeutic agents. In contrast to existing human three-dimensional skin models, the generation of these murine models is not constrained by a limited supply of material and does not depend on in vitro expansion and/or genetic manipulations that could result in inadvertent genotypic and phenotypic alterations.

Epidermal barrier disorders and corneodesmosome defects

Corneodesmosomes are modified desmosomes present in the stratum corneum (SC). They are crucial for SC cohesion and, thus, constitute one of the pivotal elements of the functional protective barrier of human skin. Expression of corneodesmosomes and, notably, the process of their degradation are probably altered during several dermatoses leading to the disruption of the permeability barrier or to abnormal, often compensative, SC accumulation. These different situations are reviewed in the present paper.

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.

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".

Inflammation, Cancer and Oxidative Lipoxygenase Activity are Intimately Linked

Cancer and inflammation are intimately linked due to specific oxidative processes in the tumor microenvironment. Lipoxygenases are a versatile class of oxidative enzymes involved in arachidonic acid metabolism. An increasing number of arachidonic acid metabolites is being discovered and apart from their classically recognized pro-inflammatory effects, anti-inflammatory effects are also being described in recent years. Interestingly, these lipid mediators are involved in activation of pro-inflammatory signal transduction pathways such as the nuclear factor κB (NF-κB) pathway, which illustrates the intimate link between lipid signaling and transcription factor activation. The identification of the role of arachidonic acid metabolites in several inflammatory diseases led to a significant drug discovery effort around arachidonic acid metabolizing enzymes. However, to date success in this area has been limited. This might be attributed to the lack of selectivity of the developed inhibitors and to a lack of detailed understanding of the functional roles of arachidonic acid metabolites in inflammatory responses and cancer. This calls for a more detailed investigation of the activity of arachidonic acid metabolizing enzymes and development of more selective inhibitors.

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.

Eicosanoids and Keratinocytes in Wound Healing

Significance: Eicosanoids are biologically active lipid mediators derived from arachidonic acid that are important in injury and inflammatory responses. Cyclooxygenase-1 and cyclooxygenase-2 mediate the production of prostanoids, whereas 5-lipoxygenase mediates the production of leukotrienes and hydroxyeicosatetraenoic acids. These lipid mediators have traditionally been known to recruit cells of the immune system to a site of injury and inflammation. However, they also interact with various cells that are resident to the wound bed, including modulation of keratinocyte activity. Recent Advances: Recent work has identified multiple prostanoid and leukotriene receptors on keratinocytes, indicating that eicosanoids directly interact with them. Recent work also shows that keratinocytes are capable of producing prostanoids and leukotrienes. Critical Issues: Much of the critical work has been performed in cell culture and mouse in vivo models. This has greatly expanded our understanding of the eicosanoid interactions with keratinocytes and wound healing in general. However, few of these in vivo models have been able to critically evaluate keratinocyte migration and re-epithelialization. Future Directions: As research continues in this exciting field, the cellular pathways stimulated by the eicosanoids will become better defined. Future research with excisional wound models in mice and pigs and ex vivo human skin models will better isolate the contribution of eicosanoid-mediated effects on keratinocyte migration and re-epithelialization.

Epidermal barriers

The epidermis functions as a physical barrier to the external environment and works to prevent loss of water from the skin. Numerous factors have been implicated in the formation of epidermal barriers, such as cornified envelopes, corneocytes, lipids, junctional proteins, proteases, protease inhibitors, antimicrobial peptides, and transcription factors. This review illustrates human diseases (ichthyoses) and animal models in which the epidermal barrier is disrupted or dysfunctional at steady state owing to ablation of one or more of the above factors. These diseases and animal models help us to understand the complicated mechanisms of epidermal barrier formation and give further insights on epidermal development.

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.

Interactions between α-tocopherol, polyunsaturated fatty acids, and lipoxygenases during embryogenesis

α-Tocopherol is a lipid-soluble antioxidant that is specifically required for reproduction and embryogenesis. However, since its discovery, α-tocopherol's specific biologic functions, other than as an antioxidant, and the mechanism(s) mediating its requirement for embryogenesis remain unknown. As an antioxidant, α-tocopherol protects polyunsaturated fatty acids (PUFAs) from lipid peroxidation. α-Tocopherol is probably required during embryonic development to protect PUFAs that are crucial to development, specifically arachidonic (ARA) and docosahexaenoic (DHA) acids. Additionally, ARA and DHA are metabolized to bioactive lipid mediators via lipoxygenase enzymes, and α-tocopherol may directly protect, or it may mediate the production and/or actions of, these lipid mediators. In this review, we discuss how α-tocopherol (1) prevents the nonspecific, radical-mediated peroxidation of PUFAs, (2) functions within a greater antioxidant network to modulate the production and/or function of lipid mediators derived from 12- and 12/15-lipoxygenases, and (3) modulates 5-lipoxygenase activity. The application and implication of such interactions are discussed in the context of α-tocopherol requirements during embryogenesis.

Genetic ablation and short-duration inhibition of lipoxygenase results in increased macroautophagy

12/15-lipoxygenase (12/15-LOX) is involved in organelle homeostasis by degrading mitochondria in maturing red blood cells and by eliminating excess peroxisomes in liver. Furthermore, 12/15-LOX contributes to diseases by exacerbating oxidative stress-related injury, notably in stroke. Nonetheless, it is unclear what the consequences are of abolishing 12/15-LOX activity. Mice in which the alox15 gene has been ablated do not show an obvious phenotype, and LOX enzyme inhibition is not overtly detrimental. We show here that liver histology is also unremarkable. However, electron microscopy demonstrated that 12/15-LOX knockout surprisingly leads to increased macroautophagy in the liver. Not only macroautophagy but also mitophagy and pexophagy were increased in hepatocytes, which otherwise showed unaltered fine structure and organelle morphology. These findings were substantiated by immunofluorescence showing significantly increased number of LC3 puncta and by Western blotting demonstrating a significant increase for LC3-II protein in both liver and brain homogenates of 12/15-LOX knockout mice. Inhibition of 12/15-LOX activity by treatment with four structurally different inhibitors had similar effects in cultured HepG2 hepatoma cells and SH-SY5Y neuroblastoma cells with significantly increased autophagy discernable already after 2 hours. Hence, our study reveals a link between ablation or inhibition of 12/15-LOX and stimulation of macroautophagy. The enhanced macroautophagy may be related to the known tissue-protective effects of LOX ablation or inhibition under various diseased conditions caused by oxidative stress and ischemia. This could provide an important cleaning mechanism of cells and tissues to prevent accumulation of damaged mitochondria and other cellular components.

Role of lipids in the formation and maintenance of the cutaneous permeability barrier

The major function of the skin is to form a barrier between the internal milieu and the hostile external environment. A permeability barrier that prevents the loss of water and electrolytes is essential for life on land. The permeability barrier is mediated primarily by lipid enriched lamellar membranes that are localized to the extracellular spaces of the stratum corneum. These lipid enriched membranes have a unique structure and contain approximately 50% ceramides, 25% cholesterol, and 15% free fatty acids with very little phospholipid. Lamellar bodies, which are formed during the differentiation of keratinocytes, play a key role in delivering the lipids from the stratum granulosum cells into the extracellular spaces of the stratum corneum. Lamellar bodies contain predominantly glucosylceramides, phospholipids, and cholesterol and following the exocytosis of lamellar lipids into the extracellular space of the stratum corneum these precursor lipids are converted by beta glucocerebrosidase and phospholipases into the ceramides and fatty acids, which comprise the lamellar membranes. The lipids required for lamellar body formation are derived from de novo synthesis by keratinocytes and from extra-cutaneous sources. The lipid synthetic pathways and the regulation of these pathways are described in this review. In addition, the pathways for the uptake of extra-cutaneous lipids into keratinocytes are discussed. 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.

Discovery of a novel activator of 5-lipoxygenase from an anacardic acid derived compound collection

Lipoxygenases (LOXs) and cyclooxygenases (COXs) metabolize poly-unsaturated fatty acids into inflammatory signaling molecules. Modulation of the activity of these enzymes may provide new approaches for therapy of inflammatory diseases. In this study, we screened novel anacardic acid derivatives as modulators of human 5-LOX and COX-2 activity. Interestingly, a novel salicylate derivative 23a was identified as a surprisingly potent activator of human 5-LOX. This compound showed both non-competitive activation towards the human 5-LOX activator adenosine triphosphate (ATP) and non-essential mixed type activation against the substrate linoleic acid, while having no effect on the conversion of the substrate arachidonic acid. The kinetic analysis demonstrated a non-essential activation of the linoleic acid conversion with a KA of 8.65 μM, αKA of 0.38μM and a β value of 1.76. It is also of interest that a comparable derivative 23d showed a mixed type inhibition for linoleic acid conversion. These observations indicate the presence of an allosteric binding site in human 5-LOX distinct from the ATP binding site. The activatory and inhibitory behavior of 23a and 23d on the conversion of linoleic compared to arachidonic acid are rationalized by docking studies, which suggest that the activator 23a stabilizes linoleic acid binding, whereas the larger inhibitor 23d blocks the enzyme active site.

Formation and functions of the corneocyte lipid envelope (CLE)

Corneocytes in mammalian stratum corneum are surrounded by a monolayer of covalently bound ω-OH-ceramides that form the corneocyte (-bound) lipid envelope (CLE). We review here the structure, composition, and possible functions of this structure, with insights provided by inherited and acquired disorders of lipid metabolism. 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.

Phospho-ΔNp63α regulates AQP3, ALOX12B, CASP14 and CLDN1 expression through transcription and microRNA modulation

Cisplatin-induced and ATM-phosphorylated (p)-ΔNp63α regulates the expression of epidermal differentiation and skin barrier regulators (AQP3, CASP14, ALOX12B, and CLDN1) in squamous cell carcinoma (SCC) cells by dual transcriptional and post-transcriptional mechanisms. We found that p-ΔNp63α bound to target gene promoters, and regulated the activity of the tested promoters in vitro. P-ΔNp63α was shown to upregulate miR-185-5p and downregulate let7-5p, which subsequently modulated AQP3, CASP14, ALOX12B and CLDN1 through their respective 3'-untranslated regions. The introduction of miR-185-5p into resistant SCC-11M cells, which are unable to phosphorylate ΔNp63α, render these cells more sensitive to cisplatin treatment. Further studies of the AQP3, CASP14, ALOX12B, and CLDN1 contributions to chemoresistance may assist in developing novel microRNA-based therapies for human SCC.

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.

The role of sphingolipid metabolism in cutaneous permeability barrier formation

The epidermal permeability barrier of mammalian skin is localized in the stratum corneum. Corneocytes are embedded in an extracellular, highly ordered lipid matrix of hydrophobic lipids consisting of about 50% ceramides, 25% cholesterol and 15% long and very long chain fatty acids. The most important lipids for the epidermal barrier are ceramides. The scaffold of the lipid matrix is built of acylceramides, containing ω-hydroxylated very long chain fatty acids, acylated at the ω-position with linoleic acid. After glucosylation of the acylceramides at Golgi membranes and secretion, the linoleic acid residues are replaced by glutamate residues originating from proteins exposed on the surface of corneocytes. Removal of their glucosyl residues generates a hydrophobic surface on the corneocytes used as a template for the formation of extracellular lipid layers of the water permeability barrier. Misregulation or defects in the formation of extracellular ceramide structures disturb barrier function. Important anabolic steps are the synthesis of ultra long chain fatty acids, their ω-hydroxylation, and formation of ultra long chain ceramides and glucosylceramides. The main probarrier precursor lipids, glucosylceramides and sphingomyelins, are packed in lamellar bodies together with hydrolytic enzymes such as glucosylceramide-β-glucosidase and acid sphingomyelinase and secreted into the intercelullar space between the stratum corneum and stratum granulosum. Inherited defects in the extracellular hydrolytic processing of the probarrier acylglucosylceramides impair epidermal barrier formation and cause fatal diseases: such as prosaposin deficiency resulting in lack of lysosomal lipid binding and transfer proteins, or the symptomatic clinical picture of the "collodion baby" in the absence of glucocerebrosidase. 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.

The important role of epidermal triacylglycerol metabolism for maintenance of the skin permeability barrier function

Survival in a terrestrial, dry environment necessitates a permeability barrier for regulated permeation of water and electrolytes in the cornified layer of the skin (the stratum corneum) to minimize desiccation of the body. This barrier is formed during cornification and involves a cross-linking of corneocyte proteins as well as an extensive remodeling of lipids. The cleavage of precursor lipids from lamellar bodies by various hydrolytic enzymes generates ceramides, cholesterol, and non-esterified fatty acids for the extracellular lipid lamellae in the stratum corneum. However, the important role of epidermal triacylglycerol (TAG) metabolism during formation of a functional permeability barrier in the skin was only recently discovered. Humans with mutations in the ABHD5/CGI-58 (α/β hydrolase domain containing protein 5, also known as comparative gene identification-58, CGI-58) gene suffer from a defect in TAG catabolism that causes neutral lipid storage disease with ichthyosis. In addition, mice with deficiencies in genes involved in TAG catabolism (Abhd5/Cgi-58 knock-out mice) or TAG synthesis (acyl-CoA:diacylglycerol acyltransferase-2, Dgat2 knock-out mice) also develop severe skin permeability barrier dysfunctions and die soon after birth due to increased dehydration. As a result of these defects in epidermal TAG metabolism, humans and mice lack ω-(O)-acylceramides, which leads to malformation of the cornified lipid envelope of the skin. In healthy skin, this epidermal structure provides an interface for the linkage of lamellar membranes with corneocyte proteins to maintain permeability barrier homeostasis. This review focuses on recent advances in the understanding of biochemical mechanisms involved in epidermal neutral lipid metabolism and the generation of a functional skin permeability barrier. 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.

Matriptase regulates proliferation and early, but not terminal, differentiation of human keratinocytes

Genetic defects in matriptase are linked to two congenital ichthyoses: autosomal recessive ichthyosis with hypotrichosis (ARIH, OMIM 610765) and ichthyosis, follicular atrophoderma, hypotrichosis, and hypohidrosis (IFAH, OMIM 602400). Mouse models with matriptase deficiency indicate an involvement of matriptase in suprabasal keratinocytes in the maintenance of the epidermal barrier. In contrast to what has been reported for mouse skin, we show that in human skin matriptase is primarily expressed in the basal and spinous keratinocytes, but not in the more differentiated keratinocytes of the granular layer. In addition, matriptase zymogen activation was predominantly detected in the basal cells. Furthermore, by using skin organotypic cultures as a model system to monitor the course of human epidermal differentiation, we found elevated matriptase zymogen activation during early stages of epidermal differentiation, coupled with a loss of matriptase expression in the late stages of this process. We also show here that matriptase deficiency in HaCaT cells modestly reduces cell proliferation and temporally affects calcium-induced expression of differentiation markers. These collective data suggest that, unlike mouse matriptase, human matriptase may be involved in the regulation of keratinocyte growth and early differentiation, rather than terminal differentiation, providing mechanistic insights into the pathology of the two congenital ichthyoses: ARIH and IFAH.

The stratum corneum: the rampart of the mammalian body

BACKGROUND

The stratum corneum (SC) is the outermost region of the epidermis and plays key roles in cutaneous barrier function in mammals. The SC is composed of 'bricks', represented by flattened, protein-enriched corneocytes, and 'mortar', represented by intercellular lipid-enriched layers. As a result of this 'bricks and mortar' structure, the SC can be considered as a 'rampart' that encloses water and solutes essential for physiological homeostasis and that protects mammals from physical, chemical and biological assaults.

STRUCTURES AND FUNCTIONS

The corneocyte cytoskeleton contains tight bundles of keratin intermediate filaments aggregated with filaggrin monomers, which are subsequently degraded into natural moisturizing compounds by various proteases, including caspase 14. A cornified cell envelope is formed on the inner surface of the corneocyte plasma membrane by transglutaminase-catalysed cross-linking of involucrin and loricrin. Ceramides form a lipid envelope by covalently binding to the cornified cell envelope, and extracellular lamellar lipids play an important role in permeability barrier function. Corneodesmosomes are the main adhesive structures in the SC and are degraded by certain serine proteases, such as kallikreins, during desquamation.

CLINICAL RELEVANCE

The roles of the different SC components, including the structural proteins in corneocytes, extracellular lipids and some proteins associated with lipid metabolism, have been investigated in genetically engineered mice and in naturally occurring hereditary skin diseases, such as ichthyosis, ichthyosis syndrome and atopic dermatitis in humans, cattle and dogs.

Kallikrein-related peptidase 5 functions in proteolytic processing of profilaggrin in cultured human keratinocytes

Filaggrin protein is synthesized in the stratum granulosum of the skin and contributes to the formation of the human skin barrier. Profilaggrin is cleaved by proteolytic enzymes and converted to functional filaggrin, but its processing mechanism remains not fully elucidated. Kallikrein-related peptidase 5 (KLK5) is a major serine protease found in the skin, which is secreted from lamellar granules following its expression in the stratum granulosum and activated in the extracellular space of the stratum corneum. Here, we searched for profilaggrin-processing protease(s) by partial purification of epidermal extracts and found KLK5 as a possible candidate. We used high performance liquid chromatography coupled with electrospray tandem mass spectrometry to show that KLK5 cleaves profilaggrin. Furthermore, based on a proximity ligation assay, immunohistochemistry, and immunoelectron microscopy analysis, we reveal that KLK5 and profilaggrin co-localize in the stratum granulosum in human epidermis. KLK5 knockdown in normal cultured human epidermal keratinocytes resulted in higher levels of profilaggrin, indicating that KLK5 potentially functions in profilaggrin cleavage.

Autosomal recessive congenital ichthyosis

The term autosomal recessive congenital ichthyosis (ARCI) refers to a group of rare disorders of keratinization classified as nonsyndromic forms of ichthyosis. This group was traditionally divided into lamellar ichthyosis (LI) and congenital ichthyosiform erythroderma (CIE) but today it also includes harlequin ichthyosis, self-healing collodion baby, acral self-healing collodion baby, and bathing suit ichthyosis. The combined prevalence of LI and CIE has been estimated at 1 case per 138 000 to 300 000 population. In some countries or regions, such as Norway and the coast of Galicia, the prevalence may be higher due to founder effects. ARCI is genetically highly heterogeneous and has been associated with 6 genes to date: TGM1, ALOXE3, ALOX12B, NIPAL4, CYP4F22, and ABCA12. In this article, we review the current knowledge on ARCI, with a focus on clinical, histological, ultrastructural, genetic, molecular, and treatment-related aspects.

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.

Systematic analysis of rat 12/15-lipoxygenase enzymes reveals critical role for spinal eLOX3 hepoxilin synthase activity in inflammatory hyperalgesia

Previously, we observed significant increases in spinal 12-lipoxygenase (LOX) metabolites, in particular, hepoxilins, which contribute to peripheral inflammation-induced tactile allodynia. However, the enzymatic sources of hepoxilin synthase (HXS) activity in rats remain elusive. Therefore, we overexpressed each of the 6 rat 12/15-LOX enzymes in HEK-293T cells and measured by LC-MS/MS the formation of HXB3, 12-HETE, 8-HETE, and 15-HETE from arachidonic acid (AA) at baseline and in the presence of LOX inhibitors (NDGA, AA-861, CDC, baicalein, and PD146176) vs. vehicle-treated and mock-transfected controls. We detected the following primary intrinsic activities: 12-LOX (Alox12, Alox15), 15-LOX (Alox15b), and HXS (Alox12, Alox15). Similar to human and mouse orthologs, proteins encoded by rat Alox12b and Alox12e possessed minimal 12-LOX activity with AA as substrate, while eLOX3 (encoded by Aloxe3) exhibited HXS without 12-LOX activity when coexpressed with Alox12b or supplemented with 12-HpETE. CDC potently inhibited HXS and 12-LOX activity in vitro (relative IC50s: CDC, ~0.5 and 0.8 μM, respectively) and carrageenan-evoked tactile allodynia in vivo. Notably, peripheral inflammation significantly increased spinal eLOX3; intrathecal pretreatment with either siRNA targeting Aloxe3 or an eLOX3-selective antibody attenuated the associated allodynia. These findings implicate spinal eLOX3-mediated hepoxilin synthesis in inflammatory hyperesthesia and underscore the importance of developing more selective 12-LOX/HXS inhibitors.

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.

Aloxe3 knockout mice reveal a function of epidermal lipoxygenase-3 as hepoxilin synthase and its pivotal role in barrier formation

Loss-of-function mutations in the lipoxygenase (LOX) genes ALOX12B and ALOXE3 are the second most common cause of autosomal recessive congenital ichthyosis. The encoded proteins, 12R-LOX and epidermal LOX-3 (eLOX-3), act in sequence to convert fatty acid substrates via R-hydroperoxides to specific epoxyalcohol derivatives and have been proposed to operate in the same metabolic pathway during epidermal barrier formation. Here, we show that eLOX-3 deficiency in mice results in early postnatal death, associated with similar but somewhat less severe barrier defects and morphological changes than reported earlier for the 12R-LOX-knockout mice. Skin lipid analysis demonstrated that the severity of barrier failure is related to the loss of covalently bound ceramides in both 12R-LOX- and eLOX-3-null mice, confirming a proposed functional linkage of the LOX pathway to ceramide processing and formation of the corneocyte lipid envelope. Furthermore, analysis of free oxygenated fatty acid metabolites revealed strongly reduced levels of hepoxilin metabolites in eLOX-3-deficient epidermis, indicating an additional function of eLOX-3 in mammalian skin as a hepoxilin synthase linked to the 12S-LOX pathway.

EGFR regulation of epidermal barrier function

Keratinocyte terminal differentiation is the process that ultimately forms the epidermal barrier that is essential for mammalian survival. This process is controlled, in part, by signal transduction and gene expression mechanisms, and the epidermal growth factor receptor (EGFR) is known to be an important regulator of multiple epidermal functions. Using microarray analysis of a confluent cell density-induced model of keratinocyte differentiation, we identified 2,676 genes that are regulated by epidermal growth factor (EGF), a ligand of the EGFR. We further discovered, and separately confirmed by functional assays, that EGFR activation abrogates all of the known essential processes of keratinocyte differentiation by 1) decreasing the expression of lipid matrix biosynthetic enzymes, 2) regulating numerous genes forming the cornified envelope, and 3) suppressing the expression of tight junction proteins. In organotypic cultures of skin, EGF acted to impair epidermal barrier integrity, as shown by increased transepidermal water loss. As defective epidermal differentiation and disruption of barrier function are primary features of many human skin diseases, we used bioinformatic analyses to identify genes that are known to be associated with skin diseases. Compared with non-EGF-regulated genes, EGF-regulated genes were significantly enriched for skin disease genes. These results provide a systems-level understanding of the actions of EGFR signaling to inhibit keratinocyte differentiation, providing new insight into the role of EGFR imbalance in skin pathogenesis.

Ichthyin/NIPAL4 localizes to keratins and desmosomes in epidermis and Ichthyin mutations affect epidermal lipid metabolism

Autosomal recessive congenital ichthyosis (ARCI) is a group of disorders characterized by abnormal desquamation of the skin and a disrupted epidermal water barrier. Ichthyin/NIPAL4 gene mutations have been identified in a subgroup of ARCI patients, but the role of ichthyin in epidermis remains elusive. In order to obtain new insights concerning the characteristics of ichthyin and the ARCI pathogenesis, we studied the expression and localization of ichthyin and related epidermal components in cultured keratinocytes and skin sections from patients with Ichthyin mutations and healthy controls. We observed an up-regulation of Ichthyin mRNA levels after in vitro differentiation of keratinocytes from both a patient with Ichthyin mutations and controls. Confocal and electron microscopy analyses of immunolabeled skin sections revealed that ichthyin localizes to desmosomes and keratins in both patients with mutant Ichthyin and controls, with an increased immunolabeling in patients. Nile red lipid analysis of skin sections exposed intra-cellular lipid accumulations in cells of the granular and cornified layers in patients but not in controls, consistent with the pathognomonic lipid membrane structures previously identified in epidermis from patients. Our combined findings indicate that ichthyin is associated with keratins and desmosomes in epidermis and is involved in lipid metabolism, possibly through processing of lamellar bodies. These results provide new clues to the understanding of the epidermal water barrier and the pathogenesis in ARCI.

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.