12R-lipoxygenase deficiency disrupts epidermal barrier function

Stereocontrol of arachidonic acid oxygenation by vertebrate lipoxygenases: newly cloned zebrafish lipoxygenase 1 does not follow the Ala-versus-Gly concept

Animal lipoxygenases (LOXs) are classified according to their specificity of arachidonic acid oxygenation, and previous sequence alignments suggested that S-LOXs contain a conserved Ala at a critical position at the active site but R-LOXs carry a Gly instead. Here we cloned, expressed, and characterized a novel LOX isoform from the model vertebrate Danio rerio (zebrafish) that carries a Gly at this critical position, classifying this enzyme as putative arachidonic acid R-LOX. Surprisingly, the almost exclusive arachidonic acid oxygenation product was 12S-H(p)ETE (hydro(pero)xyeicosatetraenoic acid), and extensive mutation around Gly-410 failed to induce R-lipoxygenation. This finding prompted us to explore the importance of the corresponding amino acids in other vertebrate S-LOXs. We found that Ala-to-Gly exchange in human 15-LOX2 and human platelet 12-LOX induced major alterations in the reaction specificity with an increase of specific R-oxygenation products. For mouse 5-LOX and 12/15-LOX from rabbits, men, rhesus monkeys, orangutans, and mice, only minor alterations in the reaction specificity were observed. For these enzymes, S-HETE (hydroxyeicosatetraenoic acid) isomers remained the major oxygenation products, whereas chiral R-HETEs contributed only 10-30% to the total product mixture. Taken together these data indicate that the Ala-versus-Gly concept may not always predict the reaction specificity of vertebrate LOX isoforms.

Updated molecular genetics and pathogenesis of ichthiyoses

Research into the molecular genetics and pathomechanisms of ichthyoses have advanced considerably, resulting in the identification of several causative genes and molecules underlying the disease. In 2009, the First Ichthyosis Consensus Conference was held to establish a consensus for the nomenclature and classification of inherited ichthyoses, by which an international consensus for the classification of inherited ichthyosis was achieved. In this review, the pathogeneses of various ichthyoses are summarized based on their revised classification and terminology. Skin barrier defects are involved in the pathogenesis of various types of ichthyosis. The known causative molecules underlying ichthyosis include ABCA12, lipoxygenase-3, 12R-lipoxygenase, CYP4F22, ichthyin and steroid sulfatase, all of which are thought to be related to the intercellular lipid layers. ABCA12 is a known keratinocyte lipid transporter associated with lipid transport in lamellar granules and a loss of ABCA12 function leads to defective lipid transport in the keratinocytes, resulting in the most severe, harlequin ichthyosis phenotype. Other causative molecules for ichthyoses are transglutaminase 1, keratins and filaggrin. Transglutaminase 1 plays a role in cornified cell envelope formation. Keratins 1, 10 and 2 are involved in the keratin network of suprabasal keratinocytes and filaggrin is essential for the formation of keratohyalin granules. It is important to obtain information concerning genetic defects and to elucidate ichthyotic disease pathomechanisms for the establishment of an effective therapy and beneficial genetic counseling, including a prenatal diagnosis for families affected by ichthyotic disease.

Lipoxygenases mediate the effect of essential fatty acid in skin barrier formation: a proposed role in releasing omega-hydroxyceramide for construction of the corneocyte lipid envelope

A barrier to water loss is vital to maintaining life on dry land. Formation of the mammalian skin barrier requires both the essential fatty acid linoleate and the two lipoxygenases 12R-lipoxygenase (12R-LOX) and epidermal lipoxygenase-3 (eLOX3), although their roles are poorly understood. Linoleate occurs in O-linoleoyl-ω-hydroxyceramide, which, after hydrolysis of the linoleate moiety, is covalently attached to protein via the free ω-hydroxyl of the ceramide, forming the corneocyte lipid envelope, a scaffold between lipid and protein that helps seal the barrier. Here we show using HPLC-UV, LC-MS, GC-MS, and (1)H NMR that O-linoleoyl-ω-hydroxyceramide is oxygenated in a regio- and stereospecific fashion by the consecutive actions of 12R-LOX and eLOX3 and that these products occur naturally in pig and mouse epidermis. 12R-LOX forms 9R-hydroperoxy-linoleoyl-ω-hydroxyceramide, further converted by eLOX3 to specific epoxyalcohol (9R,10R-trans-epoxy-11E-13R-hydroxy) and 9-keto-10E,12Z esters of the ceramide; an epoxy-ketone derivative (9R,10R-trans-epoxy-11E-13-keto) is the most prominent oxidized ceramide in mouse skin. These products are absent in 12R-LOX-deficient mice, which crucially display a near total absence of protein-bound ω-hydroxyceramides and of the corneocyte lipid envelope and die shortly after birth from transepidermal water loss. We conclude that oxygenation of O-linoleoyl-ω-hydroxyceramide is required to facilitate the ester hydrolysis and allow bonding of the ω-hydroxyceramide to protein, providing a coherent explanation for the roles of multiple components in epidermal barrier function. Our study uncovers a hitherto unknown biochemical pathway in which the enzymic oxygenation of ceramides is involved in building a crucial structure of the epidermal barrier.

Mammalian soluble epoxide hydrolase is identical to liver hepoxilin hydrolase

Hepoxilins are lipid signaling molecules derived from arachidonic acid through the 12-lipoxygenase pathway. These trans-epoxy hydroxy eicosanoids play a role in a variety of physiological processes, including inflammation, neurotransmission, and formation of skin barrier function. Mammalian hepoxilin hydrolase, partly purified from rat liver, has earlier been reported to degrade hepoxilins to trioxilins. Here, we report that hepoxilin hydrolysis in liver is mainly catalyzed by soluble epoxide hydrolase (sEH): i) purified mammalian sEH hydrolyses hepoxilin A₃ and B₃ with a V(max) of 0.4-2.5 μmol/mg/min; ii) the highly selective sEH inhibitors N-adamantyl-N'-cyclohexyl urea and 12-(3-adamantan-1-yl-ureido) dodecanoic acid greatly reduced hepoxilin hydrolysis in mouse liver preparations; iii) hepoxilin hydrolase activity was abolished in liver preparations from sEH(-/-) mice; and iv) liver homogenates of sEH(-/-) mice show elevated basal levels of hepoxilins but lowered levels of trioxilins compared with wild-type animals. We conclude that sEH is identical to previously reported hepoxilin hydrolase. This is of particular physiological relevance because sEH is emerging as a novel drug target due to its major role in the hydrolysis of important lipid signaling molecules such as epoxyeicosatrienoic acids. sEH inhibitors might have undesired side effects on hepoxilin signaling.

14S,21R-dihydroxydocosahexaenoic acid remedies impaired healing and mesenchymal stem cell functions in diabetic wounds

Treatment of diabetes-impaired wound healing remains a major unresolved medical challenge. Here, we identified suppressed formation of a novel reparative lipid mediator 14S,21R-dihydroxydocosa-4Z,7Z,10Z,12E,16Z,19Z-hexaenoic acid (14S,21R-diHDHA) in cutaneous wounds of diabetic db/db mice. These results indicate that diabetes impedes the biosynthetic pathways of 14S,21R-diHDHA in skin wounds. Administration of exogenous 14S,21R-diHDHA to wounds in diabetic animals rescued healing and angiogenesis. When db/db mesenchymal stem cells (MSCs) were administered together with 14S,21R-diHDHA to wounds in diabetic animals, they coacted to accelerate wound re-epithelialization, granulation tissue formation, and synergistically improved vascularization. In the pivotal cellular processes of angiogenesis, 14S,21R-diHDHA enhanced VEGF release, vasculature formation, and migration of db/db dermal microvascular endothelial cells (DMVECs), as well as remedied paracrine angiogenic functions of db/db MSCs, including VEGF secretion and the promotion of DMVEC migration and vasculature formation. Our results show that 14S,21R-diHDHA activates the p38 MAPK pathway in wounds, db/db MSCs, and DMVECs. Overall, the impeded formation of 14S,21R-diHDHA described in this study suggests that diabetes could affect the generation of pro-healing lipid mediators in wound healing. By restoring wound healing and MSC functions, 14S,21R-diHDHA is a new lead for the development of better therapeutics used in treating wounds of diabetics.

The role of filaggrin in the atopic diathesis

Atopic dermatitis (AD) is an inflammatory disease characterized by pruritic skin lesions, immunodysregulation, disrupted epidermal barrier function and IgE-mediated sensitization to food and environmental allergens. Identification of the aetiology of AD has become increasingly a priority, as it is clear that the disease burden exceeds AD alone, with many children suffering severe, multi-system and occasionally life-threatening allergic disease. Previous approaches to understanding AD have centred on mechanisms in the adaptive immune system, often with an emphasis on the Th1-Th2 paradigm. Recently, the conceptual focus has increasingly shifted to include a primary defect in the epithelial barrier as a threshold event in moderate-to-severe AD. Familial aggregation of the disease is well established through many family studies of AD, asthma and allergic rhinitis, suggesting a significant heritable component. The identification of loss-of-function mutations in the filaggrin (FLG) gene, whose product is a key structural protein in the outermost layer of the epidermis in up to 50% of patients with AD, provides a significant insight into explaining disease initiation and points to a complex secondary interplay of environmental and immunological sequelae once barrier disruption is established. The elucidation of the environmental, genetic and immunobiological modifiers of this structural molecule may also direct our understanding of the pathomechanisms and endotypes central to the atopic diathesis. The recent identification of a murine model for FLG-AD, with the detection of a homozygous frame-shift mutation in the Flg gene in flaky-tail (ft/ft) mice, stands to rapidly accelerate our understanding of mechanisms and therapeutic intervention points in AD. Refining the molecular understanding of AD and its subtypes will allow for specific diagnostic, treatment and ultimately, preventative algorithms, and has opened an exciting new world of investigative challenges and collaborations.

Revised nomenclature and classification of inherited ichthyoses: results of the First Ichthyosis Consensus Conference in Sorèze 2009

BACKGROUND

Inherited ichthyoses belong to a large, clinically and etiologically heterogeneous group of mendelian disorders of cornification, typically involving the entire integument. Over the recent years, much progress has been made defining their molecular causes. However, there is no internationally accepted classification and terminology.

OBJECTIVE

We sought to establish a consensus for the nomenclature and classification of inherited ichthyoses.

METHODS

The classification project started at the First World Conference on Ichthyosis in 2007. A large international network of expert clinicians, skin pathologists, and geneticists entertained an interactive dialogue over 2 years, eventually leading to the First Ichthyosis Consensus Conference held in Sorèze, France, on January 23 and 24, 2009, where subcommittees on different issues proposed terminology that was debated until consensus was reached.

RESULTS

It was agreed that currently the nosology should remain clinically based. "Syndromic" versus "nonsyndromic" forms provide a useful major subdivision. Several clinical terms and controversial disease names have been redefined: eg, the group caused by keratin mutations is referred to by the umbrella term, "keratinopathic ichthyosis"-under which are included epidermolytic ichthyosis, superficial epidermolytic ichthyosis, and ichthyosis Curth-Macklin. "Autosomal recessive congenital ichthyosis" is proposed as an umbrella term for the harlequin ichthyosis, lamellar ichthyosis, and the congenital ichthyosiform erythroderma group.

LIMITATIONS

As more becomes known about these diseases in the future, modifications will be needed.

CONCLUSION

We have achieved an international consensus for the classification of inherited ichthyosis that should be useful for all clinicians and can serve as reference point for future research.

Dioxygenase activity of epidermal lipoxygenase-3 unveiled: typical and atypical features of its catalytic activity with natural and synthetic polyunsaturated fatty acids

Epidermal lipoxygenase-3 (eLOX3) exhibits hydroperoxide isomerase activity implicated in epidermal barrier formation, but its potential dioxygenase activity has remained elusive. We identified herein a synthetic fatty acid, 9E,11Z,14Z-20:3ω6, that was oxygenated by eLOX3 specifically to the 9S-hydroperoxide. Reaction showed a pronounced lag phase, which suggested that eLOX3 is deficient in its activation step. Indeed, we found that high concentrations of hydroperoxide activator (e.g. 65 μM) overcame a prolonged lag phase (>1 h) and unveiled a dioxygenase activity with arachidonic acid; the main products were the 5-, 9-, and 7-hydroperoxyeicosatetraenoic acids (HPETEs). These were R/S mixtures (ranging from ∼50:50 to 73:27), and as the bis-allylic 7-HPETE can be formed only inside the enzyme active site, the results indicate there is oxygen availability along either face of the reacting fatty acid radical. That the active site oxygen supply is limited is implied from the need for continuous re-activation, as carbon radical leakage leaves the enzyme in the unactivated ferrous state. An Ala-to-Gly mutation, known to affect the positioning of O(2) in the active site of other lipoxygenase enzymes, led to more readily activated reaction and a significant increase in the 9R- over the 5-HPETE. Activation and cycling of the ferric enzyme are thus promoted using the 9E,11Z,14Z-20:3ω6 substrate, by continuous hydroperoxide activation, or by the Ala-to-Gly mutation. We suggest that eLOX3 represents one end of a spectrum among lipoxygenases where activation is inefficient, favoring hydroperoxide isomerase cycling, with the opposite end represented by readily activated enzymes in which dioxygenase activity is prominent.

Redox control in mammalian embryo development

The development of an embryo constitutes a complex choreography of regulatory events that underlies precise temporal and spatial control. Throughout this process the embryo encounters ever changing environments, which challenge its metabolism. Oxygen is required for embryogenesis but it also poses a potential hazard via formation of reactive oxygen and reactive nitrogen species (ROS/RNS). These metabolites are capable of modifying macromolecules (lipids, proteins, nucleic acids) and altering their biological functions. On one hand, such modifications may have deleterious consequences and must be counteracted by antioxidant defense systems. On the other hand, ROS/RNS function as essential signal transducers regulating the cellular phenotype. In this context the combined maternal/embryonic redox homeostasis is of major importance and dysregulations in the equilibrium of pro- and antioxidative processes retard embryo development, leading to organ malformation and embryo lethality. Silencing the in vivo expression of pro- and antioxidative enzymes provided deeper insights into the role of the embryonic redox equilibrium. Moreover, novel mechanisms linking the cellular redox homeostasis to gene expression regulation have recently been discovered (oxygen sensing DNA demethylases and protein phosphatases, redox-sensitive microRNAs and transcription factors, moonlighting enzymes of the cellular redox homeostasis) and their contribution to embryo development is critically reviewed.

Molecular enzymology of lipoxygenases

Lipoxygenases (LOXs) are lipid peroxidizing enzymes, implicated in the pathogenesis of inflammatory and hyperproliferative diseases, which represent potential targets for pharmacological intervention. Although soybean LOX1 was discovered more than 60years ago, the structural biology of these enzymes was not studied until the mid 1990s. In 1993 the first crystal structure for a plant LOX was solved and following this protein biochemistry and molecular enzymology became major fields in LOX research. This review focuses on recent developments in molecular enzymology of LOXs and summarizes our current understanding of the structural basis of LOX catalysis. Various hypotheses explaining the reaction specificity of different isoforms are critically reviewed and their pros and cons briefly discussed. Moreover, we summarize the current knowledge of LOX evolution by profiling the existence of LOX-related genomic sequences in the three kingdoms of life. Such sequences are found in eukaryotes and bacteria but not in archaea. Although the biological role of LOXs in lower organisms is far from clear, sequence data suggests that this enzyme family might have evolved shortly after the appearance of atmospheric oxygen on earth.

Reactive oxygen species: A radical role in development?

Reactive oxygen species (ROS), mostly derived from mitochondrial activity, can damage various macromolecules and consequently cause cell death. This ROS activity has been characterized in vitro, and correlative evidence suggests a role in various pathological conditions. In addition to this passive ROS activity, ROS also participate in cell signaling processes, though the relevance of this function in vivo is poorly understood. Throughout development, elevated cell activity is probably accompanied by highly active metabolism and, consequently, the production of large amounts of ROS. To allow proper development, cells must protect themselves from these potentially damaging ROS. However, to what degree ROS could participate as signaling molecules controlling fundamental and developmentally relevant cellular processes such as proliferation, differentiation, and death is an open question. Here we discuss why available data do not yet provide conclusive evidence on the role of ROS in development, and we review recent methods to detect ROS in vivo and genetic strategies that can be exploited specifically to resolve these uncertainties.

Eicosanoids in inflammation and cancer: the role of COX-2

Eicosanoids, a family of oxygenated metabolites of eicosapolyenoic fatty acids, such as arachidonic acid, formed via the lipoxygenase, cyclooxygenase (COX) and epoxygenase pathways, play an important role in the regulation of various pathophysiological processes, including inflammation and cancer. COX-2, the inducible isoform of COX, has emerged as the key enzyme regulating inflammation, and promises to play a considerable role in cancer. Although NSAIDs have been in use for centuries, the COX-2 selective inhibitors - coxibs - have emerged as potent anti-inflammatory drugs with fewer gastric side effects. As COX-2 plays a major role in neoplastic transformation and cancer growth, by downregulating apoptosis and promoting angiogenesis, invasion and metastasis, coxibs have a potential role in the prevention and treatment of cancer. Recent studies indicate their possible application in overcoming drug resistance by downregulating the expression of MDR-1. However, the cardiac side effects of some of the coxibs have limited their application in treating various inflammatory disorders and warrant the development of COX-2 inhibitors without side effects. This review will focus on the role of COX-2 in inflammation and cancer, with an emphasis on novel approaches to the development of COX-2 inhibitors without side effects.

Self-improvement of keratinocyte differentiation defects during skin maturation in ABCA12-deficient harlequin ichthyosis model mice

Harlequin ichthyosis (HI) is caused by loss-of-function mutations in the keratinocyte lipid transporter ABCA12. The patients often die in the first 1 or 2 weeks of life, although HI survivors' phenotypes improve within several weeks after birth. In order to clarify the mechanisms of phenotypic recovery, we studied grafted skin and keratinocytes from Abca12-disrupted (Abca12(-/-)) mice showing abnormal lipid transport. Abca12(-/-) neonatal epidermis showed significantly reduced total ceramide amounts and aberrant ceramide composition. Immunofluorescence and immunoblotting of Abca12(-/-) neonatal epidermis revealed defective profilaggrin/filaggrin conversion and reduced protein expression of the differentiation-specific molecules, loricrin, kallikrein 5, and transglutaminase 1, although their mRNA expression was up-regulated. In contrast, Abca12(-/-) skin grafts kept in a dry environment exhibited dramatic improvements in all these abnormalities. Increased transepidermal water loss, a parameter representing barrier defect, was remarkably decreased in grafted Abca12(-/-) skin. Ten-passage sub-cultured Abca12(-/-) keratinocytes showed restoration of intact ceramide distribution, differentiation-specific protein expression and profilaggrin/filaggrin conversion, which were defective in primary-cultures. Using cDNA microarray analysis, lipid transporters including four ATP-binding cassette transporters were up-regulated after sub-culture of Abca12(-/-) keratinocytes compared with primary-culture. These results indicate that disrupted keratinocyte differentiation during the fetal development is involved in the pathomechanism of HI and, during maturation, Abca12(-/-) epidermal keratinocytes regain normal differentiation processes. This restoration may account for the skin phenotype improvement observed in HI survivors.

Krankheitsspezifische Hautmodelle

In den letzten Jahren wurden Hautmodelle besonders für die Behandlung von Verbrennungsopfern, aber auch zur molekularen und zellbiologischen Aufklärung von dermatologischen und immunologischen Erkrankungen und Untersuchung von Kosmetika und Pharmaka entwickelt. Kommerzielle Modelle, als alternative Methode für Tierversuche in der kosmetischen und chemischen Industrie entwickelt, sind vielfältig erhältlich und entsprechend den Richtlinien von OECD (Organisation for Economic Co-operation and Development) und ECVAM (European Centre for the Validation of Alternative Methods) validiert. Hautmodelle beinhalten 2 Kompartimente, ein Dermisäquivalent, welches meist aus Typ-I-Kollagen und darin enthaltenen Fibroblasten besteht, und ein voll differenzierbares Epidermisäquivalent. Bis heute existieren nur wenige Hautmodelle, die spezifische monogene Hautkrankheiten simulieren oder in der Haut sichtbare Krankheitszeichen einer systemischen Autoimmunerkrankung darstellen. Wir haben kürzlich ein solches Modell für kongenitale Ichthyosen entwickelt, das hinsichtlich seiner physikalischen, physiologischen und zellbiologischen Eigenschaften validiert wird und zur Erprobung neuer Therapien geeignet ist.

LC-MS/MS analysis of epoxyalcohols and epoxides of arachidonic acid and their oxygenation by recombinant CYP4F8 and CYP4F22

CYP4F22 and CYP4F8 are expressed in epidermis, and mutations of CYP4F22 are associated with lamellar ichthyosis. Epoxyalcohols (HEETs) and epoxides (EETs) of 20:4n-6 appear to be important for the water permeability barrier of skin. Our aim was to study the MS/MS spectra and fragmentation of these compounds and to determine whether they were oxidized by CYP4F22 or CYP4F8 expressed in yeast. HEETs were prepared from 15-hydroperoxyeicosatetraenoic acid (15-HPETE), 12-HPETE, and their [(2)H(8)]labeled isotopomers, and separated by normal phase-HPLC with MS/MS analysis. CYP4F22 oxygenated 20:4n-6 at C-18, whereas metabolites of HEETs could not be identified. CYP4F8 formed omega3 hydroxy metabolites of HEETs derived from 12R-HPETE with 11,12-epoxy-10-hydroxy configuration, but not HEETs derived from 15S-HPETE. 8,9-EET and 11,12-EET were also subject to omega3 hydroxylation by CYP4F8. We conclude that CYP4F8 and CYP4F22 oxidize 20:4n-6 and that CYP4F8 selectively oxidizes 8,9-EET, 11,12-EET, and 10,11R,12R-HEET at the omega3 position.

Genotypic and clinical spectrum of self-improving collodion ichthyosis: ALOX12B, ALOXE3, and TGM1 mutations in Scandinavian patients

Infants born with autosomal recessive congenital ichthyosis (ARCI) are often encapsulated in a collodion membrane, which shows a lamellar or erythrodermic type of ichthyosis upon shedding. However, some babies show a nearly normal underlying skin after several weeks, a phenotype called "self-healing collodion baby" (SHCB). Mutations in two genes, TGM1 and ALOX12B, have previously been implicated in the etiology of SHCB, but the full genotypic spectrum remains to be determined. DNA sequencing in 11 Swedish and 4 Danish SHCB patients showed ALOX12B mutations in eight cases, ALOXE3 mutations in three cases, and TGM1 mutations in one case. In three patients, we could not find mutations in any of the known ARCI genes. In all cases, a spontaneous shedding of the collodion membrane occurred 2-4 weeks after birth. When re-examined at 2-37 years of age, the patients showed skin xerosis, a mild or focal scaling, palmar hyperlinearity with keratoderma, and a frequent appearance of red cheeks and anhidrosis. Thus, we propose replacing SHCB with the term "self-improving collodion ichthyosis" (SICI). In conclusion, ALOX12B mutations are the leading cause of SICI in Scandinavia, followed by ALOXE3 mutations, which have not been previously associated with this variant of ARCI.

Filaggrin in the frontline: role in skin barrier function and disease

Recently, loss-of-function mutations in FLG, the human gene encoding profilaggrin and filaggrin, have been identified as the cause of the common skin condition ichthyosis vulgaris (which is characterised by dry, scaly skin). These mutations, which are carried by up to 10% of people, also represent a strong genetic predisposing factor for atopic eczema, asthma and allergies. Profilaggrin is the major component of the keratohyalin granules within epidermal granular cells. During epidermal terminal differentiation, the approximately 400 kDa profilaggrin polyprotein is dephosphorylated and rapidly cleaved by serine proteases to form monomeric filaggrin (37 kDa), which binds to and condenses the keratin cytoskeleton and thereby contributes to the cell compaction process that is required for squame biogenesis. Within the squames, filaggrin is citrullinated, which promotes its unfolding and further degradation into hygroscopic amino acids, which constitute one element of natural moisturising factor. Loss of profilaggrin or filaggrin leads to a poorly formed stratum corneum (ichthyosis), which is also prone to water loss (xerosis). Recent human genetic studies strongly suggest that perturbation of skin barrier function as a result of reduction or complete loss of filaggrin expression leads to enhanced percutaneous transfer of allergens. Filaggrin is therefore in the frontline of defence, and protects the body from the entry of foreign environmental substances that can otherwise trigger aberrant immune responses.

Inhibition of 12-LOX and COX-2 reduces the proliferation of human epidermoid carcinoma cells (A431) by modulating the ERK and PI3K-Akt signalling pathways

Eicosanoids, the oxygenated metabolites of arachidonic acid (AA), mediate a variety of human diseases, such as cancer, inflammation and arthritis. To evaluate the role of eicosanoids in epidermoid carcinoma, the expression of AA metabolizing enzymes, such as lipoxygenases (LOXs) and cyclooxygenases (COXs), was analysed in a human epidermoid carcinoma cell line (A431). These studies revealed overexpression of 12-R-LOX and COX-2 in A431 cells. Baicalein (a 12-LOX inhibitor) and celecoxib (a COX-2 inhibitor) significantly reduced thymidine incorporation, whereas 12-(R)-HETE and 12-(S)-HETE (12-LOX metabolites) and PGE(2) (COX-2 metabolite) significantly enhanced thymidine incorporation, suggesting a role for these enzymes in the regulation of A431 cell proliferation. Further studies on the mechanism of cell death by baicalein and celecoxib revealed that the induction of apoptosis in A431 cells was associated with reduction in the Bcl-2/Bax ratio, release of cytochrome c, activation of caspase-3 and PARP cleavage. The apoptosis induced by baicalein and celecoxib was mediated by down regulation of ERK and PI3K-Akt pathways. Further, 12-(R)-HETE, 12-(S)-HETE and PGE(2) upregulated the p-ERK and p-Akt levels, suggesting the involvement of ERK and Akt pathways in the 12-LOX- and COX-2-mediated regulation of growth in A431 cells. Our findings suggest that 12-R-LOX and COX-2 play a critical role in the regulation of growth in epidermoid carcinoma and that their inhibitors may be of potential therapeutic importance.

Filaggrin haploinsufficiency is highly penetrant and is associated with increased severity of eczema: further delineation of the skin phenotype in a prospective epidemiological study of 792 school children

Background

Null mutations within the filaggrin gene (FLG) cause ichthyosis vulgaris and are associated with atopic eczema. However, the dermatological features of filaggrin haploinsufficiency have not been clearly defined.

Objectives

This study investigated the genotype–phenotype association between detailed skin phenotype and FLG genotype data in a population-based cohort of children.

Methods

Children (n= 792) aged 7–9 years were examined by a dermatologist. Features of ichthyosis vulgaris, atopic eczema and xerosis were recorded and eczema severity graded using the Three Item Severity score. Each child was genotyped for the six most prevalent FLG null mutations (R501X, 2282del4, R2447X, S3247X, 3702delG, 3673delC). Fisher’s exact test was used to compare genotype frequencies in phenotype groups; logistic regression analysis was used to estimate odds ratios and penetrance of the FLG null genotype and a permutation test performed to investigate eczema severity in different genotype groups.

Results

Ten children in this cohort had ichthyosis vulgaris, of whom five had mild–moderate eczema. The penetrance of FLG null mutations with respect to flexural eczema was 55·6% in individuals with two mutations, 16·3% in individuals with one mutation and 14·2% in wild-type individuals. Summating skin features known to be associated with FLG null mutations (ichthyosis, keratosis pilaris, palmar hyperlinearity and flexural eczema) showed a penetrance of 100% in children with two FLG mutations, 87·8% in children with one FLG mutation and 46·5% in wild-type individuals (P< 0·0001, Fisher exact test). FLG null mutations were associated with more severe eczema (P= 0·0042) but the mean difference was only 1–2 points in severity score. Three distinct patterns of palmar hyperlinearity were observed and these are reported for the first time.

Conclusions

Filaggrin haploinsufficiency appears to be highly penetrant when all relevant skin features are included in the analysis. FLG null mutations are associated with more severe eczema, but the effect size is small in a population setting.

Structural basis for pH-dependent alterations of reaction specificity of vertebrate lipoxygenase isoforms

Lipoxygenases have been classified according to their specificity of fatty acid oxygenation and for several plant enzymes pH-dependent alterations in the product patterns have been reported. Assuming that the biological role of mammalian lipoxygenases is based on the formation of specific reaction products, pH-dependent alterations would impact enzymes' functionality. In this study we systematically investigated the pH-dependence of vertebrate lipoxygenases and observed a remarkable stability of the product pattern in the near physiological range for the wild-type enzyme species. Site-directed mutagenesis of selected amino acids and alterations in the substrate concentrations induced a more pronounced pH-dependence of the reaction specificity. For instance, for the V603H mutant of the human 15-lipoxygenase-2 8-lipoxygenation was dominant at acidic pH (65%) whereas 15-H(p)ETE was the major oxygenation product at pH 8. Similarly, the product pattern of the wild-type mouse 8-lipoxygenase was hardly altered in the near physiological pH range but H604F exchange induced strong pH-dependent alterations in the positional specificity. Taken together, our data suggest that the reaction specificities of wild-type vertebrate lipoxygenase isoforms are largely resistant towards pH alterations. However, we found that changes in the assay conditions (low substrate concentration) and introduction/removal of a critical histidine at the active site impact the pH-dependence of reaction specificity for some lipoxygenase isoforms.

Mammalian epoxide hydrolases in xenobiotic metabolism and signalling

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

Molecular analysis of 250 patients with autosomal recessive congenital ichthyosis: evidence for mutation hotspots in ALOXE3 and allelic heterogeneity in ALOX12B

In recent years several new genes for autosomal recessive congenital ichthyosis (ARCI) have been identified. However, little is known about the molecular epidemiology and pathophysiology of this genetically and clinically heterogeneous group of severe disorders of keratinization. ARCI is characterized by intense scaling of the whole integument often associated with erythema. We and others have shown that mutations in ALOX12B and ALOXE3, coding for the lipoxygenases 12R-LOX and eLOX-3 predominantly synthesized in the epidermis, can underlie this rare condition. Here we have surveyed a large group of 250 patients with ARCI for mutations in these two genes. We have identified 11 different previously unreported mutations in ALOX12B and ALOXE3 in 21 ARCI patients from 19 unrelated families and demonstrated that mutations in the two genes are the second most common cause for ARCI in this cohort of patients. Examination of the molecular data revealed allelic heterogeneity for ALOX12B and two mutational hotspots in ALOXE3. Functional analysis of all missense mutations and a splice site mutation demonstrated that complete loss of function of the enzymes underlies the phenotype. Our findings further establish the pivotal role of the 12-lipoxygenase pathway during epidermal differentiation.

Development of an ichthyosiform phenotype in Alox12b-deficient mouse skin transplants

12R-lipoxygenase (12R-LOX) represents a key enzyme of a recently identified eicosanoid pathway in the skin that plays an essential role in the establishment and/or maintenance of the epidermal barrier function. Genetic studies show that loss-of-function mutations in ALOX12B, encoding 12R-LOX, and in ALOXE3, encoding another closely related LOX involved in this pathway, are the second most common cause for autosomal recessive congenital ichthyosis (ARCI). To investigate the pathomechanism of ARCI and the function of 12R-LOX, we recently generated a 12R-LOX knockout model. 12R-LOX-deficient mice die rapidly after birth from severe barrier dysfunction without exhibiting an obvious cutaneous phenotype. Thus, we analyzed the adult phenotype of 12R-LOX(-/-) skin transplanted onto nude mice. 12R-LOX(-/-) skin develops an ichthyosiform appearance with thickening of the epidermis, hyperproliferation, hypergranulosis, focal parakeratosis, and severe hyperkeratosis. The adult mutant mouse skin phenotype closely reproduces the ichthyosis phenotype seen in patients with ALOX12B mutations. Western blot analysis revealed restoration of profilaggrin processing that used to be disturbed in neonatal mutant skin and overexpression of filaggrin, involucrin, and repetin. The results indicate that 12R-LOX knockout mice may represent a useful animal model for a detailed analysis of mechanisms involved in ARCI forms that are associated with impaired LOX metabolism.

Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation

COUP-TF-interacting protein 2 (CTIP2; also known as Bcl11b) is a transcription factor that plays key roles in the development of the central nervous and immune systems. CTIP2 is also highly expressed in the developing epidermis, and at lower levels in the dermis and in adult skin. Analyses of mice harboring a germline deletion of CTIP2 revealed that the protein plays critical roles in skin during development, particularly in keratinocyte proliferation and late differentiation events, as well as in the development of the epidermal permeability barrier. At the core of all of these actions is a relatively large network of genes, described herein, that is regulated directly or indirectly by CTIP2. The analysis of conditionally null mice, in which expression of CTIP2 was ablated specifically in epidermal keratinocytes, suggests that CTIP2 functions in both cell and non-cell autonomous contexts to exert regulatory influence over multiple phases of skin development, including barrier establishment. Considered together, our results suggest that CTIP2 functions as a top-level regulator of skin morphogenesis.

Computational analysis of R and S isoforms of 12-lipoxygenases: homology modeling and docking studies

The present study is aimed at predicting human 12R-LOX structure by constructing a homology model. Based upon Blast results, rabbit reticulocyte 15-lipoxygenase 1LOX (protein data bank) was considered as a template for homology modeling. The 3D model was generated with Modeler in InsightII and further refined using AMBER. Further to understand the relationship of protein structure with stereo specificity, a comparative analysis of 12R-LOX model was done with that of 12S-LOX homology model to identify differences in the binding site topology and interacting residues. The large insertion of 31-aa seen in 12R-LOX is located beyond the N-terminal barrel and is accommodated on the outside of the protein without disruption of the overall tertiary structure. The 31-aa region includes SH3 domain binding PXXP motif, seven prolines and five arginines. The docking of the substrate, arachidonic acid was also performed. Our results show that the Gly441 and substrate orientation within the active site play an important role in stereo specificity of 12R-LOX.

ABCA12 maintains the epidermal lipid permeability barrier by facilitating formation of ceramide linoleic esters

Harlequin ichthyosis is a congenital scaling syndrome of the skin in which affected infants have epidermal hyperkeratosis and a defective permeability barrier. Mutations in the gene encoding a member of the ABCA transporter family, ABCA12, have been linked to harlequin ichthyosis, but the molecular function of the protein is unknown. To investigate the activity of ABCA12, we generated Abca12 null mice and analyzed the impact on skin function and lipid content. Abca12-/- mice are born with a thickened epidermis and die shortly after birth, as water rapidly evaporates from their skin. In vivo skin proliferation measurements suggest a lack of desquamation of the skin cells, rather than enhanced proliferation of basal layer keratinocytes, accounts for the 5-fold thickening of the Abca12-/- stratum corneum. Electron microscopy revealed a loss of the lamellar permeability barrier in Abca12-/- skin. This was associated with a profound reduction in skin linoleic esters of long-chain omega-hydroxyceramides and a corresponding increase in their glucosyl ceramide precursors. Because omega-hydroxyceramides are required for the barrier function of the skin, these results establish that ABCA12 activity is required for the generation of long-chain ceramide esters that are essential for the development of normal skin structure and function.

Ichthyosis, follicular atrophoderma, and hypotrichosis caused by mutations in ST14 is associated with impaired profilaggrin processing

Congenital ichthyosis encompasses a heterogeneous group of disorders of cornification. Isolated forms and syndromic ichthyosis can be differentiated. We have analyzed two consanguineous families from the United Arab Emirates and Turkey with an autosomal recessive syndrome of diffuse congenital ichthyosis, patchy follicular atrophoderma, generalized and diffuse nonscarring hypotrichosis, marked hypohidrosis, and woolly hair (OMIM 602400). By genome-wide analysis, we found a homozygous interval on chromosome 11q24-q25 and obtained a LOD score of 4.0 at D11S910. We identified a homozygous splice-site mutation in the Arab patients and a frame-shift deletion in the Turkish patient in the gene suppression of tumorigenicity-14 (ST14). The product of ST14, matriptase, is a type II transmembrane serine protease synthesized in most human epithelia. Two missense mutations in ST14 were recently described in patients with a phenotype of ichthyosis and hypotrichosis, indicating diverse activities of matriptase in the epidermis and hair follicles. Here we have further demonstrated the loss of matriptase in differentiated patient keratinocytes, reduced proteolytic activation of prostasin, and disturbed processing of profilaggrin. As filaggrin monomers play a pivotal role in epidermal barrier formation, these findings reveal the link between congenital disorders of keratinization and filaggrin processing in the human skin.

12(R)-Hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid derivative, is an activator of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is a ligand-regulated transcription factor that can be activated by structurally diverse chemicals, ranging from environmental carcinogens to dietary metabolites. Evidence supporting a necessary role for the AHR in normal biology has been established; however, identification of key endogenous ligand/activator remains to be established. Here, we report the ability of 12(R)-hydroxy-5(Z),8(Z),10(E), 14(Z)-eicosatetraenoic acid [12(R)-HETE], an arachidonic acid metabolite produced by either a lipoxygenase or cytochrome P-450 pathway, to act as a potent indirect modulator of the AHR pathway. In contrast, structurally similar HETE isomers failed to demonstrate significant activation of the AHR. Electrophoretic mobility shift assays, together with ligand competition binding experiments, have demonstrated the inability of 12(R)-HETE to directly bind or directly activate the AHR to a DNA binding species in vitro. However, cell-based xenobiotic-responsive element-driven luciferase reporter assays indicate the ability of 12(R)-HETE to modulate AHR activity, and quantitation of induction of an AHR target gene confirmed 12(R)-HETE's ability to activate AHR-mediated transcription, even at high nanomolar concentrations in human hepatoma (HepG2)- and keratinocyte (HaCaT)-derived cell lines. One explanation for these results is that a metabolite of 12(R)-HETE is acting as a direct ligand for the AHR. However, several known metabolites failed to exhibit AHR activity. The ability of 12(R)-HETE to activate AHR target genes required receptor expression. These results indicate that 12(R)-HETE can serve as a potent activator of AHR activity and suggest that in normal and inflammatory disease conditions in skin, 12(R)-HETE is produced, perhaps leading to AHR activation.

Modeling atopic dermatitis with increasingly complex mouse models

Atopic dermatitis (AD) is a chronic relapsing inflammatory skin disorder that affects approximately 15% of children in the United States. A complex disorder, AD is characterized by both skin barrier impairment and immunologic abnormalities, including decreased innate immune function and a polarized adaptive immune response. Mouse models have demonstrated the complex interdependence of immune cell-keratinocyte interactions and teased apart gene-environment relationships in a controlled setting. In this issue, Nagelkerken et al. present a mouse model with transgenic expression of apolipoprotein C1 that disrupts the skin lipid barrier and manifests many hallmark features of AD.

Pathogenesis of permeability barrier abnormalities in the ichthyoses: inherited disorders of lipid metabolism

Many of the ichthyoses are associated with inherited disorders of lipid metabolism. These disorders have provided unique models to dissect physiologic processes in normal epidermis and the pathophysiology of more common scaling conditions. In most of these disorders, a permeability barrier abnormality "drives" pathophysiology through stimulation of epidermal hyperplasia. Among primary abnormalities of nonpolar lipid metabolism, triglyceride accumulation in neutral lipid storage disease as a result of a lipase mutation provokes a barrier abnormality via lamellar/nonlamellar phase separation within the extracellular matrix of the stratum corneum (SC). Similar mechanisms account for the barrier abnormalities (and subsequent ichthyosis) in inherited disorders of polar lipid metabolism. For example, in recessive X-linked ichthyosis (RXLI), cholesterol sulfate (CSO(4)) accumulation also produces a permeability barrier defect through lamellar/nonlamellar phase separation. However, in RXLI, the desquamation abnormality is in part attributable to the plurifunctional roles of CSO(4) as a regulator of both epidermal differentiation and corneodesmosome degradation. Phase separation also occurs in type II Gaucher disease (GD; from accumulation of glucosylceramides as a result of to beta-glucocerebrosidase deficiency). Finally, failure to assemble both lipids and desquamatory enzymes into nascent epidermal lamellar bodies (LBs) accounts for both the permeability barrier and desquamation abnormalities in Harlequin ichthyosis (HI). The barrier abnormality provokes the clinical phenotype in these disorders not only by stimulating epidermal proliferation, but also by inducing inflammation.

Nutritionally essential fatty acids and biologically indispensable cyclooxygenases

The study of cyclooxygenases (COXs), targets of aspirin and related drugs, is rooted in the discovery of essential fatty acids (EFAs). There are two COXs that convert EFAs, primarily arachidonic acid, to prostaglandins. Each COX is involved with distinct biologies. COX-1 expression is constitutive while COX-2 is inducible. The two COXs might have evolved partly to permit prostaglandin formation at different tissue sites. However, COX-2 is sometimes induced in cells already expressing COX-1, and in these instances, COX-2 functions while COX-1 is latent. This can occur because of unique biochemical properties of COX-2 that enable cells to form prostaglandins when arachidonic acid comprises a small fraction of available fatty acids and the concentrations of peroxides that are necessary for COX to function are low.

Hepoxilin A3 (HXA3) synthase deficiency is causative of a novel ichthyosis form

Non-bullous congenital ichthyosis erythroderma (NCIE) and lamellar ichthyosis (LI) are characterized by mutations in 12R-lipoxygenase (12R-LOX) and/or epidermal lipoxygenase 3 (eLOX3) enzymes. The eLOX3 lacks oxygenase activity, but is capable of forming hepoxilin-type products from arachidonic acid-derived hydroperoxide from 12R-LOX, termed 12R-hydroperoxyeicosa-5,8,10,14-tetraenoic acid (12R-HpETE). Mutations in either of two enzymes lead to NCIE or LI. Moreover, 12R-LOX-deficient mice exhibit severe phenotypic water barrier dysfunctions. Here, we demonstrate that 12R-HpETE can also be transformed to 8R-HXA(3) by hepoxilin A(3) (HXA(3)) synthase (12-lipoxygenase), which exhibits oxygenase activity. We also presented a novel form of ichthyosis in a patient, termed hepoxilin A(3) synthase-linked ichthyosis (HXALI), whose scales expressed high levels of 12R-LOX, but were deficient of HXA(3) synthase.

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

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

Thematic review series: skin lipids. The role of epidermal lipids in cutaneous permeability barrier homeostasis

The permeability barrier is required for terrestrial life and is localized to the stratum corneum, where extracellular lipid membranes inhibit water movement. The lipids that constitute the extracellular matrix have a unique composition and are 50% ceramides, 25% cholesterol, and 15% free fatty acids. Essential fatty acid deficiency results in abnormalities in stratum corneum structure function. The lipids are delivered to the extracellular space by the secretion of lamellar bodies, which contain phospholipids, glucosylceramides, sphingomyelin, cholesterol, and enzymes. In the extracellular space, the lamellar body lipids are metabolized by enzymes to the lipids that form the lamellar membranes. The lipids contained in the lamellar bodies are derived from both epidermal lipid synthesis and extracutaneous sources. Inhibition of cholesterol, fatty acid, ceramide, or glucosylceramide synthesis adversely affects lamellar body formation, thereby impairing barrier homeostasis. Studies have further shown that the elongation and desaturation of fatty acids is also required for barrier homeostasis. The mechanisms that mediate the uptake of extracutaneous lipids by the epidermis are unknown, but keratinocytes express LDL and scavenger receptor class B type 1, fatty acid transport proteins, and CD36. Topical application of physiologic lipids can improve permeability barrier homeostasis and has been useful in the treatment of cutaneous disorders.

The hepoxilin connection in the epidermis

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