Diversity of mouse lipoxygenases: identification of a subfamily of epidermal isozymes exhibiting a differentiation-dependent mRNA expression pattern

Murine Alox8 versus the human ALOX15B ortholog: differences and similarities

Human arachidonate 15-lipoxygenase type B is a lipoxygenase that catalyzes the peroxidation of arachidonic acid at carbon-15. The corresponding murine ortholog however has 8-lipoxygenase activity. Both enzymes oxygenate polyunsaturated fatty acids in S-chirality with singular reaction specificity, although they generate a different product pattern. Furthermore, while both enzymes utilize both esterified fatty acids and fatty acid hydro(pero)xides as substrates, they differ with respect to the orientation of the fatty acid in their substrate-binding pocket. While ALOX15B accepts the fatty acid "tail-first," Alox8 oxygenates the free fatty acid with its "head-first." These differences in substrate orientation and thus in regio- and stereospecificity are thought to be determined by distinct amino acid residues. Towards their biological function, both enzymes share a commonality in regulating cholesterol homeostasis in macrophages, and Alox8 knockdown is associated with reduced atherosclerosis in mice. Additional roles have been linked to lung inflammation along with tumor suppressor activity. This review focuses on the current knowledge of the enzymatic activity of human ALOX15B and murine Alox8, along with their association with diseases.

12-Hydroxyeicosatetraenoic acid is the only enzymatically produced HETE increased under brain ischemia

Hydroxyeicosatetraenoic acids (HETE) are dramatically increased under brain ischemia and significantly affect post-ischemic recovery. However, the exact mechanism of HETE increase and their origin under ischemia are poorly understood. HETE might be produced de novo through lipoxygenase (LOX) -dependent synthesis with possible esterification into a lipid storage pool, or non-enzymatically through free radical oxidation of esterified arachidonic acid (20:4n6). Because HETE synthesized through LOX exhibit stereospecificity, chiral analysis allows separation of enzymatic from non-enzymatic pools. In the present study, we analyzed free HETE stereoisomers at 30 sec, 2 min, and 10 min of ischemia. Consistent with previous reports, we demonstrated a significant, gradual increase in all analyzed HETE over 10 min of brain ischemia, likely attributed to release of the esterified pool. The R/S ratio for 5-HETE, 8-HETE, and 15-HETE was not different from a racemic standard mix, indicating their non-enzymatic origin, which was in opposition to the inflamed tissue used as a positive control in our study. However, 12(S)-HETE was the predominant isoform under ischemia, indicating that ∼90 % of 12-HETE are produced enzymatically. These data demonstrate, for the first time, that 12-LOX is the major LOX isoform responsible for the enzymatic formation of the inducible HETE pool under ischemia. We also confirmed the requirement for enzyme inactivation with high-energy focused microwave irradiation (MW) for accurate HETE quantification and validated its application for chiral HETE analysis. Together, our data suggest that 12-LOX and HETE-releasing enzymes are promising targets for HETE level modulation upon brain ischemia.

Identification and In Silico Analysis of a Homozygous Nonsense Variant in TGM1 Gene Segregating with Congenital Ichthyosis in a Consanguineous Family

Background and Objectives: Lamellar ichthyosis is a rare skin disease characterized by large, dark brown plate-like scales on the entire body surface with minimum or no erythema. This phenotype is frequently associated with a mutation in the TGM1 gene, encoding the enzyme transglutaminase 1 which plays a catalytic role in the formation of the cornified cell envelop. The present study aimed to carry out clinical and genetic characterization of the autosomal recessive lamellar ichthyosis family from Balochistan. Materials and Methods: A consanguineous family with lamellar ichthyosis was enrolled from Balochistan, Pakistan. PCR amplification of all the exons and splice site junctions of the TGM1 gene followed by Sanger sequencing was performed on the genomic DNA. The identified variant was checked by In silico prediction tools to evaluate the effect of the variant on protein. Results: Sanger sequencing identified a homozygous nonsense variant c.131G >A (p.Trp44*) in the TGM1 gene that segregated in the autosomal recessive mode of inheritance in the family. The identified variant results in premature termination of transcribed mRNA and is predicted to cause a truncated or absent translation product transglutaminase-1 (TGase-1) accompanied by loss of catalytic activity, causing a severe clinical phenotype of lamellar ichthyosis in the patients. Conclusions: Here, we report a consanguineous lamellar ichthyosis family with a homozygous nonsense variant in the TGM1 gene. The variant is predicted as pathogenic by different In silico prediction tools.

Arachidonate 15-lipoxygenase type B: Regulation, function, and its role in pathophysiology

As a lipoxygenase (LOX), arachidonate 15-lipoxygenase type B (ALOX15B) peroxidizes polyenoic fatty acids (PUFAs) including arachidonic acid (AA), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and linoleic acid (LA) to their corresponding fatty acid hydroperoxides. Distinctive to ALOX15B, fatty acid oxygenation occurs with positional specificity, catalyzed by the non-heme iron containing active site, and in addition to free PUFAs, membrane-esterified fatty acids serve as substrates for ALOX15B. Like other LOX enzymes, ALOX15B is linked to the formation of specialized pro-resolving lipid mediators (SPMs), and altered expression is apparent in various inflammatory diseases such as asthma, psoriasis, and atherosclerosis. In primary human macrophages, ALOX15B expression is associated with cellular cholesterol homeostasis and is induced by hypoxia. Like in inflammation, the role of ALOX15B in cancer is inconclusive. In prostate and breast carcinomas, ALOX15B is attributed a tumor-suppressive role, whereas in colorectal cancer, ALOX15B expression is associated with a poorer prognosis. As the biological function of ALOX15B remains an open question, this review aims to provide a comprehensive overview of the current state of research related to ALOX15B.

The enzymology of human eicosanoid pathways: the lipoxygenase branches

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

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

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

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

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

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

Inhibition of imiquimod-induced psoriasis-like dermatitis in mice by herbal extracts from some Indian medicinal plants

Psoriasis is a chronic autoimmune human skin disorder that is characterized by excessive proliferation of keratinocytes, scaly plaques, severe inflammation and erythema. The pathophysiology of psoriasis involves interplay between epidermal keratinocytes, T lymphocytes, leukocytes and vascular endothelium. Increased leukocyte recruitment and elevated levels of cytokines, growth factors and genetic factors like interleukin (IL)-1β, IL-6, IL-17, IL-22, IL-23, tumour necrosis factor (TNF)-α, interferon (IFN)-γ, transforming growth factor (TGF)-β, toll-like receptor (TLR)-2, signal transducer and activator of transcription (STAT-3), 15-lipoxygenase (LOX)-2, coiled-coil alpha-helical rod protein 1 (CCHCR1), steroidogenic acute regulatory protein (StAR) and vitamin D receptor (VDR) are the most critical factors governing the exacerbation of psoriasis. In the present study, an attempt was made to elucidate the preventive role of herbal extracts of four dermo-protective Ayurvedic plants, Tinospora cordifolia (TC), Curcuma longa (CL), Celastrus paniculatus (CP) and Aloe vera (AV), against psoriasis-like dermatitis. Parkes (P) strain mice were initially induced with psoriasis-like dermatitis using topical application of imiquimod (IMQ, 5 %), followed by subsequent treatment with the herbal extracts to examine their curative effect on the psoriasis-like dermatitis-induced mice. The extracts were orally/topically administered to mice according to their ED/LD50 doses. Phenotypical observations, histological examinations, and semi-quantitative reverse transcription PCR (RT-PCR) analyses of the skin and blood samples of the control, IMQ-treated and herbal extract-treated psoriasis-like dermatitis-induced mice lead to the conclusion that the combination extract from all the plants was instrumental in downregulating the overexpressed cytokines, which was followed by the CL extract. Moreover, lesser yet positive response was evident from CP and TC extracts. The results suggest that these plants can prove to have tremendous preventive potential against the disease and can open the way to new therapeutic strategies for psoriasis treatment.

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

BACKGROUND

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

OBJECTIVES

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

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.

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.

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.

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.

p63 directly induces expression of Alox12, a regulator of epidermal barrier formation

Epidermal development and differentiation are tightly controlled processes that culminate in the formation of the epidermal barrier. A critical regulator of different stages of epidermal development and differentiation is the transcription factor p63. More specifically, we previously demonstrated elsewhere that p63 is required for both the commitment to stratification and the commitment to terminal differentiation. We now demonstrate that DeltaNp63alpha, the predominantly expressed p63 isoform in postnatal epidermis, also plays a role in the final stages of epidermal differentiation, namely the formation of the epidermal barrier. We found that DeltaNp63alpha contributes to epidermal barrier formation by directly inducing expression of ALOX12, a lipoxygenase which contributes to epidermal barrier function. Our data demonstrate that DeltaNp63alpha directly interacts with the promoter of Alox12 in the developing epidermis. Furthermore, we found that the induction of Alox12 expression by DeltaNp63alpha depends on intact p63 binding sites in the Alox12 promoter. Finally, we found that DeltaNp63alpha can induce Alox12 expression only in differentiating keratinocytes, consistent with the role of ALOX12 in epidermal barrier formation.

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.

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.

A mouse mutation in the 12R-lipoxygenase, Alox12b, disrupts formation of the epidermal permeability barrier

Nonbullous congenital ichthyosiform erythroderma (NCIE) is a nonsyndromic form of autosomal recessive congenital ichthyosis characterized by hyperkeratosis and a disruption in the epidermal permeability barrier. Identification of mutations in two lipoxygenases (LOXs), ALOX12B (12R-LOX) and ALOXE3 (eLOX3), and further functional studies implicate ALOX12B and ALOXE3 in the etiology of NCIE. Here, we report a mutation in Alox12b in the recessive ethylnitrosurea-induced mouse mutant, mummy (Alox12b(mmy-Bei)). mummy mutants have red, scaly skin and die perinatally. Histologically, mummy mutants display defects in the epidermis. We mapped mummy to a 1.9 Mb interval on Chr. 11 containing Alox12b (12R-LOX), Aloxe3 (eLOX3) and Alox15b (8-LOX). Sequencing of all three genes identified a nonsense mutation in the catalytic domain of Alox12b. We demonstrate that mummy mutants have a disrupted epidermal permeability barrier and that the nonsense mutation in mummy abolishes the enzyme activity of 12R-LOX. The mummy mutant provides a mouse model for LOX-mediated NCIE and is the first described mouse mutant affecting epidermal barrier formation identified by forward genetics.

12R-lipoxygenase deficiency disrupts epidermal barrier function

12R-lipoxygenase (12R-LOX) and the epidermal LOX-3 (eLOX-3) constitute a novel LOX pathway involved in terminal differentiation in skin. This view is supported by recent studies showing that inactivating mutations in 12R-LOX and eLOX-3 are linked to the development of autosomal recessive congenital ichthyosis. We show that 12R-LOX deficiency in mice results in a severe impairment of skin barrier function. Loss of barrier function occurs without alterations in proliferation and stratified organization of the keratinocytes, but is associated with ultrastructural anomalies in the upper granular layer, suggesting perturbance of the assembly/extrusion of lamellar bodies. Cornified envelopes from skin of 12R-LOX–deficient mice show increased fragility. Lipid analysis demonstrates a disordered composition of ceramides, in particular a decrease of ester-bound ceramide species. Moreover, processing of profilaggrin to monomeric filaggrin is impaired.

This study indicates that the 12R-LOX–eLOX-3 pathway plays a key role in the process of epidermal barrier acquisition by affecting lipid metabolism, as well as protein processing.

Role of epidermis-type lipoxygenases for skin barrier function and adipocyte differentiation

12R-lipoxygenase (12R-LOX) and epidermis-type LOX-3 (eLOX-3) are novel members of the multigene family of mammalian LOX. A considerable gap exists between the identification of these enzymes and their biologic function. Here, we present evidence that 12R-LOX and eLOX-3, acting in sequence, and eLOX-3 in combination with another, not yet identified LOX are critically involved in terminal differentiation of keratinocytes and adipocytes, respectively. Mutational inactivation of 12R-LOX and/or eLOX-3 has been found to be associated with development of an inherited ichthyosiform skin disorder in humans and genetic ablation of 12R-LOX causes a severe impairment of the epidermal lipid barrier in mice leading to post-natal death of the animals. In preadipocytes, a LOX-dependent PPARgamma activating ligand is released into the cell supernatant early upon induction of differentiation and available evidence indicates that this ligand is an eLOX-3-derived product. In accordance with this data is the observation that forced expression of eLOX-3 enhances adipocyte differentiation.

Concepts in molecular dermatotoxicology

In the recent years, molecular research has successfully elucidated some of the major mechanisms through which environmental noxae damage human skin. From this knowledge, novel concepts for skin protection have been developed. Here, we provide a brief overview of some of the most exciting and intriguing concepts in molecular dermatotoxicology.

Mutation spectrum and functional analysis of epidermis-type lipoxygenases in patients with autosomal recessive congenital ichthyosis

Autosomal-recessive congenital ichthyosis (ARCI) is a clinically and genetically heterogeneous group of severe hereditary keratinization disorders characterized by intense scaling of the whole integument, and differences in color and shape. It is often associated with erythema. To date, six loci for ARCI have been mapped. Mutations in ALOXE3 and ALOX12B on chromosome 17p13, which code for two different epidermal lipoxygenases, were recently found in patients with ichthyosiform erythroderma from Turkey, France, and North Africa. Here we describe molecular and clinical findings in 17 families with ARCI originating from Central Europe, Turkey, and the Indian subcontinent, with mutations in ALOXE3 or ALOX12B. We identified 11 novel point mutations in ALOX12B (one nonsense mutation and 10 missense mutations) and four different inactivating mutations in ALOXE3. The gene products of ALOX12B and ALOXE3, the epidermal lipoxygenases 12R-LOX and eLOX3, respectively, are preferentially synthesized in the skin. They act in sequence to convert arachidonic acid via 12(R)-HPETE to the corresponding epoxyalcohol, 8(R)-hydroxy-11(R),12(R)-epoxyeicosatrienoic acid. To assess the impairment of enzyme activity, we expressed the mutated genes in vitro and determined the activity of the recombinant proteins toward their genuine substrates. All but one of the recombinant mutants were enzymatically inactive. The characterization of disease-causing mutations in ALOXE3 and ALOX12B and the resulting ARCI phenotypes did not result in clear diagnostic criteria; however, we found a first correlation between the genetic findings and the clinical presentation of ichthyosis.

12-Lipoxygenase metabolism in mouse distal convoluted tubule cells

BACKGROUND

Several lines of evidence point to the 12-lipoxygenase (12-LOX) family as important mediators in hypertension, diabetes, and other cardiovascular diseases. The kidney has been a main focus for research of the role of this pathway in several disease models. While most of the studies have focused on mesangial or vascular cells, less is known about 12-LOX regulation at the renal tubular level. The aim of the study was to characterize the expression and regulation by hormones of the family of 12-LOX in mouse distal convoluted tubule at the molecular level.

METHODS

An immortalized mouse distal convoluted tubule (mDCT) cell line was used. mRNA and protein levels were assessed by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot, respectively, while 12(S)-HETE production was evaluated by enzyme-linked immunosorbent assay (ELISA). Cells were challenged with aldosterone, angiotensin II, 8Br-cAMP, and vasopressin.

RESULTS

We showed that both platelet (P) and leukocyte (L)-type 12-LOX are expressed in the mDCT cell line, as well as in distal tubules of human kidneys. The production of 12(S)-HETE by mDCT cells was increased in response to cAMP (by two-fold) and by vasopressin (by 1.5-fold). In contrast, neither aldosterone nor angiotensin II exerted appreciable effects on 12(S)-HETE production. The mRNA and protein levels of P-12LOX and L-12LOX were not changed by the different hormones, suggesting that they may act by modulating enzyme activity. We further have demonstrated that this mDCT cell line also expressed the recently cloned 12(R)-LOX.

CONCLUSION

mDCT cells show an active 12-LOX metabolism that appears to be modulated by cAMP and vasopressin.

Complicating the role of p53 in aging

The p53 tumor suppressor protein plays a pivotal role in integrating various DNA damage response pathways and has been shown to be mutated in a variety of human cancers. In an effort to study the effects of a mutant p53 protein in a mouse model we generated a p53 targeting vector with a mutation in codon 245, equivalent to the mutational hot spot (codon 248) in humans. However, due to an aberrant gene targeting event in ES cells, we developed a p53 mutant mouse model that expressed a truncated p53 transcript that lacked the first six exons while retaining the intended mutation in exon 7. This mouse model was shown to exhibit serendipitous phenotypes that resembled premature aging as well as increased resistance to spontaneous tumors. Based on the genetic and molecular information available at that time, we hypothesized that the truncated p53 allele (m-allele) and its effect on wt p53 activity might be responsible for the observed phenotypes. However, the availability of the mouse genome data has allowed us to further characterize the genetic deletion present in the p53+/m mouse model. Our analyses indicate that there are 24 genes (including the p53 truncation) deleted in the p53+/m mouse model. These results suggest that the p53 tumor suppressor protein may not be solely responsible for the various phenotypes exhibited by p53+/m mouse model [corrected]

Upregulation of 8-lipoxygenase in the dermatitis of IkappaB-alpha-deficient mice

Neonatal mice deficient in IkappaB-alpha, an inhibitor of the ubiquitous transcription factor NF-kappaB, develop severe and widespread dermatitis shortly after birth. In humans, inflammatory skin disorders such as psoriasis are associated with accumulation in the skin of the unusual arachidonic acid metabolite 12R-hydroxyeicosatetraenoic acid (12R-HETE), a product of the enzyme 12R-lipoxygenase. To examine the etiology of the murine IkappaB-alpha-deficient skin phenotype, we investigated the expression of lipoxygenases and the metabolism of exogenous arachidonic acid in the skin. In the IkappaB-alpha-deficient animals, the major lipoxygenase metabolite was 8S-HETE, formed together with a minor amount of 12S-HETE; 12R-HETE synthesis was undetectable. Skin from the wild-type littermates formed 12S-HETE as the almost exclusive lipoxygenase metabolite. Upregulation of 8S-lipoxygenase (8-LOX) in IkappaB-alpha-deficient mice was confirmed at the transcriptional and translational level using ribonuclease protection assay and western analysis. In immunohistochemical studies, increased expression of 8-LOX was detected in the stratum granulosum of the epidermis. In the stratum granulosum, 8-LOX may be involved in the terminal differentiation of keratinocytes. Although mouse 8S-lipoxygenase and human 12R-lipoxygenase are not ortholog genes, we speculate that in mouse and humans the two different enzymes may fulfill equivalent functions in the progression of inflammatory dermatoses.

Identification and Characterization of a Phorbol Ester-responsive Element in the Murine 8S-Lipoxygenase Gene

Murine 8S-lipoxygenase (8S-LOX) is a 12-O-tetradecanoylphorbol-13-acetate (TPA)-inducible lipoxygenase. That is, it is not detected in normal mouse skin, however, a significant increase in expression is detected in the skin of TPA promotion-sensitive strains of mice after TPA treatment. In this study, we found TPA-induced 8S-LOX mRNA expression is a result of increased transcription in SSIN primary keratinocytes and further investigated transcriptional regulation of 8S-LOX expression by cloning its promoter. The cloned 8S-LOX promoter ( approximately 2 kb) in which a transcription initiation site was mapped at -27 from the ATG has neither a TATA box nor a CCAAT box. However, the promoter was highly responsive to TPA in TPA promotion-sensitive SSIN but not in TPA promotion-resistant C57BL/6J primary keratinocytes. We then identified a Sp1 binding site located -77 to -68 from the ATG that is a TPA-responsive element (TRE) of the promoter and that Sp1, Sp2, and Sp3 proteins bind to the TRE. We also found that the binding of these proteins to the TRE was significantly increased by TPA treatment and inhibition of the binding by mithramycin A decreased TPA-induced promoter activity as well as 8S-LOX mRNA expression. These data suggest that increased binding of Sp1, Sp2, and Sp3 to the TRE of the 8S-LOX promoter is a mechanism by which TPA induces 8S-LOX expression in keratinocytes.

Differentiation-dependent regulation of secreted phospholipases A2 in murine epidermis

The action of secreted phospholipases A2 in skin is thought to be essential for epidermal barrier homeostasis. The incomplete knowledge of presence and functions of the novel secreted phospholipase A2 subtypes in skin prompted us to explore their expression in epidermis and primary keratinocytes from murine neonatal skin. We detected secreted phospholipases A2-IB, -IIA, -IIC, -IID, -IIE, -IIF, -V, -X, and -XII. To study secreted phospholipase A2 expression during epidermal differentiation, primary keratinocytes from the basal, suprabasal, and upper differentiated layers of neonatal mouse epidermis were obtained by density gradient centrifugation. mRNA for secreted phospholipases A2-IB, -IIE, -IIF, -V, and -XII-1 are mainly expressed in the upper differentiated layers, whereas the most prominent enzymes in the basal and suprabasal layers are secreted phospholipases A2-IIA, -IID, and -X. The mRNA for secreted phospholipase A2-IIC was found in all fractions. Immunohistochemical analysis in mouse skin sections reflected the mRNA distribution patterns in the different epidermal cell fractions. After in vitro induction of keratinocyte differentiation by increasing the calcium concentration of the medium, secreted phospholipases A2-IB, -IIE, -IIF, -V, and -XII-1 were upregulated, whereas secreted phospholipases A2-IIA, -IIC, -IID, and -X were mainly expressed in proliferating keratinocytes. The specific secreted phospholipase A2 expression profile in the skin suggests a distinct function for each enzyme in the epidermis.

Lipoxygenase-catalyzed formation of R-configuration hydroperoxides

Prototypical lipoxygenases (LOXs) of animals and plants synthesize hydroperoxy fatty acids of the S stereoconfiguration, yet enzymes forming R-configuration products are found in both the animal and plant kingdoms. R-LOX are widespread in aquatic invertebrates, in some of which their R-HETE products have a defined role in reproductive function. A 12R-LOX has been found recently in humans and mice. The human 12R-LOX product, 12R-HETE, appears to be involved in the pathophysiology of psoriasis and other proliferative skin diseases; a role in normal skin development is implied from the spatial and temporal expression patterns of the 12R-LOX in the mouse embryo. In plants, there are few reports of R-LOX activity and in higher plants this is limited to enzymes that catalyze a significant degree of non-specific oxygenation. There are no obvious amino acid sequence motifs characterizing R-LOXs; and in the phylogenetic tree of the LOX superfamily, the R-LOXs do not group into a specific branch of genes. The mechanistic basis of stereocontrol over the oxygenation reaction performed by LOXs may relate to a changed binding orientation of the fatty acid substrate or to the direction of attack by molecular oxygen. A potentially relevant precedent for switching of R- and S-oxygenation specificity was described recently in studies of prostaglandin C-15 oxygenation during cycloxygenase catalysis; single amino acid changes can invert the oxygenation stereospecificity at C-15. In this case, the evidence suggests that R/S switching can occur with the substrate binding in the normal conformation.

Arachidonate 8(S)-lipoxygenase

The recently identified mouse 8(S)-lipoxygenase almost exclusively directs oxygen insertion into the 8(S) position of arachidonic acid and, with lower efficiency, into the 9(S) position of linoleic acid. The protein of 677 amino acids displays 78% sequence identity to human 15(S)-lipoxygenase-2 which is considered to be its human orthologue. The 8(S)-lipoxygenase gene, Alox15b, consisting of 14 exons and spanning 14.5 kb is located within a gene cluster of related epidermis-type lipoxygenases at the central region of mouse chromosome 11. 8(S)-Lipoxygenase is predominantly expressed in stratifying epithelia of mice, constitutively in the hair follicle, forestomach, and foot-sole and inducible in the back skin with strain-dependent variations. The expression is restricted to terminally differentiating keratinocytes, in particular the stratum granulosum and 8(S)-lipoxygenase activity seems to be involved in terminal differentiation of mouse epidermis. Tumor-specific up-regulation of 8(S)-lipoxygenase expression and activity indicate a critical role of this enzyme in malignant progression during tumor development in mouse skin.

Lipoxygenase activity in altered gravity

Lipoxygenases are a family of enzymes which dioxygenate unsaturated fatty acids, thus initiating lipoperoxidation of membranes or the synthesis of signalling molecules, or inducing structural and metabolic changes in the cell. This activity is the basis for the critical role of lipoxygenases in a number of pathophysiological conditions, both in animals and plants. We review the effects of microgravity on the catalytic efficiency of purified soybean (Glycine max) lipoxygenase-1, as well as the modulation of the activity and expression of 5-lipoxygenase in human erythroleukemia K562 cells subjected to altered gravity. We also outline the molecular properties of the lipoxygenase family and discuss its possible involvement in space-related processes, such as apoptosis (programmed cell death) and immuno-depression. Finally, we discuss the modulation of cyclooxygenase activity and expression in K562 cells exposed to altered gravity, because cyclooxygenase catalyzes the oxidation of arachidonate through a pathway different from that catalyzed by lipoxygenase activity.

15-Lipoxygenase-2 expression in benign and neoplastic sebaceous glands and other cutaneous adnexa

15-Lipoxygenase-2 has a limited tissue distribution in epithelial tissues, with mRNA detected in skin, cornea, lung, and prostate. It was originally cloned from human hair rootlets. In this study the distribution of 15-lipoxygenase-2 was characterized in human skin using immunohistochemistry and in situ hybridization. Strong uniform 15-lipoxygenase-2 in situ hybridization (n = 6) and immunostaining (n = 16) were observed in benign cutaneous sebaceous glands, with expression in differentiated secretory cells. Strong 15-lipoxygenase-2 immunostaining was also observed in secretory cells of apocrine and eccrine glands. Variable reduced immunostaining was observed in skin-derived sebaceous neoplasms (n = 8). In the eyelid, Meibomian glands were uniformly negative for 15-lipoxygenase-2 in all cases examined (n = 9), and sebaceous carcinomas apparently derived from Meibomian glands were also negative (n = 12). The mechanisms responsible for differential expression in cutaneous sebaceous vs eyelid Meibomian glands remain to be established. In epidermis, positive immunostaining was observed in the basal cell layer in normal skin, whereas five examined basal cell carcinomas were negative. Thus, the strongest 15-lipoxygenase-2 expression is in the androgen regulated secretory cells of sebaceous, apocrine, and eccrine glands. This compares with the prostate, in which 15-lipoxygenase-2 is expressed in differentiated prostate secretory cells (and reduced in the majority of prostate adenocarcinomas). The product of 15-lipoxygenase-2, 15-hydroxyeicosatetraenoic acid, may be a ligand for the nuclear receptor peroxisome proliferator activated receptor-gamma, which is expressed in sebocytes, and contribute to secretory differentiation in androgen regulated tissues such as prostate and sebaceous glands.

A Gene Cluster Encoding Human Epidermis-type Lipoxygenases at Chromosome 17p13.1: Cloning, Physical Mapping, and Expression

Epidermis-type lipoxygenases, a distinct subclass within the multigene family of mammalian lipoxygenases (LOX), comprise recently discovered novel isoenzymes isolated from human and mouse skin including human 15-LOX-2, human and mouse 12R-LOX, mouse 8S-LOX, and mouse e-LOX-3. We have isolated the human homologue of mouse e-LOX-3. The cDNA of 3362 bp encodes a 711-amino-acid protein displaying 89% sequence identity with the mouse protein and exhibiting the same unusual structural feature, i.e., an extra segment of 41 amino acids, which can be located beyond the N-terminal beta-barrel domain at the surface of the C-terminal catalytic domain. The gene encoding e-LOX-3, ALOXE3, was found to be part of a gene cluster of approximately 100 kb on human chromosome 17p13.1 containing in addition the 12R-LOX gene, ALOX12B, the 15-LOX-2 gene, ALOX15B, and a novel 15-LOX pseudogene, ALOX15P. ALOXE3 and ALOX12B are arranged in a head-to-tail fashion separated by 8.5 kb. The genes are split into 15 exons and 14 introns spanning 22 and 15 kb, respectively. ALOX15P was found on the opposite DNA strand directly adjacent to the 3'-untranslated region of ALOX12B. ALOX15B is located in the same orientation 25 kb downstream of ALOX12B, and is composed of 14 exons and 13 introns spanning a total of 9.7 kb of genomic sequence. RT-PCR analysis demonstrated a predominant expression of ALOXE3, ALOX12B, and ALOX15B in skin.

Enzymic characterization of epidermis-derived 12-lipoxygenase isoenzymes

Substrate selectivity and other enzymic characteristics of two epidermis-derived lipoxygenases (LOXs), the epidermis-type (e) (12S)-LOX and (12R)-LOX, were compared with those of the platelet-type (p) (12S)-LOX. In contrast with p(12S)-LOX, e(12S)-LOX and (12R)-LOX exhibited no or very low reactivity towards the customary substrates linoleic acid and arachidonic acid but metabolized the corresponding fatty acid methyl esters, which, in contrast, were not accepted as substrates by p(12S)-LOX. Other esters of arachidonic acid and linoleic acid, including propan-2-yl and cholesterol esters, 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine, 1-palmitoyl-2-linoleyl-sn-glycero-3-phosphoethanolamine, and ceramide 1 carrying an omega-linoleic acid ester, were not metabolized by these three LOX isoenzymes. Among various polyunsaturated fatty acids the isomeric eicosatrienoic acids were found to be oxygenated by e(12S)-LOX but not by (12R)-LOX. 4,7,10,13,16,19-Docosahexaenoic acid as a substrate was restricted to p(12S)-LOX. Variations in the pH and the Ca(2+) content of the incubation medium affected the catalytic potential only slightly. Whereas (12R)-LOX activity increased in the presence of Ca(2+) and with an acidic pH, Ca(2+) had no effect on p(12S)-LOX and e(12S)-LOX; an acidic pH decreased the catalytic activity of the latter two. However, the catalytic activity of the epidermis-type isoenzymes, but not of p(12S)-LOX, was found to be markedly increased in the presence of DMSO. Under these conditions, e(12S)-LOX and (12R)-LOX oxygenated 4,7,10,13,16,19-docosahexaenoic acid to 14-hydroxy-4,7,10,12,16,19-docosahexaenoic acid and 13-hydroxy-4,7,10,14,16,19-docosahexaenoic acid respectively. In addition, (9R)-hydroxyoctadeca-10,12-dienoic acid methyl ester was generated from linoleic acid methyl ester by (12R)-LOX. Independently of the substrate, the catalytic activity of e(12S)-LOX and (12R)-LOX was always at most 2% of that of p(12S)-LOX with arachidonic acid as substrate.