Genomic and cDNA cloning of a novel mouse lipoxygenase gene

Who is the real 12-HETrE?

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

Evolutionary aspects of lipoxygenases and genetic diversity of human leukotriene signaling

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

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.

Arachidonate 12-lipoxygenases

Arachidonate 12-lipoxygenase introduces a molecular oxygen at carbon 12 of arachidonic acid to generate a 12-hydroperoxy derivative. The enzymes generate 12-hydroperoxy derivatives with either S- or R-configurations. There are three isoforms of 12S-lipoxygenases named after the cells where they were first identified; platelet, leukocyte and epidermis. The leukocyte-type enzyme is widely distributed among cells, but the tissue distribution varies substantially from species to species. The platelet and epidermal enzymes are present in only a relatively limited number of cell types. Although the structures and enzymatic properties of the three isoforms of 12S-lipoxygenases have been elucidated, the physiological roles of the 12S-lipoxygenases are not yet fully understood. There are important roles for the enzymes and their products in several biological systems including those involved in atherosclerosis and neurotransmission.

Hepoxilin B3 and its enzymatically formed derivative trioxilin B3 are incorporated into phospholipids in psoriatic lesions

In previous studies we observed that normal human epidermis forms 12-oxo-eicosatetraenoic acid (12-oxo-ETE) and hepoxilin B3 (HxB3) as major eicosanoids, both being elevated in psoriasis. We also observed that normal epidermis, in a reaction probably catalyzed by 12-lipoxygenase, only synthesize one of the two possible 10-hydroxy epimers of HxB3. We have now extended these previous studies investigating further transformation of HxB3 into trioxilin B3 (TrXB3) and esterification of both into phospholipids. Phospholipids were extracted from normal epidermis and from psoriatic scales. A combination of high performance liquid chromatography and gas chromatography-mass spectrometry analysis demonstrated the occurrence of HxB3 and TrXB3 in the phospholipids of psoriatic lesions. Alkaline- and phospholipase-A2-mediated hydrolysis of the phospholipids yielded similar quantities of both HxB3 and TrXB3 indicating their preference for the sn-2 position of glycerophospholipids. The thin layer chromatography analysis of the phospholipid classes after incubation of epidermal cells with [14C]-labeled HxB3, TrXB3, 12-hydroxy-eicosatetraenoic acid (12-HETE), 12-oxo-ETE, or 15-HETE showed that 12-HETE was the most esterified (12-HETE >15-HETE > TrXB3 > 12-oxo-ETE > HxB3). HxB3 and TrXB3 were mainly esterified in phosphatidyl-choline and phosphatidyl-ethanolamine. HxB3 was also enzymatically converted into TrXB3 in vitro. HxB3 epoxide hydrolase-like activity was not observed when boiled tissue was incubated with [14C]-HxB3, this activity being located in the cytosol fraction (100,000 x g supernatant) of fresh tissue. These findings suggest that in vivo some part of HxB3 is transformed into TrXB3 and both compounds are partially incorporated into the phospholipids.

Lipoxygenase in Human Tumor Cells

Tumor cell proliferation and metastasis proceed via a network of interdependent molecular events with a vast array of molecular players and signal transduction mechanisms differing in various types of human tumors. In the sequence of events necessary for carcinogenesis, arachidonate metabolites have been documented to play a significant role at several steps. Arachidonate metabolism in human cells occurs via several enzymatic pathways, including enzymes such as cyclo-oxygenases and lipoxygenases. This review pays particular attention to one member of the lipoxygenase family of enzymes, namely 12-lipoxygenase, since an arachidonate metabolite generated via 12-lipoxygenase action, 12(S)-HETE, has been shown to elicit various prometastatic effects of tumor cells in vivo and in vitro. We focus especially on mechanisms of activation and modulation of 12-lipoxygenase expression in human tumor cells, since various tumor cells express 12-lipoxygenase or are responsive to metabolites derived from 12-lipoxygenase action, thus offering a potential for successful therapeutic intervention against such tumors.

Characterization of the murine epidermal 12/15-lipoxygenase

The murine lipoxygenase (LO) family consists of at least seven members classified according to the HETE (hydroxyeicosatetraenoic acid) metabolite generated during arachidonic acid metabolism and the site of tissue expression. At present there are four 12-lipoxygenases that are functionally distinct, vary in cell and tissue distribution, catalytic activity and each are products of separate, linked genes. They are "platelet-type" 12-LO (P-12LO), "leukocyte-type" 12-LO (L-12LO), "epidermal-type" 12-LO (e-12LO) and the most recently discovered 12(R)-LO. In this report we characterize e-12LO, which was overexpressed in the baculovirus/insect cell expression system. The enzyme functions as a dual specificity 12/15-lipoxygenase with a 12-HETE/15-HETE product ratio of approximately 6:1 with arachidonic acid as substrate. Several other polyunsaturated fatty acids served as substrates for e-12LO such as gamma-linolenic, dihomo-gamma-linolenic and eicosapentaenoic acids. A green fluorescent protein/e-12LO fusion protein was localized to the cytosol of transfected HEK 293 cells. The e-12LO gene was expressed in mouse oocytes and early embryos. Western blot analysis revealed high level expression in postnatal day 3 mouse epidermal lysates. Together these data suggest that e-12LO plays a role in normal epidermal function and as yet an undiscovered role in early development.

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

By using reverse transcription-polymerase chain reaction technology (RT-PCR) and Northern blot analysis, the tissue-specific mRNA expression patterns of seven mouse lipoxygenases (LOX)--including 5S-, 8S-, three isoforms of 12S-, 12R-LOX, and a LOX of an as-of-yet unknown specificity, epidermis-type LOX-3 (e-LOX-3)--were investigated in NMRI mice. Among the various tissues tested epidermis and forestomach were found to express the broadest spectrum of LOX. With the exception of 5S- and platelet-type 12S-LOX (p12S-LOX) the remaining LOX showed a preference to exclusive expression in stratifying epithelia of the mouse, in particular the integumental epidermis. The expression of the individual LOX in mouse epidermis was found to depend on the state of terminal differentiation of the keratinocytes. mRNA of epidermis-type 12S-LOX (e12S-LOX) was detected in all layers of neonatal and adult NMRI mouse skin, whereas expression of p12S-LOX, 12R-LOX, and e-LOX-3 was restricted to suprabasal epidermal layers of neonatal and adult mice. 8S-LOX mRNA showed a body-site-dependent expression in that it was detected in stratifying epithelia of footsole and forestomach but not in back skin epidermis. In the latter, 8S-LOX mRNA was strongly induced upon treatment with phorbol esters. With the exception of e12S-LOX and p12S-LOX, the isozymes that are preferentially expressed in stratifying epithelia are structurally related and may be grouped together into a distinct subgroup of epidermis-type LOX.

Stereoselective biosynthesis of hepoxilin B3 in human epidermis

We previously reported that normal human epidermis forms 12-oxo-eicosatetraenoic acid and hepoxilin B3 as major eicosanoids and that hepoxilins and trioxilins are dramatically elevated in psoriatic lesions. We also observed that normal epidermis only synthesized one of the two possible 10-hydroxy- epimers of hepoxilin B3, suggesting its enzymatic origin. This study investigated the enzymatic pathways involved in the formation of hepoxilin B3 in human epidermis. Human epidermal fragments or cell fractions were incubated with [14C]-arachidonic acid or authentic 12(S)-hydroperoxyeicosatetraenoic acid. Products were analyzed by high-performance liquid chromatography, gas chromatography-mass spectrometry or a combination of both techniques. Esculetin and nordihydroguaiaretic acid inhibited formation of hepoxilin B3, 12-oxo-eicosatetraenoic acid, trioxilins, and 12-hydroxyeicosatetraenoic acid in a concentration-dependent manner. 12-Lipoxygenase activity was mainly located in the microsomal fraction (100,000 x g pellet) and 12-hydroxyeicosatetraenoic acid, hepoxilin B3, and 12-oxo-eicosatetraenoic acid were formed. The hepoxilin B3-synthesizing activity was not observed in subcellular fractions incubated with authentic 12(S)-hydroperoxyeicosatetraenoic acid, although it was located at least in the microsomal fraction when incubated with arachidonic acid. Similar results were obtained using preparations of recombinant platelet-type 12-lipoxygenase that yielded 12-oxo-eicosatetraenoic acid and hepoxilin B3 in addition to 12-hydroxyeicosatetraenoic acid, when incubated with arachidonic acid but not when incubated with 12-hydroperoxyeicosatetraenoic acid. Nevertheless, recombinant 12-lipoxygenase produced a lower ratio of 12-oxo-eicosatetraenoic acid and hepoxilin B3-12-hydroxyeicosatetraenoic acid than epidermis. Our results support the concept that 12-lipoxygenase catalyzes the formation of hepoxilin B3 and 12-oxo-eicosatetraenoic acid.

cDNA cloning, genomic structure, and chromosomal localization of a novel murine epidermis-type lipoxygenase

Using a combination of degenerate PCR technique and conventional screening procedures, we isolated a cDNA encoding a novel lipoxygenase, termed epidermis-type lipoxygenase-3 (e-LOX-3, gene symbol Aloxe3), from mouse skin. Aloxe3 mRNA is expressed in the stratified epithelia of skin, tongue, and forestomach. The cDNA encodes a protein of 711 amino acids with a calculated molecular mass of 80.6 kDa. The amino acid sequence shows approximately 54% identity to the recently identified 12(R)-lipoxygenase. Sequence comparison revealed a segment of 41 amino acid residues localized near the boundary between the N- and the C-terminal domain sequences of the molecule, a structural feature that is also characteristic of 12(R)-lipoxygenase, suggesting that these two epidermis-derived lipoxygenases may be members of a novel structural class of mammalian lipoxygenases. The novel lipoxygenase gene is divided into 15 exons and 14 introns, spanning 22.3 kb of genomic DNA. By interspecific backcross analysis, the novel gene was localized to the central region of mouse chromosome 11.

Tissue-specific translational regulation of alternative rabbit 15-lipoxygenase mRNAs differing in their 3'-untranslated regions

By screening a rabbit reticulocyte library, an alternative 15-LOX transcript of 3.6 kb (15-LOX mRNA2) was detected containing a 1019 nt longer 3'-untranslated region (UTR2) than the main 2.6 kb mRNA (15-LOX mRNA1). In anaemic animals, northern blotting showed that 15-LOX mRNA2 was predominantly expressed in non-erythroid tissues, whereas 15-LOX mRNA1 was exclusively expressed in red blood cells and bone marrow. The 15-LOX 3'-UTR2 mRNA2 contained a novel 8-fold repetitive CU-rich motif, 23 nt in length (DICE2). This motif is related but not identical to the 10-fold repetitive differentiation control element (DICE1) of 19 nt residing in the 15-LOX UTR1 mRNA1. DICE1 was shown to interact with human hnRNP proteins E1 and K, thereby inhibiting translation. From tissues expressing the long 15-LOX mRNA2, two to three unidentified polypeptides with molecular weights of 53-55 and 90-93 kDa which bound to DICE2 were isolated by RNA affinity chromatography. A 93 kDa protein from lung cytosol, which was selected by DICE2 binding, was able to suppress translational inhibition of 15-LOX mRNA2, but not of 15-LOX mRNA1, by hnRNP E1. A possible interaction between DICE1/DICE2 cis / trans factors in translational control of 15-LOX synthesis is discussed. Furthermore, the 3'-terminal part of the highly related rabbit leukocyte-type 12-LOX gene was analysed. Very similar repetitive CU-rich elements of the type DICE1 (20 repeats) and DICE2 (nine repeats) were found in the part corresponding to the 3'-UTR of the mRNA.

Expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

From a rabbit reticulocyte library a full length cDNA was isolated which predicted a novel lipoxygenase (LOX) sharing 99% identical amino acids with the rabbit 15-lipoxygenase. HPLC product analysis of the bacterially expressed protein identified it as a leukocyte-type 12-lipoxygenase (1.12-LOX). This proves the co-expression of a 15-lipoxygenase and a 1.12-lipoxygenase in one mammalian species. Among the six amino acids that are different to rabbit 15-lipoxygenase, leucine 353 is shown to be the primary determinant for 12-positional specificity. In the 3'-untranslated region of the 12-LOX-mRNA a CU-rich, 20-fold repetitive element has been found, closely related to the differentiation control element (DICE) of the rabbit 15-LOX-mRNA which is organized by ten repeats of 19 bases. By genomic PCR the 3'-terminal part of the gene for the novel 12-lipoxygenase containing the introns 10-13 has been amplified and sequenced. The introns were very similar in length to the corresponding 15-lipoxygenase introns with 89% to 95% identical nucleotide sequences. By screening a rabbit reticulocyte library an alternative 15-lipoxygenase transcript of 3.6 kb has been detected containing a 1019 nucleotides longer 3'-untranslated region (UTR2) than the main 2.6 kb mRNA. The determination of the tissue distribution by Northern blotting showed that the 3.6 kb mRNA2 was only expressed in non-erythroid tissues, whereas the 2.6 kb mRNA1 was exclusively expressed in reticulocytes. The only cell type which has been found to express the 1.12-lipoxygenase abundantly are monocytes. The results indicate that the expression of 1.12-lipoxygenase and 15-lipoxygenase is highly regulated. The UTR2 of the 15-LOX-mRNA2 contained a novel eight-fold repetitive CU-rich motif of 23 bases length which is related but not identical to the DICE of 19 bases in the UTR1. The analysis of a genomic recombinant of the complete 9.0 kb Alox15 gene confirmed that UTR1 and UTR2 are not interrupted by an additional intron.

Murine 12(R)-lipoxygenase: functional expression, genomic structure and chromosomal localization

A cDNA, recently cloned (by Krieg et al. (1998)) from mouse skin, was shown to encode a 12(R)-lipoxygenase. When expressed in HEK cells, the recombinant protein converted methyl arachidonate into the corresponding 12-HETE ester which was shown to be the R-enantiomer by chiral phase chromatography. Neither arachidonic acid nor linoleic acid were substrates for the recombinant protein. The structure of the 12(R)-lipoxygenase gene is unique among all animal lipoxygenases in that it is divided into 15 exons and 14 introns spanning approximately 12.5 kb. By interspecific backcross analysis, the 12(R)-lipoxygenase gene was localized to the central region of mouse chromosome 11.

Simultaneous expression of leukocyte-type 12-lipoxygenase and reticulocyte-type 15-lipoxygenase in rabbits

In rabbit reticulocytes an arachidonic acid 15-lipoxygenase (15-LOX) is expressed at high yield. Rescreening a rabbit reticulocyte cDNA library for alternative 15-LOX transcripts, a full length cDNA which encodes a novel lipoxygenase was isolated. The predicted amino acid sequence of this enzyme shared a high degree (99%) of identity with the reticulocyte-type 15-lipoxygenase. Among the six amino acid residues different in both enzymes a Phe-Leu exchange was detected at position 353. Recently, site-directed mutagenesis studies have revealed that this amino acid exchange converts a 15-lipoxygenase to a 12-lipoxygenase. In fact, when the novel enzyme was expressed in Escherichia coli, mainly 12-lipoxygenation of arachidonic acid was observed. The recombinant enzyme exhibited a rather broad substrate specificity. Various C-18 and C-20 polyenoic fatty acids and even complex substrates such as biomembranes were effectively oxygenated. Thus, the novel enzyme may be classified as leukocyte-type 12-lipoxygenase. Genomic polymerase chain reaction of the 3' region of the leukocyte-type 12-lipoxygenase gene indicated that introns 10 to 13 differed to about 10% from the corresponding sequences of the 15-lipoxygenase gene although their size and the intron-exon organization were very similar. In the 3'-untranslated region of the novel mRNA a C+U-rich, 20-fold repetitive element was found which appears to be highly related to the differentiation control element of the 15-lipoxygenase mRNA. Activity assays with a variety of cells and tissues prepared from normal rabbits suggested that only peripheral monocytes abundantly express the enzyme, suggesting a tissue-specific regulation of gene expression. These data indicate for the first time the co-expression of two separate genes for a reticulocyte-type 15-lipoxygenase and for a leukocyte-type 12-lipoxygenase in one species. This is of importance for the implication of both enzymes in red blood cell development and atherogenesis.

Occurrence of hepoxilins and trioxilins in psoriatic lesions

We recently found that normal human epidermis produces relatively high amounts of hepoxilins and trioxilins in vitro. Therefore, the aim of this study was to demonstrate the presence of these compounds in psoriatic lesions. Extracts from scales of patients with chronic stable plaque psoriasis were analyzed by a combination of high performance liquid chromatography and gas chromatography-mass spectrometry techniques. We found that the levels of hepoxilin B3 were more than 16-fold higher in psoriatic scales than in normal epidermis (3.2+/-2.3 and < 0.2 ng per mg, respectively), whereas hepoxilin A3 was not detected in any sample. Trioxilins were semiquantitated and referred to 12-hydroxyeicosatetraenoic acid, ratios of trioxilins A3 and B3 12-hydroxyeicosatetraenoic acid in psoriatic lesions were 0.65+/-0.23 and 0.32+/-0.28, respectively, and they were not detected in normal epidermis. The presence of a great amount of trioxilin A3 strongly suggests that hepoxilin A3 was present in psoriatic lesions and it was totally degraded to trioxilin A3 during the analysis procedure. Our results demonstrate that hepoxilins and trioxilins are produced by human skin in vivo and that the levels of these compounds are increased in psoriasis. The reported biologic activities of hepoxilins indicate that they could amplify and maintain the inflammatory response. Our results reinforce the idea that these compounds could play a role as mediators in the inflammatory response in skin, particularly in psoriasis.

cDNA cloning of a 8-lipoxygenase and a novel epidermis-type lipoxygenase from phorbol ester-treated mouse skin

Using a combination of PCR cloning and conventional screening procedures, we isolated from phorbol ester-treated mouse epidermis two full length cDNA clones encoding novel lipoxygenases. One of the cDNAs turned out to be identical to the recently cloned 8-lipoxygenase [Jisaka et al., J. Biol. Chem. 272 (1997) 24 410-24 416], the open reading frame of the second one corresponded to a protein of 701 amino acids with a calculated molecular mass of 80.6 kDa. The amino acid sequence showed 50.8% identity to human 15-lipoxygenase 2, approximately 40% to 5-lipoxygenase and 35% to 12- and 15-lipoxygenases. A unique structural feature is the insertion of 31 amino acid residues in the amino-terminal part of the molecule. Based on these data, we conclude that this epidermis-derived cDNA encodes a novel lipoxygenase isoform termed provisionally epidermis-type lipoxygenase 2 (e-LOX 2).

Molecular cloning and functional expression of a phorbol ester-inducible 8S-lipoxygenase from mouse skin

One of the effects of topical application of phorbol ester to mouse skin is the induction of an 8S-lipoxygenase in association with the inflammatory response. Here we report the molecular cloning and characterization of this enzyme. The cDNA was isolated by polymerase chain reaction from mouse epidermis and subsequently from a mouse epidermal cDNA library. The cDNA encodes a protein of 677 amino acids with a calculated molecular mass of 76 kDa. The amino acid sequence has 78% identity to a 15S-lipoxygenase cloned recently from human skin and approximately 40% identity to other mammalian lipoxygenases. When expressed in vaccinia virus-infected Hela cells, the mouse enzyme converts arachidonic acid exclusively to 8S-hydroperoxyeicosatetraenoic acid while linoleic acid is converted to 9S-hydroperoxy-linoleic acid in lower efficiency. Phorbol ester treatment of mouse skin is associated with strong induction of 8S-lipoxygenase mRNA and protein. By Northern analysis, expression of 8S-lipoxygenase mRNA was also detected in brain. Immunohistochemical analysis of phorbol ester-treated mouse skin showed the strongest reaction to 8S-lipoxygenase in the differentiated epidermal layer, the stratum granulosum. The inducibility may be a characteristic feature of the mouse 8S-lipoxygenase and its human 15S-lipoxygenase homologue.

Application of an optimised reverse transcriptase-polymerase chain reaction method to determine the cDNA nucleotide sequence of porcine basic fibroblast growth factor

In the present investigation, oligonucleotide primers of high hybridisation stringency have been used in combination with optimised reverse transcriptase-polymerase chain reaction (RT-PCR) methods for the determination of the cDNA sequence corresponding to porcine FGF-2 mRNA present in brain and uterine tissue. Application of these optimised methods have overcome previous limitations associated with the low abundance of the porcine FGF-2 mRNA, and allowed as little as 100 micrograms of tissue to be employed to generate the complete cDNA nucleotide sequences as well as to provide specific template fragments selected for their suitability in subsequent ligation and mutagenesis studies with conventional expression vectors. Comparisons of the cDNA nucleotide and the deduced amino-acid sequence of porcine FGF-2 and the known FGF-2s from other species have indicated nucleotide sequence homologies of 95.5% with the bovine, 94.7% with the human and 88.7% with the rat FGF-2 cDNA whilst amino-acid sequence homologies of 100% with the bovine, 98.7% with the human and 96.8% with the rat FGF-2, respectively, were found. Based on these investigations, application of analogous strategies and methods with low abundance mRNAs related to other members of this family of growth factors, as well as very low abundance mRNAs of other protein growth factor, in the pig should now be readily realised.

Functional expression and cellular localization of a mouse epidermal lipoxygenase

Three distinct murine lipoxygenase genes have been functionally characterized: 5-lipoxygenase (Chen, X.-S., Naumann, T. A., Kurre, U. , Jenkins, N. A., Copeland, N. G., and Funk, C. D. (1995) J. Biol. Chem. 270, 17993-17999), platelet-type 12-lipoxygenase and leukocyte-type 12-lipoxygenase (Chen, X.-S., Kurre, U., Jenkins, N. A., Copeland, N. G., and Funk, C. D. (1994) J. Biol. Chem. 269, 13979-13987). Here, we describe the cloning and functional characterization of a fourth lipoxygenase gene in mice. Using a polymerase chain reaction-based approach together with partial sequence information from a genomic clone, we isolated a novel lipoxygenase cDNA from the RNA of 3-6-day-old mouse epidermis. The open reading frame predicts a 662-amino acid lipoxygenase that displays 60% identity with both murine 12-lipoxygenase isozymes and 40% identity to 5-lipoxygenase; the sequence is identical to a genomic sequence reported recently (van Dijk, K. W., Steketee, K., Havekes, L., Frants, R., and Hofker, M. (1995) Biochim. Biophys. Acta 1259, 4-8). A full-length clone was expressed in human embryonic kidney 293 cells and homogenates from disrupted cells produced 12-hydroxyeicosatetraenoic acid (12-HETE) and minor amounts of 15-HETE from arachidonic acid. Chiral phase analysis indicated that the 12-HETE is exclusively the 12S enantiomer. In situ hybridization revealed highly specific expression of epidermal lipoxygenase in differentiated keratinocytes of the epidermis and in restricted regions of the root sheath and bulb of hair follicles. High expression was also detected in conjunctiva of the eyelid and in cells of Meibomian and preputial (sebaceous) glands. A 2. 4-kilobase mRNA was detected in mouse epidermis by Northern blot analysis and its abundance was not affected by phorbol ester treatment. The epidermal lipoxygenase gene (Aloxe) resides on mouse chromosome 11 closely linked with the two 12-lipoxygenase genes (Alox12p and Alox12l).