Aspirin-induced asthma: advances in pathogenesis, diagnosis, and management

In some asthmatic individuals, aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs) that inhibit cyclooxygen-ase 1 (COX-1) exacerbate the condition. This distinct clinical syndrome, called aspirin-induced asthma (AIA), is characterized by an eosinophilic rhinosinusitis, nasal polyposis, aspirin sensitivity, and asthma. There is no in vitro test for the disorder, and diagnosis can be established only by provocation challenges with aspirin or NSAIDs. Recent major advances in the molecular biology of eicosanoids, exemplified by the cloning of 2 cysteinyl leukotriene receptors and the discovery of a whole family of cyclooxygenase enzymes, offer new insights into mechanisms operating in AIA. The disease runs a protracted course even if COX-1 inhibitors are avoided, and the course is often severe, many patients requiring systemic corticosteroids to control their sinusitis and asthma. Aspirin and NSAIDs should be avoided, but highly specific COX-2 inhibitors, known as coxibs, are well tolerated and can be safely used. Aspirin desensitization, followed by daily aspirin treatment, is a valuable therapeutic option in most patients with AIA, particularly those with recurrent nasal polyposis or overdependence on systemic corticosteroids.

Recent investigations into the lipoxygenase pathway of plants

The plant lipoxygenase (LOX) pathway is in many respects the equivalent of the 'arachidonic acid cascade' in animals. The LOX-catalyzed dioxygenation of the plant fatty acids, linoleic and linolenic acids, is followed by metabolism of the resulting fatty acid hydroperoxides by other enzymes. Although the physiological functions of the end-products do not appear to be fully defined at this time, hormonal and anti-fungal activities have been reported.

HDL and the inflammatory response induced by LDL-derived oxidized phospholipids

Oxidation of low density lipoprotein (LDL) phospholipids containing arachidonic acid at the sn-2 position occurs when a critical concentration of "seeding molecules" derived from the lipoxygenase pathway is reached in LDL. When this critical concentration is reached, the nonenzymatic oxidation of LDL phospholipids produces a series of biologically active, oxidized phospholipids that mediate the cellular events seen in the developing fatty streak. Normal high density lipoprotein (HDL) contains at least 4 enzymes as well as apolipoproteins that can prevent the formation of the LDL-derived oxidized phospholipids or inactivate them after they are formed. In the sense that normal HDL can prevent the formation of or inactivate these inflammatory LDL-derived oxidized phospholipids, normal HDL is anti-inflammatory. HDL from mice that are genetically predisposed to diet-induced atherosclerosis became proinflammatory when the mice are fed an atherogenic diet, injected with LDL-derived oxidized phospholipids, or infected with influenza A virus. Mice that were genetically engineered to be hyperlipidemic on a chow diet and patients with coronary atherosclerosis, despite normal lipid levels, also had proinflammatory HDL. It is proposed that LDL-derived oxidized phospholipids and HDL may be part of a system of nonspecific innate immunity and that the detection of proinflammatory HDL may be a useful marker of susceptibility to atherosclerosis.

Lipoxygenases: occurrence, functions and catalysis

Lipid peroxidation is common to all biological systems, both appearing in developmentally and environmentally regulated processes. Products are hydroperoxy polyunsaturated fatty acids and metabolites derived there from collectively named oxylipins. They may either originate from chemical oxidation or are synthesized by the action of various enzymes, such as lipoxygenases (LOXes). Signalling compounds such as jasmonates, antimicrobial and antifungal compounds such as leaf aldehydes or divinyl ethers, and a plant-specific blend of volatiles including leaf alcohols are among the numerous products. Cloning of many LOXes and other key enzymes metabolizing oxylipins, as well as analyses by reverse genetic approaches and metabolic profiling revealed new insights on oxylipin functions, new reactions and the first hints on enzyme mechanisms. These aspects are reviewed with respect to function of specific LOX forms and on the development of new models on their substrate and product specificity.

Arachidonic acid metabolites as intracellular modulators of the G protein-gated cardiac K+ channel

Arachidonic acid is released from cell membranes in response to receptor-dependent as well as receptor-independent stimulation in various cells, including cardiac myocytes. Arachidonic acid is converted to prostaglandins by cyclooxygenase and to leukotrienes by 5-lipoxygenase, metabolites which are very biologically active and modulate cellular functions such as platelet aggregation, smooth muscle contraction and neural excitation. The molecular mechanisms underlying their modulations are, however, still badly understood. Here, we report that the 5-lipoxygenase metabolites of arachidonic acid activate the pertussis toxin-sensitive G protein-gated muscarinic K+ channel (IK.ACh): arachidonic acid activation of IK.ACh was prevented by the lipoxygenase inhibitors, nordihydroguaiaretic acid and AA-861; leukotriene A4 and C4 activated IK.ACh. The activation occurred in pertussis toxin-treated atrial cells and ceased when inside-out patches were formed but the patches were still susceptible to stimulation by GTP and to inhibition by GDP-beta-S. These results indicate that arachidonic acid metabolites may stimulate the G-protein in a receptor-independent way.

The diversity of the lipoxygenase family. Many sequence data but little information on biological significance

Lipoxygenases form a family of lipid peroxidising enzymes, which oxygenate free and esterified polyenoic fatty acids to the corresponding hydroperoxy derivatives. They are widely distributed in both the plant and animal kingdoms. During the last couple of years more and more lipoxygenase isoforms have been discovered but for most of them the biological significance remains unclear. This review attempts to classify the currently known mammalian lipoxygenase isoforms and critically reviews the concepts for their biological importance.

Lipoxygenase contributes to the oxidation of lipids in human atherosclerotic plaques

Oxidized LDL is present in human atherosclerotic lesions, but the mechanisms responsible for oxidation in vivo have not been definitively demonstrated. Circumstantial evidence has implicated the enzyme 15-lipoxygenase as a contributor to the formation of oxidized lipids in this disease. To assess whether oxidized lipids are indeed formed by the action of 15-lipoxygenase on polyunsaturated fatty acids (PUFAs) in vivo, we have used a sensitive and specific method (chiral phase HPLC) to analyze the lipid oxidation products present in human atherosclerotic lesions. Human 15-lipoxygenase is an omega-6 lipoxygenase that has previously been shown to oxidize esterified PUFA in a stereospecific manner, forming predominantly cholesteryl hydroperoxy-octadecadienoate (13(S)-HPODE) from cholesteryl linoleate substrate in LDL. This property allows its activity to be distinguished from nonenzymatic oxidation, which results in the formation of equal quantities of the S and R stereoisomers of the same oxidation product. A total of 80 specimens of human atherosclerotic plaque were analyzed. Esterified, oxidized linoleate was purified from human atherosclerotic lesions and from LDL oxidized by copper, and the chirality of these oxidation products was compared. There was significantly greater stereospecificity of oxidation in the oxidized linoleate from human atherosclerotic lesions. Even greater stereospecificity was detected in the HPODE derived from cholesteryl ester, purified from human lesions. Cholesteryl HPODE is the primary oxidation product from cholesteryl linoleate, the major esterified PUFA that accumulates in atherosclerotic vessels. Cholesteryl HPODE and its reduced form, cholesteryl hydroxy-octadecadienoate, were detected in all lesions analyzed. Neither the stereospecificity of oxidation nor the percentage of available substrate oxidized to primary oxidation products was correlated with the stage of disease of the lesions examined. We conclude that 15-lipoxygenase contributes to the formation of oxidized lipids in human atherosclerotic lesions.

Essential fatty acids and immune response

The implication that essential fatty acids (EFA) can affect immune response was based on the observation that EFA deficiency can accentuate or improve symptoms of certain autoimmune diseases in animals, and that supplementation of linoleic acid to animals reversed such effects. Furthermore, treatment of animals with cyclooxygenase inhibitors abrogated the effect of linoleic acid. Administration of cyclooxygenase inhibitors to animals enhanced both cell-mediated and humoral immune responses. In vitro studies have shown that prostaglandin E (PGE) group inhibits both T and B lymphocyte functions; it is suggested that effects of EFA on immune response are, in part, mediated through eicosanoids. Growing evidence now suggests that the PGE group of prostaglandins can serve as a negative feedback modulator of immune response. However, in vitro effects of other cyclooxygenase-derived products, such as PGI2 and thromboxane A2 (TXA2) have not been well established, perhaps because of their instability in aqueous media. Unlike the PGE group, some of lipoxygenase-derived products of arachidonic acid have shown immunostimulatory effects, as assessed by lymphokine production in vitro. Whether such effects can be seen in vivo remains to be determined. Some lipoxygenase-derived products with strong chemotactic action may indirectly influence immune response by modulating the population of antigen-presenting macrophages in tissues. Thus, the net effect of eicosanoids synthesized in macrophages on modulating immune response may depend on relative amounts of cyclooxygenase-derived products as compared with lipoxygenase-derived products. Macrophages are the major source of eicosanoids among immunocompetent cells. The profile of eicosanoids, produced in vitro by macrophages, varies with type of stimuli and anatomical sites. It can also be affected by the fatty acid composition of tissue lipids, which in turn can be modified by the composition of dietary EFA. Whether manipulating dietary EFA can modulate immune response in normal humans and animals needs to be determined.

Involvement of lipoxygenase-dependent production of fatty acid hydroperoxides in the development of the hypersensitive cell death induced by cryptogein on tobacco leaves

Lipid peroxidation was investigated in relation with the hypersensitive reaction in cryptogein-elicited tobacco leaves. A massive production of free polyunsaturated fatty acid (PUFA) hydroperoxides dependent on a 9-lipoxygenase (LOX) activity was characterized during the development of leaf necrosis. The process occurred after a lag phase of 12 h, was accompanied by the concomitant increase of 9-LOX activity, and preceded by a transient accumulation of LOX transcripts. Free radical-mediated lipid peroxidation represented 10% of the process. Inhibition and activation of the LOX pathway was shown to inhibit or to activate cell death, and evidence was provided that fatty acid hydroperoxides are able to mimic leaf necrotic symptoms. Within 24 h, about 50% of leaf PUFAs were consumed, chloroplast lipids being the major source of PUFAs. The results minimize the direct participation of active oxygen species from the oxidative burst in membrane lipid peroxidation. They suggest, furthermore, the involvement of lipase activity to provide the free PUFA substrates for LOX. The LOX-dependent peroxidative pathway, responsible for tissue necrosis, appears as being one of the features of hypersensitive programmed cell death.

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.

Postprandial plasma lipid hydroperoxides: a possible link between diet and atherosclerosis

There is increasing evidence implicating a dietary source of plasma lipid peroxides that become elevated in the postprandial state. This phenomenon may be a contributing factor to the correlation found between postprandial hyperlipidemia and increased risk of cardiovascular disease. Using a newly developed method for measuring lipid hydroperoxides directly in plasma, a pilot study was performed which revealed that lipid hydroperoxides are indeed elevated following a fatty meal. Lipid hydroperoxides increased within 2-4 h after the meal and returned to basal levels, corresponding to the usual postprandial hyperlipidemia. A marked suppression of postprandial hydroperoxides was found when a meal was consumed with wine, suggesting that these hydroperoxides can be formed and then absorbed during the digestive process.

Disruption of a maize 9-lipoxygenase results in increased resistance to fungal pathogens and reduced levels of contamination with mycotoxin fumonisin

Plant oxylipins, produced via the lipoxygenase (LOX) pathway, function as signals in defense and development. In fungi, oxylipins are potent regulators of mycotoxin biosynthesis and sporogenesis. Previous studies showed that plant 9-LOX-derived fatty acid hydroperoxides induce conidiation and mycotoxin production. Here, we tested the hypothesis that oxylipins produced by the maize 9-LOX pathway are required by pathogens to produce spores and mycotoxins and to successfully colonize the host. Maize mutants were generated in which the function of a 9-LOX gene, ZmLOX3, was abolished by an insertion of a Mutator transposon in its coding sequence, which resulted in reduced levels of several 9-LOX-derived hydroperoxides. Supporting our hypothesis, conidiation and production of the mycotoxin fumonisin B1 by Fusarium verticillioides were drastically reduced in kernels of the lox3 mutants compared with near-isogenic wild types. Similarly, conidia production and disease severity of anthracnose leaf blight caused by Colletotrichum graminicola were significantly reduced in the lox3 mutants. Moreover, lox3 mutants displayed increased resistance to southern leaf blight caused by Cochliobolus heterostrophus and stalk rots caused by both F. verticillioides and C. graminicola. These data strongly suggest that oxylipin metabolism mediated by a specific plant 9-LOX isoform is required for fungal pathogenesis, including disease development and production of spores and mycotoxins.

The role of cyclooxygenase and lipoxygenase in cancer chemoprevention

The involvement of prostaglandins (PGs) and other eicosanoids in the development of human cancer has been known for over two decades. Importantly, an increase in PG synthesis may influence tumor growth in human beings and experimental animals, and numerous studies have illustrated the effect of PG synthesis on carcinogen metabolism, tumor cell proliferation and metastatic potential. PGs produced by cyclooxygenases (COXs) are represented by a large series of compounds that mainly enhance cancer development and progression, acting as carcinogens or tumor promoters, with profound effects on carcinogenesis. Further investigations suggest that arachidonic acid (AA) metabolites derived from lipoxygenase (LOX) pathways play an important role in growth-related signal transduction, implying that intervention through these pathways should be useful for arresting cancer progression. We discuss here the implications of COX and LOX in colon, pancreatic, breast, prostate, lung, skin, urinary bladder and liver cancers. Select inhibitors of COX and LOX are described, including nonsteroidal antiinflammatory drugs (NSAIDs), selective COX-2 inhibitors, curcumin, tea, silymarin and resveratrol, as well as a method useful for evaluating inhibitors of COX. Although a substantial amount of additional work is required to yield a better understanding of the role of COX and LOX in cancer chemoprevention, it is clear that beneficial therapeutic effects can be realized through drug-mediated modulation of these metabolic pathways.

Oxylipin profiling reveals the preferential stimulation of the 9-lipoxygenase pathway in elicitor-treated potato cells

Lipoxygenases are key enzymes in the synthesis of oxylipins and play an important role in the response of plants to wounding and pathogen attack. In cultured potato cells treated with elicitor from Phytophthora infestans, the causal agent of late blight disease, transcripts encoding a linoleate 9-lipoxygenase and a linoleate 13-lipoxygenase accumulate. However, lipoxygenase activity assays and oxylipin profiling revealed only increased 9-lipoxygenase activity and formation of products derived therefrom, such as 9-hydroxy octadecadienoic acid and colneleic acid. Furthermore, the 9-lipoxygenase products 9(S),10(S),11(R)-trihydroxy-12(Z)-octadecenoic and 9(S),10(S),11(R)-trihydroxy-12(Z),15(Z)-octadecadienoic acid were identified as novel, elicitor-inducible oxylipins in potato, suggesting a role of these compounds in the defense response against pathogen attack. Neither 13-lipoxygenase activity nor 13-lipoxygenase products were detected in higher amounts in potato cells after elicitation. Thus, formation of products by the 9-lipoxygenase pathway, including the enzymes hydroperoxide reductase, divinyl ether synthase, and epoxy alcohol synthase, is preferentially stimulated in cultured potato cells in response to treatment with P. infestans elicitor. Moreover, elicitor-induced accumulation of desaturase transcripts and increased phospholipase A(2) activity after elicitor treatment suggest that substrates for the lipoxygenase pathway might be provided by de novo synthesis and subsequent release from lipids of the endomembrane system.

Bile salt stimulation of colonic epithelial proliferation. Evidence for involvement of lipoxygenase products

Prostaglandin E2 (PGE2) and several other prostaglandins synthesized by colon suppress the proliferative activity of colonic epithelium. However, bile salts stimulate colonic epithelial proliferation despite the actions of bile salts to enhance the release of arachidonate and consequent colonic synthesis of PGE2. The current study was conducted to assess whether bile salt-induced increases in colonic formation of arachidonate metabolites other than PGE2 were linked to the stimulation of the proliferative activity of colonic epithelium. Within 10 min of addition, deoxycholate markedly stimulated the in vitro release of [14C]arachidonate from prelabeled rat colon. When given in vivo by intracolonic instillation deoxycholate (10 mumol) increased colonic accumulation of immunoreactive prostaglandin E (PGE), thromboxane B2 (TXB2), and the lipoxygenase product 12-hydroxyeicosatetraenoic acid (12-HETE) by two to fourfold over control in 30 min. This effect of intracolonic deoxycholate was followed by a ninefold increase in mucosal ornithine decarboxylase activity (4 h), and a subsequent two to threefold increase in [3H]thymidine [( 3H]Thd) incorporation into DNA of either mucosal scrapings or isolated pools of proliferative colonic epithelial cells (24 h). Intracolonic instillation of indomethacin (50 mumol) suppressed to low or undetectable levels both basal colonic accumulation of PGE and TXB2 and the increases in each parameter induced by subsequent instillation of deoxycholate. By contrast, indomethacin enhanced accumulation of 12-HETE in both control colons and those subsequently exposed to deoxycholate. The increases in 12-HETE induced by indomethacin alone were correlated with stimulation of mucosal ornithine decarboxylase activity and [3H]Thd incorporation into mucosal DNA. Indomethacin also enhanced the increases in these parameters induced by deoxycholate. Intracolonic instillation of phenidone (25-100 mumol) suppressed accumulation of PGE, TXB2, and 12-HETE in control colons and the increases in these parameters induced by a subsequent instillation of deoxycholate. Phenidone alone did not alter mucosal ornithine decarboxylase activity or [3H]thymidine incorporation into mucosal DNA. However, phenidone suppressed or abolished increases in these parameters induced by a subsequent instillation of deoxycholate. 4-(2-[IH-imidazol-1-yl]ethoxy) benzoic acid hydrochloride UK 37,248, which selectively reduced colonic TXB2 to undetectable levels without altering PGE or 12-HETE, had no effect on control or deoxycholate-induced increases in mucosal ornithine decarboxylase activity or [3H]Thd incorporation into DNA. Neither indomethacin nor phenidone altered the increases in [(14)C]arachidonate release induced in vitro by deoxycholate. Chenodeoxycholate and cholate also stimulated [(14)C]arachidonate release from colon in vitro within 10 min, and increased colonic 12-HETE (30 min) and mucosal ornithine decarboxylase activity (4 h) upon intracolonic installation. Prior installation of phenidone inhibited the increases in both 12-HETE and ornithine decarboxylase activity induced by these bile salts. The results support a role for bile salt-induced increases in colonic accumulation of lipoxygenase products, as reflected by 12-HETE, in the subsequent stimulation of the proliferative activity of colonic epithelium.

Structural biology of mammalian lipoxygenases: Enzymatic consequences of targeted alterations of the protein structure

Lipoxygenases form a heterogeneous family of lipid peroxidizing enzymes, which have been implicated in the pathogenesis of diseases with major health political relevance (bronchial asthma, atherosclerosis, cancer, and osteoporosis). The crystal structures of one mammalian lipoxygenase and of two plant isoenzymes have been solved and the structural bases of important enzyme properties (reaction specificity, membrane binding, and suicidal inactivation) have been investigated in the past. This review will briefly summarize our current understanding on the structural biology of the most important mammalian lipoxygenase isoforms and will also address selected mechanistic features of the lipoxygenase reaction.

Stereospecific inhibition of PGH2-induced platelet aggregation by lipoxygenase products of icosaenoic acids

Mono-hydroxylated fatty acids were prepared from the three prostaglandin precursors (20:3, 20:4 and 20:5) through the platelet 12-lipoxygenase or the soybean 15-lipoxygenase and were purified by HPLC. The inhibition of PGH2-induced human platelet aggregation by these hydroxy derivatives was compared. Other hydroxy derivatives of arachidonic acid of physiological importance were also tested in that respect. We have found that 12- or 15- hydroxy-icosaenoic acids are the most potent inhibitors. As compared to 12- or 15-hydroxy -20:4 (12- or 15-HETE), 5-HETE was about three fold less potent. We have also found that leukotriene B4 (5S, 12R-diHETE) is completely devoid of inhibitory activity while its isomer 5S, 12S-diHETE shares the activity of every mono-hydroxy-icosaenoic acids which are also S derivatives. We conclude that hydroxy derivatives of icosaenoic acids can inhibit PGH2-induced platelet aggregation by structural analogy and that they need a S configuration. These findings point out a possible negative feed back modulation of platelet aggregation by the lipoxygenase products of arachidonic acid and other icosaenoic acids which can arise in platelets subsequently to dietary manipulations.

Arachidonic acid metabolism in airway epithelial cells

Airway epithelial cells carry out their physiologic role in part by activating phospholipase-fatty acid oxygenation pathways. Recent discoveries include the facts that (a) airway epithelial cells contain abundant stores of fatty acid substrates, including arachidonic acid, for oxygenation, (b) the cells release arachidonic acid upon activation of specific phospholipases, (c) the cells contain novel cyclooxygenases and lipoxygenases at high levels relative to other cell types, and (d) some of the arachidonate metabolites have potent biologic effects on airway end organs such as smooth muscle, nerves, mucus glands, and epithelial cells themselves. Studies of arachidonate metabolism in airway epithelial cells have often been done on a heterogeneous cell population of basal, ciliated, and goblet cells, so information on individual cell types and alterations during cellular differentiation is still poorly defined. Potential cell-cell interaction via transcellular synthesis of eicosanoids also requires further study. Each of these aims would be aided by the use of cultured airway epithelial cells, but the culture system has proven problematic in preserving the oxygenation phenotype of the original tissue. The same access of the epithelial cells to inhaled agents will permit lipid-modifying drugs to be delivered to them, and much of this therapeutic potential is still unexplored. Therefore, determining the factors that regulate arachidonic acid metabolism in airway epithelial cells is still a fundamental goal for unraveling the role of arachidonate products in airway function and for altering eicosanoid production in the airway.

Lipid Peroxidation during the Hypersensitive Response in Potato in the Absence of 9-Lipoxygenases

Hypersensitive cell death is an important defense reaction of plants to pathogen infection and is accompanied by lipid peroxidation processes. These may occur non-enzymatically by the action of reactive oxygen species or may be catalyzed by enzymes such as alpha-dioxygenases, lipoxygenases, or peroxidases. Correlative data showing increases in 9-lipoxygenase products in hyper-sensitively reacting cells have so far suggested that a large part of lipid peroxidation is mediated by a specific set of 9-lipoxygenases. To address the significance of 9-lipoxygenases for this type of pathogen response in potato, RNA interference constructs of a specific pathogen-induced potato 9-lipoxygenase were transferred to potato plants. Significantly reduced 9-lipoxygenase transcript levels were observed in transgenic plants after pathogen treatment. In addition, 9-lipoxygenase activity was hardly detectable, and levels of 9-lipoxygenase-derived oxylipins were reduced up to 12-fold after pathogen infection. In contrast to wild type plants, high levels of non-enzymatically as well as 13-lipoxygenase-derived oxylipins were present in 9-lipoxygenase-deficient plants. From this we conclude that during the normal hypersensitive response in potato, lipid peroxidation may occur as a controlled and directed process that is facilitated by the action of a specific 9-lipoxygenase. If 9-lipoxygenase-mediated formation of hydroperoxides is repressed, autoxidative lipid peroxidation processes and 13-lipoxygenase-mediated oxylipins synthesis become prominent. The unaltered timing and extent of necrosis formation suggests that the origin of lipid hydroperoxides does not influence pathogen-induced cell death in potato.

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).