Epidermal arachidonate lipoxygenase

In vivo stimulation of 12(S)-lipoxygenase in the rat skin by bradykinin and platelet activating factor: formation of 12(S)-HETE and hepoxilins, and actions on vascular permeability

In this study we set out to investigate whether the inflammatory agents, bradykinin (BK) and platelet activating factor (PAF), affect the lipoxygenase pathway in rat skin in vivo and whether the main products so formed may be involved in the inflammatory actions of these agents. In vitro preparations of epidermis were also investigated to determine whether lipoxygenases are stimulated by these agents. We also investigated the actions of arachidonic acid and 12(S)-HPETE as substrates for the lipoxygenases. Our results indicated that 12-lipoxygenase is actively and selectively stimulated in a dose-dependent way in both preparations by the administration of BK and PAF; the main product, 12-HETE, was shown by chiral analysis to be exclusively of the S-configuration, indicating that 12(S)-lipoxygenase was present in the rat skin and was stimulated by these inflammatory agents. Hepoxilins were also formed but to a lesser extent in both in vivo and in vitro preparations. In separate experiments, 12(S)-HETE administered intradermally on its own (40 ng/site), increased vascular permeability as also seen with bradykinin (100 ng/site) and PAF (10 ng/site). However, unlike previously observed with hepoxilin A3 administration, 12(S)-HETE did not stimulate the action of BK on vascular permeability, suggesting that the two compounds may have different mechanisms of action to enhance inflammation. These observations suggest that the vascular permeability and plasma extravasation observed with both inflammatory agents (BK and PAF) may be mediated at least in part through the activation of 12(S)-lipoxygenase, resulting in enhanced formation of 12(S)-HETE which causes acute inflammation.

Lipoxygenase activity in heart cells

Arachidonic acid (AA) metabolism via the lipoxygenase (LOX) pathway in rat hearts and in cultured rat cardiomyocytes was investigated using 1-[14C]AA. LOX activity was detected in the microsomal fraction, in the high speed supernatant prepared from rat hearts and in rat cardiomyocyte supernatant. LOX products from all fractions comigrated in thin layer chromatography as 12-hydroxyeicosatetraenoic acid (12-HETE) and 15-HETE. Enzyme linked immunosorbent assay for 12-HETE showed its formation by the microsomal fraction, the ammonium sulfate (AS) pellet, and by rat cardiomyocyte supernatant, while radioimmunoassay for 15-HETE showed its formation only by the AS pellet. The properties of LOX in each fraction are reported here.

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).

Interaction between beta-carotene and lipoxygenase in human skin

beta-Carotene is widely used in skin care therapy. Its effects on skin are unclear, but actions on lipid peroxidation pathways may be an important element of any protection activities it exerts. This study examines the possible effects of Beta-carotene on enzymatic lipid peroxidation by lipoxygenase in human skin, using in vitro and ex vivo models. The effect of Beta-carotene on lipid peroxidation in human skin were studied in skin homogenates and in a semi-in vivo model of skin penetration, using [1-14C]-arachidonic acid or [1-14C]-linoleic acid as substrate. When relatively low concentrations (about 0.3 microM) of beta-carotene were added to epidermal homogenates, the major metabolites of arachidonic acid (12-hydroxy-cis-5,8,14, trans-10-eicosatetraenoic acid and 15-hydroxy-cis-5,8,11, trans-13-eicosatetraenoic acid) and of linoleic acid (13-hydroxy-cis-9, trans-11-octadeca dienoic acid and 9-hydroxy-trans-10, cis-12-octadeca dienoic acid) were significantly decreased. Following [1-14C]-linoleic acid penetration through the semi in vivo model layers, the skin surface was the main site in which the major linoleate product, 13-hydroxy-cis-9, trans-11-octadeca dienoic acid was detected. Furthermore, its level was inhibited by up to 80%, compared with the control, when beta-carotene was added to the system. The data presented in this study suggest possible interactions between beta-carotene and human epidermal lipoxygenase. Beta-carotene may effect lipid peroxidation in human skin, either as a free radical scavenger or as a specific lipoxygenase inhibitor.

Novel 10-substituted antipsoriatic anthrones as inhibitors of epidermal 12-lipoxygenase and lipid peroxidation in membranes

The ability of novel 10-substituted anthrones to inhibit 12-lipoxygenase (12-LO) in mouse epidermal homogenate and lipid peroxidation in both bovine brain phospholipid liposomes and erythrocyte ghosts was investigated, and compared with their ability to inhibit 5-lipoxygenase (5-LO) in bovine leukocytes. The compounds were fairly potent inhibitors of epidermal 12-LO, in addition to their strong inhibitory effects against leukocyte 5-LO. Although the antipsoriatic drug, anthralin, predominantly inhibited epidermal 12-LO, the novel derivatives were more selective 5-LO inhibitors. Compounds with free phenolic groups in the attached aromatic ring were also potent inhibitors of nonenzymatic lipid peroxidation in both sources of lipid substrate. This property was not correlated with their ability to inhibit the 5- and 12-LO pathways, suggesting that their mechanism of 5-/12-LO inhibition is not simply due to scavenging of peroxyl radicals generated at the active site of the enzymes. The compounds are dual-purpose inhibitors and may play a protective role against oxidative damage to psoriatic skin, in addition to their antiinflammatory 5-LO and 12-LO inhibitory properties.

Esterification of 12(S)-hydroxy-5,8,10,14-eicosatetraenoic acid into the phospholipids of human peripheral blood mononuclear cells: inhibition of the proliferative response

12-hydroxy-eicosatetraenoic acid (12-HETE), the lipoxygenase metabolite of arachidonic acid produced by activated platelets, has been shown to accumulate in peripheral blood mononuclear cells (PBMC) of elderly people. 12-HETE being antimitogenic for lymphocytes, its accumulation in blood cells might be involved in the well-known decline in immune function which accompanies aging. Because HETEs have been shown to be rapidly metabolized and/or incorporated into cellular lipids in a variety of cell types, we have investigated the uptake, metabolism, and intracellular distribution of exogenous 12-HETE by human PBMC. [3H]-12-HETE was dose and time dependently incorporated by PBMC and also metabolized to more polar products. These polar metabolites were mainly released extracellularly and only marginally esterfied in phospholipids. Although [3H]-12-HETE radiolabel was preferentially associated with phosphatidylcholine, especially after prolonged labeling incubations or following successive short labeling pulses, a substantial amount of radiolabel was also found associated with phosphatidylinositol (20-50% of the labeled phospholipids). The stability of 12-HETE in the phospholipid pool was comparable to that reported for most other cell types, with 50% of the initial radiolabel being still present after 18 hr. Upon exposure to mitogenic activation, 12-HETE-labeled PBMC released unmodified 12-HETE from phosphatidylinositol. In addition, 12-HETE dose dependently inhibited the proliferative response of PBMC to Con A stimulation. These results suggest that 12-HETE esterification in phospholipids might lead to the generation of unusual lipid second messengers with impaired capacity to transduce activation signals, thus decreasing lymphocyte function.

Lipoxygenase activity in rat dermis and epidermis: partial purification and characterization

Lipoxygenase (LOX) activity in epidermis and dermis was distributed among microsomal and cytosolic fractions. The main products of polyunsaturated fatty acid metabolism were 12-hydroperoxy-cis-5,8,14, trans-10-eicosatetraenoic acid (12-HPETE), 15-hydroperoxy-cis-5,8,11, trans-13-eicosatetraenoic acid (15-HPETE) and 13-hydroxy-cis-9, trans-11-octadecadienoic acid (13-HOD). Enzyme activities were isolated from rat dermis and epidermis by ammonium sulphate precipitation, hydrophobic chromatography and gel filtration. In the dermis, activity was found at a molecular mass of 68 kDa, a pI of 4.6 and a Km of 50 microM. This activity was inhibited by known LOX inhibitors. The main reaction products indicated that this was 15-LOX. In the epidermis, activity was found in a fraction with a molecular mass of 68 kDa, a pI of 4.6 and a Km of 80 microM. Activity was inhibited by known LOX inhibitors whereas the reaction products indicated that this was 12-LOX. LOX activity in rat skin may involve one enzyme with dual regional specificities or may comprise two different enzymes.

Epidermis contains platelet-type 12-lipoxygenase that is overexpressed in germinal layer keratinocytes in psoriasis

Human epidermal cells exhibited none of the cytosolic lipoxygenase activity that is prominent in mucosal epithelial cells, but instead contained a microsomal activity that converted arachidonic acid to 12-hydroxyeicosatetraenoic acid (12-HETE). Identification of the extractable 12-HETE-forming activity as a 12-lipoxygenase (distinct from cytochrome P-450) included (S)-12-stereospecificity of product formation, trapping of 12-hydroperoxyeicosatetraenoic acid as an intermediate reaction product, and lack of NADPH dependence for activity. Epidermal cell poly(A)+ RNA contained high levels of a 2.3-kb mRNA that selectively hybridized with human platelet 12-lipoxygenase cDNA, and partial cDNA sequence of this mRNA indicated identity to platelet 12-lipoxygenase. The epidermal 12-lipoxygenase was not recognized by antibodies against the leukocyte-type 12- and 15-lipoxygenases (found in leukocytes, reticulocytes, and mucosal epithelial cells) but was detected by an antiplatelet 12-lipoxygenase antibody. The epidermal 12-lipoxygenase antigen was selectively expressed in germinal layer keratinocytes in healthy and psoriatic skin, and these layers exhibited hyperplasia and increased immunostaining in inflamed psoriatic skin. Together with previous results, these observations indicate that 1) epidermis generates 12-HETE by either cytochrome P-450 or lipoxygenase-based mechanisms depending on reaction conditions, and 2) 12-lipoxygenases (originally described in hematopoietic cell types) may be expressed in at least two distinct isoforms in epithelial barriers in humans, and in the case of the skin, a microsomal (platelet-type) 12-lipoxygenase is selectively overexpressed in germinal layer keratinocytes during psoriatic inflammation.

Lipoxygenase activity in the brain regions of young chicks: isolation and some properties

1. The lipoxygenase (LOX) oxygenation pathway of arachidonic acid was investigated in the cerebellum and cerebral hemispheres of young chicks. 2. Lipoxygenase products consisted mainly of 15-hydroxyeicosatetraenoic acid (15-HETE), accompanied by the 15-hydroperoxy analog (15-HPETE) and the 5-HETE product. 3. The yield of 15-HETE was 3 times greater in the cerebellar system than in the cerebrum. 4. PLA2 activity of the cerebellum was twice that of the cerebrum. 5. Affinity chromatography revealed 2 brain fractions with LOX activity which were assayed with either linoleic or arachidonic acid as substrate. 6. The fraction eluted with 0.2 M sodium acetate pH 5.0, produced a higher yield and enrichment of LOX activity than the eluate obtained with 0.1 M Tris-HCl buffer (pH 8.0). 7. A considerably higher yield and enrichment of the enzyme was achieved when the starting material was the cerebellum, compared to the cerebrum. 8. The optimal pH for both purified fractions from cerebrum and cerebellum was 6.5, with either linoleic or arachidonic acid as substrate. 9. The cerebral LOX yielded Michaelis-Menten kinetics when linoleic acid was the substrate, while the corresponding plots for the cerebellar enzyme were sigmoidal. 10. Arachidonic acid as substrate produced sigmoidal plots, except at pH 5.0, where Michaelis-Menten kinetics were observed. 11. These results and the elevated activities of PLA2 and 15-LOX could be significant in relation to the special vulnerability of the cerebellum in chick nutritional encephalomalacia.

Defect of epidermal 12(S)-hydroxyeicosatetraenoic acid receptors in psoriasis

12-hydroxyeicosatetraenoic acid (12-HETE) is assumed to play a central role in the pathophysiology of psoriasis. Since its effects in skin are mediated by specific high-affinity receptors, we studied the receptor characteristics in cultured epidermal cells from involved and apparently healthy skin of psoriasis patients by radioligand binding assay. Involved and uninvolved psoriatic epidermal cells showed a fourfold decrease in the number of 12-HETE binding sites as compared with normal healthy individuals and patients with atopic dermatitis, while receptor affinity remained unchanged. The decrease in receptor number was evident in psoriatic cells even in long-term culture and was not due to receptor down-regulation, defective response to interferon gamma or to protease degradation of receptor protein. The decrease in the number of 12-HETE receptors detectable even in clinically normal psoriatic skin functionally leads to diminished 12-HETE uptake and may thus represent a primary central molecular defect in the pathophysiology of the disease.

Stimulation by carrageenan of arachidonate 12-lipoxygenase activity in dog gingival tissue

At 4 h after injection of carrageenan into the gingiva, the 12-lipoxygenase activity of the gingival homogenate was markedly increased. Activity in the cytosol and microsomal fractions was markedly increased when assessed as the specific activity based on nmol/min/mg of protein, and in the cytosol fraction as the percentage distribution of total activity. The 12-lipoxygenase activity in the homogenate from carrageenan-treated gingiva was not affected by either EDTA or calcium ion, or a combination of the two. 12-lipoxygenase activity in both carrageenan-treated and untreated gingiva was inhibited dose-dependently by AA861, a striking difference from its effect on platelet 12-lipoxygenase. There was a marked increase of 12-lipoxygenase activity in experimentally inflamed gingiva compared to the non-inflamed gingiva.

Regulation of 12(S)-hydroxyeicosatetraenoic acid (12(S)-HETE) binding sites on human epidermal cells by interferon-gamma

We recently detected specific high-affinity binding sites for 12(S)-HETE, the main arachidonic acid metabolite in skin, on epidermal cells. The putative receptor is involved in keratinocyte chemotaxis toward 12(S)-HETE, which points to its participation in wound healing. In an effort to further characterize the 12(S)-HETE receptor, we investigated its regulation by various cytokines. Of the tested cytokines, only interferon (IFN)-gamma led to a massive induction of the 12(S)-HETE receptors. The effect was dose and time dependent and blocked by cycloheximide. The up-regulation of 12(S)-HETE receptors by IFN-gamma may represent an amplification mechanism of the assumed role of 12(S)-HETE in skin wound repair.

Influence of eicosanoids on fibroblast chemotaxis and protein synthesis in vitro

Metabolism of fibroblasts plays a key role in wound healing, fibrosis, rheumatoid arthritis, and similar physiological and pathological processes. The regulatory influence of eicosanoids, an important class of inflammatory mediators, on fibroblast metabolism, in these processes is, to date, unclear. The aim of this study was to investigate the effect of some eicosanoids on chemotaxis and protein synthesis of fibroblasts in vitro. Of twelve eicosanoids tested, only 5(S)-HETE, LTB4, and 12(S)-HETE were active as chemo-attractants for fibroblasts. 5(S)-HETE was the most potent attractant. It exerted its maximal activity at 10(-10) mol/l. 12(S)-HETE and LTB4 caused similar dose dependent fibroblast chemotaxis with a maximum of activity at 10(-7) M and 5 x 10(-8) M, respectively. Hydroxylation of LTB4 on C20 or methylation of the carboxy group of 12(S)-HETE decreased reactivity of the parent compounds only slightly. Eicosanoid induced chemotaxis could be antagonized by 12(S)-HETE but not by the proteinaceous chemoattractants fibronectin, PDGF, or EGF. Receptors for peptide and eicosanoid mediated chemotaxis are thus different. Inhibition of collagen synthesis was observed in the presence of 5(S)-HETE and 12(S)-HETE while total protein synthesis was unaffected by 12(S)-HETE and augmented by 5(S)-HETE. These data suggest that certain eicosanoids specifically regulate fibroblast activities in wound healing and similar events of connective tissue reorganization.

Effects of calcium ionophore on vitamin E-deficient rat muscle

Damage to skeletal muscles may be mediated via free radicals or intracellular calcium overload. To look for inter-relationships between these pathways we have examined the effect of intracellular Ca overload on muscles from rats fed on either a vitamin E-deficient or vitamin E-sufficient diet and assessed the non-enzymic lipid peroxidation in these muscles by examining the production of thiobarbituric acid reactive substances by homogenates. Vitamin E-deficient muscles were more susceptible to Ca-induced intracellular enzyme efflux and this was acutely corrected by supplementation of the external medium with 230 mumol alpha-tocopherol/l. Vitamin E-deficient muscles showed increased levels of basal lipid peroxides and were more susceptible to iron-catalysed lipid peroxidation. Addition of the Ca ionophore A23187 increased lipid peroxidation in vitamin E-deficient muscle homogenates, but had the opposite effect in vitamin E-sufficient muscles. These results demonstrate that vitamin E-deficient muscle has an increased susceptibility to intracellular Ca overload, but that this effect cannot be explained by a direct stimulatory effect of the ionophore on non-enzymic lipid peroxidation.

Blockade by lipoxygenase inhibitors of Ca2+-dependent insulin secretion from permeabilized rat islets. A molecular mechanism distinct from that of alpha 2-adrenergic agonists

To evaluate the regulation and effects of pancreatic islet lipoxygenase, adult rat islets were permeabilized, using digitonin or staphylococcal alpha-toxin, and then were studied in a medium simulating an intracellular milieu at fixed ambient concentrations of Ca2+. Permeabilized islets retained 12-lipoxygenase activity, as indicated by conversion of tritiated arachidonic acid to a predominant peak of [3H]12-hydroxyeicosatetraenoic acid (12-HETE); this activity was inhibited (89-98%) by the lipoxygenase blockers nordihydroguaiaretic acid (35 microM), BW755c (250 microM) or ETYA (35 microM). Lesser amounts of compounds coeluting with 15- and 11-HETE (but little or no 5-HETE) were formed; however, 11-HETE (and possibly some 15-HETE) was probably synthesized (at least in part) via cyclooxygenase, as suggested by the partial synthesis blockade induced by 50 microM ibuprofen. The production of 12-HETE did not require the presence of Ca2+, Mg2+ or ATP; it also was not stimulated by addition of cyclic AMP, a phorbol ester, or calmodulin. However, it was augmented modestly by provision of a basal cytosolic free Ca2+ concentration of 60-80 nM, with no further increase at physiologically elevated levels of 260-530 nM. Elevations in cytosolic free Ca2+ concentrations induced insulin release which was inhibited by cooling, epinephrine or protein kinase inhibitors and, therefore, was exocytotic in nature. Lipoxygenase inhibitors blocked this insulinotropic effect of calcium at submaximal or saturating Ca2+ concentrations (with or without its potentiation by 12-O-tetradecanoylphorbol-13-acetate, an activator of protein kinase C) by 53-82%. However, they did not reduce the Ca2+-independent secretory effects (at subnanomolar Ca2+ concentrations) of the phorbol ester alone. Similar results were seen using dibutyryl cyclic AMP to activate protein kinase A. The alpha 2-adrenergic agonists epinephrine or clonidine inhibited Ca2+-, TPA- or cyclic AMP-induced insulin release without reducing HETE formation. We conclude that (1) islet lipoxygenase is constitutively expressed and is not physiologically regulated by alpha 2-adrenergic agonism, Ca2+ or protein kinases; (2) lipoxygenase modulates insulin release; HETE production is not merely an epiphenomenon reflecting the activation (or inhibition) of exocytotic secretion; (3) islet lipoxygenase inhibitors reduce insulin secretion, at least in part, by blocking the direct effects of Ca2+ on exocytosis and/or its synergism with Ca2+-binding proteins such as protein kinase C; and (4) these same inhibitors do not directly poison protein kinase C or A, or the exocytotic apparatus.(ABSTRACT TRUNCATED AT 400 WORDS)

Arachidonic acid metabolism in HEL/30 murine epidermal cell line

The established mouse epidermis-derived cell line HEL/30 was incubated in the presence of 3H arachidonic acid (AA) for 1 h. After medium removal, cells were reincubated with fresh medium in the presence or absence of the calcium ionophore A23187 and tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate (TPA). The AA metabolites formed were extracted from cell-free medium and analyzed using TLC and HPLC. The distribution of the recovered radioactivity showed PGE2, 15-hydroxy-eicosatetraenoic acid (15-HETE), and leukotriene B4 (LTB4), as major products of AA metabolism. The presence of calcium ionophore A23187 increased the release of radioactivity, without affecting the profile of metabolites present in the medium. TPA elicited a preferential increase of cycloxygenase metabolism, this effect being reversed by indomethacin. 5,8,11,14-eicosatetraynoic acid (ETYA) almost completely inhibited LT and HETE formation in A23187 and TPA-treated cells. The results show that HEL/30 cells are able to metabolize AA via both cyclo- and lipoxygenase pathways and that these activities can be modified by chemical means. This cell line might be a suitable tool for studying the involvement of arachidonic acid cascade in cell response to exogenous stimuli.

Conversion of linoleic acid and arachidonic acid by skin epidermal lipoxygenases

Two different lipoxygenases have been identified in human and rat epidermis. One lipoxygenase has a (n-9)-specificity, converts arachidonic acid into 12-hydroxyeicosatetraenoic acid (12-HETE), and has been described by several investigators. Linoleic acid is not a substrate for this enzyme. The other lipoxygenase, with (n-6)-specificity, converts arachidonic acid into 15-HETE and linoleic acid into 13-hydroxyoctadecadienoic acid (13-HOD). Especially the latter lipoxygenase is thought to be involved in the regulation of the differentiation of the skin cells into a proper water-barrier layer. Linoleate is supposed to be the physiological substrate; this fatty acid is especially present in characteristic sphingolipids with unique structures.

Cytochemical expression of epidermal peroxidase and cytochrome oxidase activities in pathological skin conditions of man

The cytochemical expression of epidermal peroxidase and cytochrome oxidase activity was recently well documented in normal human skin. We report here its expression in basal and squamous cell carcinomas, actinic keratoses, psoriasis, allergic contact dermatitis, seborrheic keratoses, and autosomal dominant ichthyosis vulgaris. The two enzyme activities were evaluated using the diaminobenzidine method. If present, the two enzymes were always localized in the same organelles as in normal epidermis: endogenous peroxidase in the nuclear envelope and endoplasmic reticulum, and cytochrome oxidase in mitochondria. In basal and squamous carcinomas, actinic keratoses and psoriasis, the keratinocytes lost their peroxidase activity, but maintained their cytochrome oxidase activity. In seborrheic keratoses, allergic contact dermatitis and ichthyosis vulgaris, the cytochrome oxidase activity was greatly reduced or abolished in keratinocytes, Langerhans' cells, and melanocytes, whereas the peroxidase activity was present as in normal epidermis. These results indicate that the two peroxidatic enzymes studied are not interrelated and alternatively suppressed by different cellular dysfunctions.

Biochemical studies on a 12-lipoxygenase in human uterine cervix

Human uterine cervix possesses a high 12-lipoxygenase activity; this enzyme has been isolated in a purified form from the squamous epithelial region of human cervix and its major properties have been investigated. Enzyme activity was present in all subcellular fractions obtained by centrifugation; the highest specific activity was associated with the microsome fraction (160,000 X g pellet). Purification of the enzyme was achieved by acetone precipitation, ion exchange chromatography on CM-cellulose and affinity chromatography on linoleyl-aminoethyl-Sepharose. The product from the incubation of sodium [1-14C]arachidonate with crude enzyme extracts co-chromatographed with authentic 12-hydroxyeicosatetraenoic acid, but the purified enzyme gave a product that behaved like the 12-hydroperoxy derivative. The enzyme had optimum activity at pH 6.5, a Km of 15 microM for arachidonic acid and was stimulated by ATP and Ca2+. Enzyme activity was inhibited by esculetin, nordihydroguaiaretic acid, eicosatetraynoic acid, detergents at concentrations greater than 0.1% (w/v) and preincubation of substrate with GSH and GSH peroxidase. The occurrence of a high 12-lipoxygenase activity is discussed in relation to the specific physiological functions of this tissue.

Arachidonic acid metabolism by isolated epidermal basal and differentiated keratinocytes from the hairless mouse

The metabolism of arachidonic acid was studied using basal and differentiated keratinocytes as well as sebaceous cells isolated from hairless mice. These disassociated cells metabolized arachidonic acid predominantly to the prostaglandin H synthase products prostaglandins E2 and D2. 12-Hydroxyheptadecatrienoic acid (HHT), prostaglandin F2 alpha, thromboxane B2 and 6-ketoprostaglandin F1 alpha were also detected. Smaller amounts of the lipoxygenase products 5-, 12- and 15-hydroxyeicosatetraenoic acids (HETEs) were also detected. The major lipoxygenase product observed was 12-HETE. No leukotrienes or dihydroxy fatty acids were observed. The identity of the metabolites was established using several high-pressure liquid chromatography solvent systems. The biosynthesis of prostaglandins E2 and D2 was very rapid and was inhibited by the addition of indomethacin to the cells. The mixed population of keratinocytes and sebaceous cells were separated into enriched fractions by metrizamide gradients and elutriation techniques. The small, undifferentiated cells had high prostaglandin H synthase and 12-lipoxygenase activity. The basal cell-enriched fractions had the highest activity. With increasing differentiation of the cells, decreased biosynthetic activity was observed. These results indicate that undifferentiated keratinocytes, that is, the basal cells, may be an important source of prostaglandins and 12-HETE but are not a source of leukotrienes for the hairless mouse. It also suggests a role for keratinocyte-derived eicosanoids in the normal physiology of epidermal differentiation.

Stereochemical difference between 12-hydroxy-5,8,10,14-eicosatetraenoic acid in platelets and psoriatic lesions

Stereochemical analysis of 12-hydroxy-5,8,10,14-eicosatetraenoic acid derived from the lesional scale of patients with psoriasis is reported. Resolution of the C-12 hydroxyl enantiomers was carried out by high pressure liquid chromatography of their diastereomeric methyl ester dehydroabietyl urethane derivatives. The 'psoriasis derived" compound was shown to be stereochemically distinct from the platelet 12(S)-enantiomer as its derivative co-chromatographed with the 12(R)-diastereomer.

Some properties of lipoxygenase activities in cytosol and microsomal fractions of mouse epidermal homogenate

Lipoxygenase activity in microsomal fraction of mouse epidermal homogenate was characterized in comparison with cytosol lipoxygenase activity. The major activity was identified as 12-lipoxygenase in microsomal fraction as well as in cytosol fraction by the analyses with high-performance liquid chromatography and gas chromatography-mass spectrometry. Apparent Km values of cytosol and microsomal 12-lipoxygenase for arachidonic acid were 5.0 microM and 6.2 microM respectively. Apparent Vmax values were 14 pmol/min/mg protein for the cytosol enzyme and 32 pmol/min/mg protein for the microsomal enzyme. Net activities of cytosol and microsomal 12-lipoxygenase were 214 and 109 pmol/min/g wet tissue, respectively. Both cytosol and microsomal lipoxygenase activities were neither dependent on calcium nor ATP. Carbon monoxide failed to affect these enzyme activities. There were considerable differences either in the effect of glutathione or in the sensitivities toward several lipoxygenase inhibitors, indicating that the cytosol and the microsomal 12-lipoxygenase activities are derived from two different enzymes. Alternatively, the differences could be attributable to the different microenvironments of these enzymes.

Lipoxygenase in trout gill tissue acting on arachidonic, eicosapentaenoic and docosahexaenoic acids

Lipoxygenase activity was characterized in the gill tissue of fresh-water trout. Incubation of arachidonic acid with gill preparations yielded 12-hydroxyeicosatetraenoic acid as the major product, suggesting a 12-lipoxygenase. Eicosapentaenoic acid was similarly converted to the 12-hydroxyeicosapentaenoic acid. Both arachidonic acid and docosahexaenoic acid were converted with equal apparent velocities and affinities into single monohydroxy derivatives. Analyses of the hydroxy product of docosahexaenoic acid were consistent with 14-hydroxydocosahexaenoic acid. This enzyme activity was localized to the cytosolic fraction and displayed a broad pH optimum around pH 7. The enzyme was insensitive to the cyclooxygenase inhibitors indomethacin and aspirin but activity was strongly inhibited in the presence of the lipoxygenase inhibitors, SnCl2 (5 mM), esculetin (10 microM) and eicosatetraynoic acid (100 microM).

Arachidonic acid metabolism in cultured mouse keratinocytes

We attempted to characterize the general features of arachidonate metabolism in cultured mouse keratinocytes. The cells labeled with [3H]arachidonate were stimulated by 12-O-tetradecanoylphorbol-13-acetate (TPA), ionophore A23187, and fetal bovine serum (FBS). Common to the three substances, phosphatidylinositol, phosphatidylethanolamine, and phosphatidylcholine almost equally served as sources of arachidonate liberated by the action of phospholipase A2. The stimulation of phospholipase A2 action was observed in the order of A23187 greater than FBS greater than TPA. When stimulated by TPA or A23187, the radioactivity released into the extracellular medium was mostly found in prostaglandin (PG) E2. Formation of other PGs and hydroxyeicosatetraenoate (HETE) was extremely limited. In the case of stimulation by FBS, however, the released radioactivity was mainly associated with non-converted arachidonate. FBS also inhibited the TPA- and A23187-induced conversion of arachidonate to PGE2. Phospholipid degradation induced by the three stimulators was similarly dependent on extracellular Ca2+. The stimulation by FBS and A23187 was suppressed by calmodulin antagonists, though the effect of A23187 was much more sensitive to the antagonists when compared to that of FBS. We observed more than additive effects of the three stimulators when tested together.

Formation of 11-hydroxyeicosatetraenoic acid and 15-hydroxyeicosatetraenoic acid in human umbilical arteries is catalyzed by cyclooxygenase

Human umbilical arteries convert arachidonic acid into three hydroxy-eicosatetraenoic acids as well as 6-ketoprostaglandin F1 alpha, prostaglandins E2, F2 alpha and D2 and thromboxane B2. Two of these hydroxy derivatives of arachidonic acid were purified by reverse-phase HPLC and identified by GC-MS as 11-hydroxyeicosatetraenoic acid (11-HETE) and 15-hydroxyeicosatetraenoic acid (15-HETE) while a third, presumed dihydroxy derivative has not yet been identified. Both the cyclooxygenase and HETE synthesizing activities were found to be localized mainly in the microsomal fraction (100 000 X g pellet) (51 and 61% of total, respectively), and approx. 25% of both activities was found in the 10 000 X g pellet. The formation of these HETEs was inhibited by the cyclooxygenase inhibitors indomethacin and aspirin but not by the lipoxygenase inhibitor nordihydroguaiaretic acid. Production of immunoreactive 15-HETE as well as 6-ketoprostaglandin F1 alpha were also decreased significantly when arterial segments were incubated in the presence of either indomethacin or aspirin. Indomethacin inhibited the formation of both prostanoids and HETEs by microsomes in a concentration-dependent and time-dependent manner. The ID50 values for indomethacin against HETE synthesizing activity and against cyclooxygenase were 4.5 and 3.8 microM, respectively. The inactivation constants were found to be 0.09 and 0.08 min-1 for HETE synthesizing activity and cyclooxygenase, respectively. These two microsomal activities were solubilized in parallel with Tween-20. Incubation with three distinct monoclonal antibodies against different epitopes on cyclooxygenase precipitated both cyclooxygenase and HETE synthesizing activity. Each of these activities was recovered in the immune pellets. These studies demonstrate that in human umbilical arteries 11-HETE, 15-HETE and a presumed di-HETE are the products of cyclooxygenase.

Inhibition of teleocidin-caused epidermal ornithine decarboxylase induction by phospholipase A2-, cyclooxygenase- and lipoxygenase-inhibitors

Teleocidin (5 micrograms/mouse), a potent tumor promoting indole alkaloid from Streptomyces, induced epidermal ornithine decarboxylase (ODC) in CD-1 mice. Teleocidin-caused ODC induction was inhibited by the treatment of indomethacin (2 mumol/mouse), a selective cyclooxygenase inhibitor, and p-bromophenacyl bromide (BPB) (30 mumol/mouse), a phospholipase A2 inhibitor. Teleocidin-caused ODC induction inhibited by indomethacin was completely restored by concurrent application of prostaglandin E2 (PGE2) (140 nmol/mouse). On the other hand, teleocidin-caused ODC induction inhibited by BPB was not restored by the treatment of mice with PGE2, but partially restored by the treatment with arachidonic acid (1 mumol/mouse). Treatment of mice with lipoxygenase inhibitors such as BW755C (30 mumol/mouse), nordihydroguaiaretic acid (NDGA) (30 mumol/mouse), quercetin (10 mumol/mouse), and 2,3,5-trimethyl-6-(12-hydroxy-5,10-dodecadiynyl)-1,4-benzoquinone (AA861) (10 mumol/mouse) clearly suppressed ODC induction by teleocidin. Moreover, both NDGA (30 mumol/mouse) and quercetin (10 mumol/mouse) inhibited the restoring effect of PGE2. Therefore, our present results suggest that arachidonate metabolites, i.e., not only cyclooxygenase product(s) but also lipoxygenase product(s), are involved in the mechanism of ODC induction by teleocidin.

Subcellular localization and some properties of lipoxygenase activity in human blood platelets

Lipoxygenase activity was measured in human platelet subcellular fractions. From a sonicated platelet preparation, a granule fraction, mixed membranes (surface and intracellular) and cytosol fractions were separated by differential centrifugation. With respect to activities in the sonicated preparation, the lipoxygenase was slightly enriched in both the cytosol and mixed-membrane fractions and consistently de-enriched in the granule fractions. Approx. 65% and 20% of the total cell enzyme activity were found in the cytosol and mixed membranes respectively, with only 8% present in the granule fraction. Additionally we measured the lipoxygenase activity in purified surface- and intracellular-membrane subfractions prepared from the mixed membranes by free-flow electrophoresis. There was a slight enrichment in activity in the intracellular membrane fraction compared with that in the mixed membranes, and a depletion of activity in the surface membranes. Characterization of the enzyme activity, i.e. time course, pH-dependence, Ca2+-dependence, Vmax. and Km for arachidonic acid, and the carbon-position specificity for this acid, failed to reveal any significant differences between the membrane-bound and soluble forms of the lipoxygenase. These findings suggest that in human platelets the same lipoxygenase is associated with the membranes as in the cytosol and that the membrane-bound activity predominates in intracellular membrane elements.