Metabolism of linoleic acid and other essential fatty acids in the epidermis of the rat

Modulation of phorbol ester-associated events in epidermal cells by linoleate and arachidonate

To elucidate the events elicited by the skin tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA), which are modulated by linoleic acid (LA) and arachidonic acid (AA), the activity of these fatty acids in cultured mouse epidermal cells was compared. Approximately 94% of either exogenous radiolabelled fatty acid was incorporated into the total phospholipid pool over 15 h. The relative distribution among the phospholipid classes differed, however, such that approximately 70% of phospholipid-associated [14C]-LA was found in phosphatidylcholine, compared to approximately 30% for [14C]AA. Phosphatidylethanolamine and phosphatidylinositol/phosphatidylserine contained 17 and 13% of the phospholipid [14C]LA, and 34 and 30% of [14C]AA, respectively. Prostaglandin (PG) E2 production was low but similar in unstimulated cultures prelabelled with either [14C]LA or [14C]AA. However, in cultures treated with TPA (1.6 microM), [14C]AA-prelabelling resulted in approximately three times the amount of [14C]PGE2 compared with cultures prelabelled with [14C]LA. Cultured cells were found to contain significant delta 6 desaturase activity, which may enable conversion of LA to AA, and thus may account for the observed PGE2 production from [14C]LA treated cells. AA-Supplemented (1.6 microM) cultures supported approximately twice the induction of ornithine decarboxylase activity by TPA compared with cultures treated with 1.8 microM LA. Activation of partially purified protein kinase C was similar for either fatty acid tested over a 10-300 microM dose range. Overall, the results suggest that LA does not have the same biological activity as AA with regard to several TPA-associated events known to be important in skin tumor promotion.(ABSTRACT TRUNCATED AT 250 WORDS)

The interaction between beta-carotene and lipoxygenase in plant and animal systems

The effect of beta-carotene (BC) on the activity of lipoxygenase (LOX) from plant and animal sources has been examined. Soybean lipoxygenase L-2 activity towards linoleate was inhibited by BC by a maximum of 70% at pH 6.5, whereas L-1 activity was little affected at pH 9.0. Lineweaver-Burk plots indicated that BC inhibited LOX activity by mixed competitive/non-competitive mechanisms. Other hydrophobic compounds also inhibited LOX activity; oleic acid and retinol were competitive inhibitors whereas tocopherol acetate and 5,8,11,14-eicosatetraynoic acid (ETYA) were non-competitive inhibitors. Binding studies with L-2 LOX bound to Sepharose indicated BC-binding and inhibition with the immobilized LOX. Activity of LOX from animal sources was also inhibited by BC both towards linoleate and arachidonate.

Suppression of endotoxin mitogenicity of spleen cells by lipoxygenase inhibitors and its reversal by 13-hydroxyoctadecadienoic acid

The effects of several inhibitors of lipoxygenases were investigated in murine spleen cell cultures activated with endotoxin (lipopolysaccharide). It was found that these inhibitors interfere with the proliferative response of the cultures. Indomethacin, a specific cyclooxygenase inhibitor, had no such effect. Endotoxin induced the synthesis of tumour necrosis factor alpha in spleen cells which was prevented by treatment with a lipoxygenase inhibitor. The inhibition of the mitogenic effect of endotoxin could be reversed by addition of 13-hydroxyoctadecadienoic acid. This was not the case with leukotriene B4 and C4 or 15-hydroxyeicosatetraenoic acid. In contrast, these substances had inhibitory effects on the mitogenicity of spleen cells. It is suggested that 13-hydroxyoctadecadienoic acid is involved in the development of the mitogenic reaction, possibly on the level of tumour necrosis factor alpha production of macrophages present in the cultures.

Oxidative metabolism of N-phenyllinoleamide by human nasal polyps

N-phenyllinoleamide (NPLA), the anilide of linoleic acid, has been associated with the epidemiology of Toxic Oil Syndrome, but no data are available on its metabolism. On account of the similarity in chemical structure between the linoleic acid and NPLA, the aim of this study has been to investigate the oxidative metabolism of this xenobiotic by the human nasal polyp, a tissue with elevated 15-lipoxygenase activity. For this purpose, tissue homogenates have been incubated for 2 h with NPLA (0.1 mM) spiked with either N-(ring G-3H)PLA (0.2 microCi/ml) or N-P(1-14C)LA (0.05 microCi/ml). Gas chromatographic/mass spectrometric analysis of the high performance liquid radiochromatographic fractions shows that the 9,12,13-trihydroxy, 12,13-epoxy-11-hydroxy and 13-hydroxy NPLA derivatives are the major metabolites. These results revealed that NPLA metabolites are chemical structures related to the linoleic acid derivatives, some of which may show biological activity.

Epidermal HMG CoA reductase activity in essential fatty acid deficiency: barrier requirements rather than eicosanoid generation regulate cholesterol synthesis

We showed previously that the activity of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, the rate-limiting enzyme of cholesterol biosynthesis, increases after both barrier disruption with organic solvents and in essential fatty acid deficiency (EFAD). Here, we treated EFAD hairless mice with linoleic acid, columbinic acid (C18: 3, n-6, trans; not metabolizable to known regulatory eicosanoids), prostaglandin E2 (PGE2), or latex occlusion, and determined transepidermal water loss (TEWL), epidermal protein content, and epidermal HMG CoA reductase activity. Increased TEWL rates in EFAD were accompanied by increased HMG CoA reductase activity (+130%, n = 6, p less than 0.01) and protein content (+69%; n = 6, p less than 0.025). Artificial restoration of the barrier by occlusion reduced the increase in enzyme activity and protein content toward normal, but barrier function, measured immediately after removal of the latex wrap, deteriorated further (TEWL: two-fold greater than EFAD unoccluded; p less than 0.01). Topical applications of either linoleate or columbinate (but not PGE2), normalized barrier function, HMG CoA reductase activity, and protein content. These results show that a) barrier function modulates HMG CoA reductase activity; b) reduction of cholesterol synthesis with occlusion results in a further deterioration in barrier function, suggesting that increased synthesis is a protective homeostatic response; and c) the barrier abnormality reflects a requirement for specific fatty acids for the barrier rather than resulting from epidermal hyperplasia or decreased prostaglandin generation.

Profiling assay for lipoxygenase products of linoleic and arachidonic acid by gas chromatography-mass spectrometry

A method for determination of the lipoxygenase products of linoleic acid (9- and 13-hydroxyoctadecadienoic acid; 9-HODE, 13-HODE) and of arachidonic acid (5-, 8-, 9-, 11-, 12-, and 15-hydroxyeicosatetraenoic acid; 5-, 8-, 9-, 11-, 12-, and 15-HETE) is described. The method combines solid-phase extraction, derivatization to the corresponding fully hydrogenated methylester/trimethylsilylether derivatives and capillary gas chromatography coupled with electron impact mass spectrometry. Each regioisomeric HODE and HETE shows a unique pair of mass spectrometric fragment ions originating from fission of the fatty acid carbon chain at the hydroxylated position. The carboxyl-terminal fragment is used for quantification relative to a carboxyl-18O2-labeled analogue added as internal standard and the methyl-terminal fragment is monitored for confirmation. The assay can be extended for quantification of the complete hydroxylation profile of linoleic and arachidonic acid. Applications of this assay are demonstrated for the quantification of HODEs and HETEs in normal, hyperplastic, and neoplastic mouse epidermis. In mouse epidermis papilloma, the tissue levels of 8- and 12-HETE were found to be increased by one to two orders of magnitude compared to levels in normal epidermis.

In vivo activation of an omega-6 oxygenase in human skin

To test the hypothesis that an epidermal fatty acid oxygenase is activated in vivo under physiologic conditions, surface lipids from normal human skin were analyzed for oxygenase products. With high-performance liquid chromatography on reversed-phase and straight-phase chiral columns and gas-liquid chromatography/mass spectrometry, these lipids were found to contain free 13-hydroxyoctadeca-9Z,11E-dienoic acid and 9-hydroxyoctadeca-10E,12Z-dienoic acid. The 13-hydroxyoctadecadienoic acid was present as a stereoisomeric mixture, with an average S/R ratio of 2.2, and exceeded the concentration of 9-hydroxyoctadecadienoic acid by a factor of 2. These observations and others indicate that the 13-hydroxyoctadecadienoic acid was derived mostly from an omega-6 oxygenase (probably 15-lipoxygenase) which is activated in vivo in normal skin.

Induction of epidermal hyperproliferation by topical n-3 polyunsaturated fatty acids on guinea pig skin linked to decreased levels of 13-hydroxyoctadecadienoic acid (13-hode)

Reversal of essential fatty acid deficiency (EFA) induced epidermal hyperproliferation was recently suggested to require linoleic acid and an active lipoxygenase product. Because the nature of this lipoxygenase product is unknown, we employed a model of n-3 polyunsaturated fatty acid (PUFA) induced hyperproliferation in guinea pig skin to test a possible reversal of the hyperproliferation by an oxidative metabolite of linoleic acid. Topical applications of two n-3 PUFA: 0.5% of eicosapentaenoic acid (20:5n-3) and/or of docosahexaenoic acid (22:6n-3) for 5 d induced severe epidermal hyperproliferation. Development of the epidermal hyperproliferation paralleled a marked decrease in the major epidermal linoleic acid lipoxygenase product (13-hydroxyoctadecadienoic acid; 13-HODE). The application of 0.1% of 13-HODE to the n-3 PUFA-induced guinea pig hyperproliferative skin resulted in the restoration of normal epidermal histology and reversal of hyperproliferation as determined by epidermal uptake of 3H-thymidine. These data support the view that 13-HODE may represent the endogenous cutaneous mediator necessary for full restoration of cutaneous symptoms of essential fatty acid deficiency. Furthermore, the topical use of n-3 PUFA for the disruption of normal metabolism of skin n-6 EFA (linoleic acid) does serve as a useful tool for further investigations into the regulatory mechanisms of in vivo epidermal proliferation/differentiation.

Murine keratinocyte cultures grown at the air/medium interface synthesize stratum corneum lipids and "recycle" linoleate during differentiation

In a recent investigation we showed that murine keratinocyte cultures grown at the air/medium interface in the presence of dermis exhibit morphologic differentiation comparable to that seen in vivo, including the formation of lamellar granules and stratum corneum intercellular lipid lamellae. In the present study, lifted cultures were found to more closely reproduce the lipid composition of the parent epidermal tissue than submerged cultures grown on plastic. In addition, the specific fatty acid profile of individual lipid classes in lifted cultures was, in general, remarkably well maintained in vitro. Acylceramides, which are highly enriched in linoleic acid in vivo, remained enriched in vitro; however, the linoleic acid content of the cultures was substantially lower than that in vivo, confirming previous reports of the relative essential fatty acid deficiency of standard culture media. As the lifted cultures differentiated over time, the lipid composition changed to reflect the formation of a stratum corneum with its different complement of lipids. Label from [U-14C]linoleic acid was specifically incorporated into linoleate-containing lipids during short pulses in both submerged and lifted cultures. Changes in label distribution over a long chase period in lifted cultures indicated that linoleate was transferred from phospholipids to ceramides, providing evidence for the "recycling" of essential fatty acids in epidermis.

Essential fatty acids in clinical dermatology

A deficiency of essential fatty acid intake can produce severe cutaneous abnormalities but is exceedingly rare in clinical practice. Recent research has shown that abnormalities in essential fatty acid metabolism may play a role in atopic eczema, acne, and psoriasis. Therapeutic innovations have already resulted from this knowledge, and more are likely to emerge.

Increased plasma triglyceride secretion in EFA-deficient rats fed diets with or without saturated fat

Metabolic responses to essential fatty acid-deficiency in rats include an increased rate of triglyceride secretion into the plasma, a large reduction in the HDL1 plasma lipoprotein concentration, and increased concentrations of liver triacylglycerols and cholesteryl esters. Because of differences in the types of EFA-deficient diets used, it is not clear whether these responses were solely due to the absence of EFA from the diet or whether saturated fat, or differences in acyl group chain length in this fat, might be responsible. Therefore, we fed rats diets differing only in amounts and kinds of fat, and measured triacylglycerol secretion rates and liver concentrations of triacylglycerols and cholesteryl esters, for comparison with our earlier measurements of plasma high density lipoprotein subpopulations in rats fed exactly the same diets. The purified diets contained either no fat, 5% by weight hydrogenated coconut oil, 5% hydrogenated cottonseed oil, or each of these three diets supplemented with 1% safflower oil, or 5% corn oil. We also fed some rats a nonpurified stock diet for comparison with literature reports. The present results indicate that the metabolic responses to essential fatty acid deficiency described above are definitely due to essential fatty acid-deficiency and not to the presence or chain length of acyl groups in saturated fat in the diet.

Effects of dietary trans-fatty acids on reproductive performance of Wistar rats

1. Wistar rats were fed for three successive generations on a semi-purified diet, in which the fat was provided by butter, sunflower oil, rapeseed oil or hydrogenated vegetable fat, differing in the content of cis,cis-18:2 and trans-18:1 fatty acids. Effects of these fats on the composition of adipose tissue and reproductive performance were studied. Fatty acids were analysed using high-performance liquid chromatography. 2. The fatty acid pattern of adipose tissue was closely related to dietary fat composition and, established in the first generation, did not change significantly in successive generations of rats. 3. Hydrogenated fat adversely affected litter size, sperm morphology and regularity of oestrous cycle, and prolonged the period of gestation in experimental animals. Differences observed between the generations were not significant. 4. Hydrogenated fat decreased the level of serum testosterone in males, but the differences observed in levels of serum progesterone in females were not apparently related to the dietary trans-fatty acids.

Free sphingosines in porcine epidermis

Sphingosines and phytosphingosines serve as intermediates in the synthesis of ceramides and glucosylceramides, which are prominent components of mammalian epidermis. In the present study, we have investigated the possibility that free sphingoid bases also may be present in epidermal tissue. Samples of pig epidermis were trypsinized to separate the stratum corneum from the unkeratinized portion of the epidermis. After drying, the lipids were extracted and analyzed by thin-layer chromatography using ninhydrin to detect free amino groups. Both the stratum corneum and the unkeratinized epidermal material contained a ninhydrin-positive material with the same mobility as the sphingosine standard. Quantitation of the chromatograms by photodensitometry indicated that free sphingosine bases account for 0.44% by weight of the total stratum corneum lipid and 0.09% of the lipid in the viable portion of the epidermis. To further identify this material, it was treated with 1-fluoro-2,4-dinitrobenzene, which resulted in the production of an intensely yellow N-2,4-dinitrophenyl derivative with the same mobility as N-2,4-dinitrophenylsphingosine on thin-layer chromatography. Oxidation of the isolated dinitrophenyl derivative with lead tetraacetate produced a mixture of aldehydes which were analyzed by gas-liquid chromatography. This analysis indicates that the free sphingoid bases from the stratum corneum consist of a mixture of mainly 16- through 20-carbon sphingenines and sphinganines, the most abundant components being d17:0, d17:1, d18:1 and d20:1. The production of these free sphingosine bases may be significant in regulating epidermal differentiation.

omega-Hydroxyacid derivatives in the epidermis of several mammalian species

1. omega-Hydroxyacids from the acylceramides and acylglucosylceramides of mammalian epidermis were examined by thin-layer and gas-liquid chromatography to determine their degree of unsaturation and chain-length distributions. The species examined included human (Homo sapiens), pig (Sus scrofa), mouse (Mus musculus) and rat (Rattus rattus). 2. Within a species, the omega-hydroxyacids from the acylceramide and acylglucosylceramide were essentially identical. 3. The human omega-hydroxyacids proved to be mainly saturated with C30:0 being the major entity. 4. The pig contained similar saturated, monoenoic and small amounts of dienoic omega-hydroxyacids, with C30:0, C32:1 being the major entities. 5. The mouse and rat contained C32:0 and C34:1 as the major components.

Mediators produced by the endothelial cell

This review discusses the role of three mediators, synthesized by vascular endothelial cells, that help to keep the surface of the normal endothelium nonthrombogenic. The first is prostacyclin, a product of arachidonic acid metabolism discovered in 1976. This labile prostanoid, with a half-life of approximately 3 minutes, relaxes vascular smooth muscle and inhibits the aggregation of blood platelets. Prostacyclin and its analogues are currently being tested clinically for use in cardiovascular diseases such as primary pulmonary hypertension. The second mediator discussed is endothelium-derived relaxing factor (EDRF), discovered in 1980, which also relaxes smooth muscle and inhibits the aggregation and adhesion of platelets. Substances that stimulate the release of EDRF include acetylcholine, bradykinin, and adenosine 5'-diphosphate. EDRF is even more labile than prostacyclin, with a half-life of about 6 seconds, and it has recently been identified as nitric oxide. Prostacyclin and EDRF are released together following stimulation of endothelial receptors and synergize to inhibit platelet aggregation. 13-Hydroxy-9,11-octadecadienoic acid, a third suggested mediator, is not released but acts from inside the cell to make the endothelial surface nonadhesive for circulating blood cells. It is proposed that these three mediators form the endothelial defense mechanism against blood-borne cells and chemicals and that breakdown of this barrier results in diseases such as hypertension and atherosclerosis.

The effect of solvent extraction on the lipids of the stratum corneum in relation to observed immediate whitening of the skin

Irritant contact dermatitis along with an increased transepidermal water loss can result from exposing the skin to solvents. A study of the interaction of various solvents with human stratum corneum was made using thin-layer chromatography. Comparison of 10 solvents (trichloroethylene, toluene, dimethylsulfoxide, n-hexane, 1,2-propanediol, carbon tetrachloride, methyl ethyl ketone, ethanol, acetone, distilled water) showed that 3 solvents (dimethylsulfoxide, 1,2 propanediol, distilled water) did not extract lipids from intact human stratum corneum. The findings are discussed in relation to the visual appearance of skin (whitening) after exposure to solvents.

Hydroxyacid derivatives in human epidermis

The principal objective of this investigation was to examine the omega-hydroxyacid derivatives in the cornified layer of human skin. Toward this end, sunburn peelings were collected, and the lipids were extracted with chloroform/methanol. Comparison with authentic standards by thin layer chromatography (TLC) indicated the presence of both N-(omega-acyloxy)acylsphingosine (acylceramide) and omega-acyloxy fatty acid (acyl acid), which accounted for 1.7% and 2.0% of the total lipid, respectively, as estimated by photodensitometry of the charred chromatograms. Each of these lipids was isolated by preparative TLC, hydrolyzed, and the resulting fragments were further analyzed by TLC and by gas liquid chromatography (GLC) of appropriate derivatives. In both of these lipids, the predominant omega-hydroxyacid proved to be the 30-carbon saturated species (56-59% of the total), while linoleate was the major ester-linked fatty acid. Linoleate was more abundant in the acyl acid (38.0%) than in acylceramide (21.4%). These findings represent the first demonstration of acyl acid in human skin and support the proposition that acyl acid, like the other linoleate-rich omega-hydroxyacid derivatives, functions as a molecular rivet in maintaining epidermal structure and function.

Effect of epidermal acylglucosylceramides and acylceramides on the morphology of liposomes prepared from stratum corneum lipids

Epidermal acylglucosylceramides (AGC) and acylceramides (AC) cause aggregation and stacking of stratum corneum lipid liposomes formed from a lipid mixture containing epidermal ceramides (40%), cholesterol (25%), palmitic acid (25%), and cholesteryl sulfate (10%). This demonstrates the ability of these sphingolipids to hold adjacent bilayers in close apposition and their roles in the assembly of lamellar structures in the epidermis. However, AGC and AC in their hydrogenated form also caused aggregation and stacking of the stratum corneum lipid liposomes. This throws into doubt the proposed structural specificity of linoleate in the function of AGC and AC as molecular rivets in the assembly of the epidermal lamellar granules and the stratum corneum intercellular lamellae, respectively.

The epidermal permeability barrier

The permeability barrier of the skin which prevents transcutaneous water loss and penetration of harmful drugs from the environment is localized in the horny layer of the epidermis. Multiple lipid bilayers obstructing the intercellular space of the stratum corneum fulfill this function. In contrast to cellular membranes consisting predominantly of phospholipids, these lamellae contain mostly ceramides, cholesterol and free fatty acids. The lamellae are derived from the contents of lamellar granules (LGs) which are synthesized in the viable epidermal layers by the keratinocytes. LGs display stacks of small disks each of which represents a flattened vesicle or liposome. Prior to terminal differentiation, the disks are exocytosed into the intercellular space and fused to form uninterrupted sheetlike lamellae. The singular lipid composition of LG-disks and of stratum corneum-lamellae reflects the multistage process of barrier formation. It also renders these structures well suited to provide for a barrier function.

Regulation of lipid synthesis in relation to keratinocyte differentiation capacity

Cultured keratinocytes and squamous carcinoma cells provide a useful model system for studying the processes involved in the regulation of differentiation, as the differentiation capacity of the cells can be modulated experimentally by changing the extracellular calcium concentration. Furthermore, the squamous carcinoma cell lines exhibit a defect in their differentiation capacity which they express to different extents. In this paper, the effect of external lipoproteins has been studied on lipid synthesis in normal keratinocytes and three squamous carcinoma cell (SCC) lines which showed a decreasing capacity to differentiate in the order of normal keratinocytes greater than SCC-12F2 greater than SCC-15 greater than SCC-4. The ability of the cells to form cornified envelopes was taken as a measure of differentiation capacity. The rate of total lipid synthesis as well as the phospholipid-neutral lipid ratio decreased in the order SCC-4 greater than SCC-15 greater than SCC-12F2 greater than or equal to normal keratinocytes, clearly correlating with the differentiation capacity of the cells. Because of the high rate of phospholipid synthesis and the low rate of ceramide synthesis, it is concluded that, under these in vitro conditions used, the maturation of keratinocytes proceeds to a lesser extent than that seen under in vivo conditions. In proliferating cells, in which the low-density lipoprotein (LDL) receptor is operative to a high extent, the rate of lipogenesis, especially that of neutral lipids, responded dramatically to changes of extracellular lipoprotein concentration. In the presence of lipoproteins a marked decrease of cholesterol and triacylglycerol synthesis and an increase of cholesterol ester synthesis has been observed. On the other hand, in differentiating cells lipogenesis appeared to be independent of extracellular lipoproteins, due to the absence of the LDL uptake mechanism, the only exception being the synthesis of triacylglycerols, the rate of which could be modulated to a certain extent by extracellular lipoproteins. The results presented here demonstrate a close inverse relationship between the regulation of lipogenesis by extracellular lipoproteins and the ability of the cells to differentiate.

Effects of essential fatty acid deficiency on epidermal O-acylsphingolipids and transepidermal water loss in young pigs

Linoleate-rich O-acylglucosylceramides and acylceramides are thought to be of major significance for the physical structure and function of the epidermal permeability barrier. In the present investigation, the effects of a linoleate-free diet on O-acylsphingolipids and their associated functions were investigated. Starting at 5 days of age, male pigs were fed diets containing 12% of either lard or hydrogenated coconut oil. Transepidermal water loss was measured with an electrolytic water analyzer at weekly intervals. Pigs were killed at intervals, and epidermal lipids were isolated and analyzed. Fatty acid compositions were determined by gas-liquid chromatography (GLC). Within 2-3 weeks, pigs on the diet containing coconut oil began to display biochemical and physiological symptoms of essential fatty acid deficiency. Within 2 months, this group had extremely scaly skin and transepidermal water loss was elevated to five times that of controls. The progressive increase in transepidermal water loss correlated with replacement of linoleate by oleate in both acylceramide and acylglucosylceramide. The formation of lamellar granules and intercellular lipid sheets in the stratum corneum was not impaired in essential fatty acid deficiency as judged by electron microscopy. These results suggest that the linoleic acid normally found in the O-acylsphingolipids is not essential for formation of the epidermal membrane system. Rather, it appears that the nature of the ester-linked fatty acid in the O-acylsphingolipids regulates the permeability of this membrane system.

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.

Apparent in vivo retroconversion of dietary arachidonic to linoleic acid in essential fatty acid-deficient rats

Essential fatty acid-deficient rats were fed ethyl [U-14C]arachidonate (308 dpm/nmol) and when a decrease in the transepidermal water loss was seen, the epidermal sphingolipids, acylglucosylceramide and acylceramide were isolated. [14C]Linoleic acid (approx. 130 dpm/nmol) was present in both lipid classes, while the substrate was only detected in the former. These results intimate that in vivo retroconversion of arachidonic to linoleic acid can be induced in the rat.

Lipid composition of cultured murine keratinocytes

The current study was undertaken to determine whether a previously reported murine culture system is an acceptable model for the study of epidermal lipid metabolism. The lipid composition of primary neonatal mouse keratinocyte cultures was determined and compared with that of freshly isolated keratinocytes and whole epidermis. 14C-Labeled arachidonic acid (AA) and linoleic acid (LA) were added to cultures and the incorporation into specific lipids was assessed. The lipid composition of the cultures indicated that they were partially differentiated, which parallels the well-known incomplete keratinization seen in many keratinocyte culture systems. Of particular importance, the LA-rich uniquely epidermal lipids which may be of importance in water barrier function, acylglucosylceramide (AGC) and acylceramide (AC), were made by the cultures. Fatty acid analysis of total lipid, phospholipid, and AGC extracts revealed a significant decrease in LA content compared with the parent epidermis; this may have resulted from the low level of LA in fetal bovine serum, which was the serum source for these cultures. Labeled AA and LA were incorporated into the lipids of cultured keratinocytes in distinct patterns that were consistent with the fatty acid content of the lipids. Both AGC and AC showed preferential uptake of LA compared with AA. There was minimal labeling of non-linoleate-containing lipids and a low degree of conversion of labeled LA to AA. Considering the grossly different environment of the in vitro system compared with the in vivo state, the overall lipid composition was remarkably well maintained. Keratinocyte cultures should be of great value in the study of epidermal lipid metabolism.

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.

Metabolic profile of linoleic acid in porcine leukocytes through the lipoxygenase pathway

Porcine neutrophilic leukocytes were found to contain a lipoxygenase which converted linoleic acid into 13-hydroxy-9,11-octadecadienoic acid (n-6 specificity), arachidonic acid into 12-hydroxy-5,8,10,14-eicosatetraenoic acid (n - 9 specificity) and 5-hydroxy-6,8,11,14-eicosatetraenoic acid into 5,12-dihydroxy-6,8,10,14-eicosatetraenoic acid. This lipoxygenase was partially purified and it appeared that its substrate specificity and other properties were quite different from the 12-lipoxygenase of blood platelets. Incubations of intact or broken porcine leukocytes with added linoleic acid revealed the formation of not only 13-hydroxy-9,11-octadecadienoic acid but also of substantial amounts of epoxyhydroxy and trihydroxy isomers. These products from linoleate, collectively described by the name 'octadecanoids' were characterized in detail by a combination of chemical, chromatographic and mass spectrometric techniques. The phospholipids of porcine leukocytes contain more than twice as much linoleate than arachidonate (22 vs. 8%). In accordance with this fatty acid composition we found that in the stimulated neutrophil the endogenous production of octadecanoids often surpassed that of the eicosanoids. Lipoxygenation of endogenously liberated linoleic acid was especially pronounced when a suspension of leukocytes in citrated plasma was recalcified and allowed to clot.

Identification, isolation and characterization of epidermal lipids containing linoleic acid

Three specific linoleate-rich lipids have been identified in pig epidermis and are referred to as O-acylglucosyl ceramide, O-acyl ceramide, and O-acyl acid. The acid moiety is up to 70% linoleic acid and linked via the hydroxyl group of a omega-OH long-chain fatty acid, which itself is linked to sphingosine or glucosyl sphingosine. The identification of O-acyl ceramide confirmed the findings of another group, whereas the structural configuration of the O-acylglucosyl ceramide is different to previous reports. The identification of an O-acyl acid in epidermis is novel. Our evidence allows us to speculate that a hydroxylated derivative of the O-acyl ceramide may be intimately involved in the permeability barrier of skin, perhaps providing sufficient polarity to maintain a lamellar phase in the intercellular space of the lower stratum corneum region, and that it is specifically in this form that linoleic acid is involved in skin barrier function. Alternatively, or concurrently, this species may act as a signal for late keratinisation events.