Essential fatty acids/eicosanoid biosynthesis in the skin: biological significance

Omega-3 polyunsaturated fatty acids: photoprotective macronutrients

Ultraviolet radiation (UVR) in sunlight has deleterious effects on skin, while behavioural changes have resulted in people gaining more sun exposure. The clinical impact includes a year-on-year increase in skin cancer incidence, and topical sunscreens alone provide an inadequate measure to combat overexposure to UVR. Novel methods of photoprotection are being targeted as additional measures, with growing interest in the potential for systemic photoprotection through naturally sourced nutrients. Omega-3 polyunsaturated fatty acids (n-3 PUFA) are promising candidates, showing potential to protect the skin from UVR injury through a range of mechanisms. In this review, we discuss the biological actions of n-3 PUFA in the context of skin protection from acute and chronic UVR overexposure and describe how emerging new technologies such as nutrigenomics and lipidomics assist our understanding of the contribution of such nutrients to skin health.

Essential fatty acid metabolism and its modification in atopic eczema

Research from the 1930s to the 1950s established that a deficit of n-6 essential fatty acids (EFAs) leads to an inflammatory skin condition in both animals and humans. In a common inherited skin condition, atopic dermatitis (eczema), there was evidence of low blood EFA concentrations and of a therapeutic response to exceptionally high doses of linoleic acid. More recently, it has been established that there is no deficit of linoleic acid in atopic eczema. Concentrations of linoleic acid instead tend to be elevated in blood, milk, and adipose tissue of patients with atopic eczema, whereas concentrations of linoleic acid metabolites are substantially reduced. This suggests reduced conversion of linoleic acid to gamma-linolenic acid (GLA). In most but not all studies, administration of GLA has been found to improve the clinically assessed skin condition, the objectively assessed skin roughness, and the elevated blood catecholamine concentrations of patients with atopic eczema. Atopic eczema may be a minor inherited abnormality of EFA metabolism.

Basal transepidermal water loss is increased in platelet-type 12-lipoxygenase deficient mice

The roles of fatty acids in the skin have been under investigation since early reports of the phenotypic abnormalities of mice fed a diet deficient in essential fatty acids. Little is known about the functional significance of fatty acid metabolism by lipoxygenases in epidermis. Here, we have examined the role of platelet-type 12-lipoxygenase which converts arachidonic acid to the oxygenated metabolite 12-hydroperoxyeicosatetraenoic acid, in the skin using platelet-type 12-lipoxygenase-deficient mice generated by gene targeting. Platelet-type 12-lipoxygenase in wild-type mice was localized to the stratum granulosum by immunohistochemical analysis. Platelet-type 12-lipoxygenase-deficient mice lacked immunodetectable platelet-type 12-lipoxygenase in platelets and epidermis, appeared grossly normal, and exhibited an increase in basal transepidermal water loss without alteration in basal mitotic activity. Water loss and mitotic activity in mice with an acetone-disrupted membrane barrier were normal. No defect in ultrastructural properties or content of major fatty acids in dorsal skin or ear inflammation response was apparent in platelet-type 12-lipoxygenase-deficient mice. These results indicate that the platelet-type 12-lipoxygenase pathway in mice is partly responsible for normal permeability barrier function but the mechanism awaits further elucidation.

Double-blind, multicentre analysis of the efficacy of borage oil in patients with atopic eczema

Although gamma-linolenic acid (GLA) has been shown to correct deficiencies in skin lipids associated with reduced delta-6-desaturase activity which should result in improvement of dysregulation of inflammation and immunity in atopic eczema, clinical studies with evening primrose oil containing 10% GLA have yielded contradictory results. We have therefore examined the effect of a higher percentage (at least 23%) GLA-containing borage oil in adults with stable atopic eczema of moderate severity in a double-blind, multicentre study. One hundred and sixty patients were randomized to take daily either 500 mg of borage oil-containing capsules or the bland lipid miglyol as a placebo over a 24-week period. Use of topical diflucortolone-21-valerate cream was allowed as rescue medication, with the amount used until response being defined as primary, and clinical improvement as secondary efficacy criteria. Although several clinical symptoms improved compared with placebo, the overall response to borage oil did not reach statistical significance. Significant differences in favour of borage oil were, however, observed in a subgroup excluding patients who failed to show increased erythrocyte dihomo-gamma-linolenic acid levels and in whom adherence to inclusion criteria and the study protocol were questionable. GLA metabolites increased in borage oil-treated patients only, and serum IgE showed a trend to decrease on overall and subgroup analysis. No substance-related adverse effects were observed. This study shows no overall efficacy of GLA-containing borage oil in atopic eczema, with steroid use being the primary response parameter, although it suggests that a subgroup of patients may benefit from this well-tolerated treatment.

Effects of naturally-occurring flavonoids and biflavonoids on epidermal cyclooxygenase and lipoxygenase from guinea-pigs

Although there have been numerous topical applications of plant extracts having flavonoids known as anti-inflammatory compounds, only a few studies were reported concerning effects of flavonoids on epidermal cyclooxygenase/lipoxygenase. In this investigation, effects of naturally occurring flavonoids on epidermal cyclooxygenase/lipoxygenase were studied using five selected derivatives: flavanone, apigenin (flavone), quercetin (flavonol), amentoflavone and ginkgetin (biflavone) because eicosanoids generated in the epidermis are believed to be involved in various biological activities of the skin. Microsomal and cytosolic fractions were obtained from guinea-pig epidermal homogenate by centrifugation and used as a source for cyclooxygenase and lipoxygenase. It was found that quercetin inhibited both cyclooxygenase and lipoxygenase, being more potent against lipoxygenase, while flavanone and apigenin did not show any inhibition. Amentoflavone, one of the biflavones tested, showed potent and selective inhibitory activity on cyclooxygenase (IC50 = 3 microM) which was comparable to indomethacin (IC50 = 1 microM). In contrast, structurally similar ginkgetin possessed weak inhibitory activity on cyclooxygenase. The in vivo effects of these flavonoids on the normal and diseased skin remain to be studied.

Dermal inflammation in primates, mice, and guinea pigs: attenuation by second-generation leukotriene B4 receptor antagonist, SC-53228

Granulocyte infiltration is a prominent feature of human psoriasis. Psoriatic lesional skin contains abnormally high amounts of immunoreactive leukotriene B4 (LTB4), a potent granulocyte chemotaxin in vivo and in vitro. SC-53228 [(+)-(S)-7-(3-}2-(cyclopropylmethyl)-3-methoxy-4- [(methylamino)carbonyl]phenoxy}propoxy}-3,4-dihydro-8-propyl-2H-1- benzopyran-2-propanoic acid], a second-generation LTB4 receptor antagonist, was tested topically and orally in phorbol ester-induced dermal inflammation in three species. Skin inflammation was induced by topical application of phorbol-12-myristate-13-acetate-(PMA/TPA) and assessed by ear thickness, levels of the neutrophil marker enzyme myeloperoxidase (MPO) and histological examination. In mice, SC-53228 inhibited inflammation with a topical ED50 value of 200 +/- 18 micrograms. When applied to guinea pigs, SC-53228 (100 micrograms) inhibited the MPO increase by 86%, while 1000 micrograms abrogated inflammation in rhesus macaques with no plasma accumulation of the drug. A 1% gel formulation was also efficacious in guinea pig PMA-induced epidermal inflammation. Furthermore, single oral dose administration to mice was efficacious (ED50 < 2.5 mg/kg) as was multidose administration to rhesus macaques. PMA-induced skin inflammation possesses some of the attributes of human psoriasis and an agent such as SC-53228 may have utility in the medical management of this condition.

Antiinflammatory effects of second-generation leukotriene B4 receptor antagonist, SC-53228: impact upon leukotriene B4- and 12(R)-HETE-mediated events

Leukotriene B4 (LTB4) and 12(R)-hydroxyeicosatetraenoic acid [12(R)-HETE] are proinflammatory products of arachidonic acid metabolism that have been implicated as mediators in a number of inflammatory diseases. When injected intradermally into the guinea pig. LTB4 and 12(R)-HETE elicit a dose-dependent migration (chemotaxis) of neutrophils (PMNs) into the injection sites as assessed by the presence of a neutrophil marker enzyme myeloperoxidase. SC-41930 (7-[3-(4-acetyl-3-methoxy-2-propylphenoxy)propoxyl]-3,4-dihy dro-8-propyl-2H - 1-benzopyran-2-carboxylic acid), a first-generation LTB4 receptor antagonist, inhibited the chemotactic actions of LTB4 when given orally with an ED50 value of 1.7 mg/kg. The second-generation LTB4 receptor antagonist, SC-53228 [(+)-(S)-7-(3-(2-(cyclopropylmethyl)-3-methoxy-4- [(methylamino)carbonyl]phenoxy)propoxy)-3,4-dihydro-8-propyl-2H-1- benzopyran-2-propanoic acid], inhibited LTB4-induced chemotaxis when given intragastrically with an ED50 value of 0.07 mg/kg. Furthermore, SC-53228 inhibited 12(R)-HETE-induced granulocyte chemotaxis with an oral ED50 value of 5.8 mg/kg. When dosed orally over a range of 0.03-100 mg/kg, SC-53228 gave Cmax plasma concentrations of 0.015-41.1 micrograms/ml. SC-53228 inhibited LTB4-primed membrane depolarization of human neutrophils with an IC50 value of 34 nM. As a potent LTB4 receptor antagonist, SC-53228 may well have application in the medical management of disease states such as asthma, rheumatoid arthritis, inflammatory bowel disease, contact dermatitis, and psoriasis, in which LTB4 and/or 12(R)-HETE are implicated as inflammatory mediators.