Metabolism of polyunsaturated fatty acids by skin epidermal enzymes: generation of antiinflammatory and antiproliferative metabolites

In the skin epidermis, the metabolism of polyunsaturated fatty acids (PUFAs ) is highly active. Dietary deficiency of linoleic acid (LA), the major 18- carbon n-6 PUFA in normal epidermis, results in a characteristic scaly skin disorder and excessive epidermal water loss. Because of the inability of n ormal skin epidermis to desaturate LA to gamma-linolenic acid, it is transf ormed by epidermal 15-lipoxygenase to mainly 13-hydroxyoctadecadienoic acid , which functionally exerts antiproliferative properties in the tissue. In contrast, compared with LA, arachidonic acid (AA) is a relatively minor 20- carbon n-6 PUFA in the skin and is metabolized via the cyclooxygenase pathw ay, predominantly to the prostaglandins E-2, F-2 alpha, and D-2. AA is also metabolized via the 15-lipoxygenase pathway, predominantly to 15-hydroxyei cosate traenoic acid. At low concentrations, the prostaglandins function to modulate normal skin physiologic processes, whereas at high concentrations they induce inflammatory processes. PUFAs derived from other dietary oils are also transformed mainly into monohydroxy fatty acids. For instance, epi dermal 15-lipoxygenase transforms dihomo-gamma-linolenic acid (20:3n-6) to 15-hydroxyeicosatrienoic acid, eicosapentaenoic acid (20:5n-3) to 15-hydrox yeicosapentaenoic acid, and docosahexaenoic acid (22:6n-3) to 17-hydroxydoc osahexaenoic acid, respectively. These monohydroxy acids exhibit antiinflam matory properties in vitro. Thus, supplementation of diets with appropriate purified vegetable oils, fish oil, or both may generate local cutaneous an tiinflammatory and antiproliferative metabolites which could serve as less toxic in vivo monotherapies or as adjuncts to standard therapeutic regimens for the management of inflammatory skin disorders.