Lipoxygenases Mediate the Effect of Essential Fatty Acid in Skin Barrier Formation A PROPOSED ROLE IN RELEASING OMEGA-HYDROXYCERAMIDE FOR CONSTRUCTION OF THE CORNEOCYTE LIPID ENVELOPE

A barrier to water loss is vital to maintaining life on dry land. Formation of the mammalian skin barrier requires both the essential fatty acid linoleate and the two lipoxygenases 12R-lipoxygenase (12R-LOX) and epidermal lipoxygenase-3 (eLOX3), although their roles are poorly understood. Linoleate occurs in O-linoleoyl-omega-hydroxyceramide, which, after hydrolysis of the linoleate moiety, is covalently attached to protein via the free omega-hydroxyl of the ceramide, forming the corneocyte lipid envelope, a scaffold between lipid and protein that helps seal the barrier. Here we show using HPLC-UV, LC-MS, GC-MS, and H-1 NMR that O-linoleoyl-omega-hydroxyceramide is oxygenated in a regio- and stereospecific fashion by the consecutive actions of 12R-LOX and eLOX3 and that these products occur naturally in pig and mouse epidermis. 12R-LOX forms 9R-hydroperoxy-linoleoyl-omega-hydroxyceramide, further converted by eLOX3 to specific epoxyalcohol (9R, 10R-trans-epoxy-11E-13R-hydroxy) and 9-keto-10E, 12Z esters of the ceramide; an epoxy-ketone derivative (9R, 10R-trans-epoxy-11E-13-keto) is the most prominent oxidized ceramide in mouse skin. These products are absent in 12R-LOX-deficient mice, which crucially display a near total absence of protein-bound omega-hydroxyceramides and of the corneocyte lipid envelope and die shortly after birth from transepidermal water loss. We conclude that oxygenation of O-linoleoyl-omega-hydroxyceramide is required to facilitate the ester hydrolysis and allow bonding of the omega-hydroxyceramide to protein, providing a coherent explanation for the roles of multiple components in epidermal barrier function. Our study uncovers a hitherto unknown biochemical pathway in which the enzymic oxygenation of ceramides is involved in building a crucial structure of the epidermal barrier.