Aloxe3 knockout mice reveal a function of epidermal lipoxygenase-3 as hepoxilin synthase and its pivotal role in barrier formation.

Loss-of-function mutations in the lipoxygenase (LOX) genes ALOX12B and ALOXE3 are the second most common cause of autosomal recessive congenital ichthyosis. The encoded proteins, 12R-LOX and epidermal LOX-3 (eLOX-3), act in sequence to convert fatty acid substrates via R-hydroperoxides to specific epoxyalcohol derivatives and have been proposed to operate in the same metabolic pathway during epidermal barrier formation. Here, we show that eLOX-3 deficiency in mice results in early postnatal death, associated with similar but somewhat less severe barrier defects and morphological changes than reported earlier for the 12R-LOX-knockout mice. Skin lipid analysis demonstrated that the severity of barrier failure is related to the loss of covalently bound ceramides in both 12R-LOX- and eLOX-3-null mice, confirming a proposed functional linkage of the LOX pathway to ceramide processing and formation of the corneocyte lipid envelope. Furthermore, analysis of free oxygenated fatty acid metabolites revealed strongly reduced levels of hepoxilin metabolites in eLOX-3-deficient epidermis, indicating an additional function of eLOX-3 in mammalian skin as a hepoxilin synthase linked to the 12S-LOX pathway.

Development of an ichthyosiform phenotype in Alox12b-deficient mouse skin transplants.

12R-lipoxygenase (12R-LOX) represents a key enzyme of a recently identified eicosanoid pathway in the skin that plays an essential role in the establishment and/or maintenance of the epidermal barrier function. Genetic studies show that loss-of-function mutations in ALOX12B, encoding 12R-LOX, and in ALOXE3, encoding another closely related LOX involved in this pathway, are the second most common cause for autosomal recessive congenital ichthyosis (ARCI). To investigate the pathomechanism of ARCI and the function of 12R-LOX, we recently generated a 12R-LOX knockout model. 12R-LOX-deficient mice die rapidly after birth from severe barrier dysfunction without exhibiting an obvious cutaneous phenotype. Thus, we analyzed the adult phenotype of 12R-LOX(-/-) skin transplanted onto nude mice. 12R-LOX(-/-) skin develops an ichthyosiform appearance with thickening of the epidermis, hyperproliferation, hypergranulosis, focal parakeratosis, and severe hyperkeratosis. The adult mutant mouse skin phenotype closely reproduces the ichthyosis phenotype seen in patients with ALOX12B mutations. Western blot analysis revealed restoration of profilaggrin processing that used to be disturbed in neonatal mutant skin and overexpression of filaggrin, involucrin, and repetin. The results indicate that 12R-LOX knockout mice may represent a useful animal model for a detailed analysis of mechanisms involved in ARCI forms that are associated with impaired LOX metabolism.