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.

Peroxiredoxin 6 is required for blood vessel integrity in wounded skin.

Peroxiredoxin 6 (Prdx6) is a cytoprotective enzyme with largely unknown in vivo functions. Here, we use Prdx6 knockout mice to determine its role in UV protection and wound healing. UV-mediated keratinocyte apoptosis is enhanced in Prdx6-deficient mice. Upon skin injury, we observe a severe hemorrhage in the granulation tissue of knockout animals, which correlates with the extent of oxidative stress. At the ultrastructural level endothelial cells appear highly damaged, and their rate of apoptosis is enhanced. Knock-down of Prdx6 in cultured endothelial cells also increases their susceptibility to oxidative stress, thus confirming the sensitivity of this cell type to loss of Prdx6. Wound healing studies in bone marrow chimeric mice demonstrate that Prdx6-deficient inflammatory and endothelial cells contribute to the hemorrhage phenotype. These results provide insight into the cross-talk between hematopoietic and resident cells at the wound site and the role of reactive oxygen species in this interplay.

12R-lipoxygenase deficiency disrupts epidermal barrier function.

12R-lipoxygenase (12R-LOX) and the epidermal LOX-3 (eLOX-3) constitute a novel LOX pathway involved in terminal differentiation in skin. This view is supported by recent studies showing that inactivating mutations in 12R-LOX and eLOX-3 are linked to the development of autosomal recessive congenital ichthyosis. We show that 12R-LOX deficiency in mice results in a severe impairment of skin barrier function. Loss of barrier function occurs without alterations in proliferation and stratified organization of the keratinocytes, but is associated with ultrastructural anomalies in the upper granular layer, suggesting perturbance of the assembly/extrusion of lamellar bodies. Cornified envelopes from skin of 12R-LOX-deficient mice show increased fragility. Lipid analysis demonstrates a disordered composition of ceramides, in particular a decrease of ester-bound ceramide species. Moreover, processing of profilaggrin to monomeric filaggrin is impaired. This study indicates that the 12R-LOX-eLOX-3 pathway plays a key role in the process of epidermal barrier acquisition by affecting lipid metabolism, as well as protein processing.

Role of epidermis-type lipoxygenases for skin barrier function and adipocyte differentiation.

12R-lipoxygenase (12R-LOX) and epidermis-type LOX-3 (eLOX-3) are novel members of the multigene family of mammalian LOX. A considerable gap exists between the identification of these enzymes and their biologic function. Here, we present evidence that 12R-LOX and eLOX-3, acting in sequence, and eLOX-3 in combination with another, not yet identified LOX are critically involved in terminal differentiation of keratinocytes and adipocytes, respectively. Mutational inactivation of 12R-LOX and/or eLOX-3 has been found to be associated with development of an inherited ichthyosiform skin disorder in humans and genetic ablation of 12R-LOX causes a severe impairment of the epidermal lipid barrier in mice leading to post-natal death of the animals. In preadipocytes, a LOX-dependent PPARgamma activating ligand is released into the cell supernatant early upon induction of differentiation and available evidence indicates that this ligand is an eLOX-3-derived product. In accordance with this data is the observation that forced expression of eLOX-3 enhances adipocyte differentiation.