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  • The skin permeability barrier is

    2024-04-17

    The skin permeability barrier is important for the prevention of loss of water and electrolytes from inside the body and for protection against the invasion of external pathogens, allergens, and harmful compounds. Multi-layered lipids, called lipid lamellae, are present in the outermost cell layer of the epidermis (stratum corneum), where they have a pivotal function in skin permeability barrier formation [22]. The major constituents of lipid lamellae are ceramides (about 50%), cholesterol (about 25%), and free fatty acids (about 15%) [23], [24]. Ceramides form the backbones of sphingolipids, and a variety of ceramide species are present in the epidermis [22], [24] (Fig. S1). Acylceramide is the epidermis-specific ceramide and is especially important for skin permeability barrier formation [22]. Normal ceramide consists of a long-chain base and an amide-linked fatty acid, whereas acylceramide contains an extra linoleic Beauvericin attached to the ω-carbon of the amide-linked fatty acid [22], [24] (Fig. S1). Mutations in genes involved in acylceramide synthesis cause autosomal recessive congenital ichthyosis, which is characterized by severe skin permeability barrier defects, scaly skin, and hyperkeratosis [24], [25]. Considering the importance of lipids in skin permeability barrier formation and the regulatory roles of lipids on TRPV3 activity, it is likely that changes in lipid metabolism contribute to OS pathology. However, lipidomics analyses of skin tissues of OS patients or OS model animals have not been performed so far, although transcriptome and proteome analyses have already been reported [7]. Here, we performed comprehensive comparative lipidomics analyses of skin samples from non-lesional areas of wild type (WT) and Ht rats to reveal the lipid metabolism underlying the OS pathology.
    Methods
    Results
    Discussion To understand the molecular mechanism underlying OS syndrome, we performed comprehensive lipidomics analyses and detected approximately 600 lipid species using unaffected skin from OS model (Ht) and WT rats. We found large decreases in the levels of 15-LOX metabolites and dihydroceramides in the unaffected skin area of Ht rats (Fig. 2). Although dihydroceramides exist only at low levels in most mammalian tissues, they are exceptionally abundant in hair [29]. Among dihydroceramide species, we observed the largest decrease in d18:0/20:0 dihydroceramide in Ht rats (Fig. 2G). The ceramide synthase CERS4 is mainly involved in the production of d18:0/20:0 dihydroceramide [22], and Cers4 knockout mice exhibit the hypotrichosis phenotype [40], suggesting the importance of d18:0/20:0 dihydroceramide in hair production and/or maintenance. Real time RT-PCR analyses revealed that the expression of Degs1, which is involved in conversion of dihydroceramide to ceramide, increased slightly in Ht rats, whereas expression of Cers4 was nearly unchanged (Fig. 5). The slight increase in Degs1 expression may not be enough to explain the large decreases in dihydroceramide levels that we found in Ht rats. Rather, it is likely that the large decreases in dihydroceramides in the unaffected skin area of Ht rats were instead due to hypotrichosis (Fig. 1A). It has been reported that activation of TRPV3 in outer root sheath keratinocytes of hair follicles induces elevated intracellular Ca2+ concentrations, which in turn inhibited proliferation and induced apoptosis of the keratinocytes [41]. 15-LOXs catalyze the production of fatty acid hydroperoxides, which are rapidly reduced to hydroxides, including 15-HETrE derived from dihomo-γ-linolenic acid, 15-HETE from arachidonic acid, 15-HEPE from eicosapentaenoic acid, and 17-HDoHE from docosahexaenoic acid (Fig. S3). We found that all these hydroxides were almost undetectable in the unaffected skin area of Ht rats (Fig. 2F). The decreases in the 15-LOX metabolites were consistent with the decreased expression of Alox15 and Alox15b, the genes involved Beauvericin in their synthesis (Fig. 5). Human ALOX15B is expressed in prostate, lung, skin, and cornea [42]. In skin, ALOX15B expression was detected in sebaceous, apocrine, and eccrine glands and in the basal layer of the epidermis [43]. In prostate, ALOX15B was found to be involved in differentiation, senescence, and growth of prostate epithelial cells [44].