Localization of epidermal sphingolipid synthesis and serine palmitoyl transferase activity: alterations imposed by permeability barrier requirements.

Sphingolipids, the predominant lipid species in mammalian stratum corneum play, a central role in permeability barrier homeostatis. Prior studies have shown that the epidermis synthesizes abundant sphingolipids, a process regulated by barrier requirements, and that inhibition of sphingolipid synthesis interferes with barrier homeostasis. To investigate further the relationship between epidermal sphingolipid metabolism and barrier function, we localized sphingolipid synthetic activity in murine epidermis under basal conditions, and following acute (acetone treatment) or chronic (essential fatty acid deficiency, EFAD) barrier perturbation, using dithiothreitol and/or the staphylococcal epidermolytic toxin to isolate the lower from the outer epidermis. Under basal conditions, both the activity of serine palmitoyl transferase (SPT), the rate-limiting enzyme of sphingolipid synthesis, and the rates of 3H-H2O incorporation into sphingolipids were nearly equivalent in the lower and the outer epidermis. Following acute barrier perturbation, SPT activity increased significantly in both the lower (35%; P < 0.05) and the outer epidermal layers (60%; P < 0.01). The rates of 3H-H2O incorporation into each major sphingolipid family, including ceramides, glucosylceramides and sphingomyelin, increased significantly in both the lower and the outer epidermis of treated flanks after acute barrier disruption. Finally, SPT activity was modestly elevated (20%; P < 0.01) in the lower but not in the outer epidermis of EFAD animals. These studies demonstrate the ability of both lower and outer epidermal cells to generate sphingolipids, and that permeability barrier homeostatic mechanisms appear to differentially regulate SPT activity and sphingolipid synthesis in the lower and the outer epidermis in response to acute and chronic barrier perturbation.(ABSTRACT TRUNCATED AT 250 WORDS)