The formation of a permeability barrier between the external environment and the host is essential for survival. To provide this barrier keratinocytes undergo a complex pathway of differentiation, which culminates in keratinocyte cornification and the formation of extracellular lipid enriched lamellar membranes in the stratum corneum. The mechanisms that coordinately regulate the parallel formation of the corneocytes and lamellar membranes are unknown. The extracellular lamellar membranes are derived from the exocytosis of lamellar bodies and to synthesize lamellar bodies the keratinocyte must have abundant quantities of cholesterol, fatty acids and ceramides. These lipids could serve as signaling molecules and thereby coordinately regulate the formation of the stratum corneum. Fatty acids activate PPARs and studies have shown that PPAR activation stimulates keratinocyte differentiation. Cholesterol is converted to oxysterols that activate LXR and studies have shown that LXR activation also stimulates keratinocyte differentation. Additionally, PPAR and LXR activation also facilitates the formation of the lipid enriched lamellar membranes. Ceramides, via a number of mechanisms also stimulate keratinocyte differentiation. Recently, studies have shown that ceramides by increasing PPAR delta also increase the expression of ABCA12, which would facilitate the formation of lamellar bodies. Finally, keratinocytes accumulate a large quantity of cholesterol sulfate, which plays a key role in regulating desquamation. Cholesterol sulfate has also been shown to stimulate keratinocyte differentiation. Thus, cholesterol, cholesterol sulfate, fatty acids and ceramides all stimulate keratinocyte differentiation and thereby could coordinately regulate the formation of the stratum corneum.
Keywords: LXR; PPAR; differentiation; epidermis; keratinocytes; permeability barrier.