Peroxisome proliferator-activated receptors and skin development

PPARs are nuclear hormone receptors. PPAR subtypes (alpha, gamma, delta, the latter a xPPARbeta homologue) were initially investigated in skin because of their known role in regulating lipid metabolism. Studies adding specific PPAR ligand activators to cultured skin or skin cells are compatible with the concepts that PPARalpha activation mediates early lipogenic steps common to the function of both skin epidermal cells (keratinocytes) and sebaceous cells (sebocytes), PPARgamma activation plays a unique role in stimulating sebocyte lipogenesis, and PPARdelta activation may contribute to lipid biosynthesis in both sebocytes and keratinocytes under certain circumstances. Epidermal keratinocytes appear to express small amounts of PPARalpha and PPARdelta mRNA and a trace of PPARgamma mRNA which is up-regulated with differentiation. Sebocytes express all subtypes; PPARgamma gene expression excedes that in epidermis. The emerging data on PPAR protein expression suggests that epidermis normally expresses predominantly PPARalpha, while sebocytes express more PPARgamma than PPARalpha. These expression patterns may change during hyperplasia, differentiation and inflammation. Gene disruption studies in mice are compatible with a contribution of PPARalpha to skin barrier function, suggest that PPARgamma is necessary for sebocyte differentiation, and indicate that PPARdelta can ameliorate inflammatory responses in skin. PPARs appear to play a role in keratinocyte synthesis of the lipids that they export to the intercellular space to form the skin permeability barrier. They also appear to be important for sebocyte formation of the intracellular fused lipid droplets that constitute the holocrine secretion of the sebaceous gland. In addition, they may play roles in keratinocyte growth and differentiation and the inhibition of skin inflammation by diverse mechanisms not necessarily related to fat metabolism.