Functions of the Peroxisome Proliferator-Activated Receptor (PPAR) α and β in Skin Homeostasis, Epithelial Repair, and Morphogenesis

The three peroxisome proliferator-activated receptors (PPARα, PPARβ, and PPARγ) are ligand-activated transcription factors belonging to the nuclear hormone receptor superfamily. They are regarded as being sensors of physiological levels of fatty acids and fatty acid derivatives. In the adult mouse skin, they are found in hair follicle keratinocytes but not in interfollicular epidermis keratinocytes. Skin injury stimulates the expression of PPARα and PPARβ at the site of the wound. Here, we review the spatiotemporal program that triggers PPARβ expression immediately after an injury, and then gradually represses it during epithelial repair. The opposing effects of the tumor necrosis factor-α and transforming growth factor-β-1 signalling pathways on the activity of the PPARβ promoter are the key elements of this regulation. We then compare the involvement of PPARβ in the skin in response to an injury and during hair morphogenesis, and underscore the similarity of its action on cell survival in both situations.