PPARD and psoriasis: In addition, PPARα production was decreased in dermatitis lesions, whereas PPARβ/δ production, which is upregulated by central mediators of the inflammatory response, such as tumor necrosis factor alpha and interferon gamma, is increased in the epidermis in atopic dermatitis and psoriasis lesions compared with in normal areas of the skin (Westergaard et al., 2003; Romanowska et al., 2008).