Epidermal peroxisome proliferator-activated receptor γ as a target for ultraviolet B radiation

Ultraviolet B radiation ( UVB) is a pro-oxidative stressor with profound effects on skin in part through its ability to stimulate cytokine production. Peroxisome proliferator-activated receptor gamma (PPARgamma) has been shown to regulate inflammatory processes and cytokine release in various cell types. Since the oxidized glycerophospholipid 1-hexadecyl-2-azelaoyl glycerophosphocholine (azPC) has been shown to be a potent PPARgamma agonist, this study was designed to assess whether the PPARgamma system is a target for UVB irradiation and involved in UVB-induced inflammation in epidermal cells. The present studies demonstrated the presence of PPARgamma mRNA and functional protein in human keratinocytes and epithelial cell lines HaCaT, KB, and A431. The treatment of epidermal cells with the PPARgamma-specific agonist ciglitazone or azPC augmented cyclooxygenase-2 expression and enzyme activity induced by phorbol 12-myristate-13-acetate or interleukin-1beta. Lipid extracts from the cell homogenate of UVB-irradiated, but not control, cells contained a PPARgamma-agonistic activity identified by reporter assay, and this activity up-regulated cyclooxygenase-2 expression induced by phorbol 12-myristate-13-acetate. Subjecting purified 1-hexadecyl-2-arachidonoyl-glycerophosphocholine to UVB irradiation generated a PPARgamma-agonistic activity, among which the specific PPARgamma agonist azPC was identified by mass spectrometry. These findings suggested that UVB-generated PPARgamma-agonistic activity was due to the free radical mediated nonenzymatic cleavage of endogenous glycerophosphocholines. Treatment with the specific PPARgamma antagonist GW9662 or expression of a dominant-negative PPARgamma mutant in KB cells inhibited UVB-induced epidermal cell prostaglandin E-2 production. These findings suggested that UVB-generated PPARgamma activity is necessary for the optimal production of epidermal prostaglandins. These studies demonstrated that epithelial cells contain a functional PPARgamma system, and this system is a target for UVB through the production of novel oxidatively modified endogenous phospholipids.