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

Abstract

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 γ (PPARγ) 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 PPARγ agonist, this study was designed to assess whether the PPARγ system is a target for UVB irradiation and involved in UVB-induced inflammation in epidermal cells. The present studies demonstrated the presence of PPARγ mRNA and functional protein in human keratinocytes and epithelial cell lines HaCaT, KB, and A431. The treatment of epidermal cells with the PPARγ-specific agonist ciglitazone or azPC augmented cyclooxygenase-2 expression and enzyme activity induced by phorbol 12-myristate-13-acetate or interleukin-1β. Lipid extracts from the cell homogenate of UVB-irradiated, but not control, cells contained a PPARγ-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 PPARγ-agonistic activity, among which the specific PPARγ agonist azPC was identified by mass spectrometry. These findings suggested that UVB-generated PPARγ-agonistic activity was due to the free radical mediated non-enzymatic cleavage of endogenous glycerophosphocholines. Treatment with the specific PPARγ antagonist GW9662 or expression of a dominant-negative PPARγ mutant in KB cells inhibited UVB-induced epidermal cell prostaglandin E 2 production. These findings suggested that UVB-generated PPARγ activity is necessary for the optimal production of epidermal prostaglandins. These studies demonstrated that epithelial cells contain a functional PPARγ system, and this system is a target for UVB through the production of novel oxidatively modified endogenous phospholipids.