Mice lacking epidermal PPARγ exhibit a marked augmentation in photocarcinogenesis associated with increased UVB-induced apoptosis, inflammation and barrier dysfunction.

Recent studies suggest that peroxisome proliferator-activated receptor gamma (PPARγ) agonists may have cancer chemopreventive activity. Other studies have shown that loss of epidermal PPARγ results in enhanced chemical carcinogenesis in mice via unknown mechanisms. However, ultraviolet B (UVB) exposure represents the primary etiological agent for skin cancer formation and the role of PPARγ in photobiology and photocarcinogenesis is unknown. In previous studies, we demonstrated that UVB irradiation of cells results in the formation of oxidized glycerophosphocholines that exhibit PPARγ ligand activity. We therefore hypothesized that PPARγ would prove to be a chemopreventive target in photocarcinogenesis. We first showed that UVB irradiation of mouse skin causes generation of PPARγ agonist species in vivo. We then generated SKH-1 hairless, albino mice deficient in epidermal Pparg (Pparg-/-(epi)) using a cytokeratin 14 driven Cre-LoxP strategy. Using a chronic model of UVB photocarcinogenesis, we next showed that Pparg-/-(epi) mice exhibit an earlier onset of tumor formation, increased tumor burden and tumor progression. Increased tumor burden in Pparg-/-(epi) mice was accompanied by a significant increase in epidermal hyperplasia and p53 positive epidermal cells in surrounding skin lacking tumors. After acute UVB irradiation, Pparg-/-(epi) mice exhibited an augmentation of both UVB-induced Caspase 3/7 activity and inflammation. Increased apoptosis and inflammation was also observed after treatment with the PPARγ antagonist GW9662. With chronic UVB irradiation, Pparg-/-(epi) mice exhibited a sustained increase in erythema and transepidermal water loss relative to wildtype littermates. This suggests that PPARγ agonists could have possible chemopreventive activity in non-melanoma skin cancer.

The peroxisome proliferator-activated receptor gamma system regulates ultraviolet B-induced prostaglandin e(2) production in human epidermal keratinocytes.

Studies using PPARgamma agonists in mouse skin have suggested that peroxisome proliferator-activated receptor gamma (PPARgamma) is irrelevant to cutaneous photobiology. However, in several epithelial cell lines, ultraviolet B (UVB) has been shown to induce the nonenzymatic production of oxidized phospholipids that act as PPARgamma agonists. UVB is also a potent inducer of prostaglandin E(2) (PGE(2)) production and COX-2 expression in keratinocytes and PPARgamma is coupled to increased PGE(2) production in other cell lines. In this current study, we demonstrate that PPARgamma agonists, but not PPARalpha or PPARbeta/delta agonists, induce PGE(2) production and COX-2 expression in primary human keratinocytes (PHKs). Importantly, PPARgamma agonist-induced COX-2 expression and PGE(2) production were partially inhibited by the PPARgamma antagonist, GW9662, indicating that both PPARgamma-dependent and -independent pathways are likely involved. GW9662 also suppressed UVB and tert-butylhydroperoxide- (TBH-) induced PGE(2) production in PHKs and intact human epidermis and partially inhibited UVB-induced COX-2 expression in PHKs. These findings provide evidence that PPARgamma is relevant to cutaneous photobiology in human epidermis.