Peroxisome proliferator-activated receptor-β/δ inhibits epidermal cell proliferation by down-regulation of kinase activity

Recent work has shown that peroxisome proliferator-activated receptor β (PPARβ) attenuates cell proliferation and skin carcinogenesis, and this is due in part to regulation of ubiquitin C expression. In these studies, the role of PPARβ in modulating ubiquitin-dependent protein kinase Cα (PKCα) levels and phosphorylation signaling pathways was evaluated. Intracellular phosphorylation analysis showed that phosphorylated PKCα and other kinases were lower in wild-type mouse skin treated with 12-O-tetradecanoylphorbol-13-acetate (TPA) as compared with PPARβ-null mouse skin. No differences in expression levels of other PKC isoforms present in skin were observed. Lower ubiquitination of PKCα was found in TPA-treated PPARβ-null skin as compared with wild-type, and inhibition of ubiquitin-dependent proteasome degradation prevented TPA-induced down-regulation of PKCα. The activity of PKCα and downstream signaling kinases is enhanced, and expression of cyclooxygenase-2 (COX-2) is significantly greater, in PPARβ-null mouse skin in response to TPA compared with wild-type mouse skin. Inhibition of PKCα or COX-2 reduced cell proliferation in TPA-treated PPARβ-null keratinocytes in a dose-dependent manner, whereas it only slightly influenced cell proliferation in wild-type keratinocytes. Combined, these studies provide strong evidence that PPARβ attenuates cell proliferation by modulating PKCα/Raf1/MEK/ERK activity that may be due in part to reduced ubiquitin-dependent turnover of PKCα.