PPARγ is an important transcription factor in 1α, 25-dihydroxyvitamin D3-induced involucrin expression

Background: 1 alpha,25-Dihydroxyvitamin D3 (1 alpha,25(OH)(2)O-3), the active form of vitamin D, suppresses keratinocyte proliferation, promotes keratinocyte differentiation, and induces involucrin expression. Peroxisome proliferation-activated receptors (PPARs) are ligand -activated transcription factors. It has been reported that PPARs stimulate keratinocyte differentiation and regulate the expression of differentiation molecules. Objective: Keratinocytes treated with 1 alpha,25(OH)(2)D-3 induced PPAR-gamma, which was followed by increased involucrin expression. In this study, we investigated whether PPAR-gamma is involved in the 1 alpha,25(OH)(2)D-3-induced involucrin expression in human keratinocytes. Methods: Subconfluent keratinocytes were treated with 10(-7) M 1 alpha,25(OH)(2)D-3 for the indicated times, and PPAR and involucrin mRNA expression were determined by real-time RT-PCR. The levels of PPARs, involucrin, p38, and phospho-p38 proteins were assayed by Western blotting, and the DNA binding activities of PPAR gamma and AP-1 were investigated by etectrophoretic mobility shift assays (EMSA). To examine the rote of PPAR-gamma in 1 alpha,25(OH)(2)D-3 responses, recombinant adenovirus carrying a dominant-negative form of PPAR gamma (Axdn-PPAR gamma) was constructed and transfected into kerati-nocytes. The p38 inhibitor SB203580 was added to the cultures to evaluate the involvement of p38 in involucrin expression. Results: 1 alpha,25(OH)(2)D-3 induced PPAR gamma expression and stimulated PPAR gamma activity. The introduction of dn-PPAR gamma inhibited the expression of involucrin mRNA and protein induced by 1 alpha,25(OH)(2)D-3, and suppressed AP-1 DNA binding activity. 1 alpha,25(OH)(2)D-3 also triggered the phosphorylation of p38, which contributes to involucrin induction. Moreover, dn-PPAR gamma prevented the 1 alpha,25(OH)(2)D-3-induced phosphorylation of p38. Conclusions: These results suggest that PPAR gamma regulates involucrin expression by controlling the AP-1 signal and p38 activation in 1 alpha,25(OH)(2)D-3-induced keratinocyte differentiation. (C) 2007 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved.