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

SummaryBackground1α,25-Dihydroxyvitamin D3 (1α,25(OH)2D3), 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.ObjectiveKeratinocytes treated with 1α,25(OH)2D3 induced PPARγ, which was followed by increased involucrin expression. In this study, we investigated whether PPARγ is involved in the 1α,25(OH)2D3-induced involucrin expression in human keratinocytes.MethodsSubconfluent keratinocytes were treated with 10−7 M 1α,25(OH)2D3 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 PPARγ and AP-1 were investigated by electrophoretic mobility shift assays (EMSA). To examine the role of PPARγ in 1α,25(OH)2D3 responses, recombinant adenovirus carrying a dominant-negative form of PPARγ (Axdn-PPARγ) was constructed and transfected into keratinocytes. The p38 inhibitor SB203580 was added to the cultures to evaluate the involvement of p38 in involucrin expression.Results1α,25(OH)2D3 induced PPARγ expression and stimulated PPARγ activity. The introduction of dn-PPARγ inhibited the expression of involucrin mRNA and protein induced by 1α,25(OH)2D3, and suppressed AP-1 DNA binding activity. 1α,25(OH)2D3 also triggered the phosphorylation of p38, which contributes to involucrin induction. Moreover, dn-PPARγ prevented the 1α,25(OH)2D3-induced phosphorylation of p38.ConclusionsThese results suggest that PPARγ regulates involucrin expression by controlling the AP-1 signal and p38 activation in 1α,25(OH)2D3-induced keratinocyte differentiation.