The effect of phytosphingosine on the transcriptional activity of PPARs and the expression of PPARγ gene. PPRE-tk-Luc was transiently transfected (A) or cotransfected with 10 ng of the pCMX-mPPAR expression vector into HaCaT cells (B) as described in “Materials and Methods.” Transfected cells were treated with WY14643, prostacyclin (PGI2), troglitazone (TGZ), or phytosphingosine (PS) as indicated and assayed 24 h later for luciferase activity. The corresponding β-gal activity was used to normalize luciferase activity. HaCaT cells were treated with indicated concentrations of phytosphingosine (PS) or 10 µM troglitazone (TGZ) for 24 h (C) or treated with 5 µM PS for the indicated time periods (D). At the end of incubation, total RNA was isolated and mRNA levels of PPARγ were measured by quantitative real-time RT-PCR analysis as described in “Materials and Methods.” The mRNA level of PPARγ in vehicle control or control at 0 h was set at 1, and the relative expression levels are presented as fold induction compared with that of each control. Data represent the mean ± SD of 3 or 4 independent experiments. *Significant at P < 0.05. **Significant at P < 0.01.