Farnesol stimulates differentiation in epidermal keratinocytes via PPAR alpha

The isoprenoids farnesol and juvenile hormone III (JH), metabolites of the cholesterol biosynthetic pathway, have been shown to stimulate fetal epider mal development in rodents. In this study we determined whether this effect might be attributed to a direct induction of keratinocytes differentiation and examined the mechanisms responsible for these effects. Rates of cornif ied envelope formation, a marker of keratinocyte terminal differentiation, as well as protein and mRNA levels of two proteins required for cornified e nvelope formation, involucrin (INV) and transglutaminase, increased 2- to 3 -fold in normal human keratinocytes (NHK) treated with either farnesol or J H, even at low calcium concentrations (0.03 mM), which otherwise inhibit di fferentiation. In contrast, neither cholesterol nor mevalonate affected INV or transglutaminase mRNA levels, Effects of farnesol and JH on INV and tra nsglutaminase mRNA levels were additive with high calcium concentrations (1 .2 mM) that independently stimulate keratinocyte differentiation. In contra st, keratinocyte DNA synthesis was inhibited by these compounds. Both farne sol and JH stimulated INV and transglutaminase promoter activity, suggestin g regulation at the transcriptional level. A series of truncation and delet ion experiments revealed a farnesol-responsive region (-2452 to -1880 base pairs (bp)) in the INV gene. This region contained an AP-1 site. A single b ase pair mutation of the AP-1 site at -2116 to -2110 bp abolished farnesol responsiveness, identical to effects by peroxisome proliferator-activated r eceptor (PPAR alpha) activators. Farnesoid X-activated receptor mRNA was no t detected in NHK, but farnesol treatment increased activities of both a PP AR response element and PPAR alpha mRNA levels in NHK, Furthermore, the inc rease in PPRE activity by farnesol was dependent upon PPAR alpha in CV-1 ce lls. Finally, topical applications of farnesol increased mRNA and protein l evels of the differentiation-specific genes, profilaggrin and loricrin, det ermined by immunohistochemistry and in situ hybridization, in wild-type but not in PPAR alpha-/- murine epidermis, These findings suggest a novel role for selected isoprenoid cholesterol intermediates in the regulation of dif ferentiation-specific gene transcription and a convergence of PPAR alpha wi th the cholesterol synthetic pathway.