Modulation of keratinocyte gene expression and differentiation by PPAR-selective ligands and tetradecylthioacetic acid

Peroxisome proliferator-activated receptors (PPARs) are pleiotropic regulat ors of growth and differentiation of many cell types. We have performed a c omprehensive analysis of the expression of PPARs, transcriptional cofactors , and marker genes during differentiation of normal human keratinocytes usi ng a combination of reverse transcriptase polymerase chain reaction, Northe rn and Western blotting, and immunohistochemistry. PPAR delta was the predo minant PPAR subtype in human keratinocytes and highly expressed in basal ce lls and suprabasal cells. Induction of PPAR alpha and PPAR gamma expression was linked to differentiation, and accordingly, expression of PPAR alpha a nd PPAR gamma was in essence confined to suprabasal cells. Differentiation was not accompanied by significant changes in the expression of the coactiv ators CREB-binding protein, p300, steroid receptor coactivator 1, or the co repressors nuclear receptor corepressor and silence mediator for retinoid a nd thyroid hormone receptors, We critically evaluated the effects of select ive PPAR ligands and a synthetic fatty acid analog, tetradecylthioacetic ac id. Tetradecylthioacetic acid activated all human PPAR subtypes in the rank ing order PPAR delta >> PPAR alpha > PPAR gamma. All selective PPAR ligands marginally induced transglutaminase-l expression with the PPAR delta -sele ctive ligand L165041 being the most potent. The PPAR alpha- and PPAR gamma -selective ligands Wy14643 and BRL49653 had negligible effect on involucrin expression, whereas a dose-dependent induction was observed with L165041, Simultaneous addition of L165041 and BRL49653 synergistically induced stron g involucrin expression. Additionally, L165041 potently induced CD36 mRNA e xpression. Administration of tetradecylthioacetic acid resulted in a dramat ic decrease in proliferation and a robust upregulation of the expression of involucrin and transglutaminase. Our results indicate that tetradecylthioa cetic acid may affect keratinocyte gene expression and differentiation via PPAR-dependent and PPAR-independent pathways, and that the latter play an i mportant role.