Modulation of aryl hydrocarbon receptor (AHR)-dependent signaling by peroxisome proliferator-activated receptor β/δ (PPARβ/δ) in keratinocytes

Whether peroxisome proliferator-activated receptor beta/delta (PPAR beta/delta) reduces skin tumorigenesis by altering aryl hydrocarbon receptor (AHR)-dependent activities was examined. Polycyclic aromatic hydrocarbons (PAH) increased expression of cytochrome P4501A1 (CYP1A1), CYP1B1 and phase II xenobiotic metabolizing enzymes in wild-type skin and keratinocytes. Surprisingly, this effect was not found in Ppar beta/delta-null skin and keratinocytes. Ppar beta/delta-null keratinocytes exhibited decreased AHR occupancy and histone acetylation on the Cyp1a1 promoter in response to a PAH compared with wild-type keratinocytes. Bisulfite sequencing of the Cyp1a1 promoter and studies using a DNA methylation inhibitor suggest that PPAR beta/delta promotes demethylation of the Cyp1a1 promoter. Experiments with human HaCaT keratinocytes stably expressing shRNA against PPAR beta/delta also support this conclusion. Consistent with the lower AHR-dependent activities in Ppar beta/delta-null mice compared with wild-type mice, 7,12-dimethylbenz[a] anthracene (DMBA)-induced skin tumorigenesis was inhibited in Ppar beta/delta-null mice compared with wild-type. Results from these studies demonstrate that PPAR beta/delta is required to mediate complete carcinogenesis by DMBA. The mechanisms underlying this PPAR beta/delta-dependent reduction of AHR signaling by PAH are not due to alterations in the expression of AHR auxiliary proteins, ligand binding or AHR nuclear translocation between genotypes, but are likely influenced by PPAR beta/delta-dependent demethylation of AHR target gene promoters including Cyp1a1 that reduces AHR accessibility as shown by reduced promoter occupancy. This PPAR beta/delta/AHR crosstalk is unique to keratinocytes and conserved between mice and humans.