Komplexität der Transkriptionsregulation durch PPARβ/δ

Peroxisome-proliferator activated receptor β/δ (PPARβ/δ) is a ligand-inducible transcription factor that plays an essential role in lipid metabolism and energy homoeostasis and has been connected to different cellular processes like differentiation, proliferation and apoptosis. In the first part of the thesis, three different classes of target genes were identified in human myofibroblasts by expression profiling and genome-wide chromatin immunoprecipitation analysis (ChIP-sequencing). Class I genes are repressed by PPARβ/δ and show strong and rapid induction by specific agonists. Class II genes exhibit no PPARβ/δ-mediated repression. Their induction by agonists is comparably weak and slower, and their expression is strongly repressed by antagonists. The third class encompasses genes whose expression is ligand-independent, but correlates with PPARβ/δ levels. Surprisingly, PPARβ/δ binding of class III genes occurs in the absence of detectable PPAR response elements (PPREs). Taken together, these analyses led to the definition of different classes of target genes that are distinguished by their mechanism of regulation. PPARβ/δ does not only play a key role in the regulation of metabolic pathways, but also modulates inflammatory processes and has essential functions in tumor stroma, indicating a functional interaction between PPARβ/δ and cytokine signaling. In the second part of this thesis, transcriptional profiling of human myofibroblasts revealed a functional interaction of PPARβ/δ and transforming growth factor β (TGFβ) signaling pathways, and showed that a subset of genes are cooperatively activated by TGFβ and PPARβ/δ. Two different enhancer regions mediating synergistic activation were identified in the angiopoetin-like 4 (ANGPTL4) gene, which was used as a model. A TGFβ responsive enhancer located ∼8.5 kb upstream of the transcriptional start site (TSS) is regulated by a mechanism involving SMAD3, ETS1, RUNX2 and AP-1 transcription factors that interact with multiple contiguous binding sites. A second enhancer (PPAR-E), consisting of three adjacent PPREs, is located in the third intron ∼3.5 kb downstream of the TSS. The PPAR-E is strongly activated by all three PPAR subtypes, with a novel type of PPRE motif playing a central role. Although the PPAR-E is not regulated by TGFβ, it interacts with SMAD3, ETS1, RUNX2 and AP-1 in vivo, providing a possible mechanistic explanation for the observed synergism.