Chromatin regulation by the nuclear receptor PPARbeta/delta
Zusammenfassung der Projektergebnisse
PPARβ/δ is transcription factor and member of the nuclear receptor superfamily that can both activate and repress transcription depending on the availability of ligands (canonical regulation), but can also regulate target genes in a ligand-independent fashion (noncanonical regulation) by poorly understood mechanisms. The molecular basis of these distinct target gene responses was the main topic of the proposed project with two major goals: (i) elucidating PPARβ/δ interactions at the chromatin level, and (ii) studying the functional significance of novel interaction partners previously identified by “two-hybrid” screen in yeast. While the original hypothesis that long-range chromatin interactions may play an essential role in mediating PPARβ/δ-mediated effects could not (yet) be confirmed, other avenues led to new discoveries regarding PPARβ/δ-regulated transcription. Regarding the question how ligand-dependent effects by PPARβ/δ are brought about we identified two fundamentally different mechanisms. First, we found that a large subgroup of PPRE-dependent target genes is ligand-dependent in all cells types. These genes are usually associated with metabolic functions, in particular lipid oxidation, but also encompass ANGPTL4, for which we were able to show essential function in cancer cell invasion (10) and in human ovarian cancer progression (25). Another subgroup of canonically regulated, PPARβ/δ-RXR occupied genes is characterized by a cell type dependent ligand response and frequently associated with cell type-specific functions, exemplified by CD300A in macrophages (26). Second, we identified a large group of genes regulated by PPARβ/δ without direct DNA contact (non-canonical regulation), which fall into two groups according to heir ligand response, i.e. ligand-independent or repressed by agonist. The latter we coined inverse ligand regulation, which we found to be frequently associated with cell type-specific regulatory effects. This novel mechanism we explored in detail in human macrophages by delineating the global PPARβ/δ-regulated signaling network. Besides cell type-independent, canonical target genes with metabolic and immune regulatory functions we identified a large number of inflammation-associated NFκB and STAT1 target genes that are repressed by agonists. Comprehensive functional and bioinformatics analyses demonstrated that PPARβ/δ agonists induce a unique macrophage activation state with strong anti-inflammatory but also specific immune stimulatory components, pointing to a context-dependent function of PPARβ/δ in immune regulation. Furthermore, we discovered an unexpected cytoplasmic function of PPARβ/δ. Silencing of PPARβ/δ expression interfered with the expression of a large subset of IL-1β-induced NFκB target genes in HeLa cells, preceded by an inhibition of the phosphorylation of TAK1, IκBα and the NFκB p65. PPARβ/δ binds to the small heat-shock protein HSP27, a known positive modulator of TAK1-mediated IL-1β signaling, and physically interacts with the endogenous cytoplasmic TAK1/TAB1 complex. Taken together, our observations point to a role for PPARβ/δ in the assembly of a cytoplasmic multi-protein complex with TAK1, TAB1 and HSP27, and thereby enhances the NFκB response to IL-1β in specific cell types. Our findings pave the way to investigate novel PPARβ/δ-mediated signaling pathways and mechanisms of transcriptional regulation in further detail, with the goal of establishing an integrated picture of the common and cell type-specific roles of PPARβ/δ in metabolism, immune regulation and cancer.
Projektbezogene Publikationen (Auswahl)
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(2013). In vivo studies of PPAR-chromatin interactions: chromatin immunoprecipitation for single-locus and genome-wide analyses. Meth. Mol. Biol. 952, 175-185
Adhikary,T. and Müller, R.
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(2013). Inverse PPARβ/δ agonists suppress oncogenic signaling to the ANGPTL4 gene and inhibit cancer cell invasion. Oncogene 32,5241-5252
Adhikary, T., Brandt, D.T., Kaddatz, K., Stockert, J., Finkernagel, F., Naruhn, S., Meissner, W., Obert, J., Lieber, S., Scharfe, M., Jarek, M., Toth, P.M., Scheer, F., Diederich, W.E., Reinartz, S., Grosse, R., Müller-Brüsselbach, S. and Müller, R.
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(2013). Regulation of TAK1/TAB1-mediated IL-1β signalling by cytoplasmic PPARβ/δ. PLoS ONE 8(4):e63011
Stockert, J., Wolf, A., Kaddatz, K., Schnitzer, E., Finkernagel, F., Meissner, W., Müller-Brüsselbach, S., Kracht, M. and Müller, R.
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(2015). Deregulation of PPARβ/δ target genes in tumor-associated macrophages by fatty acid ligands in the ovarian cancer microenvironment. Oncotarget 6, 13416-13433
Schumann, T., Adhikary, T., Wortmann, A., Finkernagel, F, Lieber, S., Schnitzer, E., Legrand, N., Schober, Y., Nockher, W.A., Toth, P.M., Diederich, W.E., Nist, A., Stiewe, T., Wagner, U., Reinartz, S., Müller-Brüsselbach, S. and Müller, R.
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(2015). The controversial role of PPARβ/δ in cancer. Trends Endocrinol. Metab.
Peters, J.M., Gonzalez, F.J. and Müller, R.
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(2015). The inverse agonist DG172 triggers a PPARβ/δ-independent myeloid lineage shift and promotes GM- CSF/IL-4-induced dendritic cell differentiation. Mol. Pharmacol. 87, 162-173
Lieber, S., Scheer, F., Finkernagel, F., Meissner, W., Giehl, G., Brendel, C., Diederich, W., Müller-Brüsselbach, S. and Müller, R.
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(2015). The transcriptional PPARβ/δ network in human macrophages defines a unique agonist-induced activation state. Nucl. Acids Res. 43, 5033-5051
Adhikary, T., Wortmann, A., Schumann, T., Finkernagel, F., Lieber, S., Roth, K., Toth, P.M., Diederich, W.E., Nist, A., Stiewe, T., Kleinesudeik, L., Reinartz, S., Müller-Brüsselbach, S. and Müller R.
