Untersuchungen zur Transaktivierung und Degradation des Nukleären Hormonrezeptors PPARγ

Nuclear hormone receptors (NHRs) play a key role in many physiological and pathological processes in the human organism. They act as transcription factors whose activity is regulated by ligands. Peroxisome proliferator activated receptor gamma (PPARγ) is a member of the NHR-family and regulates several crucial biological processes such as adipogenesis, systemic insulin-sensitivity, fat metabolism, tumor biology and inflammation. PPARγ regulates genes by binding together with its heterodimeric partner retinoic X receptor (RXR) to promotor-regions. Insulin-sensitizing drugs, the thiazolidinediones (TZDs) like for example rosiglitazone, now used in the treatment of diabetes type 2, are highly potent synthetic PPARγ-ligands. But activation of PPARγ by ligands also leads to ubiquitination and proteasomal degradation of the receptor. Regarding the many different effects of PPARγ it would be of highly biological and therapeutical significance to manipulate the activity of this transcription factor selectively. Therefore it is crucial to understand the process of the activation of PPARγ in greater depth. This study addresses the question if ligand-dependent transcriptional activity and degradation are linked to each other. For RXR several point mutants have been reported whose transactivation and degradation happen independently. So in this study analogous PPARγ-point mutants were cloned and characterized. It was seen that several PPARγ-point mutants showed a reduced affinity to PPARγ-ligand. With optimal ligand concentration all PPARγ-point mutants showed a highly reduced transcriptional activity. Furthermore only transcriptional active PPARγ-allels were degraded ligand-dependent. Transcriptional inactive PPARγ-mutants were not degraded but even stabilized.We can conclude that for PPARγ-transcriptional activity and degradation have to be linked to each other. In this study it was also looked into the regulation of PPARγ-transcriptional activity by the proteasome. Several studies assessing the transcriptional activity of PPARγ by using exogenous reporter assays found that inhibition of the proteasome leads to an increased transactivation of PPARγ. Here it was shown that ligand-dependent expression of endogenous PPARγ-target-genes is inhibited by inhibition of proteasomal activity. Therefore in contrast to the other studies we can conclude that transcriptional activation of the investigated target genes by PPARγ needs the activity of the proteasome.