| Abstract |
SHP (Small heterodimer Partner) is an orphan hormone nuclear receptor, which forms heterodimers with other nuclear receptors and inhibits their activity. SHP is an atypical member of the nuclear receptor superfamily, as it does not contain the conserved DNA binding domain and therefore works as a transcriptional corepressor. In the first part of the thesis, I studied the molecular mechanism involved in SHP-dependent repression. The results show that SHP can interact physically and functionally with the major histone deacetylase HDAC1 and the euchromatic K9 methyltransferase G9. Moreover, SHP binds specifically to hypoacetylated and K9 methylated histone H3 and is localized to euchromatic territories of the nucleus. The results point to a multistep mechanism in SHP-depensent transcriptional repression, which includes histone deacetylation, followed by K9 methylation of histone H3 and stable association of SHP itself with chromatin. SHP is an important component of the feedback regulatory cascade, which controls the conversion of cholesterol to bile acids. SHP is transcriptionally induced by the nuclear receptor FXR under high concentration of bile acids and represses genes involved in bile acid biosynthesis and transport. In order to identify the bona fide molecular targets of SHP and study its biological role in liver physiology, we performed global gene expression profiling combined with chromatin immunoprecipitation assays in transgenic mice constitutively expressing SHP in the liver. The results presented in the second part of the thesis show that SHP affects genes involved in diverse biological pathways, and in particular, several key genes involved in consecutive steps of cholesterol degradation, bile acid conjugation, transport and lipogenic pathways. Sustained expression of SHP leads to the depletion of hepatic bile acid pool and a concomitant accumulation of triglycerides in the liver. The mechanism responsible for this phenotype includes SHP-mediated direct repression of downstream target genes and the bile acid sensor FXRa, and an indirect activation of PPARγ and SREBP-1c genes. Moreover, the molecular mechanism of SHP transcriptional repression was studied in vivo. The results demonstrate the role of histone deacetylation and K9 methylation of H3 on SHP-mediated repression and provide evidence for the existence of distinct gene-specific mechanisms by which SHP mediates differential transcriptional repression.
|
Μεταγραφική ρύθμιση ηπατικών μεταβολικών μονοπατιών από τον ορφανό πυρηνικό υποδοχέα SHP
