Co-factors necessary for PPAR mediated transactivation of endogenous target genes

 

The three PPAR subtypes alpha, beta/delta and gamma are very important transcriptional regulators of glucose and lipid metabolism. Even though the different PPAR subtypes activate genes through similar DR-1 conserved DNA motifs (PPREs), activation of the PPARs in vivo leads to opposite physiological scenarios. PPARa and PPARd are transcriptional regulators of fatty acid oxidation and ketogenesis, whereas PPAR? controls genes involved in lipid storage. Consequently, there must be PPAR subtype specific molecular determinants that secure PPAR selective recognition and activation of target promoters in a given cell type. In vitro experiments suggest that the different PPAR subtypes might have dissimilar binding preference for some PPAR target sites and may also have different affinity for some transcriptional co-factors. However the molecular mechanisms behind PPAR subtype specific activation of endogenous target gene in different cell types are elusive.

To mutually compare the ability of the PPAR subtypes to activate endogenous target genes in a given cell, PPARa, PPARb/d and PPARg2 were HA tagged and rapidly, equally and synchronously expressed using adenoviral delivery. Within a few hours after adenoviral delivery the PPARs establish transcriptional active complexes on genomic target loci and launch immediate activation even of silent target genes. Direct comparison of the PPAR subtypes in a given cell line reveals that they selectively occupy genomic target promoters and in correlation show subtype specific activation of target genes. Accumulating evidence suggests that transcriptional co-factors can function as master regulators for nuclear receptors and impose promoter selectivity. To study co-factor necessity for PPAR mediated transactivation of endogenous target genes, specific co-factors reported to be involved in PPAR signalling were knocked down using lentiviral delivered shRNA expression. Interestingly, knockdown of well known PPAR interacting co-factors like TRAP220 and SRC-1 had a highly promoter specific effect on target gene expression.