Abstract 17772: Dimer Conversion of PPARalpha/RXR To PPARalpha/Sirt1 Suppresses Fatty Acid Oxidation Genes
A decline in fatty acid oxidation (FAO) is a hallmark of the failing heart. The PPARα/RXR dimer transcribes genes involved in FAO through a DNA binding element called direct repeat 1 (DR1). However, how PPARα targets are downregulated in the failing heart is not well understood. Here we report that the PPARα dimerization partner is switched from RXR to Sirt1 on a subset of DR1s, thereby suppressing FAO activity and gene expression during pressure overload (PO). Although PPARα was capable of binding to all DR1s we tested, individual DR1s exhibited distinct scaffolding abilities for promoting PPARα/RXR heterodimerization. DR1s with a strong scaffolding ability represented typical DR1 sequences, whereas those with a weak scaffolding ability contained degenerate imperfect DR1 sequences. PPAR targets harboring typical DR1s, such as Cpt1b and CD36, were resistant against PO-induced downregulation and upregulated in PPARα-overexpressing (Tg-PPARα) mice. On the other hand, PPAR targets with imperfect DR1s were readily suppressed by PO, an effect that was exacerbated in Tg-PPARα mice but normalized in mice with heterozygous knockout of Sirt1 and PPARα (PPARα+/-) (relative mRNA vs Wild type (WT) sham, Cpt2: WT TAC 0.7, PPARα+/- sham 1.1, PPARα+/- TAC 1.05*, Mcad: WT TAC 0.49, PPARα+/- sham 1.1, PPARα+/- TAC 0.77*, p<0.05 vs WT TAC). PO-induced impairment of FAO is exacerbated in Tg-PPARα mice but normalized in PPARα+/- (relative FAO activity vs WT sham, WT TAC 0.50, PPARα+/- Sham 0.88, PPARα+/- TAC 0.89*, p<0.05 vs WT TAC). Thus, PPARα negatively regulates FAO through suppression of genes harboring imperfect DR1s during PO. PPARα/Sirt1-induced transcriptional suppression through imperfect DR1s was normalized by RXR overexpression, indicating that RXR counteracts the transcriptional suppression. RXR competitively inhibited the interaction between PPARα and Sirt1, which is further inhibited in the presence of a typical DR1 oligonucleotide due to enhancement of PPARα/RXR dimerization. The occupancies of PPARα/Sirt1 on the flanking regions of imperfect DR1s were increased by PO, accompanied by histone deacetylation and reduced PPARα/RXR occupancies. These results suggest that the dimer transition from PPARα/RXR to PPARα/Sirt1 downregulates PPAR targets.
- © 2013 by American Heart Association, Inc.