Abstract 544: A Novel Peroxisome Proliferator Response Element Modulates LDL-receptor Transcription in Response to PPAR Agonists.

Hepatic LDL receptor (LDLR) binds to and internalizes circulating LDL-cholesterol thereby facilitating its clearance from the serum. LDLR is regulated primarily at the transcriptional level by a sterol-regulatory element (SRE) in its proximal promoter region which is the site of action of SRE-binding protein (SREBP1/2). Additionally, SRE-dependent modulation of LDLR by peroxisome proliferator-activated receptor gamma (PPARγ) has also been previously described. However whether additional promoter elements directly regulate LDLR transcription is not very well understood. We observed that treatment with the PPARδ agonist L165041 elevated luciferase-reported activity of human LDLR promoter as well as the mRNA and protein expression of LDLR in both, human and mouse cell lines. The induction of LDLR mRNA in HepG2 cells was dose-dependent with maximal expression observed at 20uM. We also observed that the half-life of LDLR mRNA was similar in control and L165041 treated samples, suggesting that the induction was a result of transactivation of mRNA synthesis. Combinatorial treatment of HepG2 cells with L165041 and rosuvastain (RSV) demonstrated additive effects of the two compounds on LDLR mRNA as well as protein expression. Transcription factor prediction programs indicated presence of a single PPAR-response element (PPRE)-like sequence motif in the human LDLR promoter, located at -768 to -752 bases upstream of the translation start site. Luciferase reporter analyses indicated that mutation or deletion of this PPRE-like element resulted in attenuation of L165041 mediated upregulation of LDLR promoter luciferase activity. Electrophoretic mobility shift assays further confirmed the binding of PPARδ/RXR heterodimers to the LDLR-PPRE site. Taken together, our studies identify a novel PPRE in the proximal promoter of LDLR and suggest that combinatorial treatment of statin with PPAR agonist may have advantageous effects on LDLR expression.