Fibrates increase human REV-ERB alpha expression in liver via a novel peroxisome proliferator-activated receptor response element

Fibrates are widely used hypolipidemic drugs that act by modulating the exp ression of genes involved in lipid and lipoprotein metabolism. Whereas the activation of gene transcription by fibrates occurs via the nuclear recepto r peroxisome proliferator-activated receptor-alpha (PPAR alpha) interacting with response elements consisting of a direct repeat of the AGGTCA motif s paced by one nucleotide (DR1), the mechanisms of negative gene regulation b y fibrates and PPAR alpha are largely unknown. In the present study, we dem onstrate that fibrates induce the expression of the nuclear receptor Rev-er b alpha, a negative regulator of gene transcription. Fibrates increase Rev- erb alpha mRNA levels both in primary human hepatocytes and in HepG2 hepato blastoma cells. In HepG2 cells, fibrates furthermore induce Rev-erb alpha p rotein synthesis rates. Transfection studies with reporter constructs drive n by the human Rev-erb alpha promoter revealed that fibrates induce Rev-erb alpha expression at the transcriptional level via PPAR alpha. Site-directe d mutagenesis experiments identified a PPAR response element that coincides with the previously identified Rev-erb alpha negative autoregulatory Rev-D R2 element. Electromobility shift assay experiments indicated that PPAR alp ha binds as heterodimer with 9-cis-retinoic acid receptor to a subset of DR 2 elements 5' flanked by an A/T-rich sequence such as in the Rev-DR2. PPAR alpha and Rev-erb alpha bind with similar affinities to the Rev-DR2 site. I n conclusion, these data demonstrate human Rev-erb alpha as a PPAR alpha ta rget gene and identify a subset of DR2 sites as novel PPAR alpha response e lements. Finally, the PPAR alpha and Rev-erb alpha signaling pathways cross talk through competition for binding to those response elements.