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

Fibrates are widely used hypolipidemic drugs that act by modulating the expression of genes involved in lipid and lipoprotein metabolism. Whereas the activation of gene transcription by fibrates occurs via the nuclear receptor peroxisome proliferator-activated receptor-alpha (PPAR alpha) interacting with response elements consisting of a direct repeat of the AGGTCA motif spaced by one nucleotide (DR1), the mechanisms of negative gene regulation by fibrates and PPAR alpha are largely unknown. In the present study, we demonstrate that fibrates induce the expression of the nuclear receptor Rev-erb alpha, a negative regulator of gene transcription. Fibrates increase Rev-erb alpha mRNA levels both in primary human hepatocytes and in HepG2 hepatoblastoma cells. In HepG2 cells, fibrates furthermore induce Rev-erb alpha protein synthesis rates. Transfection studies with reporter constructs driven by the human Rev-erb alpha promoter revealed that fibrates induce Rev-erb alpha expression at the transcriptional level via PPAR alpha. Site-directed mutagenesis experiments identified a PPAR response element that coincides with the previously identified Rev-erb alpha negative autoregulatory Rev-DR2 element. Electromobility shift assay experiments indicated that PPAR alpha binds as heterodimer with 9-cis-retinoic acid receptor to a subset of DR2 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. In conclusion, these data demonstrate human Rev-erb alpha as a PPAR alpha target gene and identify a subset of DR2 sites as novel PPAR alpha response elements. Finally, the PPAR alpha and Rev-erb alpha signaling pathways crosstalk through competition for binding to those response elements.