PPARα blocks glucocorticoid receptor α-mediated transactivation but cooperates with the activated glucocorticoid receptor α for transrepression on NF-κB

Glucocorticoid receptor α (GRα) and peroxisome proliferator-activated receptor α (PPARα) are transcription factors with clinically important immune-modulating properties. Either receptor can inhibit cytokine gene expression, mainly through interference with nuclear factor κB (NF-κB)-driven gene expression. The present work aimed to investigate a functional cross-talk between PPARα- and GRα-mediated signaling pathways. Simultaneous activation of PPARα and GRα dose-dependently enhances transrepression of NF-κB-driven gene expression and additively represses cytokine production. In sharp contrast and quite unexpectedly, PPARα agonists inhibit the expression of classical glucocorticoid response element (GRE)-driven genes in a PPARα-dependent manner, as demonstrated by experiments using PPARα wild-type and knockout mice. The underlying mechanism for this transcriptional antagonism relies on a PPARα-mediated interference with the recruitment of GRα, and concomitantly of RNA polymerase II, to GRE-driven gene promoters. Finally, the biological relevance of this phenomenon is underscored by the observation that treatment with the PPARα agonist fenofibrate prevents glucocorticoid-induced hyperinsulinemia of mice fed a high-fat diet. Taken together, PPARα negatively interferes with GRE-mediated GRα activity while potentiating its antiinflammatory effects, thus providing a rationale for combination therapy in chronic inflammatory disorders