PPARδ and PGC1α act cooperatively to induce haem oxygenase-1 and enhance vascular endothelial cell resistance to stress

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear receptor superfamily of ligand-activated transcriptional regulators. PPAR delta has an established role in metabolism, wound healing, and angiogenesis. However, little is known about its function in endothelial homeostasis. We investigated the role of PPAR delta and its co-activator, PPAR gamma co-activator 1 alpha (PGC1 alpha), in vasculoprotection against oxidant-induced injury via induction of haem oxygenase-1. En face confocal microscopy of murine aortas demonstrated that the PPAR delta-selective ligand GW501516 induced endothelial haem oxygenase-1 expression. In vitro PPAR delta ligands induced a significant increase in haem oxygenase-1 mRNA, protein, and enzyme activity, resulting in enhanced human endothelial cell protection against cellular stress induced by hydrogen peroxide or leptin. Moreover, adenoviral-mediated overexpression of haem oxygenase-1 increased PPAR delta promoter activity and mRNA levels, amplifying the effect of PPAR delta ligands through a positive feedback loop. Mutation of PPAR response element binding sites in the haem oxygenase-1 promoter/enhancer region revealed haem oxygenase-1 to be a direct PPAR delta target gene. Inhibition of either haem oxygenase-1 or PPAR delta abrogated PPAR delta ligand-induced endothelial cytoprotection. Furthermore, siRNA depletion of PGC1 alpha demonstrated that this co-regulator acts as an essential PPAR delta transcriptional co-activator for haem oxygenase-1 induction by PPAR delta ligands and its subsequent cytoprotective actions. We have identified an important relationship between PPAR delta, PGC1 alpha, and haem oxygenase-1, demonstrating that haem oxygenase-1 induction plays an important role in cytoprotective actions of PPAR delta ligands in vascular endothelium. In light of the protective effects of haem oxygenase-1 against atherogenesis, we suggest that PPAR delta represents a potentially important therapeutic target in the vasculature.