Ligand-activated PPARβ efficiently represses the induction of LXR-dependent promoter activity through competition with RXR

Angiopoietin-like protein 3 (angptl3), a member of the vascular endothelial growth factor family, was shown to play an important role in regulating lipid metabolism. To elucidate the mechanism by which PPARβ represses angptl3 promoter activity, reporter constructs were prepared and transfection analysis carried out. PPARβ repressed angptl3-Luc promoter activity and activation of PPARβ by L-165041, a PPARβ-specific ligand, increased the extent of repression. The repression by L-165041 was lost in angptl3-Luc plasmids having a deleted or mutated LXRα binding site (DR4). PPARβL405R, deficient in RXRα binding, had no effect on angptl3-Luc promoter activity. PPARβ did not repress the activity of GAL4-LXRα which activates of GAL4DBD TK-Luc independent of RXR. Addition of RXRα completely abolished the repression of angptl3-Luc activity by PPARβ. Mammalian two-hybrid analysis revealed that PPARβ ligand binding enhanced the dissociation of the LXRα-RXRα heterodimer. Gel shift assays also indicated that PPARβ ligand binding increased dissociation of LXRα/RXRα binding to a DR4 oligonucleotide probe; addition of RXRα restored the binding lost by addition of PPARβ. Collectively, these results suggest that the binding of PPARβ-specific ligand enhances the affinity between RXRα and activated PPARβ and thus may regulate angptl3 gene expression through a DR4 element by competing with LXRα for RXRα.