Ligand-mediated regulation of peroxisome proliferator-activated receptor (PPAR) beta/delta: a comparative analysis of PPAR-selective agonists and all-trans retinoic acid.

Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily that modulate target gene expression in response to natural fatty acid ligands and synthetic agonists. It is noteworthy that all trans-retinoic acid (atRA) has recently been reported to act as a ligand for PPARbeta/delta, to activate its transcriptional activity, and, in contrast to the "classic" function of atRA, to stimulate cell proliferation (Schug et al., 2007). Here, we report that in contrast to synthetic PPARbeta/delta agonists, atRA failed to induce the transcriptional activity of PPARbeta/delta using different types of reporter gene assays. Likewise, atRA did not affect the expression of the bona fide PPARbeta/delta target genes ADRP and ANGPTL4 but strongly increased expression of the retinoic acid target gene CYP26A under the identical experimental conditions. Consistent with these observations, atRA did not compete with established PPARbeta/delta agonists in a ligand binding assay, and atRA did not enable the interaction of PPARbeta/delta with a coactivator peptide in a time-resolved fluorescence resonance energy transfer assay in vitro. These results are in sharp contrast to the effect of established PPARbeta/delta agonists in both in vitro assays. Taken as a whole, these data strongly suggest that atRA does not function as a ligand of PPARbeta/delta in any of the experimental systems tested and that the previously reported atRA effects are more likely to reflect an uncharacterized and less direct mechanism.