Structural requirements and cell-type specificity for ligand activation of peroxisome proliferator-activated receptors.

The mammalian peroxisome proliferator-activated receptor (PPAR) family consists of three different subtypes, PPARalpha, hNUC1/PPARdelta and PPARgamma. Selective agonists have been identified for PPARalpha and PPARgamma but not for hNUC1, and consequently little is known about the genes that are controlled by this receptor. Using ligand-dependent transcription assays in COS-7 cells, we screened a variety of PPAR activating agents to identify a selective activator of hNUC1. We found that the potent peroxisome proliferator, Wy-14643, and the PPARgamma-selective thiazolidinedione, BRL 49653, were poor activators of hNUC1 (EC50s of > 100 microM). Short chain fatty acids (FAs) appeared more selective for PPARalpha than for hNUC1, whereas the very long chain FA, erucic acid (C22:1) was more selective for hNUC1. Using erucic acid as a probe, we conducted a topological similarity search of the Merck Chemical Collection and identified a fatty acid-like compound, L-631,033 4-(2-acetyl-6-hydroxyundecyl) cinnamic acid, that was a selective activator of hNUC1 (EC50 of 2 microM), but was much less selective for PPARalpha or PPARgamma (EC50s of > 100 microM). Structure-function analysis of PPAR activation by L-631,033 structural analogues showed that receptor selectivity depends on the position of the carboxyl group relative to the phenyl ring on the molecule. Transfection experiments in several cell types: an osteoblastic cell line (MB 1.8), a mouse liver cell line (ML-457), rat aortic smooth muscle cells (RSMCs) and COS-7 cells revealed differences in the activation profile of specific ligands. The most notable differences were observed in RSMCs, where transactivation by L-631,033 and Wy-14643, but not by BRL 49653, was markedly reduced, and in MB 1.8 cells, where oleic acid failed to activate PPARs. These findings identify certain structural features in PPAR-activating agents that modulate PPAR activation, and suggest that as with other nuclear receptors, activation is cell-type specific.