Fatty acids have recently been demonstrated to activate peroxisome proliferator-activated receptors (PPARs) but specific structural requirements of fatty acids to produce this response have not yet been determined. Importantly, it has hitherto not been possible to show specific binding of these compounds to PPAR. To test whether a common PPAR binding metabolite might be formed, we tested the effects of long-chain ω-3 polyunsaturated fatty acids, differentially β-oxidizable fatty acids and inhibitors of fatty acid metabolism. We determined the activation of a reporter gene by a chimaeric receptor encompassing the DNA binding domain of the glucocorticoid receptor and the ligand binding domain of PPAR. The ω-3 unsaturated fatty acids were slightly more potent PPAR activators in vitro than saturated fatty acids. The peroxisomal proliferation-inducing, non-β-oxidizable, tetradecylthioacetic acid activated PPAR to the same extent as the strong peroxisomal proliferator WY 14,643, whereas the homologous β-oxidizable tetradecylthiopropionic acid was only as potent as a non-substituted fatty acid. Cyclooxygenase inhibitors, radical scavengers or cytochrome P450 inhibitors did not affect activation of PPAR. In conclusion, β-oxidation is apparently not required for the formation of the PPAR-activating molecule and this moiety might be a fatty acid, its ester with CoA, or a further derivative of the activated fatty acid prior to β-oxidation of the acyl-CoA ester. These data should aid understanding of signal transduction via PPAR and the identification of a receptor ligand.
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