PPARdelta (peroxisome proliferator-activated receptor delta) is a regulator of lipid metabolism and has been shown to induce fatty acid oxidation (FAO). PPARdelta transgenic and knock-out mice indicate an involvement of PPARdelta in regulating mitochondrial biogenesis and oxidative capacity; however, the precise mechanisms by which PPARdelta regulates these pathways in skeletal muscle remain unclear. In this study, we determined the effect of selective PPARdelta agonism with the synthetic ligand, GW501516, on FAO and mitochondrial gene expression in vitro and in vivo. Our results show that activation of PPARdelta by GW501516 led to a robust increase in mRNA levels of key lipid metabolism genes. Mitochondrial gene expression and function were not induced under the same conditions. Additionally, the activation of Pdk4 transcription by PPARdelta was coactivated by PGC-1alpha. PGC-1alpha, but not PGC-1beta, was essential for full activation of Cpt-1b and Pdk4 gene expression via PPARdelta agonism. Furthermore, the induction of FAO by PPARdelta agonism was completely abolished in the absence of both PGC-1alpha and PGC-1beta. Conversely, PGC-1alpha-driven FAO was independent of PPARdelta. Neither GW501516 treatment nor knockdown of PPARdelta affects PGC-1alpha-induced mitochondrial gene expression in primary myotubes. These results demonstrate that pharmacological activation of PPARdelta induces FAO via PGC-1alpha. However, PPARdelta agonism does not induce mitochondrial gene expression and function. PGC-1alpha-induced FAO and mitochondrial biogenesis appear to be independent of PPARdelta.

Gut microbiota regulate physiological functions in various hosts, such as energy metabolism and immunity. Lactic acid bacteria, including Lactobacillus plantarum, have a specific polyunsaturated fatty acid saturation metabolism that generates multiple fatty acid species, such as hydroxy fatty acids, oxo fatty acids, conjugated fatty acids, and trans-fatty acids. How these bacterial metabolites impact host physiology is not fully understood. Here, we investigated the ligand activity of lactic acid bacteria-produced fatty acids in relation to nuclear hormone receptors expressed in the small intestine. Our reporter assays revealed two bacterial metabolites of γ-linolenic acid (GLA), 13-hydroxy-cis-6,cis-9-octadecadienoic acid (γHYD), and 13-oxo-cis-6,cis-9-octadecadienoic acid (γKetoD) activated peroxisome proliferator-activated receptor delta (PPARδ) more potently than GLA. We demonstrate that both γHYD and γKetoD bound directly to the ligand-binding domain of human PPARδ. A docking simulation indicated that four polar residues (T289, H323, H449, and Y473) of PPARδ donate hydrogen bonds to these fatty acids. Interestingly, T289 does not donate a hydrogen bond to GLA, suggesting that bacterial modification of GLA introducing hydroxy and oxo group determines ligand selectivity. In human intestinal organoids, we determined γHYD and γKetoD increased the expression of PPARδ target genes, enhanced fatty acid β-oxidation, and reduced intracellular triglyceride accumulation. These findings suggest that γHYD and γKetoD, which gut lactic acid bacteria could generate, are naturally occurring PPARδ ligands in the intestinal tract and may improve lipid metabolism in the human intestine.