PPARδ agonism induces a change in fuel metabolism and activation of an atrophy programme, but does not impair mitochondrial function in rat skeletal muscle

PPAR alpha agonism impairs mitochondrial function, but the effect of PPAR delta agonism on mitochondrial function is equivocal. Furthermore, PPAR alpha and delta agonism increases muscle fatty acid oxidation, potentially via activation of FOXO1 signalling and PDK4 transcription. Since FOXO1 activation has also been suggested to increase transcription of MAFbx and MuRF-1, and thereby the activation of ubiquitin-proteasome mediated muscle proteolysis, this raises the possibility that muscle fuel selection and the induction of a muscle atrophy programme could be regulated by a single common signalling pathway. We therefore investigated the effect of PPAR delta (delta) agonist, GW610742, administration on muscle mitochondrial function, fuel regulation, and atrophy and growth related signalling pathways in vivo. Twenty-four male Wistar rats received vehicle or GW610742 (5 and 100 mg per kg body mass (bm)) orally for 6 days. Soleus muscle was used to determine maximal rates of ATP production (MRATP) in isolated mitochondria, gene and protein expression, and enzyme activities. MRATP were unchanged by GW610742. Muscle PDK2 and PDK4 mRNA expression increased with GW610742 (100 mg (kg bm)(-1)) compared to vehicle (P < 0.05), and was paralleled by a twofold increase in PDK4 protein expression (P < 0.05). The activity of beta-hydroxyacyl-CoA dehydrogenase increased with GW610742 (P < 0.05). Muscle MuRF1 and MAFbx mRNA expression was increased by GW610742 (100 mg (kg bm)(-1)) compared to vehicle (P < 0.05), and was matched by increased protein expression (P < 0.001), whilst Akt1 protein declined (P < 0.05). There was no effect of GW610742 on 20S proteasome activity and mRNA expression, or the muscle DNA:protein ratio. GW610742 switched muscle fuel metabolism towards decreased carbohydrate use and enhanced lipid utilization, but did not induce mitochondrial dysfunction. Furthermore, GW610742 initiated a muscle atrophy programme, possibly via changes in the Akt1/FOXO/MAFbx and MuRF1 signalling pathway.