Uncoupling protein-3 mRNA up-regulation in C2C12 myotubes after etomoxir treatment

Uncoupling proteins (UCPs) are mitochondrial membrane proton transporters that uncouple respiration from oxidative phosphorylation by dissipating the proton gradient across the membrane. Treatment of C2C12 myotubes for 24 h with 40 μM etomoxir, an irreversible inhibitor of carnitine palmitoyltransferase I (CPT-I), up-regulated uncoupling protein 3 (UCP-3) mRNA levels (2-fold induction), whereas UCP-2 mRNA levels were not modified. Etomoxir treatment also caused a 2.5-fold induction in M-CPT-I (muscle-type CPT-I) mRNA levels. In contrast, other well-known peroxisome proliferator-activated receptor α (PPARα) target genes, such as acyl-CoA oxidase and medium-chain acyl-CoA dehydrogenase, were not affected, suggesting that this transcription factor was not involved in the effects of etomoxir. Since it has been reported that CPT-I inhibition by etomoxir leads to a further increase in ceramide synthesis, we test the possibility that ceramides were involved in the changes reported. Similarly to etomoxir, addition of 20 μM C2-ceramide to C2C12 myotubes for 3, 6 and 9 h resulted in increased UCP-3 and M-CPT-I mRNA levels. These results indicate that the effects on UCP-3 mRNA levels could be mediated by increased ceramide synthesis.