Up-regulation of uncoupling protein 3 gene expression by fatty acids and agonists for PPARs in L6 myotubes

Uncoupling protein 3 (UCP3), which uncouples electron transport from ATP synthesis, is expressed at high levels in the skeletal muscle, an important organ in glucose and lipid metabolism. Because several reports proposed that fatty acids induced UCP3 gene expression in skeletal muscle in vivo, in the present study we examined the regulation of UCP3 gene expression by various fatty acids using L6 myotubes. UCP3 gene expression was increased in L6 myotubes by various fatty acids or by α-bromopalmitate, a nonmetabolized derivative of palmitic acid. Because fatty acids are also known as agonists for PPARs, we examined the involvement of PPARs in the regulation of the UCP3 gene expression. L-165041, a PPARδ agonist, increased UCP3 gene expression in L6 myotubes, whereas neither Wy 14,643, a PPARα agonist, nor Pioglitazone, a PPARγ agonist, increased it. Therefore, we conclude that UCP3 gene expression is increased by the activation of PPARδ in L6 myotubes and postulate that PPARδ mediates at least some part of the increased UCP3 gene expression by fatty acids in skeletal muscle in vivo.