Functional relationship between MyoD and peroxisome proliferator-activated receptor-dependent regulatory pathways in the control of the human uncoupling protein-3 gene transcription.

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Uncoupling protein-3 (UCP3) gene is a member of the mitochondrial carrier superfamily preferentially expressed in skeletal muscle and up-regulated by fatty acids. Peroxisome proliferator-activated receptor (PPAR)alpha and PPARdelta (also known as PPARbeta) mediate human UCP3 gene regulation by fatty acids through a direct-repeat (DR-1) element in the promoter. DR-1 mutation renders UCP3 promoter unresponsive to PPAR ligand in vitro and consistently blocks gene induction by fatty acids in vivo. Although they act through separate sites in the promoter, MyoD and PPAR-dependent regulatory pathways are functionally connected: only in the presence of MyoD, does UCP3 become sensitive to PPAR ligand-dependent regulation. MyoD controls UCP3 promoter activity through a noncanonical Ebox site located in the proximal region, close to transcription initiation site. Moreover, acetylation processes play a crucial role in the control of UCP3 gene regulation. The coactivator p300 protein enhances PPAR ligand-mediated regulation whereas a mutant form devoid of histone acetylase activity blocks the response of the promoter to fatty acids. Conversely, histone deacetylase-1 blunts MyoD-dependent expression of the UCP3 promoter and reduces PPAR-dependent responsiveness. A mutated form of MyoD unable to be acetylated has a lower transactivation capacity on the human UCP3 promoter with respect to wild-type MyoD. It is concluded that MyoD and PPAR-dependent pathways mediate human UCP3 gene regulation and that acetylase activity elicited by coregulators is implicated in the functional interaction between these regulatory pathways. Therefore the convergence of MyoD and PPAR-dependent pathways provides a molecular mechanism for skeletal muscle specificity and fatty acid regulation of human UCP3 gene.