TI - Regulation of the transcriptional activity of the peroxisome proliferator-activated receptor alpha by phosphorylation of a ligand -independent trans-activating domain . AB - The peroxisome proliferator-activated receptors ( PPARs ) are a subgroup of nuclear receptors activated by fatty acids and eicosanoids . In addition , they are subject to phosphorylation by insulin , resulting in the activation of PPARalpha , while inhibiting PPARgamma under certain conditions . However , it was hitherto unclear whether the stimulatory effect of insulin on PPARalpha was direct and by which mechanism it occurs . We now demonstrate that amino acids 1-92 of hPPARalpha contain an activation function ( AF ) -1-like domain , which is further activated by insulin through a pathway involving the mitogen -activated protein kinases p42 and p44 . Further analysis of the amino - terminal region of PPARalpha revealed that the insulin-induced trans-activation occurs through the phosphorylation of two mitogen -activated protein kinase sites at positions 12 and 21 , both of which are conserved across evolution . The characterization of a strong AF-1 region in PPARalpha , stimulating transcription one-fourth as strongly as the viral protein VP16 , is compatible with the marked basal transcriptional activity of this isoform in transfection experiments . However , it is intriguing that the activity of this AF-1 region is modulated by the phosphorylation of two serine residues , both of which must be phosphorylated in order to activate transcription . This is in contrast to PPARgamma2 , which was previously shown to be phosphorylated at a single site in a motif that is not homologous to the sites now described in PPARalpha . Although the molecular details involved in the phosphorylation -dependent enhancement of the transcriptional activity of PPARalpha remain to be elucidated , we demonstrate that the effect of insulin on the AF-1 region of PPARalpha can be mimicked by the addition of triiodothyronine receptor beta1 , a strong binder of corepressor proteins . In addition , a triiodothyronine receptor beta1 mutant deficient in interacting with corepressors is unable to activate PPARalpha . These observations suggest that the AF-1 region of PPARalpha is partially silenced by corepressor proteins , which might interact in a phosphorylation -dependent manner .