Regulation of the transcriptional activity of the peroxisome proliferator-activated receptor by phosphorylation of a ligand-independent trans-activating domain

Abstract

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 PPARα, while inhibiting PPARγ under certain conditions. However, it was hitherto unclear whether the stimulatory effect of insulin on PPARα was direct and by which mechanism it occurs. We now demonstrate that amino acids 1-92 of hPPARα 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-terminaI region of PPARα 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 PPARα, 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 PPARγ2, which was previously shown to be phosphorylated at a single site in a motif that is not homologous to the sites now described in PPARα. Although the molecular details involved in the phosphorylation- dependent enhancement of the transcriptional activity of PPARα remain to be elucidated, we demonstrate that the effect of insulin on the AF-1 region of PPARα can be mimicked by the addition of triiodothyronine receptor β1, a strong binder of corepressor proteins. In additions a triiodothyronine receptor β1 mutant deficient in interacting with corepressors is unable to activate pPARα. These observations suggest that the AF-1 region of PPARα is partially silenced by corepressor proteins, which might interact in a phosphorylation-dependent manner.