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

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 PPAR alpha, while inhibiting PPAR gamma under certain conditions. However, it was hitherto unclear whether the stimulatory effect of insulin on PPAR alpha was direct and by which mechanism it occurs. We now demonstrate that amino acids 1-92 of hPPAR alpha 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 PPAR alpha 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 PPAR alpha, 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 PPAR gamma 2, which was previously shown to be phosphorylated at a single site in a motif that is not homologous to the sites now described in PPAR alpha. Although the molecular details involved in the phosphorylation-dependent enhancement of the transcriptional activity of PPAR alpha remain to be elucidated, we demonstrate that the effect of insulin on the AF-1 region of PPAR alpha can be mimicked by the addition of triiodothyronine receptor beta 1, a strong binder of corepressor proteins. In addition, a triiodothyronine receptor beta 1 mutant deficient in interacting with corepressors is unable to activate PPAR alpha, These observations suggest that the AF-1 region of PPAR alpha is partially silenced by corepressor proteins, which might interact in a phosphorylation-dependent manner.