Transcriptional Activation by Peroxisome Proliferator-activated Receptor γ Is Inhibited by Phosphorylation at a Consensus Mitogen-activated Protein Kinase Site

The nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) regulates transcription in response to prostanoid and thiazolidinedione ligands and promotes adipocyte differentiation. The amino-terminal A/B domain of this receptor contains a consensus mitogen-activated protein kinase site in a region common to PPARγ1 and -γ2 isoforms. The A/B domain of human PPARγ1 was phosphorylated in vivo, and this was abolished either by mutation of serine 84 to alanine (S84A) or coexpression of a phosphoprotein phosphatase. In vitro, this domain was phosphorylated by ERK2 and JNK, and this was markedly reduced in the S84A mutant. A wild type Gal4-PPARγ(A/B) chimera exhibited weak constitutive transcriptional activity. Remarkably, this was significantly enhanced in the S84A mutant fusion. Ligand-dependent activation by full-length mouse PPARγ2 was also augmented by mutation of the homologous serine in the A/B domain to alanine. The nonphosphorylatable form of PPARγ was also more adipogenic. Thus, phosphorylation of a mitogen-activated protein kinase site in the A/B region of PPARγ inhibits both ligand-independent and ligand-dependent transactivation functions. This observation provides a potential mechanism whereby transcriptional activation by PPARγ may be modulated by growth factor or cytokine-stimulated signal transduction pathways involved in adipogenesis. The nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) regulates transcription in response to prostanoid and thiazolidinedione ligands and promotes adipocyte differentiation. The amino-terminal A/B domain of this receptor contains a consensus mitogen-activated protein kinase site in a region common to PPARγ1 and -γ2 isoforms. The A/B domain of human PPARγ1 was phosphorylated in vivo, and this was abolished either by mutation of serine 84 to alanine (S84A) or coexpression of a phosphoprotein phosphatase. In vitro, this domain was phosphorylated by ERK2 and JNK, and this was markedly reduced in the S84A mutant. A wild type Gal4-PPARγ(A/B) chimera exhibited weak constitutive transcriptional activity. Remarkably, this was significantly enhanced in the S84A mutant fusion. Ligand-dependent activation by full-length mouse PPARγ2 was also augmented by mutation of the homologous serine in the A/B domain to alanine. The nonphosphorylatable form of PPARγ was also more adipogenic. Thus, phosphorylation of a mitogen-activated protein kinase site in the A/B region of PPARγ inhibits both ligand-independent and ligand-dependent transactivation functions. This observation provides a potential mechanism whereby transcriptional activation by PPARγ may be modulated by growth factor or cytokine-stimulated signal transduction pathways involved in adipogenesis.