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

Abstract

The nuclear receptor peroxisome proliferator-activated receptor ␥ (PPAR␥) 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 PPAR␥1 and -␥2 isoforms. The A/B domain of human PPAR␥1 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-PPAR␥(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 PPAR␥2 was also augmented by mutation of the homologous serine in the A/B domain to alanine. The nonphosphorylatable form of PPAR␥ was also more adipogenic. Thus, phosphorylation of a mitogen-activated protein kinase site in the A/B region of PPAR␥ inhibits both ligand-independent and ligand-dependent transactivation functions. This observation provides a potential mechanism whereby transcriptional activation by PPAR␥ may be modulated by growth factor or cytokinestimulated signal transduction pathways involved in adipogenesis.

Haploid Inactivation of the Amplified-in-Breast Cancer 3 Coactivator Reduces the Inhibitory Effect of Peroxisome Proliferator-Activated Receptor γ and Retinoid X Receptor on Cell Proliferation and Accelerates Polyoma Middle-T Antigen-Induced Mammary Tumorigenesis in Mice

“…It is known that PyMT activates the ras pathway that stimulates mitogen-activated protein (MAP) kinase (58). Several studies have shown that MAP kinase directly phosphorylates PPAR␥, and the phosphorylated PPAR␥ has greatly reduced transcriptional activity and ability to promote cell differentiation (43,59,60). A combined effect of the MAP kinase activation by PyMT and AIB3 haplodeficiency on the reduction of PPAR␥ transcriptional activity may additively alter the function of PPAR␥ on cell differentiation and promote PyMT-induced mammary tumorigenesis.…”

PPARs and adipocyte function

“…Phosphorylation of PPAR 2 at serine residue 112 within the AF1 domain by growth factor (extra-cellular signal-regulated kinases 1/2) or stress-induced (p38/Jun NH 2 -terminal kinase 1/2) kinases represses PPAR transcriptional activity [34]. Indeed, over-expression of a phosphorylation defective PPAR mutant (in which serine 112 is mutated to alanine [PPAR S112A]) in mouse NIH-3T3 fibroblasts or 3T3-L1 pre-adipocytes results in enhanced adipogenesis compared with wild-type protein [12,35]. In vivo, homozygous PPAR S112A knock-in mice are protected from high-fat diet-induced insulin resistance and display decreased adipocyte size, elevated serum adiponectin, and reduced FFA levels [36].…”

Lessons from human mutations in PPARγ

“…24 However, the increased adiposity was postulated to have resulted from a gain-of-function, with increased receptor transactivation due to P115Q, inferred because phosphorylation of Ser114 reduced the ability of the receptor to mediate adipocyte differentiation and lipid accumulation. 25,26 Heterozygosity for either PPARg V290M or P467L was initially reported in nonlipodystrophic subjects with severe insulin resistance. 27 However, later re-evaluation of subjects with PPARg V290M or P467L showed loss of subcutaneous limb fat and atrophy of buttock fat, confirming that mutant PPARg causes lipodystrophy.…”