Regulation of Peroxisome Proliferator-activated Receptor γ Activity by Mitogen-activated Protein Kinase

Abstract

Adipocyte differentiation is regulated both positively and negatively by external growth factors such as insulin, platelet-derived growth factor (PDGF), and epidermal growth factor (EGF). A key component of the adipocyte differentiation process is PPAR␥, peroxisomal proliferator-activated receptor ␥. To determine the relationship between PPAR␥ activation and growth factor stimulation in adipogenesis, we investigated the effects of PDGF and EGF on PPAR␥1 activity. PDGF treatment decreased ligand-activated PPAR␥1 transcriptional activity in a transient reporter assay. In vivo [ 32 P]orthophosphate labeling experiments demonstrated that PPAR␥1 is a phosphoprotein that undergoes EGF-stimulated MEK/mitogen-activated protein (MAP) kinase-dependent phosphorylation. Purified PPAR␥1 protein was phosphorylated in vitro by recombinant activated MAP kinase. Examination of the PPAR␥1 sequence revealed a single MAP kinase consensus recognition site at Ser 82 . Mutation of Ser 82 to Ala inhibited both in vitro and in vivo phosphorylation and growth factor-mediated transcriptional repression. Therefore, phosphorylation of PPAR␥1 by MAP kinase contributes to the reduction of PPAR␥1 transcriptional activity by growth factor treatment.Peroxisome proliferator-activated receptors (PPARs) 1 are members of the nuclear hormone receptor superfamily (1). These receptors heterodimerize with retinoic acid-like receptor, RXR, and become transcriptionally active when bound to ligand. The three PPAR isoforms (␣, ␦, and ␥) differ in their C-terminal ligand binding domains, and each appears to bind and respond to a specific subset of agents including hypolipidemic drugs, long chain fatty acids, aracadonic acid metabolites, and antidiabetic thiazolidinediones (2-4). PPAR␥ is expressed predominantly in mouse white and brown fat, with lower levels in liver, whereas PPAR␣ is present in heart, kidney, and liver (5, 6). PPAR␦ expression is ubiquitous (7,8).Ectopic expression of either PPAR␣ or PPAR␥ in NIH-3T3 cells is sufficient to induce adipocyte differentiation in the presence of PPAR␥ activators (9, 10). The rapid induction of PPAR␥ during adipocyte differentiation and its enriched expression in adipose tissues suggest that PPAR␥ is responsible for the initiation and maintenance of the adipocyte phenotype in vivo (9). Previously two isotypes of PPAR␥ (PPAR␥1 and PPAR␥2) have been identified in 3T3-L1 adipocytes (11). Zhu et al. (12) have demonstrated that these two isotypes are derived from a single PPAR␥ gene by alternative promoter usage and RNA splicing. However, thus far, no functional difference has been found between the two isotypes.Adipogenesis is a complex process; multiple hormones and factors regulate the conversion of progenitor cells to adipocytes. Insulin and/or insulin-like growth factor enhance the ability of PPAR ligand to induce differentiation of both 3T3-L1-and PPAR␥-overexpressing cell lines (9, 13). In contrast, growth factors such as epidermal growth factor (EGF), platelet-derived growth factor (PDGF), and fibroblast gro...

15-Lipoxygenase-1 Metabolites Down-regulate Peroxisome Proliferator-activated Receptor γ via the MAPK Signaling Pathway

“…The addition of exogenous 13-(S)-HODE, at lower concentrations than used to observe binding to PPAR␥, up-regulates both the EGF-and serum-dependent MAPK pathway and subsequently PPAR␥ phosphorylation. The addition of a MEK inhibitor results in the inhibition of MAPK activity and subsequently PPAR␥ phosphorylation, which supports the conclusion that PPAR␥ phosphorylation is mediated by MAPK as reported in fat cells (29). This phenomenon is specific for linoleic acid metabolites as 13-(S)-HODE, 13-(R)-HODE, and 13-(S)-HpODE all have the same effect, while 15-(S)-HETE, an arachidonic acid metabolite, was inactive and in fact appeared to inhibit phosphorylation.…”

Transcriptional Activity of Peroxisome Proliferator-activated Receptor γ Is Modulated by SUMO-1 Modification

“…It has been reported that phosphorylation of Ser-112, adjacent to the sumoylation site, as revealed by this work, on PPAR␥ by mitogen-activated protein kinase significantly inhibited both ligand-independent and ligand-dependent transcriptional activation by PPAR␥ (15). Mutation analysis of the phosphorylation site revealed that this phosphorylation-mediated transcriptional repression was not due to a reduced capacity to make PPAR␥⅐RXR␣ complexes or the impairment of recognition of its DNA binding site (14). An AF-1 domain of PPAR␥ may be negatively regulated by phosphorylation and sumoylation.…”

Adipogenesis

“…PPARg can also be SUMOylated in a ligand-dependent manner in the LBD at Lys-365 (Pascual et al 2005). Another regulatory covalent modification of PPARg is phosphorylation at Ser-112 by mitogen-activated protein kinases (MAPKs) such as p44/p42 (ERKs 1 and 2) and c-Jun amino-terminal kinase (JNK), which results in transcriptional inactivation of PPARg (Camp and Tafuri 1997). Although it is widely accepted that phosphorylation of PPARg2 inhibits its activity, a point mutation at Ser-112 does not disrupt the ability of PPARg2 to confer adipogenesis (Rangwala et al 2003).…”