Phosphorylation of PPARγ via active ERK1/2 leads to its physical association with p65 and inhibition of NF‐κβ

Abstract

Peroxisome proliferator-activated receptors (PPAR) are novel nuclear receptors and PPARgamma ligands have been shown to produce pro-apoptotic effects in many cancer cell types, including colon cancer. PPARgamma ligands exert their effect through PPARgamma-dependent (genomic) and PPARgamma-independent (non-genomic) mechanisms. Recent evidence suggests that PPARgamma ligands exert their pro-apoptotic effects in part by directly antagonizing the NF-kappabeta pathway as well as through activation of the MAP kinase pathway. In this report, we have demonstrated that ciglitazone, a member of the thiazoldinedione class of PPARgamma ligands induces HT-29 colon cancer cells to undergo apoptosis and prior to apoptosis, ciglitazone exposure results in a transient phosphorylation of PPARgamma. This phosphorylation of PPARgamma was mediated through the ciglitazone-induced activation of Erk1/2. PPARgamma phosphorylation affected the genomic pathway by being inhibitory to PPARgamma-DNA binding and PPRE transcriptional activity, as well as the non-genomic pathway by increasing the physical interaction of PPARgamma with p65, leading to the inhibition of NF-kappabeta. Ciglitazone induced phosphorylation of PPARgamma through the MAP kinase pathway provides a potential regulatory mechanism for PPARgamma's physical interaction with p65, leading to inhibition of NF-kappabeta and subsequent apoptosis.

PPARγ ameliorated LPS induced inflammation of HEK cell line expressing both human Toll-like receptor 4 (TLR4) and MD2

“…This cell reporter system enabled us to examine this phenomenon without interfering undesired cellular components. Previous studies have indicated suppressive property of PPARc on NF-jB in various tissues (Sasaki et al 2005;Chen et al 2003;Remels et al 2009). One suggestive mechanism is inhibition of the IrB kinases (Sasaki et al 2005).…”

“…One suggestive mechanism is inhibition of the IrB kinases (Sasaki et al 2005). An alternative mechanism is phosphorylation of PPARc which physically interacts with p65 and subsequently decreases the transcriptional activity of NF-jB (Chen et al 2003). Therefore, further experiments are required to define which mechanism mediates ameliorative effects of PPARc on inflammation.…”

Peroxisome Proliferator‐Activated Receptors: Potential Therapeutic Targets in Lung Disease?

“…75,76 ERK-mediated PPARg phosphorylation is observed in response to ciglitazone, epidermal growth factor (EGF), and platelet-derived growth factor (PDGF). 40 The effect of this phosphorylation is twofold. First, it inhibits PPARg/RXR binding to DNA, and second, it increases the association of PPARg with NF-kB.…”

“…7,39 These gene-independent effects are mediated by direct binding of PPARs to transcription factors, including activator protein-1 (AP-1), pregnancyspecific-beta-1-glycoprotein (SP-1), nuclear factor-kappaB (NF-kB), and signal transducers and activators of transcription-1 (STAT-1). 6,7,25,[40][41][42][43] Binding to these transcription factors, in turn, prevents their activation and binding to DNA promoter elements. Additionally, PPARs can inhibit transcriptional activity by binding to and sequestering proteins that act as coactivators of these transcription factors (e.g., CREB, a coactivator of AP-1).…”

“…First, it inhibits PPARg/RXR binding to DNA, and second, it increases the association of PPARg with NF-kB. 40 In effect, this would lead to decreased expression of both proteins regulated through PPRE promoter elements and those regulated by NF-kB.…”