Opposite Action of Peroxisome Proliferator-activated Receptor-γ in Regulating Renal Inflammation FUNCTIONAL SWITCH BY ITS LIGAND

Peroxisome proliferator-activated receptor-gamma (PPAR gamma) agonists, a new class of antidiabetic agents, have been shown to possess antiinflammatory activity. In this study, we investigated the molecular mechanism by which PPAR gamma agonists inhibit proinflammatory cytokine expression in rat glomerular mesangial cells. Both natural and synthetic PPAR gamma agonists potently inhibited RANTES (regulated upon activation, normal T cell expressed and secreted) and monocyte chemoattractant protein- 1 expression induced by TNF-alpha in mesangial cells, which was dependent on NF-kappa B signaling. However, PPAR gamma agonists had little effect on TNF-alpha-triggered I kappa B alpha phosphorylation and its subsequent degradation, p65 phosphorylation, and nuclear translocation. In the absence of PPAR gamma ligand, TNF-alpha induced a physical interaction between nuclear p65 and PPAR gamma, as demonstrated by co-immunoprecipitation. Such an interaction was mediated by the C-terminal region of p65. Activation of PPAR gamma by its agonist prevented PPAR gamma p65 complex formation. Chromatin immunoprecipitation assay revealed that TNF-alpha induced p65 binding to the cis-acting kappa B elements in rat RANTES promoter, whereas disruption of PPAR gamma p65 by its agonist blocked p65 interaction with its cognate kappa B sites. Knockdown of PPAR gamma via siRNA strategy completely abolished TNF-alpha-mediated p65 binding to kappa B sites and negated RANTES induction, suggesting that unliganded PPAR gamma is obligatory for NF-kappa B signaling. Consistently, overexpression of PPAR gamma in the absence of its ligand sensitized mesangial cells to TNF-alpha stimulation. These results uncover a paradoxical action of the unliganded and ligand-activated PPAR gamma in regulating NF-kappa B signaling and demonstrate PPAR gamma ligand as a molecular switch that controls its ability to modulate inflammatory responses in opposite directions.