Activation of Retinoic X Receptor and Peroxisome Proliferator-Activated Receptor–γ Inhibits Nitric Oxide and Tumor Necrosis Factor–α Production in Rat Kupffer Cells

Abstract

Activators of peroxisome proliferator–activated receptor γ (PPARγ), which forms a heterodimer with retinoic X receptor (RXR), inhibit the production of certain inflammatory mediators. To clarify the role of the PPARγ:RXR signaling pathway in Kupffer cells, we studied the effect of an RXR agonist and PPARγ agonist on LPS–induced nitric oxide (NO) and tumor necrosis factor–α (TNF–α) production. An RXR–specific agonist, Ro47–5944, and a PPARγ–specific agonist, AD4833 (pioglitazone hydrochloride), each inhibited LPS–induced NO and TNF–α production. The combined treatment of Ro47–5944 and AD4833 resulted in enhanced inhibition, and suppressed the mRNA levels of NO and TNF–α. PPARγ:RXR activation did not affect the level of LPS–induced phosphorylation of c–jun N–terminal kinase and p38 mitogen–activated protein kinase. PPARγ:RXR activation also did not affect nuclear factor kappa B (NF–κB) nuclear translocation nor NF–κB and activator protein 1 (AP–1) activation in the electrophoretic mobility–shift assay. Finally, PPARγ:RXR activation suppressed the LPS–induced promoter activity of the NF–κB–luciferase reporter gene in RAW 264.7 cells. These data imply that PPARγ:RXR activation suppresses LPS–induced NO and TNF–α production in Kupffer cells, and that this inhibition occurred at the transcriptional level. Although no consensus PPARγ:RXR–responsive element in the promoter regions of the inducible isoform of nitric oxide synthase (iNOS) and TNF–α genes was found, PPARγ:RXR may interfere with NF–κB and AP–1 transcriptional activity. Our data also suggest a potential therapeutic approach for moderating hepatic injury such as endotoxin shock in which Kupffer cell activation has been implicated.