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.
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