PPAR Agonists Amplify iNOS Expression While Inhibiting NF-κB

Abstract

Abstract. In acute inflammation, the transcription factor NF-B is activated and increases the expression of multiple proinflammatory genes. Agonists of peroxisome proliferator activated receptors (PPAR) have been reported to exert antiinflammatory effects in various systems. In keeping with such an antiinflammatory role, it was found that several PPAR agonists, including Wy14,643, clofibrate, carbaprostacyclin, and ciglitazone inhibited NF-B activity and increased IB␣ levels in cytokine-stimulated mesangial cells (MC). Activation of NF-B has been found to be crucial to the cytokine-elicited expression of inducible nitric oxide synthase (iNOS). Despite the inhibitory effect of PPAR agonists on NF-B activity, this study provides experimental data demonstrating that these agonists amplify cytokine-elicited NO generation in MC, potentiating iNOS protein expression approximately threefold. The upregulation of iNOS expression occurred at the mRNA level and apparently did not result from iNOS mRNA stabilization.Clofibrate and ciglitazone amplified the cytokine-elicited stimulation of a 16-Kb human iNOS promoter construct in stably transfected MC, suggesting that PPAR agonists potentiate iNOS induction through transcriptional mechanisms. MC express all three PPAR proteins. However, iNOS potentiation did not correlate with increased PPAR activity. In addition, Wy14,643-induced amplification of cytokine-elicited iNOS levels also occurred in RAW264.7 macrophages and in human epithelial Caco-2 and HT-29 cells. The observation that these epithelial cell lines express an inactive, truncated PPAR␣ variant suggests that a classical PPAR␣ agonist, such as Wy14,643, may act through PPAR␣-independent mechanisms. In conclusion, these results show that, despite reducing NF-B activity, PPAR agonists may amplify the expression of certain NF-B-dependent genes that are relevant to the inflammatory process, like iNOS.

PPARα agonists inhibit nitric oxide production by enhancing iNOS degradation in LPS‐treated macrophages

“…There are, however, only two previous reports on the effects of PPARa agonists on iNOS expression and NO production in macrophages showing contradictory results (Colville-Nash et al, 1998;Cernuda-Morolló n et al, 2002). Colville-Nash et al (1998) found that a selective PPARa ligand WY14643 reduced interferon-g and LPS-induced NO production in RAW 264.7 macrophages.…”

“…PPARg agonists were proposed to inhibit the action of inflammatory transcription factors nuclear factor kappa B (NF-kB), activator protein 1 and signal transducer and activator of transcription 1 (STAT1) (Ricote et al, 1998;Chen et al, 2003). The effects of PPARa agonists on NO production and iNOS expression in macrophages have been less studied (Colville-Nash et al, 1998;Cernuda-Morolló n et al, 2002). The results of the two studies were contradictory and the mechanisms of action were not investigated in detail.…”

Recognition of apoptotic cells by macrophages activates the peroxisome proliferator-activated receptor-γ and attenuates the oxidative burst

“…Again, this hypothesis will be tested in the future. The controversy about whether RAW264.7 macrophages indeed express PPARg has been settled by showing that these cells express PPARg mRNA 30 and protein 31 as well as its activation in response to LPS/IFNg. 32 Our data on EMSA analysis, reporter activity and elimination of PPARg effects by transduction of a d/n PPARg mutant corroborated these findings.…”

Identification of Novel Protein Targets for Modification by 15-Deoxy-Δ12,14-Prostaglandin J2 in Mesangial Cells Reveals Multiple Interactions with the Cytoskeleton

“…Rat MC were obtained as reported earlier (18). Cells were grown in RPMI 1640 supplemented with 10% FBS, 2 mM glutamine, 100 U/ml penicillin, and 100 g/ml streptomycin.…”