PPAR Agonists Amplify iNOS Expression While Inhibiting NF-κB

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Abstract
In acute inflammation, the transcription factor NF-κB is activated and increases the expression of multiple pro-inflammatory 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-κB activity and increased IκBα levels in cytokine-stimulated mesangial cells (MC). Activation of NF-κB 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-κB 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 PPARα variant suggests that a classical PPARα agonist, such as Wy14,643, may act through PPARα-independent mechanisms. In conclusion, these results show that, despite reducing NF-κB activity, PPAR agonists may amplify the expression of certain NF-κB–dependent genes that are relevant to the inflammatory process, like iNOS.

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