Attenuation of colonic inflammation by PPARγ in intestinal epithelial cells:: Effect on Toll-like receptor pathway

The peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor highly expressed in the colon and playing an anti-inflammatory role through inhibition of the NF-kappa B pathway. Toll-like receptor 4 (TLR4) has been known to mediate LPS-induced cellular signaling through activation of NF-kappa B pathway in intestinal epithelial cells. The aims of this study were to evaluate attenuation of inflammation by PPAR gamma in intestinal epithelial cells and to study the possible relation between PPAR gamma and TLR4. HT-29 human epithelial cells were stimulated with LPS (20 mu g/ml) and PPAR gamma ligand, 15d-PGJ(2) (10 mu M), or with LPS (20 mu g/ml) alone for 24 hr. COX-2, IL-8, TLR4, and PPAR gamma mRNA expression was assessed by RT-PCR. IL-8 protein levels and TLR4 protein expression were analyzed by ELISA and Western blot, respectively. To evaluate the action mechanisms of PPAR gamma ligand, Western blot analysis for I kappa B alpha degradation was performed. Costimulation with LPS and PPAR gamma ligand in comparison to LPS stimulation alone (1) decreased COX-2, IL-8 mRNA expression and IL-8 protein secretion, (2) decreased TLR4 mRNA and protein expression, and (3) decreased PPAR gamma mRNA expression. PPAR gamma ligand delayed LPS-induced I kappa B alpha degradation. These findings suggest that PPAR-gamma ligands suppress inflammation in intestinal epithelial cells. PPAR gamma and TLR, these two antagonistic signaling pathways in intestinal epithelial cells may be partially cross-linked.