Activation of bile salt nuclear receptor FXR is repressed by pro-inflammatory cytokines activating NF-κB signaling in the intestine

Hyperactivation of NF-kappa B is a key factor in the pathophysiology of inflammatory bowel disease (IBD). We previously showed that the bile salt nuclear Farnesoid X Receptor (FXR) counter-regulates intestinal inflammation, possibly via repression of NF-kappa B. Here, we examine whether mutual antagonism between NF-kappa B and FXR exists. FXR and its target genes IBABP and FGF15/19 expression were determined in HT29 colon carcinoma cells and ex vivo in intestinal specimens of wild type (WT) and Fxr-ko mice, treated with/without FXR ligands (GW4064/INT-747) and inflammatory stimuli (TNF alpha/IL-1 beta). In addition, EXR activation was studied in vivo in WT and Fxr-ko mice with DSS-colitis. The involvement of NF-kappa B in decreasing FXR activity was investigated by reporter assays and Glutathione S-transferase pulldown assays. FXR target gene expression was highly reduced by inflammatory stimuli in all model systems, while FXR mRNA expression was unaffected. In line with these results, reporter assays showed reduced FXR transcriptional activity upon TNF alpha/IL-1 beta stimulation. We show that this reduction in FXR activity is probably mediated by NF-kappa B, since overexpression of NF-kappa B subunits p50 and/or p65 also lead to inhibition of FXR activity. Finally, we report that p65 and p50 physically interact with FXR in vitro. Conclusions: Together, these results indicate that intestinal inflammation strongly reduces FXR activation, probably via NF-kappa B-dependent tethering of FXR. Therefore, FXR not only inhibits inflammation, but also is targeted by the inflammatory response itself. This could result in a vicious cycle where reduced FXR activity results in less repression of inflammation, contributing to development of chronic intestinal inflammation. This article is part of a Special Issue entitled: Translating nuclear receptors from health to disease. (C) 2011 Elsevier B.V. All rights reserved.