Farnesoid X Receptor activation protects against intestinal inflammation: potential mechanisms and therapeutic implications

Abstract

Inflammatory bowel disease (IBD) is a chronic intestinal inflammatory disorder, characterized by dysregulation of the mucosal immune system and compromised intestinal epithelial barrier. In IBD the immunological balance between pro-inflammatory and anti-inflammatory mediators is severely impaired and shifted towards the pro-inflammatory side. The nuclear transcription factor kappa B is a key regulator in this shift. The bile salt nuclear Farnesoid X Receptor (FXR) is a member of the nuclear receptors (NRs) superfamily, mainly expressed in the ileum and liver. Once activated by bile salts, it regulates transcription of genes involved in bile salt homeostasis. Moreover it is also implicated in liver regeneration, carcinogenesis, energy expenditure, as well as lipid and glucose homeostasis. In this thesis, we aimed to explore the role of FXR in maintaining a healthy gut and the potential therapeutic implications of FXR pharmacological activation for IBD. Functional cross-talk between NF-?B and several NRs exists. NRs were shown to have anti-inflammatory properties, by interfering with NF-?B function and activated NF-?B is described to inhibit several NRs activity. We have recently found that pharmacological FXR activation decreases the severity of inflammation and preserves the intestinal barrier integrity in two murine colitis models. Moreover, it decreases mRNA expression of pro-inflammatory genes and increases expression of antibacterial defense genes. Moreover FXR activation inhibits inflammatory signalling in various primary human immune cell types and in lamina propria mononuclear cells from IBD patients. In another study we show that there is mutual crosstalk between FXR and NF-?B in different model systems. Firstly, the pro-inflammatory cytokine TNF? decreases FXR target gene expression in enterocyte-like cells. This finding was confirmed in ileal specimens of WT mice cultured ex vivo, and in vivo in mice with severe intestinal inflammation. Finally, ileal expression of the FXR target gene SHP is 50% lower in patients with one of the IBD phenotype, Crohn’s disease. Taken together, these findings show that FXR is not only an active player in inhibition of inflammation, but also is a target of the inflammatory response itself. The classical mechanism by which FXR regulates gene transcription is via transactivation. Beside that, we provide evidence for a novel FXR role in transrepressing NF-?B signaling. We started dissecting the mechanisms for classical transactivation and transrepression of FXR, by identification and characterization of a novel phosphorylation site on FXR-S224. Disruption of this site revealed that it is essential for FXR target gene transactivation, but not for transrepression of NF-?B signalling. These findings potentially advance drug design for FXR, since selective FXR ligands which do not result in S224 phosphorylation may be useful to treat hepatic/intestinal inflammation without interfering with metabolism, finally reducing potential side effects. The results described in this thesis indicate that FXR is an important player in the counter-regulation of intestinal inflammation. Although the exact mechanism is still unclear, there are strong indications that modulation of FXR expression/activity may be transferred from bench to bedside, by using it as a drug target for intestinal inflammatory disorders.