Abstract
Abstract Background Inflammatory Bowel Diseases (IBD) lack therapeutic options capable of inducing long-term remission in a cost-efficient manner. Repurposing strategies have allowed already established receptors and ligands to be tested as alternative pharmacotherapy for treatment of IBD. Agonists of the aryl hydrocarbon receptor (AhR), a ligand-dependent transcription factor traditionally associated with xenobiotic toxicity, have been explored as potential anti-inflammatory drugs to treat IBD, but specific mechanisms have yet to be elucidated. Like AhR, the peroxisome proliferator-activated receptor gamma (PPARg), responsible for control of lipid metabolism, has been evidenced as a key player in control of inflammation. In this study, we have explored a hypothetical crosstalk between PPARg and AhR in attenuating intestinal inflammation. Methods Bioinformatics analyses of a GEO RNAseq dataset of gut macrophages from IBD patients were used to assess correlations between PPARg and AhR. In vitro, macrophage-differentiated THP-1 cells and Caco-2 gut epithelial cells were seeded in Transwell® inserts to generate a gut-like co-culture model. Cells were treated with 100 ng/mL lipopolysaccharide (LPS) for 24 h in order to mimic a phenotype of intestinal inflammation, and also with combinations of AhR/PPARg agonists and antagonists at concentrations of 10 µM each. Proprietary AhR agonists were also tested. AhR translocation and cytokine secretion in macrophages as well as monolayer integrity and expression of tight junction proteins (e.g., claudin-2) in epithelial cells were assessed. Results Principal component analysis and Pearson correlation tests of the RNAseq dataset revealed that PPARg and AhR correlate with each other during intestinal homeostasis, but their expression patterns diverge during gut inflammation. In our co-culture model, blocking PPARg in THP-1 macrophages with a specific antagonist (GW9662) prevented AhR agonist 6-formylindolo[3,2-b]carbazole (FICZ) from inducing AhR nuclear translocation and secreting interleukin (IL)-6. In Caco-2 gut epithelial cells, GW9662 blocked the anti-inflammatory effects of FICZ on monolayer disruption and claudin-2 expression. These effects of FICZ were confirmed to be AhR-dependent, as they were prevented under treatment with the AhR antagonist CH223191. Proprietary AhR ligands also inhibited IL-6 secretion, which was shown to be the most dominant parameter in terms of converging anti-inflammatory effects of both PPARg and AhR. Conclusion Our data points to a pivotal cross-talk between AhR and PPARg, where PPARg is likely recruited downstream of AhR activation and contributes to the anti-inflammatory effects of FICZ, especially inhibition of IL-6 secretion.
Published Version
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