Alhagi honey polysaccharide ameliorates ulcerative colitis by modulating gut microbiota–tryptophan metabolism via AhR activation

Alhagi honey (AH) is produced in arid and hot areas of Central Asia, and its polysaccharides (AP) are widely known for their activity in the treatment of intestinal diseases such as diarrhea. However, the therapeutic potential and mechanism of AP in ulcerative colitis (UC) remain unclear. Here, AH polysaccharide-2 (AP2), a polysaccharide with the highest content in AP, was isolated and evaluated for its effects on dextran sulfate sodium (DSS)-induced UC in mice. AP2 was found to alleviate UC symptoms and regulate gut microbiota dysbiosis by decreasing Helicobacter levels and increasing Lactobacillus levels. Analysis of PICRUSt2 predicted that AP2 may regulate carbohydrate and amino acid metabolism, and metabolomic analysis confirmed that AP2 promotes the metabolism of tryptophan to produce kynurenic acid (kyna). Moreover, kyna acted as an aryl hydrocarbon receptor (AhR) ligand, which activated AhR to increase the expression of the tight junction proteins claudin-1 and occludin. Interestingly, AP2 showed similar effects in protecting the intestinal barrier and alleviating colitis as the AhR agonist 6-formylindolo[3,2-b]carbazole, and the AhR antagonist CH223191 partially blocked the therapeutic effect of AP2 in UC mice, indicating that the anti-UC effect of AP2 was AhR dependent. These findings demonstrate that AP2 alleviates UC by regulating the gut microbiota and promoting tryptophan metabolism to generate kyna-activated AhR. The insights gained from this study could help in the future development of AP2 as a drug candidate or functional food for the treatment of UC.