Radix Aucklandiae inhibits M1 macrophage polarization to alleviate ulcerative colitis by modulating gut microbial production of indole-3-aldehyde.

Abstract

BACKGROUND: Radix Aucklandiae (RA) is a traditional Chinese medicine used to treat ulcerative colitis (UC). However, the underlying mechanism remains unclear. PURPOSE: This research sought to elucidate the mechanism of action of RA in UC in terms of the regulation of gut microbiota and M1 macrophage polarization. STUDY DESIGN: The effects of RA on UC were studied through in vivo experiments conducted on UC mice, complemented by in vitro studies using RAW264.7 cells. METHODS: 3 % dextran sulfate sodium was used to induce UC in mice and treated with RA. Histopathological examination and colonic mucosal inflammation were evaluated after treatment. Changes in M1 macrophages in the colonic tissues were determined using immunofluorescence staining. Gut microbiota in feces was detected using 16S rRNA sequencing analysis. A metabolomic assay was used to determine the levels of tryptophan metabolites in the feces. To evaluate the effect of indole-3-aldehyde (I3A), both UC mouse and RAW264.7 cell models were used. RT-PCR was used to quantify the mRNA levels of TNF-α, IL-6, IL-1β, and aryl hydrocarbon receptor (AHR). The binding between I3A and AHR was assessed using the surface plasmon resonance assay. RESULTS: RA improved ulcerative injury, decreased inflammatory cytokines levels in colonic tissues, and reduced M1 macrophages levels in mice. RA also regulated the composition of gut microbiota and the levels of tryptophan metabolites. RA and its primary components costunolide and dehydrocostuslactone, upregulated Lactobacillus reuteri, thereby promoting the production of tryptophan metabolite I3A. RA-elevated L. reuteri and I3A significantly reduced M1 macrophage level and alleviated UC in mice. Moreover, I3A directly inhibited M1 macrophage polarization and reduced TNF-α, IL-6, and IL-1β levels. I3A could strongly bind to AHR, thereby activating its signaling pathways and reducing the inflammatory responses in M1 macrophages. CONCLUSION: RA can treat UC through inhibiting the pro-inflammatory responses of M1 macrophages by increasing intestinal L. reuteri-mediated production of I3A.