Gut Microbial Metabolite Indole-3-Acetic Acid Exerts Protective Effects by Regulating Orbital Fibroblast Fibrosis in Thyroid Eye Disease

Purpose: To profile the gut microbiota and plasma metabolites of patients with thyroid eye disease (TED) and to investigate the potential regulatory role and mechanisms of the tryptophan metabolite indole-3-acetic acid (IAA) in this disorder. Methods: The clinical study enrolled 70 patients with TED and 76 controls. Fecal and plasma samples were collected for 16S rRNA sequencing and metabolomic analyses, respectively. For the in vitro study, orbital fibroblasts (OFs) were treated with transforming growth factor beta 1 (TGF-β1) to establish a fibrosis model and with adipogenic differentiation medium to establish an adipogenic differentiation model. The effects of IAA on cell viability, migration, fibrosis, and adipogenic differentiation were assessed. The molecular mechanism by which IAA regulates OF fibrosis was explored using transcriptome sequencing and in vitro experiments. Results: Compared with controls, patients with TED exhibited gut microbiota dysbiosis and disrupted plasma tryptophan metabolism. Moreover, IAA levels were significantly reduced and were negatively correlated with proptosis and serum thyrotropin receptor antibody (TRAb) levels. For the in vitro study, IAA inhibited OF migration, fibrotic phenotype, and adipogenic differentiation. Regarding its anti-fibrotic effect, IAA activated aryl hydrocarbon receptor (AHR) signaling and significantly reduced TGF-β1-induced phosphorylation of Smad2/3 proteins. Administration of the AHR antagonist CH-223191 attenuated the activation of AHR signaling in OFs and reduced the inhibitory effect of IAA on TGF-β1-induced Smad2/3 phosphorylation and fibrotic protein expression. Conclusions: Patients with TED exhibit gut microbiota dysbiosis and disrupted plasma tryptophan metabolism. The tryptophan metabolite IAA alleviates OF fibrosis via AHR signaling, suggesting a potential protective role in treating TED.