Interplay between gut microbiota and tryptophan metabolism in type 2 diabetic mice treated with metformin

Tryptophan (TRP) metabolites have been identified as potent biomarkers for complications of type 2 diabetes mellitus (T2DM). However, it remains unclear whether the therapeutic effect of metformin in T2DM is related to the modulation of TRP metabolic pathway. This study aims to investigate whether metformin affects TRP metabolism in T2DM mice through the gut microbiota. A liquid chromatography-tandem mass spectrometry method was established to determine 16 TRP metabolites in the serum, colon content, urine, and feces of T2DM mice, and the correlations between metabolites and the T2DM mice gut microbiota were performed. The method demonstrated acceptable linearity (R-2 > 0.996), with the limit of quantification ranging from 0.29 to 69.444 nmol/L for 16 analytes, and the limit of detection ranging from 0.087 to 20.833 nmol/L. In T2DM mice, metformin treatment effectively restored levels of indole-3-lactic acid (ILA), indole-3-propionic acid (IPA), and the ILA/IPA ratio, along with several aryl hydrocarbon receptor ligands in the serum, with a notable impact in the colon but not in the urine. This restoration was accompanied by a shift in the relative abundance of Dubosiella, Turicibacter, RF39, Clostridia_UCG-014, and Alistipes. Spearman's correlation analysis revealed positive correlations between Turicibacter and Alistipes with IPA and indole-3-acetic acid. Conversely, these genera displayed negative correlations with ILA and kynurenine. In addition, our study revealed the presence of endogenous indole pathway in germ-free mice, and the impact of metformin on endogenous TRP metabolism in T2DM mice cannot be disregarded. Further research is needed to investigate the regulation of TRP metabolism by metformin.