Evaluation of the effects of metformin on gut functions and microbiota and their contribution to improving glucose tolerance in diabetic mice

Although the mechanism of action of the antidiabetic drug metformin is still a matter of discussions, increasing evidence points to a pivotal role of the gut. Aiming to clarify whether metformin-induced changes in the intestinal tract directly contribute to metabolic improvement, we evaluated the effects of escalating doses (from 50 to 200 mg/kg/day) of metformin orally administered for 4 weeks in mice made glucose intolerant by ten weeks of high fat high sucrose diet. Several intestinal parameters were studied, including caecal microbiota composition and bile acids profile, ileal FXR signaling, abundance of GLP1-producing cells and goblet cells and blood metabolome. Metformin restored glucose tolerance, fasting insulinemia and HOMA-IR index in a dose-dependent manner. Only a subset of gut-related effects, including mucus production and GLP-1 expression, exhibited a parallel dose-response relationship, suggesting a possible contribution to the observed metabolic improvements. In contrast, other changes, including ileal Fxr-Fgf15 inhibition and hepatic ceramide reduction did not scale with dose, suggesting are they are not the main drivers of metformin dose-dependent effects on glycemic control. We also pointed out marked differential sensitivity of gut bacteria to metformin supporting complex interactions of the drug with the microbial ecosystem. Finally, metformin enhanced the proliferation of intestinal epithelium, resulting in increased length of ileal villi. Altogether, this study offers new insights into the metformin mechanism of action and revealed potential novel microbial biomarkers and targets for enhancing its therapeutic efficacy. • Oral administration of metformin restored glucose tolerance and fasting insulinemia in a dose-dependent manner in glucose intolerant mice. • Similar dose-response effect was observed for metformin-induced mucus production, endocrine differentiation and GLP-1 expression in the colon , and for the abundance of several bacterial taxa in the caecum. • In contrast, ileal Fxr-Fgf15 inhibition and hepatic ceramide reduction did not scale with dose, suggesting are they are not the main drivers of metformin dose-dependent effects on glycemic control. • The sensitivity to metformin of gut bacteria was higly variable supporting complex interactions of the drug with the microbial ecosystem