Novel mechanism whereby metformin improves glucose homeostasis: TXNIP-GLUT1 axis modulation enhances intestinal glucotonic effects.

Metformin is widely used as a first-line therapy for type 2 diabetes mellitus. However, the molecular mechanisms by which it modulates intestinal glucose metabolism remain incompletely defined. Here metformin was orally administered to male C57BL/6 mice, followed by intraperitoneal glucose tolerance testing and fluorine-18 fluorodeoxyglucose tracing to evaluate glucose homeostasis. To investigate changes in intestinal glucose metabolism, IEC6 and Caco-2 cell lines were used for in vitro analysis, with organoid experiments conducted for further validation. qRT-PCR, western blotting, flow cytometry and immunohistochemistry were performed to elucidate the effects of metformin on glucose metabolism pathways. Metformin enhanced glucose uptake and excretion in the distal intestine, particularly in the ileum and colon. Mechanistically, metformin upregulated the expression and membrane localization of glucose transporter 1 (GLUT1) by downregulating thioredoxin-interacting protein (TXNIP) expression. Consistently, intestinal-specific overexpression of TXNIP abolished metformin-induced improvements in glucose tolerance, while pharmacological inhibition of GLUT1 similarly negated metformin's glucose-lowering effects. Our findings identified intestinal glucose excretion, mediated through the intestinal TXNIP-GLUT1 regulatory axis, as a previously unrecognized contributor to metformin's glucoregulatory action. These results highlight a novel intestinal mechanism underlying metformin's efficacy and provide insights for potential therapeutic strategies beyond traditional glucose regulation.