Metformin is widely used for the treatment of type 2 diabetes. Although it reduces hepatic glucose production, clinical studies show that metformin may reduce plasma dipeptidyl peptidase-4 activity and increase circulating levels of glucagon-like peptide 1 (GLP-1). We examined whether metformin exerts glucoregulatory actions via modulation of the incretin axis.

Metformin action was assessed in Glp1r ^−/−, Gipr ^−/−, Glp1r:Gipr ^−/−, Pparα (also known as Ppara)^−/− and hyperglycaemic obese wild-type mice with or without the GLP-1 receptor (GLP1R) antagonist exendin(9-39). Experimental endpoints included glucose tolerance, plasma insulin levels, gastric emptying and food intake. Incretin receptor expression was assessed in isolated islets from metformin-treated wild-type and Pparα ^−/− mice, and in INS-1 832/3 beta cells with or without peroxisome proliferator-activated receptor (PPAR)-α or AMP-activated protein kinase (AMPK) antagonists.

In wild-type mice, metformin acutely increased plasma levels of GLP-1, but not those of gastric inhibitory polypeptide or peptide YY; it also improved oral glucose tolerance and reduced gastric emptying. Metformin significantly improved oral glucose tolerance despite loss of incretin action in Glp1r ^−/−, Gipr ^−/− and Glp1r ^−/− :Gipr ^−/− mice, and in wild-type mice fed a high-fat diet and treated with exendin(9-39). Levels of mRNA transcripts for Glp1r, Gipr and Pparα were significantly increased in islets from metformin-treated mice. Metformin directly increased Glp1r expression in INS-1 beta cells via a PPAR-α-dependent, AMPK-independent mechanism. Metformin failed to induce incretin receptor gene expression in islets from Pparα ^−/− mice.

As metformin modulates multiple components of the incretin axis, and enhances expression of the Glp1r and related insulinotropic islet receptors through a mechanism requiring PPAR-α, metformin may be mechanistically well suited for combination with incretin-based therapies.