Metformin-induced glucagon-like peptide-1 secretion contributes to the actions of metformin in type 2 diabetes

Abstract

Background: Metformin reduces plasma glucose and has been shown to increase glucagon-like peptide 1 (GLP-1) secretion. Whether this is a direct action of metformin on GLP-1 release, and whether some of the glucose-lowering effect of metformin occurs due to GLP-1 release, is unknown. The current study investigated metformin-induced GLP-1 secretion and its contribution to the overall glucose-lowering effect of metformin and underlying mechanisms in patients with type 2 diabetes.

Methods: Twelve patients with type 2 diabetes were included in this placebo-controlled, double-blinded study. On 4 separate days, the patients received metformin (1,500 mg) or placebo suspended in a liquid meal, with subsequent i.v. infusion of the GLP-1 receptor antagonist exendin9-39 (Ex9-39) or saline. During 240 minutes, blood was sampled. The direct effect of metformin on GLP-1 secretion was tested ex vivo in human ileal and colonic tissue with and without dorsomorphin-induced inhibiting of the AMPK activity.

Results: Metformin increased postprandial GLP-1 secretion compared with placebo (P = 0.014), and the postprandial glucose excursions were significantly smaller after metformin + saline compared with metformin + Ex9-39 (P = 0.004). Ex vivo metformin acutely increased GLP-1 secretion (colonic tissue, P < 0.01; ileal tissue, P < 0.05), but the effect was abolished by inhibition of AMPK activity.

Conclusions: Metformin has a direct and AMPK-dependent effect on GLP-1-secreting L cells and increases postprandial GLP-1 secretion, which seems to contribute to metformin's glucose-lowering effect and mode of action.

Trial registration: NCT02050074 (https://ichgcp.net/clinical-trials-registry/NCT02050074).

Funding: This study received grants from the A.P. Møller Foundation, the Novo Nordisk Foundation, the Danish Medical Association research grant, the Australian Research Council, the National Health and Medical Research Council, and Pfizer Inc.

Keywords: Diabetes; Endocrinology; Metabolism.

Conflict of interest statement

Conflict of interest: Funding was received by RLY and DJK from Pfizer Inc. through an Emerging Science Grant. AL and MJ were employees of Pfizer Inc. during this study.

Figures

Figure 1. Time courses and incremental AUCs (iAUC) of postprandial plasma concentrations of glucagon-like peptide 1 (GLP-1) and glucagon.

Time courses (0–240 minutes) and iAUCs of postprandial GLP-1 (A) and glucagon (B), after receiving a single dose of metformin (1,500 mg) or placebo with subsequent i.v. infusion of saline or exendin9-39 (Ex9-39), respectively, in 12 patients with type 2 diabetes. Data are means ± SEM. *P < 0.05, **P < 0.01. Comparisons performed by 1-way repeated measures ANOVA with Tukey’s multiple comparison post hoc test.

Figure 2. Time courses and incremental AUCs (iAUC) of postprandial plasma concentrations of glucose and the C-peptide/glucose ratio.

Time courses (0–240 minutes) and iAUCs of postprandial glucose (A) and C-peptide/glucose ratio (B), after receiving a single dose of metformin (1,500 mg) or placebo with subsequent i.v. infusion of saline or exendin9-39 (Ex9-39), respectively, in 12 patients with type 2 diabetes. Data are means ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001. Comparisons performed by 1-way repeated measures ANOVA with Tukey’s multiple comparison post hoc test.

Figure 3. Metformin-induced secretion in human L cells.

(A) Colonic epithelial preparations readily secrete glucagon-like peptide 1 (GLP-1) in response to high (70 mM) external K+ or to a combination of 3-isobutyl-1-methylxanthine (IBMX) and forskolin (FSK) (n = 22). (B and C) Metformin (10 μM) increases GLP-1 release after 15 minutes in epithelial tissue from human colon (n = 46) (B) and ileum (n = 10) (C). (D) Metformin-induced GLP-1 release is blocked by the AMP-activated protein kinase (AMPK) inhibitor dorsomorphin (n = 18). (E) Metformin-induced GLP-1 release is blocked by the serotonin transporter (SERT) inhibitor fluoxetine and the plasma membrane monoamine transporter (PMAT) lopinavir (n = 18). Bar graph data are means ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001 compared with respective control groups. Comparisons performed by paired 1-way ANOVA and paired Student’s t test.

Figure 4. CONSORT diagram of the clinical study.

The flow chart shows the number of patients who were screened, allocated to treatment, completed the study, and included in the final analysis. Ex9-39, exendin9-39.