Abstract
AIM: To examine how circulating glucagon-like peptide-1 (GLP-1) concentrations during liraglutide treatment relate to its therapeutic actions on glucose and weight, and to study the effects of liraglutide on other proglucagon-derived peptides (PGDPs), including endogenous GLP-1, glucagon-like peptide-2, glucagon, oxyntomodulin, glicentin and major proglucagon fragment, which also regulate metabolic and weight control. MATERIALS AND METHODS: Adults who were overweight/obese (body mass index 27-40 kg/m(2) ) with prediabetes were randomized to liraglutide (1.8 mg/day) versus placebo for 14 weeks. We used specific assays to measure exogenous (liraglutide, GLP-1 agonist [GLP-1A]) and endogenous (GLP-1E) GLP-1, alongside five other PGDP concentrations during a mixed meal tolerance test (MMTT) completed at baseline and at week 14 (liraglutide, n = 16; placebo, n = 19). Glucose during MMTT, steady-state plasma glucose (SSPG) concentration for insulin resistance and insulin secretion rate (ISR) were previously measured. MMTT area-under-the-curve (AUC) was calculated for ISR, glucose and levels of PGDPs. RESULTS: Participants on liraglutide versus placebo had significantly (P ≤ .004) decreased weight (mean -3.6%, 95% CI [-5.2% to -2.1%]), SSPG (-32% [-43% to -22%]) and glucose AUC (-7.0% [-11.5% to -2.5%]) and increased ISR AUC (30% [16% to 44%]). GLP-1A AUC at study end was significantly (P ≤ .04) linearly associated with % decrease in weight (r = -0.54) and SSPG (r = -0.59) and increase in ISR AUC (r = 0.51) in the liraglutide group. Treatment with liraglutide significantly (P ≤ .005) increased exogenous GLP-1A AUC (median 310 vs. 262 pg/mL × 8 hours at baseline but decreased endogenous GLP-1E AUC [13.1 vs. 24.2 pmol/L × 8 hours at baseline]), as well as the five other PGDPs. Decreases in the PGDPs processed in the intestines are independent of weight loss, indicating a probable direct effect of GLP-1 receptor agonists to decrease their endogenous production in contrast to weight loss-dependent changes in glucagon and major proglucagon fragment that are processed in pancreatic alpha cells. CONCLUSIONS: Circulating GLP-1A concentrations, reflecting liraglutide levels, predict improvement in weight, insulin action and secretion in a linear manner. Importantly, liraglutide also downregulates other PGDPs, normalization of the levels of which may provide additional metabolic and weight loss benefits in the future.