Boscolo, Federica

Hyperglycemia in prediabetes is accompanied by increased amino acid (AA) concentrations and impaired suppression of glucagon secretion by glucose. It is unknown whether an abnormal response to AA contributes or hepatic steatosis exacerbates α-cell dysfunction. Therefore, lean participants and participants with obesity (with and without hepatic steatosis) were studied on two occasions in random order using a graded glucose infusion. Saline or an AA mixture was infused. The glucagon secretion rate (GSR) was derived from peripheral glucagon concentrations, and the change in glucose required to reduce GSR by 50% (G50) was calculated for each participant. AAs were measured during the experiment. On both occasions, experimental conditions produced a progressive rise in insulin secretion and an inverse exponential suppression of glucagon. G50 did not differ between groups in response to glucose alone. AA infusion did not change G50 in lean participants. G50 increased during AA infusion in participants with obesity with and without hepatic steatosis, leading us to report these participants as a single group. The relative change in AA concentrations across study days was generally similar between groups. We conclude that despite a normal response to glucose, obesity increases α-cell responses to AA in humans. Article Highlights The aim of this study was to understand the pathogenesis of α-cell dysfunction in prediabetes. The primary hypothesis was that α-cell responsivity to glucose (reduction of the glucagon secretion rate by 50%) is impaired by amino acid (AA) infusion. A secondary hypothesis was that this abnormal response is exacerbated by hepatic steatosis. AA infusion impaired α-cell suppression in response to rising glucose in participants with obesity, but not in lean participants, despite a normal response to glucose alone. This abnormal response was unaffected by the presence or absence of hepatic steatosis. It seems likely that an abnormal α-cell response to AAs precedes an abnormal response to glucose alone.

OBJECTIVE Genetic variation in the glucagon-like peptide 1 receptor (GLP1R) has been implicated in type 2 diabetes (T2D) risk and treatment response. We investigated whether GLP1R R131Q, which was previously reported as a T2D-protective variant in genome-wide association studies, alters receptor signaling, β-cell function, and response to glucagon-like peptide 1 receptor agonists (GLP-1RAs). RESEARCH DESIGN AND METHODS Trajectory of β-cell function was evaluated in a 20-year community-based prospective cohort in Korea (n = 6,373), where participants underwent biennial 2-h 75-g oral glucose tolerance tests, and disposition index was used as surrogate marker. Pharmacogenetic response to GLP-1RAs was assessed in a hospital-based T2D cohort in Korea (n = 177). Hyperglycemic clamp studies (n = 17), human islet experiments (n = 21), and in vitro assays were conducted to characterize GLP1R R131Q effects on receptor signaling and islet function. RESULTS GLP1R R131Q was associated with slower decline in disposition index in individuals without T2D, with reductions from baseline of 30% in homozygous carriers, 35% in heterozygotes, and 37% in wild-type individuals. In people with T2D, each allele conferred an additional 0.53% or 5.8 mmol/mol reduction in HbA1c after 6 months of GLP-1RA treatment (P = 5.8 × 10−4). Hyperglycemic clamp and human islet experiments demonstrated allele-dependent enhancement of GLP-1RA–stimulated insulin secretion (P = 0.050 and P = 0.037, respectively). In vitro, GLP1R R131Q increased GLP-1RA–stimulated cAMP production with directional observations consistent with pathway bias. CONCLUSIONS GLP1R R131Q is a gain-of-function variant associated with preservation of β-cell function and enhanced glycemic response to GLP-1RA treatment, supporting further investigation as a candidate pharmacogenetic marker for precision diabetes care.

In this study, we propose two new mathematical models able to quantify glucagon secretion during a graded glucose infusion, in the presence and absence of amino acids. The models provide an index of α-cell responsivity, G 50 , i.e., the glucose increase required to suppress glucose secretion rate (GSR) by 50%. Results show that the presence of amino acids reduced α-cell responsivity to glucose.

In this study, we propose a new mathematical model able to quantify C-peptide secretion and β-cell responsivity in response to a glucagon bolus in individuals without and with type 2 diabetes. We show that individuals with type 2 diabetes exhibit reduced β-cell responsivity to glucagon.