Sweet taste receptor signaling in beta cells mediates fructose-induced potentiation of glucose-stimulated insulin secretion

Abstract

Postprandial insulin release is regulated by glucose, but other circulating nutrients may target beta cells and potentiate glucose-stimulated insulin secretion via distinct signaling pathways. We demonstrate that fructose activates sweet taste receptors (TRs) on beta cells and synergizes with glucose to amplify insulin release in human and mouse islets. Genetic ablation of the sweet TR protein T1R2 obliterates fructose-induced insulin release and its potentiating effects on glucose-stimulated insulin secretion in vitro and in vivo. TR signaling in beta cells is triggered, at least in part, in parallel with the glucose metabolic pathway and leads to increases in intracellular calcium that are dependent on the activation of phospholipase C (PLC) and transient receptor potential cation channel, subfamily M, member 5 (TRPM5). Our results unveil a pathway for the regulation of insulin release by postprandial nutrients that involves beta cell sweet TR signaling.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Ablation of sweet TR protein, T1R2, obliterates fructose-induced calcium responses and insulin release in mouse islets. (A) Static insulin release at different glucose (“G”; in mM) concentrations with or without fructose (“F”; in mM) in WT islets (quadruplicate batches of 10 islets from n = 8 mice). (**P < 0.01, paired Student t test.) (B–E) Representative single-cell traces of calcium responses in dispersed primary beta cells from WT and T1R2−/− mice treated with glucose (“G”; in mM) and combinations of fructose (“F”; in mM), and tolbutamide as shown. (F) Area under the curve (AUC) calculated by using 10 to 20 single-cell calcium traces per mouse islet isolation (n = 6 per genotype). (***P < 0.001, ANOVA with Tukey posttest.) (G) Static insulin release in response to increasing concentrations of fructose (“F”; in mM) in the presence of glucose (8.3 mM) in islets from WT and T1R2−/− mice (n = 6 per group). Data are expressed as relative units (RU) of fold insulin change from baseline (8.3 mM glucose; set at value 1) using paired experiments. Baseline secretion at 8.3 mM glucose was similar in WT and T1R2−/− islets (5.4 ± 0.4 μg/L and 5.9 ± 0.6 μg/L, respectively). *P < 0.05 and ***P < 0.001 vs. F10 WT; ^P < 0.05 and ^^^P < 0.001 vs. F0 WT; ###P < 0.001 vs. corresponding WT; two-way ANOVA with Bonferroni posttest. (H) Static GSIS at various glucose (“G”; in mM) levels in isolated islets from WT and T1R2−/− mice (n = 6 per genotype).

Fig. 2.

A bolus of fructose induces rapid insulin release in vivo mediated by islet sweet TRs. (A and B) Plasma insulin in response to a bolus of fructose (1.0 g/kg) or saline solution injected at time 0 in WT and T1R2−/− mice (n = 8 per treatment). (C and D) Plasma glucose in response to a bolus of fructose or saline solution in WT and T1R2−/− mice. (E) Area under the curve (AUC) calculated between 0 and 10 min (vertical lines). (***P < 0.001, ANOVA with Tukey posttest.) (F) Blood glucose levels during IPGTT in age-matched WT and T1R2−/− male mice (n = 8 per treatment; Materials and Methods). Inset: Area under the curve (AUC). (G) Plasma insulin levels after 5 h fasting in WT and T1R2−/− mice (n = 12 per genotype).

Fig. 3.

Fructose potentiates GSIS dependent on sweet TRs. (A) Static insulin release of WT and T1R2−/− islets (quadruplicate batches of 10 islets from six mice per genotype) incubated at 8.3 mM glucose (WT, 4.05 ± 0.50 μg/L; T1R2−/−, 4.07 ± 0.44 μg/L) and then transferred to media with glucose (“G”) and fructose (“F”) as shown. Data are expressed as relative units (RU) of insulin fold change from baseline (8.3 mM glucose; set at value 1) using paired experiments. (*P < 0.05 and ***P < 0.001, ANOVA with Tukey posttest.) (B and D) Plasma insulin in response to a bolus of glucose (glu; 0.5 g/kg) and/or fructose (fru; 0.3 g/kg; injected at time 0) in WT and T1R2−/− mice (n = 8 per treatment). Inset: Area under the curve (AUC) calculated between 0 and 10 min (vertical lines). (**P < 0.01 and ***P < 0.001, ANOVA with Tukey posttest.) (C and E) Plasma glucose in response to a bolus of glucose and/or fructose (injected at time 0) in WT and T1R2−/− mice.

Fig. 4.

PLC activation is essential for calcium and insulin responses in beta cell TR signaling. (A) Representative single-cell trace of calcium responses in dispersed primary beta cells from WT mice treated with glucose (“G”; in mM) and combinations of fructose (“F”; in mM) and U73122 (U7; 2.0 μM) as shown. (B) Static insulin release in response to glucose (“G”; in mM) or fructose (“F”; in mM) with or without U73122 (U7; 5.0 μM) in WT islets (quadruplicate batches of 10 islets from six mice). (*P < 0.05, Student t test.) (C) Average of traces (n = 4–6 cells per trace, total of six independent experiments) showing relative change in PLC activity (left axis) or intracellular calcium (right axis) from baseline (set at 0) in MIN6 beta cells treated (vertical line) with fructose (10.0 mM) in the presence of glucose (8.3 mM). (D–F) Average of traces (n = 4–6 cells per trace, total of six independent experiments) showing relative change in PLC activity from baseline (set at 0) in MIN6 beta cells treated (vertical line) with fructose (“F”; 10.0 mM) in the presence of glucose (“G”; 3.0 mM or 8.3 mM), nifedipine (Nif; 1.0 μM), or no extracellular calcium (Ca2+ free). Experiments were performed at the presence of 8.3 mM glucose unless otherwise stated.

Fig. 5.

Ablation of TRPM5 abolishes the effects of TR signaling in mouse islets. (A and B) Representative traces of calcium response in WT and TRPM5−/− primary beta cells treated (vertical line) with fructose (“F”; 10.0 mM) in the presence of 8.3 mM glucose (quadruplicate batches of 10 islets from six mice per genotype). (C) Area under the curve (AUC) calculated using 10 to 20 single-cell calcium traces from six mice per treatment. (***P < 0.001, ANOVA with Tukey posttest.) (D) Static insulin release in response to fructose (“F”; 10 mM) in the presence of 8.3 mM glucose in WT and TRPM5−/− islets (n = 6 per treatment). (***P < 0.001, ANOVA with Tukey posttest.)

Fig. 6.

Fructose-mediated potentiation of GSIS in human islets is abolished by pharmacological inhibition of sweet TRs. (A) T1R gene expression in human islets (n = 5 donors) measured by quantitative real-time RT-PCR. Arbitrary units (AU) shown normalized to 18s rRNA. (B) Insulin release in human islet preparations in the presence of glucose (n = 8 donors). (C) Static insulin release in response to fructose (“F”; in mM) or saccharin (“S”; in mM) at the presence of 5.5 mM glucose in human islets (n = 6 donors). Data are expressed as relative units (RU) of fold insulin change from baseline (5.5 mM glucose; set at value 1) using paired experiments. (**P < 0.01 and ***P < 0.001, Student t test.) (D and F) Static GSIS in human islets (n = 6 donors per condition) incubated at 5.5 mM glucose and then transferred to media with glucose (“G”; 5.5 mM or 11.0 mM) and fructose (“F”; 3.0 mM) as shown, with or without lactisole (2.0 mM). Data are expressed as relative units (RU) of fold insulin change from baseline (5.5 mM glucose; set at value 1) using paired experiments. (*P < 0.05 and ***P < 0.001, ANOVA with Tukey posttest.) (E) Static insulin release in response to fructose (“F”; in mM) with or without lactisole (2.0 mM) in the presence of 5.5 mM glucose (n = 6 donors). Data are expressed as fold insulin change relative to release at 5.5 mM glucose (set at value 1) using paired experiments. (**P < 0.01, Student t test.)

Fig. P1.

Fructose potentiates glucose-stimulated insulin secretion dependent on sweet taste receptors on beta cells. (A) Static insulin release of WT (unmutated) and T1R2−/− (lacking T1R2 receptor molecules) mouse islets (quadruplicate batches of 10 islets from n = 6 mice per group) incubated at 8.3 mM glucose (WT, 4.05 ± 0.50 μg/L; T1R2−/−, 4.07 ± 0.44 μg/L) and then transferred to media with glucose (G) and fructose (F) as shown. Data are expressed as relative units (RU) of insulin fold change from baseline (8.3 mM glucose; set at value 1) using paired experiments (*P < 0.05 and ***P < 0.001, ANOVA with Tukey posttest). (B and C) Plasma insulin in response to an i.v. dose of glucose (glu; 0.5 g/kg) and/or fructose (fru; 0.3 g/kg; injected at time 0) in WT and T1R2−/− mice (n = 8 per treatment). Inset: Area under the curve (AUC) calculated between 0 and 10 min (shown in vertical lines). **P < 0.01 and ***P < 0.001, ANOVA with Tukey post-test.