Sweet taste receptors regulate basal insulin secretion and contribute to compensatory insulin hypersecretion during the development of diabetes in male mice

Abstract

β-Cells rapidly secrete insulin in response to acute increases in plasma glucose but, upon further continuous exposure to glucose, insulin secretion progressively decreases. Although the mechanisms are unclear, this mode of regulation suggests the presence of a time-dependent glucosensory system that temporarily attenuates insulin secretion. Interestingly, early-stage β-cell dysfunction is often characterized by basal (ie, fasting) insulin hypersecretion, suggesting a disruption of these related mechanisms. Because sweet taste receptors (STRs) on β-cells are implicated in the regulation of insulin secretion and glucose is a bona fide STR ligand, we tested whether STRs mediate this sensory mechanism and participate in the regulation of basal insulin secretion. We used mice lacking STR signaling (T1R2(-/-) knockout) and pharmacologic inhibition of STRs in human islets. Mouse and human islets deprived of STR signaling hypersecrete insulin at short-term fasting glucose concentrations. Accordingly, 5-hour fasted T1R2(-/-) mice have increased plasma insulin and lower glucose. Exposure of isolated wild-type islets to elevated glucose levels reduced STR expression, whereas islets from diabetic (db/db) or diet-induced obese mouse models show similar down-regulation. This transcriptional reprogramming in response to hyperglycemia correlates with reduced STR function in these mouse models, leading to insulin hypersecretion. These findings reveal a novel mechanism by which insulin secretion is physiologically regulated by STRs and also suggest that, during the development of diabetes, STR function is compromised by hyperglycemia leading to hyperinsulinemia. These observations further suggest that STRs might be a promising therapeutic target to prevent and treat type 2 diabetes.

Figures

Figure 1.

STRs regulate basal insulin secretion during fasting. A and B, Static insulin secretion in response to various glucose concentrations (G; mM) in mouse and human islets. Insulin secretion in mouse islets was assessed after 1.5 hours incubation at constant glucose as shown and expressed as percent of islet insulin content (quadruplicate batches of 10 islets per condition from n = 6 mice per group). Student's t test: ***, P < .001. Insulin secretion in human islets was assessed after 1.5 hours incubation at constant glucose with or without lactisole (1 mM) (quadruplicate batches of 10 islets per condition from n = 8 human preparations). Paired Student's t test: **, P < .01; ****, P < .0001. C, Plasma glucose (left panel) and insulin (right panel) in fed or 5-hour fasted WT and T1R2−/− mice (n = 16–24). Student's t test: *, P < .05; **, P < .01. D, Insulin sensitivity index assessed by calculating the ratio of 5-hour fasted plasma insulin to glucose levels for each mouse. Student's t test: **, P < .01. E, Assessment of stimulated insulin secretion in WT and T1R2−/− mice in vivo using hyperglycemic clamp. Plasma glucose was clamped at 250 mg/dL (circles; left axis) with appropriate adjustments on the glucose infusion rates (squares; right axis) for 4 hours (n = 8 per group). F, Plasma levels of C-peptide (pM) in response to the hyperglycemic clamp as a surrogate measurement of insulin secretion by β-cells. G and H, Blood glucose levels during an IPGTT and assessment of the insulinogenic index (increment in plasma insulin/increment in plasma glucose) during the first 30 minutes of the IPGTT in overnight-fasted age-matched WT and T1R2−/− male mice (n = 10–14 per group) as described in Materials and Methods. I, Static insulin secretion in response to increasing concentrations of glucose (G; mM) in WT and T1R2−/− islets. Islets were equilibrated at 3.0 mM glucose (G3.0) and then transferred to glucose concentrations as shown (G; mM) to assess GSIS expressed as percent of islet insulin content (quadruplicate batches of 10 islets per condition from n = 4 mice).

Figure 2.

STRs alter insulin secretion during sustained exposure to physiological glucose. A, Static insulin secretion in response to sustained exposure at constant glucose (G; mM) in WT islets. Insulin secretion was assessed after incubation for 3 hours and 24 hours at constant glucose (8.3, 16.7, and 25.0 mM; quadruplicate batches of 10 islets per condition from n = 6–9 mice). Paired Student's t test: **, P < .01. B, Static insulin secretion in response to sustained exposure at baseline glucose in WT and T1R2−/− islets. Insulin secretion was assessed after incubation for 3 hours and 24 hours at constant glucose (8.3 mM) (quadruplicate batches of 10 islets per condition from n = 8–9 mice). Data are expressed as percent of islet insulin content using paired experiments. Paired Student's t test: *, P < .05.

Figure 3.

STR expression and function is down-regulated in islets from hyperglycemic mouse models. A, STR signaling gene expression in MIN6 cells cultured in low (G5.0) or high (25.0) glucose (mM) for 24 hours measured by quantitative real-time RT-PCR. Relative arbitrary units (AU) shown normalized to 18S rRNA (n = 12). Student's t test: **, P < .01; ***, P < .001. B, STR signaling gene expression in isolated WT islets cultured in low (G8.3) or high (G11.0) glucose (mM) for 48 hours measured by quantitative real-time RT-PCR. Arbitrary units (AU) shown normalized to 18S rRNA (n = 10–12 mice). Student's t test: **, P < .01; ***, P < .001. C, Static insulin secretion in isolated WT islets cultured in low (G8.3) or high (G11.0) glucose (mM) for 48 hours. Insulin secretion was assessed after 3 hours incubation at constant glucose and expressed as percent of islet insulin content (8.3 mM; quadruplicate batches of 10 islets per condition from n = 6 mice). Student's t test: *, P < .05. D, STR gene expression (t1r2 and t1r3) in isolated islets from db/db and HFD mice compared with controls measured by quantitative real-time RT-PCR. Arbitrary units (AU) shown normalized to 18S rRNA (n = 6–8 mice). Student's t test: *, P < .05; **, P < .01; ***, P < .001. E, Assessment of fructose-induced insulin secretion as a STR functional assay. Static insulin release at 8.3 mM glucose (G) with or without 10.0 mM fructose (F) in islets from db/db (left panel) and HFD (right panel) mice compared with controls. Data are expressed as relative units (RU) of insulin fold change from 8.3 mM glucose alone (set at value 1) using paired experiments. (Quadruplicate batches of 10 islets from n = 5 mice per group). Paired Student's t test: *, P < .05; **, P < .01.

Figure 4.

Reduced STR function in diabetic and obese mouse islets correlates with basal insulin hypersecretion and islet hypersensitization. A, Static insulin secretion in response to glucose (G; mM) in db/db (left panel; n = 6) and HFD (right panel; n = 8–10) islets compared with controls. Basal insulin secretion was assessed after incubation for 1 hour at constant glucose (8.3 mM or 16.7 mM) and expressed as percent of islet insulin content (quadruplicate batches of 10 islets per condition). Student's t test: *, P < .05; **, P < .01. B, Static insulin secretion in response to sustained exposure to glucose in islets from db/db (n = 5) and HFD mice (n = 9). Insulin secretion was assessed after 3 hours and 24 hours incubation at constant glucose (8.3 mM; quadruplicate batches of 10 islets per condition). Data are expressed as relative units (RU) of insulin fold change from 3-hour baseline value (set at value 1) using paired experiments. Paired Student's t test: *, P < .05.