Repair of diverse diabetic defects of β-cells in man and mouse by pharmacological glucokinase activation

Efficacy and safety of dorzagliatin for type 2 diabetes mellitus: A meta-analysis and trial sequential analysis

Objective

To evaluate the efficacy and safety of dorzagliatin in the treatment of type 2 diabetes mellitus (T2DM) by using meta-analysis and trial sequential analysis (TSA).

Method

Search for clinical trials of dorzagliatin for T2DM in eight databases, with a time limit of build to July 2022. The included studies that met the requirements were carried out for meta-analysis and TSA.

Results

In terms of efficacy endpoints, meta-analysis showed that dorzagliatin decreased glycated hemoglobin A1c(HbA1c) [mean difference (MD) -0.65%, 95% confidence interval (CI) -0.76 ~ -0.54, P < 0.00001], fasting plasma glucose (FPG) (MD -9.22 mg/dL, 95% CI -9.99 ~ -8.44, P < 0.00001), 2 h postprandial glucose (2h-PPG) (MD -48.70 mg/dL, 95% CI -55.45 ~ -41.96, P < 0.00001), homeostasis model assessment 2 of insulin resistance (HOMA2-IR) (MD -0.07, 95% CI -0.14 ~ -0.01, P = 0.03) and increased homeostasis model assessment 2 of ß-cells function (HOMA2-β) (MD 2.69, 95% CI 1.06 ~ 4.31, P = 0.001) compared with placebo. And TSA revealed that the benefits observed for the current information set were conclusive, except for HOMA2-IR. In comparison with placebo, dorzagliatin increased triglyceride(TG) (MD 0.43 mmol/L, 95% CI 0.30 ~ 0.56, P < 0.00001), total cholesterol (TC) (MD 0.13 mmol/L, 95% CI 0.05 ~ 0.21, P = 0.001), body weight (MD 0.38 kg, 95% CI 0.12-0.63, P = 0.004) and body mass index (BMI) (MD 0.14 kg/m², 95% CI 0.05-0.24, P = 0.003), while low density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), systolic blood pressure (SBP) and diastolic blood pressure (DBP) were comparable. And TSA demonstrated that TG, TC, body weight, and BMI were conclusive. In terms of safety endpoints, dorzagliatin increased total adverse events (AEs) [risk ratio (RR) 1.56, 95% CI 1.06 ~ 2.30, P = 0.03], while serious AEs, hyperlipidemia, and hypoglycaemia were all comparable. And TSA indicated that the results need to be confirmed by additional studies. Harbord regression showed no publication bias.

Conclusion

Dorzagliatin was effective in lowering glycemia, reducing insulin resistance and improving islet ß-cells function without affecting blood pressure, LDL-C, and HDL-C. Although dorzagliatin caused a mild increase in TG and TC, it did not increase the incidence of hyperlipidemia, and the small increases in body weight and BMI were not clinically significant enough. In terms of safety, the total AEs caused by dorzagliatin may be a cumulative effect of single AEs, with no drug-related adverse event being reported at a higher incidence than placebo alone. Dorzagliatin's serious AEs, hyperlipidemia, and hypoglycemia are comparable to that of placebo, and dorzagliatin has a good safety profile.

Systematic review registration

https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=371802 identifier: CRD42022371802.

Lipotoxicity in a Vicious Cycle of Pancreatic Beta Cell Exhaustion

Hyperlipidemia is a common metabolic disorder in modern society and may precede hyperglycemia and diabetes by several years. Exactly how disorders of lipid and glucose metabolism are related is still a mystery in many respects. We analyze the effects of hyperlipidemia, particularly free fatty acids, on pancreatic beta cells and insulin secretion. We have developed a computational model to quantitatively estimate the effects of specific metabolic pathways on insulin secretion and to assess the effects of short- and long-term exposure of beta cells to elevated concentrations of free fatty acids. We show that the major trigger for insulin secretion is the anaplerotic pathway via the phosphoenolpyruvate cycle, which is affected by free fatty acids via uncoupling protein 2 and proton leak and is particularly destructive in long-term chronic exposure to free fatty acids, leading to increased insulin secretion at low blood glucose and inadequate insulin secretion at high blood glucose. This results in beta cells remaining highly active in the “resting” state at low glucose and being unable to respond to anaplerotic signals at high pyruvate levels, as is the case with high blood glucose. The observed fatty-acid-induced disruption of anaplerotic pathways makes sense in the context of the physiological role of insulin as one of the major anabolic hormones.

Targeting human Glucokinase for the treatment of type 2 diabetes: an overview of allosteric Glucokinase activators

Diabetes mellitus is a worldwide impacting disorder and the ratio through which the number of diabetic patients had increased worldwide, puts medical professionals to serious stress for its effective management. Due to its polygenic origin and involvement of multiple genes to its pathophysiology, leads to understanding of this ailment more complex. It seems that current interventions, such as dietary changes, life style changes and drug therapy such as oral hypoglycaemics and insulin, are unable to halt the trend. There are various novel and emerging targets on which the researchers are paying attention to combat with this ailment successfully. Human glucokinase (GK) enzyme is one of these novel and emerging targets for management of diabetes. Its availability in the pancreas and liver cells makes this target more lucrative. GK's presence in the pancreatic and hepatic cells plays a very important function for the management of glucose homoeostasis. Small molecules that activate GK allosterically provide an alternative strategy for restoring/improving glycaemic regulation, especially in type 2 diabetic patients. Although after enduring many setbacks in the development of the GK activators, interest has been renewed especially due to introduction of novel dual acting GK activator dorzagliatin, and a novel hepato-selective GK activator, TTP399. This review article has been formulated to discuss importance of GK in glucose homeostasis, recent updates on small molecules of GK activators, clinical status of GK activators and challenges in development of GK activators.

What Is the Metabolic Amplification of Insulin Secretion and Is It (Still) Relevant?

The pancreatic beta-cell transduces the availability of nutrients into the secretion of insulin. While this process is extensively modified by hormones and neurotransmitters, it is the availability of nutrients, above all glucose, which sets the process of insulin synthesis and secretion in motion. The central role of the mitochondria in this process was identified decades ago, but how changes in mitochondrial activity are coupled to the exocytosis of insulin granules is still incompletely understood. The identification of ATP-sensitive K⁺-channels provided the link between the level of adenine nucleotides and the electrical activity of the beta cell, but the depolarization-induced Ca²⁺-influx into the beta cells, although necessary for stimulated secretion, is not sufficient to generate the secretion pattern as produced by glucose and other nutrient secretagogues. The metabolic amplification of insulin secretion is thus the sequence of events that enables the secretory response to a nutrient secretagogue to exceed the secretory response to a purely depolarizing stimulus and is thus of prime importance. Since the cataplerotic export of mitochondrial metabolites is involved in this signaling, an orienting overview on the topic of nutrient secretagogues beyond glucose is included. Their judicious use may help to define better the nature of the signals and their mechanism of action.

Ethnopharmacological perspectives of glucokinase activators in the treatment of diabetes mellitus

Traditional medicinal plants have wide-reaching utilisation in the treatment of diabetes especially in developing countries where medical resources are meagre. Traditionally used anti-diabetic plants act by numerous mechanisms, however, only a few of them act through activation of the glucokinase enzyme. Glucokinase is a key regulatory enzyme in glucose metabolism thereby controls glucose homeostasis and insulin secretion. The present review significantly analyses the knowledge about various plant-based glucokinase activators including numerous phytochemicals which modulate the activity and gene expression of glucokinase and would provide data support and perspective regarding future research in the discovery and development of different plant-derived glucokinase activators.

Glucokinase activation as antidiabetic therapy: effect of nutraceuticals and phytochemicals on glucokinase gene expression and enzymatic activity

Diabetes represents an important public health problem. Recently, new molecular targets have been identified and exploited to treat this disease. Due to its pivotal role in glucose homeostasis, glucokinase (GCK) is a promising target for the development of novel antidiabetic drugs; however, pharmacological agents that modulate GCK activity have been linked to undesirable side-effects, limiting its use. Interestingly, plants might be a valuable source of new therapeutic compounds with GCK-activating properties and presumably no adverse effects. In this review, we describe biochemical characteristics related to the physiological and pathological importance of GCK, as well as the mechanisms involved in its regulation at different molecular levels. Posteriorly, we present a compendium of findings supporting the potential use of nutraceuticals and phytochemicals in the management of diabetes through modulation of GCK expression and activity. Finally, we propose critical aspects to keep in mind when designing experiments to evaluate GCK modulation properly.

Nutrient sensing in pancreatic islets: lessons from congenital hyperinsulinism and monogenic diabetes

Pancreatic beta cells sense changes in nutrients during the cycles of fasting and feeding and release insulin accordingly to maintain glucose homeostasis. Abnormal beta cell nutrient sensing resulting from gene mutations leads to hypoglycemia or diabetes. Glucokinase (GCK) plays a key role in beta cell glucose sensing. As one form of congenital hyperinsulinism (CHI), activating mutations of GCK result in a decreased threshold for glucose-stimulated insulin secretion and hypoglycemia. In contrast, inactivating mutations of GCK result in diabetes, including a mild form (MODY2) and a severe form (permanent neonatal diabetes mellitus (PNDM)). Mutations of beta cell ion channels involved in insulin secretion regulation also alter glucose sensing. Activating or inactivating mutations of ATP-dependent potassium (K_ATP ) channel genes result in severe but completely opposite clinical phenotypes, including PNDM and CHI. Mutations of the other ion channels, including voltage-gated potassium channels (K_v 7.1) and voltage-gated calcium channels, also lead to abnormal glucose sensing and CHI. Furthermore, amino acids can stimulate insulin secretion in a glucose-independent manner in some forms of CHI, including activating mutations of the glutamate dehydrogenase gene, HDAH deficiency, and inactivating mutations of K_ATP channel genes. These genetic defects have provided insight into a better understanding of the complicated nature of beta cell fuel-sensing mechanisms.

Quercetin potentiates transdifferentiation of bone marrow mesenchymal stem cells into the beta cells in vitro

PURPOSE

Type 1 diabetes is an autoimmune disease caused by the destruction of β-cells in the pancreas. Bone marrow mesenchymal stem cells are multipotent and easy accessible adult stem cells that may provide options in the treatment of type 1 diabetes. Injured pancreatic extract can promote the differentiation of rat bone marrow mesenchymal stem cells into β-cells. We aimed to observe the effect of quercetin in differentiation and insulin secretion in β-cells.

METHODS

Bone marrow mesenchymal stem cells were obtained from the tibiae of rats. Cell surface markers were analyzed by flow cytometry. The cells were treated with rat injured pancreatic extract and quercetin for 2 weeks. Insulin secretion was measured by ELISA. Insulin expression and some islet factors were evaluated by RT-PCR. PDX1, a marker for β-cell function and differentiation, was evaluated by both immunocytochemistry and Western blot. β-cell count was determined by stereology and cell count assay.

RESULTS

ELISA showed significant differences in insulin secretion in the cells treated with RIPE + 20 μM quercetin (0.55 ± 0.01 µg/L) compared with the cells treated with RIPE alone (0.48 ± 0.01 µg/L) (P = 0.026). RT-PCR results confirmed insulin expression in both groups. PDX1 protein was detected in both groups by Western blot and immunocytochemistry. Stereology results showed a significant increase in β-cell number in the RIPE + quercetin-treated cells (47 ± 2.0) when compared with RIPE treatment alone (44 ± 2.5) (P = 0.015).

CONCLUSIONS

Quercetin has a strengthening effect on the differentiation of rat bone marrow mesenchymal stem cells into β-cells and increases insulin secretion from the differentiated β-cells in vitro.

Safety, tolerability, pharmacokinetics, and pharmacodynamics of novel glucokinase activator HMS5552: results from a first-in-human single ascending dose study

Background

HMS5552, a novel fourth-generation glucokinase (GK) activator, has demonstrated promising effects on glycemic control in preclinical models of type 2 diabetes. This single ascending dose study was conducted to investigate the safety, tolerability, pharmacokinetics (PK), and pharmacodynamics (PD) of HMS5552 during its first-in-human exposure.

Methods

Sixty healthy subjects were enrolled. In each of six dose-cohorts (5, 10, 15, 25, 35, and 50 mg), ten subjects were randomized with eight subjects receiving the same cohort-dose of HMS5552 and two receiving placebo. Plasma HMS5552 exposure, glucose, and insulin were measured repeatedly during fasting and after a standardized meal. Assessment included safety, PK, and PD endpoints.

Results

HMS5552 showed dose-proportional increases in area under the curve 0 to the last quantifiable concentration (AUC_0–t) and maximum plasma concentration (C_max). Slopes estimated by linear regression for AUC_0–t and C_max were ~1.0 (0.932 and 0.933, respectively). Geometric mean elimination half-life ranged from 4.48 to 7.51 hours and apparent clearance ranged from 11.5 to 13.1 L/h across all doses. No significant sex effect was observed in PK parameters. HMS5552 also demonstrated dose-related PD responses in terms of maximum glucose change from baseline (%) and mean glucose area under effect curve 0–4 hours change from baseline (%) (P<0.001). Fifteen adverse events were reported by nine subjects (ten with HMS5552 and five with the placebo). All adverse events were mild in intensity and resolved without any treatment.

Conclusion

This first-in-human single ascending dose study provided predicted PK of HMS5552 with dose-proportional increases in AUC_0–t and C_max, as well as dose-related glucose-lowering effects over the range of 5–50 mg in healthy subjects. HMS5552 at doses up to 50 mg in healthy subjects was safe and well-tolerated.

A new compound heterozygosis for inactivating mutations in the glucokinase gene as cause of permanent neonatal diabetes mellitus (PNDM) in double-first cousins

BACKGROUND

Permanent neonatal diabetes mellitus (PNDM) is a rare disorder, characterized by uncontrolled hyperglycemia diagnosed during the first 6 months of life. In general, PNDM has a genetic origin and most frequently it results from heterozygous mutations in KCNJ11, INS and ABCC8 genes. Homozygous or compound heterozygous inactivating mutations in GCK gene as cause of PNDM are rare. In contrast, heterozygosis for GCK inactivating mutations is frequent and results in the maturity-onset diabetes of young (MODY), manifested by a mild fasting hyperglycemia usually detected later in life. Therefore, as an autosomal recessive disorder, GCK-PNDM should be considered in families with history of glucose intolerance or MODY in first relatives, especially when consanguinity is suspected.

RESULTS

Here we describe two patients born from non-consanguineous parents within a family. They presented low birth weight with persistent hyperglycemia during the first month of life. Molecular analyses for KCNJ11, INS, ABCC8 did not show any mutation. GCK gene sequencing, however, revealed that both patients were compound heterozygous for two missense combined in a novel GCK-PNDM genotype. The p.Asn254His and p.Arg447Gly mutations had been inherited from their mothers and fathers, respectively, as their mothers are sisters and their fathers are brothers. Parents had been later diagnosed as having GCK-MODY.

CONCLUSIONS

Mutations' in silico analysis was carried out to elucidate the role of the amino acid changes on the enzyme structure. Both p.Asn254His and p.Arg447Gly mutations appeared to be quite damaging. This is the first report of GCK-PNDM in a Brazilian family.

Present status of clinical deployment of glucokinase activators

Glucokinase is one of four members of the hexokinase family of enzymes. Its expression is limited to the major organs (such as the pancreas, liver, brain and the gastrointestinal tract) that are thought to have an integrated role in glucose sensing. In the liver, phosphorylation of glucose by glucokinase promotes glycogen synthesis, whereas in the β-cells, it results in insulin release. Studies of glucokinase-linked genetically-modified mice and mutations in humans have illustrated the important roles played by glucokinase in whole-body glucose homeostasis, and suggest that the use of pharmacological agents that augment glucokinase activity could represent a viable treatment strategy in patients with type 2 diabetes. Since 2003, many glucokinase activators (GKAs) have been developed, and their ability to lower the blood glucose has been shown in several animal models of type 2 diabetes. Also, we and others have shown in mouse models that GKAs also have the effect of stimulating the proliferation of β-cells. However, the results of recent phase II trials have shown that GKAs lose their efficacy within several months of use, and that their use is associated with a high incidence of hypoglycemia; furthermore, patients treated with GKAs frequently developed dyslipidemia. A better understanding of the role of glucokinase in metabolic effects is required to resolve several issues identified in clinical trials.

Caffeic acid, naringenin and quercetin enhance glucose-stimulated insulin secretion and glucose sensitivity in INS-1E cells

AIMS

Caffeic acid, naringenin and quercetin are naturally occurring phenolic compounds (PCs) present in many plants as secondary metabolites. The aim of this study was to investigate their effect on glucose-stimulated insulin secretion (GSIS) in INS-1E cells and to explore their effect on expression of genes involved in β-cell survival and function under normoglycaemic and glucotoxic conditions.

METHODS

For acute studies, INS-1E cells were grown in 11 mM glucose (72 h) and then incubated with the PCs (1 h) with 3.3/16.7 mM glucose; whereas, for chronic studies, the cells were grown in 11 mM glucose (72 h) with/without the PCs, and then incubated with 3.3/16.7 mM glucose (1 h); thereafter, GSIS was measured. For GSIS and gene expression studies (GES) under glucotoxic conditions, two sets of cells were grown in 11/25 mM glucose with/without the PCs (72 h): one was used for GES, using real time RT-PCR, and the other was exposed to 3.3/16.7 mM glucose, followed by measurement of GSIS.

RESULTS

The study demonstrated that the PCs can enhance GSIS under hyperglycaemic and glucotoxic conditions in INS-1E cells. Moreover, these compounds can differentially, yet distinctly change the expression profile of genes [Glut2 (glucose transporter 2), Gck (glucokinase), Ins1 (insulin 1), Ins2, Beta2 (neurogenic differentiation protein 1), Pdx1 (pancreatic and duodenal homeobox protein 1), Akt1 (RAC-α serine/threonine-protein kinase encoding gene), Akt2 (RAC-β serine/threonine-protein kinase encoding gene), Irs1 (insulin receptor substrate 1), Acc1 (acetyl CoA carboxylase 1), Bcl2 (β-cell lymphoma 2 protein), Bax (Bcl-2 associated X protein), Casp3 (Caspase 3), Hsp70 (heat shock protein 70), and Hsp90] involved in β-cell stress, survival and function.

CONCLUSION

The results indicate that the PCs tested enhance GSIS and glucose sensitivity in INS-1E cells. They also modulate gene expression profiles to improve β-cell survival and function during glucotoxicity.

Control of insulin secretion by cytochrome C and calcium signaling in islets with impaired metabolism

The aim of the study was to assess the relative control of insulin secretion rate (ISR) by calcium influx and signaling from cytochrome c in islets where, as in diabetes, the metabolic pathways are impaired. This was achieved either by culturing isolated islets at low (3 mm) glucose or by fasting rats prior to the isolation of the islets. Culture in low glucose greatly reduced the glucose response of cytochrome c reduction and translocation and ISR, but did not affect the response to the mitochondrial fuel α-ketoisocaproate. Unexpectedly, glucose-stimulated calcium influx was only slightly reduced in low glucose-cultured islets and was not responsible for the impairment in glucose-stimulated ISR. A glucokinase activator acutely restored cytochrome c reduction and translocation and ISR, independent of effects on calcium influx. Islets from fasted rats had reduced ISR and cytochrome c reduction in response to both glucose and α-ketoisocaproate despite normal responses of calcium. Our data are consistent with the scenario where cytochrome c reduction and translocation are essential signals in the stimulation of ISR, the loss of which can result in impaired ISR even when calcium response is normal.

Many faces of monogenic diabetes

Monogenic diabetes represents a heterogeneous group of disorders resulting from defects in single genes. Defects are categorized primarily into two groups: disruption of β-cell function or a reduction in the number of β-cells. A complex network of transcription factors control pancreas formation, and a dysfunction of regulators high in the hierarchy leads to pancreatic agenesis. Dysfunction among factors further downstream might cause organ hypoplasia, absence of islets of Langerhans or a reduction in the number of β-cells. Many transcription factors have pleiotropic effects, explaining the association of diabetes with other congenital malformations, including cerebellar agenesis and pituitary agenesis. Monogenic diabetes variants are classified conventionally according to age of onset, with neonatal diabetes occurring before the age of 6 months and maturity onset diabetes of the young (MODY) manifesting before the age of 25 years. Recently, certain familial genetic defects were shown to manifest as neonatal diabetes, MODY or even adult onset diabetes. Patients with neonatal diabetes require a thorough genetic work-up in any case, and because extensive phenotypic overlap exists between monogenic, type 2, and type 1 diabetes, genetic analysis will also help improve diagnosis in these cases. Next generation sequencing will facilitate rapid screening, leading to the discovery of digenic and oligogenic diabetes variants, and helping to improve our understanding of the genetics underlying other types of diabetes. An accurate diagnosis remains important, because it might lead to a change in the treatment of affected subjects and influence long-term complications.

Normal glucose metabolism in carnivores overlaps with diabetes pathology in non-carnivores

Carnivores, such as the dolphin and the domestic cat, have numerous adaptations that befit consumption of diets with high protein and fat content, with little carbohydrate content. Consequently, nutrient metabolism in carnivorous species differs substantially from that of non-carnivores. Important metabolic pathways known to differ between carnivores and non-carnivores are implicated in the development of diabetes and insulin resistance in non-carnivores: (1) the hepatic glucokinase (GCK) pathway is absent in healthy carnivores yet GCK deficiency may result in diabetes in rodents and humans, (2) healthy dolphins and cats are prone to periods of fasting hyperglycemia and exhibit insulin resistance, both of which are risk factors for diabetes in non-carnivores. Similarly, carnivores develop naturally occurring diseases such as hemochromatosis, fatty liver, obesity, and diabetes that have strong parallels with the same disorders in humans. Understanding how evolution, environment, diet, and domestication may play a role with nutrient metabolism in the dolphin and cat may also be relevant to human diabetes.

Expression of Ins1 and Ins2 genes in mouse fetal liver

A possible cure for diabetes is explored by using non-pancreatic cells such as fetal hepatocytes. The expression of insulin and transcription factors for insulin is investigated in mouse fetal liver. We detected mRNAs for insulin I (Ins1) and insulin II (Ins2) and proinsulin- and mature insulin-positive cells in mouse fetal liver by reverse transcription plus the polymerase chain reaction and immunohistochemistry. Glucagon, somatostatin and pancreatic polypeptide were not expressed throughout development. Mouse Ins2 and Ins1 promoters were transiently activated in mouse fetal hepatocytes of embryonic days 13.5 and 16.5, respectively. Pancreatic and duodenal homeobox 1 (Pdx1) mRNA was not expressed during development of the liver. In contrast, mRNAs and proteins of neurogenic differentiation (NeuroD)/β cell E-box transactivator 2 (Beta2) and v-maf musculoaponeurotic fibrosarcoma oncogene homolog (MafA) were almost simultaneously expressed with insulin genes in the liver. Ins2 and Ins1 promoters were activated in hepatoma cells by the transfection of the expression vector for NeuroD/Beta2 alone and by the combination of NeuroD/Beta2 and MafA, respectively. These results indicate that the expression of NeuroD/Beta2 and MafA is linked temporally with the transcription of Ins2 and Ins1 genes in mouse fetal liver and suggest the potential usage of fetal hepatocytes to make insulin-producing β cells by introducing transcription factors.

GKAs for diabetes therapy: why no clinically useful drug after two decades of trying?

Results of basic biochemical and physiological research, strongly endorsed by findings in human pathophysiology and genetics, had characterized the glucose phosphorylating enzyme glucokinase as a critical player in normal glucose homeostasis, diabetes mellitus, and hyperinsulinemic hypoglycemia, and identified the enzyme as a promising new drug target. R&D initiated in the early 1990s and directed at this target discovered glucokinase activators (GKAs) as a new class of potentially antidiabetic drugs. GKAs were characterized as nonessential allosteric activators that increase glucose affinity and V(max) of the enzyme, thus stimulating glucose metabolism in glucokinase expressing tissue, of foremost functional significance in the insulin producing pancreatic beta cells and the liver. The results of preclinical testing of GKAs by many pharmaceutical companies demonstrated uniformly high hypoglycemic efficacy in normal and diabetic animals. GKAs were also highly effective in Phase I trials in patients with type 2 diabetes mellitus (T2DM). However, results of a recent Phase II trial were less encouraging because patients developed hyperlipidemia and vascular hypertension, and the drug lost efficacy within several months. This outcome is prompting a reappraisal of the GKA strategy. In this opinion article, the 'pros and cons' of the strategy to use these compounds in diabetes management are critically reexamined and suggestions are made that might facilitate progress of GKA R&D that could still result in a novel antidiabetic medicine.

Regulation of glucagon secretion in normal and diabetic human islets by γ-hydroxybutyrate and glycine

Paracrine signaling between pancreatic islet β-cells and α-cells has been proposed to play a role in regulating glucagon responses to elevated glucose and hypoglycemia. To examine this possibility in human islets, we used a metabolomic approach to trace the responses of amino acids and other potential neurotransmitters to stimulation with [U-(13)C]glucose in both normal individuals and type 2 diabetics. Islets from type 2 diabetics uniformly showed decreased glucose stimulation of insulin secretion and respiratory rate but demonstrated two different patterns of glucagon responses to glucose: one group responded normally to suppression of glucagon by glucose, but the second group was non-responsive. The non-responsive group showed evidence of suppressed islet GABA levels and of GABA shunt activity. In further studies with normal human islets, we found that γ-hydroxybutyrate (GHB), a potent inhibitory neurotransmitter, is generated in β-cells by an extension of the GABA shunt during glucose stimulation and interacts with α-cell GHB receptors, thus mediating the suppressive effect of glucose on glucagon release. We also identified glycine, acting via α-cell glycine receptors, as the predominant amino acid stimulator of glucagon release. The results suggest that glycine and GHB provide a counterbalancing receptor-based mechanism for controlling α-cell secretory responses to metabolic fuels.