Impact of lack of suppression of glucagon on glucose tolerance in humans

Rise in fasting and dynamic glucagon levels in children and adolescents with obesity is moderate in subjects with impaired fasting glucose but accentuated in subjects with impaired glucose tolerance or type 2 diabetes

Background

Fasting levels of glucagon are known to be elevated in youth and adults with type 2 diabetes mellitus (T2D). Children and adolescents with obesity were previously reported to show increasing fasting and post-glucose-challenge hyperglucagonemia across the spectrum of glucose tolerance, while no data are available in those with impaired fasting glucose (IFG).

Materials and methods

Individuals from the Beta-JUDO study population (Uppsala and Salzburg 2010-2016) (n=101, age 13.3 ± 2.8, m/f =50/51) were included (90 with overweight or obesity, 11 with normal weight). Standardized OGTT were performed and plasma glucose, glucagon and insulin concentrations assessed at baseline, 5, 10, 15, 30, 60, 90 and 120 minutes. Patients were grouped according to their glycemic state in six groups with normal glucose metabolism (NGM) and normal weight (NG-NW), NGM with obesity or overweight (NG-O), impaired glucose tolerance (IGT), impaired fasting glucose (IFG), IGT+IFG and T2D, and in two groups with NGM and impaired glucose metabolism (IGM), for statistical analysis.

Results and conclusion

Glucagon concentrations were elevated in young normoglycemic individuals with overweight or obesity (NG-O) compared to normoglycemic individuals with normal weight. Glucagon levels, fasting and dynamic, increased with progressing glycemic deterioration, except in IFG, where levels were comparable to those in NG-O. All glycemic groups showed an overall suppression of glucagon during OGTT. An initial increase of glucagon could be observed in T2D. In T2D, glucagon showed a strong direct linear correlation with plasma glucose levels during OGTT. Glucagon in adolescents, as in adults, may play a role in the disease progression of T2D.

Hepatic glucagon action: beyond glucose mobilization

Glucagon's ability to promote hepatic glucose production has been known for over a century, with initial observations touting this hormone as a diabetogenic agent. However, glucagon receptor agonism [when balanced with an incretin, including glucagon-like peptide 1 (GLP-1) to dampen glucose excursions] is now being developed as a promising therapeutic target in the treatment of metabolic diseases, like metabolic dysfunction-associated steatotic disease/metabolic dysfunction-associated steatohepatitis (MASLD/MASH), and may also have benefit for obesity and chronic kidney disease. Conventionally regarded as the opposing tag-team partner of the anabolic mediator insulin, glucagon is gradually emerging as more than just a "catabolic hormone." Glucagon action on glucose homeostasis within the liver has been well characterized. However, growing evidence, in part thanks to new and sensitive "omics" technologies, has implicated glucagon as more than just a "glucose liberator." Elucidation of glucagon's capacity to increase fatty acid oxidation while attenuating endogenous lipid synthesis speaks to the dichotomous nature of the hormone. Furthermore, glucagon action is not limited to just glucose homeostasis and lipid metabolism, as traditionally reported. Glucagon plays key regulatory roles in hepatic amino acid and ketone body metabolism, as well as mitochondrial turnover and function, indicating broader glucagon signaling consequences for metabolic homeostasis mediated by the liver. Here we examine the broadening role of glucagon signaling within the hepatocyte and question the current dogma, to appreciate glucagon as more than just that "catabolic hormone."

The integrated incretin effect is reduced by both glucose intolerance and obesity in Japanese subjects

Introduction

Incretin-based drugs are extensively utilized in the treatment of type 2 diabetes (T2D), with remarkable clinical efficacy. These drugs were developed based on findings that the incretin effect is reduced in T2D. The incretin effect in East Asians, whose pancreatic β-cell function is more vulnerable than that in Caucasians, however, has not been fully examined. In this study, we investigated the effects of incretin in Japanese subjects.

Methods

A total of 28 Japanese subjects (14 with normal glucose tolerance [NGT], 6 with impaired glucose tolerance, and 8 with T2D) were enrolled. Isoglycemic oral (75 g glucose tolerance test) and intravenous glucose were administered. The numerical incretin effect and gastrointestinally-mediated glucose disposal (GIGD) were calculated by measuring the plasma glucose and entero-pancreatic hormone concentrations.

Results and discussion

The difference in the numerical incretin effect among the groups was relatively small. The numerical incretin effect significantly negatively correlated with the body mass index (BMI). GIGD was significantly lower in participants with T2D than in those with NGT, and significantly negatively correlated with the area under the curve (AUC)-glucose, BMI, and AUC-glucagon. Incretin concentrations did not differ significantly among the groups. We demonstrate that in Japanese subjects, obesity has a greater effect than glucose tolerance on the numerical incretin effect, whereas GIGD is diminished in individuals with both glucose intolerance and obesity. These findings indicate variances as well as commonalities between East Asians and Caucasians in the manifestation of incretin effects on pancreatic β-cell function and the integrated capacity to handle glucose.

Lowest Glucagon/Highest C-Peptide in Oral Glucose Tolerance Test: Clinical Utility in Monitoring Glucose Control in Type 2 Diabetes Mellitus

Purpose

Understanding factors that influence blood glucose levels in patients with type 2 diabetes mellitus (T2DM) is crucial for managing hyperglycemia. Currently, there is no standardized interpretation method for glucagon levels in oral glucose tolerance test (OGTT). This study aims to assess the relationship between the lowest glucagon/highest C-peptide ratio (Lglc/Hcp) in OGTT and glucose control levels in T2DM.

Patients and Methods

Clinical data from 120 patients with T2DM were examined to compare the correlations of Lglc/Hcp and other pancreatic islet function-associated indices with fasting blood glucose (G0), glucose at 120 minutes in OGTT (G120), hemoglobin A1c (HbA1c), and the area under the glucose curve in OGTT (AUCglu). Additionally, the study investigated difference in Lglc/Hcp between patient groups based on the highest blood glucose levels (Hglu) in OGTT (Hglu ≥ 16.7 mmol/L vs Hglu < 16.7 mmol/L).

Results

The generalized linear model suggested that Lglc/Hcp significantly correlated with G0 (B = 0.85, P < 0.001), G120(B = 1.46, P < 0.001), HbA1c (B = 0.67, P < 0.001), and AUCglu (B = 3.46, P < 0.001). This correlation surpassed C-peptide and glucagon-related parameters, even after adjusting for confounding factors. Furthermore, Lglc/Hcp was notably higher in patients with Hglu ≥ 16.7 mmol/L compared to those with Hglu < 16.7 mmol/L (Z = -3.71, p < 0.001).

Conclusion

Lglc/Hcp in OGTT closely relates to blood glucose control in patients with T2DM, potentially reflecting the overall pancreatic islet function in regulating glucose levels. Moreover, inhibiting glucagon secretion may be a crucial consideration for patients requiring insulin treatment.

An extended minimal model of OGTT: estimation of α- and β-cell dysfunction, insulin resistance, and the incretin effect

Loss of insulin sensitivity, α- and β-cell dysfunction, and impairment in incretin effect have all been implicated in the pathophysiology of type 2 diabetes (T2D). Parsimonious mathematical models are useful in quantifying parameters related to the pathophysiology of T2D. Here, we extend the minimum model developed to describe the glucose-insulin-glucagon dynamics in the isoglycemic intravenous glucose infusion (IIGI) experiment to the oral glucose tolerance test (OGTT). The extended model describes glucose and hormone dynamics in OGTT including the contribution of the incretin hormones, glucose-dependent insulinotropic polypeptide (GIP), and glucagon-like peptide-1 (GLP-1), to insulin secretion. A new function describing glucose arrival from the gut is introduced. The model is fitted to OGTT data from eight individuals with T2D and eight weight-matched controls (CS) without diabetes to obtain parameters related to insulin sensitivity, β- and α-cell function. The parameters, i.e., measures of insulin sensitivity, a1, suppression of glucagon secretion, k1, magnitude of glucagon secretion, γ2, and incretin-dependent insulin secretion, γ3, were found to be different between CS and T2D with P values < 0.002, <0.017, <0.009, <0.004, respectively. A new rubric for estimating the incretin effect directly from modeling the OGTT is presented. The average incretin effect correlated well with the experimentally determined incretin effect with a Spearman rank test correlation coefficient of 0.67 (P < 0.012). The average incretin effect was found to be different between CS and T2D (P < 0.032). The developed model is shown to be effective in quantifying the factors relevant to T2D pathophysiology.NEW & NOTEWORTHY A new extended model of oral glucose tolerance test (OGTT) has been developed that includes glucagon dynamics and incretin contribution to insulin secretion. The model allows the estimation of parameters related to α- and β-cell dysfunction, insulin sensitivity, and incretin action. A new function describing the influx of glucose from the gut has been introduced. A new rubric for estimating the incretin effect directly from the OGTT experiment has been developed. The effect of glucose dose was also investigated.

Differential contribution of alpha and beta cell dysfunction to impaired fasting glucose and impaired glucose tolerance

AIMS/HYPOTHESIS

People with isolated impaired fasting glucose (IFG) have normal beta cell function. We hypothesised that an increased glucose threshold for beta cell secretion explains IFG.

METHODS

We used graded glucose infusion to examine the relationship of insulin secretion rate (ISR) and glucagon secretion rate (GSR) with rising glucose. We studied 39 non-diabetic individuals (53 ± 2 years, BMI 30 ± 1 kg/m2), categorised by fasting glucose and glucose tolerance status. After an overnight fast, a variable insulin infusion was used to maintain glucose at ~4.44 mmol/l (07:00 to 08:30 hours). At 09:00 hours, graded glucose infusion commenced at 1 mg kg-1 min-1 and doubled every 60 min until 13:00 hours. GSR and ISR were calculated by nonparametric deconvolution from concentrations of glucagon and C-peptide, respectively.

RESULTS

The relationship of ISR with glucose was linear and the threshold for insulin secretion in isolated IFG did not differ from that in people with normal fasting glucose and normal glucose tolerance. GSR exhibited a single-exponential relationship with glucose that could be characterised by G50, the change in glucose necessary to suppress GSR by 50%. G50 was increased in IFG compared with normal fasting glucose regardless of the presence of impaired or normal glucose tolerance.

CONCLUSIONS/INTERPRETATION

These data show that, in non-diabetic humans, alpha cell dysfunction contributes to the pathogenesis of IFG independently of defects in insulin secretion. We also describe a new index that quantifies the suppression of glucagon secretion by glucose.

Insulin secretion and action and the response of endogenous glucose production to a lack of glucagon suppression in nondiabetic subjects

Type 2 diabetes is a disease characterized by impaired insulin secretion and defective glucagon suppression in the postprandial period. We examined the effect of impaired glucagon suppression on glucose concentrations and endogenous glucose production (EGP) at different degrees of insulin secretory impairment. The contribution of anthropometric characteristics, peripheral, and hepatic insulin action to this variability was also examined. To do so, we studied 54 nondiabetic subjects on two occasions in which endogenous hormone secretion was inhibited by somatostatin, with glucagon infused at a rate of 0.65 ng/kg/min, at 0 min to prevent a fall in glucagon (nonsuppressed day) or at 120 min to create a transient fall in glucagon (suppressed day). Subjects received glucose (labeled with [3-³H]-glucose) infused to mimic the systemic appearance of 50-g oral glucose. Insulin was infused to mimic a prandial insulin response in 18 subjects, another 18 received 80% of the dose, and the remaining 18 received 60%. EGP was measured using the tracer-dilution technique. Decreased prandial insulin resulted in greater % increase in peak glucose but not in integrated glucose concentrations attributable to nonsuppressed glucagon. The % change in integrated EGP was unaffected by insulin dose. Multivariate regression analysis, adjusted for age, sex, weight, and insulin dose, did not show a relationship between the EGP response to impaired suppression of glucagon and insulin action as measured at the time of screening by oral glucose tolerance. A similar analysis for hepatic insulin action also did not show a relationship with the EGP response. These data indicate that the effect of impaired glucagon suppression on EGP is independent of anthropometric characteristics and insulin action.NEW & NOTEWORTHY In prediabetes, anthropometric characteristics as well as insulin action do not alter the hepatic response to glucagon. The postprandial suppression or lack of suppression of glucagon secretion is an important factor governing postprandial glucose tolerance independent of insulin secretion.

Regulation of α-cell glucagon secretion: The role of second messengers

Glucagon is a potent glucose‐elevating hormone that is secreted by pancreatic α‐cells. While well‐controlled glucagon secretion plays an important role in maintaining systemic glucose homeostasis and preventing hypoglycaemia, it is increasingly apparent that defects in the regulation of glucagon secretion contribute to impaired counter‐regulation and hyperglycaemia in diabetes. It has therefore been proposed that pharmacological interventions targeting glucagon secretion/signalling can have great potential in improving glycaemic control of patients with diabetes. However, despite decades of research, a consensus on the precise mechanisms of glucose regulation of glucagon secretion is yet to be reached. Second messengers are a group of small intracellular molecules that relay extracellular signals to the intracellular signalling cascade, modulating cellular functions. There is a growing body of evidence that second messengers, such as cAMP and Ca²⁺, play critical roles in α‐cell glucose‐sensing and glucagon secretion. In this review, we discuss the impact of second messengers on α‐cell electrical activity, intracellular Ca²⁺ dynamics and cell exocytosis. We highlight the possibility that the interaction between different second messengers may play a key role in the glucose‐regulation of glucagon secretion.

Insulin Pulse Characteristics and Insulin Action in Non-diabetic Humans

OBJECTIVE

Pulsatile insulin secretion is impaired in diseases such as type 2 diabetes that are characterized by insulin resistance. This has led to the suggestion that changes in insulin pulsatility directly impair insulin signaling. We sought to examine the effects of pulse characteristics on insulin action in humans, hypothesizing that a decrease in pulse amplitude or frequency is associated with impaired hepatic insulin action.

METHODS

We studied 29 nondiabetic subjects on two occasions. On 1 occasion, hepatic and peripheral insulin action was measured using a euglycemic clamp. The deuterated water method was used to estimate the contribution of gluconeogenesis to endogenous glucose production. On a separate study day, we utilized nonparametric stochastic deconvolution of frequently sampled peripheral C-peptide concentrations during fasting to reconstruct portal insulin secretion. In addition to measuring basal and pulsatile insulin secretion, we used approximate entropy to measure orderliness and Fourier transform to measure the average, and the dispersion of, insulin pulse frequencies.

RESULTS

In univariate analysis, basal insulin secretion (R2 = 0.16) and insulin pulse amplitude (R2 = 0.09) correlated weakly with insulin-induced suppression of gluconeogenesis. However, after adjustment for age, sex, and weight, these associations were no longer significant. The other pulse characteristics also did not correlate with the ability of insulin to suppress endogenous glucose production (and gluconeogenesis) or to stimulate glucose disappearance.

CONCLUSIONS

Overall, our data demonstrate that insulin pulse characteristics, considered independently of other factors, do not correlate with measures of hepatic and peripheral insulin sensitivity in nondiabetic humans.

Impaired Suppression of Glucagon in Obese Subjects Parallels Decline in Insulin Sensitivity and Beta-Cell Function

AIM

To examine the relationship between plasma glucagon levels and insulin sensitivity and insulin secretion in obese subjects.

METHODS

Suppression of plasma glucagon was examined in 275 obese Hispanic Americans with varying glucose tolerance. All subjects received a 2-hour oral glucose tolerance test (OGTT) and a subset (n = 90) had euglycemic hyperinsulinemic clamp. During OGTT, we quantitated suppression of plasma glucagon concentration, Matsuda index of insulin sensitivity, and insulin secretion/insulin resistance (disposition) index. Plasma glucagon suppression was compared between quartiles of insulin sensitivity and beta-cell function.

RESULTS

Fasting plasma glucagon levels were similar in obese subjects with normal glucose tolerance (NGT), prediabetes, and type 2 diabetes (T2D), but the fasting glucagon/insulin ratio decreased progressively from NGT to prediabetes to T2D (9.28 ± 0.66 vs 6.84 ± 0.44 vs 5.84 ± 0.43; P < 0.001). Fasting and 2-hour plasma glucagon levels during OGTT progressively increased and correlated positively with severity of insulin resistance (both Matsuda index and euglycemic hyperinsulinemic clamp). The fasting glucagon/insulin ratio declined with worsening insulin sensitivity and beta-cell function, and correlated with whole-body insulin sensitivity (Matsuda index, r = 0.81; P < 0.001) and beta-cell function (r = 0.35; P < 0.001). The glucagon/insulin ratio also correlated and with beta-cell function during OGTT at 60 and 120 minutes (r = -0.47; P < 0.001 and r = -0.32; P < 0.001).

CONCLUSION

Insulin-mediated suppression of glucagon secretion in obese subjects is impaired with increasing severity of glucose intolerance and parallels the severity of insulin resistance and beta-cell dysfunction.

Increased alpha and beta cell mass during mouse pregnancy is not dependent on transdifferentiation

Maternal pancreatic beta-cell mass (BCM) increases during pregnancy to compensate for relative insulin resistance. If BCM expansion is suboptimal, gestational diabetes mellitus can develop. Alpha-cell mass (ACM) also changes during pregnancy, but there is a lack of information about α-cell plasticity in pregnancy and whether α- to β-cell transdifferentiation can occur. To investigate this, we used a mouse model of gestational glucose intolerance induced by feeding low-protein (LP) diet from conception until weaning and compared pregnant female offspring to control diet-fed animals. Control and LP pancreata were collected for immunohistochemical analysis and serum glucagon levels were measured. In order to lineage trace α- to β-cell conversion, we utilized transgenic mice expressing yellow fluorescent protein behind the proglucagon gene promoter (Gcg-Cre/YFP) and collected pancreata for histology at various gestational timepoints. Alpha-cell proliferation increased significantly at gestational day (GD) 9.5 in control pregnancies resulting in an increased ACM at GD18.5, and this was significantly reduced in LP animals. Despite these changes, serum glucagon was higher in LP mice at GD18.5. Pregnant Gcg-Cre/YFP mice showed no increase in the abundance of insulin+YFP+glucagon- cells (phenotypic β-cells). A second population of insulin+YFP+glucagon+ cells was identified which also did not alter during pregnancy. However, there was an altered anatomical distribution within islets with fewer insulin+YFP+glucagon- cells but more insulin+YFP+glucagon+ cells being present in the islet mantle at GD18.5. These findings demonstrate that dynamic changes in ACM occur during normal pregnancy and were altered in glucose-intolerant pregnancies.

Assessment of individual and standardized glucagon kinetics in healthy humans

Impaired glucose tolerance arises out of impaired postprandial insulin secretion and delayed suppression of glucagon. These defects occur early and independently in the pathogenesis of prediabetes. Quantification of the contribution of α-cell dysfunction to glucose tolerance has been lacking because knowledge of glucagon kinetics in humans is limited. Therefore, in a series of experiments examining the interaction of glucagon suppression with insulin secretion we studied 51 nondiabetic subjects (age = 54 ± 13 yr, BMI = 28 ± 4 kg/m²). Glucose was infused to mimic the systemic appearance of an oral challenge. Somatostatin was used to inhibit endogenous hormone secretion. 120 min after the start of the experiment, glucagon was infused at 0.65 ng/kg/min. The rise in glucagon concentrations was used to estimate its kinetic parameters [volume of distribution (V_d), half-life (t_1/2), and clearance rate (CL)]. A single-exponential model provided the best fit for the data, and individualized kinetic parameters were estimated: V_d = 8.2 ± 2.7 L, t_1/2 = 4 ± 1.1 min, CL = 1.4 ± 0.33 L/min. Stepwise linear regression was used to correlate V_d with BMI and sex (R²_adj = 0.44), whereas CL similarly correlated with lean body mass or BSA (both R² = 0.28). This enabled the development of a population-based model using anthropometric characteristics to predict V_d and CL. These data demonstrate that it is feasible to derive glucagon kinetic parameters from anthropometric characteristics, thereby enabling quantitation of the rate of glucagon appearance in the systemic circulation in large populations.NEW & NOTEWORTHY State-of-the-art measurement of insulin secretion in humans is accomplished by deconvolution of peripheral C-peptide concentrations using population-derived parameters of C-peptide kinetics. In contrast, knowledge of the kinetic parameters of glucagon in humans is lacking so that measurement of glucagon secretion to date is largely qualitative. This series of experiments enabled measurement of glucagon kinetics in 51 subjects, and subsequently, stepwise linear regression was used to correlate these parameters with anthropometric characteristics. This enabled the development of a population-based model using anthropometric characteristics to predict the volume of distribution and the rate of clearance. This is a necessary first step in the development of a model to quantitate of glucagon secretion and action (and its contribution to glucose tolerance) in large populations.

Nonalcoholic Fatty Liver Disease Impairs the Liver-Alpha Cell Axis Independent of Hepatic Inflammation and Fibrosis

Nonalcoholic fatty liver disease (NAFLD) is associated with impaired hepatic actions of glucagon and insulin. Glucagon and amino acids are linked in an endocrine feedback circuit, the liver-alpha cell axis, that may be disrupted by NAFLD. We investigated how NAFLD severity affects glucagon and insulin resistance in individuals with obesity and whether bariatric surgery improves these parameters. Plasma and liver biopsies from 33 individuals with obesity (collectively, OBE) were obtained before and 12 months after bariatric surgery (Roux-en-Y gastric bypass [RYGB] or sleeve gastrectomy [SG]). Nine healthy control individuals (collectively, CON) undergoing cholecystectomy were used as a comparison group. The NAFLD activity score (NAS) was used to subdivide study participants into the following groups: OBE-no steatosis, OBE+steatosis, and nonalcoholic steatohepatitis (NASH) and/or grade 2 fibrosis (Fib) (OBE-NASH-Fib). Measurements of amino acids by targeted metabolomics and glucagon were performed. Glucagon, amino acids (P < 0.05), and the glucagon-alanine index, a validated surrogate marker of glucagon resistance, were increased in OBE by 60%, 56%, and 61%, respectively, when compared with CON but irrespective of NAFLD severity. In contrast, markers of hepatic insulin resistance increased concomitantly with NAS. Hyperglucagonemia resolved in OBE-no steatosis and OBE+steatosis but not in OBE-NASH-Fib (median, 7.0; interquartile range, 5.0-9.8 pmol/L), regardless of improvement in insulin resistance and NAS. The type of surgery that participants underwent had no effect on metabolic outcomes. Conclusion: Glucagon resistance to amino acid metabolism exists in individuals with NAFLD independent of NAS severity. Patients with NASH showed persistent hyperglucagonemia 12 months after bariatric surgery, indicating that a disrupted liver-alpha cell may remain in NAFLD despite major improvement in liver histology.

Management of Type 2 Diabetes: Current Strategies, Unfocussed Aspects, Challenges, and Alternatives

Type 2 diabetes mellitus (T2DM) accounts for >90% of the cases of diabetes in adults. Resistance to insulin action is the major cause that leads to chronic hyperglycemia in diabetic patients. T2DM is the consequence of activation of multiple pathways and factors involved in insulin resistance and β-cell dysfunction. Also, the etiology of T2DM involves the complex interplay between genetics and environmental factors. This interplay can be governed efficiently by lifestyle modifications to achieve better management of diabetes. The present review aims at discussing the major factors involved in the development of T2DM that remain unfocussed during the anti-diabetic therapy. The review also focuses on lifestyle modifications that are warranted for the successful management of T2DM. In addition, it attempts to explain flaws in current strategies to combat diabetes. The employability of phytoconstituents as multitargeting molecules and their potential use as effective therapeutic adjuvants to first line hypoglycemic agents to prevent side effects caused by the synthetic drugs are also discussed.

Novel Insights Into Effects of Cortisol and Glucagon on Nocturnal Glucose Production in Type 2 Diabetes

CONTEXT

The effect of physiological changes in night-time cortisol and glucagon on endogenous glucose production (EGP) and nocturnal glycemia are unknown.

OBJECTIVE

To determine the effects of changes in cortisol and glucagon on EGP during the night.

DESIGN

Two overnight protocols were conducted. In Protocol 1, endogenous cortisol was blocked with metyrapone and hydrocortisone infused either at constant (constant) or increasing (variable) rates to mimic basal or physiological nocturnal cortisol concentrations. In Protocol 2, endogenous glucagon was blocked with somatostatin and exogenous glucagon was infused at either basal or elevated rates to mimic nocturnal glucagon concentrations observed in nondiabetic (ND) and type 2 diabetes (T2D) individuals. EGP was measured using [3-3H] glucose and gluconeogenesis estimated with 2H2O in all studies.

SETTING

Mayo Clinic Clinical Research Trials Unit, Rochester, MN, US.

PARTICIPANTS

In Protocol 1, 34 subjects (17 ND and 17 T2D) and in Protocol 2, 39 subjects (21 ND and 18 T2D) were studied.

MAIN OUTCOME MEASURES

Endogenous glucose production.

RESULTS

EGP, gluconeogenesis, and glycogenolysis were higher with variable than with constant cortisol at 7 am in T2D subjects. In contrast, nocturnal EGP did not differ in ND subjects between variable and constant cortisol. While elevated glucagon increased EGP, glycogenolysis, and gluconeogenesis in ND, the data in T2D subjects indicated that EGP and gluconeogenesis but not glycogenolysis were higher during the early part of the night.

CONCLUSION

Nocturnal hyperglucagonemia, but not physiological rise in cortisol, contributes to nocturnal hyperglycemia in T2D due to increased gluconeogenesis.

Fasting glucagon concentrations are associated with longitudinal decline of β-cell function in non-diabetic humans

PURPOSE

Abnormal glucagon concentrations are a feature of prediabetes but it is uncertain if α-cell dysfunction contributes to a longitudinal decline in β-cell function. We therefore sought to determine if a decline in β-cell function is associated with a higher nadir glucagon in the postprandial period or with higher fasting glucagon.

METHODS

This was a longitudinal study in which 73 non-diabetic subjects were studied on 2 occasions 6.6 ± 0.3 years apart using a 2-hour, 7-sample oral glucose tolerance test. Disposition Index (DI) was calculated using the oral minimal model applied to the measurements of glucose, insulin, C-peptide concentrations during the studies. We subsequently examined the relationship of glucagon concentrations at baseline with change in DI (used as a measure of β-cell function) after adjusting for changes in weight and the baseline value of DI.

RESULTS

After adjusting for covariates, nadir postprandial glucagon concentrations were not associated with changes in β-cell function as quantified by DI. On the other hand, fasting glucagon concentrations during the baseline study were inversely correlated with longitudinal changes in DI.

CONCLUSIONS

Defects in α-cell function, manifest as elevated fasting glucagon, are associated with a subsequent decline in β-cell function. It remains to be ascertained if abnormal α-cell function contributes directly to loss of β-cell secretory capacity in the pathogenesis of type 2 diabetes.

Optimizing Postprandial Glucose Management in Adults With Insulin-Requiring Diabetes: Report and Recommendations

Faster-acting insulins, new noninsulin drug classes, more flexible insulin-delivery systems, and improved continuous glucose monitoring devices offer unprecedented opportunities to improve postprandial glucose (PPG) management and overall care for adults with insulin-treated diabetes. These developments led the Endocrine Society to convene a working panel of diabetes experts in December 2018 to assess the current state of PPG management, identify innovative ways to improve self-management and quality of life, and align best practices to current and emerging treatment and monitoring options. Drawing on current research and collective clinical experience, we considered the following issues for the ∼200 million adults worldwide with type 1 and insulin-requiring type 2 diabetes: (i) the role of PPG management in reducing the risk of diabetes complications; (ii) barriers preventing effective PPG management; (iii) strategies to reduce PPG excursions and improve patient quality of life; and (iv) education and clinical tools to support endocrinologists in improving PPG management. We concluded that managing PPG to minimize or prevent diabetes-related complications will require elucidating fundamental questions about optimal ways to quantify and clinically assess the metabolic dysregulation and consequences of the abnormal postprandial state in diabetes and recommend research strategies to address these questions. We also identified practical strategies and tools that are already available to reduce barriers to effective PPG management, optimize use of new and emerging clinical tools, and improve patient self-management and quality of life.

Higher glucagon-to-insulin ratio is associated with elevated glycated hemoglobin levels in type 2 diabetes patients

BACKGROUND/AIMS

The importance of α-cell dysfunction in the pathogenesis of type 2 diabetes has re-emerged recently. However, data on whether relative glucagon excess is present in clinical settings are scarce. We aimed to investigate associations between glucagon-to-insulin ratio and various metabolic parameters.

METHODS

A total of 451 patients with type 2 diabetes naïve to insulin treatment were recruited. Using glucagon-to-insulin ratio, we divided subjects into quartiles according to both fasting and postprandial glucagon-to-insulin ratios.

RESULTS

The mean age of the subjects was 58 years, with a mean body mass index of 25 kg/m2 . The patients in the highest quartile of glucagon-to-insulin ratio had higher glycated hemoglobin (HbA1c) levels. HbA1c levels were positively correlated with both fasting and postprandial glucagon-to-insulin ratios. Subjects in the highest quartile of postprandial glucagon-to-insulin ratio were more likely to exhibit uncontrolled hyperglycemia, even after adjusting for confounding factors (odds ratio, 2.730; 95% confidence interval, 1.236 to 6.028; p for trend < 0.01).

CONCLUSION

Hyperglucagonemia relative to insulin could contribute to uncontrolled hyperglycemia in type 2 diabetes patients.

Current perspective on the role of insulin and glucagon in the pathogenesis and treatment of type 2 diabetes mellitus

According to the World Health Organization, 422 million adults worldwide live with diabetes mellitus (DM), a significant portion of whom have type 2 diabetes. The discovery of insulin as a key regulator of glucose metabolism has revolutionized our understanding of DM and provided several therapeutic avenues. Most studies have so far predominantly focused on the role of insulin in type 2 diabetes. However, the balance between insulin and glucagon is essential in ensuring glucose homeostasis. In this review, we begin by evaluating the principal differences between insulin and glucagon with regard to their mechanism and control of their secretion. Next, we discuss their mode of action and effects on metabolism. We further explore how the two hormones impact the natural history of type 2 diabetes. Finally, we outline how current and emerging pharmacological agents attempt to exploit the properties of insulin and glucagon to benefit patients with type 2 diabetes.

Metabolic effects of glucagon in humans

Diabetes is a common metabolic disorder that involves glucose, amino acids, and fatty acids. Either insulin deficiency or insulin resistance may cause diabetes. Insulin deficiency causes type 1 diabetes and diabetes associated with total pancreatectomy. Glucagon produces insulin resistance. Glucagon-induced insulin resistance promotes type 2 diabetes and diabetes associated with glucagonoma. Further, glucagon-induced insulin resistance aggravates the metabolic consequences of the insulin-deficient state. A major metabolic effect of insulin is the accumulation of glucose as glycogen in the liver. Glucagon opposes hepatic insulin action and enhances the rate of gluconeogenesis, increasing hepatic glucose output. In order to support gluconeogenesis, glucagon promotes skeletal muscle wasting to supply amino acids as gluconeogenic precursors. Glucagon promotes hepatic fatty acid oxidation to supply energy required to sustain gluconeogenesis. Hepatic fatty acid oxidation generates β-hydroxybutyrate and acetoacetate (ketogenesis). Prospective studies reveal that elevated glucagon secretion at baseline occurs in healthy subjects who develop impaired glucose tolerance at follow-up compared with subjects who maintain normal glucose tolerance, suggesting a relationship between elevated glucagon secretion and development of impaired glucose tolerance. Prospective studies have identified animal protein consumption as an independent risk factor for type 2 diabetes and cardiovascular disease. Animal protein intake activates glucagon secretion inducing sustained elevations in plasma glucagon. Glucagon is a major hormone that causes insulin resistance. Insulin resistance is an established cardiovascular risk factor additionally to its pathogenic role in diabetes. Glucagon may be a potential link between animal protein intake and the risk of developing type 2 diabetes and cardiovascular disease.

What Can Diabetes-Associated Genetic Variation in TCF7L2 Teach Us About the Pathogenesis of Type 2 Diabetes?

Type 2 diabetes mellitus (T2DM) is a polygenic metabolic disorder characterized by hyperglycemia occurring as a result of impaired insulin secretion and/or insulin resistance. Among the various genetic factors associated with T2DM, a common genetic variant within the transcription factor 7-like 2 locus (TCF7L2) confers the greatest genetic risk for development of the disease. However, the mechanism(s) by which TCF7L2 predisposes to diabetes remain uncertain. Here we review the current literature pertaining to the potential mechanisms by which TCF7L2 confers risk of T2DM, using genetic variation as a probe to understand the pathogenesis of the disease.

Impaired Insulin Action Is Associated With Increased Glucagon Concentrations in Nondiabetic Humans

CONTEXT

Abnormal glucagon concentrations contribute to hyperglycemia, but the mechanisms of α-cell dysfunction in prediabetes are unclear.

OBJECTIVE

We sought to determine the relative contributions of insulin secretion and action to α-cell dysfunction in nondiabetic participants across the spectrum of glucose tolerance.

DESIGN

This was a cross-sectional study. A subset of participants (n = 120) was studied in the presence and absence of free fatty acid (FFA) elevation, achieved by infusion of Intralipid (Baxter Healthcare, Deerfield, IL) plus heparin, to cause insulin resistance.

SETTING

An inpatient clinical research unit at an academic medical center.

PARTICIPANTS

A total of 310 nondiabetic persons participated in this study.

INTERVENTIONS

Participants underwent a seven-sample oral glucose tolerance test. Subsequently, 120 participants were studied on two occasions. On one day, infusion of Intralipid plus heparin raised FFA. On the other day, participants received glycerol as a control.

MAIN OUTCOME MEASURE(S)

We examined the relationship of glucagon concentration with indices of insulin action after adjusting for the effects of age, sex, and weight. Subsequently, we sought to determine whether an acute decrease in insulin action, produced by FFA elevation, altered glucagon concentrations in nondiabetic participants.

RESULTS

Fasting glucagon concentrations correlated positively with fasting insulin and C-peptide concentrations and inversely with insulin action. Fasting glucagon was not associated with any index of β-cell function in response to an oral challenge. As expected, FFA elevation decreased insulin action and also raised glucagon concentrations.

CONCLUSIONS

In nondiabetic participants, glucagon secretion was altered by changes in insulin action.

The New Biology and Pharmacology of Glucagon

In the last two decades we have witnessed sizable progress in defining the role of gastrointestinal signals in the control of glucose and energy homeostasis. Specifically, the molecular basis of the huge metabolic benefits in bariatric surgery is emerging while novel incretin-based medicines based on endogenous hormones such as glucagon-like peptide 1 and pancreas-derived amylin are improving diabetes management. These and related developments have fostered the discovery of novel insights into endocrine control of systemic metabolism, and in particular a deeper understanding of the importance of communication across vital organs, and specifically the gut-brain-pancreas-liver network. Paradoxically, the pancreatic peptide glucagon has reemerged in this period among a plethora of newly identified metabolic macromolecules, and new data complement and challenge its historical position as a gut hormone involved in metabolic control. The synthesis of glucagon analogs that are biophysically stable and soluble in aqueous solutions has promoted biological study that has enriched our understanding of glucagon biology and ironically recruited glucagon agonism as a central element to lower body weight in the treatment of metabolic disease. This review summarizes the extensive historical record and the more recent provocative direction that integrates the prominent role of glucagon in glucose elevation with its under-acknowledged effects on lipids, body weight, and vascular health that have implications for the pathophysiology of metabolic diseases, and the emergence of precision medicines to treat metabolic diseases.

Differential circulating concentrations of adipokines, glucagon and adropin in a clinical population of lean, overweight and diabetic cats

BACKGROUND

Dyslipidemia, dysregulated adipokine secretion and alteration in glucagon and adropin concentrations are important obesity-related factors in the pathophysiology of human Type 2 diabetes; however, their roles in the pathophysiology of feline diabetes mellitus are relatively unknown. Here, we determined the concentrations of circulating leptin, adiponectin, pro-inflammatory cytokines, glucagon, adropin, triglycerides, and cholesterol, in non-diabetic lean and overweight cats and newly diagnosed diabetic cats. Client-owned cats were recruited and assigned into 3 study groups: lean, overweight and diabetic. Fasting blood samples were analyzed in lean, overweight and diabetic cats at baseline and 4 weeks after consumption of high protein/low carbohydrate standardized diet.

RESULTS

Serum concentrations of triglycerides were greater in diabetics at baseline and were increased in both diabetic and overweight cats at 4 weeks. Plasma leptin concentrations were greater in diabetic and overweight at baseline and 4 weeks, whereas adiponectin was lower in diabetics compared to lean and overweight cats at baseline and 4 weeks. Diabetics had greater baseline plasma glucagon concentrations compared to lean, lower adropin than overweight at 4 weeks, and lower IL-12 concentrations at 4 weeks than baseline.

CONCLUSIONS

Our results suggest that feline obesity and diabetes mellitus are characterized by hypertriglyceridemia and hyperleptinemia; however, diabetic cats have significantly lower adiponectin and adropin compared to overweight cats. Thus, despite having similar body condition, overweight and diabetic cats have differential circulating concentrations of adiponectin and adropin.

A Plethora of GLP-1 Agonists: Decisions About What to Use and When

Incretin-based therapies are important addition to our armamentarium for the treatment of type 2 diabetes (T2DM). There are six Glucagon-like peptide-1 receptor agonists (GLP-1RAs) which have received regulatory approval for clinical use. The short-acting GLP-1RAs include exenatide twice daily, liraglutide once daily, and lixisenatide once daily. The approved long-acting GLP-1RAs are administered weekly and are exenatide, albiglutide, and dulaglutide. Although all of these therapies lower hemoglobin A1C (HbA1C), there also are unique features of GLP-1RAs that have been made manifest from clinical trial data with regard to weight-loss efficacy, fasting and post-prandial glucose control, cardiovascular safety and protection, and gastrointestinal and injection adverse effects. It is imperative to consider these features when tailoring the choice of a GLP-1RA to patient specific characteristics.

Mechanisms Underlying the Pathogenesis of Isolated Impaired Glucose Tolerance in Humans

CONTEXT

Prediabetes is a heterogeneous disorder classified on the basis of fasting glucose concentrations and 2-hour glucose tolerance.

OBJECTIVE

We sought to determine the relative contributions of insulin secretion and action to the pathogenesis of isolated impaired glucose tolerance (IGT).

DESIGN

The study consisted of an oral glucose tolerance test and a euglycemic clamp performed in two cohorts matched for anthropometric characteristics and fasting glucose but discordant for glucose tolerance.

SETTING

An inpatient clinical research unit at an academic medical center.

PATIENTS OR OTHER PARTICIPANTS

Twenty-five subjects who had normal fasting glucose (NFG) and normal glucose tolerance (NGT) and 19 NFG/IGT subjects participated in this study.

INTERVENTION(S)

Subjects underwent a seven-sample oral glucose tolerance test and a 4-hour euglycemic, hyperinsulinemic clamp on separate occasions. Glucose turnover during the clamp was measured using tracers, and endogenous hormone secretion was inhibited by somatostatin.

MAIN OUTCOME MEASURES

We sought to determine whether hepatic glucose metabolism, specifically the contribution of gluconeogenesis to endogenous glucose production, differed between subjects with NFG/NGT and those with NFG/IGT.

RESULTS

Endogenous glucose production did not differ between groups before or during the clamp. Insulin-stimulated glucose disappearance was lower in NFG/IGT (24.6 ± 2.2 vs 35.0 ± 3.6 μmol/kg/min; P = .03). The disposition index was decreased in NFG/IGT (681 ± 102 vs 2231 ± 413 × 10^-14 dL/kg/min² per pmol/L; P < .001).

CONCLUSIONS

We conclude that innate defects in the regulation of glycogenolysis and gluconeogenesis do not contribute to NFG/IGT. However, insulin-stimulated glucose disposal is impaired, exacerbating defects in β-cell function.

TCF7L2 Genotype and α-Cell Function in Humans Without Diabetes

The diabetes-associated allele in TCF7L2 increases the rate of conversion to diabetes; however, the mechanism by which this occurs remains elusive. We hypothesized that the diabetes-associated allele in this locus (rs7903146) impairs insulin secretion and that this defect would be exacerbated by acute free fatty acid (FFA)-induced insulin resistance. We studied 120 individuals of whom one-half were homozygous for the diabetes-associated allele TT at rs7903146 and one-half were homozygous for the protective allele CC. After a screening examination during which glucose tolerance status was determined, subjects were studied on two occasions in random order while undergoing an oral challenge. During one study day, FFA was elevated by infusion of Intralipid plus heparin. On the other study day, subjects received the same amount of glycerol as present in the Intralipid infusion. β-Cell responsivity indices were estimated with the oral C-peptide minimal model. We report that β-cell responsivity was slightly impaired in the TT genotype group. Moreover, the hyperbolic relationship between insulin secretion and β-cell responsivity differed significantly between genotypes. Subjects also exhibited impaired suppression of glucagon after an oral challenge. These data imply that a genetic variant harbored within the TCF7L2 locus impairs glucose tolerance through effects on glucagon as well as on insulin secretion.

Physiology of proglucagon peptides: role of glucagon and GLP-1 in health and disease

The preproglucagon gene (Gcg) is expressed by specific enteroendocrine cells (L-cells) of the intestinal mucosa, pancreatic islet α-cells, and a discrete set of neurons within the nucleus of the solitary tract. Gcg encodes multiple peptides including glucagon, glucagon-like peptide-1, glucagon-like peptide-2, oxyntomodulin, and glicentin. Of these, glucagon and GLP-1 have received the most attention because of important roles in glucose metabolism, involvement in diabetes and other disorders, and application to therapeutics. The generally accepted model is that GLP-1 improves glucose homeostasis indirectly via stimulation of nutrient-induced insulin release and by reducing glucagon secretion. Yet the body of literature surrounding GLP-1 physiology reveals an incompletely understood and complex system that includes peripheral and central GLP-1 actions to regulate energy and glucose homeostasis. On the other hand, glucagon is established principally as a counterregulatory hormone, increasing in response to physiological challenges that threaten adequate blood glucose levels and driving glucose production to restore euglycemia. However, there also exists a potential role for glucagon in regulating energy expenditure that has recently been suggested in pharmacological studies. It is also becoming apparent that there is cross-talk between the proglucagon derived-peptides, e.g., GLP-1 inhibits glucagon secretion, and some additive or synergistic pharmacological interaction between GLP-1 and glucagon, e.g., dual glucagon/GLP-1 agonists cause more weight loss than single agonists. In this review, we discuss the physiological functions of both glucagon and GLP-1 by comparing and contrasting how these peptides function, variably in concert and opposition, to regulate glucose and energy homeostasis.

LKB1 and AMPKα1 are required in pancreatic alpha cells for the normal regulation of glucagon secretion and responses to hypoglycemia

AIMS/HYPOTHESIS

Glucagon release from pancreatic alpha cells is required for normal glucose homoeostasis and is dysregulated in both Type 1 and Type 2 diabetes. The tumour suppressor LKB1 (STK11) and the downstream kinase AMP-activated protein kinase (AMPK), modulate cellular metabolism and growth, and AMPK is an important target of the anti-hyperglycaemic agent metformin. While LKB1 and AMPK have emerged recently as regulators of beta cell mass and insulin secretion, the role of these enzymes in the control of glucagon production in vivo is unclear.

METHODS

Here, we ablated LKB1 (αLKB1KO), or the catalytic alpha subunits of AMPK (αAMPKdKO, -α1KO, -α2KO), selectively in ∼45% of alpha cells in mice by deleting the corresponding flox'd alleles with a preproglucagon promoter (PPG) Cre.

RESULTS

Blood glucose levels in male αLKB1KO mice were lower during intraperitoneal glucose, aminoimidazole carboxamide ribonucleotide (AICAR) or arginine tolerance tests, and glucose infusion rates were increased in hypoglycemic clamps (p < 0.01). αLKB1KO mice also displayed impaired hypoglycemia-induced glucagon release. Glucose infusion rates were also elevated (p < 0.001) in αAMPKα1 null mice, and hypoglycemia-induced plasma glucagon increases tended to be lower (p = 0.06). Glucagon secretion from isolated islets was sensitized to the inhibitory action of glucose in αLKB1KO, αAMPKdKO, and -α1KO, but not -α2KO islets.

CONCLUSIONS/INTERPRETATION

An LKB1-dependent signalling cassette, involving but not restricted to AMPKα1, is required in pancreatic alpha cells for the control of glucagon release by glucose.

Pancreatic α-Cell Dysfunction in Type 2 Diabetes: Old Kids on the Block

Type 2 diabetes (T2D) has been known as 'bi-hormonal disorder' since decades ago, the role of glucagon from α-cell has languished whereas β-cell taking center stage. Recently, numerous findings indicate that the defects of glucagon secretion get involve with development and exacerbation of hyperglycemia in T2D. Aberrant α-cell responses exhibit both fasting and postprandial states: hyperglucagonemia contributes to fasting hyperglycemia caused by inappropriate hepatic glucose production, and to postprandial hyperglycemia owing to blunted α-cell suppression. During hypoglycemia, insufficient counter-regulation response is also observed in advanced T2D. Though many debates still remained for exact mechanisms behind the dysregulation of α-cell in T2D, it is clear that the blockade of glucagon receptor or suppression of glucagon secretion from α-cell would be novel therapeutic targets for control of hyperglycemia. Whereas there have not been remarkable advances in developing new class of drugs, currently available glucagon-like peptide-1 and dipeptidyl peptidase-IV inhibitors could be options for treatment of hyperglucagonemia. In this review, we focus on α-cell dysfunction and therapeutic potentials of targeting α-cell in T2D.

Recent Progress in the Use of Glucagon and Glucagon Receptor Antago-nists in the Treatment of Diabetes Mellitus

Glucagon is an important pancreatic hormone, released into blood circulation by alpha cells of the islet of Langerhans. Glucagon induces gluconeogenesis and glycogenolysis in hepatocytes, leading to an increase in hepatic glucose production and subsequently hyperglycemia in susceptible individuals. Hyperglucagonemia is a constant feature in patients with T2DM. A number of bioactive agents that can block glucagon receptor have been identified. These glucagon receptor antagonists can reduce the hyperglycemia associated with exogenous glucagon administration in normal as well as diabetic subjects. Glucagon receptor antagonists include isoserine and beta-alanine derivatives, bicyclic 19-residue peptide BI-32169, Des-His1-[Glu9] glucagon amide and related compounds, 5-hydroxyalkyl-4-phenylpyridines, N-[3-cano-6- (1,1 dimethylpropyl)-4,5,6,7-tetrahydro-1-benzothien-2-yl]-2-ethylbutamide, Skyrin and NNC 250926. The absorption, dosage, catabolism, excretion and medicinal chemistry of these agents are the subject of this review. It emphasizes the role of glucagon in glucose homeostasis and how it could be applied as a novel tool for the management of diabetes mellitus by blocking its receptors with either monoclonal antibodies, peptide and non-peptide antagonists or gene knockout techniques.

Glucagon and type 2 diabetes: the return of the alpha cell

In normal physiology, glucagon from pancreatic alpha cells plays an important role in maintaining glucose homeostasis via its regulatory effect on hepatic glucose production. Patients with type 2 diabetes suffer from fasting and postprandial hyperglucagonemia, which stimulate hepatic glucose production and, thus, contribute to the hyperglycemia characterizing these patients. Although this has been known for years, research focusing on alpha cell (patho)physiology has historically been dwarfed by research on beta cells and insulin. Today the mechanisms behind type 2 diabetic hyperglucagonemia are still poorly understood. Preclinical and clinical studies have shown that the gastrointestinal hormone glucose-dependent insulinotropic polypeptide (GIP) might play an important role in this pathophysiological phenomenon. Furthermore, it has become apparent that suppression of glucagon secretion or antagonization of the glucagon receptor constitutes potentially effective treatment strategies for patients with type 2 diabetes. In this review, we focus on the regulation of glucagon secretion by the incretin hormones glucagon-like peptide-1 (GLP-1) and GIP. Furthermore, potential advantages and limitations of suppressing glucagon secretion or antagonizing the glucagon receptor, respectively, in the treatment of patients with type 2 diabetes will be discussed.

A novel GPR40 agonist, CNX-011-67, suppresses glucagon secretion in pancreatic islets under chronic glucolipotoxic conditions in vitro

BACKGROUND

Elevated glucose concentrations lead to increased insulin secretion and suppression of glucagon secretion. In fact, insulin is a physiological inhibitor of glucagon secretion. Type 2 diabetes mellitus (T2DM) patients have defects in insulin secretion. In addition to this, lack of suppression of glucagon secretion under elevated glucose concentrations is also observed in T2DM patients. We have earlier shown that GPR40 activation by CNX-011-67 stimulates glucose stimulated insulin secretion (GSIS). Here we extended our studies to examine the impact of GPR40 activation by CNX-011-67 on glucagon secretion from intact islets under both normal and glucolipotoxic conditions.

FINDINGS

Glucagon secretion from intact rat islets was suppressed under elevated glucose concentration. Activation of GPR40 by CNX-011-67 further suppressed glucagon secretion. Culturing islets under chronic glucolipotoxic (GL) conditions, we have observed increased high glucose mediated glucagon secretion and content which were reduced with GPR40 activation by CNX-011-67. Interestingly, expression of pre-proglucagon gene (GCG) remained unchanged under glucolipotoxicity in the presence or absence of GPR40 activation.

CONCLUSION

Activation of GPR40 by CNX-011-67 can reduce glucagon secretion from pancreatic islets.

Alteration of de novo glucose production contributes to fasting hypoglycaemia in Fyn deficient mice

Previous studies have demonstrated that glucose disposal is increased in the Fyn knockout (FynKO) mice due to increased insulin sensitivity. FynKO mice also display fasting hypoglycaemia despite decreased insulin levels, which suggested that hepatic glucose production was unable to compensate for the increased basal glucose utilization. The present study investigates the basis for the reduction in plasma glucose levels and the reduced ability for the liver to produce glucose in response to gluconeogenic substrates. FynKO mice had a 5-fold reduction in phosphoenolpyruvate carboxykinase (PEPCK) gene and protein expression and a marked reduction in pyruvate, pyruvate/lactate-stimulated glucose output. Remarkably, de novo glucose production was also blunted using gluconeogenic substrates that bypass the PEPCK step. Impaired conversion of glycerol to glucose was observed in both glycerol tolerance test and determination of the conversion of (13)C-glycerol to glucose in the fasted state. α-glycerol phosphate levels were reduced but glycerol kinase protein expression levels were not changed. Fructose-driven glucose production was also diminished without alteration of fructokinase expression levels. The normal levels of dihydroxyacetone phosphate and glyceraldehyde-3-phosphate observed in the FynKO liver extracts suggested normal triose kinase function. Fructose-bisphosphate aldolase (aldolase) mRNA or protein levels were normal in the Fyn-deficient livers, however, there was a large reduction in liver fructose-6-phosphate (30-fold) and fructose-1,6-bisphosphate (7-fold) levels as well as a reduction in glucose-6-phosphate (2-fold) levels. These data suggest a mechanistic defect in the allosteric regulation of aldolase activity.

Serotonin 2C receptors in pro-opiomelanocortin neurons regulate energy and glucose homeostasis

Energy and glucose homeostasis are regulated by central serotonin 2C receptors. These receptors are attractive pharmacological targets for the treatment of obesity; however, the identity of the serotonin 2C receptor-expressing neurons that mediate the effects of serotonin and serotonin 2C receptor agonists on energy and glucose homeostasis are unknown. Here, we show that mice lacking serotonin 2C receptors (Htr2c) specifically in pro-opiomelanocortin (POMC) neurons had normal body weight but developed glucoregulatory defects including hyperinsulinemia, hyperglucagonemia, hyperglycemia, and insulin resistance. Moreover, these mice did not show anorectic responses to serotonergic agents that suppress appetite and developed hyperphagia and obesity when they were fed a high-fat/high-sugar diet. A requirement of serotonin 2C receptors in POMC neurons for the maintenance of normal energy and glucose homeostasis was further demonstrated when Htr2c loss was induced in POMC neurons in adult mice using a tamoxifen-inducible POMC-cre system. These data demonstrate that serotonin 2C receptor-expressing POMC neurons are required to control energy and glucose homeostasis and implicate POMC neurons as the target for the effect of serotonin 2C receptor agonists on weight-loss induction and improved glycemic control.

Current insights and new perspectives on the roles of hyperglucagonemia in non-insulin-dependent type 2 diabetes

Type 2 diabetes is well recognized as a noninsulin-dependent diabetic disease. Clinical evidence indicates that the level of circulating insulin may be normal, subnormal, and even elevated in type 2 diabetic patients. Unlike type 1 diabetes, the key problem for type 2 diabetes is not due to the absolute deficiency of insulin secretion, but because the body is no longer sensitive to insulin. Thus, insulin resistance is increased and the sensitivity to insulin is reset, so increasing levels of insulin are required to maintain body glucose and metabolic homeostasis. How insulin resistance is increased and what factors contribute to its development in type 2 diabetes remain incompletely understood. Overemphasis of insulin deficiency alone may be too simplistic for us to understand how type 2 diabetes is developed and should be treated, since glucose metabolism and homeostasis are tightly controlled by both insulin and glucagon. Insulin acts as a YIN factor to lower blood glucose level by increasing cellular glucose uptake, whereas glucagon acts as a YANG factor to counter the action of insulin by increasing glucose production. Furthermore, other humoral factors other than insulin and glucagon may also directly or indirectly contribute to increased insulin resistance and the development of hyperglycemia. The purpose of this article is to briefly review recently published animal and human studies in this field, and provide new insights and perspectives on recent debates as to whether hyperglucagonemia and/or glucagon receptors should be targeted to treat insulin resistance and target organ injury in type 2 diabetes.

Metabolic effects of bariatric surgery in patients with moderate obesity and type 2 diabetes: analysis of a randomized control trial comparing surgery with intensive medical treatment

OBJECTIVE

To evaluate the effects of two bariatric procedures versus intensive medical therapy (IMT) on β-cell function and body composition.

RESEARCH DESIGN AND METHODS

This was a prospective, randomized, controlled trial of 60 subjects with uncontrolled type 2 diabetes (HbA1c 9.7 ± 1%) and moderate obesity (BMI 36 ± 2 kg/m(2)) randomized to IMT alone, IMT plus Roux-en-Y gastric bypass, or IMT plus sleeve gastrectomy. Assessment of β-cell function (mixed-meal tolerance testing) and body composition was performed at baseline and 12 and 24 months.

RESULTS

Glycemic control improved in all three groups at 24 months (N = 54), with a mean HbA1c of 6.7 ± 1.2% for gastric bypass, 7.1 ± 0.8% for sleeve gastrectomy, and 8.4 ± 2.3% for IMT (P < 0.05 for each surgical group versus IMT). Reduction in body fat was similar for both surgery groups, with greater absolute reduction in truncal fat in gastric bypass versus sleeve gastrectomy (-16 vs. -10%; P = 0.04). Insulin sensitivity increased significantly from baseline in gastric bypass (2.7-fold; P = 0.004) and did not change in sleeve gastrectomy or IMT. β-Cell function (oral disposition index) increased 5.8-fold in gastric bypass from baseline, was markedly greater than IMT (P = 0.001), and was not different between sleeve gastrectomy versus IMT (P = 0.30). At 24 months, β-cell function inversely correlated with truncal fat and prandial free fatty acid levels.

CONCLUSIONS

Bariatric surgery provides durable glycemic control compared with intensive medical therapy at 2 years. Despite similar weight loss as sleeve gastrectomy, gastric bypass uniquely restores pancreatic β-cell function and reduces truncal fat, thus reversing the core defects in diabetes.

A computational systems analysis of factors regulating α cell glucagon secretion

Glucagon, a peptide hormone secreted from the α-cells of the pancreatic islets, is critical for blood glucose homeostasis. We reviewed the literature and employed a computational systems analysis of intracellular metabolic and electrical regulation of glucagon secretion to better understand these processes. The mathematical model of α-cell metabolic parameters is based on our previous model for pancreatic β-cells. We also formulated an ionic model for action potentials that incorporates Ca ( 2+) , K (+) , Na (+) and Cl (-) currents. Metabolic and ionic models are coupled to the equations describing Ca ( 2+) homeostasis and glucagon secretion that depends on activation of specific voltage-gated Ca ( 2+) channels. Paracrine and endocrine regulations were analyzed with an emphasis on their effects on a hyperpolarization of membrane potential. This general model simulates and gives insight into the mechanisms of regulation of glucagon secretion under a wide range of experimental conditions. We also reviewed and analyzed dysfunctional mechanisms in α-cells to determine key pharmacological targets for modulating glucagon secretion in type 1 and 2 diabetes.

Minireview: Glucagon in the pathogenesis of hypoglycemia and hyperglycemia in diabetes

Pancreatic islet α-cell glucagon secretion is critically dependent on pancreatic islet β-cell insulin secretion. Normally, a decrease in the plasma glucose concentration causes a decrease in β-cell insulin secretion that signals an increase in α-cell glucagon secretion during hypoglycemia. In contrast, an increase in the plasma glucose concentration, among other stimuli, causes an increase in β-cell insulin secretion that signals a decrease, or at least no change, in α-cell glucagon secretion after a meal. In absolute endogenous insulin deficiency (i.e. in type 1 diabetes and in advanced type 2 diabetes), however, β-cell failure results in no decrease in β-cell insulin secretion and thus no increase in α-cell glucagon secretion during hypoglycemia and no increase in β-cell insulin secretion and thus an increase in α-cell glucagon secretion after a meal. In type 1 diabetes and advanced type 2 diabetes, the absence of an increment in glucagon secretion, in the setting of an absent decrement in insulin secretion and an attenuated increment in sympathoadrenal activity, in response to falling plasma glucose concentrations plays a key role in the pathogenesis of iatrogenic hypoglycemia. In addition, there is increasing evidence that, in the aggregate, suggests that relative hyperglucagonemia, in the setting of deficient insulin secretion, plays a role in the pathogenesis of hyperglycemia in diabetes. If so, abnormal glucagon secretion is involved in the pathogenesis of both hypoglycemia and hyperglycemia in diabetes.

The effects of TAK-875, a selective G protein-coupled receptor 40/free fatty acid 1 agonist, on insulin and glucagon secretion in isolated rat and human islets

G protein-coupled receptor 40 (GPR40)/free fatty acid 1 (FFA1) is a G protein-coupled receptor involved in free fatty acid-induced insulin secretion. To analyze the effect of our novel GPR40/FFA1-selective agonist, [(3S)-6-({2',6'-dimethyl-4'-[3-(methylsulfonyl)propoxy]biphenyl-3-yl}methoxy)-2,3-dihydro-1-benzofuran-3-yl]acetic acid hemi-hydrate (TAK-875), on insulin and glucagon secretion, we performed hormone secretion assays and measured intracellular Ca²⁺ concentration ([Ca²⁺](i)) in both human and rat islets. Insulin and glucagon secretion were measured in static and dynamic conditions by using groups of isolated rat and human pancreatic islets. [Ca²⁺](i) was recorded by using confocal microscopy. GPR40/FFA1 expression was measured by quantitative polymerase chain reaction. In both human and rat islets, TAK-875 enhanced glucose-induced insulin secretion in a glucose-dependent manner. The stimulatory effect of TAK-875 was similar to that produced by glucagon-like peptide-1 and correlated with the elevation of β-cell [Ca²⁺](i). TAK-875 was without effect on glucagon secretion at both 1 and 16 mM glucose in human islets. These data indicate that GPR40/FFA1 influences mainly insulin secretion in a glucose-dependent manner. The β-cell-specific action of TAK-875 in human islets may represent a therapeutically useful feature that allows plasma glucose control without compromising counter-regulation of glucagon secretion, thus minimizing the risk of hypoglycemia.

The alpha-cell as target for type 2 diabetes therapy

Glucagon is the main secretory product of the pancreatic alpha-cells. The main function of this peptide hormone is to provide sustained glucose supply to the brain and other vital organs during fasting conditions. This is exerted by stimulation of hepatic glucose production via specific G protein-coupled receptors in the hepatocytes. Type 2 diabetic patients are characterized by elevated glucagon levels contributing decisively to hyperglycemia in these patients. Accumulating evidence demonstrates that targeting the pancreatic alpha-cell and its main secretory product glucagon is a possible treatment for type 2 diabetes. Several lines of preclinical evidence have paved the way for the development of drugs, which suppress glucagon secretion or antagonize the glucagon receptor. In this review, the physiological actions of glucagon and the role of glucagon in type 2 diabetic pathophysiology are outlined. Furthermore, potential advantages and limitations of antagonizing the glucagon receptor or suppressing glucagon secretion in the treatment of type 2 diabetes are discussed with a focus on already marketed drugs and drugs in clinical development. It is concluded that the development of novel glucagon receptor antagonists are confronted with several safety issues. At present, available pharmacological agents based on the glucose-dependent glucagonostatic effects of GLP-1 represent the most favorable way to apply constraints to the alpha-cell in type 2 diabetes.

Effects of exenatide twice daily versus sitagliptin on 24-h glucose, glucoregulatory and hormonal measures: a randomized, double-blind, crossover study

AIM

To compare exenatide and sitagliptin glucose and glucoregulatory measures in subjects with type 2 diabetes.

METHODS

An 8-week, double-blind, randomized, crossover, single-centre study. Eighty-six subjects (58% female, body mass index 35 ± 5 kg/m², haemoglobin A1c 8.3 ± 1.0%) received either exenatide 10 µg (subcutaneous) twice daily or sitagliptin 100 mg (oral) daily for 4 weeks and crossed to the other therapy for an additional 4 weeks. Main outcome was time-averaged glucose during the 24-h inpatient visits.

RESULTS

Both treatments decreased average 24-h glucose, but exenatide had a greater effect [between-group difference: -0.67 mmol/l, 95% confidence interval (CI): -0.9 to -0.4 mmol/l]. Both treatments decreased 2-h postprandial glucose (PPG), area under the curve of glucose above 7.8 mmol/l (140 mg/dl) and 11 mmol/l (200 mg/dl) and increased the time spent with glucose between 3.9 and 7.8 mmol/l (70 and 140 mg/dl) during 24 h, but exenatide had a significantly greater effect (p < 0.05). Both treatments decreased postprandial serum glucagon, with exenatide having a greater effect (p < 0.005). Both treatments decreased fasting blood glucose to a similar degree (p = 0.766). Sitagliptin increased, while exenatide decreased, postprandial intact glucagon-like peptide-1. Both drugs improved homeostasis model assessment of β-cell function (HOMA-B), with exenatide having a significantly greater effect (p = 0.005). Both exenatide and sitagliptin decreased 24-h caloric intake, with exenatide having a greater effect (p < 0.001). There was no episode of major hypoglycaemia. Adverse events were mild to moderate and mostly gastrointestinal in nature with exenatide. No study withdrawals were due to an adverse event.

CONCLUSION

Compared to sitagliptin, exenatide showed significantly lower average 24-h glucose, 2-h PPG, glucagon, caloric intake and improved HOMA-B.

Optimizing reduction in basal hyperglucagonaemia to repair defective glucagon counterregulation in insulin deficiency

In health, the pancreatic islet cells work as a network with highly co-ordinated signals over time to balance glycaemia within a narrow range. In type 1 diabetes (T1DM), with autoimmune destruction of the β-cells, lack of insulin is considered the primary abnormality and is the primary therapy target. However, replacing insulin alone does not achieve adequate glucose control and recent studies have focused on controlling the endogenous glucagon release as well. In T1DM, glucagon secretion is disordered but not absolutely deficient; it may be excessive postprandially yet it is characteristically insufficient and delayed in response to hypoglycaemia. We review our system-level analysis of the pancreatic endocrine network mechanisms of glucagon counterregulation (GCR) and their dysregulation in T1DM and focus on possible use of α-cell inhibitors (ACIs) to manipulate the glucagon axis to repair the defective GCR. Our results indicate that the GCR abnormalities are of 'network origin'. The lack of β-cell signalling is the primary deficiency that contributes to two separate network abnormalities: (i) absence of a β-cell switch-off trigger and (ii) increased intraislet basal glucagon. A strategy to repair these abnormalities with ACI is proposed, which could achieve better control of glycaemia with reduced hypoglycaemia risk.

CrbpI modulates glucose homeostasis and pancreas 9-cis-retinoic acid concentrations

Cellular retinol-binding protein type I (CrbpI), encoded by Rpb1, serves as a chaperone of retinol homeostasis, but its physiological effects remain incompletely understood. We show here that the Rbp1(-/-) mouse has disrupted retinoid homeostasis in multiple tissues, with abnormally high 9-cis-retinoic acid (9cRA), a pancreas autacoid that attenuates glucose-stimulated insulin secretion. The Rbp1(-/-) pancreas has increased retinol and intense ectopic expression of Rpb2 mRNA, which encodes CrbpII: both would contribute to increased β-cell 9cRA biosynthesis. 9cRA in Rbp1(-/-) pancreas resists postprandial and glucose-induced decreases. Rbp1(-/-) mice have defective islet expression of genes involved in glucose sensing and insulin secretion, as well as islet α-cell infiltration, which contribute to reduced glucose-stimulated insulin secretion, high glucagon secretion, an abnormally high rate of gluconeogenesis, and hyperglycemia. A diet rich in vitamin A (as in a standard chow diet) increases pancreas 9cRA and impairs glucose tolerance. Crbp1 attenuates the negative impact of vitamin A (retinol) on glucose tolerance, regardless of the dietary retinol content. Rbp1(-/-) mice have an increased rate of fatty acid oxidation and resist obesity when fed a high-fat diet. Thus, glucose homeostasis and energy metabolism rely on Rbp1 expression and its moderation of pancreas retinol and of the autacoid 9cRA.

The separate and combined impact of the intestinal hormones, GIP, GLP-1, and GLP-2, on glucagon secretion in type 2 diabetes

Type 2 diabetes mellitus (T2DM) is associated with reduced suppression of glucagon during oral glucose tolerance test (OGTT), whereas isoglycemic intravenous glucose infusion (IIGI) results in normal glucagon suppression in these patients. We examined the role of the intestinal hormones glucose-dependent insulinotropic polypeptide (GIP), glucagon-like peptide-1 (GLP-1), and glucagon-like peptide-2 (GLP-2) in this discrepancy. Glucagon responses were measured during a 3-h 50-g OGTT (day A) and an IIGI (day B) in 10 patients with T2DM [age (mean ± SE), 51 ± 3 yr; body mass index, 33 ± 2 kg/m(2); HbA(1c), 6.5 ± 0.2%]. During four additional IIGIs, GIP (day C), GLP-1 (day D), GLP-2 (day E) and a combination of the three (day F) were infused intravenously. Isoglycemia during all six study days was obtained. As expected, no suppression of glucagon occurred during the initial phase of the OGTT, whereas significantly (P < 0.05) lower plasma levels of glucagon during the first 30 min of the IIGI (day B) were observed. The glucagon response during the IIGI + GIP + GLP-1 + GLP-2 infusion (day F) equaled the inappropriate glucagon response to OGTT (P = not significant). The separate GIP infusion (day C) elicited significant hypersecretion of glucagon, whereas GLP-1 infusion (day D) resulted in enhancement of glucagon suppression during IIGI. IIGI + GLP-2 infusion (day E) resulted in a glucagon response in the midrange between the glucagon responses to OGTT and IIGI. Our results indicate that the intestinal hormones, GIP, GLP-1, and GLP-2, may play a role in the inappropriate glucagon response to orally ingested glucose in T2DM with, especially, GIP, acting to increase glucagon secretion.

Differential contribution of dietary fat and monosaccharide to metabolic syndrome in the common marmoset (Callithrix jacchus)

There is a critical need for animal models to study aspects type 2 diabetes (T2D) pathogenesis and prevention. While the rhesus macaque is such an established model, the common marmoset has added benefits including reduced zoonotic risks, shorter life span, and a predisposition to birth twins demonstrating chimerism. The marmoset as a model organism for the study of metabolic syndrome has not been fully evaluated. Marmosets fed high-fat or glucose-enriched diets were followed longitudinally to observe effects on morphometric and metabolic measures. Effects on pancreatic histomorphometry and vascular pathology were examined terminally. The glucose-enriched diet group developed an obese phenotype and a prolonged hyperglycemic state evidenced by a rapid and persistent increase in mean glycosylated hemoglobin (HgbA1c) observed as early as week 16. In contrast, marmosets fed a high-fat diet did not maintain an obese phenotype and demonstrated a delayed increase in HgbA1) that did not reach statistical significance until week 40. Consumption of either diet resulted in profound pancreatic islet hyperplasia suggesting a compensation for increased insulin requirements. Although the high-fat diet group developed atherosclerosis of increased severity, the presence of lesions correlated with glucose intolerance only in the glucose-enriched diet group. The altered timing of glucose dysregulation, differential contribution to obesity, and variation in vascular pathology suggests mechanisms of effect specific to dietary nutrient content. Feeding nutritionally modified diets to common marmosets recapitulates aspects of metabolic disease and represents a model that may prove instrumental to elucidating the contribution of nutrient excess to disease development.

Liver enzymes are associated with hepatic insulin resistance, insulin secretion, and glucagon concentration in healthy men and women

OBJECTIVE

The pathophysiological mechanisms to explain the association between risk of type 2 diabetes and elevated concentrations of γ-glutamyltransferase (GGT) and alanineaminotransferase (ALT) remain poorly characterized. We explored the association of liver enzymes with peripheral and hepatic insulin resistance, insulin secretion, insulin clearance, and glucagon concentration.

RESEARCH DESIGN AND METHODS

We studied 1,309 nondiabetic individuals from the Relationship between Insulin Sensitivity and Cardiovascular disease (RISC) study; all had a euglycemic-hyperinsulinemic clamp and an oral glucose tolerance test (OGTT) with assessment of insulin secretion and hepatic insulin extraction. The hepatic insulin resistance index was calculated in 393 individuals.

RESULTS

In both men and women, plasma concentrations of GGT and ALT were inversely related with insulin sensitivity (M/I) (all P < 0.01). Likewise, the hepatic insulin resistance index was positively correlated with both GGT (r = 0.37, P < 0.0001, men; r = 0.36, P < 0.0001, women) and ALT (r = 0.25, P = 0.0005, men; r = 0.18, P = 0.01, women). These associations persisted in multivariable models. Increased GGT and ALT were significantly associated with higher insulin secretion rates and with both reduced endogenous clearance of insulin and hepatic insulin extraction during the OGTT (P = 0.0005 in men; P = 0.003 in women). Plasma fasting glucagon levels increased over ALT quartiles (men, quartile 4 vs. quartile 1 11.2 ± 5.1 vs. 9.3 ± 3.8 pmol/L, respectively, P = 0.0002; women, 9.0 ± 4.3 vs. 7.6 ± 3.1, P = 0.001).

CONCLUSIONS

In healthy individuals, increased GGT and ALT were biomarkers of both systemic and hepatic insulin resistance with concomitant increased insulin secretion and decreased hepatic insulin clearance. The novel finding of a positive correlation between ALT and fasting glucagon level concentrations warrants confirmation in type 2 diabetes.

Dual elimination of the glucagon and GLP-1 receptors in mice reveals plasticity in the incretin axis

Disordered glucagon secretion contributes to the symptoms of diabetes, and reduced glucagon action is known to improve glucose homeostasis. In mice, genetic deletion of the glucagon receptor (Gcgr) results in increased levels of the insulinotropic hormone glucagon-like peptide 1 (GLP-1), which may contribute to the alterations in glucose homeostasis observed in Gcgr-/- mice. Here, we assessed the contribution of GLP-1 receptor (GLP-1R) signaling to the phenotype of Gcgr-/- mice by generating Gcgr-/-Glp1r-/- mice. Although insulin sensitivity was similar in all genotypes, fasting glucose was increased in Gcgr-/-Glp1r-/- mice. Elimination of the Glp1r normalized gastric emptying and impaired intraperitoneal glucose tolerance in Gcgr-/- mice. Unexpectedly, deletion of Glp1r in Gcgr-/- mice did not alter the improved oral glucose tolerance and increased insulin secretion characteristic of that genotype. Although Gcgr-/-Glp1r-/- islets exhibited increased sensitivity to the incretin glucose-dependent insulinotropic polypeptide (GIP), mice lacking both Glp1r and the GIP receptor (Gipr) maintained preservation of the enteroinsular axis following reduction of Gcgr signaling. Moreover, Gcgr-/-Glp1r-/- islets expressed increased levels of the cholecystokinin A receptor (Cckar) and G protein-coupled receptor 119 (Gpr119) mRNA transcripts, and Gcgr-/-Glp1r-/- mice exhibited increased sensitivity to exogenous CCK and the GPR119 agonist AR231453. Our data reveal extensive functional plasticity in the enteroinsular axis via induction of compensatory mechanisms that control nutrient-dependent regulation of insulin secretion.