Effects of acute NEFA manipulation on incretin-induced insulin secretion in participants with and without type 2 diabetes

GLP1R and GIPR expression and signaling in pancreatic alpha cells, beta cells and delta cells

Glucagon-like peptide-1 receptor (GLP1R) and glucose-dependent insulinotropic polypeptide receptor (GIPR) are transmembrane receptors involved in insulin, glucagon and somatostatin secretion from the pancreatic islet. Therapeutic targeting of GLP1R and GIPR restores blood glucose levels in part by influencing beta cell, alpha cell and delta cell function. Despite the importance of the incretin-mimetics for diabetes therapy, our understanding of GLP1R and GIPR expression patterns and signaling within the islet remain incomplete. Here, we present the evidence for GLP1R and GIPR expression in the major islet cell types, before addressing signaling pathway(s) engaged, as well as their influence on cell survival and function. While GLP1R is largely a beta cell-specific marker within the islet, GIPR is expressed in alpha cells, beta cells, and (possibly) delta cells. GLP1R and GIPR engage Gs-coupled pathways in most settings, although the exact outcome on hormone release depends on paracrine communication and promiscuous signaling. Biased agonism away from beta-arrestin is an emerging concept for improving therapeutic efficacy, and is also relevant for GLP1R/GIPR dual agonism. Lastly, dual agonists exert multiple effects on islet function through GIPR > GLP1R imbalance, increased GLP1R surface expression and cAMP signaling, as well as beneficial alpha cell-beta cell-delta cell crosstalk.

Glucagon-like peptide-1 secretion in people with versus without type 2 diabetes: a systematic review and meta-analysis of cross-sectional studies

AIMS/HYPOTHESIS

The aim of this systematic review was to synthesise the study findings on whether GLP-1 secretion in response to a meal tolerance test is affected by the presence of type 2 diabetes (T2D). The influence of putative moderators such as age, sex, meal type, meal form, and assay type were also explored.

METHODS

A literature search identified 32 relevant studies. The sample mean and SD for fasting GLP-1TOTAL and GLP-1TOTAL iAUC were extracted and used to calculate between-group standardised mean differences (SMD), which were meta-analysed using a random-effects model to derive pooled estimates of Hedges' g and 95 % prediction intervals (PI).

RESULTS

Pooled across 18 studies, the overall SMD in GLP-1TOTAL iAUC between individuals with T2D (n = 270, 1047 ± 930 pmol·L-1·min) and individuals without T2D (n = 402, 1204 ± 937 pmol·L-1·min) was very small, not statistically significant and heterogenous across studies (g = -0.15, p = 0.43, PI: -1.53, 1.23). Subgroup analyses demonstrated an effect of assay type whereby Hedges' g for GLP-1 iAUC was greater in individuals with, versus those without T2D when using ELISA or Mesoscale (g = 0.67 [moderate], p = 0.009), but not when using RIA (g = -0.30 [small], p = 0.10). Pooled across 30 studies, the SMD in fasting GLP-1TOTAL between individuals with T2D (n = 580, 16.2 ± 6.9 pmol·L-1) versus individuals without T2D (n = 1363, 12.4 ± 5.7 pmol·L-1) was small and heterogenous between studies (g = 0.24, p = 0.21, PI: -1.55, 2.02).

CONCLUSIONS

Differences in fasting GLP-1TOTAL and GLP-1TOTAL iAUC between individuals with, versus those without T2D were generally small and inconsistent between studies. Factors influencing study heterogeneity such as small sample sizes and poor matching of groups may help to explain the wide prediction intervals observed. Considerations to improve comparisons of GLP-1 secretion in T2D and potential mediating factors more important than T2D diagnosis per se are outlined.

PROSPERO ID

CRD42020195612.

Recent Pharmacological Options in Type 2 Diabetes and Synergic Mechanism in Cardiovascular Disease

Diabetes Mellitus is a multifactorial disease with a critical impact worldwide. During prediabetes, the presence of various inflammatory cytokines and oxidative stress will lead to the pathogenesis of type 2 diabetes. Furthermore, insulin resistance and chronic hyperglycemia will lead to micro- and macrovascular complications (cardiovascular disease, heart failure, hypertension, chronic kidney disease, and atherosclerosis). The development through the years of pharmacological options allowed us to reduce the persistence of chronic hyperglycemia and reduce diabetic complications. This review aims to highlight the specific mechanisms with which the new treatments for type 2 diabetes reduce oxidative stress and insulin resistance and improve cardiovascular outcomes.

Loss of the Incretin Effect in Type 2 Diabetes: A Systematic Review and Meta-analysis

CONTEXT

Loss of the incretin effect (IE) in type 2 diabetes (T2D) contributes to hyperglycemia and the mechanisms underlying this impairment are unclear.

OBJECTIVE

To quantify the IE impairment in T2D and to investigate the factors associated with it using a meta-analytic approach.

METHODS

PubMed, Scopus, and Web-of-Science were searched. Studies measuring IE by the gold-standard protocol employing an oral glucose tolerance test (OGTT) and an intravenous glucose infusion at matched glucose levels were selected. We extracted IE, sex, age, body mass index (BMI), and hemoglobin A1c, fasting values, and area under curve (AUC) of glucose, insulin, C-peptide, glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide 1 (GLP-1). In subjects with T2D, we also recorded T2D duration, age at diagnosis, and the percentage of subjects taking antidiabetic medications.

RESULTS

The IE weighted mean difference between subjects with T2D and those with normal glucose tolerance (NGT) was -27.3% (CI -36.5% to -18.1%; P < .001; I2 = 86.6%) and was affected by age (P < .005). By meta-regression of combined NGT and T2D data, IE was inversely associated with glucose tolerance (lower IE in T2D), BMI, and fasting GIP (P < .05). By meta-regression of T2D studies only, IE was associated with the OGTT glucose dose (P < .0001). IE from insulin was larger than IE from C-peptide (weighted mean difference 11.2%, CI 9.2-13.2%; P < .0001; I2 = 28.1%); the IE difference was inversely associated with glucose tolerance and fasting glucose.

CONCLUSION

The IE impairment in T2D vs NGT is consistent though considerably variable, age being a possible factor affecting the IE difference. Glucose tolerance, BMI, and fasting GIP are independently associated with IE; in subjects with T2D only, the OGTT dose is a significant covariate.

Fixed-ratio combination of insulin glargine plus lixisenatide (iGlarLixi) improves ß-cell function in people with type 2 diabetes

AIM

Multiple studies support the efficacy of combining a glucagon-like peptide 1 receptor agonist (GLP-1RA) with basal insulin in people with type 2 diabetes inadequately controlled on dual/triple oral therapy. Fixed-ratio combinations of basal insulin + GLP-1RA represent a further advance to facilitate management. We assessed the impact of fixed-ratio combination basal insulin + GLP-1RA treatment on β-cell function.

MATERIALS AND METHODS

We analysed data from 351 participants in the LixiLan-G trial (NCT02787551) randomized to receive iGlarLixi (insulin glargine 100 U/ml + lixisenatide) or to continue daily/weekly GLP-1RA, both on top of metformin. Participants received a 2-h meal tolerance test before randomization and at study end (26 weeks), with timed plasma glucose and C-peptide determinations. β-cell function parameters were resolved using mathematical modelling.

RESULTS

In the GLP-1RA group (n = 162), both body weight and glycated haemoglobin decreased at week 26, yet none of the insulin secretion/β-cell function parameters changed significantly. In contrast, in the iGlarLixi group (n = 189), glycated haemoglobin decreased significantly more than in the GLP-1RA group (p < .0001) despite an increase in body weight (+1.7 ± 3.9 kg, p < .0001). Fasting and stimulated insulin secretion decreased at Week 26 (both p < .0001 vs. GLP-1RA), while β-cell glucose sensitivity increased by a median 35% (p = .0032 vs. GLP-1RA). The incremental meal tolerance test glucose area showed a larger reduction with iGlarLixi versus GLP-1RA (p < .0001).

CONCLUSIONS

In people with type 2 diabetes on metformin, 26-week treatment with iGlarLixi resulted in a marked improvement in β-cell function concomitant with sparing of endogenous insulin release and a reduction in meal absorption.

The molecular link between oxidative stress, insulin resistance, and type 2 diabetes: A target for new therapies against cardiovascular diseases

Type 2 Diabetes Mellitus (T2D) is a chronic disease with a pandemic incidence whose pathogenesis has not yet been clarified. Raising evidence highlighted the role of oxidative stress in inducing insulin resistance, pancreatic beta-cell dysfunction, and leading to cardiovascular disease (CVD). Therefore, understanding the link between oxidative stress, T2D and CVD may help to further understand the pathological processes beyond this association, to personalize the algorithm of the cure, and to find new therapeutic targets. Here, we discussed the role of oxidative stress and the decrease of antioxidant defenses in the pathogenesis of T2D. Furthermore, some aspects of hypoglycemic therapies and their potential role as antioxidant agents were examined, which might be pivotal in preventing CVD in T2D patients.

Effect of free fatty acids on insulin secretion, insulin sensitivity and incretin effect - a narrative review

Deleterious effects of free fatty acids, FFAs, on insulin sensitivity are observed in vivo studies in humans. Mechanisms include impaired insulin signaling, oxidative stress, inflammation, and mitochondrial dysfunction, but the effects on insulin secretion are less well known. Our aim was to review the relationship of increased FFAs with insulin resistance, secretion and mainly with the incretin effect in humans. Narrative review. Increased endogenous or administered FFAs induce insulin resistance. FFAs effects on insulin secretion are debatable; inhibition and stimulation have been reported, depending on the type and duration of lipids exposition and the study subjects. Chronically elevated FFAs seem to decrease insulin biosynthesis, glucose-stimulated insulin secretion and β-cell glucose sensitivity. Lipids infusion decreases the response to incretins with unchanged incretin levels in volunteers with normal glucose tolerance. In contrast, FFAs reduction by acipimox did not restore the incretin effect in type-2 diabetes, probably due to the dysfunctional β-cell. Possible mechanisms of FFAs excess on incretin effect include reduction of the expression and levels of GLP-1 (glucagon like peptide-1) receptor, reduction of connexin-36 expression thus the coordinated secretory activity in response to GLP-1, and GIP (glucose-dependent insulinotropic polypeptide) receptors downregulation in islets cells. Increased circulating FFAs impair insulin sensitivity. Effects on insulin secretion are complex and controversial. Deleterious effects on the incretin-induced potentiation of insulin secretion were reported. More investigation is needed to better understand the extent and mechanisms of β-cell impairment and insulin resistance induced by increased FFAs and how to prevent them.

Brain substrate metabolism and ß-cell function in humans: A positron emission tomography study

AIMS

Recent clinical studies have shown enhanced brain glucose uptake during clamp and brain fatty acid uptake in insulin-resistant individuals. Preclinical studies suggest that the brain may be involved in the control of insulin secretion. The aim of this study was to investigate whether brain metabolism assessed as brain glucose and fatty acid uptake is associated with the parameters of β-cell function in humans.

MATERIALS AND METHODS

We analysed cross-sectional data of 120 subjects across a wide range of BMI and insulin sensitivity. Brain glucose uptake (BGU) was measured during euglycaemic-hyperinsulinaemic clamp (n = 67) and/or during fasting (n = 45) using [18F]-fluorodeoxyglucose (FDG) positron emission tomography (PET). In another group of subjects (n = 34), brain fatty acid uptake was measured using [18F]-fluoro-6-thia-heptadecanoic acid (FTHA) PET during fasting. The parameters of β-cell function were derived from OGTT modelling. Statistical analysis was performed with whole-brain voxel-based statistical parametric mapping.

RESULTS

In non-diabetics, BGU during euglycaemic hyperinsulinaemic clamp correlated positively with basal insulin secretion rate (r = 0.51, P = .0008) and total insulin output (r = 0.51, P = .0008), whereas no correlation was found in type 2 diabetics. BGU during clamp correlated positively with potentiation in non-diabetics (r = 0.33, P = .02) and negatively in type 2 diabetics (r = -0.61, P = .02). The associations in non-diabetics were not explained with whole-body insulin sensitivity or BMI. No correlations were found between baseline (fasting) BGU and basal insulin secretion rate, whereas baseline brain fatty acid uptake correlated directly with basal insulin secretion rate (r = 0.39, P = .02) and inversely with potentiation (r = -0.36, P = .04).

CONCLUSIONS

Our study provides coherent, though correlative, evidence that, in humans, the brain may be involved in the control of insulin secretion independently of insulin sensitivity.

Breakfast consumption modulates postprandial glycaemic, insulinaemic and NEFA response in pre-diabetic Asian males

Breakfast consumption is associated with a variety of nutritional and lifestyle-related health outcomes. The objective of the present study was to investigate how the consumption of breakfast affected blood glucose, insulin and NEFA profiles. A lower postprandial blood glucose, insulin and NEFA response is associated with a lower risk of development of metabolic diseases. In a randomised crossover non-blind design, thirteen pre-diabetic Chinese adult males (BMI 26·7 (sd 4·2) kg/m2) attended two sessions where they either consumed a high-glycaemic index breakfast or no breakfast consumption. Changes in glycaemic response over 27 h periods were measured using the Medtronic MiniMed iProTM2 continuous glucose monitoring system. Blood samples were collected using a peripheral venous catheter at fixed intervals for 3 h after the test meal and 3 h after standardised lunch consumption. Postprandial glucose, insulin and NEFA response was calculated as total AUC and incremental AUC using the trapezoidal rule that ignored the area under the baseline. It was found that breakfast consumption significantly decreased postprandial glucose, insulin and NEFA excursion response at lunch time (P = 0·001). Consumption of breakfast attenuated blood glucose profiles by minimising glycaemic excursions and reduced both insulinaemic and NEFA responses in pre-diabetic Asian males during the second meal. This simple dietary intervention may be a novel approach to help improve subsequent lunch glycaemic responses in Asians at high risk of developing diabetes.

Recent Insights Into Mechanisms of β-Cell Lipo- and Glucolipotoxicity in Type 2 Diabetes

The deleterious effects of chronically elevated free fatty acid (FFA) levels on glucose homeostasis are referred to as lipotoxicity, and the concurrent exposure to high glucose may cause synergistic glucolipotoxicity. Lipo- and glucolipotoxicity have been studied for over 25 years. Here, we review the current evidence supporting the role of pancreatic β-cell lipo- and glucolipotoxicity in type 2 diabetes (T2D), including lipid-based interventions in humans, prospective epidemiological studies, and human genetic findings. In addition to total FFA quantity, the quality of FFAs (saturation and chain length) is a key determinant of lipotoxicity. We discuss in vitro and in vivo experimental models to investigate lipo- and glucolipotoxicity in β-cells and describe experimental pitfalls. Lipo- and glucolipotoxicity adversely affect many steps of the insulin production and secretion process. The molecular mechanisms underpinning lipo- and glucolipotoxic β-cell dysfunction and death comprise endoplasmic reticulum stress, oxidative stress and mitochondrial dysfunction, impaired autophagy, and inflammation. Crosstalk between these stress pathways exists at multiple levels and may aggravate β-cell lipo- and glucolipotoxicity. Lipo- and glucolipotoxicity are therapeutic targets as several drugs impact the underlying stress responses in β-cells, potentially contributing to their glucose-lowering effects in T2D.

Effect of liraglutide on estimates of lipolysis and lipid oxidation in obese patients with stable coronary artery disease and newly diagnosed type 2 diabetes: A randomized trial

Elevated levels of non-esterified fatty acids (NEFA) play a role in insulin resistance, impaired beta-cell function and they are a denominator of the abnormal atherogenic lipid profile that characterizes obese patients with type 2 diabetes (T2DM). We hypothesized that the GLP-1 receptor agonist liraglutide, in combination with metformin, would reduce lipolysis. In a randomized, double-blind, placebo-controlled, cross-over trial, 41 T2DM patients with coronary artery disease were randomized and treated with liraglutide-metformin vs placebo-metformin during 12- + 12-week periods with a wash-out period of at least 2 weeks before and between the intervention periods. NEFA kinetics were estimated using the Boston Minimal Model of NEFA metabolism, with plasma NEFA and glucose levels measured during a standard 180-minute frequently sampled intravenous glucose tolerance test. Liraglutide-metformin reduced estimates of lipolysis. Furthermore, placebo-metformin increased estimates of lipid oxidation, while treatment with liraglutide eliminated this effect. We conclude that liraglutide exerts a clinically relevant reduction in estimates of lipolysis and lipid oxidation which is explained, in part, by improved insulin secretion, as revealed by an intravenous glucose tolerance test.

The diabetes pandemic and associated infections: suggestions for clinical microbiology

There are 425 million people with diabetes mellitus in the world. By 2045, this figure will grow to over 600 million. Diabetes mellitus is classified among noncommunicable diseases. Evidence points to a key role of microbes in diabetes mellitus, both as infectious agents associated with the diabetic status and as possible causative factors of diabetes mellitus. This review takes into account the different forms of diabetes mellitus, the genetic determinants that predispose to type 1 and type 2 diabetes mellitus (especially those with possible immunologic impact), the immune dysfunctions that have been documented in diabetes mellitus. Common infections occurring more frequently in diabetic vs. nondiabetic individuals are reviewed. Infectious agents that are suspected of playing an etiologic/triggering role in diabetes mellitus are presented, with emphasis on enteroviruses, the hygiene hypothesis, and the environment. Among biological agents possibly linked to diabetes mellitus, the gut microbiome, hepatitis C virus, and prion-like protein aggregates are discussed. Finally, preventive vaccines recommended in the management of diabetic patients are considered, including the bacillus calmette-Guerin vaccine that is being tested for type 1 diabetes mellitus. Evidence supports the notion that attenuation of immune defenses (both congenital and secondary to metabolic disturbances as well as to microangiopathy and neuropathy) makes diabetic people more prone to certain infections. Attentive microbiologic monitoring of diabetic patients is thus recommendable. As genetic predisposition cannot be changed, research needs to identify the biological agents that may have an etiologic role in diabetes mellitus, and to envisage curative and preventive ways to limit the diabetes pandemic.