Single-Cell Transcriptome Profiling of Human Pancreatic Islets in Health and Type 2 Diabetes.

Metformin has been the mainstay of therapy for diabetes mellitus for many years; however, the mechanistic aspects of metformin action remained ill-defined. Recent advances revealed that this drug, in addition to its glucose-lowering action, might be promising for specifically targeting metabolic differences between normal and abnormal metabolic signalling. The knowledge gained from dissecting the principal mechanisms by which metformin works can help us to develop novel treatments. The centre of metformin's mechanism of action is the alteration of the energy metabolism of the cell. Metformin exerts its prevailing, glucose-lowering effect by inhibiting hepatic gluconeogenesis and opposing the action of glucagon. The inhibition of mitochondrial complex I results in defective cAMP and protein kinase A signalling in response to glucagon. Stimulation of 5'-AMP-activated protein kinase, although dispensable for the glucose-lowering effect of metformin, confers insulin sensitivity, mainly by modulating lipid metabolism. Metformin might influence tumourigenesis, both indirectly, through the systemic reduction of insulin levels, and directly, via the induction of energetic stress; however, these effects require further investigation. Here, we discuss the updated understanding of the antigluconeogenic action of metformin in the liver and the implications of the discoveries of metformin targets for the treatment of diabetes mellitus and cancer.

Metformin—mode of action and clinical implications for diabetes and cancer

Dipeptidyl-peptidase IV/CD26 (DPP IV) is a cell-surface protease belonging to the prolyloligopeptidase family. It selectively removes the N-terminal dipeptide from peptides with proline or alanine in the second position. Apart from its catalytic activity, it interacts with several proteins, for instance, adenosine deaminase, the HIV gp120 protein, fibronectin, collagen, the chemokine receptor CXCR4, and the tyrosine phosphatase CD45. DPP IV is expressed on a specific set of T lymphocytes, where it is up-regulated after activation. It is also expressed in a variety of tissues, primarily on endothelial and epithelial cells. A soluble form is present in plasma and other body fluids. DPP IV has been proposed as a diagnostic or prognostic marker for various tumors, hematological malignancies, immunological, inflammatory, psychoneuroendocrine disorders, and viral infections. DPP IV truncates many bioactive peptides of medical importance. It plays a role in glucose homeostasis through proteolytic inactivation of...

Dipeptidyl-peptidase IV from bench to bedside: an update on structural properties, functions, and clinical aspects of the enzyme DPP IV.

Oxidative stress is increased in metabolic syndrome and type 2 diabetes mellitus (T2DM) and this appears to underlie the development of cardiovascular disease, T2DM and diabetic complications. Increased oxidative stress appears to be a deleterious factor leading to insulin resistance, dyslipidemia, β-cell dysfunction, impaired glucose tolerance and ultimately leading to T2DM. Chronic oxidative stress, hyperglycemia and dyslipidemia are particularly dangerous for β-cells from lowest levels of antioxidant, have high oxidative energy requirements, decrease the gene expression of key β-cell genes and induce cell death. If β-cell functioning is impaired, it results in an under production of insulin, impairs glucose stimulated insulin secretion, fasting hyperglycemia and eventually the development of T2DM.

Oxidative stress, insulin resistance, dyslipidemia and type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is a very complex and multifactorial metabolic disease characterized by insulin resistance and β cell failure leading to elevated blood glucose levels Hyperglycemia is suggested to be the main cause of diabetic complications, which not only decrease life quality and expectancy, but are also becoming a problem regarding the financial burden for health care systems Therefore, and to counteract the continually increasing prevalence of diabetes, understanding the pathogenesis, the main risk factors, and the underlying molecular mechanisms may establish a basis for prevention and therapy In this regard, research was performed revealing further evidence that oxidative stress has an important role in hyperglycemia-induced tissue injury as well as in early events relevant for the development of T2DM The formation of advanced glycation end products (AGEs), a group of modified proteins and/or lipids with damaging potential, is one contributing factor On the one hand it has been reported that AGEs increase reactive oxygen species formation and impair antioxidant systems, on the other hand the formation of some AGEs is induced per se under oxidative conditions Thus, AGEs contribute at least partly to chronic stress conditions in diabetes As AGEs are not only formed endogenously, but also derive from exogenous sources, ie, food, they have been assumed as risk factors for T2DM However, the role of AGEs in the pathogenesis of T2DM and diabetic complications—if they are causal or simply an effect—is only partly understood This review will highlight the involvement of AGEs in the development and progression of T2DM and their role in diabetic complications

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Advanced Glycation End Products and Oxidative Stress in Type 2 Diabetes Mellitus

Aims Glucagon-like peptide-1 (GLP-1) and gastric inhibitory polypeptide (GIP) are important insulinotropic hormones that enhance the insulin secretory response to feeding. Their potential for treating Type 2 diabetes is limited by short biological half-life owing to degradation by dipeptidyl peptidase IV (DPP IV). We investigated the acute effects of metformin on DPP IV activity in Type 2 diabetes to elucidate inhibition of DPP IV as a possible mechanism of action. Methods Eight fasting subjects with Type 2 diabetes (5M/3F, age 53.1 +/- 4.2 years, BMI 36.8 +/- 1.8 kg/m(2), glucose 8.9 +/- 1.2 mmol/l, HbA(1c) 7.8 +/- 0.6%) received placebo or metformin 1 g orally 1 week apart in a random, crossover design. Results Following metformin, DPP IV activity was suppressed compared with placebo (AUC(0-6 h) 3230 +/- 373 vs. 5764 +/- 504 nmol ml/l, respectively, P = 0.001). Circulating glucose, insulin and total GLP-1 were unchanged. Metformin also concentration-dependently inhibited endogenous DPP IV activity in vitro in plasma from Type 2 diabetic subjects. Conclusion Oral metformin effectively inhibits DPP IV activity in Type 2 diabetic patients, suggesting that the drug may have potential for future combination therapy with incretin hormones.

Inhibition of dipeptidyl peptidase IV activity by oral metformin in Type 2 diabetes

The presence and biological significance of circulating glycated insulin has been evaluated by high-pressure liquid chromatography (HPLC), electrospray ionization mass spectrometry (ESI-MS), radioimmunoassay (RIA), receptor binding, and hyperinsulinemic-euglycemic clamp techniques. ESI-MS analysis of an HPLC-purified plasma pool from four male type 2 diabetic subjects (HbA(1c) 8.1 +/- 0.2%, plasma glucose 8.7 +/- 1.3 mmol/l [means +/- SE]) revealed two major insulin-like peaks with retention times of 14-16 min. After spectral averaging, the peak with retention time of 14.32 min exhibited a prominent triply charged (M+3H)(3+) species at 1,991.1 m/z, representing monoglycated insulin with an intact M(r) of 5,970.3 Da. The second peak (retention time 15.70 min) corresponded to native insulin (M(r) 5,807.6 Da), with the difference between the two peptides (162.7 Da) representing a single glucitol adduct (theoretical 164 Da). Measurement of glycated insulin in plasma of type 2 diabetic subjects by specific RIA gave circulating levels of 10.1 +/- 2.3 pmol/l, corresponding to approximately 9% total insulin. Biological activity of pure synthetic monoglycated insulin (insulin B-chain Phe(1)-glucitol adduct) was evaluated in seven overnight-fasted healthy nonobese male volunteers using two-step euglycemic-hyperinsulinemic clamps (2 h at 16.6 micro g x kg(-1) x min(-1), followed by 2 h at 83.0 micro g x kg(-1) x min(-1); corresponding to 0.4 and 2.0 mU x kg(-1) x min(-1)). At the lower dose, the exogenous glucose infusion rates required to maintain euglycemia during steady state were significantly lower with glycated insulin (P < 0.01) and approximately 70% more glycated insulin was required to induce a similar rate of insulin-mediated glucose uptake. Maximal responses at the higher rates of infusion were similar for glycated and control insulin. Inhibitory effects on endogenous glucose production, insulin secretion, and lipolysis, as indicated by measurements of C-peptide, nonesterified free fatty acids, and glycerol, were also similar. Receptor binding to CHO-T cells transfected with human insulin receptor and in vivo metabolic clearance revealed no differences between glycated and native insulin, suggesting that impaired biological activity is due to a postreceptor effect. The present demonstration of glycated insulin in human plasma and related impairment of physiological insulin-mediated glucose uptake suggests a role for glycated insulin in glucose toxicity and impaired insulin action in type 2 diabetes.

https://diabetes.diabetesjournals.org/content/diabetes/52/2/492.full.pdf

Demonstration of Glycated Insulin in Human Diabetic Plasma and Decreased Biological Activity Assessed by Euglycemic-Hyperinsulinemic Clamp Technique in Humans

Background and Purpose

G-protein coupled receptor (GPR)55 is a novel lipid sensing receptor activated by both cannabinoid endogenous ligands (endocannabinoids) and other non-cannabinoid lipid transmitters. This study assessed the effects of various GPR55 agonists on glucose homeostasis.



Experimental Approach

Insulin secretion and changes in intracellular Ca2+ and cAMP in response to glucose and a range of GPR55 agonists [endogenous ligands (OEA, PEA), chemically synthetic cannabidiol (CBD) analogues (Abn-CBD, 0–1602), an analogue of rimonabant (AM-251) and antagonist (CBD)] were investigated in clonal BRIN-BD11 cells and mouse pancreatic islets. Cytotoxicity was assessed by LDH release, cellular localization by double-staining immunohistochemistry and in vivo effects assessed in mice.



Key Results

The most potent and selective GPR55 agonist was the synthetic CBD analogue, Abn-CBD (pEC50 10.33), maximum stimulation of 67% at 10−4 mol·L−1 (P < 0.001) in BRIN-BD11 cells. AM-251 (pEC50 7.0), OEA (pEC50 7.0), 0–1602 (pEC50 7.3) and PEA (pEC50 6.0) stimulated insulin secretion. Results were corroborated by islet studies, with no cytotoxic effects. Concentration-dependent insulin secretion by GPR55 agonists was glucose-sensitive and accompanied by elevations of [Ca2+]i (P < 0.01–P < 0.001) and cAMP (P < 0.05–P < 0.01). GPR55 agonists exhibited insulinotropic and glucose lowering activity in vivo. GPR55 was expressed on BRIN-BD11 cells and confined to islet beta cells with no distribution on alpha cells.



Conclusion and Implications

These results demonstrate GPR55 is distributed in pancreatic beta cells and is a strong activator of insulin secretion, with glucose-lowering effects in vivo. Development of agents agonizing the GPR55 receptor may have therapeutic potential in the treatment of type 2 diabetes.

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Evaluation of the insulin releasing and antihyperglycaemic activities of GPR55 lipid agonists using clonal beta-cells, isolated pancreatic islets and mice.

Effects of antidiabetic drugs on dipeptidyl peptidase IV activity: nateglinide is an inhibitor of DPP IV and augments the antidiabetic activity of glucagon-like peptide-1.

Aine McKillop

Papers

Inhibition of dipeptidyl peptidase IV activity by oral metformin in Type 2 diabetes

Demonstration of Glycated Insulin in Human Diabetic Plasma and Decreased Biological Activity Assessed by Euglycemic-Hyperinsulinemic Clamp Technique in Humans

Evaluation of the insulin releasing and antihyperglycaemic activities of GPR55 lipid agonists using clonal beta-cells, isolated pancreatic islets and mice.

Effects of antidiabetic drugs on dipeptidyl peptidase IV activity: nateglinide is an inhibitor of DPP IV and augments the antidiabetic activity of glucagon-like peptide-1.

Decreased dipeptidyl peptidase-IV activity and glucagon-like peptide-1(7-36)amide degradation in type 2 diabetic subjects