Islet dysfunction in non-insulin-dependent diabetes mellitus

Recent Insights into Beta-cell Exocytosis in Type 2 Diabetes

As one of the leading causes of morbidity and mortality worldwide, diabetes affects an estimated 422 million adults, and it is expected to continue expanding such that by 2050, 30% of the U.S. population will become diabetic within their lifetime. Out of the estimated 422 million people currently afflicted with diabetes worldwide, about 5% have type 1 diabetes (T1D), while the remaining ~95% of diabetics have type 2 diabetes (T2D). Type 1 diabetes results from the autoimmune-mediated destruction of functional β-cell mass, whereas T2D results from combinatorial defects in functional β-cell mass plus peripheral glucose uptake. Both types of diabetes are now believed to be preceded by β-cell dysfunction. T2D is increasingly associated with numerous reports of deficiencies in the exocytosis proteins that regulate insulin release from β-cells, specifically the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. SNARE protein's functionality is further regulated by a variety of accessory factors such as Sec1/Munc18 (SM), double C2-domain proteins (DOC2), and additional interacting proteins at the cell surface that influence the fidelity of insulin release. As new evidence emerges about the detailed mechanisms of exocytosis, new questions and controversies have come to light. This emerging information is also contributing to dialogue in the islet biology field focused on how to correct the defects in insulin exocytosis. Herein we present a balanced review of the role of exocytosis proteins in T2D, with thoughts on novel strategies to protect functional β-cell mass.

Risk of any hypoglycaemia with newer antihyperglycaemic agents in patients with type 2 diabetes: A systematic review and meta-analysis

OBJECTIVES

For patients with type 2 diabetes, newer antihyperglycaemic agents (AHA), including the dipeptidyl peptidase IV inhibitors (DPP4i), glucagon-like peptide-1 receptor agonists (GLP1RA) and sodium glucose co-transporter 2 inhibitors (SGLT2i) offer a lower risk of hypoglycaemia relative to sulfonylurea or insulin. However, it is not clear how AHA compare to placebo on risk of any hypoglycaemia. This study evaluates the risk of any and severe hypoglycaemia with AHA and metformin relative to placebo.

DESIGN

A systematic review and meta-analysis was conducted of randomized, placebo-controlled trials ≥12 weeks in duration. MEDLINE, Embase and the Cochrane Library were searched up to April 16, 2019. Studies allowing use of other diabetes medications were excluded. Mantel-Haenszel risk ratio with 95% confidence intervals were used to pool estimates based on class of AHA and number of concomitant therapies used.

PATIENTS

Eligible studies enrolled patients with type 2 diabetes ≥18 years of age.

RESULTS

144 studies met our inclusion criteria. Any hypoglycaemia was not increased with AHA when used as monotherapy (DPP4i (RR 1.12; 95% CI 0.81-1.56), GLP1RA (1.77; 0.91-3.46), SGLT2i (1.34; 0.83-2.15)), or as add-on to metformin (DPP4i (0.95; 0.67-1.35), GLP1RA (1.24; 0.80-1.91), SGLT2i (1.29; 0.91-1.83)) or as triple therapy (1.13; 0.67-1.91). However, metformin monotherapy (1.73; 1.02-2.94) and dual therapy initiation (3.56; 1.79-7.10) was associated with an increased risk of any hypoglycaemia. Severe hypoglycaemia was rare not increased for any comparisons.

CONCLUSIONS

Metformin and the simultaneous initiation of dual therapy, but not AHA used alone or as single add-on combination therapy, was associated with an increased risk of any hypoglycaemia relative to placebo.

Profound defects in β-cell function in screen-detected type 2 diabetes are not improved with glucose-lowering treatment in the Early Diabetes Intervention Program (EDIP)

BACKGROUND

Few studies have measured the ability of interventions to affect declining β-cell function in screen-detected type 2 diabetes. The Early Diabetes Intervention Programme (ClinicalTrials.gov NCT01470937) was a randomized study based on the hypothesis that improving postprandial glucose excursions with acarbose would slow the progression of fasting hyperglycaemia in screen-detected type 2 diabetes. In the Early Diabetes Intervention Programme, the effect of acarbose plus lifestyle advice on progression of fasting hyperglycaemia over a 5-year period was not greater than that of placebo. However, there was an early glucose-lowering effect of the trial. The objective of the current secondary analysis was to describe β-cell function changes in response to glucose lowering.

METHODS

Participants were overweight adult subjects with screen-detected type 2 diabetes. β-cell function was measured using hyperglycaemic clamps and oral glucose tolerance testing. The primary outcome was the change in β-cell function from baseline to year 1, the time point where the maximal glucose-lowering effect was seen.

RESULTS

At baseline, participants exhibited markedly impaired first-phase insulin response. Despite significant reductions in weight, fasting plasma glucose (PG) and 2-h PG, there was no clinically significant improvement in the first-phase insulin response. Late-phase insulin responses declined despite beneficial glycaemic effects of interventions.

CONCLUSIONS

Insulin secretion is already severely impaired in early, screen-detected type 2 diabetes. Effective glucose-lowering intervention with acarbose was not sufficient to improve insulin secretion or halt the decline of β-cell function.

Oral bacterial communities in individuals with type 2 diabetes who live in southern Thailand

Type 2 diabetes mellitus is increasingly common in Thailand and elsewhere. In the present investigation, the bacteriological composition of saliva and supragingival plaque in Thai diabetics with and without active dental caries and in nondiabetics was determined by differential culture and eubacterial DNA profiling. Potential associations between fasting blood sugar and glycosylated hemoglobin (biomarkers of current and historical glucose control, respectively) with decayed, missing, and filled teeth and with salivary Streptococcus and Lactobacillus counts were also investigated. The incidence of active dental caries was greater in the Thai diabetics than in nondiabetics, and the numbers of total streptococci and lactobacilli were significantly higher in supragingival plaque from diabetics than in nondiabetics. Lactobacillus counts in the saliva and supragingival plaque of diabetics with active caries were significantly higher than those in diabetics without active caries. Oral eubacterial DNA profiles of diabetic versus nondiabetic individuals and of diabetics with active caries versus those without active caries could not be readily differentiated through cluster analysis or multidimensional scaling. The elevated caries incidence in the Thai diabetics was positively associated with numbers of bacteria of the acidogenic/acid-tolerant genera Streptococcus and Lactobacillus. Lactobacillus bacterial numbers were further elevated in diabetics with active caries, although salivary eubacterial DNA profiles were not significantly altered.

Islet-cell-to-cell communication as basis for normal insulin secretion

The emergence of pancreatic islets has necessitated the development of a signalling system for the intra- and inter-islet coordination of beta cells. With evolution, this system has evolved into a complex regulatory network of partially cross-talking pathways, whereby individual cells sense the state of activity of their neighbours and, accordingly, regulate their own level of functioning. A consistent feature of this network in vertebrates is the expression of connexin (Cx)-36-made cell-to-cell channels, which cluster at gap junction domains of the cell membrane, and which adjacent beta cells use to share cytoplasmic ions and small metabolites within individual islets. This chapter reviews what is known about Cx36, and the mechanism whereby this beta-cell connexin significantly regulates insulin secretion. It further outlines other less established functions of the protein and evaluates its potential relevance for the development of novel therapeutic approaches to diabetes.

Insulinotropic agent ID-1101 (4-hydroxyisoleucine) activates insulin signaling in rat

ID-1101 (4-hydroxyisoleucine), an amino acid extracted from fenugreek seeds, exhibits an interesting glucose-dependent insulin-stimulating activity. The present study was undertaken to investigate a possible extrapancreatic effect of ID-1101 on insulin signaling and action besides its previously described insulinotropic action. Insulin-sensitizing effects of ID-1101 were investigated in rat in vivo by three different approaches: 1) using euglycemic hyperinsulinemic clamps in two different rat models of insulin resistance, i.e., Zucker fa/fa rats and rats fed a sucrose-lipid diet; 2) measuring liver and muscle phosphatidylinositol (PI) 3-kinase activity after an acute injection of ID-1101 in normal and insulin-resistant diabetic rats; and 3) after chronic treatment in two rat models of insulin resistance. Euglycemic hyperinsulinemic clamp experiments revealed that ID-1101 can improve insulin resistance through an increase of peripheral glucose utilization rate in sucrose-lipid-fed rats and by decreasing hepatic glucose production in Zucker fa/fa rats. Moreover, we demonstrated that a single injection of ID-1101 activates the PI 3-kinase activity in liver and muscle from normal rats but also in muscle from diabetic rats. Finally, chronic ID-1101 treatment significantly reduced insulinemia in type 2 diabetic rats and reduced the progression of hyperinsulinemia in insulin-resistant obese Zucker fa/fa rats. These findings clearly demonstrate that ID-1101 can reduce insulin resistance through activation of the early steps of insulin signaling in peripheral tissues and in liver. In summary, ID-1101, besides its insulinotropic effect, directly improves insulin sensitivity, making it a potentially very valuable therapeutic agent for diabetes treatment.

Association of paraoxonase 1 polymorphism with beta-cell function: a case of molecular heterosis

Paraoxonase 1 (PON1) is an antioxidant enzyme that inhibits the oxidative modification of LDL. Since PON1 has been shown to express in pancreatic islets and oxidative stress plays a role in beta-cell dysfunction, we investigated the impact of the PON1 polymorphisms on beta-cell function. This study included 84 healthy and glucose-tolerant white subjects who underwent an oral glucose tolerance test. Beta-cell function (1stPHS and 2ndPHS) was estimated according to the formulae published by Stumvoll et al. The relationship of the L55M and Q192R polymorphisms to beta-cell function was examined. Allelic frequency in this population was 0.37 for the M allele and 0.30 for the R allele. For the L55M polymorphism, the LM genotype had the lowest 1stPHS (P = 0.009) and 2ndPHS (P = 0.007), indicating molecular heterosis. Multivariate analyses confirmed that the L55M polymorphism was an independent determinant for 1stPHS (P = 0.016) and 2ndPHS (P = 0.009), after adjustment for covariates. For the Q192M polymorphism, no difference was noted in 1stPHS and 2ndPHS. We observed the L55M polymorphism of the PON1 gene as an independent determinant for beta-cell function in glucose-tolerant whites. Our observations suggest that this polymorphism could play a role in beta-cell dysfunction.

Insulin resistance is a poor predictor of type 2 diabetes in individuals with no family history of disease

In normoglycemic offspring of two type 2 diabetic parents, low insulin sensitivity (S(I)) and low insulin-independent glucose effectiveness (S(G)) predict the development of diabetes one to two decades later. To determine whether low S(I), low S(G,) or low acute insulin response to glucose are predictive of diabetes in a population at low genetic risk for disease, 181 normoglycemic individuals with no family history of diabetes (FH-) and 150 normoglycemic offspring of two type 2 diabetic parents (FH+) underwent i.v. glucose tolerance testing (IVGTT) between the years 1964-82. During 25 +/- 6 years follow-up, comprising 2,758 person years, the FH- cohort (54 +/- 9 years) had an age-adjusted incidence rate of type 2 diabetes of 1.8 per 1,000 person years, similar to that in other population-based studies, but significantly lower than 16.7 for the FH+ cohort. Even when the two study populations were subdivided by initial values of S(I) and S(G) derived from IVGTT's performed at study entry, there was a 10- to 20-fold difference in age-adjusted incidence rates for diabetes in the FH- vs. FH+ individuals with low S(I) and low S(G). The acute insulin response to glucose was not predictive of the development of diabetes when considered independently or when assessed as a function of S(I), i.e., the glucose disposition index. These data demonstrate that low glucose disposal rates are robustly associated with the development of diabetes in the FH+ individuals, but insulin resistance per se is not sufficient for the development of diabetes in individuals without family history of disease and strongly suggest a familial factor, not detectable in our current measures of the dynamic responses of glucose or insulin to an IVGTT is an important risk factor for type 2 diabetes. Low S(I) and low S(G), both measures of glucose disposal, interact with this putative familial factor to result in a high risk of type 2 diabetes in the FH+ individuals, but not in the FH- individuals.

Isulinotropic properties of Nigella sativa oil in Streptozotocin plus Nicotinamide diabetic hamster

The present study was designed to investigate the possible insulinotropic properties of Nigella sativa L. (N. sativa) oil in Streptozotocin plus Nicotinamide-induced diabetes mellitus in hamsters. Nicotinamide was injected intraperitoneally 15min before injection of Streptozotocin intravenously. Oral treatment with N. sativa oil began 4 weeks after induction of diabetes. Serum insulin was measured by enzymeimmunoassay. Islets insulin was stained using anti-insulin monoclonal antibody. Significant decrease in blood glucose level together with significant increase in serum insulin level were observed after treatment with N. sativa oil for 4 weeks. Big areas with positive immuno-reactivity for the presence of insulin were observed in the pancreases from N. sativa oil-treated group compared to non-treated one using immunohistochemical staining. Therefore, our data show that the hypoglycemic effect of N. sativa oil in Streptozotocin plus Nicotinamide diabetic hamsters resulted, at least partly, from a stimulatory effect on beta cell function with consequent increase in serum insulin level. These results indicate that N. sativa oil has insulinotropic properties in type 2-like model.

Noninsulin-dependent diabetes mellitus occurs in mice ectopically expressing the human Axl tyrosine kinase receptor

The axl tyrosine kinase receptor is aberrantly expressed on myeloid cells of many individuals afflicted with chronic myelogenous leukemia (CML) and other myeloid leukemias. Although previous studies demonstrated this kinase to have oncogenic potential, it is not known whether axl actively participates in the onset and/or progression of CML. We addressed this question by generating transgenic mice possessing constitutive ectopic expression of human axl throughout cells of the myeloid hematopoietic lineage through the use of the granulocyte colony-stimulating factor (GCSF) receptor promoter. The transgenics did not exhibit hematopoietic malignancies, but did exhibit phenotypic characteristics associated with noninsulin-dependent diabetes mellitus (NIDDM) including hyperglycemia and hyperinsulinemia, severe insulin resistance, progressive obesity, hepatic lipidosis, and pancreatic islet dysplasia. The obese-diabetes phenotype was similar to that observed in the agouti and melanocortin-4(-/-) mutants, however the axl transgenics were not hyperphagic. Axl transgenic animals expressed elevated serum tumor necrosis factor (TNF)-alpha levels that were further enhanced upon in vitro lipopolysaccharide (LPS) stimulation of peripheral blood. Administration of the axl ligand, gas6, to peripheral transgenic blood samples eliminated excessive TNF-alpha production in response to LPS stimulation. As a means to better understand axl-gas6 biology, transgenic animals were produced which systemically expressed the gas6-binding axl proteolytic cleavage product. A more severe NIDDM phenotype occurred in these mice. The observed phenotypes may be related to the axl receptor or proteolytic cleavage product competing with related axl family receptors for binding of the gas6 ligand. We conclude that axl expression in myeloid cells in itself does not lead to the onset or progression of leukemia and suggest that ectopic axl expression affects endogenous modulation of TNF-alpha production indirectly resulting in the NIDDM phenotype.

4-Hydroxyisoleucine: experimental evidence of its insulinotropic and antidiabetic properties

We have recently shown in vitro that 4-hydroxyisoleucine (4-OH-Ile), an amino acid extracted from fenugreek seeds, potentiates insulin secretion in a glucose-dependent manner. The present study was designed to investigate whether 4-OH-Ile could exert in vivo insulinotropic and antidiabetic properties. For this purpose, intravenous or oral glucose tolerance tests (IVGTTs and OGTTs, respectively) were performed not only in normal animals but also in a type II diabetes rat model. During IVGTT in normal rats or OGTT in normal dogs, 4-OH-Ile (18 mg/kg) improved glucose tolerance. The lactonic form of 4-OH-Ile was ineffective in normal rats. In non-insulin-dependent diabetic (NIDD) rats, a single intravenous administration of 4-OH-Ile (50 mg/kg) partially restored glucose-induced insulin response without affecting glucose tolerance; a 6-day subchronic administration of 4-OH-Ile (50 mg/kg, daily) reduced basal hyperglycemia, decreased basal insulinemia, and slightly, but significantly, improved glucose tolerance. In vitro, 4-OH-Ile (200 microM) potentiated glucose (16.7 mM)-induced insulin release from NIDD rat-isolated islets. So, the antidiabetic effects of 4-OH-Ile on NIDD rats result, at least in part, from a direct pancreatic B cell stimulation.

Insulin sensitization in diabetic rat liver by an antihyperglycemic agent

This study aimed to demonstrate directly that the thiazolidinedione pioglitazone acts as an insulin sensitizer. We tested the hypothesis that pioglitazone treatment of diabetic rats alters liver function such that responsiveness of selected genes to subsequent insulin regulation is enhanced. Although flux through gluconeogenic/glycolytic pathways involves regulation of many enzymes, we presently report the effects of insulin on expression of two key enzymes in these metabolic pathways, ie, phosphoenolpyruvate carboxykinase (PEPCK) and glucokinase (GK). Rats were either studied as nondiabetic controls or injected with streptozotocin as a model for insulin-deficient diabetes. Diabetic animals were treated without or with pioglitazone and subsequently examined for acute responses to insulin. Pioglitazone treatment of diabetic animals significantly enhanced the effects of insulin to reverse elevated blood glucose. Although the mean level of liver mRNA transcripts encoding PEPCK was increased to nearly 300% in diabetic animals as compared with nondiabetic controls (100%), it was significantly lower in pioglitazone-treated diabetic rats (119% of control) than in diabetic rats without pioglitazone (223% of control) after insulin treatment. By contrast, mRNA transcripts encoding GK were not detectable in diabetic animals, but were increased markedly by insulin treatment in all animal groups. Insulin-enhanced expression of GK was significantly greater in liver from animals that were treated earlier with pioglitazone (291% of control) than in liver from those that were untreated (214% of control). An amplified acute response of liver to insulin thus established pioglitazone as an insulin sensitizer. Our findings further showed that such sensitization can be developed even in the insulin-deficient state.(ABSTRACT TRUNCATED AT 250 WORDS)

The genetics and pathophysiology of type II and gestational diabetes

The development of both type II diabetes and gestational diabetes is probably governed by a complex and variable interaction of genes and environment. Molecular genetics has so far failed to identify discrete gene mutations accounting for metabolic changes in NIDDM. Both beta cell dysfunction and insulin resistance are operative in the manifestation of these disorders. Specific and sensitive immunoradiometric assays found fasting hyperproinsulinemia and first-phase hypoinsulinemia early in the natural history of the disorder. A lack of specificity of early radioimmunoassays for insulin resulted in measuring not only insulin but also proinsulins, leading to overestimation of insulin and misleading conclusions about its role in diabetes. The major causes of insulin resistance are the genetic deficiency of glycogen synthase activation, compounded by additional defects due to metabolic disorders, receptor downregulation, and glucose transporter abnormalities, all contributing to the impairment in muscle glucose uptake. The liver is also resistant to insulin in NIDDM, reflected in persistent hepatic glucose production despite hyperglycemia. Insulin resistance is present in many nondiabetics, but in itself is insufficient to cause type II diabetes. Gestational diabetes is closely related to NIDDM, and the combination of insulin resistance and impaired insulin secretion is of importance in its pathogenesis.