Jean F. Vettorazzi

Enhanced glucose-induced intracellular signaling promotes insulin hypersecretion: pancreatic beta-cell functional adaptations in a model of genetic obesity and prediabetes.

Obesity is associated with insulin resistance and is known to be a risk factor for type-2 diabetes. In obese individuals, pancreatic beta-cells try to compensate for the increased insulin demand in order to maintain euglycemia. Most studies have reported that this adaptation is due to morphological changes. However, the involvement of beta-cell functional adaptations in this process needs to be clarified. For this purpose, we evaluated different key steps in the glucose-stimulated insulin secretion (GSIS) in intact islets from female ob/ob obese mice and lean controls. Obese mice showed increased body weight, insulin resistance, hyperinsulinemia, glucose intolerance and fed hyperglycemia. Islets from ob/ob mice exhibited increased glucose-induced mitochondrial activity, reflected by enhanced NAD(P)H production and mitochondrial membrane potential hyperpolarization. Perforated patch-clamp examination of beta-cells within intact islets revealed several alterations in the electrical activity such as increased firing frequency and higher sensitivity to low glucose concentrations. A higher intracellular Ca(2+) mobilization in response to glucose was also found in ob/ob islets. Additionally, they displayed a change in the oscillatory pattern and Ca(2+) signals at low glucose levels. Capacitance experiments in intact islets revealed increased exocytosis in individual ob/ob beta-cells. All these up-regulated processes led to increased GSIS. In contrast, we found a lack of beta-cell Ca(2+) signal coupling, which could be a manifestation of early defects that lead to beta-cell malfunction in the progression to diabetes. These findings indicate that beta-cell functional adaptations are an important process in the compensatory response to obesity.

Augmented β-Cell Function and Mass in Glucocorticoid-Treated Rodents Are Associated with Increased Islet Ir-β/AKT/mTOR and Decreased AMPK/ACC and AS160 Signaling

Glucocorticoid (GC) therapies may adversely cause insulin resistance (IR) that lead to a compensatory hyperinsulinemia due to insulin hypersecretion. The increased β-cell function is associated with increased insulin signaling that has the protein kinase B (AKT) substrate with 160 kDa (AS160) as an important downstream AKT effector. In muscle, both insulin and AMP-activated protein kinase (AMPK) signaling phosphorylate and inactivate AS160, which favors the glucose transporter (GLUT)-4 translocation to plasma membrane. Whether AS160 phosphorylation is modulated in islets from GC-treated subjects is unknown. For this, two animal models, Swiss mice and Wistar rats, were treated with dexamethasone (DEX) (1 mg/kg body weight) for 5 consecutive days. DEX treatment induced IR, hyperinsulinemia, and dyslipidemia in both species, but glucose intolerance and hyperglycemia only in rats. DEX treatment caused increased insulin secretion in response to glucose and augmented β-cell mass in both species that were associated with increased islet content and increased phosphorylation of the AS160 protein. Protein AKT phosphorylation, but not AMPK phosphorylation, was found significantly enhanced in islets from DEX-treated animals. We conclude that the augmented β-cell function developed in response to the GC-induced IR involves inhibition of the islet AS160 protein activity.

Chronic use of pravastatin reduces insulin exocytosis and increases β-cell death in hypercholesterolemic mice.

We have previously demonstrated that hypercholesterolemic LDL receptor knockout (LDLr(-/-)) mice secrete less insulin than wild-type mice. Removing cholesterol from isolated islets using methyl-beta-cyclodextrin reversed this defect. In this study, we hypothesized that in vivo treatment of LDLr(-/-) mice with the HMGCoA reductase inhibitor pravastatin would improve glucose-stimulated insulin secretion. Female LDLr(-/-) mice were treated with pravastatin (400mg/L) for 1-3 months. Isolated pancreatic islets were assayed for insulin secretion rates, intracellular calcium oscillations, cholesterol levels, NAD(P)H and SNARE protein levels, apoptosis indicators and lipidomic profile. Two months pravastatin treatment reduced cholesterol levels in plasma, liver and islets by 35%, 25% and 50%, respectively. Contrary to our hypothesis, pravastatin treatment increased fasting and fed plasma levels of glucose and decreased markedly (40%) fed plasma levels of insulin. In addition, ex vivo glucose stimulated insulin secretion was significantly reduced after two and three months (36-48%, p<0.05) of pravastatin treatment. Although reducing insulin secretion and insulinemia, two months pravastatin treatment did not affect glucose tolerance because it improved global insulin sensitivity. Pravastatin induced islet dysfunction was associated with marked reductions of exocytosis-related SNARE proteins (SNAP25, Syntaxin 1A, VAMP2) and increased apoptosis markers (Bax/Bcl2 protein ratio, cleaved caspase-3 and lower NAD(P)H production rates) observed in pancreatic islets from treated mice. In addition, several oxidized phospholipids, tri- and diacylglycerols and the proapoptotic lipid molecule ceramide were identified as markers of pravastatin-treated islets. Cell death and oxidative stress (H2O2 production) were confirmed in insulin secreting INS-1E cells treated with pravastatin. These results indicate that chronic treatment with pravastatin impairs the insulin exocytosis machinery and increases β-cell death. These findings suggest that prolonged use of statins may have a diabetogenic effect.

Acute Exercise Improves Insulin Clearance and Increases the Expression of Insulin-Degrading Enzyme in the Liver and Skeletal Muscle of Swiss Mice.

The effects of exercise on insulin clearance and IDE expression are not yet fully elucidated. Here, we have explored the effect of acute exercise on insulin clearance and IDE expression in lean mice. Male Swiss mice were subjected to a single bout of exercise on a speed/angle controlled treadmill for 3-h at approximately 60–70% of maximum oxygen consumption. As expected, acute exercise reduced glycemia and insulinemia, and increased insulin tolerance. The activity of AMPK-ACC, but not of IR-Akt, pathway was increased in the liver and skeletal muscle of trained mice. In an apparent contrast to the reduced insulinemia, glucose-stimulated insulin secretion was increased in isolated islets of these mice. However, insulin clearance was increased after acute exercise and was accompanied by increased expression of the insulin-degrading enzyme (IDE), in the liver and skeletal muscle. Finally, C2C12, but not HEPG2 cells, incubated at different concentrations of 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) for 3-h, showed increased expression of IDE. In conclusion, acute exercise increases insulin clearance, probably due to an augmentation of IDE expression in the liver and skeletal muscle. The elevated IDE expression, in the skeletal muscle, seems to be mediated by activation of AMPK-ACC pathway, in response to exercise. We believe that the increase in the IDE expression, comprise a safety measure to maintain glycemia at or close to physiological levels, turning physical exercise more effective and safe.

Anti-contractile effects of perivascular adipose tissue in thoracic aorta from rats fed a high-fat diet: role of aerobic exercise training

The aim of the present study was to evaluate the effects of aerobic exercise training on perivascular adipose tissue (PVAT) function in thoracic aorta from rats fed a high‐fat diet. Aortic vascular reactivity was performed in sedentary (SD), trained (TR), sedentary high‐fat diet (SD‐HF), and trained high‐fat diet (TR‐HF) male Wistar rats in the absence (PVAT−) or in the presence (PVAT+) of thoracic PVAT. We also measured circulatory concentrations of leptin and tumour necrosis factor alpha (TNF‐α), as well as the protein expressions of TNF‐α receptor 1 (TNFR1) and inducible nitric oxide synthase (iNOS) on PVAT. In the SD‐HF group, the body weight, epididymal fat pad, thoracic PVAT, circulatory triglycerides, insulin, leptin and TNF‐α were increased when compared with the SD group, whereas exercise training reduced these values in TR‐HF group. The relaxing response curves to acetylcholine and sodium nitroprusside were not modified by either intervention (high‐fat diet or exercise training) or the presence of PVAT. The presence of PVAT had an anti‐contractile effect in response to serotonin in all groups. In SD‐HF group, the increased magnitude of anti‐contractile effects was in parallel with an up‐regulation of iNOS protein expression in PVAT without alteration in TNFR1. Exercise training was effective in normalizing the vascular reactivity in rings PVAT+ and in reducing the iNOS protein expression. Exercise training prevented the PVAT–induced alteration in thoracic aorta from rats fed a high‐fat diet.

Nighttime light exposure enhances Rev-erbα-targeting microRNAs and contributes to hepatic steatosis

OBJECTIVE The exposure to artificial light at night (ALAN) disrupts the biological rhythms and has been associated with the development of metabolic syndrome. MicroRNAs (miRNAs) display a critical role in fine-tuning the circadian system and energy metabolism. In this study, we aimed to assess whether altered miRNAs expression in the liver underlies metabolic disorders caused by disrupted biological rhythms. RESULTS We found that C3H/HePas mice exposed to ALAN developed obesity, and hepatic steatosis, which was paralleled by decreased expression of Rev-erbα and up-regulation of its lipogenic targets ACL and FAS in liver. Furthermore, the expression of Rev-erbα-targeting miRNAs, miR-140-5p, 185-5p, 326-5p and 328-5p were increased in this group. Consistently, overexpression of these miRNAs in primary hepatocytes reduced Rev-erbα expression at the mRNA and protein levels. Importantly, overexpression of Rev-erbα-targeting miRNAs increased mRNA levels of Acly and Fasn. CONCLUSION Thus, altered miRNAs profile is an important mechanism underlying the disruption of the peripheral clock caused by exposure to ALAN, which could lead to hepatic steatosis.

The effect of taurine supplementation on glucose homeostasis: the role of insulin-degrading enzyme.

Taurine is a semi-essential sulfur-containing amino acid derived from the metabolism of methionine and cysteine. Taurine controls several biological processes, including glucose homeostasis. The effects of taurine supplementation on insulin secretion and its effects on peripheral organs have been previously described. However, there are no data describing the effects of this amino acid on insulin clearance and the expression of insulin-degrading enzyme (IDE).

Cortistatin regulates glucose-induced electrical activity and insulin secretion in mouse pancreatic beta-cells

Although there is growing evidence that cortistatin regulates several functions in different tissues, its role in the endocrine pancreas is not totally known. Here, we aim to study the effect of cortistatin on pancreatic beta-cells and glucose-stimulated insulin secretion (GSIS). Exposure of isolated mouse islets to cortistatin inhibited GSIS. This effect was prevented using a somatostatin receptor antagonist. Additionally, cortistatin hyperpolarized the membrane potential and reduced glucose-induced action potentials in isolated pancreatic beta-cells. Cortistatin did not modify ATP-dependent K+ (KATP) channel activity. In contrast, cortistatin increased the activity of a small conductance channel with characteristics of G protein-coupled inwardly rectifying K+ (GIRK) channels. The cortistatin effects on membrane potential and GSIS were largely reduced in the presence of a GIRK channel antagonist and by down-regulation of GIRK2 with small interfering RNA. Thus, cortistatin acts as an inhibitory signal for glucose-induced electrical activity and insulin secretion in the mouse pancreatic beta-cell.

Glucose intolerance in monosodium glutamate obesity is linked to hyperglucagonemia and insulin resistance in α cells: ARAUJO et al.

Obesity predisposes to glucose intolerance and type 2 diabetes (T2D). This disease is often characterized by insulin resistance, changes in insulin clearance, and β‐cell dysfunction. However, studies indicate that, for T2D development, disruptions in glucagon physiology also occur. Herein, we investigated the involvement of glucagon in impaired glycemia control in monosodium glutamate (MSG)‐obese mice. Male Swiss mice were subcutaneously injected daily, during the first 5 days after birth, with MSG (4 mg/g body weight [BW]) or saline (1.25 mg/g BW). At 90 days of age, MSG‐obese mice were hyperglycemic, hyperinsulinemic, and hyperglucagonemic and had lost the capacity to increase their insulin/glucagon ratio when transitioning from the fasting to fed state, exacerbating hepatic glucose output. Furthermore, hepatic protein expressions of phosphorylated (p)‐protein kinase A (PKA) and cAMP response element‐binding protein (pCREB), and of phosphoenolpyruvate carboxykinase (PEPCK) enzyme were higher in fed MSG, before and after glucagon stimulation. Increased pPKA and phosphorylated hormone‐sensitive lipase content were also observed in white fat of MSG. MSG islets hypersecreted glucagon in response to 11.1 and 0.5 mmol/L glucose, a phenomenon that persisted in the presence of insulin. Additionally, MSG α cells were hypertrophic displaying increased α‐cell mass and immunoreactivity to phosphorylated mammalian target of rapamycin (pmTOR) protein. Therefore, severe glucose intolerance in MSG‐obese mice was associated with increased hepatic glucose output, in association with hyperglucagonemia, caused by the refractory actions of glucose and insulin in α cells and via an effect that may be due to enhanced mTOR activation.