Increased secretion of insulin and proliferation of islet β-cells in rats with mesenteric lymph duct ligation

Background &; aimsIt has been suggested that intestinal lymph flow plays an important role in insulin secretion and glucose metabolism after meals. In this study, we investigated the influence of ligation of the mesenteric lymph duct on glucose metabolism and islet β-cells in rats.MethodsMale Sprague–Dawley rats (10 weeks old) were divided into two groups: one underwent ligation of the mesenteric lymph duct above the cistern (ligation group), and the other underwent a sham operation (sham group). After 1 and 2 weeks, fasting plasma concentrations of glucose, insulin, triglyceride, glucose-dependent insulinotropic polypeptide (GIP), and the active form of glucagon-like peptide-1 (GLP-1) were measured. At 2 weeks after the operation, the oral glucose tolerance test (OGTT) and intravenous glucose tolerance test (IVGTT) were performed. After the rats had been sacrificed, the insulin content of the pancreas was measured and the proliferation of β-cells was assessed immunohistochemically using antibodies against insulin and Ki-67.ResultsDuring the OGTT, the ligation group showed a significant decrease in the plasma glucose concentration at 120 min (p < 0.05) and a significant increase in the plasma insulin concentration by more than 2-fold at 15 min (p < 0.01). On the other hand, the plasma GIP concentration was significantly decreased at 60 min (p < 0.01) in the ligated group, while the active form of GLP-1 showed a significantly higher level at 90 min (1.7-fold; p < 0.05) and 120 min (2.5-fold; p < 0.01). During the IVGTT, the plasma insulin concentration in the ligation group was significantly higher at 2 min (more than 1.4-fold; p < 0.05). Immunohistochemistry showed that the ratios of β-cell area/acinar cell area and β-cell area/islet area, and also β-cell proliferation, were significantly higher in the ligation group than in the sham group (p < 0.05, p < 0.01 and p < 0.01, respectively). The insulin content per unit wet weight of pancreas was also significantly increased in the ligation group (p < 0.05).ConclusionsIn rats with ligation of the mesenteric lymph duct, insulin secretion during the OGTT or IVGTT was higher, and the insulin content and β-cell proliferation in the pancreas were also increased. Our data show that mesenteric lymph duct flow has a role in glucose metabolism.     

Central infusion of leptin improves insulin resistance and suppresses β-cell function,but not β-cell mass,primarily through the sympathetic nervous system in a type 2 diabetic rat model

AimsWe investigated whether hypothalamic leptin alters β-cell function and mass directly via the sympathetic nervous system (SNS) or indirectly as the result of altered insulin resistant states.Main methodsThe 90% pancreatectomized male Sprague Dawley rats had sympathectomy into the pancreas by applying phenol into the descending aorta (SNSX) or its sham operation (Sham). Each group was divided into two sections, receiving either leptin at 300 ng/kg bw/h or artificial cerebrospinal fluid (aCSF) via intracerebroventricular (ICV) infusion for 3 h as a short-term study. After finishing the infusion study, ICV leptin (3 μg/kg bw/day) or ICV aCSF (control) was infused in rats fed 30 energy % fat diets by osmotic pump for 4 weeks. At the end of the long-term study, glucose-stimulated insulin secretion and islet morphometry were analyzed.Key findingsAcute ICV leptin administration in Sham rats, but not in SNSX rats, suppressed the first- and second-phase insulin secretion at hyperglycemic clamp by about 48% compared to the control. Regardless of SNSX, the 4-week administration of ICV leptin improved glucose tolerance during oral glucose tolerance tests and insulin sensitivity at hyperglycemic clamp, compared to the control, while it suppressed second-phase insulin secretion in Sham rats but not in SNSX rats. However, the pancreatic β-cell area and mass were not affected by leptin and SNSX, though ICV leptin decreased individual β-cell size and concomitantly increased β-cell apoptosis in Sham rats.SignificanceLeptin directly decreases insulin secretion capacity mainly through the activation of SNS without modulating pancreatic β-cell mass.     

The mTORC2/PKC pathway sustains compensatory insulin secretion of pancreatic β cells in response to metabolic stress

BackgroundCompensation of the pancreatic β cell functional mass in response to metabolic stress is key to the pathogenesis of Type 2 Diabetes. The mTORC2 pathway governs fuel metabolism and β cell functional mass. It is unknown whether mTORC2 is required for regulating metabolic stress-induced β cell compensation.MethodsWe challenged four-week-old β-cell-specific Rictor (a key component of mTORC2)-knockout mice with a high fat diet (HFD) for 4 weeks and measured metabolic and pancreatic morphological parameters. We performed ex vivo experiments to analyse β cell insulin secretion and electrophysiology characteristics. Adenoviral-mediated overexpression and lentiviral-ShRNA-mediated knocking down proteins were applied in Min6 cells and cultured primary mouse islets.ResultsβRicKO mice showed a significant glucose intolerance and a reduced plasma insulin level and an unchanged level β cell mass versus the control mice under HFD. A HFD or palmitate treatment enhanced both glucose-induced insulin secretion (GIIS) and the PMA (phorbol 12-myristate 13-acetate)-induced insulin secretion in the control islets but not in the βRicKO islets. The KO β cells showed similar glucose-induced Ca^2 + influx but lower membrane capacitance increments versus the control cells. The enhanced mTORC2/PKC proteins levels in the control HFD group were ablated by Rictor deletion. Replenishing PKCα by overexpression of PKCα-T638D restored the defective GIIS in βRicKO islets.ConclusionsThe mTORC2/Rictor pathway modulates β cell compensatory GIIS under nutrient overload mediated by its phosphorylation of PKCα.General significanceThis study suggests that the mTORC2/PKC pathway in β cells is involved in the pathogenesis of T2D.     

Islet NADPH oxidase activity modulates β-cell mass and endocrine function in rats with fructose-induced oxidative stress

BackgroundIslet NADPH oxidase activity is modulated by glucose and other insulin secretagogues and it might be part of the regulatory mechanism of insulin secretion. We studied its modulatory role of islet NADPH oxidase upon β-cell function in rats with fructose-induced oxidative stress.MethodsNormal rats were fed for 3 weeks with a standard diet, a fructose-rich diet or both diets plus apocynin. We measured plasma glucose, insulin, triacylglycerol and lipid peroxidation levels and the homeostasis model assessment-insulin resistance (HOMA-IR) and HOMA-β indexes, and performed an oral glucose tolerance test. β-cell volume density and the number of islets per mm² were determined by immunomorphometric analysis of the pancreas. Insulin secretion, glucose metabolism, glucokinase and NADPH oxidase activities were studied in islets isolated from each experimental group.ResultsFructose-fed rats had increased plasma triacylglycerol, insulin and lipid peroxidation levels associated with an insulin resistance state; the reactive higher secretion was unable to cope with the increased demand of insulin, leading to an impaired glucose tolerance. They also have a lower number of islets per area unit with a decreased β-cell volume density. All these alterations were prevented by blocking NADPH oxidase activity with apocynin.ConclusionFructose-induced changes are partly mediated by modulation of NADPH oxidase activity.General significanceThe metabolic dysfunctions and enhanced oxidative stress measured in fructose-fed rats resemble those recorded in human prediabetes; thus, successful strategies employed in this model could be later used to prevent the progression of this state towards type 2 diabetes in human beings.     

Taurine supplementation prevents morpho-physiological alterations in high-fat diet mice pancreatic β-cells

Taurine (Tau) is involved in beta (β)-cell function and insulin action regulation. Here, we verified the possible preventive effect of Tau in high-fat diet (HFD)-induced obesity and glucose intolerance and in the disruption of pancreatic β-cell morpho-physiology. Weaning Swiss mice were distributed into four groups: mice fed on HFD diet (36 % of saturated fat, HFD group); HTAU, mice fed on HFD diet and supplemented with 5 % Tau; control (CTL); and CTAU. After 19 weeks of diet and Tau treatments, glucose tolerance, insulin sensitivity and islet morpho-physiology were evaluated. HFD mice presented higher body weight and fat depots, and were hyperglycemic, hyperinsulinemic, glucose intolerant and insulin resistant. Their pancreatic islets secreted high levels of insulin in the presence of increasing glucose concentrations and 30 mM K⁺. Tau supplementation improved glucose tolerance and insulin sensitivity with a higher ratio of Akt phosphorylated (pAkt) related to Akt total protein content (pAkt/Akt) following insulin administration in the liver without altering body weight and fat deposition in HTAU mice. Isolated islets from HTAU mice released insulin similarly to CTL islets. HFD intake induced islet hypertrophy, increased β-cell/islet area and islet and β-cell mass content in the pancreas. Tau prevented islet and β-cell/islet area, and islet and β-cell mass alterations induced by HFD. The total insulin content in HFD islets was higher than that of CTL islets, and was not altered in HTAU islets. In conclusion, for the first time, we showed that Tau enhances liver Akt activation and prevents β-cell compensatory morpho-functional adaptations induced by HFD.     

Sexual dimorphism,weight gain and glucose intolerance in a B- and T-cell deficient mouse model

BackgroundEstrogen is thought to aid maintenance of insulin sensitivity potentially through modulation of a counter-regulatory mechanism that interferes with the contribution of adaptive and innate immune systems to visceral fat deposition. We evaluated the impact of estrogen on long-term high fat diet (HFD) intake in B- and T-cell deficient and immunocompetent animals comparatively.MethodsA total of 16 BALB and 16 SCID mice, 8 of each sex and strain, were randomized to receive low fat diet, 4.1% fat or HFD, 35% fat, such that there was a group of both each sex and each strain receiving each diet. Biweekly levels of adiponectin, leptin and insulin levels were assessed and a glucose tolerance test (GTT) was performed after 13 weeks.ResultsUnlike their male counterparts, HFD-fed SCID females neither gained weight, nor became insulin resistant. Meanwhile, in the HFD-fed BALB groups both males and females gained weight similarly, but remarkable sexual dimorphism was nonetheless observed. The females had notable higher adiponectin levels as compared to males (10–60 μg/mL vs. 6–10 μg/mL respectively) causing the adiponectin-to-leptin (A/L) ratio to reach 80 one week after HFD initiation. The A/L dropped to 10, still higher than males, by week 13, but dropped to 2 by the end of the study in agreement with inverse insulin trends. None of the HFD-fed female groups developed insulin resistance (IR) by week 13, while all male counterparts had. Similar results were observed in the HFD-fed SCID groups whereby the females did not develop IR and had a higher A/L; however, adiponectin levels were comparable between groups (5–11 μg/mL).ConclusionsThe present study provides lacking evidence indicating that estrogen may be sufficient to prevent weight gain and development of glucose intolerance in high-fat fed B- and T-cell deficient mice.     

Different roles of zinc plus arachidonic acid on insulin sensitivity between high fructose- and high fat-fed rats

AimsThis study was to determine the effects of zinc plus arachidonic acid (ZA) treatment on the insulin action in the specific ZA target organs using hyperinsulinemic euglycemic clamp method.Main methods18 Sprague–Dawley rats weighing ~ 130 g were divided into 3 groups of 6 rats and treated them with 1) normal rat chow, 2) high fructose (60.0%) diet only, or 3) the same fructose diet plus drinking water containing 10 mg zinc plus 50 mg arachidonic acid (AA)/L. In a separate study, male Wistar rats weighing ~ 250 g were fed normal rat chow (n = 4) or high fat (66.5%) diet with drinking water containing zero (n = 9) or 10 mg AA plus 20 mg zinc /L (n = 9). After 4 week treatment, insulin action was assessed using the hyperinsulinemic eguglycemic clamp technique.Key findingsHigh fructose feeding impaired suppression of hepatic glucose output by insulin compared to controls during the clamp procedure (4.39 vs. 2.35 mg/kg/min; p < 0.05). However, ZA treatment in high fructose-fed rats showed a significant improvement of hepatic insulin sensitivity compared to non-treatment controls (4.39 vs. 2.18 mg/kg/min; p < 0.05). Glucose infusion rates in Wistar rats maintained on a high fat diet (HFD) were significantly lower compared to control rats (22.8 ± 1.3 vs. 31.9 ± 1.4 mg/kg/min; p < 0.05). ZA treatment significantly improved (~ 43%) peripheral tissue insulin sensitivity in HFD fed animals (26.7 ± 1.3 [n = 9] vs. 22.8 ± 1.3 mg/kg/min; p < 0.05).SignificanceThese data demonstrate that ZA treatment is effective in improving glucose utilization in hyperglycemic rats receiving either a high-fructose or a high-fat diet.     

Effects of angiotensin II AT1 receptor inhibition and exercise training on insulin action in rats on high-fat diet

AimThis study was to determine whether combination of the angiotensin II AT1 receptor blocker (ARB), candesartan cilexetil, and exercise training can prevent the development of high-fat diet-induced insulin resistance.Main methodsF344/NSlc rats were fed normal chow diet or high-fat (HF) diet for 7 weeks. The HF-fed rats were either administered candesartan cilexetil (5 mg·kg^? 1·day^? 1), exercise-trained, or received a combination of these 2 treatments.Key findingsOral glucose tolerance tests (OGTT) showed that combined treatment with candesartan cilexetil and exercise increased glucose tolerance as compared with each treatment alone in HF-fed rats. Moreover, euglycemic–hyperinsulinemic clamp analysis showed improvement in glucose infusion rate with exercise training or candesartan cilexetil treatment alone, and further improvement was observed with the combination treatment. Systolic blood pressure improved with candesartan cilexetil but not with exercise alone. Finally, Glut-4 protein expression in soleus muscle was decreased with HF diet, and the expression was increased by exercise and not candesartan cilexetil treatment.SignificanceThese results suggest that the combination of candesartan cilexetil and exercise training improves insulin resistance as compared with each treatment alone.     

The effect of isosteviol on hyperglycemia and dyslipidemia induced by lipotoxicity in rats fed with high-fat emulsion

AimsThe aim of present study was to investigate the effects of isosteviol on hyperglycemia and hyperlipidemia in rats fed with high-fat emulsion (HFE).Main methodsHyperglycemia and hyperlipidemia in rats was induced by daily ingestion of HFE for 14 days. Isosteviol (0.2, 1.0, or 5.0 mg/kg/day) was orally administered for 7 days, with rosiglitazone maleate (5.0 mg/kg/day) used as the positive control. The levels of fasting serum glucose (FSG), fasting serum insulin (FSI), total cholesterol (TC), triglyceride (TG), high density lipoprotein (HDL), and low density lipoprotein (LDL) in serum were assayed. Intravenous glucose tolerance test (IVGTT) was performed with serum glucose and insulin levels monitored. The effect of the supplement of palmitate in HFE on the activity of isosteviol was investigated. Ultrastructural changes in islet β-cells and peroxisome proliferator-activated receptor α (PPARα) mRNA expression profile were determined.Key findingsFSG, FSI, TC and LDL levels and insulin resistance index (IRI) were decreased and HDL level was increased by all doses of isosteviol. During IVGTT, serum glucose levels were decreased by isosteviol and no significant differences were observed in insulin release between isosteviol-treated and control groups. The effects of isosteviol were attenuated by palmitate. Damage to pancreatic islet cells was partially attenuated, and expression profile of hepatic PPARα mRNA was enhanced by isosteviol.SignificanceAntihyperglycemic effects of isosteviol could enhance utilization of glucose in the periphery and reduce β-cell damage induced by dyslipidemia. Modulating-lipidemic effects of isosteviol might be related to the potential enhancement of liver PPARα mRNA expression.     

Central visfatin potentiates glucose-stimulated insulin secretion and β-cell mass without increasing serum visfatin levels in diabetic rats

IntroductionOur previous study revealed that plasma visfatin levels were lower in pregnant women with gestational diabetes (GDM) than non-GDM independent of prepreganacy BMI. We examined whether central visfatin modulates energy and glucose homeostasis via altering insulin resistance, insulin secretion or islet morphometry in diabetic rats.MethodsPartial pancreatectomized, type 2 diabetic, rats were interacerbroventricularly infused with visfatin (100 ng/rat/day, Px-VIS), visfatin + visfatin antagonist, CHS-828 (100 μg/rat/day, Px-VIS-ANT), or saline (control, Px-Saline) via osmotic pump, respectively, for 4 weeks.ResultsCentral visfatin improved insulin signaling (pAkt → pFOXO-1) but not pSTAT3 in the hypothalamus. Central visfatin did not alter serum visfatin levels in diabetic rats whereas the levels were higher in non-diabetic rats than diabetic rats. Body weight at the 2nd week was lowered in the Px-VIS group due to decreased food intake in the first two weeks compared to the Px-Saline group and energy expenditure was not significantly different among the treatment groups of diabetic rats. Visfatin antagonist treatment nullified the central visfatin effect. Px-VIS increased whole body glucose disposal rates in euglycemic hyperinsulinemic clamp compared to Px-Saline and lowered hepatic glucose output, whereas Px-VIS-ANT blocked the visfatin effect on insulin resistance (P < 0.05). In hyperglycemic clamp study, the area under the curve of insulin in first and second phase were significantly higher in the Px-VIS group than the Px-Saline group without modifying insulin sensitivity at the hyperglycemic state, whereas the increase in serum insulin levels was blocked in the Px-VIS-ANT group. Central visfatin also increased β-cell mass by increasing β-cell proliferation.ConclusionsCentral visfatin improved glucose homeostasis by increasing insulin secretion and insulin sensitivity at euglycemia through the hypothalamus in diabetic rats. Therefore, visfatin is a positive modulator of glucose homeostasis by delivering the hypothalamic signals into the peripheries.     

Altered adipocyte progenitor population and adipose-related gene profile in adipose tissue by long-term high-fat diet in mice

AimsHigh-fat diet (HFD) is associated with adipose inflammation, which contributes to key components of metabolic abnormalities. The expanded adipose tissue mass associated with obesity is the result of hyperplasia and hypertrophy of adipocytes. In this study, we investigated the effects of long-term HFD on adipocyte progenitor cell (APC) population and adipose-specific gene profiles in both white and brown adipose, and the role of perivascular adipose in the alteration of vascular function in response to HFD.Main methodsMale C57BL/6 mice were fed a standard normal diet (ND) or HFD for about 8 months. Glucose metabolism was assessed by an intraperitoneal glucose tolerance test. APC population and adipose-related gene profile were evaluated, and vascular function was measured in the presence or absence of perivascular adipose. Adiponectin and AMPK activity were also investigated.Key findingsHFD induced insulin resistance and glucose intolerance, and resulted in a decrease in APC population in brown, but not in white adipose tissue, when compared with animals fed a ND, with differential alterations of white and brown adipocyte-specific gene expression in brown and white adipose. Additionally, HFD led to altered vascular function in arteries in the presence of perivascular adipose tissue, which is associated with increased superoxide production. Adiponectin and AMPK activity were significantly decreased in response to long-term HFD.SignificanceThese findings suggest that long-term high-fat intake differentially alters adipocyte progenitor population and adipose-related gene expression in adipose tissue, and adiponectin-AMPK signaling might be involved. In addition, HFD induces changes in perivascular adipose-mediated vascular function.     

Improvement in Pancreatic β-Cell Function with Vitamin D and Calcium Supplementation in Vitamin D-Deficient Nondiabetic Subjects

ObjectiveThere are varied reports on the effect of vitamin D supplementation on β-cell function and plasma glucose levels. The objective of this study was to examine the effect of vitamin D and calcium supplementation on β-cell function and plasma glucose levels in subjects with vitamin D deficiency.MethodsNondiabetic subjects (N = 48) were screened for their serum 25-hydroxyvitamin D (25-OHD), albumin, creatinine, calcium, phosphorus, alkaline phosphatase, and intact parathyroid hormone (PTH) status. Subjects with 25-OHD deficiency underwent a 2-hour oral glucose tolerance test. Cholecalciferol (9,570 international units [IU]/day; tolerable upper intake level, 10,000 IU/day; according to the Endocrine Society guidelines for vitamin D supplementation) and calcium (1 g/day) were supplemented.ResultsThirty-seven patients with 25-OHD deficiency participated in the study. The baseline and postvitamin D/calcium supplementation and the difference (corrected) were: serum calcium, 9 ± 0.33 and 8.33 ± 1.09 mg/dL (− 0.66 ± 1.11 mg/dL); 25-OHD, 8.75 ± 4.75 and 36.83 ± 18.68 ng/mL (28.00 ± 18.33 ng/mL); PTH, 57.9 ± 29.3 and 36.33 ± 22.48 pg/mL (− 20.25 ± 22.45 pg/mL); fasting plasma glucose, 78.23 ± 7.60 and 73.47 ± 9.82 mg/dL (− 4.88 ± 10.65 mg/dL); and homeostasis model assessment-2–percent β-cell function C-peptide secretion (HOMA-2–%B C-PEP), 183.17 ± 88.74 and 194.67 ± 54.71 (11.38 ± 94.27). Significant differences were observed between baseline and post-vitamin D/calcium supplementation serum levels of corrected calcium (Z, − 3.751; P < .0001), 25-OHD (Z, − 4.9; P < .0001), intact PTH (Z, − 4.04; P < .0001), fasting plasma glucose (Z, − 2.7; P < .007), and HOMA-2–%B C-PEP (Z, − 1.923; P < .05) as determined by Wilcoxon signed rank test. Insulin resistance as measured by HOMA was unchanged.ConclusionOptimizing serum 25-OHD concentrations and supplementation with calcium improves fasting plasma glucose levels and β-cell secretory reserve. Larger randomized control studies are needed to determine if correction of 25-OHD deficiency will improve insulin secretion and prevent abnormalities of glucose homeostasis. (Endocr Pract. 2014;20:129-138)     

L-histidine sensing by calcium sensing receptor inhibits voltage-dependent calcium channel activity and insulin secretion in β-cells

AimsOur goal was to test the hypothesis that the histidine-induced activation of calcium sensing receptor (CaR) can regulate calcium channel activity of L-type voltage dependent calcium channel (VDCC) due to increased spatial interaction between CaR and VDCC in β-cells and thus modulate glucose-induced insulin secretion.Main methodsRat insulinoma (RINr1046-38) insulin-producing β-cells were cultured in RPMI-1640 medium on 25 mm diameter glass coverslips in six-well culture plates in a 5% CO₂ incubator at 37 °C. The intracellular calcium concentration, [Ca²⁺]_i, was determined by ratio fluorescence microscopy using Fura-2AM. The spatial interactions between CaR and L-type VDCC in β-cells were measured by immunofluorescence confocal microscopy using a Nikon C1 laser scanning confocal microscope. The insulin release was determined by enzyme-linked immunosorbent assay (ELISA).Key findingsThe addition of increasing concentrations of L-histidine along with 10 mM glucose resulted in 57% decrease in [Ca²⁺]_i. The confocal fluorescence imaging data showed 5.59 to 8.62-fold increase in colocalization correlation coefficient between CaR and VDCC in β-cells exposed to L-histidine thereby indicating increased membrane delimited spatial interactions between these two membrane proteins. The insulin ELISA data showed 54% decrease in the 1st phase of glucose-induced insulin secretion in β-cells exposed to increasing concentrations of L-histidine.SignificanceL-histidine-induced increased spatial interaction of CaR with VDCC can inhibit calcium channel activity of VDCC and consequently regulate glucose-induced insulin secretion by β-cells. The L-type VDCC could therefore be a potential therapeutic target in diabetes.     

Chronic insulin infusion induces reversible glucose intolerance in lean rats yet ameliorates glucose intolerance in obese rats

BackgroundAlthough insulin resistance (IR) is a key factor in the pathogenesis of type 2 diabetes (T2D), the precise role of insulin in the development of IR remains unclear. Therefore, we investigated whether chronic basal insulin infusion is causative in the development of glucose intolerance.MethodsNormoglycemic lean rats surgically instrumented with i.v. catheters were infused with insulin (3 mU/kg/min) or physiological saline for 6 weeks. At infusion-end, plasma insulin levels along with glucose tolerance were assessed.ResultsSix weeks of insulin infusion induced glucose intolerance and impaired insulin response in healthy rats. Interestingly, the effects of chronic insulin infusion were completely normalized following 24 h withdrawal of exogenous insulin and plasma insulin response to glucose challenge was enhanced, suggesting improved insulin secretory capacity. As a result of this finding, we assessed whether the effects of insulin therapy followed by a washout could ameliorate established glucose intolerance in obese rats. Obese rats were similarly instrumented and infused with insulin or physiological saline for 7 days followed by 24 h washout. Seven day-insulin therapy in obese rats significantly improved glucose tolerance, which was attributed to improved insulin secretory capacity and improved insulin signaling in liver and skeletal muscle.ConclusionModerate infusion of insulin alone is sufficient to cause glucose intolerance and impair endogenous insulin secretory capacity, whereas short-term, intensive insulin therapy followed by insulin removal effectively improves glucose tolerance, insulin response and peripheral insulin sensitivity in obese rats.General significanceNew insight into the link between insulin and glucose intolerance may optimize T2D management.     

DA-1229, a novel and potent DPP4 inhibitor, improves insulin resistance and delays the onset of diabetes

AimTo characterize the pharmacodynamic profile of DA-1229, a novel dipeptidyl peptidase (DPP) 4 inhibitor.Main methodsEnzyme inhibition assays against DPP4, DPP8 and DPP9. Antidiabetic effects of DA-1229 in HF-DIO mice and young db/db mice.Key findingsDA-1229 was shown to potently inhibit the DPP4 enzyme in human and murine soluble forms and the human membrane-bound form with IC₅₀ values of 0.98, 3.59 and 1.26 nM, respectively. As a reversible and competitive inhibitor, DA-1229 was more selective to human DPP4 (6000-fold) than to human DPP8 and DPP9. DA-1229 (0.1–3 mg/kg) dose-dependently inhibited plasma DPP4 activity, leading to increased levels of plasma GLP-1 and insulin, and thereby lowering blood glucose levels in mice. In high fat diet-fed (HF) mice, a single oral dose of 100 mg/kg of DA-1229 reduced plasma DPP4 activity by over 80% during a 24 h period. Long-term treatment with DA-1229 for 8 weeks revealed significant improvements in glucose intolerance and insulin resistance, accompanied by significant body weight reduction. However, it remains unclear whether there is a direct causal relationship between DPP4 inhibition and body weight reduction. In young db/db mice, the DA-1229 treatment significantly reduced blood glucose excursions for the first 2 weeks, resulting in significantly lower levels of HbA1c at the end of the study. Furthermore, the pancreatic insulin content of the treatment group was significantly higher than that of the db/db control.SignificanceDA-1229 as a novel and selective DPP4 inhibitor improves the insulin sensitivity in HF mice and delays the onset of diabetes in young db/db mice.     

Comparative effects of intraduodenal fat and glucose on the gut-incretin axis in healthy males

BackgroundThe interaction of nutrients with the small intestine stimulates the secretion of numerous enteroendocrine hormones that regulate postprandial metabolism. However, differences in gastrointestinal hormonal responses between the macronutrients are incompletely understood. In the present study, we compared blood glucose and plasma hormone concentrations in response to standardised intraduodenal (ID) fat and glucose infusions in healthy humans.MethodsIn a parallel study design, 16 healthy males who received an intraduodenal fat infusion were compared with 12 healthy males who received intraduodenal glucose, both at a rate of 2 kcal/min over 120 min. Venous blood was sampled at frequent intervals for measurements of blood glucose, and plasma total and active glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), insulin and glucagon.ResultsPlasma concentrations of the incretin hormones (both total and active GLP-1 and GIP) and glucagon were higher, and plasma insulin and blood glucose concentrations lower, during intraduodenal fat, when compared with intraduodenal glucose, infusion (treatment by time interaction: P < 0.001 for each).ConclusionsCompared with glucose, intraduodenal fat elicits substantially greater GLP-1, GIP and glucagon secretion, with minimal effects on blood glucose or plasma insulin in healthy humans. These observations are consistent with the concept that fat is a more potent stimulus of the ‘gut-incretin’ axis than carbohydrate.     

4Pi microscopy reveals an impaired three-dimensional mitochondrial network of pancreatic islet β-cells,an experimental model of type-2 diabetes

Insulin production in pancreatic β-cells is critically linked to mitochondrial oxidative phosphorylation. Increased ATP production triggered by blood glucose represents the β-cells' glucose sensor. Type-2 diabetes mellitus results from insulin resistance in peripheral tissues and impaired insulin secretion. Pathology of diabetic β-cells might be reflected by the altered morphology of mitochondrial network. Its characterization is however hampered by the complexity and density of the three-dimensional (3D) mitochondrial tubular networks in these cell types. Conventional confocal microscopy does not provide sufficient axial resolution to reveal the required details; electron tomography reconstruction of these dense networks is still difficult and time consuming. However, mitochondrial network morphology in fixed cells can also be studied by 4Pi microscopy, a laser scanning microscopy technique which provides an ～ 7-fold improved axial resolution (～ 100 nm) over conventional confocal microscopy. Here we present a quantitative study of these networks in insulinoma INS-1E cells and primary β-cells in Langerhans islets. The former were a stably-transfected cell line while the latter were transfected with lentivirus, both expressing mitochondrial matrix targeted redox-sensitive GFP. The mitochondrial networks and their partial disintegration and fragmentation are revealed by carefully created iso-surface plots and their quantitative analysis. We demonstrate that β-cells within the Langerhans islets from diabetic Goto Kakizaki rats exhibited a more disintegrated mitochondrial network compared to those from control Wistar rats and model insulinoma INS-1E cells. Standardization of these patterns may lead to development of morphological diagnostics for Langerhans islets, for the assessment of β-cell condition, before their transplantations.     

Hydroxytyrosol prevents metabolic impairment reducing hepatic inflammation and restoring duodenal integrity in a rat model of NAFLD

The potential mechanisms of action of polyphenols in nonalcoholic fatty liver disease (NAFLD) are overlooked. Here, we evaluate the beneficial therapeutic effects of hydroxytyrosol (HT), the major metabolite of the oleuropein, in a nutritional model of insulin resistance (IR) and NAFLD by high-fat diet. Young male rats were divided into three groups receiving (1) standard diet (STD; 10.5% fat), (2) high-fat diet (HFD; 58.0% fat) and (3) HFD + HT (10 mg/kg/day by gavage). After 5 weeks, the oral glucose tolerance test was performed, and at 6th week, blood sample and tissues (liver and duodenum) were collected for following determinations. The HT-treated rats showed a marked reduction in serum AST, ALT and cholesterol and improved glucose tolerance and insulin sensitivity, reducing homeostasis model assessment index. HT significantly corrected the metabolic impairment induced by HFD, increasing hepatic peroxisome proliferator-activated receptor PPAR-α and its downstream-regulated gene fibroblast growth factor 21, the phosphorylation of acetyl-CoA carboxylase and the mRNA carnitine palmitoyltransferase 1a. HT also reduced liver inflammation and nitrosative/oxidative stress decreasing the nitrosylation of proteins, reactive oxygen species production and lipid peroxidation. Moreover, HT restored intestinal barrier integrity and functions (fluorescein isothiocyanate-dextran permeability and mRNA zona occludens ZO-1). Our data demonstrate the beneficial effect of HT in the prevention of early inflammatory events responsible for the onset of IR and steatosis, reducing hepatic inflammation and nitrosative/oxidative stress and restoring glucose homeostasis and intestinal barrier integrity.