Lack of preservation of insulin gene expression by a glucagon-like peptide 1 agonist or a dipeptidyl peptidase 4 inhibitor in an in vivo model of glucolipotoxicity

Effect on glycemia in rats with type 2 diabetes induced by streptozotocin: low-frequency electro-pulse needling stimulated Weiwanxiashu (EX-B 3) and Zusanli (ST 36)

OBJECTIVE

To investigate the effect of low frequency electro-pulse acupuncture (EA) on blood glucose in rats with streptozotocin-induced type 2 diabetes, and the possible mechanism underlying the action.

METHODS

Rat models were established with high fat feeding and intraperitoneal injection of streptozotocin (STZ) (30 mg/kg). Rats with a random blood glucose > 16.7 mmol/L and blood glucose at 2 h-point of oral glucose tolerance test (OGTT) > 11.1 mmol/L were included as diabetic rats, and randomly divided into model group, EA Weiwanxiashu (EX-B 3) group, EA Zusanli (ST 36) group, glimepiride group, and EA non-acupoint group (n = 12). EA (2 Hz continuous wave, 2 mA, 20 min/day, 6 days/week, 4 weeks) and intra-gastric administration of glimepiride were applied as interventions. With fasting blood glucose and OGTT tested at the end of the intervention, the study observed the patterns of hypoglycemic effects. For mechanism study, it observes hematoxylin and eosin staining and Masson staining of pancreas paraffin sections, protein expression of glucagon- like peptide 1 receptor (GLP-1R) in the pancreas and skeletal muscle, glucose transporter 4 (GLUT4) protein expression in skeletal muscle membrane, to detect whether EA controls blood glucose via regulation of GLP-1R.

RESULTS

EA Weiwanxiashu (EX-B 3) significantly increased model rats' pancreas GLP-1R, and GLUT4 of skeletal muscle membrane; the therapy significantly decreased model rats' skeletal muscle GLP-1R, restored pancreas morphology, and reduced fasting blood glucose and insulin resistance indices.

CONCLUSION

EA Weiwanxiashu (EX-B 3) alone has significant effect on glycemia. EA Weiwanxiashu (EX-B 3) plus glimepiride further strengthen the effect. The regulation of the GLP-1R in pancreas and skeletal muscle might be mechanism underpinning the effect.

Glucagon-like peptide-1 analog-mediated protection against cholesterol-induced apoptosis via mammalian target of rapamycin activation in pancreatic βTC-6 cells -1mTORβTC-6

BACKGROUND

Glucagon-like peptide-1 (GLP-1) has been shown to protect pancreatic β-cells against glucolipotoxicity via activation of the Akt pathway. The present study investigated the protective effects of the GLP-1 analog liraglutide against cholesterol-induced lipotoxicity and the mechanisms involved.

METHODS

The mouse βTC-6 pancreatic β-cell line was preincubated for 30 min with 10 nmol/L liraglutide alone or in combination with the mammalian target of rapamycin (mTOR) inhibitor rapamycin (1 μmol/L) before being exposed to 5 mmol/L cholesterol for 6 h. 4',6'-Diamidino-2-phenylindole (DAPI) staining and Western blot analyses were used to assess the effects of liraglutide on cholesterol-induced apoptosis and the phosphorylation of Akt and mTOR.

RESULTS

Cholesterol significantly promoted cell apoptosis and attenuated the phosphorylation of Akt and mTOR, effects that were significantly reversed by liraglutide. Furthermore, rapamycin pretreatment alone significantly increased cholesterol-induced apoptosis compared with cholesterol-treated cells without any other pretreatment.

CONCLUSIONS

The data indicate that mTOR signaling is an essential mediator in the protection of pancreatic β-cells against cholesterol-induced apoptosis by a GLP-1 analog.

Sitagliptin attenuates sympathetic innervation via modulating reactive oxygen species and interstitial adenosine in infarcted rat hearts

We investigated whether sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor, attenuates arrhythmias through inhibiting nerve growth factor (NGF) expression in post-infarcted normoglycemic rats, focusing on adenosine and reactive oxygen species production. DPP-4 bound adenosine deaminase has been shown to catalyse extracellular adenosine to inosine. DPP-4 inhibitors increased adenosine levels by inhibiting the complex formation. Normoglycemic male Wistar rats were subjected to coronary ligation and then randomized to either saline or sitagliptin in in vivo and ex vivo studies. Post-infarction was associated with increased oxidative stress, as measured by myocardial superoxide, nitrotyrosine and dihydroethidium fluorescent staining. Measurement of myocardial norepinephrine levels revealed a significant elevation in vehicle-treated infarcted rats compared with sham. Compared with vehicle, infarcted rats treated with sitagliptin significantly increased interstitial adenosine levels and attenuated oxidative stress. Sympathetic hyperinnervation was blunted after administering sitagliptin, as assessed by immunofluorescent analysis and western blotting and real-time quantitative RT-PCR of NGF. Arrhythmic scores in the sitagliptin-treated infarcted rats were significantly lower than those in vehicle. Ex vivo studies showed a similar effect of erythro-9-(2-hydroxy-3-nonyl) adenine (an adenosine deaminase inhibitor) to sitagliptin on attenuated levels of superoxide and NGF. Furthermore, the beneficial effects of sitagliptin on superoxide anion production and NGF levels can be reversed by 8-cyclopentyl-1,3-dipropulxanthine (adenosine A₁ receptor antagonist) and exogenous hypoxanthine. Sitagliptin protects ventricular arrhythmias by attenuating sympathetic innervation via adenosine A₁ receptor and xanthine oxidase-dependent pathways, which converge through the attenuated formation of superoxide in the non-diabetic infarcted rats.

Mechanism of the beneficial and protective effects of exenatide in diabetic rats

Glucagon-like peptide 1 (GLP1) agonists are promising therapeutic agents in the treatment of diabetes mellitus. This study examines the mechanism of the protective effects of exenatide in experimental diabetes, employing four groups of ten rats each, in which two groups were streptozotocin-induced diabetic and two were control groups. One control and one diabetic group were treated with exenatide (1 μg/kg body weight (BW)) for 10 weeks. Blood plasma was taken for biochemical analyses while pancreatic tissue was taken for immunofluorescence and immunoelectron microscopy studies and real-time PCR to examine the expression of genes. The results show that exenatide improved BW gain and reduced blood glucose in diabetic rats compared with controls. Similarly, exenatide enhanced insulin release from the pancreatic fragments and improved liver and kidney functions and lipid profile in diabetic rats compared with controls. Exenatide not only induced significant increases in serum insulin level but also elevated the number of insulin-, GLP1- and exenatide-positive cells compared with untreated controls. Exenatide also elevated the number of catalase- and glutathione reductase-positive cells in diabetic rat pancreas compared with controls. Exenatide caused significant elevation in the expressions of pancreatic duodenal homeobox-1, heat shock protein-70, glutathione peroxidase, insulin receptor and GLP1 receptor genes in the pancreas of both control and diabetic rats compared with untreated animals. The results have demonstrated that exenatide can exert its beneficial and protective effects by elevating the levels of endogenous antioxidants and genes responsible for the survival, regeneration and proliferation of pancreatic β-cell.

A model of chronic nutrient infusion in the rat

Chronic exposure to excessive levels of nutrients is postulated to affect the function of several organs and tissues and to contribute to the development of the many complications associated with obesity and the metabolic syndrome, including type 2 diabetes. To study the mechanisms by which excessive levels of glucose and fatty acids affect the pancreatic beta-cell and the secretion of insulin, we have established a chronic nutrient infusion model in the rat. The procedure consists of catheterizing the right jugular vein and left carotid artery under general anesthesia; allowing a 7-day recuperation period; connecting the catheters to the pumps using a swivel and counterweight system that enables the animal to move freely in the cage; and infusing glucose and/or Intralipid (a soybean oil emulsion which generates a mixture of approximately 80% unsaturated/20% saturated fatty acids when infused with heparin) for 72 hr. This model offers several advantages, including the possibility to finely modulate the target levels of circulating glucose and fatty acids; the option to co-infuse pharmacological compounds; and the relatively short time frame as opposed to dietary models. It can be used to examine the mechanisms of nutrient-induced dysfunction in a variety of organs and to test the effectiveness of drugs in this context.