In vitro simulation of calorie restriction-induced decline in glucose and insulin leads to increased insulin-stimulated glucose transport in rat skeletal muscle

Protective effects of calorie restriction on insulin resistance and islet function in STZ-induced type 2 diabetes rats

Background

Caloric restriction (CR) has become increasingly attractive in the treatment of type 2 diabetes mellitus (T2DM) because of the increasingly common high-calorie diet and sedentary lifestyle. This study aimed to evaluate the role of CR in T2DM treatment and further explore its potential molecular mechanisms.

Methods

Sixty male Sprague–Dawley rats were used in this study. The diabetes model was induced by 8 weeks of high-fat diet (HFD) followed by a single dose of streptozotocin injection (30 mg/kg). Subsequently, the diabetic rats were fed HFD at 28 g/day (diabetic control) or 20 g/day (30% CR regimen) for 20 weeks. Meanwhile, normal rats fed a free standard chow diet served as the vehicle control. Body mass, plasma glucose levels, and lipid profiles were monitored. After diabetes-related functional tests were performed, the rats were sacrificed at 10 and 20 weeks, and glucose uptake in fresh muscle was determined. In addition, western blotting and immunofluorescence were used to detect alterations in AKT/AS160/GLUT4 signaling.

Results

We found that 30% CR significantly attenuated hyperglycemia and dyslipidemia, leading to alleviation of glucolipotoxicity and thus protection of islet function. Insulin resistance was also markedly ameliorated, as indicated by notably improved insulin tolerance and homeostatic model assessment for insulin resistance (HOMA-IR). However, the improvement in glucose uptake in skeletal muscle was not significant. The upregulation of AKT/AS160/GLUT4 signaling in muscle induced by 30% CR also attenuated gradually over time. Interestingly, the consecutive decrease in AKT/AS160/GLUT4 signaling in white adipose tissue was significantly reversed by 30% CR.

Conclusion

CR (30%) could protect islet function from hyperglycemia and dyslipidemia, and improve insulin resistance. The mechanism by which these effects occurred is likely related to the upregulation of AKT/AS160/GLUT4 signaling.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12986-021-00575-y.

Vitamin E supplementation and caloric restriction promotes regulation of insulin secretion and glycemic homeostasis by different mechanisms in rats

Vitamin E and caloric restriction have antioxidant effects in mammals. The aim of this study was to evaluate effects of vitamin E supplementation and caloric restriction upon insulin secretion and glucose homeostasis in rats. Male Wistar rats were distributed among the following groups: C, control group fed ad libitum; R, food quantity reduction of 40%; CV, control group supplemented with vitamin E [30 mg·kg^-1·day^-1]; and RV, food-restricted group supplemented with vitamin E. The experiments ran for 21 days. Glucose tolerance and insulin sensitivity was higher in the CV, R, and RV groups. Insulin secretion stimulated with different glucose concentrations was lower in the R and RV groups, compared with C and CV. In the presence of glucose and secretagogues, insulin secretion was higher in the CV group and was lower in the R and RV groups. An increase in insulin receptor occurred in the fat pad and muscle tissue of groups CV, R, and RV. Levels of hepatic insulin receptor and phospho-Akt protein were higher in groups R and RV, compared with C and CV, while muscle phospho-Akt was increased in the CV group. There was a reduction in hepatic RNA levels of the hepatocyte growth factor gene and insulin degrading enzyme in the R group, and increased levels of insulin degrading enzyme in the CV and RV groups. Thus, vitamin E supplementation and caloric restriction modulate insulin secretion by different mechanisms to maintain glucose homeostasis.

Effects of Calorie Restriction and Fiber Type on Glucose Uptake and Abundance of Electron Transport Chain and Oxidative Phosphorylation Proteins in Single Fibers from Old Rats

Calorie restriction (CR; reducing calorie intake by ~40% below ad libitum) can increase glucose uptake by insulin-stimulated muscle. Because skeletal muscle is comprised of multiple, heterogeneous fiber types, our primary aim was to determine the effects of CR (initiated at 14 weeks old) and fiber type on insulin-stimulated glucose uptake by single fibers of diverse fiber types in 23-26-month-old rats. Isolated epitrochlearis muscles from AL and CR rats were incubated with [3H]-2-deoxyglucose ± insulin. Glucose uptake and fiber type were determined for single fibers dissected from the muscles. We also determined CR-effects on abundance of several key metabolic proteins in single fibers. CR resulted in: (a) significantly (p < .05 to .001) greater glucose uptake by insulin-stimulated type I, IIA, IIB, IIBX, and IIX fibers; (b) significantly (p < .05 to .001) reduced abundance of several mitochondrial electron transport chain (ETC) and oxidative phosphorylation (OxPhos) proteins in type I, IIA, and IIBX but not IIB and IIX fibers; and (c) unaltered hexokinase II abundance in each fiber type. These results demonstrate that CR can enhance glucose uptake in each fiber type of rat skeletal muscle in the absence of upregulation of the abundance of hexokinase II or key mitochondrial ETC and OxPhos proteins.

Dietary restriction improves systemic and muscular oxidative stress in type 2 diabetic Goto-Kakizaki rats

Type 2 diabetes is a heterogeneous metabolic disease characterized by insulin resistance and β-cell dysfunction leading to hyperglycaemia and dyslipidaemia. Dietary intervention seems to improve some of these cellular complications, namely insulin resistance. Our aim was to evaluate the effects of dietary restriction on systemic and skeletal muscle oxidative stress and insulin resistance in normal Wistar rats and Goto-Kakizaki (GK) rats, a non-obese type 2 diabetic animal model. Four-month-old normal and diabetic rats were separated in four groups. One group of each strain was maintained with ad libitum standard diet, and the other group was submitted to a dietary restriction (50% of control animals daily food intake), during 2 months. Metabolic profile, insulin resistance indexes and muscle lipids were determined. Oxidative stress parameters were also measured at systemic and muscle levels: protein carbonyl, 8-hydroxy-2'-deoxyguanosine and free 8-isoprostane. Dietary restriction improved lipid profile in both strains and urinary free 8-isoprostane and plasma carbonyl compounds in diabetic rats. An improvement of muscle triglycerides accumulation and 8-isoprostane concentration and a reduction of insulin resistance were also observed in GK rats. Our data show that dietary restriction ameliorates systemic and skeletal muscle oxidative stress state in type 2 diabetes, which is associated with improved insulin resistance.

Beneficial effects of dietary restriction in type 2 diabetic rats: the role of adipokines on inflammation and insulin resistance

Inflammation plays an important role in diabetes mellitus and its complications. In this context, the negative cross-talk between adipose tissue and skeletal muscle leads to disturbances in muscle cell insulin signalling and induces insulin resistance. Because several studies have shown that energy restriction brings some benefits to diabetes, the aim of the present study was to evaluate the effects of dietary restriction on systemic and skeletal muscle inflammatory biomarkers, such C-reactive protein, adipokines and cytokines, and in insulin resistance in Goto-Kakizaki rats. This is an animal model of spontaneous non-obese type 2 diabetes with strongly insulin resistance and without dyslipidaemia. Animals were maintained during 2 months of dietary restriction (50 %) and were killed at 6 months of age. Some biochemical determinations were done using ELISA and Western blot. Data from the present study demonstrate that in Goto-Kakizaki rats the dietary restriction improved insulin resistance, NEFA levels and adipokine profile and ameliorated inflammatory cytokines in skeletal muscle. These results indicate that dietary restriction in type 2 diabetes enhances adipose tissue metabolism leading to an improved skeletal muscle insulin sensitivity.