Aim: Dysfunctional adipocytes and β-cells are crucial in the advancement of obesity-induced type 2 diabetes (T2D). Calorie restriction (CR) improves insulin sensitivity and fasting blood glucose (FBG) levels, while γ-aminobutyric acid (GABA), a neurotransmitter secreted by β-cells, exerts regenerative effects. Methods: High-fat diet (HFD) and streptozotocin (STZ) induced T2D mouse model was used to assess the effect of CR and GABA combination therapy. The mice were fed with CR diet (30% reduction of HFD) and treated with GABA (2.5 mg/kg i.p) daily for 5 weeks. Assessment of FBG, body weight (BW), insulin sensitivity, glucose tolerance, and estimation of plasma insulin and lipid profile were carried out by standard methods. The mRNA expression of target genes [liver (glucoregulatory enzymes), adipose tissue (lipid metabolism markers) and skeletal muscle (mitochondrial biogenesis markers)] was assessed by SYBR Green-qPCR. Oxygen consumption rate for mitochondrial complex I, II and III was measured in skeletal muscle using Oxytherm Clark-type oxygen electrode. The effect of monotherapies and combination treatment on pancreatic β-cell regeneration and apoptosis was analysed by IHC. Results: CR+GABA group showed significant reduction in FBG levels, improved lipid profile and whole-body insulin responsiveness as indicated by increased insulin levels, insulin sensitivity and glucose tolerance compared to the monotherapies. The transcript expression profile of glucoregulatory enzymes in the liver showed significant reduction in G6Pase, PEPCK, Glycogen Phosphorylase and increased GCK expression in CR+GABA group as compared to the monotherapies The ACC and ATG expression was up-regulated in adipose tissue; and significant increase in SIRT-1, PGC-1α and TFAM expression with up-regulated mitochondrial complex I and III activities was observed in CR+GABA group as compared to the monotherapies. Percentage of BrdU/Insulin and PDX1/Ngn3/Insulin co-positive cells was significantly higher in CR+GABA treated group than the monotherapy and HFD+STZ groups. Finally, there was a significant reduction in TUNEL/Insulin co-positive cells suggesting reduced apoptosis in CR+GABA treated group. Conclusion: Our results suggest that CR in combination with GABA ameliorates T2D in HFD+STZ treated mice, by GABA induced β-cell regeneration and CR mediated insulin sensitivity.
A mitochondrial DNA variant at position 16189 is associated with type 2 diabetes mellitus in Asians
