Mechanisms by which a Very-Low-Calorie Diet Reverses Hyperglycemia in a Rat Model of Type 2 Diabetes

Abstract

Caloric restriction rapidly reverses type 2 diabetes (T2D), but the mechanism(s) of this reversal are poorly understood. Here we show that 3 days of a very-low-calorie diet (VLCD, one-quarter their typical intake) lowered plasma glucose and insulin concentrations in a rat model of T2D without altering body weight. The lower plasma glucose was associated with a 30% reduction in hepatic glucose production resulting from suppression of both gluconeogenesis from pyruvate carboxylase (V_PC), explained by a reduction in hepatic acetyl-CoA content, and net hepatic glycogenolysis. In addition, VLCD resulted in reductions in hepatic triglyceride and diacylglycerol content and PKCɛ translocation, associated with improved hepatic insulin sensitivity. Taken together, these data show that there are pleotropic mechanisms by which VLCD reverses hyperglycemia in a rat model of T2D, including reduced DAG-PKCɛ-induced hepatic insulin resistance, reduced hepatic glycogenolysis, and reduced hepatic acetyl-CoA content, PC flux, and gluconeogenesis.

Keywords: T2D; acetyl-CoA; caloric restriction; gluconeogenesis; glycogenolysis; type 2 diabetes; very low calorie diet; very-low-calorie diet.

Fig. 1. Caloric restriction ameliorates fasting hyperglycemia and reduces ectopic hepatic lipid accumulation

(A) Body weight after 3 days of caloric restriction (or ad lib caloric intake in the controls). (B)–(D) Fasting plasma glucose, insulin, and non-esterified fatty acid concentrations. (E)–(F) Plasma and liver triglyceride concentrations. (G) Liver membrane diacylglycerol. (H) PKCε membrane/cytosol ratio. In all panels, data are the mean±S.E.M. of n=6 per group, with comparisons by t-test. *P<0.05, **P<0.01, ***P<0.001. See also Fig. S1 and Table S1.

Fig. 2. Caloric restriction lowers the rate of hepatic glucose production due to contributions from both net hepatic glycogenolysis and pyruvate carboxylase (V_PC) flux and improves hepatic insulin sensitivity

(A) Glucose production from glycogenolysis, glycerol, and oxaloacetate. The asterisk in the T2D + VLCD bar indicates P<0.05 comparing gluconeogenesis from OAA between T2D and T2D + VLCD rats. (B) Gluconeogenic enzyme protein expression, normalized to GAPDH. (C) Endogenous glucose turnover under basal and hyperinsulinemic-euglycemic clamp conditions. Comparisons between basal and clamp were performed using the 2-tailed paired Student’s t-test. (D) Suppression of hepatic glucose production in the clamp. (E) Ratio of phosphorylated Akt2 to total Akt2. In all panels, *P<0.05, **P<0.01, ***P<0.001. Data are the mean±S.E.M. of n=6 per group, with comparisons by the 2-tailed unpaired Student’s t-test unless otherwise specified. See also Fig. S2.

Fig. 3. Caloric restriction lowers hepatic acetyl-CoA content but does not alter substrate oxidation or lipolysis

(A)–(B) Hepatic long-chain and acetyl-CoA. (C) Whole-body βOHB turnover. (D)–(E) Whole-body palmitate and glycerol turnover. (F) Liver V_CS flux. (G) Hepatic triglyceride export. (H) Hepatic membrane C18:1 C18:0 DAG enrichment following a mixed-meal tolerance with [1-¹³C]triolein. n=7 T2D-small meal, 8 T2D + VLCD-small meal, 7 T2D-large meal, and 7 T2D + VLCD-large meal. **P<0.01, ***P<0.001, referring to the comparison of rats within the same group (control or VLCD) fed a small vs. large meal. In all panels, groups were compared by t-test, and *P<0.05. Unless otherwise specified, the data are the mean±S.E.M. of n=6 per group. See also Fig. S3.

Fig. 4. Increased hepatic acetyl-CoA content and increased rates of net hepatic glycogenolysis contribute to increased rates hepatic glucose production in T2D rats

(A) Acetyl-CoA content in T2D + VLCD rats (copied from Fig. 3B for comparison) and rats treated with an infusion of acetate. In panels (A) and (C), comparisons were performed using the 2-tailed unpaired Student’s t-test. (B) EGP before and after acetate infusion. Data in panels (B) and (D) were compared by the 2-tailed paired Student’s t-test. (C) Liver glycogen in T2D rats (copied from Fig. S2) and T2D rats treated with a glycogen phosphorylase inhibitor. (D) EGP before and after treatment with the phosphorylase inhibitor. In all panels, *P<0.05, ***P<0.001. Data are the mean±S.E.M. of n=6 per group. See also Fig. S4.