Defective lipid delivery modulates glucose tolerance and metabolic response to diet in apolipoprotein E-deficient mice

Abstract

Objective: Apolipoprotein E (ApoE) regulates plasma lipid levels via modulation of lipolysis and serving as ligand for receptor-mediated clearance of triglyceride (TG)-rich lipoproteins. This study tested the impact of modulating lipid delivery to tissues on insulin responsiveness and diet-induced obesity.

Research design and methods: ApoE(+/+) and apoE(-/-) mice were placed on high-fat-high-sucrose diabetogenic diet or control diet for 24 weeks. Plasma TG clearance, glucose tolerance, and tissue uptake of dietary fat and glucose were assessed.

Results: Plasma TG clearance and lipid uptake by adipose tissue were impaired, whereas glucose tolerance was improved in control diet-fed apoE(-/-) mice compared with apoE(+/+) mice after an oral lipid load. Fat mass was reduced in apoE(-/-) mice compared with apoE(+/+) mice under both dietary conditions. The apoE(-/-) mice exhibited lower body weight and insulin levels than apoE(+/+) mice when fed the diabetogenic diet. Glucose tolerance and uptake by muscle and brown adipose tissue (BAT) was also improved in mice lacking apoE when fed the diabetogenic diet. Indirect calorimetry studies detected no difference in energy expenditure and respiratory quotient between apoE(+/+) and apoE(-/-) mice on control diet. Energy expenditure and uncoupling protein-1 expression in BAT were slightly but not significantly increased in apoE(-/-) mice on diabetogenic diet.

Conclusions: These results demonstrated that decreased lipid delivery to insulin-sensitive tissues improves insulin sensitivity and ameliorates diet-induced obesity.

FIG. 1

Analysis of plasma lipoproteins in apoE^−/− and apoE^+/+ mice. Pooled plasma (0.25 ml) from five apoE^+/+ (A) and five apoE^−/− (B) mice maintained on control diet (○) or diabetogenic diet (■) was applied to two FPLC Superose columns, and 0.5-ml fractions were collected for cholesterol determinations. According to calibration with standard lipoproteins, the peak fractions 7–9 represent VLDL, fractions 15–22 are mainly IDL/LDL particles, and fractions 20–26 contain mainly large HDL₁. The second peak (fractions 30–37) is mainly HDL. C: Immunoblot analysis of apoAI and apoE in fractions 20–26 from apoE^+/+ and apoE^−/− mice.

FIG. 2

Glucose and lipid tolerance tests in apoE^−/− and apoE^+/+ mice. The apoE^−/− (filled symbols) and apoE^+/+ (open symbols) mice were maintained on control (n = 20; circles) or diabetogenic (n = 8; squares) diets. ipGTT was performed after a 6-h fasting period (A). Alternatively, the mice were fasted overnight (B–D) before determination of plasma TG levels in response to oral fat load (B) or ipGTT in the presence (C) or absence (D) of an oral lipid load. Calculations of area under curve (AUC) of the data presented in B–D are included in the insets to each figure. Data are expressed as means ± SE. *P < 0.05; **P < 0.005.

FIG. 3

Tissue uptake of deoxy-[³H]glucose by apoE^−/− and apoE^+/+ mice. A: 2-deoxy-[³H]glucose uptake by heart, skeletal muscle, gonadal (GAT), SAT, and BAT of diabetogenic diet–fed apoE^+/+ (□) and apoE^−/− (■) mice 15 min after intraperitoneal injection of the radiolabeled tracer. B and C: Tissue uptake of deoxy-[³H]glucose by control diet–fed apoE^+/+ (□) and apoE^−/− (■) mice with or without an oral fat load, respectively. Data are expressed as means ± SE for n = 5 mice per group. *P < 0.05; **P < 0.005.

FIG. 4

Whole-body fat, as percentage of whole-body weight (BW) at week 16 of the study (A) and body weight distribution (B) in apoE^−/− (filled bars) and apoE^+/+ (open bars) mice on control (circles) and diabetogenic diet (squares). Data are expressed as means ± SE for n = 8 mice per group. *P < 0.05; **P < 0.005.

FIG. 5

Energy expenditure in apoE^−/− (red tracings) and apoE^+/+ (black tracings) mice on control diet (CD) (A) and diabetogenic diet (DD) (B) per kilogram lean body mass during dark (gray shaded areas) and light (white shaded areas) phases at week 16 of the study. C and D: Energy intake per gram lean body mass and gram body weight, respectively, of apoE^+/+ and apoE^−/− mice fed control diet or diabetogenic diet. E: UCP-1 protein levels normalized to β-actin levels in BATs of apoE^+/+ and apoE^−/− mice maintained on the diabetogenic diet at week 16 of the study. a and b denote differences between the bars at P < 0.05.