Fibroblast growth factor 21 controls glycemia via regulation of hepatic glucose flux and insulin sensitivity

Abstract

Fibroblast growth factor 21 (FGF21) is a novel metabolic regulator shown to improve glycemic control. However, the molecular and functional mechanisms underlying FGF21-mediated improvements in glycemic control are not completely understood. We examined FGF21 effects on insulin sensitivity and glucose fluxes upon chronic (daily injection for 8 d) and acute (6 h infusion) administration in ob/+ and ob/ob mice. Results show that chronic FGF21 ameliorated fasting hyperglycemia in ob/ob mice via increased glucose disposal and improved hepatic insulin sensitivity. Acute FGF21 suppressed hepatic glucose production, increased liver glycogen, lowered glucagon, and improved glucose clearance in ob/+ mice. These effects were blunted in ob/ob mice. Neither chronic nor acute FGF21 altered skeletal muscle or adipose tissue glucose uptake in either genotype. In conclusion, FGF21 has potent glycemic effects caused by hepatic changes in glucose flux and improved insulin sensitivity. Thus, these studies define mechanisms underlying anti-hyperglycemic actions of FGF21 and support its therapeutic potential.

Figure 1

Blood glucose (A), insulin (B), and NEFA (C) from hyperinsulinemic (10 mU · kg⁻¹ · min⁻¹)-euglycemic (∼8.0 mmol · liter⁻¹) clamps in conscious, 5-h fasted ob/+ and ob/ob mice (n = 11–13 in each group). Ob/ob mice were sc injected with vehicle or FGF21 (1 mg · kg⁻¹ · d⁻¹) for 8 d, and jugular vein catheters were implanted 5 d before study. Vehicle-treated ob/+ were studied as an insulin-sensitive control. Blood samples were taken at indicated time points from the cut tail to measure blood glucose (D). The GIR (E) was adjusted as needed to maintain euglycemia. Mice were euthanized using a bolus of sodium pentobarbital at t = 120 min, and tissues were dissected and quickly frozen. All mice were male and studied at 9–10 wk of age. Data are presented as mean ± se, and the steady-state period was defined as t = 80–120 min during the clamp. Statistical significance was established at P < 0.05. *, Comparison to ob/+ mice; †, comparison between ob/ob mice treated with vehicle or FGF21; #, comparison within a group between basal and clamp conditions.

Figure 2

EndoR_a (A), R_d (B), and glucose clearance (C) during a 120-min hyperinsulinemic (10 mU · kg⁻¹ · min⁻¹)-euglycemic (∼8.0 mmol · liter⁻¹) clamp in conscious, 5-h fasted ob/+ and ob/ob mice (n = 11–13 in each group). Ob/ob mice were sc injected with vehicle or FGF21 (1 mg · kg⁻¹ · d⁻¹) for 8 d, and jugular vein catheters were implanted 5 d before study. Vehicle-treated ob/+ were studied as an insulin-sensitive control. [3-³H]Glucose (0.1 μCi · min⁻¹) was infused starting at t = −90 min, and plasma samples were taken from the cut tail at time points indicated in each panel. Mice were euthanized at t = 120 min using sodium pentobarbital. D–F, R_g in the gastrocnemius (D), heart (E), and white adipose tissue (WAT; F) was assessed using 2-DG at t = 78 min; G–I, R_g values normalized to clamp insulin, respectively. Mice were male, studied at 9–10 wk of age, and fasted 5 h before study. *, Differences between basal and clamp; †, differences compared with ob/+ mice; #, differences compared with vehicle-treated ob/ob mice.

Figure 3

Hepatic Akt phosphorylation at serine 473 normalized to total Akt protein content in liver (A), heart (B), gastrocnemius (C), and adipose (D) after 120 min hyperinsulinemic (10 mU · kg⁻¹ · min⁻¹)-euglycemic (∼8.0 mmol · liter⁻¹) clamps in conscious, 5-h fasted ob/+ and ob/ob mice (n = 11–13 in each group). Mice were sc injected with either vehicle or FGF21 (1 mg · kg⁻¹ · d⁻¹) for 8 d before study. Liver glycogen (E), percentage of R_d in liver glycogen (F), and triglyceride (TG; G) were measured after the clamp using enzymatic techniques and incorporation of [3-³H]glucose into glycogen. Mice were euthanized at t = 120 min using sodium pentobarbital. Mice were male, studied at 9–10 wk of age, and fasted 5 h before study. *, Differences compared with ob/+ mice; #, differences compared with vehicle-treated ob/ob mice. WAT, White adipose tissue.

Figure 4

Whole blood glucose (A), GIR (B), glucagon (C), and insulin (D) in ob/+ and ob/ob mice infused with FGF21 (1 ng · kg⁻¹ · min⁻¹) or vehicle for a total of 6 h (n = 7–9 in each group). Inset in E shows rescaled insulin levels for ob/+ mice. Samples were taken at time points indicated in each panel, and t = 0 is the mean of t = −15 and −5 min samples. Figures show data for the first 5 h before the bolus infusion of 2-DG at t = 318 min to measure tissue-specific glucose uptake. Glycogen (F) and triglyceride (G) were measured after euthanasia via a bolus of pentobarbital. Jugular vein catheters used for infusion purposes were surgically implanted in all mice 5 d before study, and blood samples were taken from the cut tail. All mice were male, studied at 9–10 wk of age, and fasted for 5 h before study. Data are presented as mean ± se, and statistical significance is established at P < 0.05. *, Differences between vehicle- and FGF21-infused ob/+ mice; #, differences between vehicle- and FGF21-infused ob/ob mice; †, differences compared with basal values.

Figure 5

EndoR_a (A), R_d (B), and glucose clearance (C) in ob/+ and ob/ob mice infused with FGF21 (1 ng · kg⁻¹ · min⁻¹) or vehicle for 6 h (n = 7–9 in each group). Time points are data from t = 0–300 min before the bolus infusion of 2-DG at t = 318 min. [3-³H]Glucose (0.1 μCi · min⁻¹) was infused starting at t = −90 min, and plasma samples were taken from the cut tail at time points indicated in each panel where t = 0 min is the mean of t = −15 and −5 min samples. R_g in the gastrocnemius (D), heart (E), and white adipose tissue (WAT; F) was assessed using 2-DG. Hepatic Akt phosphorylation at serine 473 (p-Akt for 473) normalized to total Akt protein content in liver and heart is shown in G and H, respectively. Mice were euthanized at t = 360 min using sodium pentobarbital. Mice were male, studied at 9–10 wk of age, and fasted 5 h before study. *, Differences between vehicle (VEH)- and FGF21-infused ob/+ mice; †, differences compared with basal values.