Postnatal PPARdelta activation and myostatin inhibition exert distinct yet complimentary effects on the metabolic profile of obese insulin-resistant mice

Abstract

Background: Interventions for T2DM have in part aimed to mimic exercise. Here, we have compared the independent and combined effects of a PPARdelta agonist and endurance training mimetic (GW501516) and a myostatin antibody and resistance training mimetic (PF-879) on metabolic and performance outcomes in obese insulin resistant mice.

Methodology/principal findings: Male ob/ob mice were treated for 6 weeks with vehicle, GW501516, PF-879, or GW501516 in combination with PF-879. The effects of the interventions on body composition, glucose homeostasis, glucose tolerance, energy expenditure, exercise capacity and metabolic gene expression were compared at the end of study. GW501516 attenuated body weight and fat mass accumulation and increased the expression of genes of oxidative metabolism. In contrast, PF-879 increased body weight by driving muscle growth and altered the expression of genes involved in insulin signaling and glucose metabolism. Despite their differences, both interventions alone improved glucose homeostasis. Moreover, GW501516 more effectively improved serum lipids, and PF-879 uniquely increased energy expenditure, exercise capacity and adiponectin levels. When combined the robust effects of GW501516 and/or PF-879 on body weight, adiposity, muscle mass, glycemia, serum lipids, energy expenditure and exercise capacity were highly conserved.

Conclusions/significance: The data, for the first time, demonstrate postnatal inhibition of myostatin not only promotes gains in muscle mass similar to resistance training,but improves metabolic homeostasis. In several instances, these effects were either distinct from or complimentary to those of GW501516. The data further suggest that strategies to increase muscle mass, and not necessarily oxidative capacity, may effectively counter insulin resistance and T2DM.

Figure 1. PPARδ activation and myostatin inhibition exert distinct effects on body composition.

Male ob/ob mice were treated for 6 weeks with either vehicle (veh, open bars), PPARδ agonist GW501516 (grey bars), myostatin antibody PF-879 (hatched bars), or GW501516 in combination with PF-879 (black bars) (n = 10 mice/group). Changes in body weight (A), fat mass (B) and lean mass (C) from study start to termination are summarized. Abdominal visceral (D) and subcutaneous fat (E) were quantified for a single transverse slice acquired at the 3^rd lumbar vertebrae by computed tomography. Muscle weights were determined at the end of study (F). Data are represented as mean +/− SEM. *, # and § represent p<0.05, 0.01 and 0.001, respectively. See also Figure S1.

Figure 2. PPARδ activation increases mitochondrial activity and myostatin inhibition increases muscle fiber size in ob/ob mice.

Male mice were treated for 6 weeks with either vehicle (veh, open bars), PPARδ agonist GW501516 (grey bars), myostatin antibody PF-879 (hatched bars), or GW501516 in combination with PF-879 (black bars). Representative cross-sectional images, the mean fiber area and size distribution of muscle fibers in the quadriceps muscle are presented in A, B and C, respectively (n = 5/group). Fold changes in quadriceps MHC mRNA expression levels relative to vehicle-treated mice are summarized (n = 5/group)(D). Citrate synthase activity was measured in soleus and tibialis anterior muscles (E). Data are represented as mean +/− SEM *, # and § indicate p<0.05, 0.01 and 0.001, respectively.

Figure 3. PPARδ activation and myostatin inhibition improve glucose homeostasis and tolerance in ob/ob mice.

Following 6 weeks administration of vehicle (Veh), GW501516, PF-879, or GW501516 concurrent with PF-879, non-fasted (A) and fasted (B) glucose concentrations were measured. Blood glucose concentrations before and 2 hours after a bolus of glucose were measured following an overnight fast (C) and area under the glucose curve was calculated (D). Changes in glucose concentrations in response to a bolus of insulin were measured following a 4 hr fast (E). Data are represented as mean +/− SEM (n = 10/group). *, # and § denote p<0.05, 0.01 and 0.001, respectively.

Figure 4. Myostatin inhibition increases oxygen consumption and energy expenditure in ob/ob mice.

Indirect calorimetry was performed following 6 weeks administration of vehicle (veh), GW501516, PF-879, or GW501516 in combination with PF-879 to quantify oxygen consumption (VO₂) (A) and carbon dioxide production to calculate the respiratory exchange ratio (B) and energy expenditure (C) in ob/ob mice. Data are represented as mean +/− SEM (n = 8/group). *, # and § represent p<0.05, 0.01 and 0.001, respectively.

Figure 5. PPARδ activation and myostatin inhibition improve the lipid profile and alter adiponectin in ob/ob mice.

Serum lipids were analyzed following 6 weeks treatment with vehicle (veh), GW501516, PF-879, or GW501516 in combination with PF-879. Free fatty acids (A), high density lipoproteins (HDL) (B) and triglycerides (C) are illustrated. Circulating adiponectin levels (D) and glucagon concentrations (E) are also summarized. Data are represented as mean +/− SEM (n = 10/group). *, # and § indicate p<0.05, 0.01 and 0.001, respectively. See also Figure S2.

Figure 6. Myostatin inhibition increases habitual physical activity and improves performance in ob/ob mice.

After 6 weeks of treatment with vehicle (veh), GW501516, PF-879, or GW501516 in combination with PF-879, habitual physical activity was monitored and summed for a 48 hour period (A) (n = 8/group). At study end, the time (B) and distance (C) mice were able to comply with a performance protocol on a motorized treadmill was also measured (n = 10/group). Data are represented as mean +/− SEM. * represents p<0.05.

Figure 7. The effects of PPARδ activation and myostatin inhibition on metabolic gene expression in ob/ob mice.

Following 6 weeks of treatment with either vehicle (Veh), GW501516, PF-879 or GW501516 plus PF-879, mRNA was isolated from skeletal muscle, liver and adipose tissue (A, B and C, respectively) of ob/ob mice and quantified by a low-density array card. Fold changes compared to vehicle were calculated. Data are represented as mean +/− SEM (n = 8/group). *, # and § represent differences in expression of >20% relative to control and p<0.05, 0.01 and 0.001, respectively. See also Table S1.