Adiponectin is sufficient, but not required, for exercise-induced increases in the expression of skeletal muscle mitochondrial enzymes

Abstract

Adiponectin (Ad) has been proposed to be a regulator of mitochondrial biogenesis in skeletal muscle, and necessary for exercise-induced increases in mitochondrial content. We first confirmed that Ad could acutely increase the expression of mitochondrial proteins during a 10 h incubation in isolated soleus and extensor digitorum longus (EDL) muscles. Next, we further examined the role of Ad as a regulator of mitochondrial content using Ad knockout (AdKO) mice. The AdKO animals showed no differences in resting VO2, respiratory exchange ratio, or in time to exhaustion during exercise when compared to wild-type (WT) mice. There was a reduction in resting palmitate oxidation in isolated soleus from AdKO animals (-23%, P < 0.05) but not EDL, and 5-aminoimidazole-4-carboxamide (AICAR)-stimulated palmitate oxidation was similar in both genotypes regardless of muscle. There were no differences in protein markers of mitochondrial content (COX4, CORE1, CS, PDHE1α) in red and white gastrocnemius between WT and AdKO animals. A single bout of treadmill running increased the phosphorylation of AMP-activated protein kinase (AMPK) and the mRNA expression of mitochondrial proteins in red and white gastrocnemius in both WT and AdKO animals, with no differences between genotypes. Finally, 8 weeks of chronic exercise training increased the protein content of mitochondrial markers similarly (∼25-35%) in red gastrocnemius from both WT and AdKO mice. Collectively, our results demonstrate that the absence of Ad is not accompanied by reductions in mitochondrial protein content, or a reduction in aerobic exercise capacity. We conclude that Ad is not required for the maintenance of mitochondrial content, or for exercise-induced increases in skeletal muscle mitochondrial proteins.

Figure 1. Representative blots confirming the absence of adiponectin protein in AdKO animals

Tissues included are eWAT (epididymal white adipose tissue), RG (red gastrocnemius), and serum. WT, wild-type; KO, adiponectin knockout mice.

Figure 2. Intraperitoneal glucose (A), insulin (B) and pyruvate (C) tolerance tests

Data are expressed as mean + SE, n = 9–10. a, Significantly different from WT, P < 0.05. WT, wild-type mice; AdKO, adiponectin knockout mice.

Figure 3. Protein content of mitochondrial proteins in red (A) and white (B) gastrocnemius muscle

Data are expressed relative to WT samples and arew presented as mean + SE, n = 7–8. WT, wild-type mice; AdKO, adiponectin knockout mice.

Figure 4. Basal (unstimulated) and AICAR-stimulated palmitate oxidation in isolated soleus (A) and EDL (B)

Data are expressed as mean + SE, n = 9–10. Upper-case letter indicates significance between AICAR and control conditions. a, statistically different from corresponding wild-type condition, P < 0.05. A, statistically different from corresponding basal (unstimulated) condition. WT, wild-type mice; AdKO, adiponectin knockout mice.

Figure 5. Gene expression following a 10 h incubation of soleus (A) and EDL (B) with Ad

Data are expressed as a fold change over control (no Ad) conditions and are presented as mean + SE, n = 8–9. a, statistically different from non-Ad condition, P < 0.05.

Figure 6. Phosphorylation of AMPKα and p38 MAP kinase immediately following exercise in red (A and C) and white (B and D) gastrocnemius

Data are expressed relative to WT control (no exercise) samples within a muscle type and are presented as mean + SE, n = 7–8. A, statistically different from corresponding sedentary condition, P < 0.05. WT, wild-type mice; AdKO, adiponectin knockout mice. pAMPK as shown is not expressed relative to total AMPK. Normalization of pAMPK/p38 to total AMPK/p38 produced equivocal results (data not shown).

Figure 7. Gene expression immediately following exercise in red (A) and white (B) gastrocnemius

Data are expressed as a fold change over control (no exercise) samples and are presented as mean + SE, n = 7–8. A, statistically different from corresponding sedentary condition, P < 0.05. WT, wild-type mice; AdKO, adiponectin knockout mice.

Figure 8. Protein content of mitochondrial markers following 8 weeks of chronic exercise training in red (A) and white (B) gastrocnemius

Data are expressed relative to wild-type sedentary animals and are presented as mean + SE, n = 8–10. A, significantly different from corresponding sedentary condition, P < 0.05. a, Significantly different from wild-type. WT, wild-type mice; AdKO, adiponectin knockout mice.