Role of adiponectin in human skeletal muscle bioenergetics

Abstract

Insulin resistance is associated with impaired skeletal muscle oxidation capacity and reduced mitochondrial number and function. Here, we report that adiponectin signaling regulates mitochondrial bioenergetics in skeletal muscle. Individuals with a family history of type 2 diabetes display skeletal muscle insulin resistance and mitochondrial dysfunction; adiponectin levels strongly correlate with mtDNA content. Knockout of the adiponectin gene in mice is associated with insulin resistance and low mitochondrial content and reduced mitochondrial enzyme activity in skeletal muscle. Adiponectin treatment of human myotubes in primary culture induces mitochondrial biogenesis, palmitate oxidation, and citrate synthase activity, and reduces the production of reactive oxygen species. The inhibition of adiponectin receptor expression by siRNA, or of AMPK by a pharmacological agent, blunts adiponectin induction of mitochondrial function. Our findings define a skeletal muscle pathway by which adiponectin increases mitochondrial number and function and exerts antidiabetic effects.

Figures

Figure 1

Comparison of skeletal muscle mitochondrial bioenergetics in insulin-resistant subjects with at least two first-degree relatives with type 2 diabetes (FH+, ■) and insulin-sensitive subjects with no family history of type 2 diabetes (FH-, □) A) Mitochondrial content was assessed by mtDNA copy number. B) Enzymatic quantification of mitochondrial oxidative capacity in the skeletal muscle of Mexican Americans revealed reduced citrate synthase (TCA cycle) and β-hydroxyacyl-CoA dehydrogenase (β-HAD; β oxidation) activity in FH+ patients. C) Insulin sensitivity assessed by euglycemic-hyper-insulinemic clamp (M value, mg glucose · kg-1 · min-1) versus mtDNA copy number levels in skeletal muscle. D) mtDNA copy number versus AdipoR1 mRNA expression in skeletal muscle. Data represented mean ± SEM; n = 8, FH+; n = 10, FH- subjects.

Figure 2

Molecular characterization of 12-week-old wt and adiponectin KO mice A) Gastrocnemius (Gastroc) and Soleus muscle from mice was used in qPCR analysis of mtDNA copy number and mitochondrial enzyme activity. Analysis of mtDNA copy number (A), cytochrome c oxidase (B), and citrate synthase enzyme activity (C) in KO mice revealed reduced mitochondrial bioenergetics. Data represented mean ± SEM; wt, n = 5; KO, n = 5. *p < 0.05 compared to wt mice. Protein content of myosin chain 1 and troponin-1 (slow) in gastrocnemius muscle (D). Immunoblotting was undertaken in six mice (wt, n = 3; KO, n = 3) and data are shown as a representative blot.

Figure 3

Effects of adiponectin treatment on mitochondrial number and oxidative capacity in primary human myotubes A and B) Primary human myotubes were treated for 48 hr with 0.1, 0.25, and 0.5 μg/ml of gAD and mitochondrial content measured using mtDNA copy number (A) and MitoTracker Green (B). C) Myotubes were treated with 0.25μg/ml gAD for 48 hr and cells subsequently incubated with 100 nM MitoTracker Green FM for 30 min. Mitochondrial mass is indicated by green fluorescence. Images were acquired within 5 min using a conventional wide-field microscope fitted with an ×40 Nikon plan-apo objective. D) Effects of gAD treatment on fatty acid oxidation. Data represented as mean ± SEM; n = 5. *p < 0.05 compared to control. E) Dose-dependent induction in AMPK-P and total PGC1α protein by 0.1, 0.25, and 0.5 μg/ml of gAD.

Figure 4

Effects of adiponectin treatment on mitochondrial ROS production in primary human myotubes To analyze ROS levels in myotubes, cells were incubated with DCFDA (A) (indicator of hydrogen peroxide) and (B) Tetramethylrhodamine ethyl ester (TMRE) for estimation of membrane potential. C) mRNA expression of SOD2 in myotubes treated with 0.1, 0.25, and 0.5 μg/ml of gAD for 48 hr. D) Total SOD activity (Cu/Zn-, Mn-, and Fe-SOD) in cells treated with 0.25 μg/ ml of gAD, mock-siRNA, AdipoR1/R2-specific siRNA, and AICAR. Data represented as mean ± SEM; n = 5. *p < 0.05 compared to control. E) The presence of ROS in myotubes treated with 0.25 μg/ml of gAD for 48 hr. Red fluorescence indicates the presence of ROS.

Figure 5

Effects of suppression of AdipoR1 or AdipoR2 expression by siRNA and pharmacological inhibition of AMPK activity using ara-A in primary human myotubes A and B) mitochondrial content (A) and citrate synthase activity (B) in myotubes treated with siRNA duplex and ara-A compound. Data represented mean ± SEM; n = 5. *p < 0.05 compared to control. #p < 0.05 compared to mock-siRNA. C) Phosphorylation and total amount of AMP-activated protein kinase kinase (AMPKK) (a and b), AMP-activated protein kinase (AMPK) (c and d), and total amount of peroxisome proliferative activated receptor γ coactivator 1-α (PGC1α)(e), cytochrome c oxidase (f), cAMP response element binding protein (CREB) (g), Myocyte enhancer factor 2C (MEF2C) (h), and β-actin protein (i) in primary human myotubes transfected with siRNA duplex or ara-A and incubated with 0.25 μg/ml gAD for 48 hr in duplicate cell incubations.

Figure 6

Adiponectin signaling and the regulation of mitochondrial function in skeletal muscle Adiponectin binding to its AdipoRs activates AMP-activated protein kinase (AMPK) and stimulates the phosphorylation of ACC2 and fatty-acid oxidation (Yamauchi et al., 2002, 2003). Adiponectin also activates peroxisome proliferative activated receptor-α (PPARα) stimulating transcription of genes in the fatty-acid oxidation pathway and decreasing triglyceride content in muscle (Yamauchi et al., 2001, 2002) and improving insulin sensitivity. Independent of changes in transcription and mitochondrial mass, the improvements in lipid oxidation occur in less than 6 hr (Yamauchi et al., 2002). Adiponectin activation of AMPK by upstream kinase AMPK kinase (AMPKK) activates transcription of myocyte enhancer factor 2C (MEF2C) and phosphorylation of peroxisome proliferative activated receptor γ coactivator 1-α (PGC1α), which in turn increases mitochondrial content, oxidative capacity, and oxidative-fiber type composition. Central to the development of mitochondrial dysfunction is reactive oxygen species (ROS) production, which reacts with DNA, protein, and lipids leading to oxidative damage. ROS production is inversely related to mitochondrial content. Activation of the adiponectin pathway reduces the generation of ROS by two processes: (1) Increased mitochondrial content, which in turn decreases the workload for each mitochondrion leading to reduced membrane potential (←Δψ) and lower ROS production; and (2) adiponectin increases PGC1α activity which increases the transcription/activity of the antioxidant enzyme SOD2 that decreases super oxide radical (O2 •-) (Maassen et al., 2002).