Fast/Glycolytic muscle fiber growth reduces fat mass and improves metabolic parameters in obese mice

Abstract

In contrast to the well-established role of oxidative muscle fibers in regulating whole-body metabolism, little is known about the function of fast/glycolytic muscle fibers in these processes. Here, we generated a skeletal muscle-specific, conditional transgenic mouse expressing a constitutively active form of Akt1. Transgene activation led to muscle hypertrophy due to the growth of type IIb muscle fibers, which was accompanied by an increase in strength. Akt1 transgene induction in diet-induced obese mice led to reductions in body weight and fat mass, resolution of hepatic steatosis, and improved metabolic parameters. Akt1-mediated skeletal muscle growth opposed the effects of a high-fat/high-sucrose diet on transcript expression patterns in the liver and increased hepatic fatty acid oxidation and ketone body production. Our findings indicate that an increase in fast/glycolytic muscle mass can result in the regression of obesity and metabolic improvement through its ability to alter fatty acid oxidation in remote tissues.

Figure 1. Conditional Activation of Akt1 in Skeletal Muscle Induces Reversible Type IIb Muscle Hypertrophy

(A) Top: schematic of doxycycline (DOX) treatment time course. Bottom: representative gross appearance of control and double-transgenic (DTG) mice after 2 weeks of transgene induction and after 2 weeks of induction followed by 2 weeks of repression. This experiment was initiated in DTG mice at 8 weeks of age. (B) Top: time course of transgene expression following the addition of DOX (induction) and the subsequent removal of DOX (repression). Representative blots of gastrocnemius muscle are shown. Bottom: time course of gastrocnemius muscle weight changes resulting from transgene induction followed by repression (n = 4–12 in each group). *p < 0.05 versus day 0; #p < 0.05 versus day 14. (C) Gastrocnemius, soleus, and extensor digitorum longus (EDL) muscle weight in control (Cont) and DTG mice at 2 weeks after Akt1 activation (n = 6 in each group). (D) Histological analysis of myofiber growth. Top: hematoxylin and eosin (H&E) staining of gastrocnemius muscle sections. Scale bar = 100 μm. Bottom left: distribution of mean cross-sectional areas (CSAs) of muscle fibers. Bottom right: mean CSA of muscle fibers (n = 6 in each group). (E) Top: western blot analysis of transgene expression after 2 and 6 weeks of DOX treatment. Bottom: gastrocnemius muscle weight at 2, 3, 4, and 6 weeks after DOX treatment (n = 6–12 in each group). (F) Left: representative images of gastrocnemius muscle sections stained with MHC type I, IIa, or IIb antibody (green) and laminin (red) from control and DTG mice 2 weeks after Akt1 activation. Right: distribution of mean CSAs of type IIb muscle fibers from control and DTG mice after 2 weeks of DOX treatment. (G) Forelimb grip strength of control mice and DTG mice after 2 weeks of DOX treatment (n = 6 in each group). (H) Results of forced treadmill exercise test on MCK-rtTA single-TG control mice and DTG mice after 2 weeks of DOX treatment (n = 4 in each group). Results are presented as mean ± SEM. *p < 0.05 versus control; N.S., not significant.

Figure 2. Akt1-Mediated Muscle Growth Reverses Diet-Induced Obesity

(A) Body weight of control and DTG mice fed a normal diet or a high-fat/high-sucrose (HF/HS) diet (n = 12 in each group). *p < 0.05 versus HF/HS-fed control mice. (B) Representative gross appearance and ventral view of HF/HS-fed control and DTG mice at the 4 week time point of DOX treatment. (C) Representative MRI images (1 mm axial slice) of each group of mice at the 4 week time point of DOX treatment, observed at the level of the right renal pelvis. Bright regions correspond to fat; dark regions represent lean body mass. (D) Gastrocnemius muscle and inguinal and subcutaneous fat-pad weight in control and DTG mice after 6 weeks of DOX treatment (n = 12 in each group). *p < 0.05; #p < 0.01. (E) Histological analysis of H&E-stained gastrocnemius muscle (top) and inguinal white adipose tissue (WAT, bottom) sections. Scale bars = 200 μm. MCK-rtTA single-transgenic mice were used as a control. Results are presented as mean ± SEM.

Figure 3. Akt1-Mediated Muscle Growth Normalizes Metabolic Parameters in Diet-Induced Obese Mice

(A) Blood glucose and serum insulin, glucagon, and leptin levels in normal or HF/HS-fed control and DTG mice after 6 weeks of DOX treatment (n = 7–12 in each group). (B) Glucose (left) and insulin (right) tolerance tests in the different experimental groups of mice (n = 12 in each group). *p < 0.05 versus HF/HS-fed control mice. (C) Glucose uptake into skeletal muscle in vivo (n = 6 in each group). (D) Serum lactate levels in the different experimental groups of mice (n = 7 in each group). MCK-rtTA single-transgenic mice were used as a control. Results are presented as mean ± SEM. *p < 0.05; #p < 0.01.

Figure 4. Akt1-Mediated Muscle Growth Counteracts the Metabolic Consequences of Excess Adiposity

(A) Body weight and inguinal and subcutaneous fat-pad weight in control and DTG mice fed a normal diet or a HF/HS diet for the indicated periods. DTG mice fed a HF/HS diet for 12 weeks and treated with DOX for 4 weeks (starting at the 8 week time point) had body weight and fat mass equivalent to control mice fed a HF/HS diet for 4 weeks. (B) Glucose tolerance test results in the different experimental groups of mice. (C) Serum leptin levels in the different experimental groups of mice. (D) Representative sections of liver stained with oil red O. Scale bars = 100 μm. MCK-rtTA single-transgenic mice were used as a control. Results are presented as mean ± SEM (n = 6–9 in each group). *p < 0.05; N.S., not significant.

Figure 5. Akt1-Mediated Muscle Growth Has No Effect on Food Intake, Decreases Ambulatory Activity, and Increases Metabolic Rate

(A) Food consumption of control and DTG mice fed a HF/HS diet was monitored before and over 6 weeks of DOX treatment (n = 12 in each group). (B) Ambulatory activity levels of control and DTG mice fed a HF/HS diet were determined by infrared beam interruption (n = 6 in each group). (C) Top: trace of whole-body O₂ consumption (VO₂) determined using a metabolic measuring system for 24 hr in the absence of food in control and DTG mice 4 weeks after Akt1 activation (n = 6 in each group). Bottom left: average VO₂ in control and DTG mice fed a HF/HS diet. Bottom right: respiratory exchange ratio (RER) in control and DTG mice. For all experiments, mice were fed a HF/HS diet for 8 weeks prior to DOX treatment. MCK-rtTA single-transgenic mice were used as a control. Results are presented as mean ± SEM. *p < 0.05.

Figure 6. Akt1-Mediated Changes in Muscle Gene Expression

(A) Representative gene set enrichment analysis (GSEA) plots highlighting expression differences in gastrocnemius muscle after 3 weeks of Akt1 transgene activation. Control and DTG mice were fed a HF/HS diet for 8 weeks prior to DOX treatment. Plots are shown for starch and sucrose metabolism, nuclear receptors, fatty acid metabolism, and oxidative phosphorylation gene sets (false discovery rate < 25%). Heat maps show the expression value differences for the top subset of genes that are represented in each gene set. Red corresponds to a high expression value; blue indicates a lower expression value (n = 4 in each group). (B) Relative mRNA expression levels of PGC-1α, PPARα, PPARδ, hexokinase 2 (HK2), phosphofructokinase (Pfk), and lactate dehydrogenase A (LDHA) as measured by qRT-PCR. (C) Relative protein expression levels of PGC-1α, PPARα, and PPARδ as determined by western blot analysis. MCK-rtTA single-transgenic mice were used as a control. Results are presented as mean ± SEM. *p < 0.05.

Figure 7. Akt1-Mediated Muscle Hypertrophy Promotes Fatty Acid Metabolism in Liver

(A) Comparison of transcripts significantly (p < 0.05) upregulated (red) or downregulated (blue) in liver under different experimental conditions (n = 4 in each group). Left column: comparison between control mice fed a HF/HS diet (CH) and control mice fed a normal diet (CN). Comparing these experimental conditions, 635 transcripts were upregulated and 646 transcripts were downregulated by the HF/HS diet. Right column: comparison between DTG mice fed a HF/HS diet (DH) and control mice fed a HF/HS diet (CH). 861 (67%) of the 1281 transcript expression changes in liver caused by the HF/HS diet were reversed by muscle growth. (B) Relative mRNA expression levels of PEPCK, G6Pase, PGC-1α, HNF4α, CPT1, and SCD1 as measured by qRT-PCR in livers of control mice fed a normal diet (CN), control mice fed a HF/HS diet (CH), and DTG mice fed a HF/HS diet (DH) at the 3 week time point after DOX treatment (n = 4). (C) Akt1-mediated muscle growth (3 weeks) promotes β-oxidation of palmitic acid in liver tissue (n = 8 in each group). (D) Serum (left) and urine (right) ketone body levels in the different experimental groups of mice (n = 9–12 in each group). MCK-rtTA single-transgenic mice were used as a control. Results are presented as mean ± SEM. *p < 0.05; #p < 0.01.