Increased secretion and expression of myostatin in skeletal muscle from extremely obese women

OBJECTIVE

Obesity is associated with endocrine abnormalities that predict the progression of insulin resistance to type 2 diabetes. Because skeletal muscle has been shown to secrete proteins that could be used as biomarkers, we characterized the secreted protein profile of muscle cells derived from extremely obese (BMI 48.8 +/- 14.8 kg/m(2); homeostasis model assessment [HOMA] 3.6 +/- 1.0) relative to lean healthy subjects (BMI 25.7 +/- 3.2 kg/m(2); HOMA 0.8 +/- 0.2).

RESEARCH DESIGN AND METHODS

We hypothesized that skeletal muscle would secrete proteins that predict the severity of obesity. To test this hypothesis, we used a "bottom-up" experimental design using stable isotope labeling by amino acids in culture (SILAC) and liquid chromatography/mass spectometry/mass spectometry (LC-MS/MS) to both identify and quantify proteins secreted from cultured myotubes derived from extremely obese compared with healthy nonobese women.

RESULTS

Using SILAC, we discovered a 2.9-fold increase in the secretion of myostatin from extremely obese human myotubes. The increased secretion and biological activity of myostatin were validated by immunoblot (3.16 +/- 0.18, P < 0.01) and a myoblast proliferation assay using conditioned growth medium. Myostatin was subsequently shown to increase in skeletal muscle (23%, P < 0.05) and plasma (35%, P < 0.05) and to correlate (r(2) = 0.6, P < 0.05) with the severity of insulin resistance.

CONCLUSIONS

Myostatin is a potent antianabolic regulator of muscle mass that may also play a role in energy metabolism. These findings show that increased expression of myostatin in skeletal muscle with obesity and insulin resistance results in elevated circulating myostatin. This may contribute to systemic metabolic deterioration of skeletal muscle with the progression of insulin resistance to type 2 diabetes.

Skeletal muscle lipid oxidation and obesity: influence of weight loss and exercise

Obesity is associated with a decrement in the ability of skeletal muscle to oxidize lipid. The purpose of this investigation was to determine whether clinical interventions (weight loss, exercise training) could reverse the impairment in fatty acid oxidation (FAO) evident in extremely obese individuals. FAO was assessed by incubating skeletal muscle homogenates with [1-(14)C]palmitate and measuring (14)CO(2) production. Weight loss was studied using both cross-sectional and longitudinal designs. Muscle FAO in extremely obese women who had lost weight (decrease in body mass of approximately 50 kg) was compared with extremely obese and lean individuals (BMI of 22.8 +/- 1.2, 50.7 +/- 3.9, and 36.5 +/- 3.5 kg/m(2) for lean, obese, and obese after weight loss, respectively). There was no difference in muscle FAO between the extremely obese and weight loss groups, and FAO was depressed (-45%; P < or = 0.05) compared with the lean subjects. Muscle FAO also did not change in extremely obese women (n = 8) before and 1 yr after a 55-kg weight loss. In contrast, 10 consecutive days of exercise training increased (P < or = 0.05) FAO in the skeletal muscle of lean (+1.7-fold), obese (+1.8-fold), and previously extremely obese subjects after weight loss (+2.6-fold). mRNA content for PDK4, CPT I, and PGC-1alpha corresponded with FAO in that there were no changes with weight loss and an increase with physical activity. These data indicate that a defect in the ability to oxidize lipid in skeletal muscle is evident with obesity, which is corrected with exercise training but persists after weight loss.

Primary cell cultures in the study of human muscle metabolism

Skeletal muscle plays an important role in whole-body metabolism. Some research has used cell cultures raised from human biopsy tissue to study mechanisms that regulate skeletal muscle metabolism. The purpose of the current paper is to provide evidence indicating the efficacy of primary human skeletal muscle cell cultures as a tool to study substrate regulation and control in human tissue.

GRB14, GPD1, and GDF8 as potential network collaborators in weight loss-induced improvements in insulin action in human skeletal muscle

Obesity is associated with insulin resistance in skeletal muscle; accordingly, weight loss dramatically improves insulin action. We sought to identify molecular remodeling of muscle commensurate with weight loss that could explain improvements in insulin action. Muscle from morbidly obese women was studied before and after gastric bypass surgery. Gastric bypass surgery significantly reduced body mass by approximately 45% and improved insulin action. We then assessed mRNA profiles using a stringent statistical analysis (statistical concordance with three probe set algorithms), with validation in a cross-sectional study of lean (n = 8) vs. morbidly obese (n = 8) muscle. Growth factor receptor-bound protein 14 (GRB14), glycerol-3-phosphate dehydrogenase 1 (GPD1), and growth differentiation factor 8 (GDF8; myostatin) significantly decreased approximately 2.4-, 2.2-, and 2.4-fold, respectively, after weight loss (gastric bypass). Increased expression of these transcripts was associated with increased obesity in the cross-sectional group (lean vs. morbidly obese muscle). Each transcript was validated by real-time quantitative RT-PCR assays in both study groups. Using Ingenuity Pathway Analysis, we show that all three transcripts are involved in the same regulatory network including AKT1, IGF1, TNF, PPARG, and INS. These results suggest that GRB14, GPD1, and GDF8 are weight loss-responsive genes in skeletal muscle and that the observed transcriptional modulation of these would be expected to improve insulin signaling, decrease triglyceride synthesis, and increase muscle mass, respectively, with weight loss. Thus our data provide a possible regulatory pathway involved in the development of insulin resistance in the morbidly obese state, and improvement of insulin resistance with weight loss.

Skeletal muscle fat oxidation is increased in African-American and white women after 10 days of endurance exercise training

OBJECTIVE

Obesity is associated with lower rates of skeletal muscle fatty acid oxidation (FAO), which is linked to insulin resistance. FAO is reduced further in obese African-American (AAW) vs. white women (CW) and may also be lower in lean AAW vs. CW. In lean CW, endurance exercise training (EET) elevates the oxidative capacity of skeletal muscle. Therefore, we determined whether EET would elevate skeletal muscle FAO similarly in AAW and CW with a lower lipid oxidative capacity.

RESEARCH METHODS AND PROCEDURES

In vitro rates of FAO were assessed in rectus abdominus muscle strips using [1- 14C] palmitate (Pal) from lean AAW [BMI = 24.2 +/- 0.9 (standard error) kg/m2] and CW (23.6 +/- 0.8 kg/m2) undergoing voluntary abdominal surgery. Lean AAW (22 +/- 0.9 kg/m(2)) and CW (24 +/- 0.8 kg/m2) and obese AAW (36 +/- 1.2 kg/m2) and CW (40 +/- 1.3 kg/m2) underwent 10 consecutive days of EET on a cycle ergometer (60 min/d, 75% peak oxygen uptake). FAO was measured in vastus lateralis homogenates as captured 14CO2 using [1- 14C] Pal, palmitoyl-CoA (Pal-CoA), and palmityl-carnitine (Pal-Car).

RESULTS

Muscle strip experiments showed suppressed rates of FAO (p = 0.03) in lean AAW vs. CW. EET increased the rates of skeletal muscle Pal oxidation (p = 0.05) in both lean AAW and CW. In obese subjects, Pre-EET Pal (but not Pal-CoA or Pal-Car) oxidation was lower (p = 0.05) in AAW vs. CW. EET increased Pal oxidation 100% in obese AAW (p < 0.05) and 59% (p < 0.05) in obese CW. Similar increases (p < 0.05) in post-EET FAO were observed for Pal-CoA and Pal-Car in both groups.

DISCUSSION

Both lean and obese AAW possess a lower capacity for skeletal muscle FAO, but EET increases FAO similarly in both AAW and CW. These data suggest the use of EET for treatment against obesity and diabetes for both AAW and CW.

Glucose Transporter Expression in Skeletal Muscle of Endurance-Trained Individuals

Two recently identified glucose transporters, GLUT8 and GLUT12, are expressed in human skeletal muscle and may be involved in insulin-mediated sugar transport.

PURPOSE

The purpose of this study was to measure GLUT8 and GLUT12 mRNA levels in endurance-trained versus sedentary individuals in an effort to determine the effect of repeated days of contractile activity on gene expression.

METHODS

GLUT 4, 8, and 12 mRNA were measured in biopsies from the vastus lateralis using quantitative real-time PCR in endurance-trained (N=16, age=22.0+/-0.9 yr, VO(2 max) (L.min(-1))=4.13+/-0.25) and sedentary (N=15, age=21.3+/-0.8 yr, VO(2 max) (L.min(-1))=3.21+/-0.24) subjects.

RESULTS

GLUT12 mRNA was lower (40+/-14%, P<0.05) in the exercise-trained compared with the sedentary subjects. There was no difference between groups in GLUT8 mRNA content. mRNA of the insulin-sensitive glucose transporter (GLUT4) was 78+/-27% (P<0.05) higher in skeletal muscle from endurance-trained compared with sedentary individuals.

CONCLUSION

These findings suggest an isoform-specific effect on the mRNA of the glucose transporters in human skeletal muscle with repeated days of contractile activity.

Elevated stearoyl-CoA desaturase-1 expression in skeletal muscle contributes to abnormal fatty acid partitioning in obese humans

Obesity and type 2 diabetes are strongly associated with abnormal lipid metabolism and accumulation of intramyocellular triacylglycerol, but the underlying cause of these perturbations are yet unknown. Herein, we show that the lipogenic gene, stearoyl-CoA desaturase 1 (SCD1), is robustly up-regulated in skeletal muscle from extremely obese humans. High expression and activity of SCD1, an enzyme that catalyzes the synthesis of monounsaturated fatty acids, corresponded with low rates of fatty acid oxidation, increased triacylglycerol synthesis and increased monounsaturation of muscle lipids. Elevated SCD1 expression and abnormal lipid partitioning were retained in primary skeletal myocytes derived from obese compared to lean donors, implying that these traits might be driven by epigenetic and/or heritable mechanisms. Overexpression of human SCD1 in myotubes from lean subjects was sufficient to mimic the obese phenotype. These results suggest that elevated expression of SCD1 in skeletal muscle contributes to abnormal lipid metabolism and progression of obesity.

Fat as an endocrine organ: influence of exercise

The prevalence of diabetes and obesity continues to increase. It is therefore important to identify the pathophysiology underlying these disorders. An inability of insulin to stimulate glucose uptake, i.e., insulin resistance, appears to be a common link between diabetes and obesity. The identification of various adipocyte-secreted cytokines (adipocytokines) that influence satiety, energy balance, and insulin sensitivity provide a novel target for the treatment of these disorders. Adipocytokines are differentially expressed with obesity and diabetes, making them a strong candidate for linking insulin resistance to these pathological conditions. This review explores the role of adipocytokines in insulin action and examines the effect of exercise training on adipocytokine content.

Glucose Uptake in Muscle Cell Cultures from Endurance-Trained Men

PURPOSE

To examine noninsulin- (basal) and insulin-mediated glucose uptake in human skeletal muscle cells from endurance-trained and sedentary individuals.

METHODS

Muscle biopsies (vastus lateralis) were obtained from competitive, endurance-trained athletes (N=12; VO2peak 64.9+/-2.3 mL.kg-1.min-1) and their sedentary counterparts (N=8; VO2peak 51.8+/-2.2 mL.kg-1.min-1), and isolated satellite cells allowed to proceed to myotubes.

RESULTS

The myotubes exhibited a dose response for glucose uptake with increasing insulin concentrations; maximal glucose uptake was approximately 1.5-fold over basal. In relation to exercise training status, basal glucose uptake was significantly (P<0.05) elevated by approximately 75% in the endurance-trained versus sedentary men (20.1+/-2.1 vs 11.9+/-1.9 pmol.mg protein-1.min-1, respectively). This difference persisted at insulin concentrations of 10 and 1000 etaM, although the relative increase in insulin-mediated glucose uptake (fold increase over basal) did not differ between the sedentary and endurance-trained cells.

CONCLUSIONS

These data suggest that cultured skeletal muscle cells from endurance-trained athletes may differ in respect to basal glucose uptake.

Weight Loss and Exercise: Implications for Muscle Lipid Metabolism and Insulin Action

Implications for Muscle Lipid Metabolism and An accumulation of intramuscular lipid has been reported with obesity and linked with insulin resistance. The purpose of this paper is to discuss: 1) mechanisms that may be responsible for intramuscular lipid accumulation with obesity, and 2) the effects of common interventions (weight loss or exercise) for obesity on skeletal muscle lipid metabolism and intramuscular lipid content. Data suggest that the skeletal muscle of morbidly obese humans is characterized by the preferential partitioning of lipid toward storage rather than oxidation. This phenotype may, in part, contribute to increased lipid deposition in both muscle and adipose tissue, and promote the development of morbid obesity and insulin resistance. Weight loss intervention decreases intramuscular lipid content, which may contribute to improved insulin action. On the other hand, exercise training improves insulin action and increases fatty acid oxidation in the skeletal muscle of obese/morbidly obese individuals. In summary, the accumulation of intramuscular lipid appears to be detrimental in terms of inducing insulin resistance; however, the accumulation of lipid can be reversed with weight loss. The mechanism(s) by which exercise enhances insulin action remains to be determined.

Skeletal muscle lipid metabolism with obesity

The objectives of this study were to 1). examine skeletal muscle fatty acid oxidation in individuals with varying degrees of adiposity and 2). determine the relationship between skeletal muscle fatty acid oxidation and the accumulation of long-chain fatty acyl-CoAs. Muscle was obtained from normal-weight [n = 8; body mass index (BMI) 23.8 +/- 0.58 kg/m(2)], overweight/obese (n = 8; BMI 30.2 +/- 0.81 kg/m(2)), and extremely obese (n = 8; BMI 53.8 +/- 3.5 kg/m(2)) females undergoing abdominal surgery. Skeletal muscle fatty acid oxidation was assessed in intact muscle strips. Long-chain fatty acyl-CoA concentrations were measured in a separate portion of the same muscle tissue in which fatty acid oxidation was determined. Palmitate oxidation was 58 and 83% lower in skeletal muscle from extremely obese (44.9 +/- 5.2 nmol x g(-1) x h(-1)) patients compared with normal-weight (71.0 +/- 5.0 nmol x g(-1) x h(-1)) and overweight/obese (82.2 +/- 8.7 nmol x g(-1) x h(-1)) patients, respectively. Palmitate oxidation was negatively (R = -0.44, P = 0.003) associated with BMI. Long-chain fatty acyl-CoA content was higher in both the overweight/obese and extremely obese patients compared with normal-weight patients, despite significantly lower fatty acid oxidation only in the extremely obese. No associations were observed between long-chain fatty acyl-CoA content and palmitate oxidation. These data suggest that there is a defect in skeletal muscle fatty acid oxidation with extreme obesity but not overweight/obesity and that the accumulation of intramyocellular long-chain fatty acyl-CoAs is not solely a result of reduced fatty acid oxidation.