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.

. Skeletal muscle lipid metabolism with obesity. Am J Physiol Endocrinol Metab 284: E741-E747, 2003. First published December 27, 2002 10.1152/ajpendo.00514.2002The 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 ϭ 8; body mass index (BMI) 23.8 Ϯ 0.58 kg/m 2 ], overweight/obese (n ϭ 8; BMI 30.2 Ϯ 0.81 kg/m 2 ), and extremely obese (n ϭ 8; BMI 53.8 Ϯ 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 Ϯ 5.2 nmol ⅐ g Ϫ1 ⅐ h Ϫ1 ) patients compared with normal-weight (71.0 Ϯ 5.0 nmol ⅐ g Ϫ1 ⅐ h Ϫ1 ) and overweight/obese (82.2 Ϯ 8.7 nmol ⅐ g Ϫ1 ⅐ h Ϫ1 ) patients, respectively. Palmitate oxidation was negatively (R ϭ Ϫ0.44, P ϭ 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.long-chain fatty acyl-coenzyme A; intramyocellular triacylglycerol; fatty acids THE PREVALENCE OF OVERWEIGHT/OBESITY and insulin resistance is continually increasing and is associated with increased risk for the development of non-insulin-dependent diabetes mellitus (NIDDM), hypertension, and cardiovascular disease (5,11,24). The cellular mechanisms responsible for insulin resistance with overweight and obesity are not yet clear. Data have shown that intramyocellular triacylglycerols (IMTG) are increased with obesity and NIDDM (14,19,21). In addition, the accumulation of IMTG is associated with skeletal muscle insulin resistance (3,13,15,19,23,28,29,31,36,39). It is believed, however, that the accumulation of IMTG is not the direct cause of the development of insulin resistance but that IMTG is an inert marker for the presence of other lipid intermediates (diacylglycerol, fatty acyl-CoAs, or ceramide, etc.), which have been directly linked to defects in insulin signaling (8,17,25,32,37).To date, the mechanism(s) responsible for the accretion of IMTG and intermediates of lipid metabolism in intact skeletal muscle are not evident. Two possibilities include an increase in lipid synthesis and/or a reduction in fatty acid oxidation, both of which may res...

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 Ϯ 14.8 kg/m 2 ; homeostasis model assessment [HOMA] 3.6 Ϯ 1.0) relative to lean healthy subjects (BMI 25.7 Ϯ 3.2 kg/m 2 ; HOMA 0.8 Ϯ 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 Ϯ 0.18, P Ͻ 0.01) and a myoblast proliferation assay using conditioned growth medium. Myostatin was subsequently shown to increase in skeletal muscle (23%, P Ͻ 0.05) and plasma (35%, P Ͻ 0.05) and to correlate (r 2 ϭ 0.6, P Ͻ 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. Diabetes 58: [30][31][32][33][34][35][36][37][38] 2009 O besity and type 2 diabetes are associated with endocrine abnormalities that are either precipitated by or precede the onset of peripheral insulin resistance (1). These include changes in circulating proteins and peptides that produce endothelial dysfunction, low-grade inflammation, and a prothrombotic state, all of which contribute to increased cardiovascular risk (2-4). Secreted proteins or the "secretome" constitute an important class of biologically active molecules that are released into circulation where they facilitate cross-talk between organ systems. Because secreted proteins are also involved in the progression of cardiovascular disease and cancer, there is significant interest in mining the secretome for novel biological markers (5). Whereas endocrine organs specialize in the secretion of proteins into circulation, there is mounting evidence that adipose tissue and skeletal muscle constitutively or intermittently secrete bioactive proteins (6,7). In this study, we hypothesized that skeletal muscle of extremely obese and insulinresistant women would secrete proteins into circulation that act as prognostic or diagnostic biomarkers of obesityass...

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 CO2 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 ϳ50 kg) was compared with extremely obese and lean individuals (BMI of 22.8 Ϯ 1.2, 50.7 Ϯ 3.9, and 36.5 Ϯ 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 (Ϫ45%; P Յ 0.05) compared with the lean subjects. Muscle FAO also did not change in extremely obese women (n ϭ 8) before and 1 yr after a 55-kg weight loss. In contrast, 10 consecutive days of exercise training increased (P Յ 0.05) FAO in the skeletal muscle of lean (ϩ1.7-fold), obese (ϩ1.8-fold), and previously extremely obese subjects after weight loss (ϩ2.6-fold). mRNA content for PDK4, CPT I, and PGC-1␣ 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. extreme obesity; fat oxidation; gastric bypass surgery; mitochondria; physical activity OBESITY IS ONE OF THE LEADING CAUSES of preventable death in the United States and is associated (6) with conditions such as insulin resistance, the metabolic syndrome, and type 2 diabetes. A metabolic disturbance evident with obesity is a decrement in the ability of skeletal muscle to oxidize lipid. An impairment in lipid oxidation has been observed when fatty acid oxidation (FAO) is measured in the whole body (16, 36), skeletal muscle homogenates (19), or skeletal muscle strips (13) from obese or extremely obese [body mass index (BMI) Ն40 kg/m 2 ] individuals. This decrease in FAO is also retained in primary skeletal muscle cells raised in culture from extremely obese donors (12). Such data indicate a relatively consistent impairment in the ability of human skeletal muscle to oxidize lipid with obesity, particularly in extremely obese patients. This defect may be a critical component of comorbidities seen with obesity, because a reduction in FAO can partition lipid toward ectopic storage within the muscle cell, which may in turn induce insulin resistance (9,15,22,30,31). In addition, a decrement in the ability to oxidize lipid has been linked (38) with weight gain and a propensity toward obesity. It is thus important to elucidate effective treatments that can reverse and/or compensate for the impairment in lipid oxidation seen in skeletal muscle with obesity.Exercise training and w...

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.