Obese Skeletal Muscle Research Articles

PKCβ knockout blocks insulin stimulated IRS1ser307 phosphorylation

Insulin resistance is marked by a reduction of insulin stimulated glucose uptake and insulin signal transduction. In incubated muscle from obese patients we observed decreases in insulin activation of insulin receptor substrate 1 (IRS1) tyrosine phosphorylation. We also found increased protein kinase C beta (PKCβ) activity and IRS1ser312 (mouse ser307) phosphorylation, both of which are suspected as mechanisms of decreased insulin signal transduction in insulin resistance. Recently, we showed that pre-incubating obese skeletal muscle with AICAR reduced insulin stimulation of both PKCβ and IRS1ser312 phosphorylation. The purpose of this study was to further investigate this relationship by examining the effect of insulin on IRS1 serine phosphorylation and insulin signaling activity in PKCβ knockout mice (PKCβKO). Gastrocnemius muscles from PKCβKO and wild type (WT) littermates treated with and without insulin were analyzed. Insulin increased phosphorylation of IRS1ser307 by 58% over basal in WT. This increase was abolished in the PKCβKO. Inversely, insulin stimulated IRS1 tyrosine phosphorylation was significantly higher in PKCβKO compared to WT (305% over basal vs. 111% respectively). Insulin stimulated IRS1 association of PI3K was also increased in PKCβKO vs. WT (172% vs. 86% respectively). These data indicate that insulin stimulated IRS1ser307 phosphorylation is indeed mediated by PKCβ. The project was supported by a grant from the NIH (R01 DK046121).

Skeletal Muscle Fatty Acid Oxidation and Obesity

Obesity, which is associated with insulin resistance and diabetes, is increasing at epidemic rates. Skeletal muscle from morbidly obese individuals displays a distinct phenotype that is characterized by decreased fatty acid oxidation (FAO) and increased lipid storage. It is thus important to determine the effect of obesity interventions (i.e. weight loss, exercise) on these defects in obese skeletal muscle. PURPOSE To examine the effect of surgically induced weight loss (via gastric bypass) and aerobic exercise on skeletal muscle lipid metabolism. METHODS Skeletal muscle FAO was assessed using whole muscle homogenates obtained from the vastus lateralis. Rate of 14CO2 production and label incorporation into acid soluble metabolites was determined from incubating homogenates with 1−14C palmitate. Female participants were examined prior to and 1 yr post gastric bypass (post-GOP) surgery (n=2 longitudinally, n=6 cross-sectional), a period when the participants are weight stable. The effect of 10 consecutive days of exercise at 75% VO2peak for 60min/day was examined in formerly morbidly obese, weight matched controls, and lean controls. RESULTS Weight loss (∼100 lbs) had no effect on fatty acid oxidation in skeletal muscle in subjects studied before and after weight loss in both the longitudinal and cross-sectional comparisons. Specifically, the cross sectional comparison of post-GOP subjects to never-morbidly obese weight matched controls demonstrated reduced (∼46%) FAO in post-GOP subjects. Exercise training significantly increased FAO in lean (52.51±10.3 to 73.94±10.7 ηM/g protein/min, 40%), obese (41.96±5.7 to 70.92±6.36) and formerly obese individuals (27.98±5.4 to 79.0±20, 163%). CONCLUSIONS Weight loss has no effect on skeletal muscle FAO which supports the notion that reduced FAO in skeletal muscle contributes to obesity and insulin resistance. Exercise corrects the defect in skeletal muscle FAO observed with obesity.

Muscle Lipid Oxidation Increases with Endurance Training in Obese Individuals

2259 The purpose of this investigation was to examine the effects of short-term endurance exercise on skeletal muscle fatty acid oxidation in morbidly obese individuals. Recent work from our group data has demonstrated a decreased ability for morbidly obese skeletal muscle to oxidize fat. PURPOSE: To explored whether exercise training would be an effective intervention for ameliorating this defect. METHODS: Nine obese (BMI = 40.15 ± 0.91, %Fat = 43.06 ± 1.95, age = 32.56 ± 2.23) and nine lean (BMI = 22.51 ± 0.78, % Fat = 21.97 ± 1.33, age = 27.67 ± 2.02) sedentary females women participated. Fatty acid oxidation was measured in a whole muscle homogenate preparationto measure oxidation at different steps of the metabolic pathway. from muscle biopsies of the vastus lateralis using [1-14C] palmitate. Exercise training consisted of ten After an initial biopsy of the vastus lateralis, participants completed 10 consecutive days of stationary cycling (1 hr/day) at 70–75% of VO2peak. RESULTS: Participants completed a second biopsy 14-16 hours after the last exercise session. Body weight was maintained throughout the study. Oxidation increased a similar extent in both morbidly obese (41.96 ± 5.7 to 70.92 ± 6.36, ηM×g protein-1×min-1) and lean (47.180 ± 9.44 to 77.50 ± 10.47, ηM×g protein-1×min-1) skeletal muscle samples (∼66 and ∼64%, respectively). These changes in lipid oxidation occurred without body mass loss. Also, the obese individuals exercised at a lower absolute workload (VO2) due to their lower initial VO2max. CONCLUSION: These findings suggest that morbidly obese individuals It appears morbidly obese individuals are as responsive to exercise as lean individuals in terms of enhancing muscle lipid oxidation.

Glucose transport and phosphorylation in skeletal muscle in obesity: insight from a muscle-specific positron emission tomography model.

A controversial area in understanding the contribution of obesity to skeletal muscle insulin resistance is the distribution of control of glucose metabolism across proximal steps of glucose delivery, trans-membrane transport, and intracellular trapping via phosphorylation. Dynamic positron emission tomography (PET) imaging of skeletal muscle [(18)F]2-deoxy-2-D-glucose ((18)F-FDG) uptake provides an in vivo method for assessment of these steps in humans. In the current study we have examined the application of a four-compartment skeletal muscle-specific model for assessment of (18)F-FDG metabolism that takes interstitial (18)F-FDG kinetics into account and compared this to the classic three-compartment model in lean and obese volunteers. We assessed the effects of insulin infusions at three rates (0, 40, and 120 mU/m(2).min). In comparison with the classic model, the skeletal muscle-specific model reveals more clearly definable effects of insulin on transmembrane glucose transport and an impairment of this response in obesity. Compared with the classic model for assessment of (18)F-FDG metabolism, both the skeletal muscle-specific and the classic model indicate that, with respect to distribution of control, glucose phosphorylation has an important effect at low to moderate levels of insulin stimulation in both lean and obese subjects.

Skeletal Muscle Triglycerides

The composition and biochemistry of skeletal muscle are altered in obesity and type 2 diabetes mellitus (DM) as compared to nonobese individuals. In health, skeletal muscle has a clear capacity to utilize both carbohydrate and lipid fuels and to transition between these in response to hormonal, chiefly insulin, and substrate signals. This metabolic flexibility is key for the major role that skeletal muscle can have in overall fuel balance. In obesity and type 2 DM, there is a loss of this plasticity and, instead, there is metabolic inflexibility. Rates of lipid oxidation do not suppress effectively in response to insulin, but neither do rates of lipid oxidation effectively increase during the transition to fasting conditions. An important morphological characteristic of skeletal muscle in obesity and type 2 DM is an increased content of triglyceride. The accretion of fat within muscle tissues appears to strongly correlate with insulin resistance and may not be simply a passive process, paralleling fat storage in other tissues. Instead, and of particular metabolic interest, a concept is emerging that biochemical characteristics of skeletal muscle in obese individuals dispose to fat accumulation in muscle. An effort to modify skeletal muscle in individuals with obesity and type 2 DM so that the capacity for fat oxidation and metabolic flexibility is improved should be among the goals of treatment for these disorders.

Muscle triglyceride and insulin resistance.

Skeletal muscle contains the majority of the body's glycogen stores and a similar amount of readily accessible energy as intramyocellular triglyceride (imTG). While a number of factors have been considered to contribute to the pathogenesis of insulin resistance (IR) in obesity and type 2 diabetes mellitus (DM), this review will focus on the potential role of skeletal muscle triglyceride content. In obesity and type 2 DM, there is an increased content of lipid within and around muscle fibers. Changes in muscle in fuel partitioning of lipid, between oxidation and storage of fat calories, almost certainly contribute to accumulation of imTG and to the pathogenesis of both obesity and type 2 DM. In metabolic health, skeletal muscle physiology is characterized by the capacity to utilize either lipid or carbohydrate fuels, and to effectively transition between these fuels. We will review recent findings that indicate that in type 2 DM and obesity, skeletal muscle manifests inflexibility in the transition between lipid and carbohydrate fuels. This inflexibility in fuel selection by skeletal muscle appears to be related to the accumulation of imTG and is an important aspect of IR of skeletal muscle in obesity and type 2 DM.

Functional and histological characteristics of skeletal muscle and the effects of leptin in the genetically obese (ob/ob) mouse.

Skeletal muscle mass in genetically obese (ob/ob) mice displays a reduced mass compared with their normal lean counterpart mice. However, the functional capacity of the available skeletal muscle mass in these animals has not yet been determined. To investigate the properties of skeletal muscle in ob/ob mice and determine the effects of leptin administration on skeletal muscle in these mice. Following 4 weeks of i.p. leptin administration (or control treatment) anaesthetized ob/ob and lean mice had their extensor digitorum longus and soleus muscles removed, and standard measures of isometric contractile properties and fatigability were performed. Histochemistry was used to determine fibre type proportions and individual fibre areas of all muscles. Leptin had no effect on the morphology or function of ob/ob skeletal muscle despite reducing body mass in ob/ob mice. Force production was unaltered in obese mice. However, a significant prolongation of contraction and relaxation times were evident. Obese skeletal muscle was also more fatigue resistant. Fibre proportions displayed a more slow type profile in ob/ob skeletal muscle, and in conjunction with previous work a reduced ability to hypertrophy. Skeletal muscle from obese mice is morphologically and functionally different from lean mouse skeletal muscle. Obese muscle is very similar to skeletal muscle from aged mice, and the specific contractile properties examined appear to be determined by the fibre make-up of these muscles.

Markers of capacity to utilize fatty acids in human skeletal muscle: relation to insulin resistance and obesity and effects of weight loss.

A number of biochemical defects have been identified in glucose metabolism within skeletal muscle in obesity, and positive effects of weight loss on insulin resistance are also well established. Less is known about the capacity of skeletal muscle for the metabolism of fatty acids in obesity-related insulin resistance and of the effects of weight loss, though it is evident that muscle contains increased triglyceride. The current study was therefore undertaken to profile markers of human skeletal muscle for fatty acid metabolism in relation to obesity, in relation to the phenotype of insulin-resistant glucose metabolism, and to examine the effects of weight loss. Fifty-five men and women, lean and obese, with normal glucose tolerance underwent percutaneous biopsy of vastus lateralis skeletal muscle for determination of HADH, CPT, heparin-releasable (Hr) and tissue-extractable (Ext) LPL, CS, COX, PFK, and GAPDH enzyme activities, and content of cytosolic and plasma membrane FABP. Insulin sensitivity was measured using the euglycemic clamp method. DEXA was used to measure FM and FFM. In skeletal muscle of obese individuals, CPT, CS, and COX activities were lower while, conversely, they had a higher or similar content of FABP(C) and FABP(PM) than in lean individuals. Hr and Ext LPL activities were similar in both groups. In multivariate and simple regression analyses, there were significant correlations between insulin resistance and several markers of FA metabolism, notably, CPT and FABP(PM). These data suggest that in obesity-related insulin resistance, the metabolic capacity of skeletal muscle appears to be organized toward fat esterification rather than oxidation and that dietary-induced weight loss does not correct this disposition.

Overexpression of muscle uncoupling protein 2 content in human obesity associates with reduced skeletal muscle lipid utilization.

Uncoupling proteins (UCP) may influence thermogenesis. Since skeletal muscle plays an important role in energy homeostasis and substrate oxidation, this study was undertaken to test the hypotheses that skeletal muscle UCP2 content is altered in obesity and could be linked to basal energy expenditure, insulin sensitivity, or substrate oxidation within skeletal muscle under postabsorptive (fasting) conditions. To examine these possibilities, limb basal energy expenditure and respiratory quotient (bRQ) were measured in 18 obese nondiabetic (Ob) and lean individuals (L). Total body fat (%) ranged from 11% to 46%. In addition, insulin-stimulated rates of glucose disposal (Rd) were measured under euglycemic hyperinsulinemic conditions. Biopsy of vastus lateralis muscle was used to measure cytochrome c oxidase (COX) enzyme activity and UCP2 content. Whereas low muscle COX activity was found in the Ob compared to L (6.9+/-1.6 vs. 9.6+/-1.2 U/g; P<0.001), skeletal muscle UCP2 content in Ob was significantly higher than in L (48+/-9 vs. 33+/-12 arbitrary units/g; P<0.05). Moreover, UCP2 content was positively correlated with percent of total body fat (r=0.57; P<0. 05) and bRQ (r=0.59; P<0.01), but not with visceral fat (r=0.17; P=0. 49), basal energy expenditure (r=0.07; P=0.79) or Rd (r=-0.23; P=0. 34). In summary, these results indicate that if development of obesity in humans is mediated by defective expression of UCP2 within skeletal muscle, then this effect is not observed in people with established obesity. The present study also suggests that skeletal muscle UCP2 content is not related to basal energy expenditure or insulin sensitivity in humans. However, the increased content of UCP2 within skeletal muscle in obesity appears to coincide with a reduced postabsorptive lipid utilization by muscle.

Lilly lecture 1995. Glucose transport: pivotal step in insulin action.

The effect of insulin to acutely stimulate glucose uptake into muscle and adipose tissue is essential for normal glucose homeostasis. The GLUT4 glucose transporter is a major mediator of this action, and insulin recruits GLUT4 from an intracellular pool to the plasma membrane. An important pathologic feature of obesity, NIDDM, and to a lesser extent IDDM is resistance to insulin-stimulated glucose uptake. Investigations of the mechanisms have revealed tissue-specific regulation of GLUT4 with decreased gene expression in adipose cells but not in skeletal muscle. This has led to the hypothesis that alterations in the trafficking of the GLUT4 vesicle or in the exposure or activation of the GLUT4 transporter may cause insulin resistance in skeletal muscle in obesity and diabetes. Exercise training increases GLUT4 expression in muscle in association with enhanced glucose tolerance in vivo. Transgenic mice have been created to investigate other approaches to improve insulin action on glucose transport. Overexpression of GLUT4 in adipocytes of transgenic mice increases the proportion of GLUT4 on the plasma membrane and enhances insulin sensitivity in vivo. Altering insulin signaling by overexpressing p21ras in adipocytes of transgenic mice results in increased GLUT4 on the plasma membrane in the absence of insulin and increases insulin sensitivity in vitro and in vivo. Thus, glucose transport is a pivotal step in whole-body insulin action. Strategies to increase the number of GLUT4 transporters that are functionally inserted in the plasma membrane in muscle and adipocytes may lead to new therapies to treat or prevent NIDDM.

Skeletal muscle content of membrane glycoprotein PC-1 in obesity. Relationship to muscle glucose transport.

Membrane glycoprotein PC-1, an inhibitor of insulin signaling, produces insulin resistance when overexpressed in cells transfected with PC-1 cDNA. In the present study, we determined whether PC-1 plays a role in the insulin resistance of skeletal muscle in obesity. Rectus abdominus muscle biopsies were taken from patients undergoing elective surgery. Subjects included both NIDDM patients (n = 14) and nondiabetic patients (n = 34) across a wide range of BMI values (19.5-90.1). Insulin-stimulated glucose transport was measured in incubated muscle strips, and PC-1 content, enzymatic activity, and insulin receptor content were measured in solubilized muscle extracts. Increasing BMI correlated with both an increase in the content of PC-1 in muscle (r = 0.55, P < 0.001) and a decrease in insulin stimulation of muscle glucose transport (r = -0.58, P = 0.008). NIDDM had no effect on either PC-1 content or glucose transport for any given level of obesity. Insulin stimulation of muscle glucose transport was negatively related to muscle PC-1 content (r = -0.68, P = 0.001) and positively related to insulin receptor content (r = 0.60, P = 0.005). Multivariate analysis indicated that both skeletal muscle PC-1 content and insulin receptor content, but not BMI, were independent predictors of insulin-stimulated glucose transport. Muscle PC-1 content accounted for 42% and insulin receptor content for 17% of the variance in glucose transport values. These studies raise the possibility that increased expression of PC-1 and a decreased insulin receptor content in skeletal muscle may be involved in the insulin resistance of obesity.

Insulin and Na-dependent alanine transport in skeletal muscle of obese Zucker (fa/fa) rats

Obese Zucker (fa/fa) rat skeletal muscle is characterized by a reduced rate of muscle protein deposition possibly due to alterations in amino acid transport. The purpose of the present study was to investigate alanine transport in plasma membrane vesicles from skeletal muscle of lean and obese Zucker rats, facilitating the study of alanine transport independent of cellular metabolism. Initial rates of alanine transport were measured in the presence and absence of Na using a rapid filtration technique, and the properties of membranes from control and maximally insulin-treated lean and obese Zucker rats were studied. For lean rats, the maximal stimulation (Vmax) for Na-dependent alanine transport was 207 pmol.mg-1.s-1, and the half-maximal affinity constant (K1/2) was 2.3 mM. Insulin treatment increased the Vmax to 387 pmol.mg-1.s-1 with no changes in K1/2. For the obese rats, the Vmax for Na-dependent alanine transport was 248 pmol.mg-1.s-1, and the K1/2 was 2.8 mM. These values were not changed by insulin treatment. Thus Na-dependent alanine transport in obese rat skeletal muscle is resistant to stimulation by insulin; this alteration may contribute to the abnormal muscle protein metabolism observed in these animals.

Defective insulin receptor tyrosine kinase in human skeletal muscle in obesity and type 2 (non-insulin-dependent) diabetes mellitus.

The tyrosine kinase activity of the insulin receptor was investigated in skeletal muscle biopsies from insulin-resistant males with obesity or with Type 2 (non-insulin-dependent) diabetic males who were lean or overweight. The kinase activity of the receptor from all three groups of insulin-resistant subjects was 40% less when compared to the activity of lean control subjects. This alteration was present in the absence of changes in the level of the insulin receptor on its insulin binding characteristics. We conclude that the tyrosine kinase activity of the skeletal muscle insulin receptor is defective in obesity and Type 2 diabetes, and that this alteration contributes to the insulin-resistant characteristics of both disorders.

Studies of insulin insensitivity in soleus muscles of obese mice

The isolated mouse soleus muscle is a suitable system to measure specific insulin binding and insulin effects. Studies in obese mice have pointed to discrete sites of insulin resistance of skeletal muscle in obesity: (1) A decrease in the number of insulin receptors, which may result in diminished insulin sensitivity (i.e., impaired responses to submaximally stimulating doses of insulin); and (2) Alterations that lay apart from, or beyond, the insulin receptor: thus, glucose transport (and/or phosphorylation) appears to be intrinsically altered and the stimulation by insulin of glycogen synthesis is markedly depressed. These alterations are responsible for the marked resistance to maximally stimulating doses of insulin. The serum from a patient with the syndrome of insulin resistance and acanthosis nigricans contains antibodies that inhibit insulin binding and exert insulin-like effects in muscle; this serum is, however, less effective than insulin in maximally stimulating glycogen synthesis, which suggests some differences in their mechanisms of action.

Insulin binding and effects in isolated soleus muscle of lean and obese mice.

To get some insight into the mechanisms of insulin resistance in obesity, insulin binding and biological effects were investigated in soleus muscles isolated from normal and obese mice. Basal and insulin-stimulated 2-deoxyglucose uptake were measured at the steady state of insulin binding. The results were consistent with the concept of spare receptors, i.e., maximal insulin effect was achieved when only about 20% of total receptors was occupied. When similar studies were applied to muscles of gold thioglucose obese or genetically obese (ob/ob) mice, and compared to lean controls: a) insulin binding was decreased; b) the insulin dose-response curve of 2-deoxyglucose uptake was shifted to the right; c) maximally insulin-stimulated 2-deoxyglucose uptake, glycolysis, and glycogen synthesis were markedly decreased. Insulin binding and effects returned toward normal after a 40-h fast in obese mice. These results point to two loci for the insulin resistance of skeletal muscle in obesity: 1) a decrease in the number of insulin receptors, which results in a diminished insulin sensitivity; and 2) one or more alterations beyond receptor that are responsible for the decreased responsiveness of the tissue to insulin and appear to play a major role in the insulin resistance of muscle in obesity.