Fasting plasma lactate as a possible early clinical marker for metabolic disease risk

BACKGROUND AND AIM

Elevated fasting plasma lactate concentrations are evident in individuals with metabolic diseases. However, it has yet to be determined if these associations exist in a young, healthy population as a possible early marker for metabolic disease risk. The purpose of this study was to determine if indices of the metabolic syndrome are related to plasma lactate concentrations in this population.

METHODS

Fifty (29 ± 7 yr) men (n = 19) and women (n = 31) classified as overweight (26.4 ± 1.8 kg/m2) participated in this observational study. Blood pressure and blood metabolites were measured after an overnight fast. Lactate was also measured before and after a three-day eucaloric high-fat (70 %) diet. The homeostatic model assessment for insulin resistance (HOMA-IR) was calculated as a measure of insulin resistance. Visceral adipose tissue mass was determined via dual X-ray absorptiometry.

RESULTS

Triglycerides (r = 0.55, p=<0.0001), HOMA-IR (r = 0.53, p=<0.0001), and systolic and diastolic (both, r = 0.36, p = 0.01) blood pressures associated with fasting plasma lactate. No differences in visceral adipose tissue existed between the sexes (p = 0.41); however, the relationship between visceral adipose tissue and lactate existed only in females (r = 0.59, p = 0.02) but not in males (p = 0.53). Fasting lactate and HOMA-IR increased in males (p = 0.01 and p = 0.02, respectively), but not females, following a three-day high-fat diet.

CONCLUSION

Indices of the metabolic syndrome associated with fasting plasma lactates in young relatively healthy individuals. Fasting lactate also increased in a sex-specific manner after a three-day high fat diet. Thus, lactate could become a clinical marker for metabolic disease risk.

Greater reliance on glycolysis is associated with lower mitochondrial substrate oxidation and insulin sensitivity in infant myogenic MSCs

Individuals with insulin resistance and obesity display higher skeletal muscle production of nonoxidized glycolytic products (i.e., lactate), and lower complete mitochondrial substrate oxidation to CO2. These findings have also been observed in individuals without obesity and are associated with an increased risk for metabolic disease. The purpose of this study was to determine if substrate preference is evident at the earliest stage of life (birth) and to provide a clinical blood marker (lactate) that could be indicative of a predisposition for metabolic disease later. We used radiolabeled tracers to assess substrate oxidation and insulin sensitivity of myogenically differentiated mesenchymal stem cells (MSCs), a proxy of infant skeletal muscle tissue, derived from umbilical cords of full-term infants. We found that greater production of nonoxidized glycolytic products (lactate, pyruvate, alanine) is directly proportional to lower substrate oxidation and insulin sensitivity in MSCs. In addition, we found an inverse relationship between the ratio of complete glucose oxidation to CO2 and infant blood lactate at 1 mo of age. Collectively, considering that higher lactate was associated with lower MSC glucose oxidation and has been shown to be implicated with metabolic disease, it may be an early indicator of infant skeletal muscle phenotype.NEW & NOTEWORTHY In infant myogenically differentiated mesenchymal stem cells, greater production of nonoxidized glycolytic products was directly proportional to lower substrate oxidation and insulin resistance. Glucose oxidation was inversely correlated with infant blood lactate. This suggests that innate differences in infant substrate oxidation exist at birth and could be associated with the development of metabolic disease later in life. Clinical assessment of infant blood lactate could be used as an early indicator of skeletal muscle phenotype.

Ockham's razor and the metabolic syndrome

The broad effects of bariatric/metabolic surgery on virtually every tissue and organ system remain unexplained. Weight loss, although a major factor, does not fully account for the rapid, full, and durable remission of type 2 diabetes, return of islet function, reduction of the prevalence of cancers, increase in gray matter of the brain, and decrease in all-cause mortality. This review supports the thesis that the metabolic syndrome is not a group of separate diseases but rather multiple expressions of a shared defect in the utilization of carbohydrates and lipids. That error is probably caused by a dysmetabolic signal from the foregut, stimulated by food, that limits entry of 2-carbon fragments into the tricarboxylic acid cycle, the accumulation of lactate and, in turn, increases in glucose and insulin. Surgery limits that signal by reducing contact between food and foregut mucosa. Speciation of that signal(s) may offer a new pathway for drug development.

The association between lactate and muscle aerobic substrate oxidation: Is lactate an early marker for metabolic disease in healthy subjects?

Fasting plasma lactate concentrations are elevated in individuals with metabolic disease. The aim of this study was to determine if the variance in fasting lactate concentrations were associated with factors linked with cardiometabolic health even in a young, lean cohort. Young (age 22 ± 0.5; N = 30) lean (BMI (22.4 ± 0.4 kg/m²) women were assessed for waist‐to‐hip ratio, aerobic capacity (VO₂peak), skeletal muscle oxidative capacity (near infrared spectroscopy; fat oxidation from muscle biopsies), and fasting glucose and insulin (HOMA‐IR). Subjects had a mean fasting lactate of 0.9 ± 0.1 mmol/L. The rate of deoxygenation of hemoglobin/myoglobin (R² = .23, p = .03) in resting muscle and skeletal muscle homogenate fatty acid oxidation (R² = .72, p = .004) were inversely associated with fasting lactate. Likewise, cardiorespiratory fitness (time to exhaustion during the VO₂peak test) was inversely associated with lactate (R² = .20, p = .05). Lactate concentration was inversely correlated with HDL:LDL (R² = .57, p = .02) and positively correlated with the waist to hip ratio (R² = .52, p = .02). Plasma lactate was associated with various indices of cardiometabolic health. Thus, early determination of fasting lactate concentration could become a common biomarker used for identifying individuals at early risk for metabolic diseases.

Type 2 Diabetes Modifies Skeletal Muscle Gene Expression Response to Gastric Bypass Surgery

INTRODUCTION

Roux-en-Y gastric bypass (RYGB) is an effective treatment for type 2 diabetes mellitus (T2DM) that can result in remission of clinical symptoms, yet mechanisms for improved skeletal muscle health are poorly understood. We sought to define the impact of existing T2DM on RYGB-induced muscle transcriptome changes.

METHODS

Vastus lateralis biopsy transcriptomes were generated pre- and 1-year post-RYGB in black adult females with (T2D; n = 5, age = 51 ± 6 years, BMI = 53.0 ± 5.8 kg/m²) and without (CON; n = 7, 43 ± 6 years, 51.0 ± 9.2 kg/m²) T2DM. Insulin, glucose, and HOMA-IR were measured in blood at the same time points. ANCOVA detected differentially expressed genes (p < 0.01, fold change < |1.2|), which were used to identify enriched biological pathways.

RESULTS

Pre-RYGB, 95 probes were downregulated with T2D including subunits of mitochondrial complex I. Post-RYGB, the T2D group had normalized gene expression when compared to their non-diabetic counterparts with only three probes remaining significantly different. In the T2D, we identified 52 probes upregulated from pre- to post-RYGB, including NDFUB7 and NDFUA1.

CONCLUSION

Black females with T2DM show extensive downregulation of genes across aerobic metabolism pathways prior to RYGB, which resolves 1 year post-RYGB and is related to improvements in clinical markers. These data support efficacy of RYGB for improving skeletal muscle health, especially in patients with T2DM.

Plasma Lactate as a Marker for Metabolic Health

Blood lactate concentrations traditionally have been used as an index of exercise intensity or clinical hyperlactatemia. However, more recent data suggest that fasting plasma lactate can also be indicative of the risk for subsequent metabolic disease. The hypothesis presented is that fasting blood lactate accumulation reflects impaired mitochondrial substrate use, which in turn influences metabolic disease risk.

Impaired glucose partitioning in primary myotubes from severely obese women with type 2 diabetes

The purpose of this study was to determine whether intramyocellular glucose partitioning was altered in primary human myotubes derived from severely obese women with type 2 diabetes. Human skeletal muscle cells were obtained from lean nondiabetic and severely obese Caucasian females with type 2 diabetes [body mass index (BMI): 23.6 ± 2.6 vs. 48.8 ± 1.9 kg/m², fasting glucose: 86.9 ± 1.6 vs. 135.6 ± 12.0 mg/dL, n = 9/group]. 1-[¹⁴C]-Glucose metabolism (glycogen synthesis, glucose oxidation, and nonoxidized glycolysis) and 1- and 2-[¹⁴C]-pyruvate oxidation were examined in fully differentiated myotubes under basal and insulin-stimulated conditions. Tricarboxylic acid cycle intermediates were determined via targeted metabolomics. Myotubes derived from severely obese individuals with type 2 diabetes exhibited impaired insulin-mediated glucose partitioning with reduced rates of glycogen synthesis and glucose oxidation and increased rates of nonoxidized glycolytic products, when compared with myotubes derived from the nondiabetic individuals (P < 0.05). Both 1- and 2-[¹⁴C]-pyruvate oxidation rates were significantly blunted in myotubes from severely obese women with type 2 diabetes compared with myotubes from the nondiabetic controls. Lastly, concentrations of tricarboxylic acid cycle intermediates, namely, citrate (P < 0.05), cis-aconitic acid (P = 0.07), and α-ketoglutarate (P < 0.05), were lower in myotubes from severely obese women with type 2 diabetes. These data suggest that intramyocellular insulin-mediated glucose partitioning is intrinsically altered in the skeletal muscle of severely obese women with type 2 diabetes in a manner that favors the production of glycolytic end products. Defects in pyruvate dehydrogenase and tricarboxylic acid cycle may be responsible for this metabolic derangement associated with type 2 diabetes.

Altered tricarboxylic acid cycle flux in primary myotubes from severely obese humans

BACKGROUND/OBJECTIVE

The partitioning of glucose toward glycolytic end products rather than glucose oxidation and glycogen storage is evident in skeletal muscle with severe obesity and type 2 diabetes. The purpose of the present study was to determine the possible mechanism by which severe obesity alters insulin-mediated glucose partitioning in human skeletal muscle.

SUBJECTS/METHODS

Primary human skeletal muscle cells (HSkMC) were isolated from lean (BMI = 23.6 ± 2.6 kg/m2, n = 9) and severely obese (BMI = 48.8 ± 1.9 kg/m2, n = 8) female subjects. Glucose oxidation, glycogen synthesis, non-oxidized glycolysis, pyruvate oxidation, and targeted TCA cycle metabolomics were examined in differentiated myotubes under basal and insulin-stimulated conditions.

RESULTS

Myotubes derived from severely obese subjects exhibited attenuated response of glycogen synthesis (20.3%; 95% CI [4.7, 28.8]; P = 0.017) and glucose oxidation (5.6%; 95% CI [0.3, 8.6]; P = 0.046) with a concomitant greater increase (23.8%; 95% CI [5.7, 47.8]; P = 0.004) in non-oxidized glycolytic end products with insulin stimulation in comparison to the lean group (34.2% [24.9, 45.1]; 13.1% [8.6, 16.4], and 2.9% [-4.1, 12.2], respectively). These obesity-related alterations in glucose partitioning appeared to be linked with reduced TCA cycle flux, as 2-[14C]-pyruvate oxidation (358.4 pmol/mg protein/min [303.7, 432.9] vs. lean 439.2 pmol/mg protein/min [393.6, 463.1]; P = 0.013) along with several TCA cycle intermediates, were suppressed in the skeletal muscle of severely obese individuals.

CONCLUSIONS

These data suggest that with severe obesity the partitioning of glucose toward anaerobic glycolysis in response to insulin is a resilient characteristic of human skeletal muscle. This altered glucose partitioning appeared to be due, at least in part, to a reduction in TCA cycle flux.

Time course metabolome of Roux-en-Y gastric bypass confirms correlation between leptin, body weight and the microbiome

Roux-en-Y gastric bypass (RYGB) is an effective way to lose weight and reverse type 2 diabetes. We profiled the metabolome of 18 obese patients (nine euglycemic and nine diabetics) that underwent RYGB surgery and seven lean subjects. Plasma samples from the obese patients were collected before the surgery and one week and three months after the surgery. We analyzed the metabolome in association to five hormones (Adiponectin, Insulin, Ghrelin, Leptin, and Resistin), four peptide hormones (GIP, Glucagon, GLP1, and PYY), and two cytokines (IL-6 and TNF). PCA showed samples cluster by surgery time and many microbially driven metabolites (indoles in particular) correlated with the three months after the surgery. Network analysis of metabolites revealed a connection between carbohydrate (mannosamine and glucosamine) and glyoxylate and confirms glyoxylate association to diabetes. Only leptin and IL-6 had a significant association with the measured metabolites. Leptin decreased immediately after RYGB (before significant weight loss), whereas IL-6 showed no consistent response to RYGB. Moreover, leptin associated with tryptophan in support of the possible role of leptin in the regulation of serotonin synthesis pathways in the gut. These results suggest a potential link between gastric leptin and microbial-derived metabolites in the context of obesity and diabetes.

High Incomplete Skeletal Muscle Fatty Acid Oxidation Explains Low Muscle Insulin Sensitivity in Poorly Controlled T2D

CONTEXT

Almost 50% of type 2 diabetic (T2D) patients are poorly controlled [glycated hemoglobin (HbA1c) ≥ 7%]; however, the mechanisms responsible for progressively worsening glycemic control are poorly understood. Lower skeletal muscle mitochondrial respiratory capacity is associated with low insulin sensitivity and the development of T2D.

OBJECTIVE

We investigated if skeletal muscle insulin sensitivity (SI) was different between well-controlled T2D (WCD) and poorly controlled T2D (PCD) and if the difference was associated with differences resulting from mitochondrial respiratory function.

DESIGN

Vastus lateralis muscle mitochondrial respiration, mitochondrial content, mitochondrial enzyme activity, and fatty acid oxidation (FAO) were measured. SI and the acute response to glucose (AIRg) were calculated by MINMOD analysis from glucose and insulin obtained during a modified, frequently sampled, intravenous glucose tolerance test.

RESULTS

SI and AIRg were lower in PCD than WCD. Muscle incomplete FAO was greater in PCD than WCD and greater incomplete FAO was associated with lower SI and higher HbA1c. Hydroxyacyl-coenzyme A dehydrogenase expression and activity were greater in PCD than WCD. There was no difference in maximal mitochondrial respiration or content between WCD and PCD.

CONCLUSION

The current results suggest that greater skeletal muscle incomplete FAO in poorly controlled T2D is due to elevated β oxidation and is associated with worsening muscle SI.

Circulating adipocyte-derived exosomal MicroRNAs associated with decreased insulin resistance after gastric bypass

OBJECTIVE

Exosomes from obese adipose contain dysregulated microRNAs linked to insulin signaling, as compared with lean controls, providing a direct connection between adiposity and insulin resistance. This study tested the hypotheses that gastric bypass surgery and its subsequent weight loss would normalize adipocyte-derived exosomal microRNAs associated with insulin signaling and the associated metabolome related to glucose homeostasis.

METHODS

African American female subjects with obesity (N = 6; age: 38.5 ± 6.8 years; BMI: 51.2 ± 8.8 kg/m² ) were tested before and 1 year after surgery. Insulin resistance (HOMA), serum metabolomics, and global microRNA profiles of circulating adipocyte-derived exosomes were evaluated via ANCOVA and correlational analyses.

RESULTS

One year postsurgery, patients showed decreased BMI (-18.6 ± 5.1 kg/m² ; P < 0.001), ameliorated insulin resistance (HOMA: 1.94 ± 0.6 presurgery, 0.49 ± 0.1 postsurgery; P < 0.001), and altered metabolites including branched chain amino acids (BCAA). Biological pathway analysis of predicted mRNA targets of 168 surgery-responsive microRNAs (P < 0.05) identified the insulin signaling pathway (P = 1.27E-10; 52/138 elements), among others, in the data set. The insulin signaling pathway was also a target of 10 microRNAs correlated to changes in HOMA (P < 0.05; r > 0.4), and 48 microRNAs correlated to changes in BCAA levels.

CONCLUSIONS

These data indicate that circulating adipocyte-derived exosomes are modified following gastric bypass surgery and correlate to improved postsurgery insulin resistance.

Effects of meal size on the release of GLP-1 and PYY after Roux-en-Y gastric bypass surgery in obese subjects with or without type 2 diabetes

Changes in gastrointestinal peptide release may play an important role in improving glucose control and reducing body weight following Roux-en-Y gastric bypass (RYGB), but the impact of low caloric intake on gut peptide release post-surgery has not been well characterized. The purpose of this study was to assess the relationships between low caloric intake and gut peptide release and how they were altered by RYGB. Obese females including ten normoglycemic (ON) and ten with type 2 diabetes mellitus (T2DM) (OD) were studied before, 1 week, and 3 months after RYGB. Nine lean, normoglycemic women were studied for comparison. Subjects were given three separate mixed meal challenges (MMCs; 75, 150, and 300 kcal). Plasma glucagon-like peptide 1 (GLP-1) and peptide YY (PYY) were analyzed. Prior to surgery, only minimal increases in GLP-1 and PYY were observed in response to the MMCs. After surgery, the peak GLP-1 concentration was progressively elevated in response to increasing meal sizes. The meal sizes had a statistically significant impact on elevation of GLP-1 incremental areas under the curve (ΔAUC) in both ON and OD at 1 week and 3 months post-surgery visits (p < 0.05 for all comparisons). The PYY ∆AUC was also significantly increased in a meal size-dependent manner in both ON and OD at both post-surgery visits (p < 0.05 for all comparisons). Meal sizes as small as 75-300 kcal, which cause minimal stimulation in GLP-1 or PYY release in the subjects before RYGB, are sufficient to provide statistically significant, meal size-dependent increases in the peptides post-RYGB both acutely and after meaningful weight loss occurred.

Hyperinsulinemic syndrome: the metabolic syndrome is broader than you think

BACKGROUND

Type 2 diabetes mellitus (T2DM) is characterized by hyperinsulinemia. In 2011 we showed that gastric bypass (RYGB) corrects these high levels even though insulin resistance remains high, ie, the operation "dissociates" hyperinsulinemia from insulin resistance. RYGB produces reversal of T2DM along with other diseases associated with the metabolic syndrome. This observation led us to examine whether these illnesses also were characterized by hyperinsulinemia.

METHODS

A systematic review was performed to determine whether hyperinsulinemia was present in disorders associated with the metabolic syndrome. We reviewed 423 publications. 58 were selected because of appropriate documentation of insulin measurements. Comparisons were based on whether the studies reported patients as having increased versus normal insulin levels for each metabolic disorder.

RESULTS

The presence (+) or absence (-) of hyperinsulinemia was documented in these articles as follows: central obesity (4+ vs 0-), diabetes (5+ vs 0-), hypertension (9+ vs 1-), dyslipidemia (2+ vs 0-), renal failure (4+ vs 0-), nonalcoholic fatty liver disease (5+ vs 0-), polycystic ovary syndrome (7+ vs 1-), sleep apnea (7+ vs 0-), certain cancers (4+ vs 1-), atherosclerosis (4+ vs 0-), and cardiovascular disease (8+ vs 0-). Four articles examined insulin levels in the metabolic syndrome as a whole (4+ vs 0-).

CONCLUSION

These data document that disorders linked to the metabolic syndrome are associated with high levels of insulin, suggesting that these diseases share a common etiology that is expressed by high levels of insulin. This leads us to propose the concept of a "hyperinsulinemic syndrome" and question the safety of insulin as a chronic therapy for patients with T2DM.

GLP-1 response to a mixed meal: what happens 10 years after Roux-en-Y gastric bypass (RYGB)?

BACKGROUND

Oral meal consumption increases glucagon-like peptide 1 (GLP-1) release which maintains euglycemia by increasing insulin secretion. This effect is exaggerated during short-term follow-up of Roux-en-y gastric bypass (RYGB). We examined the durability of this effect in patient with type 2 diabetes (T2DM) >10 years after RYGB.

METHODS

GLP-1 response to a mixed meal in the 10-year post-RYGB group (n = 5) was compared to lean (n = 9), obese (n = 6), and type 2 diabetic (n = 10) controls using a cross-sectional study design. Analysis of variance (ANOVA) was used to evaluate GLP-1 response to mixed meal consumption from 0 to 300 min, 0-20 min, 20-60 min, and 60-300 min, respectively. Weight, insulin resistance, and T2DM were also assessed.

RESULTS

GLP-1 response 0-300 min in the 10-year post-RYGB showed a statistically significant overall difference (p =  0.01) compared to controls. Furthermore, GLP-1 response 0-20 min in the 10-year post-RYGB group showed a very rapid statistically significant rise (p = 0.035) to a peak of 40 pM. GLP-1 response between 20 and 60 min showed a rapid statistically significant (p = 0.041) decline in GLP-1 response from ~40 pM to 10 pM. GLP-1 response in the 10-year post-RYGB group from 60 to 300 min showed no statistically significant difference from controls. BMI, HOMA, and fasting serum glucose before and >10 years after RYGB changed from 59.9 → 40.4, 8.7 → 0.88, and 155.2 → 87.6 mg/dl, respectively, and were statistically significant (p < 0.05).

CONCLUSIONS

An exaggerated GLP-1 response was noted 10 years after RYGB, strongly suggesting a durability of this effect. This phenomenon may play a key role in maintaining type 2 diabetes remission and weight loss after RYGB.

Roux-en-Y gastric bypass corrects hyperinsulinemia implications for the remission of type 2 diabetes

CONTEXT

Roux-en-Y gastric bypass (RYGB) has been shown to induce rapid and durable reversal of type 2 diabetes.

OBJECTIVE

The aim of the study was to investigate a possible mechanism for the remission of type 2 diabetes after RYGB.

DESIGN

A cross-sectional, nonrandomized, controlled study was conducted. Surgery patients were studied before RYGB and 1 wk and 3 months after surgery.

SETTING

This study was conducted at East Carolina University.

SUBJECTS

Subjects were recruited into three groups: 1) lean controls with no surgery [body mass index (BMI) < 25 kg/m²; n = 9], 2) severely obese type 2 diabetic patients (BMI > 35 kg/m²; n = 9), and 3) severely obese nondiabetic patients (BMI > 35 kg/m²; n = 9).

INTERVENTION

Intervention was RYGB.

RESULTS

One week after RYGB, diabetes was resolved despite continued insulin resistance (insulin sensitivity index was approximately 50% of lean controls) and reduced insulin secretion during an iv glucose tolerance test (acute insulin response to glucose was approximately 50% of lean controls). Fasting insulin decreased and was no different from lean control despite continued elevated glucose in the type 2 diabetic patients compared with lean.

CONCLUSIONS

After RYGB, fasting insulin decreases to levels like those of lean control subjects and diabetes is reversed (fasting blood glucose < 125 mg/dl). This leads us to propose that 1) exclusion of food from the foregut corrects hyperinsulinemia and 2) fasting insulin is dissociated from the influence of fasting glucose, insulin resistance, and BMI. The mechanisms for reversal of diabetes in the face of reduced insulin remain a paradox.

Is there a metabolic program in the skeletal muscle of obese individuals?

Severe obesity (BMI ≥ 40 kg/m(2)) is associated with multiple defects in skeletal muscle which contribute to insulin resistance and a reduction in fatty acid oxidation (FAO) in this tissue. These metabolic derangements are retained in human skeletal muscle cells raised in culture. Together, these findings are indicative of a dysfunctional global metabolic program with severe obesity which is of an epigenetic or genetic origin. Weight loss via gastric bypass surgery can "turn off" and/or correct components of this metabolic program as insulin sensitivity is restored; however, the impairment in FAO in skeletal muscle remains evident. Physical activity can improve FAO and insulin action, indicating that this patient population is not exercise resistant and that exercise offers a pathway to circumvent the abnormal program. Findings presented in this review will hopefully increase the understanding of and aid in preventing and/or treating the severely obese condition.

Lipid partitioning, incomplete fatty acid oxidation, and insulin signal transduction in primary human muscle cells: effects of severe obesity, fatty acid incubation, and fatty acid translocase/CD36 overexpression

CONTEXT

Intracellular lipid partitioning toward storage and the incomplete oxidation of fatty acids (FA) have been linked to insulin resistance.

OBJECTIVE

To gain insight into how intracellular lipid metabolism is related to insulin signal transduction, we examined the effects of severe obesity, excess FA, and overexpression of the FA transporter, FA translocase (FAT)/CD36, in primary human skeletal myocytes.

DESIGN, SETTING, AND PATIENTS

Insulin signal transduction, FA oxidation, and metabolism were measured in skeletal muscle cells harvested from lean and severely obese women. To emulate the obesity phenotype in our cell culture system, we incubated cells from lean individuals with excess FA or overexpressed FAT/CD36 using recombinant adenoviral technology.

RESULTS

Complete oxidation of FA was significantly reduced, whereas total lipid accumulation, FA esterification into lipid intermediates, and incomplete oxidation were up-regulated in the muscle cells of severely obese subjects. Insulin signal transduction was reduced in the muscle cells from severely obese subjects compared to lean controls. Incubation of muscle cells from lean subjects with lipids reduced insulin signal transduction and increased lipid storage and incomplete FA oxidation. CD36 overexpression increased FA transport capacity, but did not impair complete FA oxidation and insulin signal transduction in muscle cells from lean subjects.

CONCLUSIONS

Cultured myocytes from severely obese women express perturbations in FA metabolism and insulin signaling reminiscent of those observed in vivo. The obesity phenotype can be recapitulated in muscle cells from lean subjects via exposure to excess lipid, but not by overexpressing the FAT/CD36 FA transporter.

Peroxisome proliferator-activated receptor-gamma coactivator-1alpha overexpression increases lipid oxidation in myocytes from extremely obese individuals

OBJECTIVE

To determine whether the obesity-related decrement in fatty acid oxidation (FAO) in primary human skeletal muscle cells (HSkMC) is linked with lower mitochondrial content and whether this deficit could be corrected via overexpression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha).

RESEARCH DESIGN AND METHODS

FAO was studied in HSkMC from lean (BMI 22.4 +/- 0.9 kg/m(2); N = 12) and extremely obese (45.3 +/- 1.4 kg/m(2); N = 9) subjects. Recombinant adenovirus was used to increase HSkMC PGC-1alpha expression (3.5- and 8.0-fold), followed by assessment of mitochondrial content (mtDNA and cytochrome C oxidase IV [COXIV]), complete ((14)CO(2) production from labeled oleate), and incomplete (acid soluble metabolites [ASM]) FAO, and glycerolipid synthesis.

RESULTS

Obesity was associated with a 30% decrease (P < 0.05) in complete FAO, which was accompanied by higher relative rates of incomplete FAO ([(14)C]ASM production/(14)CO(2)), increased partitioning of fatty acid toward storage, and lower (P < 0.05) mtDNA (-27%), COXIV (-35%), and mitochondrial transcription factor (mtTFA) (-43%) protein levels. PGC-1alpha overexpression increased (P < 0.05) FAO, mtDNA, COXIV, mtTFA, and fatty acid incorporation into triacylglycerol in both lean and obese groups. Perturbations in FAO, triacylglycerol synthesis, mtDNA, COXIV, and mtTFA in obese compared with lean HSkMC persisted despite PGC-1alpha overexpression. When adjusted for mtDNA and COXIV content, FAO was equivalent between lean and obese groups.

CONCLUSION

Reduced mitochondrial content is related to impaired FAO in HSkMC derived from obese individuals. Increasing PGC-1alpha protein levels did not correct the obesity-related absolute reduction in FAO or mtDNA content, implicating mechanisms other than PGC-1alpha abundance.

Metabolic profiling of muscle contraction in lean compared with obese rodents

Interest in the pathophysiological relevance of intramuscular triacylglycerol (IMTG) accumulation has grown from numerous studies reporting that abnormally high glycerolipid levels in tissues of obese and diabetic subjects correlate negatively with glucose tolerance. Here, we used a hindlimb perfusion model to examine the impact of obesity and elevated IMTG levels on contraction-induced changes in skeletal muscle fuel metabolism. Comprehensive lipid profiling was performed on gastrocnemius muscles harvested from lean and obese Zucker rats immediately and 25 min after 15 min of one-legged electrically stimulated contraction compared with the contralateral control (rested) limbs. Predictably, IMTG content was grossly elevated in control muscles from obese rats compared with their lean counterparts. In muscles of obese (but not lean) rats, contraction resulted in marked hydrolysis of IMTG, which was then restored to near resting levels during 25 min of recovery. Despite dramatic phenotypical differences in contraction-induced IMTG turnover, muscle levels of diacylglycerol (DAG) and long-chain acyl-CoAs (LCACoA) were surprisingly similar between groups. Tissue profiles of acylcarnitine metabolites suggested that the surfeit of IMTG in obese rats fueled higher rates of fat oxidation relative to the lean group. Muscles of the obese rats had reduced lactate levels immediately following contraction and higher glycogen resynthesis during recovery, consistent with a lipid-associated glucose-sparing effect. Together, these findings suggest that contraction-induced mobilization of local lipid reserves in obese muscles promotes beta-oxidation, while discouraging glucose utilization. Further studies are necessary to determine whether persistent oxidation of IMTG-derived fatty acids contributes to systemic glucose intolerance in other physiological settings.

Lipid-induced insulin resistance is prevented in lean and obese myotubes by AICAR treatment

The molecular mechanisms of obesity-associated insulin resistance are becoming increasingly clear, and the effects of various lipid molecules, such as diacylglycerol and ceramide, on the insulin signal are being actively explored. To better understand the divergent response to lipid exposure between lean and obese, we incubated primary human muscle cells from lean [body mass index (BMI) <25 kg/m(2)] and morbidly obese (BMI >40 kg/m(2)) subjects with the saturated fatty acid palmitate. Additionally, given that AMPK-activating drugs are widely prescribed for their insulin-sensitizing effects, we sought to determine whether 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside (AICAR)-stimulated AMPK activation could prevent or reverse the deleterious effects of lipid on insulin signaling. We found that a 1-h palmitate incubation in lean myotubes reduced (P < 0.05) insulin-stimulated phosphoprotein kinase B (Akt), Akt substrate 160 (AS160), and inhibitory factor kappaBalpha (IkappaBalpha) mass, all of which were prevented with AICAR inclusion. With a longer incubation, we observed that myotubes from morbidly obese individuals appear to be largely resistant to the detrimental effects of 16 h lipid exposure as was evident, in contrast to the lean, by the absence of a reduction in insulin-stimulated insulin receptor substrate (IRS)-1 Tyr phosphorylation, phospho-Akt, and phospho-AS160 (P < 0.05). Furthermore, 16 h lipid exposure significantly reduced IkappaBalpha levels and increased phosphorylation of c-Jun NH(2)-terminal kinase (JNK) and IRS1-Ser(312) in lean myotubes only (P < 0.05). Despite a divergent response to lipid between lean and obese myotubes, AICAR inclusion improved insulin signaling in all myotubes. These findings suggest an important role for regular exercise in addition to offering a potential mechanism of action for oral AMPK-activating agents, such as thiazolidinediones and metformin.

Metformin Improves Insulin Signaling in Obese Rats via Reduced IKKbeta Action in a Fiber-Type Specific Manner

Metformin is a widely used insulin-sensitizing drug, though its mechanisms are not fully understood. Metformin has been shown to activate AMPK in skeletal muscle; however, its effects on the inhibitor of kappaB kinasebeta (IKKbeta) in this same tissue are unknown. The aim of this study was to (1) determine the ability of metformin to attenuate IKKbeta action, (2) determine whether changes in AMPK activity are associated with changes in IKKbeta action in skeletal muscle, and (3) examine whether changes in AMPK and IKKbeta function are consistent with improved insulin signaling. Lean and obese male Zuckers received either vehicle or metformin by oral gavage daily for four weeks (four groups of eight). Proteins were measured in white gastrocnemius (WG), red gastrocnemius (RG), and soleus. AMPK phosphorylation increased (P < .05) in WG in both lean (57%) and obese (106%), and this was supported by an increase in phospho-ACC in WG. Further, metformin increased IkappaBalpha levels in both WG (150%) and RG (67%) of obese rats, indicative of reduced IKKbeta activity (P < .05), and was associated with reduced IRS1-pSer(307) (30%) in the WG of obese rats (P < .02). From these data we conclude that metformin treatment appears to exert an inhibitory influence on skeletal muscle IKKbeta activity, as evidenced by elevated IkappaBalpha levels and reduced IRS1-Ser(307) phosphorylation in a fiber-type specific manner.

Regulation of GLUT4 expression in denervated skeletal muscle

Denervation by sciatic nerve resection causes decreased muscle glucose transporter 4 (GLUT4) expression, but little is known about the signaling events that cause this decrease. Experiments were designed to test the hypothesis that decreased GLUT4 expression in denervated muscle occurs because of decreased calcium/CaMK activity, which would then lead to decreased activation of the transcription factors myocyte enhancer factor 2 (MEF2) and GLUT4 enhancer factor (GEF), which are required for normal GLUT4 expression. GLUT4 mRNA was elevated in mice expressing constitutively active CaMK isoform IV (CaMKIV) and decreased by denervation. Denervation decreased GEF binding to the promoter and the content of GEF in the nucleus, but there was no change in either MEF2 binding or MEF2 protein content. Expression of a MEF2-dependent reporter gene did not change in denervated skeletal muscle. To determine the domains of the GLUT4 promoter that respond to denervation, transgenic mice expressing the chloramphenicol acetyl transferase (CAT) reporter gene driven by different lengths of the human GLUT4 promoter were denervated. Using several different promoter/reporter gene constructs, we found that all areas of the GLUT4 promoter were truncated or missing, except for the MEF2 binding domain and the basal promoter. All of the GLUT4 promoter/CAT reporter constructs evaluated responded normally to denervation. Our data lead us to conclude that decreased CaMK activity is not the reason for decreased GLUT4 content in denervated muscle and that negative control of GLUT4 expression is not mediated through the MEF2 or GEF-binding domains. These findings indicate that withdrawal of a GEF- or MEF2-dependent signal is not likely a major determinant of the denervation effect on GLUT4 expression. Thus, the response to denervation may be mediated by other elements present in the basal promoter of the GLUT4 gene.

Mechanism for improved insulin sensitivity after gastric bypass surgery

CONTEXT

Surgical treatments of obesity have been shown to induce rapid and prolonged improvements in insulin sensitivity.

OBJECTIVE

The aim of the study was to investigate the effects of gastric bypass surgery and the mechanisms that explain the improvement in insulin sensitivity.

DESIGN

We performed a cross-sectional, nonrandomized, controlled study.

SETTING

This study was conducted jointly between the Departments of Exercise Science and Physiology at East Carolina University in Greenville, North Carolina.

SUBJECTS

Subjects were recruited into four groups: 1) lean [body mass index (BMI) < 25 kg/m(2); n = 93]; 2) weight-matched (BMI = 25 to 35 kg/m(2); n = 310); 3) morbidly obese (BMI > 35 kg/m(2); n = 43); and 4) postsurgery patients (BMI approximately 30 kg/m(2); n = 40). Postsurgery patients were weight stable 1 yr after surgery.

MAIN OUTCOME MEASURES

Whole-body insulin sensitivity, muscle glucose transport, and muscle insulin signaling were assessed.

RESULTS

Postsurgery subjects had insulin sensitivity index values that were similar to the lean and higher than morbidly obese and weight-matched control subjects. Glucose transport was higher in the postsurgery vs. morbidly obese and weight-matched groups. IRS1-pSer(312) in the postsurgery group was lower than morbidly obese and weight-matched groups. Inhibitor kappaBalpha was higher in the postsurgery vs. the morbidly obese and weight-matched controls, indicating reduced inhibitor of kappaB kinase beta activity.

CONCLUSIONS

Insulin sensitivity and glucose transport are greater in the postsurgery patients than predicted from the weight-matched group, suggesting that improved insulin sensitivity after bypass is due to something other than, or in addition to, weight loss. Improved insulin sensitivity is related to reduced inhibitor of kappaB kinase beta activity and enhanced insulin signaling in muscle.

Artificial selection for high-capacity endurance running is protective against high-fat diet-induced insulin resistance

Elevated oxidative capacity, such as occurs via endurance exercise training, is believed to protect against the development of obesity and diabetes. Rats bred both for low (LCR)- and high (HCR)-capacity endurance running provide a genetic model with inherent differences in aerobic capacity that allows for the testing of this supposition without the confounding effects of a training stimulus. The purpose of this investigation was to determine the effects of a high-fat diet (HFD) on weight gain patterns, insulin sensitivity, and fatty acid oxidative capacity in LCR and HCR male rats in the untrained state. Results indicate chow-fed LCR rats were heavier, hypertriglyceridemic, less insulin sensitive, and had lower skeletal muscle oxidative capacity compared with HCR rats. Upon exposure to an HFD, LCR rats gained more weight and fat mass, and their insulin resistant condition was exacerbated, despite consuming similar amounts of metabolizable energy as chow-fed controls. These metabolic variables remained unaltered in HCR rats. The HFD increased skeletal muscle oxidative capacity similarly in both strains, whereas hepatic oxidative capacity was diminished only in LCR rats. These results suggest that LCR rats are predisposed to obesity and that expansion of skeletal muscle oxidative capacity does not prevent excess weight gain or the exacerbation of insulin resistance on an HFD. Elevated basal skeletal muscle oxidative capacity and the ability to preserve liver oxidative capacity may protect HCR rats from HFD-induced obesity and insulin resistance.

Expression of genes regulating malonyl-CoA in human skeletal muscle

In humans and animal models, increased intramuscular lipid (IML) stores have been implicated in insulin resistance. Malonyl-CoA plays a critical role in cellular lipid metabolism both by serving as a precursor in the synthesis of lipids and by inhibiting lipid oxidation. In muscle, Malonyl-CoA acts primarily as a negative allosteric regulator of carnitine palmitoyl transferase-1 (CPT1) activity, thereby blocking the transport of long chain fatty acyl CoAs into the mitochondria for oxidation. In muscle, increased malonyl-CoA, decreased muscle CPT1 activity, and increased IML have all been reported in obesity. In order to determine whether malonyl-CoA synthesis might be under transcriptional as well as biochemical regulation, we measured mRNA content of several key genes that contribute to the cellular metabolism of malonyl-CoA in muscle biopsies from lean to morbidly obese subjects. Employing quantitative real-time PCR, we determined that expression of mitochondrial acetyl-CoA carboxylase 2 (ACC2) was increased by 50% with obesity (P < 0.05). In both lean and obese subjects, expression of mitochondrial ACC2 was 20-fold greater than that of cytoplasmic ACC1, consistent with their hypothesized roles in synthesizing malonyl-CoA from acetyl-CoA for CPT1 regulation and lipogenesis, respectively. In addition, in both lean and obese subjects, expression of malonyl-CoA decarboxylase was approximately 40-fold greater than fatty acid synthase, consistent with degradation, rather than lipogenesis, being the primary fate of malonyl-CoA in human muscle. No other genes showed signs of increased mRNA content with obesity, suggesting that there may be selective transcriptional regulation of malonyl-CoA metabolism in human obesity.

PPAR-alpha agonism improves whole body and muscle mitochondrial fat oxidation, but does not alter intracellular fat concentrations in burn trauma children in a randomized controlled trial

Background

Insulin resistance is often associated with increased levels of intracellular triglycerides, diacylglycerol and decreased fat β-oxidation. It was unknown if this relationship was present in patients with acute insulin resistance induced by trauma.

Methods

A double blind placebo controlled trial was conducted in 18 children with severe burn injury. Metabolic studies to assess whole body palmitate oxidation and insulin sensitivity, muscle biopsies for mitochondrial palmitate oxidation, diacylglycerol, fatty acyl Co-A and fatty acyl carnitine concentrations, and magnetic resonance spectroscopy for muscle and liver triglycerides were compared before and after two weeks of placebo or PPAR-α agonist treatment.

Results

Insulin sensitivity and basal whole body palmitate oxidation as measured with an isotope tracer increased significantly (P = 0.003 and P = 0.004, respectively) after PPAR-α agonist treatment compared to placebo. Mitochondrial palmitate oxidation rates in muscle samples increased significantly after PPAR-α treatment (P = 0.002). However, the concentrations of muscle triglyceride, diacylglycerol, fatty acyl CoA, fatty acyl carnitine, and liver triglycerides did not change with either treatment. PKC-θ activation during hyper-insulinemia decreased significantly following PPAR-α treatment.

Conclusion

PPAR-α agonist treatment increases palmitate oxidation and decreases PKC activity along with reduced insulin sensitivity in acute trauma, However, a direct link between these responses cannot be attributed to alterations in intracellular lipid concentrations.

The molecular mechanism linking muscle fat accumulation to insulin resistance

Skeletal muscle insulin resistance is a co-morbidity of obesity and a risk factor for the development of type 2 diabetes mellitus. Insulin resistance is associated with the accumulation of intramyocellular lipids. Intramyocellular triacylglycerols do not appear to be the cause of insulin resistance but are more likely to be a marker of other lipid intermediates such as fatty acyl-CoA, ceramides or diacylglycerols. Fatty acyl-CoA, ceramides and diacylglycerols are known to directly alter various aspects of the insulin signalling cascade. Insulin signalling is inhibited by the phosphorylation of serine and threonine residues at the levels of the insulin receptor and insulin receptor substrate 1. Protein kinase C is responsible for the phosphorylation of the serine and threonine residues. Fatty acyl-CoA and diacylglycerols are known to activate protein kinase C. The cause of the intramyocellular accumulation of fatty acyl-CoA and diacylglycerols is unclear at this time. Reduced fatty acid oxidation does not appear to be responsible, as fatty acyl-CoA accumulates in skeletal muscle with a normal fatty acid oxidative capacity. Other potential mechanisms include oversupply of lipids to muscle and/or up regulated fatty acid transport.

Lipid metabolism, exercise and insulin action

Skeletal muscle constitutes 40% of body mass and takes up 80% of a glucose load. Therefore, impaired glucose removal from the circulation, such as that which occurs in obesity and type 2 diabetes, is attributable in large part to the insulin resistance in muscle. Recent research has shown that fatty acids, derived from adipose tissue, can interfere with insulin signalling in muscle. Hence, insulin-stimulated GLUT4 translocation to the cell surface is impaired, and therefore, the rate of glucose removal from the circulation into muscle is delayed. The mechanisms provoking lipid-mediated insulin resistance are not completely understood. In sedentary individuals, excess intramyocellular accumulation of triacylglycerols is only modestly associated with insulin resistance. In contrast, endurance athletes, despite accumulating large amounts of intramyocellular triacylglycerols, are highly insulin sensitive. Thus it appears that lipid metabolites, other than triacylglycerols, interfere with insulin signalling. These metabolites, however, are not expected to accumulate in athletic muscles, as endurance training increases the capacity for fatty acid oxidation by muscle. These observations, and others in severely obese individuals and type 2 diabetes patients, suggest that impaired rates of fatty acid oxidation are associated with insulin resistance. In addition, in obesity and type 2 diabetes, the rates of fatty acid transport into muscle are also increased. Thus, excess intracellular lipid metabolite accumulation, which interferes with insulin signalling, can occur as a result of impaired rates of fatty acid oxidation and/or increased rates of fatty acid transport into muscle. Accumulation of excess intramyocellular lipid can be avoided by exercise, which improves the capacity for fatty acid oxidation.

Contraction of insulin-resistant muscle normalizes insulin action in association with increased mitochondrial activity and fatty acid catabolism

Acute exercise can reverse muscle insulin resistance, but the mechanism(s) of action are unknown. With the use of a hindlimb perfusion model, we have found that acute contraction restores insulin-stimulated glucose uptake in muscle of obese Zucker rats to levels witnessed in lean controls. Previous reports have suggested that obesity-related insulin resistance stems from lipid oversupply and tissue accumulation of toxic lipid intermediates that impair insulin signaling. We reasoned that contraction might activate hydrolysis and oxidation of intramuscular lipids, thus alleviating "lipotoxicity" and priming the muscle for enhanced insulin action. Indeed, analysis of mitochondrial-derived acyl-carnitine esters suggested that contraction caused robust increases in beta-oxidative flux and mitochondrial oxidation. As predicted, contraction decreased intramuscular triacylglycerol content; however, diacylglycerol and long chain acyl-CoAs, lipid intermediates presumed to trigger insulin resistance, were either unchanged or increased. In muscles from obese animals, insulin-stimulated tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 remained impaired after contraction, whereas phosphorylation of the downstream signaling protein, AS160, was partially restored. These results suggest that acute exercise enables diabetic muscle to circumvent upstream defects in insulin signal transduction via mechanisms that are more tightly coupled to increased mitochondrial energy metabolism than the lowering of diacylglycerol and long chain acyl-CoA.

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.