Lipid Accumulation In Skeletal Muscle Research Articles

Enhanced ECCD36 Signaling Promotes Skeletal Muscle Insulin Resistance in Female Mice.

Consumption of a Western diet (WD) increases CD36 expression in vascular, hepatic, and skeletal muscle tissues promoting lipid metabolic disorders and insulin resistance. We further examined the role of endothelial cell specific CD36 (ECCD36) signaling in contributing to skeletal muscle lipid metabolic disorders, insulin resistance, and their underlying molecular mechanisms. Female ECCD36 wild type (ECCD36+/+) and knock out (ECCD36-/-) mice, aged six weeks, were provided with either a WD or a standard chow diet for a duration of 16 weeks. ECCD36+/+ WD mice were characterized by elevated fasting plasma glucose and insulin levels, increased homeostatic model assessment for insulin resistance, and glucose intolerance that were blunted in ECCD36-/- mice. Improved insulin sensitivity in ECCD36-/- mice was characterized by increased phosphoinositide 3-kinases/protein kinase B signaling that further augmented glucose transporter type 4 expression and glucose uptake. Meanwhile, 16 weeks of WD feeding also increased skeletal muscle free fatty acid (FFA) and lipid accumulation, without any observed changes in plasma FFA levels. These lipid metabolic disorders were blunted in ECCD36-/- mice. Moreover, ECCD36 also mediated in vitro palmitic acid-induced lipid accumulation in cultured ECs, subsequently leading to the release of FFAs into the culture media. Furthermore, consumption of a WD increased FFA oxidation, mitochondrial dysfunction, impaired mitochondrial respiratory, skeletal muscle fiber type transition, and fibrosis. These WD-induced abnormalities were blunted in ECCD36-/- mice. These findings demonstrate that endothelial specific ECCD36 signaling participates in skeletal muscle FFA uptake, ectopic lipid accumulation, mitochondrial dysfunction, insulin resistance, and associated skeletal muscle dysfunction in diet-induced obesity.

Dapagliflozin improves skeletal muscle insulin sensitivity through SIRT1 activation induced by nutrient deprivation state

Lipid peroxidation and mitochondrial damage impair insulin sensitivity in skeletal muscle. Sirtuin-1 (SIRT1) protects mitochondria and activates under energy restriction. Dapagliflozin (Dapa) is an antihyperglycaemic agent that belongs to the sodium-glucose cotransporter-2 (SGLT2) inhibitors. Evidence shows that Dapa can induce nutrient deprivation effects, providing additional metabolic benefits. This study investigates whether Dapa can trigger nutrient deprivation to activate SIRT1 and enhance insulin sensitivity in skeletal muscle. We treated diet-induced obese (DIO) mice with Dapa and measured metabolic parameters, lipid accumulation, oxidative stress, mitochondrial function, and glucose utilization in skeletal muscle. β-hydroxybutyric acid (β-HB) was intervened in C2C12 myotubes. The role of SIRT1 was verified by RNA interference. We found that Dapa treatment induced nutrient deprivation state and reduced lipid deposition and oxidative stress, improved mitochondrial function and glucose tolerance in skeletal muscle. The same positive effects were observed after β-HB intervening for C2C12 myotubes, and the promoting effects on glucose utilization were diminished by SIRT1 RNA interference. Thus, Dapa promotes a nutrient deprivation state and enhances skeletal muscle insulin sensitivity via SIRT1 activation. In this study, we identified a novel hypoglycemic mechanism of Dapa and the potential mechanistic targets.

Impaired Suppression of Plasma Lipid Extraction and its Partitioning Away from Muscle by Insulin in Humans with Obesity.

Humans with obesity and insulin resistance exhibit lipid accumulation in skeletal muscle, but the underlying biological mechanisms responsible for the accumulation of lipid in the muscle of these individuals remain unknown. We investigated how plasma insulin modulates the extraction of circulating triglycerides (TGs) and non-esterified fatty acids (NEFAs) from dietary and endogenous sources in the muscle of lean, insulin-sensitive humans (Lean-IS) and contrasted these responses to those in humans with obesity and insulin resistance (Obese-IR). The studies were performed in a postprandial state associated with steady-state plasma TG concentrations. The arterio-venous blood sampling technique was employed to determine the extraction of circulating lipids across the forearm muscle before and after insulin infusion. We distinguished kinetics of TGs and NEFAs from dietary sources across muscle from those from endogenous sources by incorporating stable isotope-labeled triolein in ingested fat. Plasma insulin rapidly suppressed the extraction of plasma TGs from endogenous, but not dietary, sources in the Lean-IS, but same response was absent in the Obese-IR. Furthermore, in the muscle of Lean-IS, plasma insulin decreased the extraction of circulating NEFAs from both dietary and endogenous sources, but in Obese-IR subjects this response was absent for NEFAs from dietary sources. Partitioning of circulating lipids away from the skeletal muscle when plasma insulin increases, such as during the postprandial period, is impaired in humans with obesity and insulin resistance. Trial Registration: ClinicalTrials.gov ( NCT01860911 ).

Inflammation-associated intramyocellular lipid alterations in human pancreatic cancer cachexia.

Cancer cachexia is a multifactorial metabolic syndrome characterized by systemic inflammation and ongoing skeletal muscle loss resulting in weakness, poor quality of life, and decreased survival. Whereas lipid accumulation in skeletal muscle is associated with cancer cachexia as well as the prognosis of cancer patients, surprisingly little is known about the nature of the lipids that accumulate in the muscle during cachexia, and whether this is related to inflammation. We aimed to identify the types and distributions of intramyocellular lipids in patients with and without cancer cachexia. Rectus abdominis muscle biopsies were collected during surgery of patients with pancreatic ductal adenocarcinoma (n=10 without cachexia, n=20 cachectic without inflammation (CRP<10mg/L), n=10 cachectic with inflammation (CRP≥10mg/L). L3-CT scans were analysed to assess body composition based on validated thresholds in Hounsfield units (HU). Muscle sections were stained with Oil-Red O and H&E to assess general lipid accumulation and atrophy. Untargeted lipidomic analyses were performed on laser-microdissected myotubes using LC-MS/MS. The spatial distribution of intramyocellular lipids with differential abundance between groups was visualized by mass-spectrometry imaging. Genes coding for inflammation markers and enzymes involved in de novo ceramide synthesis were studied by qPCR. Muscle radiation attenuation was lower in cachectic patients with inflammation (median 24.3 [18.6-30.8] HU) as compared with those without inflammation (34.2 [29.3-38.7] HU, P=0.033) or no cachexia (37.4 [33.9-42.9] HU, P=0.012). Accordingly, intramyocellular lipid content was lower in non-cachectic patients (1.9 [1.6-2.1]%) as compared with those with cachexia with inflammation (5.5 [4.5-7.3]%, P=0.002) or without inflammation (4.8 [2.6-6.0]%, P=0.017). Intramyocellular lipid accumulation was associated with both local IL-6 mRNA levels (rs=0.57, P=0.015) and systemic CRP levels (rs=0.49, P=0.024). Compared with non-cachectic subjects, cachectic patients had a higher relative abundance of intramyocellular glycerophospholipids and a lower relative abundance of glycerolipids. Furthermore, increases in several intramyocellular lipids such as SM(d36:1), PC(34:1), and TG(48:1) were found in cachectic patients with inflammation and correlated with specific cachexia features. Altered intramyocellular lipid species such as PC(34:1), LPC(18:2), and TG(48:1) showed an uneven distribution in muscle sections of cachectic and non-cachectic patients, with areas featuring abundance of these lipids next to areas almost devoid of them. Intramyocellular lipid accumulation in patients with cachexia is associated with both local and systemic inflammation, and characterized by changes in defined lipid species such as glycerolipids and glycerophospholipids.

Insulin resistance and dyslipidemia in low-birth-weight goat kids.

Low birth weight (LBW) impairs the development and health of livestock by affecting postnatal growth performance and metabolic health in adulthood. Previous studies on indigenous goats in southwest China showed that LBW goat kids had higher mortality and morbidity rates, including hepatic dyslipidemia and liver damage. However, the mechanism of insulin resistance affecting lipid metabolism under LBW conditions remains unclear. In this study, we conducted in vivo glucose-insulin metabolic studies, measured biochemical parameters, and analyzed related regulatory pathways. Both glucose tolerance tests and insulin tolerance tests indicated insulin resistance in LBW goat kids compared to controls (p < 0.05). The marker of insulin resistance, homeostasis model assessment (HOMA), was 2.85-fold higher in LBW than in control goats (p < 0.01). Additionally, elevated levels of free fatty acids in both plasma and skeletal muscle were observed in LBW goats compared to normal birth weight (NBW) goats (p < 0.05). Transcriptome analysis revealed impairments in lipid metabolism and insulin signaling in LBW goats. The observed lipid accumulation was associated with the upregulation of genes linked to fatty acid uptake and transport (FABP3), fatty acid oxidation (PPARA), triacylglycerol synthesis (LPIN1 and DGAT1), oxidative stress (ANKRD2), and insulin resistance (PGC1α). Furthermore, the insulin receptor substrate 2 (IRS2) was lower in the liver of LBW goat kids (p < 0.05). While there was no change in insulin function in skeletal muscle, LBW may lead to lipid accumulation in skeletal muscle by interfering with insulin function in the liver. These findings collectively impact the health and growth performance of livestock.

Gromwell (Lithospermum erythrorhizon) Attenuates High-Fat-Induced Skeletal Muscle Wasting by Increasing Protein Synthesis and Mitochondrial Biogenesis.

Gromwell (Lithospermum erythrorhizon, LE) can mitigate obesity-induced skeletal muscle atrophy in C2C12 myotubes and high-fat diet (HFD)-induced obese mice. The purpose of this study was to investigate the anti-skeletal muscle atrophy effects of LE and the underlying molecular mechanism. C2C12 myotubes were pretreated with LE or shikonin, and active component of LE, for 24 h and then treated with 500 μM palmitic acid (PA) for an additional 24 h. Additionally, mice were fed a HFD for 8 weeks to induced obesity, and then fed either the same diet or a version containing 0.25% LE for 10 weeks. LE attenuated PA-induced myotubes atrophy in differentiated C2C12 myotubes. The supplementation of LE to obese mice significantly increased skeletal muscle weight, lean body mass, muscle strength, and exercise performance compared with those in the HFD group. LE supplementation not only suppressed obesity-induced skeletal muscle lipid accumulation, but also downregulated TNF-α and atrophic genes. LE increased protein synthesis in the skeletal muscle via the mTOR pathway. We observed LE induced increase of mitochondrial biogenesis and upregulation of oxidative phosphorylation related genes in the skeletal muscles. Furthermore, LE increased the expression of peroxisome proliferator-activated receptor-gamma coactivator-1 alpha and the phosphorylation of adenosine monophosphate-activated protein kinase. Collectively, LE may be useful in ameliorating the detrimental effects of obesity-induced skeletal muscle atrophy through the increase of protein synthesis and mitochondrial biogenesis of skeletal muscle.

Nutritional regulation of skeletal muscle energy metabolism, lipid accumulation and meat quality in pigs.

The quality of pork determines consumers' purchase intention, which directly affects the economic value of pork. Minimizing the proportion of inferior pork and producing high quality pork are the ultimate goals of the pig industry. Muscle energy metabolism, serving as a regulative hub in organism energy expenditure and storage as a fat deposit, is compatible with myofiber type composition, affecting meat color, intramuscular fat content, tenderness, pH values and drip loss. Increasing data illustrate that dietary nutrients and bioactive ingredients affect muscle energy metabolism, white adipose browning and fat distribution, and myofiber type composition in humans, and rodents. Recently, some studies have shown that modulating muscle energy metabolism and lipid accumulation through nutritional approaches could effectively improve meat quality. This article reviews the progress and development in this field, and specifically discusses the impacts of dietary supply of amino acids, lipids, and gut microbiota as well as maternal nutrition on skeletal muscle energy metabolism, lipid accumulation and meat quality of pigs, so as to provide comprehensive overview with respect to effective avenues for improving meat quality.

Endothelial MRs mediate Western diet-induced lipid disorders and skeletal muscle insulin resistance in females.

Consumption of a Western diet (WD) consisting of excess fat and carbohydrates activates the renin-angiotensin-aldosterone system, which has emerged as an important risk factor for systemic and tissue insulin resistance. We recently discovered that activated mineralocorticoid receptors (MRs) in diet-induced obesity induces CD36 expression, increases ectopic lipid accumulation, and results in systemic and tissue insulin resistance. Here, we further investigated whether endothelial cell (EC) specific MR (ECMR) activation participates in WD-induced ectopic skeletal muscle lipid accumulation, insulin resistance and dysfunction. Six-week-old female ECMR knockout (ECMR-/-) and wild type (ECMR+/+) mice were fed either a WD or a chow diet for 16 weeks. ECMR-/- mice were found to have decreased WD-induced in vivo glucose intolerance and insulin resistance at 16-weeks. Improved insulin sensitivity was accompanied by increased glucose transporter type 4 (Glut4) expression in conjunction with improved soleus insulin metabolic signaling in phosphoinositide 3-kinases/protein kinase B and endothelial nitric oxide synthase activation. Additionally, ECMR-/- also blunted WD-induced increases in CD36 expression and associated elevations in soleus free fatty acid, total intramyocellular lipid content, oxidative stress, and soleus fibrosis. Moreover, in vitro and in vivo activation of ECMR increased EC derived exosomal CD36 that was further taken up by skeletal muscle cells, leading to increased skeletal muscle CD36 levels. These findings indicate that in the context of an obesogenic WD, enhanced ECMR signaling increases EC derived exosomal CD36 resulting in increased uptake and elevated concentrations of CD36 in skeletal muscle cells, contributing to increased lipid metabolic disorders and soleus insulin resistance.

Cabergoline counteracts adipose and skeletal muscle lipid accumulation through promoting oxidative pathway: an in vivo and in vitro preclinical study

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Effects of Triheptanoin on Mitochondrial Respiration and Glycolysis in Cultured Fibroblasts from Neutral Lipid Storage Disease Type M (NLSD-M) Patients

Neutral lipid storage disease type M (NLSD-M) is an ultra-rare, autosomal recessive disorder that causes severe skeletal and cardiac muscle damage and lipid accumulation in all body tissues. In this hereditary pathology, the defective action of the adipose triglyceride lipase (ATGL) enzyme induces the enlargement of cytoplasmic lipid droplets and reduction in the detachment of mono- (MG) and diglycerides (DG). Although the pathogenesis of muscle fiber necrosis is unknown, some studies have shown alterations in cellular energy production, probably because MG and DG, the substrates of Krebs cycle, are less available. No tests have been tried with medium-chain fatty acid molecules to evaluate the anaplerotic effect in NLSD cells. In this study, we evaluated the in vitro effect of triheptanoin (Dojolvi®), a highly purified chemical triglyceride with seven carbon atoms, in fibroblasts obtained from five NLSD-M patients. Glycolytic and mitochondrial functions were determined by Seahorse XF Agylent Technology, and cellular viability and triglyceride content were measured through colorimetric assays. After the addition of triheptanoin, we observed an increase in glycolysis and mitochondrial respiration in all patients compared with healthy controls. These preliminary results show that triheptanoin is able to induce an anaplerotic effect in NLSD-M fibroblasts, paving the way towards new therapeutic strategies.

Congenital adiponectin deficiency mitigates high-fat-diet-induced obesity in gonadally intact male and female, but not in ovariectomized mice

Epidemiological literature indicates that women are less susceptible to type II diabetes (T2D) than males. The general consensus is that estrogen is protective, whereas its deficiency in post-menopause is associated with adiposity and impaired insulin sensitivity. However, epidemiological data suggests that males are more prone to developing T2D, and at a lower BMI, compared to females during post-menopausal years; suggesting that another factor, other than estrogen, protects females. We proposed to determine if adiponectin (APN) serves as this protective factor. An initial experiment was performed in which gonadally intact male and female mice were fed either a purified low-fat diet (LFD) or high-fat diet (HFD) (40% kcals from fat) for 16 weeks. An additional group of HFD ovariectomy (OVX) mice were included to assess estrogen deficiency’s impact on obesity. Body composition, adipose tissue inflammation, ectopic lipid accumulation as well as glucose metabolism and insulin resistance were assessed. In corroboration with previous data, estrogen deficiency (OVX) exacerbated HFD-induced obesity in female mice. However, despite a higher body fat percentage and a similar degree of hepatic and skeletal muscle lipid accumulation, female OVX HFD-fed mice exhibited enhanced insulin sensitivity relative to HFD-fed males. Therefore, a subsequent HFD experiment was performed utilizing male and female (both gonadally intact and OVX) APN deficient mice (APN−/−) and wildtype littermates to determine if APN is the factor which protects OVX females from the similar degree of metabolic dysfunction as males in the setting of obesity. Indirect calorimetry was used to determine observed phenotype differences. APN deficiency limited adiposity and mitigated HFD-induced insulin resistance and adipose tissue inflammation in gonadally intact male and female, but not in OVX mice. Using indirect calorimetry, we uncovered that slight, but non-statistically significant differences in food intake and energy expenditure leading to a net difference in energy balance likely explain the reduced body weight exhibited by male APN-deficient mice. In conclusion, congenital APN deficiency is protective against obesity development in gonadally intact mice, however, in the setting of estrogen deficiency (OVX) this is not true. These findings suggest that gonadal status dictates the protective effects of congenital APN deficiency in the setting of HFD-induced obesity.

Mineralocorticoid Receptors Mediate Diet-Induced Lipid Infiltration of Skeletal Muscle and Insulin Resistance.

Excess circulating lipids increase total intramyocellular (IMC) lipid content and ectopic fat storage, resulting in lipotoxicity and insulin resistance in skeletal muscle. Consumption of a diet high in fat and refined sugars-a Western diet (WD)-has been shown to activate mineralocorticoid receptors (MRs) and promote insulin resistance. However, our understanding of the precise mechanisms by which enhanced MR activation promotes skeletal muscle insulin resistance remains unclear. In this study, we investigated the mechanisms by which enhanced MR signaling in soleus muscle promotes ectopic skeletal muscle lipid accumulation and related insulin resistance. Six-week-old C57BL/6J mice were fed either a mouse chow diet or a WD with or without spironolactone (1 mg/kg/day) for 16 weeks. Spironolactone attenuated 16 weeks of WD-induced in vivo glucose intolerance and insulin resistance, and improved soleus insulin metabolic signaling. Improved insulin sensitivity was accompanied by increased glucose transporter 4 (Glut4) expression in conjunction with decreased soleus free fatty acid and IMC lipid content, as well as CD36 expression. Additionally, spironolactone prevented WD-induced soleus mitochondria dysfunction. Furthermore, MR signaling also mediated WD/aldosterone-induced reductions in soleus microRNA (miR)-99a, which was identified to negatively target CD36 and prevented palmitic acid-induced increases in CD36 expression, lipid droplet formation, mitochondria dysfunction, and insulin resistance in C2C12 cells. These data indicate that inhibition of MR activation with spironolactone prevented diet-induced abnormal expression of miR-99a, which had the capacity to reduce CD36, leading to reduced IMC lipid content and improved soleus mitochondria function and insulin sensitivity.

Transcription factor CREB3 is a potent regulator of high-fat diet-induced obesity and energy metabolism.

The endoplasmic reticulum senses alterations to cellular homeostasis that activates the unfolded protein response (UPR). UPR proteins are known to aid in regulating glucose and lipid metabolism. CREB3 is a UPR-associated transcription factor whose potential role in regulating energy metabolism remains unclear. Eight-week-old wild-type (WT) and Creb3+/- mice were placed on control and high-fat diets (HFD) for 8 weeks, and metabolic phenotypes characterized by weekly weighing, indirect calorimetry, body composition scans, glucose tolerance tests, plasma analysis, tissue lipid quantifications and gene/protein expression analysis. HFD weight gain in Creb3+/- males was reduced by 34% (p < 0.0001) and females by 39.5% (p = 0.014) from their WT counterparts. No differences were found in HFD food intake or total fecal lipids between genotypes. Creb3+/- mice had increased energy expenditure and respiratory exchange ratios (p = 0.002) relative to WT. Creb3+/- mice had significant reductions in absolute fat and lean tissue, while Creb3+/- females had significant reductions in body fat% and increased lean% composition (p < 0.0001) compared to WT females. Creb3+/- mice were protected from HFD-induced basal hyperglycemia (males p < 0.0001; females p = 0.0181). Creb3+/- males resisted HFD-induced hepatic lipid accumulation (p = 0.025) and glucose intolerance compared to WT (p < 0.0001) while Creb3+/- females were protected from lipid accumulation in skeletal muscle (p = 0.001). Despite the metabolic differences of Creb3+/- mice on HFD, lipid plasma profiles did not significantly differ from WT. Fasted Creb3+/- mice additionally revealed upregulation of hepatic energy expenditure and gluconeogenic genes such as Pgc-1a and Gr (glucocorticoid receptor) (p < 0.05), respectively. Reduced expression of CREB3 increased energy expenditure and the respiratory exchange ratio, and protected mice from HFD-induced weight gain, basal hyperglycemia, and sex-specific tissue lipid accumulation. We postulate that CREB3 is a novel key regulator of diet-induced obesity and energy metabolism that warrants further investigation as a potential therapeutic target in metabolic disorders.

Sesamol counteracts on metabolic disorders of middle-aged alimentary obese mice through regulating skeletal muscle glucose and lipid metabolism.

BackgroundGlobally, obesity is a significant public problem, especially when aging. Sesamol, a phenolic lignan present in sesame seeds, might have a positive effect on high-fat diet (HFD)-induced obesity associated with aging.ObjectiveThe purpose of current research study was to explore salutary effects and mechanisms of sesamol in treating alimentary obesity and associated metabolic syndrome in middle-aged mice.MethodsC57BL/6J mice aged 4–6 weeks and 6–8 months were assigned to the young normal diet group, middle-aged normal diet group, middle-aged HFD group, and middle-aged HFD + sesamol group. At the end of experiment, glucose tolerance test and insulin tolerance test were performed; the levels of lipids and oxidative stress-related factors in the serum and skeletal muscle were detected using chemistry reagent kits; lipid accumulation in skeletal muscle was observed by oil red O staining; the expressions of muscular glucose and lipid metabolism associated proteins were measured by Western blotting.ResultsSesamol decreased the body weight and alleviated obesity-associated metabolism syndrome in middle-aged mice, such as glucose intolerance, insulin resistance, dyslipidemia, and oxidative stress. Moreover, muscular metabolic disorders were attenuated after treatment with sesamol. It increased the expression of glucose transporter type-4 and down-regulated the protein levels of pyruvate dehydrogenase kinase isozyme 4, implying the increase of glucose uptake and oxidation. Meanwhile, sesamol decreased the expression of sterol regulatory element binding protein 1c and up-regulated the phosphorylation of hormone-sensitive lipase and the level of carnitine palmityl transferase 1α, which led to the declined lipogenesis and the increased lipolysis and lipid oxidation. In addition, the SIRT1/AMPK signaling pathway was triggered by sesamol, from which it is understood how sesamol enhances glucose and lipid metabolism.ConclusionsSesamol counteracts on metabolic disorders of middle-aged alimentary obese mice through regulating skeletal muscle glucose and lipid metabolism, which might be associated with the stimulation of the SIRT1/AMPK pathway.

Sesamol Reverses Myofiber-Type Conversion in Obese States via Activating the SIRT1/AMPK Signal Pathway.

Obesity can evoke changes of skeletal muscle structure and function, which are characterized by the conversion of myofiber from type I to type II, leading to a vicious cycle of metabolic disorders. Reversing the muscle fiber-type conversion in obese states is a novel strategy for treating those with obesity. Sesamol, a food ingredient compound isolated from sesame seeds, exerted potential antiobesity effects. The present research aimed to explore the therapeutic effects of sesamol on obesity-related skeletal muscle-fiber-type conversion and elucidate the underlying molecular mechanisms through utilizing a high-fat-diet-induced obese C57BL/6J mice model and palmitic acid-exposed C2C12 myotubes. The results showed that sesamol attenuated obesity-related metabolic disturbances, elevated exercise endurance of obese mice, and decreased lipid accumulation and insulin resistance in skeletal muscle. After the treatment with sesamol, the muscular mitochondrial content and biogenesis were increased, accompanied by the enzyme activities and myosin heavy-chain isoform changed from type II fiber to type I fiber. Mechanistic studies revealed that the effects of sesamol on reversing skeletal muscle-fiber-type conversion in obese states were associated with the stimulation of the muscular sirtuin 1 (SIRT1)/AMP-activated protein kinase (AMPK) signal pathway, and these effects could be inhibited by a specific inhibitor of SIRT1, EX-527. In conclusion, our research provided novel evidence that sesamol could regulate myofiber-type conversion to treat obesity and obesity-related metabolic disorders by stimulating the muscular SIRT1/AMPK signal pathway.

Insulin Resistance: From Mechanisms to Therapeutic Strategies.

Insulin resistance is the pivotal pathogenic component of many metabolic diseases, including type 2 diabetes mellitus, and is defined as a state of reduced responsiveness of insulin-targeting tissues to physiological levels of insulin. Although the underlying mechanism of insulin resistance is not fully understood, several credible theories have been proposed. In this review, we summarize the functions of insulin in glucose metabolism in typical metabolic tissues and describe the mechanisms proposed to underlie insulin resistance, that is, ectopic lipid accumulation in liver and skeletal muscle, endoplasmic reticulum stress, and inflammation. In addition, we suggest potential therapeutic strategies for addressing insulin resistance.

Dietary lysine restriction induces lipid accumulation in skeletal muscle through an increase in serum threonine levels in rats

We previously reported that dietary amino acid restriction induces the accumulation of triglycerides (TAG) in the liver of growing rats. However, differences in TAG accumulation in individual cell types or other tissues were not examined. In this study, we show that TAG also accumulates in the muscle and adipose tissues of rats fed a low amino acid (low-AA) diet. In addition, dietary lysine restriction (low-Lys) induces lipid accumulation in muscle and adipose tissues. In adjusting the nitrogen content to that of the control diet, we found that glutamic acid supplementation to the low-AA diet blocked lipid accumulation, but supplementation with the low-Lys diet did not, suggesting that a shortage of nitrogen caused lipids to accumulate in the skeletal muscle in the rats fed a low-AA diet. Serum amino acid measurement revealed that, in rats fed a low-Lys diet, serum lysine levels were decreased, while serum threonine levels were significantly increased compared with the control rats. When the threonine content was restricted in the low-Lys diet, TAG accumulation induced by the low-Lys diet was completely abolished in skeletal muscle. Moreover, in L6 myotubes cultured in medium containing high threonine and low lysine, fatty acid uptake was enhanced compared with that in cells cultured in control medium. These findings suggest that the increased serum threonine in rats fed a low-Lys diet resulted in lipid incorporation into skeletal muscle, leading to the formation of fatty muscle tissue. Collectively, we propose conceptual hypothesis that “amino-acid signal” based on lysine and threonine regulates lipid metabolism.

Acetyl-CoA carboxylase 2 inhibition reduces skeletal muscle bioactive lipid content and attenuates progression of type 2 diabetes in Zucker diabetic fatty rats

Intramyocellular lipid (IMCL) accumulation in skeletal muscle is closely associated with development of insulin resistance. In particular, diacylglycerol and ceramide are currently considered as causal bioactive lipids for impaired insulin action. Recently, inhibition of acetyl-CoA carboxylase 2 (ACC2), which negatively modulates mitochondrial fatty acid oxidation, has been shown to reduce total IMCL content and improve whole-body insulin resistance. This study aimed to investigate whether ACC2 inhibition-induced compositional changes in bioactive lipids, especially diacylglycerol and ceramide, within skeletal muscle contribute to the improved insulin resistance. In skeletal muscle of normal rats, treatment of the ACC2 inhibitor compound 2e significantly decreased both diacylglycerol and ceramide levels while having no significant impact on other lipid metabolite levels. In skeletal muscle of Zucker diabetic fatty (ZDF) rats, which exhibited greater lipid accumulation than that of normal rats, compound 2e significantly decreased diacylglycerol and ceramide levels corresponding to reduced long chain acyl-CoA pools. Additionally, in the lipid metabolomics study, ZDF rats treated with compound 2e also showed improved diabetes-related metabolic disturbance, as reflected by delayed hyperinsulinemia as well as upregulated gene expression associated with diabetic conditions in skeletal muscle. These metabolic improvements were strongly correlated with the bioactive lipid reductions. Furthermore, long-term treatment of compound 2e markedly improved whole-body insulin resistance, attenuated hyperglycemia and delayed insulin secretion defect even at severe diabetic conditions. These findings suggest that ACC2 inhibition decreases diacylglycerol and ceramide accumulation within skeletal muscle by enhancing acyl-CoA breakdown, leading to attenuation of lipid-induced insulin resistance and subsequent diabetes progression.

Skeletal muscle and fiber type-specific intramyocellular lipid accumulation in obese mice.

ABSTRACTIn obesity, accumulation of lipid droplets in skeletal muscle fibers and a shift towards fast muscle fiber types can both contribute to insulin resistance. However, it is not yet clear how intramyocellular lipid accumulation and fiber type changes are associated. Therefore, we investigated to what extent the lipids accumulated in a fiber type-specific manner in the functionally similar fast-, intermediate- and slow-twitch gastrocnemius, plantaris, and soleus muscles, respectively, in high-fat diet-induced obese 54-week-old female C57BL/6JOlaHsd mice (n = 9) compared to control standard-diet-treated lean mice (n = 9). A high-fat diet was administered for 26 weeks. Fiber-type specific intramyocellular lipid content analysis and muscle fiber typing were performed using histochemical analysis of lipids with Sudan Black and immunohistochemical analysis of myosin heavy chains on serial sections of skeletal muscles. Compared to the lean mice, the lipid accumulation was most prominent in types 2a and 2x/d fibers (p < 0.05) of fast-twitch gastrocnemius and intermediate plantaris muscles in the obese mice, while in slow-twitch soleus muscle, there was no significant lipid accumulation in the obese animals. Furthermore, the slow-twitch soleus muscle of the obese mice with no significant change in muscle fiber diameters exhibited the most pronounced shift towards fast-type myosin heavy chain isoform expression (p < 0.05). In contrast, the fast-twitch and intermediate-twitch gastrocnemius and plantaris muscles, respectively, in which the muscle fiber diameters increased (p < 0.05), were more resistant toward myosin heavy chain expression changes. In conclusion, we demonstrated both muscle- and fiber-type specificity in intramyocellular lipid accumulation in obese mice, suggesting that in obesity, similar muscle fiber types in different muscles accumulate lipids differentially.

Inhibition of Lipid Accumulation in Skeletal Muscle and Liver Cells: A Protective Mechanism of Bilirubin Against Diabetes Mellitus Type 2.

Ectopic lipid accumulation in skeletal muscle and liver drives the pathogenesis of diabetes mellitus type 2 (DMT2). Mild hyperbilirubinaemia has been repeatedly suggested to play a role in the prevention of DMT2 and is known for its capacity to shape an improved lipid phenotype in humans and in animals. To date, the effect of bilirubin on lipid accumulation in tissues that are prone to ectopic lipid deposition is unclear. Therefore, we analyzed the effect of bilirubin on lipid accumulation in skeletal muscle and liver cell lines. C2C12 skeletal mouse muscle and HepG2 human liver cells were treated with physiological concentrations of free fatty acids (FFA) (0.5 mM and 1 mM) and unconjugated bilirubin (UCB) (17.1 and 55 µM). The intracellular presence of UCB upon exogenous UCB administration was confirmed by HPLC and the lipid accumulation was assessed by using Nile red. Exposure of both cell lines to UCB significantly reduced lipid accumulation by up to 23% (p ≤ 0.001) in HepG2 and by up to 17% (p ≤ 0.01) in C2C12 cells at 0.5 and 5 h under hypoglycaemic conditions. Simultaneously, UCB slightly increased FFA uptake in HepG2 cells after 0.5 and 5 h and in C2C12 cells after 12 h as confirmed by gas chromatographic analyses of the remaining FFA content in the incubation media. The effects of UCB on lipid accumulation and uptake were abolished in the presence of higher glucose concentrations. Monitoring the uptake of a radiolabeled glucose analogue [18F]FDG: (2-deoxy-2-[18F]fluoro-D-glucose) into both cell types further indicated higher glucose consumption in the presence of UCB. In conclusion, our findings show that UCB considerably decreases lipid accumulation in skeletal muscle and liver cells within a short incubation time of max. 5 h which suggests that mildly elevated bilirubin levels could lower ectopic lipid deposition, a major key element in the pathogenesis of DMT2.