Reduced expression of adipose triglyceride lipase enhances tumor necrosis factor alpha-induced intercellular adhesion molecule-1 expression in human aortic endothelial cells via protein kinase C-dependent activation of nuclear factor-kappaB

We examined the effects of adipose triglyceride lipase (ATGL) on the initiation of atherosclerosis. ATGL was recently identified as a rate-limiting triglyceride (TG) lipase. Mutations in the human ATGL gene are associated with neutral lipid storage disease with myopathy, a rare genetic disease characterized by excessive accumulation of TG in multiple tissues. The cardiac phenotype, known as triglyceride deposit cardiomyovasculopathy, shows massive TG accumulation in both coronary atherosclerotic lesions and the myocardium. Recent reports show that myocardial triglyceride content is significantly higher in patients with prediabetes or diabetes and that ATGL expression is decreased in the obese insulin-resistant state. Therefore, we investigated the effect of decreased ATGL activity on the development of atherosclerosis using human aortic endothelial cells. We found that ATGL knockdown enhanced monocyte adhesion via increased expression of TNFα-induced intercellular adhesion molecule-1 (ICAM-1). Next, we determined the pathways (MAPK, PKC, or NFκB) involved in ICAM-1 up-regulation induced by ATGL knockdown. Both phosphorylation of PKC and degradation of IκBα were increased in ATGL knockdown human aortic endothelial cells. In addition, intracellular diacylglycerol levels and free fatty acid uptake via CD36 were significantly increased in these cells. Inhibition of the PKC pathway using calphostin C and GF109203X suppressed TNFα-induced ICAM-1 expression. In conclusion, we showed that ATGL knockdown increased monocyte adhesion to the endothelium through enhanced TNFα-induced ICAM-1 expression via activation of NFκB and PKC. These results suggest that reduced ATGL expression may influence the atherogenic process in neutral lipid storage diseases and in the insulin-resistant state.     

Adipocyte triglyceride lipase expression in human obesity

We have investigated the gene and protein expression of adipose triglyceride lipase (ATGL) and triglyceride (TG) lipase activity from subcutaneous and visceral adipose tissue of lean and obese subjects. Visceral and subcutaneous adipose tissue was obtained from 16 age-matched lean and obese subjects during abdominal surgery. Tissues were analyzed for mRNA expression of lipolytic enzymes by real-time quantitative PCR. ATGL protein content was assessed by Western blot and TG lipase activity by radiometric assessment. Subcutaneous and visceral adipose tissue of obese subjects had elevated mRNA expression of PNPLA2 (ATGL) and other lipases including PNPLA3, PNPLA4, CES1, and LYPLAL1 (P < 0.05). Surprisingly, ATGL protein expression and TG lipase activity were reduced in subcutaneous adipose tissue of obese subjects. Immunoprecipitation of ATGL reduced total TG lipase activity in adipose lysates by 70% in obese and 83% in lean subjects. No significant differences in the ATGL activator CGI-58 mRNA levels (ABHD5) were associated with obesity. These data demonstrate that ATGL is important for efficient TG lipase activity in humans. They also demonstrate reduced ATGL protein expression and TG lipase activity despite increased mRNA expression of ATGL and other novel lipolytic enzymes in obesity. The lack of correlation between ATGL protein content and in vitro TG lipase activity indicates that small decrements in ATGL protein expression are not responsible for the reduction in TG lipase activity observed here in obesity, and that posttranslational modifications may be important.     

Piromelatine decreases triglyceride accumulation in insulin resistant 3T3-L1 adipocytes: Role of ATGL and HSL

Piromelatine, a novel investigational multimodal sleep medicine, is developed for the treatment of patients with primary and co-morbid insomnia. Piromelatine has been shown to inhibit weight gain and improve insulin sensitivity in high-fat/high-sucrose-fed (HFHS) rats. Considering that piromelatine has also been implicated in lowering of triglyceride levels in HFHS rats, this work elucidated whether this effect involves in the regulation of adipose triglyceride lipase (ATGL) and hormone-sensitive lipase (HSL) in triglyceride (TG) metabolism. In this study, we investigated the effects of piromelatine and MT2 receptors inhibition on TG content, insulin-stimulated glucose uptake, and the expressions of ATGL and HSL in 3T3-L1 adipocytes preincubated in high glucose and high insulin (HGI) conditions. Our results showed that culturing 3T3-L1 adipocytes under HGI conditions increased triglyceride accumulation with concomitant decrease of ATGL and HSL expression, inducing insulin resistance in 3T3-L1 adipocytes. We also found that triglyceride accumulation was significantly inhibited and the levels of ATGL/HSL increased after melatonin or piromelatine treatment. The effects of melatonin/piromelatine (10 nM) were counteracted by pretreatment with the relatively selective MT2 receptor antagonist luzindole (100 nM). In this study, our data demonstrate that piromelatine reverses high glucose and high insulin-induced triglyceride accumulation in 3T3-L1 adipocytes, possibly through up-regulating of ATGL and HSL expression via a melatonin-dependent manner.     

Tesaglitazar, a dual PPAR{alpha}/{gamma} agonist, ameliorates glucose and lipid intolerance in obese Zucker rats

Insulin resistance, impaired glucose tolerance, high circulating levels of free fatty acids (FFA), and postprandial hyperlipidemia are associated with the metabolic syndrome, which has been linked to increased risk of cardiovascular disease. We studied the metabolic responses to an oral glucose/triglyceride (TG) (1.7/2.0 g/kg lean body mass) load in three groups of conscious 7-h fasted Zucker rats: lean healthy controls, obese insulin-resistant/dyslipidemic controls, and obese rats treated with the dual peroxisome proliferator-activated receptor alpha/gamma agonist, tesaglitazar, 3 mumol.kg(-1).day(-1) for 4 wk. Untreated obese Zucker rats displayed marked insulin resistance, as well as glucose and lipid intolerance in response to the glucose/TG load. The 2-h postload area under the curve values were greater for glucose (+19%), insulin (+849%), FFA (+53%), and TG (+413%) compared with untreated lean controls. Treatment with tesaglitazar lowered fasting plasma glucose, improved glucose tolerance, substantially reduced fasting and postload insulin levels, and markedly lowered fasting TG and improved lipid tolerance. Fasting FFA were not affected, but postprandial FFA suppression was restored to levels seen in lean controls. Mechanisms of tesaglitazar-induced lowering of plasma TG were studied separately using the Triton WR1339 method. In anesthetized, 5-h fasted, obese Zucker rats, tesaglitazar reduced hepatic TG secretion by 47%, increased plasma TG clearance by 490%, and reduced very low-density lipoprotein (VLDL) apolipoprotein CIII content by 86%, compared with obese controls. In conclusion, the glucose/lipid tolerance test in obese Zucker rats appears to be a useful model of the metabolic syndrome that can be used to evaluate therapeutic effects on impaired postprandial glucose and lipid metabolism. The present work demonstrates that tesaglitazar ameliorates these abnormalities and enhances insulin sensitivity in this animal model.     

Adipose triglyceride lipase is a TG hydrolase of the small intestine and regulates intestinal PPARα signaling

Adipose triglyceride lipase (ATGL) is the rate-limiting enzyme mediating triglyceride (TG) hydrolysis. The lack of ATGL results in TG accumulation in multiple tissues, underscoring the critical role of ATGL in maintaining lipid homeostasis. Recent evidence suggests that ATGL affects TG metabolism via activation of peroxisome proliferator-activated receptor α (PPARα). To investigate specific effects of intestinal ATGL on lipid metabolism we generated mice lacking ATGL exclusively in the intestine (ATGLiKO). We found decreased TG hydrolase activity and increased intracellular TG content in ATGLiKO small intestines. Intragastric administration of [³H]trioleate resulted in the accumulation of radioactive TG in the intestine, whereas absorption into the systemic circulation was unchanged. Intraperitoneally injected [³H]oleate also accumulated within TG in ATGLiKO intestines, indicating that ATGL mobilizes fatty acids from the systemic circulation absorbed by the basolateral side from the blood. Down-regulation of PPARα target genes suggested modulation of cholesterol absorption by intestinal ATGL. Accordingly, ATGL deficiency in the intestine resulted in delayed cholesterol absorption. Importantly, this study provides evidence that ATGL has no impact on intestinal TG absorption but hydrolyzes TGs taken up from the intestinal lumen and systemic circulation. Our data support the role of ATGL in modulating PPARα-dependent processes also in the small intestine.     

Alpha-lipoic acid increases insulin sensitivity by activating AMPK in skeletal muscle

Triglyceride accumulation in skeletal muscle contributes to insulin resistance in obesity. We recently showed that alpha-lipoic acid (ALA) reduces body weight and prevents the development of diabetes in diabetes-prone obese rats by reducing triglyceride accumulation in non-adipose tissues. AMP-activated protein kinase (AMPK) is a major regulator of cellular energy metabolism. We examined whether ALA lowers triglyceride accumulation in skeletal muscle by activating AMPK. Alpha2-AMPK activity was decreased in obese rats compared to control rats. Administration of ALA to obese rats increased insulin-stimulated glucose disposal in whole body and in skeletal muscle. ALA also increased fatty acid oxidation and activated AMPK in skeletal muscle. Adenovirus-mediated administration of dominant negative AMPK into skeletal muscle prevented the ALA-induced increases in fatty acid oxidation and insulin-stimulated glucose uptake. These results suggest that ALA-induced improvement of insulin sensitivity is mediated by activation of AMPK and reduced triglyceride accumulation in skeletal muscle.     

Adipose triglyceride lipase (ATGL) expression in human skeletal muscle is type I (oxidative) fiber specific

Accumulation of triacylglycerol (TAG) and lipid intermediates in skeletal muscle plays an important role in the etiology of insulin resistance and type 2 diabetes mellitus. Disturbances in skeletal muscle lipid turnover and lipolysis may contribute significantly to this. So far, knowledge on the regulation of muscle lipolysis is limited. Recently the identification of a new lipase was reported: adipose triglyceride lipase (ATGL). ATGL deficient animals show significant lipid accumulation in skeletal muscle, which may indicate that ATGL plays a pivotal role in skeletal muscle lipolysis. However, until now, it is still unknown whether ATGL protein is expressed in human skeletal muscle. Therefore, the aim of the present study was to investigate whether ATGL is expressed at the protein level in human skeletal muscle, and to examine whether its expression is fiber-type specific. To accomplish this, we established an imunohistochemical and immunofluorescent staining procedure to study ATGL protein expression in relation to fiber type in human vastus lateralis muscle of eight male subjects (BMI range: 21.0–34.5 kg/m² and age: 38–59 years). In the present paper we report for the first time that ATGL protein is indeed expressed in human skeletal muscle. Moreover, ATGL is exclusively expressed in type I (oxidative) muscle fibers, suggesting a pivotal role for ATGL in intramuscular fatty acid handling, lipid storage and breakdown.     

Hepatic overexpression of hormone-sensitive lipase and adipose triglyceride lipase promotes fatty acid oxidation, stimulates direct release of free fatty acids, and ameliorates steatosis

Hepatic steatosis is often associated with insulin resistance and obesity and can lead to steatohepatitis and cirrhosis. In this study, we have demonstrated that hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL), two enzymes critical for lipolysis in adipose tissues, also contribute to lipolysis in the liver and can mobilize hepatic triglycerides in vivo and in vitro. Adenoviral overexpression of HSL and/or ATGL reduced liver triglycerides by 40-60% in both ob/ob mice and mice with high fat diet-induced obesity. However, these enzymes did not affect fasting plasma triglyceride and free fatty acid levels or triglyceride and apolipoprotein B secretion rates. Plasma 3-beta-hydroxybutyrate levels were increased 3-5 days after infection in both HSL- and ATGL-overexpressing male mice, suggesting an increase in beta-oxidation. Expression of genes involved in fatty acid transport and synthesis, lipid storage, and mitochondrial bioenergetics was unchanged. Mechanistic studies in oleate-supplemented McA-RH7777 cells with adenoviral overexpression of HSL or ATGL showed that reduced cellular triglycerides could be attributed to increases in beta-oxidation as well as direct release of free fatty acids into the medium. In summary, hepatic overexpression of HSL or ATGL can promote fatty acid oxidation, stimulate direct release of free fatty acid, and ameliorate hepatic steatosis. This study suggests a direct functional role for both HSL and ATGL in hepatic lipid homeostasis and identifies these enzymes as potential therapeutic targets for ameliorating hepatic steatosis associated with insulin resistance and obesity.     

Myotubes from Severely Obese Type 2 Diabetic Subjects Accumulate Less Lipids and Show Higher Lipolytic Rate than Myotubes from Severely Obese Non-Diabetic Subjects

About 80% of patients with type 2 diabetes are classified as overweight. However, only about 1/3 of severely obese subjects have type 2 diabetes. This indicates that several severely obese individuals may possess certain characteristics that protect them against type 2 diabetes. We therefore hypothesized that this apparent paradox could be related to fundamental differences in skeletal muscle lipid handling. Energy metabolism and metabolic flexibility were examined in human myotubes derived from severely obese subjects without (BMI 44±7 kg/m²) and with type 2 diabetes (BMI 43±6 kg/m²). Lower insulin sensitivity was observed in myotubes from severely obese subjects with type 2 diabetes. Lipolysis rate was higher, and oleic acid accumulation, triacylglycerol content, and fatty acid adaptability were lower in myotubes from severely obese subjects with type 2 diabetes compared to severely obese non-diabetic subjects. There were no differences in lipid distribution and mRNA and protein expression of the lipases HSL and ATGL, the lipase cofactor CGI-58, or the lipid droplet proteins PLIN2 and PLIN3. Glucose and oleic acid oxidation were also similar in cells from the two groups. In conclusion, myotubes established from severely obese donors with established type 2 diabetes had lower ability for lipid accumulation and higher lipolysis rate than myotubes from severely obese donors without diabetes. This indicates that a difference in intramyocellular lipid turnover might be fundamental in evolving type 2 diabetes.     

Phosphorylation of adipose triglyceride lipase Ser404 is not related to 5'-AMPK activation during moderate-intensity exercise in humans

Intramyocellular triacylglycerol provides fatty acid substrate for ATP generation in contracting muscle. The protein adipose triglyceride lipase (ATGL) is a key regulator of triacylglycerol lipolysis and whole body energy metabolism at rest and during exercise, and ATGL activity is reported to be enhanced by 5'-AMP-activated protein kinase (AMPK)-mediated phosphorylation at Ser(406) in mice. This is a curious observation, because AMPK activation reduces lipolysis in several cell types. We investigated whether the phosphorylation of ATGL Ser(404) (corresponding to murine Ser(406)) was increased during exercise in human skeletal muscle and with pharmacological AMPK activation in myotubes in vitro. In human experiments, skeletal muscle and venous blood samples were obtained from recreationally active male subjects before and at 5 and 60 min during exercise. ATGL Ser(404) phosphorylation was not increased from rest during exercise, but ATGL Ser(404) phosphorylation correlated with myosin heavy chain 1 expression, suggesting a possible fiber type dependency. ATGL Ser(404) phosphorylation was not related to increases in AMPK activity, and immunoprecipitation experiments indicated no interaction between AMPK and ATGL. Rather, ATGL Ser(404) phosphorylation was associated with protein kinase A (PKA) signaling. ATGL Ser(406) phosphorylation in C(2)C(12) myotubes was unaffected by 5-aminoimidazole-4-carboxaminde-1-β-d-ribofuranoside, an AMPK activator, and the PKA activator forskolin. Our results demonstrate that ATGL Ser(404) phosphorylation is not increased in mixed skeletal muscle during moderate-intensity exercise and that AMPK does not appear to be an activating kinase for ATGL Ser(404/406) in skeletal muscle.     

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.     

Potential mechanisms and consequences of cardiac triacylglycerol accumulation in insulin-resistant rats

The accumulation of intracellular triacylglycerol (TG) is highly correlated with muscle insulin resistance. However, it is controversial whether the accumulation of TG is the result of increased fatty acid supply, decreased fatty acid oxidation, or both. Because abnormal fatty acid metabolism is a key contributor to the pathogenesis of diabetes-related cardiovascular dysfunction, we examined fatty acid and glucose metabolism in hearts of insulin-resistant JCR:LA-cp rats. Isolated working hearts from insulin-resistant rats had glycolytic rates that were reduced to 50% of lean control levels (P < 0.05). Cardiac TG content was increased by 50% (P < 0.05) in the insulin-resistant rats, but palmitate oxidation rates remained similar between the insulin-resistant and lean control rats. However, plasma fatty acids and TG levels, as well as cardiac fatty acid-binding protein (FABP) expression, were significantly increased in the insulin-resistant rats. AMP-activated protein kinase (AMPK) plays a major role in the regulation of cardiac fatty acid and glucose metabolism. When activated, AMPK increases fatty acid oxidation by inhibiting acetyl-CoA carboxylase (ACC) and reducing malonyl-CoA levels, and it decreases TG content by inhibiting glycerol-3-phosphate acyltransferase (GPAT), the rate-limiting step in TG synthesis. The activation of AMPK also stimulates cardiac glucose uptake and glycolysis. We thus investigated whether a decrease in AMPK activity was responsible for the reduced cardiac glycolysis and increased TG content in the insulin-resistant rats. However, we found no significant difference in AMPK activity. We also found no significant difference in various established downstream targets of AMPK: ACC activity, malonyl-CoA levels, carnitine palmitoyltransferase I activity, or GPAT activity. We conclude that hearts from insulin-resistant JCR:LA-cp rats accumulate substantial TG as a result of increased fatty acid supply rather than from reduced fatty acid oxidation. Furthermore, the accumulation of cardiac TG is associated with a reduction in insulin-stimulated glucose metabolism.     

Adipose triglyceride lipase plays a key role in the supply of the working muscle with fatty acids

FAs are mobilized from triglyceride (TG) stores during exercise to supply the working muscle with energy. Mice deficient for adipose triglyceride lipase (ATGL-ko) exhibit defective lipolysis and accumulate TG in adipose tissue and muscle, suggesting that ATGL deficiency affects energy availability and substrate utilization in working muscle. In this study, we investigated the effect of moderate treadmill exercise on blood energy metabolites and liver glycogen stores in mice lacking ATGL. Because ATGL-ko mice exhibit massive accumulation of TG in the heart and cardiomyopathy, we also investigated a mouse model lacking ATGL in all tissues except cardiac muscle (ATGL-ko/CM). In contrast to ATGL-ko mice, these mice did not accumulate TG in the heart and had normal life expectancy. Exercise experiments revealed that ATGL-ko and ATGL-ko/CM mice are unable to increase circulating FA levels during exercise. The reduced availability of FA for energy conversion led to rapid depletion of liver glycogen stores and hypoglycemia. Together, our studies suggest that ATGL-ko mice cannot adjust circulating FA levels to the increased energy requirements of the working muscle, resulting in an increased use of carbohydrates for energy conversion. Thus, ATGL activity is required for proper energy supply of the skeletal muscle during exercise.     

The ameliorating effects of metformin on disarrangement ongoing in gastrocnemius muscle of sarcopenic and obese sarcopenic mice

Sarcopenia and obese sarcopenia are increasingly prevalent chronic diseases with multifactorial pathogenesis, and no approved therapeutic drug to date. In the established sarcopenic mice models, muscle weakness, ectopic lipid deposition, and inflammatory responses in both serum and gastrocnemius muscle were observed, which were even deteriorated in obese sarcopenic models. With metformin intervention for 5 months, metformin exhibited benefits and restoring effects on gastrocnemius muscle of sarcopenic mice, but less effective on that of obese sarcopenic mice, as reflected in the increased percentage of muscle mass and enlarged fiber cross-sectional area, enhanced grip strength and exercise capacities, as well as the ameliorated ectopic lipid deposition and partially restored level of TNF-α, IL-1β, IL-6, MCP-1 and IL-1α, which may be via the activation of phospho-AMPKα (Thr172). The significant up-regulated mRNA and protein level of lipolysis related proteins like hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) may contribute to the ameliorated ectopic lipid deposition with metformin intervention. The uptake of free fatty acid may be also inhibited in obese sarcopenic mice with metformin administration, as reflected in down-regulated mRNA and protein level of fatty acid transporter CD36. Furthermore, NF-κB signaling pathway was involved in the anti-inflammatory effect of metformin. These findings suggest that metformin treatment may be conducive to the prevention of age-related sarcopenia by regulating lipid metabolism in skeletal muscle, i.e. enhanced lipolysis and attenuated hyper-inflammatory responses, which may be AMPK-dependent processes. Moreover, high-fat diet would aggravate the damage to ageing in skeletal muscles and reduced their reactivity to metformin.     

Functional Cardiac Lipolysis in Mice Critically Depends on Comparative Gene Identification-58

Efficient catabolism of cellular triacylglycerol (TG) stores requires the TG hydrolytic activity of adipose triglyceride lipase (ATGL). The presence of comparative gene identification-58 (CGI-58) strongly increased ATGL-mediated TG catabolism in cell culture experiments. Mutations in the genes coding for ATGL or CGI-58 in humans cause neutral lipid storage disease characterized by TG accumulation in multiple tissues. ATGL gene mutations cause a severe phenotype especially in cardiac muscle leading to cardiomyopathy that can be lethal. In contrast, CGI-58 gene mutations provoke severe ichthyosis and hepatosteatosis in humans and mice, whereas the role of CGI-58 in muscle energy metabolism is less understood. Here we show that mice lacking CGI-58 exclusively in muscle (CGI-58KOM) developed severe cardiac steatosis and cardiomyopathy linked to impaired TG catabolism and mitochondrial fatty acid oxidation. The marked increase in ATGL protein levels in cardiac muscle of CGI-58KOM mice was unable to compensate the lack of CGI-58. The addition of recombinant CGI-58 to cardiac lysates of CGI-58KOM mice completely reconstituted TG hydrolytic activities. In skeletal muscle, the lack of CGI-58 similarly provoked TG accumulation. The addition of recombinant CGI-58 increased TG hydrolytic activities in control and CGI-58KOM tissue lysates, elucidating the limiting role of CGI-58 in skeletal muscle TG catabolism. Finally, muscle CGI-58 deficiency affected whole body energy homeostasis, which is caused by impaired muscle TG catabolism and increased cardiac glucose uptake. In summary, this study demonstrates that functional muscle lipolysis depends on both CGI-58 and ATGL.     

Effect of α-Lipoic Acid on Lipid Profile in Rats Fed a High-Fructose Diet

This study investigated the effect of administration of α-lipoic acid (LA) on lipid metabolism in high fructose–fed insulin-resistant rats. High-fructose feeding (60 g/100 g diet) to normal rats resulted in a significant increase in the concentrations of cholesterol, triglycerides (TGs), free fatty acids (FFAs), and phospholipids in plasma, liver, kidney, and skeletal muscle. Reduced activities of lipoprotein lipase (LPL) and lecithin cholesterol acyl transferase (LCAT) and increased activity of the lipogenic enzyme hydroxymethylglutaryl–coenzyme A (HMG-CoA) reductase were observed in plasma and liver. High-density lipoprotein cholesterol (HDL-C) was significantly lowered and very low-density lipoprotein cholesterol (VLDL-C) and low-density lipoprotein cholesterol (LDL-C) were significantly elevated. Treatment with LA (35 mg/kg body weight intraperitoneal) reduced the effects of fructose. The rats showed near-normal levels of lipid components on plasma and tissues. Activities of key enzymes of lipid metabolism were also restored to normal values. Cholesterol distribution in the plasma lipoproteins was normalized, resulting in a favorable lipid profile. This study demonstrates that LA can alter lipid metabolism in fructose-fed insulin-resistant rats and may have implications in the treatment of insulin resistance.     

Cardiac-specific overexpression of perilipin 5 provokes severe cardiac steatosis via the formation of a lipolytic barrier

Cardiac triacylglycerol (TG) catabolism critically depends on the TG hydrolytic activity of adipose triglyceride lipase (ATGL). Perilipin 5 (Plin5) is expressed in cardiac muscle (CM) and has been shown to interact with ATGL and its coactivator comparative gene identification-58 (CGI-58). Furthermore, ectopic Plin5 expression increases cellular TG content and Plin5-deficient mice exhibit reduced cardiac TG levels. In this study we show that mice with cardiac muscle-specific overexpression of perilipin 5 (CM-Plin5) massively accumulate TG in CM, which is accompanied by moderately reduced fatty acid (FA) oxidizing gene expression levels. Cardiac lipid droplet (LD) preparations from CM of CM-Plin5 mice showed reduced ATGL- and hormone-sensitive lipase-mediated TG mobilization implying that Plin5 overexpression restricts cardiac lipolysis via the formation of a lipolytic barrier. To test this hypothesis, we analyzed TG hydrolytic activities in preparations of Plin5-, ATGL-, and CGI-58-transfected cells. In vitro ATGL-mediated TG hydrolysis of an artificial micellar TG substrate was not inhibited by the presence of Plin5, whereas Plin5-coated LDs were resistant toward ATGL-mediated TG catabolism. These findings strongly suggest that Plin5 functions as a lipolytic barrier to protect the cardiac TG pool from uncontrolled TG mobilization and the excessive release of free FAs.     

Irisin ameliorates hepatic glucose/lipid metabolism and enhances cell survival in insulin-resistant human HepG2 cells through adenosine monophosphate-activated protein kinase signaling

Irisin is a newly identified myokine that promotes the browning of white adipose tissue, enhances glucose uptake in skeletal muscle and modulates hepatic metabolism. However, the signaling pathways involved in the effects on hepatic glucose and lipid metabolism have not been resolved. This study aimed to examine the role of irisin in the regulation of hepatic glucose/lipid metabolism and cell survival, and whether adenosine monophosphate-activated protein kinase (AMPK), a master metabolic regulator in the liver, is involved in irisin’s actions. Human liver-derived HepG2 cells were cultured in normal glucose-normal insulin (NGNI) or high glucose-high insulin (HGHI/insulin-resistant) condition. Hepatic glucose and lipid metabolism was evaluated by glucose output and glycogen content or triglyceride accumulation assays, respectively. Our results showed that irisin stimulated phosphorylation of AMPK and acetyl-CoA-carboxylase (ACC) via liver kinase B1 (LKB1) rather than Ca²⁺/calmodulin-dependent protein kinase kinase β (CaMKKβ) in HepG2 cells. Irisin ameliorated hepatic insulin resistance induced by HGHI condition. Irisin reduced hepatic triglyceride content and glucose output, but increased glycogen content, with those effects reversed by dorsomorphin, an AMPK inhibitor. Furthermore, irisin also stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation and promoted cell survival in an AMPK-dependent manner. In conclusion, our data indicate that irisin ameliorates dysregulation of hepatic glucose/lipid metabolism and cell death in insulin-resistant states via AMPK activation. These findings reveal a novel irisin-mediated protective mechanism in hepatic metabolism which provides a scientific basis for irisin as a potential therapeutic target for the treatment of insulin resistance and type 2 diabetes mellitus.     

Adipose Triglyceride Lipase (ATGL) Clone, Expression Pattern, and Regulation by Different Lipid Sources and Lipid Levels in Large Yellow Croaker (Pseudosciaena crocea R.)

Adipose triglyceride lipase (ATGL) was recently identified as a triglyceride (TG)-specific lipase. In this study, we first obtained from large yellow croaker fish a 1,820-bp (GenBank ID: HQ916211) ATGL cDNA fragment with a 141-bp 5′UTR, a 1,485-bp open reading frame, and a 194-bp 3′UTR. The predicted fish ATGL had 494 amino acids (GenBank ID: ADY89608) and a calculated molecular weight of 55.1 kDa. ATGL was highly expressed in liver and, to a lesser degree, in heart, muscle, and abdominal fat. ATGL gene expression was high at 4.5 g and then decreased at 157.9 g and increased again at 474.2 g. The effects of lipid levels and lipid sources on ATGL expression in vivo were also investigated. A high-lipid diet decreased ATGL expression in fish significantly (P?<?0.01). Fish in soybean oil group exhibited significantly lower ATGL expression than fish in the fish oil and beef tallow groups (P?<?0.01). These data enhance our understanding of ATGL in fish lipid metabolism.     

Increased collagen content in insulin-resistant skeletal muscle

Oversupply and underutilization of lipid fuels are widely recognized to be strongly associated with insulin resistance in skeletal muscle. Recent attention has focused on the mechanisms underlying this effect, and defects in mitochondrial function have emerged as a potential player in this scheme. Because evidence indicates that lipid oversupply can produce abnormalities in extracellular matrix composition and matrix changes can affect the function of mitochondria, the present study was undertaken to determine whether muscle from insulin-resistant, nondiabetic obese subjects and patients with type 2 diabetes mellitus had increased collagen content. Compared with lean control subjects, obese and type 2 diabetic subjects had reduced muscle glucose uptake (P<0.01) and decreased insulin stimulation of tyrosine phosphorylation of insulin receptor substrate-1 and its ability to associate with phosphatidylinositol 3-kinase (P<0.01 and P<.05). Because it was assayed by total hydroxyproline content, collagen abundance was increased in muscle from not only type 2 diabetic patients but also nondiabetic obese subjects (0.26+/-0.05, 0.57+/-0.18, and 0.67+/- 0.20 microg/mg muscle wet wt, lean controls, obese nondiabetics, and type 2 diabetics, respectively), indicating that hyperglycemia itself could not be responsible for this effect. Immunofluorescence staining of muscle biopsies indicated that there was increased abundance of types I and III collagen. We conclude that changes in the composition of the extracellular matrix are a general characteristic of insulin-resistant muscle.