Two phases of palmitate-induced insulin resistance in skeletal muscle: impaired GLUT4 translocation is followed by a reduced GLUT4 intrinsic activity

Interplay of fatty acids, insulin and exercise in vascular health

Fatty acid metabolism, exercise, and insulin action play critical roles in maintaining vascular health, especially relevant in metabolic disorders such as obesity, type 2 diabetes, and cardiovascular disease. Insulin, a vasoactive hormone, induces arterial vasodilation throughout the arterial tree, increasing arterial compliance and enhancing tissue perfusion. These effects, however, are impaired in individuals with obesity and type 2 diabetes, and evidence suggests that vascular insulin resistance contributes to the pathogenesis of type 2 diabetes and its cardiovascular complications. Elevated plasma levels of free fatty acids in people with insulin resistance engender vascular inflammation, endothelial dysfunction, and vascular insulin resistance. Importantly, these effects are both functionally and structurally dependent, with saturated fatty acids as the primary culprits, while polyunsaturated fatty acids may support insulin sensitivity and endothelial function. Exercise enhances fatty acid oxidation, reduces circulating free fatty acids, and improves insulin sensitivity, thereby mitigating lipotoxicity and promoting endothelial function. Additionally, exercise induces beneficial vascular adaptations. This review examines the complex interplay among fatty acid metabolism, exercise training-induced vascular adaptations, and insulin-mediated vascular changes, highlighting their collective impact on vascular health and underlying mechanisms in both healthy and insulin-resistant states. It also explores the therapeutic potential of targeted exercise prescriptions and fatty acid-focused dietary strategies for enhancing vascular health, emphasizing tailored interventions to maximize metabolic benefits. Future research should investigate the pathways linking fatty acid metabolism to vascular insulin resistance, with a focus on how exercise and dietary modifications can be personalized to enhance vascular insulin sensitivity, optimize vascular health, and reduce the risks of type 2 diabetes and associated cardiovascular complications.

Arginine and Lysine Supplementation Potentiates the Beneficial β-Hydroxy ß-Methyl Butyrate (HMB) Effects on Skeletal Muscle in a Rat Model of Diabetes

Skeletal muscle is the key tissue for maintaining protein and glucose homeostasis, having a profound impact on the development of diabetes. Diabetes causes deleterious changes in terms of loss of muscle mass, which will contribute to reduced glucose uptake and therefore progression of the disease. Nutritional approaches in diabetes have been directed to increase muscle glucose uptake, and improving protein turnover has been at least partially an oversight. In muscle, β-hydroxy β-methyl butyrate (HMB) promotes net protein synthesis, while arginine and lysine increase glucose uptake, albeit their effects on promoting protein synthesis are limited. This study evaluates if the combination of HMB, lysine, and arginine could prevent the loss of muscle mass and function, reducing the progression of diabetes. Therefore, the combination of these ingredients was tested in vitro and in vivo. In muscle cell cultures, the supplementation enhances glucose uptake and net protein synthesis due to an increase in the amount of GLUT4 transporter and stimulation of the insulin-dependent signaling pathway involving IRS-1 and Akt. In vivo, using a rat model of diabetes, the supplementation increases lean body mass and insulin sensitivity and decreases blood glucose and serum glycosylated hemoglobin. In treated animals, an increase in GLUT4, creatine kinase, and Akt phosphorylation was detected, demonstrating the synergic effects of the three ingredients. Our findings showed that nutritional formulations based on the combination of HMB, lysine, and arginine are effective, not only to control blood glucose levels but also to prevent skeletal muscle atrophy associated with the progression of diabetes.

A systematic review on anti-diabetic plant essential oil compounds: Dietary sources, effects, molecular mechanisms, and safety

Type 2 diabetes mellitus (T2DM) is a multifaceted metabolic syndrome defined through the dysfunction of pancreatic β-cells driven by a confluence of genetic and environmental elements. Insulin resistance, mediated by interleukins and other inflammatory elements, is one of the key factors contributing to the progression of T2DM. Many essential oils derived from dietary plants are beneficial against various chronic diseases. We reviewed the anti-diabetic properties of dietary plant-derived essential oil compounds, with a focus on their molecular mechanisms by modulating specific signaling pathways and other critical inflammatory mediators involved in insulin resistance. High-quality literature published in the last 12 years, from 2010 to 2022, was collected from the Scopus, Web of Science, PubMed, and Embase databases using the search terms "dietary plants," "essential oils," "anti-diabetic," "insulin resistance," "antihyperglycemic," "T2DM," "anti-diabetic essential oils," and anti-diabetic mechanism." According to the results, the essential oil compounds, including cinnamaldehyde, carvacrol, zingerone, sclareol, zerumbone, myrtenol, thujone, geraniol, citral, eugenol, thymoquinone, thymol, citronellol, α-terpineol, and linalool have been demonstrated to contain strong anti-diabetic effects via modulating various signal transduction pathways linked to glucose metabolism. Additionally, in diabetes-related animal models, they can also considerably reduce the expression of TNF-α, IL-1β, IL-4, IL-6, iNOS, and COX-2. The main signaling molecules regulated by these compounds include AMPK, GLUT4, Caspase-3, PPARγ, PPARα, NF-κB, p-IκBα, MyD88, MCP-1, SREBP-1c, AGEs, RAGE, VEGF, Nrf2/HO-1, and SIRT-1. They can also significantly inhibit the generation of TBARS and MDA, reduce oxidative stress, increase insulin levels, adiponectin, and glycoprotein enzymes, boost antioxidant enzymes like SOD, CAT, and GPx, as well as reduce glutathione and vital glycolytic enzymes. Besides, they can significantly lower the levels of liver enzymes and lipid profile markers. Moreover, most essential oil compounds are generally safe based on animal studies. In conclusion, dietary plant-derived essential oil compounds have potential anti-diabetic effects by influencing different signaling pathways and molecular targets linked to glucose metabolism, and should be safe and beneficial against diabetes and related complications.

α-Tubulin acetylation on Lysine 40 controls cardiac glucose uptake

Our group previously demonstrated that an excess of nutrients, as observed in diabetes, provokes an increase in cardiac protein acetylation responsible for a reduced insulin-stimulated translocation of the glucose transporter GLUT4 to the plasma membrane. The acetylated proteins involved in this event have yet not been identified. α-Tubulin is a promising candidate as a major cytoskeleton component involved, among other things, in the translocation of GLUT4-containing vesicles from their intracellular pools towards the plasma membrane. Moreover, α-tubulin is known to be acetylated, Lys40 (K40) being its best characterized acetylated residue. The present work sought to evaluate the impact of α-tubulin K40 acetylation on cardiac glucose entry, with a particular interest in GLUT4 translocation. First, we observed that a mouse model of high-fat diet-induced obesity presented an increase in cardiac α-tubulin K40 acetylation level. Next, we showed that treatment of insulin-sensitive primary cultured adult rat cardiomyocytes with tubacin, a specific tubulin acetylation inducer, reduced insulin-stimulated glucose uptake and GLUT4 translocation. Conversely, decreasing α-tubulin K40 acetylation by expressing a non-acetylable dominant form of α-tubulin (mCherry α-tubulin K40A mutant) remarkably intensified insulin-induced glucose transport. Finally, mCherry α-tubulin K40A expression similarly improved glucose transport in insulin-resistant cardiomyocytes or after AMP-activated protein kinase activation. Taken together, our study demonstrates that modulation of α-tubulin K40 acetylation level affects glucose transport in cardiomyocytes, offering new putative therapeutic insights regarding modulation of glucose metabolism in insulin-resistant and diabetic hearts.

In Vitro and In Vivo Antidiabetic Potential of Monoterpenoids: An Update

Diabetes mellitus (DM) is a chronic metabolic condition characterized by persistent hyperglycemia due to insufficient insulin levels or insulin resistance. Despite the availability of several oral and injectable hypoglycemic agents, their use is associated with a wide range of side effects. Monoterpenes are compounds extracted from different plants including herbs, vegetables, and fruits and they contribute to their aroma and flavor. Based on their chemical structure, monoterpenes are classified into acyclic, monocyclic, and bicyclic monoterpenes. They have been found to exhibit numerous biological and medicinal effects such as antipruritic, antioxidant, anti-inflammatory, and analgesic activities. Therefore, monoterpenes emerged as promising molecules that can be used therapeutically to treat a vast range of diseases. Additionally, monoterpenes were found to modulate enzymes and proteins that contribute to insulin resistance and other pathological events caused by DM. In this review, we highlight the different mechanisms by which monoterpenes can be used in the pharmacological intervention of DM via the alteration of certain enzymes, proteins, and pathways involved in the pathophysiology of DM. Based on the fact that monoterpenes have multiple mechanisms of action on different targets in in vitro and in vivo studies, they can be considered as lead compounds for developing effective hypoglycemic agents. Incorporating these compounds in clinical trials is needed to investigate their actions in diabetic patients in order to confirm their ability in controlling hyperglycemia.

Putative Role of Protein Palmitoylation in Cardiac Lipid-Induced Insulin Resistance

In the heart, inhibition of the insulin cascade following lipid overload is strongly associated with contractile dysfunction. The translocation of fatty acid transporter CD36 (SR-B2) from intracellular stores to the cell surface is a hallmark event in the lipid-overloaded heart, feeding forward to intracellular lipid accumulation. Yet, the molecular mechanisms by which intracellularly arrived lipids induce insulin resistance is ill-understood. Bioactive lipid metabolites (diacyl-glycerols, ceramides) are contributing factors but fail to correlate with the degree of cardiac insulin resistance in diabetic humans. This leaves room for other lipid-induced mechanisms involved in lipid-induced insulin resistance, including protein palmitoylation. Protein palmitoylation encompasses the reversible covalent attachment of palmitate moieties to cysteine residues and is governed by protein acyl-transferases and thioesterases. The function of palmitoylation is to provide proteins with proper spatiotemporal localization, thereby securing the correct unwinding of signaling pathways. In this review, we provide examples of palmitoylations of individual signaling proteins to discuss the emerging role of protein palmitoylation as a modulator of the insulin signaling cascade. Second, we speculate how protein hyper-palmitoylations (including that of CD36), as they occur during lipid oversupply, may lead to insulin resistance. Finally, we conclude that the protein palmitoylation machinery may offer novel targets to fight lipid-induced cardiomyopathy.

Elaidate, a trans fatty acid, suppresses insulin signaling for glucose uptake in a manner distinct from that of stearate

The dietary intake of elaidate (elaidic acid), a trans-fatty acid, is associated with the development of various diseases. Since elaidate is a C18 unsaturated fatty acid with a steric structure similar to that of a C18 saturated fatty acid (stearate), we previously revealed that insulin-dependent glucose uptake was impaired in adipocytes exposed to elaidate prior to and during differentiation similar to stearate. However, it is still unknown whether the mechanism of impairment of insulin-dependent glucose uptake due to elaidate is similar to that of stearate. Here, we indicate that persistent exposure to elaidate has particular effects on insulin signaling and GLUT4 dynamics. Insulin-induced accumulation of Akt at the plasma membrane (PM) and elevations of phosphorylated Akt and AS160 levels in whole cells were suppressed in adipocytes persistently exposed to 50 μM elaidate. Interestingly, persistent exposure to the same concentration of stearate has no effect on the phosphorylated Akt and AS160 levels. When cells were exposed to these fatty acids, elaidate suppressed insulin-induced fusion, but not translocation, of GLUT4 storage vesicles in the PM, whereas stearate did not suppress the fusion and translocation of GLUT4 storage, indicating that elaidate has suppressive effects on the accumulation of Akt and fusion of GLUT4 storage vesicles and that both elaidate and stearate vary in the mechanisms by which they impair insulin-dependent glucose uptake.

Unacylated ghrelin stimulates fatty acid oxidation to protect skeletal muscle against palmitate-induced impairment of insulin action in lean but not high-fat fed rats

Background

Ghrelin is a gut hormone that spikes in circulation before mealtime. Recent findings suggest that both ghrelin isoforms stimulate skeletal muscle fatty acid oxidation, lending to the possibility that it may regulate skeletal muscle’s handling of meal-derived substrates. It was hypothesized in the current study that ghrelin may preserve muscle insulin response during conditions of elevated saturated fatty acid (palmitate) availability by promoting its oxidation.

Methods and results

Soleus muscle strips were isolated from male rats to determine the direct effects of ghrelin isoforms on fatty acid oxidation, glucose uptake and insulin signaling. We demonstrate that unacylated ghrelin (UnAG) is the more potent stimulator of skeletal muscle fatty acid oxidation. Both isoforms of ghrelin generally protected muscle from impaired insulin-mediated phosphorylation of AKT Ser⁴⁷³ and Thr³⁰⁸, as well as downstream phosphorylation of AS160 Ser⁵⁸⁸ during high palmitate exposure. However, only UnAG was able to preserve insulin-stimulated glucose uptake during exposure to high palmitate concentrations. The use of etomoxir, an irreversible inhibitor of carnitine palmitoyltransferase (CPT-1) abolished this protection, strongly suggesting that UnAG’s stimulation of fatty acid oxidation may be essential to this protection. To our knowledge, we are also the first to investigate the impact of a chronic high-fat diet on ghrelin’s actions in muscle. Following 6 wks of a high-fat diet, UnAG was unable to preserve insulin-stimulated signaling or glucose transport during an acute high palmitate exposure. UnAG was also unable to further stimulate 5′ AMP-activated protein kinase (AMPK) or fatty acid oxidation during high palmitate exposure. Corticotropin-releasing hormone receptor-2 (CRF-2R) content was significantly decreased in muscle from high-fat fed animals, which may partially account for the loss of UnAG’s effects.

Conclusions

UnAG is able to protect muscle from acute lipid exposure, likely due to its ability to stimulation fatty acid oxidation. This effect is lost in high-fat fed animals, implying a resistance to ghrelin at the level of the muscle. The underlying mechanisms accounting for ghrelin resistance in high fat-fed animals remain to be discovered.

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Saturated lipids acutely impair muscle insulin signaling and glucose transport.

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Ghrelin isoforms consistently protect insulin signaling from lipid detriment.

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Unacylated ghrelin more potently stimulates fat oxidation, preserving glucose transport.

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Muscle of chronic high fat-fed rats may be resistant to ghrelin’s metabolic effects.

Role of l-carnitine and oleate in myogenic differentiation: implications for myofiber regeneration

[Purpose]

Myogenic progenitors play a critical role in injury-induced myofiber regeneration. The purpose of this study was to characterize the effects of oleate and L-carnitine on the overall behavior of proliferating myogenic progenitors (myoblasts) and its link to the mitochondrial biogenic process.

[Methods]

C2C12 myoblasts were cultured either with no treatment, oleate, L-carnitine, or their mixture. Proliferating myoblasts were investigated under a phase-contrast microscope. Myonuclei and myosin heavy chain were stained with DAPI and MF20 antibody, respectively, in differentiated myotubes and visualized under florescence microscopy. Mitochondrial biogenic markers and porin were assessed by qRT-PCR or immunoblotting.

[Results]

Increased proliferation rate was observed in myoblasts conditioned with oleate or a mixture of oleate and L-carnitine in contrast to that in non-treated (NT) and L-carnitine-treated myoblasts. Myoblast viability was not statistically different among all tested groups. Fusion index and myotube width were greater in oleate- or L-carnitine-conditioned myotubes than those in NT myotubes, with the greatest effect seen in myotubes conditioned with the mixture. The gene expressions of Pgc1-α, Nrf1, and Tfam were the greatest in myotubes conditioned with the mixture, whereas the level of Ncor1 expression was lower compared to those of the other groups. Protein level of porin was the greatest in myotubes conditioned with the mixture, followed by that of individually treated myotubes with oleate and L-carnitine.

[Conclusion]

These results provide a critical piece of cellular evidence that combined treatment of oleate and L-carnitine could serve as a potential therapeutic strategy to facilitate biological activation of myogenic progenitors.

Preventive effect of oleate on palmitate-induced insulin resistance in skeletal muscle and its mechanism of action

Insulin resistance in skeletal muscle is a feature associated with exposure to an excess of saturated fatty acids such as palmitate. Oleic acid has been shown to blunt palmitate-induced insulin resistance in muscle cells. However, there is no literature available regarding the effect of oleic acid on palmitate-induced insulin resistance in intact muscle. Therefore, this study investigated the effect of oleic acid on palmitate-induced insulin resistance in rat soleus muscle and its underlying mechanisms. For these purposes, oleic acid (1 mM) was administered for 12 h in the absence or presence of palmitate (2 mM). At the end of the experiment, plasmalemmal GLUT4, the phosphorylation of AS160 and Akt-2, and the total expression of these signaling proteins were examined. We found that treatment with palmitate for 12 h reduced insulin-stimulated GLUT4 translocation and the phosphorylation of AS160 and Akt-2. However, the administration of oleic acid fully restored insulin-stimulated GLUT4 translocation (P < 0.05), as well as AS160 and Akt-2 phosphorylation (P < 0.05) despite the continuous presence of palmitate. Wortmannin, an inhibitor of PI3-K, only slightly prevented the oleic acid-induced improvements in insulin-stimulated GLUT4 translocation, and AS160 phosphorylation. However, this treatment completely inhibited the oleic acid-induced improvement in insulin-stimulated Akt-2 phosphorylation. In contrast, the oleic acid-induced improvement in insulin signaling was not affected by compound C, an AMPK specific inhibitor. In conclusion, the results clearly indicate that oleic acid administration alleviates palmitate-induced insulin resistance by promoting GLUT4 translocation in muscle, at least in part, by activating the PI3K pathway.

Glucose Uptake Measurement and Response to Insulin Stimulation in In Vitro Cultured Human Primary Myotubes

Skeletal muscle is the largest glucose deposit in mammals and largely contributes to glucose homeostasis. Assessment of insulin sensitivity of muscle cells is of major relevance for all studies dedicated to exploring muscle glucose metabolism and characterizing metabolic alterations. In muscle cells, glucose transporter type 4 (GLUT4) proteins translocate to the plasma membrane in response to insulin, thus allowing massive entry of glucose into the cell. The ability of muscle cells to respond to insulin by increasing the rate of glucose uptake is one of the standard readouts to quantify muscle cell sensitivity to insulin. Human primary myotubes are a suitable in vitro model, as the cells maintain many features of the donor phenotype, including insulin sensitivity. This in vitro model is also suitable for the test of any compounds that could impact insulin responsiveness. Measurements of the glucose uptake rate in differentiated myotubes reflect insulin sensitivity. In this method, human primary muscle cells are cultured in vitro to obtain differentiated myotubes, and glucose uptake rates with and without insulin stimulation are measured. We provide a detailed protocol to quantify passive and active glucose transport rates using radiolabeled [³H] 2-deoxy-D-Glucose ([³H]2dG). Calculation methods are provided to quantify active basal and insulin-stimulated rates, as well as stimulation fold.

Effect of resistance exercise under conditions of reduced blood insulin on AMPKα Ser485/491 inhibitory phosphorylation and AMPK pathway activation

Insulin stimulates skeletal muscle glucose uptake via activation of the protein kinase B/Akt (Akt) pathway. Recent studies suggest that insulin downregulates AMP-activated protein kinase (AMPK) activity via Ser485/491 phosphorylation of the AMPK α-subunit. Thus lower blood insulin concentrations may induce AMPK signal activation. Acute exercise is one method to stimulate AMPK activation; however, no study has examined the relationship between blood insulin levels and acute resistance exercise-induced AMPK pathway activation. Based on previous findings, we hypothesized that the acute resistance exercise-induced AMPK pathway activation would be augmented by disruptions in insulin secretion through a decrease in AMPKα Ser485/491 inhibitory phosphorylation. To test the hypothesis, 10-wk-old male Sprague-Dawley rats were administered the toxin streptozotocin (STZ; 55 mg/kg) to destroy the insulin secreting β-cells. Three days postinjection, the right gastrocnemius muscle from STZ and control rats was subjected to resistance exercise by percutaneous electrical stimulation. Animals were killed 0, 1, or 3 h later; activation of the Akt/AMPK and downstream pathways in the muscle tissue was analyzed by Western blotting and real-time PCR. Notably, STZ rats showed a significant decrease in basal Akt and AMPKα Ser485/491 phosphorylation, but substantial exercise-induced increases in both AMPKα Thr172 and acetyl-CoA carboxylase (ACC) Ser79 phosphorylation were observed. Although no significant impact on resistance exercise-induced Akt pathway activation or glucose uptake was found, resistance exercise-induced peroxisome proliferator-activated receptor (PPAR)-γ coactivator-1 α (PGC-1α) gene expression was augmented by STZ treatment. Collectively, these data suggest that circulating insulin levels may regulate acute resistance exercise-induced AMPK pathway activation and AMPK-dependent gene expression relating to basal AMPKα Ser485/491 phosphorylation.

Cholinergic activation suppresses palmitate-induced macrophage activation and improves acylation stimulating protein resistance in co-cultured adipocytes

Acylation-stimulating protein (ASP), produced through activation of the alternative complement immune system, modulates lipid metabolism. Using a trans-well co-culture cell model, the mitigating role of α7-nicotinic acetylcholine receptor (α7nAChR)-mediated cholinergic pathway on ASP resistance was evaluated. ASP signaling in adipocytes via its receptor C5L2 and signaling intermediates Gαq, Gβ, phosphorylated protein kinase C-α, and protein kinase C-ζ were markedly suppressed in the presence of TNFα or medium from palmitate-treated RAW264.7 macrophages, indicating ASP resistance. There was no direct effect of α7nAChR activation in 3T3-L1 cell culture. However, α7nAChR activation almost completely reversed the ASP resistance in adipocytes co-cultured with palmitate-treated RAW264.7 macrophages. Further, α7nAChR activation could suppress the production of pro-inflammatory molecules TNFα and interleukin-6 produced from palmitate-treated co-cultured macrophages. These results suggest that macrophages play a significant role in the pathogenesis of ASP resistance and α7nAChR activation secondarily improves adipose ASP resistance through suppression of inflammation in macrophages. Impact statement 1. Adipocyte-macrophage interaction in acylation-stimulating protein (ASP) resistance 2. Lipotoxicity induced inflammatory response in ASP resistance 3. A vicious circle between lipotoxicity and inflammatory response in ASP resistance 4. Cholinergic modulation of inflammatory response in adipocyte and macrophage.

Influence of trans fatty acids on glucose metabolism in soleus muscle of rats fed diets enriched in or deprived of linoleic acid

PURPOSE

Industrial trans fatty acid (TFA) intake leads to impaired glucose metabolism. However, the overall effects reported are inconsistent and vary with the dietary FA composition and TFA isomer type and levels. We investigated TFA effects on glucose uptake, incorporation and oxidation, and glycogen synthesis in incubated soleus muscle under basal conditions or after treatment with insulin and/or palmitate.

METHODS

Male Wistar rats were fed either linoleic acid (LA)-enriched (⁺LA) or LA-deprived (^-LA) diet, supplemented (⁺LA + TFA or ^-LA + TFA) or not with TFA, for 60 days. Soleus muscle glucose metabolism was assessed in the absence or presence of insulin and/or palmitic acid.

RESULTS

Under basal conditions, TFA enhanced glucose uptake and oxidation regardless of the LA status. Both TFA-supplemented groups had lower insulin response to glucose metabolism. Under insulin-stimulated conditions, TFA prevented the palmitate inhibition of muscle glucose uptake and metabolism in the ⁺LA + TFA group.

CONCLUSION

Dietary TFA enhanced glucose utilization in incubated soleus muscle under basal conditions and prevented the palmitate-induced inhibition in insulin-stimulated conditions. However, TFA reduced the insulin response to glucose uptake and metabolism. The effects mentioned above were influenced by the FA profile modifications induced by the dietary LA levels, suggesting that lipid metabolization and incorporation into plasma membrane are important determining factors of glucose metabolism and insulin sensitivity.

Acupuncture Alters Expression of Insulin Signaling Related Molecules and Improves Insulin Resistance in OLETF Rats

To determine effect of acupuncture on insulin resistance in Otsuka Long-Evans Tokushima Fatty (OLETF) rats and to evaluate expression of insulin signaling components. Rats were divided into three groups: Sprague-Dawley (SD) rats, OLETF rats, and acupuncture+OLETF rats. Acupuncture was subcutaneously applied to Neiguan (PC6), Zusanli (ST36), and Sanyinjiao (SP6); in contrast, acupuncture to Shenshu (BL23) was administered perpendicularly. For Neiguan (PC6) and Zusanli (ST36), needles were connected to an electroacupuncture (EA) apparatus. Fasting blood glucose (FPG) was measured by glucose oxidase method. Plasma fasting insulin (FINS) and serum C peptide (C-P) were determined by ELISA. Protein and mRNA expressions of insulin signaling molecules were determined by Western blot and real-time RT-PCR, respectively. OLETF rats exhibit increased levels of FPG, FINS, C-P, and homeostasis model assessment-estimated insulin resistance (HOMA-IR), which were effectively decreased by acupuncture treatment. mRNA expressions of several insulin signaling related molecules IRS1, IRS2, Akt2, aPKCζ, and GLUT4 were decreased in OLETF rats compared to SD controls. Expression of these molecules was restored back to normal levels upon acupuncture administration. PI3K-p85α was increased in OLETF rats; this increase was also reversed by acupuncture treatment. Acupuncture improves insulin resistance in OLETF rats, possibly via regulating expression of key insulin signaling related molecules.

Targeting surface voids to counter membrane disorders in lipointoxication-related diseases

Saturated fatty acids (SFA), which are abundant in the so-called western diet, have been shown to efficiently incorporate within membrane phospholipids and therefore impact on organelle integrity and function in many cell types. In the present study, we have developed a yeast-based two-step assay and a virtual screening strategy to identify new drugs able to counter SFA-mediated lipointoxication. The compounds identified here were effective in relieving lipointoxication in mammalian β-cells, one of the main targets of SFA toxicity in humans. In vitro reconstitutions and molecular dynamics simulations on bilayers revealed that these molecules, albeit according to different mechanisms, can generate voids at the membrane surface. The resulting surface defects correlate with the recruitment of loose lipid packing or void-sensing proteins required for vesicular budding, a central cellular process that is precluded under SFA accumulation. Taken together, the results presented here point at modulation of surface voids as a central parameter to consider in order to counter the impacts of SFA on cell function.

The effect of palmitate supplementation on gene expression profile in proliferating myoblasts

High-fat diet, exposure to saturated fatty acids, or the presence of adipocytes in myoblast microenvironment affects skeletal muscle growth and function. The aim of the present study was to investigate the effect of palmitate supplementation on transcriptomic profile of mouse C2C12 myoblasts. Global gene expression was evaluated using whole mouse genome oligonucleotide microarrays, and the results were validated through qPCR. A total of 4047 genes were identified as differentially expressed, including 3492 downregulated and 555 upregulated genes, during a 48-h exposure to palmitate (0.1 mmol/l). Functional classification showed the involvement of these genes in several processes which regulate cell growth. In conclusion, the addition of palmitate modifies the expression of genes associated with (1) myoblast responsiveness to hormones and growth factors, (2) cytokine and growth factor expression, and (3) regulation of cell-cell and cell-matrix communication. Such alterations can affect myoblast growth and differentiation; however, further studies in this field are required.

N-3PUFA differentially modulate palmitate-induced lipotoxicity through alterations of its metabolism in C2C12 muscle cells

Excessive energy intake leads to fat overload and the formation of lipotoxic compounds mainly derived from the saturated fatty acid palmitate (PAL), thus promoting insulin resistance (IR) in skeletal muscle. N-3 polyunsaturated fatty acids (n-3PUFA) may prevent lipotoxicity and IR. The purpose of this study was to examine the differential effects of n-3PUFA on fatty acid metabolism and insulin sensitivity in muscle cells. C2C12 myotubes were treated with 500 μM of PAL without or with 50 μM of alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) for 16 h. PAL decreased insulin-dependent AKT activation and glucose uptake and increased the synthesis of ceramides and diglycerides (DG) derivatives, leading to protein kinase Cθ activation. EPA and DHA, but not ALA, prevented PAL-decreased AKT activation but glucose uptake was restored to control values by all n-3PUFA vs. PAL. Total DG and ceramide contents were decreased by all n-3PUFA, but only EPA and DHA increased PAL β-oxidation, decreased PAL incorporation into DG and reduced protein kinase Cθ activation. EPA and DHA emerge as better candidates than ALA to improve fatty acid metabolism in skeletal muscle cells, notably via their ability to increase mitochondrial β-oxidation.

Palmitate exerts opposite effects on proliferation and differentiation of skeletal myoblasts

The purpose of the study was to examine mechanisms controlling cell cycle progression/arrest and differentiation of mouse C2C12 myoblasts exposed to long-chain saturated fatty acid salt, palmitate. Treatment of proliferating myoblasts with palmitate (0.1 mmol/l) markedly decreased myoblast number. Cyclin A and cyclin D1 levels decreased, whereas total p21 and p21 complexed with cyclin-dependent kinase-4 (cdk4) increased in myoblasts treated with palmitate. In cells induced to differentiation addition of palmitate augmented the level of cyclin D3, the early (myogenin) and late (α-actinin, myosin heavy chain) markers of myogenesis, and caused an increase of myotube diameter. In conclusion, exposure to palmitate inhibits proliferation of myoblasts through a decrease in cyclin A and cyclin D1 levels and an increase of p21-cdk4 complex formation; however, it promotes cell cycle exit, myogenic differentiation and myotube growth.

Suppression of phosphatase and tensin homolog protects insulin-resistant cells from apoptosis

In the present study, a glucosamine-induced model of insulin-resistant skeletal muscle cells was established in order to investigate the effect of inhibition of phosphatase and tensin homolog (PTEN)/5'-adenosine monophosphate-activated protein kinase (AMPK) on these cells. The glucosamine-induced insulin-resistant skeletal muscle cells were produced and the rate of glucose uptake was measured using the glucose oxidase-peroxidase method. The expression levels of PTEN and phosphorylated PTEN (p-PTEN) were assessed using western blotting. Glucose transporter 4 (GLUT4) translocation was detected by immunofluorescence. Cell apoptosis was evaluated using flow cytometry. Following insulin stimulation, the rate of glucose uptake was significantly reduced in the cells with glucosamine-induced insulin-resistance in comparison with those in the control group. The expression and translocation of GLUT4 were reduced in the insulin-resistant muscle cells. By contrast, the expression of PTEN and p-PTEN as well as apoptosis were significantly increased. Following treatment with bisperoxopicolinatooxovanadate (BPV) or metformin in the insulin-resistant skeletal muscle cells, there was an increase in the rate of glucose uptake, an increase in GLUT4 expression and its translocation, a reduction in the expression of PTEN and p-PTEN, and a decrease in cell apoptosis compared with untreated insulin-resistant cells. Glucosamine may be used to produce an effective model of insulin-resistant skeletal muscle cells. Cells with glucosamine-induced insulin-resistance exhibited a reduced expression of GLUT4 and dysfunction in GLUT4 translocation, as well as increased activation of PTEN and increased cell apoptosis. Inhibition of PTEN or its upstream regulator, AMPK, protects glucosamine-induced insulin-resistant skeletal muscle cells from apoptosis.

The neuroenergetics of stress hormones in the hippocampus and implications for memory

Acute stress causes rapid release of norepinephrine (NE) and glucocorticoids (GCs), both of which bind to hippocampal receptors. This release continues, at varying concentrations, for several hours following the stressful event, and has powerful effects on hippocampally-dependent memory that generally promote acquisition and consolidation while impairing retrieval. Several studies have characterized the brain's energy usage both at baseline and during memory processing, but there are few data on energy requirements of memory processes under stressful conditions. Because memory is enhanced by emotional arousal such as during stress, it is likely that molecular memory processes under these conditions differ from those under non-stressful conditions that do not activate the hypothalamic-pituitary-adrenal (HPA) axis. Mobilization of peripheral and central energy stores during stress may increase hippocampal glucose metabolism that enhances salience and detail to facilitate memory enhancement. Several pathways activated by the HPA axis affect neural energy supply and metabolism, and may also prevent detrimental damage associated with chronic stress. We hypothesize that alterations in hippocampal metabolism during stress are key to understanding the effects of stress hormones on hippocampally-dependent memory formation. Second, we suggest that the effects of stress on hippocampal metabolism are bi-directional: within minutes, NE promotes glucose metabolism, while hours into the stress response GCs act to suppress metabolism. These bi-directional effects of NE and GCs on glucose metabolism may occur at least in part through direct modulation of glucose transporter-4. In contrast, chronic stress and prolonged elevation of hippocampal GCs cause chronically suppressed glucose metabolism, excitotoxicity and subsequent memory deficits.

Amelioration of palmitate-induced metabolic dysfunction in L6 muscle cells expressing low levels of receptor-interacting protein 140

We have shown that reduced expression of receptor-interacting protein 140 (RIP140) alters the regulation of fatty-acid (FA) oxidation in muscle. To determine whether a high level of FA availability alters the effects of RIP140 on metabolic regulation, L6 myotubes were transfected with or without RNA interference oligonucleotide sequences to reduce RIP140 expression, and then incubated with high levels of palmitic acid, with or without insulin. High levels of palmitate reduced basal (53%-58%) and insulin-treated (24%-44%) FA uptake and oxidation, and increased basal glucose uptake (88%). In cells incubated with high levels of palmitate, low RIP140 increased basal FA uptake and insulin-treated FA oxidation and glucose uptake, and decreased basal glucose uptake and insulin-treated FA uptake. Under basal conditions, low RIP140 increased the mRNA content of FAT/CD36 (159%) and COX4 (61%), as well as the protein content of Nur77 (68%), whereas the mRNA expression of FGF21 (50%) was decreased, as was the protein content of CPT1b (35%) and FGF21 (44%). Under insulin-treated conditions, low RIP140 expression increased the mRNA content of MCAD (84%) and Nur77 (84%), as well as the protein content of Nur77 (23%). Thus, a low level of RIP140 restores the rates of FA uptake in the basal state, in part via a reduction in upstream insulin signaling. Our data also indicate that the protein expression of Nur77 may be modulated by RIP140 when muscle cells are metabolically challenged by high levels of palmitate.

PALMITIC AND OLEIC ACIDS AND THEIR ROLE IN PATHOGENESIS OF ATHEROSCLEROSIS

On the basis of phylogenetic theory of general pathology, the cause of a noninfectious disease whose occurrence in a population is more than 5–7% is an impaired biological function or reaction to the environment. From the general biology viewpoint, high mortality rate related to cardio-vascular diseases and atherosclerosis (intercellular deficiency of polyenic fatty acids (PFA)) is just extinction of the Homo sapiens population upon adaptation to new environmental factors. The biological function of throphology (feeding) and biological reaction of exotrophy (external feeding) are impaired in several aspects, the major of which is nonphysiologically high dietary content of saturated fatty acids, primarily, of palmitic fatty acid (FA). The lipoprotein system formed at early stages of phylogenesis cannot transport and provide physiological deposition of great amounts of palmitic FA, which leads to the development of an adaption (compensatory) and accumulation disease. This results in hypermipidemia, impaired bioavailability of PFA to cells, compesatory production of humoral mediators from ω-9 eicosatrienoic mead FA, disorders in physiological parameters of cell plasma membrane and integral proteins, nonphysiological conformation of apoВ-100 in lipoproteins, formation of ligandless lipoproteins (biological litter) and impairments in the biological function of endoecology, utilization of ligandless lipoproteins in arterial intima by phylogenetically early macrophages that do not hydrolyze polyenic cholesterol esters, increase in the intensity of the biological reaction of inflammation, and destructive and inflammatory lesions in arterial intima of an atheromatosis or atherothrombosis type. Atheromatous masses are catabolites of PFA which were not internalized by phylogenetically late cells via receptor-mediated pathway.

TBC1D1 reduces palmitate oxidation by inhibiting β-HAD activity in skeletal muscle

In skeletal muscle the Rab-GTPase-activating protein TBC1D1 has been implicated in the regulation of fatty acid oxidation by an unknown mechanism. We determined whether TBC1D1 altered fatty acid utilization via changes in protein-mediated fatty acid transport and/or selected enzymes regulating mitochondrial fatty acid oxidation. We also determined the effects of TBC1D1 on glucose transport and oxidation. Electrotransfection of mouse soleus muscles with TBC1D1 cDNA increased TBC1D1 protein after 2 wk (P<0.05), without altering its paralog AS160. TBC1D1 overexpression decreased basal palmitate oxidation (-22%) while blunting 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR)-stimulated palmitate oxidation (-18%). There was a tendency to increase fatty acid esterification (+10 nmol·g(-1)·60 min(-1), P=0.07), which reflected the reduction in fatty acid oxidation (-12 nmol·g(-1)·60 min(-1)). Concomitantly, basal (+21%) and AICAR-stimulated glucose oxidation (+8%) were increased in TBC1D1-transfected muscles relative to their respective controls (P<0.05), independent of changes in GLUT4 and glucose transport. The reductions in TBC1D1-mediated fatty acid oxidation could not be attributed to changes in the transporter FAT/CD36, muscle mitochondrial content, CPT1 expression or the expression and phosphorylation of AS160, acetyl-CoA carboxylase, or AMPK. However, TBC1D1 overexpression reduced β-HAD enzyme activity (-18%, P<0.05). In conclusion, TBC1D1-mediated reduction of muscle fatty acid oxidation appears to occur via inhibition of β-HAD activity.

Conditioning increases the gain of contraction-induced sarcolemmal substrate transport in ultra-endurance racing sled dogs

Endurance exercise relies on transsarcolemmal flux of substrates in order to avoid depletion of intramuscular reserves. Previous studies of endurance trained sled dogs have shown a remarkable capacity of these dogs to adapt rapidly to endurance exercise by decreasing the utilization of intramuscular reserves. The current study tested the hypothesis that the dogs' glycogen-sparing phenotype is due to increased sarcolemmal transport of glucose and fatty acids. Basal and exercise-induced transport of glucose and fatty acids into sarcolemmal vesicles was evaluated in racing sled dogs prior to and after 7 months of exercise conditioning. Sarcolemmal substrate transport capacity was measured using sarcolemmal vesicles and radiolabelled substrates, and transporter abundance was measured using Western blot quantification in whole muscle homogenates and the sarcolemmal vesicle preparations. Conditioning resulted in increased basal and exercise-induced transport of both glucose and palmitate. Neither acute exercise nor conditioning resulted in changes in muscle content of GLUT4 or FAT/CD36, but conditioning did result in decreased abundance of both transporters in the sarcolemmal vesicles used for the basal transport assays, and this decrease was further amplified in the vesicles used for the exercise-induced transport assays. These results demonstrate conditioning-induced increases in sarcolemmal transport of oxidizable substrates, as well as increased gain of exercise-induced sarcolemmal transport of these substrates. These results further indicate that increased sarcolemmal transport of oxidizable substrates may be due to either an increased intrinsic capacity of the existing transporters or to a different population of transporters from those investigated.

Chronic muscle stimulation improves insulin sensitivity while increasing subcellular lipid droplets and reducing selected diacylglycerol and ceramide species in obese Zucker rats

AIMS/HYPOTHESIS

Although insulin resistance has been associated with accumulations of specific intramuscular fatty acids and altered subcellular localisation of lipid droplets, these concepts remain controversial. Therefore, we aimed to identify specific intramuscular fatty acids and subcellular lipid localisations associated with improved insulin sensitivity following chronic muscle contraction.

METHODS

In lean and insulin-resistant obese Zucker rats the tibialis anterior muscle was stimulated (6 h/day for 6 days). Thereafter, muscles were examined for insulin sensitivity, intramuscular lipid droplet localisation and triacylglycerol (TAG), diacylglycerol (DAG) and ceramide fatty acid composition.

RESULTS

In lean and obese animals, regardless of muscle type, chronic muscle contraction improved muscle insulin sensitivity and increased intramuscular levels of total and most C14-C22 TAG fatty acids (p < 0.05). Therefore, accumulation in subcellular lipid droplet compartments reflected the oversupply of lipids within muscle. In contrast, improvements in insulin sensitivity induced by muscle contraction were associated with reductions in specific DAG and ceramide species that were not uniform in red and white muscle of obese rats. However, these reductions were insufficient to fully normalise insulin sensitivity, indicating that other mechanisms are involved.

CONCLUSIONS/INTERPRETATION

Reductions in 18 C length DAG and ceramide species were the most consistent in red and white muscle and therefore may represent therapeutic targets for improving insulin sensitivity.

Impact of insulin deprivation and treatment on sphingolipid distribution in different muscle subcellular compartments of streptozotocin-diabetic C57Bl/6 mice

Insulin deprivation in type 1 diabetes (T1D) individuals increases lipolysis and plasma free fatty acids (FFA) concentration, which can stimulate synthesis of intramyocellular bioactive lipids such as ceramides (Cer) and long-chain fatty acid-CoAs (LCFa-CoAs). Ceramide was shown to decrease muscle insulin sensitivity, and at mitochondrial levels it stimulates reactive oxygen species production. Here, we show that insulin deprivation in streptozotocin diabetic C57BL/6 mice increases quadriceps muscle Cer content, which was correlated with a concomitant decrease in the body fat and increased plasma FFA, glycosylated hemoglobin level (%Hb A1c), and muscular LCFa-CoA content. The alternations were accompanied by an increase in protein expression in LCFa-CoA and Cer synthesis (FATP1/ACSVL5, CerS1, CerS5), a decrease in the expression of genes implicated in muscle insulin sensitivity (GLUT4, GYS1), and inhibition of insulin signaling cascade by Aktα and GYS3β phosphorylation under acute insulin stimulation. Both the content and composition of sarcoplasmic fraction sphingolipids were most affected by insulin deprivation, whereas mitochondrial fraction sphingolipids remained stable. The observed effects of insulin deprivation were reversed, except for content and composition of LCFa-CoA, CerS protein expression, GYS1 gene expression, and phosphorylation status of Akt and GYS3β when exogenous insulin was provided by subcutaneous insulin implants. Principal component analysis and Pearson's correlation analysis revealed close relationships between the features of the diabetic phenotype, the content of LCFa-CoAs and Cers containing C18-fatty acids in sarcoplasm, but not in mitochondria. Insulin replacement did not completely rescue the phenotype, especially regarding the content of LCFa-CoA, or proteins implicated in Cer synthesis and muscle insulin sensitivity. These persistent changes might contribute to muscle insulin resistance observed in T1D individuals.

Fatty acid metabolism, energy expenditure and insulin resistance in muscle

Fatty acids (FAs) are essential elements of all cells and have significant roles as energy substrates, components of cellular structure and signalling molecules. The storage of excess energy intake as fat in adipose tissue is an evolutionary advantage aimed at protecting against starvation, but in much of today's world, humans are faced with an unlimited availability of food, and the excessive accumulation of fat is now a major risk for human health, especially the development of type 2 diabetes (T2D). Since the first recognition of the association between fat accumulation, reduced insulin action and increased risk of T2D, several mechanisms have been proposed to link excess FA availability to reduced insulin action, with some of them being competing or contradictory. This review summarises the evidence for these mechanisms in the context of excess dietary FAs generating insulin resistance in muscle, the major tissue involved in insulin-stimulated disposal of blood glucose. It also outlines potential problems with models and measurements that may hinder as well as help improve our understanding of the links between FAs and insulin action.

Modulation of visceral fat adipokine secretion by dietary fatty acids and ensuing changes in skeletal muscle inflammation

Given the link between obesity and insulin resistance, the role of adipose-derived factors in communicating with skeletal muscle to affect its function is important. We sought to determine if high fat diets modulate visceral adipose tissue (VAT) adipokines with subsequent effects on skeletal muscle inflammation and insulin sensitivity. Rats were fed (i) low fat (LF), (ii) high saturated fatty acid (SFA), or (iii) high SFA with n-3 polyunsaturated fatty acid (SFA/n-3 PUFA) diets for 4 weeks. VAT-derived adipokines were measured in adipose conditioned medium (ACM) after 72 h. Next, skeletal muscles from LF-fed rats were incubated for 8 h in (i) control buffer (CON), (ii) CON with 2 mmol·L(-1) palmitate (PALM, positive control), (iii) ACM from LF, (iv) ACM from SFA, or (v) ACM from SFA/n-3 PUFA. ACM from rats fed SFA and SFA/n-3 PUFA had increased (P ≤ 0.05) interleukin-6 (IL-6) (+31%) and monocyte chemoattractant protein-1 (MCP-1) (+30%). Adiponectin was decreased (-29%, P ≤ 0.05) in ACM from SFA, and this was prevented in SFA/n-3 PUFA ACM. Toll-like receptor 4 (TLR4) gene expression was increased (P ≤ 0.05) in PALM soleus muscle (+356%) and all ACM groups (+175%-191%). MCP-1 gene expression was elevated (P ≤ 0.05) in PALM soleus muscle (+163%) and soleus muscle incubated in ACM from animals fed SFA (+159%) and SFA/n-3 PUFA (+151%). Glucose transport was impaired (P ≤ 0.05) in PALM muscles but preserved in ACM groups. Acute exposure of muscle to fatty acid modulated adipokines affects skeletal muscle inflammatory gene expression but not insulin sensitivity.

CD36 inhibition prevents lipid accumulation and contractile dysfunction in rat cardiomyocytes

An increased cardiac fatty acid supply and increased sarcolemmal presence of the long-chain fatty acid transporter CD36 are associated with and contribute to impaired cardiac insulin sensitivity and function. In the present study we aimed at preventing the development of insulin resistance and contractile dysfunction in cardiomyocytes by blocking CD36-mediated palmitate uptake. Insulin resistance and contractile dysfunction were induced in primary cardiomyocytes by 48 h incubation in media containing either 100 nM insulin (high insulin; HI) or 200 μM palmitate (high palmitate; HP). Under both culture conditions, insulin-stimulated glucose uptake and Akt phosphorylation were abrogated or markedly reduced. Furthermore, cardiomyocytes cultured in each medium displayed elevated sarcolemmal CD36 content, increased basal palmitate uptake, lipid accumulation and decreased sarcomere shortening. Immunochemical CD36 inhibition enhanced basal glucose uptake and prevented elevated basal palmitate uptake, triacylglycerol accumulation and contractile dysfunction in cardiomyocytes cultured in either medium. Additionally, CD36 inhibition prevented loss of insulin signalling in cells cultured in HP, but not in HI medium. In conclusion, CD36 inhibition prevents lipid accumulation and lipid-induced contractile dysfunction in cardiomyocytes, but probably independently of effects on insulin signalling. Nonetheless, pharmacological CD36 inhibition may be considered as a treatment strategy to counteract impaired functioning of the lipid-loaded heart.

Overexpression of AMP-activated protein kinase or protein kinase D prevents lipid-induced insulin resistance in cardiomyocytes

During lipid oversupply, the heart becomes insulin resistant, as exemplified by defective insulin-stimulated glucose uptake, and will develop diastolic dysfunction. In the healthy heart, not only insulin, but also increased contractile activity stimulates glucose uptake. Upon increased contraction both AMP-activated protein kinase (AMPK) and protein kinase D (PKD) are activated, and mediate the stimulation of glucose uptake into cardiomyocytes. Therefore, each of these kinases is a potential therapeutic target in the diabetic heart because they may serve to bypass defective insulin-stimulated glucose uptake. To test the preventive potential of these kinases against loss of insulin-stimulated glucose uptake, AMPK or PKD were adenovirally overexpressed in primary cultures of insulin resistant cardiomyocytes for assaying substrate uptake, insulin responsiveness and lipid accumulation. To induce insulin resistance and lipid loading, rat primary cardiomyocytes were cultured in the presence of high insulin (100 nM; HI) or high palmitate (palmitate/BSA: 3/1; HP). HI and HP each reduced insulin responsiveness, and increased basal palmitate uptake and lipid storage. Overexpression of each of the kinases prevented loss of insulin-stimulated glucose uptake. Overexpression of AMPK also prevented loss of insulin signaling in HI- and HP-cultured cardiomyocytes, but did not prevent lipid accumulation. In contrast, overexpression of PKD prevented lipid accumulation, but not loss of insulin signaling in HI- and HP-cultured cardiomyocytes. In conclusion, AMPK and PKD prevent loss of insulin-stimulated glucose uptake into cardiomyocytes cultured under insulin resistance-inducing conditions through different mechanisms. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Role of PRAS40 in Akt and mTOR signaling in health and disease

The proline-rich Akt substrate of 40 kDa (PRAS40) acts at the intersection of the Akt- and mammalian target of rapamycin (mTOR)-mediated signaling pathways. The protein kinase mTOR is the catalytic subunit of two distinct signaling complexes, mTOR complex 1 (mTORC1) and mTORC2, that link energy and nutrients to the regulation of cellular growth and energy metabolism. Activation of mTOR in response to nutrients and growth factors results in the phosphorylation of numerous substrates, including the phosphorylations of S6 kinase by mTORC1 and Akt by mTORC2. Alterations in Akt and mTOR activity have been linked to the progression of multiple diseases such as cancer and type 2 diabetes. Although PRAS40 was first reported as substrate for Akt, investigations toward mTOR-binding partners subsequently identified PRAS40 as both component and substrate of mTORC1. Phosphorylation of PRAS40 by Akt and by mTORC1 itself results in dissociation of PRAS40 from mTORC1 and may relieve an inhibitory constraint on mTORC1 activity. Adding to the complexity is that gene silencing studies indicate that PRAS40 is also necessary for the activity of the mTORC1 complex. This review summarizes the regulation and potential function(s) of PRAS40 in the complex Akt- and mTOR-signaling network in health and disease.

In vivo, fatty acid translocase (CD36) critically regulates skeletal muscle fuel selection, exercise performance, and training-induced adaptation of fatty acid oxidation

For ~40 years it has been widely accepted that (i) the exercise-induced increase in muscle fatty acid oxidation (FAO) is dependent on the increased delivery of circulating fatty acids, and (ii) exercise training-induced FAO up-regulation is largely attributable to muscle mitochondrial biogenesis. These long standing concepts were developed prior to the recent recognition that fatty acid entry into muscle occurs via a regulatable sarcolemmal CD36-mediated mechanism. We examined the role of CD36 in muscle fuel selection under basal conditions, during a metabolic challenge (exercise), and after exercise training. We also investigated whether CD36 overexpression, independent of mitochondrial changes, mimicked exercise training-induced FAO up-regulation. Under basal conditions CD36-KO versus WT mice displayed reduced fatty acid transport (-21%) and oxidation (-25%), intramuscular lipids (less than or equal to -31%), and hepatic glycogen (-20%); but muscle glycogen, VO(2max), and mitochondrial content and enzymes did not differ. In acutely exercised (78% VO(2max)) CD36-KO mice, fatty acid transport (-41%), oxidation (-37%), and exercise duration (-44%) were reduced, whereas muscle and hepatic glycogen depletions were accelerated by 27-55%, revealing 2-fold greater carbohydrate use. Exercise training increased mtDNA and β-hydroxyacyl-CoA dehydrogenase similarly in WT and CD36-KO muscles, but FAO was increased only in WT muscle (+90%). Comparable CD36 increases, induced by exercise training (+44%) or by CD36 overexpression (+41%), increased FAO similarly (84-90%), either when mitochondrial biogenesis and FAO enzymes were up-regulated (exercise training) or when these were unaltered (CD36 overexpression). Thus, sarcolemmal CD36 has a key role in muscle fuel selection, exercise performance, and training-induced muscle FAO adaptation, challenging long held views of mechanisms involved in acute and adaptive regulation of muscle FAO.

Caffeine-stimulated fatty acid oxidation is blunted in CD36 null mice

AIM

The increase in skeletal muscle fatty acid metabolism during exercise has been associated with the release of calcium. We examined whether this increase in fatty acid oxidation was attributable to a calcium-induced translocation of the fatty acid transporter CD36 to the sarcolemma, thereby providing an enhanced influx of fatty acids to increase their oxidation.

METHODS

Calcium release was triggered by caffeine (3 mm) to examine fatty acid oxidation in intact soleus muscles of WT and CD36-KO mice, while fatty acid transport and mitochondrial fatty acid oxidation were examined in giant vesicles and isolated mitochondria, respectively, from caffeine-perfused hindlimb muscles of WT and CD36-KO mice. Western blotting was used to examine calcium-induced signalling.

RESULTS

In WT, caffeine stimulated muscle palmitate oxidation (+136%), but this was blunted in CD36-KO mice (-70%). Dantrolene inhibited (WT) or abolished (CD36-KO) caffeine-induced palmitate oxidation. In muscle, caffeine-stimulated palmitate oxidation was not attributable to altered mitochondrial palmitate oxidation. Instead, in WT, caffeine increased palmitate transport (+55%) and the translocation of fatty acid transporters CD36, FABPpm, FATP1 and FATP4 (26-70%) to the sarcolemma. In CD36-KO mice, caffeine-stimulated FABPpm, and FATP1 and 4 translocations were normal, but palmitate transport was blunted (-70%), comparable to the reductions in muscle palmitate oxidation. Caffeine did not alter the calcium-/calmodulin-dependent protein kinase II phosphorylation but did increase the phosphorylation of AMPK and acetyl-CoA carboxylase comparably in WT and CD36-KO.

CONCLUSION

These studies indicate that sarcolemmal CD36-mediated fatty acid transport is a primary mediator of the calcium-induced increase in muscle fatty acid oxidation.

Differential regulation of stimulated glucose transport by free fatty acids and PPARα or -δ agonists in cardiac myocytes

Stimulation of glucose transport in response to insulin or metabolic stress is an important determinant of cardiac myocyte function and survival, particularly during ischemia-reperfusion episodes. The impact of dyslipidemia and its consequence PPAR activation on stimulated glucose transport in cardiac myocytes remains unknown. Isolated adult rat cardiac myocytes were chronically exposed to free fatty acids (FFA) or PPAR agonists. Insulin- (ISGT) and oligomycin-stimulated glucose transport (OSGT) and related cell signaling were analyzed. Exposure of cardiac myocytes to FFA reduced both ISGT and OSGT. Exposure to either PPARα or PPARδ agonists, but not to a PPARγ agonist, reduced ISGT but not OSGT and increased fatty acid oxidation (FAO). The reduction in ISGT was associated with impaired insulin signaling and, in the case of PPAR stimulation, overexpression of SOCS-3, a protein known to hinder proximal insulin signaling. In contrast, the reduction of OSGT could not be explained by a reduced activity of the cellular energy-sensing system, as assessed from the maintained phosphorylation state of AMPK. Inhibition of FAO at the level of mitochondrial acylcarnitine uptake restored OSGT but not ISGT. Seemingly paradoxically, further stimulation of FAO with PPARα or PPARδ agonists also restored OSGT but not ISGT. Together, these results suggest that inhibition of OSGT occurs downstream of energy gauging and is caused by some intermediate(s) of fatty acid oxidation, which does not appear to be acylcarnitines. The results indicate that the mechanisms underlying FFA-mediated inhibition of ISGT and OSGT differ remarkably.

Muscle-specific knock-out of NUAK family SNF1-like kinase 1 (NUAK1) prevents high fat diet-induced glucose intolerance

NUAK1 is a member of the AMP-activated protein kinase-related kinase family. Recent studies have shown that NUAK1 is involved in cellular senescence and motility in epithelial cells and fibroblasts. However, the physiological roles of NUAK1 are poorly understood because of embryonic lethality in NUAK1 null mice. The purpose of this study was to elucidate the roles of NUAK1 in adult tissues. We determined the tissue distribution of NUAK1 and generated muscle-specific NUAK1 knock-out (MNUAK1KO) mice. For phenotypic analysis, whole body glucose homeostasis and muscle glucose metabolism were examined. Quantitative phosphoproteome analysis of soleus muscle was performed to understand the molecular mechanisms underlying the knock-out phenotype. Nuak1 mRNA was preferentially expressed in highly oxidative tissues such as brain, heart, and soleus muscle. On a high fat diet, MNUAK1KO mice had a lower fasting blood glucose level, greater glucose tolerance, higher insulin sensitivity, and higher concentration of muscle glycogen than control mice. Phosphoproteome analysis revealed that phosphorylation of IRS1 Ser-1097 was markedly decreased in NUAK1-deficient muscle. Consistent with this, insulin signaling was enhanced in the soleus muscle of MNUAK1KO mice, as evidenced by increased phosphorylation of IRS1 Tyr-608, AKT Thr-308, and TBC1D4 Thr-649. These observations suggest that a physiological role of NUAK1 is to suppress glucose uptake through negative regulation of insulin signaling in oxidative muscle.

Subcellular lipid droplet distribution in red and white muscles in the obese Zucker rat

AIMS/HYPOTHESIS

Little is known about the subcellular distribution of lipids in insulin-resistant skeletal muscle. However, it has recently been suggested that lipid accumulation in the subsarcolemmal region directly contributes to insulin resistance. Therefore we hypothesised that regional differences in lipid distribution in insulin-resistant muscle may be mediated by: (1) a reduction in fatty acid trafficking into mitochondria; and/or (2) a regional increase in the enzymes regulating lipid synthesis.

METHODS

Transmission electron microscopy was used to quantify lipid droplet and mitochondrial abundance in the subsarcolemmal and intermyofibrillar compartments in red and white muscles from lean and obese Zucker rats. To estimate rates of lipid trafficking into mitochondria, the metabolic fate of radiolabelled palmitate was determined. Key enzymes of triacylglycerol synthesis were also determined in each subcellular region.

RESULTS

Subsarcolemmal-compartmentalised lipids represented a small absolute fraction of the overall lipid content in muscle, as regardless of fibre composition (red/white) or phenotype (lean/obese), lipid droplets were more prevalent in the intermyofibrillar region, whereas insulin-resistant white muscles were devoid of subsarcolemmal-compartmentalised lipid droplets. While, in obese animals, lipid droplets accumulated in both subcellular regions, in red muscle of these animals lipids only appeared to be trafficked away from intermyofibrillar mitochondria, a process that cannot be explained by regional differences in the abundance of triacylglycerol esterification enzymes.

CONCLUSIONS/INTERPRETATION

Lipid accumulation in the subsarcolemmal region is not necessary for insulin resistance. In the intermyofibrillar compartment, the diversion of lipids away from mitochondria in insulin-resistant animals probably contributes to lipid accumulation in this subcellular area.

Palmitate-activated macrophages confer insulin resistance to muscle cells by a mechanism involving protein kinase C θ and ε

Background

Macrophage-derived factors contribute to whole-body insulin resistance, partly by impinging on metabolically active tissues. As proof of principle for this interaction, conditioned medium from macrophages treated with palmitate (CM-PA) reduces insulin action and glucose uptake in muscle cells. However, the mechanism whereby CM-PA confers this negative response onto muscle cells remains unknown.

Methodology/Principal Findings

L6-GLUT4myc myoblasts were exposed for 24 h to palmitate-free conditioned medium from RAW 264.7 macrophages pre-treated with 0.5 mM palmitate for 6 h. This palmitate-free CM-PA, containing selective cytokines and chemokines, inhibited myoblast insulin-stimulated insulin receptor substrate 1 (IRS1) tyrosine phosphorylation, AS160 phosphorylation, GLUT4 translocation and glucose uptake. These effects were accompanied by a rise in c-Jun N-terminal kinase (JNK) activation, degradation of Inhibitor of κBα (IκBα), and elevated expression of proinflammatory cytokines in myoblasts. Notably, CM-PA caused IRS1 phosphorylation on Ser1101, and phosphorylation of novel PKCθ and ε. Co-incubation of myoblasts with CM-PA and the novel and conventional PKC inhibitor Gö6983 (but not with the conventional PKC inhibitor Gö6976) prevented PKCθ and ε activation, JNK phosphorylation, restored IκBα mass and reduced proinflammatory cytokine production. Gö6983 also restored insulin signalling and glucose uptake in myoblasts. Moreover, co-silencing both novel PKC θ and ε isoforms in myoblasts by RNA interference, but not their individual silencing, prevented the inflammatory response and restored insulin sensitivity to CM-PA-treated myoblasts.

Conclusions/Clinical Significance

The results suggest that the block in muscle insulin action caused by CM-PA is mediated by novel PKCθ and PKCε. This study re-establishes the participation of macrophages as a relay in the action of fatty acids on muscle cells, and further identifies PKCθ and PKCε as key elements in the inflammatory and insulin resistance responses of muscle cells to macrophage products. Furthermore, it portrays these PKC isoforms as potential targets for the treatment of fatty acid-induced, inflammation-linked insulin resistance.

Supplementation of pyruvate prevents palmitate-induced impairment of glucose uptake in C2 myotubes

Elevated fatty acid levels have been thought to contribute to insulin resistance. Repression of the glucose transporter 4 (GLUT4) gene as well as impaired GLUT4 translocation may be a mediator for fatty acid-induced insulin resistance. This study was initiated to determine whether palmitate treatment repressed GLUT4 expression, whether glucose/fatty acid metabolism influenced palmitate-induced GLUT4 gene repression (PIGR), and whether attempts to prevent PIGR restored palmitate-induced impairment of glucose uptake (PIIGU) in C2 myotubes. Not only stimulators of fatty acid oxidation, such as bezafibrate, AICAR, and TOFA, but also TCA cycle substrates, such as pyruvate, leucine/glutamine, and α-ketoisocaproate/monomethyl succinate, significantly prevented PIGR. In particular, supplementing with pyruvate through methyl pyruvate resulted in nearly complete prevention of PIIGU, whereas palmitate treatment reduced the intracellular pyruvate level. These results suggest that pyruvate depletion plays a critical role in PIGR and PIIGU; thus, pyruvate supplementation may help prevent obesity-induced insulin resistance in muscle cells.

Absence of fatty acid transporter CD36 protects against Western-type diet-related cardiac dysfunction following pressure overload in mice

Cardiac patients often are obese and have hypertension, but in most studies these conditions are investigated separately. Here, we aimed at 1) elucidating the interaction of metabolic and mechanophysical stress in the development of cardiac dysfunction in mice and 2) preventing this interaction by ablation of the fatty acid transporter CD36. Male wild-type (WT) C57Bl/6 mice and CD36(-/-) mice received chow or Western-type diet (WTD) for 10 wk and then underwent a sham surgery or transverse aortic constriction (TAC) under anesthesia. After a 6-wk continuation of the diet, cardiac function, morphology, lipid profiles, and molecular parameters were assessed. WTD administration affected body and organ weights of WT and CD36(-/-) mice, but it affected only plasma glucose and insulin concentrations in WT mice. Cardiac lipid concentrations increased in WT mice receiving WTD, decreased in CD36(-/-) on chow, and remained unchanged in CD36(-/-) receiving WTD. TAC induced cardiac hypertrophy in WT mice on chow but did not affect cardiac function and cardiac lipid concentrations. WTD or CD36 ablation worsened the outcome of TAC. Ablation of CD36 protected against the WTD-related aggravation of cardiac functional and structural changes induced by TAC. In conclusion, cardiac dysfunction and remodeling worsen when the heart is exposed to two stresses, metabolic and mechanophysical, at the same time. CD36 ablation prevents the metabolic stress resulting from a WTD. Thus, metabolic conditions are a critical factor for the compromised heart and provide new targets for metabolic manipulation in cardioprotection.

Palmitate alters the rhythmic expression of molecular clock genes and orexigenic neuropeptide Y mRNA levels within immortalized, hypothalamic neurons

The control of energy homeostasis within the hypothalamus is under the regulated control of homeostatic hormones, nutrients and the expression of neuropeptides that alter feeding behavior. Elevated levels of palmitate, a predominant saturated fatty acid in diet and fatty acid biosynthesis, alter cellular function. For instance, a key mechanism involved in the development of insulin resistance is lipotoxicity, through increased circulating saturated fatty acids. Although many studies have begun to determine the underlying mechanisms of lipotoxicity in peripheral tissues, little is known about the effects of excess lipids in the brain. To determine these mechanisms we used an immortalized, clonal, hypothalamic cell line, mHypoE-44, to demonstrate that palmitate directly alters the expression of molecular clock components, by increasing Bmal1 and Clock, or by decreasing Per2, and Rev-erbα, their mRNA levels and altering their rhythmic period within individual neurons. We found that these neurons endogenously express the orexigenic neuropeptides NPY and AgRP, thus we determined that palmitate administration alters the mRNA expression of these neuropeptides as well. Palmitate treatment causes a significant increase in NPY mRNA levels and significantly alters the phase of rhythmic expression. We explored the link between AMPK and the expression of neuropeptide Y using the AMPK inhibitor compound C and the AMP analog AICAR. AMPK inhibition decreased NPY mRNA. AICAR also elevated basal NPY, but prevented the palmitate-mediated increase in NPY mRNA levels. We postulate that this palmitate-mediated increase in NPY and AgRP synthesis may initiate a detrimental positive feedback loop leading to increased energy consumption.

Disrupted fat distribution and composition due to medium-chain triglycerides in mice with a β-oxidation defect

BACKGROUND

Because of the enhanced recognition of inherited long-chain fatty acid oxidation disorders by worldwide newborn screening programs, an increasing number of asymptomatic patients receive medium-chain triglyceride (MCT) supplements to prevent the development of cardiomyopathy and myopathy.

OBJECTIVE

MCT supplementation has been recognized as a safe dietary intervention, but long-term observations into later adulthood are still not available. We investigated the consequences of a prolonged MCT diet on abdominal fat distribution and composition and on liver fat.

DESIGN

Mice with very-long-chain acyl-coenzyme A dehydrogenase deficiency (VLCAD(-/-)) were supplemented for 1 y with a diet in which MCTs replaced long-chain triglycerides without increasing the total fat content. The dietary effects on abdominal fat accumulation and composition were analyzed by in vivo (1)H- and (13)C-magnetic resonance spectroscopy (9.4 Tesla).

RESULTS

After 1 y of MCT supplementation, VLCAD(-/-) mice accumulated massive visceral fat and had a dramatic increase in the concentration of serum free fatty acids. Furthermore, we observed a profound shift in body triglyceride composition, ie, concentrations of physiologically important polyunsaturated fatty acids dramatically decreased. (1)H-Magnetic resonance spectroscopy analysis and histologic evaluation of the liver also showed pronounced fat accumulation and marked oxidative stress.

CONCLUSION

Although the MCT-supplemented diet has been reported to prevent the development of cardiomyopathy and skeletal myopathy in fatty acid oxidation disorders, our data show that long-term MCT supplementation results in a severe clinical phenotype similar to that of nonalcoholic steatohepatitis and the metabolic syndrome.

Skeletal muscle fatty acid oxidation is not directly associated with AMPK or ACC2 phosphorylation

Rescue of palmitate-induced insulin resistance has been linked with improvements in fatty acid oxidation, but importantly, not always with concurrently altered AMPK or ACC2 phosphorylation. Therefore, we examined the interrelationships among AMPK, ACC2, and fatty acid oxidation under 12 controlled conditions in isolated muscle. Incubation of soleus muscle (0–12 h) did not alter fatty acid oxidation, but did increase AMPK and ACC2 phosphorylation (24%–30%). Muscle incubation with palmitate (2 mmol·L–1) inhibited palmitate oxidation (∼55%), but paradoxically, this was associated with increased AMPK and ACC2 phosphorylation (∼50%). Addition of an AMPK activator (thujone) to control (no palmitate) muscle increased AMPK and ACC2 phosphorylation (∼25%) but did not alter palmitate oxidation. Addition of AMPK inhibitors, compound C (50 µmol·L–1) or adenine 9-β-d-arabinofuranoside (Ara; 2.5 mmol·L–1), to thujone-treated muscles (no palmitate) did not alter palmiate oxidation but reduced AMPK phosphorylation (32%–42%), while ACC2 phosphorylation remained above basal level (+14%–18%). Finally, in palmitate-treated muscle, thujone increased AMPK (+100%) and ACC2 phosphorylation (+52%) and restored palmitate oxidation. Compound C or Ara, administered along with thujone in palmitate-treated muscle, only partly blunted palmitate oxidation recovery despite inhibiting AMPK phosphorylation (–22%), although ACC2 phosphorylation remained upregulated (+33%). Among these experiments, AMPK phosphorylation and ACC2 phosphorylation were positively correlated. However, AMPK phosphorylation was not correlated with palmitate oxidation, and unexpectedly, palmitate oxidation was negatively correlated with ACC2 phosphorylation. Our study, in accordance with a growing body of evidence, indicates that neither AMPK phosphorylation nor ACC2 phosphorylation is by itself an appropriate marker of fatty acid oxidation, and further serves to question their regulatory role.

Lack of downstream insulin-mimetic effects of visfatin/eNAMPT on glucose and fatty acid metabolism in skeletal muscles

AIM

Recent studies regarding downstream effects of visfatin/eNAMPT in skeletal muscles have attracted much attention as the previous reports suggested this adipokine may exert insulin-mimetic effects. However, studies in vivo present conflicting data and are still controversial. In this present work, we sought to investigate whether visfatin/eNAMPT is able to reproduce insulin effects on glucose transport and lipid metabolism.

METHODS

  We have used isolated skeletal muscles with different fibre type composition (Soleus and EDL) to examine glucose transport, GLUT-4 translocation, phosphorylation of insulin signalling pathway proteins, as well as the key parameters for fatty acid metabolism.

RESULTS

We found that, in vitro exposure to visfatin/eNAMPT increased skeletal muscle glucose transport but only in EDL (+20%) and not in Soleus muscle (+5%). Interestingly, classical insulin signalling pathways were not significantly involved in this process. Concomitantly, visfatin/eNAMPT exerted no significant effects on muscle's fatty acids (FA) metabolism as no change in either palmitate oxidation or esterification was observed. Importantly, combined insulin and visfatin effects were not found, suggesting non-additivity.

CONCLUSION

Our data suggest that visfatin/eNAMPT plays a rather limited role in regulating skeletal muscle glucose transport and FA metabolism.

An oral lipid challenge and acute intake of caffeinated coffee additively decrease glucose tolerance in healthy men

Lipid-induced insulin resistance has been investigated primarily with i.v. infusions, and caffeine-induced insulin resistance, with alkaloid caffeine. The effects of orally consumed lipids and coffee have not been established and to our knowledge have never been simultaneously investigated. The goals of this study were to determine whether an oral lipid challenge and caffeinated coffee would disrupt glucose homeostasis and to characterize their respective incretin responses. It was hypothesized that oral ingestion of saturated lipids would impair glucose tolerance and that caffeinated coffee would further hinder glucose management. Ten young, healthy males participated in 5 trials in a randomized, cross-over design. At time 0 h, they underwent an oral fat tolerance test (OFTT: 1 g lipid/kg body weight) or consumed water, followed 5 h later by caffeinated (5 mg/kg) coffee, decaffeinated coffee, or water. At 6 h, volunteers underwent an oral glucose tolerance test (OGTT). Consumption of the OFTT increased glucose concentrations (P < 0.05) after a subsequent OGTT. At 7 h, caffeinated coffee produced the highest glucose concentrations (P < 0.05). Glucagon-like peptide-1 active (GLP-1a) and glucose-dependent insulinotropic polypeptide (GIP) were both increased for up to 6 h in all OFTT trials (P < 0.05). Compared to all other treatments, caffeinated and decaffeinated coffee produced higher GLP-1a response at 6.25 h (P < 0.05), whereas only caffeinated coffee increased GIP secretion (P < 0.05). These results show that oral consumption of lipids and caffeinated coffee can independently and additively decrease glucose tolerance. Incretin hormones could explain at least in part this impaired glucose homeostasis.

A single prior bout of exercise protects against palmitate-induced insulin resistance despite an increase in total ceramide content

Ceramide accumulation has been implicated in the impairment of insulin-stimulated glucose transport in skeletal muscle following saturated fatty acid (FA) exposure. Importantly, a single bout of exercise can protect against acute lipid-induced insulin resistance. The mechanism by which exercise protects against lipid-induced insulin resistance is not completely known but may occur through a redirection of FA toward triacylglycerol (TAG) and away from ceramide and diacylglycerol (DAG). Therefore, in the current study, an in vitro preparation was used to examine whether a prior bout of exercise could confer protection against palmitate-induced insulin resistance and whether the pharmacological [50 μM fumonisin B(1) (FB1)] inhibition of ceramide synthesis in the presence of palmitate could mimic the protective effect of exercise. Soleus muscle of sedentary (SED), exercised (EX), and SED in the presence of FB1 (SED+FB1) were incubated with or without 2 mM palmitate for 4 h. This 2-mM palmitate exposure impaired insulin-stimulated glucose transport (-28%, P < 0.01) and significantly increased ceramide, DAG, and TAG accumulation in the SED group (P < 0.05). A single prior bout of exercise prevented the detrimental effects of palmitate on insulin signaling and caused a partial redistribution of FA toward TAG (P < 0.05). However, the net increase in ceramide content in response to palmitate exposure in the EX group was not different compared with SED, despite the maintenance of insulin sensitivity. The incubation of soleus from SED rats with FB1 (SED+FB1) prevented the detrimental effects of palmitate and caused a redirection of FA toward TAG accumulation (P < 0.05). Therefore, this research suggests that although inhibiting ceramide accumulation can prevent the detrimental effects of palmitate, a single prior bout of exercise appears to protect against palmitate-induced insulin resistance, which may be independent of changes in ceramide content.

{beta}-Hydroxybutyrate inhibits insulin-mediated glucose transport in mouse oxidative muscle

Blood ketone body levels increase during starvation and untreated diabetes. Here we tested the hypothesis that ketone bodies directly inhibit insulin action in skeletal muscle. We investigated the effect of d,l-beta-hydroxybutyrate (BOH; the major ketone body in vivo) on insulin-mediated glucose uptake (2-deoxyglucose) in isolated mouse soleus (oxidative) and extensor digitorum longus (EDL; glycolytic) muscle. BOH inhibited insulin-mediated glucose uptake in soleus (but not in EDL) muscle in a time- and concentration-dependent manner. Following 19.5 h of exposure to 5 mM BOH, insulin-mediated (20 mU/ml) glucose uptake was inhibited by approximately 90% (substantial inhibition was also observed in 3-O-methylglucose transport). The inhibitory effect of BOH was reproduced with d- but not l-BOH. BOH did not significantly affect hypoxia- or AICAR-mediated (activates AMP-dependent protein kinase) glucose uptake. The BOH effect did not require the presence/utilization of glucose since it was also seen when glucose in the medium was substituted with pyruvate. To determine whether the BOH effect was mediated by oxidative stress, an exogenous antioxidant (1 mM tempol) was used; however, tempol did not reverse the BOH effect on insulin action. BOH did not alter the levels of total tissue GLUT4 protein or insulin-mediated tyrosine phosphorylation of the insulin receptor and insulin receptor substrate-1 but blocked insulin-mediated phosphorylation of protein kinase B by approximately 50%. These data demonstrate that BOH inhibits insulin-mediated glucose transport in oxidative muscle by inhibiting insulin signaling. Thus ketone bodies may be potent diabetogenic agents in vivo.

Increased levels of peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1alpha) improve lipid utilisation, insulin signalling and glucose transport in skeletal muscle of lean and insulin-resistant obese Zucker rats

AIMS/HYPOTHESIS

Reductions in peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1alpha) levels have been associated with the skeletal muscle insulin resistance. However, in vivo, the therapeutic potential of PGC-1alpha has met with failure, as supra-physiological overexpression of PGC-1alpha induced insulin resistance, due to fatty acid translocase (FAT)-mediated lipid accumulation. Based on physiological and metabolic considerations, we hypothesised that a modest increase in PGC-1alpha levels would limit FAT upregulation and improve lipid metabolism and insulin sensitivity, although these effects may differ in lean and insulin-resistant muscle.

METHODS

Pgc-1alpha was transfected into lean and obese Zucker rat muscles. Two weeks later we examined mitochondrial biogenesis, intramuscular lipids (triacylglycerol, diacylglycerol, ceramide), GLUT4 and FAT levels, insulin-stimulated glucose transport and signalling protein phosphorylation (thymoma viral proto-oncogene 2 [Akt2], Akt substrate of 160 kDa [AS160]), and fatty acid oxidation in subsarcolemmal and intermyofibrillar mitochondria.

RESULTS

Electrotransfection yielded physiologically relevant increases in Pgc-1alpha (also known as Ppargc1a) mRNA and protein ( approximately 25%) in lean and obese muscle. This induced mitochondrial biogenesis, and increased FAT and GLUT4 levels, insulin-stimulated glucose transport, and Akt2 and AS160 phosphorylation in lean and obese animals, while bioactive intramuscular lipids were only reduced in obese muscle. Concurrently, PGC-1alpha increased palmitate oxidation in subsarcolemmal, but not in intermyofibrillar mitochondria, in both groups. In obese compared with lean animals, the PGC-1alpha-induced improvement in insulin-stimulated glucose transport was smaller, but intramuscular lipid reduction was greater.

CONCLUSIONS/INTERPRETATIONS

Increases in PGC-1alpha levels, similar to those that can be induced by physiological stimuli, altered intramuscular lipids and improved fatty acid oxidation, insulin signalling and insulin-stimulated glucose transport, albeit to different extents in lean and insulin-resistant muscle. These positive effects are probably attributable to limiting the PGC-1alpha-induced increase in FAT, thereby preventing bioactive lipid accumulation as has occurred in transgenic PGC-1alpha animals.