Primary defects in lipolysis and insulin action in skeletal muscle cells from type 2 diabetic individuals

Time-dependent reduction in oxidative capacity among cultured myotubes from spinal cord injured individuals

BACKGROUND

Skeletal muscle adapts in reaction to contractile activity to efficiently utilize energy substrates, primarily glucose and free fatty acids (FA). Inactivity leads to atrophy and a change in energy utilization in individuals with spinal cord injury (SCI). The present study aimed to characterize possible inactivity-related differences in the energy metabolism between skeletal muscle cells cultured from satellite cells isolated 1- and 12-months post-SCI.

METHODS

To characterize inactivity-related disturbances in spinal cord injury, we studied skeletal muscle cells isolated from SCI subjects. Cell cultures were established from biopsy samples from musculus vastus lateralis from subjects with SCI 1 and 12 months after the injury. The myoblasts were proliferated and differentiated into myotubes before fatty acid and glucose metabolism were assessed and gene and protein expressions were measured.

RESULTS

The results showed that glucose uptake was increased, while oleic acid oxidation was reduced at 12 months compared to 1 month. mRNA expressions of PPARGC1α, the master regulator of mitochondrial biogenesis, and MYH2, a determinant of muscle fiber type, were significantly reduced at 12 months. Proteomic analysis showed reduced expression of several mitochondrial proteins.

CONCLUSION

In conclusion, skeletal muscle cells isolated from immobilized subjects 12 months compared to 1 month after SCI showed reduced fatty acid metabolism and reduced expression of mitochondrial proteins, indicating an increased loss of oxidative capacity with time after injury.

Myokine Secretion following an Aerobic Exercise Intervention in Individuals with Type 2 Diabetes with or without Exercise Resistance

Type 2 diabetes (T2D) is characterized by muscle metabolic dysfunction that exercise can minimize, but some patients do not respond to an exercise intervention. Myokine secretion is intrinsically altered in patients with T2D, but the role of myokines in exercise resistance in this patient population has never been studied. We sought to determine if changes in myokine secretion were linked to the response to an exercise intervention in patients with T2D. The participants followed a 10-week aerobic exercise training intervention, and patients with T2D were grouped based on muscle mitochondrial function improvement (responders versus non-responders). We measured myokines in serum and cell-culture medium of myotubes derived from participants pre- and post-intervention and in response to an in vitro model of muscle contraction. We also quantified the expression of genes related to inflammation in the myotubes pre- and post-intervention. No significant differences were detected depending on T2D status or response to exercise in the biological markers measured, with the exception of modest differences in expression patterns for certain myokines (IL-1β, IL-8, IL-10, and IL-15). Further investigation into the molecular mechanisms involving myokines may explain exercise resistance with T2D; however, the role in metabolic adaptations to exercise in T2D requires further investigation.

Elucidating type 2 diabetes mellitus risk factor by promoting lipid metabolism with gymnemagenin: An in vitro and in silico approach

Introduction: Adipose tissue functions as a key endocrine organ which releases multiple bioactive substances and regulate obesity-linked complications. Dysregulation of adipocyte differentiation, triglyceride metabolism, adipokines production and lipid transport contributes to impaired lipid metabolism resulting in obesity, insulin resistance and type 2 diabetes. Gymnema sylvestre plant is frequently used in Ayurveda for treatment of diabetes and obesity. Gymnemagenin is a major bioactive compound of Gymnema sylvestre. The present study was undertaken to elucidate the role of gymnemagenin in lipid metabolism by in vitro and computational approaches. Methods: A panel of twelve genes viz., Fasn, Lipe, Lpl, Pparg, Plin2, Cidea, Scd1, Adipoq, Lep, Ccl2, Fabp4, and Slc2a4, essential in lipid metabolism were selected and gene expression pattern and triglyceride content were checked in adipocytes (3T3L1 cells) with/without treatment of gymnemagenin by Real time PCR and colorimetric estimation, respectively. Mode of action of gymnemagenin on Pparg and Fabp4 was accomplished by computational studies. Gene set enrichment and network pharmacology were performed by STRING and Cytoscape. Molecular docking was performed by AutoDock vina by POAP pipeline. Molecular dynamics, MM-PBSA were done by Gromacs tool. Results: In vitro study showed that gymnemagenin improved triglyceride metabolism by up regulating the expression of lipase genes viz., Lipe and Lpl which hydrolyse triglyceride. Gymnemagenin also up regulated the expression of anti-inflammatory gene Adipoq. Importantly, gymnemagenin treatment up regulated the expression of Pparg gene and the downstream target genes (Plin2, Cidea, and Scd1) which are associated with adipogenesis. However, gymnemagenin has no effect on expression of Fabp4, codes for a lipid transport protein. In silico study revealed that gymnemagenin targeted 12 genes were modulating 6 molecular pathways involved in diabetes and obesity. Molecular docking and dynamics revealed that gymnemagenin stably bind to active site residue of Pparg and failed to bind to Fabp4 active site compared to its standard molecules throughout 100 ns MD production run. Gymnemagenin scored binding free energy of -177.94 and -25.406 kJ/mol with Pparg and Fabp4, respectively. Conclusion: Gymnemagenin improved lipid metabolism by increasing triglyceride hydrolysis (lipolysis), up regulating the crucial gene of adipogenesis and increasing the expression of anti-inflammatory adipokine proving its therapeutic importance as anti-obesity and anti-diabetic phytocompound.

In vitro skeletal muscle models for type 2 diabetes

Type 2 diabetes mellitus, a metabolic disorder characterized by abnormally elevated blood sugar, poses a growing social, economic, and medical burden worldwide. The skeletal muscle is the largest metabolic organ responsible for glucose homeostasis in the body, and its inability to properly uptake sugar often precedes type 2 diabetes. Although exercise is known to have preventative and therapeutic effects on type 2 diabetes, the underlying mechanism of these beneficial effects is largely unknown. Animal studies have been conducted to better understand the pathophysiology of type 2 diabetes and the positive effects of exercise on type 2 diabetes. However, the complexity of in vivo systems and the inability of animal models to fully capture human type 2 diabetes genetics and pathophysiology are two major limitations in these animal studies. Fortunately, in vitro models capable of recapitulating human genetics and physiology provide promising avenues to overcome these obstacles. This review summarizes current in vitro type 2 diabetes models with focuses on the skeletal muscle, interorgan crosstalk, and exercise. We discuss diabetes, its pathophysiology, common in vitro type 2 diabetes skeletal muscle models, interorgan crosstalk type 2 diabetes models, exercise benefits on type 2 diabetes, and in vitro type 2 diabetes models with exercise.

Pathogenesis of sarcopenia and the relationship with fat mass: descriptive review

Age-associated obesity and muscle atrophy (sarcopenia) are intimately connected and are reciprocally regulated by adipose tissue and skeletal muscle dysfunction. During ageing, adipose inflammation leads to the redistribution of fat to the intra-abdominal area (visceral fat) and fatty infiltrations in skeletal muscles, resulting in decreased overall strength and functionality. Lipids and their derivatives accumulate both within and between muscle cells, inducing mitochondrial dysfunction, disturbing β-oxidation of fatty acids, and enhancing reactive oxygen species (ROS) production, leading to lipotoxicity and insulin resistance, as well as enhanced secretion of some pro-inflammatory cytokines. In turn, these muscle-secreted cytokines may exacerbate adipose tissue atrophy, support chronic low-grade inflammation, and establish a vicious cycle of local hyperlipidaemia, insulin resistance, and inflammation that spreads systemically, thus promoting the development of sarcopenic obesity (SO). We call this the metabaging cycle. Patients with SO show an increased risk of systemic insulin resistance, systemic inflammation, associated chronic diseases, and the subsequent progression to full-blown sarcopenia and even cachexia. Meanwhile in many cardiometabolic diseases, the ostensibly protective effect of obesity in extremely elderly subjects, also known as the 'obesity paradox', could possibly be explained by our theory that many elderly subjects with normal body mass index might actually harbour SO to various degrees, before it progresses to full-blown severe sarcopenia. Our review outlines current knowledge concerning the possible chain of causation between sarcopenia and obesity, proposes a solution to the obesity paradox, and the role of fat mass in ageing.

Primary skeletal muscle cells from chronic kidney disease patients retain hallmarks of cachexia in vitro

BACKGROUND

Skeletal muscle wasting and dysfunction are common characteristics noted in people who suffer from chronic kidney disease (CKD). The mechanisms by which this occurs are complex, and although progress has been made, the key underpinning mechanisms are not yet fully elucidated. With work to date primarily conducted in nephrectomy-based animal models, translational capacity to our patient population has been challenging. This could be overcome if rationale developing work could be conducted in human based models with greater translational capacity. This could be achieved using cells derived from patient biopsies, if they retain phenotypic traits noted in vivo.

METHODS

Here, we performed a systematic characterization of CKD derived muscle cells (CKD; n = 10; age: 54.40 ± 15.53 years; eGFR: 22.25 ± 13.22 ml/min/1.73 m² ) in comparison with matched controls (CON; n = 10; age: 58.66 ± 14.74 years; eGFR: 85.81 ± 8.09 ml/min/1.73 m² ). Harvested human derived muscle cells (HDMCs) were taken through proliferative and differentiation phases and investigated in the context of myogenic progression, inflammation, protein synthesis, and protein breakdown. Follow up investigations exposed HDMC myotubes from each donor type to 0, 0.4, and 100 nM of IGF-1 in order to investigate any differences in anabolic resistance.

RESULTS

Harvested human derived muscle cells isolated from CKD patients displayed higher rates of protein degradation (P = 0.044) alongside elevated expression of both TRIM63 (2.28-fold higher, P = 0.054) and fbox32 (6.4-fold higher, P < 0.001) in comparison with CONs. No differences were noted in rates of protein synthesis under basal conditions (P > 0.05); however, CKD derived cells displayed a significant degree of anabolic resistance in response to IGF-1 stimulation (both doses) in comparison with matched CONs (0.4 nm: P < 0.001; 100 nM: P < 0.001).

CONCLUSIONS

In summary, we report for the first time that HDMCs isolated from people suffering from CKD display key hallmarks of the well documented in vivo phenotype. Not only do these findings provide further mechanistic insight into CKD specific cachexia, but they also demonstrate this is a reliable and suitable model in which to perform targeted experiments to begin to develop novel therapeutic strategies targeting the CKD associated decline in skeletal muscle mass and function.

SENP2 is vital for optimal insulin signaling and insulin-stimulated glycogen synthesis in human skeletal muscle cells

Sentrin-specific protease (SENP) 2 has been suggested as a possible novel drug target for the treatment of obesity and type 2 diabetes mellitus after observations of a palmitate-induced increase in SENP2 that lead to increased fatty acid oxidation and improved insulin sensitivity in skeletal muscle cells from mice. However, no precedent research has examined the role of SENP2 in human skeletal muscle cells. In the present work, we have investigated the impact of SENP2 on fatty acid and glucose metabolism as well as insulin sensitivity in human skeletal muscle using cultured primary human myotubes. Acute (4 ​h) oleic acid oxidation was reduced in SENP2-knockdown (SENP2-KD) cells compared to control cells, with no difference in uptake. After prelabeling (24 ​h) with oleic acid, total lipid content and incorporation into triacylglycerol was decreased, while incorporation into other lipids, as well as complete oxidation and β-oxidation was increased in SENP2-KD cells. Basal glucose uptake (i.e., not under insulin-stimulated conditions) was higher in SENP2-KD cells, whereas oxidation was similar to control myotubes. Further, basal glycogen synthesis was not different in SENP2-KD myotubes, but both insulin-stimulated glycogen synthesis and Akt^Ser473 phosphorylation was completely blunted in SENP2-KD cells. In conclusion, SENP2 plays an important role in fatty acid and glucose metabolism in human myotubes. Interestingly, it also appears to have a pivotal role in regulating myotube insulin sensitivity. Future studies should examine the role of SENP2 in regulation of insulin sensitivity in other tissues and in vivo, defining the potential for SENP2 as a drug target.

Non-cell autonomous mechanisms control mitochondrial gene dysregulation in polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a common endocrine disorder associated with insulin resistance and impaired energy metabolism in skeletal muscle, the aetiology of which is currently unclear. Here, we mapped the gene expression profile of skeletal muscle from women with PCOS and determined if cultured primary myotubes retain the gene expression signature of PCOS in vivo. Transcriptomic analysis of vastus lateralis biopsies collected from PCOS women showed lower expression of genes associated with mitochondrial function, while the expression of genes associated with the extracellular matrix was higher compared to controls. Altered skeletal muscle mRNA expression of mitochondrial-associated genes in PCOS was associated with lower protein expression of mitochondrial complex II-V, but not complex I, with no difference in mitochondrial DNA content. Transcriptomic analysis of primary myotube cultures established from biopsies did not display any differentially expressed genes between controls and PCOS. Comparison of gene expression profiles in skeletal muscle biopsies and primary myotube cultures showed lower expression of mitochondrial and energy metabolism-related genes in vitro, irrespective of the group. Together, our results show that the altered mitochondrial-associated gene expression in skeletal muscle in PCOS is not preserved in cultured myotubes, indicating that the in vivo extracellular milieu, rather than genetic or epigenetic factors, may drive this alteration. Dysregulation of mitochondrial-associated genes in skeletal muscle by extracellular factors may contribute to the impaired energy metabolism associated with PCOS.

Skeletal muscle energy metabolism in obesity

Comparing energy metabolism in human skeletal muscle and primary skeletal muscle cells in obesity, while focusing on glucose and fatty acid metabolism, shows many common changes. Insulin-mediated glucose uptake in skeletal muscle and primary myotubes is decreased by obesity, whereas differences in basal glucose metabolism are inconsistent among studies. With respect to fatty acid metabolism, there is an increased uptake and storage of fatty acids and a reduced complete lipolysis, suggesting alterations in lipid turnover. In addition, fatty acid oxidation is decreased, probably at the level of complete oxidation, as β -oxidation may be enhanced in obesity, which indicates mitochondrial dysfunction. Metabolic changes in skeletal muscle with obesity promote metabolic inflexibility, ectopic lipid accumulation, and formation of toxic lipid intermediates. Skeletal muscle also acts as an endocrine organ, secreting myokines that participate in interorgan cross talk. This review highlights interventions and some possible targets for treatment through action on skeletal muscle energy metabolism. Effects of exercise in vivo on obesity have been compared with simulation of endurance exercise in vitro on myotubes (electrical pulse stimulation). Possible pharmaceutical targets, including signaling pathways and drug candidates that could modify lipid storage and turnover or increase mitochondrial function or cellular energy expenditure through adaptive thermogenic mechanisms, are discussed.

Decoration of myocellular lipid droplets with perilipins as a marker for in vivo lipid droplet dynamics: A super-resolution microscopy study in trained athletes and insulin resistant individuals

In many different cell types neutral lipids can be stored in lipid droplets (LDs). Nowadays, LDs are viewed as dynamic organelles, which store and release fatty acids depending on energy demand (LD dynamics). Proteins like perilipin 2 (PLIN2) and PLIN5 decorate the LD membrane and are determinants of LD lipolysis and fat oxidation, thus affecting LD dynamics. Trained athletes and type 2 diabetes (T2D) patients both have high levels of intramyocellular lipid (IMCL). While IMCL content scales negatively with insulin resistance, athletes are highly insulin sensitive in contrast to T2D patients, the so-called athlete's paradox. Differences in LD dynamics may be an underlying factor explaining the athlete's paradox. We aimed to quantify PLIN2 and PLIN5 content at individual LDs as a reflection of the ability to switch between fatty acid release and storage depending on energy demand. Thus, we developed a novel fluorescent super-resolution microscopy approach and found that PLIN2 protein abundance at the LD surface was higher in T2D patients than in athletes. Localization of adipocyte triglyceride lipase (ATGL) to the LD surface was lower in LDs abundantly decorated with PLIN2. While PLIN5 abundance at the LD surface was similar in athletes and T2D patients, we have observed previously that the number of PLIN5 decorated LDs was higher in athletes, indicating more LDs in close association with mitochondria. Thus, in athletes interaction of LDs with mitochondria was more pronounced and LDs have the protein machinery to be more dynamic, while in T2D patients the LD pool is more inert. This observation contributes to our understanding of the athlete's paradox.

Substrate oxidation in primary human skeletal muscle cells is influenced by donor age

Primary human myotubes represent an alternative system to intact skeletal muscle for the study of human diseases related to changes in muscle energy metabolism. This work aimed to study if fatty acid and glucose metabolism in human myotubes in vitro were related to muscle of origin, donor gender, age, or body mass index (BMI). Myotubes from a total of 82 donors were established from three different skeletal muscles, i.e., musculus vastus lateralis, musculus obliquus internus abdominis, and musculi interspinales, and cellular energy metabolism was evaluated. Multiple linear regression analyses showed that donor age had a significant effect on glucose and oleic acid oxidation after correcting for gender, BMI, and muscle of origin. Donor BMI was the only significant contributor to cellular oleic acid uptake, whereas cellular glucose uptake did not rely on any of the variables examined. Despite the effect of age on substrate oxidation, cellular mRNA expression of pyruvate dehydrogenase kinase 4 (PDK4) and peroxisome proliferator–activated receptor gamma coactivator 1 alpha (PPARGC1A) did not correlate with donor age. In conclusion, donor age significantly impacts substrate oxidation in cultured human myotubes, whereas donor BMI affects cellular oleic acid uptake.

Obesity-Associated Hypermetabolism and Accelerated Senescence of Bone Marrow Stromal Stem Cells Suggest a Potential Mechanism for Bone Fragility

Obesity is associated with increased risk for fragility fractures. However, the cellular mechanisms are unknown. Using a translational approach combining RNA sequencing and cellular analyses, we investigated bone marrow stromal stem cells (BM-MSCs) of 54 men divided into lean, overweight, and obese groups on the basis of BMI. Compared with BM-MSCs obtained from lean, obese BM-MSCs exhibited a shift of molecular phenotype toward committed adipocytic progenitors and increased expression of metabolic genes involved in glycolytic and oxidoreductase activity. Interestingly, compared with paired samples of peripheral adipose tissue-derived stromal cells (AT-MSCs), insulin signaling of obese BM-MSCs was enhanced and accompanied by increased abundance of insulin receptor positive (IR+) and leptin receptor positive (LEPR+) cells in BM-MSC cultures. Their hyper-activated metabolic state was accompanied by an accelerated senescence phenotype. Our data provide a plausible explanation for the bone fragility in obesity caused by enhanced insulin signaling leading to accelerated metabolic senescence of BM-MSCs.

Emerging Pharmacological Targets for the Treatment of Nonalcoholic Fatty Liver Disease, Insulin Resistance, and Type 2 Diabetes

Type 2 diabetes (T2D) is characterized by persistent hyperglycemia despite hyperinsulinemia, affects more than 400 million people worldwide, and is a major cause of morbidity and mortality. Insulin resistance, of which ectopic lipid accumulation in the liver [nonalcoholic fatty liver disease (NAFLD)] and skeletal muscle is the root cause, plays a major role in the development of T2D. Although lifestyle interventions and weight loss are highly effective at reversing NAFLD and T2D, weight loss is difficult to sustain, and newer approaches aimed at treating the root cause of T2D are urgently needed. In this review, we highlight emerging pharmacological strategies aimed at improving insulin sensitivity and T2D by altering hepatic energy balance or inhibiting key enzymes involved in hepatic lipid synthesis. We also summarize recent research suggesting that liver-targeted mitochondrial uncoupling may be an attractive therapeutic approach to treat NAFLD, nonalcoholic steatohepatitis, and T2D.

Myokine Regulation of Insulin Secretion: Impact of Inflammation and Type 2 Diabetes

Skeletal muscle (SkM) secretes protein factors (myokines) that can exert multiple actions. To study the control of myokine regulation of β-cell function, SkM biopsies were taken from non-diabetic (ND) and Type 2 diabetic (T2D) subjects and satellite cells cultured to myotubes (MT). MT were also treated with lipopolysaccharide (infectious inflammation – II) or a combination of glucose (10 mM), insulin (120 pM), and palmitate (0.4 mM) (metabolic inflammation – MI) to model the inflammatory and metabolic conditions seen in vivo with T2D. Conditioned media (CM) was collected from MT after 24 h and used to treat INS-1 cells for 24 h. Cell viability, total insulin content, glucose-stimulated insulin secretion (GSIS) and maximal (IBMX-stimulated) IS (IS_max) were monitored. Under baseline conditions, CM from ND and T2D MT had no effects on INS-1 cell viability, insulin content, GSIS, or IS_max. After exposure to II, CM from ND-MT augmented GSIS in INS-1 cells by 100 ± 25% over control (p < 0.05); T2D-CM had no effect. After exposure to MI, T2D-CM suppressed GSIS by 35 ± 5% (p < 0.05); ND-CM was without effect. Under either of these conditions cell viability, total insulin content and IS_max were unaffected. Effects of CM on GSIS were lost after CM was boiled. Both augmentation of GSIS by ND-CM from II-treated MT, and suppression by T2D-CM from MI-treated MT, were inhibited by wortmannin, Ro 31-8220, and SB203580. In summary: (1) ND-MT are able to augment GSIS when stressed, (2) T2D-MT responding to a diabetic-like environment secrete myokines that suppress GSIS, (3) Unknown protein factors exert effects specifically on GSIS, possibly through PI-3K, PKC, and/or p38 MAPK. In T2D, both insulin resistance and a suppression of adaptive increased insulin secretion are intrinsic properties of SkM that can contribute to the full T2D phenotype.

Primary defects in lipid handling and resistance to exercise in myotubes from obese donors with and without type 2 diabetes

Several studies have shown that human primary myotubes retain the metabolic characteristic of their donors in vitro. We have demonstrated, along with other researchers, a reduced lipid turnover and fat oxidation rate in myotubes derived from obese donors with and without type 2 diabetes (T2D). Because exercise is known to increase fat oxidative capacity in skeletal muscle, we investigated if in vitro exercise could restore primary defects in lipid handling in myotubes of obese individuals with and without T2D compared with lean nondiabetic donors. Primary myotubes cultures were derived from biopsies of lean, obese, and T2D subjects. One single bout of long-duration exercise was mimicked in vitro by electrical pulse stimulation (EPS) for 24 h. Lipid handling was measured using radiolabeled palmitate, metabolic gene expression by real-time qPCR, and proteins by Western blot. We first showed that myotubes from obese and T2D donors had increased uptake and incomplete oxidation of palmitate. This was associated with reduced mitochondrial respiratory chain complex II, III, and IV protein expression in myotubes from obese and T2D subjects. EPS stimulated palmitate oxidation in lean donors, while myotubes from obese and T2D donors were refractory to this effect. Interestingly, EPS increased total palmitate uptake in myotubes from lean donors while myotubes from T2D donors had a reduced rate of palmitate uptake into complex lipids and triacylglycerols. Novelty Myotubes from obese and T2D donors are characterized by primary defects in palmitic acid handling. Both obese and T2D myotubes are partially refractory to the beneficial effect of exercise on lipid handling.

Role of myokines in the development of skeletal muscle insulin resistance and related metabolic defects in type 2 diabetes

Due to its mass, skeletal muscle is the major site of glucose uptake and an important tissue in the development of type 2 diabetes (T2D). Muscles of patients with T2D are affected with insulin resistance and mitochondrial dysfunction, which result in impaired glucose and fatty acid metabolism. A well-established method of managing the muscle metabolic defects occurring in T2D is physical exercise. During exercise, muscles contract and secrete factors called myokines which can act in an autocrine/paracrine fashion to improve muscle energy metabolism. In patients with T2D, plasma levels as well as muscle levels (mRNA and protein) of some myokines are upregulated, while others are downregulated. The signalling pathways of certain myokines are also altered in skeletal muscle of patients with T2D. Taken together, these findings suggest that myokine secretion is an important factor contributing to the development of muscle metabolic defects during T2D. It is also of interest considering that lack of physical activity is closely linked to the occurrence of this disease. The causal relationships between sedentary behavior, factors secreted by skeletal muscle at rest and during contraction and the development of T2D remain to be elucidated. Many myokines shown to influence muscle energy metabolism still have not been characterized in the context of T2D in skeletal muscle specifically. The purpose of this review is to highlight what is known and what remains to be determined regarding myokine secretion in patients with T2D to uncover potential therapeutic targets for the management of this disease.

Decreased RNF41 expression leads to insulin resistance in skeletal muscle of obese women

CONTEXT

Toll-like receptor 4 (TLR4) activation contributes to obesity-associated insulin resistance in skeletal muscles (SM). TLR4 signaling involves two pathways: the myeloid differentiation primary response gene 88 (MyD88) leading to inflammatory cytokines production and the toll/interleukin-1 receptor domain-containing adapter-inducing interferon (IFN) I (TRIF)-dependent pathways leading to type 1 interferon (IFNI) and interferon stimulated genes (ISG) expression. The E3 ubiquitin ligase RNF41 allows the preferential activation of the TRIF-IFNI pathway; however, its role in insulin response has not been reported.

METHODS

We measured RNF41 level and IFNI pathway activation (ISG expression) in SM biopsies of obese insulin sensitive (OIS) and obese insulin resistant (OIR) women. Then we isolated and differentiated in myotubes, primary human SM cell progenitors from OIS and OIR SM biopsies. We modulated RNF41 and ISG expression in these myotubes and investigated their effects on insulin response.

RESULTS

RNF41 expression is down-regulated in vivo in OIR SM and myotubes compared to OIS SM and myotubes. TLR4 activation with palmitate induces TRIF-IFNI pathway and ISG in OIS myotubes but not in OIR myotubes. Inhibition of RNF41 expression with siRNF41 in OIS myotubes treated with palmitate attenuates insulin response, IFNI pathway activation and ISG induction, mimicking OIR phenotype. Further, overexpression of RNF41 in OIR myotubes increases insulin response and ISG expression. Exposure to IFNI or to its inducer polyinosinic-polycytidylic acid, restores ISG expression and insulin sensitivity in OIR myotubes and OIS myotubes transfected with siRNF41.

CONCLUSION

Our results identify RNF41 as essential to IFNI pathway activation in order to maintain muscle insulin sensitivity during human obesity.

Natural killer cell metabolism

Natural killer (NK) cells are a critical component in the innate immune response against disease. NK cell function is tightly regulated by specific cytokine and activation/inhibitory receptor signalling, leading to diverse effector responses. Like all living cells, energy metabolism is a fundamental requirement for NK cell activation and survival. There is growing evidence that distinct functional profiles of NK cells are determined by alterations to cellular metabolic pathways. In this review, we summarise current literature that has explored NK cell metabolism to provide insight into how metabolic regulation controls NK cell function. We focus on metabolism pathways induced by different NK cell stimuli, metabolic regulatory proteins, and nutrient and hormonal levels in health and disease which impact on NK cell metabolic and functional activity.

Sarcopenic obesity or obese sarcopenia: A cross talk between age-associated adipose tissue and skeletal muscle inflammation as a main mechanism of the pathogenesis

Sarcopenia, an age-associated decline in skeletal muscle mass coupled with functional deterioration, may be exacerbated by obesity leading to higher disability, frailty, morbidity and mortality rates. In the combination of sarcopenia and obesity, the state called sarcopenic obesity (SOB), some key age- and obesity-mediated factors and pathways may aggravate sarcopenia. This review will analyze the mechanisms underlying the pathogenesis of SOB. In obese adipose tissue (AT), adipocytes undergo hypertrophy, hyperplasia and activation resulted in accumulation of pro-inflammatory macrophages and other immune cells as well as dysregulated production of various adipokines that together with senescent cells and the immune cell-released cytokines and chemokines create a local pro-inflammatory status. In addition, obese AT is characterized by excessive production and disturbed capacity to store lipids, which accumulate ectopically in skeletal muscle. These intramuscular lipids and their derivatives induce mitochondrial dysfunction characterized by impaired β-oxidation capacity and increased reactive oxygen species formation providing lipotoxic environment and insulin resistance as well as enhanced secretion of some pro-inflammatory myokines capable of inducing muscle dysfunction by auto/paracrine manner. In turn, by endocrine manner, these myokines may exacerbate AT inflammation and also support chronic low grade systemic inflammation (inflammaging), overall establishing a detrimental vicious circle maintaining AT and skeletal muscle inflammation, thus triggering and supporting SOB development. Under these circumstances, we believe that AT inflammation dominates over skeletal muscle inflammation. Thus, in essence, it redirects the vector of processes from "sarcopenia→obesity" to "obesity→sarcopenia". We therefore propose that this condition be defined as "obese sarcopenia", to reflect the direction of the pathological pathway.

Exercise in vivo marks human myotubes in vitro: Training-induced increase in lipid metabolism

BACKGROUND AND AIMS

Physical activity has preventive as well as therapeutic benefits for overweight subjects. In this study we aimed to examine effects of in vivo exercise on in vitro metabolic adaptations by studying energy metabolism in cultured myotubes isolated from biopsies taken before and after 12 weeks of extensive endurance and strength training, from healthy sedentary normal weight and overweight men.

METHODS

Healthy sedentary men, aged 40-62 years, with normal weight (body mass index (BMI) < 25 kg/m2) or overweight (BMI ≥ 25 kg/m2) were included. Fatty acid and glucose metabolism were studied in myotubes using [14C]oleic acid and [14C]glucose, respectively. Gene and protein expressions, as well as DNA methylation were measured for selected genes.

RESULTS

The 12-week training intervention improved endurance, strength and insulin sensitivity in vivo, and reduced the participants' body weight. Biopsy-derived cultured human myotubes after exercise showed increased total cellular oleic acid uptake (30%), oxidation (46%) and lipid accumulation (34%), as well as increased fractional glucose oxidation (14%) compared to cultures established prior to exercise. Most of these exercise-induced increases were significant in the overweight group, whereas the normal weight group showed no change in oleic acid or glucose metabolism.

CONCLUSIONS

12 weeks of combined endurance and strength training promoted increased lipid and glucose metabolism in biopsy-derived cultured human myotubes, showing that training in vivo are able to induce changes in human myotubes that are discernible in vitro.

Intramyocellular fat storage in metabolic diseases

Over the past decades, obesity and its metabolic co-morbidities such as type 2 diabetes (T2D) developed to reach an endemic scale. However, the mechanisms leading to the development of T2D are still poorly understood. One main predictor for T2D seems to be lipid accumulation in "non-adipose" tissues, best known as ectopic lipid storage. A growing body of data suggests that these lipids may play a role in impairing insulin action in metabolic tissues, such as liver and skeletal muscle. This review aims to discuss recent literature linking ectopic lipid storage and insulin resistance, with emphasis on lipid deposition in skeletal muscle. The link between skeletal muscle lipid content and insulin sensitivity, as well as the mechanisms of lipid-induced insulin resistance and potential therapeutic strategies to alleviate lipotoxic lipid pressure in skeletal muscle will be discussed.

Perilipin 5 fine-tunes lipid oxidation to metabolic demand and protects against lipotoxicity in skeletal muscle

Lipid droplets (LD) play a central role in lipid homeostasis by controlling transient fatty acid (FA) storage and release from triacylglycerols stores, while preventing high levels of cellular toxic lipids. This crucial function in oxidative tissues is altered in obesity and type 2 diabetes. Perilipin 5 (PLIN5) is a LD protein whose mechanistic and causal link with lipotoxicity and insulin resistance has raised controversies. We investigated here the physiological role of PLIN5 in skeletal muscle upon various metabolic challenges. We show that PLIN5 protein is elevated in endurance-trained (ET) subjects and correlates with muscle oxidative capacity and whole-body insulin sensitivity. When overexpressed in human skeletal muscle cells to recapitulate the ET phenotype, PLIN5 diminishes lipolysis and FA oxidation under basal condition, but paradoxically enhances FA oxidation during forskolin- and contraction- mediated lipolysis. Moreover, PLIN5 partly protects muscle cells against lipid-induced lipotoxicity. In addition, we demonstrate that down-regulation of PLIN5 in skeletal muscle inhibits insulin-mediated glucose uptake under normal chow feeding condition, while paradoxically improving insulin sensitivity upon high-fat feeding. These data highlight a key role of PLIN5 in LD function, first by finely adjusting LD FA supply to mitochondrial oxidation, and second acting as a protective factor against lipotoxicity in skeletal muscle.

Adaptations to excess choline in insulin resistant and Pcyt2 deficient skeletal muscle

It was hypothesized that choline supplementation in insulin resistant (IR) CTP:phosphoethanolamine cytidylyltransferase deficient (Pcyt2^+/-) mice would ameliorate muscle function by remodeling glucose and fatty acid (FA) metabolism. Pcyt2^+/- mice either received no treatment or were allowed access to 2 mg/mL choline in drinking water for 4 weeks. Skeletal muscle was harvested from choline treated and untreated mice. Lipid analysis and metabolic gene expression and signaling pathways were compared between untreated Pcyt2^+/- mice, treated Pcyt2^+/- mice, and Pcyt2^+/+ mice. The major positive effect of choline supplementation on IR muscle was the reduction of glucose utilization for FA and triglyceride (TAG) synthesis and increased muscle glucose storage as glycogen. Choline reduced the expression of genes for FA and TAG formation (Scd1, Fas, Srebp1c, Dgat1/2), upregulated the genes for FA oxidation (Cpt1, Pparα, Pgc1α), and had minor effects on phospholipid and lipolysis genes. Pcyt2^+/- muscle had reduced insulin signaling (IRS1), autophagy (LC3), and choline transport (CTL1) proteins that were restored by choline treatment. Additionally, choline activated AMPK and Akt while inhibiting mTORC1 phosphorylation. These data established that choline supplementation could restore muscle glucose metabolism by reducing lipogenesis and improving mitochondrial and intracellular signaling for protein and energy metabolism in insulin resistant Pcyt2 deficient mice.