Mitochondrial dysfunction in insulin resistance: differential contributions of chronic insulin and saturated fatty acid exposure in muscle cells. Biosci Rep. 2012;32:465-478. [PMID: 22742515 DOI: 10.1042/BSR20120034]

Cracking the code of the cardiovascular enigma: hPSC-derived endothelial cells unveil the secrets of endothelial dysfunction

Endothelial dysfunction is a central contributor to the development of most cardiovascular diseases and is characterised by the reduced synthesis or bioavailability of the vasodilator nitric oxide together with other abnormalities such as inflammation, senescence, and oxidative stress. The use of patient-specific and genome-edited human pluripotent stem cell-derived endothelial cells (hPSC-ECs) has shed novel insights into the role of endothelial dysfunction in cardiovascular diseases with strong genetic components such as genetic cardiomyopathies and pulmonary arterial hypertension. However, their utility in studying complex multifactorial diseases such as atherosclerosis, metabolic syndrome and heart failure poses notable challenges. In this review, we provide an overview of the different methods used to generate and characterise hPSC-ECs before comprehensively assessing their effectiveness in cardiovascular disease modelling and high-throughput drug screening. Furthermore, we explore current obstacles that will need to be overcome to unleash the full potential of hPSC-ECs in facilitating patient-specific precision medicine. Addressing these challenges holds great promise in advancing our understanding of intricate cardiovascular diseases and in tailoring personalised therapeutic strategies.

High-fat diet causes mitochondrial damage and downregulation of mitofusin-2 and optic atrophy-1 in multiple organs

High-fat consumption promotes the development of obesity, which is associated with various chronic illnesses. Mitochondria are the energy factories of eukaryotic cells, maintaining self-stability through a fine-tuned quality-control network. In the present study, we evaluated high-fat diet (HFD)-induced changes in mitochondrial ultrastructure and dynamics protein expression in multiple organs. C57BL/6J male mice were fed HFD or normal diet (ND) for 24 weeks. Compared with ND-fed mice, HFD-fed mice exhibited increased body weight, cardiomyocyte enlargement, pulmonary fibrosis, hepatic steatosis, renal and splenic structural abnormalities. The cellular apoptosis of the heart, liver, and kidney increased. Cellular lipid droplet deposition and mitochondrial deformations were observed. The proteins related to mitochondrial biogenesis (TFAM), fission (DRP1), autophagy (LC3 and LC3-II: LC3-I ratio), and mitophagy (PINK1) presented different changes in different organs. The mitochondrial fusion regulators mitofusin-2 (MFN2) and optic atrophy-1 (OPA1) were consistently downregulated in multiple organs, even the spleen. TOMM20 and ATP5A protein were enhanced in the heart, skeletal muscle, and spleen, and attenuated in the kidney. These results indicated that high-fat feeding caused pathological changes in multiple organs, accompanied by mitochondrial ultrastructural damage, and MFN2 and OPA1 downregulation. The mitochondrial fusion proteins may become promising targets and/or markers for treating metabolic disease.

Glucagon-like peptide-1 receptor agonist, semaglutide attenuates chronic liver disease-induced skeletal muscle atrophy in diabetic mice

A glucagon-like peptide-1 receptor agonist (GLP-1RA) has recently been established as a pharmacological option for the treatment of type 2 diabetes. Recent studies have demonstrated the molecular role of GLP-1R in skeletal muscle homeostasis; however, the therapeutic efficacy of semaglutide, a GLP-1RA, on skeletal muscle atrophy in chronic liver disease (CLD) under diabetic conditions remains unclear. In the present study, semaglutide effectively inhibited psoas muscle atrophy and suppressed declines in grip strength in a diethoxycarbonyl-1,4-dihydrocollidine (DDC) diet-fed diabetic KK-A^y mouse model. Moreover, semaglutide inhibited ubiquitin-proteosome-mediated skeletal muscle proteolysis and promoted myogenesis in palmitic acid (PA)-stimulated C2C12 murine myocytes. Mechanistically, this effect of semaglutide on skeletal muscle atrophy was mediated by multiple functional pathways. First, semaglutide protected against hepatic injury in mice accompanied by increased production of insulin-like growth factor 1 and reduced accumulation of reactive oxygen species (ROS). These effects were associated with decreased proinflammatory cytokines and ROS accumulation, leading to the suppression of ubiquitin-proteosome muscle degradation. Moreover, semaglutide inhibited the amino acid starvation-related stress signaling that was activated under chronic liver injury, resulting in the recovery of the mammalian target of rapamycin activity in the skeletal muscle of DDC-diet fed KK-A^y mice. Second, semaglutide improved skeletal muscle atrophy by directly stimulating GLP-1R in myocytes. Semaglutide induced cAMP-mediated activation of PKA and AKT, enhanced mitochondrial biogenesis, and reduced ROS accumulation, thereby resulting in inhibition of NF-κB/myostatin-mediated ubiquitin-proteosome degradation and the augmentation of heat-shock factor-1-mediated myogenesis. Collectively, semaglutide may have potential as a new therapeutic strategy for CLD-related skeletal muscle wasting.

Mitochondrial DNA Polymorphism in HV1 and HV2 Regions and 12S rDNA in Perimenopausal Hypertensive Women

Estrogens enhance cellular mitochondrial activity. The diminution of female hormones during menopause may have an effect on the mitochondrial genome and the expression of mitochondrial proteins. Hence, oxidative stress and the pro-inflammatory state contribute to the formation of systemic illnesses including arterial hypertension (AH). This study aimed to determine the types and frequency of mutations in the mitochondrial DNA (mtDNA) nucleotide sequence in the hypervariable regions 1 and 2 (HV1 and HV2) and the 12S RNA coding sequence of the D-loop in postmenopausal women with hypertension. In our study, 100 women were investigated, 53 of whom were postmenopausal and 47 of whom were premenopausal (53.9 ± 3.7 years vs. 47.7 ± 4.2 years, respectively). Of those studied, 35 premenopausal and 40 postmenopausal women were diagnosed with AH. A medical checkup with 24 h monitoring of blood pressure (RR) and heart rate was undertaken (HR). The polymorphism of the D-loop and 12S rDNA region of mtDNA was examined. Changes in the nucleotide sequence of mtDNA were observed in 23% of the group of 100 women. The changes were identified in 91.3% of HV1 and HV2 regions, 60.9% of HV1 segments, 47.5% of HV2 regions, and 43.5% of 12S rDNA regions. The frequency of nucleotide sequence alterations in mtDNA was substantially higher in postmenopausal women (34%) than in premenopausal women (10.6%), p = 0.016. A higher frequency of changes in HV1 + HV2 sections in postmenopausal women (30.2%) compared to the premenopausal group (10.6%) was detected, p = 0.011. Only postmenopausal women were found to have modifications to the HV2 segment and the 12S rDNA region. After menopause, polymorphism in the mtDNA region was substantially more frequent in women with arterial hypertension than before menopause (p = 0.030; 37.5% vs. 11.5%). Comparable findings were observed in the HV2 and HV1 regions of the AH group (35% vs. 11.5%), p = 0.015, in the HV1 segment (25% vs. 11.5%), p = 0.529, and in the HV2 segment, 12S rDNA (25% vs. 0%). More than 80% of all changes in nucleotide sequence were homoplasmic. The mtDNA polymorphisms of the nucleotide sequence in the HV1 and HV2 regions, the HV2 region alone, and the 12S RNA coding sequence were associated with estrogen deficiency and a more severe course of arterial hypertension, accompanied by symptoms of adrenergic stimulation.

CBL/CAP Is Essential for Mitochondria Respiration Complex I Assembly and Bioenergetics Efficiency in Muscle Cells

CBL is rapidly phosphorylated upon insulin receptor activation. Mice whole body CBL depletion improved insulin sensitivity and glucose clearance; however, the precise mechanisms remain unknown. We depleted either CBL or its associated protein SORBS1/CAP independently in myocytes and assessed mitochondrial function and metabolism compared to control cells. CBL- and CAP-depleted cells showed increased mitochondrial mass with greater proton leak. Mitochondrial respiratory complex I activity and assembly into respirasomes were reduced. Proteome profiling revealed alterations in proteins involved in glycolysis and fatty acid degradation. Our findings demonstrate CBL/CAP pathway couples insulin signaling to efficient mitochondrial respiratory function and metabolism in muscle.

Elevated LDL-C, high blood pressure, and low peak V ˙ O2 associate with platelet mitochondria function in children-The Arkansas Active Kids Study

Purpose: To evaluate the association of platelet (PL) mitochondria respiration with markers of cardiovascular health in children ages 7-10 years. Methods: PL mitochondrial respiration (n = 91) was assessed by high resolution respirometry (HRR): Routine (R) respiration, complex (C) I linked respiration (CI), and maximal uncoupled electron transport capacity of CII (CII_E) were measured. The respiratory control ratio (RCR) was calculated as the ratio of maximal oxidative phosphorylation capacity of CI and CI leak respiration (P_CI/L_CI). Peak V ˙ O₂ (incremental bike test) and body composition (dual-energy X-ray absorptiometry) were measured. Multiple generalized linear regression analysis was used to model the association of measures by HRR with variables of interest: adiposity, low-density lipoprotein (LDL-C) and triglyceride (TG) status (normal vs. elevated) HOMA2-IR, blood pressure status (normal vs. high), and demographics. Results: R and CI-linked respiration positively associated with adiposity, high blood pressure (HBP), and peak V ˙ O₂. R and CI-linked respiration had inverse association with age and elevated LDL-C. CII_E was higher in children with elevated LDL-C (log-β = -0.54, p = 0.010). HBP and peak V ˙ O₂ interacted in relation to RCR (log-β = -0.01, p = 0.028). Specifically, RCR was lowest among children with HBP and low aerobic capacity (i.e., mean peak V ˙ O₂ -1SD). HOMA2-IR did not associate with measures of PL mitochondria respiration. Conclusion: In PL, R and CI-linked mitochondrial respiration directly associate with adiposity, peak V ˙ O₂ and HBP. Elevated LDL-C associates with lower CI-linked respiration which is compensated by increasing CII respiration. PL bioenergetics phenotypes in children associate with whole-body metabolic health status.

Multi-omics study identifies novel signatures of DNA/RNA, amino acid, peptide, and lipid metabolism by simulated diabetes on coronary endothelial cells

Coronary artery endothelial cells (CAEC) exert an important role in the development of cardiovascular disease. Dysfunction of CAEC is associated with cardiovascular disease in subjects with type 2 diabetes mellitus (T2DM). However, comprehensive studies of the effects that a diabetic environment exerts on this cellular type are scarce. The present study characterized the molecular perturbations occurring on cultured bovine CAEC subjected to a prolonged diabetic environment (high glucose and high insulin). Changes at the metabolite and peptide level were assessed by Liquid Chromatography–Mass Spectrometry (LC–MS²) and chemoinformatics. The results were integrated with published LC–MS²-based quantitative proteomics on the same in vitro model. Our findings were consistent with reports on other endothelial cell types and identified novel signatures of DNA/RNA, amino acid, peptide, and lipid metabolism in cells under a diabetic environment. Manual data inspection revealed disturbances on tryptophan catabolism and biosynthesis of phenylalanine-based, glutathione-based, and proline-based peptide metabolites. Fluorescence microscopy detected an increase in binucleation in cells under treatment that also occurred when human CAEC were used. This multi-omics study identified particular molecular perturbations in an induced diabetic environment that could help unravel the mechanisms underlying the development of cardiovascular disease in subjects with T2DM.

Hypoxia-induced HIF1α activation regulates small extracellular vesicle release in human embryonic kidney cells

Extracellular vesicles (EVs) are membrane enclosures released by eukaryotic cells that carry bioactive molecules and serve to modulate biological responses in recipient cells. Both increased EV release and altered EV composition are associated with the development and progression of many pathologies including cancer. Hypoxia, a feature of rapidly growing solid tumours, increases the release of EVs. However, the molecular mechanisms remain unknown. The hypoxia inducible factors (HIFs) are transcription factors that act as major regulators of the cellular adaptations to hypoxia. Here, we investigated the requirement of HIF pathway activation for EV release in Human Embryonic Kidney Cells (HEK293). Time course experiments showed that EV release increased concomitantly with sustained HIF1α and HIF2α activation following the onset of hypoxia. shRNA mediated knock-down of HIF1α but not HIF2α abrogated the effect of hypoxia on EV release, suggesting HIF1α is involved in this process. However, stabilization of HIF proteins in normoxic conditions through: (i) heterologous expression of oxygen insensitive HIF1α or HIF2α mutants in normoxic cells or (ii) chemical inhibition of the prolyl hydroxylase 2 (PHD2) repressor protein, did not increase EV release, suggesting HIF activation alone is not sufficient for this process. Our findings suggest HIF1α plays an important role in the regulation of EV release during hypoxia in HEK293 cells, however other hypoxia triggered mechanisms likely contribute as stabilization of HIF1α alone in normoxia is not sufficient for EV release.

In vitro models of insulin resistance: Mitochondrial coupling is differently affected in liver and muscle cells

Mitochondrial dysfunction in diabetes is a widely studied topic, but inconsistency in literature data suggests a need for valid and reproducible models that will help to clarify this interaction. We aimed to establish insulin resistance models using chronic high insulin treatment in two cell types: myocytes and hepatocytes, characterise them in terms of mitochondrial function and compare them to the widely used palmitate-induced model of insulin resistance. We found that insulin lowered phosphorylation of Akt while not affecting cell viability, ROS production, mitochondrial morphology or respiration, and caused decrease in mitochondrial coupling only in muscle but not in liver cells.

Comparing the effects of palmitate, insulin, and palmitate-insulin co-treatment on myotube metabolism and insulin resistance

Previous studies have shown various metabolic stressors such as saturated fatty acids (SFA) and excess insulin promote insulin resistance in metabolically meaningful cell types (such as skeletal muscle). Additionally, these stressors have been linked with suppressed mitochondrial metabolism, which is also a common characteristic of skeletal muscle of diabetics. This study characterized the individual and combined effects of excess lipid and excess insulin on myotube metabolism and related metabolic gene and protein expression. C2C12 myotubes were treated with either 500 μM palmitate (PAM), 100 nM insulin (IR), or both (PAM-IR). qRT-PCR and western blot were used to measure metabolic gene and protein expression, respectively. Oxygen consumption was used to measure mitochondrial metabolism. Glycolytic metabolism and insulin-mediated glucose uptake were measured via extracellular acidification rate. Cellular lipid and mitochondrial content were measured using Nile Red and NAO staining, respectively. IR and PAM-IR treatments led to reductions in p-Akt expression. IR treatment reduced insulin mediated glucose metabolism while PAM and PAM-IR treatment showed increases with concurrent reductions in mitochondrial metabolism. All three treatments showed suppression in mitochondrial metabolism. PAM and PAM-IR also showed increases in glycolytic metabolism. While PAM and PAM-IR significantly increased lipid content, expression of inflammatory and lipogenic proteins were unaltered. Lastly, PAM-IR reduced BCAT2 protein expression, a regulator of BCAA metabolism. Both stressors independently reduced insulin signaling, mitochondrial function, and cell metabolism, however, only PAM-IR co-treatment significantly reduced the expression of regulators of metabolism not seen with individual stressors, suggesting an additive effect of stressors on metabolic programming.

Cell models for studying muscle insulin resistance

Type 2 diabetes is one of the most prevalent chronic diseases in the world today. Insulin resistance - a reduced responsiveness of tissues to insulin - is a hallmark of type 2 diabetes pathology. Skeletal muscle plays a pivotal role in glucose homeostasis - it is responsible for the majority of insulin-mediated glucose disposal and thus is one of the tissues most affected by insulin resistance. To study the molecular mechanisms of a disease, researchers often turn to cell models - they are inexpensive, easy to use, and exist in a controlled environment with few unknown variables. Cell models for exploring muscle insulin resistance are constructed using primary cell cultures or immortalised cell lines and treating them with fatty acids, high insulin or high glucose concentrations. The choice of cell culture, concentration and duration of the treatment and the methods for measuring insulin sensitivity, in order to confirm the model, are rarely discussed. Choosing an appropriate and physiologically relevant model for a particular topic of interest is required in order for the results to be reproducible, relevant, comparable and translatable to more complex biological systems. Cell models enable research that would otherwise be inaccessible but, especially when studying human disease, they do not serve a purpose if they are not in line with the biological reality. This review aims to summarise and critically evaluate the most commonly used cell models of muscle insulin resistance: the rationale for choosing these exact treatments and conditions, the protocols for constructing the models and the measurable outcomes used for confirming insulin resistance in the cells.

Anti-diabetic effects of Grifola frondosa bioactive compound and its related molecular signaling pathways in palmitate-induced C2C12 cells

ETHNOPHARMACOLOGICAL RELEVANCE

Grifola frondosa (GF), a high value medicinal mushroom, is popularly consumed as traditional medicines and health foods in China and Japan. It is a herbal medicine traditionally used for treating inflammation, cancer and diabetes.

AIM OF THE STUDY

This study aimed to examine the anti-diabetic effects of a GF bioactive compound ergosterol peroxide (EPO), and its mechanism(s) of action in palmitate (PA)-induced C2C12 cells.

MATERIALS AND METHODS

EPO was isolated and purified from GF fruiting bodies, and used to test for anti-diabetic activity in PA-induced murine C2C12 skeletal muscle cells through measuring glucose uptake, intracellular ROS production, and expressions of MAPKs, IRS-1, PI3K, Akt and GLUT-4 proteins.

RESULTS

EPO significantly up-regulated glucose absorption and increased cell growth. At 5 μM, EPO significantly enhanced glucose uptake and decreased ROS formation, as well as up-regulated the expression of IRS-1, p-IRS-1, PI3K, Akt, p-Akt, and GLUT-4 proteins in PA-induced cells, while their p-JNK and p-p38 expression were down-regulated. GLUT-4 siRNA treatment effectively down-regulated the EPO-induced absorption of glucose and inhibited the expression of GLUT-4.

CONCLUSION

These results suggest that the anti-diabetic effect of GF was from its bioactive compound EPO through the inhibition of ROS production, up-regulation of glucose absorption, and modulation of PI3K/Akt, MAPKs and GLUT-4 signaling transduction pathways.

C2C12 cell model: its role in understanding of insulin resistance at the molecular level and pharmaceutical development at the preclinical stage

OBJECTIVES

The myoblast cell line, C2C12, has been utilised extensively in vitro as an examination model in understanding metabolic disease progression. Although it is indispensable in both preclinical and pharmaceutical research, a comprehensive review of its use in the investigation of insulin resistance progression and pharmaceutical development is not available.

KEY FINDINGS

C2C12 is a well-documented model, which can facilitate our understanding in glucose metabolism, insulin signalling mechanism, insulin resistance, oxidative stress, reactive oxygen species and glucose transporters at cellular and molecular levels. With the aid of the C2C12 model, recent studies revealed that insulin resistance has close relationship with various metabolic diseases in terms of disease progression, pathogenesis and therapeutic management. A holistic, safe and effective disease management is highly of interest. Therefore, significant efforts have been paid to explore novel drug compounds and natural herbs that can elicit therapeutic effects in the targeted sites at both cellular (e.g. mitochondria, glucose transporter) and molecular level (e.g. genes, signalling pathway).

SUMMARY

The use of C2C12 myoblast cell line is meaningful in pharmaceutical and biomedical research due to their expression of GLUT-4 and other features that are representative to human skeletal muscle cells. With the use of the C2C12 cell model, the impact of drug delivery systems (nanoparticles and quantum dots) on skeletal muscle, as well as the relationship between exercise, pancreatic β-cells and endothelial cells, was discovered.

Loss of Caveolin-1 Is Associated with a Decrease in Beta Cell Death in Mice on a High Fat Diet

Elevated free fatty acids (FFAs) impair beta cell function and reduce beta cell mass as a consequence of the lipotoxicity that occurs in type 2 diabetes (T2D). We previously reported that the membrane protein caveolin-1 (CAV1) sensitizes to palmitate-induced apoptosis in the beta pancreatic cell line MIN6. Thus, our hypothesis was that CAV1 knock-out (CAV1 KO) mice subjected to a high fat diet (HFD) should suffer less damage to beta cells than wild type (WT) mice. Here, we evaluated the in vivo response of beta cells in the pancreatic islets of 8-week-old C57Bl/6J CAV1 KO mice subjected to a control diet (CD, 14% kcal fat) or a HFD (60% kcal fat) for 12 weeks. We observed that CAV1 KO mice were resistant to weight gain when on HFD, although they had high serum cholesterol and FFA levels, impaired glucose tolerance and were insulin resistant. Some of these alterations were also observed in mice on CD. Interestingly, KO mice fed with HFD showed an adaptive response of the pancreatic beta cells and exhibited a significant decrease in beta cell apoptosis in their islets compared to WT mice. These in vivo results suggest that although the CAV1 KO mice are metabolically unhealthy, they adapt better to a HFD than WT mice. To shed light on the possible signaling pathway(s) involved, MIN6 murine beta cells expressing (MIN6 CAV) or not expressing (MIN6 Mock) CAV1 were incubated with the saturated fatty acid palmitate in the presence of mitogen-activated protein kinase inhibitors. Western blot analysis revealed that CAV1 enhanced palmitate-induced JNK, p38 and ERK phosphorylation in MIN6 CAV1 cells. Moreover, all the MAPK inhibitors partially restored MIN6 viability, but the effect was most notable with the ERK inhibitor. In conclusion, our results suggest that CAV1 KO mice adapted better to a HFD despite their altered metabolic state and that this may at least in part be due to reduced beta cell damage. Moreover, they indicate that the ability of CAV1 to increase sensitivity to FFAs may be mediated by MAPK and particularly ERK activation.

Fibroblast growth factor 19 alleviates palmitic acid-induced mitochondrial dysfunction and oxidative stress via the AMPK/PGC-1α pathway in skeletal muscle

Obesity-induced fat ectopic deposition results in mitochondrial dysfunction and oxidative stress in skeletal muscle, which could impair the quality and function of the skeletal muscle. Human fibroblast growth factor 19 (FGF19) acts as a vital metabolic regulator of bile acid synthesis and metabolic homeostasis. Recent studies have shown that FGF19 regulates skeletal muscle mass through the enlargement of muscle fiber size and protects muscles from atrophy. However, the role of FGF19 in regulating mitochondrial function and the antioxidant response in skeletal muscle remains unknown. Therefore, we investigated the effect of FGF19 on palmitic acid (PA)-induced mitochondrial dysfunction and oxidative stress in C2C12 cells. In this study, we found that FGF19 can increase the mRNA and protein expression levels of mitochondrial biogenesis regulators (PGC-1α, Nrf-1, and TFAM) and antioxidant response regulators (Nrf-2 and HO-1), alleviating PA-induced mitochondrial dysfunction and oxidative stress. However, the regulatory effect of FGF19 was blocked by Compound C, an AMP-activated protein kinase (AMPK) inhibitor, and siRNA knockdown of PGC-1a. Taken together, these findings indicate that FGF19 might promote mitochondrial biogenesis and antioxidant response via the AMPK/PGC-1α pathway, attenuating the effect of PA on mitochondrial dysfunction and oxidative stress; therefore, FGF19 might be a potential therapeutic target for the effects of obesity on skeletal muscle.

Aging-related liver degeneration is associated with increased bacterial endotoxin and lipopolysaccharide binding protein levels

Aging is a risk factor in the development of many diseases, including liver-related diseases. The two aims of the present study were 1) to determine how aging affects liver health in mice in the absence of any interventions and 2) if degenerations observed in relation to blood endotoxin levels are critical in aging-associated liver degeneration. Endotoxin levels and markers of liver damage, mitochondrial dysfunction, insulin resistance, and apoptosis as well as the Toll-like receptor 4 (Tlr-4) signaling cascade were studied in liver tissue and blood, respectively, of 3- and 24-mo-old male C57BL/6J mice. In a second set of experiments, 3- to 4-mo-old and 14-mo-old female lipopolysaccharide-binding protein (LBP)^-/- mice and littermates fed standard chow, markers of liver damage, insulin resistance, and mitochondrial dysfunction were assessed. Plasma activity of aspartate aminotransferase and histological signs of hepatic inflammation and fibrosis were significantly higher in old C57BL/6J mice than in young animals. The number of neutrophils, CD8α-positive cells, and mRNA expression of markers of apoptosis were also significantly higher in livers of old C57BL/6J mice compared with young animals, being also associated with a significant induction of hepatic Tlr-4 and LBP expression as well as higher endotoxin levels in peripheral blood. Compared with age-matched littermates, LBP^-/- mice display less signs of senescence in liver. Taken together, our data suggest that, despite being fed standard chow, old mice developed liver inflammation and beginning fibrosis and that bacterial endotoxin may play a critical role herein.NEW & NOTEWORTHY Old age in mice is associated with marked signs of liver degeneration, hepatic inflammation, and fibrosis. Aging-associated liver degeneration is associated with elevated bacterial endotoxin levels and an induction of lipopolysaccharide-binding protein (LBP) and Toll-like receptor 4-dependent signaling cascades in liver tissue. Furthermore, in old aged LBP^-/- mice, markers of senescence seem to be lessened, supporting the hypothesis that bacterial endotoxin levels might be critical in aging-associated decline of liver.

Xenobiotic mediated diabetogenesis: Developmental exposure to dichlorvos or atrazine leads to type 1 or type 2 diabetes in Drosophila

The increased incidence of diabetes to the magnitude of a global epidemic is attributed to non-traditional risk factors, including exposure to environmental chemicals. However, the contribution of xenobiotic exposure during the development of an organism to the etiology of diabetes is not fully addressed. Developing stages are more susceptible to chemical insult, but knowledge on the consequence of the same to the onset of diabetes is residual. In this context, by using Drosophila melanogaster having conserved Insulin/Insulin growth factor-like signaling (IIS) as well as glucose homeostasis as a model, we evaluated the potential of developmental exposure to dichlorvos (DDVP, an organophosphorus pesticide) or atrazine (herbicide) to cause diabetes in exposed organisms. Flies exposed to DDVP during their development display insulin deficiency or type 1 diabetes (T1D) while those exposed to atrazine show insulin resistance or type 2 diabetes (T2D), suggesting that exposure to these xenobiotics during organismal development can result in diabetes and that different mechanisms underlie pesticide mediated diabetes. We show that oxidative stress-mediated c-Jun N-terminal kinase (JNK) signaling activation underlies insulin resistance in flies exposed to atrazine during their development while DDVP-mediated T1D involves activation of caspase-mediated cell death pathway. Mitigation of oxidative stress through over-expression of SOD2 in atrazine (20μg/ml) exposed flies, revealed significantly decreased oxidative stress levels and reduced phosphorylation of JNK. Moreover, glucose and Akt phosphorylation levels in SOD2 over-expression flies exposed to atrazine were comparable to those in controls, suggesting restoration in insulin sensitivity. Therefore, exposure to xenobiotics during development is a common risk factor for the development of type 1 or type 2 diabetes. Accordingly, the present study cautions against the use of such diabetogenic pesticides. Also, mitigation of oxidative stress or anti-oxidant supplementation could be a potential therapy for xenobiotic mediated type 2 diabetes.

Metabolic implications of organelle-mitochondria communication

Cellular organelles are not static but show dynamism-a property that is likely relevant for their function. In addition, they interact with other organelles in a highly dynamic manner. In this review, we analyze the proteins involved in the interaction between mitochondria and other cellular organelles, especially the endoplasmic reticulum, lipid droplets, and lysosomes. Recent results indicate that, on one hand, metabolic alterations perturb the interaction between mitochondria and other organelles, and, on the other hand, that deficiency in proteins involved in the tethering between mitochondria and the ER or in specific functions of the interaction leads to metabolic alterations in a variety of tissues. The interaction between organelles is an emerging field that will permit to identify key proteins, to delineate novel modulation pathways, and to elucidate their implications in human disease.

Investigating the Role of Mitochondria in Type 2 Diabetes - Lessons from Lipidomics and Proteomics Studies of Skeletal Muscle and Liver

Mitochondrial dysfunction is discussed as a key player in the pathogenesis of type 2 diabetes mellitus (T2Dm), a highly prevalent disease rapidly developing as one of the greatest global health challenges of this century. Data however about the involvement of mitochondria, central hubs in bioenergetic processes, in the disease development are still controversial. Lipid and protein homeostasis are under intense discussion to be crucial for proper mitochondrial function. Consequently proteomics and lipidomics analyses might help to understand how molecular changes in mitochondria translate to alterations in energy transduction as observed in the healthy and metabolic diseases such as T2Dm and other related disorders. Mitochondrial lipids integrated in a tool covering proteomic and functional analyses were up to now rarely investigated, although mitochondrial lipids might provide a possible lynchpin in the understanding of type 2 diabetes development and thereby prevention. In this chapter state-of-the-art analytical strategies, pre-analytical aspects, potential pitfalls as well as current proteomics and lipidomics-based knowledge about the pathophysiological role of mitochondria in the pathogenesis of type 2 diabetes will be discussed.

Cbl downregulation increases RBP4 expression in adipocytes of female mice

Obesity leads to adipose tissue dysfunction, insulin resistance and diabetes. Adipose tissue produces adipokines that contribute to regulate insulin sensitivity. In turn, insulin stimulates the production and release of some adipokines. Casitas-b-lymphoma proteins (c-Cbl, Cbl-b and Cbl3) are intracellular adaptor signalling proteins that are rapidly phosphorylated by activation of tyrosine kinase receptors. c-Cbl is rapidly phosphorylated by insulin in adipocytes. Here, we tested the hypothesis that Cbl signalling regulates adipokine expression in adipose tissue. We determined the adipokine profile of WAT of Cbl-/- and Cbl+/+ mice in the C57BL6 background. Female Cbl-/- mice exhibited altered expression of adiponectin, leptin and RBP4 in visceral adipose tissue, while no significant changes were seen in male mice. TNFα and IL6 levels were unaffected by Cbl depletion. RBP4 expression was unchanged in liver. Adipose tissue of Cbl-/- animals showed increased basal activation of extracellular regulated kinases (ERK1/2) compared to Cbl+/+. c-Cbl knockdown in 3T3L1 adipocytes also increased basal ERK phosphorylation and RBP4 expression. Inhibition of ERK1/2 phosphorylation in Cbl-depleted 3T3L1 adipocytes or in adipose tissue explants of Cbl-/- mice reduced RBP4 mRNA. 17β-Estradiol increased RBP4 mRNA in adipocytes. Cbl depletion did not change ER expression but increased phosphorylation of ERα at S118, a target site for ERK1/2. ERK1/2 inhibition reduced phosphoER and RBP4 levels. These findings suggest that Cbl contributes to regulate RBP4 expression in adipose of female mice through ERK1/2-mediated activation of ERα. Since Cbl signalling is compromised in diabetes, these data highlight a novel mechanism that upregulates RBP4 locally.

RAGE-NF-κB-PPARγ Signaling is Involved in AGEs-Induced Upregulation of Amyloid-β Influx Transport in an In Vitro BBB Model

The receptor for advanced glycation end products (RAGE) at the blood-brain barrier (BBB) is critical for regulation of amyloid-β (Aβ) homeostasis in the diabetic brain. In this study, we used an in vitro BBB model consisting of mouse brain capillary endothelial cells (MBCECs) to investigate whether advanced glycation end products (AGEs) increase Aβ influx transport across the BBB and the underlying mechanisms. We found that AGEs induced Aβ influx transport across the BBB in concentration- and time-dependent manner, accompanied by increased RAGE expression and nuclear factor-kappa B p65 (NF-κB p65), and decreased nuclear peroxisome proliferator-activated receptor γ (PPARγ). Blockade of RAGE with its antibody and inhibition of NF-κB signaling with PDTC as well as activation of PPARγ with rosiglitazone significantly decreased Aβ transport across the BBB from the periphery to the brain. These treatments also pronouncedly suppressed AGEs-induced increases in RAGE expression and nuclear NF-κB p65 and reversed the decrease in nuclear PPARγ. These results suggest that RAGE-NF-κB-PPARγ signaling is involved in regulation of AGEs-induced influx transport of Aβ across the BBB and targeting the signaling pathway could serve as a novel strategy to modify such Aβ transport.

Improvement of mitochondrial function by celastrol in palmitate-treated C2C12 myotubes via activation of PI3K-Akt signaling pathway

Compelling evidences posited that high level of saturated fatty acid gives rise to mitochondrial dysfunction and inflammation in the development of insulin resistance in skeletal muscle. Celastrol is a pentacyclic triterpenoid derived from the root extracts of Tripterygium wilfordii that possesses potent anti-inflammatory properties in a number of animal models with metabolic diseases. However, the cellular mechanistic action of celastrol in alleviating obesity-induced insulin resistance in skeletal muscle remains largely unknown. Therefore, the present investigation evaluated the attributive properties of celastrol at different concentrations (10, 20, 30 and 40nM) on insulin resistance in C2C12 myotubes evoked by palmitate. We demonstrated that celastrol improved mitochondrial functions through significant enhancement of intracellular ATP content, mitochondrial membrane potential, citrate synthase activity and decrease of mitochondrial superoxide productions. Meanwhile, augmented mitochondrial DNA (mtDNA) content with suppressed DNA oxidative damage were observed following celastrol treatment. Celastrol significantly enhanced fatty acid oxidation rate and increased the level of tricarboxylic acid (TCA) cycle intermediates in palmitate-treated cells. Further analysis revealed that the improvement of glucose uptake activity in palmitate-loaded myotubes was partly mediated by celastrol via activation of PI3K-Akt insulin signaling pathway. Collectively, these findings provided evidence for the first time that the protection from palmitate-mediated insulin resistance in C2C12 myotubes by celastrol is likely associated with the improvement of mitochondrial functions-related metabolic activities.

Metabolomics allows the discrimination of the pathophysiological relevance of hyperinsulinism in obese prepubertal children

BACKGROUND/OBJECTIVES

Insulin resistance (IR) is the cornerstone of the obesity-associated metabolic derangements observed in obese children. Targeted metabolomics was employed to explore the pathophysiological relevance of hyperinsulinemia in childhood obesity in order to identify biomarkers of IR with potential clinical application.

SUBJECTS/METHODS

One hundred prepubertal obese children (50 girls/50 boys, 50% IR and 50% non-IR in each group), underwent an oral glucose tolerance test for usual carbohydrate and lipid metabolism determinations. Fasting serum leptin, total and high molecular weight-adiponectin and high-sensitivity C-reactive protein (CRP) levels were measured and the metabolites showing significant differences between IR and non-IR groups in a previous metabolomics study were quantified. Enrichment of metabolic pathways (quantitative enrichment analysis) and the correlations between lipid and carbohydrate metabolism parameters, adipokines and serum metabolites were investigated, with their discriminatory capacity being evaluated by receiver operating characteristic (ROC) analysis.

RESULTS

Twenty-three metabolite sets were enriched in the serum metabolome of IR obese children (P<0.05, false discovery rate (FDR)<5%). The urea cycle, alanine metabolism and glucose-alanine cycle were the most significantly enriched pathways (P_FDR<0.00005). The high correlation between metabolites related to fatty acid oxidation and amino acids (mainly branched chain and aromatic amino acids) pointed to the possible contribution of mitochondrial dysfunction in IR. The degree of body mass index-standard deviation score (BMI-SDS) excess did not correlate with any of the metabolomic components studied. In the ROC analysis, the combination of leptin and alanine showed a high IR discrimination value in the whole cohort (area under curve, AUC_ALL=0.87), as well as in boys (AUC_M=0.84) and girls (AUC_F=0.91) when considered separately. However, the specific metabolite/adipokine combinations with highest sensitivity were different between the sexes.

CONCLUSIONS

Combined sets of metabolic, adipokine and metabolomic parameters can identify pathophysiological relevant IR in a single fasting sample, suggesting a potential application of metabolomic analysis in clinical practice to better identify children at risk without using invasive protocols.

Skeletal muscle insulin resistance: role of mitochondria and other ROS sources

At present, obesity is one of the most important public health problems in the world because it causes several diseases and reduces life expectancy. Although it is well known that insulin resistance plays a pivotal role in the development of type 2 diabetes mellitus (the more frequent disease in obese people) the link between obesity and insulin resistance is yet a matter of debate. One of the most deleterious effects of obesity is the deposition of lipids in non-adipose tissues when the capacity of adipose tissue is overwhelmed. During the last decade, reduced mitochondrial function has been considered as an important contributor to 'toxic' lipid metabolite accumulation and consequent insulin resistance. More recent reports suggest that mitochondrial dysfunction is not an early event in the development of insulin resistance, but rather a complication of the hyperlipidemia-induced reactive oxygen species (ROS) production in skeletal muscle, which might promote mitochondrial alterations, lipid accumulation and inhibition of insulin action. Here, we review the literature dealing with the mitochondria-centered mechanisms proposed to explain the onset of obesity-linked IR in skeletal muscle. We conclude that the different pathways leading to insulin resistance may act synergistically because ROS production by mitochondria and other sources can result in mitochondrial dysfunction, which in turn can further increase ROS production leading to the establishment of a harmful positive feedback loop.

FOXO1 and GSK-3β Are Main Targets of Insulin-Mediated Myogenesis in C2C12 Muscle Cells

Myogenesis and muscle hypertrophy account for muscle growth and adaptation to work overload, respectively. In adults, insulin and insulin-like growth factor 1 stimulate muscle growth, although their links with cellular energy homeostasis are not fully explained. Insulin plays critical role in the control of mitochondrial activity in skeletal muscle cells, and mitochondria are essential for insulin action. The aim of this study was to elucidate molecular mechanism(s) involved in mitochondrial control of insulin-dependent myogenesis. The effects of several metabolic inhibitors (LY294002, PD98059, SB216763, LiCl, rotenone, oligomycin) on the differentiation of C2C12 myoblasts in culture were examined in the short-term (hours) and long-term (days) experiments. Muscle cell viability and mitogenicity were monitored and confronted with the activities of selected genes and proteins expression. These indices focus on the roles of insulin, glycogen synthase kinase 3 beta (GSK-3β) and forkhead box protein O1 (FOXO1) on myogenesis using a combination of treatments and inhibitors. Long-term insulin (10 nM) treatment in “normoglycemic” conditions led to increased myogenin expression and accelerated myogenesis in C2C12 cells. Insulin-dependent myogenesis was accompanied by the rise of mtTFA, MtSSB, Mfn2, and mitochondrially encoded Cox-1 gene expressions and elevated levels of proteins which control functions of mitochondria (kinase—PKB/AKT, mitofusin 2 protein—Mfn-2). Insulin, via the phosphatidylinositol 3-kinase (PI3-K)/AKT-dependent pathway reduced transcription factor FOXO1 activity and altered GSK-3β phosphorylation status. Once FOXO1 and GSK-3β activities were inhibited the rise in Cox-1 gene action and nuclear encoded cytochrome c oxidase subunit IV (COX IV) expressions were observed, even though some mRNA and protein results varied. In contrast to SB216763, LiCl markedly elevated Mfn2 and COX IV protein expression levels when given together with insulin. Thus, inhibition of GSK-3β activity by insulin alone or together with LiCl raised the expression of genes and some proteins central to the metabolic activity of mitochondria resulting in higher ATP synthesis and accelerated myogenesis. The results of this study indicate that there are at least two main targets in insulin-mediated myogenesis: notably FOXO1 and GSK-3β both playing apparent negative role in muscle fiber formation.

Overnourishment during lactation induces metabolic and haemodynamic heart impairment during adulthood

AIM

In this study, the effects of postnatal overfeeding on heart energy homoeostasis and cardiac haemodynamics in adult male Swiss mice were examined.

METHODS AND RESULTS

During the suckling period, the mice were divided into four groups of control or overfed pups in combination with baseline or ischaemia/reperfusion treatments (control group baseline, CGBL; overfed group baseline, OGBL; control group ischaemia/reperfusion, CGIR; and overfed group ischaemia/reperfusion, OGIR). End diastolic pressure (EDP), heart contraction speed (Max dP/dt), relaxation speed (Min dP/dt), isovolumetric relaxation time (Tau) and frequency by beats per minute (BPM) were measured. During baseline and ischaemia/reperfusion, key proteins such as AKT1, AKT2, AKT3, pAKT, adenosine monophosphate-activated protein kinase (AMPK), pAMPK, insulin receptor beta (IRβ), protein tyrosine phosphatase 1B (PTP1B), insulin receptor substrate 1 (IRS1), fatty acid binding protein (FABP), CD36, phosphoinositide 3-kinase (PI3K) and peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α) were studied. The expression of atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), carnitine palmitoyltransferase 1 (CPT1) and uncoupling protein 3 (UCP3) was studied as a marker of cardiac hypertrophy and energetic metabolism. Cardiac fibrosis was analyzed by quantifying collagen deposition, which is increased in the OGBL and OGIR groups compared with the control groups.

CONCLUSIONS

The OGBL group showed reduced EDP compared with the CGBL group and high Max dP/dt compared with the OGBL group. Ischaemia/reperfusion increased EDP and Min dP/dt in the intragroup comparison. By contrast, Tau and frequency were not significantly different among groups. The OGIR mice showed significant alterations in heart metabolism proteins, including AKT2, pAKT/AKT1, pAKT/AKT2, AMPK, pAMPK/AMPK, PTP1B, IRS1, FABP and CD36. Furthermore, alterations in ANP, BNP, CPT1 and UCP3 messenger RNA (mRNA) expression indicated hypertrophy and reduction in their efficiency, such that exclusive overnutrition in childhood induces a long-term effect on haemodynamics, metabolism and heart remodelling.

Mitofusin 2 as a driver that controls energy metabolism and insulin signaling

SIGNIFICANCE

Mitochondrial dynamics is a complex process that impacts on mitochondrial biology.

RECENT ADVANCES

Recent evidence indicates that proteins participating in mitochondrial dynamics have additional cellular roles. Mitofusin 2 (Mfn2) is a potent modulator of mitochondrial metabolism with an impact on energy metabolism in muscle, liver, and hypothalamic neurons. In addition, Mfn2 is subjected to tight regulation. Hence, factors such as proinflammatory cytokines, lipid availability, or glucocorticoids block its expression, whereas exercise and increased energy expenditure promote its upregulation.

CRITICAL ISSUES

Importantly, Mfn2 controls cell metabolism and insulin signaling by limiting reactive oxygen species production and by modulation of endoplasmic reticulum stress. In this connection, it is critical to understand precisely the molecular mechanisms involved in the global actions of Mfn2.

FUTURE DIRECTIONS

Future directions should concentrate into the analysis of those mechanisms, and to fully demonstrate that Mfn2 represents a cellular hub that senses the metabolic and hormonal milieu and drives the control of metabolic homeostasis.

Glucagon-like peptide-1 suppresses palmitic acid induced L02 cell apoptosis through JNK pathway

AIM: To investigate the potential effects of glucagon-like peptide-1 (GLP-1) on palmitic acid (PA) induced apoptosis of hepatic L02 cells and the underlying mechanism.

METHODS: L02 cells were stimulated with different levels (0.125, 0.250 and 0.500 mmol/L) of PA for different durations (12, 24 and 48 h) in the presence or absence of GLP-1. Cell Counting Kit-8 (CCK-8) assay was used to analyze the inhibitory effects on growth of L02 cells, and terminal-deoxynucleoitidyl transferase mediated nick end labeling (TUNEL) assay was used to observe the apoptotic rate of the cells. The expression of c-Jun N-terminal kinase (JNK), c-Jun, p-JNK and p-c-Jun proteins was detected by Western blot assay.

RESULTS: The growth of the L02 cells was significantly inhibited by PA in vitro, and PA induced the expression of p-JNK and p-c-Jun. GLP-1 suppressed the activation of JNK and c-Jun induced by PA.

CONCLUSION: PA can inhibit the proliferation of L02 cells and induce cell apoptosis. The JNK signaling pathway is probably involved in the mechanism of PA induced apoptosis.

Interleukin-15 modulates adipose tissue by altering mitochondrial mass and activity

Interleukin-15 (IL-15) is an immunomodulatory cytokine that affects body mass regulation independent of lymphocytes; however, the underlying mechanism(s) involved remains unknown. In an effort to investigate these mechanisms, we performed metabolic cage studies, assessed intestinal bacterial diversity and macronutrient absorption, and examined adipose mitochondrial activity in cultured adipocytes and in lean IL-15 transgenic (IL-15tg), overweight IL-15 deficient (IL-15-/-), and control C57Bl/6 (B6) mice. Here we show that differences in body weight are not the result of differential activity level, food intake, or respiratory exchange ratio. Although intestinal microbiota differences between obese and lean individuals are known to impact macronutrient absorption, differing gut bacteria profiles in these murine strains does not translate to differences in body weight in colonized germ free animals and macronutrient absorption. Due to its contribution to body weight variation, we examined mitochondrial factors and found that IL-15 treatment in cultured adipocytes resulted in increased mitochondrial membrane potential and decreased lipid deposition. Lastly, IL-15tg mice have significantly elevated mitochondrial activity and mass in adipose tissue compared to B6 and IL-15-/- mice. Altogether, these results suggest that IL-15 is involved in adipose tissue regulation and linked to altered mitochondrial function.

Unraveling biochemical pathways affected by mitochondrial dysfunctions using metabolomic approaches

Mitochondrial dysfunction(s) (MDs) can be defined as alterations in the mitochondria, including mitochondrial uncoupling, mitochondrial depolarization, inhibition of the mitochondrial respiratory chain, mitochondrial network fragmentation, mitochondrial or nuclear DNA mutations and the mitochondrial accumulation of protein aggregates. All these MDs are known to alter the capacity of ATP production and are observed in several pathological states/diseases, including cancer, obesity, muscle and neurological disorders. The induction of MDs can also alter the secretion of several metabolites, reactive oxygen species production and modify several cell-signalling pathways to resolve the mitochondrial dysfunction or ultimately trigger cell death. Many metabolites, such as fatty acids and derived compounds, could be secreted into the blood stream by cells suffering from mitochondrial alterations. In this review, we summarize how a mitochondrial uncoupling can modify metabolites, the signalling pathways and transcription factors involved in this process. We describe how to identify the causes or consequences of mitochondrial dysfunction using metabolomics (liquid and gas chromatography associated with mass spectrometry analysis, NMR spectroscopy) in the obesity and insulin resistance thematic.

Melatonin prevents mitochondrial dysfunction and insulin resistance in rat skeletal muscle

Melatonin has a number of beneficial metabolic actions and reduced levels of melatonin may contribute to type 2 diabetes. The present study investigated the metabolic pathways involved in the effects of melatonin on mitochondrial function and insulin resistance in rat skeletal muscle. The effect of melatonin was tested both in vitro in isolated rats skeletal muscle cells and in vivo using pinealectomized rats (PNX). Insulin resistance was induced in vitro by treating primary rat skeletal muscle cells with palmitic acid for 24 hr. Insulin-stimulated glucose uptake was reduced by palmitic acid followed by decreased phosphorylation of AKT which was prevented my melatonin. Palmitic acid reduced mitochondrial respiration, genes involved in mitochondrial biogenesis and the levels of tricarboxylic acid cycle intermediates whereas melatonin counteracted all these parameters in insulin-resistant cells. Melatonin treatment increases CAMKII and p-CREB but had no effect on p-AMPK. Silencing of CREB protein by siRNA reduced mitochondrial respiration mimicking the effect of palmitic acid and prevented melatonin-induced increase in p-AKT in palmitic acid-treated cells. PNX rats exhibited mild glucose intolerance, decreased energy expenditure and decreased p-AKT, mitochondrial respiration, and p-CREB and PGC-1 alpha levels in skeletal muscle which were restored by melatonin treatment in PNX rats. In summary, we showed that melatonin could prevent mitochondrial dysfunction and insulin resistance via activation of CREB-PGC-1 alpha pathway. Thus, the present work shows that melatonin play an important role in skeletal muscle mitochondrial function which could explain some of the beneficial effects of melatonin in insulin resistance states.

Trimetazidine prevents palmitate-induced mitochondrial fission and dysfunction in cultured cardiomyocytes

Metabolic and cardiovascular disease patients have increased plasma levels of lipids and, specifically, of palmitate, which can be toxic for several tissues. Trimetazidine (TMZ), a partial inhibitor of lipid oxidation, has been proposed as a metabolic modulator for several cardiovascular pathologies. However, its mechanism of action is controversial. Given the fact that TMZ is able to alter mitochondrial metabolism, we evaluated the protective role of TMZ on mitochondrial morphology and function in an in vitro model of lipotoxicity induced by palmitate. We treated cultured rat cardiomyocytes with BSA-conjugated palmitate (25 nM free), TMZ (0.1-100 μM), or a combination of both. We evaluated mitochondrial morphology and lipid accumulation by confocal fluorescence microscopy, parameters of mitochondrial metabolism (mitochondrial membrane potential, oxygen consumption rate [OCR], and ATP levels), and ceramide production by mass spectrometry and indirect immunofluorescence. Palmitate promoted mitochondrial fission evidenced by a decrease in mitochondrial volume (50%) and an increase in the number of mitochondria per cell (80%), whereas TMZ increased mitochondrial volume (39%), and decreased mitochondrial number (56%), suggesting mitochondrial fusion. Palmitate also decreased mitochondrial metabolism (ATP levels and OCR), while TMZ potentiated all the metabolic parameters assessed. Moreover, pretreatment with TMZ protected the cardiomyocytes from palmitate-induced mitochondrial fission and dysfunction. TMZ also increased lipid accumulation in cardiomyocytes, and prevented palmitate-induced ceramide production. Our data show that TMZ protects cardiomyocytes by changing intracellular lipid management. Thus, the beneficial effects of TMZ on patients with different cardiovascular pathologies can be related to modulation of the mitochondrial morphology and function.

Palmitic acid and oleic acid differentially regulate choline transporter-like 1 levels and glycerolipid metabolism in skeletal muscle cells

Choline is an essential nutrient required for the biosynthesis of membrane lipid phosphatidylcholine (PtdCho). Here we elucidate the mechanism of how palmitic acid (PAM) and oleic acid (OLA) regulate choline transporter-like protein 1 (CTL1/SLC44A1) function. We evaluated the mechanism of extracellular and intracellular transport of choline, and their contribution to PtdCho and other glycerolipid-diacylglycerol (DAG) and triacylglycerol (TAG) homeostasis in differentiated skeletal muscle cells. PAM reduces total and plasma membrane CTL1/SLC44A1 protein by lysosomal degradation, and limits the choline uptake while increasing DAG and TAG synthesis. OLA maintains total and plasma membrane CTL1/SLC44A1, but increases PtdCho synthesis more than PAM. OLA does not increase the rate of DAG synthesis, but does increase TAG content. Thus, the CTL1/SLC44A1 presence at the plasma membrane regulates choline requirements in accordance with the type of fatty acid. The increased PtdCho and TAG turnover by OLA stimulates cell growth and offers a specific protection mechanism from the excess of intracellular DAG and autophagy. This protection was present after OLA treatments, but not after PAM treatments. The mitochondrial choline uptake was reduced by both FA; however, the regulation is complex and guided not only by the presence of the mitochondrial CTL1/SLC44A1 protein but also by the membrane potential and general mitochondrial function.

The Stimulatory Effect of Essential Fatty Acids on Glucose Uptake Involves Both Akt and AMPK Activation in C2C12 Skeletal Muscle Cells

Essential fatty acid (EFA) is known to be required for the body to function normally and healthily. However, the effect of EFA on glucose uptake in skeletal muscle has not yet been fully investigated. In this study, we examined the effect of two EFAs, linoleic acid (LA) and α-linolenic acid (ALA), on glucose uptake of C2C12 skeletal muscle cells and investigated the mechanism underlying the stimulatory effect of polyunsaturated EFAs in comparison with monounsaturated oleic acid (OA). In palmitic acid (PA)-induced insulin resistant cells, the co-treatment of EFAs and OA with PA almost restored the PA-induced decrease in the basal and insulin-stimulated 2-NBDG (fluorescent D-glucose analogue) uptake, respectively. Two EFAs and OA significantly protected PA-induced suppression of insulin signaling, respectively, which was confirmed by the increased levels of Akt phosphorylation and serine/threonine kinases (PKCθ and JNK) dephosphorylation in the western blot analysis. In PA-untreated, control cells, the treatment of 500 µM EFA significantly stimulated 2-NBDG uptake, whereas OA did not. Phosphorylation of AMP-activated protein kinase (AMPK) and one of its downstream molecules, acetyl-CoA carboxylase (ACC) was markedly induced by EFA, but not OA. In addition, EFA-stimulated 2-NBDG uptake was significantly inhibited by the pre-treatment of a specific AMPK inhibitor, adenine 9-β-D-arabinofuranoside (araA). These data suggest that the restoration of suppressed insulin signaling at PA-induced insulin resistant condition and AMPK activation are involved at least in the stimulatory effect of EFA on glucose uptake in C2C12 skeletal muscle cells.

The mechanisms linking adiposopathy to type 2 diabetes

Obesity is defined as excessive accumulation of body fat in proportion to body size. When obesity occurs, the functions of adipose tissue may be deregulated, which is termed as adiposopathy. Adiposopathy is an independent risk factor for many diseases, including diabetes and cardiovascular diseases. In overweight or obese subjects with adiposopathy, hyperlipidemia exerts lipotoxicity in pancreatic islet and liver and induces pancreatic β cell dysfunction and liver insulin resistance, which are the decisive factors causing type 2 diabetes. Moreover, adipokines have been shown to play important roles in the regulation of glucose homeostasis. When adiposopathy occurs, abnormal changes in the serum adipokine profile correlate with the development and progression of pancreatic β cell dysfunction and insulin resistance in peripheral tissue. The current paper briefly discusses the latest findings regarding the effects of adiposopathy-related lipotoxicity and cytokine toxicity on the development of type 2 diabetes.

Age-dependent effects of high fat-diet on murine left ventricles: role of palmitate

Obesity-associated heart disease results in myocardial lipid accumulation leading to lipotoxicity. However, recent studies are suggestive of protective effects of high-fat diets (HFD). To determine whether age results in differential changes in diet-induced obesity, we fed young and old (3 and 18 months) male C57Bl/6 mice control diet, low-fat diet (both 10 kcal% fat) or HFD (45 kcal% fat) for 16 weeks, after which we analyzed LV function, mitochondrial changes, and potential modifiers of myocardial structure. HFD or age did not change LV systolic function, although a mildly increased BNP was observed in all old mice. This was associated with increased myocardial collagen, triglyceride, diacylglycerol, and ceramide content as well as higher caspase 3 activation in old mice with highest levels in old HFD mice. Pyruvate-dependent respiration and mitochondrial biogenesis were reduced in all old mice and in young HFD mice. Activation of AMPK, a strong inducer of mitochondrial biogenesis, was reduced in both HFD groups and in old control or LFD mice. Cardiomyocytes from old rats demonstrated significantly reduced AMPK activation, impaired mitochondrial biogenesis, higher ceramide content, and reduced viability after palmitate (C16:0) in vitro, while no major deleterious effects were observed in young cardiomyocytes. Aged but not young cardiomyocytes were unable to respond to higher palmitate with increased fatty acid oxidation. Thus, HFD results in cardiac structural alterations and accumulation of lipid intermediates predominantly in old mice, possibly due to the inability of old cardiomyocytes to adapt to high-fatty acid load.

Identification of novel type 2 diabetes candidate genes involved in the crosstalk between the mitochondrial and the insulin signaling systems

Type 2 Diabetes (T2D) is a highly prevalent chronic metabolic disease with strong co-morbidity with obesity and cardiovascular diseases. There is growing evidence supporting the notion that a crosstalk between mitochondria and the insulin signaling cascade could be involved in the etiology of T2D and insulin resistance. In this study we investigated the molecular basis of this crosstalk by using systems biology approaches. We combined, filtered, and interrogated different types of functional interaction data, such as direct protein–protein interactions, co-expression analyses, and metabolic and signaling dependencies. As a result, we constructed the mitochondria-insulin (MITIN) network, which highlights 286 genes as candidate functional linkers between these two systems. The results of internal gene expression analysis of three independent experimental models of mitochondria and insulin signaling perturbations further support the connecting roles of these genes. In addition, we further assessed whether these genes are involved in the etiology of T2D using the genome-wide association study meta-analysis from the DIAGRAM consortium, involving 8,130 T2D cases and 38,987 controls. We found modest enrichment of genes associated with T2D amongst our linker genes (p = 0.0549), including three already validated T2D SNPs and 15 additional SNPs, which, when combined, were collectively associated to increased fasting glucose levels according to MAGIC genome wide meta-analysis (p = 8.12×10⁻⁵). This study highlights the potential of combining systems biology, experimental, and genome-wide association data mining for identifying novel genes and related variants that increase vulnerability to complex diseases.

It has been shown that the crosstalk between insulin signaling and the mitochondria may be involved in the etiology of type 2 diabetes. In order to characterize the molecular basis of this crosstalk, we mined and filtered several interaction databases of different natures, including protein–protein interactions, gene co-expression, signaling, and metabolic pathway interactions, to identify reliable direct and indirect interactions between insulin signaling cascade and mitochondria genes. This allowed us to identify 286 genes that are associated simultaneously with insulin signaling and mitochondrial genes and therefore could act as a molecular bridge between both systems. We performed in vitro and in vivo experiments where the insulin signaling or the mitochondrial function were disrupted, and we found deregulation of these connecting genes. Finally, we found that common variants in genomic regions where these genes lie are enriched for genetic associations with type 2 diabetes and glycemic traits according to large genome-wide association meta-analyses. In summary, we reconstructed the network implicated in the crosstalk between the mitochondria and the insulin signaling and provide a list of genes connecting both systems. We also propose new potential type 2 diabetes candidate genes.