High-fat diet-mediated lipotoxicity and insulin resistance is related to impaired lipase expression in mouse skeletal muscle

Exercise ameliorates lipid droplet metabolism disorder by the PLIN2-LIPA axis-mediated lipophagy in mouse model of non-alcoholic fatty liver disease

Excessive hepatic lipid droplets (LDs) accumulation-induced lipid metabolism disorder contributes to the development of non-alcoholic fatty liver disease (NAFLD). Exercise is a promising therapeutic strategy for NAFLD. However, the mechanism by which exercise ameliorates NAFLD through regulating the catabolism of hepatic LDs remains unclear. In the present study, we investigated the effect of perilipin2 (PLIN2)-lysosomal acid lipase (LIPA) axis mediating exercise-triggered lipophagy in a high-fat diet (HFD)-induced NAFLD mouse model. Our results showed that exercise could reduce HFD-induced hepatic LDs accumulation and change the expression of lipolysis-related enzymes. Moreover, exercise upregulated the expression of microtubule associated protein 1 light chain 3 (LC3) and autophagy-related proteins, and downregulated sequestosome 1 (P62) expression and promoted autophagosomes formation. Interestingly, exercise downregulated PLIN2 expression, upregulated LIPA expression, and increased the activity of hepatic LIPA and serum levels of LIPA in the NAFLD mouse model. Further mechanistic studies demonstrated that adenosine monophosphate-activated protein kinase (AMPK) activator-5-Aminoimidazole-4-carboxamide ribonucleoside (AICAr) treatment significantly increased mRNA levels and protein expression of LIPA and LC3II and decreased levels of PLIN2 and P62 in palmitic acid (PA)-treated HepG2 cells. PLIN2 silencing and LIPA overexpression notably increased the mRNA level and protein expression of LC3II and decreased the mRNA level and protein expression of p62, respectively. In summary, our findings reveal novel insights into the effect of exercise on improving lipid droplet metabolism disorder in NAFLD. Enhancing the PLIN2-LIPA axis-mediated lipophagy may be one of the key mechanisms involved in NAFLD alleviation by exercise.

The macrophage STING-YAP axis controls hepatic steatosis by promoting the autophagic degradation of lipid droplets

BACKGROUND AND AIMS

The hallmark of NAFLD or hepatic steatosis is characterized by lipid droplet (LD) accumulation in hepatocytes. Autophagy may have profound effects on lipid metabolism and innate immune response. However, how innate immune activation may regulate the autophagic degradation of intracellular LDs remains elusive.

APPROACH AND RESULTS

A mouse model of a high-fat diet-induced NASH was used in the myeloid-specific stimulator of interferon genes (STING) knockout or STING/yes-associated protein (YAP) double knockout mice. Liver injury, lipid accumulation, lipid droplet proteins, autophagic genes, chromatin immunoprecipitation coupled with massively parallel sequencing, and RNA-Seq were assessed in vivo and in vitro . We found that high-fat diet-induced oxidative stress activates STING and YAP pathways in hepatic macrophages. The acrophage STING deficiency (myeloid-specific STING knockout) enhances nuclear YAP activity, reduces lipid accumulation, and increases autophagy-related proteins ATG5, ATG7, and light chain 3B but diminishes LD protein perilipin 2 expression. However, disruption of STING and YAP (myeloid STING and YAP double knockout) increases serum alanine aminotransferase and triglyceride levels and reduces β-fatty acid oxidation gene expression but augments perilipin 2 levels, exacerbating high-fat diet-induced lipid deposition. Chromatin immunoprecipitation coupled with massively parallel sequencing reveals that macrophage YAP targets transmembrane protein 205 and activates AMP-activated protein kinase α, which interacts with hepatocyte mitofusin 2 and induces protein disulfide isomerase activation. Protein disulfide isomerase activates hypoxia-inducible factor-1α signaling, increases autophagosome colocalization with LDs, and promotes the degradation of perilipin 2 by interacting with chaperone-mediated autophagy chaperone HSC70.

CONCLUSIONS

The macrophage STING-YAP axis controls hepatic steatosis by reprogramming lipid metabolism in a transmembrane protein 205/mitofusin 2/protein disulfide isomerase-dependent pathway. These findings highlight the regulatory mechanism of the macrophage STING-driven YAP activity on lipid control.

Lactobacillus rhamnosus GG administration partially prevents diet-induced insulin resistance in rats: a comparison with its heat-inactivated parabiotic

Insulin resistance and type 2 diabetes are obesity-related health alterations, featuring an ever-increasing prevalence. Besides inadequate feeding patterns, gut microbiota alterations stand out as potential contributors to these metabolic disturbances. The aim of this study was to investigate whether the administration of a probiotic (Lactobacillus rhamnosus GG) effectively prevents diet-induced insulin resistance in rats and to compare these potential effects with those exerted by its heat-inactivated parabiotic. For this purpose, 34 male Wistar rats were fed a standard or a high-fat high-fructose diet, alone or supplemented with viable or heat-inactivated Lactobacillus rhamnosus GG. The body and white adipose tissue weight increases, induced by the obesogenic diet, were prevented by probiotic and parabiotic administration. The trend towards higher basal glucose levels and significantly higher serum insulin concentration observed in the non-treated animals fed with the obesogenic diet were effectively reverted by both treatments. Similar results were also found for serum adiponectin and leptin, whose levels were brought back by the probiotic and parabiotic administration to values similar to those of the control animals. Noteworthily, parabiotic administration significantly reduced skeletal muscle triglyceride content and activated CPT-1b compared to the non-treated animals. Finally, both treatments enhanced Akt and AS160 phosphorylation in the skeletal muscle compared to the non-treated animals; however, only parabiotic administration increased GLUT-4 protein expression in this tissue. These results suggest that heat-inactivated Lactobacillus rhamnosus GG seem to be more effective than its probiotic of origin in preventing high-fat high-fructose diet-induced insulin resistance in rats.

Estradiol replacement improves high-fat diet-induced insulin resistance in ovariectomized rats

The role of 17β-estradiol (E2) in high-fat diet (HFD)-induced alteration of the protein kinase B (Akt) signaling pathway in ovariectomized (OVX) rats is unclear. Therefore, we examined whether chronic estrogen replacement restores HFD-induced impairment in insulin sensitivity by its effects concomitant with alterations in the Akt isoform 2 (Akt2) and Akt substrate of 160 kDa (AS160) phosphorylation in muscles of OVX rats. Nine-week-old female Wistar rats underwent ovariectomy under anesthesia; after 4 weeks, subcutaneous implantation of either E2 or placebo (PL) pellets was performed, and HFD feeding was initiated. Intravenous glucose tolerance tests were performed to assess insulin sensitivity. Following insulin injection into rats' portal vein, the liver and gastrocnemius muscle were dissected for insulin signaling analysis. We observed that HFD increased energy intake and body weight in the PL group; however, it was temporarily decreased in the E2 group. Adipose tissue accumulation was larger in HFD-fed rats than in normal chow diet (NCD)-fed rats in the PL group; however, this difference was not observed in the E2 group. HFD reduced insulin sensitivity in the PL group only. In vivo insulin stimulation increased Akt2 phosphorylation in the muscles of NCD-fed rats in both groups. In contrast, HFD affected insulin-stimulated phosphorylation of Akt2 and AS160 in the muscles of rats in the PL group but not in the E2 group. Our data suggest that E2 replacement improves HFD-induced insulin resistance, and this effect is accompanied by the alterations in the Akt2 and AS160 phosphorylation in insulin-stimulated muscles of OVX rats.

Short-term physical inactivity induces diacylglycerol accumulation and insulin resistance in muscle via lipin1 activation

Physical inactivity impairs muscle insulin sensitivity. However, its mechanism is unclear. To model physical inactivity, we applied 24-h hind-limb cast immobilization (HCI) to mice with normal or high-fat diet (HFD) and evaluated intramyocellular lipids and the insulin signaling pathway in the soleus muscle. Although 2-wk HFD alone did not alter intramyocellular diacylglycerol (IMDG) accumulation, HCI alone increased it by 1.9-fold and HCI after HFD further increased it by 3.3-fold. Parallel to this, we found increased protein kinase C ε (PKCε) activity, reduced insulin-induced 2-deoxyglucose (2-DOG) uptake, and reduced phosphorylation of insulin receptor β (IRβ) and Akt, key molecules for insulin signaling pathway. Lipin1, which converts phosphatidic acid to diacylglycerol, showed increase of its activity by HCI, and dominant-negative lipin1 expression in muscle prevented HCI-induced IMDG accumulation and impaired insulin-induced 2-DOG uptake. Furthermore, 24-h leg cast immobilization in human increased lipin1 expression. Thus, even short-term immobilization increases IMDG and impairs insulin sensitivity in muscle via enhanced lipin1 activity.NEW & NOTEWORTHY Physical inactivity impairs muscle insulin sensitivity. However, its mechanism is unclear. To model physical inactivity, we applied 24-h hind-limb cast immobilization to mice with normal or high-fat diet and evaluated intramyocellular lipids and the insulin signaling pathway in the soleus muscle. We found that even short-term immobilization increases intramyocellular diacylglycerol and impairs insulin sensitivity in muscle via enhanced lipin1 activity.

Identification of new enterosynes using prebiotics: roles of bioactive lipids and mu-opioid receptor signalling in humans and mice

OBJECTIVE

The enteric nervous system (ENS) plays a key role in controlling the gut-brain axis under normal and pathological conditions, such as type 2 diabetes. The discovery of intestinal actors, such as enterosynes, able to modulate the ENS-induced duodenal contraction is considered an innovative approach. Among all the intestinal factors, the understanding of the role of gut microbes in controlling glycaemia is still developed. We studied whether the modulation of gut microbiota by prebiotics could permit the identification of novel enterosynes.

DESIGN

We measured the effects of prebiotics on the production of bioactive lipids in the intestine and tested the identified lipid on ENS-induced contraction and glucose metabolism. Then, we studied the signalling pathways involved and compared the results obtained in mice to human.

RESULTS

We found that modulating the gut microbiota with prebiotics modifies the actions of enteric neurons, thereby controlling duodenal contraction and subsequently attenuating hyperglycaemia in diabetic mice. We discovered that the signalling pathway involved in these effects depends on the synthesis of a bioactive lipid 12-hydroxyeicosatetraenoic acid (12-HETE) and the presence of mu-opioid receptors (MOR) on enteric neurons. Using pharmacological approaches, we demonstrated the key role of the MOR receptors and proliferator-activated receptor γ for the effects of 12-HETE. These findings are supported by human data showing a decreased expression of the proenkephalin and MOR messanger RNAs in the duodenum of patients with diabetic.

CONCLUSIONS

Using a prebiotic approach, we identified enkephalin and 12-HETE as new enterosynes with potential real beneficial and safety impact in diabetic human.

Dietary Olive Oil Intake Improves Running Endurance with Intramuscular Triacylglycerol Accumulation in Mice

Olive oil is a functional food shown to have a variety of bioactive effects. Therefore, we expect it to be a novel functional food with an exercise-mimetic effect on skeletal muscles. This study aimed to investigate the effect of olive oil on the endurance capacity and muscle metabolism in mice. Mice fed a 7% (w/w) olive oil diet for eight weeks showed improved treadmill running endurance and increased intramuscular triacylglycerol (IMTG) accumulation in the gastrocnemius muscle compared to soybean oil diet-fed controls. The increase in running endurance with olive oil intake was independent of the muscle fiber type. To elucidate underlying the mechanism of elevated IMTG levels, we examined the expression levels of the genes related to lipid metabolism. We found that the expression of diacylglycerol O-acyltransferase1 (DGAT1) was significantly upregulated in the muscle of olive oil diet-fed mice. In addition, the olive oil diet-fed mice showed no metabolic impairment or differences in growth profiles compared to the controls. These results suggest that dietary olive oil intake affects muscle metabolism and muscle endurance by increasing energy accumulation.

Plasma lipidomics profile in pregnancy and gestational diabetes risk: a prospective study in a multiracial/ethnic cohort

INTRODUCTION

Disruption of lipid metabolism is implicated in gestational diabetes (GDM). However, prospective studies on lipidomics and GDM risk in race/ethnically diverse populations are sparse. Here, we aimed to (1) identify lipid networks in early pregnancy to mid-pregnancy that are associated with subsequent GDM risk and (2) examine the associations of lipid networks with glycemic biomarkers to understand the underlying mechanisms.

RESEARCH DESIGN AND METHODS

This study included 107 GDM cases confirmed using the Carpenter and Coustan criteria and 214 non-GDM matched controls from the National Institute of Child Health and Human Development Fetal Growth Studies-Singleton cohort, untargeted lipidomics data of 420 metabolites (328 annotated and 92 unannotated), and information on glycemic biomarkers in maternal plasma at visit 0 (10-14 weeks) and visit 1 (15-26 weeks). We constructed lipid networks using weighted correlation network analysis technique. We examined prospective associations of lipid networks and individual lipids with GDM risk using linear mixed effect models. Furthermore, we calculated Pearson's partial correlation for GDM-related lipid networks and individual lipids with plasma glucose, insulin, C-peptide and glycated hemoglobin at both study visits.

RESULTS

Lipid networks primarily characterized by elevated plasma diglycerides and short, saturated/low unsaturated triglycerides and lower plasma cholesteryl esters, sphingomyelins and phosphatidylcholines were associated with higher risk of developing GDM (false discovery rate (FDR) <0.05). Among individual lipids, 58 metabolites at visit 0 and 96 metabolites at visit 1 (40 metabolites at both time points) significantly differed between women who developed GDM and who did not (FDR <0.05). Furthermore, GDM-related lipid networks and individual lipids showed consistent correlations with maternal glycemic markers particularly in early pregnancy at visit 0.

CONCLUSIONS

Plasma lipid metabolites in early pregnancy both individually and interactively in distinct networks were associated with subsequent GDM risk in race/ethnically diverse US women. Future research is warranted to assess lipid metabolites as etiologic markers of GDM.

Pyrroloquinoline Quinone Modifies Lipid Profile, but Not Insulin Sensitivity, of Palmitic Acid-Treated L6 Myotubes

Pyrroloquinoline quinone (PQQ) is a novel stimulator of mitochondrial biogenesis and cellular energy metabolism. This is the first study investigating regulatory mechanisms and metabolic responses underlying PQQ’s action in palmitate-exposed L6 myotubes. Particularly, we assessed alterations in lipid content and composition, expression of metabolic enzymes, and changes in glucose transport. The experiments were conducted using muscle cells subjected to short (2 h) and prolonged (24 h) incubation with PQQ in a sequence of pre- and post-palmitic acid (PA) exposure. We demonstrated the opposite effects of 2 and 24 h treatments with PQQ on lipid content, i.e., a decline in the level of free fatty acids and triacylglycerols in response to short-time PQQ incubation as compared to increases in diacylglycerol and triacylglycerol levels observed after 24 h. We did not demonstrate a significant impact of PQQ on fatty acid transport. The analysis of metabolic enzyme expression showed that the vast majority of PQQ-dependent alterations cumulated in the PA/PQQ 24 h group, including elevated protein amount of peroxisome proliferator activated receptor γ co-activator 1α (PGC-1α), sirtuin-1 (SIRT1), phosphorylated 5′AMP-activated protein kinase (pAMPK), carnitine palmitoyltransferase I (CPT1), citrate synthase (CS), fatty acid synthase (FAS), and serine palmitoyltransferase, long chain base subunit 1 (SPT1). In conclusion, the results mentioned above indicate PQQ-dependent activation of both fatty acid oxidation and lipid synthesis in order to adapt cells to palmitic acid-rich medium, although PQQ did not attenuate insulin resistance in muscle cells.

Novel Insights and Mechanisms of Lipotoxicity-Driven Insulin Resistance

A large number of studies reported an association between elevated circulating and tissue lipid content and metabolic disorders in obesity, type 2 diabetes (T2D) and aging. This state of uncontrolled tissue lipid accumulation has been called lipotoxicity. It was later shown that excess lipid flux is mainly neutralized within lipid droplets as triglycerides, while several bioactive lipid species such as diacylglycerols (DAGs), ceramides and their derivatives have been mechanistically linked to the pathogenesis of insulin resistance (IR) by antagonizing insulin signaling and action in metabolic organs such as the liver and skeletal muscle. Skeletal muscle and the liver are the main sites of glucose disposal in the body and IR in these tissues plays a pivotal role in the development of T2D. In this review, we critically examine recent literature supporting a causal role of DAGs and ceramides in the development of IR. A particular emphasis is placed on transgenic mouse models with modulation of total DAG and ceramide pools, as well as on modulation of specific subspecies, in relation to insulin sensitivity. Collectively, although a wide number of studies converge towards the conclusion that both DAGs and ceramides cause IR in metabolic organs, there are still some uncertainties on their mechanisms of action. Recent studies reveal that subcellular localization and acyl chain composition are determinants in the biological activity of these lipotoxic lipids and should be further examined.

An adiponectin-S1P autocrine axis protects skeletal muscle cells from palmitate-induced cell death

Background

The prevalence of type 2 diabetes, obesity and their various comorbidities have continued to rise. In skeletal muscle lipotoxicity is well known to be a contributor to the development of insulin resistance. Here it was examined if the small molecule adiponectin receptor agonist AdipoRon mimicked the effect of adiponectin to attenuate palmitate induced reactive oxygen species (ROS) production and cell death in L6 skeletal muscle cells.

Methods

L6 cells were treated ±0.1 mM PA, and ± AdipoRon, then assays analyzing reactive oxygen species (ROS) production and cell death, and intracellular and extracellular levels of sphingosine-1 phosphate (S1P) were conducted. To determine the mechanistic role of S1P gain (using exogenous S1P or using THI) or loss of function (using the SKI-II) were conducted.

Results

Using both CellROX and DCFDA assays it was found that AdipoRon reduced palmitate-induced ROS production. Image-IT DEAD, MTT and LDH assays all indicated that AdipoRon reduced palmitate-induced cell death. Palmitate significantly increased intracellular accumulation of S1P, whereas in the presence of AdipoRon there was increased release of S1P from cells to extracellular medium. It was also observed that direct addition of extracellular S1P prevented palmitate-induced ROS production and cell death, indicating that S1P is acting in an autocrine manner. Pharmacological approaches to enhance or decrease S1P levels indicated that accumulation of intracellular S1P correlated with enhanced cell death.

Conclusion

This data indicates that increased extracellular levels of S1P in response to adiponectin receptor activation can activate S1P receptor-mediated signaling to attenuate lipotoxic cell death. Taken together these findings represent a possible novel mechanism for the protective action of adiponectin.

The Release of Adipose Stromal Cells from Subcutaneous Adipose Tissue Regulates Ectopic Intramuscular Adipocyte Deposition

Ectopic lipid deposition (ELD) is defined by excess fat storage in locations not classically associated with adipose tissue (AT) storage. ELD is positively correlated with insulin resistance and increased risk of metabolic disorders. ELD appears as lipid droplets or adipocytes, whose cell origin is unknown. We previously showed that subcutaneous AT (ScAT) releases adipocyte progenitors into the circulation. Here, we demonstrate that triggering or preventing the release of adipocyte precursors from ScAT directly promoted or limited ectopic adipocyte formation in skeletal muscle in mice. Importantly, obesity-associated metabolic disorders could be mimicked by causing adipocyte precursor release without a high-fat diet. Finally, during nutrient overload, adipocyte progenitors exited ScAT, where their retention signals (CXCR4/CXCL12 axis) were greatly decreased, and further infiltrated skeletal muscles. These data provide insights into the formation of ELD associated with calorie overload and highlight adipocyte progenitor trafficking as a potential target in the treatment of metabolic diseases.

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.

High-Intensity Interval Training Restores Glycolipid Metabolism and Mitochondrial Function in Skeletal Muscle of Mice With Type 2 Diabetes

High-intensity interval training has been reported to lower fasting blood glucose and improve insulin resistance of type 2 diabetes without clear underlying mechanisms. The purpose of this study was to investigate the effect of high-intensity interval training on the glycolipid metabolism and mitochondrial dynamics in skeletal muscle of high-fat diet (HFD) and one-time 100 mg/kg streptozocin intraperitoneal injection-induced type 2 diabetes mellitus (T2DM) mice. Our results confirmed that high-intensity interval training reduced the body weight, fat mass, fasting blood glucose, and serum insulin of the T2DM mice. High-intensity interval training also improved glucose tolerance and insulin tolerance of the T2DM mice. Moreover, we found that high-intensity interval training also decreased lipid accumulation and increased glycogen synthesis in skeletal muscle of the T2DM mice. Ultrastructural analysis of the mitochondria showed that mitochondrial morphology and quantity were improved after 8 weeks of high-intensity interval training. Western blot analysis showed that the expression of mitochondrial biosynthesis related proteins and mitochondrial dynamics related proteins in high-intensity interval trained mice in skeletal muscle were enhanced. Taken together, these data suggest high-intensity interval training improved fasting blood glucose and glucose homeostasis possibly by ameliorating glycolipid metabolism and mitochondrial dynamics in skeletal muscle of the T2DM mice.

The role of adipose triglyceride lipase in lipid and glucose homeostasis: lessons from transgenic mice

The ability of mammals to store and draw on fat reserves has been a driving force throughout evolution in an environment with intermittent nutrient availability. The discovery of adipose triglyceride lipase (ATGL) as a triglyceride lipase provided a heightened understanding of the mechanisms governing mobilization of fat reserves from adipose tissue. ATGL catalyses the initial step in adipose triglyceride lipolysis, working in concert with other enzymes to mobilize triglyceride for energy production. In addition to the role of ATGL in adipose tissue triglyceride mobilization, ATGL plays crucial roles in regulating lipid homeostasis in other tissues. These roles have been characterized primarily using transgenic mice with tissue-specific ATGL ablation. For example, the global ATGL knockout induces a severe cardiac defect that results in premature mortality that is mimicked by inducible cardiomyocyte-specific ATGL knockout. Global- and adipose-specific ATGL ablation induces a whole-body shift from lipid metabolism to glucose metabolism to satisfy metabolic demand primarily facilitated by an increase in glucose uptake by skeletal muscle. Generation of liver-specific ATGL knockouts has implicated hepatic lipolysis as a critical component of normal liver function. Analysis of β-cell ATGL knockouts implicates the necessity of pancreatic ATGL in insulin secretion. The objective of this review is to discuss the contributions of ATGL to systemic lipid- and glucose-homeostasis discovered through the study of transgenic mice.

Plin5/p-Plin5 Guards Diabetic CMECs by Regulating FFAs Metabolism Bidirectionally

Background

Hyper-free fatty acidemia (HFFA) impairs cardiac capillaries, as well as type 2 diabetes mellitus (T2DM). Perilipin 5 (Plin5) maintains metabolic balance of free fatty acids (FFAs) in high oxidative tissues via the states of nonphosphorylation and phosphorylation. However, when facing to T2DM-HFFA, Plin5's role in cardiac microvascular endothelial cells (CMECs) is not defined.

Methods

In mice of WT or Plin5^−/−, T2DM models were rendered by high-fat diet combined with intraperitoneal injection of streptozocin. CMECs isolated from left ventricles were incubated with high glucose (HG) and high FFAs (HFFAs). Plin5 phosphorylation was stimulated by isoproterenol. Plin5 expression was knocked down by small interfering RNA (siRNA). We determined cardiac function by small animal ultrasound, apoptotic rate by flow cytometry, microvessel quantity by immunohistochemistry, microvascular integrity by scanning electron microscopy, intracellular FFAs by spectrophotometry, lipid droplets (LDs) by Nile red staining, mRNAs by quantitative real-time polymerase chain reaction, proteins by western blots, nitric oxide (NO) and reactive oxygen species (ROS) by fluorescent dye staining and enzyme-linked immunosorbent assay kits.

Results

In CMECs, HFFAs aggravated cell injury induced by HG and activated Plin5 expression. In mice with T2DM-HFFA, Plin5 deficiency reduced number of cardiac capillaries, worsened structural incompleteness, and enhanced diastolic dysfunction. Moreover, in CMECs treated with HG-HFFAs, both ablation and phosphorylation of Plin5 reduced LDs content, increased intracellular FFAs, stimulated mitochondrial β-oxidation, added ROS generation, and reduced the expression and activity of endothelial nitric oxide synthase (eNOS), eventually leading to increased apoptotic rate and decreased NO content, all of which were reversed by N-acetyl-L-cysteine.

Conclusion

Plin5 preserves lipid balance and cell survival in diabetic CMECs by regulating FFAs metabolism bidirectionally via the states of nonphosphorylation and phosphorylation.

Reassessing the Role of Diacylglycerols in Insulin Resistance

Skeletal muscle (SM) insulin resistance (IR) plays an important role in the burden of obesity, particularly because it leads to glucose intolerance and type 2 diabetes. Among the mechanisms thought to link IR to obesity is the accumulation, in muscle cells, of different lipid metabolites. Diacylglycerols (DAGs) are subject of particular attention due to reported interactions with the insulin signaling cascade. Given that SM accounts for the majority of insulin-stimulated glucose uptake, this review integrates recent observational and mechanistic works with the sole focus on questioning the role of DAGs in SM IR. Particular attention is given to the subcellular distributions and specific structures of DAGs, highlighting future research directions towards reaching a consensus on the mechanistic role played by DAGs.

Beneficial effects of murtilla extract and madecassic acid on insulin sensitivity and endothelial function in a model of diet-induced obesity

Infusions of murtilla leaves exhibit antioxidant, analgesic, and anti-inflammatory properties. Several compounds that are structurally similar to madecassic acid (MA), a component of murtilla leaf extract (ethyl acetate extract, EAE), have been shown to inhibit protein tyrosine phosphatase 1B (PTP1P). The aim of this study was to evaluate if EAE and two compounds identified in EAE (MA and myricetin [MYR]) could have a beneficial effect on systemic and vascular insulin sensitivity and endothelial function in a model of diet-induced obesity. Experiments were performed in 5-week-old male C57BL6J mice fed with a standard (LF) or a very high-fat diet (HF) for 4 weeks and treated with EAE, MA, MYR, or the vehicle as control (C). EAE significantly inhibited PTP1B. EAE and MA, but not MYR, significantly improved systemic insulin sensitivity in HF mice and vascular relaxation to Ach in aorta segments, due to a significant increase of eNOS phosphorylation and enhanced nitric oxide availability. EAE, MA, and MYR also accounted for increased relaxant responses to insulin in HF mice, thus evidencing that the treatments significantly improved aortic insulin sensitivity. This study shows for the first time that EAE and MA could constitute interesting candidates for treating insulin resistance and endothelial dysfunction associated with obesity.

Update on the synergistic effect of HSL and insulin in the treatment of metabolic disorders

Hormone-sensitive lipase (HSL) is one of the three lipases in adipose tissue present during periods of energy demand. HSL is tightly controlled by insulin regulation via the central and peripheral systems. The suppressive effects of insulin on HSL are also associated with complex crosstalk with other pathways in the metabolic network. Because impaired insulin action is the driving force behind the pathogenesis of diabetes and other metabolic complications, elucidation of the intricate relationships between HSL and insulin may provide an in-depth understanding of these pandemic diseases and potentially identify strategies to inhibit disease development. Insulin not only differentially regulates HSL isoform transcription but also post-transcriptionally affects HSL phosphorylation by stimulating PKA and endothelin (ET-1), and controls its expression indirectly via regulating the activity of growth hormone (GH). In addition, a rapid elevation of HSL levels was detected after insulin injection in patients, which suggests that the inhibitory effects of insulin on HSL can be overridden by insulin-induced hypoglycemia. Conversely, individuals with hereditary HSL deficiency, and animals with experimental HSL deletion, showed major disruptions in mRNA/protein expression in insulin signaling pathways, ultimately leading to insulin resistance, diabetes, and fatty liver. Notably, HSL inactivation could cause insulin-independent fatty liver, while insulin resistance induced by HSL deficiency may further aggravate disease progression. The common beliefs that HSL is the overall rate-limiting enzyme in lipolysis and that insulin is an inhibitor of HSL have been challenged by recent discoveries; therefore, a renewed examination of their relationships is required. In this review, by analyzing current data related to the role of, and mutual regulation between, HSL and insulin and discussing unanswered questions and disparities in different lines of studies, the authors intend to shed light on our understanding of lipid metabolism and provide a rational basis for future research in drug development.

ZAG alleviates HFD-induced insulin resistance accompanied with decreased lipid depot in skeletal muscle in mice

Over the past two decades, intramuscular lipids have been viewed as a cause of insulin resistance due to their ability to suppress insulin-stimulated glucose uptake in skeletal muscle. Zinc-α2-glycoprotein (ZAG) is an adipokine involved in lipolysis of white adipose tissue (WAT). To investigate the action of ZAG on insulin resistance induced by a high-fat diet (HFD), which affects the intramuscular fat, mice were divided into three groups, normal diet, HFD, and ZAG treatment under HFD (HFZ). The results showed that the insulin sensitivity of ZAG-treated mice was significantly improved. The body weight, WAT weight, and intramuscular fat were significantly decreased in the HFZ group compared with the HFD group. The lipolytic enzymes, including phosphorylation of hormone-sensitive lipase and adipose triglyceride lipase, were significantly upregulated in the skeletal muscle of mice that received the ZAG treatment compared with the HFD group. Insulin signaling proteins, such as phosphorylation of insulin receptor substrate 1 and cell membrane glucose transporter type 4, were also significantly increased in the skeletal muscle of the ZAG-treated group. Furthermore, a metabolic rate study showed that ZAG overexpression increases the respiratory exchange ratio and heat production. In vitro, ZAG treatment promotes glucose uptake and decreases intracellular lipids in C2C12 myotubes. Taken together, these data showed that overexpression of ZAG alleviates HFD-induced insulin resistance in mice, along with decreasing the lipid content of skeletal muscle.

An 8-Week Ketogenic Diet Alternated Interleukin-6, Ketolytic and Lipolytic Gene Expression, and Enhanced Exercise Capacity in Mice

Adjusting dietary fat intake is reported to affect mitochondrial biogenesis and fatty acid oxidation (FAO), and thus may enhance exercise capacity. However, a high-fat diet where carbohydrate intake is not limited enough also makes it difficult for athletes to maintain weight, and may fail to force the body to utilize fat. As such, a low-carbohydrate, high-fat, ketogenic diet (KD) may be viable. We have previously reported that an eight-week KD enhances exercise capacity, and suggested the mechanism to be enhanced lipolysis and ketolysis. In the present study, we investigated how an eight-week KD alters mRNA expression during fatty acid mobilization, FAO and ketolysis. We found that an eight-week KD may remodel the lipid metabolism profile, thus contributing to influence exercise capacity. We also found that ketolysis, lipolysis and FAO adaptations may contribute to enhanced exhaustive exercise performance. Along with enhanced FAO capacity during exhaustive exercise, a KD may also alter IL-6 synthesis and secretion profile, thus contribute to fatty acid mobilization, ketolysis, lipolysis and preventing muscle damage. Both the lipid metabolism response and IL-6 secretion appeared to be muscle fiber specific. Taken together, the previous and present results reveal that an eight-week KD may enhance exercise performance by up-regulating ketolysis and FAO ability. Therefore, a KD may have the potential to prevent muscle damage by altering IL-6 secretion profile, indicating that a KD may be a promising dietary approach in endurance athletes, sports, and for injury prevention.

Role of ABCA1 on membrane cholesterol content, insulin-dependent Akt phosphorylation and glucose uptake in adult skeletal muscle fibers from mice

The ATP-binding cassette transporter A1 (ABCA1) promotes cellular cholesterol efflux, leading to cholesterol binding to the extracellular lipid-free apolipoprotein A-I. ABCA1 regulates lipid content, glucose tolerance and insulin sensitivity in adipose tissue. In skeletal muscle, most GLUT4-mediated glucose transport occurs in the transverse tubule, a system composed by specialized cholesterol-enriched invaginations of the plasma membrane. We have reported that insulin resistant mice have higher cholesterol levels in transverse tubule from adult skeletal muscle. These high levels correlate with decreased GLUT4 trafficking and glucose uptake; however, the role of ABCA1 on skeletal muscle insulin-dependent glucose metabolism remains largely unexplored. Here, we evaluated the functional role of the ABCA1 on insulin-dependent signaling pathways, glucose uptake and cellular cholesterol content in adult skeletal muscle. Male mice were fed for 8 weeks with normal chow diet (NCD) or high fat diet (HFD). Compared to NCD-fed mice, ABCA1 mRNA levels and protein content were lower in muscle homogenates from HFD-fed mice. In Flexor digitorum brevis muscle from NCD-fed mice, shABCA1-RFP in vivo electroporation resulted in 65% reduction of ABCA1 protein content, 1.6-fold increased fiber cholesterol levels, 74% reduction in insulin-dependent Akt (Ser⁴⁷³) phosphorylation, total suppression of insulin-dependent GLUT4 translocation and decreased 2-NBDG uptake compared to fibers electroporated with the scrambled plasmid. Pre-incubation with methyl-β cyclodextrin reestablished both GLUT4 translocation and 2-NBDG transport. Based on the present results, we suggest that decreased ABCA1 contributes to the anomalous cholesterol accumulation and decreased glucose transport displayed by skeletal muscle membranes in the insulin resistant condition.

Vernonia cinerea water extract improves insulin resistance in high-fat diet-induced obese mice

Vernonia cinerea (V cinerea) is a plant distributed in grassy areas in Southeast Asia and has several pharmacological effects, including antidiabetic activity. However, the information available regarding the effect of V cinerea on insulin resistance in high-fat diet (HFD)-induced obese mice is not yet determined. We hypothesized that V cinerea water extract (VC) improves insulin sensitivity in HFD-induced obese mice by modulating both phosphatidylinositol-3-kinase (PI3K) and adenosine monophosphate-activated protein kinase (AMPK) pathways in liver, skeletal muscle, and adipose tissue. Obesity was induced in mice from the Institute for Cancer Research by feeding an HFD 188.28 kJ (45 kcal % lard fat) for 12 weeks. During the last 6 weeks of the HFD, obese mice were treated with VC (250 and 500 mg/kg). We found that VC at both doses significantly reduced the hyperglycemia, hyperinsulinemia, hyperleptinemia, and hyperlipidemia. Obese mice treated with VC could increase serum adiponectin but reduce the proinflammatory cytokines, tumor necrosis factor-α, and monocyte chemoattractant protein-1. The extracts decreased triglyceride storage in liver and skeletal muscle of obese mice. The average size of fat cells was smaller in VC-treated groups than that of the HFD group. The protein expressions of PI3K and AMPK pathways in liver, skeletal muscle, and adipose tissue were upregulated (increased phosphorylation of PI3K, protein kinase B, AMPK, and acetyl-CoA carboxylase) by VC treatment. Furthermore, the glucose transporter 4 was increased in muscle and adipose tissue in obese mice treated with VC. These data indicate that VC treatment stimulates phosphorylation of PI3K and AMPK pathways in liver, muscle, and adipose tissue. Stimulating these pathways may improve impaired glucose and lipid homeostasis in an HFD-induced obesity mouse model. Based on these findings, it appears that VC has potential as a functional food or therapeutic agent in management of insulin resistance related diseases, such as type 2 diabetes mellitus.

Exercise training improves intramuscular triglyceride lipolysis sensitivity in high-fat diet induced obese mice

Background

The purpose of this study was to determine whether regular exercise training enhances intramuscular triglyceride (IMTG) lipolysis sensitivity during consumption of a continued high-fat diet by exploring changes in biochemical factors activated by IMTG lipolysis.

Methods

Male C57BL/6 mice aged 4 weeks were randomly divided into a high-fat diet group (HF) to induce obesity for 6 weeks and a control (CO) group. Thereafter, the HF group was divided into a high-fat diet group (HF) and high-fat diet + training group (HFT). The HFT group was trained on an animal treadmill 40 min/day, 5 days/week for 8 weeks. PKA, Plin5, p-Plin5, CGI-58, ATGL, and HSL were analyzed to investigate IMTG sensitivity by western blotting.

Results

PKA, CGI-58, and HSL protein levels in the HF group were significantly lower than those in the CO group (p < 0.05). However, PKA, CGI-58, and HSL protein levels in the HFT group were significantly higher than those in the HF group, and ATGL and p-Plin5 protein levels as well as the p-Plin5/Plin5 ratio in the HFT group were significantly higher than those in the HF group (p < 0.05). In addition, the HF group showed a significantly higher IMTG volume than the CO and HFT groups (p < 0.05).

Conclusions

These results suggest that in an obese mouse model, 8 weeks of treadmill exercise contributes to decreased IMTG volume by activating lipolysis factors, such as PKA, PLIN5, CGI-58, and lipases. Therefore, regular exercise training may play an important role in obesity treatment by increasing IMTG lipolysis sensitivity.

Perilipin 5 Deletion Unmasks an Endoplasmic Reticulum Stress-Fibroblast Growth Factor 21 Axis in Skeletal Muscle

Lipid droplets (LDs) are critical for the regulation of lipid metabolism, and dysregulated lipid metabolism contributes to the pathogenesis of several diseases, including type 2 diabetes. We generated mice with muscle-specific deletion of the LD-associated protein perilipin 5 (PLIN5, Plin5^MKO ) and investigated PLIN5's role in regulating skeletal muscle lipid metabolism, intracellular signaling, and whole-body metabolic homeostasis. High-fat feeding induced changes in muscle lipid metabolism of Plin5^MKO mice, which included increased fatty acid oxidation and oxidative stress but, surprisingly, a reduction in inflammation and endoplasmic reticulum (ER) stress. These muscle-specific effects were accompanied by whole-body glucose intolerance, adipose tissue insulin resistance, and reduced circulating insulin and C-peptide levels in Plin5^MKO mice. This coincided with reduced secretion of fibroblast growth factor 21 (FGF21) from skeletal muscle and liver, resulting in reduced circulating FGF21. Intriguingly, muscle-secreted factors from Plin5^MKO , but not wild-type mice, reduced hepatocyte FGF21 secretion. Exogenous correction of FGF21 levels restored glycemic control and insulin secretion in Plin5^MKO mice. These results show that changes in lipid metabolism resulting from PLIN5 deletion reduce ER stress in muscle, decrease FGF21 production by muscle and liver, and impair glycemic control. Further, these studies highlight the importance for muscle-liver cross talk in metabolic regulation.

Metabolomics signatures associated with an oral glucose challenge in pregnant women

AIM

The oral glucose tolerance test (OGTT), widely used as a gold standard for gestational diabetes mellitus (GDM) diagnosis, provides a broad view of glucose pathophysiology in response to a glucose challenge. We conducted the present study to evaluate metabolite changes before and after an oral glucose challenge in pregnancy; and to examine the extent to which metabolites may serve to predict GDM diagnosis in pregnant women.

METHODS

Peruvian pregnant women (n=100) attending prenatal clinics (mean gestation 25 weeks) participated in the study with 23% of them having GDM diagnosis. Serum samples were collected immediately prior to and 2-hours after administration of a 75-g OGTT. Targeted metabolic profiling was performed using a LC-MS based metabolomics platform. Changes in metabolite levels were evaluated using paired Student's t-tests and the change patterns were examined at the level of pathways. Multivariate regression procedures were used to examine metabolite pairwise differences associated with subsequent GDM diagnosis.

RESULTS

Of the 306 metabolites detected, the relative concentration of 127 metabolites were statistically significantly increased or decreased 2-hours after the oral glucose load (false discovery rate [FDR] corrected P-value<0.001). We identified relative decreases in metabolites in acylcarnitines, fatty acids, and diacylglycerols while relative increases were noted among bile acids. In addition, we found that C58:10 triacylglycerol (β=-0.08, SE=0.04), C58:9 triacylglycerol (β=-0.07, SE=0.03), adenosine (β=0.70, SE=0.32), methionine sulfoxide (β=0.36, SE=0.13) were significantly associated with GDM diagnosis even after adjusting for age and body mass index.

CONCLUSIONS

We identified alterations in maternal serum metabolites, representing distinct cellular and metabolic pathways including fatty acid metabolism, in response to an oral glucose challenge. These findings offer novel perspectives on the pathophysiological mechanisms underlying GDM.

Microbial Regulation of Glucose Metabolism and Insulin Resistance

Type 2 diabetes is a combined disease, resulting from a hyperglycemia and peripheral and hepatic insulin resistance. Recent data suggest that the gut microbiota is involved in diabetes development, altering metabolic processes including glucose and fatty acid metabolism. Thus, type 2 diabetes patients show a microbial dysbiosis, with reduced butyrate-producing bacteria and elevated potential pathogens compared to metabolically healthy individuals. Furthermore, probiotics are a known tool to modulate the microbiota, having a therapeutic potential. Current literature will be discussed to elucidate the complex interaction of gut microbiota, intestinal permeability and inflammation leading to peripheral and hepatic insulin resistance. Therefore, this review aims to generate a deeper understanding of the underlying mechanism of potential microbial strains, which can be used as probiotics.

Chronic Intermittent Hypoxia Impairs Insulin Sensitivity but Improves Whole-Body Glucose Tolerance by Activating Skeletal Muscle AMPK

Obstructive sleep apnea syndrome is a highly prevalent disease resulting in transient respiratory arrest and chronic intermittent hypoxia (cIH). cIH is associated with insulin resistance and impaired metabolic homeostasis in rodents and humans, but the exact underlying mechanisms remain unclear. In the current study, we investigated the effects of 2 weeks of cIH (1-min cycle, fraction of inspired oxygen 21-5%, 8 h/day) on whole-body insulin sensitivity and glucose tolerance in lean mice. Although food intake and body weight were reduced compared with normoxia, cIH induced systemic insulin resistance in a hypoxia-inducible factor 1-independent manner and impaired insulin signaling in liver, white adipose tissue, and skeletal muscle. Unexpectedly, cIH improved whole-body glucose tolerance independently of changes in body weight and glucose-induced insulin response. This effect was associated with elevated phosphorylation of Thr172-AMPK and Ser237-TBC1 domain family member 1 (TBC1D1) in skeletal muscle, suggesting a tissue-specific AMPK-dependent increase in TBC1D1-driven glucose uptake. Remarkably, although food intake, body weight, and systemic insulin sensitivity were still affected, the improvement in glucose tolerance by cIH was abolished in muscle-specific AMPKα1α2-deficient mice. We conclude that cIH impairs insulin sensitivity while improving whole-body glucose tolerance by promoting specific activation of the skeletal muscle AMPK pathway.

Maternal butyrate supplementation induces insulin resistance associated with enhanced intramuscular fat deposition in the offspring

Maternal nutrition is important for the risk of the offspring to develop insulin resistance and adiposity later in life. The study was undertaken to determine effects of maternal butyrate supplementation on lipid metabolism and insulin sensitivity in the offspring skeletal muscle. The offspring of rats, fed a control diet or a butyrate diet (1% sodium butyrate) throughout gestation and lactation, was studied at weaning and at 60 days of age. The offspring of dams fed a butyrate diet had higher HOMA-insulin resistance and impaired glucose tolerance. This was associated with elevated mRNA and protein expressions of lipogenic genes and decreased amounts of lipolytic enzyme. Simultaneously, enhanced acetylation of histone H3 lysine 9 and histone H3 lysine 27 modification on the lipogenic genes in skeletal muscle of adult offspring was observed. Higher concentration of serum insulin and intramuscular triglyceride in skeletal muscle of offspring from the butyrate group at weaning were accompanied by increasing levels of lipogenic genes and enrichment of acetylation of histone H3 lysine 27. Maternal butyrate supplementation leads to insulin resistance and ectopic lipid accumulation in skeletal muscle of offspring, indicating the importance of short chain fatty acids in the maternal diet on lipid metabolism.

Identification of morin as an agonist of imidazoline I-3 receptor for insulin secretion in diabetic rats

Morin is a flavonoid contained in guava that is known to reduce hyperglycemia in diabetics. Morin has been demonstrated to increase plasma insulin. However, the mechanism(s) remains unknown. The present study is designed to investigate the effect of morin on the imidazoline receptor (I-R) that regulates insulin secretion. We used Chinese hamster ovary (CHO) cells transfected with an I-R expression construct (NISCH-CHO-K1 cells) to identify the direct effect of morin on the I-R. Moreover, the imidazoline I₃ receptor (I-3R) is known to be present in pancreatic β cells and involved in insulin secretion. Therefore, we applied a specific antagonist (KU14R) to block I-3R in diabetic rats. Additionally, the effect of morin on insulin secretion was characterized in isolated pancreatic islets. Morin decreased blood glucose levels by increasing plasma insulin levels in diabetic rats. In CHO cells expressing an I-R, morin increased calcium influx in a dose-dependent manner. Additionally, KU14R dose-dependently inhibited the morin-induced effects, including hypoglycemia and the increase in insulin secretion and plasma C-peptide levels, in diabetic rats. Furthermore, morin enhanced insulin secretion from isolated pancreatic islets, and this effect was also dose-dependently inhibited by KU14R. Phospholipase C (PLC) is known to couple with the I-R, and a PLC inhibitor dose-dependently attenuated the insulin secretion induced by morin in isolated pancreatic islets. Taken together, these data suggest that morin can activate I-3R to enhance insulin secretion. Therefore, it would be useful to develop morin into a treatment for diabetic disorders.

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.

Lipid droplet dynamics and insulin sensitivity upon a 5-day high-fat diet in Caucasians and South Asians

A 5-day High-Fat High-Calorie diet (HFHC-diet) reduces insulin-stimulated glucose disposal (Rd) in South Asian, but not Caucasian healthy lean males. We aimed to investigate if differences in myocellular lipid handling are underlying this differential response. A two-step hyperinsulinemic-euglycemic clamp and muscle biopsies were performed in 12 healthy lean Caucasian and South Asian males (BMI < 25 kg/m², 19-25 years) before and after a 5-day HFHC-diet (regular diet + 375 mL cream/day; 1275 kcal/day; 94% fat). Triglyceride extractions and Western Blots for lipid droplet and mitochondrial proteins were performed. Intramyocellular lipid content and HFHC-diet response were similar between ethnicities (group effect: P = 0.094; diet effect: +~30%, P = 0.044). PLIN5 protein content increased upon the HFHC-diet (P = 0.031) and tended to be higher in South Asians (0.87 ± 0.42 AU vs. 1.35 ± 0.58 AU, P = 0.07). 4-HNE tended to increase in South Asians upon the HFHC-diet (interaction effect: P = 0.057). In Caucasians ΔPLIN5 content correlated with ΔR_d (Caucasians: r = 0.756, P = 0.011; South Asians: r = -0.085, P = 0.816), while in South Asians Δ4-HNE associated with ΔPLIN5 content (Caucasians: r = 0.312, P = 0.380; South Asians: r = 0.771, P = 0.003). These data indicate that in Caucasians, PLIN5 may be protective against HFHC-diet induced insulin resistance, which for reasons not yet understood is not observed in South Asians, who possess increased lipid peroxidation levels.

Transient Receptor Potential Canonical 3 (TRPC3) Channels Are Required for Hypothalamic Glucose Detection and Energy Homeostasis

The mediobasal hypothalamus (MBH) contains neurons capable of directly detecting metabolic signals such as glucose to control energy homeostasis. Among them, glucose-excited (GE) neurons increase their electrical activity when glucose rises. In view of previous work, we hypothesized that transient receptor potential canonical type 3 (TRPC3) channels are involved in hypothalamic glucose detection and the control of energy homeostasis. To investigate the role of TRPC3, we used constitutive and conditional TRPC3-deficient mouse models. Hypothalamic glucose detection was studied in vivo by measuring food intake and insulin secretion in response to increased brain glucose level. The role of TRPC3 in GE neuron response to glucose was studied by using in vitro calcium imaging on freshly dissociated MBH neurons. We found that whole-body and MBH TRPC3-deficient mice have increased body weight and food intake. The anorectic effect of intracerebroventricular glucose and the insulin secretory response to intracarotid glucose injection are blunted in TRPC3-deficient mice. TRPC3 loss of function or pharmacological inhibition blunts calcium responses to glucose in MBH neurons in vitro. Together, the results demonstrate that TRPC3 channels are required for the response to glucose of MBH GE neurons and the central effect of glucose on insulin secretion and food intake.

Muscle Arnt/Hif1β Is Dispensable in Myofiber Type Determination, Vascularization and Insulin Sensitivity

Aryl Hydrocarbon Receptor Nuclear Translocator/ hypoxia-inducible factor 1 beta (ARNT/ HIF1β), a member of bHLH-PAS family of transcriptional factors, plays a critical role in metabolic homeostasis, insulin resistance and glucose intolerance. The contributions of ARNT in pancreas, liver and adipose tissue to energy balance through gene regulation have been described. Surprisingly, the impact of ARNT signaling in the skeletal muscles, one of the major organs involved in glucose disposal, has not been investigated, especially in type II diabetes. Here we report that ARNT is expressed in the skeletal muscles, particularly in the energy-efficient oxidative slow-twitch myofibers, which are characterized by increased oxidative capacity, mitochondrial content, vascular supply and insulin sensitivity. However, muscle-specific deletion of ARNT did not change myofiber type distribution, oxidative capacity, mitochondrial content, capillarity, or the expression of genes associated with these features. Consequently, the lack of ARNT in the skeletal muscle did not affect weight gain, lean/fat mass, insulin sensitivity and glucose tolerance in lean mice, nor did it impact insulin resistance and glucose intolerance in high fat diet-induced obesity. Therefore, skeletal muscle ARNT is dispensable for controlling muscle fiber type and metabolic regulation, as well as diet-induced weight control, insulin sensitivity and glucose tolerance.

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.

Exenatide treatment increases serum irisin levels in patients with obesity and newly diagnosed type 2 diabetes

OBJECTIVE

Irisin is a myokine secreted by skeletal muscle during exercise. Abnormal serum irisin levels are associated with obesity and type 2 diabetes (T2D). This study investigated the changes in serum irisin in the obese patients with newly diagnosed T2D following glucagon-like peptide-1 (GLP-1) receptor agonist (exenatide) treatment.

METHODS

Fifty-four obese patients with T2D were treated with exenatide for 12weeks. The control group included 54 age-, sex-, and body mass index (BMI)-matched subjects with normal glucose tolerance.

RESULTS

Patients with T2D had lower irisin than the control group (38.06 [29.29-53.79] vs. 58.01 [43.07-87.79] ng/mL, P<0.01]. Serum irisin was negatively associated with BMI (r=-0.178, P<0.05), fasting blood glucose (FBG; r=-0.170, P<0.05), and glycosylated hemoglobin (HbA1c; r=-0.189, P<0.01) in patients with T2D. Exenatide treatment markedly increased serum irisin by 19.28ng/mL (12.59-25.98) compared to baseline (P<0.01). Increased irisin was significantly correlated with decreased FBG and HbA1c after exenatide treatment (FBG: r=-0.35; HbA1c: r=-0.37; both P<0.05).

CONCLUSIONS

Exenatide treatment significantly increased irisin in patients with T2D. Post-treatment changes in irisin were correlated with decreases in FBG and HbA1c. The upregulation of irisin might be a novel mechanism for the beneficial effects of exenatide in type 2 diabetic patients.

α-Melanocyte stimulating hormone promotes muscle glucose uptake via melanocortin 5 receptors

Objective

Central melanocortin pathways are well-established regulators of energy balance. However, scant data exist about the role of systemic melanocortin peptides. We set out to determine if peripheral α-melanocyte stimulating hormone (α-MSH) plays a role in glucose homeostasis and tested the hypothesis that the pituitary is able to sense a physiological increase in circulating glucose and responds by secreting α-MSH.

Methods

We established glucose-stimulated α-MSH secretion using humans, non-human primates, and mouse models. Continuous α-MSH infusions were performed during glucose tolerance tests and hyperinsulinemic-euglycemic clamps to evaluate the systemic effect of α-MSH in glucose regulation. Complementary ex vivo and in vitro techniques were employed to delineate the direct action of α-MSH via the melanocortin 5 receptor (MC5R)–PKA axis in skeletal muscles. Combined treatment of non-selective/selective phosphodiesterase inhibitor and α-MSH was adopted to restore glucose tolerance in obese mice.

Results

Here we demonstrate that pituitary secretion of α-MSH is increased by glucose. Peripheral α-MSH increases temperature in skeletal muscles, acts directly on soleus and gastrocnemius muscles to significantly increase glucose uptake, and enhances whole-body glucose clearance via the activation of muscle MC5R and protein kinase A. These actions are absent in obese mice, accompanied by a blunting of α-MSH-induced cAMP levels in skeletal muscles of obese mice. Both selective and non-selective phosphodiesterase inhibition restores α-MSH induced skeletal muscle glucose uptake and improves glucose disposal in obese mice.

Conclusion

These data describe a novel endocrine circuit that modulates glucose homeostasis by pituitary α-MSH, which increases muscle glucose uptake and thermogenesis through the activation of a MC5R-PKA-pathway, which is disrupted in obesity.

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Glucose stimulates α-MSH release from the pituitary.

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Systemic α-MSH drives glucose disposal and thermogenesis in skeletal muscles.

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α-MSH acts on MC5R expressed on skeletal muscles and activate cAMP-PKA pathway.

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The combined treatment of nonselective or selective PDE 4 inhibitor and α-MSH ameliorates glucose intolerance in obese mice.

PPARα-ATGL pathway improves muscle mitochondrial metabolism: implication in aging

Adipose triglyceride lipase (ATGL) maintains an optimum mitochondrial function putatively by generating cognate ligands for peroxisome proliferator-activated receptor α (PPARα), which, together with PPARγ coactivator-1α (PGC1α), regulate muscle mitochondrial biogenesis. However, the cross-talk between ATGL and PPARα in skeletal muscle mitochondrial metabolism and its implication in chronological aging is poorly understood. The role of ATGL in muscle mitochondrial metabolism was studied by overexpressing and depleting the gene and studying its downstream effect in cultured myotubes and in murine skeletal muscle. We found that PPARα directly induces ATGL expression during myogenesis. Overexpression of ATGL significantly enhanced while depletion of ATGL attenuated mitochondrial oxidative phosphorylation and fatty acid oxidation without alteration in mitochondrial content, and it rendered PPARα and PGC1α redundant in promoting mitochondrial oxidative function. However, ATGL did not alter PPARα-dependent lipid accumulation and insulin sensitivity. In middle-aged rats, ATGL expression was higher and correlated with PPARα expression and sustained fatty acid oxidation in oxidative soleus muscle. Fenofibrate feeding further induced ATGL expression selectively in this muscle compartment. These findings illustrate that PPARα and ATGL constitute a regulatory pathway in skeletal muscle, suggesting their role as a mitochondrial metabolic reserve.-Biswas, D., Ghosh, M., Kumar, S., Chakrabarti, P. PPARα-ATGL pathway improves muscle mitochondrial metabolism: implication in aging.

Lipids, lysosomes, and autophagy

Lipids are essential components of a cell providing energy substrates for cellular processes, signaling intermediates, and building blocks for biological membranes. Lipids are constantly recycled and redistributed within a cell. Lysosomes play an important role in this recycling process that involves the recruitment of lipids to lysosomes via autophagy or endocytosis for their degradation by lysosomal hydrolases. The catabolites produced are redistributed to various cellular compartments to support basic cellular function. Several studies demonstrated a bidirectional relationship between lipids and lysosomes that regulate autophagy. While lysosomal degradation pathways regulate cellular lipid metabolism, lipids also regulate lysosome function and autophagy. In this review, we focus on this bidirectional relationship in the context of dietary lipids and provide an overview of recent evidence of how lipid-overload lipotoxicity, as observed in obesity and metabolic syndrome, impairs lysosomal function and autophagy that may eventually lead to cellular dysfunction or cell death.

Exercise-like effects by Estrogen-related receptor-gamma in muscle do not prevent insulin resistance in db/db mice

Dissecting exercise-mimicking pathways that can replicate the benefits of exercise in obesity and diabetes may lead to promising treatments for metabolic disorders. Muscle estrogen-related receptor gamma (ERRγ) is induced by exercise, and when over-expressed in the skeletal muscle mimics exercise by stimulating glycolytic-to-oxidative myofiber switch, mitochondrial biogenesis and angiogenesis in lean mice. The objective of this study was to test whether muscle ERRγ in obese mice mitigates weight gain and insulin resistance. To do so, ERRγ was selectively over-expressed in the skeletal muscle of obese and diabetic db/db mice. Muscle ERRγ over-expression successfully triggered glycolytic-to-oxidative myofiber switch, increased functional mitochondrial content and boosted vascular supply in the db/db mice. Despite aerobic remodeling, ERRγ surprisingly failed to improve whole-body energy expenditure, block muscle accumulation of triglycerides, toxic diacylglycerols (DAG) and ceramides or suppress muscle PKCε sarcolemmal translocation in db/db mice. Consequently, muscle ERRγ did not mitigate impaired muscle insulin signaling or insulin resistance in these mice. In conclusion, obesity and diabetes in db/db mice are not amenable to selective ERRγ-directed programming of classic exercise-like effects in the skeletal muscle. Other biochemical pathways or integrated whole-body effects of exercise may be critical for resisting diabetes and obesity.

G0/G1 Switch Gene 2 controls adipose triglyceride lipase activity and lipid metabolism in skeletal muscle

OBJECTIVE

Recent data suggest that adipose triglyceride lipase (ATGL) plays a key role in providing energy substrate from triglyceride pools and that alterations of its expression/activity relate to metabolic disturbances in skeletal muscle. Yet little is known about its regulation. We here investigated the role of the protein G0/G1 Switch Gene 2 (G0S2), recently described as an inhibitor of ATGL in white adipose tissue, in the regulation of lipolysis and oxidative metabolism in skeletal muscle.

METHODS

We first examined G0S2 protein expression in relation to metabolic status and muscle characteristics in humans. We next overexpressed and knocked down G0S2 in human primary myotubes to assess its impact on ATGL activity, lipid turnover and oxidative metabolism, and further knocked down G0S2 in vivo in mouse skeletal muscle.

RESULTS

G0S2 protein is increased in skeletal muscle of endurance-trained individuals and correlates with markers of oxidative capacity and lipid content. Recombinant G0S2 protein inhibits ATGL activity by about 40% in lysates of mouse and human skeletal muscle. G0S2 overexpression augments (+49%, p < 0.05) while G0S2 knockdown strongly reduces (-68%, p < 0.001) triglyceride content in human primary myotubes and mouse skeletal muscle. We further show that G0S2 controls lipolysis and fatty acid oxidation in a strictly ATGL-dependent manner. These metabolic adaptations mediated by G0S2 are paralleled by concomitant changes in glucose metabolism through the modulation of Pyruvate Dehydrogenase Kinase 4 (PDK4) expression (5.4 fold, p < 0.001). Importantly, downregulation of G0S2 in vivo in mouse skeletal muscle recapitulates changes in lipid metabolism observed in vitro.

CONCLUSION

Collectively, these data indicate that G0S2 plays a key role in the regulation of skeletal muscle ATGL activity, lipid content and oxidative metabolism.

Adipose tissue depot specific differences of PLIN protein content in endurance trained rats

Adipose tissue is classified as either white (WAT) or brown (BAT) and differs not only by anatomical location but also in function. WAT is the main source of stored energy and releases fatty acids in times of energy demand, whereas BAT plays a role in regulating non-shivering thermogenesis and oxidizes fatty acids released from the lipid droplet. The PLIN family of proteins has recently emerged as being integral in the regulation of fatty acid storage and release in adipose tissue. Previous work has demonstrated that PLIN protein content varies among adipose tissue depots, however an examination of endurance training-induced depot specific changes in PLIN protein expression has yet to be done. Male Sprague-dawley rats (n = 10) underwent 8-weeks of progressive treadmill training (18-25 m/min for 30-60 min at 10% incline) or remained sedentary as control. Following training, under isoflurane induced anesthesia epidydmal (eWAT), inguinal subcutaneous (iWAT) and intrascapular brown adipose tissue (BAT) was excised, and plasma was collected. Endurance training resulted in an increase in BAT PLIN5 and iWAT PLIN3 content, while there was no difference in PLIN protein content in endurance trained eWAT. Interestingly, endurance training resulted in a robust increase in ATGL and CGI-58 in eWAT alone. Together these results suggest the potential of a depot specific function of PLIN3 and PLIN5 in adipose tissue in response to endurance training.

Citrus flavonoid, naringenin, increases locomotor activity and reduces diacylglycerol accumulation in skeletal muscle of obese ovariectomized mice

SCOPE

Estrogen deficiency has been associated with central obesity, muscle loss and metabolic syndrome in postmenopausal women. This study assessed naringenin accumulation in tissues and investigated the hypothesis that naringenin reverses diet-induced metabolic disturbances in obese ovariectomized mice.

METHODS AND RESULTS

In study 1, we measured naringenin concentrations in plasma, liver, perigonadal and subcutaneous adipose tissues, and muscle of ovariectomized C57BL/6J female mice after 11 weeks of naringenin supplementation. Naringenin accumulated 5-12 times more in mice fed a 3% naringenin diet than in mice fed a 1% naringenin diet. In study 2, ovariectomized mice were fed a high-fat diet (60 kcal% fat) for 11 weeks and half of the mice were then supplemented with 3% naringenin for another 11 weeks. Dietary naringenin suppressed weight gain, lowered hyperglycemia and decreased intra-abdominal adiposity evaluated by magnetic resonance imaging. Naringenin-fed mice exhibited elevated locomotor activity monitored by infrared beam breaks, maintained muscle mass and reduced muscle diacylglycerol content. Real-time PCR analysis in muscle revealed decreased mRNA level for genes involved in de novo lipogenesis, lipolysis and triglyceride synthesis/storage.

CONCLUSION

Long-term 3% naringenin supplementation resulted in significant naringenin accumulation in plasma and tissues, associated with attenuated metabolic dysregulation and muscle loss in obese ovariectomized mice.

Defective Natriuretic Peptide Receptor Signaling in Skeletal Muscle Links Obesity to Type 2 Diabetes

Circulating natriuretic peptide (NP) levels are reduced in obesity and predict the risk of type 2 diabetes (T2D). Since skeletal muscle was recently shown as a key target tissue of NP, we aimed to investigate muscle NP receptor (NPR) expression in the context of obesity and T2D. Muscle NPRA correlated positively with whole-body insulin sensitivity in humans and was strikingly downregulated in obese subjects and recovered in response to diet-induced weight loss. In addition, muscle NP clearance receptor (NPRC) increased in individuals with impaired glucose tolerance and T2D. Similar results were found in obese diabetic mice. Although no acute effect of brain NP (BNP) on insulin sensitivity was observed in lean mice, chronic BNP infusion improved blood glucose control and insulin sensitivity in skeletal muscle of obese and diabetic mice. This occurred in parallel with a reduced lipotoxic pressure in skeletal muscle due to an upregulation of lipid oxidative capacity. In addition, chronic NP treatment in human primary myotubes increased lipid oxidation in a PGC1α-dependent manner and reduced palmitate-induced lipotoxicity. Collectively, our data show that activation of NPRA signaling in skeletal muscle is important for the maintenance of long-term insulin sensitivity and has the potential to treat obesity-related metabolic disorders.

Development of PPAR-agonist GW0742 as antidiabetic drug: study in animals

Background

The development of new drugs for the treatment of diabetes mellitus (DM) is critically important. Insulin resistance (IR) is one of the main problems associated with type-2 DM (T2DM) seen in clinics. GW0742, a selective peroxisome proliferator-activated receptor (PPAR)-δ agonist, has been shown to ameliorate metabolic abnormalities including IR in skeletal muscle in mice fed high-fructose corn syrup. However, the influence of GW0742 on systemic insulin sensitivity has still not been elucidated. Therefore, it is important to investigate the effect of GW0742 on systemic IR in diabetic rats for the development of new drugs.

Methods

The present study used a T2DM animal model to compare the effect of GW0742 on IR using homeostasis model assessment-IR (HOMA-IR) and hyperinsulinemic euglycemic clamping. Additionally, the insulinotropic action of GW0742 was investigated in type-1 DM (T1DM) rats. Changes in the protein expression of glucose transporter 4 (GLUT4) and phosphoenolpyruvate carboxykinase (PEPCK) in skeletal muscle and in liver, respectively, were also identified by Western blots.

Results

GW0742 attenuated the increased HOMA-IR in diabetic rats fed a fructose-rich diet. This action was blocked by GSK0660 at the dose sufficient to inhibit PPAR-δ. Improvement of IR by GW0742 was also characterized in diabetic rats using hyperinsulinemic euglycemic clamping. Additionally, an increase of insulin sensitivity due to GW0742 was observed in these diabetic rats. Moreover, GW0742 reduced the hyperglycemia in T1DM rats lacking insulin. Western blotting analysis indicated that GW0742 reversed the decrease in GLUT4 and markedly reduced the increased PEPCK in liver.

Conclusion

The data showed that GW0742 has the ability to improve glucose homeostasis in diabetic rats through activation of PPAR-δ. Therefore, PPAR-δ is a good target for the development of antidiabetic drugs in the future.