Treatment of type 2 diabetes with the designer cytokine IC7Fc.

The gp130 receptor cytokines IL-6 and CNTF improve metabolic homeostasis but have limited therapeutic use for the treatment of type 2 diabetes. Accordingly, we engineered the gp130 ligand IC7Fc, in which one gp130-binding site is removed from IL-6 and replaced with the LIF-receptor-binding site from CNTF, fused with the Fc domain of immunoglobulin G, creating a cytokine with CNTF-like, but IL-6-receptor-dependent, signalling. Here we show that IC7Fc improves glucose tolerance and hyperglycaemia and prevents weight gain and liver steatosis in mice. In addition, IC7Fc either increases, or prevents the loss of, skeletal muscle mass by activation of the transcriptional regulator YAP1. In human-cell-based assays, and in non-human primates, IC7Fc treatment results in no signs of inflammation or immunogenicity. Thus, IC7Fc is a realistic next-generation biological agent for the treatment of type 2 diabetes and muscle atrophy, disorders that are currently pandemic.

The impact of a single dose of whey protein on glucose flux and metabolite profiles in normoglycemic males: insights into glucagon and insulin biology.

Protein ingestion concurrently stimulates euglycemic glucagon and insulin secretion, a response that is particularly robust with rapidly absorbing proteins. Previously, we have shown that ingestion of repeated doses of rapidly absorbing whey protein equally stimulated endogenous glucose production (EGP) and glucose disposal (Rd), thus explaining the preservation of euglycemia. Here, we aimed to determine if a smaller single dose of whey could elicit a large enough glucagon and insulin response to stimulate glucose flux. Therefore, in normoglycemic young adult males (n = 10; age ∼26; BMI ∼25), using [6,6-2H2] glucose tracing and quantitative targeted metabolite profiling, we determined the metabolic response to a single 25 g "standard" dose of whey protein. Whey protein ingestion did not alter glycemia, but increased circulating glucagon (peak 4-fold basal), insulin (peak 6-fold basal), amino acids, and urea while also reducing free fatty acid (FFA) and glycerol concentrations. Interestingly, the postprandial insulin response was driven by both a stimulation of insulin secretion and marked reduction in hepatic insulin clearance. Whey protein ingestion resulted in a modest stimulation of EGP and Rd, both peaking at ∼20% above baseline 1 h after protein ingestion. These findings demonstrate that the ingestion of a single standard serving of whey protein can induce a euglycemic glucagon and insulin response that stimulates glucose flux. We speculate on a theory that could potentially explain how glucagon and insulin synergistically provide hardwired control of nitrogen and glucose homeostasis.NEW & NOTEWORTHY Protein ingestion concurrently stimulates glucagon and insulin secretion. Here we show that in normoglycemic males, ingestion of a single "standard" 25 g serving of rapidly absorbing whey protein drives a sufficiently large glucagon and insulin response, such that it simultaneously increases endogenous glucose production and glucose disposal. We speculate on a novel theory that could potentially explain how the antagonistic/synergistic actions of glucagon and insulin simultaneously provide tight control of glucose and nitrogen homeostasis.

Autophagy is not involved in lipid accumulation and the development of insulin resistance in skeletal muscle.

OBJECTIVE: To investigate the effect of high fat diet-induced insulin resistance on autophagy markers in the liver and skeletal muscle of mice in the fasted state and following an oral glucose bolus. METHODS: Forty C57BL/6J male mice were fed either a high fat, high sucrose (HFSD, n = 20) or standard chow control (CON, n = 20) diet for 16 weeks. Upon trial completion, mice were gavaged with water or glucose and skeletal muscle and liver were collected 15 min post gavage. Protein abundance and gene expression of autophagy markers and activation of related signalling pathways were assessed. RESULTS: Compared to CON, the HFSD intervention increased LC3B-II and p62/SQSTM1 protein abundance in the liver which is indicative of elevated autophagosome content via reduced clearance. These changes coincided with inhibitory autophagy signalling through elevated p-mTOR S2448 and p-ULK1S758. HFSD did not alter autophagy markers in skeletal muscle. Administration of an oral glucose bolus had no effect on autophagy markers or upstream signalling responses in either tissue regardless of diet. CONCLUSION: HFSD induces tissue-specific autophagy impairments, with autophagosome accumulation indicating reduced lysosomal clearance in the liver. In contrast, autophagy markers were unchanged in skeletal muscle, indicating that autophagy is not involved in the development of skeletal muscle insulin resistance.

Insulin resistance in type 1 diabetes managed with metformin (INTIMET): Study protocol of a double-blind placebo-controlled, randomised trial.

BACKGROUND: Insulin resistance is an under-recognised metabolic defect and cardiovascular risk factor in Type 1 diabetes. Whether metformin improves hepatic, muscle or adipose tissue insulin sensitivity has not been studied in adults with Type 1 diabetes. We initiated the INTIMET study (INsulin resistance in Type 1 diabetes managed with METformin), a double-blind randomised, placebo-controlled trial to measure the effect of metformin on tissue-specific insulin resistance in adults with Type 1 diabetes. METHODS: We will study 40 adults aged 20-55 years with Type 1 diabetes (HbA1c ≤ 80 mmol/mol [9.5%], fasting C-peptide <0.3 nmol/L) and 20 age-, gender- and body mass index (BMI)-matched controls. Insulin sensitivity will be determined by the two-step hyperinsulinaemic-euglycaemic clamp method with deuterated glucose to document liver, muscle and adipose insulin sensitivity. Subjects with Type 1 diabetes will be randomised to metformin extended-release 1500 mg daily or matched placebo for 26 weeks. The primary outcome is change in hepatic insulin sensitivity, assessed by change in basal rate of appearance (Ra) of glucose and suppression of endogenous glucose production (EGP) during the low-dose stage of the clamp. CONCLUSION: The INTIMET study is the first clinical trial to quantify the impact of metformin on liver, muscle and adipose insulin resistance in adults with Type 1 diabetes. This study may identify factors that predict an individual's response to metformin in Type 1 diabetes. TRIAL REGISTRATION: ACTRN12619001440112.

The Effects of Early-Onset Pre-Eclampsia on Placental Creatine Metabolism in the Third Trimester.

Creatine is a metabolite important for cellular energy homeostasis as it provides spatio-temporal adenosine triphosphate (ATP) buffering for cells with fluctuating energy demands. Here, we examined whether placental creatine metabolism was altered in cases of early-onset pre-eclampsia (PE), a condition known to cause placental metabolic dysfunction. We studied third trimester human placentae collected between 27-40 weeks' gestation from women with early-onset PE (n = 20) and gestation-matched normotensive control pregnancies (n = 20). Placental total creatine and creatine precursor guanidinoacetate (GAA) content were measured. mRNA expression of the creatine synthesizing enzymes arginine:glycine aminotransferase (GATM) and guanidinoacetate methyltransferase (GAMT), the creatine transporter (SLC6A8), and the creatine kinases (mitochondrial CKMT1A & cytosolic BBCK) was assessed. Placental protein levels of arginine:glycine aminotransferase (AGAT), GAMT, CKMT1A and BBCK were also determined. Key findings; total creatine content of PE placentae was 38% higher than controls (p < 0.01). mRNA expression of GATM (p < 0.001), GAMT (p < 0.001), SLC6A8 (p = 0.021) and BBCK (p < 0.001) was also elevated in PE placentae. No differences in GAA content, nor protein levels of AGAT, GAMT, BBCK or CKMT1A were observed between cohorts. Advancing gestation and birth weight were associated with a down-regulation in placental GATM mRNA expression, and a reduction in GAA content, in control placentae. These relationships were absent in PE cases. Our results suggest PE placentae may have an ongoing reliance on the creatine kinase circuit for maintenance of cellular energetics with increased total creatine content and transcriptional changes to creatine synthesizing enzymes and the creatine transporter. Understanding the functional consequences of these changes warrants further investigation.

Mechanisms of hyperinsulinaemia in apparently healthy non-obese young adults: role of insulin secretion, clearance and action and associations with plasma amino acids.

AIMS/HYPOTHESIS: This study aimed to examine the metabolic health of young apparently healthy non-obese adults to better understand mechanisms of hyperinsulinaemia. METHODS: Non-obese (BMI < 30 kg/m2) adults aged 18-35 years (N = 254) underwent a stable isotope-labelled OGTT. Insulin sensitivity, glucose effectiveness and beta cell function were determined using oral minimal models. Individuals were stratified into quartiles based on their insulin response during the OGTT, with quartile 1 having the lowest and quartile 4 the highest responses. RESULTS: Thirteen per cent of individuals had impaired fasting glucose (IFG; n = 14) or impaired glucose tolerance (IGT; n = 19), allowing comparisons across the continuum of insulin responses within the spectrum of normoglycaemia and prediabetes. BMI (~24 kg/m2) was similar across insulin quartiles and in those with IFG and IGT. Despite similar glycaemic excursions, fasting insulin, triacylglycerols and cholesterol were elevated in quartile 4. Insulin sensitivity was lowest in quartile 4, and accompanied by increased insulin secretion and reduced insulin clearance. Individuals with IFG had similar insulin sensitivity and beta cell function to those in quartiles 2 and 3, but were more insulin sensitive than individuals in quartile 4. While individuals with IGT had a similar degree of insulin resistance to quartile 4, they exhibited a more severe defect in beta cell function. Plasma branched-chain amino acids were not elevated in quartile 4, IFG or IGT. CONCLUSIONS/INTERPRETATION: Hyperinsulinaemia within normoglycaemic young, non-obese adults manifests due to increased insulin secretion and reduced insulin clearance. Individual phenotypic characterisation revealed that the most hyperinsulinaemic were more similar to individuals with IGT than IFG, suggesting that hyperinsulinaemic individuals may be on the continuum toward IGT. Furthermore, plasma branched-chain amino acids may not be an effective biomarker in identifying hyperinsulinaemia and insulin resistance in young non-obese adults.

Modest changes to glycemic regulation are sufficient to maintain glucose fluxes in healthy young men following overfeeding with a habitual macronutrient composition.

Currently, it is unclear whether short-term overfeeding in healthy people significantly affects postprandial glucose regulation, as most human overfeeding studies have utilized induced experimental conditions such as the euglycemic-hyperinsulinemic clamp technique to assess glucoregulation. The aim of this study was to quantify glucose fluxes [rates of meal glucose appearance (Ra), disposal (Rd), and endogenous glucose production (EGP)] in response to 5 and 28 days of overfeeding (+45% energy) while maintaining habitual macronutrient composition (31.0 ± 1.9% fat, 48.6 ± 2.2% carbohydrate, 16.7 ± 1.4% protein) in healthy, lean young men. Meal tolerance testing was combined with the triple-stable isotope glucose tracer approach. Visceral adipose volume increased by ~15% with 5 days of overfeeding, while there was no further change at 28 days. In contrast, body mass (+1.6 kg) and fat mass (+1.3 kg) were significantly increased only after 28 days of overfeeding. Fasting EGP, Rd, and insulin were increased at 5 but unchanged after 28 days. Postprandial glucose and insulin responses were unaltered by 5 days of overfeeding but were modestly increased after 28 days (P < 0.05). However, meal Ra and glucose Rd were significantly increased after both 5 and 28 days of overfeeding (P < 0.05). Despite this, overfeeding did not lead to alterations to postprandial EGP suppression. Thus, in contrast to findings from euglycemic-hyperinsulinemic clamp studies, chronic overfeeding did not affect the ability to suppress EGP or stimulate Rd under postprandial conditions. Rather, glucose flux was appropriately maintained following 28 days of overfeeding through modest increases in postprandial glycemia and insulinemia.

Placental creatine metabolism in cases of placental insufficiency and reduced fetal growth.

Creatine is a metabolite involved in cellular energy homeostasis. In this study, we examined placental creatine content, and expression of the enzymes required for creatine synthesis, transport and the creatine kinase reaction, in pregnancies complicated by low birthweight. We studied first trimester chorionic villus biopsies (CVBs) of small for gestational age (SGA) and appropriately grown infants (AGA), along with third trimester placental samples from fetal growth restricted (FGR) and healthy gestation-matched controls. Placental creatine and creatine precursor (guanidinoacetate-GAA) levels were measured. Maternal and cord serum from control and FGR pregnancies were also analyzed for creatine concentration. mRNA expression of the creatine transporter (SLC6A8); synthesizing enzymes arginine:glycine aminotransferase (GATM) and guanidinoacetate methyltransferase (GAMT); mitochondrial (mtCK) and cytosolic (BBCK) creatine kinases; and amino acid transporters (SLC7A1 & SLC7A2) was assessed in both CVBs and placental samples. Protein levels of AGAT (arginine:glycine aminotransferase), GAMT, mtCK and BBCK were also measured in placental samples. Key findings; total creatine content of the third trimester FGR placentae was 43% higher than controls. The increased creatine content of placental tissue was not reflected in maternal or fetal serum from FGR pregnancies. Tissue concentrations of GAA were lower in the third trimester FGR placentae compared to controls, with lower GATM and GAMT mRNA expression also observed. No differences in the mRNA expression of GATM, GAMT or SLC6A8 were observed between CVBs from SGA and AGA pregnancies. These results suggest placental creatine metabolism in FGR pregnancies is altered in late gestation. The relevance of these changes on placental bioenergetics should be the focus of future investigations.

Urinary sodium is positively associated with urinary free cortisol and total cortisol metabolites in a cross-sectional sample of Australian schoolchildren aged 5-12 years and their mothers.

High Na intake and chronically elevated cortisol levels are independently associated with the development of chronic diseases. In adults, high Na intake is associated with high levels of urinary cortisol. We aimed to determine the association between urinary Na and K and urinary cortisol in a cross-sectional sample of Australian schoolchildren and their mothers. Participants were a sample of Australian children (n 120) and their mothers (n 100) recruited through primary schools. We assessed Na, K, free cortisol and cortisol metabolites in one 24 h urine collection. Associations between 24 h urinary electrolytes and 24 h urinary cortisol were assessed using multilevel mixed-effects linear regression models. In children, urinary Na was positively associated with urinary free cortisol (ß=0·31, 95 % CI 0·19, 0·44) and urinary cortisol metabolites (ß=0·006, 95 % CI 0·002, 0·010). Positive associations were also observed between urinary K and urinary free cortisol (ß=0·65, 95 % CI 0·23, 1·07) and urinary cortisol metabolites (ß=0·02, 95 % CI 0·03, 0·031). In mothers, urinary Na was positively associated with urinary free cortisol (ß=0·23, 95 % CI 0·01, 0·50) and urinary cortisol metabolites (ß=0·008, 95 % CI 0·0007, 0·016). Our findings show that daily Na and K intake were positively associated with cortisol production in children and their mothers. Investigation of the mechanisms involved and the potential impact of Na reduction on cortisol levels in these populations is warranted.

Endogenous glucose production after sequential meals in humans: evidence for more prolonged suppression after ingestion of a second meal.

Single-meal studies have shown that carbohydrate ingestion causes rapid and persistent suppression of endogenous glucose production (EGP). However, little is known about the regulation of EGP under real-life eating patterns in which multiple carbohydrate-containing meals are consumed throughout the day. Therefore, we aimed to characterize the regulation of EGP in response to sequential meals, specifically during the breakfast-lunch transition. Nine healthy individuals (5 men, 4 women; 32 ± 2 yr; 25.0 ± 1.4 kg/m2) ingested two identical mixed meals, each containing 25 g of glucose, separated by 4 h, and EGP was determined by the variable infusion tracer-clamp approach. EGP was rapidly suppressed after both meals, with the pattern and magnitude of suppression being similar over the initial 75-min postmeal period. However, EGP suppression was more transient after breakfast compared with lunch, with EGP returning to basal rates 3 h after breakfast. In contrast, EGP remained in a suppressed state for the entire 4-h postlunch period. This occurred despite each meal eliciting similar plasma glucose and insulin responses. However, there was greater suppression of plasma glucagon levels after lunch, likely contributing to this response. These findings highlight the potential for distinct regulation of EGP with each meal of the day and suggest that EGP may be in a suppressed state for much of the day, since EGP did not return to basal rates even after a lunch meal containing a modest amount of carbohydrate.

Measurement of postprandial glucose fluxes in response to acute and chronic endurance exercise in healthy humans.

The effect of endurance exercise on enhancing insulin sensitivity and glucose flux has been well established with techniques such as the hyperinsulinemic clamp. Although informative, such techniques do not emulate the physiological postprandial state, and it remains unclear how exercise improves postprandial glycaemia. Accordingly, combining mixed-meal tolerance testing and the triple-stable isotope glucose tracer approach, glucose fluxes [rates of meal glucose appearance (Ra), disposal (Rd), and endogenous glucose production (EGP)] were determined following acute endurance exercise (1 h cycling; ~70% V̇o2max) and 4 wk of endurance training (cycling 5 days/wk). Training was associated with a modest increase in V̇o2max (~7%, P < 0.001). Postprandial glucose and insulin responses were reduced to the same extent following acute and chronic training. Interestingly, this was not accompanied by changes to rates of meal Ra, Rd, or degree of EGP suppression. Glucose clearance (Rd relative to prevailing glucose) was, however, enhanced with acute and chronic exercise. Furthermore, the duration of EGP suppression was shorter with acute and chronic exercise, with EGP returning toward fasting levels more rapidly than pretraining conditions. These findings suggest that endurance exercise influences the efficiency of the glucoregulatory system, where pretraining rates of glucose disposal and production were achieved at lower glucose and insulin levels. Notably, there was no influence of chronic training over and above that of a single exercise bout, providing further evidence that glucoregulatory benefits of endurance exercise are largely attributed to the residual effects of the last exercise bout.

Lysine post-translational modification of glyceraldehyde-3-phosphate dehydrogenase regulates hepatic and systemic metabolism.

Reciprocal regulation of hepatic glycolysis and gluconeogenesis contributes to systemic metabolic homeostasis. Recent evidence from lower order organisms has found that reversible post-translational modification of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), particularly acetylation, contributes to the reciprocal regulation of glycolysis/gluconeogenesis. However, whether this occurs in mammalian hepatocytes in vitro or in vivo is unknown. Several proteomics studies have identified 4 lysine residues in critical regions of mammalian GAPDH that are altered by multiple post-translational modifications. In FAO hepatoma cells, mutation of all 4 lysine residues (4K-R GAPDH) to mimic their unmodified state reduced GAPDH glycolytic activity and glycolytic flux and increased gluconeogenic GAPDH activity and glucose production. Hepatic expression of 4K-R GAPDH in mice increased GAPDH gluconeogenic activity and the contribution of gluconeogenesis to endogenous glucose production in the unfed state. Consistent with the increased reliance on the energy-consuming gluconeogenic pathway, plasma free fatty acids and ketones were elevated in mice expressing 4K-R GAPDH, suggesting enhanced lipolysis and hepatic fatty acid oxidation. In normal mice, food withholding and refeeding, as well as hormonal regulators of reciprocal glycolysis/gluconeogenesis, such as insulin, glucagon, and norepinephrine, had no effect on global GAPDH acetylation. However, GAPDH acetylation was reduced in obese and type 2 diabetic db/db mice. These findings show that post-translational modification of GAPDH lysine residues regulates hepatic and systemic metabolism, revealing an unappreciated role for hepatic GAPDH in substrate selection and utilization.-Bond, S. T., Howlett, K. F., Kowalski, G. M., Mason, S., Connor, T., Cooper, A., Streltsov, V., Bruce, C. R., Walder, K. R., McGee, S. L. Lysine post-translational modification of glyceraldehyde-3-phosphate dehydrogenase regulates hepatic and systemic metabolism.

Skeletal muscle-specific overexpression of heat shock protein 72 improves skeletal muscle insulin-stimulated glucose uptake but does not alter whole body metabolism.

AIMS: The induction of heat shock protein 72 (Hsp72) via heating, genetic manipulation or pharmacological activation is metabolically protective in the setting of obesity-induced insulin resistance across mammalian species. In this study, we set out to determine whether the overexpression of Hsp72, specifically in skeletal muscle, can protect against high-fat diet (HFD)-induced obesity and insulin resistance. MATERIALS AND METHODS: An Adeno-Associated Viral vector (AAV), designed to overexpress Hsp72 in skeletal muscle only, was used to study the effects of increasing Hsp72 levels on various metabolic parameters. Two studies were conducted, the first with direct intramuscular (IM) injection of the AAV:Hsp72 into the tibialis anterior hind-limb muscle and the second with a systemic injection to enable body-wide skeletal muscle transduction. RESULTS: IM injection of the AAV:Hsp72 significantly improved skeletal muscle insulin-stimulated glucose clearance in treated hind-limb muscles, as compared with untreated muscles of the contralateral leg when mice were fed an HFD. Despite this finding, systemic administration of AAV:Hsp72 did not improve body composition parameters such as body weight, fat mass or percentage body fat, nor did it lead to an improvement in fasting glucose levels or glucose tolerance. Furthermore, no differences were observed for other metabolic parameters such as whole-body oxygen consumption, energy expenditure or physical activity levels. CONCLUSIONS: At the levels of Hsp72 over-expression reported herein, skeletal muscle-specific Hsp72 overexpression via IM injection has the capacity to increase insulin-stimulated glucose clearance in this muscle. However, upon systemic injection, which results in lower muscle Hsp72 overexpression, no beneficial effects on whole-body metabolism are observed.

Does maternal-fetal transfer of creatine occur in pregnant sheep?

Our aim was to determine the disposition of creatine in ovine pregnancy and whether creatine is transferred across the placenta from mother to fetus. Pregnant ewes received either 1) a continuous intravenous infusion of creatine monohydrate or saline from 122 to 131 days gestation, with maternal and fetal arterial blood and amniotic fluid samples collected daily for creatine analysis and fetal tissues collected at necropsy at 133 days for analysis of creatine content, or 2) a single systemic bolus injection of [13C]creatine monohydrate at 130 days of gestation, with maternal and fetal arterial blood, uterine vein blood, and amniotic fluid samples collected before and for 4 h after injection and analyzed for creatine, creatine isotopic enrichment, and guanidinoacetic acid (GAA; precursor of creatine) concentrations. Presence of the creatine transporter-1 (SLC6A8) and l-arginine:glycine amidinotransferase (AGAT; the enzyme synthesizing GAA) proteins were determined by Western blots of placental cotyledons. The 10-day creatine infusion increased maternal plasma creatine concentration three- to fourfold (P < 0.05) without significantly changing fetal arterial, amniotic fluid, fetal tissues, or placental creatine content. Maternal arterial 13C enrichment was increased (P < 0.05) after bolus [13C]creatine injection without change of fetal arterial 13C enrichment. SLC6A8 and AGAT proteins were identified in placental cotyledons, and GAA concentration was significantly higher in uterine vein than maternal artery plasma. Despite the presence of SLC6A8 protein in cotyledons, these results suggest that creatine is not transferred from mother to fetus in near-term sheep and that the ovine utero-placental unit releases GAA into the maternal circulation.

Overexpression of sphingosine kinase 1 in liver reduces triglyceride content in mice fed a low but not high-fat diet.

Hepatic insulin resistance is a major risk factor for the development of type 2 diabetes and is associated with the accumulation of lipids, including diacylglycerol (DAG), triacylglycerols (TAG) and ceramide. There is evidence that enzymes involved in ceramide or sphingolipid metabolism may have a role in regulating concentrations of glycerolipids such as DAG and TAG. Here we have investigated the role of sphingosine kinase (SphK) in regulating hepatic lipid levels. We show that mice on a high-fat high-sucrose diet (HFHS) displayed glucose intolerance, elevated liver TAG and DAG, and a reduction in total hepatic SphK activity. Reduced SphK activity correlated with downregulation of SphK1, but not SphK2 expression, and was not associated with altered ceramide levels. The role of SphK1 was further investigated by overexpressing this isoform in the liver of mice in vivo. On a low-fat diet (LFD) mice overexpressing liver SphK1, displayed reduced hepatic TAG synthesis and total TAG levels, but with no change to DAG or ceramide. These mice also exhibited no change in gluconeogenesis, glycogenolysis or glucose tolerance. Similarly, overexpression of SphK1 had no effect on the pattern of endogenous glucose production determined during a glucose tolerance test. Under HFHS conditions, normalization of liver SphK activity to levels observed in LFD controls did not alter hepatic TAG concentrations. Furthermore, DAG, ceramide and glucose tolerance were also unaffected. In conclusion, our data suggest that SphK1 plays an important role in regulating TAG metabolism under LFD conditions.

ATGL-mediated triglyceride turnover and the regulation of mitochondrial capacity in skeletal muscle.

Emerging evidence indicates that skeletal muscle lipid droplets are an important control point for intracellular lipid homeostasis and that regulating fatty acid fluxes from lipid droplets might influence mitochondrial capacity. We used pharmacological blockers of the major triglyceride lipases, adipose triglyceride lipase (ATGL) and hormone-sensitive lipase, to show that a large proportion of the fatty acids that are transported into myotubes are trafficked through the intramyocellular triglyceride pool. We next tested whether increasing lipolysis from intramyocellular lipid droplets could activate transcriptional responses to enhance mitochondrial and fatty acid oxidative capacity. ATGL was overexpressed by adenoviral and adenoassociated viral infection in C2C12 myotubes and the tibialis anterior muscle of C57Bl/6 mice, respectively. ATGL overexpression in C2C12 myotubes increased lipolysis, which was associated with increased peroxisome proliferator-activated receptor (PPAR)-∂ activity, transcriptional upregulation of some PPAR∂ target genes, and enhanced mitochondrial capacity. The transcriptional responses were specific to ATGL actions and not a generalized increase in fatty acid flux in the myotubes. Marked ATGL overexpression (20-fold) induced modest molecular changes in the skeletal muscle of mice, but these effects were not sufficient to alter fatty acid oxidation. Together, these data demonstrate the importance of lipid droplets for myocellular fatty acid trafficking and the capacity to modulate mitochondrial capacity by enhancing lipid droplet lipolysis in vitro; however, this adaptive program is of minor importance when superimposing the normal metabolic stresses encountered in free-moving animals.

In vivo cardiac glucose metabolism in the high-fat fed mouse: Comparison of euglycemic-hyperinsulinemic clamp derived measures of glucose uptake with a dynamic metabolomic flux profiling approach.

RATIONALE: Cardiac metabolism is thought to be altered in insulin resistance and type 2 diabetes (T2D). Our understanding of the regulation of cardiac substrate metabolism and insulin sensitivity has largely been derived from ex vivo preparations which are not subject to the same metabolic regulation as in the intact heart in vivo. Studies are therefore required to examine in vivo cardiac glucose metabolism under physiologically relevant conditions. OBJECTIVE: To determine the temporal pattern of the development of cardiac insulin resistance and to compare with dynamic approaches to interrogate cardiac glucose and intermediary metabolism in vivo. METHODS AND RESULTS: Studies were conducted to determine the evolution of cardiac insulin resistance in C57Bl/6 mice fed a high-fat diet (HFD) for between 1 and 16 weeks. Dynamic in vivo cardiac glucose metabolism was determined following oral administration of [U-(13)C] glucose. Hearts were collected after 15 and 60 min and flux profiling was determined by measuring (13)C mass isotopomers in glycolytic and tricarboxylic acid (TCA) cycle intermediates. Cardiac insulin resistance, determined by euglycemic-hyperinsulinemic clamp, was evident after 3 weeks of HFD. Despite the presence of insulin resistance, in vivo cardiac glucose metabolism following oral glucose administration was not compromised in HFD mice. This contrasts our recent findings in skeletal muscle, where TCA cycle activity was reduced in mice fed a HFD. Similar to our report in muscle, glucose derived pyruvate entry into the TCA cycle in the heart was almost exclusively via pyruvate dehydrogenase, with pyruvate carboxylase mediated anaplerosis being negligible after oral glucose administration. CONCLUSIONS: Under experimental conditions which closely mimic the postprandial state, the insulin resistant mouse heart retains the ability to stimulate glucose metabolism.

Application of dynamic metabolomics to examine in vivo skeletal muscle glucose metabolism in the chronically high-fat fed mouse.

RATIONALE: Defects in muscle glucose metabolism are linked to type 2 diabetes. Mechanistic studies examining these defects rely on the use of high fat-fed rodent models and typically involve the determination of muscle glucose uptake under insulin-stimulated conditions. While insightful, they do not necessarily reflect the physiology of the postprandial state. In addition, most studies do not examine aspects of glucose metabolism beyond the uptake process. Here we present an approach to study rodent muscle glucose and intermediary metabolism under the dynamic and physiologically relevant setting of the oral glucose tolerance test (OGTT). METHODS AND RESULTS: In vivo muscle glucose and intermediary metabolism was investigated following oral administration of [U-(13)C] glucose. Quadriceps muscles were collected 15 and 60 min after glucose administration and metabolite flux profiling was determined by measuring (13)C mass isotopomers in glycolytic and tricarboxylic acid (TCA) cycle intermediates via gas chromatography-mass spectrometry. While no dietary effects were noted in the glycolytic pathway, muscle from mice fed a high fat diet (HFD) exhibited a reduction in labelling in TCA intermediates. Interestingly, this appeared to be independent of alterations in flux through pyruvate dehydrogenase. In addition, our findings suggest that TCA cycle anaplerosis is negligible in muscle during an OGTT. CONCLUSIONS: Under the dynamic physiologically relevant conditions of the OGTT, skeletal muscle from HFD fed mice exhibits alterations in glucose metabolism at the level of the TCA cycle.

The regulation of glucose metabolism: implications and considerations for the assessment of glucose homeostasis in rodents.

The incidence of insulin resistance and type 2 diabetes (T2D) is increasing at alarming rates. In the quest to understand the underlying causes of and to identify novel therapeutic targets to treat T2D, scientists have become increasingly reliant on the use of rodent models. Here, we provide a discussion on the regulation of rodent glucose metabolism, highlighting key differences and similarities that exist between rodents and humans. In addition, some of the issues and considerations associated with assessing glucose homeostasis and insulin action are outlined. We also discuss the role of the liver vs. skeletal muscle in regulating whole body glucose metabolism in rodents, emphasizing the importance of defective hepatic glucose metabolism in the development of impaired glucose tolerance, insulin resistance, and T2D.

Bile acids mediate fructose-associated liver tumour growth in mice.

High fructose diets are associated with an increased risk of liver cancer. Previous studies in mice suggest increased lipogenesis is a key mechanism linking high fructose diets to liver tumour growth. However, these studies administered fructose to mice at supraphysiological levels. The aim of this study was to determine whether liver tumour growth and lipogenesis were altered in mice fed fructose at physiological levels. To test this, we injected male C57BL/6 mice with the liver carcinogen diethylnitrosamine and then fed them diets without fructose or fructose ranging from 10 to 20 % total calories. Results showed mice fed diets with ≥15 % fructose had significantly increased liver tumour numbers (2-4-fold) and total tumour burden (∼7-fold) vs mice fed no-fructose diets. However, fructose-associated tumour burden was not associated with lipogenesis. Conversely, unbiased metabolomic analyses revealed bile acids were elevated in the sera of mice fed a 15 % fructose diet vs mice fed a no-fructose diet. Using a syngeneic ectopic liver tumour model, we show that ursodeoxycholic acid, which decreases systemic bile acids, significantly reduced liver tumour growth in mice fed the 15 % fructose diet but not mice fed a no-fructose diet. These results point to a novel role for systemic bile acids in mediating liver tumour growth associated with a high fructose diet. Overall, our study shows fructose intake at or above normal human consumption (≥15 %) is associated with increased liver tumour numbers and growth and that modulating systemic bile acids inhibits fructose-associated liver tumour growth in mice.