Mechanism for the stimulation of gluconeogenesis by fatty acids in perfused rat liver

Small molecules targeting selective PCK1 and PGC-1α lysine acetylation cause anti-diabetic action through increased lactate oxidation

Small molecules selectively inducing peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1α acetylation and inhibiting glucagon-dependent gluconeogenesis causing anti-diabetic effects have been identified. However, how these small molecules selectively suppress the conversion of gluconeogenic metabolites into glucose without interfering with lipogenesis is unknown. Here, we show that a small molecule SR18292 inhibits hepatic glucose production by increasing lactate and glucose oxidation. SR18292 increases phosphoenolpyruvate carboxykinase 1 (PCK1) acetylation, which reverses its gluconeogenic reaction and favors oxaloacetate (OAA) synthesis from phosphoenolpyruvate. PCK1 reverse catalytic reaction induced by SR18292 supplies OAA to tricarboxylic acid (TCA) cycle and is required for increasing glucose and lactate oxidation and suppressing gluconeogenesis. Acetylation mimetic mutant PCK1 K91Q favors anaplerotic reaction and mimics the metabolic effects of SR18292 in hepatocytes. Liver-specific expression of PCK1 K91Q mutant ameliorates hyperglycemia in obese mice. Thus, SR18292 blocks gluconeogenesis by enhancing gluconeogenic substrate oxidation through PCK1 lysine acetylation, supporting the anti-diabetic effects of these small molecules.

A ketone monoester drink reduces postprandial blood glucose concentrations in adults with type 2 diabetes: a randomised controlled trial

AIMS/HYPOTHESIS

The aim of the present study was to conduct a randomised, placebo-controlled, double-blind, crossover trial to determine whether pre-meal ketone monoester ingestion reduces postprandial glucose concentrations in individuals with type 2 diabetes.

METHODS

In this double-blind, placebo-controlled, crossover design study, ten participants with type 2 diabetes (age 59±1.7 years, 50% female, BMI 32±1 kg/m2, HbA1c 54±2 mmol/mol [7.1±0.2%]) were randomised using computer-generated random numbers. The study took place at the Nutritional Physiology Research Unit, University of Exeter, Exeter, UK. Using a dual-glucose tracer approach, we assessed glucose kinetics after the ingestion of a 0.5 g/kg body mass ketone monoester (KME) or a taste-matched non-caloric placebo before a mixed-meal tolerance test. The primary outcome measure was endogenous glucose production. Secondary outcome measures were total glucose appearance rate and exogenous glucose appearance rate, glucose disappearance rate, blood glucose, serum insulin, β-OHB and NEFA levels, and energy expenditure.

RESULTS

Data for all ten participants were analysed. KME ingestion increased mean ± SEM plasma beta-hydroxybutyrate from 0.3±0.03 mmol/l to a peak of 4.3±1.2 mmol/l while reducing 2 h postprandial glucose concentrations by ~18% and 4 h postprandial glucose concentrations by ~12%, predominately as a result of a 28% decrease in the 2 h rate of glucose appearance following meal ingestion (all p<0.05). The reduction in blood glucose concentrations was associated with suppressed plasma NEFA concentrations after KME ingestion, with no difference in plasma insulin concentrations between the control and KME conditions. Postprandial endogenous glucose production was unaffected by KME ingestion (mean ± SEM 0.76±0.15 and 0.88±0.10 mg kg-1 min-1 for the control and KME, respectively). No adverse effects of KME ingestion were observed.

CONCLUSIONS/INTERPRETATION

KME ingestion appears to delay glucose absorption in adults with type 2 diabetes, thereby reducing postprandial glucose concentrations. Future work to explore the therapeutic potential of KME supplementation in type 2 diabetes is warranted.

TRIAL REGISTRATION

ClinicalTrials.gov NCT05518448.

FUNDING

This project was supported by a Canadian Institutes of Health Research (CIHR) Project Grant (PJT-169116) and a Natural Sciences and Engineering Research Council (NSERC) Discovery Grant (RGPIN-2019-05204) awarded to JPL and an Exeter-UBCO Sports Health Science Fund Project Grant awarded to FBS and JPL.

Hepatic malonyl-CoA synthesis restrains gluconeogenesis by suppressing fat oxidation, pyruvate carboxylation, and amino acid availability

Acetyl-CoA carboxylase (ACC) promotes prandial liver metabolism by producing malonyl-CoA, a substrate for de novo lipogenesis and an inhibitor of CPT-1-mediated fat oxidation. We report that inhibition of ACC also produces unexpected secondary effects on metabolism. Liver-specific double ACC1/2 knockout (LDKO) or pharmacologic inhibition of ACC increased anaplerosis, tricarboxylic acid (TCA) cycle intermediates, and gluconeogenesis by activating hepatic CPT-1 and pyruvate carboxylase flux in the fed state. Fasting should have marginalized the role of ACC, but LDKO mice maintained elevated TCA cycle intermediates and preserved glycemia during fasting. These effects were accompanied by a compensatory induction of proteolysis and increased amino acid supply for gluconeogenesis, which was offset by increased protein synthesis during feeding. Such adaptations may be related to Nrf2 activity, which was induced by ACC inhibition and correlated with fasting amino acids. The findings reveal unexpected roles for malonyl-CoA synthesis in liver and provide insight into the broader effects of pharmacologic ACC inhibition.

Hepatic glucose metabolism in the steatotic liver

The liver is central in regulating glucose homeostasis, being the major contributor to endogenous glucose production and the greatest reserve of glucose as glycogen. It is both a target and regulator of the action of glucoregulatory hormones. Hepatic metabolic functions are altered in and contribute to the highly prevalent steatotic liver disease (SLD), including metabolic dysfunction-associated SLD (MASLD) and metabolic dysfunction-associated steatohepatitis (MASH). In this Review, we describe the dysregulation of hepatic glucose metabolism in MASLD and MASH and associated metabolic comorbidities, and how advances in techniques and models for the assessment of hepatic glucose fluxes in vivo have led to the identification of the mechanisms related to the alterations in glucose metabolism in MASLD and comorbidities. These fluxes can ultimately increase hepatic glucose production concomitantly with fat accumulation and alterations in the secretion and action of glucoregulatory hormones. No pharmacological treatment has yet been approved for MASLD or MASH, but some antihyperglycaemic drugs approved for treating type 2 diabetes have shown positive effects on hepatic glucose metabolism and hepatosteatosis. A deep understanding of how MASLD affects glucose metabolic fluxes and glucoregulatory hormones might assist in the early identification of at-risk individuals and the use or development of targeted therapies.

Endogenous renal adiponectin drives gluconeogenesis through enhancing pyruvate and fatty acid utilization

Adiponectin is a secretory protein, primarily produced in adipocytes. However, low but detectable expression of adiponectin can be observed in cell types beyond adipocytes, particularly in kidney tubular cells, but its local renal role is unknown. We assessed the impact of renal adiponectin by utilizing male inducible kidney tubular cell-specific adiponectin overexpression or knockout mice. Kidney-specific adiponectin overexpression induces a doubling of phosphoenolpyruvate carboxylase expression and enhanced pyruvate-mediated glucose production, tricarboxylic acid cycle intermediates and an upregulation of fatty acid oxidation (FAO). Inhibition of FAO reduces the adiponectin-induced enhancement of glucose production, highlighting the role of FAO in the induction of renal gluconeogenesis. In contrast, mice lacking adiponectin in the kidney exhibit enhanced glucose tolerance, lower utilization and greater accumulation of lipid species. Hence, renal adiponectin is an inducer of gluconeogenesis by driving enhanced local FAO and further underlines the important systemic contribution of renal gluconeogenesis.

Fatty acid challenge shifts cellular energy metabolism in a substrate-specific manner in primary bovine neonatal hepatocytes

Adipose tissue mobilization increases circulating fatty acid (FA) concentrations, leads to increased hepatic FA uptake, and influences hepatic metabolism. Our objective was to trace carbon flux through metabolic pathways in primary bovine neonatal hepatocytes challenged with FA, and to examine the effect of FA challenge on oxidative stress. Primary bovine neonatal hepatocytes were isolated from 4 Holstein bull calves and maintained for 24 h before treatment with either 0 or 1 mM FA cocktail. After 21 h, either [1-14C]C16:0 or [2-14C]sodium pyruvate was added to measure complete and incomplete oxidation and cellular glycogen. Cellular and media triglyceride (TG), and glucose and ß-hydroxybutyrate (BHB) export were quantified, as well as reactive oxygen species and cellular glutathione (GSH/GSSH). Fatty acid treatment increased cellular, but not media TG, and although complete oxidation of [1-14C]C16:0 was not affected by FA, BHB export was increased. Reactive oxygen species were increased with FA treatment and GSSH was marginally increased such that the ratio of GSH:GSSG was marginally decreased. Glucose export increased, and cellular glycogen marginally increased with FA treatment while [2-14C]sodium pyruvate oxidation was decreased. These data suggest that FA treatment shifts cellular energy metabolism in a substrate-specific manner, spares pyruvate carbon from oxidation, and stimulates glucose synthesis.

Roles of malate and aspartate in gluconeogenesis in various physiological and pathological states

Gluconeogenesis, a pathway for glucose synthesis from non-carbohydrate substances, begins with the synthesis of oxaloacetate (OA) from pyruvate and intermediates of citric acid cycle in hepatocyte mitochondria. The traditional view is that OA does not cross the mitochondrial membrane and must be shuttled to the cytosol, where most enzymes involved in gluconeogenesis are compartmentalized, in the form of malate. Thus, the possibility of transporting OA in the form of aspartate has been ignored. In the article is shown that malate supply to the cytosol increases only when fatty acid oxidation in the liver is activated, such as during starvation or untreated diabetes. Alternatively, aspartate synthesized from OA by mitochondrial aspartate aminotransferase (AST) is transported to the cytosol in exchange for glutamate via the aspartate-glutamate carrier 2 (AGC2). If the main substrate for gluconeogenesis is an amino acid, aspartate is converted to OA via urea cycle, therefore, ammonia detoxification and gluconeogenesis are simultaneously activated. If the main substrate is lactate, OA is synthesized by cytosolic AST, glutamate is transported to the mitochondria through AGC2, and nitrogen is not lost. It is concluded that, compared to malate, aspartate is a more suitable form of OA transport from the mitochondria for gluconeogenesis.

Is type 2 diabetes an adiposity-based metabolic disease? From the origin of insulin resistance to the concept of dysfunctional adipose tissue

In the last decades of the past century, a remarkable amount of research efforts, money and hopes was generated to unveil the basis of insulin resistance that was believed to be the primary etiological factor in the development of type 2 diabetes. From the Reaven's insulin resistance syndrome to the DeFronzo's triumvirate (skeletal muscle, liver and beta-cell) and to Kahn's discovery (among many others) of insulin receptor downregulation and autophosphorylation, an enthusiastic age of metabolic in vivo and in vitro research took place, making the promise of a resolutory ending. However, from many published data (those of insulin receptoropathies and lipodystrophies, the genome-wide association studies results, the data on reversibility of type 2 diabetes after bariatric surgery or very-low-calorie diets, and many others) it appears that insulin resistance is not a primary defect but it develops secondarily to increased fat mass. In particular, it develops from a mismatch between the surplus caloric intake and the storage capacity of adipose tissue. On this basis, we propose to change the today's definition of type 2 diabetes in adiposity-based diabetes.Level of Evidence as a narrative review a vast array of studies have been included in the analysis, ranging from properly designed randomized controlled trials to case studies; however, the overall conclusion may be regarded as level IV.

Ins and Outs of the TCA Cycle: The Central Role of Anaplerosis

The reactions of the tricarboxylic acid (TCA) cycle allow the controlled combustion of fat and carbohydrate. In principle, TCA cycle intermediates are regenerated on every turn and can facilitate the oxidation of an infinite number of nutrient molecules. However, TCA cycle intermediates can be lost to cataplerotic pathways that provide precursors for biosynthesis, and they must be replaced by anaplerotic pathways that regenerate these intermediates. Together, anaplerosis and cataplerosis help regulate rates of biosynthesis by dictating precursor supply, and they play underappreciated roles in catabolism and cellular energy status. They facilitate recycling pathways and nitrogen trafficking necessary for catabolism, and they influence redox state and oxidative capacity by altering TCA cycle intermediate concentrations. These functions vary widely by tissue and play emerging roles in disease. This article reviews the roles of anaplerosis and cataplerosis in various tissues and discusses how they alter carbon transitions, and highlights their contribution to mechanisms of disease. Expected final online publication date for the Annual Review of Nutrition, Volume 41 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Regulation of Postabsorptive and Postprandial Glucose Metabolism by Insulin-Dependent and Insulin-Independent Mechanisms: An Integrative Approach

Glucose levels in blood must be constantly maintained within a tight physiological range to sustain anabolism. Insulin regulates glucose homeostasis via its effects on glucose production from the liver and kidneys and glucose disposal in peripheral tissues (mainly skeletal muscle). Blood levels of glucose are regulated simultaneously by insulin-mediated rates of glucose production from the liver (and kidneys) and removal from muscle; adipose tissue is a key partner in this scenario, providing nonesterified fatty acids (NEFA) as an alternative fuel for skeletal muscle and liver when blood glucose levels are depleted. During sleep at night, the gradual development of insulin resistance, due to growth hormone and cortisol surges, ensures that blood glucose levels will be maintained within normal levels by: (a) switching from glucose to NEFA oxidation in muscle; (b) modulating glucose production from the liver/kidneys. After meals, several mechanisms (sequence/composition of meals, gastric emptying/intestinal glucose absorption, gastrointestinal hormones, hyperglycemia mass action effects, insulin/glucagon secretion/action, de novo lipogenesis and glucose disposal) operate in concert for optimal regulation of postprandial glucose fluctuations. The contribution of the liver in postprandial glucose homeostasis is critical. The liver is preferentially used to dispose over 50% of the ingested glucose and restrict the acute increases of glucose and insulin in the bloodstream after meals, thus protecting the circulation and tissues from the adverse effects of marked hyperglycemia and hyperinsulinemia.

Pyruvate-Carboxylase-Mediated Anaplerosis Promotes Antioxidant Capacity by Sustaining TCA Cycle and Redox Metabolism in Liver

The hepatic TCA cycle supports oxidative and biosynthetic metabolism. This dual responsibility requires anaplerotic pathways, such as pyruvate carboxylase (PC), to generate TCA cycle intermediates necessary for biosynthesis without disrupting oxidative metabolism. Liver-specific PC knockout (LPCKO) mice were created to test the role of anaplerotic flux in liver metabolism. LPCKO mice have impaired hepatic anaplerosis, diminution of TCA cycle intermediates, suppressed gluconeogenesis, reduced TCA cycle flux, and a compensatory increase in ketogenesis and renal gluconeogenesis. Loss of PC depleted aspartate and compromised urea cycle function, causing elevated urea cycle intermediates and hyperammonemia. Loss of PC prevented diet-induced hyperglycemia and insulin resistance but depleted NADPH and glutathione, which exacerbated oxidative stress and correlated with elevated liver inflammation. Thus, despite catalyzing the synthesis of intermediates also produced by other anaplerotic pathways, PC is specifically necessary for maintaining oxidation, biosynthesis, and pathways distal to the TCA cycle, such as antioxidant defenses.

Glucose metabolism is differentially altered by choline and methionine in bovine neonatal hepatocytes

Choline and methionine serve essential roles in the liver that may interact with glucose metabolism. Our objectives were to quantify glucose export, cellular glycogen, and expression of genes controlling oxidation and gluconeogenesis in primary bovine neonatal hepatocytes exposed to increasing concentrations of choline chloride (CC) and D,L-methionine (DLM) with or without fatty acids (FA). Primary hepatocytes isolated from 3 Holstein calves were maintained as monolayer cultures for 24 h before treatment with CC (61, 128, 2028, 4528 μmol/L) and DLM (16, 30, 100, 300 μmol/L) with or without a 1 mmol/L FA cocktail in a factorial design. After 24 h, media was harvested to quantify glucose, β-hydroxybutyrate (BHB), and cells harvested to quantify glycogen, DNA, and gene expression. No interactions between CC and DLM were detected. The potential two-way interaction between CC or DLM and FA was partitioned into three contrasts when P ≤ 0.20: linear without FA, linear with FA, difference of slope. Fatty acids did not affect glucose or cellular glycogen but increased pyruvate carboxylase (PC), cytosolic and mitochondrial phosphoenolpyruvate carboxykinase (PEPCKc, PEPCKm), and glucose-6-phosphatase (G6PC) expression. Increasing CC decreased glucose but increased cellular glycogen. Expression of PC and PEPCKc was increased by CC during FA treatment. Increasing DLM did not affect metabolites or PC expression, although PEPCKc was marginally decreased. Methionine did not affect G6PC, while CC had a marginal quadratic effect on G6PC. Oxidative and gluconeogenic enzymes appear to respond to FA in primary bovine neonatal hepatocytes. Increased PC and PEPCKc by CC during FA treatment suggest increased gluconeogenic capacity. Changes in G6PC may have shifted glucose-6-phosphate towards cellular glycogen; however, subsequent examination of G6PC protein is needed. Unaltered PC and marginally decreased PEPCKc suggest increased oxidative capacity with DLM, although BHB export was unaltered. The differential regulation supports unique effects of CC and DLM within bovine hepatocytes.

Treatment approach to type 2 diabetes: Past, present and future

Type 2 diabetes mellitus (DM) is a lifelong metabolic disease, characterized by hyperglycaemia which gradually leads to the development and progression of vascular complications. It is recognized as a global burden disease, with substantial consequences on human health (fatality) as well as on health-care system costs. This review focuses on the topic of historical discovery and understanding the complexity of the disease in the field of pathophysiology, as well as development of the pharmacotherapy beyond insulin. The complex interplay of insulin secretion and insulin resistance developed from previously known "ominous triumvirate" to "ominous octet" indicate the implication of multiple organs in glucose metabolism. The pharmacological approach has progressed from biguanides to a wide spectrum of medications that seem to provide a beneficial effect on the cardiovascular system. Despite this, we are still not achieving the target treatment goals. Thus, the future should bring novel antidiabetic drug classes capable of acting on several levels simultaneously. In conclusion, given the raising burden of type 2 DM, the best present strategy that could contribute the most to the reduction of morbidity and mortality should be focused on primary prevention.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Tetrahydrocurcumin ameliorates free fatty acid-induced hepatic steatosis and improves insulin resistance in HepG2 cells

Elevated levels of free fatty acids (FFAs) in the liver, resulting from either increased lipolysis or imbalanced FFAs flux, is a key pathogenic factor of hepatic steatosis. This study was conducted to examine the therapeutic effect of tetrahydrocurcumin (THC), a naturally occurring curcuminoid and a metabolite of curcumin, on oleic acid (OA)-induced steatosis in human hepatocellular carcinoma cells and to elucidate the underlying mechanism. HepG2 cells were incubated with OA to induce steatosis, and then treated with various concentrations of THC. The results showed that THC treatment significantly decreased lipid accumulation in OA-treated HepG2 cells, possibly, by inhibiting the expression of the lipogenic proteins, sterol regulatory element-binding protein 1 (SREBP-1c), peroxisome proliferator-activated receptor gamma (PPARγ), fatty acid synthase (FAS), and fatty acid-binding protein 4 (FABP4). Moreover, THC attenuated OA-induced hepatic lipogenesis in an adenosine monophosphate–activated protein kinase (AMPK)-dependent manner, which was reversed by pretreatment with an AMPK inhibitor. THC promoted lipolysis and upregulated the expression of genes involved in β-oxidation. Glucose uptake and insulin signaling impaired in HepG2 cells incubated with OA were abated by THC treatment, including phosphorylation of the insulin receptor substrate 1 (IRS-1)/phosphoinositide 3-kinase (PI3K)/Akt and downstream signaling pathways, forkhead box protein O1 (FOXO1) and glycogen synthase kinase 3 β (GSK3β), which are involved in gluconeogenesis and glycogen synthesis, respectively. Altogether, these results demonstrated the novel therapeutic benefit of THC against hepatic steatosis and, consequently, a potential treatment for non-alcoholic fatty liver disease (NAFLD).

Glucocorticoid-Induced Metabolic Disturbances Are Exacerbated in Obese Male Mice

The purpose of this study was to determine the effects of glucocorticoid-induced metabolic dysfunction in the presence of diet-induced obesity. C57BL/6J adult male lean and diet-induced obese mice were given dexamethasone, and levels of hepatic steatosis, insulin resistance, and lipolysis were determined. Obese mice given dexamethasone had significant, synergistic effects on fasting glucose, insulin resistance, and markers of lipolysis, as well as hepatic steatosis. This was associated with synergistic transactivation of the lipolytic enzyme adipose triglyceride lipase. The combination of chronically elevated glucocorticoids and obesity leads to exacerbations in metabolic dysfunction. Our findings suggest lipolysis may be a key player in glucocorticoid-induced insulin resistance and fatty liver in individuals with obesity.

In vivo studies on the mechanism of methylene cyclopropyl acetic acid and methylene cyclopropyl glycine-induced hypoglycemia

Exposure to the toxins methylene cyclopropyl acetic acid (MCPA) and methylene cyclopropyl glycine (MCPG) of unripe ackee and litchi fruit can lead to hypoglycemia and death; however, the molecular mechanisms by which MCPA and MCPG cause hypoglycemia have not been established in vivo To determine the in vivo mechanisms of action of these toxins, we infused them into conscious rodents and assessed rates of hepatic gluconeogenesis and ketogenesis, hepatic acyl-CoA and hepatic acetyl-CoA content, and hepatocellular energy charge. MCPG suppressed rates of hepatic β-oxidation as reflected by reductions in hepatic ketogenesis, reducing both short- and medium-chain hepatic acyl-CoA concentrations. Hepatic acetyl-CoA content decreased, and hepatic glucose production was inhibited. MCPA also suppressed β-oxidation of short-chain acyl-CoAs, rapidly inhibiting hepatic ketogenesis and hepatic glucose production, depleting hepatic acetyl-CoA content and ATP content, while increasing other short-chain acyl-CoAs. Utilizing a recently developed positional isotopomer NMR tracer analysis method, we demonstrated that MCPA-induced reductions in hepatic acetyl-CoA content were associated with a marked reduction of hepatic pyruvate carboxylase (PC) flux. Taken together, these data reveal the in vivo mechanisms of action of MCPA and MCPG: the hypoglycemia associated with ingestion of these toxins can be ascribed mostly to MCPA- or MCPG-induced reductions in hepatic PC flux due to inhibition of β-oxidation of short-chain acyl-CoAs by MCPA or inhibition of both short- and medium-chain acyl-CoAs by MCPG with resultant reductions in hepatic acetyl-CoA content, with an additional contribution to hypoglycemia through reduced hepatic ATP stores by MCPA.

Testosterone is protective against impaired glucose metabolism in male intrauterine growth-restricted offspring

Placental insufficiency alters the intrauterine environment leading to increased risk for chronic disease including impaired glucose metabolism in low birth weight infants. Using a rat model of low birth weight, we previously reported that placental insufficiency induces a significant increase in circulating testosterone in male intrauterine growth-restricted offspring (mIUGR) in early adulthood that is lost by 12 months of age. Numerous studies indicate testosterone has a positive effect on glucose metabolism in men. Female growth-restricted littermates exhibit glucose intolerance at 6 months of age. Thus, the aim of this paper was to determine whether mIUGR develop impaired glucose metabolism, and whether a decrease in elevated testosterone levels plays a role in its onset. Male growth-restricted offspring were studied at 6 and 12 months of age. No impairment in glucose tolerance was observed at 6 months of age when mIUGR exhibited a 2-fold higher testosterone level compared to age-matched control. Fasting blood glucose was significantly higher and glucose tolerance was impaired with a significant decrease in circulating testosterone in mIUGR at 12 compared with 6 months of age. Castration did not additionally impair fasting blood glucose or glucose tolerance in mIUGR at 12 months of age, but fasting blood glucose was significantly elevated in castrated controls. Restoration of elevated testosterone levels significantly reduced fasting blood glucose and improved glucose tolerance in mIUGR. Thus, our findings suggest that the endogenous increase in circulating testosterone in mIUGR is protective against impaired glucose homeostasis.

Regulation of hepatic glucose metabolism in health and disease

The liver is crucial for the maintenance of normal glucose homeostasis - it produces glucose during fasting and stores glucose postprandially. However, these hepatic processes are dysregulated in type 1 and type 2 diabetes mellitus, and this imbalance contributes to hyperglycaemia in the fasted and postprandial states. Net hepatic glucose production is the summation of glucose fluxes from gluconeogenesis, glycogenolysis, glycogen synthesis, glycolysis and other pathways. In this Review, we discuss the in vivo regulation of these hepatic glucose fluxes. In particular, we highlight the importance of indirect (extrahepatic) control of hepatic gluconeogenesis and direct (hepatic) control of hepatic glycogen metabolism. We also propose a mechanism for the progression of subclinical hepatic insulin resistance to overt fasting hyperglycaemia in type 2 diabetes mellitus. Insights into the control of hepatic gluconeogenesis by metformin and insulin and into the role of lipid-induced hepatic insulin resistance in modifying gluconeogenic and net hepatic glycogen synthetic flux are also discussed. Finally, we consider the therapeutic potential of strategies that target hepatosteatosis, hyperglucagonaemia and adipose lipolysis.

The mitochondrial pyruvate carrier mediates high fat diet-induced increases in hepatic TCA cycle capacity

Objective

Excessive hepatic gluconeogenesis is a defining feature of type 2 diabetes (T2D). Most gluconeogenic flux is routed through mitochondria. The mitochondrial pyruvate carrier (MPC) transports pyruvate from the cytosol into the mitochondrial matrix, thereby gating pyruvate-driven gluconeogenesis. Disruption of the hepatocyte MPC attenuates hyperglycemia in mice during high fat diet (HFD)-induced obesity but exerts minimal effects on glycemia in normal chow diet (NCD)-fed conditions. The goal of this investigation was to test whether hepatocyte MPC disruption provides sustained protection from hyperglycemia during long-term HFD and the differential effects of hepatocyte MPC disruption on TCA cycle metabolism in NCD versus HFD conditions.

Method

We utilized long-term high fat feeding, serial measurements of postabsorptive blood glucose and metabolomic profiling and ¹³C-lactate/¹³C-pyruvate tracing to investigate the contribution of the MPC to hyperglycemia and altered hepatic TCA cycle metabolism during HFD-induced obesity.

Results

Hepatocyte MPC disruption resulted in long-term attenuation of hyperglycemia induced by HFD. HFD increased hepatic mitochondrial pyruvate utilization and TCA cycle capacity in an MPC-dependent manner. Furthermore, MPC disruption decreased progression of fibrosis and levels of transcript markers of inflammation.

Conclusions

By contributing to chronic hyperglycemia, fibrosis, and TCA cycle expansion, the hepatocyte MPC is a key mediator of the pathophysiology induced in the HFD model of T2D.

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Hepatic MPC disruption protects from hyperglycemia during long-term HFD.

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HFD increases TCA cycle metabolite pool capacity and flux.

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Hepatic MPC disruption abrogates HFD-induced TCA cycle expansion.

Integrated Regulation of Hepatic Lipid and Glucose Metabolism by Adipose Triacylglycerol Lipase and FoxO Proteins

Metabolism is a highly integrated process that is coordinately regulated between tissues and within individual cells. FoxO proteins are major targets of insulin action and contribute to the regulation of gluconeogenesis, glycolysis, and lipogenesis in the liver. However, the mechanisms by which FoxO proteins exert these diverse effects in an integrated fashion remain poorly understood. We report that FoxO proteins also exert important effects on intrahepatic lipolysis and fatty acid oxidation via the regulation of adipose triacylglycerol lipase (ATGL), which mediates the first step in lipolysis, and its inhibitor, the G0/S1 switch 2 gene (G0S2). We also find that ATGL-dependent lipolysis plays a critical role in mediating diverse effects of FoxO proteins in the liver, including effects on gluconeogenic, glycolytic, and lipogenic gene expression and metabolism. These results indicate that intrahepatic lipolysis plays a critical role in mediating and integrating the regulation of glucose and lipid metabolism downstream of FoxO proteins.

Combined intervention with pioglitazone and n-3 fatty acids in metformin-treated type 2 diabetic patients: improvement of lipid metabolism

Background

The marine n-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) exert numerous beneficial effects on health, but their potency to improve treatment of type 2 diabetic (T2D) patients remains poorly characterized. We aimed to evaluate the effect of a combination intervention using EPA + DHA and the insulin-sensitizing drug pioglitazone in overweight/obese T2D patients already treated with metformin.

Methods

In a parallel-group, four-arm, randomized trial, 69 patients (66 % men) were assigned to 24-week-intervention using: (i) corn oil (5 g/day; Placebo), (ii) pioglitazone (15 mg/day; Pio), (iii) EPA + DHA concentrate (5 g/day, containing ~2.8 g EPA + DHA; Omega-3), or (iv) pioglitazone and EPA + DHA concentrate (Pio& Omega-3). Data from 60 patients were used for the final evaluation. At baseline and after intervention, various metabolic markers, adiponectin and cytokines were evaluated in serum using standard procedures, EPA + DHA content in serum phospholipids was evaluated using shotgun lipidomics and mass spectrometry, and hyperinsulinemic-euglycemic clamp and meal test were also performed. Indirect calorimetry was conducted after the intervention. Primary endpoints were changes from baseline in insulin sensitivity evaluated using hyperinsulinemic-euglycemic clamp and in serum triacylglycerol concentrations in fasting state. Secondary endpoints included changes in fasting glycemia and glycated hemoglobin (HbA_1c), changes in postprandial glucose, free fatty acid and triacylglycerol concentrations, metabolic flexibility assessed by indirect calorimetry, and inflammatory markers.

Results

Omega-3 and Pio& Omega-3 increased EPA + DHA content in serum phospholipids. Pio and Pio& Omega-3 increased body weight and adiponectin levels. Both fasting glycemia and HbA_1c were increased by Omega-3, but were unchanged by Pio& Omega-3. Insulin sensitivity was not affected by Omega-3, while it was improved by Pio& Omega-3. Fasting triacylglycerol concentrations and inflammatory markers were not significantly affected by any of the interventions. Lipid metabolism in the meal test and metabolic flexibility were additively improved by Pio& Omega-3.

Conclusion

Besides preventing a modest negative effect of n-3 fatty acids on glycemic control, the combination of pioglitazone and EPA + DHA can be used to improve lipid metabolism in T2D patients on stable metformin therapy.

Trial registration

EudraCT number 2009-011106-42.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-015-0047-9) contains supplementary material, which is available to authorized users.

Detrimental and protective fat: body fat distribution and its relation to metabolic disease

Obesity is linked to numerous comorbidities that include, but are not limited to, glucose intolerance, insulin resistance, dyslipidemia, and cardiovascular disease. Current evidence suggests, however, obesity itself is not an exclusive predictor of metabolic dysregulation but rather adipose tissue distribution. Obesity-related adverse health consequences occur predominately in individuals with upper body fat accumulation, the detrimental distribution, commonly associated with visceral obesity. Increased lower body subcutaneous adipose tissue, however, is associated with a reduced risk of obesity-induced metabolic dysregulation and even enhanced insulin sensitivity, thus, storage in this region is considered protective. The proposed mechanisms that causally relate the differential outcomes of adipose tissue distribution are often attributed to location and/or adipocyte regulation. Visceral adipose tissue effluent to the portal vein drains into the liver where hepatocytes are directly exposed to its metabolites and secretory products, whereas the subcutaneous adipose tissue drains systemically. Adipose depots are also inherently different in numerous ways such as adipokine release, immunity response and regulation, lipid turnover, rate of cell growth and death, and response to stress and sex hormones. Proximal extrinsic factors also play a role in the differential drive between adipose tissue depots. This review focuses on the deleterious mechanisms postulated to drive the differential metabolic response between central and lower body adipose tissue distribution.

Lower body adipose tissue removal decreases glucose tolerance and insulin sensitivity in mice with exposure to high fat diet

It has been postulated that the protective effects of lower body subcutaneous adipose tissue (LBSAT) occur via its ability to sequester surplus lipid and thus serve as a "metabolic sink." However, the mechanisms that mediate this protective function are unknown thus this study addresses this postulate. Ad libitum, chow-fed mice underwent Sham-surgery or LBSAT removal (IngX, inguinal depot removal) and were subsequently provided chow (Chow; typical adipocyte expansion) or high fat diet (HFD; enhanced adipocyte expansion) for 5 weeks. Primary outcome measures included glucose tolerance and subsequent insulin response, muscle insulin sensitivity, liver and muscle triglycerides, adipose tissue gene expression, and circulating lipids and adipokines. In a follow up study the consequences of extended experiment length post-surgery (13 wks) or pre-existing glucose intolerance were examined. At 5 wks post-surgery IngX in HFD-fed mice reduced glucose tolerance and muscle insulin sensitivity and increased circulating insulin compared with HFD Sham. In Chow-fed mice, muscle insulin sensitivity was the only measurement reduced following IngX. At 13 wks circulating insulin concentration of HFD IngX mice continued to be higher than HFD Sham. Surgery did not induce changes in mice with pre-existing glucose intolerance. IngX also increased muscle, but not liver, triglyceride concentration in Chow- and HFD-fed mice 5 wks post-surgery, but chow group only at 13 wks. These data suggest that the presence of LBSAT protects against triglyceride accumulation in the muscle and HFD-induced glucose intolerance and muscle insulin resistance. These data suggest that lower body subcutaneous adipose tissue can function as a "metabolic sink."

Metabolic syndrome in pediatrics

The ever growing prevalence of childhood obesity is being accompanied by an increase in the pediatric population of diseases once believed to be exclusive of the adulthood such as the metabolic syndrome (MS). The MS has been defined as the link between insulin resistance, hypertension, dyslipidemia, impaired glucose tolerance, and other metabolic abnormalities associated with an increased risk of atherosclerotic cardiovascular diseases in adults. In this review, we will discuss the peculiar aspects of the pediatric MS and the role of novel molecules and biomarkers in its pathogenesis.

CEACAM1 loss links inflammation to insulin resistance in obesity and non-alcoholic steatohepatitis (NASH)

Mounting epidemiological evidence points to an association between metabolic syndrome and non-alcoholic steatohepatitis (NASH), an increasingly recognized new epidemic. NASH pathologies include hepatocellular ballooning, lobular inflammation, hepatocellular injury, apoptosis, and hepatic fibrosis. We will review the relationship between insulin resistance and inflammation in visceral obesity and NASH in an attempt to shed more light on the pathogenesis of these major metabolic diseases. Moreover, we will identify loss of the carcinoembryonic antigen-related cell adhesion molecule 1 as a unifying mechanism linking the immunological and metabolic abnormalities in NASH.

Impaired amino acid metabolism contributes to fasting-induced hypoglycemia in fatty acid oxidation defects

The importance of mitochondrial fatty acid β-oxidation (FAO) as a glucose-sparing process is illustrated by patients with inherited defects in FAO, who may present with life-threatening fasting-induced hypoketotic hypoglycemia. It is unknown why peripheral glucose demand outpaces hepatic gluconeogenesis in these patients. In this study, we have systematically addressed the fasting response in long-chain acyl-CoA dehydrogenase-deficient (LCAD KO) mice. We demonstrate that the fasting-induced hypoglycemia in LCAD KO mice was initiated by an increased glucose requirement in peripheral tissues, leading to rapid hepatic glycogen depletion. Gluconeogenesis did not compensate for the increased glucose demand, which was not due to insufficient hepatic glucogenic capacity but rather caused by a shortage in the supply of glucogenic precursors. This shortage in supply was explained by a suppressed glucose-alanine cycle, decreased branched-chain amino acid metabolism and ultimately impaired protein mobilization. We conclude that during fasting, FAO not only serves to spare glucose but is also indispensable for amino acid metabolism, which is essential for the maintenance of adequate glucose production.

Visceral fat and metabolic inflammation: the portal theory revisited

Abdominal (central) obesity strongly correlates with (hepatic) insulin resistance and type 2 diabetes. Among several hypotheses that have been formulated, the 'portal theory' proposes that the liver is directly exposed to increasing amounts of free fatty acids and pro-inflammatory factors released from visceral fat into the portal vein of obese patients, promoting the development of hepatic insulin resistance and liver steatosis. Thus, visceral obesity may be particularly hazardous in the pathogenesis of insulin resistance and type 2 diabetes. Herein, we will critically review existing evidence for a potential contribution of portally drained free fatty acids and/or cytokines to the development of hepatic insulin resistance.

Elevated NEFA levels impair glucose effectiveness by increasing net hepatic glycogenolysis

AIMS/HYPOTHESIS

Acute hyperglycaemia rapidly suppresses endogenous glucose production (EGP) in non-diabetic individuals, mainly by inhibiting glycogenolysis. Loss of this 'glucose effectiveness' contributes to fasting hyperglycaemia in type 2 diabetes. Elevated NEFA levels characteristic of type 2 diabetes impair glucose effectiveness, although the mechanism is not fully understood. Therefore we examined the impact of increasing NEFA levels on the ability of hyperglycaemia to regulate pathways of EGP.

METHODS

We performed 4 h 'pancreatic clamp' studies (somatostatin; basal glucagon/growth hormone/insulin) in seven non-diabetic individuals. Glucose fluxes (D-[6,6-(2)H(2)]glucose) and hepatic glycogen concentrations ((13)C magnetic resonance spectroscopy) were quantified under three conditions: euglycaemia, hyperglycaemia and hyperglycaemia with elevated NEFA (HY-NEFA).

RESULTS

EGP was suppressed by hyperglycaemia, but not by HY-NEFA. Hepatic glycogen concentration decreased ~14% with prolonged fasting during euglycaemia and increased by ~12% with hyperglycaemia. In contrast, raising NEFA levels in HY-NEFA caused a substantial ~23% reduction in hepatic glycogen concentration. Moreover, rates of gluconeogenesis were decreased with hyperglycaemia, but increased with HY-NEFA.

CONCLUSIONS/INTERPRETATION

Increased NEFA appear to profoundly blunt the ability of hyperglycaemia to inhibit net glycogenolysis under basal hormonal conditions.

Metabolic alterations following visceral fat removal and expansion: Beyond anatomic location

Increased visceral adiposity is a risk factor for metabolic disorders such as dyslipidemia, hypertension, insulin resistance and type 2 diabetes, whereas peripheral (subcutaneous) obesity is not. Though the specific mechanisms which contribute to these adipose depot differences are unknown, visceral fat accumulation is proposed to result in metabolic dysregulation because of increased effluent, e.g., fatty acids and/or adipokines/cytokines, to the liver via the hepatic portal vein. Pathological significance of visceral fat accumulation is also attributed to adipose depot/adipocyte-specific characteristics, specifically differences in structural, physiologic and metabolic characteristics compared with subcutaneous fat. Fat manipulations, such as removal or transplantation, have been utilized to identify location dependent or independent factors that play a role in metabolic dysregulation. Obesity-induced alterations in adipose tissue function/intrinsic characteristics, but not mass, appear to be responsible for obesity-induced metabolic dysregulation, thus “quality” is more important than “quantity.” This review summarizes the implications of obesity-induced metabolic dysfunction as it relates to anatomic site and inherent adipocyte characteristics.

Pathophysiology and Long-Term Management of the Metabolic Syndrome

The metabolic syndrome has been characterized by a cluster of abnormalities that include obesity, hyperglycemia, dyslipidemia, and hypertension. Other conditions associated with this syndrome include microalbuminuria, inflammation, a prothrombotic state, and a fatty liver. Together, these abnormalities lead to an environment where the risk of developing both type 2 diabetes and atherosclerotic cardiovascular disease are greatly enhanced. Recognition of this syndrome by practitioners, early treatment, and long-term management are crucial for disease prevention. Successful treatment requires the introduction of lifestyle changes initially and pharmacotherapy subsequently if lifestyle changes are not sufficient.

Mechanism of Free Fatty Acid Effects on Hepatocyte Insulin Receptor Binding and Processing

We determined whether the palmitate effects on hepatocyte insulin receptor binding and post-receptor trafficking were mediated by accelerated mitochondrial beta-oxidation or accumulation of intracellular fatty acyl-CoA derivatives and possibly protein acylation. Preincubation of hepatocytes with moderate concentrations of palmitate (0.5 mM) resulted in a 23% decline in cell-surface binding and proportional decreases in receptor-mediated insulin internalization and degradation. Brief pretreatment of hepatocytes with the carnitine palmityltransferase-I inhibitor, methyl palmoxirate (MP), prevented 70% of the palmitate effects. At higher palmitate concentrations (2.0 mM), cell-surface binding was reduced by 34%, whereas internalization of the receptor complex was reduced by 78%. These effects were only partially prevented by MP pretreatment. Receptor-mediated insulin degradation increased by 34% and was uninfluenced by MP pretreatment. Octanoate, which is rapidly shunted into mitochondrial oxidation, produced a dose-dependent reduction in insulin binding, with proportional decreases in internalization and degradation. Similarly preincubation with 2.0 mM oleate, which, unlike palmitate, is not known to produce protein acylation, resulted in proportional decreases in insulin receptor binding and receptor-mediated internalization and degradation. High concentrations of octanoate or oleate (2.0 mM) did not reproduce the additive post-receptor effects of palmitate. We conclude that the receptor and post-receptor effects of moderate palmitate concentrations are closely linked to accelerated fatty acid oxidation. The post-receptor effects observed at higher concentrations involve other mechanisms, possibly relating to intracellular levels of palmityl-CoA derivatives.

PKM2 phosphorylates histone H3 and promotes gene transcription and tumorigenesis

Tumor-specific pyruvate kinase M2 (PKM2) is essential for the Warburg effect. In addition to its well-established role in aerobic glycolysis, PKM2 directly regulates gene transcription. However, the mechanism underlying this nonmetabolic function of PKM2 remains elusive. We show here that PKM2 directly binds to histone H3 and phosphorylates histone H3 at T11 upon EGF receptor activation. This phosphorylation is required for the dissociation of HDAC3 from the CCND1 and MYC promoter regions and subsequent acetylation of histone H3 at K9. PKM2-dependent histone H3 modifications are instrumental in EGF-induced expression of cyclin D1 and c-Myc, tumor cell proliferation, cell-cycle progression, and brain tumorigenesis. In addition, levels of histone H3 T11 phosphorylation correlate with nuclear PKM2 expression levels, glioma malignancy grades, and prognosis. These findings highlight the role of PKM2 as a protein kinase in its nonmetabolic functions of histone modification, which is essential for its epigenetic regulation of gene expression and tumorigenesis.

Fatty acids-stress attenuates gluconeogenesis induction and glucose production in primary hepatocytes

BACKGROUND

Hepatic gluconeogenesis tightly controls blood glucose levels in healthy individuals, yet disorders of fatty acids (FAs) oxidation are characterized by hypoglycemia. We studied the ability of free-FAs to directly inhibit gluconeogenesis, as a novel mechanism that elucidates the hypoglycemic effect of FAs oxidation defects.

METHODS

Primary rat hepatocytes were pre-treated with FAs prior to gluconeogenic stimuli with glucagon or dexamethasone and cAMP.

RESULTS

Pre-treatment with 1 mM FAs (mixture of 2:1 oleate:palmitate) for 1 hour prior to gluconeogenic induction, significantly decreases the induced expression of the gluconeogenic genes phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6pase) as well as the induced glucose production by the cells. The inhibitory effect of FAs upon gluconeogenesis is abolished when pre-treatment is elongated to 18 hours, allowing clearance of FAs into triglycerides by the cells. Replacement of palmitate with the non-metabolic fatty acid 2-bromopalmitate inhibits esterification of FAs into triglycerides. Accordingly, the increased exposure to unesterified-FAs allows their inhibitory effect to be extended even when pre-treatment is elongated to 18 hours. Similar changes were caused by FAs to the induction of peroxisome-proliferator-activated receptor-γ coactivator 1α (PGC1α) expression, indicating this transcriptional coactivator as the mediating link of the effect. This inhibitory effect of FAs upon gluconeogenic induction is shown to involve reduced activation of cAMP response element-binding (CREB) transcription factor.

CONCLUSIONS

The present results demonstrate that free-FAs directly inhibit the induced gluconeogenic response in hepatocytes. Hence, high levels of free-FAs may attenuate hepatic gluconeogenesis, and liver glucose output.

Elevated TCA cycle function in the pathology of diet-induced hepatic insulin resistance and fatty liver

The manner in which insulin resistance impinges on hepatic mitochondrial function is complex. Although liver insulin resistance is associated with respiratory dysfunction, the effect on fat oxidation remains controversial, and biosynthetic pathways that traverse mitochondria are actually increased. The tricarboxylic acid (TCA) cycle is the site of terminal fat oxidation, chief source of electrons for respiration, and a metabolic progenitor of gluconeogenesis. Therefore, we tested whether insulin resistance promotes hepatic TCA cycle flux in mice progressing to insulin resistance and fatty liver on a high-fat diet (HFD) for 32 weeks using standard biomolecular and in vivo (2)H/(13)C tracer methods. Relative mitochondrial content increased, but respiratory efficiency declined by 32 weeks of HFD. Fasting ketogenesis became unresponsive to feeding or insulin clamp, indicating blunted but constitutively active mitochondrial β-oxidation. Impaired insulin signaling was marked by elevated in vivo gluconeogenesis and anaplerotic and oxidative TCA cycle flux. The induction of TCA cycle function corresponded to the development of mitochondrial respiratory dysfunction, hepatic oxidative stress, and inflammation. Thus, the hepatic TCA cycle appears to enable mitochondrial dysfunction during insulin resistance by increasing electron deposition into an inefficient respiratory chain prone to reactive oxygen species production and by providing mitochondria-derived substrate for elevated gluconeogenesis.

Profound hypoglycemia in starved, ghrelin-deficient mice is caused by decreased gluconeogenesis and reversed by lactate or fatty acids

When mice are subjected to 7-day calorie restriction (40% of normal food intake), body fat disappears, but blood glucose is maintained as long as the animals produce ghrelin, an octanoylated peptide that stimulates growth hormone secretion. Mice can be rendered ghrelin-deficient by knock-out of the gene encoding either ghrelin O-acyltransferase, which attaches the required octanoate, or ghrelin itself. Calorie-restricted, fat-depleted ghrelin O-acyltransferase or ghrelin knock-out mice fail to show the normal increase in growth hormone and become profoundly hypoglycemic when fasted for 18-23 h. Glucose production in Goat(-/-) mice was reduced by 60% when compared with similarly treated WT mice. Plasma lactate and pyruvate were also low. Injection of lactate, pyruvate, alanine, or a fatty acid restored blood glucose in Goat(-/-) mice. Thus, when body fat is reduced by calorie restriction, ghrelin stimulates growth hormone secretion, which allows maintenance of glucose production, even when food intake is eliminated. In humans with anorexia nervosa or kwashiorkor, ghrelin and growth hormone are known to be elevated, just as they are in fat-depleted mice. We suggest that these two hormones prolong survival in starved humans as they do in mice.

Transplantation of non-visceral fat to the visceral cavity improves glucose tolerance in mice: investigation of hepatic lipids and insulin sensitivity

AIMS/HYPOTHESIS

Intra-abdominal transplantation of non-visceral adipose tissue in rodents, simulating increased abdominal fat in obesity, paradoxically improves glucose tolerance and insulin sensitivity. We hypothesised that this improvement is due to transplant-induced enhanced uptake of fatty acids by adipose tissue, thus reducing fatty acid flux into, and triacylglycerol storage in, the liver.

METHODS

In Experiment 1, mice were sham-operated or received heterologous epididymal white adipose tissue (WAT; EWAT) or visceral WAT (VWAT) transplantation to the portal and splanchnic circulation regions in the visceral cavity. In Experiment 2, inguinal WAT (IWAT) or EWAT was removed and subsequently transplanted to the visceral cavity of the same mouse (autotransplant). IWAT and EWAT autotransplants were repeated in Experiment 3 and compared with heterotransplants.

RESULTS

Heterotransplantation of VWAT did not alter glucose tolerance, whereas auto- or hetero-transplantation of EWAT or IWAT significantly improved glucose tolerance. Transplantation-induced improvements in glucose tolerance 4 weeks after surgery coincided with decreased liver triacylglycerol, decreased portal plasma lipids and increased hepatic insulin sensitivity. By 8 weeks, these changes were apparent only in mice with autotransplantation. Heterologous EWAT transplantation-induced glucose improvement persisted without altered liver metabolism.

CONCLUSIONS/INTERPRETATION

Increases in visceral fat, via transplantation of visceral or non-visceral adipose tissue, is not a major risk factor for glucose intolerance. In fact, there are dynamic metabolic improvements following transplantation that include decreased portal lipids and improved liver metabolism, but these improvements are transient under certain circumstances.

Metabolic syndrome in pediatrics: old concepts revised, new concepts discussed

The worldwide epidemic of childhood obesity in the last decades is responsible for the occurrence in pediatrics of disorders once mainly found in adults, such as the metabolic syndrome. A key factor in the pathogenesis of metabolic syndrome is insulin resistance, a phenomenon occurring mainly in obese subjects with a general resistance to the insulin effect only on carbohydrates metabolism. Given that the metabolic syndrome is driven by obesity, the prevalence of the latter will strongly influence the prevalence of metabolic syndrome. This article addresses the causes of metabolic syndrome and the relevance of obesity in the pediatric population.

Removal of intra-abdominal visceral adipose tissue improves glucose tolerance in rats: role of hepatic triglyceride storage

Epidemiological studies have demonstrated a strong link between increased visceral fat and metabolic syndrome. In rodents, removal of intra-abdominal but non-visceral fat improves insulin sensitivity and glucose homeostasis, though previous studies make an imprecise comparison to human physiology because actual visceral fat was not removed. We hypothesize that nutrient release from visceral adipose tissue may have greater consequences on metabolic regulation than nutrient release from non-visceral adipose depots since the latter drains into systemic but not portal circulation. To assess this we surgically decreased visceral white adipose tissue (~0.5 g VWATx) and compared the effects to removal of non-visceral epididymal fat (~4 g; EWATx), combination removal of visceral and non-visceral fat (~4.5 g; EWATx/VWATx) and sham-operated controls, in chow-fed rats. At 8 weeks after surgery, only the groups with visceral fat removed had a significantly improved glucose tolerance, although 8 times more fat was removed in EWATx compared with VWATx. This suggests that mechanisms controlling glucose metabolism are relatively more sensitive to reductions in visceral adipose tissue mass. Groups with visceral fat removed also had significantly decreased hepatic lipoprotein lipase (LPL) and triglyceride content compared with controls, while carnitine palmitoyltransferase (CPT-1A) was decreased in all fat-removal groups. In a preliminary experiment, we assessed the opposite hypothesis; i.e., we transplanted excess visceral fat from a donor rat to the visceral cavity (omentum and mesentery), which drains into the hepatic portal vein, of a recipient rat but observed no major metabolic effect. Overall, our results indicate surgical removal of intra-abdominal fat improves glucose tolerance through mechanism that may be mediated by reductions in liver triglyceride.

The effects of long- or medium-chain fat diets on glucose tolerance and myocellular content of lipid intermediates in rats

Accumulation of triacylglycerols (TAGs) and acylcarnitines in skeletal muscle upon high-fat (HF) feeding is the resultant of fatty acid uptake and oxidation and is associated with insulin resistance. As medium-chain fatty acids (MCFAs) are preferentially β-oxidized over long-chain fatty acids, we examined the effects of medium-chain TAGs (MCTs) and long-chain TAGs (LCTs) on muscle lipid storage and whole-body glucose tolerance. Rats fed a low-fat (LF), HFLCT, or an isocaloric HFMCT diet displayed a similar body weight gain over 8 weeks of treatment. Only HFLCT increased myocellular TAG (42.3 ± 4.9, 71.9 ± 6.7, and 48.5 ± 6.5 µmol/g for LF, HFLCT, and HFMCT, respectively, P < 0.05) and long-chain acylcarnitine content (P < 0.05). Neither HF diet increased myocellular diacylglycerol (DAG) content. Intraperitoneal (IP) glucose tolerance tests (1.5 g/kg) revealed a significantly decreased glucose tolerance in the HFMCT compared to the HFLCT-fed rats (802 ± 40, 772 ± 18, and 886 ± 18 area under the curve for LF, HFLCT, and HFMCT, respectively, P < 0.05). Finally, no differences in myocellular insulin signaling after bolus insulin injection (10 U/kg) were observed between LF, HFLCT, or HFMCT-fed rats. These results show that accumulation of TAGs and acylcarnitines in skeletal muscle in the absence of body weight gain do not impede myocellular insulin signaling or whole-body glucose intolerance.

Dynamic modeling of methylprednisolone effects on body weight and glucose regulation in rats

Influences of methylprednisolone (MPL) and food consumption on body weight (BW), and the effects of MPL on glycemic control including food consumption and the dynamic interactions among glucose, insulin, and free fatty acids (FFA) were evaluated in normal male Wistar rats. Six groups of animals received either saline or MPL via subcutaneous infusions at the rate of 0.03, 0.1, 0.2, 0.3 and 0.4 mg/kg/h for different treatment periods. BW and food consumption were measured twice a week. Plasma concentrations of MPL and corticosterone (CST) were determined at animal sacrifice. Plasma glucose, insulin, and FFA were measured at various times after infusion. Plasma MPL concentrations were simulated by a two-compartment model and used as the driving force in the pharmacodynamic (PD) analysis. All data were modeled using ADAPT 5. The MPL treatments caused reduction of food consumption and body weights in all dosing groups. The steroid also caused changes in plasma glucose, insulin, and FFA concentrations. Hyperinsulinemia was achieved rapidly at the first sampling time of 6 h; significant elevations of FFA were observed in all drug treatment groups; whereas only modest increases in plasma glucose were observed in the low dosing groups (0.03 and 0.1 mg/kg/h). Body weight changes were modeled by dual actions of MPL: inhibition of food consumption and stimulation of weight loss, with food consumption accounting for the input of energy for body weight. Dynamic models of glucose and insulin feedback interactions were extended to capture the major metabolic effects of FFA: stimulation of insulin secretion and inhibition of insulin-stimulated glucose utilization. These models of body weight and glucose regulation adequately captured the experimental data and reflect significant physiological interactions among glucose, insulin, and FFA. These mechanism-based PD models provide further insights into the multi-factor control of this essential metabolic system.

A general introduction to the biochemistry of mitochondrial fatty acid β-oxidation

Over the years, the mitochondrial fatty acid β-oxidation (FAO) pathway has been characterised at the biochemical level as well as the molecular biological level. FAO plays a pivotal role in energy homoeostasis, but it competes with glucose as the primary oxidative substrate. The mechanisms behind this so-called glucose-fatty acid cycle operate at the hormonal, transcriptional and biochemical levels. Inherited defects for most of the FAO enzymes have been identified and characterised and are currently included in neonatal screening programmes. Symptoms range from hypoketotic hypoglycaemia to skeletal and cardiac myopathies. The pathophysiology of these diseases is still not completely understood, hampering optimal treatment. Studies of patients and mouse models will contribute to our understanding of the pathogenesis and will ultimately lead to better treatment.

Metabolic syndrome in pediatrics: old concepts revised, new concepts discussed

The worldwide epidemic of childhood obesity in the last decades is responsible for the occurrence in pediatrics of disorders once mainly found in adults, such as the metabolic syndrome. A key factor in the pathogenesis of metabolic syndrome is insulin resistance, a phenomenon occurring mainly in obese subjects with a general resistance to the insulin effect only on carbohydrates metabolism. Given that the metabolic syndrome is driven by obesity, the prevalence of the latter will strongly influence the prevalence of metabolic syndrome. This article addresses the causes of metabolic syndrome and the relevance of obesity in the pediatric population.