Mechanisms modifying glucose oxidation in diabetes mellitus

Cuproptosis: mechanisms and links with cancers

Cuproptosis was a copper-dependent and unique kind of cell death that was separate from existing other forms of cell death. The last decade has witnessed a considerable increase in investigations of programmed cell death, and whether copper induced cell death was an independent form of cell death has long been argued until mechanism of cuproptosis has been revealed. After that, increasing number of researchers attempted to identify the relationship between cuproptosis and the process of cancer. Thus, in this review, we systematically detailed the systemic and cellular metabolic processes of copper and the copper-related tumor signaling pathways. Moreover, we not only focus on the discovery process of cuproptosis and its mechanism, but also outline the association between cuproptosis and cancers. Finally, we further highlight the possible therapeutic direction of employing copper ion ionophores with cuproptosis-inducing functions in combination with small molecule drugs for targeted therapy to treat specific cancers.

Metabolic, structural and biochemical changes in diabetes and the development of heart failure

Diabetes contributes to the development of heart failure through various metabolic, structural and biochemical changes. The presence of diabetes increases the risk for the development of cardiovascular disease (CVD), and since the introduction of cardiovascular outcome trials to test diabetic drugs, the importance of improving our understanding of the mechanisms by which diabetes increases the risk for heart failure has come under the spotlight. In addition to the coronary vasculature changes that predispose individuals with diabetes to coronary artery disease, diabetes can also lead to cardiac dysfunction independent of ischaemic heart disease. The hyperlipidaemic, hyperglycaemic and insulin resistant state of diabetes contributes to a perturbed energy metabolic milieu, whereby the heart increases its reliance on fatty acids and decreases glucose oxidative rates. In addition to changes in cardiac energy metabolism, extracellular matrix remodelling contributes to the development of cardiac fibrosis, and impairments in calcium handling result in cardiac contractile dysfunction. Lipotoxicity and glucotoxicity also contribute to impairments in vascular function, cardiac contractility, calcium signalling, oxidative stress, cardiac efficiency and lipoapoptosis. Lastly, changes in protein acetylation, protein methylation and DNA methylation contribute to a myriad of gene expression and protein activity changes. Altogether, these changes lead to decreased cardiac efficiency, increased vulnerability to an ischaemic insult and increased risk for the development of heart failure. This review explores the above mechanisms and the way in which they contribute to cardiac dysfunction in diabetes.

Astaxanthin Exerts Anabolic Effects via Pleiotropic Modulation of the Excitable Tissue

Astaxanthin is a lipid-soluble carotenoid influencing lipid metabolism, body weight, and insulin sensitivity. We provide a systematic analysis of acute and chronic effects of astaxanthin on different organs. Changes by chronic astaxanthin feeding were analyzed on general metabolism, expression of regulatory proteins in the skeletal muscle, as well as changes of excitation and synaptic activity in the hypothalamic arcuate nucleus of mice. Acute responses were also tested on canine cardiac muscle and different neuronal populations of the hypothalamic arcuate nucleus in mice. Dietary astaxanthin significantly increased food intake. It also increased protein levels affecting glucose metabolism and fatty acid biosynthesis in skeletal muscle. Inhibitory inputs innervating neurons of the arcuate nucleus regulating metabolism and food intake were strengthened by both acute and chronic astaxanthin treatment. Astaxanthin moderately shortened cardiac action potentials, depressed their plateau potential, and reduced the maximal rate of depolarization. Based on its complex actions on metabolism and food intake, our data support the previous findings that astaxanthin is suitable for supplementing the diet of patients with disturbances in energy homeostasis.

Unrestricted Feed Intake Induces β-Cell Death and Impairs Insulin Secretion in Broiler Breeder Hens

Simple Summary

Ad-feed intake caused transient hyperinsulinemia, but ultimately impaired insulin secretion and glucose clearance leading to hyperglycemia in broiler breeder hens. The impairments were operated at insulin gene expression and at pyruvate anaplerosis for ATP supply for insulin release. Lipotoxicity, inflammation, and cell apoptosis in the β-islets contributed to impaired insulin secretion in Ad-hens.

Abstract

Past studies regarding to insulin secretion and glucose disposal in chickens were focused on rapidly growing juvenile broilers and may not reflect glucose/insulin physiology in adulthood. The study aimed to assess insulin secretion and glucose disposal in respect to restricted (R) vs. ad libitum (Ad) feed intake for obesity development in broiler breeder hens. Hens at age of 26 weeks were continued on R rations, or allowed Ad-feed intake up to 45 weeks. Results from prandial changes and glucose tolerance test suggested that Ad-feed intake to 45 weeks impaired insulin secretion and glucose clearance, and, thus, caused hyperglycemia in accompany with transient hyperinsulinemia at age of 33 weeks (p < 0.05). The alterations were shown operating at both transcript and protein level of insulin gene expression per se and at ATP supply for insulin release as evidenced by consistent changes of enzyme expression and activity in pyruvate anaplerosis in the β-islets (p < 0.05). Ad-feed intake also increased β-islet triacylglycerol and ceramide accumulation and provoked interleukin-1β (IL-1β) production (p < 0.05), which were further manifested by a detrimental increase of caspase 3/7 activity and cell apoptosis (p < 0.05). Results support the conclusion that release to Ad-feed intake in broiler breeder hens transiently induced hyperinsulinemia along rapid bodyweight gain and adiposity, but later provoked lipotoxicity and inflammation leading to β-cell apoptosis and ultimately impaired insulin secretion and glucose disposal.

Mitochondrial Utilization of Competing Fuels Is Altered in Insulin Resistant Skeletal Muscle of Non-obese Rats (Goto-Kakizaki)

Aim

Insulin-resistant skeletal muscle is characterized by metabolic inflexibility with associated alterations in substrate selection, mediated by peroxisome-proliferator activated receptor δ (PPARδ). Although it is established that PPARδ contributes to the alteration of energy metabolism, it is not clear whether it plays a role in mitochondrial fuel competition. While nutrient overload may impair metabolic flexibility by fuel congestion within mitochondria, in absence of obesity defects at a mitochondrial level have not yet been excluded. We sought to determine whether reduced PPARδ content in insulin-resistant rat skeletal muscle of a non-obese rat model of T2DM (Goto-Kakizaki, GK) ameliorate the inhibitory effect of fatty acid (i.e., palmitoylcarnitine) on mitochondrial carbohydrate oxidization (i.e., pyruvate) in muscle fibers.

Methods

Bioenergetic function was characterized in oxidative soleus (S) and glycolytic white gastrocnemius (WG) muscles with measurement of respiration rates in permeabilized fibers in the presence of complex I, II, IV, and fatty acid substrates. Mitochondrial content was measured by citrate synthase (CS) and succinate dehydrogenase activity (SDH). Western blot was used to determine protein expression of PPARδ, PDK isoform 2 and 4.

Results

CS and SDH activity, key markers of mitochondrial content, were reduced by ∼10-30% in diabetic vs. control, and the effect was evident in both oxidative and glycolytic muscles. PPARδ (p < 0.01), PDK2 (p < 0.01), and PDK4 (p = 0.06) protein content was reduced in GK animals compared to Wistar rats (N = 6 per group). Ex vivo respiration rates in permeabilized muscle fibers determined in the presence of complex I, II, IV, and fatty acid substrates, suggested unaltered mitochondrial bioenergetic function in T2DM muscle. Respiration in the presence of pyruvate was higher compared to palmitoylcarnitine in both animal groups and fiber types. Moreover, respiration rates in the presence of both palmitoylcarnitine and pyruvate were reduced by 25 ± 6% (S), 37 ± 6% (WG) and 63 ± 6% (S), 57 ± 8% (WG) compared to pyruvate for both controls and GK, respectively. The inhibitory effect of palmitoylcarnitine on respiration was significantly greater in GK than controls (p < 10^-3).

Conclusion

With competing fuels, the presence of fatty acids diminishes mitochondria ability to utilize carbohydrate derived substrates in insulin-resistant muscle despite reduced PPARδ content.

Pyruvate is an effective substitute for glutamate in regulating porcine nitrogen excretion

This study was performed to determine if pyruvate, which acts as a critical intermediate in energy metabolism, can substitute the role of glutamate as a metabolic fuel and effectively reduce nitrogen excretion in pigs. First, the experiment in vitro was carried out to investigate the effects of culturing porcine small intestinal epithelial cell line with pyruvate on the oxidation. Then, barrows weighing 40 kg were used in the experiment investigating the changes of nitrogen balance in response to addition of pyruvate to low-protein diets. Last, barrows (40 kg), which were surgically fitted with permanent catheters in the mesenteric vein, portal vein, hepatic vein, and carotid artery, were used to investigate the effects of supplementing low-protein diets with calcium pyruvate on the net portal fluxes of amino acids (AAs) and the consumption of AAs in the liver. The results showed that culturing cells with sodium pyruvate significantly reduced the number of glutamate oxidation (P < 0.05). Addition of calcium pyruvate to low-protein diets significantly reduced urinary nitrogen excretion from 13.2 g/d (18.0% crude protein, CP) to 10.3 g/d (15.0% CP) or 7.80 g/d (13.5% CP) and total nitrogen excretion from 22.5 g/d (18.0% CP) to 17.8 g/d (15.0% CP) or 14.2 g/d (13.5% CP) (P < 0.05), without obviously negative effects on the nitrogen retention (P > 0.05). Addition of calcium pyruvate to low-protein diets significantly decreased essential AA consumption rate in the liver (P < 0.05). This diet modification reduced the net portal fluxes of NH3, glycine, and alanine, as well as urea production rate in the liver (P < 0.05). The results indicated that pyruvate is an effective substitute for glutamate as a supplement in low-protein diets, reducing porcine nitrogen excretion and nitrogen consumption.

Reactive oxygen species enhance mitochondrial function, insulin sensitivity and glucose uptake in skeletal muscle of senescence accelerated prone mice SAMP8

Whereas reactive oxygen species (ROS) can have opposite impacts on insulin signaling, they have mainly been associated with mitochondrial dysfunction in skeletal muscle. We analyzed the relationship between these three features in skeletal muscle of senescence accelerated mice (SAM) prone (P8), which are characterized by enhanced oxidative stress compared to SAM resistant (R1). Oxidative stress, ROS production, antioxidant system, mitochondrial content and functioning, as well as in vitro and in vivo insulin signaling were investigated in gastrocnemius and quadriceps muscles. In SAMP8 compared to SAMR1, muscle content in carbonylated proteins was two-fold (p < 0.01) and ROS production by xanthine oxidase 70% (p < 0.05) higher. Furthermore, insulin-induced Akt phosphorylation measured in vivo and ex vivo as well as muscle glucose uptake measured ex vivo were significantly higher (p < 0.05). Mitochondrial respiration evidenced uncoupling and higher respiration rates with substrates of complexes II and IV, in agreement with higher maximal activity of complexes II and IV (+ 18% and 62%, respectively, p < 0.05). By contrast, maximal activity of complex I was 22% lower (p < 0.05). All strain differences were corrected after 6 months of N-acetylcysteine (NAC) treatment, thus supporting the involvement of high ROS production in these differences. In conclusion in muscle of SAMP8 compared to SAMR1, high ROS production is associated to higher insulin sensitivity and glucose uptake but to lower mitochondrial complex I activity. These conflicting adaptations, with regards to the resulting imbalance between NADH production and use, were associated with intrinsic adjustments in the mitochondrial respiration chain (mitochondrial uncoupling, enhanced complexes II and IV activity). We propose that these bioenergetics adaptations may help at preserving muscle metabolic flexibility of SAMP8.

Glucose oxidation positively regulates glucose uptake and improves cardiac function recovery after myocardial reperfusion

Myocardial reperfusion decreases glucose oxidation and uncouples glucose oxidation from glycolysis. Therapies that increase glucose oxidation lessen myocardial ischemia-reperfusion (I/R) injury. However, the regulation of glucose uptake during reperfusion remains poorly understood. We found that glucose uptake was remarkably diminished in the myocardium following reperfusion in Sprague-Dawley rats as detected by ¹⁸F-labeled and fluorescent-labeled glucose analogs, even though GLUT1 was upregulated by threefold and GLUT4 translocation remained unchanged compared with those of sham-treated rats. The decreased glucose uptake was accompanied by suppressed glucose oxidation. Interestingly, stimulating glucose oxidation by inhibition of pyruvate dehydrogenase kinase 4 (PDK4), a rate-limiting enzyme for glucose oxidation, increased glucose uptake and alleviated I/R injury. In vitro data in neonatal myocytes showed that PDK4 overexpression decreased glucose uptake, whereas its knockdown increased glucose uptake, suggesting that PDK4 has a role in regulating glucose uptake. Moreover, inhibition of PDK4 increased myocardial glucose uptake with concomitant enhancement of cardiac insulin sensitivity following myocardial I/R. These results showed that the suppressed glucose oxidation mediated by PDK4 contributes to the reduced glucose uptake in the myocardium following reperfusion, and enhancement of glucose uptake exerts cardioprotection. The findings suggest that stimulating glucose oxidation via PDK4 could be an efficient approach to improve recovery from myocardial I/R injury.

Metabolic adaptations in the adipose tissue that underlie the body fat mass gain in middle-aged rats

Little is known about adipocyte metabolism during aging process and whether this can influence body fat redistribution and systemic metabolism. To better understand this phenomenon, two animal groups were studied: young-14 weeks old-and middle-aged-16 months old. Periepididymal (PE) and subcutaneous (SC) adipocytes were isolated and tested for their capacities to perform lipolysis and to incorporate D-[U-(14)C]-glucose, D-[U-(14)C]-lactate, and [9,10(n)-(3)H]-oleic acid into lipids. Additionally, the morphometric characteristics of the adipose tissues, glucose tolerance tests, and biochemical determinations (fasting glucose, triglycerides, insulin) in blood were performed. The middle-aged rats showed adipocyte (PE and SC) hypertrophy and glucose intolerance, although there were no significant changes in fasting glycemia and insulin. Furthermore, PE tissue revealed elevated rates (+50 %) of lipolysis during beta-adrenergic-stimulation. There was also an increase (+62 %) in the baseline rate of glucose incorporation into lipids in the PE adipocytes, while these PE cells were almost unresponsive to insulin stimulation and less responsive (a 34 % decrease) in the SC tissue. Also, the capacity of oleic acid esterification was elevated in baseline state and with insulin stimulus in the PE tissue (+90 and 82 %, respectively). Likewise, spontaneous incorporation of lactate into lipids in the PE and SC tissues was higher (+100 and 11 %, respectively) in middle-aged rats. We concluded that adipocyte metabolism of middle-aged animals seems to strongly favor cellular hypertrophy and increased adipose mass, particularly the intra-abdominal PE fat pad. In discussion, we have interpreted all these results as a metabolic adaptations to avoid the spreading of fat that can reach tissues beyond adipose protecting them against ectopic fat accumulation. However, these adaptations may have the potential to lead to future metabolic dysfunctions seen in the senescence.

In Vivo Exposures to Particulate Matter Collected from Saudi Arabia or Nickel Chloride Display Similar Dysregulation of Metabolic Syndrome Genes

Particulate matter (PM) exposures have been linked to mortality, low birth weights, hospital admissions, and diseases associated with metabolic syndrome, including diabetes mellitus, cardiovascular disease, and obesity. In a previous in vitro and in vivo study, data demonstrated that PM(10μm) collected from Jeddah, Saudi Arabia (PMSA), altered expression of genes involved in lipid and cholesterol metabolism, as well as many other genes associated with metabolic disorders. PMSA contains a relatively high concentration of nickel (Ni), known to be linked to several metabolic disorders. In order to evaluate whether Ni and PM exposures induce similar gene expression profiles, mice were exposed to 100 μg/50 μl PM(SA) (PM-100), 50 μg/50 μl nickel chloride (Ni-50), or 100 μg/50 μl nickel chloride (Ni-100) twice per week for 4 wk and hepatic gene expression changes were determined. Ultimately, 55 of the same genes were altered in all 3 exposures. However, where the two Ni groups differed markedly was in the regulation (up or down) of these genes. Ni-100 and PM-100 groups displayed similar regulations, whereby 104 of the 107 genes were similarly modulated. Many of the 107 genes are involved in metabolic syndrome and include ALDH4A1, BCO2, CYP1A, CYP2U, TOP2A. In addition, the top affected pathways, such as fatty acid α-oxidation, and lipid and carbohydrate metabolism, are involved in metabolic diseases. Most notably, the top diseased outcome affected by these changes in gene expression was cardiovascular disease. Given these data, it appears that Ni and PM(SA) exposures display similar gene expression profiles, modulating the expression of genes involved in metabolic disorders.

Mangiferin stimulates carbohydrate oxidation and protects against metabolic disorders induced by high-fat diets

Excessive dietary fat intake causes systemic metabolic toxicity, manifested in weight gain, hyperglycemia, and insulin resistance. In addition, carbohydrate utilization as a fuel is substantially inhibited. Correction or reversal of these effects during high-fat diet (HFD) intake is of exceptional interest in light of widespread occurrence of diet-associated metabolic disorders in global human populations. Here we report that mangiferin (MGF), a natural compound (the predominant constituent of Mangifera indica extract from the plant that produces mango), protected against HFD-induced weight gain, increased aerobic mitochondrial capacity and thermogenesis, and improved glucose and insulin profiles. To obtain mechanistic insight into the basis for these effects, we determined that mice exposed to an HFD combined with MGF exhibited a substantial shift in respiratory quotient from fatty acid toward carbohydrate utilization. MGF treatment significantly increased glucose oxidation in muscle of HFD-fed mice without changing fatty acid oxidation. These results indicate that MGF redirects fuel utilization toward carbohydrates. In cultured C2C12 myotubes, MGF increased glucose and pyruvate oxidation and ATP production without affecting fatty acid oxidation, confirming in vivo and ex vivo effects. Furthermore, MGF inhibited anaerobic metabolism of pyruvate to lactate but enhanced pyruvate oxidation. A key target of MGF appears to be pyruvate dehydrogenase, determined to be activated by MGF in a variety of assays. These findings underscore the therapeutic potential of activation of carbohydrate utilization in correction of metabolic syndrome and highlight the potential of MGF to serve as a model compound that can elicit fuel-switching effects.

Remodeling of energy metabolism and absence of electrophysiological changes in the heart of obese hyperleptinemic mice. New insights into the pleiotropic role of leptin

Dietary treatment with high-fat diets (HFD) triggers diabetes and hyperleptinemia, concomitantly with a partial state of leptin resistance that affects hepatic and adipose tissue but not the heart. In this context, characterized by widespread steatosis, cardiac lipid content remains unchanged. As previously reported, HFD-evoked hyperleptinemia could be a pivotal element contributing to increase fatty-acid (FA) metabolism in the heart and to prevent cardiac steatosis. This metabolic adaptation might theoretically reduce energy efficiency in cardiomyocytes and lead to cardiac electrophysiological remodeling. Therefore the aim of the current study has been to investigate the impact of long-term HFD on cardiac metabolism and electrophysiological properties of the principal ionic currents responsible of the action potential duration in mouse cardiomyocytes. Male C57BL/6J mice were fed a control (10 kcal% from fat) or HFD (45 kcal% from fat) during 32 weeks. Quantification of enzymatic activities regulating mitochondrial uptake of pyruvate and FA showed an increase of both carnitine-palmitoyltransferase and citrate synthase activities together with a decrease of lactate dehydrogenase and pyruvate dehydrogenase activities. Increased expression of uncoupling protein-3, Mn-, and Cu/Zn-superoxide dismutases and catalase were also detected. Total glutathione/oxidized glutathione ratios were unaffected by HFD. These data suggest that HFD triggers adaptive mechanisms aimed at (i) facilitating FA catabolism, and (ii) preventing oxidative stress. All these changes did not affect the duration of action potentials in cardiomyocytes and only slightly modified electrocardiographic parameters.

Mitochondrial signal transduction in pancreatic β-cells

In the endocrine fraction of the pancreas, the task of the beta-cell is to continuously and perfectly adjust insulin secretion to fluctuating blood glucose levels, thereby maintaining glycemia and nutrient homeostasis. This glucose sensing coupled to insulin exocytosis depends on transduction of metabolic signals into intracellular messengers recognized by the exocytotic machinery. Central to this metabolism-secretion coupling, mitochondrial signal transduction refers to both integration and generation of metabolic signals, connecting glucose sensing to insulin exocytosis. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, in the stimulation of insulin release. This review describes the role of mitochondria in metabolic signal transduction regulating insulin secretion.

Mitochondrial dysfunction in pancreatic β cells

In pancreatic β cells, mitochondria play a central role in coupling glucose metabolism to insulin exocytosis, thereby ensuring strict control of glucose-stimulated insulin secretion. Defects in mitochondrial function impair this metabolic coupling, and ultimately promote apoptosis and β cell death. Various factors have been identified that may contribute to mitochondrial dysfunction. In this review we address the emerging concept of complex links between these factors. We also discuss the role of the mitochondrial genome and mutations associated with diabetes, the effect of oxidative stress and reactive oxygen species, the sensitivity of mitochondria to lipotoxicity, and the adaptive dynamics of mitochondrial morphology. Better comprehension of the molecular mechanisms contributing to mitochondrial dysfunction will help drive the development of effective therapeutic approaches.

Activation of SIRT1 protects pancreatic β-cells against palmitate-induced dysfunction

Sirtuin 1 (SIRT1), a nicotinamide adenosine dinucleotide-dependent histone deacetylase, is an important regulator of energy homeostasis in response to nutrient availability. In pancreatic β-cells, SIRT1 has been shown to up-regulate insulin secretion in response to glucose stimulation. However, the potential roles of SIRT1 in islet β-cells against lipotoxicity remain poorly understood. Here, we demonstrated that SIRT1 mRNA and protein expressions were markedly reduced in the islets isolated from rats infused with 20% Intralipid for 24h. Long-term exposure to 0.4mmol/L palmitate also decreased SIRT1 expression in cultured INS-1 cells and isolated rat islets, which was prevented by 10μmol/L resveratrol, a SIRT1 agonist. In addition, resveratrol improved glucose-stimulated insulin secretion decreased by palmitate, which was abrogated by EX527, a specific SIRT1 inhibitor. Furthermore, inhibition of SIRT1 activity by EX527 or a knockdown of SIRT1 suppressed insulin promoter activity, along with decreased insulin, v-maf musculoaponeurotic fibrosarcoma oncogene homolog A (MafA), and NK6 homeodomain 1 (NKX6.1) mRNA expressions. Activation of SIRT1 by resveratrol or overexpression of SIRT1 antagonized palmitate-inhibited insulin transcriptional activity. SIRT1 overexpression exerted an additive effect on pancreatic and duodenal homeobox 1 (PDX1)-stimulated insulin promoter activity and abolished forkhead box O1 protein (FOXO1)-decreased insulin transcriptional activity. Resveratrol reversed FOXO1 nuclear translocation induced by palmitate. Our findings indicate that SIRT1 protects against palmitate-induced β-cell dysfunction.

Metabolic sensing and the brain: who, what, where, and how?

Unique subpopulations of specialized metabolic sensing neurons reside in a distributed network throughout the brain and respond to alterations in ambient levels of various metabolic substrates by altering their activity. Variations in local brain substrate levels reflect their transport across the blood- and cerebrospinal-brain barriers as well as local production by astrocytes. There are a number of mechanisms by which such metabolic sensing neurons alter their activity in response to changes in substrate levels, but it is clear that these neurons cannot be considered in isolation. They are heavily dependent on astrocyte and probably tanycyte metabolism and function but also respond to hormones (e.g. leptin and insulin) and cytokines that cross the blood-brain barrier from the periphery as well as hard-wired neural inputs from metabolic sensors in peripheral sites such as the hepatic portal vein, gastrointestinal tract, and carotid body. Thus, these specialized neurons are capable of monitoring and integrating multiple signals from the periphery as a means of regulating peripheral energy homeostasis.

Different roles of zinc plus arachidonic acid on insulin sensitivity between high fructose- and high fat-fed rats

AIMS

This study was to determine the effects of zinc plus arachidonic acid (ZA) treatment on the insulin action in the specific ZA target organs using hyperinsulinemic euglycemic clamp method.

MAIN METHODS

18 Sprague-Dawley rats weighing ~130 g were divided into 3 groups of 6 rats and treated them with 1) normal rat chow, 2) high fructose (60.0%) diet only, or 3) the same fructose diet plus drinking water containing 10mg zinc plus 50mg arachidonic acid (AA)/L. In a separate study, male Wistar rats weighing ~250 g were fed normal rat chow (n=4) or high fat (66.5%) diet with drinking water containing zero (n=9) or 10mg AA plus 20mg zinc /L (n=9). After 4 week treatment, insulin action was assessed using the hyperinsulinemic eguglycemic clamp technique.

KEY FINDINGS

High fructose feeding impaired suppression of hepatic glucose output by insulin compared to controls during the clamp procedure (4.39 vs. 2.35 mg/kg/min; p<0.05). However, ZA treatment in high fructose-fed rats showed a significant improvement of hepatic insulin sensitivity compared to non-treatment controls (4.39 vs. 2.18 mg/kg/min; p<0.05). Glucose infusion rates in Wistar rats maintained on a high fat diet (HFD) were significantly lower compared to control rats (22.8 ± 1.3 vs. 31.9 ± 1.4 mg/kg/min; p<0.05). ZA treatment significantly improved (~43%) peripheral tissue insulin sensitivity in HFD fed animals (26.7 ± 1.3 [n=9] vs. 22.8 ± 1.3mg/kg/min; p<0.05).

SIGNIFICANCE

These data demonstrate that ZA treatment is effective in improving glucose utilization in hyperglycemic rats receiving either a high-fructose or a high-fat diet.

Regulation of fasting fuel metabolism by toll-like receptor 4

OBJECTIVE

Toll-like receptor 4 (TLR4) has been reported to induce insulin resistance through inflammation in high-fat-fed mice. However, the physiological role of TLR4 in metabolism is unknown. Here, we investigated the involvement of TLR4 in fasting metabolism.

RESEARCH DESIGN AND METHODS

Wild-type and TLR4 deficient (TLR4(-/-)) mice were either fed or fasted for 24 h. Glucose and lipid levels in circulation and tissues were measured. Glucose and lipid metabolism in tissues, as well as the expression of related enzymes, was examined.

RESULTS

Mice lacking TLR4 displayed aggravated fasting hypoglycemia, along with normal hepatic gluconeogenesis, but reversed activity of pyruvate dehydrogenase complex (PDC) in skeletal muscle, which might account for the fasting hypoglycemia. TLR4(-/-) mice also exhibited higher lipid levels in circulation and skeletal muscle after fasting and reversed expression of lipogenic enzymes in skeletal muscle but not liver and adipose tissue. Adipose tissue lipolysis is normal and muscle fatty acid oxidation is increased in TLR4(-/-) mice after fasting. Inhibition of fatty acid synthesis in TLR4(-/-) mice abolished hyperlipidemia, hypoglycemia, and PDC activity increase, suggesting that TLR4-dependent inhibition of muscle lipogenesis may contribute to glucose and lipid homeostasis during fasting. Further studies showed that TLR4 deficiency had no effect on insulin signaling and muscle proinflammatory cytokine production in response to fasting.

CONCLUSIONS

These data suggest that TLR4 plays a critical role in glucose and lipid metabolism independent of insulin during fasting and identify a novel physiological role for TLR4 in fuel homeostasis.

Palmitic acid acutely inhibits acetylcholine- but not GLP-1-stimulated insulin secretion in mouse pancreatic islets

Fatty acids, acetylcholine, and GLP-1 enhance insulin secretion in a glucose-dependent manner. However, the interplay between glucose, fatty acids, and the neuroendocrine regulators of insulin secretion is not well understood. Therefore, we studied the acute effects of PA (alone or in combination with glucose, acetylcholine, or GLP-1) on isolated cultured mouse islets. Two different sets of experiments were designed. In one, a fixed concentration of 0.5 mM of PA bound to 0.15 mM BSA was used; in the other, a PA ramp from 0 to 0.5 mM was applied at a fixed albumin concentration of 0.15 mM so that the molar PA/BSA ratio changed within the physiological range. At a fixed concentration of 0.5 mM, PA markedly inhibited acetylcholine-stimulated insulin release, the rise of intracellular Ca(2+), and enhancement of cAMP production but did not influence the effects of GLP-1 on these parameters of islet cell function. 2-ADB, an IP(3) receptor inhibitor, reduced the effect of acetylcholine on insulin secretion and reversed the effect of PA on acetylcholine-stimulated insulin release. Islet perfusion for 35-40 min with 0.5 mM PA significantly reduced the calcium storage capacity of ER measured by the thapsigargin-induced Ca(2+) release. Oxygen consumption due to low but not high glucose was reduced by PA. When a PA ramp from 0 to 0.5 mM was applied in the presence of 8 mM glucose, PA at concentrations as low as 50 microM significantly augmented glucose-stimulated insulin release and markedly reduced acetylcholine's effects on hormone secretion. We thus demonstrate that PA acutely reduces the total oxygen consumption response to glucose, glucose-dependent acetylcholine stimulation of insulin release, Ca(2+), and cAMP metabolism, whereas GLP-1's actions on these parameters remain unaffected or potentiated. We speculate that acute emptying of the ER calcium by PA results in decreased glucose stimulation of respiration and acetylcholine potentiation of insulin secretion.

Obesity and type 2 diabetes: slow down!--Can metabolic deceleration protect the islet beta cell from excess nutrient-induced damage?

Islet beta-cell dysfunction is a characteristic and the main cause of hyperglycaemia of Type 2 diabetes. Understanding the mechanisms that cause beta-cell dysfunction will lead to better therapeutic outcomes for patients with Type 2 diabetes. Chronic fatty acid exposure of susceptible islet beta-cells causes dysfunction and death and this is associated with increased reactive oxygen species production leading to oxidative stress and increased endoplasmic reticulum stress. We present the hypothesis that metabolic deceleration can reduce both oxidative and endoplasmic reticulum stress and lead to improved beta-cell function and viability when exposed to a deleterious fat milieu. This is illustrated by the C57BL/6J mouse which is characterised by reduced insulin secretion and glucose intolerance associated with a mutation in nicotinamide nucleotide transhydrogenase (Nnt) but is resistant to obesity induced diabetes. On the other hand the DBA/2 mouse has comparatively higher insulin secretion and better glucose tolerance associated with increased Nnt activity but is susceptible to obesity-induced diabetes, possibly as a result of increased oxidative stress. We therefore suggest that in states of excess nutrient load, a reduced ability to metabolise this load may protect both the function and viability of beta-cells. Strategies that reduce metabolic flux when beta-cells are exposed to nutrient excess need to be considered when treating Type 2 diabetes.

Role of mitochondria in beta-cell function and dysfunction

Pancreatic beta-cells are poised to sense glucose and other nutrient secretagogues to regulate insulin exocytosis, thereby maintaining glucose homeostasis. This process requires translation of metabolic substrates into intracellular messengers recognized by the exocytotic machinery. Central to this metabolism-secretion coupling, mitochondria integrate and generate metabolic signals, thereby connecting glucose recognition to insulin exocytosis. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, to the stimulation of insulin release. This review describes the mitochondrion-dependent pathways of regulated insulin secretion. Mitochondrial defects, such as mutations and reactive oxygen species production, are discussed in the context of beta-cell failure that may participate to the etiology of diabetes.

FATP1 localizes to mitochondria and enhances pyruvate dehydrogenase activity in skeletal myotubes

Fatty acid transport protein 1 (FATP1) has been previously immunolocalized in intracellular compartments. Here we show that FATP1 localizes to the mitochondria in cultured myotubes, by immunoblots of subcellular fractions and immunocytology of the fusion protein FATP1-GFP. FATP1 strongly stimulates CO(2) production from glucose whereas nonmitochondrial metabolism of glucose is only slightly enhanced. FATP1 raises the activity and activates the pyruvate dehydrogenase (PDH) complex and the pyruvate decarboxylase PDH-E1 catalytic subunit, without changing E2, E3BP or E1alpha and increasing E1beta protein content. These data reveals the localization and points to a regulatory function of FATP1 in myotube mitochondria.

Gene expression regulated by pioglitazone and exenatide in normal and diabetic rat islets exposed to lipotoxicity

BACKGROUND

Hyperlipidaemia has been suggested to contribute by pro-apoptotic actions to the loss of beta-cell mass, its secretory defects, and thereby impaired beta-cell function in type 2 diabetes. Treatment of genetically diabetic rats and also type 2 diabetic patients with pioglitazone, a PPAR-gamma agonist, lowers fasting levels of plasma glucose and triglycerides, and has been suggested to protect beta-cells against diabetic lipotoxicity in vitro and in vivo. Another recently launched anti-diabetic drug, exenatide, an incretin mimetic, has been shown to stimulate insulin secretion, growth, and proliferation of pancreatic beta-cells and to protect them against apoptosis. We aimed to investigate global alterations in beta-cell gene expression under lipotoxic conditions and the influence of in vitro treatment with pioglitazone and exenatide.

METHODS

Global gene expression profiling was thus performed to characterize genes differently regulated by palmitate, pioglitazone, and exenatide in isolated islets from non-diabetic Wistar rats and type 2 diabetic Goto-Kakizaki (GK) rats.

RESULTS

Gene expression profiling revealed significant changes in islet mRNAs involved in control of several aspects of beta-cell function, e.g. epigenetic regulation of gene expression, cell differentiation and morphogenesis, also metabolism, response to stimulus, transport, and signal transduction. Pioglitazone and exenatide appear to significantly impact epigenetic processes, e.g. stable alterations in gene expression potential, which arise during development and cell proliferation. Bcl2-like 1 (Bcl2l1), an anti-apoptotic protein, and Bcl2 modifying factor (Bmf), a pro-apoptotic protein, were both down-regulated by pioglitazone and exenatide in the presence of palmitate in diabetic GK islets. In contrast, Bmf was downregulated by pioglitazone in the presence of palmitate in non-diabetic Wistar islets. Exposure of non-diabetic Wistar islets to palmitate led to a reduction in the expression of PPAR beta/delta. This suggests that palmitate may increase the accumulation of triglycerides by reducing PPAR signalling. Moreover, treatment with either pioglitazone or exenatide restored and increased the expression of PPAR beta/delta in non-diabetic Wistar islets.

CONCLUSIONS

Taking into account that these drugs target different components of the epigenetic machinery, our findings suggest that they might participate in restoring normal gene activity in dysfunctional islets and that additive benefits may occur. Whether such events contribute to the beta-cell sparing, proliferative, and anti-apoptotic effects of these drugs in diabetes remains to be elucidated.

Reducing plasma free fatty acids by acipimox improves glucose tolerance in high-fat fed mice

To study whether free fatty acids (FFAs) contribute to glucose intolerance in high-fat fed mice, the derivative of nicotinic acid, acipimox, which inhibits lipolysis, was administered intraperitoneally (50 mg kg(-1)) to C57BL/6J mice which had been on a high-fat diet for 3 months. Four hours after administration of acipimox, plasma FFA levels were reduced to 0.46 +/- 0.06 mmol L(-1) compared with 0.88 +/- 0.10 mmol L(-1) in controls (P < 0.001). At this point, the glucose elimination rate after an intravenous glucose load (1 g kg(-1)) was markedly improved. Thus, the elimination constant (KG) for the glucose disposal between 1 and 50 min after the glucose challenge was increased from 0.54 +/- 0.01% min-1 in controls to 0.66 +/- 0.01% min-1 by acipimox (P < 0.001). In contrast, the acute insulin response to glucose (1-5 min) was not significantly different between the groups, although the area under the insulin for the entire 50-min period after glucose administration was significantly reduced by acipimox from 32.1 +/- 2.9 to 23.9 +/- 1.2 nmol L(-1) x 50 min (P = 0.036). This, however, was mainly because of lower insulin levels at 20 and 50 min because of the lowered glucose levels. In contrast, administration of acipimox to mice fed a normal diet did not affect plasma levels of FFA or the glucose elimination or insulin levels after the glucose load. It is concluded that reducing FFA levels by acipimox in glucose intolerant high-fat fed mice improves glucose tolerance mainly by improving insulin sensitivity making the ambient islet function adequate, suggesting that increased FFA levels are of pathophysiological importance in this model of glucose intolerance.

Tissue specificity of mitochondrial glutamate pathways and the control of metabolic homeostasis

Glutamate is implicated in numerous metabolic and signalling functions that vary according to specific tissues. Glutamate metabolism is tightly controlled by activities of mitochondrial enzymes and transmembrane carriers, in particular glutamate dehydrogenase and mitochondrial glutamate carriers that have been identified in recent years. It is remarkable that, although glutamate-specific enzymes and transporters share similar properties in most tissues, their regulation varies greatly according to particular organs in order to achieve tissue specific functions. This is illustrated in this review when comparing glutamate handling in liver, brain, and pancreatic beta-cells. We describe the main cellular glutamate pathways and their specific functions in different tissues, ultimately contributing to the control of metabolic homeostasis at the organism level.

Molecular mechanisms of diabetes reversibility after bariatric surgery

OBJECTIVE

Insulin resistance is a strong biological marker of both obesity and type 2 diabetes. Abnormal fat deposition within skeletal muscle has been identified as a mechanism of obesity-associated insulin resistance. Biliopancreatic diversion (BPD), inducing a massive lipid malabsorption, leads to a reversion of type 2 diabetes. To elucidate the mechanisms of diabetes reversibility, the expression of genes involved in glucose and free fatty acids (FFAs) metabolism was investigated in skeletal muscle biopsies from obese, type 2 diabetic subjects. Peripheral insulin sensitivity and insulin secretion was also measured.

SUBJECTS

Eight Caucasian obese diabetic patients (BMI 52.1+/-1.85 kg/m(2)) were studied before and 3 years after BPD.

MEASUREMENTS

The mRNA levels were estimated by quantitative real-time reverse transcription polymerase chain reaction (RT-PCR), insulin sensitivity by the euglycemic-hyperinsulinemic clamp and insulin secretion using a model describing the relationship between insulin secretion and glucose concentration.

RESULTS

Whole-body glucose uptake (M), normalized by fat-free mass, significantly increased in post-obese subjects (P<0.0001). Total insulin output decreased (P<0.05) in association with a significant improvement of beta-cells glucose sensitivity (P<0.05). mRNA levels of FABP3 (P<0.05), FACL (P<0.05), ACC2 (P<0.05), HKII (P<0.05) and PDK4 (P<0.05) were significantly decreased, while SREBP1c mRNA increased (P<0.05) after BPD.

CONCLUSION

Reversibility of type 2 diabetes after BPD is dependent on the improvement of skeletal muscle insulin sensitivity, mediated by changes in the expression of genes regulating glucose and fatty acid metabolism in response to nutrient availability.

High-fat/low-carbohydrate diet reduces insulin-stimulated carbohydrate oxidation but stimulates nonoxidative glucose disposal in humans: An important role for skeletal muscle pyruvate dehydrogenase kinase 4

AIM

The aim of this report was to study the effect of high-fat (HF)/low-carbohydrate (CHO) diet on regulation of substrate metabolism in humans.

METHODS

Ten healthy men consumed either a HF (75% energy as fat) or control (35%) diet for 6 d in random order. On d 7, blood glucose disappearance rate (Rd) was determined before and during a hyperinsulinemic euglycemic clamp. Substrate oxidation was determined by indirect calorimetry. Muscle biopsies were obtained prediet, postdiet, and postclamps.

RESULTS

Rd was similar under basal conditions but slightly elevated (approximately 10%, P < 0.05) during the last 30 min of the clamp after the HF diet. HF diet reduced CHO oxidation under basal (by approximately 40%, P < 0.05) and clamp conditions (by approximately 20%, P < 0.05), increased insulin-mediated whole-body nonoxidative glucose disposal (by 30%, P < 0.05) and muscle glycogen storage (by approximately 25%, P < 0.05). Muscle pyruvate dehydrogenase complex activity was blunted under basal and clamp conditions after HF compared with control (P < 0.05) and was accompanied by an approximately 2-fold increase (P < 0.05) in pyruvate dehydrogenase kinase 4 (PDK4) mRNA and protein expression.

CONCLUSION

Short-term HF/low-CHO dietary intake did not induce whole-body insulin resistance, but caused a shift in im glucose metabolism from oxidation to glycogen storage. Insulin-stimulated CHO oxidation and muscle pyruvate dehydrogenase complex activity were blunted after the HF diet. Up-regulation of muscle PDK4 expression was an early molecular adaptation to these changes, and we showed for the first time in healthy humans, unlike insulin-resistant individuals, that insulin can suppress PDK4 but not PDK2 gene expression in skeletal muscle.

Longitudinal study of physiologic insulin resistance and metabolic changes of puberty

Cross-sectional studies have shown that 1) adolescents are insulin resistant compared with prepubertal children and adults, 2) pubertal insulin resistance is likely mediated by growth hormone (GH), and 3) pubertal insulin resistance is associated with increased fat oxidation and decreased glucose oxidation. The aim of this study was to assess the validity of these cross-sectional observations by performing a longitudinal study in normal children during the prepubertal and pubertal periods. Nine healthy, normal weight, prepubertal children underwent hyperinsulinemic-euglycemic and hyperglycemic clamp studies for evaluation of insulin sensitivity and insulin secretion. Children had repeat evaluations during puberty. Consistent with cross-sectional observations, this longitudinal study demonstrated that during puberty: 1) insulin sensitivity decreased by approximately 50%, 2) the decrease in insulin sensitivity was compensated by a doubling in insulin secretion, and 3) the decrease in insulin sensitivity was independent of changes in percentage of body fat. Puberty was associated with increased total body lipolysis and decreased glucose oxidation. A novel observation is the demonstration of approximately 50% decrease in adiponectin levels at the pubertal time point. These metabolic changes are proposed to be partially mediated by increased GH secretion and are consistent with the Randle cycle of competition between glucose and fat oxidation.

Insulin regulation of skeletal muscle PDK4 mRNA expression is impaired in acute insulin-resistant states

We previously showed that insulin has a profound effect to suppress pyruvate dehydrogenase kinase (PDK) 4 expression in rat skeletal muscle. In the present study, we examined whether insulin's effect on PDK4 expression is impaired in acute insulin-resistant states and, if so, whether this change is accompanied by decreased insulin's effects to stimulate Akt and forkhead box class O (FOXO) 1 phosphorylation. To induce insulin resistance, conscious overnight-fasted rats received a constant infusion of Intralipid or lactate for 5 h, while a control group received saline infusion. Following the initial infusions, each group received saline or insulin infusion (n = 6 or 7 each) for an additional 5 h, while saline, Intralipid, or lactate infusion was continued. Plasma glucose was clamped at basal levels during the insulin infusion. Compared with the control group, Intralipid and lactate infusions decreased glucose infusion rates required to clamp plasma glucose by approximately 60% (P < 0.01), confirming the induction of insulin resistance. Insulin's ability to suppress PDK4 mRNA level was impaired in skeletal muscle with Intralipid and lactate infusions, resulting in two- to threefold higher PDK4 mRNA levels with insulin (P < 0.05). Insulin stimulation of Akt and FOXO1 phosphorylation was also significantly decreased with Intralipid and lactate infusions. These data suggest that insulin's effect to suppress PDK4 gene expression in skeletal muscle is impaired in insulin-resistant states, and this may be due to impaired insulin signaling for stimulation of Akt and FOXO1 phosphorylation. Impaired insulin's effect to suppress PDK4 expression may explain the association between PDK4 overexpression and insulin resistance in skeletal muscle.

The relationship between peripheral glucose utilisation and insulin sensitivity in the regulation of hepatic glucose production: studies in normal and alloxan-diabetic dogs

BACKGROUND

Hepatic glucose overproduction (HGP) of diabetes could be primary or could occur in response to the metabolic needs of peripheral (skeletal muscle (SkM)) tissues. This question was tested in normal and diabetic dogs.

METHODS

HGP, SkM glucose uptake (Rd(tissue)), metabolic clearance of glucose (MCRg) and glycolytic flux (GF(exog)), and SkM biopsies were measured in the same dogs before and after alloxan-induced diabetes. Normal dogs were exposed to (1) an extended 20-h fast, (2) low- and high-dose glucose infusions (GINF) at basal insulinaemia, and chronic diabetic dogs were exposed to (3) hyperglycaemia, (4) phlorizin-induced normoglycaemia, and (5) poor and good diabetic control.

RESULTS

(1) Prolonged fast: HGP, Rd(tissue), and GF(exog) fell in parallel (p < 0.05). (2) Low-dose GINF: plasma glucose, insulin, Rd(tissue), MCRg, and GF(exog) were unchanged, but HGP fell by approximately 40%, paralleling the supplemental GINF. (3) High-dose GINF at basal insulin: plasma glucose doubled and synchronous changes in HGP, Rd(tissue), MCRg, and GF(exog) occurred; IC(glucose), G6P, and glycogen were unchanged. (4) Hyperglycaemic diabetes: HGP was raised (p < 0.05), matching urinary glucose loss (UGL) and decreased MCR(g), and maintaining normal basal Rd(tissue) and GF(exog). SkM IC(glucose) was increased and glycogen decreased (both p < 0.05). (5) Phlorizin-induced normoglycaemia in diabetic dogs: HGP rose, matching the increased UGL, while maintaining normal Rd(tissue) and GF(exog). Intramuscular substrates normalised. (6) Whole body and SkM metabolism normalised with correction of the insulin resistance and good diabetic control.

CONCLUSION

HGP reflects whether SkM is in a state of relative glucose 'excess' or absolute/relative glucose 'deprivation'.

In beta-cells, mitochondria integrate and generate metabolic signals controlling insulin secretion

Pancreatic beta-cells are unique neuroendocrine cells displaying the peculiar feature of responding to nutrients, principally glucose, as primary stimulus. This requires translation of a metabolic substrate into intracellular messengers recognized by the exocytotic machinery. Central to this signal transduction mechanism, mitochondria integrate and generate metabolic signals, thereby coupling glucose recognition to insulin secretion. In response to a glucose rise, nucleotides and metabolites are generated by mitochondria and participate, together with cytosolic calcium, to the stimulation of insulin exocytosis. This review describes the mitochondrion-dependent pathways of regulated insulin secretion. In particular, importance of cataplerotic and anaplerotic processes is discussed, with special attention to the mitochondrial enzyme glutamate dehydrogenase. Mitochondrial defects, such as mutations and reactive oxygen species production, are presented in the context of beta-cell failure in the course of type 2 diabetes.

The role of saturation of fat depots in the pathogenesis of insulin resistance

Insulin resistance is the earliest observable abnormality in individuals who are predisposed to, and who later develop type 2 diabetes mellitus. We hypothesize that saturation of the subcutaneous fat depot is the primary event in the pathophysiology of insulin resistance in the majority of patients and postulate that this seminal event may lead to the development of hypertension, hypertriglyceridemia and depressed HDL levels (i.e., the metabolic syndrome). Our hypothesis has the following clinical implications: (1) differing responses to weight loss may be seen with regards to insulin resistance depending on the size of the fat depot; individuals with small fat depots having to maintain an extremely low body mass to preserve an insulin sensitive phenotype while individuals with a large fat depot may become insulin sensitive even when still clinically obese with some amount of weight loss; (2) peroxisome proliferator activated receptor gamma agonists, such as thiazoledinediones which expand the subcutaneous fat depot, may be especially useful in improving insulin resistance in individuals with small fat depots; (3) expanding alternate storage sites for triglycerides by a variety of techniques, such as resistance training-induced muscle hypertrophy, may also improve insulin resistance; (4) drugs, such as beta 3 adrenergic receptor agonists which promote lipolysis and have been suggested as possible agents in the treatment of obesity may actually increase insulin resistance by releasing free fatty acids into the circulation. Similarly, inhibitors of the beta oxidation of free fatty acids (e.g., carnitine palmitoyl transferase inhibitors) may also actually cause insulin resistance by sparing fat from oxidation and thus worsening fat depot saturation and (5) liposuction, by reducing the size of the subcutaneous fat depot may actually worsen insulin resistance, thus increasing the risk of type 2 diabetes mellitus.

Down-regulation of pyruvate dehydrogenase phosphatase in obese subjects is a defect that signals insulin resistance

OBJECTIVE

The objective of this study was to determine whether down-regulation of pyruvate dehydrogenase phosphatase (PDP) is responsible for poorly active pyruvate dehydrogenase (PDH) in circulating lymphocytes (CLs) of obese subjects (ObS), and if so, whether it improves when their plasma insulin rises.

RESEARCH METHODS AND PROCEDURES

PDH activity was compared in lysed CLs of 10 euglycemic ObS and 10 sex- and age-matched controls before and during plasma insulin enhancement in an oral glucose tolerance test. It was evaluated without (PDHa) or with Mg/Ca or Mg at various concentrations to assess PDP1 or PDP2 activities or with Mg/Ca and exogenous PDP to determine total PDH activity (PDHt), which is an indirect measure of the amount of PDH. The insulin sensitivity index was calculated, and PDP1 and PDP2 mRNA was sought in the CLs.

RESULTS

At T0 in ObS, PDHt was normal, whereas PDHa and PDP1 activity was below normal at all Mg/Ca concentrations. PDP2 activity was undetectable in both groups. PDP1 and PDP2 mRNA was identified, and insulin sensitivity index and PDHa were directly correlated. During the oral glucose tolerance test, plasma insulin rose considerably more in ObS than in controls; PDHa and PDP1 activity also increased but remained significantly below normal, and PDHt was unvaried in both groups.

DISCUSSION

PDP1 is down-regulated in CLs of ObS because it is poorly sensitive to Mg/Ca; this defect is attenuated when plasma insulin is greatly enhanced.

ATP-sensitive K+ channel signaling in glucokinase-deficient diabetes

As the rate-limiting controller of glucose metabolism, glucokinase represents the primary beta-cell "glucose sensor." Inactivation of both glucokinase (GK) alleles results in permanent neonatal diabetes; inactivation of a single allele causes maturity-onset diabetes of the young type 2 (MODY-2). Similarly, mice lacking both alleles (GK(-/-)) exhibit severe neonatal diabetes and die within a week, whereas heterozygous GK(+/-) mice exhibit markedly impaired glucose tolerance and diabetes, resembling MODY-2. Glucose metabolism increases the cytosolic [ATP]-to-[ADP] ratio, which closes ATP-sensitive K(+) channels (K(ATP) channels), leading to membrane depolarization, Ca(2+) entry, and insulin exocytosis. Glucokinase insufficiency causes defective K(ATP) channel regulation, which may underlie the impaired secretion. To test this prediction, we crossed mice lacking neuroendocrine glucokinase (nGK(+/-)) with mice lacking K(ATP) channels (Kir6.2(-/-)). Kir6.2 knockout rescues perinatal lethality of nGK(-/-), although nGK(-/-)Kir6.2(-/-) animals are postnatally diabetic and still die prematurely. nGK(+/-) animals are diabetic on the Kir6.2(+/+) background but only mildly glucose intolerant on the Kir6.2(-/-) background. In the presence of glutamine, isolated nGK(+/-)Kir6.2(-/-) islets show improved insulin secretion compared with nGK(+/-)Kir6.2(+/+). The significant abrogation of nGK(-/-) and nGK(+/-) phenotypes in the absence of K(ATP) demonstrate that a major factor in glucokinase deficiency is indeed altered K(ATP) signaling. The results have implications for understanding and therapy of glucokinase-related diabetes.

beta-Agonist stimulation produces changes in cardiac AMPK and coronary lumen LPL only during increased workload

Given the importance of lipoprotein lipase (LPL) in cardiac and vascular pathology, the objective of the present study was to investigate whether the beta-agonist isoproterenol (Iso) influences cardiac LPL. Incubation of quiescent cardiomyocytes with Iso for 60 min had no effect on basal, intracellular, or heparin-releasable (HR)-LPL activity. Similarly, Iso did not change HR-LPL in Langendorff isolated hearts that do not beat against an afterload. In the intact animal, LPL activity at the vascular lumen increased significantly in the Iso-treated group, together with a substantial increase in rate-pressure product. This LPL increase was likely via mechanisms regulated by activation of AMP-activated protein kinase (AMPK) and inactivation of acetyl-CoA carboxylase (ACC280). In glucose-perfused hearts, simply switching from Langendorff to the isolated working heart (that beats against an afterload) induced increases in AMPK and ACC280 phosphorylation and enhanced HR-LPL activity. Provision of insulin and albumin-bound palmitic acid to the working heart was able to reverse these effects. In these hearts, introduction of Iso to the buffer perfusate duplicated the effects seen when this beta-agonist was given in vivo. Our data suggest that Iso can influence HR-LPL only during conditions of increased workload, mechanical performance and excessive energy expenditure, and likely in an AMPK-dependent manner.

Imaging of myocardial metabolism

There is compelling evidence that alterations in myocardial substrate use play a key role in a variety of normal and abnormal cardiac conditions such as aging, left ventricular hypertrophy, and diabetic heart disease. However, it is unclear whether the metabolic changes are adaptive or maladaptive. Development of transgenic models targeting key aspects of myocardial substrate use, such as uptake, oxidation, and storage, is accelerating our understanding of the metabolic perturbations of cardiac disease. However, whether the metabolic phenotype in these models is relevant to the human condition is frequently unknown. The importance of altered myocardial metabolism in the pathogenesis of cardiac disease is underscored by the current robust development of novel therapeutics that target myocardial substrate use. Currently, magnetic resonance spectroscopy, single photon emission computed tomography, and positron emission tomography are the 3 methods available to image myocardial substrate metabolism. In this review the role of metabolic imaging in the study of specific cardiac disease processes will be discussed. Both the current and future capabilities of metabolic imaging to furthering our understanding of cardiac disease are highlighted.

PDH kinase inhibitors: a novel therapy for Type II diabetes?

The pyruvate dehydrogenase multienzyme complex catalyses the oxidative decarboxylation of pyruvate, which is an important regulatory step in oxidative metabolism. Phosphorylation of the E1 (pyruvate decarboxylase) subunit on one of three specific serine residues results in loss of enzyme activity. Four dedicated PDHK (pyruvate dehydrogenase kinase) isoenzymes have been identified, each of which display a distinct tissue-specific expression profile, and have differential regulatory properties. Thus PDHK play a key role in controlling the balance between glucose and lipid oxidation according to substrate supply. Increasing glucose oxidation by inhibiting PDHK may be an effective mechanism to increase glucose utilization; additionally, increasing pyruvate oxidation may further contribute to lowering of glucose level by decreasing the supply of gluconeogenic substrates. A number of PDHK inhibitors are now available to enable this mechanism to be evaluated as a therapy for diabetes. The isoenzyme selectivity profile of AZD7545 and related compounds will be described and evidence for their non-ATP-competitive mode of action presented. These compounds increase PDH activity in vivo, and when dosed chronically, improve glycaemic control in Zucker rats. Furthermore, glucose lowering has been demonstrated in the hyperglycaemic Zucker diabetic fatty rat. This result supports the hypothesis that inhibition of PDHK may be an effective therapy for Type II diabetes.

The cellular fate of glucose and its relevance in type 2 diabetes

Type 2 diabetes is a complex disorder with diminished insulin secretion and insulin action contributing to the hyperglycemia and wide range of metabolic defects that underlie the disease. The contribution of glucose metabolic pathways per se in the pathogenesis of the disease remains unclear. The cellular fate of glucose begins with glucose transport and phosphorylation. Subsequent pathways of glucose utilization include aerobic and anaerobic glycolysis, glycogen formation, and conversion to other intermediates in the hexose phosphate or hexosamine biosynthesis pathways. Abnormalities in each pathway may occur in diabetic subjects; however, it is unclear whether perturbations in these may lead to diabetes or are a consequence of the multiple metabolic abnormalities found in the disease. This review is focused on the cellular fate of glucose and relevance to human type 2 diabetes.

Insulin resistance and the pathogenesis of nonalcoholic fatty liver disease

The prevalence of obesity has reached epidemic proportions in most of the western world. Current estimates suggest that 22.5%of the population of the United States suffers from obesity and is at risk for development of obesity-related complications, including hypertension, coronary artery disease, diabetes, hyperlipidemia,increased predisposition for various cancers, and nonalcoholic fatty liver disease. Fatty liver disease is currently the most common abnormality observed in hepatology practice. Since it was first reported in the 1980s in obese diabetic females, our understanding of nonalcoholic fatty liver (NAFL) and nonalcoholic steatohepatitis (NASH) has undergone significant metamorphosis. It is now universally accepted that insulin resistance and subsequent hyperinsulinemia are key factors that lead to both NAFL and NASH.This article reviews the role of insulin resistance in the genesis of these conditions.

Insulin suppresses PDK-4 expression in skeletal muscle independently of plasma FFA

Starvation and experimental diabetes induce a stable increase in pyruvate dehydrogenase kinase (PDK) activity in skeletal muscle, which is largely due to a selective upregulation of PDK-4 expression. Increased free fatty acid (FFA) level has been suggested to be responsible for the upregulation. Because these metabolic states are also characterized by insulin deficiency, the present study was designed to examine whether insulin has a significant effect to regulate PDK mRNA expression in rat skeletal muscle. In study 1, overnight-fasted rats received an infusion of saline or insulin for 5 h (n = 6 each). During the insulin infusion, plasma glucose was clamped at basal levels (euglycemic hyperinsulinemic clamp). A third group (n = 6) received Intralipid infusion during the clamp to prevent a fall in plasma FFA. PDK-2 mRNA level in gastrocnemius muscle was not altered by insulin or FFA (i.e., Intralipid infusion). In contrast, PDK-4 mRNA level was decreased 72% by insulin (P < 0.05), and Intralipid infusion prevented only 20% of the decrease. PDK-4 protein level was decreased approximately 20% by insulin (P < 0.05), but this effect was not altered by Intralipid infusion. In study 2, overnight-fasted rats were refed or received an infusion of saline or nicotinic acid (NA, 30 micromol/h) for 5 h (n = 5 each). During the refeeding and NA infusion, plasma FFA levels were similarly (i.e., 60-70% vs. saline control) lowered. Muscle PDK-4 mRNA level decreased 77% after the refeeding (P < 0.05) but not after the NA infusion. In conclusion, the present data indicate that insulin had a profound effect to suppress PDK-4 expression in skeletal muscle and that, contrary to previous suggestions, circulating FFA had little impact on PDK-4 mRNA expression, at least within 5 h.

Influence of insulin on glucose metabolism and energy expenditure in septic patients

Introduction

It is recognized that administration of insulin with glucose decreases catabolic response in sepsis. The aim of the present study was to compare the effects of two levels of insulinaemia on glucose metabolism and energy expenditure in septic patients and volunteers.

Methods

Glucose uptake, oxidation and storage, and energy expenditure were measured, using indirect calorimetry, in 20 stable septic patients and 10 volunteers in a two-step hyperinsulinaemic (serum insulin levels 250 and 1250 mIU/l), euglycaemic (blood glucose concentration 5 mmol/l) clamp. Differences between steps of the clamp (from serum insulin 1250 to 250 mIU/l) for all parameters were calculated for each individual, and compared between septic patients and volunteers using the Wilcoxon nonpaired test.

Results

Differences in glucose uptake and storage were significantly less in septic patients. The differences in glucose oxidation between the groups were not statistically significant. Baseline energy expenditure was significantly higher in septic patients, and there was no significant increase in either step of the clamp in this group; when comparing the two groups, the differences between steps were significantly greater in volunteers.

Conclusion

A hyperdynamic state of sepsis leads to a decrease in glucose uptake and storage in comparison with healthy volunteers. An increase in insulinaemia leads to an increase in all parameters of glucose metabolism, but the increases in glucose uptake and storage are significantly lower in septic patients. A high level of insulinaemia in sepsis increases glucose uptake and oxidation significantly, but not energy expenditure, in comparison with volunteers.