Defective Insulin Signaling Research Articles

Muscle insulin resistance: assault by lipids, cytokines and local macrophages

The present review outlines possible mechanisms by which high fatty acids, associated with high-fat diet and obesity, impose insulin resistance on glucose uptake into skeletal muscle. It is well established that muscle insulin resistance arises in conditions of high-fatty acid availability, and correlates with accumulation of triglycerides within skeletal muscle fibres. However, it is debated whether triglycerides or other lipid metabolites such as diacylglycerols and ceramides are directly responsible. These lipid metabolites can activate serine kinases that impair insulin signalling. Accumulation of acylcarnitines and reactive oxygen species could be additional causative agents of insulin resistance. Further, the precise defects in insulin signalling in muscle caused by high intramuscular lipid (i.e. lipotoxicity) remain unclear. In parallel, proinflammatory activation within the adipose tissue of obese and high-fat fed animals or humans causes muscle insulin resistance, and is ascribed to circulating inflammatory cytokines. Recent evidence also shows proinflammatory macrophages infiltrating muscle tissue and/or intermuscular adipose tissue, and there is growing evidence that fatty acids trigger macrophages to secrete factors that directly impair insulin actions. These factors are postulated to activate stress-signalling pathways in muscle that act on the same insulin-signalling components affected by lipotoxicity. Altered intramuscular lipid metabolism, circulating cytokines, and inflammatory macrophage infiltration of muscle tissue have been recently linked to muscle insulin resistance provoked by fatty acids. Each is analysed separately in this review, but they may act simultaneously and synergistically to render skeletal muscle insulin-resistant.

Mineralocorticoid Receptor Antagonists and the Metabolic Syndrome

Key components of the metabolic syndrome (MetS), ie, obesity and insulin resistance, are associated with increased aldosterone production and mineralocorticoid receptor (MR) activation. Both MetS and hyperaldosteronism are proinflammatory and pro-oxidative states associated with cardiovascular disease. This review discusses emerging data that MR activation may contribute to abnormalities seen in MetS. In view of these data, MR antagonists may be beneficial in MetS, not only by controlling hypertension but also by reversing inflammation, oxidative stress, and defective insulin signaling at the cellular-molecular level. Clinical trials have demonstrated benefits of MR antagonists in heart failure, hypertension, and diabetic nephropathy, but additional trials are needed to demonstrate the clinical significance of MR blockade in MetS.

Autophagy: A Potential Link between Obesity and Insulin Resistance

Dysregulation of autophagy contributes to aging and to diseases such as neurodegeneration, cardiomyopathy, and cancer. The paper by Yang et al. (2010) in this issue of Cell Metabolism indicates that defective autophagy may also underlie impaired insulin sensitivity in obesity and that upregulating autophagy can combat insulin resistance.

Defective Hepatic Autophagy in Obesity Promotes ER Stress and Causes Insulin Resistance

Autophagy is a homeostatic process involved in the bulk degradation of cytoplasmic components, including damaged organelles and proteins. In both genetic and dietary models of obesity, we observed a severe downregulation of autophagy, particularly in Atg7 expression levels in liver. Suppression of Atg7 both in vitro and in vivo resulted in defective insulin signaling and elevated ER stress. In contrast, restoration of the Atg7 expression in liver resulted in dampened ER stress, enhanced hepatic insulin action, and systemic glucose tolerance in obese mice. The beneficial action of Atg7 restoration in obese mice could be completely prevented by blocking a downstream mediator, Atg5, supporting its dependence on autophagy in regulating insulin action. Our data demonstrate that autophagy is an important regulator of organelle function and insulin signaling and that loss of autophagy is a critical component of defective insulin action seen in obesity.

Early Hepatic Insulin Resistance Precedes the Onset of Diabetes in Obese C57BLKS-db/db Mice

OBJECTIVETo identify metabolic derangements contributing to diabetes susceptibility in the leptin receptor–deficient obese C57BLKS/J-db/db (BKS-db) mouse strain.RESEARCH DESIGN AND METHODSYoung BKS-db mice were used to identify metabolic pathways contributing to the development of diabetes. Using the diabetes-resistant B6-db strain as a comparison, in vivo and in vitro approaches were applied to identify metabolic and molecular differences between the two strains.RESULTSDespite higher plasma insulin levels, BKS-db mice exhibit lower lipogenic gene expression, rate of lipogenesis, hepatic triglyceride and glycogen content, and impaired insulin suppression of gluconeogenic genes. Hepatic insulin receptor substrate (IRS)-1 and IRS-2 expression and insulin-stimulated Akt-phosphorylation are decreased in BKS-db primary hepatocytes. Hyperinsulinemic-euglycemic clamp studies indicate that in contrast to hepatic insulin resistance, skeletal muscle is more insulin sensitive in BKS-db than in B6-db mice. We also demonstrate that elevated plasma triglyceride levels in BKS-db mice are associated with reduced triglyceride clearance due to lower lipase activities.CONCLUSIONSOur study demonstrates the presence of metabolic derangements in BKS-db before the onset of β-cell failure and identifies early hepatic insulin resistance as a component of the BKS-db phenotype. We propose that defects in hepatic insulin signaling contribute to the development of diabetes in the BKS-db mouse strain.

Insulin resistance as a mechanism of preserved myocardial function in heart failure animals fed a high fat diet

Heart failure (HF) rats fed a high saturated fat diet (SAT) have improved left ventricular contractile function compared to HF animals fed normal chow (NC) diets. Both HF and SAT are individually associated with the development of insulin resistance (IR), but the role of IR in HF+SAT is unknown. We hypothesize that the cardioprotective effect seen in HF+SAT results from defects in insulin signaling that decrease glucose uptake in IR myocardium. Rats underwent coronary ligation or sham (SH) surgery & were fed NC or SAT for 8 weeks. Cardiac contractility (peak positive dP/dt) was preserved in HF+SAT animals versus HF+NC. SH+SAT, HF+NC & HF+SAT animals have abnormal glucose tolerance tests & elevated insulin levels compared to SH+NC, indicating peripheral IR. MicroPET data shows HF+SAT animals have myocardial IR relative to all other groups. Protein targets of insulin signaling (IRS‐1, Akt, pAkt, GLUT4) are currently being analyzed. HF+SAT animals are IR in both peripheral tissue & the myocardium, while SH+SAT & HF+NC animals are IR in the periphery only. This correlates to preserved contractile function in HF+SAT versus HF+NC. We predict that insulin signaling targets such as pAkt and GLUT4 are downregulated, which may be the mechanism by which myocardial insulin resistance preserves contractile function in HF+SAT animals. (Grant # NHLBI HL‐081857)

Attenuated mTOR Signaling and Enhanced Autophagy in Adipocytes from Obese Patients with Type 2 Diabetes

Type 2 diabetes (T2D) is strongly linked to obesity and an adipose tissue unresponsive to insulin. The insulin resistance is due to defective insulin signaling, but details remain largely unknown. We examined insulin signaling in adipocytes from T2D patients, and contrary to findings in animal studies, we observed attenuation of insulin activation of mammalian target of rapamycin (mTOR) in complex with raptor (mTORC1). As a consequence, mTORC1 downstream effects were also affected in T2D: feedback signaling by insulin to signal-mediator insulin receptor substrate-1 (IRS1) was attenuated, mitochondria were impaired and autophagy was strongly upregulated. There was concomitant autophagic destruction of mitochondria and lipofuscin particles, and a dependence on autophagy for ATP production. Conversely, mitochondrial dysfunction attenuated insulin activation of mTORC1, enhanced autophagy and attenuated feedback to IRS1. The overactive autophagy was associated with large numbers of cytosolic lipid droplets, a subset with colocalization of perlipin and the autophagy protein LC3/atg8, which can contribute to excessive fatty acid release. Patients with diagnoses of T2D and overweight were consecutively recruited from elective surgery, whereas controls did not have T2D. Results were validated in a cohort of patients without diabetes who exhibited a wide range of insulin sensitivities. Because mitochondrial dysfunction, inflammation, endoplasmic-reticulum stress and hypoxia all inactivate mTORC1, our results may suggest a unifying mechanism for the pathogenesis of insulin resistance in T2D, although the underlying causes might differ.

Genetic variants regulating insulin receptor signalling are associated with the severity of liver damage in patients with non-alcoholic fatty liver disease

Background/aimsThe aim of this study was to assess the effect of functional ENPP1(ectoenzyme nucleotide pyrophosphate phosphodiesterase 1)/PC-1 (plasma cell antigen-1) and IRS-1 (insulin receptor substrate-1) polymorphisms influencing insulin receptor activity...

Curcumin Inhibits Hepatic Protein-Tyrosine Phosphatase 1B and Prevents Hypertriglyceridemia and Hepatic Steatosis in Fructose-Fed Rats

High consumption of dietary fructose is an important contributory factor in the development of hepatic steatosis in insulin or leptin resistance. We investigated the effects of curcumin on fructose-induced hypertriglyceridemia and liver steatosis and explored its preventive mechanisms in rats. Curcumin reduced serum insulin and leptin levels in fructose-fed rats. This compound could increase phosphorylation of insulin receptor and insulin receptor substrate 1 to enhance Akt and extracellular signal-regulated kinase1/2 (ERK1/2) activation in the liver of fructose-fed rats. Moreover, curcumin increased phosphorylation of hepatic janus-activated kinase-signal transducer 2 and subsequently also stimulated Akt and ERK1/2 activation in this model. Suppression of curcumin on leptin signaling overstimulation in tyrosine1138 phosphorylation of the long form of leptin receptor and signal transducer and activator of transcription 3 resulted in down-regulation of suppressor of cytokine signaling 3 in the liver of fructose-fed rats. Thus, improvement of insulin and leptin signaling transduction and subsequently elevation of peroxisome proliferator-activated receptor alpha expression by curcumin led to reduction of very-low-density lipoprotein overproduction and triglyceride hypersynthesis. Furthermore, overexpression and hyperactivity of hepatic protein tyrosine phosphatase 1B (PTP1B) associated with defective insulin and leptin signaling were observed in fructose-fed rats. Additionally, curcumin was found to significantly reduce hepatic PTP1B expression and activity in this model. Our data indicate that the mechanisms by which curcumin protects against fructose-induced hypertriglyceridemia and hepatic steatosis are its inhibition on PTP1B and subsequently improvement of insulin and leptin sensitivity in the liver of rats. This PTP1B inhibitory property may be a promising therapeutic strategy for curcumin to treat fructose-induced hepatic steatosis driven by hepatic insulin and leptin resistance.

Insulin resistance in polycystic ovary syndrome: maker or marker?

Insulin resistance plays a central role in the pathophysiology of polycystic ovary syndrome (PCOS). However, this finding is not universal in PCOS, and many women with insulin resistance do not develop this syndrome, leading to much controversy. Insulin resistance is more prevalent in certain phenotypes of PCOS, but tests of insulin resistance are not included in any of the currently used diagnostic criteria to define PCOS. Several genetic and environmental factors and their interactions appear to modulate the presence, degree and expression of insulin resistance. Tissue-specific defects in insulin signaling have been described, and different tissues contribute to insulin resistance differently in PCOS. Our understanding of the syndrome may benefit from greater insight into the mechanisms of the complex inter-relationship between insulin resistance and PCOS.

Pathogenesis of insulin resistance in skeletal muscle.

Insulin resistance in skeletal muscle is manifested by decreased insulin-stimulated glucose uptake and results from impaired insulin signaling and multiple post-receptor intracellular defects including impaired glucose transport, glucose phosphorylation, and reduced glucose oxidation and glycogen synthesis. Insulin resistance is a core defect in type 2 diabetes, it is also associated with obesity and the metabolic syndrome. Dysregulation of fatty acid metabolism plays a pivotal role in the pathogenesis of insulin resistance in skeletal muscle. Recent studies have reported a mitochondrial defect in oxidative phosphorylation in skeletal muscle in variety of insulin resistant states. In this review, we summarize the cellular and molecular defects that contribute to the development of insulin resistance in skeletal muscle.

Neonatal exendin-4 treatment reduces oxidative stress and prevents hepatic insulin resistance in intrauterine growth-retarded rats

Intrauterine growth retardation (IUGR) has been linked to the development of type 2 diabetes in adulthood. We have developed an IUGR model in the rat whereby the animals develop diabetes later in life. Previous studies demonstrate that administration of the long-acting glucagon-like-peptide-1 agonist, exendin-4, during the neonatal period prevents the development of diabetes in IUGR rats. IUGR animals exhibit hepatic insulin resistance early in life (prior to the onset of hyperglycemia), characterized by blunted suppression of hepatic glucose production (HGP) in response to insulin. Basal HGP is also significantly higher in IUGR rats. We hypothesized that neonatal administration of exendin-4 would prevent the development of hepatic insulin resistance. IUGR and control rats were given exendin-4 on days 1-6 of life. Hyperinsulinemic-euglycemic clamp studies showed that Ex-4 significantly reduced basal HGP by 20% and normalized insulin suppression of HGP in IUGR rats. While Ex-4 decreased body weight and fat content in both Control and IUGR animals, these differences were only statistically significant in Controls. Exendin-4 prevented development of oxidative stress in liver and reversed insulin-signaling defects in vivo, thereby preventing the development of hepatic insulin resistance. Defects in glucose disposal and suppression of hepatic glucose production in response to insulin were reversed. Similar results were obtained in isolated Ex-4-treated neonatal hepatocytes. These results indicate that exposure to exendin-4 in the newborn period reverses the adverse consequences of fetal programming and prevents the development of hepatic insulin resistance.

Mitochondrial dysfunction enhances the migration of vascular smooth muscles cells via suppression of Akt phosphorylation

Background Atherosclerosis is one of the major complications of diabetes, which may result from insulin resistance via mitochondrial dysfunction. Although a strong association between insulin resistance and cardiovascular disease has been suggested, it is not clear yet whether stress-inducing factors damage mitochondria and insulin signaling pathway in cardiovascular tissues. Methods We investigated whether stress-induced mitochondrial dysfunction might alter the insulin/Akt signaling pathway in A10 rat vascular smooth muscle cells (VSMC). Results The treatment of oxidized low density lipoprotein (oxLDL) decreased ATP contents, mitochondrial respiration activity, mRNA expressions of OXPHOS subunits and IRS-1/2 and insulin-mediated phosphorylations of Akt and AMP-activated protein kinase (AMPK). Similarly, dideoxycytidine (ddC), the mtDNA replication inhibitor, or rotenone, OXPHOS complex I inhibitor, inhibited the insulin-mediated pAkt while increased pAMPK regardless of insulin. Reciprocally, an inhibitor of Akt, triciribine (TCN), decreased cellular ATP contents. Overexpression of Akt dominant positive reversed the oxLDL- or ddC-mediated ATP decrease but AMPK activator did not. Akt activation also normalized the aberrant VSMC migration induced by ddC. Conclusions Defective insulin signaling and mitochondrial function may collectively contribute to developing cardiovascular disease. General significance Akt may be a possible therapeutic target for treating insulin resistance-associated atherosclerosis.

Role of aldosterone and angiotensin II in insulin resistance: an update

The role of the Renin-Angiotensin-Aldosterone system (RAAS) on the development of insulin resistance and cardiovascular disease is an area of growing interest. Most of the deleterious actions of the RAAS on insulin sensitivity appear to be mediated through activation of the Angiotensin II (Ang II) Receptor type 1 (AT(1)R) and increased production of mineralocorticoids. The underlying mechanisms leading to impaired insulin sensitivity remain to be fully elucidated, but involve increased production of reactive oxygen species and oxidative stress. Both experimental and clinical studies also implicate aldosterone in the development of insulin resistance, hypertension, endothelial dysfunction, cardiovascular tissue fibrosis, remodelling, inflammation and oxidative stress. There is abundant evidence linking aldosterone, through non-genomic actions, to defective intracellular insulin signalling, impaired glucose homeostasis and systemic insulin resistance not only in skeletal muscle and liver but also in cardiovascular tissue. Blockade of the different components of the RAAS, in particular Ang II and AT(1)R, results in attenuation of insulin resistance, glucose homeostasis, as well as decreased cardiovascular disease morbidity and mortality. These beneficial effects go beyond to those expected with isolated control of hypertension. This review focuses on the role of Ang II and aldosterone in the pathogenesis of insulin resistance, as well as in clinical relevance of RAAS blockade in the prevention and treatment of the metabolic syndrome and cardiovascular disease.

Dissecting the role of insulin resistance in the metabolic syndrome

Over 20 years ago, insulin resistance was postulated to play a central role in the pathogenesis of the metabolic syndrome. However, this has been difficult to prove, leading to a great deal of controversy within the field. Recent studies on mice and humans with genetic defects in insulin signaling have allowed us, for the first time, to dissect which features of the metabolic syndrome can be caused by insulin resistance. Liver insulin receptor knockout mice show that hepatic insulin resistance can produce hyperglycemia, increased apolipoprotein B secretion and atherosclerosis, and increased biliary cholesterol secretion and cholesterol gallstones. Many of these changes may be due to disinhibition of the transcription factor, forkhead box O1. Yet, neither liver insulin receptor knockout mice nor humans with insulin receptor mutations develop the hypertriglyceridemia or hepatic steatosis associated with the metabolic syndrome. These data point to a central role for insulin resistance in the pathogenesis of the metabolic syndrome, as hyperglycemia, atherosclerosis, and cholesterol gallstones can all be caused by insulin resistance. However, hypertriglyceridemia and hepatic steatosis are not due directly to insulin resistance and should be considered pathogenically distinct features of the metabolic syndrome.

A useful list of spontaneously arising animal models of obesity and diabetes

TO THE EDITOR: over the years, we have had the opportunity to edit books and compendia that characterize diverse animal models of obesity and diabetes. Most of those animal models have been either selected through inbreeding or characterized following spontaneously arising mutations. It is now timely to provide, in a summary manner, a concise list of such models as an update and reference for future research. The major defects are described in the references cited. In some cases, insulin resistance has been linked to impaired insulin signaling at various levels, including negative feedback at the level of the insulin receptor substrate (IRS)-1 (5, 15, 24). However, in most cases the mechanism of the diabetogenic changes has not been exhaustively investigated. We hope that this list will guide studies geared at elucidating specific defects in the signal flow and how these studies may explain the metabolic failures leading to the diabetic proneness of the models as well as the mechanism of the ensuing complications. These animal models should also enable the discovery of therapeutic modalities with relevance to human diabetes and should continue to be a useful tool along with specific target-generated transgenic and knockout animals so amply used to understand metabolism and energy balance. Obesity and diabetes in mice with mutations in leptin or leptin receptor genes. Since the discovery that ob is a mutation in the leptin structural gene and db is a mutation in the leptin receptor gene, the nomenclature for these mutations has been changed to reflect their molecular basis. The Lep ob mutation on chromosome 6 was discovered in the Jackson Laboratory, Bar Harbor, ME, and recognized by marked obesity and hyperphagia. This mutation was subsequently transferred to the B6 inbred strain background. On this genetic background, the mutation produces juvenile-onset obesity, hyperinsulinemia, and insulin resistance with mild hyperglycemia and a sustained hyperplasia of the pancreatic -cells. The Lepr db mutation is a recessive mutation on chromosome 4 that occurred spontaneously in the C57BLKS/J inbred strain. The obesity/diabetes syndrome is associated with progressively severe hyperglycemia and correlated with pancreatic cell necrosis and islet atrophy at the end stage. The ob mutation is predominantly obese and exhibits only mild hyperglycemia. The current genetic nomenclature for these mice is as follows: Lep obJ , common name “obese” gene product; leptin or leptin mRNA and Lepr db-IJ , common name “diabetic” gene product leptin receptor or leptin receptor mRNA. Detailed information on these strains can be found in Chua et al. (2). Zucker diabetic fatty rat with a leptin receptor defect. The Zucker diabetic fatty (ZDF) rat exhibits leptin receptor defects. This type of obesity, although associated with insulin resistance, is unlike common forms of human obesity. The ZDF rat was developed into a reproducible type 2 diabetic model at Indiana University (16) from a Zucker rat colony (lepr fa )i n which certain individuals exhibited a propensity to diabetes. The male rat is characterized by hyperinsulinemia and hyperglycemia at 6‐7 wk of age, with glucose reaching levels of 500 mg/dl and insulin levels dropping successively. The female rat requires a high-fat diet for the expression of diabetes. The ZDF rat carries a genetic defect in -cell transcription that is independent of the leptin receptor mutation, causing obesity and insulin resistance likely to be inherited in the -cell gene. Goto-Kakizaki rat with impaired -cell mass and function due to polygenic inheritance. The Goto-Kakizaki rat is a nonobese substrain of Wistar rat origin with inherited chronic hyperglycemia. It was selected through a group of eight generation-inbreeding Wistar rats displaying high glucose levels during a glucose tolerance test. They present “starfish-shaped” islet abnormalities and pancreatic hormone deficiencies, resembling the polygenic basis of human type 2 diabetes (14). New Zealand obese mouse. This is a model of obesity, glucose intolerance, and metabolic syndrome of polygenic nature. This animal exhibits hepatic and peripheral leptin insensitivity, insulin resistance, impaired insulin secretion, hypercholesteremia, and hypertension (6). It displays classic features of obesity, including excessive body weight hyperphagia and reduced energy expenditure. Such obesity is responsible for its impaired glucose metabolism. JCR:LA-cp rat: exhibiting metabolic syndrome with microand macrovascular disease. The prediabetic state in the JCR: LA-cp rat is characterized by abdominal obesity, hypertriglyceridemia and insulin resistance, and a marked damage to the vascular system, which is associated with atherosclerosis, vasculopathy, and ischemic end-stage disease (19). It is a unique model of the obesity/insulin resistance syndrome with cardiovascular implications of polygenic derivation. SHROB rat: a model of metabolic syndrome. The spontaneously obese SHROB (Koletzky) rat is an overtly nondiabetic rat with the primary and secondary characteristics associated with the human metabolic syndrome, including insulin resistance. It exhibits a single recessive trait, a nonsense mutation causing loss of hypothalamic leptin receptors designated as fa k (12), and its insulin-signaling defects were initially reported by

Disruption of glucose homeostasis and induction of insulin resistance by elevated free fatty acids in human L02 hepatocytes

Free fatty acids (FFA) have been implicated as an important causative link between obesity, insulin resistance, and Type 2 diabetes. However, the underlying mechanisms especially for FFA-mediated hepatic insulin resistance are not fully elucidated. Here, we investigated the impaired sites in insulin signaling pathways and mechanisms of insulin resistance induced by elevated FFA in L02 hepatocytes. L02 cells were cultured in Dulbecco's modified eagle medium containing various concentrations of palmitic acid (PA) for 24 h followed by 10(-7) mol/l insulin stimulation. In some experiments, cells were pre-treated with enzymatic inhibitor Wortmannin (10(-6) mol/l). Glucose levels in medium, cytosolic glycogen contents, and phosphoenolpyruvate carboxykinase (PEPCK) activity were measured. Protein level of insulin receptor substrate (IRS)-2 and phosphorylated Akt were detected by Western blot analysis. L02 cells treated with high levels of PA exhibited increased glucose levels, whereas hepatic glycogen contents were decreased in a dose-dependent manner as compared to the control cells. There was a significant attenuation of IRS- 2 protein expression in the cells cultured with PA, and Wortmannin intervention exhibited different IRS-2 protein level with or without PA treatment. In accordance with the reduced IRS-2 level, the insulin-stimulated phosphorylation of Akt was diminished in the PA-treated cells. Basal PEPCK activity and insulin- regulated PEPCK activity were overstimulated in the cells incubated with PA. These data indicate high levels of FFA can disrupt glucose homeostasis, inflict some defects in insulin signaling, and induce insulin resistance in L02 cells.

Lipid and insulin infusion-induced skeletal muscle insulin resistance is likely due to metabolic feedback and not changes in IRS-1, Akt, or AS160 phosphorylation

Type 2 diabetes is characterized by hyperlipidemia, hyperinsulinemia, and insulin resistance. The aim of this study was to investigate whether acute hyperlipidemia-induced insulin resistance in the presence of hyperinsulinemia was due to defective insulin signaling. Hyperinsulinemia (approximately 300 mU/l) with hyperlipidemia or glycerol (control) was produced in cannulated male Wistar rats for 0.5, 1 h, 3 h, or 5 h. The glucose infusion rate required to maintain euglycemia was significantly reduced by 3 h with lipid infusion and was further reduced after 5 h of infusion, with no difference in plasma insulin levels, indicating development of insulin resistance. Consistent with this finding, in vivo skeletal muscle glucose uptake (31%, P < 0.05) and glycogen synthesis rate (38%, P < 0.02) were significantly reduced after 5 h compared with 3 h of lipid infusion. Despite the development of insulin resistance, there was no difference in the phosphorylation state of multiple insulin-signaling intermediates or muscle diacylglyceride and ceramide content over the same time course. However, there was an increase in cumulative exposure to long-chain acyl-CoA (70%) with lipid infusion. Interestingly, although muscle pyruvate dehydrogenase kinase 4 protein content was decreased in hyperinsulinemic glycerol-infused rats, this decrease was blunted in muscle from hyperinsulinemic lipid-infused rats. Decreased pyruvate dehydrogenase complex activity was also observed in lipid- and insulin-infused animals (43%). Overall, these results suggest that acute reductions in muscle glucose metabolism in rats with hyperlipidemia and hyperinsulinemia are more likely a result of substrate competition than a significant early defect in insulin action or signaling.

Mechanisms and consequences of macrophage apoptosis in atherosclerosis

Macrophage apoptosis is an important feature of atherosclerotic plaque development. Research directed at understanding the functional consequences of macrophage death in atherosclerosis has revealed opposing roles for apoptosis in atherosclerotic plaque progression. In early lesions, macrophage apoptosis limits lesion cellularity and suppresses plaque progression. In advanced lesions, macrophages apoptosis promotes the development of the necrotic core, a key factor in rendering plaques vulnerable to disruption and in acute lumenal thrombosis. The first section of this review will examine the role of phagocytic clearance of apoptotic macrophages, a process known as efferocytosis, in the dichotomous roles of macrophage apoptosis in early vs. advanced lesions. The second section will focus on the molecular and cellular mechanisms that are thought to govern macrophage death during atherosclerosis. Of particular interest is the complex and coordinated role that the endoplasmic reticulum (ER) stress pathway and pattern recognition receptors (PRRs) may play in triggering macrophage apoptosis.

Involvement of TNF-α in abnormal adipocyte and muscle sortilin expression in obese mice and humans

Insulin resistance is caused by numerous factors including inflammation. It is characterised by defective insulin stimulation of adipocyte and muscle glucose transport, which requires the glucose transporter GLUT4 translocation towards the plasma membrane. Defects in insulin signalling can cause insulin resistance, but alterations in GLUT4 trafficking could also play a role. Our goal was to determine whether proteins controlling GLUT4 trafficking are altered in insulin resistance linked to obesity. Using real-time RT-PCR, we searched for selected transcripts that were differentially expressed in adipose tissue and muscle in obese mice and humans. Using various adipocyte culture models and in vivo mice treatment, we searched for the involvement of TNF-alpha in these alterations in obesity. Sortilin mRNA and protein were downregulated in adipose tissue from obese db/db and ob/ob mice, and also in muscle. Importantly, sortilin mRNA was also decreased in morbidly obese human diabetic patients. Sortilin and TNF-alpha (also known as TNF) mRNA levels were inversely correlated in mice and human adipose tissues. TNF-alpha decreased sortilin mRNA and protein levels in cultured mouse and human adipocytes, an effect partly prevented by the peroxisome proliferator-activated receptor gamma activator rosiglitazone. TNF-alpha also inhibited adipocyte and muscle sortilin mRNA when injected to mice. Sortilin, an essential player in adipocyte and muscle glucose metabolism through the control of GLUT4 localisation, is downregulated in obesity and TNF-alpha is likely to be involved in this defect. Chronic low-grade inflammation in obesity could thus contribute to insulin resistance by modulating proteins that control GLUT4 trafficking.