Recent advances in our understanding of insulin action and insulin resistance

Increased risk of Alzheimer's disease in Type II diabetes: insulin resistance of the brain or insulin-induced amyloid pathology?

Type II diabetes mellitus (DM2) is associated with an increased risk of cognitive dysfunction and dementia. The increased risk of dementia concerns both Alzheimer's disease and vascular dementia. Although some uncertainty remains into the exact pathogenesis, several mechanisms through which DM2 may affect the brain have now been identified. First, factors related to the ‘metabolic syndrome’, a cluster of metabolic and vascular risk factors (e.g. dyslipidaemia and hypertension) that is closely linked to DM2, may be involved. A number of these risk factors are predictors of cerebrovascular disease, accelerated cognitive decline and dementia. Secondly, hyperglycaemia may be involved, through adverse effects of potentially ‘toxic’ glucose metabolites on the brain and its vasculature. Thirdly, insulin itself may be involved. Insulin can directly modulate synaptic plasticity and learning and memory, and disturbances in insulin signalling pathways in the periphery and in the brain have recently been implicated in Alzheimer's disease and brain aging. Insulin also regulates the metabolism of β-amyloid and tau, the building blocks of amyloid plaques and neurofibrillary tangles, the neuropathological hallmarks of Alzheimer's disease. In this paper, the evidence for the association between DM2 and dementia and for each of these underlying mechanisms will be reviewed, with emphasis on the role of insulin itself.

Mechanism of feedback regulation of insulin receptor substrate-1 phosphorylation in primary adipocytes

Serine and threonine phosphorylation of IRS-1 (insulin receptor substrate-1) has been reported to decrease its ability to be tyrosine-phosphorylated by the insulin receptor. Insulin itself may negatively regulate tyrosine phosphorylation of IRS-1 through a PI3K (phosphoinositide 3-kinase)-dependent feedback pathway. In the present study, we examined the regulation and role of IRS-1 serine phosphorylation in the modulation of IRS-1 tyrosine phosphorylation in physiologically relevant cells, namely freshly isolated primary adipocytes. We show that insulin-stimulated phosphorylation of Ser312 and Ser616 in IRS-1 was relatively slow, with maximal phosphorylation achieved after 20 and 5 min respectively. The effect of insulin on phosphorylation of both these sites required the activation of PI3K and the MAPKs (mitogen-activated protein kinases) ERK1/2 (extracellular-signal-regulated kinase 1 and 2), but not the activation of mTOR (mammalian target of rapamycin)/p70S6 kinase, JNK (c-Jun N-terminal kinase) or p38MAPK. Although inhibition of PI3K and ERK1/2 both substantially decreased insulin-stimulated phosphorylation of Ser312 and Ser616, only wortmannin enhanced insulin-stimulated tyrosine phosphorylation of IRS-1. Furthermore, inhibition of mTOR/p70S6 kinase, JNK or p38MAPK had no effect on insulin-stimulated IRS-1 tyrosine phosphorylation. The differential effect of inhibition of ERK1/2 on insulin-stimulated IRS-1 phosphorylation of Ser312/Ser616 and tyrosine indicates that these events are independent of each other and that phosphorylation of Ser312/Ser616 is not responsible for the negative regulation of IRS-1 tyrosine phosphorylation mediated by PI3K in primary adipocytes.

Evaluation of glucose transport and its regulation by insulin in human monocytes using flow cytometry

BACKGROUND

We investigated the effects of insulin on glucose transport in human monocytes using flow cytometry, a method with several advantages over previously used techniques. We hypothesized that monocytes could be used as tools to study insulin action at the cellular level and facilitate the investigation of mechanisms that lead to insulin resistance.

METHODS

Blood was withdrawn from 38 healthy subjects. The expression of glucose transporter (GLUT) isoforms in plasma membrane and the rates of glucose transport were determined with and without insulin (10 to 1,000 mU/L). Anti-CD14 phycoerythrin monoclonal antibody was used for monocyte gating. GLUT isoforms were determined after staining cells with specific antisera to GLUT1, GLUT3, and GLUT4. Glucose transport was monitored with 6-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-6-deoxyglucose (NBDG).

RESULTS

Insulin increased the uptake of NBDG (median effective dose 20 mU/L) and the expression of GLUT3 and GLUT4 isoforms in the plasma membrane (median effective doses 20 and 35 mU/L, respectively) but had no effect on GLUT1. Maximal effects were always reached at 100 mU/L of insulin.

CONCLUSIONS

Monocytes may be a valid model system to study the effects of insulin on glucose transport. Further, flow cytometry is suitable for this investigation and can be used as an alternative to radiotracer methods.

A biochemical and functional characterization of diet-induced brain insulin resistance

While considerable research has examined diminished insulin responses within peripheral tissues, comparatively little has been done to examine the effects of this metabolic disruption upon the CNS. The present study employed biochemical and electrophysiological assays of acutely prepared brain slices to determine whether neural insulin resistance is a component of the metabolic syndrome observed within the fructose-fed (FF) hamster. The tyrosine phosphorylation levels of the insulin receptor (IR) and insulin receptor substrate 1 (IRS-1) in response to insulin were significantly reduced within FF hamsters. Also, insulin-mediated phosphorylation of both residues necessary for activation of the serine-threonine kinase Akt/PKB, a key effector of insulin signaling, was markedly decreased. Elevated levels of the protein tyrosine phosphatase 1B, which dephosphorylates the IR and IRS-1, were also observed within the cerebral cortex and hippocampus of FF hamsters. Examination of whether a nutritionally induced compromise of neural insulin signaling altered synaptic function revealed a significant attenuation of insulin-induced long-term depression, but no effect upon either paired-pulse facilitation or electrically induced long-term potentiation. Collectively, our results demonstrate, for the first time, that nutritionally induced insulin resistance significantly affects the neural insulin signaling pathway, and suggest that brain insulin resistance may contribute to cognitive impairment.

Molecular mechanism(s) of burn-induced insulin resistance in murine skeletal muscle: role of IRS phosphorylation

Hyperglycemia, glucose intolerance and elevated insulin levels frequently occur in burned patients; however, the mechanism(s) for this insulin resistance has not been fully elucidated. One possible mechanism could involve alterations in the phosphorylation of serine 307 of the insulin receptor substrate-1 (IRS-1) via activation of stress kinase enzymes, including SAPK/JNK. In the present study we examined the time course of the effect of burn injury to mice on: levels of IRS-1 protein, phosphorylation of serine 307 of IRS-1, SAPK/JNK kinase levels and activity and Akt kinase activity in hind limb skeletal muscle. Burn injury produced a reduction in hind limb muscle mass 24 h after injury, and, which persisted for 168 h. At 24 h after injury, there was a dramatic ( approximately 9-fold) increase in phosphorylation of IRS-1 serine 307 followed by a more moderate elevation thereafter. Total IRS-1 protein was slightly elevated at 24 h after injury and decreased to levels below sham treated animals at the later times. Burn injury did not appear to change total SAPK/JNK protein content, however, enzyme activity was increased for 7 days after injury. Akt kinase activity was decreased in skeletal muscle following burn injury; providing a biochemical basis for burn-induced insulin resistance. These findings are consistent with the hypothesis that burn-induced insulin resistance may be related, at least in part, to alterations in the phosphorylation of key proteins in the insulin signaling cascade, including IRS-1, and that changes in stress kinases, such as SAPK/JNK produced by burn injury, may be responsible for these changes in phosphorylation.

Relation of Body Mass Index and Insulin Resistance to Cardiovascular Risk Factors, Inflammatory Factors, and Oxidative Stress During Adolescence

Background— This study assessed the relation of fatness and insulin resistance and their interaction with cardiovascular risk factors, inflammatory factors, and oxidative stress in thin and heavy adolescents.

Methods and Results— Euglycemic insulin clamp studies were performed on 295 (169 male, 126 female) adolescents (mean±SE age, 15±0.1 years). Comparisons were made between (1) heavy and thin adolescents; (2) insulin-sensitive and insulin-resistant adolescents; and (3) thin insulin-sensitive (T-IS), thin insulin-resistant (T-IR), heavy insulin-sensitive (H-IS), and heavy insulin-resistant (H-IR) adolescents. Summed z scores were used to determine clustering of risk factors (fasting insulin, triglycerides, HDL-C, and systolic blood pressure [SBP]) among the groups. SBP, triglycerides, and fasting insulin were significantly higher and HDL-C significantly lower in the heavy adolescents. Fasting insulin and triglycerides were significantly higher and HDL-C significantly lower in the insulin-resistant adolescents. Among the 4 groups, the risk factors and cluster score followed a pattern of risk as follows: T-IS<T-IR<H-IS<H-IR, with H-IR significantly greater than the other groups and showing an interaction between fatness and insulin resistance.

Conclusions— These results show the significant association of both fatness and insulin resistance and their significant interaction with cardiovascular risk factors in adolescence. The finding that insulin resistance may be acting interactively with fatness suggests that interventions directed at insulin resistance in addition to weight loss may be required to alter early development of cardiovascular risk.

Uncoupling insulin signalling by serine/threonine phosphorylation: a molecular basis for insulin resistance

Insulin resistance refers to a decreased capacity of circulating insulin to regulate nutrient metabolism. Recent studies reveal that agents that induce insulin resistance exploit phosphorylation-based negative feedback control mechanisms otherwise utilized by insulin itself to uncouple the insulin receptor from its downstream effectors and thereby terminate insulin signal transduction. This article focuses on the Ser/Thr protein kinases which phosphorylate insulin receptor substrates and the major Ser sites that are phosphorylated, as key elements in the uncoupling of insulin signalling and the induction of an insulin resistance state.

Outcomes with the polymer-based paclitaxel-eluting TAXUS stent in patients with diabetes mellitus

OBJECTIVES

We sought to determine the safety and efficacy of polymer-regulated site-specific delivery of paclitaxel in patients with diabetes mellitus undergoing stent implantation.

BACKGROUND

Percutaneous coronary intervention in patients with diabetes is associated with high rates of restenosis and repeat revascularization due to excessive neointimal proliferation, a process that may be blunted with the site-specific delivery of paclitaxel.

METHODS

In the TAXUS-IV trial, 1,314 patients were prospectively randomized to the slow rate-release polymer-based paclitaxel-eluting TAXUS stent or the bare-metal EXPRESS stent (Boston Scientific Corp., Natick, Massachusetts). Medically treated diabetes was present in 318 patients (24%), 105 of whom required insulin.

RESULTS

Among patients with diabetes, the TAXUS stent, compared to the bare-metal stent, reduced the rate of 9-month binary angiographic restenosis by 81% (6.4% vs. 34.5%, p < 0.0001), and reduced the 12-month rates of target lesion revascularization by 65% (7.4% vs. 20.9%, p = 0.0008), target vessel revascularization by 53% (11.3% vs. 24%, p < 0.004), and composite major adverse cardiac events by 44% (15.6% vs. 27.7%, p = 0.01). The one-year rates of cardiac death (1.9% vs. 2.5%), myocardial infarction (3.2% vs. 6.4%), and subacute thrombosis (0.6% vs. 1.2%) were comparable between the paclitaxel-eluting and control stents, respectively. In the insulin-requiring subgroup, the TAXUS stent reduced angiographic restenosis by 82% (7.7% vs. 42.9%, p = 0.0065), and reduced the one-year rate of target lesion revascularization by 68% (6.2% vs. 19.4%, p = 0.07), a relative reduction similar to patients without diabetes.

CONCLUSIONS

The site-specific delivery of paclitaxel after coronary stent implantation is highly effective in reducing clinical and angiographic restenosis in patients with diabetes mellitus.

Expression, purification, and enzymatic characterization of the dual specificity mitogen-activated protein kinase phosphatase, MKP-4

Mitogen-activated protein kinase phosphatase-4 (MKP-4) is a dual specificity phosphatase, which acts as a negative regulator of insulin-stimulated pathways. Here, we describe expression, purification, and biochemical characterization of MKP-4. We used the Baculovirus expression system and purification with a combination of affinity and gel filtration chromatography to generate pure MKP-4 and MKP-4/p38 complex. Both MKP-4 and the MKP-4/p38 complex exhibited moderate activity toward the surrogate substrates p-nitrophenyl phosphate, 6, 8-difluoro-4-methylumbelliferyl phosphate, and 3-O-methylfluorescein phosphate. The phosphatase activity could be inhibited by peroxovanate, a potent inhibitor of protein tyrosine phosphatases. We further determined kinetic parameters for the MKP-4 and the MKP-4/p38 by using spectrophotometric and fluorescence intensity methods. The MKP-4/p38 complex was found to provide substantially higher phosphatase activity than MKP-4 alone, similar to what has been shown for MKP-3. Our data allow the configuration of screens for modulators of MKP-4 activity.

Impaired glucose tolerance and impaired fasting glucose--a review of diagnosis, clinical implications and management

The diagnostic categories of impaired glucose tolerance (IGT) and impaired fasting glucose (IFG) were stablished in an effort to identify populations at risk for developing type 2 diabetes mellitus (T2DM). Both IGT and IFG are associated with increased risk of developing T2DM, but recent analyses found that the thresholds of risk vary among different populations and an even lower diagnostic threshold of IFG may be appropriate. IGT has been linked with an increased risk of cardiovascular events and some analyses have demonstrated an increased mortality risk compared with patients with normal glucose tolerance. In contrast, a continuum of increased risk of microvascular manifestations of T2DM has been demonstrated with IFG but an association of IFG with cardiovascular events has not been well established. Although both IGT and IFG are associated with resistance to insulin and increased insulin secretion, they do not identify the identical patient populations and are not equivalent in predicting development of T2DM or cardiovascular events. IFG and IGT have been associated with other features of insulin resistance, including dyslipidaemia, hypertension, abdominal obesity, microalbuminuria, endothelial dysfunction, and markers of inflammation and hypercoagulability, traits collectively referred to as the metabolic syndrome. Analyses of combinations of these components have also been associated with progression to T2DM, cardiovascular disease and increased mortality. The foundation of treatment for IGT, IFG, and the metabolic syndrome is lifestyle modification, including both dietary change and routine exercise. To date, several clinical trials have found that lifestyle modification is the most efficacious strategy to prevent progression to T2DM. Alternative treatments include pharmacotherapy with metformin or acarbose, both of which have been demonstrated to decrease the development of T2DM. Ongoing clinical trials are evaluating newer pharmacotherapies, including angiotensin converting enzyme inhibitors, angiotensin receptor antagonists, metglitinides and thiazolidinediones, to prevent both T2DM and cardiovascular events. In combination with lifestyle modification, these therapies offer hope for effective prevention of T2DM and its consequences in high-risk patients.

Insulin acts via mitogen-activated protein kinase phosphorylation in rabbit blastocysts

The addition of insulin during in vitro culture has beneficial effects on rabbit preimplantation embryos leading to increased cell proliferation and reduced apoptosis. We have previously described the expression of the insulin receptor (IR) and the insulin-responsive glucose transporters (GLUT) 4 and 8 in rabbit preimplantation embryos. However, the effects of insulin on IR signaling and glucose metabolism have not been investigated in rabbit embryos. In the present study, the effects of 170 nM insulin on IR, GLUT4 and GLUT8 mRNA levels, Akt and Erk phosphorylation, GLUT4 translocation and methyl glucose transport were studied in cultured day 3 to day 6 rabbit embryos. Insulin stimulated phosphorylation of the mitogen-activated protein kinase (MAPK) Erk1/2 and levels of IR and GLUT4 mRNA, but not phosphorylation of the phosphatidylinositol 3-kinase-dependent protein kinase, Akt, GLUT8 mRNA levels, glucose uptake or GLUT4 translocation. Activation of the MAPK signaling pathway in the absence of GLUT4 translocation and of a glucose transport response suggest that in the rabbit preimplantation embryo insulin is acting as a growth factor rather than a component of glucose homeostatic control.

Ser/Thr phosphorylation of IRS proteins: a molecular basis for insulin resistance

S6K1, like other serine and threonine kinases activated by insulin (such as mTOR and PKCzeta), has recently been shown to participate in negative feedback mechanisms aimed at terminating insulin signaling through IRS (insulin receptor substrate) phosphorylation. Such homeostatic mechanisms can also be activated by excess nutrients or inducers of insulin resistance (such as fatty acids and proinflammatory cytokines) to produce an insulin-resistant state that often leads to the development of diabetes. Identification of the specific kinases involved in such insulin resistance pathways can help lead to the rational design of novel therapeutic agents for treating insulin resistance and type 2 diabetes.

Lessons learned from gene expression profile studies of aging and caloric restriction

To examine molecular events associated with aging and its retardation by caloric restriction (CR), we have employed high-density oligonucleotide microarrays to define transcriptional patterns in mouse tissues, including skeletal muscle, brain, heart, and adipose. Aging results in a differential gene expression pattern specific to each tissue, and most alterations can be completely or partially prevented by CR. Transcriptional patterns of tissues from calorie-restricted animals suggest that CR retards the aging process by reducing endogenous damage and by inducing metabolic shifts associated with specific transcriptional profiles. These studies demonstrate that DNA microarrays can be used in aging research to generate panels of hundreds of transcriptional biomarkers, providing a new tool to measure biological age on a tissue-specific basis and to evaluate interventions designed to mimic the effects of CR.

Serine phosphorylation proximal to its phosphotyrosine binding domain inhibits insulin receptor substrate 1 function and promotes insulin resistance

Ser/Thr phosphorylation of insulin receptor substrate (IRS) proteins negatively modulates insulin signaling. Therefore, the identification of serine sites whose phosphorylation inhibit IRS protein functions is of physiological importance. Here we mutated seven Ser sites located proximal to the phosphotyrosine binding domain of insulin receptor substrate 1 (IRS-1) (S265, S302, S325, S336, S358, S407, and S408) into Ala. When overexpressed in rat hepatoma Fao or CHO cells, the mutated IRS-1 protein in which the seven Ser sites were mutated to Ala (IRS-1(7A)), unlike wild-type IRS-1 (IRS-1(WT)), maintained its Tyr-phosphorylated active conformation after prolonged insulin treatment or when the cells were challenged with inducers of insulin resistance prior to acute insulin treatment. This was due to the ability of IRS-1(7A) to remain complexed with the insulin receptor (IR), unlike IRS-1(WT), which underwent Ser phosphorylation, resulting in its dissociation from IR. Studies of truncated forms of IRS-1 revealed that the region between amino acids 365 to 430 is a main insulin-stimulated Ser phosphorylation domain. Indeed, IRS-1 mutated only at S408, which undergoes phosphorylation in vivo, partially maintained the properties of IRS-1(7A) and conferred protection against selected inducers of insulin resistance. These findings suggest that S408 and additional Ser sites among the seven mutated Ser sites are targets for IRS-1 kinases that play a key negative regulatory role in IRS-1 function and insulin action. These sites presumably serve as points of convergence, where physiological feedback control mechanisms, which are triggered by insulin-stimulated IRS kinases, overlap with IRS kinases triggered by inducers of insulin resistance to terminate insulin signaling.

Glucose, insulin and the brain: modulation of cognition and synaptic plasticity in health and disease: a preface

The brain has long been viewed as an insulin-insensitive organ. Following the demonstration of insulin receptors in the brain, this assumption has been challenged, and a whole new field of research has emerged. Insulin appears to play a role in brain physiology, and disturbances of cerebral insulin signalling and glucose homeostasis are implicated in brain pathology. This special issue of the European Journal of Pharmacology provides an overview of research into the involvement of glucose and insulin in the modulation of cognition and synaptic plasticity in health and in disease. In three sections, the effects of glucose and insulin on the brain will be discussed from a physiological perspective, in relation to brain ageing and Alzheimer's disease, and in relation to diabetes mellitus, respectively. These subjects will be covered from various angles, from the level of molecular neurosciences, to the level of applied neuropsychology.

Reduced incidence of new-onset diabetes mellitus after renal transplantation with 3-hydroxy-3-methylglutaryl-coenzyme a reductase inhibitors (statins)

Statins have anti-inflammatory effects, modify endothelial function and improve peripheral insulin resistance. We hypothesized that statins influence the development of new-onset diabetes mellitus in renal transplant recipients. The records of all previously non-diabetic adults who received an allograft in Toronto between January 1, 1999 and December 31, 2001 were reviewed with follow-up through December 31, 2002. All patients receiving cyclosporine or tacrolimus, mycophenolate mofetil and prednisone were included. New-onset diabetes was diagnosed by the Canadian Diabetic Association criteria: fasting plasma glucose > or =7.0 mmol/L or 2-h postprandial glucose > or =11.1 mmol/L on more than two occasions. Statin use prior to diabetes development was recorded along with other variables. Cox proportional hazards models analyzing statin use as a time-dependent covariate were performed. Three hundred fourteen recipients met study criteria, of whom 129 received statins. New-onset diabetes incidence was 16% (n = 49). Statins (p = 0.0004, HR 0.238[0.109-0.524]) and ACE inhibitors/ARB (p = 0.01, HR 0.309[0.127-0.750]) were associated with decreased risk. Prednisone dose (p = 0.0001, HR 1.007[1.003-1.010] per 1 mg/d at 3 months), weight at transplant (p = 0.02, HR 1.022[1.003-1.042] per 1 kg), black ethnicity (p = 0.02, HR 1.230[1.023-1.480]) and age > or =45 years (p = 0.01, HR 2.226[1.162-4.261]) were associated with increased diabetes. Statin use is associated with reduced new-onset diabetes development after renal transplantation.

Mechanisms of hemorrhage-induced hepatic insulin resistance: role of tumor necrosis factor-alpha

Hemorrhage, sepsis, burn injury, surgical trauma and critical illness all induce insulin resistance. Recently we found that trauma and hemorrhage acutely induced hepatic insulin resistance in the rat. However, the mechanisms of this hemorrhage-induced acute hepatic insulin resistance are unknown. Here we report on the mechanisms of this hepatic insulin resistance. Protein levels and phosphorylation of the insulin receptor and insulin receptor substrate-1/2 (IRS-1/2) were measured, as was the association between IRS-1/2 and phosphatidylinositol 3-kinase (PI3K). Also examined were the hepatic expression of TNFalpha and TNFalpha-induced serine phosphorylation of IRS-1. Insulin receptor and IRS-1/2 protein levels and insulin-induced tyrosine phosphorylation of the insulin receptor were unaltered. In contrast, insulin-induced tyrosine phosphorylation of IRS-1/2 and association between IRS-1/2 and PI3K were dramatically reduced after hemorrhage. Hepatic levels of TNFalpha mRNA and protein were increased as was phosphorylation of IRS-1 serine 307 after hemorrhage. Our data provide the first evidence that compromised IRS-1/2 tyrosine phosphorylation and their association with PI3K contribute to hemorrhage-induced acute hepatic insulin resistance. Increased local TNFalpha may play a role in inducing this hepatic insulin resistance after trauma and hemorrhage.

Lipoprotein transport in the metabolic syndrome: pathophysiological and interventional studies employing stable isotopy and modelling methods

The accompanying review in this issue of Clinical Science [Chan, Barrett and Watts (2004) Clin. Sci. 107, 221–232] presented an overview of lipoprotein physiology and the methodologies for stable isotope kinetic studies. The present review focuses on our understanding of the dysregulation and therapeutic regulation of lipoprotein transport in the metabolic syndrome based on the application of stable isotope and modelling methods. Dysregulation of lipoprotein metabolism in metabolic syndrome may be due to a combination of overproduction of VLDL [very-LDL (low-density lipoprotein)]-apo (apolipoprotein) B-100, decreased catabolism of apoB-containing particles and increased catabolism of HDL (high-density lipoprotein)-apoA-I particles. These abnormalities may be consequent on a global metabolic effect of insulin resistance, partly mediated by depressed plasma adiponectin levels, that collectively increases the flux of fatty acids from adipose tissue to the liver, the accumulation of fat in the liver and skeletal muscle, the hepatic secretion of VLDL-triacylglycerols and the remodelling of both LDL (low-density lipoprotein) and HDL particles in the circulation. These lipoprotein defects are also related to perturbations in both lipolytic enzymes and lipid transfer proteins. Our knowledge of the pathophysiology of lipoprotein metabolism in the metabolic syndrome is well complemented by extensive cell biological data. Nutritional modifications may favourably alter lipoprotein transport in the metabolic syndrome by collectively decreasing the hepatic secretion of VLDL-apoB and the catabolism of HDL-apoA-I, as well as by potentially increasing the clearance of LDL-apoB. Several pharmacological treatments, such as statins, fibrates or fish oils, can also correct the dyslipidaemia by diverse kinetic mechanisms of action, including decreased secretion and increased catabolism of apoB, as well as increased secretion and decreased catabolism of apoA-I. The complementary mechanisms of action of lifestyle and drug therapies support the use of combination regimens in treating dyslipoproteinaemia in subjects with the metabolic syndrome.

The potential biological mechanisms of arsenic-induced diabetes mellitus

Although epidemiologic studies carried out in Taiwan, Bangladesh, and Sweden have demonstrated a diabetogenic effect of arsenic, the mechanisms remain unclear and require further investigation. This paper reviewed the potential biological mechanisms of arsenic-induced diabetes mellitus based on the current knowledge of the biochemical properties of arsenic. Arsenate can substitute phosphate in the formation of adenosine triphosphate (ATP) and other phosphate intermediates involved in glucose metabolism, which could theoretically slow down the normal metabolism of glucose, interrupt the production of energy, and interfere with the ATP-dependent insulin secretion. However, the concentration of arsenate required for such reaction is high and not physiologically relevant, and these effects may only happen in acute intoxication and may not be effective in subjects chronically exposed to low-dose arsenic. On the other hand, arsenite has high affinity for sulfhydryl groups and thus can form covalent bonds with the disulfide bridges in the molecules of insulin, insulin receptors, glucose transporters (GLUTs), and enzymes involved in glucose metabolism (e.g., pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase). As a result, the normal functions of these molecules can be hampered. However, a direct effect on these molecules caused by arsenite at physiologically relevant concentrations seems unlikely. Recent evidence has shown that treatment of arsenite at lower and physiologically relevant concentrations can stimulate glucose transport, in contrary to an inhibitory effect exerted by phenylarsine oxide (PAO) or by higher doses of arsenite. Induction of oxidative stress and interferences in signal transduction or gene expression by arsenic or by its methylated metabolites are the most possible causes to arsenic-induced diabetes mellitus through mechanisms of induction of insulin resistance and beta cell dysfunction. Recent studies have shown that, in subjects with chronic arsenic exposure, oxidative stress is increased and the expression of tumor necrosis factor alpha (TNFalpha) and interleukin-6 (IL-6) is upregulated. Both of these two cytokines have been well known for their effect on the induction of insulin resistance. Arsenite at physiologically relevant concentration also shows inhibitory effect on the expression of peroxisome proliferator-activated receptor gamma (PPARgamma), a nuclear hormone receptor important for activating insulin action. Oxidative stress has been suggested as a major pathogenic link to both insulin resistance and beta cell dysfunction through mechanisms involving activation of nuclear factor-kappaB (NF-kappaB), which is also activated by low levels of arsenic. Although without supportive data, superoxide production induced by arsenic exposure can theoretically impair insulin secretion by interaction with uncoupling protein 2 (UCP2), and oxidative stress can also cause amyloid formation in the pancreas, which could progressively destroy the insulin-secreting beta cells. Individual susceptibility with respect to genetics, nutritional status, health status, detoxification capability, interactions with other trace elements, and the existence of other well-recognized risk factors of diabetes mellitus can influence the toxicity of arsenic on organs involved in glucose metabolism and determine the progression of insulin resistance and impaired insulin secretion to a status of persistent hyperglycemia or diabetes mellitus. In conclusions, insulin resistance and beta cell dysfunction can be induced by chronic arsenic exposure. These defects may be responsible for arsenic-induced diabetes mellitus, but investigations are required to test this hypothesis.

Physiopathologie de l’insulinorésistance

AMONG THE BIOLOGICAL MEDIATORS OF INSULIN RESISTANCE: two compounds released by the adipocyte are found, such as free fatty acids and tumor necrosis factor-alpha. They are incriminated in the deleterious role of visceral adiposity on the metabolic parameters. INTRA-CELL CORTISOL: Attention is also focused on the potential implication of cortisol in the genesis of metabolic syndrome, because cortisol is a potent antagonist of the effect of insulin and its presence in excess enhances visceral obesity and insulin resistance. GENETIC ASPECTS: Although no major locus has yet been identified, recent findings of several mutations or polymorphisms in genes acting in different regulation systems (adiponectin, PPARgamma2) also provide an interesting insight into the pathogenesis of this syndrome. Moreover, there is growing epidemiological evidence that intra-uterine factors could induce a so-called programming of the individual that may, at least in part, account for the difficulties encountered by the classical genetic approach.

Nelfinavir-induced insulin resistance is associated with impaired plasma membrane recruitment of the PI 3-kinase effectors Akt/PKB and PKC-zeta

AIMS/HYPOTHESIS

Chronic exposure of 3T3-L1 adipocytes to the HIV protease inhibitor nelfinavir induces insulin resistance, recapitulating key metabolic alterations of adipose tissue in the lipodystrophy syndrome induced by these agents. Our goal was to identify the defect in the insulin signal transduction cascade leading to nelfinavir-induced insulin resistance.

METHODS

Fully differentiated 3T3-L1 adipocytes were exposed to 30 micro mol/l nelfinavir for 18 h, after which the amount, the phosphorylation and the localisation of key proteins in the insulin signalling cascade were evaluated.

RESULTS

Insulin-induced interaction of phosphatidylinositol 3'-kinase (PI 3-kinase) with IRS proteins was normal in cells treated with nelfinavir, as was IRS-1-associated PI 3-kinase activity. Yet insulin-induced phosphorylation of Akt/protein kinase B (PKB), p70S6 kinase and extracellular signal-regulated kinase 1/2 was significantly impaired. This could not be attributed to increased protein phosphatase 2A activity or to increased expression of phosphoinositide phosphatases (SHIP2 or PTEN). However, insulin failed to induce translocation of the PI 3-kinase effectors Akt/PKB and protein kinase C-zeta (PKC-zeta) to plasma membrane fractions of nelfinavir-treated adipocytes.

CONCLUSIONS/INTERPRETATION

We therefore conclude that nelfinavir induces a defect in the insulin signalling cascade downstream of the activation of PI 3-kinase. This defect manifests itself by impaired insulin-mediated recruitment of Akt/PKB and PKC-zeta to the plasma membrane.

Degradation of pro-insulin-receptor proteins by proteasomes

BACKGROUND

Type-2 diabetes is characterized by hyperinsulinemia, peripheral insulin resistance, and diminished tyrosine phosphorylation activity. It has been recently shown that proteasomes are implicated in the degradation of the insulin receptor substrate-1 (IRS-1) but not in that of the insulin receptor (IR). However, it is unknown whether proteasomes are involved in pro-IR degradation.

METHODS

We used CHO-IR and the 3T3-L1 cells treated with insulin at different concentrations and compared the proteasome activity of IRS-1, IR, and pro-IR degradation either in presence or in absence of lactacystin.

RESULTS

A total of 100 nM of insulin allowed degradation of IRS-1 after 6 h of incubation. At 1,000 nM of insulin, pro-IR degradation began at 1 h of incubation, similar to IRS-1 degradation. Surprisingly, at a higher concentration (10 microM) of insulin, a drastic decrease of proteins was observed from the first minute of incubation. This activity was blocked by lactacystin, a specific proteasome inhibitor.

CONCLUSIONS

According to these results, we propose that pro-IR is degraded by proteasomes.

Human placental growth hormone increases expression of the p85 regulatory unit of phosphatidylinositol 3-kinase and triggers severe insulin resistance in skeletal muscle

The insulin resistance of normal pregnancy is necessary to divert fuels to the fetus to meet fetal growth demands and is mediated by placental hormones. We recently demonstrated that human placental GH (hPGH) can trigger severe insulin resistance in transgenic (TG) mice. In this study we sought to elucidate the cellular mechanisms by which hPGH interferes with insulin signaling in muscle in TG mice. Insulin-stimulated GLUT-4 translocation to the plasma membrane (PM) was reduced in the TG compared with wild-type (WT) mice (P = 0.05). Insulin receptor (IR) levels were modestly reduced by 19% (P < 0.01) in TG mice, but there were no changes in phosphorylation of IR or IR substrate-1 (IRS-1) between WT and TG mice. A singular finding was a highly significant increase in the p85 alpha regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase; P < 0.001), yet a reduced ability of insulin to stimulate IRS-1-associated PI 3-kinase activity (P < 0.05). Although the levels of the p110 catalytic subunit protein of PI 3-kinase and IRS-1 were unchanged in the TG mice, insulin's ability to stimulate p110 association with IRS-1 was markedly reduced (P < 0.0001). We demonstrate a unique mechanism of insulin resistance and suggest that hPGH may contribute to the insulin resistance of normal pregnancy by increasing the expression of the p85 alpha monomer, which competes in a dominant negative fashion with the p85-p110 heterodimer for binding to IRS-1 protein.

Nonalcoholic fatty liver disease in patients with type 2 diabetes

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) is reported commonly in patients with type 2 diabetes mellitus (DM), which has been suggested as a risk factor for the progressive form of NAFLD, or nonalcoholic steatohepatitis. The aim of this study was to assess the outcome of patients with NAFLD and DM.

METHODS

A cohort of patients with NAFLD was identified, and patients with other causes of liver disease (alcohol, medication, etc.) were excluded. Clinical, pathological, and mortality data were available for this cohort. Patients were categorized and compared according to the presence or absence of DM.

RESULTS

Of 132 patients with NAFLD, 44 patients (33%) had an established diagnosis of DM. Patients with DM were older and had greater serum glucose and triglyceride levels and a greater aspartate aminotransferase-alanine aminotransferase ratio. Liver biopsy specimens from patients with DM showed more vacuolated nuclei and acidophilic bodies. Cirrhosis (histological or clinical) occurred in 25% of patients with DM (11 of 44 patients) and NAFLD compared with only 10.2% (9 of 88 patients) of patients without DM with NAFLD (P = 0.04). After adjusting for potential confounders (age, body mass index, and the presence of cirrhosis), both overall mortality (risk ratio [RR], 3.30; 95% confidence interval [CI], 1.76-6.18; P = 0.002) and mortality related to liver disease (RR, 22.83; 95% CI, 2.97-175.03; P = 0.003) were greater in diabetic patients with NAFLD. Markers of hepatic dysfunction (low albumin level, high total bilirubin level, and prolonged prothrombin time) were the only independent predictors of increased mortality.

CONCLUSIONS

Patients with NAFLD and DM are at risk for the development of an aggressive outcome, such as cirrhosis and mortality. This study supports the potential role of insulin resistance in the development of poor clinical outcomes in patients with NAFLD.

Antiproliferative coatings for the treatment of coronary heart disease:. what are the targets and which are the tools?

Since the advent of percutaneous coronary intervention (PCI) for stenosing coronary disease, restenosis has remained a clinical problem. Despite the emergence and evolution of coronary stents, the rate of restenosis following PCI is still 10-20%, and above 50% in high risk subgroups. With increased understanding of the pathophysiology of this process, a number of potential therapeutic targets have been identified, allowing the development of novel therapies against restenosis, which can now be delivered locally using stent platforms. Some of the reported clinical trial data utilizing drug-eluting stents (DES) have produced such profound reductions in clinical and angiographic restenosis that we have been tempted to believe we are on the brink of eradicating this process completely. As the initial excitement subsides, however, there is a need to decide whether these tools will remain effective in real-world interventional practice. In this article we review the pathophysiology of the restenotic process, and the biological targets of the DES therapies currently available in clinical practice. We attempt to define clinical target populations for DES therapy, and assess the impact on outcomes thus far. We consider the advantages that newly emergent stent coatings might offer, and whether targeting specific patient subgroups with unique antiproliferative agents may provide the best chance of limiting restenosis in high risk subgroups. Finally, we consider future strategies to prevent restenosis, with a movement away from the antiproliferative approach, and toward accelerating endothelialization.

Adipose tissue compartments and insulin resistance in overweight-obese Caucasian men

We examined the association between insulin resistance and adipose tissue compartments in overweight/obese men. Total intra-abdominal, intraperitoneal, retroperitoneal, total subcutaneous, anterior subcutaneous and posterior subcutaneous abdominal adipose tissue (PSAAT) masses (total intra-abdominal fat TIAATM, IPATM, RPATM, TSAATM, ASAATM and PSAATM, respectively) were quantified in 51 overweight/obese men using magnetic resonance imaging (MRI). Total adipose tissue mass (TATM) was also determined using bioelectrical impedance. Insulin resistance was estimated by homeostasis model assessment (HOMA) score. In univariate regression analysis, all fat compartments, with the exception of RPATM, were significantly and positively correlated with the HOMA score. The positive correlation between HOMA score and both IPATM and anterior subcutaneous adipose tissue mass was independent of obesity status. After adjusting total for TATM, only IPATM was significantly correlated with HOMA score (partial r=0.38, P<0.01). In stepwise regression, IPATM was the best predictor of HOMA score. In multivariate regression models including age, obesity status, non-esterified fatty acid (NEFAs) levels, triglycerides and energy expenditure, IPATM remained a significant positive predictor (P<0.05) of HOMA score. In overweight/obese men intraperitoneal AT is the fat compartment that best predicts the degree of insulin resistance. This association appears to be independent of age, total body fat mass and obesity status.

Adipose tissue energy metabolism: altered gene expression profile of mice subjected to long-term caloric restriction

We investigated the influences of short-term and lifespan-prolonging long-term caloric restriction (LCR) on gene expression in white adipose tissue (WAT). Over 11,000 genes were examined using high-density oligonucleotide microarrays in four groups of 10- to 11-month-old male C57Bl6 mice that were either fasted for 18 h before death (F), subjected to short-term caloric restriction for 23 days (SCR), or LCR for 9 months and compared with nonfasted control (CO) mice. Only a few transcripts of F and SCR were differentially expressed compared with CO mice. In contrast, 345 transcripts of 6,266 genes found to be expressed in WAT were altered significantly by LCR. The expression of several genes encoding proteins involved in energy metabolism was increased by LCR. Further, many of the shifts in gene expression after LCR are known to occur during adipocyte differentiation. Selected LCR-associated alterations of gene expression were supported by quantitative reverse transcriptase-polymerase chain reaction, histology, and histochemical examinations. Our data provide new insights on the metabolic state associated with aging retardation by LCR.

Suppression of apolipoprotein AI gene expression in HepG2 cells by TNF alpha and IL-1beta

Plasma inflammatory cytokines are elevated in obese subjects as well as in those with type 2 diabetes. This presumably results in systemic insulin resistance, characterized by a pro-atherogenic plasma lipid profile and reduced apolipoprotein AI (apoAI) protein levels. To determine how cytokine-mediated insulin resistance suppresses apoAI gene expression, we investigated the effect of tumor necrosis factor alpha (TNF alpha) and interleukin-1beta (IL-1beta) on apoAI protein, mRNA, and transcriptional activity in the human hepatoma cell line HepG2. ApoAI secretion was suppressed in a dose-dependent manner in HepG2 cells treated with both cytokines. ApoAI protein levels were 2892+/-22.0, 2263+/-117, 2458+/-25.0, 3401+/-152, 2333+/-248, 1520+/-41.5 and 956.0+/-11.0 arbitrary units (AU) in cells treated with 0, 0.3, 1.0, 3.0, 10, 30, and 100 ng/ml TNF alpha, achieving statistical significance in the 30 and 100 ng/ml range (P<0.0009). ApoAI protein levels were 4055+/-360, 3697+/-101, 3347+/-327, 1561+/-33.0, 1581+/-182, 810.0+/-59.5, and 1766+/-717 AU in cells treated with similar doses of IL-1beta, achieving statistical significance within the range of 3-100 ng/ml (P<0.02). ApoAI mRNA levels were suppressed 50.8% in HepG2 cells treated with 30 ng/ml TNF alpha for 24 h (P<0.05), and remained suppressed for up to 96 h. Similarly, treatment of cells with 30 ng/ml IL-1beta for 24 h, resulted in 42.9% reduction in apoAI mRNA levels (P<0.05) and remained suppressed for up to 96 h. In order to determine if the effect of TNF alpha and IL-1beta occurs at the transcriptional level, HepG2 cells were transfected with a chloramphenicol acetyltransferase (CAT) reporter gene plasmid containing the full-length apoAI promoter, and after 24 h, treated with TNF alpha (30 ng/ml), IL-1beta (30 ng/ml), or both cytokines. CAT activity was suppressed by both cytokines (24.0+/-1.9% acetylation in control cells vs. 5.6+/-1.2% (P<0.0004), 10.2+/-1.5% (P<0.0006), and 3.9+/-0.9% acetylation (P<0.0002) in cells treated with TNF alpha, IL-1beta, and the combination of both cytokines, respectively) suggesting that cytokine-mediated suppression occurs at the transcriptional level. Using a series of apoAI deletion constructs, the cytokine response element was mapped between nucleotides -325 and -186 (relative to the transcriptional start site). This region contains a previously identified and characterized cis-element, site A, which binds several different transcription factors. Finally, electrophoretic mobility shift assays (EMSA) showed that TNF alpha treatment of HepG2 cells is associated with reduced nuclear factor binding to site A. These studies suggest that inflammatory cytokines down-regulate apoAI expression at least partly through inhibition of binding of the nuclear factors to site A of the apoAI promoter.

Thiazolidinediones in type 2 diabetes mellitus: current clinical evidence

Type 2 diabetes mellitus is characterised by insulin resistance as well as progressive pancreatic beta cell dysfunction. The cornerstone of current oral blood-glucose lowering therapy consists of metformin, which primarily lowers hepatic glucose production, and the sulphonylureas that act by stimulating pancreatic beta-cells to secrete insulin. Recently, a novel class of agents, the thiazolidinediones, has been introduced that favourably influence insulin sensitivity and possibly also pancreatic beta-cell function. The thiazolidinediones are synthetic ligands that bind to the nuclear peroxisome proliferator-activated receptor-gamma and exert their action by activating transcription of genes that, among others, regulate adipocyte differentiation and adipogenesis as well as glucose and lipid metabolism. To date, the precise mechanisms underlying the actions of thiazolidinediones are largely unknown. When given as monotherapy or in combination with sulphonylureas, metformin or insulin in patients with type 2 diabetes, the currently available thiazolidinediones (rosiglitazone and pioglitazone) ameliorate glycaemic control, by lowering fasting and postprandial blood glucose levels, and improve insulin sensitivity in placebo-controlled trials. They seem to have differential effects on dyslipidaemia in patients with type 2 diabetes; rosiglitazone increases total cholesterol as well as high-density lipoprotein (HDL) and low-density lipoprotein cholesterol levels and affects plasma triglyceride levels depending on the baseline values, whereas pioglitazone lowers triglycerides and increases HDL cholesterol levels. The adverse events of both agents that occur with greater frequency than in patients treated with placebo are fluid retention and oedema. As demonstrated, mainly in preclinical studies to date, rosiglitazone and pioglitazone possess beneficial effects on other cardiovascular risk factors associated with the insulin resistance syndrome. Thus, these agents were shown to decrease blood pressure, enhance myocardial function and fibrinolysis, as well as possess anti-inflammatory and other beneficial vascular effects. Long-term efficacy and surveillance of this promising class of drugs in patients, however, still need to be demonstrated in outcome trials.

Voies de signalisation de l’insuline : mécanismes affectés dans l’insulino-résistance

Insulin has a major anabolic function leading to storage of lipidic and glucidic substrates. All its effects result from insulin binding to a specific membrane receptor which is expressed at a high level on the 3 insulin target tissues: liver, adipose tissue and muscles. The insulin receptor exhibits a tyrosine-kinase activity which leads, first, to receptor autophosphorylation and then to tyrosine phosphorylation of substrates proteins, IRS proteins in priority. This leads to the formation of macromolecular complexes close to the receptor. The two main transduction pathways are the phosphatidylinositol 3 kinase pathway activating protein kinase B which is involved in priority in metabolic effects, and the MAP kinase pathway involved in nuclear effects, proliferation and differentiation. However, in most cases, a specific effect of insulin requires the participation of the two pathways in a complex interplay which could explain the pleiotropy and the specificity of the insulin signal. The negative control of the insulin signal can result from hormone degradation or receptor dephosphorylation. However, the major negative control results from phosphorylation of serine/threonine residues on the receptor and/or IRS proteins. This phosphorylation is activated in response to different signals involved in insulin resistance, hyperinsulinism, TNFalpha or increased free fatty acids from adipose tissue, which are transformed inside the cell in acyl-CoA. A deleterious role for molecules issued from the adipose tissue is postulated in the resistance to insulin of the liver and muscles present in type 2 diabetes, obesity and metabolic syndrome.

Physiological and pathophysiological functions of the ecto-nucleotide pyrophosphatase/phosphodiesterase family

The ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) multigene family contains five members. NPP1-3 are type II transmembrane metalloenzymes characterized by a similar modular structure composed of a short intracellular domain, a single transmembrane domain and an extracellular domain containing a conserved catalytic site. The short intracellular domain of NPP1 has a basolateral membrane-targeting signal while NPP3 is targeted to the apical surface of polarized cells. NPP4-5 detected by database searches have a predicted type I membrane orientation but have not yet been functionally characterized. E-NPPs have been detected in almost all tissues often confined to specific substructures or cell types. In some cell types, NPP1 expression is constitutive or can be induced by TGF-beta and glucocorticoids, but the signal transduction pathways that control expression are poorly documented. NPP1-3 have a broad substrate specificity which may reflect their role in a host of physiological and biochemical processes including bone mineralization, calcification of ligaments and joint capsules, modulation of purinergic receptor signalling, nucleotide recycling, and cell motility. Abnormal NPP expression is involved in pathological mineralization, crystal depositions in joints, invasion and metastasis of cancer cells, and type 2 diabetes. In this review we summarize the present knowledge on the structure and the physiological and biochemical functions of E-NPP and their contribution to the pathogenesis of diseases.

Extracellular matrix proteins modulate endocytosis of the insulin receptor

Internalization of the insulin receptor (IR) is a highly regulated multi-step process whose underlying molecular basis is not fully understood. Here we undertook to study the role of extracellular matrix (ECM) proteins in the modulation of IR internalization. Employing Chinese hamster ovary cells that overexpress IR (CHO-T cells), our results indicate that IR internalization proceeds unaffected even when Tyr phosphorylation of IR substrates, such as IRS-1, is impaired (e.g. in CHO-T cells overexpressing IRS-1 whose pleckstrin-homology domain has been deleted or in CHO-T cells that overexpress the PH/PTB domain of IRS-1). In contrast, IR internalization is affected by the context of the ECM proteins to which the cells adhere. Hence, IR internalization was inhibited 40-60% in CHO-T cells adherent onto galectin-8 (an ECM protein and an integrin ligand of the galectin family) when compared with cells adherent onto fibronectin, collagen, or laminin. Cells adherent to galectin-8 manifested a unique cytoskeletal organization, which involved formation of cortical actin and generation of F-actin microspikes that contrasted with the prominent stress-fibers formed when cells adhered to fibronectin. To better establish a role for actin filament organization in IR endocytosis, this process was assayed in CHO-T cells (adherent onto fibronectin), whose actin filaments were disrupted upon treatment with latrunculin B. Latrunculin B did not affect insulin-induced Tyr phosphorylation of IR or its ability to phosphorylate its substrates; still, a 30-50% reduction in the rate of IR internalization was observed in cells treated with latrunculin B. Treatment of cells with nocodazole, which disrupts formation of microtubules, did not affect IR internalization. These results indicate that proper actin, but not microtubular, organization is a critical requirement for IR internalization and suggest that integrin-mediated signaling pathways emitted upon cell adhesion to different extracellular matrices and the altered cytoskeletal organizations generated thereof affect the itinerary of the insulin receptor.

Rapid induction of insulin resistance in opioid mu-receptor knock-out mice

Role of opioid mu-receptor that has been showed to involve in the insulin-dependent diabetic rats in the induction of insulin resistance remains unclear. The present study is performed to clarify this point. The wild-type mice or opioid mu-receptor knockout mice were employed to induce insulin resistance by feeding with fructose-rich chow or by the repeated intraperitoneal injections of long-acting form of human insulin at 0.5 IU/kg three times daily. The basal plasma glucose concentration was not markedly changed in fructose-fed mice regardless the presence of opioid mu-receptor or not. However, the plasma glucose lowering activity of tolbutamide (10.0 mg/kg) disappeared rapidly in opioid mu-receptor knockout mice receiving fructose-rich chow as compared to that in wild type group. In opioid mu-receptor knockout mice, the elevation of plasma glucose concentration and the loss of plasma glucose lowering activity of tolbutamide were observed at 15 days after insulin injection. However, similar change was obtained at 21 days later of insulin injection in wild-type mice, showing that decrease of insulin action was more markedly in opioid mu-receptor knockout mice. Our results indicated that opioid mu-receptor is related to the delay of insulin resistance induced by fructose-fed method or insulin repeated injection.

Modulation of insulin-stimulated degradation of human insulin receptor substrate-1 by Serine 312 phosphorylation

Ser/Thr phosphorylation of insulin receptor substrate-1 (IRS-1) is a negative regulator of insulin signaling. One potential mechanism for this is that Ser/Thr phosphorylation decreases the ability of IRS-1 to be tyrosine-phosphorylated by the insulin receptor. An additional mechanism for modulating insulin signaling is via the down-regulation of IRS-1 protein levels. Insulin-induced degradation of IRS-1 has been well documented, both in cells as well as in patients with diabetes. Ser/Thr phosphorylation of IRS-1 correlates with IRS-1 degradation, yet the details of how this occurs are still unknown. In the present study we have examined the potential role of different signaling cascades in the insulin-induced degradation of IRS-1. First, we found that inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin block the degradation. Second, knockout cells lacking one of the key effectors of this cascade, the phosphoinositide-dependent kinase-1, were found to be deficient in the insulin-stimulated degradation of IRS-1. Conversely, overexpression of this enzyme potentiated insulin-stimulated IRS-1 degradation. Third, concurrent with the decrease in IRS-1 degradation, the inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin also blocked the insulin-stimulated increase in Ser(312) phosphorylation. Most important, an IRS-1 mutant in which Ser(312) was changed to alanine was found to be resistant to insulin-stimulated IRS-1 degradation. Finally, an inhibitor of c-Jun N-terminal kinase, SP600125, at 10 microm did not block IRS-1 degradation and IRS-1 Ser(312) phosphorylation yet completely blocked insulin-stimulated c-Jun phosphorylation. Further, insulin-stimulated c-Jun phosphorylation was not blocked by inhibitors of the phosphatidylinositol 3-kinase and mammalian target of rapamycin, indicating that c-Jun N-terminal kinase is unlikely to be the kinase phosphorylating IRS-1 Ser(312) in response to insulin. In summary, our results indicate that the insulin-stimulated degradation of IRS-1 via the phosphatidylinositol 3-kinase pathway is in part dependent upon the Ser(312) phosphorylation of IRS-1.

Differential regulation of lipoprotein kinetics by atorvastatin and fenofibrate in subjects with the metabolic syndrome

The metabolic syndrome is characterized by insulin resistance and abnormal apolipoprotein AI (apoAI) and apolipoprotein B-100 (apoB) metabolism that may collectively accelerate atherosclerosis. The effects of atorvastatin (40 mg/day) and micronised fenofibrate (200 mg/day) on the kinetics of apoAI and apoB were investigated in a controlled cross-over trial of 11 dyslipidemic men with the metabolic syndrome. ApoAI and apoB kinetics were studied following intravenous d(3)-leucine administration using gas-chromatography mass spectrometry with data analyzed by compartmental modeling. Compared with placebo, atorvastatin significantly decreased (P < 0.001) plasma concentrations of cholesterol, triglyceride, LDL cholesterol, VLDL apoB, intermediate-density lipoprotein (IDL) apoB, and LDL apoB. Fenofibrate significantly decreased (P < 0.001) plasma triglyceride and VLDL apoB and elevated HDL(2) cholesterol (P < 0.001), HDL(3) cholesterol (P < 0.01), apoAI (P = 0.01), and apoAII (P < 0.001) concentrations, but it did not significantly alter LDL cholesterol. Atorvastatin significantly increased (P < 0.002) the fractional catabolic rate (FCR) of VLDL apoB, IDL apoB, and LDL apoB but did not affect the production of apoB in any lipoprotein fraction or in the turnover of apoAI. Fenofibrate significantly increased (P < 0.01) the FCR of VLDL, IDL, and LDL apoB but did not affect the production of VLDL apoB. Relative to placebo and atorvastatin, fenofibrate significantly increased the production (P < 0.001) and FCR (P = 0.016) of apoAI. Both agents significantly lowered plasma triglycerides and apoCIII concentrations, but only atorvastatin significantly lowered (P < 0.001) plasma cholesteryl ester transfer protein activity. Neither treatment altered insulin resistance. In conclusion, these differential effects of atorvastatin and fenofibrate on apoAI and apoB kinetics support the use of combination therapy for optimally regulating dyslipoproteinemia in the metabolic syndrome.

Impaired insulin action on phosphatidylinositol 3-kinase activity and glucose transport in skeletal muscle of pancreatic cancer patients

INTRODUCTION

Glucose intolerance or overt diabetes occurs in 80% of patients with pancreatic cancer (PC). This associated metabolic disorder includes peripheral insulin resistance, which may be caused by factors produced by the PC. The mechanism underlying PC-associated insulin resistance has not been clearly defined.

AIM

To characterize basal and insulin-stimulated glucose transport, phosphatidylinositol (PI) 3-kinase activity, and glucose transporter 4 (GLUT4) in skeletal muscles of PC patients.

METHODOLOGY

Skeletal muscle samples were obtained from the abdominal wall of 17 PC patients during surgery. Control muscles were sampled in the same way from 11 donors undergoing abdominal surgery for benign diseases. PI 3-kinase activity, glucose transport, and GLUT4 were assessed in vitro in these muscles.

RESULTS

In the presence of physiologic concentrations of insulin, glucose transport and PI 3-kinase activity were significantly decreased in the PC group compared with controls. At supraphysiologic insulin concentrations, glucose transport was significantly decreased but PI 3-kinase activity was normalized. In the absence of insulin, these parameters were not significantly different between PC and control groups. Muscle GLUT4 contents were similar between PC and control groups.

CONCLUSION

Defects in insulin-mediated PI 3-kinase activity and glucose transport contribute to the insulin resistance in patients with PC.

Lower blood glucose, hyperglucagonemia, and pancreatic alpha cell hyperplasia in glucagon receptor knockout mice

Glucagon, the counter-regulatory hormone to insulin, is secreted from pancreatic alpha cells in response to low blood glucose. To examine the role of glucagon in glucose homeostasis, mice were generated with a null mutation of the glucagon receptor (Gcgr(-/-)). These mice display lower blood glucose levels throughout the day and improved glucose tolerance but similar insulin levels compared with control animals. Gcgr(-/-) mice displayed supraphysiological glucagon levels associated with postnatal enlargement of the pancreas and hyperplasia of islets due predominantly to alpha cell, and to a lesser extent, delta cell proliferation. In addition, increased proglucagon expression and processing resulted in increased pancreatic glucogen-like peptide 1 (GLP-1) (1-37) and GLP-1 amide (1-36 amide) content and a 3- to 10-fold increase in circulating GLP-1 amide. Gcgr(-/-) mice also displayed reduced adiposity and leptin levels but normal body weight, food intake, and energy expenditure. These data indicate that glucagon is essential for maintenance of normal glycemia and postnatal regulation of islet and alpha and delta cell numbers. Furthermore, the lean phenotype of Gcgr(-/-) mice suggests glucagon action may be involved in the regulation of whole body composition.

Interaction of iron, insulin resistance, and nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD) has emerged as a ubiquitous liver disorder with occasional serious overtones. Although diabetes and obesity were initially held culpable, insulin resistance (IR) is now considered the fundamental operative mechanism. IR is probably the "first step" in nonalcoholic steatohepatitis (NASH). Oxidative stress may be the elusive "second" of possibly multiple steps in the progression of steatosis to fibrosing steatohepatitis. Because hepatic iron promotes oxidative stress, it was mooted as a contributory cofactor in NASH. This proposal was strengthened by an association with hepatic fibrosis. Subsequent studies have shown neither a significant increase in hepatic iron nor an association between hepatic iron and any of the histologic determinants in NASH. Likewise, the increased prevalence of hemochromatosis gene (HFE) mutations in some studies appears to be largely irrelevant to the development of hepatic fibrosis. Excess hepatic iron may occur in insulin resistance-associated iron overload (IRHIO), characterized by hyperferritinemia with normal to mild increases in transferrin saturation. Although patients with IRHIO have a high prevalence of IR-related metabolic disorders, the relationship of IRHIO to NASH is unclear. A recent study showed improvement in insulin sensitivity with the use of venesection in patients with NAFLD, but this approach cannot be implemented without extensive review.

Insulin resistance and prostate cancer risk

Because high waist-to-hip ratio (WHR) and high serum insulin levels have been reported to be associated with an increased risk of prostate cancer, we assessed the relationship between insulin resistance and prostate cancer risk in Chinese men. We measured fasting serum glucose and insulin levels in 128 case and 306 control subjects and used the homeostasis model assessment to derive indices of insulin sensitivity and resistance. Relative to men in the lowest tertiles, men in the highest tertile of insulin sensitivity had a reduced risk of prostate cancer (odds ratio [OR] = 0.35, 95% confidence interval [CI] = 0.21 to 0.60), but men in the highest tertile of insulin resistance had an increased risk of prostate cancer (OR = 2.78, 95% CI = 1.63 to 4.72). Considering insulin resistance and WHR together, the effect of insulin resistance was apparent in all tertiles of WHR, with men in the highest tertile of insulin resistance and WHR having the highest risk (OR = 8.21, 95% CI = 2.84 to 23.70). The associations between prostate cancer risk and insulin sensitivity or resistance were independent of total caloric intake and serum levels of insulin-like growth factors, sex hormones, and sex hormone-binding globulin. Because of the retrospective design of this study, the role of insulin resistance in prostate cancer needs to be confirmed in prospective studies.

Increased frequency of the G972R variant of the insulin receptor substrate-1 (irs-1) gene among girls with a history of precocious pubarche

To test the hypothesis that lower sex hormone-binding globulin (SHBG) concentrations are associated with heterozygosity for the G972R variant of the IRS-1 gene among adolescent girls with a history of precocious pubarche (PP) and hyperinsulinemic ovarian hyperandrogenism.Association study. Academic research environment. Adolescent girls with a history of PP and healthy adolescent female control subjects. Determine body mass index; measure serum androgen, insulin-like growth factor (IGF)-binding protein 1, lipids, IGF-1, and SHBG concentrations; perform glucose tolerance tests; and assay for G972R variant of the IRS-1 gene. Serum androgen, IGFBP-1, and SHBG concentrations; IRS-1 genotypes.Twenty-five of 54 (45%) girls with a history of PP developed hyperinsulinemic ovarian hyperandrogenism at adolescence. Frequency of heterozygosity for G972 was 31% among girls with a history of PP, 40% among girls with hyperinsulinemic ovarian hyperandrogenism, and 19% among healthy control subjects. Sex hormone-binding globulin concentrations were lower among girls heterozygous for G972R variant. Predictors of progression from PP to hyperinsulinemic ovarian hyperandrogenism included chronological age, insulin, low-density lipoprotein cholesterol, and IGFBP-1 concentrations. The low mean SHBG concentration found among G972R carriers suggests that this variant may be a minor locus associated with development of hyperinsulinemic insulin resistance and ovarian androgen excess in girls with a history of PP.

Regulatory effects of HMG CoA reductase inhibitor and fish oils on apolipoprotein B-100 kinetics in insulin-resistant obese male subjects with dyslipidemia

Hepatic accumulation of lipid substrates perturbs apolipoproteinB-100 (apoB) metabolism in insulin-resistant, obese subjects and may account for increased risk of cardiovascular disease. In a placebo-controlled trial, we examined the independent and combined effects of decreasing cholesterol synthesis with atorvastatin (40 mg/day) and triglyceride synthesis with fish oils (4 g/day) on apoB kinetics. The subjects were 48 viscerally obese, insulin-resistant men with dyslipidemia who were studied in a fasted state. We found that atorvastatin significantly decreased plasma apoB-containing lipoproteins (P < 0.001, main effect) through increases in the fractional catabolic rate (FCR) of VLDL-, IDL-, and LDL-apoB (P < 0.01). Fish oils significantly decreased plasma levels of triglycerides and VLDL-apoB (P < 0.001), decreased the VLDL-apoB secretion rate (P < 0.01), but increased the conversion of VLDL to LDL (P < 0.001). Compared with placebo, combined treatment with atorvastatin and fish oils decreased VLDL-apoB secretion (P < 0.03) and increased the FCR of apoB in each lipoprotein fraction (P < 0.03) and the percent conversion of VLDL to LDL (P < 0.05). None of the treatments altered insulin resistance. In conclusion, in visceral obesity, atorvastatin increased hepatic clearance of all apoB-containing lipoproteins, whereas fish oils decreased hepatic secretion of VLDL-apoB. The differential effects of atorvastatin and fish oils on apoB kinetics support their combined use in correcting defective apoB metabolism in obese, insulin-resistant subjects.

Serum interleukin 2 levels in patients with rheumatoid arthritis and correlation with insulin sensitivity

Interleukin 2 (IL-2), a Th1 lymphocyte-derived cytokine, is thought to play an important role in the pathogenesis of type 2 diabetes and rheumatoid arthritis (RA). The aim of our study was to evaluate changes in serum IL-2 levels and their correlation with glucose metabolism abnormalities, such as insulin resistance, in patients with RA. Thirty-six subjects with varying degrees of disease activity and 20 healthy age-, sex- and body mass index-matched control individuals were evaluated. Patients with any causes of peripheral insulin resistance were excluded. After a 12-h overnight fast, fasting insulin levels, homeostatic model assessment-insulin resistance (HOMA-IR) estimated insulin sensitivity, and serum IL-2 levels were significantly higher in all patients with RA than in the control individuals. Fasting insulin, HOMA-IR scores and IL-2 levels were correlated in the RA group. This study showed that patients with RA have altered IL-2 regulation, and that there was a significant correlation between serum IL-2 levels and insulin sensitivity.

Waging war on physical inactivity: using modern molecular ammunition against an ancient enemy

A hypothesis is presented based on a coalescence of anthropological estimations of Homo sapiens' phenotypes in the Late Paleolithic era 10,000 years ago, with Darwinian natural selection synergized with Neel's idea of the so-called thrifty gene. It is proposed that humans inherited genes that were evolved to support a physically active lifestyle. It is further postulated that physical inactivity in sedentary societies directly contributes to multiple chronic health disorders. Therefore, it is imperative to identify the underlying genetic and cellular/biochemical bases of why sedentary living produces chronic health conditions. This will allow society to improve its ability to effect beneficial lifestyle changes and hence improve the overall quality of living. To win the war against physical inactivity and the myriad of chronic health conditions produced because of physical inactivity, a multifactorial approach is needed, which includes successful preventive medicine, drug development, optimal target selection, and efficacious clinical therapy. All of these approaches require a thorough understanding of fundamental biology and how the dysregulated molecular circuitry caused by physical inactivity produces clinically overt disease. The purpose of this review is to summarize the vast armamentarium at our disposal in the form of the extensive scientific basis underlying how physical inactivity affects at least 20 of the most deadly chronic disorders. We hope that this information will provide readers with a starting point for developing additional strategies of their own in the ongoing war against inactivity-induced chronic health conditions.

Insulin resistance, polycystic ovary syndrome, and type 2 diabetes mellitus

OBJECTIVE

To review the definition and prevalence of two insulin resistance (IR)-associated phenotypes, polycystic ovary syndrome (PCOS) and type 2 diabetes mellitus, as well as the risk and nature of their simultaneous presentation.

DESIGN

Review of published literature.

RESULT(S)

Insulin resistance affects between 10% and 25% of the general population. Two common disorders frequently associated with IR are PCOS, affecting 4% to 6% of reproductive-aged women, and type 2 diabetes mellitus, which is observed in about 2% to 6% of similarly aged women. Overall, about 50% to 70% of women with PCOS and 80% to 100% of patients with type 2 diabetes mellitus have variable degrees of IR. Insulin resistance and its secondary hyperinsulinemia appear to underlie many of the endocrine features of PCOS in a large proportion of such patients. The risk of type 2 diabetes mellitus among PCOS patients is 5- to 10-fold higher than normal. In turn, the risk of PCOS among reproductive-aged type 2 diabetes mellitus patients appears to be similarly increased.

CONCLUSION(S)

It remains to be determined whether PCOS and type 2 diabetes mellitus represent no more than different clinical manifestations of the same IR syndrome, with their phenotypic differences due to the presence or absence of a coincidental genetic defect at the level of the ovary or pancreas, respectively, or representing the result of etiologically different subtypes of IR syndromes.

Clinical review 145: Pleiotropic effects of statins: lipid reduction and beyond

There is accumulating evidence that statins have beneficial effects that are independent of their classical actions on lipoproteins. These effects include reductions in inflammation in the vasculature, kidney, and bone. Potential beneficial effects of these agents include enhancement of nitric oxide production in vasculature and the kidney. These agents appear to reduce bone fractures and may improve insulin sensitivity and reduce the likelihood of persons progressing from impaired glucose tolerance to type II diabetes. Potential beneficial pleiotropic effects of statins are covered in this review.