Alter Insulin Action Research Articles

Effect of cooling heat-stressed dairy cows during the dry period on insulin response

Heat stress (HT) during the dry period affects hepatic gene expression and adipose tissue mobilization during the transition period. In addition, it is postulated that HT may alter insulin action on peripheral tissues. Our objective was to evaluate the effect of cooling heat-stressed cows during the dry period on insulin effects on peripheral tissues during the transition period. Cows were dried off 46d before expected calving and assigned to 1 of 2 treatments: HT (n = 16) or cooling (CL, n = 16). During the dry period, the average temperature-humidity index was 78, but CL cows were cooled with sprinklers and fans, whereas HT cows were not. After calving, all cows were housed and managed under the same conditions. Rectal temperatures were measured twice daily (0730 and 1430h) and respiration rate recorded 3 times weekly during the dry period. Dry matter intake was recorded daily from dry-off to 42d relative to calving (DRC). Body weight and body condition score were measured weekly from dry-off to 42 DRC. Milk yield and composition were recorded daily to 42wk postpartum. Glucose tolerance tests (GTT) and insulin challenges (IC) were performed at dry-off, −14, 7, and 28 DRC in a subset of cows (HT, n = 8; CL, n = 8). Relative to HT, CL cows had lower rectal temperatures (39.3 vs. 39.0°C) in the afternoon and respiration rate (69 vs. 48 breath/min). Cows from the cooling treatment tended to consume more feed than HT cows prepartum and postpartum. Compared with HT, CL cows gained more weight before calving but lost more weight and body condition in early lactation. Cows from the cooling treatment produced more milk than HT cows (34.0 vs. 27.7kg/d), but treatments did not affect milk composition. Treatments did not affect circulating insulin and metabolites prepartum, but CL cows had decreased glucose, increased nonesterified fatty acid, and tended to have lower insulin concentrations in plasma postpartum compared with HT cows. Cooling prepartum HT cows did not affect the insulin responses to GTT and IC during the transition period and glucose responses to GTT and IC at −14 and 28 DRC were not affected by treatments. At 7 DRC, CL cows tended to have slower glucose clearance to GTT and weaker glucose response to IC relative to HT cows. Cows from the cooling treatment had stronger nonesterified fatty acid responses to IC postpartum but not prepartum compared with HT. In conclusion, cooling heat-stressed dairy cows in the dry period reduced insulin effects on peripheral tissues in early lactation but not in the dry period.

Brain meets body: the blood-brain barrier as an endocrine interface.

The blood-brain barrier (BBB) separates the central nervous system (CNS) from the peripheral tissues. However, this does not prevent hormones from entering the brain, but shifts the main control of entry to the BBB. In general, steroid hormones cross the BBB by transmembrane diffusion, a nonsaturable process resulting in brain levels that reflect blood levels, whereas thyroid hormones and many peptides and regulatory proteins cross using transporters, a saturable process resulting in brain levels that reflect blood levels and transporter characteristics. Protein binding, brain-to-blood transport, and pharmacokinetics modulate BBB penetration. Some hormones have the opposite effect within the CNS than they do in the periphery, suggesting that these hormones cross the BBB to act as their own counterregulators. The cells making up the BBB are also endocrine like, both responding to circulating substances and secreting substances into the circulation and CNS. By dividing a hormone's receptors into central and peripheral pools, the former of which may not be part of the hormone's negative feed back loop, the BBB fosters the development of variable hormone resistance syndromes, as exemplified by evidence that altered insulin action in the CNS can contribute to Alzheimer's disease. In summary, the BBB acts as a regulatory interface in an endocrine-like, humoral-based communication between the CNS and peripheral tissues.

The Insulin-Mediated Modulation of Visually Evoked Magnetic Fields Is Reduced in Obese Subjects

BackgroundInsulin is an anorexigenic hormone that contributes to the termination of food intake in the postprandial state. An alteration in insulin action in the brain, named “cerebral insulin resistance”, is responsible for overeating and the development of obesity.Methodology/Principal FindingsTo analyze the direct effect of insulin on food-related neuronal activity we tested 10 lean and 10 obese subjects. We conducted a magnetencephalography study during a visual working memory task in both the basal state and after applying insulin or placebo spray intranasally to bypass the blood brain barrier. Food and non-food pictures were presented and subjects had to determine whether or not two consecutive pictures belonged to the same category.Intranasal insulin displayed no effect on blood glucose, insulin or C-peptide concentrations in the periphery; however, it led to an increase in the components of evoked fields related to identification and categorization of pictures (at around 170 ms post stimuli in the visual ventral stream) in lean subjects when food pictures were presented. In contrast, insulin did not modulate food-related brain activity in obese subjects.Conclusions/SignificanceWe demonstrated that intranasal insulin increases the cerebral processing of food pictures in lean whereas this was absent in obese subjects. This study further substantiates the presence of a “cerebral insulin resistance” in obese subjects and might be relevant in the pathogenesis of obesity.

Cardiac Complications of Obesity

Obesity is an independent risk factor for the development of heart failure. Although the mechanisms are multifactorial and include increased coronary atherosclerosis and ischemic heart disease, increased hypertension and obstructive sleep apnea, there is growing evidence that intrinsic defects within the cardiomyocyte might contribute to the increased vulnerability of the heart in these subjects. Given the close association between obesity, insulin resistance and type 2 diabetes, it is not always straightforward to distinguish between those changes that can truly be attributable to obesity, versus those that are secondary to systemic metabolic changes such as insulin resistance or impaired glucose tolerance. Obesity and insulin resistance are associated with changes in myocardial substrate utilization (increased FA uptake and oxidation and decreased glucose utilization), mitochondrial dysfunction (decreased oxidative phosphorylation and mitochondrial uncoupling), oxidative stress, increased apoptosis, accumulation of toxic lipid intermediates (lipotoxicity), altered myocardial insulin action and altered myocardial calcium handling, which impair cardiomyocyte function.

FoxO1 Links Hepatic Insulin Action to Endoplasmic Reticulum Stress

Forkhead box O1 (FoxO1) is a transcription factor that mediates the inhibitory effect of insulin on target genes in hepatic metabolism. Hepatic FoxO1 activity is up-regulated to promote glucose production during fasting and is suppressed to limit postprandial glucose excursion after meals. Increased FoxO1 activity augments the expression of insulin receptor (IR) and IR substrate (IRS)2, which in turn inhibits FoxO1 activity in response to reduced insulin action. To address the underlying physiology of such a feedback loop for regulating FoxO1 activity, we delivered FoxO1-ADA by adenovirus-mediated gene transfer into livers of adult mice. FoxO1-ADA is a constitutively active allele that is refractory to insulin inhibition, allowing us to determine the metabolic effect of a dislodged FoxO1 feedback loop in mice. We show that hepatic FoxO1-ADA production resulted in significant induction of IR and IRS2 expression. Mice with increased FoxO1-ADA production exhibited near glycogen depletion. Unexpectedly, hepatic FoxO1-ADA production elicited a profound unfolded protein response, culminating in the induction of hepatic glucose-regulated protein 78 (GRP78) expression. These findings were recapitulated in primary human and mouse hepatocytes. FoxO1 targeted GRP78 gene for trans-activation via selective binding to an insulin responsive element in the GRP78 promoter. This effect was counteracted by insulin. Our studies underscore the importance of an IR and IRS2-dependent feedback loop to keep FoxO1 activity in check for maintaining hepatic glycogen homeostasis and promoting adaptive unfolded protein response in response to altered metabolism and insulin action. Excessive FoxO1 activity, resulting from a dislodged FoxO1 feedback loop in insulin resistant liver, is attributable to hepatic endoplasmic reticulum stress and metabolic abnormalities in diabetes.

Interventional Studies for Polycystic Ovarian Syndrome in Children and Adolescents

Polycystic ovarian syndrome (PCOS) is characterized by chronic anovulation, clinical and/or biochemical hyperandrogenism, which can be associated with altered insulin action. Symptoms usually begin around menarche, but onset after puberty may also occur as a result of environmental modifiers such as weight gain. The consequences of PCOS extend beyond the reproductive axis; there is a substantial risk for development of metabolic and cardiovascular abnormalities similar to the metabolic syndrome. Currently, the treatment is targeted to the patient's primary complaint such as hirsutism, restoration of regular menses or pregnancy. Pharmacological agents available for the treatment of hirsutism include androgen suppressors and peripheral androgen blockers. Recently, our understanding of the role of insulin resistance has led to the use of insulin-sensitizing medications as first-choice therapy. In conjunction with weight reduction and exercise, a pharmacologic reduction in insulin levels by either metformin or thiazolidinediones ameliorates both hyperinsulinemia and hyperandrogenism.

Insulin-mediated cortical activity in the slow frequency range is diminished in obese mice and promotes physical inactivity

There is evidence from mouse models and humans that alterations in insulin action in the brain are accompanied by an obese phenotype; however, the impact of insulin with regard to behavioural aspects such as locomotion is unknown. To address insulin action in the brain with regard to cortical activity in distinct frequency bands and the behavioural consequences, the insulin signalling pathway was followed from the receptor to electrical activity and locomotion. Western blot analysis, electrocorticograms with intracerebroventricular (i.c.v.) application of insulin, and measurements of locomotor activity were performed in lean and obese, as well as Toll-like receptor (TLR) 2/4-deficient, mice. We show that insulin application i.c.v. into lean mice was accompanied by a profound increase in cortical activity in the slow frequency range, while diet-induced obese mice displayed insulin resistance. In parallel, insulin administered i.c.v. increased locomotor activity in lean mice, whereas a phosphatidylinositol-3 (PI3) kinase inhibitor or obesity abolished insulin-mediated locomotion. A potential candidate that links insulin signalling to locomotion is the Kv1.3 channel that is activated by PI3-kinase. Pharmacological inhibition of Kv1.3 channels that bypassed insulin receptor activation promoted activity. Moreover, mice deficient in TLR2/4-dependent signalling displayed an increase in cortical activity in the slow frequency range that was correlated with improved spontaneous and insulin-mediated locomotor activity. Our data provide functional evidence for a direct effect of insulin on brain activation patterns in the slow frequency bands and locomotor activity in lean mice, while in obese mice, insulin-mediated locomotion is blunted and further aggravates physical inactivity.

Intramuscular Lipid Metabolism in the Insulin Resistance of Smoking

OBJECTIVESmoking decreases insulin action and increases the risk of type 2 diabetes in humans. Mechanisms responsible for smoking-induced insulin resistance are unclear. We hypothesized smokers would have increased intramuscular triglyceride (IMTG) and diacylglycerol (DAG) concentration and decreased fractional synthesis rate (FSR) compared with nonsmokers.RESEARCH DESIGN AND METHODSNonsmokers (n = 18, aged 20 ± 0.5 years, BMI 22 ± 0.4 kg/m2, body fat 20 ± 2%, 0 cigarettes per day) and smokers (n = 14, aged 21 ± 0.7 years, BMI 23 ± 0.4 kg/m2, body fat 20 ± 3%, 18 ± 0.7 cigarettes per day) were studied in a fasted condition after a standardized diet. [U-13C]palmitate was infused during 4 h of rest followed by a skeletal muscle biopsy and intravenous glucose tolerance test.RESULTSSmokers were less insulin sensitive (Si) compared with nonsmokers (Si 5.28 ± 0.5 nonsmokers vs. 3.74 ± 0.3 smokers 10−4 · μU−1 · ml−1, P = 0.03). There were no differences in IMTG or DAG concentration (IMTG 24.2 ± 3.4 nonsmokers vs. 27.2 ± 5.9 smokers μg/mg dry wt, DAG 0.34 ± 0.02 nonsmokers vs. 0.35 ± 0.02 smokers μg/mg dry wt) or IMTG FSR between groups (0.66 ± 0.1 nonsmokers vs. 0.55 ± 0.09 smokers %/hr). Intramuscular lipid composition was different, with increased percent saturation of IMTG (32.1 ± 1.2 nonsmokers vs. 35.2 ± 1.0 smokers %, P = 0.05) and DAG (52.8 ± 1.7 nonsmokers vs. 58.8 ± 2.2 smokers %, P = 0.04) in smokers. Smokers had significantly decreased peroxisome proliferator–activated receptor-γ (1.76 ± 0.1 nonsmokers vs. 1.42 ± 0.11 smokers arbitrary units [AU], P = 0.03) and increased monocyte chemotactic protein-1 (3.11 ± 0.41 nonsmokers vs. 4.83 ± 0.54 smokers AU, P = 0.02) mRNA expression compared with nonsmokers. We also found increased insulin receptor substrate-1 Ser636 phosphorylation in smokers compared with nonsmokers (0.73 ± 0.08 nonsmokers vs. 1.14 ± 0.09 smokers AU, P = 0.002).CONCLUSIONSThese data suggest: 1) IMTG concentration and turnover are not related to alterations in insulin action in smokers compared to nonsmokers, 2) increased saturation of IMTG and DAG in skeletal muscle may be related to insulin action, and 3) basal inhibition of insulin receptor substrate-1 may decrease insulin action in smokers.

Contribution of Impaired Myocardial Insulin Signaling to Mitochondrial Dysfunction and Oxidative Stress in the Heart

Diabetes-associated cardiac dysfunction is associated with mitochondrial dysfunction and oxidative stress, which may contribute to left ventricular dysfunction. The contribution of altered myocardial insulin action, independent of associated changes in systemic metabolism, is incompletely understood. The present study tested the hypothesis that perinatal loss of insulin signaling in the heart impairs mitochondrial function. In 8-week-old mice with cardiomyocyte deletion of insulin receptors (CIRKO), inotropic reserves were reduced, and mitochondria manifested respiratory defects for pyruvate that was associated with proportionate reductions in catalytic subunits of pyruvate dehydrogenase. Progressive age-dependent defects in oxygen consumption and ATP synthesis with the substrate glutamate and the fatty acid derivative palmitoyl-carnitine were observed. Mitochondria also were uncoupled when exposed to palmitoyl-carnitine, in part as a result of increased reactive oxygen species production and oxidative stress. Although proteomic and genomic approaches revealed a reduction in subsets of genes and proteins related to oxidative phosphorylation, no reductions in maximal activities of mitochondrial electron transport chain complexes were found. However, a disproportionate reduction in tricarboxylic acid cycle and fatty acid oxidation proteins in mitochondria suggests that defects in fatty acid and pyruvate metabolism and tricarboxylic acid flux may explain the mitochondrial dysfunction observed. Impaired myocardial insulin signaling promotes oxidative stress and mitochondrial uncoupling, which, together with reduced tricarboxylic acid and fatty acid oxidative capacity, impairs mitochondrial energetics. This study identifies specific contributions of impaired insulin action to mitochondrial dysfunction in the heart.

Dipeptidyl Peptidase-4 Inhibition by Vildagliptin and the Effect on Insulin Secretion and Action in Response to Meal Ingestion in Type 2 Diabetes

OBJECTIVE—The purpose of this study was to determine the mechanism by which dipeptidyl peptidase-4 inhibitors lower postprandial glucose concentrations.RESEARCH DESIGN AND METHODS—We measured insulin secretion and action as well as glucose effectiveness in 14 subjects with type 2 diabetes who received vildagliptin (50 mg b.i.d.) or placebo for 10 days in random order separated by a 3-week washout. On day 9 of each period, subjects ate a mixed meal. Insulin sensitivity (SI), glucose effectiveness, and β-cell responsivity indexes were estimated using the oral glucose and C-peptide minimal models. At 300 min 0.02 unit/kg insulin was administered intravenously.RESULTS—Vildagliptin reduced postprandial glucose concentrations (905 ± 94 vs. 1,008 ± 104 mmol/6 h, P = 0.02). Vildagliptin did not alter net SI (7.71 ± 1.28 vs. 6.41 ± 0.84 10−4 dl · kg−1 · min−1 · μU−1 · ml−1, P = 0.13) or glucose effectiveness (0.019 ± 0.002 vs. 0.018 ± 0.002 dl · kg−1 · min−1, P = 0.65). However, the net β-cell responsivity index was increased (35.7 ± 5.2 vs. 28.9 ± 5.2 10−9 min−1, P = 0.03) as was total disposition index (381 ± 48 vs. 261 ± 35 10−14 dl · kg−1 · min−2 · pmol−1 · l−1, P = 0.006). Vildagliptin lowered postprandial glucagon concentrations (27.0 ± 1.1 vs. 29.7 ± 1.5 μg · l−1 · 6 h−1, P = 0.03), especially after administration of exogenous insulin (81.5 ± 6.4 vs. 99.3 ± 5.6 ng/l, P = 0.02).CONCLUSIONS—Vildagliptin lowers postprandial glucose concentrations by stimulating insulin secretion and suppressing glucagon secretion but not by altered insulin action or glucose effectiveness. A novel observation is that vildagliptin alters α-cell responsiveness to insulin administration, but the significance of this action is as yet unclear.

Environmental Contaminants as Risk Factors for Developing Diabetes

The contribution of exposure to persistent organic pollutants (POPs) to the incidence of diabetes has received little attention until recently. A number of reports have emerged, however, concerning elevated diabetes in persons occupationally exposed to dioxin. United States (US) Air Force personnel in Vietnam who sprayed Agent Orange containing dioxin as a contaminant had elevated rates of diabetes, leading to US government compensation for diabetes in these veterans. Recent studies in populations exposed to polychlorinated biphenyls (PCBs) and chlorinated pesticides found a dose-dependent elevated risk of diabetes. An elevation in risk of diabetes in relation to levels of several POPs has been demonstrated by two different groups using the National Health and Nutrition Examination Survey (NHANES), a random sampling of US citizens. The strong associations seen in quite different studies suggest the possibility that exposure to POPs could cause diabetes. One striking observation is that obese persons that do not have elevated POPs are not at elevated risk of diabetes, suggesting that the POPs rather than the obesity per se is responsible for the association. Although a specific mechanism is not known, most POPs induce a great number and variety of genes, including several that alter insulin action. Because diabetes is a dangerous disease that is increasing in frequency throughout the world, further study of the possibility that exposure to POPs contributes to the etiology of diabetes is critical.

Muscle Mitochondrial ATP Synthesis and Glucose Transport/Phosphorylation in Type 2 Diabetes

BackgroundMuscular insulin resistance is frequently characterized by blunted increases in glucose-6-phosphate (G-6-P) reflecting impaired glucose transport/phosphorylation. These abnormalities likely relate to excessive intramyocellular lipids and mitochondrial dysfunction. We hypothesized that alterations in insulin action and mitochondrial function should be present even in nonobese patients with well-controlled type 2 diabetes mellitus (T2DM).Methods and FindingsWe measured G-6-P, ATP synthetic flux (i.e., synthesis) and lipid contents of skeletal muscle with 31P/1H magnetic resonance spectroscopy in ten patients with T2DM and in two control groups: ten sex-, age-, and body mass-matched elderly people; and 11 younger healthy individuals. Although insulin sensitivity was lower in patients with T2DM, muscle lipid contents were comparable and hyperinsulinemia increased G-6-P by 50% (95% confidence interval [CI] 39%–99%) in all groups. Patients with diabetes had 27% lower fasting ATP synthetic flux compared to younger controls (p = 0.031). Insulin stimulation increased ATP synthetic flux only in controls (younger: 26%, 95% CI 13%–42%; older: 11%, 95% CI 2%–25%), but failed to increase even during hyperglycemic hyperinsulinemia in patients with T2DM. Fasting free fatty acids and waist-to-hip ratios explained 44% of basal ATP synthetic flux. Insulin sensitivity explained 30% of insulin-stimulated ATP synthetic flux.ConclusionsPatients with well-controlled T2DM feature slightly lower flux through muscle ATP synthesis, which occurs independently of glucose transport /phosphorylation and lipid deposition but is determined by lipid availability and insulin sensitivity. Furthermore, the reduction in insulin-stimulated glucose disposal despite normal glucose transport/phosphorylation suggests further abnormalities mainly in glycogen synthesis in these patients.

Transfer to Sulphonylurea Therapy in Adult Subjects With Permanent Neonatal Diabetes Due to KCNJ11-Activating Mutations

Activating mutations in the KCJN11 gene encoding in the ATP-sensitive K+ channel (KATP channel) subunit Kir6.2 were reported (1) as the most common cause of permanent neonatal diabetes (PND). Recently, it has been shown that most subjects with Kir6.2 mutations could be switched from insulin to sulfonylurea and that such treatment is both safe and highly effective, at least in the short term (2,3). Notably, the majority of reported successfully transferred patients were children. Data on adults are very scarce, and there are few mutation carriers transferred off insulin (2,4). Moreover, some adult subjects are unable to switch from insulin to sulfonylurea (2). We have recently identified four adult carriers of a Kir6.2 mutation and provided evidence that they, before the sulfonylurea exposure, were characterized by decreased insulin sensitivity (5). Here, we report their successful transfer to sulfonylurea. To dissect the genetic background of PND in Poland, the Nationwide Registry has recently been established. Four adult subjects with Kir6.2-activating mutations were identified by the end of 2005. Three subjects carried the R210H mutation, while one individual had the K170N substitution. They were included in the current project, which aimed to switch from insulin to sulfonylurea and to assess, by hyperinsulinemic-euglycemic clamp, whether the alteration in insulin action occurs after this transfer. The study protocol and informed-consent procedures were approved by the ethics committee of Jagiellonian University. The project was conducted according to the …

Activation of liver X receptors promotes lipid accumulation but does not alter insulin action in human skeletal muscle cells

The aim of this study was to investigate the effects of liver X receptor (LXR) activation on lipid metabolism and insulin action in human skeletal muscle cells prepared from control subjects and from patients with type 2 diabetes. Cultured myotubes were obtained from muscle biopsies of 11 lean, healthy control subjects and ten patients with type 2 diabetes. The mRNA levels of LXR isoforms and lipogenic genes were estimated by RT-quantitative PCR, and the effects of LXR agonists on insulin action were evaluated by assays of protein kinase B serine 473 phosphorylation and glycogen synthesis. Both LXRalpha and LXRbeta were expressed in human skeletal muscle and adipose tissue and there was no difference in their mRNA abundance in tissues from patients with type 2 diabetes compared with control subjects. In cultured muscle cells, LXR activation by T0901317 strongly increased expression of the genes encoding lipogenic enzymes, including sterol regulatory element binding protein 1c, fatty acid synthase and stearoyl-CoA desaturase 1, and also promoted triglyceride accumulation in the presence of a high glucose concentration. Importantly, these effects on lipid metabolism did not affect protein kinase B activation by insulin. Furthermore, LXR agonists did not modify insulin action in muscle cells from patients with type 2 diabetes. These data suggest that LXR agonists may lead to increased utilisation of lipids and glucose in muscle cells without affecting the mechanism of action of insulin. However, the long-term consequences of triglyceride accumulation in muscle should be evaluated before the development of effective LXR-based therapeutic agents.

Review article: adipocytokines and insulin resistance

Insulin resistance has been implicated as one possible factor that links visceral obesity to unfavourable metabolic and cardiovascular consequences. However, the mechanism whereby adipose tissue causes alterations in insulin action remains unclear. White adipose tissue is secreting several hormones, particularly leptin and adiponectin, and a variety of other protein signals: the adipocytokines. They include proteins involved in the regulation of energy balance, lipid and glucose metabolism as well as angiogenesis, vascular and blood pressure regulation. Visceral obesity and inflammation within white adipose tissue may be a crucial step contributing to the emergence of insulin resistance, type 2 diabetes and atherosclerosis. A growing list of adipocytokines involved in inflammation (IL-1beta, IL-6, IL-8, IL-10, TNF-alpha, TGF-beta,) and the acute-phase response (serum amyloid A, PAI-1) have been found to be increased in the metabolic syndrome. It is, however, unclear as to the extent adipose tissue contributes quantitatively to the elevated circulating levels of these factors in obesity and how they may affect the insulin-dependent tissues. This review describes the role of the currently known adipocytokines and hormones released by adipose tissue in generating the insulin resistance state and the chronic inflammatory profile which frequently goes together with visceral obesity.

Severe impairment in liver insulin signaling fails to alter hepatic insulin action in conscious mice

Insulin exerts its potent effects on hepatic glucose fluxes via direct and indirect mechanisms. Whereas a liver-specific insulin receptor (IR) knockout (LIRKO) mouse exhibits glucose intolerance as well as insulin resistance, it is unclear whether a more acute decrease in the expression of hepatic IR would be sufficient to induce hepatic insulin resistance. Here we report that the downregulation of hepatic IR expression by up to 95% does not modify hepatic insulin action. The i.p. administration (2 injections over 1 week) of an antisense oligodeoxynucleotide (ASO) directed to reduce insulin expression downregulated hepatic IR expression in C57BL6J mice. A high dose of IR-ASO decreased IR protein approximately 95%, while a control-ASO failed to modify IR expression. At this dose, the IR-ASO also decreased IR expression in adipose tissue but did not significantly decrease IR expression in hypothalamus or skeletal muscle. Insulin action was assessed with insulin clamp studies in conscious mice. The rate of glucose infusion during the clamp studies was comparable in control-ASO- and IR-ASO-treated mice. Importantly, the depletion of liver IR protein markedly impaired downstream insulin signaling in the liver, but it failed to modify the rate of glucose production. Thus, near ablation of liver IR does not alter insulin action on glucose production.

Severe impairment in liver insulin signaling fails to alter hepatic insulin action in conscious mice

Insulin exerts its potent effects on hepatic glucose fluxes via direct and indirect mechanisms. Whereas a liver-specific insulin receptor (IR) knockout (LIRKO) mouse exhibits glucose intolerance as well as insulin resistance, it is unclear whether a more acute decrease in the expression of hepatic IR would be sufficient to induce hepatic insulin resistance. Here we report that the downregulation of hepatic IR expression by up to 95% does not modify hepatic insulin action. The i.p. administration (2 injections over 1 week) of an antisense oligodeoxynucleotide (ASO) directed to reduce insulin expression downregulated hepatic IR expression in C57BL6J mice. A high dose of IR-ASO decreased IR protein approximately 95%, while a control-ASO failed to modify IR expression. At this dose, the IR-ASO also decreased IR expression in adipose tissue but did not significantly decrease IR expression in hypothalamus or skeletal muscle. Insulin action was assessed with insulin clamp studies in conscious mice. The rate of glucose infusion during the clamp studies was comparable in control-ASO– and IR-ASO–treated mice. Importantly, the depletion of liver IR protein markedly impaired downstream insulin signaling in the liver, but it failed to modify the rate of glucose production. Thus, near ablation of liver IR does not alter insulin action on glucose production.

Insulin signaling in heart muscle: Lessons from genetically engineered mouse models

The heart is an insulin-responsive organ, and disorders of insulin action, such as diabetes and obesity, can have profound effects on cardiac performance. Insulin signaling influences numerous functions within the heart, such as metabolic substrate preference, cell size, and the response of the heart to ischemia and hypertrophy. Because the systemic consequences of altered insulin action can have significant but indirect effects on the heart, the generation of mice with altered expression of insulin receptors and key components of the insulin-signal transduction pathways in cardiomyocytes have led to interesting and occasionally surprising new insights into the regulation of cardiac biology by insulin.

Development of whole-body and skeletal muscle insulin resistance after one day of hindlimb suspension

Hindlimb suspension (HS) of rats is a model of simulated weightlessness and induces dynamic alterations in insulin action. In the present study, the effect of acute (1-day) HS on whole-body glucose tolerance and insulin action on skeletal muscle glucose transport was assessed in juvenile, female Sprague-Dawley rats. Compared to weight-bearing control rats, 1-day HS animals displayed significantly decreased glucose tolerance and diminished whole-body insulin sensitivity. Glucose transport activity in the 1-day unweighted soleus muscle was significantly decreased ( P < .05) compared to weight-bearing control muscles both in the absence and presence of insulin (2 mU/mL). Insulin-mediated glucose transport activity in the extensor digitorum longus (EDL) muscles also tended ( P = .09) to be lower. There was no change in the protein expression of insulin receptor β-subunit (IR-β), insulin receptor substrate-1 (IRS-1), IRS-2, the p85 subunit of phosphatidylinositol-3 kinase (PI3-kinase), Akt, and glucose transporter protein 4 (GLUT-4). The activities of these proteins were also unchanged, as insulin-stimulated IR-β tyrosine phosphorylation, IRS-1 tyrosine phosphorylation, IRS-1-associated p85, and Akt serine phosphorylation were similar to controls. However, basal Akt phosphorylation was significantly depressed ( P < .05) in the 1-day HS soleus. In addition, the protein expression and basal phosphorylation of the stress-activated p38 mitogen-activated protein kinase (p38 MAPK) were significantly elevated ( P < .05) in the 1-day unweighted soleus. These results indicate that the development of insulin resistance in the 1-day unweighted soleus is not due to impaired functionality of elements involved in the IR/IRS-1/PI3-kinase/Akt signaling pathway. However, activation of p38 MAPK may play a role in this response.

Alteration in insulin action: role of IRS-1 serine phosphorylation in the retroregulation of insulin signalling

Insulin resistance, when combined with impaired insulin secretion, contributes to the development of type 2 diabetes. Insulin resistance is characterised by a decrease in insulin effect on glucose transport in muscle and adipose tIssue. Tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1) and its binding to phosphatidylinositol 3-kinase (PI 3-kinase) are critical events in the insulin signalling cascade leading to insulin-stimulated glucose transport. Modification of IRS-1 by serine phosphorylation could be one of the mechanisms leading to a decrease in IRS-1 tyrosine phosphorylation, PI 3-kinase activity and glucose transport. Recent findings demonstrate that "diabetogenic" factors such as FFA, TNFalpha, hyperinsulinemia and cellular stress, increase the serine phosphorylation of IRS-1 and identified Ser307/612/632 as phosphorylated sites. Moreover, several kinases able to phosphorylate these serine residues have been identified. These exciting results suggest that serine phosphorylation of IRS-1 is a possible hallmark of insulin resistance in biologically insulin responsive cells or tIssues. Identifying the pathways by which "diabetogenic" factors activate IRS-1 kinases and defining the precise role of serine phosphorylation events in IRS-1 regulation represent important goals. Such studies may enable rational drug design to selectively inhibit the activity of the relevant enzymes and generate a novel class of therapeutic agents for type 2 diabetes.