Activation Of Insulin Signaling Pathway Research Articles

Sustained Action of Ceramide on the Insulin Signaling Pathway in Muscle Cells: IMPLICATION OF THE DOUBLE-STRANDED RNA-ACTIVATED PROTEIN KINASE

In vivo, ectopic accumulation of fatty acids in muscles leads to alterations in insulin signaling at both the IRS1 and Akt steps. However, in vitro treatments with saturated fatty acids or their derivative ceramide demonstrate an effect only at the Akt step. In this study, we adapted our experimental procedures to mimic the in vivo situation and show that the double-stranded RNA-dependent protein kinase (PKR) is involved in the long-term effects of saturated fatty acids on IRS1. C2C12 or human muscle cells were incubated with palmitate or directly with ceramide for short or long periods, and insulin signaling pathway activity was evaluated. PKR involvement was assessed through pharmacological and genetic studies. Short-term treatments of myotubes with palmitate, a ceramide precursor, or directly with ceramide induce an inhibition of Akt, whereas prolonged periods of treatment show an additive inhibition of insulin signaling through increased IRS1 serine 307 phosphorylation. PKR mRNA, protein, and phosphorylation are increased in insulin-resistant muscles. When PKR activity is reduced (siRNA or a pharmacological inhibitor), serine phosphorylation of IRS1 is reduced, and insulin-induced phosphorylation of Akt is improved. Finally, we show that JNK mediates ceramide-activated PKR inhibitory action on IRS1. Together, in the long term, our results show that ceramide acts at two distinct levels of the insulin signaling pathway (IRS1 and Akt). PKR, which is induced by both inflammation signals and ceramide, could play a major role in the development of insulin resistance in muscle cells.

Bilirubin Increases Insulin Sensitivity in Leptin-Receptor Deficient and Diet-Induced Obese Mice Through Suppression of ER Stress and Chronic Inflammation

Obesity-induced endoplasmic reticulum (ER) stress causes chronic inflammation in adipose tissue and steatosis in the liver, and eventually leads to insulin resistance and type 2 diabetes (T2D). The goal of this study was to understand the mechanisms by which administration of bilirubin, a powerful antioxidant, reduces hyperglycemia and ameliorates obesity in leptin-receptor-deficient (db/db) and diet-induced obese (DIO) mouse models. db/db or DIO mice were injected with bilirubin or vehicle ip. Blood glucose and body weight were measured. Activation of insulin-signaling pathways, expression of inflammatory cytokines, and ER stress markers were measured in skeletal muscle, adipose tissue, and liver of mice. Bilirubin administration significantly reduced hyperglycemia and increased insulin sensitivity in db/db mice. Bilirubin treatment increased protein kinase B (PKB/Akt) phosphorylation in skeletal muscle and suppressed expression of ER stress markers, including the 78-kDa glucose-regulated protein (GRP78), CCAAT/enhancer-binding protein (C/EBP) homologous protein, X box binding protein (XBP-1), and activating transcription factor 4 in db/db mice. In DIO mice, bilirubin treatment significantly reduced body weight and increased insulin sensitivity. Moreover, bilirubin suppressed macrophage infiltration and proinflammatory cytokine expression, including TNF-α, IL-1β, and monocyte chemoattractant protein-1, in adipose tissue. In liver and adipose tissue of DIO mice, bilirubin ameliorated hepatic steatosis and reduced expression of GRP78 and C/EBP homologous protein. These results demonstrate that bilirubin administration improves hyperglycemia and obesity by increasing insulin sensitivity in both genetically engineered and DIO mice models. Bilirubin or bilirubin-increasing drugs might be useful as an insulin sensitizer for the treatment of obesity-induced insulin resistance and type 2 diabetes based on its profound anti-ER stress and antiinflammatory properties.

Evidence for a regulatory role of Cullin-RING E3 ubiquitin ligase 7 in insulin signaling

Dysfunctional regulation of signaling pathways downstream of the insulin receptor plays a pivotal role in the pathogenesis of insulin resistance and type 2 diabetes. In this study we report both in vitro and in vivo experimental evidence for a role of Cullin-RING E3 ubiquitin ligase 7 (CRL7) in the regulation of insulin signaling and glucose homeostasis. We show that Cul7−/− mouse embryonic fibroblasts displayed enhanced AKT and Erk MAP kinase phosphorylation upon insulin stimulation. Depletion of CUL7 by RNA interference in C2C12 myotubes led to increased activation of insulin signaling pathways and cellular glucose uptake, as well as a reduced capacity of these cells to execute insulin-induced degradation of insulin receptor substrate 1 (IRS1). In vivo, heterozygosity of either Cul7 or Fbxw8, both key components of CRL7, resulted in elevated PI3 kinase/AKT activation in skeletal muscle tissue upon insulin stimulation when compared to wild-type controls. Finally, Cul7+/− or Fbxw8+/− mice exhibited enhanced insulin sensitivity and plasma glucose clearance. Collectively, our findings point to a yet unrecognized role of CRL7 in insulin-mediated control of glucose homeostasis by restraining PI3 kinase/AKT activities in skeletal muscle cells.

Grape skin extract protects against programmed changes in the adult rat offspring caused by maternal high-fat diet during lactation

Maternal overnutrition during suckling period is associated with increased risk of metabolic disorders in the offspring. We aimed to assess the effect of Vitis vinifera L. grape skin extract (ACH09) on cardiovascular and metabolic disorders in adult male offspring of rats fed a high-fat (HF) diet during lactation. Four groups of female rats were fed: control diet (7% fat), ACH09 (7% fat plus 200 mg kg−1 d−1 ACH09 orally), HF (24% fat), and HF+ACH09 (24% fat plus 200 mg kg−1 d−1 ACH09 orally) during lactation. After weaning, all male offspring were fed a control diet and sacrificed at 90 or 180 days old. Systolic blood pressure was increased in adult offspring of HF-fed dams and ACH09 prevented the hypertension. Increased adiposity, plasma triglyceride, glucose levels and insulin resistance were observed in offspring from both ages, and those changes were reversed by ACH09. Expression of insulin cascade proteins IRS-1, AKT and GLUT4 in the soleus muscle was reduced in the HF group of both ages and increased by ACH09. The plasma oxidative damage assessed by malondialdehyde levels was increased, and nitrite levels decreased in the HF group of both ages, which were reversed by ACH09. In addition, ACH09 restored the decreased plasma and mesenteric arteries antioxidant activities of superoxide dismutase, catalase and glutathione peroxidase in the HF group. In conclusion, the treatment of HF-fed dams during lactation with ACH09 provides protection from later-life hypertension, body weight gain, insulin resistance and oxidative stress. The protective effect ACH09 may involve NO synthesis, antioxidant action and activation of insulin-signaling pathways.

Retinol binding protein 4 affects the adipogenesis of porcine preadipocytes through insulin signaling pathways

Retinol binding protein 4 (RBP4), a novel cytokine, is mainly secreted by hepatocytes and adipocytes. RBP4 reportedly induces insulin resistance and RBP4 secretion is increased in the adipocytes of animals or humans with type 2 diabetes, obesity, and metabolic syndrome, but its role in preadipocyte differentiation remains unclear. In this study, we investigated the effect of RBP4 on the differentiation of porcine preadipocytes into adipocytes. The results suggest that RBP4 significantly suppresses the differentiation of porcine preadipocytes into adipocytes, including those treated with the hormone cocktail methylisobutylxanthine-dexamethasone-insulin. RBP4 also weakened the activity of normal threonine 308, the phosphorylation of serine/threonine kinase AKT, and downstream insulin signaling, including the mammalian target of rapamycin (mTOR) and β-catenin. Moreover, the activation of insulin signaling mediated by knockdown RBP4 in porcine preadipocytes was recovered in the suppression of LY294002. RBP4 also had a suppressive effect on the differentiation of porcine preadipocytes by decreasing the activation of insulin signaling pathways.

Hydrogen peroxide induces activation of insulin signaling pathway via AMP-dependent kinase in podocytes

Podocytes are cells that form the glomerular filtration barrier in the kidney. Insulin signaling in podocytes is critical for normal kidney function. Insulin signaling is regulated by oxidative stress and intracellular energy levels. We cultured rat podocytes to investigate the effects of hydrogen peroxide (H2O2) on the phosphorylation of proximal and distal elements of insulin signaling. We also investigated H2O2-induced intracellular changes in the distribution of protein kinase B (Akt). Western blots showed that H2O2 (100μM) induced rapid, transient phosphorylation of the insulin receptor (IR), the IR substrate-1 (IRS1), and Akt with peak activities at 5min (Δ 183%, P<0.05), 3min (Δ 414%, P<0.05), and 10min (Δ 35%, P<0.05), respectively. Immunostaining cells with an Akt-specific antibody showed increased intensity at the plasma membrane after treatment with H2O2>. Furthermore, H2O2 inhibited phosphorylation of the phosphatase and tensin homologue (PTEN; peak activity at 10min; Δ −32%, P<0.05) and stimulated phosphorylation of the AMP-dependent kinase alpha subunit (AMPKα; 78% at 3min and 244% at 10min). The stimulation of AMPK was abolished with an AMPK inhibitor, Compound C (100μM, 2h). Moreover, Compound C significantly reduced the effect of H2O2 on IR phosphorylation by about 40% (from 2.07±0.28 to 1.28±0.12, P<0.05). In addition, H2O2 increased glucose uptake in podocytes (from 0.88±0.04 to 1.29±0.12nmol/min/mg protein, P<0.05), and this effect was attenuated by Compound C.Our results suggested that H2O2 activated the insulin signaling pathway and glucose uptake via AMPK in cultured rat podocytes. This signaling may play a potential role in the prevention of insulin resistance under conditions associated with oxidative stress.

Effect of flavonol glycosides from Cinnamomum osmophloeum leaves on adiponectin secretion and phosphorylation of insulin receptor- β in 3T3-L1 adipocytes

Ethnopharmacological relevance Cinnamomum osmophloeum is used for various ethnomedical conditions in Taiwan including diabetic complications. Aim of the study The aim of present study was to identify the anti-diabetic compounds from C. osmophloeum leaves and evaluate the preliminary molecular basis for their insulin-like effects. Materials and methods Silica gel column chromatographic purification of MeOH extract from leaves of C. osmophloeum resulted in the isolation of a two kaempferol glycosides CO-1 and CO-2. These two compounds were evaluated for their effects on adiponectin secretion, tyrosine phosphorylation of insulin receptor (IR)- β and glucose transporter 4 (GLUT4) in differentiated mouse 3T3-L1 adipocytes, and the results were compared with the reference drug insulin. Results The compound CO-1 at a concentration of 5 μM was able to act as an insulin-mimetic in terms of its ability to increase adiponectin secretion by 12.2-fold, while CO-2 has no such effect up to 20 μM tested. Furthermore, 5 μM of CO-1 and 20 μM of CO-2 showed potential to increase the phosphorylation of IR β by 2.3- and 2.1-fold, respectively, in addition to their positive effect on GLUT4 translocation. CO-1 and CO-2 stimulated GLUT4 translocation are reduced by phosphatidylinositol-3-kinase (PI3-K) inhibitor. Conclusion The present study indicates that the insulin-like anti-diabetic mechanism of constituents from C. osmophloeum leaves in part due to enhanced adiponectin secretion, and activation of insulin signaling pathway leading to GLUT4 translocation which involved phosphorylation of IR and activation of PI3-K.

Insulin signaling activation protects dentate neurons from oxygen‐glucose deprivation in organotypic hippocampal slice cultures

A link between diabetes and ischemia has been well accepted. However, the physiological and pathophysiological roles of insulin in the brain are not fully understood. In the present study, we used organotypic hippocampal slice cultures as an in vitro model to study the role of insulin signaling in cell survival/death in oxygen‐glucose deprivation (OGD)‐induced injury. We found that, as compared to the CA1‐3 regions, the dentate gyrus (DG) neuronal survival was more sensitive to inhibition of PI3K/AKt signaling under basal conditions. Dentate neuronal sensitivity to PI3K/Akt signaling activation was inversely related to their vulnerability to OGD‐induced injury. Insulin conferred neuroprotection to DG neurons via activation of PI3K/Akt signaling. In contrast, CA1 and CA3 neurons were less sensitive to PI3K/Akt signaling but more vulnerable to OGD. MAPK inhibitors reduced CA neuronal injury from OGD when PI3K signaling was inhibited. In addition, in the presence of insulin, CA1 neurons were partly protected from OGD by these MAPK inhibitors, and were also protected by NMDA receptor blocker CPP, suggesting that an activity‐dependent activation of MAPK signaling may contribute to CA1 neuronal vulnerability. CA2 sub‐region was distinctive in its response to glutamate, OGD, and insulin, compared to other CA sub‐regions. CA2 neurons were sensitive to the protective effects of insulin from OGD, but more resistant to those of glutamate. Our results suggest that PI3K and MAPK mediated survival/death signaling may play a role in sub‐regional vulnerability to cerebral ischemia. We speculate that activation of insulin signaling pathways contributes to neuronal repair under pathophysiological conditions such as diabetes and stroke.

Successful Intensive Insulin Treatment of Type 1 Diabetic Dogs Leads to Restoration of Peripheral Leukocyte Insulin Signaling Gene Expression and Enzyme Activities

The purpose of this study was to investigate whether intensive insulin treatment of dogs suffering from type 1 diabetes mellitus, resulting in tight glycemic control, could be reflected by changes in peripheral leukocyte metabolism. Specifically, plasma metabolites and enzyme activities were assessed. In addition, quantitative RT-PCR was used to determine changes in insulin signaling gene (insulin receptor substrate (IRS)-1, IRS-2 and phosphatidylinositol 3-kinase (PI3-K) P85alpha) mRNA levels in peripheral leukocytes. Lastly, leukocyte enzymes involved in cellular energy metabolism were examined for changes in glucose utilization. Our results indicated that intensive insulin treatment was successful in type 1 DM dogs, leading to tight glycemic control. The mean glucose concentration and glycated albumin percentage significantly decreased to 156 mg/dl and 15.6%, respectively, following treatment. In peripheral leukocytes, the IRS-2 and PI3-K p85alpha mRNA levels significantly increased, and a significant increase in pyruvate kinase and pyruvate carboxylase activity, two enzymes involved in cellular energy metabolism, was also observed post treatment. Therefore, the observed changes in insulin signaling pathway activity and cellular energy metabolism enzyme activity in peripheral leukocytes are considered to be characteristics of amelioration of glucose metabolism by insulin action. As such, peripheral leukocytes are sufficiently sensitive to monitor for improving glycemic control during intensive insulin treatment of type 1 DM dogs. Blood cells such as leukocytes are much more readily available than muscle or adipose tissue for studies in dogs.

Nutrigenomic Studies of Effects of Chlorella on Subjects with High-Risk Factors for Lifestyle-Related Disease

In order to clarify the physiological effects of Chlorella intake on subjects with high-risk factors for lifestyle-related diseases, we conducted Chlorella ingestion tests on 17 subjects with high-risk factors for lifestyle-related diseases and 17 healthy subjects over a 16-week period, including a 4-week post-observation period. We conducted blood biochemical tests and analyzed gene expression profile in whole blood cells in the peripheral blood before and after Chlorella intake. We confirmed that in both groups, Chlorella intake resulted in noticeable reductions in body fat percentage, serum total cholesterol, and fasting blood glucose levels. Through gene expression analysis, we found that gene expression profiles varied with Chlorella intake and identified many genes that exhibited behavior such that after the completion of the intake period, expression levels returned to pre-intake expression ones. Among these were genes related to signal transduction molecules, metabolic enzymes, receptors, transporters, and cytokines. A difference in expression level was found between the two groups at the start of the tests, and we were able to identify genes with noticeable variance in expression level resulting from Chlorella intake in the high-risk factor group. These included genes involved in fat metabolism and insulin signaling pathways, which suggests that these pathways could be physiologically affected by Chlorella intake. There were clear variations in the expression profiles of genes directly related to uptake of glucose resulting from Chlorella intake, indicating that the activation of insulin signaling pathways could be the reason for the hypoglycemic effects of Chlorella.

Defective insulin signaling pathway and increased glycogen synthase kinase‐3 activity in the brain of diabetic mice: Parallels with Alzheimer's disease and correction by insulin

We have evaluated the effect of peripheral insulin deficiency on brain insulin pathway activity in a mouse model of type 1 diabetes, the parallels with Alzheimer's disease (AD), and the effect of treatment with insulin. Nine weeks of insulin-deficient diabetes significantly impaired the learning capacity of mice, significantly reduced insulin-degrading enzyme protein expression, and significantly reduced phosphorylation of the insulin-receptor and AKT. Phosphorylation of glycogen synthase kinase-3 (GSK3) was also significantly decreased, indicating increased GSK3 activity. This evidence of reduced insulin signaling was associated with a concomitant increase in tau phosphorylation and amyloid beta protein levels. Changes in phosphorylation levels of insulin receptor, GSK3, and tau were not observed in the brain of db/db mice, a model of type 2 diabetes, after a similar duration (8 weeks) of diabetes. Treatment with insulin from onset of diabetes partially restored the phosphorylation of insulin receptor and of GSK3, partially reduced the level of phosphorylated tau in the brain, and partially improved learning ability in insulin-deficient diabetic mice. Our data indicate that mice with systemic insulin deficiency display evidence of reduced insulin signaling pathway activity in the brain that is associated with biochemical and behavioral features of AD and that it can be corrected by insulin treatment.

Potential impact of carbohydrate and fat intake on pathological left ventricular hypertrophy

Currently, a high carbohydrate/low fat diet is recommended for patients with hypertension; however, the potentially important role that the composition of dietary fat and carbohydrate plays in hypertension and the development of pathological left ventricular hypertrophy (LVH) has not been well characterized. Recent studies demonstrate that LVH can also be triggered by activation of insulin signaling pathways, altered adipokine levels, or the activity of peroxisome proliferator-activated receptors (PPARs), suggesting that metabolic alterations play a role in the pathophysiology of LVH. Hypertensive patients with high plasma insulin or metabolic syndrome have a greater occurrence of LVH, which could be due to insulin activation of the serine-threonine kinase Akt and its downstream targets in the heart, resulting in cellular hypertrophy. PPARs also activate cardiac gene expression and growth and are stimulated by fatty acids and consumption of a high fat diet. Dietary intake of fats and carbohydrate and the resultant effects of plasma insulin, adipokine, and lipid concentrations may affect cardiomyocyte size and function, particularly in the setting of chronic hypertension. This review discusses potential mechanisms by which dietary carbohydrates and fats ca affect cardiac growth, metabolism, and function, mainly in the context of pressure overload-induced LVH.

Enhancement of insulin signaling pathway in adipocytes by oxovanadium(IV) complexes

Recently, we have found that some oxovanadium(IV) complexes are potent insulin-mimetic compounds for treating both type I and type II diabetic animals. However, the functional mechanism of oxovanadium(IV) complexes is not fully understood. In this report, we have shown that oxovanadium(IV)–picolinate complexes such as VO(pa) 2, VO(3mpa) 2, and VO(6mpa) 2 act on the insulin signaling pathway in 3T3-L1 adipocytes. Among them, VO(3mpa) 2 was found to be the highest potent activator in inducing not only the phosphotyrosine levels of both IRβ and IRS but also the activation of downstream kinases in the insulin receptor, such as Akt and GSK3β, which in turn translocated the insulin-dependent GLUT4 to the plasma membrane. Then, we examined whether or not oxovanadium(IV)–picolinates exhibit the hypoglycemic activity in STZ-induced diabetic mice, and found that VO(3mpa) 2 is more effective than the others in improving the hyperglycemia of the animals. Our present data indicate that both activation of insulin signaling pathway, which follows the GLUT4 translocation to the plasma membrane, and enhancement of glucose utilization by oxovanadium(IV) complexes cause the hypoglycemic effect in diabetic animals.

Subcellular localization of glucose transporter 4 in the hypothalamic arcuate nucleus of ob/ob mice under basal conditions

Glucose transporter (GLUT) 4 plays an important role in insulin-induced glucose uptake in skeletal muscle and white adipose tissue. Although GLUT4 is abundant in the hypothalamus as well as in these peripheral tissues, little is known about the role of GLUT4 in the hypothalamus. In this study, we examined the subcellular localization of GLUT4 and the activation of insulin signaling pathways in the hypothalamic arcuate nucleus of ob/ob mice under basal conditions. The expression of GLUT4 in the arcuate nucleus of ob/ob mice was higher than that in lean mice. Interestingly, GLUT4 on the plasma membrane increased significantly in neurons of the arcuate nucleus of ob/ob mice when compared to that in lean mice. Because serum insulin levels of ob/ob mice were very high, we hypothesized that insulin strongly stimulates GLUT4 translocation in the arcuate nucleus of ob/ob mice. Unexpectedly, tyrosine phosphorylation of IR and insulin receptor substrate-1 (IRS-1) was faint in the hypothalamus of lean and ob/ob mice. In addition, phosphorylation of IRS-1 at Ser 307 in the hypothalamus of ob/ob mice was higher when compared to that in lean mice, suggesting that insulin signaling is impaired by phosphorylation of IRS-1 at Ser 307 in the hypothalamus of ob/ob mice. However, serine phosphorylation of Akt in the arcuate nucleus of ob/ob mice increased significantly when compared to that in lean mice. Furthermore, the expression of brain-derived neurotrophic factor, an activator of PI3K-Akt pathway in neurons, increased significantly in the ventromedial hypothalamus of ob/ob mice. We discuss the possibility of novel pathways which induce the translocation of GLUT4 in the arcuate nucleus of ob/ob mice.

Microvascular recruitment is an early insulin effect that regulates skeletal muscle glucose uptake in vivo.

Insulin increases glucose disposal into muscle. In addition, in vivo insulin elicits distinct nitric oxide synthase-dependent vascular responses to increase total skeletal muscle blood flow and to recruit muscle capillaries (by relaxing resistance and terminal arterioles, respectively). In the current study, we compared the temporal sequence of vascular and metabolic responses to a 30-min physiological infusion of insulin (3 mU. min(-1). kg(-1), euglycemic clamp) or saline in rat skeletal muscle in vivo. We used contrast-enhanced ultrasound to continuously quantify microvascular volume. Insulin recruited microvasculature within 5-10 min (P < 0.05), and this preceded both activation of insulin-signaling pathways and increases in glucose disposal in muscle, as well as changes in total leg blood flow. Moreover, l-NAME (N(omega)-nitro-l-arginine-methyl ester), a specific inhibitor of nitric oxide synthase, blocked this early microvascular recruitment (P < 0.05) and at least partially inhibited early increases in muscle glucose uptake (P < 0.05). We conclude that insulin rapidly recruits skeletal muscle capillaries in vivo by a nitric oxide-dependent action, and the increase in capillary recruitment may contribute to the subsequent glucose uptake.

Selective stimulation of collagen synthesis in the presence of costimulatory insulin signaling by connective tissue growth factor in scleroderma fibroblasts.

To examine the mechanism of collagen induction by connective tissue growth factor (CTGF), a profibrotic cytokine overexpressed in the skin of patients with systemic sclerosis (SSc). Dermal fibroblasts from 7 SSc patients and 7 matched healthy adult donors were stimulated with CTGF in the presence or absence of the culture-medium supplement, insulin-transferrin-selenium (ITS). Expression of collagen protein was analyzed by a (3)H-proline incorporation assay. To identify the signaling pathways mediating CTGF induction of collagen, pharmacologic inhibitors were used, including rottlerin, a protein kinase C delta (PKC delta) inhibitor. Collagen levels in both SSc and normal fibroblasts were increased after treatment with transforming growth factor beta in serum-free medium, whereas no stimulation was observed following addition of CTGF. In the presence of ITS, CTGF (2.5 ng/ml) potently stimulated collagenous protein levels in SSc cell lines (n = 5); however, CTGF was not stimulatory in the majority of normal fibroblasts (n = 6). ITS alone induced collagen levels in normal fibroblasts to the levels observed in SSc skin fibroblasts, thereby diminishing the hallmark difference in basal collagen levels in these cell types. Insulin was the ITS component responsible for promoting the basal and CTGF stimulation of collagenous proteins. Rottlerin, the PKC delta inhibitor, down-regulated collagen synthesis in normal and SSc fibroblasts cultured in ITS, and inhibited the stimulatory effects of CTGF in cooperation with insulin or of insulin (500 ng/ml) alone. Increased responsiveness of SSc fibroblasts to CTGF-mediated collagen synthesis requires the costimulatory activation of insulin signaling pathways to induce matrix production. Blockade of this effect via rottlerin may suggest that PKC delta is a downstream signaling molecule necessary for CTGF stimulation of collagen synthesis.

Reduction of protein tyrosine phosphatase 1B increases insulin-dependent signaling in ob/ob mice.

Protein tyrosine phosphatase 1B (PTP1B) is a negative regulator of insulin receptor (IR) signal transduction and a drug target for treatment of type 2 diabetes. Using PTP1B antisense oligonucleotides (ASOs), effects of decreased PTP1B levels on insulin signaling in diabetic ob/ob mice were examined. Insulin stimulation, prior to sacrifice, resulted in no significant activation of insulin signaling pathways in livers from ob/ob mice. However, in PTP1B ASO-treated mice, in which PTP1B protein was decreased by 60% in liver, similar stimulation with insulin resulted in increased tyrosine phosphorylation of the IR and IR substrate (IRS)-1 and -2 by threefold, fourfold, and threefold, respectively. IRS-2-associated phosphatidylinositol 3-kinase activity was also increased threefold. Protein kinase B (PKB) serine phosphorylation was increased sevenfold in liver of PTP1B ASO-treated mice upon insulin stimulation, while phosphorylation of PKB substrates, glycogen synthase kinase (GSK)-3alpha and -3beta, was increased more than twofold. Peripheral insulin signaling was increased by PTP1B ASO, as evidenced by increased phosphorylation of PKB in muscle of insulin-stimulated PTP1B ASO-treated animals despite the lack of measurable effects on muscle PTP1B protein. These results indicate that reduction of PTP1B is sufficient to increase insulin-dependent metabolic signaling and improve insulin sensitivity in a diabetic animal model.

Apoptosis in the beta cells: cause or consequence of insulin secretion defect in diabetes?

Pancreatic g -cell dysfunction and insulin resistance are two interrelated defects in the pathophysiology of type 2 diabetes. Defects in peripheral insulin action precede the development of glucose intolerance, as the pancreas compensates for insulin resistance by increasing insulin production and secretion. This may be achieved by enhancing cellular secretory capacity or by increasing g -cell mass. Over time, the pancreatic secretion of insulin becomes inadequate for the extent of insulin resistance, and the levels of fasting and postprandial glucose rise leading to the onset of frank hyperglycemia, which leads to reduction in g -cell function and survival through a process referred to as glucose toxicity. There is increasing evidence that apoptosis is the main mode of pancreatic g -cell death not only in type 1 but also in type 2 diabetes. Recently, studies in knockout mice, human and rat islets, and pancreatic g -cell lines demonstrated that defective insulin signaling in g -cells might play an important pathophysiological role by affecting both secretory function and cell survival. The purpose of this review is to present recent advances in understanding of the interrelationship between molecular mechanisms underlying defects in insulin secretion and g -cell survival in type 2 diabetes caused by impaired activation of insulin signaling pathways.