Insulin resistance induced by high glucose and high insulin precedes insulin receptor substrate 1 protein depletion in human adipocytes

Nutri-stress, mitochondrial dysfunction, and insulin resistance-role of heat shock proteins

Excess nutrient flux into the cellular energy system results in a scenario of cellular metabolic stress in diseases involving insulin resistance, such as type 2 diabetes, referred to as nutri-stress and results in cellular bioenergetic imbalance, which leads to insulin resistance and disease. Under nutri-stress, the heat shock response system is compromised due to metabolic abnormalities that disturb energy homeostasis. Heat shock proteins (HSPs) are the chief protectors of intracellular homeostasis during stress. Heat shock response (HSR) impairment contributes to several metabolic pathways that aggravate chronic hyperglycaemia and insulin resistance, highlighting a central role in disease pathogenesis. This article discusses the role of nutri-stress-related molecular events in causing insulin resistance and the nature of the roles played by heat shock proteins in some of the crucial checkpoints of the molecular networks involved in insulin resistance. Ample evidence suggests that the heat shock machinery regulates critical pathways in mitochondrial function and energy metabolism and that cellular energy status highly influences it. Weakening of HSPs, therefore, leads to loss of their vital cytoprotective functions, propagating nutri-stress in the system. Further research into the mechanistic roles of HSPs in metabolic homeostasis will help widen our understanding of lifestyle diseases, their onset, and complications. These inducible proteins may be crucial to attenuating lifestyle risk factors and disease management.

d-Allulose Ameliorates Hyperglycemia Through IRE1α Sulfonation-RIDD-Sirt1 Decay Axis in the Skeletal Muscle

Aims: The skeletal muscle maintains glucose disposal via insulin signaling and glucose transport. The progression of diabetes and insulin resistance is critically influenced by endoplasmic reticulum (ER) stress. d-Allulose, a low-calorie sugar substitute, has shown crucial physiological activities under conditions involving hyperglycemia and insulin resistance. However, the molecular mechanisms of d-allulose in the progression of diabetes have not been fully elucidated. Here, we evaluated the effect of d-allulose on hyperglycemia-associated ER stress responses in human skeletal myoblasts (HSkM) and db/db diabetic and high-fat diet-fed mice. Results: d-allulose effectively controlled glycemic markers such as insulin and hemoglobin A1c (HbA1c), showing anti-diabetic effects by inhibiting the disruption of insulin receptor substrate (IRS)-1 tyrosine phosphorylation and glucose transporter 4 (GLUT4) expression, in which the phosphatidylinositol-3 kinase (PI3K)/protein kinase B (Akt) pathway is involved. The levels of glucose dysmetabolism-based NADPH oxidase, such as NADPH-dependent oxidoreductase (Nox) 4, were highly increased, and their interaction with IRE1α and the resultant sulfonation-regulated IRE1-dependent decay (RIDD)-Sirt1 decay were also highly increased under diabetic conditions, which were controlled with d-allulose treatment. Skeletal muscle cells grown with a high glucose medium supplemented with d-allulose showed controlled IRE1α sulfonation-RIDD-Sirt1 decay, in which Nox4 was involved. Innovation and Conclusion: The study observations indicate that d-allulose contributes to the muscular glucose disposal in the diabetic state where ER-localized Nox4-induced IRE1α sulfonation results in the decay of Sirt1, a core factor for controlling glucose metabolism. Antioxid. Redox Signal. 37, 229-245.

Rosemary extract activates AMPK, inhibits mTOR and attenuates the high glucose and high insulin-induced muscle cell insulin resistance

Impaired action of insulin in skeletal muscle, termed insulin resistance, leads to increased blood glucose levels resulting in compensatory increase in insulin levels. The elevated blood glucose and insulin levels exacerbate insulin resistance and contribute to the pathogenesis of type 2 diabetes mellitus. In previous studies we found attenuation of free fatty acid-induced muscle cell insulin resistance by rosemary extract (RE). In the present study we investigated the effects of RE on high glucose (HG) and high insulin (HI)-induced muscle cell insulin resistance. Exposure of L6 myotubes to 25 mmol/L glucose and 100 nmol/L insulin for 24 h, to mimic hyperglycemia and hyperinsulinemia, abolished the acute insulin-stimulated glucose uptake, increased the serine phosphorylation of IRS-1 and the phosphorylation/activation of mTOR and p70S6K. Treatment with RE significantly improved the insulin-stimulated glucose uptake and increased the acute insulin-stimulated tyrosine phosphorylation while reducing the HG+HI-induced serine phosphorylation of IRS-1 and phosphorylation of mTOR and p70S6K. Additionally, treatment with RE significantly increased the phosphorylation of AMPK, its downstream effector ACC and the plasma membrane GLUT4 levels. Our data indicate a potential of RE to counteract muscle cell insulin resistance and more studies are required to investigate its effectiveness in vivo. Novelty: RE phosphorylated muscle cell AMPK and ACC under both normal and HG+HI conditions. The HG+HI-induced serine phosphorylation of IRS-1 and activation of mTOR and p70S6K were attenuated by RE. RE restored the insulin-stimulated glucose uptake by enhancing GLUT4 glucose transporter translocation to plasma membrane.

Dietary Energy Partition: The Central Role of Glucose

Humans have developed effective survival mechanisms under conditions of nutrient (and energy) scarcity. Nevertheless, today, most humans face a quite different situation: excess of nutrients, especially those high in amino-nitrogen and energy (largely fat). The lack of mechanisms to prevent energy overload and the effective persistence of the mechanisms hoarding key nutrients such as amino acids has resulted in deep disorders of substrate handling. There is too often a massive untreatable accumulation of body fat in the presence of severe metabolic disorders of energy utilization and disposal, which become chronic and go much beyond the most obvious problems: diabetes, circulatory, renal and nervous disorders included loosely within the metabolic syndrome. We lack basic knowledge on diet nutrient dynamics at the tissue-cell metabolism level, and this adds to widely used medical procedures lacking sufficient scientific support, with limited or nil success. In the present longitudinal analysis of the fate of dietary nutrients, we have focused on glucose as an example of a largely unknown entity. Even most studies on hyper-energetic diets or their later consequences tend to ignore the critical role of carbohydrate (and nitrogen disposal) as (probably) the two main factors affecting the substrate partition and metabolism.

Jejunal Insulin Signalling Is Increased in Morbidly Obese Subjects with High Insulin Resistance and Is Regulated by Insulin and Leptin

Little is known about the jejunal insulin signalling pathways in insulin resistance/diabetes states and their possible regulation by insulin/leptin. We study in jejunum the relation between insulin signalling and insulin resistance in morbidly obese subjects with low (MO-low-IR) or with high insulin resistance (MO-high-IR), and with type 2 diabetes treated with metformin (MO-metf-T2DM)), and the effect of insulin/leptin on intestinal epithelial cells (IEC). Insulin receptor substrate-1 (IRS1) and the catalytic p110β subunit (p110β) of phosphatidylinositol 3-kinase (PI3K) were higher in MO-high-IR than in MO-low-IR. The regulatory p85α subunit of PI3K (p85α)/p110β ratio was lower in MO-high-IR and MO-metf-T2DM than in MO-low-IR. Akt-phosphorylation in Ser473 was reduced in MO-high-IR compared with MO-low-IR. IRS1 and p110-β were associated with insulin and leptin levels. The improvement of body mass index (BMI) and HOMA-IR (homeostasis model assessment of insulin resistance index) after bariatric surgery was associated with a higher IRS1 and a lower p85α/p110β ratio. IEC (intestinal epithelial cells) incubation with a high glucose + insulin dose produced an increase of p85α and p110β. High dose of leptin produced an increase of IRS1, p85α and p110β. In conclusion, despite the existence of insulin resistance, the jejunal expression of genes involved in insulin signalling was increased in MO-high-IR. Their expressions were regulated mainly by leptin. IRS1 and p85α/p110β ratio was associated with the evolution of insulin resistance after bariatric surgery.

Baicalein improves glucose metabolism in insulin resistant HepG2 cells

Insulin resistance (IR) is the primary pathogenesis of Type 2 diabetes mellitus (T2DM). Scutellaria baicalensis Georgi is a traditional Chinese herbal medicine, often used in the clinical treatment of T2DM. Baicalein which is considered to have anti-IR effects is one of its active ingredients. IR-induced HepG2 cells were used to investigate the effect of baicalein on glucose metabolism and insulin-signaling pathway, using metformin as a positive control. We found that the use of both baicalein and metformin increased the glucose consumption of IR cells, as well as increasing the pyruvate kinase (PK) and glucokinase (GCK) activity. Also increased was the expression levels of insulin receptor (InsR), insulin receptor substrate-1 (IRS-1), phosphoinositide 3-kinase (PI3K), protein kinase B (AKT) pathway and glucose transporter 2 (GLUT2). Reduced expression levels were found in that of glucose 6 phosphatase (G6Pase) and phosphoenolpyruvate carboxykinase (PEPCK) mRNA. The results confirmed that baicalein (10^-6 and 10^-5 mol/L) promotes glucose uptake and glycolysis, inhibits gluconeogenesis of hepatocytes to improve glucose metabolism, and may be as a result from regulation of InsR/IRS-1/PI3K/AKT pathway. Additionally, baicalein has large concentration range on inhibiting IR, and at lower concentrations has strong anti-IR hepatocyte activity.

Cellular Stresses and Stress Responses in the Pathogenesis of Insulin Resistance

Insulin resistance (IR), a key component of the metabolic syndrome, precedes the development of diabetes, cardiovascular disease, and Alzheimer's disease. Its etiological pathways are not well defined, although many contributory mechanisms have been established. This article summarizes such mechanisms into the hypothesis that factors like nutrient overload, physical inactivity, hypoxia, psychological stress, and environmental pollutants induce a network of cellular stresses, stress responses, and stress response dysregulations that jointly inhibit insulin signaling in insulin target cells including endothelial cells, hepatocytes, myocytes, hypothalamic neurons, and adipocytes. The insulin resistance-inducing cellular stresses include oxidative, nitrosative, carbonyl/electrophilic, genotoxic, and endoplasmic reticulum stresses; the stress responses include the ubiquitin-proteasome pathway, the DNA damage response, the unfolded protein response, apoptosis, inflammasome activation, and pyroptosis, while the dysregulated responses include the heat shock response, autophagy, and nuclear factor erythroid-2-related factor 2 signaling. Insulin target cells also produce metabolites that exacerbate cellular stress generation both locally and systemically, partly through recruitment and activation of myeloid cells which sustain a state of chronic inflammation. Thus, insulin resistance may be prevented or attenuated by multiple approaches targeting the different cellular stresses and stress responses.

Insulin/IGF-1 enhances intestinal epithelial crypt proliferation through PI3K/Akt, and not ERK signaling in obese humans

The intestinal epithelium is continuously regenerated through proliferation and differentiation of stem cells located in the intestinal crypts. Obesity affects this process and results in greater stem cell proliferation and altered tissue growth and function. Obesity-induced high levels of insulin and insulin-like growth factor-1 in the stem cell niche are found to impact proliferation in rodents indicating that insulin and insulin-like growth factor-1 receptors may play a role in modulating intestinal epithelial stem cell proliferation. To determine whether insulin or insulin-like growth factor-1 can induce proliferation in human intestinal epithelial stem cells, and if two downstream insulin and insulin-like growth factor-1 receptor signaling pathways, PI3K/Akt and ERK, are involved, we used primary small intestinal epithelial crypts isolated from obese humans and investigated (1) the effect of insulin or insulin-like growth factor-1 on crypt proliferation, and (2) the effect of insulin and insulin-like growth factor-1 signaling inhibitors on insulin or insulin-like growth factor-1-induced proliferation. We found that insulin and insulin-like growth factor-1 enhanced the proliferation of crypt cells, including intestinal epithelial stem cells. Inhibition of the PI3K/Akt pathway attenuated insulin and insulin-like growth factor-1-induced proliferation, but inhibition of the ERK pathway had no effect. These results suggest that the classical metabolic PI3K pathway and not the canonical proliferation ERK pathway is involved in the insulin/insulin-like growth factor-1-induced increase in crypt proliferation in obese humans, which may contribute to abnormal tissue renewal and function. Impact statement This study investigates if insulin or insulin-like growth factor-1 (IGF-1) induces intestinal epithelial proliferation in humans, and if insulin and IGF-1 receptor signaling is involved in this process in obesity. Although obesity-induced high levels of insulin and IGF-1 in the stem cell niche are found to impact the proliferation of intestinal epithelial stem cells in rodents, we are the first to investigate this effect in humans. We found that insulin and IGF-1 enhanced the proliferation of intestinal crypts (including stem cells and other crypt cells) isolated from obese humans, and PI3K/Akt, and not ERK signaling was involved in insulin or IGF-1-induced proliferation. The imbalance in signaling between PI3K/Akt and ERK pathways may point to a pathway-specific impairment in insulin/IGF-1 receptor signaling. We propose that this may contribute to reciprocal relationships between insulin/IGF-1 receptor resistance and intestinal epithelial proliferation that leads to abnormal tissue renewal and function.

Combined Hyperglycemia- and Hyperinsulinemia-Induced Insulin Resistance in Adipocytes Is Associated With Dual Signaling Defects Mediated by PKC-ζ

A hyperglycemic and hyperinsulinemic environment characteristic of type 2 diabetes causes insulin resistance. In adipocytes, defects in both insulin sensitivity and maximum response of glucose transport have been demonstrated. To investigate the molecular mechanisms, freshly isolated rat adipocytes were incubated in control (5.6 mM glucose, no insulin) and high glucose (20 mM)/high insulin (100 nM) (HG/HI) for 18 hours to induce insulin resistance. Insulin-resistant adipocytes manifested decreased sensitivity of glucose uptake associated with defects in insulin receptor substrate (IRS)-1 Tyr phosphorylation, association of p85 subunit of phosphatidylinositol-3-kinase, Akt Ser473 and Thr308 phosphorylation, accompanied by impaired glucose transporter 4 translocation. In contrast, protein kinase C (PKC)-ζ activity was augmented by chronic HG/HI. Inhibition of PKC-ζ with a specific cell-permeable peptide reversed the signaling defects and insulin sensitivity of glucose uptake. Transfection of dominant-negative, kinase-inactive PKC-ζ blocked insulin resistance, whereas constitutively active PKC-ζ recapitulated the defects. The HG/HI incubation was associated with stimulation of IRS-1 Ser318 and Akt Thr34 phosphorylation, targets of PKC-ζ. Transfection of IRS-1 S318A and Akt T34A each partially corrected insulin signaling, whereas combined transfection of both completely normalized insulin signaling. In vivo hyperglycemia/hyperinsulinemia in rats for 48 hours similarly resulted in activation of PKC-ζ and increased phosphorylation of IRS-1 Ser318 and Akt Thr34. These data indicate that impairment of insulin signaling by chronic HG/HI is mediated by dual defects at IRS-1 and Akt mediated by PKC-ζ.

Effects of propylene glycol or elevated luteinizing hormone during follicle development on ovulation, fertilization, and early embryo development

Seventeen nonlactating Holstein cows were superovulated in a Latin-square designed experiment to determine the effects of increased propylene glycol (PROP) and luteinizing hormone (LH) during antral follicle development on ovarian function, fertilization, and early embryo quality. PROP was orally drenched every 4 h for 7 days to induce hyperinsulinemia and associated metabolic changes. LH concentrations were altered by increasing LH (3-fold) during last 2 days of superovulation. Treatment groups were as follows: (1) control-oral drenching with water plus low-LH preparation; (2) high LH(HLH)-water plus HLH preparation; (3) PROP-drenching with PROP plus low LH; (4) PROP/HLH-PROP plus HLH. PROP increased glucose (P < 0.05) and insulin (P < 0.02) concentrations at all time points analyzed. Neither PROP nor LH affected numbers of follicles > 9 mm at time of gonadotropin-releasing hormone-induced LH surge, although percentage of these follicles that ovulated was decreased by both PROP (P = 0.002) and LH (P = 0.048). In addition, PROP tended (P = 0.056) to decrease total number of ovulations. PROP reduced (P = 0.028) fertilization rate, while LH tended (P = 0.092) to increase fertilization rate. There was no effect of either PROP or LH on any measure of embryo quality including percentage of embryos that were degenerate, quality 1, or quality 1 and 2 of total structures collected or fertilized structures. These results indicate that acute elevation in insulin during the preovulatory follicular wave can decrease percentage of large follicles that ovulate, particularly when combined with increased LH, and reduce fertilization of ovulated oocytes.

Glucose Uptake Measurement and Response to Insulin Stimulation in In Vitro Cultured Human Primary Myotubes

Skeletal muscle is the largest glucose deposit in mammals and largely contributes to glucose homeostasis. Assessment of insulin sensitivity of muscle cells is of major relevance for all studies dedicated to exploring muscle glucose metabolism and characterizing metabolic alterations. In muscle cells, glucose transporter type 4 (GLUT4) proteins translocate to the plasma membrane in response to insulin, thus allowing massive entry of glucose into the cell. The ability of muscle cells to respond to insulin by increasing the rate of glucose uptake is one of the standard readouts to quantify muscle cell sensitivity to insulin. Human primary myotubes are a suitable in vitro model, as the cells maintain many features of the donor phenotype, including insulin sensitivity. This in vitro model is also suitable for the test of any compounds that could impact insulin responsiveness. Measurements of the glucose uptake rate in differentiated myotubes reflect insulin sensitivity. In this method, human primary muscle cells are cultured in vitro to obtain differentiated myotubes, and glucose uptake rates with and without insulin stimulation are measured. We provide a detailed protocol to quantify passive and active glucose transport rates using radiolabeled [³H] 2-deoxy-D-Glucose ([³H]2dG). Calculation methods are provided to quantify active basal and insulin-stimulated rates, as well as stimulation fold.

MicroRNA 21 is up-regulated in adipose tissue of obese diabetic subjects

We investigated miR21 expression in omental (OAT) and subcutaneous adipose tissue (SAT) from 16 obese subjects undergoing bariatric surgery. Patients were divided into two age- and BMI-matched groups according to the presence of type 2 diabetes (T2D). miR21 was not differently expressed in OAT and SAT. However, miR21 expression was two folds greater in adipose tissue in patients with T2D. Accordingly, in primary cultures of adipocytes from non diabetic overweight subjects, miR21 expression increased after 24-h exposure to high glucose and insulin. In conclusion, miR21 appears linked to insulin-resistance deterioration within its pathophysiologic progression from obesity to T2D.

Irisin ameliorates hepatic glucose/lipid metabolism and enhances cell survival in insulin-resistant human HepG2 cells through adenosine monophosphate-activated protein kinase signaling

Irisin is a newly identified myokine that promotes the browning of white adipose tissue, enhances glucose uptake in skeletal muscle and modulates hepatic metabolism. However, the signaling pathways involved in the effects on hepatic glucose and lipid metabolism have not been resolved. This study aimed to examine the role of irisin in the regulation of hepatic glucose/lipid metabolism and cell survival, and whether adenosine monophosphate-activated protein kinase (AMPK), a master metabolic regulator in the liver, is involved in irisin's actions. Human liver-derived HepG2 cells were cultured in normal glucose-normal insulin (NGNI) or high glucose-high insulin (HGHI/insulin-resistant) condition. Hepatic glucose and lipid metabolism was evaluated by glucose output and glycogen content or triglyceride accumulation assays, respectively. Our results showed that irisin stimulated phosphorylation of AMPK and acetyl-CoA-carboxylase (ACC) via liver kinase B1 (LKB1) rather than Ca(2+)/calmodulin-dependent protein kinase kinase β (CaMKKβ) in HepG2 cells. Irisin ameliorated hepatic insulin resistance induced by HGHI condition. Irisin reduced hepatic triglyceride content and glucose output, but increased glycogen content, with those effects reversed by dorsomorphin, an AMPK inhibitor. Furthermore, irisin also stimulated extracellular signal-regulated kinase (ERK) 1/2 phosphorylation and promoted cell survival in an AMPK-dependent manner. In conclusion, our data indicate that irisin ameliorates dysregulation of hepatic glucose/lipid metabolism and cell death in insulin-resistant states via AMPK activation. These findings reveal a novel irisin-mediated protective mechanism in hepatic metabolism which provides a scientific basis for irisin as a potential therapeutic target for the treatment of insulin resistance and type 2 diabetes mellitus.

Effects of glucose and insulin on secretion of amyloid-β by human adipose tissue cells

OBJECTIVE

Obesity and type 2 diabetes mellitus are risk factors for developing Alzheimer disease. Overlapping patterns of metabolic dysfunction may be common molecular links between these complex diseases. Amyloid-β (Aβ) precursor protein and associated β- and γ-secretases are expressed in adipose tissue. Aβ precursor protein is up-regulated with obesity and correlated to insulin resistance. Aβ may be secreted by adipose tissue, its production may be regulated through metabolic pathways, and Aβ may exert effects on adipose tissue insulin receptor signaling.

METHODS

Human stromal-vascular cells and differentiated adipocytes were cultured with different combinations of glucose and insulin and then assayed for Aβ in conditioned media. Aβ was measured in vivo using adipose tissue microdialysis.

RESULTS

Aβ secretion was increased by glucose and insulin in vitro. Adipose tissue microdialysates contained Aβ. Adipocytes treated with Aβ had decreased expression of insulin receptor substrate-2 and reduced Akt-1 phosphorylation.

CONCLUSIONS

Aβ was made by adipose tissue cells in vitro at concentrations similar to in vivo measurements. Regulation of Aβ production by glucose and insulin and effects of Aβ on the insulin receptor pathway suggest similar cellular mechanisms may exist between neuronal dysfunction in Alzheimer disease and adipose dysfunction in type 2 diabetes.

Selective Insulin Resistance in the Kidney

Insulin resistance has been characterized as attenuation of insulin sensitivity at target organs and tissues, such as muscle and fat tissues and the liver. The insulin signaling cascade is divided into major pathways such as the PI3K/Akt pathway and the MAPK/MEK pathway. In insulin resistance, however, these pathways are not equally impaired. For example, in the liver, inhibition of gluconeogenesis by the insulin receptor substrate (IRS) 2 pathway is impaired, while lipogenesis by the IRS1 pathway is preserved, thus causing hyperglycemia and hyperlipidemia. It has been recently suggested that selective impairment of insulin signaling cascades in insulin resistance also occurs in the kidney. In the renal proximal tubule, insulin signaling via IRS1 is inhibited, while insulin signaling via IRS2 is preserved. Insulin signaling via IRS2 continues to stimulate sodium reabsorption in the proximal tubule and causes sodium retention, edema, and hypertension. IRS1 signaling deficiency in the proximal tubule may impair IRS1-mediated inhibition of gluconeogenesis, which could induce hyperglycemia by preserving glucose production. In the glomerulus, the impairment of IRS1 signaling deteriorates the structure and function of podocyte and endothelial cells, possibly causing diabetic nephropathy. This paper mainly describes selective insulin resistance in the kidney, focusing on the proximal tubule.

Effects of high glucose on caveolin-1 and insulin signaling in 3T3-L1 adipocytes

Adipocytes exposed to high glucose concentrations exhibit impaired metabolic function, including an increase of oxidative and proinflammatory factors that might favor the development of insulin resistance. Caveolin-1 (Cav-1) is a key mediator of the insulin transduction pathway whose expression is significantly enhanced during adipocyte differentiation. In this work, we studied the effects of high glucose concentration on the regulation of Cav-1 expression and activation and its relation to the insulin signaling pathway during the adipogenic process and in long-term differentiated adipocytes. Both, long-term high glucose exposure during adipogenesis and short-term glucose incubation of mature adipocytes, promoted triglyceride accumulation in 3T3-L1 cells. The short-term exposure of mature adipocytes to high glucose significantly reduced the sensitivity to insulin of Cav-1, insulin receptor (IR) and potein kinase B (AKT-2) phosphorylation, as well as insulin-induced deoxyglucose uptake. Adipocytes differentiated in the presence of high glucose lost Cav-1 and IR response to insulin-stimulated phosphorylation, but maintained the insulin sensitivity of AKT-2 phosphorylation and deoxyglucose uptake. Although long-term high glucose exposure increased DNA methylation in Cav-1 promoter, Cav-1 expression was not affected. Moreover, these cells showed an increase of Cav-1, IR and AKT-2 protein content, pointing to an adaptive response induced by the long-term high glucose exposure.

Transmembrane tumor necrosis factor-alpha sensitizes adipocytes to insulin

Transmembrane TNF-α (tmTNF-α) acts both as a ligand, delivering 'forward signaling' via TNFR, and as a receptor, transducing 'reverse signaling'. The contradiction of available data regarding the effect of tmTNF-α on insulin resistance may be due to imbalance in both signals. Here, we demonstrated that high glucose-induced impairment of insulin-stimulated glucose uptake by 3T3-L1 adipocytes was concomitant with decreased tmTNF-α expression and increased soluble TNF-α (sTNF-α) secretion. However, when TACE was inhibited, preventing the conversion of tmTNF-α to sTNF-α, this insulin resistance was partially reversed, indicating a salutary role of tmTNF-α. Treatment of 3T3-L1 adipocytes with exogenous tmTNF-α promoted insulin-induced phosphorylation of IRS-1 and Akt, facilitated GLUT4 expression and membrane translocation, and increased glucose uptake while addition of sTNF-α resulted in the opposite effect. Furthermore, tmTNF-α downregulated the production of IL-6 and MCP-1 via NF-κB inactivation, as silencing of A20, an inhibitor for NF-κB, by siRNA, abolished this effect of tmTNF-α. However, tmTNF-α upregulated adiponectin expression through the PPAR-γ pathway, as inhibition of PPAR-γ by GW9662 abrogated both tmTNF-α-induced adiponectin transcription and glucose uptake. Our data suggest that tmTNF-α functions as an insulin sensitizer via forward signaling.

Female fetus is associated with greater maternal insulin resistance in pregnancy

AIM

To explore the hypothesis that female fetus is associated with greater maternal insulin resistance during pregnancy.

METHODS

In a singleton pregnancy cohort study (n = 299), we compared maternal insulin resistance according to fetal sex, based on plasma biomarkers from a 50-g 1-h oral glucose tolerance test at 24-28 weeks gestation. The primary outcome was plasma glucose-to-insulin ratio. Other outcomes included plasma proinsulin-to-insulin ratio, and insulin, proinsulin, leptin, adiponectin and insulin-like growth factor I and II concentrations.

RESULTS

After adjusting for maternal race, age, parity, education, pre-pregnancy BMI, smoking and alcohol use, history of gestational diabetes, and gestational age at blood sampling, plasma insulin concentrations were significantly higher (mean ± sd: 66.4 ± 50.5 vs. 51.0 ± 46.1 mU/l; adjusted P = 0.001), and glucose-to-insulin ratios significantly lower (2.60 ± 2.03 vs. 3.77 ± 4.98 mg/dl/mU/l; adjusted P = 0.002) in women bearing a female vs those bearing a male fetus, despite similar glucose levels (116.4 ± 27.2 vs. 117.0 ± 31.9 mg/dl; adjusted P = 0.92).There were no significant differences in proinsulin-to-insulin ratios, or leptin, adiponectin, insulin-like growth factor I and insulin-like growth factor II concentrations by fetal sex.

CONCLUSION

Female fetus may be associated with greater maternal insulin resistance during pregnancy.

The detrimental effects of acute hyperglycemia on myocardial glucose uptake

AIMS

Although acute hyperglycemic (AHG) episodes are linked to lower glucose uptake, underlying mechanisms remain unclear. We hypothesized that AHG triggers reactive oxygen species (ROS) production and increases non-oxidative glucose pathway (NOGP) activation, i.e. stimulation of advanced glycation end products (AGE), polyol pathway (PP), hexosamine biosynthetic pathway (HBP), PKC; thereby decreasing cardiac glucose uptake.

MAIN METHODS

H9c2 cardiomyoblasts were exposed to 25 mM glucose for 24h vs. 5.5mM controls ± modulating agents during the last hour of glucose exposure: a) antioxidant #1 for mitochondrial ROS (250 μM 4-OHCA), b) antioxidant #2 for NADPH oxidase-generated ROS (100 μM DPI), c) NOGP inhibitors - 100 μM aminoguanidine (AGE), 5 μM chelerythrine (PKC); 40 μM DON (HBP); and 10 μM zopolrestat (PP). ROS levels (mitochondrial, intracellular) and glucose uptake were evaluated by flow cytometry.

KEY FINDINGS

AHG elevated ROS, activated NOGPs and blunted glucose uptake. Transketolase activity (pentose phosphate pathway [PPP] marker) did not change. Respective 4-OHCA and DPI treatment blunted ROS production, diminished NOGP activation and normalized glucose uptake. NOGP inhibitory studies identified PKCβII as a key downstream player in lowering insulin-mediated glucose uptake. When we employed an agent (benfotiamine) known to shunt flux away from NOGPs (into PPP), it decreased ROS generation and NOGP activation, and restored glucose uptake under AHG conditions.

SIGNIFICANCE

This study demonstrates that AHG elicits maladaptive events that function in tandem to reduce glucose uptake, and that antioxidant treatment and/or attenuation of NOGP activation (PKC, polyol pathway) may limit the onset of insulin resistance.

The Chinese herbal medicine FTZ attenuates insulin resistance via IRS1 and PI3K in vitro and in rats with metabolic syndrome

BACKGROUND

Insulin resistance plays an important role in the development of metabolic syndrome (MS). Fu Fang Zhen Zhu Tiao Zhi formula (FTZ), a Chinese medicinal decoction, has been used to relieve hyperlipidemia, atherosclerosis and other symptoms associated with metabolic disorders in the clinic.

METHODS

To evaluate the effect of FTZ on insulin resistance, HepG2 cells were induced with high insulin as a model of insulin resistance and treated with FTZ at one of three dosages. Next, the levels of glucose content, insulin receptor substrate1 (IRS1) protein expression and phosphatidylinositol 3-kinase (PI3K) subunit p85 mRNA expression were measured. Alternatively, MS was induced in rats via gavage feeding of a high-fat diet for four consecutive weeks followed by administration of FTZ for eight consecutive weeks. Body weight and the plasma levels of lipids, insulin and glucose were evaluated. Finally, the expression of PI3K p85 mRNA in adipose tissue of rats was measured.

RESULTS

Our results revealed that FTZ attenuated glucose content and up-regulated the expression of PI3K p85 mRNA and IRS1 protein in insulin-resistant HepG2 cells in vitro. Moreover, FTZ reduced body weight and the plasma concentrations of triacylglycerol, cholesterol, fasting glucose and insulin in insulin resistant MS rats. FTZ also elevated the expression of PI3K p85 mRNA in the adipose tissues of MS rats.

CONCLUSION

FTZ attenuated MS symptoms by decreasing the plasma levels of glucose and lipids. The underlying mechanism was attenuation of the reduced expression of PI3K p85 mRNA and IRS1 protein in both insulin-resistant HepG2 cells and MS rats.

Insulin stimulates translocation of human GLUT4 to the membrane in fat bodies of transgenic Drosophila melanogaster

The fruit fly Drosophila melanogaster is an excellent model system for studies of genes controlling development and disease. However, its applicability to physiological systems is less clear because of metabolic differences between insects and mammals. Insulin signaling has been studied in mammals because of relevance to diabetes and other diseases but there are many parallels between mammalian and insect pathways. For example, deletion of Drosophila Insulin-Like Peptides resulted in 'diabetic' flies with elevated circulating sugar levels. Whether this situation reflects failure of sugar uptake into peripheral tissues as seen in mammals is unclear and depends upon whether flies harbor the machinery to mount mammalian-like insulin-dependent sugar uptake responses. Here we asked whether Drosophila fat cells are competent to respond to insulin with mammalian-like regulated trafficking of sugar transporters. Transgenic Drosophila expressing human glucose transporter-4 (GLUT4), the sugar transporter expressed primarily in insulin-responsive tissues, were generated. After expression in fat bodies, GLUT4 intracellular trafficking and localization were monitored by confocal and total internal reflection fluorescence microscopy (TIRFM). We found that fat body cells responded to insulin with increased GLUT4 trafficking and translocation to the plasma membrane. While the amplitude of these responses was relatively weak in animals reared on a standard diet, it was greatly enhanced in animals reared on sugar-restricted diets, suggesting that flies fed standard diets are insulin resistant. Our findings demonstrate that flies are competent to mobilize translocation of sugar transporters to the cell surface in response to insulin. They suggest that Drosophila fat cells are primed for a response to insulin and that these pathways are down-regulated when animals are exposed to constant, high levels of sugar. Finally, these studies are the first to use TIRFM to monitor insulin-signaling pathways in Drosophila, demonstrating the utility of TIRFM of tagged sugar transporters to monitor signaling pathways in insects.

The association of adipose-derived dimethylarginine dimethylaminohydrolase-2 with insulin sensitivity in experimental type 2 diabetes mellitus

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide synthase (NOS), which can be hydrolyzed by dimethylarginine-dimethylaminohydrolase (DDAH). It has been reported that adipocytes can produce DDAH/ADMA, but its role remains unknown. In the present study, we examined the effects of adipocyte-derived DDAH/ADMA on insulin sensitivity using animal and cell models. Results showed that in adipose tissue of high fat diet-fed diabetic rats, as well as in high glucose (25 mM) plus insulin (100 nM)-treated 3T3-L1 adipocytes, expression levels of insulin receptor substance-1 (IRS-1), glucose transporter-4 (GLUT-4), and DDAH isoform-2 (DDAH-2) were down-regulated compared with control, although DDAH-1 expression showed no significant changes. We also observed that nitric oxide bioavailability, DDAH and NOS activities were subsequently decreased, while the local ADMA content was elevated in diabetic adipose tissue. Transfection of human DDAH-2 gene into high glucose- and insulin-treated 3T3-L1 adipocytes significantly ameliorated DDAH activity, reduced ADMA contents, and up-regulated the mRNA expression levels of IRS-1 and GLUT-4. These findings suggested that in the development of type 2 diabetes mellitus, local DDAH-2 in adipocytes might play an important role in regulating insulin sensitivity.

Effect of glucocorticoid-induced insulin resistance on follicle development and ovulation

Polycystic ovarian syndrome (PCOS) is characterized by hyperandrogenemia, polycystic ovaries, and menstrual disturbance and a clear association with insulin resistance. This research evaluated whether induction of insulin resistance, using dexamethasone (DEX), in a monovular animal model, the cow, could produce an ovarian phenotype similar to PCOS. In all of these experiments, DEX induced insulin resistance in cows as shown by increased glucose, insulin, and HOMA-IR (homeostasis model assessment of insulin resistance). Experiment 1: DEX induced anovulation (zero of five DEX vs. four of four control cows ovulated) and decreased circulating estradiol (E2). Experiment 2: Gonadotropin-releasing hormone (GnRH) was administered to determine pituitary and follicular responses during insulin resistance. GnRH induced a luteinizing hormone (LH) surge and ovulation in both DEX (seven of seven) and control (seven of seven) cows. Experiment 3: E2 was administered to determine hypothalamic responsiveness after induction of an E2 surge in DEX (eight of eight) and control (eight of eight) cows. An LH surge was induced in control (eight of eight) but not DEX (zero of eight) cows. All control (eight of eight) but only two of eight DEX cows ovulated within 60 h of E2 administration. Experiment 4: Short-term DEX was initiated 24 h after induced luteal regression to determine if DEX could acutely block ovulation before peak insulin resistance was induced, similar to progesterone (P4). All control (five of five), no P4-treated (zero of six), and 50% of DEX-treated (three of six) cows ovulated by 96 h after luteal regression. All anovular cows had reduced circulating E2. These data are consistent with DEX creating a lesion in hypothalamic positive feedback to E2 without altering pituitary responsiveness to GnRH or ovulatory responsiveness of follicles to LH. It remains to be determined if the considerable insulin resistance and the reduced follicular E2 production induced by DEX had any physiological importance in the induction of anovulation.

Prostaglandin F2α facilitates collagen synthesis in cardiac fibroblasts via an F-prostanoid receptor/protein kinase C/Rho kinase pathway independent of transforming growth factor β1

Accumulation of collagen I and III in the myocardium is a prominent feature of interstitial fibrosis. Prostaglandin F(2α) (PGF(2α)) facilitates fibrosis by increasing collagen synthesis. However, the underlying mechanisms mediating the effect of PGF(2α) on collagen expression in cardiac fibroblasts are not yet fully elucidated. We measured the mRNA and protein levels of collagen I and III by quantitative real-time PCR and ELISA, respectively. Activation of signaling pathways was determined by western blot analysis. In primary rat cardiac fibroblasts, treatment with PGF(2α) stimulated both the mRNA and protein levels of collagen I and III, and pretreatment with the F-prostanoid (FP) receptor antagonist AL-8810, protein kinase C inhibitor LY-333531, and Rho kinase inhibitor Y-27632 significantly inhibited PGF(2α)-induced collagen I and III expression. FP receptor, protein kinase C, and Rho kinase were activated with PGF(2α) treatment. PGF(2α) may be an important regulator in the synthesis of collagen I and III via an FP receptor/protein kinase C/Rho kinase cascade in cardiac fibroblasts, which might be a new therapeutic target for myocardial fibrosis.

Effects of hyperinsulinemia on lipolytic function of three-dimensional adipocyte/endothelial co-cultures

The increased incidence of both type 2 diabetes mellitus and obesity has prompted the need to develop physiologically relevant adipose tissue models for controlled study of both normal and diseased adipose functions. Insulin resistance, characteristic of both type 2 diabetes mellitus and obesity, is often preceded by hyperinsulinemia. We propose here a three-dimensional (3D) co-culture adipose tissue model to study the effects of high insulin exposure, which enabled the study of physiological cell responses to hyperinsulinemic conditions. Two-dimensional adipocyte studies were initially conducted to establish a baseline control in which insulin levels were established. Adipocytes and endothelial cells were subsequently co-cultured on 3D porous silk fibroin scaffolds in normal or high insulin concentrations, and their physiological responses were assessed with respect to lipogenesis and lipolysis. High insulin levels stimulated both an increase in triglyceride accumulation and a decrease in lipolysis levels compared to that of normal insulin conditions. In contrast, adipocyte monocultures did not exhibit any differences between insulin levels. The ability of this 3D system to elicit physiological responses to hyperinsulinemia in co-culture serves as a significant step forward in adipose tissue engineering. The development of physiologically relevant 3D in vitro adipose tissue models presents promise for the study of disease mechanisms as well as in assessing therapeutic treatments.

Insulin-like growth factor-I-stimulated insulin receptor substrate-1 negatively regulates Src homology 2 domain-containing protein-tyrosine phosphatase substrate-1 function in vascular smooth muscle cells

Vascular smooth muscle cells maintained in normal (5.6 mm) glucose respond to insulin-like growth factor-I (IGF-I) with increased protein synthesis but do not proliferate. In contrast, hyperglycemia alters responsiveness to IGF-I, resulting in increased SHPS-1 phosphorylation and assembly of a signaling complex that enhances MAPK and phosphatidylinositol 3-kinase pathways. Hyperglycemia also reduces the basal IRS-1 concentration and IGF-I-stimulated IRS-1-linked signaling. To determine if failure to down-regulate IRS-1 alters vascular smooth muscle cell (VSMC) responses to IGF-I, we overexpressed IRS-1 in VSMCs maintained in high glucose. These cultures showed reduced SHPS-1 phosphorylation, transfer of SHP-2 to SHPS-1, and impaired Shc and MAPK phosphorylation and cell proliferation in response to IGF-I. In vitro studies demonstrated that SHPS-1 was a substrate for type I IGF receptor (IGF-IR) and that IRS-1 competitively inhibited SHPS-1 phosphorylation. Exposure of VSMC cultures to a peptide that inhibited IRS-1/IGF-IR interaction showed that IRS-1 binding to IGF-IR impairs SHPS-1 phosphorylation in vivo. IRS-1 also sequestered SHP-2. Expression of an IRS-1 mutant (Y1179F/Y1229F) reduced IRS-1/SHP-2 association, and exposure of cells expressing the mutant to the inhibitory peptide enhanced SHPS-1 phosphorylation and SHP-2 transfer. This result was confirmed by expressing an IRS-1 mutant that had both impaired binding to IGF-IR and to SHP-2 IGF-I increased SHPS-1 phosphorylation, SHP-2 association with SHPS-1, Shc MAPK phosphorylation, and proliferation in cells expressing the mutant. We conclude that IRS-1 is an important factor for maintaining VSMCs in the non-proliferative state and that its down-regulation is a component of the VSMC response to hyperglycemic stress that results in an enhanced response to IGF-I.

Insulin therapy and type 2 diabetes: management of weight gain

The potential for insulin-related weight gain in patients with type 2 diabetes presents a therapeutic dilemma and frequently leads to delays in the initiation of insulin therapy. It also poses considerable challenges when treatment is intensified. Addressing insulin-related weight gain is highly relevant to the prevention of metabolic and cardiovascular consequences in this high-risk population with type 2 diabetes. In addition to lifestyle changes (eg, diet and exercise) and available medical interventions to minimize the risk of weight gain with insulin treatment, familiarity with the weight gain patterns of different insulins may help deal with this problem. The use of basal insulin analogs may offer advantages over conventional human insulin preparations in terms of more physiologic time-action profiles, reduced risk of hypoglycemia, and reduced weight gain.

Changes in insulin sensitivity and insulin release in relation to glycemia and glucose tolerance in 6,414 Finnish men

OBJECTIVE

We evaluated insulin sensitivity and insulin secretion across the entire range of fasting (FPG) and 2-h plasma glucose (PG), and we investigated the differences in insulin sensitivity and insulin release in different glucose tolerance categories.

RESEARCH DESIGN AND METHODS

A total of 6,414 Finnish men (aged 57 +/- 7 years, BMI 27.0 +/- 3.9 kg/m2) from our ongoing population-based METSIM (Metabolic Syndrome in Men) study were included. Of these subjects, 2,168 had normal glucose tolerance, 2,859 isolated impaired fasting glucose (IFG), 217 isolated impaired glucose tolerance (IGT), 701 a combination of IFG and IGT, and 469 newly diagnosed type 2 diabetes.

RESULTS

The Matsuda index of insulin sensitivity decreased substantially within the normal range of FPG (-17%) and 2-h PG (-37%) and was approximately -65 and -53% in the diabetic range of FPG and 2-h PG, respectively, compared with the reference range (FPG and 2-h PG <5.0 mmol/l). Early-phase insulin release declined by only approximately -5% within the normal range of FPG and 2-h PG but decreased significantly in the diabetic range of FPG (by 32-70%) and 2-h PG (by 33-51%). Changes in insulin sensitivity and insulin secretion in relation to hyperglycemia were independent of obesity. The predominant feature of isolated IGT was impaired peripheral insulin sensitivity. Isolated IFG was characterized by impaired early and total insulin release.

CONCLUSIONS

Peripheral insulin sensitivity was already decreased substantially at low PG levels within the normoglycemic range, whereas impairment in insulin secretion was observed mainly in the diabetic range of FPG and 2-h PG. Obesity did not affect changes in insulin sensitivity or insulin secretion in relation to hyperglycemia.

Metabolic stress in insulin's target cells leads to ROS accumulation - A hypothetical common pathway causing insulin resistance

The metabolic syndrome is a cluster of cardiovascular risk factors, and visceral adiposity is a central component that is also strongly associated with insulin resistance. Both visceral obesity and insulin resistance are important risk factors for the development of type 2 diabetes. It is likely that adipose tissue, particularly in the intra-abdominal depot, is part of a complex interplay involving several tissues and that dysregulated hormonal, metabolic and neural signalling within and between organs can trigger development of metabolic disease. One attractive hypothesis is that many factors leading to insulin resistance are mediated via the generation of abnormal amounts of reactive oxygen species (ROS). There is much evidence supporting that detrimental effects of glucose, fatty acids, hormones and cytokines leading to insulin resistance can be exerted via such a common pathway. This review paper mainly focuses on metabolic and other 'stress' factors that affect insulin's target cells, in particular adipocytes, and it will highlight oxidative stress as a potential unifying mechanism by which these stress factors promote insulin resistance and the development and progression of type 2 diabetes.