Diverse signaling pathways in the cellular actions of insulin

Dietary Flavonoids in the Prevention of T2D: An Overview

Type 2 diabetes (T2D) is a progressive metabolic disease that is increasing in prevalence globally. It is well established that insulin resistance (IR) and a progressive decline in functional β-cell mass are hallmarks of developing T2D. Obesity is a leading pathogenic factor for developing IR. Constant IR will progress to T2D when β-cells are unable to secret adequate amounts of insulin to compensate for decreased insulin sensitivity. Recently, a considerable amount of research has been devoted to identifying naturally occurring anti-diabetic compounds that are abundant in certain types of foods. Flavonoids are a group of polyphenols that have drawn great interest for their various health benefits. Results from many clinical and animal studies demonstrate that dietary intake of flavonoids might be helpful in preventing T2D, although cellular and molecular mechanisms underlying these effects are still not completely understood. This review discusses our current understanding of the pathophysiology of T2D and highlights the potential anti-diabetic effects of flavonoids and mechanisms of their actions.

Hyperglycemia- and hyperinsulinemia-induced insulin resistance causes alterations in cellular bioenergetics and activation of inflammatory signaling in lymphatic muscle

Insulin resistance is a well-known risk factor for obesity, metabolic syndrome (MetSyn) and associated cardiovascular diseases, but its mechanisms are undefined in the lymphatics. Mesenteric lymphatic vessels from MetSyn or LPS-injected rats exhibited impaired intrinsic contractile activity and associated inflammatory changes. Hence, we hypothesized that insulin resistance in lymphatic muscle cells (LMCs) affects cell bioenergetics and signaling pathways that consequently alter contractility. LMCs were treated with different concentrations of insulin or glucose or both at various time points to determine insulin resistance. Onset of insulin resistance significantly impaired glucose uptake, mitochondrial function, oxygen consumption rates, glycolysis, lactic acid, and ATP production in LMCs. Hyperglycemia and hyperinsulinemia also impaired the PI3K/Akt while enhancing the ERK/p38MAPK/JNK pathways in LMCs. Increased NF-κB nuclear translocation and macrophage chemoattractant protein-1 and VCAM-1 levels in insulin-resistant LMCs indicated activation of inflammatory mechanisms. In addition, increased phosphorylation of myosin light chain-20, a key regulator of lymphatic muscle contraction, was observed in insulin-resistant LMCs. Therefore, our data elucidate the mechanisms of insulin resistance in LMCs and provide the first evidence that hyperglycemia and hyperinsulinemia promote insulin resistance and impair lymphatic contractile status by reducing glucose uptake, altering cellular metabolic pathways, and activating inflammatory signaling cascades.-Lee, Y., Fluckey, J. D., Chakraborty, S., Muthuchamy, M. Hyperglycemia- and hyperinsulinemia-induced insulin resistance causes alterations in cellular bioenergetics and activation of inflammatory signaling in lymphatic muscle.

The action of antidiabetic plants of the canadian james bay cree traditional pharmacopeia on key enzymes of hepatic glucose homeostasis

We determined the capacity of putative antidiabetic plants used by the Eastern James Bay Cree (Canada) to modulate key enzymes of gluconeogenesis and glycogen synthesis and key regulating kinases. Glucose-6-phosphatase (G6Pase) and glycogen synthase (GS) activities were assessed in cultured hepatocytes treated with crude extracts of seventeen plant species. Phosphorylation of AMP-dependent protein kinase (AMPK), Akt, and Glycogen synthase kinase-3 (GSK-3) were probed by Western blot. Seven of the seventeen plant extracts significantly decreased G6Pase activity, Abies balsamea and Picea glauca, exerting an effect similar to insulin. This action involved both Akt and AMPK phosphorylation. On the other hand, several plant extracts activated GS, Larix laricina and A. balsamea, far exceeding the action of insulin. We also found a significant correlation between GS stimulation and GSK-3 phosphorylation induced by plant extract treatments. In summary, three Cree plants stand out for marked effects on hepatic glucose homeostasis. P. glauca affects glucose production whereas L. laricina rather acts on glucose storage. However, A. balsamea has the most promising profile, simultaneously and powerfully reducing G6Pase and stimulating GS. Our studies thus confirm that the reduction of hepatic glucose production likely contributes to the therapeutic potential of several antidiabetic Cree traditional medicines.

Effect of a novel proteoglycan PTP1B inhibitor from Ganoderma lucidum on the amelioration of hyperglycaemia and dyslipidaemia in db/db mice

Protein tyrosine phosphatase 1B (PTP1B) is implicated in the negative regulation of the insulin signalling pathway by dephosphorylating the insulin receptor (IR) and IR substrates. Ganoderma lucidum has traditionally been used for the treatment of diabetes in Chinese medicine; however, its anti-diabetic potency and mechanism in vivo is still unclear. Our previously published study reported a novel proteoglycan PTP1B inhibitor, named Fudan-Yueyang-Ganoderma lucidum (FYGL) from G. lucidum, with a half-maximal inhibitory concentration (IC₅₀) value of 5·12 (sem 0·05) μg/ml, a protein:polyglycan ratio of 17:77 and 78 % glucose in polysaccharide, and dominant amino acid residues of aspartic acid, glycine, glutamic acid, alanine, serine and threonine in protein. FYGL is capable of decreasing plasma glucose in streptozotocin-induced diabetic mice with a high safety of median lethal dose (LD₅₀) of 6 g/kg. In the present study, C57BL/6 db/db diabetic mice were trialed further using FYGL as well as metformin for comparison. Oral treatment with FYGL in db/db diabetic mice for 4 weeks significantly (P < 0·01 or 0·05) decreased the fasting plasma glucose level, serum insulin concentration and the homeostasis model assessment of insulin resistance. FYGL also controlled the biochemistry indices relative to type 2 diabetes-accompanied lipidaemic disorders. Pharmacology research suggests that FYGL decreases the plasma glucose level by the mechanism of inhibiting PTP1B expression and activity, consequently, regulating the tyrosine phosphorylation level of the IR β-subunit and the level of hepatic glycogen, thus resulting in the improvement of insulin sensitivity. Therefore, FYGL is promising as an insulin sensitiser for the therapy of type 2 diabetes and accompanied dyslipidaemia.

Insulin-stimulated translocation of glucose transporter (GLUT) 12 parallels that of GLUT4 in normal muscle

CONTEXT

GLUT4 is the predominant glucose transporter isoform expressed in fat and muscle. In GLUT4 null mice, insulin-stimulated glucose uptake into muscle was diminished but not eliminated, suggesting that another insulin-sensitive system was present.

OBJECTIVE

This study was intended to determine whether insulin caused GLUT12 translocation in muscle.

DESIGN

Six normal volunteers had muscle biopsies before and after euglycemic insulin infusions.

SETTING

Infusions and biopsies were performed in an outpatient clinic.

PARTICIPANTS

Subjects were nonobese, young adults with no family history of diabetes.

MAIN OUTCOME MEASURES

GLUT12, GLUT4, and GLUT1 proteins were quantified in muscle biopsy fractions. Cultured myoblasts were used to determine whether GLUT12 translocation was phosphatidyl inositol-3 kinase (PI3-K)-dependent.

INTERVENTION

Insulin was infused at 40 mU/m(2) x min for 3 h.

RESULTS

In human muscle, insulin caused a shift of a portion of GLUT12 from intracellular low-density microsomes to the plasma membrane (PM) fraction (17% in PM at baseline, 38% in PM after insulin). Insulin increased GLUT4 in PM from 13 to 42%. GLUT1 was predominantly in the PM fractions at baseline and did not change significantly after insulin. L6 myoblasts in culture also expressed and translocated GLUT12 in response to insulin, but inhibiting PI3-K prevented the translocation of GLUT12 and GLUT4.

CONCLUSIONS

Insulin causes GLUT12 to translocate from an intracellular location to the plasma membrane in normal human skeletal muscle. Translocation of GLUT12 in cultured myoblasts was dependent on activation of PI3-K. GLUT12 may have evolutionarily preceded GLUT4 and now provides redundancy to the dominant GLUT4 system in muscle.

Rab GTPase-activating protein AS160 is a major downstream effector of protein kinase B/Akt signaling in pancreatic beta-cells

OBJECTIVE

Protein kinase B/Akt plays a central role in beta-cells, but little is known regarding downstream Akt substrates in these cells. Recently, Rab GTPase-activating protein AS160, a substrate of Akt, was shown to be involved in insulin modulation of GLUT4 trafficking in skeletal muscle and adipose tissue. The aim of this study was to investigate the expression and potential role of AS160 in beta-cells.

RESEARCH DESIGN AND METHODS

AS160 mRNA expression was measured in mouse and human islets and fluorescence-activated cell sorted beta-cells and compared in islets from control subjects versus individuals with type 2 diabetes. For knockdown experiments, transformed mouse insulin-secreting MIN6B1 cells were transfected with pSUPER-GFP plasmid encoding a small hairpin RNA against insulin receptor substrate (IRS)-2, AS160, or a negative control. Primary mouse islet cells were transfected with AS160 small interfering RNA.

RESULTS

AS160 was expressed in human and mouse pancreatic beta-cells and phosphorylated after glucose stimulation. AS160 mRNA expression was downregulated in pancreatic islets from individuals with type 2 diabetes. In MIN6B1 cells, glucose induced phosphorylation of Akt and AS160, and this was mediated by insulin receptor/IRS-2/phosphatidylinositol 3-kinase independently of changes in cytosolic Ca(2+). Knockdown of AS160 resulted in increased basal insulin secretion, whereas glucose-stimulated insulin release was abolished. Furthermore, beta-cells with decreased AS160 showed increased apoptosis and loss of glucose-induced proliferation.

CONCLUSIONS

This study shows for the first time that AS160, previously recognized as a key player in insulin signaling in skeletal muscle and adipose tissue, is also a major effector of protein kinase B/Akt signaling in the beta-cell.

Carboxy-terminal modulator protein induces Akt phosphorylation and activation, thereby enhancing antiapoptotic, glycogen synthetic, and glucose uptake pathways

Carboxy-terminal modulator protein (CTMP) was identified as binding to the carboxy terminus of Akt and inhibiting the phosphorylation and activation of Akt. In contrast to a previous study, we found CTMP overexpression to significantly enhance Akt phosphorylation at both Thr(308) and Ser(473) as well as the kinase activity of Akt, while phosphatidylinositol 3-kinase (PI3-kinase) activity was unaffected. Translocation of Akt to the membrane fraction was also markedly increased in response to overexpression of CTMP, with no change in the whole cellular content of Akt. Furthermore, the phosphorylations of GSK-3beta and Foxo1, well-known substrates of Akt, were increased by CTMP overexpression. On the other hand, suppression of CTMP with small interfering RNA partially but significantly attenuated this Akt phosphorylation. The cellular activities reportedly mediated by Akt activation were also enhanced by CTMP overexpression. UV-B-induced apoptosis of HeLa cells was significantly reversed not only by overexpression of the active mutant of Akt (myr-Akt) but also by that of CTMP. Increases in glucose transport activity and glycogen synthesis were also induced by overexpression of either myr-Akt or CTMP in 3T3-L1 adipocytes. Taking these results into consideration, it can be concluded that CTMP induces translocation of Akt to the membrane and thereby increases the level of Akt phosphorylation. As a result, CTMP enhances various cellular activities that are principally mediated by the PI3-kinase/Akt pathway.

Identification of insulin signaling elements in human beta-cells: autocrine regulation of insulin gene expression

Although many studies using rodent islets and insulinoma cell lines have been performed to determine the role of insulin in the regulation of islet function, the autocrine effect of insulin on insulin gene expression is still controversial, and no consensus has yet been achieved. Because very little is known about the insulin signaling pathway in human islets, we used single-cell RT-PCR to profile the expression of genes potentially involved in the insulin signaling cascade in human beta-cells. The detection of mRNAs for insulin receptor (IR)A and IRB; insulin receptor substrate (IRS)-1 and IRS-2; phosphoinositide 3-kinase (PI3K) catalytic subunits p110alpha, p110beta, PI3KC2alpha, and PI3KC2gamma; phosphoinositide-dependent protein kinase-1; protein kinase B (PKB)alpha, PKBbeta, and PKBgamma in the beta-cell population suggests the presence of a functional insulin signaling cascade in human beta-cells. Small interfering RNA-induced reductions in IR expression in human islets completely suppressed glucose-stimulated insulin gene expression, suggesting that insulin regulates its own gene expression in human beta-cells. Defects in this regulation may accentuate the metabolic dysfunction associated with type 2 diabetes.

Rosiglitazone ameliorates abnormal expression and activity of protein tyrosine phosphatase 1B in the skeletal muscle of fat-fed, streptozotocin-treated diabetic rats

Protein tyrosine phosphatase 1B (PTP1B) acts as a physiological negative regulator of insulin signaling by dephosphorylating the activated insulin receptor (IR). Here we examine the role of PTP1B in the insulin-sensitizing action of rosiglitazone (RSG) in skeletal muscle and liver. Fat-fed, streptozotocin-treated rats (10-week-old), an animal model of type II diabetes, and age-matched, nondiabetic controls were treated with RSG (10 micromol kg(-1) day(-1)) for 2 weeks. After RSG treatment, the diabetic rats showed a significant decrease in blood glucose and improved insulin sensitivity. Diabetic rats showed significantly increased levels and activities of PTP1B in the skeletal muscle (1.6- and 2-fold, respectively) and liver (1.7- and 1.8-fold, respectively), thus diminishing insulin signaling in the target tissues. We found that the decreases in insulin-stimulated glucose uptake (55%), tyrosine phosphorylation of IRbeta-subunits (48%), and IR substrate-1 (IRS-1) (39%) in muscles of diabetic rats were normalized after RSG treatment. These effects were associated with 34 and 30% decreases in increased PTP1B levels and activities, respectively, in skeletal muscles of diabetic rats. In contrast, RSG did not affect the increased PTP1B levels and activities or the already reduced insulin-stimulated glycogen synthesis and tyrosine phosphorylation of IRbeta-subunits and IRS-2 in livers of diabetic rats. RSG treatment in normal rats did not significantly change PTP1B activities and levels or protein levels of IRbeta, IRS-1, and -2 in diabetic rats. These data suggest that RSG enhances insulin activity in skeletal muscle of diabetic rats possibly by ameliorating abnormal levels and activities of PTP1B.

Hypoglycemic effect of Astragalus polysaccharide and its effect on PTP1B

AIM

To examine the effects of Astragalus polysaccharide (APS), a component of an aqueous extract of Astragalus membranaceus roots, on protein tyrosine phosphatase 1B (PTP1B), a negative regulator of insulin-receptor (IR) signal transduction, and its potential role in the amelioration of insulin resistance.

METHODS

Ten-week-old fat-fed streptozotocin (STZ)-treated rats, an animal model of type II diabetes mellitus (TIIDM), were treated with APS (400 mg/kg p.o.) for 5 weeks. Insulin sensitivity was identified by the insulin-tolerance test. Further analyses on the possible changes in insulin signaling occurring in skeletal muscle and liver were performed by immunoprecipitation or Western blotting. PTP1B activity was measured by an assay kit.

RESULTS

The diabetic rats responded to APS with a significant decrease in body weight, plasma glucose, and improved insulin sensitivity. The activity and expression of PTP1B were elevated in the skeletal muscle and liver of TIIDM rats. Thus the insulin signaling in target tissues was diminished. APS reduced both PTP1B protein level and activity in the muscle, but not in the liver of TIIDM rats. Insulin-induced tyrosine phosphorylation of the IR beta-subunit and insulin receptor substrate-1 (IRS-1) were increased in the muscle, but not in the liver of APS-treated TIIDM rats. There was no change in the activity or expression of PTP1B in APS-treated normal rats, and blood insulin levels did not change in TIIDM rats after treatment with APS.

CONCLUSION

APS enables insulin-sensitizing and hypoglycemic activity at least in part by decreasing the elevated expression and activity of PTP1B in the skeletal muscles of TIIDM rats.

Glucose mediates transcriptional repression of the human angiotensin type-1 receptor gene: role for a novel cis-acting element

Human angiotensin type 1 receptor (hAT1R) gene is regulated by hormones, second messengers, and both pathophysiological and developmental states. The focus of the present study was to determine the role of glucose in the trans-repression of hAT1R gene transcription and to identify the functional cis-acting response element(s). Serial deletions of the hAT1R promoter region indicated that an area between -1717 and -1543 base pairs upstream of the 5' end of the cDNA sequence has a glucose responsive regulatory element (GluRE) to down-regulate the gene expression. Further analysis revealed a putative 29-bp (5'-AACTGATTTTTGTATATTGATCTTGTATT-3') repressor element located between -1582 and -1610 bp was necessary for transcriptional repression. Removal of this region from promoter construct abolished repression of the hAT1R gene transcription in human proximal tubule epithelial cells (hPTECs). Using mobility shift assays, we demonstrated DNA binding activity to the labeled repressor element in hPTEC nuclear extracts. Additional studies demonstrated increased DNA binding activity to the labeled repressor element in nuclear extracts treated with high glucose (25 mM). Southwestern analysis identified two GluRE binding proteins of 34 and 36 kDa in glucose-treated extracts. Glucose-induced activity of the repressor trans-acting factor(s) reached a maximum at 4 h, which correlated with decreased transcriptional activity of the hAT1R gene, suggesting that glucose can down-regulate the transcription of the hAT1R gene through the repressor element. Furthermore, insertion of the glucose response element into heterologous SV40 promoter (SV40) chloramphenicol acetyl transferase (CAT) vector showed orientation/distance-independent repression of SV40 promoter-mediated CAT activity in hPTECs. Our results show that the glucose response factor(s) acts as trans-acting factor(s) binding to the cis-acting repressor element in the hAT1R promoter, which may participate in the control of basal transcription as well as glucose-mediated transcriptional inhibition of the hAT1R gene.

Actions of PPARgamma agonism on adipose tissue remodeling, insulin sensitivity, and lipemia in absence of glucocorticoids

Peroxisome proliferator-activated receptor gamma (PPARgamma) agonists improve insulin sensitivity and lipemia partly through enhancing adipose tissue proliferation and capacity for lipid retention. The agonists also reduce local adipose glucocorticoid production, which may in turn contribute to their metabolic actions. This study assessed the effects of a PPARgamma agonist in the absence of glucocorticoids (adrenalectomy, ADX). Intact, ADX, and intact pair-fed (PF) rats were treated with the PPARgamma agonist rosiglitazone (RSG) for 2 wk. RSG increased inguinal (subcutaneous) white (50%) and brown adipose tissue (6-fold) weight but not that of retroperitoneal (visceral) white adipose tissue. ADX but not PF reduced fat accretion in both inguinal and retroperitoneal adipose depots but did not affect brown adipose mass. RSG no longer increased inguinal weight in ADX and PF rats but increased brown adipose mass, albeit less so than in intact rats. RSG increased cell proliferation in white (3-fold) and brown adipose tissue (6-fold), as assessed microscopically and by total DNA, an effect that was attenuated but not abrogated by ADX. RSG reduced the expression of the glucocorticoid-activating enzyme 11beta-hydroxysteroid dehydrogenase 1 (11beta-HSD1) in all adipose depots. RSG improved insulin sensitivity (reduction in fasting insulin and homeostasis model assessment of insulin resistance, both -50%) and triacylglycerolemia (-75%) regardless of the glucocorticoid status, these effects being fully additive to those of ADX and PF. In conclusion, RSG partially retained its ability to induce white and brown adipose cell proliferation and brown adipose fat accretion and further improved insulin sensitivity and lipemia in ADX rats, such effects being therefore independent from the PPARgamma-mediated modulation of glucocorticoids.

Bisphenol A affects glucose transport in mouse 3T3-F442A adipocytes

Recently, environmental chemicals have appeared in daily human life, and these chemicals have been incidentally taken in by humans. The serum concentrations of some of these chemicals have been found to be associated with the onset and incidence rate of diabetes mellitus. It has been suggested that one of the environmental chemicals, bisphenol A (BPA), has hormone-like activity. It has also been demonstrated that some hormones affect insulin resistance and fat distribution in the body. To study the effects of these environmental chemicals on glucose metabolism, the effect of BPA on glucose transport in mouse 3T3-F442A adipocytes was investigated. The 3T3-F442A adipocytes were incubated with various concentrations of BPA in a medium. Deoxyglucose uptake assay was performed with and without insulin. Immunoblot analysis was performed with a glucose transporter (GLUT) 4-specific antibody and antiphosphotyrosine antibody. The BPA treatment enhanced basal and insulin-stimulated glucose uptake, and caused an increased amount of GLUT4 protein. Thus, the enhanced glucose uptake resulting from the BPA treatment was at least partially due to the increased amount of GLUT4. Tyrosine phosphorylation of insulin receptor substrate-1 with insulin stimulation was not significantly affected. In conclusion, it was demonstrated that BPA, one of the chemicals that we intake incidentally, affects the glucose transport in adipocytes, and also that the environmental chemicals may be identified as one of the environmental factors that affect diabetes and obesity.

Selective glycogen synthase kinase 3 inhibitors potentiate insulin activation of glucose transport and utilization in vitro and in vivo

Insulin resistance plays a central role in the development of type 2 diabetes, but the precise defects in insulin action remain to be elucidated. Glycogen synthase kinase 3 (GSK-3) can negatively regulate several aspects of insulin signaling, and elevated levels of GSK-3 have been reported in skeletal muscle from diabetic rodents and humans. A limited amount of information is available regarding the utility of highly selective inhibitors of GSK-3 for the modification of insulin action under conditions of insulin resistance. In the present investigation, we describe novel substituted aminopyrimidine derivatives that inhibit human GSK-3 potently (K(i) < 10 nmol/l) with at least 500-fold selectivity against 20 other protein kinases. These low molecular weight compounds activated glycogen synthase at approximately 100 nmol/l in cultured CHO cells transfected with the insulin receptor and in primary hepatocytes isolated from Sprague-Dawley rats, and at 500 nmol/l in isolated type 1 skeletal muscle of both lean Zucker and ZDF rats. It is interesting that these GSK-3 inhibitors enhanced insulin-stimulated glucose transport in type 1 skeletal muscle from the insulin-resistant ZDF rats but not from insulin-sensitive lean Zucker rats. Single oral or subcutaneous doses of the inhibitors (30-48 mg/kg) rapidly lowered blood glucose levels and improved glucose disposal after oral or intravenous glucose challenges in ZDF rats and db/db mice, without causing hypoglycemia or markedly elevating insulin. Collectively, our results suggest that these selective GSK-3 inhibitors may be useful as acute-acting therapeutics for the treatment of the insulin resistance of type 2 diabetes.

The role of calcium in the regulation of glucose uptake in isolated working rat heart

Catecholamines or ischemia may increase myocardial glucose uptake by an increase in intracellular calcium. We tested the hypothesis that increasing or decreasing extracellular calcium supply would change glucose uptake. Hearts were perfused for 60 min at a physiological workload with Krebs-Henseleit buffer containing glucose (5 mM) and oleate (0.4 mM; bound to 1% BSA). Calcium concentration was 2.5 mM. In group A (control; n = 12), insulin (1 mU/ml) was added at 30 min. In Group B (n = 7), the calcium concentration was increased to 5.0 and 7.5 mM at 20 min and 40 min, respectively. In Group C (n = 7), verapamil was added at 20 min (0.25 microM) and 40 min (1.0 microM) to decrease calcium influx. In group D (n = 7), EDTA was added at 20 min (0.5 mM) and at 40 min (1.5 mM) to decrease the free extracellular calcium. Glucose uptake was measured by 3H2O production from [2-3H]glucose and cardiac work was measured simultaneously. Cardiac power in group B was 8.24 +/- 0.60 mW at 2.5 mM calcium, 9.45 +/- 0.50 mW at 5 mM calcium and 7.99 +/- 0.99 mW at 7.5 mM calcium (n.s.). The addition of verapamil decreased contractile function in a dose-dependent manner (8.50 +/- 0.74 vs. 3.11 +/- 0.84 vs. 1.48 +/- 0.39 mW, p < 0.01) suggesting that verapamil decreased cytosolic calcium concentration. A similar dose-dependent reduction in contractile performance was observed in the EDTA group (8.44 +/- 0.81 vs. 7.42 +/- 0.96 vs. 4.03 +/- 1.32 mW, p < 0.01). Glucose uptake was 1.35 +/- 0.11 micromol/min/g dry weight under control conditions. Glucose uptake increased threefold with the addition of insulin. Increasing extracellular [Ca2+] did not affect glucose uptake. Decreasing Ca2+ availability showed a trend towards a decrease in glucose uptake (n.s.), which was minor compared to the decrease in contractile function. We conclude that extracellular calcium does not regulate glucose uptake in the isolated working rat heart in the presence of glucose and fatty acids as substrates. The trend of decreased glucose uptake when calcium supply was limited may be due to dramatically reduced energy demand and not directly due to changes in calcium.

Effect of short-term exercise training on insulin-stimulated PI 3-kinase activity in middle-aged men

The purpose of this study was to determine whether the improved insulin action with short-term exercise training in middle-aged individuals is associated with enhanced phosphatidylinositol (PI) 3-kinase activity in skeletal muscle. Nine men of ages 50-70 yr were studied before and after 7 consecutive days of supervised exercise (60 min/day, 70% peak O2 consumption). Insulin sensitivity was measured with a euglycemic hyperinsulinemic glucose clamp in the sedentary condition and 15-17 h after the final exercise session. Anti-phosphotyrosine-associated PI 3-kinase activity was determined from muscle samples obtained in the fasted condition and after 60 min of insulin infusion during the clamp. With exercise, the glucose infusion rate increased (P < 0.001) by 33%, indicating enhanced insulin action (mean +/- SE, 6.6 +/- 0.6 vs. 8.7 +/- 0.8 mg x kg(-1) x min(-1)). Short-term exercise training did not, however, increase insulin-stimulated (insulin stimulated/fasting) PI 3-kinase activity (1.8 +/- 0.8 vs. 1.8 +/- 0.7-fold stimulation with insulin pre- vs. posttraining, respectively). There was also no change in insulin-stimulated protein kinase B activity (1.3 +/- 0.1 vs. 1.4 +/- 0.2-fold stimulation with insulin) with training. These data suggest that insulin action is enhanced with short-term exercise training via an adaptation distal to PI 3-kinase in middle-aged, insulin-resistant individuals.

Skeletal muscle of stroke-prone spontaneously hypertensive rats exhibits reduced insulin-stimulated glucose transport and elevated levels of caveolin and flotillin

Insulin resistance is of major pathogenic importance in several common human disorders, but the underlying mechanisms are unknown. The stroke-prone spontaneously hypertensive (SHRSP) rat is a model of human insulin resistance and is characterized by reduced insulin-mediated glucose disposal and defective fatty acid metabolism in isolated adipocytes (Collison et al. [Diabetes 49:2222-2226, 2000]). In this study, we have examined skeletal muscle and cultured skeletal muscle myoblasts for defects in insulin action in the male SHRSP rat model compared with the normotensive, insulin-sensitive control strain, Wistar-Kyoto (WKY). We show that skeletal muscle from SHRSP animals exhibits a marked decrease in insulin-stimulated glucose transport compared with WKY animals (fold increase in response to insulin: 1.4 +/- 0.15 in SHRSP, 2.29 +/- 0.22 in WKY; n = 4, P = 0.02), but the stimulation of glucose transport in response to activation of AMP-activated protein kinase was similar between the two strains. Similar reductions in insulin-stimulated glucose transport were also evident in myoblast cultures from SHRSP compared with WKY cultures. These differences were not accounted for by a reduction in cellular GLUT4 content. Moreover, analysis of the levels and subcellular distribution of insulin receptor substrates 1 and 2, the p85alpha subunit of phosphatidylinositol 3'-kinase, and protein kinase B (PKB)/cAKT in skeletal muscle did not identify any differences between the two strains; the insulin-dependent activation of PKB/cAKT was not different between the two strains. However, the total cellular levels of caveolin and flotillin, proteins implicated in insulin signal transduction/compartmentalization, were markedly elevated in skeletal muscles from SHRSP compared with WKY animals. Increased cellular levels of the soluble N-ethylmaleimide attachment protein receptor (SNARE) proteins syntaxin 4 and vesicle-associated membrane protein (VAMP)-2 were also observed in the insulin-resistant SHRSP strain. Taken together, these data suggest that the insulin resistance observed in the SHRSP is manifest at the level of skeletal muscle, that muscle cell glucose transport exhibits a blunted response to insulin but unchanged responses to activation of AMP-activated protein kinase, that alterations in key molecules in both GLUT4 trafficking and insulin signal compartmentalization may underlie these defects in insulin action, and that the insulin resistance of these muscles appears to be of genetic origin rather than a paracrine or autocrine effect, since the insulin resistance is also observed in cultured myoblasts over several passages.

Insulin alters heterogeneous nuclear ribonucleoprotein K protein binding to DNA and RNA

The interaction of the multimodular heterogeneous nuclear ribonucleoprotein (hnRNP) K protein with many of its protein and nucleic acid partners is regulated by extracellular signals. Acting as a docking platform, K protein could link signal-transduction pathways to DNA- and RNA-directed processes such as transcription, mRNA processing, transport, and translation. Treatment of hepatocyte culture with insulin increased K protein tyrosine phosphorylation. Insulin altered K protein interaction with RNA and DNA in vitro. Administration of insulin into mice had similar effects on K protein in liver. Coimmunoprecipitations of RNA with K protein revealed preferential in vivo K protein binding of a subset of transcripts, including the insulin-inducible c-fos mRNA. These results suggest a class of insulin pathways that signal nucleic acid-directed processes that involve K protein.

In vivo effects of insulin and bis(maltolato)oxovanadium (IV) on PKB activity in the skeletal muscle and liver of diabetic rats

In this study, the in vivo effects of insulin and chronic treatment with bis(maltolato)oxovanadium (IV) (BMOV) on protein kinase B (PKB) activity were examined in the liver and skeletal muscle from two animal models of diabetes, the STZ-diabetic Wistar rat and the fatty Zucker rat. Animals were treated with BMOV in the drinking water (0.75-1 mg/ml) for 3 (or 8) weeks and sacrificed with or without insulin injection. Insulin (5 U/kg, i.v.) increased PKBalpha activity more than 10-fold and PKBbeta activity more than 3-fold in both animal models. Despite the development of insulin resistance, insulin-induced activation of PKBalpha was not impaired in the STZ-diabetic rats up to 9 weeks of diabetes, excluding a role for PKBalpha in the development of insulin resistance in type 1 diabetes. Insulin-induced PKBalpha activity was markedly reduced in the skeletal muscle of fatty Zucker rats as compared to lean littermates (fatty: 7-fold vs. lean: 14-fold). In contrast, a significant increase in insulin-stimulated PKBalpha activity was observed in the liver of fatty Zucker rats (fatty: 15.7-fold vs. lean: 7.6-fold). Chronic treatment with BMOV normalized plasma glucose levels in STZ-diabetic rats and decreased plasma insulin levels in fatty Zucker rats but did not have any effect on basal or insulin-induced PKBalpha and PKBbeta activities. In conclusion (i) in STZ-diabetic rats PKB activity was normal up to 9 weeks of diabetes; (ii) in fatty Zucker rats insulin-induced activation of PKBalpha (but not PKBbeta) was markedly altered in both tissues; (iii) changes in PKBalpha activity were tissue specific; (iv) the glucoregulatory effects of BMOV were independent of PKB activity.

Overexpression of SH2-containing inositol phosphatase 2 results in negative regulation of insulin-induced metabolic actions in 3T3-L1 adipocytes via its 5'-phosphatase catalytic activity

Phosphatidylinositol (PI) 3-kinase plays an important role in various metabolic actions of insulin including glucose uptake and glycogen synthesis. Although PI 3-kinase primarily functions as a lipid kinase which preferentially phosphorylates the D-3 position of phospholipids, the effect of hydrolysis of the key PI 3-kinase product PI 3,4,5-triphosphate [PI(3,4,5)P3] on these biological responses is unknown. We recently cloned rat SH2-containing inositol phosphatase 2 (SHIP2) cDNA which possesses the 5'-phosphatase activity to hydrolyze PI(3,4,5)P3 to PI 3,4-bisphosphate [PI(3,4)P2] and which is mainly expressed in the target tissues of insulin. To study the role of SHIP2 in insulin signaling, wild-type SHIP2 (WT-SHIP2) and 5'-phosphatase-defective SHIP2 (Delta IP-SHIP2) were overexpressed in 3T3-L1 adipocytes by means of adenovirus-mediated gene transfer. Early events of insulin signaling including insulin-induced tyrosine phosphorylation of the insulin receptor beta subunit and IRS-1, IRS-1 association with the p85 subunit, and PI 3-kinase activity were not affected by expression of either WT-SHIP2 or Delta IP-SHIP2. Because WT-SHIP2 possesses the 5'-phosphatase catalytic region, its overexpression marked by decreased insulin-induced PI(3,4,5)P3 production, as expected. In contrast, the amount of PI(3,4,5)P3 was increased by the expression of Delta IP-SHIP2, indicating that Delta IP-SHIP2 functions in a dominant-negative manner in 3T3-L1 adipocytes. Both PI(3,4,5)P3 and PI(3,4)P2 were known to possibly activate downstream targets Akt and protein kinase C lambda in vitro. Importantly, expression of WT-SHIP2 inhibited insulin-induced activation of Akt and protein kinase C lambda, whereas these activations were increased by expression of Delta IP-SHIP2 in vivo. Consistent with the regulation of downstream molecules of PI 3-kinase, insulin-induced 2-deoxyglucose uptake and Glut4 translocation were decreased by expression of WT-SHIP2 and increased by expression of Delta IP-SHIP2. In addition, insulin-induced phosphorylation of GSK-3beta and activation of PP1 followed by activation of glycogen synthase and glycogen synthesis were decreased by expression of WT-SHIP2 and increased by the expression of Delta IP-SHIP2. These results indicate that SHIP2 negatively regulates metabolic signaling of insulin via the 5'-phosphatase activity and that PI(3,4,5)P3 rather than PI(3,4)P2 is important for in vivo regulation of insulin-induced activation of downstream molecules of PI 3-kinase leading to glucose uptake and glycogen synthesis.

Time course evaluation of protein synthesis and glucose uptake after acute resistance exercise in rats

The temporal pattern for changes in rates of protein synthesis and glucose uptake after resistance exercise, especially relative to each other, is not known. Male Sprague-Dawley rats performed acute resistance exercise (n = 7) or remained sedentary (n = 7 per group), and the following were assessed in vivo 1, 3, 6, 12 and 24 h later: rates of protein synthesis, rates of glucose uptake, phosphatidylinositol 3-kinase (PI3-kinase) activity, and p70(S6k) activity. Rates of protein synthesis in mixed gastrocnemius muscle did not increase until 12 h after exercise (e.g., at 12 h, sedentary = 138 +/- 4 vs. exercised = 178 +/- 6 nmol phenylalanine incorporated x g muscle(-1) x h(-1), mean +/- SE, P < 0.05), whereas at 6 h after exercise rates of glucose uptake were significantly elevated (sedentary = 0.18 +/- 0.020 vs. exercised = 0.38 +/- 0.024 micromol glucose 6-phosphate incorporated x kg muscle(-1) x min(-1), P < 0.05). At 24 h after exercise, rates of protein synthesis were still elevated, whereas glucose uptake had returned to basal levels. Arterial insulin concentrations were not different between groups at any time. Non-insulin-stimulated activities of PI3-kinase and p70(S6k) were higher at 6, 12, and 24 h after exercise (P < 0.05), and, generally, these occurred when rates of protein synthesis (12 and 24 h) and glucose uptake were elevated (6 and 12 but not 24 h) by exercise. These data suggest that regulators of protein synthesis and glucose uptake may respond to the same contraction-generated signals with different kinetics or that they respond to different intra- or extracellular signals that are generated by exercise.

Effects of bis(maltolato) oxovanadium (IV) on protein serine kinases in skeletal muscle of streptozotocin-diabetic rats

The in vivo effects of bis(maltolato)oxovanadium (IV) (BMOV) on the activity of protein serine kinases in skeletal muscle of STZ-diabetic Wistar rats were studied. BMOV was administered to STZ-diabetic rats at a concentration of 0.75 mg/ml for 8 weeks. Chronic BMOV treatment completely normalized plasma glucose levels in the diabetic animals after 8 weeks of treatment. Insulin-stimulated ERK-1 and ERK-2 activity was markedly increased in STZ-diabetic rats. Chronic BMOV treatment normalized the activity of ERK-2 in the diabetic treated animals, whereas the activity of ERK-1 was unaffected. In contrast to ERK-1 and ERK-2, the activity of the ribosomal S6 kinase p90rsk was decreased in STZ-diabetic rats. BMOV treatment restored the activity to normal levels. Basal p70 S6K activity was increased about 2.5-fold in the untreated diabetic group and no further increase in activity was observed after insulin stimulation. BMOV treatment did not correct the changes in p70 S6K activity in either the basal or insulin-stimulated states. In conclusion (i) the activity of ERK-1, ERK-2 and p90rsk were altered in skeletal muscle of STZ-diabetic rats; (ii) the glucoregulatory effects of BMOV were accompanied by concurrent improvement in the activities of ERK-2 and p90rsk; and (iii) there appears to be a dissociation between the activation of ERK-2 and p90rsk, suggesting that the regulation of p90rsk may be much more complex in vivo.

Osmotic shock inhibits insulin signaling by maintaining Akt/protein kinase B in an inactive dephosphorylated state

We have previously reported that insulin and osmotic shock stimulate an increase in glucose transport activity and translocation of the insulin-responsive glucose transporter isoform GLUT4 to the plasma membrane through distinct pathways in 3T3L1 adipocytes (D. Chen, J. S. Elmendorf, A. L. Olson, X. Li, H. S. Earp, and J. E. Pessin, J. Biol. Chem. 272:27401-27410, 1997). In investigations of the relationships between these two signaling pathways, we have now observed that these two stimuli are not additive, and, in fact, osmotic shock pretreatment was found to completely prevent any further insulin stimulation of glucose transport activity and GLUT4 protein translocation. In addition, osmotic shock inhibited the insulin stimulation of lipogenesis and glycogen synthesis. This inhibition of insulin-stimulated downstream signaling occurred without any significant effect on insulin receptor autophosphorylation or tyrosine phosphorylation of insulin receptor substrate 1 (IRS1). Furthermore, there was no effect on either the insulin-stimulated association of the p85 type I phosphatidylinositol (PI) 3-kinase regulatory subunit with IRS1 or phosphotyrosine antibody-immunoprecipitated PI 3-kinase activity. In contrast, osmotic shock pretreatment markedly inhibited the insulin stimulation of protein kinase B (PKB) and p70S6 kinase activities. In addition, the dephosphorylation of PKB was prevented by pretreatment with the phosphatase inhibitors okadaic acid and calyculin A. These data support a model in which osmotic shock-induced insulin resistance of downstream biological responses results from an inhibition of insulin-stimulated PKB activation.

Insulin increases near-membrane but not global Ca2+ in isolated skeletal muscle

It has long been debated whether changes in Ca2+ are involved in insulin-stimulated glucose uptake in skeletal muscle. We have now investigated the effect of insulin on the global free myoplasmic Ca2+ concentration and the near-membrane free Ca2+ concentration ([Ca2+]mem) in intact, single skeletal muscle fibers from mice by using fluorescent Ca2+ indicators. Insulin has no effect on the global free myoplasmic Ca2+ concentration. However, insulin increases [Ca2+]mem by approximately 70% and the half-maximal increase in [Ca2+]mem occurs at an insulin concentration of 110 microunits per ml. The increase in [Ca2+]mem by insulin persists when sarcoplasmic reticulum Ca2+ release is inhibited but is lost by perfusing the fiber with a low Ca2+ medium or by addition of L-type Ca2+ channel inhibitors. Thus, insulin appears to stimulate Ca2+ entry into muscle cells via L-type Ca2+ channels. Wortmannin, which inhibits insulin-mediated activation of glucose transport in isolated skeletal muscle, also inhibits the insulin-mediated increase in [Ca2+]mem. These data demonstrate a new facet of insulin signaling and indicate that insulin-mediated increases in [Ca2+]mem in skeletal muscle may underlie important actions of the hormone.

Insulin stimulation of rat ventricular K+ currents depends on the integrity of the cytoskeleton

1. The effect of insulin on K+ currents was studied with enzymatically dispersed ventricular myocytes from insulin-deficient (type I) diabetic rats. Diabetic conditions were induced by a single intravenous injection of streptozotocin (100 mg kg-1) given 8-13 days before the experiments. Measurements of plasma glucose and insulin levels confirmed the diabetic status of the animals. 2. A Ca2+-independent transient outward K+ current, It, and a slowly inactivating, quasi-steady-state current, Iss, which are depressed in diabetic myocytes, could be restored by exposure to 1, 10 or 100 nM insulin. This was only observed after a delay of 5-6 h, although an insulin exposure of only 1 h was sufficient to initiate its stimulatory action on It and Iss. The stimulatory effect of insulin on these K+ currents was prevented by 2 microM cycloheximide, which in itself had no direct effect on these currents. 3. Disruption of the actin microfilament network with 1 microM cytochalasin D (CD) also prevented the stimulatory effect of 100 nM insulin on both It and Iss. Since CD was added 1 h after insulin, inhibitory effects on insulin signalling were ruled out. Adding CD (1 microM) 5-9 h after insulin, when currents were already augmented, had no effect (up to 50 min exposure). Incubating control cells for 6-10 h with 1 microM CD had no effect on any of the currents measured. 4. Stabilization of the actin network by pre-exposure to 2.5 microM phalloidin restored the stimulatory effect of insulin, in the continued presence of CD, ruling out any effects of CD on components other than the cytoskeleton. 5. The stimulatory effect of insulin was also prevented by incubating cells with insulin in the presence of the microtubule-disrupting agent colchicine (5 microM). 6. These results suggest that the insulin-mediated augmentation of K+ currents in diabetic myocytes requires protein synthesis, possibly of K+ channels, as well as an intact cytoskeleton. The possibility that newly formed channels translocate to the plasma membrane in a process dependent on different elements of the cytoskeleton is discussed.

Thiazolidinediones and insulin resistance: peroxisome proliferatoractivated receptor gamma activation stimulates expression of the CAP gene

c-Cbl-associated protein (CAP) is a signaling protein that interacts with both c-Cbl and the insulin receptor that may be involved in the specific insulin-stimulated tyrosine phosphorylation of c-Cbl. The restricted expression of CAP in cells metabolically sensitive to insulin suggests an important potential role in insulin action. The expression of CAP mRNA and proteins are increased in 3T3-L1 adipocytes by the insulin sensitizing thiazolidinedione drugs, which are activators of the peroxisome proliferator-activated receptor gamma (PPARgamma). The stimulation of CAP expression by PPARgamma activators results from increased transcription. This increased expression of CAP was accompanied by a potentiation of insulin-stimulated c-Cbl tyrosine phosphorylation. Administration of the thiazolidinedione troglitazone to Zucker (fa/fa) rats markedly increased the expression of the major CAP isoform in adipose tissue. This effect was sustained for up to 12 weeks of treatment and accompanied the ability of troglitazone to prevent the onset of diabetes and its complications. Thus, CAP is the first PPARgamma-sensitive gene identified that participates in insulin signaling and may play a role in thiazolidinedione-induced insulin sensitization.

Effect of nutritional state on the formation of a complex involving insulin receptor IRS-1, the 52 kDa Src homology/collagen protein (Shc) isoform and phosphatidylinositol 3'-kinase activity

The Src homology and collagen protein (Shc) is tyrosine phosphorylated in response to insulin; however, evidence for its interaction with insulin receptor (IR) in normal tissues is missing. Interactions between IR, Shc and regulatory subunits of the phosphatidylinositol 3'-kinase (PI 3'-kinase) were characterized in the present study in liver and muscles of chickens submitted to various nutritional states. A chicken liver Shc cDNA fragment encoding a 198 amino acid long fragment, including the phosphotyrosine binding domain was sequenced. It shows 89% homology with the corresponding human homologue. The amounts of the three Shc isoforms (66, 52 and 46 kDa) and Shc messenger were not altered by the nutritional state. Shc tyrosine phosphorylation was decreased by fasting in both liver and muscle. Importantly, Shc was immunoprecipitated by IR antibody (mostly the 52 kDa isoform) or by alphaIRS-1(mostly the 46 kDa isoform). IR-Shc association was decreased by fasting and restored by refeeding. In liver, alphaShc immunoprecipitated the three forms of regulatory subunits of PI 3'-kinase and a PI 3'-kinase activity which was decreased by fasting. In muscle, alphaShc immunoprecipitated only the p85 isoform; the associated PI 3'-kinase activity was not altered by the nutritional state. Conversely, in both tissues anti-p85 antibody precipitated only the 52 kDa Shc isoform. In liver, antibodies to insulin receptor substrate-1 (alphaIRS-1), Shc or IR immunoprecipitated the three regulatory subunits of PI 3'-kinase and an equal PI 3'-kinase activity, without any residual activity left in the supernatants, suggesting the presence of a large complex involving IR, IRS-1, Shc (mainly the 52 kDa isoform) and PI 3'-kinase activity. The presence of another complex containing IRS-1 and the 46 kDa Shc isoform, but no PI 3'-kinase activity, is suggested.

Phosphatidylinositol 4-kinase, but not phosphatidylinositol 3-kinase, is present in GLUT4-containing vesicles isolated from rat skeletal muscle

Insulin stimulates the rate of glucose transport into muscle and adipose cells by translocation of glucose transporter (GLUT4)-containing vesicles from an intracellular storage pool to the surface membrane. This event is mediated through the insulin receptor substrates (IRSs), which in turn activate phosphatidylinositol (PI) 3-kinase isoforms. It has been suggested that insulin causes attachment of PI 3-kinases to the intracellular GLUT4-containing vesicles in rat adipose cells. Furthermore, it has also been shown that GLUT4-containing vesicles in adipose cells contain a PI 4-kinase. In the present study we investigate whether GLUT4-containing vesicles isolated from rat skeletal muscle display PI 3-kinase and/or PI 4-kinase activities. Insulin stimulation caused a rapid increase (5-15-fold increase compared with control) in the intracellular cytosolic IRS-1-associated PI-3 kinase activity. This PI 3-kinase activity was also present in a membrane preparation containing the insulin-regulatable pool of GLUT4 transporters. However, when GLUT4-containing vesicles were isolated by immunoprecipitation from basal and insulin-stimulated (3 min) skeletal muscle, the vesicles displayed PI 4-kinase, but not PI 3-kinase, activity. Insulin did not regulate the PI 4-kinase activity in the GLUT4-containing vesicles. In conclusion, GLUT4-containing vesicles from rat skeletal muscle contain a PI 4-kinase, but not a PI 3-kinase. It is suggested that, in skeletal muscle, insulin causes activation of the IRS/PI 3-kinase complex in an intracellular membrane compartment associated closely with the GLUT4-containing vesicles, but not in the GLUT4-containing vesicles themselves.

Divergent effects of exercise on metabolic and mitogenic signaling pathways in human skeletal muscle

The molecular signaling mechanisms by which muscle contractions lead to changes in glucose metabolism and gene expression remain largely undefined. We assessed whether exercise activates MAP kinase proteins (ERK1/2, SEK1, and p38 MAP kinase) as well as Akt and PYK2 in skeletal muscle from healthy volunteers obtained during and after one-leg cycle ergometry at approximately 70% VO2max. Exercise led to a marked increase in ERK1/2 phosphorylation, which rapidly decreased to resting levels upon recovery. Exercise increased phosphorylation of SEK1 and p38 MAP kinase to a lesser extent than ERK1/2. In contrast to ERK1/2, p38 MAP kinase phosphorylation was increased in nonexercised muscle upon cessation of exercise. Phosphorylation of the transcription factor CREB was increased in nonexercised muscle upon cessation of exercise. Exercise did not activate Akt or increase tyrosine phosphorylation of PYK2. Thus, exercise has divergent effects on parallel MAP kinase pathways, of which only p38 demonstrated a systemic response. However, our data do not support a role of Akt or PYK2 in exercise/contraction-induced signaling in human skeletal. Activation of the different MAP kinase pathways by physical exercise appears to be important in the regulation of transcriptional events in skeletal muscle.

Type I and II models of diabetes produce different modifications of K+ currents in rat heart: role of insulin

1. Several K+ currents were measured and compared in enzymatically dispersed ventricular myocytes from control and diabetic rats. 2. Diabetic conditions were established either with a single intravenous injection of streptozotocin (STZ, 100 mg kg-1; 6-14 days duration) or by feeding with a fructose-enriched diet for 4-10 weeks. Both groups became hyperglycaemic, with the former having decreased and the latter having elevated levels of plasma insulin. These conditions therefore mimic type I (insulin-dependent) and type II (non-insulin-dependent) diabetes mellitus, respectively. 3. As reported previously, a Ca(2+)-independent transient outward K+ current, I(t), was attenuated in the type I model. This was not observed in the type II model. The two models differed greatly in the changes observed in a quasi-steady-state K+ current denoted Iss. In the STZ model Iss was substantially attenuated, whereas in the fructose-fed model it was augmented. In both models, the background inwardly rectifying current, IK1, was unchanged. Concomitantly, there was a substantial prolongation of the action potential in the STZ model but not in the fructose-fed model. 4. Incubation of control myocytes with insulin (100 nM) for 5-9 h caused a significant augmentation of Iss, with no effect on I(t) or on IK1. Incubation of myocytes from STZ-diabetic rats with insulin reversed the attenuation of I(t), but not of Iss. 5. The effect of insulin was not blocked by wortmannin, an inhibitor of phosphatidylinositol 3-kinase. However, inhibition of the mitogen-activated protein kinase pathway with PD98059 prevented restoration of I(t). Insulin action on I(t) may therefore involve changes in transcription or expression of channel proteins, rather than changes in cellular metabolism.

A novel, multifuntional c-Cbl binding protein in insulin receptor signaling in 3T3-L1 adipocytes

The protein product of the c-Cbl proto-oncogene is prominently tyrosine phosphorylated in response to insulin in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. After insulin-dependent tyrosine phosphorylation, c-Cbl specifically associates with endogenous c-Crk and Fyn. These results suggest a role for tyrosine-phosphorylated c-Cbl in 3T3-L1 adipocyte activation by insulin. A yeast two-hybrid cDNA library prepared from fully differentiated 3T3-L1 adipocytes was screened with full-length c-Cbl as the target protein in an attempt to identify adipose-specific signaling proteins that interact with c-Cbl and potentially are involved in its tyrosine phosphorylation in 3T3-L1 adipocytes. Here we describe the isolation and the characterization of a novel protein that we termed CAP for c-Cbl-associated protein. CAP contains a unique structure with three adjacent Src homology 3 (SH3) domains in the C terminus and a region showing significant sequence similarity with the peptide hormone sorbin. Both CAP mRNA and proteins are expressed predominately in 3T3-L1 adipocytes and not in 3T3-L1 fibroblasts. CAP associates with c-Cbl in 3T3-L1 adipocytes independently of insulin stimulation in vivo and in vitro in an SH3-domain-mediated manner. Furthermore, we detected the association of CAP with the insulin receptor. Insulin stimulation resulted in the dissociation of CAP from the insulin receptor. Taken together, these data suggest that CAP represents a novel c-Cbl binding protein in 3T3-L1 adipocytes likely to participate in insulin signaling.

Phosphorylation of insulin receptor substrate 1 by glycogen synthase kinase 3 impairs insulin action

The phosphorylation of insulin receptor substrate 1 (IRS-1) on tyrosine residues by the insulin receptor (IR) tyrosine kinase is involved in most of the biological responses of insulin. IRS-1 mediates insulin signaling by recruiting SH2 proteins through its multiple tyrosine phosphorylation sites. The phosphorylation of IRS-1 on serine/threonine residues also occurs in cells; however, the particular protein kinase(s) promoting this type of phosphorylation are unknown. Here we report that glycogen synthase kinase 3 (GSK-3) is capable of phosphorylating IRS-1 and that this modification converts IRS-1 into an inhibitor of IR tyrosine kinase activity in vitro. Expression of wild-type GSK-3 or an "unregulated" mutant of the kinase (S9A) in CHO cells overexpressing IRS-1 and IR, resulted in increased serine phosphorylation levels of IRS-1, suggesting that IRS-1 is a cellular target of GSK-3. Furthermore, insulin-induced tyrosine phosphorylation of IRS-1 and IR was markedly suppressed in cells expressing wild-type or the S9A mutant, indicating that expression of GSK-3 impairs IR tyrosine kinase activity. Taken together, our studies suggest a new role for GSK-3 in attenuating insulin signaling via its phosphorylation of IRS-1 and may provide new insight into mechanisms important in insulin resistance.

Insulin stimulates tyrosine phosphorylation of the proto-oncogene product of c-Cbl in 3T3-L1 adipocytes

We report here that the product of the c-Cbl proto-oncogene is prominently tyrosine phosphorylated in response to insulin in 3T3-L1 adipocytes. The tyrosine phosphorylation of c-Cbl reaches a maximum within 1-2 min after stimulation by insulin and gradually declines thereafter. The tyrosine phosphorylation of c-Cbl was also observed after treatment of 3T3-L1 adipocytes with epidermal growth factor, whereas platelet-derived growth factor had no effect. After insulin-dependent tyrosine phosphorylation, c-Cbl specifically associates with fusion proteins containing the Src homology 2 (SH2) domains of Crk and the Fyn tyrosine kinase, but not with fusion proteins containing the SH2 domains of either the p85 subunit of phosphatidylinositol 3'-kinase or the tyrosine phosphatase SHPTP2/Syp. Furthermore insulin stimulates the association of c-Cbl with endogenous c-Crk and Fyn in intact 3T3-L1 adipocytes. The tyrosine phosphorylation of c-Cbl is regulated during adipocyte differentiation. Neither insulin-like growth factor 1 nor insulin stimulated the tyrosine phosphorylation of c-Cbl in 3T3-L1 fibroblasts. Moreover, c-Cbl is not tyrosine phosphorylated in response to insulin in cells expressing high levels of the human insulin receptor, or in hepatocytes, despite comparable levels of c-Cbl expression. These results suggest that c-Cbl might have a novel function in the regulation of insulin receptor intracellular signalling in 3T3-L1 adipocytes.