JAK1-dependent phosphorylation of insulin receptor substrate-1 (IRS-1) is inhibited by IRS-1 serine phosphorylation

C2C12 cell model: its role in understanding of insulin resistance at the molecular level and pharmaceutical development at the preclinical stage

OBJECTIVES

The myoblast cell line, C2C12, has been utilised extensively in vitro as an examination model in understanding metabolic disease progression. Although it is indispensable in both preclinical and pharmaceutical research, a comprehensive review of its use in the investigation of insulin resistance progression and pharmaceutical development is not available.

KEY FINDINGS

C2C12 is a well-documented model, which can facilitate our understanding in glucose metabolism, insulin signalling mechanism, insulin resistance, oxidative stress, reactive oxygen species and glucose transporters at cellular and molecular levels. With the aid of the C2C12 model, recent studies revealed that insulin resistance has close relationship with various metabolic diseases in terms of disease progression, pathogenesis and therapeutic management. A holistic, safe and effective disease management is highly of interest. Therefore, significant efforts have been paid to explore novel drug compounds and natural herbs that can elicit therapeutic effects in the targeted sites at both cellular (e.g. mitochondria, glucose transporter) and molecular level (e.g. genes, signalling pathway).

SUMMARY

The use of C2C12 myoblast cell line is meaningful in pharmaceutical and biomedical research due to their expression of GLUT-4 and other features that are representative to human skeletal muscle cells. With the use of the C2C12 cell model, the impact of drug delivery systems (nanoparticles and quantum dots) on skeletal muscle, as well as the relationship between exercise, pancreatic β-cells and endothelial cells, was discovered.

Quantitative analysis of JAK binding using isothermal titration calorimetry and surface plasmon resonance

Janus Kinases (JAKs) are the key effector kinases that initiate intracellular signalling cascades in response to cytokines and growth factors. As such, a large number of cytoplasmic proteins interact with JAKs both as substrates and as components of regulatory machinery designed to ensure correct activation and termination of signalling. In vitro techniques such as Isothermal Titration Calorimety and Surface Plasmon Resonance are valuable methods to verify and quantify the interaction between JAK and potential binding partners or substrates. Here we describe protocols that exploit both of these in vitro techniques in order to more fully understand the intracellular JAK signalling network.

Photodynamic therapy activated signaling from epidermal growth factor receptor and STAT3: Targeting survival pathways to increase PDT efficacy in ovarian and lung cancer

Patients with serosal (pleural or peritoneal) spread of malignancy have few definitive treatment options and consequently have a very poor prognosis. We have previously shown that photodynamic therapy (PDT) can be an effective treatment for these patients, but that the therapeutic index is relatively narrow. Here, we test the hypothesis that EGFR and STAT3 activation increase survival following PDT, and that inhibiting these pathways leads to increased PDT-mediated direct cellular cytotoxicity by examining BPD-PDT in OvCa and NSCLC cells. We found that BPD-mediated PDT stimulated EGFR tyrosine phosphorylation and nuclear translocation, and that EGFR inhibition by erlotinib resulted in reduction of PDT-mediated EGFR activation and nuclear translocation. Nuclear translocation and PDT-mediated activation of EGFR were also observed in response to BPD-mediated PDT in multiple cell lines, including OvCa, NSCLC and head and neck cancer cells, and was observed to occur in response to porfimer sodium-mediated PDT. In addition, we found that PDT stimulates nuclear translocation of STAT3 and STAT3/EGFR association and that inhibiting STAT3 signaling prior to PDT leads to increased PDT cytotoxicity. Finally, we found that inhibition of EGFR signaling leads to increased PDT cytotoxicity through a mechanism that involves increased apoptotic cell death. Taken together, these results demonstrate that PDT stimulates the nuclear accumulation of both EGFR and STAT3 and that targeting these survival pathways is a potentially promising strategy that could be adapted for clinical trials of PDT for patients with serosal spread of malignancy.

Reciprocal interference between insulin and interferon-alpha signaling in hepatic cells: a vicious circle of clinical significance?

Insulin resistance (IR) is common in chronic hepatitis C (CHC) and associates with reduced virological response to pegylated-interferon (PEG-IFN)/ribavirin therapy, but the underlying mechanisms are still unclear. We have previously shown that, in CHC patients, insulin plasma levels are inversely related to antiviral effect induced by PEG-IFN. Therefore, we investigated the in vitro effect of insulin on interferon alpha (IFN-α) intracellular signaling as well as that of IFN-α on insulin signaling. HepG2 cells, preincubated with or without insulin, were stimulated with IFN-α2b and messenger RNA (mRNA) and protein expression of IFN-stimulated genes (ISGs) were measured at different timepoints. The role of intracellular suppressors of cytokine signaling 3 (SOCS3) was evaluated with the small interfering RNA (siRNA) strategy. To assess the effect of IFN-α on insulin signaling, HepG2 were preincubated with or without IFN before addition of insulin and cells were then analyzed for IRS-1 and for Akt/PKB Ser473 phosphorylation. Insulin (100 and 1000 nM) significantly reduced in a dose-dependent fashion IFN-induced gene expression of PKR (P=0.017 and P=0.0017, respectively), MxA (P=0.0103 and P=0.00186), and 2'-5' oligoadenylatesynthetase 1 (OAS-1) (P=0.002 and P=0.006). Insulin also reduced IFN-α-induced PKR protein expression. Although insulin was confirmed to increase SOCS3 expression, siRNA SOCS3 did not restore ISG expression after insulin treatment. IFN-α was found to reduce, in a dose-dependent fashion, IRS-1 gene expression as well as Akt/PKB Ser473 phosphorylation induced by insulin.

CONCLUSION

These results provide evidence of reciprocal interference between insulin and IFN-α signaling in liver cells. These findings may contribute to understand the role of insulin in CHC: IR might be favored by endogenous cytokines including IFN-α, and the resulting hyperinsulinemia then reduces the antiviral response to exogenous IFN in a vicious circle of clinical significance.

Interferon-gamma regulation of intestinal epithelial permeability

The maintenance and regulation of the barrier function of the epithelial lining of the intestine are important homeostatic events, serving to allow selective absorption from the gut lumen while simultaneously limiting the access of bacteria into the mucosa. Interferon-gamma is a pleiotrophic cytokine produced predominantly by natural kill cells and CD4+ T cells that under normal circumstances, and particularly during infection or inflammation, will be a component of the intestinal milieu. Use of colon-derived epithelial cell lines and, to a less extent, murine in vivo analyses, have revealed that interferon-gamma (IFN-gamma) can increase epithelial permeability as gauged by markers of paracellular permeability and bacterial transcytosis, with at least a portion of the bacteria using the transcellular permeation pathway. In this review, we describe the main characteristics of epithelial permeability and then focus on the ability of IFN-gamma to increase epithelial permeability, and the mechanism(s) thereof.

Important genetic checkpoints for insulin resistance in salt-sensitive (S) Dahl rats

Despite the marked advances in research on insulin resistance (IR) in humans and animal models of insulin resistance, the mechanisms underlying high salt-induced insulin resistance remain unclear. Insulin resistance is a multifactorial disease with both genetic and environmental factors (such as high salt) involved in its pathogenesis. High salt triggers insulin resistance in genetically susceptible patients and animal models of insulin resistance. One of the mechanisms by which high salt might precipitate insulin resistance is through its ability to enhance an oxidative stress-induced inflammatory response that disrupts the insulin signaling pathway. The aim of this hypothesis is to discuss two complementary approaches to find out how high salt might interact with genetic defects along the insulin signaling and inflammatory pathways to predispose to insulin resistance in a genetically susceptible model of insulin resistance. The first approach will consist of examining variations in genes involved in the insulin signaling pathway in the Dahl S rat (an animal model of insulin resistance and salt-sensitivity) and the Dahl R rat (an animal model of insulin sensitivity and salt-resistance), and the putative cellular mechanisms responsible for the development of insulin resistance. The second approach will consist of studying the over-expressed genes along the inflammatory pathway whose respective activation might be predictive of high salt-induced insulin resistance in Dahl S rats.

Variations in genes encoding the insulin receptor substrates -1 and/or -2 (IRS-1, -2) and/or genes encoding the glucose transporter (GLUTs) proteins have been found in patients with insulin resistance. To better understand the combined contribution of excessive salt and genetic defects to the etiology of the disease, it is essential to investigate the following question:

Question 1: Do variations in genes encoding the IRS -1 and -2 and/or genes encoding the GLUTs proteins predict high salt-induced insulin resistance in Dahl S rats?

A significant amount of evidence suggested that salt-induced oxidative stress might predict an inflammatory response that upregulates mediators of inflammation such as the nuclear factor- kappa B (NF-kappa B), the tumor necrosis factor-alpha (TNF-α) and the c-Jun Terminal Kinase (JNK). These inflammatory mediators disrupt the insulin signaling pathway and predispose to insulin resistance. Therefore, the following question will be thoroughly investigated:

Question 2: Do variations in genes encoding the NF-kappa B, the TNF-α and the JNK, independently or in synergy, predict an enhanced inflammatory response and subsequent insulin resistance in Dahl S rats in excessive salt environment?

Finally, to better understand the combined role of these variations on glucose metabolism, the following question will be addressed:

Question 3: What are the functional consequences of gene variations on the rate of glucose delivery, the rate of glucose transport and the rate of glucose phosphorylation in Dahl S rats?

The general hypothesis is that "high-salt diet in combination with defects in candidate genes along the insulin signaling and inflammatory pathways predicts susceptibility to high salt-induced insulin resistance in Dahl S rats".

Insulin receptor substrate (IRS)-1 regulates murine embryonic stem (mES) cells self-renewal

Mouse embryonic stem (mES) cells are pluripotent cells that can be propagated in vitro with leukemia inhibitory factor (LIF) and serum. Intracellular signaling by LIF is principally mediated by activation of STAT-3, although additional pathways for self-renewal have been described. Here, we identified a novel role for Insulin receptor substrate-1 (IRS-1) as a critical factor in mES cells self-renewal and differentiation. IRS-1 is expressed and tyrosyl phosphorylated during mES cells self-renewal. Differentiation of mES cells, by LIF withdrawal, is associated with a marked reduction in IRS-1 expression. Targeting of IRS-1 by si-IRS-1 results in a severe reduction of Oct-4 protein expression and alkaline phosphatase activity, markers of undifferentiated mES cells. IRS-1 targeting does not interfere with LIF-induced STAT-3 phosphorylation, but negatively affects protein kinase B (PKB/AKT) and glycogen synthase kinase-3 (GSK-3beta) phosphorylation, which are downstream effectors of the LIF-mediated PI3K signaling cascade. Targeting of IRS-1 also results in a marked down regulation of Id-1 and Id-2 proteins expression, which are important components for self-renewal of ES cells. Conversely, over expression of IRS-1 inhibits mES cell differentiation. Taken together, these results suggest that expression and activity of IRS-1 are critical to the maintenance of the self-renewal program in mES cells.

Historical developments in the research of interferon receptors

Interferons (IFNs) were discovered 50 years ago independently by Isaacs and Lindemann and by Nagata and Kojima. When it was later realized that IFNs are active at very low concentrations, research began to determine how their powerful effects were generated from such a small initial signal. It has since been established that interferons, as well as all other cytokines, employ cell surface receptors to translate their presence in the serum to a potent cellular response to a viral infection. These receptor complexes are composed of multiple distinct glycosylated transmembrane polypeptides, a number of protein tyrosine kinases, and interact transiently with a large variety of other proteins including transcription factors, phosphatases, signaling repressors, and adaptor proteins coupling the receptor to alternative signaling pathways. Three major receptor complexes exist that are exclusive to each of three major classes of interferon. Even though the effects of each major class of interferon vary physiologically, each receptor complex interacts with its ligand in similar ways and activates similar signaling cascades. In this mini-review, we take a historical perspective at the major events in the characterization of interferon receptors, discussing interesting results that still need to be explained.

Type 2 Diabetes Impairs Insulin Receptor Substrate-2-Mediated Phosphatidylinositol 3-Kinase Activity in Primary Macrophages to Induce a State of Cytokine Resistance to IL-4 in Association with Overexpression of Suppressor of Cytokine Signaling-3

Chronic elevation of proinflammatory markers in type 2 diabetes (T2D) is well defined, but the role of anti-inflammatory cytokines in T2D is less clear. In this study, we report that normal IL-4-dependent elaboration of IL-1 receptor antagonist (IL-1RA) requires IRS-2-mediated PI3K activity in primary macrophages. We also show that macrophages isolated from obese/diabetic db/db mice have impaired IRS-2-mediated PI3K activity and constitutively overexpress suppressor of cytokine signaling (SOCS)-3, which impairs an important IL-4 anti-inflammatory function. Peritoneal proinflammatory cytokine levels were examined in diabese (db/db) mice, and IL-6 was found to be nearly 7-fold higher than in nondiabese (db/+) control mice. Resident peritoneal macrophages were isolated from db/db mice and were found to constitutively overexpress IL-6 and were unable to elaborate IL-1RA in response to IL-4-like db/+ mouse macrophages. Inhibition of PI3K with wortmannin or blockage of IRS-2/PI3K complex formation with a cell permeable IRS-2-derived tyrosine phosphopeptide inhibited IL-4-dependent IL-1RA production in db/+ macrophages. Examination of IL-4 signaling in db/db macrophages revealed that IL-4-dependent IRS-2/PI3K complex formation and IRS-2 tyrosine phosphorylation was reduced compared with db/+ macrophages. SOCS-3/IL-4 receptor complexes, however, were increased in db/db mouse macrophages compared with db/+ mice macrophages as was db/db mouse macrophage SOCS-3 expression. These results indicate that in the db/db mouse model of T2D, macrophage expression of SOCS-3 is increased, and impaired IL-4-dependent IRS-2/PI3K formation induces a state of IL-4 resistance that disrupts IL-4-dependent production of IL-1RA.

Oncogenic K-Ras signals through epidermal growth factor receptor and wild-type H-Ras to promote radiation survival in pancreatic and colorectal carcinoma cells

Pancreatic and colorectal carcinomas frequently express oncogenic/mutant K-Ras that contributes to both tumorigenesis and clinically observed resistance to radiation treatment. We have previously shown that farnesyltransferase inhibitors (FTI) radiosensitize many pancreatic and colorectal cancer cell lines that express oncogenic K-ras at doses that inhibit the prenylation and activation of H-Ras but not K-Ras. In the present study, we have examined the mechanism of FTI-mediated radiosensitization in cell lines that express oncogenic K-Ras and found that wild-type H-Ras is a contributor to radiation survival in tumor cells that express oncogenic K-Ras. In these experiments, inhibiting the expression of oncogenic K-Ras, wild-type H-Ras, or epidermal growth factor receptor (EGFR) led to similar levels of radiosensitization as treatment with the FTI tipifarnib. Treatment with the EGFR inhibitor gefitinib led to similar levels of radiosensitization, and the combinations of tipifarnib or gefitinib plus inhibition of K-Ras, H-Ras, or EGFR expression did not provide additional radiosensitization compared with tipifarnib or gefitinib alone. Finally, supplementing culture medium with the EGFR ligand transforming growth factor alpha was able to reverse the radiosensitizing effect of inhibiting K-ras expression. Taken together, these findings suggest that EGFR-activated H-Ras signaling is initiated by oncogenic K-Ras to promote radiation survival in pancreatic and colorectal cancers.

Growth retardation alters the epigenetic characteristics of hepatic dual specificity phosphatase 5

Uteroplacental insufficiency leads to intrauterine growth retardation (IUGR) and adult onset insulin resistance in both humans and rats. IUGR rat liver is characterized by persistent changes in histone 3 lysine 9 and lysine 14 acetylation, which may induce postnatal changes in gene expression. We hypothesized that it would be possible to identify hepatic genes whose epigenetic characteristics and mRNA levels are altered due to IUGR using chromatin immunoprecipitation (ChIP) coupled with random primed differential display polymerase chain reaction (PCR). One of the isolated sequences identified contained exon 2 of the dual specificity phosphatase-5 gene (DUSP5). IUGR affected hepatic DUSP5 mRNA levels and exon 2 DNA methylation into adulthood in the rat. DUSP5 dephosphorylates Erk1 and Erk2 within the MAPK signaling cascade, which in turn affects serine 612 phosphorylation of insulin receptor substrate-1 (p612 IRS-1). In adult rat liver, IUGR increased Erk1/Erk2 phosphorylation and p612 IRS-1 phosphorylation. Increased serine phosphorylation of hepatic IRS-1 may contribute to the insulin resistance that characterizes these animals. We conclude that intrauterine growth retardation induced by uteroplacental insufficiency 1) affects the hepatic epigenetic characteristics and mRNA of the DUSP-5 and 2) increases hepatic insulin receptor substrate-1 phosphorylation at serine 612 in adult rats.

Angiotensin II modulates tyr-phosphorylation of IRS-4, an insulin receptor substrate, in rat liver membranes

Angiotensin II (Ang II), a major regulator of blood pressure, is also involved in the control of cellular proliferation and hypertrophy and might exhibit additional actions in vivo by modulating the signaling of other hormones. As hypertension and Insulin (Ins) resistance often coexist and are risk factors for cardiovascular diseases, Ang II and Insulin signaling cross-talk may have an important role in hypertension development. The effect of Ins on protein tyrosine phosphorylation was assayed in rat liver membrane preparations, a rich source of Ins receptors. Following stimulation, Ins (10(-7) M) induced tyr-phosphorylation of different proteins. Insulin consistently induced tyr-phosphorylation of a 160 kDa protein (pp160) with maximum effect between 1 and 3 min. The pp160 protein was identified by anti-IRS-4 but not by anti-IRS-1 antibody. Pre-stimulation with Ang II (10(-7) M) diminishes tyr-phosphorylation level of pp160/IRS-4 in a dose-dependent manner. Okadaic acid, the PP1A and PP2A Ser/Thr phosphatase inhibitor, increases pp160 phosphorylation induced by Ins and prevents the inhibitory effect of Ang II pre-stimulation. Genistein, a tyrosine kinase inhibitor, diminishes tyr-phosphorylation level of IRS-4. PI3K inhibitors Wortmanin and LY294002, both increase tyr-phosphorylation of IRS-4, either in the presence of Ins alone or combined with Ang II. These results suggest that Ins and Ang II modulate IRS-4 tyr-phosphorylation in a PI3K-dependent manner. In summary, we showed that Ins induces tyr-phosphorylation of IRS-4, an effect modulated by Ang II. Assays performed in the presence of different inhibitors points toward a PI3K involvement in this signaling pathway.

Alternative signaling pathways regulating type I interferon-induced apoptosis

Type I interferons (IFNs) are pleiotropic cytokines that exert multiple effects on normal and tumor cells. These effects are supposedly mediated through the stimulation of several signal transduction pathways by type I IFNs. These include the well-studied canonical Jak-Stat pathway, largely responsible for the antiviral and growth-inhibitory activities of IFNs, as well as the phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase (MAPK) pathways, whose importance in IFN-induced biologic outcomes has not been precisely established. One of the effects of type I IFNs on tumor cells is the induction of programmed cell death, apoptosis, which has been studied extensively over the last decade and has been suggested to be an important effector mechanism for IFN's antitumor effects in the treatment of cancer. The aim of this review is to summarize and discuss the recent data in the field of type I IFN-induced apoptosis, with special emphasis on the molecular mechanisms of apoptosis and on the role of alternative noncanonical signaling pathways stimulated by type I IFNs in this process.

The PI3' Kinase Pathway in Interferon Signaling

Interferons (IFNs) are pleiotropic cytokines with important immunomodulatory, antitumor, antiviral, and growth inhibitory properties. Consistent with the multiplicity of their effects, the IFNs activate several cellular cascades after they bind to their receptors. Although the Jak-Stat pathway is the most studied pathway, it appears that additional signaling cascades play roles in IFN signaling. The phosphatidylinositol 3'-kinase (PI3'K) pathway has emerged as one of the critical players in IFN signal transduction and is the focus of this review. This pathway is activated distinctively from the Stat pathway but plays important roles in the regulation of IFN-dependent gene transcription and initiation of mRNA translation.

Insulin-induced stimulation of JNK and the PI 3-kinase/mTOR pathway leads to phosphorylation of serine 318 of IRS-1 in C2C12 myotubes

Increased serine/threonine phosphorylation of insulin receptor substrate-1 (IRS-1) is associated with cellular insulin resistance. We have recently identified serine 318 (Ser318) as a novel protein kinase C-zeta (PKC-zeta)-dependent phosphorylation site within IRS-1. As other kinases may phosphorylate at this serine residue as well, we aimed to identify such kinases in the present study. In C2C12 myotubes, exposure to insulin or phorbol ester markedly increased Ser318 phosphorylation. In contrast, high glucose, tumor necrosis factor-alpha, and free fatty acids did not provoke Ser318 phosphorylation. JNK and the PI 3-kinase/mTOR pathway were found to be implicated in insulin-induced Ser318 phosphorylation, but not in TPA-stimulated phosphorylation that was, at least partly, mediated by classical or novel PKC. In conclusion, with JNK and the PI 3-kinase/mTOR pathway as mediators of insulin-induced Ser318 phosphorylation, we have identified kinases that have previously been reported to play key roles in phosphorylation of other serine residues in IRS-1.

Mechanisms of type-I- and type-II-interferon-mediated signalling

Interferons are cytokines that have antiviral, antiproliferative and immunomodulatory effects. Because of these important properties, in the past two decades, major research efforts have been undertaken to understand the signalling mechanisms through which these cytokines induce their effects. Since the original discovery of the classical JAK (Janus activated kinase)-STAT (signal transducer and activator of transcription) pathway of signalling, it has become clear that the coordination and cooperation of multiple distinct signalling cascades - including the mitogen-activated protein kinase p38 cascade and the phosphatidylinositol 3-kinase cascade - are required for the generation of responses to interferons. It is anticipated that an increased understanding of the contributions of these recently identified pathways will advance our current thinking about how interferons work.

Interferon α-induced Apoptosis in Tumor Cells Is Mediated through the Phosphoinositide 3-Kinase/Mammalian Target of Rapamycin Signaling Pathway

Interferon (IFN) alpha induces a caspase-dependent apoptosis that is associated with activation of the proapoptotic Bak and Bax, loss of mitochondrial membrane potential, and release of cytochrome c. In addition to the onset of the classical Jak-STAT pathway, IFNalpha also induced phosphoinositide 3-kinase (PI3K) activity. Pharmacological inhibition of PI3K activity by Ly294002 disrupted IFN-induced apoptosis upstream of mitochondria. Inhibition of mTOR by rapamycin or by overexpression of a kinase dead mutant of mTOR, efficiently blocked IFNalpha-induced apoptosis. A PI3K and mTOR-dependent phosphorylation of p70S6 kinase and 4E-BP1 repressor was induced by IFNalpha treatment of cells and was strongly inhibited by Ly294002 or rapamycin. The activation of Jak-STAT signaling upon IFNalpha stimulation was not affected by abrogating PI3K/mTOR pathway. Neither was the expression of several IFNalpha target genes affected, nor the ability of IFNalpha to protect against virus-induced cell death affected by inhibition of the PI3K/mTOR pathway. These data demonstrate that an intact PI3K/mTOR pathway is necessary for the ability of IFNalpha to induce apoptosis, whereas activation of the Jak-STAT pathway alone appears to be insufficient for this specific IFNalpha-induced effect.

Insulin receptor substrate-2-dependent interleukin-4 signaling in macrophages is impaired in two models of type 2 diabetes mellitus

We have shown previously that hyperinsulinemia inhibits interferon-alpha-dependent activation of phosphatidylinositol 3-kinase (PI3-kinase) through mammalian target of rapamycin (mTOR)-induced serine phosphorylation of insulin receptor substrate (IRS)-1. Here we report that chronic insulin and high glucose synergistically inhibit interleukin (IL)-4-dependent activation of PI3-kinase in macrophages via the mTOR pathway. Resident peritoneal macrophages (PerMPhis) from diabetic (db/db) mice showed a 44% reduction in IRS-2-associated PI3-kinase activity stimulated by IL-4 compared with PerMPhis from heterozygote (db/+) control mice. IRS-2 from db/db mouse PerMPhis also showed a 78% increase in Ser/Thr-Pro motif phosphorylation without a difference in IRS-2 mass. To investigate the mechanism of this PI3-kinase inhibition, 12-O-tetradecanoylphorbol-13-acetate-matured U937 cells were treated chronically with insulin (1 nm, 18 h) and high glucose (4.5 g/liter, 48 h). In these cells, IL-4-stimulated IRS-2-associated PI3-kinase activity was reduced by 37.5%. Importantly, chronic insulin or high glucose alone did not impact IL-4-activated IRS-2-associated PI3-kinase. Chronic insulin + high glucose did reduce IL-4-dependent IRS-2 tyrosine phosphorylation and p85 association by 54 and 37%, respectively, but did not effect IL-4-activated JAK/STAT signaling. When IRS-2 Ser/Thr-Pro motif phosphorylation was examined, chronic insulin + high glucose resulted in a 92% increase in IRS-2 Ser/Thr-Pro motif phosphorylation without a change in IRS-2 mass. Pretreatment of matured U937 cells with rapamycin blocked chronic insulin + high glucose-dependent IRS-2 Ser/Thr-Pro motif phosphorylation and restored IL-4-dependent IRS-2-associated PI3-kinase activity. Taken together these results indicate that IRS-2-dependent IL-4 signaling in macrophages is impaired in models of type 2 diabetes mellitus through a mechanism that relies on insulin/glucose-dependent Ser/Thr-Pro motif serine phosphorylation mediated by the mTOR pathway.

Insulin attenuates leptin-induced STAT3 tyrosine-phosphorylation in a hepatoma cell line

Leptin, the 16 kDa protein product of the ob gene, is secreted by adipocytes. The long form leptin receptor (ObRb) is expressed at high levels in the hypothalamus, and regulates appetite and energy expenditure. The fact that serum concentration of leptin is correlated with body mass index (BMI) suggests reduced sensitivity to leptin. Even though hyperinsulinemia and hyperleptinemia could coexist in obese humans, little is known about the interaction of insulin and leptin. In this study, we examined the effect of insulin on leptin signaling using Huh 7 cells transiently transfected with ObRb cDNA. Insulin inhibits leptin-induced STAT3 phosphorylation in a time- and dose-dependent manner without affecting Janus tyrosine kinases (JAKs) JAK2 phosphorylation. Okadaic acid prevents the inhibitory effect of insulin on leptin-induced STAT3 activation.

Vanadate and rapamycin synergistically enhance insulin-stimulated glucose uptake

Tyrosine dephosphorylation, serine phosphorylation, and proteasomal degradation of insulin receptor substrates (IRSs) are implicated in the negative regulation of insulin action. Here we show that simultaneous inhibition of IRS-1 tyrosine dephosphorylation and proteasomal degradation synergistically augments insulin-responsive glucose uptake. L6 skeletal muscle cells (L6 cells) were treated with inhibitors of protein-tyrosine phosphatases, proteasomal degradation, and mammalian target of rapamycin (mTOR), and the effects of insulin on glucose uptake, IRS-1 tyrosine phosphorylation, phosphatidylinositol (PI) 3-kinase activity, and IRS-1 mass were examined. Pretreatment of L6 cells with sodium orthovanadate (Na(3)VO(4)) plus the mTOR inhibitor rapamycin caused a 5-fold increase in insulin-responsive glucose uptake at 2 hours when compared to insulin alone. Evaluation of IRS-1 associated PI 3-kinase activity, IRS-1-associated p85 mass, and IRS-1 tyrosine phosphorylation showed that 2 hours after insulin addition they were reduced by 70% from maximal activity. Likewise, IRS-1 mass was reduced by 50%. When L6 cells were pretreated with Na(3)VO(4) plus the proteasome inhibitor MG-132 or the mTOR inhibitor rapamycin prior to insulin addition, IRS-1 mass loss as well as IRS-1/PI-3 kinase complex decay was blocked at 2 hours and PI 3-kinase activity was increased 2.5-fold and 4-fold, respectively, over insulin alone. Finally, treatment of L6 cells with subtherapeutic amounts of vanadyl sulfate and rapamycin induced a synergistic 3-fold increase in insulin-induced glucose uptake at 2 hours. These findings indicate that vanadium and rapamycin synergize to enhance glucose uptake by preventing IRS-1 mass loss and IRS-1/PI 3-kinase complex decay and may offer a new approach to enhance glucose transport in diabetes.

Role of leucine in protein metabolism during exercise and recovery

Exercise produces changes in protein and amino acid metabolism. These changes include degradation of the branched-chain amino acids, production of alanine and glutamine, and changes in protein turnover. One of the amino acid most affected by exercise is the branched-chain amino acid leucine. Recently, there has been an increased understanding of the role of leucine in metabolic regulations and remarkable new findings about the role of leucine in intracellular signaling. Leucine appears to exert a synergistic role with insulin as a regulatory factor in the insulin/ phosphatidylinositol-3 kinase (PI3-K) signal cascade. Insulin serves to activate the signal pathway, while leucine is essential to enhance or amplify the signal for protein synthesis at the level of peptide initiation. Studies feeding amino acids or leucine soon after exercise suggest that post-exercise consumption of amino acids stimulates recovery of muscle protein synthesis via translation regulations. This review focuses on the unique roles of leucine in amino acid metabolism in skeletal muscle during and after exercise.

Methylglyoxal enhances cisplatin-induced cytotoxicity by activating protein kinase Cdelta

The cytotoxic side effects of anti-neoplastic drugs are increased in patients with either type 1 or type 2 diabetes mellitus by a mechanism that is not clearly defined. We report that the circulating glucose metabolite, methylglyoxal (MGO), enhances cisplatin-induced apoptosis by activating protein kinase Cdelta (PKCdelta). We found that treatment of myeloma cells with the antioxidant N-acetylcysteine completely blocked cisplatin-dependent intracellular GSH oxidation, reactive oxygen species (ROS) generation, poly(ADP-ribose) polymerase cleavage, and apoptosis. Importantly, co-treatment of cells with the reactive carbonyl MGO and cisplatin increased apoptosis by 90% over the expected additive effect of combined MGO and cisplatin treatment. This same synergism was also observed when ROS generation was examined. MGO and cisplatin increased PKCdelta activity by 4-fold, and this effect was blocked by the PKCdelta inhibitor rottlerin but not by NAC. Furthermore, rottlerin blocked combined MGO and cisplatin-induced ROS generation and apoptosis. Finally, MGO and cisplatin induced c-Abl activation and c-Abl:PKCdelta association. Rottlerin blocked c-Abl activation, but the c-Abl inhibitor STI-571 increased MGO and cisplatin-induced apoptosis by 50%. Taken together these data indicate that MGO synergistically enhances cisplatin-induced apoptosis through activation of PKCdelta and that PKCdelta is critical to both cell death and cell survival pathways. These findings suggest that in the patient with diabetes mellitus heightened oxidative stress can enhance the cytotoxicity of agents that induce DNA damage.

IL-10 inhibits apoptosis of promyeloid cells by activating insulin receptor substrate-2 and phosphatidylinositol 3'-kinase

IL-10 is well known to be a potent inhibitor of the synthesis of proinflammatory cytokines, but noninflammatory hemopoietic cells also express IL-10Rs. Here we show that IL-10 directly affects progenitor myeloid cells by protecting them from death following the removal of growth factors. Murine factor-dependent cell progenitors cultured in the absence of growth factors were 43 +/- 1% apoptotic after 12 h. Addition of IL-10 at a concentration as low as 100 pg/ml significantly reduced the apoptotic population to 32 +/- 3%. At 10 ng/ml, IL-10 caused a 4-fold reduction in the apoptotic population (11 +/- 1%). The anti-apoptotic activity of IL-10 was significantly inhibited with a neutralizing IL-10R Ab. Factor-dependent cell progenitor promyeloid cells expressed functional IL-10Rs, as assessed by precipitation of a 110-kDa protein with an Ab to the IL-10R and by the ability of IL-10 to activate Jak1 and Tyk2 and to phosphorylate tyrosine 705 on Stat-3. IL-10 increased tyrosyl phosphorylation of insulin receptor substrate-2 and stimulated the enzymatic activity of both phosphatidylinositol 3'-kinase and Akt. The anti-apoptotic activity of IL-10 was blocked by inhibition of phosphatidylinositol 3'-kinase. Wortmannin and LY294002 also totally inhibited activation of extracellular signal-related kinase (ERK)1/2 by IL-10. Direct inhibition of ERK1/2 with the mitogen-activated protein kinase/ERK kinase inhibitor PD98059 partially, but significantly, impaired the anti-apoptotic activity of IL-10. These data establish that activation of the IL-10R promotes survival of progenitor myeloid cells. This survival-promoting activity is totally due to IL-10 stimulating the insulin receptor substrate-2/PI 3-kinase/Akt pathway, which increases the anti-apoptotic activity of ERK1/2.

The role of the insulin receptor substrate-1 in the differentiation of rat hippocampal neuronal cells

H19-7/IGF-IR cells are rat hippocampal cells expressing a human IGF-I receptor, which differentiate to a neuronal phenotype when stimulated by IGF-I at 39 degrees C. H19-7/IGF-IR cells have low levels of expression of insulin receptor substrate-l (IRS-1), a major substrate of the IGF-IR. IGF-I induces serine-phosphorylation and down-regulation of the endogenous IRS-1 upon differentiation of H19-7/IGF-IR cells. The profound influence of IRS-1 on differentiation of H19-7/IGF-IR cells was confirmed by transfecting these cells with a plasmid expressing mouse IRS-1. Over-expression of wild type IRS-1 in H19-7/IGF-IR cells abolishes IGF-I-induced differentiation at 39 degrees C. A mutant of IRS-1 lacking the PTB domain loses the ability to inhibit the differentiation program. H19-7/IGF-IR/IRS-1 cells at 39 degrees C show a stronger and prolonged activation of Akt, when compared to H19-7/IGF-IR cells. The role of Akt in the inhibition of the differentiation program was confirmed by using the inhibitor of Class I PI3 kinases LY29400, which restores IGF-I-induced differentiation of H19-7/IGF-IR/IRS-1 cells. H19-7/IGF-IR/IRS-1 cells show a strong reduction in MAP kinases signaling, which is related to the superactivation of Akt. This was confirmed by expressing in H19-7/IGF-IR cells a constitutively active Akt, which inhibited MAP kinases activation in these cells. These experiments confirm the importance of MAPK in the mechanism of IGF-I-mediated differentiation of H19-7/IGF-IR cells

Liver-specific igf-1 gene deletion leads to muscle insulin insensitivity

Insulin and insulin-like growth factors (IGFs) mediate a variety of signals involved in mammalian development and metabolism. To study the metabolic consequences of IGF-I deficiency, we used the liver IGF-I-deficient (LID) mouse model. The LID mice show a marked reduction (approximately 75%) in circulating IGF-I and elevated growth hormone (GH) levels. Interestingly, LID mice show a fourfold increase in serum insulin levels (2.2 vs. 0.6 ng/ml in control mice) and abnormal glucose clearance after insulin injection. Fasting blood glucose levels and those after a glucose tolerance test were similar between the LID mice and their control littermates. Thus, the high levels of circulating insulin enable the LID mice to maintain normoglycemia in the presence of apparent insulin insensitivity. Insulin-induced autophosphorylation of the insulin receptor and tyrosine phosphorylation of insulin receptor substrate (IRS)-1 were absent in muscle, but were normal in liver and white adipose tissue of the LID mice. In contrast, IGF-I-induced autophosphorylation of its cognate receptor and phosphorylation of IRS-1 were normal in muscle of LID mice. Thus, the insulin insensitivity seen in the LID mice is muscle specific. Recombinant human IGF-I treatment of the LID mice caused a reduction in insulin levels and an increase in insulin sensitivity. Treatment of the LID mice with GH-releasing hormone antagonist, which reduces GH levels, also increased insulin sensitivity. These data provide evidence of the role of circulating IGF-I as an important component of overall insulin action in peripheral tissues.

CD45 negatively regulates monocytic cell differentiation by inhibiting phorbol 12-myristate 13-acetate-dependent activation and tyrosine phosphorylation of protein kinase Cdelta

The protein-tyrosine phosphatase CD45 is expressed on all monocytic cells, but its function in these cells is not well defined. Here we report that CD45 negatively regulates monocyte differentiation by inhibiting phorbol 12-myristate 13-acetate (PMA)-dependent activation of protein kinase C (PKC) delta. We found that antisense reduction of CD45 in U937 monocytic cells (CD45as cells) increased by 100% the ability of PMA to enlarge cell size, increase cell cytoplasmic process width and length, and induce surface expression of CD11b. In addition, reduction in CD45 expression caused the duration of peak PMA-induced MEK and extracellular signal-regulated kinase (ERK) 1/2 activity to increase from 5 min to 30 min while leading to a 4-fold increase in PMA-dependent PKCdelta activation. Importantly, PMA-dependent tyrosine phosphorylation of PKCdelta was also increased 4-fold in CD45as cells. Finally, inhibitors of MEK (PD98059) and PKCdelta (rottlerin) completely blocked PMA-induced monocytic cell differentiation. Taken together, these data indicate that CD45 inhibits PMA-dependent PKCdelta activation by impeding PMA-dependent PKCdelta tyrosine phosphorylation. Furthermore, this blunting of PKCdelta activation leads to an inhibition of PKCdelta-dependent activation of ERK1/2 and ERK1/2-dependent monocyte differentiation. These findings suggest that CD45 is a critical regulator of monocytic cell development.

Src homology 3 domain-dependent interaction of Nck-2 with insulin receptor substrate-1

Insulin receptor substrate-1 (IRS-1) is a multi-domain protein that mediates signal transduction from receptors for insulin and other growth factors to a variety of downstream molecules through both tyrosine-phosphorylation-dependent and -independent interactions. While the tyrosine-phosphorylation-dependent interactions mediated by IRS-1 have been well characterized, the molecular basis underlying the tyrosine-phosphorylation-independent IRS-1 interactions is largely unknown. We previously detected, in an in vitro binding assay, interactions of Nck-2 Src homology (SH) 3 domains with IRS-1. We show here that IRS-1 associates with Nck-2 in vivo. Additionally, we have investigated the molecular basis underlying the IRS-1-Nck-2 complex formation. We have found that (i) mutations at the highly conserved tryptophan within the Nck-2 SH3 domains markedly reduced the association with IRS-1, (ii) interactions mediated by multiple SH3 domains enhance the complex formation of Nck-2 with IRS-1, (iii) deletion of either the phosphotyrosine-binding/Shc and IRS-1 NPXY-binding (PTB/SAIN) domains or the Pre-C-terminal domain of IRS-1, but not the pleckstrin homology (PH) domain, reduced the Nck-2 binding, (iv) PTB/SAIN domains or the Pre-C-terminal domain alone is capable of interacting with Nck-2, and (v) the IRS-1-Nck-2 interaction occurs in the absence of other proteins and therefore is direct. These results establish that IRS-1 is a bona fide target of the Nck-2 SH3 domains and reveal that IRS-1 forms a complex with Nck-2 via direct interactions mediated by multiple domains from both binding partners.

Fes mediates the IL-4 activation of insulin receptor substrate-2 and cellular proliferation

Although Jak kinases are essential for initiating cytokine signaling, the role of other nonreceptor tyrosine kinases in this process remains unclear. We have examined the role of Fes in IL-4 signaling. Examination of Jak1-deficient cell lines demonstrates that Jak1 is required for the activation of Fes by IL-4. Experiments studying signaling molecules activated by IL-4 receptor suggest that IL-4 signaling can be subdivided into Fes-dependent and Fes-independent pathways. Overexpression of kinase-inactive Fes blocks the IL-4 activation of insulin receptor substrate-2, but not STAT6. Fes appears to be a downstream kinase from Jak1/Jak3 in this process. Further examination of downstream signaling demonstrates that kinase-inactive Fes inhibits the recruitment of phosphoinositide 3-kinase to the activated IL-4 receptor complex and decreases the activation of p70(S6k) kinase in response to IL-4. This inhibition correlates with a decrease in IL-4-induced proliferation. In contrast, mutant Fes does not inhibit the activation of Akt by IL-4. These data demonstrate that signaling pathways activated by IL-4 require different tyrosine kinases. This differential requirement predicts that specific kinase inhibitors may permit the disruption of specific IL-4-induced functions.

Protein kinase C-zeta phosphorylates insulin receptor substrate-1 and impairs its ability to activate phosphatidylinositol 3-kinase in response to insulin

Protein kinase C-zeta (PKC-zeta) is a serine/threonine kinase downstream from phosphatidylinositol 3-kinase in insulin signaling pathways. However, specific substrates for PKC-zeta that participate in the biological actions of insulin have not been reported. In the present study, we identified insulin receptor substrate-1 (IRS-1) as a novel substrate for PKC-zeta. Under in vitro conditions, wild-type PKC-zeta (but not kinase-deficient mutant PKC-zeta) significantly phosphorylated IRS-1. This phosphorylation was reversed by treatment with the serine-specific phosphatase, protein phosphatase 2A. In addition, the overexpression of PKC-zeta in NIH-3T3(IR) cells caused significant phosphorylation of cotransfected IRS-1 as demonstrated by [(32)P]orthophosphate labeling experiments. In rat adipose cells, endogenous IRS-1 coimmunoprecipitated with endogenous PKC-zeta, and this association was increased 2-fold upon insulin stimulation. Furthermore, the overexpression of PKC-zeta in NIH-3T3(IR) cells significantly impaired insulin-stimulated tyrosine phosphorylation of cotransfected IRS-1. Importantly, this was accompanied by impaired IRS-1-associated phosphatidylinositol 3-kinase activity. Taken together, our results raise the possibility that IRS-1 is a novel physiological substrate for PKC-zeta. Because PKC-zeta is located downstream from IRS-1 and phosphatidylinositol 3-kinase in established insulin signaling pathways, PKC-zeta may participate in negative feedback pathways to IRS-1 similar to those described previously for Akt and GSK-3.

Frap-dependent serine phosphorylation of IRS-1 inhibits IRS-1 tyrosine phosphorylation

We have previously shown that interferon-alpha (IFN alpha)-dependent tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) is impaired by serine phosphorylation of IRS-1 due to the reduced ability of serine phosphorylated IRS-1 to serve as a substrate for Janus kinase 1 (JAK1). Here we report that FKBP12-rapamycin-associated protein (FRAP) is a physiologic IRS-1 kinase that blocks IFN alpha signaling by serine phosphorylating IRS-1. We found that both FRAP and insulin-activated p70 S6 kinase (p70(s6k)) serine phosphorylated IRS-1 between residues 511 and 772 (IRS-1(511-772)). Importantly, only FRAP-dependent IRS-1(511-772) serine phosphorylation inhibited by 50% subsequent JAK1-dependent tyrosine phosphorylation of IRS-1. Furthermore, treatment of U266 cells with the FRAP inhibitor rapamycin increased IFN alpha-dependent tyrosine phosphorylation by twofold while reducing constitutive IRS-1 serine phosphorylation within S/T-P motifs by 80%. Taken together, these data indicate that FRAP, but not p70(s6k), is a likely physiologic IRS-1 serine kinase that negatively regulates JAK1-dependent IRS-1 tyrosine phosphorylation and suggests that FRAP may modulate IRS-dependent cytokine signaling.

Mechanisms of regulation of cell adhesion and motility by insulin receptor substrate-1 in prostate cancer cells

LNCaP cells are human prostatic cancer cells that have a frame-shift mutation of the tumor suppressor gene PTEN and do not express the insulin receptor substrate-1 (IRS-1), a major substrate of the type 1 insulin-like growth factor receptor (IGF-IR). Ectopic expression of IRS-1 in LNCaP cells increases cell adhesion and decreases cell motility by an IGF-I-independent mechanism. We show now that these effects of IRS-1 are accompanied by serine phosphorylation of IRS-1 and are inhibited by inhibitors of phosphatidylinositol 3-kinase (PI3K). We have confirmed the requirement for PI3K activity and serine phosphorylation by the use of IRS-1 mutants, expressed in LNCaP cells. Serine phosphorylation inhibits IGF-I-induced tyrosyl phosphorylation of IRS-1, which is restored by the expression of wild-type PTEN or by inhibition of PI3K activity. Finally, IRS-1 in LNCaP cells co-immunoprecipitates with integrin alpha 5 beta 1, and the association is again IGF-I-independent. We conclude that in LNCaP cells, IRS-1 is serine phosphorylated by PI3K, generating effects that are different, and even opposite, from those generated by IGF-I.

Gab1 phosphorylation: a novel mechanism for negative regulation of HGF receptor signaling

Signal transduction by HGF receptor, the tyrosine kinase encoded by the MET oncogene, switches on a genetic program called 'invasive growth' inducing epithelial cell dissociation, migration, growth, and ultimately leading to differentiation into branched tubular structures. Sustained tyrosine phosphorylation of the downstream adaptor protein Gab1 is required for the HGF response. Here we show that serine/threonine phosphorylation of Gab1 provides a control mechanism for negative regulation. Treatment with okadaic acid, a potent inhibitor of the serine/threonine protein phosphatases PP1 and PP2A, was followed by activation of a number of serine/threonine kinases, hyper-phosphorylation in serine and threonine of Gab1 and severe inhibition of the HGF-induced biological responses. Under these conditions, Gab1 was found to be concomitantly hypo-phosphorylated in tyrosine, and thus endowed with reduced ability to recruit SH2 containing signal transducers such as PI3 kinase. Among the serine-threonine kinases activated by PP1 and PP2A inhibition, we found that PKC-alpha and PKC-beta1 are required for negative regulation of Gab1. These data provide a novel negative mechanism for the HGF receptor signaling pathways and highlight a potentially useful target for inhibitors of invasive growth.

Serine/threonine phosphorylation of IRS-1 triggers its degradation: possible regulation by tyrosine phosphorylation

Insulin receptor substrate (IRS)-1 protein expression is markedly reduced in many insulin-resistant states, although the mechanism for this downregulation is unclear. In this study, we have investigated the early events in the insulin pathway that trigger the degradation of IRS-1. Incubation of the adipocytes with insulin induced a fast electrophoretic mobility shift of IRS-1 and a subsequent degradation of the protein. Wortmannin and rapamycin blocked this mobility shift of IRS-1, maintained the insulin-induced tyrosine phosphorylation of IRS-1, and blocked its degradation. In contrast, a glycogen synthase kinase 3 inhibitor, a mitogen-activated protein kinase/extracellular-regulated kinase inhibitor, and various protein kinase C inhibitors had no effect. Incubation with okadaic acid increased the serine/threonine phosphorylation of IRS-1 and its degradation, mimicking insulin, and its effect was prevented by the proteasome inhibitor lactacystin, as well as by rapamycin. Treatment of the cells with the tyrosine phosphatase inhibitor orthovanadate in the presence of insulin or okadaic acid partially inhibited the degradation of IRS-1. We propose that a rapamycin-dependent pathway participates as a negative regulator of IRS-1, increasing its serine/threonine phosphorylation, which triggers degradation. Thus, regulation of serine/threonine versus tyrosine phosphorylation may modulate IRS-1 degradation, affecting insulin sensitivity.