The role of insulin receptor kinase domain autophosphorylation in receptor-mediated activities. Analysis with insulin and anti-receptor antibodies

Evidence of disturbed insulin signaling in animal models of Alzheimer's disease

Since glucose reuptake by neurons is mostly independent of insulin, it has been an intriguing question whether insulin has or not any roles in the brain. Consequently, the identification of insulin receptors in the central nervous system has fueled investigations of insulin functions in the brain. It is also already known that insulin can influence glucose reuptake by neurons, mostly during activities that have the highest energy demand. The identification of high density of insulin receptors in the hippocampus also suggests that insulin may present important roles related to memory. In this context, studies have reported worse performance in cognitive tests among diabetic patients. In addition, alterations in the regulation of central insulin pathways have been observed in the brains of Alzheimer's disease (AD) patients. In fact, some authors have proposed AD as a third type of diabetes and recently, our group proposed insulin resistance as a common link between different AD hypotheses. Therefore, in the present narrative review, we intend to revise and gather the evidence of disturbed insulin signaling in experimental animal models of AD.

Insulin Resistance as a Common Link Between Current Alzheimer's Disease Hypotheses

Almost 115 years ago, Alois Alzheimer described Alzheimer's disease (AD) for the first time. Since then, many hypotheses have been proposed. However, AD remains a severe health public problem. The current medical approaches for AD are limited to symptomatic interventions and the complexity of this disease has led to a failure rate of approximately 99.6%in AD clinical trials. In fact, no new drug has been approved for AD treatment since 2003. These failures indicate that we are failing in mimicking this disease in experimental models. Although most studies have focused on the amyloid cascade hypothesis of AD, the literature has made clear that AD is rather a multifactorial disorder. Therefore, the persistence in a single theory has resulted in lost opportunities. In this review, we aim to present the striking points of the long scientific path followed since the description of the first AD case and the main AD hypotheses discussed over the last decades. We also propose insulin resistance as a common link between many other hypotheses.

Role of Skeletal Muscle in Insulin Resistance and Glucose Uptake

The skeletal muscle is the largest organ in the body, by mass. It is also the regulator of glucose homeostasis, responsible for 80% of postprandial glucose uptake from the circulation. Skeletal muscle is essential for metabolism, both for its role in glucose uptake and its importance in exercise and metabolic disease. In this article, we give an overview of the importance of skeletal muscle in metabolism, describing its role in glucose uptake and the diseases that are associated with skeletal muscle metabolic dysregulation. We focus on the role of skeletal muscle in peripheral insulin resistance and the potential for skeletal muscle-targeted therapeutics to combat insulin resistance and diabetes, as well as other metabolic diseases like aging and obesity. In particular, we outline the possibilities and pitfalls of the quest for exercise mimetics, which are intended to target the molecular mechanisms underlying the beneficial effects of exercise on metabolic disease. We also provide a description of the molecular mechanisms that regulate skeletal muscle glucose uptake, including a focus on the SNARE proteins, which are essential regulators of glucose transport into the skeletal muscle. © 2020 American Physiological Society. Compr Physiol 10:785-809, 2020.

Guava leaves extract ameliorates STZ induced diabetes mellitus via activation of PI3K/AKT signaling in skeletal muscle of rats

Blood glucose homeostasis and insulin signaling pathway regulation take a vital role in the management of diabetes mellitus. Our present was designed to explore the mechanism of the blood homeostasis, regulation of oxidative stress and insulin signaling pathway by guava leaf extract (GLE). Diabetes mellitus was induced in male albino Wistar by streptozotocin (STZ) (Single dose-40 mg/kg b.w.). As an extension STZ rats received GLE (GLE; 200 mg/kg b.w). At the end of the study the lipid peroxidation products, antioxidants, insulin signaling genes were analyzed. Treatment with GLE resulted in decreased plasma and skeletal muscle lipid peroxidation markers, increased antioxidants, and improved insulin signaling genes. GLE treatment helps to maintain blood homeostasis alleviates oxidative stress and regulates the insulin signaling genes in skeletal muscle. Overall the results suggest GLE treatment regulates blood glucose, inhibits oxidative stress, and importantly it regulates insulin signaling pathway genes in skeletal muscle. Further studies on the GLE role in other important pathways can add additional strength to the claim that GLE is a strong anti-diabetic candidate.

Dissecting PUGNAc-mediated inhibition of the pro-survival action of insulin

Previous studies utilizing PUGNAc, the most widely used β-N-acetylglucosaminidase (OGA) inhibitor to increase global O-N-acetylglucosamine (GlcNAc) levels, have reported a variety of effects including insulin resistance as a direct result of elevated O-GlcNAc levels. The notion of OGA inhibition causing insulin resistance was not replicated in studies in which elevated global O-GlcNAc levels were achieved using two other OGA inhibitors. Related to insulin action, work by others has suggested that O-GlcNAc elevation may inhibit the anti-apoptotic action of insulin. Thus, we examined the pro-survival action of insulin upon serum deprivation in the presence of PUGNAc as well as two selective OGA inhibitors (GlcNAcstatin-g and Thiamet-G), and a selective lysosomal hexosaminidase inhibitor (INJ2). We established that PUGNAc inhibits the pro-survival action of insulin but this effect is not recapitulated by the selective OGA inhibitors suggesting that elevation in O-GlcNAc levels alone is not responsible for PUGNAc's effect on the anti-apoptotic action of insulin. Further, we demonstrate that a selective hexosaminidase A/B (HexA/B) inhibitor does not impact insulin action suggesting that PUGNAc's effect is not due to inhibition of lysosomal hexosaminidase. Finally, we tested a combination of selective OGA and lysosomal hexosaminidase inhibitors but were not able to recapitulate the inhibition of insulin action generated by PUGNAc alone. These results strongly suggest that the defect in insulin action upon PUGNAc treatment does not derive from its inhibition of OGA or HexA/B, and that there is an unknown target of PUGNAc that is the likely culprit in inhibiting the protective effect of insulin from apoptosis.

Novel method demonstrates differential ligand activation and phosphatase-mediated deactivation of insulin receptor tyrosine-specific phosphorylation

Insulin receptor signaling is a complex cascade leading to a multitude of intracellular functional responses. Three natural ligands, insulin, IGF1 and IGF2, are each capable of binding with different affinities to the insulin receptor, and result in variable biological responses. However, it is likely these affinity differences alone cannot completely explain the myriad of diverse cellular outcomes. Ligand binding initiates activation of a signaling cascade resulting in phosphorylation of the IR itself and other intracellular proteins. The direct catalytic activity along with the temporally coordinated assembly of signaling proteins is critical for insulin receptor signaling. We hypothesized that determining differential phosphorylation among individual tyrosine sites activated by ligand binding or dephosphorylation by phosphatases could provide valuable insight into insulin receptor signaling. Here, we present a sensitive, novel immunoassay adapted from Meso Scale Discovery technology to quantitatively measure changes in site-specific phosphorylation levels on endogenous insulin receptors from HuH7 cells. We identified insulin receptor phosphorylation patterns generated upon differential ligand activation and phosphatase-mediated deactivation. The data demonstrate that insulin, IGF1 and IGF2 elicit different insulin receptor phosphorylation kinetics and potencies that translate to downstream signaling. Furthermore, we show that insulin receptor deactivation, regulated by tyrosine phosphatases, occurs distinctively across specific tyrosine residues. In summary, we present a novel, quantitative and high-throughput assay that has uncovered differential ligand activation and site-specific deactivation of the insulin receptor. These results may help elucidate some of the insulin signaling mechanisms, discriminate ligand activity and contribute to a better understanding of insulin receptor signaling. We propose this methodology as a powerful approach to characterize agonists and antagonists of the insulin receptor and can be adapted to serve as a platform to evaluate ligands of alternate receptor systems.

Insulin receptor activation with transmembrane domain ligands

Complementary surfaces are buried when peptide hormones, growth factors, or cytokines bind and activate cellular receptors. Although these extended surfaces provide high affinity and specificity to the interactions, they also present great challenges to the design of small molecules that might either mimic or antagonize the process. We show that the insulin receptor (IR) and downstream signals can be activated by targeting a site outside of its ligand-binding domain. A 24-residue peptide having the IR transmembrane (TM) domain sequence activates IR, but not related growth factor receptors, through specific interactions with the receptor TM domain. Like insulin-dependent activation, IR-TM requires that IR have a competent ATP-binding site and kinase activation loop. IR-TM also activates mutated receptors from patients with severe insulin resistance, which do not respond to insulin. These results show that IR can be activated through a pathway that bypasses its canonical ligand-binding domain.

Insulin receptor membrane retention by a traceable chimeric mutant

Background

The insulin receptor (IR) regulates glucose homeostasis, cell growth and differentiation. It has been hypothesized that the specific signaling characteristics of IR are in part determined by ligand-receptor complexes localization. Downstream signaling could be triggered from the plasma membrane or from endosomes. Regulation of activated receptor's internalization has been proposed as the mechanism responsible for the differential isoform and ligand-specific signaling.

Results

We generated a traceable IR chimera that allows the labeling of the receptor at the cell surface. This mutant binds insulin but fails to get activated and internalized. However, the mutant heterodimerizes with wild type IR inhibiting its auto-phosphorylation and blocking its internalization. IR membrane retention attenuates AP-1 transcriptional activation favoring Akt activation.

Conclusions

These results suggest that the mutant acts as a selective dominant negative blocking IR internalization-mediated signaling.

Cannabinoids induce pancreatic β-cell death by directly inhibiting insulin receptor activation

Cannabinoid 1 (CB1) receptors have been previously detected in pancreatic β cells, where they attenuate insulin action. We now report that CB1 receptors form a heteromeric complex with insulin receptors and the heterotrimeric guanosine triphosphate-binding protein α subunit Gα(i). Gα(i) inhibited the kinase activity of the insulin receptor in β cells by directly binding to the activation loop in the tyrosine kinase domain of the receptor. Consequently, phosphorylation of proapoptotic protein Bad was reduced and its apoptotic activity was stimulated, leading to β-cell death. Pharmacological blockade or genetic deficiency of CB1 receptors enhanced insulin receptor signaling after injury, leading to reduced blood glucose concentrations and activation of Bad, which increased β-cell survival. These findings provide direct evidence of physical and functional interactions between CB1 and insulin receptors and suggest a mechanism whereby peripherally acting CB1 receptor antagonists improve insulin action in insulin-sensitive tissues independent of the other metabolic effects of CB1 receptors.

Insulin receptor kinase-independent signaling via tyrosine phosphorylation of phosphatase PHLPP1

Most insulin responses correlate well with insulin receptor (IR) Tyr kinase activation; however, critical exceptions to this concept have been presented. Specific IR mutants and stimulatory IR antibodies demonstrate a lack of correlation between IR kinase activity and specific insulin responses in numerous independent studies. IR conformation changes in response to insulin observed with various IR antibodies define an IR kinase-independent signal that alters the C-terminus. IR-related receptors in lower eukaryotes that lack a Tyr kinase point to an alternative mechanism of IR signaling earlier in evolution. However, the implied IR kinase-independent signaling mechanism remained obscure at the molecular level. Here we begin to define the molecular basis of an IR-dependent but IR kinase-independent insulin signal that is equally transmitted by a kinase-inactive mutant IR. This insulin signal results in Tyr phosphorylation and catalytic activation of phosphatase PHLPP1 via a PI 3-kinase-independent, wortmannin-insensitive signaling pathway. Dimerized SH2B1/PSM is a critical activator of the IR kinase and the resulting established insulin signal. In contrast it is an inhibitor of the IR kinase-independent insulin signal and disruption of SH2B1/PSM dimer binding to IR potentiates this signal. Dephosphorylation of Akt2 by PHLPP1 provides an alternative, SH2B1/PSM-regulated insulin-signaling pathway from IR to Akt2 of opposite polarity and distinct from the established PI 3-kinase-dependent signaling pathway via IRS proteins. In combination, both pathways should allow the opposing regulation of Akt2 activity at two phosphorylation sites to specifically define the insulin signal in the background of interfering Akt-regulating signals, such as those controlling cell proliferation and survival.

Molecular mechanisms of differential intracellular signaling from the insulin receptor

Binding of insulin to the insulin receptor (IR) leads to a cascade of intracellular signaling events, which regulate multiple biological processes such as glucose and lipid metabolism, gene expression, protein synthesis, and cell growth, division, and survival. However, the exact mechanism of how the insulin-IR interaction produces its own specific pattern of regulated cellular functions is not yet fully understood. Insulin analogs, anti-IR antibodies as well as synthetic insulin mimetic peptides that target the two insulin-binding regions of the IR, have been used to study the relationship between different aspects of receptor binding and function as well as providing new insights into the structure and function of the IR. This review focuses on the current knowledge of activation of the IR and how activation of the IR by different ligands initiates different cellular responses. Investigation of differential activation of the IR may provide clues to the molecular mechanisms of how the insulin-receptor interaction controls the specificity of the downstream signaling response. Differences in the kinetics of ligand-interaction with the IR, the magnitude of the signal as well as its subcelllar location all play important roles in determining/eliciting the different biological responses. Additional studies are nevertheless required to dissect the precise molecular mechanisms leading to the differential signaling from the IR.

Activation of the insulin receptor (IR) by insulin and a synthetic peptide has different effects on gene expression in IR-transfected L6 myoblasts

Single-chain peptides have been recently produced that display either mimetic or antagonistic properties against the insulin and IGF-1 (insulin-like growth factor 1) receptors. We have shown previously that the insulin mimetic peptide S597 leads to significant differences in receptor activation and initiation of downstream signalling cascades despite similar binding affinity and in vivo hypoglycaemic potency. It is still unclear how two ligands can initiate different signalling responses through the IR (insulin receptor). To investigate further how the activation of the IR by insulin and S597 differentially activates post-receptor signalling, we studied the gene expression profile in response to IR activation by either insulin or S597 using microarray technology. We found striking differences between the patterns induced by these two ligands. Most remarkable was that almost half of the genes differentially regulated by insulin and S597 were involved in cell proliferation and growth. Insulin either selectively regulated the expression of these genes or was a more potent regulator. Furthermore, we found that half of the differentially regulated genes interact with the genes involved with the MAPK (mitogen-activated protein kinase) pathway. These findings support our signalling results obtained previously and confirm that the main difference between S597 and insulin stimulation resides in the activation of the MAPK pathway. In conclusion, we show that insulin and S597 acting via the same receptor differentially affect gene expression in cells, resulting in a different mitogenicity of the two ligands, a finding which has critical therapeutic implications.

SOCS proteins causing trouble in insulin action

First discovered as inhibitors of cytokine signalling, the suppressor of cytokine signalling (SOCS) proteins have appeared, over recent years, as potent repressors of other signalling pathways including the one induced by insulin. SOCS-1 and SOCS-3 have been extensively studied both in vitro and in vivo in the context of insulin action. It has been shown that these two SOCS members are able to inhibit the insulin signalling pathway by three different mechanisms: (1) inhibition of tyrosine phosphorylation of insulin receptor substrate (IRS) proteins because of competition at the docking site on the insulin receptor (IR), (2) induction of the proteasomal degradation of the IRS and (3) inhibition of the IR kinase. A key feature of the SOCS proteins is that they are induced regulators. Indeed, expression of SOCS proteins is virtually absent in basal conditions, but is rapidly and robustly induced in response to several stimuli such as hormones, cytokines and growth factors. A significant correlation between SOCS-3 expression and insulin resistance has been demonstrated in vivo. Interestingly, the level of SOCS-3 expression is strikingly enhanced in insulin-sensitive tissues from both patients and animal models with type 2 diabetes and insulin resistance. While it remains to be established whether the increased expression of SOCS is a cause or a consequence of insulin resistance, a large body of observations supports a role for SOCS proteins in the disease process found in states with insulin resistance.

Molecular mechanisms of insulin resistance: serine phosphorylation of insulin receptor substrate-1 and increased expression of p85alpha: the two sides of a coin

Initial attempts to unravel the molecular mechanism of insulin resistance have strongly suggested that a defect responsible for insulin resistance in the majority of patients lies at the postreceptor level of insulin signaling. Subsequent studies in insulin-resistant animal models and humans have consistently demonstrated a reduced strength of insulin signaling via the insulin receptor substrate (IRS)-1/phosphatidylinositol (PI) 3-kinase pathway, resulting in diminished glucose uptake and utilization in insulin target tissues. However, the nature of the triggering event(s) remains largely enigmatic. Two separate, but likely, complementary mechanisms have recently emerged as a potential explanation. First, it became apparent that serine phosphorylation of IRS proteins can reduce their ability to attract PI 3-kinase, thereby minimizing its activation. A number of serine kinases that phosphorylate serine residues of IRS-1 and weaken insulin signal transduction have been identified. Additionally, mitochondrial dysfunction has been suggested to trigger activation of several serine kinases, leading to a serine phosphorylation of IRS-1. Second, a distinct mechanism involving increased expression of p85alpha has also been found to play an important role in the pathogenesis of insulin resistance. Conceivably, a combination of both increased expression of p85alpha and increased serine phosphorylation of IRS-1 is needed to induce clinically apparent insulin resistance.

31P NMR study on the autophosphorylation of insulin receptors in the plasma membrane

A nonradioactive 31P nuclear magnetic resonance (NMR) spectroscopy protocol has been developed and used to investigate in vitro autophosphorylation of insulin receptors. Optimum experimental conditions have been explored, and the effects of Mn2+ and phosphocreatine (PCr) on the determination of the phosphorylation reaction have been assayed. The method was used to monitor the time courses of the phosphorylation reaction in solution. The results from this NMR study were in agreement with observations of insulin receptor phosphorylation made by using Western blotting.

Gene therapy of the brain

Many chronic neurologic diseases do not respond to small molecule therapeutics, and have no effective long-term therapy. Gene therapy offers the promise of an effective cure for both genetic and acquired brain disease. However, the limiting problem in brain gene therapy is delivery to brain followed by regulation of the expression of the transgene. Present day gene vectors do not cross the blood-brain barrier (BBB). Consequently, brain gene therapy requires craniotomy and the local injection of a viral gene vector. However, there are few brain disorders that can be effectively treated with local injection. Most applications of gene therapy require global expression in the brain of the exogenous gene, and this can only be achieved with a noninvasive delivery through the BBB—the trans-vascular route to brain. An additional consideration is the potential toxicity of all viral and nonviral approaches, which may either integrate into the host genome and cause insertional mutagenesis or cause inflammation in the brain. Nonviral, noninvasive gene therapy of the brain is now possible with the development of a new approach to targeting therapeutic genes to the brain following an IV administration. This approach utilizes genetically engineered molecular Trojan horses, which ferry the gene across the BBB and into neurons. Global and reversible expression of therapeutic genes in the human brain without surgery and without viral vectors is now possible.

The APS adapter protein couples the insulin receptor to the phosphorylation of c-Cbl and facilitates ligand-stimulated ubiquitination of the insulin receptor

The APS adapter protein is rapidly tyrosine-phosphorylated following insulin stimulation. In insulin-stimulated 3T3-L1 adipocytes, APS co-precipitated with phosphorylated c-Cbl. In CHO.T-APS cells overexpressing the insulin receptor and APS, APS co-precipitated with c-Cbl but not in CHO.T cells which do not express APS. APS-mediated recruitment of c-Cbl to the insulin receptor led to rapid ubiquitination of the insulin receptor beta-subunit in CHO. T-APS but not in parental CHO.T cells. These results suggest that the function of APS is to facilitate coupling of the insulin receptor to c-Cbl in order to catalyse the ubiquitination of the receptor and initiation of internalisation or degradation.

Insulin-like growth factor-I (IGF-I)-dependent activation of pp42/44 mitogen-activated protein kinase occurs independently of IGF-I receptor kinase activation and IRS-1 tyrosine phosphorylation

The proliferation and metabolism of H4IIE hepatoma cells is apparently mediated through the insulin receptor. These cells, however, also have high-affinity binding sites for insulin-like growth factor-I (IGF-I). Addition of insulin to H4IIE cells increased RNA synthesis, DNA synthesis and cell number. IGF-I, on the other hand, was ineffective at concentrations equivalent to the lowest effective insulin dose, although stimulation was observed with concentrations 100-fold higher. Similar results were obtained when glucose uptake was measured. Western blot analysis demonstrated that tyrosine phosphorylation patterns produced by insulin and IGF-I differed. In particular, phosphorylation of insulin receptor substrate-1 (IRS-1) was evident after treatment with insulin, but not after treatment with IGF-I. Correspondingly, insulin, but not IGF-I, stimulated receptor tyrosine kinase activity. In contrast with these results, both insulin and IGF-I induced mitogen-activated protein (MAP) kinase phosphorylation and activity at a concentration of 10 nM. The correlation between insulin-dependent and IGF-I-dependent MAP kinase activation was confirmed by Western blot analysis of phosphorylated MAP kinase kinase (MEK). These results suggest that phosphorylation of IRS-1 is essential for both cell proliferation and glucose metabolism, but is uncoupled from the MAP kinase cascade. Furthermore, stimulation of MEK and MAP kinase is independent of receptor tyrosine kinase activity.

Differential modulation of the tyrosine phosphorylation state of the insulin receptor by IRS (insulin receptor subunit) proteins

In response to insulin, tyrosine kinase activity of the insulin receptor is stimulated, leading to autophosphorylation and tyrosine phosphorylation of proteins including insulin receptor subunit (IRS)-1, IRS-2, and Shc. Phosphorylation of these proteins leads to activation of downstream events that mediate insulin action. Insulin receptor kinase activity is requisite for the biological effects of insulin, and understanding regulation of insulin receptor phosphorylation and kinase activity is essential to understanding insulin action. Receptor tyrosine kinase activity may be altered by direct changes in tyrosine kinase activity, itself, or by dephosphorylation of the insulin receptor by protein-tyrosine phosphatases. After 1 min of insulin stimulation, the insulin receptor was tyrosine phosphorylated 8-fold more and Shc was phosphorylated 50% less in 32D cells containing both IRS-1 and insulin receptors (32D/IR+IRS-1) than in 32D cells containing only insulin receptors (32D/IR), insulin receptors and IRS-2 (32D/IR+IRS-2), or insulin receptors and a form of IRS-1 that cannot be phosphorylated on tyrosine residues (32D/IR+IRS-1F18). Therefore, IRS-1 and IRS-2 appeared to have different effects on insulin receptor phosphorylation and downstream signaling. Preincubation of cells with pervanadate greatly decreased protein-tyrosine phosphatase activity in all four cell lines. After pervanadate treatment, tyrosine phosphorylation of insulin receptors in insulin-treated 32D/IR, 32D/ IR+IRS-2, and 32D/IR+IRS-1F18 cells was markedly increased, but pervanadate had no effect on insulin receptor phosphorylation in 32D/IR+IRS-1 cells. The presence of tyrosine-phosphorylated IRS-1 appears to increase insulin receptor tyrosine phosphorylation and potentially tyrosine kinase activity via inhibition of protein-tyrosine phosphatase(s). This effect of IRS-1 on insulin receptor phosphorylation is unique to IRS-1, as IRS-2 had no effect on insulin receptor tyrosine phosphorylation. Therefore, IRS-1 and IRS-2 appear to function differently in their effects on signaling downstream of the insulin receptor. IRS-1 may play a major role in regulating insulin receptor phosphorylation and enhancing downstream signaling after insulin stimulation.

Autophosphorylation of KDR in the kinase domain is required for maximal VEGF-stimulated kinase activity and receptor internalization

We have previously reported the identification of four autophosphorylation sites on the KDR VEGF receptor. Two of these sites (tyrosines 951 and 996) are located in the receptor's kinase insert domain, and two (tyrosines 1054 and 1059) are located in the catalytic domain. In order to clarify the functional significance of these sites, we made DNA constructs in which tyrosine codons were replaced with those for phenylalanine, and expressed the DNA constructs in 293 cells. VEGF binding to cells expressing the native receptor led to a rapid increase in receptor and PLC-gamma phosphorylation, and a slower increase in the phosphorylation of p125FAK and paxillin. VEGF binding to KDR(Y951F) and KDR(Y996F) expressing cells resulted in phosphorylation of all cellular substrates tested, although the level of PLCgamma phosphorylation was decreased for KDR(Y996F). The decreased level of PLCgamma phosphorylation was not because PLCgamma-containing SH2 domains bind to the Y996 autophosphorylation site. We conclude that there exists receptor autophosphorylation sites not previously identified which allow for signaling via PLCgamma, as well as p125FAK and paxillin. VEGF binding to cells expressing KDR mutated at both tyrosine's 1054 and 1059 activated receptor autophosphorylation but at a level which was only 10% of that seen for cells expressing native receptor. Tyrosine phosphorylation of cell signaling proteins was not observed in KDR(Y1054,1059) expressing cells. Utilizing an in vitro assay which directly measures receptor catalytic activity allowed us to determine that the tyrosine kinase activity of the native receptor was significantly greater than that for the double mutant. We conclude from this result that VEGF-induced autophosphorylation at tyrosines 1054 and 1059 is a required step for allowing maximal KDR kinase activity. Maximal rates of receptor kinase activity is required for VEGF-induced receptor internalization, as internalization was delayed in the KDR(Y1054,1059F) expressing cells when compared to cells expressing native receptor.

Insulin receptor has tyrosine kinase activity toward Shc in rat liver

Insulin induces tyrosine phosphorylation of Shc in cell cultures and in insulin-sensitive tissues of the intact rat. However, the ability of insulin receptor (IR) tyrosine kinase to phosphorylate Shc has not been previously demonstrated. In the present study, we investigated insulin-induced IR tyrosine kinase activity towards Shc. Insulin receptor was immunoprecipitated from liver extracts, before and after a very low dose of insulin into the portal vein, and incubated with immunopurified Shc from liver of untreated rats. The kinase assay was performed in vitro in the presence of exogenous ATP and the phosphorylation level was quantified by immunoblotting with antiphosphotyrosine antibody. The results demonstrate that Shc interacted with insulin receptor after infusion of insulin, and, more important, there was insulin receptor kinase activity towards immunopurified Shc. The description of this pathway in animal tissue may have an important role in insulin receptor tyrosine kinase activity toward mitogenic transduction pathways.

p125Fak focal adhesion kinase is a substrate for the insulin and insulin-like growth factor-I tyrosine kinase receptors

The focal adhesion kinase p125(Fak) is a widely expressed cytosolic tyrosine kinase, which is involved in integrin signaling and in signal transduction of a number of growth factors. In contrast to tyrosine kinase receptors such as the platelet-derived growth factor and the hepatocyte growth factor receptors, which induce p125(Fak) phosphorylation, insulin has been shown to promote its dephosphorylation. In this study, we compared p125(Fak) phosphorylation in insulin-stimulated cells maintained in suspension or in an adhesion state. We found that, in nonattached cells, insulin promotes p125(Fak) phosphorylation, whereas dephosphorylation occurred in attached cells. This was observed in Rat-1 fibroblasts overexpressing the insulin receptor, as well as in Hep G2 hepatocytes and in 3T3-L1 adipocytes expressing more natural levels of insulin receptors. Insulin-induced p125(Fak) phosphorylation correlated with an increase in paxillin phosphorylation, indicating that p125(Fak) kinase activity may be stimulated by insulin. Mixing of purified insulin or insulin-like growth factor-I (IGF-I) receptors with p125(Fak) resulted in an increase in p125(Fak) phosphorylation. Using a kinase-deficient p125(Fak) mutant, we found that this protein is a direct substrate of the insulin and IGF-I receptor tyrosine kinases. This view is supported by two additional findings. (i) A peptide corresponding to p125(Fak) sequence comprising amino acids 568-582, which contains tyrosines 576 and 577 of the kinase domain regulatory loop, is phosphorylated by the insulin receptor; and (ii) p125(Fak) phosphorylation by the insulin receptor is prevented by addition of this peptide. Finally, we observed that p125(Fak) phosphorylation by the receptor results in its activation. Our results show that the nature of the cross-talk between the insulin/IGF-I receptors and p125(Fak) is dependent on the cell architecture, and hence the interaction of the insulin/IGF-I signaling system with the integrin system will vary accordingly.

A role for nuclear factor kappaB in the antiapoptotic function of insulin

We previously reported that insulin activates nuclear factor kappaB (NF-kappaB) in Chinese hamster ovary (CHO)-R cells overexpressing wild-type insulin receptors (IRs) through a pathway requiring IR tyrosine kinase and Raf-1 kinase activities. We now investigated whether the activation of NF-kappaB by insulin could serve an antiapoptotic function. Insulin (10(-9)-10(-7) M) inhibited apoptosis induced by serum withdrawal in CHO-R cells in a concentration-dependent manner. Insulin antiapoptotic signaling: (i) was dependent on IR number and IR tyrosine kinase activity since it was reduced in parental CHO cells and was abolished in CHO-Y2 cells overexpressing IRs mutated at Tyr1162/1163; (ii) was, like insulin activation of NF-kappaB, dependent on Raf-1 but not on mitogen-activated protein kinase activity since both processes were decreased by the dominant-negative Raf-1 mutant Raf-C4 whereas they persisted in mitogen-activated protein kinase-depleted cells; and (iii) required NF-kappaB activation since it was decreased by proteasome inhibitors and the dominant-negative IkappaB-alpha (A32/36) mutant and was mimicked by overexpression of the NF-kappaB c-Rel subunit. We also show that insulin antiapoptotic signaling but not insulin activation of NF-kappaB involved phosphatidylinositol 3-kinase (PI 3-kinase), as supported by the inhibition of the former but not of the latter process by the PI 3-kinase inhibitor LY294002. Inhibition of both NF-kappaB and PI 3-kinase totally abolished insulin antiapoptotic signaling. Thus insulin exerts a specific antiapoptotic function which is dependent on IR tyrosine kinase activity and is mediated by both a Raf-1-dependent pathway that leads to NF-kappaB activation and a PI 3-kinase-dependent pathway.

Reduced insulin receptor signaling in the obese spontaneously hypertensive Koletsky rat

Insulin resistance is associated with both obesity and hypertension. However, the cellular mechanisms of insulin resistance in genetic models of obese-hypertension have not been identified. The objective of the present study was to investigate the effects of genetic obesity on a background of inherited hypertension on initial components of the insulin signal transduction pathway and glucose transport in skeletal muscle and liver. Oral glucose tolerance testing in SHROB demonstrated a sustained postchallenge elevation in plasma glucose at 180 and 240 min compared with lean spontaneously hypertensive rat (SHR) littermates, which is suggestive of glucose intolerance. Fasting plasma insulin levels were elevated 18-fold in SHROB. The rate of insulin-stimulated 3-O-methylglucose transport was reduced 68% in isolated epitrochlearis muscles from the SHROB compared with SHR. Insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit and insulin receptor substrate-1 (IRS-1) in intact skeletal muscle of SHROB was reduced by 36 and 23%, respectively, compared with SHR, due primarily to 32 and 60% decreases in insulin receptor and IRS-1 protein expression, respectively. The amounts of p85 alpha regulatory subunit of phosphatidylinositol-3-kinase and GLUT-4 protein were reduced by 28 and 25% in SHROB muscle compared with SHR. In the liver of SHROB, the effect of insulin on tyrosine phosphorylation of IRS-1 was not changed, but insulin receptor phosphorylation was decreased by 41%, compared with SHR, due to a 30% reduction in insulin receptor levels. Our observations suggest that the leptin receptor mutation fak imposed on a hypertensive background results in extreme hyperinsulinemia, glucose intolerance, and decreased expression of postreceptor insulin signaling proteins in skeletal muscle. Despite these changes, hypertension is not exacerbated in SHROB compared with SHR, suggesting these metabolic abnormalities may not contribute to hypertension in this model of Syndrome X.

Tyr624 and Tyr628 in insulin receptor substrate-2 mediate its association with the insulin receptor

In addition to the pleckstrin homology domain and the phosphotyrosine binding domain in insulin receptor substrate (IRS)-1 and IRS-2, a region between amino acids 591 and 786 in IRS-2 (IRS-2-(591-786)) binds to the insulin receptor. Based on peptide competition studies, this region interacts with the phosphorylated regulatory loop of the insulin receptor; we designate this region the kinase regulatory loop binding (KRLB) domain. Two tyrosine residues in the KRLB domain at positions 624 and 628 are crucial for this interaction. Phosphorylation of tyrosine residues in the KRLB domain by the insulin receptor inhibits the binding to the receptor. These results reveal a novel mechanism regulating the interaction of the insulin receptor and IRS-2 that may distinguish the signal of IRS-2 from IRS-1.

Autophosphorylation of activation loop tyrosines regulates signaling by the TRK nerve growth factor receptor

Many receptor tyrosine kinases possess an "activation loop" containing three similarly placed tyrosine autophosphorylation sites. To examine their roles in the TRK NGF receptor, these residues (Tyr-670, Tyr-674, and Tyr-675) were mutated singly and in all combinations to phenylalanine and stably expressed in Trk-deficient PC12nnr5 cells. All mutant receptors showed significantly diminished nerve growth factor (NGF)-stimulated autophosphorylation, indicating impaired catalytic activity. NGF-induced neurite outgrowth exhibited dose-responsive behavior when transfectants were compared by relative receptor expression and exhibited a functional hierarchy: wild type > Y670F >/= Y674F >> Y675F >/= YY670/674FF = YY670/675FF >> YY674/675FF > YYY670/674/675FFF. NGF-induced tyrosine phosphorylation of Shc, ERKs, and SNT and immediate early gene inductions generally paralleled neurogenic potential. However, activation of phosphatidylinositol 3'-kinase and tyrosine phosphorylation of phospholipase Cgamma-1 was essentially abolished. The latter effect appears due to selective inability of the mutated TRKs to autophosphorylate the tyrosine residue (Tyr-785) required for binding phospholipase Cgamma-1 and indicates that the "activation loop" tyrosines participate in NGF-dependent changes in receptor conformation. Our findings stress the importance that expression levels play in assessing the consequences of receptor mutations and that all three activation loop tyrosines have roles regulating both overall and specific NGF-mediated signaling through TRK.

Human GRB-IRbeta/GRB10. Splice variants of an insulin and growth factor receptor-binding protein with PH and SH2 domains

cDNA clones encoding human (h) Grb7 and a previously unknown protein with high homology to hGrb-IR and mGrb10 (where m indicates mouse) were found by screening expressed sequence tag data bases. hGrb7 mRNA expression is greatest in pancreas and restricted to a few other tissues. The second protein termed hGrb-IRbeta/Grb10 contains an intact PH domain and lacks the 80-residue mGrb10 insertion. Expression is greatest in pancreas and muscle but occurs in nearly all tissues. hGrb-IRbeta/Grb10 and hGrb-IR likely arise as alternative mRNA splicing products of a common gene. Reverse transcriptase-coupled polymerase chain reaction shows both mRNAs in muscle. In cells, Grb-IRbeta/Grb10 protein translocates from cytosol to membrane upon insulin stimulation, most likely due to direct interactions with the insulin receptor. These interactions are mediated by the SH2 domain and additional regions of the protein. Studies with mutated receptors and synthetic phosphopeptides show that the hGrb-IRbeta/Grb10 SH2 domain binds at least two sites in the insulin receptor: the kinase activation loop > the juxtamembrane site. hGrb-IRbeta/Grb10 also binds a 135-kDa phosphoprotein in unstimulated 3T3-L1 adipocytes; binding is reduced upon insulin stimulation. In addition, the c-Abl SH3 domain binds Grb-IR/Grb10, whereas Fyn, phosphatidylinositol 3-kinase p85, and Grb2 SH3 domains do not. The site of c-Abl SH3 domain interaction is highly conserved within the Grb-IR/Grb10/Grb7/Grb14 family. hGrb-IRbeta/Grb10 also binds platelet-derived growth factor and epidermal growth factor receptors, suggesting a broader role in the signaling pathways of numerous receptors. We conclude that hGrb-IRbeta/Grb10 is a widely expressed, PH and SH2 domain-containing, SH3 domain-binding protein that functions downstream from activated insulin and growth factor receptors.

The early intracellular signaling pathway for the insulin/insulin-like growth factor receptor family in the mammalian central nervous system

Several studies support the idea that the polypeptides belonging to the family of insulin and insulin-like growth factors (IGFs) play an important role in brain development and continue to be produced in discrete areas of the adult brain. In numerous neuronal populations within the olfactory bulb, the cerebral and cerebellar cortex, the hippocampus, some diencephalic and brainstem nuclei, the spinal cord and the retina, specific insulin and IGF receptors, as well as crucial components of the intracellular receptor signaling pathway have been demonstrated. Thus, mature neurons are endowed with the cellular machinery to respond to insulin and IGF stimulation. Studies in vitro and in vivo, using normal and transgenic animals, have led to the hypothesis that, in the adult brain, IGF-I not only acts as a trophic factor, but also as a neuromodulator of some higher brain functions, such as long-term potentiation and depression. Furthermore, a trophic effect on certain neuronal populations becomes clearly evident in the ischemic brain or neurodegenerative disorders. Thus, the analysis of the early intracellular signaling pathway for the insulin/IGF receptor family in the brain is providing us with new intriguing findings on the way the mammalian brain is sculpted and operates.

Role of the insulin receptor C-terminal acidic domain in the modulation of the receptor kinase by polybasic effectors

Basic polymers such as polylysine have been found to activate insulin receptor autophosphorylation and kinase activity toward substrates. It was suggested that acidic receptor domains may be involved in the interaction of the receptor with these basic effectors. In a previous study, we have shown that the receptor acid-rich C-terminal sequence, including residues 1270-1280, is involved in the regulation of the receptor kinase activity. Moreover, this domain may be the site of interaction with histone, which is a modulator of the receptor kinase. In this study, we investigated whether the insulin receptor domain comprising amino acids 1270-1280 is involved in the interaction with polybasic effectors. We used anti-peptide serum directed to this sequence, and basic activators such as polylysine, polyarginine and protamine sulfate. Our antibodies inhibit polylysine-induced receptor autophosphorylation, whereas they have no effect on receptor phosphorylation stimulated by concanavalin A which is a non-basic activator of the insulin receptor. Polylysine-induced receptor aggregation was blocked by the antibodies (Fab fragments or whole Ig), indicating that competition occurs between the antibody and polylysine at the level of their binding site to the receptor. Finally, we observed a direct interaction of the 125I-peptide corresponding to receptor sequence 1270-1280 with the basic polymers in dot-blot experiments. Interestingly, the peptide did not bind spermine, a basic molecule which is not an activator of the insulin receptor kinase. Our data indicate that the insulin receptor C-terminal acidic domain including residues 1270-1280 is involved in the interaction of polylysine and other polybasic molecules with the receptor. Since this receptor region has been implicated in the regulation of the receptor kinase activity, we propose that interaction of basic effectors with this domain may be responsible for their activating properties.

Glucose-induced tyrosine phosphorylation of p125 in beta cells and pancreatic islets. A novel proximal signal in insulin secretion

In this study, we demonstrate that stimulation of beta cells with carbachol and glucose causes increased tyrosine phosphorylation of a 125-kDa protein concurrently with increased insulin secretion. The effect was observed in two different insulin-secreting cell lines and in rat pancreatic islets. Tyrosine phosphorylation was largely calcium independent and occurred within 2 min after stimulation of beta cells with glucose and the muscarinic agonist carbachol. In islets, the effect of glucose was greatly diminished by the addition of mannoheptulose, a seven-carbon sugar that inhibits glucokinase, suggesting that glucose metabolism is required for tyrosine phosphorylation of the protein to occur. Neither insulin nor insulin-like growth factor I significantly increased tyrosine phosphorylation of the 125-kDa protein, suggesting that it was not an autocrine effect. Depolarization of beta cells with glyburide or 50 m potassium dramatically increased insulin secretion but had no significant effect on tyrosine phosphorylation. Addition of phorbol ester caused a less than 2-fold increase in tyrosine phosphorylation, whereas the calcium ionophore A23187 had no effect. Among the various fuel secretagogues tested, only -glucose stimulated tyrosine phosphorylation, both alone and in combination with carbachol. Finally, the tyrosine kinase inhibitor AG879 inhibited both tyrosine phosphorylation and insulin secretion in a dose-dependent manner. Taken together, these data demonstrate the presence of a novel signaling pathway in glucose-induced insulin secretion: tyrosine phosphorylation of beta cell p125, which is a proximal step in insulin secretion. Our current working hypothesis is that glucose stimulation of beta cell p125 tyrosine phosphorylation is an essential step for insulin secretion.

Insulin receptor structural requirements for the formation of a ternary complex with IRS-1 and PI 3-kinase

Insulin receptor substrate-1 (IRS-1) is a major substrate of the insulin receptor tyrosine kinase and is an intermediate in insulin signaling. Phosphotyrosyl-IRS-1 binds to other signaling proteins including phosphatidylinositol 3-kinase (PI 3-kinase). We examined the role of three insulin receptor tyrosine autophosphorylation domains in association of the receptor with IRS-1. Our data support the idea that tyrosine phosphorylation of the insulin receptor juxtamembrane domain is necessary for receptor association with IRS-1. We provide evidence that the kinase regulatory domain, which is part of a loop structure at the mouth of the catalytic cleft, when mutated to replace Tyr1146, Tyr1150, and Tyr1151 with phenylalanine can bind receptor substrates without tyrosine phosphorylation of residues in the receptor juxtamembrane region. In addition, our data show that the amount of PI 3-kinase directly associated with the insulin receptor C-terminus is low when compared to the PI 3-kinase associating with IRS-1. We also demonstrate that a substantial amount (approximately 25%) of the IRS-1 associated PI 3-kinase is associated with the insulin receptor in a ternary complex of insulin receptor/IRS-1/PI 3-kinase.

Intermolecular phosphorylation between insulin holoreceptors does not stimulate substrate kinase activity

We photocoupled benzoylphenylalanineB25, B29 epsilon-biotin insulin (BBpa-insulin) to native insulin receptors to obtain a uniform receptor population with covalently bound, non-dissociable ligand. We employed BBpa-insulin-bound and autophosphorylated (activated) receptor to phosphorylate substrate insulin receptor under conditions where the substrate receptor never interacts with insulin. The substrate receptor becomes phosphorylated in this inter-receptor fashion and reaches a phosphorylation state 50% of the maximal obtainable by autophosphorylation. However, this phosphorylation does not activate the substrate receptor to any measurable degree. We conclude that intermolecular phosphorylation of the insulin holoreceptors is unlikely to be of physiological significance.

The role of the tyrosine kinase domain of the insulin-like growth factor-I receptor in intracellular signaling, cellular proliferation, and tumorigenesis

Insulin and insulin-like growth factor (IGF-I) receptors are heterotetrameric proteins consisting of two alpha-and two beta-subunits and members of the transmembrane tyrosine kinase receptors. Specific ligand binding to the receptor triggers a cascade of intracellular events, which begins with autophosphorylation of several tyrosine residues of the beta-subunit of the receptor. The triple cluster in the tyrosine kinase domain of the beta-subunit is the earliest and major autophosphorylation site. Previous studies have shown that substitutions of these three tyrosines by phenylalanines of both insulin and IGF-I receptors practically abolish any activation of cellular signaling pathways. We have studied the effect of double tyrosine mutations on IGF-I induced receptor autophosphorylation, activation of Shc and IRS-1 pathways, and cell proliferation and tumorigenicity. Substitution of tyrosines 1131/1135 blocks any detectable autophosphorylation, whereas substitution of tyrosines 1131/1136 or 1135/1136 only reduces autophosphorylation levels in some clones by approximately 50%. Nevertheless, all the cells expressing IGF-I receptors with double tyrosine substitutions demonstrated markedly reduced signaling through Shc and IRS-1 pathways. In addition, they were unable to respond to IGF-I-stimulated cell growth in culture, and tumor formation in nude mice was abrogated. These data suggest that the presence of tyrosine 1131 or 1135 essential for receptor autophosphorylation, whereas the presence of each of these tyrosines is necessary for a fully functional receptor.

Differential activation of mitogen-activated protein kinase and S6 kinase signaling pathways by 12-O-tetradecanoylphorbol-13-acetate (TPA) and insulin. Evidence for involvement of a TPA-stimulated protein-tyrosine kinase

AG-18, an inhibitor of protein-tyrosine kinases, was employed to study the role of tyrosine-phosphorylated proteins in insulin- and phorbol ester-induced signaling cascades. When incubated with Chinese hamster ovary cells overexpressing the insulin receptor, AG-18 reversibly inhibited insulin-induced tyrosine phosphorylation of insulin receptor substate-1, with minimal effects either on receptor autophosphorylation or on phosphorylation of Shc64. Under these conditions, AG-18 inhibited insulin-stimulated phosphorylation of the ribosomal protein S6, while no inhibition of insulin-induced activation of mitogen-activated protein kinase (MAPK) kinase or MAPK was detected. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of MAPK kinase and MAPK and phosphorylation of S6 were inhibited by AG-18. This correlated with inhibition of TPA-stimulated tyrosine phosphorylation of several proteins, the most prominent ones being pp114 and pp120. We conclude that Tyr-phosphorylated insulin receptor substrate-1 is the main upstream regulator of insulin-induced S6 phosphorylation by p70s6k, whereas MAPK signaling seems to be activated in these cells primarily through the adaptor molecule Shc. In contrast, TPA-induced S6 phosphorylation is mediated by the MAPK/p90rsk cascade. A key element of this TPA-stimulated signaling pathway is an AG-18-sensitive protein-tyrosine kinase.

Insulin-induced egr-1 expression in Chinese hamster ovary cells is insulin receptor and insulin receptor substrate-1 phosphorylation-independent. Evidence of an alternative signal transduction pathway

Insulin's effects primarily are initiated by insulin binding to its plasma membrane receptor and the sequential tyrosine phosphorylation of the insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1). However, studies suggest some insulin effects, including those at the nucleus, may not be regulated by this pathway. The present study compared the levels of insulin binding, insulin receptor and IRS-1 tyrosine phosphorylation, and phosphatidylinositol 3'-kinase activity to immediate early gene c-fos and egr-1 mRNA expression in Chinese hamster ovary (CHO) cells expressing only neomycin-resistant plasmid (CHONEO), overexpressing wild type human insulin receptor (CHOHIRc) or ATP binding site-mutated insulin receptors (CHOA1018K). Insulin binding in CHONEO cells was markedly lower than that in other cell types. 10 nM insulin significantly increased tyrosine phosphorylation of insulin receptor and IRS-1 in CHOHIRc cells. Phosphorylation of insulin receptor and IRS-1 in CHONEO and CHOA1018K cells was not detected in the presence or absence of insulin. Similarly, insulin increased phosphatidylinositol 3-kinase activity only in CHOHIRc cells. As determined by Northern blot, nuclear run-on analysis, and in situ hybridization, insulin induced c-fos mRNA expression, through transcription, in CHOHIRc cells but not in CHONEO and CHOA1018K cells, consistent with previous reports. In contrast, all three cell types showed a similar insulin dose-dependent increase of egr-1 mRNA expression through transcription. These data indicated that insulin-induced egr-1 mRNA expression did not correlate with the levels of insulin binding to insulin receptor or phosphorylation of insulin receptor and IRS-1. These results suggest that different mechanisms are involved in induction of c-fos and egr-1 mRNA expression by insulin, the former by the more classic insulin receptor tyrosine kinase pathway and the latter by a yet to be determined alternative signal transduction pathway.

Insulin activates nuclear factor kappa B in mammalian cells through a Raf-1-mediated pathway

We examined the effect of insulin on nuclear factor kappa B (NF-kappa B) activity in Chinese ovary (CHO) cells overexpressing wild-type (CHO-R cells) or -defective insulin receptors mutated at Tyr1162 and Tyr1163 autophosphorylation sites (CHO-Y2 cells). In CHO-R cells, insulin caused a specific, time-, and concentration-dependent activation of NF-kappa B. The insulin-induced DNA-binding complex was identified as the p50/p65 heterodimer. Insulin activation of NF-kappa B: 1) was related to insulin receptor number and tyrosine kinase activity since it was markedly reduced in parental CHO cells which proved to respond to insulin growth factor-1 and phorbol 12-myristate 13-acetate (PMA) activation, and was dramatically decreased in CHO-Y2 cells; 2) persisted in the presence of cycloheximide and was blocked by pyrrolidine dithiocarbamate, aspirin and sodium salicylate, three compounds interfering with I kappa B degradation and/or NF-kappa B.I kappa B complex dissociation; 3) was independent of both PMA-sensitive and atypical (zeta) protein kinases C; and 4) was dependent on Raf-1 kinase activity since insulin-stimulated NF-kappa B DNA binding activity was inhibited by 8-bromo-cAMP, a Raf-1 kinase inhibitor. Moreover, insulin activation of NF-kappa B-driven luciferase reporter gene expression was blocked in CHO-R cells expressing a Raf-1 dominant negative mutant. This is the first evidence that insulin activates NF-kappa B in mammalian cells through a post-translational mechanism requiring both insulin receptor tyrosine kinase and Raf-1 kinase activities.

Evidence for a differential interaction of SHC and the insulin receptor substrate-1 (IRS-1) with the insulin-like growth factor-I (IGF-I) receptor in the yeast two-hybrid system

Using the yeast two-hybrid system, a genetic assay for studying protein-protein interactions, we have examined and compared the interaction of the insulin-like growth factor-I receptor (IGF-IR) and the insulin receptor (IR) with their two known substrates p52Shc and the insulin receptor substrate-1 (IRS-1). We also mapped the specific domains of the IGF-IR and p52Shc participating in these interactions. Our findings can be summarized as follows: (i) the tyrosine kinase activity of the IGF-IR is essential for the interaction with p52Shc and IRS-1, (ii) p52Shc and IRS-1 bind to the IGF-IR in the NPEY-juxtamembrane motif, (iii) contrary to p52Shc, IRS-1 binds also to the major autophosphorylation sites (Tyr-1131, -1135, and -1136) of the IGF-IR, and (iv) the amino-terminal domain of p52Shc is required for its association with the IR and the IGF-IR. We propose that (i) the IGF-IR and the IR share at least in part the same molecular mechanism underlying their interplay with their two substrates, p52Shc and IRS-1, and (ii) IRS-1 interacts with the IGF-IR in a fashion that is different from that used by p52Shc. Finally, our data highlight the crucial role of the juxtamembrane domain in signaling by both the IR and the IGF-IR.

Mitogen-activated protein kinase kinase inhibition does not block the stimulation of glucose utilization by insulin

Insulin stimulates the activity of mitogen-activated protein kinase (MAPK) via its upstream activator, MAPK kinase (MEK), a dual specificity kinase that phosphorylates MAPK on threonine and tyrosine. The potential role of MAPK activation in insulin action was investigated with the specific MEK inhibitor PD98059. Insulin stimulation of MAPK activity in 3T3-L1 adipocytes (2.7-fold) and L6 myotubes (1.4-fold) was completely abolished by pretreatment of cells with the MEK inhibitor, as was the phosphorylation of MAPK and pp90Rsk, and the transcriptional activation of c-fos. Insulin receptor autophosphorylation on tyrosine residues and activation of phosphatidylinositol 3'-kinase were unaffected. Pretreatment of cells with PD98059 had no effect on basal and insulin-stimulated glucose uptake, lipogenesis, and glycogen synthesis. Glycogen synthase activity in extracts from 3T3-L1 adipocytes and L6 myotubes was increased 3-fold and 1.7-fold, respectively, by insulin. Pretreatment with 10 microM PD98059 was without effect. Similarly, the 2-fold activation of protein phosphatase 1 by insulin was insensitive to PD98059. These results indicate that stimulation of the MAPK pathway by insulin is not required for many of the metabolic activities of the hormone in cultured fat and muscle cells.

Tyrosine phosphorylation of insulin receptor substrate-1 in vivo depends upon the presence of its pleckstrin homology region

To characterize the structural basis for the interactions between the insulin receptor (IR) and its major substrate, insulin receptor substrate-1 (IRS-1), a segment of the NH2-terminal region of IRS-1 (Pro5-Pro65) was deleted. This region contains the first four conserved boxes of a pleckstrin homology (PH) domain, located at the NH2-terminal part of IRS-1. COS-7 cells were then cotransfected with the genes coding for IR and a wild-type (WT) or a mutated form of IRS-1. IRS-1 delta PH underwent significantly reduced insulin-dependent tyrosine phosphorylation compared with WT IRS-1. The reduced in vivo tyrosine phosphorylation of IRS-1 delta PH was accompanied by reduced association between IRS-1 delta PH and its downstream effector p85 regulatory subunit of phosphatidylinositol-3 kinase. In contrast, both WT IRS-1 and IRS-1 delta PH underwent comparable insulin-dependent tyrosine phosphorylation in vitro when incubated with partially purified insulin receptor kinase. These findings suggest that the overall structure of IRS-1 is not altered by deletion of its PH domain and that the PH domain is not the main site for protein-protein interactions between the insulin receptor and IRS-1, at least in vitro. In conclusion, the PH region might facilitate in vivo binding of IRS-1 to membrane phospholipids or other cellular constituents in close proximity to the IR, whereas the actual interactions with the IR are presumably mediated through other domains of the IRS-1 molecule. This could account for the fact that partial deletion of the PH domain selectively impairs the in vivo interactions between the insulin receptor and IRS-1, whereas their in vitro interactions remain unaffected.

Thiol-specific biotinylation of the insulin receptor in permeabilized cells enhances receptor function

We examined the reactivity of insulin receptor sulfhydryls to biotinylation in Chinese hamster ovary cells that express high levels of human insulin receptors (CHO/HIRc cells). Following the biotinylation reaction, the insulin receptor was purified by immunoprecipitation, and resolved by SDS-polyacrylamide gel electrophoresis before electrotransfer to membranes. The use of enzyme-linked streptavidin in conjunction with a chemiluminescent technique allowed the detection of thiol-biotinylated receptor beta-subunit, with no modification of the alpha-subunit. In cells expressing large numbers of IGF-1 receptors, the same technique enabled the detection of thiol-biotinylated IGF-1 receptors as well. Thiol-alkylation of intact CHO/HIRc cells with an impermeant reagent did not impair the ability of maleimidodibutyrylbiocytin (MBB) to biotinylate sulfhydryls on the receptor beta-subunit after cell permeabilization with digitonin. In contrast, thiol-alkylation of digitonin-permeabilized cells prevented MBB-induced receptor biotinylation. The basal and insulin-activated insulin receptors exhibited a comparable reactivity to MBB. Furthermore, the use of affinity purification on monomeric avidin-agarose enabled us to learn that the biotinylation reaction was near-quantitative. MBB had no effect on insulin binding nor on receptor autophosphorylation and insulin-dependent receptor kinase activity. However, basal levels of receptor kinase activity were significantly elevated by thiol-biotinylation. Further, in the presence of vanadate, MBB retained the ability to enhance receptor kinase activity in permeabilized cells, consistent with the notion that this increased exogenous substrate phosphorylation was not accounted for by inactivation of protein tyrosine phosphatases. The dephosphorylation of thiol-biotinylated, 32P-labeled insulin receptors by particulate protein tyrosine phosphatases was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

Combination of insulinomimetic agents H2O2 and vanadate enhances insulin receptor mediated tyrosine phosphorylation of IRS-1 leading to IRS-1 association with the phosphatidylinositol 3-kinase

To analyze the mechanism of action of the insulinomimetic agents H2O2, vanadate, and pervanadate (H2O2 and vanadate), CHO cells or CHO cells that overexpress wild-type or mutant insulin receptor and/or the insulin receptor substrate (IRS-1) were used. H2O2 or vanadate treatment alone had little or no effect on tyrosine phosphorylation of cellular proteins; however, pervanadate treatment dramatically enhanced tyrosine phosphorylation of a number of proteins including the insulin receptor and IRS-1. However, the insulin receptor and IRS-1 coimmunoprecipitate from insulin-treated but not from pervanadate-treated cells. Pervanadate-induced tyrosine phosphorylation of the insulin receptor led to an increase in insulin receptor tyrosine kinase activity toward IRS-1 in vivo and IRS-1 peptides in vitro equal to that induced by insulin treatment. Pervanadate-enhanced phosphorylation of IRS-1 led to a fifteenfold increase in IRS-1-associated phosphatidylinositol (PtdIns) 3-kinase activity. However, insulin receptor-associated PtdIns 3-kinase activity from pervanadate-treated cells was not detectable, while insulin receptor-associated PtdIns 3-kinase activity from insulin-treated cells was 20% of the IRS-1-associated activity. Thus, pervanadate but not H2O2 or vanadate alone under these conditions mimics many of insulin actions, but pervanadate treatment does not induce insulin receptor/IRS-1 association.

Mechanisms of enhanced transmembrane signaling by an insulin receptor lacking a cytoplasmic beta-subunit domain

We have recently characterized a mutant insulin receptor (HIR delta 978) in which the insulin receptor beta-subunit was truncated at amino acid residue 978. Compared with parental Rat1 cells, the cells expressing the truncated receptor exhibited enhanced sensitivity to insulin's biologic actions. All of these effects are now extended to transcriptional events, since we now show enhanced sensitivity to insulin stimulation of c-fos mRNA expression. These effects were insulin-specific, since insulin-like growth factor-1 stimulation of glucose incorporation into glycogen, alpha-aminoisobutyric acid uptake, and thymidine incorporation into DNA were normal. In addition, the truncated receptor exhibited enhanced sensitivity only in vivo, but not in vitro, since the kinase activity of wheat germ agglutinin-purified receptor preparations was comparable between HIR delta 978 and parental Rat1 insulin receptors. Parental rat endogenous insulin-like growth factor-1 receptors and transfected human insulin receptors form hybrid receptors as well as homologous tetrameric receptors. The normal heterotetrameric receptors possess kinase activity in vivo leading to tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and its association with the p85 regulatory subunit of phosphatidyl inositol 3-kinase. Interestingly, preincubation with human-specific anti-insulin receptor antibody abolished the increased insulin sensitivity in glucose incorporation into glycogen in HIR delta 978 cells. Furthermore, microinjection of anti-IRS-1 antibody into HIR delta 978 cells inhibited insulin stimulation of DNA synthesis. In summary: 1) truncated receptors on the cell surface confer enhanced insulin sensitivity in vivo; 2) the normal heterotetrameric receptors are functionally active and couple to IRS-1 efficiently; and 3) IRS-1 is an important molecule transmitting insulin's biological signals in HIR delta 978 cells.

Effect of phosphotyrosyl-IRS-1 level and insulin receptor tyrosine kinase activity on insulin-stimulated phosphatidylinositol 3, MAP, and S6 kinase activities

The role of tyrosine phosphorylation of the insulin receptor substrate 1 (IRS-1) was studied utilizing parental CHO cells or CHO cells that overexpress IRS-1, the insulin receptor, or both IRS-1 and the insulin receptor. Insulin stimulation of these four cell lines led to progressive levels of IRS-1 tyrosine phosphorylation of one, two, four, and tenfold. Maximal insulin-stimulated IRS-1 associated PtdIns 3'-kinase activit in these cells was 1-, 1.5-, 3-, and 3-fold, while insulin sensitivity, as determined by ED50, was 1-, 2.5-, 10-, and 10-fold. Both sensitivity and maximal response paralleled the increased level of phosphotyrosyl-IRS-1; however, the increased level of phosphotyrosyl-IRS-1 seen in CHO/IR/IRS-1 cells did not further increase these responses. Likewise, maximal insulin-stimulated MAP kinase activity in these cell lines increased in parallel with IRS-1 tyrosine phosphorylation except in the CHO/IR/IRS-1 cell lines with activity levels of one-, five-, nine-, and ninefold. However, insulin sensitivity of the MAP and S6 kinases and maximal insulin-stimulated S6 kinase activity was not changed by a twofold increase in phosphotyrosyl-IRS-1, but an increase was observed with insulin-stimulated receptor autophosphorylation and kinase activity in CHO/IR cells which led to a tenfold increase in insulin receptor autophosphorylation and a fourfold increase in IRS-1 tyrosine phosphorylation. Thus, these three kinase activities may be differentially coupled to the activation of the insulin receptor kinase activity via IRS-1 and other possible cellular substrates.