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

Insulin effects on glucose tolerance, hypermetabolic response, and circadian-metabolic protein expression in a rat burn and disuse model

Insulin controls hyperglycemia after severe burns, and its use opposes the hypermetabolic response. The underlying molecular mechanisms are poorly understood, and previous research in this area has been limited because of the inadequacy of animal models to mimic the physiological effects seen in humans with burns. Using a recently published rat model that combines both burn and disuse components, we compare the effects of insulin treatment vs. vehicle on glucose tolerance, hypermetabolic response, muscle loss, and circadian-metabolic protein expression after burns. Male Sprague-Dawley rats were assigned to three groups: cage controls (n = 6); vehicle-treated burn and hindlimb unloading (VBH; n = 11), and insulin-treated burn and hindlimb unloading (IBH; n = 9). With the exception of cage controls, rats underwent a 40% total body surface area burn with hindlimb unloading, then IBH rats received 12 days of subcutaneous insulin injections (5 units·kg(-1)·day(-1)), and VBH rats received an equivalent dose of vehicle. Glucose tolerance testing was performed on day 14, after which blood and tissues were collected for analysis. Body mass loss was attenuated by insulin treatment (VBH = 265 ± 17 g vs. IBH = 283 ± 14 g, P = 0.016), and glucose clearance capacity was increased. Soleus and gastrocnemius muscle loss was decreased in the IBH group. Insulin receptor substrate-1, AKT, FOXO-1, caspase-3, and PER1 phosphorylation was altered by injury and disuse, with levels restored by insulin treatment in almost all cases. Insulin treatment after burn and during disuse attenuated the hypermetabolic response, increased glucose clearance, and normalized circadian-metabolic protein expression patterns. Therapies aimed at targeting downstream effectors may provide the beneficial effects of insulin without hypoglycemic risk.

Inhibition of glycogen synthase kinase 3 improves insulin action and glucose metabolism in human skeletal muscle

Glycogen synthase kinase (GSK)-3 has been implicated in the regulation of multiple cellular physiological processes in skeletal muscle. Selective cell-permeable reversible inhibitors (INHs) of GSK-3 (CT98014 and CHIR98023 [Chiron, Emeryville, CA] and LiCl) were used to evaluate the role of GSK-3 in controlling glucose metabolism. Acute treatment (30 min) of cultured human skeletal muscle cells with either INH resulted in a dose-dependent activation of glycogen synthase (GS) with a maximally effective concentration of approximately 2 micromol/l. The maximal acute effect of either INH on GS (103 +/- 25% stimulation over basal) was greater than the maximal insulin response (48 +/- 9%, P < 0.05 vs. INH); LiCl was as effective as insulin. The GSK-3 inhibitor effect, like that of insulin, was on the activation state (fractional velocity [FV]) of GS. Cotreatment of muscle cells with submaximal doses of INH and insulin resulted in an additive effect on GS FV (103 +/- 10% stimulation, P < 0.05 vs. either agent alone). Glucose incorporation into glycogen was also acutely stimulated by INH. While prolonged (6-24 h) insulin exposure led to desensitization of GS, INH continued to activate GS FV for at least 24 h. Insulin and LiCl acutely activated glucose uptake, whereas INH stimulation of glucose uptake required more prolonged exposure, starting at 6 h and continuing to 24 h. Chronic (4-day) treatment with INH increased both basal (154 +/- 32% of control) and insulin-stimulated (219 +/- 74%) glucose uptake. Upregulation of uptake activity occurred without any change in total cellular GLUT1 or GLUT4 protein content. Yet the same chronic treatment resulted in a 65 +/- 6% decrease in GSK-3 protein and a parallel decrease (61 +/- 11%) in GSK-3 total activity. Together with the INH-induced increase in insulin-stimulated glucose uptake, there was an approximately 3.5-fold increase (P < 0.05) in insulin receptor substrate (IRS)-1 protein abundance. Despite upregulation of IRS-1, maximal insulin stimulation of Akt phosphorylation was unaltered by INH treatment. The results suggest that selective inhibition of GSK-3 has an impact on both GS and glucose uptake, including effects on insulin action, using mechanisms that differ from and are additive to those of insulin.

An assay method for evaluating chemical selectivity of agonists for insulin signaling pathways based on agonist-induced phosphorylation of a target peptide

An optical method for evaluating the physiologically relevant agonist and antagonist selectivity of an insulin signaling pathway based on an insulin-dependent on/off switching of phosphorylation of a target peptide via insulin receptor is described. Insulin receptor serves as a binding for insulin and a given insulin receptor-binding peptide as a target for an insulin-receptor complex. Upon binding of insulin to its receptor, the insulin receptor undergoes autophosphorylation which enables the receptor to have a kinase activity and phosphorylate various substrates. The phosphorylated tyrosine in the substrate was measured with a monoclonal anti-phosphotyrosine antibody. As the target substrate for insulin receptor, a Y939 peptide consisting of 12 amino acid residues derived from insulin receptor substrate 1 (IRS-1) was used. The present assay method involves different sequential steps: (1) immobilization of a biotin-coupled Y939 peptide on an avidin coated 96-well plate via biotin-avidin complexation; (2) insulin-dependent phosphorylation of the Y939 peptide by the insulin receptor; (3) enzymatic reaction and absorptiometric assay of the phosphorylated Y939 peptide using the anti-phosphotyrosine antibody labeled with horseradish peroxidase. An insulin-dependent absorbance was observed for insulin concentrations from 1.0 x 10(-10) to 1.0 x 10(-7) M, and it leveled off. The observed absorbance was explained to be due to an increase in the phosphorylated Y939 peptide caused by insulin and its receptor complexation. No signal was, however, induced by both vanadyl and vanadate ions at concentrations up to 1.0 x 10(-4) M; these results and previous intact cell level data taken together led to the conclusion that these ions did not induce phosphorylation of the Y939 peptide. Upon addition of tyrphostin 25, a specific inhibitor for insulin receptor kinase activity, phosphorylation of the Y939 peptide in the presence of 1.0 microM insulin was competitively inhibited over 1.0 x 10(-4) M tyrphostin 25. The present system thus exhibited "physiologically more relevant" agonist and antagonist selectivity, the principle of which is based in part on the insulin signal transduction rather than simply relying on the binding assay. The potential use of the present method for evaluating the selectivity of a wide range of agonists and antagonists toward the insulin signaling pathways is discussed.

Insulin signal transduction by a mutant human insulin receptor lacking the NPEY sequence. Evidence for an alternate mitogenic signaling pathway that is independent of Shc phosphorylation

The cytoplasmic juxtamembrane domain of the human insulin receptor (hIR) contains a single copy of the tetrameric amino acid sequence Asn-Pro-Glu-Tyr (NPEY) (residues 969-972 in the exon 11-containing B-isoform), which exists in the context of NPXY. In this study, we examined the role of NPEY972 in mediating insulin signal transduction and cellular biological effects. Transfected Chinese hamster ovary cell lines expressing either the wild-type hIR-B isoform (hIR.WT) or a mutant receptor lacking the NPEY972 sequence (hIRDeltaNPEY) and control Chinese hamster ovary.Neo cells were used to comparatively analyze the following insulin effects: in vivo receptor tyrosine autophosphorylation and kinase activity, signal transduction to downstream signaling molecules, and stimulation of glycogen and DNA synthesis. The results showed that in comparison to hIR.WT, the hIRDeltaNPEY mutant demonstrated the following: (a) normal insulin-mediated receptor tyrosine phosphorylation, but approximately 50% reduction in phosphorylation of p185-(insulin receptor substrate-1) and binding of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase), (b) an enhanced stimulation of PI 3-kinase enzymatic activity, (c) a complete inability to phosphorylate Shc, (d) minimal impairment of insulin sensitivity for glycogen synthesis, and (e) an augmented response to insulin-stimulated DNA synthesis via a high capacity, low sensitivity pathway. These results demonstrate the following: 1) the NPEY972 sequence is important but not absolutely essential for coupling of hIR kinase to insulin receptor substrate-1 and p85 or for mediating insulin's metabolic and mitogenic effects, 2) the NPEY972 sequence is necessary for Shc phosphorylation, and 3) the absence of Shc phosphorylation releases the constraints on maximal insulin-stimulated mitogenic response, thus indicating that alternate signaling pathway(s) exist for this insulin action. This alternate pathway appears to be associated with enhanced activation of PI 3-kinase and is of high capacity and low sensitivity.

CSF-1 receptor/insulin receptor chimera permits CSF-1-dependent differentiation of 3T3-L1 preadipocytes

A chimeric growth factor receptor (CSF1R/IR) was constructed by splicing cDNA sequences encoding the extracellular ligand binding domain of the human colony stimulating factor-1 (CSF-1) receptor to sequences encoding the transmembrane and cytoplasmic domains of the human insulin receptor. The addition of CSF-1 to cells transfected with the CSF1R/IR chimera cDNA stimulated the tyrosine phosphorylation of a protein that was immunoprecipitated by an antibody directed against the carboxyl terminus of the insulin receptor. Phosphopeptide maps of the 32P-labeled CSF1R/IR protein revealed the same pattern of phosphorylation observed in 32P-labeled insulin receptor beta subunits. CSF-1 stimulated the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and Shc in cells expressing the CSF1R/IR chimera. Lipid accumulation and the expression of a differentiation-specific marker demonstrated that 3T3-L1 preadipocytes undergo CSF-1-dependent differentiation when transfected with the CSF1R/IR chimera cDNA but not when transfected with the expression vector alone. A 12-amino acid deletion within the juxtamembrane region of the CSF1R/IR (CSF1R/IRDelta960) blocked CSF-1-stimulated phosphorylation of IRS-1 and Shc but did not inhibit CSF-1-mediated differentiation of 3T3-L1 preadipocytes. These observations indicate that adipocyte differentiation can be initiated by intracellular pathways that do not require tyrosine phosphorylation of IRS-1 or Shc.

Cloning of the chicken insulin receptor substrate 1 gene

The action of insulin, IGF-1, and IGF-2 is mediated via two receptor tyrosine kinases, the insulin and IGF-1 receptors. Upon ligand binding, these receptors become active kinases, undergoing autophosphorylation and phosphorylating cellular substrates, including insulin receptor substrate-1 (IRS-1). IRS-1 acts as a docking protein and mediates multiple interactions among other proteins, resulting in transduction of the metabolic and mitogenic signals. The IRS-1 gene has been cloned from four species (human, rat, mouse, and frog). In the present study, the chicken IRS-1 gene was cloned. Chicken, as is true of birds in general, have a higher fasting and fed blood glucose than do mammals. Chicken IRS-1 DNA sequence encodes a 1240 amino acid protein. The most conserved regions were the IRS homology-2 (IH-2), the pleckstrin homology, and the shc and IRS-1 NPXY-binding (SAIN) domains. Twelve of the cIRS-1 tyrosine residues are in sequence motifs that, when phosphorylated, could interact with proteins containing SH2 domains. All twelve of these motifs were conserved. IRS-1 mRNA is expressed during embryogenesis in chicken and persists after hatching. In LMH cells, derived from a chicken hepatoma, two bands were tyrosine phosphorylated in an insulin-dependent manner: IRS-1 (approximately 180 kDa) and the insulin receptor beta subunit (approximately 95 kDa). Chicken IRS-1 is structurally and functionally similar to its human homolog, despite the difference in blood glucose levels and the evolutionary distance between birds and mammals.

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.