Differences in the sites of phosphorylation of the insulin receptor in vivo and in vitro

Prolonged Early Exposure to a High-Fat Diet Augments the Adverse Effects on Neurobehavior and Hippocampal Neuroplasticity: Involvement of Microglial Insulin Signaling

High-fat diet (HFD) consumption may contribute to the high prevalence of cognitive-emotional issues in modern society. Mice fed a HFD for a prolonged period develop more severe neurobehavioral disturbances when first exposed to a HFD in the juvenile period than in adulthood, suggesting an initial age-related difference in the detrimental effects of long-term HFD feeding. However, the mechanism underlying this difference remains unclear. Here, male C57BL/6J mice initially aged 4 (IA4W) or 8 (IA8W) weeks were fed a control diet (CD) or HFD for 6 months and then subjected to metabolic, neurobehavioral, and histomorphological examinations. Although the detrimental effects of long-term HFD feeding on metabolism and neurobehavior were observed in mice of both ages, IA4W-HFD mice showed significant cognitive inflexibility accompanied by significantly greater levels of anxiety-like behavior than age-matched controls. Hippocampal neuroplasticity and microglial phenotype were altered by HFD feeding, whereas significant morphological alterations were more frequently observed in IA4W-HFD mice than in IA8W-HFD mice. Additionally, significantly increased hippocampal microglial engulfment of postsynaptic proteins and elevated phospho-insulin-receptor levels were observed in IA4W-HFD, but not in IA8W-HFD, mice. These findings suggest that aberrant microglia-related histomorphological changes in the hippocampus underlie the exacerbated detrimental neurobehavioral effects of prolonged early HFD exposure and indicate that enhanced insulin signaling might drive microglial dysfunction after prolonged early HFD exposure.

Hypoglycemic effects of dendrobium officinale leaves

Introduction: Numerous studies have demonstrated that the stems of D. officinale have the effect of lowering blood glucose, but the leaves of D. officinale have seldom been investigated. In this study, we mainly studied the hypoglycemic effect and mechanism of D. officinale leaves. Methods: Initially in vivo, male C57BL/6 mice were administered either standard feed (10 kcal% fat) or high-fat feed (60 kcal% fat) along with either normal drinking water or drinking water containing 5 g/L water extract of D. officinale leaves (EDL) for 16 weeks, and changes in body weight, food intake, blood glucose, etc., were monitored weekly. Next in vitro, C2C12 myofiber precursor cells which were induced to differentiate into myofibroblasts and cultured with EDL to detect the expression of insulin signaling pathway related proteins. HEPA cells were also cultured with EDL to detect the expression of hepatic gluconeogenesis or hepatic glycogen synthesis related proteins. Eventually after separating the components from EDL by ethanol and 3 kDa ultrafiltration centrifuge tube, we conducted animal experiments using the ethanol-soluble fraction of EDL (ESFE), ethanol-insoluble fraction of EDL (EIFE), ESFE with a molecular weight of >3 kDa (>3 kDa ESFE), and ESFE with a molecular weight of <3 kDa (<3 kDa ESFE) for intensive study. Results: The results in vivo revealed that the mice fed the high-fat diet exhibited significantly decreased blood glucose levels and significantly increased glucose tolerance after the EDL treatment, whereas the mice fed the low-fat diet did not. The results in vitro showed that EDL activated the expression of protein kinase B (AKT), the phosphorylation of AKT, and the expression of downstream GSK3β in the insulin signaling pathway. EDL treatment of HEPA cells confirmed that EDL did not affect hepatic gluconeogenesis or hepatic glycogen synthesis. In the experiment of studying the composition of EDL, we found that the >3 kDa ESFE displayed the effect of lowering blood glucose. In summary, the effect of EDL in lowering blood glucose may bethanole achieved by activating the insulin signaling pathway to increase insulin sensitivity, and the main functional substance was contained within the >3 kDa ESFE. Discussion: The findings of this study represent a reference point for further exploration of the hypoglycemic effects of D. officinale leaves and may assist in both the identification of new molecular mechanisms to improve insulin sensitivity and the isolation of monomeric substances that lower blood glucose. Furthermore, the obtained results may provide a theoretical basis for the development of hypoglycemic drugs with D. officinale leaves as the main component.

Insulin on the brain: The role of central insulin signalling in energy and glucose homeostasis

Insulin signals to the brain where it coordinates multiple physiological processes underlying energy and glucose homeostasis. This review explores where and how insulin interacts within the brain parenchyma, how brain insulin signalling functions to coordinate energy and glucose homeostasis and how this contributes to the pathogenesis of metabolic disease.

Insulin action in the brain: Roles in energy and glucose homeostasis

A growing body of evidence from research in rodents and humans has identified insulin as an important neuoregulatory peptide in the brain, where it coordinates diverse aspects of energy balance and peripheral glucose homeostasis. This review discusses where and how insulin interacts within the brain and evaluates the physiological and pathophysiological consequences of central insulin signalling in metabolism, obesity and type 2 diabetes.

Anti-obesity and hypolipidemic effects of Rheum undulatum in high-fat diet-fed C57BL/6 mice through protein tyrosine phosphatase 1B inhibition

Protein tyrosine phosphatase 1B (PTP1B) is important in the regulation of metabolic diseases and has emerged as a promising signaling target. Previously, we reported the PTP1B inhibitory activity of Rheum undulatum (RU). In the present study, we investigated the metabolic regulatory effects of RU in a high-fat diet (HFD) model. RU treatment significantly blocked body weight gain, which was accompanied by a reduction of feed efficiency. In addition, it led to a reduction of liver weight mediated by overexpression of PPARα and CPT1 in the liver, and an increase in the expression of adiponectin, aP2, and UCP3 in adipose tissue responsible for the reduction of total and LDL-cholesterol levels. Chrysophanol and physcion from RU significantly inhibited PTP1B activity and strongly enhanced insulin sensitivity. Altogether, our findings strongly suggest that 2 compounds are novel PTP1B inhibitors and might be considered as anti-obesity agents that are effective for suppressing body weight gain and improving lipid homeostasis.

Phosphoinositides: Key modulators of energy metabolism

Phosphoinositides are key players in many trafficking and signaling pathways. Recent advances regarding the synthesis, location and functions of these lipids have dramatically improved our understanding of how and when these lipids are generated and what their roles are in animal physiology. In particular, phosphoinositides play a central role in insulin signaling, and manipulation of PtdIns(3,4,5)P₃levels in particular, may be an important potential therapeutic target for the alleviation of insulin resistance associated with obesity and the metabolic syndrome. In this article we review the metabolism, regulation and functional roles of phosphoinositides in insulin signaling and the regulation of energy metabolism. This article is part of a Special Issue entitled Phosphoinositides.

The Structural Basis of Action of Vanadyl (VO2+) Chelates in Cells

Much emphasis has been given to vanadium compounds as potential therapeutic reagents for the treatment of diabetes mellitus. Thus far, no vanadium compound has proven efficacious for long-term treatment of this disease in humans. Therefore, in review of the research literature, our goal has been to identify properties of vanadium compounds that are likely to favor physiological and biochemical compatibility for further development as therapeutic reagents. We have, therefore, limited our review to those vanadium compounds that have been used in both in vivo experiments with small, laboratory animals and in in vitro studies with primary or cultured cell systems and for which pharmacokinetic and pharmacodynamics results have been reported, including vanadium tissue content, vanadium and ligand lifetime in the bloodstream, structure in solution, and interaction with serum transport proteins. Only vanadyl (VO²⁺) chelates fulfill these requirements despite the large variety of vanadium compounds of different oxidation states, ligand structure, and coordination geometry synthesized as potential therapeutic agents. Extensive review of research results obtained with use of organic VO²⁺-chelates shows that the vanadyl chelate bis(acetylacetonato)oxidovanadium(IV) [hereafter abbreviated as VO(acac)₂], exhibits the greatest capacity to enhance insulin receptor kinase activity in cells compared to other organic VO²⁺-chelates, is associated with a dose-dependent capacity to lower plasma glucose in diabetic laboratory animals, and exhibits a sufficiently long lifetime in the blood stream to allow correlation of its dose-dependent action with blood vanadium content. The properties underlying this behavior appear to be its high stability and capacity to remain intact upon binding to serum albumin. We relate the capacity to remain intact upon binding to serum albumin to the requirement to undergo transcytosis through the vascular endothelium to gain access to target tissues in the extravascular space. Serum albumin, as the most abundant transport protein in the blood stream, serves commonly as the carrier protein for small molecules, and transcytosis of albumin through capillary endothelium is regulated by a Src protein tyrosine kinase system. In this respect it is of interest to note that inorganic VO²⁺ has the capacity to enhance insulin receptor kinase activity of intact 3T3-L1 adipocytes in the presence of albumin, albeit weak; however, in the presence of transferrin no activation is observed. In addition to facilitating glucose uptake, the capacity of VO²⁺- chelates for insulin-like, antilipolytic action in primary adipocytes has also been reviewed. We conclude that measurement of inhibition of release of only free fatty acids from adipocytes stimulated by epinephrine is not a sufficient basis to ascribe the observations to purely insulin-mimetic, antilipolytic action. Adipocytes are known to contain both phosphodiesterase-3 and phosphodiesterase-4 (PDE3 and PDE4) isozymes, of which insulin antagonizes lipolysis only through PDE3B. It is not known whether the other isozyme in adipocytes is influenced directly by VO²⁺- chelates. In efforts to promote improved development of VO²⁺- chelates for therapeutic purposes, we propose synergism of a reagent with insulin as a criterion for evaluating physiological and biochemical specificity of action. We highlight two organic compounds that exhibit synergism with insulin in cellular assays. Interestingly, the only VO²⁺- chelate for which this property has been demonstrated, thus far, is VO(acac)₂.

Insulin down-regulates specific activity of ATP-binding cassette transporter A1 for high density lipoprotein biogenesis through its specific phosphorylation

Insulin resistance/hyperinsulinism is one of the major risks for atherosclerotic vascular diseases and low HDL may be involved in pathogenesis. We examined direct effects of insulin on HDL biosynthesis focusing on the activity of ATP-binding cassette transporter A1 (ABCA1) in culture cells and in experimental animals. Insulin impairs HDL biosynthesis through modulation of ABCA1 activity by two different mechanisms. Insulin enhances degradation of ABCA1. However, even after this effect was cancelled by blocking its specific signal, insulin still reduces HDL biogenesis. This effect was found due to phosphorylation of ABCA1 that leads to decrease of its specific activity. We identified a novel insulin-specific phosphorylation site Tyr1206 of ABCA1 to regulate its specific activity. The observation in a rat model of insulin resistance was consistent with these results. The findings demonstrate a new mechanism for regulation of ABCA1 activity and provide new insights into the link between development of atherosclerosis, and insulin resistance/hyperinsulinism.

Activation of the PI3K/Akt signal transduction pathway and increased levels of insulin receptor in protein repair-deficient mice

Protein L-isoaspartate (D-aspartate) O-methyltransferase is an enzyme that catalyses the repair of isoaspartyl damage in proteins. Mice lacking this enzyme (Pcmt1-/- mice) have a progressive increase in brain size compared with wild-type mice (Pcmt1+/+ mice), a phenotype that can be associated with alterations in the PI3K/Akt signal transduction pathway. Here we show that components of this pathway, including Akt, GSK3beta and PDK-1, are more highly phosphorylated in the brains of Pcmt1-/- mice, particularly in cells of the hippocampus, in comparison with Pcmt1+/+ mice. Examination of upstream elements of this pathway in the hippocampus revealed that Pcmt1-/- mice have increased activation of insulin-like growth factor-I (IGF-I) receptor and/or insulin receptor. Western blot analysis revealed an approximate 200% increase in insulin receptor protein levels and an approximate 50% increase in IGF-I receptor protein levels in the hippocampus of Pcmt1-/- mice. Higher levels of the insulin receptor protein were also found in other regions of the adult brain and in whole tissue extracts of brain, liver, heart and testes of both juvenile and adult Pcmt1-/- mice. There were no significant differences in plasma insulin levels for adult Pcmt1-/- mice during glucose tolerance tests. However, they did show higher peak levels of blood glucose, suggesting a mild impairment in glucose tolerance. We propose that Pcmt1-/- mice have altered regulation of the insulin pathway, possibly as a compensatory response to altered glucose uptake or metabolism or as an adaptive response to a general accumulation of isoaspartyl protein damage in the brain and other tissues.

Increased cell proliferation and granule cell number in the dentate gyrus of protein repair-deficient mice

Recent studies have demonstrated that mice lacking protein L-isoaspartate (D-aspartate) O-methyltransferase (Pcmt1-/- mice) have alterations in the insulin-like growth factor-I (IGF-I) and insulin receptor pathways within the hippocampal formation as well as other brain regions. However, the cellular localization of these changes and whether the alterations might be associated with an increase in cell number within proliferative regions, such as the dentate gyrus, were unknown. In this study, stereological methods were used to demonstrate that these mice have an increased number of granule cells in the granule cell layer and hilus of the dentate gyrus. The higher number of granule cells was accompanied by a greater number of cells undergoing mitosis in the dentate gyrus, suggesting that an increase in neuronal cell proliferation occurs in this neurogenic zone of adult Pcmt1-/- mice. In support of this, increased doublecortin labeling of immature neurons was detected in the subgranular zone of the dentate gyrus. In addition, double immunofluorescence studies demonstrated that phosphorylated IGF-I/insulin receptors in the subgranular zone were localized on immature neurons, suggesting that the increased activation of one or both of these receptors in Pcmt1-/- mice could contribute to the growth and survival of these cells. We propose that deficits in the repair of isoaspartyl protein damage leads to alterations in metabolic and growth-receptor pathways, and that this model may be particularly relevant for studies of neurogenesis that is stimulated by cellular damage.

Substrates for protein kinase CK2 in insulin receptor preparations from rat liver membranes: identification of a 210-kDa protein substrate as the dimeric form of endoplasmin

Chromatography of extracts from rat liver membranes on wheat-germ lectin-Sepharose resulted in a partial resolution of the insulin receptor from other phosphorylatable proteins. Among the latter, a protein (p210, with an apparent M(r) of 210 kDa on SDS/PAGE under nonreducing conditions) was found to be phosphorylated by protein kinase CK2 on Thr and Ser residues. Under reducing conditions p210 was resolved into two phosphopolypeptides with apparent M(r) of 95 and 105 kDa. Neither the 95-kDa nor the 105-kDa polypeptides were recognized by antibodies against the beta-subunit of the insulin receptor. Both polypeptides gave identical phosphopeptide maps after protease V8 digestion and contained the same N-terminal amino acid sequence. This sequence coincided with that of endoplasmin, and both polypeptides as well as p210 were recognized by antibodies against this protein. This shows that p210 corresponds to the dimeric form of rat liver endoplasmin. DEAE-Sepharose chromatography of p210 preparations removed most other contaminating proteins and revealed the presence of a protein kinase activity that coeluted with p210. This protein kinase possessed the properties (substrate specificity and inhibition by heparin) that are characteristic of the protein kinase CK2 enzymes. Furthermore, phosphoamino acid analysis and phosphopeptide maps of the 95/105-kDa polypeptides phosphorylated either by the endogenous protein kinase or by exogenous protein kinase CK2 gave similar results. The phosphorylation of p210/endoplasmin by protein kinase CK2 and its coelution gives support to the involvement of this protein kinase in membrane-associated processes.

Isoproterenol inhibits insulin-stimulated tyrosine phosphorylation of the insulin receptor without increasing its serine/threonine phosphorylation

The effect of a beta-adrenergic agonist (isoproterenol) on the tyrosine kinase activity of the insulin receptor was studied in intact adipocytes. Isoproterenol treatment rapidly (5 min) inhibited the insulin-induced autophosphorylation of the insulin receptor on tyrosine residues in intact adipocytes. The effect of insulin on the phosphorylation of cellular proteins on tyrosine residues was also inhibited by isoproterenol. In order to understand the mechanism responsible for this inhibition, two-dimensional phosphopeptide mapping of the insulin receptor was performed. The pattern of phosphorylation of the insulin receptor in freshly isolated adipocytes showed marked differences from that previously observed in cultured cells overexpressing insulin receptors. These differences include a larger proportion of receptors being phosphorylated on the three tyrosines from the kinase domain and no apparent phosphorylation of the two tyrosines close to the C-terminus after insulin stimulation. Isoproterenol markedly inhibited the effect of insulin on the phosphorylation of the three tyrosines from the kinase domain. However, this inhibition was not associated with an increase in the phosphorylation of serine/threonine peptides. Thus, this direct analysis of insulin receptor phosphorylation sites in intact adipocytes does no support the idea that beta-adrenegic agents inhibit the tyrosine kinase activity of the receptor through a serine/threonine phosphorylation-dependent mechanism.

Localization of the insulin-like growth factor I receptor binding sites for the SH2 domain proteins p85, Syp, and GTPase activating protein

Potential signaling substrates for the insulin-like growth factor I (IGF-I) receptor are SH2 domain proteins including the p85 subunit of phosphatidylinositol 3-kinase, the tyrosine phosphatase Syp, GTPase activating protein (GAP), and phospholipase C-gamma (PLC-gamma). In this study, we demonstrate an association between the IGF-I receptor and p85, Syp, and GAP, but not with PLC-gamma in lysates of cells overexpressing the human IGF-I receptor. We further investigated these interactions using glutathione S-transferase (GST) fusion proteins containing the amino-terminal SH2 domains of p85 or GAP, or both SH2 domains of Syp or PLC-gamma to precipitate the IGF-I receptor from purified receptor preparations and from whole cell lysates. p85-, Syp-, and GAP-GSTs precipitated the IGF-I receptor, whereas the PLC-gamma-GST did not. Using phosphopeptides corresponding to IGF-I receptor phosphorylation sites, we determined that the p85- and Syp-GST association with the IGF-I receptor could be inhibited by a carboxyl-terminal peptide containing pY1316 and that the GAP-GST association could be inhibited by a NPXY domain peptide. The GAP-GST binding site was confirmed by showing that a mutant IGF-I receptor with a deletion of the NPXY domain including tyrosine 950 was poorly precipitated by the GAP-GST. We conclude that p85 and Syp may bind directly to the IGF-I receptor at tyrosine 1316, and that GAP may bind to the IGF-I receptor at and PLC-gamma was not evident. p85, Syp, and GAP are potential modulators of IGF-I receptor signal transduction.

Functional identification of three major phosphoproteins in endocytic fractions from rat liver. A comparative in vivo and in vitro study

Liver plasma membranes originating from the sinusoidal, lateral and canalicular domains and 'early' and 'late' endosomes were prepared from rats injected with [32P]orthophosphate. The phosphorylated polypeptides in these subcellular fractions, resolved by gel electrophoresis, were analysed and compared with those obtained by in vitro phosphorylation of the fractions by endogenous protein kinases. The polypeptides phosphorylated in vitro were different in plasma membranes, endosomes and lysosomes. Three of the major phosphoproteins in the endocytic membranes were shown to be the polymeric immunoglobulin receptor, the beta subunit of the insulin receptor and the 550-kDa low-density-lipoprotein-receptor-related protein (LRP). An additional 35-kDa polypeptide of unknown function was a major phosphorylated component and thus emerges as a candidate marker protein of hepatic endosomes. Phosphoserine was shown to be the major amino acid phosphorylated in vitro in the phosphoproteins of endocytic membranes. The subcellular distribution in liver tissue of protein kinase activity was also investigated and activity shown to be recovered mainly in blood-sinusoidal and lateral plasma membranes; bile canalicular plasma membranes and endosomes contained low protein kinase activities. The results show that receptor phosphorylation is an 'early' event in endocytosis and the trafficking of ligands that is sustained especially in early endosomes in liver, and emphasizes the biochemical and thus functional distinctiveness of the plasma membrane and the endosomal and lysosomal compartments with regard to their population of phosphorylated proteins.

Insulin resistance in rats harboring growth hormone-secreting tumors: decreased receptor number but increased kinase activity in liver

Growth hormone (GH) is a potent antagonist of insulin action, and this resistance occurs primarily at a post-binding step(s). To elucidate the underlying mechanisms, the effects of chronic GH excess on the structure and function of insulin receptors partially purified from the liver were examined in rats harboring GH-secreting tumors. Insulin resistance was established in this animal model of GH hypersecretion by a hyperinsulinemic euglycemic clamp. Specific binding of 125I-insulin and receptor number were reduced in tumor animals by 40% and 62%, respectively, reflecting downregulation of the insulin receptor by hyperinsulinemia in these animals. Receptors from tumor animals showed a 50% increase in beta-subunit phosphorylation and in the kinase activity toward the synthetic polypeptide Glu4:Tyr1 when measured in vitro in the absence of insulin; however, the incremental stimulation by insulin (170 nmol/L) of the phosphorylation of either the beta-subunit or Glu4:Tyr1 was not different between control and experimental animals. There was no difference between the two groups in Glu4:Tyr1 phosphorylation measured after immunodepletion of receptors by antibodies to the insulin receptor, indicating that the observed alteration in the kinase activity of tumor animals was intrinsic to the insulin receptor. Exposure to chronic GH excess did not alter insulin receptor structure, as evidenced by electrophoretic mobility under reducing and nonreducing conditions. The enhanced basal kinase activity of the receptor from tumor animals may reflect a more highly phosphorylated state of the receptor (and hence elevated enzyme activity) in these animals due to elevated serum insulin levels. These results demonstrate that the hepatic insulin resistance in rats chronically exposed to GH excess is not due to impaired insulin receptor kinase activity.

Protein kinase C and insulin receptor beta-subunit serine phosphorylation in cultured foetal rat hepatocytes

In digitonin-permeabilized cultured foetal hepatocytes, insulin receptor beta-subunit was highly phosphorylated on serine residues in the presence of [gamma-32P]ATP and Ca2+, a process enhanced after short exposure to insulin with no detectable insulin receptor autophosphorylation. By contrast with this situation, experiments performed with isolated foetal insulin receptors revealed an insulin stimulation of both serine phosphorylation and tyrosine autophosphorylation. In permeabilized cells, insulin receptor beta-subunit phosphorylation was increased after a 2-min exposure to phorbol 12-myristate 13-acetate (PMA) prior to applying the permeabilization/phosphorylation step, while it was inhibited by chronic treatment with PMA leading to protein kinase C (PKC) down modulation. The PKC specific inhibitor, GF109203X, strikingly reduced basal and insulin-enhanced phosphorylation of insulin receptor beta-subunit in permeabilized cells, but failed to exert any effect with isolated receptors. Labelling of glycogen from [U-14C]glucose determined 1 h after a 10-min transitory exposure to insulin and/or modulators of PKC activity showed that PMA prevented insulin glycogenic response, whereas GF109203X was ineffective. Thus, although not directly responsible for insulin receptor serine phosphorylation in cultured foetal hepatocytes, PKC physiologically regulates this process which may inhibit insulin receptor tyrosine kinase activity. This regulation is independent of the antagonistic effect of PMA-activated PKC on insulin glycogenic response.

Substrate specificities of catalytic fragments of protein tyrosine phosphatases (HPTP beta, LAR, and CD45) toward phosphotyrosylpeptide substrates and thiophosphotyrosylated peptides as inhibitors

The transmembrane PTPase HPTP beta differs from its related family members in having a single rather than a tandemly duplicated cytosolic catalytic domain. We have expressed the 354-amino acid, 41-kDa human PTP beta catalytic fragment in Escherichia coli, purified it, and assessed catalytic specificity with a series of pY peptides. HPTP beta shows distinctions from the related LAR PTPase and T cell CD45 PTPase domains: it recognizes phosphotyrosyl peptides of 9-11 residues from lck, src, and PLC gamma with Km values of 2, 4, and 1 microM, some 40-200-fold lower than the other two PTPases. With kcat values of 30-205 s-1, the catalytic efficiency, kcat/Km, of the HPTP beta 41-kDa catalytic domain is very high, up to 5.7 x 10(7) M-1 s-1. The peptides corresponding to PLC gamma (766-776) and EGFR (1,167-1,177) phosphorylation sites were used for structural variation to assess pY sequence context recognition by HPTP beta catalytic domain. While exchange of the alanine residue at the +2 position of the PLC gamma (Km of 1 microM) peptide to lysine or aspartic acid showed little or no effect on substrate affinity, replacement by arginine increased the Km 35-fold. Similarly, the high Km value of the EGFR pY peptide (Km of 104 microM) derives largely from the arginine residue at the +2 position of the peptide, since arginine to alanine single mutation at the -2 position of the EGFR peptide decreased the Km value 34-fold to 3 microM. Three thiophosphotyrosyl peptides have been prepared and act as substrates and competitive inhibitors of these PTPase catalytic domains.

Resin immobilized synthetic peptides used to characterize phosphorylation and antigenic properties of insulin receptor autophosphorylation domains

To develop a common strategy in peptide design for kinase assay, antibody production and affinity purification, we investigated phosphorylation and antigenic properties of peptides immobilized on an aminated polyacrylic resin (Expansin) corresponding to autophosphorylation domains of the insulin receptor tyrosine kinase. Immobilized peptides (1143-1155) and peptide (1314-1330), designated p1151 and p1322, respectively, were good substrates for the insulin receptor with Km of 0.74 and 0.78 mM. By contrast, peptide (952-963), designated p960, was poorly phosphorylated. p1151 showed distinctive behaviour as a substrate, displaying a higher basal phosphorylation, a leftward shift of the insulin dose-response curve (ED50 = 0.7 ng mL-1 insulin compared to 20 ng mL-1 for other substrates) and an inhibition by 90% of receptor autophosphorylation (ID50 = 0.5 mM). Similar substrate behaviour was observed with another tyrosine kinase, the pp60c-src. Antibodies against P1151 and p1322 have comparable reactivity in ELISA, but the antibody against p960 was poor. While purified immunoglobulins (IgG) against both p1151 and p1322 were inhibitors of receptor autophosphorylation and kinase, in immunoprecipitation the IgG against p1151 mainly interacted with the phosphorylated receptor and that against p1322 with non-phosphorylated forms. Functional mapping of the receptor with oligoclonal 1322-antibody revealed inhibition of phosphate transfer to exogenous substrate poly(Glu,Tyr) (4:1) but not towards immobilized p1151. These data provide further support for the distinctive features of endogenous phosphorylation domain 1151. We conclude that immobilized peptides on polyacrylic resin offer a major new potential for use in kinase assays, immunization, immunoabsorbent techniques and purification of well defined oligoclonal antibodies.

Insulin receptor beta-subunit serine phosphorylation in permeabilized cultured fetal rat hepatocytes

Regulation of cellular protein phosphorylation by insulin was investigated after short exposure at 37 degrees C prior to applying the permeabilization/phosphorylation step in the presence of digitonin and [gamma-32P]ATP for 30 min at 4 degrees C. The results revealed major 32P incorporation into a limited number of membrane polypeptides exhibiting a molecular mass of 95, 58 and 51 kDa. Phosphorylation of 95 kDa protein was selectively inhibited with Ca(2+)-free EGTA-containing permeabilization/phosphorylation buffer and became predominant in the presence of Ca2+. Considering in particular its immunoprecipitation by a monoclonal antibody directed against insulin receptor, the 32P-labeled 95 kDa protein represented the beta-subunit of the insulin receptor. Its phosphorylation was transiently stimulated after exposure to insulin (35% after 2 min), and concerned mostly serine residues under both basal and stimulated conditions. Vanadate had a similar effect and both agents favored glycogenesis, whereas heparin which inhibited 95 kDa protein phosphoseryl phosphorylation had an opposite effect on glycogenesis. These results suggest a biological role for the membrane-associated phosphoseryl-protein kinase(s) and phosphatase(s) acting on the insulin receptor beta-subunit in cultured fetal hepatocytes.

NMR analysis of regioselectivity in dephosphorylation of a triphosphotyrosyl dodecapeptide autophosphorylation site of the insulin receptor by a catalytic fragment of LAR phosphotyrosine phosphatase

An autophosphorylation site in the activated insulin receptor tyrosine kinase domain has three tyrosines phosphorylated when fully activated. To begin to examine recognition of triphosphotyrosyl sites by protein tyrosine phosphatases in possible control of signal transduction a triphosphotyrosyl dodecapeptide TRDIpYETDpYpYRK corresponding to residues 1,142-1,153 of the insulin receptor was prepared and incubated with the 40-kDa catalytic domain of the human PTPase LAR. To assess regioselectivity of recognition, the three diphosphotyrosyl regioisomers, and the three monophosphotyrosyl regioisomers were prepared and assayed. All seven peptides were PTPase substrates. To identify any preferences in dephosphorylation at pY5, pY9, or pY10, 1H-NMR analyses were conducted during enzyme incubations and distinguishing fingerprint regions determined for each of the seven phosphotyrosyl peptides. LAR PTPase shows strong preference for dephosphorylation first at pY5 (at tri-, di-, and monophosphotyrosyl levels). Initially this regioselectivity gives the Y5(pY9)(pY10) diphospho regioisomer, followed by equal dephosphorylation at pY9 or pY10 to give the corresponding monophosphoryl species on the way to fully dephosphorylated product. The NMR methodology is applicable to other peptides with multiple sites of phosphorylation that undergo attack by any phosphatase.

Kinase inhibition by a phosphorylated peptide corresponding to the major insulin receptor autophosphorylation domain

We studied the inhibitory effect of non-phosphorylated and triphosphorylated synthetic peptides, corresponding to amino acids 1143-1155 of the insulin proreceptor (domain 1151) on autophosphorylation and kinase of the insulin receptor. Tyrosine-phosphorylated peptides were synthesized using the N-(9-fluorenylmethoxycarbonyl)-O-dibenzylphosphono-L- tyrosine. The triphosphorylated peptide (1151-P3) and the non-phosphorylated peptide (1151-NP), respectively, inhibited insulin receptor autophosphorylation by 65% and 70%, in a dose-dependent and additive manner. When the receptor was pre-phosphorylated for 1 min with [gamma-32P]ATP, 1151-P3 decreased autophosphorylation to 60% of maximum, whereas 1151-NP had no further effect. In both non-activated and preactivated receptors, 1151-P3 inhibition of receptor autophosphorylation was prevented by adding 2 mM vanadate. Kinase activity towards exogenous substrate poly(Glu4, Tyr) was dose-dependently inhibited by both analogues. This effect was independent of the state of receptor phosphorylation or the addition of vanadate. Since 1151-P3 inhibited the exogenous kinase without altering receptor endogenous autophosphorylation after the addition of vanadate, we investigated 1151-NP and 1151-P3 competition for the phosphorylation of a resin-immobilized 1151 peptide. While 1151-NP (at 2 mM) was highly competitive, inhibiting phosphate incorporation by 70%, 1151-P3 caused a four-fold increase in the phosphorylation of 1151-NP--resin. The receptor underwent conformational changes during autophosphorylation and an antibody directed against a peptide corresponding to amino acids 1314-1330 of the proreceptor (1322Ab) was previously shown to immunoprecipitate specifically the non-phosphorylated receptor forms. Nevertheless, the 1322Ab immunoprecipitated a fully autophosphorylated receptor in the presence of 1151-NP, but not of 1151-P3, thus suggesting a conformational change induced by the non-phosphorylated peptide. In conclusion, kinase inhibition was still observed after the addition of phosphate groups to three 1151-peptide tyrosines, but the peptide effect on receptor autophosphorylation, phosphorylation of homologous 1151-NP--resin and conformational changes induced in the receptor was altered dramatically. These data may provide a basis for further understanding the role of tyrosine phosphorylation in insulin receptor kinase activation or regulation.

Effect of glucagon on insulin receptor phosphorylation in intact liver cells

Evidence is presented that incubation of rat liver cells with glucagon leads to an increase in the phosphorylation of specific serine residues within insulin receptors, particularly in the presence of insulin. However, no changes in either the tyrosine phosphorylation of the receptors or the tyrosine kinase activity towards a synthetic peptide substrate was detected.

The insulin receptor: signalling mechanism and contribution to the pathogenesis of insulin resistance

The insulin receptor is a heterotetrameric structure consisting of two alpha-subunits of Mr 135 kilodalton on the outside of the plasma membrane connected by disulphide bonds to beta-subunits of Mr 95 kilodalton which are transmembrane proteins. Insulin binding to the alpha-subunit induces conformational changes which are transduced to the beta-subunit. This leads to the activation of a tyrosine kinase activity which is intrinsic to the cytoplasmatic domains of the beta-subunit. Activation of the tyrosine kinase activity of the insulin receptor represents an essential step in the transduction of an insulin signal across the plasma membrane of target cells. Signal transduction on the post-kinase level is not yet understood in detail, possible mechanisms involve phosphorylation of substrate proteins at tyrosine residues, activation of serine kinases, the interaction with G-proteins, phospholipases and phosphatidylinositol kinases. Studies in multiple insulin-resistant cell models have demonstrated that an impaired response of the tyrosine kinase to insulin stimulation is one potential mechanism causing insulin resistance. An impairment of the insulin effect on tyrosine kinase activation in all major target tissues of insulin, in particular the skeletal muscle was demonstrated in Type 2 (non-insulin-dependent) diabetic patients. There is no evidence that the impaired tyrosine kinase response in the skeletal muscle is a primary defect, however, it is likely that this abnormality of insulin signal transduction contributes significantly to the pathogenesis of the insulin-resistant state in Type 2 diabetes.

Muscle insulin resistance in uremic humans: glucose transport, glucose transporters, and insulin receptors

To determine the cellular basis for insulin resistance observed in patients with uremia, we investigated insulin action in vivo and in vitro using skeletal muscle obtained from patients with chronic renal failure. Uremic subjects had significantly reduced rates of insulin-stimulated glucose disposal, as determined by a 3-h intravenous glucose tolerance test and using the hyperinsulinemic euglycemic clamp technique. Hepatic glucose production was similar before (control, 76.2 +/- 6.3 vs. uremic, 74.2 +/- 6.9 mg.kg-1.min-1) and during insulin infusion at 40 mU.m-2.min-1 (control, -60.9 +/- 6.6 vs. uremic, -53.9 +/- 6.3 mg.kg-1.min-1). In incubated human skeletal muscle fiber strips, basal 2-deoxy-D-glucose transport was unchanged in uremic subjects compared with controls. However, the increase in insulin-stimulated glucose transport was significantly reduced by 50% in muscles from uremic patients (P = 0.012). In partially purified insulin receptors prepared from skeletal muscle, 125I-labeled insulin binding, beta-subunit receptor autophosphorylation, and tyrosine kinase activity were all unchanged in uremic subjects. The abundance of insulin-sensitive (muscle/fat, GLUT-4) glucose transporter protein measured by Western blot using Mab 1F8 or polyclonal antisera was similar in muscles of control and uremic patients. These findings suggest that the insulin resistance observed in skeletal muscle of uremic patients cannot be attributed to defects in insulin receptor function or depletion of the GLUT-4 glucose transporter protein. An alternative step in insulin-dependent activation of the glucose transport process may be involved.

Epidermal growth factor stimulated phosphorylation of a 120-kilodalton endogenous substrate protein in rat hepatocytes

Endogenous substrates of the EGF receptor have been described in transformed cells; however, little is known about substrates in normal tissue. To characterize epidermal growth factor (EGF) receptor phosphorylation and search for endogenous substrates in normal rat hepatocytes, cells were labeled with [32P]orthophosphate, and phosphotyrosine-containing proteins were sought by using a high-affinity, specific anti-phosphotyrosine antibody. Exposure of 32P-labeled freshly isolated hepatocytes to 1 microgram/mL EGF caused phosphorylation of several proteins of Mr 185K, 160K, and 120K. The 185- and 160-kDa proteins (pp185 and pp160) were identified as the intact and proteolyzed forms of the EGF receptor by virtue of their immunoprecipitation with anti-EGF receptor antibody. This antibody failed to recognize the 120-kDa phosphoprotein (pp120). The phosphopeptide map derived from pp120 was by trypsinization and HPLC separation different from that of pp185, further indicating that pp120 is distinct from the EGF receptor. This pp120 was also immunologically distinct from the pp120 substrate of the insulin receptor kinase and from ATP-citrate lyase. Phosphoamino acid analysis revealed pp120 to be phosphorylated on both tyrosine and serine residues. Autophosphorylation of EGF receptor and phosphorylation of pp120 were almost maximal within 1 min of EGF stimulation. The dose-response curves for phosphorylation of the EGF receptor and pp120 were identical (ED50 = 30 ng/mL) and were superimposable with the fractional occupancy of the EGF receptor. In A431 cells, a transformed cell line whose growth is inhibited by EGF, EGF produced a decrease in pp120 phosphorylation. These data suggest that pp120 is an endogenous substrate for the EGF receptor in hepatocytes whose phosphorylation may be closely related to EGF stimulation of cell growth.

Insulin-sensitive tyrosine kinase: relationship with in vivo insulin action in humans

To investigate the relationship of insulin receptor kinase with insulin resistance in humans, we studied insulin-sensitive tyrosine kinase activity in muscle biopsies taken from 20 Pima Indians [14 nondiabetics, 6 with non-insulin-dependent mellitus (NIDDM)] during euglycemic clamps, at insulin concentrations of approximately 68 microU/ml (low dose) and approximately 1,170 microU/ml (high dose). In the nondiabetics, the low dose, insulin-induced kinase activation in vivo was 1.5-fold the activity in the fasting state (P less than 0.05), whereas in the diabetics, the kinase activity actually decreased by 40% relative to fasting (P less than 0.05). The difference in delta-kinase in vivo was significant (P less than 0.01) between the two groups. Similarly, the kinase activation in vitro in response to 1 nM insulin was lower in diabetic subjects compared with nondiabetics (P less than 0.01). These data indicate that, in NIDDM, both in vitro and in vivo insulin-stimulated tyrosine kinase activity is impaired. Among nondiabetics, the kinase sensitivity to insulin, calculated as the ratio of the kinase activity at 1 nM insulin in vitro to the kinase activity at 100 nM insulin, was positively correlated with plasma insulin concentrations 2 h after an oral glucose load (r = 0.69, P less than 0.01). Thus, in nondiabetic subjects with insulin resistance, insulin activation of the kinase is not reduced, but the kinase sensitivity to insulin increases with increasing plasma insulin levels. Therefore, the site of insulin resistance in nondiabetic subjects is distal to the insulin receptor kinase. Furthermore, it is possible that circulating insulin, by increasing the kinase sensitivity to insulin, is a determinant of the receptor kinase activity.

Activation of liver and muscle insulin receptor tyrosine kinase activity during in vivo insulin administration in rats

We have studied autophosphorylation and tyrosine kinase activity of the insulin receptor purified from liver and muscle of fasted rats before and after infusion of insulin (100 mU/h) during a 2.5 h glucose clamp. Recovery of insulin receptors and insulin binding to the solubilised receptors was unaffected by the glucose clamp. Autophosphorylation of the insulin receptor beta subunit was increased in liver receptors prepared from rats at the end of the glucose clamp compared to rats in the basal state both in the absence of insulin in vitro (109% increase, p less than 0.001) and after in vitro stimulation with 10(-7) mol/l insulin (clamped vs fasted; 96% increase, p less than 0.001). Insulin (10(-7) mol/l) stimulated autophosphorylation was also increased in muscle receptor preparations from clamped rats compared with rats in the basal state (58% increase, p less than 0.05). In both liver and muscle receptors, the clamp increased the amount of [32P]-phosphate incorporated into the beta subunit without changing the sensitivity of the insulin stimulation. HPLC analysis of the tryptic phosphopeptides derived from the beta subunit after insulin stimulated autophosphorylation of liver receptors revealed an increase of 32P in all phosphorylation sites without any change in the overall pattern. Tyrosine kinase activity of liver and muscle insulin receptors from clamped rats was also increased approximately twofold (p less than 0.05) when analysed using a synthetic substrate (poly Glu4 Tyr1). Our results support the notion that the insulin receptor exists in an active an inactive form, and that elevated plasma insulin concentrations increases the proportion of active receptors.

Intrinsic kinase activity of the insulin receptor

Since the identification of the insulin receptor by insulin-binding activity almost two decades ago, our understanding of the structure and function of the insulin receptor has progressed tremendously. The importance of the intrinsic tyrosine protein kinase activity of the insulin receptor is implied by the fact that the insulin receptor belongs to a family of receptor tyrosine kinases which play a role in growth control, by experiments demonstrating the intimate association of normal kinase activity and insulin action, and by evidence that the intrinsic kinase activity can be regulated under certain conditions. There are still some major gaps in our knowledge concerning the structure/function of the insulin receptor such as how activation of the intrinsic kinase activity of the receptor leads to altered cellular physiology. The kinase may phosphorylate endogenous substrates or autophosphorylation may simply alter beta subunit conformation so it can then interact with an effector system (i.e. a serine kinase) directly, or indirectly through a G-protein. The truth may lie somewhere between these two pathways.

The in vivo phosphorylation sites of bovine alpha B-crystallin

Phosphate content determinations established that in alpha B-crystallin two phosphate groups can be present in vivo in bovine lenses. Comparison of tryptic digests of phosphorylated and unphosphorylated alpha B chains, revealed the location of the two phosphorylation sites in tryptic peptides T2 and T3. Thermolytic digestion and gas-phase sequencing demonstrated that Ser-19 and Ser-45 are the in vivo phosphorylation sites of bovine alpha B-crystallin. This pattern of phosphorylation differs from the previously reported in vitro obtained results.

Identification of phosphorylation sites for adenosine 3',5'-cyclic phosphate dependent protein kinase on the voltage-sensitive sodium channel from Electrophorus electricus

The voltage-sensitive sodium channel from the electroplax of Electrophorus electricus is selectively phosphorylated by the catalytic subunit of cyclic-AMP-dependent protein kinase (protein kinase A) but not by protein kinase C. Under identical limiting conditions, the protein was phosphorylated 20% as rapidly as the synthetic model substrate kemptamide. A maximum of 1.7 +/- 0.6 equiv of phosphate is incorporated per mole. Phosphoamino acid analysis revealed labeled phosphoserine and phosphothreonine at a constant ratio of 3.3:1. Seven distinct phosphopeptides were identified among tryptic fragments prepared from radiolabeled, affinity-purified protein and resolved by HPLC. The three most rapidly labeled fragments were further purified and sequenced. Four phosphorylated amino acids were identified deriving from three consensus phosphorylation sites. These were serine 6, serine 7, and threonine 17 from the amino terminus and a residue within 47 amino acids of the carboxyl terminus, apparently serine 1776. The alpha-subunits of brain sodium channels, like the electroplax protein, are readily phosphorylated by protein kinase A. However, these are also phosphorylated by protein kinase C and exhibit a markedly different pattern of incorporation. Each of three brain alpha-subunits displays an approximately 200 amino acid segment between homologous repeat domains I and II, which is missing from the electroplax and skeletal muscle proteins [Noda et al. (1986) Nature (London) 320, 188; Kayano et al. (1988) FEBS Lett. 228, 1878; Trimmer et al. (1989) Neuron 3, 33]. Most of the phosphorylation of the brain proteins occurs on a cluster of consensus phosphorylation sites located in this segment. This contrasts with the pattern of highly active sites on the amino and carboxyl termini of the electroplax protein. The detection of seven labeled tryptic phosphopeptides compared to the maximal labeling stoichiometry of approximately 2 suggests that many of the acceptor sites on the protein may be blocked by endogenous phosphorylation.

Insulin resistance and diabetes, mechanism and possible intervention

The issue of the peripheral resistance to insulin action has been getting a lot of attention over the last decade. The reason for this is that insulin is a major regulatory hormone and is involved in the metabolism of carbohydrates, lipids, protein and ions. To understand the pathophysiology of insulin resistance it is necessary to elucidate the methods for the assessment of insulin resistance and the molecular mechanism of insulin action. Insulin action is impaired in pathologic and physiologic states such as diabetes mellitus and obesity as well as in some rare syndromes. Further understanding of the pathophysiology of the impaired action of insulin improves the chances of defining new ways of treatment to improve the sensitivity to insulin action.

Quantitative dissociation of glucose transport stimulation and insulin receptor tyrosine kinase activation in isolated adipocytes with a covalent insulin dimer (B29,B29'-suberoyl-insulin)

The covalent insulin dimer B29,B29'-suberoyl-insulin was investigated for its effects on insulin receptor binding, insulin receptor tyrosine kinase activity and glucose transport in isolated adipose cells. The dimer stimulated glucose transport (initial 3-O-methylglucose uptake rate) to the same extent as insulin did (basal rate, 35 +/- 3 pmol/sec/microliter lipid; insulin, 380 +/- 27; B29,B29'-suberoyl-insulin, 369 +/- 24, means +/- S.E.), although at higher concentrations (EC50 1.94 +/- 0.64 nM versus 0.1 +/- 0.02 with insulin). In contrast, the dimer only partially (23%) mimicked insulin's effect on phosphate incorporation into insulin receptors immunoprecipitated after equilibration of cells with [32P]phosphate. Similarly, insulin receptor tyrosine kinase as assessed by receptor autophosphorylation and phosphorylation of the substrate poly-(Glu/Tyr) was not fully activated by treatment of cells with the insulin dimer (31 and 42% of the effect of insulin, respectively) in concentrations which maximally activate glucose transport and give rise to full insulin receptor occupancy (5 X 10(-7) M). Further, the dimer activated the receptor tyrosine kinase in solubilized purified insulin receptor preparations from adipose cells to only 25% of the effect of insulin (EC50 32.0 +/- 16 versus 1.9 +/- 1.0 nM with insulin) in spite of full receptor occupancy. Binding of the dimer to insulin receptors followed single site binding kinetics, indicating that the derivative is unable to induce negative cooperativity of the insulin receptor. It is concluded that a partial phosphorylation of insulin receptors and a submaximal tyrosine kinase activation are sufficient for full stimulation of glucose transport in the adipocyte. Further, it is suggested that negative cooperativity of the insulin receptor and activation of its tyrosine kinase require a similar conformational change of the receptor protein.

Glyburide but not ciglitazone enhances insulin action in the liver independent of insulin receptor kinase activation

To test the hypothesis that sulfonylureas enhance insulin action by activating the insulin receptor tyrosine kinase, the effects of glyburide, a second generation sulfonylurea, and ciglitazone, a nonsulfonylurea hypoglycemic agent, were determined in primary cultures of rat hepatocytes on insulin action and insulin receptor structure and function. Twenty hours of preincubation with glyburide (1 microgram/mL) resulted in increased insulin (1 X 10(-7) mol/L) stimulation of [14C] acetate incorporation into lipids and [14C] alpha-aminoisobutyric acid uptake without any change in basal activity. Ciglitazone (1 microgram/mL) was without any effect. Glyburide's actions were mediated without altering the following: (1) 125I-insulin binding; (2) the electrophoretic mobility of the affinity labeled alpha-subunit or the autophosphorylated beta-subunit of the insulin receptor; and (3) the insulin-stimulated insulin receptor kinase activity using histone or the beta-subunit of the insulin receptor as phosphoacceptors. These data suggest that the action of sulfonylureas is distal to the insulin receptor tyrosine kinase. Ciglitazone in vitro is ineffective in the liver, which suggests the peripheral tissues as the possible site of action.

Influence of insulin on phosphate uptake by brush border membranes from human placenta

Regulation of phosphate transport by insulin was investigated in brush border membranes from human placenta at term. At 22 degrees C, a 45 min incubation of the total tissue with 10(-6) M insulin significantly decreased both the initial rate and the peak of sodium-dependent phosphate uptake by the corresponding brush border membranes. In contrast, Na+ transport was not influenced by the hormone. Increasing the insulin concentration from 0 to 10(-5) M resulted in a dose-dependent inhibition of phosphate uptake with half-maximal effect at 1.1 x 10(-9) M. The hormone decreased PO4 transport by decreasing the affinity of the carrier for the substrate (Km = 0.180 +/- 0.010 mM and 0.215 +/- 0.015 mM in absence and presence of 10(-6) M insulin respectively, P less than 0.05). The inhibitory effect of insulin required the presence of Mn2+ whereas neither Mn2+ nor insulin alone had any influence on PO4 uptake. It is therefore assumed that receptor phosphorylation, which needs the presence of Mn2+, is an intermediate step of insulin action on PO4 uptake by the subsequently isolated brush border membranes. In contrast, insulin had no effect on PO4 uptake when the membranes were directly incubated with the hormone prior to the transport measurement, suggesting that an intracellular messenger is needed for the inhibitory effect. This messenger is not cAMP since insulin at 10(-6) M concentration has no effect on cAMP content of the total placental tissue.(ABSTRACT TRUNCATED AT 250 WORDS)