Mutation of tyrosine residues 1162 and 1163 of the insulin receptor affects hormone and receptor internalization

Sodium tungstate activates glycogen synthesis through a non-canonical mechanism involving G-proteins

Tungstate treatment ameliorates experimental diabetes by increasing liver glycogen deposition through an as yet unidentified mechanism. The signalling mechanism of tungstate was studied in CHOIR cells and primary cultured hepatocytes. This compound exerted its pro-glycogenic effects through a new G-protein-dependent and Tyr-Kinase Receptor-independent mechanism. Chemical or genetic disruption of G-protein signalling prevented the activation of the Ras/ERK cascade and the downstream induction of glycogen synthesis caused by tungstate. Thus, these findings unveil a novel non-canonical signalling pathway that leads to the activation of glycogen synthesis and that could be exploited as an approach to treat diabetes.

Differential endocytosis and signaling dynamics of insulin receptor variants IR-A and IR-B

Insulin signaling comprises a complex cascade of events, playing a key role in the regulation of glucose metabolism and cellular growth. Impaired response to insulin is the hallmark of diabetes, whereas upregulated insulin activity occurs in many cancers. Two splice variants of the insulin receptor (IR) exist in mammals: IR-A, lacking exon 11, and full-length IR-B. Although considerable biochemical data exist on insulin binding and downstream signaling, little is known about the dynamics of the IR itself. We created functional IR transgenes fused with visible fluorescent proteins for use in combination with biotinamido-caproyl insulin and streptavidin quantum dots. Using confocal and structured illumination microscopy, we visualized the endocytosis of both isoforms in living and fixed cells and demonstrated a higher rate of endocytosis of IR-A than IR-B. These differences correlated with higher and sustained activation of IR-A in response to insulin and with distinctive ERK1/2 activation profiles and gene transcription regulation. In addition, cells expressing IR-B showed higher AKT phosphorylation after insulin stimulation than cells expressing IR-A. Taken together, these results suggest that IR signaling is dependent on localization; internalized IRs regulate mitogenic activity, whereas metabolic balance signaling occurs at the cell membrane.

The antidiabetic agent sodium tungstate activates glycogen synthesis through an insulin receptor-independent pathway

Sodium tungstate is a powerful antidiabetic agent when administered orally. In primary cultured hepatocytes, tungstate showed insulin-like actions, which led to an increase in glycogen synthesis and accumulation. However, this compound did not significantly alter the insulin receptor activation state or dephosphorylation rate in cultured cells (CHO-R) or in primary hepatocytes, in either short or long term treatments. In contrast, at low concentrations, tungstate induced a transient strong activation of extracellular signal-regulated kinases 1 and 2 (ERK1/2) after 5-10 min of treatment, in a similar way to insulin. Moreover, this compound did not significantly delay or inhibit the dephosphorylation of ERK1/2. ERK1/2 activation triggered a cascade of downstream events, which included the phosphorylation of p90rsk and glycogen synthase-kinase 3beta. Experiments with a specific inhibitor of ERK1/2 activation and kinase assays indicate that these proteins were directly involved in the stimulation of glycogen synthase and glycogen synthesis induced by tungstate without a direct involvement of protein kinase B (PKB/Akt). These results show a direct involvement of ERK1/2 in the mechanism of action of tungstate at the hepatic level.

Insulin receptor tyrosine kinase activity and phosphorylation of tyrosines 1162 and 1163 are required for insulin-increased prolactin gene expression

Insulin treatment increased prolactin gene expression in GH4 cells, a rat pituitary tumor cell line, through the endogenous insulin receptor. However, insulin regulation of transfected plasmids required the expression of cotransfected insulin receptor. Prolactin-CAT expression was increased 12-fold in cells transfected with wild type insulin receptor, but insulin did not increase prolactin gene expression when a kinase negative mutant of the ATP binding site (K1030R) was expressed. Thus, receptor kinase activity was required for signaling to gene transcription. Mutation of tyrosine 1158 did not reduce insulin-increased prolactin-CAT expression while individual mutations of tyrosine 1162 and tyrosine 1163 each reduced insulin-increased prolactin-CAT expression by 50% and a triple mutant of tyrosines 1158/1162/1163 was inactive. Thus, mutation of tyrosine 1162 and 1163 was also sufficient to inactivate signaling by the insulin receptor. Insulin-stimulated auto phosphorylation occurred in all mutants in vitro except the ATP binding site mutant. However, the ability of mutant insulin receptors to mediate insulin-increased prolactin-CAT expression correlated with the substrate-specific catalytic activity of the receptors. This suggested that phosphorylation of these tyrosines was important for substrate access to the catalytic domain of the receptor.

Combined use of insulin and endothelin-1 causes decrease of protein expression of beta-subunit of insulin receptor, insulin receptor substrate-1, and insulin-stimulated glucose uptake in rat adipocytes

Previously, we reported that insulin-stimulated glucose uptake (ISGU) can be inhibited by endothelin (ET-1). However, the mechanism by which ET-1 impairs ISGU in adipocytes remains unclear. This study investigated the effects of ET-1 on insulin action in rat adipocytes in order to elucidate the molecular mechanism of action of ET-1 on ISGU. The results show that ISGU was increased fivefold after 3-h treatment with 1 nM insulin. Treatment with 100 nM ET-1 had no effect on basal glucose uptake. However, ET-1 inhibited approximately 25% of ISGU and 20% of insulin binding after 3-h treatment in the presence of 1 nM insulin. Expression of the beta-subunit of the insulin receptor (IRbeta) and the insulin receptor substrate-1 (IRS-1) in adipocytes was not significantly affected by 1 nM insulin or by 100 nM ET-1, even after 3-h treatment. However, expressions of IRbeta and IRS-1 were dramatically decreased in a dose- and time-dependent manner when adipocytes were treated with both insulin and ET-1. Approximately 50% of IRbeta and 65% of IRS-1 expression levels were suppressed when adipocytes were simultaneously treated with both 1 nM insulin and 100 nM ET-1 for 3 h. These results suggest that the inhibitory effect of ET-1 on ISGU may be mediated via the insulin receptor and suppression of IRbeta/IRS-1 expression.

Insulin induction of protein kinase C alpha expression is independent of insulin receptor Tyr1162/1163 residues and involves mitogen-activated protein kinase kinase 1 and sustained activation of nuclear p44MAPK

We examined the effect of insulin on protein kinase C alpha (PKCalpha) expression and the implication of the mitogen-activated protein kinase kinase 1 mitogen-activated protein kinase (MAPK) pathway in this effect. PKCalpha expression was measured by quantitative RT-PCR and Western blotting using Chinese hamster ovary (CHO) cells overexpressing human insulin receptors of the wild type (CHO-R) or insulin receptors mutated at Tyr1162/1163 autophosphorylation sites (CHO-Y2). In CHO-R cells, insulin caused a time- and concentration-dependent increase in PKCalpha messenger RNA, with a maximum at 6 h and 10-(8)M insulin. This increase involved a transcriptional mechanism, as it was not due to stabilization of PKCalpha messenger RNA and was associated with a similar increase in the immunoreactive PKCalpha level. Insulin induction of PKCalpha expression involved the MEK1MAPK pathway, as it was 1) almost completely suppressed by the potent MEK1 inhibitor PD98059, 2) mimicked by the dominant-active MEK1 (S218D/S222D) mutant, and 3) associated with sustained MAPK activation. In CHO-Y2 cells in which the early phase of MAPK activation by insulin was lost and only the late and sustained phase of activation was observed, insulin signaling of PKCalpha expression was preserved and again involved the MEK1-MAPK pathway. Moreover, we show that in both CHO-R and CHO-Y2 cells, insulin stimulation of PKCalpha gene expression was associated with prolonged activation of nuclear p44MAPK. These results indicate that induction of PKCalpha gene expression by insulin is independent of Tyr1162/1163 autophosphorylation sites and correlates with sustained activation of p44MAPK at the nuclear level.

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.

Biological effects of encapsulated insulin on transfected Chinese hamster ovary cells

Oral administration of insulin incorporated into the wall of isobutylcyanoacrylate nanocapsule to diabetic rats induces a long-lasting normalization of their fasting glycaemia. In this study, we examined the biological action of encapsulated insulin on DNA and glycogen syntheses in Chinese hamster ovary cells transfected with the human insulin receptor gene. In the 10(-11) mol/l-10(-9) mol/l concentration range, encapsulated insulin elicited responses comparable to those induced by native insulin: at 10(-9) mol/l, the rates of glycogen and DNA synthesis were enhanced by factors 3 and 2.5, respectively. Encapsulated insulin at 10(-7) mol/l evoked receptor desensitization although it did not induce receptor down-regulation and did not alter receptor recycling for up to 6 h. Chloroquine decreased the action of native insulin on glycogen synthesis, but did not affect the dose-response characteristics of encapsulated insulin. Acid-washing of the cells after 1 h of stimulation decreased maximal insulin responsiveness and provoked a dose response curve for encapsulated insulin similar to that of the native hormone. Direct measurement of effective insulin binding activity showed that encapsulated insulin (at 10(-8) and 10(-7) mol/l) was withdrawn from the incubation medium 5-8 times less efficiently than native insulin. These data are in agreement with previous results showing that the polymeric wall protects encapsulated insulin from degradation. Persistence of intact encapsulated insulin inside and outside the cell may result in modifying signalling events and thus be responsible for the observed cellular desensitization.

Insulin receptor mutation at tyrosines 1162 and 1163 alters both receptor serine phosphorylation and desensitization

Chinese hamster ovary (CHO) cells expressing human insulin receptor (hIR) of the wild-type (CHO R) or hIR mutated at tyrosines 1162 and 1163 (CHO Y2) were compared for agonist-induced receptor phosphorylation of serine/threonine residues and receptor desensitization. Relative to CHO R cells, CHO Y2 cells exhibited a marked decrease in their response to insulin and 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) for hIR phosphorylation on serine residues. Moreover, the tyr1162,1163 mutant hIR could not be normally phosphorylated by purified protein kinase C (PKC) in vitro. Finally, in contrast to CHO R cells, CHO Y2 cells were refractory to PMA-induced IR desensitization for subsequent activation by insulin of exogenous tyrosine kinase and glycogen synthesis. These results strongly suggest that the replacement of tyrosines 1162 and 1163 by phenylalanine residues changes the IR beta-subunit conformation and thus impedes phosphorylation of the IR at crucial serine residues and prevents PMA-induced desensitization. This supports the hypothesis that IR serine phosphorylation and desensitization are related.

Association of the transmembrane TGF-alpha precursor with a protein kinase complex

A variety of growth factors including transforming growth factor-alpha (TGF-alpha) are synthesized as transmembrane precursors. The short cytoplasmic domain of the transmembrane TGF-alpha precursor lacks any apparent motif associated with signal transduction. However, the sequence conservation of this cytoplasmic domain and its abundance of cysteine residues, reminiscent of the cytoplasmic domains of CD4 and CD8, suggest a biological function. In this study, we showed that transmembrane TGF-alpha was rapidly internalized after interaction with a specific antibody and that this internalization was greatly decreased when the COOH-terminal 31 amino acids were removed. Chemical cross-linking experiments revealed two associated proteins of 86 and 106 kD which coimmunoprecipitated with the TGF-alpha precursor. The association of p86 was dependent on the presence of the COOH-terminal cytoplasmic 31 amino acids of the TGF-alpha precursor, whereas p106 still remained associated when this segment was deleted. In addition, p106 was tyrosine-phosphorylated and exposed on the cell surface. The protein complex associated with transmembrane TGF-alpha displayed kinase activities towards tyrosine, serine, and threonine residues. These activities were not associated with transmembrane TGF-alpha when the COOH-terminal segment was truncated. The association of a protein kinase complex with transmembrane TGF-alpha may provide the basic elements for a "reverse" mode of signaling through the cytoplasmic domain of this growth factor, which may lead to two-directional communication during ligand-receptor interaction.

Insulin receptor autophosphorylation sites tyrosines 1162 and 1163 control both insulin-dependent and insulin-independent receptor internalization pathways

We previously reported that Chinese hamster ovary (CHO) cell lines overexpressing mutated human insulin receptors (hIRs) in which the tyrosine residues 1162 and 1163 were replaced by phenylalanines (CHO-Y2) exhibited a marked defect in hormone-induced receptor internalization as compared to CHO transfectants overexpressing wild-type hIRs (CHO-R). These two cell lines are now used to compare the role of tyrosines 1162-1163 in basal and ligand-stimulated receptor internalization as well as in receptor turnover. We show here that (1) in CHO-Y2 cells, basal endocytosis, like insulin-induced internalization, was markedly altered despite normal receptor turnover and (2) in both CHO-R and CHO-Y2 cells, basal receptor endocytosis was altered by tunicamycin, an inhibitor of protein N-glycosylation, whereas insulin-induced internalization was not. These results support a role for tyrosines 1162-1163 of the IR beta-subunit major autophosphorylation domain in both basal and ligand-stimulated receptor endocytosis and provide evidence that the two processes follow distinct pathways.

LH-RH analogue carrying a cytotoxic radical is internalized by rat pituitary cells in vitro

The binding and internalization of a cytotoxic analogue of luteinizing hormone-releasing hormone (LH-RH), T-98 (agonist [D-Lys6]LH-RH linked to glutaryl-2-(hydroxymethyl)anthraquinone), by rat anterior pituitary cells was investigated. Analogue T-98 was bound to pituitary membrane binding sites for LH-RH with a high affinity (Kd = 1.2 nM) and was 17 times more potent in releasing luteinizing hormone (LH) from superfused rat pituitary cells than LH-RH. The labeling of this cytotoxic LH-RH analogue was carried out both with radioactive (125I) and nonradioactive iodine. Monoiodination of the Tyr5 residue of T-98 did not significantly affect its binding affinity but greatly decreased its LH-releasing activity to about 3% of the original value. Di-iodination in the same position lowered binding affinity twenty-threefold and further diminished LH-releasing potency. [125I]T-98 was found to bind very strongly to polystyrene, which precluded the use of regular tissue culture plasticware in our experiments. In pituitary cells cultured in glass vials, binding and internalization of [125I]T-98 were observed, which were time and temperature dependent, and which could be inhibited by excess unlabeled analogue. No enzymatic degradation of labeled T-98 was detected in the culture medium during the incubation. Our results indicate that T-98 is internalized by pituitary gonadotropes through receptor-mediated endocytosis. Because this new class of compounds was designed as anticancer drugs, our findings also suggest that this cytotoxic LH-RH agonist may also be internalized by LH-RH receptors present in breast, prostate, ovarian, and other tumors.

Two steps of insulin receptor internalization depend on different domains of the beta-subunit

The internalization of signaling receptors such as the insulin receptor is a complex, multi-step process. The aim of the present work was to determine the various steps in internalization of the insulin receptor and to establish which receptor domains are implicated in each of these by the use of receptors possessing in vitro mutations. We find that kinase activation and autophosphorylation of all three regulatory tyrosines 1146, 1150, and 1151, but not tyrosines 1316 and 1322 in the COOH-terminal domain, are required for the ligand-specific stage of the internalization process; i.e., the surface redistribution of the receptor from microvilli where initial binding occurs to the nonvillous domain of the cell. Early intracellular steps in insulin signal transduction involving the activation of phosphatidylinositol 3'-kinase are not required for this redistribution. The second step of internalization consists in the anchoring of the receptors in clathrin-coated pits. In contrast to the first ligand specific step, this step is common to many receptors including those for transport proteins and occurs in the absence of kinase activation and receptor autophosphorylation, but requires a juxta-membrane cytoplasmic segment of the beta-subunit of the receptor including a NPXY sequence. Thus, there are two independent mechanisms controlling insulin receptor internalization which depend on different domains of the beta-subunit.

Insulin-induced surface redistribution regulates internalization of the insulin receptor and requires its autophosphorylation

The role of insulin-induced receptor autophosphorylation in its internalization was analyzed by comparing 125I-labeled insulin (125I-insulin) internalization in Chinese hamster ovary (CHO) cell lines transfected with normal (CHO.T) or mutated insulin receptors. In four cell lines with a defect of insulin-induced autophosphorylation, 125I-insulin internalization was impaired. By contrast, in CHO.T cells and in two other CHO cell lines with amino acid deletions or insertions that do not perturb autophosphorylation, 125I-insulin internalization was not affected. A morphological analysis showed that the inhibition is linked to the ligand-specific surface redistribution in which the insulin-receptor complexes leave microvilli and concentrate on nonvillous segments of the membrane where endocytosis occurs.