Insulin receptor kinase following internalization in isolated rat adipocytes

[Involvement of the endosomal compartment in cellular insulin signaling]

The insulin receptor and insulin signaling proteins downstream the receptor reside in different subcellular compartments and undergo redistribution within the cell upon insulin activation. Endocytosis of the insulin-receptor complex, by mediating ligand degradation and receptor dephosphorylation, is generally viewed as a mechanism which attenuates or arrests insulin signal transduction. However, several observations suggest that insulin receptor endocytosis and/or recruitement of insulin signaling proteins to endosomes are also involved in a positive regulation of insulin signaling: (1) upon internalization, the insulin receptor remains transiently phosphorylated and activated; (2) in insulin-stimulated cells or tissues, signaling proteins of the PI3K/Akt and Ras/Raf/Mek/Erk pathways are recruited to endosomes or other intracellular compartments, in which they undergo phosphorylation and/or activation; and (3) depletion or overexpression of proteins involved in the regulation of membrane trafficking and endocytosis interfere with insulin signaling. These observations support a spatial and temporal regulation of insulin signal transduction and reinforce the concept that, as for other membrane signaling receptors, endocytosis and signaling are functionally linked.

Model-based hypothesis testing of key mechanisms in initial phase of insulin signaling

Type 2 diabetes is characterized by insulin resistance of target organs, which is due to impaired insulin signal transduction. The skeleton of signaling mediators that provide for normal insulin action has been established. However, the detailed kinetics, and their mechanistic generation, remain incompletely understood. We measured time-courses in primary human adipocytes for the short-term phosphorylation dynamics of the insulin receptor (IR) and the IR substrate-1 in response to a step increase in insulin concentration. Both proteins exhibited a rapid transient overshoot in tyrosine phosphorylation, reaching maximum within 1 min, followed by an intermediate steady-state level after approximately 10 min. We used model-based hypothesis testing to evaluate three mechanistic explanations for this behavior: (A) phosphorylation and dephosphorylation of IR at the plasma membrane only; (B) the additional possibility for IR endocytosis; (C) the alternative additional possibility of feedback signals to IR from downstream intermediates. We concluded that (A) is not a satisfactory explanation; that (B) may serve as an explanation only if both internalization, dephosphorylation, and subsequent recycling are permitted; and that (C) is acceptable. These mechanistic insights cannot be obtained by mere inspection of the datasets, and they are rejections and thus stronger and more final conclusions than ordinary model predictions.

Insulin is a central player in maintaining energy balance in our bodies and in type 2 diabetes, where the effect of insulin on its target tissues is diminished. Insulin acts on cells by binding to specific insulin receptors (IRs) at the cell surface. This triggers a series of events, including attachment of phosphate to IR, activation of downstream proteins that eventually mediate the signal to specific targets in the cell, and internalization of IR to the inner cytosolic part of the cell. The importance, time relations, and interactions between these events are not fully understood. We have collected experimental time-series and developed a novel analysis method based on mathematical modeling to gain insights into these initial aspects of how insulin controls cells. The main conclusion is that either IR internalization and the subsequent recycling back to the cell surface or feedbacks from downstream proteins (or both) must be significantly active during the first few minutes of insulin action. These conclusions could not have been reached from the experimental data through conventional biological reasoning, and this work thus illustrates the power of modeling to improve our understanding of biological systems.

Interaction between the insulin receptor and Grb14: A dynamic study in living cells using BRET

Grb14 is a molecular adaptor that binds to the activated insulin receptor (IR) and negatively regulates insulin signaling. We have studied the dynamics of interaction of the IR with Grb14, in real time, in living HEK cells, using bioluminescence resonance energy transfer (BRET). Insulin rapidly and dose-dependently stimulated this interaction. Removing insulin from the incubation medium only resulted in a modest decrease in BRET signal, indicating that the interaction between the IR and Grb14 can remain long after insulin stimulus has disappeared. BRET saturation experiments indicated that insulin markedly increases the affinity between IR and Grb14, resulting in recruitment of the adaptor to the activated IR. In addition, using both BRET and co-immunoprecipitation experiments, we demonstrated that insulin induced the dimerization of Grb14, most likely as a result of simultaneous binding of two Grb14 molecules on the activated IR. We also investigated the relationships between IR, Grb14 and the protein tyrosine phosphatase PTP1B. We observed that insulin-induced BRET between the IR and PTP1B was markedly reduced by Grb14, suggesting that Grb14 regulated this interaction in living cells. Using site-specific antibodies against phosphorylated tyrosines of the insulin receptor, we showed that Grb14 protected the three tyrosines of the kinase loop from dephosphorylation by PTP1B, while favouring dephosphorylation of tyrosine 972. This resulted in decreased IRS-1 binding to the IR and decreased activation of the ERK pathway. Our work suggests that Grb14 may regulate signalling through the insulin receptor by controlling its tyrosine-dephosphorylation in a site-specific manner.

Dynamics of the interaction between the insulin receptor and protein tyrosine-phosphatase 1B in living cells

The dynamics of the interaction of the insulin receptor with a substrate-trapping mutant of protein-tyrosine phosphatase 1B (PTP1B) were monitored in living human embryonic kidney cells using bioluminescence resonance energy transfer (BRET). Insulin dose-dependently stimulates this interaction, which could be followed in real time for more than 30 minutes. The effect of insulin on the BRET signal could be detected at early time-points (30 seconds), suggesting that in intact cells the tyrosine-kinase activity of the insulin receptor is tightly controlled by PTP1B. Interestingly, the basal (insulin-independent) interaction of the insulin receptor with PTP1B was much weaker with a soluble form of the tyrosine-phosphatase than with the endoplasmic reticulum (ER)-targeted form. Inhibition of insulin-receptor processing using tunicamycin suggests that the basal interaction occurs during insulin-receptor biosynthesis in the ER. Therefore, localization of PTP1B in this compartment might be important for the regulation of insulin receptors during their biosynthesis.

Dynamics of protein-tyrosine phosphatases in rat adipocytes

Protein-tyrosine phosphatases (PTPases) play a key role in maintaining the steady-state tyrosine phosphorylation of the insulin receptor (IR) and its substrate proteins such as insulin receptor substrate 1 (IRS-1). However, the PTPase(s) that inactivate IR and IRS-1 under physiological conditions remain unidentified. Here, we analyze the subcellular distribution in rat adipocytes of several PTPases thought to be involved in the counterregulation of insulin signaling. We found that the transmembrane enzymes, protein-tyrosine phosphatase (PTP)-alpha and leukocyte common antigen-related (LAR), were detected predominantly in the plasma membrane and to a lesser extent in the heavy microsomes, a distribution similar to that of insulin receptor. PTP-1B and IRS-1 were present in light microsomes and cytosol, whereas SHPTP2/Syp was exclusively cytosolic. Insulin induced a redistribution of PTP-alpha from the plasma membrane to the heavy microsomes in a parallel fashion with the receptor. The distribution of PTP-1B in the light microsomes from resting adipocytes was similar to that of IRS-1 as determined by sucrose velocity gradient fractionation. Analysis of the catalytic activity of partially purified rat adipocyte PTP-alpha and LAR and recombinant PTP-1B showed that all three PTPases dephosphorylate IR. When a mix of IR/IRS-1 was used as a substrate, PTP-1B was particularly effective in dephosphorylating IRS-1. Considering that IR and IRS-1 can be dephosphorylated in internal membrane compartments from rat adipocytes (Kublaoui, B., Lee, J., and Pilch, P.F. (1995) J. Biol. Chem. 270, 59-65) and that PTP-alpha and PTP-1B are the respective PTPases in these fractions, we conclude that these PTPases are responsible for the counterregulation of insulin signaling there, whereas both LAR and PTP-alpha may act upon cell surface insulin receptors.

Synergistic role of the phosphatidylinositol 3-kinase and mitogen-activated protein kinase cascade in the regulation of insulin receptor trafficking

To examine the molecular mechanism of insulin receptor trafficking, we investigated the intracellular signaling molecules that regulate this process in Rat1 fibroblasts overexpressing insulin receptors. Cellular localization of insulin receptors was assessed by confocal laser microscopy with indirect immunofluorescence staining. Insulin receptors were visualized diffusely in the basal state. Insulin treatment induced the change of insulin receptor localization to perinuclear compartment. This insulin-induced insulin receptor trafficking was not affected by treatment of the cells with PI3-kinase inhibitor (wortmannin), whereas treatment with MEK [mitogen-activated protein (MAP) kinase-Erk kinase] inhibitor (PD98059) partly inhibited the process in a dose-dependent manner. Interestingly, treatment with both wortmannin and PD98059 almost completely inhibited insulin receptor trafficking. The functional importance of PI3-kinase and MAP kinase in the trafficking process was directly assessed by using single cell microinjection analysis. Microinjection of p85-SH2 and/or catalytically inactive MAP kinase ([K71A]Erk1) GST fusion protein gave the same results as treatment with wortmannin and PD98059. Furthermore, to determine the crucial step for the requirement of PI3-kinase and MAP kinase pathways, the effect of wortmannin and PD98059 on insulin receptor endocytosis was studied. Insulin internalization from the plasma membrane and subsequent insulin degradation were not affected by treatment with wortmannin and PD98059. In contrast, insulin receptor down-regulation from the cell surface and insulin receptor degradation, after prolonged incubation with insulin, were markedly impaired by the treatment. These results suggest that PI3-kinase and MAP kinase pathways synergistically regulate insulin receptor trafficking at a step subsequent to the receptor internalization.

Chloroquine extends the lifetime of the activated insulin receptor complex in endosomes

Insulin signal transduction, initiated by binding of insulin to its receptor at the plasma membrane, activates the intrinsic receptor tyrosine kinase and leads to internalization of the activated ligand-receptor complex into endosomes. This study addresses the role played by the activated insulin receptor within hepatic endosomes and provides evidence for its central role in insulin-stimulated events in vivo. Rats were treated with chloroquine, an acidotrophic agent that has been shown previously to inhibit endosomal insulin degradation, and then with insulin. Livers were removed and fractionated by density gradient centrifugation to obtain endosomal and plasma membrane preparations. Chloroquine treatment increased the amount of receptor-bound insulin in endosomes at 2 min after insulin injection by 93% as determined by exclusion from G-50 columns and by 90% as determined by polyethylene glycol precipitation (p < 0.02). Chloroquine treatment also increased the insulin receptor content of endosomes after insulin injection (integrated over 0-45 min) by 31% when compared with controls (p < 0.05). Similarly, chloroquine increased both insulin receptor phosphotyrosine content and its exogenous tyrosine kinase activity after insulin injection (64%; p < 0.01 and 96% and p < 0. 001, respectively). In vivo chloroquine treatment was without any observable effect on insulin binding to plasma membrane insulin receptors, nor did it augment insulin-stimulated receptor autophosphorylation or kinase activity in the plasma membrane. Concomitant with its effects on endosomal insulin receptors, chloroquine treatment augmented insulin-stimulated incorporation of glucose into glycogen in diaphragm (p < 0.001). These observations are consistent with the hypothesis that chloroquine-dependent inhibition of endosomal insulin receptor dissociation and subsequent degradation prolongs the half-life of the active endosomal receptor and potentiates insulin signaling from this compartment.

Insulin stimulation of human adipocytes activates the kinase of only a fraction of the insulin receptors

The degree of insulin receptor kinase activation by in situ stimulation was studied in isolated human adipocytes. Although maximal in situ stimulation increased the kinase activity approximately 10-fold, this activity could again be doubled by subsequent activation in a cell-free system. To investigate how in situ stimulation resulted in incomplete activation, receptors binding or not binding to anti-phosphotyrosine antibody (alpha-PY) were studied separately. Even after maximal insulin stimulation of the cells, approximately 50% of the receptors did not bind to alpha-PY and had low kinase activity. In the cell-free system, however, these receptors reached activity levels similar to the other receptors, suggesting that they were intact and that factors in their cellular environment had prevented their activation. The activity of the alpha-PY-binding receptors could only be slightly increased in the cell-free system, suggesting that almost complete activation had been attained in situ. In situ stimulation with increasing insulin concentrations increased the number of activated receptors rather than their individual activity. We conclude that factors in the in situ environment prevent insulin activation of approximately 50% of the insulin receptors in human adipocytes and might therefore be important regulators of insulin signaling.

Insulin internalization and other signaling pathways in the pleiotropic effects of insulin

Insulin is the major anabolic hormone in humans and affects multiple cellular processes. Insulin rapidly regulates short-term effects on carbohydrate, lipid, and protein metabolism and is also a potent growth factor controlling cell proliferation and differentiation. The metabolic and growth-related effects require insulin binding to its receptor and receptor phosphorylation. Evidence suggests these events result in subsequent substrate phosphorylation and activation of multiple signaling pathways involving Src homology domain-containing proteins and the internalization of the insulin:receptor complex. The role of insulin internalization in insulin action is largely speculative. For more than two decades, extensive investigation has been carried out by numerous laboratories of the mechanisms by which insulin causes its pleiotropic responses and the cellular processing of insulin receptors. This chapter reviews our current knowledge of the phosphorylation signaling pathways activated by insulin and presents evidence that substrates other than insulin receptor substrate-1 are involved in insulin's regulation of immediate-early gene expression. We also review the mechanisms involved in insulin internalization and present evidence that internalization may play a key role in insulin action through both signal transduction processes and translocation of insulin to the cell cytoplasm and nucleus.

Intracellular signal transduction: The role of endosomes

Polypeptide hormones, growth factors, and other biologically significant molecules are specifically internalized by target cells. Exposure of cells to these ligands results in the formation of ligand-receptor complexes on the cell surface and subsequent internalization of these complexes into the endosomal apparatus (endosomes, or ENs). The study of ENs has identified several important functions for this unique cellular organelle. These include the dissociation of ligand from receptor and receptor recycling to the cell surface and the degradation of some internalized ligands, as well as the delivery of others to lysosomes. More recently, it has become apparent that ENs fulfill another critical role, that of signal transduction. In this article, we review the evidence substantiating this role for ENs and propose three models by which ENs participate in cell signaling.

The B isoform of the insulin receptor signals more efficiently than the A isoform in HepG2 cells

We have demonstrated previously that dexamethasone treatment of HepG2 cells caused an enhancement of insulin's metabolic effects (Kosaki, A., and Webster, N. J. (1993) J. Biol. Chem. 268, 21990-21996). This correlated with increased expression of the mRNA encoding the B isoform of the insulin receptor (IR). In the present study, we have demonstrated that dexamethasone treatment caused in addition an enhancement in insulin-stimulated immediate-early gene expression (c-fos and egr-1). Dexamethasone treatment caused an increase in in vivo IR autophosphorylation and insulin receptor substrate-1 (IRS-1) phosphorylation both early events in the insulin signaling pathway. Furthermore, the IRS-1 phosphorylation was distinctly left shifted, although the level of IRS-1 protein was unchanged. Total cellular tyrosine phosphatase activity was unaltered when assayed with 32P-labeled IR and IRS-1. Studies in vitro on wheat-germ agglutinin-purified receptors showed that the B isoform of the IR had increased kinase activity both toward itself and the exogenous substrates poly.glu4:tyr1 and recombinant IRS-1 protein. In addition, two-dimensional tryptic phosphopeptide maps indicated that the B isoform has an additional phosphopeptide that is not seen for the A isoform. In conclusion, it appears that the B isoform of the IR signals more efficiently than the A isoform in HepG2 cells.

Binding of human, porcine and bovine insulin to insulin receptors from human brain, muscle and adipocytes and to expressed recombinant alternatively spliced insulin receptor isoforms

Previous studies have suggested that human and porcine insulin exert identical effects on blood glucose and counter-regulatory hormones but elicit different neurophysiological reactions. A major goal of the present study was to investigate whether this could be caused by different relative affinities of the insulins from different species to insulin receptors from the brain compared to other tissues. Insulin receptors isolated from human brain, muscle or adipocytes as well as from cultured cells over-expressing either of the human insulin receptor isoforms (exon 11- or exon 11 +) were immobilized to microwells coated with monoclonal anti-insulin receptor antibody. Subsequently the binding of human, porcine and bovine insulin was measured. While the receptors derived from the different tissues had different affinities for insulin, there were no tissue-specific differences in the relative binding of the insulins of the three species. The insulins of the three species were also not different with regard to their binding to the receptor isoforms. Finally, in human brain homogenates no differences in the degradation rates for human, porcine and bovine insulin were detected. Thus, our data do not support the hypothesis that different neurophysiological reactions during hypoglycaemia due to human or porcine insulin are caused by differences of the binding of the insulins to human brain insulin receptors or their degradation in the human brain.

Dynamics of signaling during insulin-stimulated endocytosis of its receptor in adipocytes

Insulin causes rapid insulin receptor autophosphorylation, receptor endocytosis, and phosphorylation of its principle substrate (IRS-1). Using rat adipocytes, we studied the dynamics of receptor autophosphorylation, the kinase activity, and the IRS-1 phosphorylation state relative to the subcellular localization of these proteins. After 2 min of insulin exposure, the specific phosphotyrosine content of the insulin receptor in the internal membranes (IM) peaks at a level 5-6-fold higher than the plasma membrane (PM) receptor and then declines after 5-8 min to a level similar to the PM receptor. The exogenous kinase activity of these receptors exactly mirrored their phosphotyrosine content. The distribution of IRS-1 is 80% cytosolic, 20% IM-associated, and essentially undetectable in the PM. The phosphorylation state of IRS-1 in the IM parallels that of the insulin receptor, but cytosolic IRS-1 phosphorylation remains constant. Insulin-dependent GLUT4 translocation to the PM occurs after the peak of IRS-1 phosphorylation. The data are consistent with the hypothesis that insulin action may be mediated by receptor internalization and interaction with its substrate(s) associated with internal membranes. A small fraction of phosphorylated insulin receptors is sufficient for signal transduction. The dephosphorylation of the insulin receptor and IRS-1 in the IM appears to be a concerted process, possibly mediated by the same enzyme.

Functional properties of a heterozygous mutation (Arg1174-->Gln) in the tyrosine kinase domain of the insulin receptor from a type A insulin resistant patient

We analysed the biochemical properties of insulin receptors of a Type A insulin resistant patient with a single heterozygous point mutation substituting Gln for Arg1174. Insulin binding capacity and affinity to Epstein-Barr virus transformed lymphocytes was normal. Quantitative analysis of autophosphorylation and substrate phosphorylation of soluble insulin receptors isolated from patient cells revealed no differences in the basal state whereas in the presence of insulin autophosphorylation activity was only 30% of control receptors. The stimulation of substrate phosphorylation and down-regulation of receptors on patient cells after chronic exposure to insulin was diminished when compared to controls. We conclude that the heterozygous Arg1174 mutation does not perturb basal kinase activity but specifically interferes with the kinase activation by insulin and that the mutation has a dominant negative effect on the wild type kinase.

Characterization of the hepatic insulin receptor undergoing internalization through clathrin-coated vesicles and endosomes

Administration of insulin to rats caused a transient increase in the amount of hepatic insulin receptor present in clathrin-coated vesicles and endosomes. However, the total 'in vitro' insulin stimulated tyrosine kinase activity of the receptor present in endosomes did not vary when expressed per mg of protein and decreased when expressed per beta-subunit content. A decrease in the endogenous phospho tyrosine content of the receptor beta-subunit was observed in endosomes in response to insulin. This indicates that a fraction of the internalized receptor is dephosphorylated in endosomes, which renders it unable to become stimulated by insulin 'in vitro'.

A 180,000 molecular weight glycoprotein substrate of the insulin receptor tyrosine kinase is present in human placenta and in rat liver, muscle, heart and brain plasma membrane preparations

Cell signalling for insulin may include insulin receptor tyrosine kinase catalysing the phosphorylation of one or more cell proteins. Since temporally the insulin receptor will encounter plasma membrane proteins first, we have studied the in vitro phosphorylation of purified plasma membrane preparations. Two proteins were immunoprecipitated with anti-phosphotyrosine antibody from rat liver, muscle, heart and brain membranes and from human placenta membranes: the insulin receptor (detected as a phosphorylated-beta-subunit) and a 180,000 molecular weight protein (pp180). pp180 is a monomeric glycoprotein that in the absence of dithiothreitol migrated in denaturing gels like a 150,000 molecular weight protein. pp180 was a substrate for the insulin receptor: (i) receptor and pp180 phosphorylation followed a similar insulin dose-response, although fold-stimulation of autophosphorylation was greater; and (ii) removal of insulin receptors with monoclonal antibodies prevented subsequent pp180 phosphorylation. Insulin-activated receptors increased the extent, but not the rate, of pp180 phosphorylation; the increased phosphate was incorporated into tyrosine and appeared to do so in three or four of pp180's 12 tryptic phosphopeptides. Some data suggest that pp180 is the same protein in each of the tested tissues. The occurrence of pp180, an insulin receptor substrate, in plasma membranes of several insulin responsive tissues suggests that it has a role in insulin signalling.

Insulin stimulates the tyrosine phosphorylation of a 165 kDa protein that is associated with microsomal membranes of rat adipocytes

The beta-subunit of the insulin receptor possesses an insulin-stimulatable protein tyrosine kinase activity. It has been widely postulated that this activity may mediate the transduction of the insulin signal by phosphorylation of cellular substrates involved in the mechanism of insulin action. We have identified, by immunoblotting with antiphosphotyrosine antibodies, a 165 kDa protein in rat adipocytes that is rapidly phosphorylated in response to insulin. Phosphorylation of this protein (pp165) occurs within 5-10 s of exposure to 10 nM insulin, suggesting that it may be a direct substrate for the insulin receptor. This protein was recovered in an intracellular membrane that fractionates with the low-density microsomes. Using discontinuous sucrose density-gradient centrifugation, pp165-containing vesicles were separated from other vesicles of the low-density microsomes including the glucose transporter-containing vesicles, indicating that pp165 is probably not a regulatory component of the vesicles that translocate glucose transporters in response to insulin. However, pp165 may be involved in conveying receptor activation at the cell surface to an intracellular site of insulin action.

Effects of hypercortisolemia and diabetes on skeletal muscle insulin receptor function in vitro and in vivo

Activation of skeletal muscle insulin receptor tyrosine kinase in vitro and in vivo was studied in two rat models of insulin resistance: insulinopenic diabetes and hypercortisolemia. In control rats, intravenous insulin administration resulted in dose-dependent in vivo activation of the muscle insulin receptor kinase towards histone H2b. Half-maximal and maximal activation were observed 5 min after injecting 0.1 and 0.5 U insulin/100 g, respectively. Diabetes (7 days) induced with streptozotocin did not affect insulin binding affinity of solubilized muscle receptors but depressed receptor kinase activation in vivo by 52 or 40% after intravenous insulin administration (0.1 or 2 U/100 g, respectively). Cortisone treatment (5 days) resulting in weight loss, hyperglycemia, and hyperinsulinemia did not affect the number, insulin binding affinity, or kinase activity of solubilized receptors activated with insulin in vitro or in vivo. It is concluded that impaired insulin receptor tyrosine kinase activation was demonstrated in vivo in rats with insulinopenic diabetes and that glucocorticoid-induced insulin resistance probably reflects postreceptor defect(s) in muscle.