Receptor aggregation is necessary for activation of the soluble insulin receptor kinase

Anti-Insulin Receptor Antibodies Improve Hyperglycemia in a Mouse Model of Human Insulin Receptoropathy

Loss-of-function mutations in both alleles of the human insulin receptor gene (INSR) cause extreme insulin resistance (IR) and usually death in childhood, with few effective therapeutic options. Bivalent antireceptor antibodies can elicit insulin-like signaling by mutant INSR in cultured cells, but whether this translates into meaningful metabolic benefits in vivo, wherein the dynamics of insulin signaling and receptor recycling are more complex, is unknown. To address this, we adopted a strategy to model human insulin receptoropathy in mice, using Cre recombinase delivered by adeno-associated virus to knockout endogenous hepatic Insr acutely in floxed Insr mice (liver insulin receptor knockout [L-IRKO] + GFP), before adenovirus-mediated add back of wild-type (WT) or mutant human INSR Two murine anti-INSR monoclonal antibodies, previously shown to be surrogate agonists for mutant INSR, were then tested by intraperitoneal injections. As expected, L-IRKO + GFP mice showed glucose intolerance and severe hyperinsulinemia. This was fully corrected by add back of WT but not with either D734A or S350L mutant INSR. Antibody injection improved glucose tolerance in D734A INSR-expressing mice and reduced hyperinsulinemia in both S350L and D734A INSR-expressing animals. It did not cause hypoglycemia in WT INSR-expressing mice. Antibody treatment also downregulated both WT and mutant INSR protein, attenuating its beneficial metabolic effects. Anti-INSR antibodies thus improve IR in an acute model of insulin receptoropathy, but these findings imply a narrow therapeutic window determined by competing effects of antibodies to stimulate receptors and induce their downregulation.

Insulin in the brain: its pathophysiological implications for States related with central insulin resistance, type 2 diabetes and Alzheimer's disease

Although the brain has been considered an insulin-insensitive organ, recent reports on the location of insulin and its receptors in the brain have introduced new ways of considering this hormone responsible for several functions. The origin of insulin in the brain has been explained from peripheral or central sources, or both. Regardless of whether insulin is of peripheral origin or produced in the brain, this hormone may act through its own receptors present in the brain. The molecular events through which insulin functions in the brain are the same as those operating in the periphery. However, certain insulin actions are different in the central nervous system, such as hormone-induced glucose uptake due to a low insulin-sensitive GLUT-4 activity, and because of the predominant presence of GLUT-1 and GLUT-3. In addition, insulin in the brain contributes to the control of nutrient homeostasis, reproduction, cognition, and memory, as well as to neurotrophic, neuromodulatory, and neuroprotective effects. Alterations of these functional activities may contribute to the manifestation of several clinical entities, such as central insulin resistance, type 2 diabetes mellitus (T2DM), and Alzheimer's disease (AD). A close association between T2DM and AD has been reported, to the extent that AD is twice more frequent in diabetic patients, and some authors have proposed the name "type 3 diabetes" for this association. There are links between AD and T2DM through mitochondrial alterations and oxidative stress, altered energy and glucose metabolism, cholesterol modifications, dysfunctional protein O-GlcNAcylation, formation of amyloid plaques, altered Aβ metabolism, and tau hyperphosphorylation. Advances in the knowledge of preclinical AD and T2DM may be a major stimulus for the development of treatment for preventing the pathogenic events of these disorders, mainly those focused on reducing brain insulin resistance, which is seems to be a common ground for both pathological entities.

Impact of receptor clustering on ligand binding

Background

Cellular response to changes in the concentration of different chemical species in the extracellular medium is induced by ligand binding to dedicated transmembrane receptors. Receptor density, distribution, and clustering may be key spatial features that influence effective and proper physical and biochemical cellular responses to many regulatory signals. Classical equations describing this kind of binding kinetics assume the distributions of interacting species to be homogeneous, neglecting by doing so the impact of clustering. As there is experimental evidence that receptors tend to group in clusters inside membrane domains, we investigated the effects of receptor clustering on cellular receptor ligand binding.

Results

We implemented a model of receptor binding using a Monte-Carlo algorithm to simulate ligand diffusion and binding. In some simple cases, analytic solutions for binding equilibrium of ligand on clusters of receptors are provided, and supported by simulation results. Our simulations show that the so-called "apparent" affinity of the ligand for the receptor decreases with clustering although the microscopic affinity remains constant.

Conclusions

Changing membrane receptors clustering could be a simple mechanism that allows cells to change and adapt its affinity/sensitivity toward a given stimulus.

Dynamic association of human insulin receptor with lipid rafts in cells lacking caveolae

Cholesterol-sphingolipid rich plasma membrane domains, known as rafts, have emerged as important regulators of signal transduction. The adipocyte insulin receptor (IR) is localized to and signals via caveolae that are formed by polymerization of caveolins. Caveolin binds to IR and stimulates signalling. We report that, in liver-derived cells lacking caveolae, autophosphorylation of the endogenous IR is dependent on raft lipids, being compromised by acute cyclodextrin-mediated cholesterol depletion or by antibody clustering of glycosphingolipids. Moreover, we provide evidence that IR becomes recruited to detergent-resistant domains upon ligand binding and that clustering of GM2 ganglioside inhibits IR signalling apparently by excluding the ligand-bound IR from these domains. Our results indicate that, in cells derived from liver, an important insulin target tissue, caveolae are not required for insulin signalling. Rather, the dynamic recruitment of the ligand-bound IR into rafts may serve to regulate interactions in the initiation of the IR signalling cascade.

Insulin receptors and insulin action in the brain: review and clinical implications

Insulin receptors are known to be located on nerve cells in mammalian brain. The binding of insulin to dimerized receptors stimulates specialized transporter proteins that mediate the facilitated influx of glucose. However, neurons possess other mechanisms by which they obtain glucose, including transporters that are not insulin-dependent. Further, insulin receptors are unevenly distributed throughout the brain (with particularly high density in choroid plexus, olfactory bulb and regions of the striatum and cerebral cortex). Such factors imply that insulin, and insulin receptors, might have functions within the central nervous system in addition to those related to the supply of glucose. Indeed, invertebrate insulin-related peptides are synthesized in brain and serve as neurotransmitters or neuromodulators. The present review summarizes the structure, distribution and function of mammalian brain insulin receptors and the possible implications for central nervous system disorders. It is proposed that this is an under-studied subject of investigation.

Insulin receptor binding kinetics: modeling and simulation studies

Biological actions of insulin regulate glucose metabolism and other essential physiological functions. Binding of insulin to its cell surface receptor initiates signal transduction pathways that mediate cellular responses. Thus, it is of great interest to understand the mechanisms underlying insulin receptor binding kinetics. Interestingly, negative cooperative interactions are observed at high insulin concentrations while positive cooperativity may be present at low insulin concentrations. Clearly, insulin receptor binding kinetics cannot be simply explained by a classical bimolecular reaction. Mature insulin receptors have a dimeric structure capable of binding two molecules of insulin. The binding affinity of the receptor for the second insulin molecule is significantly lower than for the first bound insulin molecule. In addition, insulin receptor aggregation occurs in response to ligand binding and aggregation may also influence binding kinetics. In this study, we develop a mathematical model for insulin receptor binding kinetics that explicitly represents the divalent nature of the insulin receptor and incorporates receptor aggregation into the kinetic model. Model parameters are based upon published data where available. Computer simulations with our model are capable of reproducing both negative and positive cooperativity at the appropriate insulin concentrations. This model may be a useful tool for helping to understand the mechanisms underlying insulin receptor binding and the coupling of receptor binding to downstream signaling events.

Non-receptor cytosolic protein tyrosine kinases from various rat tissues

Adipocytic-cytosolic non-receptor protein tyrosine kinase (CytPTK) when activated can substitute for the insulin receptor tyrosine kinase (InsRTK), in manifesting several insulin effects in insulin-receptor independent fashion. Our aims here were to utilize PolyGlu4Tyr, a good experimental exogenous substrate for protein tyrosine kinases (PTKs) in general, for studying qualitative and quantitative parameters of CytPTKs extracted from different tissue cytosols. At the same time, we would search for a unique specific marker specifically characterizing CytPTKs. High speed supernatants of spleen, thymus, smooth muscle, lung and kidney were found to be rich in CytPTK activities. Their specific activities being 6- to 13-fold that of liver or adipose cytosols. Brain, testis and adrenal cytosols were an intermediate source of CytPTK activity, whereas CytPTK activity of heart and skeletal muscle was low. It was also evaluated that the capacity of the cytosol to phosphorylate PolyGlu4Tyr is 15-50% that of the non-stimulated Triton X-100 extractable plasma membrane PTKs. Fractionation of the cytosols on superose 12 column revealed several CytPTKs within the same tissue, their peaks ranging between 30 and 450 kDa. Immunoblotting analysis showed Fyn and Lyn were present in most tissue cytosols. Upon immunoprecipitation, however, with anti-Fyn or anti-Lyn, negligible amounts (< 2%) of the total cellular CytPTK were precipitated. Thus, these general markers of CytPTKs comprise only a minor proportion of the total intracellular PolyGlu4Tyr phosphorylating capacity. To see whether a specific marker for CytPTK could be detected, we also examined the requirement of CytPTKs for divalent ions, their preferred phosphate donor and their sensitivity to inhibition by known PTK inhibitors. We found that the order of reactivity with divalent cations was Co2+ > Mn2+ > Mg2+, while Zn2+ and Ca2+ did not support CytPTK activity. The best phosphate donor was ATP (ED50 = 5 microM), but other nucleoside 3-phosphates could substitute for ATP at high concentrations. With respect to these parameters, no basic difference exists between cytosolic and plasma-membrane PTKs. The PTK inhibitors, genestein and quercetin, inhibited both cytosolic and membranal PTKs at micromolar concentrations. In contrast, staurosporine was a potent inhibitor of CytPTKs (IC50 5-20 nM) and a poor inhibitor of membranal PTKs (IC50 10-40 microM). One of the conclusions we can draw from this study is that tissue cytosols contain PolyGlu4Tyr phosphorylating capacity in quantities greater than previously assumed and that the low level of phosphotyrosine found in cells is not the result of limited intracellular levels of CytPTKs.

Guanosine 5'-(gamma-thio) triphosphate (GTP gamma S) inhibits phosphorylation of insulin receptor and a novel GTP-binding protein, Gir, from human placenta

A novel 66 kDa GTP-binding protein, designated Gir, has been partially purified along with insulin receptor (IR) from human placenta. This protein binds 8-azido-GTP, is ADP-ribosylated by pertussis toxin, phosphorylated by IR tyrosine kinase and cross-reacts with antibodies against synthetic peptides from the GTP-binding domain of Gz alpha(P960). Phosphorylation of IR-beta subunit and Gir by IR tyrosine kinase was almost completely inhibited by 100 microM GTP gamma S, > 75% by 50 microM and 20-30% by 1 microM, while GDP at these concentrations had no significant effect on the phosphorylation. IR tyrosine kinase phosphorylated Gir at the tyrosine residues. These studies indicate regulation of IR tyrosine kinase activity by guanosine phosphates and involvement of Gir in insulin action.

Expression of functional insulin-like growth factor-I receptors by human amnion cells

OBJECTIVES

The purpose of the study was to investigate whether amnion cells contain functional insulin-like growth factor-I receptors.

STUDY DESIGN

To test whether human amnion cells contain insulin-like growth factor-I receptors, radioligand binding studies, affinity cross-linking studies, and Northern blot analysis were conducted in primary amnion cells and in an immortal amnion cell line (WISH). To test whether the insulin-like growth factor-I receptors on amnion cells are functional, cytochalasin B-inhibitable 2-deoxyglucose uptake was measured after stimulating the cells with insulin-like growth factor-I.

RESULTS

Radioligand binding studies demonstrated that primary amnion cells and WISH cells contained a single class of high-affinity receptors with an apparent dissociation constant of 0.18 +/- 0.04 nmol/L and a receptor concentration of 79 +/- 26.2 fmol/mg protein and dissociation constant of 0.44 +/- 0.03 nmol/L and a receptor concentration of 33.3 +/- 6.45 fmol per 106 cells, respectively. Affinity cross-linking studies revealed two major insulin-like growth factor-I binding sites, 135 and 270 kd. Both primary amnion cells and WISH cells exhibited cytochalasin B-inhibitable tritiated 2-deoxyglucose uptake in response to insulin-like growth factor-I treatment. Finally, treatment of WISH cells caused tyrosine phosphorylation of three proteins (molecular weight, 116, 95.4, and 83.5 kd) was observed by Western blotting with antiphosphotyrosine antibodies.

CONCLUSION

These results provide the first evidence that human amnion epithelial cells contain functional high-affinity insulin-like growth factor-I receptors that mediate glucose transport.

Aggregation-induced activation of the epidermal growth factor receptor protein tyrosine kinase

Various agents are able to stimulate the EGF receptor protein tyrosine kinase in the absence of ligand binding. To characterize their mechanism of action, we investigated their effects on the kinase activity of the intracellular domain of the EGF receptor (EGFR-IC). EGFR-IC (67 kDa) lacking the extracellular domain and transmembrane segment of the EGF receptor, but retaining kinase and autophosphorylation domains, was produced and purified as a soluble, cytoplasmic protein from Sf9 insect cells infected with a recombinant baculovirus. EGFR-IC was able to undergo autophosphorylation in a manner similar to full-length EGFR. Synthetic substrate peptides showed similar affinity to EGFR-IC as to the full-length receptor. The activity of the EGFR-IC was found to be dependent on divalent cations, Mn2+ being a more potent activator than Mg2+. Agents capable of aggregating the kinase by direct interaction (cross-linking antibodies, polycations) or through altering the surrounding solvent structure and thereby decreasing protein solubility [ammonium sulfate, poly(ethylene glycol), 2-methyl-2,4-pentanediol] activated the kinase in a manner which correlated with their ability to precipitate the EGFR intracellular domain. The widely different chemical nature of these agents suggests that they do not act by direct interaction with specific allosteric regulatory sites, but rather by facilitating the interactions between kinase molecules. These results support the hypothesis that full-length receptor aggregation itself, induced by ligand binding to the extracellular domain, results in intracellular domain interactions and the activation of kinase activity.

Monoclonal antibody to the human insulin receptor, but not insulin, stimulates S6 kinase via human insulin receptors mutated at three major tyrosine autophosphorylation sites

Studies were carried out to examine the role of the major insulin receptor tyrosine autophosphorylation sites in stimulation of S6 kinase activity. For these studies, we employed HTC rat hepatoma cells transfected with and expressing human insulin receptors. In cells transfected with and expressing a large number of normal human insulin receptors (HTC-IR cells), the sensitivity of cells to insulin to stimulate S6 kinase was increased tenfold when compared to untransfected wild type HTC cells (HTC-WT cells). However, in cells transfected with and expressing a large number of mutated human insulin receptors where the tyrosines at three major autophosphorylation sites (1158, 1162, and 1163) were mutated to phenylalanines (HTC-F3 cells), there was no change in insulin sensitivity when compared to HTC-WT cells. We next studied the effect of a human-specific monoclonal antibody to the human insulin receptor, MA-5, on S6 kinase activation. In HTC-WT cells, MA-5 did not interact with endogenous rat insulin receptors and thus did not stimulate S6 kinase. In HTC-IR cells expressing normal human insulin receptors, MA-5 stimulated S6 kinase. Interestingly, MA-5, unlike insulin, was also able to stimulate S6 kinase in HTC-F3 cells expressing mutated receptors. In order to further understand the signaling mechanisms by MA-5 and insulin, two potential intermediate protein kinases were investigated. Neither insulin nor MA-5 appears to activate either microtubule-associated protein 2 (MAP-2) kinase or protein kinase C in these cells. These studies suggest therefore that: 1) insulin and MA-5 may signal S6 kinase activation by independent mechanisms that do not employ either MAP-2 kinase or protein kinase C; and 2) under certain circumstances, S6 kinase appears to be activated by mechanisms that are independent of insulin receptor tyrosine autophosphorylation.

Regulation of biological functions by an insulin receptor monoclonal antibody in insulin receptor beta-subunit mutants

We investigated the effects of MA-5, a human-specific monoclonal antibody to the insulin receptor alpha-subunit, on transmembrane signaling in cell lines transfected with and expressing both normal human insulin receptors and receptors mutated in their beta-subunit tyrosine kinase domains. In cell lines expressing normal human insulin receptors, MA-5 stimulated three biological functions: aminoisobutyric acid (AIB) uptake, thymidine incorporation, and S6 kinase activation. Under conditions where these biological functions were stimulated, there was no detectable stimulation of receptor tyrosine kinase. We then combined the use of this monoclonal antibody with cells expressing insulin receptors with mutations in the beta-subunit tyrosine kinase domain; two of ATP binding site mutants V1008 (Gly----Val) and M1030 (Lys----Met) and one triple-tyrosine autophosphorylation site mutant F3 (Tyr----Phe at 1158, 1162, and 1163). In cells expressing V1008 receptors, none of the three biological functions of insulin was stimulated. In cells expressing M1030 receptors, AIB uptake was stimulated to a small, but significant, extent whereas the other two functions were not. In cells expressing F3 receptors, AIB uptake and S6 kinase activation, but not thymidine incorporation, were fully stimulated. The data suggest, therefore, that (1) activation of insulin receptor tyrosine kinase may not be a prerequisite for signaling of all the actions of insulin and (2) there may be multiple signal transduction pathways to account for the biological actions of insulin.

Crystals of the complex between human growth hormone and the extracellular domain of its receptor

Single crystals suitable for high-resolution diffraction studies have been grown of the human growth hormone (hGH) complexed to the extracellular domain of its cloned receptor from the human liver (hGHbp), using the technique of repeat seeding. The crystals are in space group P2(1)2(1)2, with a = 145.8 A, b = 68.6 A, c = 76.0 A, and diffract to at least 2.7 A resolution on a rotating anode X-ray source. Analysis of the composition of these crystals showed the stoichiometry of the complex to be hGH: (hGHbp)2. This finding, coupled with biochemical data on the complex in solution, indicates that the biologically significant dimerization of the growth hormone receptor is mediated through a single hormone molecule. Structure determination of the complex is currently being completed.

Increased hepatic insulin proreceptor-to-receptor ratio in diabetes: a possible processing defect

Hepatic insulin proreceptors and receptors were studied in control and in ketotic diabetic rats 2-4 wk after streptozotocin treatment. Solubilized preparations were partially purified by wheat germ agglutinin-agarose (WGA) and lentil lectin agarose (LLA) chromatography to enrich eluates in insulin receptors and proreceptors, respectively. After phosphorylation with [gamma-32P]ATP, an approximately 190-kDa glycoprotein was identified in LLA eluates as the insulin proreceptor, based on insulin dose-dependent tyrosine autophosphorylation, immunoprecipitation with insulin receptor-specific antibodies, and high-mannose glycosylation. Mature approximately 95 kDa phosphorylated beta-subunits were present in both LLA and WGA eluates. LLA also showed phosphorylated partially processed beta-subunits (approximately 85 kDa) and proreceptors (approximately 190 kDa). Proreceptors comprised less than 1% of the total yield of hepatic insulin receptors. The incorporation of 32P into proreceptors (per gram liver or DNA) was 4.7- or 4.5-fold greater in diabetic vs. control rats, whereas receptor labeling increased only 1.8- or 1.5-fold in diabetic rats. beta-Subunit autophosphorylation per receptor was identical in control and diabetic rats. The phosphorylation data suggested a diabetes-associated 2.6-fold increase in proreceptor-to-receptor ratios. When assessed by cross-linking with 125I-labeled insulin or by immunoblotting, proreceptor-to-receptor ratios were increased 1.5- and 3.1-fold, respectively, in diabetic rats. The data suggest that uncontrolled diabetes may alter insulin receptor processing.

Basic polycations activate the insulin receptor kinase and a tightly associated serine kinase

The effects of cationic polyamino acids on phosphorylation of the insulin and insulin-like growth factor 1 receptor kinases were studied and the following observations were made. (a) Polylysine stimulated both tyrosine and serine phosphorylation of the insulin receptor and of additional proteins present in lectin-purified membrane preparations from rat liver. (b) Polylysine synergized with insulin to enhance phosphorylation of the insulin receptor and of additional proteins (pp40 and pp110). (c) Polylysine effects were more pronounced upon increasing the polylysine chain length. (d) The effect of polylysine was biphasic with an optimum at 100 micrograms/ml. (e) Polylysine was found ineffective in stimulating the phosphorylation of immobilized insulin receptors. Taken together, these findings support the notion that the action of polylysine involves conformational changes and presumably aggregation of soluble receptors. The same effects of polylysine were obtained with highly purified insulin receptor preparations. Under these conditions polylysine enhanced both serine and tyrosine phosphorylation of the insulin receptor, suggesting that polylysine stimulates the activity of the insulin receptor kinase, and of a serine kinase that is tightly associated with the insulin receptor.

Analysis of gain-of-function mutations of the lin-12 gene of Caenorhabditis elegans

Certain cell fate decisions are specified by cell-cell interactions during the development of the nematode Caenorhabditis elegans. For example, in a wild-type hermaphrodite gonad, two cells, Z1.ppp and Z4.aaa, have the potential to become the anchor cell (AC). Intercellular communication establishes their fates and ensures that only one cell becomes the AC, while the other becomes a ventral uterine precursor cell (VU). One component of this intercellular communication seems to be the 'AC-to-VU' signal from the presumptive AC that causes the other cell to become a VU. Genetic and developmental studies indicated that the lin-12 gene specifies the fates of Z1.ppp and Z4.aaa. Molecular studies suggest that lin-12 directly participates in their communications, perhaps acting as the receptor for the 'AC-to-VU' signal. Here, we report the molecular lesions associated with lin-12 gain-of-function mutations, cell isolation experiments, and genetic studies of an unusual lin-12 allele. These data suggest that self-association of the putative lin-12-encoded receptor leads to its activation, and that certain gain-of-function mutations result in ligand-independent activation.

Antiphosphotyrosine antibodies modulate insulin receptor kinase activity and insulin action

Insulin elicits the autophosphorylation of the beta-subunit of its receptor on tyrosine residues: this effect appears to be the earliest post-binding event involved in insulin action. In the present study we have raised highly specific antibodies to phosphotyrosine residues, and we have taken advantage of these antibodies to further evaluate the role of the insulin receptor tyrosine kinase in the generation of insulin's biological responses. Using a cell-free phosphorylation assay, we show here that these antibodies increase the tyrosine kinase activity of the receptor, and its phosphorylation on tyrosine residues. In contrast, the antibodies do not interfere with dephosphorylation of the insulin receptor. Introduction of the same antibodies in living Fao hepatoma cells enhances the effect of insulin on both glucose transport and aminoacid uptake. As a whole our data indicate that the insulin receptor kinase is involved in the generation of an early (glucose transport) and late (aminoacid uptake) response to insulin. Further, conformational changes in phosphotyrosine containing domains of the insulin receptor appear to modulate insulin's biological effects. Finally, the injection of antibodies in intact cells provides us with a novel and promising tool to search for cellular substrates for the insulin receptor tyrosine kinase.

The insulin receptor: structure and function

Promising progress in understanding the molecular basis of insulin action has been achieved by demonstrating that the insulin receptor is an insulin-sensitive tyrosine kinase. Here we discuss the structure of this receptor kinase and compare it with receptors for related growth factors. We review the known modes to regulate the receptor kinase activity, either through its autophosphorylation (on tyrosine residues) or through its phosphorylation by other kinases (on serine and threonine residues). We discuss the role of the receptor kinase activity in hormone signal transduction in light of results indicating a reduced kinase activity in insulin-resistant states. Finally, studies to identify natural substrates for the insulin receptor kinase are presented. The possible physiological role of these phosphorylated substrates in mediating insulin action is evaluated.

gp140, the EBV/C3d receptor (CR2) of human B lymphocytes, is involved in cell-free phosphorylation of p120, a nuclear ribonucleoprotein

gp140, the EB/C3d receptor (EBV/C3dR; CR2), is a membrane site involved in human B cell regulation. Cross-linking of this receptor on the cell surface by its specific ligands led to the enhancement of B cell proliferation in synergy with T cell factors. In vitro activation of human peripheral B lymphocytes by cross-linking membrane immunoglobulins with anti-mu antibody induced EBV/C3dR phosphorylation. These studies were pursued by analyzing cell-free phosphorylation of EBV/C3dR isolated from Raji cell fractions, and immobilized on OKB7, a monoclonal anti-EBV/C3dR antibody. Three EBV/C3dR-related antigens which could be cell-free phosphorylated were detected: gp140, the EBV/C3dR, p130 and p120. gp140, the mature form of EBV/C3dR, was isolated from plasma membrane and from purified nuclei. p130 was identified as an intracellular intermediate of EBV/C3dR glycosylation, localized in low-density microsomes. Phosphoamino acid analysis of EBV/C3dR allowed the detection of phosphotyrosine and phosphoserine residues. These data suggest that EBV/C3dR could carry an autophosphorylation activity and could be associated to serine kinases. Using polyclonal anti-p120 antibody and anti-120 kDa nuclear ribonucleoprotein monoclonal antibody (mAb), p120 was identified as a nuclear ribonucleoprotein antigenically not related to EBV/C3dR. Detection of p120 on EBV/C3dR, immobilized on OKB7, was due to interactions between both antigens, instead of anti-EBV/C3dR mAb cross-reactivity with p120. Cell-free phosphorylation of p120 was under the control of EBV/C3dR. However, it is not yet established whether other nuclear or membrane components were involved in the control of p120 cell-free phosphorylation by EBV/C3dR. From the data presented herein, we propose that phosphorylation of a 120-kDa nuclear ribonucleoprotein by EBV/C3dR-associated kinases could represent a crucial step in in vivo regulation of human B cell activation.

After insulin binds

Three recent advances pertinent to the mechanism of insulin action include (i) the discovery that the insulin receptor is an insulin-dependent protein tyrosine kinase, functionally related to certain growth factor receptors and oncogene-encoded proteins, (ii) the molecular cloning of the insulin proreceptor complementary DNA, and (iii) evidence that the protein tyrosine kinase activity of the receptor is essential for insulin action. Efforts are now focusing on the physiological substrates for the receptor kinase. Experience to date suggests that they will be rare proteins whose phosphorylation in intact cells may be transient. The advantages of attempting to dissect the initial biochemical pathway of insulin action include the wealth of information about the metabolic consequences of insulin action and the potential for genetic analysis in Drosophila and in man.

Protein kinase activity of the partially purified insulin receptor from human adipocytes

Insulin receptors were partially purified by wheatgerm agglutinin chromatography from adipocytes of fasted healthy female subjects. The partially purified receptors showed binding characteristics similar to those of intact calls with an apparent affinity for insulin (half maximal binding) of 1.6 X 10(-9) mol/l. Insulin receptor alpha- and beta-subunits were identified by affinity labelling and phosphorylation with (gamma-32p)ATP, respectively. The electrophoretic mobility was 135 K for the alpha-subunit, and 97.5 K for the beta-subunit. The intrinsic tyrosine kinase activity of the insulin receptor was demonstrated by autophosphorylation of receptors purified by immunoprecipitation, and by phosphorylation of a synthetic substrate: poly(Glu, Tyr (4:1]. The kinase was activated by insulin in a dose-dependent manner with half maximal stimulation at 8 X 10(-10) mol/l. The Km value for ATP was 50 mumol/l. The dose-response relationship between percentage maximal kinase activation and fractional receptor occupancy by insulin was sigmoidal with half maximal effect when 35% of receptors are occupied. It is suggested that positively cooperation interactions between the receptor monomers are involved in stimulation of kinase activity and receptor autophosphorylation by insulin.

Stimulus-secretion coupling mechanisms in mast cells

We have investigated several aspects of the complex sequence of events, transmitting the antigen-induced signal of cross-linking immunoglobulin E (IgE) resident in the membrane surface of mast cells into the signals yielding the final process of mediator release. Already the initial phase of this cascade still requires a better understanding. Namely, we are still missing a clear physical description of the effective stimulus-producing antigen-IgE complex in terms of size and spatial requirements. We are investigating this problem on a well-defined cell line (rat basophilic leukemia-RBL-2H3) using synthetic divalent haptens and a monoclonal IgE. A subsequent phase following IgE aggregation is a transient increase in the free calcium concentration in these cells' cytosol. The source of the Ca2+ and the way by which it enters the cytosol are studied predominantly by examining antigen-induced channel activity in the cells' membrane allowing Ca2+ influx from the exterior medium. Finally, we have observed that under certain experimental conditions, antigen-induced degranulation can be achieved even without a rise in cytosolic free calcium. In our search for alternative second messengers, we examine the potential candidacy of the cytosolic Na+/H+ balance. So far, we have found that antigen-stimulated secretion does require extracellular sodium and involves changes in its cytosolic pH. However, further studies are required to clarify its possible role as a coupling element.

Further characterization of the low and high affinity binding components of the thyrotropin receptor

Following cross-linking with disuccinimidyl suberate and analysis by SDS-PAGE and autoradiography, both the high- and low-affinity TSH binding components exhibited two similar 125I-TSH-labeled bands, with Mr values of 80,000 and 68,000. IgG fractions from patients with Graves' disease inhibited 125I-TSH binding to both components, while normal IgG had no effect. Although not entirely conclusive, these results suggest that the high- and low-affinity components share similar subunit composition and antigenic determinants.