Identification of epitopes on the human insulin receptor reacting with rabbit polyclonal antisera and mouse monoclonal antibodies

How insulin-like growth factor I binds to a hybrid insulin receptor type 1 insulin-like growth factor receptor

Monomers of the insulin receptor and type 1 insulin-like growth factor receptor (IGF-1R) can combine stochastically to form heterodimeric hybrid receptors. These hybrid receptors display ligand binding and signaling properties that differ from those of the homodimeric receptors. Here, we describe the cryoelectron microscopy structure of such a hybrid receptor in complex with insulin-like growth factor I (IGF-I). The structure (ca. 3.7 Å resolution) displays a single IGF-I ligand, bound in a similar fashion to that seen for IGFs in complex with IGF-1R. The IGF-I ligand engages the first leucine-rich-repeat domain and cysteine-rich region of the IGF-1R monomer (rather than those of the insulin receptor monomer), consistent with the determinants for IGF binding residing in the IGF-1R cysteine-rich region. The structure broadens our understanding of this receptor family and assists in delineating the key structural motifs involved in binding their respective ligands.

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A cryo-EM structure of IGF-I bound to a hybrid IR/IGF-1R ectodomain is presented

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The structure is congruent to those of the single-liganded homodimeric receptors

Kinetics of Blood-Brain Barrier Transport of Monoclonal Antibodies Targeting the Insulin Receptor and the Transferrin Receptor

Biologic drugs are large molecule pharmaceuticals that do not cross the blood-brain barrier (BBB), which is formed by the brain capillary endothelium. Biologics can be re-engineered for BBB transport as IgG fusion proteins, where the IgG domain is a monoclonal antibody (MAb) that targets an endogenous BBB transporter, such as the insulin receptor (IR) or transferrin receptor (TfR). The IR and TfR at the BBB transport the receptor-specific MAb in parallel with the transport of the endogenous ligand, insulin or transferrin. The kinetics of BBB transport of insulin or transferrin, or an IRMAb or TfRMAb, can be quantified with separate mathematical models. Mathematical models to estimate the half-time of receptor endocytosis, MAb or ligand exocytosis into brain extracellular space, or receptor recycling back to the endothelial luminal membrane were fit to the brain uptake of a TfRMAb or a IRMAb fusion protein in the Rhesus monkey. Model fits to the data also allow for estimates of the rates of association of the MAb in plasma with the IR or TfR that is embedded within the endothelial luminal membrane in vivo. The parameters generated from the model fits can be used to estimate the brain concentration profile of the MAb over time, and this brain exposure is shown to be a function of the rate of clearance of the antibody fusion protein from the plasma compartment.

Probing Structure and Function of Alkali Sensor IRR with Monoclonal Antibodies

To study the structure and function of the pH-regulated receptor tyrosine kinase insulin receptor-related receptor (IRR), а member of the insulin receptor family, we obtained six mouse monoclonal antibodies against the recombinant IRR ectodomain. These antibodies were characterized in experiments with exogenously expressed full-length IRR by Western blotting, immunoprecipitation, and immunocytochemistry analyses. Utilizing a previously obtained set of IRR/IR chimeras with swapped small structural domains and point amino acid substitutions, we mapped the binding sites of the obtained antibodies in IRR. Five of them showed specific binding to different IRR domains in the extracellular region, while one failed to react with the full-length receptor. Unexpectedly, we found that 4D5 antibody can activate IRR at neutral pH, and 4C2 antibody can inhibit activation of IRR by alkali. Our study is the first description of the instruments of protein nature that can regulate activity of the orphan receptor IRR and confirms that alkali-induced activation is an intrinsic property of this receptor tyrosine kinase.

The beta secretase BACE1 regulates the expression of insulin receptor in the liver

Insulin receptor (IR) plays a key role in the control of glucose homeostasis; however, the regulation of its cellular expression remains poorly understood. Here we show that the amount of biologically active IR is regulated by the cleavage of its ectodomain, by the β-site amyloid precursor protein cleaving enzyme 1 (BACE1), in a glucose concentration-dependent manner. In vivo studies demonstrate that BACE1 regulates the amount of IR and insulin signaling in the liver. During diabetes, BACE1-dependent cleavage of IR is increased and the amount of IR in the liver is reduced, whereas infusion of a BACE1 inhibitor partially restores liver IR. We suggest the potential use of BACE1 inhibitors to enhance insulin signaling during diabetes. Additionally, we show that plasma levels of cleaved IR reflect IR isoform A expression levels in liver tumors, which prompts us to propose that the measurement of circulating cleaved IR may assist hepatic cancer detection and management.

Targeting nanoparticles across the blood-brain barrier with monoclonal antibodies

Development of therapeutics for brain disorders is one of the more difficult challenges to be overcome by the scientific community due to the inability of most molecules to cross the blood-brain barrier (BBB). Antibody-conjugated nanoparticles are drug carriers that can be used to target encapsulated drugs to the brain endothelial cells and have proven to be very promising. They significantly improve the accumulation of the drug in pathological sites and decrease the undesirable side effect of drugs in healthy tissues. We review the systems that have demonstrated promising results in crossing the BBB through receptor-mediated endocytic mechanisms for the treatment of neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

The "thrifty" gene encoding Ahsg/Fetuin-A meets the insulin receptor: Insights into the mechanism of insulin resistance

Ahsg (fetuin-A) is a 55-59kDa phosphorylated glycoprotein synthesized in the adult predominantly by hepatocytes, from which it enters the circulation. When dysregulated, this glycoprotein operates to influence the clinical sequelae of insulin resistance-type 2 diabetes and cardiovascular disease. The pathological sequelae likely arise from two separable molecular "faces" of Ahsg-one acting at the level of the insulin receptor and a second face influencing ectopic biomineralization in the intima. A detailed understanding of these two functional faces of Ahsg is not yet clear for lack of structural studies. Ahsg has a physiological role in the biomineralization of bone, which when dysregulated can lead to ectopic calcification of soft tissues in the vasculature. Ahsg has a second physiological function in regulating how insulin signals through its receptor, a transmembrane tyrosine kinase. Dysregulation of this "face" of Ahsg results in morbid sequelae such as impaired glucose disposal and fatty liver. Ahsg binds to tandem fibronectin type 3 (Fn3) domains present in the 194 amino acid residue extracellular portion of the β-subunit of the insulin receptor, distant from the high-affinity pocket formed by two complementing α-subunits where insulin binds. Only two proteins are known to bind directly to the insulin receptor ectodomain - insulin and Ahsg - the former turns on the receptor's intrinsic tyrosine kinase (TK) activity, and the latter shuts it down. Recent X-ray crystallographic studies of the ectodomain of the insulin receptor now sharpen our understanding of the receptor's extracellular α-subunit and linked β-subunit. Ahsg genotype and its circulating level have been correlated with body morphometrics (obese versus lean and visceral adiposity) in epidemiological studies enrolling thousands of patients. Epidemiological studies from the clinic reveal high levels of circulating Ahsg in insulin resistance and diabetes. This review endeavors to explain how one protein can mediate diverse pathologies, but specifically addresses its metabolic "face" blunting insulin receptor activity, an action leading to insulin resistance.

Glucose binds to the insulin receptor affecting the mutual affinity of insulin and its receptor

Insulin activity is sensitive to glucose concentration but the mechanisms are still unclear. An unexamined possibility is that the insulin receptor (IR) is sensitive to glucose concentration. We demonstrate here that insulin-like peptides derived from the IR bind glucose at low millimolar, and cytochalasin B at low micromolar, concentrations; several insulin-like IR peptides bind insulin at nanomolar Kd; and this binding is antagonized by increasing glucose concentrations. In addition, glucose and cytochalasin B bind to IR isolated from rat liver and increasing glucose decreases insulin binding to this IR preparation. The presence of GLUT 1 in our IR preparation suggests the possibility of additional glucose-mediated allosteric control. We propose a model in which glucose binds to insulin, the IR, and GLUT; insulin binds to the IR; and the IR binds to GLUT. This set of interactions produces an integrated system of insulin-dependent interactions that is highly sensitive to glucose concentration.

Autoreactive T-cell receptor (Vbeta/D/Jbeta) sequences in diabetes are homologous to insulin, glucagon, the insulin receptor, and the glucagon receptor

The hypervariable (Vbeta/D/Jbeta) regions of T-cell receptors (TCR) have been sequenced in a variety of autoimmune diseases by various investigators. An analysis of some of these sequences shows that TCR from both human diabetics and NOD mice mimic insulin, glucagon, the insulin receptor, and the glucagon receptor. Such similarities are not found in the TCR produced in other human autoimmune diseases. These data may explain how insulin, glucagon, and their receptors are targets of autoimmunity in diabetes and also suggest that TCR mimicking insulin and its receptor may be targets of anti-insulin autoantibodies. Such intra-systemic mimicry of self-proteins also raises complex questions about how "self" and "nonself" are regulated during TCR production, especially in light of the complementarity of insulin for its receptor and glucagon for its receptor. The data presented here suggest that some TCR may be complementary to other TCR in autoimmune diseases, a possibility that is experimentally testable. Such complementarity, if it exists, could either serve to down-regulate the clones bearing such TCR or, alternatively, trigger an intra-immune system civil war between them.

High-affinity insulin binding: insulin interacts with two receptor ligand binding sites

The interaction of insulin with its receptor is complex. Kinetic and equilibrium binding studies suggest coexistence of high- and low-affinity binding sites or negative cooperativity. These phenomena and high-affinity interactions are dependent on the dimeric structure of the receptor. Structure-function studies of insulin analogs suggest insulin has two receptor binding sites, implying a bivalent interaction with the receptor. Alanine scanning studies of the secreted recombinant receptor implicate the L1 domain and a C-terminal peptide of the receptor alpha subunit as components of one ligand binding site. Functional studies suggest that the first and second type III fibronectin repeats of the receptor contain a second ligand binding site. We have used structure-directed alanine scanning mutagenesis to identify determinants in these domains involved in ligand interactions. cDNAs encoding alanine mutants of the holo-receptor were transiently expressed in 293 cells, and the binding properties of the expressed receptor were determined. Alanine mutations of Lys(484), Leu(552), Asp(591), Ile(602), Lys(616), Asp(620), and Pro(621) compromised affinities for insulin 2-5-fold. With the exception of Asp(620), none of these mutations compromised the affinity of the recombinant secreted receptor for insulin, indicating that the perturbation of the interaction is at the site of mutation and not an indirect effect on the interaction with the binding site of the secreted receptor. These residues thus form part of a novel ligand binding site of the insulin receptor. Complementation experiments demonstrate that insulin interacts in trans with both receptor binding sites to generate high-affinity interactions.

The location and characterisation of the O-linked glycans of the human insulin receptor

O-linked glycosylation is a post-translational and post-folding event involving exposed S/T residues at beta-turns or in regions with extended conformation. O-linked sites are difficult to predict from sequence analyses compared to N-linked sites. Here we compare the results of chemical analyses of isolated glycopeptides with the prediction using the neural network prediction method NetOGlyc3.1, a procedure that has been reported to correctly predict 76% of O-glycosylated residues in proteins. Using the heavily glycosylated human insulin receptor as the test protein six sites of mucin-type O-glycosylation were found at residues T744, T749, S757, S758, T759, and T763 compared to the three sites (T759 and T763- correctly, T756- incorrectly) predicted by the neural network method. These six sites occur in a 20 residue segment that begins nine residues downstream from the start of the insulin receptor beta-chain. This region which also includes N-linked glycosylation sites at N742 and N755, is predicted to lack secondary structure and is followed by residues 765-770, the known linear epitope for the monoclonal antibody 18-44.

IGF-I/insulin hybrid receptors in human endothelial cells

Vascular complications are common in diabetes. IGF-I receptors (IGF-IR) and insulin receptors (IR) in endothelial cells might respond to altered levels of IGF-I and insulin, resulting in altered endothelial function in diabetes. We therefore studied IGF-IR and IR gene expression, ligand binding, receptor protein, and phosphorylation in human umbilical vein endothelial cells (HUVEC). IGF-IR mRNA was more abundant than IR mRNA in freshly isolated HUVEC (IGF-IR/IR ratio 7.1 +/- 1.5) and in cultured HUVEC (ratio 3.5 +/- 0.51). Accordingly, specific binding of (125)I-IGF-I (0.64 +/- 0.25%) was higher than that of (125)I-insulin (0.25 +/- 0.09%). Protein was detected for both receptors and IGF-I/insulin hybrid receptors. IGF-IR phosphorylation was stimulated by 10(-10) to 10(-8) M IGF-I. IR were activated by 10(-9) to 10(-8) M insulin and IGF-I. We conclude that HUVEC express more IGF-IR than IR, and also express hybrid receptors. Both IGF-I and insulin phosphorylate their own receptors but only IGF-I seems to phosphorylate hybrid receptors.

Deletion of V335 from the L2 domain of the insulin receptor results in a conformationally abnormal receptor that is unable to bind insulin and causes Donohue's syndrome in a human subject

An infant with Donohue's syndrome (leprechaunism) was found to be homozygous for an in-frame trinucleotide deletion within the insulin receptor gene resulting in the deletion of valine 335. When transiently transfected into Chinese hamster ovary cells, mutant receptor was produced in a mature form, but at significantly lower levels compared with wild-type receptor. Cell surface biotinylation experiments revealed that significant amounts of the DeltaV335 receptor were expressed on the cell surface. Despite this, cells expressing this receptor showed no significant insulin binding or ligand-induced receptor autophosphorylation. Although the DeltaV335 receptor was capable of being immunoprecipitated with antibodies directed against the beta-subunit of the receptor, the mutant receptor could not be recognized by a panel of antibodies directed against different epitopes of the alpha-subunit, suggesting that the loss of V335 results in a major conformational alteration in the receptor alpha-subunit. This would be predicted by the positioning of V335 at a critical location within a strand that provides the main rigid scaffold for the two beta-sheet faces of the L2 domain of the receptor. The severe biochemical and clinical consequences of this novel mutation, which occur despite substantial expression on the cell surface, emphasize the crucial role of the L2 domain in ligand binding by the insulin receptor.

Molecular complementarity III. peptide complementarity as a basis for peptide receptor evolution: a bioinformatic case study of insulin, glucagon and gastrin

Dwyer has suggested that peptide receptors evolved from self-aggregating peptides so that peptide receptors should incorporate regions of high homology with the peptide ligand. If one considers self-aggregation to be a particular manifestation of molecular complementarity in general, then it is possible to extend Dwyer's hypothesis to a broader set of peptides: complementary peptides that bind to each other. In the latter case, one would expect to find homologous copies of the complementary peptide in the receptor. Thirteen peptides, 10 of which are not known to self-aggregate (amylin, ACTH, LHRH, angiotensin II, atrial natriuretic peptide, somatostatin, oxytocin, neurotensin, vasopressin, and substance P), and three that are known to self-aggregate (insulin, glucagon, and gastrin), were chosen. In addition to being self-aggregating, insulin and glucagon are also known to bind to each other, making them a mutually complementary pair. All possible combinations of the 13 peptides and the extracellular regions of their receptors were investigated using bioinformatic tools (a total of 325 combinations). Multiple, statistically significant homologies were found for insulin in the insulin receptor; insulin in the glucagon receptor; glucagon in the glucagon receptor; glucagon in the insulin receptor; and gastrin in gastrin binding protein and its receptor. Most of these homologies are in regions or sequences known to contribute to receptor binding of the respective hormone. These results suggest that the Dwyer hypothesis for receptor evolution may be generalizable beyond self-aggregating to complementary peptides. The evolution of receptors may have been driven by small molecule complementarity augmented by modular evolutionary processes that left a "molecular paleontology" that is still evident in the genome today. This "paleontology" may allow identification of peptide receptor sites.

Prodrug based optimal drug delivery via membrane transporter/receptor

The carrier-mediated absorption of drugs and prodrugs across epithelial and endothelial barriers is emerging as a novel trend in biotherapeutics. This review examines the important advances in this field in the past decade. The feasibility of drug absorption of the parent drug or the appropriately modified prodrug via these transporters is discussed in detail. Several successful examples of synthesis of prodrugs recognised by the targeted transporters are described. The applicability of this approach in translocating drugs across the almost impenetrable blood-brain barrier (BBB) has also been examined.

Furin-mediated processing in the early secretory pathway: sequential cleavage and degradation of misfolded insulin receptors

Improperly folded membrane proteins are retained in the endoplasmic reticulum and then diverted to a degradative pathway by a network of molecular chaperones and intracellular proteases. Here we report that mutant insulin proreceptors (Pro(62)) retained in the early secretory pathway undergo proteolytic cleavage at a tetrabasic concensus site for the subtilisin-like protease furin (SPC 1), generating two unstable proteolytic intermediates of 80/120 kDa corresponding to alpha (135 kDa) and beta (90 kDa) subunits. These are degraded more rapidly than the uncleaved proreceptor protein. Site-directed mutagenesis of the normal RKRR processing site prevented cleavage. Use of inhibitors and furin-deficient cell lines confirmed that furin is responsible for proreceptor cleavage; furin overexpression increased the degradation of mutant but not wild-type receptors. Together, these results suggest that processing and degradation occur sequentially for mutant proreceptors.

Properties of an insulin receptor with an IGF-1 receptor loop exchange in the cysteine-rich region

The insulin receptor (IR) and the insulin-like growth factor-I receptor (IGF-1R) show differential binding of insulin and IGFs. The specificity determinants for IGF-1 binding are known to be located in the cysteine-rich (Cys-rich) region between residues 223 and 274 of human IGF-1R, which includes a loop that protrudes into the putative ligand binding site. In this report we have replaced residues 260-277 of human IR with residues 253-266 of the human IGF-1R to produce an IR-based, cysteine loop exchange chimaera, termed hIR-Cys loop exchange (CLX), in which all 14 amino acid residues in the exchanged loop differ from wild-type insulin receptor. This loop exchange had a detrimental effect on the efficiency of pro-receptor processing and on the binding of the mouse monoclonal antibody 83-7. However, this antibody, which binds hIR but not hIGF-1R, was still capable of immunoprecipitating the mature chimaeric receptor, indicating that the conformational epitope recognised by this antibody is not primarily determined by the loop region exchanged. The loop exchange did not significantly affect the ability of insulin to displace bound radiolabelled insulin, but increased the capacity of IGF-1 to competitively displace labelled insulin by at least 10 fold.

Structural domains of the insulin receptor and IGF receptor required for dimerisation and ligand binding

We investigated structural requirements for dimerisation and ligand binding of insulin/IGF receptors. Soluble receptor fragments consisting of N-terminal domains (L1/CYS/L2, L1/CYS/L2/F0) or fibronectin domains (F0/F1/F2, F1/F2) were expressed in CHO cells. Fragments containing F0 or F1 domains were secreted as disulphide-linked dimers, and those consisting of L1/CYS/L2 domains as monomers. None of these proteins bound ligand. However, when a peptide of 16 amino acids from the alpha-subunit C-terminus was fused to the C-terminus of L1/CYS/L2, the monomeric insulin and IGF receptor constructs bound their respective ligands with affinity only 10-fold lower than native receptors.

Single-molecule imaging of human insulin receptor ectodomain and its Fab complexes

The insulin receptor (IR) is a four-chain, transmembrane dimer held together by disulfide bonds. To gain information about the molecular envelope and the organization of its domains, single-molecule images of the IR ectodomain and its complexes with three Fabs have been analyzed by electron microscopy. The data indicate that the IR ectodomain resembles a U-shaped prism of approximate dimensions 90 x 80 x 120 A. The width of the cleft (assumed membrane-distal) between the two side arms is sufficient to accommodate ligand. Fab 83-7, which recognizes the cys-rich region of IR, bound halfway up one end of each side arm in a diametrically opposite manner, indicating a twofold axis of symmetry normal to the membrane surface. Fabs 83-14 and 18-44, which have been mapped respectively to the first fibronectin type III domain (residues 469-592) and residues 765-770 in the insert domain, bound near the base of the prism at opposite corners. These images, together with the data from the recently determined 3D structure of the first three domains of the insulin-like growth factor type I receptor, suggest that the IR dimer is organized into two layers with the L1/cys-rich/L2 domains occupying the upper (membrane distal) region of the U-shaped prism and the fibronectin type III domains and the insert domains located predominantly in the membrane-proximal region.

Members of the insulin receptor family contain three fibronectin type III domains

The insulin receptor is a large transmembrane dimer, comprised of several domains. Detailed 3D structural information is available for the L1-cys-rich-L2 domains in the extracellular region (ectodomain) and for the tyrosine kinase catalytic domain in the cytoplasmic portion of the receptor. In addition, previous sequence analyses have identified two fibronectin type III domains in the C-terminal half of each ectodomain monomer. In this report, evidence is provided to show that a third fibronectin type III module exists between the L2 domain and the two previously described fibronectin type III domains.

Partial activation of the insulin receptor kinase domain by juxtamembrane autophosphorylation

Increased enzymatic activity of receptor tyrosine kinases occurs after trans-phosphorylation of one or two tyrosines in the activation loop, located near the catalytic cleft. Partial activation of the insulin receptor's kinase domain was observed at dilute concentrations of kinase, suggesting that cis-autophosphorylation was occurring. Autophosphorylation during partial activation mapped to the juxtamembrane (JM) tyrosines and not to activation loop tyrosines. Furthermore, a double JM Tyr-to-Phe mutant kinase (JMY2F) did not undergo partial activation but catalyzed substrate phosphorylation at a very low rate. Steady-state kinetics of peptide phosphorylation were determined with and without JM autophosphorylation. The JMY2F mutant was used to prevent concurrent cis-autophosphorylation and therefore to approximate the basal state apoenzyme in the kinetic analysis. Partial activation was dominated by a decreased Michaelis constant for peptide substrate, from KM,PEP >/= 2.5 mM in the basal state to 0.2 mM in the partially activated state; the KM,ATP remained virtually unchanged at approximately 1 mM, and kcat increased from 180 to 600 min-1. The high KM,PEP suggests weak binding of peptide substrates to the apoenzyme. This was confirmed by Ki > 1 mM for peptide substrates used as inhibitors of JM autophosphorylation. The absence of comparably large changes in kcat and KM,ATP suggests that the JM region is primarily a strong barrier to the peptide entry step of trans-phosphorylation reactions. The JM region therefore functions as an intrasteric inhibitor in the basal state of the insulin receptor's kinase domain.

Neutralising antibodies to the granulocyte colony-stimulating factor receptor recognise both the immunoglobulin-like domain and the cytokine receptor homologous domain

To define regions of the granulocyte colony-stimulating factor (G-CSF) receptor that are important for ligand binding, neutralising monoclonal antibodies to the human receptor have been produced. Eleven antibodies recognised six different receptor epitopes. Antibodies from three of the epitope groups were able to detect the receptor by western blotting but did not inhibit G-CSF binding. The other three antibody groups inhibited G-CSF binding either completely (groups 1 and 2) or partially (group 3). All the antibodies inhibited proliferation of BA/F3 cells expressing the G-CSF receptor to varying extents. By using human-marine chimeric receptors, the binding sites of the antibodies were mapped to the immunoglobulin-like domain (groups 1 and 3), the cytokine receptor homologous domain (group 2) or the fibronectin type III domains (groups 4 to 6). These results show that the immunoglobulin-like and cytokine receptor homologous domains of the receptor are important for ligand binding and subsequent signalling.

Altered basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from NIDDM patients compared with control subjects

To measure possible changes in basal and insulin-stimulated phosphotyrosine phosphatase (PTPase) activity in skeletal muscle from insulin-resistant individuals, soluble and particulate muscle fractions were prepared from biopsies taken before and after a 3-h hyperinsulinaemic euglycaemic clamp in eight non-insulin-dependent diabetic (NIDDM) patients and nine control subjects. We used a sensitive sandwich-immunofluorescence assay and the human insulin receptor as the substrate. PTPase activity was expressed as percentage of dephosphorylation of phosphotyrosyl-residues in immobilized insulin receptors per 2 h incubation time per 83 micrograms and 19 micrograms muscle fraction protein (soluble and particulate fraction, respectively). In the diabetic soluble muscle fractions, the basal PTPase activity was decreased compared with that of control subjects (11.5 +/- 5.5 vs 27.5 +/- 3.3, p < 0.04, mean +/- SEM). In the particulate muscle fractions from the control subjects, PTPase activity was increased after 3 h hyperinsulinaemia (20.0 +/- 3.2 vs 30.2 +/- 3.6, p < 0.03) and in the corresponding soluble fractions PTPase activity seemed decreased (27.5 +/- 3.3 vs 19.9 +/- 5.9, NS). No effect of insulin on PTPase activity was found in NIDDM patients (25.1 +/- 4.1 vs 27.2 +/- 5.2, 11.5 +/- 5.5 vs 15.1 +/- 4.5 [particulate and soluble fractions], NS). In conclusion, we found that the basal PTPase activity in soluble muscle fractions was decreased in NIDDM patients; furthermore, insulin stimulation was unable to increase PTPase activities in the particulate fractions, as opposed to the effect of insulin in control subjects.

Fusion of insulin receptor ectodomains to immunoglobulin constant domains reproduces high-affinity insulin binding in vitro

A unique feature of the insulin receptor is that it is dimeric in the absence of ligand. Dimerization of two adjacent transmembrane domain (TMD) alpha helices has been shown to be critical in receptor kinase activation. Moreover, previous work has suggested that the TMD is involved in stabilizing the high-affinity binding site; soluble receptors expressed after simple truncation at the ectodomain-TMD junction have reduced affinity for insulin. To further examine this issue, we have replaced the TMD and intracellular domain of the soluble human insulin receptor (HIRs) with constant domains from immunoglobulin Fc and lambda subunits (HIRs-Fc and HIRs-lambda). Studies of receptor biosynthesis and binding characteristics were performed following transient transfection of receptor cDNAs into human embryonal kidney 293 cells. Each hybrid receptor was initially synthesized as a single chain proreceptor, followed by cleavage into alpha- and beta-Fc or beta-lambda subunits. The majority of secreted protein appeared in the cell medium as fully processed heterotetramer. Fc fragments released from HIRs-Fc by papain digestion and analyzed by nonreducing SDS-polyacrylamide gel electrophoresis were dimeric. Furthermore, dissociation constants for both chimeras were similar to those for the full-length holoreceptor (wild-type receptor, Kd1 = 200 pM and Kd2 = 2 nM; HIRs-Fc, Kd1 = 200 pM and Kd2 = 40 nM; and HIRs-lambda, Kd1 = 200 pM and Kd2 = 5 nM). These results extend previous observations that dimerization of the membrane-proximal ectodomain is necessary to maintain an intact high-affinity insulin-binding site.

Alanine-scanning mutagenesis of a C-terminal ligand binding domain of the insulin receptor alpha subunit

A recent affinity labeling study has suggested that amino acids 704-717 of the C terminus of the insulin receptor represent a contact site for insulin. To determine whether these amino acids are part of a ligand binding site, we have performed alanine-scanning mutagenesis of this region. Mutant cDNAs encoding recombinant secreted receptors were transiently expressed in 293 EBNA cells, and their insulin binding properties were evaluated. Of the 14 residues in this region only 4 amino acids, Asp-707, Val-712, Pro-716, and Arg-717, could be mutated to alanine without compromising insulin binding. The reduction in affinity resulting from the individual mutation of the remaining amino acids varied from an increase in Kd to 3.69 x 10(-9) M (Asn-711) to greater than 10(-6) M (Thr-704, Phe-705, Glu-706, and His-710); the Kd of native secreted recombinant receptor is 0.56 x 10(-9) M.

Decreased skeletal muscle phosphotyrosine phosphatase (PTPase) activity towards insulin receptors in insulin-resistant Zucker rats measured by delayed Europium fluorescence

In order to measure the phosphotyrosine phosphatase (PTPase) activity in small muscle biopsies, a sandwich-immunofluorescence assay was developed using the phosphorylated human insulin receptor as a substrate, a C-terminal insulin receptor antibody as catching antibody and Europium-labelled anti-phosphotyrosine as detecting antibody. Soluble and particulate muscle fractions were prepared from soleus muscle of obese, diabetic (fa/fa) Zucker rats and their lean littermates (Fa/-). In the soluble muscle fractions of the obese (fa/fa) rats PTPase activity was significantly reduced compared to control (Fa/-) rats (45.2 +/- 2.6% vs 61.3 +/- 4.7%, p < 0.02). This reduction was completely prevented by 24 days of metformin treatment which decreased plasma glucose and plasma insulin levels. In particulate muscle fractions, however, no difference in PTPase activity was found among any groups of rats examined. These results show that the alterations in soluble PTPase activity in the insulin-resistant, diabetic Zucker rat vary with the abnormality in glucose homeostasis.

Antibody binding to the juxtamembrane region of the insulin receptor alters receptor affinity

The effect of three antibodies that interact with distinct regions of the insulin receptor (the alpha subunit (83-7), the juxtamembrane region near tyrosine 960 (960) or the carboxy terminal region of the beta subunit (CT-1) on insulin binding was examined. Detergent-solubilized insulin receptors from IM-9 cells immobilized on Sepharose beads by 960 antisera bound 2-3 times more 125I-insulin tracer (25-60 pM) than receptors immobilized with either 83-7 or CT-1. Pre-incubation of solubilized receptors with either 83-7 or 960 resulted in equivalent depletion (90%) of insulin binding activity from solubilized IM-9 cell extracts, suggesting that both antibodies were in excess and capable of binding a similar population of receptors. Antibody 960, but not CT-1 or 83-7, also increased insulin binding 2 fold to solubilized receptors precipitated with polyethylene glycol. To determine whether the altered binding observed with antibody 960 was due to increased affinity of the receptor for insulin or appearance of more insulin binding sites, binding studies were performed over a wide range of insulin concentrations. Analysis of the resulting binding curves indicated that 960 increased the affinity of the receptor for insulin 3 fold over control (kd = 0.3 nM for 960, and 0.9 nM for 83-7, respectively). The antibody 960 also specifically increased insulin binding to intact, saponin-permeabilized IM-9 cell membranes. These results indicate that binding of 960 antibody to the juxtamembrane region of the insulin receptor alters the affinity of the receptor for insulin. Since tyrosine 960 in the juxtamembrane region has been suggested to play a role in receptor signalling, changes in receptor conformation in this region that are likely to account for the change in affinity may play a role in signal transduction.

The insulin receptor: structure, function, and signaling

The insulin receptor is a member of the ligand-activated receptor and tyrosine kinase family of transmembrane signaling proteins that collectively are fundamentally important regulators of cell differentiation, growth, and metabolism. The insulin receptor has a number of unique physiological and biochemical properties that distinguish it from other members of this large well-studied receptor family. The main physiological role of the insulin receptor appears to be metabolic regulation, whereas all other receptor tyrosine kinases are engaged in regulating cell growth and/or differentiation. Receptor tyrosine kinases are allosterically regulated by their cognate ligands and function as dimers. In all cases but the insulin receptor (and 2 closely related receptors), these dimers are noncovalent, but insulin receptors are covalently maintained as functional dimers by disulfide bonds. The initial response to the ligand is receptor autophosphorylation for all receptor tyrosine kinases. In most cases, this results in receptor association of effector molecules that have unique recognition domains for phosphotyrosine residues and whose binding to these results in a biological response. For the insulin receptor, this does not occur; rather, it phosphorylates a large substrate protein that, in turn, engages effector molecules. Possible reasons for these differences are discussed in this review. The chemistry of insulin is very well characterized because of possible therapeutic interventions in diabetes using insulin derivatives. This has allowed the synthesis of many insulin derivatives, and we review our recent exploitation of one such derivative to understand the biochemistry of the interaction of this ligand with the receptor and to dissect the complicated steps of ligand-induced insulin receptor autophosphorylation. We note possible future directions in the study of the insulin receptor and its intracellular signaling pathway(s).

Binding of the Ras activator son of sevenless to insulin receptor substrate-1 signaling complexes

Signal transmission by insulin involves tyrosine phosphorylation of a major insulin receptor substrate (IRS-1) and exchange of Ras-bound guanosine diphosphate for guanosine triphosphate. Proteins containing Src homology 2 and 3 (SH2 and SH3) domains, such as the p85 regulatory subunit of phosphatidylinositol-3 kinase and growth factor receptor-bound protein 2 (GRB2), bind tyrosine phosphate sites on IRS-1 through their SH2 regions. Such complexes in COS cells were found to contain the heterologously expressed putative guanine nucleotide exchange factor encoded by the Drosophila son of sevenless gene (dSos). Thus, GRB2, p85, or other proteins with SH2-SH3 adapter sequences may link Sos proteins to IRS-1 signaling complexes as part of the mechanism by which insulin activates Ras.

The native alpha 2 beta 2 tetramer is the only subunit structure of the insulin receptor in intact cells and purified receptor preparations

The native subunit structure of the insulin receptor was reinvestigated by two-dimensional nonreducing/reducing gel electrophoresis. Human insulin receptor expressed in murine fibroblasts was found to be a single oligomer, the alpha 2 beta 2 heterotetramer. The structure was assessed using receptor metabolically labeled with [35S]methionine, and using receptor autophosphorylation at two levels of purification: the insulin affinity-purified receptor and the more commonly used wheat germ agglutinin-Sepharose-enriched fraction from whole membrane extracts. Lower molecular weight oligomers and free subunits were observed only upon heating the sample prior to electrophoresis. This artifact of sample handling was dependent upon three factors: (i) temperature, (ii) time of heating, and (iii) impurities typically present in partially purified receptor preparations. We conclude that the alpha 2 beta 2 tetramer is the only insulin receptor subunit structure native in intact cells and subsequently isolated from cell membranes.

Biosynthesis of the precursor of a soluble human insulin receptor ectodomain in insect Sf9 cells infected with a recombinant baculovirus

In contrast to transfected mammalian cells, insect Sf9 cells infected with a recombinant Baculovirus inefficiently process and secrete a soluble derivative of the extracellular domain of the human insulin receptor. The high-mannose form of the receptor precursor that accumulates intracellularly is not grossly aberrant or malfolded, as its interaction with a diverse panel of monoclonal antibodies are comparable to secreted precursor and proteolytically processed receptor, both of which bear partially trimmed oligosaccharide chains. Thus the inefficient step in the biosynthesis of this protein in Sf9 cells is either at, or just preceding, the trimming of its high-mannose oligosaccharide chains.