Specificity of insulin and insulin-like growth factor I receptors investigated using chimeric mini-receptors. Role of C-terminal of receptor alpha subunit

Insulin receptor-inspired soluble insulin binder

The insulin receptor (IR) is a 320 kDa membrane receptor tyrosine kinase mediating the pleiotropic actions of insulin, leading to phosphorylation of several intracellular substrates including serine/threonine-protein kinase (AKT1), and IR autophosphorylation. Structural details of the IR have been recently revealed. A high-binding insulin site, L1 (Kd =2 nM), consists of two distant domains in the primary sequence of the IR. Our design simplified the L1 binding site and transformed it into a soluble insulin binder (sIB). The sIB, a 17 kDa protein, binds insulin with 38 nM affinity. The sIB competes with IR for insulin and reduces by more than 50% phosphorylation of AKT1 in HEK 293 T cells, with similar effects on IR autophosphorylation. The sIB represents a new tool for research of insulin binding and signaling properties.

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

The brain as an insulin-sensitive metabolic organ

Background

The brain was once thought of as an insulin-insensitive organ. We now know that the insulin receptor is present throughout the brain and serves important functions in whole-body metabolism and brain function. Brain insulin signaling is involved not only in brain homeostatic processes but also neuropathological processes such as cognitive decline and Alzheimer's disease.

Scope of review

In this review, we provide an overview of insulin signaling within the brain and the metabolic impact of brain insulin resistance and discuss Alzheimer's disease, one of the neurologic diseases most closely associated with brain insulin resistance.

Major conclusions

While brain insulin signaling plays only a small role in central nervous system glucose regulation, it has a significant impact on the brain's metabolic health. Normal insulin signaling is important for mitochondrial functioning and normal food intake. Brain insulin resistance contributes to obesity and may also play an important role in neurodegeneration.

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Insulin is an important regulator of brain metabolism.

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Brain insulin signaling helps regulate whole body metabolism.

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Brain insulin signaling may play an important role in the pathogenesis of Alzheimer's disease.

Insulin-Like Growth Factor I Regulation and Its Actions in Skeletal Muscle

The insulin-like growth factor (IGF) pathway is essential for promoting growth and survival of virtually all tissues. It bears high homology to its related protein insulin, and as such, there is an interplay between these molecules with regard to their anabolic and metabolic functions. Skeletal muscle produces a significant proportion of IGF-1, and is highly responsive to its actions, including increased muscle mass and improved regenerative capacity. In this overview, the regulation of IGF-1 production, stability, and activity in skeletal muscle will be described. Second, the physiological significance of the forms of IGF-1 produced will be discussed. Last, the interaction of IGF-1 with other pathways will be addressed. © 2019 American Physiological Society. Compr Physiol 9:413-438, 2019.

A thing of beauty: Structure and function of insulin's "aromatic triplet"

The classical crystal structure of insulin was determined in 1969 by D.C. Hodgkin et al. following a 35-year program of research. This structure depicted a hexamer remarkable for its self-assembly as a zinc-coordinated trimer of dimer. Prominent at the dimer interface was an "aromatic triplet" of conserved residues at consecutive positions in the B chain: PheB24 , PheB25 and TyrB26 . The elegance of this interface inspired the Oxford team to poetry: "A thing of beauty is a joy forever" (John Keats as quoted by Blundell, T.L., et al. Advances in Protein Chemistry 26:279-286 [1972]). Here, we revisit this aromatic triplet in light of recent advances in the structural biology of insulin bound as a monomer to fragments of the insulin receptor. Such co-crystal structures have defined how these side chains pack at the primary hormone-binding surface of the receptor ectodomain. On receptor binding, the B-chain β-strand (residues B24-B28) containing the aromatic triplet detaches from the α-helical core of the hormone. Whereas TyrB26 lies at the periphery of the receptor interface and may functionally be replaced by a diverse set of substitutions, PheB24 and PheB25 engage invariant elements of receptor domains L1 and αCT. These critical contacts were anticipated by the discovery of diabetes-associated mutations at these positions by Donald Steiner et al. at the University of Chicago. Conservation of PheB24 , PheB25 and TyrB26 among vertebrate insulins reflects the striking confluence of structure-based evolutionary constraints: foldability, protective self-assembly and hormonal activity.

Insulin Receptor Isoforms in Physiology and Disease: An Updated View

The insulin receptor (IR) gene undergoes differential splicing that generates two IR isoforms, IR-A and IR-B. The physiological roles of IR isoforms are incompletely understood and appear to be determined by their different binding affinities for insulin-like growth factors (IGFs), particularly for IGF-2. Predominant roles of IR-A in prenatal growth and development and of IR-B in metabolic regulation are well established. However, emerging evidence indicates that the differential expression of IR isoforms may also help explain the diversification of insulin and IGF signaling and actions in various organs and tissues by involving not only different ligand-binding affinities but also different membrane partitioning and trafficking and possibly different abilities to interact with a variety of molecular partners. Of note, dysregulation of the IR-A/IR-B ratio is associated with insulin resistance, aging, and increased proliferative activity of normal and neoplastic tissues and appears to sustain detrimental effects. This review discusses novel information that has generated remarkable progress in our understanding of the physiology of IR isoforms and their role in disease. We also focus on novel IR ligands and modulators that should now be considered as an important strategy for better and safer treatment of diabetes and cancer and possibly other IR-related diseases.

IGF1 Receptor Targeted Theranostic Nanoparticles for Targeted and Image-Guided Therapy of Pancreatic Cancer

Overcoming resistance to chemotherapy is a major and unmet medical challenge in the treatment of pancreatic cancer. Poor drug delivery due to stromal barriers in the tumor microenvironment and aggressive tumor biology are additional impediments toward a more successful treatment of pancreatic cancer. In attempts to address these challenges, we developed IGF1 receptor (IGF1R)-directed, multifunctional theranostic nanoparticles for targeted delivery of therapeutic agents into IGF1R-expressing drug-resistant tumor cells and tumor-associated stromal cells. These nanoparticles were prepared by conjugating recombinant human IGF1 to magnetic iron oxide nanoparticles (IONPs) carrying the anthracycline doxorubicin (Dox) as the chemotherapeutic payload. Intravenously administered IGF1-IONPs exhibited excellent tumor targeting and penetration in an orthotopic patient-derived xenograft (PDX) model of pancreatic cancer featuring enriched tumor stroma and heterogeneous cancer cells. IGF1R-targeted therapy using the theranostic IGF1-IONP-Dox significantly inhibited the growth of pancreatic PDX tumors. The effects of the intratumoral nanoparticle delivery and therapeutic responses in the orthotopic pancreatic PDX tumors could be detected by magnetic resonance imaging (MRI) with IONP-induced contrasts. Histological analysis showed that IGF1R-targeted delivery of Dox significantly inhibited cell proliferation and induced apoptotic cell death of pancreatic cancer cells. Therefore, further development of IGF1R-targeted theranostic IONPs and MRI-guided cancer therapy as a precision nanomedicine may provide the basis for more effective treatment of pancreatic cancer.

Theoretical and computational studies of peptides and receptors of the insulin family

Synergistic interactions among peptides and receptors of the insulin family are required for glucose homeostasis, normal cellular growth and development, proliferation, differentiation and other metabolic processes. The peptides of the insulin family are disulfide-linked single or dual-chain proteins, while receptors are ligand-activated transmembrane glycoproteins of the receptor tyrosine kinase (RTK) superfamily. Binding of ligands to the extracellular domains of receptors is known to initiate signaling via activation of intracellular kinase domains. While the structure of insulin has been known since 1969, recent decades have seen remarkable progress on the structural biology of apo and liganded receptor fragments. Here, we review how this useful structural information (on ligands and receptors) has enabled large-scale atomically-resolved simulations to elucidate the conformational dynamics of these biomolecules. Particularly, applications of molecular dynamics (MD) and Monte Carlo (MC) simulation methods are discussed in various contexts, including studies of isolated ligands, apo-receptors, ligand/receptor complexes and intracellular kinase domains. The review concludes with a brief overview and future outlook for modeling and computational studies in this family of proteins.

Flexibility in the insulin receptor ectodomain enables docking of insulin in crystallographic conformation observed in a hormone-bound microreceptor

Insulin binding to the insulin receptor (IR) is the first key step in initiating downstream signaling cascades for glucose homeostasis in higher organisms. The molecular details of insulin recognition by IR are not yet completely understood, but a picture of hormone/receptor interactions at one of the epitopes (Site 1) is beginning to emerge from recent structural evidence. However, insulin-bound structures of truncated IR suggest that crystallographic conformation of insulin cannot be accommodated in the full IR ectodomain due to steric overlap of insulin with the first two type III fibronectin domains (F1 and F2), which are contributed to the insulin binding-pocket by the second subunit in the IR homodimer. A conformational change in the F1-F2 pair has thus been suggested. In this work, we present an all-atom structural model of complex of insulin and the IR ectodomain, where no structural overlap of insulin with the receptor domains (F1 and F2) is observed. This structural model was arrived at by flexibly fitting parts of our earlier insulin/IR all-atom model into the simulated density maps of crystallized constructs combined with conformational sampling from apo-IR solution conformations. Importantly, our experimentally-consistent model helps rationalize yet unresolved Site.

Optimizing IGF-I for skeletal muscle therapeutics

It is virtually undisputed that IGF-I promotes cell growth and survival. However, the presence of several IGF-I isoforms, vast numbers of intracellular signaling components, and multiple receptors results in a complex and highly regulated system by which IGF-I actions are mediated. IGF-I has long been recognized as one of the critical factors for coordinating muscle growth, enhancing muscle repair, and increasing muscle mass and strength. How to optimize this panoply of pathways to drive anabolic processes in muscle as opposed to aberrant growth in other tissues is an area that deserves focus. This review will address how advances in the bioavailability, potency, and tissue response of IGF-I can provide new potential directions for skeletal muscle therapeutics.

The complexity of the IGF1 gene splicing, posttranslational modification and bioactivity

The insulinlike growth factor-I (IGF-I) is an important factor which regulates a variety of cellular responses in multiple biological systems. The IGF1 gene comprises a highly conserved sequence and contains six exons, which give rise to heterogeneous mRNA transcripts by a combination of multiple transcription initiation sites and alternative splicing. These multiple transcripts code for different precursor IGF-I polypeptides, namely the IGF-IEa, IGF-IEb and IGF-IEc isoforms in humans, which also undergo posttranslational modifications, such as proteolytic processing and glycosylation. IGF-I actions are mediated through its binding to several cell-membrane receptors and the IGF-I domain responsible for the receptor binding is the bioactive mature IGF-I peptide, which is derived after the posttranslational cleavage of the pro-IGF-I isoforms and the removal of their carboxy-terminal E-peptides (that is, the Ea, Eb and Ec). Interestingly, differential biological activities have been reported for the different IGF-I isoforms, or for their E-peptides, implying that IGF-I peptides other than the IGF-I ligand also possess bioactivity and, thus, both common and unique or complementary pathways exist for the IGF-I isoforms to promote biological effects. The multiple peptides derived from IGF-I and the differential expression of its various transcripts in different conditions and pathologies appear to be compatible with the distinct cellular responses observed to the different IGF-I peptides and with the concept of a complex and possibly isoform-specific IGF-I bioactivity. This concept is discussed in the present review, in the context of the broad range of modifications that this growth factor undergoes which might regulate its mechanism(s) of action.

Functional collaboration of insulin-like growth factor-1 receptor (IGF-1R), but not insulin receptor (IR), with acute GH signaling in mouse calvarial cells

GH signals through the GH receptor (GHR), a cytokine receptor linked to Janus kinase 2 (JAK2). GH activates signal transducer and activator of transcription 5 (STAT5), causing expression of genes including IGF-I. IGF-I binds IGF-I receptor (IGF-IR), a heterotetrameric (α2-β2) tyrosine kinase growth factor receptor similar to insulin receptor (IR). In addition to this GH -> GHR -> IGF-I -> IGF-IR pathway, GH induces a complex including GHR, JAK2, and IGF-IR and deletion of floxed IGF-1R in primary murine calvarial cells with Cre-recombinase-expressing adenovirus (Ad-Cre) desensitizes cells to GH for STAT5 activation and IGF-I mRNA accumulation. Diminished GH-induced STAT5 phosphorylation in Ad-Cre-treated cells is rescued by adenoviruses encoding either IGF-IR or IGF-IR lacking the β-chain intracellular domain. Reasoning that IGF-IR's extracellular portion (α or extracellular β) mediates functional interaction with GH signaling, we pursued reconstitution studies. Although structurally related to IGF-IR, IR expressed adenovirally did not rescue GH-induced STAT5 phosphorylation in Ad-Cre-treated cells. We thus created chimeras, swapping homologous IR extracellular regions into IGF-IR. IR and IGF-IR possess N-terminal L1, cysteine-rich (CR), and L2 α-chain domains. We created Ad-IGF-IR/IR-L1 and Ad-IGF-IR/IR-L1-CR-L2, in which L1 alone or L1, CR, and L2 of IR replace corresponding IGF-IR regions, respectively. Ad-IGF-IR/IR-L1, but not Ad-IGF-IR/IR-L1-CR-L2, rescued GH-induced STAT5 phosphorylation in Ad-Cre-treated cells. Additionally, medium containing a soluble IGF-IR (including only L1-CR-L2) dampened GH-induced STAT5 phosphorylation in calvarial cells and two other GH-responsive cell lines. Thus, an extracellular determinant(s), likely in CR-L2, specifically allows IGF-IR to collaborate with GHR and JAK2 for robust GH-induced acute STAT5 phosphorylation.

All-atom structural models of insulin binding to the insulin receptor in the presence of a tandem hormone-binding element

Insulin regulates blood glucose levels in higher organisms by binding to and activating insulin receptor (IR), a constitutively homodimeric glycoprotein of the receptor tyrosine kinase (RTK) superfamily. Therapeutic efforts in treating diabetes have been significantly impeded by the absence of structural information on the activated form of the insulin/IR complex. Mutagenesis and photo-crosslinking experiments and structural information on insulin and apo-IR strongly suggest that the dual-chain insulin molecule, unlike the related single-chain insulin-like growth factors, binds to IR in a very different conformation than what is displayed in storage forms of the hormone. In particular, hydrophobic residues buried in the core of the folded insulin molecule engage the receptor. There is also the possibility of plasticity in the receptor structure based on these data, which may in part be due to rearrangement of the so-called CT-peptide, a tandem hormone-binding element of IR. These possibilities provide opportunity for large-scale molecular modeling to contribute to our understanding of this system. Using various atomistic simulation approaches, we have constructed all-atom structural models of hormone/receptor complexes in the presence of CT in its crystallographic position and a thermodynamically favorable displaced position. In the "displaced-CT" complex, many more insulin-receptor contacts suggested by experiments are satisfied, and our simulations also suggest that R-insulin potentially represents the receptor-bound form of hormone. The results presented in this work have further implications for the design of receptor-specific agonists/antagonists.

How insulin engages its primary binding site on the insulin receptor

Insulin receptor signalling has a central role in mammalian biology, regulating cellular metabolism, growth, division, differentiation and survival. Insulin resistance contributes to the pathogenesis of type 2 diabetes mellitus and the onset of Alzheimer's disease; aberrant signalling occurs in diverse cancers, exacerbated by cross-talk with the homologous type 1 insulin-like growth factor receptor (IGF1R). Despite more than three decades of investigation, the three-dimensional structure of the insulin-insulin receptor complex has proved elusive, confounded by the complexity of producing the receptor protein. Here we present the first view, to our knowledge, of the interaction of insulin with its primary binding site on the insulin receptor, on the basis of four crystal structures of insulin bound to truncated insulin receptor constructs. The direct interaction of insulin with the first leucine-rich-repeat domain (L1) of insulin receptor is seen to be sparse, the hormone instead engaging the insulin receptor carboxy-terminal α-chain (αCT) segment, which is itself remodelled on the face of L1 upon insulin binding. Contact between insulin and L1 is restricted to insulin B-chain residues. The αCT segment displaces the B-chain C-terminal β-strand away from the hormone core, revealing the mechanism of a long-proposed conformational switch in insulin upon receptor engagement. This mode of hormone-receptor recognition is novel within the broader family of receptor tyrosine kinases. We support these findings by photo-crosslinking data that place the suggested interactions into the context of the holoreceptor and by isothermal titration calorimetry data that dissect the hormone-insulin receptor interface. Together, our findings provide an explanation for a wealth of biochemical data from the insulin receptor and IGF1R systems relevant to the design of therapeutic insulin analogues.

Insulin action on H292 bronchial carcinoma cells as compared to normal bronchial epithelial cells

Inhaled insulin may contribute to bronchial carcinoma due to IGF-I receptor activation by high local concentrations. Therefore, effects of insulin and IGF-I on human bronchial carcinoma cells (H292) and normal bronchial epithelium cells (HBE) were studied. TGF-β was included since it also influences carcinoma progression. H292 and HBE cells expressed both the insulin receptor and the IGF-I receptor; in H292 cells an additional, shorter, splicing variant (IR-A) of the insulin receptor was present. Insulin receptor expression was around four to five times higher in H292 than in HBE cells at mRNA and protein levels. Insulin and TGF-β exerted contrary actions on proliferation and gene expression in H292 cells. Genes regulated by insulin, IGF-I, and TGF-β were linked to inflammation, cell adhesion, muscle contraction and differentiation. Insulin and IGF-I also suppressed DNA repair genes. EC(50) for insulin-induced proliferation was around 5 nM in H292 and around 30 nM HBE cells. The EC(50) values for gene expression ranged from 9 to 90 nM in both cell types, dependent on the gene studied. In H292 cells, the proliferative response was much stronger if TGF-β was present. In HBE cells this interaction of insulin and TGF-β was not observed, and changes in gene expression were mostly lower by at least 10-fold as compared to H292. All in all, the insulin effects in H292 were generally much stronger than in HBE cells and - with regard to proliferation - occurred at lower concentrations. Thus, insulin will hardly induce cancer from normal bronchial cells but may favour progression of pre-existing tumours.

Insulin receptor-related receptor as an extracellular alkali sensor

The insulin receptor-related receptor (IRR), an orphan receptor tyrosine kinase of the insulin receptor family, can be activated by alkaline media both in vitro and in vivo at pH >7.9. The alkali-sensing property of IRR is conserved in frog, mouse, and human. IRR activation is specific, dose-dependent and quickly reversible and demonstrates positive cooperativity. It also triggers receptor conformational changes and elicits intracellular signaling. The pH sensitivity of IRR is primarily defined by its L1F extracellular domains. IRR is predominantly expressed in organs that come in contact with mildly alkaline media. In particular, IRR is expressed in the cell subsets of the kidney that secrete bicarbonate into urine. Disruption of IRR in mice impairs the renal response to alkali loading attested by development of metabolic alkalosis and decreased urinary bicarbonate excretion in response to this challenge. We therefore postulate that IRR is an alkali sensor that functions in the kidney to manage metabolic bicarbonate excess.

Overall mortality in diabetes mellitus: where do we stand today?

Life expectancy for a patient with type 2 diabetes remains substantially shorter than an equivalent individual without diabetes, largely because of a greater risk of cardiovascular disease. Diabetes is also associated with an increased incidence of many types of cancer, suggesting that malignancy may also contribute to higher rates of mortality. Hyperglycemia is one of the key risk factors for diabetes-associated macro- and microvascular disease, and as such, intensive glycemic control is associated with improved outcomes for patients, including a reduction in this risk of death from any cause, when initiated early in the disease course. Recent trials in patients with more advanced disease have failed to demonstrate a mortality benefit with intensive glycemic control, although this may reflect their short observation period. Intensive multifactorial therapy, including lifestyle intervention and control of hyperglycemia, hypertension, lipids, thrombosis, and microalbuminuria, is likely to be the best strategy against diabetes-associated macrovascular mortality. However, analysis of the Action to Control Cardiovascular Risk in Diabetes (ACCORD) trial indicates that there may be a subpopulation of patients who are unable to achieve glycemic targets with intensive therapy and that aggressive intensification of treatment in this group may increase mortality risk. It remains to be determined whether the relationship between diabetes and malignancy is causal or whether they share common risk factors. Current recommendations for a healthy lifestyle based on good diet, physical exercise, and weight management in order to control diabetes-related complications are likely to apply in reducing the risk of many forms of cancer and should be advocated for all patients.

All-atom structural models for complexes of insulin-like growth factors IGF1 and IGF2 with their cognate receptor

Type 1 insulin-like growth factor receptor (IGF1R) is a membrane-spanning glycoprotein of the insulin receptor family that has been implicated in a variety of cancers. The key questions related to molecular mechanisms governing ligand recognition by IGF1R remain unanswered, partly due to the lack of testable structural models of apo or ligand-bound receptor complexes. Using a homology model of the IGF1R ectodomain IGF1RDeltabeta, we present the first experimentally consistent all-atom structural models of IGF1/IGF1RDeltabeta and IGF2/IGF1RDeltabeta complexes. Our explicit-solvent molecular dynamics (MD) simulation of apo-IGF1RDeltabeta shows that it displays asymmetric flexibility mechanisms that result in one of two binding pockets accessible to growth factors IGF1 and IGF2, as demonstrated via an MD-assisted Monte Carlo docking procedure. Our MD-generated ensemble of structures of apo and IGF1-bound IGF1RDeltabeta agrees reasonably well with published small-angle X-ray scattering data. We observe simultaneous contacts of each growth factor with sites 1 and 2 of IGF1R, suggesting cross-linking of receptor subunits. Our models provide direct evidence in favor of suggested electrostatic complementarity between the C-domain (IGF1) and the cysteine-rich domain (IGF1R). Our IGF1/IGF1RDeltabeta model provides structural bases for the observation that a single IGF1 molecule binds to IGF1RDeltabeta at low concentrations in small-angle X-ray scattering studies. We also suggest new possible structural bases for differences in the affinities of insulin, IGF1, and IGF2 for their noncognate receptors.

Structural resolution of a tandem hormone-binding element in the insulin receptor and its implications for design of peptide agonists

The C-terminal segment of the human insulin receptor alpha-chain (designated alphaCT) is critical to insulin binding as has been previously demonstrated by alanine scanning mutagenesis and photo-cross-linking. To date no information regarding the structure of this segment within the receptor has been available. We employ here the technique of thermal-factor sharpening to enhance the interpretability of the electron-density maps associated with the earlier crystal structure of the human insulin receptor ectodomain. The alphaCT segment is now resolved as being engaged with the central beta-sheet of the first leucine-rich repeat (L1) domain of the receptor. The segment is alpha-helical in conformation and extends 11 residues N-terminal of the classical alphaCT segment boundary originally defined by peptide mapping. This tandem structural element (alphaCT-L1) thus defines the intact primary insulin-binding surface of the apo-receptor. The structure, together with isothermal titration calorimetry data of mutant alphaCT peptides binding to an insulin minireceptor, leads to the conclusion that putative "insulin-mimetic" peptides in the literature act at least in part as mimics of the alphaCT segment as well as of insulin. Photo-cross-linking by novel bifunctional insulin derivatives demonstrates that the interaction of insulin with the alphaCT segment and the L1 domain occurs in trans, i.e., these components of the primary binding site are contributed by alternate alpha-chains within the insulin receptor homodimer. The tandem structural element defines a new target for the design of insulin agonists for the treatment of diabetes mellitus.

In vitro metabolic and mitogenic signaling of insulin glargine and its metabolites

Background

Insulin glargine (Lantus®) is a long-acting basal insulin analog that demonstrates effective day-long glycemic control and a lower incidence of hypoglycemia than NPH insulin. After subcutaneous injection insulin glargine is partly converted into the two main metabolites M1 ([Gly^A21]insulin) and M2 ([Gly^A21,des-Thr^B30]insulin). The aim of this study was to characterize the glargine metabolites in vitro with regard to their insulin receptor (IR) and IGF-1 receptor (IGF1R) binding and signaling properties as well as their metabolic and mitogenic activities.

Methods

The affinity of human insulin, insulin glargine and its metabolites to the IR isoforms A and B or IGF1R was analyzed in a competitive binding assay using SPA technology. Receptor autophosphorylation activities were studied via In-Cell Western in CHO and MEF cells overexpressing human IR-A and IR-B or IGF1R, respectively. The metabolic response of the insulins was studied as stimulation of lipid synthesis using primary rat adipocytes. Thymidine incorporation in Saos-2 cells was used to characterize the mitogenic activity.

Conclusions

The binding of insulin glargine and its metabolites M1 and M2 to the IR were similar and correlated well with their corresponding autophosphorylation and metabolic activities in vitro. No differences were found towards the two IR isoforms A or B. Insulin glargine showed a higher affinity for IGF1R than insulin, resulting in a lower EC₅₀ value for autophosphorylation of the receptor and a more potent stimulation of thymidine incorporation in Saos-2 cells. In contrast, the metabolites M1 and M2 were significantly less active in binding to and activation of the IGF1R and their mitogenicity in Saos-2 cells was equal to human insulin. These findings strongly support the idea that insulin glargine metabolites contribute with the same potency as insulin glargine to blood glucose control but lead to significantly reduced growth-promoting activity.

A thermodynamic study of ligand binding to the first three domains of the human insulin receptor: relationship between the receptor alpha-chain C-terminal peptide and the site 1 insulin mimetic peptides

The C-terminal segment of the insulin receptor (IR) alpha-chain plays a critical role in insulin binding. This 16-residue peptide together with the central beta-sheet of the receptor L1 domain forms one of the insulin binding surfaces of the IR monomer. Here we use isothermal titration calorimetry to assay directly the binding of the IR alphaCT peptide to an IR construct (IR485) consisting of the three N-terminal domains of the receptor monomer. Our measurements show further that the binding of the IR alphaCT peptide to IR485 competes with the binding of a prototypical "Site 1" insulin mimetic peptide to the same receptor fragment. The competitive nature of their binding appears to be reflected in a previously undetected sequence similarity between the IR alphaCT peptide and the Site 1 mimetic peptide. In contrast, a prototypical "Site 2" peptide has very limited affinity for IR485. Taken together, these results complement our recent observation that there is a possible structural relationship between these mimetic peptides and insulin itself. They also add support to the view that the segment of unexplained electron density lying on the surface of the central beta-sheet of the L1 domain in the IR ectodomain crystal structure arises from the IR alphaCT peptide. Finally, we show that mutation of the critical IR alphaCT peptide residue Phe714 to alanine does not affect the peptide's affinity for IR485 and conclude that the resultant loss of insulin binding with this mutation results from loss of interaction of the phenylalanine side chain with insulin.

The insulin-like growth factor (IGF)-I E-peptides modulate cell entry of the mature IGF-I protein

Insulin-like growth factor (IGF)-I is a critical protein for cell development and growth. Alternative splicing of the igf1 gene gives rise to multiple isoforms. In rodents, proIGF-IA and proIGF-IB have different carboxy-terminal extensions called the E-peptides (EA and EB) and upon further posttranslational processing, produce the identical mature IGF-I protein. Rodent EB has been reported to have mitogenic and motogenic effects independent of IGF-I. However, effects of EA or EB on mature IGF-I, or whether proIGF-IA and proIGF-IB have different properties, have not been addressed. To determine whether the presence of EA or EB affected the distribution and stability of mature IGF-I protein, transient transfections of cDNAs encoding murine IGF-IA, IGF-IB, and mature IGF-I were performed in C2C12 cells, a skeletal muscle cell line. IGF-I secretion was measured by enzyme-linked immunosorbent assay of the media, and did not differ between expression of proIGF-IA, proIGF-IB, or mature IGF-I expression. Next, epitope-tagged constructs were transfected to determine cellular distribution of IGF-I, EA, and EB in the cells throughout the culture. IGF-I was detected in significantly fewer nontransfected cells in cultures transfected with mature IGF-I compared with transfection of proIGF-IA or proIGF-IB. These results demonstrate that EA and EB are not required for IGF-I secretion but that they increase cell entry of IGF-I from the media. This study provides evidence that the EA and EB may modulate IGF-I in addition to having independent activity.

Ligand-induced activation of the insulin receptor: a multi-step process involving structural changes in both the ligand and the receptor

Current models of insulin binding to the insulin receptor (IR) propose (i) that there are two binding sites on the surface of insulin which engage with two binding sites on the receptor and (ii) that ligand binding involves structural changes in both the ligand and the receptor. Many of the features of insulin binding to its receptor, namely B-chain helix interactions with the leucine-rich repeat domain and A-chain residue interactions with peptide loops from another part of the receptor, are also seen in models of relaxin and insulin-like peptide 3 binding to their receptors. We show that these principles can likely be extended to the group of mimetic peptides described by Schäffer and coworkers, which are reported to have no sequence identity with insulin. This review summarizes our current understanding of ligand-induced activation of the IR and highlights the key issues that remain to be addressed.

A novel approach to identify two distinct receptor binding surfaces of insulin-like growth factor II

Very little is known about the residues important for the interaction of insulin-like growth factor II (IGF-II) with the type 1 IGF receptor (IGF-1R) and the insulin receptor (IR). Insulin, to which IGF-II is homologous, is proposed to cross-link opposite halves of the IR dimer through two receptor binding surfaces, site 1 and site 2. In the present study we have analyzed the contribution of IGF-II residues equivalent to insulin's two binding surfaces toward the interaction of IGF-II with the IGF-1R and IR. Four "site 1" and six "site 2" analogues were produced and analyzed in terms of IGF-1R and IR binding and activation. The results show that Val(43), Phe(28), and Val(14) (equivalent to site 1) are critical to IGF-1R and IR binding, whereas mutation to alanine of Gln(18) affects only IGF-1R and not IR binding. Alanine substitutions at Glu(12), Asp(15), Phe(19), Leu(53), and Glu(57) analogues resulted in significant (>2-fold) decreases in affinity for both the IGF-1R and IR. Furthermore, taking a novel approach using a monomeric, single-chain minimized IGF-1R we have defined a distinct second binding surface formed by Glu(12), Phe(19), Leu(53), and Glu(57) that potentially engages the IGF-1R at one or more of the FnIII domains.

A comparative structural bioinformatics analysis of the insulin receptor family ectodomain based on phylogenetic information

The insulin receptor (IR), the insulin-like growth factor 1 receptor (IGF1R) and the insulin receptor-related receptor (IRR) are covalently-linked homodimers made up of several structural domains. The molecular mechanism of ligand binding to the ectodomain of these receptors and the resulting activation of their tyrosine kinase domain is still not well understood. We have carried out an amino acid residue conservation analysis in order to reconstruct the phylogeny of the IR Family. We have confirmed the location of ligand binding site 1 of the IGF1R and IR. Importantly, we have also predicted the likely location of the insulin binding site 2 on the surface of the fibronectin type III domains of the IR. An evolutionary conserved surface on the second leucine-rich domain that may interact with the ligand could not be detected. We suggest a possible mechanical trigger of the activation of the IR that involves a slight 'twist' rotation of the last two fibronectin type III domains in order to face the likely location of insulin. Finally, a strong selective pressure was found amongst the IRR orthologous sequences, suggesting that this orphan receptor has a yet unknown physiological role which may be conserved from amphibians to mammals.

Structural insights into ligand-induced activation of the insulin receptor

The current model for insulin binding to the insulin receptor proposes that there are two binding sites, referred to as sites 1 and 2, on each monomer in the receptor homodimer and two binding surfaces on insulin, one involving residues predominantly from the dimerization face of insulin (the classical binding surface) and the other residues from the hexamerization face. High-affinity binding involves one insulin molecule using its two surfaces to make bridging contacts with site 1 from one receptor monomer and site 2 from the other. Whilst the receptor dimer has two identical site 1-site 2 pairs, insulin molecules cannot bridge both pairs simultaneously. Our structures of the insulin receptor (IR) ectodomain dimer and the L1-CR-L2 fragments of IR and insulin-like growth factor receptor (IGF-1R) explain many of the features of ligand-receptor binding and allow the two binding sites on the receptor to be described. The IR dimer has an unexpected folded-over conformation which places the C-terminal surface of the first fibronectin-III domain in close juxtaposition to the known L1 domain ligand-binding surface suggesting that the C-terminal surface of FnIII-1 is the second binding site involved in high-affinity binding. This is very different from previous models based on three-dimensional reconstruction from scanning transmission electron micrographs. Our single-molecule images indicate that IGF-1R has a morphology similar to that of IR. In addition, the structures of the first three domains (L1-CR-L2) of the IR and IGF-1R show that there are major differences in the two regions governing ligand specificity. The implications of these findings for ligand-induced receptor activation will be discussed. This review summarizes the key findings regarding the discovery and characterization of the insulin receptor, the identification and arrangement of its structural domains in the sequence and the key features associated with ligand binding. The remainder of the review deals with a description of the receptor structure and how it explains much of the large body of biochemical data in the literature on insulin binding and receptor activation.

An investigation of the ligand binding properties and negative cooperativity of soluble insulin-like growth factor receptors

To investigate the interaction of the insulin-like growth factor (IGF) ligands with the insulin-like growth factor type 1 receptor (IGF-1R), we have generated two soluble variants of the IGF-1R. We have recombinantly expressed the ectodomain of IGF-1R or fused this domain to the constant domain from the Fc fragment of mouse immunoglobulin. The ligand binding properties of these soluble IGF-1Rs for IGF-I and IGF-II were investigated using conventional ligand competition assays and BIAcore biosensor technology. In ligand competition assays, the soluble IGF-1Rs both bound IGF-I with similar affinities and a 5-fold lower affinity than that seen for the wild type receptor. In addition, both soluble receptors bound IGF-II with similar affinities to the wild type receptor. BIAcore analyses showed that both soluble IGF-1Rs exhibited similar ligand-specific association and dissociation rates for IGF-I and for IGF-II. The soluble IGF-1R proteins both exhibited negative cooperativity for IGF-I, IGF-II, and the 24-60 antibody, which binds to the IGF-1R cysteine-rich domain. We conclude that the addition of the self-associating Fc domain to the IGF-1R ectodomain does not affect ligand binding affinity, which is in contrast to the soluble ectodomain of the IR. This study highlights some significant differences in ligand binding modes between the IGF-1R and the insulin receptor, which may ultimately contribute to the different biological activities conferred by the two receptors.

Characterization of insulin/IGF hybrid receptors: contributions of the insulin receptor L2 and Fn1 domains and the alternatively spliced exon 11 sequence to ligand binding and receptor activation

The IR (insulin receptor) and IGFR (type I insulin-like growth factor receptor) are found as homodimers, but the respective pro-receptors can also heterodimerize to form insulin-IGF hybrid receptors. There are conflicting data on the ligand affinity of hybrids, and especially on the influence of different IR isoforms. To investigate further the contribution of individual ligand binding epitopes to affinity and specificity in the IR/IGFR family, we generated hybrids incorporating both IR isoforms (A and B) and IR/IGFR domain-swap chimaeras, by ectopic co-expression of receptor constructs in Chinese hamster ovary cells, and studied ligand binding using both radioligand competition and bioluminescence resonance energy transfer assays. We found that IR-A-IGFR and IR-B-IGFR hybrids bound insulin with similar relatively low affinity, which was intermediate between that of homodimeric IR and homodimeric IGFR. However, both IR-A-IGFR and IR-B-IGFR hybrids bound IGF-I and IGF-II with high affinity, at a level comparable with homodimeric IGFR. Incorporation of a significant fraction of either IR-A or IR-B into hybrids resulted in abrogation of insulin- but not IGF-I-stimulated autophosphorylation. We conclude that the sequence of 12 amino acids encoded by exon 11 of the IR gene has little or no effect on ligand binding and activation of IR-IGFR hybrids, and that hybrid receptors bind IGFs but not insulin at physiological concentrations regardless of the IR isoform they contained. To reconstitute high affinity insulin binding within a hybrid receptor, chimaeras in which the IGFR L1 or L2 domains had been replaced by equivalent IR domains were co-expressed with full-length IR-A or IR-B. In the context of an IR-A-IGFR hybrid, replacement of IR residues 325-524 (containing the L2 domain and part of the first fibronectin domain) with the corresponding IGFR sequence increased the affinity for insulin by 20-fold. We conclude that the L2 and/or first fibronectin domains of IR contribute in trans with the L1 domain to create a high affinity insulin-binding site within a dimeric receptor.

The insulin and EGF receptor structures: new insights into ligand-induced receptor activation

The insulin receptor (IR) and epidermal growth factor receptor (EGFR; also known as ErbB) families exhibit similarities in the composition of their ectodomains. The past five years have seen structures determined for all members of the EGFR family including some complexes with ligand or monoclonal antibody fragments. These structures have led to a clearer understanding of their mechanism of activation and inhibition. By contrast, obtaining equivalent understanding of the IR family has lagged behind. However, within the past year, structures of partial and complete ectodomains of the IR have been published that show that the extracellular region of the receptor adopts an unexpected 'inverted V' conformation relative to the cell membrane. This is very different from the folded-over (tethered) conformation of the unactivated EGFR and provides insight into the potential mechanism of activation of the IR.

Structure of the insulin receptor ectodomain reveals a folded-over conformation

The insulin receptor is a phylogenetically ancient tyrosine kinase receptor found in organisms as primitive as cnidarians and insects. In higher organisms it is essential for glucose homeostasis, whereas the closely related insulin-like growth factor receptor (IGF-1R) is involved in normal growth and development. The insulin receptor is expressed in two isoforms, IR-A and IR-B; the former also functions as a high-affinity receptor for IGF-II and is implicated, along with IGF-1R, in malignant transformation. Here we present the crystal structure at 3.8 A resolution of the IR-A ectodomain dimer, complexed with four Fabs from the monoclonal antibodies 83-7 and 83-14 (ref. 4), grown in the presence of a fragment of an insulin mimetic peptide. The structure reveals the domain arrangement in the disulphide-linked ectodomain dimer, showing that the insulin receptor adopts a folded-over conformation that places the ligand-binding regions in juxtaposition. This arrangement is very different from previous models. It shows that the two L1 domains are on opposite sides of the dimer, too far apart to allow insulin to bind both L1 domains simultaneously as previously proposed. Instead, the structure implicates the carboxy-terminal surface of the first fibronectin type III domain as the second binding site involved in high-affinity binding.

The first three domains of the insulin receptor differ structurally from the insulin-like growth factor 1 receptor in the regions governing ligand specificity

The insulin receptor (IR) and the type-1 insulin-like growth factor receptor (IGF1R) are homologous multidomain proteins that bind insulin and IGF with differing specificity. Here we report the crystal structure of the first three domains (L1-CR-L2) of human IR at 2.3 A resolution and compare it with the previously determined structure of the corresponding fragment of IGF1R. The most important differences seen between the two receptors are in the two regions governing ligand specificity. The first is at the corner of the ligand-binding surface of the L1 domain, where the side chain of F39 in IR forms part of the ligand binding surface involving the second (central) beta-sheet. This is very different to the location of its counterpart in IGF1R, S35, which is not involved in ligand binding. The second major difference is in the sixth module of the CR domain, where IR contains a larger loop that protrudes further into the ligand-binding pocket. This module, which governs IGF1-binding specificity, shows negligible sequence identity, significantly more alpha-helix, an additional disulfide bond, and opposite electrostatic potential compared to that of the IGF1R.

Mapping of the insulin-like growth factor II binding site of the Type I insulin-like growth factor receptor by alanine scanning mutagenesis

The Type I insulin-like growth factor receptor is a physiological receptor for insulin-like growth factor II (IGF-II). To characterize the molecular basis of the receptor's ligand binding properties, we have examined the effects of alanine mutations of residues in the ligand binding site of the receptor on its affinity for IGF-II. The functional epitope for IGF-II comprises residues in the N-terminal L1 domain and residues at the C-terminus of the alpha subunit. Cysteine rich domain residues do not appear to be critical for IGF-II binding.

Comparison of the Functional Insulin Binding Epitopes of the A and B Isoforms of the Insulin Receptor

The human insulin receptor is expressed as two isoforms that are generated by alternate splicing of its mRNA; the B isoform has 12 additional amino acids (718-729) encoded by exon 11 of the gene. The isoforms have been reported to have different ligand binding properties. To further characterize their insulin binding properties, we have performed structure-directed alanine-scanning mutagenesis of a major insulin binding site of the receptor, formed from the receptor L1 domain (amino acids 1-470) and amino acids 705-715 at the C terminus of the alpha subunit. Alanine mutants of each isoform were transiently expressed as recombinant secreted extracellular domain in 293 cells, and their insulin binding properties were evaluated by competitive binding assays. Mutation of Arg(86) and Phe(96) of each isoform resulted in receptors that were not secreted. The Kds of unmutated receptors were almost identical for both isoforms. Several new mutations compromising insulin binding were identified. In L1, mutation of Leu(37) decreased affinity 20- to 40-fold and mutations of Val(94), Glu(97), Glu(120), and Lys(121) 3 to 10-fold for each isoform. A number of mutations produced differential effects on the two isoforms. Mutation of Asn(15) in the L1 domain and Phe(714) at the C terminus of the alpha subunit inactivated the A isoform but only reduced the affinity of the B isoform 40- to 60-fold. At the C terminus of the alpha subunit, mutations of Asp(707), Val(713), and Val(715) produced 7- to 16-fold reductions in affinity of the A isoform but were without effect on the B isoform. In contrast, alanine mutations of Tyr(708) and Asn(711) inactivated the B isoform but only reduced the affinities of the A isoform 11- and 6-fold, respectively. In conclusion, alanine-scanning mutagenesis of the insulin receptor A and B isoforms has identified several new side chains contributing to insulin binding and indicates that the energetic contributions of certain side chains differ in each isoform, suggesting that different molecular mechanisms are used to obtain the same affinity.

Functional reconstitution of insulin receptor binding site from non-binding receptor fragments

We have previously shown that a minimized insulin receptor (IR) consisting of the first 468 amino acids of the insulin receptor fused to 16 amino acids from the C terminus of the alpha-subunit (CT domain) bound insulin with nanomolar affinity (Kristensen, C., Wiberg, F. C., Schäffer, L., and Andersen, A. S. (1998) J. Biol. Chem. 273, 17780-17786). In the present study, we show that a smaller construct that has the first 308 residues fused to the CT domain also binds insulin. Insulin receptor fragments consisting of the first 468 or 308 residues did not bind insulin. However, when these fragments were mixed with a synthetic peptide corresponding to the CT domain, insulin binding was detectable. At concentrations of 10 microm CT peptide, insulin binding was fully reconstituted yielding apparent affinities of 9-11 nm. To further investigate the minimum requirement for the length of the N terminus of IR, we tested smaller receptor fragments for insulin binding in the presence of the CT peptide and found that a fragment consisting of the first 255 amino acids of IR was able to fully reconstitute the insulin binding site, yielding an apparent affinity of 11 +/- 4 nm for insulin.

Role of insulin receptor dimerization domains in ligand binding, cooperativity, and modulation by anti-receptor antibodies

To define the structures within the insulin receptor (IR) that are required for high affinity ligand binding, we have used IR fragments consisting of four amino-terminal domains (L1, cysteine-rich, L2, first fibronectin type III domain) fused to sequences encoded by exon 10 (including the carboxyl terminus of the alpha-subunit). The fragments contained one or both cysteine residues (amino acids 524 and 682) that form disulfides between alpha-subunits in native IR. A dimeric fragment designated IR593.CT (amino acids 1-593 and 704-719) bound (125)I-insulin with high affinity comparable to detergent-solubilized wild type IR and mIR.Fn0/Ex10 (amino acids 1-601 and 650-719) and greater than that of dimeric mIR.Fn0 (amino acids 1-601 and 704-719) and monomeric IR473.CT (amino acids 1-473 and 704-719). However, neither IR593.CT nor mIR.Fn0 exhibited negative cooperativity (a feature characteristic of the native insulin receptor and mIR.Fn0/Ex10), as shown by failure of unlabeled insulin to accelerate dissociation of bound (125)I-insulin. Anti-receptor monoclonal antibodies that recognize epitopes in the first fibronectin type III domain (amino acids 471-593) and inhibit insulin binding to wild type IR inhibited insulin binding to mIR.Fn0/Ex10 but not IR593.CT or mIR.Fn0. We conclude the following: 1) precise positioning of the carboxyl-terminal sequence can be a critical determinant of binding affinity; 2) dimerization via the first fibronectin domain alone can contribute to high affinity ligand binding; and 3) the second dimerization domain encoded by exon 10 is required for ligand cooperativity and modulation by antibodies.

Alanine scanning mutagenesis of a type 1 insulin-like growth factor receptor ligand binding site

The high resolution crystal structure of an N-terminal fragment of the IGF-I receptor, has been reported. While this fragment is itself devoid of ligand binding activity, mutational analysis has indicated that its N terminus (L1, amino acids 1-150) and the C terminus of its cysteine-rich domain (amino acids 190-300) contain ligand binding determinants. Mutational analysis also suggests that amino acids 692-702 from the C terminus of the alpha subunit are critical for ligand binding. A fusion protein, formed from these fragments, binds IGF-I with an affinity similar to that of the whole extracellular domain, suggesting that these are the minimal structural elements of the IGF-I binding site. To further characterize the binding site, we have performed structure directed and alanine-scanning mutagenesis of L1, the cysteine-rich domain and amino acids 692-702. Alanine mutants of residues in these regions were transiently expressed as secreted recombinant receptors and their affinity was determined. In L1 alanine mutants of Asp(8), Asn(11), Tyr(28), His(30), Leu(33), Leu(56), Phe(58), Arg(59), and Trp(79) produced a 2- to 10-fold decrease in affinity and alanine mutation of Phe(90) resulted in a 23-fold decrease in affinity. In the cysteine-rich domain, mutation of Arg(240), Phe(241), Glu(242), and Phe(251) produced a 2- to 10-fold decrease in affinity. In the region between amino acids 692 and 702, alanine mutation of Phe(701) produced a receptor devoid of binding activity and alanine mutations of Phe(693), Glu(693), Asn(694), Leu(696), His(697), Asn(698), and Ile(700) exhibited decreases in affinity ranging from 10- to 30-fold. With the exception of Trp(79), the disruptive mutants in L1 form a discrete epitope on the surface of the receptor. Those in the cysteine-rich domain essential for intact affinity also form a discrete epitope together with Trp(79).

Hormone-triggered conformational changes within the insulin-receptor ectodomain: requirement for transmembrane anchors

Interaction between two alphabeta half-receptors within the (alphabeta)(2) holoreceptor complex is required for insulin binding with high affinity and for insulin-triggered changes of size and shape. To understand the underlying structure-function relationship, two truncated receptor constructs have been characterized. Reduction in the Stokes radius and increase in the sedimentation coefficient, which are characteristic for wild-type receptors, were entirely lacking for the recombinant human insulin receptor (HIR) ectodomain (HIR-ED). Stokes radii of about 5.8 nm and sedimentation coefficients of 10.2 S were found for both insulin-bound and free HIR-EDs. However, attaching the membrane anchors to the ectodomain, as with the recombinant membrane-anchored ectodomain (HIR-MAED) construct, was sufficient to restore not only high-affinity hormone binding but also the marked insulin-inducible alterations in hydrodynamic properties. The Stokes radii of HIR-MAED complexes, as assessed by non-denaturing PAGE, decreased upon insulin binding from 9.5 nm to 7.9 nm. In parallel, the sedimentation coefficient was increased from 9.0 S to 9.8 S. CD and fluorescence spectroscopy of HIR-MAED revealed only minor insulin-induced changes in the secondary structure. Similarity with wild-type receptors has also been demonstrated by the differential insertion of insulin-bound and free HIR-MAED complexes into artificial bilayer membranes of Triton X-114. The results are consistent with a model of receptor function that ensures a global insulin-triggered reorientation of subdomains within the ectodomain moieties while the secondary structure is essentially retained. For the rearrangement of such subdomains, the transmembrane anchors confer essential structural constraints on the receptor ectodomain.

Ligand-induced conformational change in the minimized insulin receptor

Within the class of insulin and insulin-like growth factor receptors, detailed information about the molecular recognition event at the hormone-receptor interface is limited by the absence of suitable co-crystals. We describe the use of a biologically active insulin derivative labeled with the NBD fluorophore (B29NBD-insulin) to characterize the mechanism of reversible 1:1 complex formation with a fragment of the insulin receptor ectodomain. The accompanying 40 % increase in the fluorescence quantum yield of the label provides the basis for a dynamic study of the hormone-receptor binding event. Stopped-flow fluorescence experiments show that the kinetics of complex formation are biphasic comprising a bimolecular binding event followed by a conformational change. Displacement with excess unlabeled insulin gave monophasic kinetics of dissociation. The rate data are rationalized in terms of available experiments on mutant receptors and the X-ray structure of a non-binding fragment of the receptor of the homologous insulin-like growth factor (IGF-1).

A new secreted insect protein belonging to the immunoglobulin superfamily binds insulin and related peptides and inhibits their activities

Insulin and related peptides are key hormones for the regulation of growth and metabolism. Here we describe a novel high affinity insulin-related peptide-binding protein (IBP) secreted from cells of the insect Spodoptera frugiperda. This IBP is composed of two Ig-like C2 domains, has a molecular mass of 27 kDa, binds human insulin with an affinity of 70 pm, and inhibits insulin signaling through the insulin receptor. The binding protein also binds insulin-like growth factors I and II, proinsulin, mini-proinsulin, and an insulin analog lacking the last 8 amino acids of the B-chain (des-octa peptide insulin) with high affinity, whereas an insulin analog with a Asp-B10 mutation bound with only 1% of the affinity of human insulin. This binding profile suggests that IBP recognizes a region that is highly conserved in the insulin superfamily but distinct from the classical insulin receptor binding site. The closest homologue of the Spodoptera frugiperda binding protein is the essential gene product IMP-L2, found in Drosophila, where it is implicated in neural and ectodermal development (Garbe, J. C., Yang, E., and Fristrom, J. W. (1993) Development 119, 1237-1250). Here we show that the IMP-L2 protein also binds insulin and related peptides, offering a possible functional explanation to the IMP-L2 null lethality.

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.