Binding affinities of insulin-like growth factor-I (IGF-I) fusion proteins to IGF binding protein 1 and IGF-I receptor are not correlated with mitogenic activity

Soluble expression of IGF1 fused to DsbA in SHuffle™ T7 strain: optimization of expression and purification by Box-Behnken design

Production of insulin-like growth factor 1 (IGF1) in Escherichia coli mostly results in the formation of inclusion bodies. In the present study, IGF1 was fused to disulfide bond oxidoreductase A (DsbA) and expressed in SHuffle™ T7 strain, in order to obtain correctly folded protein. Soluble expression and IMAC purification of DsbA-IGF1 were optimized by applying the Box-Behnken design of response surface methodology. The optimization greatly increased concentration of soluble protein from 317 to 2600 mg/L, and IMAC yield from 400 to 1900 mg/L. Results of ANOVA showed induction OD600 and temperature had significant effects on the soluble protein expression while isopropyl-β-d thiogalactoside, in the concentrations tested, displayed no significant effect. Moreover, the three parameters of the binding buffer including, pH, concentration of NaCl, and imidazole displayed significant effects on the IMAC yield. Then, purified DsbA-IGF1 was cleaved by human rhinovirus 3C protease, and authentic IGF1 was obtained in flow through of a subtractive IMAC. Final polishing of the protein by reversed-phase HPLC yielded IGF1 with purity of 96%. The quality attributes of purified IGF1 such as purity, identity, molecular size, molecular weight, secondary structure, and biological activity were assessed and showed to be comparable to the standard IGF1. The final yield of purified IGF1 was estimated to be 120 ± 18 mg from 1 L of the culture. Our results demonstrated a simple and easily scalable strategy for production of large amounts of bioactive IGF1 by rational designing soluble protein expression, and further optimization of expression and purification methods.

The IGF-Trap: Novel Inhibitor of Carcinoma Growth and Metastasis

The IGFI receptor promotes malignant progression and has been recognized as a target for cancer therapy. Clinical trials with anti-IGFIR antibodies provided evidence of therapeutic efficacy but exposed limitations due in part to effects on, and the compensatory function of, the insulin receptor system. Here, we report on the production, characterization, and biologic activity of a novel, IGF-targeting protein (the IGF-Trap) comprising a soluble form of hIGFIR and the Fc portion of hIgG1. The IGF-Trap has a high affinity for hIGFI and hIGFII but low affinity for insulin, as revealed by surface plasmon resonance. It efficiently blocked IGFIR signaling in several carcinoma cell types and inhibited tumor cell proliferation, migration, and invasion in vitro. In vivo, the IGF-Trap showed favorable pharmacokinetic properties and could suppress the growth of established breast carcinoma tumors when administered therapeutically into tumor-bearing mice, improving disease-free survival. Moreover, IGF-Trap treatment markedly reduced experimental liver metastasis of colon and lung carcinoma cells, increasing tumor cell apoptosis and reducing angiogenesis. Finally, when compared with an anti-IGFIR antibody or IGF-binding protein-1 that were used at similar or higher concentrations, the IGF-Trap showed superior therapeutic efficacy to both inhibitors. Taken together, we have developed a targeted therapeutic molecule with highly potent anticancer effects that could address limitations of current IGFIR-targeting agents.

Production of functional human insulin-like growth factor binding proteins (IGFBPs) using recombinant expression in HEK293 cells

Insulin-like growth factor binding proteins (IGFBPs) display many functions in humans including regulation of the insulin-like growth factor (IGF) signaling pathway. The various roles of human IGFBPs make them attractive protein candidates in drug discovery. Structural and functional knowledge on human proteins with therapeutic relevance is needed to design and process the next generation of protein therapeutics. In order to conduct structural and functional investigations large quantities of recombinant proteins are needed. However, finding a suitable recombinant production system for proteins such as full-length human IGFBPs, still remains a challenge. Here we present a mammalian HEK293 expression method suitable for over-expression of secretory full-length human IGFBP-1 to -7. Protein purification of full-length human IGFBP-1, -2, -3 and -5 was conducted using a two-step chromatography procedure and the final protein yields were between 1 and 12mg protein per liter culture media. The recombinant IGFBPs contained PTMs and exhibited high-affinity interactions with their natural ligands IGF-1 and IGF-2.

Alanine scanning of a putative receptor binding surface of insulin-like growth factor-I

Current evidence supports a binding model in which the insulin molecule contains two binding surfaces, site 1 and site 2, which contact the two halves of the insulin receptor. The interaction of these two surfaces with the insulin receptor results in a high affinity cross-linking of the two receptor alpha subunits and leads to receptor activation. Evidence suggests that insulin-like growth factor-I (IGF-I) may activate the IGF-I receptor in a similar mode. So far IGF-I residues structurally corresponding to the residues of the insulin site 1 together with residues in the C-domain of IGF-I have been found to be important for binding of IGF-I to the IGF-I receptor (e.g. Phe(23), Tyr(24), Tyr(31), Arg(36), Arg(37), Val(44), Tyr(60), and Ala(62)). However, an IGF-I second binding surface similar to site 2 of insulin has not been identified yet. In this study, we have analyzed whether IGF-I residues corresponding to the six residues of the insulin site 2 have a role in high affinity binding of IGF-I to the IGF-I receptor. Six single-substituted IGF-I analogues were produced, each containing an alanine substitution in one of the following positions (corresponding insulin residues in parentheses): Glu(9) (His(B10)), Asp(12) (Glu(B13)), Phe(16) (Leu(B17)), Asp(53) (Ser(A12)), Leu(54) (Leu(A13)), and Glu(58) (Glu(A17)). In addition, two analogues with 2 and 3 combined alanine substitutions were also produced (E9A,D12A IGF-I and E9A,D12A,E58A IGF-I). The results show that introducing alanine in positions Glu(9), Asp(12), Phe(16), Leu(54), and Glu(58) results in a significant reduction in IGF-I receptor binding affinity, whereas alanine substitution at position 53 had no effect on IGF-I receptor binding. The multiple substitutions resulted in a 33-100-fold reduction in IGF-I receptor binding affinity. These data suggest that IGF-I, in addition to the C-domain, uses surfaces similar to those of insulin in contacting its cognate receptor, although the relative contribution of the side chains of homologous residues varies.

Ligand rebinding: self-consistent mean-field theory and numerical simulations applied to surface plasmon resonance studies

Rebinding of dissociated ligands from cell surface proteins can confound quantitative measurements of dissociation rates important for characterizing the affinity of binding interactions. This can be true also for in vitro techniques such as surface plasmon resonance (SPR). We present experimental results using SPR for the interaction of insulin-like growth factor-I (IGF-I) with one of its binding proteins, IGF binding protein-3 (IGFBP-3), and show that the dissociation, even with the addition of soluble heparin in the dissociation phase, does not exhibit the expected exponential decay characteristic of a 1:1 binding reaction. We thus consider the effect of (multiple) rebinding events and, within a self-consistent mean-field approximation, we derive the complete mathematical form for the fraction of bound ligands as a function of time. We show that, except for very low association rate and surface coverage, this function is nonexponential at all times, indicating that multiple rebinding events strongly influence dissociation even at early times. We compare the mean-field results with numerical simulations and find good agreement, although deviations are measurable in certain cases. Our analysis of the IGF-I-IGFBP-3 data indicates that rebinding is prominent for this system and that the theoretical predictions fit the experimental data well. Our results provide a means for analyzing SPR biosensor data where rebinding is problematic and a methodology to do so is presented.

Decorin, a Novel Player in the Insulin-like Growth Factor System

Decorin is a multifunctional proteoglycan that is expressed by sprouting endothelial cells. Its expression supports capillary formation and cell survival. Previously, it was shown that some effects of decorin are mediated by protein kinase B and the cyclin-dependent kinase inhibitor, p21. However, the cell surface receptor responsible for these effects was unknown. We demonstrate that decorin binds to the insulin-like growth factor-I (IGF-I) receptor on endothelial cells with an affinity in the nanomolar range (K(D) = 18 nm), which is comparable with IGF-I (K(D) = 1.2 nm). Furthermore, decorin can bind IGF-I itself, but with a lower affinity (K(D) = 190 nm) than classical IGF-I-binding proteins. Decorin addition causes IGF-I receptor phosphorylation and activation, which is followed by receptor down-regulation. These effects are caused by the core protein of decorin, and the binding region could be mapped to the N terminus of the molecule. The physiological relevance of the decorin/IGF-I receptor interaction was corroborated in two animal models (e.g. inflammatory angiogenesis in the cornea and unilateral ureteral obstruction). In both models the IGF-I receptor was up-regulated in decorin-deficient mice compared with controls and the up-regulation could not compensate the decorin deficiency in the disease models. These data indicate that decorin is an important player in the IGF system and its loss cannot fully be compensated in different types of diseases.

Effect of receptor phosphorylation on the binding between IRS-1 and IGF-1R as revealed by surface plasmon resonance biosensor

A receptor binding assay based on the surface plasmon resonance (SPR) biosensor technique was developed to study the interaction between insulin-like growth factor-1 receptor (IGF-1R) and its intracellular substrate protein insulin receptor substrate-1 (IRS-1). The sensor surface was modified with anti-IGF-1R (alpha-subunit) monoclonal antibodies for the capturing of the receptor-containing membrane fragments from cell lysates. The IGF-1R was successfully immobilized on the sensor surface with binding capability for its intracellular substrates. SPR measurements showed that the tyrosine phosphorylation of IGF-1R induced by its extracellular ligand insulin-like growth factor-1 caused the receptor to bind with IRS-1 10 times faster than the unactivated receptor. As a result, the affinity constants of IRS-1 to phosphorylated and unphosphorylated IGF-1R were (8.06+/-5.18)x10(9) M(-1) and (9.81+/-4.61)x10(8) M(-1), respectively.

Hydrogen exchange/electrospray ionization mass spectrometry studies of structural features of proteins and protein/protein interactions

The rate at which amide hydrogens located at the peptide backbone in protein/protein complexes undergo hydrogen/deuterium exchange is highly dependent on whether the amide groups participate in binding. Here, a new mass spectrometric method is presented in which this effect is utilized for the characterization of protein/ligand binding sites. The information obtained is which region within the protein participates in binding. The method includes hydrogen/deuterium exchange of receptor and ligand protein amide protons, binding, and back exchange. After this procedure those backbone amide groups that participate in protein binding are protected from back exchange and therefore still deuterated. These regions were then identified by peptic proteolysis, fast microbore high-performance liquid chromatography separation, and electrospray ionization mass spectrometry. The approach has been applied to the investigation of structural features of insulin-like growth factor I (IGF-I) and the interaction of insulin-like growth factor I with IGF-I binding protein 1. The data show that the approach can provide information on the location of the hydrophobic core of IGF-1 and on two regions that are mainly involved in binding to IGF-I binding protein 1. The data are consistent with results obtained with other approaches. The amount of sample required for one experiment is in the subnanomolar range.

The insulin-like growth factor (IGF)binding protein 1 binding epitope on IGF-I probed by heteronuclear NMR spectroscopy and mutational analysis

NMR spectroscopy studies and biosensor interaction analysis of native and site-directed mutants of insulin-like growth factor I (IGF-I) was applied to identify the involvement of individual residues in IGF-I binding to IGF-binding protein 1 (IGFBP-1). Backbone NMR chemical shifts were found to be affected by IGFBP-1 binding in the following residues: Pro2, Glu3, Cys6, Gly7, Gly19, Pro28-Gly30, Gly32, Arg36, Arg37, Gln40-Gly42, Pro63, Lys65, Pro66, and Lys68-Ala70. Three IGF-I arginine side chains were identified by NMR to participate in IGFBP-1 binding. All IGF-I arginine residues were replaced by alanines, using site-directed mutagenesis, in four single substituted variants, IGF-I(R21A), IGF-I(R50A), IGF-I(R55A), and IGF-I(R56A), and one double replacement mutant, IGF-I(R36A/R37A). Biosensor interaction analysis binding studies demonstrate the involvement of Arg36-Arg37 and Arg50 in IGFBP-1 binding, while experiments with the IGF-I receptor implicate Arg21, Arg36-Arg37, and Arg56 as part of the receptor binding epitope. These overlapping binding surfaces explain why IGF-I receptor and IGFBP-1 binding to IGF-I is competitive. The C terminus of free, but not IGFBP-1-bound, IGF-I is found to exist in two distinct, NMR-detectable conformations at 30 degreesC. One possible explanation for this structural heterogeneity could be cis-trans isomerization of the Cys6-Cys48 disulfide bond.