Molecular interactions in the insulin-like growth factor (IGF) axis: a surface plasmon resonance (SPR) based biosensor study

Insulin Growth Factor Binding Protein 7 (IGFBP7)-Related Cancer and IGFBP3 and IGFBP7 Crosstalk

The insulin/insulin-like growth factors (IGFs) have crucial tasks in the growth, differentiation, and proliferation of healthy and pernicious cells. They are involved in coordinated complexes, including receptors, ligands, binding proteins, and proteases. However, the systems can become dysregulated in tumorigenesis. Insulin-like growth factor-binding protein 7 (IGFBP7) is a protein belonging to the IGFBP superfamily (also termed GFBP-related proteins). Numerous studies have provided evidence that IGFBP3 and IGFBP7 are involved in a variety of cancers, including hepatocellular carcinoma (HCC), breast cancer, gastroesophageal cancer, colon cancer, prostate cancer, among many others. Still, very few suggest an interaction between these two molecules. In studying several cancer types in our laboratories, we found that both proteins share some crucial signaling pathways. The objective of this review is to present a comprehensive overview of the relationship between IGFBP7 and cancer, as well as highlighting IGFBP3 crosstalk with IGFBP7 reported in recent studies.

Proteins adsorbed to a polysulfone hemodialysis membrane under heparin and citrate anticoagulation regimens

The study aim was to compare molecular-level effects (blood-dialyzer interactions) of heparin and citrate anticoagulation using proteome-wide analysis of biofilm adsorbed to dialysis membrane. Ten patients receiving maintenance hemodialysis were examined in a crossover design under three different anticoagulation regimens, namely citrate, heparin, and anticoagulation-free (control). Following a regular hemodialysis session (4 hours, polysulfone membrane), dialyzers were flushed and the surface biofilm eluted by acetic acid. Protein composition of the eluates was determined by 2-dimensional gel electrophoresis and resulting patterns compared between regimens. Proteins responsible for the difference were identified by mass spectrometry. Citrate anticoagulation was associated with significantly less protein adsorption to the membrane than heparin (2.2 [1.1-2.9] mg vs. 6.5 [2.9-11.6] mg, P = 0.009). Among the proteins identified as major discriminators between citrate and the other regimens, fibrin α-chain fragments of molecular weight below 40 kDa prevailed. In these fragments, an analysis of the amino acid sequence has been performed by comparison with the UniProt database. It showed missing α-chain cross-links. On the contrary, heparin prevented adsorption and cleavage of several heparin-binding proteins; especially complement factor H-related protein 3, insulin-like growth factor binding proteins (2, 4, and 5), and chemerin. Compared to heparin, citrate is associated with less protein adsorption and imperfectly crosslinked fibrin clot formation. Membrane adsorptive properties are significantly modified by the anticoagulation regimen.

Understanding the Key to Targeting the IGF Axis in Cancer: A Biomarker Assessment

Type 1 insulin like growth factor receptor (IGF-1R) targeted therapies showed compelling pre-clinical evidence; however, to date, this has failed to translate into patient benefit in Phase 2/3 trials in unselected patients. This was further complicated by the toxicity, including hyperglycemia, which largely results from the overlap between IGF and insulin signaling systems and associated feedback mechanisms. This has halted the clinical development of inhibitors targeting IGF signaling, which has limited the availability of biopsy samples for correlative studies to understand biomarkers of response. Indeed, a major factor contributing to lack of clinical benefit of IGF targeting agents has been difficulty in identifying patients with tumors driven by IGF signaling due to the lack of predictive biomarkers. In this review, we will describe the IGF system, rationale for targeting IGF signaling, the potential liabilities of targeting strategies, and potential biomarkers that may improve success.

Targeting Insulin-Like Growth Factor Binding Protein-3 Signaling in Triple-Negative Breast Cancer

Insulin-like growth factor binding protein-3 (IGFBP-3) is a key regulatory molecule of the IGF axis and can function in a tissue-specific way as both a tumor suppressor and promoter. Triple-negative breast cancer (TNBC) has high tumor expression of IGFBP-3 associated with markers of poor prognosis and, although accounting for 15-20% of all breast cancers, is responsible for disproportionate rates of morbidity and mortality. Because they lack estrogen and progesterone receptors and overexpression of HER2, TNBC are resistant to treatments that target these molecules, making the development of new therapies an important goal. In addition to frequent high expression of IGFBP-3, these tumors also express EGFR highly, but targeting EGFR signaling alone in TNBC has been of little success. Identification of a functional growth-stimulatory interaction between EGFR and IGFBP-3 signaling prompted investigation into cotargeting these pathways as a novel therapy for TNBC. This involves inhibition of both EGFR kinase activity and a mediator of IGFBP-3's stimulatory bioactivity, sphingosine kinase-1 (SphK1), and has shown promise in a preclinical setting. Functional interaction between EGFR and IGFBP-3 may also promote chemoresistance in TNBC, and delineating the mechanisms involved may identify additional targets for development of therapies in cancers that express both IGFBP-3 and EGFR.

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.

Heparin-binding mechanism of the IGF2/IGF-binding protein 2 complex

IGF1 and IGF2 are potent stimulators of diverse cellular activities such as differentiation and mitosis. Six IGF-binding proteins (IGFBP1-IGFBP6) are primary regulators of IGF half-life and receptor availability. Generally, the binding of IGFBPs inhibits IGF receptor activation. However, it has been shown that IGFBP2 in complex with IGF2 (IGF2/IGFBP2) stimulates osteoblast function in vitro and increases skeletal mass in vivo. IGF2 binding to IGFBP2 greatly increases the affinity for 2- or 3-carbon O-sulfated glycosaminoglycans (GAGs), e.g. heparin and heparan sulfate, which is hypothesized to preferentially and specifically target the IGF2/IGFBP2 complex to the bone matrix. In order to obtain a more detailed understanding of the interactions between the IGF2/IGFBP2 complex and GAGs, we investigated heparin-binding properties of IGFBP2 and the IGF2/IGFBP2 complex in a quantitative manner. For this study, we mutated key positively charged residues within the two heparin-binding domains (HBDs) in IGFBP2 and in one potential HBD in IGF2. Using heparin affinity chromatography, we demonstrate that the two IGFBP2 HBDs contribute differentially to GAG binding in free IGFBP2 and the IGF2/IGFBP2 protein complex. Moreover, we identify a significant contribution from the HBD in IGF2 to the increased IGF2/IGFBP2 heparin affinity. Using molecular modeling, we present a novel model for the IGF2/IGFBP2 interaction with heparin where all three proposed HBDs constitute a positively charged and surface-exposed area that would serve to promote the increased heparin affinity of the complex compared with free intact IGFBP2.

IGF binding proteins in cancer: mechanistic and clinical insights

The six members of the family of insulin-like growth factor (IGF) binding proteins (IGFBPs) were originally characterized as passive reservoirs of circulating IGFs, but they are now understood to have many actions beyond their endocrine role in IGF transport. IGFBPs also function in the pericellular and intracellular compartments to regulate cell growth and survival - they interact with many proteins, in addition to their canonical ligands IGF-I and IGF-II. Intranuclear roles of IGFBPs in transcriptional regulation, induction of apoptosis and DNA damage repair point to their intimate involvement in tumour development, progression and resistance to treatment. Tissue or circulating IGFBPs might also be useful as prognostic biomarkers.

Phosphorylation of IGFBP-1 at discrete sites elicits variable effects on IGF-I receptor autophosphorylation

We previously demonstrated that hypoxia and leucine deprivation cause hyperphosphorylation of IGF-binding protein-1 (IGFBP-1) at discrete sites that markedly enhanced IGF-I affinity and inhibited IGF-I-stimulated cell growth. In this study we investigated the functional role of these phosphorylation sites using mutagenesis. We created three IGFBP-1 mutants in which individual serine (S119/S169/S98) residues were substituted with alanine and S101A was recreated for comparison. The wild-type (WT) and mutant IGFBP-1 were expressed in Chinese hamster ovary cells and IGFBP-1 in cell media was isolated using isoelectric-focusing-free-flow electrophoresis. BIACore analysis indicated that the changes in IGF-I affinity for S98A and S169A were moderate, whereas S119A greatly reduced the affinity of IGFBP-1 for IGF-I (100-fold, P < .0001). Similar results were obtained with S101A. The IGF-I affinity changes of the mutants were reflected in their ability to inhibit IGF-I-induced receptor autophosphorylation. Employing receptor-stimulation assay using IGF-IR-overexpressing P6 cells, we found that WT-IGFBP-1 inhibited IGF-IRβ autophosphorylation (~2-fold, P < .001), possibly attributable to sequestration of IGF-I. Relative to WT, S98A and S169A mutants did not inhibit receptor autophosphorylation. S119A, on the other hand, greatly stimulated the receptor (2.3-fold, P < .05). The data with S101A matched S119A. In summary, we show that phosphorylation at S98 and S169 resulted in milder changes in IGF-I action; nonetheless most dramatic inhibitory effects on the biological activity of IGF-I were due to IGFBP-1 phosphorylation at S119. Our results provide novel demonstration that IGFBP-1 phosphorylation at S119 can enhance affinity for IGF-I possibly through stabilization of the IGF-IGFBP-1 complex. These data also propose that the synergistic interaction of distinct phosphorylation sites may be important in eliciting more pronounced effects on IGF-I affinity that needs further investigation.

Modification of the zonal elution method for detection of transient protein-protein interactions involving ligand exchange

A new affinity chromatography method was developed by modifying a zonal elution method. The new method targets transient protein-protein interactions, such as those encountered during direct ligand transfer between the ligand transporter and its cognate receptor. A ligand-loaded transport protein is immobilized on the solid support, and a plug containing a putative receptor is flowed through the column. Elution profiles of proteins not interacting with the immobilized transporter can be approximated with a simple Gaussian curve, while the elution profiles of cognate receptors show significant delay and exhibit complex shape. Ligand transfer from the immobilized transporter molecules to the receptors is verified by both UV absorbance measurements and mass spectrometry. The sensitivity of the method is demonstrated using retinoic acid (RA) transfer from various isoforms of cellular RA binding proteins (CRABPs) and RA receptor γ (RARγ). Although these interactions have been hypothesized long ago to proceed via direct mechanism (i.e., via transient docking of the receptor and the transporter), the existing biophysical techniques failed to detect the presence of the transporter-receptor complexes. However, the modified zonal elution method provides unequivocal evidence of direct interaction between RARγ and one of the CRABP isoforms (CRABP II) during the ligand transfer to the receptor.

IGFBP5 induces cell adhesion, increases cell survival and inhibits cell migration in MCF-7 human breast cancer cells

Maintenance of tissue boundaries is crucial for control of metastasis. We describe a new signalling pathway in which epithelial cell disruption can be minimised and thereby restricts epithelial-mesenchymal transgressions. This involves the release of insulin-like growth factor (IGF)-binding protein 5 (IGFBP5) from apoptotic cells, which increases the adhesion of epithelial cells on mesenchymal but not epithelial extracellular matrix (ECM), and involves the direct interaction of IGFBP5 and α2β1 integrins. IGFBP5 also induced cell adhesion to vitronectin in the absence of αVβ3 integrin, the vitronectin receptor, again through an α2β1-integrin-dependent action, suggesting that IGFBP5 can induce spreading on matrices, even in the absence of the integrins normally used in this process. Using IGFBP5 mutants we demonstrate that the effect is IGF-independent but requires the heparin-binding domain in the C-terminus of IGFBP5. A truncated mutant containing only the C-terminal of IGFBP5 also induced adhesion. Adhesion induced by IGFBP5 was dependent on Cdc42 and resulted in activation of integrin-linked kinase (ILK) and Akt. Consistent with these changes, IGFBP5 facilitated prolonged cell survival in nutrient-poor conditions and decreased phosphorylation of the stress-activated kinase p38 MAPK (MAPK14). Whereas IGFBP5 enhanced adhesion, it inhibited cell migration, although this was not evident using the truncated C-terminal mutant, suggesting that effects of IGFBP5 on adhesion and migration involve different mechanisms. We anticipate that these responses to IGFBP5 would reduce the metastatic potential of cells.

Signalling pathways of insulin-like growth factors (IGFs) and IGF binding protein-3

Although the insulin-like growth factor (IGF) system is essential for normal growth and development, its dysregulation has been implicated in a range of pathological states. The peptide growth factors IGF-I and IGF-II exert their effects by binding to cell-surface heterotetrameric tyrosine kinase receptors and activating multiple intracellular signalling cascades, leading to changes in the expression of proteins essential for cell proliferation, survival and differentiation. The IGF system comprises multiple ligands, receptors and high-affinity IGF binding proteins (IGFBPs), with added complexity arising from crosstalk between its receptors and other key growth-regulatory pathways such as those activated by steroid hormones, integrins and other receptor tyrosine kinases. The IGFBPs are also increasingly recognised for their intrinsic growth-regulatory activity, and the ability of IGFBP-3 to modulate signalling pathways of nuclear hormone and growth factor receptors, as well as novel receptors, is believed to play a role both in normal physiology and in disease.

Protein-protein interactions: principles, techniques, and their potential role in new drug development

A vast network of genes is inter-linked through protein-protein interactions and is critical component of almost every biological process under physiological conditions. Any disruption of the biologically essential network leads to pathological conditions resulting into related diseases. Therefore, proper understanding of biological functions warrants a comprehensive knowledge of protein-protein interactions and the molecular mechanisms that govern such processes. The importance of protein-protein interaction process is highlighted by the fact that a number of powerful techniques/methods have been developed to understand how such interactions take place under various physiological and pathological conditions. Many of the key protein-protein interactions are known to participate in disease-associated signaling pathways, and represent novel targets for therapeutic intervention. Thus, controlling protein-protein interactions offers a rich dividend for the discovery of new drug targets. Availability of various tools to study and the knowledge of human genome have put us in a unique position to understand highly complex biological network, and the mechanisms involved therein. In this review article, we have summarized protein-protein interaction networks, techniques/methods of their binding/kinetic parameters, and the role of these interactions in the development of potential tools for drug designing.

Binding between insulin-like growth factor 1 and insulin-like growth factor-binding protein 3 is not influenced by glucose or 2-deoxy-D-glucose

A recent report (Zhong, D., Xiong, L., Liu, T., Liu, X., Liu, X., Chen, J., Sun, S. Y., Khuri, F. R., Zong, Y., Zhou, Q., and Zhou, W. (2009) J. Biol. Chem. 284, 23225-23233) details that 2-deoxy-D-glucose (2-DG), a well known inhibitor of glycolysis and a candidate antineoplastic agent, also induces insulin-like growth factor 1 receptor (IGF-1R) signaling through the inhibition of insulin-like growth factor 1-insulin-like growth factor-binding protein 3 (IGF-1-IGFBP-3) complex formation. Zhong et al. hypothesized that disrupted IGF-1/IGFBP-3 binding by 2-DG led to increased free IGF-1 concentrations and, consequently, activation of IGF-1R downstream pathways. Because their report suggests unprecedented off-target effects of 2-DG, this has profound implications for the fields of metabolism and oncology. Using ELISA, surface plasmon resonance, and novel "intensity-fading" mass spectrometry, we now provide a detailed characterization of complex formation between IGF-1 and IGFBP-3. All three of these independent methods demonstrated that there was no effect of glucose or 2-DG on the interaction between IGF-1 and IGFBP-3. Furthermore, we show examples of 2-DG exposure associated with reduced rather than increased IGF-1R and AKT activation, providing further evidence against a 2-DG increase in IGF-1R activation by IGF-1-IGFBP-3 complex disruption.

Grading the commercial optical biosensor literature-Class of 2008: 'The Mighty Binders'

Optical biosensor technology continues to be the method of choice for label-free, real-time interaction analysis. But when it comes to improving the quality of the biosensor literature, education should be fundamental. Of the 1413 articles published in 2008, less than 30% would pass the requirements for high-school chemistry. To teach by example, we spotlight 10 papers that illustrate how to implement the technology properly. Then we grade every paper published in 2008 on a scale from A to F and outline what features make a biosensor article fabulous, middling or abysmal. To help improve the quality of published data, we focus on a few experimental, analysis and presentation mistakes that are alarmingly common. With the literature as a guide, we want to ensure that no user is left behind.

Separation of the molecular forms of the insulin-like growth factor (IGF)-Binding proteins by affinity chromatography

Association of IGFBP-1, IGFBP-2 and IGFBP-3 with other proteins in human serum and placental cell membranes was investigated using affinity chromatography matrix with immobilized antibodies. Circulating IGFBP-1 was found to be predominantly bound to alpha(2)-macroglobulin and not in the binary complex with its ligand, IGFBP-2 complexes and/or polymers were detected, which was not acknowledged before, and IGFBP-3 molecular forms were differentiated into those that form binary/ternary complexes and those that form stable associations with other serum proteins. As for placental membranes, both IGFBP-1 dimers and high molecular mass IGFBP-1 associations, most probably with alpha(2)-macroglobulin, were recognized and resolved.

Heparan sulfate regulates ADAM12 through a molecular switch mechanism

The disintegrin and metalloproteases (ADAMs) are emerging as therapeutic targets in human disease, but specific drug design is hampered by potential redundancy. Unlike other metzincins, ADAM prodomains remain bound to the mature enzyme to regulate activity. Here ADAM12, a protease that promotes tumor progression and chondrocyte proliferation in osteoarthritic cartilage, is shown to possess a prodomain/catalytic domain cationic molecular switch, regulated by exogenous heparan sulfate and heparin but also endogenous cell surface proteoglycans and the polyanion, calcium pentosan polysulfate. Sheddase functions of ADAM12 are regulated by the switch, as are proteolytic functions in placental tissue and sera of pregnant women. Moreover, human heparanase, an enzyme also linked to tumorigenesis, can promote ADAM12 sheddase activity at the cell surface through cleavage of the inhibitory heparan sulfate. These data present a novel concept that might allow targeting of ADAM12 and suggest that other ADAMs may have specific regulatory activity embedded in their prodomain and catalytic domain structures.