Role of N- and C-terminal residues of insulin-like growth factor (IGF)-binding protein-3 in regulating IGF complex formation and receptor activation

The role of IGFBP-3 in tumor development and progression: enlightenment for diagnosis and treatment

IGFBP-3 is aberrantly expressed in many tumor types, and its serum and tumor tissue levels provide auxiliary information for assessing the degree of tumor malignancy and patient prognosis, making it a potential therapeutic target for human malignancies and conferring it remarkable clinical value for determining patient prognosis. In this review, we provide a comprehensive overview of the aberrant expression, diverse biological effects, and clinical implications of IGFBP-3 in tumors and its role as a potential prognostic marker and therapeutic target for tumors. In addition, we summarize the signaling pathways through which IGFBP-3 exerts its effects. IGFBP-3 comprises an N-terminal, an intermediate region, and a C-terminal structural domain, each exerting different biological effects in several tumor cell types in an IGF-dependent/non-independent manner. IGFBP-3 shares an intricate relationship with the tumor microenvironment, thereby affecting tumor growth. Overall, IGFBP-3 is an essential regulatory factor that mediates tumor occurrence and progression. Gaining deeper insights into the fundamental characteristics of IGFBP-3 and its role in various tumor types will provide new perspectives and allow for the development of novel strategies for cancer diagnosis, treatment, and prognostic evaluation.

Signaling Pathways of the Insulin-like Growth Factor Binding Proteins

The 6 high-affinity insulin-like growth factor binding proteins (IGFBPs) are multifunctional proteins that modulate cell signaling through multiple pathways. Their canonical function at the cellular level is to impede access of insulin-like growth factor (IGF)-1 and IGF-2 to their principal receptor IGF1R, but IGFBPs can also inhibit, or sometimes enhance, IGF1R signaling either through their own post-translational modifications, such as phosphorylation or limited proteolysis, or by their interactions with other regulatory proteins. Beyond the regulation of IGF1R activity, IGFBPs have been shown to modulate cell survival, migration, metabolism, and other functions through mechanisms that do not appear to involve the IGF-IGF1R system. This is achieved by interacting directly or functionally with integrins, transforming growth factor β family receptors, and other cell-surface proteins as well as intracellular ligands that are intermediates in a wide range of pathways. Within the nucleus, IGFBPs can regulate the diverse range of functions of class II nuclear hormone receptors and have roles in both cell senescence and DNA damage repair by the nonhomologous end-joining pathway, thus potentially modifying the efficacy of certain cancer therapeutics. They also modulate some immune functions and may have a role in autoimmune conditions such as rheumatoid arthritis. IGFBPs have been proposed as attractive therapeutic targets, but their ubiquity in the circulation and at the cellular level raises many challenges. By understanding the diversity of regulatory pathways with which IGFBPs interact, there may still be therapeutic opportunities based on modulation of IGFBP-dependent signaling.

Doxorubicin induced ROS-dependent HIF1α activation mediates blockage of IGF1R survival signaling by IGFBP3 promotes cardiac apoptosis

Doxorubicin (Dox) causes the generation of intracellular reactive oxygen species (ROS) and inactivates insulin-like growth factor 1 (IGF1) signaling, leading to cardiomyocyte apoptosis. IGF-binding protein 3 (IGFBP3) is the most abundant circulating IGF1 carrier protein with high affinity, which has been reported to mediate ROS-induced apoptosis. Hypoxia-inducible factor 1α (HIF1A), an upstream protein of IGFBP3 is regulated by prolyl hydroxylase domain (PHD) through hydroxylation. In this study, we investigated the role of IGFBP3, HIF1A, and PHD in Dox-induced cardiac apoptosis.Cells challenged with 1 μM Dox for 24 h increased ROS generation, augmented intracellular and secreted IGFBP3 levels, and reduced IGF1 signaling. Further, we showed that Dox enhanced the extracellular association of IGF1 with IGFBP3. Moreover, echocardiography parameters, especially ejection fraction (EF) and fractional shortening (FS) were significantly reduced in ventricle tissue of Dox challenged rats. Notably, siRNA approach against IGFBP3 or an anti-IGFBP3 antibody rescued Dox-induced cardiac apoptosis, mitochondrial ROS, and the decrease in the IGF1 signaling activity. Furthermore, silencing HIF1A either using siRNA or inhibitor downregulated intracellular IGFBP3, rescued apoptosis, mitochondrial generation, and reduction in IGF1 signaling. Finally, western blot data revealed that ROS scavenger reversed Dox-induced cardiac apoptosis, increased levels of HIF1A and secreted IGFBP3, and decreased IGF1 survival signaling and PHD expression.These findings suggest that Dox-induced ROS generation suppressed PHD, which might stabilize nuclear HIF1A protein, leading to increased IGFBP3 expression and secretion. This in turn results in enhanced extracellular association of the latter with IGF1 and blocks IGF1 pro-survival signaling and may result in inducing cardiac apoptosis.

Insulin-Like Growth Factor Binding Protein-3 (IGFBP-3): Unraveling the Role in Mediating IGF-Independent Effects Within the Cell

Insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), one of the six members of the IGFBP family, is a key protein in the IGF pathway. IGFBP-3 can function in an IGF-dependent as well as in an IGF-independent manner. The IGF-dependent roles of IGFBP-3 include its endocrine role in the delivery of IGFs from the site of synthesis to the target cells that possess IGF receptors and the activation of associated downstream signaling. IGF-independent role of IGFBP-3 include its interactions with the proteins of the extracellular matrix and the proteins of the plasma membrane, its translocation through the plasma membrane into the cytoplasm and into the nucleus. The C-terminal domain of IGFBP-3 has the ability to undergo cell penetration therefore, generating a short 8-22-mer C-terminal domain peptides that can be conjugated to drugs or genes for effective intracellular delivery. This has opened doors for biotechnological applications of the molecule in molecular medicine. The aim of this this review is to summarize the complex roles of IGFBP-3 within the cell, including its mechanisms of cellular uptake and its translocation into the nucleus, various molecules with which it is capable of interacting, and its ability to regulate IGF-independent cell growth, survival and apoptosis. This would pave way into understanding the modus operandi of IGFBP-3 in regulating IGF-independent processes and its pleiotropic ability to bind with potential partners thus regulating several cellular functions implicated in metabolic diseases, including cancer.

Multiple Growth Factor Targeting by Engineered Insulin-like Growth Factor Binding Protein-3 Augments EGF Receptor Tyrosine Kinase Inhibitor Efficacy

Resistance to cancer therapy is a challenge because of innate tumor heterogeneity and constant tumor evolution. Since the pathway of resistance cannot be predicted, combination therapies may address this progression. We discovered that in addition to IGF1 and IGF2, IGFBP-3 binds bFGF, HGF, neuregulin, and PDGF AB with nanomolar affinity. Because growth factors drive resistance, simultaneous inhibition of multiple growth factor pathways may improve the efficacy of precision therapy. Growth factor sequestration by IGFBP-3-Fc enhances the activity of EGFR inhibitors by decreasing cell survival and inhibiting bFGF, HGF, and IGF1 growth factor rescue and also potentiates the activity of other cancer drugs. Inhibition of tumor growth in vivo with adjuvant IGFBP-3-Fc with erlotinib versus erlotinib after treatment cessation supports that the combination reduces cell survival. Inhibition of multiple growth factor pathways may postpone resistance and extend progression-free survival in many cancer indications.

In silico interaction of insulin-like growth factor binding protein 3 with insulin-like growth factor 1

Insulin-like growth factor binding protein-3 (IGFBP-3) is a vital protein exist in circulation which interacts with high affinity to insulin-like growth factor (IGFs) altering their activities. Therefore, the interaction between IGFs and IGFBP-3 has a key role altering large spectrum of activities such as cell cycle progression, proliferation and apoptosis. Despite decades of research, the crystal structure of IGFBP-3 has not been identified possibly due to some technical challenge in its crystallizing. The three-dimensional (3D) structure of IGFBP-3 was predicted using homology modeling, Phyre2, and molecular dynamic. Its interaction with IGF-1 was also identified by HADDOCK software. IGFBP-3 has the most identity with other IGFBPs in N and C-domain; however, its linker domain has lower identity. Our data predicted that IGF-1 structurally interacts with N-domain and linker domain of IGFBP-3. Some conserved residues of IGFBP-3 such as Glu33, Arg36, Gly39, Arg60, Arg66, Asn109, and Ile146 interacts with Glu3, Asp12, Phe16, Gly19, Asp20, Arg21, and Glu58 of IGF-1. In addition, our data predict that the linker domain has a loop structure which covers post translational modification and interacts with IGF-1. The phosphorylation of Ser111 in linker domain, which previously has been shown to induce apoptosis make a repulsive force interrupting this interaction to IGF-1, which enables IGFBP-3 to induce apoptosis. The present study suggests that the linker domain has a key role in recognition of IGFBP-3 with IGF-1.

Understanding Insulin Endocrinology in Decapod Crustacea: Molecular Modelling Characterization of an Insulin-Binding Protein and Insulin-Like Peptides in the Eastern Spiny Lobster, Sagmariasus verreauxi

The insulin signalling system is one of the most conserved endocrine systems of Animalia from mollusc to man. In decapod Crustacea, such as the Eastern spiny lobster, Sagmariasus verreauxi (Sv) and the red-claw crayfish, Cherax quadricarinatus (Cq), insulin endocrinology governs male sexual differentiation through the action of a male-specific, insulin-like androgenic gland peptide (IAG). To understand the bioactivity of IAG it is necessary to consider its bio-regulators such as the insulin-like growth factor binding protein (IGFBP). This work has employed various molecular modelling approaches to represent S. verreauxi IGFBP and IAG, along with additional Sv-ILP ligands, in order to characterise their binding interactions. Firstly, we present Sv- and Cq-ILP2: neuroendocrine factors that share closest homology with Drosophila ILP8 (Dilp8). We then describe the binding interaction of the N-terminal domain of Sv-IGFBP and each ILP through a synergy of computational analyses. In-depth interaction mapping and computational alanine scanning of IGFBP_N' highlight the conserved involvement of the hotspot residues Q₆₇, G₇₀, D₇₁, S₇₂, G₉₁, G₉₂, T₉₃ and D₉₄. The significance of the negatively charged residues D₇₁ and D₉₄ was then further exemplified by structural electrostatics. The functional importance of the negative surface charge of IGFBP is exemplified in the complementary electropositive charge on the reciprocal binding interface of all three ILP ligands. When examined, this electrostatic complementarity is the inverse of vertebrate homologues; such physicochemical divergences elucidate towards ligand-binding specificity between Phyla.

The role and regulation of IGFBP-1 phosphorylation in fetal growth restriction

Fetal growth restriction (FGR) increases the risk of perinatal complications and predisposes the infant to developing metabolic, cardiovascular, and neurological diseases in childhood and adulthood. The pathophysiology underlying FGR remains poorly understood and there is no specific treatment available. Biomarkers for early detection are also lacking. The insulin-like growth factor (IGF) system is an important regulator of fetal growth. IGF-I is the primary regulator of fetal growth, and fetal circulating levels of IGF-I are decreased in FGR. IGF-I activity is influenced by a family of IGF binding proteins (IGFBPs), which bind to IGF-I and decrease its bioavailability. During fetal development the predominant IGF-I binding protein in fetal circulation is IGFBP-1, which is primarily secreted by the fetal liver. IGFBP-1 binds IGF-I and thereby inhibits its bioactivity. Fetal circulating levels of IGF-I are decreased and concentrations of IGFBP-1 are increased in FGR. Phosphorylation of human IGFBP-1 at specific sites markedly increases its binding affinity for IGF-I, further limiting IGF-I bioactivity. Recent experimental evidence suggests that IGFBP-1 phosphorylation is markedly increased in the circulation of FGR fetuses suggesting an important role of IGFBP-1 phosphorylation in the regulation of fetal growth. Understanding of the significance of site-specific IGFBP-1 phosphorylation and how it is regulated to contribute to fetal growth will be an important step in designing strategies for preventing, managing, and/or treating FGR. Furthermore, IGFBP-1 hyperphosphorylation at unique sites may serve as a valuable biomarker for FGR.

Investigation of alanine mutations affecting insulin-like growth factor (IGF) I binding to IGF binding proteins

Binding properties of wild type (WT) and six single amino acid substituted variants (E3A, E9A, D12A, D20A, F23A, and E58A) of insulin-like growth factor I (IGF-I) were analyzed with respect to their binding details to IGF binding proteins (IGFBPs) by molecular dynamics (MD) simulations. The binding sites and binding interactions on IGF-I and IGFBPs are screened and compared with the static X-ray structure. Electrostatic interaction is the primary driving force of the interaction between IGF-I and IGFBPs. Mutation may cause the rearrangement of binding sites, however, the unfolding of protein induced by mutation is not obvious in this work. We also provide the detailed picture of binding factors. And the results show that, whether the unfolding of helix occurs or not, the Ala mutation will change the molecular atmosphere of the binding interface by the rearrangement of conformation, and further affects the binding residues and binding interactions.

Glucose intolerance in aging male IGFBP-3 transgenic mice: differential effects of human IGFBP-3 and its mutant IGFBP-3 devoid of IGF binding ability

We have reported a reduction of insulin secretion and glucose intolerance in young mice overexpressing human IGFBP-3 (phosphoglycerate kinase [PGK]BP3) or its mutant Gly56/Gly80/Gly81-IGFBP-3 (PGKmutBP3) under the PGK promoter. Here, we investigated changes in glucose and lipid homeostasis with age in PGKBP3 and PGKmutBP3 mice compared with wild-type mice. Body weight, glucose tolerance, insulin tolerance, visceral fat, interscapular brown adipose tissue (BAT), serum lipids, and pancreas histology were examined at age 3, 6, and 12 months. Murine IGFBP-3 was similar in all mouse genotypes and decreased with age in parallel with total IGF-1. Visceral fat and BAT masses increased in PGKmutBP3 mice, but not in PGKBP3 mice. Glucose tolerance was impaired in both PGKBP3 and PGKmutBP3 mice. However, PGKBP3 mice had increased expression of uncoupling protein-1 in BAT and reduced adiposity, and continued to have smaller pancreatic β-cell mass and reduced insulin secretion through age 12 months. In contrast, PGKmutBP3 mice developed insulin resistance with age in association with pancreatic β-cell hyperplasia, impaired expression of uncoupling protein-1 in BAT, and increased adiposity. In addition, both PGKBP3 and PGKmutBP3 mice had elevated glycerol in the circulation, but only PGKBP3 mice had elevated free fatty acids and only PGKmutBP3 mice had elevated triglycerides. Estimated free IGF-1 did not increase with age in transgenic mice, as it did in wild-type mice. Thus, overexpression of human IGFBP-3 or its mutant devoid of IGF binding ability leads to glucose intolerance with, however, different effects on insulin secretion, insulin sensitivity, and lipid homeostasis in aging mice.

Involvement of insulin-like growth factor binding protein-3 in peroxisome proliferator-activated receptor gamma-mediated inhibition of breast cancer cell growth

We have previously reported that insulin-like growth factor binding protein-3 (IGFBP-3), a protein with dichotomous effects on both cell proliferation and cell survival, interacts with peroxisome proliferator-activated receptor gamma (PPARγ) and inhibits adipogenic PPARγ signaling. We now show that IGFBP-3 and PPARγ interact in breast cancer cells, through amino- and carboxyl-terminal residues of IGFBP-3. IGFBP-3 and the PPARγ ligands, rosiglitazone or 15-deoxy-Δ(12,14)-prostaglandin J2, separately inhibited the proliferation of MCF-7, MDA-MB-231 and MDA-MB-468 breast cancer cells. However, growth inhibition by IGFBP-3 and PPARγ ligand combined was greater than by either alone. Two IGFBP-3 mutants with reduced PPARγ binding caused no growth inhibition when used alone and abolished the inhibitory effect of rosiglitazone when used in combination with PPARγ ligand. Cell growth inhibition by PPARγ ligands was substantially blocked by IGFBP-3 siRNA and restored by exogenous IGFBP-3. We conclude that the interaction between IGFBP-3 and PPARγ is important for the growth-inhibitory effect of PPARγ ligands in human breast cancer cells, suggesting that IGFBP-3 expression by breast tumors may regulate their sensitivity toward PPARγ ligands.

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.

Regulation of retinal endothelial cell apoptosis through activation of the IGFBP-3 receptor

The goal of this study was to investigate whether insulin-like growth factor binding protein-3 receptor (IGFBP-3 receptor) is required for IGFBP-3 to inhibit retinal endothelial cell (REC) apoptosis. REC were grown in normal glucose (5 mM) or high glucose medium (25 mM) for 3 days. Once cells reached confluence, they were transfected with an endothelial- specific IGFBP-3 plasmid DNA (non-IGF binding; IGFBP-3 NB) at 1 μg/ml for 24 h. Cell proteins were extracted and analyzed for IGFBP-3 receptor expression by Western blotting or use in coimmunoprecipitation or co-localization experiments for detection of IGFBP-3 and IGFBP-3 receptor binding. REC were also transfected with or without IGFBP-3 receptor siRNA before IGFBP-3NB plasmid DNA transfection. Cell lysates were processed for a cell death ELISA, a cleaved caspase 3 ELISA, and Western blotting to measure key pro- and anti-apoptotic markers: Bcl-xL, Bax, Cytochrome C and Akt. The IGFBP-3 receptor is present on REC. Overexpression of IGFBP-3 in REC significantly increased protein levels of IGFBP-3 receptor (p < 0.05). Significant increases in cell death were found in cells transfected with IGFBP-3 receptor siRNA versus not treated samples (p < 0.05). Data suggest that IGFBP-3 inhibits retinal endothelial cell death through activation of an IGFBP-3 receptor in a hyperglycemic environment. This is the first demonstration of the involvement of IGFBP-3 receptor in inhibition of REC cell death. Future studies will investigate the mechanism by which IGFBP-3 receptor may inhibit retinal endothelial cell death.

Insulin-like growth factor binding protein-3 (IGFBP-3): Novel ligands mediate unexpected functions

In addition to its important role in the regulation of somatic growth by acting as the major circulating transport protein for the insulin-like growth factors (IGFs), IGF binding protein-3 (IGFBP-3) has a variety of intracellular ligands that point to its function within major signaling pathways. The discovery of its interaction with the retinoid X receptor has led to the elucidation of roles in regulating the function of several nuclear hormone receptors including retinoic acid receptor-α, Nur77 and vitamin D receptor. Its interaction with the nuclear hormone receptor peroxisome proliferator-activated receptor-γ is believed to be involved in regulating adipocyte differentiation, which is also modulated by IGFBP-3 through an interaction with TGFβ/Smad signaling. IGFBP-3 can induce apoptosis alone or in conjunction with other agents, and in different systems can activate caspases -8 and -9. At least two unrelated proteins (LRP1 and TMEM219) have been designated as receptors for IGFBP-3, the latter with a demonstrated role in inducing caspase-8-dependent apoptosis. In contrast, IGFBP-3 also has demonstrated roles in survival-related functions, including the repair of DNA double-strand breaks through interaction with the epidermal growth factor receptor and DNA-dependent protein kinase, and the induction of autophagy through interaction with GRP78. The ability of IGFBP-3 to modulate the balance between pro-apoptotic and pro-survival sphingolipids by regulating sphingosine kinase 1 and sphingomyelinases may be integral to its role at the crossroads between cell death and survival in response to a variety of stimuli. The pleiotropic nature of IGFBP-3 activity supports the idea that IGFBP-3 itself, or pathways with which it interacts, should be investigated as targets of therapy for a variety of diseases.

Protection of blood retinal barrier and systemic vasculature by insulin-like growth factor binding protein-3

Previously, we showed that insulin growth factor (IGF)-1 binding protein-3 (IGFBP-3), independent of IGF-1, reduces pathological angiogenesis in a mouse model of the oxygen-induced retinopathy (OIR). The current study evaluates novel endothelium-dependent functions of IGFBP-3 including blood retinal barrier (BRB) integrity and vasorelaxation. To evaluate vascular barrier function, either plasmid expressing IGFBP-3 under the regulation of an endothelial-specific promoter or a control plasmid was injected into the vitreous humor of mouse pups (P1) and compared to the non-injected eyes of the same pups undergoing standard OIR protocol. Prior to sacrifice, the mice were given an injection of horseradish peroxidase (HRP). IGFBP-3 plasmid-injected eyes displayed near-normal vessel morphology and enhanced vascular barrier function. Further, in vitro IGFBP-3 protects retinal endothelial cells from VEGF-induced loss of junctional integrity by antagonizing the dissociation of the junctional complexes. To assess the vasodilatory effects of IGFBP-3, rat posterior cerebral arteries were examined in vitro. Intraluminal IGFBP-3 decreased both pressure- and serotonin-induced constrictions by stimulating nitric oxide (NO) release that were blocked by L-NAME or scavenger receptor-B1 neutralizing antibody (SRB1-Ab). Both wild-type and IGF-1-nonbinding mutant IGFBP-3 (IGFBP-3NB) stimulated eNOS activity/NO release to a similar extent in human microvascular endothelial cells (HMVECs). NO release was neither associated with an increase in intracellular calcium nor decreased by Ca²⁺/calmodulin-dependent protein kinase II (CamKII) blockade; however, dephosphorylation of eNOS-Thr⁴⁹⁵ was observed. Phosphatidylinositol 3-kinase (PI3K) activity and Akt-Ser⁴⁷³ phosphorylation were both increased by IGFBP-3 and selectively blocked by the SRB1-Ab or PI3K blocker LY294002. In conclusion, IGFBP-3 mediates protective effects on BRB integrity and mediates robust NO release to stimulate vasorelaxation via activation of SRB1. This response is IGF-1- and calcium-independent, but requires PI3K/Akt activation, suggesting that IGFBP-3 has novel protective effects on retinal and systemic vasculature and may be a therapeutic candidate for ocular complications such as diabetic retinopathy.

Insulin-like growth factor binding proteins: a structural perspective

Insulin-like growth factor binding proteins (IGFBP-1 to -6) bind insulin-like growth factors-I and -II (IGF-I and IGF-II) with high affinity. These binding proteins maintain IGFs in the circulation and direct them to target tissues, where they promote cell growth, proliferation, differentiation, and survival via the type 1 IGF receptor. IGFBPs also interact with many other molecules, which not only influence their modulation of IGF action but also mediate IGF-independent activities that regulate processes such as cell migration and apoptosis by modulating gene transcription. IGFBPs-1 to -6 are structurally similar proteins consisting of three distinct domains, N-terminal, linker, and C-terminal. There have been major advances in our understanding of IGFBP structure in the last decade and a half. While there is still no structure of an intact IGFBP, several structures of individual N- and C-domains have been solved. The structure of a complex of N-BP-4:IGF-I:C-BP-4 has also been solved, providing a detailed picture of the structural features of the IGF binding site and the mechanism of binding. Structural studies have also identified features important for interaction with extracellular matrix components and integrins. This review summarizes structural studies reported so far and highlights features important for binding not only IGF but also other partners. We also highlight future directions in which structural studies will add to our knowledge of the role played by the IGFBP family in normal growth and development, as well as in disease.

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.

Free insulin-like growth factor binding protein-3 (IGFBP-3) reduces retinal vascular permeability in association with a reduction of acid sphingomyelinase (ASMase)

PURPOSE

To examine the effect of free insulin-like growth factor (IGF) binding protein-3 (IGFBP-3), independent of the effect of insulin-like growth factors, in modulating retinal vascular permeability.

METHODS

We assessed the ability of a form of IGFBP-3 that does not bind IGF-1 (IGFBP-3NB), to regulate the blood retinal barrier (BRB) using two distinct experimental mouse models, laser-induced retinal vessel injury and vascular endothelial growth factor (VEGF)-induced retinal vascular permeability. Additionally, in vitro studies were conducted. In the animal models, BRB permeability was quantified by intravenous injection of fluorescein labeled serum albumin followed by digital confocal image analysis of retinal flat-mounts. Claudin-5 and vascular endothelial-cadherin (VE-cadherin) localization at interendothelial junctions was studied by immunofluorescence. In vitro changes in transendothelial electrical resistance (TEER) and flux of fluorescent dextran in bovine retinal endothelial monolayers (BREC) were measured after IGFBP-3NB treatment. Acid (ASMase) and neutral (NSMase) sphingomyelinase mRNA levels and activity were measured in mouse retinas.

RESULTS

Four days postinjury, laser-injured mouse retinas injected with IGFBP-3NB plasmid demonstrated reduced vascular permeability compared with retinas of laser-injured mouse retinas injected with control plasmid. IGFBP-3NB administration resulted in a significant decrease in laser injury-associated increases in ASMase and NSMase mRNA and activity when compared with laser alone treated mice. In vivo, intravitreal injection of IGFBP-3NB reduced vascular leakage associated with intravitreal VEGF injection. IGFBP-3NB partially restored VEGF-induced in vivo permeability and dissociation of claudin-5 and VE-cadherin at junctional complexes. When IGFBP-3NB was applied basally to bovine retinal endothelial cells (BREC) in vitro, TEER increased and macromolecular flux decreased.

CONCLUSIONS

Intravitreal administration of IGFBP-3NB preserves junctional integrity in the presence of VEGF or laser injury by reducing BRB permeability in part by modulating sphingomyelinase levels.

Nuclear export and mitochondrial and endoplasmic reticulum localization of IGF-binding protein 3 regulate its apoptotic properties

Tumor suppression by IGF-binding protein 3 (IGFBP3) may occur in an IGF-independent manner, in addition to its role as a regulator of IGF bioavailability. After secretion, IGFBP3 is internalized, rapidly localized to the nucleus, and is later detected in the cytoplasm. We identified a putative nuclear export sequence (NES) in IGFBP3 between amino acids 217 and 228, analogous to the leucine-rich NES sequence of p53 and HIV Rev. Mutation of the NES prevents nucleocytoplasmic shuttling of IGFBP3 and blocks its ability to induce apoptosis. Targeting of IGFBP3 to the mitochondria and endoplasmic reticulum (ER) was confirmed by co-localization with organelle markers using fluorescence confocal microscopy and subcellular fractionation. Mitochondrial targeting was also demonstrated in vivo in IGFBP3-treated prostate cancer xenografts. These results show that IGFBP3 shuttles from the nucleus to the mitochondria and ER, and that nuclear export is essential for its effects on prostate cancer apoptosis.

Activity of human pregnancy insulin-like growth factor binding protein-3: determination by reconstituting recombinant complexes

During pregnancy, IGF binding protein-3 (IGFBP-3) is completely proteolyzed to fragments with low affinities for IGFs but appears to transport IGFs normally in high-molecular-mass complexes. We previously reported that synthetic isolated amino- and carboxyl-terminal domains of IGFBP-3 cooperate to bind IGFs, and we investigated whether this is the mechanism whereby proteolyzed IGFBP-3 fragments bind IGFs normally in pregnancy serum. Two fragments of IGFBP-3 have been isolated from pregnancy serum, one with the same N-terminal sequence as intact IGFBP-3 (GASSG) and the other with an N-terminal sequence (160)KVDYE. Recombinant forms of these proteins, IGFBP-3(1-159) and IGFBP-3(160-264), have been synthesized and characterized, demonstrating that although the fragments individually have greatly reduced affinity for IGF complex formation, when combined they cooperate to form complexes with IGF with or without the acid-labile subunit, inhibit IGF transport across endothelial cell monolayers and inhibit IGF-I-induced IGF type I receptor phosphorylation. It is proposed that proteolysis of IGFBP-3 into two discrete complementary fragments does not significantly increase IGF bioavailability, consistent with previous findings that proteolyzed IGFBP-3 in pregnancy serum is functionally normal and circulates as part of the IGF ternary complexes.

The insulin-like growth factor family: molecular mechanisms, redox regulation, and clinical implications

Insulin-like growth factor (IGF)-induced signaling networks are vital in modulating multiple fundamental cellular processes, such as cell growth, survival, proliferation, and differentiation. Aberrations in the generation or action of IGF have been suggested to play an important role in several pathological conditions, including metabolic disorders, neurodegenerative diseases, and multiple types of cancer. Yet the exact mechanism involved in the pathogenesis of these diseases by IGFs remains obscure. Redox pathways involving reactive oxygen species (ROS) and reactive nitrogen species (RNS) contribute to the pathogenetic mechanism of various diseases by modifying key signaling pathways involved in cell growth, proliferation, survival, and apoptosis. Furthermore, ROS and RNS have been demonstrated to alter IGF production and/or action, and vice versa, and thereby have the ability to modulate cellular functions, leading to clinical manifestations of diseases. In this review, we provide an overview on the IGF system and discuss the potential role of IGF-1/IGF-1 receptor and redox pathways in the pathophysiology of several diseases.

Inhibition of adipocyte differentiation by insulin-like growth factor-binding protein-3

Insulin-like growth factor-binding protein-3 (IGFBP-3) interacts with the type II nuclear receptors retinoid X receptor (RXR)alpha and retinoic acid receptor-alpha and modulates their transcriptional activity. Peroxisome proliferator-activated receptor (PPAR)gamma, a related nuclear receptor that dimerizes with RXRalpha, plays an important role in adipocyte differentiation. IGFBP-3 is regulated during adipocyte differentiation, but its role in this process is unknown. We demonstrate that IGFBP-3 interferes with the PPARgamma-dependent processes of adipocyte differentiation and maintenance of the gene expression characteristic of mature adipocytes. Treatment of adipocytes with exogenous IGFBP-3, but not an IGFBP-3 mutant that does not bind RXRalpha or PPARgamma, decreased markers of adipocyte differentiation, PPARgamma, and resistin but increased the preadipocyte marker plasminogen activator inhibitor-1. Furthermore, expression of human IGFBP-3, but not the IGFBP-3 mutant, by preadipocytes inhibited preadipocyte differentiation as determined by gene markers and lipid accumulation. IGFBP-3 interacted with PPARgamma in vitro and in 3T3-L1 adipocyte lysates and inhibited PPARgamma heterodimerization with RXRalpha in vitro. Wild-type IGFBP-3, but not mutant IGFBP-3, blocked ligand-induced transactivation of PPAR response element in 3T3-L1 cells. The observation that IGFBP-3 inhibits adipocyte differentiation and impacts on the PPARgamma system suggests a role for IGFBP-3 in the pathogenesis of obesity and insulin resistance.

Perspectives in mammalian IGFBP-3 biology: local vs. systemic action

Insulin-like growth factor (IGF) binding protein (IGFBP)-3 has traditionally been defined by its role as a binding protein and its association with IGF delivery and availability. Development of non-IGF binding IGFBP-3 analogs and the use of cell lines devoid of type 1 IGF receptors (IGF-R) have led to critical advances in the field of IGFBP-3 biology. These studies show that IGFBP-3 has IGF-independent roles in inhibiting cell proliferation in cancer cell lines. Nuclear transcription factor, retinoid X receptor (RXR)-alpha, and IGFBP-3 functionally interact to reduce prostate tumor growth and prostate-specific antigen in vivo. Moreover, IGFBP-3 inhibits insulin-stimulated glucose uptake into adipocytes independent of IGF. The purpose of this review is to highlight IGFBP-3 as a novel effector molecule and not just another "binding protein" by discussing its IGF-independent actions on metabolism and cell growth. Although this review presents studies that assume the role of IGFBP-3 as either an endocrine or autocrine/paracrine molecule, these systems may not exist as distinct entities, justifying the examination of IGFBP-3 in an integrated model. Also, we provide an overview of factors that regulate IGFBP-3 availability, including its production, methylation, and ubiquitination. We conclude with the role of IGFBP-3 in whole body systems and possible future applications of IGFBP-3 in physiology.

Phylogenetic analysis of the insulin-like growth factor binding protein (IGFBP) and IGFBP-related protein gene families

Insulin-like growth factor (IGF) activity is regulated by six high affinity binding proteins (IGFBPs) and possibly by some of the nine IGFBP-related proteins (IGFBP-rPs). To determine the phylogenetic relationship of this proposed gene superfamily, we conducted maximum likelihood (ML) and Bayesian inference analyses on a matrix of amino acid sequences from a diversity of vertebrate species. A single most likely phylogram, ML bootstrap, and Bayesian consensus tree of 10,000,000 generations revealed a monophyletic IGFBP lineage independent of the IGFBP-rPs. The IGFBPs segregated into three distinct clades: IGFBP-1, -3, and -6. Subsequent gene duplication events within the IGFBP-1 and -3 clades resulted in the production and divergence of IGFBP-2 and -4 within the IGFBP-1 clade and IGFBP-5 in the IGFBP-3 clade. By contrast, the IGFBP-rPs were distributed paraphyletically into two clades: IGFBP-rP1, 5, and 6 in one clade and the CCN family (IGFBP-rP2-4,7-9) in another. A recently identified IGFBP-3 homolog in rainbow trout localized to the IGFBP-2 subclade. Subsequence analysis identified a RGD motif common to IGFBP-2 orthologs, but did not identify the nuclear localization sequence present in IGFBP-3 and -5 homologs. The putative trout IGFBP-3 was 36-55% identical to different IGFBP-2 proteins, but only 24-27% identical to IGFBP-3 proteins. These results suggest that the IGFBPs and IGFBP-rPs are at best distantly related and that the limited similarities likely resulted from exon shuffling. They also suggest that rainbow trout, and possibly other salmonids, possess two IGFBP-2 paralogs as the putative trout IGFBP-3 is misannotated.

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

This review describes a comprehensive analysis of a surface plasmon resonance (SPR)-based biosensor study of molecular interactions in the insulin-like growth factor (IGF) molecular axis. In this study, we focus on the interaction between the polypeptide growth factors IGF-I and IGF-II with six soluble IGF binding proteins (IGFBP 1-6), which occur naturally in various biological fluids. We have describe the conditions required for the accurate determination of kinetic rate constants for these interactions and highlight the experimental and theoretical pitfalls, which may be encountered in the early stages of such a study. We focus on IGFBP-5 and describe a site-directed mutagenesis study, which examines the contribution of various residues in the protein to high affinity interaction with IGF-I and -II. We analyse the interaction of IGFBP-5 (and IGFBP-3) with heparin and other biomolecules and describe experiments, which were designed to monitor multi-protein complex formation in this molecular axis.

Molecular basis of the interaction between IGFBP-3 and retinoid X receptor: role in modulation of RAR-signaling

IGFBP-3 interacts with the retinoid X receptor-alpha (RXRalpha) and retinoic acid receptor-alpha (RARalpha) and thereby interferes with the formation of RXR:RAR heterodimers. Here we identify the domains in RXRalpha and IGFBP-3 that participate in this interaction. When different regions of RXRalpha were expressed independently, we found that only the DNA-binding domain (C-domain) bound IGFBP-3. Residues in the second Zn-finger loop (Gln49, Arg52), which contribute to C-domain dimerization on DR1 response elements, proved essential to IGFBP-3 binding. In complementary studies, we found that residues within the N-terminal domain of IGFBP-3 (Thr58, Arg60) and motifs in its C-terminal domain ((220)LysLysLys, (228)LysGlyArgLysArg) were required for interaction with RXRalpha and RARalpha. Unlike wild-type IGFBP-3, the non-retinoid receptor-binding mutants of IGFBP-3 were unable to attenuate all-trans-retinoic acid-induced transactivation of the RAR response element by RXR:RAR heterodimers. We conclude that residues in both the N- and C-terminal domains of IGFBP-3 are involved in binding the retinoid receptors, and that this interaction is essential to the modulation of RAR-signaling by IGFBP-3.

Promotion of cancer cell migration: an insulin-like growth factor (IGF)-independent action of IGF-binding protein-6

A family of six high affinity IGF-binding proteins (IGFBPs 1-6) plays an important role in modulating IGF activities. Recent studies suggest that some IGFBPs may have IGF-independent effects, including induction of apoptosis and modulation of cell migration. However, very little is known about possible IGF-independent actions of IGFBP-6. We have generated a non-IGF-binding IGFBP-6 mutant by substituting Ala for four amino acid residues (Pro(93)/Leu(94)/Leu(97)/Leu(98)) in its N-domain IGF-binding site. A >10,000-fold loss of binding affinity for IGF-I and IGF-II was observed using charcoal solution binding assay, BIAcore biosensor, and ligand blotting. Wild-type and mutant IGFBP-6, as well as IGF-II, induced cell migration in RD rhabdomyosarcoma and LIM 1215 colon cancer cells. Cell migration was mediated by the C-domain of IGFBP-6. Transient p38 phosphorylation was observed in RD cells after treatment with IGFBP-6, whereas no change was seen in phospho-ERK1/2 levels. Phospho-JNK was not detected. IGFBP-6-induced cell migration was inhibited by SB203580, an inhibitor of p38 MAPK, and PD98059, an inhibitor of ERK1/2 MAPK activation. In contrast, SP600125, a JNK MAPK inhibitor, had no effect on migration. Knockdown of p38 MAPK using short interfering RNA blocked IGFBP-6-induced migration of RD cells. These results indicate that p38 MAPK is involved in IGFBP-6-induced IGF-independent RD cell migration.

Regulation of insulin-like growth factor (IGF) bioactivity by sequential proteolytic cleavage of IGF binding protein-4 and -5

The biological activity of IGF-I and -II is controlled by six binding proteins (IGFBPs), preventing the IGFs from interacting with the IGF receptor. Proteolytic cleavage of IGFBPs is one mechanism by which IGF can be released to bind the receptor. The IGFBPs are usually studied individually, although the presence of more than one of the IGFBPs in most tissues suggests a cooperative function. Thus, the IGFBPs are part of regulatory networks with proteolytic enzymes in one end and the IGF receptor in the other end. We have established a model system that allows analysis of the dynamics between IGF, IGFBP-4 and -5, the IGF receptor, and the proteolytic enzyme PAPP-A, which specifically cleaves both IGFBP-4 and -5. We demonstrate different mechanisms of IGF release from IGFBP-4 and -5: cooperative binding to IGF is observed for the proteolytic fragments of IGFBP-5, but not fragments of IGFBP-4. Furthermore, we find that PAPP-A-mediated IGF-dependent cleavage of IGFBP-4 is inhibited by IGFBP-5, which sequesters IGF from IGFBP-4, and that cleavage of both IGFBP-4 and -5 is required for the release of bioactive IGF. Finally, we show that cell surface-localized proteolysis of IGFBP-4 represents the final regulatory step of efficient IGF delivery to the receptor. Our data define a regulatory system in which molar ratios between the IGFBPs and IGF and between the different IGFBPs, sequential proteolytic cleavage of the IGFBPs, and surface association of the activating proteinase are key elements in the regulation of IGF receptor stimulation.

The aminoterminal insulin-like growth factor (IGF) binding domain of IGF binding protein-3 cannot be functionally substituted by the structurally homologous domain of CCN3

IGF binding proteins (IGFBPs) are a family of structurally homologous proteins that bind IGFs with high affinities and can modulate IGF activity. The IGF binding site has been shown to comprise residues in both the aminoterminal and carboxyterminal domains. In recent years several proteins including members of the CCN (connective tissue growth factor, Cyr61, and nephroblastoma overexpressed) family were recognized as having structural homology in their aminoterminal domains to the IGFBPs. Despite their low or undetectable IGF binding ability, a proposal was made to rename them as IGFBP-related proteins. To test whether the aminoterminal domain of a CCN protein can fulfill the high-affinity IGF binding function of an IGFBP, we created a chimera in which the aminoterminal domain of IGFBP-3 was substituted with the aminoterminal domain of CCN3 (previously known as Nov). The CCN3-IGFBP-3 chimera bound IGFs and inhibited IGF activity very weakly, similar to CCN3 itself. Although structurally similar, the aminoterminal domain of CCN3 is unable to replace the aminoterminal domain of IGFBP-3 in forming a high-affinity IGF-binding site. These results argue against a direct role of CCN3 in the regulation of IGF bioavailability and indicate that the nomenclature of IGFBP-related proteins (which implies functional relationship to the classical IGFBPs) is inappropriate for CCN proteins.

Structure, dynamics and heparin binding of the C-terminal domain of insulin-like growth factor-binding protein-2 (IGFBP-2)

Insulin-like growth factor-binding protein-2 (IGFBP-2) is the largest member of a family of six proteins (IGFBP-1 to 6) that bind insulin-like growth factors I and II (IGF-I/II) with high affinity. In addition to regulating IGF actions, IGFBPs have IGF-independent functions. The C-terminal domains of IGFBPs contribute to high-affinity IGF binding, and confer binding specificity and have overlapping but variable interactions with many other molecules. Using nuclear magnetic resonance (NMR) spectroscopy, we have determined the solution structure of the C-terminal domain of IGFBP-2 (C-BP-2) and analysed its backbone dynamics based on 15N relaxation parameters. C-BP-2 has a thyroglobulin type 1 fold consisting of an alpha-helix, a three-stranded anti-parallel beta-sheet and three flexible loops. Compared to C-BP-6 and C-BP-1, structural differences that may affect IGF binding and underlie other functional differences were found. C-BP-2 has a longer disordered loop I, and an extended C-terminal tail, which is unstructured and very mobile. The length of the helix is identical with that of C-BP-6 but shorter than that of C-BP-1. Reduced spectral density mapping analysis showed that C-BP-2 possesses significant rapid motion in the loops and termini, and may undergo slower conformational or chemical exchange in the structured core and loop II. An RGD motif is located in a solvent-exposed turn. A pH-dependent heparin-binding site on C-BP-2 has been identified. Protonation of two histidine residues, His271 and His228, seems to be important for this binding, which occurs at slightly acidic pH (6.0) and is more significant at pH 5.5, but is largely suppressed at pH 7.4. Possible preferential binding of IGFBP-2 and its C- domain fragments to glycosaminoglycans in the acidic extracellular matrix (ECM) of tumours may be related to their roles in cancer.

Insulin-like growth factor binding protein-3 in extracellular matrix stimulates adhesion of breast epithelial cells and activation of p44/42 mitogen-activated protein kinase

IGF-binding protein-3 (IGFBP-3) is a multifunctional protein that regulates the potent mitogenic and antiapoptotic effects of IGF-I and IGF-II and exerts bioactivity independent of modulating IGF receptor activation. Previous studies have shown that in solution, IGFBP-3 binds constituent proteins of the extracellular matrix (ECM) such as fibronectin and collagen and is present in ECM deposited by fibroblasts in vitro; however, binding of IGFBP-3 to matrix has not been characterized, nor has its function in this environment been investigated. In this study, we show that IGFBP-3 binds to ECM deposited by human breast epithelial and cancer cells and neonatal human fibroblasts. IGF-I and heparin blocked binding of IGFBP-3 to matrix when added with the binding protein but were unable to displace IGFBP-3 already bound to the matrix. IGF-I bound to matrix-immobilized IGFBP-3 with approximately 25-fold reduced affinity compared with IGFBP-3 in solution. Mutation of the C-terminal basic domain of IGFBP-3 (228KGRKR-->MDGEA) resulted in markedly reduced binding to matrix compared with wild-type IGFBP-3, whereas mutation of the adjacent consensus heparin-binding domain (220KKK-->HSR) had relatively little effect. In the presence of matrix-bound IGFBP-3, adhesion of breast epithelial cells was increased by approximately 25%, and activation of the signaling pathway intermediate p44/42 MAPK was enhanced greater than 3-fold. These results indicate a previously unrecognized and potentially important role for IGFBP-3 in the extracellular matrix.

Paradoxical effects of the phage display-derived peptide antagonist IGF-F1-1 on insulin-like growth factor-1 receptor signaling

The insulin-like growth factor binding proteins (IGFBPs) represent a unique class of IGF antagonists regulating the bioavailability of the IGFs extracellularly. Accordingly, they represent an important class of proteins for cancer therapeutics and chemoprevention. IGF-F1-1 is a cyclic hexadecapeptide identified by high throughput phage display that binds to the IGFBP-binding domain on IGF-1. It acts as an IGFBP-mimetic, capable of inhibiting IGF-1 binding to the IGFBPs. To further examine the utility of IGF-F1-1 as an IGF-1 antagonist we tested its ability to inhibit IGFBP-2 and IGFBP-3 binding to IGF-1, (125)I-IGF-1 binding to IGF-1Rs and to block IGF-1 induced Akt activation, cell cycle changes and [(3)H]thymidine incorporation in MCF-7 cells. These biological activities were inhibited by treatment with IGFBP-2, wortmannin or the IGF-1R tyrosine kinase inhibitor, NVP-AEW541, but not by IGF-F1-1. Our findings confirm previous studies indicating that IGF-F1-1 is a weak antagonist of IGF-1 binding to the IGFBPs and the IGF-1R and suggest that it does not effectively inhibit downstream events stimulated by IGF-1. We further demonstrated that IGF-F1-1 treatment of MCF-7 cells results in the paradoxical activation of Akt, S-phase transition and [(3)H]thymidine incorporation. These results suggest that IGF-F1-1 is a weak agonist, exhibiting mitogenic actions. IGF-F1-1 may act in conjunction with IGF-1 at the IGF-1R or independently of IGF-1 at the IGF-1R or another receptor.

Insulin-like growth factor-binding protein-5 (IGFBP-5): a critical member of the IGF axis

The six members of the insulin-like growth factor-binding protein family (IGFBP-1-6) are important components of the IGF (insulin-like growth factor) axis. In this capacity, they serve to regulate the activity of both IGF-I and -II polypeptide growth factors. The IGFBPs are able to enhance or inhibit the activity of IGFs in a cell- and tissue-specific manner. One of these proteins, IGFBP-5, also has an important role in controlling cell survival, differentiation and apoptosis. In this review, we report on the structural and functional features of the protein which are important for these effects. We also examine the regulation of IGFBP-5 expression and comment on its potential role in tumour biology, with special reference to work with breast cancer cells.

Enhanced insulin-like growth factor-I (IGF-I) cell association at reduced pH is dependent on IGF binding protein-3 (IGFBP-3) interaction

The cellular microenvironment impacts how signals are transduced by cells and plays a key role in tissue homeostasis. Although pH is generally well regulated, there are a number of situations where acidosis occurs and our work addresses how low pH impacts cell association of insulin-like growth factor-I (IGF-I) in the presence of IGF binding protein-3 (IGFBP-3). We have previously shown that IGF-I cell binding was enhanced in the presence of IGFBP-3 at low pH and now show that this binding is IGFBP-mediated as it is inhibited by Y60L-IGF-I, a mutant with reduced affinity for the IGF receptor (IGF-IR), and unaffected by insulin, which binds but not IGFBPs. Using surface plasmon resonance (SPR), we show that direct binding between IGF-I and IGFBP-3 is pH sensitive. Despite this, the key step in the process appears to be IGFBP-3 cell surface association as Long-R(3)-IGF-I, a mutant with reduced affinity for IGFBPs, shows a similar increase in cell association at pH 5.8 in the presence of IGFBP-3 but does not exhibit pH-dependent binding by SPR. Further, analysis indicates a large increase in low-affinity binding sites for IGF-I in the presence of IGFBP-3 and an elimination of IGF-I enhanced binding when a non-cell associating mutant of IGFBP-3 is added in place of IGFBP-3. That the IGFBP-3-mediated binding localizes IGF-I away from IGF-IR is suggested by triton-solubility testing and indicates additional complexities to IGF-I regulation by IGFBP-3. Identifying the pH-dependent binding partner(s) for IGFBP-3 is a necessary next step in deciphering this process.

Cumulative mutagenesis of the basic residues in the 201-218 region of insulin-like growth factor (IGF)-binding protein-5 results in progressive loss of both IGF-I binding and inhibition of IGF-I biological action

We have reported previously that mutation of two conserved nonbasic amino acids (G203 and Q209) within the highly basic 201-218 region in the C-terminal domain of IGF-binding protein-5 (IGFBP-5) decreases binding to IGFs. This study reveals that cumulative mutagenesis of the 10 basic residues in this region, to create the C-Term series of mutants, ultimately results in a 15-fold decrease in the affinity for IGF-I and a major loss in heparin binding. We examined the ability of mutants to inhibit IGF-mediated survival of MCF-7 cells and were able to demonstrate that this depended not only upon the affinity for IGF-I, but also the kinetics of this interaction, because IGFBP-5 mutants with similar affinity constants (K(D)) values, but with different association (Ka) and dissociation (Kd) rate values, had markedly different inhibitory properties. In contrast, the affinity for IGF-I provided no predictive value in terms of the ability of these mutants to enhance IGF action when bound to the substratum. Instead, these C-Term mutants appeared to enhance the actions of IGF-I by a combination of increased dissociation of IGF-IGFBP complexes from the substratum, together with dissociation of IGF-I from IGFBP-5 bound to the substratum. These effects of the IGFBPs were dependent upon binding to IGF-I, because a non-IGF binding mutant (N-Term) was unable to inhibit or enhance the actions of IGF-I. These results emphasize the importance of the kinetics of association/dissociation in determining the enhancing or inhibiting effects of IGFBP-5 and demonstrate the ability to generate an IGFBP-5 mutant with exclusively IGF-enhancing activity.

Survey of the year 2004 commercial optical biosensor literature

The year 2004 represents a milestone for the biosensor research community: in this year, over 1000 articles were published describing experiments performed using commercially available systems. The 1038 papers we found represent an approximately 10% increase over the past year and demonstrate that the implementation of biosensors continues to expand at a healthy pace. We evaluated the data presented in each paper and compiled a 'top 10' list. These 10 articles, which we recommend every biosensor user reads, describe well-performed kinetic, equilibrium and qualitative/screening studies, provide comparisons between binding parameters obtained from different biosensor users, as well as from biosensor- and solution-based interaction analyses, and summarize the cutting-edge applications of the technology. We also re-iterate some of the experimental pitfalls that lead to sub-optimal data and over-interpreted results. We are hopeful that the biosensor community, by applying the hints we outline, will obtain data on a par with that presented in the 10 spotlighted articles. This will ensure that the scientific community at large can be confident in the data we report from optical biosensors.