Localization of an insulin-like growth factor (IGF) binding site of bovine IGF binding protein-2 using disulfide mapping and deletion mutation analysis of the C-terminal domain

Characterization of Insulin-Like Growth Factor Binding Protein 7 (Igfbp7) and Its Potential Involvement in Shell Formation and Metamorphosis of Pacific Abalone, Haliotis discus hannai

Insulin-like growth factor binding proteins (IGFBPs) are secreted proteins that play an important role in IGF regulation of growth and development of vertebrate and invertebrates. In this study, the IGFBP7 gene was cloned and characterized from mantle tissues of H. discus hannai, and designated as Hdh IGFBP7. The full-length cDNA sequence transcribed from the Hdh IGFBP7 gene was 1519-bp long with an open reading frame of 720-bp corresponding to a putative polypeptide of 239 amino acids. The molecular mass of its mature protein was approximately 23.44 KDa with an estimated isoelectric point (pI) of 5.35, and it shared significant homology with IGFBP7 gene of H. madaka. Hdh IGFBP7 has a characteristic IGFBP N-terminal domain (22–89 aa), a kazal-type serine proteinase inhibitor domain (77–128), and an immunoglobulin-like C2 domain (144–223). Furthermore, twelve cysteine residues and a signature motif of IGFBPs (XCGCCXXC) were found in its N-terminal domain. Phylogenetic analysis revealed that Hdh IGFBP7 was aligned with IGFBP7 of H. madaka. Tissue distribution analysis showed that the mRNA of Hdh IGFBP7 was expressed in all examined tissues, with the highest expression level observed in the mantle and gill tissues. The expression level of Hdh IGFBP7 mRNA was relatively higher at the juvenile stage during its metamorphosis period. In situ hybridization showed that Hdh IGFBP7 transcript was expressed in epithelial cells of the dorsal mantle pallial and mucus cells of the branchial epithelium in gill. These results provide basic information for future studies on the role of IGFBP7 in IGF regulation of shell growth, development and metamorphosis of abalone.

IGFBP2: integrative hub of developmental and oncogenic signaling network

Insulin-like growth factor (IGF) binding protein 2 (IGFBP2) was discovered and identified as an IGF system regulator, controlling the distribution, function, and activity of IGFs in the pericellular space. IGFBP2 is a developmentally regulated gene that is highly expressed in embryonic and fetal tissues and markedly decreases after birth. Studies over the last decades have shown that in solid tumors, IGFBP2 is upregulated and promotes several key oncogenic processes, such as epithelial-to-mesenchymal transition, cellular migration, invasion, angiogenesis, stemness, transcriptional activation, and epigenetic programming via signaling that is often independent of IGFs. Growing evidence indicates that aberrant expression of IGFBP2 in cancer acts as a hub of an oncogenic network, integrating multiple cancer signaling pathways and serving as a potential therapeutic target for cancer treatment.

Molecular cloning and expression pattern of IGFBP-2a in black porgy (Acanthopagrus schlegelii) and evolutionary analysis of IGFBP-2s in the species of Perciformes

Insulin-like growth factor-binding protein-2 (IGFBP-2) plays a key role in regulating growth and development by its affinity with insulin-like growth factors (IGFs). In this study, we cloned the coding sequence (CDS) of IGFBP-2a from the black porgy (Acanthopagrus schlegelii) muscle and identified that the full-length CDS of IGFBP-2a was 882 bp. Real-time quantitative PCR revealed that IGFBP-2a was most abundant in the liver of the black porgy and backcross breed (F1♀×black porgy♂) but remained lower in each tested tissue in self-cross breed (F1♀×F1♂). In addition, the IGFBP-2a expression in the liver of three breeds showed a negative correlation with their growth rates, indicating that the IGFBP-2a played a growth-inhibiting role in the three breeds. We further identified 810 bp IGFBP-2b gene from the draft genome of black porgy. Finally, we examined the IGFBP-2a and IGFBP-2b genes by scanning the genomes of the species of Perciformes and found the IGFBP-2 gene duplication took place earlier than the divergence of perciform species. Interestingly, six positively selected sites were detected in both Perciformes IGFBP-2 genes, although both genes were identified to be under purifying selection. Specially, these positively selected sites were located in the functional domains, suggesting these sites played key roles in the growth of Perciformes. Our study partially explains the molecular basis for the prepotency in black porgy hybrids, which will provide guidance for their cultivation in the future.

Insulin-like growth factor system in cancer: novel targeted therapies

Insulin-like growth factors (IGFs) are essential for growth and survival that suppress apoptosis and promote cell cycle progression, angiogenesis, and metastatic activities in various cancers. The IGFs actions are mediated through the IGF-1 receptor that is involved in cell transformation induced by tumour. These effects depend on the bioavailability of IGFs, which is regulated by IGF binding proteins (IGFBPs). We describe here the role of the IGF system in cancer, proposing new strategies targeting this system. We have attempted to expand the general viewpoint on IGF-1R, its inhibitors, potential limitations of IGF-1R, antibodies and tyrosine kinase inhibitors, and IGFBP actions. This review discusses the emerging view that blocking IGF via IGFBP is a better option than blocking IGF receptors. This can lead to the development of novel cancer therapies.

IGFBP-2 - taking the lead in growth, metabolism and cancer

The activity of the Insulin-like Growth Factors (IGFs) ligands elicited via their receptors and transduced by various intracellular signal pathways is modulated by the IGF Binding Proteins (IGFBPs). Among all the IGFBPs, IGFBP-2 has been implicated in the regulation of IGF activity in most tissue and organs. Besides binding to IGFs in the circulation these IGF-regulatory activities of IGFBP-2 involve interactions with components of the extracellular matrix, cell surface proteoglycans and integrin receptors. In addition to these local peri-cellular activities, IGFBP-2 exerts other key functions within the nucleus, where IGFBP-2 directly or indirectly promotes transcriptional activation of specific genes. All of these IGFBP-2 activities, intrinsic or dependent on IGFs, contribute to its functional roles in growth/development, metabolism and malignancy as evidenced by studies in IGFBP-2 animal models and also by many in vitro studies. Finally, preclinical studies have demonstrated that IGFBP-2 administration can be beneficial in improving metabolic responses (inhibition of adipogenesis and enhanced insulin sensitivity), while blockade of IGFBP-2 appears to be an effective approach to inhibiting tumour growth and metastasis.

Molecular cloning, expression profiles and promoter analysis of insulin-like growth factor binding protein-4 (IGFBP-4) in Japanese flounder (Paralichthys olivaceus)

We cloned and characterized cDNA sequence of insulin-like growth factor binding protein-4 (IGFBP-4) from Japanese flounder (Paralichthys olivaceus). The 1493 bp full-length cDNA sequence contained an open reading frame (ORF) of 780 bp, which encoded a protein of 259 amino acids. The deduced amino acid sequences included a putative signal peptide of 28 amino acid residues resulting in a mature protein of 231 amino acids. Twenty cysteine residues and two conserved IGFBPs motif (GCGCCXXC and CWCV) were found in the N- and C-terminal domain. In the over 13 kbp genomic sequence, four exons, three introns, and 5'-/3'-flanking sequences were identified. Sequence alignment and phylogenetic analysis showed that Japanese flounder IGFBP-4 was indeed the ortholog of the human IGFBP-4 gene and shared high identities with other teleost IGFBP-4 genes. The promoter region was also analyzed and several potential transcription factor (TF) binding sites were determined which may modulate the IGFBP-4 expression. Quantitative real-time PCR analysis revealed that IGFBP-4 mRNA was observed in various tissues, with intestine showing the highest expression. The maternal transcripts of IGFBP-4 gene existed in the early embryonic stages and then increased in the following stages until hatching, suggesting that IGFBP-4 may be involved in the fish early development. The expression level of IGFBP-4 mRNA was relatively higher at 3 days post hatching (dph) and 15 dph, and gradually decreased during the metamorphosis period. All these results indicated that IGFBP-4 plays a significant role in IGF regulating vertebrate growth and development.

Exogenous administration of protease-resistant, non-matrix-binding IGFBP-2 inhibits tumour growth in a murine model of breast cancer

BACKGROUND

Insulin-like growth factors (IGF-I and IGF-II) signal via the type 1 IGF receptor (IGF-1R) and IGF-II also activates the insulin receptor isoform A (IR-A). Signalling via both receptors promotes tumour growth, survival and metastasis. In some instances IGF-II action via the IR-A also promotes resistance to anti-IGF-1R inhibitors. This study assessed the efficacy of two novel modified IGF-binding protein-2 (IGFBP-2) proteins that were designed to sequester both IGFs. The two modified IGFBP-2 proteins were either protease resistant alone or also lacked the ability to bind extracellular matrix (ECM).

METHODS

The modified IGFBP-2 proteins were tested in vitro for their abilities to inhibit cancer cell proliferation and in vivo to inhibit MCF-7 breast tumour xenograft growth.

RESULTS

Both mutants retained low nanomolar affinity for IGF-I and IGF-II (0.8-2.1-fold lower than IGFBP-2) and inhibited cancer cell proliferation in vitro. However, the combined protease resistant, non-matrix-binding mutant was more effective in inhibiting MCF-7 tumour xenograft growth and led to inhibition of angiogenesis.

CONCLUSIONS

By removing protease cleavage and matrix-binding sites, modified IGFBP-2 was effective in inhibiting tumour growth and reducing tumour angiogenesis.

Insulin-like growth factor binding protein-2 (IGFBP-2) in Japanese flounder, Paralichthys olivaceus: molecular cloning, expression patterns and hormonal regulation during metamorphosis

In this study, we cloned and characterized cDNA sequences of two insulin-like growth factor binding protein-2 (IGFBP-2a and IGFBP-2b) from Japanese flounder, Paralichthys olivaceus. The full-length cDNA of IGFBP-2a is 1,046 bp long and consists an open frame (ORF) of 876 bp, a 5'-untranslated region (UTR) of 125 bp and a 3'-UTR of 45 bp. IGFBP-2b is 1,067 bp, including a 5'-UTR of 53 bp, a 3'-UTR of 198 bp and an ORF of 816 bp. Real-time quantitative PCR results revealed that IGFBP-2a -2b mRNA was expressed in all detected tissues. Interestingly, the levels of IGFBP-2a mRNA in all detected tissues were higher in female than male, but IGFBP-2b was precisely the opposite. At different embryonic stages, the levels of IGFBP-2a mRNA were typically higher than IGFBP-2b. After hatching, IGFBP-2a mRNA was gradually decreased to a relatively lower level. However, the expression of IGFBP-2b mRNA was increased after hatching, including 3, 7, 10, 14, 17, 20 and 23 days post-hatching (dph), and it presents a higher level until 29 (metamorphic climax), 36 (post-climax) and 41 dph (the end of metamorphosis). In levothyroxine sodium salt (T4, the main form of thyroid hormone in animals)-treated and thiourea (TU)-treated larvae, the expressions of IGFBP-2a had not visibly changed, except in T4-treated 17 dph larvae. The expressions of IGFBP-2b mRNA were distinctly increased from 17 to 23 dph, but suddenly dropped to a lower level in and after 29 dph. However, the levels of IGFBP-2b mRNA during metamorphosis were greatly down-regulated after TU treatment. These results provided basic information for further studies on the role of IGF system in flatfish development and metamorphosis.

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.

Defining the disulfide bonds of insulin-like growth factor-binding protein-5 by tandem mass spectrometry with electron transfer dissociation and collision-induced dissociation

The six high-affinity insulin-like growth factor-binding proteins (IGFBPs) comprise a conserved family of secreted molecules that modulate IGF actions by regulating their half-life and access to signaling receptors, and also exert biological effects that are independent of IGF binding. IGFBPs are composed of cysteine-rich amino- (N-) and carboxyl- (C-) terminal domains, along with a cysteine-poor central linker segment. IGFBP-5 is the most conserved IGFBP, and contains 18 cysteines, but only 2 of 9 putative disulfide bonds have been mapped to date. Using a mass spectrometry (MS)-based strategy combining sequential electron transfer dissociation (ETD) and collision-induced dissociation (CID) steps, in which ETD fragmentation preferentially induces cleavage of disulfide bonds, and CID provides exact disulfide linkage assignments between liberated peptides, we now have definitively mapped 5 disulfide bonds in IGFBP-5. In addition, in conjunction with ab initio molecular modeling we are able to assign the other 4 disulfide linkages to within a GCGCCXXC motif that is conserved in five IGFBPs. Because of the nature of ETD fragmentation MS experiments were performed without chemical reduction of IGFBP-5. Our results not only establish a disulfide bond map of IGFBP-5 but also define a general approach that takes advantage of the specificity of ETD and the scalability of tandem MS, and the predictive power of ab initio molecular modeling to characterize unknown disulfide linkages in proteins.

Insulin-like growth factors (IGFs), IGF receptors, and IGF-binding proteins: roles in skeletal muscle growth and differentiation

The insulin-like growth factor (IGF) signaling pathway consists of multiple IGF ligands, IGF receptors, and IGF-binding proteins (IGFBPs). Studies in a variety of animal and cellular systems suggest that the IGF signaling pathway plays a key role in regulating skeletal muscle growth, differentiation, and in maintaining homeostasis of the adult muscle tissues. Intriguingly, IGFs stimulate both myoblast proliferation and differentiation, which are two mutually exclusive biological events during myogenesis. Both of these actions are mediated through the same IGF-1 receptor. Recent studies have shed new insights into the molecular mechanisms underlying these paradoxical actions of IGFs in muscle cells. In this article, we provide a brief review of our current understanding of the IGF signaling system and discuss recent findings on how local oxygen availability and IGFBPs act to specify IGF actions in muscle cells.

Common carp (Cyprinus carpio) insulin-like growth factor binding protein-2 (IGFBP-2): molecular cloning, expression profiles, and hormonal regulation in hepatocytes

In the present study, we cloned IGFBP-2 cDNA from common carp (Cyprinus carpio) liver. The 1879 bp full-length cDNA encodes 274 amino acid residues containing a putative signal peptide of 22 residues. Two IGFBP-2 transcripts with estimated sizes of 2.2 and 1.5 kb have been detected with Northern blot analysis in liver. Relatively high levels of IGFBP-2 mRNA were observed in all regions of brain, liver, pituitary, ovary and testis. Intermediate levels were observed in white muscle, thymus gland and head kidney, while in retina, heart and other tissues IGFBP-2 mRNA levels were very low. A significant level of IGFBP-2 mRNA was firstly detected at lens formation stage, and it continued to increase to the highest level at blood cycling stage, and fell to a relatively high level until hatching. The expression pattern of IGFBP-2 mRNA was similar during different stages of testis and ovary. At recrudescing stage the expression level was extremely low, but it sharply increased to a high level at matured stage, and finally brought back to the very low level at regressed stage. Hepatocytes IGFBP-2 mRNA was greatly reduced by growth hormone but increased by insulin. PD-98059 and LY-294002, the specific inhibitor of MEK and PI3K, increased IGFBP-2 mRNA expression level and completely blocked the inhibitory effect of GH. It is suggested that the MAPK and PI3 kinase-signaling pathways were involved in the decrease of IGFBP-2 mRNA expression induced by GH in primary cultured hepatocytes.

Duplication of the IGFBP-2 gene in teleost fish: protein structure and functionality conservation and gene expression divergence

Background

Insulin-like growth factor binding protein-2 (IGFBP-2) is a secreted protein that binds and regulates IGF actions in controlling growth, development, reproduction, and aging. Elevated expression of IGFBP-2 is often associated with progression of many types of cancers.

Methodology/Principal Findings

We report the identification and characterization of two IGFBP-2 genes in zebrafish and four other teleost fish. Comparative genomics and structural analyses suggest that they are co-orthologs of the human IGFBP-2 gene. Biochemical assays show that both zebrafish igfbp-2a and -2b encode secreted proteins that bind IGFs. These two genes exhibit distinct spatiotemporal expression patterns. During embryogenesis, IGFBP-2a mRNA is initially detected in the lens, then in the brain boundary vasculature, and subsequently becomes highly expressed in the liver. In the adult stage, liver has the highest levels of IGFBP-2a mRNA, followed by the brain. Low levels of IGFBP-2a mRNA were detected in muscle and in the gonad in male adults only. IGFBP-2b mRNA is detected initially in all tissues at low levels, but later becomes abundant in the liver. In adult males, IGFBP-2b mRNA is only detected in the liver. In adult females, it is also found in the gut, kidney, ovary, and muscle. To gain insights into how the IGFBP-2 genes may have evolved through partitioning of ancestral functions, functional and mechanistic studies were carried out. Expression of zebrafish IGFBP-2a and -2b caused significant decreases in the growth and developmental rates and their effects are comparable to that of human IGFBP-2. IGFBP-2 mutants with altered IGF binding-, RGD-, and heparin-binding sites were generated and their actions examined. While mutating the RGD and heparin binding sites had little effect, altering the IGF binding site abolished its biological activity.

Conclusions/Significance

These results suggest that IGFBP-2 is a conserved regulatory protein and it inhibits growth and development primarily by binding to and inhibiting IGF actions in vivo. The duplicated IGFBP-2 genes may provide additional flexibility in the regulation of IGF activities.

Cooperativity of the N- and C-terminal domains of insulin-like growth factor (IGF) binding protein 2 in IGF binding

A family of six insulin-like growth factor (IGF) binding proteins (IGFBP-1-6) binds IGF-I and IGF-II with high affinity and thus regulates their bioavailability and biological functions. IGFBPs consist of N- and C-terminal domains, which are highly conserved and cysteine-rich, joined by a variable linker domain. The role of the C-domain in IGF binding is not completely understood in that C-domain fragments have very low or even undetectable IGF binding affinity, but loss of the C-domain dramatically disrupts IGF binding by IGFBPs. We recently reported the solution structure and backbone dynamics of the C-domain of IGFBP-2 (C-BP-2) and identified a pH-dependent heparin binding site [Kuang, Z., Yao, S., Keizer, D. W., Wang, C. C., Bach, L. A., Forbes, B. E., Wallace, J. C., and Norton, R. S. (2006) Structure, dynamics and heparin binding of the C-terminal domain of insulin-like growth factor-binding protein-2 (IGFBP-2), J. Mol. Biol. 364, 690-704]. Here, we have analyzed the molecular interactions among the N-domain of IGFBP-2 (N-BP-2), C-BP-2, and IGFs using cross-linking and nuclear magnetic resonance (NMR) spectroscopy. The binding of C-BP-2 to the IGF-I.N-BP-2 binary complex was significantly stronger than the binding of C-BP-2 to IGF-I alone, switching from intermediate exchange to slow exchange on the NMR time scale. A conformational change or stabilization of the IGF-I Phe49-Leu54 region and the Phe49 aromatic ring upon binding to the N-domains, as well as an interdomain interaction between N-BP-2 and C-BP-2 (which is also detectable in the absence of ligand), may contribute to this cooperativity in IGF binding. Glycosaminoglycan binding by IGFBPs can affect their IGF binding although the effects appear to differ among different IGFBPs; here, we found that heparin bound to the IGF-I.N-BP-2.C-BP-2 ternary complex, but did not cause it to dissociate.

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.

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.

Insulin-like growth factor binding protein-2: contributions of the C-terminal domain to insulin-like growth factor-1 binding

Signaling by the insulin-like growth factor (IGF)-1 receptor (IGF-1R) has been implicated in the promotion and aggressiveness of breast, prostate, colorectal, and lung cancers. The IGF binding proteins (IGFBPs) represent a class of natural IGF antagonists that bind to and sequester IGF-1/2 from the IGF-1R, making them attractive candidates as therapeutics for cancer prevention and control. Recombinant human IGFBP-2 significantly attenuated IGF-1-stimulated MCF-7 cell proliferation with coaddition of 20 or 100 nM IGFBP-2 (50 or 80% inhibition, respectively). We previously identified IGF-1 contact sites both upstream and downstream of the CWCV motif (residues 247-250) in human IGFBP-2 (J Biol Chem 276:2880-2889, 2001). To further test their contributions to IGFBP-2 function, the single tryptophan in human IGFBP-2, Trp-248, was selectively cleaved with 2-(2'nitrophenylsulfenyl)-3-methyl-3 bromoindolenine (BNPS-skatole) and the BNPS-skatole products IGFBP-2(1-248) and IGFBP-2(249-289) as well as IGFBP-2(1-190) were expressed as glutathione S-transferase-fusion proteins and purified. Based on competition binding analysis, deletion of residues 249 to 289 caused an approximately 20-fold decrease in IGF-1 binding affinity (IGFBP-2 EC50 = 0.35 nM and IGFBP-2(1-248) = 7 nM). Removal of the remainder of the C-terminal domain had no further effect on affinity (IGFBP-2(1-190) EC50 = 9.2 nM). In kinetic assays, IGFBP-2(1-248) and IGFBP-2(1-190) exhibited more rapid association and dissociation rates than full-length IGFBP-2. These results confirm that regions upstream and downstream of the CWCV motif participate in IGF-1 binding. They further support the development of full-length IGFBP-2 as a cancer therapeutic.

Structure and properties of the C-terminal domain of insulin-like growth factor-binding protein-1 isolated from human amniotic fluid

Insulin-like growth factor (IGF)-binding protein-1 (IGFBP-1) regulates the activity of the insulin-like growth factors in early pregnancy and is, thus, thought to play a key role at the fetal-maternal interface. The C-terminal domain of IGFBP-1 and three isoforms of the intact protein were isolated from human amniotic fluid, and sequencing of the four N-terminal polypeptide chains showed them to be highly pure. The addition of both intact IGFBP-1 and its C-terminal fragment to cultured fibroblasts has a similar stimulating effect on cell migration, and therefore, the domain has a biological activity on its own. The three-dimensional structure of the C-terminal domain was determined by x-ray crystallography to 1.8 Angstroms resolution. The fragment folds as a thyroglobulin type I domain and was found to bind the Fe(2+) ion in the crystals through the only histidine residue present in the polypeptide chain. Iron (II) decreases the binding of intact IGFBP-1 and the C-terminal domain to IGF-II, suggesting that the metal binding site is close to or part of the surface of interaction of the two molecules.

Structural basis for the regulation of insulin-like growth factors by IGF binding proteins

Insulin-like growth factor binding proteins (IGFBPs) control the extracellular distribution, function, and activity of IGFs. Here, we report an X-ray structure of the binary complex of IGF-I and the N-terminal domain of IGFBP-4 (NBP-4, residues 3-82) and a model of the ternary complex of IGF-I, NBP-4, and the C-terminal domain (CBP-4, residues 151-232) derived from diffraction data with weak definition of the C-terminal domain. These structures show how the IGFBPs regulate IGF signaling. Key features of the structures include (1) a disulphide bond ladder that binds to IGF and partially masks the IGF residues responsible for type 1 IGF receptor (IGF-IR) binding, (2) the high-affinity IGF-I interaction site formed by residues 39-82 in a globular fold, and (3) CBP-4 interactions. Although CBP-4 does not bind individually to either IGF-I or NBP-4, in the ternary complex, CBP-4 contacts both and also blocks the IGF-IR binding region of IGF-I.

Roles of insulin-like growth factor (IGF) binding proteins in regulating IGF actions

The insulin-like growth factor (IGF) system is an evolutionarily conserved signaling pathway that is composed of two IGF ligands, two IGF receptors, and six IGF binding proteins. Studies in a variety of species suggest that the IGF signaling system plays a fundamental role in regulating embryonic growth and differentiation as well as in maintaining homeostasis in the adults. In extracellular fluids, IGFs are present in a complex with an IGF-binding protein (IGFBP). These IGFBPs are traditionally thought to function as carrier proteins and regulate circulating IGF turnover, transport, and distribution. Locally expressed IGFBPs can also inhibit and/or potentiate IGF activities. Recent studies have shown that some IGFBPs, in particular IGFBP-3 and -5, possess intrinsic biological activities and can act through IGF-independent mechanisms. In this article, we provide a brief overview of our current understanding of the IGF signaling system with particular reference to IGFBPs.

Diversity of human insulin-like growth factor (IGF) binding protein-2 fragments in plasma: primary structure, IGF-binding properties, and disulfide bonding pattern

The insulin-like growth factor binding proteins (IGFBPs) play a major role in the regulation of the effects and the bioavailability of the insulin-like growth factors (IGFs). IGFs are released from IGFBP-IGF complexes by proteolysis of IGFBPs generating fragments with reduced ligand-binding properties. To identify naturally occurring fragments of IGFBP-2, a peptide library generated from human hemofiltrate was immunologically screened. Purification of immunoreactive IGFBP-2 fragments was performed by consecutive chromatographic steps. A total of 18 different IGFBP-2 fragments was isolated and characterized. The peptides exhibited different N-terminal amino acid residues that were located in the variable midregion of IGFBP-2. Four major cleavage sites were determined to be between Tyr103 and Gly104, Leu152 and Ala153, Arg156 and Glu157, and Gln165 and Met166. The resulting fragments were further processed by amino and/or carboxy peptidases and comprised 37-185 amino acid residues. Ligand blotting, solution binding assays, and BIAcore analyses revealed that all tested fragments retained low IGF-binding capacity. The most abundant fragment IGFBP-2 (167-279) showed 10% of IGF-II binding compared to recombinant human (rh)IGFBP-2. Furthermore, the disulfide bonding pattern of the C-terminal domain of rhIGFBP-2 was defined, indicating linkages between cysteine residues 191-225, 236-247, and 249-270. This study provides the most comprehensive molecular characterization of human IGFBP-2 fragments formed in vivo, exhibiting both residual IGF-binding capacities and the integrin-binding sequence.

Peri/nuclear localization of intact insulin-like growth factor binding protein-2 and a distinct carboxyl-terminal IGFBP-2 fragment in vivo

Insulin-like growth factor binding protein-2 (IGFBP-2) as one of the most important IGFBPs has never been assessed in the intracellular compartment in vivo. Since there is evidence for novel intracellular functions of distinct IGFBPs, we investigated the presence of IGFBP-2 inside the cell. In peri/nuclear fractions of various tissues isolated from IGFBP-2 transgenic and non-transgenic mice we were able to show the presence of intact IGFBP-2. In addition, we demonstrate the presence of a highly conserved carboxyl-terminal IGFBP-2 fragment in the peri/nuclear fraction by using different peptide-induced antibodies. In pancreatic sections, confocal microscopy revealed the presence of IGFBP-2 on the nuclear surface but not within the nucleus. Our findings suggest novel functions of intact IGFBP-2 and IGFBP-2 fragments within the cell.

Synthesis and characterization of biotinylated forms of insulin-like growth factor-1: topographical evaluation of the IGF-1/IGFBP-2 AND IGFBP-3 interface

Activation of the insulin-like growth factor-1 (IGF)-1 receptor signaling pathways by IGF-1 and IGF-2 results in mitogenic and anabolic effects. The bioavailability of the IGFs is regulated by six soluble binding proteins, the insulin-like growth factor binding proteins (IGFBPs), which bind with approximately 0.1 nM affinity to the IGFs and often serve as endogenous antagonists of IGF action. To identify key domains of IGF-1 involved in the interaction with IGFBP-2 and IGFBP-3, we employed IGF-1 selectively biotinylated on residues Gly 1, Lys 27, Lys 65, and Lys 68. All monobiotinylated species of IGF-1 exhibited high affinity ( approximately 0.1-0.2 nM) for IGFBP-2 and IGFBP-3 in solid-phase-binding assays. However, different labeling intensities were observed in ligand blot analysis of IGFBP-2 and IGFBP-3. The N(epsilon)(Lys65/68)(biotin)-IGF-1 (N(epsilon)(Lys65/68b)-IGF-1) probe exhibited the highest signal intensity, while N(alpha)(Gly1b)-IGF-1 and N(epsilon)(Lys27b)-IGF-1 demonstrated significantly lower signals. When taken together, these results suggest that, once bound to IGFBP-2 or IGFBP-3, the biotin moieties of N(alpha)(Gly1b)-IGF-1 and N(epsilon)(Lys27b)-IGF-1 are inaccessible to NeutrAvidin-peroxidase, the secondary binding component. Ligand blots using IGF-1 derivatized with a long chain form of the N-hydroxysuccinimide biotin (NHS-biotin) to yield N(alpha)(Gly1)(LC-biotin)-IGF-1 and N(epsilon)(Lys27)(LC-biotin)-IGF-1 demonstrated increased signal intensity compared with their NHS-biotin counterparts. In BIAcore analysis, IGFBP-2 and IGFBP-3 bound only to the N(epsilon)(Lys65/68b)-IGF-1-coated flowcell of a biosensor chip, confirming the inaccessibility of Gly 1 and Lys 27 when IGF-1 is bound to IGFBP-2 and IGFBP-3. These data confirm the involvement of the IGFBP-binding domain on IGF-1 in binding to IGFBP-2 and IGFBP-3 and support involvement of the IGF-1R-binding domain in IGFBP binding.

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

Insulin-like growth factor-binding protein-3 (IGFBP-3), the major IGFBP in the circulation, sequesters IGF in a stable ternary complex with the acid-labile subunit. The high affinity IGF-binding site is proposed to reside within an N-terminal hydrophobic domain in IGFBP-3, but C-terminal residues have also been implicated in the homologous protein IGFBP-5. We have mutated in various combinations Leu(77), Leu(80), and Leu(81) in the N terminus and Gly(217) and Gln(223) in the C terminus of IGF-BP-3. All mutants retained immunoreactivity toward a polyclonal IGFBP-3 antibody, whereas IGF ligand blotting showed that all of the mutants had reduced binding to IGFs. Both solution IGF binding assays and BIAcore analysis indicated that mutations to the N-terminal region caused greater reduction in IGF binding activity than C-terminal mutations. The combined N- and C-terminal mutants showed undetectable binding to IGF-I but retained <10% IGF-II binding activity. Reduced ternary complex formation was seen only in mutants that had considerably reduced IGF-I binding, consistent with previous studies indicating that the binary IGF.IGFBP-3 complex is required for acid-labile subunit binding. Decreased IGF binding was also reflected in the inability of the mutants to inhibit IGF-I signaling in IGF receptor overexpressing cells. However, when present in excess, IGFBP-3 analogs defined as non-IGF-binding by biochemical assays could still inhibit IGF signaling. This suggests that residual binding activity of IGFBP-3 mutants may still be sufficient to inhibit IGF biological activity and questions the use of such analogs to study IGF-independent effects of IGFBP-3.

What's new in the IGF-binding proteins?

Since their initial discovery over 25 years ago as IGF carrier proteins, the insulin-like growth factor binding protein (IGFBP) family has grown to six members, ranging in size from 216 to 289 amino acids. The assumption over the years has been that this family of proteins, having higher affinities for IGF-I and IGF-II than does the IGF-IR, serves to block access of these ligands to the receptor. Although the need for such regulatory proteins is consistent with the constitutive secretion of IGFs from many cell types, it is not surprising that additional functions have begun to be uncovered for these proteins. This review will examine new and old actions of the IGFBPs from a biochemical and cell biological perspective.

C-terminal domain of insulin-like growth factor (IGF) binding protein-6: structure and interaction with IGF-II

IGFs are important mediators of growth. IGF binding proteins (IGFBPs) 1-6 regulate IGF actions and have IGF-independent actions. The C-terminal domains of IGFBPs contribute to high-affinity IGF binding and modulation of IGF actions and confer some IGF-independent properties, but understanding how they achieve this has been constrained by the lack of a three-dimensional structure. We therefore determined the solution structure of the C-domain of IGFBP-6 using nuclear magnetic resonance (NMR). The domain consists of a thyroglobulin type 1 fold comprising an alpha-helix followed by a loop, a three-stranded antiparallel beta-sheet incorporating a second loop, and finally a disulfide-bonded flexible third loop. The IGF-II binding site on the C-domain was identified by examining NMR spectral changes upon complex formation. It consists of a largely hydrophobic surface patch involving the alpha-helix, the first beta-strand, and the first and second loops. The site was confirmed by mutagenesis of several residues, which resulted in decreased IGF binding affinity. The IGF-II binding site lies adjacent to surfaces likely to be involved in glycosaminoglycan binding of IGFBPs, which might explain their decreased IGF affinity when bound to glycosaminoglycans, and nuclear localization. Our structure provides a framework for understanding the roles of IGFBP C-domains in modulating IGF actions and conferring IGF-independent actions, as well as ultimately for the development of therapeutic IGF inhibitors for diseases including cancer.

Binding site for the C-domain of insulin-like growth factor (IGF) binding protein-6 on IGF-II; implications for inhibition of IGF actions

Insulin-like growth factors (IGFs) are important mediators of growth and IGF-binding proteins (IGFBPs) 1-6 regulate IGF actions. As IGFBP C-terminal domains contribute to high-affinity IGF binding, we have defined the binding site for the C-domain of IGFBP-6 on IGF-II using NMR. This site lies adjacent to and between the binding sites for the IGFBP N-domain and IGF-I receptor (IGFIR), which have previously been found on opposite sides of the IGF molecule. The C-domain is therefore likely to interfere with IGF binding to the IGFIR, providing a structural basis for the potent inhibitory effects of intact IGFBPs on IGF actions.

Role of extracellular matrix in insulin-like growth factor (IGF) binding protein-2 regulation of IGF-II action in normal human osteoblasts

Insulin-like growth factor binding protein-2 (IGFBP-2) in its native form had little affinity for extracellular matrix (ECM) derived from human or rat osteoblastic cells. However, in the presence of IGFs, IGFBP-2 binding to ECM was markedly enhanced, with IGF-II being more effective than IGF-I. IGF-II-enhanced binding of IGFBP-2 to ECM was specific for IGFBP-2 of the six known IGFBPs. In the presence of IGF-II, IGFBP-2 bound with high affinity to heparin-Sepharose, but not to type I collagen, fibronectin, or laminin. Furthermore, heparin and heparan sulfate, but not chondroitin sulfate, inhibited IGFBP-2/IGF-II binding to ECM. High salt (100 mM NaCl) inhibited, while CaCl(2) enhanced binding of IGFBP-2/IGF-II to ECM. In the presence of ECM, IGFBP-2/IGF-II was as effective as IGF-II alone in stimulating [3H]thymidine and [3H]proline incorporation and in inhibiting apoptosis in cultured human osteoblasts. On the other hand, IGFBP-2 was a potent inhibitor of IGF-II action in human breast and ovarian carcinoma cells. There was no difference between soluble and ECM-associated IGFBP-2 in affinity for IGF-I and IGF-II. These data suggest a unique mechanism for targeting an anabolic IGFBP-2/IGF-II complex in bone.

Zinc partitions insulin-like growth factors (IGFs) from soluble IGF binding protein (IGFBP)-5 to the cell surface receptors of BC3H-1 muscle cells

Zinc (Zn(2+)) is a multifunctional micronutrient. The list of functions for this micronutrient expanded with the recent discovery that Zn(2+) retains insulin-like growth factors binding proteins (IGFBPs) on the surface of cultured cells, lowers the affinity of cell-associated IGFBPs, and increases the affinity of the cell surface insulin-like growth factor (IGF)-type 1 receptor (IGF-1R). However, currently there is no information concerning the effect of Zn(2+) on soluble IGFBPs. In the current study, the soluble IGFBP-5 secreted by BC(3)H-1 cells is shown to bind approximately 50% more [(125)I]-IGF-II than [(125)I]-IGF-I at pH 7.4. Zn(2+) is shown to depress the binding of both IGF-I and IGF-II to soluble secreted IGFBP-5; [(125)I]-IGF-I binding is affected more so than [(125)I]-IGF-II binding. Zn(2+) acts by lowering the affinity (K(a)) of IGFBP-5 for the IGFs. Scatchard plots are non-linear indicating the presence of high and low affinity binding sites; Zn(2+) affects only binding to the high affinity site. In contrast, Zn(2+) increases the affinity by which either [(125)I]-IGF-I or [(125)I]-R(3)-IGF-I binds to the IGF-1R, but depresses [(125)I]-IGF-II binding to the IGF-type 2 receptor (IGF-2R) on BC(3)H-1 cells. By depressing the association of the IGFs with soluble IGFBPs, Zn(2+) is shown to repartition either [(125)I]-IGF-I or [(125)I]-IGF-II from soluble IGFBP-5 onto cell surface IGF receptors. Zn(2+) was active at physiological doses depressing IGF binding to IGFBP-5 and the IGF-2R at 15-20 microM. Hence, a novel mechanism is further characterized by which the trace micronutrient Zn(2+) could regulate IGF activity.

Zinc alters the kinetics of IGF-II binding to cell surface receptors and binding proteins

The growth of most tissues is markedly depressed as a result of zinc deficiency by uncharacterized mechanisms that clearly involve the insulin-like growth factor (IGF) system. Herein, we describe the mechanism by which zinc (Zn2+) maintains IGF-II in an active form by directly regulating IGF-II binding to IGF-binding proteins (IGFBPs) and the type 1 IGF receptor (IGF-1R). The specificity of Zn2+ effects was confirmed by using other cations that can (Cd2+ and Au3+) or cannot (La3+) mimic Zn2+ actions. Human fibroblasts, glioblastoma cells, and murine myoblasts were used to determine the kinetics of IGF-II binding to cell surface IGFBP-3, IGFBP-5, and the IGF-1R, respectively. Zn2+, Cd2+, and Au3+, but not La3+, decreased total binding and the affinity for [125I]IGF-II association with IGFBP-3 and IGFBP-5. These effects were a result of lowered rate of ligand association without affecting rate of dissociation. In contrast, Zn2+ enhanced [125I]IGF-II binding to the IGF-1R by enhancing the rate of ligand association and decreasing the rate of dissociation. Our previous work had shown that Zn2+ acts at physiological levels to alter IGF binding. Together with the current work, these findings imply that Zn2+ acts in vivo to prevent secreted IGF-II from binding to IGFBP-3 and IGFBP- 5, thus maintaining IGF-II in an "active state," i.e., readily available for IGF-1R association.

Specific amino acid substitutions determine the differential contribution of the N- and C-terminal domains of insulin-like growth factor (IGF)-binding protein-5 in binding IGF-I

We have previously reported that two highly conserved amino acids in the C-terminal domain of rat insulin-like growth factor-binding protein (IGFBP)-5, Gly(203) and Gln(209), are involved in binding to insulin-like growth factor (IGF)-1. Here we report that mutagenesis of both amino acids simultaneously (C-Term mutant) results in a cumulative effect and an even greater reduction in IGF-I binding: 30-fold measured by solution phase IGF binding assay and 10-fold by biosensor analysis. We compared these reductions in ligand binding to the effects of specific mutations of five amino acids in the N-terminal domain (N-Term mutant), which had previously been shown by others to cause a very large reduction in IGF-I binding (). Our results confirm this as the major IGF-binding site. To prove that the mutations in either N- or C-Term were specific for IGF-I binding, we carried out CD spectroscopy and showed that these alterations did not lead to gross conformational changes in protein structure for either mutant. Combining these mutations in both domains (N+C-Term mutant) has a cumulative effect and leads to a 126-fold reduction in IGF-I binding as measured by biosensor. Furthermore, the equivalent mutations in the C terminus of rat IGFBP-2 (C-Term 2) also results in a significant reduction in IGF-I binding, suggesting that the highly conserved Gly and Gln residues have a conserved IGF-I binding function in all six IGFBPs. Finally, although these residues lie within a major heparin-binding site in IGFBP-5 and -3, we also show that the mutations in C-Term have no effect on heparin binding.

Cellular actions of the insulin-like growth factor binding proteins

In addition to their roles in IGF transport, the six IGF-binding proteins (IGFBPs) regulate cell activity in various ways. By sequestering IGFs away from the type I IGF receptor, they may inhibit mitogenesis, differentiation, survival, and other IGF-stimulated events. IGFBP proteolysis can reverse this inhibition or generate IGFBP fragments with novel bioactivity. Alternatively, IGFBP interaction with cell or matrix components may concentrate IGFs near their receptor, enhancing IGF activity. IGF receptor-independent IGFBP actions are also increasingly recognized. IGFBP-1 interacts with alpha(5)beta(1) integrin, influencing cell adhesion and migration. IGFBP-2, -3, -5, and -6 have heparin-binding domains and can bind glycosaminoglycans. IGFBP-3 and -5 have carboxyl-terminal basic motifs incorporating heparin-binding and additional basic residues that interact with the cell surface and matrix, the nuclear transporter importin-beta, and other proteins. Serine/threonine kinase receptors are proposed for IGFBP-3 and -5, but their signaling functions are poorly understood. Other cell surface IGFBP-interacting proteins are uncharacterized as functional receptors. However, IGFBP-3 binds and modulates the retinoid X receptor-alpha, interacts with TGFbeta signaling through Smad proteins, and influences other signaling pathways. These interactions can modulate cell cycle and apoptosis. Because IGFBPs regulate cell functions by diverse mechanisms, manipulation of IGFBP-regulated pathways is speculated to offer therapeutic opportunities in cancer and other diseases.

Use of mutagenesis to probe IGF-binding protein structure/function relationships

The IGF-binding proteins (IGFBPs) are multifunctional proteins that modulate IGF actions. To determine whether specific domains within these proteins account for specific functions, we and other laboratories have used in vitro mutagenesis. Prior experiments that used a variety of techniques had identified discrete regions within each protein that were proposed to account for specific functions. Alterations of these regions by substituting charged residues with neutral residues or hydrophobic residues with nonhydrophobic residues as well as domain swapping, i.e., substituting a domain from one specific form of IGFBP for the homologous domain in another form, has resulted in the elucidation of the functions of many of these specific sequences. Because the areas of protein sequence that are altered involve a limited number of amino acids, they generally do not alter the conformation of the entire protein; therefore, these specific substitutions can often be correlated with the functional changes that occur after mutagenesis. Mutants have been particularly useful for performing functional analyses in which the purified mutant protein is added to a biological test system. In some cases it has been possible to overexpress the mutagenized protein and determine whether the constitutively synthesized, mutant form of IGFBP has altered functional activity. These results have revealed that discrete regions of IGFBP sequence can mediate important and specific functional properties of these proteins.

Intact IGF-binding protein-4 and -5 and their respective fragments isolated from chronic renal failure serum differentially modulate IGF-I actions in cultured growth plate chondrocytes

Impairment of longitudinal growth among children with chronic renal failure (CRF) may be partly attributable to the inhibition of insulin-like growth factor (IGF) activity by an excess amount of high-affinity IGF-binding proteins (IGFBP). Elevated levels of immunoreactive IGFBP-4 in CRF serum are inversely correlated with the standardized heights of these children, whereas levels of IGFBP-5, which circulates mainly as proteolyzed fragments, are positively correlated with growth parameters. To delineate the respective effects of these IGFBP on growth cartilage, the biologic effects of intact and fragmented forms of IGFBP-4 and IGFBP-5 on rat growth plate chondrocytes in primary cultures were characterized. Intact IGFBP-4 and IGFBP-5 and the amino-terminal fragment IGFBP-5(1-169) were recombinant proteins; the carboxy-terminal fragments IGFBP-5(144-252) and IGFBP-4(136-237) and the amino-terminal fragment IGFBP-4(1-122) were purified to homogeneity from CRF hemofiltrates. Intact IGFBP-4 and, to a lesser extent, IGFBP-4(1-122) inhibited IGF-I-induced cell proliferation. In contrast, intact IGFBP-5 was stimulatory in the absence or presence of exogenous IGF-I, whereas the amino-terminal fragment IGFBP-5(1-169) was inhibitory. Studies on the mechanism by which IGFBP-4 and IGFBP-5 exert opposite effects on chondrocyte proliferation demonstrated that intact IGFBP-4 prevented the binding of (125)I-IGF-I to chondrocytes, whereas intact IGFBP-5 enhanced ligand binding and was able to bind specifically to the cell membrane. These data suggest that intact IGFBP-4 and, to a lesser extent, IGFBP-4(1-122) act exclusively as growth-inhibitory binding proteins in the growth cartilage. IGFBP-5, however, can either stimulate (if it remains intact) or inhibit (if amino-terminal forms predominate) IGF-I-stimulated chondrocyte proliferation.

The Twisted gastrulation family of proteins, together with the IGFBP and CCN families, comprise the TIC superfamily of cysteine rich secreted factors

AIMS

To analyse the similarities between the Twisted gastrulation (TSG) proteins known to date; in addition, to determine phylogenetic relations among the TSG proteins, and between the TSGs and other protein families--the CCN (for example, CCN2 (CTGF), CCN1 (CYR61), and CCN3 (NOV)) and IGFBP (insulin-like growth factor binding protein) families.

METHODS

TBLASTN and FASTA3 were used to identify new tsg genes and relatives of the TSG family. The sequences were aligned with ClustalW. The predictions of sites for signal peptide cleavage, post-translational modifications, and putative protein domains were carried out with software available at various databases. Unrooted phylogenetic trees were calculated using the UPGMA method.

RESULTS

Several tsg genes from vertebrates and invertebrates were compared. Alignment of protein sequences revealed a highly conserved family of TSG proteins present in both vertebrates and invertebrates, whereas the slightly less well conserved IGFBP and CCN proteins are apparently present only in vertebrates. The TSG proteins display strong homology among themselves and they are composed of a putative signal peptide at the N-terminus followed by a cysteine rich (CR) region, a conserved domain devoid of cysteines, a variable midregion, and a C-terminal CR region. The most striking similarity between the TSGs and the IGFBP and CCN proteins occurs in the N-terminal conserved cysteine rich domain and the characteristic 5' cysteine rich domain(s), spacer region, and 3' cysteine rich domain structure.

CONCLUSION

The family of highly conserved TSG proteins, together with the IGFBP and CCN families, constitute an emerging multigene superfamily of secreted cysteine rich factors. The TSG branch of the superfamily appears to pre-date the others because it is present in all species examined, whereas the CCN and IGFBP genes are found only in vertebrates.

Insulin-like growth factor binding proteins (IGFBPs) as potential physiological substrates for human kallikreins hK2 and hK3

Insulin-like growth factors (IGFs) are important growth regulators of both normal and malignant prostate cells. Their action is regulated by six insulin-like growth factor binding proteins (IGFBPs). The proteolytic cleavage of IGFBPs by various proteases decreases dramatically their affinity for their ligands and therefore enhances the bioavailability of IGFs. To elucidate the putative biological role of prostatic kallikreins hK2 and hK3 (prostate-specific antigen) in tumour progression, we analyzed the degradation of IGFBP-2, -3, -4 and -5 by these two tissue kallikreins. We found that hK3, already characterized as an IGFBP-3 degrading protease, cleaved IGFBP-4 but not IGFBP-2 and -5, whereas hK2 cleaved all of the IGFBPs much more effectively, and at concentrations far lower than those reported for other IGFBP-degrading proteases. The proteolytic patterns after cleavage of IGFBPs by hK2 and hK3 were similar and were not modified in the presence of IGF-I. Heparin, but not other glycosaminoglycans, enhanced dramatically the ability of hK3 but not hK2 to degrade IGFBP-3 and IGFBP-4. More importantly, the IGFBP fragments generated by hK2 and hK3 had no IGF-binding capacity, as assessed by Western ligand blotting. Our results suggest that the prostatic kallikreins hK2 and hK3 may influence specifically the tumoral growth of prostate cells through the degradation of IGFBPs, to increase IGF bioavailability.

BIAcore analysis of bovine insulin-like growth factor (IGF)-binding protein-2 identifies major IGF binding site determinants in both the amino- and carboxyl-terminal domains

In the absence of a complete tertiary structure to define the molecular basis of the high affinity binding interaction between insulin-like growth factors (IGFs) and IGF-binding proteins (IGFBPs), we have investigated binding of IGFs by discrete amino-terminal domains (amino acid residues 1-93, 1-104, 1-132, and 1-185) and carboxyl-terminal domains (amino acid residues 96-279, 136-279, and 182-284) of bovine IGFBP-2 (bIGFBP-2). Both halves of bIGFBP-2 bound IGF-I and IGF-II in BIAcore studies, albeit with different affinities ((1-132)IGFBP-2, K(D) = 36.3 and 51.8 nm; (136-279)IGFBP-2HIS, K(D) = 23.8 and 16.3 nm, respectively). The amino-terminal half appears to contain components responsible for fast association. In contrast, IGF binding by the carboxyl-terminal fragment results in a more stable complex as reflected by its K(D). Furthermore, des(1-3)IGF-I and des(1-6)IGF-II exhibited reduced binding affinity to (1-279)IGFBP-2HIS, (1-132)IGFBP-2, and (136-279)IGFBP-2HIS biosensor surfaces compared with wild-type IGF. A charge reversal at positions 3 and 6 of IGF-I and IGF-II, respectively, affects binding interactions with the amino-terminal fragment and full-length bIGFBP-2 but not the carboxyl-terminal fragment.

Synthesis and characterization of insulin-like growth factor (IGF)-1 photoprobes selective for the IGF-binding proteins (IGFBPS). photoaffinity labeling of the IGF-binding domain on IGFBP-2

Elevated insulin-like growth factor (IGF)-1 levels are prognostic for the development of prostate and breast cancers and exacerbate the complications of diabetes. In each case, perturbation of the balance between IGF-1/2, the IGF-1 receptor, and the IGF-binding proteins (IGFBPs) leads to elevated IGF-1 sensitivity. Blockade of IGF action in these diseases would be clinically significant. Unfortunately, effective IGF antagonists are currently unavailable. The IGFBPs exhibit high affinity and specificity for the IGFs and serve as natural IGF antagonists, limiting their mitogenic/anti-apoptotic effects. As an initial step in designing IGFBP-based agents that antagonize IGF action, we have begun to analyze the structure of the IGF-binding site on IGFBP-2. To this end, two IGF-1 photoprobes, N(alphaGly1)-(4-azidobenzoyl)-IGF-1 (abG(1)IGF-1) and N(alphaGly1)-([2-6-(biotinamido)-2(p-azidobenzamido)hexanoamido]ethyl-1,3'-dithiopropionoyl)-IGF-1 (bedG(1)IGF-1), selective for the IGFBPs were synthesized by derivatization of the alpha-amino group of Gly(1), known to be part of the IGFBP-binding domain. Mass spectrometric analysis of the reduced, alkylated, and trypsin-digested abG(1)IGF-1.recombinant human IGFBP-2 (rhIGFBP-2) complex indicated photoincorporation near the carboxyl terminus of rhIGFBP-2, between residues 266 and 287. Mass spectrometric analysis of avidin-purified tryptic peptides of the bedG(1)IGF-1.rhIGFBP-2 complex revealed photoincorporation within residues 212-227. Taken together, these data indicate that the IGFBP-binding domain on IGF-1 contacts the distal third of IGFBP-2, providing evidence that the IGF-1-binding domain is located within the C terminus of IGFBP-2.

Contribution of residues A54 and L55 of the human insulin-like growth factor-II (IGF-II) A domain to Type 2 IGF receptor binding specificity

The underlying specificity of the interaction between insulin-like growth factor-II (IGF-II) and mammalian Type 2 insulin-like growth factor/cation-independent mannose 6 phosphate receptor (IGF2R) is not understood. We have mutated residues A54 and L55 of IGF-II in the second A domain helix to arginine (found in the corresponding positions of IGF-I) and measured IGF2R binding. There is a 4- and 3.3-fold difference in dissociation constants for A54R IGF-II and L55R IGF-II, respectively, and a 6.6-fold difference for A54R L55R IGF-II compared with IGF-II as measured by BlAcore analysis using purified rat IGF2R. This is also confirmed using cross-linking and soluble rat placental membrane receptor binding assays. Binding to the type I IGF receptor (IGF1R) and IGF binding protein-2 (IGFBP-2) is not altered. We can, therefore, conclude that residues at positions 54 and 55 in IGF-II are important for and equally contribute to IGF2R binding.

O-glycosylation of insulin-like growth factor (IGF) binding protein-6 maintains high IGF-II binding affinity by decreasing binding to glycosaminoglycans and susceptibility to proteolysis

Insulin-like growth factor binding protein-6 (IGFBP-6) is an O-linked glycoprotein which specifically inhibits insulin-like growth factor (IGF)-II actions. The effects of O-glycosylation of IGFBP-6 on binding to glycosaminoglycans and proteolysis, both of which reduce the IGF binding affinity of other IGFBPs were studied. Binding of recombinant human nonglycosylated (n-g) IGFBP-6 to a range of glycosaminoglycans in vitro was approximately threefold greater than that of glycosylated (g) IGFBP-6. When bound to glycosaminoglycans, IGFBP-6 had approximately 10-fold reduced binding affinity for IGF-II. Exogenously added n-gIGFBP-6 but not gIGFBP-6 also bound to partially purified rat PC12 phaeochromocytoma membranes. Binding of n-gIGFBP-6 was inhibited by increasing salt concentrations, which is typical of glycosaminoglycan interactions. O-glycosylation also protected human IGFBP-6 from proteolysis by chymotrypsin and trypsin. Proteolysis decreased the binding affinity of IGFBP-6 for IGF-II, even with a relatively small reduction in apparent molecular mass as observed with chymotrypsin. Analysis by ESI-MS of IGFBP-6 following limited chymotryptic digestion showed that a 4.5-kDa C-terminal peptide was removed and peptide bonds involved in the putative high affinity IGF binding site were cleaved. The truncated, multiply cleaved IGFBP-6 remained held together by disulphide bonds. In contrast, trypsin cleaved IGFBP-6 in the mid-region of the molecule, resulting in a 16-kDa C-terminal peptide which did not bind IGF-II. These results indicate that O-glycosylation inhibits binding of IGFBP-6 to glycosaminoglycans and cell membranes and inhibits its proteolysis, thereby maintaining IGFBP-6 in a high-affinity, soluble form and so contributing to its inhibition of IGF-II actions.

Substitutions for hydrophobic amino acids in the N-terminal domains of IGFBP-3 and -5 markedly reduce IGF-I binding and alter their biologic actions

Insulin-like growth factor-binding protein-3 and -5 (IGFBP-3 and -5) have been shown to bind insulin-like growth factor-I and -II (IGF-I and -II) with high affinity. Previous studies have proposed that the N-terminal region of IGFBP-5 contains a hydrophobic patch between residues 49 and 74 that is required for high affinity binding. These studies were undertaken to determine if mutagenesis of several of these residues resulted in a reduction of the affinity of IGFBP-3 and -5 for IGF-I. Substitutions for residues 68, 69, 70, 73, and 74 in IGFBP-5 (changing one charged residue, Lys(68), to a neutral one and the four hydrophobic residues to nonhydrophobic residues) resulted in an approximately 1000-fold reduction in the affinity of IGFBP-5 for IGF-I. Substitutions for homologous residues in IGFBP-3 also resulted in a >1000-fold reduction in affinity. The physiologic consequence of this reduction was that IGFBP-3 and -5 became very weak inhibitors of IGF-I-stimulated cell migration and DNA synthesis. Likewise, the ability of IGFBP-5 to inhibit IGF-I-stimulated receptor phosphorylation was attenuated. These changes did not appear to be because of alterations in protein folding induced by mutagenesis, because the IGFBP-5 mutant was fully susceptible to proteolytic cleavage by a specific IGFBP-5 protease. In summary, residues 68, 69, 70, 73, and 74 in IGFBP-5 appear to be critical for high affinity binding to IGF-I. Homologous residues in IGFBP-3 are also required, suggesting that they form a similar binding pocket and that for both proteins these residues form an important component of the core binding site. The availability of these mutants will make it possible to determine if there are direct, non-IGF-I-dependent effects of IGFBP-3 and -5 on cellular physiologic processes in cell types that secrete IGF-I.

Insulin-like growth factor (IGF)-binding proteins: interactions with IGFs and intrinsic bioactivities

The insulin-like growth factor (IGF)-binding proteins (IGFBPs) are a family of six homologous proteins with high binding affinity for IGF-I and IGF-II. Information from NMR and mutagenesis studies is advancing knowledge of the key residues involved in these interactions. IGF binding may be modulated by IGFBP modifications, such as phosphorylation and proteolysis, and by cell or matrix association of the IGFBPs. All six IGFBPs have been shown to inhibit IGF action, but stimulatory effects have also been established for IGFBP-1, -3, and -5. These generally involve a decrease in IGFBP affinity and may require cell association of the IGFBP, but precise mechanisms are unknown. The same three IGFBPs have well established effects that are independent of type I IGF receptor signaling. IGFBP-1 exerts these effects by signaling through alpha(5)beta(1)-integrin, whereas IGFBP-3 and -5 may have specific cell-surface receptors with serine kinase activity. The regulation of cell sensitivity to inhibitory IGFBP signaling may play a role in the growth control of malignant cells.

Partial IGF affinity of circulating N- and C-terminal fragments of human insulin-like growth factor binding protein-4 (IGFBP-4) and the disulfide bonding pattern of the C-terminal IGFBP-4 domain

Within the IGF axis, the insulin-like growth factor-binding proteins (IGFBPs) are known to play a pivotal role in cell proliferation and differentiation. Defined proteolysis of the IGFBPs is proposed to be an essential mechanism for regulating IGF bioavailability. The generated IGFBP fragments in part exhibit different IGF-dependent and -independent biological activities. Characterizing naturally occurring forms of IGFBPs in human plasma, we identified both a N- and a C-terminal fragment of IGFBP-4 by means of immunoreactivity screening. As a source for peptide isolation, we used large amounts of human hemofiltrate obtained from patients with chronic renal failure. Purification of the IGFBP-4 peptides from hemofiltrate was performed by consecutive cation-exchange and reverse-phase chromatographic steps. Mass spectrometric and sequence analysis revealed an M(r) of 13 233 for the purified N-terminal fragment spanning residues Asp(1)-Phe(122) of IGFBP-4 and an M(r) of 11 344 for the C-terminal fragment extending from Lys(136) to Glu(237). Proteolytic digestion and subsequent biochemical analysis showed that the six cysteines of the C-terminal IGFBP-4 fragment are linked between residues 153-183, 194-205, and 207-228 (disulfide bonding pattern, 1-2, 3-4, and 5-6). Plasmon resonance spectroscopy, ligand blot analysis, and saturation and displacement studies demonstrated a very low affinity of the C-terminal IGFBP-4 fragment for the IGFs (IGF-II, K(d) = 690 nM; IGF-I, K(d) > 60 nM), whereas the N-terminal fragment retained significant IGF binding properties (IGF-II, K(d) = 17 nM; IGF-I, K(d) = 5 nM). This study provides the first molecular characterization of circulating human IGFBP-4 fragments formed in vivo exhibiting an at least 5-fold decrease in the affinity of the N-terminal IGFBP-4 fragment for the IGFs and a very low IGF binding capacity of the C-terminal fragment.

Insulin-like growth factor binding protein 2 is a growth inhibitory protein conserved in zebrafish

Fish serum contains several specific binding proteins for insulin-like growth factors (IGFBPs). The structure and physiological function of these fish IGFBPs are unknown. Here we report the complete primary sequence of a zebrafish IGFBP deduced from cDNA clones isolated by library screening and rapid amplification of cDNA ends. The full-length 1,757-bp cDNA encodes a protein of 276 aa, which contains a putative 22-residue signal peptide and a 254-residue mature protein. The mature zebrafish IGFBP has a predicted molecular size of 28,440 Da and shows high sequence identity with human IGFBP-2 (52%). The sequence identities with other human IGFBPs are <37%. Chinese hamster ovary cells stably transfected with the zebrafish IGFBP-2 cDNA secreted a 31-kDa protein, which bound to IGF-I and IGF-II with high affinity, but did not bind to Des(1-3)IGF-I or insulin. Northern blot analyses revealed that the zebrafish IGFBP-2 transcript is a 1.8-kb band expressed in many embryonic and adult tissues. In adult zebrafish, IGFBP-2 mRNA levels were greatly reduced by growth hormone treatment but increased by prolonged fasting. When overexpressed or added to cultured zebrafish and mammalian cells, the zebrafish IGFBP-2 significantly inhibited IGF-I-stimulated cell proliferation and DNA synthesis. These results indicate that zebrafish IGFBP-2 is a negative growth regulator acting downstream in the growth hormone-IGF-I axis.

The insulin-like growth factor-binding protein (IGFBP) superfamily

Over the last decade, the concept of an IGFBP family has been well accepted, based on structural similarities and on functional abilities to bind IGFs with high affinities. The existence of other potential IGFBPs was left open. The discovery of proteins with N-terminal domains bearing striking structural similarities to the N terminus of the IGFBPs, and with reduced, but demonstrable, affinity for IGFs, raised the question of whether these proteins were "new" IGFBPs (22, 23, 217). The N-terminal domain had been uniquely associated with the IGFBPs and has long been considered to be critical for IGF binding. No other function has been confirmed for this domain to date. Thus, the presence of this important IGFBP domain in the N terminus of other proteins must be considered significant. Although these other proteins appear capable of binding IGF, their relatively low affinity and the fact that their major biological actions are likely to not directly involve the IGF peptides suggest that they probably should not be classified within the IGFBP family as provisionally proposed (22, 23). The conservation of this single domain, so critical to high-affinity binding of IGF by the six IGFBPs, in all of the IGFBP-rPs, as well, speaks to its biological importance. Historically, and perhaps, functionally, this has led to the designation of an "IGFBP superfamily". The classification and nomenclature for the IGFBP superfamily, are, of course, arbitrary; what is ultimately relevant is the underlying biology, much of which still remains to be deciphered. The nomenclature for the IGFBP related proteins was derived from a consensus of researchers working in the IGFBP field (52). Obviously, a more general consensus on nomenclature, involving all groups working on each IGFBP-rP, has yet to be reached. Further understanding of the biological functions of each protein should help resolve the nomenclature dilemma. For the present, redesignating these proteins IGFBP-rPs simplifies the multiple names already associated with each IGFBP related protein, and reinforces the concept of a relationship with the IGFBPs. Beyond the N-terminal domain, there is a lack of structural similarity between the IGFBP-rPs and IGFBPs. The C-terminal domains do share similarities to other internal domains found in numerous other proteins. For example, the similarity of the IGFBP C terminus to the thyroglobulin type-I domain shows that the IGFBPs are also structurally related to numerous other proteins carrying the same domain (87). Interestingly, the functions of the different C-terminal domains in members of the IGFBP superfamily include interactions with the cell surface or ECM, suggesting that, even if they share little sequence similarities, the C-terminal domains may be functionally related. The evolutionary conservation of the N-terminal domain and functional studies support the notion that IGFBPs and IGFBP-rPs together form an IGFBP superfamily. A superfamily delineates between closely related (classified as a family) and distantly related proteins. The IGFBP superfamily is therefore composed of distantly related families. The modular nature of the constituents of the IGFBP superfamily, particularly their preservation of an highly conserved N-terminal domain, seems best explained by the process of exon shuffling of an ancestral gene encoding this domain. Over the course of evolution, some members evolved into high-affinity IGF binders and others into low-affinity IGF binders, thereby conferring on the IGFBP superfamily the ability to influence cell growth by both IGF-dependent and IGF-independent means (Fig. 10). A final word, from Stephen Jay Gould (218): "But classifications are not passive ordering devices in a world objectively divided into obvious categories. Taxonomies are human decisions imposed upon nature--theories about the causes of nature's order. The chronicle of historical changes in classification provides our finest insight into conceptual revolutions