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

Characterization of endogenously circulating IGFBP-4 fragments-Novel biomarkers for cardiac risk assessment

BACKGROUND

Recent findings show that circulating N- and C-terminal fragments of IGF-binding protein-4 (NT-IGFBP-4 and CT-IGFBP-4) can be utilized as biomarkers for cardiac risk assessment in acute coronary syndrome (ACS) patients. The fragments are thought to be the products of pregnancy-associated plasma protein A (PAPP-A)-dependent proteolysis. Two immunoassays for the measurement of IGFBP-4 fragments have been proposed. However, properties of the endogenous IGFBP-4 fragments that could influence the performance of the immunoassays were still not investigated.

METHODS

NT- and CT-IGFBP-4 were extracted from pooled ACS plasma using affinity purification, and their concentrations were measured using sandwich immunoassays utilizing antibodies specific to their proteolytic neo-epitopes or internal epitopes. The extracted fragments were characterized by Western blots (WB) and mass-spectrometry. ACS plasma samples were analyzed by size exclusion chromatography (SEC).

RESULTS

Immunoassays utilizing the neo-epitope-specific and the internal epitope-specific antibodies measured equal concentrations of the analyte in the endogenous IGFBP-4 fragments preparations. Only the 18 kDa NT-IGFBP-4 and 14 kDa CT-IGFBP-4 were detected in the WB analysis. Using mass-spectrometry, peaks corresponding to intact non-truncated and non-modified NT-IGFBP-4 (14626 Da) and CT-IGFBP-4 (11346 Da) were observed. The absence of complexed forms of IGFBP-4 in patients' plasma was demonstrated using SEC.

CONCLUSIONS

Endogenous NT- and CT-IGFBP-4 from ACS patients' plasma correspond to the PAPP-A-derived IGFBP-4 fragments and do not undergo any truncation, modification, or complex formation in the patients' blood. Because of the demonstrated intact state of the circulating IGFBP-4 fragments, the neo-epitope-specific immunoassays perform reliably, allowing further clinical validation of these novel biomarkers.

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.

Insulin-like growth factors in normal and diseased kidney

This article reviews the physiology of the insulin-like growth factor (IGF) system in the kidney and the changes and potential role of this system in selected renal diseases. The potential therapeutic uses of recombinant human IGF-I for the treatment of acute and chronic kidney failure are briefly discussed.

N-terminal and C-terminal fragments of IGFBP-4 as novel biomarkers for short-term risk assessment of major adverse cardiac events in patients presenting with ischemia

OBJECTIVES

Pregnancy Associated Plasma Protein A (PAPP-A)-derived N- and C-terminal fragments of IGF-binding protein-4 (NT- and CT-IGFBP-4) released from vulnerable atherosclerotic plaques are proposed to be used for cardiovascular risk assessment.

DESIGN AND METHODS

NT- and CT-IGFBP-4 were measured by novel immunoassays in EDTA-plasma of 180 patients admitted to the emergency department with symptoms of myocardial ischemia but without ST-segment elevation. Six-month incidence of major adverse cardiac events (MACE), including myocardial infarction, cardiac death, percutaneous coronary interventions, and coronary artery bypass grafting was recorded.

RESULTS

Sixteen patients met the endpoint. NT- and CT-IGFBP-4 were strong predictors of MACE: area under ROC curve (AUC) 0.856 and 0.809, respectively. NT-IGFBP-4 concentrations≥214μg/L and CT-IGFBP-4 concentrations≥124μg/L were associated with increased risk of future MACE: adjusted hazard ratio 13.79 and 7.93, respectively.

CONCLUSIONS

IGFBP-4 fragments can be utilized as biomarkers for MACE prediction in patients with suspected myocardial ischemia.

Insulin-like growth factor binding protein-2: NMR analysis and structural characterization of the N-terminal domain

The insulin-like growth factor binding proteins are 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. IGFBP-2, the largest member of this family, is over-expressed in many cancers and has been proposed as a possible target for the development of novel anti-cancer therapeutics. The IGFBPs have a common architecture consisting of conserved N- and C-terminal domains joined by a variable linker domain. The solution structure and dynamics of the C-terminal domain of human IGFBP-2 have been reported (Kuang Z. et al. J. Mol. Biol. 364, 690-704, 2006) but neither the N-domain (N-BP-2) nor the linker domain have been characterised. Here we present NMR resonance assignments for human N-BP-2, achieved by recording spectra at low protein concentration using non-uniform sampling and maximum entropy reconstruction. Analysis of secondary chemical shifts shows that N-BP-2 possesses a secondary structure similar to that of other IGFBPs. Although aggregation hampered determination of the solution structure for N-BP-2, a homology model was generated based on the high degree of sequence and structure homology exhibited by the IGFBPs. This model was consistent with experimental NMR and SAXS data and displayed some unique features such as a Pro/Ala-rich non-polar insert, which formed a flexible solvent-exposed loop on the surface of the protein opposite to the IGF-binding interface. NMR data indicated that this loop could adopt either of two alternate conformations in solution - an entirely flexible conformation and one containing nascent helical structure. This loop and an adjacent poly-proline sequence may comprise a potential SH3 domain interaction site for binding to other proteins.

Proteins on the catwalk: modelling the structural domains of the CCN family of proteins

The CCN family of proteins (CCN1, CCN2, CCN3, CCN4, CCN5 and CCN6) are multifunctional mosaic proteins that play keys roles in crucial areas of physiology such as angiogenesis, skeletal development tumourigenesis, cell proliferation, adhesion and survival. This expansive repertoire of functions comes through a modular structure of 4 discrete domains that act both independently and in concert. How these interactions with ligands and with neighbouring domains lead to the biological effects is still to be explored but the molecular structure of the domains is likely to play an important role in this. In this review we have highlighted some of the key features of the individual domains of CCN family of proteins based on their biological effects using a homology modelling approach.

Insulin-like growth factor (IGF) binding protein-4 is both a positive and negative regulator of IGF activity in vivo

IGFs are required for normal prenatal and postnatal growth. Although actions of IGFs can be modulated by a family of IGF-binding proteins (IGFBPs) in vitro, these studies have identified a complicated pattern of stimulatory and inhibitory IGFBP effects, so that understanding relevant aspects of IGFBP action in vivo has been limited. Here we have produced a null mutation of one specific IGFBP, IGFBP-4, which is coexpressed with IGF-II early in development. Surprisingly, mutation of IGFBP-4, believed from in vitro studies to be exclusively inhibitory, leads to a prenatal growth deficit that is apparent from the time that the IGF-II growth deficit first arises, which strongly suggests that IGFBP-4 is required for optimal IGF-II-promoted growth during fetal development. Mice encoding a mutant IGFBP-4 protease (pregnancy-associated plasma protein-A), which facilitates IGF-II release from an inactive IGF-II/IGFBP-4 complex in vitro, are even smaller than IGFBP-4 mutant mice. However, the more modest IGFBP-4 growth deficit is completely restored in double IGFBP-4/pregnancy-associated plasma protein-A-deficient mice. Taken together these results indicate not only that IGFBP-4 functions as a local reservoir to optimize IGF-II actions needed for normal embryogenesis, but also establish that IGFBP-4 proteolysis is required to activate most, if not all, IGF-II mediated growth-promoting activity.

Use of multidimensional separation protocols for the purification of trace components in complex biological samples for proteomics analysis

The routine detection of low abundance components in complex samples for detailed proteomics analysis continues to be a challenge. Whilst the potential of multidimensional chromatographic fractionation for this purpose has been proposed for some years, and was used effectively for the purification to homogeneity of trace components in bulk biological samples for N-terminal sequence analysis, its practical application in the proteomics arena is still limited. This article reviews some of the recent data using these approaches, including the use of microaffinity purification as part of multidimensional protocols for downstream proteomics analysis.

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.

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 4 in Children with Acute Lymphoblastic Leukemia

Insulin-like growth factor-binding protein 4 (IGFBP-4) is a potent inhibitor of IGF-mediated cell proliferation. To investigate the functional relevance of IGFBP-4 in leukemia, we measured plasma IGFBP-4 levels and messenger RNA expression in leukemic cell clones of patients with acute lymphoblastic leukemia (ALL) and in control subjects. The IGFBP-4 levels of ALL patients at diagnosis were significantly lower than the levels of healthy control subjects. We evaluated the patients at diagnosis and after 33 days of chemotherapy and found plasma IGFBP-4 levels at day 33 to be significantly lower than the levels at diagnosis. There was no correlation of plasma IGFBP-4 level with age, sex, immunophenotype, or ALL risk group, and there was no correlation of IGFBP-4 level with plasma IGF-I, IGF-II, IGFBP-1, IGFBP-2, and IGFBP-3 levels. Gene expression analysis of the leukemic blast population at diagnosis revealed that the leukemic clones did not significantly contribute to systemic IGFBP-4 levels. The decrease in plasma IGFBP-4 levels during chemotherapy represents an indirect effect, probably caused by the chemotherapeutic effects on IGFBP-4-expressing cells of the liver and other organs. In addition, IGFBP-4 gene expression was investigated in 13 human immune cell-related cell lines by reverse transcription-polymerase chain reaction analysis. IGFBP-4 was exclusively expressed in cell lines derived either from B-cells or from myelomonocytic cells, whereas IGFBP-4 was not expressed in T-cell lines.

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.

Growth hormone/insulin-like growth factor system in children with chronic renal failure

Disturbances of the somatotropic hormone axis play an important pathogenic role in growth retardation and catabolism in children with chronic renal failure (CRF). The apparent discrepancy between normal or elevated growth hormone (GH) levels and diminished longitudinal growth in CRF has led to the concept of GH insensitivity, which is caused by multiple alterations in the distal components of the somatotropic hormone axis. Serum levels of IGF-I and IGF-II are normal in preterminal CRF, while in end-stage renal disease (ESRD) IGF-I levels are slightly decreased and IGF-II levels slightly increased. In view of the prevailing elevated GH levels in ESRD, these serum IGF-I levels appear inadequately low. Indeed, there is both clinical and experimental evidence for decreased hepatic production of IGF-I in CRF. This hepatic insensitivity to the action of GH may be partly the consequence of reduced GH receptor expression in liver tissue and partly a consequence of disturbed GH receptor signaling. The actions and metabolism of IGFs are modulated by specific high-affinity IGFBPs. CRF serum has an IGF-binding capacity that is increased by seven- to tenfold, leading to decreased IGF bioactivity of CRF serum despite normal total IGF levels. Serum levels of intact IGFBP-1, -2, -4, -6 and low molecular weight fragments of IGFBP-3 are elevated in CRF serum in relation to the degree of renal dysfunction, whereas serum levels of intact IGFBP-3 are normal. Levels of immunoreactive IGFBP-5 are not altered in CRF serum, but the majority of IGFBP-5 is fragmented. Decreased renal filtration and increased hepatic production of IGFBP-1 and -2 both contribute to high levels of serum IGFBP. Experimental and clinical evidence suggests that these excessive high-affinity IGFBPs in CRF serum inhibit IGF action in growth plate chondrocytes by competition with the type 1 IGF receptor for IGF binding. These data indicate that growth failure in CRF is mainly due to functional IGF deficiency. Combined therapy with rhGH and rhIGF-I is therefore a logical approach.

Interaction of Insulin-like Growth Factor II (IGF-II) with Multiple Plasma Proteins

In the circulation, most of the insulin-like growth factors (IGFs), IGF-binding proteins (IGFBPs), and IGFBP proteases are bound in high molecular mass complexes of > or =150 kDa. To investigate molecular interactions between proteins involved in IGF.IGFBP complexes, Cohn fraction IV of human plasma was subjected to IGF-II affinity chromatography followed by reversed-phase high pressure liquid chromatography and analysis of bound proteins. Mass spectrometry and Western blotting revealed the presence of IGFBP-3, IGFBP-5, transferrin, plasminogen, prekallikrein, antithrombin III, and the soluble IGF-II/mannose 6-phosphate receptor in the eluate. Furthermore, an IGFBP-3 protease cleaving also IGFBP-2 but not IGFBP-4 was co-purified from the IGF-II column. Inhibitor studies and IGFBP-3 zymography have demonstrated that the 92-kDa IGFBP-3 protease belongs to the class of serine-dependent proteases. IGF-II ligand blotting and surface plasmon resonance spectrometry have been used to identify plasminogen as a novel high affinity IGF-II-binding protein capable of binding to IGFBP-3 with 50-fold higher affinity than transferrin. In combination with transferrin, the overall binding constant of plasminogen/transferrin for IGF-II was reduced 7-fold. Size exclusion chromatography of the IGF-II matrix eluate revealed that transferrin, plasminogen, and the IGFBP-3 protease are present in different high molecular mass complexes of > or =440 kDa. The present data indicate that IGFs, low and high affinity IGFBPs, several IGFBP-associated proteins, and IGFBP proteases can interact, which may result in the formation of binary, ternary, and higher molecular weight complexes capable of modulating IGF binding properties and the stability of IGFBPs.

Synthesis of the blood circulating C-terminal fragment of insulin-like growth factor (IGF)-binding protein-4 in its native conformation. Crystallization, heparin and IGF binding, and osteogenic activity

Insulin-like growth factor-binding proteins play a critical role in a wide variety of important physiological processes. It has been demonstrated that both an N-terminal and a C-terminal fragment of insulin-like growth factor-binding protein-4 exist and accumulate in the circulatory system, these fragments accounting for virtually the whole amino acid sequence of the protein. The circulating C-terminal fragment establishes three disulfide bridges, and the binding pattern of these has recently been defined. Here we show that the monodimensional 1H NMR spectrum of the C-terminal fragment is typical of a protein with a relatively close packed tertiary structure. This fragment can be produced in its native conformation in Escherichia coli, without the requirement of further refolding procedures, when synthesis is coupled to its secretion from the cell. The recombinant protein crystallizes with the unit cell parameters of a hexagonal system. Furthermore, it binds strongly to heparin, acquiring a well defined oligomeric structure that interacts with insulin-like growth factors, and promotes bone formation in cultures of murine calvariae.

IGF-binding protein-3 fragments in plasma of a child with acute renal failure

The insulin-like growth factors (IGF) -I and -II promote cellular growth and differentiation of various organs. Their growth-stimulating effects are modulated by a family of six IGF-binding proteins (IGFBPs). Altered patterns of intact and fragmented IGFBPs have been reported in serum and urine of children with chronic renal failure (CRF), and it has been suggested that this may contribute to the growth failure observed in these patients. In the present study, a rapid and comprehensive method is presented to analyze IGFBPs and IGFBP fragments in the plasma of a child with acute renal failure (ARF) who had undergone plasmapheresis. The plasma IGF-I and IGFBP-3 levels were drastically reduced. Plasmapheresis filtrate (3 l) was fractionated by cation-exchange chromatography and reversed-phase high-performance liquid chromatography. The fractions obtained were tested by ligand and immunoblotting. In addition to IGFBP-1 and -4 fragments, the majority of IGF-binding polypeptides were IGFBP-3 immunoreactive. N-terminal sequence analysis of a 17-kDa polypeptide revealed the isolation of a C-terminal fragment of IGFBP-3 starting with Lys 160. The IGF-II-binding polypeptide pattern in the ARF plasma resembles the pattern in hemofiltrate from CRF patients, suggesting that similar or identical proteases are involved in IGFBP-3 fragmentation and common mechanisms may lead to the accumulation of the fragments in both diseases.

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.

Up date on IGFBP-4: regulation of IGFBP-4 levels and functions, in vitro and in vivo

Of the six known high affinity insulin-like growth factor binding-proteins (IGFBPs), IGFBP-4 appears to be unique in that it is the only IGFBP that functions mostly like a traditional binding protein. In this regard, none of the IGF independent effects that have been ascribed for other IGFBPs have been described for IGFBP-4. However, recent in vitro and in vivo studies, in particular the recent identification of pregnancy-associated plasma protein-A as a major IGFBP-4 protease, are consistent with the idea that IGFBP-4 is an extremely important component of IGF system in several tissues including gonads and bone. In this review, we have provided an update on IGFBP-4 research and we have summarized our current understanding of the regulation of levels and actions of IGFBP-4 and proteolytic fragments both in vitro and in vivo.

Substrate specificity of the metalloproteinase pregnancy-associated plasma protein-A (PAPP-A) assessed by mutagenesis and analysis of synthetic peptides: substrate residues distant from the scissile bond are critical for proteolysis

Human pregnancy-associated plasma protein-A (PAPP-A) cleaves insulin-like growth factor (IGF) binding protein-4 (IGFBP-4), causing a dramatic reduction in its affinity for IGF-I and -II. Through this mechanism, PAPP-A is a regulator of IGF bioactivity in several systems, including the human ovary and the cardiovascular system. PAPP-A belongs to the metzincin superfamily of zinc metalloproteinases, and is the founding member of a fifth metzincin family, the pappalysins. Herein, we first determined that PAPP-A cleaves IGFBP-4 at a single site (Met-135/Lys-136), and we analysed the influence of ionic strength, pH and zinc ion concentration on the cleavage reaction. Secondly, we sought to delineate the role of substrate residues in PAPP-A-mediated cleavage by the construction and analysis of 30 IGFBP-4 mutants in which various residues were replaced by alanine, by the analysis of eight mutants of IGFBP-5 (found recently to be a second PAPP-A substrate), and by cleavage analysis of synthetic peptides derived from IGFBP-4. Our data reveal a complex mode of substrate recognition and/or binding, pointing at important roles for several basic residues located up to 16 residues N-terminal to the scissile bond. An unexpected parallel can be drawn with an intracellular enzyme, the mitochondrial processing peptidase, that may help us to understand properties of the pappalysins. Further, proteinase-resistant variants of IGFBP-4 and -5, presented here, will be useful tools for the study of proteolysis in cell-based systems, and our finding that a synthetic peptide can be cleaved by PAPP-A provides the basis for development of quantitative assays for the investigation of PAPP-A enzyme kinetics.

In vivo processed fragments of IGF binding protein-2 copurified with bioactive IGF-II

Proteolysis of insulin-like growth factor binding proteins (IGFBPs), the major carrier of insulin-like growth factors (IGFs) in the circulation, is an essential mechanism to regulate the bioavailability and half-live of IGFs. Screening for peptides in human hemofiltrate, stimulating the survival of PC-12 cells, resulted in the isolation of C-terminal IGFBP-2 fragments and intact IGF-II co-eluting during the chromatographic purification procedure. The IGFBP-2 fragments exhibited molecular masses of 12.7 and 12.9kDa and started with Gly169 and Gly167, respectively. The fragments were able to bind both IGFs. The stimulatory effect of the purified fraction on the survival of the PC-12 cells could be assigned exclusively to IGF-II, since it was abolished by the addition of neutralizing IGF-II antibodies. We suggest that in the circulation IGF-II is not only complexed with intact IGFBP but also with processed IGFBP-2 fragments not impairing the biological activity of IGF-II.

Differential effects of insulin-like growth factor binding proteins-1, -2, -3, and -6 on cultured growth plate chondrocytes

BACKGROUND

In children with chronic renal failure (CRF), impairment of longitudinal growth is in part due to excess amounts of circulating high-affinity insulin-like growth factor binding proteins (IGFBPs) that might decrease or prevent insulin-like growth factor (IGF) binding to its signaling receptor. However, it appears from the clinical studies that various IGFBPs may have contrasting effects on longitudinal growth. Because of the potential importance of the IGFBPs as modulators of longitudinal growth in pediatric CRF, the aim of the present study was to investigate the biological effects of IGFBP-1, -2, -3, and -6 on cultured growth plate chondrocytes that express the type 1 IGF receptor.

METHODS

The effects of exogenous IGFBPs on IGF-independent and IGF-dependent proliferation of rat growth plate chondrocytes in primary culture were investigated. Proliferation was assessed by colony formation of agarose-stabilized long-term suspension cultures and by the [3H]thymidine assay. The effects of IGFBPs on IGF-I binding and the binding of IGFBPs to chondrocytes were assessed by binding studies with radiolabeled proteins in monolayer culture.

RESULTS

Intact IGFBP-1, IGFBP-2 and IGFBP-6 inhibited in equimolar concentration the IGF-I- and IGF-II-stimulated DNA synthesis and cell proliferation, whereas the biological activity of IGFBP-3 was complex. It had an IGF-independent antiproliferative effect and also inhibited IGF-dependent chondrocyte proliferation under coincubation conditions, whereas under preincubation conditions IGFBP-3 enhanced IGF-I-responsiveness. Studies on the mechanism by which IGFBP-3 potentiated IGF activity demonstrated that under preincubation conditions IGFBP-3 is capable to associate with the cell membrane and to facilitate IGF-I cell surface binding.

CONCLUSIONS

Intact IGFBP-1, IGFBP-2 and IGFBP-6 act exclusively as growth inhibitors on IGF-dependent proliferation of growth plate chondrocytes. IGFBP-3, however, can either inhibit IGF-independent and IGF-dependent cell proliferation, or enhance IGF responsiveness of chondrocytes dependent on the temporal relationship to the IGF exposure.

Isolation and characterization of circulating fragments of the insulin-like growth factor binding protein-3

Proteolysis of insulin-like growth factor binding protein-3 (IGFBP-3), the major carrier of IGFs in the circulation, is an essential mechanism to regulate IGF bioavailability. To analyze naturally occurring IGFBP-3 fragments a peptide library established from human hemofiltrate was screened. Three IGFBP-3 fragments were detected with apparent molecular masses of 34, 16, and 11 kDa. Mass spectrometric and sequence analysis identified the 16 and 11 kDa peptides as glycosylated and non-glycosylated N-terminal fragments spanning residues Gly1-Ala98 of IGFBP-3. Both the circulating forms and those secreted from IGFBP-3(1-98) overexpressing cells bound IGF. Additionally, two smaller fragments (IGFBP-3(139-157) and IGFBP-3(139-159)) were identified in the hemofiltrate. The data indicate that proteolysis of circulating IGFBP-3 occurs in the variable domain at residues alanine 98, phenylalanine 138, glutamine 157, and tyrosine 159.

Transferrin binds insulin-like growth factors and affects binding properties of insulin-like growth factor binding protein-3

In the circulation, most of the insulin-like growth factors (IGFs) are bound to a ternary 150 kDa complex with IGF-binding protein (IGFBP)-3 and the acid labile subunit. In the current study, we identify transferrin (Tf) by mass spectrometry, and immunoprecipitation as a component of a major IGF-binding fraction separated from human plasma. IGF ligand blotting, cross-linkage experiments and surface plasmon resonance spectrometry have been used to demonstrate the capability of Tf to bind IGFs specifically. In combination with Tf, IGFBP-3 showed a 5-fold higher affinity for IGF-II than IGFBP-3 alone. The data suggest that Tf may play an important role in regulating IGF/IGFBP-3 functions.

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.

Survey of the year 2000 commercial optical biosensor literature

We have compiled a comprehensive list of the articles published in the year 2000 that describe work employing commercial optical biosensors. Selected reviews of interest for the general biosensor user are highlighted. Emerging applications in areas of drug discovery, clinical support, food and environment monitoring, and cell membrane biology are emphasized. In addition, the experimental design and data processing steps necessary to achieve high-quality biosensor data are described and examples of well-performed kinetic analysis are provided.

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.

A new radioimmunoassay to measure rat insulin-like growth factor binding protein-4 (IGFBP-4) in serum, wound fluid and conditioned media

Insulin-like growth factor binding protein-4 (IGFBP-4) is, like the other five IGFBPs, a critical regulator of the activity of insulin-like growth factor-I (IGF-I) and IGF-II. Because of the possible role of IGFBP-4 in multiple systems including reproduction, pregnancy, bone formation and wound healing, we developed a radioimmunoassay (RIA) to measure the concentration of IGFBP-4 in the serum, extracellular fluid and conditioned media of cells of rodents so that these animal models could be better exploited. Rat IGFBP-4 was expressed in Escherichia coli and purified to homogeneity. The recombinant material was used to raise a rabbit polyclonal antibody against rat IGFBP-4 and for iodination and standards. The RIA developed was sensitive to less than 1 ng/mL of rat IGFBP-4 and IGF-I did not interfere. There was no cross-reactivity with other rat IGFBPs on immuno-Western analysis of serum and wound fluid. IGFBP-4 from mouse serum did cross-react in our assay; however, serum from horse, pig or human showed no cross-reaction. Human IGFBP-1 and IGFBP-3 showed a very weak cross-reactivity. Serum IGFBP-4 levels showed no gender difference but did reveal a significant 66% increase in older rats. During the course of wound healing, which is IGF-I dependent, IGFBP-4 showed no changes. In conclusion an RIA for rat IGFBP-4 has been developed that specifically measures the concentration of IGFBP-4 in the sera, extracellular fluid and conditioned media of rodents. With this assay, the role of IGFBP-4 and IGFs in growth, development and the function of multiple systems can be further investigated using rodent models.