Substitution of specific amino acids in insulin-like growth factor (IGF) binding protein 5 alters heparin binding and its change in affinity for IGF-I response to heparin

Pitavastatin Regulates Ang II Induced Proliferation and Migration via IGFBP-5 in VSMC

Angiotensin II (Ang II), a key mediator of hypertensive, causes structural changes in the arteries (vascular remodeling), which involve alterations in cell growth, vascular smooth muscle cell (VSMC) hypertrophy. Ang II promotes fibrotic factor like IGFBP5, which mediates the profibrotic effects of Ang II in the heart and kidneys, lung and so on. The purpose of this study was to identify the signaling pathway of IGFBP5 on cell proliferation and migration of Ang II-stimulated VSMC. We have been interested in Ang II-induced IGFBP5 and were curious to determine whether a Pitavastatin would ameliorate the effects. Herein, we investigated the question of whether Ang II induced the levels of IGFBP5 protein followed by proliferation and migration in VSMC. Pretreatment with the specific Angiotensin receptor type 1 (AT1) inhibitor (Losartan), Angiotensin receptor type 2 (AT2) inhibitor (PD123319), MAPK inhibitor (U0126), ERK1/2 inhibitor (PD98059), P38 inhibitor (SB600125) and PI3K inhibitor (LY294002) resulted in significantly inhibited IGFBP5 production, proliferation, and migration in Ang II-stimulated VSMC. In addition, IGFBP5 knockdown resulted in modulation of Ang II induced proliferation and migration via IGFBP5 induction. In addition, Pitavastatin modulated Ang II induced proliferation and migration in VSMC. Taken together, our results indicated that Ang II induces IGFBP5 through AT1, ERK1/2, P38, and PI3K signaling pathways, which were inhibited by Pitavastatin. These findings may suggest that Pitavastatin has an effect on vascular disease including hypertension.

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.

The IGFBP7 homolog Imp-L2 promotes insulin signaling in distinct neurons of the Drosophila brain

In Drosophila, Insulin-like peptide 2 (Dilp-2) is expressed by insulin-producing cells in the brain, and is secreted into the hemolymph to activate insulin signaling systemically. Within the brain, however, a more local activation of insulin signaling may be required to couple behavioral and physiological traits to nutritional inputs. We show that a small subset of neurons in the larval brain has high Dilp-2-mediated insulin signaling activity. This local insulin signaling activation is accompanied by selective Dilp-2 uptake and depends on the expression of the Imaginal morphogenesis protein-late 2 (Imp-L2) in the target neurons. We suggest that Imp-L2 acts as a licensing factor for neuronal IIS activation through Dilp-2 to further increase the precision of insulin activity in the brain.

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.

Duplicated zebrafish insulin-like growth factor binding protein-5 genes with split functional domains: evidence for evolutionarily conserved IGF binding, nuclear localization, and transactivation activity

Insulin-like growth factor binding protein (IGFBP)-5 is a secreted protein that binds to IGF and modulates IGF actions. IGFBP-5 is also found in the nucleus of mammalian cells and has transactivation activity. The structural basis of this transactivation activity and its role in mediating IGF-independent actions are not clear. Here we report that there are 2 igfbp-5 genes in zebrafish and other teleost fish. In zebrafish, igfbp-5a and -5b are expressed in spatially restricted, mostly nonoverlapping domains during early development. The IGF binding site is conserved in both zebrafish IGFBP-5s, and they are both secreted and capable of IGF binding. Both proteins contain a consensus bipartite nuclear localization signal and were found in the nucleus when introduced into cultured cells. Although zebrafish IGFBP-5b possesses transactivation activity, zebrafish IGFBP-5a lacks this activity. Mutational analysis demonstrated that 2 unique amino acids in positions 22 and 56 of IGFBP-5a are responsible for its lack of transactivation activity. These findings suggest that the duplicated zebrafish IGFBP-5s have evolved divergent regulatory mechanisms and distinct biological properties by partitioning of ancestral structural domains and provide new evidence for a conserved role of the IGF binding, nuclear localization, and transactivation domain of this multifunctional IGFBP.

Molecular characterization, tissue distribution patterns and nutritional regulation of IGFBP-1, -2, -3 and -5 in yellowtail, Seriola quinqueradiata

Insulin-like growth factor-binding proteins (IGFBPs) play a vital role in regulating the biological activities of IGFs. In this study, we cloned and determined full-length cDNA sequences of yellowtail IGFBP-1, -2, -3 and -5. Their tissue distribution was determined by real-time quantitative RT-PCR, which revealed that IGFBP-1, -2, -3 and -5 are widely distributed in yellowtail tissues. In yellowtail, both IGFBP-1 and -2 are highly expressed in the liver, IGFBP-3 is predominantly expressed in the heart and skin, with the lowest expression in the liver, and IGFBP-5 is highly expressed in the liver and kidneys. The widespread tissue expression of the yellowtail IGFBPs suggests that they may act in an autocrine and/or paracrine manner in the regulation of IGF activity. The effects of nutritional deprivation on yellowtail IGFBPs and IGF-I were also examined. During a 15-day starvation period, significant elevation was observed in hepatic yellowtail IGFBP-1. Refeeding restored its level to that of the control. No significant change was observed in the hepatic yellowtail IGFBP-2 mRNA levels in starved fish compared with control fish during the starvation period. Interestingly, during the early period of food deprivation, a significant increase was observed in hepatic yellowtail IGFBP-3 and -5 mRNA levels, concomitant to the significant elevation in hepatic IGF-I mRNA from day 3 to day 9. The unexpected increase in growth stimulatory IGFBPs and IGF-I during nutritional deprivation may represent a species-specific response to changes in nutritional condition.

Insulin-like growth factor (IGF) binding protein-5 blocks skeletal muscle differentiation by inhibiting IGF actions

Signaling through the IGF-I receptor by locally produced IGF-I or -II is critical for normal skeletal muscle development and repair after injury. In most tissues, IGF action is modulated by IGF binding proteins (IGFBPs). IGFBP-5 is produced by muscle cells, and previous studies have suggested that when overexpressed it may either facilitate or inhibit IGF actions, and thus potentially enhance or diminish IGF-mediated myoblast differentiation or survival. To resolve these contradictory observations and discern the mechanisms of action of IGFBP-5, we studied its effects in cultured muscle cells. Purified wild-type (WT) mouse IGFBP-5 or a variant with diminished extracellular matrix binding (C domain mutant) each prevented differentiation at final concentrations as low as 3.5 nm, whereas analogs with reduced IGF binding (N domain mutant) were ineffective even at 100 nm. None of the IGFBP-5 variants altered cell number. An IGF-I analog (R(3)IGF-I) with diminished affinity for IGFBPs promoted full muscle differentiation in the presence of IGFBP-5(WT), showing that IGFBP-5 interferes with IGF-dependent signaling pathways in myoblasts. When IGFBP-5(WT) or variants were overexpressed by adenovirus-mediated gene transfer, concentrations in muscle culture medium reached 500 nm, and differentiation was inhibited, even by IGFBP-5(N). As 200 nm of purified IGFBP-5(N) prevented activation of the IGF-I receptor by 10 nm IGF-II as effectively as 2 nm of IGFBP-5(WT), our results not only demonstrate that IGFBP-5 variants with reduced IGF binding affinity impair muscle differentiation by blocking IGF actions, but underscore the need for caution when labeling effects of IGFBPs as IGF independent because even low-affinity analogs may potently inhibit IGF-I or -II if present at high enough concentrations in biological fluids.

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

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

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

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

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.

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

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

Insulin-like growth factor binding protein-2 binding to extracellular matrix plays a critical role in neuroblastoma cell proliferation, migration, and invasion

IGF binding proteins (IGFBPs) modulate IGF cellular bioavailability and may directly regulate tumor growth and invasion. We have previously shown that IGFBP-2 binds and localizes IGF-I to the pericellular matrix and have provided some evidence suggesting that the heparin binding domain (HBD) or the arginine-glycine-aspartic acid (RGD) integrin binding motif may be involved in these interactions. However, the precise mechanisms involved remain to be elucidated. We therefore mutated the HBD or RGD sequence of IGFBP-2 and investigated consequent effects on extracellular matrix (ECM) binding, IGF-induced proliferation, and migration of neuroblastoma cells. IGFBP-2 and its arginine-glycine-glutamic acid (RGE) mutant similarly bound ECM components, whereas binding of mutant HBD-IGFBP-2 to each of the ECM substrates was markedly reduced by 70-80% (P < 0.05). IGF-I (100 ng/ml) increased incorporation of 3H-thymidine in neuroblastoma SK-N-SHEP cells by approximately 30%, an effect blunted by exogenously added native or either mutant IGFBP-2. Overexpression of IGFBP-2 and its RGE mutant potently promoted SHEP cell proliferation (5-fold), whereas SHEP cell proliferation was negligible when HBD-IGFBP-2 was overexpressed. Addition or overexpression of IGFBP-2 and its RGE mutant potently (P < 0.05) enhanced SHEP cell migration/invasion through the ECM. However, overexpression of the HBD-IGFBP-2 mutant potently inhibited (50-60%) SHEP cell invasion through ECM. Thus, IGFBP-2, which binds to the ECM, enhances proliferation and metastatic behavior of neuroblastoma cells, functions that directly or indirectly use the HBD but not the integrin binding sequence. Our novel findings thus point to a key role for the HBD of IGFBP-2 in the control and regulation of neuroblastoma growth and invasion.

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.

Expression and characterization of a serine protease that preferentially cleaves insulin-like growth factor binding protein-5

Insulin-like growth factor binding proteins (IGFBPs) play important roles in regulating the functions of insulin-like growth factors (IGFs). Because IGFBPs have very high affinity for IGF-I and IGF-II, they can regulate the amount of each growth factor that is able to bind to cell surface receptors, therefore, factors that alter IGFBP affinity have the capacity to regulate IGF actions. Protease activities that are present in cell culture systems and physiologic fluids have been shown to degrade IGFBP-5. Previously, a region of sequence in a serine protease was identified that was homologous with the N-terminal 90 amino acids of members of the IGFBP family and with members of the CCN family of proteins. In a prior study, the protease was expressed in human kidney cultured cells and the cell culture supernatants were shown to cleave IGFBP-5, however, it is unknown whether the purified protease would cleave IGFBP-5 and whether it would also cleave other specific forms of IGFBPs. In this study, we expressed this protease in an insect cell expression system, purified it to homogeneity and tested its capacity to cleave IGFBP-5. The expressed protease preferentially cleaved IGFBP-5, and it had minimal activity toward other forms of IGFBPs. The proteolytic activity of this IGFBPase is inhibited by serine protease inhibitors including PMSF and 3,4-dichloroisocoumarin, as well as by divalent metal ions such as, Zn and Cu. Mutation of the active site serine resulted in a major reduction in IGFBP-5 cleavage. The protease binds to heparin and its ability to degrade IGFBP-5 is blocked in the presence of heparin. Inhibition of the activity of the protease following its secretion by B104 cells resulted in inhibition of IGFBP-5 proteolysis and IGF-I stimulation of protein synthesis. Northern blotting revealed that the transcript was expressed in multiple human tissues, including placenta, uterus, prostate, testis, spinal cord, brain, liver, small intestine, thyroid, and spleen. The highest expression was in uterus and placenta, suggesting a possible role of sex steroids in regulating its expression. Understanding the mechanism of how cleavage of IGFBP-5 by this protease alters its activity will help to further our understanding of the biologic actions of the IGFs.

The role of IGFBP-5 in mammary gland development and involution

Insulin-like growth factor-I (IGF-I) plays an important role as a survival factor during mammary gland development and remodelling during involution of the mature/lactating mammary gland, and elevated concentrations have been associated with increased risk of breast cancer. The actions of IGF-I are modulated by a family of binding proteins (IGFBPs) and we have shown that IGFBP-5 is associated with cell death in the mammary gland and more recently provided the first evidence that it is causally related to apoptosis of the mammary gland. A transgenic mouse expressing IGFBP-5 on a mammary-specific promoter led to impaired mammary development involving inhibition of IGF-signalling and involving members of the Bcl-2 family. Subsequent studies in vitro and in vivo using exogenous IGFBP-5 treatment have added support to this concept. Although the effects of IGFBP-5 did appear to involve inhibition of IGF action, a role for IGF-independent effects cannot be ruled out. Such IGF-independent effects involve potential interactions with components of the extracellular matrix involved in tissue remodelling including plasminogen activator inhibitor-1 (PAI-1). In addition, intracellular events involving nuclear localisation of IGFBP-5 have been shown to have the ability to inhibit cell proliferation. Thus, IGFBP-5 seems important for regulating both apoptosis and cell proliferation in the mammary gland during development and post-lactation involution.

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.

Fibroblast growth factor-2 over-rides insulin-like growth factor-I induced proliferation and cell survival in human neuroblastoma cells

The insulin-like growth factor (IGF) system is a key regulator of cell growth, survival and differentiation, and these functions are co-modulated by other growth factors including fibroblast growth factor-2 (FGF-2). To investigate IGF/FGF interactions in neuronal cells, we employed neuroblastoma cells (SK-N-MC). In serum free conditions proliferation of the SK-N-MC cells was promoted by IGF-I (25 ng/ml), but blunted by FGF-2 (50 ng/ml). IGF-I-induced proliferation was abolished in the presence of FGF-2 even when IGF-I was used at 100 ng/ml. In addition to our previously described FGF-2 induced proteolytic cleavage of IGFBP-2, we found that FGF-2 increased IGFBP-6 levels in conditioned medium (CM) without affecting IGFBP-6 mRNA abundance. Modulation of IGFBP-2 and -6 levels were not significant mechanisms involved in the blockade of IGF-I action since the potent IGF-I analogues [QAYL]IGF-I and des(1-3)IGF-I (minimal IGFBP affinity) were unable to overcome FGF-2 inhibition of cell proliferation. FGF-2 treated cells showed morphological differentiation expressing the TUJ1 neuronal marker while cells treated with IGF-I alone showed no morphological change. When IGF-I was combined with FGF-2, however, cell morphology was indistinguishable from that seen with FGF-2 alone. FGF-2 inhibited proliferation and enhanced differentiation was also associated with a 70% increase in cell death. Although IGF-I alone was potently anti-apoptotic (60% decreased), IGF-I was unable to prevent apoptosis when administrated in combination with FGF-2. Gene-array analysis confirmed FGF-2 activation of the intrinsic and extrinsic apoptotic pathways and blockade of IGF anti-apoptotic signaling. FGF-2, directly and indirectly, overcomes the proliferative and anti-apoptotic activity of IGF-I by complex mechanisms, including enhancement of differentiation and apoptotic pathways, and inhibition of IGF-I induced anti-apoptotic signalling. Modulation of IGF binding protein abundance by FGF-2 does not play a significant role in inhibition of IGF-I induced mitogenesis.

Fibronectin Binds Insulin-like Growth Factor-binding Protein 5 and Abolishes Its Ligand-dependent Action on Cell Migration

Insulin-like growth factor-binding protein 5 (IGFBP-5) is a secreted protein that binds to insulin-like growth factors (IGFs) and modulates IGF actions on cell proliferation, differentiation, survival, and motility. IGFBP-5 also regulates these cellular events through IGF-independent mechanisms. To elucidate the molecular mechanisms governing these diverse actions of IGFBP-5, we screened a human cDNA library by a yeast two-hybrid system using IGFBP-5 as bait and identified fibronectin (FN) as a potential IGFBP-5-interacting partner. The complex formation of IGFBP-5 and FN was established by glutathione S-transferase pull-down, solution, and solid phase binding assays using glutathione S-transferase-IGFBP-5 and native IGFBP-5 in vitro and by co-immunoprecipitation in vivo. Binding assay using deletion mutants indicated that the IGFBP-5 C domain binds to the 10th and 11th type I repeats of FN. IGFBP-5 potentiated IGF-I-induced cell migration in FN-null, but not in wild-type, mouse embryonic cells. When FN was reintroduced either as an adhesive substrate or in solution to the FN-null cells, the potentiating effect of IGFBP-5 on IGF-I-induced cell migration was abolished. Binding of IGFBP-5 to FN had no effect on the ability of IGFBP-5 to bind IGF-I, but it increased the proteolytic degradation of IGFBP-5. Inhibition of IGFBP-5 proteolysis restored the potentiating effect of IGFBP-5. These results suggest that FN and IGFBP-5 bind to each other, and this binding negatively regulates the ligand-dependent action of IGFBP-5 by triggering IGFBP-5 proteolysis.

Regulation of vascular smooth muscle cell responses to insulin-like growth factor (IGF)-I by local IGF-binding proteins

Insulin-like growth factor (IGF)-I is a pleiotropic hormone that regulates vascular smooth muscle cell (VSMC) migration, proliferation, apoptosis, and differentiation. These actions are mediated by the IGF-I receptor. How activation of the same receptor by the same ligand leads to these diverse cellular responses is not well understood. Here we describe a novel mechanism specifying VSMC responses to IGF-I stimulation, distinctive for the pivotal roles of local IGF-binding proteins (IGFBPs). The role of local IGFBPs was indicated by comparing the activities of IGF-I and des-1-3-IGF-I, an IGF-I analog with reduced binding affinity to IGFBPs. Compared with IGF-I, des-1-3-IGF-I was more potent in stimulating DNA synthesis but much less potent in inducing directed migration of VSMCs. When the effects of individual IGFBPs were tested, IGFBP-2 and IGFBP-4 were found to inhibit IGF-I-stimulated DNA synthesis and migration. IGFBP-5 had an inhibitory effect on IGF-I-stimulated DNA synthesis, but it strongly potentiated IGF-I-induced VSMC migration. By using a non-IGF-binding IGFBP-5 mutant and an IGF-I-neutralizing antibody, it was demonstrated that IGFBP-5 also stimulates VSMC migration in an IGF-independent manner. This effect of IGFBP-5 was inhibited by soluble heparin and by treating cells with heparinase. Mutation of the heparin-binding motif of IGFBP-5 reduced its migration promoting activity. These findings suggest that local IGFBPs are important determinants of cellular responses to IGF-I stimulation, and a key player in this paradigm is IGFBP-5. IGFBP-5 not only modulates IGF-I actions, but it also stimulates cell migration by interacting with cell-surface heparan sulfate proteoglycans.

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.

Insulin-like growth factor (IGF)-binding protein-3 mutants that do not bind IGF-I or IGF-II stimulate apoptosis in human prostate cancer cells

Insulin-like growth factor (IGF)-binding protein-3 (IGFBP-3) can stimulate apoptosis and inhibit cell proliferation directly and independently of binding IGFs or indirectly by forming complexes with IGF-I and IGF-II that prevent them from activating the IGF-I receptor to stimulate cell survival and proliferation. To date, IGF-independent actions only have been demonstrated in a limited number of cells that do not synthesize or respond to IGFs. To assess the general importance of IGF-independent mechanisms, we have generated human IGFBP-3 mutants that cannot bind IGF-I or IGF-II by substituting alanine for six residues in the proposed IGF binding site, Ile(56)/Tyr(57)/Arg(75)/Leu(77)/Leu(80)/Leu(81), and expressing the 6m-hIGFBP-3 mutant construct in Chinese hamster ovary cells. Binding of both IGF-I and IGF-II to 6m-hIGFBP-3 was reduced >80-fold. The nonbinding 6m-hIGFBP-3 mutant still was able to inhibit DNA synthesis in a mink lung epithelial cell line in which inhibition by wild-type hIGFBP-3 previously had been shown to be exclusively IGF-independent. 6m-hIGFBP-3 only can act by IGF-independent mechanisms since it is unable to form complexes with the IGFs that inhibit their action. We next compared the ability of wild-type and 6m-hIGFBP-3 to stimulate apoptosis in serum-deprived PC-3 human prostate cancer cells. PC-3 cells are known to synthesize and respond to IGF-II, so that IGFBP-3 could potentially act by either IGF-dependent or IGF-independent mechanisms. In fact, 6m-hIGFBP-3 stimulated PC-3 cell death and stimulated apoptosis-induced DNA fragmentation to the same extent and with the same concentration dependence as wild-type hIGFBP-3. These results indicate that IGF-independent mechanisms are major contributors to IGFBP-3-induced apoptosis in PC-3 cells and may play a wider role in the antiproliferative and antitumorigenic actions of IGFBP-3.

Signaling through the Smad pathway by insulin-like growth factor-binding protein-3 in breast cancer cells. Relationship to transforming growth factor-beta 1 signaling

We previously demonstrated in T47D cells transfected to express the transforming growth factor-beta receptor type II (TGF-betaRII) that insulin-like growth factor binding protein-3 (IGFBP-3) could stimulate Smad2 and Smad3 phosphorylation, potentiate TGF-beta1-stimulated Smad phosphorylation, and cooperate with exogenous TGF-beta1 in cell growth inhibition (Fanayan, S., Firth, S. M., Butt, A. J., and Baxter, R. C. (2000) J. Biol. Chem. 275, 39146-39151). This study further explores IGFBP-3 signaling through the Smad pathway. Like TGF-beta1, natural and recombinant IGFBP-3 stimulated the time- and dose-dependent phosphorylation of TGF-betaR1 as well as Smad2 and Smad3. This effect required the presence of TGF-betaRII. IGFBP-3 mutated in carboxyl-terminal nuclear localization signal residues retained activity in TGF-betaR1 and Smad phosphorylation, whereas IGFBP-5 was inactive. Immunoneutralization of endogenous TGF-beta1 suggested that TGF-beta1 was not essential for IGFBP-3 stimulation of this pathway, but it increased the effect of IGFBP-3. IGFBP-3, like TGF-beta1, elicited a rapid decline in immunodetectable Smad4 and Smad4.Smad2 complexes. IGFBP-3 and nuclear localization signal mutant IGFBP-3 stimulated the activation of the plasminogen activator inhibitor-1 promoter but was not additive with TGF-beta, suggesting that this end point is not a direct marker of the IGFBP-3 effect on cell proliferation. This study defines a signaling pathway for IGFBP-3 from a cell surface receptor to nuclear transcriptional activity, requiring TGF-betaRII but not dependent on the nuclear translocation of IGFBP-3. The precise mechanism by which IGFBP-3 interacts with the TGF-beta receptor system remains to be established.

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.

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.

Heparin therapy for ulcerative colitis? Effects and mechanisms

Despite intensive medical treatment with steroids and immunosuppressants, acute colitis is still associated with a colectomy rate of up to 15%. Following the observation that a patient with severe steroid-resistant colitis went into remission when treated with heparin for a deep vein thrombosis, there have been a number of reports on the use of heparin in acute ulcerative colitis. Although small and uncontrolled, these studies consistently demonstrate the beneficial effects of heparin, with surprisingly few side-effects in a disease characterized by mucosal haemorrhage. The mechanisms by which heparin may ameliorate ulcerative colitis remain unclear. A simple anticoagulant effect may be responsible, but similar effects are not seen with warfarin. As a result of their intense negative charge, the glycosaminoglycans that constitute heparin have diverse biological effects. These include potent anti-inflammatory actions, in vitro and in vivo, and the potentiation of the activity of the peptide growth factors necessary for mucosal regeneration and repair. This review summarizes the clinical reports on heparin treatment for ulcerative colitis and explores the mechanisms by which this novel form of treatment may exert its effects.

Free insulin-like growth factor-I and breast cancer risk

Insulin-like growth factor-I (IGF-I) has mitogenic and anti-apoptotic effects on breast cancer cells. Epidemiologic studies have shown that high plasma levels of IGF-I and low levels of IGF binding protein (BP)-3 are associated with increased risk of breast cancer in premenopausal women. The actions of IGF-I are mediated through the IGF-I receptor (IGF-IR) and are regulated by IGFBPs. In circulation, most of the IGF-I binds to IGFBP-3, and binding of IGF-I to IGFBP-3 inhibits the actions of IGF-I. Since free IGF-I, which does not bind to IGFBPs, can readily cross the endothelial barrier to interact with IGF-IR, circulating free IGF-I is thought to be more relevant to the biologic activity of IGF-I. To examine the association of free IGF-I with breast cancer, we compared free IGF-I levels between 40 newly diagnosed breast cancer patients and 40 age- and race-matched healthy controls. Plasma levels of free IGF-I, total IGF-I and IGF-II, as well as total, intact and fragment IGFBP-3, were measured using commercial immunoassay kits. The association between IGF-I and breast cancer was examined using the conditional logistic regression analysis. Analysis of correlation (Spearman) showed that free IGF-I was correlated with total IGF-I and IGFBP-3 but not with IGF-II. The odds ratios for breast cancer patients having high plasma IGF-I (> or = median) after adjusting for menopausal status and IGFBP-3 were 2.00 (p < o r = 0.376) for total IGF-I and 6.31 (p < or = 0.047) for free IGF-I. A high ratio of IGF-I to IGFBP-3 was also associated with breast cancer (p < 0.05). No association was found for IGF-II, nor for total, intact and fragment IGFBP-3. The findings of this study suggest that measuring free IGF-I in circulation is more useful than measuring total IGF-I with respect to evaluation of an association between IGF-I and breast cancer risk.

NMR 15N relaxation of the insulin-like growth factor (IGF)-binding domain of IGF binding protein-5 (IGFBP-5) determined free in solution and in complex with IGF-II

15N NMR relaxation rates of mini-IGFBP-5, an N-terminal insulin-like growth factor binding domain of the insulin-like growth factor binding protein 5 (IGFBP-5), were analysed at three field strengths using the Lipari-Szabo procedure (see below) and reduced spectral density methods. Isotropic and anisotropic Lipari-Szabo models were analysed and an analytical formula for the overall correlation time for anisotropic molecules is presented. Mini-IGFBP-5 was found to be mainly rigid on fast ps time scales except for 11 unstructured flexible residues at the C-terminus. The insulin-like growth factor binding loop in the apo-protein exhibits small amounts of flexibility on fast time scales (ps to ns) but several loop residues show significant exchange broadening. These loop residues display no exchange broadening in the complex of IGF-II/mini-IGFBP-5. The isotropic overall tumbling time in solution at 31 degrees C of mini-IGFBP-5 complexed to IGF-II is tauc = 18.4 +/- 0.2 ns indicating a strong tendency for aggregation.

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.

Differential effect of fasting on IGF-BPs in serum of young and adult rats and its implication to impaired skin GAG content

During fasting or aging of animals there is a decreased content of skin glycosaminoglycans (GAGs). It has been found that the skin of adult rats contains about 60% of GAGs found in the skin of young animals. Fasting of both groups of animals (young and adult) resulted in decrease of GAG content. However, GAG content in the skin of fasted young rats decreased by 30% and in fasted adult rats by 15% only, compared to fed animals, respectively. The mechanism for the phenomena is not known. We considered insulin-like growth factor-I (IGF-I) as a potential candidate involved in regulation of GAG biosynthesis in both experimental models of animals. Adult rat sera were found to contain about 75% of IGF-I recovered from young rat sera. Fasting of both groups of animals resulted in dramatic decrease in serum IGF-I levels to about 50% of initial values. Since IGF-I activity and IGF-I serum half-life depends on the level of specific IGF-binding proteins (IGFBPs) we determined (i) relationship between main groups of IGFBPs, namely high molecular weight binding proteins (HMWBPs) and low molecular weight binding proteins (LMWBPs) and (ii) the amounts of IGF-I bound to respective proteins in the sera of all experimental animals. Control young rat serum was found to contain about 90% of HMWBPs and about 10% of LMWBPs as determined by ligand binding assay. In contrast, control adult rat serum contained about 60% of HMWBPs and about 40% of LMWBPs. Fasting of both groups of animals resulted in significant increase in serum levels of LMWBPs. Control young rat serum was found to contain about 8% IGF-I bound to LMWBPs while serum of control adult rats contained 18% IGF-I bound to these proteins. In sera of fasted young animals however, about 75% of the bound IGF-I was recovered from LMWBPs (about 60% of total serum IGF-I) while in sera of fasted adult animals only about 56% of the bound IGF-I was recovered from LMWBPs (about 50% of total serum IGF-I). Evidence was provided that during fasting of both groups of animals there is a significant decrease in serum BP-3 and dramatic increase in serum BP-1 concentrations, compared to respective controls. However, the concentration of BP-1 in serum of fasted young rats was increased by about 60 fold while in serum of fasted adult rats only by about 10 fold, compared to respective control animals. Negative correlation between skin GAG content and LMWBPs derived IGF-I during fasting of young (r = -0.943, p < 0.001) and adult ( r = -0.571, p < 0.01) rats was found. The data presented suggest that the effects of aging and fasting on decreased skin GAG content may be due to induction of LMWBPs that are known to (i) inhibit IGF-I dependent function and (ii) increase clearance of IGF-I from circulation. However, the effects of fasting are distinct in respect to young and adult rats suggesting that mechanisms involved in regulation of IGF-I bioactivity during aging are more complex that during fasting.

Identification of multiple, differentially expressed messenger RNAs in dermal fibroblasts from patients with systemic sclerosis

OBJECTIVE

To simultaneously identify several genes whose expression is altered in dermal fibroblasts from patients with systemic sclerosis (SSc).

METHODS

Total RNA was prepared from fibroblasts derived from clinically affected and unaffected skin of patients with SSc. The RNA samples were analyzed using differential-display reverse transcription-polymerase chain reaction (DDRT-PCR). Complementary DNA (cDNA) fragments corresponding to differentially expressed messenger RNAs (mRNAs) were eluted, cloned, and sequenced. The differential expression of the corresponding mRNAs was confirmed by ribonuclease protection assay.

RESULTS

We identified 21 differentially expressed mRNAs. Their corresponding cDNAs were sequenced and the sequences obtained were compared with those of known genes entered into the EMBL/GenBank database. Three of the sequences corresponded to transcripts of yet-unidentified genes. Some of the mRNAs shared partial homology with extracellular matrix components, cellular receptors, enzymes, and nuclear factors. Others corresponded to known mRNAs such as those of fibronectin, fibronectin receptor, laminin receptor homolog, beta-tubulin, insulin-like growth factor binding protein 5, KIAA0179 protein, and protease nexin 1.

CONCLUSION

The application of DDRT-PCR to scleroderma research has identified many mRNAs whose altered expression in scleroderma has not yet been described, thus providing new information for further investigation and potential targets for the development of novel therapies.

Insulin-like growth factor binding protein-5 proteolytic activity in ovine articular chondrocyte culture

We have previously demonstrated that ovine articular chondrocytes synthesise and release insulin-like growth factor binding protein-5 (IGFBP-5) which subsequently undergoes proteolysis in the tissue culture medium. The IGFBP-5 proteolytic activity has now been characterised and its substrate specificity analysed using recombinant IGFBP-5 and purified chondrocyte-derived IGFBPs. Iodinated human recombinant IGFBP-5 was incubated with chondrocyte culture or conditioned medium in the presence or absence of various inhibitors. Serine protease inhibitors aprotinin and heparin effectively inhibited the breakdown of IGFBP-5. Furthermore, insulin-like growth factor-I (IGF-I) but not its structural analogues with reduced affinity for IGFBP-5, was also able to partially protect IGFBP-5 from degradation indicating that the association of IGF with the binding protein was required for the inhibition of the proteolytic activity. The inflammatory cytokine interleukin-1 did not have any effect on IGFBP-5 proteolysis. The proteolytic activity appears to be IGFBP-5-specific since the incubation of chondrocyte-derived IGFBPs with chondrocyte conditioned medium resulted in the loss of IGFBP-5 while the levels of the other two IGFBPs (IGFBP-2 and a 24 kDa IGFBP) remained unchanged. In conclusion, we show that IGFBP-5 is specifically cleaved by a serine protease released by primary cultures of ovine articular chondrocytes and also demonstrate the ability of IGF-I to inhibit the proteolytic activity both in cell culture and in cell-free conditions.

Structure of the IGF-binding domain of the insulin-like growth factor-binding protein-5 (IGFBP-5): implications for IGF and IGF-I receptor interactions

Binding proteins for insulin-like growth factors (IGFs) IGF-I and IGF-II, known as IGFBPs, control the distribution, function and activity of IGFs in various cell tissues and body fluids. Insulin-like growth factor-binding protein-5 (IGFBP-5) is known to modulate the stimulatory effects of IGFs and is the major IGF-binding protein in bone tissue. We have expressed two N-terminal fragments of IGFBP-5 in Escherichia coli; the first encodes the N-terminal domain of the protein (residues 1-104) and the second, mini-IGFBP-5, comprises residues Ala40 to Ile92. We show that the entire IGFBP-5 protein contains only one high-affinity binding site for IGFs, located in mini-IGFBP-5. The solution structure of mini-IGFBP-5, determined by nuclear magnetic resonance spectroscopy, discloses a rigid, globular structure that consists of a centrally located three-stranded anti-parallel beta-sheet. Its scaffold is stabilized further by two inside packed disulfide bridges. The binding to IGFs, which is in the nanomolar range, involves conserved Leu and Val residues localized in a hydrophobic patch on the surface of the IGFBP-5 protein. Remarkably, the IGF-I receptor binding assays of IGFBP-5 showed that IGFBP-5 inhibits the binding of IGFs to the IGF-I receptor, resulting in reduction of receptor stimulation and autophosphorylation. Compared with the full-length IGFBP-5, the smaller N-terminal fragments were less efficient inhibitors of the IGF-I receptor binding of IGFs.

Insulin-like growth factor-binding protein 5 complexes with the acid-labile subunit. Role of the carboxyl-terminal domain

We have recently shown that insulin-like growth factor (IGF)-binding protein 5 forms ternary complexes with IGF-I or IGF-II and the acid-labile subunit (ALS) (Twigg, S. M., and Baxter, R. C. (1998) J. Biol. Chem. 273, 6074-6079). Because IGF-binding protein 3 (IGFBP-3) binds to ALS through its basic carboxyl-terminal domain, we tested whether a homologous region present in IGFBP-5 is involved in IGFBP-5 binding to ALS. Chimeric peptides were generated by carboxyl-terminal domain interchange between recombinant human IGF-BP-5 and IGFBP-6, producing two IGFBP peptides designated 5-5-6 and 6-6-5. Determined by immunoprecipitation and by Superose chromatography, 6-6-5 formed ternary complexes, albeit less potently than IGF-BP-5. In contrast, 5-5-6, like IGFBP-6, did not form ternary complexes by these methods. Whereas 6-6-5, like IGFBP-6, had a marked preference for binary complex formation with IGF-II rather than IGF-I, it formed ternary complexes more efficiently with IGF-I, like IGF-BP-5. The glycosaminoglycans heparin and heparan sulfate bind to IGFBP-5 through its basic carboxyl-terminal domain. At high concentrations, these glycosaminoglycans inhibited ALS binding to binary complexed IGF-BP-5. In addition, in the absence of IGFs, IGFBP-5, a synthetic peptide representing the basic carboxyl-terminal sequence IGFBP-5(201-218), and the corresponding IGFBP-3 basic sequence IGFBP-3(215-232), competed weakly for ALS binding to covalent IGF-IGFBP-5 complex, as did a random-sequence synthetic peptide with the same composition as IGFBP-5(201-218). These findings are consistent with the basic carboxyl-terminal domain on IGFBP-5 being the principal site in IGFBP-5 that binds to ALS.

Binding of insulin-like growth factor (IGF)-binding protein-5 to smooth-muscle cell extracellular matrix is a major determinant of the cellular response to IGF-I

Insulin-like growth factor-binding protein-5 (IGFBP-5) has been shown to bind to fibroblast extracellular matrix (ECM). Extracellular matrix binding of IGFBP-5 leads to a decrease in its affinity for insulin-like growth factor-I (IGF-I), which allows IGF-I to better equilibrate with IGF receptors. When the amount of IGFBP-5 that is bound to ECM is increased by exogenous addition, IGF-I's effect on fibroblast growth is enhanced. In this study we identified the specific basic residues in IGFBP-5 that mediate its binding to porcine smooth-muscle cell (pSMC) ECM. An IGFBP-5 mutant containing alterations of basic residues at positions 211, 214, 217, and 218 had the greatest reduction in ECM binding, although three other mutants, R214A, R207A/K211N, and K202A/R206N/R207A, also had major decreases. In contrast, three other mutants, R201A/K202N/R206N/R208A, and K217N/R218A and K211N, had only minimal reductions in ECM binding. This suggested that residues R207 and R214 were the most important for binding, whereas alterations in K211 and R218, which align near them, had minimal effects. To determine the effect of a reduction in ECM binding on the cellular replication response to IGF-I, pSMCs were transfected with the mutant cDNAs that encoded the forms of IGFBPs with the greatest changes in ECM binding. The ECM content of IGFBP-5 from cultures expressing the K211N, R214A, R217A/R218A, and K202A/R206N/R207A mutants was reduced by 79.6 and 71.7%, respectively, compared with cells expressing the wild-type protein. In contrast, abundance of the R201A/K202N/R206N/R208A mutant was reduced by only 14%. Cells expressing the two mutants with reduced ECM binding had decreased DNA synthesis responses to IGF-I, but the cells expressing the R201A/K202N/R206N/R208A mutant responded well to IGF-I. The findings suggest that specific basic amino acids at positions 207 and 214 mediate the binding of IGFBP-5 to pSMC/ECM. Smooth-muscle cells that constitutively express the mutants that bind weakly to ECM are less responsive to IGF-I, suggesting that ECM binding of IGFBP-5 is an important variable that determines cellular responsiveness.

Alanine screening mutagenesis establishes tyrosine 60 of bovine insulin-like growth factor binding protein-2 as a determinant of insulin-like growth factor binding

The determinants of insulin-like growth factor (IGF) binding to its binding proteins (IGFBPs) are poorly characterized in terms of important residues in the IGFBP molecule. We have previously used tyrosine iodination to implicate Tyr-60 in the IGF-binding site of bovine IGFBP-2 (Hobba, G. D., Forbes, B. E., Parkinson, E. J., Francis, G. L., and Wallace, J. C. (1996) J. Biol. Chem. 271, 30529-30536). In this report, we show that the mutagenic replacement of Tyr-60 with either Ala or Phe reduced the affinity of bIGFBP-2 for IGF-I (4.0- and 8.4-fold, respectively) and for IGF-II (3.5- and 4.0-fold, respectively). Although adjacent residues Val-59, Thr-61, Pro-62, and Arg-63 are well conserved in IGFBP family members, Ala substitution for these residues did not reduce the IGF affinity of bIGFBP-2. Kinetic analysis of the bIGFBP-2 mutants on IGF biosensor chips in the BIAcore instrument revealed that Tyr-60 --> Phe bIGFBP-2 bound to the IGF-I surface 3.0-fold more slowly than bIGFBP-2 and was released 2.6-fold more rapidly than bIGFBP-2. We therefore propose that the hydroxyl group of Tyr-60 participates in a hydrogen bond that is important for the initial complex formation with IGF-I and the stabilization of this complex. In contrast, Tyr-60 --> Ala bIGFBP-2 associated with the IGF-I surface 5.0-fold more rapidly than bIGFBP-2 but exhibited an 18.4-fold more rapid release from this surface compared with bIGFBP-2. Thus both the aromatic nature and the hydrogen bonding potential of the tyrosyl side chain of Tyr-60 are important structural determinants of the IGF-binding site of bIGFBP-2.

Regulatory Actions of Insulin-like Growth Factor-binding Proteins

The six insulin-like growth factor-binding proteins (IGFBPs) are important regulators of insulin-like growth factor (IGF) action. Circulating high molecular weight complexes that contain IGF and IGFBP-3 restrict IGF bioavailability, and excess IGFBPs inhibit IGF action by forming biologically inactive complexes. IGFs can be released from these complexes by proteolysis. Potentiation of IGF activity might occur under specific circumstances, and involves the slow dissociation of IGFs from IGFBP complexes localized in the pericellular space, whose affinity has been reduced by dephosphorylation or association with the cell surface or extracellular matrix. Several IGFBPs or IGFBP fragments also have activities that do not involve IGFs or IGF receptors. The mechanisms by which IGFBPs regulate IGF action and exert their independent actions will be examined.

Overexpression of insulin-like growth factor-binding protein-4 (IGFBP-4) in smooth muscle cells of transgenic mice through a smooth muscle alpha-actin-IGFBP-4 fusion gene induces smooth muscle hypoplasia

Insulin-like growth factor I (IGF-I) has been postulated to function as a smooth muscle cell (SMC) mitogen and to play a role in the pathogenesis of bladder hypertrophy, estrogen-induced uterine growth, and restenosis after arterial angioplasty. IGF-binding protein-4 (IGFBP-4) inhibits IGF-I action in vitro and is the most abundant IGFBP in the rodent arterial wall. To explore the function of this binding protein in vivo, transgenic mouse lines were developed harboring fusion genes consisting of a rat IGFBP-4 complementary DNA cloned downstream of either a -724 bp fragment of the mouse smooth muscle alpha-actin 5'-flanking region (SMP2-BP-4) or -1074 bp, 63 bp of 5'-untranslated region, and 2.5 kb of intron 1 of smooth muscle alpha-actin (SMP8-BP-4). SMP2-BP-4 mice expressed low levels of the exogenous IGFBP-4 messenger RNA (mRNA), which was not specifically targeted to SMC-rich tissue environments, and were therefore not analyzed further. Six SMP8-BP-4 transgenic lines derived from separate founders were characterized. Mating of hemizygous SMP8-BP-4 mice with controls produced about 50% transgenic offspring, with equal sex distribution. Expression of IGFBP-4 mRNA in nontransgenic littermates was maximal in liver and kidney. By contrast, transgenic IGFBP-4 mRNA expression, distinguished because of a smaller transcript size, was confined to SMC-containing tissues, with the following hierarchy: bladder > aorta > stomach = uterus. There was no transgene expression in skeletal muscle, brain, or cardiac myocytes. The abundance of IGFBP-4 measured by Western ligand blotting or by immunoblotting, was 8- to 10-fold higher in aorta and bladder of SMP8-BP-4 mice than in their nontransgenic littermates, with no change in plasma IGFBP-4 levels. Transgenic mice exhibited a significant reduction in wet weight of SMC-rich tissues, including bladder, intestine, aorta, uterus, and stomach, with no change in total body or carcass weight. In situ hybridization showed that transgene expression was targeted exclusively to the muscular layers of the arteries, veins, bladder, ureter, stomach, intestine, and uterus. Overexpression of IGFBP-4 was associated with SMC hypoplasia, a reciprocal phenotype to that of transgenic mice overexpressing IGF-I under control of the same promoter (SMP8-IGF-I). Double transgenic mice derived from mating SMP8-BP-4 with SMP8-IGF-I animals showed a modest decrease in wet weight at selected SMC tissues. Although we cannot exclude that the effects of IGFBP-4 may be IGF independent, these data suggest that IGFBP-4 is a functional antagonist of IGF-I action on SMC in vivo.

Protease-resistant form of insulin-like growth factor-binding protein 5 is an inhibitor of insulin-like growth factor-I actions on porcine smooth muscle cells in culture

IGFs are pleiotrophic mitogens for porcine smooth muscle cells (pSMC) in culture. The effects of IGFs on cells are modulated by various insulin-like growth factor-binding proteins (IGFBP). IGFBP-5 is synthesized by pSMC and binds to the extracellular matrix. However, IGFBP-5 is also secreted into conditioned medium of cultured cells and is cleaved into fragments by a concomitantly produced protease. These fragments have reduced affinity for the IGFs and cleavage makes it difficult to assess the role of intact IGFBP-5. To study the consequence of accumulation of intact IGFBP-5 in medium, we determined the cleavage site in IGFBP-5 and prepared a protease resistant mutant. Amino acid sequencing of purified IGFBP-5 fragments suggested Arg138-Arg139 as the primary cleavage site. Arg138-Arg139-->Asn138-Asn139 mutations were introduced to create protease-resistant IGFBP-5, which has the same affinity for IGF-I as the native protein. This mutant IGFBP-5 remained intact even after 24 h of incubation and it inhibited several IGF-I actions when added to pSMC culture medium. The mutant IGFBP-5 (500 ng/ml) decreased IGF-I stimulated cellular DNA synthesis by 84%, protein synthesis by 77%, and it inhibited IGF-I stimulated migration of pSMC by 77%. It also inhibited IGF-I stimulation of IRS-1 phosphorylation. In contrast, the same amount of native IGFBP-5 did not inhibit IGF-I actions. The significance of inhibitory effects of the protease resistant IGFBP-5 was further demonstrated in pSMC transfected with mutant or native IGFBP-5 cDNAs. The mutant IGFBP-5 accumulated in culture medium of transfected cells, while native IGFBP-5 was degraded into fragments, PSMC overexpressing the mutant IGFBP-5 also responded poorly to IGF-I compared with mock transfected cells. IGF-I (5 ng/ml) increased [35S]methionine incorporation into control cells by 36% above the basal level, but it did not significantly change (4%) in pSMC cultures that were producing the mutant IGFBP-5. In conclusion, the accumulation of protease-resistant IGFBP-5 in the medium was inhibitory to IGF-I actions on pSMC. This suggests that proteolysis can prevent IGFBP-5 from acting as an inhibitor of IGF-I-stimulated effects and that it serves as an important mechanism for regulating cellular responsiveness to IGF-I.

Insulin-like growth factor (IGF)-binding protein-5-(201-218) region regulates hydroxyapatite and IGF-I binding

Insulin-like growth factor-binding protein-5 (IGFBP-5), the major bone IGFBP, modifies the biological activity of IGFs within the osteoblastic pericellular environment. Because glycosaminoglycans modulate IGFBP-5 binding to osteoblast organic extracellular matrix (ECM), we assessed whether the heparin binding domain of IGFBP-5, IGFBP-5-(102-218), modifies the interaction of IGFBP-5 with the inorganic bone ECM hydroxyapatite (HA). Synthetic IGFBP-5-(201-218) peptide increased the binding of IGFBP-5 to HA as well as the binding of IGF-I to HA-bound IGFBP-5. This action was specific for the heparin-binding domain, because IGFBP-5-(130-138), IGFBP-5-(138-152), and IGFBP-5-(1-169) were without effect. IGFBP-5-(201-218) was found to bind directly to IGFBP-5 and cause a threefold enhancement of the IGF-I binding affinity for IGFBP-5, whether IGFBP-5 was bound to HA or was in a matrix-free fluid phase. Heparin inhibited the binding of IGFBP-5 to HA and blocked the interaction of IGFBP-5 with IGFBP-5-(201-218) in the fluid phase, suggesting that the primary heparin-binding domain of IGFBP-5 specifically enhances the binding of IGFBP-5 to HA and increases IGF-I binding to IGFBP-5.

Plasmin degradation of insulin-like growth factor-binding protein-5 (IGFBP-5): regulation by IGFBP-5-(201-218)

Using the major bone insulin-like growth factor-binding protein (IGFBP) IGFBP-5, we took a mechanistic approach in evaluating the role of the heparin-binding domain of IGFBP-5 in regulating plasmin (Pm) proteolysis of IGFBP-5. Using synthetic IGFBP-5 peptide fragments, we determined that the heparin-binding domain, IGFBP-5-(208-218), inhibits Pm proteolysis of intact IGFBP-5. The mechanism of action of IGFBP-5-(201-218) was by inhibiting Pm binding to substrate IGFBP-5. IGFBP-5-(201-218) action was independent of site of proteolysis, fluid, or solid phase interaction. In addition, IGFBP-5-(201-218) was found to inhibit plasminogen (Pg) activation to Pm IGFBP-5-(201-218) did not directly inhibit the activity of Pm, urokinase Pg activator (PA), or tissue-type PA but acted as a competitive inhibitor of Pg activation by PA, which is in contrast to the stimulating effect of heparin on Pg activation. These data indicate that the heparin-binding domain contains the serine protease (Pg-to-Pm) binding site region of IGFBP-5, and that this region, which is presumed to represent a Pm-induced proteolytic product of IGFBP-5, is capable of regulating Pm action.

Insulin-like growth factor binding protein-2 binds to cell surface proteoglycans in the rat brain olfactory bulb

A family of six insulin-like growth factor binding proteins (IGFBPs) bind IGF-I and modulate its biological activity. IGFBPs may bind to macromolecules on the cell surface or pericellular extracellular matrix, and this interaction may modulate their effect on IGF activity. To date, little is known about the specificity of IGFBPs in the regulation of IGF action in the brain. We therefore explored whether IGFBPs were associated with cell membrane or extracellular matrix components in the rat brain. IGF-I binding sites with the characteristics of an IGFBP were found in the olfactory bulb mitral cell layer. This IGFBP was identified as IGFBP-2 by immunoprecipitation of both solubilized membrane preparations and cross-linked 125I-IGF: IGFBP complexes. While binding of IGFBP-2 to cell membranes was unaffected by RGD-containing peptide, it was inhibited by high salt concentration, suggesting interaction with proteoglycans. IGFBP-2 bound in vitro to the glycosaminoglycans chondroitin-4 and -6-sulfate, keratan sulfate, and heparin. IGFBP-2 also bound the proteoglycan aggrecan, an effect reduced by digestion of its glycosaminoglycans. Binding of IGFBP-2 to chondroitin-6-sulfate decreased the binding affinity of IGFBP-2 for IGF-I approximately 3-fold. Finally, an IGFBP-2 antibody coimmunoprecipitated IGFBP-2 and an approximately 200 kDa proteoglycan containing chondroitin-sulfate side chains from the rat olfactory bulb, providing definitive evidence for IGFBP-2 binding to olfactory bulb proteoglycans. These findings indicate that IGFBP-2 binds to proteoglycans in cell membranes of the rat olfactory bulb. Because we have previously shown that IGFs are highly expressed in the rat olfactory bulb, cell associated IGFBP-2 may have an important role in directing IGFs to specific sites in this brain region.

Insulin-like growth factor binding protein-5 binds to plasminogen activator inhibitor-I

Insulin-like growth factor binding protein-5 (IGFBP-5) has been shown to bind to the extracellular matrix (ECM) of both fibroblasts and smooth muscle cells. The ECM-IGFBP-5 interaction is mediated in part by binding to heparan sulfate containing proteoglycans. Because proteoglycans may not be the only components of ECM that bind to IGFBP-5, we have determined its ability to bind to other ECM proteins. When a partially purified mixture of the proteins that were present in fibroblast conditioned medium was purified by IGFBP-5 affinity chromatography, a 55-kDa protein was eluted. Amino acid sequencing of the amino terminal 28 amino acids showed that it was human plasminogen activator inhibitor-1 (PAI-1). To determine if this interaction was specific, purified human PAI-1 was incubated with IGFBP-5 and the IGFBP-5/PAI-1 complex immunoprecipitated with anti-PAI-1 antiserum. When the precipitate was analyzed by immunoblotting using anti-IGFBP-5 antiserum, the intensity of the IGFBP-5 band was substantially increased compared with controls that did not contain human PAI-1. A synthetic IGFBP-5 peptide that contained the amino acid sequence between positions 201 and 218 inhibited IGFBP-5/PAI-1 interaction. Coincubation of IGFBP-5 mutants that contained substitutions for specific basic residues located between positions 201 and 218 with PAI-1 indicated that some of these amino acids were important for binding. Two mutants that contained neutral substitutions for specific basic amino acids within the glycosaminoglycan binding domain had reduced binding to PAI-1. In contrast, three other mutants that also had substitutions for charged residues in the same region had no reduction in binding. Heparin and heparan sulfate inhibited the IGFBP-5/PAI-1 interaction; however, several other glycosaminoglycans had no effect. PAI-1 was determined to be an important ECM component for binding because approximately 27% of total ECM binding could be inhibited with anti-PAI-1 antiserum. Competitive binding studies with unlabeled IGFBP-5 showed that the dissociation constant of PAI-1 for IGFBP-5 was 9.1 x 10(-8) M. In summary, IGFBP-5 binds specifically to plasminogen activator inhibitor-1. Because this is present in the extracellular matrix of several cell types, it may be one of the important binding components of ECM. PAI-1 binding partially protects IGFBP-5 from proteolysis, suggesting that it is one of the ECM components that is involved in mediating this effect.