Carboxyl-terminal structure of basic fibroblast growth factor significantly contributes to its affinity for heparin

Anti-tumor and anti-metastatic activity of the FGF2 118-126 fragment dependent on the loop structure

Fibroblast Growth Factor /FGF Receptor 1 (FGF2/FGFR1) system regulates the growth and metastasis of different cancers. Inhibition of this signaling pathway is an attractive target for cancer therapy. Here, we aimed to reproduce the 118-126 fragment of FGF2 to interfere with the FGF2-FGFR1 interaction. To determine whether the loop structure affects the function of this fragment, we compared cyclic (disulfide-bonded) and linear peptide variants. The cyclic peptide (referred to as BGF1) effectively inhibited the FGF2-induced proliferation of HUVECs, 4T1 mammary carcinoma, U87 glioblastoma, and SKOV3 ovarian carcinoma cells. It led to apoptosis induction in HUVECs, whereas the linear peptide (referred to as BGF2) was ineffective. In a murine 4T1 tumor model, BGF1 inhibited tumor growth more effectively than Avastin and increased animals' survival without causing weight loss, but the linear peptide BGF2 had no significant anti-tumor effects. According to immunohistochemical studies, the anti-tumor properties of BGF1 were associated with suppression of tumor cell proliferation (Ki-67 expression), angiogenesis (CD31 expression), and apoptosis induction (as was shown by increased p53 expression and TUNEL staining and decreased Bcl-2 expression). The potential of BGF1 to suppress tumor invasion was indicated by quantitative analysis of the metastasis-related proteins, including FGFR1, pFGFR1, NF-κB, p-NF-κB, MMP-9, E-cadherin, N-cadherin, and Vimentin, and supported by small animal positron emission tomography (PET) used 18Fluorodeoxyglucose (18F-FDG). These results demonstrate that the functional properties of the 118-126 region of FGF2 depend on the loop structure and the peptide derived from this fragment encourages further preclinical investigations.

Coreceptor Functions of Cell Surface Heparan Sulfate Proteoglycans

Receptor-ligand interactions play an important role in many biological processes by triggering specific cellular responses. These interactions are frequently regulated by coreceptors that facilitate, alter, or inhibit signaling. Coreceptors work in parallel with other specific and accessory molecules to coordinate receptor-ligand interactions. Cell surface heparan sulfate proteoglycans (HSPGs) function as unique coreceptors because they can bind to many ligands and receptors through their HS and core protein motifs. Cell surface HSPGs are typically expressed in abundance of the signaling receptors and, thus, are capable of mediating the initial binding of ligands to the cell surface. HSPG coreceptors do not possess kinase domains or intrinsic enzyme activities and, for the most part, binding to cell surface HSPGs does not directly stimulate intracellular signaling. Because of these features, cell surface HSPGs primarily function as coreceptors for many receptor-ligand interactions. Given that cell surface HSPGs are widely conserved, they likely serve fundamental functions to preserve basic physiological processes. Indeed, cell surface HSPGs can support specific cellular interactions with growth factors, morphogens, chemokines, extracellular matrix (ECM) components, and microbial pathogens and their secreted virulence factors. Through these interactions, HSPG coreceptors regulate cell adhesion, proliferation, migration and differentiation, and impact the onset, progression, and outcome of pathophysiological processes, such as development, tissue repair, inflammation, infection, and tumorigenesis. This review seeks to provide an overview of the various mechanisms of how cell surface HSPGs function as coreceptors.

Heparan sulfate in angiogenesis: a target for therapy

Heparan sulfate (HS), a long linear polysaccharide of alternating disaccharide residues, interacts with a wide variety of proteins, including many angiogenic factors. The involvement of HS in signaling of pro-angiogenic factors (e.g. vascular endothelial growth factor and fibroblast growth factor 2), as well as interaction with anti-angiogenic factors (e.g. endostatin), warrants its role as an important modifier of (tumor) angiogenesis. This review summarizes our current understanding of the role of HS in angiogenic growth factor signaling, and discusses therapeutic strategies to target HS and modulate angiogenesis.

Diversification of the structural determinants of fibroblast growth factor-heparin interactions: implications for binding specificity

The functions of a large number (>435) of extracellular regulatory proteins are controlled by their interactions with heparan sulfate (HS). In the case of fibroblast growth factors (FGFs), HS binding determines their transport between cells and is required for the assembly of high affinity signaling complexes with their cognate FGF receptor. However, the specificity of the interaction of FGFs with HS is still debated. Here, we use a panel of FGFs (FGF-1, FGF-2, FGF-7, FGF-9, FGF-18, and FGF-21) spanning five FGF subfamilies to probe their specificities for HS at different levels as follows: binding parameters, identification of heparin-binding sites (HBSs) in the FGFs, changes in their secondary structure caused by heparin binding and structures in the sugar required for binding. For interaction with heparin, the FGFs exhibit K(D) values varying between 38 nM (FGF-18) and 620 nM (FGF-9) and association rate constants spanning over 20-fold (FGF-1, 2,900,000 M(-1) s(-1) and FGF-9, 130,000 M(-1) s(-1)). The canonical HBS in FGF-1, FGF-2, FGF-7, FGF-9, and FGF-18 differs in its size, and these FGFs have a different complement of secondary HBS, ranging from none (FGF-9) to two (FGF-1). Differential scanning fluorimetry identified clear preferences in these FGFs for distinct structural features in the polysaccharide. These data suggest that the differences in heparin-binding sites in both the protein and the sugar are greatest between subfamilies and may be more restricted within a FGF subfamily in accord with the known conservation of function within FGF subfamilies.

Direct and allosteric inhibition of the FGF2/HSPGs/FGFR1 ternary complex formation by an antiangiogenic, thrombospondin-1-mimic small molecule

Fibroblast growth factors (FGFs) are recognized targets for the development of therapies against angiogenesis-driven diseases, including cancer. The formation of a ternary complex with the transmembrane tyrosine kinase receptors (FGFRs), and heparan sulphate proteoglycans (HSPGs) is required for FGF2 pro-angiogenic activity. Here by using a combination of techniques including Nuclear Magnetic Resonance, Molecular Dynamics, Surface Plasmon Resonance and cell-based binding assays we clarify the molecular mechanism of inhibition of an angiostatic small molecule, sm27, mimicking the endogenous inhibitor of angiogenesis, thrombospondin-1. NMR and MD data demonstrate that sm27 engages the heparin-binding site of FGF2 and induces long-range dynamics perturbations along FGF2/FGFR1 interface regions. The functional consequence of the inhibitor binding is an impaired FGF2 interaction with both its receptors, as demonstrated by SPR and cell-based binding assays. We propose that sm27 antiangiogenic activity is based on a twofold-direct and allosteric-mechanism, inhibiting FGF2 binding to both its receptors.

Molecular level interaction of the human acidic fibroblast growth factor with the antiangiogenic agent, inositol hexaphosphate

Acidic fibroblast growth factor (FGF1) regulates a wide array of important biological phenomena such as angiogenesis, cell differentiation, tumor growth, and neurogenesis. Generally, FGFs are known for their strong affinity for the glycosaminoglycan heparin, as a prerequisite for recognition of a specific tyrosine kinase on the cell surface and are responsible for the cell signal transduction cascade. Inositol hexaphosphate (IP6) is a natural antioxidant and is known for its antiangiogenic role, in addition to its ability to control tumor growth. In the present study, we investigated the interaction of IP6 with the acidic fibroblast growth factor (FGF1) using various biophysical techniques including isothermal calorimetry, circular dichroism, and multidimensional NMR spectroscopy. Herein, we have reported the three-dimensional solution structure of the FGF1-IP6 complex. These data show that IP6 binds FGF1 and enhances its thermal stability. In addition, we also demonstrate that IP6 acts as an antagonist to acidic fibroblast growth factor by inhibiting its receptor binding and subsequently decreasing the mitogenic activity. The inhibition likely results in the ability of IP6 to antagonize the angiogenic and mitogenic activity of FGF1.

Identification of heparin-binding sites in proteins by selective labeling

Heparan sulfate proteoglycans are key regulators of complex molecular networks due to the interaction of their sugar chains with a large number of partner proteins, which in humans number more than 200 (Ori, A., Wilkinson, M. C., and Fernig, D. G. (2008) The heparanome and regulation of cell function: structures, functions and challenges. Front. Biosci. 13, 4309-4338). We developed a method to selectively label residues involved in heparin binding that matches the requirements for medium/high throughput applications called the "Protect and Label" strategy. This is based on the protection against chemical modification given by heparin/heparan sulfate to the residues located in the heparin-binding site. Thus, analysis of fibroblast growth factor-2 bound to heparin and incubated with N-hydroxysuccinimide acetate showed that lysines involved in the sugar binding are protected against chemical modification. Moreover following release from heparin, the protected lysine side chains may be specifically labeled with N-hydroxysuccinimide biotin. After protein digestion, the biotinylated peptides were readily isolated and identified by MALDI-Q-TOF mass spectrometry. The analysis of labeled peptides obtained from three well characterized heparin-binding proteins with very different heparin-binding sites, fibroblast growth factor-2, platelet factor-4, and pleiotrophin demonstrates the success of this new approach, which thus provides a rapid and reliable procedure to identify heparin-binding sites.

Structural basis for fibroblast growth factor receptor activation

FGF signaling plays a ubiquitous role in human biology as a regulator of embryonic development, homeostasis and regenerative processes. In addition, aberrant FGF signaling leads to diverse human pathologies including skeletal, olfactory, and metabolic disorders as well as cancer. FGFs execute their pleiotropic biological actions by binding, dimerizing and activating cell surface FGF receptors (FGFRs). Proper regulation of FGF-FGFR binding specificity is essential for the regulation of FGF signaling and is achieved through primary sequence variations among the 18 FGFs and seven FGFRs. The severity of human skeletal syndromes arising from mutations that violate FGF-FGFR specificity is a testament to the importance of maintaining precision in FGF-FGFR specificity. The discovery that heparin/heparan sulfate (HS) proteoglycans are required for FGF signaling led to numerous models for FGFR dimerization and heralded one of the most controversial issues in FGF signaling. Recent crystallographic analyses have led to two fundamentally different models for FGFR dimerization. These models differ in both the stoichiometry and minimal length of heparin required for dimerization, the quaternary arrangement of FGF, FGFR and heparin in the dimer, and in the mechanism of 1:1 FGF-FGFR recognition and specificity. In this review, we provide an overview of recent structural and biochemical studies used to differentiate between the two crystallographic models. Interestingly, the structural and biophysical analyses of naturally occurring pathogenic FGFR mutations have provided the most compelling and unbiased evidences for the correct mechanisms for FGF-FGFR dimerization and binding specificity. The structural analyses of different FGF-FGFR complexes have also shed light on the intricate mechanisms determining FGF-FGFR binding specificity and promiscuity and also provide a plausible explanation for the molecular basis of a large number craniosynostosis mutations.

Electrostatic Contributions to Protein Retention in Ion-Exchange Chromatography. 2. Proteins with Various Degrees of Structural Differences

The relation of protein structure to retention provides a framework within which to investigate chromatographic adsorption mechanisms. Protein sets with varying degrees of structural differences were studied to relate variations in protein properties to retention behavior. To explore molecular contributions to protein adsorption in ion-exchange chromatography, protein-adsorbent electrostatic interactions were modeled using a continuum approach. The calculations qualitatively capture the chromatographic differentiation of closely related subtilisin variants. Descriptions of the electrostatic interactions of FGF-1 vs FGF-2 with cation exchangers were obtained, and aid in rationalizing differences in experimental retention trends across a set of adsorbents based on different adsorption mechanisms linked to the adsorbent structure. Comparative calculations for proteins with differences in local or overall arginine-lysine composition, including subtilisin variants G166R/G166K and lysozyme/cytochrome c, suggest that continuum electrostatics is not adequate to capture the full quantitative characteristics of the chromatographic retention of proteins. To allow more accurate description of retention, additional molecular interactions, specifically hydration effects, must be incorporated in the model.

Insertional-fusion of basic fibroblast growth factor endowed ribonuclease 1 with enhanced cytotoxicity by steric blockade of inhibitor interaction

Basic fibroblast growth factor (bFGF) was inserted in the middle of human ribonuclease 1 (RNase1) sequence at an RNase inhibitor (RI)-binding site (Gly89) by a new gene fusion technique, insertional-fusion. The resultant insertional-fusion protein (CL-RFN89) was active both as bFGF and as RNase. Furthermore, it acquired an additional ability of evading RI through steric blockade of RI-binding caused by fused bFGF domain. As a result, CL-RFN89 showed stronger growth inhibition on B16/BL6 melanoma cells than an RI-sensitive tandem fusion protein. Thus, the insertional-fusion technique increases accessible positions for gene fusion on RNase, resulting in construction of a potent cytotoxic RNase.

Effect of amino-acid substitutions on Alzheimer's amyloid-beta peptide-glycosaminoglycan interactions

One of the major clinical features of Alzheimer's disease is the presence of extracellular amyloid plaques that are associated with glycosaminoglycan-containing proteoglycans. It has been proposed that proteoglycans and glycosaminoglycans facilitate amyloid fibril formation and/or stabilize these aggregates. Characterization of proteoglycan-protein interactions has suggested that basic amino acids in a specific conformation are necessary for glycosaminoglycan binding. Amyloid-beta peptide (Abeta) has a cluster of basic amino acids at the N-terminus (residues 13-16, His-His-Gln-Lys), which are considered critical for glycosaminoglycan interactions. To understand the molecular recognition of glycosaminoglycans by Abeta, we have examined a series of synthetic peptides with systematic alanine substitutions. These include: His13-->Ala, His14-->Ala, Lys16-->Ala, His13His14Lys16-->Ala and Arg5His6-->Ala. Alanine substitutions result in differences in both the secondary and fibrous structure of Abeta1-28 as determined by circular dichroism spectroscopy and electron microscopy. The results demonstrate that the His-His-Gln-Lys region of Abeta, and in particular His13, is an important structural domain, as Ala substitution produces a dysfunctional folding mutant. Interaction of the substituted peptides with heparin and chondroitin sulfate glycosaminoglycans demonstrate that although electrostatic interactions contribute to binding, nonionic interactions such as hydrogen bonding and van der Waals packing play a role in glycosaminoglycan-induced Abeta folding and aggregation.

Inhibition of cell growth by a fused protein of human ribonuclease 1 and human basic fibroblast growth factor

Pancreatic-type RNases are considered to have cytotoxic potential due to their ability to degrade RNA molecules when they enter the cytosol. However, most of these RNases show little cytotoxicity because cells have no active uptake mechanism for these RNases and because the ubiquitous cytoplasmic RNase inhibitor is considered to play a protective role against the endocytotic leak of RNases from the outside of cells. To study the cytotoxic potential of RNase toward malignant cells targeting growth factor receptors, the C-terminus of human RNase 1 was fused to the N-terminus of human basic fibroblast growth factor (bFGF). This RNase-FGF fused protein effectively inhibited the growth of mouse melanoma cell line B16/BL6 with high levels of cell surface FGF receptor. This effect appeared to result from prolongation of the overall cell cycle rather than the killing of cells or specific arrest in a particular phase of the cell cycle. Thus, human RNase 1 fused to a ligand of cell surface molecules, such as the FGF receptor, is shown to be an effective candidate for a selective cell targeting agent with low toxic effects on normal cell types.

Anabolic effect of aminoterminally truncated fibroblast growth factor 4 (FGF4) on bone

Fibroblast growth factor 4 (FGF4), a member of the FGF family, plays several important roles in bone development during embryogenesis. Systemic administration of FGF4 increases bone mass in rats, which suggests the potential therapeutic usefulness of this growth factor in treatment for osteopenia and in bone regeneration. We investigated the length of FGF4 required to exert its anabolic effects, because this information may be useful in developing new molecules to mimic the effects of FGF4. Because the active site of FGF family molecules is in the carboxylterminal region, we produced aminoterminally truncated recombinant human FGF4s (rhFGF4s) of different sizes. Human FGF4 cDNA containing almost the full length of the coding region (573 bp, 191 amino acid residues) was inserted into pUC18 vector and then deleted from the 5' end using the ExoIII system. Each of the deleted FGF4 cDNAs was subcloned into a pET29(+) expression vector. Differently sized recombinant proteins were expressed in the BL21(DE3)pLysS Escherichia coli strain and then purified. The growth-stimulative effects on NIH3T3 cells of each recombinant protein were examined by means of MTT colorimetric assay. Full-length and the shortened recombinant proteins, which stimulated NIH3T3 cell growth, were then subcutaneously administered into male ddY mice (6 weeks old) every day for 2 weeks. Bone mineral density (BMD) was measured using dual-energy X-ray absorptiometry (DEXA) and peripheral quantitative computed tomography (pQCT). The rhFGF4 of 134 amino acid residues, the region homologous to other members of the FGF family, exerted a growth-stimulative effect on NIH3T3 cells comparable to the full-length version of FGF4; however, the shortest version, with 111 amino acid residues, showed a limited growth-stimulative effect. Systemic administration of the rhFGF4 of 134 amino acid residues increased the bone mineral density (BMD) of femurs at a dose of 0.1 mg/kg, which was comparable to that of the full-length rhFGF4. DEXA analysis, pQCT analysis, soft X-ray photos, and contact microradiographs revealed an increase in femoral trabecular bone in FGF4-treated animals; an increase in bone formation was also evident upon histomorphometric analysis. These results indicate that the region of FGF4 that is homologous to other FGF family members provides a sufficient anabolic effect in bone and that this recombinant protein is potentially useful as a therapeutic agent in bone.

The C-terminal region of fibroblast growth factor-1 is crucial for its biological activity and high level protein expression in mammalian cells

We have studied the role of the carboxyl(C)-terminus of fibroblast growth factor(FGF)-1 using prokaryotic and eukaryotic expression systems. The full-length FGF-1 protein and its mutants lacking 6- and 9-amino acids at the C-terminus IFGF-1 (Cdel6) and FGF-1 (Cdel9), respectively] could be expressed in E. coli cells at the similar levels. The deletion mutants bound very weakly to FGF receptor and to heparin, and did not stimulate DNA synthesis in BALB/c3T3 cells. In contrast to E. coli cells, in NIH3T3 transfectants and L6 transfectants, the protein expression level of FGF-1 (Cdel6) was significantly lower than that of FGF-1, and longer C-terminal deletions further decreased the protein expression levels. However, the level of transcripts in the transfectants and the level of translates in in vitro system were equivalent for all the FGF-1 constructs. Treatment with proteasome inhibitors of the NIH3T3 transfectants expressing FGF-1(Cdel6) increased the protein level six-fold. The results indicate that the C-terminus of FGF-I is crucial for its biological activity and high-level expression in mammalian cells and suggest that deletion of the C-terminus of FGF-1 induces its post-translational degradation by proteasome system.

Fibroblast growth factors and their receptors

Fibroblast growth factors (FGFs) represent a group of polypeptide mitogens eliciting a wide variety of responses depending upon the target cell type. The knowledge of the cell surface receptors mediating the effects of FGFs has recently expanded remarkably. The complexity of the FGF family and the FGF-induced responses is reflected in the diversity and redundancy of the FGF receptors. In this review, a number of biochemical characteristics and biological properties of the FGF family and its receptors are described and their expression both in normal tissues and in tumours is discussed. Finally we speculate on the targetting of growth inhibition agents to tumours through FGF receptors.

Interactions of putative heparin-binding domains of basic fibroblast growth factor and its receptor, FGFR-1, with heparin using synthetic peptides

We have examined structure-function relationships that have been proposed to account for the heparin-binding properties of basic fibroblast growth factor and its receptor, FGFR-1, using synthetic peptides, DNA synthesis assays and binding assays in a resonant mirror biosensor. The results suggest that the interaction of FGFR-1 with heparin may not be physiologically relevant and that the site of interaction of the polysaccharide on bFGF is more complex than has been anticipated.

Expression and analysis of heparin-binding regions of the amyloid precursor protein of Alzheimer's disease

Deletion mutagenesis studies have suggested that there are two domains within APP which bind heparan sulphate. These domains have been cloned and expressed in the yeast Pichia pastoris. Both recombinant proteins bound to heparin. One domain (APP316-447) was further characterised by binding studies with peptides encompassing this region. Peptides homologous to APP316-346 and APP416-447 were found to bind heparin. Circular dichroism studies show that APP416-447 shifted towards an alpha-helical conformation in the presence of heparin. This study suggests that heparin-binding domains may lie within regions high in alpha-helical structure.

A 10-amino acid sequence of fibroblast growth factor 2 is sufficient for its mitogenic activity on neural progenitor cells

During development of the central nervous system, neurons and glia are generated from immature neural progenitor cells (NPCs). Basic fibroblast growth factor (FGF-2) is a mitogen for these cells both in vitro and in vivo. However, it is not known whether other members of the FGF family have similar mitogenic effects on NPCs. We have found that FGF-4, in addition to FGF-2, is a mitogen for NPCs isolated from fetal and adult central nervous systems. Other family members have no proliferative effects on these cells. FGFs transduce signals to the cytoplasm through a family of transmembrane tyrosine kinase receptors (FGFR-1-4) or their isoforms. The high-affinity receptor binding sites are found in two regions of the FGF-2 molecule. We have examined the involvement of these sites in mitogenic signaling. Synthetic peptides corresponding to sequences in FGF-2 receptor binding sites were examined in [3H]thymidine incorporation assays for their agonist or antagonist activity. A 10-aa sequence present in the first receptor binding domain has been found to act as an antagonist, blocking the mitogenic effects of FGF-2. Chemical crosslinking studies using 125I-labeled FGF-2 showed specific reduction in binding of radiolabeled FGF-2 to its receptors present on the membranes of NPCs. The identification of this sequence will assist in the study of pathways involved in signal transduction for mitogenesis in these cells and elucidate the role of FGF-2 and FGF-4 during normal development and in the pathogenesis of disease.

Lectin-like characteristics of recombinant human interleukin-1beta recognizing glycans of the glycosylphosphatidylinositol anchor

We found that 35S-labeled recombinant human interleukin-1beta (rhIL-1beta) binds phosphatidylinositol-specific phospholipase C-treated human placental alkaline phosphatase, phosphatidylinositol-specific phospholipase C-treated trypanosome surface variant glycoproteins, and urinary uromodulin immobilized on plates or immobilized on CNBr-activated Sepharose 4B. The interaction between rhIL-1beta and these glycoproteins was lectin-like, since it was inhibited in the presence of specific saccharides, i.e. mannose 6-phosphate or synthetic Ac-NH.CH2.CH2. PO4--->6Manalpha1-->(+/-2Manalpha1-->+/-6Manalpha1-->) propyl at about 1 microM. On the other hand, a wide variety of compounds including biantennary sugar chains derived from these glycoproteins as well as ethanolamine phosphate, inositol phosphate, mannose 6-sulfate, mannose 1-phosphate, glucose 6-phosphate, and mannitol 6-phosphate did not show any inhibitory effect at concentrations up to 1 mM. These results indicate that rhIL-1beta interacts with these glycoproteins via the mannose 6-phosphate diester of glycans on the glycosylphosphatidylinositol (GPI) anchor. Furthermore, when monolayers of polarized Madin-Darby canine kidney cells on polycarbonate filter membranes were incubated with 35S-rhIL-1beta in either the apical or basolateral chamber, 35S-interleukin-1beta was found to bind specifically to the apical membranes with a Ka value of 4.6 x 10(7) M-1, and the specific interaction was inhibited by 1 microM mannose 6-phosphate. Since the mannose 6-phosphate diester moiety exists only in the GPI glycans on plasma membranes, it was evident that interleukin-1beta can directly interact with the mannose 6-phosphate diester component of the intact glycan of GPI anchors on plasma membranes.

Identification of binding domains for basic fibroblast growth factor in proteoglycan macrophage colony-stimulating factor

We recently demonstrated that proteoglycan macrophage colony-stimulating factor (PG-M-CSF) binds basic fibroblast growth factor (bFGF) and neutralizes the biological activity of bFGF. In this study, we identified the binding sites of PG-M-CSF for bFGF. We examined the binding of bFGF to overlapping 12-mer peptides with the sequence of the putative binding region. High affinity binding was detected at two peaks; one consisted of the three adjacent peptides, 212-223, 213-224 and 214-225 and the other, of the three adjacent peptides, 246-257, 247-258 and 248-259. The synthetic peptide (212VDPGSAKQRPPRST225) did not inhibit bFGF binding to another peptide (246PQPRPSVGAFNPGM259), and vice versa. However, both peptides inhibited the bFGF-induced but not platelet-derived growth factor-induced stimulation of DNA synthesis in murine Balb/c 3T3 cells.

Stimulation of fibroblast growth factor receptor-1 occupancy and signaling by cell surface-associated syndecans and glypican

The formation of distinctive basic FGF-heparan sulfate complexes is essential for the binding of bFGF to its cognate receptor. In previous experiments, cell-surface heparan sulfate proteoglycans extracted from human lung fibroblasts could not be shown to promote high affinity binding of bFGF when added to heparan sulfate-deficient cells that express FGF receptor-1 (FGFR1) (Aviezer, D., D. Hecht, M. Safran, M. Eisinger, G. David, and A. Yayon. 1994. Cell 79:1005-1013). In alternative tests to establish whether cell-surface proteoglycans can support the formation of the required complexes, K562 cells were first transfected with the IIIc splice variant of FGFR1 and then transfected with constructs coding for either syndecan-1, syndecan-2, syndecan-4 or glypican, or with an antisense syndecan-4 construct. Cells cotransfected with receptor and proteoglycan showed a two- to three- fold increase in neutral salt-resistant specific 125I-bFGF binding in comparison to cells transfected with only receptor or cells cotransfected with receptor and anti-syndecan-4. Exogenous heparin enhanced the specific binding and affinity cross-linking of 125I-bFGF to FGFR1 in receptor transfectants that were not cotransfected with proteoglycan, but had no effect on this binding and decreased the yield of bFGFR cross-links in cells that were cotransfected with proteoglycan. Receptor-transfectant cells showed a decrease in glycophorin A expression when exposed to bFGF. This suppression was dose-dependent and obtained at significantly lower concentrations of bFGF in proteoglycan-cotransfected cells. Finally, complementary cell-free binding assays indicated that the affinity of 125I-bFGF for an immobilized FGFR1 ectodomain was increased threefold when the syndecan-4 ectodomain was coimmobilized with receptor. Equimolar amounts of soluble syndecan-4 ectodomain, in contrast, had no effect on this binding. We conclude that, at least in K562 cells, syndecans and glypican can support bFGF-FGFR1 interactions and signaling, and that cell-surface association may augment their effectiveness.

Interaction of angiogenic basic fibroblast growth factor with endothelial cell heparan sulfate proteoglycans. Biological implications in neovascularization

Basic fibroblast growth factor is an angiogenic molecule involved in several physiological and pathological processes, including wound repair, embryonic development, and tumor growth. In vitro, basic fibroblast growth factor induces an "angiogenic phenotype" in endothelial cells, which includes chemotaxis, mitogenesis, protease production, beta-integrin expression, and tube formation in three-dimensional gels. It acts by binding to specific tyrosine kinase receptors and to cell-associated heparan sulfate proteoglycans. The physiological significance of the interaction with cell-associated and soluble heparan sulfate proteoglycans is manyfold. Heparan sulfate proteoglycans protect basic fibroblast growth factor from inactivation in the extracellular environment and modulate its bioavailability. At the cell surface, soluble and cell-associated heparan sulfate proteoglycans may play different roles in modulating the dimerization of the growth factor and its interaction with tyrosine kinase receptors. Finally, they affect the internalization and the intracellular fate of basic fibroblast growth factor, suggesting that growth factor slash proteoglycan complexes are involved in intracellular delivery. The bioavailability and the biological activity of basic fibroblast growth factor on endothelial cells strictly depend on the glycosaminoglycan milieu of the extracellular environment. Hence the angiogenic activity of the growth factor in vivo might be modulated by using exogenous glycosaminoglycans. The capacity of glycosaminoglycans to bind to and to influence the biological activity of basic fibroblast growth factor depends on size, degree of sulfation, and disaccharide composition. In the present paper we discuss the physiological significance and the biochemical bases of the interaction of the growth factor with heparan sulfate proteoglycans and exogenous glycosaminoglycans with a view to the possible therapeutic use of heparin-related oligosaccharides as basic fibroblast growth factor agonists or antagonists in angiogenesis-dependent diseases.

Glu-96 of basic fibroblast growth factor is essential for high affinity receptor binding. Identification by structure-based site-directed mutagenesis

The importance of basic fibroblast growth factor (bFGF) in several pathophysiological processes has stimulated interest in the design of receptor antagonists to mitigate such effects. Of key importance in this connection is the characterization of the functional binding epitopes of the growth factor for its receptor. Based on peptide mapping and molecular dynamics calculations of the three-dimensional structure of basic fibroblast growth factor, we employed site-directed mutagenesis to investigate the effect of altering residues at positions 107, 109-114, and 96 on bFGF on receptor binding affinity. All muteins were cloned and expressed in Escherichia coli, purified to homogeneity employing heparin-Sepharose columns, and evaluated for receptor binding affinity. We found that replacement of residues at positions 107 and 109-114 by alanine or phenylalanine had little effect on receptor binding affinities compared with wild type bFGF, in agreement with previous evidence that bFGF residues 109-114 comprise a low affinity binding site. By contrast, substitution of Glu-96 with alanine yielded a molecule having about 0.1% of the affinity of the wild type bFGF. The affinity of the corresponding lysine and glutamine muteins was 0.3 and 10%, respectively, emphasizing the importance of a negative charge at this position. Our findings are consistent with the view that residues 106-115 on bFGF represent a low affinity binding site on bFGF. In addition, we identify Glu-96 as a crucial residue for binding to fibroblast growth factor receptor-1.

Purification of recombinant human basic fibroblast growth factor: stability of selective sorbents under cleaning in place conditions

Human basic fibroblast growth factor (bFGF) was produced from recombinant Escherichia coli by high-cell-density cultivation. In order to develop a purification strategy for large-scale purification, chromatographic sorbents with different anionic functional groups were compared in terms of selectivity for bFGF and stability under cleaning in place (CIP) conditions. Heparin-Sepharose CL-6B, Fractogel EMD-SO3- 650 (S) and SP-Sepharose (high performance) were found suitable for this purpose with decreasing selectivity in that order. Each sorbent was treated eight times under CIP conditions employing both 0.2 and 1.0 M NaOH, in order to study modifications of these sorbents. Heparin-Sepharose displayed more than 50% loss of capacity after the first CIP treatment and decreasing selectivity with each cycle. Both cation exchangers displayed almost constant results regarding selectivity and capacity. The Fractogel EMD-SO3- exhibited only slightly lower selectivity for bFGF than Heparin-Sepharose and the highest capacity of all sorbents tested. Agglomeration of bFGF at low salt concentrations was a serious problem. By direct application of pooled fractions from Fractogel EMD-SO3- onto Heparin-Sepharose a highly pure product was obtained; however, the recovery after Heparin-Sepharose was only 30%.

An amino-terminal fragment peptide of acidic fibroblast growth factor modulates synaptic transmission in rat hippocampal slices

Effects of acidic fibroblast growth factor (aFGF) fragments such as aminoterminal aFGF (1-15) and carboxyl-terminal aFGF (114-140) on synaptic transmission were investigated in rat hippocampal slices. Stimulation was applied to Schaffer collateral/commissural afferents, and evoked population spikes were recorded in the CA1 pyramidal cell layer. Continuous perfusion of slices with aFGF (1-15) slightly decreased the basal amplitude of population spikes and significantly increased the paired-pulse facilitation. When brief tetanic stimulation (7 pulses at 100 Hz) was applied 30 min after the perfusion of aFGF (1-15), aFGF (1-15)-treated slices enhanced the magnitude of short-term potentiation after the tetanus and facilitated a generation of long-term potentiation. These effects of aFGF (1-15) were dose-dependent. Perfusion of slices with aFGF (114-140) had no effect on the basal spike amplitude, paired-pulse facilitation, and short-term potentiation. Both aFGF (1-15) and aFGF (114-140) had no effect on the DNA synthesis-stimulating activity in BALB/c 3T3-L1 cells. The results suggest that aFGF (1-15), which is not involved in mitogenic activity, is implicated in a modulatory mechanism of synaptic plasticity.

Tumor cells secrete an angiogenic factor that stimulates basic fibroblast growth factor and urokinase expression in vascular endothelial cells

Culture medium conditioned by human SK-Hep1 hepatoma cells or mouse S180 sarcoma cells rapidly up-regulates endothelial cell expression of basic fibroblast growth factor (bFGF) and induces formation of capillary-like structures by vascular endothelial cells grown on three-dimensional fibrin gels (in vitro angiogenesis). Incubation of endothelial cells with the tumor cell-conditioned media also results in increased expression of urokinase plasminogen activator (uPA), a key component of the proteolytic system required for cell invasion and capillary formation. Although the tumor cell-conditioned media contain no bFGF, addition of anti-recombinant bFGF IgG abolishes the up-regulation of uPA and blocks in vitro angiogenesis. This indicates that both the increase in uPA production and formation of capillary-like structures are mediated by endogenous bFGF expressed by the endothelial cells. Both the bFGF/uPA-inducing activity and the angiogenic activity of SK-Hep1 cell-conditioned medium copurify with a relatively acid-resistant peptide that has moderate affinity for heparin and M(r) < 18 kDa > 3.5 kDa. Known cytokines with similar biochemical features do not possess the same biological activity. These findings indicate that angiogenesis can be mediated by endothelial cell bFGF through an autocrine mechanism and that the bFGF-inducing peptide may represent a novel tumor-derived angiogenic factor that modulates in endothelial cells the concerted expression of cytokines and proteolytic enzymes required for capillary formation.

Diminished heparin binding of a basic fibroblast growth factor mutant is associated with reduced receptor binding, mitogenesis, plasminogen activator induction, and in vitro angiogenesis

Using modeling of heparin-fibroblast growth factor interactions, we replaced four basic residues of basic fibroblast growth factor (FGF-2) with neutral glutamine residues by site-specific mutagenesis to give the mutants K128Q, K138Q, K128Q-K138Q, R129Q, K134Q, and R129Q-K134Q. The FGF mutants were characterized for their receptor and heparin binding affinities, mitogenic and cell proliferation activities, and their ability to induce plasminogen activator (PA) production and in vitro angiogenesis by cultured endothelial cells. Heparin binding properties and biological activities of the three mutants involving R129 and K134 remained essentially unchanged; however, significant changes for three mutants involving K128 and K138 were found. The KD values for heparin binding for K128Q and K138Q mutants were increased about 10-fold, and that for the K128Q-K138Q double mutant was increased by about 100-fold. The mutant K128Q-K138Q required a 10-fold higher concentration of heparin to promote binding to heparan sulfate proteoglycan (HSPG)-deficient CHO cells transfected with fibroblast growth factor receptor-1 (FGFR1) or to induce DNA synthesis in HSPG-deficient myeloid cells transfected with FGFR1. Binding affinities of the mutants to cell surface receptors on BHK-21 cells, however, were similar to that of wild-type FGF-2. In endothelial cell proliferation assays the activities of K128Q and K128Q-K138Q were about 10-fold lower than that of the wild-type protein, whereas the K138Q mutant exhibited wild-type activity. In addition, the K128Q-K138Q mutant displayed a markedly lowered capacity to induce PA activity in cultured endothelial cells and to form capillary-like structures in an in vitro angiogenesis model.(ABSTRACT TRUNCATED AT 250 WORDS)

Heparitinase inhibition of mesoderm induction and gastrulation in Xenopus laevis embryos

We have examined the involvement of proteoglycan molecules in the induction of mesodermal tissue in Xenopus laevis embryos. Blastocoelic injections of the enzyme heparitinase at early blastula stages lead to gastrulation defects and to failures in the development of anterior embryonic structures. The period of sensitivity of embryos to this treatment suggests a possible role for these molecules during mesoderm induction. We show that heparan sulfate proteoglycans (HSPGs) and chondroitin sulfate proteoglycans are the predominant sulfated glycoconjugates synthesized in early Xenopus embryos and that HSPGs are degraded by blastocoelic injections of heparitinase. Further, bFGF induction of mesoderm in explants of Xenopus stage 8 embryonic animal cap tissue is blocked by heparitinase but not by Chondroitinase ABC, using three separate criteria of mesoderm induction. Since HSPGs present in blastula animal cap cells are digested by heparitinase under the culture conditions used in the mesoderm-induction assay, we suggest that cell-surface heparan sulfate proteoglycans are required for basic fibroblast growth factor-mediated mesoderm induction.

Nonenzymatic glycosylation in vitro and in bovine endothelial cells alters basic fibroblast growth factor activity. A model for intracellular glycosylation in diabetes

Intracellular sugars are more reactive glycosylating agents than glucose. In vitro nonezymatic glycosylation of basic fibroblast growth factor (bFGF) by fructose, glucose-6-phosphate (G6P), or glyceraldehyde-3-phosphate (G3P) reduced high affinity heparin-binding activity of recombinant bFGF by 73, 77, and 89%, respectively. Mitogenic activity was reduced 40, 50, and 90%. To investigate the effects of bFGF glycosylation in GM7373 endothelial cells, we first demonstrated that GLUT-1 transporters were not downregulated by increased glucose concentration. In 30 mM glucose, the rate of glucose transport increased 11.6-fold, and the intracellular glucose concentration increased sixfold at 24 h and fivefold at 168 h. The level of total cytosolic protein modified by advanced glycosylation end-products (AGEs) was increased 13.8-fold at 168 h. Under these conditions, mitogenic activity of endothelial cell cytosol was reduced 70%. Anti-bFGF antibody completely neutralized the mitogenic activity at both 5 and 30 nM glucose, demonstrating that all the mitogenic activity was due to bFGF. Immunoblotting and ELISA showed that 30 mM glucose did not decrease detectable bFGF protein, suggesting that the marked decrease in bFGF mitogenic activity resulted from posttranslational modification of bFGF induced by elevated glucose concentration. Cytosolic AGE-bFGF was increased 6.1-fold at 168 h. These data are consistent with the hypothesis that nonenzymatic glycosylation of intracellular protein alters vascular cell function.

Characterization of cDNA encoding basic fibroblast growth factor of the marsupial Monodelphis domestica

We have isolated and characterized a 1,593-bp cDNA containing the coding region of the basic fibroblast growth factor (BFGF) gene of a marsupial, the opossum Monodelphis domestica. The encoded protein is 156 amino acids long. The BFGF gene of M. domestica is 82-87% identical to the BFGF genes of placental mammals at the nucleotide level and 92-93% identical to these genes at the level of the amino acids encoded. Regions of the BFGF molecule important in heparin binding, high-affinity receptor binding, and biologic function are highly conserved between placental mammals and this marsupial. There are several AUG and CUG codons in the 5' region of the marsupial cDNA that may serve as alternate sites of translation initiation; use of these sites would produce amino-terminally extended BFGF proteins. Amino-terminal extensions of BFGF in other species serve as nuclear localization signals. Conserved A+T-rich motifs in the 3' untranslated region of the marsupial mRNA probably serve to regulate mRNA stability. The high degree of evolutionary conservation of BFGF in mammals suggests that the molecule plays an important role in normal growth and development and that stringent control of its activity is essential.

Isolation of peptides that inhibit binding of basic fibroblast growth factor to its receptor from a random phage-epitope library

Basic fibroblast growth factor (bFGF) is known to bind to its cell-surface receptors with high affinity and in a heparin-dependent manner. In an attempt to predict the receptor recognition site on bFGF we screened phage-epitope libraries with monoclonal antibodies DG2 and DE6, which inhibit bFGF binding to its receptor. On the affinity-isolated phages, we identified several peptide sequences as the putative antibody-binding epitopes on bFGF. The identified library epitopes shared the consensus sequence Pro-(Pro/Ser)-Gly-His-(Tyr/Phe)-Lys, corresponding to two continuous protein sequences of bFGF: Pro-Pro-Gly-His-Phe-Lys and Arg-Thr-Gly-Gln-Tyr-Lys at amino acids 13-18 and 120-125 of bFGF, respectively. Synthetic peptides of the corresponding phage epitopes or of the above bFGF sequences specifically inhibited binding of the antibodies to bFGF, blocked binding of bFGF to its high-affinity receptor, and inhibited basal and bFGF-induced proliferation of vascular endothelial cells at submicromolar peptide concentrations. The potent inhibition of bFGF binding and biological activity by peptides recognized by the antibodies suggests that these sequences are functionally involved in receptor binding and may constitute part of the receptor-binding determinants on bFGF.

Structural studies of the binding of the anti-ulcer drug sucrose octasulfate to acidic fibroblast growth factor

BACKGROUND

The anti-ulcer drug sucrose octasulfate (SOS) binds to fibroblast growth factors (FGFs), proteins which stimulate the growth and differentiation of several cell types, including stomach epithelial cells. It is believed that SOS stabilizes FGFs against acid denaturation in the stomach, thus enhancing their ability to stimulate healing of ulcerated tissue. SOS binds to the same site on FGF as heparin and other proteoglycans; in vivo, FGF must bind to cell-surface proteoglycans or to heparin before it can interact with FGF receptors and stimulate growth. The details of this process are not understood.

RESULTS

We report the crystal structure of a 1:1 complex between acidic FGF (aFGF) and SOS at 2.7 A resolution. SOS binds to a positively charged region of aFGF, largely composed of residues 112-127, and makes contacts primarily with Lys112, Arg116, Lys118, and Arg122. This region is also important in binding heparin. The overall conformation of aFGF is not changed by binding SOS, although the positions of some side chains in the binding site shift by as much as 6 A.

CONCLUSION

The SOS-FGF crystal structure is consistent with the model that SOS stabilizes FGF by neutralizing several positively charged residues that would destabilize the native structure by electrostatic repulsion. On the basis of this structure, we provide a model for the complex of heparin with an FGF dimer. Such interactions may facilitate FGF receptor dimerization, which may be important in receptor signaling.

Production of autostimulatory growth factors by the human carcinoma line, RPMI 2650

The human carcinoma line RPMI 2650 produces autocrine factors; they are detected by the ability of RPMI 2650 conditioned medium (CM) to stimulate growth in soft agar of RPMI 2650 cells plated at low density. The autocrine activity in crude CM can be fractionated by ultrafiltration into a lower molecular weight (MW) fraction (R1-30), which concentrates molecules in the 1000-30,000 Da range; and a higher MW fraction (R30) with molecules greater than 30,000 Da in a more concentrated form. R1-30 is labile to acid, base, and heat treatment, whereas R30 is stable to (and sometimes activated by) these treatments. Boiling of R30, however, renders it labile to acid, base, and trypsin treatments. CM can be separated into a weakly heparin-binding fraction (with stability properties similar, but not identical, to R1-30), and a non-heparin binding fraction (with stability properties similar to R30). RPMI 2650 cells secrete transforming growth factor (TGF)alpha- and TGF beta-like molecules, but the R1-30 fraction can be distinguished from these TGFs, and from most other known growth factors, by its unusual combination of acid lability and weak affinity for heparin. Since the R30/non-heparin binding fraction is rendered labile by boiling or acid treatment, it may represent a bound or conformationally stable form of a growth factor.

Structure of a heparan sulphate oligosaccharide that binds to basic fibroblast growth factor

Binding of basic fibroblast growth factor (bFGF) to the extracellular matrix of cultured bovine aorta smooth muscle cells is likely to be mediated via heparan sulphate, since not only exogenous addition of heparan sulphate to the culture medium but also pretreatment of the cells with heparitinase (but not chondroitinase ABC) resulted in loss of binding. Comparison of the affinity of bFGF to various glycosaminoglycan-conjugated gels showed a direct and specific binding of bFGF to heparan sulphate. Heparan sulphate also bound to a bFGF affinity gel. However, the proportion of heparan sulphate bound varied depending on the source of the HS (more than 90% and 45% with pig aorta heparan sulphate and mouse EHS tumour heparan sulphate respectively). The bound heparan sulphate had the ability to protect bFGF from proteolytic digestion, but the unbound heparan sulphate did not. The results suggest the presence in the bound heparan sulphate of a specific structure involved in binding. Limited digestion with heparitinase I of porcine aorta heparan sulphate yielded 13% oligosaccharides bound to the gel, of which the smallest were octasaccharides. Analysis of a hexadecasaccharide fraction which was obtained at the highest yield among the bound oligosaccharides was performed by h.p.l.c. of the deamination products obtained with nitrous acid and the unsaturated disaccharide products formed by heparitinase digestion. Comparison of the disaccharide unit compositions exhibited a marked difference in IdoA(2SO4)GlcNSO3 and IdoA(2SO4)GlcNSO3(6SO4) units between the bound and unbound hexadecasaccharides. The amounts measured were 3 mol and 1 mol per mol of the former and 0.4 mol and 0.6 mol per mol of the latter. It is likely that the binding of bFGF to heparan sulphate may require the domain structure of the heparan sulphate to be composed of clustering IdoA(2SO4)-GlcNSO3 units.

Stabilization of basic fibroblast growth factor with dextran sulfate

Dextran sulfate protected bFGF from heat and acid inactivation and from proteolytic degradation. The protective effect was stronger than that of heparin which is known as a stabilizer of bFGF. Dextran sulfate and bFGF formed a high molecular weight complex via ionic interaction when mixed together in aqueous solution. The complex was dissociated when the ionic strength was increased and the protective effect was completely abolished. Successive digestion of bFGF with Staphylococcus aureus V8 protease and pepsin followed by affinity chromatography on an immobilized dextran sulfate column and reversed-phase high performance liquid chromatography yielded three positively charged fragment peptides, Tyr24-Phe30, Tyr106-Trp114 and Tyr124-Leu138. These results suggest that dextran sulfate stabilizes bFGF by binding close to the putative heparin binding sites of the bFGF molecule.

Proteoglycans in cell regulation

Proteoglycans are a diverse group of proteins carrying one or more glycosaminoglycan side chains linked to the protein as O-glycosides. Our appreciation of these structures has matured from a curiosity about unusual structural glycoproteins, to confer upon them a central role in cell biology. The major classes of glycosaminoglycans are heparan sulfate and heparin, chondroitin and dermatan sulfates, keratan sulfate and hyaluronic acid. The latter is unique in that it does not contain sulfate residues, and appears to be synthesized, at least sometimes, free of a carrier protein. There is now a wealth of information on the ability of these structures to influence the growth and development of cells and tissues. Many direct and specific effects of proteoglycans will undoubtedly be found, and there are likely to be indirect effects of the glycosaminoglycans relating to their polyelectrolyte nature. Convincing arguments that biological activity resides in certain proteoglycan core proteins are also appearing. The following discussion concerns the role of proteoglycans in the regulation and action of autocrine and polypeptide growth factors, direct mitogenic and antimitogenic actions of glycosaminoglycans, the role of these structures in regulating gene expression, and the biological activities of proteoglycan core proteins. The probable role of proteoglycans in normal glomerular cell function, and in progressive renal disease, will be presented as a harbinger of the significant role we can expect them to play in diagnosis and therapy in the near future.

Structure/activity relationships in basic FGF

Although the FGFs have been subject to extensive biological studies, only limited progress has been made so far in determining the critical elements of structure-activity relationships in the FGFs. Among the recognized structural elements with potential to affect the biological activity of FGFs are the cysteine residues, and the heparin- and receptor-binding domains. These features have been studied using a variety of experimental approaches, but the available data are inconclusive. For example, ambiguity regarding the presence of a disulfide structure in FGFs was not resolved until the availability of x-ray crystal structure data. Furthermore, the functionally important heparin- and receptor-binding domains have been poorly characterized, with some interpretations being controversial. In this report, we describe a novel fragment of basic FGF (bFGF) with high biological activity [Ser78,96-bFGF(70-153)]. This fragment was generated by pronase treatment of heparin-bound recombinant Glu3,5Ser78,96-bFGF mutant and is active in vitro at an ED50 of about 100 ng/ml. The structure of the fragment and the manner by which it was generated provide additional insight into important aspects of structure-activity relationships in FGFs. Specifically, we conclude that (a) the cysteines in our bFGF mutant do not form a disulfide bond, (b) the high-affinity heparin binding of bFGF critically depends on an intact 3-dimensional structure of the growth factor rather than on specific heparin-binding sequence domains, and (c) the bFGF sequence between residues 70 and 122 is important for high biological activity.

Tumor cytotoxic factor/hepatocyte growth factor from human fibroblasts: cloning of its cDNA, purification and characterization of recombinant protein

Two different forms of cDNA for F-TCF were isolated from cDNA library prepared with mRNA from human embryonic lung fibroblast, IMR-90 cells. One of them was completely identical to the cDNA for placenta type hepatocyte growth factor (HGF) and the other one was a variant cDNA for the HGF with a deletion of 15 base pairs in the coding region. The cDNAs were expressed in CHO cells and recombinant proteins were purified and characterized. The deleted form of recombinant F-TCF (rF-TCF) was slightly lower in heparin affinity than the intact form. Both rF-TCFs showed almost same dose-response curves for cytotoxicity on Sarcoma 180 or Meth A sarcoma cells. Dose-response curves for the stimulation of DNA synthesis in rat hepatocytes were also almost same before reaching maximal activity at 12.5 ng/ml but significantly different at higher concentrations. The deleted form of rF-TCF maintained maximal activity in the dose range of 12.5 to 100 ng/ml, although the intact form decreased the activity dose-dependently at more than 25 ng/ml. This suggests that the deletion of five amino acids results in a conformational change which alters heparin binding and hepatocyte growth stimulating activities.

Requirement of heparan sulfate for bFGF-mediated fibroblast growth and myoblast differentiation

Basic fibroblast growth factor (bFGF) binds to heparan sulfate proteoglycans at the cell surface and to receptors with tyrosine kinase activity. Prevention of binding between cell surface heparan sulfate and bFGF (i) substantially reduces binding of fibroblast growth factor to its cell-surface receptors, (ii) blocks the ability of bFGF to support the growth of Swiss 3T3 fibroblasts, and (iii) induces terminal differentiation of MM14 skeletal muscle cells, which is normally repressed by fibroblast growth factor. These results indicate that cell surface heparan sulfate is directly involved in bFGF cell signaling.

Three-dimensional structure of human basic fibroblast growth factor, a structural homolog of interleukin 1 beta

The three-dimensional structure of the 146-residue form of human basic fibroblast growth factor (bFGF), expressed as a recombinant protein in yeast, has been determined by x-ray crystallography to a resolution of 1.8 A. bFGF is composed entirely of beta-sheet structure, comprising a three-fold repeat of a four-stranded antiparallel beta-meander. The topology of bFGF is identical to that of interleukin 1 beta, showing that although the two proteins share only 10% sequence identity, bFGF, interleukin 1, and their homologs comprise a family of structurally related mitogenic factors. Analysis of the three-dimensional structure in light of functional studies of bFGF suggests that the receptor binding site and the positively charged heparin binding site correspond to adjacent but separate loci on the beta-barrel.

A broad-spectrum human lung fibroblast-derived mitogen is a variant of hepatocyte growth factor

A heparin-binding mitogen was isolated from conditioned medium of human embryonic lung fibroblasts. It exhibited broad target-cell specificity whose pattern was distinct from that of any known growth factor. It rapidly stimulated tyrosine phosphorylation of a 145-kDa protein in responsive cells, suggesting that its signaling pathways involved activation of a tyrosine kinase. Purification identified a major polypeptide with an apparent molecular mass of 87 kDa under reducing conditions. Partial amino acid sequence analysis and cDNA cloning revealed that it was a variant of hepatocyte growth factor, a mitogen thought to be specific for hepatic cells and structurally related to plasminogen. Recombinant expression of the cDNA in COS-1 cells established that it encoded the purified growth factor. Its site of synthesis and spectrum of targets imply that this growth factor may play an important role as a paracrine mediator of the proliferation of melanocytes and endothelial cells, as well as cells of epithelial origin.