Solution structure of human acidic fibroblast growth factor and interaction with heparin-derived hexasaccharide

CD44 Glycosylation as a Therapeutic Target in Oncology

The interaction of non-kinase transmembrane glycoprotein CD44 with ligands including hyaluronic acid (HA) is closely related to the occurrence and development of tumors. Changes in CD44 glycosylation can regulate its binding to HA, Siglec-15, fibronectin, TM4SF5, PRG4, FGF2, collagen and podoplanin and activate or inhibit c-Src/STAT3/Twist1/Bmi1, PI3K/AKT/mTOR, ERK/NF-κB/NANOG and other signaling pathways, thereby having a profound impact on the tumor microenvironment and tumor cell fate. However, the glycosylation of CD44 is complex and largely unknown, and the current understanding of how CD44 glycosylation affects tumors is limited. These issues must be addressed before targeted CD44 glycosylation can be applied to treat human cancers.

Synthesis of Novel Suramin Analogs With Anti-Proliferative Activity via FGF1 and FGFRD2 Blockade

A promising approach in cancer therapy is the inhibition of cell proliferation using small molecules. In this study, we report the synthesis of suramin derivatives and their applications. We used NMR spectroscopy and docking simulations to confirm binding sites and three-dimensional models of the ligand-protein complex. The WST-1 assay was used to assess cell viability and cell proliferation in vitro to evaluate the inhibition of protein-protein interactions and to investigate the anti-proliferative activities in a breast cancer cell line. All the suramin derivatives showed anti-proliferative activity by blocking FGF1 binding to its receptor FGFRD2. The dissociation constant was measured by fluorescence spectroscopy. The suramin compound derivatives synthesized herein show potential as novel therapeutic agents for their anti-proliferative activity via the inhibition of protein-protein interactions. The cytotoxicity of these suramin derivatives was lower than that of the parent suramin compound, which may be considered a significant advancement in this field. Thus, these novel suramin derivatives may be considered superior anti-metastasis molecules than those of suramin.

NMR Characterization of the Interactions Between Glycosaminoglycans and Proteins

Glycosaminoglycans (GAGs) constitute a considerable fraction of the glycoconjugates found on cellular membranes and in the extracellular matrix of virtually all mammalian tissues. The essential role of GAG-protein interactions in the regulation of physiological processes has been recognized for decades. However, the underlying molecular basis of these interactions has only emerged since 1990s. The binding specificity of GAGs is encoded in their primary structures, but ultimately depends on how their functional groups are presented to a protein in the three-dimensional space. This review focuses on the application of NMR spectroscopy on the characterization of the GAG-protein interactions. Examples of interpretation of the complex mechanism and characterization of structural motifs involved in the GAG-protein interactions are given. Selected families of GAG-binding proteins investigated using NMR are also described.

Regenerative responses of rabbit corneal endothelial cells to stimulation by fibroblast growth factor 1 (FGF1) derivatives, TTHX1001 and TTHX1114

Utilising rabbit corneal endothelial cells (CEC) in three different paradigms, two human FGF1 derivatives (TTHX1001 and TTHX1114), engineered to exhibit greater stability, were tested as proliferative agents. Primary CECs and mouse NIH 3T3 cells treated with the two FGF1 derivatives showed equivalent EC₅₀ ranges (3.3-24 vs.1.9-16. ng/mL) and, in organ culture, chemically lesioned corneas regained half of the lost endothelial layer in three days after treatment with the FGF1 derivatives as compared to controls. In vivo, following cryolesioning, the CEC monolayer, as judged by specular microscopy, regenerated 10-11 days faster when treated with TTHX1001. Over two weeks, all treated eyes showed clearing of opacity about twice that of untreated controls. In all three rabbit models, both FGF1 derivatives were effective in inducing CEC proliferation over control conditions, supporting the prediction that these stabilised FGF1 derivatives can potentially regenerate corneal endothelial deficits in humans.

Suramin derivatives play an important role in blocking the interaction between FGF1 and FGFRD2 to inhibit cell proliferation

The inhibition of protein function by small compounds plays a critical role in controlling cell proliferation. We report on a new class of small molecule (NCTU-Alan-2026) inhibitors for cell proliferation. NCTU-Alan-2026 blocks the interaction between FGF1 and its receptor FGF1R2D2. Extensive NMR studies combined with fluorescence experiments provided the specific mechanism of how NCTU-Alan-2026 could inhibit cell proliferation. We describe an innovative therapeutic approach for anti-proliferation and demonstrate an example of inhibition of small molecules by blocking the protein-protein interaction. We found that the compound NCTU-Alan-2026 blocked the interaction between the two proteins FGF1 and FGF1R2D2 and inhibited cell proliferation. The toxicity of NCTU-Alan-2026 is lower than that of suramin. Thus, NCTU-Alan-2026 could be a better drug than suramin in the treatment of cancer.

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.

Influence of heparin mimetics on assembly of the FGF.FGFR4 signaling complex

Fibroblast growth factor (FGF) signaling regulates mammalian development and metabolism, and its dysregulation is implicated in many inherited and acquired diseases, including cancer. Heparan sulfate glycosaminoglycans (HSGAGs) are essential for FGF signaling as they promote FGF.FGF receptor (FGFR) binding and dimerization. Using novel organic synthesis protocols to prepare homogeneously sulfated heparin mimetics (HM), including hexasaccharide (HM(6)), octasaccharide (HM(8)), and decasaccharide (HM(10)), we tested the ability of these HM to support FGF1 and FGF2 signaling through FGFR4. Biological assays show that both HM(8) and HM(10) are significantly more potent than HM(6) in promoting FGF2-mediated FGFR4 signaling. In contrast, all three HM have comparable activity in promoting FGF1.FGFR4 signaling. To understand the molecular basis for these differential activities in FGF1/2.FGFR4 signaling, we used NMR spectroscopy, isothermal titration calorimetry, and size-exclusion chromatography to characterize binding interactions of FGF1/2 with the isolated Ig-domain 2 (D2) of FGFR4 in the presence of HM, and binary interactions of FGFs and D2 with HM. Our data confirm the existence of both a secondary FGF1.FGFR4 interaction site and a direct FGFR4.FGFR4 interaction site thus supporting the formation of the symmetric mode of FGF.FGFR dimerization in solution. Moreover, our results show that the observed higher activity of HM(8) relative to HM(6) in stimulating FGF2.FGFR4 signaling correlates with the higher affinity of HM(8) to bind and dimerize FGF2. Notably FGF2.HM(8) exhibits pronounced positive binding cooperativity. Based on our findings we propose a refined symmetric FGF.FGFR dimerization model, which incorporates the differential ability of HM to dimerize FGFs.

Effect of FGF-binding protein 3 on vascular permeability

Fibroblast growth factor-binding protein 1 (FGF-BP1 is BP1) is involved in the regulation of embryonic development, tumor growth, and angiogenesis by mobilizing endogenous FGFs from their extracellular matrix storage. Here we describe a new member of the FGF-BP family, human BP3. We show that the hBP3 protein is secreted from cells, binds to FGF2 in vitro and in intact cells, and inhibits FGF2 binding to heparin. To determine the function of hBP3 in vivo, hBP3 was transiently expressed in chicken embryos and resulted in > 50% lethality within 24 h because of vascular leakage. The onset of vascular permeability was monitored by recording the extravasation kinetics of FITC-labeled 40-kDa dextran microperfused into the vitelline vein of 3-day-old embryos. Vascular permeability increased as early as 8 h after expression of hBP3. The increased vascular permeability caused by hBP3 was prevented by treatment of embryos with PD173074, a selective FGFR kinase inhibitor. Interestingly, a C-terminal 66-amino acid fragment (C66) of hBP3, which contains the predicted FGF binding domain, still inhibited binding of FGF2 to heparin similar to full-length hBP3. However, expression of the C66 fragment did not increase vascular permeability on its own, but required the administration of exogenous FGF2 protein. We conclude that the FGF binding domain and the heparin binding domain are necessary for the hBP3 interaction with endogenous FGF and the activation of FGFR signaling in vivo.

Interaction of anti-aggregation agent dimethylethylammonium propane sulfonate with acidic fibroblast growth factor

Prevention of aggregation is critical for analyzing protein structure. Non-detergent sulfobetaines (NDSBs) are known to prevent protein aggregation, but the molecular mechanisms of their anti-aggregation effect are poorly understood. To elucidate the underlying mechanisms, we analyzed the effects of dimethylethylammonium propane sulfonate (NDSB-195) on acidic fibroblast growth factor (aFGF). NDSB-195 (0.5M) increased both aggregation and denaturation temperatures of aFGF by 4 degrees C. Chemical shift perturbation analyses indicated that many affected residues were located at the junction between a beta-strand (or 3(10)-helix) and a loop, irrespective of the chemical properties of the residue. The apparent dissociation constants of the interaction ranged from 0.04 to 3M, indicating weak interactions between NDSB and protein molecules.

Stability of angiogenic agents, ginsenoside Rg1 and Re, isolated from Panax ginseng: In vitro and in vivo studies

The study was designed to investigate the stability of ginsenoside Rg(1) (Rg(1)) and Re (Re), two natural herbal compounds isolated from Panax ginseng, based on their activity to promote angiogenesis in vitro and in vivo. After being treated at different temperatures, pHs, and solvent species for distinct durations, the remaining activities of Rg(1) and Re on human umbilical vein endothelial cell (HUVEC) proliferation, migration, and tube formation were examined in vitro. Additionally, the remaining activity of each treated test agent, mixed in a growth factor-reduced Matrigel, in stimulating angiogenesis was evaluated subcutaneously in a mouse model. Basic fibroblast growth factor (bFGF) was used as a control. It was found in vitro that HUVEC proliferation, migration in a Transwell plate, and tube formation on Matrigel were all significantly enhanced in the presence of bFGF, Rg(1), or Re. However, after being treated at different temperatures, pHs, or solvent species, the remaining activity of bFGF on HUVEC behaviors reduced significantly. This observation was more significant with increasing the duration of treatment. In contrast, the activities of Rg(1) and Re remained unchanged throughout the entire course of the study. The in vivo results observed on day 7 after implantation showed that the blank control (Matrigel alone) was slightly vascularized. In contrast, the density of neo-vessels in the Matrigel plug mixed with bFGF, Rg(1), or Re was significantly enhanced. However, after being treated, the density of neo-vessels was significantly reduced in the Matrigel plug mixed with bFGF, while those of Rg(1) and Re remained unchanged. The aforementioned results suggested that Rg(1) and Re could be a novel group of nonpeptide angiogenic agents with a superior stability and may be used for the management of tissue regeneration.

Strategies for targeting protein-protein interactions with synthetic agents

The development of small-molecule modulators of protein-protein interactions is a formidable goal, albeit one that possesses significant potential for the discovery of novel therapeutics. Despite the daunting challenges, a variety of examples exists for the inhibition of two large protein partners with low-molecular-weight ligands. This review discusses the strategies for targeting protein-protein interactions and the state of the art in the rational design of molecules that mimic the structures and functions of their natural targets.

Solution NMR structure of a human FGF-1 monomer, activated by a hexasaccharide heparin-analogue

The 3D structure of a complex formed by the acidic fibroblast growth factor (FGF-1) and a specifically designed synthetic heparin hexasaccharide has been determined by NMR spectroscopy. This hexasaccharide can substitute natural heparins in FGF-1 mitogenesis assays, in spite of not inducing any apparent dimerization of the growth factor. The use of this well defined synthetic heparin analogue has allowed us to perform a detailed NMR structural analysis of the heparin-FGF interaction, overcoming the limitations of NMR to deal with the high molecular mass and heterogeneity of the FGF-1 oligomers formed in the presence of natural heparin fragments. Our results confirm that glycosaminoglycans induced FGF-1 dimerization either in a cis or trans disposition with respect to the heparin chain is not an absolute requirement for biological activity.

Backbone dynamics of a biologically active human FGF-1 monomer, complexed to a hexasaccharide heparin-analogue, by 15N NMR relaxation methods

The binding site and backbone dynamics of a bioactive complex formed by the acidic fibroblast growth factor (FGF-1) and a specifically designed heparin hexasaccharide has been investigated by HSQC and relaxation NMR methods. The comparison of the relaxation data for the free and bound states has allowed showing that the complex is monomeric, and still induces mutagenesis, and that the protein backbone presents reduced motion in different timescale in its bound state, except in certain points that are involved in the interaction with the fibroblast growth factor receptor (FGFR).

Understanding the mechanism of the antimitogenic activity of suramin

Fibroblast growth factors (FGFs) play crucial roles in the regulation of key cellular processes such as angiogenesis, differentiation, and tumor growth. Suramin, a polysulfonated naphthylurea, is known to be a potent inhibitor of FGF-induced angiogenesis. Using isothermal titration calorimetry, we demonstrate that human acidic fibroblast growth factor (hFGF-1) binds to suramin with high affinity in the nanomolar range. The suramin:hFGF-1 binding stoichiometry is estimated to be 2:1. Size-exclusion chromatography data reveal that suramin oligomerizes hFGF-1 to form a stable tetramer. Thermal unfolding experiments monitored by steady state fluorescence, and limited trypsin digestion analysis data suggest that suramin-induced oligomerization of hFGF-1 occurs in two steps. The first step involves the binding of suramin at specific sites on the protein. Two molecules of suramin appear to bind simultaneously to one molecule of hFGF-1. Binding of suramin possibly involves formation of solvent-exposed nonpolar surfaces in hFGF-1. In the second step, FGF appears to oligomerize through coalescence of the solvent-accessible nonpolar surfaces. Results of the NMR experiments reveal that suramin binds to residues in the heparin binding pocket as well as to residues involved in FGF receptor binding. On the basis of the results of this study, we propose a model to explain the molecular mechanism(s) underlying the antimitogenic activity of suramin. To our knowledge, this is the first study in which suramin interaction sites on FGF have been characterized.

Laminin alpha5 chain metastasis- and angiogenesis-inhibiting peptide blocks fibroblast growth factor 2 activity by binding to the heparan sulfate chains of CD44

Recently, we reported that the laminin alpha5 synthetic peptide A5G27 (RLVSYNGIIFFLK, residues 2,892-2,904) binds to the CD44 receptor of B16-F10 melanoma cells via the glycosaminoglycans on CD44 and inhibits tumor cell migration, invasion, and angiogenesis in a dominant-negative manner. Here, we have identified the potential mechanism of A5G27 activity using WiDr human colorectal carcinoma cells. WiDr cells bound to the laminin A5G27 peptide via the heparin-like and chondroitin sulfate B glycosaminoglycan side chains of CD44. Cell binding to fibroblast growth factor (FGF2) was blocked by laminin peptide A5G27 but not by either a scrambled version of this peptide or by another laminin peptide known to bind cell surface proteoglycans. FGF2 signaling involving tyrosine phosphorylation was also blocked by laminin peptide A5G27 but was not affected by peptide controls. Finally, we have shown that peptide A5G27 directly blocks FGF2 binding to heparin. Peptide A5G27 has sequence homology to a region on FGF2 that binds heparin and the FGF receptor and is important in FGF2 central cavity formation. We conclude that peptide A5G27 inhibits metastasis and angiogenesis by blocking FGF2 binding to the heparan sulfate side chains of CD44 variant 3, thus decreasing FGF2 bioactivity.

Molecular Mechanism of Inhibition of Nonclassical FGF-1 Export,

Fibroblast growth factor (FGF-1) lacks a signal sequence and is exported by an unconventional release mechanism. The nonclassical export of FGF-1 has been shown to be inhibited by an anti-allergic and anti-inflammatory drug, amlexanox (AMX). We investigate the molecular mechanism(s) underlying the inhibitory action of AMX on the release of FGF-1, using a variety of biophysical techniques including multidimensional NMR spectroscopy. AMX binds to FGF-1 and enhances its conformational stability. AMX binds to locations close to Cys30 and sterically blocks Cu(2+)-induced oxidation, leading to the formation of the homodimer of FGF-1. AMX-induced inhibition of the formation of the FGF-1 homodimer is observed both under cell-free conditions and in living cells. Results of this study suggest a novel approach for the design of drugs against FGF-1-mediated disorders.

Conformational flexibility of a synthetic glycosylaminoglycan bound to a fibroblast growth factor. FGF-1 recognizes both the (1)C(4) and (2)S(O) conformations of a bioactive heparin-like hexasaccharide

The first direct NMR determination of the conformation of a conformationally flexible heparin-like hexasaccharide bound to a key receptor, FGF-1, is described. The determination has been based on the use of a 13C-labeled protein and a regular 12C sugar. FGF-1 recognizes several conformations of the iduronic moieties of the hexasaccharide. Therefore, this case is different than that described for the controversial recognition of heparin-like saccharides by AT-III, which seems to recognize just one conformation of the iduronic acid residues.

Vascular leakage in chick embryos after expression of a secreted binding protein for fibroblast growth factors

Fibroblast growth factors (FGFs) have been implicated in a variety of physiologic and pathologic processes from embryonic development to tumor growth and angiogenesis. FGFs are immobilized in the extracellular matrix of different tissues and require release from this storage site to trigger a response. Secreted FGF-binding proteins (FGF-BPs) can release immobilized FGFs, enhance the activity of locally stored FGFs and can thus serve as an angiogenic switch molecule in cancer. Here, we report on the effect of human FGF-BP transgene expression in chicken embryos. To establish the transgenic model, plasmid-based reporter vectors expressing luciferase, beta-galactosidase or green fluorescent protein were introduced through different routes into 4- to 5-day-old embryos grown outside their egg shell on top of the yolk sac. This allows for easy manipulation and continuous observation of phenotypic effects. Expression of human FGF-BP induced dose-dependent vascular permeability, hemorrhage and embryonic lethality. Light and electron microscopic studies indicate that this hemorrhage results from compromised microvascular structure. An FGF-1 expression vector with an added secretory signal mimicked this vascular leakiness phenotype whereas wild-type FGF-1 required coexpression of a threshold amount of FGF-BP. This model is a powerful tool for real-time monitoring of the effects of transient transgene expression during embryogenesis.

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.

Sequence swapping does not result in conformation swapping for the beta4/beta5 and beta8/beta9 beta-hairpin turns in human acidic fibroblast growth factor

The beta-turn is the most common type of nonrepetitive structure in globular proteins, comprising ~25% of all residues; however, a detailed understanding of effects of specific residues upon beta-turn stability and conformation is lacking. Human acidic fibroblast growth factor (FGF-1) is a member of the beta-trefoil superfold and contains a total of five beta-hairpin structures (antiparallel beta-sheets connected by a reverse turn). beta-Turns related by the characteristic threefold structural symmetry of this superfold exhibit different primary structures, and in some cases, different secondary structures. As such, they represent a useful system with which to study the role that turn sequences play in determining structure, stability, and folding of the protein. Two turns related by the threefold structural symmetry, the beta4/beta5 and beta8/beta9 turns, were subjected to both sequence-swapping and poly-glycine substitution mutations, and the effects upon stability, folding, and structure were investigated. In the wild-type protein these turns are of identical length, but exhibit different conformations. These conformations were observed to be retained during sequence-swapping and glycine substitution mutagenesis. The results indicate that the beta-turn structure at these positions is not determined by the turn sequence. Structural analysis suggests that residues flanking the turn are a primary structural determinant of the conformation within the turn.

Heparin-binding domains in vascular biology

Heparin is a major anticoagulant with activity mediated primarily through its interaction with antithrombin (AT). Heparan sulfate (HS), structurally related to heparin, binds a wide range of proteins of different functionality, taking part in various physiological and pathological processes. The heparin-AT complex, the most well understood facet of anticoagulation, serves as a prototypical example of the important role of heparin/HS in vascular biology. Extensive studies have identified common structural features in heparin/HS-binding sites of proteins. These include the elucidation of consensus sequences in proteins, patterns of clusters of basic and nonbasic residues, and common spatial arrangements of basic amino acids in the heparin-binding sites. Although these studies have provided valuable information, heparin/HS-binding proteins differ widely in structure. The prediction of heparin/HS-binding proteins from sequence information is not currently possible, and elucidation of protein-binding sites requires the individual study of each glycosaminoglycan-protein complex. Thus, x-ray crystallography and site-directed mutagenesis experiments are among the most powerful tools, providing accurate structural information, facilitating the characterization of heparin-protein complexes. Heparin and structurally related heparan sulfate bind a large number of proteins, taking part in a wide range of biological processes, particularly ones involved in vascular biology. Heparin-binding domains share certain common structural features, but there is no absolute dependency on specific sequences or protein folds.

Relative retention of the fibroblast growth factors FGF-1 and FGF-2 on strong cation-exchange sorbents

The isocratic retention of two heparin-binding fibroblast growth factors, FGF-1 (acidic FGF) and FGF-2 (basic FGF), was compared on a set of six preparative strong cation-exchange adsorbents. The FGFs comprise a solute pair that are structurally equivalent, yet differ in protein parameters of potential importance in cation-exchange chromatography, such as isoelectric point, net charge, and the number and distribution of basic amino acids. The cation-exchange adsorbents comprise a diverse set of materials in common use for protein purification, with physical and chemical properties that have been characterized and described previously. Isocratic k' values for the two proteins obtained on each adsorbent at several different [NaCl] are compared with one another and with corresponding data for hen egg lysozyme, which is also strongly retained on cation-exchangers. Of the six adsorbents examined, three showed strong retention of both FGFs, with equivalent k' values for FGF-1 and FGF-2. Three others, which showed weaker overall retention for the FGF pair, showed much larger retention differences between FGF-1 and FGF-2. The trends in retention order among the stationary phases are very similar to those seen previously with other unrelated proteins. However, retention differences between the two FGFs, and between the FGFs and lysozyme, do not correlate well with simple charge properties such as net charge, indicating, as in some previous studies, the importance of local regions on the protein surface in determining retention. These observations are interpreted in terms of the structural features of the proteins and the physicochemical properties of the adsorbents.

The Crystal Structure of Fibroblast Growth Factor (FGF) 19 Reveals Novel Features of the FGF Family and Offers a Structural Basis for Its Unusual Receptor Affinity†,‡

The 22 members of the FGF family have been implicated in cell proliferation, differentiation, survival, and migration. They are required for both development and maintenance of vertebrates, demonstrating an exquisite pattern of affinities for both protein and proteoglycan receptors. FGF19, one of the most divergent human FGFs, is unique in binding solely to one receptor, FGFR4. We have used molecular replacement to solve the crystal structure of FGF19 at 1.3 A resolution using five superimposed FGF structures as the search model. The structure shows that two novel disulfide bonds found in FGF19, one of which appears to be conserved among several of the other FGFs, stabilize extended loops. The key heparin-binding loops of FGF19 have radically different conformations and charge patterns, compared to other FGFs, correlating with the unusually low affinity of FGF19 for heparin. A model for the complex of FGF19 with FGFR4 demonstrates that unique sequences in both FGF19 and FGFR4 are key to the formation of the complex. The structure therefore offers a clear explanation for the unusual affinity of FGF19 for FGFR4 alone.

Structurally homologous all beta-barrel proteins adopt different mechanisms of folding

Acidic fibroblast growth factors from human (hFGF-1) and newt (nFGF-1) (Notopthalamus viridescens) are 16-kDa, all beta-sheet proteins with nearly identical three-dimensional structures. Guanidine hydrochloride (GdnHCl)-induced unfolding of hFGF-1 and nFGF-1 monitored by fluorescence and far-UV circular dichroism (CD) shows that the FGF-1 isoforms differ significantly in their thermodynamic stabilities. GdnHCl-induced unfolding of nFGF-1 follows a two-state (Native state to Denatured state(s)) mechanism without detectable intermediate(s). By contrast, unfolding of hFGF-1 monitored by fluorescence, far-UV circular dichroism, size-exclusion chromatography, and NMR spectroscopy shows that the unfolding process is noncooperative and proceeds with the accumulation of stable intermediate(s) at 0.96 M GdnHCl. The intermediate (in hFGF-1) populated maximally at 0.96 M GdnHCl has molten globule-like properties and shows strong binding affinity to the hydrophobic dye, 1-Anilino-8-naphthalene sulfonate (ANS). Refolding kinetics of hFGF-1 and nFGF-1 monitored by stopped-flow fluorescence reveal that hFGF-1 and nFGF-1 adopts different folding mechanisms. The observed differences in the folding/unfolding mechanisms of nFGF-1 and hFGF-1 are proposed to be either due to differential stabilizing effects of the charged denaturant (Gdn(+) Cl(-)) on the intermediate state(s) and/or due to differences in the structural interactions stabilizing the native conformation(s) of the FGF-1 isoforms.

Introduction of a chemical constraint in a short peptide derived from human acidic fibroblast growth factor elicits mitogenic structural determinants

Fibroblast growth factors (FGFs) are regulatory proteins associated with a number of physiological and pathological states. On the basis of data suggesting a functional role for specific regions of human acidic FGF (aFGF), a linear peptide encompassing residues 99-108 (peptide1) and its cyclic analogue (peptide 2) were synthesized and their functional and structural features were investigated. While peptide 1 is inactive on Balb/c 3T3 fibroblasts, peptide 2 is mitogenic with ED(50) of approximately 50 microM. Moreover, peptide 1 is not able to inhibit the binding of human aFGF to cellular receptors whereas peptide 2 exhibits significant inhibitory activity. The NMR-derived solution conformers indicated the presence, only in peptide 2, of structural elements that we believe are related to its ability to emulate the biological activity of the native protein. These results suggest that the expression of mitogenic activity in short peptides, besides the presence of specific amino acids, requires the existence of stable structural features. In addition, they indicate that the introduction of chemical restraints in peptides can provide novel possibilities for the development of receptor agonists or antagonists.

Investigation of the structural stability of the human acidic fibroblast growth factor by hydrogen-deuterium exchange

The conformational stability of the human acidic fibroblast growth factor (hFGF-1) is investigated using amide proton exchange and temperature-dependent chemical shifts, monitored by two-dimensional NMR spectroscopy. The change in free energy of unfolding (DeltaG(u)) of hFGF-1 is estimated to be 5.00 +/- 0.09 kcal.mol(-)(1). Amide proton-exchange rates of 74 residues (in hFGF-1) have been unambiguously measured, and the exchange process occurs predominately according to the conditions of the EX2 limit. The exchange rates of the fast-exchanging amide protons exposed to the solvent have been measured using the clean SEA-HSQC technique. The amide proton protection factor and temperature coefficient estimates show reasonably good correlation. Residues in beta-strands II and VI appear to constitute the stability core of the protein. Among the 12 beta-strands constituting the beta-barrel architecture of hFGF-1, beta-strand XI, located in the heparin binding domain, exhibits the lowest average protection factor value. Amide protons involved in the putative folding nucleation site in hFGF-1, identified by quench-flow NMR studies, do not represent the slow-exchanging core. Residues in portions of hFGF-1 experiencing high conformational flexibility mostly correspond to those involved in receptor recognition and binding.

Characterization of the structure and dynamics of a near-native equilibrium intermediate in the unfolding pathway of an all beta-barrel protein

The structure and dynamics of equilibrium intermediate in the unfolding pathway of the human acidic fibroblast growth factor (hFGF-1) are investigated using a variety of biophysical techniques including multidimensional NMR spectroscopy. Guanidinium hydrochloride (GdnHCl)-induced unfolding of hFGF-1 proceeds with the accumulation of a stable intermediate state. The transition from the intermediate state to the unfolded state(s) is cooperative without the accumulation of additional intermediate(s). The intermediate state induced maximally in 0.96 m GdnHCl is found to be obligatory in the folding/unfolding pathway of hFGF-1. Most of the native tertiary structure interactions are preserved in the intermediate state. (1)H-(15)N chemical shift perturbation data suggest that the residues in the C-terminal segment including those located in the beta-strands IX, X, and XI undergo the most discernible structural change(s) in the intermediate state in 0.96 m GdnHCl. hFGF-1 in the intermediate state (0.96 m GdnHCl) does not bind to its ligand, sucrose octasulfate. Limited proteolytic digestion experiments and hydrogen-deuterium exchange monitored by (15)N heteronuclear single quantum coherence (HSQC) spectra show that the conformational flexibility of the protein in the intermediate state is significantly higher than in the native conformation. (15)N spin relaxation experiments show that many residues located in beta-strands IX, X, and XI exhibit conformational motions in the micro- to millisecond time scale. Analysis of (15)N relaxation data in conjunction with the amide proton exchange kinetics suggests that residues in the beta-strands II, VIII, and XII possibly constitute the stability core of the protein in the near-native intermediate state.

Structure and stability of an acidic fibroblast growth factor from Notophthalmus viridescens

The three-dimensional solution structure of an acidic fibroblast growth factor (nFGF-1) from the newt (Notophthalmus viridescens) is determined using multidimensional NMR techniques. Complete assignment of all the atoms ((1)H, (15)N, and (13)C) has been achieved using a variety of triple resonance experiments. 50 structures were calculated using hybrid distance geometry-dynamical simulated annealing technique with a total of 1359 constraints. The atomic root mean square distribution for the backbone atoms in the structured region is 0.60 A. The secondary structural elements include 12 beta-strands arranged antiparallely into a beta-barrel structure. The protein (nFGF-1) exists in a monomeric state upon binding to the ligand, sucrose octa sulfate (SOS), in a stoichiometric ratio of 1:1. The SOS binding site consists of a dense cluster of positively charged residues located at the C-terminal end of the molecule. The conformational stabilities of nFGF-1 and its structural and functional homologue from the human source (hFGF-1) are drastically different. The differential stabilities of nFGF-1 and hFGF-1 are attributed to the differences in the number of hydrogen bonds and the presence of solvent inaccessible cavities in the two proteins.

Identification of rare partially unfolded states in equilibrium with the native conformation in an all beta-barrel protein

Human acidic fibroblast growth factor 1 (hFGF-1) is an all beta-barrel protein, and the secondary structural elements in the protein include 12 antiparallel beta-strands arranged into a beta-trefoil fold. In the present study, we investigate the stability of hFGF-1 by hydrogen-deuterium exchange as a function of urea concentration. Urea-induced equilibrium unfolding of hFGF-1 monitored by fluorescence and CD spectroscopy suggests that the protein unfolds by a two-state (native to denatured) mechanism. Hydrogen exchange in hFGF-1, under the experimental conditions used, occurs by the EX2 mechanism. In contrast to the equilibrium unfolding events monitored by optical probes, native state hydrogen exchange data show that the beta-trefoil architecture of hFGF-1 does not behave as a single cooperative unit. There are at least two structurally independent units with differing stabilities in hFGF-1. Beta-strands I, II, III, VI, VII, X, XI, and XII fit into the global unfolding isotherm. By contrast, residues in beta-strands IV, V, VIII, and IX exchange by the subfolding isotherm and could be responsible for the occurrence of high-energy partially unfolded state(s) in hFGF-1. There appears to be a broad continuum of stabilities among the four beta-strands (beta-strands IV, V, VIII, and IX) constituting the subglobal folding unit. The slow exchanging residues in hFGF-1 do not represent the folding nucleus of the protein.

Effect of heparin chain length on the interaction with tissue factor pathway inhibitor (TFPI)

Tissue factor pathway inhibitor (TFPI) is a heparin-binding protein involved in the extrinsic blood coagulation system. In order to elucidate the minimal size of heparin chain required for the interaction with TFPI, we prepared a series of heparin-derived oligosaccharides with tailored chain length ranged from disaccharide to eicosasaccharide after the successive treatments of heparin, including partial N-desulphation, deaminative cleavage with nitrous acid and gel-filtration. Affinity chromatography study of each oligosaccharide fraction using TFPI as the ligand indicated that increasing the degree of polymerisation causes increased affinity, and that a remarkable change in the affinity occurs between the decamers and dodecamers. Measurement of factor Xa inhibitory activity of TFPI in the presence of each oligosaccharide fraction indicated that the fractions shorter than dodecamers only slightly enhanced the TFPI activity for factor Xa inhibition, while the fractions larger than octadecamers had an effect comparable to full-length heparin. These were compatible to the results from the kinetic analyses of the interaction between TFPI and heparin-derived oligosaccharide with an evanescent wave-based biosensor system, IAsys, using a TFPI C-terminal peptide as the ligand.

Oligomerization of acidic fibroblast growth factor is not a prerequisite for its cell proliferation activity

Oligomerization of fibroblast growth factors (FGFs) induced on binding to heparin or heparan sulfate proteoglycan is considered to be crucial for receptor activation and initiation of biological responses. To gain insight into the mechanism of activation of the receptor by FGFs, in the present study we investigate the effect(s) of interaction of a heparin analog, sucrose octasulfate (SOS), on the structure, stability, and biological activities of a recombinant acidic FGF from Notophthalmus viridescens (nFGF-1). SOS is found to bind to nFGF-1 and significantly increase the thermodynamic stability of the protein. Using a variety of techniques such as size-exclusion chromatography, sedimentation velocity, and multidimensional nuclear magnetic resonance (NMR) spectroscopy, it is shown that binding of SOS to nFGF-1 retains the protein in its monomeric state. In its monomeric state (complexed to SOS), n-FGF-1 shows significant cell proliferation activity. (15)N and (1)H chemical shift perturbation and the intermolecular nuclear Overhauser effects (NOEs) between SOS and nFGF-1 reveal that the ligand binds to the dense, positively charged cluster located in the groove enclosed by beta-strands 10 and 11. In addition, molecular modeling based on the NOEs observed for the SOS-nFGF-1 complex, indicates that SOS and heparin share a common binding site on the protein. In conclusion, the results of the present study clearly show that heparin-induced oligomerization of nFGF-1 is not mandatory for its cell proliferation activity.

Nature of Interaction between basic fibroblast growth factor and the antiangiogenic drug 7,7-(carbonyl-bis[imino-N-methyl-4,2-pyrrolecarbonylimino[N-methyl-4,2-pyrrole]-carbonylimino])-bis-(1,3-naphtalene disulfonate). II. Removal of polar interactions affects protein folding

Fibroblast growth factor-2 (basic FGF), a potent inducer of angiogenesis, and the naphthalene sulfonic distamycin A derivative, 7,7-(carbonyl-bis[imino-N-methyl-4,2-pyrrolecarbonylimino[N-methyl-4,2-pyrrole]-carbonylimino])-bis-(1,3-naphtalene disulfonate) (PNU145156E), which exhibits in vivo antiangiogenic activity, form a tight reversible (1:1) complex. PNU145156E binds to the heparin and the selenate-binding sites on bFGF. The cis bFGF-heparin (2:1) complex, essential for the activation of the angiogenic process, is thus prevented. The nature of the forces involved in bFGF:PNU145156E complex, using the wild-type and the K128Q, K138Q, K134Q, and K128Q-K138Q point mutated bFGFs was sought. Based on thermodynamic analysis of the complexation constants, protein temperature stability profiles by ultraviolet absorption, circular dichroism measurements, fluorescence Förster energy-transfer, and anisotropy studies, in harmony with the published x-ray crystallographic structure, the following molecular interactions are proposed: reduced coulombic interactions, hence loosening of the complex by the removal of charged polar groups from the bFGF-heparin binding cleft resulted in decreased binding constants and in a change in the binding mode from polar to nonpolar. Concomitantly, upon mutation, the protein was rendered more compact, less flexible, and less aqueously exposed compared with the wild type. These were further pronounced with the double mutant: weaker dominantly nonpolar protein-drug interactions were accompanied by conspicuous folding. With heparin, however, wild-type bFGF forms a tighter complex with a more compact structure.

Synthesis of sulfated colominic acids and their interaction with fibroblast growth factors

Colominic acid, an alpha (2-->8)-linked poly(sialic acid), was sulfated and characterized by NMR spectroscopy. During sulfation, the secondary hydroxyl group at C-4 had almost the same reactivity as the primary hydroxyl group at C-9, while the secondary hydroxyl group at C-7 was hardly substituted. Analysis by molecular modeling suggested that the lack of substitution at C-7 was due to a steric hindrance. A mobility shift assay indicated that FGF-2 bound to the sulfated colominic acid. Synthetic sulfated colominic acid potentiated the mitogenic activity of FGFs for fibroblasts in the same manner as heparin. Sulfated colominic acid with a low degree of sulfation was able to potentiate FGF activity. Regardless of the degree of sulfation, sulfated colominic acid-induced cytotoxicity was not observed. It was suggested that the carboxyl groups in sulfated colominic acid cooperate with the sulfate groups to reinforce the interaction with FGFs and to reduce the cytotoxicity of sulfated colominic acids.

Order out of complexity--protein structures that interact with heparin

Many proteins of widely differing functionality and structure are capable of binding heparin. Structural characterisations of the many types of such complexes are being reported in ever-increasing number and at improved resolution. Several crystal structures of complexes formed through the interaction of heparin-derived oligosaccharides with one or more protein partners have been described.

Structures of heparin-derived disaccharide bound to cobra cardiotoxins: context-dependent conformational change of heparin upon binding to the rigid core of the three-fingered toxin

Glycosaminoglycans (GAGs) have been suggested to be a potential target for cobra cardiotoxin (CTX) with high affinity and specificity via a cationic belt at the concave surface of the polypeptide. The interaction of GAGs, such as high-molecular weight heparin, with CTXs not only can induce aggregation of CTX molecules but also can enhance their penetration into membranes. The binding of short chain heparin, such as a heparin-derived disaccharide [DeltaUA2S(1-->4)-alpha-D-GlcNS6S], to CTX A3 from Taiwan cobra (Naja atra), however, will not induce aggregation and was, therefore, investigated by high-resolution (1)H NMR. A novel heparin binding site on the convex side of the CTX, near the rigid disulfide bond-tightened core region of Cys38, was identified due to the observation of intermolecular NOEs between the protein and carbohydrate. The derived carbohydrate conformation using complete relaxation and conformational exchange matrix analysis (CORCEMA) of NOEs indicated that the glycosidic linkage conformation and the ring conformation of the unsaturated uronic acid in the bound state depended significantly on the charge context of CTX molecules near the binding site. Specifically, comparative binding studies of several heparin disaccharide homologues with two CTX homologues (CTX Tgamma from Naja nigricollis and CTX A3) indicated that the electrostatic interaction of N-sulfate of glucosamine with NH(3)(+)zeta of Lys12 and of the 2-O-sulfate of the unsaturated uronic acid with NH(3)(+)zeta of Lys5 played an important role. These results also suggest a model on how the CTX-heparin interaction may regulate heparin-induced aggregation of the toxin via the second heparin binding site.

The analysis of heparin-protein interactions using evanescent wave biosensor with regioselectively desulfated heparins as the ligands

Evanescent wave biosensor has been recently employed as a powerful tool for analyses of macromolecular interactions. In the present study, evanescent wave biosensor analysis was developed to analyze the heparin-protein interaction using as ligands a series of heparin derivatives regioselectively desulfated by chemical methods, particularly to evaluate the effect of each sulfate group of heparin. The method for immobilizing heparin on the cuvette of the evanescent wave biosensor equipment was optimized to obtain the high response required for accurate measurement. The best result was achieved when the amino group introduced at the reducing end of heparin was coupled with carboxymethyl dextran on the surface of the cuvette using glycolchitosan as a multivalent linker. The established system appeared to describe well the interactions of heparin with such proteins as acidic and basic fibroblast growth factors and tissue factor pathway inhibitor.

Thermodynamic characterization of the human acidic fibroblast growth factor: evidence for cold denaturation

The thermodynamic parameters characterizing the conformational stability of the human acidic fibroblast growth factor (hFGF-1) have been determined by isothermal urea denaturation and thermal denaturation at fixed concentrations of urea using fluorescence and far-UV CD circular dichroism (CD) spectroscopy. The equilibrium unfolding transitions at pH 7.0 are adequately described by a two-state (native <--> unfolded state) mechanism. The stability of the protein is pH-dependent, and the protein unfolds completely below pH 3.0 (at 25 degrees C). hFGF-1 is shown to undergo a two-state transition only in a narrow pH range (pH 7.0-8.0). Under acidic (pH <6.0) and basic (pH >8.0) conditions, hFGF-1 is found to unfold noncooperatively, involving the accumulation of intermediates. The average temperature of maximum stability is determined to be 295.2 K. The heat capacity change (DeltaC(p)()) for the unfolding of hFGF-1 is estimated to be 2.1 +/- 0.5 kcal.mol(-1).K(-1). Temperature denaturation experiments in the absence and presence of urea show that hFGF-1 has a tendency to undergo cold denaturation. Two-dimensional (1)H-(15)N HSQC spectra of hFGF-1 acquired at subzero temperatures clearly show that hFGF-1 unfolds under low-temperature conditions. The significance of the noncooperative unfolding under acidic conditions and the cold denaturation process observed in hFGF-1 are discussed in detail.

Importance of basic residues and quaternary structure in the function of MIP-1 beta: CCR5 binding and cell surface sugar interactions

Chemokines direct immune cells toward sites of infection by establishing a gradient across the extracellular matrix of the tissue. This gradient is thought to be stabilized by ligation of chemokines to sulfated polysaccharides known as glycosaminoglycans (GAGs) that are found on the surface of endothelial and other cells as well as in the tissue matrix. GAGs interact with chemokines and in some cases cause them to aggregate. The interaction between cell surface GAGs and chemokines has also been postulated to play a role in the anti-HIV activity of some chemokines, including MIP-1beta. Since many proteins interact with GAGs by utilizing basic residues, we mutated R18, K45, R46, and K48 in MIP-1beta to investigate the role of these residues in GAG binding and CCR5 function. We find that no single amino acid substitution alone has a dramatic effect on heparin binding, although change at R46 has a moderate effect. However, binding to heparin is completely abrogated in a mutant (K45A/R46A/K48A) in which the entire "40's loop" has been neutralized. A functional study of these mutants reveals that the charged residues in this 40's loop, particularly K48 and R46, are critical mediators of MIP-1beta binding to its receptor CCR5. However, despite the partially overlapping function of the residues in the 40's loop in binding to both CCR5 and heparin, the presence of cell surface sugars does not appear to be necessary for the ability of MIP-1beta to function on its receptor CCR5, as enzymatic removal of GAGs from cells results in little effect on MIP-1beta activity. Because the means by which the chemokine gradient transmits information to the recruited cells is not well defined, we also mutated the basic residues in MIP(9), a truncated form of MIP-1beta that is impaired in its ability to dimerize, to probe whether the quaternary structure of this chemokine influences its ability to bind heparin. None of the truncated variants bound as well as the full-length proteins containing the same mutation, suggesting that the MIP-1beta dimer participates in heparin binding.

Structural events during the refolding of an all beta-sheet protein

The refolding kinetics of the 140-residue, all beta-sheet, human fibroblast growth factor (hFGF-1) is studied using a variety of biophysical techniques such as stopped-flow fluorescence, stopped-flow circular dichroism, and quenched-flow hydrogen exchange in conjunction with multidimensional NMR spectroscopy. Urea-induced unfolding of hFGF-1 under equilibrium conditions reveals that the protein folds via a two-state (native <--> unfolded) mechanism without the accumulation of stable intermediates. However, measurement of the unfolding and refolding rates in various concentrations of urea shows that the refolding of hFGF-1 proceeds through accumulation of kinetic intermediates. Results of the quenched-flow hydrogen exchange experiments reveal that the hydrogen bonds linking the N- and C-terminal ends are the first to form during the refolding of hFGF-1. The basic beta-trefoil framework is provided by the simultaneous formation of beta-strands I, IV, IX, and X. The other beta-strands comprising the beta-barrel structure of hFGF-1 are formed relatively slowly with time constants ranging from 4 to 13 s.

15N NMR relaxation studies of free and ligand-bound human acidic fibroblast growth factor

15N NMR relaxation data have been used to characterize the backbone dynamics of the human acidic fibroblast growth factor (hFGF-1) in its free and sucrose octasulfate (SOS)-bound states. (15)N longitudinal (R(1)), transverse (R(2)) relaxation rates and (1H)-(15)N steady-state nuclear Overhauser effects were obtained at 500 and 600 MHz (at 25 degrees C) for all resolved backbone amide groups using (1)H- detected two-dimensional NMR experiments. Relaxation data were fit to the extended model free dynamics for each NH group. The overall correlation time (tau(m)) for the free and SOS-bound forms were estimated to be 10.4 +/- 1.07 and 11.1 +/- 1.35 ns, respectively. Titration experiments with SOS reveals that the ligand binds specifically to the C-terminal domain of the protein in a 1:1 ratio. Binding of SOS to hFGF-1 is found to induce a subtle conformational change in the protein. Significant conformational exchange (R(ex)) is observed for several residues in the free form of the protein. However, in the SOS-bound form only three residues exhibit significant R(ex) values, suggesting that the dynamics on the micro- to millisecond time scale in the free form is coupled to the cis-trans-proline isomerization. hFGF-1 is a rigid molecule with an average generalized parameter (S(2)) value of 0.89 +/- 0.03. Upon binding to SOS, there is a marked decrease in the overall flexibility (S(2) = 0.94 +/- 0.02) of the hFGF-1 molecule. However, the segment comprising residues 103-111 shows increased flexibility in the presence of SOS. Significant correlation is found between residues that show high flexibility and the putative receptor binding sites on the protein.

Heparin and heparan sulfate: biosynthesis, structure and function

Heparin and heparan sulfate glycosaminoglycans are acidic complex polysaccharides found on the cell surface and in the extracellular matrix. Recent progress has uncovered a virtual explosion of important roles of these biopolymers in fundamental biological processes. Advances in the understanding of biosynthesis and structure and the development of novel analytical methods for composition and sequence analysis have provided remarkable insights into structure/function relationships of these complex and once elusive polysaccharides.

Identification and characterization of an equilibrium intermediate in the unfolding pathway of an all beta-barrel protein

The guanidinium hydrochloride (GdnHCl)-induced unfolding of an all beta-sheet protein, the human acidic fibroblast growth factor (hFGF-1), is studied using a variety of biophysical techniques including multidimensional NMR spectroscopy. The unfolding of hFGF-1 in GdnHCl is shown to involve the formation of a stable equilibrium intermediate. Size exclusion chromotagraphy using fast protein liquid chromatography shows that the intermediate accumulates maximally at 0.96 m GdnHCl. 1-Anilinonapthalene 8-sulfonate binding, one-dimensional (1)H NMR, and limited proteolytic digestion experiments suggest that the intermediate has characteristics resembling a molten globule state. Chemical shift perturbation and hydrogen-deuterium exchange monitored by (1)H-(15)N heteronuclear single quantum coherence spectra reveal that profound structural changes in the intermediate state (in 0.96 m GdnHCl) occur in the C-terminal, heparin binding region of the protein molecule. Additionally, results of the stopped flow fluorescence experiments suggest that the kinetic refolding of hFGF-1 proceeds through the accumulation of an intermediate at low concentrations of the denaturant. To our knowledge, the present study is the first report wherein an equilibrium intermediate is characterized in detail in an all beta-barrel protein.