Five PDGF B amino acid substitutions convert PDGF A to a PDGF B-like transforming molecule

A review of platelet derived growth factor playing pivotal role in bone regeneration

This article is focused on the literature review and study of recent advances in the field of bone grafting, which involves platelet-derived growth factor (PDGF) as one of the facilitating factors in bone regeneration. This article includes a description of the mechanism of PDGF for use in surgeries where bone grafting is required, which promotes future application of PDGF for faster bone regeneration or inhibition of bone growth if required as in osteosarcoma. The important specific activities of PDGF include mitogenesis (increase in the cell populations of healing cells), angiogenesis (endothelial mitoses into functioning capillaries), and macrophage activation (debridement of the wound site and a second phase source of growth factors for continued repair and bone regeneration). Thus PDGF can be utilized in wound with bone defect to conceal the wound with repair of bony defect.

Vaccination with platelet-derived growth factor B kinoids inhibits CCl₄-induced hepatic fibrosis in mice

Platelet-derived growth factor B (PDGF-B) plays an essential role in hepatic fibrosis. Inhibition of the PDGF-B signaling in chronically injured livers might represent a potential therapeutic measure for hepatic fibrosis. In this study, we assessed the effects of vaccination against PDGF-B on CCl₄-induced liver fibrosis in BALB/c mice. The PDGF-B kinoid immunogens were prepared by cross-linking two PDGF-B-derived B-cell epitope peptides [PDGF-B¹⁶-(23-38) and PDGF-B¹⁶-(72-83)] to ovalbumin and keyhole limpet hemocyanin, respectively. Enzyme-linked immunosorbent assay, Western blotting, and NIH3T3 cell proliferation assay verified that immunization with the PDGF-B kinoids elicited the production of high levels of neutralizing anti-PDGF-B autoantibodies. The vaccination markedly alleviated CCl₄-induced hepatic fibrosis, as indicated by the lessened morphological alternations and reduced hydroxyproline contents in the mouse livers. Moreover, immunohistochemical staining for proliferating cell nuclear antigen, α-smooth muscle actin, and desmin demonstrated that neutralization of PDGF-B inhibited both the proliferation and the activation of hepatic stellate cells in the fibrotic mouse livers. Taken together, this study demonstrated that vaccination with PDGF-B kinoids significantly suppressed CCl₄-induced hepatic fibrosis in mice. Our results suggest that vaccination against PDGF-B might be developed into an effective, convenient, and safe therapeutic measure for the treatment of hepatic fibrosis.

Structures of a platelet-derived growth factor/propeptide complex and a platelet-derived growth factor/receptor complex

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) are prototypic growth factors and receptor tyrosine kinases which have critical functions in development. We show that PDGFs share a conserved region in their prodomain sequences which can remain noncovalently associated with the mature cystine-knot growth factor domain after processing. The structure of the PDGF-A/propeptide complex reveals this conserved, hydrophobic association mode. We also present the structure of the complex between PDGF-B and the first three Ig domains of PDGFRbeta, showing that two PDGF-B protomers clamp PDGFRbeta at their dimerization seam. The PDGF-B:PDGFRbeta interface is predominantly hydrophobic, and PDGFRs and the PDGF propeptides occupy overlapping positions on mature PDGFs, rationalizing the need of propeptides by PDGFs to cover functionally important hydrophobic surfaces during secretion. A large-scale structural organization and rearrangement is observed for PDGF-B upon receptor binding, in which the PDGF-B L1 loop, disordered in the structure of the free form, adopts a highly specific conformation to form hydrophobic interactions with the third Ig domain of PDGFRbeta. Calorimetric data also shows that the membrane-proximal homotypic PDGFRalpha interaction, albeit required for activation, contributes negatively to ligand binding. The structural and biochemical data together offer insights into PDGF-PDGFR signaling, as well as strategies for PDGF-antagonism.

A synthetic, bioactive PDGF mimetic with binding to both alpha-PDGF and beta-PDGF receptors

A multi-domain peptide, PAB2-1c, was designed and synthesized as a bioactive mimic of PDGF. PBA2-1c bound to both alpha- and beta-PDGF receptors as determined by surface plasmon resonance (SPR). The equilibrium dissociation constant (Kd) of binding to alpha-PDGF receptors by PAB2-1c (1.7 x 10(-8) M) compared favorably rhPDGF-AA (1.34 x 10(-8) M). Binding to -PDGF receptor by PAB2-1c (2.2 x 10(-8) M) was less favorable than, that of recombinant human PDGFBB (1.59 x 10(-9) M). Interestingly, PBA2-1c bound to these two receptors with similar affinity suggesting that, PBA2-1c was not PDGF receptor selective. In a murine myoblast cell line C2C12, PBA2-1c increased the tyrosine phosphorylation on PDGF receptors and the phosphorylation of AKT and ERK1/2 in a concentration-related manner. PBA2-1c also stimulated an increase in cell proliferation, cell migration, and collagen gel contraction. In these cell-based assays, PAB2-1c was effective at 1 microg/ml or lesser. The results support the hypothesis that PBA2-1c is a mimetic of PDGF, although it has a more promiscuous receptor interaction.

Structure-activity studies on a library of potent calix[4]arene-based PDGF antagonists that inhibit PDGF-stimulated PDGFR tyrosine phosphorylation

Platelet-derived growth factor (PDGF) and its receptor PDGFR are required for tumor growth and angiogenesis, so disruption of the PDGF-PDGFR interaction should lead to starvation of tumors and reduction of tumor growth. Potent PDGF antagonists have been discovered through the synthesis of a series of calix[4]arene-based compounds that are designed to bind to the three-loop region of PDGF. The effect of lower-rim alkylation, linker and number of interacting head groups on the calix[4]arene scaffold on PDGF affinity and cellular activity has been investigated.

Thyroid-stimulating hormone and thyroid-stimulating hormone receptor structure-function relationships

This review focuses on recent advances in the structure-function relationships of thyroid-stimulating hormone (TSH) and its receptor. TSH is a member of the glycoprotein hormone family constituting a subset of the cystine-knot growth factor superfamily. TSH is produced by the pituitary thyrotrophs and released to the circulation in a pulsatile manner. It stimulates thyroid functions using specific membrane TSH receptor (TSHR) that belongs to the superfamily of G protein-coupled receptors (GPCRs). New insights into the structure-function relationships of TSH permitted better understanding of the role of specific protein and carbohydrate domains in the synthesis, bioactivity, and clearance of this hormone. Recent progress in studies on TSHR as well as studies on the other GPCRs provided new clues regarding the molecular mechanisms of receptor activation. Such advances are a result of extensive site-directed mutagenesis, peptide and antibody approaches, detailed sequence analyses, and molecular modeling as well as studies on naturally occurring gain- and loss-of-function mutations. This review integrates expanding information on TSH and TSHR structure-function relationships and summarizes current concepts on ligand-dependent and -independent TSHR activation. Special emphasis has been placed on TSH domains involved in receptor recognition, constitutive activity of TSHR, new insights into the evolution of TSH bioactivity, and the development of high-affinity TSH analogs. Such structural, physiological, pathophysiological, evolutionary, and therapeutic implications of TSH-TSHR structure-function studies are frequently discussed in relation to concomitant progress made in studies on gonadotropins and their receptors.

Platelet-derived growth factor-BB and basic fibroblast growth factor directly interact in vitro with high affinity

Platelet-derived growth factor-BB (PDGF-BB) and basic fibroblast growth factor (bFGF) are potent growth factors active on many cell types. The present study indicates that they directly interact in vitro. The interaction was investigated with overlay experiments, surface plasmon resonance experiments, and solid-phase immunoassays by immobilizing one factor or the other and by steady-state fluorescence analysis. The interaction observed was specific, dose-dependent, and saturable, and the bFGF/PDGF-BB binding stoichiometry was found to be 2:1. K(D)(1) for the first step equilibrium and the overall K(D) values were found to be in the nanomolar and in the picomolar range, respectively. Basic FGF/PDGF-BB interaction was strongly reduced as a function of time of PDGF-BB proteolysis. Furthermore, docking analysis suggested that the PDGF-BB region interacting with bFGF may overlap, at least in part, with the PDGF-BB receptor-binding site. This hypothesis was supported by surface plasmon resonance experiments showing that an anti-PDGF-BB antibody, known to inhibit PDGF-BB binding with its receptor, strongly reduced bFGF/PDGF-BB interaction, whereas a control antibody was ineffective. According to these data, the observed bFGF.PDGF-BB complex formation might explain, at least in part, previous observations showing that PDGF-BB chemotactic and mitogenic activity on smooth muscle cells are strongly inhibited in the presence of bFGF.

Design of growth factor antagonists with antiangiogenic and antitumor properties

This review describes our recent efforts in the development of novel therapies for cancer. Our primary approach is to design synthetic agents that antagonize the function of growth factors that are critically involved in oncogenesis and angiogenesis. We achieve this by designing synthetic molecules that can recognize the exterior surface of the growth factor and so block the interaction with its receptor tyrosine kinase. A key step is the construction of synthetic agents that contain a large (> 400A2) and functionalized surface area to recognize a complementary surface on the target growth factor. In the course of this work we have discovered a molecule, GFB-111, that binds to PDGF, prevents it from binding to its receptor tyrosine kinase, blocks PDGF-induced receptor autophosphorylation, activation of Erk1 and Erk2 kinases and DNA synthesis. The binding affinity for PDGF is high (IC50=250 nM) and selective over EGF, IGF-1, aFGF, bFGF and HRGbeta. In nude mouse models GFB-111 also shows significant inhibition of tumor growth and angiogenesis.

Human thyroid-stimulating hormone: structure-function analysis

This article provides the reader with an overview of methodological strategies to investigate structure-function relationships of human thyroid-stimulating hormone (hTSH). Various aspects of hTSH production, purification, and characterization described here in more detail are not only relevant to studies on other members of the glycoprotein hormone family, but also applicable to studies of other glycosylated proteins. Knowledge of structure-function relationships of specific hTSH domains is important for a better understanding of the molecular mechanisms of its action. New insights from such studies permit the design of glycoprotein hormone analogs with specific pharmacological properties and potential clinical applications.

Predimerization of recombinant platelet-derived growth factor receptor extracellular domains increases antagonistic potency

Platelet-derived growth factor (PDGF) is a dimeric growth factor acting through tyrosine kinase alpha- and beta-receptors. In both receptors, the extracellular parts are composed of five Ig-like domains. Functional mapping of the extracellular part of the receptors have shown that ligand-binding occurs to Ig-like domains 2 and 3 and that Ig-like domain 4 is involved in receptor-receptor interactions. Recombinant GST-fusion proteins of PDGF alpha-receptor Ig-like domains 1-4 and beta-receptor Ig-like domains 1-3 (alphaRD1-4-GST and betaRD1-3-GST) were generated and compared with their cleaved counterparts (alphaRD1-4 and betaRD1-3) with regard to their ability to block PDGF binding to cell surface receptors. In the case of both the alpha- and the beta-receptors, 100-1000-fold lower concentrations of the GST-fusion proteins were required, as compared to the cleaved forms, for inhibition of PDGF binding to cell surface receptors. alphaRD1-4-GST and betaRD1-3-GST, in contrast to alphaRD1-4 and betaRD1-3, were shown to occur as ligand independent dimers. Covalently cross-linked alphaRD1-4 dimers displayed a 50-fold increased potency as compared to alphaRD1-4. We thus conclude that the dimeric nature of alphaRD1-4-GST and betaRD1-3-GST is responsible for the high antagonistic potency of the fusion proteins.

Mechanism of action and in vivo role of platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a major mitogen for connective tissue cells and certain other cell types. It is a dimeric molecule consisting of disulfide-bonded, structurally similar A- and B-polypeptide chains, which combine to homo- and heterodimers. The PDGF isoforms exert their cellular effects by binding to and activating two structurally related protein tyrosine kinase receptors, denoted the alpha-receptor and the beta-receptor. Activation of PDGF receptors leads to stimulation of cell growth, but also to changes in cell shape and motility; PDGF induces reorganization of the actin filament system and stimulates chemotaxis, i.e., a directed cell movement toward a gradient of PDGF. In vivo, PDGF has important roles during the embryonic development as well as during wound healing. Moreover, overactivity of PDGF has been implicated in several pathological conditions. The sis oncogene of simian sarcoma virus (SSV) is related to the B-chain of PDGF, and SSV transformation involves autocrine stimulation by a PDGF-like molecule. Similarly, overproduction of PDGF may be involved in autocrine and paracrine growth stimulation of human tumors. Overactivity of PDGF has, in addition, been implicated in nonmalignant conditions characterized by an increased cell proliferation, such as atherosclerosis and fibrotic conditions. This review discusses structural and functional properties of PDGF and PDGF receptors, the mechanism whereby PDGF exerts its cellular effects, and the role of PDGF in normal and diseased tissues.

Role of immunoglobulin-like domains 2-4 of the platelet-derived growth factor alpha-receptor in ligand-receptor complex assembly

Platelet-derived growth factor (PDGF) is a dimeric protein that exerts its effects through tyrosine kinase alpha- and beta-receptors. The extracellular part of each receptor is composed of five Ig-like domains. Recombinant forms of alpha-receptor domains 1-4 (alphaRD1-4), 1-3 (alphaRD1-3), and 1 and 2 (alphaRD1-2) were prepared after expression in Chinese hamster ovary cells and were used to study the assembly of soluble ligand-receptor complexes. When incubated with micromolar concentrations of PDGF, both alphaRD1-3 and alphaRD1-4 formed complexes of 1:2 molar composition, i.e. one dimeric PDGF molecule bound two soluble receptors. alphaRD1-3, in contrast to alphaRD1-4, formed detectable 1:1 complexes under conditions of ligand excess. alphaRD1-4 displayed an increased ability to form 1:2 complexes as compared with alphaRD1-3 under conditions of limiting concentrations of ligand. We thus conclude that Ig-like domain 4-mediated receptor-receptor interactions contribute to 1:2 PDGF.alphaRD1-4 complex formation. Since alphaRD1-4 and alphaRD1-3 were equipotent in blocking binding of subnanomolar concentrations of PDGF to cell-surface receptors, we also conclude that this effect is predominantly achieved through formation of Ig-like domain 4-independent 1:1 ligand-receptor complexes. Finally, since alphaRD1-2 bound PDGF-BB with high affinity, whereas PDGF-AA was bound only with low affinity, we conclude that Ig-like domain 3 of the PDGF alpha-receptor contains epitopes of particular importance for PDGF-AA binding and that most of the PDGF-BB-binding epitopes reside in Ig-like domains 1 and 2.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

The crystal structure of vascular endothelial growth factor (VEGF) refined to 1.93 A resolution: multiple copy flexibility and receptor binding

BACKGROUND

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific angiogenic and vasculogenic mitogen. VEGF also plays a role in pathogenic vascularization which is associated with a number of clinical disorders, including cancer and rheumatoid arthritis. The development of VEGF antagonists, which prevent the interaction of VEGF with its receptor, may be important for the treatment of such disorders. VEGF is a homodimeric member of the cystine knot growth factor superfamily, showing greatest similarity to platelet-derived growth factor (PDGF). VEGF binds to two different tyrosine kinase receptors, kinase domain receptor (KDR) and Fms-like tyrosine kinase 1 (Flt-1), and a number of VEGF homologs are known with distinct patterns of specificity for these same receptors. The structure of VEGF will help define the location of the receptor-binding site, and shed light on the differences in specificity and cross-reactivity among the VEGF homologs.

RESULTS

We have determined the crystal structure of the receptor-binding domain of VEGF at 1.93 A resolution in a triclinic space group containing eight monomers in the asymmetric unit. Superposition of the eight copies of VEGF shows that the beta-sheet core regions of the monomers are very similar, with slightly greater differences in most loop regions. For one loop, the different copies represent different snapshots of a concerted motion. Mutagenesis mapping shows that this loop is part of the receptor-binding site of VEGF.

CONCLUSIONS

A comparison of the eight independent copies of VEGF in the asymmetric unit indicates the conformational space sampled by the protein in solution; the root mean square differences observed are similar to those seen in ensembles of the highest precision NMR structures. Mapping the receptor-binding determinants on a multiple sequence alignment of VEGF homologs, suggests the differences in specificity towards KDR and Flt-1 may derive from both sequence variation and changes in the flexibility of binding loops. The structure can also be used to predict possible receptor-binding determinants for related cystine knot growth factors, such as PDGF.

Identification of a cyclic peptide inhibitor of platelet-derived growth factor-BB receptor-binding and mitogen-induced DNA synthesis in human fibroblasts

Peptides corresponding to residues from Loops I and III of platelet-derived growth factor-BB (PDGF-BB) were examined for their potential to act as PDGF antagonists. We have identified two peptides which directly stimulated DNA synthesis in human dermal fibroblasts and a cyclic peptide which inhibited PDGF-induced DNA synthesis. The inhibitory action of cyclic PDGF-BB(73-81), on DNA synthesis was shown to be restricted to cells which express PDGF receptors. Also cyclic PDGF-BB(73-81) specifically competed for 125I-labelled PDGF-BB but not for 125I-labelled EGF binding to their respective cellular receptors. The cyclic peptide therefore provides a minimum structure to investigate PDGF/receptor interactions and our findings confirm the importance of the loop configuration of PDGF-BB(73-81) in the native molecule. The cyclic peptide may constitute a basis for developing more potent inhibitors of PDGF action.

Immunoglobulin-like domain 4-mediated receptor-receptor interactions contribute to platelet-derived growth factor-induced receptor dimerization

Platelet-derived growth factor (PDGF) is a dimeric growth factor that activates its tyrosine kinase receptor by inducing receptor dimerization. In this study, we investigated if receptor-receptor interactions, in addition to ligand-receptor interactions, contribute to the ligand-induced dimerization of the PDGF receptors. Analysis of two deletion mutants of the PDGF alpha-receptor indicated a role for Ig-like domain 4 in ligand-receptor or receptor-receptor interactions. When the fourth Ig-like domain of the PDGF alpha-receptor instead was replaced with the corresponding sequence of the stem cell factor receptor, the binding of PDGF-AA and -BB was not affected, nor was the ability to form homodimeric receptor complexes. This indicates that Ig-like domain 4 does not participate in ligand-receptor interactions. However, the chimeras did not form heterodimers with wild-type PDGF alpha- or beta-receptors. Together, these findings suggest that Ig-like domain 4 mediates specific receptor-receptor interactions. This notion was also supported by the finding that a soluble form of Ig-like domain 4 of the PDGF alpha-receptor acted as a PDGF alpha-receptor antagonist. We conclude that specific receptor-receptor interactions contribute to PDGF receptor dimerization in vivo and that complementary epitopes in Ig-like domain 4 mediate these interactions. Our experiments also identify Ig-like domain 4 as a target for PDGF antagonists.

Inhibitory DNA ligands to platelet-derived growth factor B-chain

We have identified a group of DNA molecules that bind to platelet-derived growth factor (PDGF)-AB with subnanomolar affinity from a randomized DNA library using in vitro selection. Individual ligands cloned from the affinity-enriched pool bind to PDGF-AB and PDGF-BB with comparably high affinity (Kd approximately 10(-10) M) and to PDGF-AA with lower affinity (> 10(-8) M), indicating specific recognition of the PDGF B-chain in the context of the hetero- or homodimer. The consensus secondary structure motif for most of the high-affinity ligands is a three-way helix junction with a three-nucleotide loop at the branch point. Photo-cross-linking experiments with 5-iodo-2'-deoxyuridine-substituted ligands establish a point contact between a thymidine nucleotide in the helix junction loop region and phenylalanine 84 of the PDGF-B chain. Representative minimal DNA ligands inhibit the binding of 125I-PDGF-BB but not of 125I-PDGF-AA to PDGF alpha- or beta-receptors expressed in porcine aortic endothelial (PAE) cells in a concentration-dependent manner with half-maximal effects of approximately 1 nM. The same ligands also exhibit a similar inhibitory effect on PDGF-BB-dependent [3H]thymidine incorporation in PAE cells expressing the PDGF beta-receptors. These DNA ligands represent a novel class of specific and potent antagonists of PDGF-BB and, by inference, PDGF-AB.

Identification of three amino acid residues in the B-chain of platelet-derived growth factor with different importance for binding to PDGF alpha- and beta-receptors

The B-chain homodimer isoform of platelet-derived growth factor (PDGF) binds with high affinity both to alpha- and to beta-receptors. In order to localize amino acid residues in PDGF-BB of differential importance for the binding to the two receptors, PDGF-BB mutants were analyzed in which single amino acid residues were changed to alanine residues. We found that Phe-118 in loop 1 of the PDGF B-chain is crucial for binding to both receptors, and that the surrounding amino acids, Asn-117 and Leu-119, appear to be important primarily for binding to the beta-receptor. In contrast, Lys-161 in loop 3 was found to be more important for binding to alpha-receptors than beta-receptors. Previous studies have shown that the receptor binding epitope of PDGF-BB is composed mainly of loops 1 and 3; the findings of the present study show that the alpha- and beta-receptors interact with different amino acid residues in these regions.

Identification of vascular endothelial growth factor determinants for binding KDR and FLT-1 receptors. Generation of receptor-selective VEGF variants by site-directed mutagenesis

Vascular endothelial growth factor (VEGF) expression in various cell types is induced by hypoxia and other stimuli. VEGF mediates endothelial cell proliferation, angiogenesis, vascular growth, and vascular permeability via the endothelial cell receptors, kinase insert domain-containing receptor (KDR)/fetal liver kinase 1 (Flk-1) and FLT-1. Alanine-scanning mutagenesis was used to identify a positively charged surface in VEGF that mediates binding to KDR/Flk-1. Arg82, Lys84 and His86, located in a hairpin loop, were found to be critical for binding KDR/Flk-1, while negatively charged residues, Asp63, Glu64, and Glu67, were associated with FLT-1 binding. A VEGF model based on PDGFb indicated these positively and negatively charged regions are distal in the monomer but are spatially close in the dimer. Mutations within the KDR site had minimal effect on FLT-1 binding, and mutants deficient in FLT-1 binding did not affect KDR binding. Endothelial cell mitogenesis was abolished in mutants lacking KDR affinity; however, FLT-1 deficient mutants induced normal proliferation. These results suggest dual sets of determinants in the VEGF dimer that cross-link cell surface receptors, triggering endothelial cell growth and angiogenesis. Furthermore, this mutational analysis implicates KDR, but not FLT-1, in VEGF induction of endothelial cell proliferation.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

Insect prothoracicotropic hormone: a new member of the vertebrate growth factor superfamily

Prothoracicotropic hormone (PTTH) is a brain neurosecretory protein that controls insect development. PTTH of the silkmoth Bombyx mori is a homodimeric protein, the subunit of which consists of 109 amino acids. Clear-cut sequence similarity to any other proteins has not been observed. By disulfide-bond pattern analysis and modeling of the PTTH structure based on the known three-dimensional (3D) structures of growth factor family with cystine-knot motif, we propose that the PTTH protomer adopts the fold unique to the structural superfamily of the growth factors, beta-nerve growth factor (beta-NGF), transforming growth factor-beta 2 (TGF-beta 2), and platelet-derived growth factor-BB (PDGF-BB). The insect neurohormone PTTH appears to be a member of the growth factor superfamily, sharing a common ancestral gene with the three vertebrate growth factors, beta-NGF, TGF-beta 2 and PDGF-BB.

Structural role of extracellular domain 1 of alpha-platelet-derived growth factor (PDGF) receptor for PDGF-AA and PDGF-BB binding

The purpose of this study was to bacterially express, purify, and refold combinations of the extracellular immunoglobulin (Ig)-like domains (2-3, 1-3, and 1-5) of the human alpha-platelet-derived growth factor receptor (alpha PDGFR) to characterize molecular interactions with its ligand, platelet-derived growth factor (PDGF). The far UV circular dichroism spectroscopy of the alpha-PDGFR extracellular domains (ECDs) revealed a predominantly beta-sheet protein, with a structure consistent with folded Ig-like domains. The addition of PDGF-BB to these ECD types changed the conformation of all three types with a decrease in mean residue ellipticity in the following rank order: 1-5 = 1-3 > 2-3. In striking contrast, addition of PDGF-AA to these ECD types markedly changed the conformation of ECD 2-3, by an increased mean residue ellipticity but no changes were observed for ECDs 1-3 and 1-5. PDGF-AA bound to the immobilized ECD types 2-3, 1-3, and 1-5 at concentrations of 20, 11, and 7.5 nM, respectively. In contrast, PDGF-BB bound the ECD types 2-3, 1-3, and 1-5 at concentrations of 3, 3, and 2.2 nM, respectively. Scatchard analysis of binding studies using labeled ECDs indicated that PDGF-BB bound ECD 1-3 and ECD 2-3 with KD values of 74 and 72 nM, respectively. While, PDGF-AA bound ECD 1-3 and ECD 2-3 with KD values of 33 and 87 nM, respectively. Therefore, our results indicated that the loss of ECD 1 impaired the binding affinity of alpha PDGFR ECD 1-3 toward PDGF-AA without having a similar effect on PDGF-BB binding. Together all of our data suggest that ECD 1 is differentially required for proper orientation of PDGF-AA but not PDGF-BB binding determinant within ECDs 2 and 3.

Platelet-derived growth factor in human glioma

Platelet-derived growth factor (PDGF) is a 30 kDa protein consisting of disulfide-bonded dimers of A- and B-chains. PDGF receptors are of two types, alpha- and beta-receptors, which are members of the protein-tyrosine kinase family of receptors. The receptors are activated by ligand-induced dimerization, whereby the receptors become phosphorylated on tyrosine residues. These form attachment sites for signalling molecules, which inter alia activate the Ras.Raf pathway. PDGF has important functions in development and is required for a proper timing of oligodendrocyte differentiation. The v-sis oncogene of simian sarcoma virus (SSV) is a retroviral homolog of the B-chain gene, and induces transformation by an autocrine activation of PDGF receptors at the cell surface. SSV induces malignant glioma in experimental animals, suggesting a role for autocrine PDGF in glioma development. PDGF and PDGF receptors are frequently coexpressed in human glioma cell lines. Specific and nonspecific PDGF antagonists block the growth of some glioma cell lines in vitro and in vivo, suggesting that autocrine PDGF is involved in transformation and tumorigenesis. In situ studies of human gliomas show overexpression of alpha-receptors in glioma cells of high-grade tumors. In a few cases, overexpression is caused by receptor amplification. Since high-grade glioma cells also express the PDGF A-chain, an autocrine activation of the alpha-receptor may drive the proliferation of glioma cells in vivo.

Involvement of loop 2 of platelet-derived growth factor-AA and -BB in receptor binding

Platelet-derived growth factor (PDGF) is a disulfide-bonded antiparallel dimer of A- and B-polypeptide chains. Each subunit contains two loops (loops 1 and 3) which point in the same direction, and which are located close to a region (loop 2) from the other subunit of the dimer. Previous studies have shown that epitopes in loops 1 and 3 are important for binding to PDGF alpha- and beta-receptors. The aim of the present investigation was to determine the importance of loop 2 for receptor interactions. PDGF A- and B-chain cDNA:s were mutated in the loop 2 regions and transfected into COS cells. Analyses of conditioned media of such cell cultures revealed that PDGF B-chain mutated in the loop 2 region lost its ability to compete with 125I-PDGF for binding to PDGF beta-receptors, but retained 2-5% of its binding of alpha-receptors. The A-chain binds only to alpha-receptors; 2-5% of this binding was also retained after mutation of the loop 2 region. In conclusion, the loop 2 region of PDGF is important for receptor binding, but appears to be more important for binding to the PDGF beta-receptors than to the alpha-receptors.

Production of platelet-derived growth factor receptor (PDGFR-beta) in E. coli. Mapping ligand binding domain

Portions of the extracellular domain of the platelet-derived growth factor receptor beta (PDGFR-beta) were expressed as fusion proteins with a hexa His tag in E. coli. Following purification by Ni chelate chromatography, the recombinant receptors were tested in cross-competition studies with 125I-labelled PDGF-AA and -BB. Although of lower affinity than the native receptor (IC50 values of 10(-8) M) the recombinant molecules retained ligand binding specificity and neutralized the mitogenic effect of PDGF-BB. These data indicate that the ligand binding region lies within the first four immunoglobulin-like domains on PDGFR-beta. This E. coli expression system could be further used as a rapid and economical means to produce mutated receptors and map the ligand binding domain.

Platelet-derived growth factor AB heterodimer interchain interactions influence secretion as well as receptor binding and activation

Platelet-derived growth factor (PDGF) is a disulfide-linked dimer comprised of two related polypeptide chains. To investigate the effects of an inactivating lesion introduced into one chain of the nascent PDGF dimer, approaches were developed to optimize synthesis, assembly, secretion, and purification of heterodimers between normal PDGF A and wild-type or mutant PDGF B. PDGF AB heterodimers were released into culture fluids less efficiently than PDGF AA, but to a greater degree than the cell-associated PDGF BB. These results suggest that interactions between two chains influence PDGF secretion. Analysis of heterodimers between PDGF A and disabled PDGF B mutants on cells that express either alpha or beta PDGFRs demonstrated that the impaired biologic activity of the mutant PDGF B chain was ameliorated with respect to binding and triggering of alpha PDGFRs. In cells that expressed both receptor types, heterodimers of mutant PDGF B and wild-type PDGF A gained substantially in their ability to recruit and trigger alpha, but not beta, PDGFRs. Partial rescue of impaired PDGF B mutant chain function by dimer formation with a wild-type PDGF A chain implies that interchain interactions markedly affect PDGFR binding and activation.