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

Targeting the PDGF signaling pathway in the treatment of non-malignant diseases

Platelet-derived growth factor (PDGF) is a family of mesenchymal mitogens with important functions during the embryonal development and in the control of tissue homeostasis in the adult. The PDGF isoforms exert their effects by binding to α-and β-tyrosine kinase receptors. Overactivity of PDGF signaling has been linked to the development of certain malignant and non-malignant diseases, including atherosclerosis and various fibrotic diseases. Different types of PDGF antagonists have been developed, including inhibitory monoclonal antibodies and DNA aptamers against PDGF isoforms and receptors, and receptor tyrosine kinase inhibitors. Beneficial effects have been recorded using such inhibitors in preclinical models and in patients with certain malignant as well as non-malignant diseases. The present communication summarizes the use of PDGF antagonists in the treatment of non-malignant diseases.

PDGF Receptors as Targets in Tumor Treatment

Signaling through platelet-derived growth factor (PDGF) receptors contributes to multiple tumor-associated processes. The recent introduction of clinically useful PDGF inhibitors have the last years validated PDGF receptors in malignant and stromal cells as relevant cancer drug targets. Mutational activation of PDGF receptor signaling in malignant cells has been described in some rare tumor types such as dermatofibrosarcoma protuberans, a subset of GISTs, and some hematologic malignancies. Furthermore, expression of PDGF receptors on pericytes is a common characteristic of solid tumors. The clinical efficacy of novel multikinase inhibitors, such as sunitinib and sorafenib, most likely involves targeting of PDGF receptor-dependent pericytes. Preclinical studies suggest that targeting of stromal PDGF receptors might also constitute a novel strategy to enhance tumor drug uptake. Finally, recent studies have implied both pro- and antimetastatic effects of PDGF receptors on malignant and stromal cells. The studies on the roles of PDGF receptors in cancer signaling are thus presently in a dynamic phase where collaborations between oncologists, pathologists, and tumor biologists are predicted to be highly productive.

Inhibitory effect of soluble PDGF-beta receptor in culture-activated hepatic stellate cells

Following liver injury, hepatic stellate cells undergo phenotypic transformation with acquisition of myofibroblast-like features, characterized by increased cell proliferation, motility, contractility, and extracellular matrix production. Activation of hepatic stellate cells is regulated by several cytokines and growth factors, including platelet-derived growth factor B-chain, a potent mitogen for HSC, overexpressed during hepatic fibrogenesis. This pleiotropic mediator exerts cellular effects by binding to specific receptors, inducing receptor dimerization and tyrosine-autophosphorylation. Activated receptor phosphotyrosines recruit signal transduction molecules, initiating various signaling pathways. We produced a soluble PDGFbeta-receptor (sPDGFRbeta) consisting of an extracellular domain connected to the IgG-Fc part of human immunoglobulin heavy chain. This soluble, chimeric receptor inhibits PDGF signaling and PDGF-induced proliferation in culture-activated hepatic stellate cells. Furthermore, sPDGFR decreased collagen type I (alphaI) mRNA expression and inhibits autocrine-looping in PDGF-BB mRNA production. In summary, sPDGFRbeta clearly shows effective inhibitory properties in early HSC activation, suggesting potential therapeutic impact for anti-PDGF intervention in liver fibrogenesis.

Protein detection using proximity-dependent DNA ligation assays

The advent of in vitro DNA amplification has enabled rapid acquisition of genomic information. We present here an analogous technique for protein detection, in which the coordinated and proximal binding of a target protein by two DNA aptamers promotes ligation of oligonucleotides linked to each aptamer affinity probe. The ligation of two such proximity probes gives rise to an amplifiable DNA sequence that reflects the identity and amount of the target protein. This proximity ligation assay detects zeptomole (40 x 10(-21) mol) amounts of the cytokine platelet-derived growth factor (PDGF) without washes or separations, and the mechanism can be generalized to other forms of protein analysis.

An extracellular ligand increases the specific activity of the receptor-like protein tyrosine phosphatase DEP-1

Cellular growth, differentiation and migration is regulated by protein tyrosine phosphorylation. Receptor-like protein tyrosine phosphatases are thus likely to be key regulators of vital cellular processes. The regulation of these enzymes is in general poorly understood. Ligands have been identified only for a small subset of the receptor-like protein tyrosine phosphatases and in no case has upregulation of the specific activity by extracellular ligands been demonstrated. Prompted by earlier findings of ligands for receptor-like protein tyrosine phosphatases in extracellular matrix we investigated if Matrigel, a preparation of extracellular matrix proteins, contained modulators of the specific activity of the receptor-like protein tyrosine phosphatase DEP-1. Matrigel stimulation of cells increased the specific activity of immunoprecipitated DEP-1. Also, incubation of immunoprecipitated DEP-1 with Matrigel led to an increase in DEP-1 activity, which was blocked by soluble DEP-1 extracellular domain. Finally, immunoprecipitated DeltaECD-DEP-1, a mutant form of DEP-1 lacking most of the extracellular domain, failed to respond to Matrigel stimulation. These experiments identify Matrigel as a source of DEP-1 agonist(s) and provide the first evidence for upregulation of the specific activity of receptor-like protein tyrosine phosphatases by extracellular ligands.

Involvement of platelet-derived growth factor in disease: development of specific antagonists

Platelet-derived growth factor (PDGF) is a family of dimeric isoforms that stimulates, e.g., growth, chemotaxis and cell shape changes of various connective tissue cell types and certain other cells. The cellular effects of PDGF isoforms are exerted through binding to two structurally related tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation. This enables a number of SH2 domain containing signal transduction molecules to bind to the receptors, thereby initiating various signaling pathways. PDGF isoforms have important roles during the embryonic development, particularly in the formation of connective tissue in various organs. In the adult, PDGF stimulates wound healing. Overactivity of PDGF has been implicated in certain disorders, including fibrotic conditions, atherosclerosis, and malignancies. Different kinds of PDGF antagonists are currently being developed and evaluated in different animal disease models, as well as in clinical trials.