The antiangiogenic factor thrombospondin 1 (TSP-1) binds with high affinity to several heparin-binding angiogenic factors, including fibroblast growth factor 2 (FGF-2), vascular endothelial growth factor (VEGF), and hepatocyte growth factor/ scatter factor (HGF/SF). The aim of this study was to investigate whether TSP-1 affects FGF-2 association with the extracellular matrix (ECM) and its bioavailability. TSP-1 prevented the binding of free FGF-2 to endothelial cell ECM. It also IntroductionAngiogenesis, sprouting of new blood vessels from pre-existing ones, is a crucial event in physiologic and pathologic processes, including tumor growth and metastasis. 1 Angiogenesis is a complex process, regulated by pro-and antiangiogenic factors, that involves dynamic interaction between a variety of cells, growth factors, and the extracellular matrix (ECM). 2 As in other morphogenic processes, the ECM acts not just as a structural support, but also as a direct modulator of cellular functions. Matrix components and bioactive fragments released by limited proteolysis can directly regulate endothelial cell functions. 3 In addition, the matrix contributes to angiogenesis by acting as a reservoir for angiogenic factors. Growth factors are stored in the matrix through binding to heparansulfate proteoglycans (HSPGs). Binding to HSPGs is reversible, 4 and biologically active factors can be released from the matrix by different agents. 5,6 Fibroblast growth factor 2 (FGF-2, basic FGF) is the most extensively studied example of matrix-stored angiogenic factor. 5,7 As do other members of the FGF family, FGF-2 has a high affinity for the glycosaminoglycan heparin and for HSPGs. Cell-surface HSPGs are required for the formation of an active FGF/FGF receptor signaling complex, 8 for internalization, and hence for internalization-dependent activities such as endothelial cell proliferation. 7,9 Conversely, matrix-associated HSPGs allow the storage of FGF-2 within the ECM. 10 Matrix-associated FGF-2 can then act locally, or be released as a soluble, biologically active factor. 6 Mobilization of active FGF-2 from the matrix is an important mechanism of induction of angiogenesis. Biologically active FGF-2 is released by heparin, 11 matrix-degrading proteases, 12 heparanase, 13 and the FGF-binding protein (FGF-BP). 14 Conversely, other endogenous or pharmacologic agents that prevent FGF-2 interaction with HSPGs exert an antiangiogenic activity, as in the case of platelet factor 4 (PF-4), 15 a soluble syndecan ectodomain, 16 suramin, 17 chemically modified heparins, and heparin-mimicking polyanionic compounds. [18][19][20] Other important angiogenic factors, including vascular endothelial growth factor (VEGF) and hepatocyte growth factor/scatter factor (HGF/SF), bind to HSPGs that again contribute to growth factor binding to cell receptors and to the ECM. [21][22][23] Thrombospondin 1 (TSP-1) is the most studied member of a family of at least 5 related proteins. 24,25 TSP-1 is a matricellular molecule, a modular glycoprotein composed of mult...

Thrombospondin-1, an antiangiogenic matricellular protein, binds with high affinity to the angiogenic fibroblast growth factor-2, affecting its bioavailability and activity. The present work aimed at further locating the fibroblast growth factor-2 binding site of thrombospondin-1 and investigating its activity, using recombinant thrombospondin-1 proteins. Only recombinant constructs containing the thrombospondin-1 type III repeats bound fibroblast growth factor-2, whereas other domains, including the known anti-angiogenic type I repeats, were inactive. Binding was specific and inhibited by the anti thrombospondin-1 monoclonal antibody B5.2. Surface plasmon resonance analysis on BIAcore revealed a binding affinity (K(d)) of 310nM for the type III repeats and 11nM for intact thrombospondin-1. Since the type III repeats bind calcium, the effect of calcium on thrombospondin-1 binding to fibroblast growth factor-2 was investigated. Binding was modulated by calcium, as thrombospondin-1 or the type III repeats bound to fibroblast growth factor-2 only in calcium concentrations <0.3mM. The type III repeats inhibited binding of fibroblast growth factor-2 to endothelial cells, fibroblast growth factor-2-induced endothelial cell proliferation in vitro and angiogenesis in the chorioallantoic membrane assay in vivo, thus indicating the antiangiogenic activity of the domain. In conclusion, this study demonstrates that the fibroblast growth factor-2 binding site of thrombospondin-1 is located in the type III repeats. The finding that this domain is active in inhibiting angiogenesis indicates that the type III repeats represent a novel antiangiogenic domain of thrombospondin-1.

Endogenous inhibitors of angiogenesis, such as thrombospondin-1 (TSP-1), are promising sources of therapeutic agents to treat angiogenesis-driven diseases, including cancer. TSP-1 regulates angiogenesis through different mechanisms, including binding and sequestration of the angiogenic factor fibroblast growth factor-2 (FGF-2), through a site located in the calcium binding type III repeats. We hypothesized that the FGF-2 binding sequence of TSP-1 might serve as a template for the development of inhibitors of angiogenesis. Using a peptide array approach followed by binding assays with synthetic peptides and recombinant proteins, we identified a FGF-2 binding sequence of TSP-1 in the 15-mer sequence DDDDDNDKIPD-DRDN. Molecular dynamics simulations, taking the full flexibility of the ligand and receptor into account, and nuclear magnetic resonance identified the relevant residues and conformational determinants for the peptide-FGF interaction. This information was translated into a pharmacophore model used to screen the NCI2003 small molecule databases, leading to the identification of three small molecules that bound FGF-2 with affinity in the submicromolar range. The lead compounds inhibited FGF-2-induced endothelial cell proliferation in vitro and affected angiogenesis induced by FGF-2 in the chicken chorioallantoic membrane assay. These small molecules, therefore, represent promising leads for the development of antiangiogenic agents. Altogether, this study demonstrates that new biological insights obtained by integrated multidisciplinary approaches can be used to develop small molecule mimics of endogenous proteins as therapeutic agents.Inhibitors of angiogenesis, aimed at preventing the deregulated formation of new blood vessels, are emerging as a successful approach to treat an array of diseases, including cancer (1). Antiangiogenic strategies are designed to reestablish the balance between angiogenic factors and endogenous inhibitors, restoring the physiologically quiescent condition of the vasculature (2). A strategy to achieve this is to exploit the activity of endogenous inhibitors of angiogenesis (3).Thrombospondin-1 (TSP-1) 2 was the first endogenous inhibitor of angiogenesis identified (4, 5). Of the five members that constitute the TSP family in mammals, the homotrimeric TSP-1 and TSP-2 share domain organization and the ability to inhibit angiogenesis. Like other thrombospondins, TSP-1 is a multi-modular protein. Each monomer consists of an N-terminal globular domain followed by the coiled-coil oligomerization domain, a von Willebrand factor type C repeat, three properdin-like type I repeats, two TSP-type epidermal growth factor (EGF)-like or type II repeats, and a signature domain comprising a third EGF-like repeat, the calcium binding wire or type III repeats, and the lectin-like C-terminal globular domain (6).TSP-1 has been classified functionally as a matricellular protein, i.e. an extracellular protein that acts to regulate cell interactions with the environment (7). TSP-1 binds to a variety of ...

AbstrAct:Inhibitors of angiogenesis are an important addition to conventional chemotherapy. Among different "druggable" angiogenic factors, fibroblast growth factor-2 (FGF-2) is an attractive target for novel therapies because of its intricated involvement in tumor neovascularization, tumor cell proliferation and migration, and the acquisition of resistance to antiangiogenic therapies. FGF-2 bioavailability and activity is affected by several natural ligands, including the endogenous inhibitor of angiogenesis thrombospondin-1 (TSP-1). We hypothesized that the FGF-2-binding sequence of TSP-1 might serve as a template for the development of non-peptide inhibitors of angiogenesis. Computational biology and nuclear magnetic resonance spectroscopy approaches, major investigative tools in the characterizations of protein-protein interaction (PPI), were used to map the residues at the TSP-1/FGF-2 interface. The translation of this three-dimensional information into a pharmacophore model allowed screening a small molecule databases, identifying three FGF-2-binding, antiangiogenic small molecules, mimetic of TSP-1. Pharmacophore-based approaches are thus feasible tools to exploit naturally occurring PPI, by generating a set of lead compounds mimetic of endogenous proteins, as a starting point for the development of novel therapeutic agents.