SCATTER FACTOR BINDS TO THROMBOSPONDIN AND OTHER EXTRACELLULAR-MATRIXCOMPONENTS

Scatter factor (SF) is an angiogenic growth factor that stimulates mot ility and invasion of carcinoma cells. SF is present in the extracellu lar matrix (ECM) of breast cancers, where it might act to promote tumo r cell invasion and angiogenesis. To investigate how SF is incorporate d into the ECM, we studied the binding of SF to various ECM components using a solid-phase binding assay based on the SF enzyme-linked immun osorbent assay. We found that SF binds to a variety of EMC molecules, with different binding capacities. The highest SF binding capacities w ere observed for thrombospondin-1 (TSP-1), fibronectin (Fn), and hepar an sulfate proteoglycan, although SF did not bind to albumin. Mature t wo-chain SF and precursor single-chain SF bound approximately equally well to TSP-1 and Fn. Moreover, two SF alpha-chain peptides (NK1 and N K2) both bound to TSP-1 and Fn, suggesting that the whole SF molecule is not required for binding. Based on binding competition assays, TSP- 1 exhibited higher affinity for SF than did nine other ECM molecules, including Fn and heparan sulfate proteoglycan. Although heparin in sol ution potently inhibited the binding of SF to TSP-1-coated surfaces, e ven very high concentrations of heparin could not elute SF already bou nd to TSP-1. SF binding was modulated by binding interactions among EC M molecules (TSP-1-Fn, TSP-1-collagen I, and Fn-collagen I), suggestin g that the matrix capacity to bind SF depends upon its exact compositi on. SF bound in a dose-dependent fashion to ECMs secreted by three hum an breast carcinoma cell lines. Binding of SF to matrices from all thr ee cell lines was significantly inhibited by preincubation of the matr ices with antibodies against TSP-1, whereas antibodies against several other ECM components were less effective or ineffective in inhibiting SF binding. In addition, TSP-1 markedly inhibited chemotaxis of micro vascular endothelial cells toward SF and SF-induced angiogenesis in th e rat cornea neovascularization assay. Our findings suggest that 1) SF interacts with a variety of ECM components, 2) high affinity SF-TSP-1 interactions may mediate the binding of SF to the breast cancer matri x, and 3) the SF-TSP-1 interaction may contribute to modulation of ang iogenesis. Possible implications of these findings for tumor angiogene sis are discussed.