How does thrombospondin-1 (Tsp-1) signaling selectively inhibit the proliferation of immature endothelial cells without affecting mature endothelial cells participating in the formation of blood capillaries?

ascular endothelial cells are an established source of thrombospondin 1 (TSP1), a multifunctional extracellular matrix molecule. TSP1 appears to play an important role in modulating endothelial cell functions such as proliferation, migration, and capillary morphogenesis. In addition, TSP1 has recently been reported to potently inhibit angiogenesis both in vitro and in vivo. To better understand the mechanism underlying the antiangiogenic property of TSP1, endogenous TSP1 production was disrupted in bovine aortic endothelial cells (BAEC) by stable transfection with a vector expressing a TSP1 antisense RNA. Stable transfectants in which the antisense vector caused a decrease in TSP1 production were assayed for their ability to form capillary-like cords on gelled basement membrane matrix and for their responsiveness to the angiogenic/chemotactic mediator basic fibroblast growth factor. BAEC in which TSP1 production was disrupted exhibited a ten-fold increase over control BAEC in chemotactic activity to basic fibroblast growth factor and a twofold increase over control cells in the number of capillary-like cords that formed on gelled basement membrane matrix. Thus, the down-regulation of endogenous TSP1 appears to facilitate endothelial cell chemotaxis and capillary morphogenesis. These studies suggest that the modulation of TSP1 production is an important component of the angiogenic response, and support the idea that soluble TSP1 inhibits angiogenesis by interfering with endothelial cell chemotaxis and capillary formation.