Angiogenesis is an essential component for tumor development regulated by both pro-angiogenic and anti-angiogenic factors. It is a multi-step process that includes endothelial cell proliferation, migration, basement membrane degradation and new lumen organization. Naturally occurring inhibitors of angiogenesis are found in mammalian tissues where they help maintain the quiescence of the normal vasculature. Thrombospondin 1 (TSP1) is one such inhibitor of angiogenesis that limits vessel density in normal tissues and curtails tumor growth. Thrombospondins are a family of extracellular matrix proteins that have diverse effects on cell adhesion, motility, proliferation and survival. Two members of this family, TSP1 and TSP2 act as inhibitors of angiogenesis. TSP1 is a large, multifunctional glycoprotein that influences cellular phenotype and the structure of the extracellular matrix. It inhibits angiogenesis through direct effects on endothelial cell migration and survival and through indirect effects on growth factor mobilization.TSP1 binds to a wide variety of integrin and non-integrin cell surface receptors including CD36, CD47 and HSPG. Receptor binding sites on TSP1 are dispersed throughout the molecule, with most domains binding multiple receptors. In some cases, TSP1 can also bind to multiple receptors concurrently. The induction of apoptosis by TSP1 in endothelial cells requires the sequential activation of CD36, the Src-family tyrosine kinase p59Fyn, caspase 3-like proteases and p38 MAPK. While caspase 3 is directly involved in the apoptotic process, p38 MAPK leads to activation of genes that lead to apoptosis.
In tumors, TSP1 secreted by stromal cells and some tumor cells directly inhibits endothelial cell migration and survival, stimulating endothelial cell apoptosis. The indirect effects of TSP1 on tumor growth result from its ability to activate TGF-β and inhibit extracellular MMPs. The generation of active TGF-β by TSP1 in the stroma suppresses the growth of tumor cells that remain responsive to TGF-β, while TSP1 inhibition of the formation of active MMP9 results in decreased amounts of pro-angiogenic factor VEGF liberated from the extracellular matrix. Furthermore, binding of TSP1 to CD47 is found to be required for inhibition of NO-stimulated vascular cell responses and cGMP signaling leading to angiogenesis. Thus, TSP1 inhibits angiogenesis directly by inducing endothelial cell apoptotic pathways while indirect effects on tumor cells involve activation of TGF-β and inhibition VEGF-activated survival pathways.