Molecular basis for the regulation of angiogenesis by thrombospondin-1 and -2

Abstract

Thrombospondins TSP-1 and TSP-2 are potent endogenous inhibitors of angiogenesis. They inhibit angiogenesis through direct effects on endothelial cell migration, proliferation, survival, and apoptosis and by antagonizing the activity of VEGF. Several of the membrane receptor systems and signal transduction molecules that mediate the effects of TSP-1 and TSP-2 have been elucidated. TSP-1 and TSP-2 exert their direct effects through CD36, CD47, and integrins. Recent data indicate that CD36 and β1 integrins collaborate to transmit the signals that are initiated by TSP-1 and TSP-2. Furthermore, these receptors appear to associate with VEGFR2 to form a platform for the integration of positive and negative signals for angiogenesis. Cross talk between pro- and antiangiogenic signal transduction pathways may enable TSP-1 and TSP-2 to inhibit angiogenesis by antagonizing survival pathways while also activating apoptotic pathways. CD36 and CD47 are both involved in the suppression of nitric oxide (NO). Advances in understanding of the molecular regulation of angiogenesis by TSP have paved the way for innovations in experimental treatment of cancers and will likely continue to offer vast avenues for discovery in other disease processes as well.

Figure 1.

Schematic representation of the role of TSP-1 in the tumor microenvironment. TSP-1 affects angiogenesis through direct effects on endothelial cells and by antagonizing VEGF function. TSP-1 also suppresses the level of circulating endothelial cells (CEC).

Figure 2.

Signaling pathways that mediate the induction of endothelial apoptosis by TSP-1.

Figure 3.

Inhibition of NO signaling by thrombospondin-1. TSP-1 inhibits myristate uptake by competing for binding sites on CD36. Decreased myristalation leads to decreased recruitment of Src to the plasma membrane.