ANTIANGIOGENESIS ß1 Integrins Are Receptors for Thrombospondin 1

Thrombospondin 1 (TSP1) is an endogenous antiangiogenic protein, and it is known that some of these effects are mediated by binding of the type-1 repeat (TSR) domains to the cell surface receptor CD36. However, Short et al. show that TSP1 and a TSR-containing peptide inhibited migration of human umbilical vein endothelial cells (HUVEC), which do not express CD36, in response to vascular endothelial growth factor (VEGF). Fluorescence-activated cell sorting (FACS) indicated that ß1 integrins were abundant on HUVEC, and antibodies against ß1 integrins that activated ß1 integrins signaling inhibited cell migration in a dose-dependent manner. Concentrations of activating antibodies against ß1 integrins that were too low to inhibit migration directly interfered with the inhibitory activity of the TSR peptide, but antibodies against ß3 integrins and nonactivating antibodies against ß1 integrins did not. Small interfering RNAs (siRNAs) were used to decrease ß1 or ß3 integrins, and individually these did not block VEGF-induced migration; however, siRNA treatment to reduce ß1 integrin did block the inhibitory effect of the TSR peptide. Immunoblotting of HUVEC proteins bound to beads coated with TSR peptide indicated a direct interaction between TSR and 5 ß1 integrins. Antibodies against 5 or 3 blocked the TSR peptide inhibition of VEGF-induced migration. Both antagonists of phospholipase C- (PLC-) and of phosphoinositide 3-kinase (PI3K) inhibited migration. TSR and the PLC- antagonist had additive effects, whereas TSR and the PI3K inhibitor did not. The p85 subunit of PI3K was found in a complex with ß1 integrin; thus, PI3K appears to be involved in the inhibitory effects of ß1 signaling on migration. Finally, the involvement of the ß1 integrins in mediating TSP1 inhibitory effects was present in cells that do express the CD36 receptor for TSR. In cells that do express CD36, antibodies to CD36 blocked inhibition of migration by the TSR peptide; however, migration in response to full-length TSP1 was not blocked by the antibody to CD36 but was blocked by the ß1 integrin antibody. Thus, inhibition of migration by TSP1 appears to involve both CD36 and ß1 integrins and can be mediated solely by ß1 integrins in cells lacking CD36. Understanding the mechanisms of inhibitors of angiogenesis may aid in designing such inhibitors for cancer therapy.

S. M. Short, A. Derrien, R. P. Narsimhan, J. Lawler, D. E. Inger, B. R. Zetter, Inhibition of endothelial cell migration by thrombospondin-1 type-1 repeats is mediated by ß1 integrins. J. Cell Biol. 168, 643-653 (2005). [Abstract] [Full Text]

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