THROMBOSPONDIN-1 AND TYPE-I REPEAT PEPTIDES OF THROMBOSPONDIN-1 SPECIFICALLY INDUCE APOPTOSIS OF ENDOTHELIAL-CELLS

Thrombospondin 1 (TSP1) inhibits angiogenesis and modulates endothelia l cell adhesion, motility, and growth. The antiproliferative activity of TSP1 is mimicked by synthetic peptides derived from the type I repe ats of TSP1 that antagonize fibroblast growth factor 2 and activate la tent transforming growth factor beta. These TSP1 analogues induced pro grammed cell death in bovine aortic endothelial cells based on morphol ogical changes, assessment of DNA fragmentation, and internucleosomal DNA cleavage. Intact TSP1 also induced DNA fragmentation The endotheli al cell response was specific because no DNA fragmentation was induced in MDA-MB-435S breast carcinoma cells, although TSP1 and the peptide conjugates inhibited the growth of both cell types. Apoptosis did not depend on activation of latent transforming growth factor beta because peptides lacking the activating sequence RFK were active. Apoptosis w as not sensitive to inhibitors of ceramide generation but was inhibite d by the phosphatase inhibitor vanadate. Induction of DNA fragmentatio n by the peptides nas decreased when endothelial cell cultures reached confluence. Growth of the cells on a fibronectin substrate also suppr essed induction of apoptosis by TSP1 or the peptides. Differential sen sitivities to kinase inhibitors suggest that apoptosis and inhibition of proliferation are mediated by distinct signal transduction pathways . These results demonstrate that induction of apoptosis by the TSP1 an alogues is not a general cytotoxic effect and is conditional on a lack of strong survival-promoting signals, such as those provided by a fib ronectin matrix. The antitumor activity of TSP1 may therefore result f rom an increased sensitivity to apoptosis in endothelial cells adjacen t to a provisional matrix during formation of vascular beds in tumors expressing TSP1.