Thrombospondin-1, PECAM-1, and regulation of angiogenesis

Thrombospondin-l (TSP1) is a multidomain glycoprotein expressed by many cel l types. It is a multifunctional protein with important roles in regulation of vascular cell functions. Mutation or loss of tumor suppressor genes res ults in down regulation of TSP1 expression during malignant transformation. Thus, suggesting that down regulation of TSP1 may contribute to developmen t of the tumor angiogenic phenotype and perhaps tumor metastasis. TSP1 was demonstrated to be a natural inhibitor of angiogenesis. Peptides from proco llagen-like domain and type 1 repeats of TSP1, like whole TSP1, inhibit the angiogenic response to a variety of angiogenic stimuli in vivo and endothe lial cell (EC) migration in vitro by directly acting on ECs. The molecular mechanisms which mediate these inhibitory effects of TSP1 and its peptides are not understood. TSP1 expression is down regulated in the Polyoma middle T transformed mouse brain ECs (bEND.3). This may remove the TSP1 inhibitor y effects allowing ECs to rapidly proliferate in culture and form hemangiom as in vivo. Re-expression of TSP1 in bEND.3 cells restores a normal phenoty pe and suppresses their ability to form hemangiomas. This is mediated by mo dulating expression of several genes in concert favoring a differentiated s tate of endothelium. TSP1 transfected bEND.3 cells down regulate expression of PECAM-1, a multifunctional endothelial cell adhesion molecule with esse ntial roles in angiogenesis. A similar phenotype to that of TSP1 transfecte d cells was observed when endogenous PECAM-1 levels were down regulated by anti-sense transfection of bEND.3 cells. The anti-sense PECAM-1 transfected cells turn on expression of endogenous TSP1 and its angioinhibitory recept or, CD36. Expression of other genes with potential roles in regulation of E C phenotype were also affected in patterns very similar to those observed i n TSP1 transfected bEND.3 cells. Therefore, it appears that a reciprocal re lationship exists between TSP1 and PECAM-1 such that they are constituents of a "switch" that regulates in concert many components of the angiogenic a nd differentiated phenotype of ECs.