THROMBOSPONDIN-1 EXPRESSION IN TRANSFORMED ENDOTHELIAL-CELLS RESTORESA NORMAL PHENOTYPE AND SUPPRESSES THEIR TUMORIGENESIS

Murine endothelial cells are readily transformed in a single step by t he polyomavirus oncogene encoding middle-sized tumor antigen. These ce lls (bEND.3) form tumors (hemangiomas) in mice which are lethal in new born animals. The bEND.3 cells rapidly proliferate in culture and expr ess little or no thrombospondin 1 (TS1). To determine the role of TS1 in regulation of endothelial cell phenotype, we stably transfected bEN D.3 cells with a human TS1 expression vector. The cells expressing hum an TS1 were readily identified by their altered morphology and exhibit ed a slower growth rate and lower saturation density than the parental bEND.3 cells. The TS1-expressing cells also formed aligned cords of t ells instead of clumps or cysts in Matrigel. Moreover, while the bEND. 3 cells formed large tumors in nude mice within 48 hr, the TS1-express ing cells failed to form tumors even after 1 month. The TS1-transfecte d cells expressed transforming growth factor beta mRNA and bioactivity at levels similar to those of the parental or vector-transfected bEND .3 cells, indicating that the effects of TS1 expression are not due to the activation of transforming growth factor beta by TS1. TS1 express ion resulted in a >100-fold decrease in net fibrinolytic (urokinase-ty pe plasminogen activator, uPA) activity due to more plasminogen-activa tor inhibitor 1 and less uPA secretion. TS1 thus appears to be an impo rtant regulator of endothelial cell phenotype required for maintaining the quiescent, differentiated state.