EVIDENCE THAT TENASCIN AND THROMBOSPONDIN-1 MODULATE SPROUTING OF ENDOTHELIAL-CELLS

Cultured endothelial cells undergo a reversible transition from a rest ing (cobblestone) phenotype to an angiogenic (sprouting) phenotype. Th is transition mimics the early events of angiogenesis. We have previou sly reported that the addition of exogenous xylosides inhibits endothe lial cell sprouting and modifies the extracellular matrix (ECM) synthe sised by the cells. We have now investigated whether endothelial sprou ting is mediated by the nature of the extracellular matrix in contact with the cells. Accordingly, cell-free matrices deposited by bovine ao rtic endothelial cells (BAEC) were isolated. These matrices were produ ced under conditions in which the formation of the sprouting phenotype was permitted (controls) or inhibited (by the addition of exogenous x ylosides). BAEC were then plated on these matrices and grown under con ditions which promote sprouting. Sprouting proceeded normally on contr ol matrices, whereas it was inhibited when the cells were grown on mat rices deposited in the presence of xylosides. The composition of the p ermissive and inhibitory matrices was then analysed. Inhibitory matric es contained reduced levels of tenascin and increased levels of thromb ospondin-1 by comparison to the permissive matrices. In contrast, no d ifferences were detected in the relative levels of laminin. The roles of tenascin and thrombospondin-1 in endothelial sprouting were confirm ed using specific antibodies. Immunolocalisation studies revealed the presence of both proteins in sprouting cells. Antibodies to tenascin i nhibited the formation of sprouting cells on permissive matrices and o n gelatin-coated dishes without affecting cell growth. Tenascin synthe sis was increased when sprouting cells were present in the cultures. A ntibodies to thrombospondin-1 stimulated sprouting on inhibitory matri ces. These results suggest that the transition from a resting to a spr outing phenotype is promoted by tenascin and inhibited by thrombospond in-1.