Adhesion-induced intracellular signalling in endothelial cells depends on the nature of the matrix

The adhesion of a human microvascular endothelial cell line to its own matr ix was studied in comparison with adhesion of the same cells to fibronectin or thrombospondin-l. These endothelial cells adhered preferentially to the ir matrix whereas an equal cell number was attached to fibronectin or throm bospondin-1. The adhesion of cells to thrombospondin-1 was mediated by the N-terminal heparin binding domain of thrombospondin-l as shown by the use o f a recombinant fragment, N18. Cells adhering to their matrix displayed a m orphology and a cytoskeleton organization very similar to that observed in vivo with an apical immunostaining for actin stress fibers and a fine basal labeling for vinculin. Cells on fibronectin were extensively spread and ra pidly assembled stress fibers and focal contacts. Cells adherent to thrombo spondin-l presented large lamellae rich in actin but devoid of vinculin and only few actin fibers were observed. Depending on the substratum used, adh ering endothelial cells displayed also different tyrosine phosphorylation p atterns on electrophoresis. Our observations indicate that endothelial cell s adhering to their matrix present an activation state intermediate between that induced by a "hyperadhesive" protein like fibronectin and that genera ted by a moderate, indeed anti-adhesive, protein like thrombospondin-1.