Fibrin II induces endothelial cell capillary tube formation

Abstract

We studied the formation of capillary tubes by endothelial cells which were sandwiched between two fibrin gels under serum-free conditions. After formation of the overlying fibrin gel, the endothelial cell monolayer rearranged into an extensive net of capillary tubes. Tube formation was apparent at 5 h and was fully developed by 24 h. The capillary tubes were vacuolated, and both intracellular and intercellular lumina were present. Maximal tube formation was observed with fibrin 11 (which lacks both fibrinopeptide A and B), minimal tube formation with fibrin 1 (which lacks only fibrinopeptide A), and complete absence of tube formation with fibrin 325 (which lacks the NH2-terminal β15-42 sequence, in addition to fibrinopeptides A and B). The inability of fibrin 325 to stimulate capillary tube formation supports the idea that β15-42 plays an important role in this process, and its importance was confirmed by the finding that exogenous soluble β15-42 inhibited fibrin II-induced capillary tube formation. This effect was specific for fibrin, since β15-42 did not inhibit tube formation by endothelial cells sandwiched between collagen gels. The interaction of the apical surface of the endothelial cell with the overlying fibrin II gel, as opposed to the underlying fibrin gel upon which the cells were seeded, was necessary for capillary tube formation. These studies suggest that the β15- 42 sequence of fibrin interacts with a component of the apical cell surface and that this interaction plays a fundamental role in the induction of endothelial capillary tube formation.