Modulation of angiogenesis in vitro by laminin-entactin complex

Laminin is a cross-shaped glycoprotein whose inner cross-region binds to the glycoprotein entactin (nidogen) forming a stable complex extractable from basement membrane matrices with chelating agents. In this study we evaluated the effect of the laminin-entactin complex on angiogenesis in serum-free collagen gel culture of rat aorta. Laminin-entactin stimulated or inhibited angiogenesis depending on its concentration in the gel. Stimulatory concentrations of laminin-entactin (30 to 300 micrograms/ml) promoted an increase in the number and length of microvessels. A similar effect was observed with purified laminin. Elongation of microvessels was also obtained with the laminin fragments E1' and E8 and with entactin, all of which have binding sites for endothelial cells. By contrast the E4 fragment of laminin, which has no cellular binding sites, failed to promote microvascular elongation. Inhibitory concentrations of laminin-entactin (3000 micrograms/ml) allowed formation of only a few stubby endothelial sprouts. Laminin-entactin promoted dose-dependent stabilization of microvessels preventing their regression. Microvessels stabilized by laminin-entactin were surrounded by thick patches of basement membrane-like material, whereas untreated microvessels prone to regression had a highly attenuated basement membrane. The angiogenic effect of laminin-entactin was enhanced by exogenous bFGF. However, bFGF was unable to stimulate angiogenesis over control values when incorporated in gels containing a high concentration of laminin-entactin. Our results indicate that laminin and its supramolecular complex laminin-entactin play an important modulatory role in angiogenesis. The dose-dependent effects of the laminin-entactin complex suggest that the basement membrane is a dynamic regulator of angiogenesis whose function varies depending on the concentration of its molecular components.