In vitro angiogenic and proteolytic properties of bovine lymphatic endothelial cells.

The aim of the present study was to determine whether angiogenic cytokines, which induce neovascularization in the blood vascular system, might also be operative in the lymphatic system. In an assay of spontaneous in vitro angiogenesis, endothelial cells isolated from bovine lymphatic vessels retained their histotypic morphogenetic properties by forming capillary-like tubes. In a second assay, in which endothelial cells could be induced to invade a three-dimensional collagen gel within which they formed tube-like structures, lymphatic endothelial cells responded to basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) in a manner similar to what has previously been observed with endothelial cells derived from the blood vascular system. Finally, since angiogenesis is believed to require extracellular proteolytic activity, we investigated the effects of bFGF and VEGF on lymphatic endothelial cell proteolytic properties by focussing on the plasminogen activator (PA) system. bFGF and VEGF increased urokinase, urokinase receptor, and tissue-type PA expression. This was accompanied by an increase in PA inhibitor-1, which is thought to play an important permissive role in angiogenesis by protecting the extracellular matrix against excessive proteolytic degradation. Taken together, these results demonstrate that with respect to in vitro morphogenetic and proteolytic properties, lymphatic endothelial cells respond to the previously described angiogenic factors, bFGF and VEGF, in a manner very similar to what has been described for endothelial cells derived from the blood vascular system.