Functional roles for PECAM-1 (CD31) and VE-cadherin (CD144) in tube assembly and lumen formation in three-dimensional collagen gels

Various In vitro models have been described that emulate one or more of the processes involved in angiogenesis in vivo. In the present study endotheli al cells were cultured in three-dimensional type I collagen lattices in the presence of a mixture of basic fibroblast growth factor, vascular endothel ial cell growth factor, and phorbol myristate acetate. Under these conditio ns, the endothelial cells rapidly assemble into an interconnected network o f tube-like structures with a high frequency of intercellular canals or lum ens, The formation of the networks and lumens was completely blocked by cyc loheximide and by actinomycin D. Monoclonal antibodies directed against CD3 1 or vascular endothelial cadherin (VE-cadherin) inhibited the formation of endothelial tubes. A subtle difference in the morphology of cells treated with anti-CD31 versus anti-VE-cadherin was noted; namely, cells incubated i n the presence of CD31 antibodies were rounded or formed attenuated tube-li ke structures, both of which were characterized by a single, large intra- o r intercellular vacuole. In contrast, tube formation by cells incubated in the presence of VE-cadherin antibodies was also impaired and, most notably, demonstrated a reduction in either vacuole formation or vacuole fusion, de pending upon the monoclonal antibody used. We suggest that the two endothel ial-junction-associated proteins, CD31 and VE-cadherin, play different role s in the process of tube formation. CD31 appears to be required for cell el ongation, migration, and/or invasion in the gels as well as for cell-cell a ssociation to form the network structures. VE-cadherin also appears to be r equired for cell-cell association, but additionally appears to play some ro le in the process of vacuolization or vacuole fusion leading to intercellul ar lumen formation.