The most serious property of cancer is its ability to invade surrounding tissue and spread to new organ systems. Blood-borne metastasis is a highly complex and multifaceted process by which successful tumor cell emboli finally arrest at and invade through the microvessel basal lamina and associated connective tissue matrix. Destruction of the basal lamina appears to require the production of degradative enzymes by the invading tumor cells. We are examining some of these events, using an in vitro model consisting of human microvascular endothelial cells (MEC) and human tumor cells. Pure populations of MEC can be readily cultured as confluent monolayers and have been shown to maintain their differentiated characteristics, even after serial passage. The subendothelial matrix has been characterized biochemically and found to contain type IV collagen as the major component. The type IV collagen is cross-linked by covalent bond to form high molecular weight aggregates. In addition, laminin, heparan sulfate proteoglycan, and thrombospondin are also present. When cultured under appropriate conditions, the MEC elaborate a continuous basal lamina-like structure that exhibit lamina rara and lamina densa regions. The ability of tumor cells with high and low metastatic potential to solubilize and degrade the basal lamina-associated macromolecules (type IV collagen, laminin, and proteoglycan) of the subendothelial matrix by the activity of proteases and glycosidases will be assayed and directly compared with their similar activities on extracellular matrices and basement membranes produced by other cell types, including fibroblasts, smooth muscle cells, and epithelial cells. Tissue explants of human primary and metastatic tumors also will be assayed for their degradative and invasive behavior toward the MEC and their basal lamina. (A)