Invasion of an artificial blood vessel wall by human fibrosarcoma cells

Artificial blood vessel walls constructed by the addition of bovine arterial endothelial cells to multilayers of rat smooth muscle cells were used as substrates for the human fibrosarcoma cell line HT1080. The extracellular matrix proteins elaborated by the smooth muscle cells were prelabeled with [3H]-proline; therefore, their subsequent digestion could be followed by the appearance of radioactivity in the culture medium. The fibrosarcoma cells rapidly hydrolyzed smooth muscle multilayers in the absence of endothelial cells, but an endothelial layer markedly retarded the destructive ability of the tumor cells. The protective effect of the endothelium was not due to a lack of penetration of this cell layer, since HT1080 cells were observed by light and electron microscopy to be in the subendothelial area 24 hr after plating. Subsequently, the tumor cells multiplied in the region between the endothelial and smooth muscle layers and, although their degradative ability was retarded, they were ultimately capable of destroying the structure. Endothelial cells also inhibited hydrolysis of the smooth muscle layers if added simultaneously or up to 1 week after HT1080 cells, but the degree of inhibition was not as great as that seen with a preestablished endothelial layer. Measurable inhibition of tumor cell degradative activity was observed at fibrosarcoma:endothelial cell ratios of 25:1, demonstrating the potency of endothelial cells in modulating this aspect of the invasive phenotype. Although the HT1080 cells only slowly degraded the preexisting matrix proteins in artificial vessel wall cultures, they interfered with the production of new connective tissue proteins which occurred in control cultures. These experiments therefore suggest that endothelial cells have profound effects on tumor cell proteolytic activity, and the significance of these observations to tumor cell extravasation in vivo is discussed.