Roles of cell adhesion molecules in tumor angiogenesis induced by cotransplantation of cancer and endothelial cells to nude rats

Roles of cell adhesion molecules mediating the interaction of cancer and endothelial cells in tumor angiogenesis were investigated using new in vitro and in vivo model systems with a cultured murine endothelial cell line (F-2) and human cultured epidermoid cancer cells (A431). The A431 cells exhibited typical in vitro cell adhesion to the endothelial F-2 cells. The initial step of adhesion was mediated by sialyl Lewis(x) (Le(x)) and sialyl Le(a), the carbohydrate determinants expressed on the cancer cells, and E-selectin expressed constitutively on F-2 cells. Prolonged culture led to the implantation of cancer cells into the monolayer of the F-2 cells, which was mediated mainly by alpha(3)beta(1)-integrin. F-2 cells cultured on Matrigel showed evident tube formation, and coculture of F-2 cells with A431 cells led to the formation of A431 cell nests constantly surrounded by tube-like networks consisting of F-2 cells. This in vitro morphogenesis was inhibited by the addition of anti-sialyl Le(x)/Le(a) or anti-beta(1)-integrin antibodies, which led to the formation of cancer cell aggregates that were independent from the F-2 cell networks. This in vitro morphological appearance was exactly reproduced in the in vivo tumors, which were formed when the mixture of A431 and F-2 cells at the ratio of 10:1 were cotransplanted s.c. into the back of nude rats. The tumors of A431 supplemented with F-2 cells were profoundly vascularized throughout by the tubular structures formed by F-2 cells, the lumen of which contained the host rat blood cells. The tumor mass thus formed was an average 5.8-fold as large as control A431 tumors that were grown without F-2 cells. The co-injection of anti-Le(x)/Le(a) or anti-beta(1)-integrin antibodies produced a marked reduction in the size of A431 tumors, which were not vascularized and accompanied an independent tiny remnant clump of F-2 cells. The size of these A431 tumors did not differ significantly from those of control A431 tumors raised without F-2 cells. These results indicate that the interaction of tumor cells and endothelial cells in orderly tumor angiomorphogenesis is highly dependent on the action of cell adhesion molecules mediating the adhesion of cancer cells to endothelial cells, inhibition of which remarkably retards tumor growth and angiogenesis.