Active interaction of human A375 melanoma cells with the lymphatics in vivo

We have used the avian chorioallantoic membrane (CAM) to study the interact ion of tumor cells with the lymphatics in vivo. The vascular endothelial gr owth factor-C (VEGF-C) has been shown to be lymphangiogenic. We have theref ore grown VEGF-C-expressing human A375 melanoma cells on the CAM. These tum ors induced numerous lymphatics at the invasive front, and compressed or de stroyed VEGF receptor (R)-3-positive lymphatics were observed within the so l id tumors. The lymphatics in the CAM and in the A375 melanomas could also be demonstrated with an antibody against Prox 1, a highly specific marker of lymphatic endothelial cells. Proliferation studies revealed a BrdU label ing index of 11.6% of the lymphatic endothelial cells in the tumors and at their margins. A great number of melanoma cells invaded the lymphatics. Suc h interactions were not observed with VEGF-C-negative Malme 3 M melanoma ce lls. Lymphangiogenesis was inhibited to some extent when A375 melanoma cell s were transfected with cDNA encoding soluble VEGFR-3 (sflt4), and the BrdU labeling index of the lymphatics in these tumors was 3.9%. Invasion of lym phatics and growth of blood vascular capillaries were not inhibited by the transfection. Therefore, tumor-induced lymphangiogenesis seems to be depend ent to some extent on VEGF-C/flt4 interactions, but invasion of lymphatics seems to be a distinct mechanism.