Vascular endothelial growth factor receptor 3 (VEGFR-3) is a major inducer of lymphangiogenic signalling and seems to be involved also in angiogenesis. Since both processes are closely linked with tumor metastasis this study investigated the expression of VEGFR-3 in tumor-associated vessels in colorectal carcinomas and evaluated its relevance for lymphogenous and hematogenous metastasis. In a comparative study between microvascular endothelial cells isolated from the tumor (HCTEC) and the corresponding non-neoplastic tissue (HCMEC) from five patients with colorectal cancer VEGFR-3 expression was measured using a specific ELISA. The expression pattern was individually different, with cases showing reduced, elevated and unchanged protein levels. Under hypoxic culture conditions (3% O2 for 24 h), which are more realistic for the tumor situation, the levels remained unchanged. In contrast, hypoxia exposure of macrovascular human umbilical vein endothelial cells (HUVEC) led to a consistent downregulation of VEGFR-3 protein. These data indicate a ‘hypoxia-restistant’ behaviour of VEGFR-3 in colonic microvasculature. Using immunohistochemistry the endothelial expression pattern of VEGFR-3 in 74 non-metastatic, lymphogenously-metastatic and hematogenously-metastatic colorectal carcinoma specimens was assessed. Positive VEGFR-3 expression was highly significantly associated with those cases showing distant metastasis (p=0.0003). In contrast, significant differences in the expression of VEGFR-3 between non-metastatic tumors and carcinomas with lymph node metastasis were not found. The majority of the detectable intratumoral VEGFR-3-positive vessels were of blood vascular origin (CD31 positive, D2-40 negative). Whereas intratumoral lymphatic vessels were collapsed, VEGFR-3 positive peritumoral lymphatic vessels had mostly open lumina. These morphological observations provide evidence for a predominant significance of VEGFR-3-positive, possibly angiogenesis-mediated, tumor-associated blood vessels in hematogenous metastasis of colorectal cancer. In addition, due to their patency VEGFR-3-positive peritumoral, but not intratumoral lymphatics could be the vascular substrate functionally mediating lymphogenous metastasis.

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