Background: For the successful therapeutic use of inhibitors of the vascular endothelial growth factor receptor (VEGFR) pathway detailed knowledge of the mechanisms leading to tumor progression is indispensable. The main goal of this study was to determine the relevance of the VEGFR-2 activating pathway for colon carcinoma (CC) metastasis. The initial event is ligand-induced receptor activation through tyrosine autophosphorylation.
Methods: VEGFR-2, its ligands VEGF-C and VEGF-D and the phosphorylated (activated) receptor forms pVEGFR-2(Tyr1175) and pVEGFR-2(Tyr1214) were investigated immunohistochemically in different tumor compartments (intratumoral (zone 1) - invasive front (zone 2) - extratumoral soft tissue (zone 3)) and various cell types (tumor cells, inflammatory cells, macro- and microvasculature) in 84 non-metastatic, lymphogenous-metastatic and haematogenous-metastatic CC.
Results: VEGF-D produced by tumor cells has an autocrine affinity for its receptor VEGFR-2. In tumor budding regions VEGF-D-induced receptor activation by autophosphorylation at Tyr1214 seems to be a possible initial event of the VEGFR-2-mediated signaling pathway, but without effect on metastatic behaviour. In inflammatory cells of almost all CC VEGFR-2 phosphorylation at Tyr 1175 and Tyr 1214 was detectable without accompanying receptor expression, suggesting receptor activation without cell surface expression. Peritumoral inflammatory cells also expressed paracrine acting VEGF-C. The autocrine VEGF-D/VEGFR-2 signaling axis and receptor autophosphorylation at Tyr1214 appear to be main events for capillaries in all three tumor zones and for small vessels in zone 1 and 2. Independent of the metastatic status a large number of cases with capillary immunopositivity in the angiogenically active invasive front was documented, especially for VEGF-D, VEGFR-2 and pVEGFR-2(Tyr1214). VEGFR-2 positive extratumoral capillaries were significantly more common in distant metastatic CC. In all tumor compartments the investigated biomolecules were also detected in different frequencies in the macrovasculature, which is responsible for sufficient tumor vascularization. In addition, vascular paracrine-acting VEGF-C production was widely detected, but without zone and vessel-type dependence.
Conclusions: The VEGFR-2 activating pathway is closely involved in tumor cell-associated, vessel-mediated and immuno-inflammatory processes in colon carcinoma and appears to contribute to tumor survival and growth as well as maintenance of the infiltrative phenotype rather than to promote metastasis.