An autocrine loop directed by the vascular endothelial growth factor promotes invasiveness of human melanoma cells

The vascular endothelial growth factor-A (VEGF-A) is a cytokine that promotes angiogenesis through the activation of two tyrosine kinase receptors, VEGFR-1 and VEGFR-2, on vascular endothelial cells. Moreover, several experimental evidences indicate that VEGF-A may also play a role in tumor progression by acting on neoplastic cells expressing VEGFRs. In this study we show that human melanoma cells that simultaneously produce VEGF-A and express VEGFRs exhibit a higher spontaneous ability to invade the extracellular matrix (ECM) than melanoma cells not expressing either VEGF-A or VEGFRs. Exposure of VEGFR expressing melanoma cells to exogenous VEGF-A further increases their ability to invade the ECM. Moreover, an inhibitor of VEGFR tyrosine kinase activity is able to abrogate VEGF-A-induced stimulation of ECM invasion. A cell clone (13443/N2) derived from a VEGF-A responsive melanoma cell line and expressing high levels of VEGFR-2 invades the ECM eight-fold more efficiently than a cell clone derived from the same cell line and expressing extremely low levels of the receptor. Exposure of 13443/N2 cells to VEGF-E, which selectively binds and activates VEGFR-2, increases their ability to invade the ECM. Finally, the expression of the VEGF-A mRNA antisense sequence in 13443/N2 cells markedly reduces the release of VEGF-A and ECM invasion. In conclusion, our data show for the first time that a VEGF-A-driven autocrine loop promotes human melanoma cell ability to invade the ECM, and strongly support the hypothesis that activation of VEGFR-2 plays a primary role in this process.