Hypoxia-induced metastasis of human melanoma cells: involvement of vascular endothelial growth factor-mediated angiogenesis

Tumour cells exposed to hypoxia have been shown to up-regulate the expressi on of vascular endothelial growth factor (VEGF). The purpose of the present work was to investigate whether hypoxia-induced VEGF up-regulation can res ult in increased metastatic efficiency of human melanoma cells. Two melanom a lines, one showing high (A-07) and the other showing low (D-12) VEGF secr etion under aerobic conditions, were included in the study. Cell cultures w ere exposed to hypoxia (oxygen concentrations < 10 ppm) in vitro and metast atic efficiency, i.e. lung colonization efficiency, as well as transplantab ility and angiogenic potential were assessed in BALB/c-nu/nu mice. Both cel l lines showed significantly increased VEGF secretion under hypoxic conditi ons as measured by enzyme-linked immunosorbent assay. The D-12 cells showed increased metastatic efficiency, transplantability and angiogenic potentia l following exposure to hypoxia. The metastatic efficiency increased with t he duration of the hypoxia treatment and decreased with the time after reox ygenation. The A-07 cells on the other hand showed unchanged metastatic eff iciency, transplantability and angiogenic potential following exposure to h ypoxia. Both cell lines showed significantly decreased metastatic efficienc y and angiogenic potential in mice treated with neutralizing antibody again st VEGF. These results suggest that (a) VEGF is a limiting factor for the r ate of angiogenesis in low but not in high VEGF-expressing melanomas under normoxic conditions and (b) transient hypoxia might promote the development of metastases in low VEGF-expressing melanomas by upregulating the express ion of VEGF and hence enhancing the angiogenic potential of the tumour cell s.