HYPOXIA-INDUCED ANGIOGENESIS AND VASCULAR ENDOTHELIAL GROWTH-FACTOR SECRETION IN HUMAN-MELANOMA

Tumour cells exposed to hypoxia in vitro can show increased expression of several selected genes, including the gene encoding the vascular e ndothelial growth factor (VEGF), suggesting that hypoxia followed by r eoxygenation might promote the malignant progression of tumours. An in vitro/in vivo study was conducted to investigate whether hypoxia can increase the angiogenic potential of tumour cells through increased VE GF secretion. Four human melanoma cell lines (A-07, D-12, R-18, U-25) were included in the study. Cell cultures were exposed to hypoxia (oxy gen concentration <10 p.p.m.) in vitro using the steel chamber method. Rate of VEGF secretion was measured in vitro in aerobic and hypoxic c ell cultures by ELISA. Angiogenesis was assessed in vivo using the int radermal angiogenesis assay. Aliquots of cells harvested from aerobic cultures or cultures exposed to hypoxia for 24 h were inoculated intra dermally in the flanks of adult female BALB/c-nu/nu mice. Tumours deve loped and angiogenesis was quantified by scoring the number of capilla ries in the dermis oriented towards the tumours. The number of tumour- oriented capillaries did not differ significantly between tumours from hypoxic and aerobic cultures for A-07 and U-25, whereas tumours from hypoxic cultures showed a larger number of tumour-oriented capillaries than tumours from aerobic cultures for D-12 and R-18. The VEGF secret ion under aerobic conditions and the absolute increase in VEGF secreti on induced by hypoxia were lower for D-12 and R-18 than for A-07 and U -25, whereas the relative increase in VEGF secretion induced by hypoxi a was higher for D-12 and R-18 than for A-07 and U-25. VEGF is not a l imiting factor in the angiogenesis of some tumours under normoxic cond itions. Hypoxia can increase the angiogenic potential of tumour cells by increasing the secretion of VEGF, but only of tumour cells showing low VEGF secretion under normoxia. Transient hypoxia might promote the malignant progression of tumours by temporarily increasing the angiog enic potential of tumour cells showing low VEGF expression under normo xic conditions.