Background: Vascular endothelial growth factor (VEGF) is a specific endothelial cell mitogen with potent angiogenic properties. In tumors, VEGF has been localized to the most necrotic and ischemic areas of the tissues, suggesting that local hypoxia is a potent inducer of VEGF production. Initial experiments in vitro confirmed the stimulatory effect of hypoxia on VEGF expression. The extent of this response and the mechanisms involved in oxygen sensing are poorly characterized.
Experimental design: Confluent monolayers of malignant cell lines or primary cultures of fibroblast or endothelial cells were exposed to hypoxia or incubated with either cobalt chloride, a stimulator of erythropoietin gene expression, or sodium azide, an inhibitor of oxydative phosphorylation. VEGF expression was analyzed by Northern blot or RNase protection assays. The expression VEGF in vivo was studied in animals subjected to hypobaric hypoxia or functional anemia.
Results: Hypoxia greatly stimulated VEGF expression in tumor cell lines and primary fibroblast cultures. Endothelial cells, that expressed very low constitutive levels of VEGF, were resistant to hypoxic stimulation. RNase protection analysis showed that hypoxia primarily stimulated the induction of smaller and medium VEGF isoforms, i.e., the same ones expressed under normal conditions. The stimulatory effect of hypoxia on VEGF could be reproduced in vitro by cobalt chloride but not with sodium azide. In vivo, both hypoxia and anemia were found to be potent inducers of VEGF expression in several organs including heart, brain, liver, kidney, and muscle. As in vitro, cobalt was also found to be a potent stimulator of VEGF in vivo.
Conclusions: Hypoxia is a potent inducer of VEGF expression in malignant as well as normal cultured cells. It is also a stimulator of VEGF expression in vivo. The VEGF gene appears to respond to hypoxia like the erythropoietin gene, and the mechanism of oxygen sensing probably is mediated by a heme-containing protein.