HYPOXIC STIMULATION OF VASCULAR ENDOTHELIAL GROWTH-FACTOR EXPRESSION IN-VITRO AND IN-VIVO

BACKGROUND: Vascular endothelial growth factor (VEGF) is a specific en dothelial cell mitogen with potent angiogenic properties. In tumors, V EGF has been localized to the most necrotic and ischemic areas of the tissues, suggesting that local hypoxia is a potent inducer of VEGF pro duction. Initial experiments in vitro confirmed the stimulatory effect of hypoxia on VEGF expression. The extent of this response and the me chanisms involved in oxygen sensing are poorly characterized. EXPERIME NTAL DESIGN: Confluent monolayers of malignant cell lines or primary c ultures 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 phosphoryl ation. VEGF expression was analyzed by Northern blot or RNase protecti on assays. The expression VEGF in vivo was studied in animals subjecte d 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 leve ls of VEGF, were resistant to hypoxic stimulation. RNase protection an alysis showed that hypoxia primarily stimulated the induction of small er and medium VEGF isoforms, i.e., the same ones expressed under norma l conditions. The stimulatory effect of hypoxia on VEGF could be repro duced in vitro by cobalt chloride but not with sodium azide. In vivo, both hypoxia and anemia were found to be potent inducers of VEGF expre ssion in several organs including heart, brain, liver, kidney, and mus cle. As in vitro, cobalt was also found to be a potent stimulator of V EGF in vivo. CONCLUSIONS: Hypoxia is a potent inducer of VEGF expressi on in malignant as well as normal cultured cells. It is also a stimula tor of VEGF expression in vivo. The VEGF gene appears to respond to hy poxia like the erythropoietin gene, and the mechanism of oxygen sensin g probably is mediated by a heme-containing protein.