HYPOXIC INDUCTION OF HUMAN VASCULAR ENDOTHELIAL GROWTH-FACTOR EXPRESSION THROUGH C-SRC ACTIVATION

ANGIOGENESIS the formation of new microvasculature by capillary sprout ing, is crucial for tumour development(1). Hypoxic regions of solid tu mours produce the powerful and directly acting angiogenic protein VEGF /VPF (vascular endothelial growth factor/vascular permeability factor) (2-6). We now investigate the signal transduction pathway involved in hypoxic induction of VEGF expression. Hypoxia is known to induce a tyr osine kinase cascade that results in the activation of nitrogen-fixati on genes in Rhizobium meliloti(7), and activation of tyrosine kinases is critical in signalling triggered by growth factors and ultraviolet light. We show here that genistein, an inhibitor of protein tyrosine k inases blocks VEGF induction. Hypoxia increases the kinase activity of pp60(c-src) and its phosphorylation on tyrosine 416 but does not acti vate Fyn or Yes. Expression of either a dominant-negative mutant form of c-Src or of Raf-1 markedly reduces VEGF induction. VEGF induction b y hypoxia in c-src(-) cells is impaired, although there is a compensat ory activation of Fyn. Our results provide an insight into hypoxia-tri ggered intracellular signalling, define VEGF as a new downstream targe t for c-Src, and suggest a role for c-Src in promoting angiogenesis.