Vascular endothelial growth factor (
VEGF) is an endothelial cell-specific
mitogen, which also enhances vascular permeability. Because this
angiogenic factor has been suggested to play a role in
brain tumor biology, we have begun to investigate the regulation of
VEGF expression in cultures of rat type I astrocytes. In this report, we have focused on the influence of
hypoxia on
VEGF expression. Under standard in vitro conditions (21% O2)
VEGF expression in astrocytes in barely detectable by northern analysis. However, after exposure to 0.2% O2 for as little as 3 h
VEGF mRNA levels are markedly increased reaching a maximum by approximately 8 h of exposure. Treatment of astrocytes with CoCl2 or
desferrioxamine results in a similar induction of
VEGF, suggesting that the
oxygen sensor regulating
VEGF expression in astrocytes is a
heme-containing molecule. Although acute treatment with TPA (6 h) induces
VEGF expression, chronic exposure to TPA (24 h) to deplete PKC activity does not reduce the
hypoxia-induced
VEGF expression. These data indicate that
VEGF induction in astrocytes can proceed through PKC-dependent and -independent pathways. Furthermore, chronic exposure to TPA or treatment with
herbimycin A results in the enhancement of the
hypoxia-mediated increase in
VEGF mRNA levels. These results suggest that PKC and
herbimycin-sensitive
tyrosine kinase may serve as negative regulators of the
hypoxia-activated signal transduction pathway that leads to the induction of
VEGF expression. However, treatment of astrocytes with the nonspecific
kinase inhibitors H7 and H8 reduced the level of
VEGF induction by
hypoxia, indicating that some type of
kinase activity is required in this signaling pathway.