The overexpression in
tumor cells of (proto)-oncogenic
receptor tyrosine kinases such as
epidermal growth factor receptor (EGFR) or ErbB2/neu (also known as HER-2) is generally thought to contribute to the development of solid
tumors primarily through their effects on promoting uncontrolled cell proliferation. However, agents that antagonize the function of the
protein products encoded by these (proto)-oncogenes are known to behave in vivo in a cytotoxic-like manner. This implies that such oncogenes may regulate critical cell survival functions, including angiogenesis. The latter could occur as a consequence of regulation of relevant
growth factors by such oncogenes. We therefore sought to determine whether EGFR or ErbB2/neu may contribute to
tumor angiogenesis by examining their effects on the expression of vascular endothelial cell
growth factor (
VEGF)/
vascular permeability factor (VPF), one of the most important of all known inducers of
tumor angiogenesis. We found that in vitro treatment of EGFR-positive A431 human
epidermoid carcinoma cells, which are known to be heavily dependent on
VEGF/VPF in vivo as an angiogenesis
growth factor, with the
C225 anti-EGFR
neutralizing antibody caused a dose-dependent inhibition of
VEGF protein expression. Prominent suppression of
VEGF/VPF expression in vivo, as well as a significant reduction in
tumor blood vessel counts, were also observed in established A431
tumors shortly after injection of the antibody as few as four times into nude mice. Transformation of NIH 3T3 fibroblasts with mutant ErbB2/neu, another EGFR-like oncogenic
tyrosine kinase, resulted in a significant induction of
VEGF/VPF, and the magnitude of this effect was further elevated by
hypoxia. Moreover, treatment of ErbB2/neu-positive SKBR-3 human
breast cancer cells in vitro with a specific neutralizing anti-ErbB2/neu
monoclonal antibody (4D5) resulted in a dose-dependent reduction of
VEGF/VPF
protein expression. Taken together, the results suggest that oncogenic properties of EGFR and ErbB2/neu may, at least in part, be mediated by stimulation of
tumor angiogenesis by up-regulating potent angiogenesis
growth factors such as
VEGF/VPF. These genetic changes may cooperate with epigenetic/environmental effects such as
hypoxia to maximally stimulate
VEGF/VPF expression. Therapeutic disruption of EGFR or ErbB2/neu
protein function in vivo may therefore result in partial suppression of angiogenesis, a feature that could enhance the therapeutic index of such agents in vivo and endow them with anti-
tumor effects, the magnitude of which may be out of proportion with their observed
cytostatic effects in monolayer tissue culture.