Resistance to
VEGF inhibitors is emerging as a major clinical problem. Notch signaling has been implicated in
tumor angiogenesis. Therefore, to investigate mechanisms of resistance to
angiogenesis inhibitors, we transduced human
glioblastoma cells with retroviruses encoding Notch
delta-like ligand 4 (DLL4), grew them as
tumor xenografts and then treated the murine hosts with the
VEGF-A inhibitor
bevacizumab. We found that DLL4-mediated
tumor resistance to
bevacizumab in vivo. The large vessels induced by DLL4-Notch signaling increased
tumor blood supply and were insensitive to
bevacizumab. However, blockade of Notch signaling by
dibenzazepine, a γ-
secretase inhibitor, disrupted the large vessels and abolished the
tumor resistance. Multiple molecular mechanisms of resistance were shown, including decreased levels of
hypoxia-induced
VEGF and increased levels of the
VEGF receptor VEGFR1 in the
tumor stroma, decreased levels of VEGFR2 in large blood vessels, and reduced levels of VEGFR3 overall. DLL4-expressing
tumors were also resistant to a VEGFR targeting multikinase inhibitor. We also observed activation of other pathways of
tumor resistance driven by DLL4-Notch signaling, including the FGF2-FGFR and EphB4-EprinB2 pathways, the inhibition of which reversed
tumor resistance partially. Taken together, our findings show the importance of classifying mechanisms involved in angiogenesis in
tumors, and how combination
therapy to block DLL4-Notch signaling may enhance the efficacy of
VEGF inhibitors, particularly in DLL4-upregulated
tumors, and thus provide a rational base for the development of novel strategies to overcome antiangiogenic resistance in the clinic.