Tumour growth requires accompanying expansion of the host vasculature, with tumour progression often correlated with vascular density. Vascular endothelial growth factor (VEGF) is the best-characterized inducer of tumour angiogenesis. We report that VEGF dynamically regulates tumour endothelial expression of Delta-like ligand 4 (Dll4), which was previously shown to be absolutely required for normal embryonic vascular development. To define Dll4 function in tumour angiogenesis, we manipulated this pathway in murine tumour models using several approaches. Here we show that blockade resulted in markedly increased tumour vascularity, associated with enhanced angiogenic sprouting and branching. Paradoxically, this increased vascularity was non-productive-as shown by poor perfusion and increased hypoxia, and most importantly, by decreased tumour growth-even for tumours resistant to anti-VEGF therapy. Thus, VEGF-induced Dll4 acts as a negative regulator of tumour angiogenesis; its blockade results in a striking uncoupling of tumour growth from vessel density, presenting a novel therapeutic approach even for tumours resistant to anti-VEGF therapies.

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Anti-angiogenesis therapies have emerged as important treatment options for several types of tumours. To date, these therapies have focused on blocking the vascular endothelial growth factor (VEGF) pathway. A recent series of papers have shown that one ligand for the Notch receptors, Delta-like ligand 4 (DLL4), is normally induced by VEGF and is a negative-feedback regulator that restrains vascular sprouting and branching. Consistent with this role, the deletion or inhibition of DLL4 results in excessive, non-productive angiogenesis. This unrestrained angiogenesis unexpectedly and paradoxically decreases tumour growth, even in tumours resistant to anti-VEGF therapies. Can too much angiogenesis be bad for tumours but good for patients?

Development of the vascular system depends on the highly coordinated actions of a variety of angiogenic regulators. Several of these regulators are members of the tyrosine kinase superfamily, including VEGF receptors and angiopoietin receptors, Tie1 and Tie2. Tyrosine kinase signaling is counter-regulated by the activity of tyrosine phosphatases, including vascular endothelial protein tyrosine phosphatase (VE-PTP), which has previously been shown to modulate Tie2 activity. We generated mice in which VE-PTP is replaced with a reporter gene. We confirm that VE-PTP is expressed in endothelium and also show that VE-PTP is highly expressed in the developing outflow tract of the heart and later is expressed in developing heart valves. Vasculogenesis occurs normally in mice lacking VE-PTP; however, angiogenesis is abnormal. Angiogenic defects in VE-PTP-null mice were most pronounced in the yolk sac and include a complete failure to elaborate the primitive vascular scaffold into higher-order branched arteries, veins, and capillaries. VE-PTP continues to be expressed into adulthood in the vasculature and heart valves, suggesting later roles in vascular development or homeostasis. VE-PTP is also expressed in the vasculature of growing tumors, suggesting that VE-PTP may be a new potential target for angiogenic therapies.gene targeting ͉ tyrosine kinase ͉ Tie2

Background: Delta-like ligand 4 (Dll4) is a Notch ligand that is upregulated by hypoxia and vascular endothelial growth factor-A (VEGF-A) and is reported to have a role in tumor angiogenesis. Evidence from xenograft studies suggests that inhibiting Dll4–Notch signalling may overcome resistance to anti-VEGF therapy. The aim of this study was to characterise the expression of Dll4 in colon cancer and to assess whether it is associated with markers of hypoxia and prognosis. Method: In all, 177 colon cancers were represented in tissue microarrays. Immunohistochemistry was performed using validated antibodies against Dll4, VEGF, hypoxia-inducible factor (HIF)-1 α , HIF-2 α , prolyl hydroxylase (PHD)1, PHD2, PHD3 and carbonic anhydrase 9 (CA9). Results: The expression of Dll4 was observed preferentially in the endothelium of 71% (125 out of 175) of colon cancers, but not in the endothelium adjacent to normal mucosa (none out of 107, P <0.0001). The expression of VEGF was significantly associated with HIF-2 α ( P <0.0001) and Dll4 ( P =0.010). Only HIF-2 α had a significant multivariate prognostic effect (hazard ratio 1.61, 95% confidence interval 1.01–2.57). Delta-like ligand 4 was also expressed by neoplastic cells, particularly neoplastic goblet cells. Conclusion: Endothelial expression of Dll4 is not a prognostic factor, but is significantly associated with VEGF. Assessing endothelial Dll4 expression may be critical in predicting response to anti-VEGF therapies.