VEGF and Dll4-Notch pathways affect each other. VEGF increases Notch signalling components Dll4 expression in vivo and vitro. The expression of Dll4 in cultured endothelial cells was increased by VEGF treatments [
38] (Fig.
2). Dll4 was strongly expressed on the front of growing vessels in vascularised tumors and Dll4 expression in tumor vessels was rapidly decreased after blocking VEGF [
39]. In murine model of developing retina, blockade of VEGF significantly induced the decrease of sprouting and Dll4 expression on the vessels [
22]. Dll4-Notch signalling can alter expressions of three VEGF receptors. It has been demonstrated that VEGFR1 expression may be increased by Notch signalling. The decrease of VEGFR1 expression was significantly induced in Dll4 heterozygous mice in which the Notch signaling activation was reduced [
22]. In addition, activated Dll4-Notch signalling induced the increase of expressions of VEGFR1 and soluble VEGFR1 in cultured endothelial cells [
40]. Conversely, Notch signalling can provide negative feedback to reduce the activity of the VEGF/VEGFR2 pathway. In vitro, it has been observed that VEGFR2 expression was decreased following activation of Notch in cultured endothelial cells [
41]. In Dll4 heterozygous mice, it also has been demonstrated that VEGFR2 expression were increased in vessels [
22]. VEGF induces Dll4 expression in tip cells, which in turn decreased VEGFR2 in stalk cells, thus the differentiation of tip and stalk cells were regulated differently in this way [
42]. Notch signalling pathway could affect VEGFR3 expression through regulating VEGFR3 promoters. In addition, Notch signalling also alter VEGF responsiveness in human and murine endothelial cells through regulations of VEGF receptors expressions [
43].
Hu and colleagues have found that combining Dll4-targeted siRNA with VEGF inhibition bevacizumab was more effective in inhibiting angiogenesis in preclinical models of cancer, and patients with tumors after treatment with anti-VEGF therapy had lower Dll4 expression [
36]. Thus, targeting Dll4 in combination with VEGF inhibition potentially improves outcome of ovarian cancer treatments.
Moreover, there is a link between Notch signalling pathway and Nitric oxide/soluble guanylyl cyclase signalling. Nitric oxide (NO) produced by tumor, stromal and endothelial cells promotes proliferation and survival of ovarian cancer cells, mediating by soluble guanylyl cyclase (sGC). NO also promotes tumor angiogenesis at low concentrations, and it has been confirmed that Notch activation augments nitric oxide/soluble guanylyl cyclase signalling in immortalized ovarian surface epithelial cells and ovarian cancer cells [
44], thus a combination of soluble guanylyl cyclase and Notch inhibition may also be a more effective combination in inhibiting angiogenesis in ovarian cancer.