Delta Like 4 Expressing Bone Marrow-Derived Endothelial Progenitor Cells Regulate Tumour Angiogenesis

Bone marrow-derived endothelial progenitor cells (BM-EPCs) have been implicated in adult neoangiogenesis and consequently used as therapies for human pathologies with endothelial damage. The administration of these cells in human patients temporally improves endothelial function, although the engraftment of these cells in newly formed vessels is inefficient. Conversely, therapeutic stratagies to block EPC contribution during tumor angiogenesis have been proposed. In this work, we analysed the role of the Notch/Delta signalling pathway in EPC function during tumour neoangiogenesis, by regulating the expression of Notch ligand, delta-like 4 (Dll4) in these cells. Sublethally irradiated NOD-SCID mice received WT, Dll4^+/− (Dll4 heterozygous mice) or Dll4 SiRNA-treated BM-EPCs and were subcutaneously inoculated with well established Human or murine tumor xenografts. Tumours growing in Dll4-depleted EPCs transplanted mice presented increased microvessel density when compared with WT EPCs transplanted mice or non-transplanted controls, regardless of VEGF expression. Although with increased vessel number, tumours of Dll4^+/− EPC transplanted mice presented increased hypoxia and decreased tumour cell proliferation, suggesting an impairment in vessel function. In addition, these tumours present a diminished expression of PDGF, a vessel stabilizing factor, and increased expression of Ang2, known as a vessel destabilizing factor. We next verified whether the vessel destabilization observed in tumors after Dll4-depleted EPCs transplant might be due to a diferential endothelial differentiation or incorporation of EPCs in the tumour vasculature. In order to answer this question we quantified the incorporation of WT and Dll4-depleted EPCs in tumour vessels. Accordingly to our results, the presence of Dll4-depleted EPCs was reduced compared to WT EPCs, suggesting that Dll4-depleted EPCs might have reduced capacity to adhere to the renewing tumor vasculature, or to the underlying basement membrane. To test this, we used an in vitro endothelial differentiation assay, and observed a defect on the adhesion of of Dll4-depleted EPCs to extracellular matrix, which was correlated with a reduced expression of integrin subunits a3 and b1. These results suggest that the reduction of Dll4 on EPCs reduces integrin expression interfering with their ability to adhere, incorporate and stabilize the tumor vasculature during tumor neoangiogenesis. Therefore, EPCs have a major role in vessel stabilization in active neoangiogenic sites by the regulation of Dll4 expression. We propose that targeting the Notch/Dll4 pathway on EPCs, modulating vessel stability, may have therapeutic potential.