The DELTA like-4 ligand (DLL4) belongs to the highly conserved NOTCH family and is specifically expressed in the endothelium. DLL4 regulates crucial processes in vascular growth, including endothelial cell (EC) sprouting and arterial specification. Its expression is increased by VEGF-A. In the present study, we show that VEGF-induced DLL4 expression depends on NOTCH activation. VEGF-induced DLL4 expression was prevented by the blockage of NOTCH signaling with ␥-secretase or ADAM inhibitors in human cardiac microvascular ECs. Similar to VEGF-A, recombinant DLL4 itself stimulated NOTCH signaling and resulted in up-regulation of DLL4, suggesting a positive feed-forward mechanism. These effects were abrogated by NOTCH inhibitors but not by inhibition of VEGF signaling. NOTCH activation alone suffices to induce DLL4 expression as illustrated by the positive effect of NOTCH intracellular domain (NICD)-1 or -4 overexpression. To discriminate between NICD/RBP-J and FOXC2-regulated DLL4 expression, DLL4 promoter activity was assessed in promoter deletion experiments. NICD induced promoter activity was dependent on RBP-J site but independent of the FOXC2 binding site. Accordingly, constitutively active FOXC2 did not affect DLL4 expression. The notion that the positive feed-forward mechanism might propagate NOTCH activation to neighboring ECs was supported by our observation that DLL4-eGFP-transfected ECs induced DLL4 expression in nontransfected cells in their vicinity. In summary, our data provide evidence for a mechanism by which VEGF or ligand-induced NOTCH signaling up-regulates DLL4 through a positive feedforward mechanism. By this mechanism, DLL4 could propagate its own expression and enable synchronization of NOTCH expression and signaling between ECs.The NOTCH family encompasses a fundamental signaling pathway involving four receptors (NOTCH-1, -2, -3, and -4) and, in vertebrates, five cognate ligands (DLL-1, -3, and -4, and JAGGED-1 and -2). Cell-cell contact is a prerequisite for NOTCH signaling as all members are membrane bound. The first activation step upon receptor-ligand interaction involves cleavage of the extracellular domain of the NOTCH receptor by ADAM (a disintegrin and metalloprotease). Subsequently, the ␥-secretase complex instigates a second proteolytic cleavage resulting in the release of the NOTCH intracellular domain (NICD) 2 into the cytoplasm. Next, the NICD translocates to the nucleus, where it associates with the Recombinant signal binding protein for immunoglobulin kappa J region (RBP-J) CBF1/RBP-J, Su(H), Lag1 (CSL) transcription factor to initiate the transcription of its downstream targets basic helix-loop-helix proteins HES (hairy/enhancer of split) and hairy related transcription factors (HRT, HEY, and