| Delta‑like protein 4 (DLL4) is an endothelial‑enriched Notch ligand of the Delta/Serrate/Lag family that presents an extracellular DSL motif and tandem EGF‑like repeats to engage Notch receptors on adjacent cells, where ligand‑induced receptor cleavage generates the Notch intracellular domain and drives transcriptional programs that sculpt vascular patterning. DLL4 expression is largely restricted to arterial and actively sprouting endothelium, with highest levels in tip and stalk cells of angiogenic vessels, and is strongly induced by pro‑angiogenic cues such as VEGF and hypoxia‑regulated HIF‑1α, placing DLL4/Notch signaling downstream of VEGF receptor activation in the hierarchy of angiogenic control. At the sprouting front, DLL4 expressed on emerging tip cells activates Notch in neighboring stalk cells, down‑regulates VEGF receptor expression, limits excessive tip‑cell formation, and enforces a hierarchical branching pattern that yields a functional, perfused vascular network; genetic or pharmacologic blockade of DLL4/Notch signaling disrupts this lateral inhibition, increases endothelial proliferation and branching, and produces dense but poorly perfused, non‑productive vasculature. DLL4 is up‑regulated in the vasculature of many solid cancers, including colorectal, breast, renal, pancreatic, and glioblastoma models, and that forced DLL4 expression in tumor or stromal compartments reduces vessel density but enlarges lumen size and improves vascular function, lowering hypoxia and apoptosis and supporting tumor growth, whereas soluble DLL4 decoys or anti‑DLL4 antibodies increase vessel number yet impair perfusion and suppress tumor expansion, including in settings that are resistant to anti‑VEGF therapy. Across preclinical models, high DLL4 levels in tumor endothelium correlate with more mature vasculature and shorter patient survival, while DLL4 blockade consistently drives hyper‑sprouting and functional collapse of the tumor vasculature, identifying DLL4/Notch as a central negative regulator of angiogenic output that modulates and intersects with VEGF signaling rather than simply mirroring it. |
