Endothelial Notch1 Activity Facilitates Metastasis.

Endothelial cells (ECs) provide angiocrine factors orchestrating tumor progression. Here, we show that activated Notch1 receptors (N1ICD) are frequently observed in ECs of human carcinomas and melanoma, and in ECs of the pre-metastatic niche in mice. EC N1ICD expression in melanoma correlated with shorter progression-free survival. Sustained N1ICD activity induced EC senescence, expression of chemokines and the adhesion molecule VCAM1. This promoted neutrophil infiltration, tumor cell (TC) adhesion to the endothelium, intravasation, lung colonization, and postsurgical metastasis. Thus, sustained vascular Notch signaling facilitates metastasis by generating a senescent, pro-inflammatory endothelium. Consequently, treatment with Notch1 or VCAM1-blocking antibodies prevented Notch-driven metastasis, and genetic ablation of EC Notch signaling inhibited peritoneal neutrophil infiltration in an ovarian carcinoma mouse model.

No evidence for a functional role of bi-directional Notch signaling during angiogenesis.

The Delta-Notch pathway is a signal exchanger between adjacent cells to regulate numerous differentiation steps during embryonic development. Blood vessel formation by sprouting angiogenesis requires high expression of the Notch ligand DLL4 in the leading tip cell, while Notch receptors in the trailing stalk cells are activated by DLL4 to achieve strong Notch signaling activity. Upon ligand binding, Notch receptors are cleaved by ADAM proteases and gamma-secretase. This releases the intracellular Notch domain that acts as a transcription factor. There is evidence that also Notch ligands (DLL1, DLL4, JAG1, JAG2) are processed upon receptor binding to influence transcription in the ligand-expressing cell. Thus, the existence of bi-directional Delta-Notch signaling has been proposed. We report here that the Notch ligands DLL1 and JAG1 are processed in endothelial cells in a gamma-secretase-dependent manner and that the intracellular ligand domains accumulate in the cell nucleus. Overexpression of JAG1 intracellular domain (ICD) as well as DLL1-ICD, DLL4-ICD and NOTCH1-ICD inhibited endothelial proliferation. Whereas NOTCH1-ICD strongly repressed endothelial migration and sprouting angiogenesis, JAG1-ICD, DLL1-ICD and DLL4-ICD had no significant effects. Consistently, global gene expression patterns were only marginally affected by the processed Notch ligands. In addition to its effects as a transcription factor, NOTCH1-ICD promotes cell adhesion to the extracellular matrix in a transcription-independent manner. However, JAG1-ICD, DLL1-ICD and DLL4-ICD did not influence endothelial cell adhesion. In summary, reverse signaling of Notch ligands appears to be dispensable for angiogenesis in cellular systems.

Integrin cytoplasmic domain-associated protein-1 attenuates sprouting angiogenesis.

RATIONALE: The ICAP1 (integrin cytoplasmic domain-associated protein-1) is a specific intracellular binding protein of beta1-integrins and the cerebral cavernous malformation (CCM) protein CCM1. ICAP1 recruits CCM1 to the cell membrane and activates CCM1 by changing its conformation. Because CCM1 plays a critical role for cardiovascular development, we hypothesized that its activator ICAP1 is involved in vascular differentiation. OBJECTIVE: The objective of this study was to define the role of ICAP1 in endothelial cells. METHODS AND RESULTS: Loss of ICAP1 in primary human endothelial cells causes excessive angiogenic branching and network formation in vitro (3D sprouting angiogenesis) and in vivo (xenotransplantation of ICAP1-silenced human endothelial cells). ICAP1 increases cell motility and the initial formation of capillary sprouts but prevents vessel outgrowth. ICAP1 inhibits Rho kinase activity and ERK (extracellular signal-regulated kinase) phosphorylation and induces expression of the cell cycle inhibitors p21 and p27, leading to less endothelial proliferation. However, ICAP1 promotes endothelial survival and AKT phosphorylation. Global gene expression analyses revealed that the ICAP1 effects are mediated by strong activation of DELTA-NOTCH signaling. Active NOTCH1 or silencing of the NOTCH ligand DLL4 phenocopy the ICAP1 effects and blockade of NOTCH cleavage rescues the ICAP1-mediated defects in endothelial cells. Both ICAP1 and NOTCH1 reduce the expression of ESM1 (endothelial cell-specific molecule-1), and silencing of ESM1 disturbs vascular endothelial growth factor- or fibroblast growth factor 2-induced sprouting angiogenesis. CONCLUSIONS: In this study, we identified ICAP1 as a novel regulator to prevent excessive sprouting angiogenesis.

Cerebral cavernous malformation protein CCM1 inhibits sprouting angiogenesis by activating DELTA-NOTCH signaling.

Cerebral cavernous malformations (CCM) are frequent vascular abnormalities caused by mutations in one of the CCM genes. CCM1 (also known as KRIT1) stabilizes endothelial junctions and is essential for vascular morphogenesis in mouse embryos. However, cellular functions of CCM1 during the early steps of the CCM pathogenesis remain unknown. We show here that CCM1 represents an antiangiogenic protein to keep the human endothelium quiescent. CCM1 inhibits endothelial proliferation, apoptosis, migration, lumen formation, and sprouting angiogenesis in primary human endothelial cells. CCM1 strongly induces DLL4-NOTCH signaling, which promotes AKT phosphorylation but reduces phosphorylation of the mitogen-activated protein kinase ERK. Consistently, blocking of NOTCH activity alleviates CCM1 effects. ERK phosphorylation is increased in human CCM lesions. Transplantation of CCM1-silenced human endothelial cells into SCID mice recapitulates hallmarks of the CCM pathology and serves as a unique CCM model system. In this setting, the multikinase inhibitor Sorafenib can ameliorate loss of CCM1-induced excessive microvascular growth, reducing the microvessel density to levels of normal wild-type endothelial cells. Collectively, our data suggest that the origin of CCM lesions is caused by perturbed Notch signaling-induced excessive capillary sprouting, which can be therapeutically targeted.