Preclinical validation of a novel metastasis-inhibiting Tie1 function-blocking antibody.

The angiopoietin (Ang)-Tie pathway has been intensely pursued as candidate second-generation anti-angiogenic target. While much of the translational work has focused on the ligand Ang2, the clinical efficacy of Ang2-targeting drugs is limited and failed to improve patient survival. In turn, the orphan receptor Tie1 remains therapeutically unexplored, although its endothelial-specific genetic deletion has previously been shown to result in a strong reduction in metastatic growth. Here, we report a novel Tie1 function-blocking antibody (AB-Tie1-39), which suppressed postnatal retinal angiogenesis. During primary tumor growth, neoadjuvant administration of AB-Tie1-39 strongly impeded systemic metastasis. Furthermore, the administration of AB-Tie1-39 in a perioperative therapeutic window led to a significant survival advantage as compared to control-IgG-treated mice. Additional in vivo experimental metastasis and in vitro transmigration assays concurrently revealed that AB-Tie1-39 treatment suppressed tumor cell extravasation at secondary sites. Taken together, the data phenocopy previous genetic work in endothelial Tie1 KO mice and thereby validate AB-Tie1-39 as a Tie1 function-blocking antibody. The study establishes Tie1 as a therapeutic target for metastasis in a perioperative or neoadjuvant setting.

Caspase-8 modulates physiological and pathological angiogenesis during retina development.

During developmental angiogenesis, blood vessels grow and remodel to ultimately build a hierarchical vascular network. Whether, how, cell death signaling molecules contribute to blood vessel formation is still not well understood. Caspase-8 (Casp-8), a key protease in the extrinsic cell death-signaling pathway, regulates cell death via both apoptosis and necroptosis. Here, we show that expression of Casp-8 in endothelial cells (ECs) is required for proper postnatal retina angiogenesis. EC-specific Casp-8-KO pups (Casp-8ECKO) showed reduced retina angiogenesis, as the loss of Casp-8 reduced EC proliferation, sprouting, and migration independently of its cell death function. Instead, the loss of Casp-8 caused hyperactivation of p38 MAPK downstream of receptor-interacting serine/threonine protein kinase 3 (RIPK3) and destabilization of vascular endothelial cadherin (VE-cadherin) at EC junctions. In a mouse model of oxygen-induced retinopathy (OIR) resembling retinopathy of prematurity (ROP), loss of Casp-8 in ECs was beneficial, as pathological neovascularization was reduced in Casp-8ECKO pups. Taking these data together, we show that Casp-8 acts in a cell death-independent manner in ECs to regulate the formation of the retina vasculature and that Casp-8 in ECs is mechanistically involved in the pathophysiology of ROP.

Endothelial Tie1-mediated angiogenesis and vascular abnormalization promote tumor progression and metastasis.

The endothelial tyrosine kinase receptor Tie1 remains poorly characterized, largely owing to its orphan receptor status. Global Tie1 inactivation causes late embryonic lethality, thereby reflecting its importance during development. Tie1 also plays pivotal roles during pathologies such as atherosclerosis and tumorigenesis. In order to study the contribution of Tie1 to tumor progression and metastasis, we conditionally deleted Tie1 in endothelial cells at different stages of tumor growth and metastatic dissemination. Tie1 deletion during primary tumor growth in mice led to a decrease in microvessel density and an increase in mural cell coverage with improved vessel perfusion. Reduced angiogenesis and enhanced vascular normalization resulted in a progressive increase of intratumoral necrosis that caused a growth delay only at later stages of tumor progression. Concomitantly, surgical removal of the primary tumor decreased the number of circulating tumor cells, reduced metastasis, and prolonged overall survival. Additionally, Tie1 deletion in experimental murine metastasis models prevented extravasation of tumor cells into the lungs and reduced metastatic foci. Taken together, the data support Tie1 as a therapeutic target by defining its regulatory functions during angiogenesis and vascular abnormalization and identifying its role during metastasis.

The Orphan Receptor Tie1 Controls Angiogenesis and Vascular Remodeling by Differentially Regulating Tie2 in Tip and Stalk Cells.

Tie1 is a mechanistically poorly characterized endothelial cell (EC)-specific orphan receptor. Yet, Tie1 deletion is embryonic lethal and Tie1 has been implicated in critical vascular pathologies, including atherosclerosis and tumor angiogenesis. Here, we show that Tie1 does not function independently but exerts context-dependent effects on the related receptor Tie2. Tie1 was identified as an EC activation marker that is expressed during angiogenesis by a subset of angiogenic tip and remodeling stalk cells and downregulated in the adult quiescent vasculature. Functionally, Tie1 expression by angiogenic EC contributes to shaping the tip cell phenotype by negatively regulating Tie2 surface presentation. In contrast, Tie1 acts in remodeling stalk cells cooperatively to sustain Tie2 signaling. Collectively, our data support an interactive model of Tie1 and Tie2 function, in which dynamically regulated Tie1 versus Tie2 expression determines the net positive or negative effect of Tie1 on Tie2 signaling.