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      Anti-angiogenic Targets: Angiopoietin and Angiopoietin Receptors

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          Abstract

          Tumor blood vessel formation (angiogenesis) is essential for tumor growth and metastasis. Two main endothelial ligand–receptor pathways regulating angiogenesis are vascular endothelial growth factor (VEGF) receptor and angiopoietin-TIE receptor pathways. The angiopoietin-TIE pathway is required for the remodeling and maturation of the blood and lymphatic vessels during embryonic development after VEGF and VEGF-C mediated development of the primary vascular plexus. Angiopoietin-1 (ANGPT1) stabilizes the vasculature after angiogenic processes, via tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (TIE2) activation. In contrast, ANGPT2 is upregulated at sites of vascular remodeling. ANGPT2 is secreted by activated endothelial cells in inflammation, promoting vascular destabilization. ANGPT2 has been found to be expressed in many human cancers. Intriguingly, in preclinical models inhibition of ANGPT2 has provided promising results in preventing tumor angiogenesis, tumor growth, and metastasis, making it an attractive candidate to target in tumors. However, until now the first ANGPT2 targeting therapies have been less effective in clinical trials than in experimental models. Additionally, in preclinical models combined therapy against ANGPT2 and VEGF or immune checkpoint inhibitors has been superior to monotherapies, and these pathways are also targeted in early clinical trials. In order to improve current anti-angiogenic therapies and successfully exploit ANGPT2 as a target for cancer treatment, the biology of the angiopoietin-TIE pathway needs to be profoundly clarified.

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          Angiopoietin-2 sensitizes endothelial cells to TNF-alpha and has a crucial role in the induction of inflammation.

          The angiopoietins Ang-1 and Ang-2 have been identified as ligands of the receptor tyrosine kinase Tie-2 (refs. 1,2). Paracrine Ang-1-mediated activation of Tie-2 acts as a regulator of vessel maturation and vascular quiescence. In turn, the antagonistic ligand Ang-2 acts by an autocrine mechanism and is stored in endothelial Weibel-Palade bodies from where it can be rapidly released upon stimulation. The rapid release of Ang-2 implies functions of the angiopoietin-Tie system beyond its established role during vascular morphogenesis as a regulator of rapid vascular responses. Here we show that mice deficient in Ang-2 (encoded by the gene Angpt2) cannot elicit an inflammatory response in thioglycollate-induced or Staphylococcus aureus-induced peritonitis, or in the dorsal skinfold chamber model. Recombinant Ang-2 restores the inflammation defect in Angpt2(-/-) mice. Intravital microscopy showed normal TNF-alpha-induced leukocyte rolling in the vasculature of Angpt2(-/-)mice, but rolling cells did not firmly adhere to activated endothelium. Cellular experiments showed that Ang-2 promotes adhesion by sensitizing endothelial cells toward TNF-alpha and modulating TNF-alpha-induced expression of endothelial cell adhesion molecules. Together, these findings identify Ang-2 as an autocrine regulator of endothelial cell inflammatory responses. Ang-2 thereby acts as a switch of vascular responsiveness exerting a permissive role for the activities of proinflammatory cytokines.
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            Kinetics of vascular normalization by VEGFR2 blockade governs brain tumor response to radiation: role of oxygenation, angiopoietin-1, and matrix metalloproteinases.

            The recent landmark Phase III clinical trial with a VEGF-specific antibody suggests that antiangiogenic therapy must be combined with cytotoxic therapy for the treatment of solid tumors. However, there are no guidelines for optimal scheduling of these therapies. Here we show that VEGFR2 blockade creates a "normalization window"--a period during which combined radiation therapy gives the best outcome. This window is characterized by an increase in tumor oxygenation, which is known to enhance radiation response. During the normalization window, but not before or after it, VEGFR2 blockade increases pericyte coverage of brain tumor vessels via upregulation of Ang1 and degrades their pathologically thick basement membrane via MMP activation.
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              Dual angiopoietin-2 and VEGFA inhibition elicits antitumor immunity that is enhanced by PD-1 checkpoint blockade

              Pathological angiogenesis is a hallmark of cancer and a therapeutic target. Vascular endothelial growth factor A (VEGFA) and angiopoietin-2 (ANGPT2; also known as ANG2) are proangiogenic cytokines that sustain tumor angiogenesis and limit antitumor immunity. We show that combined ANGPT2 and VEGFA blockade by a bispecific antibody (A2V) provided superior therapeutic benefits, as compared to the single agents, in both genetically engineered and transplant tumor models, including metastatic breast cancer (MMTV-PyMT), pancreatic neuroendocrine tumor (RIP1-Tag2), and melanoma. Mechanistically, A2V promoted vascular regression, tumor necrosis, and antigen presentation by intratumoral phagocytes. A2V also normalized the remaining blood vessels and facilitated the extravasation and perivascular accumulation of activated, interferon-γ (IFNγ)-expressing CD8+ cytotoxic T lymphocytes (CTLs). Whereas the antitumoral activity of A2V was, at least partly, CTL-dependent, perivascular T cells concurrently up-regulated the expression of the immune checkpoint ligand programmed cell death ligand 1 (PD-L1) in tumor endothelial cells. IFNγ neutralization blunted this adaptive response, and PD-1 blockade improved tumor control by A2V in different cancer models. These findings position immune cells as key effectors of antiangiogenic therapy and support the rationale for cotargeting angiogenesis and immune checkpoints in cancer therapy.
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                Author and article information

                Contributors
                +4949+49 761 206-1700 , +4949+49 761 206-1705 , marme@tumorbio.uni-freiburg.de
                elina.kiss@helsinki.fi
                pipsa.saharinen@helsinki.fi
                Journal
                978-3-319-33673-2
                10.1007/978-3-319-33673-2
                Tumor Angiogenesis
                Tumor Angiogenesis
                A Key Target for Cancer Therapy
                978-3-319-33671-8
                978-3-319-33673-2
                05 July 2019
                : 227-250
                Affiliations
                Tumor Biology Center, Freiburg, Baden-Württemberg Germany
                [2 ]GRID grid.7737.4, ISNI 0000 0004 0410 2071, Translational Cancer Biology Program, Research Programs Unit, Biomedicum Helsinki, , University of Helsinki, ; Helsinki, Finland
                [3 ]GRID grid.7737.4, ISNI 0000 0004 0410 2071, Translational Cancer Biology Program, Research Programs Unit and Medicum Biomedicum Helsinki, , University of Helsinki, ; Helsinki, Finland
                [4 ]GRID grid.452042.5, ISNI 0000 0004 0442 6391, Wihuri Research Institute, ; Helsinki, Finland
                Article
                4
                10.1007/978-3-319-33673-2_4
                7124104
                15873ea5-ae2b-4492-aa7a-a43c079c553c
                © Springer Nature Switzerland AG 2019

                This article is made available via the PMC Open Access Subset for unrestricted research re-use and secondary analysis in any form or by any means with acknowledgement of the original source. These permissions are granted for the duration of the World Health Organization (WHO) declaration of COVID-19 as a global pandemic.

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                © Springer Nature Switzerland AG 2019

                angiogenesis,angiopoietin,angiopoietin-2,angpt,ang,tie1,tie2,tek,tumor metastasis,anti-angiogenic therapy

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