SnapShot : Angiopoietins and Their Functions

Peer reviewe

Association of Angiopoietin-2 and Ki-67 Expression with Vascular Density and Sunitinib Response in Metastatic Renal Cell Carcinoma

The Angiopoietin-2 (Ang2, Angpt2) growth factor is a context-dependent antagonist/agonist ligand of the endothelial Tie2 receptor tyrosine kinase and known to promote tumour angiogenesis and metastasis. Angiopoietin antagonists have been tested in clinical cancer trials in combination with VEGF-based anti-angiogenic therapy, including sunitinib, which is widely used as a first-line therapy for metastatic renal cell carcinoma (mRCC). However, little is known about Ang2 protein expression in human tumours and the correlation of tumour Ang2 expression with tumour vascularization, tumour cell proliferation and response to anti-angiogenic therapies. Here, we evaluated, using immunohistochemistry, the expression of Ang2, CD31 and the cell proliferation marker Ki-67 in the primary kidney cancer from 136 mRCC patients, who received first-line sunitinib after nephrectomy. Ang2 protein expression was restrained to RCC tumour vessels, and correlated with tumour vascularization and response to sunitinib. High pre-therapeutic Ang2 expression, and more strongly, combined high expression of both Ang2 and CD31, were associated with a high clinical benefit rate (CBR). Low cancer Ki-67 expression, but not Ang2 or CD31 expression, was associated with favourable progression-free (PFS) and overall survival (OS) as compared to patients with high Ki-67 expression (PFS 6.5 vs. 10.6 months, P = 0.009; OS, 15.7 vs. 28.5 months, P = 0.015). In summary, in this study to investigate endothelial Ang2 in mRCC patients treated with first-line sunitinib, high cancer Ang2 expression was associated with the CBR, but not PFS or OS, whereas low Ki-67 expression was significantly associated with long PFS and OS.Peer reviewe

KSHV infection of endothelial precursor cells with lymphatic characteristics as a novel model for translational Kaposi's sarcoma studies

Author summaryKaposi's sarcoma herpesvirus (KSHV) is the etiologic agent of Kaposi's sarcoma (KS). The main proliferative component of KS, spindle cells, express markers of lymphatic and blood endothelium. Endothelial precursor cells, which are circulating endothelial colony forming cells (ECFCs), have been proposed as the source of spindle cells. Here we examined both blood and lymphatic ECFCs infected with KSHV. Lymphatic ECFCs are readily infected by KSHV, maintain the viral episomes and show modest transformation of the cells, which the infected blood ECFCs and all uninfected ECFCs failed to show. The lymphatic ECFCs express SOX18, which supported the maintenance of high copy numbers of KSHV genomes. The KSHV-infected lymphatic ECFCs persisted in vivo and recapitulated the phenotype of KS tumor cells such as high number of viral genome copies and spindling morphology. These KS tumor cell hallmarks were significantly reduced by SOX18 chemical inhibition using a small molecule SM4 treatment. These data suggest that KSHV-infected lymphatic ECFCs could be the progenitors of KS spindle cells and are a promising model for the translational studies to develop new therapies for KS.Kaposi's sarcoma herpesvirus (KSHV) is the etiologic agent of Kaposi's sarcoma (KS), a hyperplasia consisting of enlarged malformed vasculature and spindle-shaped cells, the main proliferative component of KS. While spindle cells express markers of lymphatic and blood endothelium, the origin of spindle cells is unknown. Endothelial precursor cells have been proposed as the source of spindle cells. We previously identified two types of circulating endothelial colony forming cells (ECFCs), ones that expressed markers of blood endothelium and ones that expressed markers of lymphatic endothelium. Here we examined both blood and lymphatic ECFCs infected with KSHV. Lymphatic ECFCs are significantly more susceptible to KSHV infection than the blood ECFCs and maintain the viral episomes during passage in culture while the blood ECFCs lose the viral episome. Only the KSHV-infected lymphatic ECFCs (K-ECFCLY) grew to small multicellular colonies in soft agar whereas the infected blood ECFCs and all uninfected ECFCs failed to proliferate. The K-ECFCLYs express high levels of SOX18, which supported the maintenance of high copy number of KSHV genomes. When implanted subcutaneously into NSG mice, the K-ECFCLYs persisted in vivo and recapitulated the phenotype of KS tumor cells with high number of viral genome copies and spindling morphology. These spindle cell hallmarks were significantly reduced when mice were treated with SOX18 inhibitor, SM4. These data suggest that KSHV-infected lymphatic ECFCs can be utilized as a KSHV infection model for in vivo translational studies to test novel inhibitors representing potential treatment modalities for KS.Peer reviewe

Multiple angiopoietin recombinant proteins activate the Tie1 receptor tyrosine kinase and promote its interaction with Tie2

The Tie1 receptor tyrosine kinase was isolated over a decade ago, but so far no ligand has been found to activate this receptor. Here, we have examined the potential of angiopoietins, ligands for the related Tie2 receptor, to mediate Tie1 activation. We show that a soluble Ang1 chimeric protein, COMP-Ang1, stimulates Tie1 phosphorylation in endothelial cells with similar kinetics and angiopoietin dose dependence when compared with Tie2. The phosphorylation of overexpressed Tie1 was weakly induced by COMP-Ang1 also in transfected cells that do not express Tie2. When cotransfected, Tie2 formed heteromeric complexes with Tie1, enhanced Tie1 activation, and induced phosphorylation of a kinase-inactive Tie1 in a ligand-dependent manner. Tie1 phosphorylation was also induced by native Ang1 and Ang4, although less efficiently than with COMP-Ang1. In conclusion, we show that Tie1 phosphorylation is induced by multiple angiopoietin proteins and that the activation is amplified via Tie2. These results should be important in dissecting the signal transduction pathways and biological functions of Tie1

Targeting beta 1-integrin inhibits vascular leakage in endotoxemia

Loss of endothelial integrity promotes capillary leakage in numerous diseases, including sepsis, but there are no effective therapies for preserving endothelial barrier function. Angiopoietin-2 (ANGPT2) is a context-dependent regulator of vascular leakage that signals via both endothelial TEK receptor tyrosine kinase (TIE2) and integrins. Here, we show that antibodies against beta 1-integrin decrease LPS-induced vascular leakage in murine endotoxemia, as either a preventative or an intervention therapy. beta 1-integrin inhibiting antibodies bound to the vascular endotheliumin vivo improved the integrity of endothelial cell-cell junctions and protected mice from endotoxemia-associated cardiac failure, without affecting endothelial inflammation, serum proinflammatory cytokine levels, or TIE receptor signaling. Moreover, conditional deletion of a single allele of endothelial beta 1-integrin protected mice from LPS-induced vascular leakage. In endothelial mono-layers, the inflammatory agents thrombin, lipopolysaccharide (LPS), and IL-1 beta decreased junctional vascular endothelial (VE)-cadherin and induced actin stress fibers via beta 1- and alpha 5-integrins and ANGPT2. Additionally, beta 1-integrin inhibiting antibodies prevented inflammation-induced endothelial cell contractility and monolayer permeability. Mechanistically, the inflammatory agents stimulated ANGPT2-dependent translocation of alpha 5 beta 1-integrin into tensin-1-positive fibrillar adhesions, which destabilized the endothelial monolayer. Thus, beta 1-integrin promotes endothelial barrier disruption during inflammation, and targeting beta 1-integrin signaling could serve as a novel means of blocking pathological vascular leak.Peer reviewe

VEGF-C is required for intestinal lymphatic vessel maintenance and lipid absorption

Vascular endothelial growth factor C (VEGF-C) binding to its tyrosine kinase receptor VEGFR-3 drives lymphatic vessel growth during development and in pathological processes. Although the VEGF-C/VEGFR-3 pathway provides a target for treatment of cancer and lymphedema, the physiological functions of VEGF-C in adult vasculature are unknown. We show here that VEGF-C is necessary for perinatal lymphangiogenesis, but required for adult lymphatic vessel maintenance only in the intestine. Following Vegfc gene deletion in adult mice, the intestinal lymphatic vessels, including the lacteal vessels, underwent gradual atrophy, which was aggravated when also Vegfd was deleted. VEGF-C was expressed by a subset of smooth muscle cells adjacent to the lacteals in the villus and in the intestinal wall. TheVegfc-deleted mice showed defective lipid absorption and increased fecal excretion of dietary cholesterol and fatty acids. When fed a high-fat diet, the Vegfc-deficient mice were resistant to obesity and had improved glucose metabolism. Our findings indicate that the lymphangiogenic growth factors provide trophic and dynamic regulation of the intestinal lymphatic vasculature, which could be especially important in the dietary regulation of adiposity and cholesterol metabolism.Peer reviewe

Opposing actions of angiopoietin-2 on Tie2 signaling and FOXO1 activation

Angiopoietin-2 (ANG2) regulates blood vessel remodeling in many pathological conditions through differential effects on Tie2 signaling. While ANG2 competes with ANG1 to inhibit Tie2, it can paradoxically also promote Tie2 phosphorylation (p-Tie2). A related paradox is that both inactivation and overactivation of Tie2 can result in vascular remodeling. Here, we reconciled these opposing actions of ANG2 by manipulating conditions that govern its actions in the vasculature. ANG2 drove vascular remodeling during Mycoplasma pulmonis infection by acting as a Tie2 antagonist, which led to p-Tie2 suppression, forkhead box O1 (FOXO1) activation, increased ANG2 expression, and vessel leakiness. These changes were exaggerated by anti-Tie2 antibody, inhibition of PI3K signaling, or ANG2 overexpression and were reduced by anti-ANG2 antibody or exogenous ANG1. In contrast, under pathogen-free conditions, ANG2 drove vascular remodeling by acting as an agonist, promoting high p-Tie2, low FOXO1 activation, and no leakage. Tie1 activation was strong under pathogen-free conditions, but infection or TNF-α led to Tie1 inactivation by ectodomain cleavage and promoted the Tie2 antagonist action of ANG2. Together, these data indicate that ANG2 activation of Tie2 supports stable enlargement of normal nonleaky vessels, but reduction of Tie1 in inflammation leads to ANG2 antagonism of Tie2 and initiates a positive feedback loop wherein FOXO1-driven ANG2 expression promotes vascular remodeling and leakage

Endothelial destabilization by angiopoietin-2 via integrin beta 1 activation

Peer reviewe