Angiopoietin-2 Interferes with Anti-VEGFR2-Induced Vessel Normalization and Survival Benefit in Mice Bearing Gliomas

Edwin L. Steele Laboratory, Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts 02114, USA.
Clinical Cancer Research (Impact Factor: 8.72). 07/2010; 16(14):3618-27. DOI: 10.1158/1078-0432.CCR-09-3073
Source: PubMed


In brain tumors, cerebral edema is a significant source of morbidity and mortality. Recent studies have shown that inhibition of vascular endothelial growth factor (VEGF) signaling induces transient vascular normalization and reduces cerebral edema, resulting in a modest survival benefit in glioblastoma patients. During anti-VEGF treatment, circulating levels of angiopoietin (Ang)-2 remained high after an initial minor reduction. It is not known, however, whether Ang-2 can modulate anti-VEGF treatment of glioblastoma. Here, we used an orthotopic glioma model to test the hypothesis that Ang-2 is an additional target for improving the efficacy of current anti-VEGF therapies in glioma patients.
To recapitulate high levels of Ang-2 in glioblastoma patients during anti-VEGF treatment, Ang-2 was ectopically expressed in U87 glioma cells. Animal survival and tumor growth were assessed to determine the effects of Ang-2 and anti-VEGF receptor 2 (VEGFR2) treatment. We also monitored morphologic and functional vascular changes using multiphoton laser scanning microscopy and immunohistochemistry.
Ectopic expression of Ang-2 had no effect on vascular permeability, tumor growth, or survival, although it resulted in higher vascular density, with dilated vessels and reduced mural cell coverage. On the other hand, when combined with anti-VEGFR2 treatment, Ang-2 destabilized vessels without affecting vessel regression and compromised the survival benefit of VEGFR2 inhibition by increasing vascular permeability. VEGFR2 inhibition normalized tumor vasculature whereas ectopic expression of Ang-2 diminished the beneficial effects of VEGFR2 blockade by inhibiting vessel normalization.
Cancer treatment regimens combining anti-VEGF and anti-Ang-2 agents may be an effective strategy to improve the efficacy of current anti-VEGF therapies.

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    • "Interestingly, some studies indicated that Ang-2 had no significant effect on tumor growth. For example, Sung-Suk Chae et al. [23] showed that ectopic expression of Ang-2 had no effect on U87 glioma tumor growth. Study from H Yoshiji et al. [24] also indicated that overexpression of Ang-2 did not increase hepatocellular carcinoma development. "
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    ABSTRACT: The overexpression of angiopoietin-2 (Ang-2) has both pro-tumorigenic and anti-tumorigenic effects. However, the mechanisms of this protein's dual effects are poorly understood, and it remains unclear how Ang-2 cooperates with vascular endothelial growth factor (VEGF). In the current study, we investigated the effects of Ang-2 overexpression on nasopharyngeal carcinoma growth in the presence of different levels of VEGF. Ang-2 was introduced into the CNE2 cell line by liposome transfection, and the expression of endogenous VEGF was inhibited by microRNA-mediated RNA interference. CNE2 cells expressing varying levels of Ang-2 and VEGF were injected subcutaneously into the flanks of nude mice. Tumor growth was measured, and vessels from the harvested tumors were analyzed. The overexpression of Ang-2 had no obvious effect on CNE2 tumor growth in the presence of endogenous VEGF but significantly inhibited CNE2 tumor growth when the expression of endogenous VEGF was silenced, and the Ang-2/VEGF ratio is negatively correlated with tumor growth. Ang-2 overexpression decreased the percentage of alpha-SMA-positive cells around the tumor vessels but reduced the microvessel density only in the absence of VEGF. Our results indicate that the effects of Ang-2 on nasopharyngeal carcinoma are highly dependent on the level of VEGF expression, Ang-2/VEGF ratio may offer a novel therapeutic approach for treating human cancer.
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    • "In addition to the above-mentioned adverse effects of anti-VEGF treatment, pre-clinical and clinical reports suggest that cancer cells may develop resistance to anti-angiogenic therapy by different mechanisms that include (1) switch to a pro-migratory phenotype, (2) up-regulation of other pro-angiogenic molecules [151] and (3) the increased recruitment of myeloid cells that support tumor growth (reviewed in [7, 66, 129]). As such, novel therapies are currently under development in both academia and pharmaceutical companies that include (1) novel VEGF inhibitors such as the VEGF trap [46, 55], (2) drugs that target the Tie2/Angiopoietin signaling pathway (reviewed in [17, 58]), (3) double inhibition of VEGF and angiopoietins [12, 18, 73], (4) double inhibition of VEGF and c-met [130, 154], (5) inhibition of PlGF [30, 40] and (6) drugs that block myeloid cell recruitment and/or their polarization [91], among others. However, it has been suggested that intratumoral vessel are heterogenous and that, for example, VEGF-dependent and -independent vessels may co-exist within a given tumor [137]. "
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    • "However, this strategy also seems counterintuitive: it is a widely held belief that antiangiogenic therapy eradicates tumor vasculature, thus depriving the tumor of oxygen and nutrients. However, preclinical models have shown that antiangiogenic therapy may transiently “normalize” the tumor vasculature to make it more efficient for oxygen delivery, thereby providing a window of opportunity for enhanced sensitivity to radiation treatment [3], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Since there are no guidelines for optimal scheduling of antiangiogenic therapy and radiation therapy, the timing of the administration of antiangiogenic agents, relative to the delivery of ionizing radiation, would be critical for optimizing the antitumor effect of these treatments. "
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