Arjan W. Griffioen

Galectin-1 is essential in tumor angiogenesis and is a target for antiangiogenesis therapy

We describe that galectin-1 (gal-1) is a receptor for the angiogenesis inhibitor anginex, and that the protein is crucial for tumor angiogenesis. gal-1 is overexpressed in endothelial cells of different human tumors. Expression knockdown in cultured endothelial cells inhibits cell proliferation and migration. The importance of gal-1 in angiogenesis is illustrated in the zebrafish model, where expression knockdown results in impaired vascular guidance and growth of dysfunctional vessels. The role of gal-1 in tumor angiogenesis is demonstrated in gal-1-null mice, in which tumor growth is markedly impaired because of insufficient tumor angiogenesis. Furthermore, tumor growth in gal-1-null mice no longer responds to antiangiogenesis treatment by anginex. Thus, gal-1 regulates tumor angiogenesis and is a target for angiostatic cancer therapy.

Convergence and amplification of toll-like receptor (TLR) and receptor for advanced glycation end products (RAGE) signaling pathways via high mobility group B1 (HMGB1)

Sustained proinflammatory responses in rheumatoid arthritis, atherosclerosis, and diabetic retinopathy, as well as in cancer, are often associated with increased angiogenesis that contributes to tissue disruption and disease progression. High mobility group B1 (HMGB1) has been recognized as a proinflammatory cytokine and more recently, as a proangiogenic factor. HMGB1 can either be passively released from necrotic cells or actively secreted in response to angiogenic and inflammatory signals. HMGB1 itself may signal through the receptor for advanced glycation end products (RAGE), and via toll-like receptors, TLR2 and TLR4. Activation of these receptors results in the activation of NFκB, which induces the upregulation of leukocyte adhesion molecules and the production of proinflammatory cytokines and angiogenic factors in both hematopoietic and endothelial cells, thereby promoting inflammation. Interestingly, HMGB1 seems to be involved in a positive feedback mechanism, that may help to sustain inflammation and angiogenesis in several pathological conditions, thereby contributing to disease progression. Endothelial cells express HMGB1, as well as the receptors RAGE, TLR2, and TLR4, and in diverse pathologies HMGB1 and its receptors are overexpressed. Furthermore, HMGB1-induced signaling can activate NFκB, which can subsequently induce the expression of HMGB1 receptors. Thus, HMGB1 can mediate amplification of inflammation and angiogenesis through increased secretion of HMGB1 and increased expression of the receptors it can interact with. In this review, we discuss signaling cascades that HMGB1 can induce via TLRs and RAGE, as well as its contribution to pathologies involving endothelial cells.

MR molecular imaging and fluorescence microscopy for identification of activated tumor endothelium using a bimodal lipidic nanoparticle

In oncological research, there is a great need for imaging techniques that specifically identify angiogenic blood vessels in tumors on the basis of differences in the expression level of biomolecular markers. In the angiogenic cascade, different cell surface receptors, including the αvβ3‐integrin, are strongly expressed on activated endothelial cells. In the present study, we aimed to image angiogenesis by detecting the expression of αvβ3 in tumor bearing mice with a combination of magnetic resonance imaging (MRI) and fluorescence microscopy. To that end, we prepared MR‐detectable and fluorescent liposomes, which carry ∼700 αvβ3‐specific RGD peptides per liposome. RGD competition experiments and RAD‐conjugated liposomes were used as controls for specificity. In vivo, both RAD liposomes and RGD liposomes gave rise to signal increase on T1‐weighted MR images. It was established by the use of ex vivo fluorescence microscopy that RGD liposomes and RAD liposomes accumulated in the tumor by different mechanisms. RGD liposomes were specifically associated with activated tumor endothelium, while RAD liposomes were located in the extravascular compartment. This study demonstrates that MR molecular imaging of angiogenesis is feasible by using a targeted contrast agent specific for the αvβ3‐integrin, and that the multimodality imaging approach gave insight into the exact mechanism of accumulation in the tumor.

Monocyte/macrophage infiltration in tumors: modulators of angiogenesis

The role of a tumor immune infiltrate in cancer progression and metastasis has been debated frequently. Although often considered to be associated with improved prognosis and leading to the enhanced survival of cancer patients, inflammatory cells have also been described to assist the tumor’s capabilities to progress, proliferate, and metastasize. Tumor‐associated macrophages (TAMs), for example, have been shown to be symbiotically related to tumor cells: Tumor cells recruit TAMs and provide them with survival factors, and TAMs in turn produce a variety of angiogenic factors in response to the tumor microenvironment. This review will describe the composition of an immune infiltrate in tumors and the angiogenic and angiostatic properties of the cells present. Special emphasis will be on the angiogenesis‐associated activities of TAMs. The development of immunotherapy and gene therapy using TAMs to mediate tumor cytotoxicity or to deliver gene constructs will be discussed as well. As immunotherapy has so far not been as effective as anticipated, a combination therapy in which angiostatic agents are used as well is put forward as a novel strategy to treat cancer.

Scheduling of Radiation with Angiogenesis Inhibitors Anginex and Avastin Improves Therapeutic Outcome via Vessel Normalization

Purpose: To test whether a direct antiangiogenic peptide (anginex) and a vascular endothelial growth factor antibody (bevacizumab, Avastin) can transiently normalize vasculature within tumors to improve oxygen delivery, alleviate hypoxia, and increase the effect of radiation therapy. Experimental Design: Tumor oxygenation levels, microvessel density and pericyte coverage were monitored in three different solid tumor models (xenograft human ovarian carcinoma MA148, murine melanoma B16F10, and murine breast carcinoma SCK) in mice. Multiple treatment schedules were tested in these models to assess the influence on the effect of radiation therapy. Results: In all three tumor models, we found that tumor oxygenation levels, monitored daily in real time, were increased during the first 4 days of treatment with both anginex and bevacizumab. From treatment day 5 onward, tumor oxygenation in treated mice decreased significantly to below that in control mice. This “tumor oxygenation window” occurred in all three tumor models varying in origin and growth rate. Moreover, during the treatment period, tumor microvessel density decreased and pericyte coverage of vessels increased, supporting the idea of vessel normalization. We also found that the transient modulation of tumor physiology caused by either antiangiogenic therapy improved the effect of radiation treatment. Tumor growth delay was enhanced when single dose or fractionated radiotherapy was initiated within the tumor oxygenation window as compared with other treatment schedules. Conclusions: The results are of immediate translational importance because the clinical benefits of bevacizumab therapy might be increased by more precise treatment scheduling to ensure radiation is given during periods of peak radiosensitivity. The oxygen elevation in tumors by non–growth factor–mediated peptide anginex suggests that vessel normalization might be a general phenomenon of agents directed at disrupting the tumor vasculature by a variety of mechanisms.

Organometallic Ruthenium(II) Arene Compounds with Antiangiogenic Activity

The antimetastatic ruthenium(II) compounds [Ru(η(6)-p-cymene)Cl(2)(PTA)] (PTA = 1,3,5-triaza-7-phosphaadamantane) (RAPTA-C) and [Ru(η(6)-toluene)Cl(2)(PTA)] (RAPTA-T), as well as their analogues [Ru(η(6)-p-cymene)Cl(2)(DAPTA)] (DAPTA = (3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane)) (DAPTA-C) and [Ru(η(6)-toluene)Cl(2)(DAPTA)] (DAPTA-T), respectively, were tested in in vitro bioassays for endothelial cell function. All compounds showed low toxicity profiles and similar dose-dependent antiproliferative effects in endothelial cells at ≥100 μg/mL (∼200 μM). EC migration, measured 6 h after drug exposure, was also efficiently inhibited (ED(50) of ∼300 μg/mL, ∼500 μM, for all compounds). Since no cytostatic effect was noted, the inhibition of proliferation was considered mainly to consist of antiangiogenic activity. RAPTA-T and DAPTA-C were also tested in vivo in the chicken chorioallantoic membrane (CAM) assay and found to inhibit CAM development. Importantly, effective prevention of revascularization of the CAM after vaso-occlusive photodynamic therapy was observed. The reported ruthenium complexes show promising antimetastatic activity involving inhibition of angiogenesis and therefore are attractive agents for development of anticancer therapies based on combination of chemo- and angiostatic treatments.

Tumor cells secrete galectin-1 to enhance endothelial cell activity.

Tumor angiogenesis is a key event in cancer progression. Here, we report that tumors can stimulate tumor angiogenesis by secretion of galectin-1. Tumor growth and tumor angiogenesis of different tumor models are hampered in galectin-1-null (gal-1(-/-)) mice. However, tumor angiogenesis is less affected when tumor cells express and secrete high levels of galectin-1. Furthermore, tumor endothelial cells in gal-1(-/-) mice take up galectin-1 that is secreted by tumor cells. Uptake of galectin-1 by cultured endothelial cells specifically promotes H-Ras signaling to the Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase (Erk) kinase (Mek)/Erk cascade and stimulates endothelial cell proliferation and migration. Moreover, the activation can be blocked by galectin-1 inhibition as evidenced by hampered membrane translocation of H-Ras.GTP and impaired Raf/Mek/Erk phosphorylation after treatment with the galectin-1-targeting angiogenesis inhibitor anginex. Altogether, these data identify galectin-1 as a proangiogenic factor. These findings have direct implications for current efforts on galectin-1-targeted cancer therapies.

Anti-angiogenesis therapy can overcome endothelial cell anergy and promote leukocyte-endothelium interactions and infiltration in tumors

Tumor escape from immunity, as well as the failure of several anti‐cancer vaccination and cellular immunotherapy approaches, is suggested to be due to the angiogenesis‐mediated suppression of endothelial cell (EC) adhesion molecules involved in leukocyte‐vessel wall interactions. We hypothesized that inhibition of angiogenesis would overcome this escape from immunity. We investigated this in vivo by means of intravital microscopy and ex vivo by immunohistochemistry in two mouse tumor models. Angiogenesis inhibitors anginex, endostatin, and angiostatin, and the chemotherapeutic agent paclitaxel were found to significantly stimulate leukocyte‐vessel wall interactions by circumvention of EC anergy in vivo, i.e., by the up‐regulation of endothelial adhesion molecules in tumor vessels. This was confirmed by in vitro studies of cultured EC at the protein and mRNA levels. The new angiostatic designer peptide anginex was most potent at overcoming EC anergy; the enhanced leukocyte‐vessel interactions led to an increase in the numbers of tumor infiltrating leukocytes. While anginex inhibited tumor growth and microvessel density significantly, the amount of infiltrated leukocytes (CD45), as well as the number of CD8+ cytotoxic T lymphocytes, was enhanced markedly. The current results suggest that immunotherapy strategies can be improved by combination with anti‐angiogenesis.‐Dirkx, A. E. M., oude Egbrink, M. G. A., Castermans, K., van der Schaft, D. W. J., Thijssen, V. L. J. L., Dings, R. P. M., Kwee, L., Mayo, K. H., Wagstaff, J., Bouma‐ter Steege, J. C. A., Griffioen, A. W. Anti‐angiogenesis therapy can overcome endothelial cell anergy and promote leukocyteendothelium interactions and infiltration in tumors. FASEB J. 20, 621–630 (2006)

Anginex, a designed peptide that inhibits angiogenesis.

Novel beta-sheet-forming peptide 33-mers, betapep peptides, have been designed by using a combination approach employing basic folding principles and incorporating short sequences from the beta-sheet domains of anti-angiogenic proteins. One of these designed peptides (betapep-25), named anginex, was observed to be potently anti-angiogenic. Anginex specifically inhibits vascular endothelial cell proliferation and induces apoptosis in these cells, as shown by flow-cytometric detection of sub-diploid cells, TUNEL (terminal deoxyribonucleotidyl transferase-mediated dUTP-nick-end labelling) analysis and cell morphology. Anginex also inhibits endothelial cell adhesion to and migration on different extracellular matrix components. Inhibition of angiogenesis in vitro is demonstrated in the sprout-formation assay and in vivo in the chick embryo chorio-allantoic membrane angiogenesis assay. Comparison of active and inactive betapep sequences allows structure-function relationships to be deduced. Five hydrophobic residues and two lysines appear to be crucial to activity. This is the first report of a designed peptide having a well-defined biological function as a novel cytokine, which may be an effective anti-angiogenic agent for therapeutic use against various pathological disorders, such as neoplasia, rheumatoid arthritis, diabetic retinopathy and restenosis.

Magnetic and fluorescent nanoparticles for multimodality imaging

The development of nanoparticulate contrast agents is providing an increasing contribution to the field of diagnostic and molecular imaging. Such agents provide several advantages over traditional compounds. First, they may contain a high payload of the contrast-generating material, which greatly improves their detectability. Second, multiple properties may be easily integrated within one nanoparticle to allow its detection with several imaging techniques or to include therapeutic qualities. Finally, the surface of such nanoparticles may be modified to improve circulation half-lives or to attach targeting groups. Magnetic resonance imaging and optical techniques are highly complementary imaging methods. Combining these techniques would therefore have significant advantages and may be realized through the use of nanoparticulate contrast agents. This review gives a survey of the different types of fluorescent and magnetic nanoparticles that have been employed for both magnetic resonance and optical imaging studies.