Mechanisms of angiogenesis in gliomas

Gliomas are the most frequent primary tumors of the central nervous system in adults. Glioblastoma muhiforme, the most aggressive form of astrocytic tumors, displays a rapid progression that is accompanied by particular poor prognosis of patients. Intense angiogenesis is a distinguishing pathologic characteristic of these tumors and in fact, glioblastomas are of the most highly vascularized malignant tumors. For this reason, research and therapy strategies have focused on derstanding the mechanisms leading to the origin of tumor angiogenic blood vessels in order to develop new approaches that effectively block angiogenesis and cause tumor regression.     

Role of hematopoietic lineage cells as accessory components in blood vessel formation

In adults, the vasculature is normally quiescent, due to the dominant influence of endogenous angiogenesis inhibitors over angiogenic stimuli. However, blood vessels in adults retain the capacity for brisk initiation of angiogenesis, the growth of new vessels from pre-existing vessels, during tissue repair and in numerous diseases, including inflammation and cancer. Because of the role of angiogenesis in tumor growth, many new cancer therapies are being conducted against tumor angiogenesis. It is thought that these anti-angiogenic therapies destroy the tumor vessels, thereby depriving the tumor of oxygen and nutrients. Therefore, a better understanding of the molecular mechanisms in the process of sprouting angiogenesis may lead to more effective therapies not only for cancer but also for diseases involving abnormal vasculature. It is widely believed that after birth, endothelial cells (EC) in new blood vessels are derived from resident EC of pre-existing vessels. However, evidence is now emerging that cells derived from the bone marrow may also contribute to postnatal angiogenesis. Most studies have focused initially on the contribution of endothelial progenitor cells in this process. However, we have proposed a concept in which cells of the hematopoietic lineage are mobilized and then entrapped in peripheral tissues, where they function as accessory cells that promote the sprouting of resident EC by releasing angiogenic signals. Most recently we found that hematopoietic cells play major roles in tumor angiogenesis by initiating sprouting angiogenesis and also in maturation of blood vessels in the fibrous cap of tumors. Therefore, manipulating these entrapment signals may offer therapeutic opportunities to stimulate or inhibit angiogenesis.     

Antiangiogenic cancer therapy

Like most embryonic tissues, tumors have the ability to build up their own blood vessel networks. However, the architecture of tumor vessels is fundamentally different from that found in healthy tissues. Tumor vessels are usually irregular, heterogeneous, leaky, and poorly associated with mural cells. Endothelial cells in tumor vessels are also disorganized and express imbalanced surface molecules. These unusual features may provide some molecular and structural basis for selective inhibition or even destruction of tumor vessels by angiogenesis inhibitors. In animal tumor models, several angiogenesis inhibitors seem to inhibit tumor angiogenesis specifically without obvious effects on the normal vasculature. As a result, these inhibitors produced potent antitumor effects in mice. Excited by these preclinical studies, more than 60 angiogenesis inhibitors are being evaluated for their anticancer effects in human patients. Although the ultimate outcome of antiangiogenic clinical trials remains to be seen, several early observations have reported some disappointing results. These early clinical data have raised several important questions. Can we cure human cancers with angiogenesis inhibitors? Have we found the ideal angiogenesis inhibitors for therapy? What is the difference between angiogenesis in an implanted mouse tumor and in a spontaneous human tumor? What are the molecular mechanisms of these angiogenesis inhibitors? Should angiogenesis inhibitors be used alone or in combinations with other existing anticancer drugs? In this review, we will discuss these important issues in relation to ongoing antiangiogenic clinical trials.     

Anti-angiogenic Treatment Strategies for Malignant Brain Tumors

The use of angiogenesis inhibitors may offer novel strategies in brain tumor therapy. In contrast to traditional cancer treatments that attack tumor cells directly, angiogenesis inhibitors target at the formation of tumor-feeding blood vessels that provide continuous supply of nutrients and oxygen.With respect to brain tumor therapy, inhibitors of angiogenesis display unique features that are unknown to conventional chemotherapeutic agents. The most important features are independence of the blood–brain barrier, cell type specificity, and reduced resistance. Malignant brain tumors, especially malignant gliomas, are among the most vascularized tumors known. Despite multimodal therapeutic approaches, the prognosis remains dismal. Thus, angiogenesis inhibitors may be highly effective drugs against these tumors. In a clinical setting, they could be applied in the treatment of multiple tumors or postsurgically as an adjuvant therapy to prevent recurrence.This article provides an overview of current anti-angiogenic treatment strategies with emphasis on substances already in clinical trials or candidate substances for clinical trials. The cellular and molecular basis of these substances is reviewed.     

Mouse models for studying angiogenesis and lymphangiogenesis in cancer

The formation of new blood vessels (angiogenesis) is required for the growth of most tumors. The tumor microenvironment also induces lymphangiogenic factors that promote metastatic spread. Anti-angiogenic therapy targets the mechanisms behind the growth of the tumor vasculature. During the past two decades, several strategies targeting blood and lymphatic vessels in tumors have been developed. The blocking of vascular endothelial growth factor (VEGF)/VEGF receptor-2 (VEGFR-2) signaling has proven effective for inhibition of tumor angiogenesis and growth, and inhibitors of VEGF-C/VEGFR-3 involved in lymphangiogenesis have recently entered clinical trials. However, thus far anti-angiogenic treatments have been less effective in humans than predicted on the basis of pre-clinical tests in mice. Intrinsic and induced resistance against anti-angiogenesis occurs in patients, and thus far the clinical benefit of the treatments has been limited to modest improvements in overall survival in selected tumor types. Our current knowledge of tumor angiogenesis is based mainly on experiments performed in tumor-transplanted mice, and it has become evident that these models are not representative of human cancer. For an improved understanding, angiogenesis research needs models that better recapitulate the multistep tumorigenesis of human cancers, from the initial genetic insults in single cells to malignant progression in a proper tissue environment. To improve anti-angiogenic therapies in cancer patients, it is necessary to identify additional molecular targets important for tumor angiogenesis, and to get mechanistic insight into their interactions for eventual combinatorial targeting. The recent development of techniques for manipulating the mammalian genome in a precise and predictable manner has opened up new possibilities for the generation of more reliable models of human cancer that are essential for the testing of new therapeutic strategies. In addition, new imaging modalities that permit visualization of the entire mouse tumor vasculature down to the resolution of single capillaries have been developed in pre-clinical models and will likely benefit clinical imaging.     

Heterogeneity of tumor endothelial cells

Tumor blood vessels play important roles in tumor progression and metastasis. Thus, targeting tumor blood vessels is an important strategy for cancer therapy. Tumor endothelial cells (TECs) are the main targets of anti‐angiogenic therapy. Although tumor blood vessels generally sprout from pre‐existing vessels and have been thought to be genetically normal, they display a markedly abnormal phenotype, including morphological changes. The degree of angiogenesis is determined by the balance between the positive and negative regulating molecules that are released by tumor and host cells in the microenvironment. Reportedly, tumor blood vessels are heterogeneous with TECs differing from normal endothelial cells (in contrast to the conventional view). We recently compared characteristics of different TECs isolated from highly and low metastatic tumors. We found TECs from highly metastatic tumors had more proangiogenic phenotypes than those from low metastatic tumors. Elucidating the variety of TEC phenotypes and identifying TEC molecular signatures should lead to more complete understanding of the mechanisms of tumor progression, discovery of new therapeutic targets, and development of biomarkers. This review considers current studies on TEC heterogeneity and discusses the therapeutic implications of these findings.     

The roles of chemokine CXCL12 in embryonic and brain tumor angiogenesis

The formation of blood vessels in embryos and tumors are different processes but under the control of common molecular mechanisms. Chemokine CXCL12 involved in both embryonic and tumor angiogenesis. In this review, we summarize recent advances in understanding the roles of CXCL12 in brain tumor angiogenesis/vasculogenesis. CXCL12 and its cognate receptors are abnormally induced in brain tumors, in particular in tumor cells and endothelium. Pathologically enhanced CXCL12 signaling may promote the formation of new vessels through recruiting circulating endothelial progenitor cells or directly enhancing the migration/growth of endothelial cells. Therefore, CXCL12 signaling represents an important mechanism that regulates brain tumor angiogenesis/vasculogenesis and may provide potential targets for anti-angiogenic therapy in malignant gliomas.     

Targeting tumor vasculature with homing peptides from phage display

Tumor vasculature expresses a number of molecular markers at much lower levels than those seen in the blood vessels of normal tissues, and in some cases, such markers are undetectable. The presence of these markers relates to angiogenesis; the same markers are shared by all blood vessels undergoing angiogenesis. The endothelial cells, pericytes and smooth muscle cells, and the vascular extracellular matrix in angiogenic vessels can each express such markers. Molecularly, they represent vascular growth factor receptors, cell adhesion proteins and their receptors. Screening of phage display libraries for peptides that home to tumor vasculature when injected into mice has recently provided a new tool for analyzing the distinguishing features of tumor vasculature. Tumor-homing peptides isolated in this manner, as well as an antibody against a form of fibronectin expressed in tumor blood vessels, have been found to serve as targeting devices to concentrate drugs and other therapeutic materials to tumors in in vivo models. Such a targeting strategy can therefore potentially improve the efficacy of drugs and reduce their side effects.     

VEGF receptor signaling in tumor angiogenesis

The growth of human tumors and development of metastases depend on the de novo formation of blood vessels. The formation of new blood vessels is tightly regulated by specific growth factors that target receptor tyrosine kinases (RTKs). Vascular endothelial growth factor (VEGF) and the Flk-1/KDR RTK have been implicated as the key endothelial cell-specific factor signaling pathway required for pathological angiogenesis, including tumor neovascularization. Inhibition of the VEGF tyrosine kinase signaling pathway blocks new blood vessel formation in growing tumors, leading to stasis or regression of tumor growth. Advances in understanding the biology of angiogenesis have led to the development of several therapeutic modalities for the inhibition of the VEGF tyrosine kinase signaling pathway. A number of these modalities are under investigation in clinical studies to evaluate their potential to treat human cancers.     

Endothelial Cells of Tumor Vessels: Abnormal but not Absent

The question of whether some blood vessels in tumors of non-vascular origin are lined by cancer cells has been discussed for many years because of the relevance to metastasis, access of drugs to tumor cells, and the effectiveness of angiogenesis inhibitors. Most evidence favoring the existence of tumor cell-lined vessels has come from observations of standard histopathological tissue sections or from transmission and scanning electron microscopic studies. However, it has been difficult to determine convincingly just how abundant these vessels are in tumors. On the one hand, virtually the entire microvasculature is supposedly lined by tumor cells in aggressive uveal melanomas, assuming the presence of vasculogenic mimicry where tumor cells masquerading as endothelial cells create the channels for blood flow. On the other hand, morphometric studies using immunohistochemistry and green fluorescent protein-transfected tumor cells suggest that human colon cancer cells constitute only 3% of the vessel surface in tumors grown orthotopically in mice. This commentary weighs evidence that cancer cells are located in the wall of tumor vessels and discusses the pitfalls in identifying such vessels. Published data along with new observations illustrate the challenges of making an unequivocal identification of tumor cells in vessel walls. Taken together, current evidence suggests that cancer cells contribute at most only a small proportion of the lining of blood vessels in tumors and may be migrating through vessel walls or exposed by defects in the endothelium. Even in aggressive uveal melanomas, blood flow probably occurs mainly through channels lined by endothelial cells, not tumor cells, and most existing data do not support a functionally significant contribution of vasculogenic mimicry. Innovative new approaches that distinguish pleomorphic tumor cells from abnormal endothelial cells in vessel walls will help to define the incidence and importance of tumor cell-lined blood vessels in drug delivery and metastasis via the bloodstream.     

Molecular Mechanisms of Tumor Angiogenesis and Tumor Progression

The formation of new blood vessels (angiogenesis) is crucial for the growth and persistence of primary solid tumors and their metastases. Furthermore, angiogenesis is also required for metastatic dissemination, since an increase in vascular density will allow easier access of tumor cells to the circulation. Induction of angiogenesis precedes the formation of malignant tumors, and increased vascularization seems to correlate with the invasive properties of tumors and thus with the malignant tumor phenotype. In the last few years, the discovery and characterization of tumor-derived angiogenesis modulators greatly contributed to our understanding of how tumors regulate angiogenesis. However, although angiogenesis appears to be a rate-limiting event in tumor growth and metastatic dissemination, a direct connection between the induction of angiogenesis and the progression to tumor malignancy is less well understood. In this review, we discuss the most recent observations concerning the modulation of angiogenesis and their implications in tumor progression, as well as their potential impact on cancer therapy.     

A new family of angiogenic factors

Angiogenesis is the production of new blood vessels from pre-existing ones. This process is tightly regulated by a series of pro- and anti-angiogenic molecules in normal physiology and when this equilibrium is broken serious consequences may arise. Solid tumors are characterized by a fast growth that eventually pushes cells away from their natural source of oxygen and nutrients from the capillaries. To survive in this hypoxic environment, tumor cells secrete a variety of pro-angiogenic molecules that would elicit proliferation of new blood vessels, thus re-establishing oxygen and nutrient supply. Blockade of angiogenesis may provide a rational approach to managing tumor growth and novel strategies are being developed. The identification of new targets is of paramount importance in the search for a clinically proficient anti-angiogenic therapy. The adrenomedullin family of peptides and gastrin-releasing peptide (GRP) are newly identified pro-angiogenic molecules, secreted by the tumors, whose inhibition results in a considerable reduction of angiogenesis and of tumor growth in animal models. The recent identification of small molecules that reduce the angiogenic effect of these peptides opens new avenues for the development of new anti-tumorigenic drugs.     

Twenty years after：the beautiful hypothesis andthe ugly facts

The limited clinical beneifts from current antiangiogenic therapy for cancer patients have triggered some critical thoughts and insightful investigations aiming to further elucidate the relationship between vessels and cancer. Tumors need blood perfusion but there are mounting evidences that angiogenesis alone does not explain it in all the neoplasms. In this editorial, for a special issue on tumor and vessels published in theChinese Journal of Cancer, we brielfy introduce the history of the evidences that solid tumors can sometimes obtain blood perfusion by alter?native approaches other than sprouting angiogenesis, i.e., vessel co?option and vasculogenic mimicry. This editorial provides also the links to several most recently published discoveries and hypotheses on tumor interaction with blood vessels.     

Current concepts of tumor-induced angiogenesis

Tumor induced angiogenesis is responsible for the nutrition of the growing tumor and can also increase the probability of hematogenous tumor dissemination. Data obtained from morphological analysis of tumor angiogenesis can contribute to the development of new anti-angiogenic therapies. Based on in vitro and in vivo observations several models of angiogenesis were introduced, explaining the mechanism of lumen formation and the timing of basement membrane depositon. (1) Lumen is formed either by cell body curving or by fusion of intracellular vacuoles of nonpolarized endothelial cells. New basement membrane is deposited after lumen formation. (2) Slit-like lumen is immediately formed by migrating polarized endothelial cells. Basement membrane is continuously deposited during endothelial cell migration, only cellular processes of the endothelial cell migrating on the tip of the growing capillary are free of deposited basement membrane material. (3) Development of transluminal bridges in larger vessels - a process called intussusceptive growth - leads to the division of the vessels. These models, however, describe angiogenesis in tissues rich in connective tissue. Different processes of angiogenesis take place in organs - such as liver, lungs, adrenals, which are the most frequent sites of metastasis - having high vessel density without sufficient space for capillary sprouting. In the case of liver metastases of Lewis lung carcinoma the proliferation of endothelial cells was elicited only by direct contact between tumor and endothelial cells, leading to the development of large convoluted vessels inside the metastases. These vessels were continuous with the sinusoidal system, suggesting that these metastases have dual blood supply. This observation, among others, is in contrast to the generally accepted view that liver tumors have arterial blood supply. The increasing number of data demonstrating the dual or venous blood supply of liver metastases should be taken into consideration in the therapy of liver metastasis.     

VEGF as a therapeutic target in cancer

Tumors require nutrients and oxygen in order to grow, and new blood vessels, formed by the process of angiogenesis, provide these substrates. The key mediator of angiogenesis is vascular endothelial growth factor (VEGF), which is induced by many characteristics of tumors, most importantly hypoxia. Therefore, VEGF is an appealing target for anticancer therapeutics. In addition, VEGF is easy to access as it circulates in the blood and acts directly on endothelial cells. VEGF-mediated angiogenesis is rare in adult humans (except wound healing and female reproductive cycling), and so targeting the molecule should not affect other physiological processes. Tumor blood vessels, formed under the influence of VEGF, are disorganized, tortuous and leaky with high interstitial pressure, reducing access for chemotherapies. Inhibiting VEGF would reduce the vessel abnormality and increase the permeability of the tumor to chemotherapies. Several approaches to targeting VEGF have been investigated. The most common strategies have been receptor-targeted molecules and VEGF-targeting molecules. The disadvantage of receptor-targeted approaches is that the VEGF receptors also bind different members of the VEGF super-family and affect systems other than angiogenesis. The best-studied and most advanced approach to VEGF inhibition is the humanized monoclonal antibody bevacizumab (Avastin), which is the only anti-angiogenic agent approved for treatment of cancer.     

Tumor angiogenesis: A physiological process or genetically determined?

Continued tumor growth is dependent upon the growth of new blood vessels. This commentary reviews the mechanisms whereby tumors become vascularized and examines whether tumor angiogenesis is solely an example of a normal physiologic process or is part of the genetic program of the tumor. The likelihood that neovascularization of tumors combines both of these components, that is, utilizing tumor-specific elements as well as capacities common to all cells, is discussed.     

Tumor angiogenesis: cause or consequence of cancer?

Both tumors and normal tissues need a blood supply for oxygen, nutrients, and waste removal. However, whereas normal vasculature is hierarchically assembled into efficient networks of arteries, capillaries, and veins, the blood vessels of tumors are a mess-chaotic, leaky, inefficient, and barely making do. Why the difference? Do tumor vessels lack the signals to mature or, instead, is their maturation actively suppressed? What triggers and maintains tumor vasculature? In a recent study using a switchable Myc-driven mouse tumor model, we addressed these fundamental questions. We identified the inflammatory cytokine interleukin-1beta as an essential initiating trigger of vascular endothelial growth factor-dependent angiogenesis. Here, we consider how kinetic studies using regulatable forms of Myc or other oncogenes can shed new light on the way tumors initiate and maintain their aberrant blood supplies.     

Vascular endothelial growth factor in ovarian cancer

Tumor growth requires nutrients and oxygen. Both nutrients and oxygen are provided via the vasculature. Thus, when a tumor increases in volume, new blood vessels must form and invade the expanding tumor. This process, called angiogenesis, has theoretical significance in the context of ovarian cancer for two reasons. First, the process of angiogenesis and vessel regression occurs in a tightly controlled way as part of normal ovarian function. This suggests that at least some ovarian cells are primed to produce the paracrine stimulus needed for new blood vessel growth and that, on tranformation, this capability is present early in tumor development. Second, the characteristically large size of ovarian tumors indicates that angiogenesis is mandatory to sustain the tumor. In this article, we review the experimental and clinical correlative data that support the hypothesis that ovarian cancers are highly angiogenic. Because a critical component of angiogenesis is the paracrine and autocrine production of vascular endothelial cell growth factor, there is substantial focus on this topic.     

Anti-VEGF/VEGFR therapy for cancer: reassessing the target

Judah Folkman recognized that new blood vessel formation is important for tumor growth and proposed antiangiogenesis as a novel approach to cancer therapy. Discovery of vascular permeability factor VEGF-A as the primary tumor angiogenesis factor prompted the development of a number of drugs that targeted it or its receptors. These agents have often been successful in halting tumor angiogenesis and in regressing rapidly growing mouse tumors. However, results in human cancer have been less impressive. A number of reasons have been offered for the lack of greater success, and, here, we call attention to the heterogeneity of the tumor vasculature as an important issue. Human and mouse tumors are supplied by at least 6 well-defined blood vessel types that arise by both angiogenesis and arterio-venogenesis. All 6 types can be generated in mouse tissues by an adenoviral vector expressing VEGF-A(164). Once formed, 4 of the 6 types lose their VEGF-A dependency, and so their responsiveness to anti-VEGF/VEGF receptor therapy. If therapies directed against the vasculature are to have a greater impact on human cancer, targets other than VEGF and its receptors will need to be identified on these resistant tumor vessels.     

Magnetic resonance imaging applications in the evaluation of tumor angiogenesis

Angiogenesis, the growth of new blood vessels, is a critical component in the development of solid tumors. Over the last decade, progress in the study of the biology of angiogenesis has led to identification of a large number of molecules that promote, participate, and regulate the growth of new vessels in normal tissue and in tumors. Consequently, many new targets for suppression of angiogenesis have been identified and are now at various stages of development and evaluation in clinical trials. Magnetic resonance imaging (MRI) provides an attractive tool for in vivo analysis of the basic biology of angiogenesis, for preclinical evaluation of the activity of a number of potential antiangiogenic agents, as well as for clinical detection, diagnosis, and prognosis. One of the features of MRI is the wide range of physiologic parameters by which angiogenesis can be imaged. This review presents the biological basis of angiogenesis with emphasis on characteristics of the neovasculature that can be used for imaging, followed by an overview of the MRI approaches that are being evaluated for the analysis of tumor angiogenesis.