In situ expression of angiopoietins in astrocytomas identifies angiopoietin-2 as an early marker of tumor angiogenesis

Differences in vasculature between pilocytic and anaplastic astrocytomas of childhood

BACKGROUND

The clinical manifestations of childhood pilocytic astrocytoma (PA) and anaplastic astrocytoma (AA) markedly differ, especially in the time to progression and prognosis. Because of the aggressive course and poor survival rate of AA, one would expect it to be associated with a high angiogenic index. Counterintuitively, we often find higher microvessel density counts in PA than in AA.

PROCEDURE

We examined the differences in type or density of microvasculature between the two neoplasms. To differentiate established, mature vessels from immature growing ones, we used antibodies to Factor VIII (FVIII) to stain endothelial cells (ECs) of blood vessels and alpha-smooth muscle actin (alpha-SMA) antibodies to stain vessels supported by adventitia.

RESULTS

We found that large, mature, alpha-SMA-positive vessels predominated in PA, and small, immature, alpha-SMA-negative vessels in AA. The vessel maturation index was 54.5% for PA, and 6.1% for AA. Immunostaining with vascular endothelial growth factor (VEGF) and anti-flt-1/VEGF receptor-1 antibodies showed distinct tissue patterns. VEGF immunoreactivity occurred mainly in the processes of the tumor astrocytes in PA; the opposite was observed in AA. flt-1/VEGFR-1 was detected in the tumor cells of AA but not in those of PA.

CONCLUSIONS

We propose that the predominance of small, alpha-SMA-negative vessels in AA represents immature, unstable vasculature with a potentially greater susceptibility to anti-angiogenic therapy. The expression of both flt-1 and VEGF by AA tumor cells also suggests a possible autocrine growth-promoting function for VEGF in addition to its role as paracrine pro-angiogenic growth factor for activated ECs, thus making anti-angiogenesis an attractive therapeutic target in the treatment of AA.

Angiogenesis in Nervous System Disorders

Angiogenesis is a crucial requirement for embryonal development and new vessel formation during adult life. Various disease processes such as cancer, ischemia, vascular malformations, and inflammatory processes also depend on pathological angiogenesis. A better understanding of the complex and coordinated interactions among various angiogenic pathways involved in pathological angiogenesis is necessary to improve our therapeutic approaches to the various disease processes observed in the central nervous system. This review summarizes the current understanding of the role of principal angiogenic factors relevant to neurosurgical abnormalities.

Infectious angiogenesis: Bartonella bacilliformis infection results in endothelial production of angiopoetin-2 and epidermal production of vascular endothelial growth factor

Pathological angiogenesis, the development of a microvasculature by neoplastic processes, is a critical component of the development of tumors. The role of oncogenes in the induction of angiogenesis has been extensively studied in benign and malignant tumors. However, the role of infection in inducing angiogenesis is not well understood. Verruga peruana is a clinical syndrome caused by the bacterium Bartonella bacilliformis, and is characterized by the development of hemangioma-like lesions, in which bacteria colonize endothelial cells. To gain insight into how this bacteria induces angiogenesis in vivo, we performed in situ hybridization of clinical specimens of verruga peruana for the angiogenesis factors vascular endothelial growth factor (VEGF), its receptors VEGFR1 and VEGFR2, and angiopoietin-2. High-level expression of angiopoietin-2 and VEGF receptors was observed in the endothelium of verruga peruana. Surprisingly, the major source of VEGF production in verruga peruana is the overlying epidermis. Infection of cultured endothelium with B. bacilliformis also resulted in induction of angiopoetin-2 in vitro. These findings imply a collaboration between infected endothelium and overlying epidermis to induce angiogenesis.

The enigmatic role of angiopoietin-1 in tumor angiogenesis

A tumor vasculature is highly unstable and immature, characterized by a high proliferation rate of endothelial cells, hyper-permeability, and chaotic blood flow. The dysfunctional vasculature gives rise to continual plasma leakage and hypoxia in the tumor, resulting in constant on-sets of inflammation and angiogenesis. Tumors are thus likened to wounds that will not heal. The lack of functional mural cells, including pericytes and vascular smooth muscle cells, in tumor vascular structure contributes significantly to the abnormality of tumor vessels. Angiopoietin-1 (Ang1) is a physiological angiogenesis promoter during embryonic development. The function of Ang1 is essential to endothelial cell survival, vascular branching, and pericyte recruitment. However, an increasing amount of experimental data suggest that Ang1-stimulated association of mural cells with endothelial cells lead to stabilization of newly formed blood vessels. This in turn may limit the otherwise continuous angiogenesis in the tumor, and consequently give rise to inhibition of tumor growth. We discuss the enigmatic role of Ang1 in tumor angiogenesis in this review.

New non-angiogenesis dependent pathways for tumour growth

The current wisdom is that tumours are endowed with an angiogenic capability and that their growth, invasion and metastasis are angiogenesis-dependent. This article summarises the literature concerning recent histomorphological studies that indicate that some tumours may be vascularised without significant angiogenesis, probably by using existing vessels, a process later described as vascular co-option, or even by forming vascular channels on their own through a non-endothelial cell process called "vascular mimicry". Moreover, the possibility that bone marrow-derived stem cells may also be a source of endothelial precursor cells recruited for tumour-induced neovascularisation, is reviewed. In fact, it has been assumed that the additional endothelial cells required to construct new tumour vessels come from the division and proliferation of local endothelial cells and that endothelial cells incorporated into sites of neovascularisation, including tumour-induced new blood vessels, may be derived from these precursor cells. Finally, lymphoangiogenesis as a mechanism of de novo formation of lymphatics, favouring the metastatic dissemination of tumour cells, is summarised. Potential therapeutic applications are also discussed.

Expression of Tie-2 in human peripheral and autonomic nervous system

Tie-2, a tyrosine kinase receptor, is essential for vascular integrity by regulating cellular adhesion between pericytes and endothelial cells. The aim of this study was to identify sites of expression of Tie-2 other than the vasculature. Tie-2 expression was first detected in human colon by Western blotting and reverse-transcription-polymerase chain reaction (RT-PCR) in tissue extracts. The presence of the Tie-2 mRNA and protein was detected by immunohistochemistry and in situ hybridization in cells of the colon myenteric and submucosal plexus, in both neuronal and Schwann cells. Tie-2 protein was also found in the nervous system of the female urogenital tract. In the human sciatic nerve and schwannoma, RT-PCR, Western blotting and immunohistochemistry analysis further confirmed the presence of Tie-2 mRNA and protein in non-autonomic peripheral nervous tissue. In conclusion, using several approaches and tissues we have demonstrated the presence of Tie-2 in human peripheral and autonomic nervous tissue, suggesting a role for Tie-2 in neural tissue. Thus, attempts to disrupt the tumour vessels by manipulation of the Tie-2 system in tumours may result in side-effects in peripheral nerves.

Altered expression of angiopoietins during blood-brain barrier breakdown and angiogenesis

Angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) belong to a novel family of endothelial growth factors that function as ligands for the endothelial-specific receptor tyrosine kinase, Tie-2. Ang-1 reduces endothelial permeability of noncerebral vessels and has a major role in vascular stabilization and maturation, whereas Ang-2 is thought to be an endogenous antagonist of the action of Ang-1 at Tie-2. Expression of these ligands at the mRNA and protein level were studied during both blood-brain barrier (BBB) breakdown and cerebral angiogenesis occurring in the rat cortical cold-injury model by RT-PCR analysis and immunohistochemistry respectively, during a time course of 6 hours to 6 days. In addition, immunohistochemical detection of fibronectin was used to detect BBB breakdown at the lesion site and dual labeling was used to determine whether the vessels demonstrating BBB breakdown expressed endothelial Ang-1 or Ang-2. Endothelial Ang-1 and Tie-2 proteins were present in all cerebral vessels of normal brain including those of the choroid plexuses, whereas both these proteins as well as Ang-2 were present in choroid plexus epithelium and in ependymal cells, suggesting that angiopoietins have an autocrine effect on these cell types as well. In contrast, in the early phase after injury during the known period of BBB breakdown, increased Ang-2 mRNA and protein and decreased endothelial Ang-1 and Tie-2 proteins were observed. Two to 6 days after injury, the progressive increase in Ang-1 mRNA and protein and the decrease in Ang-2 coincided with cerebrovascular angiogenesis. Confocal microscopy showed colocalization of both Ang-1 and Ang-2 in endothelium of lesion vessels, and our observation of colocalization of Ang-1 and Ang-2 in polymorphonuclear leukocytes and macrophages has not been reported previously. This study demonstrates that Ang-1 is an important factor in maintaining normal homeostasis in the brain. Thus Ang-1 therapy may have therapeutic potential in reducing BBB breakdown and the ensuing edema after massive brain injury.

Angiopoietins in angiogenesis and beyond

The angiopoietin (Ang) family of growth factors includes four members, all of which bind to the endothelial receptor tyrosine kinase Tie2. Two of the Angs, Ang-1 and Ang-4, activate the Tie2 receptor, whereas Ang-2 and Ang-3 inhibit Ang-1-induced Tie2 phosphorylation. While genetic models have underscored the importance of Angs in the developing cardiovascular system, other studies have demonstrated that Ang-1 promotes endothelial cell survival, sprouting and tube formation. More recently, a new aspect of the biology of this class of growth factors has emerged, namely the ability of Ang-1 to reduce inflammation. This review presents an outline of Angs and their receptors, examining their structure, expression, signalling, regulation and biological significance and comments on the role and potential usefulness of Angs in medicine.

Inhibition of rat corneal angiogenesis by a nuclease-resistant RNA aptamer specific for angiopoietin-2

Angiopoietin-2 (Ang2) appears to be a naturally occurring antagonist of the endothelial receptor tyrosine kinase Tie2, an important regulator of vascular stability. Destabilization of the endothelium by Ang2 is believed to potentiate the actions of proangiogenic growth factors. To investigate the specific role of Ang2 in the adult vasculature, we generated a nuclease-resistant RNA aptamer that binds and inhibits Ang2 but not the related Tie2 agonist, angiopoietin-1. Local delivery of this aptamer but not a partially scrambled mutant aptamer inhibited basic fibroblast growth factor-mediated neovascularization in the rat corneal micropocket angiogenesis assay. These in vivo data directly demonstrate that a specific inhibitor of Ang2 can act as an antiangiogenic agent.

Cost of migration: invasion of malignant gliomas and implications for treatment

Tumors of glial origin consist of a core mass and a penumbra of invasive, single cells, decreasing in numbers towards the periphery and still detectable several centimeters away from the core lesion. Several decades ago, the diffuse nature of malignant gliomas was recognized by neurosurgeons when super-radical resections using hemispherectomies failed to eradicate these tumors. Local invasiveness eventually leads to regrowth of a recurrent tumor predominantly adjacent to the resection cavity, which is not significantly altered by radiation or chemotherapy. This raises the question of whether invasive glioma cells activate cellular programs that render these cells resistant to conventional treatments. Clinical and experimental data demonstrate that glioma invasion is determined by several independent mechanisms that facilitate the spread of these tumors along different anatomic and molecular structures. A common denominator of this cellular behavior may be cell motility. Gene-expression profiling showed upregulation of genes related to motility, and functional studies demonstrated that cell motility contributes to the invasive phenotype of malignant gliomas. There is accumulating evidence that invasive glioma cells show a decreased proliferation rate and a relative resistance to apoptosis, which may contribute to chemotherapy and radiation resistance. Interestingly, interference with cell motility by different strategies results in increased susceptibility to apoptosis, indicating that this dynamic relationship can potentially be exploited as an anti-invasive treatment paradigm. In this review, we discuss mechanisms of glioma invasion, characteristics of the invasive cell, and consequences of this cellular phenotype for surgical resection, oncologic treatments, and future perspectives for anti-invasive strategies.

Angiopoietin-2 expression in breast cancer correlates with lymph node invasion and short survival

Angiogenic factors produced by tumor cells are essential for tumor growth and metastasis. In our study, the expression of Angiopoietin-1 (ANG1) and Angiopoietin-2 (ANG2) mRNA in archival human breast cancer tumor samples and in 6 breast cancer cell lines was investigated. Total RNA from biopsies of 38 breast cancer patients was extracted and ANG1 and ANG2 mRNA expression was measured by means of quantitative real-time RT-PCR (Taqman). Matching data with available clinicopathologic and biochemical data revealed a significant association between ANG2 expression and axillary lymph node invasion. Univariate and multivariate survival analysis, by means of Kaplan-Meier method and Cox's proportional hazards model, showed significant and independent association between ANG2 mRNA level and both disease-free (p < 0.0001) and overall survival (p < 0.0003). An important fact is that, notwithstanding the small number of cases examined, this association was confirmed also in the group of lymph node-negative patients (DFS, p < 0.003; OS, p < 0.020). Immunohistochemical analysis demonstrated that Ang2 is expressed by both tumor cells and endothelial elements. Expression in tumor cells was confirmed by studying a panel of human breast carcinoma cell lines in culture by RT-PCR. In ZR75.1 and T47D cells, expression of ANG2 mRNA was increased up to 10-fold by treatment with estrogen within 24 hr. Although preliminary, these data suggest a possible role of ANG2 as a prognostic factor for primary breast cancer.

Tumour-Induced Angiogenesis: A Review

Angiogenesis, the formation of new blood vessels, has become a broad subject and is a very active area for current research. This paper describes the main biological events involved in angiogenesis and their importance in cancer progression. In the first section, a fundamental overview of tumour biology is presented. In the second section, the biology of healthy blood vessels is described and, in the third section, the mechanisms of cell migration and proliferation, which are crucial to angiogenesis, are discussed. In the fourth section, a detailed account of tumour-induced angiogenesis is given, whilst the pro- and anti-angiogenic factors involved are reviewed in the fifth section. Finally, the processes of tumour invasion and metastasis are examined in the sixth section.

Anti-angiogenic agents for the treatment of brain tumors

It is accepted that novel therapeutic approaches are needed for the majority of patients with malignant brain tumors. The vascularity of many primary brain tumors and the encouraging preclinical studies suggest that antiangiogenic agents have the potential to become an important component of multimodality treatment of patients with brain tumors. The understanding of the biology of angiogenesis is improving rapidly, offering the hope for more specific vascular targeting of brain tumor neovasculature. Neuroimaging techniques evaluating the angiogenic process and the impact of antiangiogenic agents will be an important tool for the rapid development of these novel therapeutic agents.

Mechanisms and future directions for angiogenesis-based cancer therapies

Targeting angiogenesis represents a new strategy for the development of anticancer therapies. New targets derived from proliferating endothelial cells may be useful in developing anticancer drugs that prolong or stabilize the progression of tumors with minimal systemic toxicities. These drugs may also be used as novel imaging and radiommunotherapeutic agents in cancer therapy. In this review, the mechanisms and control of angiogenesis are discussed. Genetic and proteomic approaches to defining new potential targets on tumor vasculature are then summarized, followed by discussion of possible antiangiogenic treatments that may be derived from these targets and current clinical trials. Such strategies involve the use of endogenous antiangiogenic agents, chemotherapy, gene therapy, antiangiogenic radioligands, immunotherapy, and endothelial cell-based therapies. The potential biologic end points, toxicities, and resistance mechanisms to antiangiogenic agents must be considered as these therapies enter clinical trials.

Angiopoietin-2 is required for postnatal angiogenesis and lymphatic patterning, and only the latter role is rescued by Angiopoietin-1

VEGF and Angiopoietin-1 requisitely collaborate during blood vessel development. While Angiopoietin-1 obligately activates its Tie2 receptor, Angiopoietin-2 can activate Tie2 on some cells, while it blocks Tie2 activation on others. Our analysis of mice lacking Angiopoietin-2 reveals that Angiopoietin-2 is dispensable for embryonic vascular development but is requisite for subsequent angiogenic remodeling. Unexpectedly, mice lacking Angiopoietin-2 also exhibit major lymphatic vessel defects. Genetic rescue with Angiopoietin-1 corrects the lymphatic, but not the angiogenesis, defects, suggesting that Angiopoietin-2 acts as a Tie2 agonist in the former setting, but as an antagonist in the latter setting. Our studies define a vascular growth factor whose primary role is in postnatal angiogenic remodeling and also demonstrate that members of the VEGF and Angiopoietin families collaborate during development of the lymphatic vasculature.

Tight junctions of the blood-brain barrier: development, composition and regulation

1. The blood-brain barrier is essential for the maintenance and regulation of the neural microenvironment. The main characteristic features of blood-brain barrier endothelial cells are an extremely low rate of transcytotic vesicles and a restrictive paracellular diffusion barrier. 2. Endothelial blood-brain barrier tight junctions differ from epithelial tight junctions, not only by distinct morphological and molecular properties, but also by the fact that endothelial tight junctions are more sensitive to microenvironmental than epithelial factors. 3. Many ubiquitous molecular tight junction components have been identified and characterized including claudins, occludin, ZO-1, ZO-2, ZO-3, cingulin and 7H6. Signaling pathways involved in tight junction regulation include G-proteins, serine-, threonine- and tyrosine-kinases, extra and intracellular calcium levels, cAMP levels, proteases and cytokines. Common to most of these pathways is the modulation of cytoskeletal elements and the connection of tight junction transmembrane molecules to the cytoskeleton. Additionally, crosstalk between components of the tight junction- and the cadherin-catenin system of the adherens junction suggests a close functional interdependence of the two cell-cell contact systems. 4. Important new molecular aspects of tight junction regulation were recently elucidated. This review provides an integration of these new results.

Microvessel morphology and vascular endothelial growth factor expression in human colonic carcinoma with or without metastasis

We quantified microvessel morphology and vascular endothelial growth factor (VEGF) expression in human colonic carcinoma with or without metastasis. The cancerous growth and the noncancerous section of surgical specimens from 36 patients with colorectal carcinoma (14 without metastasis and 22 with metastasis) were studied. Tissue slices immunostained with CD34 were processed for microvessel counts (per mm(2)), the mean diameter of microvessels (microm), and the mean spatial direction of microvessels (degree), defined by the angle between the longitudinal axis of microvessels and the direction perpendicular to the surface of the mucosa. Tissue slices immunostained with anti-VEGF antibody were processed for total epithelial cell counts (per mm(2)), VEGF-positive cell counts (per mm(2)), and VEGF-positive ratio (%). Carcinoma without metastasis had significantly larger microvessel counts (213 +/- 77, p < 0.01), larger microvessel diameter (7.99 +/- 1.77, p < 0.05), and larger spatial direction (47.2 +/- 8.3, p < 0.01) than normal tissue (144 +/- 49 for microvessel counts; 7.03 +/- 0.90 for microvessel diameter; 39.5 +/- 6.6 for spatial direction). Compared with carcinoma without metastasis, carcinoma with metastasis had a significantly larger microvessel diameter (9.75 +/- 2.65, p < 0.03) and lower microvessel counts (180 +/- 92, p = 0.51). Carcinoma without metastasis had a significantly larger VEGF-positive cell count (1276 +/- 805, p < 0.05) and larger VEGF-positive ratio (53.6 +/- 39.3, p < 0.05) than normal tissue (571 +/- 553 for VEGF-positive cell counts; 24.6 +/- 23.2 for VEGF-positive ratio). Carcinoma with metastasis had a significantly lower total cell count (1443 +/- 237, p < 0.001) and lower VEGF-positive cell count (716 +/- 463, p < 0.05) than carcinoma without metastasis. With tumor progression, microvessel diameter significantly increased and microvessel counts decreased, which can be in part explained by VEGF expression. The microvessel diameter seems to be the dominant parameter responsible for cancer cell intravasation as the first step of metastasis.

Mechanisms of normal and tumor-derived angiogenesis

Often those diseases most evasive to therapeutic intervention usurp the human body's own cellular machinery or deregulate normal physiological processes for propagation. Tumor-induced angiogenesis is a pathological condition that results from aberrant deployment of normal angiogenesis, an essential process in which the vascular tree is remodeled by the growth of new capillaries from preexisting vessels. Normal angiogenesis ensures that developing or healing tissues receive an adequate supply of nutrients. Within the confines of a tumor, the availability of nutrients is limited by competition among actively proliferating cells, and diffusion of metabolites is impeded by high interstitial pressure (Jain RK. Cancer Res 47: 3039-3051, 1987). As a result, tumor cells induce the formation of a new blood supply from the preexisting vasculature, and this affords tumor cells the ability to survive and propagate in a hostile environment. Because both normal and tumor-induced neovascularization fulfill the essential role of satisfying the metabolic demands of a tissue, the mechanisms by which cancer cells stimulate pathological neovascularization mimic those utilized by normal cells to foster physiological angiogenesis. This review investigates mechanisms of tumor-induced angiogenesis. The strategies used by cancer cells to develop their own blood supply are discussed in relation to those employed by normal cells during physiological angiogenesis. With an understanding of blood vessel growth in both normal and abnormal settings, we are better suited to design effective therapeutics for cancer.

Microtumor growth initiates angiogenic sprouting with simultaneous expression of VEGF, VEGF receptor-2, and angiopoietin-2

Tumors have been thought to initiate as avascular aggregates of malignant cells that only later induce vascularization. Recently, this classic concept of tumor angiogenesis has been challenged by the suggestion that tumor cells grow by co-opting preexisting host vessels and thus initiate as well-vascularized tumors without triggering angiogenesis. To discriminate between these two mechanisms, we have used intravital epifluorescence microscopy and multi-photon laser scanning confocal microscopy to visualize C6 microglioma vascularization and tumor cell behavior. To address the mechanisms underlying tumor initiation, we assessed the expression of VEGF, VEGF receptor-2 (VEGFR-2), and angiopoietin-2 (Ang-2), as well as endothelial cell proliferation. We show that multicellular aggregates (<< 1 mm(3)) initiate vascular growth by angiogenic sprouting via the simultaneous expression of VEGFR-2 and Ang-2 by host and tumor endothelium. Host blood vessels are not co-opted by tumor cells but rather are used as trails for tumor cell invasion of the host tissue. Our data further suggest that the established microvasculature of growing tumors is characterized by a continuous vascular remodeling, putatively mediated by the expression of VEGF and Ang-2. The results of this study suggest a new concept of vascular tumor initiation that may have important implications for the clinical application of antiangiogenic strategies.

Microtumor growth initiates angiogenic sprouting with simultaneous expression of VEGF, VEGF receptor-2, and angiopoietin-2

Tumors have been thought to initiate as avascular aggregates of malignant cells that only later induce vascularization. Recently, this classic concept of tumor angiogenesis has been challenged by the suggestion that tumor cells grow by co-opting preexisting host vessels and thus initiate as well-vascularized tumors without triggering angiogenesis. To discriminate between these two mechanisms, we have used intravital epifluorescence microscopy and multi-photon laser scanning confocal microscopy to visualize C6 microglioma vascularization and tumor cell behavior. To address the mechanisms underlying tumor initiation, we assessed the expression of VEGF, VEGF receptor-2 (VEGFR-2), and angiopoietin-2 (Ang-2), as well as endothelial cell proliferation. We show that multicellular aggregates (<< 1 mm(3)) initiate vascular growth by angiogenic sprouting via the simultaneous expression of VEGFR-2 and Ang-2 by host and tumor endothelium. Host blood vessels are not co-opted by tumor cells but rather are used as trails for tumor cell invasion of the host tissue. Our data further suggest that the established microvasculature of growing tumors is characterized by a continuous vascular remodeling, putatively mediated by the expression of VEGF and Ang-2. The results of this study suggest a new concept of vascular tumor initiation that may have important implications for the clinical application of antiangiogenic strategies.

Microtumor growth initiates angiogenic sprouting with simultaneous expression of VEGF, VEGF receptor-2, and angiopoietin-2

Tumors have been thought to initiate as avascular aggregates of malignant cells that only later induce vascularization. Recently, this classic concept of tumor angiogenesis has been challenged by the suggestion that tumor cells grow by co-opting preexisting host vessels and thus initiate as well-vascularized tumors without triggering angiogenesis. To discriminate between these two mechanisms, we have used intravital epifluorescence microscopy and multi-photon laser scanning confocal microscopy to visualize C6 microglioma vascularization and tumor cell behavior. To address the mechanisms underlying tumor initiation, we assessed the expression of VEGF, VEGF receptor-2 (VEGFR-2), and angiopoietin-2 (Ang-2), as well as endothelial cell proliferation. We show that multicellular aggregates (<< 1 mm(3)) initiate vascular growth by angiogenic sprouting via the simultaneous expression of VEGFR-2 and Ang-2 by host and tumor endothelium. Host blood vessels are not co-opted by tumor cells but rather are used as trails for tumor cell invasion of the host tissue. Our data further suggest that the established microvasculature of growing tumors is characterized by a continuous vascular remodeling, putatively mediated by the expression of VEGF and Ang-2. The results of this study suggest a new concept of vascular tumor initiation that may have important implications for the clinical application of antiangiogenic strategies.

Stabilization of breast cancer xenograft tumour neovasculature by angiopoietin-1

Angiopoietin-1 is a promoter of physiological vasculogenesis and angiogenesis because it induces vascular branching and smooth muscle recruitment to newly formed blood vessels. However, angiopoietin-1 expression in tumours appears to be uncommon, and angiopoietin-1 overexpression in cancer cells has been reported to lead to inhibition of xenograft tumour growth. We report here that angiopoietin-1 overexpression resulted in stabilization of tumour edge-associated blood vessels, as it prevented vessel dilation and dissociation of smooth muscle cells from existing vessels. In addition, angiopoietin-1 stimulated an infiltration of mesenchymal cells into the tumours, such that the coverage of microvessels by pericytes increased markedly, and the cancer cells were separated into small masses by the host stroma. The rates of both cancer cell proliferation and apoptosis decreased significantly in the presence of angiopoietin-1. Tie2, the receptor for angiopoietin-1, was found to be present in vascular smooth muscle cells in culture in addition to endothelial cells. These findings suggest that a vascular stabilization effect of angiopoietin-1 accounts for the inhibition of tumour growth.

Androgens modulate the balance between VEGF and angiopoietin expression in prostate epithelial and smooth muscle cells

BACKGROUND

The vasculature of the prostate responds to androgens. Androgens most likely affect the vasculature indirectly by modulating the expression of angiogenic factors in the cells of the prostate. Most studies to date have examined the production of angiogenic factors by the prostate luminal epithelium. Here we examine the effects of androgen on production of three angiogenic factors, vascular endothelial growth factor (VEGF), angiopoietin-1, and angiopoietin-2, by the three major cell types in the prostate.

METHODS

The ability of androgen to modulate VEGF, angiopoietin-1, and angiopoietin-2 production in cultured mouse prostate luminal epithelial, basal epithelial, and smooth muscle cells (SMCs) was assessed by Western blot and RT-PCR.

RESULTS

The production of VEGF was modulated by androgens in both luminal epithelial and prostate SMCs but not in basal epithelial cells. However, in prostate luminal epithelial cell cultures, VEGF was predominately secreted apically, suggesting that in vivo most of the epithelium-derived VEGF is unavailable to the underlying blood vessels. In addition, prostate luminal epithelial cells produced angiopoietin-2, an angiogenesis inhibitor. In contrast, prostate SMCs produced angiopoietin-1, a positive modulator of angiogenesis. Synthesis of the angiopoietins did not respond to androgen treatment.

CONCLUSIONS

Prostate smooth muscle may play an important role in regulating vascular responses to androgen.

Analysis of mural cell recruitment to tumor vessels

BACKGROUND

Tumor blood vessels are both structurally and functionally abnormal compared with normal vessels. A limited support of mural cells may contribute to these abnormalities. Here, we characterized mural cell recruitment in 2 mouse tumor models and addressed the question of why tumor vessels fail to recruit a proper coat of mural cells.

METHODS AND RESULTS

We studied mural cell recruitment to the vasculature of 2 transplantable mouse tumor models, T241 fibrosarcoma and KRIB osteosarcoma. We found that both tumors formed a vessel network with heterogeneous and highly abnormal organization of mural cells. Transplantation of tumors to mice expressing lacZ in mural cells demonstrated that these cells were host-derived. Although tumor vessel endothelium expressed PDGF-B, an embryonic mitogen for mural cells, only very few PDGFRbeta-positive cells were found to be associated with the developing tumor vasculature, suggesting a limited pool of recruitable mural cells. We tested whether exogenous mural cells could be recruited to tumor vessels by injecting mixtures of T241 tumor cells and embryonic mesenchymal cells isolated from mice expressing lacZ in mural cells. In the tumors that arose, lacZ-positive cells were efficiently recruited to the tumor vessels.

CONCLUSIONS

T241 and KRIB tumors show a similar highly abnormal organization of vessel-associated mural cells. T241 tumor vessels seem highly capable of recruiting exogenously added mural cells. The sparse mural cell coat of tumor vessels may result from a limited pool of mural cells available for recruitment.

Genetic and biologic progression in astrocytomas and their relation to angiogenic dysregulation

Infiltrative astrocytic neoplasms are the most common malignancies of the central nervous system. They remain clinically problematic because of their involvement of brain structures critical to proper cognitive, behavioral, and motor function; their widely invasive properties, which make them difficult to resect totally; and their nearly inevitable biologic progression in spite of adjuvant therapy. Glioblastoma multiforme (GBM, World Health Organization grade IV), the most malignant form of infiltrating astrocytoma, can present as a high-grade lesion from the outset (so-called de novo GBM) or can evolve from a lower grade precursor lesion (secondary GBM). Molecular genetic investigations suggest that GBM is best regarded as a clinicopathologic entity composed of multiple molecular genetic subsets. Molecular alterations associated with progression to GBM and that define genetic subsets include epidermal growth factor receptor amplifications, p53 mutations, retinoblastoma pathway alterations [most commonly, p16(CDKN2A) losses], and chromosome 10 alterations, including PTEN mutations. Despite the wide range of genetic events that ultimately lead to GBM, the vascular changes that evolve are remarkably similar. Microvascular hyperplasia is spatially and temporally associated with pseudopalisading necrosis in GBM and is believed to be driven by hypoxia-induced expression of proangiogenic cytokines such vascular endothelial growth factor. In addition, genetic alterations in GBM are thought to contribute directly or indirectly to angiogenic dysregulation. Both p53 mutations and genetic losses on chromosome 10 may tip the balance toward an angiogenic phenotype through upregulation of proangiogenic factors and/or downregulation of angiogenesis inhibitors. Understanding genetic events and their relation to angiogenic regulation in astrocytic neoplasms may eventually lead to therapies that are specifically directed at molecularly defined subsets of these diseases.

Angiopoietin-1, angiopoietin-2 and Tie-2 in tumour and non-tumour tissues during growth of experimental melanoma

Tumour progression is dependent on the formation of new vessels in tumour tissue. Tumour cells produce a variety of factors that influence vessel growth and maintenance both in tumour and tumour-adjacent tissues. Angiopoietin-1 (Ang-1), angiopoietin-2 (Ang-2) and their tyrosine kinase receptor Tie-2 have been shown to play an important role in the processes of growth and remodelling of normal as well as tumour vessels. We studied gene expression of the angiogenic factors Ang-1 and Ang-2 and of their tyrosine kinase receptor Tie-2 in the tumour and non-tumour tissues of mice bearing the experimental melanoma B16. Using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR we measured Ang-1, Ang-2 and Tie-2 mRNA levels in the tumour, bone marrow, liver and spleen. Melanoma tissue overexpressed Ang-2 mRNA compared with spleen, liver and bone marrow of normal mice, suggesting its role during melanoma progression. On the other hand, there was a significant decrease in Ang-2 mRNA level in bone marrow cells collected on days 5 and 10 of tumour growth compared with the expression of Ang-2 mRNA in the bone marrow of normal mice and those collected on days 15 and 20 of tumour growth. These data demonstrate, for the first time, an ectopic effect of the tumour on the gene coding for an angiogenic factor, and also suggest that tumour growth may influence angiogenesis and/or vasculogenesis in distant organs.

Angiogenic profile of childhood primitive neuroectodermal brain tumours/medulloblastomas

Primitive neuroectodermal brain tumours (PNET) including medulloblastomas (PNET/MB) are the most common malignant brain tumours of childhood. Similar to many other brain tumours, PNET/MB often show marked neovascularisation. To determine which angiogenic factors contribute to PNET/MB angiogenesis, we examined the expression of eight angiogenic factors (vascular endothelial growth factors (VEGF, VEGF-B, VEGF-C), basic fibroblast growth factor (bFGF), angiopoetins (Ang-1, Ang-2), transforming growth factor (TGF-alpha), and platelet-derived endothelial growth factor (PDGF-A)) by semi-quantitative reverse transcriptase polymerase chain reaction (RT-PCR) in six PNET cell lines and 28 primary PNET/MB. Expression levels of angiogenic factors were compared with microvessel density, TrkC mRNA expression, clinical variables and survival outcomes. Our results indicate that all PNET/MB tested produce a wide range of angiogenic factors that are, individually or together, likely to play a direct role in PNET/MB tumour growth. This suggests that anti-angiogenesis approaches targeting VEGF alone may be insufficient in PNET/MB.

Differential expression of angiopoietin-1 and angiopoietin-2 in colon carcinoma. A possible mechanism for the initiation of angiogenesis

BACKGROUND

Angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2) are important regulators of endothelial cell (EC) survival. Current models suggest that an increase in Ang-2 expression in ECs leads to the initiation of angiogenesis. The authors hypothesized that the imbalance of Ang-1 and Ang-2 activities in colon carcinoma leads to a net gain in Ang-2 function.

METHODS

Reverse transcriptase-polymerase chain reaction (RT-PCR) analyses and immunofluorescent double-staining were performed to examine human colon carcinoma cell lines, surgical specimens, normal mucosa, and liver metastases for the expression of Ang-1 and Ang-2.

RESULTS

RT-PCR analyses revealed that 7 of 18 colon carcinoma cell lines expressed Ang-1, and 14 of 18 colon carcinoma cell lines expressed Ang-2 (P < 0.05). Of the surgical specimens from patients with colon carcinoma, 6 of 11 specimens expressed Ang-1, and 11 of 11 specimens expressed Ang-2 (P < 0.05). However, Ang-1 and Ang-2 were expressed with relative equal frequency in normal mucosa (P = 0.62). Immunofluorescent staining (n = 20 specimens) revealed the presence of Ang-2 protein in normal mucosa and tumor epithelium, but Ang-1 was expressed only in normal mucosa. A similar pattern was found for hepatic colorectal metastases. Double staining for Ang-1 or Ang-2 and cytokeratin-22 (an epithelial marker) demonstrated that Ang-1 was produced by uninvolved, normal colonic epithelium, whereas Ang-2 was produced by normal and malignant colonic epithelium.

CONCLUSIONS

In patients with colon carcinoma, Ang-2 is expressed ubiquitously in tumor epithelium, whereas expression of Ang-1 in tumor epithelium is rare. The net gain of Ang-2 activity is possibly an initiating factor for tumor angiogenesis.

HER2 signaling--induced microvessel dismantling

BACKGROUND

The human epidermal growth factor receptor 2 protein (HER2) signaling in breast cancer imparts a metastatic advantage to the cell, likely by regulating gene expression. The HER2 signaling up-regulates angiopoietin-2 (Ang-2), which disrupts endothelial cell (EC) adherens junctions. We postulated that HER2 signaling may facilitate angioinvasion by disrupting microvessel integrity.

METHODS

Rat microvessels, embedded in collagen, were grown into capillary networks and cocultured with MCF-7 or HER2 overexpressing MCF-7 (HER) to test for microvessel breakdown. We quantitated this effect by determining the cumulative length of intact microvessels. Other experiments used Herceptin- or heregulin beta 1-pretreated MCF-7 cells to modulate HER2 signaling, or soluble Tie-2/Fc receptor fusion protein (sTie2) to sequester tumor-cell released Ang-2.

RESULTS

The MCF-7 cells induced a time-dependent loss of microvessel integrity. At 12 hours, HER cells induced a 90% reduction in cumulative length (P <.05). Pretreatment with Herceptin reduced whereas heregulin beta 1 augmented microvessel dismantling (P <.01). Sequestration of Ang-2 significantly, though not dramatically, reduced the MCF-7 cell induction of microvessel dismantling (P <.01).

CONCLUSIONS

We show that HER2 signaling in breast cancer cells leads to induction of microvessel dismantling, which may open a portal for angioinvasion. It appears that Ang-2 affects this mechanism, although other factors also function in microvessel dismantling.

Evidence of increased endothelial cell turnover in brain arteriovenous malformations

OBJECTIVE

We hypothesized that human brain arteriovenous malformations (BAVMs) are nonstatic vascular lesions with active angiogenesis or vascular remodeling. To test this hypothesis, we assessed endothelial cell turnover in BAVMs.

METHODS

We identified nonresting endothelial cells by use of immunohistochemistry for the Ki-67 antigen. From archived paraffin blocks, we selected BAVM vessels without intravascular thrombosis or embolic material in areas nonadjacent to the nidus edge. For controls, we used 50- to 100-microm diameter cortical vessels from temporal lobe cortex removed for epilepsy treatment. The Ki-67 index was calculated as a percentage of Ki-67-positive endothelial cells. The data were analyzed by the nonparametric Mann-Whitney test and reported as mean +/- standard deviation.

RESULTS

Thirty-seven specimens that met the above criteria were selected. There were 26 +/- 15 vessels counted in each BAVM specimen versus 18 +/- 5 in each control cortex (n = 5). The mean Ki-67 index was higher for BAVM vessels than control cortical vessels (0.7 +/- 0.6 versus 0.1 +/- 0.2%; P = 0.005), which represented an approximately seven-fold increase in the number of nonresting endothelial cells. In the BAVM group, there was a trend for younger patients to have a wider variation and higher Ki-67 index than older patients; no trend was evident in the control group.

CONCLUSION

Compared with control vessels, BAVM vessels have higher endothelial cell turnover, which suggests the presence of active angiogenesis or vascular remodeling in BAVMs.

Altered Expression of Angiopoietins and Tie2 Endothelium Receptor in Psoriasis

Psoriasis is a chronic inflammatory skin disease in which epidermal proliferation is closely associated with excessive microvascular expansion within the papillary dermis. Angiopoietins have recently been identified as the major ligands of the endothelial- specific receptor Tie2. Angiopoietin 1 induces Tie2 signaling as a receptor activator and maintains blood vessel formation, whereas angiopoietin 2 destabilizes vessels by blocking Tie2 signaling as an antagonist of angiopoietin 1 and acts with vascular endothelial growth factor to initiate angiogenesis. In this study we examined the potential role of angiopoietins and the Tie2 receptor in vascular changes of psoriasis. Angiopoietin 1, angiopoietin 2, and Tie2 were upregulated in involved psoriasis skin compared to uninvolved psoriasis skin, healthy skin, and chronic spongiotic dermatitis skin. Angiopoietin 1 was expressed by stromal cells in the highly vascularized papillary dermis of involved psoriasis skin. Angiopoietin 2 was expressed by endothelial cells in the vicinity of the proliferating epidermis that abundantly expressed vascular endothelial growth factor. Vascular endothelial growth factor and basic fibroblast growth factor, which were overexpressed in involved psoriasis skin, enhanced angiopoietin 2 and Tie2 expression in dermal microvascular endothelial cell cultures. Thus, our findings suggest that upregulation of angiopoietin 1, angiopoietin 2, and Tie2 is closely associated with the development of microvascular proliferation in psoriasis, and that the angiopoietin-Tie2 system may act coordinately with vascular endothelial growth factor and basic fibroblast growth factor to promote neovascularization in psoriasis. Moreover, successful antipsoriatic treatment was accompanied by noticeable reduction of angiopoietin 2 expression, suggesting that alteration of angiopoietin 2 expression may be particularly important in controlling vascular proliferation in the treatment of psoriasis.

Tie receptors: new modulators of angiogenic and lymphangiogenic responses

Angiogenesis is required for normal embryonic vascular development and aberrant angiogenesis contributes to several diseases, including cancer, diabetes and tissue ischaemia. What are the molecular mechanisms that regulate this important process? The Tie family of receptors and their ligands, the angiopoietins, are beginning to provide insight into how vessels make decisions such as whether to grow or regress--processes that are important not only during development but throughout an organism's life.

Expression of angiopoietin-1 in human glioblastomas regulates tumor-induced angiogenesis: in vivo and in vitro studies

To define a role for the angiopoietin/Tie2 system in astrocytoma angiogenesis, we examined the expression of angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) in these tumors by immunohistochemistry and in situ hybridization. Furthermore, we studied in vitro the effects elicited by glioblastoma cell-secreted Ang1 or by recombinant Ang1 on functions of endothelial cells (ECs). Our observations of astrocytomas show that a stage-specific induction of angiopoietins occurs and is correlated with angiogenic phases of different intensity. Ang1 expression was found in a few astrocytes scattered in the tumor at all stages of astrocytoma progression. In blood vessels, Ang1 mRNA increased progressively in high-grade glioblastomas, in which the number of vessels was higher than in low-grade tumors. Ang2 was detected in tumor cells and in ECs in high-grade astrocytomas, whereas its expression was negligible in low-grade tumors. Coculture of glioblastoma cell lines producing Ang1 with endothelium demonstrated a key role of this ligand in the control of EC network organization. We found that recombinant Ang1 in vitro induces EC spreading and reorganization of the cell monolayer into cordlike structures. These results suggest that Ang1 directly acts on ECs by modulating cell-cell and cell-matrix associations and promoting the differentiation phase of angiogenesis.

Expression and function of angiopoietin-1 in breast cancer

Angiopoietin-1 (Ang1) has been shown to act as an angiogenic promoter in embryonic angiogenesis by promoting vascular branching, pericyte recruitment and endothelial survival. We have investigated the role of Ang1 in tumour neovascularization under clinical conditions and in animal models. The expression of Ang1 in clinical breast cancer specimens was analysed by using laser-capture microdissection and reverse transcriptase-linked polymerase chain reaction (RT-PCR) on RNA isolated from the samples. Despite the expression of Ang1 in many human breast cancer cell lines, the gene was expressed in only three of 21 breast cancer clinical specimens, even though its receptor, Tie2, is abundant in the vasculature of all of these tumours. Ang1 was then overexpressed in a human breast cancer cell line (MCF-7) on its own and in conjunction with FGF1, an angiogenic factor shown to be able to increase the tumorigenicity of MCF-7 cells. High concentrations of Ang1 were produced in the conditioned media of the transfected cells (range 156-820 ng ml(-1)). However, in contrast to its physiological role as promoter of angiogenesis, overexpression of Ang1 did not enhance tumour growth, but instead caused up to a 3-fold retardation of tumour growth (P = 0.003).

Vascular-specific growth factors and blood vessel formation

A recent explosion in newly discovered vascular growth factors has coincided with exploitation of powerful new genetic approaches for studying vascular development. An emerging rule is that all of these factors must be used in perfect harmony to form functional vessels. These new findings also demand re-evaluation of therapeutic efforts aimed at regulating blood vessel growth in ischaemia, cancer and other pathological settings.

Vascular apoptosis and involution in gliomas precede neovascularization: a novel concept for glioma growth and angiogenesis

Vascular changes in gliomas were analyzed by implanting fluorescent-labeled glioma 261 cells in the brains of 28 mice. Seven animals were killed each week for 4 weeks. We investigated the expression of angiopoietin-2 (Ang-2) by in situ hybridization and compared it with the distribution of apoptotic cells identified by DNA strand breaks (using the terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick end labeling [TUNEL] method) and transmission electron microscopy (TEM). As early as 1 week after implantation, tumor cells accumulated around vessels, which expressed Ang-2 and were TUNEL negative. TEM showed tumor cells adjacent to the vascular cells "lifting up" the normal astrocytic feet processes away from the endothelial cells and disrupting normal pericytic cuffing. After 2 weeks the number of perivascular glioma cells had increased. No increase in the number of blood vessels was detected at this time. Vascular cells remained positive for Ang-2 and rare ones were TUNEL positive. TEM showed closely packed proliferating perivascular tumor cells. After 3 weeks, there was vascular involution with scant zones of tumor necrosis. Ang-2 was still detected in vascular cells, but now numerous vascular cells were TUNEL positive. In addition, TEM showed apoptotic vascular cells. After 4 weeks, there were extensive areas of tumor necrosis with pseudopalisading and adjacent angiogenesis. Ang-2 was detected in vascular cells at the edge of the tumors in the invaded brain and in vessels surrounded by tumor cells. At both 3 and 4 weeks, most of the TUNEL-positive tumor cells lacked morphological features characteristic of apoptosis and displayed features consistent with necrotic cell death as determined by TEM. Only rare tumor cells appeared truly apoptotic. In contrast, the TUNEL-positive endothelial cells and pericytes were round and shrunken, with condensed nuclear chromatin by TEM, suggesting that vascular cells were undergoing an apoptotic cell death. These results suggest that vascular cell apoptosis and involution preceded tumor necrosis and that angiogenesis is a later event in tumor progression in experimental gliomas. Moreover, Ang-2 is detected prior to the onset of apoptosis in vascular cells and could be linked to vascular involution.