Recent Advances in Angiogenesis and Antiangiogenesis

Integrin the are major extracellular matrix receptors and their functional state with respect to the affinity for extracellular matrix proteins is pivotal for their biological activities in physiologic and pathological settings. Integrins’ machinery depends on the dynamic regulation of their adhesive function in space and time. In cells, integrins exist in different conformations which determine their affinity for extracellular matrix proteins and are continuously endocytosed, trafficked through endosomal compartments, and recycled back to the plasma membrane. Therefore real-time modulation of cell - extracellular matrix adhesion can result from two interconnected phenomena: the regulation of integrin conformation and traffic in response to extracellular stimuli. This review summarizes recent data highlighting the different mechanisms by which semaphorins and their receptors plexins and neuropilins regulate integrin functions in vascular system.

Osteopontin (OPN) is a phosphorylated acidic (Arg-Gly-Asp) RGD-containing glycoprotein, which exists both as an immobilized extracellular matrix component and as a soluble molecule. The biological functions of OPN are extensively regulated on the posttranscriptional and post-translational levels and many of the signaling pathways mediated by secreted OPN are activated by ligation of the integrin and CD44 families of receptors. Such a multifaceted glycoprotein, that is expressed by numerous different cells and tissues, is expected to exert pleiotropic functions. Indeed, OPN is implicated in tumor metastases, tissue remodeling, inflammation, and cell-mediated immunity. Recently, substantial evidence suggests that OPN positively regulates angiogenesis. However, the mechanisms that define the role of this molecule in angiogenesis are incompletely understood. The following review will discuss the biochemical and biological properties of OPN in the context of its role in the modulation of angiogenesis.

Mesenchymal stem cells (MSC) are a heterogeneous subset of stromal stem cells that can be isolated from many adult tissues. They can differentiate into cells of the mesodermal lineage, such as adipocytes, osteocytes and chondrocytes, providing a promising tool for tissue repair. MSC can interact with cells of both the innate and adaptive immune systems, leading to the modulation of several effector functions. The immunoregulatory functions of human MSC, coupled with their low immunogenicity, provide a rationale for the use of allogeneic MSC to treat severe graft-versus-host disease (GvHD) and, possibly, autoimmune disorders. In addition, MSC exhibit tropism for sites of tissue damage as well as for the tumor microenvironment, where they integrate into the tumor-associated stroma supporting cancer growth. However, studies investigating the in vivo and in vitro effects mediated by MSC on tumor growth provided conflicting results, depending on the experimental model tested. This chapter reviews the role of MSC in different angiogenic processes and underlying mechanisms. In particular, we discuss the involvement of MSC in angiogenesis in ischemic brain and heart after stroke, wound healing, tumor angiogenesis and maintenance of hematopoietic stem cell niche.

Angiogenesis refers to the formation of new blood vessels from pre-existing vascular structures, i.e. capillaries and post-capillary venules. This process occurs in different conditions, such as embryo development and post-natal tissue growth, inflammation like wound healing and chronic allergies, and cancer. Both structural cells and inflammatory cells in the different tissues are involved in the mechanisms of endothelial cell proliferation, migration and activation, through the production and release of a large spectrum of pro-angiogenic mediators. These may create the specific micro-environment that favours an increased rate of tissue vascularization. In this review, we will present the most recent findings on the contribution of inflammatory cells to the development and progression of inflammation-associated angiogenesis. We will also provide some insight of the complex signaling network, which links each inflammatory cell to the surrounding scenario.

Thymus plays a key role in the development of the immune system of the organism and vasculature seems to be involved in some steps of its development. Few data are available about vasculogenesis/angiogenesis in the thymus in normal and pathological conditions, despite the architecture of the vascular tree being relatively well known. There are some particular models of expression of endothelial cell (EC) markers in the thymus. In normal conditions, CD31 and CD105 are less expressed, opposite to CD34 and factor VIII that stain almost all vessels. The aspect is different in thymoma, where both CD31 and CD105 stain intratumoral blood vessels. Microvessel density significantly increases from normal thymus to thymic involution, myasthenia gravis and thymoma. In the tumor area, most of the vessels are immature or intermediate, and their number and type correlate with progression of thymoma. Proliferative ECs, defined by the co-expression of CD34 and Ki67, were found in high number mainly in high-grade advanced-stage thymoma. Progenitor ECs were not found in the prenatal and normal postnatal thymus, but isolated ECs that co-express AC133 and Tie2 were found in high grade thymoma. Besides ECs and perivascular cells, mast cells seem to be involved in thymus angiogenesis. A strong correlation was found between mast cell number and MVD. Few data are available about the expression of angiogenic factors and their receptors in the thymus. Vascular endothelial growth factor (VEGF) is expressed by epithelial cells of the normal thymus and overexpressed by tumor cells in thymoma B3 and thymic carcinoma. The expression of VEGF correlates with the presence of immature blood vessels and Masaoka clinical stage. VEGF receptors 1 and 2 are also expressed in normal and tumoral thymus. A divergent expression of VEGFR1 and 2 is found during thymoma progression. Other growth factors, like fibroblast growth factor and platelet derived growth factor and their receptors may contribute to angiogenesis in the thymus as they are expressed with different patterns in normal and pathological conditions. Lymphatic vessels seem to be better developed than previously thought, and they were also found in the thymic medulla. On the other hand, D2-40/podoplanin stains a subset of stromal epithelial cells and the corresponding thymoma. Thymus is not only a site for angiogenesis, but also a source for angiogenic factors. From these, it was shown that thymosins are involved not only in thymus but also in systemic angiogenesis.

Zebrafish (Danio rerio) represents a powerful model system in cancer research. Recent observations have shown the possibility to exploit zebrafish to investigate tumor angiogenesis, a pivotal step in cancer progression and target for anti-tumor therapies. Experimental models have been established in zebrafish adults, juveniles, and embryos, each one with its own advantages and disadvantages. Novel genetic tools and high resolution in vivo imaging techniques are also becoming available in zebrafish. It is anticipated that zebrafish will represent an important tool for chemical discovery and gene targeting in tumor angiogenesis. This review focuses on the recently developed tumor angiogenesis models in zebrafish, with particular emphasis to tumor engrafting in zebrafish embryos.

Immunohistochemistry, flow cytometry and cell culture procedures have demonstrated the presence of circulating endothelial cells (CECs) and circulating endothelial progenitors (CEPs) in the blood of vertebrates. CECs and CEPs are currently being investigated in a variety of diseases as markers of vascular turnover or damage and, also in the case of CEPs, vasculogenesis. CEPs appear to have a “catalytic” role in different steps of cancer progression and recurrence after therapy, and there are preclinical and clinical data suggesting that CEC enumeration might be useful to select and predict clinical response in patients who are candidates for anti-angiogenic treatments. In some types of cancer, CECs and CEPs might be one of the possible hidden identities of cancer stem cells. The definition of CEC and CEP phenotype and the standardization of CEC and CEP enumeration strategies are highly desirable goals in order to exploit these cells as reliable biomarkers in oncology clinical trials.

Thymidine phosphorylase/platelet-derived endothelial cell growth factor (TP/PDECGF) has diverse functions within cells, including the regulation of thymidine levels, the mediation of angiogenesis and apoptosis and the activation of prodrugs of the cancer chemotherapeutic agent 5-fluorouracil (5FU). The purpose of this review is to provide an overview of the pro-tumor effects of TP, including the molecular mechanisms of angiogenesis stimulation and apoptosis inhibition by TP.

Angiogenesis plays a pivotal role in progression of both solid and hematologic tumors. We have focused on multiple myeloma (MM) and its bone marrow stromal cells which are not only a support for tumor cell survival, but also active inducers of angiogenesis by releasing a broad number of angiogenic cytokines. Also, stromal cells such as macrophages and mast cells can participate in blood vessels formation in MM through other processes, such as a vasculogenic mimicry. Finally, it has been discovered that hematopoietic stem and progenitor cells (HSPCs) are involved in the vasculogenesis of MM.

Promising novel antiangiogenic strategies are emerging for the treatment of cancer and the inhibition of angiogenesis might represent a powerful tool as adjuvant therapy of malignant tumors. Over the last fifteen years several reports have been published concerning the relationship between tumor progression and angiogenesis in neuroblastoma in experimental models in vitro and in vivo. Moreover, a high vascular index in neuroblastoma correlates with poor prognosis, suggesting dependence of aggressive tumor growth on active angiogenesis. Here, we present an overview of the most recent advances in antiangiogenesis in neuroblastoma, and describe tumor vascular-targeted preclinical results, as well as future perspectives.

The formation of tumor supporting vessels can be accomplished by the sprouting of preexisting vessels, i.e. the proliferation of resident endothelial cells (angiogenesis) or by vasculogenesis, i.e. the de novo formation of vessels by circulating endothelial progenitor cells (EPC) presumably deriving from the bone marrow. Cytokines and chemokines released by tumors and inflamed tissue have been shown to recruit EPC and other progenitor cells from the circulation to home to sites of active vessel and tumor growth. Therefore, EPC-based therapies might be used to target specifically malignant tumors. Incorporated autologous cells thereby function as “Trojan horses” and deliver enzymes for activation of cytotoxic agents or release antiangiogenic proteins. However, the extent of EPC incorporation and the precise mechanisms by which EPC contribute to neovessels or migrate and invade tumor tissue are still under investigation. Furthermore, cells used for therapeutic purposes, regardless of their origin, have to be produced under Good Manufacturing Practice (GMP) conditions and should be at least homogenous and unequivocally characterized to minimize potential risks of malignant transformation in individuals after transplantation. Thus, this review will summarize the current knowledge on EPC, their ex-vivo propagation, genetic modification and homing to tumors in preclinical trials.

Tumor vascular disrupting agents (VDAs) are characterized by their ability to produce a very rapid and selective shut-down of tumor blood flow sufficient to induce extensive secondary tumor cell death. This effect is brought about by efficacy against established tumor blood vessels, making their mode of action conceptually distinct from that of the anti-angiogenic agents. Three main groups of VDAs are currently in clinical trial, consisting of DMXAA (5, 6-dimethylxanthenone-4-acetic acid), tubulin binding agents including the combretastatins and junctional protein inhibitors. These agents have different primary targets but produce similar morphological and functional effects on the tumor vasculature. The signaling pathways that mediate these effects are only partially understood but, in the case of disodium combretastatin A-4 3-0-phosphate (CA-4-P), undoubtedly involve activation of the small GTP-ase Rho and Rho kinase. Innate and induced resistance mechanisms need to be investigated in order to provide new targets for improving the efficacy of VDAs, especially in combination with conventional cancer treatments. Here, we review the developmental status of, and mechanisms of action and resistance to, currently available VDAs.

Angiogenesis-driven pathologies, including cancer, are sustained by a preponderance of angiogenic factors over endogenous inhibitors of new-vessel formation. Restoring this balance represents a logical therapeutic strategy to treat these pathologies. Therefore, endogenous inhibitors of angiogenesis, and in particular thrombospondin-1 (TSP-1), are a powerful source of potential antiangiogenic tools. Different therapeutic approaches have been proposed to exploit the antiangiogenic properties of TSP-1, including TSP-1 fragments, synthetic peptides and peptidomimetics, gene therapy strategies and agents that up-regulate TSP-1 expression. This review focuses on the possibility of exploiting TSP-1 for the design of antiangiogenic agents, with particular reference on their use in antineoplastic therapies.

The paradigm for the treatment of multiple myeloma (MM) has significantly changed: therapeutic options have evolved from the introduction of melphalan and prednisone in the 1960s, high-dose chemotherapy and stem cell transplantation in the late 1980s and 1990s, to the rapid introduction of small novel molecules within the last seven years. Based on the understanding of the complex interaction of MM cells with bone marrow microenvironment; and of the role of neoangiogenesis in MM pathogensis, a number of novel therapeutic agents with anti-angiogenic properties are now available, playing a key role in the treatment of MM both in the preclinical settings and as part of clinical trials.