Angiogenesis

The concept that tumor growth depends on angiogenesis was initially proposed in the early 1970s.  However, its significant meaning had not been taken seriously for a long time. In the recent decade, due to the discovery of the effect of angiogenic factors on angiogenesis as well as the important impact of angiogenesis on tumor growth, invasion and metastasis, especially the early tumorigenesis, angiogenesis has become one of the hot fields in cancer research and has brought up a new way of thinking for the tumor treatment.

Angiogenesis refers to the growth derived from existing capillaries and the new capillary blood vessel of the post-capillary venules. Tumor angiogenesis is an extremely complex process, generally including vascular endothelium matrix degradation, vascular endothelial cell migration, and endothelial cell proliferation, endothelial cell pipeline branch formation into vascular rings and the formation of new matrix membrane and some other steps. Because this kind of tumor angiogenesis is structurally and functionally abnormal, and also the vascular matrix is imperfect, this microvascular is prone to be subject to leakage, so the tumor cells do not need to go through a complex process of invasion and instead can directly penetrate into the blood vessels to enter the bloodstream and start metastasis in the distant site. More and more studies have shown that there are rare cases on angiogenesis in benign tumor and the vascular grow slowly in benign tumor; while most malignant tumor has intensive and rapid progression of angiogenesis. Therefore, angiogenesis plays an important role in tumor progression and metastasis. Inhibition of this process will significantly prevent the development and spread of metastasis of tumor tissue (Carmeliet P, 2005).

Studies have shown that the growth and maturation of new blood vessels is a very complex and coordinated process. The formation and development of blood vessels depends on the dynamic balance of angiogenesis promoting factors and inhibitors, requiring the interaction of cells to cells, cells to cell-matrix, requiring the activation of a series of receptor and is regulated by various kinds of pro-angiogenic growth factors and angiogenesis inhibitors. With the deepened understanding on the angiogenesis and function of solid tumor, the discovery of a large number of pro-angiogenic growth factor and vascular growth inhibition factor make people aware of that the growth of tumor blood vessels is also adjustable with the VEGF and angiopotietin (Ang) secreted by the tumor cells themselves being able to induce tumor angiogenesis. Currently it has been found of over 20 kinds of angiogenic growth factors including VEGF, bFGF, IGF-1, TNF-α, TNF-β, and platelet-derived growth factor (PDGF). Wherein the VEGF and its receptor (VEGFR) is the most important pro-tumor vascular endothelial growth factor found so far. It can promote the proliferation, migration and survival of tumor vascular endothelial cells, increasing the permeability of tumor newborn blood vessel, thus playing a key role in tumor angiogenesis with the pro-angiogenic effect of VEGF165 being the strongest. VEGF165 mainly binds to VEGFR-2 and play an important function. Multiple regulatory factors can through either direct or indirect pathways to affect VEFG and signalling transduction pathway of its receptor; neuroplin-1 can bind with VEGF165 as co-receptor, thus strengthening the binding capability of VEGF165 to VEGFR-2, further promoting the tumor angiogenesis. Hypoxia induces tumor cells to produce large amounts of hypoxia inducible factor -1 (HIF-1), increasing the expression of VEGF and its receptors, thus promoting the formation of tumor newborn blood vessels. For in vitro condition, bFGF, IGF-1, TNF-α, TNF-β and PDGF a can increase the expression of tumor-specific VEGF, indicating the regulatory function of these factors on angiogenesis is partially through up-regulating the expression of VEGF to achieve. bFGF can also affect the migration of endothelial cells and promote the formation of blood vessel lumen, stimulate the endothelial cells to secrete collagenase for degradation of the matrix membrane in order to facilitate the growth of new blood vessels (Ferrara N, 2005). After the discovery of VEGF, people have also discovered the second type of angiogenic factor family, namely Ang and its receptor. So far, it has been found of four kinds of members belong to this family, namely Ang1-4 with Ang-1 being the pro-angiogenic growth factor secreted by tumor cells. The Ang receptor, Tie is a kind of endothelial cell specific tyrosine kinase receptors, mainly distributed in endothelial and hematopoietic cell surface. Ang1 ~ 4 can specifically acts on the endothelial cells with a strong pro-angiogenic effect. Tie-1, Tie-2 and the Tie-2 ligand (Angl, Ang2) play an important role in embryonic blood vessel formation and angiogenesis. Activation of Tie-2/Ang-1 signaling pathway can induce the sprouting of capillary to form branches, recruiting the pericytes and smooth muscle cells, maintaining the stability of blood vessels while activating inositol phospholipid kinase (PI3) in the endothelial cells so that the generation of apoptosis inhibitor surviving will increased to fight against the apoptosis of endothelial cells. Ang and VEGF family protein complement and coordinate with each other in the process of angiogenesis. VEGF plays a key role in the early stages of angiogenesis while Ang family plays an important role in the latter stage of the vascular maturation and further stabilization (Ribatti D, 2009; Tan A, 2010; Le Bourhis X, 2010).

[Stages and characteristics of angiogenesis]
The newly formed capillary is comprised of endothelial cells and pericyte cells, these two types of cells have the ability to form complete capillary network. For in vivo condition, accompanied with the pro-differentiation signal transduction, angiogenesis can progress according to endothelial cell activation, proliferation, migration and tube formation process such as hypoxia and other effects of endogenous or exogenous factors that trigger the tumor cells to release cytokines.

Quiescent endothelial cells can be activated by host or tumor released cytokines (I period), specific cell proliferation (II period), and flow along the fiber network of the stimulus source of the angiogenesis, forming arranging cell cord (III period), the finally vessel bud can form tube-like structures; cells enter into the stationary phase after leaving the cell cycle. Through intercellular cross-link of the intercellular vesicles which play adhesion and contact effects, eventually forming a clear lumen (IV period).

The drop angle in the extracellular matrix is an important part of the infiltration of new blood vessels. It is mainly achieved through altering the balance between proteolytic enzymes. The proteolytic and fibrinolytic effects of extracellular matrix are the two functions of the pericyte cells. Pericyte cells are also thought to be related to the generation of the growth factors and inhibitory factors. Cell adhesion receptor, through its interaction with the extracellular matrix adhesion proteins such as collagen and fibronectin, moves to the vascular cells. The extracellular matrix can also act as a growth factor, particularly as the storage pool of aFGF and bFGF.
Because of the incompleteness of the matrix membrane of the proliferated capillary, the capillary inside the tumor blood exhibits as "leaking" property. Meanwhile, vascular endothelial growth factor, through the interaction of a series of cytoplasmic vesicles connected in luminal and luminal surface, is capable of increasing the vascular permeability. In normal tissues, it may play a role in regulating the basic permeability of microvascular while for the tumor microvasculature, this property may be related to the malignant exudate and the formation of ascites.

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