The first identified form of angiogenesis is endothelial sprouting. It was originally described by Ausprunk and Folkmann in 1977. According to their hypothesis, in response to the NO mediated VEGF signal, the postcapillary venules become dilatated, cell-cell interactions get lost, the basement membrane (BM) degrades, and thus the vessel becomes fenestrated. Endothelial cells lose their polarity and migrate to the connective tissue. Then a tube is formed from the ECs, which is followed by lumen maintained by MMP2, which binds αvβ3 integrin. Growth factors are liberated from the
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BM, such as bFGF and VEGF, thus inducing migration of ECs with a maintained polarity, lumen and except from tip cells, a maintained BM. As the sprouts grow the BM on the tip is continuously synthesized, pericytes are recruited in response to bFGF and PDGF and the interaction between ECs and mural cells is stabilized by TGF-β1 and Ang1 (262). This hypothesis provides explanations missing from the former model of Ausprunk and Folkmann. These include the losening and then regaining ECs polarity, the fact that there is de- and redifferentiation in the same process and that the lumen is formed before the synthesis of the BM, although formation of BM is known to be the facilitator for lumen formation.
3.2.2.2 Vessel incorporation or co-option
In 1987 WD Thompson raised the possibility that tumors acquire their vasculature by vessel incorporation, instead of vessel ingrowth (263). This theory was proven by Josephine Holash in 1999 (264,265). Angiogenesis by vessel incorporation usually occurs,when tumors grow or metastatize into well vascularized tissues, such as lung, liver or skin. Tumor cells grow along the preexisting, well developed vessels of the host tissue, thus annexing its vasculature. The process is faster than sprouting angiogenesis, as it does not require EC proliferation. Moreover, on the periphery of the tumor these vessels provide surface for sprouting angiogenesis. Meanwhile, in response to the locally predominant antiangiogenic factors, ECs can undergo apoptosis in the centre.
This in one hand causes necrosis of the tumor mass, but on the other hand it also triggers the extravasation and metastatization of tumor cells. Maintenance of incorporated vessels is secured by the interaction of Ang1-TIE2.
3.2.2.3 Intussusceptive microvascular growth
Intussusceptive microvascular growth was first described by Sybill Patan in 1996 in a human colon adenocarcinoma model (266). This type of vessel growth is characterized by the incorporation of peritumoral capillaries, which are then separeted by connective tissue pillars. The original model of Caduff has been redrawn by Paku and his collegues in 2011. After the formation of an intraluminal endothelial bridge the BM is locally degraded, thus the EC can attach to a collagene bundle from the underlying collagene layer. The actin cytoskeleton of the EC exerts pulling force to the collagene bundle,
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which in turn is transported through the vessel lumen. Finally, connective tissue cells are immigrated to the pillars and new collagenous connective tissue is deposited (267), thus the process results in lots of vessels with big lumens, providing surface for EC sprouting. In the absence of EC proliferation, the process is faster than sprouting and does not involve permeability of the vessel wall. This suggests and some experimental data also supports, that intussusception does not rely on VEGF signalization, but instead is regulated by physiologic stimuli and cytokines, that lead to vessel maturation, such as bFGF (268), PDGF-BB (269), Ang1-TIE2 complex (270), ephrin-ephrin receptor (271).
However, the role of VEGF in the process is still contested (269,272,273).
Moreover, it is also shown, that in response to some antiangiogenic treatment, such as vatalanib or the mTOR inhibitor sirolimus, experimental tumors switch from endothelial sprouting to intussusceptive angiogenesis (274,275).
3.2.2.4 Glomeruloid angiogenesis
In 1992 Hauro Ohtani described coiled vascular structures in human gastrointestinal carcinoma. As they resemble renal glomeruli, they were called glomeruloid structures (276). These are characterized by many, tightly associated capillary loops with different thickness of BMs. The molecular mechanism behind the process is still not clear. On one hand, Sundberg et al. suggests that VEGF is a key mediator of inducing glomeruloid body formation and maintaining these vessels (277). On the other hand, it was also shown that the glomeruloid structure is created by proliferating and migrating tumor cells, which pull the capillaries and their branching points into the tumor cell nests. Thus, this type of vascular growth cannot be termed as true angiogenesis, but rather a reorganization of tumor blood vessels, which does not require EC proliferation (278). Moreover, these vessels seem to be able to provide enough oxygen and nutrients to the cells, as no necrosis was observed in tumors developing them. As a result, glomeruloid bodies are not only diagnostic markers of glioblastoma, but also poor prognostic factors in many cancer types (279).
3.2.2.5 Postnatal vasculogenesis
Postnatal vasculogenesis refers to the process, in which bone marrow derived VEGFR2+, TIE2+, AC133+, CD34+ endothelial progenitor cells (EPCs) incorporate
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into the EC layer of the tumor capillary network in response to tumor derived VEGF (280) and other proangiogenic factors. After incorporation into the EC layer, EPCs are differentiated to mature ECs. Thereafter, by producing pro-angiogenic factors, such as VEGF and PlGF, they mediate the attraction of additional EPCs to the tumor vasculature. VEGF mobilizes these cells from the bone marrow via the stromal derived factor (SDF) and its receptor, CXCR4. The process of postnatal vasculogenesis was described by Takayuki Asahara and his group both in physiologic and pathologic conditions at the end of the 1990s (281).
3.2.2.6 Vessel-like structures, formed by tumor cells
It has been shown, that not only ECs, but aggressive tumor cells can also form vessel-like structures, which facilitate tumor perfusion.
In 1941 Béla Kellner described tumor sinusoids in soft tissue sarcomas (282). These are lumens, which are covered exclusively by tumor cells, and are responsible for the transport of blood cells within the tissue.
It is also possible that tumor cells form a lumen together with ECs, without expressing endothelial or embrional markers. These structures are called mosaic vessels and were also first described in the 1940s (283). The genesis of these types of vessels is still not well understood. They are considered to be formed either by the apoptosis of incorporated ECs, which is followed by the occupacy of the lumen or invasion of the vessel by tumor cells. The process is thus thought to be mediated via Ang2 signalling.
Vasculogenic mimicry refers to the process, when tumor cells express differential markers to completely resemble BM covered ECs. The process was first described by the group of Mary J Hendrix in uveal melanoma, where highly agressive melanoma cells formed vessel-like channels and upregulated endothelial genes and genes involved in microvascular channel formation. Meanwhile, these cells downregulate classical melanoma markers. None of the main angiogenic cytokines, such as VEGF, PDGF, bFGF, TGF-β, Ang, Notch, TNF-α seem to induce the formation of these channels (284).
Different types of vascularization mechanisms in cancer are shown in Figure 11.
30 Sprouting angiogenesis
Glomeruloid agiogenesis
Vessel incorporation Intussusceptive angiogenesis
Postnatal vasculogenesis
Vasculogenic mimicry