M. Schwab (ed.), Encyclopedia of Cancer, Springer-Verlag Berlin Heidelberg, 2011
The generation of microvascular channels by genetically deregulated, aggressive tumor cells was termed “vasculogenic mimicry” (VM) to emphasize their de novo generation without participation by endothelial cells. VM is thought to represent a vascular channel formation without the involvement of endothelial cells, in contrast to angiogenesis. Vasculogenic mimicry refers to a blood supply pathway in tumors that is formed by tumor cells and that is independent of endothelial cell-lined blood vessels. Three factors are thought to govern the formation of functional and patterned microcirculation channels by VM: (1) plasticity of highly malignant tumor cells, (2) remodeling of the extracellular matrix (ECM), and (3) connection of the VM channel with host blood vessels to acquire blood supply from the host tissue. Formation of VM in tumors may have substantial impact on clinical outcome of tumor patients. Tumor patients in the presence of VM have a poorer prognosis than those without VM, and VM-targeted therapy is a perspective for tumors showing VM. At this point, VM is a new concept originally described for melanoma that needs to be studied further in detail.
Tumor angiogenesis is a key for tumor growth, invasions, and metastasis. Tumor growth is angiogenesis-dependent, and angiogenic switch is an essential step for a small and noninvasive tumor to transit into a tumor with invasive and metastatic ability. Blood vessels are assembled by two processes: (1) vasculogenesis, the reorganization of randomly distributed cells into a blood vessel network, and (2) angiogenesis, the sprouting of new vessels from preexisting vasculature in response to external chemical stimulation.
Current status of studies on VM in tumors
It is believed that VM consists of tumor cells and PAS-positive ECM on the inner wall of channels. The constituents of PAS-positive ECM are laminin, collagens IV and VI, mucopolysaccharide, and heparin sulfate glucoprotein (HSPG). Initially, PAS-positive ECM was considered as an absolutely indispensable element. However, VM channels in the absence of PAS-positive ECM were observed in a melanoma mouse model. VM is an independent blood supply pattern in tumors (Figs. 1–3). Compared with endotheliumdependent vessels, it has several characteristics as follows: (1) VM channels are lined by tumor cells but not endothelial cells. (2) There is red blood cell (RBC) leakage into tumor tissue near to endotheliumdependent vessels, while leaking RBCs are lacking in tumors with VM. (3) Necrosis and inflammatory cells are not observed in tumors undergoing VM.
Molecular mechanisms underlying VM
Compared with less aggressive melanoma cells, highly aggressive melanoma cells express higher levels of matrix metalloproteinases (MMP-1, 2, 9, and 14) and the 5g2 chain of laminin. This increases expression of MMPs and presence of the laminin receptor on the surface of tumor cells help cells adhere to more laminin. The activated MMPs cleave laminin into several short chains and eventually promote the formation of VM. Phosphoinositide-3-kinase modulates the function of MMP-14 (MT1-MMP), which activates MMP-2 with the help of tissue inhibitor of MMP-2 (TIMP2), and the activated MMP-2 then cleaves 5g2 chain into g20 and g2x chains. The two chains facilitate the formation of VM. The cleavage fragments of 5g2 can be secreted by highly malignant melanoma cells directly. VE-cadherin has been proved to be closely related to the formation of VM channels. Highly aggressive melanoma cells express VE-cadherin but less aggressive ones do not and inhibition of VM formation by downregulating the expression of the VE-cadherin gene.
Dedifferentiation of tumor cells is the key to formation of VM channels
Much information on VM has come from studies of highly malignant melanomas. Tumor cells having the ability of VM formation show an embryonic phenotype. A cDNA microarray study of 5,000 genes from a patient with poorly and highly aggressive melanoma cells revealed that there was a differential expression in 210 genes, including some genes associated with the phenotypes of endothelial and hematopoietic stem cells. Except for embryonic genotypes, cells of tumors with VM express various angiogenesis-related cytokines. Flt-1 and Tie-2 are expressed by tumor cells of naked mice bearing human inflammatory breast carcinoma cells. Ovarian cancer cells with high aggressivity express the vascular endothelial growth factor (VEGF) and other angiogenesis-related cytokines (e.g., Ang-1 and Ang-2), whereas those with low aggressivity express VEGF only. The expression of tyrosine kinase (an enzyme that catalyzes the phosphorylation of several signal transduction proteins) is upregulated in highly aggressive melanoma cells than in less aggressive melanoma cells. Aggressive melanoma cells show an increased activity of tyrosine kinase around VM channels. Epithelial cell kinase (EphA2), a tyrosine kinase receptor, is specifically expressed in highly aggressive melanoma cells. Inhibitors of tyrosine kinase activity hinder VM channel formation, and a transient knockout of EphA2 shows reduced VM channel formation.
Linearly patterned programmed cell necrosis and three-stage phenomenon
Linearly patterned programmed cell necroses (LPPCN) and three-stage phenomenon are thought to play essential roles in the blood supply for melanoma. At the early stage of tumor generation, endothelium-dependent vessels do not sprout into tumor center. Under the pressure of hypoxia, some tumor cells activate apoptosis-associated genes, and lacunas left by dissolving LPPCN cells connect with each other and form channel networks (Fig. 4). The channels coming from LPPCN cells face two opposite ends. If they connect with endotheliumdependent vessels, blood will flow into these channels lined by tumor cells and the channels will be a functional microcirculation. In contrast to this, the mass of tumor cells will undergo necrosis if these channels fail to link to endothelium-dependent vessels. Three microcirculation patterns – VM, mosaic vessels, and endothelium-dependent vessels – coexist in melanoma tissue. Angiogenesis requires the recruitment of normal endothelial cells, which may not be efficient and/or sufficient enough for sustaining aggressive tumor growth at the initial stage of rapid growth. Some tumor cells dedifferentiate, connect with other tumor cells or endothelium, and finally line the wall of tube. They are vasculogenic mimicry and mosaic vessels. Mosaic vessel may be a transition between VM channel and endothelium-dependent vessel. The three-stage phenomenon on tumor blood supply pattern assumes that there is a transformation among VM channels, mosaic vessels and endothelium-dependent blood vessels (Fig. 5). In the stage of rapid tumor growth, endothelium-dependent vessels sprout from normal tissue but are insufficient to support the rapid tumor growth. VM occurs on the base of LPPCN and acts as the major blood supply pattern for tumor growth. As tumor size becomes bigger, endothelial cells from peripheral blood home, proliferate on the wall of VM, and cover some tumor cells forming VM. Mosaic vessels appear and become the major microcirculation pattern in tumors. Finally, endothelium-dependent vessels take over the dominant role in tumor blood supply.
The effect of local tumor microenvironment on the formation of VM channels
Tumor growth and evolution are regulated by a good many factors and tumor cells display distinguished blood supply pattern and biological behavior to adapt different microenvironment. The environmental factors impacting VM channel formation include oxygen pressure, interstitial fluid pressure (IFP) in tumor tissue, pH, focal concentration of cytokines, and ECM. Hypoxia is a two-edge sword for tumor generation and development. Hypoxia and ischemia induce tumor cells to necrosis and tumor suppression, whereas hypoxia actives metastasis-related genes to promote tumor invasion. The hypoxic condition enhances the formation of VM channels, which exerts its function thoughHIF-1a and its downstream molecules. In the hypoxic environment, accumulated HIF-1a in tumor cells induces MMP-2, MMP-9, and VEGF expression and activation. MMP-2 and MMP-9 proteinases degrade the ECM components and facilitate VM formation, tumor invasion, and metastasis. VEGF secreted by tumor cells results in permeability of blood vessels, resulting in an increase and leakingout of many serum proteins. Such a milieu provides a temporary matrix for VM channel formation. Under the stimuli of hypoxia, LPPCN-associated genes can be triggered and some tumor cells undergo LPPCN. Interspaces left by dissolving cells connect with each other as networks to provide the space basement for VM. Interstitial fluid pressure (IFP) is another important factor affecting tumor microcirculation patterns. Increased IFP is a characteristic of malignant tumors because of its rapid proliferation. It is similar to hypoxia and has double impact on tumor development. High IFP inhibits both endothelial cells of blood vessels and lymphatic vessels to migrate into the tumor center, with the result of tumor hypoxia. Elevated IFP stimulates tumor cells to secret invasion-associated proteins. High IFP is a barrier for endotheliumdependent vessels sprouting into tumor tissue, but hypoxia induced by it is an inducer for VM. Moreover, the expression of MMP-2, MMP-9, integrin, selectin, and kinesin increase significantly in tumor cells growing in the microenvironment with high IFP, which promote VM formation to provide sufficient nutrition and oxygen for tumor growth.
Clinical significance of VM
VM has been observed in several human malignant tumor types, such as highly aggressive uveal melanomas, breast cancer, liver cancer, glioma, ovarian cancer, melanoma, Prostate Cancer, malignant astrocytoma, and bidirectional differentiated malignant tumors. Tumor cells lining on the inner surface of VM channels are directly exposed to blood flow, may move into the bloodstream and metastasize to other organs. VM is associated with poor prognosis in patients. Tumors with VM have a higher rate of metastasis compared with tumors without VM, and the patients have a lower 5-year survival rate. Routine antiangiogenic drugs, such as angiostatin and endostatin, which target endothelial cells, have not achieved a therapeutic effect on tumors that exhibit VM because of the absence of endothelium-dependent vessels (Fig. 6).
Advances and challenges
VM and lymphagenesis in tumors
VM, a new pattern of blood supply to the tumors, has attracted the attention of many researchers, but many phenomena unique to VM channel formation remain to be elucidated. As a functional tumor microcirculation, VM channels need to be studied with regard to their connection with endothelium-dependent vessels, their relationship with lymphatic tubes, and their dual function as vessels and lymphatic tubes. Uveal melanoma cells have a specific vortex vein but no lymphatic tube. Highly aggressive melanoma expresses lymphatic-vessel endothelial hyaluronan receptor1 (LYVE1) and VEGF-C, a lymphatic tube-related growth factor.
Given the important role of angiogenesis in tumor growth and metastasis, therapies aiming at endothelial cells represent promising antitumor strategies. As VM has a different structure from endothelium-dependent vessels, traditional antiangiogenic agents targeting at endothelial cells, such as anginex, TNP-470, and endostatin, have no remarkable effects on malignant tumors with VM. One of the distinguished features of tumors with VM is that cell adhesion molecules, tumor invasion-related proteinases and ECM synthesis and secretion-associated proteins are overexpressed by tumor cells. These molecules represent potential targets for anti-VM strategies of highly aggressive and blood metastatic tumors with VM. Suppressing tyrosine kinase activity, using a knockout EphA2 gene, downregulating VE-cadherin, using antibodies against human MMPs and the laminin 5g2 chain, and using anti-PI3K therapy are strategies that have been employed to inhibit VM.
Fig. 1. Vasculogenic mimicry (VM). Melanoma cells form VM channels, and red blood cells (RBC) flow into the channel. Necrosis and inflammatory cells are not observed in tumors undergoing VM
Fig. 2. The connection of VM channel and endothelium-dependent vessel shows VM is a functional microcirculation
Fig. 3. VM consists of tumor cells and PAS-positive ECM on the inner wall of channels. The PAS-positive patterns are lined by tumor cells, and there are red blood cells in the center of the pattern Fig. 4 LPPCN. Cells undergoing LPPCN have spindle-like figure and dark blue nuclei. They connect with each other as lines and some cells enter the wall of VM channels
Fig. 4. LPPCN. Cells undergoing aLPPCN have spindle-like figure and dark blue nuclei. They connect with each other as lines and some cells enter the wall of VM channels
Fig. 5. Three-stage phenomenon on tumor blood supply pattern. During rapid tumor growth, endothelium- dependent vessels sprouting from normal tissue cannot satisfy the need for growth.VMoccur on the base of LPPCN and acts as the major blood supply pattern for tumor growth. As endothelial cells from host microvessels migrate and endothelial progenitor cells from peripheral blood home into the wall of VM, endothelial cells cover some tumor cells forming VM. Mosaic vessels appear and become the major microcirculation pattern in tumors. Finally, endothelium-dependent vessels get the dominant role in tumor blood supply
Fig. 6. Comparison of overall survival time of patients with bidirectional differentiated malignant tumors. A Kaplan–Meier algorithm reveals that the survival time of melanoma, mesothelial sarcomas (MS), alveolar rhabdomyosarcomas (AS), and synovial sarcomas (SS) without VM are both significantly longer than that of patients with VM
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