Signalling pathways in vasculogenic mimicry

Tumor angiogenesis and vascular normalization: alternative therapeutic targets

Tumor blood vessels are a key target for cancer therapeutic management. Tumor cells secrete high levels of pro-angiogenic factors which contribute to the creation of an abnormal vascular network characterized by disorganized, immature and permeable blood vessels, resulting in poorly perfused tumors. The hypoxic microenvironment created by impaired tumor perfusion can promote the selection of more invasive and aggressive tumor cells and can also impede the tumor-killing action of immune cells. Furthermore, abnormal tumor perfusion also reduces the diffusion of chemotherapeutic drugs and radiotherapy efficiency. To fight against this defective phenotype, the normalization of the tumor vasculature has emerged as a new therapeutic strategy. Vascular normalization, by restoring proper tumor perfusion and oxygenation, could limit tumor cell invasiveness and improve the effectiveness of anticancer treatments. In this review, we investigate the mechanisms involved in tumor angiogenesis and describe strategies used to achieve vascular normalization.

Notch3 signaling-mediated melanoma-endothelial crosstalk regulates melanoma stem-like cell homeostasis and niche morphogenesis

Melanoma is among the most virulent cancers, owing to its propensity to metastasize and its resistance to current therapies. The treatment failure is largely attributed to tumor heterogeneity, particularly subpopulations possessing stem cell-like properties, ie, melanoma stem-like cells (MSLCs). Evidence indicates that the MSLC phenotype is malleable and may be acquired by non-MSLCs through phenotypic switching upon appropriate stimuli, the so-called 'dynamic stemness'. Since the phenotypic characteristics and functional integrity of MSLCs depend on their vascular niche, using a two-dimensional (2D) melanoma-endothelium co-culture model, where the MSLC niche is recapitulated in vitro, we identified Notch3 signaling pathway as a micro-environmental cue governing MSLC phenotypic plasticity via pathway-specific gene expression arrays. Accordingly, lentiviral shRNA-mediated Notch3 knockdown (KD) in melanoma cell lines exhibiting high levels of endogenous Notch3 led to retarded/abolished tumorigenicity in vivo through both depleting MSLC fractions, evinced by MSLC marker downregulation (eg, CD133 and CD271); and impeding the MSLC niche, corroborated by the attenuated tumor angiogenesis as well as vasculogenic mimicry. In contrast, Notch3 KD affected neither tumor growth nor MSLC subsets in a melanoma cell line with relatively low endogenous Notch3 expression. Thus, Notch3 signaling may facilitate MSLC plasticity and niche morphogenesis in a cell context-dependent manner. Our findings illustrate Notch3 as a molecular switch driving melanoma heterogeneity, and provide the biological rationale for Notch inhibition as a promising therapeutic option.

Vasculogenic mimicry signaling revisited: focus on non-vascular VE-cadherin

Vasculogenic mimicry (VM) is a blood supply system independent of endothelial vessels in tumor cells from different origins. It reflects the plasticity of aggressive tumor cells that express vascular cell markers and line tumor vasculature. The presence of VM is associated with a high tumor grade, short survival, invasion and metastasis. Endothelial cells (ECs) express various members of the cadherin superfamily, in particular vascular endothelial (VE-) cadherin, which is the main adhesion receptor of endothelial adherent junctions. Aberrant extra-vascular expression of VE-cadherin has been observed in certain cancer types associated with VM. In this review we focus on non-endothelial VE-cadherin as a prominent factor involved in the acquisition of tubules-like structures by aggressive tumor cells and we summarize the specific signaling pathways, the association with trans-differentiation and stem-like phenotype and the therapeutic opportunities derived from the in-depth knowledge of the peculiarities of the biology of VE-cadherin and other key components of VM.

Disrupting Tumor Angiogenesis and "the Hunger Games" for Breast Cancer

Angiogenesis, one of the hallmarks of cancers, has become an attractive target for cancer therapy since decades ago. It is broadly thought that upregulation of angiogenesis is involved in tumor progression and metastasis. Though tumor vessels are tortuous, disorganized, and leaky, they deliver oxygen and nutrients for tumor development. Based on this knowledge, many kinds of drugs targeting angiogenesis pathways have been developed, such as bevacizumab. However, the clinical outcomes of anti-angiogenesis therapies are moderate in metastatic breast cancer as well as in metastatic colorectal cancer and non-small cell lung cancer, even combined with traditional chemotherapy. In this chapter, the morphologic angiogenesis patterns and the key molecular pathways regulating angiogenesis are elaborated. The FDA-approved anti-angiogenesis drugs and current challenges of anti-angiogenesis therapy are described. The strategies to overcome the barriers will also be elucidated.

Metastasis-associated in colon cancer-1 in gastric cancer: Beyond metastasis

Metastasis-associated in colon cancer-1 (MACC1) is an oncogene that was first identified in colon cancer. The upstream and downstream of MACC1 form a delicate regulatory network that supports its tumorigenic role in cancers. Multiple functions of MACC1 have been discovered in many cancers. In gastric cancer (GC), MACC1 has been shown to be involved in oncogenesis and tumor progression. MACC1 overexpression adversely affects the clinical outcomes of GC patients. Regarding the mechanism of action of MACC1 in GC, studies have shown that it promotes the epithelial-to-mesenchymal transition and accelerates cancer metastasis. MACC1 is involved in many hallmarks of GC in addition to metastasis. MACC1 promotes vasculogenic mimicry (VM) via TWIST1/2, and VM increases the tumor blood supply, which is necessary for tumor progression. MACC1 also facilitates GC lymphangiogenesis by upregulating extracellular secretion of VEGF-C/D, indicating that MACC1 may be an important player in GC lymphatic dissemination. Additionally, MACC1 supports GC growth under metabolic stress by enhancing the Warburg effect. In conclusion, MACC1 participates in multiple biological processes inside and outside of GC cells, making it an important mediator of the tumor microenvironment.

CD44 enhances tumor aggressiveness by promoting tumor cell plasticity

Aggressive tumor cells can obtain the ability to transdifferentiate into cells with endothelial features and thus form vasculogenic networks. This phenomenon, called vasculogenic mimicry (VM), is associated with increased tumor malignancy and poor clinical outcome. To identify novel key molecules implicated in the process of vasculogenic mimicry, microarray analysis was performed to compare gene expression profiles of aggressive (VM⁺) and non-aggressive (VM⁻) cells derived from Ewing sarcoma and breast carcinoma. We identified the CD44/c-Met signaling cascade as heavily relevant for vasculogenic mimicry. CD44 was at the center of this cascade, and highly overexpressed in aggressive tumors. Both CD44 standard isoform and its splice variant CD44v6 were linked to increased aggressiveness in VM. Since VM is most abundant in Ewing sarcoma tumors functional analyses were performed in EW7 cells. Overexpression of CD44 allowed enhanced adhesion to its extracellular matrix ligand hyaluronic acid. CD44 expression also facilitated the formation of vasculogenic structures in vitro, as CD44 knockdown experiments repressed migration and vascular network formation. From these results and the observation that CD44 expression is associated with vasculogenic structures and blood lakes in human Ewing sarcoma tissues, we conclude that CD44 increases aggressiveness in tumors through the process of vasculogenic mimicry.

Melanoma educates mesenchymal stromal cells towards vasculogenic mimicry

Accumulating evidence suggests that mesenchymal stromal cells (MSCs) are recruited to the tumor, and promote tumor development and growth. The present study was performed to investigate the communication between aggressive melanoma and MSCs in vasculogenic mimicry (VM). Normal human MSCs plated on Matrigel were unable to form capillary-like structures (CLSs). By contrast, MSCs co-cultured with aggressive melanoma cell lines, namely, Mel Cher, Mel Kor and Mel P, generated CLSs. Significantly, MSCs co-cultured with poorly aggressive melanoma cells, namely, Mel Me, failed to form CLSs. To identify factors responsible for VM, the effects of vascular endothelial growth factor A (VEGFA), pro-epidermal growth factor, basic fibroblast growth factor and stromal cell-derived factor 1α on the formation of CLSs by MSCs were tested. VM was induced by the addition of VEGFA, whereas other cytokines were inefficient. To confirm the hypothesis that aggressive tumor cells can increase the vasculogenic ability of MSCs, a standard B16/F10 mouse melanoma test system was used. MSCs isolated from the adipose tissues of C57BL/6 mice with melanoma formed a vascular-like network on Matrigel, whereas MSCs from healthy mice failed to form such structures. This study provides the first direct evidence that melanoma tumors educate MSCs to engage in VM. The education may occur distantly. These findings offer promise for novel therapeutic directions in the treatment of metastatic melanoma.

miR-1236 down-regulates alpha-fetoprotein, thus causing PTEN accumulation, which inhibits the PI3K/Akt pathway and malignant phenotype in hepatoma cells

Alpha fetoprotein (AFP) is a clinical biomarker of hepatocellular carcinoma (HCC). Here, we found that miR-1236 is down-regulated, whereas AFP is highly expressed in HCC tissues and cells. We demonstrated that miR-1236 directly targets the 3′UTR of AFP and down-regulates its expression. Also, miR-1236 inhibited and AFP stimulated proliferation, migration, invasion and vasculogenic mimicry (VM) of HCC. In agreement, AFP over-expression counteracted the inhibitory effect of miR-1236. We demonstrated that AFP promoted the ubiquitination of PTEN, thus decreasing PTEN levels, while miR-1236 inhibited the PI3K/Akt pathway.

Tumor cell vascular mimicry: Novel targeting opportunity in melanoma

In 1999, the American Journal of Pathology published an article, entitled "Vascular channel formation by human melanoma cells in vivo and in vitro: vasculogenic mimicry" by Maniotis and colleagues, which ignited a spirited debate for several years and earned the journal's distinction of a "citation classic" (Maniotis et al., 1999). Tumor cell vasculogenic mimicry (VM), also known as vascular mimicry, describes the plasticity of aggressive cancer cells forming de novo vascular networks and is associated with the malignant phenotype and poor clinical outcome. The tumor cells capable of VM share the commonality of a stem cell-like, transendothelial phenotype, which may be induced by hypoxia. Since its introduction as a novel paradigm for melanoma tumor perfusion, many studies have contributed new findings illuminating the underlying molecular pathways supporting VM in a variety of tumors, including carcinomas, sarcomas, glioblastomas, astrocytomas, and melanomas. Of special significance is the lack of effectiveness of angiogenesis inhibitors on tumor cell VM, suggesting a selective resistance by this phenotype to conventional therapy. Facilitating the functional plasticity of tumor cell VM are key proteins associated with vascular, stem cell, extracellular matrix, and hypoxia-related signaling pathways--each deserving serious consideration as potential therapeutic targets and diagnostic indicators of the aggressive, metastatic phenotype. This review highlights seminal findings pertinent to VM, including the effects of a novel, small molecular compound, CVM-1118, currently under clinical development to target VM, and illuminates important molecular pathways involved in the suppression of this plastic, aggressive phenotype, using melanoma as a model.

Involvement of HMGB1 in Resistance to Tumor Vessel-Targeted, Monoclonal Antibody-Based Immunotherapy

High mobility group box 1 (HMGB1) is a member of the “danger associated molecular patterns” (DAMPs) than can localize in various compartments of the cell (from the nucleus to the cell surface) and subserve different functions accordingly. HMGB1 is implicated in maintenance of genomic stability, autophagy, immune regulation, and tumor growth. HMGB1-induced autophagy promotes tumor resistance to chemotherapy, as shown in different models of malignancy, for example, osteosarcoma, leukemia, and gastric cancer. To the best of our knowledge, there is virtually no information on the relationships between HMGB1 and resistance to immunotherapy. A recent study from our group has shed new light on this latter issue. We have demonstrated that targeting of tumor-derived endothelial cells with an anti-human CD31 monoclonal antibody in a human neuroblastoma model was unsuccessful due to a complex chain of events involving the participation of HMGB1. These results are discussed in detail since they provide the first evidence for a role of HMGB1 in resistance of tumor cells to monoclonal antibody-based immunotherapy.

Dual Receptor Recognizing Cell Penetrating Peptide for Selective Targeting, Efficient Intratumoral Diffusion and Synthesized Anti-Glioma Therapy

Cell penetrating peptides (CPPs) were widely used for drug delivery to tumor. However, the nonselective in vivo penetration greatly limited the application of CPPs-mediated drug delivery systems. And the treatment of malignant tumors is usually followed by poor prognosis and relapse due to the existence of extravascular core regions of tumor. Thus it is important to endue selective targeting and stronger intratumoral diffusion abilities to CPPs. In this study, an RGD reverse sequence dGR was conjugated to a CPP octa-arginine to form a CendR (R/KXXR/K) motif contained tandem peptide R8-dGR (RRRRRRRRdGR) which could bind to both integrin αvβ3 and neuropilin-1 receptors. The dual receptor recognizing peptide R8-dGR displayed increased cellular uptake and efficient penetration ability into glioma spheroids in vitro. The following in vivo studies indicated the active targeting and intratumoral diffusion capabilities of R8-dGR modified liposomes. When paclitaxel was loaded in the liposomes, PTX-R8-dGR-Lip induced the strongest anti-proliferation effect on both tumor cells and cancer stem cells, and inhibited the formation of vasculogenic mimicry channels in vitro. Finally, the R8-dGR liposomal drug delivery system prolonged the medium survival time of intracranial C6 bearing mice by 2.1-fold compared to the untreated group, and achieved an exhaustive anti-glioma therapy including anti-tumor cells, anti-vasculogenic mimicry and anti-brain cancer stem cells. To sum up, all the results demonstrated that R8-dGR was an ideal dual receptor recognizing CPP with selective glioma targeting and efficient intratumoral diffusion, which could be further used to equip drug delivery system for effective glioma therapy.

Linearly Patterned Programmed Cell Necrosis Induced by Chronic Hypoxia Plays a Role in Melanoma Angiogenesis

Background: Highly aggressive tumors are exposed to hypoxia and increased tumor interstitial fluid pressure (IFP) conditions which is resistant to blood supply. Physiological responses of the organism may reduce IFP through induction of orderly cell death.

Specific aims: This study demonstrates that orderly cell death provided spatial structure for early angiogenesis in the hypoxic, high-IFP tumor microenvironment and the participation of linearly patterned programmed cell necrosis (LPPCN) in nascent melanoma angiogenesis.

Methods: Animal model, laser capture microdissection, wound healing and transwell assays, three-dimensional cultures, zymography assays, western-blotting analysis, immunohistochemistry and RT-PCR were performed.

Results: This study demonstrated a special form of cell death occurring in groups of malignant tumor cells which arrayed in lines. Both features of apoptosis and necrosis can be found in this cell death pattern and were termed as LPPCN. Its role as a stimulus of tumor angiogenesis was investigated using human melanoma samples and an animal model. Computer image analysis showed that LPPCN and tumor microvessels had identical spatial distributions. It can be induced by chronic hypoxia, high IFP and subsequent calcium influx. Higher number of tumor associated macrophages (TAM) and VEGF expression were found in the tumor with LPPCN. Based on the tumor-bearing animal model, it was found that block of caspase pathway inhibited LPPCN, microvessel density and vasculogenic mimicry (VM).

Conclusions: LPPCN formation may play an important role in tumor angiogenesis due to stimulation of macrophage infiltration and HIF-1α regulation, and that inhibition of LPPCN may be a novel therapeutic strategy against tumor angiogenesis and metastasis.

Failure of anti tumor-derived endothelial cell immunotherapy depends on augmentation of tumor hypoxia

We have previously demonstrated that Tenascin-C (TNC)⁺ human neuroblastoma (NB) cells transdifferentiate into tumor-derived endothelial cells (TDEC), which have been detected both in primary tumors and in tumors formed by human NB cell lines in immunodeficient mice. TDEC are genetically unstable and may favor tumor progression, suggesting that their elimination could reduce tumor growth and dissemination. So far, TDEC have never been targeted by antibody-mediated immunotherapy in any of the tumor models investigated.

To address this issue, immunodeficient mice carrying orthotopic NB formed by the HTLA-230 human cell line were treated with TDEC-targeting cytotoxic human (h)CD31, that spares host-derived endothelial cells, or isotype-matched mAbs. hCD31 mAb treatment did not affect survival of NB-bearing mice, but increased significantly hypoxia in tumor microenvironment, where apoptotic and proliferating TDEC coexisted, indicating the occurrence of vascular remodeling.

Tumor cells from hCD31 mAb treated mice showed i) up-regulation of epithelial-mesenchymal transition (EMT)-related and vascular mimicry (VM)-related gene expression, ii) expression of endothelial (i.e. CD31 and VE-cadherin) and EMT-associated (i.e. Twist-1, N-cadherin and TNC) immunophenotypic markers, and iii) up-regulation of high mobility group box-1 (HMGB-1) expression. In vitro experiments with two NB cell lines showed that hypoxia was the common driver of all the above phenomena and that human recombinant HMGB-1 amplified EMT and TDEC trans-differentiation.

In conclusion, TDEC targeting with hCD31 mAb increases tumor hypoxia, setting the stage for the occurrence of EMT and of new waves of TDEC trans-differentiation. These adaptive responses to the changes induced by immunotherapy in the tumor microenvironment allow tumor cells to escape from the effects of hCD31 mAb.

LRIG1 inhibits hypoxia-induced vasculogenic mimicry formation via suppression of the EGFR/PI3K/AKT pathway and epithelial-to-mesenchymal transition in human glioma SHG-44 cells

Leucine-rich repeats and immunoglobulin-like domains 1 (LRIG1) is a pan-negative regulator of the epidermal growth factor receptor (EGFR) signaling pathway. The aim of this study was to investigate the underlying mechanism of LRIG1 in the regulation of vasculogenic mimicry (VM) formation in glioma cells. We constructed an enhanced green fluorescent protein plasmid (pEGFP) system, pEGFP-C1-LRIG1, for overexpression of LRIG1, and transfected it into human glioma cell line SHG-44. Under hypoxic conditions induced by CoCl2, we investigated the effects of LRIG1 overexpression on VM formation and VM-dependent malignant behaviors including migration, invasion, and proliferation. Additionally, we explored the effects of LRIG1 on the expression levels of major components of the EGFR/PI3K/AKT pathway as well as E-cadherin and vimentin. We found that LRIG1 overexpression is able to inhibit hypoxia-induced VM formation, migration, invasion, and proliferation. Furthermore, LRIG1 overexpression counteracts hypoxia-induced increase in the expression of phosphorylated EGFR (pEGFR), PI3K (pPI3K), and AKT (pAKT) and reverts hypoxia-induced alteration in E-cadherin and vimentin expression levels. In LRIG1 knockdown SHG-44 cells, however, hypoxia-induced VM formation and alteration in E-cadherin and vimentin expression levels were exacerbated. These results suggest that the inhibitory effects of LRIG1 are most likely mediated by suppression of the EGFR/PI3K/AKT pathway and epithelial-mesenchymal transition (EMT) process. Our findings provide compelling evidence implicating LRIG1 in glioma pathophysiology, suggesting that gene therapy using LRIG1 may serve as a treatment for this disease.

Histone deacetylase 3 expression correlates with vasculogenic mimicry through the phosphoinositide3-kinase / ERK-MMP-laminin5γ2 signaling pathway

Vasculogenic mimicry (VM) refers to the process by which highly aggressive tumor cells mimic endothelial cells to form vessel-like structures that aid in supplying enough nutrients to rapidly growing tumors. Histone deacetylases (HDACs) regulate the expression and activity of numerous molecules involved in cancer initiation and progression. Notably, HDAC3 is overexpressed in the majority of carcinomas. However, thus far, no data are available to support the role of HDAC3 in VM. In this study, we subjected glioma specimens to immunohistochemical and histochemical double-staining methods and found that VM and HDAC3 expression were related to the pathological grade of gliomas. The presence of VM correlated with HDAC3 expression in glioma tissues. The formation of tubular structures, as determined by the tube formation assay to evaluate VM, was impaired in U87MG cells when transfected by siRNA or treated with an HDAC3 inhibitor. Importantly, the expression of VM-related molecules such as MMP-2/14 and laminin5γ2 was also affected when HDAC3 expression was altered. Furthermore, U87MG cells were treated with a phosphoinositide 3-kinase (PI3K) inhibitor or/and ERK inhibitor and found that the PI3K and ERK signaling pathways play key roles in VM; whereas, in VM, the two signaling pathways did not act upstream or downstream from each other. Taken together, our findings showed that HDAC3 contributed to VM in gliomas, possibly through the PI3K/ERK–MMPs–laminin5γ2 signaling pathway, which could potentially be a novel therapeutic target for gliomas.

Glioblastoma vasculogenic mimicry: signaling pathways progression and potential anti-angiogenesis targets

Glioblastoma (GBM) is a highly angiogenic malignancy that is resistant to standard therapy; neo-formed vessels of this aggressive malignancy are thought to arise by sprouting of pre-existing brain capillaries. However, the conventional anti-angiogenic therapy, which seemed promising initially, shows transitory and incomplete efficacy. The discovery of vasculogenic mimicry (VM) has offered a new horizon for understanding tumor vascularization. VM is a tumor cell-constituted, matrix-embedded fluid-conducting meshwork that is independent of endothelial cells and is positively correlated with poor prognosis. Therefore, a better understanding of GBM vasculature is needed to optimize anti-angiogenic therapy. This review focuses on the signaling molecules and cascades involved in VM in relation to ongoing glioma research, as well as the clinical translational advances in GBM that have been offered by the development of optimized anti-angiogenesis treatment modalities.

Induction of Vasculogenic Mimicry Overrides VEGF-A Silencing and Enriches Stem-like Cancer Cells in Melanoma

The basis for resistance to VEGF inhibition is not fully understood despite its clinical importance. In this study, we examined the adaptive response to VEGF-A inhibition by a loss-of-function analysis using plasmid-based shRNA. Tumor xenografts that initially responded to VEGF-A inhibition underwent an adaptation in vivo, leading to acquired resistance. VEGF-A blockade in tumors was associated with HIF1α expression and an increase in CD144(+) vasculogenic mimicry (VM), leading to formation of channels displaying Tie-1 and MMP-2 upregulation. CD133(+) and CD271(+) melanoma stem-like cells (MSLC) accumulated in the perivascular niche. Tumor xenografts of melanoma cell populations that were intrinsically resistant to VEGF-A blockade did not exhibit any of these features, compared with nontarget control counterparts. Thus, melanomas that are initially sensitive to VEGF-A blockade acquire adaptive resistance by adopting VM as an alternate angiogenic strategy, thereby enriching for deposition of MSLC in the perivascular niche through an HIF1α-dependent process. Conversely, melanomas that are intrinsically resistant to VEGF-A blockade do not show any evidence of compensatory survival mechanisms that promote MSLC accumulation. Our work highlights the potential risk of anti-VEGF treatments owing to a selective pressure for an adaptive resistance mechanism that empowers the development of stem-like cancer cells, with implications for how to design combination therapies that can improve outcomes in patients.

Multiple strategies of oxygen supply in Drosophila malignancies identify tracheogenesis as a novel cancer hallmark

Angiogenesis is the term used to describe all the alterations in blood vessel growth induced by a tumour mass following hypoxic stress. The occurrence of multiple strategies of vessel recruitment favours drug resistance, greatly complicating the treatment of certain tumours. In Drosophila, oxygen is conveyed to the internal organs by the tracheal system, a closed tubular network whose role in cancer growth is so far unexplored. We found that, as observed in human cancers, Drosophila malignant cells suffer from oxygen shortage, release pro-tracheogenic factors, co-opt nearby vessels and get incorporated into the tracheal walls. We also found that the parallelisms observed in cellular behaviours are supported by genetic and molecular conservation. Finally, we identified a molecular circuitry associated with the differentiation of cancer cells into tracheal cells. In summary, our findings identify tracheogenesis as a novel cancer hallmark in Drosophila, further expanding the power of the fly model in cancer research.

Advanced research on vasculogenic mimicry in cancer

Vasculogenic mimicry (VM) is a brand-new tumour vascular paradigm independent of angiogenesis that describes the specific capacity of aggressive cancer cells to form vessel-like networks that provide adequate blood supply for tumour growth. A variety of molecule mechanisms and signal pathways participate in VM induction. Additionally, cancer stem cell and epithelial-mesenchymal transitions are also shown to be implicated in VM formation. As a unique perfusion way, VM is associated with tumour invasion, metastasis and poor cancer patient prognosis. Due to VM's important effects on tumour progression, more VM-related strategies are being utilized for anticancer treatment. Here, with regard to the above aspects, we make a review of advanced research on VM in cancer.

Inhibitory effects of B-cell lymphoma 2 on the vasculogenic mimicry of hypoxic human glioma cells

The aim of this study was to investigate the mechanisms and effects of B-cell lymphoma 2 (Bcl-2) on the vasculogenic mimicry (VM) of human glioma cells. U87 cells were cultured under hypoxic conditions and then divided into four groups: Control, 3-(5-hydroxymethyl-2-furyl)-1-benzylindazole (YC-1), ABT-737 and YC-1 + ABT-737. These groups were treated with the corresponding simulators. The expression of hypoxia-inducible factor-1α (HIF-1α), matrix metalloproteinase (MMP)-2, MMP-14 and Bcl-2 in each group was determined using a reverse transcription-quantitative polymerase chain reaction and western blot analysis. Compared with that in the control group, the mRNA and protein expression of MMP-2, MMP-14 and Bcl-2 in the YC-1 and ABT-737 groups was significantly reduced. The expression of HIF-1α, however, was only significantly reduced in the YC-1 group (P<0.05). Compared with those in the YC-1 + ABT-737 group, the expression levels of the four proteins in the YC-1 and ABT-737 groups were not significantly different, with the exception of the expression of HIF-1α in the ABT-737 group, which was significantly enhanced (P<0.05). The mRNA expression levels of HIF-1α, MMP-2 and MMP-14 in the YC-1 group were significantly different from those in the ABT-737 group (P<0.01); however, no significant difference was observed in the expression of Bcl-2. In conclusion, Bcl-2 may be an important factor in the VM formation of human malignant glioma U87 cells under hypoxic conditions. Certain functions of Bcl-2 may be attributed to the HIF-1α-MMP-2-MMP-14-VM channel, whereas other functions may be independent of the channel.

Divergence(s) in nodal signaling between aggressive melanoma and embryonic stem cells

The significant role of the embryonic morphogen Nodal in maintaining the pluripotency of embryonic stem cells is well documented. Interestingly, the recent discovery of Nodal's re-expression in several aggressive and metastatic cancers has highlighted its critical role in self renewal and maintenance of the stem cell-like characteristics of tumor cells, such as melanoma. However, the key TGFβ/Nodal signaling component(s) governing Nodal's effects in metastatic melanoma remain mostly unknown. By employing receptor profiling at the mRNA and protein level(s), we made the novel discovery that embryonic stem cells and metastatic melanoma cells share a similar repertoire of Type I serine/threonine kinase receptors, but diverge in their Type II receptor expression. Ligand:receptor crosslinking and native gel binding assays indicate that metastatic melanoma cells employ the heterodimeric TGFβ receptor I/TGFβ receptor II (TGFβRI/TGFβRII) for signal transduction, whereas embryonic stem cells use the Activin receptors I and II (ACTRI/ACTRII). This unexpected receptor usage by tumor cells was tested by: neutralizing antibody to block its function; and transfecting the dominant negative receptor to compete with the endogenous receptor for ligand binding. Furthermore, a direct biological role for TGFβRII was found to underlie vasculogenic mimicry (VM), an endothelial phenotype contributing to vascular perfusion and associated with the functional plasticity of aggressive melanoma. Collectively, these findings reveal the divergence in Nodal signaling between embryonic stem cells and metastatic melanoma that can impact new therapeutic strategies targeting the re-emergence of embryonic pathways.

Testing an unusual in vivo vessel network model: a method to study angiogenesis in the colonial tunicate Botryllus schlosseri

Tunicates are the closest relatives to vertebrates and include the only chordate species able to reproduce both sexually and asexually. The colonial tunicate Botryllus schlosseri is embedded in a transparent extracellular matrix (the tunic) containing the colonial circulatory system (CCS). The latter is a network of vessels external to zooids, limited by a simple, flat epithelium that originated from the epidermis. The CCS propagates and regenerates by remodelling and extending the vessel network through the mechanism of sprouting, which typically characterises vertebrate angiogenesis. In exploiting the characteristics of B. schlosseri as a laboratory model, we present a new experimental and analysis method based on the ability to obtain genetically identical subclones representing paired samples for the appropriate quantitative outcome statistical analysis. The method, tested using human VEGF and EGF to induce angiogenesis, shows that the CCS provides a useful in vivo vessel network model for testing the effects of specific injected solutes on vessel dynamics. These results show the potentiality of B. schlosseri CCS as an effective complementary model for in vivo studies on angiogenesis and anticancer therapy. We discuss this potentiality, taking into consideration the origin, nature, and roles of the cellular and molecular agents involved in CCS growth.

ROCK is involved in vasculogenic mimicry formation in hepatocellular carcinoma cell line

Ras homolog family member A (RhoA) and Rho-associated coiled coil-containing protein kinases 1 and 2 (ROCK1 and 2) are key regulators of focal adhesion, actomyosin contraction and cell motility. RhoA/ROCK signaling has emerged as an attractive target for the development of new cancer therapeutics. Whether RhoA/ROCK is involved in regulating the formation of tumor cell vasculogenic mimicry (VM) is largely unknown. To confirm this hypothesis, we performed in vitro experiments using hepatocellular carcinoma (HCC) cell lines. Firstly, we demonstrated that HCC cells with higher active RhoA/ROCK expression were prone to form VM channels, as compared with RhoA/ROCK low-expressing cells. Furthermore, Y27632 (a specific inhibitor of ROCK) rather than exoenzyme C3 (a specific inhibitor of RhoA) effectively inhibited the formation of tubular network structures in a dose-dependent manner. To elucidate the possible mechanism of ROCK on VM formation, real-time qPCR, western blot and immunofluorescence were used to detect changes of the key VM-related factors, including VE-cadherin, erythropoietin-producing hepatocellular carcinoma-A2 (EphA2), phosphoinositide 3-kinase (PI3K), matrix metalloproteinase (MMP)14, MMP2, MMP9 and laminin 5γ2-chain (LAMC2), and epithelial-mesenchymal-transition (EMT) markers: E-cadherin and Vimentin. The results showed that all the expression profiles were attenuated by blockage of ROCK. In addition, in vitro cell migration and invasion assays showed that Y27632 inhibited the migration and invasion capacity of HCC cell lines in a dose-dependent manner markedly. These data indicate that ROCK is an important mediator in the formation of tumor cell VM, and suggest that ROCK inhibition may prove useful in the treatment of VM in HCC.

The chicken chorioallantoic membrane model in biology, medicine and bioengineering

The chicken chorioallantoic membrane (CAM) is a simple, highly vascularized extraembryonic membrane, which performs multiple functions during embryonic development, including but not restricted to gas exchange. Over the last two decades, interest in the CAM as a robust experimental platform to study blood vessels has been shared by specialists working in bioengineering, development, morphology, biochemistry, transplant biology, cancer research and drug development. The tissue composition and accessibility of the CAM for experimental manipulation, makes it an attractive preclinical in vivo model for drug screening and/or for studies of vascular growth. In this article we provide a detailed review of the use of the CAM to study vascular biology and response of blood vessels to a variety of agonists. We also present distinct cultivation protocols discussing their advantages and limitations and provide a summarized update on the use of the CAM in vascular imaging, drug delivery, pharmacokinetics and toxicology.

Curcumin inhibits vasculogenic mimicry through the downregulation of erythropoietin-producing hepatocellular carcinoma-A2, phosphoinositide 3-kinase and matrix metalloproteinase-2

Glioblastomas (GBMs) are the most common and aggressive malignant primary brain tumors found in humans. In high-grade gliomas, vasculogenic mimicry (VM) is often detected. VM is the formation of de novo vascular networks by highly invasive tumor cells, instead of endothelial cells. An understanding of the mechanisms of VM formation will contribute to the targeted therapy of GBMs. In the present study, the efficacy of curcumin (CCM) on VM formation and its mechanisms were investigated. It was found that CCM inhibits the VM formation, proliferation, migration and invasion of human glioma U251 cells in a dose-dependent manner. Furthermore, CCM downregulated the protein and mRNA expression of erythropoietin-producing hepatocellular carcinoma-A2, phosphoinositide 3-kinase and matrix metalloproteinase-2, indicating that CCM may function through these factors for the inhibition of VM formation. These data provide novel insights into the use of CCM to antagonize VM, and may contribute to the angiogenesis-targeted therapy of malignant glioma.

Brain tumor-targeted drug delivery strategies

Despite the application of aggressive surgery, radiotherapy and chemotherapy in clinics, brain tumors are still a difficult health challenge due to their fast development and poor prognosis. Brain tumor-targeted drug delivery systems, which increase drug accumulation in the tumor region and reduce toxicity in normal brain and peripheral tissue, are a promising new approach to brain tumor treatments. Since brain tumors exhibit many distinctive characteristics relative to tumors growing in peripheral tissues, potential targets based on continuously changing vascular characteristics and the microenvironment can be utilized to facilitate effective brain tumor-targeted drug delivery. In this review, we briefly describe the physiological characteristics of brain tumors, including blood–brain/brain tumor barriers, the tumor microenvironment, and tumor stem cells. We also review targeted delivery strategies and introduce a systematic targeted drug delivery strategy to overcome the challenges.

CD133-targeted niche-dependent therapy in cancer: a multipronged approach

Cancer treatment continues to be challenged by the development of therapeutic resistances and relapses in the clinical setting, which are largely attributed to tumor heterogeneity, particularly the existence of cancer stem cells (CSCs). Thus, targeting the CSC subpopulation may represent an effective therapeutic strategy. However, despite advances in identifying and characterizing CD133(+) CSCs in various human cancers, efforts to translate these experimental findings to clinical modalities have been slow in the making, especially in light of the growing awareness of CSC plasticity and the foreseeable pitfall of therapeutically targeting CSC base sorely on a surface marker. We, and others, have demonstrated that the CD133(+) CSCs reside in complex vascular niches, where reciprocal signaling between the CD133(+) CSCs and their microenvironment may govern niche morphogenesis and homeostasis. Herein, we discuss the multifaceted functional role of the CD133(+) cells in the context of their niche, and the potential of targeting CD133 as a niche-dependent approach in effective therapy.

The interplay between hypoxia, endothelial and melanoma cells regulates vascularization and cell motility through endothelin-1 and vascular endothelial growth factor

Reciprocal growth factor exchanges between endothelial and malignant cells within the hypoxic microenvironment determine tumor progression. However, the nature of these exchanges has not yet been fully explored. We studied the mutual regulation between endothelial cells (EC), melanoma cells and hypoxia that dictate tumor aggressiveness and angiogenic activity. Here, we investigated the presence of bidirectional autocrine/paracrine endothelin (ET)-1/ET receptor (ETBR) signaling in melanoma cells, blood and lymphatic EC. In all these cells, hypoxia enhanced ET-1 expression, which in turn induced vascular endothelial growth factor (VEGF)-A and VEGF-C secretion, through the hypoxia-inducible growth factor (HIF)-1α and HIF-2α. Autocrine/paracrine exchanges of ET-1, VEGF-A and VEGF-C promoted tumor aggressiveness and morphological changes in blood and lymphatic EC. Furthermore, conditioned media from EC enhanced melanoma cell migration and vessel-like channel formation. This regulation was inhibited by ETBR blockade, by using the selective ETBR antagonist, or ETBR small interfering RNA (siRNA), and by VEGFR-2/-3 antibodies, indicating that ET-1, VEGF-A/VEGF-C, produced by melanoma cells or EC mediated inter-regulation between these cells. Interestingly, HIF-1α/HIF-2α siRNA, impaired this reciprocal regulation, demonstrating the key role of these transcriptional factors in signaling exchanges. In melanoma xenografts, the ETBR antagonist reduced tumor growth and the number of blood and lymphatic vessels. These results reveal an interplay between melanoma cells and EC mediated by ET-1 and VEGF-A/-C and coordinated by the hypoxic microenvironment through HIF-1α/2α transcriptional programs. Thus, targeting ETBR may improve melanoma treatment for tumor and EC, by inhibiting autocrine/paracrine signaling that sustains melanoma progression.

Disrupting the CXCL12/CXCR4 axis disturbs the characteristics of glioblastoma stem-like cells of rat RG2 glioblastoma

BACKGROUND

Glioblastoma stem-like cells (GSC) have been shown to promote tumor growth, tumor-associated neovascularization, therapeutic resistance, and metastasis. CXCR4 receptors have been found involved in the proliferation, metastasis, angiogenesis, and drug-resistant characteristics of glioblastoma. However, the role of CXCR4 in modulating the stem-like cell properties of rat glioblastoma remains ambiguous.

METHODS

To explore the role of the CXCL12/CXCR4 axis in maintaining rat GSC properties, we disrupted the CXCR4 signaling by using small hairpin interfering RNA (shRNA). To investigate the role of the CXCL12/CXCR4 axis in maintaining rat GSC properties, we used a spheroid formation assay to assess the stem cell self-renewal properties. A western blot analysis and PCR arrays were used to examine the genes involved in proliferation, self-renewal, and cancer drug resistance. Finally, DNA content and flow cytometry, an immunohistochemical analysis, and methylcellulose colony formation, in vitro invasive and intracranial injection xenograft assays were employed to examine the disruptive effect of CXCR4 on the characteristics of GSCs of the RG2 cell line.

RESULTS

Disrupting CXCR4 inhibited the proliferation of RG2 cells both in vitro and in vivo. The spheroid formation assay indicated that CXCR4 was vital for the self-renewal of RG2 GSCs. Disrupting the CXCL12/CXCR4 pathway also reduced the expression of GSC cell markers, including Nestin, ABCG2, and musashi (Msi), and the expression of genes involved in regulating stem cell properties, including Oct4, Nanog, maternal embryonic leucine zipper kinase (MELK), MGMT, VEGF, MMP2, and MMP9.

CONCLUSION

The chemokine receptor CXCR4 is crucial for maintaining the self-renewal, proliferation, therapeutic resistance, and angiogenesis of GSCs of rat RG2 glioblastoma.

Vessel co-option in primary human tumors and metastases: an obstacle to effective anti-angiogenic treatment?

Angiogenesis has been regarded as essential for tumor growth and progression. Studies of many human tumors, however, suggest that their microcirculation may be provided by nonsprouting vessels and that a variety of tumors can grow and metastasize without angiogenesis. Vessel co-option, where tumor cells migrate along the preexisting vessels of the host organ, is regarded as an alternative tumor blood supply. Vessel co-option may occur in many malignancies, but so far mostly reported in highly vascularized tissues such as brain, lung, and liver. In primary and metastatic lung cancer and liver metastasis from different primary origins, as much as 10–30% of the tumors are reported to use this alternative blood supply. In addition, vessel co-option is introduced as a potential explanation of antiangiogenic drug resistance, although the impact of vessel co-option in this clinical setting is still to be further explored. In this review we discuss tumor vessel co-option with specific examples of vessel co-option in primary and secondary tumors and a consideration of the clinical implications of this alternative tumor blood supply.

Both primary and metastatic tumors use preexisting host tissue vessels as their blood supply. Tumors may grow to a clinically detectable size without angiogenesis and makes them less likely to respond to drugs designed to target the abnormal vasculature produced by angiogenesis, but further studies to explore the biological and clinical implication of these co-opted vessels is needed.

p38MAPK inhibition: a new combined approach to reduce neuroblastoma resistance under etoposide treatment

Neuroblastoma (NB) is the second most common solid pediatric tumor and is characterized by clinical and biological heterogeneity, and stage-IV of the disease represents 50% of all cases. Considering the limited success of present chemotherapy treatment, it has become necessary to find new and effective therapies. In this context, our approach consists of identifying and targeting key molecular pathways associated with NB chemoresistance. This study has been carried out on three stage-IV NB cell lines with different status of MYCN amplification. Cells were exposed to a standard chemotherapy agent, namely etoposide, either alone or in combination with particular drugs, which target intracellular signaling pathways. Etoposide alone induced a concentration-dependent reduction of cell viability and, at very high doses, totally counteracted cell tumorigenicity and neurosphere formation. In addition, etoposide activated p38 mitogen-activated protein kinase (MAPK), AKT and c-Jun N-terminal kinase. Pre-treatment with SB203580, a p38MAPK inhibitor, dramatically sensibilized NB cells to etoposide, strongly reducing the dosage needed to inhibit tumorigenicity and neurosphere formation. Importantly, SB203580–etoposide cotreatment also reduced cell migration and invasion by affecting cyclooxygenase-2, intercellular adhesion molecule-1, C–X–C chemokine receptor-4 and matrix metalloprotease-9. Collectively, our results suggest that p38MAPK inhibition, in combination with standard chemotherapy, could represent an effective strategy to counteract NB resistance in stage-IV patients.

Endothelin-1 induces the transactivation of vascular endothelial growth factor receptor-3 and modulates cell migration and vasculogenic mimicry in melanoma cells

Endothelin receptor B (ET(B)R) is a G-protein-coupled receptor overexpressed in melanoma, blood, and lymphatic endothelial cells. Given that aberrant signal transduction can be mediated through cross talk between receptors, here, we explore the functional relationship between ET(B)R and the vascular endothelial growth factor receptor (VEGFR)-3 system and how this cross talk might influence the aggressive behavior of melanoma cells. The expression of VEGFR-3 and its ligands, VEGF-C and VEGF-D, significantly increased after activating ET(B)R by ET-1 in primary and metastatic melanoma cell lines. These effects, similarly to those induced by hypoxia, were mediated by hypoxia-inducible factor (HIF)-1α and HIF-2α. ET-1 caused the phosphorylation of VEGFR-3, which was accompanied by the activation of the downstream signaling molecules, such as MAPK and AKT. Inhibition of c-Src activity or silencing of the scaffold protein β-arrestin-1 reduced ET-1-induced VEGFR-3 phosphorylation, demonstrating that, upon ET-1 stimulus, β-arrestin-1 is involved with c-Src in the ET(B)R-mediated VEGFR-3 transactivation. Moreover, ET-1 in combination with VEGF-C further increased VEGFR-3, MAPK, and AKT phosphorylation and markedly promoted cell migration and vasculogenic mimicry. Dual inhibition of ET(B)R and VEGFR-3 was required for the effective inhibition of these effects, as well as for VEGFR-3 phosphorylation, demonstrating that ET(B)R cross talk with VEGFR-3 enhances cell plasticity and motility. Finally, in melanoma xenografts, ET(B)R antagonist inhibited tumor growth and the activation of the VEGF-C/VEGFR-3 axis, indicating that targeting ET(B)R may improve melanoma treatment acting directly or indirectly by impairing ET(B)R cross talk with VEGFR-3.

Nicotinamide inhibits vasculogenic mimicry, an alternative vascularization pathway observed in highly aggressive melanoma

Vasculogenic mimicry (VM) describes functional vascular channels composed only of tumor cells and its presence predicts poor prognosis in melanoma patients. Inhibition of this alternative vascularization pathway might be of clinical importance, especially as several anti-angiogenic therapies targeting endothelial cells are largely ineffective in melanoma. We show the presence of VM structures histologically in a series of human melanoma lesions and demonstrate that cell cultures derived from these lesions form tubes in 3D cultures ex vivo. We tested the ability of nicotinamide, the amide form of vitamin B3 (niacin), which acts as an epigenetic gene regulator through unique cellular pathways, to modify VM. Nicotinamide effectively inhibited the formation of VM structures and destroyed already formed ones, in a dose-dependent manner. Remarkably, VM formation capacity remained suppressed even one month after the complete withdrawal of Nicotimamid. The inhibitory effect of nicotinamide on VM formation could be at least partially explained by a nicotinamide-driven downregulation of vascular endothelial cadherin (VE-Cadherin), which is known to have a central role in VM. Further major changes in the expression profile of hundreds of genes, most of them clustered in biologically-relevant clusters, were observed. In addition, nicotinamide significantly inhibited melanoma cell proliferation, but had an opposite effect on their invasion capacity. Cell cycle analysis indicated moderate changes in apoptotic indices. Therefore, nicotinamide could be further used to unravel new biological mechanisms that drive VM and tumor progression. Targeting VM, especially in combination with anti-angiogenic strategies, is expected to be synergistic and might yield substantial anti neoplastic effects in a variety of malignancies.

Inherent phenotypic plasticity facilitates progression of head and neck cancer: endotheliod characteristics enable angiogenesis and invasion

The presence of the EMT (epithelial-mesenchymal transition), EndMT (endothelial-mesenchymal transition) and VM (vasculogenic mimicry) demonstrates the multidirectional extent of phenotypic plasticity in cancers. Previous findings demonstrating the crosstalk between head and neck squamous cell carcinoma (HNSCC) and vascular endothelial growth factor (VEGF) imply that HNSCC cells share some functional commonalities with endothelial cells. Our current results reveal that cultured HNSCC cells not only possess endothelial-specific markers, but also display endotheliod functional features including low density lipoprotein uptake, formation of tube-like structures on Matrigel and growth state responsiveness to VEGF and endostatin. HNSCC cell subpopulations are also highly responsive to transforming growth factor-β1 and express its auxiliary receptor, endoglin. Furthermore, the endotheliod characteristics observed in vitro recapitulate phenotypic features observed in human HNSCC tumors. Conversely, cultured normal human oral keratinocytes and intact or ulcerated human oral epithelia do not express comparable endotheliod characteristics, which imply that assumption of endotheliod features is restricted to transformed keratinocytes. In addition, this phenotypic state reciprocity facilitates HNSCC progression by increasing production of factors that are concurrently pro-proliferative and pro-angiogenic, conserving cell energy stores by LDL internalization and enhancing cell mobility. Finally, recognition of this endotheliod phenotypic transition provides a solid rationale to evaluate the antitumorigenic potential of therapeutic agents formerly regarded as exclusively angiostatic in scope.

Vasculogenic mimicry of HT1080 tumour cells in vivo: critical role of HIF-1α-neuropilin-1 axis

HT1080 - a human fibrosarcoma-derived cell line – forms aggressive angiogenic tumours in immuno-compromised mice. In spite of its extensive use as a model of tumour angiogenesis, the molecular event(s) initiating the angiogenic program in these cells are not known. Since hypoxia stimulates tumour angiogenesis, we examined the hypoxia-induced events evoked in these cells. In contrast to cells grown under normoxic conditions, hypoxia-primed (1% O₂) HT1080 cells formed robust tubules on growth factor-reduced matrigel and formed significantly larger tumours in xenograft models in a chetomin-sensitive manner, indicating the role of HIF-1α-mediated transcription in these processes. Immuno-histochemical analyses of tumours formed by GFP-expressing HT1080 cells clearly showed that the tumour cells themselves expressed various angiogenic markers including Neuropilin-1 (NRP-1) and formed functional vessels containing red blood cells, thereby unambiguously demonstrating the vasculogenic mimicry of HT1080 cells in vivo. Experiments performed with the HT1080 cells stably transfected with plasmid constructs expressing shNRP-1 or full-length NRP-1 clearly established that the HIF1α-mediated up-regulation of NRP-1 played a deterministic role in the process. Hypoxia-exposure resulted in an up-regulation of c-Myc and OCT3/4 and a down-regulation of KLF4 mRNAs, suggesting their involvement in the tumour formation and angiogenesis. However, silencing of NRP-1 alone, though not affecting proliferation in culture, was sufficient to abrogate the tumour formation completely; clearly establishing that the hypoxia-mediated HIF-1α-dependent up-regulation of NRP-1 is a critical molecular event involved in the vasculogenic mimicry and tumor formation by HT1080 cells in vivo.

Tumor cell vasculogenic mimicry: from controversy to therapeutic promise

In 1999, The American Journal of Pathology published an article entitled "Vascular channel formation by human melanoma cells in vivo and in vitro: vasculogenic mimicry," by Maniotis and colleagues, which ignited a spirited debate for several years and earned distinction as a citation classic. Tumor cell vasculogenic mimicry (VM) refers to the plasticity of aggressive cancer cells forming de novo vascular networks, which thereby contribute to perfusion of rapidly growing tumors, transporting fluid from leaky vessels, and/or connecting with the constitutional endothelial-lined vasculature. The tumor cells capable of VM share a plastic, transendothelial phenotype, which may be induced by hypoxia. Since VM was introduced as a novel paradigm for melanoma tumor perfusion, many studies have contributed new findings illuminating the underlying molecular pathways supporting VM in a variety of tumors, including carcinomas, sarcomas, glioblastomas, astrocytomas, and melanomas. Facilitating the functional plasticity of tumor cell VM are key proteins associated with vascular, stem cell, and hypoxia-related signaling pathways, each deserving serious consideration as potential therapeutic targets and diagnostic indicators of the aggressive, metastatic phenotype.

The involvement of Notch signaling in melanoma vasculogenic mimicry

Notch signaling plays an important role in tumor angiogenesis. Recent studies suggest that Notch signaling also regulates the progression of primary melanomas toward an aggressive phenotype. The aim of this study was to investigate the involvement of Notch signaling pathway in organization of tumor cells into capillary-like structures (CLS), the phenomenon also known as vasculogenic mimicry (VM). Here, we show that Notch signaling cascade was constitutively active in melanoma cell lines we used. Blocking Notch signaling with the γ-secretase inhibitors, DAPT, dibenzazepine or Jagged1 neutralizing antibody resulted in stabilization of CLS indicating that Notch signaling pathway attenuates melanoma VM. We further studied this phenomenon on melanomas grafted in nude mice. Compared to control, VM channels in DAPT-treated grafted melanoma became larger and more branched. DAPT-treated melanomas also exhibited an up-regulation of MMP-2 and VEGFR1, both known as VM mediators. Moreover, we did not observe necrosis in VM channels areas of DAPT-treated melanomas. These findings indicate that VM regulated by Notch signaling may present a novel target in melanoma therapy.

In vivo evolution of tumor-derived endothelial cells

The growth of a malignant tumor beyond a certain, limited size requires that it first develop an independent blood supply. In addition to providing metabolic support, this neovasculature also allows tumor cells to access the systemic circulation, thus facilitating metastatic dissemination. The neovasculature may originate either from normal blood vessels in close physical proximity to the tumor and/or from the recruitment of bone marrow-derived endothelial cell (EC) precursors. Recent studies have shown that human tumor vasculature ECs may also arise directly from tumor cells themselves and that the two populations have highly similar or identical karyotypes. We now show that, during the course of serial in vivo passage, these tumor-derived ECs (TDECs) progressively acquire more pronounced EC-like properties. These include higher-level expression of EC-specific genes and proteins, a greater capacity for EC-like behavior in vitro, and a markedly enhanced propensity to incorporate into the tumor vasculature. In addition, both vessel density and size are significantly increased in neoplasms derived from mixtures of tumor cells and serially passaged TDECs. A comparison of early- and late-passage TDECs using whole-genome single nucleotide polymorphism profiling showed the latter cells to have apparently evolved by a process of clonal expansion of a population with a distinct pattern of interstitial chromosomal gains and losses affecting a relatively small number of genes. The majority of these have established roles in vascular development, tumor suppression or epithelial-mesenchymal transition. These studies provide direct evidence that TDECs have a strong evolutionary capacity as a result of their inherent genomic instability. Consequently such cells might be capable of escaping anti-angiogenic cancer therapies by generating resistant populations.

Molecular pathways: vasculogenic mimicry in tumor cells: diagnostic and therapeutic implications

Tumor cell vasculogenic mimicry (VM) describes the functional plasticity of aggressive cancer cells forming de novo vascular networks, thereby providing a perfusion pathway for rapidly growing tumors, transporting fluid from leaky vessels, and/or connecting with endothelial-lined vasculature. The underlying induction of VM seems to be related to hypoxia, which may also promote the plastic, transendothelial phenotype of tumor cells capable of VM. Since its introduction in 1999 as a novel paradigm for melanoma tumor perfusion, many studies have contributed new insights into the underlying molecular pathways supporting VM in a variety of tumors, including melanoma, glioblastoma, carcinomas, and sarcomas. In particular, critical VM-modulating genes are associated with vascular (VE-cadherin, EphA2, VEGF receptor 1), embryonic and/or stem cell (Nodal, Notch4), and hypoxia-related (hypoxia-inducible factor, Twist1) signaling pathways. Each of these pathways warrants serious scrutiny as potential therapeutic, vascular targets, and diagnostic indicators of plasticity, drug resistance, and the aggressive metastatic phenotype.

Zinc finger E-box binding homeobox 1 promotes vasculogenic mimicry in colorectal cancer through induction of epithelial-to-mesenchymal transition

Our previous studies have shown that epithelial-mesenchymal transition (EMT) may be involved in the vasculogenic mimicry (VM) formation in hepatocellular carcinoma. Here, we hypothesize that zinc finger E-box binding homeobox 1 (ZEB1) promotes VM formation in colorectal carcinoma (CRC) by inducing EMT. We identified VM in 39 (19.2%) out of 203 CRC patients. The presence of VM was associated with aggressive biological behavior and was an unfavorable prognostic indicator. By immunohistochemical analysis, we found that the VM-positive CRC samples showed increased ZEB1 expression compared with the VM-negative samples and the ZEB1 expression occurred concomitantly with features of EMT. In vitro, knockdown of ZEB1 in poorly differentiated HCT116 CRC cells destroyed the vessel-like structures in the 3-D culture, a property associated with VM formation. Knockdown of ZEB1 resulted in restoration of epithelial phenotypes and significantly inhibited the ability to migrate and invade. In addition, ZEB1 underexpression decreased the expression of vascular endothelial (VE)-cadherin and Flk-1, which are characteristics of endothelial cells. Taken together, our results suggest that ZEB1 can promote VM formation by inducing EMT in CRC and might represent an important target in CRC.

Transforming growth factor-β is required for vasculogenic mimicry formation in glioma cell line U251MG

Both vasculogenic mimicry (VM) and transforming growth factor-β (TGFβ) are positively correlated with malignancy in glioma. Accordingly, we supposed that TGFβ might be related with VM, and aimed to detect whether TGFβ could influence VM formation in two glioma cell lines U251MG and SHG44, which were different in malignancy. We found that the VM-positive U251MG had a significantly higher TGFβ expression than the VM-negative SHG44. Downregulating TGFβ in U251MG by RNAi technology resulted in a significantly impaired VM formation, which could be rescued by rhTGFβ. However, adding rhTGFβ could not induce VM in SHG44. To investigate the possible mechanism, we detected the changes of some VM-related genes including EphA2, VE-cadherin, MMP-2, MMP-9, MT1-MMP and LAMC2 by RT-PCR and found that MT1-MMP transcript was affected by TGFβ expression. Gelatin zymography showed a declined MMP-2 activity in the TGFβ-inhibited cells. Further studies showed that MT1-MMP inhibition impaired VM formation in U251MG. Moreover, TGFβ induced MT1-MMP expression and VM formation in a dose-dependent manner. These findings indicated us that TGFβ was required for VM formation in U251MG. MT1-MMP was correlated with TGFβ-induced VM formation. Thus, TGFβ might be a potential target for VM inhibition in glioma.

The cancer stem cell niche--there goes the neighborhood?

The niche is the environment in which stem cells reside and is responsible for the maintenance of unique stem cell properties such as self-renewal and an undifferentiated state. The heterogeneous populations which constitute a niche include both stem cells and surrounding differentiated cells. This network of heterogeneity is responsible for the control of the necessary pathways that function in determining stem cell fate. The concept that cancer stem cells, a subpopulation of cells responsible for tumor initiation and formation, reside in their own unique niche is quickly evolving and it is of importance to understand and identify the processes occurring within this environment. The necessary intrinsic pathways that are utilized by this cancer stem cell population to maintain both self-renewal and the ability to differentiate are believed to be a result of the environment where cancer stem cells reside. The ability of a specific cancer stem cell niche to provide the environment in which this population can flourish is a critical aspect of cancer biology that mandates intense investigation. This review focuses on current evidence demonstrating that homeostatic processes such as inflammation, epithelial to mesenchymal transition, hypoxia and angiogenesis contribute to the maintenance and control of cancer stem cell fate by providing the appropriate signals within the microenvironment. It is necessary to understand the key processes occurring within this highly specialized cancer stem cell niche to identify potential therapeutic targets that can serve as the basis for development of more effective anticancer treatments.

Zoledronic acid inhibits vasculogenic mimicry in murine osteosarcoma cell line in vitro

BACKGROUND

To study the effects of zoledronic acid (ZA) on the vasculogenic mimicry of osteosarcoma cells in vitro.

METHODS

A Three-dimensional culture of LM8 osteosarcoma cells on a type I collagen matrix was used to investigate whether osteosarcoma cells can develop vasculogenic mimicry, and to determine the effects of ZA on this process. In addition, the cellular ultrastructural changes were observed using scanning electron microscopy and laser confocal microscopy. The effects of ZA on the translocation of RhoA protein from the cytosol to the membrane in LM8 cells were measured via immunoblotting.

RESULTS

ZA inhibited the development of vasculogenic mimicry by the LM8 osteosarcoma cells, decreased microvilli formation on the cell surface, and disrupted the F-actin cytoskeleton. ZA prevented translocation of RhoA protein from the cytosol to the membrane in LM8 cells.

CONCLUSIONS

ZA can impair RhoA membrane localization in LM8 cells, causing obvious changes in the ultrastructure of osteosarcoma cells and induce cell apoptosis, which may be one of the underlying mechanisms by which the agent inhibits the development of vasculogenic mimicry by the LM8 cells.

Antiangiogenic therapy, hypoxia, and metastasis: risky liaisons, or not?

All human cells, including cancer cells, need oxygen and nutrients to survive. A widely used strategy to combat cancer is therefore the starvation of tumor cells by cutting off the blood supply of tumors. Clinical experience indeed shows that tumor progression can be delayed by anti-angiogenic agents. However, emerging evidence indicates that in certain experimental conditions, hypoxia as a result of pruning of the tumor microvasculature can promote tumor invasion and metastasis, although these findings are contextual and debated. Genetic studies in mice unveiled that vascular-targeting strategies that avoid aggravation of tumor hypoxia or even promote tumor oxygenation might prevent such an invasive metastatic switch. In this article, we will discuss the emerging link between hypoxia signaling and the various steps of metastasis.