Targeting cancer stem cells to modulate alternative vascularization mechanisms

IFITM3 promotes glioblastoma stem cell-mediated angiogenesis via regulating JAK/STAT3/bFGF signaling pathway

Interferon-induced transmembrane protein 3 (IFITM3) has been previously verified to be an endosomal protein that prevents viral infection. Recent findings suggested IFITM3 as a key factor in tumor invasion and progression. To clarify the role and molecular mechanism of IFITM3 in Glioblastoma multiforme (GBM) progression, we investigated the expression of IFITM3 in glioma datasets culled from The Cancer Genome Atlas (TCGA) and Chinese Glioma Genome Atlas (CGGA). Primary GBM stem cells (GSCs) were cultured and identified in vitro. Loss-of-function and gain-of-function experiments were established by using shRNAs and lentiviral vectors targeting IFITM3. Co-culture system of GSCs and vascular endothelial cells was constructed in a Transwell chamber. Tube formation and spheroid-based angiogenesis assays were performed to determine the angiogenic capacity of endothelial cells. Results revealed that IFITM3 is elevated in GBM samples and predictive of adverse outcome. Mechanistically, GSCs-derived IFITM3 causes activation of Jak2/STAT3 signaling and leads to robust secretion of bFGF into tumor environment, which eventually results in enhanced angiogenesis. Taken together, these evidence indicated IFITM3 as an essential factor in GBM angiogenesis. Our findings provide a new insight into mechanism by which IFITM3 modulates GBM angiogenesis.

Snail promotes the generation of vascular endothelium by breast cancer cells

A further understanding of tumor angiogenesis is urgently needed due to the limited therapeutic efficacy of anti-angiogenesis agents. However, the origin of endothelial cells (EC) in tumors remains widely elusive and controversial. Snail has been thoroughly elucidated as a master regulator of the epithelial-mesenchymal transition (EMT), but its role in endothelium generation is not yet established. In this study, we reported a new and unexpected function of Snail in endothelium generation by breast cancer cells. We showed that high Snail-expressing breast cancer cells isolated from patients showed more endothelium generated from these cells. Expression of Snail was positively correlated with endothelial markers in breast cancer patients. The ectopic expression of Snail induced endothelial marker expression, tube formation and DiI-AcLDL uptake of breast cancer cells in vitro, and enhanced tumor growth and microvessel density in vivo. Snail-mediated endothelium generation depended on VEGF and Sox2. Mechanistically, Snail promoted the expression of VEGF and Sox2 through recruiting the p300 activator complex to these promoters. We showed the dual function of Snail in tumor initiation and angiogenesis in vivo and in vitro through activation of Sox2 and VEGF, suggesting Snail may be an ideal target for cancer therapy.

Etoposide-Bevacizumab a new strategy against human melanoma cells expressing stem-like traits

Tumors contain a sub-population of self-renewing and expanding cells known as cancer stem cells (CSCs). Putative CSCs were isolated from human melanoma cells of a different aggressiveness, Hs294T and A375 cell lines, grown under hypoxia using “sphere-forming assay”, CD133 surface expression and migration ability. We found that a cell sub-population enriched for P1 sphere-initiating ability and CD133 expression also express larger amount of VEGF-R2. Etoposide does not influence phenotype of this sub-population of melanoma cells, while a combined treatment with Etoposide and Bevacizumab significantly abolished P1 sphere-forming ability, an effect associated with apoptosis of this subset of cells. Hypoxic melanoma cells sorted for VEGF-R2/CD133 positivity also undergo apoptosis when exposed to Etoposide and Bevacizumab. When Etoposide and Bevacizumab-treated hypoxic cells were injected intravenously into immunodeficient mice revealed a reduced capacity to induce lung colonies, which also appear with a longer latency period. Hence, our study indicates that a combined exposure to Etoposide and Bevacizumab targets melanoma cells endowed with stem-like properties and might be considered a novel approach to treat cancer-initiating cells.

Hinokitiol inhibits vasculogenic mimicry activity of breast cancer stem/progenitor cells through proteasome-mediated degradation of epidermal growth factor receptor

Hinokitiol, alternatively known as β-thujaplicin, is a tropolone-associated natural compound with antimicrobial, anti-inflammatory and antitumor activity. Breast cancer stem/progenitor cells (BCSCs) are a subpopulation of breast cancer cells associated with tumor initiation, chemoresistance and metastatic behavior, and may be enriched by mammosphere cultivation. Previous studies have demonstrated that BCSCs exhibit vasculogenic mimicry (VM) activity via the epidermal growth factor receptor (EGFR) signaling pathway. The present study investigated the anti-VM activity of hinokitiol in BCSCs. At a concentration below the half maximal inhibitory concentration, hinokitiol inhibited VM formation of mammosphere cells derived from two human breast cancer cell lines. Hinokitiol was additionally indicated to downregulate EGFR protein expression in mammosphere-forming BCSCs without affecting the expression of messenger RNA. The protein stability of EGFR in BCSCs was also decreased by hinokitiol. The EGFR protein expression and VM formation capability of hinokitiol-treated BCSCs were restored by co-treatment with MG132, a proteasome inhibitor. In conclusion, the present study indicated that hinokitiol may inhibit the VM activity of BCSCs through stimulating proteasome-mediated EGFR degradation. Hinokitiol may act as an anti-VM agent, and may be useful for the development of novel breast cancer therapeutic agents.

Slug promoted vasculogenic mimicry in hepatocellular carcinoma

Vasculogenic mimicry (VM) refers to the unique capability of aggressive tumour cells to mimic the pattern of embryonic vasculogenic networks. Epithelial–mesenchymal transition (EMT) regulator slug have been implicated in the tumour invasion and metastasis of human hepatocellular carcinoma (HCC). However, the relationship between slug and VM formation is not clear. In the study, we demonstrated that slug expression was associated with EMT and cancer stem cell (CSCs) phenotype in HCC patients. Importantly, slug showed statistically correlation with VM formation. We consistently demonstrated that an overexpression of slug in HCC cells significantly increased CSCs subpopulation that was obvious by the increased clone forming efficiency in soft agar and by flowcytometry analysis. Meantime, the VM formation and VM mediator overexpression were also induced by slug induction. Finally, slug overexpression lead to the maintenance of CSCs phenotype and VM formation was demonstrated in vivo. Therefore, the results of this study indicate that slug induced the increase and maintenance of CSCs subpopulation and contributed to VM formation eventually. The related molecular pathways may be used as novel therapeutic targets for the inhibition of HCC angiogenesis and metastasis.

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.

HER2/neu expression correlates with vasculogenic mimicry in invasive breast carcinoma

Vasculogenic mimicry (VM) refers to the condition in which tumour cells mimic endothelial cells to form extracellular matrix-rich tubular channels. VM is more extensive in more aggressive tumours. The human epidermal growth factor receptor 2 (HER2) gene is amplified in 20–30% of human breast cancers and has been implicated in mediating aggressive tumour growth and metastasis. However, thus far, there have been no data on the role of HER2 in VM formation. Immunohistochemical and histochemical double-staining methods were performed to display VM in breast cancer specimens. Transfection in MCF7 cells was performed and clones were selected by G418. The three-dimensional Matrigel culture was used to evaluate VM formation in the breast cancer cell line. According to statistical analysis, VM was related to the presence of a positive nodal status and advanced clinical stage. The positive rate of VM increased with increased HER2 expression. In addition, cases with HER2 3+ expression showed significantly greater VM channel count than those in other cases. The exogenous HER2 overexpression in MCF-7 cells induced vessel-like VM structures on the Matrigel and increased the VM mediator vascular endothelial (VE) cadherin. Our data provide evidence for a clinically relevant association between HER2 and VM in human invasive breast cancer. HER2 overexpression possibly induces VM through the up-regulation of VE cadherin. Understanding the key molecular events may provide therapeutic intervention strategies for HER2+ breast cancer.

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.

Mechanisms of resistance to anti-angiogenic therapy and development of third-generation anti-angiogenic drug candidates

The concept of inhibiting tumor neovessels has taken the hurdle from the bench to the bedside and now represents an extra pillar of anticancer treatment. So far, anti-angiogenic therapy prolongs survival in the order of months in some settings while failing to induce a survival benefit in others, in part because of intrinsic refractoriness or evasive escape. This review provides an update on recent mechanisms via which tumor and stromal cells induce resistance and discusses recent evolutions in the (pre)clinical development of novel third-generation anti-angiogenic agents to overcome this problem.

Transdifferentiation of cancer stem cells into endothelial cells

Vasculogenic mimicry was first described as the unique ability of aggressive melanoma cells to express an endothelial phenotype and to form vessel-like networks in three dimensional cultures, “mimicking” the pattern of embryonic vascular networks and recapitulating the patterned networks seen in patients with aggressive tumors correlated with poor prognosis. Recent work shows the occurrence of alternative vasculogenic patterns is due to the presence of stem cell population (cancer stem cells, CSC) at least in melanoma and glioblastoma. In the present review the new perspectives to target vasculogenic mimicry for an anti-vascular treatment strategy and the possible use of AQP1 as target, are discussed. Interest in AQP1 as a target arises from the pivotal role it plays in the organisation of vascular network affecting the cytoskeleton.

Consice review: Contribution of cancer stem cells to neovascularization

Cancer stem cells (CSCs), a special subpopulation of tumor cells, are considered as tumor initiating cells. More recently, these cells have also been identified as initiators of tumor neovascularization. A better understanding of the contribution of CSCs to neovascularization should elucidate the mechanisms of cancer initiation and progression as well as establish new concepts for cancer diagnosis and treatment. In this review, we discuss the evidence for the roles of CSCs in tumor vascularization, including production of proangiogenic factors, transdifferentiation into vascular mural cells such as endothelial and smooth muscle-like cells, and formation of nonendothelium-lined vasculogenic mimicry. In addition, the potential therapeutic significance of targeting CSCs is envisaged.

Glioma stem/progenitor cells contribute to neovascularization via transdifferentiation

OBJECTIVE

Previous studies suggest that tumor cells might be the progenitor for tumor vasculature. Whether vascular tube formation from transdifferentiation of human glioma stem/progenitor cells (hGSPCs) contribute to angiogenesis of gliomas remain largely uncertain.

METHODS

hGSPCs were isolated from thirteen surgical specimens of gliomas and cultured in medium favored for stem cell growth. In vitro transdifferentiation of hGSPCs was performed under hypoxia. Expression of vascular endothelial cells (VECs) markers CD31, CD34, kinase insert domain receptor (KDR), and von Willebrand factor (vWF) were analyzed with real-time quantitative RT-PCR and immunofluorescence techniques. Vasculogenic mimicry of hGSPCs was evaluated in a tumor stem cell xenograft model in vivo. Relationships between content of hGSPCs and expression levels of both VECs markers and proangiogenic factors in large number of clinical specimens were further investigated in glioma tissue microarray.

RESULTS

In vitro, hGSPCs can transdifferentiate into VECs under hypoxia, they manifested typical "flagstone" pattern when cultivated in medium containing VEGF for a few days; when cultivated on Matrigel they were capable of forming capillary-like structures. Expression of VECs markers including CD31, CD34, KDR, and vWF were significantly up-regulated after transdifferentiation. Human leukocyte antigen (HLA) positively stained vessels were observed inside the xenograft tumors after intracerebral transplantation of hGSPCs in athymic nude mice, implied part of tumor cells with human origin were involved in formation of tumor vessels. In surgical specimens of human glioma, tumor vascular cells coexpressing the markers of early VECs (CD34) and markers of hGSPCs (ABCG2 and nestin) suggest that these vascular cells may stemmed from hGSPCs.

CONCLUSIONS

Our observations suggest the functional role of hGSPCs as endothelial progenitors, which have properties that give rise to VECs, and have the ability to form vascular endothelial tubes. However, unspecific markers (ABCG2, nestin) that stain for both endothelial as well as glioma stem cells, were found to be expressed in tumor vasculature of human specimen, and limit further interpretation of this finding.

Tumor-initiating cells and tumor vascularization

Tumor-initiating cells (TICs) with stem-like cell properties initiate and sustain progressive growth, resulting in a heterogeneous tumor mass. The survival and growth of tumors rely on the development of a vasculature to provide nutrients and oxygen. Crosstalk between TICs and vascularization may be one of the central players in the initiation, long-term maintenance, and progression of tumors. This review surveys current evidence concerning the crosstalk that occurs in tumor/stromal interactions, including genetic change, vascular niche, hypoxia, and dormancy of tumors. A better understanding of this crosstalk might help provide the basis for developing more effective therapeutic drug targets.

Endothelial-like cells derived directly from human tumor xenografts

Tumor-associated endothelial cells (TAECs) harboring various genomic abnormalities have been described in human cancers although their origins remain obscure. We generated 4 human cancer cell lines tagged with multiple markers, grew them as xenografts, and characterized their TAECs. Depending on their tumor of origin, 5-40% of TAECs reproducibly expressed all tags. Tagged TAECs (tTAECS) were morphologically, immunologically and functionally similar, although not identical, to normal endothelial cells (ECs) and contained only human chromosomes. tTAECs underwent a senescent-like proliferative arrest after several in vitro passages, but could be immortalized by telomerase, thus allowing us to show that the retention of the EC phenotype was of long-term duration. In contrast, nonimmortalized tTAECs could be propagated in vivo where they incorporated into the tumor neo-vasculature. Although consistent with previous reports that some tumor cells may undergo "vasculogenic mimicry" (VM), the tumor-derived endothelial-like cells described here appear distinctly different. Moreover, their properties and behaviors are more durable than expected for cells undergoing VM, are not the result of fusions between ECs and tumor cells, and are cell autonomous. These findings could have significant implications for therapies that target tumor angiogenesis.

Molecular regulation of vasculogenic mimicry in tumors and potential tumor-target therapy

“Vasculogenic mimicry (VM)”, is a term that describes the unique ability of highly aggressive tumor cells to express a multipotent, stem cell-like phenotype, and form a pattern of vasculogenic-like networks in three-dimensional culture. As an angiogenesis-independent pathway, VM and/or periodic acid-schiff-positive patterns are associated with poor prognosis in tumor patients. Moreover, VM is resistant to angiogenesis inhibitors. Here, we will review the advances in research on biochemical and molecular signaling pathways of VM in tumors and on potential anti-VM therapy strategy.

AQP1 is not only a water channel: It contributes to cell migration through Lin7/beta-catenin

AQPs are water channel proteins. In particular, AQP1 was demonstrated to be involved in cell migration. According to the model proposed by Verkman and collaborators, AQP drives water influx, facilitating lamellipodia extension and cell migration. Investigating the possible connection between AQP1 and cytoskeleton, our group showed that such a water channel through Lin7/beta-catenin affects the organization of the cytoskeleton and proposed a model. All together, these data appear particularly intriguing since the use of AQP1 as target might be useful to modulate angiogenesis/vasculogenic mimicry.

MR molecular imaging of tumor vasculature and vascular targets

Tumor angiogenesis and the ability of cancer cells to induce neovasculature continue to be a fascinating area of research. As the delivery network that provides substrates and nutrients, as well as chemotherapeutic agents to cancer cells, but allows cancer cells to disseminate, the tumor vasculature is richly primed with targets and mechanisms that can be exploited for cancer cure or control. The spatial and temporal heterogeneity of tumor vasculature, and the heterogeneity of response to targeting, make noninvasive imaging essential for understanding the mechanisms of tumor angiogenesis, tracking vascular targeting, and detecting the efficacy of antiangiogenic therapies. With its noninvasive characteristics, exquisite spatial resolution and range of applications, magnetic resonance imaging (MRI) techniques have provided a wealth of functional and molecular information on tumor vasculature in applications spanning from "bench to bedside". The integration of molecular biology and chemistry to design novel imaging probes ensures the continued evolution of the molecular capabilities of MRI. In this review, we have focused on developments in the characterization of tumor vasculature with functional and molecular MRI.