Signalling pathways in vasculogenic mimicry

Vessel Co-option: A Promising Therapeutic Strategy in Oral Squamous Cell Carcinoma

Background Vessel co-option is the mechanism by which cancer cells take over existing blood vessels for their own growth and metabolism. It is often associated with treatment failure and metastasis in tumors of the brain, lung, liver, and skin. Limited studies have highlighted the role of vessel co-option in oral squamous cell carcinoma (OSCC). This study aims to determine the association between vessel co-option in OSCC and the Ki67 labeling index (LI) to ascertain its prognostic role. Methodology In this retrospective study, small biopsy specimens were sent and histopathologically processed to prepare formalin-fixed paraffin-embedded blocks. Only cases diagnosed as OSCC were included in the study. Immunohistochemistry for CD34, p40, and Ki67 was performed. Results The study included 39 OSCC patients. The majority (n = 27; 69.23%) of the patients were in the age group of 31-50 years. The male-to-female ratio was 3:1. Tongue (n = 15; 38.46%) was the most common site, followed by the floor of the mouth (n = 11; 28.21%). The majority (n = 25; 64.10%) of the cases were classified as moderately differentiated squamous cell carcinoma. Microscopically, vessel co-option was noted in 71.79% (n = 28) of the cases. Immunohistochemically, Ki67 LI was >10% in 41.03% (n = 16) of the cases. A statistically significant association was found between Ki67 LI and vessel co-option (p = 0.0013). Conclusions Vessel co-option is a potential determinant of the aggressive potential of OSCC. Histopathological assessment of vessel co-option in conjunction with the Ki67 proliferative index has promising potential for assessing the prognosis of OSCC.

Emerging Paradigms in Cancer Metastasis: Ghost Mitochondria, Vasculogenic Mimicry, and Polyploid Giant Cancer Cells

The capacity of cancer cells to migrate from a primary tumor, disseminate throughout the body, and eventually establish secondary tumors is a fundamental aspect of metastasis. A detailed understanding of the cellular and molecular mechanisms underpinning this multifaceted process would facilitate the rational development of therapies aimed at treating metastatic disease. Although various hypotheses and models have been proposed, no single concept fully explains the mechanism of metastasis or integrates all observations and experimental findings. Recent advancements in metastasis research have refined existing theories and introduced new ones. This review evaluates several novel/emerging theories, focusing on ghost mitochondria (GM), vasculogenic mimicry (VM), and polyploid giant cancer cells (PGCCs).

Crosstalk between Endothelial Cells and Tumor Cells: A New Era in Prostate Cancer Progression

Prostate cancer stands as one of the most prevalent malignancies afflicting men worldwide. The tumor microenvironment plays a pivotal role in tumor progression, comprising various cell types including endothelial cells, tumor-associated fibroblasts, and macrophages. Recent accumulating evidence underscores the indispensable contribution of endothelial cells to prostate cancer development. Both endothelial cells and tumor cells release a multitude of factors that instigate angiogenesis, metastasis, and even drug resistance in prostate cancer. These factors serve as regulators within the tumor microenvironment and represent potential therapeutic targets for managing prostate cancer. In this review, we provide an overview of the crucial functions of endothelial cells in angiogenesis, metastasis, and drug resistance, and their prospective therapeutic applications in combating this disease.

Vasculogenic mimicry score identifies the prognosis and immune landscape of lung adenocarcinoma

Background: Lung cancer has a high incidence and mortality rate worldwide. Vasculogenic mimicry (VM) is a specific modality of tumor angiogenesis that could potentially be a new target for tumor therapy. The purpose of this study was to explore the role of VM-related genes in assessing the prognosis and immune landscape of lung cancer. Methods: VM-related genes were obtained from previous studies, and the expression data and clinical data of lung adenocarcinoma (LUAD) patients were obtained from the TCGA database and GEO database. We performed enrichment analysis of 24 VM-related genes and screened hub genes by constructing a protein-protein interaction network and using Cytoscape software. Subsequently, we developed the VM score based on univariate Cox regression analysis and Lasso analysis and validated the VM score on the GSE72094 dataset. In addition, we constructed a nomogram based on the VM score in the TCGA cohort. Finally, we explored the correlation between the VM score and the tumor microenvironment, immune cell infiltration, immune checkpoints, and drug sensitivity. Results: Enrichment analysis revealed that VM-related genes were associated with the HIF signaling pathway and angiogenic pathway. We developed a VM score based on 3 genes (EPHA2, LAMC2 and LOXL2) in LUAD patients. Kaplan-Meier analysis showed that the VM score was associated with poor prognosis in LUAD patients. The receiver operating characteristic curve suggested that the VM score and nomogram are valid predictors for the overall survival of LUAD patients. The VM score was significantly correlated with immune cell infiltration, such as naïve B cells, neutrophils, and eosinophils, and there was a difference in the TME between the high VM score group and the low VM score group. LUAD patients in the high VM score group were more sensitive to antitumor drugs. Conclusion: In summary, the VM score developed in this study is a valuable indicator for evaluating the prognosis and immune landscape of LUAD patients. VM may be a potential target for antitumor therapy in lung cancer.

VE-Cadherin in Cancer-Associated Angiogenesis: A Deceptive Strategy of Blood Vessel Formation

Tumor growth depends on the vascular system, either through the expansion of blood vessels or novel adaptation by tumor cells. One of these novel pathways is vasculogenic mimicry (VM), which is defined as a tumor-provided vascular system apart from endothelial cell-lined vessels, and its origin is partly unknown. It involves highly aggressive tumor cells expressing endothelial cell markers that line the tumor irrigation. VM has been correlated with high tumor grade, cancer cell invasion, cancer cell metastasis, and reduced survival of cancer patients. In this review, we summarize the most relevant studies in the field of angiogenesis and cover the various aspects and functionality of aberrant angiogenesis by tumor cells. We also discuss the intracellular signaling mechanisms involved in the abnormal presence of VE-cadherin (CDH5) and its role in VM formation. Finally, we present the implications for the paradigm of tumor angiogenesis and how targeted therapy and individualized studies can be applied in scientific analysis and clinical settings.

Vandetanib alters the tumoricidal capacity of human breast cancer stem cells via inhibiting vasculogenic capacity

Introduction

The inhibition of vascularization into tumor stroma as well as dynamic cell growth is the center of attention. Here, we aimed to examine the role of vandetanib on angiogenesis capacity of breast cancer stem cell (CSCs).

Methods

MDA-MB-231 cells were exposed to different doses of vandetanib and survival rate was monitored. Stimulatory effects of vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and epidermal growth factor (EGF) were evaluated in vandetanib-treated MDA-MB-231 cells. In vitro tubulogenesis capacity was studied on the Matrigel surface. The synergistic effects of vandetanib on cell survival were also assessed after PI3K and/or Wnt3a inhibition. Vascular endothelial (VE)-cadherin, matrix metalloproteinase-2 (MMP-2), -9, Wnt3a, and p-Akt/Akt ratio were measured using western blotting.

Results

Vandetanib reduced survival rate in a dose-dependent manner (P<0.05). Proliferative effects associated with VEGF, FGF, and EGF were blunted in these cells pre-exposed to vandetanib (P<0.05). The microcirculation pattern's triple-negative breast cancer (TNBC) was suppressed by 1, 5 µM of vandetanib (P<0.05). Hence 1, 5 µM of vandetanib potentially decreased the population of CD24- cells. 1 and 5 µM of vandetanib inhibited cell proliferation by blocking PI3K and Wnt3a pathways and decreased the p-Akt/Akt ratio, Wnta3 protein levels (P<0.05). 1 and 5 µM vandetanib combined with PI3K inhibitor diminished metastatic markers including, MMP-2, and MMP-9. The concurrent treatment (PI3K, inhibitor+ 1, 5 µM vandetanib) also considerably reduced epithelial-mesenchymal transition (EMT) markers such as VE-cadherin (P<0.05).

Conclusion

Vandetanib suppressed vasculogenic mimicry (VM) networking through blunting stemness properties, coincided with suppression of VE-cadherin in CSCs.

STAT3-EMT axis in tumors: Modulation of cancer metastasis, stemness and therapy response

Epithelial-to-mesenchymal transition (EMT) mechanism is responsible for metastasis of tumor cells and their spread to various organs and tissues of body, providing undesirable prognosis. In addition to migration, EMT increases stemness and mediates therapy resistance. Hence, pathways involved in EMT regulation should be highlighted. STAT3 is an oncogenic pathway that can elevate growth rate and migratory ability of cancer cells and induce drug resistance. The inhibition of STAT3 signaling impairs cancer progression and promotes chemotherapy-mediated cell death. Present review focuses on STAT3 and EMT interaction in modulating cancer migration. First of all, STAT3 is an upstream mediator of EMT and is able to induce EMT-mediated metastasis in brain tumors, thoracic cancers and gastrointestinal cancers. Therefore, STAT3 inhibition significantly suppresses cancer metastasis and improves prognosis of patients. EMT regulators such as ZEB1/2 proteins, TGF-β, Twist, Snail and Slug are affected by STAT3 signaling to stimulate cancer migration and invasion. Different molecular pathways such as miRNAs, lncRNAs and circRNAs modulate STAT3/EMT axis. Furthermore, we discuss how STAT3 and EMT interaction affects therapy response of cancer cells. Finally, we demonstrate targeting STAT3/EMT axis by anti-tumor agents and clinical application of this axis for improving patient prognosis.

Vitamin D Exerts Significant Antitumor Effects by Suppressing Vasculogenic Mimicry in Breast Cancer Cells

Background

Numerous clinical and experimental observations have alluded to the substantial anti-neoplastic role of vitamin D in breast cancer (BC), primarily by inducing apoptosis and affecting metastasis. Tumor progression and resistance to chemotherapy have been linked to vasculogenic mimicry (VM), which represents the endothelial-independent formation of microvascular channels by cancer cells. However, the effect of vitamin D on VM formation in BC has not been thoroughly investigated. This study examined the impact of 1α,25-dihydroxyvitamin D3 (calcitriol), the active form of vitamin D, on the expression of major factors involved in BC migration, invasion, and VM formation.

Experimental Methods

Publicly available transcriptomic datasets were used to profile the expression status of the key VM markers in vitamin D-treated BC cells. The in silico data were validated by examining the expression and activity of the key factors that are involved in tumor progression and MV formation in hormone-positive MCF-7 and aggressive triple‐negative MDA-MB-231 BC cells after treatment with calcitriol.

Results and Discussions

The bioinformatics analysis showed that tumor VM formation-enriched pathways were differentially downregulated in vitamin D-treated cells when compared with control counterparts. Treatment of BC cells with calcitriol resulted in increased expression of tissue inhibitors of metalloproteinases (TIMPs 1 and 2) and decreased content and gelatinolytic activity of matrix metalloproteinases (MMPs 2 and 9). Furthermore, calcitriol treatment reduced the expression of several pro-MV formation regulators including vascular endothelial growth factor (VEGF), tumor growth factor (TGF-β1), and amphiregulin. Eventually, this process resulted in a profound reduction in cell migration and invasion following the treatment of BC cells with calcitriol when compared to the controls. Finally, the formation of VM was diminished in the aggressive triple‐negative MDA-MB-231 cancer cell line after calcitriol treatment.

Conclusion

Our findings demonstrate that vitamin D mediates its antitumor effects in BC cells by inhibiting and curtailing their potential for VM formation.

WISP2/CCN5 Suppresses Vasculogenic Mimicry through Inhibition of YAP/TAZ Signaling in Breast Cancer Cells

Simple Summary

Breast cancer is the most frequent malignancy in women worldwide. Advanced breast cancer with distant organ metastases is considered incurable with currently available therapies. The vasculogenic mimicry (VM) process is associated with an invasive and metastatic cancer phenotype and a poor prognosis for human breast cancer patients. Our aim was to study the effect of WISP2, a matricellular protein, on VM. We found that WISP2 inhibits VM through inhibition of CYR61 protein expression and YAP-TAZ signaling. Our finding may open promising candidates for blocking VM in breast cancer.

Abstract

Vasculogenic mimicry (VM) formed by aggressive tumor cells to create vascular networks connected with the endothelial cells, plays an important role in breast cancer progression. WISP2 has been considered as a tumor suppressor protein; however, the relationship between WISP2 and VM formation remains unclear. We used the in vitro tube formation assay and in vivo immunohistochemical analysis in a mouse model, and human breast tumors were used to evaluate the effect of WISP2 on VM formation. Here we report that WISP2 acts as a potent inhibitor of VM formation in breast cancer. Enforced expression of WISP2 decreased network formation while knockdown of WISP2 increased VM. Mechanistically, WISP2 increased retention of oncogenic activators YAP/TAZ in cytoplasm, leading to decreased expression of the angiogenic factor CYR61. Studies using an in vivo mouse model and human breast tumors confirmed the in vitro cell lines data. In conclusion, our results indicate that WISP2 may play a critical role in VM and highlight the critical role of WISP2 as a tumor suppressor.

Angiogenesis in patient-derived xenografts of odontogenic myxoma

Previously, by employing 3D organotypic tissue culture and patient-derived xenograft (PDX) model, oral myxoma response to a MAPK/MEK inhibitor was observed. Gross examination of the tumour fragments obtained after 55 days of PDX grafting revealed increased capsule vascularization. Microscopic analyses showed blood capillaries intermixed with myxoma cells, but the origin of these capillaries, from mice or humans, was not established. This study aimed to investigate whether the endothelial cells observed in the myxoma PDX model are derived from the mouse or from the primary human tumour. Immunohistochemistry was performed on five tumour fragments from the PDX of myxoma after 55 days of implantation in mice. Immunopositivity for antibodies against human (HLA-ABC) and mouse (H2 Db/H2-D1) major histocompatibility complex class I (MHCI) was assessed in the endothelial cells. The endothelial cells in the PDX fragments revealed a membrane staining for the human MHCI protein in the PDX tumour and adjacent connective tissue capsule, indicating that capillaries were derived from the human tumour fragment. Considering the probable human origin of the endothelial cells from capillary blood vessels in the myxoma PDX, we conclude that this PDX model is an interesting model to study myxoma angiogenesis.

Vasculogenic Mimicry-An Overview

Vasculogenic mimicry (VM), a tumor microcirculation model found in melanoma in the last 20 years, is a vascular channel-like structure composed of tumor cells, but without endothelial cells, that stains positive for periodic acid-Schiff (PAS) and negative staining for CD31. VM provides, to the highly aggressive malignant tumor cells, adequate oxygen and nutrient supply for tumor growth and subsequent metastasis process and its presence are related to poor prognosis in patients. VM is independent of endothelial cells, which may partly explain why angiogenesis drug inhibitors have not achieved the expected success for cancer treatment.

Effect of Apatinib plus melatonin on vasculogenic mimicry formation by cancer stem cells from breast cancer cell line

BACKGROUND AND AIM

Vasculogenic mimicry (VM) is one of the most important causes of breast cancer metastasis and resistance against drugs. The cancer stem cells (CSCs) are known as essential factors for VM formation. In this study, the effects of melatonin, Apatinib, and a combination of Apatinib/melatonin on VM formation were investigated by breast CSCs from breast cancer cell line.

MATERIALS AND METHODS

The percentage of CSCs was determined in two breast cancer cell lines (MCF-7 and MDA-MB-231) by flow cytometry. The effects of Apatinib, melatonin, and a combination of Apatinib/melatonin were evaluated on proliferation and viability, migration and invasion, apoptosis, and VM formation in MDA-MB-231 cells. Moreover, expression levels of the involved proteins in cancer cell proliferation and viability, CSCs, migration and invasion, and VM formation were evaluated by real-time polymerase chain reaction (RT-PCR) and western blotting methods.

RESULTS

Results of the present study showed that melatonin and Apatinib reduced survival rate of CSCs in a dose- and time-dependent manner. Apatinib, melatonin, and a combination of Apatinib/melatonin inhibited proliferation of breast CSCs (P ≤ 0.001). Formation of VM was decreased in the MDA-MB-231 cancer cell line treated with Apatinib and combination of Apatinib/melatonin. Apatinib and combination of Apatinib/melatonin reduced invasion of breast CSCs (P ≤ 0.0001). Expression of vascular endothelial VE-cadherin, ephrinA2 receptor (EPHA2), p-PI3K/phosphoinositide-3 kinase (PI3K) and phospho-AKT (p-AKT)/AKT ratios was decreased under the influence of Apatinib and a combination of Apatinib/melatonin (P ≤ 0.01).

CONCLUSION

Apatinib or a combination of Apatinib/melatonin may be used to manage patients with breast cancer. However, further studies are needed to identify anti-cancer mechanisms of melatonin and Apatinib for better management of the patients with breast cancer.

The emerging roles of circular RNAs in vessel co-option and vasculogenic mimicry: clinical insights for anti-angiogenic therapy in cancers

Unexpected resistance to anti-angiogenic treatment prompted the investigation of non-angiogenic tumor processes. Vessel co-option (VC) and vasculogenic mimicry (VM) are recognized as primary non-angiogenic mechanisms. In VC, cancer cells utilize pre-existing blood vessels for support, whereas in VM, cancer cells channel and provide blood flow to rapidly growing tumors. Both processes have been implicated in the development of tumor and resistance to anti-angiogenic drugs in many tumor types. The morphology, but rare molecular alterations have been investigated in VC and VM. There is a pressing need to better understand the underlying cellular and molecular mechanisms. Here, we review the emerging circular RNA (circRNA)-mediated regulation of non-angiogenic processes, VC and VM.

Anti-angiogenic agents - overcoming tumour endothelial cell anergy and improving immunotherapy outcomes

Immune checkpoint inhibitors have revolutionized medical oncology, although currently only a subset of patients has a response to such treatment. A compelling body of evidence indicates that anti-angiogenic therapy has the capacity to ameliorate antitumour immunity owing to the inhibition of various immunosuppressive features of angiogenesis. Hence, combinations of anti-angiogenic agents and immunotherapy are currently being tested in >90 clinical trials and 5 such combinations have been approved by the FDA in the past few years. In this Perspective, we describe how the angiogenesis-induced endothelial immune cell barrier hampers antitumour immunity and the role of endothelial cell anergy as the vascular counterpart of immune checkpoints. We review the antitumour immunity-promoting effects of anti-angiogenic agents and provide an update on the current clinical successes achieved when these agents are combined with immune checkpoint inhibitors. Finally, we propose that anti-angiogenic agents are immunotherapies - and vice versa - and discuss future research priorities.

A bi-directional dialog between vascular cells and monocytes/macrophages regulates tumor progression

Cancer progression largely depends on tumor blood vessels as well on immune cell infiltration. In various tumors, vascular cells, namely endothelial cells (ECs) and pericytes, strongly regulate leukocyte infiltration into tumors and immune cell activation, hence the immune response to cancers. Recently, a lot of compelling studies unraveled the molecular mechanisms by which tumor vascular cells regulate monocyte and tumor-associated macrophage (TAM) recruitment and phenotype, and consequently tumor progression. Reciprocally, TAMs and monocytes strongly modulate tumor blood vessel and tumor lymphatic vessel formation by exerting pro-angiogenic and lymphangiogenic effects, respectively. Finally, the interaction between monocytes/TAMs and vascular cells is also impacting several steps of the spread of cancer cells throughout the body, a process called metastasis. In this review, the impact of the bi-directional dialog between blood vascular cells and monocytes/TAMs in the regulation of tumor progression is discussed. All together, these data led to the design of combinations of anti-angiogenic and immunotherapy targeting TAMs/monocyte whose effects are briefly discussed in the last part of this review.

The Role of Anti-angiogenesis in the Treatment Landscape of Non-small Cell Lung Cancer - New Combinational Approaches and Strategies of Neovessel Inhibition

Tumor progression depends primarily on vascular supply, which is facilitated by angiogenic activity within the malignant tissue. Non-small cell lung cancer (NSCLC) is a highly vascularized tumor, and inhibition of angiogenesis was projected to be a promising therapeutic approach. Over a decade ago, the first anti-angiogenic agents were approved for advanced stage NSCLC patients, however, they only produced a marginal clinical benefit. Explanations why anti-angiogenic therapies only show modest effects include the highly adaptive tumor microenvironment (TME) as well as the less understood characteristics of the tumor vasculature. Today, advanced methods of in-depth characterization of the NSCLC TME by single cell RNA sequencing (scRNA-Seq) and preclinical observations enable a detailed characterization of individual cancer landscapes, allowing new aspects for a more individualized inhibition of angiogenesis to be identified. Furthermore, the tumor vasculature itself is composed of several cellular subtypes, which closely interact with other cellular components of the TME, and show distinct biological functions such as immune regulation, proliferation, and organization of the extracellular matrix. With these new insights, combinational approaches including chemotherapy, anti- angiogenic and immunotherapy can be developed to yield a more target-oriented anti-tumor treatment in NSCLC. Recently, anti-angiogenic agents were also shown to induce the formation of high endothelial venules (HEVs), which are essential for the formation of tertiary lymphoid structures, and key components in triggering anti-tumor immunity. In this review, we will summarize the current knowledge of tumor-angiogenesis and corresponding anti-angiogenic therapies, as well as new aspects concerning characterization of tumor-associated vessels and the resulting new strategies for anti-angiogenic therapies and vessel inhibition in NSCLC. We will further discuss why anti-angiogenic therapies form an interesting backbone strategy for combinational therapies and how anti-angiogenic approaches could be further developed in a more personalized tumor-oriented fashion with focus on NSCLC.

Effects of miR‑93 on epithelial‑to‑mesenchymal transition and vasculogenic mimicry in triple‑negative breast cancer cells

Triple‑negative breast cancer (TNBC) is characterized by strong invasiveness, frequent local recurrence and distant metastasis, with poor prognosis. According to tumor angiogenesis theory, tumor cells can obtain blood supply not only by fusing with host blood vessels, but also by constructing a new vascular system through angiogenesis, so as to continuously obtain nutrients and oxygen supply; this is called vasculogenic mimicry (VM). In our previous study, differential expression profiles of miRNAs were examined with gene chip in TNBC and corresponding paracancer tissues, which demonstrated significant up‑regulation of microRNA (miR)‑93. Bioinformatics found that the target genes of miR‑93 were associated with cell proliferation, invasion and migration. The present study investigated the association between miR‑93, epithelial‑to‑mesenchymal transition (EMT) and VM formation in TNBC cell lines. The results indicated that miR‑93 depletion suppressed MDA‑MB‑231 cell viability, invasion and migration (P<0.001). In addition, knockdown of miR‑93 significantly upregulated the expression levels of EMT‑associated genes such as E‑cadherin and occludin, but downregulated the expression levels of vimentin and N‑cadherin in MDA‑MB‑231 cells. VM formation assay showed a significant decrease in microtubule‑forming ability of cells following miR‑93 knockdown, which was associated with the occurrence of EMT, suggesting that miR‑93 may promote the formation of VM via EMT and may be a therapeutic target for the treatment of TNBC.

A novel definition of microvessel density in renal cell carcinoma: Angiogenesis plus vasculogenic mimicry

The present study proposed the novel concept of total microvessel density (TMVD), which is the combination of the MVD and the vasculogenic mimicry (VM) status, and evaluated its clinical significance in patients with renal cell carcinoma (RCC). For that purpose, tumor samples from 183 patients with primary RCC were examined by CD34 single or periodic acid Schiff (PAS)/CD34 dual histology staining. MVD and VM were determined according to previous literature. Clinical information (tumor stage and grade, and duration of survival) was retrieved and analyzed. Survival information and VM-associated gene expression data of patients with RCC were also retrieved from The Cancer Genome Atlas (TCGA) database and the clinical significance of each individual gene was analyzed. The results indicated that MVD exhibited obvious differences among patients with RCC; however, it was not correlated with the stage/grade or length of survival in patients with RCC. In total, 81 patients (44.3%) were CD34(−)/PAS(+) and defined as VM(+), and they had a significantly shorter survival compared with that of VM(−) patients (P=0.0002). VM was not associated with MVD. TMVD was able to distinguish between patients with high and low MVD in terms of survival, thus TMVD was better compared with MVD alone at distinguishing between patients with different survival prognoses. TCGA data analysis revealed that among the VM-associated genes, nodal growth differentiation factor, caspase-3, matrix metalloproteinase-9 and galectin-3 had a statistically significant impact on the overall/disease-free survival of patients with RCC. In conclusion, the TMVD concept may be more appropriate and sensitive compared with the MVD or VM alone in predicting tumor aggressiveness and patient survival, particularly in RCC, which is a highly vascularized, VM-rich neoplasm, and certain VM formation-associated genes are negatively associated with the survival of patients with RCC.

Vascular mimicry: Triggers, molecular interactions and in vivo models

Vascular mimicry is induced by a wide array of genes with functions related to cancer stemness, hypoxia, angiogenesis and autophagy. Vascular mimicry competent (VM-competent) cells that form de novo blood vessels are common in solid tumors facilitating tumor cell survival and metastasis. VM-competent cells display increased levels of vascular mimicry selecting for stem-like cells in an O₂-gradient-dependent manner in deeply hypoxic tumor regions, while also aiding in maintaining tumor cell metabolism and stemness. Three of the principal drivers of vascular mimicry are EphA2, Nodal and HIF-1α, however, directly or indirectly many of these molecules affect VE-Cadherin (VE-Cad), which forms gap-junctions to bind angiogenic blood vessels together. During vascular mimicry, the endothelial-like functions of VM-competent cancer stem cells co-opt VE-Cad to bind cancer cells together to create cancer cell-derived blood conducting vessels. This process potentially compensates for the lack of access to blood and nutrient in avascular tumors, simultaneously providing nutrients and enhancing cancer invasion and metastasis. Current evidence also supports that vascular mimicry promotes cancer malignancy and metastasis due to the cooperation of oncogenic signaling molecules driving cancer stemness and autophagy. While a number of currently used cancer therapeutics are effective inhibitors of vascular mimicry, developing a new class of vascular mimicry specific inhibitors could allow for the treatment of angiogenesis-resistant tumors, inhibit cancer metastasis and improve patient survival. In this review, we describe the principal vascular mimicry pathways in addition to emphasizing the roles of hypoxia, autophagy and select proangiogenic oncogenes in this process.

Inhibitory effect of melatonin on hypoxia-induced vasculogenic mimicry via suppressing epithelial-mesenchymal transition (EMT) in breast cancer stem cells

Vasculogenic mimicry (VM) play an important role in breast cancer metastasis and anti- angiogenic drugs resistance. Hypoxia, the epithelial-mesenchymal transition (EMT), and cancer stem cells (CSCs) are known as essential factors for VM formation. Also, melatonin is an amino acid-derived hormone with many anti-tumor effects. Despite the antitumor effects of melatonin, its effect on VM formation in breast cancer has not been considered yet, so we investigated the effect of melatonin on VM formation through EMT process under hypoxia conditions in breast CSCs. The CSCs percentage and VM formation were determined in MCF-7 and MDA-MB-231, respectively. Also, analysis of HIF-1α expression under hypoxia in MDA-MB-231 and MCF-7 cell lines was performed using Western blot. The effect of melatonin on the VM formation, invasion, and migration was also investigated. Moreover, the effect of melatonin on the expression EMT markers was evaluated. CD44⁺ CD24^-phenotype as CSCs marker in MDA-MB-231 cell line, was 80.8%, while it was 11.1% in MCF-7 cell line. HIF-1α expression was up-regulated in the VM-positive breast cancer cell line MDA-MB-231, and consequently, affected the expression of the EMT markers E-cadherin, vimentin, snail, and MMP9. Melatonin had significant effect on EMT and formations of VM in breast CSCs. Melatonin could prevent the formation of VM by affecting the important molecules involved in the formation of VM structures and the EMT. Moreover, our data clearly showed that, melatonin represents molecule with significant anti-cancer activities that may potentially optimize the management of breast cancer through the overcoming drug resistance in anti-angiogenic drugs.

Resistance Mechanisms to Anti-angiogenic Therapies in Cancer

Tumor growth and metastasis rely on tumor vascular network for the adequate supply of oxygen and nutrients. Tumor angiogenesis relies on a highly complex program of growth factor signaling, endothelial cell (EC) proliferation, extracellular matrix (ECM) remodeling, and stromal cell interactions. Numerous pro-angiogenic drivers have been identified, the most important of which is the vascular endothelial growth factor (VEGF). The importance of pro-angiogenic inducers in tumor growth, invasion and extravasation make them an excellent therapeutic target in several types of cancers. Hence, the number of anti-angiogenic agents developed for cancer treatment has risen over the past decade, with at least eighty drugs being investigated in preclinical studies and phase I-III clinical trials. To date, the most common approaches to the inhibition of the VEGF axis include the blockade of VEGF receptors (VEGFRs) or ligands by neutralizing antibodies, as well as the inhibition of receptor tyrosine kinase (RTK) enzymes. Despite promising preclinical results, anti-angiogenic monotherapies led only to mild clinical benefits. The minimal benefits could be secondary to primary or acquired resistance, through the activation of alternative mechanisms that sustain tumor vascularization and growth. Mechanisms of resistance are categorized into VEGF-dependent alterations, non-VEGF pathways and stromal cell interactions. Thus, complementary approaches such as the combination of these inhibitors with agents targeting alternative mechanisms of blood vessel formation are urgently needed. This review provides an updated overview on the pathophysiology of angiogenesis during tumor growth. It also sheds light on the different pro-angiogenic and anti-angiogenic agents that have been developed to date. Finally, it highlights the preclinical evidence for mechanisms of angiogenic resistance and suggests novel therapeutic approaches that might be exploited with the ultimate aim of overcoming resistance and improving clinical outcomes for patients with cancer.

An Overview of Vasculogenic Mimicry in Breast Cancer

Vasculogenic mimicry (VM) is the formation of vascular channels lacking endothelial cells. These channels are lined by tumor cells with cancer stem cell features, positive for periodic acid-Schiff, and negative for CD31 staining. The term VM was introduced by Maniotis et al. (1), who reported this phenomenon in highly aggressive uveal melanomas; since then, VM has been associated with poor prognosis, tumor aggressiveness, metastasis, and drug resistance in several tumors, including breast cancer. It is proposed that VM and angiogenesis (the de novo formation of blood vessels from the established vasculature by endothelial cells, which is observed in several tumors) rely on some common mechanisms. Furthermore, it is also suggested that VM could constitute a means to circumvent anti-angiogenic treatment in cancer. Therefore, it is important to determinant the factors that dictate the onset of VM. In this review, we describe the current understanding of VM formation in breast cancer, including specific signaling pathways, and cancer stem cells. In addition, we discuss the clinical significance of VM in prognosis and new opportunities of VM as a target for breast cancer therapy.

Forskolin promotes vasculogenic mimicry and invasion via Notch‑1‑activated epithelial‑to‑mesenchymal transition in syncytiolization of trophoblast cells in choriocarcinoma

Choriocarcinoma (CC) is characterized by earlier blood metastasis compared with other female genital tumors and a high incidence of massive hemorrhage. Vasculogenic mimicry (VM) is highly associated with metastasis, and syncytiotrophoblast is involved in the formation of VM in CC. Forskolin is a typical activator of the cAMP pathway, which is involved in the syncytiolization of trophoblastic cells. In the present study, to determine the effects and mechanism of forskolin on cell invasion and VM during syncytiolization in vitro and in vivo, JEG-3 and JAR cell lines were treated with 100 µM forskolin for 48 h, and wound healing and invasion assays were used to verify cell migratory and invasive capacities. A 3D culture and tube formation assays were established to detect VM. Variation of morphology and markers of the epithelial-to-mesenchymal transition (EMT) were assessed, and the role of the Notch signaling pathway was investigated in CC cells treated with forskolin. The results of the present study demonstrated that 100 µM forskolin induced syncytiolization of trophoblastic cells and enhanced the migratory and invasive abilities of JEG-3 and JAR cell lines. In addition, the capacity of VM was significantly increased, whereas tube formation ability was decreased by forskolin in vitro and in vivo compared with the respective control groups. The cellular morphology exhibited EMT during the syncytiolization process, which was further supported by the changes in EMT marker expression, including downregulation of E-cadherin and cytokeratin and upregulation of N-cadherin, vimentin and zinc finger E-box-binding homeobox 1. The Notch-1 signaling pathway was activated to induce EMT in forskolin-induced VM process in CC cells, and VM and EMT could be reversed by using the γ-secretase inhibitor DAPT to block the Notch-1 pathway. Overall, the results of the present study demonstrated that forskolin enhanced the capacity of VM formation and metastasis through Notch-1-activated EMT in the syncytiolization of trophoblastic cells.

Celastrol Suppresses Glioma Vasculogenic Mimicry Formation and Angiogenesis by Blocking the PI3K/Akt/mTOR Signaling Pathway

Angiogenesis and vasculogenic mimicry (VM) are thought to be the predominant processes ensuring tumor blood supply during the growth and metastasis of glioblastoma (GBM). Celastrol has potential anti-glioma effects, however the mechanisms underlying these effects remain unclarified. Recent studies have shown that the PI3K/Akt/mTOR signaling pathway is closely related to angiogenesis and VM formation. In the present study, we have demonstrated, for the first time, that celastrol eliminated VM formation by blocking this signaling pathway in glioma cells. By the treatment of celastrol, tumor growth was suppressed, tight junction and basal lamina structures in tumor microvasculature were disarranged in U87 glioma orthotopic xenografts in nude mice. Periodic acid Schiff (PAS)-CD31 staining revealed that celastrol inhibited both VM and angiogenesis in tumor tissues. Additionally, celastrol reduced the expression levels of the angiogenesis-related proteins CD31, vascular endothelial growth factor receptor (VEGFR) 2, angiopoietin (Ang) 2 and VEGFA, VM-related proteins ephrin type-A receptor (EphA) 2, and vascular endothelial (VE)-cadherin. Hypoxia inducible factor (HIF)-1α, phosphorylated PI3K, Akt, and mTOR were also downregulated by treatment with celastrol. In vitro, we further demonstrated that celastrol inhibited the growth, migration, and invasion of U87 and U251 cells, disrupted VM formation, and blocked the activity of PI3K, Akt, and mTOR. Collectively, our data suggest that celastrol inhibits VM formation and angiogenesis likely by regulating the PI3K/Akt/mTOR signaling pathway.

Targeting vasculogenic mimicry by phytochemicals: A potential opportunity for cancer therapy

Vasculogenic mimicry (VM) is regarded as a process where very aggressive cancer cells generate vascular-like patterns without the presence of endothelial cells. It is considered as the main mark of malignant cancer and has pivotal role in cancer metastasis and progression in various types of cancers. On the other hand, resistance to the antiangiogenesis therapies leads to the cancer recurrence. Therefore, development of novel chemotherapies and their combinations is urgently needed for abolition of VM structures and also for better tumor therapy. Hence, identifying compounds that target VM structures might be superior therapeutic factors for cancers treatment and controlling the recurrence and metastasis. In recent times, naturally occurring compounds, especially phytochemicals have obtained great attention due to their safe properties. Phytochemicals are also capable of targeting VM structure and also their main signaling pathways. Consequently, in this review article, we illustrated key signaling pathways in VM, and the phytochemicals that affect these structures including curcumin, genistein, lycorine, luteolin, columbamine, triptolide, Paris polyphylla, dehydroeffusol, jatrorrhizine hydrochloride, grape seed proanthocyanidins, resveratrol, isoxanthohumol, dehydrocurvularine, galiellalactone, oxacyclododecindione, brucine, honokiol, ginsenoside Rg3, and norcantharidin. The recognition of these phytochemicals and their safety profile may lead to new therapeutic agents' development for VM elimination in different types of tumors.

Epigallocatechin-3-Gallate Suppresses Vasculogenic Mimicry through Inhibiting the Twist/VE-Cadherin/AKT Pathway in Human Prostate Cancer PC-3 Cells

Vasculogenic mimicry (VM) is the alternative process of forming vessel-like networks by aggressive tumor cells, and it has an important role in tumor survival, growth, and metastasis. Epigallocatechin-3-gallate (EGCG) is well known to have diverse bioactivities including anti-cancer effects. However, the efficacy of EGCG on VM is elusive. In this study, we explored whether and how EGCG affects VM in human prostate cancer (PCa) PC-3 cells. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Invasive and VM formation abilities were assessed by an invasion assay and a three-dimensional (3D) culture VM tube formation assay, respectively. Western blots were carried out. An immunofluorescence assay was performed to detect nuclear twist expression. EGCG effectively inhibited the invasive ability, as well as tubular channel formation, without affecting cell viability. EGCG significantly downregulated the expression of vascular endothelial cadherin (VE-cadherin) and its transcription factor, twist, N-cadherin, vimentin, phosphor-AKT, and AKT, but not phospho-erythropoietin-producing hepatocellular receptor A2 (EphA2) and EphA2. In addition, EGCG diminished the nuclear localization of twist. Treatment with SC79, an AKT activator, effectively rescued EGCG-inhibited VM formation. These results demonstrated for the first time that EGCG causes marked suppression of VM through inhibiting the twist/VE-cadherin/AKT pathway in human PCa PC-3 cells.

The role of vascular mimicry as a biomarker in malignant melanoma: a systematic review and meta-analysis

BACKGROUND

Vasculogenic mimicry (VM) a microvascular system consisting of non-endothelial cells that is newly formed by aggressive tumors, has been proposed as an important therapeutic target in malignant melanoma (MM). We performed a systematic literature review to evaluate the diagnostic and prognostic accuracy of VM status for overall survival of MM patients.

METHODS

The quality of the included studies was evaluated using the QUADAS-2 tool. Diagnostic capacity of VM variables, including sensitivity, specificity, positive likelihood ratio (PLR), negative likelihood ratio (NLR), diagnostic odds ratio (DOR), and the area under summary receiver operating characteristic (SROC), were pooled using Meta-DiSc software.

RESULTS

A retrospective observational study was conducted based on twelve clinical studies including 978 clinically confirmed melanoma patients with proportion (P). VM+ melanoma cells were associated with poor prognosis in 38% of MM group (P = 0.35, 95% confidence intervals (CI): 0.27-0.42, p < 0.001). The pooled sensitivity and specificity were 0.82 (95% CI: 0.79-0.84) and 0.69 (95% CI: 0.66-0.71), respectively. Furthermore, the pooled PLR, NLR, and DOR were 2.56 (95% CI: 1.94-3.93), 0.17 (95% CI: 0.07-0.42), and 17.75 (95% CI: 5.30-59.44), respectively. Furthermore, the AUC of SROC was 0.63, indicating high reliability of VM status as a biomarker. Importantly, subgroup results suggested that VM+ status is a significantly accurate prognostic biomarker when diagnosed by the CD31-/PAS+ staining methods in Asian MM samples (p < 0.001).

CONCLUSIONS

Our findings support the potential of VM status of tumors as a promising prognostic biomarker and emphasize an effective adjuvant therapeutic strategy in the prognosis of Asian MM patients.

Calcium Mobilization in Endothelial Cell Functions

Endothelial cells (ECs) constitute the innermost layer that lines all blood vessels from the larger arteries and veins to the smallest capillaries, including the lymphatic vessels. Despite the histological classification of endothelium of a simple epithelium and its homogeneous morphological appearance throughout the vascular system, ECs, instead, are extremely heterogeneous both structurally and functionally. The different arrangement of cell junctions between ECs and the local organization of the basal membrane generate different type of endothelium with different permeability features and functions. Continuous, fenestrated and discontinuous endothelia are distributed based on the specific function carried out by the organs. It is thought that a large number ECs functions and their responses to extracellular cues depend on changes in intracellular concentrations of calcium ion ([Ca²⁺]_i). The extremely complex calcium machinery includes plasma membrane bound channels as well as intracellular receptors distributed in distinct cytosolic compartments that act jointly to maintain a physiological [Ca²⁺]_i, which is crucial for triggering many cellular mechanisms. Here, we first survey the overall notions related to intracellular Ca²⁺ mobilization and later highlight the involvement of this second messenger in crucial ECs functions with the aim at stimulating further investigation that link Ca²⁺ mobilization to ECs in health and disease.

Defining the Hallmarks of Metastasis

Metastasis is the primary cause of cancer morbidity and mortality. The process involves a complex interplay between intrinsic tumor cell properties as well as interactions between cancer cells and multiple microenvironments. The outcome is the development of a nearby or distant discontiguous secondary mass. To successfully disseminate, metastatic cells acquire properties in addition to those necessary to become neoplastic. Heterogeneity in mechanisms involved, routes of dissemination, redundancy of molecular pathways that can be utilized, and the ability to piggyback on the actions of surrounding stromal cells makes defining the hallmarks of metastasis extraordinarily challenging. Nonetheless, this review identifies four distinguishing features that are required: motility and invasion, ability to modulate the secondary site or local microenvironments, plasticity, and ability to colonize secondary tissues. By defining these first principles of metastasis, we provide the means for focusing efforts on the aspects of metastasis that will improve patient outcomes.

HypoxamiRs Profiling Identify miR-765 as a Regulator of the Early Stages of Vasculogenic Mimicry in SKOV3 Ovarian Cancer Cells

Vasculogenic mimicry (VM) is a novel cancer hallmark in which malignant cells develop matrix-associated 3D tubular networks with a lumen under hypoxia to supply nutrients needed for tumor growth. Recent studies showed that microRNAs (miRNAs) may have a role in VM regulation. In this study, we examined the relevance of hypoxia-regulated miRNAs (hypoxamiRs) in the early stages of VM formation. Data showed that after 48 h hypoxia and 12 h incubation on matrigel SKOV3 ovarian cancer cells undergo the formation of matrix-associated intercellular connections referred hereafter as 3D channels-like structures, which arose previous to the apparition of canonical tubular structures representative of VM. Comprehensive profiling of 754 mature miRNAs at the onset of hypoxia-induced 3D channels-like structures showed that 11 hypoxamiRs were modulated (FC>1.5; p < 0.05) in SKOV3 cells (9 downregulated and 2 upregulated). Bioinformatic analysis of the set of regulated miRNAs showed that they might impact cellular pathways related with tumorigenesis. Moreover, overall survival analysis in a cohort of ovarian cancer patients (n = 485) indicated that low miR-765, miR-193b, miR-148a and high miR-138 levels were associated with worst patients outcome. In particular, miR-765 was severely downregulated after hypoxia (FC < 32.02; p < 0.05), and predicted to target a number of protein-encoding genes involved in angiogenesis and VM. Functional assays showed that ectopic restoration of miR-765 in SKOV3 cells resulted in a significant inhibition of hypoxia-induced 3D channels-like formation that was associated with a reduced number of branch points and patterned tubular-like structures. Mechanistic studies confirmed that miR-765 decreased the levels of VEGFA, AKT1 and SRC-α transducers and exerted a negative regulation of VEGFA by specific binding to its 3'UTR. Finally, overall survival analysis of a cohort of ovarian cancer patients (n = 1435) indicates that high levels of VEGFA, AKT1 and SRC-α and low miR-765 expression were associated with worst patients outcome. In conclusion, here we reported a novel hypoxamiRs signature which constitutes a molecular guide for further clinical and functional studies on the early stages of VM. Our data also suggested that miR-765 coordinates the formation of 3D channels-like structures through modulation of VEGFA/AKT1/SRC-α axis in SKOV3 ovarian cancer cells.

Vasculogenic Mimicry Formation Is Associated with Erythropoietin Expression but Not with Erythropoietin Receptor Expression in Cervical Squamous Cell Carcinoma

Background

Vasculogenic mimicry (VM), as an endothelium-independent cancer microcirculation, has been observed in many malignancies including cervical cancer. Erythropoietin (EPO) and erythropoietin receptor (EPO-R) could produce an angiogenic effect to promote cervical squamous cell carcinoma (CSCC) progression. However, the association between VM formation and EPO/EPO-R expression in CSCC is poorly explored.

Methods

Seventy-six paraffin-embedded CSCC samples, 25 high-grade squamous intraepithelial lesion (HSIL) samples, 20 low-grade squamous intraepithelial lesion (LSIL) samples, and 20 normal cervix samples were collected. Immunohistochemistry SP method was performed to detect EPO/EPO-R expression and CD31/periodic acid-Schiff (PAS) double staining was performed to detect VM formation. The associations of EPO/EPO-R and VM with clinicopathological parameters of CSCC were analyzed. The associations between VM formation and EPO/EPO-R expression were also analyzed.

Results

The positive expression rates of EPO and EPO-R were gradually increasing along the progression of normal cervix-LSIL-HSIL-CSCC sequence (P<0.05). EPO and EPO-R expression were not significantly associated with clinicopathological parameters of CSCC patients (P>0.05). VM was significantly associated with FIGO stage, lymphovascular space involvement, and lymph node metastasis (P<0.05). VM was positively associated with EPO expression (r=0.284, P<0.05) but was not associated with EPO-R expression (P>0.05).

Conclusion

These data suggest that increased EPO/EPO-R expression may play an important role in cervical carcinogenesis. EPO overexpression may promote VM formation in CSCC.

Angiogenesis in pancreatic cancer: current research status and clinical implications

Pancreatic cancer is one of the most lethal malignancies worldwide. Although the standard of care in pancreatic cancer has improved, prognoses for patients remain poor with a 5-year survival rate of < 5%. Angiogenesis, namely, the formation of new blood vessels from pre-existing vessels, is an important event in tumor growth and hematogenous metastasis. It is a dynamic and complex process involving multiple mechanisms and is regulated by various molecules. Inhibition of angiogenesis has been an established therapeutic strategy for many solid tumors. However, clinical outcomes are far from satisfying for pancreatic cancer patients receiving anti-angiogenic therapies. In this review, we summarize the current status of angiogenesis in pancreatic cancer research and explore the reasons for the poor efficacy of anti-angiogenic therapies, aiming to identify some potential therapeutic targets that may enhance the effectiveness of anti-angiogenic treatments.

Recruited bone marrow derived cells, local stromal cells and IL-17 at the front line of resistance development to anti-VEGF targeted therapies

Although anti-angiogenic agents targeting VEGF have shown affordable beneficial outcomes in several human cancer types, in most pre-clinical and clinical studies, these effects are transient and followed by rapid relapse and tumor regrowth. Recently, it has been suggested that recruited bone marrow derived cells (BMDCs) to the tumor-microenvironment together with stromal cells play an important role in development of resistance to anti-VEGF therapies. Additionally, acquired resistance to anti-VEGF therapies has shown to be mediated partly through overexpression of different pro-angiogenic cytokines and growth factors including G-CSF, IL-6, IL-8, VEGF and FGF by these cells. Alongside, IL-17, a pro-inflammatory cytokine, mostly secreted by infiltrated CD4⁺ T helper cells, has shown to mediate resistance to anti-VEGF therapies, through recruiting BMDCs and modulating stromal cells activities including endothelial cells, tumor associated macrophages and cancer associated fibroblasts. Here, we examined the role of BMDCs, tumor stromal cells, IL-17 and their negotiation in development of resistance to anti-VEGF targeted therapies.

[Biejiajian Pills inhibits hepatoma carcinoma cell vasculogenic mimicry by suppressing RhoA/ROCK signaling pathway]

OBJECTIVE

To observe effects of Biejiajian Pills on hepatocarcinoma (HCC) cell vasculogenic mimicry (VM) and explore the molecular mechanism by which Biejiajian Pills inhibits HCC metastasis and invasion.

METHODS

Forty male SD rats were randomly divided into 4 groups for gastric lavage of normal saline or high, moderate or low doses of Biejiajian Pills (twice daily) for 4 consecutive days. The sera were collected from the rats for treatment of cultured human HCC HepG2 cells. VM formation in the cells was detected using an image acquisition and analysis system 24 h after incubation of the cells with the sera and with the RhoA/ROCK inhibitor Y-27632(P). The expression levels of RhoA and ROCK1 in the cells were detected using Western blotting, and the contents of VE-cadherin and PI3K in the culture supernatant were determined using ELISA.

RESULTS

Treatment with the sera from Biejiajian Pills-treated rats significantly inhibited formation of VM in HepG2 cells, and the diameters of VM formed were significantly greater than those in the positive control group (P < 0.01). Y-27632 completely inhibited the formation of VM in HepG2 cells (P < 0.01). Treatments with Biejiajian Pills and Y-27632 both inhibited the expression of RhoA and ROCK1 (P < 0.05) and significantly lowered the contents of VE-cadherin and PI3K in the culture supernatant (P < 0.05).

CONCLUSIONS

Biejiajian Pills can inhibit the formation of VM in HCC cells in vitro possibly by inhibiting the RhoA/ROCK pathways and the expressions of VE-cadherin and PI3K.

In vitro and in vivo apatinib inhibits vasculogenic mimicry in melanoma MUM-2B cells

The effect of apatinib on the formation of vasculogenic mimicry (VM) was studied in a malignant melanoma cell line. MUM-2B cells cultured in three-dimensional Matrigel were treated with varying concentrations (0, 0.01, 0.05, 0.1, 0.5 μmol/L) of apatinib to test its effect on VM in vitro, followed by MTT proliferation and transwell invasion assays to determine the effect of apatinib on cell proliferation and invasion of MUM-2B cells. In vivo, we used a melanoma cancer model to test the effect of short-term apatinib (100, 200, 300 mg/kg) treatment on VM. Western blotting, immunohistochemistry staining, and CD31-PAS dual staining were performed to assess the expression of VEGFR-2, ERK-1/2, PI3K, and MMP-2, and formation of VM. The results showed apatinib-treated groups formed a lesser number of VM in 3D matrigel, while the cell viability in MTT proliferation assay and the number of migration cells in transwell invasion assay were significantly lower in apatinib-treated groups. In addition, short-term apatinib treatment inhibited angiogenesis, VM formation, and tumor growth in models of melanoma cancer. Mice in apatinib-treated groups showed a markedly reduced expression of VEGFR-2, ERK-1/2, PI3K, and MMP-2. In summary, apatinib could inhibit the expression of VEGFR-2, and downregulate the ERK1/2/PI3K/MMP-2 signaling cascade, which may be one of the underlying mechanisms by which apatinib inhibits angiogenesis and the development of VM in models of melanoma cancer, and restrains the formation of VM by MUM-2B cells. Apatinib shows inhibitory effects on cell proliferation and invasion of MUM-2B cells, which is a close relationship with the VM.

Angiogenesis and vasculogenic mimicry are inhibited by 8-Br-cAMP through activation of the cAMP/PKA pathway in colorectal cancer

Introduction

Vasculogenic mimicry (VM) describes the formation of an epithelial-independent tumor microcirculation system that differs from traditional angiogenesis. Angiogenesis and the formation of VM are closely related through the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) pathway and the epithelial–mesenchymal transition (EMT) process.

Materials and methods

In this study, 8-Br-cAMP, a cAMP analog and PKA activator, was used to activate the cAMP/PKA pathway to evaluate the effects of cAMP/PKA on angiogenesis and VM in colorectal cancer (CRC) cells. We used a syngeneic model of CRC in BALB/c mice.

Results

We discovered that treatment with 8-Br-cAMP significantly reduced tumor number compared to control mice after the 7th, 14th, and 28th days of treatment. VM was evaluated by periodic acid–schiff (PAS)–CD31 staining, and we found that VM was inhibited by 8-Br-cAMP treatment in vivo. Immunohistochemistry confirmed the inhibition of vascular endothelial growth factor (VEGF) and cAMP and the activation of PKA by 8-Br-cAMP; quantitative real-time-PCR (qRT-PCR) demonstrated that 8-Br-cAMP regulated the expression of vascular endothelial (VE)-cadherin, matrix metalloproteinase 2 (MMP2), ephrin type-A receptor 2 (EphA2), and VEGF in vivo. Experiments in vitro revealed that treatment with 8-Br-cAMP and U0126 decreased VEGF expression through PKA–ERK in CT26 cells by qRT-PCR. We further confirmed that tube formation of human umbilical vein endothelial cells was inhibited by 8-Br-cAMP in vitro.

Discussion

This study demonstrates that angiogenesis and VM are inhibited by 8-Br-cAMP treatment. Our data indicate that 8-Br-cAMP acts through the cAMP/PKA–ERK pathway and through EMT processes in CRC. These findings provide an insight into mechanisms of CRC and suggest that the cAMP/PKA–ERK pathway is a novel potential therapeutic target for the treatment of CRC.

TP53INP1 inhibits hypoxia-induced vasculogenic mimicry formation via the ROS/snail signalling axis in breast cancer

Tumour protein p53‐inducible nuclear protein 1 (TP53INP1) is a tumour suppressor associated with malignant tumour metastasis. Vasculogenic mimicry (VM) is a new tumour vascular supply pattern that significantly influences tumour metastasis and contributes to a poor prognosis. However, the molecular mechanism of the relationship between TP53INP1 and breast cancer VM formation is unknown. Here, we explored the underlying mechanism by which TP53INP1 regulates VM formation in vitro and in vivo. High TP53INP1 expression was not only negatively correlated with a poor prognosis but also had a negative relationship with VE‐cadherin, HIF‐1α and Snail expression. TP53INP1 overexpression inhibited breast cancer invasion, migration, epithelial‐mesenchymal transition (EMT) and VM formation; conversely, TP53INP1 down‐regulation promoted these processes in vitro by functional experiments and Western blot analysis. We established a hypoxia model induced by CoCl₂ and assessed the effects of TP53INP1 on hypoxia‐induced EMT and VM formation. In addition, we confirmed that a reactive oxygen species (ROS)‐mediated signalling pathway participated in TP53INP1‐mediated VM formation. Together, our results show that TP53INP1 inhibits hypoxia‐induced EMT and VM formation via the ROS/GSK‐3β/Snail pathway in breast cancer, which offers new insights into breast cancer clinical therapy.

Vasculogenic mimicry in hepatocellular carcinoma contributes to portal vein invasion

Portal vein invasion (PVI) is common in hepatocellular carcinoma (HCC) and largely contributes to tumor recurrence after radical tumor resection or liver transplantation. Vasculogenic mimicry (VM) was an independent vascular system lined with tumor cells and associated with poor prognosis of HCC. The present study was conducted to evaluate the relationship between VM and portal vein invasion. A total of 44 HCC cases receiving anatomic liver resection were included in the study and were divided into groups with and without PVI. The prevalence of VM in each group was examined by CD34-PAS dual staining. The regulatory molecules of VM formation such as Notch1, Vimentin and matrix metalloproteinases (MMPs) were investigated by immunohistochemical staining. Analysis was performed to explore the association of PVI, VM and the VM regulatory molecules. PVI was found in 40.91% (18/44) cases and VM was found in 38.64% (17/44) cases in total samples. The incidence of VM was 72.22% (13/18) in PVI group while it was 15.38% (4/26) in non-PVI group (P<0.001), VM formation was positively correlated with PVI (r=0.574, P<0.001). The VM forming regulatory molecules such as Notch1, Vimentin, MMP-2 and MMP-9 were found to be correlated with PVI in HCC patients. Taken together, our results suggested that VM formation, alone with its regulatory molecules, is the promoting factor of PVI in hepatocellular carcinoma.

M2-like tumor-associated macrophages drive vasculogenic mimicry through amplification of IL-6 expression in glioma cells

Vasculogenic mimicry (VM) has offered a new horizon for understanding tumor angiogenesis, but the mechanisms of VM in glioma progression have not been studied explicitly until now. As a significant component of immune infiltration in tumor microenvironment, macrophages have been demonstrated to play an important role in tumor growth and angiogenesis. However, whether macrophages could play a potential key role in glioma VM is still poorly understood. Herein we reported that both VM and CD163⁺ cells were associated with WHO grade and reduced patient survival, and VM channel counting was correlated to the number of infiltrated CD163⁺ cells in glioma specimens. In vitro studies of glioma cell lines implicated that M2-like macrophages (M2) promoted glioma VM. We found that conditional medium derived from M2 amplified IL-6 expression in glioma cells. Furthermore, our data indicated that IL-6 could promote glioma VM, as blocking IL-6 with neutralizing antibodies abrogated M2-mediated VM enhancement. In addition, the potent PKC inhibitor bisindolylmaleimide I could prevent M2-induced IL-6 upregulation and further inhibited glioma VM facilitation. Taken together, our results suggested that M2-like macrophages drove glioma VM through amplifying IL-6 secretion in glioma cells via PKC pathway.

Expressions of glia maturation factor-β by tumor cells and endothelia correlate with neovascularization and poor prognosis in human glioma

Glia maturation factor-β (GMF-β) has been reported to promote glial differentiation, and act as a negative prognostic indicator in certain cancers. However, its roles in glioma progression remain unclear. Since neurogenesis and vasculogenesis were proved to share some common regulators during gliomagenesis, we aim to explore the potential impact of GMF-β on tumor neovascularization and patient survival in glioma. In this study, we first detected GMF-β expression not only in tumor cells but also in microvascular endothelia by double immunohistochemical staining. Both tumoral and endothelial GMF-β expression levels were positively correlated with tumor grade and microvessel density (MVD), while negatively associated with poor prognoses of the patients. Interestingly, multivariate analysis demonstrated that endothelial GMF-β expression level was the only independent predictor of progression-free and overall survival of glioma patients. The results of in vitro angiogenesis assay showed that GMF-β knockdown significantly inhibited tubulogenesis of human U87 glioblastoma cells. Furthermore, GMF-β knockdown suppressed tumor growth and the formation of human-CD31 positive (glioma cell-derived) microvessels in a mouse orthotopic U87 glioma model. Our results demonstrated that GMF-β is an important player in glioma progression via promoting neovascularization. GMF-β may therefore be a novel prognostic marker as well as a potential therapeutic target for glioma.

Multi-targeting NGR-modified liposomes recognizing glioma tumor cells and vasculogenic mimicry for improving anti-glioma therapy

Like the anti-angiogenic strategy, anti-vascular mimicry is considered as a novel targeting strategy for glioma. In the present study, we used NGR as a targeting ligand and prepared NGR-modified liposomes containing combretastatin A4 (NGR-SSL-CA4) in order to evaluate their potential targeting of glioma tumor cells and vasculogenic mimicry (VM) formed by glioma cells as well as their anti-VM activity in mice with glioma tumor cells. NGR-SSL-CA4 was prepared by a thin-film hydration method. The in vitro targeting of U87-MG (human glioma tumor cells) by NGR-modified liposomes was evaluated. The in vivo targeting activity of NGR-modified liposomes was tested in U87-MG orthotopic tumor-bearing nude mice. The anti-VM activity of NGR-SSL-CA4 was also investigated in vitro and in vivo. The targeting activity of the NGR-modified liposomes was demonstrated by in vitro flow cytometry and in vivo biodistribution. The in vitro anti-VM activity of NGR-SSL-CA4 was indicated in a series of cell migration and VM channel experiments. NGR-SSL-CA4 produced very marked anti-tumor and anti-VM activity in U87-MG orthotopic tumor-bearing mice in vivo. Overall, the NGR-SSL-CA4 has great potential in the multi-targeting therapy of glioma involving U87-MG cells, and the VM formed by U87-MG cells as well as endothelial cells producing anti-U87-MG cells, and anti-VM formed by U87-MG cells as well as anti-endothelial cell activity.

Fibrocytes: A Novel Stromal Cells to Regulate Resistance to Anti-Angiogenic Therapy and Cancer Progression

An adequate blood supply is essential for cancer cells to survive and grow; thus, the concept of inhibiting tumor angiogenesis has been applied to cancer therapy, and several drugs are already in clinical use. It has been shown that treatment with those anti-angiogenic drugs improved the response rate and prolonged the survival of patients with various types of cancer; however, it is also true that the effect was mostly limited. Currently, the disappointing clinical results are explained by the existence of intrinsic or acquired resistance to the therapy mediated by both tumor cells and stromal cells. This article reviews the mechanisms of resistance mediated by stromal cells such as endothelial cells, pericytes, fibroblasts and myeloid cells, with an emphasis on fibrocytes, which were recently identified as the cell type responsible for regulating acquired resistance to anti-angiogenic therapy. In addition, the other emerging role of fibrocytes as mediator-producing cells in tumor progression is discussed.

Long Non-coding RNA LINC00339 Stimulates Glioma Vasculogenic Mimicry Formation by Regulating the miR-539-5p/TWIST1/MMPs Axis

Glioma is recognized as a highly angiogenic malignant brain tumor. Vasculogenic mimicry (VM) greatly restricts the therapeutic effect of anti-angiogenic tumor therapy for glioma patients. However, the molecular mechanisms of VM formation in glioma remain unclear. Here, we demonstrated that LINC00339 was upregulated in glioma tissue as well as in glioma cell lines. The expression of LINC00339 in glioma tissues was positively correlated with glioma VM formation. Knockdown of LINC00339 inhibited glioma cell proliferation, migration, invasion, and tube formation, meanwhile downregulating the expression of VM-related molecular MMP-2 and MMP-14. Furthermore, knockdown of LINC00339 significantly increased the expression of miR-539-5p. Both bioinformatics and luciferase reporter assay revealed that LINC00339 regulated the above effects via binding to miR-539-5p. Besides, overexpression of miR-539-5p resulted in decreased expression of TWIST1, a transcription factor known to play an oncogenic role in glioma and identified as a direct target of miR-539-5p. TWIST1 upregulated the promoter activities of MMP-2 and MMP-14. The in vivo study showed that nude mice carrying tumors with knockdown of LINC00339 and overexpression of miR-539-5p exhibited the smallest tumor volume through inhibiting VM formation. In conclusion, LINC00339 may be used as a novel therapeutic target for VM formation in glioma.

Collateral Damage Intended-Cancer-Associated Fibroblasts and Vasculature Are Potential Targets in Cancer Therapy

After oncogenic transformation, tumor cells rewire their metabolism to obtain sufficient energy and biochemical building blocks for cell proliferation, even under hypoxic conditions. Glucose and glutamine become their major limiting nutritional demands. Instead of being autonomous, tumor cells change their immediate environment not only by their metabolites but also by mediators, such as juxtacrine cell contacts, chemokines and other cytokines. Thus, the tumor cells shape their microenvironment as well as induce resident cells, such as fibroblasts and endothelial cells (ECs), to support them. Fibroblasts differentiate into cancer-associated fibroblasts (CAFs), which produce a qualitatively and quantitatively different extracellular matrix (ECM). By their contractile power, they exert tensile forces onto this ECM, leading to increased intratumoral pressure. Moreover, along with enhanced cross-linkage of the ECM components, CAFs thus stiffen the ECM. Attracted by tumor cell- and CAF-secreted vascular endothelial growth factor (VEGF), ECs sprout from pre-existing blood vessels during tumor-induced angiogenesis. Tumor vessels are distinct from EC-lined vessels, because tumor cells integrate into the endothelium or even mimic and replace it in vasculogenic mimicry (VM) vessels. Not only the VM vessels but also the characteristically malformed EC-lined tumor vessels are typical for tumor tissue and may represent promising targets in cancer therapy.

Evolving Significance and Future Relevance of Anti-Angiogenic Activity of mTOR Inhibitors in Cancer Therapy

mTOR inhibitors have demonstrated remarkable anti-tumor activity in experimental models, mainly by reducing cancer cell growth and tumor angiogenesis. Their use in cancer patients as monotherapy has, however, generated only limited benefits, increasing median overall survival by only a few months. Likewise, in other targeted therapies, cancer cells develop resistance mechanisms to overcome mTOR inhibition. Hence, novel therapeutic strategies have to be designed to increase the efficacy of mTOR inhibitors in cancer. In this review, we discuss the present and future relevance of mTOR inhibitors in cancer therapy by focusing on their effects on tumor angiogenesis.

The Candidate Tumor Suppressor Gene SLC8A2 Inhibits Invasion, Angiogenesis and Growth of Glioblastoma

Glioblastoma is the most frequent and most aggressive brain tumor in adults. Solute carrier family 8 member 2 (SLC8A2) is only expressed in normal brain, but not present in other human normal tissues or in gliomas. Therefore, we hypothesized that SLC8A2 might be a glioma tumor suppressor gene and detected the role of SLC8A2 in glioblastoma and explored the underlying molecular mechanism. The glioblastoma U87MG cells stably transfected with the lentivirus plasmid containg SLC8A2 (U87MG-SLC8A2) and negative control (U87MG-NC) were constructed. In the present study, we found that the tumorigenicity of U87MG in nude mice was totally inhibited by SLC8A2. Overexpression of SLC8A2 had no effect on cell proliferation or cell cycle, but impaired the invasion and migration of U87MG cells, most likely through inactivating the extracellular signal-related kinases (ERK)1/2 signaling pathway, inhibiting the nuclear translocation and DNA binding activity of nuclear factor kappa B (NF-κB), reducing the level of matrix metalloproteinases (MMPs) and urokinase-type plasminogen activator (uPA)-its receptor (uPAR) system (ERK1/2-NF-κB-MMPs/uPA-uPAR), and altering the protein levels of epithelial to mesenchymal transitions (EMT)-associated proteins E-cardherin, vimentin and Snail. In addition, SLC8A2 inhibited the angiogenesis of U87MG cells, probably through combined inhibition of endothelium-dependent and endothelium-nondependent angiogenesis (vascular mimicry pattern). Totally, SLC8A2 serves as a tumor suppressor gene and inhibits invasion, angiogenesis and growth of glioblastoma.

Vascularized Tissue-Engineered Model for Studying Drug Resistance in Neuroblastoma

Neuroblastoma is a vascularized pediatric tumor derived from neural crest stem cells that displays vasculogenic mimicry and can express a number of stemness markers, such as SOX2 and NANOG. Tumor relapse is the major cause of succumbing to this disease, and properties attributed to cancer stem-like cells (CSLC), such as drug-resistance and cell plasticity, seem to be the key mechanisms. However, the lack of controllable models that recapitulate the features of human neuroblastoma limits our understanding of the process and impedes the development of new therapies. In response to these limitations, we engineered a perfusable, vascularized in vitro model of three-dimensional human neuroblastoma to study the effects of retinoid therapy on tumor vasculature and drug-resistance.

METHODS

The in vitro model of neuroblastoma was generated using cell-sheet engineering and cultured in a perfusion bioreactor. Firstly, we stacked three cell sheets containing SKNBE(2) neuroblastoma cells and HUVEC. Then, a vascular bed made of fibrin, collagen I and HUVEC cells was placed onto a collagen-gel base with 8 microchannels. After gelling, the stacked cell sheets were placed on the vascular bed and cultured in the perfusion bioreactor (perfusion rate: 0.5 mL/min) for 4 days. Neuroblastoma models were treated with 10μM isotretionin in single daily doses for 5 days.

RESULTS

The bioengineered model recapitulated vasculogenic mimicry (vessel-like structure formation and tumor-derived endothelial cells-TECs), and contained CSLC expressing SOX2 and NANOG. Treatment with Isotretinoin destabilized vascular networks but failed to target vasculogenic mimicry and augmented populations of CSLCs expressing high levels of SOX2. Our results suggest that CSLCs can transdifferentiate into drug resistant CD31⁺-TECs, and reveal the presence of an intermediate state STEC (stem tumor-derived endothelial cell) expressing both SOX2 and CD31.

CONCLUSION

Our results reveal some roles of SOX2 in drug resistance and tumor relapse, and suggest that SOX2 could be a therapeutic target in neuroblastoma.