Search

Epidemiological relationships between cancer and cardiovascular diseases have been reported, but a molecular basis remains unclear. Some proteoglycans that strongly bind low-density-lipoprotein (LDL) are abundant both in atherosclerotic regions and in high metastatic-tumor tissue. LDL retention is crucial for the initiation of atherosclerosis, although its contribution to malignancy of cancer is not known. In our study, we show the importance of the accumulation of LDL in tumor metastasis. We demonstrated that high metastatic-tumor tissue contains high amounts of LDL and forms more oxidized LDL (ox-LDL). Interestingly, lectin-like ox-LDL receptor 1 (LOX-1), a receptor for ox-LDL and a recognized key molecule for cardiovascular diseases, was highly expressed in tumor endothelial cells (TECs). Neutrophils are important for ox-LDL formation. Since we observed the accumulation and activation of neutrophils in HM-tumors, we evaluated the involvement of LOX-1 in neutrophil migration and activation. LOX-1 induced neutrophil migration via CCL2 secretion from TECs, which was enhanced by ox-LDL. Finally, we show genetic manipulation of LOX-1 expression in TECs or tumor stroma tended to reduce lung metastasis. Thus, the LOX-1/ox-LDL axis in TECs may lead to the formation of a high metastatic-tumor microenvironment via attracting neutrophils.

The antibodies and dilutions for Immunohistochemical staining

Supplemental Figure 5 shows (A) treatment schedule of subcutaneous model, (B) therapeutic result of PK-9 subcutaneous tumor model mice (n= 5). 3 groups (PBS: non-treatment group, 11R-7_14 AAA: negative control peptide group, 11R-DB: treatment group). In the 11R-DB group, tumor growth was suppressed significantly (11R-DB vs 11R-7â,‹14AAA and PBS: *p

Supplemental Figure 2 shows, (A) analysis of expression of C16orf74 in PDAC and other cancer cell lines by qRT-PCR and (B) analysis of expression of endogenous C16orf74 protein in PDAC and NHDF by Western blot.

Supplemental Figure 4 shows the quantitative graph of AKt phosphorylation and mTOR phosphorylation. (A) AKt phosphorylation in PK-1, PK-9, PANC-1 and MIAPaCa-2 cells. (B) mTOR phosphorylation in PK-1, PK-9, PANC-1 and MIAPaCa-2 cells.

Supplemental Figure 1 shows, (A) analysis of expression of C16orf74 in C16orf74 stable and MOCK cell lines by qRT-PCR, (B) expression of C16orf74 in C16orf74 stable and MOCK by Western blotting, (C) expression of C16orf74 in C16orf74 stable clonal cell lines by Immunofluorescence. C16orf74 localizes under the cell membrane, and (D) expression of endogenous C16orf74 protein in PK-9 cells shown by Immunofluorescence. Endogenous C16orf74 localizes under the cell membrane.

Supplemental Figure 3 shows matrigel invasion assay of C16orf74 stable cell lines (Clone C16-A. C16-B. C16-C) with 11R-DB administration and no administration. In the administered group, the invasion was significantly suppressed. * P

The antibodies and dilutions for Immunofluorescence

The primary antibodies and dilutions for Western blot analysis

RT-PCR primers for construction of the expression vectors

Recent studies have demonstrated a relationship between oral bacteria and systemic inflammation. Endothelial cells (ECs), which line blood vessels, control the opening and closing of the vascular barrier and contribute to hematogenous metastasis; however, the role of oral bacteria-induced vascular inflammation in tumor metastasis remains unclear. In this study, we examined the phenotypic changes in vascular ECs following Streptococcus mutans (S. mutans) stimulation in vitro and in vivo. The expression of molecules associated with vascular inflammation and barrier-associated adhesion was analyzed. Tumor metastasis was evaluated after intravenous injection of S. mutans in murine breast cancer hematogenous metastasis model. The results indicated that S. mutans invaded the ECs accompanied by inflammation and NF-kappa B activation. S. mutans exposure potentially disrupts endothelial integrity by decreasing vascular endothelial (VE)-cadherin expression. The migration and adhesion of tumor cells were enhanced in S. mutans-stimulated ECs. Furthermore, S. mutans-induced lung vascular inflammation promoted breast cancer cell metastasis to the lungs in vivo. The results indicate that oral bacteria promote tumor metastasis through vascular inflammation and the disruption of vascular barrier function. Improving oral hygiene in patients with cancer is of great significance in preventing postoperative pneumonia and tumor metastasis.

Oral squamous cell carcinoma (OSCC) impairs functionality and sensuousness resulting in poor quality of life. Biomarkers can predict disease trajectory and lead to effective treatments. Transcriptomics have identified genes that are upregulated in tumor endothelial cells (TECs) compared with normal endothelial cells (NECs). Among them, chemokine receptor 7 (CXCR7) is highly expressed in TECs of several cancers and involved in angiogenesis of TECs. However, levels of CXCR7 in OSCC blood vessels have not been fully investigated. In this study, we analyzed the correlation between CXCR7 expression in TECs and clinicopathological factors in OSCC. Immunohistochemistry for CXCR7 and CD34 was performed on 59 OSCC tissue specimens resected between 1996 and 2008 at Hokkaido University Hospital. CXCR7 expression in blood vessels was evaluated by the ratio of CXCR7+/CD34+ blood vessels. CXCR7 expression was 42% and 19% in tumor and non-tumor parts, respectively, suggesting that CXCR7 expression is higher in TECs than in NECs. CXCR7 expression in TECs correlated with advanced T-stage and cancer stage. Overall survival and disease-free survival rates were higher in low-expressing CXCR7 patients than in high-expressing. These results suggest that CXCR7 expression in blood vessels may be a useful diagnostic and prognostic marker for OSCC patients.

Objectives In lung cancer, surgery remains the most curative treatment and limited resection is beneficial for patients with low cardiopulmonary function and low malignancy tumors. However, there are no biomarkers of low malignancy to select candidates for limited resection without compromising the outcome of treatments. Recently we identified biglycan (BGN) as a tumor endothelial cell (TEC) marker that is associated with tumor progression in various cancers. In this study, we analyzed the association between BGN expression in TECs in lung cancer and cancer progression in patients. Materials and Methods First, we performed immunohistochemistry of BGN with resected lung tumor tissues of 155 patients who had undergone thoracic surgery and analyzed the correlation between BGN‐positive vessel density in primary lung tumors and clinicopathological factors. Second, we measured the BGN levels in preoperative serum of other 46 patients with lung cancer by ELISA, and analyzed the correlation between BGN expression in tumor tissues and blood BGN levels. Results High BGN expression in the TECs was significantly associated with T factor, and was a significant negative predictor. BGN levels in preoperative serum of 46 patients with lung cancer was significantly correlated with BGN expression in the TECs. Preoperative serum BGN level was significantly lower in healthy volunteers and less invasive adenocarcinoma than in invasive adenocarcinoma and other lung carcinomas. These results suggest that low BGN level in preoperative serum in patients with lung cancer might indicate low malignancy. Conclusions BGN can be a potential biomarker for lung cancer., We found that biglycan (BGN) is expressed in tumor endothelial cells (TECs) and that BGN expression in TECs is associated with tumor progression and prognosis in lung cancer. Moreover, BGN expression in preoperative serum was significantly associated with BGN expression in TECs and with tumor malignancy.

Adipose tissue in the body is classified as white adipose tissue (WAT); a fat-accumulating tissue, or brown adipose tissue (BAT); an energy-dissipating tissue. Transforming WAT-to-BAT (browning) is a promising strategy for the treatment of obesity, since it would lead to an increase in energy expenditure. Rosiglitazone (Rosi), an agonist of the peroxisome proliferator-activated receptor γ (PPARγ), is known to be a potent browning inducer in subcutaneous WAT. However, the effectiveness of Rosi has been quite limited because of several off-target effects. The objective of this study was to develop locally administered Rosi-loaded nanoparticles (Rs-NPs) with the ability to target adipocytes to achieve the adipose tissue-specific activation of PPARγ, thus causing the browning of WAT. We prepared dual targeted Rs-NPs that were modified with a specific peptide that targets prohibitin that are expressed in adipocytes, and a cell penetrating peptide for enhancing cellular uptake and controlling intracellular trafficking. The Rs-NPs modified with a single ligand were internalized into mature adipocytes and induced browning activity in vitro but they failed to significantly affect the body weight of the diet-induced obese mice model. The dual-targeted Rs-NPs induced a strong browning activity, both in vitro and in vivo, and successfully inhibited the progression of obesity, as evidenced by the shrinkage of hypertrophied adipocytes without any detectable systemic adverse effects. Meanwhile, free Rosi aggravated hepatic steatosis and did not cause adipose tissue browning nor the inhibition of body weight gain. We conclude that the increased energy expenditure via adipose tissue browning using dual-targeted Rs-NP is a promising strategy for the treatment of obesity and its related metabolic syndrome.

This study analyzes the relationship between biglycan expression in prostate cancer and clinicopathological parameters to clarify the potential link between biglycan and prognosis and progression to castration-resistant prostate cancer (CRPC).We retrospectively analyzed 60 cases of prostate cancer patients who underwent robot-assisted laparoscopic radical prostatectomy in Hokkaido University Hospital.Biglycan was expressed in the tumor stroma but not in tumor cells. There was no significant relationship with biochemical recurrence (p = 0.5237), but the expression of biglycan was 36.1% in the group with progression to CRPC. This indicates a significant relationship with progression to CRPC (p = 0.0182). Furthermore, the expression of biglycan-positive blood vessels was significantly higher (15.9%) in the group with biochemical recurrence than in the group without biochemical recurrence (8.5%) (p = 0.0169). The biglycan-positive vessels were 28.6% in the group with progression to CRPC, which was significantly higher than that in the group without progression to CRPC (p 0.0001).This is the first study to show that stroma biglycan is a useful prognostic factor for prostate cancer.

Tumor blood vessels play important roles in tumor progression and metastasis. Targeting tumor endothelial cells (TECs) is one of the strategies for cancer therapy. We previously reported that biglycan, a small leucine-rich proteoglycan, is highly expressed in TECs. TECs utilize biglycan in an autocrine manner for migration and angiogenesis. Furthermore, TEC-derived biglycan stimulates tumor cell migration in a paracrine manner leading to tumor cell intravasation and metastasis. In this study, we explored the therapeutic effect of biglycan inhibition in the TECs of renal cell carcinoma using an in vivo siRNA delivery system known as a multifunctional envelope-type nanodevice (MEND), which contains a unique pH-sensitive cationic lipid. To specifically deliver MEND into TECs, we incorporated cyclo(Arg-Gly-Asp-D-Phe-Lys) (cRGD) into MEND because alpha(V)beta(3) integrin, a receptor for cRGD, is selective and highly expressed in TECs. We developed RGD-MEND-encapsulating siRNA against biglycan. First, we confirmed that MEND was delivered into OS-RC-2 tumor-derived TECs and induced in vitro RNAi-mediated gene silencing. MEND was then injected intravenously into OS-RC-2 tumor-bearing mice. Flow cytometry analysis demonstrated that MEND was specifically delivered into TECs. Quantitative RT-PCR indicated that biglycan was knocked down by biglycan siRNA-containing MEND. Finally, we analyzed the therapeutic effect of biglycan silencing by MEND in TECs. Tumor growth was inhibited by biglycan siRNA-containing MEND. Tumor microenvironmental factors such as fibrosis were also normalized using biglycan inhibition in TECs. Biglycan in TECs can be a novel target for cancer treatment.

The tumor microenvironment (TME) consists of various components including cancer cells, tumor vessels, cancer‐associated fibroblasts (CAFs), and inflammatory cells. These components interact with each other via various cytokines, which often induce tumor progression. Thus, a greater understanding of TME networks is crucial for the development of novel cancer therapies. Many cancer types express high levels of TGF‐β, which induces endothelial‐to‐mesenchymal transition (EndMT), leading to formation of CAFs. Although we previously reported that CAFs derived from EndMT promoted tumor formation, the molecular mechanisms underlying these interactions remain to be elucidated. Furthermore, tumor‐infiltrating inflammatory cells secrete various cytokines, including TNF‐α. However, the role of TNF‐α in TGF‐β‐induced EndMT has not been fully elucidated. Therefore, this study examined the effect of TNF‐α on TGF‐β‐induced EndMT in human endothelial cells (ECs). Various types of human ECs underwent EndMT in response to TGF‐β and TNF‐α, which was accompanied by increased and decreased expression of mesenchymal cell and EC markers, respectively. In addition, treatment of ECs with TGF‐β and TNF‐α exhibited sustained activation of Smad2/3 signals, which was presumably induced by elevated expression of TGF‐β type I receptor, TGF‐β2, activin A, and integrin αv, suggesting that TNF‐α enhanced TGF‐β‐induced EndMT by augmenting TGF‐β family signals. Furthermore, oral squamous cell carcinoma‐derived cells underwent epithelial‐to‐mesenchymal transition (EMT) in response to humoral factors produced by TGF‐β and TNF‐α‐cultured ECs. This EndMT‐driven EMT was blocked by inhibiting the action of TGF‐βs. Collectively, our findings suggest that TNF‐α enhances TGF‐β‐dependent EndMT, which contributes to tumor progression., This study showed that TGF‐β and TNF‐α cooperate to induce the endothelial‐to‐mesenchymal transition (EndMT), in which endothelial cells (ECs) acquire mesenchymal phenotypes. The ECs that have undergone EndMT, in turn, secrete TGF‐β2 and Activin by themselves. These secreted cytokines not only stabilize the mesenchymal phenotypes of ECs, but also induce the epithelial‐to‐mesenchymal transition (EMT) of epithelial cancer cells, which contributes to formation of malignant cancer cells.

Over the past 30 years, the therapeutic outcome for pancreatic ductal adenocarcinoma (PDAC) has remained stagnant due to the lack of effective treatments. We performed a genome-wide analysis to identify novel therapeutic targets for PDAC. Our analysis showed that Homo sapiens chromosome 16 open reading frame 74 (C16orf74) was upregulated in most patients with PDAC and associated with poor prognosis. Previously, we demonstrated that C16orf74 interacts with the catalytic subunit alpha of protein phosphatase 3 and plays an important role in PDAC invasion. However, the pathophysiologic function of C16orf74 is still unclear. In this study, through the analysis of C16orf74 interaction, we demonstrate a new strategy to inhibit the growth and invasion of PDAC. C16orf74 exists in the homodimer form under the cell membrane and binds integrin αVβ3 and is also involved in invasion by activating Rho family (Rac1) and MMP2. Considering that this dimeric form was found to be involved in the function of C16orf74, we designed an 11R-DB (dimer block) cell-permeable dominant-negative peptide that inhibits the dimer form of C16orf74. 11R-DB suppressed invasion and proliferation of PDAC cell lines by inhibiting phosphorylation of Akt and mTOR and also by inactivation of MMP2. 11R-DB also showed antitumor effects in an orthotopic xenograft model and peritoneal metastasis model. Thus, this study demonstrates that dimerized C16orf74, present in the cell membrane, is involved in pancreatic cancer invasion and proliferation. In addition, the C16orf74 dimer block cell-permeable peptide (11R-DB) has a potent therapeutic effect on PDAC in vitro and in vivo.

BackgroundTumor endothelial cells (TECs) perform tumor angiogenesis, which is essential for tumor growth and metastasis. Tumor cells produce large amounts of lactic acid from glycolysis; however, the mechanism underlying the survival of TECs to enable tumor angiogenesis under high lactic acid conditions in tumors remains poorly understood.MethodologyThe metabolomes of TECs and normal endothelial cells (NECs) were analyzed by capillary electrophoresis time-of-flight mass spectrometry. The expressions of pH regulators in TECs and NECs were determined by quantitative reverse transcription-PCR. Cell proliferation was measured by the MTS assay. Western blotting and ELISA were used to validate monocarboxylate transporter 1 and carbonic anhydrase 2 (CAII) protein expression within the cells, respectively. Human tumor xenograft models were used to access the effect of CA inhibition on tumor angiogenesis. Immunohistochemical staining was used to observe CAII expression, quantify tumor microvasculature, microvessel pericyte coverage, and hypoxia.ResultsThe present study shows that, unlike NECs, TECs proliferate in lactic acidic. TECs showed an upregulated CAII expression both in vitro and in vivo. CAII knockdown decreased TEC survival under lactic acidosis and nutrient-replete conditions. Vascular endothelial growth factor A and vascular endothelial growth factor receptor signaling induced CAII expression in NECs. CAII inhibition with acetazolamide minimally reduced tumor angiogenesis in vivo. However, matured blood vessel number increased after acetazolamide treatment, similar to bevacizumab treatment. Additionally, acetazolamide-treated mice showed decreased lung metastasis.ConclusionThese findings suggest that due to their effect on blood vessel maturity, pH regulators like CAII are promising targets of antiangiogenic therapy.Graphical abstract

Background: Tumor endothelial cells (TECs) reportedly exhibit altered phenotypes. We have demonstrated that TECs acquire drug resistance with the upregulation of p-glycoprotein (p-gp, ABCB1), contrary to traditional assumptions. Furthermore, p-gp expression was higher in TECs of highly metastatic tumors than in those of low metastatic tumors. However, the detailed mechanism of differential p-gp expression in TECs remains unclear. Methods: miRNA was identified in highly metastatic tumor extracellular vesicles (EVs) and the roles of miRNA in endothelial cell resistance were analyzed in vitro and in vivo. Results: In the present study, we found that treatment of highly metastatic tumor-conditioned medium induced resistance to 5-fluorouracil (5-FU) with interleukin-6 (IL-6) upregulation in endothelial cells (ECs). Among the soluble factors secreted from highly metastatic tumors, we focused on extracellular vesicles (EVs) and determined that miR-1246 was contained at a higher level in highly metastatic tumor EVs than in low metastatic tumor EVs. Furthermore, miR-1246 was transported via the EVs into ECs and induced IL-6 expression. Upregulated IL-6 induced resistance to 5-FU with STAT3 and Akt activation in ECs in an autocrine manner. Conclusions: These results suggested that highly metastatic tumors induce drug resistance in ECs by transporting miR-1246 through EVs.

The tumor microenvironment contains various components, including cancer cells, tumor vessels, and cancer-associated fibroblasts, the latter of which are comprised of tumor-promoting myofibroblasts and tumor-suppressing fibroblasts. Multiple lines of evidence indicate that transforming growth factor-β (TGF-β) induces the formation of myofibroblasts and other types of mesenchymal (non-myofibroblastic) cells from endothelial cells. Recent reports show that fibroblast growth factor 2 (FGF2) modulates TGF-β-induced mesenchymal transition of endothelial cells, but the molecular mechanisms behind the signals that control transcriptional networks during the formation of different groups of fibroblasts remain largely unclear. Here, we studied the roles of FGF2 during the regulation of TGF-β-induced mesenchymal transition of tumor endothelial cells (TECs). We demonstrated that auto/paracrine FGF signals in TECs inhibit TGF-β-induced endothelial-to-myofibroblast transition (End-MyoT), leading to suppressed formation of contractile myofibroblast cells, but on the other hand can also collaborate with TGF-β in promoting the formation of active fibroblastic cells which have migratory and proliferative properties. FGF2 modulated TGF-β-induced formation of myofibroblastic and non-myofibroblastic cells from TECs via transcriptional regulation of various mesenchymal markers and growth factors. Furthermore, we observed that TECs treated with TGF-β were more competent in promoting in vivo tumor growth than TECs treated with TGF-β and FGF2. Mechanistically, we showed that Elk1 mediated FGF2-induced inhibition of End-MyoT via inhibition of TGF-β-induced transcriptional activation of α-smooth muscle actin promoter by myocardin-related transcription factor-A. Our data suggest that TGF-β and FGF2 oppose and cooperate with each other during the formation of myofibroblastic and non-myofibroblastic cells from TECs, which in turn determines the characteristics of mesenchymal cells in the tumor microenvironment.

Going from bench to bedside is a simplified description of translational research, with the ultimate goal being to improve the health status of mankind. Tumor endothelial cells (TECs) perform angiogenesis to support the growth, establishment, and dissemination of tumors to distant organs. TECs have various features that distinguish them from normal endothelial cells, which include alterations in gene expression patterns, higher angiogenic and metabolic activities, and drug resistance tendencies. The special characteristics of TECs enhance the vulnerability of tumor blood vessels toward antiangiogenic therapeutic strategies. Therefore, apart from being a viable therapeutic target, TECs would act as a better mediator between the bench (i.e., angiogenesis research) and the bedside (i.e., clinical application of drugs discovered through research). Exploitation of TEC characteristics could reveal unidentified strategies of enhancing and monitoring antiangiogenic therapy in the treatment of cancer, which are discussed in this review.

Tumor endothelial cells (TEC) lining tumor blood vessels actively contribute to tumor progression and metastasis. In addition to tumor cells, TEC may develop drug resistance during cancer treatment, allowing the tumor cells to survive chemotherapy and metastasize. We previously reported that TECs resist paclitaxel treatment via upregulation of ABCB1. However, whether TEC phenotypes are altered by anticancer drugs remains to be clarified. Here, we show that ABCB1 expression increases after chemotherapy in urothelial carcinoma cases. The ratio of ABCB1-positive TEC before and after first-line chemotherapy in urothelial carcinoma tissues (n = 66) was analyzed by ABCB1 and CD31 immunostaining. In 42 cases (64%), this ratio increased after first-line chemotherapy. Chemotherapy elevated ABCB1 expression in endothelial cells by increasing tumor IL8 secretion. In clinical cases, ABCB1 expression in TEC correlated with IL8 expression in tumor cells after first-line chemotherapy, leading to poor prognosis. In vivo, the ABCB1 inhibitor combined with paclitaxel reduced tumor growth and metastasis compared with paclitaxel alone. Chemotherapy is suggested to cause inflammatory changes in tumors, inducing ABCB1 expression in TEC and conferring drug resistance. Overall, these findings indicate that TEC can survive during chemotherapy and provide a gateway for cancer metastasis. Targeting ABCB1 in TEC represents a novel strategy to overcome cancer drug resistance. Significance: These findings show that inhibition of ABCB1 in tumor endothelial cells may improve clinical outcome, where ABCB1 expression contributes to drug resistance and metastasis following first-line chemotherapy.

Tumor growth and metastasis are dependent on angiogenesis, which is the formation of new blood vessels. The newly formed blood vessels around the tumor supply oxygen and nutrients to the tumor, supporting its progression. Moreover, these blood vessels also serve as channels through which tumor cells metastasize to distant organs. The balance between angiogenic stimulators and inhibitors regulates angiogenesis in the tumor microenvironment. Tumor blood vessels, especially the endothelial cells lining tumor blood vessels (tumor endothelial cells [TECs]), are important targets in cancer therapy. As newly formed tumor blood vessels originate from pre-existing normal vessels, tumor blood vessels and TECs have traditionally been considered to be the same as normal ones. However, tumor blood vessels have a distinctively abnormal phenotype, including morphological alterations. Recently, it has been revealed that TECs constitute a heterogeneous population, exhibiting characteristics that are largely induced by tumor microenvironmental factors. Furthermore, TECs induce cancer progression through metastasis. In this review, we describe recent studies on TEC abnormalities related to cancer progression and consider their therapeutic implications.

Antibody detection methods for viral infections have received broad attention due to the COVID-19 pandemic. In addition, there remains an ever-increasing need to quantitatively evaluate the immune response to develop vaccines and treatments for COVID-19. Here, we report an analytical method for the rapid and quantitative detection of SARS-CoV-2 antibody in human serum by fluorescence polarization immunoassay (FPIA). A recombinant SARS-CoV-2 receptor binding domain (RBD) protein labeled with HiLyte Fluor 647 (F-RBD) was prepared and used for FPIA. When the anti-RBD antibody in human serum binds to F-RBD, the degree of polarization (P) increases by suppressing the rotational diffusion of F-RBD. The measurement procedure required only mixing a reagent containing F-RBD with serum sample and measuring the P value with a portable fluorescence polarization analyzer after 15 min incubation. We evaluated analytical performance of the developed FPIA system using 30 samples: 20 COVID-19 positive sera and 10 negative sera. The receiver operating characteristic curve drawn with the obtained results showed that this FPIA system had high accuracy for discriminating COVID-19 positive or negative serum (AUC = 0.965). The total measurement time was about 20 min, and the serum volume required for measurement was 0.25 μL. Therefore, we successfully developed the FPIA system that enables rapid and easy quantification of SARS-CoV-2 antibody. It is believed that our FPIA system will facilitate rapid on-site identification of infected persons and deepen understanding of the immune response to COVID-19.

Tumor blood vessels play an important role in tumor progression and metastasis. We previously reported that tumor endothelial cells (TEC) exhibit several altered phenotypes compared with normal endothelial cells (NEC). For example, TEC have chromosomal abnormalities and are resistant to several anticancer drugs. Furthermore, TEC contain stem cell‐like populations with high aldehyde dehydrogenase (ALDH) activity (ALDH high TEC). ALDH high TEC have proangiogenic properties compared with ALDH low TEC. However, the association between ALDH high TEC and drug resistance remains unclear. In the present study, we found that ALDH mRNA expression and activity were higher in both human and mouse TEC than in NEC. Human NEC:human microvascular endothelial cells (HMVEC) were treated with tumor‐conditioned medium (tumor CM). The ALDH high population increased along with upregulation of stem‐related genes such as multidrug resistance 1, CD90, ALP, and Oct‐4. Tumor CM also induced sphere‐forming ability in HMVEC. Platelet‐derived growth factor (PDGF)‐A in tumor CM was shown to induce ALDH expression in HMVEC. Finally, ALDH high TEC were resistant to fluorouracil (5‐FU) in vitro and in vivo. ALDH high TEC showed a higher grade of aneuploidy compared with that in ALDH low TEC. These results suggested that tumor‐secreting factor increases ALDH high TEC populations that are resistant to 5‐FU. Therefore, ALDH high TEC in tumor blood vessels might be an important target to overcome or prevent drug resistance.

Tumor metastasis is the main cause of cancer‐related death. Understanding the molecular mechanisms underlying tumor metastasis is crucial to control this fatal disease. Several molecular pathways orchestrate the complex biological cell events during a metastatic cascade. It is now well known that bidirectional interaction between tumor cells and their microenvironment, including tumor stroma, is important for tumor progression and metastasis. Tumor stromal cells, which acquire their specific characteristics in the tumor microenvironment, accelerate tumor malignancy. The formation of new blood vessels, termed as tumor angiogenesis, is a requirement for tumor progression. Tumor blood vessels supply nutrients and oxygen and also provide the route for metastasis. Tumor endothelial cells, which line tumor blood vessels, also exhibit several altered phenotypes compared with those of their normal counterparts. Recent studies have emphasized “angiocrine factors” that are released from tumor endothelial cells and promote tumor progression. During intravasation, tumor cells physically contact tumor endothelial cells and interact with them by juxtacrine and paracrine signaling. Recently, we observed that in highly metastatic tumors, tumor endothelial cells interact with tumor cells by secretion of a small leucine‐rich repeat proteoglycan known as biglycan. Biglycan from tumor endothelial cells stimulates the tumor cells to metastasize. In the present review, we highlight the role of tumor stromal cells, particularly endothelial cells, in the initial steps of tumor metastasis.