A novel gene expression profile in lymphatics associated with tumor growth and nodal metastasis

Cross‑talk between lymphangiogenesis and malignant melanoma cells: New opinions on tumour drainage and immunization (Review)

Malignant melanoma (MM) is a highly aggressive tumour that can easily metastasize through the lymphatic system at the early stages. Lymph node (LN) involvement and lymphatic vessel (LV) density (LVD) represent a harbinger of an adverse prognosis, indicating a strong link between the state of the lymphatic system and the advancement of MM. Permeable capillary lymphatic vessels are the optimal conduits for melanoma cell (MMC) invasion, and lymphatic endothelial cells (LECs) can also release a variety of chemokines that actively attract MMCs expressing chemokine ligands through a gradient orientation. Moreover, due to the lower oxidative stress environment in the lymph compared with the blood circulation, MMCs are more likely to survive and colonize. The number of LVs surrounding MM is associated with tumour-infiltrating lymphocytes (TILs), which is crucial for the effectiveness of immunotherapy. On the other hand, MMCs can release various endothelial growth factors such as VEGF-C/D-VEGFR3 to mediate LN education and promote lymphangiogenesis. Tumour-derived extracellular vesicles are also used to promote lymphangiogenesis and create a microenvironment that is more conducive to tumour progression. MM is surrounded by a large number of lymphocytes. However, both LECs and MMCs are highly plastic, playing multiple roles in evading immune surveillance. They achieve this by expressing inhibitory ligands or reducing antigen recognition. In recent years, tertiary lymphoid structures have been shown to be associated with response to anti-immune checkpoint therapy, which is often a positive prognostic feature in MM. The present review discusses the interaction between lymphangiogenesis and MM metastasis, and it was concluded that the relationship between LVD and TILs and patient prognosis is analogous to a dynamically tilted scale.

Isolation and short-term culturing of primary lymphatic endothelial cells from collecting lymphatics: A techniques study

OBJECTIVE

To develop an experimental method for routine isolation and short-term culture of primary lymphatic endothelial cells from specific collecting vessels.

METHODS

Lymphatic endothelial cell tubes (LECTs) were isolated from micro-dissected collecting vessels. LECTs were allowed to attach and grow for ~3 weeks before being passaged. Non-purified cultures were partially characterized by immunofluorescence and RT-PCR at passages 1-2.

RESULTS

The method was validated in cultures of primary lymphatic endothelial cells (LECs) from male and female mice. After 1 or 2 passages, >60% of the LECs maintained expression of Prox1. Expression of 22 different genes was assessed using RT-PCR. Prox1, Vegfr3, eNos, Cdh5, Pecam1, Cx43, Cx37, and Cx47, among others, were expressed in these short-term cultured LECs, while Myh11, Cnn1, Desmin, and Cd11b were not detected. Prox1 expression, as determined by western blotting, was similar in cultured LECs from age-matched male and female mice. Confocal imaging of intracellular calcium in cultures of primary LECs from Cdh5-GCaMP8 mice demonstrated that a functional phenotype was maintained, similar to lymphatic endothelial cells in freshly isolated vessels.

CONCLUSIONS

This method provides an innovative tool for routine isolation and study of primary LECs from specific collecting lymphatic vessels from any mouse, and in fact, from other species.

The immunoregulatory protein CD200 as a potentially lucrative yet elusive target for cancer therapy

CD200 is an immunoregulatory cell surface ligand with proven pro-tumorigenic credentials via its ability to suppress CD200 receptor (CD200R)-expressing anti-tumor immune function. This definitive role for the CD200-CD200R axis in regulating an immunosuppressive tumor microenvironment has garnered increasing interest in CD200 as a candidate target for immune checkpoint inhibition therapy. However, while the CD200 blocking antibody samalizumab is still in the early stages of clinical testing, alternative mechanisms for the pro-tumorigenic role of CD200 have recently emerged that extend beyond direct suppression of anti-tumor T cell responses and, as such, may not be susceptible to CD200 antibody blockade. Herein, we will summarize the current understanding of CD200 expression and function in the tumor microenvironment as well as alternative strategies for potential neutralization of multiple CD200 mechanisms in human cancers.

Gene expression in meningeal lymphatic endothelial cells following traumatic brain injury in mice

Meningeal lymphatic vessels transport both the cerebrospinal fluid and interstitial fluid to the deep cervical lymph nodes. Traumatic brain injury (TBI) is accompanied by meningeal injury. We hypothesized that the TBI-induced meningeal injury would damage lymphatic vessels and affect brain function. We observed altered gene expression in meningeal lymphatic endothelial cells (LECs) in a mouse model of TBI. Through flow cytometry–based cell sorting, meningeal LECs were obtained from a mouse model of controlled cortical impact 3 days after TBI. Microarray analysis, real-time polymerase chain reaction assays, and enzyme-linked immunosorbent assays were performed to determine mRNA and protein expression levels in meningeal LECs. The number of meningeal LECs was significantly lower in the injury group than in the sham group 3 days after TBI. Additionally, the mRNA expression of lymphatic vessel endothelial hyaluronan receptor 1 (a specific marker of lymphatic vessels) in meningeal LECs was significantly lower in the injury group than in the sham group. The mRNA and protein expression of FMS-like tyrosine kinase 4 and neuropilin 2 (markers of lymphangiogenesis) in meningeal LECs was significantly higher in the injury group than in the sham group. Our findings indicate that TBI is associated with the impairment of meningeal LECs and meningeal lymphangiogenesis, which implicates lymphatic vessel injury in the pathogenesis of this condition.

Lymphatic vessels in cancer

The lymphatic system, composed of initial and collecting lymphatic vessels as well as lymph nodes that are present in almost every tissue of the human body, acts as an essential transport system for fluids, biomolecules and cells between peripheral tissues and the central circulation. Consequently, it is required for normal body physiology but is also involved in the pathogenesis of various diseases, most notably cancer. The important role of tumor-associated lymphatic vessels and lymphangiogenesis in the formation of lymph node metastasis has been elucidated during the last two decades, whereas the underlying mechanisms and the relation between lymphatic and peripheral organ dissemination of cancer cells are incompletely understood. Lymphatic vessels are also important for tumor-host communication, relaying molecular information from a primary or metastatic tumor to regional lymph nodes and the circulatory system. Beyond antigen transport, lymphatic endothelial cells, particularly those residing in lymph node sinuses, have recently been recognized as direct regulators of tumor immunity and immunotherapy responsiveness, presenting tumor antigens and expressing several immune-modulatory signals including PD-L1. In this review, we summarize recent discoveries in this rapidly evolving field and highlight strategies and challenges of therapeutic targeting of lymphatic vessels or specific lymphatic functions in cancer patients.

A robust and interpretable gene signature for predicting the lymph node status of primary T1/T2 oral cavity squamous cell carcinoma

Oral cavity squamous cell carcinoma (OSCC) affects more than 30 000 individuals in the United States annually, with smoking and alcohol consumption being the main risk factors. Management of early-stage tumors usually includes surgical resection followed by postoperative radiotherapy in certain cases. The cervical lymph nodes (LNs) are the most common site for local metastasis, and elective neck dissection is usually performed if the primary tumor thickness is greater than 3.5 mm. However, postoperative histological examination often reveals that many patients with early-stage disease are negative for neck nodal metastasis, posing a pressing need for improved risk stratification to either avoid overtreatment or prevent the disease progression. To this end, we aimed to identify a primary tumor gene signature that can accurately predict cervical LN metastasis in patients with early-stage OSCC. Using gene expression profiles from 189 samples, we trained K-top scoring pairs models and identified six gene pairs that can distinguish primary tumors with nodal metastasis from those without metastasis. The signature was further validated on an independent cohort of 35 patients using real-time polymerase chain reaction (PCR) in which it achieved an area under the receiver operating characteristic (ROC) curve and accuracy of 90% and 91%, respectively. These results indicate that such signature holds promise as a quick and cost effective method for detecting patients at high risk of developing cervical LN metastasis, and may be potentially used to guide the neck treatment regimen in early-stage OSCC.

Analyzing Lymphatic Vessel Patterning in Adult Tissue

Whole-mount immunostaining allows intact tissue to be surveyed in three dimensions, avoiding the more restricted fields of view provided by visualizing thin sections. This technique is particularly useful for imaging lymphatic and blood networks by high-resolution confocal microscopy, revealing how such vessels are spatially positioned, the subcellular arrangements of individual antigens, and interactions with individual cells within the interstitium or vessel lumen. The purpose of this chapter is to provide a practical guide for obtaining images of lymphatic vessels following immunofluorescence staining, primarily in mouse skin.

Targeting Lymphangiogenesis and Lymph Node Metastasis in Liver Cancer

Increased lymphangiogenesis and lymph node metastasis, the important prognostic indicators of aggressive hepatobiliary malignancies such as hepatocellular cancer and cholangiocarcinoma, are associated with poor patient outcome. The liver produces 25% to 50% of total lymphatic fluid in the body and has a dense network of lymphatic vessels. The lymphatic system plays critical roles in fluid homeostasis and inflammation and immune response. Yet, lymphatic vessel alterations and function are grossly understudied in the context of liver pathology. Expansion of the lymphatic network has been documented in clinical samples of liver cancer; and although largely overlooked in the liver, tumor-induced lymphangiogenesis is an important player, increasing tumor metastasis in several cancers. This review aims to provide a detailed perspective on the current knowledge of alterations in the hepatic lymphatic system during liver malignancies, as well as various molecular signaling mechanisms and growth factors that may provide future targets for therapeutic intervention. In addition, the review also addresses current mechanisms and bottlenecks for effective therapeutic targeting of tumor-associated lymphangiogenesis.

Mechanisms and Clinical Significance of Tumor Lymphatic Invasion

Tumor-associated lymphatic vessels play an important role in tumor progression, mediating lymphatic dissemination of malignant cells to tumor-draining lymph nodes and regulating tumor immunity. An early, necessary step in the lymphatic metastasis cascade is the invasion of lymphatic vessels by tumor cell clusters or single tumor cells. In this review, we discuss our current understanding of the underlying cellular and molecular mechanisms, which include tumor-specific as well as normal, developmental and immunological processes “hijacked” by tumor cells to gain access to the lymphatic system. Furthermore, we summarize the prognostic value of lymphatic invasion, discuss its relationship with local recurrence, lymph node and distant metastasis, and highlight potential therapeutic options and challenges.

The lymphatic vasculature: An active and dynamic player in cancer progression

The lymphatic vasculature has been widely described and explored for its key functions in fluid homeostasis and in the organization and modulation of the immune response. Besides transporting immune cells, lymphatic vessels play relevant roles in tumor growth and tumor cell dissemination. Cancer cells that have invaded into afferent lymphatics are propagated to tumor‐draining lymph nodes (LNs), which represent an important hub for metastatic cell arrest and growth, immune modulation, and secondary dissemination to distant sites. In recent years many studies have reported new mechanisms by which the lymphatic vasculature affects cancer progression, ranging from induction of lymphangiogenesis to metastatic niche preconditioning or immune modulation. In this review, we provide an up‐to‐date description of lymphatic organization and function in peripheral tissues and in LNs and the changes induced to this system by tumor growth and progression. We will specifically focus on the reported interactions that occur between tumor cells and lymphatic endothelial cells (LECs), as well as on interactions between immune cells and LECs, both in the tumor microenvironment and in tumor‐draining LNs. Moreover, the most recent prognostic and therapeutic implications of lymphatics in cancer will be reported and discussed in light of the new immune‐modulatory roles that have been ascribed to LECs.

Role of lymphatic endothelial cells in the tumor microenvironment-a narrative review of recent advances

Background

As lymphatic vessel is a major route for solid tumor metastasis, they are considered an essential part of tumor drainage conduits. Apart from forming the walls of lymphatic vessels, lymphatic endothelial cells (LECs) have been found to play multiple other roles in the tumor microenvironment, calling for a more in-depth review. We hope that this review may help researchers gain a detailed understanding of this fast-developing field and shed some light upon future research.

Methods

To achieve an informative review of recent advance, we carefully searched the Medline database for English literature that are openly published from the January 1995 to December 2020 and covered the topic of LEC or lymphangiogenesis in tumor progression and therapies. Two different authors independently examined the literature abstracts to exclude possible unqualified ones, and 310 papers with full texts were finally retrieved.

Results

In this paper, we discussed the structural and molecular basis of tumor-associated LECs, together with their roles in tumor metastasis and drug therapy. We then focused on their impacts on tumor cells, tumor stroma, and anti-tumor immunity, and the molecular and cellular mechanisms involved. Special emphasis on lung cancer and possible therapeutic targets based on LECs were also discussed.

Conclusions

LECs can play a much more complex role than simply forming conduits for tumor cell dissemination. Therapies targeting tumor-associated lymphatics for lung cancer and other tumors are promising, but more research is needed to clarify the mechanisms involved.

Single-Cell Transcriptomic Analysis Identifies a Unique Pulmonary Lymphangioleiomyomatosis Cell

Rationale: Lymphangioleiomyomatosis (LAM) is a metastatic neoplasm of reproductive-age women associated with mutations in tuberous sclerosis complex genes. LAM causes cystic remodeling of the lung and progressive respiratory failure. The sources and cellular characteristics of LAM cells underlying disease pathogenesis remain elusive.Objectives: Identification and characterization of LAM cells in human lung and uterus using a single-cell approach.Methods: Single-cell and single-nuclei RNA sequencing on LAM (n = 4) and control (n = 7) lungs, immunofluorescence confocal microscopy, ELISA, and aptamer proteomics were used to identify and validate LAMCORE cells and secreted biomarkers, predict cellular origins, and define molecular and cellular networks in LAM.Measurements and Main Results: A unique cell type termed LAMCORE was identified, which was distinct from, but closely related to, lung mesenchymal cells. LAMCORE cells expressing signature genes included known LAM markers such as PMEL, FIGF, CTSK, and MLANA and novel biomarkers validated by aptamer screening, ELISA, and immunofluorescence microscopy. LAM cells in lung and uterus are morphologically indistinguishable and share similar gene expression profiles and biallelic TSC2 mutations, supporting a potential uterine origin for the LAMCORE cell. Effects of LAM on resident pulmonary cell types indicated recruitment and activation of lymphatic endothelial cells.Conclusions: A unique population of LAMCORE cells was identified in lung and uterus of patients with LAM, sharing close transcriptomic identity. LAM cell selective markers, secreted biomarkers, and the predicted cellular molecular features provide new insights into the signaling and transcriptional programs that may serve as diagnostic markers and therapeutic targets to influence the pathogenesis of LAM.

Timed Ang2-Targeted Therapy Identifies the Angiopoietin-Tie Pathway as Key Regulator of Fatal Lymphogenous Metastasis

Recent clinical and preclinical advances have highlighted the existence of a previously hypothesized lymphogenous route of metastasis. However, due to a lack of suitable preclinical modeling tools, its contribution to long-term disease outcome and relevance for therapy remain controversial. Here, we established a genetically engineered mouse model (GEMM) fragment-based tumor model uniquely sustaining a functional network of intratumoral lymphatics that facilitates seeding of fatal peripheral metastases. Multiregimen survival studies and correlative patient data identified primary tumor-derived Angiopoietin-2 (Ang2) as a potent therapeutic target to restrict lymphogenous tumor cell dissemination. Mechanistically, tumor-associated lymphatic endothelial cells (EC), in contrast to blood vascular EC, were found to be critically addicted to the Angiopoietin-Tie pathway. Genetic manipulation experiments in combination with single-cell mapping revealed agonistically acting Ang2-Tie2 signaling as key regulator of lymphatic maintenance. Correspondingly, acute presurgical Ang2 neutralization was sufficient to prolong survival by regressing established intratumoral lymphatics, hence identifying a therapeutic regimen that warrants further clinical evaluation. SIGNIFICANCE: Exploiting multiple mouse tumor models including a unique GEMM-derived allograft system in combination with preclinical therapy designs closely matching the human situation, this study provides fundamental insight into the biology of tumor-associated lymphatic EC and defines an innovative presurgical therapeutic window of migrastatic Ang2 neutralization to restrict lymphogenous metastasis.This article is highlighted in the In This Issue feature, p. 211.

Stromal regulation of tumor-associated lymphatics

Recent advances have identified a growing array of roles played by lymphatics in the tumor microenvironment, from providing a route of metastasis to immune modulation. The tumor microenvironment represents an exceptionally complex, dynamic niche comprised of a diverse mixture of cancer cells and normal host cells termed the stroma. This review discusses our current understanding of stromal elements and how they regulate lymphatic growth and functional properties in the tumor context.

Mechanisms of Tumor-Lymphatic Interactions in Invasive Breast and Prostate Carcinoma

During the last few years, diverse studies have shown that tumors can actively interact with the lymphatic system and promote metastases development. In order to examine the molecular mechanisms involved in this interaction, we co-cultured tumor and lymphatic endothelial cells (LEC) and subsequently analyzed the molecular alterations of LECs. Therefore, LECs were co-cultivated with either a highly or weakly metastatic breast cancer cell line using contact (mixture) and non-contact (transwell) co-cultures. mRNA profiles from LECs were subsequently analyzed for genes specifically induced by highly metastatic tumor cells (“metastatic specific”). Among the up-regulated “metastatic specific” genes, we found candidates involved in cell cycle, cell adhesion and motility (BST2, E-selectin, and HMMR), cytokines (CCL7, CXCL6, CXCL1, and CSF2) and factors of the complement system (C1R, C3, and CFB). Among the down-regulated genes, we detected the hyaluronan receptor STAB2, angiogenic factor apelin receptor (APLNR), and the glycosylation enzyme MAN1A1. In an additional prostate cancer co-culture model, we could confirm a “metastatic specific” upregulation of E-selectin and CCL7 in LECs after interaction with the prostate cancer cell lines LNCAP (highly metastatic) and DU145 (weakly metastatic). These data allowed us to identify a set of genes regulated in LECs during in vitro communication with cancer cells, which might subsequently facilitate lymphatic metastasis.

Transcriptional profiling of breast cancer-associated lymphatic vessels reveals VCAM-1 as regulator of lymphatic invasion and permeability

Tumor‐associated lymphangiogenesis and lymphatic invasion of tumor cells correlate with poor outcome in many tumor types, including breast cancer. Various explanations for this correlation have been suggested in the past, including the promotion of lymphatic metastasis and an immune‐inhibitory function of lymphatic endothelial cells (LECs). However, the molecular features of tumor‐associated lymphatic vessels and their implications for tumor progression have been poorly characterized. Here, we report the first transcriptional analysis of tumor‐associated LECs directly isolated from the primary tumor in an orthotopic mouse model of triple negative breast cancer (4T1). Gene expression analysis showed a strong upregulation of inflammation‐associated genes, including endothelial adhesion molecules such as VCAM‐1, in comparison to LECs derived from control tissue. In vitro experiments demonstrated that VCAM‐1 is not involved in the adhesion of tumor cells to LECs but unexpectedly promoted lymphatic permeability by weakening of lymphatic junctions, most likely through a mechanism triggered by interactions with integrin α4 which was also induced in tumor‐associated LECs. In line with this, in vivo blockade of VCAM‐1 reduced lymphatic invasion of 4T1 cells. Taken together, our findings suggest that disruption of lymphatic junctions and increased permeability via tumor‐induced lymphatic VCAM‐1 expression may represent a new target to block lymphatic invasion and metastasis.

What's new?

Tumor‐associated lymphatic vessels serve important roles in tumor progression and metastasis. Nonetheless, little is known about the molecular changes in these vessels that give rise to a tumor‐promoting phenotype. In this study, transcriptional analysis was performed on lymphatic endothelial cells (LECs) isolated from a mouse model of triple‐negative breast cancer. Endothelial adhesion molecules, including tumor‐induced VCAM‐1, were strongly upregulated in tumor‐associated LECs. Additional experiments showed that VCAM‐1 upregulation influences lymphatic permeability and that its inhibition attenuates lymphatic breast cancer cell invasion. The findings identify VCAM‐1 as a potential target for the blockade of lymphatic invasion of tumor cells.

TNF-derived peptides inhibit tumour growth and metastasis through cytolytic effects on tumour lymphatics

Tumour necrosis factor (TNF) is a multi-functional cytokine with profound and diverse effects on physiology and pathology. Identifying the molecular determinants underlying the functions and pathogenic effects of TNF is key to understanding its mechanisms of action and identifying new therapeutic opportunities based on this important molecule. Previously, we showed that some evolutionarily conserved peptides derived from TNF could induce cell death (e.g. apoptosis and/or necrosis), a feature of immune defence mechanisms shared by many vertebrates. In this study, we demonstrated that necrosis-inducing peptide P16 kills human glioblastoma cancer cells and primary human hepatoma or renal cancer cells isolated from patients who had not responded to standard treatments. Importantly, we show that the necrosis-inducing peptide P1516 significantly improves survival by inhibiting tumour metastasis in a 4T1 breast cancer syngeneic graft mouse model. Because the lymphatic system is an important metastatic route in many cancers, we also tested the effect of TNF-derived peptides on monolayers of primary human lymphatic endothelial cells (hDLEC) and found that they increased junctional permeability by inducing cytoskeletal reorganization, gap junction formation and cell death. Transmission electron microscopy imaging evidence, structural analysis and in-vitro liposome leakage experiments strongly suggest that this killing is due to the cytolytic nature of these peptides. P1516 provides another example of a pro-cytotoxic TNF peptide that probably functions as a cryptic necrotic factor released by TNF degradation. Its ability to inhibit tumour metastasis and improve survival may form the basis of a novel approach to cancer therapy.

The Interplay Between Lymphatic Vessels and Chemokines

Chemokines are a family of small protein cytokines that act as chemoattractants to migrating cells, in particular those of the immune system. They are categorized functionally as either homeostatic, constitutively produced by tissues for basal levels of cell migration, or inflammatory, where they are generated in association with a pathological inflammatory response. While the extravasation of leukocytes via blood vessels is a key step in cells entering the tissues, the lymphatic vessels also serve as a conduit for cells that are recruited and localized through chemoattractant gradients. Furthermore, the growth and remodeling of lymphatic vessels in pathologies is influenced by chemokines and their receptors expressed by lymphatic endothelial cells (LECs) in and around the pathological tissue. In this review we summarize the diverse role played by specific chemokines and their receptors in shaping the interaction of lymphatic vessels, immune cells, and other pathological cell types in physiology and disease.

Tumor-Associated Lymphatic Vessel Features and Immunomodulatory Functions

The lymphatic system comprises a network of lymphoid tissues and vessels that drains the extracellular compartment of most tissues. During tumor development, lymphatic endothelial cells (LECs) substantially expand in response to VEGFR-3 engagement by VEGF-C produced in the tumor microenvironment, a process known as tumor-associated lymphangiogenesis. Lymphatic drainage from the tumor to the draining lymph nodes consequently increases, powering interstitial flow in the tumor stroma. The ability of a tumor to induce and activate lymphatic growth has been positively correlated with metastasis. Much effort has been made to identify genes responsible for tumor-associated lymphangiogenesis. Inhibition of lymphangiogenesis with soluble VEGFR-3 or with specific monoclonal antibodies decreases tumor spread to LNs in rodent models. Importantly, tumor-associated lymphatics do not only operate as tumor cell transporters but also play critical roles in anti-tumor immunity. Therefore, metastatic as well as primary tumor progression can be affected by manipulating tumor-associated lymphatic remodeling or function. Here, we review and discuss our current knowledge on the contribution of LECs immersed in the tumor microenvironment as immunoregulators, as well as a possible functional remodeling of LECs subsets depending on the organ microenvironment.

Heterogeneity of tumor lymphangiogenesis: Progress and prospects

Lymphangiogenesis and increased expression of lymphangiogenic growth factors are associated with high rates of lymph node (LN) metastasis and with poor prognosis in some, but not all, solid tumors. In addition to its involvement in metastasis, lymphangiogenesis has been shown to have other roles in tumor pathogenesis, such as the niche function of tumor stem cells and regulatory functions of antitumor immune responses. In contrast, evidence has accumulated that tumor-induced lymphangiogenesis displays the heterogeneity in gene signature, structure, cellular origins and functional plasticity. This review summarizes the advances in the research on the heterogeneity of tumor lymphangiogenesis and discusses how it may contribute to functional complexity and multiplicity of lymphangiogenesis in tumor progression.

The evolving role of lymphatics in cancer metastasis

While the link between the lymphatic system and the metastatic spread of cancer is centuries old, understanding of the underlying mechanisms is still evolving. Lymphatic vessels provide a route for tumour cells to reach regional lymph nodes (LNs), which is prognostic of distant organ metastasis and poor survival. However, genomic analyses of metastatic cancer now reveal complex patterns of dissemination. The lymphatic endothelial cells lining lymphatics respond to molecular cues from the tumour microenvironment, mediating growth and remodelling of lymphatic vessels at the primary tumour, draining LNs and distant premetastatic niches. Recent studies emphasise that this not only supports metastasis but also influences antitumour immunity. Understanding the complex interactions between tumour cells, the immune system and lymphatics will be essential to inform developing therapeutic and prognostic approaches to cancer.

Tumor-associated Lymphatic Endothelial Cells Promote Lymphatic Metastasis By Highly Expressing and Secreting SEMA4C

PURPOSE

Lymphatic vessels are mainly regarded as passive conduits for the dissemination of cancer cells. In this study, we investigate whether and how the tumor-associated lymphatic vessels may play an active role in tumor metastasis.

EXPERIMENTAL DESIGN

In situ laser capture microdissection of lymphatic vessels followed by cDNA microarray analysis was used to determine the expression profiling of lymphatic endothelial cells (LEC). Gene expression levels and activity of signaling pathways were measured by real-time RT-PCR, ELISA, or immunoblotting. Lymphangiogenesis was assessed by IHC. Lymph node metastasis was measured using fluorescence imaging. The effects of SEMA4C on lymphangiogenesis in vitro were evaluated using migration assay and tube-formation assay of LECs.

RESULTS

Tumor-associated LECs are molecularly and functionally different from their normal counterparts. In addition to expressing high levels of membrane-bound SEMA4C, tumor-associated LECs also produced soluble SEMA4C (sSEMA4C). Increased serum sSEMA4C was detected in patients with breast cancer and cervical cancer. Patients with metastasis had much higher levels of serum sSEMA4C. sSEMA4C promoted lymphangiogenesis by activating PlexinB2-ERBB2 signaling in LECs, and promoted the proliferation and migration of tumor cells by activating PlexinB2-MET signaling, thus promoting lymphatic metastasis. Although the SEMA4C signaling pathways differ between LECs and tumor cells, RHOA activation was necessary for the effects of SEMA4C in both types of cells.

CONCLUSIONS

Tumor-associated LECs produce sSEMA4C to promote lymphatic metastasis of tumors. Our results suggest that SEMA4C and RHOA might be potential therapeutic targets, and that higher serum sSEMA4C could be a marker for breast cancer and cervical cancer. Clin Cancer Res; 23(1); 214-24. ©2016 AACR.

Heterogeneity in the lymphatic vascular system and its origin

Lymphatic vessels have historically been viewed as passive conduits for fluid and immune cells, but this perspective is increasingly being revised as new functions of lymphatic vessels are revealed. Emerging evidence shows that lymphatic endothelium takes an active part in immune regulation both by antigen presentation and expression of immunomodulatory genes. In addition, lymphatic vessels play an important role in uptake of dietary fat and clearance of cholesterol from peripheral tissues, and they have been implicated in obesity and arteriosclerosis. Lymphatic vessels within different organs and in different physiological and pathological processes show a remarkable plasticity and heterogeneity, reflecting their functional specialization. In addition, lymphatic endothelial cells (LECs) of different organs were recently shown to have alternative developmental origins, which may contribute to the development of the diverse lymphatic vessel and endothelial functions seen in the adult. Here, we discuss recent developments in the understanding of heterogeneity within the lymphatic system considering the organ-specific functional and molecular specialization of LECs and their developmental origin.

Esophageal adenocarcinoma and obesity: peritumoral adipose tissue plays a role in lymph node invasion

Obesity is associated with cancer risk in esophageal adenocarcinoma (EAC). Adipose tissue directly stimulates tumor progression independently from body mass index (BMI), but the mechanisms are not fully understood. We studied the morphological, histological and molecular characteristics of peritumoral and distal adipose tissue of 60 patients with EAC, to investigate whether depot-specific differences affect tumor behavior. We observed that increased adipocyte size (a hallmark of obesity) was directly associated with leptin expression, angiogenesis (CD31) and lymphangiogenesis (podoplanin); however, these parameters were associated with nodal metastasis only in peritumoral but not distal adipose tissue of patients. We treated OE33 cells with conditioned media (CM) collected from cultured biopsies of adipose tissue and we observed increased mRNA levels of leptin and adiponectin receptors, as well as two key regulator genes of epithelial-to-mesenchymal transition (EMT): alpha-smooth muscle actin (α-SMA) and E-cadherin. This effect was greater in cells treated with CM from peritumoral adipose tissue of patients with nodal metastasis and was partially blunted by a leptin antagonist. Therefore, peritumoral adipose tissue may exert a direct effect on the progression of EAC by secreting depot-specific paracrine factors, and leptin is a key player in this crosstalk.

ESAM predicts cardiovascular mortality in diabetic hemodialysis patients

AIM

To assess endothelial cell selective adhesion molecule (ESAM) as predictor of cardiovascular mortality in diabetic dialysis patients (DDPs).

METHODS

ESAM, clinical and laboratory parameters were assessed in 73 DDP. Cardiovascular mortality was recorded in a 2 years' prospective observational study.

RESULTS

Baseline ESAM was 17.1 (10.05-24.8) ng/ml and was correlated to phosphate (r = -0.42, p = 0.008), parathormone (r = -0.36, p = 0.048), albumin (r = -0.24, p = 0.048). ESAM significantly predicted cardiovascular death in univariate [HR = 1.03, 95% CI (1.006-1.054), p = 0.01] and multivariate [HR = 1.034, 95% CI (1.003-1.066), p = 0.03] Cox analysis. Time to cardiovascular death was shorter for patients with ESAM >12.44 ng/ml, p = 0.0045.

CONCLUSION

ESAM is an independent predictor of cardiovascular mortality in DDP.

Prognostic and diagnostic implications of MMP-2, MMP-9, and VEGF-α expressions in colorectal cancer

PURPOSE

Angiogenesis and tumor invasion are complex processes that are mediated by various proteins, such as vascular endothelial growth factor (VEGF-α) and the matrix-degrading enzymes metalloproteinase-2 and -9 (MMP-2, MMP-9). The aim of this study was to determine what roles MMP-2, MMP-9, and VEGF-α play in colorectal cancer (CRC) by correlating their expression levels with the cancer TNM stage, modified Dukes criteria, degree of cell differentiation, and long-term patient survival.

METHODS

The present series consisted of tissue samples obtained from 180 patients who had undergone large bowel resection during 1995 and 2005 at the Luis Antonio Hospital. Archival paraffin-embedded CRC tissue samples were used to generate tissue microarray blocks, which were immunohistochemically stained for MMP-2, MMP-9, and VEGF-α. Three different grading systems were applied to evaluate staining intensity. Chi-squared Person test and Kaplan-Meier survival curves were used, and values of p<0.05 were considered statistically significant.

RESULTS

MMP-2 expression showed a significant association with more invasive cancer stages (p<0.001) and death (p<0.041). VEGF-α expression correlated with a high TNM stage (p<0.009), the degree of cell differentiation (p<0.025) and patient death as a result of disease (p<0.035). The Kaplan-Meier survival estimated that patients with strong staining for MMP-2 (log-rank x(2)=34.09; p<0.0001), MMP-9 (log-rank x(2)=12.83; p<0.0003) and VEGF (log-rank x(2)=33.9; p<0.0001) showed a greater tendency towards death during 60 months of follow up.

CONCLUSIONS

The quantification of VEGF-α, MMP-2 and MMP-9 expression in colorectal cancer may be related to survival. These data add to the growing epidemiological and experimental evidence that VEGF-α, MMP-2 and MMP-9 may play a role in colorectal tumorigenesis.

Role of lymphatic vasculature in regional and distant metastases

In cancer, lymphatic vasculature has been traditionally viewed only as a transportation system for metastatic cells. It has now become clear that lymphatics perform many additional functions which could influence cancer progression. Lymphangiogenesis, induced at the primary tumor site and at distant sites, potently augments metastasis. Lymphatic endothelial cells (LECs) control tumor cell entry and exit from the lymphatic vessels. LECs also control immune cell traffic and directly modulate adaptive immune responses. This review highlights advances in our understanding of the mechanisms by which lymphatic vessels, and in particular lymphatic endothelium, impact metastasis.

Facilitation of endoglin-targeting cancer therapy by development/utilization of a novel genetically engineered mouse model expressing humanized endoglin (CD105)

Endoglin (ENG) is a TGF-β coreceptor and essential for vascular development and angiogenesis. A chimeric antihuman ENG (hENG) monoclonal antibody (mAb) c-SN6j (also known as TRC105) shows promising safety and clinical efficacy features in multiple clinical trials of patients with various advanced solid tumors. Here we developed a novel genetically engineered mouse model to optimize the ENG-targeting clinical trials. We designed a new targeting vector that contains exons 4-8 of hENG gene to generate novel genetically engineered mice (GEMs) expressing functional human/mouse chimeric (humanized) ENG with desired epitopes. Genotyping of the generated mice confirmed that we generated the desired GEMs. Immunohistochemical analysis demonstrated that humanized ENG protein of the GEMs expresses epitopes defined by 7 of our 8 anti-hENG mAbs tested. Surprisingly the homozygous GEMs develop normally and are healthy. Established breast and colon tumors as well as metastasis and tumor microvessels in the GEMs were effectively suppressed by systemic administration of anti-hENG mAbs. Additionally, test result indicates that synergistic potentiation of antitumor efficacy can be induced by simultaneous targeting of two distinct epitopes by anti-hENG mAbs. Sorafenib and capecitabine also showed antitumor efficacy in the GEMs. The presented novel GEMs are the first GEMs that express the targetable humanized ENG. Test results indicate utility of the GEMs for the clinically relevant studies. Additionally, we generated GEMs expressing a different humanized ENG containing exons 5-6 of hENG gene, and the homozygous GEMs develop normally and are healthy.

Targeting angiogenic pathway for chemoprevention of experimental colon cancer using C-phycocyanin as cyclooxygenase-2 inhibitor

An angiogenic pathway was studied that involved stromal tissue degradation with matrix metalloproteinases (MMPs), vesicular endothelial growth factor-A (VEGF-A), and hypoxia inducible factor-1α (HIF-1α) mediated growth regulation in a complex interaction with chemokines, such as monocyte chemoattractant protein-1 (MCP-1) and macrophage inflammatory protein-1β (MIP-1β). Gene and protein expression was studied with real-time PCR, Western immunoblot, and immunofluorescence. Morphological and histopathological analysis of tumor was done, as also the activity of MMPs and HIF-1α by gelatin zymography and ELISA. Binding interactions of proteins were studied by molecular docking. Piroxicam, a traditional NSAID and C-phycocyanin, a biliprotein from Spirulina platensis, were utilized in the chemoprevention of DMH-induced rat colon cancer. A significant number of tumors was evident in DMH treated animals, while with piroxicam and C-phycocyanin, the number and size of tumors/lesions were reduced. Colonic tissues showed severe dysplasia, tubular adenoma, and adenocarcinoma from DMH, with invasive features along with signet ring cell carcinoma. No occurrence of carcinoma was detected in either of the drug treatments or in a combination regimen. An elevated VEGF-A, MMP-2, and MMP-9 level was observed, which is required for metastasis and invasion into surrounding tissues. Drugs induced chemoprevention by down-regulating these proteins. Piroxicam docked in VEGF-A binding site of VEGF-A receptors i.e., VEGFR1 and VEGFR2, while phycocyanobilin (a chromophore of C-phycocyanin) docked with VEGFR1 alone. HIF-1α is up-regulated which is associated with increased oxygen demand and angiogenesis. MCP-1 and MIP-1β expression was also found altered in DMH and regulated by the drugs. Anti-angiogenic role of piroxicam and C-phycocyanin is well demonstrated.

Prognostic Significance of β-Catenin, E-Cadherin, and SOX9 in Colorectal Cancer: Results from a Large Population-Representative Series

Robust biomarkers that can precisely stratify patients according to treatment needs are in great demand. The literature is inconclusive for most reported prognostic markers for colorectal cancer (CRC). Hence, adequately reported studies in large representative series are necessary to determine their clinical potential. We investigated the prognostic value of three Wnt signaling-associated proteins, β-catenin, E-cadherin, and SOX9, in a population-representative single-hospital series of 1290 Norwegian CRC patients by performing immunohistochemical analyses of each marker using the tissue microarray technology. Loss of membranous or cytosolic β-catenin and loss of cytosolic E-cadherin protein expression were significantly associated with reduced 5-year survival in 903 patients who underwent major resection (722 evaluable tissue cores) independently of standard clinicopathological high-risk parameters. Pre-specified subgroup analyses demonstrated particular effect for stage IV patients for β-catenin membrane staining (P = 0.018; formal interaction test P = 0.025). Among those who underwent complete resection (714 patients, 568 evaluable), 5-year time-to-recurrence analyses were performed, and stage II patients with loss of cytosolic E-cadherin were identified as an independent high-risk subgroup (P = 0.020, formal interaction test was not significant). Nuclear β-catenin and SOX9 protein, regardless of intracellular location, were not associated with prognosis. In conclusion, the protein expression level of membranous or cytosolic β-catenin and E-cadherin predicts CRC patient subgroups with inferior prognosis.

Culturing purifies murine lymph node lymphatic endothelium

BACKGROUND

Lymph node (LN) lymphatic sinuses transport lymph, cells, and antigens from the periphery through the LN. The lymphatic endothelium lining these sinuses appears to be an important contributor to the lymph node immune response. It has been challenging to obtain sufficient LN lymphatic endothelial cells for investigation of their functions, as they are minor constituents of LNs.

METHODS AND RESULTS

A procedure was developed to purify lymphatic endothelial cells (LEC) from murine LNs, which yields large numbers of primary LN LEC. Two-dimensional in vitro cultures of dissociated LN stromal cells initially consist of multiple cell types, and then rapidly evolve to produce pure cultures of lymphatic endothelium within a few passages. One million LEC can be harvested after 4 weeks of culture, and much larger cell numbers can be obtained by continued culturing over long periods. The LEC cultures maintain endothelial morphology and expression of LEC markers, and preserve the same slow growth characteristics over at least 20 passages. The LEC cultures readily form tubes in Matrigel at early and at late passages, resembling those formed by LEC lines.

CONCLUSIONS

A simple and economical approach to obtain purified primary murine LN LEC was developed for in vitro studies of their function. The morphology, growth characteristics, and functional behavior of these cells in tube formation assays did not change between initial and long-term passages. Large numbers of these cells can be harvested after long-term passage, so that they can be studied in biochemical and biological assays.

Emerging roles of lymphatic endothelium in regulating adaptive immunity

Emerging research on the roles of stromal cells in modulating adaptive immune responses has included a new focus on lymphatic endothelial cells (LECs). LECs are presumably the first cells that come into direct contact with peripheral antigens, cytokines, danger signals, and immune cells travelling from peripheral tissues to lymph nodes. LECs can modulate dendritic cell function, present antigens to T cells on MHC class I and MHC class II molecules, and express immunomodulatory cytokines and receptors, which suggests that their roles in adaptive immunity are far more extensive than previously realized. This Review summarizes the emergent evidence that LECs are important in maintaining peripheral tolerance, limiting and resolving effector T cell responses, and modulating leukocyte function.

Mechanisms of lymphatic metastasis

Malignant tumors release growth factors such as VEGF-C to induce lymphatic vessel expansion (lymphangiogenesis) in primary tumors and in draining sentinel LNs, thereby promoting LN metastasis. Surprising recent evidence suggests that lymphatic vessels do not merely represent passive channels for tumor spread, but that they may actively promote tumor cell recruitment to LNs, cancer stem cell survival, and immune modulation. New imaging approaches allow the sensitive visualization of the earliest LN metastases and the quantitative, noninvasive measurement of the function of tumor-draining lymphatic vessels, with potential applications in the development of biomarkers for prognosis and measurement of therapeutic response.

JAM-C promotes lymphangiogenesis and nodal metastasis in non-small cell lung cancer

This study aims to investigate lymphatic metastasis-related genes in non-small cell lung carcinomas (NSCLC). NSCLC tissue was analyzed for expression of junctional adhesion molecule-C (JAM-C) protein. Our data revealed novel associations between JAM-C overexpression in primary tumors and lymphatic microvessel density (LMVD), lymph node metastasis, and poorer overall survival and recurrence-free survival. We used the highly metastatic human lung adenocarcinoma cell line Anip973 and its parental line AGZY83-a, which has a low metastatic capacity, in vivo and vitro. We found that JAM-C played an important role in different metastasis capacity of lymph node. JAM-C affected tumor growth, LNM, JAM-C, VEGF-C, vasculature, and ERK1/2 phosphorylation (p-ERK1/2). β1 integrin was involved in lymph node metastasis. Moreover, JAM-C knockdown in highly metastatic Anip973 decreased cell migration in scratch-wound assays. The JAM-C knockdown in Anip973 cells and JAM-C cDNA in AGZY83-a cells regulated the vascular endothelial growth factor C (VEGF-C) expression. Immunofluorescence showed that blocked VEGF-C expression in JAM-C shRNA Anip973 cells were restored after JAM-C treatment. JAM-C-induced VEGF-C in JAM-C cDNA AGZY83-a cells was also effectively inhibited by treatment with an antibody specifically against JAM-C. Use of media from Anip973 cells, AGZY83-a, and A549cells lung cancer cells that overexpressed or downregulated JAM-C was demonstrated to affect activity of VEGF-C-induced β1 integrin subunit or ERK activity in human dermal lymphatic endothelial cells (HDLEC) treated with VEGF-C or inhibitory antibody to JAM-C. Overall, these results indicate that JAM-C could mediate metastasis as it contributes to VEGF-C expression in cancer cells. JAM-C affects β1and ERK activation in HDLEC, thus promoting lymphangiogenesis and nodal metastasis. Our findings indicate that JAM-C may be a therapeutic target for preventing and treating lymphatic metastases.

Lymphangiogenesis and lymphatic vessel remodelling in cancer

The generation of new lymphatic vessels through lymphangiogenesis and the remodelling of existing lymphatics are thought to be important steps in cancer metastasis. The past decade has been exciting in terms of research into the molecular and cellular biology of lymphatic vessels in cancer, and it has been shown that the molecular control of tumour lymphangiogenesis has similarities to that of tumour angiogenesis. Nevertheless, there are significant mechanistic differences between these biological processes. We are now developing a greater understanding of the specific roles of distinct lymphatic vessel subtypes in cancer, and this provides opportunities to improve diagnostic and therapeutic approaches that aim to restrict the progression of cancer.

(89)Zr-labeled anti-endoglin antibody-targeted gold nanoparticles for imaging cancer: implications for future cancer therapy

AIMS

Antibody-labeled gold nanoparticles represent an attractive tool for cancer imaging and therapy. In this study, the anti-CD105 antibody was conjugated with gold nanoparticles (AuNPs) for the first time. The antibody biodistribution in mice before and after conjugation to AuNPs was studied, with a focus on tumor targeting.

MATERIALS & METHODS

Antibodies were radiolabeled with 89Zr before conjugation to AuNPs (5 nm). Immunonanoconjugates were characterized in vitro in terms of size, stability in plasma and binding to the target. Quantitative PET imaging and ICP-MS analysis assessed in vivo distribution and specific tumor targeting of tracers.

RESULTS

The tumor uptake of immunoconjugates was preserved up to 24 h after injection, with high tumor contrast and selective tumor targeting. No major tracer accumulation was observed over time in nonspecific organs. ICP-MS analysis confirmed the antibody specificity after nanoparticle conjugation.

CONCLUSION

The anti-CD105 antibody conjugation to AuNPs did not greatly affect CD105-dependent tumor uptake and the efficacy of tumor targeting for cancer detection.

HIV-1 matrix protein p17 promotes lymphangiogenesis and activates the endothelin-1/endothelin B receptor axis

OBJECTIVE

AIDS-related lymphomas are high grade and aggressively metastatic with poor prognosis. Lymphangiogenesis is essential in supporting proliferation and survival of lymphoma, as well as tumor dissemination. Data suggest that aberrant lymphangiogenesis relies on action of HIV-1 proteins rather than on a direct effect of the virus itself. HIV-1 matrix protein p17 was found to accumulate and persist in lymph nodes of patients even under highly active antiretroviral therapy. Because p17 was recently found to exert a potent proangiogenic activity by interacting with chemokine (C-X-C motif) receptors 1 and 2, we tested the prolymphangiogenic activity of the viral protein.

APPROACH AND RESULTS

Human primary lymph node-derived lymphatic endothelial cells were used to perform capillary-like structure formation, wound healing, spheroids, and Western blot assays after stimulation with or without p17. Here, we show that p17 promotes lymphangiogenesis by binding to chemokine (C-X-C motif) receptor-1 and chemokine (C-X-C motif) receptor-2 expressed on lymph node-derived lymphatic endothelial cells and activating the Akt/extracellular signal-regulated kinase signaling pathway. In particular, it was found to induce capillary-like structure formation, sprout formation from spheroids, and increase lymph node-derived lymphatic endothelial cells motility. The p17 lymphangiogenic activity was, in part, sustained by activation of the endothelin-1/endothelin receptor B axis. A Matrigel plug assay showed that p17 was able to promote the outgrowth of lymphatic vessels in vivo, demonstrating that p17 directly regulates lymphatic vessel formation.

CONCLUSIONS

Our results suggest that p17 may generate a prolymphangiogenic microenvironment and plays a role in predisposing the lymph node to lymphoma growth and metastasis. This finding offers new opportunities to identify treatment strategies in combating AIDS-related lymphomas.

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

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

Lymphatic vessels: new targets for the treatment of inflammatory diseases

The lymphatic system plays an important role in the physiological control of the tissue fluid balance and in the initiation of immune responses. Recent studies have shown that lymphangiogenesis, the growth of new lymphatic vessels and/or the expansion of existing lymphatic vessels, is a characteristic feature of acute inflammatory reactions and of chronic inflammatory diseases. In these conditions, lymphatic vessel expansion occurs at the tissue level but also within the draining lymph nodes. Surprisingly, activation of lymphatic vessel function by delivery of vascular endothelial growth factor-C exerts anti-inflammatory effects in several models of cutaneous and joint inflammation. These effects are likely mediated by enhanced drainage of extravasated fluid and inflammatory cells, but also by lymphatic vessel-mediated modulation of immune responses. Although some of the underlying mechanisms are just beginning to be identified, lymphatic vessels have emerged as important targets for the development of new therapeutic strategies to treat inflammatory conditions. In this context, it is of great interest that some of the currently used anti-inflammatory drugs also potently activate lymphatic vessels.

Is lymphatic endoglin expression a risk marker for breast cancer metastasis? Results of a pilot study

BACKGROUND

Studies have identified endoglin as a biological marker that is overexpressed on the microvessels of certain solid cancers (breast, colorectal cancer, and head and neck squamous cell cancers). There is, at present, no immunohistochemical marker that can discriminate between lymph node-negative and or lymph node-positive breast cancer tissue.

METHODS

The expression of endoglin was quantified by immunohistochemistry and assessment of microvessel density in 53 surgical specimens. These were comprised of breast tumor tissue that had not spread to the regional lymph nodes (lymph node-negative breast tumor tissue: 20 specimens), breast tumor tissue had spread to regional lymph nodes (lymph node-positive breast tumor tissue: 21 specimens), and normal breast tissue as a control (12 specimens).

RESULTS

Significant difference was observed between the expression of endoglin on microvessels of lymph node-negative and lymph node-positive breast cancer tissue (p<0.05). This significant difference was shown to be due to endoglin expression on lymphatic vessels (p<0.02), rather than on blood vessels (p>0.05).

CONCLUSIONS

These findings are the first to suggest that endoglin expression on breast tumor lymphatic vessels may have diagnostic potential as a discriminator between lymph node-negative and lymph node-positive breast cancer. Further studies would be required to confirm this.

Endothelin 1 in cancer: biological implications and therapeutic opportunities

Activation of autocrine and paracrine signalling by endothelin 1 (ET1) binding to its receptors elicits pleiotropic effects on tumour cells and on the host microenvironment. This activation modulates cell proliferation, apoptosis, migration, epithelial-to-mesenchymal transition, chemoresistance and neovascularization, thus providing a strong rationale for targeting ET1 receptors in cancer. In this Review, we discuss the advances in our understanding of the diverse biological roles of ET1 in cancer and describe the latest preclinical and clinical progress that has been made using small-molecule antagonists of ET1 receptors that inhibit ET1-driven signalling.

Vascular remodeling in cancer

The growth and dissemination of tumors rely on an altered vascular network, which supports their survival and expansion and provides accessibility to the vasculature and a route of transport for metastasizing tumor cells. The remodeling of vascular structures through generation of new vessels (for example, via tumor angiogenesis) is a well studied, even if still quite poorly understood, process in human cancer. Antiangiogenic therapies have provided insight into the contribution of angiogenesis to the biology of human tumors, yet have also revealed the ease with which resistance to antiangiogenic drugs can develop, presumably involving alterations to vascular signaling mechanisms. Furthermore, cellular and/or molecular changes to pre-existing vessels could represent subtle pre-metastatic alterations to the vasculature, which are important for cancer progression. These changes, and associated molecular markers, may forecast the behavior of individual tumors and contribute to the early detection, diagnosis and prognosis of cancer. This review, which primarily focuses on the blood vasculature, explores current knowledge of how tumor vessels can be remodeled, and the cellular and molecular events responsible for this process.

LKB1 represses focal adhesion kinase (FAK) signaling via a FAK-LKB1 complex to regulate FAK site maturation and directional persistence

Liver kinase β1 (LKB1, also known as STK11) is a serine/threonine kinase that has multiple cellular functions including the regulation of cell polarity and motility. Murine proteomic studies show that LKB1 loss causes aberrant adhesion signaling; however, the mechanistic underpinnings of this relationship are unknown. We show that cells stably depleted of LKB1 or its co-activator STRADα have increased phosphorylation of focal adhesion kinase (FAK) at Tyr(397)/Tyr(861) and enhanced adhesion to fibronectin. LKB1 associates in a complex with FAK and LKB1 accumulation at the cellular leading edge is mutually excluded from regions of activated Tyr(397)-FAK. LKB1-compromised cells lack directional persistence compared with wild-type cells, but this is restored through subsequent pharmacological FAK inhibition or depletion, showing that cell directionality is mediated through LKB1-FAK signaling. Live cell confocal imaging reveals that LKB1-compromised cells lack normal FAK site maturation and turnover, suggesting that defects in adhesion and directional persistence are caused by aberrant adhesion dynamics. Furthermore, re-expression of full-length wild-type or the LKB1 N-terminal domain repressed FAK activity, whereas the kinase domain or C-terminal domain alone did not, indicating that FAK suppression is potentially regulated through the LKB1 N-terminal domain. Based upon these results, we conclude that LKB1 serves as a FAK repressor to stabilize focal adhesion sites, and when LKB1 function is compromised, aberrant FAK signaling ensues, resulting in rapid FAK site maturation and poor directional persistence.

The new era of the lymphatic system: no longer secondary to the blood vascular system

The blood and lymphatic systems are the two major circulatory systems in our body. Although the blood system has been studied extensively, the lymphatic system has received much less scientific and medical attention because of its elusive morphology and mysterious pathophysiology. However, a series of landmark discoveries made in the past decade has begun to change the previous misconception of the lymphatic system to be secondary to the more essential blood vascular system. In this article, we review the current understanding of the development and pathology of the lymphatic system. We hope to convince readers that the lymphatic system is no less essential than the blood circulatory system for human health and well-being.

Adrenomedullin gene dosage correlates with tumor and lymph node lymphangiogenesis

Adrenomedullin (AM) is a potent lymphangiogenic factor that promotes lymphatic endothelial cell (LEC) proliferation through a pharmacologically tractable G-protein-coupled receptor. Numerous types of human cancers have increased levels of AM; however, the functional consequences of this fact have not been characterized. Therefore, we evaluated whether modulating adrenomedullin (Adm) gene dosage within tumor cells affects lymphangiogenesis. Murine Lewis lung carcinoma (LLC) cells that overexpress or underexpress Adm were injected subcutaneously into C57BL/6 mice, and tumors were evaluated for growth and vascularization. A dosage range from ∼10 to 200% of wild-type Adm expression did not affect LLC proliferation in vitro or in vivo, nor did it affect angiogenesis. Notably, the dosage of Adm markedly and significantly influenced tumor lymphangiogenesis. Reduced Adm expression in tumors decreased the proliferation of LECs and the number of lymphatic vessels, while elevated tumor Adm expression led to enlarged lymphatic vessels. Moreover, overexpression of Adm in tumors induced sentinel lymph node lymphangiogenesis and led to an increased incidence of Ki67-positive foci within the lung. These data show that tumor-secreted AM is a critical factor for driving both tumor and lymph node lymphangiogenesis. Thus, pharmacological targeting of AM signaling may provide a new avenue for inhibition of tumor lymphangiogenesis.

The CXCL12-CXCR4 chemokine pathway: a novel axis regulates lymphangiogenesis

PURPOSE

Lymphangiogenesis, the growth of lymphatic vessels, contributes to lymphatic metastasis. However, the precise mechanism underlying lymphangiogenesis remains poorly understood. This study aimed to examine chemokine/chemokine receptors that directly contribute to chemoattraction of activated lymphatic endothelial cells (LEC) and tumor lymphangiogenesis.

EXPERIMENTAL DESIGN

We used quantitative RT-PCR to analyze specifically expressed chemokine receptors in activated LECs upon stimulation of vascular endothelial growth factor-C (VEGF-C). Subsequently, we established in vitro and in vivo models to show lymphangiogenic functions of the chemokine axis. Effects of targeting the chemokine axis on tumor lymphangiogenesis and lymphatic metastasis were determined in an orthotopic breast cancer model.

RESULTS

VEGF-C specifically upregulates CXCR4 expression on lymphangiogenic endothelial cells. Moreover, hypoxia-inducible factor-1α (HIF-1α) mediates the CXCR4 expression induced by VEGF-C. Subsequent analyses identify the ligand CXCL12 as a chemoattractant for LECs. CXCL12 induces migration, tubule formation of LECs in vitro, and lymphangiogenesis in vivo. CXCL12 also stimulates the phosphorylation of intracellular signaling Akt and Erk, and their specific antagonists impede CXCL12-induced chemotaxis. In addition, its level is correlated with lymphatic vessel density in multiple cancer tissues microarray. Furthermore, the CXCL12-CXCR4 axis is independent of the VEGFR-3 pathway in promoting lymphangiogenesis. Intriguingly, combined treatment with anti-CXCL12 and anti-VEGF-C antibodies results in additive inhibiting effects on tumor lymphangiogenesis and lymphatic metastasis.

CONCLUSIONS

These results show the role of the CXCL12-CXCR4 axis as a novel chemoattractant for LECs in promoting lymphangiogenesis, and support the potential application of combined targeting of both chemokines and lymphangiogenic factors in inhibiting lymphatic metastasis.

Lymphangiogenesis and cancer

Historically, lymphatic vessels were considered passive participants in tumor metastasis by simply providing channels for tumor cells to transit to draining lymph nodes. The discovery of several key lymphatic-specific molecular markers and an increased availability of in vitro and in vivo experimental systems to study lymphatic biology have however highlighted a much more complex, active role for the lymphatic vasculature in metastatic tumor spread. This review will briefly describe the lymphatic system and lymphangiogenesis and then focus on the role of the lymphatic system in cancer metastasis. The progression of our understanding from the lymphatic system as a somewhat passive conduit for metastasis to an active participant in metastatic tumor dissemination, regulated by a complex array of lymphangiogenic factors, chemokines, and immune cell subsets, will be described.

In vitro assays using primary embryonic mouse lymphatic endothelial cells uncover key roles for FGFR1 signalling in lymphangiogenesis

Despite the importance of blood vessels and lymphatic vessels during development and disease, the signalling pathways underpinning vessel construction remain poorly characterised. Primary mouse endothelial cells have traditionally proven difficult to culture and as a consequence, few assays have been developed to dissect gene function and signal transduction pathways in these cells ex vivo. Having established methodology for the purification, short-term culture and transfection of primary blood (BEC) and lymphatic (LEC) vascular endothelial cells isolated from embryonic mouse skin, we sought to optimise robust assays able to measure embryonic LEC proliferation, migration and three-dimensional tube forming ability in vitro. In the course of developing these assays using the pro-lymphangiogenic growth factors FGF2 and VEGF-C, we identified previously unrecognised roles for FGFR1 signalling in lymphangiogenesis. The small molecule FGF receptor tyrosine kinase inhibitor SU5402, but not inhibitors of VEGFR-2 (SU5416) or VEGFR-3 (MAZ51), inhibited FGF2 mediated LEC proliferation, demonstrating that FGF2 promotes proliferation directly via FGF receptors and independently of VEGF receptors in primary embryonic LEC. Further investigation revealed that FGFR1 was by far the predominant FGF receptor expressed by primary embryonic LEC and correspondingly, siRNA-mediated FGFR1 knockdown abrogated FGF2 mediated LEC proliferation. While FGF2 potently promoted LEC proliferation and migration, three dimensional tube formation assays revealed that VEGF-C primarily promoted LEC sprouting and elongation, illustrating that FGF2 and VEGF-C play distinct, cooperative roles in lymphatic vascular morphogenesis. These assays therefore provide useful tools able to dissect gene function in cellular events important for lymphangiogenesis and implicate FGFR1 as a key player in developmental lymphangiogenesis in vivo.