Lymphatic endothelial tumors induced by intraperitoneal injection of incomplete Freund's adjuvant

Histopathological evaluation of the effectiveness of oral Eppikajutsuto treatment for lymphatic malformation

BACKGROUND

Lymphatic malformation (LM) is a congenital disease caused by lymphatic vessel malformation. Although standard therapies for LMs are sclerotherapy and/or surgical excision, a new therapy using Japanese herbal medicine Eppikajutsuto (TJ-28) has been recently reported as clinically effective. We aimed to experimentally confirm the therapeutic effectiveness of TJ-28 for LMs.

METHODS

LM lesions were generated in the mesentery and peritoneum of mice by intraperitoneal injection of Freund's incomplete adjuvant. Mice with LMs were treated by gavage or dietary administration of TJ-28 for 2 months. Formalin-fixed paraffin-embedded tissue sections of mesentery and peritoneum tissues were histologically and immunohistochemically examined by focusing on lymph nodes and perinodal lymph vessels.

RESULTS

Multiple Freund's incomplete adjuvant-associated foreign-body granulomas were formed in the mesentery and peritoneum, resulting in congestion of lymph fluid and dilatation of lymph vessels. The numbers and sizes of lymph nodes were not significantly different between TJ-28-treated and control groups. However, the luminal areas of lymphatic vessels were reduced significantly in the TJ-28 treatment group by both gavage and dietary administrations.

CONCLUSION

TJ-28 conspicuously reduced congestion of lymph fluid. This is the first histopathological evaluation of LM model mice to study the effectiveness of oral TJ-28 treatment.

Cardiac macrophage subsets differentially regulate lymphatic network remodeling during pressure overload

The lymphatic network of mammalian heart is an important regulator of interstitial fluid compartment and immune cell trafficking. We observed a remodeling of the cardiac lymphatic vessels and a reduced lymphatic efficiency during heart hypertrophy and failure induced by transverse aortic constriction. The lymphatic endothelial cell number of the failing hearts was positively correlated with cardiac function and with a subset of cardiac macrophages. This macrophage population distinguished by LYVE-1 (Lymphatic vessel endothelial hyaluronic acid receptor-1) and by resident macrophage gene expression signature, appeared not replenished by CCR2 mediated monocyte infiltration during pressure overload. Isolation of macrophage subpopulations showed that the LYVE-1 positive subset sustained in vitro and in vivo lymphangiogenesis through the expression of pro-lymphangiogenic factors. In contrast, the LYVE-1 negative macrophage subset strongly expressed MMP12 and decreased the endothelial LYVE-1 receptors in lymphatic endothelial cells, a feature of cardiac lymphatic remodeling in failing hearts. The treatment of mice with a CCR2 antagonist during pressure overload modified the proportion of macrophage subsets within the pathological heart and preserved lymphatic network from remodeling. This study reports unknown and differential functions of macrophage subpopulations in the regulation of cardiac lymphatic during pathological hypertrophy and may constitute a key mechanism underlying the progression of heart failure.

Extracellular Hsp90α Promotes Tumor Lymphangiogenesis and Lymph Node Metastasis in Breast Cancer

Early detection and discovery of new therapeutic targets are urgently needed to improve the breast cancer treatment outcome. Here we conducted an official clinical trial with cross-validation to corroborate human plasma Hsp90α as a novel breast cancer biomarker. Importantly, similar results were noticed in detecting early-stage breast cancer patients. Additionally, levels of plasma Hsp90α in breast cancer patients were gradually elevated as their clinical stages of regional lymph nodes advanced. In orthotopic breast cancer mouse models, administrating with recombinant Hsp90α protein increased both the primary tumor lymphatic vessel density and sentinel lymph node metastasis by 2 and 10 times, respectively. What is more, Hsp90α neutralizing antibody treatment approximately reduced 70% of lymphatic vessel density and 90% of sentinel lymph node metastasis. In the in vitro study, we demonstrated the role of extracellular Hsp90α (eHsp90α) as a pro-lymphangiogenic factor, which significantly enhanced migration and tube formation abilities of lymphatic endothelial cells (LECs). Mechanistically, eHsp90α signaled to the AKT pathway through low-density lipoprotein receptor-related protein 1 (LRP1) to upregulate the expression and secretion of CXCL8 in the lymphangiogenic process. Collectively, this study proves that plasma Hsp90α serves as an auxiliary diagnosis biomarker and eHsp90α as a molecular mediator promoting lymphangiogenesis in breast cancer.

Anlotinib suppresses lymphangiogenesis and lymphatic metastasis in lung adenocarcinoma through a process potentially involving VEGFR-3 signaling

Objective: Lymphatic metastasis is one of the leading causes of malignancy dispersion in various types of cancer. However, few anti-lymphangiogenic drugs have been approved for clinical use to date. Therefore, new therapies to block lymphangiogenesis are urgently required. Methods: Immunohistochemistry, immunofluorescence, Western blot, migration assays, and lymphangiogenesis and lymphatic metastasis assays were used. Results: Anlotinib, a receptor tyrosine kinase inhibitor, suppressed the rate of new metastatic lesions (31.82% in the placebo arm and 18.18% in the anlotinib arm) in patients with advanced lung adenocarcinoma who were enrolled in our ALTER-0303 study. D2-40⁺-lymphatic vessel density was strongly correlated with disease stage, metastasis, and poor prognosis in 144 Chinese patients with lung adenocarcinoma. In mice bearing A549^EGFP tumors, tumor lymphatic vessel density, tumor cell migration to lymph nodes, and the number of distant metastatic lesions were lower in the anlotinib group than in the controls. Anlotinib inhibited the growth and migration of human lymphatic endothelial cells (hLECs) and lymphangiogenesis in vitro and in vivo. Treatment of hLECs with anlotinib downregulated phosphorylated vascular endothelial growth factor receptor 3 (VEGFR-3). Conclusions: Anlotinib inhibits lymphangiogenesis and lymphatic metastasis, probably through inactivating VEGFR-3 phosphorylation. The results indicate that anlotinib may be beneficial for treatment in avoiding lymphangiogenesis and distant lymphatic metastasis in lung adenocarcinoma. (Trial registration: ALTER0303; NCT02388919; March 17, 2015.).

Adenosine A2a receptor promotes lymphangiogenesis and lymph node metastasis

The formation of new lymphatic vessels, or lymphangiogenesis, is a critical step of the tissue repair program. In pathological conditions involving chronic inflammation or tumorigenesis, this process is often dysregulated and can contribute to disease progression. Yet, lymphangiogenesis is still incompletely understood. In this study, we identified A2a adenosinergic signaling as an important regulator of inflammatory and tumor-associated lymphangiogenesis. Using Adora2a (A2a)-deficient mice, we demonstrated that A2a signaling was involved in the formation of new lymphatic vessels in the context of peritoneal inflammation. We also demonstrated that tumor-associated and sentinel lymph node lymphangiogenesis were impaired in A2a-deficient mice, protecting them from lymph node metastasis. Notably, A2a signaling in both hematopoietic and non-hematopoietic cells contributed to sentinel lymph node metastasis. In A2a-deficient tumor-draining lymph nodes, impaired lymphangiogenesis was associated with a reduced accumulation of B cells and decreased VEGF-C levels. Supporting a role for non-hematopoietic A2a signaling, we observed that primary murine lymphatic endothelial cells (LEC) predominantly expressed A2a receptor and that A2a signaling blockade altered LEC capillary tube formation in vitro. Finally, we observed that Adora2a, Nt5e and Entpd1 gene expression positively correlated with Lyve1, Pdpn and Vegfc in several human cancers, thereby supporting the notion that adenosine production and A2a receptor activation might promote lymphangiogenesis in human tumors. In conclusion, our study highlights a novel pathway regulating lymphangiogenesis and further supports the use of A2a or adenosine blocking agents to inhibit pathological lymphangiogenesis in cancers and block the dissemination of tumor cells through the lymphatic system.

Podoplanin+ tumor lymphatics are rate limiting for breast cancer metastasis

Metastatic dissemination employs both the blood and lymphatic vascular systems. Solid tumors dynamically remodel and generate both vessel types during cancer progression. Lymphatic vessel invasion and cancer cells in the tumor-draining lymph nodes (LNs) are prognostic markers for breast cancer metastasis and patient outcome, and tumor-induced lymphangiogenesis likely influences metastasis. Deregulated tumor tissue fluid homeostasis and immune trafficking associated with tumor lymphangiogenesis may contribute to metastatic spreading; however, the precise functional characterization of lymphatic endothelial cells (LECs) in tumors is challenged by the lack of specific reagents to decipher their rate-limiting role in metastasis. Therefore, we generated novel transgenic mice (PDPN promoter-driven Cre recombinase transgene [PDPN-Cre] and PDPN promoter-driven thymidine kinase transgene [PDPN-tk]) that allow for the identification and genetically controlled depletion of proliferating podoplanin (Pdpn)-expressing LECs. We demonstrate that suppression of lymphangiogenesis is successfully achieved in lymphangioma lesions induced in the PDPN-tk mice. In multiple metastatic breast cancer mouse models, we identified distinct roles for LECs in primary and metastatic tumors. Our findings support the functional contribution of primary tumor lymphangiogenesis in controlling metastasis to axillary LNs and lung parenchyma. Reduced lymphatic vessel density enhanced primary tumor lymphedema and increased the frequency of intratumoral macrophages but was not associated with a significant impact on primary tumor growth despite a marked reduction in metastatic dissemination. Our findings identify the rate-limiting contribution of the breast tumor lymphatic vessels for lung metastasis.

Fluorescent reporter transgenic mice for in vivo live imaging of angiogenesis and lymphangiogenesis

The study of lymphangiogenesis is an emerging science that has revealed the lymphatic system as a central player in many pathological conditions including cancer metastasis, lymphedema, and organ graft rejection. A thorough understanding of the mechanisms of lymphatic growth will play a key role in the development of therapeutic strategies against these conditions. Despite the known potential of this field, the study of lymphatics has historically lagged behind that of hemangiogenesis. Until recently, significant strides in lymphatic studies were impeded by a lack of lymphatic-specific markers and suitable experimental models compared to those of the more immediately visible blood vasculature. Lymphangiogenesis has also been shown to be a key phenomenon in developmental biological processes, such as cell proliferation, guided migration, differentiation, and cell-to-cell communication, making lymphatic-specific visualization techniques highly desirable and desperately needed. Imaging modalities including immunohistochemistry and in situ hybridization are limited by the need to sacrifice animal models for tissue harvesting at every experimental time point. Moreover, the processes of mounting and staining harvested tissues may introduce artifacts that can confound results. These traditional methods for investigating lymphatic and blood vasculature are associated with several problems including animal variability (e.g., between mice) when replicating lymphatic growth environments and the cost concerns of prolonged, labor-intensive studies, all of which complicate the study of dynamic lymphatic processes. With the discovery of lymphatic-specific markers, researchers have been able to develop several lymphatic and blood vessel-specific, promoter-driven, fluorescent-reporter transgenic mice for visualization of lymphatics in vivo and in vitro. For instance, GFP, mOrange, tdTomato, and other fluorescent proteins can be expressed under control of a lymphatic-specific marker like Prospero-related homeobox 1 (Prox1), which is a highly conserved transcription factor for determining embryonic organogenesis in vertebrates that is implicated in lymphangiogenesis as well as several human cancers. Importantly, Prox1-null mouse embryos develop without lymphatic vessels. In human adults, Prox1 maintains lymphatic endothelial cells and upregulates proteins associated with lymphangiogenesis (e.g., VEGFR-3) and downregulates angiogenesis-associated gene expression (e.g., STAT6). To visualize lymphatic development in the context of angiogenesis, dual fluorescent-transgenic reporters, like Prox1-GFP/Flt1-DsRed mice, have been bred to characterize lymphatic and blood vessels simultaneously in vivo. In this review, we discuss the trends in lymphatic visualization and the potential usage of transgenic breeds in hemangiogenesis and lymphangiogenesis research to understand spatial and temporal correlations between vascular development and pathological progression.

Integrin binding site within the gC1q domain orchestrates EMILIN-1-induced lymphangiogenesis

Lymphatic vessels (LVs) play a pivotal role in the control of tissue homeostasis and also have emerged as important regulators of immunity, inflammation and tumor metastasis. EMILIN-1 is the first ECM protein identified as a structural modulator of the growth and maintenance of LV; accordingly, Emilin1^-/- mice display lymphatic morphological alterations leading to functional defects as mild lymphedema, leakage and compromised lymph drainage. Many EMILIN-1 functions are exerted by the binding of its gC1q domain with the E933 residue of α4 and α9β1 integrins. To investigate the specific regulatory role of this domain on lymphangiogenesis, we generated a transgenic mouse model expressing an E933A-mutated EMILIN-1 (E1-E933A), unable to interact with α4 or α9 integrin. The mutant resulted in abnormal LV architecture with dense, tortuous and irregular networks; moreover, the number of anchoring filaments was reduced and collector valves had aberrant narrowed structures. E933A mutation also affected lymphatic function in lymphangiography assays and made the transgenic mice more prone to lymph node metastases. The finding that the gC1q/integrin interaction is crucial for a correct lymphangiogenesis response was confirmed and reinforced by functional in vitro tubulogenesis assays. In addition, ex vivo thoracic-duct ring assays revealed that E1-E933A-derived lymphatic endothelial cells had a severe reduction in sprouting capacity and were unable to organize into capillary-like structures. All these data provide evidence that the novel "regulatory structural" role of EMILIN-1 in the lymphangiogenic process is played by the integrin binding site within its gC1q domain.

Tumor necrosis factor superfamily 15 promotes lymphatic metastasis via upregulation of vascular endothelial growth factor-C in a mouse model of lung cancer

Lymphatic metastasis is facilitated by lymphangiogenic growth factor vascular endothelial growth factor-C (VEGFC) that is secreted by some primary tumors. We previously identified tumor necrosis factor superfamily 15 (TNFSF15), a blood vascular endothelium-derived cytokine, in lymphatic endothelial cells, as a key molecular modulator during lymphangiogenesis. However, the effect of TNFSF15 on tumor lymphatic metastasis and the underlying molecular mechanisms remain unclear. We report here that TNFSF15, which is known to inhibit primary tumor growth by suppressing angiogenesis, can promote lymphatic metastasis through facilitating lymphangiogenesis in tumors. Mice bearing tumors induced by A549 cells stably overexpressing TNFSF15 exhibited a significant increase in densities of lymphatic vessels and a marked enhancement of A549 tumor cells in newly formed lymphatic vessels in the primary tumors as well as in lymph nodes. Treatment of A549 cells with TNFSF15 results in upregulation of VEGFC expression, which can be inhibited by siRNA gene silencing of death domain-containing receptor-3 (DR3), a cell surface receptor for TNFSF15. In addition, TNFSF15/DR3 signaling pathways in A549 cells include activation of NF-κB during tumor lymphangiogenesis. Our data indicate that TNFSF15, a cytokine mainly produced by blood endothelial cells, facilitates tumor lymphangiogenesis by upregulating VEGFC expression in A549 cells, contributing to lymphatic metastasis in tumor-bearing mice. This finding also suggests that TNFSF15 may have potential as an indicator for prognosis evaluation.

Thoracic involvement in generalised lymphatic anomaly (or lymphangiomatosis)

Generalised lymphatic anomaly (GLA), also known as lymphangiomatosis, is a rare disease caused by congenital abnormalities of lymphatic development. It usually presents in childhood but can also be diagnosed in adults. GLA encompasses a wide spectrum of clinical manifestations ranging from single-organ involvement to generalised disease. Given the rarity of the disease, most of the information regarding it comes from case reports. To date, no clinical trials concerning treatment are available. This review focuses on thoracic GLA and summarises possible diagnostic and therapeutic approaches.

Possible diagnostic and therapeutic approaches to generalised lymphatic anomaly (lymphangiomatosis)http://ow.ly/4n4pgU

Functional Importance of a Proteoglycan Coreceptor in Pathologic Lymphangiogenesis

Supplemental Digital Content is available in the text.

Rationale:

Lymphatic vessel growth is mediated by major prolymphangiogenic factors, such as vascular endothelial growth factor (VEGF-C) and VEGF-D, among other endothelial effectors. Heparan sulfate is a linear polysaccharide expressed on proteoglycan core proteins on cell membranes and matrix, playing roles in angiogenesis, although little is known about any function(s) in lymphatic remodeling in vivo.

Objective:

To explore the genetic basis and mechanisms, whereby heparan sulfate proteoglycans mediate pathological lymphatic remodeling.

Methods and Results:

Lymphatic endothelial deficiency in the major heparan sulfate biosynthetic enzyme N-deacetylase/N-sulfotransferase-1 (Ndst1; involved in glycan-chain sulfation) was associated with reduced lymphangiogenesis in pathological models, including spontaneous neoplasia. Mouse mutants demonstrated tumor-associated lymphatic vessels with apoptotic nuclei. Mutant lymphatic endothelia demonstrated impaired mitogen (Erk) and survival (Akt) pathway signaling and reduced VEGF-C–mediated protection from starvation-induced apoptosis. Lymphatic endothelial-specific Ndst1 deficiency (in Ndst1^f/fProx1^+/CreERT2 mice) was sufficient to inhibit VEGF-C–dependent lymphangiogenesis. Lymphatic heparan sulfate deficiency reduced phosphorylation of the major lymphatic growth receptor VEGF receptor-3 in response to multiple VEGF-C species. Syndecan-4 was the dominantly expressed heparan sulfate proteoglycan in mouse lymphatic endothelia, and pathological lymphangiogenesis was impaired in Sdc4^(−/−) mice. On the lymphatic cell surface, VEGF-C induced robust association between syndecan-4 and VEGF receptor-3, which was sensitive to glycan disruption. Moreover, VEGF receptor-3 mitogen and survival signaling was reduced in the setting of Ndst1 or Sdc4 deficiency.

Conclusions:

These findings demonstrate the genetic importance of heparan sulfate and the major lymphatic proteoglycan syndecan-4 in pathological lymphatic remodeling. This may introduce novel future strategies to alter pathological lymphatic-vascular remodeling.

Local inhibition of elastase reduces EMILIN1 cleavage reactivating lymphatic vessel function in a mouse lymphoedema model

Lymphatic vasculature critically depends on the connections of lymphatic endothelial cells with the extracellular matrix (ECM), which are mediated by anchoring filaments (AFs). The ECM protein EMILIN1 is a component of AFs and is involved in the regulation of lymphatic vessel functions: accordingly, Emilin1^−/− mice display lymphatic vascular morphological alterations, leading to functional defects such as mild lymphoedema, lymph leakage and compromised lymph drainage. In the present study, using a mouse post-surgical tail lymphoedema model, we show that the acute phase of acquired lymphoedema correlates with EMILIN1 degradation due to neutrophil elastase (NE) released by infiltrating neutrophils. As a consequence, the intercellular junctions of lymphatic endothelial cells are weakened and drainage to regional lymph nodes is severely affected. The local administration of sivelestat, a specific NE inhibitor, prevents EMILIN1 degradation and reduces lymphoedema, restoring a normal lymphatic functionality. The finding that, in human secondary lymphoedema samples, we also detected cleaved EMILIN1 with the typical bands of an NE-dependent pattern of fragmentation establishes a rationale for a powerful strategy that targets NE inhibition. In conclusion, the attempts to block EMILIN1 degradation locally represent the basis for a novel ‘ECM’ pharmacological approach to assessing new lymphoedema treatments.

Immunohistochemical expression of vascular endothelial growth factor in orofacial lesions - A review

Angiogenesis is considered to be an important biological process in disease progression and tumorigenesis. Among the various factors associated with angiogenesis, vascular endothelial growth factor (VEGF) is a leading candidate. VEGF induces the proliferation, differentiation, and migration of vascular endothelial cells, increases capillary permeability, and enhances endothelial cell survival by preventing apoptosis. This article reviews and highlights the role of VEGF in health, and various oral diseases.

Fucoidan Suppresses Hypoxia-Induced Lymphangiogenesis and Lymphatic Metastasis in Mouse Hepatocarcinoma

Metastasis, the greatest clinical challenge associated with cancer, is closely connected to multiple biological processes, including invasion and adhesion. The hypoxic environment in tumors is an important factor that causes tumor metastasis by activating HIF-1α. Fucoidan, extracted from brown algae, is a sulfated polysaccharide and, as a novel marine biological material, has been used to treat various disorders in China, Korea, Japan and other countries. In the present study, we demonstrated that fucoidan derived from Undaria pinnatifida sporophylls significantly inhibits the hypoxia-induced expression, nuclear translocation and activity of HIF-1α, the synthesis and secretion of VEGF-C and HGF, cell invasion and lymphatic metastasis in a mouse hepatocarcinoma Hca-F cell line. Fucoidan also suppressed lymphangiogenesis in vitro and in vivo. In addition, accompanied by a reduction in the HIF-1α nuclear translocation and activity, fucoidan significantly reduced the levels of p-PI3K, p-Akt, p-mTOR, p-ERK, NF-κB, MMP-2 and MMP-9, but increased TIMP-1 levels. These results indicate strongly that the anti-metastasis and anti-lymphangiogenesis activities of fucoidan are mediated by suppressing HIF-1α/VEGF-C, which attenuates the PI3K/Akt/mTOR signaling pathways.

A practical and sensitive method of quantitating lymphangiogenesis in vivo

To address the inadequacy of current assays, we developed a directed in vivo lymphangiogenesis assay (DIVLA) by modifying an established directed in vivo angiogenesis assay. Silicon tubes (angioreactors) were implanted in the dorsal flanks of nude mice. Tubes contained either growth factor-reduced basement membrane extract (BME)-alone (negative control) or BME-containing vascular endothelial growth factor (VEGF)-D (positive control for lymphangiogenesis) or FGF-2/VEGF-A (positive control for angiogenesis) or a high VEGF-D-expressing breast cancer cell line MDA-MD-468LN (468-LN), or VEGF-D-silenced 468LN. Lymphangiogenesis was detected superficially with Evans Blue dye tracing and measured in the cellular contents of angioreactors by multiple approaches: lymphatic vessel endothelial hyaluronan receptor-1 (Lyve1) protein (immunofluorescence) and mRNA (qPCR) expression and a visual scoring of lymphatic vs blood capillaries with dual Lyve1 (or PROX-11 or Podoplanin)/Cd31 immunostaining in cryosections. Lymphangiogenesis was absent with BME, high with VEGF-D or VEGF-D-producing 468LN cells and low with VEGF-D-silenced 468LN. Angiogenesis was absent with BME, high with FGF-2/VEGF-A, moderate with 468LN or VEGF-D and low with VEGF-D-silenced 468LN. The method was reproduced in a syngeneic murine C3L5 tumor model in C3H/HeJ mice with dual Lyve1/Cd31 immunostaining. Thus, DIVLA presents a practical and sensitive assay of lymphangiogenesis, validated with multiple approaches and markers. It is highly suited to identifying pro- and anti-lymphangiogenic agents, as well as shared or distinct mechanisms regulating lymphangiogenesis vs angiogenesis, and is widely applicable to research in vascular/tumor biology.

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.

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.

Matrix metalloproteinase-2 governs lymphatic vessel formation as an interstitial collagenase

Lymphatic dysfunctions are associated with several human diseases, including lymphedema and metastatic spread of cancer. Although it is well recognized that lymphatic capillaries attach directly to interstitial matrix mainly composed of fibrillar type I collagen, the interactions occurring between lymphatics and their surrounding matrix have been overlooked. In this study, we demonstrate how matrix metalloproteinase (MMP)-2 drives lymphatic morphogenesis through Mmp2-gene ablation in mice, mmp2 knockdown in zebrafish and in 3D-culture systems, and through MMP2 inhibition. In all models used in vivo (3 murine models and thoracic duct development in zebrafish) and in vitro (lymphatic ring and spheroid assays), MMP2 blockage or down-regulation leads to reduced lymphangiogenesis or altered vessel branching. Our data show that lymphatic endothelial cell (LEC) migration through collagen fibers is affected by physical matrix constraints (matrix composition, density, and cross-linking). Transmission electron microscopy and confocal reflection microscopy using DQ-collagen highlight the contribution of MMP2 to mesenchymal-like migration of LECs associated with collagen fiber remodeling. Our findings provide new mechanistic insight into how LECs negotiate an interstitial type I collagen barrier and reveal an unexpected MMP2-driven collagenolytic pathway for lymphatic vessel formation and morphogenesis.

In vivo imaging of lymphatic vessels in development, wound healing, inflammation, and tumor metastasis

Lymphatic vessel growth or lymphangiogenesis occurs during embryonic development and wound healing and plays an important role in tumor metastasis and inflammatory diseases. However, the possibility of noninvasive detection and quantification of lymphangiogenesis has been lacking. Here, we present the Vegfr3(EGFPLuc) mouse model, where an EGFP-luciferase fusion protein, expressed under the endogenous transcriptional control of the Vegfr3 gene, allows the monitoring of physiological and pathological lymphangiogenesis in vivo. We show tracking of lymphatic vessel development during embryogenesis as well as lymphangiogenesis induced by specific growth factors, during wound healing and in contact hypersensitivity (CHS)--induced inflammation where we also monitor down-regulation of lymphangiogenesis by the glucocorticoid dexamethasone. Importantly, the Vegfr3-reporter allowed us to tracking tumor-induced lymphangiogenesis at the tumor periphery and in lymph nodes in association with the metastatic process. This is the first reporter mouse model for luminescence imaging of lymphangiogenesis. It should provide an important tool for studying the involvement of lymphangiogenesis in pathological processes.

Digging deeper into lymphatic vessel formation in vitro and in vivo

BACKGROUND

Abnormal lymphatic vessel formation (lymphangiogenesis) is associated with different pathologies such as cancer, lymphedema, psoriasis and graft rejection. Lymphatic vasculature displays distinctive features than blood vasculature, and mechanisms underlying the formation of new lymphatic vessels during physiological and pathological processes are still poorly documented. Most studies on lymphatic vessel formation are focused on organism development rather than lymphangiogenic events occurring in adults. We have here studied lymphatic vessel formation in two in vivo models of pathological lymphangiogenesis (corneal assay and lymphangioma). These data have been confronted to those generated in the recently set up in vitro model of lymphatic ring assay. Ultrastructural analyses through Transmission Electron Microscopy (TEM) were performed to investigate tube morphogenesis, an important differentiating process observed during endothelial cell organization into capillary structures.

RESULTS

In both in vivo models (lymphangiogenic corneal assay and lymphangioma), migrating lymphatic endothelial cells extended long processes exploring the neighboring environment and organized into cord-like structures. Signs of intense extracellular matrix remodeling were observed extracellularly and inside cytoplasmic vacuoles. The formation of intercellular spaces between endothelial cells led to tube formation. Proliferating lymphatic endothelial cells were detected both at the tips of sprouting capillaries and inside extending sprouts. The different steps of lymphangiogenesis observed in vivo are fully recapitulated in vitro, in the lymphatic ring assay and include: (1) endothelial cell alignment in cord like structure, (2) intracellular vacuole formation and (3) matrix degradation.

CONCLUSIONS

In this study, we are providing evidence for lymphatic vessel formation through tunneling relying on extensive matrix remodeling, migration and alignment of sprouting endothelial cells into tubular structures. In addition, our data emphasize the suitability of the lymphatic ring assay to unravel mechanisms underlying lymphangiogenesis.

Lymphatic endothelial heparan sulfate deficiency results in altered growth responses to vascular endothelial growth factor-C (VEGF-C)

Growth and remodeling of lymphatic vasculature occur during development and during various pathologic states. A major stimulus for this process is the unique lymphatic vascular endothelial growth factor-C (VEGF-C). Other endothelial growth factors, such as fibroblast growth factor-2 (FGF-2) or VEGF-A, may also contribute. Heparan sulfate is a linear sulfated polysaccharide that facilitates binding and action of some vascular growth factors such as FGF-2 and VEGF-A. However, a direct role for heparan sulfate in lymphatic endothelial growth and sprouting responses, including those mediated by VEGF-C, remains to be examined. We demonstrate that VEGF-C binds to heparan sulfate purified from primary lymphatic endothelia, and activation of lymphatic endothelial Erk1/2 in response to VEGF-C is reduced by interference with heparin or pretreatment of cells with heparinase, which destroys heparan sulfate. Such treatment also inhibited phosphorylation of the major VEGF-C receptor VEGFR-3 upon VEGF-C stimulation. Silencing lymphatic heparan sulfate chain biosynthesis inhibited VEGF-C-mediated Erk1/2 activation and abrogated VEGFR-3 receptor-dependent binding of VEGF-C to the lymphatic endothelial surface. These findings prompted targeting of lymphatic N-deacetylase/N-sulfotransferase-1 (Ndst1), a major sulfate-modifying heparan sulfate biosynthetic enzyme. VEGF-C-mediated Erk1/2 phosphorylation was inhibited in Ndst1-silenced lymphatic endothelia, and scratch-assay responses to VEGF-C and FGF-2 were reduced in Ndst1-deficient cells. In addition, lymphatic Ndst1 deficiency abrogated cell-based growth and proliferation responses to VEGF-C. In other studies, lymphatic endothelia cultured ex vivo from Ndst1 gene-targeted mice demonstrated reduced VEGF-C- and FGF-2-mediated sprouting in collagen matrix. Lymphatic heparan sulfate may represent a novel molecular target for therapeutic intervention.

A critical role for lymphatic endothelial heparan sulfate in lymph node metastasis

BACKGROUND

Lymph node metastasis constitutes a key event in tumor progression. The molecular control of this process is poorly understood. Heparan sulfate is a linear polysaccharide consisting of unique sulfate-modified disaccharide repeats that allow the glycan to bind a variety of proteins, including chemokines. While some chemokines may drive lymphatic trafficking of tumor cells, the functional and genetic importance of heparan sulfate as a possible mediator of chemokine actions in lymphatic metastasis has not been reported.

RESULTS

We applied a loss-of-function genetic approach employing lymphatic endothelial conditional mutations in heparan sulfate biosynthesis to study the effects on tumor-lymphatic trafficking and lymph node metastasis. Lymphatic endothelial deficiency in N-deacetylase/N-sulfotransferase-1 (Ndst1), a key enzyme involved in sulfating nascent heparan sulfate chains, resulted in altered lymph node metastasis in tumor-bearing gene targeted mice. This occurred in mice harboring either a pan-endothelial Ndst1 mutation or an inducible lymphatic-endothelial specific mutation in Ndst1. In addition to a marked reduction in tumor metastases to the regional lymph nodes in mutant mice, specific immuno-localization of CCL21, a heparin-binding chemokine known to regulate leukocyte and possibly tumor-cell traffic, showed a marked reduction in its ability to associate with tumor cells in mutant lymph nodes. In vitro modified chemotaxis studies targeting heparan sulfate biosynthesis in lymphatic endothelial cells revealed that heparan sulfate secreted by lymphatic endothelium is required for CCL21-dependent directional migration of murine as well as human lung carcinoma cells toward the targeted lymphatic endothelium. Lymphatic heparan sulfate was also required for binding of CCL21 to its receptor CCR7 on tumor cells as well as the activation of migration signaling pathways in tumor cells exposed to lymphatic conditioned medium. Finally, lymphatic cell-surface heparan sulfate facilitated receptor-dependent binding and concentration of CCL21 on the lymphatic endothelium, thereby serving as a mechanism to generate lymphatic chemokine gradients.

CONCLUSIONS

This work demonstrates the genetic importance of host lymphatic heparan sulfate in mediating chemokine dependent tumor-cell traffic in the lymphatic microenvironment. The impact on chemokine dependent lymphatic metastasis may guide novel therapeutic strategies.

Endostatin inhibits tumour lymphangiogenesis and lymphatic metastasis via cell surface nucleolin on lymphangiogenic endothelial cells

Endostatin has potent anti-endothelial and anti-angiogenic functions. Endostatin was reported to reduce lymphangiogenesis by down-regulating the level of VEGF-C in tumour tissues. However, there is little evidence for the direct function of endostatin on lymphangiogenic endothelial cells and lymphangiogenic vessels. Here, we report that cell surface nucleolin, which was reported as an endostatin receptor mediating its anti-angiogenic and anti-tumour functions, is also selectively expressed on the cell surface of lymphangiogenic endothelial cells both in vitro and in vivo. Treatment of primary mouse lymphatic endothelial cells (mLECs) by endostatin inhibits mLEC migration, tubule formation, and activation of the Erk pathway in mLECs, while neutralization of cell surface nucleolin or nucleolin knockdown results in loss of the anti-lymphatic endothelial activities of endostatin. Also, anti-nucleolin antibody or lentivirus delivered nucleolin siRNA abolishes the anti-lymphangiogenic function of endostatin in the Matrigel plug assay. Endostatin remarkably inhibits tumour-associated lymphangiogenesis, leading to reduced lymphatic metastasis. Systemic blockade of nucleolin notably abolishes the anti-lymphangiogenic and anti-lymphatic metastatic functions of endostatin. Importantly, endostatin does not affect quiescent lymphatics in normal organs, which is consistent with the lack of expression of cell surface nucleolin in quiescent lymphatics. Taken together, our results demonstrate that endostatin directly acts on lymphangiogenic endothelial cells via cell surface nucleolin, which provides a novel mechanism for the inhibition of tumour lymphangiogenesis and lymphatic metastasis by endostatin.

Lymphangiogenesis: A new player in cancer progression

Lymph node metastasis is the hallmark of colon cancer progression, and is considered one of the most important prognostic factors. Recently, there has been growing evidence that tumor lymphangiogenesis (formation of new lymphatic vessels) plays an important role in this process. Here, we review the latest findings of the role of lymphangiogenesis in colorectal cancer progression, and discuss its clinical application as a biomarker and target for new therapy. Understanding the molecular pathways that regulate lymphangiogenesis is mandatory to pave the way for the development of new therapies for cancer. In the future, tailored treatments consisting of combinations of chemotherapy, other targeted therapies, and anti-lymphangiogenesis agents will hopefully improve patient outcomes. This progression to the clinic must be guided by new avenues of research, such as the identification of biomarkers that predict response to treatment.

Multiple expressions of lymphatic markers and morphological evolution of newly formed lymphatics in lymphangioma and lymph node lymphangiogenesis

The rapid evolution of reliable technology combined with increasing number of specific markers for lymphatic endothelial cells (LECs) facilitates the investigation of lymphangiogenesis in developing and diseased tissues. Here, we injected incomplete Freund's adjuvant (IFA) peritoneally into BALB/c and nonobese diabetic (NOD) mice to induce lymphangioma and found atypical lymphatic accumulations with intervening fibrous tissue and lymphoid aggregates. Lymphatic markers, LYVE-1 and podoplanin, were used to specifically define the morphological features of the neoplastic lymphatics. The NOD mice (affected by an autoimmune disorder) had fewer and smaller lymphangiomas than the BALB/c mice. Injection of IFA in the footpad skin of the mice also disturbed draining regional lymph node lymphangiogenesis and caused enlargement of popliteal lymph nodes. Molecular analyses of the LECs indicated potential interventions for lymphangioma through vascular endothelial growth factor (VEGF)-A/-C/-D and their receptors, VEGF receptors-2/-3, and Prox-1 signaling pathways. These findings represent an important link between multiple factors and lymphatic disorders.

Does plasminogen activator inhibitor-1 drive lymphangiogenesis?

The purpose of this study is to explore the function of plasminogen activator inhibitor-1 (PAI-1) during pathological lymphangiogenesis. PAI-1, the main physiological inhibitor of plasminogen activators is involved in pathological angiogenesis at least by controlling extracellular proteolysis and by regulating endothelial cell survival and migration. Protease system's role in lymphangiogenesis is unknown yet. Thus, based on its important pro-angiogenic effect, we hypothesized that PAI-1 may regulate lymphangiogenesis associated at least with metastatic dissemination of cancer cells. To address this issue, we studied the impact of PAI-1 deficiency in various murine models of tumoral lymphangiogenesis. Wild-type PAI-1 proficient mice were used as controls. We provide for the first time evidence that PAI-1 is dispensable for tumoral lymphangiogenesis associated with breast cancers either induced by mammary carcinoma cell injection or spontaneously appearing in transgenic mice expressing the polyomavirus middle T antigen (PymT) under the control of a mouse mammary tumor virus long-terminal repeat promoter (MMTV-LTR). We also investigated inflammation-related lymphatic vessel recruitment by using two inflammatory models. PAI-1 deficiency did neither affect the development of lymphangioma nor burn-induced corneal lymphangiogenesis. These novel data suggest that vascular remodelling associated with lymphangiogenesis and angiogenesis involve different molecular determinants. PAI-1 does not appear as a potential therapeutic target to counteract pathological lymphangiogenesis.

VEGF-D deficiency in mice does not affect embryonic or postnatal lymphangiogenesis but reduces lymphatic metastasis

Vascular endothelial growth factor-D (VEGF-D) is one of the two ligands of the VEGFR-3 receptor on lymphatic endothelial cells. Gene-silencing studies in mice and Xenopus tadpoles recently showed that the role of endogenous VEGF-D in lymphatic development is moderate. By contrast, exogenous VEGF-D is capable of stimulating lymphangiogenesis. Nonetheless, its endogenous role in pathological conditions remains largely unknown. Hence, we reassessed its role in disease, using Vegf-d(null) mice. Vegf-d(null) mice were generated that, under physiological conditions, displayed normal embryonic and postnatal lymphangiogenesis and lymphatic remodelling, efficient lymphatic functioning and normal health. Vegf-d(null) mice also reponded normally in models of skin wound healing and healing of infarcted myocardium, despite enhanced expression of VEGF-D in these models in wild-type mice. In contrast, Vegf-d(null) mice displayed reduced peritumoral lymphangiogenesis and lymph node metastasis in an orthotopic pancreatic tumour model. Together, our data indicate that endogenous VEGF-D in mice is dispensible for lymphangiogenesis during development, in postnatal and adult physiology and in several pathological conditions, but significantly contributes to lymphatic metastasis.

Toll-like receptor 4 in lymphatic endothelial cells contributes to LPS-induced lymphangiogenesis by chemotactic recruitment of macrophages

The lymphatic vessel is a major conduit for immune cell transport; however, little is known about how lymphatic vessels regulate immune cell trafficking and how lymphatic vessels themselves respond to inflammation. Toll-like receptor 4 (TLR4) plays a central role in lipopolysaccharide (LPS)–induced inflammation, but the role of TLR4 in lymphatic endothelial cells (LECs) is poorly understood. Here, we found that LECs express high amounts of TLR4 in the intracellular region, and that the TLR4 of LECs is the main mediator of nuclear factor–κB (NF-κB) activation by LPS. LPS-TLR4 signaling in LECs resulted in the production of various chemokines for chemotaxis of macrophage. In addition, TLR4 in LECs actively contributed to the recruitment of macrophages to the draining lymphatic vessel. Furthermore, the macrophages that infiltrated into the lymphatic vessel induced lymphangiogenesis by secreting lymphangiogenic growth factors. These phenomena were largely attenuated not only in the mice defective in TLR4 signaling but also in the chimeric mice defective in TLR4 signaling that were recipients for bone marrow transplantation from normal TLR4-signaling mice. In conclusion, TLR4 in LECs plays an essential role in LPS-induced inflammatory lymphangiogenesis by chemotactic recruitment of macrophages.

The adhesion molecule L1 regulates transendothelial migration and trafficking of dendritic cells

The adhesion molecule L1, which is extensively characterized in the nervous system, is also expressed in dendritic cells (DCs), but its function there has remained elusive. To address this issue, we ablated L1 expression in DCs of conditional knockout mice. L1-deficient DCs were impaired in adhesion to and transmigration through monolayers of either lymphatic or blood vessel endothelial cells, implicating L1 in transendothelial migration of DCs. In agreement with these findings, L1 was expressed in cutaneous DCs that migrated to draining lymph nodes, and its ablation reduced DC trafficking in vivo. Within the skin, L1 was found in Langerhans cells but not in dermal DCs, and L1 deficiency impaired Langerhans cell migration. Under inflammatory conditions, L1 also became expressed in vascular endothelium and enhanced transmigration of DCs, likely through L1 homophilic interactions. Our results implicate L1 in the regulation of DC trafficking and shed light on novel mechanisms underlying transendothelial migration of DCs. These observations might offer novel therapeutic perspectives for the treatment of certain immunological disorders.

Modeling lymphangiogenesis in a three-dimensional culture system

A lack of appropriate in vitro models of three-dimensional lymph vessel growth hampers the study of lymphangiogenesis. We developed a lymphatic ring assay--a potent, reproducible and quantifiable three-dimensional culture system for lymphatic endothelial cells that reproduces spreading of endothelial cells from a pre-existing vessel, cell proliferation, migration and differentiation into capillaries. In the assay, mouse thoracic duct fragments are embedded in a collagen gel, leading to the formation of lumen-containing lymphatic capillaries, which we assessed by electron microscopy and immunostaining. We developed a computerized method to quantify the lymphatic network. By applying this model to gene-deficient mice, we found evidence for involvement of the matrix metalloproteinase, MMP-2, in lymphangiogenesis. The lymphatic ring assay bridges the gap between two-dimensional in vitro models and in vivo models of lymphangiogenesis, can be used to exploit the potential of existing transgenic mouse models, and rapidly identify regulators of lymphangiogenesis.

Emilin1 deficiency causes structural and functional defects of lymphatic vasculature

Lymphatic-vasculature function critically depends on extracellular matrix (ECM) and on its connections with lymphatic endothelial cells (LECs). However, the composition and the architecture of ECM have not been fully taken into consideration in studying the biology and the pathology of the lymphatic system. EMILIN1, an elastic microfibril-associated protein, is highly expressed by LECs in vitro and colocalizes with lymphatic vessels in several mouse tissues. A comparative study between WT and Emilin1-/- mice highlighted the fact that Emilin1 deficiency in both CD1 and C57BL/6 backgrounds results in hyperplasia, enlargement, and frequently an irregular pattern of superficial and visceral lymphatic vessels and in a significant reduction of anchoring filaments. Emilin1-deficient mice also develop larger lymphangiomas than WT mice. Lymphatic vascular morphological alterations are accompanied by functional defects, such as mild lymphedema, a highly significant drop in lymph drainage, and enhanced lymph leakage. Our findings demonstrate that EMILIN1 is involved in the regulation of the growth and in the maintenance of the integrity of lymphatic vessels, a fundamental requirement for efficient function. The phenotype displayed by Emilin1(-/-) mice is the first abnormal lymphatic phenotype associated with the deficiency of an ECM protein and identifies EMILIN1 as a novel local regulator of lymphangiogenesis.

Similarities and differences of human and experimental mouse lymphangiomas

Lymphangioma is a disfiguring malformation of early childhood. A mouse lymphangioma model has been established by injecting Freund's incomplete adjuvant (FIA) intraperitoneally, but has not been compared with the human disease. We show that, in accordance with studies from the 1960s, the mouse model represents an oil-granuloma, made up of CD45-positive leukocytes and invaded by blood and lymph vessels. Several markers of lymphatic endothelial cells are expressed in both mouse and human, like CD31, Prox1, podoplanin, and Lyve-1. However, the human disease affects all parts of the lymphovascular tree. We observed convolutes of lymphatic capillaries, irregularly formed collectors with signs of disintegration, and large lymph cysts. We observed VEGFR-2 and -3 expression in both blood vessels and lymphatics of the patients, whereas in mouse VEGFR-2 was confined to activated blood vessels. The experimental mouse FIA model represents a vascularized oil-granuloma rather than a lymphangioma and reflects the complexity of human lymphangioma only partially.

Investigation of intratumoural and peritumoural lymphatics expressed by podoplanin and LYVE-1 in the hybridoma-induced tumours

Tumour-associated lymphatics contribute to a key component of metastatic spread, however, the biological interaction of tumour cells with intratumoural and peritumoural lymphatics (ITLs and PTLs) has remained unclear. To address this important issue, we have focused on the morphological and molecular aspects of newly formed lymphatics (lymphangiogenesis) and pre-existing lymphatics in the intratumoural and peritumoural tissues by using a hybridoma-induced tumour model. In the present study, ITLs with very high vessel density within the tumour mass showed small and flattened contours that varied from non-solid-to-solid tumours, whereas PTLs were relatively disorganized and tortuous, and packed with a cluster of tumour cells at the tumour periphery. Lymphatic endothelial cells (LECs) both in ITLs and PTLs were expressed with LYVE-1 and podoplanin in various tumour tissues, in which initial lymphatics were extremely extended and dilated. The tumour cells were frequently detected adhering to or penetrating lymphatic walls, especially near the open junctions. In the metastatic tissues, lymphangiogenic vasculatures occurred within the tumour matrix, and collecting PTLs represented abnormal twisty valve leaflets. The Western blot and RT-PCR analysis showed local variations of LEC proliferating potentials and lymphatic involvement in metastasis by a distinct profile of the protein and mRNA expression by LYVE-1, podoplanin, Prox-1 and vascular endothelial growth factor-3 (VEGFR-3). These findings indicated that both ITLs and PTLs, including enlarged pre-existing and newly formed lymphatics, may play a crucial role in metastasis with an active tumour cell adhesion, invasion, migration and implantation.

Induction of Apoptosis and Inhibition of Cell Migration and Tube-Like Formation by Dihydroartemisinin in Murine Lymphatic Endothelial Cells

Dihydroartemisinin (DHA) is a semisynthesized agent from the artemisinin first extracted from the Chinese plant Artemisia annua. Previous studies have shown that artemisinin derivates, apart from their antimalarial activity, possess antitumor, antiangiogenic, and anti-inflammatory effects. In the present investigation, DHA was found to have a potent ability in influencing lymphatic endothelial cells (LECs) behavior. Murine LECs were isolated from benign lymphangiomas induced by intraperitoneal injection of incomplete Freund's adjuvant and identified by indirect immunofluorescence assay and fluorescence-activated cell sorting analysis to examine the expression of the specific marker VEGFR-3/Flt-4. When LECs were treated with DHA at 10 microg/ml, the growth of LECs was inhibited, and LECs showed typical apoptotic morphological features, with a higher apoptotic rate as compared with the controls. DHA also exerted a significant inhibitory effect on migration and tube-like formation of LECs in a dose-dependent manner. Quantitative RT-PCR further showed that DHA remarkably downregulated the expression of antiapoptotic bcl-2 mRNA, but upregulated that of the proapoptotic gene bax mRNA. In addition, DHA could strongly attenuate the mRNA and protein levels of VEGFR-3/Flt-4. In summary, these findings indicate that DHA may be useful as a potential lymphangiogenesis inhibitor under induction of cell apoptosis, inhibition of the migration, and formation of tube-like structures in LECs.

Site-specific induction of lymphatic malformations in a rat model for image-guided therapy

BACKGROUND

Lymphatic malformation is a common benign mass in children and adults and is representative of a derangement in lymphangiogenesis. These lesions have high recurrence rates and significant morbidity associated with surgery. Several sclerotherapy regimens have been developed clinically to treat lymphatic malformations; however, an animal model has not been developed that is adequate to test the efficacy of image-guided therapeutic interventions.

OBJECTIVE

To develop an animal model suitable for evaluation of percutaneous treatments of lymphatic malformations.

MATERIALS AND METHODS

Male Harlan Sprague-Dawley rats (n = 9) received two US-guided injections of Incomplete Freund's Adjuvant (IFA) over a 2-week period. All nine rats were injected twice into the peritoneum (IP); a subgroup (n = 3) received additional injections into the neck. Three animals that received IP injections of saline were used as controls. The injection sites were monitored for the development of lesions by high-resolution ultrasonography at 2-week intervals for 100 days. High-resolution (4.7 Tesla) magnetic resonance imaging was then performed on two animals noted to have developed masses. The rats were sacrificed and histologic examination of the identified lesions was performed, including immunohistochemical staining for vascular (CD31) and lymphatic (Flt-4 and Prox-1) endothelium.

RESULTS

All animals injected with IFA developed cystic lesions. The three animals injected at dual sites were noted to have both microcystic and macrocystic malformations in the neck and microcystic plaque-like lesions in the peritoneum. The macrocystic malformations (> or =5 mm) in the neck were detected by ultrasonography and grossly later during necropsy. Histopathologic analysis revealed the cystic spaces to be lined by lymphatic endothelium supported by a connective tissue stroma. Control animals did not exhibit detectable lesions with either ultrasonography or necropsy.

CONCLUSION

This model represents a promising tool for translational development of image-guided interventions for lymphatic malformations. It may also serve as a model for the study of lymphangiogenesis and the development of anti-lymphangiogenic therapies.

Inhibition of the mammalian target of rapamycin impedes lymphangiogenesis

Lymphatic complications are common side effects of mammalian target of rapamycin (mTOR) inhibitor-based immunosuppression in kidney transplantation. Therefore, we investigated whether the mTOR inhibitor rapamycin, besides its known antihemangiogenic effect, also impedes regenerative lymphangiogenesis. In a murine skin flap model, rapamycin impaired recovery of lymphatic flow across surgical incisions resulting in prolonged wound edema in these animals. Importantly, the antilymphangiogenic effect of rapamycin was not related to a general inhibition of wound healing as demonstrated an in vivo Matrigeltrade mark lymphangiogenesis assay and a model of lymphangioma. Rapamycin concentrations as low as 1 ng/ml potently inhibited vascular endothelial growth factor (VEGF)-C driven proliferation and migration, respectively, of isolated human lymphatic endothelial cells (LECs) in vitro. Mechanistically, mTOR inhibition impairs downstream signaling of VEGF-A as well as VEGF-C via mTOR to the p70S6 kinase in LECs. In conclusion, we provide extensive experimental evidence for an antilymphangiogenic activity of mTOR inhibition suggesting that the early use of mTOR inhibitor following tissue injury should be avoided. Conversely, the antilymphangiogenic properties of rapamycin and its derivates may provide therapeutic value for the prevention and treatment of malignancies, respectively.

Methods to study lymphatic vessel integrins

The lymphatic system plays a key role in the drainage of fluids and proteins from tissues and in the trafficking of immune cells throughout the body. Comprised of a network of capillaries, collecting vessels, and lymph nodes, the lymphatic system plays a role in the metastasis of tumor cells to distant parts of the body. Tumors induce lymphangiogenesis, the growth of new lymphatic vessels, in the peritumoral space and also within tumors and lymph nodes. Tumor lymphangiogenesis has been shown to play a role in promoting tumor metastasis. As mediators of lymphatic endothelial cell adhesion, migration, and survival, integrins play key roles in the regulation of lymphangiogenesis. Recent studies indicate that select integrins promote lymphangiogenesis during development and disease and that inhibitors or loss of expression of these integrins can block lymphangiogenesis. In this report, we describe methods to isolate and culture murine and human lymphatic endothelial cells as well as methods to analyze the expression of integrins on these cells. We also show how to assess integrin-mediated adhesion, migration, and tube formation in vitro. We demonstrate how to evaluate integrin function during lymphangiogenesis in a variety of animal models in vivo. Additionally, we show how to study lymphangiogenesis using intravital microscopy.

Molecular Profile and Proliferative Responses of Rat Lymphatic Endothelial Cells in Culture

BACKGROUND

Lymphangiogenesis plays an important role in metastasis of many solid tumors. To study lymphangiogenesis under controlled conditions, an in vitro model is needed. The goal of this work was to establish such an in vitro model by determining a molecular profile of rat mesenteric lymphatic endothelial cells (RMLEC) and characterizing their proliferative responses to angiogenic and lymphangiogenic factors, such as vascular endothelial growth factor A and C (VEGF-A and VEGF-C).

METHODS AND RESULTS

RMLEC strongly expressed most lymphatic-specific markers, including Prox-1, LYVE-1, and VEGFR-3. Proliferation of RMLEC was serum and heparin dependent. In the presence of low (2%) serum concentration, exogenously added VEGF-A and VEGFC stimulated RMLEC in a linear and dose-dependent manner. This effect was abrogated by anti-VEGF-A and VEGF-C antibodies, as well as by soluble Tie-2 and Flt-4 fusion proteins. Abrogation was reversed by VEGF-A, suggesting that this factor as an important regulator of lymphangiogenesis.

CONCLUSIONS

Cultured RMLEC preserved a molecular profile consistent with the phenotype of lymphatic endothelium in vivo and respond to either VEGF-A or VEGF-C factors. VEGFA was able to rescue RMLEC proliferation inhibited by a neutralizing VEGF-C antibody or soluble Tie-2 fusion protein. These results support the existence of cross-talk among angiogenic and lymphangiogenic factors. This work established experimental conditions that allow in vitro modeling of lymphatic endothelial responses to lymphangiogenic regulators. Preliminary results using this model suggest that VEGF-A, VEGF-C, and angiopoietins work in concert to promote lymphangiogenesis in vivo.

Effects of transforming growth factor-beta in the development of inflammatory pseudotumour-like lesions in a murine model

Alterations in transforming growth factor (TGF)-beta signalling have been frequently implicated in human cancer, and an important mechanism underlying its pro-oncogenic nature is suppression of the host antitumour immune response. Considering the immunosuppressive effect of TGF-beta, we asked whether human tumour cells, known to secrete TGF-beta in culture, would survive and grow when implanted into the peritoneal cavity of immunocompetent mice. Therefore, we developed a xenogeneic model where mice were intraperitoneally (i.p.) injected with a TGF-beta-secreting human colorectal adenocarcinoma cell line, LISP-A10. Although animals did not develop macroscopic tumours, the recovery and isolation of human tumour cells was achieved when an inflammatory environment was locally induced by the administration of complete Freund's adjuvant (CFA). This procedure significantly increased TGF-beta concentrations in the peritoneal fluid and was accompanied by impaired activation of the host-specific immune response against LISP-A10 cells. Furthermore, inflammatory lesions resembling human inflammatory pseudotumours (IPTs) were observed on the surface of i.p. organs. These lesions could be induced by either injection of LISP-A10 cells, cells-conditioned medium or recombinant TGF-beta but only after administration of CFA. In addition, host cyclooxygenase-2 and kinin receptors played an important role in the induction of TGF-beta-mediated IPT-like lesions in our experimental model.

Generation and characterization of a mouse lymphatic endothelial cell line

Lymphatic vessels, by channeling fluid and leukocytes from the periphery into lymph nodes, play a central role in the development of the immune response. Despite their importance in homeostasis and disease, the difficulties in enriching and culturing lymphatic endothelial cells limit studies of their biology. Here, we report the isolation, stabilization, and characterization of a mouse lymphatic endothelial cell line (MELC) and the generated clones thereof. Cells were isolated from benign lymphangiomas induced by intraperitoneal injections of incomplete Freund's adjuvant. The MELC line expressed molecules typical of lymphatic endothelium, including VEGFR3/Flt-4, podoplanin, Prox-1, and D6, but not LYVE-1. It also expressed CD34, ICAM-1, VCAM, and JAM-A, but not CD31, VE-cadherin, E-selectin, or CX3CL1/fractalkine (both TNFalpha-induced), at variance with vascular endothelial cells tested in parallel. The inflammatory cytokines TNFalpha and IL-4 regulated production of selected adhesion molecules (VCAM), cytokines (IL-6), and chemokines (CCL2/JE). Whole genome transcriptional profiling identified a set of 150 known genes differentially expressed in MELC versus vascular endothelial cells. Thus, the MELC line may represent an invaluable source of lymphatic endothelium.

Production of two novel monoclonal antibodies that distinguish mouse lymphatic and blood vascular endothelial cells

We produced two novel rat monoclonal antibodies (LA102 and LA5) to identify mouse lymphatic vessels and blood vessels, respectively. We characterized the two antibodies as to the morphological and functional specificities of endothelial cells of both types of vessels. The antibodies were produced by a rapid differential immunization of DA rats with collagenase- and neuraminidase-treated mouse lymphangioma tissues. LA102 specifically reacted with mouse lymphatic vessels except the thoracic duct and the marginal sinus of lymph nodes, but not with any blood vessels. In contrast, LA5 reacted with most mouse blood vessels with a few exceptions, but not with lymphatics. LA102 recognized a protein of 25-27 kDa, whereas LA5 recognized a molecule of 12-13 kDa. Neither antibody recognized any currently identified lymphatic or vascular endothelial cell antigens. Immunoelectron microscopy revealed that the antigens recognized by LA102 and LA5 were localized on both luminal and abluminal endothelial cell membranes of each vessel type. Interestingly, LA102 immunoreactivity was strongly expressed on pinocytic or transport vesicle membrane in the cytoplasm of lymphatic endothelium. Besides endothelial cells, both antibodies also recognized some types of lymphoid cells. Since, the LA102 antigen molecule is expressed on some lymphoid cells, it may play important roles in the migration of lymphoid cells and in some transport mechanisms through lymphatic endothelial cells.

Isolation and characterization of lymphatic microvascular endothelial cells from human tonsils

Human lymphatic endothelial cells (LECs) have isolated prevalently from human derma and tumors. As specialized lymphatic organs within the oropharynx, palatine tonsils are easily obtained and rich in lymphatic venules. Using a two-step purification method based on the sorting of endothelial cells with Ulex Europaeus Agglutinin 1 (UEA-1)-coated beads, followed by purification with monoclonal antibody D2-40, we successfully purified LECs from human palatine tonsils. The LECs were expanded on flasks coated with collagen type 1 and fibronectin for up to 8-10 passages and then analyzed for phenotypic and functional properties. Cultured cells retained the phenotypic pattern of the lymphatic endothelium of palatine tonsils and expressed functional VEGFR-3 molecules. In fact, stimulation with VEGFR-3 ligand, the vascular endothelium grow factor C, induced a marked increase in cell proliferation. Similarly to blood endothelial cells (BECs), LECs were able to form tube-like structure when seeded in Cultrex basement membrane extract. Comparative studies performed on LECs derived from palatine tonsils and iliac lymphatic vessels (ILVs), obtained with the same procedures, showed substantial discrepancies in the expression of various lymphatic markers. This points to the existence of micro- and macrovessel-derived LECs with different phenotypes, possibly involving different biological activities and functions. Palatine tonsil- and ILV-derived LECs may, therefore, represent new models for investigating function and biochemical properties of these lymphatic endothelia.

Lymphatic endothelium: an important interactive surface for malignant cells

Endothelial cells line the vessels which transport fluid and cells throughout the body. Although much attention has been paid to these cells in the context of the blood vascular system, endothelial cells also line lymphatic vessels. Recent progress in identifying growth factors which drive the development of lymphatic vessels and molecular markers specific for lymphatics has expanded our understanding of the role the lymphatic system plays in human pathology. Techniques for purifying populations of lymphatic endothelial cells also allow the in vitro analysis of this unique surface to explore its role in tumour metastasis, immune cell function and fluid transport. This review provides a synopsis of the recent data pertaining to the purification and culture of lymphatic endothelial cells, and the interaction of tumour cells with lymphatic endothelium.