Adenoviral expression of vascular endothelial growth factor-C induces lymphangiogenesis in the skin

Anti-lymphangiogenesis effects of a specific anti-interleukin 7 receptor antibody in lung cancer model in vivo

Interleukin 7 (IL-7) is known to promote lymphangiogenesis. To study the relationship between IL-7 and the lymphangiogenesis in lung cancer cells xenograft tumors, we investigated how IL-7 regulates lymphangiogenesis by Quantitative real-time reverse transcriptase-polymerase chain reaction, Western blot, co-immunoprecipitation, chromatin immunoprecipitation, and immunohistochemistry methods. We found that, in lung cancer cells xenograft tumors IL-7/IL-7 receptor (IL-7R) increase the expression of VEGF-D and lymphangiogenesis, induce c-Fos and c-Jun heterodimer formation, and enhance c-Fos/c-Jun DNA binding activity to regulate VEGF-D. Taken together, our results provided evidence that IL-7/IL-7R induce VEGF-D upregulation and promote lymphangiogenesis via c-Fos/c-Jun pathway in lung cancer.

Serum insulin-like growth factor 1 correlates with the risk of nodal metastasis in endocrine-positive breast cancer

Increased insulin-like growth factor (igf) signalling has been observed in breast cancer, including endocrine-responsive cancers, and has been linked to disease progression and recurrence. In particular, igf-1 has the ability to induce and promote lymphangiogenesis through the induction of vascular endothelial growth factor C (vegfc). In the present study, we analyzed serum and tumour samples from 60 patients with endocrine-positive breast cancer to determine the expression and the possible relationship of circulating igf-1, igf binding protein 3 (igfbp3), and vegfc with the presence of lymphatic metastasis and other immunohistochemical parameters. The analysis revealed a clear and significant correlation between high basal levels of igf-1, igfbp3, and vegfc and lymph node metastasis in endocrine-responsive breast cancer. In addition, expression of those molecules was significantly higher in breast cancer patients than in healthy control subjects. Those findings may enable more accurate prediction of prognosis in patients with breast cancer.

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.

Intercellular adhesion molecule-1 (ICAM-1) expression correlates with oral cancer progression and induces macrophage/cancer cell adhesion

Intercellular adhesion molecule-1 (ICAM-1) is a transmembrane glycoprotein in the immunoglobulin superfamily, which plays an important role in cell adhesion and signal transduction. Although ICAM-1 is believed to play a role in several malignancies, it is still uncertain whether or not ICAM-1 expression contributes to cancer progression. In this study, we performed clinicopathological and cell biological analyses of ICAM-1 expression in oral squamous cell carcinoma (SCC). First, we examined the ICAM-1 expression in tongue SCC immunohistochemically, and revealed that ICAM-1 was expressed predominantly at the invasive front area of tongue SCC. ICAM-1 expression at the invasive front area was correlated with invasion, lymph node metastasis and increased blood and lymphatic vessel density of the tongue SCC. The relationship between ICAM-1 expression and clinicopathological factors were consistent with the increased proliferation, invasion and cytokine-production activities of ICAM-1-transfected SCC cells. Second, we analyzed the relationship between macrophages and ICAM-1-expressing tongue SCC cells because ICAM-1 is known to act as a ligand for adhesion of immune cells. Increased ICAM-1 expression in tongue SCC was correlated with increased macrophage infiltration within SCC nests. Moreover, macrophage/SCC-cell adhesion through ICAM-1 molecule was revealed using an in vitro cell adhesion and blockade assay. These findings indicate that ICAM-1 plays an important role in tongue SCC progression, which may result from the SCC-cell activity, angiogenic activity, lymphangiogenic activity and macrophage/SCC-cell adhesion.

Craniofacial abnormalities and altered wnt and mmp mRNA expression in zebrafish embryos exposed to gasoline oxygenates ETBE and TAME

Gasoline additives ethyl tert butyl ether (ETBE) and tertiary amyl methyl ether (TAME) are used world wide, but the consequence of developmental exposure to these hydrophilic chemicals is unknown for aquatic vertebrates. The effect of ETBE and TAME on zebrafish embryos was determined following OECD 212 guidelines, and their toxicity was compared to structurally related methyl tert-butyl ether (MTBE), which is known to target developing vasculature. LC50s for ETBE and TAME were 14 mM [95% CI=10-20] and 10 mM [CI=8-12.5], respectively. Both chemicals caused dose dependent developmental lesions (0.625-10 mM), which included pericardial edema, abnormal vascular development, whole body edema, and craniofacial abnormalities. The lesions were suggestive of a dysregulation of WNT ligands and matrix metalloproteinase (MMP) protein families based on their roles in development. Exposure to 5 mM ETBE significantly (p≤0.05) decreased relative mRNA transcript levels of mmp-9 and wnt3a, while 2.5 and 5 mM TAME significantly decreased wnt3a, and wnt8a. TAME also significantly decreased mmp-2 and -9 mRNA levels at 5mM. ETBE and TAME were less effective in altering the expression of vascular endothelial growth factor-a and -c, which were the only genes tested that were significantly decreased by MTBE. This is the first study to characterize the aquatic developmental toxicity following embryonic exposure to ETBE and TAME. Unlike MTBE, which specifically targets angiogenesis, ETBE and TAME disrupt multiple organ systems and significantly alter the mRNA transcript levels of genes required for general development.

Biomarkers of Lymphatic Function and Disease

Substantial advances have accrued over the last decade in the identification of the processes that contribute to lymphatic vascular development in health and disease. Identification of distinct regulatory milestones, from a variety of genetic models, has led to a stepwise chronology of lymphatic development. Several molecular species have been identified as important tissue biomarkers of lymphatic development and function. At present, vascular endothelial growth-factor receptor (VEGFR)-3/VEGF-C/VEGF-D signaling has proven useful in the identification of clinical lymphatic metastatic potential and the assessment of cancer prognosis. Similar biomarkers, to be utilized as surrogates for the assessment of inherited and acquired diseases of the lymphatic circulation, are actively sought, and will represent a signal advance in biomedical investigation.

Roles of transcriptional network during the formation of lymphatic vessels

The lymphatic vascular system, also known as the second vascular system in vertebrates, plays crucial roles in various physiological and pathological processes. It participates in the maintenance of normal tissue fluid balance, trafficking of the immune cells and absorption of fatty acids in the gut. Furthermore, lymphatic system is associated with the pathogenesis of a number of diseases, including lymphedema, inflammatory diseases and tumour metastasis. Lymphatic vessels are comprised of lymphatic endothelial cells (LECs), which are differentiated from blood vascular endothelial cells. This review highlights recent advances in our understanding of the transcriptional control of LEC fate determination and reflects on efforts to understand the roles of transcriptional networks during this discrete developmental process.

Macrophage-mediated lymphangiogenesis: the emerging role of macrophages as lymphatic endothelial progenitors

It is widely accepted that macrophages and other inflammatory cells support tumor progression and metastasis. During early stages of neoplastic development, tumor-infiltrating macrophages (TAMs) mount an immune response against transformed cells. Frequently, however, cancer cells escape the immune surveillance, an event that is accompanied by macrophage transition from an anti-tumor to a pro-tumorigenic type. The latter is characterized by high expression of factors that activate endothelial cells, suppress immune response, degrade extracellular matrix, and promote tumor growth. Cumulatively, these products of TAMs promote tumor expansion and growth of both blood and lymphatic vessels that facilitate metastatic spread. Breast cancers and other epithelial malignancies induce the formation of new lymphatic vessels (i.e., lymphangiogenesis) that leads to lymphatic and subsequently, to distant metastasis. Both experimental and clinical studies have shown that TAMs significantly promote tumor lymphangiogenesis through paracrine and cell autonomous modes. The paracrine effect consists of the expression of a variety of pro-lymphangiogenic factors that activate the preexisting lymphatic vessels. The evidence for cell-autonomous contribution is based on the observed tumor mobilization of macrophage-derived lymphatic endothelial cell progenitors (M-LECP) that integrate into lymphatic vessels prior to sprouting. This review will summarize the current knowledge of macrophage-dependent growth of new lymphatic vessels with specific emphasis on an emerging role of macrophages as lymphatic endothelial cell progenitors (M-LECP).

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.

Methyl tert butyl ether is anti-angiogenic in both in vitro and in vivo mammalian model systems

Methyl-tertiary butyl ether (MTBE), a well known gasoline oxygenate, and US Food and Drug Administration approved gallstone treatment, has been previously shown to specifically target teleost embryonic angiogenesis. The studies reported here were to determine whether similar vascular disrupting effects occur in higher vertebrate models. Rat brain endothelial cells were isolated and allowed to form microcapillary-like tubes on Matrigel. MTBE (0.34-34.0 mm) exposure resulted in a dose-dependent reduction of tube formation, with the LOAEL at 0.34 mm, while MTBE's primary metabolite, tertiary butyl alcohol had no effect on tube formation. HUVECs, a primary cell line representing macrovascular cells, were able to form tubes on Matrigel in the presence of MTBE (1.25-80 mm), but the tubes were narrower than those formed in the absence of MTBE. In a mouse Matrigel plug implantation assay, 34.0 mm MTBE completely inhibited vessel invasion into plugs containing endothelial cell growth supplement (ECGS) compared with control plugs with ECGS alone. When timed-pregnant Fisher 344 rats were gavaged with MTBE (500-1500 mg kg(-1) ) from day 6 of organogenesis through 10 days post-parturition, no organ toxicity or histological changes in pup vasculature were observed. Results of the in vitro cell culture studies show that MTBE is anti-angiogenic at mm concentrations and has potential use as an anti-angiogenic treatment for solid tumors with minimal toxicity.

Influence of route of administration and liposomal encapsulation on blood and lymph node exposure to the protein VEGF-C156S

VEGF-C156S is a recombinant form of human vascular endothelial growth factor C (VEGF-C), which targets the receptor VEGFR-3 present in the lymphatics. VEGF-C156S has lymphangiogenic properties and may represent a potential therapeutic approach in treating the lymphatic disease lymphedema. In the present study, we tested the hypotheses that (1) subcutaneous (s.c.) injection will provide higher lymphatic exposure than intravenous (i.v.) administration of VEGF-C156S and (2) s.c. injection of liposomal (s.c. Lipo) VEGF-C156S will provide greater lymphatic exposure than nonliposomal proteins. The protein VEGF-C156S was radiolabeled with Iodine-125 by a modified chloramine-T method and encapsulated into liposomes. The protein was injected at a dose of 125 μg/kg to mice i.v. or s.c.; the liposomal preparation was administered s.c. (s.c. Lipo). Blood and lymph nodes were collected over 24 h. The mean residence time in lymph nodes after s.c. or s.c. (Lipo) administration was approximately double that following i.v. administration. The area under the concentration-time curve (AUC) ratio of lymph node-blood after s.c. administration of VEGF-C156S was more than double of the AUC ratio after i.v. administration. The results suggest that lymph node exposure of VEGF-C156S was significantly higher after s.c. administration of liposomal or nonliposomal protein as compared with i.v. administration.

Role of the lymphatics in cancer metastasis and chemotherapy applications

The lymphatic system was first described centuries ago. The recent discovery of various molecular markers has allowed for more in-depth research of the lymphatic system and its role in health and disease. The lymphatic system has recently been elucidated as playing an active role in cancer metastasis. The knowledge of the active processes involved in lymphatic metastasis provides novel treatment targets for various malignancies.

PI3K/Akt and MAPK/ERK1/2 signaling pathways are involved in IGF-1-induced VEGF-C upregulation in breast cancer

OBJECTIVE

To investigate the signaling pathways involved in insulin-like growth factor-1 (IGF-1)-induced vascular endothelial growth factor C (VEGF-C) up-regulation and lymphatic metastasis in MDA-MB-231 breast cancer cells.

METHODS

MDA-MB-231 breast cancer cells were exposed to IGF-1 with various concentrations. The expression level of VEGF-C was assessed by real-time PCR and Western blot. Akt and ERK1/2 phosphorylation was detected by Western blot. Signaling transduction inhibitors, LY294002 and PD98059, were used to block PI3K/Akt and MAPK/ERK1/2 signaling pathways, respectively.

RESULTS

IGF-1 increased the level of VEGF-C expression in a dose-dependent manner in MDA-MB-231 breast cancer cells. In addition, phosphorylation of Akt and ERK1/2 was enhanced by IGF-1. Remarkably, inhibition of Akt phosphorylation by LY294002 completely blocked the effects on IGF-1-induced VEGF-C up-regulation. Inhibition of ERK1/2 phosphorylation by PD98059 reduced IGF-1-induced VEGF-C expression.

CONCLUSION

This study identified that PI3K/Akt and MAPK/ERK1/2 signaling pathways were involved in IGF-1-induced VEGF-C up-regulation and suggested their important roles in lymphatic metastasis in breast cancer.

Photodynamic ablation of lymphatic vessels and intralymphatic cancer cells prevents metastasis

The dissemination of tumor cells to sites far from the primary tumor (metastasis) is the principal cause of death in cancer patients. Tumor-associated lymphatic vessels are a key conduit for metastatic tumor cells, which typically first colonize the lymph nodes. Although the primary tumor and affected lymph nodes can be removed during surgery, tumor cells inside lymphatic vessels are left behind. Here, we show that in-transit tumor cells inside lymphatic vessels in mice bearing mouse melanomas or human lung tumors give rise to metastases. Using photodynamic therapy with the benzoporphyrin derivative verteporfin, we selectively destroyed lymphatic vessels in mice and pigs. Destruction of tumor-associated lymphatic vessels also eradicated intralymphatic tumor cells and prevented metastasis of mouse melanoma cells and subsequent relapse. Photodynamic therapy, when combined with anti-lymphangiogenic therapy, prevented further tumor invasion of lymphatic vessels. These findings highlight the potential of targeting in-transit tumor cells in patients.

Thrombospondin 1 inhibits inflammatory lymphangiogenesis by CD36 ligation on monocytes

Lymphangiogenesis plays an important role in tumor metastasis and transplant outcome. Here, we show that thrombospondin-1 (TSP-1), a multifunctional extracellular matrix protein and naturally occurring inhibitor of angiogenesis inhibits lymphangiogenesis in mice. Compared with wild-type mice, 6-mo-old TSP-1-deficient mice develop increased spontaneous corneal lymphangiogenesis. Similarly, in a model of inflammation-induced corneal neovascularization, young TSP-1-deficient mice develop exacerbated lymphangiogenesis, which can be reversed by topical application of recombinant human TSP-1. Such increased corneal lymphangiogenesis is also detected in mice lacking CD36, a receptor for TSP-1. In these mice, repopulation of corneal macrophages with predominantly WT mice via bone marrow reconstitution ameliorates their prolymphangiogenic phenotype. In vitro, exposure of WT macrophages to TSP-1 suppresses expression of lymphangiogenic factors vascular endothelial growth factor (VEGF)-C and VEGF-D, but not of a primarily hemangiogenic factor VEGF-A. Inhibition of VEGF-C is not detected in the absence or blockade of CD36. These findings suggest that TSP-1, by ligating CD36 on monocytic cells, acts as an endogenous inhibitor of lymphangiogenesis.

Therapeutic lymphangiogenesis using stem cell and VEGF-C hydrogel

Lymphedema is a manifestation of lymphatic system insufficiency. It arises from primary lymphatic dysplasia or secondary obliteration after lymph node dissection or irradiation. Although improvement of swelling can be achieved by comprehensive non-operative therapy, treatment of this condition requires lifelong care and good compliance. Recently molecular-based treatments using VEGF-C have been investigated by several researchers. We designed the present study to determine whether the therapeutic efficacy of implanted human adipose-derived stem cells (hADSCs) could be improved by applying a gelatin hydrogel containing VEGF-C (VEGF-C hydrogel) to the site of tissue injury in a lymphedema mouse model. Four weeks after the operation, we evaluated edema and determined lymphatic vessel density at various post-operative time points. Mice treated with hADSCs and VEGF-C hydrogel showed a significantly decreased dermal edema depth compared to the groups of mice that received hADSCs only or VEGF-C hydrogel only. Immunohistochemical analysis also revealed that the hADSC/VEGF-C hydrogel group showed significantly greater lymphatic vessel regeneration than all the other groups. hADSCs were detected in the implantation sites of all mice in the hADSC/VEGF-C group, and exhibited a lymphatic endothelial differentiation phenotype as determined by co-staining PKH-labeled hADSCs for the lymphatic marker LYVE-1. Our results suggest that co-administration of hADSCs and VEGF-C hydrogel has a substantial positive effect on lymphangiogenesis.

Biomaterial guides for lymphatic endothelial cell alignment and migration

Axillary dissection during breast cancer surgery produces extensive lymphatic vessel damage that often leads to lifelong secondary lymphedema of the arm. We have developed a biodegradable material conduit for lymphatic vessel reconstruction where fibers electrospun along the conduit lumen promote endothelial cell alignment and migration in vitro. The diameter and density of the electrospun fibers were optimized for cell migration and direction on two-dimensional substrates by seeding human lymphatic endothelial cells (LECs) onto aligned fibers of varying diameters and densities, randomly oriented fibers, and film substrates with no fibers. We found that LECs became aligned in the fiber direction, with cells seeded on the randomly oriented fibers becoming oriented in random directions, whereas cells seeded on the highly aligned fibers became highly aligned. Cell migration was dependent upon fiber alignment and density, with optimal migration found on 1300 nm diameter aligned fibers of low density. Blood endothelial cells seeded on the fibers exhibited similar behavior as the LECs. Fiber alignment was preserved upon rolling the two-dimensional substrate into the tubular geometry of a lymphatic vessel. The data suggest that aligned electrospun fibers may promote endothelial migration across the conduit in a manner that is independent of lymphatic growth factors.

Regulation of developmental lymphangiogenesis by Syk(+) leukocytes

Lymphatic vessels are essential for tissue homeostasis and immune surveillance and contribute to pathological conditions. Lymphatic endothelium differentiates from veins and forms an independent vascular tree with only few connections to the venous circulation. Failure of blood and lymphatic vessel separation results in hemorrhage and edema. VEGF-C and -D are strong inducers of lymphangiogenesis and have essential (VEGF-C) and modulatory (VEGF-D) roles during developmental lymphangiogenesis. We describe here a myeloid population that is defined by expression of the tyrosine kinase Syk, comprises largely M2-polarized mononuclear cells, and robustly expresses angiogenic factors, including VEGF-C/-D and chemokines. These cells stimulate lymphangiogenesis in vivo. Deletion of Syk causes increased chemotractant production, enhanced transmigration, and accumulation in the skin. Ensuing lymphatic hyperplasia and vessel dilation cause the formation of blood-lymphatic shunts. This mechanism does not involve circulating endothelial progenitor cells and demonstrates the potential of hematopoietic cells to control developmental lymphangiogenesis.

Vascular endothelial growth factor-A and -C: expression and correlations with lymphatic metastasis and prognosis in colorectal cancer

Previous studies obtained contradicting results regarding the correlation between expression of VEGF-A, VEGF-C and colorectal cancer patients' clinicopathological features and prognosis. Moreover, the association between the growth factors' expression and lymphatic vessel invasion (LVI) with intratumoral and peritumoral difference has not been reported. In this study, 81 primary colorectal cancer samples were immunohistochemically stained for VEGF-A, VEGF-C and podoplanin. The expression of VEGF-A and VEGF-C in marginal portion was significantly higher than those in central portion (P = 0.000 for both). The expression of VEGF-A in marginal portion was correlated with lymph node metastasis (P = 0.031). The expression of VEGF-C in marginal portion was correlated with TNM stage (P = 0.045), peritumoral LVI (P = 0.048) and lymph node metastasis (P = 0.019). The group with high VEGF-A expression in marginal portion showed worse survival than the low expression group (P = 0.039). Patients with high expression of VEGF-C in the marginal portion were not significantly different from those with low VEGF-C expression (P = 0.121). Patients with high expression of both VEGF-A and VEGF-C in the marginal portion showed the worst survival (P = 0.015). In conclusion, increased expression of VEGF-A and VEGF-C in marginal portion of colorectal cancer was correlated with lymph node metastasis. VEGF-C facilitates colorectal cancer cells invade into peritumoral lymphatic vessels, and different mechanisms may exist in the invasion of tumor cells into peritumoral and intratumoral lymphatic vessels. Assessment the expression of both VEGF-A and VEGF-C in the marginal portion of tumor may help to identify patients with colorectal cancer with unfavourable overall survival.

Lymphatic development

The lymphatic system is essential for fluid homeostasis, immune responses, and fat absorption, and is involved in many pathological processes, including tumor metastasis and lymphedema. Despite its importance, progress in understanding the origins and early development of this system has been hampered by lack of defining molecular markers and difficulties in observing lymphatic cells in vivo and performing genetic and experimental manipulation of the lymphatic system. Recent identification of new molecular markers, new genes with important functional roles in lymphatic development, and new experimental models for studying lymphangiogenesis has begun to yield important insights into the emergence and assembly of this important tissue. This review focuses on the mechanisms regulating development of the lymphatic vasculature during embryogenesis.

Molecular targets for therapeutic lymphangiogenesis in lymphatic dysfunction and disease

The convergence of multiple disciplines upon the study of the lymphatic vasculature has invigorated a renaissance of research, using powerful investigative tools and an exponential growth of interest in this historically underappreciated system. Fundamental discoveries in lymphatic development have yielded relevant animal models for vexing clinical diseases that suffer from nonexistent of minimally effective treatments. Inherited and acquired lymphedema represent the current crux of research efforts to identify potential molecular therapies born from these early discoveries. The importance of the lymphatic system is, however, not limited to lymphedema but encompasses a diverse spectrum of human disease including inflammation and cancer metastasis. As the lymphatic vasculature continues to benefit from fruits of biomedical investigation, translation of mechanistic insights into targeted, rationally-conceived therapeutics will be become a reality.

Rasip1 is required for endothelial cell motility, angiogenesis and vessel formation

Ras proteins are small GTPases that regulate cellular growth and differentiation. Components of the Ras signaling pathway have been shown to be important during embryonic vasculogenesis and angiogenesis. Here, we report that Rasip1, which encodes a novel Ras-interacting protein, is strongly expressed in vascular endothelial cells throughout development, in both mouse and frog. Similar to the well-characterized vascular markers VEGFR2 and PECAM, Rasip1 is specifically expressed in angioblasts prior to vessel formation, in the initial embryonic vascular plexus, in the growing blood vessels during angiogenesis and in the endothelium of mature blood vessels into the postnatal period. Rasip1 expression is undetectable in VEGFR2 null embryos, which lack endothelial cells, suggesting that Rasip1 is endothelial specific. siRNA-mediated reduction of Rasip1 severely impairs angiogenesis and motility in endothelial cell cultures, and morpholino knockdown experiments in frog embryos demonstrate that Rasip1 is required for embryonic vessel formation in vivo. Together, these data identify Rasip1 as a novel endothelial factor that plays an essential role in vascular development.

Transgenic induction of vascular endothelial growth factor-C is strongly angiogenic in mouse embryos but leads to persistent lymphatic hyperplasia in adult tissues

Vascular endothelial growth factor-C (VEGF-C) is the quintessential lymphangiogenic growth factor that is required for the development of the lymphatic system and is capable of stimulating lymphangiogenesis in adults by activating its receptor, VEGFR-3. Although VEGF-C is a major candidate molecule for the development of prolymphangiogenic therapy for defective lymphatic vessels in lymphedema, the stability of lymph vessels generated by exogenous VEGF-C administration is not currently known. We studied VEGF-C-stimulated lymphangiogenesis in inducible transgenic mouse models in which growth factor expression can be spatially and temporally controlled without side effects, such as inflammation. VEGF-C induction in adult mouse skin for 1 to 2 weeks caused robust lymphatic hyperplasia that persisted for at least 6 months. VEGF-C induced lymphangiogenesis in numerous tissues and organs when expressed in the vascular endothelium in either neonates or adult mice. Very few or no effects were observed in either blood vessels or collecting lymph vessels. Additionally, VEGF-C stimulated lymphangiogenesis in embryos after the onset of lymphatic vessel development. Strikingly, a strong angiogenic effect was observed after VEGF-C induction in vascular endothelium at any point before embryonic day 16.5. Our results indicate that blood vessels can undergo VEGF-C-induced angiogenesis even after down-regulation of VEGFR-3 in embryos; however, transient VEGF-C expression in adults can induce long-lasting lymphatic hyperplasia with no obvious side effects on the blood vasculature.

The role of VEGF-C staining in predicting regional metastasis in melanoma

Sentinel lymph node status is the most important prognostic factor in primary melanoma. The number of melanoma-associated lymphatic vessels has been associated with sentinel lymph node status and survival. Vascular endothelial growth factor-C (VEGF-C) is found to promote tumour-associated lymphatic vessel growth. In many human neoplasms, VEGF-C expression in neoplastic cells or tumour-associated macrophages (TAMs) has been linked to lymphatic dissemination of tumour cells. Recent studies have suggested a correlation between VEGF-C expression in primary melanoma and the presence of lymph node metastasis. We performed VEGF-C immunohistochemical staining on melanoma tissues of 113 patients with known sentinel lymph node status. We showed that both high VEGF-C expression in melanoma cells and TAMs are positively associated with the presence of a positive sentinel lymph node. No correlation with Breslow thickness, Clark invasion level or ulceration could be detected. VEGF-C expression in melanoma cells was predictive of a shorter overall and disease-free survival, without being an independent predictor of survival. Our results confirm that VEGF-C expression in primary cutaneous melanoma plays a role in the lymphatic spread of the tumour.

Animal models for the molecular and mechanistic study of lymphatic biology and disease

The development of animal model systems for the study of the lymphatic system has resulted in an explosion of information regarding the mechanisms governing lymphatic development and the diseases associated with lymphatic dysfunction. Animal studies have led to a new molecular model of embryonic lymphatic vascular development, and have provided insight into the pathophysiology of both inherited and acquired lymphatic insufficiency. It has become apparent, however, that the importance of the lymphatic system to human disease extends, beyond its role in lymphedema, to many other diverse pathologic processes, including, very notably, inflammation and tumor lymphangiogenesis. Here, we have undertaken a systematic review of the models as they relate to molecular and functional characterization of the development, maturation, genetics, heritable and acquired diseases, and neoplastic implications of the lymphatic system. The translation of these advances into therapies for human diseases associated with lymphatic dysfunction will require the continued study of the lymphatic system through robust animal disease models that simulate their human counterparts.

Blocking VEGFR-3 suppresses angiogenic sprouting and vascular network formation

Angiogenesis, the growth of new blood vessels from pre-existing vasculature, is a key process in several pathological conditions, including tumour growth and age-related macular degeneration. Vascular endothelial growth factors (VEGFs) stimulate angiogenesis and lymphangiogenesis by activating VEGF receptor (VEGFR) tyrosine kinases in endothelial cells. VEGFR-3 (also known as FLT-4) is present in all endothelia during development, and in the adult it becomes restricted to the lymphatic endothelium. However, VEGFR-3 is upregulated in the microvasculature of tumours and wounds. Here we demonstrate that VEGFR-3 is highly expressed in angiogenic sprouts, and genetic targeting of VEGFR-3 or blocking of VEGFR-3 signalling with monoclonal antibodies results in decreased sprouting, vascular density, vessel branching and endothelial cell proliferation in mouse angiogenesis models. Stimulation of VEGFR-3 augmented VEGF-induced angiogenesis and sustained angiogenesis even in the presence of VEGFR-2 (also known as KDR or FLK-1) inhibitors, whereas antibodies against VEGFR-3 and VEGFR-2 in combination resulted in additive inhibition of angiogenesis and tumour growth. Furthermore, genetic or pharmacological disruption of the Notch signalling pathway led to widespread endothelial VEGFR-3 expression and excessive sprouting, which was inhibited by blocking VEGFR-3 signals. Our results implicate VEGFR-3 as a regulator of vascular network formation. Targeting VEGFR-3 may provide additional efficacy for anti-angiogenic therapies, especially towards vessels that are resistant to VEGF or VEGFR-2 inhibitors.

The role of the lymphatic circulation in the natural history and expression of cardiovascular disease

The lymphatic vasculature is essential to fluid, protein and cellular transport, and to immune responsiveness. The last decade has witnessed a virtual renaissance of investigation into the function of the lymphatic microvasculature, prompting re-consideration of its role in the genesis and progression of cardiovascular pathology. The lymphatic microvasculature of the heart and vascular wall likely participate in atherogenesis, myocardial infarction, congestive heart failure, and cardiac transplantation. Intensive exploration of lymphatic mechanisms of cardiovascular disease is likely to lead to enhanced insights and novel therapeutic approaches.

Deletion of vascular endothelial growth factor C (VEGF-C) and VEGF-D is not equivalent to VEGF receptor 3 deletion in mouse embryos

Lymphatic vessels play an important role in the regulation of tissue fluid balance, immune responses, and fat adsorption and are involved in diseases including lymphedema and tumor metastasis. Vascular endothelial growth factor (VEGF) receptor 3 (VEGFR-3) is necessary for development of the blood vasculature during early embryogenesis, but later, VEGFR-3 expression becomes restricted to the lymphatic vasculature. We analyzed mice deficient in both of the known VEGFR-3 ligands, VEGF-C and VEGF-D. Unlike the Vegfr3(-/-) embryos, the Vegfc(-/-); Vegfd(-/-) embryos displayed normal blood vasculature after embryonic day 9.5. Deletion of Vegfr3 in the epiblast, using keratin 19 (K19) Cre, resulted in a phenotype identical to that of the Vegfr3(-/-) embryos, suggesting that this phenotype is due to defects in the embryo proper and not in placental development. Interestingly, the Vegfr3(neo) hypomorphic mutant mice carrying the neomycin cassette between exons 1 and 2 showed defective lymphatic development. Overexpression of human or mouse VEGF-D in the skin, under the K14 promoter, rescued the lymphatic hypoplasia of the Vegfc(+/-) mice in the K14-VEGF-D; Vegfc(+/-) compound mice, suggesting that VEGF-D is functionally redundant with VEGF-C in the stimulation of developmental lymphangiogenesis. Our results suggest VEGF-C- and VEGF-D-independent functions for VEGFR-3 in the early embryo.

The role of vascular endothelial growth factor in wound healing

BACKGROUND

A chronic wound is tissue with an impaired ability to heal. This is often a consequence of one of the following etiologies: diabetes, venous reflux, arterial insufficiency sickle cell disease, steroids, and/or pressure. Healing requires granulation tissue depending on epithelialization and angiogenesis. Currently no growth factor is available to treat patients with impaired healing that stimulates both epithelialization and angiogenesis. The objective is to review is the multiple mechanisms of vascular endothelial growth factor (VEGF) in wound healing.

MATERIALS AND METHODS

The authors reviewed the literature on the structure and function of VEGF, including its use for therapeutic angiogenesis. Particular attention is given to the specific role of VEGF in the angiogenesis cascade, its relationship to other growth factors and cells in a healing wound.

RESULTS

VEGF is released by a variety of cells and stimulates multiple components of the angiogenic cascade. It is up-regulated during the early days of healing, when capillary growth is maximal. Studies have shown the efficacy of VEGF in peripheral and cardiac ischemic vascular disease with minimal adverse effects. Experimental data supports the hypothesis that VEGF stimulates epithelialization and collagen deposition in a wound.

CONCLUSION

VEGF stimulates wound healing through angiogenesis, but likely promotes collagen deposition and epithelialization as well. Further study of the molecule by utilizing the protein itself, or novel forms of delivery such as gene therapy, will increase its therapeutic possibilities to accelerate closure of a chronic wound.

The resolution of lymphedema by interstitial flow in the mouse tail skin

Lymphangiogenesis is considered a promising approach for increasing fluid drainage during secondary lymphedema. However, organization of lymphatics into functional capillaries may be dependent upon interstitial flow (IF). The present study was undertaken to determine the importance of lymphangiogenesis for lymphedema resolution. We created a lymphatic obstruction that produces lymphedema in mouse tail skin. The relatively scar-free skin regeneration that occurred across the obstruction allowed the progression of lymphangiogenesis to be observed and compared with the evolution of lymphedema. The role of vascular endothelial growth factor-C (VEGF-C)/VEGF receptor (VEGFR)-3 signaling in lymphedema resolution was investigated by exogenous administration of VEGF-C or neutralizing antibodies against VEGFR-3. VEGF-C protein improved lymphedema at 15 days [reducing dermal thickness from 742 ± 105 to 559 ± 141 μm with 95% confidence intervals (CIs), P < 0.05] without increasing lymphatic capillary coverage (11.6 ± 6.4% following VEGF-C treatment relative to 9.6 ± 6.2% with 95% CIs, P > 0.50). Blocking VEGFR-3 signaling did not inhibit lymphedema resolution at 25 days (dermal thickness of 462 ± 127 μm following VEGFR-3 inhibition relative to 502 ± 87 μm with 95% CIs) or inhibit IF, although VEGFR-3 blocking prevented lymphangiogenesis (reducing lymphatic coverage to 0.2 ± 0.7% relative to 8.7 ± 7.3% with 95% CIs, P < 0.005). A second mouse tail lymphedema model was employed to investigate the ability of VEGF-C to increase fluid drainage across a scar. We found that neither neutralization of VEGFR-3 nor administration of VEGF-C affected the course of skin swelling over 25 days. These findings suggest that resolution of lymphedema in the mouse tail skin may be more dependent upon IF and regeneration of the extracellular matrix across the obstruction than lymphatic capillary regeneration.

Transgenic mouse models of angiogenesis and lymphangiogenesis

The development of transgenic technologies in mice has allowed the study of the consequences of genetic alterations on angiogenesis and lymphangiogenesis. This review summarizes the murine models currently available for studies involving the manipulation of angiogenesis and lymphangiogenesis. Abnormal embryonic vascular development, resulting from defects in the formation of a primitive vascular plexus, has been observed in mice lacking vascular endothelial growth factor, vascular endothelial growth factor receptor-1 and -2, transforming growth factor-beta, fibronectin, or vascular endothelial cadherin. Defects in the expansion and remodeling of the embryonic vasculature occur in mice deficient in Tie-1, Tie-2, or angiopoietin-1, and in mice overexpressing neuropilin or angiopoietin-2. Impaired recruitment and investment of mural cells have been observed in mice with disruption of the genes encoding platelet-derived growth factor-B, platelet-derived growth factor-B receptor, and tissue factor. Gene-targeting experiments in mice have identified the EphB/ephrinB system as a critical and rate-limiting determinant of arteriovenous differentiation during embryonic vascular development. Vascular endothelial growth factor-C is necessary for the initial sprouting and migration of lymphatic endothelial cells from embryonic veins, and mice lacking vascular endothelial growth factor-C die prenatally, whereas vascular endothelial growth factor-D is dispensable for embryonic lymphatic development.

Coordinated vascular endothelial growth factor expression and signaling during skeletal myogenic differentiation

Angiogenesis is largely controlled by hypoxia-driven transcriptional up-regulation and secretion of vascular endothelial growth factor (VEGF) and its binding to the endothelial cell tyrosine receptor kinases, VEGFR1 and VEGFR2. Recent expression analysis suggests that VEGF is expressed in a cell-specific manner in normoxic adult tissue; however, the transcriptional regulation and role of VEGF in these tissues remains fundamentally unknown. In this report we demonstrate that VEGF is coordinately up-regulated during terminal skeletal muscle differentiation. We reveal that this regulation is mediated in part by MyoD homo- and hetero-dimeric transcriptional mechanisms. Serial deletions of the VEGF promoter elucidated a region containing three tandem CANNTG consensus MyoD sites serving as essential sites of direct interaction for MyoD-mediated up-regulation of VEGF transcription. VEGF-null embryonic stem (ES) cells exhibited reduced myogenic differentiation compared with wild-type ES cells, suggesting that VEGF may serve a role in skeletal muscle differentiation. We demonstrate that VEGFR1 and VEGFR2 are expressed at low levels in myogenic precursor cells and are robustly activated upon VEGF stimulation and that their expression is coordinately regulated during skeletal muscle differentiation. VEGF stimulation of differentiating C2C12 cells promoted myotube hypertrophy and increased myogenic differentiation, whereas addition of sFlt1, a VEGF inhibitor, resulted in myotube hypotrophy and inhibited myogenic differentiation. We further provide evidence indicating VEGF-mediated myogenic marker expression, mitogenic activity, migration, and prosurvival functions may contribute to increased myogenesis. These data suggest a novel mechanism whereby VEGF is coordinately regulated as part of the myogenic differentiation program and serves an autocrine function regulating skeletal myogenesis.

VEGF-C induced angiogenesis preferentially occurs at a distance from lymphangiogenesis

AIMS

Vascular endothelial growth factor-C (VEGF-C) has been shown to stimulate both angiogenesis and lymphangiogenesis in some but not all models where VEGF-C is over-expressed. Our aim was to investigate the interaction between lymphangiogenesis and angiogenesis in adult tissues regulated by VEGF-C and identify evidence of polarized growth of lymphatics driven by specialized cells at the tip of the growing sprout.

METHODS AND RESULTS

We used an adult model of lymphangiogenesis in the rat mesentery. The angiogenic effect of VEGF-C was markedly attenuated in the presence of a growing lymphatic network. Furthermore, we show that this growth of lymphatic vessels can occur both by recruitment of isolated lymphatic islands to a connected network and by filopodial sprouting. The latter is independent of polarized tip cell differentiation that can be generated all along lymphatic capillaries, independently of the proliferation status of the lymphatic endothelial cells.

CONCLUSION

These results both demonstrate a dependence of VEGF-C-mediated angiogenesis on lymphatic vascular networks and indicate that the mechanism of VEGF-C-mediated lymphangiogenesis is different from that of classical angiogenic mechanisms.

An experimental model for the study of lymphedema and its response to therapeutic lymphangiogenesis

BACKGROUND

Evaluation of the efficacy of molecular treatment strategies for lymphatic vascular insufficiency requires a suitable preclinical animal model. Ideally, the model should closely replicate the untreated human disease in its pathogenesis and pathological expression.

OBJECTIVE

We have undertaken a study of the time course of the development and resolution of acquired, experimental lymphedema and of its responses to vascular endothelial growth factor (VEGF)-C lymphangiogenesis in the mouse tail model.

STUDY DESIGN

We provoked post-surgical lymphedema in the mouse tail model and assessed the effects of exogenously administered human recombinant VEGF-C. Quantitative assessment of immune traffic function was performed through sequential in vivo bioluminescent imaging.

RESULTS

In untreated lymphedema, tail edema was sustained until day 21. Exogenous administration of human recombinant VEGF-C produced a significant decrease in volume. Untreated lymphedema in the mouse tail model was characterized by the presence of dilated cutaneous lymphatics, marked acute inflammatory changes, and hypercellularity; VEGF-C produced a substantial reversion to the normal pattern, with notable regression in the size and number of cutaneous lymphatic vessels that express lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). In vivo imaging confirmed the presence of an impairment of immune traffic in lymphedema that was ameliorated after VEGF-C administration.

CONCLUSION

The post-surgical murine tail model of lymphedema closely simulates attributes of human lymphedema and provides the requisite sensitivity to detect therapeutically induced functional and structural alterations. It can, therefore, be used as an investigative platform to assess mechanisms of disease and its responses to candidate therapies, such as therapeutic lymphangiogenesis.

Distinct vascular endothelial growth factor signals for lymphatic vessel enlargement and sprouting

Lymphatic vessel growth, or lymphangiogenesis, is regulated by vascular endothelial growth factor-C (VEGF-C) and -D via VEGF receptor 3 (VEGFR-3). Recent studies suggest that VEGF, which does not bind to VEGFR-3, can also induce lymphangiogenesis through unknown mechanisms. To dissect the receptor pathway that triggers VEGFR-3–independent lymphangiogenesis, we used both transgenic and adenoviral overexpression of placenta growth factor (PlGF) and VEGF-E, which are specific activators of VEGFR-1 and -2, respectively. Unlike PlGF, VEGF-E induced circumferential lymphatic vessel hyperplasia, but essentially no new vessel sprouting, when transduced into mouse skin via adenoviral vectors. This effect was not inhibited by blocking VEGF-C and -D. Postnatal lymphatic hyperplasia, without increased density of lymphatic vessels, was also detected in transgenic mice expressing VEGF-E in the skin, but not in mice expressing PlGF. Surprisingly, VEGF-E induced lymphatic hyperplasia postnatally, and it did not rescue the loss of lymphatic vessels in transgenic embryos where VEGF-C and VEGF-D were blocked. Our data suggests that VEGFR-2 signals promote lymphatic vessel enlargement, but unlike in the blood vessels, are not involved in vessel sprouting to generate new lymphatic vessels in vivo.

VEGF-C and VEGF-A synergistically enhance lymph node metastasis of gastric cancer

PURPOSE

Vascular endothelial growth factor C (VEGF-C) is the most important molecule in lymphangiogenesis and its relationship with lymph node metastasis has attracted considerable interest. We investigated the relationship of VEGF-C or VEGF-A with clinicopathological factors in gastric cancer patients.

METHODS

Eighty gastric cancer patients who underwent gastric resection were analyzed immunohistochemically for expression of VEGF-C and VEGF-A protein.

RESULTS

Positive immunoreactivity of VEGF-C and VEGF-A was observed in 75 (93.8%) and 41 (51.3%) patients, respectively. VEGF-A expression was significantly correlated with tumor differentiation (p=0.0017) and vascular invasion (p=0.0004). And positive relationship of VEGF-C expression was only demonstrated with tumor differentiation (p=0.0168). Interestingly, however, the frequency of lymph node metastasis was significantly higher in the patients with expression of both VEGF-C and VEGF-A (strong positive expression, p=0.036). Furthermore, the expression of both was also significantly correlated with depth of tumor invasion, tumor differentiation, lymphatic invasion, and vascular invasion.

CONCLUSION

The present results suggest that strong expression of VEGF-A in addition to VEGF-C expression is essential in lymph node metastasis, presumably because enhanced metastatic potential including lymphangiogenesis induced by both VEGF-A and VEGF-C is vital in lymph node metastasis of gastric cancer.