Molecular biology and pathology of lymphangiogenesis

Current views on the function of the lymphatic vasculature in health and disease

The lymphatic vascular system is essential for lipid absorption, fluid homeostasis, and immune surveillance. Until recently, lymphatic vessel dysfunction had been associated with symptomatic pathologic conditions such as lymphedema. Work in the last few years had led to a better understanding of the functional roles of this vascular system in health and disease. Furthermore, recent work has also unraveled additional functional roles of the lymphatic vasculature in fat metabolism, obesity, inflammation, and the regulation of salt storage in hypertension. In this review, we summarize the functional roles of the lymphatic vasculature in health and disease.

Interaction between the extracellular matrix and lymphatics: consequences for lymphangiogenesis and lymphatic function

The lymphatic system is important for body fluid balance as well as immunological surveillance. Due to the identification of new molecular markers during the last decade, there has been a recent dramatic increase in our knowledge on the molecular mechanisms involved in lymphatic vessel growth (lymphangiogenesis) and lymphatic function. Here we review data showing that although it is often overlooked, the extracellular matrix plays an important role in the generation of new lymphatic vessels as a response to physiological and pathological stimuli. Extracellular matrix-lymphatic interactions as well as biophysical characteristics of the stroma have consequences for tumor formation, growth and metastasis. During the recent years, anti-lymphangiogenesis has emerged as an additional therapeutic modality to the clinically applied anti-angiogenesis strategy. Oppositely, enhancement of lymphangiogenesis in situations of lymph accumulation is seen as a promising strategy to a set of conditions where few therapeutic avenues are available. Knowledge on the interaction between the extracellular matrix and the lymphatics may enhance our understanding of the underlying mechanisms and may ultimately lead to better therapies for conditions where reduced or increased lymphatic function is the therapeutic target.

Stimulation of lymphangiogenesis via VEGFR-3 inhibits chronic skin inflammation

The role of lymphangiogenesis in inflammation has remained unclear. To investigate the role of lymphatic versus blood vasculature in chronic skin inflammation, we inhibited vascular endothelial growth factor (VEGF) receptor (VEGFR) signaling by function-blocking antibodies in the established keratin 14 (K14)-VEGF-A transgenic (Tg) mouse model of chronic cutaneous inflammation. Although treatment with an anti-VEGFR-2 antibody inhibited skin inflammation, epidermal hyperplasia, inflammatory infiltration, and angiogenesis, systemic inhibition of VEGFR-3, surprisingly, increased inflammatory edema formation and inflammatory cell accumulation despite inhibition of lymphangiogenesis. Importantly, chronic Tg delivery of the lymphangiogenic factor VEGF-C to the skin of K14-VEGF-A mice completely inhibited development of chronic skin inflammation, epidermal hyperplasia and abnormal differentiation, and accumulation of CD8 T cells. Similar results were found after Tg delivery of mouse VEGF-D that only activates VEGFR-3 but not VEGFR-2. Moreover, intracutaneous injection of recombinant VEGF-C156S, which only activates VEGFR-3, significantly reduced inflammation. Although lymphatic drainage was inhibited in chronic skin inflammation, it was enhanced by Tg VEGF-C delivery. Together, these results reveal an unanticipated active role of lymphatic vessels in controlling chronic inflammation. Stimulation of functional lymphangiogenesis via VEGFR-3, in addition to antiangiogenic therapy, might therefore serve as a novel strategy to treat chronic inflammatory disorders of the skin and possibly also other organs.

Functional analyses of the bone marrow kinase in the X chromosome in vascular endothelial growth factor-induced lymphangiogenesis

OBJECTIVE

The goal of this study was to investigate the novel hypothesis that bone marrow kinase in the X chromosome (Bmx), an established inflammatory mediator of pathological angiogenesis, promotes lymphangiogenesis.

METHODS AND RESULTS

We have recently demonstrated a critical role for Bmx in inflammatory angiogenesis. However, the role of Bmx in lymphangiogenesis has not been investigated. Here, we show that in wild-type mice, Bmx is upregulated in lymphatic vessels in response to vascular endothelial growth factor (VEGF). In comparison with wild-type mice, Bmx-deficient mice mount weaker lymphangiogenic responses to VEGF-A and VEGF-C in 2 mouse models. In vitro, Bmx is expressed in cultured human dermal microvascular lymphatic endothelial cells. Furthermore, pharmacological inhibition and short interfering RNA mediated silencing of Bmx reduces VEGF-A and VEGF-C-induced signaling and lymphatic endothelial cell tube formation. Mechanistically, we demonstrated that Bmx differentially regulates VEGFR-2 and VEGFR-3 receptor signaling pathways: Bmx associates with and directly regulates VEGFR-2 activation, whereas Bmx associates with VEGFR-3 and regulates downstream signaling without an effect on the receptor autophosphorylation.

CONCLUSIONS

Our in vivo and in vitro results provide the first insight into the mechanism by which Bmx mediates VEGF-dependent lymphangiogenic signaling.

Protein tyrosine phosphatase PTPN14 is a regulator of lymphatic function and choanal development in humans

The lymphatic vasculature is essential for the recirculation of extracellular fluid, fat absorption, and immune function and as a route of tumor metastasis. The dissection of molecular mechanisms underlying lymphangiogenesis has been accelerated by the identification of tissue-specific lymphatic endothelial markers and the study of congenital lymphedema syndromes. We report the results of genetic analyses of a kindred inheriting a unique autosomal-recessive lymphedema-choanal atresia syndrome. These studies establish linkage of the trait to chromosome 1q32-q41 and identify a loss-of-function mutation in PTPN14, which encodes a nonreceptor tyrosine phosphatase. The causal role of PTPN14 deficiency was confirmed by the generation of a murine Ptpn14 gene trap model that manifested lymphatic hyperplasia with lymphedema. Biochemical studies revealed a potential interaction between PTPN14 and the vascular endothelial growth factor receptor 3 (VEGFR3), a receptor tyrosine kinase essential for lymphangiogenesis. These results suggest a unique and conserved role for PTPN14 in the regulation of lymphatic development in mammals and a nonconserved role in choanal development in humans.

Quantitative imaging of lymphatic function with liposomal indocyanine green

Lymphatic vessels play a major role in cancer progression and in postsurgical lymphedema, and several new therapeutic approaches targeting lymphatics are currently being developed. Thus, there is a critical need for quantitative imaging methods to measure lymphatic flow. Indocyanine green (ICG) has been used for optical imaging of the lymphatic system, but it is unstable in solution and may rapidly enter venous capillaries after local injection. We developed a novel liposomal formulation of ICG (LP-ICG), resulting in vastly improved stability in solution and an increased fluorescence signal with a shift toward longer wavelength absorption and emission. When injected intradermally to mice, LP-ICG was specifically taken up by lymphatic vessels and allowed improved visualization of deep lymph nodes. In a genetic mouse model of lymphatic dysfunction, injection of LP-ICG showed no enhancement of draining lymph nodes and slower clearance from the injection site. In mice bearing B16 luciferase-expressing melanomas expressing vascular endothelial growth factor-C (VEGF-C), sequential near-IR imaging of intradermally injected LP-ICG enabled quantification of lymphatic flow. Increased flow through draining lymph nodes was observed in mice bearing VEGF-C-expressing tumors without metastases, whereas a decreased flow pattern was seen in mice with a higher lymph node tumor burden. This new method will likely facilitate quantitative studies of lymphatic function in preclinical investigations and may also have potential for imaging of lymphedema or improved sentinel lymph detection in cancer.

Pathophysiology of oral cancer in experimental animal models: a review with focus on the role of sympathetic nerves

Global increase in incidence and mortality as well as poor prognosis of oral cancer (OC) has intensified efforts towards early detection and prevention of this disfiguring disease. Several studies have been conducted using experimental animal models to understand the pathophysiology and molecular events involved in OC. Lack of identification of specific biomarkers during the multifaceted steps of oral carcinogenesis has hindered its diagnosis and treatment. Solid stress generated by growing tumors as well as abnormalities in tumor vasculature lead to increased interstitial fluid pressure, which could obstruct therapeutic drug delivery to tumors. Furthermore, the sympathetic nervous system is known to affect angiogenesis, vessel permeability, immune responses and carcinogenesis. Recent findings indicate that, in addition to angiogenic and lymphangiogenic factors, tumor cells release neurotrophic factors that initiate innervation. Interactions between cytokines and sympathetic neurotransmitters, and their respective receptors expressed by the nerve, immune and tumor cells appear to influence tumor growth. Thus, understanding the complex signaling processes and interrelationships between vascular, nervous and immune systems during oral carcinogenesis may prove vital for successful prevention and treatment of OC. This review aims at outlining the available knowledge on pathophysiology of OC in experimental animal models including evidence from our own findings.

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.

Lymphatics in the human endometrium disappear during decidualization

BACKGROUND

The mammalian placenta plays a central role in maternal tolerance of the semi-allogeneic fetus and fluid balance between the maternal and fetal compartments. The lymphatics play a role in both these function. The aim of this study was to describe the distribution of lymphatic vessels in human decidua, with particular focus on the lymphatics that surround remodelling spiral arteries during decidualization and trophoblast invasion.

METHODS

Placental bed and non-placental bed (decidua parietalis) biopsies were obtained from 41 women undergoing elective termination of pregnancy at 6-18 weeks gestational age as well as placental bed biopsies from 5 women undergoing elective Caesarean section at term. In addition to routine haematoxylin and eosin staining, double immunohistochemical labelling was performed on serial 3-µm sections to identify lymphatic vessels in conjunction with one of the following: blood vessels, smooth muscle, epithelial and trophoblast cells or proliferating cells. Representative photomicrographs of all sections were obtained from a total of 273 areas (46 samples, average 6 range 3-15 areas per sample). Descriptive findings of the organization of lymphatics in human placental bed and decidua parietalis were made from a total of 1638 images.

RESULTS

Lymphatic vessels positive for podoplanin were abundant in non-decidualized hypersecretory endometrium at all stages of gestation. By contrast, the decidua was nearly always devoid of lymphatics. In some samples, structures that appeared to be regressing lymphatics could be observed at the boundary between non-decidualized hypersecretory and decidualized endometrium. Lymphatic vessels were notably absent from the vicinity of spiral arteries that were surrounded by decidualized stromal cells. Lymphatic vessels in non-decidualized hypersecretory endometrium appeared larger and more elongated as gestation progressed. Proliferating lymphatic vascular endothelial cells were identified in both large vessels, and in streaks of D2-40 positive cells that could have been newly forming lymphatic vessels. Placental bed lymphatics exhibited limited and variable staining with LYVE-1 at all stages of pregnancy apart from term.

CONCLUSIONS

We have made novel observations on lymphatics in the placental bed and their relationship with other structures throughout pregnancy. Endometrial stromal cell decidualization results in a loss of lymphatics, with this phenomenon being particularly apparent around the spiral arteries.

New insights into the development of lymphoid tissues

Secondary lymphoid organs are important locations for the initiation of adaptive immune responses. They develop before birth, and their formation requires interaction between lymphotoxin-α₁ß₂-expressing lymphoid-tissue inducer cells and lymphotoxin-ß receptor-expressing stromal organizer cells. Here, we discuss new insights into the earliest phases of peripheral lymph node and Peyer's patch formation that occur before lymphotoxin-ß receptor signalling and suggest a role for the developing nervous system. In addition, we discuss the differing requirements for the postnatal formation of mucosa-associated lymphoid tissues and tertiary lymphoid structures that develop at sites of chronic inflammation.

Blood vessel endothelial VEGFR-2 delays lymphangiogenesis: an endogenous trapping mechanism links lymph- and angiogenesis

Angio- and lymphangiogenesis are inherently related processes. However, how blood and lymphatic vessels regulate each other is unknown. This work introduces a novel mechanism explaining the temporal and spatial relation of blood and lymphatic vessels. Vascular endothelial growth factor-A (VEGF-A) surprisingly reduced VEGF-C in the supernatant of blood vessel endothelial cells, suggesting growth factor (GF) clearance by the growing endothelium. The orientation of lymphatic sprouting toward angiogenic vessels and away from exogenous GFs was VEGF-C dependent. In vivo molecular imaging revealed higher VEGF receptor (R)-2 in angiogenic tips compared with normal vessels. Consistently, lymphatic growth was impeded in the angiogenic front. VEGF-C/R-2 complex in the cytoplasm of VEGF-A-treated endothelium indicated that receptor-mediated internalization causes GF clearance from the extracellular matrix. GF clearance by receptor-mediated internalization is a new paradigm explaining various characteristics of lymphatics.

Vascular endothelial growth factor-D over-expressing tumor cells induce differential effects on uterine vasculature in a mouse model of endometrial cancer

BACKGROUND

It has been hypothesised that increased VEGF-D expression may be an independent prognostic factor for endometrial cancer progression and lymph node metastasis; however, the mechanism by which VEGF-D may promote disease progression in women with endometrial cancer has not been investigated. Our aim was to describe the distribution of lymphatic vessels in mouse uterus and to examine the effect of VEGF-D over-expression on these vessels in a model of endometrial cancer. We hypothesised that VEGF-D over-expression would stimulate growth of new lymphatic vessels into the endometrium, thereby contributing to cancer progression.

METHODS

We initially described the distribution of lymphatic vessels (Lyve-1, podoplanin, VEGFR-3) and VEGF-D expression in the mouse uterus during the estrous cycle, early pregnancy and in response to estradiol-17beta and progesterone using immunohistochemistry. We also examined the effects of VEGF-D over-expression on uterine vasculature by inoculating uterine horns in NOD SCID mice with control or VEGF-D-expressing 293EBNA tumor cells.

RESULTS

Lymphatic vessels positive for the lymphatic endothelial cell markers Lyve-1, podoplanin and VEGFR-3 profiles were largely restricted to the connective tissue between the myometrial circular and longitudinal muscle layers; very few lymphatic vessel profiles were observed in the endometrium. VEGF-D immunostaining was present in all uterine compartments (epithelium, stroma, myometrium), although expression was generally low. VEGF-D immunoexpression was slightly but significantly higher in estrus relative to diestrus; and in estradiol-17beta treated mice relative to vehicle or progesterone treated mice. The presence of VEGF-D over-expressing tumor cells did not induce endometrial lymphangiogenesis, although changes were observed in existing vessel profiles. For myometrial lymphatic and endometrial blood vessels, the percentage of profiles containing proliferating endothelial cells, and the cross sectional area of vessel profiles were significantly increased in response to VEGF-D in comparison to control tumor cells. In contrast, no significant changes were noted in myometrial blood vessels. In addition, examples of invading cells or tumor emboli were observed in mice receiving VEGF-D expressing 293EBNA cells.

CONCLUSIONS

These results illustrate that VEGF-D over-expression has differential effects on the uterine vasculature. These effects may facilitate VEGF-D's ability to promote endometrial cancer metastasis and disease progression.

The role of macrophages and mast cells in lymphangiogenesis and angiogenesis in cervical carcinogenesis

During carcinogenesis it is known that growth factors and cytokines from stromal and inflammatory cells from the microenvironment promote angiogenesis and lymphangiogenesis. However, the participation of macrophages and mast cells in these processes is not well understood. The aim of this study was to evaluate the relationship between mast cell and macrophage density with blood and lymphatic vessels in various stages of carcinoma of the uterine cervix. Tissue sections from archival paraffin-embedded samples from cases with cervical intraepithelial neoplasias (CIN) 1, 2, 3, carcinoma in situ, and invasive carcinoma were used. Immunohistochemical staining was done using the following antibodies: anti-LYVE-1; anti-CD31; anti-CD68, and anti-tryptase. Our results showed a significant increase in the number of macrophages in carcinoma in situ, a correlation between lymphatic vessels and macrophages in premalignant lesions CIN 2, and a correlation between mast cells and blood vessels in both CIN 2 and carcinoma in situ. In conclusion, our data underscore the importance of the recruitment of macrophages and mast cells in the development of tumor-associated blood and lymphatic capillaries.

Pigment epithelium-derived factor stimulates tumor macrophage recruitment and is downregulated by the prostate tumor microenvironment

Pigment epithelium-derived factor (PEDF) is a potent inhibitor of angiogenesis but whether it has additional effects on the tumor microenvironment is largely unexplored. We show that overexpression of PEDF in orthotopic MatLyLu rat prostate tumors increased tumor macrophage recruitment. The fraction of macrophages expressing inducible nitric oxide synthase, a marker of cytotoxic M1 macrophages, was increased, suggesting that PEDF could enhance antitumor immunity. In addition, PEDF overexpression reduced vascular growth both in the tumor and in the surrounding normal tissue, slowed tumor growth, and decreased lymph node metastasis. Contrary, extratumoral lymphangiogenesis was increased. PEDF expression is, for reasons unknown, often decreased or lost during prostate tumor progression. When AT-1 rat prostate tumor cells, expressing high levels of PEDF messenger RNA (mRNA) and protein, were injected into the prostate, PEDF is markedly downregulated, suggesting that factors in the microenvironment suppressed its expression. One such factor could be macrophage-derived tumor necrosis factor alpha (TNFalpha). A fraction of the accumulating macrophages expressed TNFalpha, and TNFalpha treatment downregulated the expression of PEDF protein and mRNA in prostate AT-1 tumor cells in vitro and in the rat ventral prostate in vivo. PEDF apparently has multiple effects in prostate tumors: it suppresses angiogenesis and metastasis, but it also causes macrophage accumulation. Accumulating macrophages may inhibit tumor growth, but they may also suppress PEDF and enhance lymph angiogenesis and, in this way, eventually enhance tumor growth.

Breast carcinomatous tumoral emboli can result from encircling lymphovasculogenesis rather than lymphovascular invasion

The canonical view of the origin of tumor lymphovascular emboli is that they usually originate from lymphovascular invasion as part of a multistep metastatic process. Recent experimental evidence has suggested that metastasis can occur earlier than previously thought and we found evidence that tumor emboli formation can result from the short-circuiting step of encircling lymphovasculogenesis. Experimentally, we used a xenograft of human inflammatory breast cancer (MARY-X), a model that exhibited florid tumor emboli, to generate tumoral spheroids in vitro. In observational studies, we chose human breast carcinoma cases where there appeared to be a possible transition of in situ carcinoma to lymphovascular emboli without intervening stromal invasion. These cases were studied by morphometry as well as IHC with tumor proliferation (Ki-67) and adhesion (E-cadherin) markers, myoepithelial (p63), as well as endothelial (podoplanin [D2-40], CD31, VEGFR-3, Prox-1) markers. Unlabelled spheroids coinjected with either GFP or RFP-human myoepithelial cells or murine embryonal fibroblasts (MEFs) gave rise to tumors which exhibited GFP/RFP immunoreactivity within the cells lining the emboli-containing lymphovascular channels. In vitro studies demonstrated that the tumoral spheroids induced endothelial differentiation of cocultured myoepithelial cells and MEFs, measured by real time PCR and immunofluorescence. In humans, the in situ clusters exhibited similar proliferation, E-cadherin immunoreactivity and size as the tumor emboli (p =.5), suggesting the possibility that the latter originated from the former. The in situclusters exhibited a loss (50%-100%) of p63 myoepithelial immunoreactivity but not E-cadherin epithelial immunoreactivity. The tumor emboli were mainly present within lymphatic channels whose dual p63/CD31, p63/D2-40 and p63/VEGFR-3 and overall weak patterns of D2-40/CD31/VEGFR-3 immunoreactivities suggested that they represented immature and newly created vasculature derived from originally myoepithelial-lined ducts. Collectively both experimental as well as observational studies suggested the possibility that these breast cancer emboli resulted from encircling lymphovasculogenesis rather than conventional lymphovascular invasion.

Lymphatic capillary hypoplasia in the skin of fetuses with increased nuchal translucency and Turner's syndrome: comparison with trisomies and controls

Fetuses with Turner's syndrome or trisomies 21, 18 and 13 show excess of skin, which can be visualized by ultrasonography as increased nuchal translucency at 11-13(+6) weeks' gestation. The objective of this study was to gain insight in the development and distribution of blood vessels, lymphatic capillaries of the cutis and lymphatic collectors of the cutis and subcutis and to study developmental changes with increasing gestation. Immunofluorescence of cryosections with 10 specific antibodies was used to investigate the nuchal skin of three fetuses with Turner syndrome's and to differentiate lymphatics, lymph capillaries (FLT4, PTN 63, LYVE1, PROX1), blood vessels (KDR, CD 31, PDPN), blood clotting activity (von Willebrand factor), basement membranes and big vessels (Laminin, Collagen Type IV). The findings were compared with those in seven fetuses with trisomy 21 and two fetuses each with trisomies 18 or 13, respectively, as well as six normal controls. Immunoreactive receptors for vascular endothelial growth factors (FLT4) were decreased in lymphatic capillaries of the skin of Turner fetuses. Accordingly, LYVE1 was scarce and PROX1 staining was less intense in the dermis of Turner fetuses. Lymphatic collectors were, however, evenly stained. In normal fetuses and in those with trisomies, lymphatic capillaries were evenly distributed. We conclude that lymphatic capillary hypoplasia might be responsible for nuchal cystic hygroma in Turner syndrome. The biological basis for increased nuchal translucency in trisomies may however be different.

H-, N- and Kras cooperatively regulate lymphatic vessel growth by modulating VEGFR3 expression in lymphatic endothelial cells in mice

Mammalian Ras, which is encoded by three independent genes, has been thought to be a versatile component of intracellular signalling. However, when, where and how Ras signalling plays essential roles in development and whether the three Ras genes have overlapping functions in particular cells remain unclear. Here, we show that the three Ras proteins dose-dependently regulate lymphatic vessel growth in mice. We find that lymphatic vessel hypoplasia is a common phenotype in Ras compound knockout mice and that overexpressed normal Ras in an endothelial cell lineage selectively causes lymphatic vessel hyperplasia in vivo. Overexpression of normal Ras in lymphatic endothelial cells leads to sustained MAPK activation, cellular viability and enhanced endothelial network formation under serum-depleted culture conditions in vitro, and knockdown of endogenous Ras in lymphatic endothelial cells impairs cell proliferation, MAPK activation, cell migration and endothelial network formation. Ras overexpression and knockdown result in up- and downregulation of vascular endothelial growth factor receptor (VEGFR) 3 expression, respectively, in lymphatic endothelial cells in vitro. The close link between Ras and VEGFR3 in vitro is consistent with the result that Ras knockout and transgenic alleles are genetic modifiers in lymphatic vessel hypoplasia caused by Vegfr3 haploinsufficiency. Our findings demonstrate a cooperative function of the three Ras proteins in normal development, and also provide a novel aspect of VEGFR3 signalling modulated by Ras in lymphangiogenesis.

Direct transcriptional regulation of neuropilin-2 by COUP-TFII modulates multiple steps in murine lymphatic vessel development

The lymphatic system plays a key role in tissue fluid homeostasis. Lymphatic dysfunction contributes to the pathogenesis of many human diseases, including lymphedema and tumor metastasis. However, the mechanisms regulating lymphangiogenesis remain largely unknown. Here, we show that COUP-TFII (also known as Nr2f2), an orphan member of the nuclear receptor superfamily, mediates both developmental and pathological lymphangiogenesis in mice. Conditional ablation of COUP-TFII at an early embryonic stage resulted in failed formation of pre-lymphatic ECs (pre-LECs) and lymphatic vessels. COUP-TFII deficiency at a late developmental stage resulted in loss of LEC identity, gain of blood EC fate, and impaired lymphatic vessel sprouting. siRNA-mediated downregulation of COUP-TFII in cultured primary human LECs demonstrated that the maintenance of lymphatic identity and VEGF-C-induced lymphangiogenic activity, including cell proliferation and migration, are COUP-TFII-dependent and cell-autonomous processes. COUP-TFII enhanced the pro-lymphangiogenic actions of VEGF-C, at least in part by directly stimulating expression of neuropilin-2, a coreceptor for VEGF-C. In addition, COUP-TFII inactivation in a mammary gland mouse tumor model resulted in inhibition of tumor lymphangiogenesis, suggesting that COUP-TFII also regulates neo-lymphangiogenesis in the adult. Thus, COUP-TFII is a critical factor that controls lymphangiogenesis in embryonic development and tumorigenesis in adults.

Axon guidance molecules in vascular patterning

Endothelial cells (ECs) form extensive, highly branched and hierarchically organized tubular networks in vertebrates to ensure the proper distribution of molecular and cellular cargo in the vertebrate body. The growth of this vascular system during development, tissue repair or in disease conditions involves the sprouting, migration and proliferation of endothelial cells in a process termed angiogenesis. Surprisingly, specialized ECs, so-called tip cells, which lead and guide endothelial sprouts, share many feature with another guidance structure, the axonal growth cone. Tip cells are motile, invasive and extend numerous filopodial protrusions sensing growth factors, extracellular matrix and other attractive or repulsive cues in their tissue environment. Axonal growth cones and endothelial tip cells also respond to signals belonging to the same molecular families, such as Slits and Roundabouts, Netrins and UNC5 receptors, Semaphorins, Plexins and Neuropilins, and Eph receptors and ephrin ligands. Here we summarize fundamental principles of angiogenic growth, the selection and function of tip cells and the underlying regulation by guidance cues, the Notch pathway and vascular endothelial growth factor signaling.

Lymphangiogenesis: Molecular mechanisms and future promise

The growth of lymphatic vessels (lymphangiogenesis) is actively involved in a number of pathological processes including tissue inflammation and tumor dissemination but is insufficient in patients suffering from lymphedema, a debilitating condition characterized by chronic tissue edema and impaired immunity. The recent explosion of knowledge on the molecular mechanisms governing lymphangiogenesis provides new possibilities to treat these diseases.

Targeting of drugs and nanoparticles to tumors

The various types of cells that comprise the tumor mass all carry molecular markers that are not expressed or are expressed at much lower levels in normal cells. These differentially expressed molecules can be used as docking sites to concentrate drug conjugates and nanoparticles at tumors. Specific markers in tumor vessels are particularly well suited for targeting because molecules at the surface of blood vessels are readily accessible to circulating compounds. The increased concentration of a drug in the site of disease made possible by targeted delivery can be used to increase efficacy, reduce side effects, or achieve some of both. We review the recent advances in this delivery approach with a focus on the use of molecular markers of tumor vasculature as the primary target and nanoparticles as the delivery vehicle.

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.

Biofluid mechanics of special organs and the issue of system control. Sixth International Bio-Fluid Mechanics Symposium and Workshop, March 28-30, 2008 Pasadena, California

In the field of fluid flow within the human body, focus has been placed on the transportation of blood in the systemic circulation since the discovery of that system; but, other fluids and fluid flow phenomena pervade the body. Some of the most fascinating fluid flow phenomena within the human body involve fluids other than blood and a service other than transport--the lymphatic and pulmonary systems are two striking examples. While transport is still involved in both cases, this is not the only service which they provide and blood is not the only fluid involved. In both systems, filtration, extraction, enrichment, and in general some "treatment" of the fluid itself is the primary function. The study of the systemic circulation has also been conventionally limited to treating the system as if it were an open-loop system governed by the laws of fluid mechanics alone, independent of physiological controls and regulations. This implies that system failures can be explained fully in terms of the laws of fluid mechanics, which of course is not the case. In this paper we examine the clinical implications of these issues and of the special biofluid mechanics issues involved in the lymphatic and pulmonary systems.

Neuropilin-2 mediates VEGF-C-induced lymphatic sprouting together with VEGFR3

If neuropilin-2 and the growth factor VEGF-C don’t come together, lymphatic vessels don’t branch apart.

Vascular sprouting is a key process-driving development of the vascular system. In this study, we show that neuropilin-2 (Nrp2), a transmembrane receptor for the lymphangiogenic vascular endothelial growth factor C (VEGF-C), plays an important role in lymphatic vessel sprouting. Blocking VEGF-C binding to Nrp2 using antibodies specifically inhibits sprouting of developing lymphatic endothelial tip cells in vivo. In vitro analyses show that Nrp2 modulates lymphatic endothelial tip cell extension and prevents tip cell stalling and retraction during vascular sprout formation. Genetic deletion of Nrp2 reproduces the sprouting defects seen after antibody treatment. To investigate whether this defect depends on Nrp2 interaction with VEGF receptor 2 (VEGFR2) and/or 3, we intercrossed heterozygous mice lacking one allele of these receptors. Double-heterozygous nrp2vegfr2 mice develop normally without detectable lymphatic sprouting defects. In contrast, double-heterozygote nrp2vegfr3 mice show a reduction of lymphatic vessel sprouting and decreased lymph vessel branching in adult organs. Thus, interaction between Nrp2 and VEGFR3 mediates proper lymphatic vessel sprouting in response to VEGF-C.

Novel aspects of corneal angiogenic and lymphangiogenic privilege

In this article, we provide the results of experimental studies demonstrating that corneal avascularity is an active process involving the production of anti-angiogenic factors, which counterbalance the pro-angiogenic/lymphangiogenic factors that are upregulated during wound healing. We also summarize pertinent published reports regarding corneal neovascularization (NV), corneal lymphangiogenesis and corneal angiogenic/lymphangiogenic privilege. We outline the clinical causes of corneal NV, and discuss the angiogenic proteins (VEGF and bFGF) and angiogenesis regulatory proteins. We also describe the role of matrix metalloproteinases MMP-2, -7, and MT1-MMP, anti-angiogenic factors, and lymphangiogenic regulatory proteins during corneal wound healing. Established and potential new therapies for the treatment of corneal neovascularization are also discussed.

Filariasis and lymphoedema

Among the causes of lymphoedema (LE), secondary LE due to filariasis is the most prevalent. It affects only a minority of the 120 million people infected with the causative organisms of lymphatic filariasis (LF), Wuchereria bancrofti and Brugia malayi/timori, but is clustered in families, indicating a genetic basis for development of this pathology. The majority of infected individuals develop filarial-specific immunosuppression that starts even before birth in cases where mothers are infected and is characterized by regulatory T-cell responses and high levels of IgG4, thus tolerating high parasite loads and microfilaraemia. In contrast, individuals with this pathology show stronger immune reactions biased towards Th1, Th2 and probably also Th17. Importantly, as for the aberrant lymph vessel development, innate immune responses that are triggered by the filarial antigen ultimately result in the activation of vascular endothelial growth factors (VEGF), thus promoting lymph vessel hyperplasia as a first step to lymphoedema development. Wolbachia endosymbionts are major inducers of these responses in vitro, and their depletion by doxycycline in LF patients reduces plasma VEGF and soluble VEGF-receptor-3 levels to those seen in endemic normals preceding pathology improvement. The search for the immunogenetic basis for LE could lead to the identification of risk factors and thus, to prevention; and has so far led to the identification of single-nucleotide polymorphisms (SNP) with potential functional relevance to VEGF, cytokine and toll-like receptor (TLR) genes. Hydrocele, a pathology with some similarity to LE in which both lymph vessel dilation and lymph extravasation are shared sequelae, has been found to be strongly associated with a VEGF-A SNP known for upregulation of this (lymph-)angiogenesis factor.

Mutations in CCBE1 cause generalized lymph vessel dysplasia in humans

Lymphedema, lymphangiectasias, mental retardation and unusual facial characteristics define the autosomal recessive Hennekam syndrome. Homozygosity mapping identified a critical chromosomal region containing CCBE1, the human ortholog of a gene essential for lymphangiogenesis in zebrafish. Homozygous and compound heterozygous mutations in seven subjects paired with functional analysis in a zebrafish model identify CCBE1 as one of few genes causing primary generalized lymph-vessel dysplasia in humans.

Lymphatic endothelial cell sphingosine kinase activity is required for lymphocyte egress and lymphatic patterning

Lymphocyte egress from lymph nodes (LNs) is dependent on sphingosine-1-phosphate (S1P), but the cellular source of this S1P is not defined. We generated mice that expressed Cre from the lymphatic vessel endothelial hyaluronan receptor 1 (Lyve-1) locus and that showed efficient recombination of loxP-flanked genes in lymphatic endothelium. We report that mice with Lyve-1 CRE-mediated ablation of sphingosine kinase (Sphk) 1 and lacking Sphk2 have a loss of S1P in lymph while maintaining normal plasma S1P. In Lyve-1 Cre⁺ Sphk-deficient mice, lymphocyte egress from LNs and Peyer's patches is blocked. Treatment with pertussis toxin to overcome Gαi-mediated retention signals restores lymphocyte egress. Furthermore, in the absence of lymphatic Sphks, the initial lymphatic vessels in nonlymphoid tissues show an irregular morphology and a less organized vascular endothelial cadherin distribution at cell–cell junctions. Our data provide evidence that lymphatic endothelial cells are an in vivo source of S1P required for lymphocyte egress from LNs and Peyer's patches, and suggest a role for S1P in lymphatic vessel maturation.

VEGF-A expression by HSV-1-infected cells drives corneal lymphangiogenesis

Inflammatory lymphangiogenesis plays a crucial role in the development of inflammation and transplant rejection. The mechanisms of inflammatory lymphangiogenesis during bacterial infection, toll-like receptor ligand administration, and wound healing are well characterized and depend on ligands for the vascular endothelial grow factor receptor (VEGFR) 3 that are produced by infiltrating macrophages. But inflammatory lymphangiogenesis in nonlymphoid tissues during chronic viral infection is unstudied. Herpes simplex virus 1 (HSV-1) infection of the cornea is a leading cause of blindness and depends on aberrant host immune responses to antigen within the normally immunologically privileged cornea. We report that corneal HSV-1 infection drives lymphangiogenesis and that corneal lymphatics persist past the resolution of infection. The mechanism of HSV-1–induced lymphangiogenesis was distinct from the described mechanisms of inflammatory lymphangiogenesis. HSV-1–elicited lymphangiogenesis was strictly dependent on VEGF-A/VEGFR-2 signaling but not on VEGFR-3 ligands. Macrophages played no role in the induction of lymphangiogenesis and were not a detectable source of VEGF-A. Rather, using VEGF-A reporter transgenic mice, we have identified infected epithelial cells as the primary source of VEGF-A during HSV-1 infection. Our results indicate that HSV-1 directly induces vascularization of the cornea through up-regulation of VEGF-A expression.

Regulation of endometrial vascular remodelling: role of the vascular endothelial growth factor family and the angiopoietin–TIE signalling system

Angiogenesis, lymphangiogenesis and vascular maturation occur on a regular, physiological basis in human endometrium. These processes form part of a continuum of vascular remodelling involving numerous regulatory factors. Key factors include vascular endothelial growth factor (VEGF)A, VEGFC and VEGFD, and their associated receptors VEGFR1, VEGFR2 and VEGFR3. A second group of vascular regulatory proteins belongs to the angiopoietin (ANG)–TIE system. Although members of the VEGF family and the ANG–TIE system are represented in the endometrium, our understanding of how these different molecules interact to regulate remodelling of the blood and lymphatic vasculature present in the endometrium is still limited. A review of the current information is provided.

Stromal collagenase in melanoma: a vascular connection

In this issue, Zigrino et al. report on the role of host-derived mouse collagenase-3 (matrix metalloproteinase (MMP)-13) in melanoma growth and metastasis using a mouse model that lacks MMP-13. The authors demonstrate that vascularization of cutaneous melanomas in these mice is impaired compared with that of controls. This study emphasizes the importance of stromal murine MMP-13, a functional homologue of human MMP-1, in tumor progression.

Lymphangiogenesis and angiogenesis: concurrence and/or dependence? Studies in inbred mouse strains

Genetic background significantly affects angiogenesis in mice. However, lymphangiogenic response to growth factors (GFs) in different strains has not been studied. We report constitutive expression of corneal lymphatics that extends beyond the limits of normal limbal vessels. In untreated corneas, the total number (P=0.006), the number above blood vessels (P=10(-8)), and the area of preexisting lymphatics (P=0.007) were significantly higher in C57BL/6 than in BALB/c mice. Normal corneas of three other strains, the nu/nu, 129E, and Black Swiss mice, showed in most parameters intermediate phenotypes. FGF-2(-/-) mice showed significantly less preexisting lymphatics than control (P=0.009), which suggests a role for this GF in lymphatic development. VEGF-A-induced corneal lymphangiogenic response was significantly higher in BALB/c mice (P=0.03), but it did not differ significantly in C57BL/6 mice, when compared to PBS-implanted control. FGFR-3 expression was higher in C57BL/6 than BALB/c mice, which suggests GF-receptor heterogeneity as a possible explanation for strain-dependent differences. The heterogeneity of preexisting lymphatic vessels in the limbal area significantly correlated with the extent of corneal lymphangiogenesis (VEGF-A: r=0.7, P=0.01; FGF-2: r=0.96, P=10(-5)) in BALB/c but not in C57BL/6 mice. Removal of conjunctival lymphatics did not affect GF-induced lymphangiogenesis. This work introduces physiological expression of lymphatics without blood vessels, which indicates that angiogenesis and lymphangiogenesis, even though intricately related, may occur independently. Furthermore, we show strain-dependence of normal and GF-induced lymphangiogenesis. These differences may affect disease development in various strains.

Identification of targets of Prox1 during in vitro vascular differentiation from embryonic stem cells: functional roles of HoxD8 in lymphangiogenesis

During lymphatic development, Prox1 plays central roles in the differentiation of blood vascular endothelial cells (BECs) into lymphatic endothelial cells (LECs), and subsequently in the maturation and maintenance of lymphatic vessels. However, the molecular mechanisms by which Prox1 elicits these functions remain to be elucidated. Here, we identified FoxC2 and angiopoietin-2 (Ang2), which play important roles in the maturation of lymphatic vessels, as novel targets of Prox1 in mouse embryonic-stem-cell-derived endothelial cells (MESECs). Furthermore, we found that expression of HoxD8 was significantly induced by Prox1 in MESECs, a finding confirmed in human umbilical vein endothelial cells (HUVECs) and human dermal LECs (HDLECs). In mouse embryos, HoxD8 expression was significantly higher in LECs than in BECs. In a model of inflammatory lymphangiogenesis, diameters of lymphatic vessels of the diaphragm were increased by adenovirally transduced HoxD8. We also found that HoxD8 induces Ang2 expression in HDLECs and HUVECs. Moreover, we found that HoxD8 induces Prox1 expression in HUVECs and that knockdown of HoxD8 reduces this expression in HDLECs, suggesting that Prox1 expression in LECs is maintained by HoxD8. These findings indicate that transcriptional networks of Prox1 and HoxD8 play important roles in the maturation and maintenance of lymphatic vessels.

Golden carbon nanotubes as multimodal photoacoustic and photothermal high-contrast molecular agents

Carbon nanotubes have shown promise as contrast agents for photoacoustic and photothermal imaging of tumours and infections because they offer high resolution and allow deep tissue imaging. However, in vivo applications have been limited by the relatively low absorption displayed by nanotubes at near-infrared wavelengths and concerns over toxicity. Here, we show that gold-plated carbon nanotubes-termed golden carbon nanotubes-can be used as photoacoustic and photothermal contrast agents with enhanced near-infrared contrast ( approximately 10(2)-fold) for targeting lymphatic vessels in mice using extremely low laser fluence levels of a few mJ cm(-2). Antibody-conjugated golden carbon nanotubes were used to map the lymphatic endothelial receptor, and preliminary in vitro viability tests show golden carbon nanotubes have minimal toxicity. This new nanomaterial could be an effective alternative to existing nanoparticles and fluorescent labels for non-invasive targeted imaging of molecular structures in vivo.

Angiogenesis and lymphangiogenesis are downregulated in primary breast cancer

BACKGROUND

Angiogenesis and lymphangiogenesis are considered to play key roles in tumour growth, progression and metastasis. However, targeting tumour angiogenesis in clinical trials showed only modest efficacy. We therefore scrutinised the concept of tumour angiogenesis and lymphangiogenesis by analysing the expression of crucial markers involved in these processes in primary breast cancer.

METHODS

We analysed the expression of angiogenic, lymphangiogenic or antiangiogenic factors, their respective receptors and specific markers for endothelial and lymphendothelial cells by quantitative real-time RT-PCR in primary breast cancer and compared the expression profiles to non-cancerous, tumour-adjacent tissues and breast tissues from healthy women.

RESULTS

We found decreased mRNA amounts of major angiogenic and lymphangiogenic factors in tumour compared to healthy tissues, whereas antiangiogenic factors were upregulated. Concomitantly, angiogenic and lymphangiogenic receptors were downregulated in breast tumours. This antiangiogenic, antilymphangiogenic microenvironment was even more pronounced in aggressive tumours and accompanied by reduced amounts of endothelial and lymphatic endothelial cell markers.

CONCLUSION

Primary breast tumours are not a site of highly active angiogenesis and lymphangiogenesis. Selection for tumour cells that survive with minimal vascular supply may account for this observation in clinical apparent tumours.

Inducible capillary formation in lymphatic endothelial cells by blocking lipid phosphate phosphatase-3 activity

Lymphangiogenesis plays critical roles under normal and/or pathological conditions; however, the molecular contributors to this event were unknown until recently. In the present study, we first employed gene chip analysis and confirmed that lipid phosphate phosphatase-3 (LPP3) expression was increased until capillary formation in the conditionally immortalized rat lymphatic endothelial cell line. Signaling responses occur when several lipids induce acute biological functions; further, lipid phosphate phosphatases (LPPs) control their functions via dephosphorylation; however, there is no report on the association between LPP3 and lymphangiogenesis. siRNA-targeted LPP3 significantly increased capillary formation of human lymphatic endothelial cells; in contrast, it decreased cell adhesion to the basement membrane matrix. Furthermore, the inducible effect of the LPP inhibitor on capillary formation was observed. For the first time, we report that LPP3 abolishes accelerated abnormal lymphangiogenesis. Blocking LPP3 activities may aid in the development of novel therapy for lymph vessel defects.

Regulation of angiogenesis in malignancies associated with Epstein–Barr virus and Kaposi’s sarcoma-associated herpes virus

Tumor angiogenesis is the process by which new blood vessels are formed within emerging or progressing malignancies. The human Epstein–Barr virus and Kaposi’s sarcoma-associated herpesvirus critically contribute to the pathogenesis of selected tumor types, including nasopharyngeal carcinoma and Kaposi’s sarcoma, respectively, where angiogenesis is robust and often disrupted. Lymphangiogenesis, the process by which new lymphatic vessels are formed, is also induced in Epstein–Barr virus and Kaposi’s sarcoma-associated herpesvirus-associated malignancies and in some cases may contribute to metastasis. Recent studies have identified a number of molecules and signaling pathways that underlie angiogenesis and lymphangiogenesis, and clarified the pivotal role of the VEGF family of proteins and their receptors. New treatment modalities that target members of this family have gained approval for clinical use in cancer. Pathogenetic steps are often difficult to dissect in many cancer types, but virus-induced malignancies provide a unique opportunity for understanding the molecular regulation of cancer progression, including angiogenesis. Dissection of viral gene contribution to tumor angiogenesis could result in a better understanding of the angiogenic process, its contribution to cancer and help in the design of rational therapies that target tumor growth and vascularization.

Activated forms of VEGF-C and VEGF-D provide improved vascular function in skeletal muscle

The therapeutic potential of vascular endothelial growth factor (VEGF)-C and VEGF-D in skeletal muscle has been of considerable interest as these factors have both angiogenic and lymphangiogenic activities. Previous studies have mainly used adenoviral gene delivery for short-term expression of VEGF-C and VEGF-D in pig, rabbit, and mouse skeletal muscles. Here we have used the activated mature forms of VEGF-C and VEGF-D expressed via recombinant adeno-associated virus (rAAV), which provides stable, long-lasting transgene expression in various tissues including skeletal muscle. Mouse tibialis anterior muscle was transduced with rAAV encoding human or mouse VEGF-C or VEGF-D. Two weeks later, immunohistochemical analysis showed increased numbers of both blood and lymph vessels, and Doppler ultrasound analysis indicated increased blood vessel perfusion. The lymphatic vessels further increased at the 4-week time point were functional, as shown by FITC-lectin uptake and transport. Furthermore, receptor activation and arteriogenic activity were increased by an alanine substitution mutant of human VEGF-C (C137A) having an increased dimer stability and by a chimeric CAC growth factor that contained the VEGF receptor-binding domain flanked by VEGF-C propeptides, but only the latter promoted significantly more blood vessel perfusion when compared to the other growth factors studied. We conclude that long-term expression of VEGF-C and VEGF-D in skeletal muscle results in the generation of new functional blood and lymphatic vessels. The therapeutic value of intramuscular lymph vessels in draining tissue edema and lymphedema can now be evaluated using this model system.