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

Effect of hepatoma H22 on lymphatic endothelium in vitro

AIM: To determine the effect of metastatic hepatoma cells on lymphangioma-derived endothelium, and to establish in vitro model systems for assessing metastasis-related response of lymphatic endothelium.

METHODS: Benign lymphangioma, induced by intraperitoneal injection of the incomplete Freund’s adjuvant in BALB/c mice, was embedded in fibrin gel or digested and then cultured in the conditioned medium derived from hepatoma H22. Light and electron microscopy, and the transwell migration assay were used to determine the effect of H22 on tissue or cell culture. Expressions of Flt-4, c-Fos, proliferating cell nuclear antigen (PCNA), and inducible nitric oxide synthase (iNOS) in cultured cells, and content of nitric oxide in culture medium were also examined.

RESULTS: The embedded lymphangioma pieces gave rise to array of capillaries, while separated cells from lymphangioma grew to a cobblestone-like monolayer. H22 activated growth and migration of the capillaries and cells, induced expressions of Flt-4, c-Fos, PCNA and iNOS in cultured cells, and significantly increased the content of NO in the culture medium.

CONCLUSION: Lymphangioma-derived cells keep the differentiated phenotypes of lymphatic endothelium, and the models established in this study are feasible for in vitro study of metastasis-related response of lymphatic endothelium.

Differential recognition and scavenging of native and truncated macrophage-derived chemokine (macrophage-derived chemokine/CC chemokine ligand 22) by the D6 decoy receptor

The promiscuous D6 receptor binds several inflammatory CC chemokines and has been recently proposed to act as a chemokine-scavenging decoy receptor. The present study was designed to better characterize the spectrum of CC chemokines scavenged by D6, focusing in particular on CCR4 ligands and analyzing the influence of NH(2)-terminal processing on recognition by this promiscuous receptor. Using D6 transfectants, it was found that D6 efficiently bound and scavenged most inflammatory CC chemokines (CCR1 through CCR5 agonists). Homeostatic CC chemokines (CCR6 and CCR7 agonists) were not recognized by D6. The CCR4 agonists CC chemokine ligand 17 (CCL17) and CCL22 bound to D6 with high affinity. CCL17 and CCL22 have no agonistic activity for D6 (chemotaxis and calcium fluxes), but were rapidly scavenged, resulting in reduced chemotactic activity on CCR4 transfectants. CD26 mediates NH(2) terminus processing of CCL22, leading to the production of CCL22 (3-69) and CCL22 (5-69) that do not interact with CCR4. These NH(2)-terminal truncated forms of CCL22 were not recognized by D6. The results presented in this study show that D6 recognizes and scavenges a wide spectrum of inflammatory CC chemokines, including the CCR4 agonists CCL22 and CCL17. However, this promiscuous receptor is not engaged by CD26-processed, inactive, CCL22 variants. By recognizing intact CCL22, but not its truncated variants, D6 expressed on lymphatic endothelial cells may regulate the traffic of CCR4-expressing cells, such as dendritic cells.

Inhibition of lymphangiogenesis-related properties of murine lymphatic endothelial cells and lymph node metastasis of lung cancer by the matrix metalloproteinase inhibitor MMI270

Based on a previous report on the effect of a matrix metalloproteinase (MMP) inhibitory compound, MMI270, in regulating tumor-induced angiogenesis, as well as recent findings concerning functional correlations among tumor metastasis, angiogenesis and lymphangiogenesis, we investigated the anti-metastatic efficacy of MMI270 in a murine model of lymph node metastasis of lung cancer, and analyzed whether this inhibitor could also regulate lymphangiogenesis-related properties of murine lymphatic endothelial cells (LECs) and invasive properties of Lewis lung cancer (LLC) cells. The observation that MMI270 led to a significant decrease in the weight of tumor-metastasized lymph nodes of mice led us to test its anti-lymphangiogenic and anti-invasive effects in vitro. Murine LECs were characterized by an in vitro tube formation assay, by semi-quantitative RT-PCR assay to examine the expression of mRNAs for flt-4, Flk-1, Tie-1, Tie-2, CD54/ICAM1, vWF, MMPs and uPA, and by western blotting to confirm the protein expression of flt-4 and CD31/PECAM. This is the first report on the expression of MMP-2, MMP-9 and MT1-MMP in murine LECs, as well as on the inhibition of their enzymatic activity, and of the invasive ability and tube-forming property of LECs by an MMP inhibitor. Furthermore, MMI270 was shown to strongly inhibit the activity of MMP-2 and -9 produced by LLC cells and the invasion of these cells through Matrigel. In summary, the present results indicate that MMI270, apart from its anti-tumor angiogenic application, might be useful as an anti-metastatic drug, on the basis of its downregulatation of both the lymphangiogenesis-related properties of LECs and the invasive properties of LLC cells in vitro.

Evidence of CXC, CC and C chemokine production by lymphatic endothelial cells

Although production of chemokines by vascular endothelial cells has been documented, there is only limited information regarding the expression of chemokines by the lymphatic endothelium. Here we used lymphatic endothelial cells (LEC) derived from experimentally induced murine lymphangiomas to investigate the pattern of chemokine expression by these cells. Histological analysis of the lymphatic hyperplasia revealed the presence of leucocytes in the tissues surrounding the lesions, suggesting the presence of chemoattractant activity. A functional chemotactic assay on human polymorphonuclear cells and on purified subpopulations of murine leucocytes using culture supernatants from LEC primary cultures confirmed the presence of chemoattractant activity. The identity of different cytokines of the CXC, CC and C subfamilies was investigated by reverse trancriptase-polymerase chain reaction on total endothelial cell RNA. Amplified fragments corresponding to KC, IP10, Mig-1, BCL, MIP-2, SLC, RANTES, MCP-1, C10, and Lptn were obtained, and confirmed by Southern blot and sequencing. In contrast, MIP-1alpha, MIP-1beta, and MIP-1gamma were not detected. Higher levels of expression were revealed by Northern blot analysis for Mig-1, MCP-1 and C10. The lymphatic endothelium-restricted production of these chemokines was also confirmed by in situ hybridization. The presence of high C10 mRNA expression levels in LEC was particularly unexpected, because the production of this molecule has been previously identified only in cells of the haematopoietic lineage. These observations represent the first detailed analysis of chemokine production by lymphatic endothelial cells and may account, in part, for the mechanism of leucocyte recruitment into the lymphatics, and of lymphocyte recirculation within the lymphatic system.

Cutting edge: scavenging of inflammatory CC chemokines by the promiscuous putatively silent chemokine receptor D6

In an effort to define the actual function of the promiscuous putatively silent chemokine receptor D6, transfectants were generated in different cell types. Engagement of D6 by inflammatory CC chemokines elicited no calcium response nor chemotaxis, but resulted in efficient agonist internalization and degradation. Also in lymphatic endothelium, where this receptor is expressed in vivo, D6 did not elicit cellular responses other than ligand internalization and degradation. In particular, no evidence was obtained for D6-mediated transcytosis of chemokines in the apical-to-basal or basal-to-apical directions. These results indicate that D6 acts as an inflammatory chemokine scavenging nonactivatory decoy receptors and suggest that in lymphatic vessels D6 may function as a gatekeeper for inflammatory CC chemokines, by clearing them and preventing excessive diffusion via afferent lymphatics to lymph nodes.

The lectin-like domain of thrombomodulin confers protection from neutrophil-mediated tissue damage by suppressing adhesion molecule expression via nuclear factor kappaB and mitogen-activated protein kinase pathways

Thrombomodulin (TM) is a vascular endothelial cell (EC) receptor that is a cofactor for thrombin-mediated activation of the anticoagulant protein C. The extracellular NH(2)-terminal domain of TM has homology to C-type lectins that are involved in immune regulation. Using transgenic mice that lack this structure (TM(LeD/LeD)), we show that the lectin-like domain of TM interferes with polymorphonuclear leukocyte (PMN) adhesion to ECs by intercellular adhesion molecule 1-dependent and -independent pathways through the suppression of extracellular signal-regulated kinase (ERK)(1/2) activation. TM(LeD/LeD) mice have reduced survival after endotoxin exposure, accumulate more PMNs in their lungs, and develop larger infarcts after myocardial ischemia/reperfusion. The recombinant lectin-like domain of TM suppresses PMN adhesion to ECs, diminishes cytokine-induced increase in nuclear factor kappaB and activation of ERK(1/2), and rescues ECs from serum starvation, findings that may explain why plasma levels of soluble TM are inversely correlated with cardiovascular disease. These data suggest that TM has antiinflammatory properties in addition to its role in coagulation and fibrinolysis.

Lymphatic versus blood vascular endothelial growth factors and receptors in humans

Three different growth factor systems have been described acting via endothelial cell-specific receptor tyrosine kinases (RTKs). These are vascular endothelial growth factors (VEGFs), angiopoietins, and ephrins. Recent studies on gene targeting suggest that they play critical roles in embryonic development and contribute to the integrity and responses to environmental factors in the adult vasculature. Coagulation, inflammation, immune response regulation, vascular tone, stromal component synthesis, and angiogenesis are all dependent on the physiological and pathological events that affect endothelial cells in the heart, arteries, veins, and lymphatic vessels. Angiogenesis, the formation of new blood vessels from preexisting ones, takes place in adults only during hormonal control of female reproduction. All other activation of angiogenesis in adulthood occurs in response to injury or pathological processes such as tumorigenesis, diabetes, or inflammatory conditions. Insufficient growth of collateral vessels is a major problem in atherosclerotic cardiovascular disease. Controlled stimulation of angiogenesis would be of therapeutic value. Lymphangiogenesis, the mechanisms involved in the development of lymphatic vessels, was studied intensively nearly a century ago, although since then it has been neglected, perhaps because, unlike the disorders of blood vessels, those of the lymphatic vessels are seldom life-threatening. Interrupting this one-way system can cause severe disorders, including liver dysfunction, genetic disease (e.g., Milroys disease), and degenerative disease (e.g., primary lymphangiosclerosis). Recently, novel growth factors, receptors, cell surface proteins, and transcription factors have been found which play a role in the lymphatic endothelium. These are VEGF-C, VEGF-D, VEGFR-3, LYVE-1, podoplanin, and Prox-1. Until recently lymphatic vessels have been difficult to study due to a lack of appropriate tools. Monoclonal antibodies raised against VEGFR-3 and against its ligands, VEGF-C and VEGF-D, have offered an insight into expression studies in tissues. In this review, we summarize the recent data on VEGFs in the human vasculature.

Isolation and characterization of dermal lymphatic and blood endothelial cells reveal stable and functionally specialized cell lineages

A plexus of lymphatic vessels guides interstitial fluid, passenger leukocytes, and tumor cells toward regional lymph nodes. Microvascular endothelial cells (ECs) of lymph channels (LECs) are difficult to distinguish from those of blood vessels (BECs) because both express a similar set of markers, such as CD31, CD34, podocalyxin, von Willebrand factor (vWF), etc. Analysis of the specific properties of LECs was hampered so far by lack of tools to isolate LECs. Recently, the 38-kD mucoprotein podoplanin was found to be expressed by microvascular LECs but not BECs in vivo. Here we isolated for the first time podoplanin(+) LECs and podoplanin(-) BECs from dermal cell suspensions by multicolor flow cytometry. Both EC types were propagated and stably expressed VE-cadherin, CD31, and vWF. Molecules selectively displayed by LECs in vivo, i.e., podoplanin, the hyaluronate receptor LYVE-1, and the vascular endothelial cell growth factor (VEGF)-C receptor, fms-like tyrosine kinase 4 (Flt-4)/VEGFR-3, were strongly expressed by expanded LECs, but not BECs. Conversely, BECs but not LECs expressed VEGF-C. LECs as well as BECs formed junctional contacts with similar molecular composition and ultrastructural features. Nevertheless, the two EC types assembled in vitro in vascular tubes in a strictly homotypic fashion. This EC specialization extends to the secretion of biologically relevant chemotactic factors: LECs, but not BECs, constitutively secrete the CC chemokine receptor (CCR)7 ligand secondary lymphoid tissue chemokine (SLC)/CCL21 at their basal side, while both subsets, upon activation, release macrophage inflammatory protein (MIP)-3alpha/CCL20 apically. These results demonstrate that LECs and BECs constitute stable and specialized EC lineages equipped with the potential to navigate leukocytes and, perhaps also, tumor cells into and out of the tissues.

Mouse LYVE-1 is an endocytic receptor for hyaluronan in lymphatic endothelium

The glycosaminoglycan hyaluronan is a key substrate for cell migration in tissues during inflammation, wound healing, and neoplasia. Unlike other matrix components, hyaluronan (HA) is turned over rapidly, yet most degradation occurs not locally but within distant lymph nodes, through mechanisms that are not yet understood. While it is not clear which receptors are involved in binding and uptake of hyaluronan within the lymphatics, one likely candidate is the lymphatic endothelial hyaluronan receptor LYVE-1 recently described in our laboratory (Banerji, S., Ni, J., Wang, S., Clasper, S., Su, J., Tammi, R., Jones, M., and Jackson, D.G. (1999) J. Cell Biol. 144, 789-801). Here we present evidence that LYVE-1 is involved in the uptake of hyaluronan by lymphatic endothelial cells using a new murine LYVE-1 orthologue identified from the EST data base. We show that mouse LYVE-1 both binds and internalizes hyaluronan in transfected 293T fibroblasts in vitro and demonstrate using immunoelectron microscopy that it is distributed equally among the luminal and abluminal surfaces of lymphatic vessels in vivo. In addition, we show by means of specific antisera that expression of mouse LYVE-1 remains restricted to the lymphatics in homozygous knockout mice lacking a functional gene for CD44, the closest homologue of LYVE-1 and the only other Link superfamily HA receptor known to date. Together these results suggest a role for LYVE-1 in the transport of HA from tissue to lymph and imply that further novel hyaluronan receptors must exist that can compensate for the loss of CD44 function.