Interactions of tumor cells with lymphatic vessels are of paramount importance for tumor progression, however, the underlying molecular mechanisms are poorly understood. Whereas enlarged lymphatic vessels are frequently observed at the periphery of malignant melanomas, it has remained unclear whether intratumoral lymphangiogenesis occurs within these tumors. Here, we demonstrate the presence of intratumoral lymphatics and enlargement of lymphatic vessels at the tumor periphery in vascular endothelial growth factor (VEGF)-C-overexpressing human melanomas transplanted onto nude mice. VEGF-C expression also resulted in enhanced tumor angiogenesis, indicating a coordinated regulation of lymphangiogenesis and angiogenesis in melanoma progression. The specific biological effects of VEGF-C were critically dependent on its proteolytic processing in vivo. Furthermore, VEGF-C induced chemotaxis of macrophages in vitro and in vivo, revealing a potential function of VEGF-C as an immunomodulator. Taken together, our results identify VEGF-C as multifunctional factor involved in regulating tumor lymphangiogenesis, angiogenesis, and immune response. Interactions of tumor cells with lymphatic vessels are of paramount importance for tumor progression, however, the underlying molecular mechanisms are poorly understood. Whereas enlarged lymphatic vessels are frequently observed at the periphery of malignant melanomas, it has remained unclear whether intratumoral lymphangiogenesis occurs within these tumors. Here, we demonstrate the presence of intratumoral lymphatics and enlargement of lymphatic vessels at the tumor periphery in vascular endothelial growth factor (VEGF)-C-overexpressing human melanomas transplanted onto nude mice. VEGF-C expression also resulted in enhanced tumor angiogenesis, indicating a coordinated regulation of lymphangiogenesis and angiogenesis in melanoma progression. The specific biological effects of VEGF-C were critically dependent on its proteolytic processing in vivo. Furthermore, VEGF-C induced chemotaxis of macrophages in vitro and in vivo, revealing a potential function of VEGF-C as an immunomodulator. Taken together, our results identify VEGF-C as multifunctional factor involved in regulating tumor lymphangiogenesis, angiogenesis, and immune response. The spread of tumor cells via lymphatic vessels to the regional lymph nodes is one of the most important indicators of tumor aggressiveness, and the extent of lymph node involvement is a major determinant for the staging and the prognosis of most human malignancies.1Witte MH Way DL Witte CL Bernas M Lymphangiogenesis: mechanisms, significance and clinical implications.Exs. 1997; 79: 65-112PubMed Google Scholar, 2Lauria R Perrone F Carlomagno C De Laurentiis M Morabito A Gallo C Varriale E Pettinato G Panico L Petrella G Bianco AR De Placido S The prognostic value of lymphatic and blood vessel invasion in operable breast cancer.Cancer. 1995; 76: 1772-1778Crossref PubMed Scopus (156) Google Scholar, 3Willis RA The Spread of Tumors in the Human Body. C. V. Mosby Co., St. Louis1952: 18-35Google Scholar Abundant, rearranged, and often dilated lymphatic vessels containing clusters of tumor cells are commonly observed at the periphery of many tumors, including malignant melanomas.1Witte MH Way DL Witte CL Bernas M Lymphangiogenesis: mechanisms, significance and clinical implications.Exs. 1997; 79: 65-112PubMed Google Scholar, 4Hartveit E Attenuated cells in breast stroma: the missing lymphatic system of the breast.Histopathology. 1990; 16: 533-543Crossref PubMed Scopus (65) Google Scholar, 5Cann SA van Netten JP Ashby TL Ashwood-Smith MJ van der Westhuizen NG Role of lymphagenesis in neovascularisation.Lancet. 1995; 346: 903Abstract Full Text PDF PubMed Scopus (10) Google Scholar, 6Deutsch A Lubach D Nissen S Neukam D Ultrastructural studies on the invasion of melanomas in initial lymphatics of human skin.J Invest Dermatol. 1992; 98: 64-67Crossref PubMed Scopus (13) Google Scholar, 7Jussila L Valtola R Partanen TA Salven P Heikkila P Matikainen MT Renkonen R Kaipainen A Detmar M Tschachler E Alitalo R Alitalo K Lymphatic endothelium and Kaposi's sarcoma spindle cells detected by antibodies against the vascular endothelial growth factor receptor-3.Cancer Res. 1998; 58: 1599-1604PubMed Google Scholar However, the presence of lymphatic vessels within tumors and the ability of tumor cells to induce lymphangiogenesis have remained controversial.8Folkman J Angiogenesis and tumor growth.N Engl J Med. 1996; 334: 921Google Scholar, 9Leu AJ Berk DA Lymboussaki A Alitalo K Jain RK Absence of functional lymphatics within a murine sarcoma: a molecular and functional evaluation.Cancer Res. 2000; 60: 4324-4327PubMed Google Scholar Whereas tumor angiogenesis involving the formation of new blood vessels has been studied extensively, the role of lymphatic vessels in tumor pathology and the molecules regulating lymphangiogenesis have remained mostly unknown. Recently, a novel member of the vascular endothelial growth factor (VEGF) family has been identified that is distinguished by its capacity to stimulate lymphangiogenesis.10Joukov V Pajusola K Kaipainen A Chilov D Lahtinen I Kukk E Saksela O Kalkkinen N Alitalo K A novel vascular endothelial growth factor, VEGF-C, is a ligand for the Flt4 (VEGFR-3) and KDR (VEGFR-2) receptor tyrosine kinases.EMBO J. 1996; 15: 290-298Crossref PubMed Scopus (1177) Google Scholar, 11Lee J Gray A Yuan J Luoh SM Avraham H Wood WI Vascular endothelial growth factor-related protein: a ligand and specific activator of the tyrosine kinase receptor Flt4.Proc Natl Acad Sci USA. 1996; 93: 1988-1992Crossref PubMed Scopus (333) Google Scholar Vascular endothelial growth factor-C (VEGF-C) stimulated lymphangiogenesis in the avian chorioallantoic membrane assay,12Oh SJ Jeltsch MM Birkenhager R McCarthy JE Weich HA Christ B Alitalo K Wilting J VEGF and VEGF-C: specific induction of angiogenesis and lymphangiogenesis in the differentiated avian chorioallantoic membrane.Dev Biol. 1997; 188: 96-109Crossref PubMed Scopus (436) Google Scholar and transgenic mice overexpressing VEGF-C in the skin were characterized by specific hyperplasia of the lymphatic network without obvious effects on blood vessels.13Jeltsch M Kaipainen A Joukov V Kukk E Lymbousssaki AXM Lakso M Alitalo K Hyperplasia of lymphatic vessels in VEGF-C transgenic mice.Science. 1997; 276: 1423-1425Crossref PubMed Scopus (1124) Google Scholar In normal adult human tissues, the VEGF-C receptor VEGFR-3 (Flt-4) has been found predominantly expressed by lymphatic endothelium.7Jussila L Valtola R Partanen TA Salven P Heikkila P Matikainen MT Renkonen R Kaipainen A Detmar M Tschachler E Alitalo R Alitalo K Lymphatic endothelium and Kaposi's sarcoma spindle cells detected by antibodies against the vascular endothelial growth factor receptor-3.Cancer Res. 1998; 58: 1599-1604PubMed Google Scholar, 14Kaipainen A Korhonen J Mustonen T van HV Fang GH Dumont D Breitman M Alitalo K Expression of the fms-like tyrosine kinase 4 gene becomes restricted to lymphatic endothelium during development.Proc Natl Acad Sci USA. 1995; 92: 3566-3570Crossref PubMed Scopus (1203) Google Scholar However, VEGF-C also binds VEGFR-2 (KDR), that is mainly expressed by activated endothelium of blood vessels, suggesting a potential function of VEGF-C in the induction of angiogenesis.10Joukov V Pajusola K Kaipainen A Chilov D Lahtinen I Kukk E Saksela O Kalkkinen N Alitalo K A novel vascular endothelial growth factor, VEGF-C, is a ligand for the Flt4 (VEGFR-3) and KDR (VEGFR-2) receptor tyrosine kinases.EMBO J. 1996; 15: 290-298Crossref PubMed Scopus (1177) Google Scholar, 15Neufeld G Cohen T Gengrinovitch S Poltorak Z Vascular endothelial growth factor (VEGF) and its receptors.FASEB J. 1999; 13: 9-22Crossref PubMed Scopus (3184) Google Scholar Indeed, VEGF-C stimulates the proliferation and migration of blood vascular endothelial cells in vitro and has been shown to increase vascular permeability in the Miles assay,10Joukov V Pajusola K Kaipainen A Chilov D Lahtinen I Kukk E Saksela O Kalkkinen N Alitalo K A novel vascular endothelial growth factor, VEGF-C, is a ligand for the Flt4 (VEGFR-3) and KDR (VEGFR-2) receptor tyrosine kinases.EMBO J. 1996; 15: 290-298Crossref PubMed Scopus (1177) Google Scholar, 11Lee J Gray A Yuan J Luoh SM Avraham H Wood WI Vascular endothelial growth factor-related protein: a ligand and specific activator of the tyrosine kinase receptor Flt4.Proc Natl Acad Sci USA. 1996; 93: 1988-1992Crossref PubMed Scopus (333) Google Scholar, 16Joukov V Sorsa T Kumar V Jeltsch M Claesson WL Cao Y Saksela O Kalkkinen N Alitalo K Proteolytic processing regulates receptor specificity and activity of VEGF-C.EMBO J. 1997; 16: 3898-3911Crossref PubMed Scopus (655) Google Scholar, 17Skobe M Brown LF Tognazzi K Ganju RK Dezube BJ Alitalo K Detmar M Vascular endothelial growth factor-C (VEGF-C) and its receptors KDR and flt-4 are expressed in AIDS-associated Kaposi's sarcoma.J Invest Dermatol. 1999; 113: 1047-1053Crossref PubMed Scopus (109) Google Scholar and to induce angiogenesis in the mouse corneal micropocket assay and in the rabbit ischemic hind limb model.18Cao Y Linden P Farnebo J Cao R Eriksson A Kumar V Qi JH Claesson-Welsh L Alitalo K Vascular endothelial growth factor C induces angiogenesis in vivo.Proc Natl Acad Sci USA. 1998; 95: 14389-14394Crossref PubMed Scopus (503) Google Scholar, 19Witzenbichler B Asahara T Murohara T Silver M Spyridopoulos I Magner M Principe N Kearney M Hu JS Isner JM Vascular endothelial growth factor-C (VEGF-C/VEGF-2) promotes angiogenesis in the setting of tissue ischemia.Am J Pathol. 1998; 153: 381-394Abstract Full Text Full Text PDF PubMed Scopus (312) Google Scholar VEGF-C mRNA expression has been demonstrated in a large number of human tumors, including malignant melanomas.20Salven P Lymboussaki A Heikkila P Jaaskela-Saari H Enholm B Aase K von Euler G Eriksson U Alitalo K Joensuu H Vascular endothelial growth factors VEGF-B and VEGF-C are expressed in human tumors.Am J Pathol. 1998; 153: 103-108Abstract Full Text Full Text PDF PubMed Scopus (295) Google Scholar, 21Valtola R Salven P Heikkila P Taipale J Joensuu H Rehn M Pihlajaniemi T Weich H deWaal R Alitalo K VEGFR-3 and its ligand VEGF-C are associated with angiogenesis in breast cancer.Am J Pathol. 1999; 154: 1381-1390Abstract Full Text Full Text PDF PubMed Scopus (491) Google Scholar, 22Kurebayashi J Otsuki T Kunisue H Mikami Y Tanaka K Yamamoto S Sonoo H Expression of vascular endothelial growth factor (VEGF) family members in breast cancer.Jpn J Cancer Res. 1999; 90: 977-981Crossref PubMed Scopus (158) Google Scholar, 23Andre T Kotelevets L Vaillant JC Coudray AM Weber L Prevot S Parc R Gespach C Chastre E Vegf, Vegf-B, Vegf-C and their receptors KDR, FLT-1 and FLT-4 during the neoplastic progression of human colonic mucosa.Int J Cancer. 2000; 86: 174-181Crossref PubMed Scopus (144) Google Scholar, 24Akagi K Ikeda Y Miyazaki M Abe T Kinoshita J Maehara Y Sugimachi K Vascular endothelial growth factor-C (VEGF-C) expression in human colorectal cancer tissues.Br J Cancer. 2000; 83: 887-891Crossref PubMed Scopus (222) Google Scholar, 25Niki T Iba S Tokunou M Yamada T Matsuno Y Hirohashi S Expression of vascular endothelial growth factors A, B, C, and D and their relationships to lymph node status in lung adenocarcinoma.Clin Cancer Res. 2000; 6: 2431-2439PubMed Google Scholar, 26Ohta Y Nozawa H Tanaka Y Oda M Watanabe Y Increased vascular endothelial growth factor and vascular endothelial growth factor-c and decreased nm23 expression associated with microdissemination in the lymph nodes in stage I non-small cell lung cancer.J Thorac Cardiovasc Surg. 2000; 119: 804-813Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar, 27Shushanov S Bronstein M Adelaide J Jussila L Tchipysheva T Jacquemier J Stavrovskaya A Birnbaum D Karamysheva A VEGF-C and VEGFR3 expression in human thyroid pathologies.Int J Cancer. 2000; 86: 47-52Crossref PubMed Scopus (43) Google Scholar, 28Bunone G Vigneri P Mariani L Buto S Collini P Pilotti S Pierotti MA Bongarzone I Expression of angiogenesis stimulators and inhibitors in human thyroid tumors and correlation with clinical pathological features.Am J Pathol. 1999; 155: l967-1976Abstract Full Text Full Text PDF PubMed Scopus (353) Google Scholar, 29Fellmer PT Sato K Tanaka R Okamoto T Kato Y Kobayashi M Shibuya M Obara T Vascular endothelial growth factor-C gene expression in papillary and follicular thyroid carcinomas.Surgery. 1999; 126: 1056-1061Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar, 30Yonemura Y Endo Y Fujita H Fushida S Ninomiya I Bandou E Taniguchi K Miwa K Ohoyama S Sugiyama K Sasaki T Role of vascular endothelial growth factor C expression in the development of lymph node metastasis in gastric cancer.Clin Cancer Res. 1999; 5: 1823-1829PubMed Google Scholar, 31Ohta Y Shridhar V Bright RK Kalemkerian GP Du W Carbone M Watanabe Y Pass HI VEGF and VEGF type C play an important role in angiogenesis and lymphangiogenesis in human malignant mesothelioma tumours.Br J Cancer. 1999; 81: 54-61Crossref PubMed Scopus (314) Google Scholar, 32Eggert A Ikegaki N Kwiatkowski J Zhao H Brodeur GM Himelstein BP High-level expression of angiogenic factors is associated with advanced tumor stage in human neuroblastomas.Clin Cancer Res. 2000; 6: 1900-1908PubMed Google Scholar Expression of its receptor VEGFR-3 has been detected in lymphatic vessels and occasionally also in blood vessels adjacent to cancer cells.7Jussila L Valtola R Partanen TA Salven P Heikkila P Matikainen MT Renkonen R Kaipainen A Detmar M Tschachler E Alitalo R Alitalo K Lymphatic endothelium and Kaposi's sarcoma spindle cells detected by antibodies against the vascular endothelial growth factor receptor-3.Cancer Res. 1998; 58: 1599-1604PubMed Google Scholar, 14Kaipainen A Korhonen J Mustonen T van HV Fang GH Dumont D Breitman M Alitalo K Expression of the fms-like tyrosine kinase 4 gene becomes restricted to lymphatic endothelium during development.Proc Natl Acad Sci USA. 1995; 92: 3566-3570Crossref PubMed Scopus (1203) Google Scholar, 21Valtola R Salven P Heikkila P Taipale J Joensuu H Rehn M Pihlajaniemi T Weich H deWaal R Alitalo K VEGFR-3 and its ligand VEGF-C are associated with angiogenesis in breast cancer.Am J Pathol. 1999; 154: 1381-1390Abstract Full Text Full Text PDF PubMed Scopus (491) Google Scholar Most recently, we have shown that VEGF-C overexpression in experimental breast cancer selectively induced intratumoral lymphangiogenesis, leading to increased tumor metastasis, without obvious effects on angiogenesis.33Skobe M Hawighorst T Jackson DG Prevo R Janes L Velasco P Riccardi L Alitalo K Claffey K Detmar M Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis.Nat Med. 2001; 7: 192-198Crossref PubMed Scopus (1522) Google Scholar In the present study we demonstrate the occurrence of intratumoral lymphangiogenesis in VEGF-C-overexpressing human melanomas transplanted onto nude mice. Moreover, VEGF-C also induced tumor angiogenesis and recruitment of macrophages. Our results indicate that the distinct biological effects of VEGF-C are critically dependent on its proteolytic processing in vivo. The human malignant melanoma cell lines MeWo,34Sordat BCM Ueyama Y Fogh J The Nude Mouse in Experimental and Clinical Research.in: Fogh J Giovanella BC Academic Press, New York1982: 95-147Google Scholar, 35Kerbel RS Man MS Dexter D A model of human cancer metastasis: extensive spontaneous and artificial metastasis of a human pigmented melanoma and derived variant sublines in nude mice.J Natl Cancer Inst. 1984; 72: 93-108Crossref PubMed Scopus (92) Google Scholar WM9, and WM239 were provided by Dr. Robert S. Kerbel (Sunnybrook Health Science Center, Toronto, Canada). Human melanoma cell lines PM-WK, RPM-MC, RPM-EP, and MM-LH36Byers HR Etoh T Lee KW Mihm MJ Gattoni CS Organ-specific metastases in immunodeficient mice injected with human melanoma cells: a quantitative pathological analysis.Melanoma Res. 1993; 3: 247-253Crossref PubMed Google Scholar were obtained from Dr. Randy Byers (Boston University Medical School, Boston, MA). The human melanoma cell lines SK-MEL-5, SK-MEL-25, SK-MEL-28, MelJuso, Colo 38, MML-1, HS695T, and MELKL-2 were obtained from the tumor bank of the German Cancer Research Center in Heidelberg, Germany. All melanoma cell lines were maintained in RPMI 1640 medium with 5% fetal bovine serum; human prostatic adenocarcinoma PC-3 cells (American Type Culture Collection, Rockville, MD) in Ham's F12 medium with 5% fetal bovine serum. All media were purchased from Life Technologies, Inc., Grand Island, NY. A human VEGF-C cDNA comprising the complete coding sequence (GenBank accession number X94216)10Joukov V Pajusola K Kaipainen A Chilov D Lahtinen I Kukk E Saksela O Kalkkinen N Alitalo K A novel vascular endothelial growth factor, VEGF-C, is a ligand for the Flt4 (VEGFR-3) and KDR (VEGFR-2) receptor tyrosine kinases.EMBO J. 1996; 15: 290-298Crossref PubMed Scopus (1177) Google Scholar was cloned into a pcDNA3.1/Zeo expression vector (Invitrogen, San Diego, CA) that contains a CMV-enhancer promoter and a Zeocin selection cassette. The sequence and the orientation of the VEGF-C gene in the construct were verified by restriction mapping and direct sequencing. Human MeWo malignant melanoma cells were transfected either with the human VEGF-C cDNA cloned into a pcDNA3.1/Zeo vector or with the vector alone using the Superfect transfection reagent (Qiagen, Chatsworth, CA). Transfected cells were selected and maintained in growth medium containing 50 μg/ml Zeocin. Stably transfected cell clones were individually expanded and analyzed for VEGF-C mRNA expression and protein secretion. Total cellular RNA was isolated from cultured cells and from tumors using the RNeasy kit (Qiagen). The isolated RNA was subjected to electrophoresis (15 μg per lane) and transferred to Hybond-N+ membranes (Amersham, Arlington Heights, IL). 32P-radiolabeled DNA probes were labeled by the random priming method (Multiprime labeling kit; Amersham, Arlington Heights, IL). The VEGF-C probe used was a fragment containing nucleotides 581 to 1634 of human VEGF-C cDNA. The probe for human VEGF hybridizes with a region of VEGF mRNA common to all known VEGF splice variants.37Berse B Brown LF Van De Water L Dvorak HF Senger DR Vascular permeability factor (vascular endothelial growth factor) gene is expressed differentially in normal tissues, macrophages, and tumors.Mol Biol Cell. 1992; 3: 211-220Crossref PubMed Scopus (866) Google Scholar A human β-actin cDNA probe (Clontech, Palo Alto, CA) was used as a control for equal RNA loading. Blots were hybridized at 65°C for 24 hours, washed at high stringency, and exposed to X-OMAT MR film (Kodak, Rochester, NY). Cells grown to 80% confluency were lysed with Izuhara buffer (50 mmol/L HEPES, 150 mmol/L NaCl, 1% Triton X-100, 30 mmol/L Na-pyrophosphate, 50 mmol/L Na-fluoride, 1 mmol/L Na-orthovanadate, pH 7.4) containing protease inhibitors (phenylmethylsulfonyl fluoride, 0.02 mol/L; leupeptin, 50 μg/ml; aprotinin, 50 μg/ml). The amount of protein was determined with the BioRad protein assay (BioRad, Hercules, CA), and 15 μg were analyzed on the gel. Conditioned media were obtained from subconfluent cells grown for 60 hours in serum-free media, concentrated 100-fold using Centricon-10 columns (Amicon, Beverly, MA), and 15 μg of protein were loaded on the gel. Tumors were snap-frozen in liquid nitrogen, lysed by homogenizing the tissue (0.5 g) in 2 ml of a buffer containing 2% sodium dodecyl sulfate, 50 mmol/L Tris (pH 7.4), and protease inhibitors as above, and 10 μl were loaded onto the gel. All samples were boiled in denaturing Laemmli sample buffer (BioRad), analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and blotted onto polyvinylidene difluoride membranes (BioRad). Filters were blocked overnight with 5% nonfat milk in phosphate-buffered saline (PBS)/0.1% Tween 20 and were incubated with a rabbit antiserum against human VEGF-C (1:1000).16Joukov V Sorsa T Kumar V Jeltsch M Claesson WL Cao Y Saksela O Kalkkinen N Alitalo K Proteolytic processing regulates receptor specificity and activity of VEGF-C.EMBO J. 1997; 16: 3898-3911Crossref PubMed Scopus (655) Google Scholar After washes, membranes were incubated with horseradish-peroxidase-conjugated anti-rabbit IgG (1:2000; Amersham), washed, and analyzed using the Amersham ECL+ chemiluminescence reagents. Receptor phosphorylation assays were performed as described,33Skobe M Hawighorst T Jackson DG Prevo R Janes L Velasco P Riccardi L Alitalo K Claffey K Detmar M Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis.Nat Med. 2001; 7: 192-198Crossref PubMed Scopus (1522) Google Scholar using antibodies against mouse VEGFR-2 (Santa Cruz Biotechnology, Santa Cruz, California) and phosphotyrosine (PY-20; ICN Biomedicals, Aurora, Ohio). Stably transfected MeWo cells (2 × 106 in 100 μl of serum-free culture medium) were injected intradermally on each side of the back of 8-week-old female Swiss/c (nu/nu) nude mice (five mice for each clone). Three clones were analyzed for each construct. Tumor growth was measured weekly using a digital caliper. Tumors were harvested when they reached a size of >1200 mm3 or after 6 to 8 weeks, and were embedded in OCT compound and frozen in liquid nitrogen or were fixed in 4% paraformaldehyde/PBS and processed for routine histology. For RNA or protein extractions, tumors were snap-frozen in liquid nitrogen. Two independent experiments with five mice in each group were performed. The 19-amino acid synthetic peptide CYPGKQAERAKWVPERRSQ, corresponding to amino acid residues 265 to 285 in the Ig-like domain 3 of the mouse VEGFR-3 extracellular domain, was used to immunize rabbits with standard techniques. The antisera were affinity purified and specific staining of lymphatic vessels was confirmed in adult mouse tissues (heart, lung, spleen, and liver). Cryosections were stained as previously described,38Skobe M Rockwell P Goldstein N Vosseler S Fusenig NE Halting angiogenesis suppresses carcinoma cell invasion.Nat Med. 1997; 3: 1222-1227Crossref PubMed Scopus (411) Google Scholar using antibodies against mouse CD31 (dilution 1/30; Pharmingen, San Diego, CA), LYVE-1 (1/300),33Skobe M Hawighorst T Jackson DG Prevo R Janes L Velasco P Riccardi L Alitalo K Claffey K Detmar M Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis.Nat Med. 2001; 7: 192-198Crossref PubMed Scopus (1522) Google Scholar, 39Prevo R Banerji S Ferguson DJ Clasper S Jackson DG Mouse LYVE-1 is an endocytic receptor for hyaluronan in lymphatic endothelium.J Biol Chem. 2001; 276: 19420-19430Crossref PubMed Scopus (418) Google Scholar VEGFR-3 (1/50; rabbit polyclonal; 1/30; R&D Systems, Minneapolis, MN), CD11b/Mac-1 (1/200; Pharmingen), or F4/80 (1/200; Serotec, Raleigh, NC). The secondary antibodies, labeled with either Texas Red or fluorescein isothiocyanate (Jackson Immuno Research, West Grove, PA) were used at the dilution 1/50. Cell nuclei were counterstained with Hoechst bisbenzimide (Sigma, St. Louis, MO) at 20 μg/ml. Specimens were examined by using a Nikon E-600 microscope (Nikon, Melville, NY). For analysis of tumor vascularization, immunohistochemical stainings were performed on 6-μm frozen sections of tumor xenografts using an antibody against mouse CD31.40Detmar M Brown LF Schön MP Elicker BM Velasco P Richard L Fukumura D Monsky W Claffey KP Jain RK Increased microvascular density and enhanced leukocyte rolling and adhesion in the skin of VEGF transgenic mice.J Invest Dermatol. 1998; 111: 1-6Crossref PubMed Scopus (467) Google Scholar Tissue sections were viewed using a Nikon E-600 microscope and images were captured with a SPOT digital camera (Diagnostic Instruments, Sterling Heights, MI). Two MeWo/control and two MeWo/VEGF-C clones were analyzed, five tumors each. On each tumor section, three randomly chosen fields were evaluated at ×60 magnification. Morphometric analysis was performed using the IPLab software (Scanalytics, Fairfax, VA) to determine vessel density, size distribution, and the relative tumor area covered by vessels.33Skobe M Hawighorst T Jackson DG Prevo R Janes L Velasco P Riccardi L Alitalo K Claffey K Detmar M Induction of tumor lymphangiogenesis by VEGF-C promotes breast cancer metastasis.Nat Med. 2001; 7: 192-198Crossref PubMed Scopus (1522) Google Scholar For analysis of macrophage densities, sections were stained with CD11b/Mac-1 antibody as described above. Three MeWo/control and three MeWo/VEGF-C clones were analyzed, three tumors each. For each tumor, the total area of the adjacent overlying skin was analyzed, and the relative area of the skin occupied by macrophages was determined using the IPLab software. The unpaired t-test was used for statistical analysis. Computed tomography was performed to determine the clearance rate of a lymphographic contrast agent from the tumors. Mice were imaged 5 weeks after tumor cell inoculation. Mice bearing MeWo/control tumors were additionally analyzed when the tumor size equaled that of the MeWo/VEGF-C tumors at 5 weeks (∼1000 mm3). Three mice carrying two tumors each were analyzed in both groups. After anesthesia with a mixture of ketamine (100 mg/kg) and xylazine (10 mg/kg), mice were injected with an iodinated lymphographic contrast agent,41Wolf GL Shore MT Bessin G McIntire GL Bacon ER Illig KJ Lymph node extraction of radiopaque nanoparticulates in the rabbit as measured in vivo with CT.Acad Radiol. 1999; 6: 55-60Abstract Full Text PDF PubMed Scopus (12) Google Scholar and imaging was performed by using a helical computed tomography scanner (TCT900S/xII; Toshiba, Tokyo, Japan). The contrast agent was administered very slowly by injection into the center of each tumor (20 μl per tumor) by using a 30-gauge needle attached to a Hamilton syringe. Mice were imaged before, immediately after, 6 hours, 24 hours, 48 hours, and 72 hours after the injection of the contrast agent. Each imaging study consisted of a helical data acquisition through the tumor and the axillary region for the assessment of axillary lymph nodes. The imaging parameters were 120-kV tube voltage, 150-mA tube current, 2-mm slice thickness, and a high-resolution imaging mode with an 80-mm field of view. Images were reconstructed onto a 512 × 512 image matrix yielding 0.05 mm3 voxels. Data analysis was performed with the image analysis program DIP Station (HIPG, Boulder, CO) to obtain the average signal intensity value for the tumor volume. Signal intensity values in computed tomography are expressed as Hounsfield units that change linearly as a function of contrast agent concentration in the tissue.42Groothuis DR Lapin GD Vriesendorp FJ Mikhael MA Patlak CS A method to quantitatively measure transcapillary transport of iodinated compounds in canine brain tumors with computed tomography.J Cereb Blood Flow Metab. 1991; 11: 939-948Crossref PubMed Scopus (28) Google Scholar A signal-time curve was created for each tumor and the rate constant was calculated using a mono-exponential model as a first approximation. The average clearance rate was then calculated for each group of animals. Macrophage accumulation was induced in Swiss/c nu/nu mice by intraperitoneal injection of 1 ml of 3% Brewer thioglycollate medium and macrophages were harvested 5 days after injection by lavage with Hanks’ balanced salt solution.43Edelson PJ Cohn ZA In Vitro Methods in Cell Mediated Tumor Immunity 1. Academic Press, New York1976: 333-340Google Scholar For flow cytometry, macrophages were detached from the tissue culture dish by scraping, incubated for 15 minutes at 4°C with antibodies to CD11b or mouse VEGFR-3 and with fluorescein isothiocyanate-labeled corresponding secondary antibodies. Chemotaxis was examined using 24-well Transwell migration chambers (8 μm pore size; Costar). Inserts were coated on the underside with 10 μg/ml of collagen type I (Collagen Corp., Palo Alto, CA) and 4 × 105 cells were added to the upper compartment in 100 μl of serum-free Dulbecco's modified Eagle's medium. VEGF-C (1 to 100 ng/ml) or the macrophage chemoattractant N-formyl-met-leu-phe (fMLP; 10−6 mol/L) were added to 600 μl of serum-free media in the lower compartment only. After 3 hours, inserts were fixed and washed as described44Senger DR Ledbetter SR Claffey KP Papadopoulos-Sergiou A Perruzzi CA Detmar M Stimulation of endothelial cell migration by vascular permeability factor/vascular endothelial growth factor through cooperative mechanisms involving the αvβ3 integrin, osteopontin, and thrombin.Am J Pathol. 1996; 149: 293-305PubMed Google Scholar and cell nuclei were stained with propidium iodide. The number of migrated cells was determined by using the IPLab software. For every insert, three fields were evaluated at ×120 magnification. All assays were performed in triplicate. In vitro proliferation rates of transfected MeWo cell clones were determined by using the BrdU labeling and detection kit (Boehringer Mannheim, Mannheim, Germany). Cells were plated in 96-well plates (5 × 103 cells/well; 8 wells/cell clone) and were allowed to proliferate for 24 hours before addition of BrdU (10 μmol/L) for 6 hours. The absorbance was determined at 405 nm using a microtiter plate reader (Titertek, Huntsville, AL). To assess the effect of VEGF or VEGF-C on MeWo cell proliferation, MeWo/control clone 5 was seeded into a 96-well plate at a density of 2.5 × 103 cells/well, and cells were treated for 5 days with 20 ng/ml of recombinant human VEGF-C16Joukov V Sorsa T Kumar V Jeltsch M Claesson WL Cao Y Saksela O Kalkkinen N Alitalo K Proteolytic processing regulates receptor specificity and activity of VEGF-C.EMBO J.