Targeting Receptor Tyrosine Kinase on Lymphatic ... - CiteSeerX

RESEARCH ARTICLE

Neoplasia . Vol. 8, No. 9, September 2006, pp. 747 – 757

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Targeting Receptor Tyrosine Kinase on Lymphatic Endothelial Cells for the Therapy of Colon Cancer Lymph Node Metastasis1 Robert B. Rebhun*, Robert R. Langley*, Kenji Yokoi *, Dominic Fan *, Jeffrey E. Gershenwald y and Isaiah J. Fidler* Departments of *Cancer Biology and ySurgical Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, TX 77030, USA Abstract Oral treatment with the dual-receptor tyrosine kinase inhibitor AEE788 effectively reduces the number of peritumoral lymphatic vessels and the incidence of lymph node metastasis in nude mice with human HT29 colon cancer cells growing in the cecum. Whether inhibition of lymph node metastasis in colon cancer can be achieved by directly targeting lymphatic endothelial cells remains unclear. Using a microsurgical approach, we generated conditionally immortalized lymphatic endothelial cell lines from the H-2Kb-tsA58 mouse mesentery and characterized these cells for the expression of lymphatic endothelial cell markers. Lymphatic endothelial cells were stimulated in culture with an array of tumor cell – produced cytokines, leading to the identification of redundant pathways for proliferation and survival. Treatment with AEE788 decreased the migration, proliferation, and survival of lymphatic endothelial cells, demonstrating that oral treatment with AEE788 effectively decreases the incidence of colon cancer lymphatic metastasis due, in part, to the direct inhibition of lymphatic endothelial cell signaling. Neoplasia (2006) 8, 747 – 757 Keywords: Lymphatic endothelial cells, lymph node metastasis, colon cancer, tyrosine kinase, lymphangiogenesis.

Introduction Colon cancer frequently metastasizes to regional lymph nodes, affecting prognosis and disease outcome [1]. In several tumor models, the formation of new lymphatic vessels from pre-existing lymphatics (i.e., lymphangiogenesis) is associated with an enhanced spread of tumor cells to regional lymph nodes [2– 6]. A rapidly expanding array of growth factors has been shown to regulate lymphangiogenesis, including the vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) family of proteins, which includes VEGF-A, VEGF-C, VEGF-D, and platelet-derived growth factor (PDGF). In addition, basic fibroblast growth factor (bFGF), hepatocyte growth factor (HGF), insulin-like growth factor (IGF)-1/2, and epidermal growth factor (EGF) also mediate lymphatic endothelial cell biology [7,8]. Im-

provements in the ability to identify lymphatic vessels have allowed more extensive studies examining the potential role of lymphatic vessels during metastasis [9]; however, the majority of these studies have been confined to the dermis. Thus, the role of lymphangiogenesis in colon cancer metastasis remains unclear. Results of histologic studies of human colon cancer have suggested that pathologic conditions, including cancer, are associated with alterations in both the density and the distribution of lymphatic vessels [10– 12]. Tumor cell invasion of lymphatic vessels has been shown to be an independent prognostic factor in colon cancer [13], and an increased density of lymphatic vessels within clinical specimens of colon cancer has been shown to be predictive of lymph node metastases, suggesting that lymphangiogenesis is essential for the spread of colon cancer to regional and distant lymph nodes [14– 16]. Another potential link between tumor-mediated lymphangiogenesis and lymph node metastasis in colon cancer is suggested by the finding that nodal metastasis correlates with the expression of lymphangiogenic growth factors by tumor cells [17 –20]. Antivascular therapy can lead to the destruction of neoplasms [21], and systemic administration of receptor tyrosine kinase inhibitors can produce antitumor effects by inhibiting receptor phosphorylation in tumors and tumor-associated endothelial cells [22 – 27]. Whether treatment of colon cancer and inhibition of lymph node metastasis can be achieved by directly targeting the lymphatic vasculature are unknown. We have previously reported that oral administration of AEE788, a dual-receptor tyrosine kinase inhibitor of the epidermal growth factor receptor (EGFR) and the vascular epidermal growth factor receptor-2 (VEGFR-2)/kinase domain receptor [28], can decrease the incidence of lymphatic metas-

Abbreviations: EOMA, murine hemangioendothelioma; MTT, 3-(4,5-dimethylthiazol-2-yl)2,5-diphenol-tetrazolium bromide Address all correspondence to: Isaiah J. Fidler, Department of Cancer Biology, The University of Texas M. D. Anderson Cancer Center, Unit 173, 1515 Holcombe Boulevard, Houston, TX 77030. E-mail: [email protected] 1 This work was supported, in part, by Cancer Center Support Core grant CA16672 and SPORE in Prostate Cancer grant CA90270 from the National Cancer Institute, National Institutes of Health. Robert Rebhun is the recipient of the American Legion Auxiliary Fellowship in Cancer Research for 2005 to 2006. Received 25 April 2006; Revised 6 July 2006; Accepted 12 July 2006. Copyright D 2006 Neoplasia Press, Inc. All rights reserved 1522-8002/06/$25.00 DOI 10.1593/neo.06322

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tasis in nude mice with orthotopic HT29 human colon carcinoma growing in the cecum [22]. However, it remained unclear whether the decrease in lymphatic metastasis was mediated by the actions of AEE788 on the tumor-associated lymphatic vasculature. In the present study, we compared peritumoral lymphatic vessel density (LVD) in control and in AEE788-treated HT29 human colon cancers growing in the cecum of nude mice to assess the potential effects of AEE788 on lymphatic endothelial cells. We developed a novel approach to isolate conditionally immortalized lymphatic endothelial cells from the mouse mesentery and examined the direct consequences of AEE788 exposure on lymphatic endothelial cell signaling, survival, proliferation, and migration.

Materials and Methods Cell Lines and Culture Conditions HT29 human colon cancer cells were maintained in minimum essential medium (MEM) supplemented with 10% fetal bovine serum (FBS), sodium pyruvate, nonessential amino acids, L-glutamine, a two-fold vitamin solution (Life Technologies, Grand Island, NY), and a penicillin/streptomycin mixture (Flow Laboratories, Rockville, MD). Cultures were maintained at 37jC in 5% CO2 and 95% air. Murine hemangioendothelioma (EOMA) and NIH-3T3 fibroblasts were maintained as described for HT29 tumor cells. Cultures were free of Mycoplasma and pathogenic murine viruses (assayed by Science Applications International Co., Frederick, MD). Brain microvascular cells were isolated as previously described [29]. Lymphatic endothelial cell lines and brain microvascular endothelial cells were maintained at 33jC in 5% CO2 and 95% air in Dulbecco’s modified Eagle’s medium (DMEM) supplemented as described above. Reagents AEE788 (Novartis Pharma, Basel, Switzerland) is an adenosine triphosphate – competitive low-molecular-weight tyrosine kinase family inhibitor of EGFR and VEGFR-2 [28]. AEE788 was diluted in dimethyl sulfoxide (DMSO) before oral dosing. The following primary antibodies were used: LYVE-1 and podoplanin (Research Diagnostics, Flanders, NJ); mouse CD31 and VEGFR-3 (PharMingen, San Diego, CA); Prox-1 (AngioBio, Del Mar, CA); VEGFR-2 (SC504) and VEGFR-3 (SC637) (Santa Cruz Biotechnology, Santa Cruz, CA); EGFR (Zymed, San Francisco, CA); phospho-EGFR (Biosource International, Inc., Carlsbad, CA); phosphoVEGFR-2/3 (Oncogene, Boston, MA); streptavidin-conjugated Alexa fluorescent 594, anti-rat Alexa fluorescent 594, and anti-rabbit Alexa 488 (Molecular Probes, Eugene, OR); phospho-Akt (Ser-473), Akt, phospho-ERK1/2 (Thr-202/Tyr204), and P44/42 MAPK (Cell Signaling Technology, Beverley, MA); SV40 large T antigen (Oncogene Research, San Diego, CA); smooth muscle actin and b-actin (Sigma-Aldrich, St. Louis, MO); and vinculin (Serotec, Raleigh, NC). Secondary reagents included peroxidase-conjugated F(abV)2 fragments from Jackson ImmunoResearch (West Grove,

PA). Anti-rabbit and anti-mouse conjugated horseradish peroxidase antibodies were purchased from Bio-Rad Laboratories (Hercules, CA). A tyramide signal amplification reagent (NEN Life Science Products, Boston, MA) was also used in the study according to the manufacturer’s instructions. Animals Male athymic nude mice (NCI-nu) were purchased from the National Cancer Institute Frederick Cancer Research and Development Center (Frederick, MD). Mice homozygous for a temperature-sensitive SV40 large T antigen (H-2Kb-tsA58 mice; CBA/caC57BL/10 hybrid) were purchased from Charles River Laboratories (Wilmington, MA). The mice were housed under specific pathogen-free conditions in accordance with institutional guidelines and the current regulations of the National Institutes of Health, the US Department of Health and Human Services, and the US Department of Agriculture. Orthotopic Implantation of Tumor Cells To produce orthotopic cecal tumors, HT29 cells were harvested from subconfluent cultures by brief exposure to 0.25% trypsin and 0.02% ethylenediaminetetraacetic acid (EDTA). Cells were washed once in serum-free medium and resuspended in Hank’s balanced salt solution (HBSS). Only single-cell suspensions with more than 90% viability were used for injection. Cells (1 106) in 50 ml of HBSS were injected orthotopically into the cecal wall of nude mice as described previously [22]. Treatment of Established Human Colon Carcinoma Tumors Growing in the Cecum of Athymic Nude Mice Treatment was performed as previously described [22]. Briefly, 2 weeks after cecal injection, 10 mice were each randomly assigned to receive one of the following treatments: 1) oral gavage of DMSO/0.5% Tween 80 diluted 1:20 with water, thrice a week, and intraperitoneal injection of phosphate-buffered saline (PBS), once a week (control group); or 2) oral gavage of AEE788 (50 mg/kg), thrice a week, and intraperitoneal injection of PBS, once a week. Necropsy Procedures and Histologic Studies Mice were euthanized with pentobarbital sodium (Abbot Laboratories, North Chicago, IL) on Day 36 of treatment, and tumors growing within the cecum were excised. Tumor tissue was embedded in an ornithine carbamyl transferase compound (Miles Laboratories, Elkhart, IN), frozen in liquid nitrogen, and stored at 70jC. All macroscopically enlarged mesenteric lymph nodes were harvested and then examined histologically for the presence of metastases. Determination of LVD Frozen sections were fixed by sequential immersions in ice-cold acetone, acetone – chloroform (50:50 vol/vol), and acetone solution (5 minutes each). Slides were rinsed with PBS and blocked with a mixture of PBS containing 5% normal horse serum and 1% normal goat serum for 30 minutes at room temperature. Tissue samples were then incubated

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overnight at 4jC with anti-mouse LYVE-1 antibody, washed, and incubated for 1 hour with anti-rabbit Alexa 488. Samples were then washed and incubated overnight at 4jC with either anti-CD31 antibody or anti – VEGFR-3 antibody, washed, and incubated at room temperature for 1 hour with the appropriate Alexa 594 antibody. Cell nuclei were stained using Hoechst 33342 (Polysciences, Inc., Warrington, PA), and the slides were mounted with a glycerol/PBS medium containing 0.1 M propyl gallate (Sigma-Aldrich) to minimize fluorescent bleaching. LVD was evaluated by two independent investigators using a modification of the method previously described [30] for determining the blood microvascular density of tumors. Briefly, regions of the tumor that stained intensely for LYVE-1 were selected by low-power (original magnification, 40 magnification) scanning of the section. The mean LVD of these ‘‘hot spots’’ was then quantified by counting the number of lymphatic vessels at higher (original magnification, 100) magnification in a minimum of three microscopic fields for each tumor sample. Immunofluorescence microscopy was performed using a Zeiss Axioplan fluorescence microscope (Carl Zeiss, Inc., Thornwood, NY) equipped with a 100-W Hg lamp and narrow bandpass excitation filters. Representative images were obtained using a cooled chargecoupled device Hamamatsu C5810 camera (Hamamatsu Photonics, Bridgewater, NJ) and Optimas software (Media Cybernetics, Silver Spring, MD). Composite figures were made using PhotoShop software (Adobe Systems Incorporated, Mountain View, CA). Expression of VEGFR-2, VEGFR-3, and EGFR in Tumor-Associated Lymphatic Vessels HT29 tumors were stained for the lymphatic endothelial cell marker LYVE-1, as described above. The slides were then blocked in rabbit serum diluted in fish gel (20 mg/ml) for 30 minutes at room temperature and then with goat anti-rabbit F(abV)2 fragments diluted in fish gel. The slides were incubated with antibodies directed against either EGFR, VEGFR2, VEGFR-3, or their respective phosphorylated forms for 24 hours at 4jC. The slides were rinsed thrice with PBS and then incubated with anti-rabbit Alexa 594 secondary antibody (1:500; Molecular Probes) for 45 minutes. The slides were rinsed and then analyzed by fluorescence microscopy. Microsurgical Isolation of Lymphatic Endothelial Cells H-2Kb -tsA58 mice were anesthetized with an intraperitoneal injection of pentobarbital sodium. The hair was clipped, and the skin was prepared for surgery in standard fashion. A ventral midline incision was made, allowing exteriorization of the cecum and associated mesentery. Isosulfan blue (100 ml; Ben Venue Laboratories, Inc., Bedford, OH) was injected into the cecal wall, allowing rapid visualization of the draining mesenteric lymph node and the primary collecting duct. A curved stainless-steel cutting needle and microforceps were used to dissect lymphatic vessels from the node and the surrounding mesentery; at this time, the mice were killed by cervical dislocation. Lymphatic vessels were placed into individual wells of a six-well plate (previously coated with 1% gelatin) containing DMEM supplemented with 10%


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FBS. Primary cultures were supported at 33jC in a mixture of 5% carbon dioxide and 95% oxygen; the medium was replaced as needed. Derivation of Clonal Populations of Mesenteric Lymphatic Endothelial Cells Cells were harvested from subconfluent cultures by overlaying the monolayer with a solution of 0.25% trypsin and 0.1% EDTA. Cells were resuspended in DMEM containing 10% FBS, and the number of detached cells was adjusted to achieve a final concentration of 5 cells/ml. Cell suspension (100 ml) was then placed into individual wells of 96-well plates. Cells were allowed to incubate overnight at 33jC and were then examined under phase-contrast microscopy to identify wells containing only one cell. Wells with only one cell were marked, the plates were returned to the incubator, and the medium was changed as necessary. Four clonal populations were randomly selected and combined for expansion and subsequent comparison with a pooled population of cells. Immunohistochemical Evaluation of Immortalized Mesenteric Lymphatic Endothelial Cells Cells were seeded onto two-chamber slides at a density of 2.5 104 cells/well in DMEM containing 10% FBS and incubated at 37jC for 72 hours. Tissue fixation was carried out by immersion in ice-cold acetone for 15 minutes, and the slides were blocked in PBS containing 5% normal horse serum and 1% normal goat serum. The slides were incubated overnight at 4jC with primary antibodies directed against LYVE-1, podoplanin, Prox-1, or VEGFR-3. The slides were rinsed thrice with PBS and then incubated with the appropriate peroxidase-conjugated secondary antibody for 1 hour at room temperature. The slides were washed and then incubated in a biotin tyramide solution for 10 minutes, and then incubated for 1 hour with streptavidin-conjugated Alexa fluorescent 594. Cell nuclei were labeled with Hoechst 33342. Fluorescent bleaching was minimized by mounting the slides with a glycerol/PBS medium containing 0.1 M propyl gallate. Immunofluorescence microscopy was performed using a Zeiss Axioplan fluorescence microscope, as described above. Composite photographs were made using PhotoShop software (Adobe Systems Incorporated). Western Blot Analysis To determine the kinetics of the expression of the temperature-sensitive SV40 large T antigen, 4 105 lymphatic endothelial cells were plated onto a series of 10-cm culture dishes in DMEM containing 10% FBS and placed in either an incubator set at 33jC for 72 hours or an incubator set at 37jC for 24, 48, or 72 hours. At each time point, cells were washed twice with PBS and then lysed with buffer [20 mM Tris – HCl (pH 8.0), 137 mM NaCl, 10% glycerol, 2 mM EDTA, 1 mM phenylmethylsulfonyl fluoride, 2 mM sodium orthovanadate, 20 mM leupeptin, and 0.15 U/ml aprotinin]. Protein concentrations were determined using the Bradford method (BioRad Laboratories), and 50 mg of total protein resolved in 8% sodium dodecyl sulfate – polyacrylamide gel electrophoresis

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under reducing conditions was transferred to polyvinylidene difluoride membranes. Membranes were blocked with 5% (weight/volume) nonfat dried milk in Tris-buffered saline (pH 7.5; 100 mM NaCl, 50 mM Tris, and 0.1% Tween 20) (Sigma-Aldrich) for 1 hour and then incubated overnight at 4jC with an antibody directed against the SV40 large Tantigen. Immunodetection was performed using the corresponding secondary horseradish peroxidase – conjugated antibody, and activity was detected using enhanced chemiluminescence (Amersham Biosciences, Piscataway, NJ). Akt and ERK1/2 Signaling in Mesenteric Lymphatic Endothelial Cells In this series of experiments, we sought to identify factors that could activate Akt and ERK1/2 in mesentery-derived lymphatic endothelial cells. Lymphatic endothelial cells were seeded onto several 10-cm dishes at a density of 4 105 cells/dish in DMEM containing 10% FBS and placed at 37jC for 54 hours, at which time the incubation medium was aspirated and replaced with serum-free DMEM. After serum starvation for 18 hours at 37jC, the incubation medium was aspirated and replaced with either fresh serum-free DMEM or serum-free DMEM supplemented with 100 ng/ml VEGF-C or VEGF-D (R&D Systems, Minneapolis, MN) for 5, 15, 30, or 45 minutes. In parallel studies, additional plates were stimulated with 20 ng/ml VEGF-A (R&D Systems), PDGF-BB, bFGF, or EGF (Invitrogen, Carlsbad, CA). At the correct time point, cells were washed twice with ice-cold PBS and lysed with buffer, and the extent of Akt and ERK1/2 activation was determined using Western blot analysis. AEE788 Inhibition of Akt and ERK1/2 Phosphorylation in Mesenteric Lymphatic Endothelial Cells Lymphatic endothelial cells were seeded onto several sixwell plates at a density of 1 105 cells/well in DMEM containing 10% FBS and placed at 37jC for 54 hours. The incubation medium was aspirated and replaced with serumfree DMEM. After serum starvation for 18 hours at 37jC, lymphatic endothelial cells were treated for with AEE788 (0, 0.25, 0.5, 1.0, 2.5, or 5.0 mM) for 120 minutes, followed by an additional 15-minute incubation in the presence or absence of growth factors. Cells were washed twice with ice-cold PBS and lysed with buffer, and protein expression was determined by Western blot analysis as described above. Cytotoxicity and Proliferation Assays Lymphatic endothelial cells were incubated at 37jC for 72 hours, plated in 96-well plates (5 103 cells per well) containing 100 ml of DMEM plus 10% FBS, and then incubated at 37jC for an additional 24 hours. The medium was aspirated and replaced with 200 ml of serum-free MEM with or without specified concentrations of AEE788, or MEM containing 5% FBS with or without AEE788. After 72 hours, the number of cells that were metabolically active was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenoltetrazolium bromide (MTT) assay [31]. Briefly, 0.42 mg/ml MTT was added to the cultures. After 2 hours, the cells were lysed with 60 ml of DMSO. MTT conversion to formazan by

metabolically active cells was measured with an MR-5000 96-well microtiter plate reader at 570 nm (Dynatech, Inc., Chantilly, VA). Assays were performed for a minimum of three times. The effect of AEE788 on cell viability was also evaluated using propidium iodide staining. For these experiments, lymphatic endothelial cells were plated in a series of 100-mm dishes at a density of 1 106 cells/dish. After a 24-hour incubation period, the medium was replaced with MEM containing 5% FBS or the same medium with either 0.5 or 5.0 mM AEE788. Additional plates were treated in the same fashion, except that FBS was not included in the medium. After a 48-hour incubation period, the cells were fixed with ice-cold 70% ethanol and placed at 20jC for 24 hours. Cells were pelleted (400 g for 5 minutes) in tubes and washed once in staining buffer (PBS with 2% fetal calf serum and 0.01% NaN3). Cells were treated with 15 mg/ml ribonuclease A (Sigma-Aldrich) for 30 minutes at 37jC. Cells were stained with 50 mg/ml propidium iodide in staining buffer for 30 minutes at room temperature. Samples were placed on ice and then analyzed by flow cytometry. Migration Assays Lymphatic endothelial cells were incubated at 37jC for 72 hours, trypsinized, and washed in MEM containing 5% FBS. Cells were washed again and resuspended in serumfree MEM. Cells (2.0 104) in 200 ml were plated on top of 24-well polycarbonate transwell migration inserts (3.0 mm pore size; Fisher Scientific, St. Louis, MO), which had been placed onto 24-well plates containing 500 ml of serum-free MEM or MEM containing 10% FBS. Additional wells contained AEE788 at concentrations of 0.5 or 5.0 mM in MEM plus 10% FBS. Plates were incubated at 37jC for 20 hours, at which time the cells from the upper compartment were manually removed with cotton swabs and the membrane was fixed and processed for cell counting. Cells that migrated to the underside of the membrane were counted under 40 magnification. All assays were performed in triplicate. Statistical Analysis Statistical analyses were performed using GraphPad Prism software (San Diego, CA). P values were calculated using one-way analysis of variance. Means determined to be significantly different ( P < .05) were then subjected to post hoc analysis using Tukey’s multiple comparison testing.

Results Immunofluorescent Staining of HT29 Tumors To determine the specificity of the lymphatic vessel staining of HT29 tumors, we performed a dual staining of tissue samples with LYVE-1 and CD31 antibodies. CD31+ vascular structures were found within and adjacent to HT29 tumors. LYVE-1+ cells were absent from HT29 tumors (Figure 1A). The peritumoral staining of irregularly shaped thin-walled vessels (consistent with lymphatic vessels) for LYVE-1 was evident and colocalized with some, but not all, CD31+ vessels



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Figure 1. Immunohistochemical analysis of HT29 human colon cancer growing orthotopically within nude mice. (A) Representative example of the colocalization of LYVE-1 (green), CD31 (red), and Hoescht (blue). The peripheral margin of the HT29 tumor is outlined in white. LYVE-1 staining is absent within the tumor, yet an abundance of CD31+ vessels is evident. (B) Representative photomicrographs of LYVE-1+ peritumoral lymphatic vessels surrounding HT29 control or AEE788treated tumors. (C) The mean peritumoral LVD of HT29 tumors in mice receiving oral treatment with control or AEE788 was determined as described in the Materials and Methods section. Oral treatment with AEE788 significantly reduced the peritumoral LVD of human HT29 tumors growing within the cecum of nude mice.

(Figure 1A). VEGFR-3 was also found to colocalize with positively stained LYVE-1 vessels (data not shown). Determination of LVD To determine the effects of AEE788 therapy on LVD, HT29 tumors growing in the cecum of nude mice were stained for the lymphatic-specific marker LYVE-1. For each tumor, intensely staining peritumoral regions were identified, and the number of LYVE-1+ vessels within these regions was counted under 100 magnification. Treatment with AEE788 significantly decreased peritumoral LVD in colon cancer tumors compared with control mice (P < .001) (Figure 1B). Microsurgical Isolation of Lymphatic Endothelial Cells Injection of isosulfan blue (100 ml) into the cecal wall permitted the rapid visualization of the mesenteric lymphatic collecting duct and associated draining lymph nodes (Figure 2, A and B). Under the guidance of a dissecting microscope, the mesenteric vessels of mice were dissected and removed using a sterile, curved, stainless-steel cutting needle (5-0) and microforceps (Figure 2, C and D). The vessels were placed into individual wells of six-well plates precoated with 1% gelatin, covered with medium containing 10% FBS, and placed in an incubator set at 33jC. After a 72-hour incubation period, we observed the attachment of some of the vessels (Figure 2E ) and the outgrowth of a homogenous population of cells with cobblestone morphology (Figure 2F ). Characterization of Mesenteric Lymphatic Endothelial Cells The immunohistochemical analysis of clonally derived lymphatic endothelial cells revealed the expression of LYVE-1,


Prox-1, podoplanin, and VEGFR-3 (Figure 2, G –K). Mesenteric lymphatic endothelial cells also internalized acetylated low-density lipoproteins and formed tube structures on Matrigel surfaces (data not shown). To assess the kinetics of the expression of SV40 large T antigen, several clonal subcultures were transferred to an incubator set at 37jC. In cells growing at 37jC, the expression of SV40 large T antigen declined in a time-dependent manner to near undetectable levels by the 72-hour time point (Figure 3A). To exclude any potential effects of SV40 large T antigen on lymphatic endothelial cell phenotype, lymphatic endothelial cells were incubated for at least 72 hours in an incubator set at 37jC. Expression Profiling of Mesenteric Lymphatic Endothelial Cells by Western Blot Analysis To determine whether lymphatic endothelial cells exhibited a phenotype distinct from that of fibroblasts or vascular endothelial cells, we examined the expression level of several proteins. Vascular endothelial cells isolated from the brain of H-2K b-tsA58 mice [29] were used as controls because the brain is an organ devoid of lymphatic vessels and any endothelial cell derived from this tissue will be of blood vascular origin. Consistent with immunohistochemical analysis, lymphatic endothelial cells expressed robust levels of VEGFR-3 and podoplanin. Moreover, the level of VEGFR-3 in lymphatic endothelial cells was higher than that observed in brain-derived endothelial cells, and VEGFR-3 was not detected in either NIH-3T3 fibroblasts or EOMA cell lines. Podoplanin was not detected in endothelioma cells and was expressed at a low level in NIH-3T3 fibroblasts. Lymphatic

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endothelial cells and brain endothelial cells expressed similar levels of podoplanin (Figure 3B). As expected, clonally derived lymphatic endothelial cells were essentially negative for a-smooth muscle actin, ruling out contamination with fibroblasts or smooth muscle cells. Effects of AEE788 on Lymphatic Endothelial Cell Proliferation, Survival, and Migration To determine whether AEE788 inhibits the proliferation and survival of mesenteric lymphatic endothelial cells, MTT assays were performed. Lymphatic endothelial cells were plated in a medium with or without serum and were treated with 0, 0.5, or 5.0 mM AEE788, as described in the Materials and Methods section. In both the presence and the absence of serum, AEE788 inhibited proliferation in a dose-dependent manner (Figure 4A). Data derived from experiments using flow cytometry and propidium iodide staining paralleled MTT results in that the incubation of lymphatic endothelial cells in concentrations of 0.5 and 5.0 mM AEE788 in serum-containing medium for 48 hours increased cell death

by 21% and 30%, respectively (data not shown). Treatment of lymphatic endothelial cells with 0.5 and 5.0 mM AEE788 in the absence of serum for 48 hours increased cell death by 30% and 60%, respectively (data not shown). In addition to affecting lymphatic endothelial cell survival, AEE788 significantly (P < .001) reduced lymphatic endothelial cell migration (Figure 4B). Akt and ERK1/2 Signaling in Mesenteric Lymphatic Endothelial Cells Previous studies have shown that stimulation of VEGFR-3 initiates the activation of Akt and ERK1/2 pathways in lymphatic endothelial cells [32]. To determine whether these signaling pathways were intact in cultured mesenteric lymphatic endothelial cells, we stimulated lymphatic endothelial cells with a broad panel of growth factors and found that VEGF-C or VEGF-D ligands resulted in the phosphorylation of Akt and ERK1/2 (Figure 5). Stimulation with PDGF-BB or EGF also produced a robust activation of Akt and ERK1/2 (Figure 5). VEGF-A (VPF) promoted the activation of Akt

Figure 2. Isolation of lymphatic endothelial cells. (A and B) Isosulfan blue injected into the cecal wall allows direct visualization of the afferent mesenteric collecting duct and mesenteric lymph nodes. The arrows indicate the afferent mesenteric collecting duct, the asterisk represents the cecal injection site, and the mesenteric lymph node is labeled L. (C and D) A curved stainless-steel cutting needle and microforceps were used to remove the vessel from the associated mesentery. (E) Attachment of a lymphatic vessel in culture yields an outgrowth of cells. (F) Confluent monolayer of clonal lymphatic endothelial cells exhibits cobblestone morphology. Representative examples of immunohistochemical staining for lymphatic markers. (G) Control. (H) LYVE-1. (I) Prox-1. (J) Podoplanin. (K) VEGFR-3.



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Figure 3. Characterization of lymphatic endothelial cells. (A) The temperate expression of SV40 large T antigen was examined by Western blot analysis, as described in the text. Cells were incubated at 37jC for 0, 24, 48, or 72 hours before lysis. By 48 hours, the expression of SV40 large T antigen had been markedly reduced. -Actin is shown as an internal loading control. (B) Cell proteins were obtained and examined by Western blot analysis, as described in the text. Clonally derived lymphatic endothelial cells express high levels of VEGFR-3 and podoplanin. In addition, lymphatic endothelial cells are essentially negative for the expression of a-smooth muscle actin (a-sm actin). Vinculin is shown as an internal loading control. LyEC, lymphatic endothelial cells; BrEC, brain endothelial cells.

Figure 5. Effect of growth factors on the phosphorylation of Akt and ERK1/2. Under serum-free conditions, there is low-level activation of Akt and ERK1/2. Addition of PDGF-BB (20 ng/ml), bFGF (20 ng/ml), VEGF-D (100 ng/ml), VEGF-C (100 ng/ml), VEGF-A (100 ng/ml), or EGF (20 ng/ml) produces varying increases in the phosphorylation of Akt and ERK1/2 over a 45-minute time course. Total Akt and total ERK1/2 are shown as loading controls. P-Akt, phosphorylated Akt; T-Akt, total Akt; P-ERK1/2, phosphorylated ERK1/2; TERK1/2, total ERK1/2.

and the transient activation of ERK1/2, whereas the actions of bFGF were more directed toward the phosphorylation of ERK1/2 than toward the activation of Akt (Figure 5). Effects of AEE788 on Mesenteric Lymphatic Endothelial Cell Survival and Proliferation Pathways Treatment of lymphatic endothelial cells with AEE788 inhibited survival and proliferation pathways in response to stimulation with EGF. AEE788 also produced various levels of inhibition in response to other lymphangiogenic growth factors, including VEGF-A, VEGF-C, VEGF-D, bFGF, and PDGF-BB (Figure 6). Figure 4. Inhibition of lymphatic endothelial cell proliferation and migration in response to treatment with AEE788. Lymphatic endothelial cells were treated as described in the text. (A) Proliferation of lymphatic endothelial cells as measured by MTT assay. Absorbance of 5 103 cells, as plated at the start of the assay, is represented with a horizontal line. (B) Migration of lymphatic endothelial cells toward a serum-free medium, a medium containing 10% FBS, or a medium containing 10% FBS with either 0.5 or 5.0 M AEE788. Data are expressed as mean ± SD and were repeated at least thrice with comparable results. *P V .001.


Expression of Total and Phosphorylated VEGFR-2, VEGFR-3, and EGFR in Peritumoral Lymphatic Vessels Next, we performed double-immunofluorescence staining to evaluate the effects of AEE788 on receptor tyrosine kinase expression and activation on peritumoral lymphatic vessels. Treatment with AEE788 had no effect on the levels

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Figure 6. AEE788 inhibits the phosphorylation of Akt and ERK1/2 within lymphatic endothelial cells. Stimulation of Akt and ERK1/2 is seen in response to treatment with various growth factors. The blockade of Akt and ERK1/2 phosphorylation is most evident in cells treated with EGF, whereas intermediate blockade was achieved in cells treated with VEGF-C, VEGF-D, or PDGF-BB. Only mild inhibition was evident in cells treated with VEGF-A or bFGF.

of EGFR, VEGFR-2, or VEGFR-3 (Figure 7A); however, AEE788 did inhibit the phosphorylation of these receptors on peritumoral lymphatic vessels associated with HT29 human colon cancer (Figure 7B).

Discussion We recently reported that oral treatment with AEE788 effectively reduced the incidence of lymph node metastasis in nude mice implanted with human HT29 colon cancer cells into the cecum [22]. Because lymphangiogenesis has been shown to enhance lymphatic metastasis in some tumor models, we questioned whether AEE788 might have had a direct effect on tumor lymphangiogenesis in orthotopically implanted colon tumors. In this study, treatment with AEE788 significantly reduced the number of peritumoral lymphatic vessels of HT29 tumors, establishing that AEE788 has a direct effect on tumor lymphangiogenesis in this tumor model. We used an anatomic dissection technique to isolate the lymphatic endothelial cells of the mouse mesentery. Similar to results generated from other laboratories [7,32,33], we noted that a variety of growth factors stimulate the activation of proliferation and survival programs in lymphatic endothelial cells. In addition, we found that in vitro treatment of mesenteric lymphatic endothelial cells with AEE788 results in the inhibition of lymphatic endothelial cell survival, proliferation, and migration in response to multiple growth factors. Taken together, these results demonstrate that oral administration of AEE788 effectively decreases the incidence of lymphatic

metastasis and that this effect is due, in part, to the direct inhibition of lymphatic endothelial cell signaling. The recent discovery of several lymphatic markers has facilitated a more extensive study of lymphangiogenesis and lymphatic metastasis [9]. For example, it is now known that activation of VEGFR-3 by VEGF-C and VEGF-D signals for survival and cell division in lymphatic endothelial cells growing in cell culture [32,34 – 38]. VEGF-C and VEGF-D have also been shown to induce lymphangiogenesis in vivo [3,5,39] and to enhance lymphatic metastasis in some tumor models [2 – 5]. Several other growth factors, including VEGFA, bFGF, PDGF-BB, HGF, IGF-1, IGF-2, and EGF, also promote lymphangiogenesis [7,8]. Indeed, the complexity of lymphatic endothelial cell biology becomes more apparent when the indirect effects of several known lymphangiogenic growth factors are considered. For example, expression levels of VEGF-C mRNA can be modulated by a variety of proteins, including IGF-1R, PDGF, EGF, transforming growth factor-b, tumor necrosis factor-a, and interleukin-1b [40 – 42]. VEGF-A has been shown to be chemotactic for macrophages capable of producing an array of cytokines and growth factors, including VEGF-C and VEGF-D [43,44]. For these reasons, it is not surprising that VEGF-A can induce lymphangiogenesis through both direct [45] and indirect [43] effects on the tumor microenvironment. Similar findings have been reported for HGF [46,47] and bFGF [48]. EGF has been shown to induce the proliferation of lymphatic endothelial cells in culture and to upregulate the expression of lymphangiogenic growth factors [7,39,49,50].



In the present report, we found that a wide variety of growth factors directly stimulated the activation of the proliferation and survival pathways in lymphatic endothelial cells from the mouse mesentery and that these signaling cascades can be inhibited by physiologically relevant doses of AEE788. Some of these cytokines appear to preferentially activate either Akt or ERK1/2 signaling. For example, the activity of bFGF was more focused on ERK1/2, whereas VEGF-A preferentially stimulated Akt signaling. Although AEE788 has been shown to preferentially inhibit EGFR and VEGFR-2 kinase activity, higher concentrations of AEE788 have been shown to quench the activity of VEGFR-3 in vitro [28]. VEGFR-2 heterodimerization with VEGFR-3 can regulate tyrosine phosphorylation sites within the cytoplasmic tail region of VEGFR-3 [51]. Studies also suggest that VEGFR-2 may be necessary for the intracellular signaling initiated by VEGFR-3 [52]. Although this study did not intend to investigate the interactions between VEGFR-2 and VEGFR-3, the generation of an immortalized lymphatic endothelial cell line

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should facilitate studies of these complex cellular interactions. Previous studies have shown that AEE788 can significantly reduce tumor microvascular density by inhibiting the activation of EGFR and VEGFR [22 – 24]. Here, we provide preliminary evidence that AEE788 can also target receptor tyrosine kinases on tumor-associated lymphatic vessels and contribute to the therapy of tumors. Because the tumor cell invasion of lymphatic vessels in colon cancer is known to be a negative indicator for survival [13], several investigations have focused their efforts on defining the role of lymphangiogenesis in this disease. Initial studies reported high expression levels of the lymphangiogenic growth factors VEGF-C and VEGF-D in human colorectal cancers, but whether the expression of these growth factors predicts lymph node metastasis is not clear [17 – 20]. LVD has been associated with lymph node metastasis in human colon cancer [14 – 16]; however, there are conflicting reports as to whether colon tumors possess intratumoral lymphatic vessels [14 – 16]. The sensitivity and the specificity

Figure 7. Double-immunofluorescence staining for total and phosphorylated VEGFR-2, VEGFR-3, and EGFR in peritumoral lymphatic vessels. (A) Samples were stained with anti – LYVE-1 antibody (green) and anti-EGFR, anti – VEGFR-2, or anti – VEGFR-3 (red) antibody, as described in the Materials and Methods section. The colocalization of LYVE-1 and receptors appears as yellow fluorescence. The expression of EGFR, VEGFR-2, and VEGFR-3 was found in peritumoral lymphatic vessels of mice treated with control or AEE788. (B) Phosphorylation of EGFR and VEGFR-2/3 in lymphatic endothelial cells was decreased by treatment with AEE788. Scale bars = 100 m.


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of routinely used lymphatic markers are variable [9]. We characterized lymphatic vessels using multiple markers, including CD31, VEGFR-3, and LYVE-1, and were unable to detect intratumoral lymphatic vessels. Our finding that HT29 human colon cancer growing orthotopically within the cecum of nude mice is devoid of lymphatic vessels is in agreement with more recent studies evaluating human colon cancer specimens [15,16]. Our findings indicate that lymphangiogenesis plays an important role in the dissemination of colon cancer into regional lymph nodes. We also demonstrate that, in addition to tumor cells and tumor-associated vascular endothelial cells, lymphatic endothelial cells represent an additional target for the therapy of colon cancer with small molecule tyrosine kinase inhibitors.

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