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1 Department of Dermatology, Klinikum der J. W. Goethe-Universität, Frankfurt am Main; 2 RBA Dermatology, Schering AG, Berlin; 3 Department of Hematology/Oncology, Krankenhaus Nordwest, Frankfurt am Main; 4 Institute of Transfusion Medicine, German Red Cross Blood Center, Frankfurt am Main; and 5 Department of Molecular Biology, Max-Planck-Institut für Physiologische und Klinische Forschung, Bad Nauheim, Germany
| ABSTRACT |
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| INTRODUCTION |
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Ample evidence supports the concept that blood-borne metastasis is significantly facilitated by interactions between disseminating tumor cells and blood platelets (3, 4, 5) . Interference by antiplatelet agents or anticoagulants in tumor-cellplatelet association potently inhibits both spontaneous and experimental metastasis. In addition, measures that lower circulating platelet counts have resulted in a decrease in distant metastases. At the cellular level, tumor-cellplatelet interactions are assumed to both stabilize tumor cell arrest and assist cancer cell survival in secondary organs. In addition, platelettumor cell aggregates can protect from immune surveillance (6) and may supply tumor cells with critical stimulatory growth factors and cytokines, which are released on platelet activation (7 , 8) .
Important molecular mechanisms of tumor-cellplatelet interactions have recently been identified (7)
. P-selectin, a member of the selectin family of adhesion molecules, has been demonstrated to mediate crucial cancer cell interactions with platelets and endothelial cells. In vitro, P-selectin binds to numerous tumor cell lines via sialylated fucosylated glycans present on mucin and nonmucin structures (9
, 10)
. P-selectin is constitutively present in the
-granules of platelets and in the WeibelPalade bodies of endothelial cells and can be rapidly translocated to the cell surface on activation to support adhesive interactions (11)
. The proposed role of P-selectin in platelettumor complex formation and metastasis was established by the use of genetically engineered P-selectin mutant mice. Both efficient tumor-cellplatelet aggregation and the development of experimental lung metastasis after i.v. injection of colon carcinoma cells were significantly diminished in P-selectin-deficient mice (12
, 13)
. These observations indicate that P-selectin is critical for lung colonization of hematogenously borne tumor cells, supporting the concept that P-selectin-mediated tumor-cellplatelet interactions assist the process of metastasis.
On the other hand, the extent to which endothelial P-selectin expression may additionally contribute to formation of metastases has yet to be elucidated. Detailed ultrastructural analyses revealed that involvement of platelets occurred only after tumor cells had arrested, suggesting that early tumor cell arrest in the capillary bed proceeds independently of platelets (14 , 15) . Although cancer cells in general seem not to follow the adhesion cascade with eventual arrest to vessel wall endothelium in a manner comparable to that seen with leukocytes in inflammation, interactions with endothelial P-selectin have previously been demonstrated to mediate adhesion of melanoma cells under flow in vitro (16 , 17) . Thus, P-selectin-mediated adhesive events between the microvasculature and tumor cells may provide critical activation signals that facilitate tumor cell survival and initial proliferation after extravasation (18) , whereas definite arrest of cancer cells is accomplished by blood flow pattern and mechanical entrapment (1) .
Several independent studies have indicated that tumor metastasis can be inhibited by the widely used anticoagulant heparin in different experimental models (19) . In addition to its functions as an anticoagulant drug, many biological effects of heparin and its derivatives have been elucidated over the past years that potentially account for their therapeutic actions in spontaneous and experimental metastasis. In particular, the effects of heparin in cancer have been attributed to interference with immune surveillance (20) , with the action of heparin- or heparan sulfate-bound angiogenic growth factors (21) , to obstruction of the activity of the extracellular matrix-degrading heparanase (22) , and to inhibition of cancer cell adhesion to platelets and vascular endothelium (19) . At the molecular level, strong evidence suggests that many of the established heparin effects are mediated by blockade of P-selectin binding (23 , 24) . This assumption has recently been substantiated in vivo by use of inflammatory and metastatic disease models (10 , 25) .
Here we show that transplantation of bone marrow (BM) from P-selectin-deficient into wild-type (wt) mice conveys inhibition of experimental lung metastasis. However, the extent to which BM-conferred lack of platelet P-selectin expression attenuated melanoma lung metastasis was significantly less than that seen in P-selectin-deficient mice. Because we demonstrate that a significant fraction of melanoma cells directly interact with postcapillary venules in a P-selectin-dependent manner, endothelial P-selectin expression may additionally contribute to formation of hematogenous metastases. Heparin is shown to inhibit both P-selectin-mediated melanoma cell rolling and lung melanoma metastasis formation in vivo, providing further evidence that endothelial P-selectin expression may represent an extra target of heparin action in experimental melanoma lung metastasis.
| MATERIALS AND METHODS |
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Cells and Culture Conditions.
The human melanoma cell lines NW624 and NW1539 were generated and cultured as described previously (26)
. The murine melanoma cell line B16F10, originally derived from spontaneous melanoma of C57BL/6 origin, was from American Type Culture Collection (Rockville, MD). B16F10 cells were cultured at 37°C and 5% CO2 in DMEM (Gibco Invitrogen Corp., Karlsruhe, Germany) supplemented with 10% fetal bovine serum (Gibco), 2 mM L-glutamine (Gibco), 100 units/ml penicillin, 100 µg/ml streptomycin, and 250 µg/ml amphotericin B (all from Sigma Chemicals, Deisenhofen, Germany). For intravital microscopy, cells were fluorescently labeled with 10 µM carboxyfluorescein diacetate, succinimidyl ester [CFDA-SE; Molecular Probes, Eugene, OR] or with 2.5 µM Vybrant DiI cell-labeling solution (Molecular Probes) for dual-color intravital microscopy according to the manufacturers instructions. For platelet isolation, blood was drawn from healthy volunteers through a 21-gauge needle. Blood was collected in 3-ml tubes containing sodium citrate (Sarstedt, Nümbrecht, Germany) and was centrifuged immediately at 110 x g for 15 min at 24°C to obtain platelet-rich plasma. Platelets were then separated from plasma proteins by gel filtration (27)
through a Sepharose 2B column (Pharmacia, Uppsala, Sweden) and activated by the thrombin receptor-activating peptide TRAP-5 at 2.5 mM (Bachem, Heidelberg, Germany). Human peripheral blood mononuclear cells were prepared by Ficoll-Paque PLUS density gradient sedimentation (Amersham Biosciences, Freiburg, Germany), stained with 2.5 µM Vybrant DiI cell-labeling solution (Molecular Probes), and adjusted to 3 x 107 cell/ml in AIM V cell culture medium (Gibco).
Experimental Lung Metastasis Assay.
To prepare B16F10 cells for inoculation, cells were harvested by brief exposure to a 0.25% trypsin0.02% EDTA solution (Sigma). Cell viability in single-cell suspensions was determined by trypan blue exclusion. A total of 3 x 105 B16F10 cells (>90% viability) in 100 µl of PBS were injected into the lateral tail vein of 812-week-old male syngeneic C57BL/6J mice (Charles River). As indicated, selected mice received commercially available unfractionated sodium heparin [12.5 or 60 IU (100 µl) in 0.9% sodium chloride; Liquemin N; Hoffmann-La Roche AG, Grenzach-Wyhlen, Germany] 30 min before inoculation or every second day starting 24 h after tail vein injection. Animals were sacrificed 11 days postinjection, and lungs were excised and fixed in phosphate-buffered 10% formaldehyde. Metastatic foci at lung surfaces were counted by two observers in a blinded fashion.
Murine BM Transplantation Model.
BM cells were obtained by flushing the femurs of donor P-selectin-deficient mice in C57BL/6J background (Charles River) as described previously (28)
. Recipient C57BL/6J mice received lethal irradiation doses at 46 weeks of age, and animals received i.v. injections containing 23 x 106 BM cells. BM cells from normal C57BL/6J mice were taken as controls. At 6 weeks post-transplantation, by which time the BM of recipient mice was reconstituted, melanoma cell inoculation was performed.
Analysis of Platelet P-Selectin Expression.
When mice were sacrificed at day 11 postinjection for analysis of lung metastases, blood was obtained by heart puncture and anticoagulated with sodium citrate (Sarstedt). Five µl of blood were immediately diluted in 50 µl of PBS containing calcium and magnesium. Platelets were activated with epinephrine (Aventis, Bad Soden, Germany) followed by staining with phycoerythrin-labeled anti-CD41 (clone MWReg30; BD Biosciences, Heidelberg, Germany) and FITC-labeled anti-CD62P-antibody (Ab; clone RB40.34; BD Biosciences) to identify platelets and P-selectin expression, respectively. Subsequently, cells were washed twice and analyzed by a BD FACScan Cytometer (Becton Dickinson, Mountain View, CA), equipped with CellQuest software for data acquisition and analysis (BD Biosciences). We collected 10,000 CD41+ cells for analysis of P-selectin expression.
Adherence of Platelets to Melanoma Cells.
The adherence assay was performed according to the description by Kim et al. (29)
. Platelets were isolated as detailed above and were subsequently fluorescently labeled with 10 µM CFDA-SE. Before seeding, B16F10 cells were stained with 2.5 µM Vybrant DiI for dual-color fluorescence microscopy; B16F10 cells were grown to 50% confluency in 6-well plates overnight, washed with PBS, and incubated with TRAP-5-activated CFDA-SE-labeled platelets (3 x 106 cells/well) in the presence of blocking P-selectin Ab (5 µg/ml; clone WAPS12.2; Pierce Biotechnology, Rockford, IL) or isotype-matched control Ab (5 µg/ml; rat IgG1, clone 43414; R&D Systems, Wiesbaden, Germany) or in the presence of unfractionated sodium heparin (5 IU/ml). After shaking for 15 min at room temperature, wells were washed twice with PBS. CFDA-SE-labeled platelets and DiI-stained B16F10 cells were visualized by fluorescence microscopy of corresponding representative fields.
Intravital Microscopy.
Intravital microscopy was performed as described previously (30
, 31)
. In brief, mice were anesthetized by i.p. injection of ketamine (Ketamin; Schwabe-Curamed, Karlsruhe, Germany) and xylazine (Rompun; Bayer AG, Leverkusen, Germany) and were placed on a homothermic blanket. The right common carotid artery was prepared microsurgically, and a catheter was inserted for retrograde injection of cells and heparin. The left ear of the mouse was gently placed between a microscope slide and a coverslip. Vascular architecture and fluorescently labeled cells were visualized during their passage through vessels under fluorescent epi-illumination by either a Zeiss-09 filter system (Zeiss, Oberkochen, Germany) for single-color intravital microscopy or a multiband filter system (XF 53; Omega Optical, Brattleboro, VT) for two-color intravital microscopy. Continuous video recordings of the microcirculation were obtained from a 1/3-inch DSP-3 charge-coupled device camera (DXC-390; Sony, Cologne, Germany) mounted on a modified (32)
Zeiss microscope (Axioteck Vario 100 HD; Zeiss) equipped with a x10 saltwater immersion objective (Nikon, Düsseldorf, Germany). Images were stored on tape by use of a Panasonic AG 7355 recorder. The velocities of cells in individual vessel segments were determined by off-line analysis of videotapes. Cells were considered noninteracting when they moved at the velocity of the mean blood flow (Vfree), whereas cells moving at lower velocities were defined as interacting or rolling, respectively. To calculate Vfree, we determined the mean velocity of at least 20 noninteracting cells for each vessel.
| RESULTS |
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| DISCUSSION |
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Earlier studies on hematogenous metastasis have indicated that circulating tumor cells generally arrest in the microcirculation of secondary organs and may extravasate with high efficiency (37) , regardless of the tumor type or the metastatic ability. Experimental in vivo observations and histological analyses revealed that the majority of cancer cells usually arrest in the first capillary bed because of size restrictions (38 , 39) . Despite efficient arrest and initial extravasation, however, significant differences are present relating to the subsequent propensity of cancer cells to survive extravasation and to proliferate to form metastatic colonies (1) . We therefore hypothesized that endothelial P-selectin-mediated contacts to cancer cells may provide critical activation signals that potentially facilitate tumor cell survival and initial proliferation after extravasation. This assumption is supported by recent experimental data showing P-selectin-dependent adhesion of malignant melanoma cells in vitro (16 , 17) . Although postnatal vasculogenesis is less likely to be involved in the initial stages of metastatic growth, integration of P-selectin-deficient endothelial progenitors may have additionally contributed to P-selectin-dependent attenuation of lung metastasis in our BM transplantation studies.
We assessed probable adhesion pathways under shear flow conditions in vivo by use of intravital microscopy and demonstrated direct interactions between melanoma cells and murine ear postcapillary venules (Fig. 3)
. A highly reproducible proportion of different melanoma cell lines showed spontaneous P-selectin-dependent rolling in skin microvessels in vivo. As anticipated, the adhesive interactions were sensitive to heparin, which is known to act as a potent P-selectin inhibitor (23
, 24)
. On the other hand, diminished rolling in P-selectin-deficient mice was not further reduced by heparin treatment (Fig. 3B)
, supporting the assumption that tumor cells make contact with murine ear postcapillary venules primarily in a P-selectin-dependent fashion. These observations are in line with results from our experimental metastasis assays, in which administration of heparin attenuated melanoma lung metastasis to levels comparable to those seen in P-selectin-deficient mice (Figs. 1
and 5)
. Similar findings were recently reported in an experimental lung metastasis model involving colon adenocarcinoma cell lines (10
, 13)
, suggesting a more general significance of P-selectin in hematogenous metastasis. Because eventual arrest of tumor cells in secondary organs seems to be accomplished primarily by mechanical factors (1)
, P-selectin-dependent contacts with target cells before entrapment may mediate intercellular signaling events. In addition to its anchoring functions, P-selectin and its ligands have been shown to act as important signal transduction receptors (18)
. In the context of leukocyte extravasation, P-selectin-dependent binding of monocytes or neutrophils was demonstrated to trigger signaling that resulted in changes in cytokine expression or the integrin activation state (40
, 41)
. Pertinent to these findings, adhesive interactions of melanoma cells with endothelial P-selectin could likely initiate signal transduction pathways that convey changes in gene expression and cytoskeletal arrangement, thereby affecting cellular survival and proliferation of single tumor cells during extravasation (37)
.
Whereas the inhibitory effects of heparin on experimentally induced metastasis have been repeatedly reported in distinct models over past years, the cellular and molecular mechanisms by which heparin mediates its actions have only recently been elucidated in greater detail (7
, 19)
. Different heparin preparations were shown to act as ligands for P-selectin, thereby blocking binding of P-selectin to its natural and tumor mucin ligands (23
, 24)
. Importantly, both tumor-cellplatelet aggregation and experimental metastasis of human carcinoma cells were seen to be significantly inhibited in P-selectin-deficient mice (29)
. In an equivalent fashion, we found that lung colonization of syngeneic B16 melanoma cells was markedly attenuated in P-selectin-deficient mice compared with wt controls (Fig. 1)
. Because L-selectin expression by leukocytes may also facilitate metastatic spread in vivo (10)
, the inhibitory effects of heparin on L-selectin binding may additionally convey the antimetastatic effects observed in our studies. However, both experimental lung metastasis formation (Fig. 5)
and melanoma cell rolling (Fig. 3)
are attenuated by heparin treatment to an extent comparable to that seen in P-selectin-deficient mice (Fig. 1)
, suggesting that the heparin effects detected in our experimental models are mediated primarily by inhibition of P-selectin-dependent binding. This assumption is also supported by recent studies on colon carcinoma cell metastasis that indicated that the effect of a single heparin dose is accounted for by blockade of P-selectin function only (10)
. Heparin not only impaired P-selectin-dependent platelettumor cell interactions but also suppressed blood-borne metastasis in secondary organs, whereas in P-selectin-deficient mice no further inhibition by heparin treatment was seen (10
, 13)
. Therefore, substantial evidence supports the concept that heparin mediates its antimetastatic effects to a great extent via inhibition of P-selectin-dependent interactions.
Activated platelets have been shown previously to facilitate leukocyteendothelial cell interactions in a P-selectin-dependent fashion, providing an additional mechanism for leukocyte delivery to the microvasculature (42
, 43)
. Evidence has been provided that leukocyteplatelet interactions may augment both direct adhesion of blood mononuclear cells to the endothelium (44)
and indirect leukocyte adhesion via bridging platelets (42)
. Pertinent to these findings, we have recently observed that activated platelets can form aggregates with leukocytes in vivo, resulting in increased rolling of leukocytes along vascular endothelium of the murine ear in a P-selectin-dependent fashion (33)
. We hypothesized that melanomaendothelial cell interactions would be enhanced in the presence of activated platelets in a comparable manner. Use of a two-color intravital microscopy that allows for discrimination between melanoma cells and platelets in vivo revealed neither considerable platelettumor cell interactions nor increased melanoma cell rolling in postcapillary venules of the murine ear (Fig. 4)
. Therefore, in contrast to platelet-mediated leukocyte delivery to skin endothelium, melanoma cell adhesive interactions with endothelium may not to be assisted by activated platelets.
Strikingly, a single dose of unfractionated heparin resulted in significant reduction of experimental blood-borne metastasis (13
, 34)
. Previous studies have also indicated that both unfractionated and different fractionated heparin preparations may exert antimetastatic effects in a dose-dependent fashion (34)
. However, the extent to which heparin may additionally affect formation and growth of lung colonies after tumor challenge has yet to be resolved. Sciumbata et al. (34)
reported a slight but insignificant prolongation of survival when heparin treatment was maintained on a twice/week cycle for 3 weeks after i.v. injection of B16 melanoma cells. The results from our heparin treatment studies clearly suggest that initiation of heparin treatment 24 h after tumor challenge has no effect on blood-borne metastasis (Fig. 4)
. Similarly, continued injection of heparin every second day after melanoma cell inoculation failed to produce additional or synergistic antimetastatic effects compared with mice treated before tumor challenge only. Thus, heparin seems to primarily interfere with the establishment of metastases rather than the initiation or maintenance of metastatic tumor growth.
Heparin comprises a complex set of glycosaminoglycan molecules that, in addition to the established anticoagulant effects, have been shown to modulate numerous cellular functions (19) . Because inhibition of P-selectin represents a key mechanism by which heparin mediates its antimetastatic as well as its anti-inflammatory properties, specific oligosaccharide structures in heparin have been identified recently that may specifically interact with P-selectin (25) . These studies indicated that inhibitory properties depend on 6-O-sulfated glucosamine residues, which are distinct from the binding site for antithrombin that confers the potent anticoagulant activity of heparin (45) . Hence, chemically modified heparins are conceivable that exert diminished anticoagulant activity but retain antimetastatic properties. Alternatively, noncarbohydrate imidazole-based selectin inhibitors, which have been identified by use of a P-selectin ELISA-based assay system, may characterize an attractive class of compounds with potential therapeutic effects on metastasis formation (46) . In addition, we have recently shown that efomycines, comprising a novel family of small molecules with inhibitory effects on selectin-dependent cell binding, exhibit considerable efficacy in different disease models of cutaneous inflammation (30) . Hence, it will be of utmost interest to investigate whether these compounds will actually interfere with P-selectin-dependent tumor cell rolling and metastasis formation in vivo.
We here provide evidence that endothelial P-selectin expression may contribute to the formation of hematogenous metastases. This assumption is supported by transplantation studies with BM from P-selectin-deficient mice, revealing that P-selectin-dependent attenuation of lung metastasis in deficient mice can be explained only in part by the lack of platelet P-selectin expression. In addition, we demonstrated that melanoma cells directly interact with postcapillary venules in a P-selectin-dependent manner, an interaction that can be fully abrogated by heparin treatment. These results thus indicate that endothelial P-selectin expression may represent an additional target of heparin action in experimental melanoma lung metastasis.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Requests for reprints: Jens Gille, Zentrum der Dermatologie, Klinikum der J. W. Goethe-Universität, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main, Germany. Phone: 49 69 63015418; Fax: 49 69 63017948; E-mail: Gille{at}em.uni-frankfurt.de
Received 4/16/03. Revised 12/30/03. Accepted 2/16/04.
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