Tumor cells contain various membrane receptors which can bind directly to platelets and mediate tumor cell-platelet binding and activate platelets (). Flow cytometry, fluorescence microscopy, and intravital microscopy have revealed the presence of platelet-tumor cell aggregates in vitro
and in vivo
Interaction of platelets, coagulation and tumor cells. Cartoon representation showing some of the molecules implicated for tumor cells and platelets to promote interaction and influence tumor cell growth and survival.
P-selectin is an adhesion receptor found in the α-granules of platelets and Weibel-Palade bodies of endothelial cells.13
After platelet activation P-selectin appears on the platelet surface and aids the recruitment of other circulating platelets and leukocytes. Chondroitin sulfate glycosaminoglycans on the surface of human MDA-MET cells and murine 4T1 cells have been shown to bind selectively P-selectin.14
It has also been suggested that platelet P-selectin recognizes sulfated galactosylceramide SM2, SM3, and SM4 on MC-38 cells and sulfatide removal results in inhibition of in vitro
platelet P-selectin binding to MC-38 cells and reduced syngenic experimental metastasis in vivo.15
Experimental metastasis and subcutaneously implanted tumor growth was reported to be reduced in P-selectin deficient mice and in an immunocompetent model with MC-38 colon carcinoma cells and B16 melanoma cells.16,17
Not only is the rate of tumor cell homing to lungs diminished in P-selectin deficient mice, but tumor cells fail to make aggregates with platelets resulting in a decreased number of metastatic nodules in the lungs of P-selectin deficient mice.16
A selectin ligand mimicry peptide, IELLQAR, has been found to have an inhibitory effect on B16 induced experimental metastasis.18
An inhibitor of sialyl Lewis X, such as AcGnG-NM, not only reduces binding of tumor cells to selectin coated surfaces, activated platelets, and TNF-α activated endothelial cells, but also diminishes experimental metastasis in SCID mice.19
Heparin inhibits the binding of P-selectin to its receptors and has been shown to inhibit experimental metastasis in syngenic mouse models.20
Human platelets also express MAPK p38α, aserine threonine kinase, and the expression of MAPK p38α is directly linked to platelet P-selectin expression.Mice lacking MAPK p38α are not viable, but heterozygousp38α+/- mice have reduced experimental metastasis with no effect on primary tumor growth.21
More recently, a role for P-selectin using models of spontaneous tumor metastasis has been presented.22
Thus, through a wide array of studies it can be concluded P-selectin facilitates direct binding to tumor cells and augments tumor metastasis.
The platelet receptor, glycoprotein (GP) Ib-IX supports adhesion of platelets on a compromised vascular wall and, as such, is a key initiator of the platelet paradigm in hemostasis.23
We reported that B16F10 mouse melanoma cell metastasis was reduced 15-fold in GP Ib-IX deficient mouse colonies suggesting an important role for adhesion in a syngenic mouse model.24
However, overexpression of the polyoma middle T antigen in mouse mammary tissue and lung metastasis were not affected by the absence of platelet GP Ib-IX in a model of spontaneous tumor formation (Jain and Ware, unpublished observation).Confounding results have been described with the administration of the anti-GP Ib-IX antibodies and the opposite effect, namely increased colonization of the lung.25
The genetic absence of the platelet collagen receptor, GP VI, is also associated with a 50% reduction in experimental metastasis.26
However, primary tumor growth and angiogenesis was not altered in GP VI deficient mice.26
Although GP Ib-IX is widely considered to be a platelet-specific complex, several studies have suggested the expression of GP Ib-IX subunits by a variety of tumor cells.27–29
In examining the expression of the major subunit of the GP Ib-IX complex, the α-subunit of GP Ib, in lysates of commonly used human tumor cell lines we have been unable to document the presence of GP Ibα () At this time we conclude the expression of GP Ib-IX by cancer cells is not a common mechanism contributing to tumor formation or metastasis.
Figure 3 Western blot analysis of human tumor cell lines for the presence of human GP Ibα antigen. Also shown are normal mouse and human platelet lysates for representative signals. The blot was reacted with an anti-α-tubulin antibody as a positive (more ...)
Von Willebrand factor (VWF) is a key major ligand for the platelet GP Ib-IX complex. Lewis lung carcinoma and B16-B6 mediated experimental metastasis was increased 2-fold and 5-fold, respectively, in VWF deficient mice.30
Lung colonization of tumor cells was increased 1–4 hr post injection tumor cells in VWF deficient mice suggesting VWF may be responsible for tumor cell clearance in the circulation. VWF deficiency did not have any effect on the growth of primary tumors. However, it has also been reported that an anti-VWF antibody protects mice from experimental metastasis in mouse models.31
It is possible that in the absence of VWF, platelet GP Ib-IX availability is increased resulting in increased experimental metastasis. More definitive experimental proof is required to test this possibility.
Integrin αIIbβ3 (GP IIb-IIIa) is the most abundant receptor on the platelet surface.It participates in hemostasis by bridging platelet/platelet interactions via the ligand, fibrinogen.32
GP IIb-IIIa inhibition by the monoclonal antibody 10E5 has been reported to diminish binding of CT26 and HCT28 cells to platelets in vitro.31
Integrin β3−/ − mice show a reduction in B16F10 melanoma induced osteolytic experimental metastasis and reduced osteolytic bone invasion, both reversed by bone marrow transplantation of β3+/+ marrow.33
Antibody inhibition of GP IIb-IIIa diminishes tumor cell adhesion on extracellular matrix under flow conditions suggesting a role for GP IIb-IIIa in platelet–tumor cell emboli extravasation.34
) a mouse-human chimeric antibody for GP IIb-IIIa has anti-angiogenic and anti-tumor properties in mouse models.35
A single treatment of this antibody in a xenograft model of SCID mice reduces experimental metastasis significantly. In addition, c7E3 also inhibits VEGF secretion from platelets in the presence of tumor cells.36
Taken together these studies suggest the major platelet integrin receptor plays a significant role in tumorigenesis at several different mechanistic levels.
The major ligand of GP IIb-IIIa, fibrinogen, is also implicated in metastasis. As a central ligand supporting platelet/platelet interactions and as a key cleavage substrate for thrombin in coagulation, fibrinogen is essential in the well characterized paradigm of hemostasis andthrombosis.37
In the realm of tumor biology, fibrinogen supports the formation of platelet-fibrinogen-tumor cell emboli as tumor cells intravasate.38
Local deposition of fibrin and fibrin products have been found in solid tumors39
and reported to support angiogenesis.40
Experimental metastasis, spontaneous hematogenous metastasis and lymphatic metastasis are significantly diminished in fibrinogen knockout mice.3,41,42
Soluble fibrin monomer infusion facilitates platelet-tumor cell adhesion in vitro
and experimental metastasis in vivo.43
Thus, fibrinogen and fibrin participate in a variety of pathways contributing to tumor cell survival and growth.