Non-tissue factor procoagulants in cancer cells

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

Generation of a monoclonal antibody that inhibits the procoagulant activity of various cancer cell lines

BACKGROUND

Tumor procoagulant is one of the factors responsible for disseminated intravascular coagulation and metastasis. The authors found procoagulant activity in LK52 human squamous cell carcinoma cells, which they designated cancer cell-derived blood coagulating activity 1 (CCA-1). A monoclonal antibody (MoAb) was generated to characterize this CCA-1 procoagulant activity. To date, antibodies that show an inhibitory effect on procoagulant activity as well as high reactivity in cancer cells are well known for their tissue factor specificity.

METHODS

Characterization of the procoagulant activity of CCA-1 was performed and an anti-CCA-1 MoAb, FS01, was generated. CCA-1 expression on the cancer cell surface was examined by flow cytometry. Procoagulant activity of various cancer cell lines and the inhibitory effect of the FS01 MoAb on this procoagulant activity was monitored by a clot timer.

RESULTS

The enzymologic character differed from that of cancer procoagulant (CP). The FS01 MoAb inhibited the procoagulant activity of CCA-1, but did not inhibit that of tissue factor. A positive correlation was observed between the expression intensity of CCA-1 and the inhibitory effect of the FS01 MoAb on the procoagulant activity of cancer cell lines. Expression of CCA-1 was observed more frequently than that of tissue factor in human cancer cell lines.

CONCLUSIONS

The FS01 MoAb generated in the current study is a new antibody that reacts with various cancer cell lines, but not with normal cells. FS01 inhibits cancer cell-derived procoagulant activity and does not react with tissue factor and CP. CCA-1, which is recognized by the FS01 MoAb, appears to play a major role in cancer cell-derived procoagulant activity.

Modulation of human endothelial cell procoagulant activity in tumour models in vitro

Several tumour-derived factors have recently been identified which induce tissue factor (TF) expression in endothelial cells in vitro. However, there is only limited evidence that endothelial cells lining tumour blood vessels express elevated procoagulant activity (PCA) in vivo. We have investigated the effects of human breast and small cell lung cancer cell lines on the PCA of human micro- and macrovessel endothelial cell monolayers using a one-stage clotting assay, as well as detection of TF mRNA by RT-PCR. Only conditioned medium from the MDA-MB-231 breast adenocarcinoma cell line produced a consistent although transient increase in endothelial cell surface PCA, which was maximal by 6-9 hr. TF mRNA was detectable in the endothelial cells after 1 hr incubation with MDA-MB-231-conditioned medium and subsequently fell below detectable levels. Following 24 hr stimulation, nearly half the endothelial cell PCA was due to the presence of TF-containing membrane vesicles shed by the MDA-MB-231 cells. Consistent with these findings, the MDA-MB-231 cell line expressed high levels of cell surface-associated TF activity. Co-culture of MDA-MB-231 and endothelial cells for up to 5 days increased (approx. 118-fold) PCA associated with endothelial cell monolayers, due mainly to sequestration of shed tumour cell vesicles. Our results suggest that induction of TF de novo is not a common feature in the supporting endothelium of these tumour types.

Thrombin increases the metastatic potential of tumor cells

Initial arrest of tumor cells in the microvasculature and their attachment to the endothelium and subendothelial matrix (SEM) are essential prerequisites for metastasis to occur. Factors mediating these interactions are viewed as important determinants of the tumor-cell metastatic phenotype. In this work we have studied the effects of thrombin, its analogs and its precursors on the adhesive properties and metastatic potential of tumor cells. We show that alpha-thrombin, the native form of the key coagulation enzyme, is capable of enhancing tumor-cell adhesion to both the endothelium and SEM components represented by fibronectin. Subclotting, physiological concentrations of alpha-thrombin produced a 2- to 5-fold increase in tumor-cell adhesion. A bell-shaped dose-response curve was observed, with maximal effect at 0.1 U/ml. Maximum effect occurred when cells were exposed to the agonist for 15 min and exposure for up to 4 hr resulted in enhanced tumor-cell adhesion. Prolonged incubation with thrombin resulted in a decline in the thrombin-enhanced adhesion which reached unstimulated control levels by 24 hr. Thrombin precursors and active-site-inhibited thrombin analogs only had minimal adhesion-enhancing activity; nitro- and exosite-alpha-thrombin, which retain a functional active site, mimicked, although to a lesser degree, the action of alpha-thrombin. Tumor-cell incubation with thrombin resulted in an upregulated cell-surface expression of the alpha11b beta 3 integrin, a receptor mediating interactions between tumor cells and endothelial cells, and between tumor cells and SEM. Antibodies against alpha 11b beta 3 integrin effectively inhibited thrombin-enhanced tumor-cell adhesion. Thrombin effects on tumor cells involved the PKC signal transduction pathway as thrombin-enhanced adhesion was inhibited by pre-incubation with PKC inhibitors and a transient PKC translocation from cytosol to membrane was observed following thrombin challenge. In vivo, thrombin-treated tumor cells demonstrated a 2-fold increase in their lung-colonizing ability. In contrast to the adhesion results, the metastasis-enhancing effects of alpha-thrombin were mimicked by a thrombin precursor (prothrombin) and thrombin analogs.

Tissue factor as a tumor procoagulant

Tissue factor is a cell surface glycoprotein responsible for initiating the extrinsic pathway of coagulation. Many tumor cell homogenates and intact tumor cells have been shown to contain tissue factor activity. Immunohistochemical studies show that many tumors associated with Trousseau's syndrome express tissue factor on their cell surfaces. Tumor cells shed membrane fragments which carry tissue factor that can account for the activation of the clotting system. Tumor cells also produce soluble substances that can induce tissue factor expression on host cells, such as endothelium and monocytes, at sites distant from the tumor. Although, all the functional TF molecules are localized on the outer cell membrane in many tumor cells, the procoagulant activity on the intact cell surface is largely dormant and can be greatly enhanced upon cell injury or damage. Tissue factor procoagulant activity on the cell surface can be modulated by alterations in the plasma membrane without loss of cell viability. Tissue factor activity on cell surfaces is largely regulated by a plasma inhibitor, tissue factor pathway inhibitor. This inhibitor binds to both functional and non-functional tissue factor/VIIa complexes on the cell surface and prevents non-functional tissue factor/VIIa complexes from becoming functional after cell injury or lysis. Heparin, but not warfarin, therapy is effective in preventing the occurrence of devastating thrombotic events in patients with Trousseau's syndrome and the reason(s) for this are still unknown.