Tissue Factor and Cancer

The notion that clotting proteins can serve more than one function (i.e., beyond their involvement in the production of a fibrin clot) is not new and is consistent with a broad view of biology that acknowledges the importance of redundant systems and overlapping functions for proteins to accomplish a variety of physiologic tasks. Indeed, thrombin, the ultimate clotting protease, is well-known as a mitogen for many cell types, acting as a growth-promoting stimulus for, among others, smooth muscle cells, endothelial cells, and, most recently, tumor cells.[1] In this issue of Seminars in Thrombosis and Hemostasis, we are pleased to present a current view of the ubiquitous, transmembrane clotting protein tissue factor (TF) as a critical player in tumor biology; not just by virtue of its well-known properties as the receptor for factor VIIa–mediated triggering of both the intrinsic and extrinsic clotting cascades, but also as a signaling protein for the regulation of tumor cell movement, angiogenesis, and metastasis.

The intracellular signaling function of TF was first described by Hans Prydz's group nearly 15 years ago, when they demonstrated that binding of factor VIIa to TF induced movement of intracellular, submembrane calcium pools in a variety of cell types, including human tumor cells.[2] [3] Soon thereafter, the ability of TF to mediate other biological functions requiring intracellular signaling captured the fancy of many investigators. A role for TF in stimulating the increased transcription of vascular endothelial growth factor (VEGF) was reported,[4] and, subsequently, the compelling story of TF interacting with protease-activated receptors (PARs) began to unfold.[5] [6] And so, TF has come into its own as a full-fledged signaling receptor, much like the tyrosine kinase molecules. Even without an ability to act enzymatically, it would appear that induction of TF gene expression in tumor cells is capable of subsequently mediating a variety of growth signals of importance.

Schaffner and Ruf open our volume with a review of “Tissue Factor and Protease-Activated Receptor Signaling in Cancer.” They discuss signaling via the cleavage and activation of the PAR-2 receptor, normally regulated by factor VIIa binding but constitutive in malignant cells, involving a “noncoagulant” form of TF, which is β1 integrin–dependent. The PAR-1 receptor, which is upregulated in most human cancers, is activated by the TF:VIIa:Xa complex and, therefore, linked to the procoagulant function of TF. These authors have reported recently the utility of monoclonal antibodies for selective inhibition, respectively, of the procoagulant or signaling functions of TF in the dissection of the relative importance in cancer (and inflammation) of these “two faces of the TF coin.”[7]

Palumbo, in his article “Mechanisms Linking Tumor Cell–Associated Procoagulant Function to Tumor Dissemination,” also discusses TF-mediated signaling. He reviews the evidence that “… the role of TF in tumor growth is quite context dependent … .” emphasizing the importance of both procoagulant active and procoagulant inactive, or “encrypted,” TF for various aspects of the natural history of tumor development. Palumbo and colleagues have provided important evidence for the interaction of multiple components of the hemostatic pathway in the support of early steps in the micrometastatic process—with an emphasis in the current review on recent evidence that hemostatic factors (platelets, fibrinogen, etc.) limit natural killer (NK) cell function.[8]

In the third contribution to this issue, Signaevsky and colleagues describe the “Role of Alternatively Spliced Tissue Factor in Pancreatic Cancer Growth and Angiogenesis.” Alternatively spliced human TF (asHTF), which results from alternative splicing that eliminates exon 5 and produces frameshifting in exon 6 of the TF gene, lacks the transmembrane and cytoplasmic domains and has a unique COOH-terminal domain. The resulting protein, asHTF, is therefore soluble and circulates in human blood but without apparent native procoagulant activity under physiologic conditions. This group from the Soff laboratory cite the presence of a Lys-Lys doublet in the 41-amino-acid peptide produced at the new C-terminus of asHTF as support for exploring additional biological functions of asHTF, and they describe a series of experiments in which the expression of asHTF is documented in the majority of pancreatic cancer cell lines and in specimens obtained from patients. Transfection of asHTF cDNA into nonproducers resulted in cells without procoagulant activity but tumors that grew more aggressively as xenografts and demonstrated nearly threefold increased angiogenesis—thus the title of their contribution.

Milsom et al provide compelling evidence for the existence of subsets of TF-producing cells within the tumor itself and in the surrounding host-derived milieu in their contribution “Diverse Roles of Tissue Factor–Expressing Cell Subsets in Tumor Progression.” The work from the Rak laboratory includes compelling evidence for oncogenic upregulation of TF in human cancer cells[9]; new studies described here implicate both host cells and tumor cells in the “formation of a provisional (TF-dependent) cancer stem cell niche.” These authors further our understanding as well of the regulation of the formation of TF-containing microvesicles, so-called microparticles (MPs), as a surrogate marker of cancer growth and progression, a theme taken up by several of our other contributors. Although the existence of cancer stem cells remains somewhat controversial, the Rak group reviews their data demonstrating that TF can be preferentially localized to a CD133-positive subset of human squamous cell carcinoma cells. Further work from this group will be followed with interest.

Pawlinski and Mackman describe the “Use of Mouse Models to Study the Role of Tissue Factor in Tumor Biology,” turning our attention to the remarkable advantages that genetically well-defined animal models provide for attacking complex biological questions. The authors have developed several mouse models with altered TF expression and, in collaboration with the Rak group, used these mice to ask important questions regarding the relative contribution of tumor cell TF versus host cell TF to tumor development, growth, metastasis, angiogenesis, and MP generation. In the current review, the authors catalogue these important models and provide guidelines for future use of conditional mutants to investigate the role of TF derived from different host cells in tumor biology.

Nadir and colleagues, in their contribution “Heparanase, Tissue Factor, and Cancer,” provide a link between the overexpression by cancer cells of heparanase, an endo-β-D-glucuronidase that cleaves (among other substrates) heparin sulfate (HS) in the extracellular matrix, and the induction of both TF and tissue factor pathway inhibitor (TFPI) in tumor cells. The authors demonstrate that either induction of endogenous expression by transfection of the heparanase gene or addition of exogenous heparanase recapitulated their findings of increased expression of TF and TFPI in a variety of human tumors. The TFPI produced, however, was inactive, likely due to its interaction with heparanase—further contributing to the hypercoagulable state characteristic of cancer and allowing TF to proceed unopposed in the tumor milieu. These novel experiments from the laboratories of Vlodavsky and Brenner provide evidence for yet another integrated pathway of importance at the cell level (believed to be mediated by nonenzymatic induction of p38 mitogen-activated protein kinase by heparanase), whereby TF is upregulated in malignant cells and can increase the likelihood of tumor growth, angiogenesis, and metastasis.

Zwicker in his review “Tissue Factor–Bearing Microparticles and Cancer” details the possible pathophysiologic role(s) of blood-borne TF MPs in cancer and the development in the Furie laboratory of a method for accurate sizing and quantification of the MPs using impedance-based flow cytometry. The authors claim an increased sensitivity of 10,000-fold over traditional light scatter flow cytometry. A case-controlled study is being conducted by the group to determine if the level of TF-bearing MPs is associated with acute venous thromboembolism in cancer patients.

Lechner and Weltermann in their contribution “Chemotherapy-Induced Thrombosis: A Role for Microparticles and Tissue Factor?” review various mechanisms that have been implicated in the pathogenesis of chemotherapy-related thromboembolism in cancer patients. Ultimately, the authors support the hypothesis that many chemotherapeutic agents increase vascular endothelial cell procoagulant MP generation. However, they were not able to demonstrate that this process resulted in increased TF expression in the MPs. The lack of well-designed, controlled clinical studies hampered their ability to extrapolate from their in vitro data to the bedside. Perhaps the case-controlled study described by Zwicker will fill this gap.

Finally, Falanga et al complete this issue by reviewing “Hypercoagulability and Tissue Factor Gene Upregulation in Hematologic Malignancies.” These authors shift the focus a bit from solid tumors and remind us that (1) thromboembolic complications are just as frequent in patients with hematologic malignancies (lymphoma and multiple myeloma, in particular); (2) molecular abnormalities in acute leukemias and in patients with polycythemia vera (PV) and essential thrombocythemia (ET) also upregulate TF; and (3) chemotherapy increases the risk for thromboembolism in many of these patients. Both the t_15–17 translocation in acute promyelocytic leukemic cells and the JAK2 mutation in PV and ET hematopoietic stem cells result in increased expression of TF, which may play an important role in promoting thromboembolic complications in these patients.

It is our hope that the articles contained in this issue will provide the reader with a good overview of the current status of this exciting field and stimulate additional work on the role of TF as a multifunctional receptor in tumor biology, perhaps worthy of targeting for therapy of cancer as have been the tyrosine kinases.

REFERENCES

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