Hemostasis and Angiogenesis in Malignancy

The association between cancer and thromboembolic diseases was first suggested by Armand Trousseau more than 130 years ago. Although initial progress was slow, it is now well recognized that most cancer patients are in a hypercoagulable state that may present with various clinical symptoms. It also has been long known that cancer growth and metastasis are intimately linked to the new growth of blood vessels (angiogenesis), but the close linkage between hemostasis and angiogenesis has only recently been appreciated. Both hemostasis and angiogenesis depend on a balance between a large number of stimulatory and inhibiting mechanisms. Under physiological conditions the inhibitor pathways appear to prevail. Under pathological conditions, however, especially in cancer, the systems become activated. This issue of Seminars in Thrombosis and Hemostasis reviews the present understanding of the interrelationship between the two systems and how they complement each other to foster tumor development, tumor growth, and metastasis.

In the first article, Wojtukiewicz and colleagues give a comprehensive overview of the two systems in health and disease and delineate which components of the extremely complex systems interact with each other. The release of angiogenesis-promoting factors and down-regulation of angiogenesis-inhibiting factors play a key role in this process. Endothelial cells undergo profound changes during angiogenesis, leading to increased permeability that allows proangiogenic stimuli to enter tissues and thus foster the formation of new blood vessels needed for malignant tumors to grow. Tissue factor released at higher levels in cancer patients not only activates the hemostasis system, but also promotes angiogenesis by multiple means, clotting-dependent or -independent. Thrombin and fibrinogen/fibrin are additional major players in this process, and a wealth of information has been gathered over the last few years that has unraveled many of the interrelated pathways. Although the entire process appears to be extremely complex, the authors have summarized the key events in a comprehensive and understandable fashion. The articles that follow provide a greater insight into the events associated with angiogenesis.

Yu and coworkers review the role of oncogenes as regulators of tissue factor (TF) expression. TF not only initiates the activation of the clotting system, but also serves as a mediator of intracellular signaling events that alter gene expression patterns and cell behavior. TF is intimately involved in tumor growth, angiogenesis and metastasis. In some cancer patients high levels of TF indicate poor prognosis. The authors describe their own work on the down-regulation of TF by oncogene-targeting agents. Such down-regulation leads to decreased expression of epidermal growth factor receptor and thus impacts angiogenesis. Targeting TF expression in cancer patients should be considered in conjunction with other therapeutic modalities.

In the third article, Fernandez and associates describe the role of TF and fibrin in tumor angiogenesis. The link between expression of TF and thromboembolism in certain cancer patients is reiterated, and the close relationship between TF and vascular endothelial growth factor (VEGF) and thus angiogenesis is discussed. There are various pathways, clotting-dependent and -independent, by which TF affects angiogenesis in tumors and thereby facilitates tumor growth and metastasis. TF appears to be a key player in these events. Also fibrin, fibrinogen, and their degradation products seem to be involved in this process. Fibrin matrices serve as binding sites for a large number of cellular elements via both integrin-mediated and non-integrin-mediated cell surface receptors. The organizational structure of fibrin also plays an important role. In addition, the functions of degradation products of fibrin and fibrinogen are extensively reviewed.

Colman delineates the role of the contact system of hemostasis in angiogenesis. For new blood vessels to develop, endothelial cells have to detach, the basement membrane has to be degraded, endothelial cells have to migrate and proliferate, and tubes have to form. The factors of the contact system are intimately involved in these processes either directly or via the activation of the fibrinolytic system with the ultimate generation of plasmin. High-molecular-weight kininogen (HK) displaces adhesive proteins and thus inhibits cell adhesion. The enzymatic degradation of the basement membrane is facilitated by plasmin that is generated by HK-mediated profibrinolytic activity, that is, the activation of prourokinase. These complex mechanisms are expertly reviewed and potential therapeutic pathways suggested.

Next, Tsopanoglou and Maragoudakis review the role of thrombin in angiogenesis. Cancer patients are in a hypercoagulable state, indicating a steadily increased generation of thrombin. The authors present evidence that thrombin activates angiogenesis by a process that is independent of fibrin formation and independent of the “active site” of the thrombin molecule. Thrombin exerts several functions in this event: it decreases the ability of endothelial cells to attach to basement membrane proteins; it potentiates VEGF-induced endothelial cell proliferation; it increases the mRNA and protein levels of certain integrins; and it increases VEGF secretion. These findings not only explain the angiogenic and tumor-promoting effects of thrombin, but also present the possibility of using antagonists of thrombin or its receptors as additional treatment modalities for cancer.

In the sixth article Engelse and coworkers describe the effects of the fibrinolytic system and matrix metalloproteinases on angiogenesis. For tumor angiogenesis to occur, the extracellular matrix that surrounds capillary sprouts and migrating tumor cells has to be proteolytically degraded. In this process, enzymes of the fibrinolytic system and members of the matrix metalloproteinases play a major role. The key enzyme of the fibrinolytic system is plasmin that is, during angiogenesis, generated from its precursor by urokinase-type plasminogen activator (u-PA). u-PA has to bind to a specific receptor that is present on many cell surfaces. Plasmin can then proteolytically degrade cell matrix proteins. In addition, the u-PA/plasmin system can directly activate growth factors. Similar proteolytic activities are generated from matrix metalloproteinases that belong to the family of Ca-/Zn-dependent endopeptidases. The two systems cooperate with each other. These systems are regulated by several antiangiogenic components, and an understanding of these interrelationships is important for future protease-based therapies.

In the next article Cao and Xue review angiostatin, an inhibitor of angiogenesis. It is absent in healthy tissues but is produced by tumors. Angiostatin is an internal proteolytic fragment of plasminogen and is related to the kringle structures found in this protein. Proteolysis is thus a prerequisite for its generation and a precondition for angiogenesis inhibition. Since several other proteins contain identical kringles as plasminogen, their proteolysis also forms inhibitors of angiogenesis. The structure of these kringles, which is critical to their function, is extensively reviewed by the authors. Tumors generate angiostatin by producing proteolytic enzymes that cleave plasminogen. The potential clinical use of angiostatin for the treatment of cancer has generated considerable interest, and numerous angiogenesis-inhibiting compounds are in various stages of testing. The authors comprehensively review the status of these trials.

Sierko and Wojtukiewicz next discuss the role of platelets in angiogenesis in cancer. After reviewing the structure and function of normal platelets, they elucidate the relationship between platelets and vascular growth. Megakaryocytes contain cytokines, VEGF and basic fibroblast growth factor (bFGF) that increase thrombopoiesis and likely explain the thrombocytosis frequently encountered in cancer patients. VEGF is also stored in platelets, thus increasing the potential for angiogenesis at sites where platelets adhere. Adhesion molecules in platelets allow them to adhere to tumor cells. VEGF derived from platelets predicts prognosis in cancer patients. VEGF also increases fenestration of endothelial cells so that many substances, including clotting-related proteins, can reach the extravascular spaces. However, platelets do not only contain pro-angiogenic substances, they also incorporate inhibitors, most notably thrombospondin.

Tang and Conti present an update on endothelial cell development, angiogenesis, and tumor neovascularization. They provide information not only on cell development, but also on the regulation of the development and on senescence. Starting with a review of the present understanding of embryonal vasculogenesis and angiogenesis, the authors discuss these mechanisms in tumor development and progression and also outline therapeutic potentials for blocking tumor neovascularization.

In the tenth article Nie and Honn review the regulation of angiogenesis in tumors by eicosonoids. Their potential role in this process was initially suspected when it was found that the use of nonsteroidal anti-inflammatory drugs inhibited angiogenesis and tumor growth. It was also observed that levels of cyclooxygenase-2 and 12-lipoxygenase were elevated in cancer patients. Subsequently it was determined that both compounds, when overexpressed in cancer cells, enhance angiogenesis and stimulate tumor growth. The authors review the mechanisms involved and discuss the potential role of inhibitors of cyclooxygenase-2 or 12-lipoxygenase in the treatment of cancer patients.

The contribution by Sargiannidou and colleagues describes the role of thrombospondin-1 (TSP-1) in cancer metastasis and angiogenesis. TSP-1 is a protein released from platelets. It has both pro- and antiangiogenic properties. By virtue of its antiplasmin activity, TSP-1 prevents tumor-platelet emboli from being proteolytically degraded. At the same time, TSP-1, bound to matrix, promotes endothelial cell apoptosis and inhibits angiogenesis. Simultaneously, angiogenesis is stimulated by proliferation of myofibroblasts that secrete proangiogenic factors. TSP-1's action is clearly dependent upon the interaction with several host- and tumor-associated proteins. TSP-1 may become an interesting target for the development of anticancer medications.

Bikfalvi and Gimenez-Gallego review the control of angiogenesis and tumor invasion by platelet factor 4 (PF 4) and some of its fragments. PF 4 is a protein released from platelets and is best known for its heparin-neutralizing properties. It apparently also impacts angiogenesis by inhibiting growth of vascular endothelial cells, endothelial cell migration, and tubulogenesis. It further inhibits angiogenesis by binding VEGF and bFGF. These various mechanisms are discussed in detail. Even some fragments of PF 4, most notably PF 4CTF, are active in blocking VEGF- and bFGF-induced angiogenesis. The authors also discuss the potential use of PF 4 or its fragments as adjunct therapies for certain cancer patients.

In the preceding articles frequent references were made to the potential use of the various angiogenesis inhibitors as cancer therapy. The last article, by Wojtukiewicz et al, comprehensively reviews this issue. Basic sciences have provided a solid foundation for the fact that hemostasis and angiogenesis are inextricably linked. Several components of the hemostasis system have been found that promote angiogenesis and thus cancer growth, while others were identified as inhibitory activities in this complex process. It is the antiangiogenic compounds that are comprehensively discussed from the viewpoint of their potential clinical use. Cancer treatment in its present form, while tremendously advanced in the last decades, still has its shortcomings and additional approaches are needed to further improve the life expectancy of cancer patients. Such a new approach appears to be at hand with the work so far accomplished with antiangiogenic compounds. The near future will undoubtedly yield exciting new avenues of cancer management.

My thanks and appreciation go to all colleagues who participated in this interesting issue for their excellent contributions that will, without doubt, give readers a practical and useful insight into an exciting new approach to cancer treatment. Special thanks go to Drs. Wojtukiewicz and Sierko for their tremendous efforts in assembling this issue.