Procoagulant Circulating Microparticles in Health and Disease - Essay Example

RUNNING HEAD: PROCOAGULANT CIRCULATING MICROPARTICLES Procoagulant Circulating Microparticles [The Writer’s name] [The name of the Institution] Procoagulant Circulating Microparticles Normal cells and cancer cells release microparticles and exosomes into their environment. Microparticles are budded off from the cell surface and are best known for their ability to support coagulation. Exosomes, which are stored in intracellular multivesicular bodies and are released when the membrane of the multivesicular body fuses with the cells plasma membrane, efficiently modulate the immune response. (Kakkar, DeRuvo, Chinswangwatanakul, Tebbutt, Williamson, 2005 p.1004-5) Dvorak and coworkers demonstrated that tumor-derived procoagulant activity (PCA) is associated with sedimentable, ultramiscroscopic plasma membrane-derived vesicles in vitro (cancer cell-conditioned culture medium) as well as in vivo (ascitis tumor fluid from animals). These vesicles, isolated by centrifugation at 100 000 · g, ranged in size from 15 and 800 nm (median 60 nm) (Dvorak, Quay, Orenstein, Bitzer, Carvalho, 2007 p. 923-4) These investigators showed that cancer cell-derived vesicles support coagulation via various mechanisms, i.e. one procoagulant activity associated with shed tumor vesicles behaved as tissue factor, and shed tumorvesicles also acted at a second step late in the clotting cascade at the level of prothrombinase generation, presumably by providing a phospholipid surface(Dvorak, Quay, Orenstein, Bitzer, Carvalho, 2007 p. 923-4) . A decade later, in 1993, from four cases of Trousseaus syndrome, i.e. cancer patients who have spontaneous recurrent or migratory episodes of venous thrombosis, arterial emboli due to nonbacterial thrombotic endocarditis, or both, it was concluded that two properties of a tumor can account for the pathogenesis of Trousseaus syndrome: The first is that the malignant cell expresses tissue factor on its external surface. The second is that the tumor cells are anatomically positioned so that cells or vesicles shed from them are exposed to the circulating blood, either directly or by their entrance into the circulatory system from the lymphatic system. (Rapaport, 2005 p. 153-61) Concurrently, other investigators concluded that a continuing entrance into the circulation of tissue factor from malignant cells is responsible for the manifestations of Trousseaus syndrome in most patients. (Rapaport, 2005 p. 153-61) Taken together, these studies demonstrate that the strong association between malignant disease and coagulation activation may– at least partially – be explained by the release of tissue factor (TF) exposing vesicles from cancer cells into the blood or other body fluids, which in turn may contribute to the low grade disseminated intravascular coagulation and thrombotic episodes which are characteristic of Trousseaus syndrome. Other potential sources of TF-exposing vesicles The true cellular origin of microparticle-associated TF in cancer patients, however, has proven surprisingly difficult to establish. Patients with disseminated breast and pancreatic cancer have increased levels of microparticle-associated TF in plasma compared with controls, and the patients with a low likelihood of survival have (in plasma) both a high microparticle-associated TF activity and increased numbers of epithelial mucin (MUC1) exposing microparticles. (Rapaport, 2005 p. 153-61) Whether or not MUC1-exposing microparticles, for example microparticles originating from tumor cells, expose TF, however, was not investigated. Surprisingly, a low number of microparticles was present that stained positive for both MUC1 and glycoprotein IIIa (CD61; integrin b3). As glycoprotein IIIa is abundantly exposed on platelets and platelet-derived microparticles, they concluded that a small part of circulating microparticles seemed to result from fusion of cellular vesicles originating from malignant epithelial cells and platelets. Whether or not these particular microparticles expose TF, however, was not investigated. Membrane fusion of cancer cell-derived vesicles with other cells (and possibly vesicles) was initially reported in the early 1980s, and this process makes it very difficult to establish the true cellular origin of microparticle-associated TF in plasma of cancer patients. (Victor, Paul, John, 2006 p. 380-10) Membrane fusion, however, is not limited to cancer cell derived vesicles. Tissue factor-exposing microparticles from monocytes also fuse with activated platelets, thereby delivering their tissue factor to the platelet surface. (Kakkar, DeRuvo, Chinswangwatanakul, Tebbutt, Williamson, 2005 p.1004-5) A recent case report of a 55-year-old patient with a giant-cell lung carcinoma showed that a minor fraction of the TF-exposing microparticles in the patients plasma expose CD14 (LPS receptor), indicating that these microparticles originate from monocytes. The cellular origin of the majority of the TFexposing microparticles, however, was not established [9]. In a recent study on 20 patients with advanced colorectal cancer, the numbers of circulating TF-exposing microparticles was found to be 2-fold increased compared with controls. On average, 52% of these (TF-exposing) microparticles exposed CD41a (glycoprotein IX; a platelet marker), 24% exposed CD45 (LCA or leukocyte-common antigen; a general marker of leukocytes) and 32% exposed CD14. (Callander, Rapaport, 2006 p. 3764-71) From this study, it was concluded that most of the TF-exposing microparticles are of platelet origin. As no double labeling experiments were performed to study possible co-localization of CD45 or CD14 with CD61, it cannot be excluded that TF is exposed on microparticles showing characteristics of both leukocytes and platelets, thus reflecting membrane fusion. Alternatively, although still controversial, given the fact that platelets themselves are important for storage of tissue factor, then also the CD61-and TF-exposing microparticles may originate from platelets directly without prior membrane fusion. Taken together, leukocytes, in particular monocytes and platelets may contribute to the pool of TF-exposing microparticles in cancer patients. Microparticles in cancer patients: a mixed double? To make the cellular origin of TF-exposing microparticles in cancer patients even more complex, the cellular origin of CD61-exposing microparticles may be questioned for two reasons. First, many cancer cells in blood are coated with platelets, presumably to escape from immune surveillance. (Callander, Rapaport, 2006 p. 3764-71) Membrane fusion may then transfer tissue factor and other membrane proteins, including glycoprotein IIb–IIIa, from cancer cells to platelets and vice versa. Second, many cancer cells express integrins, including glycoprotein IIb–IIIa, to mimick non-transformed cells such as platelets, and to promote cell adhesion [12]. Thus, staining solely with for example CD61 does not exclude the possibility that these microparticles in fact originate from cancer cells. In sum, the true cellular origin of microparticle-exposed TF in cancer patients is still not fully established. Evidence that cancer cells contribute to the circulating pool of coagulant TF in vivo In this issue of the Journal of Thrombosis and Haemostasis, a study of Davila and coworkers may shed more light on the cellular origin of plasma TF in cancer patients. (Victor, Paul, John, 2006 p. 380-10) Human pancreatic cancer cells (L3.6pl) were injected in the pancreas of immune incompetent (nude) mice. At various time intervals, mouse blood was collected and human TF antigen (ELISA) and activity (thrombin generation assay, with and without antihuman tissue factor) were determined in the cell-free plasma. Three weeks after injection, TF antigen was detectable, and the amount of TF antigen was related to the tumor weight. In addition, the mouse plasma samples were also shown to be capable of initiating (human) TF-dependent thrombin generation. This study demonstrates in a set of elegant experiments that plasma TF originates, in an animal model of cancer, at least in part from tumor cells. The authors conclude that circulating tumor-derived tissue factor exhibited PCA ex vivo, but strictly speaking they did not demonstrate this, as they did not isolate microparticles from the cell-free mouse plasma samples to study their procoagulant activity. It is striking that also in this study the levels of TF antigen and activity are somewhat discrepant. As outlined by the authors, TF activity was detected in the plasmas of tumor-bearing mice only when circulating tissue factor was present beyond certain concentration. Therefore, they conclude that circulating active tissue factor has to reach concentrations sufficient to overcome the physiological threshold of the in vivo anticoagulat systems and trigger a detectable activation of the coagulation cascade. In fact, this is precisely what we showed earlier when studying the procoagulant activity of human microparticles, isolated from pericardial wound blood of patients undergoing cardiac surgery or healthy individuals, in a rat venous stasis model. (Victor, Paul, John, 2006 p. 380-10) With regard to the cellular origin and function of TFexposing microparticles in cancer, many questions remain unanswered. The study of Davila shows directly that TF does originate from cancer cells, but it does not exclude that concurrently TF originates from other cell types. It also remains obscure to which extent exosomes expose procoagulant TF. Davila et al. describe that part of the TF activity is associated with exosomes using a 0.1-lm filter, but additional evidence is essential to support this initial and important observation. Perhaps, the two most important questions, however, are (i) the biological benefits a tumor may have to release TF-exposing microparticles, and (ii) whether membrane fusion between (TF-exposing) microparticles and cells not only transfers a procoagulant phenotype, but also transmits other TF-mediated functions to such cells, including the ability to support angiogenesis, to protect cells from apoptosis and trans smembrane signaling. Taken together, by showing that cancer cells contribute to the pool of circulating TF-exposing microparticles in vivo is a further step in our understanding of the complex relationship between tumors, TF and the prothrombotic tendency in cancer patients. TF plays a leading role in fibrin deposition during inflammatory and thrombotic disorders. Endothelial cells and monocytes are the only cells in the blood capable of expressing TF and may do so in response to agents such as bacterial lipopolysaccharide8 and activated platelets. It is suggested that monocytes and possibly PMN-leukocytes are the source of circulating TF, which is transferred to platelets by the interaction of CD15 and CD62-P (P-selectin) on platelets. In conclusion, we have demonstrated the presence of TF in washed platelets and presented evidence that platelets release functionally active TF. However, the role of platelet TF in physiological and pathological processes remains to be determined. References Callander N, Rapaport SI. Trousseaus syndrome. West J Med 2006; 158: 364–71. Dvorak HF, Quay SC, Orenstein NS, Dvorak AM, Hahn P, Bitzer AM, Carvalho AC. Tumor shedding and coagulation. Science 2007; 212: 923–4. Kakkar AK, DeRuvo N, Chinswangwatanakul V, Tebbutt S, Williamson RCN. Extrinsic-pathway coagulation activity in cancer with high factor VIIa and tissue factor. Lancet 2005; 346: 1004–5. Rapaport SI. Blood coagulation and its alterations in hemorrhagic and thrombotic disorders. West J Med 2005; 158: 153–61. Victor Hoffbrand, Paul Moss, John Pettit. Essential Haematology, 5th Edition October 2006, Blackwell Publishers Read More