doi: 10.1161/CIRCRESAHA.108.172080
© 2008 American Heart Association, Inc.
Editorials |
Annexin 2/Factor Xa–Mediated Signal Transduction
From the Department of Pathology, Duke University Medical Center, Durham, NC.
Correspondence to Salvatore V. Pizzo, Duke University Medical Center, Chair, Department of Pathology, M301 Davison Bldg, Box 3712, Durham, NC 27710-0001. E-mail pizzo001{at}mc.duke.edu
See related article, pages 457–464
Key Words: annexin 2 • factor Xa • protease-activated receptor 1 • fibrinolysis
In this issue of Circulation Research, Bhattacharjee et al1 provide evidence that annexin 2 may be important in the ability of factor Xa to induce signal transduction dependent on protease-activated receptor (PAR)-1. The significance of these observations must be considered in the context of broader questions related to thrombosis and fibrinolysis. In the first decade of the 20th century, knowledge of blood coagulation resulted in a relatively simple model. In 1905, Morowitz proposed that the coagulation cascade consists of 4 fluid phase components of blood.2 The 4 components involved in coagulation were identified as fibrinogen, Ca2+, prothrombin, and "tissue thromboplastin." He also recognized that platelets and perhaps leukocytes played a role in blood clotting.2 Tissue thromboplastin was identified as a lipid- and protein-containing component of tissues that was released during vascular injury. In the presence of Ca2+, this substance activated prothrombin. The resultant thrombin converted fibrinogen to fibrin. The discovery of "factor V" in 1943 expanded the model of coagulation beyond these 4 components and triggered a search for other coagulation factors. During the 20th century, many more such factors were discovered. Most of these received factor numbers, whereas some were known more commonly by name. The concept arose that there were 2 pathways of coagulation, namely, the intrinsic and extrinsic systems. It was believed that the intrinsic pathway was most important at the surface of damaged blood vessels, whereas the extrinsic system became activated after major vascular injury causing the release of tissue thromboplastin or, as it became known, tissue factor (TF). Both the extrinsic and intrinsic systems were seen as converging at the point where factor X was activated and this factor Xa was responsible for the activation of prothrombin. The 2 systems were recognized as protease cascades in which activation of one serine protease precursor resulted in the activation of the next serine protease zymogen and so on down the chain, until thrombin was produced. At each step, a multiplier effect was created as the catalytic activation of one protease zymogen would result in conversion of many other precursor enzyme molecules into active forms.
Problems with this scheme surfaced in the latter part of the 20th century. For example, deficiency of factor XIIa, which activated factor XI at the onset of the intrinsic system, did not cause bleeding. A deficiency of factor XI may cause bleeding, but only in some families. It was very difficult to explain such data based on the classic scheme of coagulation. During the 1980s and 1990s, new concepts to explain coagulation and thrombosis were developed that appeared to give us a better understanding of how this complex system is activated. It became evident that the separation of an intrinsic and extrinsic loop of the cascade was an artificial creation of the laboratory.3 Moreover, many of the activating steps occur not in the fluid phase but on the surface of endothelial cells, platelets, and monocyte/macrophages, among other cells. In this view, activation of factor X by factor VIIa/TF complexes (the extrinsic pathway) on the cell surface is the most critical first event in generating thrombin. Activation of the intrinsic pathway then serves as an enzymatic amplifier to generate a large burst of thrombin.
While these concepts were being developed, other investigators focused on understanding how fibrinolysis was activated and regulated. This process dissolves crosslinked fibrin plugs and provides a balance between mechanisms that form and those that remove a thrombus. Plasminogen, a serine protease zymogen, circulates in blood at a high concentration. It is converted to plasmin by one of several activators, including tissue plasminogen activator (t-PA). Both plasminogen and t-PA bind to fibrin. This assembly of the fibrinolytic system on the clot results in a several hundred–fold kinetically augmented ability of t-PA to activate plasminogen.4 t-PA is produced by endothelial cells and released from these cells during hypoxic injury.5 The presumption made by most investigators was that the process of t-PA–mediated plasminogen generation was fibrin dependent, thus localizing fibrinolysis to the clot. Other studies, however, indicated that plasmin could also be generated on cell surfaces in the absence of fibrin. In this context, annexin 2 was identified by Hajjar and colleagues as one of several cell surface-binding sites for plasminogen and t-PA.6,7 Annexin 2 is found on many cell types, including endothelial cells and monocyte/macrophages.8,9 It is part of a large family of related cell surface, multidomain proteins.8,9 Annexins are Ca2+-dependent proteins that bind membrane phospholipids. Activation of plasminogen on the cell surface is far more efficient than occurs in the absence of such an organizing template and leads to the production of more plasmin at a faster rate in a manner analogous to fibrin stimulation. The importance of annexin 2 in fibrinolysis was demonstrated in vivo by Hajjar and colleagues using homozygous annexin 2–deficient mice.10 These animals spontaneously show fibrin deposition in the microvasculature and defective removal of arterial thrombi.10 In vitro studies by these investigators further suggest a role for annexin 2 in endothelial migration through fibrin/collagen networks.9 Such migration has been recognized for many decades by pathologists as a critical step in recanalization of thrombi.
The events that trigger coagulation and fibrinolysis, moreover, occur in a broader context involving the response of cells to various proteases activated at cell surfaces. For example, it has been appreciated for many years that thrombin exhibits mitogenic effects and triggers proliferation of cells in tissue culture. Coughlin and colleagues identified a family of PARs on the cell surface.11–14 These receptors are coupled to G proteins, and their activation induces proproliferative and antiapoptotic signaling cascades.15 The relationship among the 4 known PARs is complex, and specific combinations are expressed in a cell type–specific manner. Not only does thrombin activate PARs, but TF-dependent complexes such as factor VIIa/TF and factor VIIa/factor Xa/TF complexes can also activate PARs in a process that cleaves the receptors to free a tethered receptor ligand (Figure).13–18
|
In this context, the present report by Bhattacharjee et al provides new insights into the roles that activated coagulation factors play in addition to generating a clot. In particular, these investigators demonstrate that factor Xa binding to annexin 2 can mediate PAR-1–dependent activation of extracellular signal-regulated kinase 1/2 in endothelial cells. These events, moreover, are independent of a requirement for TF in complex with factor Xa. These observations are of particular interest because annexin 2, but not TF, is constitutively expressed on endothelial cells.8,9 TF can be expressed on these cells if they are stimulated in any of a number of ways, such as by growth factors or lipopolysaccharide.3 However, TF expression by endothelial cells is variable in contrast to its constitutive expression by monocytes/macrophages.3 Thus the present observations suggest a mechanism for localizing factor Xa–dependent proliferative and antiapoptotic effects of factor Xa to healthy endothelial cells.1 One would predict an in vivo scenario in which vascular injury and thrombosis generated amounts of factor Xa that not only activated prothrombin but were also available in the vicinity of healthy endothelial cells at the margins of injury and beyond. This factor Xa in complex with annexin 2 binds to PAR-1 and promotes endothelial cell proliferation and repair of the endothelial cell surface. Concomitantly, annexin 2 in complex with plasminogen and t-PA would degrade the developing clot by generating plasmin, thus tending to localize a thrombus and further serving to protect healthy endothelial cells at the margins of injury. Coupled with these events, annexin 2 expression on the endothelial cells would also promote migration of these cells into the thrombus, which is essential for recanalization when fibrinolysis fails to remove a clot.10 It would seem, perhaps, that the roles of annexin 2 are broader than previously thought and are able to coordinate 2 significant mechanisms that protect endothelial cells from hypoxia and promote endothelial proliferation to repair a damaged vascular site. Such mechanisms may well play a role that is equally important to the role of growth factors, such as vascular endothelial growth factor, in maintaining an intact endothelium.
 |
Acknowledgments |
|---|
Sources of Funding
This laboratory is supported by a research grant from the National Heart, Lung, and Blood Institute, HL-24066.
Disclosures
None.
|
Footnotes |
|---|
The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
|
References |
|---|
 Top References |
|---|
1. Bhattacharjee G, Ahamed J, Pawlinski R, Liu C, Mackman N, Ruf W, Edgington TS. Factor Xa binding to annexin 2 mediates signal transduction via protease-activated receptor 1. Circ Res. 2008; 102: 457–464.
2. Owen CA Jr, Bowie EJW, Thompson JH Jr, eds. Introduction and historical perspective. In The Diagnosis of Bleeding Disorders. Boston: Little, Brown and Co; 1975: 3–84.
3. Mann KG, Krishnaswamy S, Lawson JH. Surface-dependent hemostasis. Semin Hematol. 1992; 29: 213–226.[Medline] [Order article via Infotrieve]
4. Hajjar KA. The molecular basis of fibrinolysis. In Nathan DG, Orkin SH, eds. Hematology of Infancy and Childhood. Philadelphia, Pa: W.B. Saunders Co; 1998: 1557–1573.
5. Levin EG, Santell L, Osborn KG. The expression of endothelial tissue plasminogen activator in vivo: a function defined by vessel size and anatomic location. J Cell Sci. 1997; 110: 139–148.[Abstract]
6. Hajjar KA, Jacovina AT, Chacko J. An endothelial cell receptor for plasminogen/tissue plasminogen activator. J Biol Chem. 1994; 269: 21191–21197.
7. Cesarman GM, Guevara CA, Hajjar KA. An endothelial cell receptor for plasminogen/tissue activator (t-PA). J Biol Chem. 1994; 269: 21198–21203.
8. Hajjar KA, Krishman S. Annexin II: a mediator of the plasmin/plasminogen activator system. Trends in Cardiovasc Med. 1999; 9: 128–138.[CrossRef]
9. Min Keeng M, Choi K-S, Kassam G, Fitzpatrick SL, Kwon M. Waisman DM. Role of annexin II tetramer in plasminogen activator. Trends in Cardiovasc Med. 1999; 9: 92–102.[CrossRef]
10. Ling Q, Jacovina AT, Deora A, Febbraio M, Simantov R, Silverstein RL, Hempstead B, Mark WH, Hajjar K. Annexin II regulates fibrin homeostasis and neoangiogenesis in vivo. J Clin Invest. 2004; 113: 38–48.[CrossRef][Medline] [Order article via Infotrieve]
11. Coughlin SR. Thrombin signaling and protease-activated receptors. Nature. 2000; 407: 258–264.[CrossRef][Medline] [Order article via Infotrieve]
12. Camerer E, Huang W, Coughlin SR. Tissue factor- and factor-X-dependent activation of protease-activated receptor 2 by factor VIIa. Proc Natl Acad Sci U S A. 2000; 97: 5255–5260.
13. Riewald M, Ruf W. Mechanistic coupling of protease signaling and initiation of coagulation by tissue factor. Proc Natl Acad Sci U S A. 2001; 98: 7742–7747.
14. Kataoka H, Hamilton JR, McKemy DD, Camerer E, Zheng YW, Cheng A, Griffin C, Coughin SR. Protease-activated receptors 1 and 4 mediate thrombin signaling in endothelial cells. Blood. 2003; 102: 3224–3231.
15. O’Brien PJ, Molino M, Kahn M, Brass LF. Protease activated receptors: theme and variations. Oncogene. 2001; 20: 1570–1581.[CrossRef][Medline] [Order article via Infotrieve]
16. Camerer E, Kataoka H, Kahn M, Lease K, Coughlin SR. Genetic evidence that protease-activated receptors mediate factor Xa signaling in endothelial cells. J Biol Chem. 2002; 277: 16081–16087.
17. Morris DR, Ding Y, Ricks TK, Gullapalli A, Wolfe BL, Trejo J. Protease-activated receptor-2 is essential for factor VIIa and Xa-induced signaling, migration, and invasion of breast cancer cells. Cancer Res. 2006; 66: 307–314.
18. Riewald M, Kravchenko VV, Petrovan RJ, O’Brien PJ, Brass LF, Ulevitch RJ, Ruf W. Gene induction by coagulation factor Xa is mediated by activation of protease-activated receptor 1. Blood. 2001; 97: 3109–3116.
Related Article:
Circ. Res. 2008 102: 457-464.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||