Plasmin-Cleaved β-2-Glycoprotein 1 Is an Inhibitor of Angiogenesis

Angiostatin was first discovered as a plasminogen fragment with antitumor/antiangiogenic property. One of the angiostatin isoforms, that is, angiostatin 4.5 (AS4.5), consisting of plasminogen kringle 1 to 4 and a most part of kringle 5, is produced by autoproteolysis and present in human plasma. ␤2-glycoprotein I (␤2GPI) is proteolytically cleaved by plasmin in its domain V (nicked ␤2GPI), resulting in binding to plasminogen. Antiangiogenic properties have been recently reported in nicked ␤2GPI as well as in intact ␤2GPI at higher concentrations. In the present study, we found significant binding of nicked ␤2GPI to AS4.5 (K D ‫؍‬ 3.27 ؋ 10 6 M ؊1 ). Via this binding, nicked ␤2GPI attenuates the antiangiogenic functions of AS4.5 in the proliferation of arterial/venous endothelial cells, in the extracellular matrix invasion and the tube formation of venous endothelial cells, and in vivo angiogenesis. In contrast, intact ␤2GPI does not bind to AS4.5 or inhibit its antiangiogenic activity. Thus, nicked ␤2GPI exerts dual effects on angiogenesis, that is, nicked ␤2GPI promotes angiogenesis in the presence of AS4.5, whereas nicked ␤2GPI inhibits angiogenesis at concentrations high enough to neutralize AS4.5. Our data suggest that plasmin-nicked ␤2GPI promotes angiogenesis by interacting with plasmin-generated AS4.5 in sites of increased fibrinolysis such as thrombus. IntroductionAngiogenesis is the formation of a new capillary network from preexisting vessels and is essential in many physiologic and pathologic states, such as reproduction, development, wound healing, tumorigenesis, rheumatoid arthritis, diabetic retinopathy, and thrombosis. 1,2 Angiogenesis is tightly controlled by activators, such as fibroblast growth factors (FGFs) and vascular endothelial growth factor (VEGF), and by inhibitors, such as thrombospondin-1, interferon-␣/␤, platelet factor-4, and angiostatin. 3 Angiostatin was discovered in urine from mice with low-metastatic Lewis lung carcinoma as a kringle-containing fragment of plasminogen. 4 This first-reported angiostatin consists of 4 kringle domains (K1-K4) and possesses antitumor/antiangiogenic properties. Later, K1 to K3 was revealed to be a more potent inhibitor of angiogenesis. 5 Although several isoforms have been reported, angiostatin 4.5 (AS4.5, also referred to as K1-K5) is the only naturally occurring isoform identified in human plasma that consists of kringles 1 to 4 and approximately 85% of kringle 5. [6][7][8] Plasma concentrations of in vivo-generated AS4.5 were measured in cancer patients who received tissue plasminogen activator and mesna in a clinical trial. 9 In this study, generation of 2 isoforms of AS4.5 in human plasma was observed, namely, Lys-AS4.5 and Glu-AS4.5. Whereas Lys-AS4.5 is the originally reported natural AS4.5, Glu-AS4.5 is a larger form that obtains intact N-terminal domain of the precursor protein. Approximately 20 nM Lys-AS4.5 was detected even in the patients before treatment. After infusion of tissue plasminogen activator with mesna, Lys-AS4.5 levels incre...

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Nicked β2-glycoprotein I binds angiostatin 4.5 (plasminogen kringle 1-5) and attenuates its antiangiogenic property

Nakagawa

Yasuda

Matsuura

et al. 2009

“…90 Proteolytic cleavage of HMWK can also generate 2-chain HMWK that inhibits angiogenesis. 91 Furthermore, cleavage of ␤ 2 GPI by plasmin 92 and polymorphonuclear neutrophil-derived proteases 93 has been shown to regulate the ability of ␤ 2 GPI to exhibit antiangiogenic and antibacterial activities. Similarly, cleavage of HRG by plasmin and other proteases has been proposed as a potential mechanism for the generation of antiangiogenic and antimicrobial fragments.…”

Section: Heparin Bindingmentioning

confidence: 99%

Histidine-rich glycoprotein: the Swiss Army knife of mammalian plasma

Poon

Patel

Davis

et al. 2011

199

209

Histidine-rich glycoprotein (HRG), also known as histidine-proline-rich glyco-protein, is an abundant and well-characterized protein of vertebrate plasma. HRG has a multidomain structure that allows the molecule to interact with many ligands, including heparin, phospholipids, plasminogen, fibrinogen, immunoglobulin G, C1q, heme, and Zn²(+). The ability of HRG to interact with various ligands simultaneously has suggested that HRG can function as an adaptor molecule and regulate numerous important biologic processes, such as immune complex/necrotic cell/pathogen clearance, cell adhesion, angiogenesis, coagulation, and fibrinolysis. The present review covers the proposed multifunctional roles of HRG with a focus on recent findings that have led to its emergence as a key regulator of immunity and vascular biology. Also included is a discussion of the striking functional similarities between HRG and other important multifunctional proteins found in plasma, such as C-reactive protein, C1q, β₂ glycoprotein I, and thrombospondin-1.

“…12 In addition, ␤ 2 -GPI also exerts antiangiogenic and antitumor activities. 13,14 Despite these activities, the main function of ␤ 2 -GPI is unknown.…”

Section: Introductionmentioning

confidence: 99%

“…12 In addition, ␤ 2 -GPI also exerts antiangiogenic and antitumor activities. 13,14 Despite these activities, the main function of ␤ 2 -GPI is unknown.␤ 2 -GPI is composed, like complement factor H and factor H-related protein 1, of consecutive short consensus repeat elements (SCRs), also called complement control protein modules, each approximately 60 amino acids in size. 15 These repeats are frequently found in proteins with complement-regulatory functions.…”

mentioning

confidence: 99%

β2-glycoprotein I, the major target in antiphospholipid syndrome, is a special human complement regulator

Gropp

Weber

Reuter

et al. 2011

The human plasma protein ␤ 2 -glycoprotein I (␤ 2 -GPI) is the major target of autoantibodies associated with antiphospholipid syndrome. However, the biologic function of this abundant protein is still unclear. Here we identify ␤ 2 -GPI as a complement regulator. ␤ 2 -GPI circulates in the plasma in an inactive circular form. On surface binding, such as to apoptotic cells, ␤ 2 -GPI changes conformation to an elongated form that acquires C3/C3b binding activities. ␤ 2 -GPI apparently changes conformation of C3, so that the regulator factor H attaches and induces subsequent degradation by the protease factor I. ␤ 2 -GPI also mediates further cleavage of C3/C3b compared with factor H alone. Our data provide important insights into innate immune regulation by plasma protein ␤ 2 -GPI, which may be exploited in the prevention and therapy of autoimmune disease antiphospholipid syndrome. Introduction␤ 2 -glycoprotein I (␤ 2 -GPI), also termed apolipoprotein H, 1 is a 50-kDa glycosylated human plasma protein with a concentration of 200 g/mL (4M) 2 that also associates with lipoprotein particles. 3 In the autoimmune disease antiphospholipid syndrome (APS), autoantibodies to ␤ 2 -GPI are identified. [4][5][6] APS is characterized by recurrent vascular thrombosis and pregnancy loss; and in pregnant women, ␤ 2 -GPI autoantibodies trigger severe complications, resulting in miscarriage, intrauterine growth restriction, and fetal death. 7-9 ␤ 2 -GPI displays anticoagulant activity and inhibits the contact activation of the intrinsic coagulation pathway, 10 platelet prothrombinase activity 11 and adenosine diphosphate-induced platelet aggregation. 12 In addition, ␤ 2 -GPI also exerts antiangiogenic and antitumor activities. 13,14 Despite these activities, the main function of ␤ 2 -GPI is unknown.␤ 2 -GPI is composed, like complement factor H and factor H-related protein 1, of consecutive short consensus repeat elements (SCRs), also called complement control protein modules, each approximately 60 amino acids in size. 15 These repeats are frequently found in proteins with complement-regulatory functions. The fifth domain of ␤ 2 -GPI has a modified structure, as it contains a 6-residue insertion and an extra 20-amino acid-long mobile tail. Together, these additional amino acids form a binding site for negatively charged, anionic phospholipids, such as phosphatidylserine or cardiolipin, 16,17 which are exposed on apoptotic or necrotic cells. Recently, Agar et al 18 identified 2 conformations of ␤ 2 -GPI: a circular inactive form of ␤ 2 -GPI in the plasma and an elongated, active one when ␤ 2 -GPI is bound to surfaces. The circular form results from internal interaction of the N-terminal SCRI with the C-terminal SCRV of ␤ 2 -GPI. This interaction is disrupted by binding of ␤ 2 -GPI to surfaces, which generates the elongated conformation. 18 The crystal structure of ␤ 2 -GPI shows the open, J-shaped form of ␤ 2 -GPI. 19,20 As the ␤ 2 -GPI protein function is unclear, we aimed to identify the role of this human plasma protein. Given t...