Myeloperoxidase Is a Negative Regulator of Phospholipid-Dependent Coagulation

Abstract

Myeloperoxidase (MPO) is a cationic heme enzyme stored in neutrophilic polymorphonuclear leukocytes (PMNs) that has recently been implicated in inflammatory cell signaling and tissue damage. Although PMNs play a critical role in both innate immunity and vascular thrombosis, no previous study has systematically investigated the effect of MPO on blood coagulation. Here, we show that PMN-derived MPO inhibits the procoagulant activity (PCA) of lipidated recombinant human tissue factor (rhTF) in a time- and concentration-dependent manner that involves, but is not entirely dependent on the enzyme's catalytic activity. Similarly, MPO together with its substrate, H2O2, inhibited the PCA of plasma microvesicles isolated from lipopolysaccharide (LPS)-stimulated whole blood, an effect additive to that of a function blocking TF antibody. Treatment of whole blood with LPS or phorbol-myristate-acetate dramatically increased MPO plasma levels, and co-incubation with 4-ABAH, a specific MPO inhibitor, significantly enhanced the PCA in plasma supernatants. MPO and MPO/H2O2 also inhibited the PCA of activated platelets and purified phospholipids (PLs), suggesting that modulation of negatively charged PLs, i.e., phosphatidylserine, rather than direct interference with the TF/FVIIa initiation complex was involved. Consistently, pretreatment of activated platelets with MPO or MPO/H2O2 attenuated the subsequent binding of lactadherin, which specifically recognizes procoagulant PS on cell membranes. Finally, endogenously released MPO regulated the PCA of THP1 cells in an autocrine manner dependent on the binding to CD11b/CD18 integrins. Collectively, these findings indicate that MPO is a negative regulator of PL-dependent coagulation and suggest a more complex role of activated PMNs in haemostasis and thrombosis.

Effect of myeloperoxidase on the anticoagulant activity of low molecular weight heparin and rivaroxaban in an in vitro tumor model

“…16 In this regard, electrostatic interactions between the highly cationic enzyme and negatively charged structures likely play a role. 17 Elevated levels of MPO in malignant tissues and increased MPO serum levels in patients with breast cancer are consistent with a triangular network formed by bidirectional relationships between cancer, inflammation, and thrombosis. [18][19][20][21] For almost two decades, parenteral anticoagulation with low molecular weight heparin (LMWH) has been the treatment of choice for cancer-associated VTE.…”

“…Interestingly, MPO also has immunomodulatory effects that are independent of its catalytic activity 16 . In this regard, electrostatic interactions between the highly cationic enzyme and negatively charged structures likely play a role 17 . Elevated levels of MPO in malignant tissues and increased MPO serum levels in patients with breast cancer are consistent with a triangular network formed by bidirectional relationships between cancer, inflammation, and thrombosis 18‐21 …”

Reactive Oxygen Species in Venous Thrombosis

“…For example, MPO, an ROS-generating enzyme, is among the molecules extruded with NETs. Contrary to this, however, a recent study shows that MPO inhibits phospholipid-dependent coagulation [79], while acquired neutrophil MPO deficiency in man is associated with thrombotic disease [80]. These data suggest a regulatory function on blood coagulation beyond the known oxidative properties of MPO.…”

Bacitracin and Rutin Regulate Tissue Factor Production in Inflammatory Monocytes and Acute Myeloid Leukemia Blasts

“…Following LPS stimulation, MVs were isolated from PPP or cell culture supernatants by double high-speed centrifugation at 16,100× g for 30 min at RT. Pelleted MVs were resuspended in PBS to one-third of their initial sample volume and analyzed for TF-specific activity by a two-stage chromogenic FXa generation assay, as previously described [41].…”