Reactive Oxygen Species in Venous Thrombosis

Abstract

Reactive oxygen species (ROS) have physiological roles as second messengers, but can also exert detrimental modifications on DNA, proteins and lipids if resulting from enhanced generation or reduced antioxidant defense (oxidative stress). Venous thrombus (DVT) formation and resolution are influenced by ROS through modulation of the coagulation, fibrinolysis, proteolysis and the complement system, as well as the regulation of effector cells such as platelets, endothelial cells, erythrocytes, neutrophils, mast cells, monocytes and fibroblasts. Many conditions that carry an elevated risk of venous thrombosis, such as the Antiphospholipid Syndrome, have alterations in their redox homeostasis. Dietary and pharmacological antioxidants can modulate several important processes involved in DVT formation, but their overall effect is unknown and there are no recommendations regarding their use. The development of novel antioxidant treatments that aim to abrogate the formation of DVT or promote its resolution will depend on the identification of targets that enable ROS modulation confined to their site of interest in order to prevent off-target effects on physiological redox mechanisms. Subgroups of patients with increased systemic oxidative stress might benefit from unspecific antioxidant treatment, but more clinical studies are needed to bring clarity to this issue.

Keywords: DVT; anticoagulation; antioxidant defenses; dietary antioxidants; oxidative stress; reactive oxygen species; redox regulation; thrombus resolution; venous thrombosis.

Figure 1

Major sources of reactive oxygen species (ROS) and antioxidant defense mechanisms. NADPH oxidases (NOX; highlighted in red) appear to be the most important source of ROS involved in processes related to the formation and resolution of venous thrombi.

Figure 2

ROS in the formation of a venous thrombus. (A) Physiologic venous circulation. (B) At sites of blood stasis, activated endothelial cells increase ROS luminally (contributing directly and indirectly to recruitment and activation of leukocytes and platelets) and abluminally (contributing directly and indirectly to the activation of mast cells and vascular smooth muscle cells). Activated cells contribute to further ROS generation and use redox signaling to exert their prothrombotic effects, including activation of the coagulation system and platelet aggregation; mast cell degranulation and tissue factor (TF) production within the vein wall. (C) Increased coagulability of blood in the presence of low flow (two of Virchow’s Triad) provides the environment for venous thrombus formation. In the forming thrombus, erythrocytes (major component) and platelets are entrapped by large amounts of cross-linked fibrin. The oxidative stress leads to oxidation of hemoglobin (Fe²⁺) to methemoglobin (Fe³⁺), resulting in a positive feedback on oxidative stress and thrombus formation. Neutrophil extracellular traps (NETs) provide an additional scaffold and contribute to further oxidative stress and thrombus formation.

Figure 3

The role of ROS in the resolution of a venous thrombus. Oxidative stress at the site of a venous thrombus (intra- and abluminal) leads to the recruitment of leukocytes (with neutrophils predominating in the early, and monocytes predominating in the later phase of resolution) and fibroblasts. ROS-mediated cell recruitment results from direct chemotactic effects, or indirectly through increased expression of chemokines and adhesion molecules. The recruited cells promote thrombus resolution through phagocytic action removing red cell mass, thrombus and vein wall remodeling including fibrin degradation and the stimulation of new ECM deposition and neovessel formation. There are conflicting reports on whether fibrinolysis is stimulated or inhibited by ROS.