Thrombin generation is usually triggered when disruption of vascular integrity allows coagulation factors in the plasma to contact extravascular tissue factor (tissue thromboplastin; coagulation factor III). Prothrombin in 1 mL of human plasma, when fully converted, forms 15 to 38 U of thrombin, which is concentrated in the resultant clot.
8 9 Thrombin is formed from prothrombin in areas of increased vascular permeability and hemorrhage at sites of blood–retinal barrier breakdown in many retinal diseases, such as ocular trauma, PVR, PDR, and AMD. Thrombosis is also enhanced by localized increases in thrombin concentration in the retinal circulation during retinal ischemia that results in increased leakage of serum components, including prothrombin.
10 Thrombin itself may induce gap formation between endothelial cells, further enhancing its own formation.
11 Thrombin stimulates VEGF in many cell types
12 13 14 and interplays with VEGF synergistically in promoting angiogenesis.
15 Despite the likely exposure of RPE cells to thrombin in retinal diseases, no study has shown that thrombin induces RPE cells to produce VEGF. Therefore, it is of pathophysiological importance to assess the role of thrombin in stimulating VEGF expression in RPE cells. Moreover, thrombin is also likely to coexist with other proangiogenic factors such as TNF-α and TGF-β, which have been detected in diseased ocular tissues.
16 17 18 19 Histopathological studies of choroidal neovascular membranes from patients with AMD have demonstrated the presence of various growth factors, including TGF-β.
20 Nevertheless, interactions of thrombin with other proangiogenic factors in stimulating VEGF secretion have not been reported. Apart from its central role in blood coagulation, thrombin also regulates other cellular functions, including those involved in wound healing and inflammation. One example is thrombin’s mitogenic effects on RPE cells.
21 The latter finding has led to thrombin’s use as a therapeutic agent.
22 23 As a mitogen of retinal cells, thrombin has been used in intraocular surgery for diabetic retinopathy and ocular trauma to control bleeding and to close macular holes. However, thrombin treatment frequently causes significant inflammation. This clinical observation is consistent with our previous observations showing the potential proinflammatory role of thrombin, TNF-α, and monocytes by stimulating RPE cells to secrete chemokines.
24 25 26 These observations underscore the potential importance of investigating whether thrombin at sites of blood–retinal barrier breakdown works in concert with leukocytes to stimulate VEGF gene expression in ocular tissues. In this study thrombin, working additively with TNF-α and monocytes and synergistically with TGF-β2, triggered multiple signaling pathways and led to enhanced VEGF gene expression in human RPE cells.