Bidirectional Pathologic Effects of Thrombin

Hemostasis is an essential reaction in organisms with circulatory systems. To stop bleeding as soon as possible, reactions of the coagulation system proceed rapidly and are regulated by positive feedback loops. However, coagulation can be harmful when induced at inappropriate sites, potentially resulting in local thromboembolisms in vital organs such as the heart and brain and systemic microthrombi in conditions such as disseminated intravascular coagulopathy. Increasingly, reports indicate that coagulation-related factors can cause serious diseases without obvious thromboembolism-associated ischemia, for example, atherosclerosis and Alzheimer disease. Thrombin is an essential coagulation factor that can also cause tissue injury, particular to endothelial cells. However, thrombin can also function as a tissue-protective factor depending on conditions. Endothelial protein C receptor (EPCR) and protease-activated receptors (PARs) regulate thrombin activity, but many details regarding the mechanisms remain unknown. This short review summarizes the bidirectional effects of thrombin signaling via EPCR and PAR1 and discusses points relevant to translating the tissue-protective effects of thrombin into clinical benefits.