Objective: High mobility group box 1 protein (HMGB1) was identified as a mediator of endotoxin lethality. We previously reported that thrombomodulin (TM), an endothelial thrombin-binding protein, bound to HMGB1, thereby protecting mice from lethal endotoxemia. However, the fate of HMGB1 bound to TM remains to be elucidated.
Methods and results: TM enhanced thrombin-mediated cleavage of HMGB1. N-terminal amino acid sequence analysis of the HMGB1 degradation product demonstrated that thrombin cleaved HMGB1 at the Arg10-Gly11 bond. Concomitant with the cleavage of the N-terminal domain of HMGB1, proinflammatory activity of HMGB1 was significantly decreased (P<0.01). HMGB1 degradation products were detected in the serum of endotoxemic mice and in the plasma of septic patients with disseminated intravascular coagulation (DIC), indicating that HMGB1 could be degraded under conditions in which proteases were activated in the systemic circulation.
Conclusions: TM not only binds to HMGB1 but also aids the proteolytic cleavage of HMGB1 by thrombin. These findings highlight the novel antiinflammatory role of TM, in which thrombin-TM complexes degrade HMGB1 to a less proinflammatory form.