Reduction in host-activated protein C levels and resultant microvascular thrombosis highlight the important functional role of protein C anticoagulant system in the pathogenesis of sepsis and septic shock. Thrombomodulin (TM) is a critical factor to activate protein C in mediating the anticoagulation and anti-inflammation effects. However, TM protein content is decreased in inflammation and sepsis, and the mechanism is still not well defined. In this report, we identified that the TM 5 untranslated region (UTR) bearing the internal ribosome entry site (IRES) element controls TM protein expression. Using RNA probe pulldown assay, HuR was demonstrated to interact with the TM 5UTR. Overexpression of HuR protein inhibited the activity of TM IRES, whereas on the other hand, reducing the HuR protein level reversed this effect. When cells were treated with IL-1, the IRES activity was suppressed and accompanied by an increased interaction between HuR and TM 5UTR. In the animal model of sepsis, we found the TM protein expression level to be decreased while concurrently observing the increased interaction between HuR and TM mRNA in liver tissue. In summary, HuR plays an important role in suppression of TM protein synthesis in IL-1 treatment and sepsis.
INTRODUCTIONThere is ample evidence that inflammation and coagulation are intricately related processes, whereby inflammation not only leads to activation of coagulation, but coagulation also markedly affects inflammation activity (Esmon, 2005;Levi and Van der Poll, 2005). Inflammation-induced coagulation contributes to vascular thrombotic disease and is also the major consequence in the pathogenesis of microvascular failure and subsequent multiple organ failure in severe sepsis (Diehl and Borgel, 2005). Both preclinical and clinical studies have suggested that excessive microvascular thrombosis during sepsis results in part from depletion of endogenous anticoagulant systems, such as the heparin-antithrombin system, the protein C anticoagulant pathway, and the tissue factor pathway inhibitor system (Haley et al., 2004).Thrombomodulin (TM) is an important anticoagulant protein present on the surface of vascular endothelial cells (Dittman and Majerus, 1990). TM forms a high-affinity complex with thrombin and results in approximately a 100-fold increase in the activation of protein C to execute anticoagulant effects (Esmon, 1993). Recent studies have shown that TM also plays an important role in attenuation of the inflammatory response (Van de Wouwer and Conway, 2004). One mechanism for TM's anti-inflammatory effect relates to the properties of activated protein C (APC). For example, APC has been found to inhibit endotoxin-induced production of tumor necrosis factor-␣ (TNF-␣), interleukin (IL)-1, IL-6, and IL-8 in cultured monocytes/macrophages (Okajima, 2001). TM is also a critical cofactor for thrombin-mediated activation of the thrombin-activatable fibrinolysis inhibitor (TAFI), which is responsible for inactivation of complement factors C3a and C5a to protect against co...