The results of this study demonstrate for the first time that (a) sRAGE, is released in the plasma within 30-45 min after severe trauma in humans; (b) the release of sRAGE in the plasma requires severe tissue injury and (c) is associated with posttraumatic coagulation abnormalities and endothelial cell and complement activation.
The first important finding of our study is that plasma levels of sRAGE are increased in severely traumatized patients within 30-45 minutes after injury. There was no significant fluid resuscitation or other potentially confounding treatment prior to blood sampling and, therefore, our findings represent the direct effects of the injury and shock on the release of sRAGE into the bloodstream. The soluble form of RAGE is devoid of the transmembrane and cytosolic domains and arises from two sources: proteolysis of the full-length receptor and alternative splicing. Recent studies have reported that RAGE is subjected to protein ectodomain shedding by metalloproteases (18
). The second form of soluble RAGE, named endogenous secretory RAGE (esRAGE), is formed by alternative splicing of RAGE mRNA (20
). The plasma levels of esRAGE are fourfold lower that sRAGE released by proteolysis, but might be expected to reflect levels of receptor expression in a more direct way than sRAGE (7
). As RAGE is activated by multiple ligands, including HMGB1, it is likely that the major function of RAGE is to propagate cellular inflammation and dysfunction due to sustained ligand-receptor interaction at sites of ligand deposition. However, soluble RAGE takes on a dominant-negative phenotype and blocks signaling of RAGE and other receptors activated by the multiple ligands of RAGE, as it has been reported in an experimental model of E. coli
-mediated acute lung injury (21
In our trauma patient population, sRAGE was elevated early after trauma and before any fluid resuscitation. Furthermore, sRAGE correlated with the severity of injury and the plasma levels of an important mediator of endothelial activation (angiopoietin-2) and complement that are both known to increase vascular permeability (22
). These results are not surprising, as a previous study has demonstrated the critical role of RAGE signaling in an experimental model of intestinal barrier dysfunction after hemorrhagic shock (24
). As we and other investigators have reported that plasma levels of Ang-2, complement and HMGB1 are elevated early after severe trauma (4
), the increase in plasma levels of sRAGE early after trauma may be interpreted as both a marker of cellular damage as well as a compensatory mechanism to modulate the severity of the inflammatory response observed after severe trauma in humans.
The second important finding of our study is that plasma levels of sRAGE correlate with coagulation abnormalities observed after severe trauma and hypovolemic shock. We found that increasing plasma levels of sRAGE correlated with increasing plasma levels of PF 1+2, a marker of thrombin activation, increasing plasma levels soluble thrombomodulin and decreasing plasma levels of protein C. Furthermore, there was a correlation between plasma levels of sRAGE and activation of fibrinolysis early after trauma. There are several potential explanations for these correlations. First, the activation of RAGE signaling by advanced glycation end-products has been shown in monocytes to increase the expression of tissue factor, a major mediator of the initiation of coagulation after tissue injury (25
), thus suggesting that the activation of RAGE signaling may contribute to the activation of coagulation in the microcirculation after trauma. Second, it has recently been reported that HMGB1, an important ligand of RAGE, is released in the plasma early after trauma (4
). It is known that the activation of RAGE signaling by its ligands induces a positive feedback on the expression of the receptor at the cell membrane (26
). Furthermore, HMGB1 has been shown to cause fibrin deposition and activation of the coagulation in healthy rat (27
). Taken together, these results indicate that RAGE and its ligands may contribute to the activation of the coagulation in the microcirculation early after trauma.
Do plasma levels RAGE measured early after trauma correlate with the later development of organ injury and outcome? Our results clearly show that there is no correlation between mortality and the plasma levels of sRAGE early after severe trauma. A previous study has reported that plasma levels of sRAGE are significantly more elevated in septic patients who ultimately died than in survivors (28
). The significance of plasma levels of sRAGE measured in trauma or septic patients is still uncertain. sRAGE levels may represent a marker of cellular damage as well as be part of a counterregulatory mechanism that modulates the severity of the posttraumatic or septic inflammatory response. Indeed it is possible that the role of sRAGE may change from a pro-inflammatory one (reflecting ligand signaling and subsequent solubilization) to an anti-inflammatory decoy effect over time after a traumatic or septic insult. Another possible reason that might explain the difference between both studies is likely related to the fact that we sampled the blood of our trauma patients within 45 min after injury while the plasma levels of sRAGE were measured later in septic patients within 24 hours after diagnosis of sepsis (4
). Indeed it is possible that our blood sampling was performed too early to detect the maximum plasma level of sRAGE after severe trauma.
Despite a lack of correlation between sRAGE levels and mortality, we found a significant correlation between plasma levels of sRAGE and the later development of acute renal failure in severely traumatized patients. Several previously published studies have reported the association between an increase in plasma levels of sRAGE and compromised renal function (29
). One hypothesis is that renal insufficiency can suppress handling and excretion of sRAGE, although the major site(s) where sRAGE is processed is currently unknown. However, the time-course of change in the renal function early after trauma using specific markers of the renal function has not be studied. Thus, it is possible that there is an early decline of the renal function after severe trauma that would explain the correlation between increase in plasma levels of sRAGE and the later diagnosis of renal dysfunction.
In summary, the results of this study demonstrate for the first time that sRAGE is released into the bloodstream early after severe trauma in humans. The release of sRAGE requires severe injury and is associated with posttraumatic coagulation abnormalities and endothelial cell and complement activation. Furthermore, we found that there was a significant relationship between plasma levels of sRAGE and the development of acute renal failure. In contrast, this relationship was not quite significant for patients who developed acute lung injury, although patients with less than 26 ventilator-free days had significantly higher plasma levels of sRAGE than those with more than 26 ventilator-free days. Finally, there was no relationship between plasma levels of sRAGE and mortality rate in trauma patients. Thus, future studies will be needed to determine the role of plasma levels of sRAGE in modulating the inflammatory response after severe trauma.