The role of thrombin in lipopolysaccharide-induced liver injury

Systemic exposure to Gram negative bacterial lipopolysaccharide (LPS) triggers a cascade of events that culminates in injury to tissues, including the liver. The hypothesis tested was that thrombin participates in LPS-induced liver damage by a receptor-mediated mechanism independent of the formation of occlusive fibrin clots. Perfusion of livers isolated from LPS-treated rats with blood containing ancrod resulted in significant release of alanine aminotransferase (ALT) which was not observed when the blood was treated with heparin. Perfusion of similar livers with buffer containing a -thrombin induced release of ALT in amounts comparable to those observed with ancrod-treated blood. These results indicate that thrombin is a critical mediator of LPS-induced liver damage and contributes to liver injury through a mechanism that is independent of clot formation. Thrombin was not cytotoxic to isolated hepatic parenchymal cells. Neither addition of LPS to the culture medium nor treatment of rats with LPS prior to hepatocyte isolation influenced this result. In the isolated, buffer-perfused liver, thrombin causes injury in a dose-dependent manner with an EC50 of approximately 0.4 nM. This concentration was consistent with a receptor-mediated event. Moreover, thrombin receptor activating peptide reproduced the damaging effect of thrombin in this system. Platelets and neutrophils (PMNs) are two cell types critical to LPS-induced liver damage and with which thrombin may interact. Platelet depletion of LPS-treated rat liver donors did not influence thrombin hepatic toxicity, but PMN depletion abolished this effect. To identify the extrahepatic factors required for LPS-induced liver injury, naive livers were perfused with buffer containing LPS, PMNs and/or thrombin. Livers were damaged maximally by the combination of all three factors. This demonstrates that the only critical, extrahepatic mediators needed for LPS-induced liver damage are thrombin and PMNs. Thrombin did not stimulate or enhance degranulation of rat PMNs, and it was not directly toxic to isolated rat hepatocytes in the presence of PMNs in vitro even after LPS exposure. Accordingly, hepatocyte killing by the PMN/thrombin combination appears to require an additional cellular and/or soluble mediator(s). These results suggest that thrombin causes tissue injury during inflammation through activation of a cellular receptor and that its role in the pathogenesis involves interactions with blood PMNs and perhaps nonparenchymal liver cells.