The early stages of alcohol-induced liver injury involve chronic inflammation. Whereas mechanisms by which this effect is mediated are not completely understood, it is hypothesized that enhanced sensitivity to circulating lipopolysaccharide (LPS) contributes to this process. It has recently been shown that ethanol induces activation of plasminogen activator inhibitor-1 (PAI-1). PAI-1 causes fibrin accumulation in liver by inhibiting degradation of fibrin (fibrinolysis). LPS also enhances fibrin accumulation by activating the coagulation cascade. It was therefore hypothesized that ethanol will synergistically increase fibrin accumulation caused by LPS, enhancing liver damage. Accordingly, the effect of ethanol pretreatment on LPS-induced liver injury and fibrin deposition was determined in mice. Ethanol enhanced liver damage caused by LPS, as determined by plasma parameters and histological indices of inflammation and damage. This effect was concomitant with a significant increase in PAI-1 expression. Extracellular fibrin accumulation caused by LPS was also robustly increased by ethanol preexposure. Coadministration of the thrombin inhibitor hirudin or the MEK (mitogen-activated protein kinase) inhibitor U0126 significantly attenuated the enhanced liver damage caused by ethanol preexposure; this protection correlated with a significant blunting of the induction of PAI-1 caused by ethanol/LPS. Furthermore, thrombin/MEK inhibition prevented the synergistic effect of ethanol on the extracellular accumulation of fibrin caused by LPS. Similar protective effects on fibrin accumulation were observed in tumor necrosis factor receptor 1 (TNFR-1)(-/-) mice or in wild-type injected with PAI-1-inactivating antibody.
Conclusion: These results suggest that enhanced LPS-induced liver injury caused by ethanol is mediated, at least in part, by fibrin accumulation in livers, mediated by an inhibition of fibrinolysis by PAI-1. These results also support the hypothesis that fibrin accumulation may play a critical role in the development of early alcohol-induced liver injury.