Anti-inflammatory and anti-apoptotic roles of endothelial cell STAT3 in alcoholic liver injury

Background: It is generally believed that the hepatoprotective effect of interleukin-6 (IL-6) is mediated via activation of signal transducer and activator of transcription 3 (STAT3) in hepatocytes. IL-6-deficient mice are more susceptible to alcohol-induced hepatocyte apoptosis and steatosis and elevation of serum alanine transaminase (ALT); however, whereas hepatocyte-specific STAT3 knockout mice are more susceptible to alcohol-induced hepatic steatosis, they have similar hepatocyte apoptosis and serum ALT after alcohol feeding compared with wild-type mice. This suggests that the hepatoprotective effect of IL-6 in alcoholic liver injury may be mediated via activation of STAT3-independent signals in hepatocytes, activation of STAT3 in nonparenchymal cells, or both. We have previously shown that IL-6 also activates STAT3 in sinusoidal endothelial cells (SECs). Thus, the purpose of this study was to investigate whether STAT3 in endothelial cells also plays a protective role in alcoholic liver injury. Methods: Wild-type and endothelial cell-specific STAT3 knockout (STAT3^E-/-) mice were pair-fed and fed ethanol containing diet for 4 weeks. Liver injury and inflammation were determined. Results: Feeding mice with ethanol-containing diet for 4 weeks induced greater hepatic injury (elevation of serum ALT) and liver weight in STAT3^E-/- mice than wild-type control groups. In addition, ethanol-fed STAT3 ^E-/- mice displayed greater hepatic inflammation and substantially elevated serum and hepatic levels of IL-6 and TNF-α compared with wild-type mice. Furthermore, ethanol-fed STAT3^E-/- mice displayed a greater abundance of apoptotic SECs and higher levels of serum hyaluronic acid than wild-type controls. Conclusions: These data suggest that endothelial cell STAT3 plays important dual functions of attenuating hepatic inflammation and SEC death during alcoholic liver injury.