Tryptophan protects hepatocytes against reactive oxygen species-dependent cell death via multiple pathways including Nrf2-dependent gene induction

Hepatocyte apoptosis plays a key role in the pathogenesis of immune-mediated hepatitis. However, the detailed mechanisms of apoptosis signaling are still unclear and effective therapeutic drugs for hepatitis have been explored. Here, we show that tryptophan (Trp) suppressed IFN-gamma-mediated hepatic apoptosis in vitro. Trp inhibited the downstream apoptotic events of mitochondria disruption, such as cell death and caspase-3 activation, while it did not influence upstream signaling including STAT1 activation and IRF1 expression. Trp suppressed reactive oxygen species (ROS) generation at the mitochondria. IFN-gamma induced ROS in mitochondria by inhibiting complex I and III, but not II. This ROS generation by IFN-gamma required de novo protein synthesis. Trp showed relatively weak direct scavenging activity but antagonized IFN-gamma against the suppression of complex I. In addition, Trp increased the expression of the Nrf2-dependent antioxidant genes NQO1, HO-1 and GCS in hepatocytes both in vitro and in vivo. Finally, the administration of Trp in an acetaminophen-induced ROS-dependent hepatitis model suppressed the liver injury in vivo. Thus, Trp protects hepatocytes from ROS-dependent cell injury via multiple pathways. This study suggests Trp as a therapeutic antioxidant drug for hepatitis and a regulator for Nrf2-dependent genes.