The function of the lysosomal degradative pathway of autophagy in cellular injury is unclear, because findings in nonhepatic cells have implicated autophagy as both a mediator of cell death and as a survival response. Autophagic function is impaired in steatotic and aged hepatocytes, suggesting that in these settings hepatocellular injury may be altered by the decrease in autophagy. To delineate the specific function of autophagy in the hepatocyte injury response, the effects of menadione-induced oxidative stress were examined in the RALA255-10G rat hepatocyte line when macroautophagy was inhibited by a short hairpin RNA (shRNA)-mediated knockdown of the autophagy gene atg5. Loss of macroautophagy sensitized cells to apoptotic and necrotic death from normally nontoxic concentrations of menadione. Loss of macroautophagy led to overactivation of the c-Jun N-terminal kinase (JNK)/c-Jun signaling pathway that induced cell death. Death occurred from activation of the mitochondrial death pathway with cellular adenosine triphosphate (ATP) depletion, mitochondrial cytochrome c release, and caspase activation. Sensitization to death from menadione occurred despite up-regulation of other forms of autophagy in compensation for the loss of macroautophagy. Chaperone-mediated autophagy (CMA) also mediated resistance to menadione. CMA inhibition sensitized cells to death from menadione through a mechanism different from that of a loss of macroautophagy, because death occurred in the absence of JNK/c-Jun overactivation or ATP depletion.
Conclusion: Hepatocyte resistance to injury from menadione-induced oxidative stress is mediated by distinct functions of both macroautophagy and CMA, indicating that impaired function of either form of autophagy may promote oxidant-induced liver injury.