Macroautophagy and Chaperone-Mediated Autophagy Are Required for Hepatocyte Resistance to Oxidant Stress

Abstract

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)/cJun 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/cJun 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 oxidantinduced liver injury. (HEPATOLOGY 2010;52:266-277)

Target of Rapamycin (TOR) and Autophagy in Chronic Hepatitis C Virus Infection: Relation to Disease Activity

“…HCV might usurp the autophagy pathway to use it as a mechanism for initiating HCV RNA replication and thereafter, it might inhibit auto lysosome maturation to protect itself from xenophagic degradation [71,76]. Autophagy facilitates inflammation resolution through suppression of inflammasome activation [77], removal of apoptotic corpses [78] and cell protection from oxidative stress through mitophagy [79]. This could explain our results as well as similar previous reports [79 -82].…”

Regulation of lipid stores and metabolism by lipophagy

“…(b) Lipotoxicity from the saturated FFA palmitate may be mediated in part by its ability to inhibit lipophagy, resulting in decreased levels of b-oxidation and palmitate metabolism, which increase cellular levels of palmitate to trigger apoptosis of atg5 sensitized hepatocytes to death from nontoxic levels of oxidative stress from the superoxide generator menadione and increased cell death from toxic levels of oxidant stress. 72 In the absence of autophagy, menadione-induced decreases in cellular rates of b-oxidation and ATP content were amplified and sustained rather than transient. These findings suggest that lipophagy-generated FFAs are critical to sustain rates of b-oxidation sufficient to maintain cellular energy homeostasis under oxidant stress.…”

Autophagy: A Crucial Moderator of Redox Balance, Inflammation, and Apoptosis in Lung Disease

“…Accumulating evidence using pro-oxidant compounds has suggested that autophagy represents a general inducible response to oxidative stress (22,144,145,154,181). Additionally, autophagy can be modulated during altered states of oxygen tension, including hyperoxia, and hypoxia, corresponding to increased or decreased pO 2 , respectively (91,171).…”

“…These effects of exogenous H 2 O 2 on autophagy could be inhibited by synthetic antioxidants (22). Autophagy can also be upregulated by ROS generated from redox-cycling compounds, such as menadione (2-methylnaphthalene-1,4-dione), which produce intracellular H 2 O 2 and O 2 -by electron transfer reactions in the mitochondria (181). In addition to macroautophagy, recent studies suggest that chaperone-mediated autophagy can be induced by menadione and may assist in the processing of oxidatively-modified proteins (72).…”