Macroautophagy hereafter referred to as
Macroautophagy (hereafter referred to as autophagy) depends on an intracellular lysosome-dependent degradation system that maintains cellular metabolism and homeostasis . Genes involved in these processes are termed autophagy-related genes (ATGs), which regulate autophagosome formation and complicated membrane reorganization in autophagy [12,13]. Although autophagy is generally a cell survival mechanism to protect angiogenic from various stresses and injuries , it can promote cell death in some cases . Notably, recently accumulating evidence has shown that excessive autophagy activation promotes ferroptotic cell death . In particular, the autophagic turnover of ferritin, namely ferritinophagy, is required for erastin-induced ferroptosis [17,18]. However, these findings are not enough to fully elucidate the molecular mechanism and regulation of ferroptosis by selective autophagy.
The liver is essential for the body's fat metabolism though its storing and processing of lipids, and it is continuously accumulating lipids and thus is persistently threatened by lipotoxicity [19,20]. In this study, we provided the first evidence that lipophagy, namely the degradation of intracellular lipid droplets (LDs) via autophagy [, , ], contributes to ferroptosis through the production of lipid peroxidation in hepatocytes. Accordingly, the genetic inhibition of lipophagy was found to limit ferroptosis in vitro and in vivo. This study provides a potential mechanistic explanation on how autophagy induces ferroptosis in hepatocytes.
Discussion The synthesis, storage, and degradation of neutral lipids such as triacylglycerols and steryl esters are dynamic processes . Impaired lipid metabolism is implicated in inflammation, immunity, and cell death . Lipid peroxidation, the process of oxidative degradation of lipids, plays a central role in the induction of ferroptosis through increased lipotoxicity [, , ]. In the current study, we evaluated the role of LDs in the process of ferroptosis, and further examined the underlying mechanisms in this molecular context. We demonstrated that lipophagy promotes lipid peroxidation in ferroptosis through decreased lipid storage and increased lipid degradation. Such mechanisms enable cells to use selective autophagy to induce cell death when the demands for lipid composition change. Prior research indicated that ferroptosis was different from other RCDs because the pharmacologic inhibition of apoptosis, necrosis, necroptosis, and autophagy by small-molecule inhibitors (e.g., Z-VAD-FMK, BOC-D-FMK, wortmannin, and necrostatin-1) fails to reverse ferroptosis activator-induced cell death . Current evidence suggests that autophagy-dependent cell death involves ferroptosis [17,18]. Genetic or pharmacologic inhibition of autophagy blocks ferroptosis in cancer and noncancer cells [17,18]. Autophagy delivers cytoplasmic material and organelles to lysosomes for degradation. Lysosomes and lysosomal proteases were found to participate in various cell death pathways. Consequently, the inhibition of signal transducer and activator of transcription 3 (STAT3)-dependent cathepsin B expression and lysosomal membrane permeabilization also protects against ferroptosis , indicating a complex interplay between these different RCDs. The role of autophagy in cell death is context-dependent. Differences in the degraded substrate can determine the pro-survival or pro-death functions of autophagy. Autophagy inhibits apoptosis and necroptosis by degrading their regulators . In contrast, ferritin degradation by autophagy (namely ferritinophagy) promotes ferroptosis through the induction of free iron release and subsequent oxidative injury [17,18]. This process requires core autophagy regulators Atg5 and Atg7, which are needed for autophagosome formation [39,40], as well as nuclear receptor coactivator 4 (NCOA4), which is the cargo receptor for autophagic ferritin degradation . Our current study found that LD degradation by autophagy (namely lipophagy) increases ferroptosis by inducing lipid release and subsequent lipid peroxidation. In addition to ATG5, this process requires RAB7A, a cargo receptor for autophagic LD degradation . In contrast, increased lipid storage by TPD52 diminished lipid peroxidation in ferroptosis. These findings indicate that the balance between synthesis, storage, and degradation can manipulate the ferroptotic process.