Non-alcoholic fatty liver disease (NAFLD) caused by obesity and excessive/unbalanced diet has become a common liver disease worldwide. According to statistical and epidemiological analysis, 1.9 billion people worldwide suffer from NAFLD, accounting for about 25% of the world’s population (Younossi et al., 2018; Younossi et al., 2019). The etiology of NAFLD is complex and accompanied by multiple complications. Therefore, the development of suitable therapies is slow, and the number of candidate drugs that have successfully entered the late clinical stage is small and challenging to develop. A healthy lifestyle remains the key to prevention and treatment (Romero-Gomez et al., 2017; Sheka et al., 2020). There is an urgency to explore practical strategies for treating NAFLD.

NAFLD is a metabolic stress-induced liver injury disease, mainly manifested by steatosis and lipid accumulation in hepatic parenchymal cells. According to the course of the disease, it is divided into simple fatty liver, steatohepatitis, liver fibrosis, and hepatocellular carcinoma (Marengo et al., 2016; Nassir, 2022). The link between the main disease features of NAFLD and the actual events leading to hepatic cell death—insulin resistance, lipid metabolism imbalance, inflammation, and oxidative stress—is increasingly recognized (Zhao et al., 2021). Programmed cell death is a gene-directed form of cell self-extinction, which is a necessary condition for the normal development of biological tissues, including apoptosis, autophagy, pyroptosis, necroptosis, ferroptosis, and cuproptosis (Lockshin, 2016; Bedoui et al., 2020; Kopeina and Zhivotovsky, 2022). Multiple studies have found that lipophagy, a form of selective autophagy, is significantly inhibited during the development of NAFLD (Chen and Lin, 2022). Alleviating lipid metabolism disorders by restoring lipophagy dysregulation is an effective strategy to prevent and treat NAFLD. Furthermore, the mammalian target of rapamycin (mTOR), an atypical serine/threonine protein kinase and a member of the phosphatidylinositol kinase-related kinase (PIKK) family, plays an important role in NAFLD as Table 1 shows. mTOR exists in two complexes, the target of rapamycin complex 1 (mTORC1) and rapamycin complex 2 (mTORC2). Among them, mTORC1 is a valuable negative core lipophagy regulator (Kim and Guan, 2015; Liu et al., 2022).

This article briefly discusses the role of mTORC1 in the formation of lipophagy, summarizes its intervention effect on the regulation of lipophagy in NAFLD, and preliminarily clarifies the relationship between mTORC1 and lipophagy and NAFLD.

The term “autophagy” was first introduced by Christian de Duve at the International Conference on Lysosomes in 1963 (Klionsky, 2008; Wen and Klionsky, 2020). More than 20 years later, Yoshinori Ohsumi gradually explored the physiological significance and molecular mechanism of autophagy, laying a solid foundation for the field of autophagy (Tsukada and Ohsumi, 1993; Ohsumi, 2014).

Autophagy is a phenomenon of cellular degradation within lysosomes of defective or aged organelles and proteins, invading pathogens and some cytoplasmic components after receiving induction signals from autophagy-related genes, and is also considered type II programmed cell death (Levine and Klionsky, 2004; Li et al., 2020b). There are three known pathways of cellular autophagy: macroautophagy, microautophagy, and molecular chaperone-mediated autophagy. In general, substrates are classified according to the size and type of the degraded substrate, the scale of their degradation, and the various mechanisms by which the substrate is transported to the lysosome (Parzych and Klionsky, 2014; Bednarczyk et al., 2018; Mizushima and Levine, 2020). Macroautophagy is present in all eukaryotes and is the dominant form of autophagy. Unless otherwise stated, the term “autophagy” generally refers to macroautophagy (Yang and Klionsky, 2009).

Lipophagy is the process in which cells selectively recognize and degrade cholesteryl esters and triglycerides in lipid droplets (LDs) by activating autophagy-associated molecules to form free cholesterol, fatty acids, and glycerol to prevent excessive accumulation of lipids (Schulze et al., 2017). The co-localization of autophagy markers and LDs markers in hepatocytes and the involvement of autophagy in the clearance of lipid droplets and triglycerides identified for the first record the presence of lipophagy (Singh et al., 2009). The series of processes involved in lipophagy is that under the stimulation of internal and external factors, a membrane vesicle with a double-layer membrane structure appears in the cytoplasm, which gradually bends into a cup-like vesicle system through continuous extension and expansion: autophagic precursors. The autophagic precursor can enclose the nearby LDs and form an autophagosome after complete closure. The outer membrane of the autophagosome fuses with the lysosome and enters the lysosome with a monolayer structure. Inside the lysosome, the monolayer structure and contents of the autophagosome can be rapidly degraded by the action of related enzymes and eventually degraded to free fatty acids and glycerol. (Kim et al., 2001; Shin, 2020; Li and Peng, 2022).

mTORC1 is a negative regulator of lipophagy, the main upstream regulatory pathways (PI3K/AKT, RAS/RAF, AMPK, Wnt, and TNF) and its function in lipophagy are shown in Figure 1.

mTORC1 inhibits lipophagy by affecting autophagosome formation. The ATG kinase complex induces autophagosome formation in yeast macrophages (Kamada et al., 2000). In mammalian cells, the complex consists of the Unc-51 serine/threonine kinase family ULK1/ULK2, ATG13, and 200 kDa adhesion point kinase family interaction proteins (FIP200) (Hara et al., 2008; Ganley et al., 2009; Kim et al., 2011). ULK1/2 is a downstream target of mTORC1, their binding interaction is influenced by nutritional status (Hosokawa et al., 2009b; Nagy et al., 2014). Under nutrient-rich conditions, mTORC1 binds to the complex and dissociates from nutrient-deficient states. When mTORC1 associates with the complex, it can inactivate ULK1/2 and ATG13 by inducing their phosphorylation. When cells are treated with the mTOR inhibitor rapamycin or are undernourished, mTORC1 dissociates from the ULK complex, stimulating the process of lipophagy (Hosokawa et al., 2009a; Jung et al., 2009).

In addition, the Class Ⅲ PI3K complex, a downstream regulator of the ULK1 complex, is also significant for the formation of autophagosomes (Longatti and Tooze, 2009). This complex consists of PIK3C3/VPS34, PIK3R4/p150, and Beclin-1 in mammalian cells (Kihara et al., 2001; Furuya et al., 2005). Beclin-1 binds to Class III PI3K to form the Beclin-1-PIK3C3/VPS34 complex, the effect of which depends mainly on the status and activity of Beclin-1 (Kang et al., 2011). For example, the anti-apoptotic protein Bcl-2 binds Beclin −1 and prevents its interaction with PIK3C3, and the Beclin −1 binding protein Rubicon inhibits PIK3C3 activity in Class III PI3K complex (Pattingre et al., 2005; Matsunaga et al., 2009). In addition, SH3GLB1/Bif-1, a positive regulator of the Class III PI3K complex, promotes autophagy by facilitating the interaction of UVRAG with Beclin-1 (Takahashi et al., 2007). AMBRA1, another positive regulator of the Class III PI3K complex, also promotes autophagosome nucleation directly by facilitating the exchange of Beclin-1 with the kinase VPS34 (Di Bartolomeo et al., 2010). Moreover, mTORC1 can directly phosphorylate AMBRA1 to inhibit lipophagy and also directly phosphorylate ATG14 in the Class III PI3K complex through several sites (Ser3, Ser223, Thr233, Ser383, and Ser440) to inhibit lipophagy (Nazio et al., 2013; Shimobayashi and Hall, 2014). PI3P generated by the Class III PI3K complex is required for autophagy in yeast and mammals (Burman and Ktistakis, 2010). For example, the ATG18 direct homologs WIPI1 and WIPI2 in mammalian cells can participate in autophagy by binding to PI3P under starvation or amino acid deficiency conditions (Polson et al., 2010). Moreover, WIPI2 has been shown to function by binding directly to ATG16-L1 in the ATG12-ATG5-ATG16-L1 complex associated with the ubiquitin-like protein binding system to form autophagosomes. mTORC1 has been shown to phosphorylate the Ser395 site of WIPI2 and induce specific interaction of WIPI2 with the E3 ubiquitin ligase HUWE1 to control autophagosome formation (Dooley et al., 2015; Wan et al., 2018).

mTORC1 also has significant effects on autophagosome and lipophagy gene related transcription factors. It has been found that under nutrient-rich conditions, mTORC1 phosphorylates ribosomal protein S6 kinase (S6K2), forming a complex with nuclear receptor co-repressor 1 (NCoR1). In addition, mTORC1 binds to and phosphorylates transcription factor EB (TFEB), the master transcription factor responsible for ATG and lysosomal genes, remaining in the cytoplasm. In contrast, in the absence of nutrients or inactivation of mTORC1, nuclear translocation of NCoR1 is inhibited. TFEB is subsequently dephosphorylated and enters the nucleus to participate in the transcription of lysosomal genes. ULK1 kinase is activated, which in turn induces lipophagy. Upon activation of lipophagy, on the one hand, peroxisome proliferator-activated receptor α (PPARα) is activated by degrading NCoR1. On the other hand, activated PPARα upregulates lipophagy through AMPK/mTOR signaling, decreasing intrahepatic lipid content and stimulating ß-oxidation in the liver and hepatocytes (Martina et al., 2012; Saito et al., 2019; Tong et al., 2019). Furthermore, studies have shown that PPARα and farnesol-like X receptor (FXR) compete for the same DNA binding site in the ATG gene promoter with opposite transcriptional outputs and that pharmacological activation of PPARα reverses the normal inhibition of lipophagy in the fed state and induces lipid degradation (Lee et al., 2014a).

mTORC1 also has downstream negative regulators of lipophagy. Koren first indicated that death-associated protein 1 (DAP1) is involved in the negative regulation of autophagy. Further studies showed that mTOR is a specific kinase of DAP1 (Koren et al., 2010; Yahiro et al., 2014). In addition, mTORC1 has also been shown to inhibit autophagosome formation through direct phosphorylation and activation of acetyltransferase p300. Acetyltransferase p300 has been identified as a direct phosphorylation substrate for mTORC1. mTOR phosphorylates and activates four serine residues (Ser2271, Ser2279, Ser2291 and Ser2315) in the C-terminal structural domain of p300 (Wan et al., 2017).

mTOR has been found to have a significant role in the terminal phase of lipophagy. mTORC1 phosphorylates the Ser498 site of UVRAG to prevent the maturation of autophagosomes (Kim et al., 2015). And during the sustained starvation response, autophagosomes formed by the fusion of autophagosomes with lysosomes can reactivate mTOR, providing negative feedback to inhibit further autophagosome formation and prevent the breakdown of more components than necessary for survival (Munson and Ganley, 2015).

mTORC1, as a central negative regulator of adipose autophagy, exerts an essential regulatory role on the initiation, formation, and end stages of lipophagy and is a highly effective therapeutic target. Moreover, restoration of lipophagy dysregulation based on mTORC1 plays different effects in different stages of NAFLD development. In the early stage of NAFLD progression, the leading role of lipophagy is to degrade LDs in hepatocytes to alleviate steatosis and thus improve NAFLD. Although lipophagy is thought to provide binding energy for the activation of hepatic fibroblasts in the later stages of NAFLD development, enhanced autophagy has also been shown to inhibit the activation of HSCs and thereby ameliorate liver fibrosis. Therefore, accurately assessing the progress stage of NAFLD and targeting the intervention of lipophagic signaling molecules in hepatocytes are more conducive to the treatment of NAFLD. Lipophagy makes essential contributions in regulating hepatic mass metabolism and maintaining energy homeostasis and also provides valuable novel ideas for the therapeutic approach of NAFLD. Multiple molecular mechanisms regulate lipophagy, and in this paper, we discuss mTORC1-centered lipophagy to provide a candidate strategy for the clinical treatment of NAFLD.