Autophagy is induced by two key kinases, mechanistic target of rapamycin kinase (mTOR) complex 1 (mTORC1) and protein kinase AMP-activated catalytic subunit (PRKA)/adenosine monophosphate-activated protein kinase (AMPK). It integrates upstream molecular signals, activates the unc-51 like autophagy activating kinase 1 (ULK1/Atg1) kinase complex, and ultimately induces autophagy [18].

AMPK is a critical kinase regulating energy homeostasis and one of the central regulators of eukaryotic cell and organism metabolism. It is responsible for the control of the input and output of cells and the regulation of the stability of cellular physiological activities [19]. Various substances in the body activate AMPK, which in turn inhibits lipogenesis, increases fatty acid oxidation, and plays a crucial role in the synthesis of cholesterol and total TG [20]. Additionally, a study revealed that AMPK regulates autophagy through the autophagy-related protein 1 (Atg1), participating in autophagy initiation and enhancing autophagosome maturation, thereby enabling the fusion of autophagosomes and lysosomes [21].

AMPK is activated through the phosphorylation by the upstream AMPK kinases (AMP-KKs). Activated AMPK regulates multiple metabolic processes like autophagy [22]. AMPK enhances autophagy through the phosphorylation of autophagy-related proteins in the mTORC1, ULK1, and PIK3C3/VPS34 complexes or indirectly by controlling the expression of autophagy-related genes downstream of transcription factors, including Forkhead box O3 (FOXO3), Transcription Factor EB (TFEB), and Bromodomain-containing protein 4 (BRD4) [23, 24].

A study revealed that zinc is an effective lipophagy promoter. Zinc-induced autophagy and lipid metabolism include the up-regulation of protein kinase-$\beta$ (Ca${}^{2+}$/CaMKK$\beta$)/AMPK pathway. It induces the transcriptional activation of key genes associated with autophagy and lipolysis, the activation of autophagy and lipid metabolisms through the up-regulation of phagocytosis in hepatocytes and the decrease of hepatic lipid accumulation [25].

A study indicated that stearoyl-CoA desaturase 1 (SCD1) inhibits hepatic steatosis by inducing AMPK-mediated lipophagy. SCD1 is a potential therapeutic target for NAFLD. SCD1 activity is enhanced in NAFLD individuals, and the deletion of the SCD1 gene decreases liver lipid synthesis [26]. AMPK activity is influenced by SCD1: the inhibition of SCD1 enhances AMPK phosphorylation and alters the expression of genes associated with adipogenesis [27]. A study showed that SCD1 inhibition induces hepatocyte lipophagy and apoptosis of human hepatocellular carcinoma (HCC) through the AMPK signaling pathway [28].

A study showed that the phosphorylation of AMPK is increased, and the expression of hepatic autophagy markers is also increased in the livers of mice fed with barley sprout extracts (BSE). Saponin is a major flavonoid in BSE and an activator of AMPK [29]. Saponins enhance the degradation of LD in the liver and the subsequent activation of fat oxidation by activating AMPK, thereby inhibiting hepatic steatosis in alcohol-fed mice [30].

Sirtuin3 (SIRT3), a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase, is an important biomolecule regulating macroautophagy and lipid metabolism [31]. SIRT3 overexpression activates macroautophagy using the AMPK-ULK1 pathway, primarily acting on LDs. Thus, it results in smaller LD and decreases lipid accumulation [32].

However, the AMPK pathway involves hepatic lipophagy and hepatic lipid metabolism. The fatty acid translocase cluster of differentiation (CD36) is a multifunctional membrane protein promoting long-chain fatty acid uptake [33]. The current study revealed that CD36 expression in mice fed a high-fat diet is increased in the liver and autophagy is reduced. CD36 is involved in fatty acid oxidation by activating the adenosine monophosphate-activated protein kinase and regulating hepatic autophagy using the AMPK pathway [34].

Moreover, other proteins including ghrelin, TGF-$\beta$-activated kinase-1 (TAK1), and 4-chlorodipicolinate complex, activate the AMPK pathway (Table 1, Ref. [18]) [35, 36, 37].

A phosphatidylinositol 3-kinase-related kinase (PIKK) protein, an mTOR is a highly conserved serine/threonine (Ser/Thr) protein kinase. The mTOR exists as two protein complexes: mTORC1 and mTORC2. They behave as central controllers regulating cellular metabolism, growth, proliferation, survival, and autophagy processes [38]. mTOR inhibitors block the mTOR signaling pathway, leading to anti-inflammatory, anti-proliferative, autophagic, and apoptosis-inducing characteristics. Unlike the AMPK pathway, mTOR activation inhibits autophagy, and its inhibition activates autophagy. Among them, rapamycin can bind to the target of mTORC1 and is an mTOR inhibitor [39].

A recent study observed that Irbesartan (IRB) inhibits p-Akt and p-mTOR by activating PPAR-$\gamma$ and p-AMPK, triggering the activation of autophagy in the liver of db/db mice [40]. According to the results, IRB may improve hyperlipidemia and hepatic steatosis by upregulating PPAR-$\gamma$ expression, blocking the AMPK/Akt/mTOR signaling pathways, and inducing liver autophagy [41].

The mitochondrial fusion protein 2 (Mfn2) induces autophagy through the PI3K/AKT/mTOR signaling pathway. Mfn2 increases the expression of LC3-II, Atg5, and Bcl-2 in liver cells and down-regulates the expression of p62 and Bax [42]. Rapamycin Mfn2 significantly inhibits the expression of p-PI3K, p-Akt, and p-mTOR in liver cells, thereby inducing lipophagy [43].

In addition, the rate-limiting enzyme of peroxisomal $\beta$-oxidation, acyl-Coenzyme A oxidase 1 palmitoyl (ACOX1) increases with fasting or with a high-fat diet (HFD). Liver-specific ACOX1 knockout (acox1-LKO) protects mice from hepatic steatosis induced by starvation or HFD by inducing lipophagy [44]. Hepatic ACOX1 deficiency decreases the total cytosolic acetyl-CoA levels [45]. The decrease of acetyl-coenzyme A weakens the acetylation of mTORC1 regulation-related protein (regulatory associated protein of mTOR complex 1, RPTOR/RAPTOR), thereby eliminating the inhibitory effect on autophagy [27].

Additionally, branched-chain amino acids (BCAAs) in the liver activate mTOR, enhance hepatocyte apoptosis, block hepatic free fatty acids (FFA)/TG transformation, inhibit lipid-induced autophagy, and increase hepatocyte lipotoxicity and susceptibility to FFA [46]. BCAA inhibits autophagy, and obstacles the self-repairing mechanism that is protective against lipotoxicity, thereby aggravating liver injury [47].

There is evidence that ezetimibe induces autophagy even under hyperactivated mTOR conditions through the AMPK-TFEB pathway. A study indicated that ezetimibe reduces hepatic steatosis, inflammation, and fibrosis by enhancing autophagy through the PRKA activation and nuclear translocation of TFEB [48].

Vacuolar protein sorting 34 (Vps34) is inside a class III PI3Kinase (PIK3C3) among mammals. Phosphatidylinositol (PI) synthesizes phosphatidylinositol 3-phosphate (PI3P), which is essential for the extension of autophagy vesicles and the recruitment of ATG proteins to autophagy vesicles. Vps34 is activated by binding to Vps15 and binds to Beclin-1 to develop the Vps34-Vps15-Beclin1 complex [49].

A recent study indicated that Vps34 can be acetylated using acetyltransferase p300. In contrast, p300-mediated acetylation can inhibit the activity of Vps34. Acetylation reduces the affinity between Vps34 and the substrate PI and hinders the formation of the Vps34-Beclin1 core complex [50]. The inactivation of p300 induces Vps34 deacetylation, PI3P production, and autophagy even in AMPK${}^{-/-}$, TSC2${}^{-/-}$, or ULK1${}^{-/-}$ cells. p300-dependent Vps34 acetylation/deacetylation is the physiological key to Vps34 activation, controlling the initiation of typical autophagy and atypical autophagy [51].

Although the amino acid sequences of Beclin 1 and Atg6/vps30 differ significantly (24.4% sequence identity and 39.1% sequence similarity), Beclin 1 is a mammalian homolog of the yeast Atg6/vps30. Beclin 1 regulates autophagy in mammalian cells like yeast by forming complexes using different proteins [52].

The content of Beclin-1 in cells determines the degree of autophagy, and it is a marker protein for autophagy formation [53]. Beclin-1 is phosphorylated by Atg1 and is the whole scaffold of the PI3K complex to enhance the localization of autophagy proteins to autophagy vesicles [54]. It selectively participates in different stages of autophagy when the PI3K complex binds to other regulatory proteins. These include the formation of autophagy vesicles by binding to Atg14 and the maturation and transport of autophagy vesicles by binding to the UV radiation resistance-associated gene (UVRAG) [55].

Sirtuin type1 (SIRT1) is a member of the family of silent information regulators and it is a nicotinamide adenine dinucleotide NAD+-dependent histone/protein deacetylase. The deacetylation of proteins and histones upregulates or downregulates gene transcription and protein function [56]. A previous study showed that the regulation of the deacetylation of SIRT1 can induce autophagy and decrease lipid accumulation in the liver [57].

Berberine stimulates SIRT1 deacetylation and induces autophagy in an autophagy protein 5-dependent manner. It also promotes gene expression and circulation of fibroblast growth factor 21 (FGF21) and ketone bodies within mouse liver in a SIRT1-dependent way. Autophagy and FGF21 activation in the liver regulate lipid storage as well as the utilization and whole-body energy metabolism [58].

Both resveratrol and caloric restriction upregulate the SIRT1-autophagy pathway to reduce hepatic steatosis in rats fed with a high-fat diet. Autophagy is later induced by the SIRT1-FOXO3 signal transduction pathway [59]. A recent study observed that the natural polyphenol resveratrol enhances the levels of cAMP, SIRT1, phosphorylated protein kinase A (pPRKA), phosphorylated AMP-activated protein kinase (pAMPK), and SIRT1 activity in HepG2 cells. Moreover, it can induce autophagy through the cAMP-PRKA-AMPK-SIRT1 signal pathway, whereas autophagy inhibition markedly abolishes resveratrol-mediated hepatic steatosis improvement [60].

Caloric restriction and resveratrol in animal studies can affect the sirtuin system. A randomized trial on healthy people revealed that the reduction of calories and supplementation with resveratrol significantly increases SIRT1 plasma levels [60]. Resveratrol increases the expression of SIRT1 mRNA in the liver of mice fed with a high-fat diet and controls the number and function of human adipocytes in a SIRT1-dependent manner [59].

However, SIRT1 also down-regulates the sterol regulatory element-binding protein (SREBP) homologs during fasting, inhibiting lipid synthesis and storage. SREBPs are steroid regulatory element binding proteins and transcription factors. A previous study revealed that SIRT1 can directly deacetylate SREBP [61]. Therefore, the modulation of SIRT1 activity may affect SREBP ubiquitination, protein stability, and target gene expression. SREBPs undergo cleavage-induced activation due to the low sterol levels in the cell, thus promoting the transcription of enzymes critical to sterol biosynthesis [62].

TGs in the liver are sequestered into the LDs, which are the main lipid storage organelles. LDs are home to proteins, the most abundant of which are the perilipins (PLINs) [7].

It has been observed that the overexpression of PLIN2 protects small LD from macroautophagy/autophagy, and PLIN2 deficiency enhances autophagy and depletes liver TG [63]. When mice are hungry, the degradation of PLIN2 and PLIN3 in hepatocytes is enhanced using autophagy. In contrast, the increase of adipose triglyceride lipase (ATGL) and LC3 is observed on the surface of LD [64]. In addition, PLIN1 can colocalized with the selective autophagy receptor p62, mediating the recognition of LD through the autophagy initiation membrane [65].

Neutral lipids in mouse liver and cultured hepatocytes are mobilized through the sequestration of perilipin 2-coated LDs in autophagosomes. This involves that LDs are delivered to the lysosomes, where triacylglycerols are hydrolyzed by lysosomal lipases [66].

The research identified that unphosphorylated perilipin 1 prevents Rab7 from docking to LDs and inhibits lipophagy. Moreover, Rab7 is recruited to lysosomes through conformational changes in perilipin 1, which are induced by phosphorylation with the association of LD with lysosomes [67].

ATGL is closely associated with lipophagy. Although ATGL is an essential molecule in direct lipid decomposition, it mediates lipid decomposition synergistically through lipophagy [68]. When lipophagy is inhibited, the increase in lipid decomposition due to ATGL overexpression is also inhibited. It is speculated that ATGL may decrease the size of LDs through lipolysis and develop the conditions for lipophagy [69].

In addition, ATGL is a selective autophagy receptor and mediates the specific recognition of LD by LC3 on the autophagy membrane through the LC3-interacting region (LIR) [70].

Autophagy stimulates lipophagy by the co-localization of ATGL and LD. Similarly, autophagy activation increases the co-localization of ATGL and LD [71]. Moreover, the correlation between ATGL and LD in autophagy-deficient cells significantly decreased, indicating that the localization ability of the lipase on LD is deficient [72]. The decrease in lipolytic activity of ATGL in autophagy-deficient cells cannot be attributed to the change in its association with LD. This is because when ATGL is reduced in hepatocytes, its binding to the autophagy inhibitor G0/G1 switch gene 2 (G0S2) decreases [70]. Moreover, its binding to the autophagy activation gene marker 58 (CGI-58) increases. These phenomena indicate that the relationship between ATGL and liver LD affects lipophagy activation [72].

Peroxisome proliferator-activated receptor-$\alpha$ (PPAR$\alpha$) is a ligand-induced nuclear receptor protein and a PPAR subtype. PPAR$\alpha$ controls liver autophagy-regulating gene transcription of autophagy-related proteins, thereby regulating fatty acid $\beta$-oxidation and autophagy [73]. Researchers speculate that PPAR$\alpha$ can link autophagy signal and gene transcription [74].

PPAR$\alpha$ is the main isomer inside the liver. PPARs are from the nuclear receptor family of ligand-activated transcription factors. PPAR ligands include endogenous lipids like FFA and lipid metabolites [75]. After ligand binding, PPAR binds to the PPAR response element inside the promoter of the target gene and heterodimerizes with another nuclear receptor, the retinoid X receptor (RXR) [76]. Several co-activators and co-repressors associate with PPAR/RXR heterodimers to regulate the transcriptional activity. PPAR/RXR modulates mitochondrial and peroxisomal beta-oxidation, fatty acid (FA) uptake, and lipolysis by regulating the expression of genes encoding these enzymes or proteins. Autophagy-lysosome-mediated lipophagy of TG in the liver is controlled by PPAR$\alpha$ [77].

A study revealed that zinc is an effective promoter of fat phagocytosis. The administration of zinc significantly reduces lipid accumulation in hepatocytes. It enhanced the release of free fatty acids, which is associated with elevated fatty acid oxidation and inhibition of fat formation, followed by autophagy activation. Zn${}^{2+}$ activates autophagy and lipid depletion by promoting the metal response element binding transcription factor (MTF)-1DNA binding within the PPAR$\alpha$ promoter region to induce the transcriptional activation of key genes associated with autophagy and lipolysis [25].

High-fat diet significantly induces hepatic steatosis in rats. In contrast, pioglitazone reverses hepatic steatosis due to a high-fat diet. Pioglitazone combined with a high-fat diet dramatically decreases the content of serum insulin and liver TG [78]. Pioglitazone significantly increases the expression of lipolysis-related proteins (ATGL, HSL), $\beta$-oxidation-related proteins (CPT-1A), and autophagy-related proteins (ATG7, LC3, LAL) in hepatocytes [79]. Therefore, the downregulation of the PPAR$\alpha$/PPAR$\gamma$ in AML12 cells can effectively decrease the expression of ATGL, CPT-1A, and LC3II induced by pioglitazone. Pioglitazone attenuates hepatic steatosis by promoting intracellular lipolysis, $\beta$-oxidation, and autophagy, depending on PPAR$\alpha$ and PPAR$\gamma$ [5].

Some researchers also identified a nuclear receptor, farnesoid X receptor (FXR), which has an opposite effect than that exerted by PPAR. PPAR$\alpha$ and FXR are activated in fasting and fed livers. Both PPAR$\alpha$ and FXR regulate liver autophagy in mice. PPAR$\alpha$ induces autophagic lipid degradation or lipophagy, while FXR strongly suppresses the induction of autophagy in the fasting state. Although PPAR$\alpha$ and FXR compete to bind to the common sites in the autophagy gene promoter, they possess an opposite regulatory effect [74].

The Rab GTPase protein family is a crucial regulator of intracellular vesicular trafficking. The Rab family members regulate a series of molecular events by cycles between active GTP-states and inactive GDP-states [80]. The Rab protein family is vital in LD metabolism. Approximately 30 Rab family proteins have been identified on the LD surface [81]. Among them, Rab7 protein is widely involved in the regulation of the autophagosome maturation and intracellular transport. Hepatocyte starvation activates Rab7 on the surface of LDs and promotes lysosomal transport to LDs [82]. Rab7 expression in the liver of rats subjected to alcohol liquid diet is also significantly reduced. These results indicate that Rab7 regulates lipid metabolism through lipophagy [83]. Rab10 is a member of the Rab family of small GTPase located on the LD surface. Activated Rab10 in starved hepatocytes is co-located on the LD surface with the autophagy proteins Lc3 and Atg16. In addition, Rab25 and Rab32 can be involved in the molecular mechanism of lipophagy [84]. However, the involvement of other members of the RAB family in regulating lipophagy requires further exploration.

It has been observed that specific neurons within the central nervous system (CNS) can remotely control lipophagy in the liver [85]. Lysosomal-associated membrane protein 3 (LAMP3) is overexpressed, activating Akt and upregulating the expression of the lipogenase FASN and SCD-1 in HepG2 cells. Additionally, the increased TG content induced by LAMP3 overexpression is attenuated by the treatment with a PI3K/Akt pathway inhibitor. These effects can regulate hepatic lipid metabolism by activating the PI3K/Akt signaling pathway [86].

Oleic acid in unsaturated fatty acids can enhance the formation of triglyceride-rich LD while inducing autophagy. Palmitic acid in saturated fatty acids is rarely converted into triglyceride-rich LD, thereby inhibiting autophagy and inducing apoptosis [87]. However, palmitic acid can induce autophagy through the protein kinase C-mediated signal transduction pathway. Moreover, palmitic acid can phosphorylate the autophagy-promoting factor Hsp27 and enhance its induced autophagy [88].

TFEB is a basic-helix-loop-helix-leucine zipper (bHLH-Zip) transcription factor in the MiT family. Many lysosomal genes are regulated by the single transcription factor TFEB, such as lipid metabolism-related genes PGC-1 $\alpha$, autophagy-related genes-Lamp1, cathepsin B, Atg9B, and LC3. TFEB plays a crucial role in the regulation of lipid homeostasis through the link of autophagy to energy metabolism at the transcriptional level [89]. TFEB expression is induced by starvation and inhibited by alcohol, while its activation is associated with the inactivation of mTORC and other TFEB dephosphorylations [90]. The transcription factor E3 also belongs to the MiT family, its function is similar to TFEB, and it is usually located in the cytoplasm. After activation, it translocates inside the nucleus and decreases the steatosis of hepatocytes by enhancing lipophagy [48].

Many other factors affect autophagy, including calcium channel blockers, phosphatidylinositol-5-phosphate 4-kinase, and thyroid hormones [91, 92]. In addition, diets rich in polyunsaturated fatty acids (PUFA), excessive exercise, and a certain degree of radiation directly or indirectly regulate lipophagy [93].

SCD1 is a crucial enzyme for controlling lipid metabolism. Sodium palmitate-treated hepatocytes had an increased SCD1 expression, AMPK inactivation, and lipophagy deficiency [108]. The inhibition of SCD1 expression in hepatocytes can enhance AMPK activity and lipophagy and reduce lipid deposition. SCD1 overexpression leads to a significant decrease in AMPK activity and lipophagy, and the lipid deposition decreased inside the hepatocytes [27]. It has been established that SCD1 controls lipophagy through AMPK and affects the lipid metabolism of primary mouse hepatocytes. Additionally, CAY10566 (a specific inhibitor of SCD1) significantly decreases hepatic steatosis and hepatic lipid droplet accumulation and promotes AMPK activity and lipophagy [109].

The average size of LD tripled by the ATGL gene knockout in AML12 cells cultured in vitro. Numerous small LDs accumulate in lysosomal acid lipase (LAL) knockout AML12 cells [110]. After ATGL+LAL knockout in the AML12 cells, the average size of LD increases by two times compared to ATGL knockout, but as opposite of LAL knockout, small LD significantly reduces [111]. These phenomena establish the synergistic effect between lipase lipolysis and lipophagy. However, the molecular mechanism between the interaction is unclear, which can be associated with the size of LD. Lipolysis may be a “tandem” process. Large LDs synthesize free fatty acids by lipase lipolysis, and lipophagocytosis occurs after esterification to synthesize smaller LD [97].

Studies on autophagy indicated that lipid metabolism can be regulated, and some external stimuli can enhance lipid metabolism in cells and lipophagy [112]. However, it is unclear whether lipid metabolism in adipocytes is promoted by the activation of lipophagy, and further research is required.