**5.1 Lipolysis**

In hepatocytes, lipolysis and lipophagy together participate in lipids mobilization, which are further degraded by β-oxidation. The lipolysis of LDs involves the coordinated response of surface proteins and corresponding enzymes. Intracellular lipolysis can be divided into neutral and acid lipolysis, depending on the pH value and the corresponding subcellular location.

Neutral lipolysis of TG occurs in the cytoplasm with neutral pH, during which the TG and CE stored in LDs are directly degraded by the consecutive action of the three neutral lipases, including patatin-like phospholipase domain-containing protein 2 (PNPLA2), LIPE/HSL (lipase E, hormone-sensitive type), and monoglyceride lipase (MGL) [52]. Studies found that ATGL encoded by the patatin-like phospholipase domain-containing protein 2 (PNPLA2) gene may participate in NAFLD, of which ATGL knockout leads to LDs accumulation, while overexpression of ATGL could alleviate hepatic steatosis and increase FAs oxidation [53, 54]. Recent study identified that despite enhancing hepatic steatosis, ATGL/PNPLA2 deficiency may decrease hepatic lipolysis and increase PPARδ, which protects hepatocyte from inflammation and ER stress [55]. HSL was considered a rate-limiting enzyme for TG hydrolysis. HSL deficiency, characterized by relatively mild forms of dyslipidemia and hepatic steatosis, manifests in a more benign phenotype than does ATGL deficiency [56]. Besides, HSL could bind to CHREBP to prevent its translocation into the nucleus and downregulate its transcriptional activity of CHREBP [57]. MGL is the rate-limiting enzyme of monoacylglycerol (MG) degradation that derives from phospholipids or TG. Study found that MGL deficiency in mice leads to MG accumulation, minor changes of plasma VLDL metabolism, and a moderate protection from diet-induced hepatic steatosis [58]. Besides, MGL inhibition could attenuate LPS-induced

inflammation in liver [59]. Hypoxia training may induce MGL expression and ameliorate hepatic steatosis [60].

Acid lipolysis, mediated by lipases such as the lipase A and lysosomal acid (LIPA/ LAL), occurs in acidic pH inside lysosome, which could hydrolyze lipids delivered into lysosomes through receptor-mediated endocytosis of lipoproteins and lipophagy and further produce free cholesterol and FAs [61]. The reduction of LAL activity causes intra-lysosomal CE accumulation and lowers free cholesterol in cytosol, which could induce transcription factor promoting lipogenesis and synthesis of cholesterol and of VLDL [62]. Clinical study has demonstrated that blood LAL levels at different stages of NAFLD evolution are gradually declined [63]. In addition, impaired LAL activity appears specific to NAFLD in the context of liver disease.
