**3.1** *De novo* **lipogenesis**

Except uptake of circulating FAs, DNL enables liver to synthesize FAs by using non-lipid precursors (such as sugars and proteins) [31]. The production of acetyl-CoA initially provides the substrate required for DNL, which is converted to malonyl-CoA by acetyl-CoA carboxylase (ACC) and malonyl-CoA is then converted to palmitate by fatty acid synthase (FASN) [32]. The transcriptional regulation of DNL is mainly regulated by two transcription factors: sterol regulatory elementbinding protein 1c (SREBP1c) and carbohydrate regulatory element-binding protein (ChREBP), which both stimulated by the activation of LXR and by nuclear translocation to activate target gene transcription [33, 34]. Studies identified that in NAFLD, SREBP1c expression is elevated, which is in agreement with hepatic TG levels, while SREBP1c knockout decreases the expression of lipogenic enzymes [33]. In addition, SREBP1c induces lipogenesis elevation and harmful lipid species accumulation, which might interfere with insulin signaling and therefore indirectly leads to the development of hepatic insulin resistance. When SREBP1c elevated, the expression of downstream targets ACC and FASN is accordingly increased in NAFLD [33, 35]. Although study demonstrated that knockout ACC1 could decrease hepatic lipid accumulation and inhibit DNL process, it may reactivity increase the expression of ACC2, which inhibits mitochondrial β-oxidation and leads to hepatic steatosis [36]. ACC1/2 inhibition could be a new option to improve hepatic steatosis in NAFLD [33]. As for another transcription factors, ChREBP participates in fructolysis, glycolysis, gluconeogenesis, and DNL pathways could mediate carbohydrate-associated DNL rather than induced by HFD [37]. Study found that high-fructose feeding could increase hepatocellular carbohydrate metabolites, expression of ChREBP target genes, and hepatic steatosis [38]. Inhibiting ChREBP expression could downregulate glucose-induced lipogenesis, which further reduces hepatic TG content and protect

steatosis [39]. On the other hand, ChREBP knockout enhances cholesterol synthesis and its lipotoxicity and therefore induces hepatic steatosis, which may indicate the hepatoprotective effect of ChREBP [40].

Synthesized FAs may undergo a series of steps including desaturation, elongation, and esterification, and therefore ultimately being stored as a form of TG or exported in the form of VLDL particles. DNL was independently associated with intrahepatic TG levels [29]. Abnormally elevated DNL, occurred in NAFLD, could lead to the excessive production of saturated FAs (like palmitate), which induces steatohepatitis [41]. Besides, elevated DNL might also lead malonyl-CoA to inhibit the activity of carnitine palmitoyl transferase 1 (CPT1) suppressing hepatic FAs oxidation and increase ceramide synthesis from palmitoyl-CoA causing mitochondrial dysfunction, oxidative stress, and cell death. The above three effects may indirectly induce intrahepatic TG accumulation. Therefore, inadequate suppression of DNL is a feature of liver lipid accumulation in NAFLD.
