Liver-specific Gene Inactivation of the Transcription Factor ATF4 Alleviates Alcoholic Liver Steatosis in Mice

Abstract

Although numerous biological functions of the activating transcription factor 4 (ATF4) have been identified, a direct effect of ATF4 on alcoholic liver steatosis has not been described previously. The aim of our current study is to investigate the role of ATF4 in alcoholic liver steatosis and elucidate the underlying mechanisms. Here, we showed that the expression of ATF4 is induced by ethanol in hepatocytes in vitro and in vivo, and liverspecific ATF4 knock-out mice are resistant to ethanol-induced liver steatosis, associated with stimulated hepatic AMP-activated protein kinase (AMPK) activity. Furthermore, adenovirus-mediated AMPK knockdown significantly reversed the suppressive effects of ATF4 deficiency on ethanol-induced liver steatosis in mice. In addition, ethanol-fed ATF4 knock-out mice exhibit AMPK-dependent inhibition of fatty acid synthase and stimulation of carnitine palmitoyltransferase 1 (CPT1) in the liver. Moreover, hepatic Tribbles homolog 3 (TRB3) expression was stimulated by ethanol in an ATF4-dependent manner, and adenovirus-mediated TRB3 knockdown blocked ATF4-dependent ethanol-induced AMPK inhibition and triglyceride accumulation in AML-12 cells. Finally, TRB3 directly interacted with AMPK to suppress its phosphorylation. Taken together, these results identify the ATF4-TRB3-AMPK axis as a novel pathway responsible for ethanol-induced liver steatosis.People consuming excessive amounts of alcohol for prolonged periods suffer from alcoholic liver disease, which encompasses fatty liver, hepatic inflammation and necrosis, progressive fibrosis, and hepatocellular carcinoma (1). Alcoholic fatty liver is the early stage of alcoholic liver disease, characterized by increased accumulation of fat in the liver, which is usually caused by increased free fatty acid (FFA) 4 uptake, augmented de novo lipogenesis, decreased ␤-oxidation, and/or impaired triglyceride (TG) export (2). Studies have identified some important regulators for ethanol-induced liver steatosis, including sirtuin 1 (SIRT1) (3), sterol regulatory element-binding protein 1 (SREBP-1) (4), peroxisome proliferator-activated receptor ␥ coactivator 1-␣ (PGC-1␣) (5), and 5Ј-adenosine monophosphate-activated protein kinase (AMPK) (6 -8).Among them, AMPK is one of the most important regulators (6 -8). AMPK is a heterotrimeric complex consisting of a catalytic ␣ subunit and two regulatory ␤/␥ subunits. It acts as a key metabolic "switch" by phosphorylating target enzymes involved in lipid metabolism, such as acetyl-CoA carboxylase (ACC) (9). Malonyl-CoA, the product of ACC, is both a precursor for fatty acid biosynthesis and a potent inhibitor of ␤-oxidation at the step regulated by carnitine palmitoyltransferase I (10). AMPK activity is suppressed under ethanol exposure (6) and activation of AMPK by its activator (5-aminoimidazole-4-carboxamide 1-␤-D-ribofuranoside) or metformin prevents ethanol-induced liver steatosis (11, 12). However, the molecular mechanisms underlying ethanol suppression of AMPK activity remain largely unknown. Activating...

Fetal alcohol spectrum disorder predisposes to metabolic abnormalities in adulthood

“…In addition to cellular stress response genes, there was an upregulation of retinol binding protein 4 (rbp4) (P = 0.009, 1% EtOH), a circulating factor that regulates glucose and lipid metabolism, in both 0.5% and 1% EtOH adults ( Figure 7J, Supplemental Tables 13 and 16, refs. [55][56][57]. Additional significantly dysregulated genes included those critical for immune function (mhc1uba, P = 6.71 × 10 -11 , 1% EtOH), glycogen storage (gyg1a, P × 10 -07 , 1% EtOH), and iron homeostasis (fth1a, slc40a1, tfa, hamp) ( Figure 7J and Supplemental Tables 13 and 16).…”

Fat Mass and Obesity Associated (FTO) Gene and Hepatic Glucose and Lipid Metabolism

“…Interestingly, ATF4 deficiency or ATF4 knockdown can also protect the liver from diet-induced and ethanol-induced steatosis in mice [44,46,56,57]. Expression of lipogenic enzyme-encoding genes and production of very low-density lipoprotein (VLDL)-triglyceride is reduced in the liver of ATF4-deficient mice [47].…”

Preemptive Activation of the Integrated Stress Response Protects Mice From Diet‐Induced Obesity and Insulin Resistance by Fibroblast Growth Factor 21 Induction

“…Several studies suggested that ISR activation contribute to pathogenesis of liver steatosis (LS). ATF4 has been shown to activate lipogenesis in hepatocytes (15)(16)(17) and increase the expression of hepatic very-low-density lipoprotein receptor, which contribute to worsened hepatic steatosis (HS). (18) In line with this, ISR inhibition by transgenic overexpression of GADD34 in the liver suppressed lipogenesis and protected mice from HS.…”