Impaired autophagic flux is associated with increased endoplasmic reticulum stress during the development of NAFLD

Abstract

The pathogenic mechanisms underlying the progression of non-alcoholic fatty liver disease (NAFLD) are not fully understood. In this study, we aimed to assess the relationship between endoplasmic reticulum (ER) stress and autophagy in human and mouse hepatocytes during NAFLD. ER stress and autophagy markers were analyzed in livers from patients with biopsy-proven non-alcoholic steatosis (NAS) or non-alcoholic steatohepatitis (NASH) compared with livers from subjects with histologically normal liver, in livers from mice fed with chow diet (CHD) compared with mice fed with high fat diet (HFD) or methionine-choline-deficient (MCD) diet and in primary and Huh7 human hepatocytes loaded with palmitic acid (PA). In NASH patients, significant increases in hepatic messenger RNA levels of markers of ER stress (activating transcription factor 4 (ATF4), glucose-regulated protein 78 (GRP78) and C/EBP homologous protein (CHOP)) and autophagy (BCN1) were found compared with NAS patients. Likewise, protein levels of GRP78, CHOP and p62/SQSTM1 (p62) autophagic substrate were significantly elevated in NASH compared with NAS patients. In livers from mice fed with HFD or MCD, ER stress-mediated signaling was parallel to the blockade of the autophagic flux assessed by increases in p62, microtubule-associated protein 2 light chain 3 (LC3-II)/LC3-I ratio and accumulation of autophagosomes compared with CHD fed mice. In Huh7 hepatic cells, treatment with PA for 8 h triggered activation of both unfolding protein response and the autophagic flux. Conversely, prolonged treatment with PA (24 h) induced ER stress and cell death together with a blockade of the autophagic flux. Under these conditions, cotreatment with rapamycin or CHOP silencing ameliorated these effects and decreased apoptosis. Our results demonstrated that the autophagic flux is impaired in the liver from both NAFLD patients and murine models of NAFLD, as well as in lipid-overloaded human hepatocytes, and it could be due to elevated ER stress leading to apoptosis. Consequently, therapies aimed to restore the autophagic flux might attenuate or prevent the progression of NAFLD.

Protective effect of quercetin on high-fat diet-induced non-alcoholic fatty liver disease in mice is mediated by modulating intestinal microbiota imbalance and related gut-liver axis activation

“…It has already been reported that HFD feeding induced the activation of the unfolded protein response (UPR) related to reticulum stress [79]. In fact, in the present study HFD increased the gene expression of the ER stress markers GRP78 and CHOP, similarly to findings shown in patients whith NAFLD [80]. Quercetin supplementation to HFD-fed mice reverted gene expression deregulation associated to this stress pathway.…”

“…The beneficial effect of quercetin on reticulum stress could be exerted by a complex mechanism underlying its modulatory effect on dysbiosis-mediated gut-liver axis activation and also by counteracting FFA and CYP2E1-dependent lipotoxicity as a result of its antioxidant activity, as previouly suggested [81]. The blockage of reticulum stress by quercetin could be relevant in the progression of steatosis to steatohepatitis, given its central role in the pathogenesis of NAFLD [80].…”

The role of ER stress in lipid metabolism and lipotoxicity

“…In livers from patients with nonalcoholic fatty liver disease or nonalcoholic steatohepatitis, there was increased ER stress gene expression with decreased autophagic flux (112). Consistently, palmitate treatment in Huh7 cells increased ER stress associated with decreased autophagy, which was restored with an autophagy inducer, rapamycin (112).…”

Augmenter of liver regeneration protects against carbon tetrachloride-induced liver injury by promoting the autophagy in mice

“…Densitometry analysis of the proteins was performed for each sample. www.impactjournals.com/oncotarget (PI3K) inhibitor, was used to block autophagy because the complex of Beclin-1 and PI3K is required for autophagic vesicle nucleation [17]. As reported in a previous study [18][19][20][21], less apoptotic cells were observed in mice treated with CCl 4 and ALR than those treated with CCl 4 only, which suggested that ALR inhibits hepatocyte apoptosis.…”