Lipid-Induced Endoplasmic Reticulum Stress Impairs Selective Autophagy at the Step of Autophagosome-Lysosome Fusion in Hepatocytes

Abstract

Blockage of hepatic autophagic degradation system occurs in obesity and is associated with the development of nonalcoholic fatty liver disease. However, the mechanism of this blockage remains unclear. We found a high-fat diet induced accumulation of autophagosomes in the mice livers. However, autophagy substrates such as p62 and ubiquitinated proteins also accumulated in the livers in this model. These findings indicate the possibility that a high-fat diet impairs autophagic flux in the liver. Then, to assess the autophagic flux in more detail, we performed analyses of autophagic flux in cultured hepatocytes exposed to monounsaturated fatty acids (FAs) or saturated FAs (SFAs). SFAs but not monounsaturated FAs suppressed degradation of contents in the autophagosomes. We analyzed each stage of the autophagy pathway (ie, autophagosome formation, autophagosome-lysosome fusion, lysosomal degradation) in cultured hepatocytes treated with monounsaturated FAs or SFAs and found that SFAs impaired autophagosome-lysosome fusion. This impairment occurred in an endoplasmic reticulum stress-dependent manner. Moreover, ubiquitin and p62-positive inclusions observed in high-fat diet-fed mice livers and SFA-treated cells were sequestered within autophagosomes. We also found that SFA-induced accumulation of Ser351-phosphorylated p62, which is indispensable for selective autophagy, further increased on administration of a lysosomal proteinase inhibitor. Although lipid-induced endoplasmic reticulum stress interferes with the autophagosome-lysosome fusion, selective autophagic sequestration of aggregated proteins is not inhibited.

Macrophage‐Specific Hypoxia‐Inducible Factor‐1α Contributes to Impaired Autophagic Flux in Nonalcoholic Steatohepatitis

“…Using bafilomycin, an inhibitor of late‐stage autophagy, we verified that the PA‐induced LC3‐II accumulation results from decreased autophagic flux rather than activation of autophagosome formation. This finding is consistent with another report demonstrating that high‐fat diet (HFD) and PA impair autophagic flux through lysosomal dysfunction in hepatocytes …”

Peretinoin, an acyclic retinoid, suppresses steatohepatitis and tumorigenesis by activating autophagy in mice fed an atherogenic high-fat diet

“…The suppression of autophagy in HF diet mice has been reported in several studies. [19–21] However, one of the differences of our model from previous reports was the long observation period (68 weeks in Ath+HF diet mice and 22 weeks in STAM™ mice) compared with the relative short observation period of previous studies (from 8 to 16 weeks). The impaired autophagy was restored by the administration of peretinoin (Figure 4 and Supplemental Figure 2), and this finding was more obvious in older mice, which would be associated with the increased autophagy observed in the liver of normal aged mice without tumors.…”

To die or not to die: death signaling in nonalcoholic fatty liver disease

“…To this end, a recent study demonstrated that palmitic acid impaired autophagy flux by preventing the late stages of autophagy. Miyagawa et al demonstrated fusion of autophagosomes and lysosomes in cultured hepatocytes after saturated FFA treatment, independently of lysosomal functions of acidification and hydrolysis [76]. Impairment of autophagic flux induced the accumulation of autophagosomes in palmitic acid-treated but not oleic acid-treated cells.…”