Inhibited Lipophagy Suppresses Lipid Metabolism in Zebrafish Liver Cells

Abstract

Lipophagy degrades lipid droplets (LDs) through the lysosomal degradative pathway, thus plays important roles in regulating lipid metabolism in mammals. However, information on the existence and functions of lipophagy in fish lipid metabolism is still limited. In the present study, we confirmed the existence of lipophagy by observing the structures of LDs sequestered in autophagic vacuoles in the zebrafish liver cell line (ZFL) via electronic microscopy. Moreover, starved cells increased the mRNA expression of the microtubule-associated protein 1A/1B light chain 3 beta ( LC3 ), which is a marker protein for autophagy and protein conversion from LC3-I to LC3-II. Inhibiting autophagy with chloroquine increased significantly the LDs content and decreased fatty acid β-oxidation and esterification activities in the ZFL cells cultured in the fed state. Furthermore, inhibiting autophagy function downregulated the mRNA expression of the genes and their proteins related to lipid metabolism. Altogether, the present study verified the existence of lipophagy and its essential regulatory roles in lipid metabolism in fish cells.

Modulation of Autophagy: A Novel “Rejuvenation” Strategy for the Aging Liver

“…Subsequently, the role of lipophagy in promoting lipid metabolism was also confirmed in zebrafish liver cells. Wang et al [ 159 ] observed the sequestered LDs in autophagic vacuoles of zebrafish liver cells by electron microscopy, thus confirming the occurrence of lipophagy. Inhibition of autophagy by chloroquine, a lysosomal acidification inhibitor that blocks the fusion of autophagosomes and lysosomes [ 160 ], significantly increased the LDs and TG content of the liver cells.…”

Prostaglandin 2α Promotes Autophagy and Mitochondrial Energy Production in Fish Hepatocytes

“…As such, sufficient energy availability in the environment results in the formation of LDs, whereas energy utilization by organisms leads to the breakdown of LDs through lipolysis or/and autophagy/lipophagy [ 40 ]. Liberated fatty acids are transported to the mitochondria for β-oxidation, the tricarboxylic acid cycle, and the mitochondrial respiratory chain, which finally produce ATP [ 40 , 41 ]. Our finding that serum starvation reduced LD levels is consistent with previous reports in mouse hepatocytes and ZFL cells [ 14 , 16 , 41 ], and this reduction is attributed to increased lipid catabolism and ATP production.…”

“…Liberated fatty acids are transported to the mitochondria for β-oxidation, the tricarboxylic acid cycle, and the mitochondrial respiratory chain, which finally produce ATP [ 40 , 41 ]. Our finding that serum starvation reduced LD levels is consistent with previous reports in mouse hepatocytes and ZFL cells [ 14 , 16 , 41 ], and this reduction is attributed to increased lipid catabolism and ATP production. Indeed, in the present study, the expression of autophagic and lipolytic marker genes or proteins, the level of ATP, and the expression of mitochondrial genes were increased ( Figure 1 ), demonstrating that serum deprivation is a universal measure for studying the mechanism of LD degradation.…”

Involvement of Autophagy in Ageing and Chronic Cholestatic Diseases

“…This further impairs autophagic activity, since it appears to prevent autophagosome acidification and to reduce the expression of proteolytic enzymes, as reported in a mouse model of genetically induced obesity [156]. The age-related reduction in autophagy efficiency also affects lipophagy, which plays an important role in lipid metabolism [157,158]. The decline of lipophagy in the aged or steatotic liver slows down the breakdown of lipids accumulated in the liver and results in a lower FFA intake for lipid metabolism, compounding the cell function impairment [38,159].…”